Tin Chemistry: Fundamentals, Frontiers and Applications

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Author: Marcel Gielen | Alwyn G. Davies | Keith Pannell | Edward Tiekink

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Tin Chemistry

Tin Chemistry: Fundamentals, Frontiers, and Applications Edited by Marcel Gielen, Alwyn Davies, Keith Pannell and Edward Tiekink © 2008 John Wiley & Sons, Ltd. ISBN: 978-0-470-51771-0

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Tin Chemistry Fundamentals, Frontiers, and Applications

Edited by

ALWYN G. DAVIES University College London, UK

MARCEL GIELEN Free University of Brussels VUB, Belgium

KEITH H. PANNELL University of Texas at El Paso, USA

EDWARD R. T. TIEKINK University of Texas at San Antonio, USA

A John Wiley and Sons, Ltd, Publication

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This edition first published 2008 C 2008 John Wiley & Sons, Ltd Registered office John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, United Kingdom For details of our global editorial offices, for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at www.wiley.com The right of the author to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books. Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The publisher is not associated with any product or vendor mentioned in this book. This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought. The Publisher and the Author make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of fitness for a particular purpose. The advice and strategies contained herein may not be suitable for every situation. In view of ongoing research, equipment modifications, changes in governmental regulations, and the constant flow of information relating to the use of experimental reagents, equipment, and devices, the reader is urged to review and evaluate the information provided in the package insert or instructions for each chemical, piece of equipment, reagent, or device for, among other things, any changes in the instructions or indication of usage and for added warnings and precautions. The fact that an organization or Website is referred to in this work as a citation and/or a potential source of further information does not mean that the author or the publisher endorses the information the organization or Website may provide or recommendations it may make. Further, readers should be aware that Internet Websites listed in this work may have changed or disappeared between when this work was written and when it is read. No warranty may be created or extended by any promotional statements for this work. Neither the Publisher nor the Author shall be liable for any damages arising herefrom. Library of Congress Cataloging-in-Publication Data Tin chemistry : fundamentals, frontiers, and applications / edited by Marcel Gielen . . . [et al.]. p. cm. Includes bibliographical references and index. ISBN 978-0-470-51771-0 (cloth : alk. paper) 1. Organotin compounds. I. Gielen, M. (Marcel), 1938– QD412.S7T56 2008 546 .686–dc22 2008010997 British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. ISBN 9780470517710 Typeset in 10/12pt Times by Aptara, New Delhi, India Printed in Great Britain by Antony Rowe Ltd, Chippenham, Wiltshire

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Dedicated to the memory of Des Cunningham, 1942–2006

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Preface List of Contributors 1

2

Introduction and Overview 1.1 Introduction Alwyn G. Davies 1.1.1 History, Occurrence, Production, and Applications 1.1.2 The Element 1.1.3 Structure and Bonding 1.1.4 Organotin Compounds, Rn Snm 1.1.5 Organotin Compounds With Electronegative Ligands, Rn SnX4-n 1.1.6 Preparation of Organotin(IV) Compounds 1.1.7 Preparation of Organotin Compounds in Lower Valence States 1.1.8 Literature References Fundamentals in Tin Chemistry 2.1 NMR Spectroscopy of Tin Compounds Bernd Wrackmeyer 2.1.1 Introduction 2.1.2 Experimental 2.1.3 Nuclear Spin Relaxation 2.1.4 Chemical Shifts δ 119 Sn 2.1.5 Indirect Nuclear Spin–Spin Coupling Constants n J (119 Sn,X) References 2.2 Tin(II) Clusters Tristram Chivers and Dana J. Eisler 2.2.1 Introduction

xvii xix 1 1 1 3 4 5 9 10 13 13 14 17 17 17 17 20 21 39 45 53 53

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2.3

2.4

2.5

2.6

2.7

2.2.2 Imidotin Cubane Clusters 2.2.3 The Seco-Cubane Sn3 (μ3 -NtBu)(μ-NtBu)(μ-NHtBu)2 2.2.4 Double-Cubane Clusters 2.2.5 Imidotin Chalcogenides 2.2.6 Phosphido– and Arsenido–Tin Clusters 2.2.7 Summary and Future Prospects References Mono-Organotin Oxo-Clusters Franc¸ois Ribot 2.3.1 Introduction 2.3.2 Molecular Structures 2.3.3 Syntheses and Formation Mechanisms 2.3.4 Conclusions Acknowledgments References Organotin Carboxylate and Sulfonate Clusters Vadapalli Chandrasekhar, Puja Singh, and Kandasamy Gopal 2.4.1 Introduction 2.4.2 Organotin Carboxylates 2.4.3 Organotin Sulfonates 2.4.4 Conclusion Acknowledgments References Macrocyclic and Supramolecular Chemistry of Organotin(IV) Compounds Herbert H¨opfl 2.5.1 Introduction 2.5.2 Metallosupramolecular Chemistry with Tin 2.5.3 Formation of Tin Macrocycles and Extended Networks 2.5.4 Conclusions and Perspectives Acknowledgments References Deltahedral Zintl Ions of Tin: Synthesis, Structure, and Reactivity Slavi C. Sevov 2.6.1 Introduction 2.6.2 Background 2.6.3 Geometry, Charge, Electron Count, and Electronic Structure 2.6.4 Reactions With Nine-Atom Deltahedral Zintl Anions of Tin 2.6.5 Solution Studies by NMR 2.6.6 Concluding Remarks Acknowledgments References Stable Stannylium Cations in Condensed Phases Joseph B. Lambert 2.7.1 Introduction 2.7.2 Pentacoordination (Trigonal Pyramids) 2.7.3 Tetracoordination: Binding with Solvent or Anion

53 60 61 62 65 66 66 69 69 70 78 89 90 90 93 93 93 105 113 114 114 117 117 117 120 133 133 133 138 138 138 140 143 148 150 150 150 152 152 154 155

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2.8

2.9

2.10

2.11

2.12

2.7.4 Tricoordination: NMR Evidence for Free Stannylium Ions 2.7.5 Tricoordination: Crystallographic Evidence and Computational Confirmation of Free Stannylium Ions 2.7.6 Summary References Preparation and Coordination Chemistry of Mono- and Bidentate Benzannulated N-Heterocyclic Stannylenes Including Some Germanium and Lead Analogs Alexander V. Zabula and F. Ekkehardt Hahn 2.8.1 Introduction 2.8.2 Stable N-Heterocyclic Stannylenes, Germylenes, and Plumbylenes 2.8.3 Complexes of Bidentate N-Heterocyclic Germylenes and Stannylenes References Stannenes, Distannenes, and Stannynes Yoshiyuki Mizuhata and Norihiro Tokitoh 2.9.1 Introduction 2.9.2 Distannenes (Sn Sn) 2.9.3 Stannenes (Sn C) 2.9.4 Silastannenes (Si Sn) and Germastannenes (Ge Sn) 2.9.5 Stannynes (Sn C) 2.9.6 Distannynes (Sn Sn) and Their Reduced Species References Tetraorganodistannoxanes: Simple Chemistry From a Personal Perspective Klaus Jurkschat 2.10.1 Introduction 2.10.2 Unsymmetrically Substituted Tetraorganodistannoxanes 2.10.3 Looking for the Third Dimension 2.10.4 Variation of R 2.10.5 Variation of Spacer Z 2.10.6 Variation of the Electronegative Substituents X and Y 2.10.7 Miscellaneous Acknowledgments References Unusual Bonds and Coordination Geometries M´onica Moya-Cabrera, Vojtech Jancik and Raymundo Cea-Olivares 2.11.1 Introduction 2.11.2 Unusual Bonds 2.11.3 Unusual Coordination Geometries References Tin(II) Heterobimetallic and Oligometallic Derivatives Muhammad Mazhar and Imtiaz-ud-Din 2.12.1 Introduction 2.12.2 General Synthetic Procedures 2.12.3 Characterization Techniques 2.12.4 Stoichiometric and Structural Aspects of Tin(II) Heterobimetallic and Oligometallic Compounds 2.12.5 Conclusion References

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156 157 158 159 160 160 160 166 174 177 177 177 183 193 195 196 198 201 201 202 209 212 212 221 221 229 229 231 231 231 241 247 251 251 254 256 258 267 267

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2.13 Computational Methods for Organotin Compounds Sarah R. Whittleton, Russell J. Boyd, and T. Bruce Grindley 2.13.1 Introduction 2.13.2 Relativistic Effects in Heavy Elements 2.13.3 Effective Core Potentials 2.13.4 Other Computational Methods Available for Tin 2.13.5 Current State of Computational Organotin Chemistry 2.13.6 Structure Prediction 2.13.7 Reaction Pathways and Mechanisms 2.13.8 Thermochemistry 2.13.9 Bond Strengths and Bond Dissociation Enthalpies 2.13.10 Spectroscopic and Related Properties 2.13.11 Conclusion Acknowledgments References 3

Materials Chemistry and Structural Chemistry of Tin Compounds 3.1 Tin Compounds For CVD (Chemical Vapor Deposition) Geraldo M. de Lima 3.1.1 Introduction 3.1.2 General Aspects of CVD 3.1.3 Organometallic Chemical Vapor Deposition (MOCVD) 3.1.4 Tin Compounds For CVD 3.1.5 Tin(IV) Oxide and Related Materials 3.1.6 Tin Sulfides 3.1.7 Tin(II) Selenide and Telluride 3.1.8 Tin (IV) Phosphide 3.1.9 Tin Alloys 3.1.10 Summary References 3.2 Class II Tin-Based Hybrid Materials Prepared From Alkynyltin Precursors Thierry Toupance 3.2.1 Introduction 3.2.2 Functionalization of Oxide Surfaces 3.2.3 Self-Assembled Tin-Based Hybrid Materials 3.2.4 Nanoporous Nanostructured Tin Dioxide Materials 3.2.5 Conclusion Acknowledgments References 3.3 Organotin Compounds as PVC Stabilizers Esen Arkis 3.3.1 Introduction 3.3.2 Types of Organotin Stabilizers 3.3.3 Tin Carboxylates 3.3.4 Tin Mercaptides 3.3.5 The Mechanism of Stabilization

269 269 269 270 271 272 272 274 276 276 277 278 278 279 285 285 285 285 287 287 287 290 291 292 292 292 293 296 296 297 301 306 309 310 310 312 312 313 314 315 315

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3.4

3.5

3.6

3.7

3.8

3.3.6 Operational Considerations 3.3.7 Evaluating Stability 3.3.8 Conclusion References Organotin Compounds as Anion-Selective Carriers in Chemical Sensors Nikos Chaniotakis 3.4.1 Introduction to Chemical Sensors 3.4.2 Potentiometric Ion Selective Electrodes (ISEs) 3.4.3 The Ionophores 3.4.4 Organotin-Mediated Anion Partitioning into Liquid Polymeric Membranes 3.4.5 Anion Selective Organotin-based ISEs 3.4.6 Conclusions References Tin Compounds as Flame Retardants and Smoke Suppressants Paul A. Cusack 3.5.1 Introduction 3.5.2 Tin Treatments For Fibers 3.5.3 Zinc Stannates 3.5.4 Recent Developments 3.5.5 Fire-Retardant Mechanism 3.5.6 Summary References Quadratic Non-Linear Optical Properties of Tin-Based Coordination Compounds Pascal G. Lacroix and Norberto Farf´an 3.6.1 Introduction 3.6.2 Basic Concepts of Quadratic Non-Linear Optics 3.6.3 Tin-Based Materials in Quadratic Non-Linear Optics 3.6.4 Concluding Remarks Acknowledgments References Monoorganotin Precursors For Hybrid Materials Bernard Jousseaume 3.7.1 Introduction 3.7.2 Functional Trialkynylorganotins 3.7.3 Bridged Ditins 3.7.4 Conclusion Acknowledgments References Organotin Polymers and Related Materials Hemant K. Sharma and Keith H. Pannell 3.8.1 Introduction 3.8.2 Synthesis of Linear Oligostannanes 3.8.3 Synthesis of Polystannanes 3.8.4 Properties of Polystannanes 3.8.5 Polymers with Tin in the Backbone

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317 320 322 323 324 324 324 326 326 329 336 337 339 339 339 340 343 346 348 348 351 351 351 352 358 359 359 361 361 361 368 374 374 374 376 376 376 378 381 386

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3.8.6 Polymers with Tin as a Pendant Group Acknowledgments References 3.9 Intermolecular Tin . . . π-Aryl Interactions: fact or artifact? A New Bonding Motif For Supramolecular Self-Assembly in Organotin Compounds Ionel Haiduc, Edward R. T. Tiekink, and Julio Zukerman-Schpector 3.9.1 Introduction 3.9.2 Discussion 3.9.3 Conclusions and Outlook Acknowledgments References

388 389 389

Medicinal/Biocidal Applications of Tin Compounds and Environmental Aspects 4.1 The Cardiovascular Activity of Organotin Compounds Mala Nath 4.1.1 Introduction 4.1.2 Cardiovascular Activity of Organotin Compounds 4.1.3 Conclusion References 4.2 Organotins: Insecticidal/Larvicidal Activities and Quantitative Structure–Activity Relationships George Eng and Xueqing Song 4.2.1 Overview: Organotins 4.2.2 Larvicidal/Insecticidal Activities 4.2.3 Quantitative Structure–Activity Relationships 4.2.4 Food For Thought References 4.3 Anti-Fungal Activity of Organotin Compounds Heloisa Beraldo and Geraldo M. de Lima 4.3.1 Introduction 4.3.2 Biological Applications of Organotin Compounds 4.3.3 Fungi and Fungal Infections 4.3.4 Mechanisms of Biological Action of Organotin Compounds 4.3.5 Structure–Activity Relationships 4.3.6 Anti-Fungal Screening 4.3.7 Conclusions References 4.4 Chemical and Biotechnological Developments in Organotin Cancer Chemotherapy Claudio Pettinari and Fabio Marchetti 4.4.1 Introduction 4.4.2 Developments in the Design of Organotin Anti-Cancer Compounds 4.4.3 Conclusion References 4.5 Impact of Organotin Compounds on the Function of Human Natural Killer Cells Margaret M. Whalen 4.5.1 Introduction

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. Beltr´an, Rosa Santillan and Norberto Farf´an 4.6.1 Introduction 4.6.2 Interaction of Organotin Compounds with Biological Systems and Mimic Xenobiotics 4.6.3 Interaction of Organotin Compounds in Real and Model Membranes 4.6.4 Roles of Organotin Compounds in Cell Function 4.6.5 Aspects of Organotins in Structural and Molecular Biology 4.6.6 Perspectives Acknowledgments References 5

Tin in Organic Synthesis 5.1 Applications of Organotin Derivatives for Carbohydrate Synthesis T. Bruce Grindley 5.1.1 Introduction 5.1.2 Preparation 5.1.3 Structures 5.1.4 Reactions References 5.2 Reactions of SE Substitution for Organostannanes in Organic Synthesis David R. Williams, and Partha P. Nag 5.2.1 Introduction 5.2.2 Mechanistic Considerations and a Predictive Model for Reactions with Aldehydes 5.2.3 Allylation Reactions of Substrate Control 5.2.4 Allylation Reactions Exhibiting α-Chelation Control 5.2.5 Allylation Reactions Exhibiting β-Chelation Control 5.2.6 Reactions of γ -(Alkoxy)allylstannanes 5.2.7 Reactions of Oxocarbenium Cations 5.2.8 Reactions of N-Acyliminium Cations 5.2.9 Reactions with α, β-Unsaturated Carbonyl Compounds 5.2.10 Reactions of Allylic Stannanes with Imines 5.2.11 Transmetalation Reactions of Allylic Stannanes 5.2.12 Reactions of Chiral Allylic Stannanes 5.2.13 Reactions of Allenylstannanes 5.2.14 Transmetalation Reactions of Allenylstannanes

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470 472 474 475 475 476 477 479 482 482 482 484 488 491 492 492 492 497 497 497 498 500 504 512 515 515 516 519 521 524 526 527 531 532 533 535 541 543 547

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5.3

5.4

5.5

5.6

5.2.15 Reactions of Propargylic Stannanes 5.2.16 Enantioselective Reactions with Chiral Lewis Acids 5.2.17 Conclusion and Future Outlook References Cross-Coupling of Organotin Compounds for Carbon Carbon Bond Formation Pablo Espinet and Miroslav Genov 5.3.1 Introduction 5.3.2 Mechanistic Aspects and Consequences 5.3.3 Catalysts and Ligands 5.3.4 Ligandless Coupling 5.3.5 Copper Effect 5.3.6 Microwave-Assisted Reactions 5.3.7 Natural Product Synthesis 5.3.8 Conclusion References Stille Cross-Coupling for the Synthesis of Natural Products Sergio Pascual and Antonio M. Echavarren 5.4.1 Introduction 5.4.2 Alkenyl–Alkenyl Stille Coupling 5.4.3 Alkenyl–Alkynyl Stille Coupling 5.4.4 Alkenyl–Aryl Stille Coupling 5.4.5 Aryl–Aryl Stille Coupling 5.4.6 sp3 –sp2 Coupling Reactions 5.4.7 Couplings for the Synthesis of Ketones 5.4.8 Summary and Outlook References New Trends in the Synthesis of Solid-Supported Organotin Reagents and Interest of their Use in Organic Synthesis in a Concept of Green Chemistry Jean-Mathieu Chr´etien, Jeremy D. Kilburn, Franc¸oise Zammattio, Erwan Le Grognec, and Jean-Paul Quintard 5.5.1 Introduction 5.5.2 Removal of Tin Residues by Partition between Two Phases 5.5.3 Solid-Supported Organotin Reagents 5.5.4 Use of Supported Organotins in Organic Synthesis 5.5.5 Perspectives and Conclusions References Palladium-Catalyzed Cascade Cyclization-Anion Capture Processes Employing Pre- and In Situ-Formed Organostannanes Ron Grigg and Visuvanathar Sridharan 5.6.1 Introduction 5.6.2 Mono-Cyclization–Anion Capture Processes 5.6.3 Bis-Cyclization–Anion Capture Processes 5.6.4 Cyclization–Anion Capture Processes Involving Relay Switches 5.6.5 Summary References

550 552 555 556 561 561 561 564 570 571 572 574 575 575 579 579 579 587 587 593 594 599 600 602 607 607 607 608 608 613 617 618 622 622 623 630 634 637 638

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5.7 Carbostannylation of Carbon Carbon Unsaturated Bonds Eiji Shirakawa 5.7.1 Introduction 5.7.3 Carbostannylation of Alkynes 5.7.4 Carbostannylation of Alkenes 5.7.5 Carbostannylation of Dienes 5.7.6 Conclusion References 5.8 Green Organotin Chemistry: an Oxymoron? David Young 5.8.1 Introduction 5.8.2 How Hazardous Are Organostannanes? 5.8.3 Removing Organotin By-Products 5.8.4 Modified Organotin Reagent for Easy Separation 5.8.5 Solid Phase Tin Reagents 5.8.6 Less Toxic Organotin Reagents 5.8.7 No-Tin Reagents 5.8.8 Conclusion References 6

Tin in Catalysis 6.1 Green Organotin Catalysts Junzo Otera, Monique Biesemans, Vanja Pinoie, Kevin Poelmans, and Rudolph Willem 6.1.1 Introduction 6.1.2 Fluorous Distannoxane Catalysts 6.1.3 Grafted Organotin Catalysts 6.1.4 Conclusion References 6.2 Organotin Catalysts for Isocyanate Reactions Werner J. Blank and Edward T. Hessell 6.2.1 Introduction 6.2.2 Mechanism of Urethane Catalysis by Tin Compounds 6.2.3 Structure of the Tin Catalyst 6.2.4 Mechanisms 6.2.5 Synergism of Tin Compounds with Amine Catalysts: DBDTL and 1,4-Diazabicyclo[2.2.2]octane 6.2.6 Mechanism of Catalysis with Blocked Isocyanates 6.2.7 Organotin Catalyst Composition 6.2.8 Catalysis with Organotin Compounds 6.2.9 Applications 6.2.10 Blocked Isocyanates 6.2.11 Catalyst Interactions 6.2.12 Polymer Synthesis 6.2.13 Organotin Replacements References

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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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Preface

The chemistry of tin has now grown to the stage where a dedicated monograph can do little more than trace the structure of the subject and provide a guide to the literature. We felt that authoritative, in-depth, reviews were now needed of those branches, both pure and applied, where developments have been most pronounced, and the present volume is the result. Each chapter, by specialists in the field, deals with one important aspect of tin chemistry, and gives a detailed account of its present standing. Both inorganic and organic aspects are covered, though progress has been most extensive in organotin chemistry which is living up to its reputation of being studied by more techniques, and finding more applications, than the organic derivatives of any other metal. Advances in techniques include sophisticated NMR methods, both in solution and the solid-state, and computational methods, and these, coupled with X-ray diffraction and other established methods, have been applied to the study of a wide variety of structures. Topics covered in the book include Sn(II) clusters, tin Zintl ions, Sn(II) heterobimetallic compounds, R3 Sn+ cations, stannylenes (R2 Sn:), stannenes (R2 Sn SnR2 and R2 Sn CR2 ), stannynes (RSn SnR), organotin oxide, carboxylate and sulfonate clusters, dendrimers and macrocycles, organotin polymers, Sn-π interactions, unusual bondings and structures, and compounds with non-linear optical properties. Non-metallurgical uses of tin reflect the biological activity of organotin compounds, and the nontoxicity of inorganic tin. Inorganic tin compounds are used in flame-retardants and smoke suppressants, and SnO2 for coating glass (though usually deposited from organotin compounds). The principal use for organotin compounds is still as a stabiliser for PVC, and a small but important application is as ionophores in sensors, and as precursors for hybrid organic-inorganic nanometric materials. The use as marine antifoulants is being phased out because of its effect on other marine life, but organotin compounds are showing promise as larvicides, insecticides, and fungicides, and, particularly, in cancer therapy. Organotin compounds find wide applications in organic synthesis, and their dominance in some homolytic mechanisms has been referred to as the tyranny of tin. In particular, tin hydrides still hold the field in ring-closing cyclisations. The established use of organotin compounds as reactants has been developed in carbon-carbon crosslinking catalysed by transition metals, in carbohydrate synthesis, in the conjugative electrophilic substitution of allylstannanes, and in the of the reaction of allyltin compounds and organotin phenoxides catalysed by lithium perchlorate. They are used as catalysts in a number of reactions,

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particularly esterification and transesterification. Relatively new applications are in carbostannylation of multiple bonds, and in the anionic capture of the intermediates from palladium-catalysed ring-closing reactions. At the same time, there can be concerns about the disposal of organotin residues, and of traces of toxic organotin residues remaining in the products, and fluorous, polymer-bound, and solid-supported organotin compounds are being increasingly used to avoid the problem. We hope that the chapters of this book will enable readers to keep abreast of these rapidly developing fields. A. G. Davies, M. Gielen, K. H. Pannell, E. R. T. Tiekink

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List of Contributors

Esen Arkis

Izmir Department Institute of Technology, Chemical Engineering Department, Turkey

Hiram I. Beltr´an Departamento de Ciencias Naturales, Divisi´on de Ciencias Naturales e Ingenier´ıa, Universidad Aut´onoma Metropolitana – Cuajimalpa, Mexico Heloisa Beraldo Inorganic Medicinal Chemistry Laboratory, Departamento de Qu´ımica – Universidade Federal de Minas Gerais, Brazil Monique Biesemans

High Resolution NMR Centre (HNMR), Vrije Universiteit Brussels, Belgium

Werner J. Blank 89 Spectacle Lane, Wilton, CT, USA Russell J. Boyd Department of Chemistry, Dalhousie University, Halifax, Canada Raymundo Cea-Olivares

Instituto de Qu´ımica, Universidad Nacional Aut´onoma de Mexico, Mexico

Vadapalli Chandrasekhar

Department of Chemistry, Indian Institute of Technology Kanpur, India

Nikos Chaniotakis Iraklion, Greece

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

Tristram Chivers Department of Chemistry, University of Calgary, Canada Jean-Mathieu Chr´etien Laboratoire de Synth`ese Organique (LSO), UFR des Sciences et des Techniques, Universit´e de Nantes, France Paul A. Cusack Tin Technology, St. Albans, UK

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List of Contributors

Alwyn G Davies

Chemistry Department, University College London, UK

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

Institute of Chemical Research of Catalonia (ICIQ), Tarragona, Spain

Dana J. Eisler Department of Chemistry, University of Calgary, Canada George Eng DC, USA

Department of Chemistry and Physics, University of the District of Columbia, Washington,

Pablo Espinet Spain

IU CINQUIMA/Qu´ımica Inorg´anica, Facultad de Ciencias, Universidad de Valladolid,

Norberto Farf´an Departamento de Qu´ımica, Facultad de Qu´ımica, Universidad Nacional Aut´onoma de Mexico, Mexico Miroslav Genov IU CINQUIMA/Qu´ımica Inorg´anica, Facultad de Ciencias, Universidad de Valladolid, Spain Kandasamy Gopal

Department of Chemistry, Indian Institute of Technology Kanpur, India

Ron Grigg Molecular Innovation, Diversity and Automated Synthesis (MIDAS) Centre, School of Chemistry, Leeds University, UK T. Bruce Grindley Department of Chemistry, Dalhousie University, Halifax, Canada Imtiaz-ud-Din

Department of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan

F. Ekkehardt Hahn Institut f¨ur Anorganische und Analytische Chemie, Westf¨alische WilhelmsUniversit¨at M¨unster, Germany Ionel Haiduc

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

Edward T. Hessell

King Industries Inc. Norwalk, CT, USA

Herbert H¨opfl Centro de Investigaciones Qu´ımicas, Universidad Aut´onoma del Estado de Morelos, Mexico Vojtech Jancik Instituto de Qu´ımica, Universidad Nacional Aut´onoma de Mexico, Mexico Bernard Jousseaume Klaus Jurkschat

Groupe Mat´eriaux, Universit´e Bordeaux 1, Talence, France

Lehrstuhl f¨ur Anorganische Chemie der Universit¨at, Dortmund, Germany

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List of Contributors

Jeremy D. Kilburn Cezary M. Kinart

xxi

School of Chemistry, University of Southampton, UK Department of Chemistry, University of Lodz, Poland

Wojciech J. Kinart Department of Organic Chemistry, University of Lodz, Poland Pascal G. Lacroix Laboratoire de Chimie de Coordination du CNRS, Toulouse, France Joseph B. Lambert Department of Chemistry, Northwestern University, IL, USA Erwan Le Grognec Laboratoire de Synth`ese Organique (LSO), UFR des Sciences et des Techniques, Universit´e de Nantes, France Fabio Marchetti Dipartimento di Scienze Chimiche, University of Camerino, Italy M. Mazhar Department of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan Yoshiyuki Mizuhata Institute for Chemical Research, Kyoto University, Japan M´onica Moya-Cabrera Mexico

Instituto de Qu´ımica, Universidad Nacional Aut´onoma de Mexico,

Partha P. Nag Department of Chemistry, Indiana University, Bloomington, IN, USA Mala Nath Department of Chemistry, Indian Institute of Technology Roorkee, India Junzo Otera Department of Applied Chemistry, Okayama University of Science, Japan Keith H. Pannell Department of Chemistry, University of Texas at El Paso, TX, USA Sergio Pascual

Institute of Chemical Research of Catalonia (ICIQ), Tarragona, Spain

Claudio Pettinari Dipartimento di Scienze Chimiche, University of Camerino, Italy Vanja Pinoie High Resolution NMR Centre (HNMR), Vrije Universiteit Brussels, Belgium Kevin Poelmans High Resolution NMR Centre (HNMR), Vrije Universiteit Brussels, Belgium Jean-Paul Quintard Laboratoire de Synth`ese Organique (LSO), UFR des Sciences et des Techniques, Universit´e de Nantes, France Fran¸cois RiBOT

Chimie de la Mati`ere Condens´ee de Paris, Universit´e Pierre et Marie Curie, Paris

Rosa Santillan Departamento de Qu´ımica, Centro de Investigaci´on y de Estudios Avanzados del Instituto Polit´ecnico Nacional, Mexico.

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List of Contributors

Slavi C. Sevov Department of Chemistry and Biochemistry, University of Notre Dame, IN, USA Hemant K. Sharma Eiji Shirakawa Puja Singh

Department of Chemistry, University of Texas at El Paso, TX, USA

Department of Chemistry, Graduate School of Science, Kyoto University, Japan

Department of Chemistry, Indian Institute of Technology Kanpur, India

Xueqing Song Department of Chemistry and Physics, University of the District of Columbia, Washington, DC, USA V. Sridrahan Molecular Innovation, Diversity and Automated Synthesis (MIDAS) Centre, School of Chemistry, Leeds University, UK Edward R.T. Tiekink Norihiro Tokitoh

Department of Chemistry, The University of Texas at San Antonio, TX, USA

Institute for Chemical Research, Kyoto University, Japan

Thierry Toupance Institut des Sciences Mol´eculaires, Groupe Mat´eriaux, University of Bordeaux 1, Institut des Sciences Mol´eculaires, France Margaret M. Whalen Department of Chemistry, Tennessee State University, Nashville, TN, USA Sarah R. Whittleton Department of Chemistry, Dalhousie University, Halifax, Canada Rudolph Willem High Resolution NMR Centre (HNMR), Vrije Universiteit Brussels, Belgium David R. Williams Bernd Wrackmeyer

Department of Chemistry, Indiana University, Bloomington, IN, USA Anorganische Chemie II, Universit¨at Bayreuth, Germany

David Young Eskitis Institute of Cell and Molecular Therapies, Griffith University, Queensland, Australia Alexander V. Zabula Institut f¨ur Anorganische und Analytische Chemie, Westf¨alische WilhelmsUniversit¨at M¨unster, Germany Fran¸coise Zammattio Laboratoire de Synth`ese Organique (LSO), UFR des Sciences et des Techniques, Universit´e de Nantes, France Julio Zukerman-Schpector

Department of Chemistry, Universidade Federal de S´ao Carlos, Brazil

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1 Introduction and Overview 1.1

Introduction

Alwyn G. Davies Chemistry Department, University College London, UK

1.1.1

History, Occurrence, Production, and Applications

Tin has been known as a metal since time immemorial, and the discovery, in about 3500 bc, that it formed a strong, hard alloy with copper, started the Bronze Age, which lasted until about 1200 bc. The abundance of tin in the Earth’s surface is about 2 ppm, significantly less than that of zinc (94 ppm), copper (63 ppm), or lead (12 ppm). The most important ore is cassiterite, SnO2 , which occurs as placer (alluvial) deposits. The breakdown of the current production of tin by area is shown in Figure 1.1.1. About 75% of the world’s production comes from China and South East Asia, and about 18% from South America, but the annual figures are sensitive to political, social, and economic factors.1 The cassiterite ore is obtained by dredging, open-cast mining, or gravel-washing, in which the ore is washed out of the deposit with high-pressure jets of water. The cassiterite has a density 2.5 times that of sand, and the ore is concentrated by gravity. It is roasted to remove arsenic and sulfur, and to convert metal sulfides into oxides, then it is reduced by smelting with coal or fuel oil in a reverberatory, rotary, or electric furnace (Equation 1.1.1). SnO2 + 2CO −−−→ Sn + 2CO2

(1.1.1)

An increasing amount of tin is also being recovered, by melting, from food and drink cans and industrial scrap. In 2005 and 2006, the total annual production of refined tin was about 350 000 tonnes. At the time of writing (April, 2008), demand exceeds supply, and the price of high-grade tin has just hit a record high of US$22 150/tonne. The applications of tin are shown in the pie chart in Figure 1.1.2. About half the production of tin is used in solders, and this is increasing with the increasing production of telecommunication and electronic equipment, and the need to eliminate lead, because of its toxicity. Tin Chemistry: Fundamentals, Frontiers, and Applications Edited by Marcel Gielen, Alwyn Davies, Keith Pannell and Edward Tiekink © 2008 John Wiley & Sons, Ltd. ISBN: 978-0-470-51771-0

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Tin Chemistry: Fundamentals, Frontiers and Applications China

37.2%

Indonesia 23.2% Malaysia

Figure 1.1.1

6.5%

Thailand

7.9%

Bolivia

4.0%

Brazil

2.5%

Peru

11.7%

Belgium

2.3%

Russia

1.1%

Other

3.7%

World production of tin

Conventional tin/lead solders have the approximate composition Sn63/Pb37 by weight, corresponding to the eutectic mixture, which is close to Sn3 Pb, with a melting point of 183 ◦ C. Lead-free solders are often composed of tin with 3–4% silver and 0.5–1% copper, and have a melting point of 215–220 ◦ C. Some 20% of the production of tin goes into tinplate, which is produced by hot-dipping or electroplating; its use in canning has reduced because of the increasing competition from aluminium cans, and protective polymer layers for steel cans. As an alloy with lead, tin has been used also in pewters, for making organ pipes, and, alloyed with copper, for making bronze. Babbitt metal, used in bearings, commonly contains about 90% of tin, together with a small amount of harder metals such as copper or antimony. About 14% of the production of tin goes into tin chemicals; a further breakdown is not possible because of the commercial sensitivity of the information. Tin tetrachloride and butyltin trichloride are used for coating glass with SnO2 (see Chapter 3.1), and float glass is produced on a molten pool of tin. The first organotin compound, diethyltin dichloride, was prepared by Frankland in 1849 by heating ethyl chloride with metallic tin, and this is often taken to mark the beginning of organometallic chemistry. The first application of organotin compounds came in about 1943, when they were used first for the stabilization of PVC against heat during processing, and a variety of industrial and biological applications were subsequently developed, although, in recent years these have been somewhat curtailed by concerns about toxicity. Solders 49.7% Tinplate 18.3% Chemicals 14.1% Brass and bronze 5.6%

Figure 1.1.2

Glass

1.8%

Other

10.4%

Applications of tin

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Introduction Table 1.1.1

3

Properties of tin

Property

Value

Property

Value

Atomic number Atomic mass Melting point Boiling point Density (white tin) Density (grey tin)

50 118.710 232 ◦ C 2625 ◦ C 5.769 g cm–3 7.280 g cm–3

Electronegativity Atomic radius Covalent radius van der Waals radius

1.96 (Pauling) 1.45 pm 1.41 pm 2.17 pm

In the last half century, there has been much more research activity in the organometallic chemistry of tin than in its inorganic chemistry, and this is reflected in the contents of this book. 1.1.2

The Element

Selected properties of the element are shown in Table 1.1.1. It is in Group 14 of the Periodic Table, with the electronic configuration [Kr] 4d10 5s2 5p2 ; its principal valence state is Sn(IV), though Sn(II) inorganic compounds are common, and many stannous organic compounds, with specially designed structures, have been prepared in recent years. Tin has 10 stable isotopes (Table 1.1.2), which is the largest number for any element, and results in very characteristic mass spectra. The 117 Sn and 119 Sn isotopes, each with spin 1/2, are used in NMR spectroscopy. The γ-active 119m Sn isotope, which is prepared by the neutron-irradiation of enriched 118 Sn, is used in M¨ossbauer spectroscopy. Metallic tin exists in two allotropes. White tin, or β-tin, is a silvery-white, electrically conducting, metal, with a distorted cubic structure. Below about 10 ◦ C, it slowly coverts into grey tin, or α-tin, with a 26% increase in volume, which creates excrescences on the surface, called tin pest or plague. α-Tin is a semiconductor with a diamond structure, with Hf = 2.51 kJ mol–1 compared with metallic tin.2

Table 1.1.2 Isotope

Tin isotopes Mass

Abundance (%)

Spin

112

111.90494

0.95

0

114

113.90296

0.65

0

115

114.90353

0.34

1/2

116

115.90211

14.24

0

117

116.90306

7.57

1/2

118

117.90179

24.01

0

119

118.90339

8.58

1/2

120

119.90213

32.97

0

122

121.90341

4.17

0

124

123.90524

5.98

0

Mass Spectrum

112

114

116

118

120

122

124

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Tin Chemistry: Fundamentals, Frontiers and Applications

White tin is inert to air at room temperature, but at 200 ◦ C it is oxidized to SnO2 . Samples for microanalysis for C, H, and N, by combustion in oxygen, are usually sealed in tin capsules. The heat of combustion of the tin (Hc = −142 kJ mol–1 ) raises the temperature from about 1000 ◦ C to 1800 ◦ C, and the SnO2 which is formed acts as an oxidation catalyst. Tin shows no reaction with water and dilute acids, but concentrated hydrochloric acid reacts to give SnCl2 and hydrogen, and concentrated sulfuric acid gives SnSO4 and SO2 . In ether, HCl gas reacts to give solvated H2 SnCl2 , which, together with HSnCl3 from HCl and SnCl2 , finds some use in organic synthesis. Hot aqueous alkali, MOH, reacts to give M2 [Sn(OH)6 ]. If electrons are added into the crystal structure, they lead to the breaking of the Sn–Sn bonds, each atom carrying an unshared electron pair, and ultimately to the formation of isolated anionic clusters with triangular faces (deltahedra), known as Zintl ions (e.g. 1 and 2; see Chapter 1.6).3 These compounds are diamagnetic and poor conductors. They can be prepared either by reduction of tin with an alkali metal or electrochemically, and are soluble in polar, basic solvents. For example, [Na+ ]4 [Sn9 ]4– can be prepared from the reaction of tin with sodium in ethylenediamine, or in the presence of a crown ether to associate with the sodium cations. The most common structure is a nine-atom cluster 2.4

Surprisingly little work appears to have been carried out on the reaction of these cluster anions with organic electrophiles, but electrically neutral organotin clusters can be prepared, usually by the reduction of organotin(II) compounds carrying bulky aryl groups. Examples are Sn5 Ar6 , with a propellane structure (3), and Sn8 Ar4 (4) and Sn8 Ar8 , with cubic structures.5

1.1.3

Structure and Bonding

Both the Sn(II) and Sn(IV) states are stable. The Sn(II) state uses mainly the 5p orbitals for bonding, leaving the unshared singlet pair in the largely 5s state, with a little p character, and compounds SnX2 (5, the stannylenes) have an XSnX angle of about 90–100◦ . These compounds are most stable when there are strongly electron-attracting ligands, which make loss of the remaining electron pair more difficult (e.g. :SnF2 , :SnCl2 ), or when the ligands X are very bulky, and sterically protect the tin against further ligation (e.g. :Sn[N(SiMe3 )2 ]2 ). Otherwise, oxidation readily occurs to the Sn(IV) state, where the tin is sp3 hybridized, and the SnX4 (8, stannane) molecule has tetrahedral symmetry. However, both the stannylenes and the stannanes have vacant 5d orbitals, which can accept one or more further ligands, The stannylenes readily form the pyramidal sp3 complexes :SnX3 (6), and the trigonal bipyramidal sp3 d complexes :SnX4 (7), and the stannanes form the trigonal bipyridamidal sp3 d complexes SnX5 (9) or octahedral sp3 d2 complexes SnX6 (10). All of these may carry charges corresponding to the charge of the new ligands X. These basic structures are often distorted, and higher coordination states are sometimes formed.

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Introduction

Sn X

Sn

X

X

X

X

X

X

Sn

X

X

X

X

X

p2

sp3

X sp3d

5

6

7

X X

Sn X

X

sp3

Sn X

X

Sn

X

X

X sp3d

sp3d2

9

10

8

5

X X

Structures of Sn(IV) compounds

Structures of Sn(II) compounds

The X groups themselves may act as these further ligands, resulting in intramolecular coordination, or, particularly in condensed phases, intermolecular association, to give oligomers or polymers. A variety of techniques are available for determining these structures [119m Sn M¨ossbauer spectroscopy, 117 Sn or 119 Sn NMR spectroscopy in the solid or liquid state (see Chapter 2.1),6 IR spectroscopy, X-ray diffraction etc.] and structural studies have been a major aspect of inorganic and organic tin chemistry. For example, in the vapor phase, Sn(II)F2 is a monomer with an FSnF angle of 94◦ (11),7 whereas, in the solid state, it exists as cyclic tetramers, held together by weaker Sn–F interactions (12).8 Within the ring, the average FSnF angle is 83.7◦ , and outside the ring it is 82.8◦ . In the gas-phase, Me3 SnCl is a ˚ in solution, the monomer is in equilibrium with oligomers, tetrahedral monomer with r SnCl 2.306(3) A; but in the crystal it is associated into a zig-zag polymer (13), with approximately trigonal bipyramidal ˚ 9 tin, and r SnCl 2.43 and 3.27 A. F Sn F F Sn F

F

F

Sn

Sn F

F Sn

Me F

Me

Cl Sn Me Me

Cl

Sn

Cl

Me Me

F

11

12

13

Some values of bond lengths and bond dissociation energies are given in Table 1.1.3, but it must be emphasized that these are only indicative values, which are dependent on the physical state. 1.1.4

Organotin Compounds, Rn Snm

The various known organotin species and their structures, where the tin is bonded to only carbon or tin, and without functional groups, are shown in Table 1.1.4. •+ Stannane radical cations SnMe•+ 4 (and SnH4 ) have been generated in frozen Freon matrices by irradiation with γ -rays. The ESR spectra, with the backing of MNDO and PM3 calculations, have been interpreted as implying distortion of the tetrahedral structure of the stannane into a C3v configuration, with an almost planar trigonal base, and one long one-electron SnMe bond. Alkylstannane radical anions can be prepared by γ -irradiation of the stannane in a matrix of Me4 Si, and arylstannane radical anions by reduction with an alkali metal. The radical anion, Me4 Sn•– , appears to have a trigonal bipyramidal structure, with the unpaired electron located in an equatorial, largely sp2 , orbital.

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Tin Chemistry: Fundamentals, Frontiers and Applications

Table 1.1.3

SnX bond lengths, dissociation energies, and stretching frequencies

Bond

r/pm

Me3 Sn H Me3 Sn Me Me3 Sn Et Me3 Sn CH CH2 Me3 Sn Ph Me3 Sn SnMe3 R2 Sn SnAr2 RSn SnR Me3 SnCl Me3 SnBr Me3 SnI Me3 SnOH a

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For (CH CH2 )4 Sn.

b

For Ph4 Sn.

171 218 220 212a 214b 277.6 ca. 280 267–307 235.1 249 272 196 c

BDE/ kJ mol–1 322 ±17 295 ±17 281 340 358 291 90c — 425 ±7 381 ±17 320 ±17 488 ±17

ν/cm–1 1846 526, 506 — — 241 192 — — 331 234 189 3620 (OH)

For R = (Me3 Si)2 CH

Hypervalent pentaorganostannate anions, such as Me5 Sn– , are formed when an organolithium compound is added to a tetraorganostannane in the presence of a ligand to solvate the lithium cation. The 119 Sn NMR signal occurs about 300 ppm upfield from that of the parent stannane. The six isomeric Phn Me5−n Sn− anions can be observed in equilibrium, and the NMR spectra imply that the anions have a trigonal bipyramidal structure, with the phenyl groups in the apical positions.10 These anions are formed particularly readily from the stannacyclopentanes and stannacyclopentadienes, perhaps because ring strain is relieved by rehybridization of the tin. The UV spectra of the oligostannanes, (R2 Sn)n , show a strong absorption maximum, with a red shift with increasing chain length. Conjugation between the σSnSn bonds produces a series of molecular orbitals analogous to the π-orbitals of a conjugated polyene. The bonding orbitals have no nodes at the midpoints of bonds, and an increasing number of nodes at the tin atoms; the antibonding orbitals have nodes at the midpoints of bonds, and again an increasing number of nodes at tin. The HOMO has a node at each tin atom, as shown in Table 1.1.4. A metal-like electronic band is formed, with the implication of useful electronic and optical properties (see Chapter 3.8).11 The three-coordinate anion Ph3 Sn– is pyramidal, with an average CSnC angle of 96.9◦ , and the electron pair in a predominantly sp3 orbital.12 The hindered SnSi-bonded anion, (But2 MeSi)3 Sn– , is less pyramidal, with average SiSnSi angles of 111.6◦ , presumably because of the increased steric hindrance.13 Prolonged attempts to characterize free organotin cations culminated in 2003, in the isolation of the tris(2,4,6-triisopropylphenyl)stannyl cation by the reaction shown in Equation (1.2) (see Chapter 2.7).14 (1.1.2) The cation is planar about the tin, with the aryl rings twisted out of the plane, propeller-fashion, by an average of 61.1◦ ; In the NMR spectrum, the value of δSn is 714. The SiSn-bonded cation (But2 MeSi)3 Sn+ is similarly planar, and shows δSn at 2653 ppm.15

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Table 1.1.4 Rn Snm

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Structures of compounds with only SnSn or SnC bonds Structure

Evidence

Name

R4 Sn

X-Ray

Stannane

R4 Sn•+

ESR

Stannane radical cation

R4 Sn•−

ESR

Stannane radical anion

R3 Sn-SnR3

X-Ray

Distannane

(R2 Sn)n

X-Ray

Oligostannane

R5 Sn−

NMR

Hypervalent stannate anion

R3 Sn−

X-Ray

Stannate anion

R3 Sn+

X-Ray

Stannylium cation (see Chapter 2.7)

R3 Sn•

ESR

Stannyl radical

R2 Sn:

X-Ray

Stannylene (see Chapter 2.8)

Cp2 Sn:

X-Ray

Stannocene

R2 Sn SnR2

X-Ray

Distannene (see Chapter 2.9)

RSn SnR

X-Ray

Distannyne (see Chapter 2.9)

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Tin Chemistry: Fundamentals, Frontiers and Applications

In a radical centered on an element X, the magnitude of the ESR hyperfine coupling to X, a(X), if X has nuclear spin, gives a measure of the s-electron density at the nucleus. In a planar, sp2 hybridized radical, X will lie in the nodal plane of the p orbital which contains the unpaired electron, and show a low coupling constant. On the other hand, a pyramidal, sp3 hybridized radical, with the unpaired electron in an sp3 orbital with a substantial amount of s character, will show a large value of a(X). Simple stannyl radicals, R3 Sn•, have large a(119 Sn) values, e.g. Me3 Sn•,161.1 mT; Ph3 Sn•,186.6 mT, implying that the radicals are pyramidal and not planar, as alkyl radicals are. This is supported by the fact that a(Sn) shows a negative temperature coefficient, and that optically active organotin compounds retain their asymmetry when they react through intermediate stannyl radicals. Only one stannyl radical has been isolated and had its structure determined by X-ray diffraction. The reaction of But2 MeSiNa with SnCl2 gives the radical (But2 MeSi)3 Sn•, which shows a(117/119 Sn) 32.9 mT, very much less than the value for the simpler organostannyl radicals. In the crystal, it has a trigonal planar structure (like the corresponding cation), which is doubtless imposed by the the steric requirements of the bulky silyl groups.15 Simple alkyl and aryl stannylenes, R2 Sn:, exist only as short-lived intermediates, but kinetic stability can be achieved with bulky substituent groups. The first to be isolated was [(Me3 Si)2 CH]2 Sn: (‘Lappert’s stannylene’).16 In the gas phase, it is a singlet monomer with the angle SiSnSi 97(2)◦ , but, in solution, the monomer is in equilibrium with the (distannene) dimer. Several other persistent stannylenes have been prepared, usually from an aryllithium compound and SnCl2 . Typical aryl groups are 2,4,6-triisopropylphenyl, 2,4,6-tri-t-butylphenyl, and 2,4,6-tristrifluoromethylphenyl. They can be regarded as having notionally sp2 hybridized tin, with a vacant 5p orbital (Table 1.1.4), and can behave as both Lewis acids and Lewis bases (see Chapter 2.9). If suitable intramolecular substitutents, particularly amino groups, are present, the stannylenes may form pyramidal (sp3 ) 3-coordinate complexes, or trigonal bipyramidal (sp3 d) complexes. Stannocene, Cp2 Sn:, similarly can be described as having sp2 hybridized tin, with the non-bonding electron pair as a phantom ligand. The angle between the planes of the cyclopentadienyl rings is 46◦ , but if the rings contain alkyl substituents, this angle decreases.17 Bis(pentamethylcyclopentadienyl)tin(II) shows an angle of 36◦ , and in the bis(pentaphenylcyclopentadienyl) compound it is zero, with parallel rings. The distannene dimer of Lappert’s stannylene, [(Me3 Si)2 CH]2 Sn Sn[CH(SiMe3 )2 ]2 , has an SnSn ˚ with a CSnC angle of 109.2◦ , and an angle of 41◦ between the SnSn bond and bond length of 2.768(1) A, the CSnC plane. The other known distannenes have a similar trans structure, with SnSn bond lengths ˚ (see Chapter 2.9). The bonding is interpreted in terms of mutual overlap of the sp2 orbital up to 3.09 A carrying the electron pair, with the vacant 5p orbital, as illustrated in Table 1.1.4. A few stannenes, R2 Sn CR 2 , have been prepared, but not enough yet to allow confident interpretation of the nature of the bonding. The distannynes, ArSn SnAr, Ar = 2,6-(2,6-Pri2 C6 H3 )2 C6 H3 , and Ar = 2,6-(2,4,6-Pri3 C6 H2 )2 C6 H3 ˚ and a CSnSn angle of 125.2◦ , and have trans-bent structures, with an SnSn bond length of 2.67 A ◦ ˚ distances 2.78–2.82 A and angles 93.6–98.0 , respectively. This bonding may be interpreted in terms of one σ -bond, and two of the type described above for the distannenes, as shown in Table 1.1.4.18,19 These bond lengths and angles, however, and indeed the mode of bonding, are very sensitive to the structure of the aryl groups. Even the introduction of an Me3 Si group into the 4-position of the phenyl ring of the former of these two compounds appears to change the structure into one closer to two singly bonded Ar2 Sn: molecules.20 Reduction of the distannyne with an alkali metal gives the radical anion and then the dianion, ˚ and a CSnSn angle of 107.5◦ . ArSn SnAr= 2K+ , with an SnSn distance of 2.78 A

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Introduction

1.1.5

9

Organotin Compounds With Electronegative Ligands, Rn SnX4-n

Some examples of organotin compounds with electronegative ligands, which are intra- or intermolecularly hypercoordinated, are given in formulae 14–19. The complex of pyridine with trimethyltin chloride, with trigonal bipyramidal coordination, as shown in 14, was the first such compound for which the structure was determined by X-ray diffraction.21 The ligand may be present as a functional group in an aromatic ring or alkyl chain on the tin (e.g. 15), and stannylenes, which behave as both Lewis acids and Lewis bases, may exhibit a similar intra-molecular association (e.g. 16).

N Me Me 2.43 Å

Sn Me Cl

14

NMe2 Sn Br

15

Ph Me

(Me3Sn)2C

N SnSn(SiMe3)3

16

The carboxylates R3 SnOCOR , in the crystal, are usually inter-molecularly bonded through the carbonyl groups into linear polymers, (e.g. 17)22 and are 4-coordinate monomers only when the R groups are bulky (e.g. phenyl, cyclohexyl, or neophyl).23,24 When difunctional organotin compounds R2 SnX2 , are hydrolyzed, the first isolable compound is often the difunctional distannoxane, XR2 SnOSnR2 X (see Chapter 2.10).25 These compounds have attracted a lot of attention because of their ability to catalyze reactions such as esterification and transesterification. When R is a simple alkyl or aryl group, these distannoxanes are associated into dimmers (e.g. 18),26 but many more complex variants on this theme have been synthesized, largely in a search for more effective catalysts. Mono-organotin oxo compounds assemble into larger oligomeric clusters (see Chapter 3.2). In the crystal, trimethyltin methoxide is a linear polymer, and dimethyltin dimethoxide is a dimer. The cyclic dialkyl-1,3,2-dioxastannolanes, which are formed from 1,2-diols, are important in regiospecific synthesis, particularly in carbohydrate chemistry. Their degree of association is determined by steric factors: in the solid state, 1,1-di-n-butyl-1,3,2-dioxastannolane is a 6-coordinate ribbon polymer, but the di-tert-butyl analog is a 5-coordinate dimer (19). Methyl 4,6-O-benzylidene-2,3-O-dibutylstannylene-αd-glucopyranoside is a 5-cordinate dimer, but methyl 4,6-O-benzylidene-2,3-O-dibutylstannylene-α-dmannopyranoside is a pentamer containing three, medial, 6-coordinate and two, terminal, 5-coordinate tin atoms.

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1.1.6

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Tin Chemistry: Fundamentals, Frontiers and Applications

Preparation of Organotin(IV) Compounds

Scheme 1.1.1 illustrates the principal routes into the major classes of organotin(IV) compounds through the nucleophilic alkylation of tin tetrachloride with an organometallic reagent. In the laboratory, this is usually a Grignard reagent or an organolithium compound, RM. The R group may be alkyl, allyl, aryl, alkenyl, or alkynyl. Unless R is very bulky, it is difficult to stop the reaction at the stage of an organotin halide, and the product is usually R4 Sn. In industry, the reaction is usually carried out with the cheaper alkylaluminium compounds, which need no ethereal solvent, and, as these are prepared by the Ziegler growth reaction between Et3 Al and ethylene, it is the organotin compounds with alkyl groups having even numbers of carbon atoms, such as butyl and octyl, that are readily available. Most commonly, organic syntheses are carried out with n-butyltin compounds, such as tri-n-butyltin hydride or di-n-butyltin dichloride, because they are cheaper and less toxic than the corresponding methyltin compounds. Under the appropriate conditions, the alkylation with R3 Al can be stopped at the stage of an intermediate alkyltin halide. Cyclopentadienyltin(IV) compounds can be prepared by treating tin tetrachloride, or an organotin chloride, with CpNa. These compounds are fluxionally σ -bonded.17 They are photosensitive, and the stannanes CpSnR3 , on irradiation with UV light, undergo homolysis to give Cp• and R3 Sn• radicals. Many additives have been investigated for promoting the direct reaction of organic halides with tin metal, but the reaction has limited use. Dimethyltin dichloride can be prepared from the reaction of methyl chloride with molten tin, but, in the laboratory, the reaction is used principally for preparing diallyl- or dibenzyl-tin dichlorides. The reaction between tin(II) halides and organic halides to give organotin trihalides, similarly finds little practical application. The tetraorganotin compounds are converted into the organotin chlorides by the Kocheshkov reaction, which involves the disproportionation reaction with SnCl4 , the basis of which is the non-additivity of bond dissociation energies caused by the interaction of RSn with RSn, RSn with SnCl, and SnCl with SnCl bonds. The chlorides can then be converted into other functional compounds Rn SnX4-n , by anion exchange. The dialkyltin dithiolates, (e.g.(n-C8 H17 )2 Sn(SCH2 CO2 CH2 CHEtBu)2 , ‘isooctylthioglycollate’), and maleates (e.g. [n-Bu2 SnOCOCH CHCO2 ]n ), are used for stabilising PVC against elimination of HCl when it is processed at high temperature. Di-n-butyltin dilaurate is used in catalyzing the formation of polyurethanes from diols and diisocyanates, though, for some purposes, stannous octoate is preferred. Hydrolysis of the halides and other functionally substituted compounds gives the hydroxides, which are often unstable and spontaneously dehydrate to give the oxides. The first hydrolysis products of the difunctional compounds R2 SnX2 are usually the difunctional distannoxanes, XR2 SnOSnR2 X, which have attracted much attention (see above). Reduction of the halides with a metal hydride such as lithium aluminium hydride, sodium borohydride, or poly(methylhydrosiloxane) gives the corresponding organotin hydrides.27 These have an important place in organic synthesis for the reduction of halides to hydrides (hydrostannolysis) and the addition to alkenes and alkynes (hydrostannation), by radical chain reactions. Further reactions may intervene between the pairs of reactions shown in Equations (1.1.3) and (1.1.4), and (1.1.4) and (1.1.5), and these reactions are particularly useful for inducing ring-closure reactions. R3 Sn• + R X −→ R3 SnX + R •

(1.1.3)

R • + R3 SnH −→ R H + R3 Sn•

(1.1.4)

R3 Sn• + C C −→ R3 SnC−C• R3 SnC−C• + R3 SnH −→ R3 SnC−CH + R3 Sn•

(1.1.5) (1.1.6)

RSnCl 3 L2

R2 SnCl 2L 2

R 2SnH 2

L

L

HO -

X-

HO -

X-

X-

HO -

R2 SnCl 2

R3 SnCl

RSnCl 3

LiAlH 4

L

LiAlH 4

RX

RSn(OH)Cl 2

RSnX3

HO -

SnCl 2

Sn

(R2SnO) n

RI

[RSn(OH)O] n

(R2SnO)n

R2 SnX 2

R 2 SnX 2

R 3 SnOSnR 3

(X = OCOR', OR', NR' 2 , SR' etc)

XR2 SnOSnR 2 X

R2 SnXCl

R3 SnOH

R3 SnX

Introduction

Scheme 1.1.1 Synthesis of organotin(IV) compounds based on alkylation of SnCl4 with an organometallic reagent, and the Kocheshkov redistribution reaction

or Pdo

base

R3 SnH

SnCl 4 200 o C

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R 3Sn C C H

R3 Sn C C

Hydrostannation

H CC

R 3 SnH

LiAlH4

R3 SnCl

C C

Pd o

BuLi, LDA, KOBut, etc M

X

R 3Sn hν

R 3SnSnR 3

R 3SnM

R3 SnX

R'X

R 3SnR' Scheme 1.1.2 compounds

Synthesis of organotin compounds based on the reactions of tin hydrides or stannylmetallic

Carbostannation of alkynes, and 1,2- and 1,3-dienes can be brought about by acyl, allyl, or alkynylstannanes in the presence of a nickel(0) catalyst.28 Reaction of the hydrides with a base or with a Pd0 compound leads to the elimination of hydrogen, and the formation of Sn Sn-bonded oligomers or polymers. The principal alternative route into the organotin(IV) manifold is based on the reaction of tin hydrides or stannylmetallic compounds, as shown in Scheme 1.1.2. The most common (radical) mechanism of hydrostannation of alkenes and alkynes with organotin hydrides is outlined, for alkenes, in Equations (1.1.5) and (1.1.6). Hydrostannation can also be catalyzed by palladium compounds. Other tin hydrides may react by a polar rather than a radical mechanism. A series of PVC stabilisers (the Estertins) have been developed by the AKZO company, and which are prepared by the reaction of HCl with metallic tin or SnCl2 , to give the solvated chlorotin hydrides H2 SnCl2 and HSnCl3 , respectively, which add, in a heterolytic Michael fashion to acrylic or propargylic esters [e.g. Equations (1.7) and (1.8)];29 the chlorides are then converted by anion exchange into the thioglycollates. H2 C CHCO2 Me Et2 O Sn + HCl −−−→ [H2 SnCl2 ] −−−−−−−−−−−−−−→ Cl2 Sn(CH2 CH2 CO2 Me)2 HC CCO2 Me Et2 O SnCl2 + HCl −−−→ [HSnCl3 ] −−−−−−−−−−−−−−→ Cl3 SnCH CHCO2 Me

(1.1.7) (1.1.8)

Tin hydrides react with strong metallic bases such as BuLi, NaH, LDA, or RMgX, by deprotonation to give the corresponding stannylmetallic compounds, R3 SnM, which will react with organic electrophiles to create new Sn C bonds [e.g. Equations (1.1.9) and (1.1.10)]. Me3 SnLi + CCl4 −→ (Me3 Sn)4 C Bu3 SnMgCl +

−→ Bu3 SnCH2 CH2 OH

(1.1.9) (1.1.10)

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Introduction

13

SnCl2 RCp2SnX

CpM

RX

R*Sn SnR*

RM M

CpSnX

HX

.. RSnCl

Cp2Sn: X2

RLi

Cp2SnX2

(R2Sn)n R2Sn ..

Cp = cyclopentadienyl

Scheme 1.1.3

SnR*2

R*2 Sn CR2

Me N R*2Sn

R*2 Sn R2 C..

MeN3

1.1.7

.. R*SnH

SnR*2 SnR*2

R* = bulky group

Preparation of lower-valence organotin compounds

Preparation of Organotin Compounds in Lower Valence States

Routes to lower-valence organotin compounds are shown in Scheme 1.1.3. Alkylation or arylation of stannous chloride gives stannylenes, R2 Sn:, which normally self react to give linear or cyclic oligomers, but if the R groups are very bulky (t-butylphenyl, or 2,6-bis(2,4,6-triisopropylphenyl)phenyl), kinetically stable stannylenes can be obtained (see Chapter 2.8). In the solid state, the stannylenes usually dimerize to give the distannenes, R*2 Sn SnR*2 , where R* denotes a bulky group. Reaction of a stannylene with a carbene gives a stannene, R*2 Sn CR2 . Reduction of chlorostannenes with potassium or sodium has recently given the first distannynes, R*Sn SnR* (see Chapter 2.9). Stannocene, Cp2 Sn: (see above), was prepared in 1956 as an air-sensitive white solid, m.p. 105 ◦ C, by the reaction of cyclopentadienyllithium with SnCl2 .30 1.1.8

Literature

There are no monographs on the inorganic chemistry of tin, but the topic is covered in Comprehensive Inorganic Chemistry (1973, E.W. Abel)31 , in both editions of Comprehensive Coordination Chemistry (1987, P.G. Harrison;32 2004, J. Parr33 ), and in Encyclopedia of Inorganic Chemistry (J.L. Wardell),34 and there is a chapter on the inorganic chemistry in both editions of Chemistry of Tin (P.G. Harrison; P.J. Smith).35,36 Organotin chemistry is much better served. It was first surveyed in Krause and von Grosse’s Die Chemie der Metal-organischen Verbindungen,37 published in 1937, and reprinted in 1965. The first generally available article in English was Ingham, Rosenberg, and Gilman’s Chemical Review,38 in 1960 – a time when the field was beginning to expand rapidly. Copies were widely circulated, and introduced the subject to a wide audience. Neumann’s Die Organische Chemie des Zinns, appeared in 1967,39 and was translated and updated as The Organic Chemistry of Tin in 1970,40 and Poller’s The Chemistry of Organotin Compounds,41

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also appeared in 1970. This was supplemented by Kocheshkov, Zemlyanskii, Sheverdina, and Panov’s Metodi Elemento-organiicheskoi Khimii, Germanii, Olovo, Svinesh (1968) for readers in Russia,42 where research into organotin chemistry was very active. By 1971, the subject could not be covered comprehensively by one author, and Organotin Chemistry, edited by Sawyer, appeared in three volumes with articles by 14 authors.43 The three editions of Comprehensive Organometallic Chemistry included chapters on tin (1982, 1995, and 2007 respectively);44–46 2000 copies of the first of these chapters were distributed, and again helped to popularize the subject. Two volumes in the series of reviews edited by Patai (1978) and by Rappoport (2002), and the two editions of Chemistry of Tin, edited by Harrison35 and by Smith,36 deal largely with organotin compounds. There is a short review in Encyclopedia of Inorganic Chemistry.47 Much of the activity in the organotin field has been directed towards the use of organotin compounds in organic synthesis. This topic was covered in Pereyre, Quintard, and Rahm’s Tin in Organic Synthesis 1987,48 and supplemented by Jousseaume and Pereyre49 in Smith’s Chemistry of Tin. Articles in Houben Weyl,50 and its successor, Science of Synthesis,51 also deal mainly with the use of organotin compounds in synthesis. Organotin compounds are covered in 25 volumes of Gmelin, which is also available on line, in a different format, and there have been two editions (1997 and 2004) of Davies’ Organotin Chemistry.52,53 This last book gives a more extensive bibliography. References 1. I am grateful to the ITRI Ltd (www.itri.co.uk) for providing current data on the production and applications of tin. 2. M. J. P. Musgrave, Proc. Roy. Soc. 227A, 503 (1963). 3. T. F. F¨assler, in Metal Clusters in Chemistry, P. Braunstein, L. A. Ora, P. R. Raithby (Eds), Wiley-VCH, Weinheim, 1999, Vol. 3, 1612–1642. 4. T. F. F¨assler, Coord. Chem. Rev. 215, 347 (2001). 5. N. Wiberg and P. Power, in Molecular Clusters of the Main Group Elements, M. Driess and H. N¨oth (Eds), Wiley-VCH, Weinheim, 200, 188–208. 6. J. C. Martins, M. Biesemans, and R. Willem, Progr. NMR Spectrosc. 36, 271 (2000). 7. R. H. Hauge, J. W. Hastie, and J. L. Margrave, J. Mol. Spectrosc. 45, 420 (1973). 8. R. C. McDonald, H. H. K. Hau, and K. Eriks, Inorg. Chem. 15, 762 (1976). 9. M. B. Hossain, J. L. Lefferts, K. C. Molloy, D. Van der Helm, and J. J. Zuckerman, Inorg. Chim. Acta 36, L409 (1979). 10. H. J. Reich, and N. H. Phillips, Pure Appl. Chem. 59, 1021 (1987). 11. L. R. Sita, K. W. Terry, and K. Shibata, J. Am. Chem. Soc. 117, 8049 (1995). 12. T. Birchall, and V. Manivannan, J. Chem. Soc., Dalton Trans. 2671 (1985). 13. T. Fukawa, M. Nakamoto, V. Y. Lee, and A. Sekiguchi, Organometallics 23, 2376 (2004). 14. J. B. Lambert, L. Lin, and S. Keinan, J. Am. Chem. Soc. 125, 6022 (2003). 15. A. Sekiguchi, T. Fukawa, V. Y. Lee, and M. Nakamoto, J. Am. Chem. Soc. 125, 9250 (2003). 16. P. J. Davidson, D. H. Harris, and M. F. Lappert, J. Chem. Soc., Dalton Trans. 2268 (1976). 17. P. Jutzi, and N. Burford, Chem. Rev. 99, 969 (1999). 18. N. Wiberg, S. K. Vasisht, G. Fischer, and P. Mayer, Z. Anorg. Allgem. Chem. 630, 1823 (2004). 19. P. P. Power, Appl. Organomet. Chem. 19, 488 (2005). 20. R. C. Fischer, L. Pu, J. C. Fettinger, M. A. Brynda, and P. P. Power, J. Am. Chem. Soc. 128, 11366 (2006). 21. I. R. M. Beattie, G. P., and Hulme, R. Chemistry & Industry, 1429 (1962). 22. M. A. Saeed, A. Badshah, M. K. Rauf, D. C. Craig, and S. Ali, Acta Cryst. E62, m469 (2006). 23. E. R. T. Tiekink, Appl. Organomet. Chem. 5, 1 (1991).

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Introduction 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53.

15

E. R. T. Tiekink, Trends in Organomet. Chem. 1, 71 (1994). A. G. Davies, J. Chem. Research, 309 (2004). P. G. Harrison, M. J. Begley, and K. C. Molloy, J. Organomet. Chem. 186, 213 (1980). A. G. Davies, J. Chem. Res. 141 (2006). Y. Nakao, E. Shirakawa, T. Tsuchimoto, and T. Hiyama, J. Organomet. Chem. 689, 3701 (2004). R. E. Hutton, and V. Oakes, in Organotin Compounds: New Chemistry and Applications, ed. J. J. Zuckerman (Ed.), American Chemical Society: Washington, 1976, 123–133. E. O. Fischer, and H. Grubert, Z. Naturforsch. 11B, 423 (1956). E. W. Abel, in Comprehensive Inorganic Chemistry, J. C. Bailar, H. J. Emeleus, R. S. Nyholm, and A. F. Trotman-Dickenson, (Eds), Pergamon, Oxford, 1973, Vol. 2, Chapter 17, 43–104. P. G. Harrison and T. Kikabbai, in Comprehensive Coordination Chemistry, G. Wilkinson, R. D. Gillard, and J. A. McCleverty (Eds), Pergamon, Oxford, 1987, Vol. 2. J. Parr, in Comprehensive Coordination Chemistry II, J. A. McCleverty and T. J. Meyer (Eds), Elsevier, Oxford, 2004, Vol. 3, 545–608. J. L. Wardell, in Encyclopedia of Inorganic Chemistry, R. B. King (Ed.), Wiley, Chichester, 2005, Vol. 9, 5590–5605. P. G. Harrison (Ed.), Chemistry of Tin. Blackie, Glasgow, 1989. P. J. Smith (Ed.), Chemistry of Tin, 2nd edn., Blackie, London, 1998. E. Krause, and A. von Grosse, Die Chemie der Metal-organischen Verbindungen. Borntraeger, Berlin, 1937. R. K. Ingham, S. D. Rosenberg, and H. Gilman, Chem. Rev. 60, 459 (1960). W. P. Neumann, Die Organische Chemie des Zinns. Ferdinand Enke Verlag, Stuttgart, 1967. W. P. Neumann, The Organic Chemistry of Tin. John Wiley & Sons, Ltd. Chichester, 1970. R. C. Poller, The Chemistry of Organotin Compounds. Logos Press, London, 1970. K. A. Kocheshkov, N. N. Zemlyansky, N. I. Sherevdina, and E. M. Panov, Metodi Elemento-organicheskoi Khimii. Germanii, Olovo, Svine. Nauka, Moscow, 1968, 162–530. A. K. Sawyer, Organotin Compounds. Marcel Dekker, New York, 1971. A. G. Davies, and P. J. Smith, in Comprehensive Organometallic Chemistry, G. Wilkinson, F. G. A. Stone, and E. W. Abel (Eds), Pergamon Press, Oxford, 1982, Vol. 2, 519–627. A. G. Davies, in Comprehensive Organometallic Chemistry II, E.W. Abel, F. G. A. Stone, and G. Wilkinson, Pergamon Press, Oxford, 1995, Vol. 2, 217–303. A. G. Davies, in Comprehensive Organometallic Chemistry III, R. H. Crabtree and D. M. Mingos (Eds), Elsevier, Oxford, 2007, Vol. 3, 809–883. J. L. Wardell and G. M. Spencer, in Encyclopedia of Inorganic Chemistry, R. B. King (Ed.), Chichester, 2005, Vol. 9, 5590–5605. M. Pereyre, J. P. Quintard, and A. Rahm, Tin in Organic Synthesis. Butterworth, London, 1987. B. Jousseaume, and M. Pereyre, in Tin, 2nd edn., P. J. Smith (Ed), Chapman and Hall, London, 1998, 290–387. G. B¨ahr and S. Pawlenko, in Houben Weyl, Methoden der Organische Chemie, Thieme, Stuttgart, 1978, Vol. 13/6. E. J. Thomas, in Science of Synthesis. Vol. 5. Compounds of Group 14 (Ge, Sn, Pb), M. G. Moloney (Ed.), Thieme, Stuttgart, 2003, 195–607. A. G. Davies, Organotin Chemistry. VCH, Weinheim, 1997. A. G. Davies, Organotin Chemistry, 2nd edn. Wiley-VCH, Weinheim, 2004.

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2 Fundamentals in Tin Chemistry 2.1

NMR Spectroscopy of Tin Compounds

Bernd Wrackmeyer Anorganische Chemie II, Universit¨at Bayreuth, D-95440 Bayreuth, Germany

2.1.1

Introduction

The element tin possesses numerous naturally occurring isotopes, of which three (115 Sn, 117 Sn, 119 Sn) are magnetically active spin-1/2 nuclei (Table 2.1.1). Owing to their fairly high natural abundance and favorable nuclear magnetic properties, 117 Sn and 119 Sn nuclei are highly attractive for NMR experiments, both in solution and in the solid state. Usually, the 119 Sn isotope is the target, although 117 Sn NMR spectra can be measured likewise if desired. 119 Sn NMR has become an extremely valuable, sometimes indispensable, routine tool in almost every field of tin chemistry. This development is aided by various sophisticated NMR techniques which have greatly enhanced the information that can be obtained from NMR measurements of spin-1/2 nuclei in general.1 The progress in 119 Sn NMR has been reviewed repeatedly.2–8 Moreover, the presence of magnetically active tin isotopes is also mirrored in the NMR spectra of other nuclei, such as 1 H, 13 C, 19 F, or 31 P to name some prominent examples. The volume of useful 119 Sn NMR data produced so far is prohibitive for a complete assessment. However, this overview attempts to give a general outline of the field, and shows applications of NMR measurements in solution and in the solid state, aiming at representative and stimulating examples that can be studied by standard experiments. In this context, NMR parameters, such as chemical shifts δ 119 Sn (in ppm; this attribute will not be further used here) and coupling constants n J (119 Sn,X) (in Hz) are of great importance. This review excludes 119 Sn NMR parameters of paramagnetic compounds, alloys, and various other inorganic solids. 2.1.2

Experimental

Referencing

With modern NMR spectrometers the definition of a fixed frequency (Table 2.1.1) as the reference for chemical shifts δ is always recommended9 when an internal reference cannot be generally applied. The Tin Chemistry: Fundamentals, Frontiers, and Applications Edited by Marcel Gielen, Alwyn Davies, Keith Pannell and Edward Tiekink © 2008 John Wiley & Sons, Ltd. ISBN: 978-0-470-51771-0

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Table 2.1.1 NMR properties of some important spin-1/2 nuclei including the magnetically active tin isotopes 115 Sn, 117 Sn and 119 Sna Nucleus 1

H C 15 N 19 F 29 Si 31 P 77 Se 103 Rh 115 Sn 117 Sn 119 Sn 125 Te 183 W 187 Os 195 Pt 199 Hg 207 Pb 13

N. A. (%) 99.985 1.108 0.37 100 4.70 100 7.58 100 0.35 7.61 8.58 6.99 14.28 1.64 33.8 16.84 22.6

RCb

γ [107 rad s−1 T−1 ]

[MHz]

26.7522 6.7283 −2.7126 25.1815 −5.3190 10.8394 5.1214 −0.8468 −8.8014 −9.589 −10.0318 −8.5087 1.1283 0.6193 5.8383 4.8458 5.6264

100.000000 25.145020 10.136767 94.094003 19.867187 40.480747 19.071523 3.160000 32.718780 35.632295 37.290665 31.549802 4.166398 2.282343 21.400000 17.910841 20.920597

5.67 103 1.00 2.19 10−2 4.73 103 2.10 3.77 102 3.02 0.180 0.707 19.9 25.7 12.8 6.08 10−2 1.15 10−3 19.9 5.68 11.9

Reference standard SiMe4 , 1% in CDCl3 SiMe4 1% in CDCl3 MeNO2 (neat) CFCl3 (neat) SiMe4 1% in CDCl3 H3 PO4 . 85 % aq Me2 Se (neat) No compound SnMe4 (neat) SnMe4 (neat) SnMe4 (neat) Me2 Te (neat) Na2 WO4 in D2 O, 1 M OsO4 (CDCl3 ) No compoundc HgMe2 (neat)d PbMe4 (neat)

a

Most data taken from Reference 9. R means receptivity relative to that of 13 C at natural abundance. δ Pt([PtCl6 ]2− ) = +4533. d (199 Hg) = 17.870535 MHz for Hg(ClO4 )2 (0.1 M in 0.1 M HClO4 ). b C

c 195

frequency (1 H) for TMS = 100.000 000 MHz can be compared with the 1 H NMR frequency of TMS for individual NMR spectrometers. Thus, it is not necessary to handle toxic SnMe4 as a chemical, and the choice of the fixed frequency (119 Sn) for neat SnMe4 (Table 2.1.1) guarantees precise reproduction of δ 119 Sn data. Other referencing techniques should be discouraged. For solid-state 119 Sn NMR spectra, indirect referencing is required,10,11 and several solid tin compounds can be used. A sharp 119 Sn NMR signal is observed for solid Sn(C6 H11 )4 (δ 119 Sn −97.3) under conditions of MAS, cross-polarization, and 1 H decoupling. This is a commercial chemical, easy to handle in air and the FID after even one pulse can be used to optimize experimental conditions. Techniques for Measurement of 119 Sn Resonances

As a result of the relatively high sensitivity to the NMR experiment (Table 2.1.1), 119 Sn resonance signals can be observed in most cases directly, using the PFT technique. If 1 H decoupling is required, it may prove necessary to use inverse gated 1 H decoupling in order to suppress the NOE (maximum NOE ηmax = −1.34) which may partially or completely cancel 119 Sn NMR signals. Frequently, there is a significant dependence of the positions of the 119 Sn NMR signals on temperature gradients in the sample. Therefore, the 1 H decoupling power should be carefully adjusted, and a constant temperature is required to ensure that the line widths of the 119 Sn NMR signals are not controlled by experimental conditions. Polarization transfer pulse sequences (INEPT 12,13 or DEPT 14 ) can be applied successfully if the approximate magnitude of coupling constants n J (119 Sn,1 H) is known (see Figure 2.1.1). Indirect detection of 119 Sn NMR signals, in general by 1 H NMR, using various types of twodimensional (2D) NMR techniques1,6 (HMQC, HMBC, HSQC) enables one to study fairly dilute solutions

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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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NMR of Tin Compounds

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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NMR of Tin Compounds

25

bis(amino)stannylenes38 (Scheme 2.1.3), the lone pair of electrons at tin is used to form the M Sn bond. Since the formally empty pz -orbital at the tin atom is still present, there is no dramatic shift of the 119 Sn resonance in comparison with the free Sn(II) amide. This is also indicated by δ 119 Sn of bis(alkoxy)stannylene complexes in the absence of further association,40 although there are no data to compare with the free ligand. However, in dimers of stannylenes or in adducts of stannylenes, a marked increase in 119 Sn nuclear shielding is observed, since the Sn-pz orbital is now used in donor–acceptor interactions.40,41

(Me3Si)2N (Me3Si)2N

But

B

Sn

N

Me3Si

8

Me2Si Sn

δ119Sn 776 36

B

N

Me2Si

10 t

Me3Si(Bu )N

SiMe3

Sn

δ119Sn 791.5 39

Me3Si(But)N

N

(Me3Si)2N (

Sn N

11

12 38

But δ119Sn 759 36

δ119Sn 885 (M = Pd) 815 (M = Pt)

9 δ119Sn 850 36

Scheme 2.1.3

Sn)3M (Me3Si)2N

Diaminostannylenes

An increasing number of tin compounds are becoming available, in which tin is involved in multiple bonding with other elements, such as carbon, silicon, germanium,42,43 or phosphorus,44a sulfur, or selenium,44b and this type of bonding causes magnetic deshielding of 119 Sn nuclei (Scheme 2.1.4). However, tin usually avoids multiple bonding, and this is reflected by δ 119 Sn data as in 15, pointing towards a greater contribution of the zwitterionic structure. But

But Me3Si

N

B

Si

Sn[CH(SiMe3)2]2 Me3Si

N

B But

N(SiMe3)2 Ar Sn

But 14

13

δ119Sn 835 43a

Pri2N

Ar = C6H3-2,6-(NMe2)2 δ119Sn 621 42b Pri2N

+ i

Sn[N(SiMe3)2]2 15

Pr 2N δ

119

Scheme 2.1.4

Sn – 44.7

Sn[N(SiMe3)2]2 i

Pr 2N 42a

Multiple bonding between tin and carbon or silicon

In stannocenes, the tin atoms formally possess the coordination number 10, and the 119 Sn magnetic shielding is high (Scheme 2.1.5), in contrast to the stannylenes. Numerous stannocenes are instructive examples, for which the molecular structures have been determined in the solid state, and the 119 Sn NMR spectra have been measured both in the solid state and in solution.

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16 Sn(η5-C5H5)2

δ119Sn solution solid-state –2199 45 –2162.8, 2224.4 45

21 Sn[η5-C5(CH2Ph)5]2

δ119Sn solution solid-state –2188 50 –2288 50

17 Sn(η5-C5H3-2,5-But2)

–2100 46

22 Sn(η5-C5Ph5)2

–

18 Sn(η5-C5Me5)2

–2129 47 –2136.6, –2140.2 47 23 Sn(η5-NC4H2-2,5-But2)2

5

i

19 Sn(η -C5Pr 5)2

–

5

t

20 Sn(η -C5Me4-SiMe2Bu )2 –2204

49

–2362

48

–2236

49

Scheme 2.1.5

24 [Sn(η

–1889

5

-C5H3-2,5-But2)]+[BF4]–

–2215 50 51

–2337.7

46

-

Stannocenes and related derivatives

The δ 119 Sn values for stannocenes are found within a narrow range, apparently independent of the structure (ring-centroid-Sn-ring-centroid angles vary from ca. 140◦ to 180◦ ), and there is no great difference between the data for solutions and the solid state.45,47,49,50 The latter fact points towards similar structures in both phases, proving that the η5 -coordination of the cyclopentadienyl rings is retained in solution. In 23, two N -pyrrolyl anions replace the cyclopentadienyl ligands, and the δ 119 Sn value of 23 is in agreement with η5 -coordination, as suggested by the solid-state structure.51 The major reason for the highly shielded 119 Sn nuclei in all these compounds (Scheme 2.1.5) appears to be the position of the tin atom (in the formal oxidation state +2) above the five-membered ring (in an apical position) with an energetically low-lying orbital for the lone pair of electrons at tin. Thus, in the cationic 24 only one cyclopentadienyl ring is coordinated to tin,46 and neither the positive charge nor the lower coordination number reduces the 119 Sn nuclear shielding. Tin(IV) compounds are known with tin coordination numbers of 3, 4, 5, 6, and in some cases >6. The search for ‘free’ triorganotin cations (stannylium ions) continues,52 and a few examples (e.g. 2553 and 2654 ) have been reported where three-coordinate tin atoms are likely to be present both in the solid state and in solution (Scheme 2.1.6). The extreme 119 Sn nuclear deshielding in the tris(silyl)stannylium ion 2654 is caused by B0 -induced currents involving σ − π* magnetic dipole-allowed transitions, for which the energy difference is much smaller when compared with the triarylstannylium ion 25. Examples of another new class of organotin compounds are also shown in Scheme 2.1.6. The positively charged tin centers in these zwitterionic species 27 and 28 are stabilized by coordination with C C bonds.55 Pri

Pri Pri

3

+ Sn

Pri 25

δ119Sn 714 53

(But2MeSi)3Sn + 26 δ119Sn 2653 54

Me

–

+ Me2Sn Pri δ

119

BEt2 27

Me

–

+ Sn+

Et2B

Et

Sn 215.4

Et

55a

Me Me 28

–

BEt2 Et

δ119Sn 165.6 55b

Scheme 2.1.6 bonds

‘Free’ stannylium cations and stannylium ions stabilized by side-on coordination with C

C

The change in coordination number from 4 to 5 is frequently observed for many tin compounds. This may be enforced by the appropriate chemistry (Scheme 2.1.7). The negative 119 Sn values are typical, although their magnitude varies with the system under investigation. Very often intra-molecular association, adduct formation or inter-molecular aggregation (e.g. in

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NMR of Tin Compounds

Me Me

Me Sn Me Me

– Ph Ph

Ph Sn Ph

29

Ph 30

δ119Sn -277 56 Δ

119

Sn -277

–

Bu3 Sn

– Me3Sn

H2 N

Me 32

27

– BEt2 Et

δ119Sn -303 56

31 δ119Sn -44 57

δ119Sn -175.6 58

Δ119Sn -174.2

Δ119Sn -37.6

Δ119Sn -291.6

Scheme 2.1.7 Anionic complexes of tetraorganotin compounds. The values (coordination shifts) refer to SnMe4 , SnPh4 , 1,1-dibutyl-1-stanna-indene, and the corresponding dihydro-1,2,5-azoniastannaboratole, respectively

a vast number of organotin carboxylates and related derivatives), sometimes as temperature-dependent equilibria, are observed using 119 Sn NMR spectroscopy, or interactions with donor solvents (e.g. DMSO) have to be taken into account. There are numerous examples of neutral or ionic tin compounds for which the structural information from X-ray analysis is confirmed by 119 Sn NMR data for the compounds in solution. Depending on the Lewis-acidity of the respective tin compound and the availability of suitable donors, the coordination number of tin may readily increase to 6 in solution or even to 7 or 8 in the solid state. Tin halides have been studied on many occasions since they are important starting materials for the synthesis of tin compounds. The δ 119 Sn NMR data for the solid state and frequently also in solution indicate inter- or intra-molecular coordination if suitable donor sites are available.59–65 However, predictions about Sn..Cl..Sn bridges should be made with great caution: see, for example, the Sn..Cl..Sn bridge in an anionic chloro complex 66 and the absence of such a bridge in 1,1 -bis(trichlorostannyl)ferrocene (δ 119 Sn23.2). In the case of the latter, the crystal structure has been determined,67 showing neither appreciable intra- or inter-molecular Sn..Cl..Sn interactions. Adducts of organotin or tin halides with Lewis bases may be fluxional, e.g. with respect to cis/trans isomerization in octahedral complexes, and in the case of dimethylsulfide adducts of SnCl4 , this process is readily analyzed by magnetization transfer experiments using 119 Sn NMR spectroscopy.68 Although weak association in the solid state is readily seen from the results of crystal structures, it may not always be obvious from solid-state 119 Sn NMR spectra, since the isotropic δ 119 Sn values are similar to those in solution. However, considering the respective principal structure and careful analysis of the tensors of the chemical shifts usually reveal such weak interactions. The dialkyltin tropolonates are instructive examples in this respect (Figure 2.1.7).69 Tin compounds with tin–chalcogen bonds are highly abundant and possess an amazing variety of structural features, in particular for Sn O bonds if the oxygen is part of a carboxylate unit or of another acid.70–80 Diorganotin oxides are polymers or trimers, depending on the steric requirements of the organyl group. In the case of sulfur and selenium, structures of trimers and dimers have been determined, and numerous NMR spectroscopic studies in solution and in the solid-state have been carried out.81,82 Substituent Effects and Effects of Cyclic Structures

The influence of various substituents on δ 119 Sn for series of tetra-coordinate tin(IV) compounds SnX4-n Yn (n = 0–4) can be graphically depicted as U-shaped curves. These substituent effects are not linear, but approximately pair-wise additive. Some representative 119 Sn NMR data are given in Table 2.1.3 (tin hydrides), Table 2.1.4 (tetraorgano. tin), Table 2.1.5 (tin halides), Table 2.1.6 (tin chalcogenides), and Table 2.1.7 (compounds with Sn N, Sn P, Sn As, Sn Sb, and Sn Bi bonds).

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Tin Chemistry: Fundamentals, Frontiers and Applications C36 08 C34

C35 05 C21

Sn2

C27

07 C28

06

δiso −202 δ119Sn (solution) −195.5 δiso −269.5 δ119Sn (solution) −233

C5

C1A C2A

01

C3A Sn1A

δ119Sn

04A

Sn1 04 03

01A

200

−200

0

−400

−600

C5A

−800

02

C1

Figure 2.1.7 Comparison of 111.9 MHz solid-state 119 Sn{1 H} CP/MAS NMR spectra and solid-state molecular structures of dimethyltin- (upper trace; MAS 5500 Hz) and dibutyltin tropolonates (lower trace; MAS 8000 Hz). In the latter, tin has coordination number 7, and the 119 Sn chemical shift anisotropy is much larger (δ11 = +195.8, δ22 = −153.4, δ33 = −850.6; η = 0.60), when compared with that for the Me2 Sn derivative (δ11 = −23.4, δ22 = −79.6, δ33 = −498; η = 0.19), where the tin atom is six-coordinate. (Adapted from Reference 69, with permission from John Wiley & Sons, Ltd.) Table 2.1.3

119

Sn NMR parameters of some tin hydridesa

Compound

δ 119 Sn

1 J (119 Sn,1 H)

Me3 SnH Bu3 SnH Ph3 SnH (CF3 )3 SnH Fe(η5 -C5 H4 -SnMe2 H)2 [b]

−104.5 −91.4 −164.5 −309.8 −102.4

1744 1604.4 1935.8 2798 1824

Bu2 Sn(Cl)H Me2 SnH2 Ph2 SnH2 (CF3 )2 SnH2 Fe(η5 -C5 H4 -SnMeH2 )2 [b]

−18.3 −225.0 −234.0 −303.1 −210.5

2178 1797 1928 2536 1876

MeSnH3 PhSnH3 H2 C CH-SnH3 H2 C C CH-SnH3 HC C-SnH3 CF3 SnH3 Fe(η5 -C5 H4 -SnH3 )2

−346.0 −320.0 −361 −338.4 −320.6 −351.6 −330.7

1852 1920

a

See References 5 and 8 for more data.

2010.3 2242.4 2207 1947

J (119 Sn,13 C) 352 912 371.8 (Me) 514.4 (C-1) 451 485 777 387 (Me) 546.6 (C-1)

488.0 517.2 653 583.2

Ref. 5 5 5 142 143 144 5 5 142 143 5 5 145 146 146 142 143

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SnNMR parameters of some tetraorganotin compounds a δ 119 Sn

Compound SnMe4 SnEt4 SnPr4 SnPri4 SnBu4 Sn(CH2 CH CH2 )4 Sn(CH2 Ph)4 SnPh4 Sn(CH CH2 )4 Sn(C CMe)4 Sn(C CSiMe3 )4 Sn(C C-Fc)4 Sn(CF3 )4 (E)-Me3 Sn-CH C(Me)Hb

0 1.4 −16.8 −43.9 −11.5 −47.9 −35.8 128.8 −157.4 −345.4 −384.9 −342.2 −350.4 −32.5

(Z)-Me3 Sn-CH C(Me)H

−51.2

(E)-Me3 Sn-CH CH-SnMe3

−52.3

(Z)-Me3 SnCH CH-SnMe3

−60.6

(Me3 Sn)2 C C CH2

−9.6

Me3 Sn-Ph

−28.6

Me3 Sn-2-thienyl

−27.5

Me3 Sn-2-pyridyl

−52.9

Me3 Sn-2-thiazolyl

−32.6

Fe(η5 -C5 H4 SnMe3 )2

−4.2

Me3 Sn-C CMe (C6 D6 )

−73.8

Me3 Sn-C C-Cl (CDCl3 )

−56.1

Me3 Sn-C C-SnMe3 (CDCl3 )

−80.9

Me3 Sn-C C-C C-SnMe3 (CDCl3 )

−59.0

Me2 Sn(CH CH2 )2

−79.4

Me2Sn

CMe2

Me2 SnPh2 Me2 Sn(C CH)2 MeSn(CH CH2 )3 MeSnPh3 c MeSn(C CMe)3 a

−161.7 −60 −154.5 −124.0 5 −98 −250.4

See ref.5,8 for further data. See Reference 231 for numerous alkenyltin compounds. c See Reference 160 for similar compounds. b

1

J (119 Sn,13 C)

336.6 320.0 316.0 308.5 314.0 264.9 258.8 531.1 519.3 1167.7 1036.0 1167.3 1001.0 352.0 (Me) 478.4 (C ) 346.9 (Me) 464.6 (C ) 337.0(Me) 409.0 (C ) 340.0(Me) 496.0(C ) 496.1 (C ) 348.2 (Me) 261.7 (C ) 347.5 (Me) 474.4 (Ph) 373.3 (Me) 395.5 (C ) 346.6 (Me) 605.5 (C ) 373.8 (Me) 461.5 (C ) 357.8(Me) 492.5 (C-1) 404.1 (Me) 502.9 (C ) 406.8 (Me) 385.7 (C ) 400.7 (Me) 388.5 (C ) 405.0 (Me) 372.6 (C ) 369.4 (Me) 466.8 (C )

2

J (119 Sn,13 C) – 23.0 30.0 15.0 20.0 48.3 37.8 35.5 <4 242.5 171.9 241.8 542 (19 F) 77.8

30.5

3

J (119 Sn,13 C)

– – 51.0 – 52.0 51.3 23.6 53.1, 11.7 (4 J ) 20.1 18.6 (29 Si)

5,104a 5, 147 5,147 5,104a 4,147 5,147 148 149 5,147 150 150 228 142 136

49.3

136

1013.0 (119 Sn)

151

491.0(119 Sn)

151

60.6

104e 5, 147

32 37.1

45.2 (C-4) 17.3 (C-5) 93.2

68.6

153 154 155

51.8

40.3

156

107.4

11.0

157

76.6 45.2 83.0

379.2 (Me)

86 47.6 (117 Sn)

157

18.5; 8.0 (4 J ) 158 19.0 (117 Sn,5 J ) 5, 104 88.0

440.0 (C )

88 5 104,157

501.7 (Me) 606.7 (C ) 617.1(Me) 873.9 (C )

Ref.

184.2

16.2

5 159

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Table 2.1.5

119

Sn NMR parameters of some tin(IV) halidesa

Compound

δ 119 Sn

1 J (119 Sn,13 C)

2 J (119 Sn,13 C) 3 J (119 Sn,13 C)

+139 −[2298 (19 F)] 124.5 − [2374 (19 F)] −40.8 − [2463 (19 F)] −261.6 − [2920 (19 F)] 16.4 −274.0 (solid) − [3000 (19 F)] −803 − [1625 (19 F)] [SnF6 ]2− 2− [PhSnF5 ] −692 − [1180, 2430 (19 F)] [Ph2 SnF3 ]− −402 − [2310, 2250 (19 F)] Ph3 SnF-OP(NMe2 )3 −272.0 − [2040 (19 F)] Me3 Sn-Cl (CDCl3 , 33 mol%) 165.7 379.0 [h] (CDCl3 , 15 mol%) 171.2 (acetone, 30.5 mol%) 119.8 Et3 Sn-Cl (CCl4 ) 155 347.3 28.1 Et34 Sn-Cl (THF) 103 300 32 Bu3 Sn-Cl (CDCl3 ) 152.0 337.0 23.2 But3 Sn-Cl 50 (PhCH2 )3 Sn-Cl 51.3 293.0 42.7 Ph3 Sn-Cl (CDCl3 ) −44.7 (HC C)3 Sn-Cl (CDCl3 ) −263.2 1312.7 268.6 (CF3 )3 Sn-Cl −255.8 1179 636 (19 F) (Bu2 SnCl)2 O −139.8 and −92 n.r. 73.2 (117 Sn) N(CH2 CH2 CH2 )3 Sn-Cl 19.5 476.6 28.9 N[CH2 CH2 N(Me)]3 Sn-Cl −180.2 9.1 −192 (solid) −577.9 [HB(3-t Bu-5-Mepz)3 ]Sn-Clb,c Me2 SnCl2 (CH2 Cl2 ) 137.0 Et2 SnCl2 (CCl4 /CH2 Cl2 ) 121.0 439.5 Et2 SnCl2 (THF) 9 586 50 Bu2 SnCl2 (CDCl3 ) 126.3 419.9 36.6 But2 SnCl2 (CCl4 ) 56 (H2 C CH)2 SnCl2 −30.2 (CDCl3 ) 750.7 −394.1 (DMSO) 1469.7 Ph2 SnCl2 (CH2 Cl2 ) −32.0 [Ph2 SnCl3 ]− −250 [Ph2 SnCl4 ]2− −425 (HC C)2 SnCl2 (CDCl3 ) −194.5 1466.1 300.6 (CF3 )2 SnCl2 −179.1 1498 773 (19 F) (Et2 N)2 SnCl2 −100.1 [(Me3 Si)2 N]2 SnCl2 −143.8 BuSnCl3 (CDCl3 ) 6.1 645.0 40.0 BuSnCl3 (DMSO) −457.1 n.r. 71.6 PhSnCl3 (CH2 Cl2 ) −65.0 1118.4 79.1 (PhCMe2 CH2 )3 Sn-F (CDCl3 ) (PhMe2 Si)3 CSnMe2 -F (Me3 Si)3 CSnPh2 -F N[CH2 CH2 N(Me)]3 Sn-F

CF3 SnCl3

−141.5

2055

966 (19 F)

26.8

Ref. 5 162 162 26c 163 163 163 164 5,8,165

64.5 31.8

38.5 23.1

85.4

120.0 210.3 125.5, 25.8 (4 J )

5,86 236 5,86 5 166 5 86 142 167 64b 26c 168 5 5 236 169 5 170 5 163 163 86 142 171 171 172 172 5,86 142

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NMR of Tin Compounds Table 2.1.5

31

Continued

Compound SnCl4 [SnCl6 ]2− Me3 Sn-Br (C6 H6 or CHCl3 ) Et3 Sn-Br Bu3 Sn-Br (CDCl3 ) Ph3 Sn-Br (CDCl3 ) Me2 SnBr2 (C6 H6 or CHCl3 ) Et2 SnBr2 (CCl4 ) Bu2 SnBr2 (neat) MeSnBr3 (C6 H6 or CHCl3 ) BuSnBr3 (CDCl3 ) BuSnBr3 (DMSO) SnBr4 (CS2 ) [SnBr6 ]2− Me3 Sn-I (C6 H6 or CHCl3 ) Bu3 Sn-I (CDCl3 ) Me2 SnI2 (C6 H6 or CHCl3 ) Bu2 SnI2 (neat) MeSnI3 (CCl4 ) SnI4 (CS2 )

δ 119 Sn

1 J (119 Sn,13 C)

2 J (119 Sn,13 C)

−150 ±2 −732 (Cl) 128 148 138.5 −59.8 70.0 96 89.7 −165 −114.7 −604.1 −638 −2070 38.6 86.9 −159 −56.1 −699.5 −1701

–

3

J (119 Sn,13 C)

Ref.

–

–

326.7

23.2

65.0

391.8

34.2

85.4

590.6 n.r.

56.8 69.7

117.8 218.9

5 173 5 5 86 5 5 5 169 5 172

313.0

23.2

64.5

347.9

33

83.6

5 5 5 86 169 5 5

a

See Reference 161 for compilations with 1 J (119 Sn,19 F). See References 5 and 8 for more data. c pz = pyrazolyl. b

The bond angles at the tin atom play an important role. This is most obvious for different ring sizes,83,85,89,90 and the nature of the elements adjacent to the tin atom will, of course, also determine structural properties. 119 Sn nuclear deshielding is typical of tin atoms in five-membered rings (Scheme 2.1.8) when compared with non-cyclic structures and particularly with six-membered or larger rings. In order to allow the assessment of the influence of four- or three-membered rings, the δ 119 Sn data set is still insufficient. It appears that 119 Sn nuclear shielding is fairly high for tin atoms in three-membered rings.84 119 Sn NMR spectroscopy, together with NMR data from other nuclei, serves most conveniently to monitor molecular rearrangements and helps with unambiguous structural assignment. An example is shown in Scheme 2.1.992 (note again the differences in the δ 119 Sn data for five- and six-membered rings). Similarly, the complex products arising from the hydrolysis of butyltin alkoxides were studied by 119 Sn NMR in the liquid and solid states,93 and monitoring of the hydrolysis of BuSnCl3 was also instructive.94 Examples of 119 Sn data for tin compounds with Sn–Group-14 element, Sn B, Sn Tl, and Sn Li bonds are given in Table 2.1.8. Although there are no typical ranges of chemical shifts (with a few exceptions, such as for the Zintl anions), the characterization of these compounds is straightforward, in general, considering the spin–spin coupling (see below) between 117/119 Sn and the magnetically active isotopes of most of these elements (including tin itself).

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Table 2.1.6

119

Sn NMR parameters of compounds with tin–chalcogen bondsa

Compound

δ 119 Sn

Me3 Sn-OH (CH2 Cl2 ) Me3 Sn-OMe (C6 H6 ) Me3 Sn-OPri (C6 H6 ) Me3 Sn-OBut (C6 H6 ) Me3 SnOPh Me3 Sn-O-SiMe3 (Me3 Sn)2 O (C6 D6 ) Me3 Sn-O-ReO3 Bu3 Sn-OMe (C6 D6 )

118 129 109 91 151.0 121.0 110.6 91.8 98.5

Bu3 Sn-OPri Bu3 Sn-OBut Bu3 Sn-OC(O)Me (Bu3 Sn)2 O (C6 H6 ) (But3 Sn)2 O (C6 H6 ) Ph3 Sn-OH (CH2 Cl2 ) (Ph3 Sn)2 O (CDCl3 )

92.1 71.7 103.8 82 −39.3 −86 −85.5

Me2 Sn(OMe)2 (C6 H6 ) Me2 Sn(OBut )2 Me2 Sn(acac)2 (CHCl3 ) Bu2 Sn(OMe)2 (CCl4 ) Bu2 Sn(OBut )2 (But2 SnO)3 (CDCl3 ) MeSn(OEt)3 Pri Sn(OPri )3 BuSn(OEt)3 (C6 H6 ) BuSn(OBut )3 (C6 H6 ) Sn(OPri )4 (CDCl3 ) Sn(OBu)4 (CDCl3 ) Sn(OBui )4 Me3 Sn-SMe (C6 H6 or CHCl3 ) Me3 Sn-SBut Me3 Sn-SC(S)NMe2 (Me3 Sn)2 S (C6 H6 ) Me3 Sn-S-Fcb Bu3 Sn-SBu Bu3 Sn-SBut Ph3 Sn-SMe (CH2 Cl2 ) (Ph3 Sn)2 S (CDCl3 ) Me2 Sn(SMe)2 (C6 H6 or CHCl3 ) Me2 Sn(SCH2 CH2 S) Me2 Sn(SCH2 CH2 CH2 S) Me2 Sn(SCH CHS) (Me2 SnS)3 (C6 H6 ) (Bu2 SnS)3 (CCl4 ) But2 Sn(SCH2 )2

−126.3 −1.8 −365 −159 −34 −84.3 −434 −217.9 (−364.2 solid) −432 −199 −596 −583, −616, −629.6 −373 90 55.5 25.0 86.5 91.3 74.1 50.4 −47 −53.7 144 193.0 (66.9 solid) 149 231 128 126.9 171 (171 solid)

1

J (119 Sn, 13 C)

2

J (119 Sn,13 C)

407.7 480.7 365.1 363.8 363.8 360.1

627.4

20.7, 57.8 (3 J ), 8.7 (4 J ) 19.5 18.4 18.5

45.2, 59.5 (3 J ), 12.2 (4 J )

866.3

41.9

n.r.

n.r.

348.9 330.7 330.8

15.7 21.2 20.8

567.4

44.2, 59.2 (3 J ), 13.1 (4 J )

372.0 445.0 (solid) 368.7 407

<5(13 C) 191 (117 Sn)

Ref. 5 5 5 5 174 175 5 176 86 71b 71b 71b 5 5 5 5,86 5 5 5 5 5 5 5 24 5 5 177 177 177 5 5 178 5 179 71b 71b 5 5,86 5 180 5 86 5 5 181

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NMR of Tin Compounds Table 2.1.6

Continued

Compound (t Bu2 SnS)2 Ph2 Sn(SMe)2 (CH2 Cl2 ) (Ph2 SnS)3 (CDCl3 ) MeSn(SMe)3 (C6 H6 or CHCl3 ) PhSn(SMe)3 (CH2 Cl2 ) (4-Me-C6 H4 Sn)4 S6 d (C6 F5 Sn)4 S6 (SCH2 CH2 S)2 Sn Sn(SPh)4 Me3 Sn-SeMe Me3 Sn-SePh Me3 Sn-Se-Fcb (Me3 Sn)2 Se fc(SnMe2 )2 Sec (Ph3 Sn)2 Se (CDCl3 ) Me2 Sn(SeMe)2 (Me2 SnSe)3 (t Bu2 SnSe)2

δ 119 Sn 124.0 38.5 16.8 167 107 86.4 46.4 277.0 (241.0 solid) 49.5 45.6 55 53.6 44.5 28.1 −78.5 57.1 42 51.2

(Ph2 SnSe)3 MeSn(SeMe)3 Sn(SeMe)4 Sn(SePh)4 Sn(Se-Fc)4 b [Sn4 Se10 ]4−d (Me3 Sn)2 Te (CH2 Cl2 ) (Me2 SnTe)3 (C6 D6 ) (But2 SnTe)e2

−44 14.8 −80.5 −132.7 −218.3 −350.1 −66.8 −195 −125.2

Sn(TePh)4 [SnTe3 ]2− [SnTe4 ]4− [Sn2 Te6 ]4−d

−570.5 −1170.1 −1823.6 −1675.0

[Sn2 Te7 ]4−

−1344.9

a

33

1

J (119 Sn, 13 C) 363

2

J (119 Sn,13 C)

Ref.

114 (117 Sn)

183 5 5 5 5 182 182 180 184 5 5 179

260.7 (117 Sn) 290.0

333.2 977.6 (77 Se) 1060 (77 Se) 364.1 (Me) 491.1 (fc) 1133(77 Se) 544.0 1213 (77 Se) 1228 (77 Se) 923 (77 Se) 1380 (77 Se) 1520 (77 Se) 1584 (77 Se) 1700 (77 Se) 2274, 1536 (77 Se) 1385 (125 Te) 3098 (125 Te) 289 2117 (125 Te) 3379 (125 Te) 4535 (125 Te) 2851 (125 Te) 3998, 1633 (125 Te) 4023, 2902, 2319 (125 Te)

19.3 60.5 (fc) 204.8(117 Sn) 44.8, 58.0 (3 J ), 11.8 (4 J ) 333 17 (117 Sn)

11.9 342 (117 Sn) 29 (117 Sn)

124a 185 5, 86 5 5 183 5 5 5 184 179 187 124a 5 183 184 187 187 188 188

See References 5 and 8 for more data; see Reference 44b for three-coordinate tin compounds with Sn S (δ 119 Sn 531) and Sn Se bonds (δ 119 Sn 440). b Fc = C5 H4 FeCp (1-ferrocenyl). c fc = 1,1’-ferrocenediyl. d For 119 Sn NMR data of chalcogenide derivatives of imidotin cage complexes, see Reference 231. e See Reference 234 for other four-membered rings (R2 SnTe)2 .

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Tin Chemistry: Fundamentals, Frontiers and Applications

Table 2.1.7

119

Sn NMR parameters of compounds with Sn N, Sn P, Sn As, Sn Sb and Sn Bi bondsa 1

Compound

δ

119

Sn

1

119

J(

13

Sn, C)

Me3 Sn-NMe2 Me3 Sn-NEt2 Me3 Sn-N(H)Ph Me3 Sn-N(Me)Ph Me3 Sn-NC4 H4 (Me3 Sn)2 NMe (Me3 Sn)2 NPh (Me3 Sn)2 NSiMe3

75.5 60 46.4 73.0 72.9 81 63.0 66

378.1

(Me3 Sn)3 N

86.3

366.2

But3 Sn-NH2 (But3 Sn)2 NH (Me3 Sn)2 NC(O)Me solid-state (C6 H11 )3 Sn-NCS Me2 Sn(NMe2 )2 Me2 Sn[N(Me)Ph]2 (MeC C)2 Sn(NEt2 )2 MeSn(NMe2 )3 MeSn[N(Me)Ph]3 PhC C-Sn[N(Me)CH2 CH2 ]3 N Sn(NMe2 )4 Sn[N(Me)Ph]4 [Sn(NCS)6 ]2− Me3 Sn-PEt2 (C6 D6 ) Me3 Sn-PBut2 (C6 D6 ) Me3 Sn-PPh2 (C6 H6 ) (Me3 Sn)2 PBut (C6 D6 ) (Me3 Sn)2 PPh (CH2 Cl2 ) (Me3 Sn)3 P (C6 D6 ) (Me3 Sn)3 P-Cr(CO)5 (Me3 Sn)3 P-W(CO)5 fc(SnMe2 )2 PPhb Me2 Sn(PPh2 )2 (2,4,6-i Pr3 -C6 H2 )2 Sn PArc [(Me3 Si)2 CH]2 Sn PArc Sn[P[Si(F)(C6 H2 −2,4,6-Pri3 )2 ]2 Cl4 Sn(PBu3 )2 (Me3 Sn)3 As (Me3 Sn)3 As-Co(CO)2 NO (Me3 Sn)3 Sb (Me3 Sn)3 Bi a

−27.9 −16.1 40.7, 126.6 0.3, −19.4 −3.2 58.8 30.5 −176.9 −15.1 −66.0 −173.9 −172 (solid) −121.8 −175.6 −842 −11.1 −37.8 −3.0 −0.1 14.2 37.2 64.2 63.4 1.3 −11.5 499.5 658.3 1551 −573 6.2 31.7 −90 −110.0

1

J (119 Sn,15 N) J (119 Sn,31 P)

other coupling constants

−26.3 +2.2 −37.2

389.6

−41.7 −83.7 113.3 138.5

n.r.

136 (14 N)

1009.6

+24.0 146.8

785.5

+87 n.r.

288.8 303

+175 150 (14 N) 665.3 792 586 816 724 829 409.5 375.5 735.4 808 2208 2295 1682 2395

68.5 (117 Sn,2 J ) 8.3 (29 Si,2 J ), (117 Sn, 2 J ) 6.5(13 C,3 J ) 186.8(117 Sn,2 J ) 16.5(1 H,2 J ) 35.0 (1 H,2 J )

2

135.9 J (119 SnC

13

C)

144 (117 Sn) 365.4 (117 Sn)

Ref. 5 5 114a 114a 5, 106a 5 114a, 119 107 107, 119 190 190 191 192 5 118 193 5 118 26c 5 118 194 5 5 5 5 5 122 122 122 195 5 44 44 196 5 198 198 5 5

See References 4 and 8 for more data; see Reference 233 for three-coordinate tin–nitrogen compounds: β-diketiminatotin(II) halides, amides, chalcogenides. fc = 1,1’-ferrocenediyl. c Ar = 2,4,6-But3 -C6 H2 . b

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NMR of Tin Compounds δ119Sn

δ119Sn SnMe2

Et

SnMe2 34 SnMe2 35

SnMe2 8.4 86 36

53.5 83

33

Et

SnMe2 S

SnMe2 S 40

Et

–42.5 83

SnMe2 133.5 37 SiMe3

Et2B

Me2Sn

SnMe2 38

87

190.0 83 231.0 86

C Se

105.8 91

Et N Me2Si SnMe2 Me2Si N 44 Et

C Se

–177.5 88

δ119Sn

SiMe3 N SnMe2 N 43 SiMe3

39

S

SiMe3

72.4 85

δ119Sn

S

SnMe2 213.2 90 41

35

73.9 91

S SnMe2 S

Scheme 2.1.8

149.0 83

42

Examples of five- and six-membered stannacycles

Numerous compounds with Sn M bonds, where M is a transition metal, have become available by various routes (e.g. insertion of SnCl2 into M Cl bonds, oxidative addition, or simply by metathesis accompanied by alkali metal halide formation). Examples are given in Table 2.1.9. For the tin coordination number 4, one observes an increase in 119 Sn nuclear shielding by a bond between tin and more heavy metals; a similar trend is well known for 13 C and 31 P nuclear shielding in C M and P M bonds, respectively. Tin Compounds With a Formal Oxidation State < +2

The tin nuclei in Zintl anions95 of the type [Sn5 ]2− , [Sn9 ]4− and others such as [Sn8 Tl]5− and [Sn9−n Pbn ]4− , and similar species containing Pt,96 are highly shielded (δ 119 Sn values range from around −1100 to around −1900). The fluxional cluster anion [Sn9 ]4− functions as a ligand in transition metal complexes (e.g. [1-M(CO)3 (η4 -Sn9 )]4− with M = Cr, Mo, W 97 ). These complexes are anionic clusters with a closo structure (bicapped quadratic antiprisms), and are rigid on the NMR timescale, in contrast to the fluxional Zintl anions. Therefore, they provide a wealth of NMR data (δ 119 Sn and coupling constants) for describing the bonding situation. On the other hand, the least shielded 119 Sn nuclei known so far belong to transition metal complexes containing trigonal-planar coordinated tin atoms in the formal oxidation state 0 (48)98 or −2 (4999

Me

Et

Me2Sn

BEt2 N

45

Me

Et

Me2Sn

BEt2 S

S

Me2Sn 47

15.2

77.3

δ11B

–9.6

40.6

2.5 –92

–147

Et

B

Et S

δ119Sn –106.5 δ14N

Scheme 2.1.9 NMR92

Et

N

N

46

Me

–344

An examples of twofold intra-molecular rearrangement monitored by

119

Sn,

11

B and

14

N

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Tin Chemistry: Fundamentals, Frontiers and Applications

Table 2.1.8 119 Sn NMR parameters of compounds with tin–silicon, –germanium, –tin, –lead, –boron, –lithium and –potassium bondsa Compound Me3 Sn-SiMe3 (Me3 Sn)4 Si (Ph3 Sn)2 SiPh2 (Me3 Sn)4 Ge (Ph3 Sn)2 GePh2 Me3 Sn-SnMe3 Me3 Sn-SnPh3 CH2 (SnMe2 SnMe2 )2 CH2 Me2 (Ph)Sn-Sn(Ph)Me2 fc(SnMe2 )2 b

δ 119 Sn −126.7 −34 −167 −25.2 −147 −113 −91.5, −153 −78.5 −120.2 −45.3

(Me3 Sn)2 SnMe2 fc(SnMe2 )3 b

−99.5, −261.7 −102.5 (C Sn) −249.3 ( Sn )

(Me3 Sn)3 SnMe (Me3 Sn)4 Sn Ph3 Sn-SnPh3 (Me2 Sn)6 c (But2 Sn)4 (100 ◦ C) [MeN(CH2 CH2 CH2 )2 SnMe]2 [Sn4 ]2− (en) [Sn9 ]4− (en) [Sn8 Tl]5− (en)

−89.5, −489.7 −80, −739 −143.6 −241.4 99.0 −101.9 −1895 −1230 −1167

Me3 Sn-PbMe3 Me3 Sn-PbBut3 Ph3 Sn-PbMe3 [Me3 Sn-BH3 ]Li (THF) Me3 Sn-B(NMe2 )2 (Me3 Sn)2 BNMe2 2,3-μ-Ph3 Sn-B5 H8 1-Ph3 Sn-B5 H8 2-Ph3 Sn-B5 H8 SnC2 Me2 B9 H9 d [Sn(C2 Me2 B9 H9 )2 ]2−d Me3 Sn-Li (THF) Bu3 SnLi / Et2 O (But CH2 )3 SnK a

−57.0 21.0 −119.5 −28.5 −150 −149 −98.3 −89.2 −87.5 −379 449 −183 −155 −221; −211 (solid)

1

J (119 Sn,13 C)

413 258.1 419

227.0 (Me) 338.9 (C-1) 244.7 (Me) 185.0 (Me) 362.9 (C-1)

162

n.r.

1

J (119 Sn,M)

Ref.

656 (29 Si) 220 (29 Si) 515 (29 Si) 724 (2 J , 117 Sn) 37.5 (73 Ge) 725 (2 J , 117 Sn) 4460 (117 Sn) 4240 (119 Sn) 4245 (117 Sn) 4153 (117 Sn) 5274 (117 Sn)

114a 131 134

2873 (119 Sn) 2875 (119 Sn) 537.2 (2 J , 117 Sn) 1733 (119 Sn) 881 (119 Sn) 4470 (117 Sn) 1176 (117 Sn) 1195 (117 Sn) 4060 (117 Sn) 1281 (117 Sn) 266 (117 Sn) 429 (117 Sn) 800 (203,205 Tl) 3570 (207 Pb) 1637 (207 Pb) 2800 (207 Pb) 554 (11 B) 953 (11 B) 657 (11 B) 1117 (11 B) 1061 (11 B)

402.5 (7 Li) 289 (39 K)

115 134 114a 114a 198 86 199 5 200 200 5 5,131 5 201 202 203 5 5 5 114 117 114b 204 204 204 205 205 205 206 206 110 208 209

See References 5 and 8 for more data; n.r. means not reported. fc = 1,1 -ferrocenediyl. c These data have been reported for the cyclic pentamer; however, the intensity pattern of 117 Sn satellites (2:2:1) correspond to the hexamer. d For other borane clusters containing tin, see e.g. Reference 208. b

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NMR of Tin Compounds Table 2.1.9

119

37

Sn NMR parameters of compounds with a transition metal–tin bonda

Compound

δ

119

Sn

[Me3 Sn-Ti(CO)6 ]− [Ph3 Sn-Ti(CO)6 ]− [(Me3 Sn)2 Zr(CO)5 ]2− [(Me3 Sn)4 Zr(CO)4 ]2− [(Ph3 Sn)4 Zr(CO)4 ]2− [(Ph3 Sn)4 Hf(CO)4 ]2− Cp2 Zr[Sn(CHSiMe3 )2 ]2 Me3 Sn-Ta(Cp)2 H2 Me3 Sn-Cr(Cp)(CO)3 Me3 Sn-Mo(Cp)(CO)3 Me3 Sn-W(Cp)(CO)3 b Cl2 Sn(PBut3 )-W(CO)5 (2-Ph2 PCH2 -C6 H4 )2 SnW(CO)5 Me3 Sn-Mn(CO)5 (Ph3 Sn)2 Mn(Cp’)(CO)2 Ph3 Sn-Mn(Cp’)(H)(CO)2 Me2 Sn[Mn(CO)5 ]2 MeSn[Mn(CO)5 ]3 Me3 Sn-Re(CO)5 Me2 Sn[Re(CO)5 ]2 (Me3 Sn)2 Fe(CO)4 (Me3 Sn)2 Fe(CO)3 CS (Me2 ClSn)2 Fe(CO)4 [Me2 SnFe(CO)4 ]2

35.0 23.5 16.4 49.5 337.0 301.0 +1677.6 53 161 121 43.1 2.9 −7.5

{Me2 Sn[Fe(CO)4 ]2 }2 Sn

123.8 (Me2 Sn) 287.7 (Sn) 1532.0 144.1 689.6 889.9 −34.6

Sn[Fe(CO)4 ]4 Me3 Sn-FeCp(CO)2 [Sn(Cl)Fe(Cp)(CO)]2 Sn[FeCp(CO)2 ]2 [Ru(SnCl3 )6 ]4− (3M HCl)

63 61.1 58.6 150 284 −89 −223 80.4 78.5 258.5 87.8

[Os(SnCl3 )5 Cl]4− (MeNO2 )c

−531.8 (eq) −581.8 (ax)

Me3 Sn-Co(CO)4 Me2 Sn[Co(CO)4 ]2 MeSn[Co(CO)4 ]3 (OEP)Rh-SnBu3 d (OEP)Rh-SnPh3 d (Bu3 Sn-μ-H)2 Rh(H)(PPh3 )2 [Rh(SnCl3 )5 ]4−

151 293 483 118.3 −121.6 −11.7 8.5

[J (119 Sn,M)] { J (119 Sn,117 Sn)} 2

– – – – {1100} – {630.0}

[150] [1470] [894.0]

J (119 Sn,13 C)

J (119 Sn,31 P)

Ref.

94.0 (2 J ) 97.0 (2 J ) 49.0 (2 J ) 92.0 (2 J ) 100.0 (2 J ) 93.0 (2 J )

– – – – – – –

210 210 211 211 212 212 213 5 5 5 5 5 214

179.0

(1 J

)

136.0(2 J , 1 H)

{349.8} {332.3} {302.0} [64.0] {1798} {1580} [36.0] [42.7] 2981 (1 J(Sn,Sn)) {2474 (cis) 13460 (trans)} [1290] {2270 (cis, eq, eq), 1614 (eq, ax), 19465 (trans, eq,eq)}

[314.2] [412.9] [102.9] [806] {3803}

274.4 (1 J ) 282.3 (1 J ) – 86.0, 48.0 28.0 (2 J ) – 42.0 (2 J ) – 95 (2 J ) 2474 (2 J cis) 13460 (trans)

n.r. n.r.

1560 (1 J ) 443.0

–

– – – – – – – –

5 215 215 5 5 5 5 216 216 217 217 217 217 217 217 218 218 219 219 5

5 5 5 221 221 223 5 (Continued )

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Tin Chemistry: Fundamentals, Frontiers and Applications

Table 2.1.9

119

Sn NMR parameters of compounds with a transition metal–tin bonda δ

Compound [Rh(H)(SnCl3 )5 ]3− [Ir(H)(SnCl3 )5 ]3− cis-(Me3 Sn)2 Pt(PPh3 )2 fcSnMe2 Pt(PPh3 )2 SnMe2 e

119

[J (119 Sn,M)] { J (119 Sn,117 Sn)} J (119 Sn,13 C) 2

Sn

77.8(ax) −13.7 (eq) −185.6 (ax) −318.8 (eq) −16.5 −27.5

[532,ax] [596,eq] [e] [8532.0] [9056.0]

cis-Me3 Sn(Me3 SnC=C)Pt- −57.5 (SnPt) [8140] (PPh3 )2 −88.9 (SnC ) 278.8 (3 J (Pt,Sn) −144.0 [15077] [etpPt-SnCl3 ]− f cis-[PtCl2 (SnCl3 )2? ]2− [Pt(SnCl3 )5 ]2−

−375 −126.4

[27627] {2936} [15791] {6057}

(Continued)

J (119 Sn,31 P)

Ref.

–

224 224 224

– −

652.3[f] 141,224 +206.0 (cis) 141 −1574.5 (trans) 111.0 (cis) 141 1606 (trans) 190.0 (cis) 225 2044.0 (trans) 226 – 227

– – – – – –

a

See References 5 and 8 for more extensive compilations. A relationship between the W Sn bond length and the magnitude of 1 J (183 W,119 Sn) has been proposed for tungstenocene–stannyl complexes.235 c For some other complexes with Os Sn bonds, see Reference 220. d OEP = 2,3,7,8,12,13,17,18-Octaethylporphyrinato ligand. e 1,3-Distanna-2-platina-[3]ferrocenophane. f etp = tris(2-diphenylphosphanoethyl)phosphane. b

(Scheme 2.1.10). This deshielding is related to the presence of an energetically low-lying π * LUMO available for B0 -induced rotation of charge, causing an increase in the paramagnetic contribution to magnetic shielding.

(OC)2(Cp)Mn

Mn(Cp)((CO)2

Cr(CO)5

Sn

Sn Mn(Cp)(CO)2

48

δ119Sn 3301

Scheme 2.1.10 Two examples of extreme metal complexes

(OC)5Cr

2-

Cr(CO)5 49

δ119Sn 3924 119

Sn nuclear magnetic deshielding in low valent tin transition

Isotope-Induced 119 Sn Chemical Shifts

There are no primary isotope shifts for the magnetically active tin nuclei.100 As expected for a heavy nucleus, such as 119 Sn, secondary isotope effects may be quite large and can be readily detected. In tributyltin tritide, the 1 1/3 H(119 Sn) value was determined as −2.7 ppm (to be compared with 1 1/2 H(119 Sn) = − 1.8 ppm),101 whereas in stannane SnH4−n Dn , the 1 1/2 H(119 Sn) values are ca.−0.403 ppm/D.102 Weak intra-molecular association can be deduced from long-range isotope effects n 1/2 H(119 Sn), e.g. in an RO(H). . . Sn fragment.103

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Numerous data have been collected for 1 12/13 C(119 Sn).104 These isotope-induced shifts possess the expected negative sign (shifts of the 119 Sn NMR signals to lower frequency for the heavier isotope) as well as a positive sign, depending on the other substituents at the tin atom. In addition to mass-dependent vibrational effects, responsible for part of the isotope-induced chemical shifts, there is also a strong contribution arising from electronic effects.105 Typically, the latter effects become increasingly important and noticeable for heavy nuclei. The application of HEED extended pulse sequences106 has opened the way to the measurement of isotope shifts 1 14/15 N(X) in natural abundance of 15 N (0.37 %). Expectedly, the isotope-induced effect of the different tin isotopes 117 Sn and 119 Sn on chemical shifts of other nuclei is very small, considering the small difference in their masses. However, precise measurements of 13 C104d,e and 15 N NMR spectra107 revealed these tiny effects on 13 C and 15 N nuclear shielding, respectively [1 117/119 Sn(13 C), 1 117/119 Sn(15 N), 1 115/119 Sn(15 N)].

2.1.5

Indirect Nuclear Spin–Spin Coupling Constants n J(119 Sn,X)

General

Indirect nuclear spin–spin coupling constants n J (119 Sn,X) can be measured either from 119 Sn NMR spectra, as splitting or as satellites (dependent on the natural abundance of X), or from X NMR spectra as 117/119 Sn satellites accompanying the central X NMR signal. The presence of two magnetically active spin-1/2 isotopes of the same element, 119 Sn and 117 Sn in fair natural abundance, allows for the measurement of 119 Sn–117 Sn spin–spin coupling, if the two tin atoms are in identical surroundings, or, additionally, of 119 Sn–119 Sn spin–spin coupling for different surroundings (Figure 2.1.8). The J values possess a sign, positive or negative, which frequently is not given, although the signs are known in the majority of cases. In case of ambiguity, the respective sign can be determined by appropriate 1D or 2D NMR experiments7 (see Figure 2.1.9 for an example). One should keep in mind that γ (119 Sn) < 0 (see Tables 2.1.1 and 2.1.2 for the signs of other γ values). Therefore, it might be more convenient to use the notation of reduced coupling constant n K (119 Sn,X), n

K (119 Sn,X) = n J (119 Sn,X) 4π 2 (γ (119 Sn) γ (X) h)−1

(2.1.2)

since J and K values possess opposite signs if γ (X) > 0. Examples for various coupling constants involving 119 Sn can be found in the Tables 2.1.3–2.1.10. One-Bond Coupling Constants 1 J(119 Sn,X)

The sign of 1 J (119 Sn,1 H) is negative (1 K (119 Sn,1 H) > 0), typical of one-bond spin–spin coupling involving 1 H. The changes in the magnitude of 1 J (119 Sn,1 H) (Table 2.1.3) appear to depend on the sum of the electronegativities of the other substituents at tin, as expected, if the concept of rehybridization108 is valid. This simple relationship might become more complicated in the case of five-coordinate tin hydrides,109 where the Sn H bond in solution might prefer, on average, either axial or equatorial positions. In the known organotin(IV) compounds, the coupling constants 1 J (119 Sn,13 C) are negative (1 K (119 Sn,13 C) > 0), although the concept of rehybridization is no longer strictly valid (see Scheme 2.1.11 for some selected 1 J (119 Sn,13 C) data). The sign of 1 J (119 Sn,13 C) changes to positive (1 K (119 Sn,13 C) < 0) in triorganotin lithium derivatives.110 In organotin(II) compounds, the influence of the lone pair of

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Figure 2.1.8 186.5 MHz 119 Sn{1 H} NMR spectrum (refocused INEPT pulse sequence with CPD 1 H decoupling) of an organometallic-substituted alkene (23 ◦ C in C6 D6 ) bearing two stannyl groups (see Figure 2.1.10 for the different line widths). The 117 Sn satellites (arrows) corresponding to 2 J ( 119 Sn,117 Sn) = 811 Hz are readily identified as part of an AX spin system, whereas the 119 Sn satellites corresponding to 2 J ( 119 Sn,119 Sn) form an AB-spin system, of which only the inner lines (asterisks) are visible.86

electrons111 at the tin atom will cause a negative sign of 1 K (119 Sn,13 C).36 The bond angle CSnC (θ) appears to be related to the magnitude of 1 J(119 Sn,13 C) for dimethyltin(IV) [|1 J (119 Sn,13 C) | = 10.7 (± 0.5) θ − 778 (± 64)] 112 or dibutyltin(IV) compounds [1 J (119 Sn,13 C) | = 9.99 (± 0.73) θ − 746 (± 100)].113 The sign of 1 J (119 Sn,29 Si) is positive in silylstannanes (1 K (119 Sn,29 Si) > 0),114 (see Scheme 2.1.12), and also a positive sign of 1 K(119 Sn,73 Ge) in (Me3 Sn)4 Ge has been determined.115 In the case of the tris(trimethylsilyl)stannyl anion, the magnitude of 1 J (119 Sn,29 Si) is dependent on the cation and the solvent.133 It is likely that sign inversion has to be considered, which was experimentally verified for 1 119 J ( Sn119 Sn) in the corresponding tris(trimethylstannyl)stannyl anion.135 In the case of polystannanes, sign inversion of 1 J (119 Sn,119 Sn) from positive (1 K (119 Sn,119 Sn) > 0) to negative (1 K (119 Sn,119 Sn) < 0) has been proposed as a result of the increase in the Sn Sn bond length, when it exceeds 285 pm.116 In the case of plumbylstannanes, positive and negative 1 K (207 Pb,119 Sn) values have been determined experimentally (e.g. 1 J (207 Pb,119 Sn) = −3570 Hz for Me3 Sn–PbMe114b and +1637 Hz for 3 Me3 Sn-PbBut3 117). Owing to the polar Sn N bond and the presence of the lone pair of electrons at the nitrogen atom, coupling constants 1 J (119 Sn,15 N) may be of either sign (Scheme 2.1.13), and in the case of triorganostannylamines, changes in sign may even have to be considered for different R groups linked

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Figure 2.1.9 Contour plots of the 50.4 MHz 15 N/ 1 H (left) and 186.5 MHz 2D119 Sn/ 1 H shift correlations of 15 N enriched (ca. 10%) PhN(SnMe3 )2 . The experiments serve for comparison of signs of reduced coupling constants K , and positive and negative tilts of the cross peaks indicate alike or different signs of K , respectively. In the formulae A to D, the arrows indicate the path of 1 H polarization transfer, and the dashed lines from the respective passive to the active nuclei show which reduced coupling constants are compared with respect to their signs. Thus, in the 15 N/ 1 H correlation, 119 Sn or 117 Sn is the passive nucleus, and in the 119 Sn/ 1 H correlation 15 N (labeled ‘C’) and 117 Sn (labeled ‘D’) are the passive nuclei. Since it is known that 2 K (Sn, 1 H Me ) < 0, all other signs (see ‘results’) follow unambiguously. (Adapted from Reference 119.)

J(119Sn,13C) [Hz] (Me) 294.0 436.6 Me3SnBut 353.5 450.9 Me3SnCH=CH2 Me3SnPh 347.5 474.4 CH Me3SnC 410.3 414.8 Me3SnH 349.5 Me3SnCl 379.0 396.3 (Me3Sn)2O Me3SnNEt2 381.0 1

1

J(119Sn,13C) [Hz]

336.6 SnMe4 320.0 SnEt4 308.5 SnPri4 258.8 Sn(CH2Ph)4 Sn(CH=CH2)4 519.3 531.1 SnPh4 Sn(C CH)4 1176.2 1001.0 Sn(CF3)4

Scheme 2.1.11

J(119Sn,13C) [Hz] (Me) 369.4 466.8 Me2Sn(CH=CH2)2 1

Me2Sn(C

CH)2

(Me3Sn)4C Me2SnH2 Me2Sn(NEt2)2 MeSn(C CH)3 MeSn(NEt2)3

503.0 316.3 485.1 469.7 630.9 592.4

576.2 108.3

833.0

Some representative examples of one-bond 119 Sn–13 C spin–spin couplings

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Me3SnSiMe3 (Me3Sn)2SiMe2 (Me3Sn)4Si

J(119Sn,29Si) [Hz]

656 114a 504 131 (Ph3Sn)2SiPh2 t 220 131 Bu 2Sn(SiPh3)2 132

Me2Sn(SiMe3)2

525

Sn(SiMe3)4

351 133

Ph2Sn(SiMe3)2

–

[Sn(SiMe3)3] [Cs(2.2.2)crypt]+

Scheme 2.1.12

515 134 357 132 520 132

Me2 Me2 Si t But2Sn SiSiMe Bu 2Sn SiMe2 2 t t Bu 2Sn SiMe2 Bu 2Sn Si Si Me2 Me2 214 189b 217 189b

355 133

Selected examples of one-bond 119 Sn–29 Si spin–spin couplings

to the nitrogen atom.114a,118,119 The magnitude of 1 J (119 Sn,15 N) in monomeric bis(amino)stannylenes is large,26,120 and a negative sign (1 K (119 Sn,15 N) < 0!) can be safely assumed, since there are lone pairs of electrons at both nitrogen and tin atoms. In any case, 119 Sn–15 N coupling may serve, together with δ 119 Sn data, as an indication of coordinative Sn–N bonding.109,121 1

Me3SnN(H)Ph Me3SnN(Me)Ph Me3SnN(SiMe3)2 1-Me3Sn-pyrrole 1-Me3Sn-indole 8-Me3Sn-carbazole Me3SnN(PMe2)Ph Me3SnN[P(S)Me2]Ph

J(119Sn,15N) [Hz]

–26.3 +2.2 –52.5 –37.2 –45.3 –49.3 +9.5 –21.0

(Me3Sn)2NBut (Me3Sn)2NPh (Me3Sn)2NSiMe3 (Me3Sn)2NGeMe3 (Me3Sn)3N (Me3Sn)2NBC8H14 (Me3Sn)2NNMe2

–38.0 –41.4 –59.0 –68.4 –83.7 –31.9 –45.5

Me2Sn[N(Me)Ph]2 +24.0 MeSn[N(Me)Ph]3 +87.0 +175.0 Sn[N(Me)Ph]4 Sn[N(But)SiMe3]2 –398.9 –366.0 Sn[N(SiMe3)2]2 But N SiMe2 Sn SiMe2 N

–382

But

Scheme 2.1.13 Sign and magnitude of various coupling constants 1 J ( 119 Sn15 N) for tin(IV)- and tin(II)nitrogen compounds

Except for the phosphane adducts of tin(IV) halides, the experimental evidence points towards a positive sign of 1 J (119 Sn,31 P) (1 K(119 Sn,31 P) < 0!) in all tin–phosphorus(III) compounds,122 a consequence of the dominant influence of the lone pair of electrons at the phosphorus atom. Even for transition metal complexes of stannylphosphanes,122 or in borane complexes of stannylphosphanes,123 the coupling sign remains positive. A negative sign of 1 K (119 Sn,77 Se)124a and 1 K (125 Te,119 Sn)124a is likely for all tin–selenium and tin– tellurium compounds,124 and this can also be assumed for 1 K (119 Sn,19 F). In contrast, a positive sign can be attributed to the 1 K (119 Sn,M) (M = transition metal nuclei) values known so far. In the case of Sn X bonds and quadrupolar nuclei X, the 119 Sn NMR signal may be broadened without resolved splitting, as a result of scalar relaxation of the second kind.125 In this case, the coupling constants can be calculated, if the relaxation time of the X nucleus is known.126 In favorable cases, the pattern of the solid-state 119 Sn NMR spectra can be analyzed with respect to the isotropic indirect nuclear spin–spin coupling (e.g. in Ph3 SnCl, 1 J (119 Sn,35 Cl) = +280 and +275 Hz).127

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Two-Bond (Geminal) Coupling Constants 2 J(119 Sn,X)

There is a wealth of data for 2 J (119 Sn,1 H) in organotin compounds, mainly for SnMe groups, and in this case a smoothly curved relationship between 2 J (119 Sn,1 H) and 1 J (119 Sn,13 CMe ) has been noted.110 The sign of 2 J (119 Sn,1 H) is positive, in general (2 K (119 Sn,1 H) < 0!), for intervening aliphatic carbon atoms, in contrast with the negative sign of 2 J (119 Sn,1 H) if the intervening carbon atom is olefinic (e.g. for all vinyl tin compounds). The general trend of data and sign of 2 J (119 Sn,13 C) data is analogous to 2 J (119 Sn,1 H). However, since the magnitude of 2 J (119 Sn,13 C) is less predictable than that of 2 J (119 Sn,1 H), it is advisable to determine the sign of 2 J (119 Sn,13 C) prior to discussion.128 This advice applies similarly to other coupling constants 2 119 J ( Sn,X), and the relevant NMR techniques have been reviewed.7 In five-membered stannacycles, the coupling pathways across two and three bonds have to be considered, and since the respective contributions are of opposite signs, in most cases, the interpretation of the magnitude of these coupling constants is not always straightforward. If the intervening nuclei are carbon atoms, the contribution from 2 J is usually small when compared with that of 3 J . For transition metal complexes with two or more SnCl3 ligands, the magnitude of the coupling constants 2 119 J ( Sn,117 Sn) across the metal can range from small values to several hundred and several thousand Hz, depending on the stereochemistry, other substituents and the intervening metal atom. As expected, a trans-coupling pathway usually leads to very large coupling constants. Three-Bond (Vicinal) Coupling Constants 3 J(119 Sn,X)

The magnitude of vicinal coupling constants depends on the dihedral angle in the respective fragment (Karplus-type dependence). This relationship holds also for 3 J (119 Sn,X) (e.g. X = 1 H, 11 B, 13 C, 15 N) in many tin compounds (see Scheme 2.1.14) for examples with X = 13 C), and has been widely exploited for structural assignments.4,5,7,85,129,136 3

Me3Sn

J(119Sn,13C) [Hz]

CH3

Me3Sn

CH2Me 64.3 130 BEt2

Me3Sn

BEt2 83.0 130 CH3

49.3 136 Me3Sn CH3 Me3Sn

CH3

Me3Sn

79.0 136 Me3Sn 106.0 (trans) 137 77.8 (cis) 137 Me3Sn

CH3

141.5 (trans) 138 BEt2 118.2 (cis) 138 2

Me2Sn

CMe2

88.0 88

Me2Sn 5

Me2Sn

H2 C

4.2 85

B

Dihedral angle Sn-C-C-CH2 close to 90°

Scheme 2.1.14

Me2Sn

63.0 86 CH2CH3 3 2 3 86 4 66.0 (3) ( J + J ) 61.0 (4) ( 2J + 3J ) 86 H2 C

68.4 85

B

Dihedral angle Sn-C-C-CH2 close to 180°

Stereochemical dependence of three-bond (vicinal) 119 Sn–13 C spin–spin couplings

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Figure 2.1.10 76.4 MHz 119 Sn[1 H} NMR spectrum of a mixture of isomers of alkenylstannanes, in which a boryl group is either in cis- or in trans-position relative to the stannyl group.130 The line widths of the 119 Sn NMR signals are a function of the residual broadening owing to partial relaxation of three-bond 119 Sn 11 B spin–spin coupling.86

So far, most signs determined for 3 K (119 Sn,X) are positive, with very few exceptions. If X is a quadrupolar nucleus such as 11 B, differential broadening of the 119 Sn NMR signals (see Figures 2.1.3 and 2.1.10 for partially relaxed scalar 119 Sn–11 B coupling) indicates the relative magnitudes of |3 J(119 Sn,11 B)|.130 Long-Range Coupling Constants n J(119 Sn,X) with n > 3

As is typical for heavy spin-1/2 nuclei with an appreciably large magnetic moment, 119 Sn–X spin–spin coupling across four or more bonds is often observed (e.g. X = 1 H, 13 C, 19 F). However, these data are diagnostic only in the case of series of analogous compounds. One problem in this context is that the sign of such coupling constants may change readily, and a second problem is the fact that these data receive only scant attention. In general, n J (119 Sn,X) data are measured if: (i) multiple equivalent coupling pathways through polycyclic structures, (ii) π -bonding systems, or (iii) a metal, or both a metal and π -bonding systems are involved. Thus, in most benzyltin compounds 5 J (119 Sn,13 C) can be measured. The same is true for 2-picolyltin derivatives.106b The π-system of allenes is also well known to mediate spin–spin couplings across four or five bonds, and various types of long-range coupling constants n J (119 Sn,1 H), n J (119 Sn,13 C) and n 119 J ( Sn,119 Sn) (n = 4,5; see Reference 139 for other n J (Sn,Sn) data) have been determined, including the respective signs.138 Two examples for Sn–Sn coupling across six and four bonds with intervening metals and π-systems are given in Scheme 2.1.15. In principle, coupling constants n J (119 Sn,X) (n > 3) have to be considered as valuable parameters for structural assignments. Moreover, in particular for X = 1 H, they can serve for improving NMR

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Me3Sn

PEt3 Pt PEt3

6J(119Sn,117Sn)

Scheme 2.1.15

SnMe3 25.0 Hz 140

Me3Sn

45

SnMe3 Pt PPh3 PPh3

4J(119Sn,117Sn)

17.5 Hz 141

Long range Sn–Sn spin–spin coupling constants in platinum(II) complexes

experiments in inverse 1 H/119 Sn HMBC experiments.1 Furthermore, various selective 1D and 2D polarization transfer experiments can be based on small coupling constants n J (119 Sn,1 H). References 1. 2. 3. 4. 5. 6.

7.

8. 9. 10. 11.

12.

13. 14. 15. 16. 17.

18. 19. 20.

J. C. Martins, M. Biesemans, and R. Willem, Progr. NMR Spectrosc. 36, 271 (2000). J. D. Kennedy and W. McFarlane, Rev. Silicon, Germanium, Tin, Lead Compounds 1, 235 (1974). P. J. Smith and A. P. Tupciauskas, Annu. Rep. NMR Spectrosc. 8, 291 (1976). J. D. Kennedy and W. McFarlane, in Multinuclear NMR, J. Mason (Ed.), Plenum Press, New York, 1987, pp. 305–333. B. Wrackmeyer, Annu. Rep. NMR Spectrosc. 16, 73 (1985) F. Kayser, M. Biesemans, M. Gielen, and R. Willem, in Physical Organometallic Chemistry – Advanced Applications of NMR to Organometallic Chemistry, M. Gielen, R. Willem, and B. Wrackmeyer (Eds.),Vol. 1, John Wiley & Sons, Ltd, Chichester, 1996, pp. 45–86. B. Wrackmeyer, in Physical Organometallic Chemistry – Advanced Applications of NMR to Organometallic Chemistry, M. Gielen, R. Willem, and B. Wrackmeyer (Eds.),Vol. 1, John Wiley & Sons, Ltd, Chichester, 1996, pp. 87–122. B. Wrackmeyer, Annu. Rep. NMR Spectrosc. 38, 203 (1999). R. K. Harris, E. D. Becker, S. M. Cabral de Menezes, R. Goodfellow, and P. Granger, Encyclopedia of Nuclear Magnetic Resonance 9, 5–19, John Wiley & Sons, Ltd, Chichester, 2002. A. Sebald, in Solid-State NMR II – NMR-Basic Principles and Progress, P. Diehl, E. Fluck, H. G¨unther, R. Kosfeld, and J. Seelig (Eds.), Vol. 31, Springer, Berlin, 1994, pp. 91–131. A. Sebald, in Physical Organometallic Chemistry – Advanced Applications of NMR to Organometallic Chemistry, M. Gielen, R. Willem, and B. Wrackmeyer (Eds.),Vol. 1, John Wiley & Sons, Ltd, Chichester, 1996, pp. 123–157. (a) G.A. Morris and R. Freeman, J. Am. Chem. Soc. 101, 760 (1979); (b) G.A. Morris, J. Am. Chem. Soc. 102, 428 (1980); (c) G.A. Morris, J. Magn. Reson. 41, 185 (1980); (d) D.P. Burum, R.R. Ernst, J. Magn. Reson. 39, 163 (1980). V. Gouron, B. Jousseaume, M. Ratier, J.-C. Lartigue, and M. Petraud, Magn. Reson. Chem. 28, 755 (1990). (a) D. T. Pegg, D. M. Doddrell, W. M. Brooks, and M. R. Bendall, J. Magn. Reson. 44, 32 (1981); (b) D. T. Pegg, D. M. Doddrell, and M. R. Bendall, J. Chem. Phys. 77, 2745 (1982). T. Parella, Magn. Reson. Chem. 36, 467 (1998). (a) G. Metz, X. Wu, and S. O. Smith, J. Magn. Reson. Series A 110, 219 (1994); (b) O. B. Peersen, X. Wu, and S. O. Smith, J. Magn. Reson. Series A 106, 127 (1994). (a) S. J. Blunden, A. Frangou, and D. G. Gillies, Org. Magn. Reson. 20, 170 (1982); (b) A. Lauksonen, and R. E. Wasylishen, J. Am. Chem. Soc. 117, 392 (1995); (c) T. B. Grindley, R. D. Curtis, R. Thangarasa, and R. E. Wasylishen, Can. J. Chem. 68, 2103 (1990); (d) T. Birchall, V. Manivannan, Can. J. Chem. 63, 2211 (1985). B. Wrackmeyer, C. Stader, K. Horchler, H. Zhou, and D. Schlosser, Inorg. Chim. Acta, 176, 205 (1990). B. Wrackmeyer, G. Kehr, and R. Boese, Chem. Ber., 125, 643 (1992). (a) A.C. de Dios, Magn. Reson. Chem. 34, 773 (1996); (b) N. Bertazzi, G. Casella, F. Ferrante, L. Pellerito, A. Rotondo, and E. Rotondo, Dalton Trans. 2007, 1440.

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S. Adams and M. Dr¨ager, J. Organomet. Chem. 323, 11 (1987). J. D. Kennedy and W. McFarlane, J. Chem. Soc. Dalton Trans. 1976, 1219. T. N. Mitchell and C. Kummetat, J. Organomet. Chem. 157, 275 (1978). T. N. Mitchell and A. Amamria, J. Organomet. Chem. 252, 47 (1983). B. Wrackmeyer, U. D¨orfler, G. Kehr, H.E. Maisel, and W. Milius, J. Organomet. Chem. 524, 169 (1996). T.N. Mitchell and C. Nettelbeck, Magn. Reson. Chem. 25, 879 (1987). B. Wrackmeyer, K. Horchler von Locquenghien, E. Kupce, and A. Sebald, Magn. Reson. Chem. 31, 45 (1993). M. Herberhold, U. Steffl, W. Milius, and B. Wrackmeyer, Chem. Eur. J. 4, 1027 (1998). R. Eujen, N. Jahn, and U. Thurmann, J. Organomet. Chem. 434, 159 (1992). H. Schumann, C. Janiak, E. Hahn, C. Kolax, J. Loebel, M.D. Rausch, J.J. Zuckerman, and M.J. Heeg, Chem. Ber. 119, 2656 (1986). T. Kawakami, T. Sugimoto, A. Baba, H. Matsuda, and N. Sonoda, J. Org. Chem. 60, 2677 (1995). T. Janati, J.-C. Guillemin, and M. Soufiaoui, J. Organomet. Chem. 486, 57 (1995). L. Lasalle, T. Janati, and J.-C. Guillemin, Chem. Commun. 1995, 699. B.E. Mann and B.F. Taylor, 13 C NMR Data of Organometallic Compounds, Academic Press, London 1981. J. Holecek, A. Lycka, K. Handlir, and M. Nadvornik, Collect. Czech Chem. Commun. 53, 571 (1988). I. Wharf, Inorg. Chim. Acta, 1989, 159, 41 (1089). B. Wrackmeyer, G. Kehr, D. Wettinger, and W. Milius, Main Group Met. Chem. 16, 433 (1993). T.N. Mitchell, A. Amamria, H. Killing, and D. Rutschow, J. Organomet. Chem. 304, 257 (1986). B. Wrackmeyer, E. Kupce, and J. K¨ummerlen, Magn. Reson. Chem. 30, 403 (1992). B. Wrackmeyer, Z. Naturforsch. Teil B, 34, 235 (1979). B. Wrackmeyer, H.E. Maisel, and H. Zhou, Main Group Met. Chem. 16, 475 (1993). I. I. Padilla-Martinez, M. de Jesus Rosalez-Hoz, R. Contreras, S. Kerschl, and B. Wrackmeyer, Chem. Ber. 127, 343 (1994). F.H. K¨ohler, W.A. Geike, and N. Hertkorn, J. Organomet. Chem. 334, 359 (1987). B. Wrackmeyer, J. Magn. Reson. 42, 287 (1981). F. H¨olzl and B. Wrackmeyer, J. Organometal. Chem. 179, 397 (1979). B. Wrackmeyer, G. Kehr, and D. Wettinger, Inorg. Chim. Acta, 220, 161 (1994). R. N. Kapoor, P. Apodaca, M. Montes, F. D. Gomez, and K. H. Pannell, Appl. Organomet. Chem. 19, 518 (2005). J. W. Emsley, L. Phillips, and V. Wray, Prog. NMR Spectrosc. 10, 716 (1977). S. S. Al-Juaid, S. M. Dhaher, C. Eaborn, P. B. Hitchcock, and J. D. Smith, J. Organomet. Chem. 325, 117 (1987). D. Dakternieks and H. Zhu, Organometallics, 11, 3820 (1992). M. Jang and A. F. Janzen, J. Fluorine Chem. 66, 129 (1994). S. .E. Johnson and C. B. Knobler, Organometallics,11, 3684 (1992). A. Lycka, J. Jirman, A. Kolonicny, and J. Holecek, J. Organomet. Chem. 333, 305 (1987). C. Picard, P. Tisnes, and L. Casaux, J. Organomet. Chem. 315, 277 (1986). D. L. Reger, S. S. Mason, J. Takats, X. W. Zhang, A. L. Rheingold, and B. S. Haggerty, Inorg. Chem. 32, 4345 (1993). J. Holecek, M. Nadvornik, K. Handlir, and A. Lycka, J. Organomet. Chem. 315, 299 (1986). J. Holecek, K. Handlir, M. Nadvornik, S.M. Teleb, and A. Lycka, J. Organomet. Chem. 339, 61 (1988). B. Wrackmeyer, G. Kehr, and H. Zhou, S. Ali, Inorg. Chim. Acta, 197, 129 (1992). V. Pejchal, J. Holecek, M. Nadvornik, and A. Lycka, Collect. Czech. Chem. Commun. 60, 1492 (1995). K. B. Dillon and A. Marshall, J. Chem. Soc. Dalton Trans. 1987, 315. M.-C. Ye and J.G. Verkade, Energy & Fuels, 8, 172 (1994). B. Wrackmeyer, U. Klaus, and W. Milius, Chem. Ber. 128, 679 (1995). W. A. Herrmann, J. G. Kuchler, J. K. Felixberger, E. Herdtweck, and W. Wagner, Angew. Chem. 100, 420 (1988); Angew. Chem. Int. Ed. 27, 394 (1988).

141. 142. 143. 144. 145. 146. 147. 148. 149. 150. 151. 152. 153. 154. 155. 156. 157. 158. 159. 160. 161. 162. 163. 164. 165. 166. 167. 168. 169. 170. 171. 172. 173. 174. 175. 176.

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C.D. Chandler, G.D. Fallon, A.J. Koplick, and B.O. West, Aust. J. Chem. 40, 1427 (1987). O.-S. Jung and Y.S. Sohn, Bull. Korean. Chem. Soc. 9, 365 (1988). M. Herberhold, M. H¨ubner, and B. Wrackmeyer, Z. Naturforsch. Teil B, 48, 940 (1993). A. G. Davies, S. D. Slater, D. C. Povey, and G. W. Smith, J. Organomet. Chem. 352, 283 (1988). P. A. Bates, M. B. Hursthouse, A. G. Davies, and S. D. Slater, J. Organomet. Chem. 363, 45 (1989). H. Berwe and A. Haas, Chem. Ber. 120, 1175 (1987). H. Puff, G. Bertram, B. Ebeling, M. Franken, R. Gattermayer, R. Hundt, W. Schuh, and R. Zimmer, J. Organomet. Chem. 379, 235 (1989). P. A. W. Dean and R. V. S. Srivastava, Inorg. Chim. Acta, 105, 1 (1985). M. Herberhold, U. Steffl, W. Milius, and B. Wrackmeyer, J. Organomet. Chem. 533, 109 (1997). J. Campbell, D. P. DiCiommo, H. P. A. Mercier, A. M. Pirani, G. J. Schrobilgen, and M. Willuhn, Inorg. Chem. 34, 6265 (1995). R. C. Burns, L. A. Devereux, P. Granger, and G. J. Schrobilgen, Inorg. Chem. 24, 2615 (1985). J. Campbell, L. A. Devereux, M. Gerken, H. P. A. Mercier, A. M. Pirani, and G. J. Schrobilgen, Inorg. Chem. 35, 2945 (1995). (a) U. Hermann, G. Reeske,M. Sch¨urmann, and F. Uhlig, Z. Anorg. Allg. Chem. 627, 453 (2001); (b) U. Hermann, M. Sch¨urmann, and F. Uhlig, J. Organomet. Chem. 585, 211 (1999). A. D¨orr, D. Gudat, D. H¨anssgen, H. Hens, and E. Stahlhut, Bull. Soc. Chim. Fr. 131, 674 (1994). S. Geetha, M. Ye, and J.G. Verkade, Inorg. Chem. 34, 6158 (1995). K. C. Molloy, K. Quill, S. J. Blunden, and R. Hill, J. Chem. Soc. Dalton Trans. 1986, 875. B. Wrackmeyer, G. Kehr, H. Zhou, and S. Ali, Main Group Met. Chem. 15, 89 (1992). S. J. Blunden, P. A. Cussack, and D. G. Gillies, J. Magn. Reson. 60, 114 (1984). M. Herberhold, U. Steffl, and B. Wrackmeyer, Z. Naturforsch. Teil B,54, 57 (1999). M. Drieß, R. Janoschek, H. Pritzkow, and U. Winkler, Angew. Chem. 107, 1746 (1995); Angew. Chem. Int. Ed. 34, 1614 (1995). H. Schumann and K.-H. K¨ohricht, J. Organomet. Chem. 373, 307 (1989). T. N. Mitchell, R. Faust, B. Fabisch, and R. Wickenkamp, Magn. Reson. Chem. 28, 82 (1990). M. Herberhold, U. Steffl, W. Milius, and B. Wrackmeyer, Angew. Chem. 108, 1927 (1996); Angew. Chem. Int. Ed. 35, 1803 (1996). M. Herberhold, U. Steffl, W. Milius, and B. Wrackmeyer, Z. Anorg. Allg. Chem. 624, 386 (1998). B. Watta, W.P. Neumann, and J. Sauer, Organometallics, 4, 1954 (1985). H. Puff, C. Bach, W. Schuh, and R. Zimmer, J. Organomet. Chem. 312, 313 (1986). K. Jurkschat, A. Tzschach, C. M¨ugge, J. Piret Meunier, M. van Meerssche, G. van Binst, C. Wynants, M. Gielen, and R. Willem, Organometallics, 7, 593 (1988). W. Biffar, H. N¨oth, H. Pommerening, R. Schwerth¨offer, W. Storch, and B. Wrackmeyer, Chem. Ber. 114, 49 (1981). D. K. Srivastava, N. P. Rath, and L. Barton, Organometallics, 11, 2263 (1992). P. Jutzi, D. Wegener, H.-G. Stammler, A. Karaulov, and M.B. Hursthouse, Inorg. Chim. Acta 198, 369 (1992). T. G¨adt, F. M. Schappacher, R. P¨ottgen, and L. Wesemann, Inorg. Chem. 46, 2864 (2007). H. J. Reich, J. P. Borst, and R. R. Dykstra, Organometallics, 13, 1 (1994). P. B. Hitchcock, M. F. Lappert, G. A. Lawless, and B. Royo, Chem. Commun. 1993, 554. J. E. Ellis and P. Yuen, Inorg. Chem. 32, 4998 (1993). J. E. Ellis, P. Yuen, and M. Jang, J. Organomet. Chem. 507, 283 (1996). J. E. Ellis, K.-M. Chi, A.-J. DiMaio, S. R. Frerichs, J. R. Stenzel, A. L. Rheingold, and B. S. Haggerty, Angew. Chem. 103, 196 (1991); Angew. Chem. Int. Ed. 30, 194 (1991). W. E. Piers, R. M. Whittal, G. Ferguson, J. F. Gallagher, R. D. J. Froese, H. J. Stronks, and P. H. Krygsman, Organometallics, 11, 4015 (1992). K. Jurkschat, H.-B. Abicht, A. Tzschach, and B. Mahieu, J. Organomet. Chem.309, C47 (1986). U. Schubert, E. Kunz, B. Harkers, J. Willnecker, and J. Meyer, J. Am. Chem. Soc. 111, 2572 (1989). W. Petz, B. Wrackmeyer, and W. Storch, Chem. Ber. 122, 2261 (1989).

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217. 218. 219. 220.

B. Wrackmeyer, B. Distler, and M. Herberhold, Z. Naturforsch. Teil B, 47, 1749 (1992). Z. Duan, D. Lei, and M. J. Hamdensmith, Polyhedron, 10, 2105 (1991). M. Moriyama, T. Aoki, S. Sjimota, and Y. Saito, J. Chem. Soc. Chem. Commun. 1982, 500. M. M. M¨ohlen, C. E. F. Rickard, W. R. Roper, G. R. Whittell, and L. J. Wright, J. Organomet. Chem. 691, 4065 (2006). T. Mizutani, T. Uesaka, and H. Ogoshi, Organometallics, 14, 341 (1995). L. Carlton and R. Weber, Inorg. Chem. 32, 4169 (1993). T. Yamakawa, S. Shinoda, Y. Saito, H. Moriyama, and P. S. Pregosin, Magn. Reson. Chem. 23, 202 (1985). Y. Obora, Y. Tsuji, K. Nishiyama, M. Ebihara, and T. Kawamura, J. Am. Chem. Soc. 118, 10922 (1996). P. Br¨uggeller, Z. Naturforsch. Teil B, 41, 1561 (1986). J. H. Nelson, W. L. Wilson, L. W. Cary, N. W. Alcock, H. J. Clase, G. S. Jas, L. Ramsey-Tassin, and J. W. Kenney, III, Inorg. Chem. 35, 883 (1996). K. R. Koch, Magn. Reson. Chem. 30, 158 (1992). B. Wrackmeyer, B. H. Kenner-Hofmann, W. Milius, P. Thoma, O. L. Tok, and M. Herberhold, Eur. J. Inorg. Chem. 2006, 101. B. Wrackmeyer, H. E. Maisel, W. Milius, and M. Herberhold, J. Organomet. Chem. 680, 271 (2003). T. Endo, F. Sasaki, H. Hara, J. Suzuki, S. Tamura, Y. Nagata, T. Iyoshi, A. Saigusa, and T. Nakano, Appl. Organomet. Chem. 21, 183 (2007). D. J. Eisler and T. Chivers, Chem. Eur. J. 12, 233 (2006). P. B. Hitchcock, J. Hu, M. F. Lappert, and J. R. Severn, Dalton Trans. 2004, 4193. T. Tajima, N. Takeda, T. Sasamori, and N. Tokitoh, Eur. J. Inorg. Chem. 2005, 4291. T. A. Mobley, R. Gandour, E. P. Gillis, K. Nti-Addae, R. Palchaudhuri, P. Rajbhandari, N. Tomson, A. Vargas, and Q. Zheng, Organometallics, 24, 3897 (2005). P. A. Chugunov, N. A. Troitskii, K. S. Nosov, M. P. Egorov, and O. M. Nefedov, Russ. Chem. Bull. Int. Ed. 53, 2327 (2004).

221. 222. 223. 224. 225. 226. 227. 228. 229. 230. 231. 232. 233. 234. 235.

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Tin(II) Clusters

2.2

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Tin(II) Clusters

Tristram Chivers and Dana J. Eisler Department of Chemistry, University of Calgary, Alberta, Canada

2.2.1

Introduction

The chemistry of tin(II) clusters has undergone remarkable development since Veith’s initial report on the synthesis of the imidotin cubane [Sn(μ3 -NR)]4 in 1979.1 A number of synthetic strategies for these compounds have been developed, and a wide array of examples have been well characterized. There are three main structural motifs observed in imidotin clusters, although the cubane cluster [Sn(μ3 -NR)]4 , I (Scheme 2.2.1), is by a wide margin the most common. The seco-cubane structure, II, in which a single corner of the cube is vacant, has only been well-characterized in a single case,2 but a number of related structures have been observed.3−6 More recently, the double-cubane clusters, III, have been reported by Wright,7−9 but have so far only been obtained when heterocyclic substituents are present on the imido-nitrogen centers. The first studies on imidotin clusters focused mainly on synthesis and structures, although a few reactivity studies were performed. The potential of the cubane clusters to act as multidentate Lewis-base ligands was recognized early on, and investigations into the coordination chemistry of these compounds have been undertaken.10,11 More recent studies have focused on the reactions of imidotin clusters with chalcogens, with the aim of generating single-source precursors that can be used for the deposition of tin chalcogenides.12,13

Scheme 2.2.1

The purpose of this chapter is to provide the reader with an overview of the significant findings in tin(II) cluster chemistry up to mid-2007. Although the majority of investigations have involved imidotin clusters, there have been important developments in the last 10 years in our knowledge of related phosphidotin clusters (see Section 2.2.6). 2.2.2

Imidotin Cubane Clusters

Synthesis of [Sn(μ3 -NR)]4

(1) Transamination reactions. Most of the known imidotin cubanes of the type [Sn(μ3 -NR)]4 have been prepared via transamination reactions between a tin amide reagent and a primary amine. The first example was reported by Veith in 1979 and involved the reaction of the cyclic diazastannylene 1 (Scheme 2.2.2) with tert-butylamine at 50 ◦ C.1 However, the initial reaction of these reagents produces either the tricyclic compound 2 or the seco-cubane 3, depending on the reaction stoichiometry (from molar ratios of 1:tBuNH2 of 2:1 or 3:4, respectively). Compounds 2 and 3 are converted to the cubane

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Scheme 2.2.2

cluster [Sn(μ3 -NtBu)]4 , 4, at high temperatures (200 ◦ C).1,14 Alternatively, the reaction of 3 with one equivalent of 1 at 210 ◦ C for 2 days produces 4. In all cases, the cubane 4 was obtained in yields ranging from 83–92%. By contrast, the reactions of 1 with less bulky amines, such as iso-propylamine or N,N dimethylhydrazine, generate the cubanes 5 and 6 in 90% yield under milder conditions (5: 70 ◦ C, 15 min.; 6: spontaneous formation under ambient conditions).15 More recently, Veith has extended this synthetic protocol to generate the series of silyl-substituted cubanes 7–11 via the reaction of 1 with the appropriate primary amines, under conditions similar to those used in the formation of 4.5 With the exception of 7, analogs of the seco-cube 3 were isolated as intermediates in each case; pyrolysis at 250 ◦ C/0.01 torr was required to produce the cubanes [Sn(μ3 -NR)]4 .5 The cubane 7 was isolated by sublimation at 200 ◦ C/0.01 torr, so the possibility of a seco-cube intermediate cannot be ruled out. Since Veith’s early work, a wide variety of imidotin cubane clusters [Sn(μ3 -NR)]4 have been reported and several different synthetic methodologies have been employed, with varying degrees of success. The synthesis of the cubanes 12 and 13 (Scheme 2.2.3), with bulky aromatic substituents on the nitrogen centers, was achieved by the reaction of Sn[N(SiMe3 )2 ]2 with the appropriate primary amine, under relatively mild conditions (13: boiling hexane, 1 h, 35% yield; 12: 50 ◦ C in the melt 5 min, 80% yield).16 It has been suggested that the versatility of the reaction of primary amines with Sn[N(SiMe3 )2 ]2 is limited, since it requires the use of relatively acidic amines,17 however, this claim has not been substantiated by any reported experimental studies. Wright and coworkers have used the powerful base Sn(NMe2 )2 to prepare the cubanes 14 and 15 in 60% and 38% yields, respectively.17 One of the advantages of using the highly reactive reagent Sn(NMe2 )2 is that it allows for the formation and isolation of thermally unstable cubanes, e.g. 15, since the synthesis can be conducted at 20 ◦ C or below.17 Wright has also prepared [Sn(μ3 -NtBu)]4 , 4, from Sn(NMe2 )2 and tBuNH2 in boiling toluene.18 Transamination of Sn(NMe2 )2 has also been employed to synthesize a series of [Sn(μ3 -NR)]4 cubanes that contain

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Scheme 2.2.3

methoxy-substituted phenyl rings, by reaction with the appropriate primary amine.3,19 However, many of these compounds were isolated in low yields (8–15%); notable exceptions are 18, 22 and 23 (82, 68, and 37% yields, respectively). Compound 24 was also obtained in 23% yield by this synthetic route.3 (2) Reactions of Sn(II) Reagents with Metal Amides and Metal Imides. A few [Sn(μ3 -NR)]4 cubanes have been synthesized by methods other than transamination reactions with primary amines. The compounds 25 and 26 (Scheme 2.3.3) were obtained in 21 and 51% yields, respectively, from the reaction of SnCl2 with [(Me3 Sn)(Me3 M)NLi·(Et2 O)]2 (M = Ge, Sn) with the elimination of Me3 SnCl at room temperature.20 The analogous trimethylsilyl-substituted cubane 7 has also been prepared by this route, using the reagent [(Me3 Sn)(Me3 Si)NLi·(Et2 O)]2 .21 The imidotin cubane 27 containing the bulky adamantyl-substituted diiso-propylphenyl substituents was prepared in 76% yield from the room temperature reaction of SnCl2 with the corresponding lithiated amine.22 The known cubane 14 was also generated in 42% yield from the reaction of SnCp2 with lithium cyclohexylamide.23 Finally, the phenylsubstituted cubane 28 was obtained in 42% yield from the reaction of SnCl2 with the magnesium imide [(THF)MgNPh]6 in boiling THF for 18 h; compound 28 was also isolated as a cocrystallized complex with MgCl2 (THF)4 , for which a crystal structure verified the presence of a cubane core.24 Synthesis of Related Imidotin Cubane Clusters

The basketane-like compound 29 (Scheme 2.2.4) was obtained as a minor by-product in the reaction of the cyclic diazastannylene, 1, with Me2 Si(NHMe)2 .25 While 29 has been well characterized, including by an X-ray structural determination, no direct synthetic route to this compound has been reported.25 The

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Scheme 2.2.4

cubane 30a, in which one imido substituent in the cluster is replaced by an oxo group was first prepared (in 44% yield) in 1980 by the stoichiometric hydrolysis of the seco-cubane 3 in boiling tert-butylamine.26 More recently, an improved synthesis of 30a (in 89% yield) from the hydrolysis of the cubane 4, prepared in situ at –78 ◦ C, in a mixture of THF and acetonitrile, was reported.27 The sulfur analog, 30b, obtained from the reaction of 3 with H2 S, has been mentioned in a review.28 However, no details of the synthetic procedure or characterization of this compound were given. Reaction Pathways for the Formation of [Sn(μ3 -NR)]4

The reaction pathway for the formation of [Sn(μ3 -NR)]4 from the cyclic diazastannylene 1 has been discussed previously by Veith.29 More recent studies on the synthesis of imidotin cubanes from Sn(NMe2 )2 have resulted in the isolation and structural characterization of important intermediates.3,4 For example, the reaction of this reagent with the bulky amines 2,6-diiso-propylaniline and 2,4,6-trimethylaniline produces the mixed amido/imido compounds 31 and 32 (Scheme 2.2.5) rather than the expected [Sn(μ3 NR)]4 cubanes. These clusters exhibit high thermal stability, as demonstrated by the observation that the known diiso-propylphenyl-substituted cubane 12 was only obtained from this reaction after refluxing in toluene for 2.5 days.4 The reluctance of the compounds 31 and 32 to proceed to the cubane structures was attributed to the high steric bulk of the aromatic substituents. A configurational isomer of these compounds, 33, was isolated during the synthesis of the cubane 21.3 The tricyclic compound 33 can be considered to consist of an [Sn(μ-NR)]2 dimer coordinated to an Sn(NMe2 )2 monomer. The possibility

Scheme 2.2.5

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Tin(II) Clusters

Scheme 2.2.6

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Possible reaction pathways for the formation of [Sn(μ3 -NR)]4

that [Sn(μ3 -NR)]4 cubanes are formed stepwise in solution, through the aggregation of [Sn(μ-NR)]2 dimers has been considered previously by Veith.6 In this context Veith carried out the synthesis of the cubane 4 in the presence of [Sn(OtBu)2 ]2 and isolated the analogous tricyclic complex 34, in which the [Sn(μ-NtBu)]2 dimer is coordinated to a Sn(OtBu)2 monomer.6 The identification of the compounds 31–34 provides significant insight into the pathways involved in the formation of tin cubanes; a possible route (pathway i in Scheme 2.2.6) has been previously proposed by Wright.3,4 A second possibility, (pathway ii in Scheme 2.2.6) is presented here for the first time, although aspects of this proposal have been discussed previously by Veith.29 The primary difference between the two pathways is in the stoichiometry of the first step. Pathway i requires a 1:1 molar ratio of reactants [tBuNH2 :Sn(NMe2 )2 ], whereas that ratio is 2:1 in pathway ii. Pathway i does not account for the formation of 33, while pathway ii does not produce complexes of the type 31 and 32. An explanation of the formation of the two isomeric structures represented by 31, 32, and 33 must involve an intramolecular rearrangement of the amido and imido substituents. For example, cleavage of two Sn–N bonds in B and formation of a new Sn–N bond to give A would be entropically driven by the loss of steric strain involved in the conversion of two four-membered rings into two six-membered rings (Scheme 2.2.6).

Reaction Chemistry

(1) Reactions of [Sn(μ3 -NR)]4 . An intriguing aspect of the chemistry of imidotin cubanes lies in their potential to act as multidentate Lewis-base donors to form coordination complexes. While this

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Scheme 2.2.7

possibility has been investigated by Veith, only a few complexes have been reported. The first example was the formation of [Sn(μ3 -NtBu)]4 ·2AlCl3 , 35 (Scheme 2.2.7).10 Treatment of the cubane 4 with AlCl3 in molar ratios ranging from 1:1 to 1:4 gave only the bis-adduct 35, which was identified by X-ray crystallography. The AlCl3 ligands in 35 exchange rapidly between the four tin centers in solution, as indicated by the observation of a single resonance for the tert-butyl substituents in the 1 H NMR spectrum.10 Transition-metal complexes of [Sn(μ3 -NR)]4 cubanes have also been reported. The reaction of Fe2 (CO)9 with [Sn(μ3 -NSiEt3 )]4 , 8, produces the complex [Sn(μ3 -NSiEt3 )]4 ·2Fe(CO)4 , 36, in 82% yield, even when an excess of Fe2 (CO)9 is used.5 The presence of only two broad signals for the ethyl substituents in the 1 H NMR spectrum suggests that the iron centers scramble in solution, cf. 35. A series of related transition-metal complexes of 4 have been mentioned in reviews, however no details have been reported for the complexes 37a–d.11,29 The reaction of the cubane 4 with an excess of SO2 results in the rapid formation of the dimeric compound 38, in which two imidotin cubanes are bridged by two sulfito ligands. The retention of the cubane structure is unexpected, since SO2 is known to insert readily into Sn–NR2 bonds.30 The dimer 38 is the only known complex in which two [Sn(μ3 -NR)]4 cubane clusters are strongly linked. The reaction of 4 with P(SiMe3 )3 was attempted with a view to preparing the phosphorus-containing cluster 39 (Scheme 2.2.7), but instead resulted in the formation of the phosphido-centered double cubane

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40, in an unspecified low yield.31 When the same reaction was carried out in the presence of 30a, the yield of 40 was increased to 14%. The solid-state structure of 40 revealed a distorted six-coordinate geometry at the phosphorus center. The phosphorus atom is strongly bound to five of the tin atoms, with Sn–P ˚ while the contact to the unique tin center is considerably distances in the range 2.580(4)–2.725(3) A, ˚ 31 longer, at 3.298(3) A. In contrast to the previous examples, a number of reactions in which the cubane core is not retained have been reported. Early work by Veith showed that the reaction of the cubane 4 with HCl results in the complete decomposition of the cluster.32 The addition of HCl to 4 readily produces the dimeric compound [(NHtBu)Sn(μ-Cl)]2 , as well as the adduct tBuNH2 ·SnCl2 and the salt [tBuNH3 ][SnCl3 ], all of which were well characterized. More recently, Wright has investigated the interaction of 4, prepared in situ, with lithiated amines and phosphines.18 The reaction of three equivalents of LiNHC10 H7 with 4 in boiling toluene produced the cubane 41, in which only a single NtBu substituent of the parent cluster has been retained. In addition, one of the tin centers has been replaced by two lithium ions, only one of which is accommodated within the anionic cubane core, while the second is present as a THF-solvated counterion. The outcome of the reaction of 4 with six equivalents of LiPHC6 H11 is even more dramatic, with all of the tert-butylimido groups being replaced by phosphinidine substituents, to give the cluster 42, which can be considered to consist of an [Sn4 (μ4 -PCy)6 ]4− tetra-anion with four lithium cations.18 Finally, Wright and coworkers have reported that the reaction of the cubane 21, prepared in situ, with KOtBu and MeN(Li)CH2 CH2 N(Li)Me produces the heteroleptic stannate complexes 43 and 44 (Scheme 2.2.8), respectively.33 (2) Reactions of Sn4 (μ3 -NR)3 (μ3 -O). An interesting substitution reaction of the oxo-cubane 30a has been reported by Wright.27 The treatment of 30a with LiNHC10 H7 produces the heteroleptic imidotin cubane 45 (Scheme 2.2.9) in which the oxo group has been replaced by an imido group. The substitution occurs exclusively at the oxygen center owing to the higher polarity of the Sn–O bonds (relative to Sn–N) and is thermodynamically driven by the formation of LiOH.27 The isolation of the cluster 45 demonstrates the potential of this synthetic route to generate mixed-substituent imidotin cubanes of the type Sn4 (μ3 -NR)3 (μ3 -NR ), which are not accessible by direct synthetic procedures. However, to date, 45 remains the only example of this type of cluster.

Scheme 2.2.8

The oxo cluster 30a has also been used to prepare a handful of metal complexes. The aluminium complex 46 was obtained in 81% yield from the reaction of 30a with 1.5 equivalents of AlMe3 ; no coordination to the tin centers was observed.26 Thermolysis of 46 resulted in the generation of the cubane 4.26 The complexes 47 and 48 were prepared in THF by the reaction of 30a with LiCl and FeCl2 , respectively, in a 3:1 molar ratio.34 Both complexes exhibit the coordination of three oxo cubanes to the metal center; strong metal–oxygen bonding is indicated by the short metal–oxygen bond distances [47,

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Scheme 2.2.9

˚ 48, Fe–O 2.026(7) A]. ˚ The oxo cubane 30a behaves as a stronger O-donor ligand than Li–O 1.94(1) A; THF, presumably as a result of the localization of negative charge on the oxygen center.34 2.2.3

The Seco-Cubane Sn3 (μ3 -NtBu)(μ-NtBu)(μ-NHtBu)2

Synthesis of seco-Sn3 (μ3 -NtBu)(μ-NtBu)(μ-NHtBu)2

The seco-cubane 3 was first prepared in 98% yield from the reaction of three equivalents of 1 with four equivalents of tert-butylamine.1 The identity of this compound was later unambiguously established by an X-ray structural determination.2 Alternatively, 3 can be obtained from the reaction of Sn[N(SiMe3 )2 ]2 with tert-butylamine.35 While this latter reaction produces 3 in lower yield (71%), it has the advantage of using the more conveniently accessible reagent Sn[N(SiMe3 )2 ]2 . Veith has reported the observation of a series of silyl-substituted analogs of 3, which were subsequently converted into the cubane clusters 8–11, although no characterization of these intermediates was given in that work (see Transamination Reactions above).5 Undoubtedly, there is considerable scope for the synthesis of seco-cubanes related to 3, provided that appropriately bulky substituents are present on the nitrogen centers. Reactions of seco-Sn3 (μ3 -NtBu)(μ-NtBu) (μ-NHtBu)2

The reaction of the seco-cubane 3 with hydrogen halides has been investigated by Veith and coworkers. The 1:1 reaction of dilute solutions of either HCl or HBr with 3 readily produces the complexes 49a and 49b (Scheme 2.2.10) in nearly quantitative yield.36 These complexes are comprised of the cationic cluster [Sn3 (μ3 -NtBu)(μ-NHtBu)3 ]+ , which is coordinated to a halide anion via three hydrogen bridges. Their identity was unambiguously established in both cases by X-ray structural determinations. Treatment of 3 with an excess of HX resulted in the complete decomposition of the cluster, giving rise to products similar to those observed in the analogous reaction with the cubane 4.32,36 The iodo compound 49c was not obtained directly from HI and 3, but by a simple anion-exchange reaction between the chloro compound 49a and sodium iodide; the bromo derivative 49b could also be

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Scheme 2.2.10

prepared similarly by reaction with sodium bromide.36 It was noted by Veith that these latter reactions are bi-phasic, suggesting the possibility that the compounds 49a–c could have potential as phase-transfer catalysts.36 The vacant corner inherent in the structure of the seco-cubane 3 provides the possibility of generating mixed-metal clusters, such as 50a,b, by further reactions with suitable metallating reagents. The reaction of 3 with the germanium and lead analogs of 1, were found to require forcing conditions to generate the closed cubane compounds, e.g. 50a, 190 ◦ C for 15 h; 50b, 210 ◦ C.37 While 50a is thermodynamically stable, scrambling occurs in the formation of the lead derivative 50b to give the heterometallic cubanes 51 and 52 (Scheme 2.2.10) and the symmetrical cubanes [M(μ3 -NtBu)]4 (M = Sn, Pb), on the basis of mass spectrometric studies.37 The presence of the two amido protons in 3 also provides the opportunity to generate heterobimetallic clusters by reaction with organoalkali reagents. The magnesiated cubane 53 is produced in 69% yield from the reaction of dibutylmagnesium with 3 in hot THF.38 The reaction of 3 with two equivalents of butyllithium at –78 ◦ C in THF generates 54 in 39% yield.13 The structure of the solvent-separated ion pair 54 is analogous to that of 41.

2.2.4

Double-Cubane Clusters

A fascinating recent development in imidotin-cluster chemistry involves the isolation of a series of double cubanes, which contain an Sn7 (μ3 -NR)8 core. Wright and coworkers have demonstrated that when pyridinyl or pyrimidinyl groups are present on the imido nitrogen centers, the unusual doublecubane clusters 55–60 are obtained (Scheme 2.2.11), rather than the [Sn(μ3 -NR)]4 cubanes.8 These clusters are comprised of two interlocked [Sn(μ3 -NR)]4 cubanes, which share one tin vertex. The central tin center is formally in the +4 oxidation state, so that these double cubanes may be viewed as involving the coordination of two [Sn3 (μ3 -NR)4 ]2− anions, such as those present in the clusters 53 and 54, to a central Sn+4 cation. The presence of both Sn(II) and Sn(IV) centers was verified by 119 Sn NMR spectroscopy.7 The deposition of tin metal was observed during the syntheses of 55–60, suggesting that

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Scheme 2.2.11

the oxidation of one Sn(II) center to Sn(IV) is accompanied by the reduction of another Sn(II) center to tin metal.7,8 The compounds 55–60 have all been prepared, albeit in low yields (8–26%), from the reaction of Sn(NMe2 )2 with the appropriate primary amine, under conditions which produce cubanes for nonheterocyclic containing amines.7,8 Thus, the formation of the double-cubane structural motif presumably arises from the nature of the heterocyclic substituents on the imido nitrogen centers. Inspection of the solid-state structures of these compounds revealed the presence of short intramolecular heterocyclic ˚ which may play a role in the formation of the douN· · ·Sn contacts, ranging from 2.812(4) to 3.188(6) A, 7 ble cubanes. The likely formation of double-cubane clusters, which do not contain heterocyclic imido substituents, by metathetical reactions of two equivalents of 53 or 54 with main-group or transition-metal tetrahalides has not yet been investigated. The reaction of a primary amine and Sn(NMe2 )2 in the presence of water generated a related double cubane, 61 (Scheme 2.2.12), in which one of the NR groups in each cubane was replaced by an oxygen atom.9 The interesting bis-amido-μ-imidoditin(II) acceptor ligand, [(NHR)Sn]2 (μ-NR), provides a bridge between the two oxygen centers.

Scheme 2.2.12

2.2.5

Imidotin Chalcogenides

The first imidotin-chalcogenide cluster containing a terminal tin–chalcogen bond to be reported was the tellurium complex 62 (Scheme 2.2.13), which was unexpectedly obtained from the reaction of

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Scheme 2.2.13

[Li2 Te(NtBu)3 ]2 with SnCl2 .39 In solution the terminal tellurium atom was found to exchange rapidly between the tin(II) and tin(IV) sites, as determined by variable temperature NMR spectroscopy, cf. 35 and 36. The discovery of compound 62 prompted investigations into reactions of imidotin clusters [Sn(μ3 -NR)]4 with chalcogens, with a view to preparing single-source precursors for the deposition of tin-chalcogenide materials, via the thermodynamically favourable elimination of tBuN=NtBu.12 Initial studies focused on the reaction of the cubane 4 with sulfur, selenium and tellurium. In the case of tellurium, only a single tin center is oxidized to give the mono-telluride 63b, even when 4 was refluxed in toluene for two days with an excess of tellurium.12 Oxidation of 4 was more facile with selenium, resulting in the generation of 64, the first example of a compound containing more than one terminal tin– chalcogen bond. The mono-sulfide 63a was prepared by the reaction of 4 with one equivalent of sulfur. Further reaction with sulfur produced only insoluble products, presumably sulfido-bridged oligomers.12 The selenide and telluride clusters 64 and 63b are fluxional in solution as a result of rapid exchange of the terminal chalcogen centers between the tin sites, cf. 62. Reactions of the seco-cubane 3 with chalcogens were also investigated. While mono-chalcogenides are readily produced in the case of sulfur, selenium and tellurium, further reaction with either sulfur or selenium produced only insoluble products.35 The solid-state structures of the mono-selenide and mono-telluride 65a and 65b (Scheme 2.2.14) revealed that one of the Sn(NtBu)2 (NHtBu) sites was oxidized in preference to the unique Sn(NtBu)(NHtBu)2 tin center. Solution-state NMR studies indicated that chalcogen exchange occurs exclusively between the two Sn(NtBu)2 (NHtBu) sites.35 In the case of selenium, in addition to 65a, the selenido-bridged dimer 66 was present in solution as indicated by 1 JSn-Se coupling constants (65a, 3220 Hz; 66, 779 Hz).

Scheme 2.2.14

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Reaction of the heterobimetallic cluster 53 with selenium or tellurium in hot THF produced the monochalcogenides 67a and 67b (Scheme 2.2.15), which each contain a terminal tin–chalcogen bond.38 In solution, chalcogen exchange occurs between the three tin sites. A dramatic increase in the susceptibility of the tin centers towards oxidation by chalcogens was observed for the anionic cluster of 54. The reaction of 54 with one or two equivalents of selenium or tellurium occurs spontaneously at room temperature, to give the corresponding mono- and di-chalcogenide clusters 68, 69, 70a and 70b. While a monomeric cluster was formed in the solid state for the mono-telluride 68, a dimeric structure was observed for the mono-selenide 69, presumably owing to the localization of anionic charge on the more electronegative selenium centers.38 The marked enhancement in the reactivity of the anionic cluster in 54 (compared to that of the seco-cubane 3) was further demonstrated by the isolation and structural characterization of the tri-chalcogenide clusters 71a and 71b in high yields (Scheme 2.2.15).13 Pure samples of the mono- and di-chalcogenides may be obtained by the reaction of the tri-chalcogenide clusters 71a or 71b with 54 in the appropriate stoichiometric ratio. This chalcogen-transfer process can be used to chalcogenate otherwise unreactive imidotin clusters. For example, although the neutral cubane 12 does not react with selenium in boiling toluene, oxidation of the tin centers to give 72 occurs readily at room temperature when the reaction is carried out in the presence of a catalytic amount of 54.38 Complex 72 is a dimer in which two [Sn(μ-NR)]2 four-membered rings are bridged by two selenido ligands.

Scheme 2.2.15

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Tin(II) Clusters

2.2.6

65

Phosphido– and Arsenido–Tin Clusters

During the past 10 years there have been significant developments in the chemistry of tin(II) clusters that incorporate phosphorus and, to a lesser extent, arsenic, primarily from the groups of Westerhausen, Wright, and Driess.40,41 The reaction of Sn[N(SiMe3 )2 ]2 with H2 ESiiPr3 (E = P, As) in a 1:1 molar ratio produces the hexagonal prisms [Sn(μ3 -ESiiPr3 )]6 (73a, E = P; 73b, E = As; Scheme 2.2.16) as blackred crystals in very high yields,42,43 whereas the corresponding reaction with H2 PSitBu3 generates the cubane [Sn(μ3 -PSitBu3 )]4 , 74.44 The phosphorus or arsenic analogs of [Sn(μ3 -NtBu)]4 are not known. When the reaction of Sn[N(SiMe3 )2 ]2 with H2 PSi(iPr3 )2 (2,4,6-iPr3 C6 H2 ) is carried out in the presence of SnCl2 ,the Sn3 P2 Cl2 cluster 75, in which SnCl2 is trapped by a dimeric [Sn(μ-PR)]2 molecule, is isolated in 44% yield.42

Scheme 2.2.16

The transmetallation of [Sn(μ3 -PSitBu3 )]4 74 with barium metal produces the cubanes 76–78 (Scheme 2.2.17) in which one, two, or three tin atoms are replaced by barium, cf. the mixed Sn–Pb clusters 50–52.45,46 The cubanes 76–78 are also accessible from the reaction of a mixture of Sn[N(SiMe3 )2 ]2 and Ba[N(SiMe3 )2 ]2 with H2 PSitBu3 . The substitution of tin by barium in these clusters results in a very large high-field shift of the RP ligand bonded to three tin atoms (δ 31 P = –529).41 Another cluster type that involves Sn(II) and an alkaline earth metal is represented by 79 (M = Ca, Ba) in which the dianion [(Me3 Si)2 PSn(μ-PSiMe3 )2 SnP(SiMe3 )2 ]2− coordinates to the divalent metal as a tetradentate ligand.47,48

Scheme 2.2.17

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Scheme 2.2.18

The reactions of the Sn(II) reagent Sn(NMe2 )2 with alkali metal primary phosphides RPH(M) have been studied extensively by Wright and coworkers.40,49−54 The structures of the phosphido tin(II) clusters obtained via this approach depend markedly on the organic substituent (R) and the alkali metal (M). For less bulky R groups and/or for the heavier alkali metals, the formation of P–P and Sn–Sn bonded complexes is commonly observed. Some examples of this behavior include the anions [{Sn(μ-PMes)}2 (μMesPPMes)]2− 80,51 [{Sn(μ-PCy)}3 ]2− 81,50 and [Sn3 (μ3 -PtBu)(μ-PtBu)3 ]2− 82 (Scheme 2.2.18).53 The ˚ in the trimeric cluster 81 are in the typical range of a Zintl anion.50 This tin–tin bond lengths (3.15–3.18 A) unusual structure is described by an electron-deficient (two-electron–three-center) bonding arrangement involving p-orbitals on the three tin atoms. The dianion in 82 is the phosphorus analog of the dianions present in the heterometallic imidotin cubanes 53 and 54. Tetraanions of the type [Sn4 (μ4 -PR)6 ]4− (R = tBu, Cy) are present in the structure of 42 (see Reactions of [Sn(μ3 -NR)]4 above).

2.2.7

Summary and Future Prospects

Our knowledge of the fundamental chemistry of tin(II) clusters has progressed significantly since Veith’s initial report on the formation of the imidotin cubane [Sn(μ3 -NtBu)]4 almost 30 years ago. In addition to the refinement of synthetic strategies to the well-known [Sn(μ3 -NR)]4 cubanes, a wide variety of other structural motifs have been established. Compared to the imidotin systems, recent investigations of phosphidotin(II) clusters have revealed significant differences in the types of cluster structures that are accessible and synthetic routes to a variety of novel, anionic clusters have been established. The reaction chemistry of these potentially versatile reagents is essentially undeveloped. The incorporation of other main group elements or transition metals into tin(II) clusters via metathetical reactions of metal halides with anions such as [Sn3 (μ3 -NtBu)(μ-NtBu)3 ]2− (E = N, P) is an obvious target. The transfer of the fundamental knowledge on tin(II) clusters described in this chapter to practical applications in materials science is an important, but worthwhile, challenge for the future. References 1. M. Veith, M.L. Sommer, and D. J¨ager, Chem. Ber., 112, 2581 (1979). 2. M. Veith, Z. Naturforsch., 35b, 20 (1980). 3. A. Bashall, N. Feeder, E.A. Harron, M. McPartlin, M.E.G. Mosquera, D. S´aez, and D.S. Wright, J. Chem. Soc., Dalton Trans., 4104 (2000). 4. R.E. Allan, M.A. Beswick, G.R. Coggan, P.R. Raithby, A.E.H. Wheatley, and D.S. Wright, Inorg. Chem., 36, 5202 (1997).

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5. M. Veith, M. Ops¨older, M. Zimmer, and V. Huch, Eur. J. Inorg. Chem., 1143 (2000). 6. M. Veith, and W. Frank, Angew. Chem. Int. Ed. Engl., 23, 158 (1984). 7. D.R. Armstrong, F. Benevelli, A.D. Bond, N. Feeder, E.A. Harron, A.D. Hopkins, M. McPartlin, D. Moncrieff, D. S´aez, E.A. Quadrelli, A.D. Woods, and D.S. Wright, Inorg. Chem., 41, 1492 (2002). 8. F. Benevelli, E.L. Doyle, E.A. Harron, N. Feeder, E.A. Quadrelli, D. S´aez, and D.S. Wright, Angew. Chem. Int. Ed., 39, 1501 (2000). 9. N. Feeder, E.A. Harron, M.E.G. Mosquera, A.D. Woods, and D.S. Wright, Chem. Commun., 1010 (2001). 10. M. Veith and W. Frank, Angew. Chem. Int. Ed. Engl., 24, 223 (1985). 11. M. Veith, Phosphorus, Sulfur, and Silicon, 41, 195 (1989). 12. T. Chivers, T.J. Clark, M. Krahn, M. Parvez, and G. Schatte, Eur. J. Inorg. Chem., 1857 (2003). 13. T. Chivers and D.J. Eisler, Angew. Chem. Int. Ed., 43, 6686 (2004). 14. M. Veith and O. Recktenwald, Z. Naturforsch., 38b, 1054 (1983). 15. M. Veith and G. Schlemmer, Chem. Ber., 115, 2141 (1982). 16. H. Chen, R.A. Bartlett, H.V. Rasika Dias, M.M. Olmstead, and P.P. Power, Inorg. Chem., 30, 3390 (1991). 17. R.E. Allan, M.A. Beswick, A.J. Edwards, M.A. Paver, M.A. Rennie, P.R. Raithby, and D.S. Wright, J. Chem. Soc., Dalton Trans., 1991 (1995). 18. R.E. Allan, M.A. Beswick, N.L. Cromhout, M.A. Paver, P.R. Raithby, A. Steiner, M. Trevithick, and D.S. Wright, Chem. Commun., 1501 (1996). 19. A. Bashall, A. Ciulli, E.A. Harron, G.T. Lawson, M. McPartlin, M.E.G. Mosquera, and D.S. Wright, J. Chem. Soc., Dalton Trans., 1046 (2002). 20. J.F. Eichler, O. Just, and W.S. Rees, Jr., Inorg. Chem., 45, 6706 (2006). 21. J.F. Eichler, O. Just, and W.S. Rees, Jr., Phosphorus, Sulfur, and Silicon, 179, 715 (2004). 22. Y. Tang, L.N. Zakharov, A.L. Rheingold, and R.A. Kemp, Inorg. Chim. Acta, 359, 775 (2006). 23. R.E. Allan, M.A. Beswick, M.K. Davies, P.R. Raithby, A. Steiner, and D.S. Wright, J. Organomet. Chem., 550, 71 (1998). 24. W.J. Grigsby, T. Hascall, J.J. Ellison, M.M. Olmstead, and P.P. Power, Inorg. Chem., 35, 3254 (1996). 25. M. Veith, M. Grosser, and O. Recktenwald, J. Organomet. Chem., 216, 27 (1981). 26. M. Veith and H. Lange, Angew. Chem. Int. Ed. Engl., 19, 401 (1980). 27. B. Gal´an, M.E.G. Mosquera, J.S. Palmer, P.R. Raithby, and D.S. Wright, J. Chem. Soc., Dalton Trans., 1043 (1999). 28. M. Veith, Chem. Rev., 90, 3 (1990). 29. M. Veith, Angew. Chem. Int. Ed. Engl., 26, 1 (1987). 30. T. Chivers, T.J. Clark, M. Parvez, and G. Schatte, Dalton Trans., 2107 (2003) 31. M.E.G. Mosquera, A.D. Hopkins, P.R. Raithby, A. Steiner, A. Rothenberger, A.D. Woods, and D.S. Wright, Chem. Commun., 327 (2001). 32. M. Veith, M. Jarczyk, and V. Huch, Chem. Ber., 121, 347 (1988). 33. A.D. Bond, E.A. Harron, G.T. Lawson, M.E.G. Mosquera, M. McPartlin, and D.S. Wright, J. Chem. Soc., Dalton Trans., 3525 (2002). 34. C. Brown, M.E.G. Mosquera, J.S. Palmer, P.R. Raithby, A. Steiner, and D.S. Wright, J. Chem. Soc., Dalton Trans., 487 (2000). 35. T. Chivers, D.J. Eisler, and J.S. Ritch, Z. Anorg. Allg. Chem., 630, 1941 (2004). 36. M. Veith, J. Fischer, T.R. Prout, M. N¨otzel, P. Hobein, and V. Huch, Inorg. Chem., 30, 4130 (1991). 37. M. Veith and M. Grosser, Z. Naturforsch., 37b, 1375 (1982). 38. D.J. Eisler and T. Chivers, Chem. Eur. J., 12, 233 (2006). 39. T. Chivers and G. Schatte, Chem. Commun., 2264 (2001). 40. F. Garc´ıa, M.L. Stead, and D.S. Wright, J. Organomet. Chem., 691, 1673 (2006). 41. M. Driess, R. E. Mulvey, and M. Westerhausen, in Molecular Clusters of the Main Group Elements, M. Driess and H. N¨oth (Eds), Wiley-VCH, Weinheim, 2004, Chapter 3.6. 42. M. Driess, S. Martin, K. Merz, V. Pintchouk, H. Pritzkow, H. Gr¨utzmacher, and M. Kaupp, Angew. Chem. Int. Ed. Engl., 36, 1894 (1997).

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43. M. Westerhausen, N. Makropoulos, H. Piotrowski, M. Warchhold, and H. N¨oth, J. Organomet. Chem., 614–615, 70 (2000). 44. M. Westerhausen, M. Krofta, N. Wiberg, J. Knozek, H. N¨oth, and A. Pfitzner, Z. Naturforsch., 53B, 1489 (1998). 45. M. Westerhausen, M. Krofta, S. Schneiderbauer, and H. Piotrowski, Z. Anorg. Allg. Chem., 1391 (2005). 46. M. Westerhausen, Dalton Trans., 4755 (2006). 47. M. Westerhausen, H.D. Hausen, and W. Schwarz, Z. Anorg. Allg. Chem., 621, 877 (1995). 48. M. Westerhausen and W. Schwarz, Z. Anorg. Allg. Chem., 622, 903 (1996). 49. A.D. Bond, A. Rothenberger, A.D. Woods, and D.S. Wright, Chem. Commun., 525 (2001). 50. P. Alvarez-Bercedo, A.D. Bond, R. Haigh, A.D. Hopkins, G.T. Lawson, M. McPartlin, D. Moncrieff, M.E.G. Mosquera, J.M. Rawson, A.D. Woods, and D.S. Wright, Chem. Commun., 1288 (2003). 51. F. Garcia, A.D. Hopkins, S.M. Humphrey, M. McPartlin, C.M. Pask, A.D. Woods, and D.S. Wright, Organometallics, 23, 4821 (2004). 52. F. Garc´ıa, A.D. Hopkins, R.A. Kowenicki, M. McPartlin, C.M. Pask, M.L. Stead, A.D. Woods, and D.S. Wright, Organometallics, 24, 1813 (2005). 53. F. Garc´ıa, J.P. Hehn, R.A. Kowenicki, M. McPartlin, C.M. Pask, A. Rothenberger, M.L. Stead, and D.S. Wright, Organometallics, 25, 3275 (2006). 54. P. Alvarez, F. Garc´ıa, J.P. Hehn, F. Kraus, G.T. Lawson, N. Korber, M.E.G. Mosquera, M. McPartlin, D. Moncrieff, C.M. Pask, A.D. Woods, and D.S. Wright, Chem. Eur. J, 13, 1078 (2007).

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Mono-Organotin Oxo-Clusters

2.3

69

Mono-Organotin Oxo-Clusters

Franc¸ois Ribot Chimie de la Mati`ere Condens´ee de Paris, Universit´e Pierre et Marie Curie-Paris6, Paris, France

2.3.1

Introduction

Metal oxo-clusters are finite polymetallic aggregates, the architecture of which is based on a metal– oxygen framework. A great diversity exists in these species. They range from purely inorganic polyoxometalates1−5 to organometallic oxides6 in which a Mx O y core has its surface protected by organic groups. Mono-organotin derivatives exhibit a rich oxo-cluster chemistry and numerous compounds with different structural types have been described. For the sake of classification, mono-organotin oxo-clusters can be divided in two main families. The first one includes all the oxo-clusters which also bear multidentate ligands (e.g. carboxylates, phosphinates, phosphonates, etc.). In these species, the architecture of the metal–oxygen framework is strongly influenced by the binding characteristics of the ligands, as well as the ligand-to-tin ratio. This first family is still a very active field of research, that has been the subject of several reviews,7-14 and thus will not be discussed here. The second family, the subject of the present chapter, contains the mono-organotin oxo-clusters which only possess oxo- and/or hydroxo- bridges and various types of terminal groups (hydroxy, halogeno, etc.), in addition to the organic residue directly bonded to each tin atom through an Sn C link. They correspond to the general formula (RSn)n Ox (OH) y Clz . A third type of compounds which can be considered to be mono-organotin oxo-clusters are polyoxometalates that include in their metal–oxygen framework one or more SnR units, isolated or connected through Sn O Sn linkages.15-23 In these derivatives, tin is not the most abundant element and the properties are mainly related to the other metal (e.g. tungsten) or to the organic group bonded to tin. These compounds, though interesting, will not be discussed here. Mono-organotin oxo-clusters have shown good potential in the field of catalysis24 and hybrid organic– inorganic materials.25-31 They are also fundamentally interesting because of their relation to organostannonic acid, RSnO(OH), a compound active in transesterification reactions,32-37 that generally is an ill-defined polymer.7,38 Indeed, these oxo-clusters share with organostannonic acid a general preparation scheme based on the hydrolysis-condensation of organotin trihalides, or similar molecular precursors,39-44 and they represent stable intermediates, or dead ends, in the synthesis of polymeric RSnO(OH). A good description of their structures and a better understanding of their formation are likely to provide significant insight into the ill-defined and still unknown structure of organostannonic acid. With its two main NMR-active isotopes, i.e. 119 Sn (I = 1/2, 8.58%, 45 = 37.290665 MHz) and 117 Sn (I = 1/2, 7.61%, 45 = 35.632295 MHz), tin is a friendly and versatile spectroscopic probe, complementary to single crystal X-ray diffraction, to study tin-based compounds, both in solution and in the solid state.46-49 In solution, the similar natural abundance of both isotopes gives generally rich satellite patterns in which the coupling between two equivalent nuclei can be observed through the n J (119 Sn–117 Sn). The chemical shift anisotropy data, that can only be obtained from solid-state 119 Sn NMR, are also valuable information to characterize the local geometry.48,49 In the context of organotin oxo-clusters, solution 119 Sn or 117 Sn NMR is one of the most valuable techniques to prove whether a given metal–oxygen framework, evidenced in a crystalline material, is preserved upon dissolution. Solid-state 119 Sn NMR, which is becoming more and more accessible, allows one to establish that a given organotin oxo-cluster is present in a solid, even an amorphous solid,29,31 and that it is not mixed with any ill-defined oxo-polymer.50 Finally, the

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comparison of solid-state and solution 119 Sn NMR data is a way to identify weak coordinative contacts and structural evolutions between the crystallized and the dissolved species.50 The aim of this chapter is to review and discuss the molecular structures, syntheses, and possible formation mechanisms of the organotin oxo-clusters that follow the formula (RSn)n Ox (OH) y Clz . 2.3.2

Molecular Structures

This first section presents the molecular structures of all the known derivatives. Many of them adopt a cage structure with the organic groups pointing outward. The drawings (Figures 2.3.1–4, 2.3.7, and 2.3.8) are all based on crystalline structures. They use the fractional coordinates that have been either published or obtained from CCDC51 or FIZ Karlsruhe.52 To ease the readability, all the organic groups are omitted and only the Sn C bonds are shown. The compounds discussed also include species without confirmatory X-ray crystal structures but sufficient spectroscopic evidence exists to unambiguously relate them to a known molecular structure. There are several ways to describe a structure and its metal–oxygen framework. One of them relies on sub-units,53 and a very common one in many organotin oxo-clusters is the so-called O-capped trimer, in which three tin atoms are connected all together with a μ3 -O and two-by-two with three μ2 -O(H/R ) (Scheme 2.3.1). For five- or six-coordinate metals this sub-unit corresponds to three distorted trigonal bipyramids or octahedra connected through a common vertex and three edges. This tri-metallic sub-unit, which is, for example, found in {[(RSn)(OH)(O2 PR2 )]3 O}(O2 PR2 ),9 {[(RSn)3 (OR )3 O]2 (HPO3 )4 }, 10,11 and {(PhSn)3 P2 W15 O59 }9- ,22 occurs in the structural chemistry of many other metal oxo-clusters, such as polyoxometalates (POMs)1–5 and titanium oxo-clusters.54

Scheme 2.3.1

Sn

O framework of the O-capped trimer sub-unit

{RSn(μ2 -OH)Cl2 (H2 O)}2

The dimer {RSn(OH)Cl2 (OH2 )}2 is the smallest mono-organotin oxo-cluster. Its molecular structure (Figure 2.3.1a) is based on two octahedra joined via two μ2 -OH bridges defining a common edge. This double bridge results in a distannoxane ring (Sn2 O2 ). The four other vertices of each octahedron are occupied by two chloride atoms (cis-position), the first carbon of the R group (equatorial position, trans to the shorter Sn OH), and one water molecule (axial position). Such a dimer has been isolated for methyl-,55 ethyl-,56 isopropyl-,57 n-butyl-,58 and isobutyltin.57 The n-butyl- and methyltin derivatives have been studied by solution 119 Sn NMR. For the n-butyltin compounds, the observation of a major resonance at −408.6 ppm (in acetone-d6 ) is consistent with six-coordinate tin atoms and with the preservation of the dimeric structure in solution.58 A similar conclusion does not seem to hold for the methyltin derivative, for which 119 Sn NMR indicates a drop in the coordination of tin (−175.9 ppm in CDCl3 ).55 Such a dimer has also been observed containing bridged organotins. Indeed, the hydrolysis of 1,3bis(trichlorostannyl)propane, in water, yields a solid where the dimers are associated by the alkyl spacer in infinite chains.59

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Figure 2.3.1 Molecular structures of {RSn(OH)Cl2 (OH2 )}2 (a),55−59 {(Me3 Si)3 CSnO(OH)}3 (b),60 {[(Me3 Si)3 CSn]4 O6 } (c),62 and {2,4,6-i Pr3 C6 H2 SnO(OH)}6 (d).63 Tin is presented as small black circles, O (or H2 O) as medium white circles, OH as medium striped circles, and Cl as large hatched circles. The same color code applies subsequently, as appropriate.

{(Me3 Si)3 CSn(μ2 -O)(OH)}3 and {(Me3 Si)3 CSn(μ2 -O)Cl}3

For tris(trimethylsilyl)methyltin, a structure based on three four-coordinate tin atoms connected through μ2 -O bridges in a Sn3 O3 cycle has been observed.60 This structure is reminiscent of the one observed for several diorganotin oxides7 and stannasiloxanes.61 The oxy-hydroxy derivative (Figure 2.3.1b) adopted an ideal chair conformation with the organic groups in equatorial positions and a cis-arrangement of the three axial hydroxy groups. The 119 Sn NMR spectra in C6 D6 ({(Me3 Si)3 CSn(μ2 -O)(OH)}3 : −156 ppm with 2 J (117 Sn,119 Sn) = 719 Hz; {(Me3 Si)3 CSn(μ2 -O)Cl}3 :−141.0 ppm with 2 J (117 Sn,119 Sn) = 829 Hz and −133.0 ppm with 2 J (117 Sn,119 Sn) = 822 Hz) are consistent with a tetrahedral coordination of the tin atoms and indicate that the structures are preserved in solution. The number of signals (one for the hydroxy derivative and two for the chloro derivative) indicates that these two compounds exhibit different conformations.60 {[(Me3 Si)3 CSn]4 (μ2 -O)6 }

Tris(trimethylsilyl)methyltin also affords a fully condensed species, the structure of which is based on four Sn3 O3 cyclo-tristannoxanes rings ‘fused’ together in a regular hetero-adamantane Sn4 O6 core with tetrahedral tin atoms and μ2 -oxo bridges only (Figure 2.3.1c).62 In such a compound, the oxo-core is completely covered by the bulky organic residues. The same compact architecture has also been reported for tris(trimethylsilyl)methyltin and n-butyltin compounds, but with sulfur or selenium instead of oxygen.62

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On the basis of 119 Sn NMR, this oxo-cluster is said to be stable in solution. However, the reported chemical shift (94.3 ppm in C6 D6 ) is quite different from those observed for other CSnO3 environments with the same organotin.60 {(2,4,6-i Pr3 C6 H2 Sn)6 (OH)4 (μ3 -O)4 (μ2 -O)2 (μ2 -OH)2 }

With the bulky 2,4,6-tris(isopropyl)phenyl group, a hexamer has been isolated.63 Its structure (Figure 2.3.1d), which contains only five-coordinate tin atoms (distorted trigonal bipyramid), is based on a Sn6 O8 twisted ribbon made of six four-membered [Sn2 O2 ] rings. The oxygen atoms are distributed in between μ3 -O, μ2 -O, and μ2 -OH bridges and terminal hydroxy groups. An intra-molecular set of hydrogen bonds ‘closes’ both ends of this bracelet-like framework. Alternatively, the metal–oxygen framework can be described by the connection of two O-capped trimers modified by the opening of one of the three μ2 -O bridges. This structure is kinetically labile in solution, as seen by the appearance of ten 119 Sn NMR signals (from −309.5 to −538.7 ppm) when the compound is dissolved in deuterated chloroform. However, this decomposition is reversible and the hexamer is quantitatively recovered upon crystallization.63 The solid-state 119 Sn NMR (CP-MAS) signal of this oxo-cluster contains only two isotropic chemical shifts (−213 and −355 ppm) while three different tin atoms exist in the structure.63 However, according to distances and angles they can be grouped in only two environments, one tin atom exhibiting a more distorted trigonal bipyramid than the two others. This stronger distortion likely explains the 140 ppm difference in the 119 Sn chemical shifts of two five-coordinate environments. {(i PrSn)9 (μ3 -O)8 (μ2 -OH)6 Cl5 }

The pyramidal cage structure of {(i PrSn)9 O8 (OH)6 Cl5 } is based on five six-coordinate (distorted octahedron) and four five-coordinate (distorted trigonal bipyramid) tin atoms linked by μ3 -oxo and μ2 -OH bridges (Figure 2.3.2).64 A terminal chloride atom is also connected to each six-coordinate tin atom. No study has ever checked if this product of hydrolysis was preserved in solution or could only be isolated in the solid state.

Figure 2.3.2 Molecular structure of {( i PrSn)9 O8 (OH)6 Cl5 }. (Reproduced from reference 64, copyright 1989, Elsevier.)

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Figure 2.3.3 Ball and stick (left) and polyhedral (right) molecular structure of {(RSn)12 O14 (OH)6 }2+ .29,50,65−69 The polyhedra around the five-coordinate tin atoms are striped.

{(RSn)12 (μ3 -O)14 (μ2 -OH)6 }2+ and Related Compounds

The macrocation {(RSn)12 O14 (OH)6 }2+ is the most documented mono-organotin oxo-cluster. Its molecular structure (Figure 2.3.3) is based on six five-coordinate (distorted square pyramid), located at the equator of this almost spherical cage, and six six-coordinate (distorted octahedron) tin atoms, linked by μ3 -O and μ2 -OH bridges.29,50,65−69 To better describe this metal–oxygen framework, it can be envisaged as being comprised of three sub-units. Two of them are O-capped trimers that contain all six-coordinate tin atoms and the hydroxy bridges. These sub-units define the positive poles of this oxo-cluster. The other sub-unit is a macro-ring containing all five-coordinate tin atoms connected through their basal oxygen atoms. The anions required to balance the positive charge always interact through hydrogen bonds with one or several μ2 -OH links. A similar molecular structure is found for the titanium(IV) oxo-cluster, Ti12 O16 (Oi Pr)16 ,70,71 and a vanadium(V/IV) oxo-cluster, {(VO)12 O12 F2 (OH)6 }6− .72 In the titanium derivative, Ti-Oi Pr moieties replace the RSn groups and four out of six μ2 -OH groups are also substituted by alkoxy groups; the last two original μ2 -OH groups are transformed into μ2 -O, to yield a neutral compound. The anionic vanadium derivative is purely inorganic, with terminal oxo groups on its surface, and incorporates a μ3 -F to replace the internal μ3 -O that connects the six-coordinate metal atoms at each pole. This similarity between a vanadium and an organotin oxo-cluster has prompted Reuter et al. to look for mixed organotin-vanadium compounds and they found several where one, two, three, or six V O groups replace the square-pyramidally coordinated organotin groups.73 Two of these mixed compounds {(i PrSn)11 (OVIV )O14 (OH)6 }Cl,73 and {(n BuSn)9 (OVV )3 O14 (OH)6 Cl2 (DMSO)2 },74 have been structurally well-characterized (Figures 2.3.4a and b). The metal–oxygen framework of the first structure does not differ from that of {(RSn)12 O14 (OH)6 }2+ , but bears a single positive charge. For the second structure, because the three [OVO4 ] units are condensed together and V O single bonds are shorter than Sn O bonds, the original metal–oxygen framework is disrupted in two places, with two μ3 -O groups transformed into μ2 -O, and allows two equatorial tin atoms to increase their coordination to six, each with a Sn Cl bond and an Sn O coordinative bond with a DMSO molecule.74 One additional difference between these two mixed organotin–vanadium oxo-clusters is found in the oxidation state of vanadium; +4 in the derivative with a single vanadium and +5 in the derivative with three vanadium atoms.

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Figure 2.3.4 Ball and stick (left) and polyhedral (right) molecular structures of {( i PrSn)11 (OV)O14 (OH)6 }+ ,73 and {( n BuSn)9 (OV)3 O14 (OH)6 Cl2 DMSO2 }.74 Additional color code: vanadium as small hatched circles. The polyhedra around the five-coordinate tin atoms are striped.

The structure of {(RSn)12 O14 (OH)6 }2+ is fully preserved in solution, where it acquired a perfect 3-fold symmetry (D3d ), as proved many times by its distinctive 119 Sn NMR signature composed of two resonances, one for each coordination, and up to three two-bonds scalar couplings in between tin atoms of different or identical coordination (Figure 2.3.5, Table 2.3.1).29,43,50,66,67,69,75,76 As presented in the Introduction, solid-state 119 Sn NMR is a complementary tool to X-ray diffraction for the study of organotin oxo-clusters. Mono-organotin derivatives generally exhibit large chemical shift anisotropies and their spectra contain many spinning side bands.29,31,50,67,75,77 The 119 Sn NMR (MAS) spectra of {(n BuSn)12 O14 (OH)6 }(OH)2 (HOi Pr)4 and {(n BuSn)12 O14 (OH)6 }(pTs)2 (Dioxane) are shown as examples (Figure 2.3.6). From such spectra, an anisotropy and an asymmetry can be measured for each chemical shift.78 For five-coordinate tin atoms (isotropic chemical shift ca. −280 ppm), the anisotropy equals 380 ± 20 ppm, while for six-coordinate tin atoms (isotropic chemical shift ca. −460 ppm), the anisotropy is smaller at 300 ± 20 ppm The asymmetry for both coordinations can range from 0.15 to

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Figure 2.3.5 Solution 119 Sn NMR spectrum of {(n BuSn)12 O14 (OH)6 }(pTs)2 in CDCl2 at 93.3 MHz. : 2 J ( 119 Sn-119/117 Sn) between a five- and a six-coordinate tin atom linked through a single oxo-bridge, : 2 J ( 119 Sn-119/117 Sn) between a five- and a six-coordinate tin atom linked through a double oxo-bridge, ∗ : 2 J ( 119 Sn-117 Sn) between two six-coordinate tin atoms linked through a double oxo-hydroxo bridge, and : 2 J ( 119 Sn-117 Sn) between two five-coordinate tin atoms linked through a double oxo-bridge.50

0.45, depending on the compound. For {(n BuSn)12 O14 (OH)6 }(pTs)2 (Dioxane), 119 Sn NMR has clearly provided evidence for the weak interaction that takes place in the solid state between some of the five-coordinate tin atoms and the oxygen atoms of the dioxane molecules. Indeed a low frequency shift of about 20 ppm is observed for the tin atoms experiencing this extra contact, characterized by an Sn O ˚ 50 Solid-state 119 Sn NMR was also used to unambiguously show the preservation distance of 3.3 A. Table 2.3.1

119

Sn NMR data for various {(RSn)12 O14 (OH)6 }X2 derivatives

{(RSn)12 O14 (OH)6 }X2

Solvent

δ(119 Sn)/ppm and [2 J (119 Sn–119 Sn)]/Hz

Ref.

R = n Bu, X = Cl R = n Bu, X = OH R = n Bu, X = OH R = n Bu, X = pTsa R = n Bu, X = O2 PPh2 R = n Bu, X = AMPSb R = n Bu, X = AcO R = Me3 SiCH2 , X = Cl R = pBST,c X = OH R = AcO(CH2 )5 , X = OH R = PentCro,d X = OH R = C6 H5 , X = OH/Cl

CD2 Cl2 C6 D6 CDCl3 CD2 Cl2 CD2 Cl2 CDCl3 CD2 Cl2 CDCl3 CDCl3 CDCl3 CDCl3 CD2 Cl2

−283.1 [425, 156], −468.1 [425, 188] −280.1 [383, 177, 40], −447.4 [383, 205, 40] −282.2 [380, 177], −449.0 [380, 205] −282.8 [423, 162, 42], −461.8 [423, 176, 42] −283.3 [412, 158], −457.7 [412, 174] −283.0 [423, 153, 42], −462.0 [423, 172, 42] −281 [410], −458 [410] −269.6 [434, 183], −460.5 [434, 191] −280.0 [unresolv.], −470.4 [unresolv.] −280.7 [373, 178], −443.7 [373, 207] −282.2 [unresolv.], −449.0 [unresolv.] −354 [420, 220], −520 [420, 246]

66 67 43 50 75 29 31 69 43 43 43 76

a c

pTs : 4-CH3 C6 H4 SO3 ; b : AMPS : H2 C CHCONHC(CH3 )2 CH2 SO3 , : pBST : 4-(H2 C CH)C6 H4 (CH2 )4 , d : PentCro : MeCH CHCO2 (CH2 )5

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Figure 2.3.6 Solid-state 119 Sn NMR MAS spectra of {( n BuSn)12 O14 (OH)6 }(OH)2 (HOi Pr)4 (a)67 and {( n BuSn)12 O14 (OH)6 }(pTs)2 (Dioxane) (b)50 at 111.9 MHz (ν M AS = 13 kHz). Isotropic chemical shifts are indicated with arrows.

of the {(n BuSn)12 O14 (OH)6 }2+ oxo-core in insoluble amorphous materials prepared from functional oxo-clusters.29,31 [Na ⊂{(i PrSn)12 (μ4 -O)4 (μ2 -OH)24 }]5+

The structure of [Na ⊂ {(i PrSn)12 O4 (OH)24 }]5+ is based on the association, around a central sodium ion, of four identical O-capped trimers ‘(RSn)3 (μ3 -O)(μ2 -OH)3 (OH)6 ,’ through μ2 -OH bridges only (Figure 2.3.7).79 The trimeric sub-units are oriented so that only two of them share an edge; all the other trimer–trimer contacts are through vertices. Such an arrangement of the trimers is also found for the polyoxometalate [PW10 V2 O40 ]5− and corresponds to the γ isomer of the Keggin’s structure.5,80

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Figure 2.3.7 Ball and stick (left) and polyhedral (right) molecular structure {( i PrSn)12 O4 (OH)24 }]5+ .79 Additional color code: sodium as small hatched circles.

of

[Na

77

⊂

There is no evidence for the stability of such species in solution and it might be only encountered in the solid state where it cocrystallizes in between infinite chains of [Ag7 I11 ]4− . {(2,4,6-i Pr3 C6 H2 Sn)8 (μ4 -O)2 (μ3 -O)8 (μ2 -O)4 (μ2 -OH)8 (SnOH)4 }

{(2,4,6-i Pr3 C6 H2 Sn)8 O14 (OH)8 (SnOH)4 } does not strictly belong to the same family because it includes four purely inorganic tin atoms which come from a dearylation process during its synthesis.63 However, its structure is worth discussing here. It can be described with a central adamantane-type inorganic core, {(HOSn)4 O6 }, capped by two identical and perpendicular organostannoxane arcs, {(2,4,6i Pr3 C6 H2 Sn)O(OH)}4 (Figure 2.3.8). The four inorganic tin atoms are six-coordinate and each bear a terminal OH. The organostannoxane arcs are based on two central six-coordinate tin atoms and two external five-coordinate ones (distorted trigonal bypiramid). An alternate description of this metal–oxygen framework can be one based on four identical O-capped trimers, each based on a six-coordinate inorganic tin, a six-coordinate aryltin moiety, and a five-coordinate one. This structure includes five different types

Figure 2.3.8 Ball and stick (left) and polyhedral (right) molecular structure of {(2,4,6Pr3 C6 H2 Sn)8 (HOSn)4 O14 (OH)8 }.63 Additional color code: inorganic tin as small hatched circles. The polyhedra around the five-coordinate tin atoms are striped.

i

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of oxygen atoms: two μ4 -O, eight μ3 -O, four μ2 -O, eight μ2 -OH, and four terminal OH. It incorporates two features not observed in any of the previously described organotin oxo-clusters: oxygen atoms between four tin atoms and face-sharing RSnO5 octahedra. The latter feature is also observed in a POM derivative [{(PhSn)2 O}2 H(α-AsW9 O33 )2 ]9− .15 Its signature 119 Sn NMR spectrum, three resonances (−574.1, −438.2, and −338.0 ppm, for SnO6 , CSnO5 , and CSnO4 environments, respectively) in a 1:1:1 ratio, is fully consistent with the stability of its metal–oxygen framework in solution.63 Solid-state 119 Sn NMR (CP-MAS), that shows three isotropic chemical shifts (−576, −448, −343 ppm) very close to those observed in solution, indicates that almost no changes take place when this organotin oxo-cluster is dissolved. 2.3.3

Syntheses and Formation Mechanisms

Syntheses of Organotin Oxo-Clusters

As with organostannonic acid, mono-organotin oxo-clusters are generally prepared by hydrolysiscondensation of molecular precursors such as organotin trihalides (RSnCl3 , RSnBr3 ), organotin trialkoxides (RSn(OR )3 ),39 and organotin trialkynides (RSn(C≡CR )3 ).40−44 Organotin trichlorides are more accessible (several are commercially available or can be prepared via Kotcheskov’s redistribution),81 but they generally yield compounds that still contain chloride, either directly bonded to tin (not hydrolyzed) or as charge compensating anion with cationic species. In some cases, already hydrolyzed/condensed precursors, of unknown structures (i.e. i PrSn(OH)2 Cl · 3/4H2 O and n BuSnO(OH)), are also used. The synthesis of {(n BuSn)12 O14 (OH)6 }2+ from n BuSnO(OH) and sulfonic acid is inspired by the significant amount of work on oxo-clusters with carboxylate, phosphinate, or phosphonate ligands.7−9 Yet the poor complexing ability of sulfonates results in the formation of an oxo-cluster that does not include any sulfonate in its architecture. The experimental conditions reported are quite diverse and they are summarized in Tables 2.3.2 and 2.3.3. There are no clear trends that allow the confident prediction of the composition of the final compounds, especially when considering that some organotin oxo-clusters are serendipitous products. The influence of the organic group appears very strong. For example, tris(trimethylsilyl)methyl, a very bulky group, yields compounds based on Sn3 O3 tristannoxane rings and only four-coordinate tin atoms.60 2,4,6Tris(isopropyl)phenyl, another bulky substituent, can yield a hexamer and a dodecamer that contain only five-coordinate and five- and six-coordinate tin atoms, respectively; in the latter, one third of the tin atoms have undergone dearylation.63 The bulkiness of the organic substituents is not the only parameter; intra- or inter-molecular interactions involving the tin atom can modify the reaction outcome. For example, while the hydrolysis-condensation of AcO(CH2 )5 Sn(C C-n Bu)3 yields the {(RSn)12 O14 (OH)6 }2+ macrocation, with AcO(CH2 )3 Sn(C C-n Bu)3 a strong intra-molecular interaction,82,83 involving the alkoxy oxygen, probably prevents the formation of the oxo-cluster and instead a soluble oxo-polymer is obtained.43 Solvent is also of importance to control the outcome of the hydrolysis-condensation reaction. With exactly the same precursor (i PrSn(OH)2 Cl · 3/4H2 O), the same amount, and the same hydrolysis procedure (atmospheric moisture), {(i PrSn)9 O8 (OH)6 Cl5 } is obtained in DMSO,64 while {(i PrSn)12 O14 (OH)6 }Cl2 is obtained in DMF or DMPU.65 The parameter with the least influence seems to be the nature of the hydrolyzable group. Indeed, the macrocation {(RSn)12 O14 (OH)6 }2+ has been obtained, with the same organic R group, from organotin trichlorides, trialkoxides, and trialkynides. Yet, organotin trichlorides need harsher conditions to be fully hydrolyzed. A comment needs to be made on the yields of reactions. Indeed, hydrolysis-condensation reactions can yield well-defined organotin oxo-clusters, but they can also yield ill-defined organotin oxo-polymers.

EtSnCl3 n BuSnCl3 i PrSnCl3 i BuSnCl3 MeSnCl3 Cl3 Sn(CH2 )3 SnCl3 (Me3 Si)3 CSnCl3 {(Me3 Si)3 CSnOCl}3 (Me3 Si)3 CSnBr3 2,4,6-i Pr3 C6 H2 Sn(C CMe)3

{EtSnOHCl2 (H2 O)}2 {n BuSnOHCl2 (H2 O)}2 {i PrSnOHCl2 (H2 O)}2 {i BuSnOHCl2 (H2 O)}2 {MeSnOHCl2 (H2 O)}2 {Sn(CH2 )3 Sn(OH)2 Cl4 (H2 O)2 } {(Me3 Si)3 CSnOCl}3 {(Me3 Si)3 CSnO(OH)}3 {[(Me3 Si)3 CSn]4 O6 } {2,4,6-i Pr3 C6 H2 SnO(OH)}6

Synthesis conditions

a

: Secondary product in the synthesis of {(2,4,6-i Pr3 C6 H2 Sn}8 O14 (OH)8 (SnOH)4 }; b : Unknown structure, prepared by the hydrolysis of i PrSnCl3 in toluene

48 94 75 75 80 –a 56 – – 20

– 10

56 58 57 57 55 59 60 60 62 63 63 64 79 63

Yield Ref.

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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Crystalsa

c

a

i

i

i

65 65 68 68 66 69 69 67 67 76 43 43 43 43 50 29

– – – – 84 85 93 67 – – – – – – 60 15

: in the absence of X-ray structure, the formation of {(RSn)12 O14 (OH)6 }2+ is confirmed by 119 Sn NMR; b : unknown structure, prepared by the hydrolysis of i PrSnCl3 in toluene; : crystallized in acetone/acetonitrile; d : crystallized in chloroform; e : crystallized in dioxane containing 0.5 % H2 O; f : crystallized in acetonitrile

Pr/Cl Pr/Cl i Pr/{(i PrSn)4 (MoO4 )4 O(OH)3 }

i

Yield Ref.

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PrSn(OH)2 Cl·3/4H2 Ob DMF, air moisture, few weeks 1 r3H2 O and 1 r2DMF PrSn(OH)2 Cl·3/4H2 Ob DMPU, air moisture, few weeks 1 r4DMPU·4H2 O i b PrSn(OH)2 Cl·3/4H2 O DMSO, (NH4 )2 MoO4 (1.25 eq.), air moisture, 1 r6DMSO few days i i Pr/{(i PrSn)4 (MoO4 )4 O(OH)3 } PrSn(OBu)3 DMF, H3 [P(Mo3 O10 )4 ] (0.12 eq.), air moisture, 1 r10DMF few days n n Bu/Cl BuSnCl3 Water, slow addition of 1M KOH till pH = 4 1 r2H2 Oc Me3 SiCH2 SnCl3 Water, slow addition of 1M KOH till pH = 4 1d Me3 SiCH2 /Cl Me3 SiCH2 /Cl Me3 SiCH2 SnCl3 Toluene/NH3 aq. (15%), Me3 SiCl (6.7 eq.) No n n i Bu/OH BuSn(Oi Pr)3 PrOH, H2 O/Sn = 10, 15 hours 1 r4i PrOH n n t t Bu/OH BuSn(O Amyl)3 AmylOH, H2 O/Sn = 10 No Ph/OH + Cl PhSn(Ot Amyl)x Cl3−x t AmylOH, H2 O/Sn = 10 No n n Bu/OH BuSn(C Cn Bu)3 CHCl3 /THF/H2 O No 4-(H2 C CH)C6 H4 (CH2 )4 /OH RSn(C Cn Bu)3 CHCl3 /THF/H2 O No AcO(CH2 )5 /OH RSn(C Cn Bu)3 CHCl3 /THF/H2 O No RSn(C Cn Bu)3 CHCl3 /THF/H2 O No MeCH CHCO2 (CH2 )5 /OH n n Bu/4-CH3 C6 H4 SO3 BuSnO(OH) toluene, R’SO3 H/Sn= 3.5, reflux 48 hours 1 rDioxanee n Bu/H2 C CHCONHC(CH3 )2 CH2 SO3 n BuSnO(OH) toluene, R’SO3 H/Sn= 3.5, reflux 24 hours 1f

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These latter species can be highly soluble in organic solvents, but, because of reorientation time, they can be 119 Sn NMR silent,43,50 especially if they are mixed with 119 Sn NMR ‘loud’ compounds, the signals of which are very narrow. Moreover, these oxo-polymers generally have a mean composition close to the one of the oxo-clusters (for n BuSnO(OH) Sn : 56.8%, C : 23.0%, and H : 4.8%; for {(n BuSn)12 O14 (OH)6 }(OH)2 Sn : 57.7%, C : 23.3%, and H : 4.7%). Accordingly, a yield simply measured from the weight of recovered solid, especially if there is an amorphous part, with the hypothesis, based on 119 Sn solution NMR, that there is no other compound can be definitely overestimated. To avoid such problems, quantitative 119 Sn solution NMR with an internal or external concentration reference should be performed to make sure that all the dissolved solid participates in the observed resonances. For instance, in the synthesis of {(n BuSn)12 O14 (OH)6 }(pTs)2 from n BuSnO(OH), the 119 Sn NMR spectrum of the crude materials looks exactly the same as the crystallized compound. However, quantitative 119 Sn NMR has shown that this crude material contains only ca. 50% {(n BuSn)12 O14 (OH)6 }(pTs)2 .50 To address these problems of yield determination, 119 Sn solid-state MAS NMR and size-exclusion chromatography can also be practical tools.50,84 Inter-conversions between different species have been reported for oxo-clusters with complexing ligands.8,9 But nothing similar has been observed for {(RSn)n Ox (OH) y Clz } compounds, except for {2,4,6-i Pr3 C6 H2 SnO(OH)}6 , that transforms into unknown species in solution, but can be quantitatively recovered upon crystallization.63 Another related case concerns the formation of {(Me3 Si)3 CSnO(OH)}3 upon treating {(Me3 Si)3 CSnOCl}3 with LiOH.60 However, this transformation proceeds without any modification of the metal–oxygen framework, which remains based on a Sn3 O3 tristannoxane ring. Changing the composition while keeping the metal–oxygen framework has also been observed several times with the substitution of the anions in {(RSn)12 O14 (OH)6 }X2 .28,30,31,75,85 This exchange reaction appears controlled by the acidity of the anions. Indeed hydroxyls can be replaced easily by many different anions [Equation (2.3.1)],31,75 and this approach has been used to prepare oxo-clusters with functional anions.28 By contrast, when the anion to exchange is derived from a strong acid (HCl or RSO3 H), the reaction with two equivalents of a weaker acid does not yield the exchange product, but the partial destruction of the metal–oxygen framework and the formation of different oxo-clusters that include complexing ligands [Equations (2.3.2) and (2.3.3)].75,85 Yet this destruction can be avoided if salts of the weak acids are used [Equation (2.3.4)].85,86 Finally, whatever the anion, if an excess of carboxylic or phosphinic acid is used, the metal–oxygen framework is transformed [Equations (2.3.5) and (2.3.6)].31,66 For Y = RSO3 , Cl, RCO2 , R2 PO2 {(n BuSn)12 O14 (OH)6 }(OH)2 + 2HY → {(n BuSn)12 O14 (OH)6 }Y2 + 2H2 O

(2.3.1)

For X = RSO3 , Cl {(n BuSn)12 O14 (OH)6 }X2 + 2RCO2 H → 5/6{(n BuSn)12 O14 (OH)6 }X2 + 1/3HX + 4/3H2 O + 1/3{n BuSnO(O2 CR)}6

(2.3.2)

For X = RSO3 , Cl {(n BuSn)12 O14 (OH)6 }X2 + 2R2 PO2 H → 7/8{(n BuSn)12 O14 (OH)6 }X2 + 1/4HX + 1/2H2 O + 1/2{[n BuSn(OH)(O2 PR2 )]3 O}(O2 PR2 )

(2.3.3)

For Y = OH, R2 PO2 {(n BuSn)12 O14 (OH)6 }(O3 SR)2 + 2Me4 NY → {(n BuSn)12 O14 (OH)6 }Y2 + 2Me4 N(O3 SR)

(2.3.4)

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For X = RSO3 , Cl, OH {(n BuSn)12 O14 (OH)6 }X2 + 12RCO2 H → 2{n BuSnO(O2 CR)}6 + 2HX + 8H2 O

(2.3.5)

{(n BuSn)12 O14 (OH)6 }X2 + 16R2 PO2 H → 4{[n BuSn(OH)(O2 PR2 )]3 O}(O2 PR2 ) + 2HX + 4H2 O (2.3.6) Because, in {(RSn)12 O14 (OH)6 }X2 , the anions do not interact directly with any tin atom but with the μ2 -OH that constitute the charged poles, the exchange of the anions or their ionic dissociation cause only small changes on the 119 Sn chemical shifts (Table 2.3.1). 1 H–1 H NOESY/ROESY (homonuclear Overhauser correlation spectroscopy) and 31 P–1 H HOESY (heteronuclear NOESY) have been used to study the ionic dissociation of {(RSn)12 O14 (OH)6 }X2 (X : O3 SR and O2 PR2 ), which depends on the solvent nature and the anion.50,75 More recently, pulsed field gradient (DOSY) 1 H NMR has also shown its remarkable versatility to probe association/dissociation and exchange of the charge compensating anions of {(RSn)12 O14 (OH)6 }X2 .86 There is another type of reaction with {(RSn)12 O14 (OH)6 }X2 that preserves its structure. Even though no species were isolated, the possible replacement of two μ2 -OH, out of six, by methanol has been clearly shown by electrospray mass spectrometry (ES-MS).66 Reflections on Possible Formation Mechanisms

The diversity observed in the molecular structure of organotin oxo-clusters is difficult to explain, but is likely related to a subtle balance between the nature of the organic group and the hydrolysis conditions (solvent, hydrolysis ratio, etc.). There is even a case where a unique reaction mixture yields two different oxo-clusters with fairly different metal–oxygen frameworks, i.e. {2,4,6-i Pr3 C6 H2 SnO(OH)}6 and {(2,4,6i Pr3 C6 H2 Sn)8 O14 (OH)8 (SnOH)4 }.63 In order to clarify this diversity, a reflection on possible formation mechanisms involving a limited number of ‘plausible intermediates’ is interesting. A leading idea in this reflection is the strong tendency for mono-organotin derivatives to increase the coordination of tin upon hydrolysis-condensation. Already in mono-organotin trialkoxides this tendency exists and tin coordination is increased by the formation of alkoxy bridges.87 Except for oxo-clusters involving the very bulky (Me3 Si)3 C group,60 the coordination of tin in all the oxo-clusters is five or six. Such coordination numbers are also classically observed for organotin oxo-clusters that contain complexing ligands.8,9 This increase of coordination, related to the Lewis acidity of mono-organotin, is achieved by the formation of bridges and adducts. The smallest organotin oxo-cluster {RSn(OH)Cl2 (OH2 )}2 combines both features, and compounds isolated for R : Me and —(CH2 )3 — show that tin can form water adducts, up to a coordination of six, prior to the hydrolysis of the first chloride.55,59 A second leading idea is the occurrence of μ3 -O bridges in many organotin oxo-clusters. These considerations lead to propose the trimers {(RSn)3 (μ3 -O)(μ2 -OH)2 (OH)2+x X3−x L y } (X : Cl, OR , C CR , and L : H2 O, ROH, THF, DMSO, etc.) as plausible intermediates. In this formula, X represents a hydrolyzable group not yet removed, and L a molecule that can bind to tin atoms through a dative bond and participate to their coordination expansion (y ≤ 3 for CNmax = 6). These molecules, being generally very labile, will not be considered in the proposed mechanisms nor drawn in the schemes and figures, to avoid their overcrowding. The formation of such trimers (Scheme 2.3.2) can be rationalized by the condensation of a dimer {RSn(μ2 -OH)(OH)X}2 and a monomer {RSn(OH)2 X}, two species likely related by a concentration-controlled association/dissociation equilibrium. The condensation of such trimers, or their derivatives, makes it possible to generate several known oxo-clusters. For instance, the ‘tilted’ face-to-face condensation of two trimers {(RSn)3 (μ3 -O)(μ2 -OH)2 (OH)5 } yields exactly the metal–oxygen framework of {2,4,6-

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Scheme 2.3.2 Proposed mechanism for the formation of {(RSn)3 ( μ3 -O) (μ2 -OH)2 (OH)2+x X3−x } (X : Cl, OR’, C CR’) i

Pr3 C6 H2 SnO(OH)}6 [Figure 2.3.9, Equation (2.3.7)]. The condensation of four trimers {(RSn)2 (HOSn)(μ3 -O)(μ2 -O)(μ2 -OH)2 (OH)3 }, derived from the previous one through an intramolecular condensation and the hydrolysis of the organic group of one of the three tin atoms (dearylation process), yields exactly the metal–oxygen framework of {(2,4,6-i Pr3 C6 H2 Sn)8 O14 (OH)8 (SnOH)4 } [Figure 2.3.10, Equations (2.3.8–10)]. The aggregation around a central sodium atom of four trimers, {(RSn)3 (μ3 -O)(μ2 -O)(μ2 -OH)2 (OH)3 }, followed by their connection

Figure 2.3.9

Proposed mechanism for the formation of {2,4,6-i Pr3 C6 H2 SnO(OH)}6 .

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Figure 2.3.10

Proposed mechanism for the formation of {(2,4,6-i Pr3 C6 H2 Sn)8 (HOSn)4 O14 (OH)8 }.

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through the conversion of each terminal hydroxy group into a bridging one (μ2 -OH) and the protonation of the four surface μ2 -O bridges, results in the formation of [Na ⊂ {(i PrSn)12 O4 (OH)24 }]5+ [Figure 2.3.11, Equation (2.3.11)]. The protonation step that turns μ2 -O into μ2 -OH is likely favoured by the delocalization of the positive charge on three adjacent μ2 -OH. However, the reason why this latter compound corresponds only to the γ -isomer of the Keggin’s structure does not appear in the proposed mechanism. In the proposed mechanisms, the trimeric intermediates for {(2,4,6-i Pr3 C6 H2 Sn)8 O14 (OH)8 (SnOH)4 } and [Na ⊂ {(i PrSn)12 O4 (OH)24 }]5+ are similar, but their organic groups are oriented differently, opposite to the face or on the face defined by the three μ2 -O/OH bridges. This difference is in line with the bulkiness of the organic groups. 2{(RSn)3 (μ3 -O)(μ2 -OH)2 (OH)5 } → {(RSn)6 (μ3 -O)4 (μ2 -O)2 (μ2 -OH)2 (OH)4 } + 4H2 O

(2.3.7)

{(RSn)3 (μ3 -O)(μ2 -OH)2 (OH)5 } → {(RSn)3 (μ3 -O)(μ2 -O)(μ2 -OH)2 (OH)3 } + H2 O

(2.3.8)

{(RSn)3 (μ3 -O)(μ2 -O)(μ2 -OH)2 (OH)3 } + H2 O → {(RSn)2 (HOSn)(μ3 -O)(μ2 -O)(μ2 -OH)2 (OH)3 } + RH (2.3.9) 4{(RSn)2 (HOSn)(μ3 -O)(μ2 -O)(μ2 -OH)2 (OH)3 } → {(RSn)8 (μ4 -O)2 (μ3 -O)8 (μ2 -O)4 (μ2 -OH)8 (SnOH)4 } + 6H2 O

(2.3.10)

4{(RSn)3 (μ3 -O)(μ2 -O)(μ2 -OH)2 (OH)3 } + Na+ + 4H+ → {Na ⊂ (RSn)12 (μ4 -O)4 (μ2 -OH)24 }5+ (2.3.11) The sole consideration of {(RSn)3 (μ3 -O)(μ2 -OH)2 (OH)2+x Cl3−x } trimers does not allow us to build the metal–oxygen framework of {(i PrSn)9 O8 (OH)6 Cl5 }. However, its construction can be rationalized with the help of three dimeric units {RSn(μ2 -OH)(OH)Cl}2 and one trimer {(RSn)3 (μ3 -O)(μ2 OH)2 (OH)2 Cl3 } [Figure 2.3.12, Equation (2.3.12)]. The dimeric intermediate considered here has already been proposed to rationalize the formation of several phosphinate-based organotin derivatives and, moreover, it has the formula of the starting material used to prepare {(i PrSn)9 O8 (OH)6 Cl5 }.13,64 The condensation of the same dimer and trimer, in which the chloride can be substituted for an alkoxy or an alkynide group, or their fully hydrolyzed versions, allows the building up of the metal oxygen framework of the macrocation {(RSn)12 O14 (OH)6 }2+ [Figure 2.3.13, Equation (2.3.13)]; again the last step here is the protonation of the μ2 -O. 3{RSn(μ2 -OH)(OH)Cl}2 + {(RSn)3 (μ3 -O)(μ2 -OH)2 (OH)3 Cl2 } → {(RSn)9 (μ3 -O)8 (μ2 -OH)6 Cl5 } +4H2 O + 3HCl

(2.3.12)

3{RSn(μ2 -OH)(OH)X}2 + 2{(RSn)3 (μ3 -O)(μ2 -O)(μ2 -OH)2 (OH)3 } + 2H+ → {(RSn)12 (μ3 -O)14 (μ2 -OH)6 }2+ + 6H2 O + 6HX

(2.3.13)

The formation of {(Me3 Si)3 CSnOCl}3 , {(Me3 Si)3 CSnO(OH)}3 , and {[(Me3 Si)3 CSn]4 O6 } with their four-coordinate tin atoms, appears completely different. The following mechanisms can be

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Figure 2.3.11

Proposed mechanism for the formation of [Na ⊂ {( i PrSn)12 O4 (OH)24 }]5+ .

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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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89

Proposed mechanism for the formation of {(Me3 Si)3 CSnOCl}3 and {(Me3 Si)3 CSnO(OH)}3

proposed (Scheme 2.3.3). Instead of condensation, a dimer {RSn(μ2 -OH)(OH)Cl}2 and a monomer {RSn(OH)2 Cl} can fuse together to yield a cyclic trimer {RSn(μ2 -OH)(OH)Cl}3 .13 Then, because of the very bulky tris(trimethylsilyl)methyl groups, which prevents any intermolecular condensation and induces a steric stress on five-coordinate tin atoms, the trimer undergoes intra-molecular dehydration to yield {RSn(μ2 -O)Cl}3 . Then, {RSn(μ2 -O)(OH)}3 is simply formed by hydrolyzing the chlorides, as observed experimentally.60 The formation of the adamantane-like core of {[(Me3 Si)3 CSn]4 O6 } is likely the result of the condensation of {RSn(μ2 -O)OH}3 with RSn(OH)x X3−x . Careful studies, especially by 119 Sn solution NMR and MS, of the solutions from which the various oxo-clusters are obtained, will be necessary to confirm these suggested mechanisms. Yet, without explaining everything, they manage to rationalize, with only a limited number of plausible intermediates, the formation of all the metal–oxygen frameworks discussed in this chapter. The main types of intermediate considered are the dimers, {(RSn)2 (μ2 -OH)2 (OH)2+x X2−x } (X : Cl, OR , C CR ) and the trimers {(RSn)3 (μ3 -O)(μ2 -OH)2 (OH)2+x X3−x } (X : Cl, OR , C CR ). 2.3.4

Conclusions

A variety of organotin oxo-clusters, with the general formula (RSn)n Ox (OH) y Clz and different molecular structures, have been reported during the last 30 years. The {(RSn)12 O14 (OH)6 }2+ cluster is the most studied, and the only one that has been evaluated for its catalytic properties24 or ability to generate advanced hybrid organic–inorganic materials.27−31 Similar work is needed on the other derivatives to determine the potentialities of this class of compounds. The diversity of structures, even if they might be derived from a limited number of intermediates, reflects the complexity that is present in ill-defined polymeric organostannonic acids. Oxo-clusters can probably help to understand these materials, but more as spectroscopic references than as ready-to-use solutions. Indeed, two very different oxo-clusters, both with bulky organic groups, have been reported with the formula RSnO(OH).60,63 The first one, {(Me3 Si)3 CSnO(OH)}3 , was even presented as the answer to an 80-year-old question about the basic structural elements of mono-organotin acids.60 However, these two oxo-clusters exhibit only four- or five-coordinate tin atoms, while solid-state 119 Sn NMR has shown that n BuSnO(OH) contains mostly (>80%) six-coordinate tin atoms that moreover present spectroscopic characteristics very similar to the ones found for the six-coordinate tin atoms of {(n BuSn)12 O14 (OH)6 }2+ .77 Hopefully, the story shall continue and this family of oxo-clusters will grow, as recently demonstrated by the discovery of two new members.63 With this perspective, a closer look at the solutions from which the oxo-clusters are recovered might be very fruitful, as the species that have been isolated so far may not be the only ones to form, but simply the only ones to crystallize/precipitate. Mono-organotin compounds with complex or functional organic groups, as well as new synthesis conditions (e.g. solvothermal or supercritical),88 could also yield metal–oxygen frameworks of yet unknown architectures. In the context of new nanostructured materials, mono-organotin derivatives with a cleavable organic group (e.g. aryl, benzyl)63,89 are especially exciting because they could yield oxo-clusters that might be subsequently

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assembled by condensation, after the cleavage of the surface Sn C bonds, to form unknown tin oxide structures. Acknowledgments The author would like to acknowledge the students, post-doctoral fellows, and colleagues who have contributed over the years to work on organotin oxo-clusters. Their names can be found in the references. CNRS, Universit´e P. et M. Curie-Paris6, and EU are also thanked for financial support. References 1. M.T. Pope, in Comprehensive Coordination Chemistry II, J.A. McCleverty, and T.J. Meyer (Eds), Elsevier, Oxford, 2004, pp. 635. 2. C.L. Hill, in Comprehensive Coordination Chemistry II, J.A. McCleverty and T.J. Meyer (Eds), Elsevier, Oxford, 2004, pp. 679. 3. Polyoxometalate Chemistry for Nano-Composite Design, T. Yamase and M.T. Pope (Eds), Kluwer Academic/Plenum Publishers, New York, 2002. 4. Polyoxometalate Chemistry: from Topology via Self-assembly to Applications, M.T. Pope and A. Müller (Eds), Kluwer Academic Publishers, Dordrecht, 2001. 5. M.T. Pope and A. Müller Angew. Chem. Int. Ed., 30, 34 (1991). 6. H.W. Roesky, I. Haiduc and N.S. Hosmane, Chem. Rev., 103, 2579 (2003). 7. V. Chandrasekhar, S. Nagendran and V. Baskar, Coord. Chem. Rev., 235, 1 (2002). 8. V. Chandrasekhar and K. Gopal, Appl. Organomet. Chem., 19, 429 (2005). 9. R.R. Holmes, Acc. Chem. Res., 22, 190 (1989). 10. V. Chandrasekhar, V. Baskar and J.J. Vittal, J. Am. Chem. Soc., 125, 2392 (2003). 11. V. Chandrasekhar, V. Baskar, K. Gopal and J.J. Vittal, Organometallics, 24, 4926 (2005). 12. K.C.K. Swamy, S. Nagabrahmanandachari and K. Raghuraman, J. Organomet. Chem., 587, 132 (1999). 13. K.C.K. Swamy, R.O. Day and R.R. Holmes, Inorg. Chem., 31, 4184 (1992). 14. S.Y. Song, J.F. Ma, J. Yang L.L. Gao and Z.M. Su, Organometallics, 26, 2125 (2007). 15. G. Sazani, M.H. Dickman and M.T. Pope, Inorg. Chem., 39, 939 (2000). 16. G. Sazani and M.T. Pope, Dalton Trans., 1989 (2004). 17. N. Belai and M.T. Pope, Polyhedron, 25, 2015 (2006). 18. S. Bareyt, S. Piligkos, B. Hasenknopf, P. Gouzerh, E. Lacote, S. Thorimbert and M. Malacria, Angew. Chem. Int. Ed., 42, 3404 (2003). 19. S. Bareyt, S. Piligkos, B. Hasenknopf, P. Gouzerh, E. Lacote, S. Thorimbert and M. Malacria, J. Am. Chem. Soc., 127, 6788 (2005). 20. F.B. Xin and M.T. Pope, Inorg. Chem., 35, 5693 (1996). 21. F.B. Xin, M.T. Pope, G.J. Long and U. Russo, Inorg. Chem., 35, 1207 (1996). 22. F.B. Xin and M.T. Pope, Organometallics, 13, 4881 (1994). 23. X.H. Wang and J.F. Liu, J. Coord. Chem., 51, 73 (2000). 24. S. Durand, K. Sakamoto, T. Fukuyama, A. Orita, J. Otera, A. Duthie, D. Dakternieks, M. Schulte, and K. Jurkschat, Organometallics, 19, 3220 (2000). 25. C. Sanchez, G.J. de Soler-Illia, F. Ribot, T. Lalot, C.R. Mayer, and V. Cabuil, Chem. Mater., 13, 3061 (2001). 26. F. Ribot and C. Sanchez, Comments Inorg. Chem., 20, 327 (1999). 27. L. Angiolini, D. Caretti, R. De Vito, F.T. Niesel, E. Salatelli, C. Carlini, F. Ribot and C. Sanchez, J. Inorg. Organomet. Polym., 7, 151 (1997). 28. F. Ribot, F. Banse, C. Sanchez, M. Lahcini and B. Jousseaume, J. Sol-Gel Sci. Technol., 8, 529 (1997). 29. F. Ribot, D. Veautier, S.J. Guillaudeu and T. Lalot, J. Mater. Chem., 15, 3973 (2005). 30. F. Ribot, A. Lafuma, C. Eychenne-Baron and C. Sanchez, Adv. Mater., 14, 1496 (2002). 31. F. Ribot, F. Banse, F. Diter and C. Sanchez, New J. Chem., 19, 1145 (1995).

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32. Z.P. Du, W.K. Kang, T. Cheng, J. Yao, and G.Y. Wang, J. Mol. Catal., A Chem., 246, 200 (2006). 33. O.A. Mascaretti, R.L.E. Furlan, C.J. Salomon and E.G. Mata, Phosphorus Sulfur Silicon Relat. Elem., 150–151, 89 (1999). 34. R.L.E. Furlan, E.G. Mata and O.A. Mascaretti, Tetrahedron Lett., 39, 2257 (1998). 35. M. Noda, Prep. Biochem. Biotechnol., 29, 333 (1999). 36. P.C. Rooney, US Patent [5,166,310] 1992. 37. S.D. Barnicki, C.E. Sumner Jr., H.C. Williams, US Patent [5,512,691] 1996. 38. J.G.A. Luitjen, Recl. Trav. Chim. Pays Bas, 85, 873 (1966). 39. D.P. Gaur, G. Srivastava, and R.C. Mehrotra, J. Organomet. Chem. 63, 221 (1973). 40. B. Jousseaume, M. Lahcini, M.-C. Rascle, F. Ribot and C. Sanchez, Organometallics, 14, 685 (1995). 41. C. Franc, B. Jousseaume, M. Linker and T. Toupance, Chem. Mater., 12, 3100 (2000). 42. H. Elhamzaoui, B. Jousseaume, H. Riague, T. Toupance, P. Dieudonne, C. Zakri, M. Maugey and H. Allouchi, J. Am. Chem. Soc., 126, 8130 (2004). 43. P. Jaumier, B. Jousscaume, M. Lahcini, F. Ribot and C. Sanchez, Chem. Commun., 369 (1998). 44. B. Jousseaume, H. Riague, T. Toupance, M. Lahcini, P. Mountford, and B.R. Tyrrell, Organometallics, 21, 4590 (2002). 45. Resonance frequency of neat tetramethyltin for a 1 H resonance frequency of 100.000 000 MHz. 46. J.C. Martins, M. Biesemans and R. Willem, Prog. Nucl. Magn. Reson. Spectrosc., 36, 271 (2000). 47. (a) B. Wrackmeyer, in Annual Reports on NMR Spectroscopy (vol. 38), G. A. Webb (Ed), Academic Press, San Diego, 1999, pp. 203–264; (b) B. Wrackmeyer, in Tin Chemistry–Fundamentals, Frontiers and Applications, A. Davies, M. Gielen, K. Pannell and E.R.T. Tiekink (Eds), Wiley, 2007, chapter 2. 48. A. Sebald, in Advanced Applications of NMR to Organometallic Chemistry, M. Gielen, R. Willem and B. Wrackmeyer (Eds), Wiley, Chichester, 1996, pp. 123–157. 49. T.N. Mitchell, in Chemistry of Tin (2nd edn), P.J. Smith (Ed), Blackie Academic & Professional, London, 1998, pp. 480–495. 50. C. Eychenne-Baron, F. Ribot, N. Steunou, C. Sanchez, F. Fayon, M. Biesemans, J.C. Martins and R. Willem, Organometallics, 19, 1940 (2000). 51. Cambridge Crystallographic Data Centre, http://www.ccdc.cam.ac.uk. 52. FIZ Karlsruhe, http://www.fiz-karlsruhe.de/home.html. 53. A sub-unit used to describe a structure does not have to exist as an isolated chemical species. It is simply a set of atoms with a specific connection scheme. 54. L. Rozes, N. Steunou, G. Fornasieri and C. Sanchez, Monatsh. Chem., 137, 501 (2006). 55. S.E. Johnson and C.B. Knobler, Organometallics, 13, 4928 (1994). 56. C. Lecomte, J. Protas and M. Devaud, Acta Crystallogr. Section B, 32, 923 (1976). 57. H. Puff and H. Reuter, J. Organomet. Chem., 364, 57 (1989). 58. R.R. Holmes, S. Shafieezad, V. Chandrasekhar, J.M. Holmes, and R.O. Day, J. Am. Chem. Soc., 110, 1174 (1988). 59. B. Zobel, A. Duthie, D. Dakternieks and E.R.T. Tiekink, Organometallics, 20, 2820 (2001). 60. J. Janssen, J. Magull and H.W. Roesky, Angew. Chem. Int. Ed., 41, 1365 (2002). 61. J. Beckmann and K. Jurkschat, Coord. Chem. Rev., 215, 267 (2001). 62. K. Wraage, T. Pape, R. Herbst-Irmer, M. Noltemeyer, H.-G. Schmidt, and H.W. Roesky, Eur. J. Inorg. Chem., 869 (1999). 63. G. Prabusankar, B. Jousseaume, T. Toupance and H. Allouchi, Angew. Chem. Int. Ed., 45, 1255 (2006). 64. H. Puff and H. Reuter, J. Organomet. Chem., 368, 173 (1989). 65. H. Puff and H. Reuter, J. Organomet. Chem., 373, 173 (1989). 66. D. Dakternieks, H. Zhu, E.R.T. Tiekink and R. Colton, J. Organomet. Chem., 476, 33 (1994). 67. F. Banse, F. Ribot, P. Toledano, J. Maquet and C. Sanchez, Inorg. Chem., 34, 6371 (1995). 68. G. Kastner and H. Reuter, Main Group Met. Chem., 23, 383 (2000). 69. J. Beckmann, K. Jurkschat, U. Kaltenbrunner, S. Rabe, M. Schuermann, D. Dakternieks, A. Duthie, and D. Mueller, Organometallics, 19, 4887 (2000).

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Tin Chemistry: Fundamentals, Frontiers and Applications V.W. Day, T.A. Eberspacher, W.G. Klemperer and C.W. Park, J. Am. Chem. Soc., 115, 8469 (1993). N. Steunou, F. Robert, K. Boubekeur, F. Ribot, and C. Sanchez, Inorg. Chim. Acta, 279, 144 (1998). A. Muller, R. Rohlfing, E. Krickemeyer, and H. Bogge, Angew. Chem. Int. Ed., 32, 909 (1993). G. Kastner and H. Reuter, J. Organomet. Chem., 598, 381 (2000). M. Izaaryene, G. Kastner and H. Reuter, Z. Kristallogr., 220, 622 (2005). F. Ribot, C. Sanchez, R. Willem, J.C. Martins and M. Biesemans, Inorg. Chem., 37, 911 (1998). F. Ribot and E. Dien, unpulished results. F. Ribot, C. Eychenne–Baron, F. Fayon, D. Massiot and B. Bresson, Main Group Met. Chem., 25, 115 (2002). Anisotropy (ζ ) and asymmetry (η) are defined as follows: ζ = σ33 –σiso and η = (σ22 –σ11 )/(σ33 –σiso ), with σiso = (σ11 + σ22 + σ33 )/3. σ11 , σ22 , and σ33 are the three components of the shielding tensor expressed in its principal axis system with the convention, |σ33 –σiso | > |σ11 –σiso | > |σ22 –σiso |. R.K. Harris, S.E. Lawrence, S.W. Oh, V.G.K. Das, J. Molecular Struct., 347, 309 (1995). H. Reuter, Angew. Chem. Int. Ed., 30, 1482 (1991). P.J. Domaille and R.L. Harlow, J. Am. Chem. Soc., 108, 2108 (1986). K. Molloy, in Chemistry of Tin (2nd edn), P.J. Smith (Ed), Blackie Academic & Professional, London, 1998, pp. 138–175. M. Biesemans, R. Willem, S. Damoun, P. Geerlings, E.R.T. Tiekink, P. Jaumier, M. Lahcini and B. Jousseaume, Organometallics, 17, 90 (1998). M. Biesemans, R. Willem, S. Damoun, P. Geerlings, M. Lahcini, P. Jaumier, and B. Jousseaume, Organometallics, 15, 2237 (1996). F. Ribot, C. Eychenne-Baron, and C. Sanchez, Phosphorus Sulfur Silicon Relat. Elem., 150–151, 41 (1999). C. Eychenne-Baron, F. Ribot and C. Sanchez, J. Organomet. Chem., 567, 137 (1998). F. Ribot, V. Escax, J.C. Martins, M. Biesemans, L. Ghys, I. Verbruggen, and R. Willem, Chem. Eur. J., 10, 1747 (2004). J.D. Kennedy, J.Chem.Soc. Perkin Trans., 2, 242 (1977). 87. G.L. Zheng, J.F. Ma, Z.M. Su, L.K. Yan, J. Yang, Y.Y. Li, and J.F. Liu, Angew. Chem. Int. Ed., 43, 2409 (2004). V. Chandrasekhar, K. Gopal, P. Sasikumar and R. Thirumoorthi, Coord. Chem. Rev., 249, 1745 (2005).

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Organotin Carboxylate and Sulfonate Clusters

2.4

93

Organotin Carboxylate and Sulfonate Clusters

Vadapalli Chandrasekhar, Puja Singh, and Kandasamy Gopal Department of Chemistry, Indian Institute of Technology, Kanpur, India

2.4.1

Introduction

Organotin carboxylates and sulfonates form an important class among the family of organotin compounds.1−3 These compounds possess rich structural diversity. A range of structures from simple mononuclear compounds to complex polynuclear cages and clusters are formed.4 The structural diversity of these compounds emanates from several features. These include: (1) The ability of tin to have varied coordination geometries and coordination numbers. (2) The versatility of the carboxylate ligand to engage in different modes of binding from monodentate to bidentate; in the case of the latter from chelating to bridging. (3) The varied coordination response of the sulfonate ligand, which varies from non-coordinating to polydentate coordination. The theme of this chapter revolves around clusters of organotin carboxylates and sulfonates. For the sake of completion other structural types are mentioned briefly. 2.4.2

Organotin Carboxylates

Among organotin carboxylates, clusters and cages are formed mainly in di- and monoorganotin compounds.2 Among triorganotin carboxylates, the predominant structures are chain and discrete structures, although some macrocycles are also known.2−3 Triorganotin Carboxylates

Triorganotin carboxylates, R3 SnO2 CR , formed generally in the reactions of R3 SnOH or (R3 Sn)2 O with a carboxylic acid R CO2 H, usually possess two main types of structures:2 (a) chain structures; (b) discrete structures. Polymeric or chain structures are the most common structural types known for triorganotin carboxylates. These are formed in three situations (Figure 2.4.1): (1) When the carboxylate ligand binds in a bridging anisobidentate manner and interconnects adjacent tin centers (Figure 2.4.1(a)); (2) When a dicarboxylate ligand bridges two tin centers (Figure 2.4.1(b)); (3) When a carboxylate ligand containing an additional coordination site in the form of a heteroatom bridges two adjacent tin centers, binding one tin atom by the heteroatom and the other by the carboxylate oxygen atom (Figure 2.4.1(c)). In all of these cases the geometry around tin is trigonal bipyramidal. The axial sites are occupied by the electronegative substituents. The O–Sn–O angles in compounds belonging to the type shown in Figure 2.4.1(a) are usually in the range of 170 to 175◦ . The two Sn–O distances are generally not equivalent; the Sn–O (carboxyl) distance is slightly longer. For example, in Me3 SnO2 CC10 H7 , the shorter ˚ 5 Owing to this bond distance variation, Sn–O distance is 2.14(1) while the longer distance is 2.57(1)A. the tin atom is displaced from the equatorial plane towards the covalently bound oxygen. Discrete structures are found generally for Ar3 SnO2 CR (Ar = Aryl). Many examples of triphenyltin carboxylates, with some exceptions, are known to adopt these structures. In these compounds the tin is

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R CO2H

Sn

O

1:2

R O

(a) R R HO2C R3SnOSnR3

CO2H

O

R

R

O

R n

R

Sn

O

R

O

1:1

Sn

O

R

O

Sn

O (b)

R

O n

R

R R Sn

O CO2H

X 1:2

O R

R

R

X O

Sn

(c) R

Figure 2.4.1

X n

Formation of chain structures in triorganotin carboxylates

bound covalently to three carbon atoms and one oxygen atom, see Figure 2.4.2. A fifth weak intramolecular Sn–O interaction is often found with the participation of the carbonyl oxygen atom. The Sn–O covalent ˚ 2 The bond in these compounds is typical of a single bond distance and varies from 2.03 to 2.12 A. 2 ˚ Sn–O coordinate bond, on the other hand varies, from 2.45 to 3.11A. In general, the preference of the carboxylate ligand to adopt a bridging coordination mode seems to drive the formation of the coordination polymer. In situations where this cannot be accomplished, discrete structures are realized.14 Other types of structures found for triorganotin carboxylates include dimeric structures, as seen for [(Ph3 SnO2 C-C6 H4 -2-Cl)2 .H2 O]15 and Ph3 Sn(O2 C-C6 H4 -2-S)2 SnPh3 .16 In the former, a water molecule blocks the fifth coordination site at tin and prevents polymer propagation. In the latter, one of the tin atoms is bound by the carboxylate ligand, while the other is bound by a thiolate ligand. Macrocyclic structures are known for some triorganotin carboxylates. Thus, n-Bu3 SnO2 C-C6 H3 -2,6-F2 is a tetranuclear macrocycle,17 while Ph3 Sn(NPG) (NPG = N-phthalyl glycinate) is a hexanuclear macrocycle.13 Representative examples of the various structural types known for triorganotin carboxylates are summarized in Figure 2.4.2. Diorganotin carboxylates

The simplest of the diorganotin carboxylates are the dicarboxylates of the type R2 Sn(O2 CR )2 .2 These compounds are formed in a 1:2 reaction between an organotin substrate such as [R2 SnO]n and a carboxylic acid R CO2 H. In general, these have monomeric structures. The tin is hexacoordinate,

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Organotin Carboxylate and Sulfonate Clusters Ph Ph Ph

O

Sn

O2N

N

O

Sn

O

O N

(b)

Ph Ph

O

N

O

Sn

O

O

Me

O

Sn

O

O

Me

O2N

Ph

HO

n

NO2

n

NO2 O

O

Ph

O

(f)

S

O Ph Ph H2O

n-Bu

O

Ph

O

Sn

O Sn

O2N

O Sn

n-Bu

O

n-Bu n-Bu Sn

O

N Ph

n-Bu n-Bu

O n-Bu

N

O

n-Bu Sn n-Bu

n-Bu O

Sn

Ph

O

Ph NO2

n-Bu n-Bu

O

Ph Ph

O

O

Ph

Ph

Me

N

O

S

N

(d)

Ph

O

Me Sn

(e)

(c)

N

N

Sn

Sn

O

Me Me

O Ph

Me

Me

H2O

(a)

Sn

NO2 Me

S

95

O

Ph

O

O

Ph

Sn

Sn Ph Ph

NO2

N O

O

Ph Ph

O O

O

(g)

O Sn

Sn

O

N O

Ph Ph

Ph Ph O O

Ph

Ph

O

O

O

Ph N

N

Sn Sn

O

O

O

Ph Ph

N

O

O (h)

Figure 2.4.2 Representative examples of various structural forms of triorganotin carboxylates: (a)–(c) represent discrete structures; 6−8 (d) represents a chain structure where the carboxylate ligand bridges two tin centers; 9 (e) represents a chain structure where a heteroatom and a carboxylate ligand bridge two tin centers; 10 (f) represents a dimeric structure; 11 and (g)–(h) represent macrocyclic structures. 12−13

skew-trapezoidal bipyramidal, with a 2C, 4O coordination environment in which the tin-bound organic groups lie over the weaker Sn←O bonds. Each carboxylate ligand binds to the tin in a chelating anisobidentate mode. Other diorganotin carboxylates are of the type [(R2 SnO2 CR )2 O]2 .2 These are formed in 1:1 reactions of [R2 SnO]n and the carboxylic acid. The solid-state structures of these compounds reveal that they are dimeric and possess ladder structures (Figure 2.4.3). This topic has been dealt with in detail in many comprehensive review articles.1−4,18−24 The reader may refer to these articles for further details. Ladder structures have been classified depending on subtle variation of the coordination of the carboxylate ligands. These are depicted in Figure 2.4.3. The common structural motif of all the ladder structures is a central four-membered Sn2 O2 ring. Some recent representative examples of [(R2 SnO2 CR )2 O]2 are summarized in Table 2.4.1.25−63 Hydroxyl-bridged dimeric structures are readily formed in the reactions of [t-Bu2 SnO]3 with carboxylic acids.64−65 Representative examples are shown in Figure 2.4.4. Among these compounds, [t-Bu2 Sn(μ-OH)(O2 CC5 H4 -Fe-C5 H5 )]2 (Figure 2.4.4(b)) is an electroactive compound54 while [t-Bu2 Sn(μ-OH)(O2 C-Fl)]2 (Fl = fluorenyl) (Figure 2.4.4(c)) is a photoactive compound.56 Some trinuclear clusters are also known among diorganotin carboxylates.66−73 These are summarized in Table 2.4.2. It can be seen that the carboxylic acids involved in the formation of these compounds are mono/dicarboxylic acids containing other heteroatoms (Table 2.4.2, entries 1–5, 8 and 9) or dicarboxylic acids without other heteroatoms (Table 2.4.2, entries 6 and 7). Among the cyclic compounds,

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O R

R

O

R

O

R

O

O O

O Sn

O

Sn

R

O

O

R

R R

R

O

O O

Sn

R

R

R

O Sn

Sn

O

R

O

O

O

R Sn

R

R

O Sn

O

Sn

Sn

Sn O

R

O O

O

O

O

O

R

Sn R R

O

R

R

R

O

O

R

Sn

R

L4

O

O

O

R

O

R

O

O

O

R L3

O

R

L2

R

Sn

Sn

O Sn

R

R

L1

O

O Sn

O

R

O

O O

R

O R

O

Sn

O

R

Sn

Sn

O

Sn

R

O

O

O

R

O

R

O

R

R

R

Sn

Sn

O

R

O

R

Sn

Sn

O

R

O

R

R

O

L5

L6

R

n

O

O Sn R

R

N

O

O O

R

Sn

Sn

R

R

O O

R Sn O

R

L7

N

N

N

Sn

HO O Sn

Sn N

O

Sn

Sn

R R R

O O

Sn

R

O O

R L8 R

OH

R

O

O

O R

R

Sn

O

R

R

O

R

O

Sn

O

R

O

N

Sn R

R

O

O

O

R

Sn O

O

R

R

O

R

O L9

N N

Figure 2.4.3 Various types of ladder structures ( L1 –L9 ) found in [(R2 SnO2 CR’)2 O]2 (see Table 2.4.1 for specific examples)

[{(n-Bu2 )2 Sn(2,5-PDC)}3 ] (2,5-PDC = 2,5-pyridinedicarboxylate, D = Lewis base, such as DMSO, H2 O)68 and [{(n-Bu2 )2 Sn(1,3-BDC)(D)}3 ] (1,3-BDC = 1,3-benzenedicarboxylate)70 (Table 2.4.2, entries 4 and 6) deserve special mention. Both of these are macrocyclic compounds formed as a result of intermolecular bridging by the dicarboxylate ligands. Among these, [{(n-Bu2 )2 Sn(2,5-PDC)}3 ] forms a three-dimensional hybrid supramolecular network which traps hydrophobic molecules in its microporous channels.68 Recently, there have been reports on other macrocyclic compounds involving diorganotin carboxylates.74−78 Some examples are shown in Figure 2.4.5. A 48-membered macrocycle containing 18

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Organotin Carboxylate and Sulfonate Clusters Table 2.4.1 L9 ]a,b Entry

R

Representative examples of diorganotin carboxylates, [(R2 SnO2 CR’)2 O]2 [Ladder types L1 to R (carboxylate ligand)

Ref.

Entry

R (carboxylate ligand)

R

Ladder L1 1.

97

Me

25

4.

S

n-C8 H17

N S

N

O

Ref.

28

S

S

N

2.

n-Bu

26

N

5.

S

n-Bu

Me2N

N 3.

Me

27

OH

6.

29

S

n-Bu

30

Fe

N

O Ladder L2

7.

n-Bu

31

OH

11.

n-Bu

N

N

O 8.

n-Bu

35

O

S

32

12.

n-Bu

33

13.

n-Bu, n-Pr

N

O PPh2

36

S

9.

n-Bu

O

10.

n-Bu

O2N

30

Fe

Ph

34

14.

n-Bu

37

Cl

Cl

O (Continued )

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Table 2.4.1 Entry

20:38

(Continued) R

R (carboxylate ligand)

n-Bu, n-Pr

Ref.

Entry

R

38

20.

n-Bu

Ref.

SH

42

N

Fe

16.

R (carboxylate ligand)

S

H3C

n-Bu

Ph

39

21.

n-Bu

40

22.

n-Bu

O2N

NO2

43

MeO 17.

n-Bu

Me

29

S Et2N

HN

S

Me 18.

n-Bu

41

N N

23.

n-Bu

44

N N

19.

n-Bu

41

Me N N

Me

Me N N Me

24.

n-Bu

45

S

N

Ladder L3 25.

n-Bu

46

O Ph

S Ladder L4 26.

n-Bu

26

28.

O

CH3

O

Me, n-Bu

N

48

Cl OMe

27.

n-Bu

47

O

N S

29.

PhCH2

49

O S

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Organotin Carboxylate and Sulfonate Clusters Table 2.4.1 Entry 30.

99

(Continued)

R

R (carboxylate ligand)

n-Bu

OH CH3 H3C

Ref.

Entry

R

50

35.

n-Bu

R (carboxylate ligand)

Ref. 53

OH

N

N 31.

n-Bu

OH CH3

50

36.

n-Bu

N

32.

Me

54

Fe

51

37.

n-Bu

55 O

33.

n-Bu

Cl

52

38.

n-Bu

56

26

39.

n-Bu

56

Cl 34.

n-Bu

N S N

Ladder L5 40.

Me

57

H3C

Ladder L6 41.

n-Bu

58

Fe Ladder L7 42.

Me

59

Me

HN

44.

Ph

Cl

Cl

61

Cl

Me 43.

Me

Me

Me

60

Me (Continued )

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Table 2.4.1 Entry

July 4, 2008

(Continued) R (carboxylate ligand)

R

Ref.

Entry

R (carboxylate ligand)

R

Ref.

Ladder L8 45.

n-C8 H17

62

N N

Ladder L9 46.

a b

PhCH2

63

These examples are derived from literature published after 2003. For older examples see references 1–4 and 18–24. See Figure 2.4.3 for structural depiction of ladder structures L1 –L9 .

tin atoms (Figure 2.4.5(a)) has been assembled in a reaction between 2-mercaptonicotinic acid and di-nbutyltin dichloride.74 The macrocycle contains an outer rim of four hexa-coordinate tin centers which are linked to a pair of central hydrolyzed hexa-tin ladders. The concerted coordination action of the multiple coordinating sites (N,S,O) assist in the assembly of the macrocycle. Other macrocycles involving hydrolyzed organostannoxane motifs include the bridging of two penta-tin organostannoxane ladders by a pair of ferrocene dicarboxylate ligands (Figure 2.4.5(c))63 or bridging a pair of tetra-tin organostannoxane ladders by 2-mercapto-4-methyl-5-thiazole acetate ligand (Figure 2.4.5(e)).77 Figure 2.4.5(b) shows an interesting macrocycle containing a central distannoxane linked to the periphery of a tetra-tin containing macrocycle.75 In this compound, the multiple coordination action of m-mercaptobenzoate influences the formation of the multinuclear cage. Macrocycles of the type shown in Figure 2.4.5(d) containing six tin centers have been realized in some instances.75−76 The use of o-hydroxy benzoates as ligands has also been productive in the formation of hexa-tin-containing macrocyclic rings (Figure 2.4.5(f)).78

t-Bu O

Sn

Me C O t-Bu t-Bu O C O

H O

H O

Sn t-Bu

O

Fe

t-Bu

O

H O

Sn

C O t-Bu

t-Bu

O H (b)

Ph

O O F3C

t-Bu

Figure 2.4.4

t-Bu

O C O

Sn

C Sn

O H (c)

O C Me

Sn O H (a)

t-Bu

t-Bu

CF3 O Sn C CF3 O Ph

Fe

t-Bu

H O

Ph Sn

O H (d)

O F3C C O F3C

Ph

Hydroxyl-bridged dinuclear derivatives.63,53,55,64

CF3

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Organotin Carboxylate and Sulfonate Clusters Table 2.4.2

Entry 1.

101

Representative examples of trinuclear diorganostannoxane macrocycles

R and X

R (carboxylate ligand)

Me and Cl

Ref. Entry 66

N

R (carboxylate ligand)

R and X

6. n-Bu and no X

70 O

O

N O

Ref.

Sn

Sn

O O

O

Ph

Ph

Sn

Sn 2.

n-Bu and Cl

N

66

7. n-Bu and no X

O

N

Ph P

O

Sn

71

Ph

P

O

O O O

Sn

Sn

3.

n-Bu and Cl

67

O N

8. Me and no X

72 O

O

O

Sn

Sn

N O

O

Sn 4.

n-Bu and H2 O Sn

O

Sn

O

O N

O

68

O

9. n-Bu and no X

N O

Ph and no X

69

N

O

Sn

O

S

Sn

73

Me O

O

Sn

Sn 5.

Sn

Me Sn

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Tin Chemistry: Fundamentals, Frontiers and Applications R O

Sn

R

R R

S

R R

R R

R

R R

Sn

R

O

Sn

Sn

R

O

O

Sn

S

O

Sn

S

R

R

N

Sn

HO

R

Sn

R

Sn

O

O

O

O

R

R

R

R

Sn

OH

R

R

O

O

R

O

Sn

Sn

Sn

R

O

R

R Sn

R

R

O Sn R

R

R

R

R

Sn

Sn O

R

O

O R

Sn

O Sn

O

Sn

O

Sn

O O

R = Me

R Sn

R

R HO

O

O Sn

O

R R (b)

R

O O

R

O

S O

O R

R R

O O

OH

R

O

R = n-Bu

O R

R

R O

R = CH2Ph ;

R Sn

R

O

R = n-Bu

R (a)

Sn O

O

O

S R

Sn

R

O

N

N

O R

S

R

O

R

Sn

R Sn

O O

S R

Sn

O S

Sn

N

S R

O

Sn

O

O R R

Sn

O

R

Sn

R

O

Sn

R R

R O

O

R

Sn O

O

R

O

O

Sn

O

Sn

R

N

Sn

O R R

O

R

O

R

R

O

S

O

Sn S

R

Sn

S

O

N

N

N

R R

=

Fe

(c-1)

O O

(c-2)

S ;

O S

=

O O (d-2)

(d-1) i-Pr

N S

O

R R O

R R

R Sn R MeO

N

Sn MeO

Sn O

O

Sn O

R

R

Sn

Sn

R

O R R O

S

S

O

S

R = Me

i-Pr

i-Pr

Sn

O

R

R

i-Pr

O

R Sn

O

R = n-Bu

Sn R

N

O

O

Sn

i-Pr

R

O

O

O

O

(e)

Figure 2.4.5

R

O

R

i-Pr

S

N

R

O

R

R

O O

O

Sn

Sn

O

OMe

O

R

Sn R R

Sn

O

O O

OMe

O

i-Pr

R

i-Pr

R R

O

S

S S

i-Pr

O

i-Pr

R i-Pr

(f)

i-Pr

Various multi-nuclear diorganotin macrocycles

R

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Organotin Carboxylate and Sulfonate Clusters R

R O

O

O

O

Sn O HO2C

R

CO2H

103

Sn O

O

(a)

R

O n

or

2:1

R O

O

Sn

O

R

[R2SnO]n

O

R Sn

O

O

(b) R

R

CO2H Sn

O

X

O Sn

X 1:2

n

O

X

O

Sn

R (c)

Figure 2.4.6

Formation of one-dimensional polymeric diorganotin carboxylates

In addition to macrocycles, one- and two-dimensional polymers are also known among diorganotin carboxylates (Figures 2.4.6 and 2.4.7). Several examples of two-dimensional polymers are known among diorganotin carboxylates (Figure 2.4.7(a–e)).3,79 The reaction of pyrazole-3,5-dicarboxylic acid with dibenzyltin dichloride affords a two-dimensional polymer (Figure 2.4.7(d)).79 The repeat unit of the twodimensional polymer contains two symmetrically related tri-tin halves. Hydrolysis of the two-dimensional polymer affords a polymeric tape containing, alternately, the hexa-tin macrocycle and a tetra-tin ladder (Figure 2.4.7(e)). Monoorganotin Carboxylates

The reaction of monoorganotin oxide-hydroxide [RSn(O)OH]n , with a carboxylic acid R CO2 H in a 1:1 stoichiometry affords hexanuclear compounds, [RSn(O)O2 CR ]6 .2,4 This reaction is very general and a number of such hexanuclear compounds, also known as drums, have been prepared (Figure 2.4.8, Table 2.4.3).56,61,80−96 In some cases drums have been formed as a result of alkyl or aryl cleavage preceding hydrolysis of R2 Sn(O2 CR )89,93 or during 1:2 condensation reaction of (R3 Sn)2 O with car2 boxylic acids.86−87 All of the drums have a central Sn6 O6 cage, which is formed by the fusion of two Sn3 O3 rings. Alternate tins are bridged by bidentate carboxylate ligands. The sides of the drum consist of six four-membered Sn2 O2 rings. The Sn–O bond distances found in these compounds, as ˚ 80 Many of the exemplified for [n-BuSn(O)O2 CFc]6 (Fc = ferrocenyl), range from 2.06 to 2.14 A. drum compounds also exhibit a rich supramolecular chemistry as a result of several non-covalent interactions.92 The generality of the drum synthesis has been utilized to prepare stannoxane-cored dendrimer-like molecules containing an interesting electroactive, photoactive, or coordinating periphery (Table 2.4.3,

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Tin Chemistry: Fundamentals, Frontiers and Applications Sn O

O

Sn

O O2N

O

Sn

O O O O2N

Sn

Sn

O

O

NO2 O

O O

Sn

O O

Sn

O

O

O

Sn

O

Bn Sn

O N

O N

N N

N Sn Bn

O

O

O

R Sn

O

Bn

N

R O

Sn

RR R

O

R n

(d)

Bn Sn

Bn n

(c) O

Bn = -CH2Ph O

Bn Bn Sn

Sn

O O

O N

N N

Bn

Bn Sn

Bn

N

O O

HO

N O

Sn Bn

Bn

O N N O

Bn (e)

Figure 2.4.7

Sn

Sn

Bn O

Sn

N

N

O

OH

Bn N

Bn O

O

O

O Bn Bn Bn O

Bn

Bn

Sn

O

HO

Sn Bn

Bn

Sn

OH

O

Sn Bn

Sn

O

O

N N H

N

Sn

Sn

O

O

Bn Sn

Sn

Bn

H N N

R R

O

HO

O

Cl

Sn

O

O

O

Bn O

Sn

Sn

O Bn

Bn Bn

N

O

R

R Bn

O

OH Sn

n

Cl

O

O

Sn

O

(b)

O

Bn Bn

N N

O

Sn

O

O

Sn

Sn

Bn

Bn

O

Sn

(a)

O

Sn

Sn O

O Sn

O

O

O

O

n

O

Sn

Sn

NO2 O

O

Sn

O

O

O

Sn

O

Sn

Sn

Sn

O

O Bn n

One- and two-dimensional polymers formed among diorganotin carboxylates3,79

O

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Organotin Carboxylate and Sulfonate Clusters

R

R C

COOH

O

O O

O

O

R

C

O O

R = n-Bu

R

R

C O

O Sn

R Sn O

O O

Sn

O O

O O

R

C R

C R

Figure 2.4.8

Sn

Sn

-6 H2O

R

O

R

Sn

6 [RSn(O)OH]n + 6

C

O

R

105

Drum

Formation of hexanuclear organostannoxane drums

entries 1–2, 9, 12–14 and 27–30).56,80−81,85 More recently, multi-porphyrin- and multi-buckminister fullerene-containing organostannoxanes have been prepared by utilizing the drum synthesis (Figure 2.4.9; Table 2.4.3).95−96 The metallated derivative of the former has been used as a catalyst for plasmid cleavage.95 Although drum compounds are the must ubiquitous forms found in monoorganotin carboxylates, hexanuclear open-chain ladder compounds (Figure 2.4.10(a) and (b))2 and trinuclear compounds (Figure 2.4.10(c))97 are also found in some instances. The hexanuclear ladders can be converted to drums by a controlled hydrolysis.2 In an interesting recent development the reaction of [n-BuSn(O)(OH)]n with 9-hydroxy-9fluorenecarboxylic acid (LH2 ) afforded a new hexanuclear cage [n-BuSn(μ-OH)(L)]6 .3H2 O.2CHCl3 ] (Figure 2.4.11).98 This cage consists of three [Sn2 (μ-OH)2 ] units which are bridged together by the tridentate carboxylate ligand, L. The six tin atoms of this cage occupy the vertices of a trigonal prism. Another interesting aspect of these cages is that they form guest-assisted three-dimensional columnar supramolecular structures when the guests are phenols. 2.4.3

Organotin Sulfonates

The sulfonate ligand is weakly coordinating in comparison to the strongly coordinating carboxylate ligand. Therefore, is not surprising that the chemistry of the organotin sulfonates is much less developed than that of the corresponding organotin carboxylates. In many instances the reaction of an organotin oxide with a sulfonic acid results in the formation of the hydrated organotin cations (Figure 2.4.12).99 Such hydrated organotin cations are quite interesting in their own right in terms of their structural and catalytic chemistry. However, these are not covered in this section nor are the monomeric organotin sulfonates with the focus being on organotin sulfonates, where at least two tin atoms are present.100−114

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Table 2.4.3 Entry

20:38

R

Representative examples of hexanuclear monoorganostannoxane drumsa R’ (carboxylate ligand)

n-Bu

Ref.

Entry

80

11.

n-Bu

83

81

12.

n-Bu

56

82

13.

n-Bu

81

14.

n-Bu

R

R’ (carboxylate ligand)

Ref.

Fe 2.

n-Bu

Fe

3.

n-Bu

4.

n-Bu

Me Me

Me

5.

Me

6.

n-Bu

7.

n-Bu

8.

n-Bu

9.

n-Bu

10.

n-Bu

85

N N Me

Me

N N

Me

n-Bu

N N

85

N N

Me

Me

82

15.

PhCH2

83

16.

PhCH2

87

82

17.

Ph

88

82

18.

Ph

56

19.

Me3 SiCH2

N

86

Me

H2N

NH2

84

20.

Ph

Cl Cl

Cl

H3C H N

61

89

90

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Organotin Carboxylate and Sulfonate Clusters Table 2.4.3 Entry 21.

107

(Continued)

R

R’ (carboxylate ligand)

PhCH2

N

Ref.

Entry

86

26.

PhCH2

91

27.

n-Bu

TTP-H2b

95

R

R’ (carboxylate ligand)

NH

Ref. 63

O 22.

PhCH2

O 23.

Me

H3C

92

28.

n-Bu

TTP-Mb

95

24.

i-Pr

H3C

93

29.

n-Bu

Ful-1b

96

25.

PhCH2

94

30.

n-Bu

Ful-2b

96

a b

NH

See Figure 2.4.8 for structural depiction of drum. See Figure 2.4.9 for structural depiction of the carboxylate ligand.

R'' N

TTP-H2

R''

TTP-M N

HN

NH

R''

R''

O

O

N

N R''

R'' =

(a)

O

N M = Cu (b-1) Pd (b-2)

R''

Ful-2 R'' O

O

O

O

O

O R'' = C16H33

(c)

Me

Ful-1 R''

O

N M

R''

R''

O

O

O

O

O

R'' O

R'' O

O O

O O

O

(d)

Figure 2.4.9 Schematic representation of the carboxylate ligands in the drum compounds containing the porphyrin or buckminister fullerene periphery

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Figure 2.4.10

Monoorganotin carboxylates with open structures R

R OH

O

OH

O

R

6 [RSn(O)OH]n

Sn

OH

OH

HO CO2H

Sn

O

O

O

Sn

OH

O O O O

6

O R

O O +

O

Sn

Sn

O OO O Sn

O

OH R = n-Bu R

Figure 2.4.11

R

Formation of hexanucular organotin trigonal prism n-Bu H2O

[n-Bu2SnO]n +

H2O SO3H

Figure 2.4.12

2+ OH2

Sn OH2

·2 SO3−

Formation of hydrated organotin cation

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Organotin Carboxylate and Sulfonate Clusters

109

Tri- and Di-Organotin Sulfonates

Polymeric or chain structures are formed among some triorganotin sulfonates (Figure 2.4.13).84,100−102 In these compounds, sulfonate ligands act as bridging bidentate ligands, similar to a carboxylate ligand, and generate a coordination polymer. A number of diorganotin sulfonates containing the four-membered distannoxane [Sn2 (μ-OH)2 ] motif are known.24 These are monomeric (Figure 2.4.14(a–e)), oligomeric (Figure 2.4.14(f)) or polymeric (Figure 2.4.14(g–i)). The sulfonate ligand in these compounds is either coordinating (Figure 2.4.14(a–c) and 2.4.14(g–i)) or non-coordinating (Figure 2.4.14(d–f)). Compounds such as {[n-Bu2 Sn(μ-OH)(O3 SC6 H3 -2,5-Me2 )]2 }n are two-dimensional polymers.103 This compound contains a [Sn2 (μ-OH)2 ] unit as its repeat unit. Such four-membered distannoxanes are linked to each other by an anisobidentate coordination action of the sulfonate ligand to afford a 20-membered macrocyclic ring. The macrocyclic rings, in turn, are linked to each other to afford a coordination polymer (Figure 2.4.15). Diorganotin sulfonates, containing motifs other than [Sn2 (μ-OH)2 ] are summarized in Figure 2.4.16. These include oligomeric and polymeric compounds.101,104−109 In all of these cases the sulfonate ligand is involved in coordination to the tin atoms. In an interesting example, the reaction of 1,5-napthalenedisulfonic acid (LH2 ) with (n-Bu3 Sn)2 O leads to a Sn–C bond cleavage to afford [n-Bu2 Sn(H2 O)3 (L)Sn(H2 O)3 n-Bu2 ]2+ [L]2− (Figure 2.4.17).102 In this compound one of the disulfonate ligands is involved in bridging two hydrated tin centers. The other disulfonate is present as the counter anion. The crystal structure of this compound shows that it possesses a three-dimensional pillared structure, formed as a result of intricate intramolecular O–H—O bonds.

Ph O

Sn

S

O

O

Ph

CF3

Ph

Sn Ph

O Ph

Sn

Ph

n

O H

CH3 = H3C

O

; (a-1)

Sn

O

Ph

Ph (b)

Sn

Ph

Ph O

Sn

O

O

Sn

S Ph

OH2

Ph

Ph

O

S O

O

Sn

n

(c)

Figure 2.4.13

O

O

(a-2)

O

S

Polymeric triorganotin sulfonates106−109

n

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n-Bu H2O

O

O H (a)

O S n-Bu O2

n-Bu

P

Sn

Sn O H (e)

Ph Et

NMe2

n-Bu

Et .2 CF3SO3-

O

4+ OH2

O H

H2O

SiMe3 SiMe3

H O Sn

Sn

OH2

O H

Me3Si

(f)

n-Bu O

H O

n

.4n CF3SO3−

O S

O

Sn

R

Sn O H

O n-Bu

S

CH2

Ph

Sn R

t-Bu .2 CF3SO3-

Sn

Sn

2+ OH2

Sn O H (d)

H O

H2O

Sn O

Sn

Me3Si

OH2

N O

t-Bu

H O

t-Bu

Ph

H O

H2O

t-Bu

CF3

2+

Et

Et

O

n-Bu

O .2 n-Bu4N+

NMe2

Me2N

Ph

O

H2O O

Me

O

O H (b)

n-Bu

Sn

n-Bu

S O2

O2 S

O 2− S

Sn

O S

O

O H (c)

O

n-Bu O

H O

O

Me

n-Bu

H O

Sn F3C

OH2

O

P

Me2N

O

CF3

n-Bu Sn

n-Bu

NMe2 n-Bu O

Me2N

O N

Sn

Sn F3C

O2 S

n-Bu

H O

Sn n-Bu

(g)

O

O n R = 2,4,6-(Me)3C6H2-; 2,5-(Me)2C6H3-

Sn

n-Bu

O

H O

Sn O

Me

n-Bu

S O Sn

O

n-Bu O O

O S

Me

Sn O H (h)

n-Bu

O n

Sn n-Bu

H O

H2O Sn

O H

n-Bu

S

O

O

O

n-Bu

n-Bu H O

Sn F 3C

Figure 2.4.14

O S

CF3

Sn

O

F 3C

n-Bu O

O O H S n-Bu O2 (i)

OH2 O2 S

n-Bu O

CF3

Sn n-Bu

n O

Diorganotin sulfonates containing four-membered [Sn2 ( μ-OH)2 ] units24

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Organotin Carboxylate and Sulfonate Clusters O

n-Bu n-Bu

n-Bu

O

H O

S Sn

Sn n-Bu

O

O H

O

OH

n-Bu

Sn

O

n-Bu

n-Bu O n

Two-dimensional coordination polymer of {[n-Bu2 Sn(μ-OH)(O3 SC6 H3 -2,5-Me2 )]2 }n Ph R

O O O S O

O O

O Sn

O

R

O

F3C

S

Sn

S

O

Sn

Sn O H (b)

Ph

O

Sn

O

CF3

O

O

Sn

O

S

F3C

Ph Ph

O O

R H2 C F

(a-1)

(c)

(a-2)

Sn

n

Ph

n-Bu Me

n-Bu

O

O

Sn

Sn

O Sn

n-Bu

Sn O

O

S

O n-Bu

O Sn

Ph

S O

F3C

Ph O O Sn

N n-Bu

Ph

S O

O Sn Ph

(f)

Me

Figure 2.4.16

Ph P O Sn Ph

Ph O n

(g)

Examples of di- and triorganotin sulfonates

S

CF3 O

n-Bu Sn

O H (e)

CF3

O

O

Sn

n-Bu

O

O

Sn

O

Ph

O

O

n-Bu O

(d)

Sn O H

n-Bu O H O

Sn

O

Ph

n-Bu

Sn O

Ph

O

n-Bu

O

N O

S

Sn

O

O

F3C

Me

n-Bu

O

n-Bu

O

O

Ph S O

n-Bu

S

Sn

O

F3C

Ph

H O

Ph

CH3

Ph

O

Sn

R O

Ph P

Ph Ph

Ph

Sn

P

Ph

Ph

Ph Ph

O

Sn

O

Ph

O

O S

O

Ph

O

S

Ph

R

O

R

n-Bu

Sn

O

Ph

Ph

H O

O

O

S

O

O R

O

Sn

O

O

O O

O

O

O

S

R

Sn

F 3C

Ph

Ph

O

R

R

n-Bu

O H

n-Bu

O

R

Sn

Sn

S

O

Sn

O

H O

O

O

OH

n-Bu

O

n-Bu

n-Bu

Sn HO

Sn

n-Bu

S

O

Figure 2.4.15

O

O

S

n-Bu

n-Bu

HO

O

O

Sn

O

n-Bu

O

O

111

n-Bu n-Bu

n

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n-Bu

H2O

S

O

n-Bu3Sn)2O + SO3H

Sn

H2O

n-Bu

O OH2

SO3–

OH2 ⋅

Sn

O

2+

n-Bu

H2O

O

O

S

OH2

n-Bu

O

SO3–

SO3H

Figure 2.4.17

Formation of dinuclear organotin cation

A similar Sn–C bond cleavage reaction as above has been observed in the reaction of (Ph3 Sn)2 O with triflic acid. The resulting product [(Ph2 Sn)2 (μ3 -O)(μ-OH)(O3 SCF3 )]2 is a tetramer (Figure 2.4.16(d)).106 Monoorganotin Sulfonates

The reaction of [n-BuSn(O)OH]n with aryl sulfonic acids leads to the formation of the dodecanuclear organooxotin cage {[(n-BuSn)12 (μ3 -O)14 (μ-OH)6 ]2+ .2RSO− 3 } (R = 4-MeC6 H4 -; 2,5-Me2 C6 H3 -; CH2 =CHC(O)-NH-C(Me)2 CH2 -) (Figure 2.4.18).110−112 Such dodecanuclear organooxotin cages with other counter anions are also known. The dodecanuclear organooxotin cage has a ‘football’ shaped geometry and comprises of two tri-tin poles and a hexa-tin equator. The three tin atoms present in the poles are linked to each other by a μ3 -O. Four μ-OH ligands bridge alternate tin atoms in the tri-tin subunit. This overall coordination leads to a coordination number of six (1C, 5O) for the tin atoms located at the poles. In contrast, the six tin atoms situated in the equator are five-coordinate (1C, 4O). Further, the equatorial belt of the football comprises of Sn2 O2 distannoxane rings. The utility of the football cages in catalyzing esterification reactions111 as well as in forming a nano building block for tin-based inorganic–organic hybrid materials has been investigated.113

R R

3 [RSn(O)OH]n + R' SO3H

Toluene Reflux

Sn

O

R

HO

Sn

R = n-Bu R

Sn Sn R

O O

O

O O O

Sn R

Sn

O

Sn

O

R

O Sn R

O O

HO

R' =

O Sn

HO

R

Sn

R

R O

2+ OH OH

Sn Sn

·2

R' SO3−

OH

R foot-ball cage

NH O

Figure 2.4.18

Formation of dodecanuclear football shaped macrocations

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Organotin Carboxylate and Sulfonate Clusters

113

R Sn

SO3H

H2O

H2O

SO3H

+

R OH

R

Sn

HO

HO

O H

O HO

R

Sn

OH2 R

HO

HO

O

O

Sn HO Sn

(a)

R OH

Sn

R ·4

R

O H O

SO3−

O

OH

O

OH O2S O

Sn

Sn

S

HO

Sn

Sn

R

O

O

Sn

OH

O

O

HO

O S

R

O OH

4+

OH2 O 2 S O Sn

HO

Sn

H O

Sn

HO

HO OH2

H O

SO3−

O S

R=

·

(b)

O O-capped cluster

R

Figure 2.4.19

Formation of cationic monoorganotin sulfonates

Recently the hydrolysis of aryltrialkyltin 2,4,6-i-Pr3 C6 H2 Sn(CCMe)3 in the presence of aryl sulfonic acids has been investigated (Figure 2.4.19).114 A decanuclear tetracationic cage {[(2,4,6i-Pr3 C6 H2 Sn)7 Sn3 (μ-OH)16 (OH)3 (μ3 -O)3 (μ4 -O)(4-MeC6 H4 SO3 )2 ]4+ .[4-MeC6 H4 SO− and 3 ]4 .3H2 O} a trinuclear O-capped cluster {[(2,4,6-i-Pr3 C6 H2 Sn)3 (μ3 -O)(μ-OH)3 (2,5-Me2 C6 H3 SO3 )3 ]+ .[2,5Me2 C6 H3 SO− 3 ].2C6 H5 Me} were isolated in these reactions (Figure 2.4.19). The latter structural type is quite commonly observed, both independently and as part of other larger organooxotin cages.2 2.4.4

Conclusion

Organotin carboxylates and sulfonates have a rich structural chemistry. In many instances complex cluster/cage structures are realized. Often the formation of such cages is the result of a partial hydrolysis, resulting in the in situ generation of hydroxide or oxide ligands, which hold multiple tin centers together through their bridging coordination action. The role of the carboxylate and sulfonate ligands is to provide structural support, in many instances by a bridging coordination mode. Depending on the type of bridging action, molecular cages/clusters or polymers (one- or two-dimensional) are formed. The presence of additional coordination sites in the carboxylate ligand provides an opportunity for enhancing the versatility of the ligand and can result in the formation of new structural types. The use of nanometric organooxotin cages in the formation of new materials is another aspect of contemporary research. Finally the predictable assembly of an organooxotin cage with a definite nuclearity and dimension is allowing this methodology to be adapted for the preparation of stannoxane-cored starburst type molecules. In such compounds the stannoxane core serves as an inert support around which a desired functional periphery is built. Instances of electroactive, photoactive, and coordination peripheries are already known. 54,56,85,95

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Acknowledgments V. C. is thankful to the Department of Science and Technology, New Delhi for funding and for a J. C. Bose Fellowship. P. S. and K. G. thank the Indian Institute of Technology Kanpur and the Council of Scientific and Industrial Research, New Delhi for financial support. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37.

Davies A. G., Organotin Chemistry, WILEY-VCH, Verlag, GmbH and Co. KGaA, Weinheim, 2004. Chandrasekhar V., Nagendran S., and Baskar V., Coord. Chem. Rev. 2002; 235: 1. Chandrasekhar V., Nagendran S., Gopal K., Sasikumar P., and Moorthi R., Coord. Chem. Rev. 2005; 249: 1745. Chandrasekhar V., Gopal K., and Thilagar P., Acc. Chem. Res. 2007; 40: 420. Begley M. J., Sowerby D. B., Kapoor P., and Kapoor R., Polyhedron 1995; 14: 1937. Yin H.-D. and Wang C-H., Appl. Organometal. Chem. 2004; 18: 411. Teoh S. G., Tan T. S., Yeap G.-Y., and Fun H.-K., Z. Kristallogr., New Cryst. Struct. 1999; 214:161. Yin H. D., Wang Q.-B.Nightmare, and Xue S.-C., J. Organomet. Chem. 2005; 690: 435. Renamy S. V., Bassene S., Diop C. A. K., Sidibe M., Diop L., Mahon M. F., and Molloy K. C., Appl. Organometal. Chem. 2004; 18: 455. Yin H., Wang C., Wang Y., and Ma C.-L., Indian J. Chem., Sect.A 2003; 42: 48. Ng S. W. and Hook J.M., Acta Crystallogr., 1999; C55: 312. Sadiq-ur-Rehman, Ali S., Badshah A., Malik A., Ahmed E., Jin G.-X., and Tiekink E. R. T., Appl. Organometal. Chem. 2004; 18: 401. Ng S. W., Das V. G. K., Pelizzi G., and Vitali F., Heteroatom. Chem. 1990; 1: 433. Willem R., Bouhdid A., Meddour A., Camacho C. C., Mercier F., Gielen M., and Biesemans M., Organometallics 1997; 16: 4377. Holmes R. R., Day R.O., Chandrasekhar V., Volano J. F., and Holmes J. M., Inorg. Chem. 1986; 25: 2490. Ng S. W., Chin K. L., Wei C., Das V. G. K., and Mak T. C. W., J. Organomet. Chem. 1989; 365: 207. Gielen M., Khloufi A. E., Biesemans M., Kayser F., Willem R., Mahieu B., Maes D., Lisgarten J. N., Wynes L., Moreira A., Chattopadhya T. K., and Palmar R. A., Organometallics 1994; 13: 2849. Holmes R, R., Acc. Chem. Res. 1989; 22: 190. Tiekink E. R. T., Appl. Organometal. Chem. 1991; 5: 1. Jain V. K., Coord. Chem. Rev. 1994; 135/136: 809. Gielen M., Biesemans M., Willem R., and Tiekink E. R. T., Eur. J. Inorg. Chem. 2004; 445. Chandrasekhar V. and Gopal K., Appl. Organometal. Chem. 2005; 19: 429. Beckmann J., Appl. Organometal. Chem. 2005; 19: 494. Chandrasekhar V., Singh P., and Gopal K., Appl. Organometal. Chem. 2007; 21: 483. Benetollo F., Lobbia G. G., Mancini M., Pellei M., Santini C., J. Organomet. Chem. 2005; 690: 1994. Ma C.-L., Han Y., and Zhang R., J. Organomet. Chem. 2004; 689: 1675. Tian G.-R., Zhang R.-F., Ma C.-L., and Ng S. W., Acta Crystallogr., 2005; E61: m2528. Yin H.-D., Gao Z.-J., Li G., Xu H.-L., and Hong M., Chin. J. Inorg. Chem. 2006; 22:157. Yin H.-D., Xue S.-C., and Liu G.-F., Acta Chim. Sinica 2004; 62: 603. Peng B., Sun L.-J., Chang W.-X., and Xie Q.-L., Chin. J. Struct. Chem. 2003; 22: 647. Chen M.-S., Kuang D.-Z., Deng Y.-F., Zhang C.-H., and Feng Y.-L., Chin. J. Inorg. Chem. 2006; 22: 367. Yin H., Xue S.-C., and Wang Q.-B., Indian J. Chem., Sect.B. 2005; 44: 1040. Tian L., Yu Q., Zheng X., Shang Z., Liu X., and Qian B., Appl. Organometal. Chem. 2005; 19: 672. Kapoor R., Gupta A., Kapoor P., and Venugopalan P., Appl. Organometal. Chem. 2003; 17: 600. Zhou Y., Jiang T., Ren S., Yu J., and Xia Z., J. Organomet. Chem. 2005; 690: 2186. Phadnis P. P., Dey S., Jain V. K., Nethaji M., and Butcher R. J., Polyhedron 2006; 25: 87. Li F.-H., Yin H.-D., Huang W.-B., and Wang Y.-W., Acta Crystallogr., Sect.E: Struct. Rep. Online 2006; 62: m919.

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Organotin Carboxylate and Sulfonate Clusters 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81.

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82. Chandrasekhar V., Gopal K., Nagendran S., Steiner A., and Zacchini S., Cryst. Growth & Des. 2006; 6: 267. 83. Chandrasekhar V., Schmid C. G., Burton S. D., Holmes J. M., Day R. O., and Holmes R. R., Inorg. Chem. 1987; 26:1050. 84. Chandrasekhar V., Baskar V., Boomishankar R., Gopal K., Zacchini S., Bickley J. F., and Steiner A., Organometallics 2003; 22: 3710. 85. Chandrasekhar V., Thilagar P., and Sasikumar P., J. Organomet. Chem. 2006; 691: 1681. 86. Yin H., Wang C., and Wang Y., Indian J. Chem., Sect.B. 2004; 43: 612. 87. Yin H., Wang C., and Wang Y. ,Indian J. Chem., Sect.B. 2004; 43: 1493. 88. Chandrasekhar V., Day R. O., and Holmes R. R., Inorg. Chem. 1985; 24: 1970. 89. Beckmann J., Dakternieks D., Duthie A., Thompson L., and Tiekink E. R. T., Acta Crystallogr., 2004; E60: m767. 90. Yin H.-D., Wang C.-H., Ma C.-L., and Fang H.-X., Chin. J. Org. Chem. 2003; 23: 291. 91. Yin H.-D., Wang C.-H., Ma C.-L., and Fang H.-X., Chin. J. Chem. 2003; 21: 452. 92. Day R. O., Chandrasekhar V., Swamy K. C. K., Holmes J. M., Burton S. D., and Holmes R. R., Inorg. Chem. 1988; 27: 2887. 93. Kuan F.-S., Dakternieks D., and Tiekink E. R. T., Acta Crystallogr., 2002; E58: m301. 94. Yin H.-D., Wang C.-H., and Ma C.-L., Chin. J. Org. Chem. 2003; 23: 470. 95. Chandrasekhar V., Nagendran S., Azhakar R., Kumar R. M., Srinivasan A., Ray K., Chandrashekar T. K., Madhavaiah C., Verma S., Priyakumar U. D., and Sastry G. N., J. Am. Chem. Soc. 2005; 127: 2410. 96. Hahn U., Gegout A., Duhayon C., Coppel Y., Saquet A., and Nierengarten J.-F. Chem. Commun 2007; 516. 97. Swamy K. C. K, Nagabrahmanandachari S., and Raghuraman K., J. Organomet. Chem. 1999; 587: 132. 98. Chandrasekhar V., Thilagar P., Bickley J. F., and Steiner A., J. Am. Chem. Soc. 2005; 127: 11556. 99. Chandrasekhar V., Boomishankar R., Singh S., Steiner A., and Zacchini S., Organometallics 2002; 21: 4575. 100. Diop C. A. K., Bassene S., Sidibe M., Sarr A. D., Diop L., Molloy K. C., Mahon M. F., and Toscano R. A,. Main Group Met. Chemistry 2002; 25: 683. 101. Beckmann J., Dakternieks D., Duthie A., and Mitchell C., Organometallics 2004; 23: 6150. 102. Chandrasekhar V., Boomishankar R., Singh S., Steiner A., and Bickley J. F., Organometallics 2003; 22: 3342. 103. Chandrasekhar V., Singh P., and Gopal K., Organometallics 2007; 26: 2833. 104. Allen FH, Lerbscher JA, Trotter J. J. Chem. Soc. (A) 1971; 2507. 105. Shankar R., Kumar M., Narula S. P., Chadha R. K., J. Organomet. Chem. 2003; 671: 35. 106. Beckmann J., Dakternieks D., Duthie A., and Mitchell C., Appl. Organometal. Chem. 2004; 18: 51. 107. Orita A., Xiang J., Sakamoto K., and Otera J., J. Organomet. Chem. 2001; 624: 287. 108. Shankar R., Kumar M., Chadha R. K., and Hundal G., Inorg. Chem. 2003; 42: 8585. 109. Beckmann J., Dakternieks D., Duthie1 A., Mitchell C., Ribot F., de la Caillerie J.B.d’E., and Revel B., Appl. Organometal. Chem. 2004; 18: 353. 110. Eychenne-Baron C., Ribot F., Steunou N., Sanchez C., Fayon F., Biesemans M., Martins J. C., and Willem R., Organometallics 2000; 19: 1940. 111. Chandrasekhar V., Boomishankar R., Gopal K., Sasikumar P., Singh S., Steiner A., and Zacchini S., E. J. Inorg. Chem. 2006; 4129. 112. Ribot F., Veautier D., Guilladeau S. J., and Lalot T., J. Mater. Chem. 2005; 15: 3975. 113. Sanchez C., Soler-Illia G. J. D. A. A. , Ribot F. , Grosso D., C. R. Chimie 2003; 6: 1131 114. Prabusankar G., Jousseaume B., Toupance T., and Allouchi H., Dalton Trans. 2007; 3121.

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Macrocyclic and Supramolecular Chemistry of Organotin(IV) Compounds

2.5

117

Macrocyclic and Supramolecular Chemistry of Organotin(IV) Compounds

Herbert H¨opfl Centro de Investigaciones Qu´ımicas, Universidad Aut´onoma del Estado de Morelos, Cuernavaca, M´exico

2.5.1

Introduction

In 2006, the Cambridge Structural Database revealed 7062 crystal structures containing tin(II) or tin(IV), of which only a very limited number has discrete macrocyclic structures or extended polymeric networks.1 Since the number of publications on such systems has increased exponentially during the last ten years, a brief overview of the progress reached so far in this field is worthwhile. The purpose of the present survey is to examine and visualize the potential that organotin(IV) complexes can have in metal-directed self-assembly of macrocycles and extended networks. Their application in the formation of cage- and cluster-type structures is only mentioned superficially, since this chemistry is described in other sections within this chapter.

2.5.2

Metallosupramolecular Chemistry with Tin

Metal-Directed Self-Assembly

The combination of metal ions and appropriate ligands to form macrocyclic, cage-like, and extended network structures has become a powerful tool for the construction of systems having cavities, pores, or channels, and is currently one of the most important topics in chemical and material sciences.2 Applications are visualized for a wide range of fields, such as ion and molecular recognition, sorption, filtration, storage, transport, catalysis, etc.2b,f,i,k,m The ultimate goal in the design of such functional materials is the generation of methods that allow control over size, shape, and function of the corresponding cavities. For this purpose metal–organic architectures prepared from tailored building blocks are ideal, since a wide range of structural modifications can be carried out. Concerning the metal ions, parameters such as atom size, coordination number, coordination geometry, Lewis acidity, etc., can be varied, either for the same element or by selection of another metal atom, giving a whole range of different molecular and supramolecular structures.2b,f Regarding the organic building blocks, the number, Lewis basicity, spatial separation, and geometric orientation of the metal-coordinating donor atoms can be modified. Furthermore, substituents attached to the organic connectors or additional ligands coordinated to the metal ion (e.g. blockers) can be employed in order to: (i) modulate the hydrophilic/hydrophobic character of the cavity walls; (ii) cover them with certain functional groups; and (iii) provide the cavities with a chiral shape.2b,c,f,m Through judicious choice of metal ions, connectors (spacers) and blockers, a large variety of discrete structures and network topologies can be prepared, i.e.: (i) zero-dimensional triangles, squares, or parallelograms; (ii) one-dimensional chains, helices, or ladders; (iii) two-dimensional sheets, grids, honeycomb-like assemblies, or so-called brick-wall structures; and (iv) three-dimensional cubic or diamandoid networks (Figure 2.5.1). The nuclearity, shape, and size of the rings or cages, which are present in the resulting supramolecular architectures, depend strongly on the donor–metal–donor and donor– connector–donor bond angles, e.g. systems involving 90◦ angles give squares and cubes, while systems with 120◦ angles give hexagons and honeycomb-like arrangements.2a,b,c,e,k

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Figure 2.5.1 Schematic representation of some of the discrete and extended supramolecular architectures that can be prepared by metal-directed self-assembly: (a) 1D polymeric chains; (b) discrete macrocyclic structures; (c) two-dimensional grid- and honeycomb-like networks; (d) three-dimensional diamandoid open frameworks

So far, mainly transition metals have been explored for metal-directed self-assembly and little is known about supramolecular architectures with main group elements,2e,2o,3 which is somewhat surprising considering the impact elements such as aluminium and silicon have in industry. Additionally, several main-group elements that form kinetically and thermally stable organometallic compounds, in particular boron, silicon, sulfur, phosphorus, and tin, are very interesting building blocks for metal-directed selfassembly, because the organic substituents attached to the metal ions can be varied almost indefinitely in size, shape, and functionality. So far, little attention has been paid to the utilization of tin(II) and tin(IV) compounds in this direction, and, therefore, only a limited number of macrocycles or coordination polymers is known. Moreover, many of these compounds have been prepared with another objective or obtained accidentally. Primary and Secondary Tin Building Blocks

As already mentioned, the supramolecular structure of coordination assemblies is ruled by geometric factors, such as the metal coordination environment and, in particular, the number and geometry of coordination sites available for the interaction with organic building blocks.

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

R

R

R X

Sn X

b)

X

X

X

R

c)

X

X X

X

Sn X

R

Sn

X

X

Sn

X R

X X

X

X

R

Sn R

Sn X

119

X

R

R X

X

X

X

X Sn R

X X

R X

Sn R

X R

Figure 2.5.2 Most common coordination environments for tin macrocycles, cages and coordination polymers: (a) monoorganotin; (b) diorganotin; (c) triorganotin derivatives. R = organic group, X = ligand with functional groups containing C, N, O, S, etc.

For tin(IV), the most frequent coordination geometries are based on tetrahedral, trigonal-bipyramidal, octahedral, and pentagonal-bipyramidal polyhedra.4 In organometallic tin(IV) complexes there is a tendency for the tin atoms to enhance their coordination number either via intra-molecular coordination or inter-molecular association. The final coordination number depends strongly on the number of organic substituents attached to the tin atom and the nature of the donor atoms of the ligands.5 Considering that the formation of metallocyclic polygons and polyhedra requires at least two free coordination sites, the number of possible bonding environments is limited. Figure 2.5.2 summarizes the coordination geometries found most frequently in organotin(IV) macrocycles and coordination polymers. For the construction of discrete macrocycles, cages, and extended assemblies, it is also possible to employ di- or oligo-nuclear metal building blocks, in which two or more metal centers are connected to form so-called secondary building units (SBUs). Examples are stannoxane-derived organotin dimers, clusters and cages (Figure 2.5.3).6 All examples shown in this figure contain at least one Sn2 O2 ring, and are frequently observed in the solid-state structures of organotin carboxylates, as illustrated by the examples shown in the same figure. Diorganotin dicarboxylates frequently associate through the combination of two skewed-trapezoidal bipyramids forming a single Sn2 O2 core (Figure 2.5.3a).7 Systems having a Sn4 O2 core are common for partially hydrolyzed diorganotin carboxylates, in which the central bis(tetraorgano)stannoxane fragment, [XR2 Sn-O-SnR2 Y]2 , generally contains additional bridges between the exo and endo tin atoms. If either X or Y is a single atom, a ladder-type structure having a chair conformation is obtained. The bridging ligands generally are carboxylates, however, systems containing alkoxides, halides, or phosphonates are also known.7 The carboxylate can function either as monodentate or bidentate bridging ligand, and all possible combinations have been described.7 With monoorganotin derivatives, a series of oligotin clusters has been described, of which the drum structure containing a Sn6 O6 core has received most attention.6b,c So far, there are few reports on macrocyclic and supramolecular assemblies using organotin(IV) (see below).

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Tin Chemistry: Fundamentals, Frontiers and Applications a)

b)

O Sn

R Sn

Sn O

Sn Y

O

Sn2O2

Sn

R

Sn

R X

Sn R

O

R Sn

O

R Y

R

X, Y = Hal-, -O2CR'', etc.

R O

O

c)

O

R R O

X

Sn

Sn4O2

Sn

O

Sn

O

O

R

O

Sn

Sn

O Sn O

R O

Sn

O

O Sn

O

Sn Sn

O Sn6O6

Figure 2.5.3 Frequent tin-containing secondary building units (SBUs) that can be used for the generation of macrocyclic or extended supramolecular assemblies: (a) Sn2 O2 dimers; (b) Sn4 O2 X2 Y2 -distannoxanes; (c) Sn6 O6 drums Methodologies for Tin-Directed Self Assembly

Considering the current state-of-the-art for metal-directed self-assembly, and the coordination facilities of tin(IV) species, three general methodologies can be proposed for the construction of supramolecular assemblies containing tin: (1) Assembly of mono-, di- or oligonuclear tin compounds via inorganic connectors; (2) Assembly of mono-, di- or oligonuclear tin compounds via organic connectors; (3) Combination of the afore-mentioned options. Since the synthetic methods employed for the generation of the supramolecular tin(IV) assemblies described herein are very similar to those known for monomeric tin(IV) complexes, in the following, mainly structural aspects in relation to the ring and network composition, as well as the cavities, if existing, are described. 2.5.3

Formation of Tin Macrocycles and Extended Networks

Application of Inorganic Connectors

Functional groups containing electronegative atoms such as fluorine, chlorine, oxygen, sulfur, and nitrogen are frequently capable of bridging tin moieties, and oligo- or polymeric structures are obtained. For R4−n Sn(IV)m+ moieties, the most common bridging functions are X = F− , Cl− , O2− , S2− , OH− , OR− , 3− 2− − − 8 CN− , RSO− 3 , PO4 , RPO3 , R2 PO2 and RCO2 .

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Macrocyclic and Supramolecular Chemistry of Organotin(IV) Compounds O− Xn−

O

E

E R

O− −

X = Hal , Chalc , OH−, OR−, SR−, CN−, N3−, SCN−, etc. n−

E = C, N

2−

+

O−

O

O

R

R

E

O−

−

M

O

−

O

121

O−

O−

O

M = V , CrVI, MoVI, WVI, TcVII,ReVII, etc. V

E = P, As E = S, Se, Te R = alkyl, aryl, OR, O−, NR2, etc.

Figure 2.5.4 Functional groups with bridging capability that might be candidates for the formation of tin macrocycles or extended networks

Of the anionic functional groups outlined in Figure 2.5.4, only some have been applied as bridging ligands for tin atoms; therefore, there might be additional promising candidates for the formation of tin macrocycles or extended networks. In functions with π-electron density, delocalization can occur, and in this case the binding energies between the donor atoms of the ligand and the tin atom will be intermediate between those of the corresponding covalent and coordinative bonds (symmetrical or isobidentate bridging). Some of the bridging ligands can have different configurations, as shown in Figure 2.5.5 for bidentate ligands. Depending on the energy differences between the different configurations, the ligands can be more or less flexible in the variation of the M–ligand–M bond angle, an aspect that is important for the formation of cyclic structures, since it makes it possible to reduce steric strain. When sulfur replaces oxygen, chelating begins to dominate over bridging, as it occurs in complexes with ligands derived from dithiophosphate derivatives9 and dithiocarbamates.10 (1) Oxygen-Containing Bidentate or Oligodentate Connectors. Although monomeric and 1D polymeric R4−n Sn(IV)m+ complexes are known for many of the ligands shown in Figure 2.5.4, so far macrocyclic and extended network structures have been reported only for a limited number, and interestingly, most of them are triorganotin derivatives. This can be explained by the requirement of a subtle balance of electronic and steric effects of the organic groups attached to the tin and ligand moieties,5 and may be, in part, also due to the relatively small number of research groups dedicated to a systematic exploration of supramolecular tin chemistry. Concerning the oxygen-containing bidentate ligands [Rn EO2+x ] y− and MOm− 4 , discrete dimeric, tetrameric and hexameric ring assemblies have been reported for organotin carboxylate,11 phosphinate,12 phosphonate monoester,13 phosphate diester,14 sulfonate,12d,15 and tungstate16 derivatives. In these macrocyclic structures, the composition of the central inorganic ring system is [SnO2 E]n or [SnO2 M]n , respectively, with n = 2 for the dimer, n = 4 for the tetramer, and n = 6 for the hexamer, thus R

R

X

X

X

M

M

M

syn-syn

Figure 2.5.5

R

X

syn-anti

M

M

X

X

M

anti-anti

Possible configurations for bidentate bridging ligands

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b)

R'

R' R

R O

O O R

S

R''

O

O

Sn O

S

R

R

R'

O

C

R'

O R R

C

R

O

R

O

Sn

O

R'

Sn

R

O

O

O

Sn

R''

R

O

O

R

C

R

Sn R

R'

O

Sn

R

O C

R

R' R = Et; R' = Me, Ph; R'' = Me

c)

R''

R = nBu; R' = 4−NO2C6H4, 2,6−C6H3F2

R'

R

R

Sn

P

Sn

P O

O

O

R

R

R

R''

R

O R

R R'

R'' R'

Sn R

O

O P

P O

R

O

R

R Sn

Sn O

R

P

O

O P

Sn O

R

R

R R

R' R''

R'' R'

R

R'

R''

R = Ph; R', R'' = Me, Ph, OMe, OPh

Figure 2.5.6 Examples of: (a) dimeric; (b) tetrameric; (c) hexameric and organotin assemblies bridged by inorganic oxygen-containing connectors

giving rings of 8, 16, and 24 members (for examples, see Figure 2.5.6). Generally, the bridging ligands adopt an approximate syn-anti conformation. In triorganotin derivatives, the tin atoms exhibit trigonalbipyramidal coordination geometries, in which the organic substituents are located in the equatorial plane. NMR spectroscopic, cryoscopic, and related studies have shown that macrocyclic triorganotin tetramers and hexamers undergo transformations fast on the NMR timescale in solution.11b,12c Two-dimensional and three-dimensional structures containing analogous and related macrocyclic ring systems are known for most of the ligand types that form discrete macrocyles.17 Additionally, there are a few examples for two-dimensional and three-dimensional coordination polymers containing sulfate, phosphate, vanadate, chromate, molybdate, and tungstate,18 indicating that these are also possible candidates for the formation of discrete macrocycles.

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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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a) S

R Y

OMe

Sn

R

R Sn S

MeO

Sn

Y

OH

R R = CH(SiMe 3)2; Y = −CH2CH2−

Ph

b) N

R

N

O

R Sn

O

Ph

O

R

N Sn

R

R

R

R O

O R

Sn

R

N R

O

O

Sn

O

N

R

t-Bu t-Bu

R

t-Bu Sn t-Bu

O

O Te

O

O

R

O O

Te

N

N Ph

Sn R

OMe

R = CH 2Ph

R

O

OH

R

c)

Y

HO

R

R S

Sn

O

O Sn

Sn

R Sn

OH

HO

S

MeO OMe S

Sn

R Sn

Sn

OMe S

OH

Sn

R

Sn

O R

R

O

R = 4−C6 H4−OMe, 4−C6H4−NMe2

N Ph

R = Me d) Cl R' R'

Sn

Cl

R R

Ga

Cl

Cl R'

Ga

R' R' R'

Ga

R

R' Cl

Ga

R Sn

R'

Cl

R R = Me, (R')2 = 1,8−C10H6

Figure 2.5.7 Figure 2.5.6

Ph

e)

R

Ph

Ph

R

Sn

Ph

Ph Sn

R

Os R Ph Sn R Ph R R R R Os R R R Ph R R Sn R R Os Os Ph Sn R R R Ph Ph Os

R R

R

Os Sn

Ph R

R R

R = CO

Discrete tin rings formed with inorganic bridging connectors different from those shown in

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R M

M C R R

N

R

M

Sn

N

N R

N

C

M C N

R

R

N C

Sn

C

R

R

R C

N

R

R

R

N C

Sn

N

R Sn R

Sn

N

C

C

N

C

125

Sn

R

N R

C

C

M M

R

C

N

Sn R

N

C

M

R

M R

C

R

R

C

N

R

N

Sn N

Sn R

C

N

R

C

M

M

C

C

N

N R

R Sn

R

R

Sn R

R N

N

C

C M

M C

R

N

R

Sn R M = Fe, Co, Ni, Cu, Mo, W, Ru, Os

N Sn

N

R

C

N C

C

R R

M

R = Me, Et, Pr, nBu, Ph, etc.

Figure 2.5.8 Trinuclear, tetranuclear, and hexanuclear macrocyclic rings present in two-dimensional and three-dimensional networks of triorganotin cyanometalates

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b)

S

S

(CH2)n H3C N N P N

N

P

Cl

O

CH3 N

O

N

Sn Cl

Cl

+

N

H

CH3 H O 2− O Sn Cl Cl

Cl

N

+

CH3

n = 2, 3

Figure 2.5.9

Mononuclear tin macrocycles

three-dimensional structure,26,27 most of them being bis(triorganotin) dicarboxylates. Open-framework structures are almost unknown so far,28 and therefore, in the following, emphasis is placed only on systems having macrocyclic structures. There have been reports on four types of macrocyclic organotin(IV) assemblies formed from neutral or anionic bidentate organic connectors: (i) mononuclear R4−n Sn(IV)m+ macrocycles; (ii) di- and oligonuclear R4−n Sn(IV)m+ macrocycles; (iii) dinuclear cryptands; and (iv) macrocycles and cryptands formed from oligonuclear secondary building blocks. (1) Mononuclear R4−n Sn(IV)m+ Macrocycles. There are only a few structural reports on tin macrocycles, in which ring closure has been achieved through the combination of an R4−n Sn(IV)m+ fragment with a single bidentate ligand (Figure 2.5.9).29 The ring size varies from 11 to 20 members and it is interesting to note that the cavities of the derivatives shown in Figure 2.5.9a are chiral, making them of potential interest for asymmetric synthesis.29a,b Mononuclear cyclooligomers have been also proposed for diorganotin poly(ethyleneglycol) derivatives.30 (2) Di- and Oligonuclear R4−n Sn(IV)m+ Macrocycles. As shown in Figure 2.5.10, neutral di- or oligonuclear R4−n Sn(IV)m+ macrocycles are obtained when connectors such as (CH2 )n , O Y O , S Y S , and related bridging groups (Y = aliphatic or aromatic bridge) are combined with two or more diorganotin (R2 Sn2+ ) fragments (Figure 2.5.10).31−34 Interestingly, organometallic macrocycles with aromatic connectors are very rare. In Figure 2.5.10a cyclophane and porphyrin-type derivatives are shown.31b,e Dicarboxylates or carboxylates containing an additional anionic function (e.g. RS− , RO− , etc.) capable of coordination to tin atoms (functionalized monocarboxylates) are ideal candidates for the construction of discrete di- and oligonuclear organotin macrocycles, as is shown by a series of organotin carboxylate macrocycles having dimeric,28,35 trimeric,36 tetrameric,37 and hexameric38 compositions (Figure 2.5.11). Dicarboxylate ligands that do not contain heteroatoms capable of coordination to the tin atoms via chelate ring formation, are coordinating in an anisobidentate manner to diorganotin moieties. The resulting diorganotin carboxylates have either a skewed-trapezoidal bipyramidal coordination environment, or a pentagonal-bipyramidal geometry, if an additional coordination site in the equatorial plane is occupied.28,36c,e Together with the spatial orientation of the coordinating functions, these geometric parameters determine the nuclearity and size of the macrocyclic ring. Both planar and corrugated macrocycles have been reported (Figure 2.5.11a). Macrocycles obtained from functionalized dicarboxylates capable of chelate ring formation are generally smaller than the analogous macrocycles obtained from the unsubstituted dicarboxylate (compare

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b)

H

CH3 Et

Cl

O

Et

Sn

O

Et

Cl

P

O

CH3

O O

P

Et

O

O O

CH3 O

O

Cl CH3

O O

Sn Cl

Et

P

Et

NH

O NMe2

P

+ N

Et

NMe2

Et

127

MeO

Bu

MeO

Bu

Cl

O

− Sn

Cl

O

Bu

OMe

Sn − Bu

OMe

O NH N +

H

Figure 2.5.10 Examples for di- and oligononuclear R4−n Sn(IV) m+ macrocycles with connectors such as: (a) (CH2 )n ; (b) O Y O ; (c) S Y S , and related bridging groups (Y = aliphatic or aromatic bridge)

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c)

Sn S

Ph S P S

tBu

S

S

tBu Sn

Sn tBu

N

S

S

S

S

tBu

P

S N

Ph

S

S Sn R R = Me, tBu

CH3

H3C

S

S

CH3

S

Sn

Sn N

N

S

S

CH3 S

S

N

N

N

N

S

S CH3 CH3

CH3

Sn

Sn

N

N S

S

N

N

CH3

S

S S

S Sn

CH3 CH3

Figure 2.5.10

(Continued)

analogous or related entries in Figures 2.5.11a and 2.5.11b). For example, when comparing the trinuclear macrocycles [nBu2 Sn(isophthalate)]3 , [nBu2 Sn(2,5-pyridinedicarboxylate)]3 , and [Me2 Sn(2,6˚ for the first complex to 8.5 and 5.0 A ˚ pyridinedicarboxylate)]3 , the Sn· · ·Sn distances decrease from 9.0 A for the latter two derivatives. However, it appears that chelate ring formation contributes in a positive manner to the formation of macrocyclic ring structures, since a relatively large number of functionalized carboxylates has given macrocyclic instead of polymeric structures (Figure 2.5.11b).

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The tetranuclear macrocycle shown in Figure 2.5.11c has been obtained by a very interesting and promising synthetic strategy, which consists of the application of a functionalized organotin complex in the self-assembly process.37 In assemblies where the coordination number can be enhanced from six to seven, frequently intermolecular association occurs in the solid state through the formation of O· · ·Sn interactions (Sn2 O2 ring formation) that can generate interesting two- or three-dimensional coordination polymers, including open frameworks.26,28 There are also reports on extended networks formed through extensive hydrogen-bonding interactions between the macrocyclic ring systems, in particular when water molecules are coordinated to the tin atoms.28,36e The trimeric di-n-butyltin macrocycles derived from

a)

Ph O

O R R

O

O

Sn

Ph

Sn

R Ph

O

O

O

O

Ph

O

O

O

O

O

P Ph

O

Sn

O

O

O

O R

R R O

R O

Sn

Ph

O Sn

R O

P

Sn

P

Ph

O

R R R

Ph

P

O Ph

R

Ph

P P

Ph

Ph

R = nBu

R = nBu O O

O O

L

R

N

L Sn

R Sn

O

R

O

N CH3

O

Sn

Sn

O O

R

O

N

O O Sn L

O O

CH3 O

N

R

O CH3 N

CH3

O

O

O

CH3 O

N

Sn

R O

R = nBu, Ph L = H 2O, DMSO

O

O

CH3

O

Figure 2.5.11 Di-, tri-, tetra- and hexanuclear macrocycles can be prepared from carboxylate-derived connectors using: (a) dicarboxylates; (b) functionalized monocarboxylates; and (c) organotin compounds functionalized with a carboxylate group

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N

R'

R'

N R

Cl

O

Sn O

R

Cl

Sn

R'

N

R

O

O

Cl

R R

R'

O

O

N

O R R'

R

Sn O

Sn

N

O

R

R

N

R Sn

N O

Sn

N O

R O

N

O

O

O R

R'

R = Me, nBu R = Et, nBu R' = H, Me

PhH2C PhH2C

O Sn

O

N HN

O

Ph

Ph

O

O

Ph

O O

N Sn

S

Sn

O O

NH N

O

N

Sn

S

CH2Ph

S

O

CH2Ph

O

Sn

O Ph R'

N Ph

R' S R'

R R'

O S

O

Sn

Sn R

R

R O R'

O

R' O

R = Me, nBu R' = H, iPr

R Sn S

R

Sn

R

R

O R'

O

O

R

R'

R

Sn

O Sn R'

S

O

O

R

O

R

S

R'

Figure 2.5.11

R'

(Continued)

S

R'

Ph O

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c) R

O

Sn

R R

Sn

R

O O

O

O

O

O

R R

Sn R

Sn

O

R R = nBu

Figure 2.5.11

(Continued)

2,5-pyridinedicarboxylic acid, [nBu2 Sn(2,5-pyridinedicarboxylate)]3 , assemble to a three-dimensional hydrogen-bonded structure that contains spherical assemblies built from eight macrocycles. These cap˚ 3 that sules are stabilized by a total of 36 hydrogen bonds and have an accessible inner cavity of 1850 A, can be occupied by organic molecules such as diphenyl- and triphenylmethane, triptycene, and sodium tetraphenylborate.28,36e Imidazole- and pyridine- derived ligands are important organic connectors and have been applied for the assembly of a large number of metallomacrocycles.2 However, with tin, only very few examples have been reported so far.39 Dithiocarbamate ligands have some structural similarities with carboxylates and their coordination chemistry has been explored extensively.10 Interestingly, their application in metallosupramolecular chemistry is almost unexplored so far, 41 and there are only three reports on structurally characterized macrocycles derived from bis(dithiocarbamates) (Figure 2.5.12).41

H N

N

H

S S S S R Sn R Sn R R S S S S

H

N

N

H

R = Me, nBu, t Bu, Cy, Ph

Figure 2.5.12

N

N S R R

S

S Sn

Sn S

S

S

S

R R S

N

N R= Me, n Bu

Organotin macrocycles derived from bis(dithiocarbamates) as organic connectors

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Tin Chemistry: Fundamentals, Frontiers and Applications a)

c)

(CH2) n-2

R

Sn

Sn

(CH2) n

O O

R'

R'

N

N

N R' N R'

P

M

P

O 2- O O Sn O O

R 2R Sn R

Ph Ph

L'

R = Ph, Cl n = 5-10

O

L'

R

(CH2) n-2

b)

Ph Ph

L'

P Ph O Ph

R' N O

M L'

P O Ph O Ph

L'

O

N O

O 2- O O Sn O

2- R Sn R R

P

M

P Ph

Ph

Ph

L'

Ph

M = Pd, Ag L ' = Br, DMF

R'

R = Cl, Br R ' = Me, Cl; (R')2 = C4H6

Figure 2.5.13

Tin cryptands obtained from

(CH2 ) Y (CH2 ) ,

O Y O

and related connectors

(3) Dinuclear Cryptands. The combination of monoorganotin moieties and (CH2 ) Y (CH2 ) or O Y O connectors gives neutral42 or charged cryptands43,44 (Figure 2.5.13), of which the latter have been applied for the inclusion of metal ions. Halogen-bridged cryptands related to the systems shown in Figure 2.5.13a have also been reported.31f Fourfold-bridged dinuclear tin cryptands are unknown so far. It is possible to modulate gradually the ring size of the cavity when changing the length of the connectors. For the cryptands shown in Figure 2.5.13a the Sn· · ·Sn distances have been varied ˚ 42 from 5.5 to 11.1 A. (4) Macrocycles and Cryptands Formed from Oligonuclear Secondary Building Blocks. Some research groups have developed synthetic methodologies that allow them to link ladder-type secondary building blocks to form either oligonuclear macrocyclic or cryptand structures. Thus, it has been possible to combine the pentanuclear stannoxane ladder shown in Figure 2.5.14a with inorganic bridging functions such as nitrate, carbonate, sulfate, hydrogen phosphate, and trifluoromethanesulfonate.45 The Sn· · ·Sn ˚ In combination with organic conseparations in the macrocyclic structures range from 5.67 to 6.33 A. nectors, such as carboxylic acid derivatives, the macrocycle ring size could be expanded up to 18.3 ˚ 45c,46 There is also an example for a bis(heptanuclear) ladder system, which, in addition to the central A. macrocycle, contains two smaller lateral macrocycles.47 A ladder-derived twin macrocycle has been also reported.48

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a) R X R

Sn

O

O

Sn

Sn O

R

O

R R

O

R R Sn

Sn

X

R

R = CH 2Ph, nBu; X = O, OH L = NO, CO, P(OH)(O), SO2, S(O)CF3, C(O)−(CH2)4−C(O), C(O)−2,4−C6 H4NO2−C(O)−, C(O)−ferrocene−C(O), etc.

O L

Sn L

Sn R Sn R

O

R

O

Cl R

R Cl

O

Sn

O

L

R

O

R

Sn R

Sn

Sn Cl

R Sn

R

R X

R

Cl

L

X O

R

Cl R Sn

R

R

O

Sn

O

Sn

R R

L

R

133

Cl Sn

L Cl

R Sn Cl

R = −CH2tBu, −CH2iPr, −CH2SiMe3 L = −(CH 2)n− with n = 1−10; 1,3−CH2−C6H4−CH2−

Figure 2.5.14 Representative examples for oligonuclear: (a) macrocycles and (b) cryptands that have been prepared from ladder-type secondary organotin building blocks linked through organic and/or inorganic connectors

Furthermore, a series of cryptands have been prepared consisting of ladders of different nuclearity (n = 4, 6, 8) that have been linked pair-wise through a varying number of (CH2 )n (n = 3 12) and related connectors.49 A triple ladder is also known.50 2.5.4

Conclusions and Perspectives

This survey has shown that during the last few years, the number of reports on tin macrocycles and coordination polymers having cavities, pores, or channels has considerably increased. The exploration of organotin-derived assemblies is particularly promising, since the organic substitutents attached to the tin atoms can be employed in different directions such as the modification of functionality, size, and shape of the cavity. Applications in host–guest chemistry, molecular recognition, filtration, sorption, storage, catalysis, transport, etc., are practically unexplored to date.

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40. (a) O.D. Fox, M.G.B. Drew, and P.D. Beer, Angew. Chem. Int. Ed., 39, 136 (2000); (b) O.D. Fox, M.G.B. Drew, E.J.S. Wilkinson, and P.D. Beer, Chem. Commun., 391 (2000); (c) P.D. Beer, N. Berry, M.G.B. Drew, O.D. Fox, M.E. Padilla-Tosta, and S. Patell, Chem. Commun., 199 (2001); (d) S.W. Lai, M.G.B. Drew, and P.D. Beer, J. Organomet. Chem., 89, 637 (2001); (e) L.H. Uppadine, J.M. Weeks, and P.D Beer, Dalton Trans., 3367 (2001); (f) M.E. Padilla-Tosta, O.D. Fox, M.G.B. Drew, and P.D. Beer, Angew. Chem. Int. Ed., 40, 4235 (2001); (g) P.D. Beer, N.G. Berry, A.R. Cowley, E.J. Hayes, E.C. Oates, and W.W.H. Wong, Chem. Commun., 2408 (2003); (h) P.D. Beer, A.G. Cheetham, M.G.B. Drew, O.D. Fox, E.J. Hayes, and T.D. Rolls, Dalton Trans., 603 (2003); (i) P.R.A. Webber, M.G.B. Drew, R. Hibbert, and P.D. Beer, Dalton Trans., 1127 (2004). 41. (a) O.-S. Jung, Y. S. Sohn, and J. A. Ibers, Inorg. Chem., 25, 2273 (1986); (b) W.H. Lee, O.-S. Jung, Y.S. Sohn, P. Kim, Bull. Korean Chem. Soc., 7, 421 (1986); (c) R. Reyes-Mart´ınez, J.A. Guerrero-Alvarez, H. H¨opfl, and H. Tlahuext, Arkivoc, 19 (2008). 42. (a) M. Newcomb, M.T. Blanda, Y. Azuma, and T.J. Delord, Chem. Commun., 1159 (1984); (b) M. Newcomb, J.H. Horner, M.T. Blanda, and P.J. Squattrito, J. Am. Chem. Soc., 111, 6294 (1989); (c) H. Horner, P.J. Squatritto, N. McGuire, J.P. Riebenspies, and M. Newcomb, Organometallics, 10, 1741 (1991). 43. (a) X. Sun, D.W. Johnson, D.L. Caulder, R.E. Powers, K.N. Raymond, and E.H. Wong, Angew. Chem. Int. Ed., 38, 1303 (1999); (b) X. Sun, D.W. Johnson, D.L. Caulder, K.N. Raymond, and E.H. Wong, J. Am. Chem. Soc., 123, 2752 (2001); (c) X. Sun, D.W. Johnson, K.N. Raymond, and E.H. Wong, Inorg. Chem., 40, 4504 (2001). 44. (a) S.V. Lindeman, Y.T. Struchkov, and Y.Z.Voloshin, J. Coord. Chem., 28, 319 (1993); (b) S.V. Lindeman, Y.T. Struchkov, and Y.Z. Voloshin, J. Coord. Chem., 34, 203 (1995); (c) Y.Z. Voloshin, V.K. Belsky, and V.V. Trachevskii, Polyhedron, 11, 1939 (1992). 45. (a) A. Orita, K. Sakamoto, H. Ikeda, J. Xiang, and J. Otera, Chem. Lett., 40 (2001); (b) C.L. Ma, J. Zhang, Q. Jiang, and R. Zhang, Inorg. Chim. Acta, 357, 2791 (2004); (c) G.-L. Zheng, J.-F. Ma, J. Yang, Y.-Y. Li, and X.-R. Hao, Chem. Eur. J., 10, 3761 (2004); (d) D. Ballivet-Tkatchenko, S. Chambrey, R. Keiski, R. Ligabue, L. Plasseraud, P. Richard, and H. Turunen, Catal. Today, 115, 80 (2006). 46. (a) R.-H. Wang, M.-C. Hong, J.-H. Luo, R. Cao, and J.-B. Weng, Eur. J. Inorg. Chem., 2082 (2002); (b) C.L. Ma, and J. Sun, Dalton Trans.,1785 (2004). 47. C.L. Ma, Q. Jiang, R. Zhang, and D.Wang, Dalton Trans., 2975 (2003). 48. R. Zhang, J. Sun, and C.L. Ma, Inorg. Chim. Acta, 357, 4322 (2004). 49. (a) M. Mehring, M. Schurmann, I. Paulus, D. Horn, K. Jurkschat, A. Orita, J. Otera, D. Dakternieks, and A. Duthie, J. Organomet. Chem., 574, 176 (1999); (b) D. Dakternieks, F.S. Kuan, A. Duthie, and E.R.T. Tiekink, Main Group Met. Chem., 23, 731 (2000); (c) B. Costisella, D. Dakternieks, K. Jurkschat, M. Mehring, I. Paulus, and M. Schurmann, Khim. Get. Soedin., SSSR (Russ.) (Chem. Heterocycl. Compd.), 1535 (2001); (d) M. Mehring, I. Paulus, B. Zobel, M. Schurmann, K. Jurkschat, A. Duthie, and D. Dakternieks, Eur. J. Inorg. Chem., 153 (2001); (e) D. Dakternieks, A. Duthie, B. Zobel, K. Jurkschat, M. Schurmann, and E.R.T. Tiekink, Organometallics, 21, 647 (2002); (f) J. Beckmann, D. Dakternieks, A. Duthie, F.S. Kuan, and E.R.T. Tiekink, Organometallics, 22, 4399 (2003); (g) J. Beckmann, D. Dakternieks, A. Duthie, F.S. Kuan, and E.R.T.Tiekink, J. Organomet. Chem., 688, 56 (2003); (h) J. Beckmann, D. Dakternieks, A. Duthie, F.S. Kuan, K. Jurkschat, M. Schurmann, and E.R.T. Tiekink New J. Chem., 28, 1268 (2004). 50. M. Mehring, M. Schurmann, H. Reuter, D. Dakternieks, and K. Jurkschat, Angew. Chem. Int. Ed., 36, 1112 (1997).

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2.6

Deltahedral Zintl Ions of Tin: Synthesis, Structure, and Reactivity

Slavi C. Sevov Department of Chemistry and Biochemistry, University of Notre Dame,USA

2.6.1

Introduction

One little-known area of chemistry is the chemistry of heavy main-group p elements, mostly metals and semimetals, in negative oxidation states. While the chemistry of Sn2+ and Sn4+ , for example, is very well studied and we all learn about it very early in our education, not many people are aware of the fact that tin atoms can accept electrons and exhibit completely different reactivity when negatively charged. For example, catenation and clustering are uncommon for cationic tin, but is the norm for anions, both in inter-metallics and molecular compounds. Thus, cyclopentadienyl-like Sn6− 5 is found in the inter-metallic compounds Na8 AeSn6 (Ae = Ba, Eu), Li9−x AeSn6+x (Ae = Ca, Eu), Li5 Ca7 Sn11 , and Li6 Eu5 Sn9 ,1 isolated tetrahedra of Sn4− 4 and alkali-metal cations constitute the structures of A4 Sn4 (A = alkali metal),2 while Na4 CaSn6 and Li2 Ln5 Sn7 (Ln = Ce, Pr, Sm, Eu) contain infinite anionic chains and heptane-like Sn16− oligomers, respectively.3,4 Notice that, as might be expected, the negative 7 oxidation states are achieved when the more electronegative p element, tin in this case, is combined with much more electropositive s or f element such as the alkali, alkaline-earth, or rare-earth metals. Notice also that, for the purpose of structure rationalization and systematic description, the formal oxidation state assignments assume complete electron transfer from the more electropositive atoms to the p element. Such polar inter-metallic compounds, called Zintl phases when electronically balanced, are known for all post-transition metals and semimetals.5 Negatively charged clusters of some of the p-metals and semimetals are also stable in solutions and can be crystallized from them as ionic molecular compounds. Known, at present, are a number of such ligand-free anionic clusters, called Zintl ions.6 Some examples are the clusters with general formulas 3− 4− 2− Pn3− 7 , E9 , E9 , and E5 where Pn = P, As, Sb and E = Si, Ge, Sn, Pb. This chapter will limit its attention to the synthesis, structure, and functionalization of molecular anionic clusters of tin. Although both nine- and five-atom clusters are known (Figure 2.6.1), the focus will be predominantly on the nine-atom 4− deltahedral clusters Sn3− 9 and Sn9 which are the most readily prepared and most extensively studied species. 2.6.2

Background

The history of the Zintl ions starts in 1891 when Joannis experimented with liquid ammonia solutions of sodium and their reactivity towards heavy p-block elements.7 He noticed that Pb and Sb dissolve in such solutions, and the original blue color (due to the solvated free electrons) changed to green for lead and to brown for antimony. Furthermore, he was able to estimate the ratio of Na : Pb as approximately 1 : 2 and this is very close to 4 : 9 (1 : 2.25) of the nowadays well-known clusters Pb4− 9 . In 1917 this ratio was measured electrolytically more precisely by Smyth and reported as 1 : 2.26.8 In the meantime, Kraus showed that tin also dissolves in such solutions with a similar color change, this time to red.9 However, there was no clear understanding of why and under what form these p elements dissolve. Speculated in some cases was the existence of metal-salt aggregates such as Na4 Sn·Sn8 , Na4 Pb·Pb8 , and Na3 Sb·Sb6 . Ten or so years later, in the 1930s, Edward Zintl conducted a series of more systematic studies of these systems.10 He carried out potentiometric titrations of liquid ammonia solutions of alkali metals with various p metal salts, typically halides. Thus, the titration of a sodium solution with lead(II) iodide revealed that the green anionic species in solution are Pb4− 9 . Zintl and coworkers also discovered that

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b)

3− Figure 2.6.1 The two known anionic clusters of tin: (a) the nine-atom Sn4− 9 and Sn9 , and (b) the five-atom 2− Sn5 . Both clusters are deltahedral, although the nine-atom species has an open square face (the bottom). It is derived from the complete ten-atom deltahedron, a bicapped square antiprism, by removal of one of the capping vertices. The five-atom cluster is a trigonal bipyramid

the same polyatomic anions could be extracted in solutions from binary alloys of the corresponding p element with an alkali metal mixed in appropriate proportions. Based on these extraction studies they proposed the existence of a number of anions in solution although, at the time, it was assumed that the alloys did not contain these anions, an assumption that was proven wrong more than 65 years later.11 3− Some of the proposed species, such as E4− 9 for E = Sn, Pb and Pn7 for Pn = As, Sb, were structurally 3− characterized later by other groups, while the existence of other proposed species, for example Pb4− 7 , Bi7 , 3− 3− 6 Pn5 , Pn3 for Pn = As, Sb, Bi, was never confirmed. Nonetheless, the recently synthesized mixedatom clusters [Bi3 M2 (CO)6 ]3− (M = Cr, Mo), [Bi3 Ni4 (CO)6 ]3− , [Sb3 Ni4 (CO)6 ]3− , and [Bi3 Ni6 (CO)9 ]3− 3− 12 suggest indirectly that the proposed Bi3− 3 and Sb3 may exist in solutions. On the other hand, species 2− 2− like the square-like Bi4 and the double-bonded oxygen-like Bi2 were not originally proposed, but have been structurally characterized.13 The ions mentioned above and, generally, all polyatomic anions of post-transition metals and semimetals are now known as Zintl ions. Among them, the deltahedral Zintl ions form a special subclass of polyatomic clusters with geometries made of triangular faces. At the time of their discovery, however, the shapes of these anions were unknown. It took more than 30 years after Zintl’s work for the first, although partial, structural report to appear. In 1970 Kummer and Diehl reported that, by dissolving an alloy of NaSn2.4−2.5 in ethylenediamine (= en) they could crystallize a compound with an overall formula Na4 Sn9 r7en.14 However, it was Corbett who, in 1975, reported the first single-crystal structure with a deltahedral cluster, that of Sn4− 9 in the compound [Na-(2,2,2-crypt)]4 Sn9 , crystallized by using 2,2,2-crypt (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo-[8.8.8]-hexacosane) as a sequestering agent of the alkali-metal cations, a method that is now the most common in crystallization of Zintl ions.15 A year later, Kummer and Diehl reported the structure of Na4 Sn9 r7en with the same Sn4− 9 deltahedral Zintl ion.16 Most of the effort during the following 25–30 years was focused primarily on improving

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the synthesis and crystallization of Zintl ions, on rationalization of subtle differences in their geometries and electronic structures, and NMR studies of their solutions. In addition to 2,2,2-crypt as a sequestering agent, 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) and even smaller crown ethers were found to help crystallization in some cases as well. The first reaction with deltahedral Zintl ions, which happened to be Sn4− 9 , was reported in 1988 by Eichhorn, Haushalter, and Pennington.17 It was a simple ligand exchange reaction in which the labile 6 ligand of a transition-metal complex was replaced by the Zintl ion. Thus, Sn4− 9 readily replaced the η 4− mesitylene in (mes)Cr(CO)3 and formed the corresponding transition-metal complex [Sn9 M(CO)3 ] . 2.6.3

Geometry, Charge, Electron Count, and Electronic Structure

The most stable clusters of tin, as well as of the rest of the elements in this group except carbon, are, by far, the nine-atom clusters. As mentioned above, Sn4− 9 was the first deltahedral cluster to be structurally characterized. While Ge9 was added very soon after,18 nine-atom clusters of lead and silicon were found respectively 20 and 30 years later.19,20 Thus, the following discussion of geometry, charges, cluster-bonding, and electronic structure is valid not only for tin clusters, but also for those of silicon, germanium, and lead. The bonding in the deltahedral clusters cannot be rationalized with simple 2-center–2-electron bonds because it is achieved through delocalized electrons. These clusters are analogous to the well-known cage-like boranes both structurally and electronically, and they similarly follow the Wade–Mingos rules for electron counting.21 Each BH in the boranes corresponds to a naked atom of group 14 where the B H bonding pair of electrons is replaced by a lone pair of electrons. For example, E2− 5 has 4− 2n + 2 = 12 cluster bonding electrons (each vertex provides two electrons) as closo-B5 H2− 5 , while E9 (E = group 14 element) is a nido-cluster with 2n + 4 = 22 cluster-bonding electrons and corresponds to a nido-B9 H4− 9 . While experimenting with different synthetic and crystallization techniques for the E4− 9 clusters, it was discovered that the same clusters, but with a charge of 3 , i.e. E3− 9 , can be crystallized as well. However, it was not clear what specific conditions lead to one or the other type. Sn3− 9 was characterized initially in [K-(2,2,2-crypt)]3 Sn9 r1.5en and then in [K-(2,2,2-crypt)]6 (Sn9 )2 r1.5en r0.5tol (tol = toluene).22 The re15 16 r ported compounds with Sn4− 9 , on the other hand, are many more: [Na-(2,2,2-crypt)]4 Sn9 , Na4 Sn9 7en, 23 24 25 r r K[K-(2,2,2-crypt)]3 Sn9 , K[K-(18-crown-6)]3 Sn9 en, Rb2 [Rb-(18-crown-6)]2 Sn9 1.5en, Cs7 [K(2,2,2-crypt)](Sn9 )2 r3en,26 [K-(12-crown-4)2 ]2 [K-(12-crown-4)]2 Sn9 r4en,27 and Li4 Sn9 r(NH3 )17 .28 Very similar lists of compounds exist for germanium and lead while for silicon, the cluster with a charge of 4 has not been reported.6 In addition to the clusters with 3 and 4 charges, a silicon cluster 29 with a charge of 2 , i.e. Si2− 9 , was also structurally characterized. Detailed studies carried out for the germanium system revealed that the nine-atom clusters with different charges are in equilibria between themselves and solvated electrons.30 The same equilibria most 3− 2− likely exist for tin, i.e. Sn4− 9 and Sn9 (and possibly Sn9 ) may coexist in solution in an equilibrium such 4− 3− as Sn9 Sn9 + e− (solv). It was realized later that the size and the shape of the available counter cations defines which of these species crystallizes from the solutions. Thus, excess of 2,2,2-crypt in solutions with K+ leads to the presence of only the very large cations [K-(2,2,2-crypt)]+ , and apparently only three such large cations can pack in a crystal lattice with a nine-atom cluster. This means that only Sn3− 9 clusters can be crystallized in these cases and, indeed, this is observed in [K-(2,2,2-crypt)]3 Sn9 r1.5en and [K-(2, 2,2-crypt)]6 (Sn9 )2 r1.5en r0.5tol.22 Deficiency of 2,2,2-crypt, on the other hand, results in the availability of small naked alkali-metal cations. This allows for packing of four cations, some naked and some sequestered, with a cluster anion and this selectively extracts Sn4− 9 in K[K-(2,2,2-crypt)]3 Sn9 and

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Cs7 [K-(2,2,2-crypt)](Sn9 )2 r3en.23,26 Apparently, the flat crown ethers have a similar size and shape effect.24,25,27 In addition, complete absence of a sequestering agent, as in Na4 Sn9 r7en and Li4 Sn9 r(NH3 )17 , logically leads to the same clusters in the solid state.16,28 Finally, the crystallization of Sn4− 9 with [Na-(2,2,2-crypt)] maybe due to the smaller size of the sodium cation and its effect on the overall size of the cryptated cation.15 Five-atom clusters with a charge of 2 are known for all heavier elements of this group,20,31 but their synthesis is quite erratic, i.e. there are no clear guidelines for exactly how to crystallize these species. The first Sn2− 5 clusters, reported by Corbett et al., were crystallized from ethylenediamine solutions of precursors, with nominal compositions anywhere between NaSn and NaSn1.7 .31a The same clusters were later crystallized from a solution of a precursor with a nominal composition Na1.7 Sn.31b Although the 4− nine-atom tin clusters Sn3− 9 and Sn9 are typically extracted from similar solutions of precursors with compositions close to NaSn2.25 , it is not clear how important the different precursor stoichiometries are. 3− Keep in mind that the average charge per tin atom in Sn2− 5 , 0.4 , is between those of Sn9 , 0.33 , and 4− Sn9 , 0.44 , and these numbers do not correlate in any way with the average charge per tin atom in the precursors (assuming complete electron transfer from Na to Sn). The specific geometry and electronic structure of the nine-atom clusters are very well suited for handling different charges with very small structural distortions and for easy inter-conversion between species with different charges. The overall shape of the clusters can be viewed as that of a tricapped trigonal prism (Figure 2.6.2) in which one, two, or three of the trigonal prismatic edges parallel to the three-fold axis (vertical in Figure 2.6.2) are elongated to some extent.6a It has been shown for germanium that lengthening and shortening of these edges involves very little energy, yet greatly affects the electronic structure of the 6a cluster, particularly the energy of the HOMO of the E4− 9 . This orbital (Figure 2.6.3) is bonding within the triangular bases of the trigonal prism, but is antibonding between them, most strongly along the same trigonal prismatic edges that change distance. Thus, one or more long edges reduce the antibonding character of the orbital and it is occupied and the HOMO for E4− 9 , a nido-cluster with 22 cluster-bonding electrons. Shortening of the edge(s) pushes the orbital higher in energy because of increasing antibonding character. It becomes occupied by only one electron in E3− 9 , a radical cluster with 23 cluster-bonding electrons. Further shortening of the edges pushes the orbital even higher in energy, to become empty

Figure 2.6.2 The overall shape of a nine-atom cluster viewed as a tricapped trigonal prism (vertical threefold axis) with one, two, or three elongated trigonal prismatic edges parallel to the three-fold axis. Shown is a cluster with one elongated edge (an open bond), which results in a pseudo-square face. This cluster is identical to the cluster in Figure 2.6.1a, but the pseudo-square face in the latter is at the bottom

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Sng4−

Sng3−

Sng2−

Figure 2.6.3 Schematic MO diagrams (left) of nine-atom tin clusters with different charges and the frontier orbital (encircled in the MOs and shown to the right) that changes energy upon cluster distortion. This orbital 3− 2− 2− 2− is the HOMO in Sn4− 9 , the SOMO in Sn9 , and the LUMO in a hypothetical Sn9 (known are Si9 and Ge9 , although the latter has never been structurally well characterized). Notice that the orbital is π -bonding within the two triangular bases of the trigonal prism (top and bottom triangles), but is σ -antibonding between them along the vertical trigonal prismatic edges. Shortening of the latter increases the antibonding character and pushes the orbital higher in energy as shown

and the LUMO for E2− 9 , a closo-cluster with 20 cluster-bonding electrons. This charge flexibility has been extensively exploited in the case of germanium clusters and it is believed to be responsible for their diverse chemistry of adding a variety of substituents, both nucleophiles and electrophiles.6a Also of interest for later discussion are the three filled orbitals below the HOMO of E4− 9 . These orbitals are shown in Figure 2.6.4 for a cluster with one elongated edge that generates a pseudo-square open face (the front face in Figure 2.6.4). This face can be capped by an additional vertex, in which case one of

Figure 2.6.4 The three filled molecular orbitals below the HOMO in E94− with the cluster shown with its pseudo-square open face in front. If an additional atom is to cap this open face, the MO shown in (a) is perfectly positioned for σ -interactions while those in (b) and (c) can participate in π -interactions

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the three orbitals has the appropriate symmetry for σ -overlap (Figure 2.6.4a), while the other two can form π -bonds (vertical and horizontal in Figures 2.6.4b and c, respectively), with suitable orbitals on the capping vertex. 2.6.4

Reactions With Nine-Atom Deltahedral Zintl Anions of Tin

17 As mentioned already, Sn4− 9 was the very first deltahedral cluster to undergo a reaction of any kind. 4− 4− The reaction, Sn9 + (mes)Cr(CO)3 → [Sn9 Cr(CO)3 ] + (mes), can be described as a simple ligand exchange reaction in which the cluster replaces a six-electron donating group at the Cr atom and forms [Sn9 Cr(CO)3 ]4− (Figure 2.6.5). The Cr(CO)3 fragment coordinates at the open pseudo-square face of the 2 Sn4− 9 cluster. Its three frontier and empty d orbitals z , xz, and yz are perfectly positioned to interact with the three filled cluster orbitals shown in Figure 2.6.4 resulting in a σ -interaction between z 2 and the orbital in Figure 2.6.4a, and π-interactions between xz and yz on Cr(CO)3 and the MOs shown in Figures 2.6.4b and c. Therefore, the cluster behaves as a six-electron donating ligand such as mesitylene, benzene, etc. Focusing on the organometallic part of the anion, these six additional electrons are needed to completing the 18-electron environment at the Cr atom. From cluster perspective, on the other hand, this reaction can be viewed as adding the missing vertex in the nido-Sn4− 9 to form the corresponding closo-species [Sn9 Cr(CO)3 ]4− . The latter has the classical shape for a closo-cluster with ten vertices, namely a bicapped square antiprism, where chromium is one of the two capping atoms. The fragment Cr(CO)3 is isolobal with CH3+ and, therefore, has three empty frontier orbitals and donates zero electrons to the cluster bonding. This means that the capped cluster retains the original 22 cluster-bonding electrons and, with its ten vertices, corresponds to a closo-species according to Wade–Mingos rules.21 Notice that, overall, the original cluster does not undergo a redox process, it just adds a vertex without loss or gain of electrons. It was shown later that similar reactions can be carried out with the heavier congeners in the Cr group, 4− Mo and W. It is now known that both Sn4− 9 and Pb9 can be derivatized with M(CO)3 where M = Cr, Mo, or W to form bicapped square antiprisms where the transition metal caps one of the squares.17,32−35 (It is somewhat surprising that no analogous [Ge9 M(CO)3 ]4− clusters have been characterized to date,

Cr

Figure 2.6.5 [Sn9 Cr(CO)3 ]4− , the first derivative of a deltahedral Zintl anion. The Cr(CO)3 fragment caps the open pseudo-square face of the original Sn4− 9 cluster and does not donate electrons for cluster bonding. The hetero-atomic cluster is a closo-species according to both shape, a bicapped square antiprism, and electron count. The three empty d orbitals z 2 , xz, and yz of Cr(CO)3 overlap with the filled orbitals shown in Figure 2.6.4 and, therefore, the cluster behaves as a six-electron donating ligand

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presumably due to subtle differences in the sizes and electronic properties of the clusters.) However, more recent reports suggest that both [Sn9 M(CO)3 ]4− and [Pb9 M(CO)3 ]4− may be fluxional in solution (below) with the possibility of the M(CO)3 occupying other positions in the cluster.33−35 Rotation of a triangular face of the cluster was suggested as a possible mechanism for such fluxionality, although such a process would be very complex and involve a series of steps, each associated with a bond cleavage or bond creation. It was later structurally proved that another isomer indeed exists for both tin and lead. The transition metal in the new isomers, η5 -[Sn9 M(CO)3 ]4− and η5 -[Pb9 M(CO)3 ]4− , occupies a position in the square antiprism and is five-coordinate (Figure 2.6.6).33,35 Apparently the two tin isomers, η4 -[Sn9 W(CO)3 ]4− and η5 -[Sn9 W(CO)3 ]4− , cocrystallize from the reaction with W(CO)3 (mes). On the other hand, only η4 -[Pb9 Mo(CO)3 ]4− is made by the reaction with the corresponding Mo(CO)3 (mes), while η5 -[Pb9 Mo(CO)3 ]4− is produced exclusively from the reaction with Mo(CO)3 (MeCN).35 Another report of NMR studies in solutions, however, questions whether the two isomers inter-convert from one to another in solution (dynamic model) or are formed irreversibly during the synthesis (static model).34 Let us look more closely at these isomers and try to rationalize some possible mechanisms. As already discussed, the open pseudo-square face of these clusters is the most electron-rich place in the free cluster, and it is logical to assume that strong electrophiles, such as M(CO)3 will coordinate there (Figure 2.6.7). At this point the new intermediate can undergo two different distortions. The lower energy path is to optimize its geometry by perfecting the two capped faces to squares via further elongation of the former long trigonal prismatic edge and shortening of the other two edges, as shown in Figure 2.6.7b. The resulting cluster is the much more common isomer η4 -[E9 M(CO)3 ]4− (Figure 2.6.7c). The second choice for the capped cluster is to undergo a more energy-demanding rearrangement that involves at least one diamond–square–diamond (DSD) transformation (Figure 2.6.7d). This involves breaking the bond between atoms 5–9, as numbered in Figure 2.6.7d, and creating a new bond between the transition metal M and atom 6. A potential second DSD may be occurring at the face made of atoms 1–2–5–6. However,

W

Figure 2.6.6 The isomer η5 -[Sn9 W(CO)3 ]4− , a bicapped square antiprism (the pseudo four-fold axis is vertical), where the transition metal is a part of the square prism and is five-coordinate with the cluster

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M caps square face

b)

c) ?

a) M caps square face (different rearrangement)

d)

e)

Figure 2.6.7 Proposed mechanisms for: (a)→(b)→(c) the formation of the more common isomer η4 [Sn9 M(CO)3 ]4− and (a)→(d)→(e) the more exotic isomer η5 -[Sn9 M(CO)3 ]4−

as already discussed, the nine-atom clusters may have one, two, or three elongated prismatic edges, and therefore atoms 1–6 may already be far apart. The described rearrangement followed by geometry optimization would produce the more exotic isomer η5 -[E9 M(CO)3 ]4− (Figure 2.6.7e). This proposed mechanism is consistent with both the dynamic and static models, i.e. it allows for inter-conversion and equilibrium between the two isomers via the same two intermediates as shown with a question mark in Figure 2.6.7. In addition to the tricarbonyl fragments, tin clusters can also be capped by ZnPh to form [Sn9 ZnPh]3− (Figure 2.6.8).36 The reaction is carried out with ZnPh2 , and the fragment caps the cluster at the same open pseudo-square face. However, unlike the zero-electron donating tricarbonyl fragments, ZnPh is isolobal with CH2+ and donates one electron to the cluster. This results in a lower charge for the resulting species,

Zn

Figure 2.6.8 The ZnPh-capped cluster [E9 ZnPh]3− for E = Si, Ge, Sn, and Pb made by reaction of the clusters with ZnPh2 . The ZnPh fragment is a one-electron donor and reduces the charge of the cluster by one, compared to clusters capped with M(CO)3 . The ZnPh fragment is also an electrophile and caps the same open pseudo-square face as M(CO)3

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i.e. 3– instead of 4–, although the number of cluster-bonding electrons remains 22. There is, however, an example of a capped germanium cluster with 21 bonding electrons, [Ge9 Ni(CO)]3− , where the capping Ni(CO) fragment is a zero-electron donor.37 It suggests that not only E4− 9 clusters can be capped, but also those with a lower charge, E3− . 9 Besides capping with organometallic fragments, the nine-atom deltahedral clusters can undergo a second type of modification, namely insertion of a transition-metal atom inside the cluster. For example, reactions of Pb4− 9 with Pt(PPh3 )4 and Ni(COD)2 (COD = 1,5-cyclooctadiene) produce Pt-centered icosahedra [Pt@Pb12 ]2− and Ni-centered bicapped square antiprisms [Ni@Pb10 ]2− , respectively.38 Clearly, the nine-atom clusters have somehow reassembled into 12- and 10-atom species. This behavior, however, seems to be specific only to lead clusters. Thus, similar reactions of Gen− 9 clusters with Ni(COD)2 39 produce [Ni@Ge9 ]3− , which is a centered version of the empty Ge3− species. It should be pointed 9 out that the inserted atoms of this group, i.e. Ni, Pd, and Pt, have a closed-shell d 10 configuration and do not contribute electrons for cluster-bonding. However, they provide orbitals that overlap with the cluster-bonding orbitals and thus contribute to the overall bonding within the cluster. Although similar centered single-cluster species of tin have not been structurally characterized yet, there is a very good proof that such insertion of a transition-metal atom occurs for them as well. Thus, reaction of tin clusters with Ni(COD)2 produces a dimer of Ni-centered Sn9 clusters fused via a common vertex.40 The anion [Ni2 Sn17 ]4− (Figure 2.6.9) is made of two tricapped trigonal prisms of tin, each of the kind with two elongated prismatic edges (shown lighter in Figure 2.6.9). The two clusters share the tin atom that caps the trigonal prismatic faces with the elongated edges and are positioned at 90◦ with respect to each other. As shown in Figure 2.6.9, the left prism has its pseudo three-fold axis vertical, while the one to the right has its axis along the viewing direction. Finally, the nine-atom tin clusters can undergo a third type of modification which is a combination of the two types already discussed, i.e. insertion of a transition-metal atom combined with capping by a fragment of a transition-metal complex. The insertion of the transition metal is most likely the first step of this process. This has been demonstrated for germanium clusters, where the reaction was carried out stepwise by first inserting an Ni atom and then using the product for capping with a NiCO fragment.37 The inserted transition metal appears to affect the overall electronic structure of the cluster in a subtle way that seems to change the cluster site for preferred coordination of capping fragments, from an open

Figure 2.6.9 The dimer [Ni2 Sn17 ] 4− made of two Ni-centered clusters fused via a common tin vertex. Each half is a tricapped trigonal prism with two elongated prismatic edges (shown lighter), and the common vertex is the atom capping the prismatic face with two elongated edges in each half. The two clusters are rotated around the Ni Sn Ni axis by 90 ◦ with respect to each other, so that the pseudo three-fold axis of the cluster to the left is vertical, while it is along the viewing direction for the cluster to the right

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Figure 2.6.10 The Pt-centered and Pt(PPh3 )-capped [Pt@(Sn9 Pt-PPh3 )]2− . The Sn9 cluster, a tricapped trigonal prism (vertical pseudo three-fold axis), with three elongated edges (shown lighter), is capped at the upper trigonal prismatic base. The capped base opens up as the capping Pt-atom is pressed towards the center of the cluster. The final geometry is very close to spherical, i.e. all atoms, including the capping Pt atom, are at similar distances to the central atom

pseudo-square face to one of the triangular bases of the tricapped trigonal prism. This is observed for all centered and capped germanium clusters [Ni@(Ge9 Ni-PPh3 )]2− ,41 [Ni@(Ge9 Ni-CO)]2− ,37 [Ni@(Ge9 NiC≡CPh)]3− ,37 [Ni@(Ge9 Ni-en)]3− ,37 and the dimer [(Ni@Ge9 )Ni(Ni@Ge9 )]4− ,39 as well as for one of the two known capped and centered tin clusters, [Pt@(Sn9 Pt-PPh3 )]2− (Figure 2.6.10).42 The capping atom in these clusters is apparently pulled towards the center of the cluster, perhaps by interactions with the central atom, to a distance from the center that is similar to those of the nine tin atoms (Figure 2.6.11a, b). This distortion, in turn, causes the capped triangular base to open so much that the corresponding three atoms are no longer in contact with each other (Figure 2.6.11c). The resulting cluster is nearly spherical, with all atoms, including the capping atom, at similar distances from the center. The range of such ˚ 42 distances in [Pt@(Sn9 Pt-PPh3 )]2− is very narrow, 2.70–2.80 A. Electronically, the centered and capped clusters are very similar to the empty clusters and can handle different numbers of cluster-bonding electrons, perhaps by small changes in their geometry. They seem to be most stable with 20 cluster-bonding electrons, a number that corresponds to a capped closo-species, according to the Wade–Mingos rules, which prescribe 2n electrons for such cases. The number of vertices n in [M@(E9 M-L)]2− is 10 and the 20 electrons are provided by the nine atoms of group 14 and the charge of 2 . It should be pointed out that despite its higher charge [Ni@(Ge9 Ni-C≡CPh)]3− has the

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Figure 2.6.11 The two types of capping observed for centered tin clusters: the path (a)→(b)→(c) represents capping of a triangular base of the tricapped trigonal prismatic cluster; the path (a)→(d)→(e) represents capping of the open pseudo-square face of the cluster as observed also for empty clusters (Figure 2.6.7). The capped triangular base in (b) opens up upon insertion of the capping atom towards the center of the cluster, in order to interact with the central atom, and this results in the cluster shown in (c)

same electron count, simply because the one extra negative charge is for the anionic ligand [C≡CPh]− .37 There are, however, two examples of centered and capped clusters that carry 21 electrons, [Ni@(Ge9 Nien)]3− and [Ni@(Sn9 Ni-CO)]3− .37,42 Again, they should be viewed as derivatives of the corresponding empty E3− 9 clusters. While the germanium product conforms to the common geometry described above and shown in Figure 2.6.10, the tin species represents the first and only centered cluster that is capped at the open pseudo-square face, as are the empty clusters (Figure 2.6.12). With just this one example it is difficult to even speculate about the reasons for the different geometry in this case. The formation of the species again most likely starts with insertion of the centering atom followed by capping of the face and pulling in the capping atom by interactions with the central atom (Figure 2.6.11a, d, e). The final geometry is not as spherical as for the species with a capped triangular face because the tin atom opposite ˚ from the center, than the rest of the atoms with distances in the capping Ni atom is further out, at 2.83 A ˚ 42 the range 2.62–2.68 A. 2.6.5

Solution Studies by NMR

The first extensive NMR studies of deltahedral Zintl ions were carried out by Rudolph and coworkers.43 Using 117 Sn, 119 Sn, and 207 Pb NMR they were the first to show that the homo- and hetero-atomic nine-atom deltahedral clusters Sn9 , Pb9 , Sn9−x Gex , and Sn9−x Pbx are fluxional in solution. The homoatomic clusters exhibit single resonances while multiple signals were observed for the hetero-atomic species due to their different stoichiometries, with one signal per stoichiometry. In addition to this, all the clusters show single spin–spin couplings of the types 117 Sn 119 Sn, 117 Sn 207 Pb, and 119 Sn 207 Pb. These observations indicate that, on the NMR timescale, all atoms of the cluster are equivalent by dynamic and

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Ni Ni

Figure 2.6.12 The Ni-centered and Ni(CO)-capped cluster [Ni@(Sn9 Ni-CO)]3− , where the open pseudosquare face of the cluster is capped, as in capped empty clusters (Figures 2.6.5 and 2.6.8). This is the only centered and capped cluster with this geometry, all others have the triangular base of the tricapped trigonal prism capped, as in Figure 2.6.10

fast intra-molecular exchange at room temperature. The absence of a signal for Sn3− 9 can be understood in light of its unpaired electron and paramagnetic character, which will broaden and greatly shift the eventual resonance. Rudolph and coworkers were also the first to explore reactions of nine atom clusters with transitionmetal complexes by NMR, specifically reactions of Sn9 and Pb9 clusters with Pt(PPh3 )4 .44 The 119 Sn and 207 Pb NMR experiments again showed single resonances for the corresponding derivatives, in addition to signals from unreacted E9 clusters. According to the authors, the observed spin–spin couplings between the cluster and 195 Pt suggested a single platinum atom per cluster. Almost 20 years later the product of this reaction with tin clusters was structurally characterized as [Pt@(Sn9 Pt-PPh3 )]2− (discussed above, Figure 2.6.10).42 It was confirmed that in solution the tin atoms are in dynamic exchange while the Pt Pt PPh3 fragment stays rigid, almost like a molten tin drop on a stick. According to the 195 Pt NMR, spin–spin coupling between Sn and Pt occurs only with the central platinum atom and this is consistent with the earlier observations for only one Pt atom per cluster, based on 119 Sn NMR alone.42 Finally, two recent NMR studies of the W(CO)3 capped tin cluster [Sn9 W(CO)3 ]4− (discussed above, Figures 2.6.5 and 2.6.6) suggest different behavior for these species33,34 While Eichhorn et al. imply dynamic behavior and therefore equilibrium between clusters with differently positioned W(CO)3 , i.e. between η4 and η5 coordination,33 the report by Schrobilgen et al. claims that such an interpretation is misleading, due to poor resolution of the NMR spectra.34 The latter group presents both 117 Sn and 119 Sn NMR spectra that show three different environments for the tin atoms and match the theoretically calculated ones for a static cluster. Similar spectra were observed for the analogous tin clusters capped with Cr(CO)3 and Mo(CO)3 .34 The same authors, on the other hand, acknowledge possible dynamic behavior for the Mo(CO)3 -capped lead cluster [Pb9 Mo(CO)3 ]4− , and migration of the Mo(CO)3 from the η4 to the η5 position.34 This was later confirmed by the structural characterization of η5 -[Pb9 Mo(CO)3 ]4− made by reaction of lead clusters with Mo(CO)3 (MeCN).35 Obviously more studies are needed in order to resolve the dispute. One strong piece of evidence in support of the exchange theory is the structurally characterized η5 -[Sn9 W(CO)3 ]4− coexisting with η4 -[Sn9 W(CO)3 ]4− in the reaction product.33

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Concluding Remarks

The nine-atom deltahedral Zintl ions of group 14 are fascinating species in many respects: unusual bonding, aesthetically pleasing and yet flexible geometry, and very rich chemistry. The observed reactivity of the tin clusters with transition-metal compounds is similar to that of germanium clusters and this suggests that many more similarities may be expected between the two systems. The chemistry of the germanium clusters has been much more extensively studied at this stage.6a It has been shown that they participate in additional reactions such as oligo- and polymerization, functionalization with main-group organometallic fragments (SnR3 , GeR3 , SbR2 , BiR2 ) and with various organic groups, addition of alkenes by reactions with alkynes, etc. All this indicates that similar reactions with tin clusters may result in the corresponding functionalized tin analogs or, perhaps, even unexpected and potentially more interesting species. Acknowledgments I would like to thank my former and current coworkers Angel Ugrinov, Jose Goicoechea, Michael Hull, and Donald Chapman for their many discoveries in the chemistry of the Zintl ions. The financial support by the National Science Foundation (CHE-0446131 and CHE-0742365) is greatly appreciated. References 1. (a) Todorov, I. and Sevov, S. C., Inorg. Chem. 2004, 43, 6490; (b) Todorov, I. and Sevov, S. C., Inorg. Chem. 2005, 44, 5361. 2. Sch¨afer, H. and Eisnemann, B., Rev. Mat. Sci. 1985, 15, 1. 3. Todorov, I. and Sevov, S. C., Inorg. Chem. 2006, 45, 4478. 4. Todorov, I. and Sevov, S. C., Inorg. Chem. 2007, 46, 4044. 5. (a) Sevov, S. C. in Intermetallic Compounds Principles and Practice: Progress, Eds. J. H. Westbrook and R. L. Fleischer, John Wiley & Sons, Ltd., Chichester, England, 2002, pp. 113–132; (b) Chemistry, Structure, and Bonding of Zintl Phases and Ions, Ed. S. M. Kauzlarich, VCH Publishers, Inc., New York, NY, 1996; (c) Pottgen, R., Z. Naturforsch. 2006, 61, 677. 6. Recent reviews: (a) Sevov, S. C. and Goicoechea, J. M., Organometallics 2006, 25, 567;. (b) F¨assler, T. F., Coord. Chem. Rev. 2001, 215, 377; (c) Corbett, J. D., Angew. Chem. Int. Ed. 2000, 39, 670; (d) Corbett, J. D., Chem. Rev. 1985, 85, 383; (e) Corbett, J. D., Struct. Bonding 1997, 87, 157. 7. (a) Joannis, A., Hebd. Seances Acad. Sci. 1891, 113, 795; (b) Joannis, A., Hebd. Seances Acad. Sci. 1892, 113, 587; (c) Joannis, A., Ann. Chim. Phys 1906, 7, 75. 8. Smyth F. H., J. Am. Chem. Soc. 1917, 39, 1299. 9. (a) Kraus, C. A., J. Am. Chem. Soc. 1907, 29, 1571; (b) Kraus, C. A., J. Am. Chem. Soc. 1922, 44, 1216; (c) Kraus, C. A., Trans. Am. Electrochem. Soc. 1924, 45, 175; (d) Kraus, C. A., J. Am. Chem. Soc. 1925, 47, 43. 10. (a) Zintl, E., Goubeau, J.and Dullenkopf, W. Z. Phys. Chem., Abt. A 1931, 154, 1; (b) Zintl, E.and Harder, A. Z. Z. Phys. Chem., Abt. A 1931, 154, 47; (c) Zintl, E. and Dullenkopf, W., Z. Phys. Chem., Abt. B 1932, 16, 183; (d) Zintl, E. and Kaiser, H., Z. Anorg. Allg. Chem. 1933, 211, 113; (e) Zintl, E., Harder, A., and Neumayr, S., Z. Phys. Chem., Abt. A 1931, 154, 92. 11. Queneau, V. and Sevov, S. C., Angew. Chem. Int. Ed. Engl. 1997, 36, 1754. 12. (a) Goicoechea, J. M., Hull, M. W., and Sevov, S. C., J. Am. Chem. Soc., in press; (b) Xu, L., Ugrinov, A., and Sevov, S. C., J. Am. Chem. Soc. 2001, 123, 4091. 13. (a) Cisar, A. and Corbett, J. D. Inorg. Chem. 1977, 16, 2482; (b) Xu, L., Bobev, S., El-Bahraoui, J., and Sevov, S. C., J. Am. Chem. Soc. 2000, 122, 1838. 14. Kummer, D. and Diehl, L. Angew. Chem., Int. Ed. Engl. 1970, 9, 895. 15. (a) Corbett, J. D. and Edwards, P. A., J. Chem. Soc., Chem. Commun. 1975, 984; (b) Corbett, J. D. and Edwards, P. A., J. Am. Chem. Soc. 1977, 99, 3313. 16. Diehl, L., Khodadadeh, K., Kummer, D., and Str¨ahle, J., Chem. Ber. 1976, 109, 3404.

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17. Eichhorn B. W., Haushalter, R. C., and Pennington, W. T., J. Am. Chem. Soc. 1988, 110, 8704. 18. Belin, C. H. E., Corbett, J. D., and Cisar, A., J. Am. Chem. Soc. 1977, 99, 7163. 19. (a) F¨assler, T. F. and Hunziker, M., Inorg. Chem. 1994, 33, 5380; (b) Campbell, J., Dixon, D. A., Mercier, H. P. A., and Schrobilgen, G. J., Inorg. Chem. 1995, 34, 5798. 20. Goicoechea, J. M. and Sevov, S. C., J. Am. Chem. Soc. 2004, 126, 6860. 21. (a) Wade, K. J., Adv. Inorg. Chem. Radiochem. 1976, 18, 1; (b) Wade, K., J. Chem. Soc. D 1971, 792; (c) Mingos, D. M. P., Nat. Phys. Sci. 1972, 99, 236; (d) Mingos, D. M. P., Acc. Chem. Res., 1984, 17, 311. 22. (a) Critchlow, S. C. and Corbett, J. D., J. Am. Chem. Soc. 1983, 105, 5715; (b) F¨assler, T. F. and Hunziker, M., Z. Anorg. Allg. Chem. 1996, 622, 837. 23. Burns, R. C. and Corbett, J. D., Inorg. Chem. 1985, 24, 1489. 24. F¨assler, T. F. and Hoffmann, R., Angew. Chem. Int. Ed. 1999, 38, 543. 25. Hauptmann, R. and F¨assler, T. F., Z. Anorg. Allg. Chem. 2002, 628, 1500. 26. Hauptmann, R., Hoffmann, R., and F¨assler, T. F., Z. Anorg. Allg. Chem. 2001, 627, 2220. 27. Ugrinov, A. and Sevov, S. C., Appl. Organomet. Chem. 2003, 17, 373. 28. Korber, N. and Fleischmann, A., J. Chem. Soc., Dalton Trans. 2001, 383. 29. Goicoechea, J. M. and Sevov, S. C., Inorg. Chem. 2005, 44, 2654. 30. Ugrinov, A. and Sevov, S. C., Chem. Eur. J. 2004, 10, 3727. 31. (a) Edwards, P. A. and Corbett, J. D., Inorg. Chem. 1977, 16, 903; (b) Birchall, T., Burns, R. C., Devereux, L. A., and Schrobilgen, G. J., Inorg. Chem. 1985, 24, 890; (c) Campbell, J. and Schrobilgen, G. J. Inorg. Chem. 1997, 36, 4078; (d) Somer, M., Carrillo-Cabrera, W., Peters, E., Peters, K., Kaupp, M., and von Schnering, H. G., Z. Anorg. Allg. Chem. 1999, 625, 37. 32. Eichhorn, B. W. and Haushalter, R. C., J. Chem. Soc., Chem. Commun. 1990, 937. 33. Kesanli, B., Fettinger, J., and Eichhorn, B. W., Chem. Eur. J. 2001, 7, 5277. 34. Campbell, J., Mercier, H. P. A., Franke, H., Santry, D. P., Dixon, D. A., and Schrobilgen, G. J. Inorg. Chem. 2002, 41, 86. 35. Yong, L., Hoffmann, S. D., and F¨assler, T. F., Eur. J. Inorg. Chem. 2005, 3663. 36. Goicoechea, J. M. and Sevov, S. C., Organometallics, 2006, 25, 4530. 37. Goicoechea, J. M. and Sevov, S. C., J. Am. Chem. Soc. 2006, 128, 4155. 38. (a) Esenturk, E. N., Fettinger, J., Lam, Y.-F., and Eichhorn, B. W. Angew. Chem. Int. Ed. 2004, 43, 2132; (b) Esenturk, E. N., Fettinger, J., and Eichhorn, B. W., Chem. Commun. 2005, 247. 39. Goicoechea, J. M. and Sevov, S. C., Angew. Chem. Int. Ed. 2005, 44, 2. 40. Esenturk, E. N., Fettinger, J., and Eichhorn, B. W., J. Am. Chem. Soc. 2006, 128, 12. 41. (a) Gardner, D. R., Fettinger, J., and Eichhorn, B. W., Angew. Chem., Int. Ed. Engl. 1996, 35, 2852; (b) Esenturk, E. N., Fettinger J., and Eichhorn B. W., Polyhedron, 2006, 25, 521. 42. Kesanli, B., Fettinger, J., Gardner, D. R., and Eichhorn, B. W., J. Am. Chem. Soc. 2002, 124, 4779. 43. (a) Rudolph, R. W., Wilson, W. L., Parker, F., Taylor, R. C., and Young, D. C. J. Am. Chem. Soc. 1978, 100, 4629; (b) Rudolph, R. W., Taylor, R. C., and Young, D. C. J. Am. Chem. Soc. 1981, 103, 2480; (c) Wilson, W. L., Rudolph, R. W., Lohr, L. L., Taylor, R. C., and Pyykko, P., Inorg. Chem. 1986, 25, 1535. 44. Teixidor, F., Luetkens, M. L., Jr., and Rudolph, R. W., J. Am. Chem. Soc. 1983, 105, 149.

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2.7

Stable Stannylium Cations in Condensed Phases

Joseph B. Lambert Department of Chemistry, Northwestern University, Evanston, Illinois , USA

2.7.1

Introduction

Tricoordinate tin, until recently, has been unknown in condensed phases, due to its high electrophilicity. Thermodynamic stability in the gas phase testifies to the kinetic roots of its instability, particularly in the liquid phase. The ideal tricoordinate geometry is planar and trigonal, as viewed in 1 from above or below, and in 2 from the side on. R

R Sn

R

Sn

R R

R

1

2

It is expected that each R–Sn–R angle is very close to 120◦ , so that they must sum to 360◦ . The empty 4p orbital should have identical, large lobes above and below the plane defined by the tin atom and its three attached atoms. Such materials have been assigned the name stannylium ions by analogy with the 1993 IUPAC recommendations for carbylium (e.g., methylium), silylium, and germylium ions.1 These terms imply cationic trivalency and are to be distinguished from methanium, silanium, and stannanium, which imply cationic pentavalency.1 Convenient general terms, without implications of coordination number or geometry, are carbocation, silyl cation, germyl cation, and stannyl cation. As will become evident, current nomenclature does not provide accurate coordination or geometric information for some cationic organotin forms. The long and tortuous path to the preparation of stable tricoordinate silicon cations (silylium ions) in condensed phases2,3 presaged the creation of analogous germanium and tin species. In hindsight, four technical breakthroughs were required for the preparation of stable silylium ions, and consequently for stable stannylium ions: (1) solvents of low nucleophilicity, capable of dissolving ionic species; (2) anions of very low nucleophilicity; (3) substituents that stabilize positive charge by resonance (mesomerism), while also sterically hindering the approach of nucleophiles; and (4) novel methodologies for generation of the cation. Although numerous solvents have long been known that are capable of creating and dissolving organic ions, for the most part they are appreciably nucleophilic. Thus classic solvolysis experiments studied carbocations in water, alcohols, and carboxylic acids, which all proved entirely inappropriate for silylium and stannylium ions because of their nucleophilic (i.e., silaphilic or stannaphilic) properties. Various so-called ‘magic acid’ formulations proved equally ineffective, because of the presence of oxygen and halogen (particularly fluoro) functionalities with strong nucleophilicity. Dipolar, aprotic (more aptly, nonhydroxylic) solvents, such as acetonitrile and hexamethylphosphoramide (HMPA) invariably led to some kind of coordination between the solvent and the silicon or tin center. Early experiments with solvents of low nucleophilicity emphasized halogenated hydrocarbons, dichloromethane in particular, which have proved successful in some cases. Probably the most important breakthrough was the observation, reported in 1992, that aromatic solvents such as benzene dissolve stannyl cations.4 Although this observation was made by Lambert and Kuhlmann in the context of stannyl cations, it also was critical in the preparation of the first stable silylium ions in condensed phases. All successful preparations of stable, free stannylium

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ions have used aromatic solvents. These solvents, however, were not successful without careful choice of substituents on tin, as well as of the anion. Much research has been carried out in many contexts to create anions with low nucleophilicity.5 In the specific context of low valent silyl cations, the anion families that have proved most successful have been perfluoroaromatics introduced by Lambert and Zhang6 and carboranes introduced by Reed and coworkers,7 reported in adjacent publications in 1993. Prior to use of these anions, unsuccessful investigations were reported with anions such as perchlorate, tetrafluoroborate, tetraphenylborate, and hexachloroantimonate. All successful preparations of stannylium cations have used perfluoroaromatic anions, particularly tetrakis(pentafluorophenyl)borate (TPFPB). Coordination with solvent or anion can generate several different geometries. When a stannylium ion coordinates with a neutral solvent molecule S, either a tetracoordinate species (3, referred to as a solvated stannyl cation) or a pentacoordinate species (4) can form. Although these materials qualify as stannyl S

S

R R

Sn

R

Sn R

R R

S

3

4

cations, they are not stannylium ions, which require tricoordination. The neutral solvent S brings in additional coordination, but does not change the overall charge. An anion A− can produce similar species (5 and 6), but with altered charge. The tetracoordinate species 5 has its formal charge neutralized by the A Sn R

R

Sn

R R

5

A

A R R

R

Sn

S

A

6

7

R R

presence of the anion. The pentacoordinate species 6 contains one solvent molecule and one anion, so charge is also neutralized. Pentacoordination also can involve a pair of anions, leading to 7, which is anionic. None of these three species, therefore, is a stannyl cation, much less a stannylium cation. It is noted that the geometry around tin in 4, 6, and 7 is trigonal with respect to the organic ligands R, with some analogy to the stannylium ion 2, but with variable charge and higher coordination. The nature of the bonding between tin and either solvent S or anion A may be more complex than implied by structures 3–7, in which only formal charge is depicted. In the first place, full bonding may not occur between the tin atom and either the solvent S or the anion A (8–12). As a result, the Sn S or the Sn A bond order decreases, as the bond length increases. Secondly, more positive charge is present on tin in these five structures (and more negative charge on the anion), compared with the fully bonded analogs 3–7. Thirdly, in forms 8 and 9, the geometry around tin flattens with respect to that in 3 or 5. Thus, whereas the sum of the R Sn R angles may be ca. 328◦ (three tetrahedral angles) for 3 and 5, it occurs variably within the range >328◦ to <360◦ for 8 and 9. Such molecules have intermediate bonding and should be considered to have some stannylium cation character. Systems 10–12 have lengthened bonds to solvent or anion within the pentacoordinated geometry, and higher positive charge density on tin, than in 4, 6, and 7. These species (10–12) are not stannylium cations, because tin has higher coordination than

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S

R

Sn

R

R

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9

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Sn

R R

R

A

R R

R

Sn

A

R R

R

Sn

S

S

A

10

11

12

R R

three. The geometry around tin, however, remains trigonal. There is no accepted nomenclature for these variants. 2.7.2

Pentacoordination (Trigonal Pyramids)

The trigonal bipyramids illustrated by 4, 6, and 7 (or 10–12) with solvent S or anion A− in the apical positions have been found often, particularly when at least one apical atom is highly electron withdrawing (O, N, F, Cl).8 Such pentacoordinate structures are not of central interest in the current context, but some examples are worth discussing. Two nuclear magnetic resonance criteria have been used extensively.9 Decreased coordination from the ideal tetragonal geometry of tetramethyltin generally moves the 119 Sn chemical shift to a larger positive value (higher frequency, lower field, deshielded), as expected for a pure, trigonal stannylium ion 1 or its partial version 8. Increased coordination, as in the various trigonal bipyramids, results in larger negative values (lower frequency even to negative values, higher field, shielded). A second important NMR criterion is the one bond 1 J (119 Sn–13 C) coupling constant, which responds to the geometry around tin. Flattening of the geometry to the extreme of sp2 hybridization, as expected either in a silylium ion 1 or in the trigonal bipyramids (4, 6, 7, 10–12), results in an increase in the coupling constant, with respect to tetrahedral models (3, 5, 8, 9).10 Thus for trigonal bipyramidal systems, we expect 119 Sn chemical shifts that move into the negative range and one-bond coupling constants with increased magnitude. One of the earliest claims for a free stannylium ion was the very simple system SnH+ 3 , formed by reaction of tin tetrahydride with fluorosulfonic acid at −78 ◦ C.11 The resulting ion exhibited a chemical shift of δ−186. The negative chemical shift, however, indicates higher, not lower, coordination. Cremer et al. modeled the system with coordination to one or two water molecules and concluded that pentacoordination was likely.12 Bagno et al. carried out more definitive calculations, not only at higher levels, relativistically, and with spin–orbit coupling, but also by modeling coordination with fluorosulfonic acid rather than water.13 They calculated the 119 Sn chemical shift for SnH3 (FSO3 H)+ 2 to be δ−142, in good agreement with the observed value. This early, proposed stannylium ion11 therefore proved to be pentacoordinate, presumably with a pair of apical fluorosulfonic acid molecules coordinated through oxygen, i.e., 4 or 10. In another early study, Edlund et al. were able to demonstrate the existence of an equilibrium between tetracoordinated (3 or 8) and pentacoordinated (4 or 10) stannyl cations.14 They prepared their materials

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by the method used by Lambert et al. in attempts to generate silylium cations, by reaction of tin hydrides with the trityl cation (R3 Sn—H + Ph3 C+ → Ph3 C—H + ‘R3 Sn+ ’). For tributyltin hydride, the chemical shift of the resulting species, with perchlorate as the anion, varied with solvent: δ 156 (CH2 Cl2 ), 54 (CH3 CN), and −43 (HMPA). With tetrafluoroborate as the anion, the respective figures were δ 220, 44, and −44. Although definitive structures could not be described, the trend clearly suggests that the species with the less coordinating solvent, dichloromethane, is tetracoordinate (3 or, more likely, 8). A second solvent molecule is definitely introduced with HMPA and is likely for acetonitrile (4 or 10). The chemical shifts for the materials formed from triphenyltin hydride were in the range δ−211 to −276 for both anions, with the lower negative end of this range for acetonitrile and the higher negative end for HMPA. These materials must be pentacoordinate. The authors also measured the 1 J (119 Sn–13 C) coupling constants for the product for tributyltin hydride with perchlorate as the anion. The values were 339 (CH2 Cl2 ), 424 (CH3 CN), and 491 (HMPA) Hz, indicating geometrical alterations towards the expected sp2 hybridization (3 → 4 or 8 →10) as the coupling constant increases. Another purported tin cation was produced by the reaction of CB11 Me12 r with Bu6 Sn2 .15 Although the authors referred to the product as Bu3 Sn+ CB11 Me− 12 , the X-ray structure revealed an infinite column of alternating anions and cations. A methyl group of the anion was present at the apical positions, above ˚ compared with a value of and below the plane of the tributyltin portion. The Sn–C distance is 2.81 A, ˚ 2.14 A for the normal Sn–C covalent bond length. These values suggest that the best representation of this species in the solid is 12. The 119 Sn chemical shift was observed to be δ 454, whereas the value expected for trialkylstannylium ions is closer to δ 1700, as discussed in the next two sections. This slightly deshielded value indicates residual bonding to the anion, again as in 12. This material thus resembles the pentacoordinate structure 12 in both the solid and solution. 2.7.3

Tetracoordination: Binding with Solvent or Anion

11 119 In the same study as claims for SnH+ Sn chemical 3 , Birchall and Manivannan also reported that the + shift for a purported SnMe3 ion was δ 322. This material was prepared by the reaction of trimethyltin hydride with fluorosulfonic acid. At the time (1985), this value could be considered unusually deshielded, so that this species clearly had at least some stannylium ion character. No further structural data, however, were available. In 1992, Lambert and Kuhlmann reported several different methods in attempts to generate stannylium ions.4 The most successful, and novel, was the reaction of tributyltin hydride with tris(pentafluorophenyl)borane, which abstracts the hydride to create an anion H(C6 F5 )3 B− with very low nucleophilicity. In a second approach to reducing the overall nucleophilicity of the environment, they used benzene for the solvent, the first example of the use of aromatic solvents in the silylium/germylium/stannylium context. The 119 Sn chemical shift for this tributyl system in benzene proved to be δ 360, the most deshielded value at the time. The perchlorate of the same substrate (prepared by hydride abstraction from tributyltin hydride with trityl perchlorate) exhibited a shift of δ 245 in CD2 Cl2 , 231 in benzene, and 150 in sulfolane. Trimethyltin hydride, when reacted with trityl perchlorate, gave a species with chemical shifts of δ 249 in CD2 Cl2 and δ 234 in benzene. The authors ambiguously called these species tricoordinated, as well as perturbed by solvent, which requires a fourth coordination. Shortly thereafter (1994), Kira, Sakurai, and coworkers reported a very similar reaction of tributhyltin hydride with trityl tetrakis[3,5-di(trifluoromethyl)phenyl]borate in CD2 Cl2 .16 The resulting species, which they called a trivalent tin cation, had a chemical shift of δ 356 at −20 ◦ C. The material decomposed above this temperature and reacted with ethyl ether at −70 ◦ C to give a more solvated species [‘Bu3 Sn(Et2 O)+ ’] with a chemical shift of δ 165 at −20 ◦ C.

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The difficulty in interpreting these results, at the time, arose from the lack of reliable points of comparison for the expected 119 Sn chemical shifts of stannylium ions. There was no doubt that values higher than 300 were deshielded to an unprecedented extent, but were these shifts sufficient to demonstrate tricoordination or trivalency? Computation in the early 1990s could not provide a reliable answer. Arshadi et al.10 bypassed the calculational problem in 1996 by publishing a remarkable empirical correlation between structurally analogous silicon and tin compounds (29 Si chemical shift vs 119 Sn chemical shift). Since reliable calculations were available for the 29 Si chemical shifts of trialkylsilylium ions, this plot could provide at least an indication of the expected 119 Sn chemical shifts of trialkylstannylium ions, which proved to be δ ca. 1700. The species observed by Birchall, Lambert, and Sakurai thus were very far from the expected chemical shift and hence from the ideal tricoordinate geometry of the stannylium ion. Since the values are deshielded to some extent, pentacoordination could be ruled out. The best description of the structures observed by all these authors, therefore, is the bond-stretched, solvent-coordinated stannyl cation 8. Lambert and Kuhlmann observed high conductivity, so that the neutral anion-coordinated variant 9 could be eliminated. Such structures (8) also apply to those reported in 1992 by Edlund et al.14 as the tetrahedral part of the equilibrium with pentacoordinate species. The most deshielded material within this category (8) was reported in 2002 by Piers and coworkers.17 They used a novel, highly fluorinated borate anion, the solvent benzene, and hydride abstraction with trityl to produce the species Bu3 Sn(benzene)+ with a 119 Sn chemical shift of δ 434. By this time, authors were referring to such species as solvated stannyl cations. The first X-ray structure of a species 8 was reported by Lambert et al. in 1995.18 Reaction of triethylstannane with trityl tetrakis(perfluorophenyl)borate (TPFPB) in benzene produced a material best represented as Et3 Sn(benzene)+ , which reacted with water to produce (Et3 Sn)2 OH+ TPFPB− . The Sn–O ˚ Whereas the Sn–C length is normal [as in (Ph3 Sn)2 O with 2.14 and Sn–C bond lengths were both 2.12 A. ˚ ˚ in (Ph3 Sn)2 O]. As calculated from A], the Sn–O bond clearly is lengthened [in comparison with 1.96 A ˚ the Pauling equation, the lengthening of 0.16 A corresponds to an Sn–O covalent bond order of only 0.54 (or ionic bond order of 0.46). The C–Sn–C angles sum to 352.8◦ , indicating a very flat tetrahedron, as in 8 rather than 3. This family of stannyl cations (8) has a flattened tetrahedral structure and somewhat deshielded 119 Sn chemical shifts (values up to δ 434). The bond between tin and the fourth coordination site is lengthened, and tin clearly has considerable positive charge density. Although one may speak of stannylium ion character, these materials, nonetheless, are not free, tricoordinate stannylium ions. 2.7.4

Tricoordination: NMR Evidence for Free Stannylium Ions

Lambert and Zhao reported preparation of a free, tricoordinate silylium ion in 1997, based on NMR chemical shift data.19 They used their technique of nucleophilic isolation provided by aromatic solvents and by the TPFPB anion. In addition, the substituents on silicon were bulked up by the use of 2,4,6trimethylphenyl (mesityl) to prevent access of even the weak solvent and anion nucleophiles to the silicon atom. Hydride abstraction had proved impossible with trimesitylsilane, because of steric hindrance to access of electrophiles. Consequently, they developed a novel method to generate the cation by using allyl as the leaving group instead of hydride. This procedure is illustrated in Equation (2.7.1) with tin instead of silicon. + Mes3 SnCH2 CH CH2 + E+ → Mes3 SnCH+ 2 CHCH2 E → Mes3 Sn + CH2 CHCH2 E

(2.7.1)

The allyl group extends outside the steric sphere imposed by the mesityl groups, so that an electrophile such as trityl has easy access to the double bond. The tetracoordinate tin intermediate shown in the equation is sterically relieved as it expels the neutral leaving group to produce the stannylium (or silylium

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or germylium) ion. The observed 29 Si chemical shift δ 225 of the silylium ion formed in this way agreed closely with that of calculations (δ 230).20 This very successful method was applied to tin, according to Equation (2.7.1), in 1999 by Lambert et al.21 Allyltributylstannane was treated with trityl TPFPB in benzene to produce a species with a 119 Sn chemical shift of only δ 263, the same value as when hydride was abstracted from tributyltin hydride. Such a species is clearly solvated, Bu3 Sn(benzene)+ (an example of 8), and possesses possibly 20% stannylium character. The butyl groups are insufficient to prohibit partial coordination with solvent. Reaction of the much bulkier allyltrimesitylstannane with electrophiles, however, produced a species with a 119 Sn chemical shift of δ 806. Such a value is much further deshielded than all previous examples. Moreover, the normal shielding effect of the aryl rings should offset some of the cationic deshielding and cause much lower values (more shielded) than for the trialkyl cases. Unfortunately, no 119 Sn chemical shift calculation was known at the time, so the authors only had recourse to the empirical correlation between silicon and tin chemical shifts.10 If this correlation is strictly linear, the expected 119 Sn chemical shift for trimesitylstannylium should be about δ 1100, as calculated from the observed 29 Si value for trimesitylsilylium of δ 225. Since this end of the plot could well be non-linear, the estimate of 1100 could be either too high or too low. The linear assumption indicated that the authors had achieved an ion with 70–80% stannylium character, but not a free stannylium ion. To test whether the chemical shift was being shielded by interaction with solvent, the authors altered the nucleophilicity of the solvent.21 They found the value of δ 806 to be invariant to changes to better donors such as toluene and p-xylene or to worse donors such as 1,2-dichlorobenzene. These results seemed to indicate little solvent interaction. They also succeeded in synthesizing allylphenyldi(2,4,6triisopropylphenyl)stannane,21 which on treatment with electrophiles produced a cation with a 119 Sn chemical shift of δ 697, more shielded than the trimesityl case. The presence of the single phenyl group, however, may permit better access of solvent, even with the two bulkier groups. In 2001, Lambert and Lin examined the duryl substituent in this context.22 It was thought that the second (meta) pair of methyl groups would provide steric buttressing to prevent the ortho methyl groups from bending away from the tin atom, thereby enhancing steric protection around tin in comparison with the effect of mesityl. The tridurylsilylium cation proved to have a nearly identical 29 Si chemical shift (δ 227) to that of trimesitylsilylium (δ 225), indicating similar silylium ion character. Treatment of allyltridurylstannane with electrophiles produced a cation with a 119 Sn chemical shift of only δ 720, more shielded than that of trimesitylstannylium (δ 806). This puzzling result seemed to indicate lower stannylium character. The NMR results alone were inconclusive. Clearly, highly deshielded chemical shifts were being observed, indicative of high stannylium ion character. In the absence of a good point of reference from reliable calculations, however, it was impossible to conclude whether these were entirely free (1) or partially solvated (8) stannylium ions. 2.7.5

Tricoordination: Crystallographic Evidence and Computational Confirmation of Free Stannylium Ions

Lambert and coworkers reported the first crystallographic evidence for a free stannylium ion in 2003.23 Their breakthrough was the synthesis of the highly crowded starting material allyltris(2,4,6triisopropylphenyl)stannane. The X-ray structure revealed a tricoordinate species of the type 1 without interaction between tin and either the solvent benzene or the anion TPFPB. No atom from the anion ˚ and no solvent was present. The sum of the three C Sn C angles was 359.9(2)◦ , was closer than 4 A, experimentally planar. The angle of twist of the aryl rings with respect to the plane was 61.1◦ , somewhat

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higher than the twist angle in trimesitylsilylium of 49.2◦ . The observed 119 Sn chemical shift in solution was δ 714, yet another value apparently short of the expected δ ca. 1100 from the empirical correlation.10 High-level calculations, however, were now possible. In the gas phase, the calculated value proved to be δ 763. The authors commented that, ‘Calculations at this level are systematically higher frequencies than are observation values.’ They concluded that the observed chemical shift was entirely in accord with a free, tricoordinate stannylium ion, unencumbered by any interaction with solvent or anion. The upper (deshielded) limit for triarylstannylium ions thus proved to be δ ca. 700–800 rather than δ ca. 1100. The 29 Si/119 Sn correlation10 must be non-linear at the high frequency end. Such a conclusion suggested that the previously observed trimesitylstannylium (δ 806), tridurylstannylium (δ 720), and phenyldi(2,4,6-triisopropylphenyl)stannylium (δ 697) ions were also entirely free. The later and even higher-level calculations of Bagno et al. confirmed this conclusion.13 They calculated a value of δ 840 for trimesitylstannylium at the scalar level and of δ 910 at the spin–orbit level. Both values are in good agreement with the observation (δ 806). In general, their calculations gave slightly more deshielded values than did those observed for all tin species. They presented a plot of calculations at the spin–orbit level vs. observation for the 199 Sn chemical shifts, which was remarkably linear for a wide range of tin structures. The value for the trimesitylstannylium ion was the deshielded extreme on this plot and fell very close to the linear fit. These calculations on molecules in the gas phase provide strong support that the observed trimesitylstannylium ion, for which no crystal structure is available, is indeed free and tricoordinate, as likely are tridurylstannylium22 and phenyldi(2,4,6-triisopropylphenyl)stannylium.21 In a subsequent study, Lambert, Herber, and their coworkers reported, by M¨ossbauer spectroscopy, that the motion of the tin atom within tris(2,4,6-triisopropylphenyl)stannylium tetrakis (pentafluorophenyl)borate is isotropic.24 Very soon after the above report of the first stable stannylium ion, Sekiguchi et al. reported the preparation of a stable stannylium ion by an entirely novel method of generation.25 They first prepared the stable free radical, tris(di-tert-butylmethylsilyl)stannyl [(t-Bu2 MeSi)3 Sn]. Treatment of this material with trityl TPFPB in benzene (thus using the same methods of nucleophilic isolation as previously developed) led to the stable tris(di-tert-butylmethylsilyl)stannylium ion. The sum of the three C–Sn–C angles was 360.0◦ , indicative of planarity and trigonality. Closest approach to atoms of the anion was greater than ˚ The 119 Sn chemical shift proved to be a very remarkable δ 2653 in dichloromethane. Deshielding 5 A. was enhanced by the silyl substituents (and by the absence of aryl substituents). The cation was modeled with trisilylstannylium ion [(H3 Si)3 Sn+ ], for which a chemical shift of δ 2841 was calculated, in good agreement, considering the usual overestimation associated with these calculations. 2.7.6

Summary

Several stable, tricoordinate aryl-substituted stannylium ions have been prepared by Lambert and coworkers,21−23 and a stable tricoordinate silyl-substituted stannylium ion has been prepared by Sekiguchi and coworkers.25 These materials were prepared respectively by the allyl-leaving group and radical oxidation methods. Both groups employed nucleophilic isolation by use of aromatic solvents and the TPFPB anion, and both groups used substituents that were both electronically stabilizing and sterically bulky to prohibit approach of solvent or anion. The cations were characterized by X-ray crystallography, NMR chemical shifts, and supporting calculations. Eleven years had elapsed since the breakthrough of aromatic solvents,4 which had been followed quickly by the development of perfluorinated anions6 and much later of the allyl leaving group method.19 Many years were required before all the pieces were in place, but there is no doubt now that tricoordinate stannylium ions may be prepared that are stable at room temperature in both liquid and solid phases.

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References 1. Paragraphs RC-82.2.2.1 and RC-82.1.1.1, A Guide to IUPAC Nomenclature of Organic Compounds (Recommendations 1993), Blackwell Scientific Publications, Oxford, UK, 1993. 2. J. B. Lambert, L. Kania, and S. Zhang, Chem. Rev., 95, 1191 (1995). 3. J. B. Lambert, Y. Zhao, and S. M. Zhang, J. Phys. Org. Chem., 14, 370 (2001). 4. J. B. Lambert and B. Kuhlmann, J. Chem. Soc., Chem. Commun., 931 (1992). 5. S. H. Strauss, Chem. Rev., 93, 927 (1993). 6. J. B. Lambert and S. Zhang, J. Chem. Soc., Chem. Commun., 383 (1993). 7. Z. Xie, D. J. Liston, T. Jelinek, V. Mitro, R. Bau, and C. A. Reed, J. Chem. Soc., Chem. Commun., 384 (1993). 8. Davies, A. G. Organotin Chemistry, 2nd ed., Wiley-VCH, Weinheim, Germany, 2004. 9. B. Wrackmeyer, Annu. Rep. NMR Spectrosc., 38, 203 (1999). 10. M. Arshadi, D. Johnels, and U. Edlund, Chem. Commun., 1279 (1996). 11. T. Birchall and V. Manivannan, J. Chem. Soc., Dalton Trans., 2671 (1985). 12. D. Cremer, L. Olsson, R. Reichel, and E. Kraka, Isr. J. Chem., 33, 369 (1993). 13. A. Bagno, G. Casella, and G. Saiella, J. Chem. Theory Comp., 21, 37 (2006). 14. U. Edlund, M. Arshadi, and J. Johnela, J. Organomet. Chem., 456, 57 (1993). 15. I. Zharov, B. T. King, Z. Havlac, A. Pardi, and J. Michl, J. Am. Chem. Soc., 122, 10253 (2000). 16. M. Kira, T. Oyamada, and H. Sakurai, J. Organomet. Chem., 471, C4 (1994). 17. L. D. Henderson, W. E. Piers, G. J. Irvine, and R. McDonald, Organometallics, 21, 340 (2002). 18. J. B. Lambert, S. M. Ciro, and C. L. Stern, J. Organomet. Chem., 499, 49 (1995). 19. J. B. Lambert and Y. Zhao, Angew. Chem., Int. Ed. Engl., 36, 400 (1997). 20. T. M¨uller, Y. Zhao, and J. B. Lambert, Organometallics, 17, 278 (1998). 21. J. B. Lambert, Y. Zhao, H. Wu, W. C. Tse, and B. Kuhlmann, J. Am. Chem. Soc., 121, 5001 (1999). 22. J. B. Lambert and L. Lin, J. Org. Chem., 66, 8537 (2001). 23. J. B. Lambert, L. Lin, S. Keinan, and T. M¨uller, J. Am. Chem. Soc., 125, 6022 (2003). 24. J. B. Lambert, L. Lin, C. Nowik, and R. H. Herber, Inorg. Chem., 43, 405 (2004). 25. A. Sekiguchi, T. Fukawa, V. Y. Lee, and M. Nakamoto, J. Am. Chem. Soc., 125, 9250 (2003).

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2.8

Preparation and Coordination Chemistry of Mono- and Bidentate Benzannulated N-Heterocyclic Stannylenes Including Some Germanium and Lead Analogs

Alexander V. Zabula and F. Ekkehardt Hahn Institut f¨ur Anorganische und Analytische Chemie, Westf¨alische Wilhelms-Universit¨at, M¨unster, Germany

2.8.1

Introduction

Stannylenes and their lighter silicon and germanium, as well as the heavier lead analogs are singlet species of divalent silicon, germanium, tin, or lead, which possess an unshared electron pair in the ground state and the vacant p-orbital of the divalent group 14 elements. Particularly in the case of tin and germanium, they are key intermediates in numerous reactions and are most likely more significant for the chemistry of these elements than carbenes are in organic chemistry. Germylenes and stannylenes were first considered as heavy analogs of carbenes in 1962 by Volpin.1 Later, Nefedov developed and expanded this description.2 Although simple germylenes and stannylenes are reactive species that can be characterized by low-temperature spectroscopic techniques, or by the products of their reactions,3 the use of sterically demanding groups or/and substitution of germanium and tin atoms with heteroatoms (N, O, S) significantly stabilizes the reactive divalent state and a great number of germylenes and stannylenes, stable at room temperature, have been isolated over the last 30 years.4 Due to their electronic structure, these analogs of carbenes are capable of forming complexes with Lewis bases and Lewis acids (Scheme 2.8.1). Like carbenes, they are also prospective ligands in transition-metal complexes.

Scheme 2.8.1 Electronic structure of dialkyl- and diamino-substituted carbene analogs and their adducts with Lewis bases and Lewis acids

2.8.2

Stable N-Heterocyclic Stannylenes, Germylenes, and Plumbylenes

The longest known stable tin(II) organyl-bis(cyclopentadienyl)tin, or stannocene, was synthesized by Fischer and Grubert in 19565 by the reaction between CpLi and SnCl2 . The molecular structure determination showed the angle between the normals to the Cp rings to measure around 145◦5 which was taken as an indication of the presence of an unshared electron pair at the tin atom. However, a strong dependence of the (R5 )Cp–Sn–Cp(R5 ) angle on the R substituents (Me, Bz, Ph) at the cyclopentadienyl rings5c,6 with

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angles up to 180◦6b was later found. The first stable dialkyl7 and diaminostannylenes A7,8 (Scheme 2.8.2), with sterically demanding groups that provide stabilization of the tin atom, were synthesized by Lappert. N-heterocyclic stannylenes (NHSn) of type B were first reported by Veith.9 These cyclic stannylenes are monomeric or dimeric in solution and in the solid state, depending on the N -substituents R. Some years thereafter the cyclic diaminostannylenes C H (Scheme 2.8.2), which contain the tin atom as part of an aromatic system have been described. They are normally obtained from the corresponding amines by a transamination reaction using Sn[N(SiMe3 )2 ]2 , or by the reaction of the lithiated amines with SnCl2 (Scheme 2.8.2).

Scheme 2.8.2

Preparation and types of N-substituted diaminostannylenes

The first benzimidazolin-2-stannylenes of type C (R = SiMe3 , SiMe2 tBu, CH2 tBu) were synthesized and characterized in 1995.10,11 The presence of bulky N ,N -substituents led to an efficient shielding of the tin(II) center. A different type of benzannulated stannylene D with a pyrido-annulated five-membered ring was prepared by Heinecke et al.12 The diaminostannylene E, which is an analog of Arduengo’s carbene,

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Scheme 2.8.3

Dimerization of NHSns with sterically demanding N,N -substituents

was isolated recently.13 This shows remarkable chemical properties, such as the transfer of the tin atom from one stannylene to a diazadiene with formation of a new stannylene of type E. The benzannulated N ,P-substituted stannylene F was described recently14 and stannylenes derived from diaminonaphthalene G15 and cationic stannylenes H16 containing a 10 π -electron ring system are also known. The empty p-orbital in N-heterocyclic diaminostannylenes normally acts as a strong Lewis acid. This property influences the aggregation behavior of NHSns. Stannylenes of type C, with sterically demanding N ,N -substituents (R = CH2 tBu) form a dimeric aggregate in the solid state with contacts between the tin atom and the aromatic six-membered ring of an adjacent molecule. The N ,N -(SiMe3 )2 -substituted benzannulated NHSn dimerizes in the presence of tetramethylethylenediamine (TMEDA) via a donor– acceptor interaction of the nitrogen atoms of TMEDA with the empty p-orbitals at two tin(II) atoms10 (Scheme 2.8.3). A different aggregation behavior has been found for NHSns with sterically non-demanding or donorfunctionalized N ,N -substituents. The NHSn 1 with N ,N -(CH3 )2 -substituents dimerizes in the solid state via two strong inter-molecular Sn N interactions11 (Figure 2.8.1). Only weak intra-molecular Sn←O and no inter-molecular interactions were observed for the monomeric N ,N -ether-functionalized stannylene 211 . The stannylene with n-propylamine substituents 3 exhibits both intra-molecular and inter-molecular Sn←N interactions, thereby forming a bimolecular aggregate in the solid state.11 119 Sn NMR spectroscopy provides useful information about the Lewis acidic properties of benzannulated stannylenes. The 119 Sn chemical shifts for various benzannulated N-heterocyclic stannylenes fall in a broad range from −5–269 ppm11 depending on the N ,N -substituents and the solvent. It is obvious that the 119 Sn chemical shift reflects the amount of electron density at the tin atom and a high-field shift would indicate a gain in electron density at the tin center. The difference in the 119 Sn chemical shift recorded in non-polar and donor solvents is indicative of the donor strengths of the N ,N -functional groups. A significant difference in the 119 Sn chemical shift has been observed for stannylene 1 in benzene (222 ppm) and THF (107 ppm) (Scheme 2.8.4). Apparently, the dimer observed for 1 in the solid state (Figure 2.8.1) breaks up in solution, and the tin center is stabilized in THF solution by coordination of THF molecules, causing the observed high-field shift. This high-field shift is not observed in the 119 Sn NMR spectrum of 1 recorded in non-coordinating benzene. For the stannylenes with donor-functionalized N ,N substituents, the difference in the 119 Sn chemical shift in THF and benzene can serve as an indication of the strengths of intra-molecular donor-acceptor interaction. The difference in the 119 Sn chemical shift is larger in 2 compared to 3. This indicates a weak interaction of the oxygen donors with the tin center in benzene, which is replaced by a Sn· · ·OTHF interaction in THF solution (Scheme 2.8.4). Essentially

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Figure 2.8.1 Molecular structures of benzannulated N-heterocyclic stannylenes with sterically nondemanding or Lewis base functionalized N,N -substituents

no dependence of the chemical shift upon the type of solvent was observed for stannylene 3 which was taken as evidence for the persistence of the Sn· · ·NMe2 donor interaction, even in THF solution.11 Another indication for the Lewis acidic properties of NHSns is provided by the reaction of dibenzotetraazafulvalenes with stannylenes.17 Electron-rich enetetramines can be cleaved into N-heterocyclic carbenes.18 Pioneering work by Lappert has demonstrated that this cleavage, in the presence of an electrophilic metal center, leads to NHC complexes.18a,19 Hahn and coworkers have used the electrophilic tin center in the NHSn 4 to cleave the dibenzotetraazafulvalene 5 with the formation of the zwitterionic stannylene-carbene adduct 6 (Scheme 2.8.5).17 Similar carbene adducts of diaminogermylenes and diaminostannylenes have been described by Schumann et al. and Lappert et al.20 and even transient diorganogermylenes have been stabilized by formation of an adduct with an N-heterocyclic carbene.21 Cyclic and non-cyclic polydentate dialkyl- and diaminostannylenes have been described in the literature, 22,23 some of which can act as chelating ligands in transition-metal complexes.23a Veith et al.

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Scheme 2.8.4 Dependence of the 119 Sn resonance for 1–3 from the solvent (values recorded in C6 D6 and [D8 ]THF in parentheses)

have shown that polycyclic bis- or tris-stannylenes undergo a transformation upon heating to cubane-type tetra-stannylenes with an Sn4 N4 core.24 Such tin(II)-containing cubanes are potential synthons for iminostannylenes R N Sn.25 Benzannulated N-heterocyclic bis-stannylenes with a CH2 CMe2 CH2 bridge and sterically demanding or donor-functionalized N -substituents, such as CH2 tBu or CH2 CH2 CH2 NMe2 have been described recently.26 The bis-stannylene with the donor-functionalized N -substituent 7 shows both an intra-molecular interaction of the amino group with the vacant p-orbital at each of the tin atoms, and an inter-molecular interaction between this orbital and the π-system of the benzene ring of an adjacent molecule. These interactions lead to a polymeric arrangement of the bis-stannylenes in the crystal lattice (Figure 2.8.2). Germanium compounds analogous to the N-heterocyclic stannylenes A H (Scheme 2.8.2) are known. The first stable diorgano- and diaminogermylenes, Ge[C(SiMe3 )3 ]2 and Ge[N(SiMe3 )2 ]2 , apart

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Scheme 2.8.5 Cleavage of the dibenzotetraazafulvalene 5 by NHSn 4 with formation of the zwitterionic stannylene-carbene adduct 6

from some germanocenes,5c,27 have been described by Lappert.7 The reactive germanium atom in the diaminogermylenes is not only stabilized by the steric bulk of the substituents at the GeII atom, but also by electron-donation from the trigonal-planar nitrogen atoms to the vacant p-orbital of the germanium atom (push-pull effect). The subsequent report9 of the first heterocyclic germylene (NHGe) I (Scheme 2.8.6) predates Arduengo’s description of the first N-heterocyclic carbene28 by almost 10 years. Later, the first stable NHGes derived from saturated imidazoles J were reported.29 The molecular structure of the first benzannulated NHGe K, with a germanium atom as a part of an aromatic system, was described in 1989,30

Figure 2.8.2 Molecular structure of the donor-functionalized bis-stannylene 7 (top) and arrangement of the molecules in the crystal lattice (bottom)

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Scheme 2.8.6

Selected N-heterocyclic germylenes

again some 10 years before its carbene analog was prepared.31 The NHGe analog of Arduengo’s carbene L was first described in 1992.32 Based on spectroscopic data and quantum-chemical calculations, it was estimated that a cyclic delocalization of the six π -electrons occurs in the planar heterocycle of L.33 It appears that aromaticity is much more pronounced in NHGes of type L than in the related N-heterocyclic silylenes. Pyrido-,12 naphtho-,34 and tropo-annulated35 N-heterocyclic germylenes have been described recently. Even cationic36 and ring-functionalized NHGes exhibiting betaine reactivity37 are known. Some bis- and polygermylenes, none of which can act as a chelating ligand, have been described.22,38 Recently novel N-heterocyclic bis-germylenes of type M, which are potentially bidentate ligands, have been prepared. In these derivatives two benzannulated germylene rings are linked together by a bridge between two ring-nitrogen atoms.39 The arrangement of the bis-germylenes in the solid state strongly depends on the type of bridge and the steric demand of the substituents at the nitrogen atoms of the heterocycle. The bis-germylene with an n-butylene bridge exists as monomer 8 (Figure 2.8.3), when the second nitrogen atom of the heterocycle is substituted with the sterically demanding neopentyl group.39 Replacement of the neopentyl group with the less bulky ethyl group leads to the bimolecular aggregate 9 exhibiting weak intermolecular Ge· · ·N interactions (Figure 2.8.3). The bis-germylene with an 1,2-(CH2 )2 (C6 H4 ) linker 10 exists in a polymeric arrangement in the crystal lattice with interactions between the empty p-orbital of the germanium atom and the aromatic system of the benzene ring of a parallel-oriented adjacent molecule (Figure 2.8.3). The bis-germylene 11 with the lutidine bridging unit exhibits a potential pincer topology in the solid state and can serve as a Ge N Ge pincer ligand.40 This bisgermylene shows an interesting intramolecular Ge· · ·Ge interaction in the solid state (Figure 2.8.3). The first N-heterocyclic benzannulated plumbylenes 12 (N ,N -substituents CH2 tBu) and 13 (N ,N substituents iBu) have recently been described. Depending on the steric bulk of the N ,N -substituents, they form two different bimolecular aggregates in the solid state (Figure 2.8.4).41 2.8.3

Complexes of Bidentate N-Heterocyclic Germylenes and Stannylenes

Complexes of germylenes and stannylenes with transition metals have attracted considerable attention, and this chemistry has been reviewed.42 Homoleptic complexes of three-coordinated platinum or palladium with Ge[N(SiMe3 )2 ]2 or Sn[N(SiMe3 )2 ]2 have been prepared by Lappert.43 They react with carbon monoxide to give the tri-nuclear clusters [M(Ge, Sn)[N(SiMe3 )2 ]2 )(CO)3 ] (M = Pt, Pd).44 Heteroleptic complexes of three-coordinated Pd with the Ge[N(SiMe3 )2 ]2 ligand and different phosphines are able to reversibly bind H2 and CO2 .45 Even an example of intramolecular C H activation has been described for the iridium(I) complex with the Ge[N(SiMe3 )2 ]2 ligand.46 The N-heterocyclic germylenes and stannylenes of type Me2 Si(NtBu)2 E (E = Ge, Sn) form complexes [M(Me2 Si(NtBu)2 E)4 (μ-X)2 ] (M = Ni, X = Br; M = Pd, Pt, X = Cl) starting from the metal halides.

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Figure 2.8.3

Figure 2.8.4

Arrangement of bis-germylenes of type M in the solid state

Molecular structures of the N-heterocyclic plumbylenes 12 and 13

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In addition, the germylene Ge[N(SiMe3 )2 ]2 is capable of replacing all the triphenylphosphine ligands in [RhCl(PPh3 )3 ] to give a rhodium(I) tetra-germanium compound (two GeII and two GeIV ).47 The dehydrohalogenating properties of dialkylgermylenes were used for the transformation of a carbene ligand into a carbyne at a ruthenium metal center.48 A naphthannulated germylene reacts with [Ni(cod)2 ] to give the homoleptic complex [Ni(NHGe)4 ]34a while the sterically more demanding NHGe J (R = tBu) in the same reaction gave the three-coordinated complex [Ni(J)3 ].32 Some adducts of stannylenes with Lewis acids, such as boron compounds49 and AlCl350 are known. More common are adducts of stannylenes with Lewis bases, such as 1,4-dioxane51 or phosphines. Stannylenes form adducts with carbenes (Scheme 2.8.5) and with carbene analogs, such as gallium(I) derivatives52 or even other stannylenes.53 In the latter adducts between two tin(II) atoms, one stannylene acts as a Lewis base and the other one as a Lewis acid. Stannylenes as ligands in transition-metal complexes act as Lewis bases. In many cases the empty porbital at the coordinated stannylene center accepts electron density from donor ligands.42 The homoleptic platinum(0) complex of diaminostannylene Sn[N(SiMe3 )2 ]2 A was obtained by substitution of the cod ligand in [Pt(cod)2 ].43,54 The reaction of Sn[N(SiMe3 )2 ]2 with [PtCl(μ-Cl)(PEt3 )]2 gave insertion of the stannylene in the Pt Cl bond, with formation of a tin(IV) complex.54 The stannylene analog of nickel tetracarbonyl described by Veith et al. exhibits a tetrahedral nickel(0) atom in the solid state, and NMR investigations show that the coordination number four is preserved in solution.55 Heteroleptic complexes of the unsaturated NHGe of type L (R = CH2 tBu) with the tetracarbonyl or tricarbonyl molybdenum complex fragments have been described.56 Upon complex formation, a shortening of Ge N bond distances within the coordinated germylene was observed. A similar shortening of the Ge N bond lengths occurs in the {Mo(CO)4 } complex 14 with a benzannulated bis-germylene (Figure 2.8.5)39 relative to the free bis-germylenes of type M. In addition, the N Ge N angle in the coordinated NHGe is larger than in the free ligand. These observations are in accordance with theoretical predictions.57 In the molybdenum complex 14, both germylene donor groups are coordinated in cis-positions, an arrangement identical to that observed for the analogous bis-carbene complex.18a Upon cooling, the protons of methylene groups in 14 become diastereotopic due to the restricted movement of the germylene ligands. At 240 K four well-resolved doublets are observed in the 1 H NMR spectrum of 14 (Figure 2.8.5).

Figure 2.8.5

Molecular structure of 14 and its variable temperature 1 H NMR spectra

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Scheme 2.8.7

169

Preparation of the homoleptic complexes 15–18 with bis-stannylene ligands

Bis-stannylenes with different bridging groups react with M0 precursors ([Ni(cod)2 ] or [Pt(nbe)3 ]) to give the homoleptic tetrakis-stannylene complexes 15–18 (Scheme 2.8.7).58 The molecular structures of 15 and 16 are depicted in Figure 2.8.6 together with the 119 Sn NMR spectra of the complexes recorded in [D8 ]THF and [D8 ]toluene. The 119 Sn chemical shift of 15 clearly depends on the solvent used for recording the NMR spectrum, with the 119 Sn resonance shifted upfield in [D8 ]THF. Only one resonance was detected for 15, both in [D8 ]THF and [D8 ]toluene. These observations are consistent with the presence of four identical tin(II) atoms in 15 with the upfield shift in [D8 ]THF caused by coordination of the solvent into the empty p-orbital at the tin atom (see also Scheme 2.8.4). The structure analysis (Figure 2.8.6, left) confirms the presence of four identical tin centers in 15, all of which are easily accessible for interactions with solvent molecules. 119 Sn NMR spectra of complex 16 exhibit two resonances in [D8 ]THF and [D8 ]toluene, one of which is significantly shifted upfield in [D8 ]THF. The molecular structure of 16 (Figure 2.8.6, right) explains this observation. Complex 16 possesses two different types of tin(II) atoms. One type (Sn1, Sn3) is sterically protected by the phenylene bridge between two stannylene units, the other type (Sn2, Sn4) is much less shielded. In non-coordinating [D8 ]toluene, two almost identical 119 Sn resonances around 450 ppm are observed. In [D8 ]THF the sterically unprotected tin atoms (Sn2 and Sn4) are coordinated by solvent molecules causing the upfield shift to 390 ppm while the other two tin atoms (Sn1 and Sn3) are inaccessible for solvent coordination. The X-ray diffraction analysis shows the nickel(0) atoms in 15 and 16 coordinated in a slightly distorted tetrahedral fashion by four stannylene donors. The Ni Sn distances fall in the normal ˚ for both complexes. range of 2.3654(15)–2.3827(14) A The platinum(0) complexes 17 and 18 also contain a tetrahedrally coordinated transition metal center. The molecular structures of 17 and 18 (Figure 2.8.7) are similar to those found for the analogous complexes 15 and 16. The sterically unprotected stannylene tin atoms in 17 are coordinated by THF molecules in the solid state (these THF molecules are not shown in Figure 2.8.7).

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Figure 2.8.6 Molecular structures of the homoleptic complexes 15 (left) and 16 (right) and their 119 Sn NMR spectra in [D8 ]THF and [D8 ]toluene

Figure 2.8.7 Molecular structures of the homoleptic platinum (0) complexes of bis-stannylenes 17 (left, coordinated THF molecules are not shown for clarity) and 18 (right)

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When [Pt(PPh3 )4 ] was reacted in the presence of O PPh3 with a bis-stannylene, a heteroleptic tetrahedral platinum(0) complex 19 with a P2 Sn2 donor set was obtained (Scheme 2.8.8). One of the two stannylene donors in 19 is coordinated by the oxygen atom of a molecule of O PPh3 (Scheme 2.8.8), again underlining the Lewis acidity of the p-orbital at the tin atom. In spite of coordination of O PPh3 to one tin atom only, both Pt Sn bond lengths are almost equidistant and only one resonance was observed ˚ compared to related in the 119 Sn NMR spectrum. The Sn1 O1 bond distance is rather short (2.270(4) A) 51 ˚ ether-adducts of stannylenes (2.527(5) A ). The nature of the M EII bond (E = C, Si, Ge, Sn) in transition-metal complexes is still the subject of discussion.57 It is believed that stannylenes are weaker π -acceptors than carbenes. In general, the σ contribution to the M E bond is believed to be larger than the π contribution, although the latter should not be neglected.57 We have compared the bond parameters in the bis-stannylene complex 20 (Figure 2.8.8, left) with those measured for the analogous bis-carbene complex 2118a and the bis-germylene complex 1439 (Figure 2.8.5). The bond parameters for the molybdenum complexes with N ,N -alkylsubstituted bis-carbenes, bis-germylenes and bis-stannylenes (Table 2.8.1) were also compared to those of the N donor-functionalized bis-stannylene 7 (Figure 2.8.2) and its molybdenum complex 2226 (Figure 2.8.8, right). The X-ray data for the bis-carbene complex 21, the bis-germylene complex 14, and the bis-stannylene analog 20 show the expected elongation for the Mo E distances (E = C: 2.324(3); E = Ge: 2.5204(6), ˚ The Mo−E distance increases by about 0.2 A ˚ from E = C 2.5189(6); E = Sn: 2.6850(3), 2.6736(4) A). ˚ → E = Ge and by another 0.15 A from E = Ge → E = Sn (Table 2.8.1). A different trend was observed ˚ for E = C → E = Ge and for the intra-heterocycle E N bond lengths. These elongated by about 0.45 A ˚ by 0.2 A for E = Ge → E = Sn. The intra-heterocycle E N bond lengths increased much more than the Mo E bond lengths.26 This could indicate that the heavier analogs of benzannulated carbenes do not act exclusively as σ -donors, but, owing to the Lewis acidity of the empty p-orbital of the stannylenes, also

Scheme 2.8.8 Preparation and molecular structure of complex 19. Phenyl groups at the phosphorus atoms have been omitted for clarity

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Table 2.8.1

Bond parameters and vibrational data for complexes 14, 20–22, and bis-stannylene 7

N

N N

E

O C

E

N

Mo C O

C O

C O

2118a (E = Ccarbene )

1439 (E = Ge)

2026 (E = Sn)

2226

r (Mo–E), A˚

2.324(3)

r (E–N), A˚ r (Sn–NMe2 ), A˚

1.385(4) 1.367(4) –

2.5204(6) 2.5189(6) 1.823(3)– 1.827(3) –

2.6850(3) 2.6736(4) 2.0182(2)– 2.0398(2) –

2.7046(6) – 2.033(2), 2.0586(2) 2.390(2)

(E–Mo–E), ◦ (N–E–N), ◦

95.77(14) 103.3(2) 1889a 2002a 1857a 1799a

89.734(12) 81.48(7) 81.25(7) 1947b 2038b 1929b –

86.87(2) 80.49(7)

ν(CO), A1 , cm−1 ν(CO), A1 , cm−1 ν(CO), B1 , cm−1 ν(CO), B2 , cm−1

90.691(2) 87.93(14), 87.73(14) 1950b 2029b 1925b 1908a

a b

1928 2008 – –

726

2.063(3)– 2.090(3) 2.561(4) 2.530(4) – 78.56(13) 78.57(12) – – – –

Infrared data Raman data (λexc = 632.8 nm)

function as π-acceptor ligands. The π -acceptor capability appears to increase in turn from carbenes, to germylenes, and to stannylenes. This assumption is corroborated by an inspection of the ν(CO) stretching modes in 21, 14, and 20. The wave numbers for the trans-CO-stretching modes increase, starting from the bis-carbene complex to the bis-germylene complex to reach a maximum for the bis-stannylene complex. This indicates reduced back-bonding to the CO ligands, which can be caused by enhanced back-bonding to the germylene and stannylene ligand or, less likely, is caused by their reduced σ -donor capacity. An evaluation of the π-acceptor capability of NHGe relative to phosphites has recently been published.59 An interesting observation was made with complex 22, containing the N -donor-functionalized bisstannylene ligand. Due to interaction of tin atom with the N -propylamine-donor (N3, Figure 2.8.8) stabilizing back-bonding from the molybdenum atom is no longer required, resulting in an expansion of ˚ relative to 20 (2.6850(3), 2.6736(4) A). ˚ Stabilization of the p-orbital the Mo Sn bond in 22 (2.7046(6) A) og the tin atom by the Me2 N donor group also leads to a reduction in the intra-heterocycle N1 Sn N2 delocalization and concurrent elongation of the intra-ring Sn N bond lengths in 22, relative to 20 (Figure 2.8.8, Table 2.8.1). The lutidine-bridged bis-stannylene 23 can act as a trapping agent for Sn O and Pb O generated in situ.60 Hydrolysis of E[N(SiMe3 )2 ]2 (E = Sn, Pb) leads to tin(II) and lead(II) monoxides which were trapped by the bis-stannylene 23 (Scheme 2.8.9). In both cases the bis-stannylene acts as a pentadentate

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Figure 2.8.8 Molecular structures of the molybdenum complexes with the N-alkyl-substituted bisstannylene 20 (right) and the N-donor-substituted bis-stannylene 22 (right)

pincer-ligand towards Pb O and Sn O. The inter-atomic distances Sn1(Sn2) O1 and Sn3(Pb) N in 24 and 25 (Scheme 2.8.9) fall in the range between donor-acceptor and covalent bond lengths; however, the choice between covalent and coordination bonds, in the case of complexes 24 and 25, seems somewhat arbitrary. The 119 Sn M¨ossbauer spectrum of complex 24 shows two absorptions for two different divalent tin atoms.

Scheme 2.8.9

Preparation and molecular structures of the complexes 24 and 25

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References 1. (a) M. E. Volpin and D. N. Kursanov, Russ. J. Gen. Chem., 32, 1118 (1962); (b) M. E. Volpin, Y. D. Koreshkov, V. G. Dulova, and D. N. Kursanov, Tetrahedron, 18, 107 (1962). 2. O. M. Nefedov and M. N. Manakov, Angew. Chem., 78, 1039 (1966); Angew. Chem., Int. Ed. Engl., 5, 1021 (1966). 3. W. P. Neumann, Chem. Rev., 91, 311 (1991); (b) S. E. Boganov, V. I. Faustov, M. P. Egorov, and O. M. Nefedov, Russ. Chem. Bull., Int. Ed., 53, 960 (2004). 4. (a) J. Barrau and G. Rima, Coord. Chem. Rev., 178–180, 593 (1998); (b) M. Weidenbruch, Eur. J. Inorg. Chem., 373 (1999); (c) N. Tokitoh and R. Okazaki, Coord. Chem. Rev., 210, 251 (2000); (d) O. K¨uhl, Coord. Chem. Rev., 248, 411 (2004). 5. (a) E. O. Fischer and H. Grubert, Z. Naturforsch., 11b, 423 (1956); (b) J. L. Atwood, W. E. Hunter, A. H. Cowley, R. A. Jones, and C. A. Stewart, J. Chem. Soc., Chem. Commun., 925 (1981); (c) H. Schumann, C. Janiak, F. E. Hahn, C. Kolax, J. Loebel, M. D. Rausch, J. J. Zuckerman, and M. J. Heeg, Chem. Ber., 119, 2656 (1986). 6. (a) P. Jutzi, F. Kohl, P. Hofmann, C. Kr¨uger, and Y.-H. Tsay, Chem. Ber., 113, 757 (1980); (b) M. J. Heeg, C. Janiak, and J. J. Zuckerman, J. Am. Chem. Soc., 106, 4259 (1984). 7. (a) D. H. Harris and M. F. Lappert, J. Chem. Soc., Chem. Commun., 895 (1974); (b) P. J. Davidson, D. H. Harris, and M. F. Lappert, J. Chem. Soc., Dalton Trans., 2268 (1976); (c) T. Fjeldberg, A. Haaland, B. E. R. Schilling, M. F. Lappert, and A. J. Thorne, J. Chem. Soc., Dalton Trans., 1551 (1986). 8. M. F. Lappert, P. P. Power, M. J. Slade, L. Hedberg, K. Hedberg, and V. Schomaker, J. Chem. Soc., Chem. Commun., 369 (1979). 9. (a) M. Veith and M. Grosser, Z. Naturforsch., 37b, 1375 (1982); (b) M. Veith, Angew. Chem., 99, 1 (1987); Angew. Chem., Int. Ed. Engl., 26, 1 (1987). 10. H. Braunschweig, B. Gehrhus, P. B. Hitchcock, and M. F. Lappert, Z. Anorg. Allg. Chem., 621, 1922 (1995). 11. F. E. Hahn, L. Wittenbecher, D. Le Van, and A. V. Zabula, Inorg. Chem., 46, 7662 (2007). 12. (a) J. Heinicke, A. Oprea, M. K. Kindermann, T. Karpati, L. Nyul´aszi, and T. Veszpr´emi, Chem.–Eur. J., 4, 541 (1998); (b) O. K¨uhl, P. L¨onnecke, and J. Heinicke, Polyhedron, 20, 2215 (2001); (c) F. Ullah, G. Bajor, T. Veszpr´emi, P. G. Jones, and J. W. Heinicke, Angew. Chem. 119, 2751 (2007); Angew. Chem., Int. Ed., 46, 2697 (2007). 13. T. Gans-Eichler, D. Gudat, and M. Nieger, Angew. Chem. 114, 1966 (2002); Angew. Chem., Int. Ed., 41, 1888 (2002). 14. F. Garcia, S. M. Humphrey, R. A. Kowenicki, E. J. L. McInnes, C. M. Pask, M. McPartlin, J. M. Rawson, M. L. Stead, A. D. Woods, and D. S. Wright, Angew. Chem. 117, 3522 (2005); Angew. Chem., Int. Ed., 44, 3456 (2005). 15. P. Bazinet, G. P. A. Yap, G. A. DiLabio, and D. S. Richenson, Inorg. Chem., 44, 4616 (2005). 16. (a) H. V. Rasika Dias, and W. Jin, J. Am. Chem. Soc., 118, 9123 (1996); (b) A. E. Ayers and H. V. Rasika Dias, Inorg. Chem., 41, 3259 (2002). 17. F. E. Hahn, L. Wittenbecher, M. K¨uhn, T. L¨ugger, and R. Fr¨ohlich, J. Organomet. Chem., 617−618, 629 (2001). 18. (a) F. E. Hahn, L. Wittenbecher, D. Le Van, and R. Fr¨ohlich, Angew. Chem. 112, 551 (2000); Angew. Chem., Int. Ed., 39, 541 (2000); (b) Y. Liu, P. E. Lindner, and D. M. Lemal, J. Am. Chem. Soc. 121, 10626 (1999); (c) R. W. Alder, M. E. Blake, L. Chaker, J. N. Harvey, F. Paolini, and J. Sch¨utz, Angew. Chem., 116, 6020 (2004); Angew. Chem., Int. Ed., 43, 5896 (2004). 19. (a) M. F. Lappert, J. Organomet. Chem., 358, 185 (1988); (b) M. F. Lappert, J. Organomet. Chem., 690, 5467 (2005). 20. (a) H. Schumann, M. Glanz, F. Girgsdies, F. E. Hahn, M. Tamm, and A. Grzegorzewski, Angew. Chem., 109, 2328 (1997); Angew. Chem., Int. Ed. Engl., 36, 2232 (1997); (b) B. Gehrhus, P. B. Hitchcock, and M. F. Lappert, J. Chem. Soc., Dalton Trans., 3094 (2000). 21. P. A. Rupar, M. C. Jennings, P. J. Ragogna, and K. M. Baines, Organometallics, 26, 4109 (2007). 22. S. Kobayashi and S. Cao, Chem. Lett., 941 (1994).

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23. (a) M. Henn, M. Sch¨urmann, B. Mahieu, P. Zanello, A. Cinquantini, and K. Jurkschat, J. Organomet. Chem., 691, 1560 (2006); (b) H. Braunschweig, C. Drost, P. B. Hitchcock, M. F. Lappert, and L. J.-M. Pierssens, Angew. Chem., 109, 285 (1997); Angew. Chem., Int. Ed. Engl., 36, 261 (1997). 24. (a) M. Veith, M.-L. Sommer, and D. J¨ager, Chem. Ber., 112, 2581 (1979); (b) M. Veith and H. Lange, Angew. Chem., 92, 408 (1980); Angew. Chem., Int. Ed. Engl., 19, 401 (1980); (c) M. Veith and O. Recktenwald, Z. Naturforsch., 36b, 144 (1981); (d) M. Veith and G. Schlemmer, Chem. Ber., 115, 2141 (1982); (e) M. Veith and O. Recktenwald, Z. Naturforsch., 38b, 1054 (1983). 25. 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). 26. F. E. Hahn, A. V. Zabula, T. Pape, A. Hepp, R. Tonner, and G. Frenking, unpublished results. 27. (a) M. Grenz, F. E. Hahn, W.-W. du Mont, and J. Pickardt, Angew. Chem., 96, 69 (1984); Angew. Chem., Int. Ed. Engl., 23, 61 (1984); (b) H. Schumann, C. Janiak, F. E. Hahn, J. Loebel, and J. J. Zuckerman, Angew. Chem., 97, 765 (1985); Angew. Chem., Int. Ed. Engl., 24, 773 (1985). 28. A. J. Arduengo III, R. L. Harlow, and M. Kline, J. Am. Chem. Soc., 113, 361 (1991). 29. A. Meller and C.-P Gr¨abe, Chem. Ber., 118, 2020 (1985). 30. (a) J. Pfeiffer, W. Maringgele, M. Noltemeyer, and A. Meller, Chem. Ber., 122, 245 (1989); (b) J. Pfeiffer, M. Noltemeyer, and A. Meller, Z. Anorg. Allg. Chem., 572, 145 (1989). 31. F. E. Hahn, L. Wittenbecher, R. Boese, and D. Bl¨aser, Chem.-Eur. J., 5, 1931 (1999). 32. W. A. Herrmann, M. Denk, J. Behm, W. Scherer, F.-R. Klingan, H. Bock, B. Solouki, and M. Wagner, Angew. Chem., 104, 1489 (1992); Angew. Chem., Int. Ed. Engl., 31, 1485 (1992). 33. (a) C. Boehme and G. Frenking, J. Am. Chem. Soc., 118, 2039 (1996); (b) L. A. Leites, S. S. Bukalov, A. V. Zabula, I. A. Garbuzova, D. F. Moser, and R. West, J. Am. Chem. Soc., 126, 4114 (2004). 34. (a) P. Bazinet, G. P. A. Yap, and D. S. Richeson, J. Am. Chem. Soc., 123, 11162 (2001); (b) J. Heinicke and A. Oprea, Heteroatom. Chem., 9, 439 (1998). 35. I. L. Fedushkin, A. A. Skatova, V. A. Chudakova, N. M. Khvoinova, A. Y. Baurin, S. Dechert, M. Hummert, and H. Schumann, Organometallics, 23, 3714 (2004). 36. M. Stender, A. D. Phillips, and P. P. Power, Inorg. Chem., 40, 5314 (2001). 37. M. Driess, S. Yao, M. Brym, and C. van W¨ullen, Angew. Chem., 118, 4455 (2006); Angew. Chem., Int. Ed., 45, 4349 (2006). 38. (a) P. B. Hitchcock, M. F. Lappert, and A. J. Thorne, J. Chem. Soc., Chem. Commun., 1587 (1990); (b) R. A. Bartlett and P. P. Power, J. Am. Chem. Soc., 112, 3660 (1990); (c) H. Braunschweig, P. B. Hitchcock, M. F. Lappert, and L. J.-M. Pierssen, Angew. Chem. 106, 1243 (1994); Angew. Chem. Int., Ed. Engl., 33, 1156 (1994). 39. A. V. Zabula, F. E. Hahn, T. Pape, and A. Hepp, Organometallics, 26,1972 (2007). 40. F. E. Hahn, A. V. Zabula, T. Pape, and A. Hepp, Eur. J. Inorg. Chem., 2405 (2007). 41. F. E. Hahn, D. Heitmann, and T. Pape, Eur. J. Inorg. Chem., 1039 (2008). 42. (a) W. Petz, Chem. Rev., 86, 1019 (1986); (b) M. F. Lappert and R. S. Rowe, Coord. Chem. Rev., 100, 267 (1990). 43. P. B. Hitchcock, M. F. Lappert, and M. C. Misra, J. Chem. Soc., Chem. Commun., 863 (1985). 44. G. K. Campbell, P. B. Hitchcock, M. F. Lappert, and M. C. Misra, J. Organomet. Chem., 289, C1 (1985). 45. (a) Z. T. Cygan, J. E. Bender IV, K. E. Litz, J. W. Kampf, and M. M. Banaszak Holl, Organometallics, 21, 5373 (2002); (b) K. E. Litz, K. Henderson, R. W. Gourley, and M. M. Banaszak Holl, Organometallics, 14, 5008 (1995). 46. S. M. Hawkins, P. B. Hitchcock, M. F. Lappert, and A. K. Rai, J. Chem. Soc., Chem. Commun., 1689 (1986). 47. (a) M. Veith, L. Stahl, and V. Huch, Inorg. Chem., 28, 3278 (1989); (b) M. Veith, A. M¨uller, L. Stahl, M. N¨otzel, M. Jarczyk, and V. Huch, Inorg. Chem., 35, 3848 (1996). 48. S. R. Caskey, M. H. Stewart, Y. J. Ahn, M. J. A. Johnson, and J. W. Kampf, Organometallics, 24, 6074 (2005). 49. C. Drost, P. B. Hitchcock, and M. F. Lappert, Organometallics, 17, 3838 (1998). 50. M. Veith and W. Frank, Angew. Chem., 97, 213 (1985); Angew. Chem., Int. Ed. Engl., 24, 223 (1985). 51. E. Hough and D. G. Nicholson, J. Chem. Soc., Dalton Trans., 1782 (1976). 52. S. P. Green, C. Jones, K.-A. Lippert, D. P. Mills, and A. Stasch, Inorg. Chem., 45, 7242 (2006).

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53. (a) W.-P. Leung, W.-H. Kwok, F. Xue, and T. C. W. Mak, J. Am. Chem. Soc., 119, 1145 (1997); (b) C. Drost, P. B. Hitchcock, and M. F. Lappert, Angew. Chem., 111, 1185 (1999); Angew. Chem., Int. Ed., 38, 1113 (1999). 54. T. A. K. Al-Allaf, C. Eaborn, P. B. Hitchcock, M. F. Lappert, and A. Pidcock, J. Chem. Soc., Chem. Commun., 548 (1985). 55. M. Veith, L. Stahl, and V. Huch, J. Chem. Soc., Chem. Commun., 359 (1990). 56. O. K¨uhl, P. L¨onnecke, and J. Heinecke, Inorg. Chem., 42, 2836 (2003). 57. C. Boehme and G. Frenking, Organometallics, 17, 5801 (1998). 58. A. V. Zabula, T. Pape, A. Hepp, F. E. Hahn, Organometallics, in press, DOI: 10.1021/om80096f. 59. O. K¨uhl, K. Lifson, and W. Langel, Eur. J. Org. Chem., 2336 (2006). 60. A. V. Zabula, T. Pape, A. Hepp, F. M. Schappacher, U. Ch. Rodewald, R. P¨ottgen, and F. E. Hahn, J. Am. Chem. Soc., 130, 5648 (2008).

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2.9

177

Stannenes, Distannenes, and Stannynes

Yoshiyuki Mizuhata and Norihiro Tokitoh Institute for Chemical Research, Kyoto University, Japan

2.9.1

Introduction

The second-row elements of the Periodic Table, such as boron, carbon, nitrogen, and oxygen are able to form multiple pπ pπ bonds, resulting in the development of a wide variety of organic chemistry. In contrast to the second-row elements, their heavier congeners, having a principal quantum number greater than two, had been thought for many years to be incapable of forming stable molecules having pπ pπ bonds, either with themselves or with other elements.1 Such a view is referred to as the ‘classical double-bond rule.’ This interpretation was rationalized by the long bond distances between heavier elements, which do not allow sufficient overlapping of np-orbitals. The breakthrough in this field was the isolation of the first stable distannene (Sn Sn, 1)2 in 1973, phosphaalkene (P C, 2)3 in 1978, and silene (Si C, 3),4 diphosphene (P P, 4),5 and disilene (Si Si, 5)6 in 1981. In all cases, introduction of bulky ligands on the central atoms prevented oligomerization, thus making such reactive species isolable as stable compounds. These sensational results provided definitive evidence that double-bonded compounds of heavier main-group elements could be isolated as stable species without oligomerization (or other side reactions) when they were ‘kinetically well-stabilized’ with bulky substituents. Subsequently, significant and exciting progress has been made in the chemistry of unsaturated compounds of heavier main-group elements,7 especially in the field of group 14 elements, by taking advantage of steric protection.

Dis

Dis

Mes

Sn Sn Dis

Dis

R

Ph P

C R

Ph

1

2 R

Me3Si

OSiMe3 Si C

Me3Si

P

Scheme 2.9.1

Mes

Mes P

Ad 3

2.9.2

Mes*

Si Mes*

4

Mes: R = Me Mes*: R = t-Bu

Si

Mes

Mes 5

Ad = 1-adamantyl Dis = CH(SiMe3)2

Double-bond compounds of heavier main group elements

Distannenes (Sn Sn)

Much attention has been focused on the chemistry of heavier congeners of alkenes, i.e., ‘dimetallenes’ (>E E<; E = Si, Ge, Sn, Pb), and a number of reports have appeared on their syntheses, structures, and properties.7–10 Experimental and theoretical studies revealed that their structures and properties are apparently different from olefins (>C C<). The dominant difference is their trans-bent structures

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R

repulsion R R

E

E

R R

R E

E R R

in-plane structure

Figure 2.9.1

trans-bent structure

Interactions of singlet metallylenes (R2 E:)

in contrast to the planar D2h structures of olefins. Theoretical studies of several model compounds support such structural features, as summarized in the following.11 When the double-bond systems, R2 E ER2 , are homolytically cleaved, the resulting R2 E: units may exist in a triplet or in a singlet state. Contrary to the carbon system, the heavier group 14 atoms have a low ability to form hybrid orbitals, therefore, they prefer the (ns)2 (np)2 valence electronic configuration in their divalent species. Since two electrons remain as a singlet pair in the ns orbital, the ground state of R2 E: is singlet, unlike the case of R2 C:, with a triplet ground state.12 As a result, severe repulsion between the closed-shell orbitals of two R2 E: units prevents their dimerization leading to the formation of R2 E ER2 in the planar form as shown in Figure 2.9.1. However, the two R2 E: units can form unique elongated double bonds in trans-bent configurations to avoid repulsion,13 where each of the R2 E: units donates a lone pair of electrons to an empty p orbital of its bonding partner, to form double donor–accepter bonds, as shown in Figure 2.9.1. In this section, the characteristics of the previously reported distannenes, classified into two types: (i) typical examples bearing two three-coordinated Sn atoms, and (ii) tristanaallene bearing formal sphybridized Sn atoms, are discussed in detail. Typical Examples

As mentioned above, the chemistry of distannenes, >Sn Sn<, the tin analogs of ethylene, has the longest history among all heavy alkene analogs of the type > 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

<5◦

21.4, 64.4 46 42 39.4 44.9 41.5 28.6 1.22(5) 83.3 ( SnCl2 ), 0 ( SnR2 )

–

41.0

θ a (deg)

73.63(6) –

10.8 0 0 0 0 0 63.2 44.62(7) 87

–

0

τ (deg)

– 503 (>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

˚ d(Sn Sn) (A)

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Selected parameters of distannenes 1, 6–17 and the related compounds 20, 21

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(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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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 C<) and distannenes (>Sn Sn<). The bonding situations of stannenes are considered to be different from those of distannenes. In this section, the previously reported stannenes are classified into three types: (i) typical examples, bearing three-coordinate tin and carbon atoms (except class 3), (ii) compounds bearing an Sn C bond in a cumulative double-bond system, and (iii) compounds bearing a Sn C bond in an aromatic ring. Typical Examples

The structures of stannenes 22–30 were determined by X-ray crystallographic or NMR spectroscopic analyses,28–36 and some of their parameters are summarized in Table 2.9.2. The molecular structures of 22 and 27 are shown in Figures 2.9.4a and b, respectively. In most cases (except 26 and 27), the reactions of a stable stannylene and carbenes were adopted for the synthesis of the corresponding stannene. The stannenes 2632 and 2733 were synthesized by dehydrofluorination of the corresponding fluorenyl(fluoro)stannane with tert-butyllithium. The stabilities of 26 and 27 are sensitive to their degree of steric protection. Although compound 26, bearing two 2,4,6-triisopropylphenyl groups, is stable below −20 ◦ C and dimerizes at room temperature to give the [2+2]-dimer 32, compound 27 bearing 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl and mesityl groups is stable even at 80 ◦ C (Scheme 2.9.9).

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Tin Chemistry: Fundamentals, Frontiers and Applications 22: R = R' = CH(SiMe3)2 t-Bu

t-Bu R

N

B Sn C

C(SiMe3)2 B

R'

R R

23: R, R' =

Sn C

SiMe2 N

t-Bu 24: R = R' = RA 25: R = RA, R' = Si(SiMe3)3

t-Bu

(Me3Si)2N (Me3Si)2N Sn C

R

R'

N(i-Pr)2

R

i-Pr

R R

t-Bu

N

Sn C RA =

N

N(i-Pr)2

Mes: R = Me Tip: R = i-Pr Tbt: R = CH(SiMe3)2

26: R = R' = Tip 27: R = Tbt, R' = Mes

i-Pr 29: R = Cl 30: R = Tip

28

Scheme 2.9.8

Stable stannenes

The stable stannenes bearing an Sn C bond shorter than the typical single-bond lengths (average value ˚ 37 are limited to the diboryl-substituted systems 22,28 24,30 and 25,31 reported by Berndt et al., and 2.14 A) tetraarylstannene 27 reported by Tokitoh et al. However, their X-ray crystallographic analysis showed that the environments around the Sn C bonds and the 119 Sn NMR signals are substantially different from each other. Although structures 22 and 25 each show a bent arrangement of the substituents (except the tin atom in 22), the tin and carbon atoms in 24 and 27 are co-planar (Figure 2.9.5). Other examples Table 2.9.2

Selected parameters of stannenes 22–31

No.

˚ d(Sn C1a ) (A)

θ(Sn, C1a , deg)

τ (deg)b p

22 23 24 25 26 27 28 29 30 31

2.025(4) – 2.032(5) 2.032(2) – 2.016(5) 2.303(9) 2.290(5) 2.379(5) 2.397(3)

5, 16 – 0, 0 13.2, 9.7 – 0, 0

61 – 36 12 – 28.5

a

68.6

835 647 374.2 not obtained 288.0 270 −44.69 −59.4 710.0

C1 is the carbon atom doubly bonded to tin atom.

E, E' E b

δ Sn (ppm)c

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

E'

θ

τ (E, E’ = Sn and C).

δ C1 (ppm)c 142 91 137 (−40 ◦ C) 133.85 133.9 150.17 180.68 177.2

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(a)

(b)

Figure 2.9.4

C

B

C

40°

82°

B Sn

Molecular structures of (a) 22 and (b) 27

C B

9° B B

B

C

33°

29°

C C

Si

C

22

C

C

36°

C

24

Figure 2.9.5

185

25

27

Stereo projections along the C Sn Bond of 22, 24, 25, and 27

Tip 26 > – 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 Gr¨utzmacher 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 Escudi´e 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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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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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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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 Escudi´e 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

Molecular structure of 56

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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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 M¨ossbauer 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

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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. Escudi´e, C. Couret, H. Ranaivonjatovo, and J. Satg´e, 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. Gr¨utzmacher, 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. Gr¨utzmacher and T. F. F¨assler, 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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14. (a) S. Masamune and L. R. Sita, J. Am. Chem. Soc., 107, 6390 (1985); (b) M. Weidenbruch, A. Sch¨afer, 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. Gr¨utzmacher, 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. F¨assler, and H. Gr¨utzmacher, 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. Escudi´e, 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. St¨urmann, W. Saak, M. Weidenbruch, A. Berndt, and D. Sclesclkewitz, Heteroatom Chem., 10, 554 (1999). 32. (a) G. Anselme, H. Ranaivonjatovo, J. Escudi´e, C. Couret, and J. Satg´e, Organometallics, 11, 2748 (1992); (b) G. Anselme, J. -P. Declercq, A. Dubourg, H. Ranaivonjatovo, J. Escudi´e, 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. Bl¨aser, and R. Boese, Chem. Ber., 128, 245 (1995). 36. A. Sch¨afer, 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. Escudi´e, S. Richelme, and J. Satg´e, 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. Gr¨utzmacher, S. Freitag, R. Herbert-Irmer, and G. S. Sheldrick, Angew. Chem. Int. Ed. Engl., 31, 437 (1992). 41. L. Baiget, H. Ranaivonjatovo, J. Escudi´e, 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. Escudi´e, H. Ranaivonjatovo, and J. Satg´e, Chem. Commun., 2621 (1996). 50. A. Sch¨afer, 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. N¨oth, 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

Klaus Jurkschat Lehrstuhl f¨ur Anorganische Chemie der Technischen Universit¨at, 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 L¨owig5 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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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

207

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

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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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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

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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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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

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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

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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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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. L¨owig, 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. P´etiaud 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. Sch¨urmann 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. Sch¨urmann, 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. Sch¨urmann, and K. Jurkschat, unpublished results.

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29. J. Beckmann, M. Henn, K. Jurkschat, M. Sch¨urmann, D. Dakternieks and A. Duthie, Organometallics, 21, 192 (2002). 30. J. Beckmann, K. Jurkschat, S. Rabe, M. Sch¨urmann and D. Dakternieks, Z. Anorg. Allg. Chem., 627, 458 (2001). 31. J. Beckmann, K. Jurkschat, B. Mahieu and M. Sch¨urmann, 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. Kasn´a, R. Jambor, M Sch¨urmann, and K. Jurkschat, J. Organomet. Chem., 692, 3555 (2007). 34. J. Beckmann, K. Jurkschat, U. Kaltenbrunner, S. Rabe, M. Sch¨urmann, D. Dakternieks, A. Duthie and D. M¨uller, Organometallics, 19, 4887 (2000). 35. U. Baumeister, D. Dakternieks, K. Jurkschat and M. Sch¨urmann, 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. Sch¨urmann, D. Suter and R. Willem, Organometallics, 21, 3819 (2002). 38. J. Beckmann, D. Dakternieks, A. Duthie, K. Jurkschat, M. Mehring, C. Mitchell and M. Sch¨urmann, 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. Sch¨urmann, 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. Sch¨urmann, and K. Jurkschat, unpublished results. 44. B. Zobel, M. Sch¨urmann, 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. Sch¨urmann, K. Jurkschat, A. Duthie and D. Dakternieks, Eur. J. Inorg. Chem., 153 (2001). 48. M. Schulte, M. Sch¨urmann, D. Dakternieks and K. Jurkschat, Chem. Commun., 1291 (1999). 49. M. Schulte, M. Mehring, I. Paulus, M. Sch¨urmann, 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. Sch¨urmann, D. Dakternieks, and K. Jurkschat, Chem. Commun., 834 (2002). 52. M. Mehring, M. Sch¨urmann, and K. Jurkschat, unpublished results. 53. J. Beckmann, D. Dakternieks, A. Duthie, F.S. Kuan, K. Jurkschat, M. Sch¨urmann and E.R.T. Tiekink, New J. Chem., 29, 1268 (2004). 54. D. Dakternieks, A. Duthie, B. Zobel, K. Jurkschat, M. Sch¨urmann and E.R.T. Tiekink, Organometallics, 21, 647 (2002). 55. D. Dakternieks, B. Zobel, K. Jurkschat, M. Sch¨urmann 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. Sch¨urmann, 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. Sch¨urmann, H. Reuter, D. Dakternieks and K. Jurkschat, Angew. Chem. Int. Ed., 36, 1112 (1997).

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Unusual Bonds and Coordination Geometries

M´onica Moya-Cabrera, Vojtech Jancik and Raymundo Cea-Olivares Instituto de Qu´ımica, Universidad Nacional Aut´onoma de M´exico

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

Molecular structure of 38

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<).89 The most important result is the Si Sn double bond length (2.42 A), ˚ 90 and Sn Sn for the first time. This value is intermediate between the typical Si Si (2.14–2.29 A) 91 ˚ (2.59–3.09 A) double bond lengths. As expected the Si Sn double bond is trans-bent, but the bending angles around the Si and Sn atoms are quite unusual: 26.2◦ for the Si atom and 9.6◦ for the Sn atom.

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C15

C13 C12

N3

A12 A11

N2 C16

N4 C11

Sn1

Sn2

C14

Sn1

N1

Ga1 N1 N2 Sn4

N3

N4

Sn3 1Å 1Å

95

Figure 2.11.7

99

Molecular structures of compounds 95 and 99

Si3

Si2 Si1

Sn1 Sn1

Ge1

1Å

1Å

100

Figure 2.11.8

101

Molecular structures of compounds 100 and 101

N5

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On the other hand, 101 is the first rather stable germastannene structurally characterized (Figure 2.11.8).92 This compound was easily obtained from the reaction of (2,4,6-triisopropylphenyl)magnesium bromide with GeCl2 • (dioxane) and SnCl2 at low temperature. Compound 101 reveals a Ge Sn bond ˚ which is about 0.12 A ˚ shorter than the average value for Ge Sn single bonds93 and, length of 2.51 A, ˚ calculated for the parent compound despite the bulky aryl groups, is even shorter than the value of 2.56 A 94 H2 Ge SnH2 . Further examples of compounds bearing unusual tin bonds with p-block elements include K[2.2.2.crypt]3 [Pb2 Sn2 ] (102),95 (4-Me-C6 H4 )3 PbSn(4-Me-C6 H4 )3 (103),96 (Me3 Sn)4 Sb2 (104),97,98 K[2.2.2.crypt]2 [Bi2 Sn2 ] (105),99 and [(Me3 Si)3 Sn]6 Bi8 (106).100 2.11.3

Unusual Coordination Geometries

The high diversity of coordination polyhedra in tin chemistry is a direct result of the existence of two oxidation states Sn(II) and Sn(IV), the presence of the empty 4 pz and 5d orbitals and depends strongly on the nature of the compound (ionic versus covalent). Compounds with the tin atom in the +IV oxidation state have mostly structural arrangements based on ideal tetrahedral, trigonal bipyramidal, or octahedral geometries.1 In the case of Sn(II), the presence of the lone electron pair results in the formation of many – mostly bent – regular and irregular molecular geometries.101 However, in some compounds, the lone electron pair is not stereochemically active, mainly due to the steric bulk of the ligands which are present. In the following pages, we will summarize the most important and unusual geometries for Sn(II) and Sn(IV) reported in the CSD crystallographic database according to the coordination number of the central tin atom.102 Compounds with Mono-Coordinated Tin Atoms

A few ionic compounds of Sn(II) with substituted cyclopentadienyl ligands, stabilized by weakly coordinating anions, such as [Cp*Sn][BF4 ] (107),103 [(t-BuMe2 Si)Me4 C5 Sn][BF4 ] (108),104 and [Cp*Sn][B(C6 F5 )4 ] (109),105 can be considered to have a coordination number of one. In the solid state, the low coordination number of tin atoms in these compounds is stabilized by weak interaction ˚ in 108 give rise to a supramolecular arwith fluoride atoms. The Sn· · ·F interactions (2.90–3.20 A) ˚ and each rangement, where four tin atoms are in the corners of a regular tetrahedron (Sn· · ·Sn 4.59 A) − face of this tetrahedron is capped by the BF4 anion (Figure 2.11.9). In stanna(car)boranes, such as 1-Sn-2-Me3 Si-3-Me-2,3-dicarbaheptaborane (110),106 1-Sn-2-Me3 Si-2,3-dicarba-closo-heptaborane (111) (Figure 2.11.9),107 or [2,7,8-(μ-H)3 {Fe(triphos)} SnB11 H11 ] (triphos = MeC(CH2 PPh2 )3 ) (112)108 prepared in non-polar solvents, the tin atom is mostly bound only to a face of the parent (car)borane and can thus be considered to be mono-coordinated. Compounds with Di-Coordinated Tin Atoms

The coordination number two is relatively common for Sn(II) compounds, but such complexes have to be stabilized by bulky ligands to prevent oligomerization. The presence and stereoactivity of the lone electron pair on the tin center in these complexes leads to bent coordination geometries. So far, only Sn(C5 Ph5 )2 (113)109 , Sn(Ci5 Pr5 )2 (114) (Figure 2.11.10)110 and Sn[C5 Me4 (Sit BuMe2 )]2 (115)111 have the linear arrangement of the cyclopentadiene rings and thus a non-stereoactive lone electron pair. The tristannaallene Sn[Sn(Sit Bu3 )2 ]2 (116), which crystallizes with two independent molecules in the asymmetric unit, has the most obtuse non-linear angles for Sn(II) observed so far (155.8 and 156.0◦ ) (Figure 2.11.2).112 Linear arrangements of tin(IV) are generally harder to achieve, because of the necessity of forming multiple bonds. However, three examples, [Cl(Me3 P)4 W Sn–C6 H3 -2,6-Mes2 ] (117) (Figure 2.11.10),

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F2 F3

B1

Sn1 B3

B2

F1 Sn

F4

Sn1 B1 Si1

F

B4

B

Si1 1Å 1Å 1Å

108

111

Figure 2.11.9 The simplified structure (left) and the supramolecular arrangement (middle) of compound 108 and (right) the molecular structure of 111

[Cl(dppe)2 W Sn–C6 H3 -2,6-Mes2 ]·3.5C6 H6 (118), and [(dppe)2 W Sn–C6 H3 -2,6-Mes2 ]+ PF6 ·5C4 H8 O (119), of such compounds containing tin tungsten triple bonds have been structurally characterized.113,114 In all three cases, the tin center is shielded by the bulky 2,6-Mes2 -C6 H3 ligand and the tungsten atom is protected either by four PMe3 molecules,113 or chelated by two diphenylphosphinoethane ligands. 114 Complex 119 is prepared by chloride abstraction from 118 using Tl+ PF− 6.

Sn1

P1

Sn1

W1 P2 P4

P3 1Å

1Å

114

Figure 2.11.10

117

Molecular structures of compounds 114 and 117

C11

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Si6

Si8

Si2

Cu1 Si5

Si1

Sn1

Si2

Sn1

243

Sn1

Si3

Si2

Si1

Si1

Si3

Si7 Si3 Si4

1Å

1Å

1Å

75

Figure 2.11.11

123

124

Molecular structures of compounds 75 and 123; and the anion fragment of 124

Compounds with Tri-Coordinated Tin Atoms

Complexes containing a tri-coordinated tin atom are quite usual. Most of these species are based on Sn(II) and have a trigonal pyramidal geometry with angles close to 90◦ due to the geometry of the orthogonal p-orbitals. Nevertheless, there are several examples known where the geometry is close to trigonal planar. The sum of the angles around the tin atom (118.8–120.8◦ ) in the monomeric complex Pd[Sn{N(SiMe3 )2 }2 ]3 (120) is exactly 360◦ , confirming a nearly regular trigonal planar coordination.115 Another such donor–acceptor compound, (2,6-Mes2 -C6 H2 ){(Me3 Si)3 Si}Sn→Cu{Si(SiMe3 )3 } (75) (Figure 2.11.11) (range of the angles around the tin center is 114.5–128.6◦ ), based on tin(II) was reported in 1999.67 The trigonal planar geometry is also characteristic for tri-coordinated tin(IV) cations, but only a few of them have the bond angles around the tin atom close to the ideal number of 120◦ . In 2003, two independent reports on the preparation of the stannylium cation in compounds of the type [R3 Sn][B(C6 F5 )4 ] came out of the groups of Lambert and M¨uller (R = 2,4,6-i Pr3 -C6 H2 ) (121)116 and Sekiguchi (R = t Bu2 MeSi) (122).117 Moreover, although the synthetic route of Lambert and M¨uller is rather complicated and goes via triarylstannyl−allyl derivatives, Segikuchi reported the unique tin radical (t Bu2 MeSi)3 Sn (123) (Figure 2.11.11) to be an intermediate in the two-step synthesis of 122.117 This radical features the closest to an ideal trigonal planar geometry around the tin atom reported to date, with a range for the Si Sn Si angles of 119.7–120.2◦ . The unpaired electron is located in the pz orbital, confirming the π -nature of this radical. Reduction of 123 with an excess of elemental potassium in the presence of [2.2.2]cryptand results in the formation of a unique anion [(t Bu2 MeSi)3 Sn]− (124) (Figure 2.11.11) with an unusually low pyramidal character.6 Compounds with Tetra-Coordinated Tin Atoms

This coordination number is the most common for tin(IV) compounds and corresponds to a tetrahedral geometry. However, it is known also for tin(II) species that display a larger variety of coordination modes. Thus, the most common are the -trigonal bipyramid and tetragonal pyramid geometries, with the lone electron pair in the apical position. An example of such a regular tetragonal pyramid is a dimeric tin(II) pyrazolate [Sn{3,5-(CF3 )2 -1,2-C3 N2 }2 ]2 (125).118 The structural polymorphy of Sn{N(Ph2 PSe)2 }2 (126) was reported recently. The minor red polymorph contains the tin center in a -trigonal bipyramidal environment (126a), whereas the major yellow polymorph has an unusual square-planar environment of the tin atom and thus, the lone electron pair is not stereochemically active (126b) (Figure 2.11.12).119,120 Replacing one of the selenium atoms with sulfur leads only to the -trigonal bipyramidal conformer.121

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N1 P2

Se1

P1

P1

Se2

N1

Se1

P2 Sn1 Se2A Se1A

P1A

P2A

Se2

N1A

Sn1 Se2A P2A N1A P1A Se1A

1Å 1Å

126a

126b

Figure 2.11.12 Polymorphs 126a (square-planar geometry around the tin center) and 126b (square-based pyramid geometry around the tin center)

The only other example of square−planar coordination around tin(II) atom, Sn{N(i Pr2 PSe)2 }2 (127), was reported in 2005.122 It seems that this coordination mode is possible only when the tin atom is coordinated to soft, highly polarizable donor atoms.119,120 Compounds with Penta-Coordinated Tin Atoms

The basic shape for this group of compounds based on tin(IV) is a trigonal bipyramid (tbp). Although this is a common coordination geometry for tin (as demonstrated by the fact that from the 1700 pentacoordinated tin species that are included in the CSD, approximately 1600 have tbp geometry around the tin atom), only catena-Ph3 SnF (128) has been reported to have a symmetry- imposed perfect tbp geometry.123 [(SnBr)·Ni(H2 O)(MeCN)(L)]+ Br− (H2 L = N,N’-1,3-propylenebis-3-methoxysalicylideneamine) (129) contains a penta-coordinated tin(II) atom in a highly distorted pseudo-octahedral geometry with an atypically wide equatorial angle of 151◦ .124 Another unusual irregular geometry around a penta-coordinated tin atom was observed in [LiSn(Sit Bu2 Me)3 ]2 (11) (Figure 2.11.13).6 Compounds with Hexa-Coordinated Tin Atoms

Coordination number six results mostly in an octahedral geometry around the tin center. Such a symmetryimposed perfect octahedron can be found, for example, in the dihydroxy-meso-tetraphenylphorphirinatotin(IV) (130) and has all X Sn X (X = N or O) angles equal to 90◦ .125 The Sn N bond lengths ˚ and are slightly longer than the Sn O bond lengths (2.02 A). ˚ The tin oxalate are 2.11 A [Me2 NH(CH2 )4 NHMe2 ][Sn2 (C2 O4 )3 ] (131) contains a tin atom coordinated by six oxygen atoms in the form of a pentagonal pyramid (Figure 2.11.14).126 The tin atom lies slightly below the plane formed by the five equatorial oxygen atoms, as a result of the presence of a stereoactive lone electron pair and thus, the geometry is properly described as a pseudo-pentagonal bipyramid.

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Si2

Sn1 Si3

Li1A Si1A Si1 Li1

Si3A Sn1A

Si2A

1Å

Figure 2.11.13

Molecular structure of compound 11

Sn1B

O2 O4

Sn1C

O1A O1 O2A

O3

O3A

O6 Sn1

O5

O4A O5A 1Å

O6A Sn1A

Figure 2.11.14

Part of the polymeric anion in 131

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Figure 2.11.15

Structure of compound 134 exhibiting only the anion

Compounds with Hepta-Coordinated Tin Atoms

The coordination number seven is relatively common for compounds containing tin in the oxidation state +IV and represents mostly pentagonal bipyramidal geometry. Most of them contain a bidentate ligand forming four- or five-membered chelates with the metal. Compounds PhSn(i Bu2 NCS2 )3 (132)127 and [Ph2 Sn{PhN(O)N(O)}2 (H2 O)] (133)128 are examples with a range for the equatorial angles of 65.5–76.3 and 68.2–76.5◦ , respectively. The geometry of the anion in [NH4 ][SnF(EDTA)] (134)129 is based on an octahedral geometry with four oxygen atoms in the equatorial plane, but one of the apical positions is split into two coordination sites occupied by two nitrogen atoms from the EDTA (Figure 2.11.15). In [SnCl(18-crown-6)][SnCl3 ] (135), the cation contains a tin(II) center in a pseudo-hexagonal bipyramidal environment.130 Compounds with Octa-Coordinated Tin Atoms

With the increasing number of donor atoms around the tin center, there are fewer and fewer structurally characterized examples of coordination species. However, two main coordination geometries have been observed for tin(IV) octa-coordinated species: hexagonal bipyramidal (hbp; mostly with highly irregular values for the equatorial angles) and tetragonal anti-prism (ta). [(Ph3 As)3 Ag][SnPh2 (NO3 )3 ] (hbp; 49.9– 72.6◦ ) (136)131 and bis(phthalocyanine)tin(IV) (ta) (137)132,133 can be considered as examples of these coordination geometries. There are no structurally authenticated examples of tin with coordination number nine. Compounds with Deca-Coordinated Tin Atoms

The coordination number ten is the highest reported so far for coordination species of tin. It is observed only in [Sn(15-crown-5)2 ][SnCl3 ]2 (138) and the geometry around the tin center is pentagonal anti−prismatic with a non-stereoactive lone electron pair (Figure 2.11.16).134 However, a disorder was detected in the crystal with a staggered conformation of the rings that leads to pentagonal-prismatic coordination.

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Figure 2.11.16

247

Structure of compound 138 exhibiting only the cation

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

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Tin(II) Heterobimetallic and Oligometallic Derivatives

Muhammad Mazhar and Imtiaz-ud-Din Department of Chemistry, Quaid-i-Azam University, Islamaba, Pakistan

2.12.1

Introduction

The synthesis and composition of tin(II) heterometallic compounds are of particular importance, as their multimetallic oxides and phosphides cover a wide range of applications due to their specific electronic, optical, and catalytic properties.1−4 The ground-state electronic configuration of tin, [Kr] 4d10 5s2 5p2 5d0 , demonstrates its natural inclination to expand the coordination sphere owing to the availability of vacant d-orbitals. Tin, either in oxidation state +II or +IV, contributes significantly to the development of high tech inorganic material. There has been a growth in the area of tin(II) chemistry since the landmark preparation of the bulky dialkyltin(II) species, Sn[CH(SiMe3 )2 ]2 by Davidson and Lappert in 1975. The preparation of divalent compounds, R2 M, of group IV elements, where R is an organic group, and M is C, Si, Ge, Sn, or Pb, is a formidable task as they are kinetically unstable so that their coordinative unsaturation leads to disproportionation or polymerization. Stannous alkoxides (methyl, ethyl), because of molecular association, are less volatile and have lower solubilities in organic solvents than their bulky analogs, which restrict their technological uses. The most widely studied tin oxide, cassiterite, has a pivotal role in the development of various optoelectronic devices including solar cells, liquid crystal displays,5−7 thermoelectronic converters,8,9 phosphors10 and solid-state devices for gas alarms for use in domestic, commercial, and industrial premises.11,12 This suggests a continuous need for research on these materials for optimization of parameters related to their performances. It has long been known13 that when two or more metal sites with different electronic configurations are taken to make a ceramic bimetallic/oligometallic material, the synergic effect of the metals can lead to enhanced activity/selectivity of the final ceramic material. The functionality of the tin oxide-based material may be increased by the presence of a second metal such as in indium–tin oxide (ITO) which is largely used in displays of various kinds, e.g. electrochemichromic cells14 and antimony-doped tin oxide, which enhances the conductivity of the material.15 The chemistry of Sn(II) alkoxides may suggest some unique structural features that impart flexible bridging tendencies between similar or dissimilar metal atoms. Thus, tin(II) heterobimetallic alkoxides could serve as single source precursors to M : SnO2 composites (where M may be any other metal). Sometimes, lower homologues of alkoxides with groups such as OEt and OPri , could suffer from disproportionation into separate homometallic components at elevated temperatures during CVD applications. Functionalized alkoxide groups like R2 NCH2 CH2 O and ROCH2 CH2 O can offer greater prospects for stabilizing the Sn–O–M moiety.16 Generally, metal β-diketonates are mostly monomeric and volatile, and are being used as precursors for metal–organic chemical vapor deposition. A number of Sn(II) derivatives have been synthesized as mono- or oligomers, depending on the steric influence on the ligand.17 The oligomerization of Sn(II) derivatives can be predicted by merely counting the electron pairs and free orbitals of the combining elements as well as considering the bulkiness of the ‘R’ substituents of the chelating ligands, as manifested by the pioneering work of Bradley and Mehrotra,18 i.e. small ligands will favor a high degree of oligomerization, whereas bulky ligands favor the formation of monomeric species. The aim and purpose of the synthesis of tin(II) heterobimetallic derivatives is to prepare such a material that could deliver both elements of a final material simultaneously, leading to formation of complex ceramic materials in a single step, and was envisaged in the deposition of SrTa2 O6 directly from [SrTa2 (OEt)6 (μ-OEt)4 (μ-bis-dmap)2 ]19 , where dmap = 1,3-bis(dimethylamino)-2-propanoate.

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This strategy was adopted to alleviate tedious techniques of mixing multicomponent precursors with different properties.20 In view of their technological demands, tin(II) heterometallic alkoxides are the most studied class of compounds and their properties can be modulated by chelating ligands like β-diketonates, carboxylates, and functionalized alkoxides for specific purposes. These compounds also facilitate the formation of oxoalkoxides, either by hydrolysis or by some side reactions. Ligand Coordination Modes

The most widely used ligands in the formation of tin(II) heterometallic compounds are those which can establish metal–oxygen bonds conveniently, namely the alkoxides (OR), functionalized alkoxides (O(CH2 )X, where X = OR, NR2 ), β-diketonates (RCOCH2 COR), and carboxylates (OOCR). The alkoxides offer many attractive synthetic routes to the formation of heterometallic species due to certain specific reasons. Firstly, the oxygen atom in the alkoxy ligand has a lone pair of electrons and occupies doubly bridging (μ2 ) or triply bridging (μ3 ) positions, so that the probability of linking two or more metals in the form of oligomeric units is increased. Secondly, metal alkoxides are more labile and can serve as better starting compounds for the synthesis of heteroleptic species, such as Sn(OR)n-x Zx (Z = RCOCH2 COR, OOCR). Thirdly, it is facile to interchange, at room temperature, a simple alkoxide group by functionalized alkoxides, O(CH2 )n X (X = OR alkoxyalcohol; or NR2 alkanolamine), thus meeting the solubility and volatility demands required for these particular types of compounds. Various coordination modes of alkoxides and functionalized alkoxides are delineated in Scheme 2.12.1. β-Diketones such as pentane-2,4-dione (acac) ionizes like weak acid in aqueous solution and the resulting anion can act as a ligand towards metal ions, forming complexes through both oxygen atoms by means of a six-membered ring (Figure 2.12.1). R O M

R

R

O

O M M M

M

Terminal

M

Bridging μ2

Bridging μ3

1a X O

O

M

M

Terminal O

X

X

M

Bridging μ2 M O

M

O MMM

Bridging μ3 M

CH2 O

X

Chelating

M X

Bridging-chelating μ2

M

M

CH2 X

Triple bridging chelating μ3

1b

Scheme 2.12.1 Coordination modes of alkoxides (1a) and functionalized alkoxides (1b) where X = OR, NR2 and M = the same or different metals21

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H H3C

C O

Figure 2.12.1

C M

CH3

O

Six-membered aromatic ring of metal β-diketonates

The high stability constant of metal β-diketonates is mainly due to the aromatic character of the sixmembered ring. β-Diketones can adopt various modes of combination for complexation with metals, which are depicted in Scheme 2.12.2. Metal carboxylates [M(O2 CR)n ]m are used as associated oxide precursors where the carboxylate acts, mostly, as a chelating, bridging, or bridging-chelating ligand (Scheme 2.12.3). R1

R1 O R2

R1 O

R2

O R2

H H

O R3

M

O

O M

R3

(1)

(3) R1

R1

M/m

R2

R3

(4)

M

M

M O

M/m O

M M/m

R2 O

M M

M

(6)

R1

O R2

O R3

(5)

R1

M/m

R2

M

O M

(7)

O M/m

O R3

M

O

O R2

H

R3

(2)

R1

R3

M

R3

M M

(8)

Scheme 2.12.2 Various coordination modes of β-diketonates; where R1 , R2 , R3 = H, alkyl, or aryl, M, M = different metal atoms and m = the oxidation state of the metal

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O R

C

R O

M

R

Chelating M

M O

C

R

O

M

Bridging

Scheme 2.12.3

M O

Terminal O

C

C

M O

Bridging-chelating

Coordination modes of carboxylate ligands

Comparative Studies of Tin(II) Complexes

Metal alkoxides, aminoalkoxides, βdiketonates, and carboxylates are the most common reactants for the synthesis of Sn(II) heterobimetallic and oligometallic complexes, due to their bridging and bridgingchelation coordination properties. Such complexes can be prepared by simple mixing of the starting materials in appropriate solvents under mild conditions of temperature. These reactions usually proceed smoothly in hydrocarbon solvents over a period of 1–2 hours with progressive dissolution of the insoluble reactants, and the complexes can be crystallized out almost quantitatively from the filtrate. Thus, metal alkoxides may undergo exchange reactions with metal β-diketonates in a simple stoichiometric ratio to yield the heterobimetallic complex. M(acac)2 + 2Ti(OPri )4 −→ 1/2[M(acac)Ti(OPri )5 ]2 + 1/2[Ti(acac)(OPri )3 ]2 M = Sn2+ , Co2+

(2.12.1)

The reactivity of tin alkoxides with carboxylates is more complex than that of β-diketonates because of the following competition reactions: (1) Substitution of alkoxide by the carboxylate. Sn(OR)n + x R CO2 H −→ Sn(OR)n-x (R CO2 )x + x ROH

(2.12.2)

(2) Generation of oxo-ligands by non-hydrolytic condensation and elimination of an ester from an unstable tin carboxylatoalkoxide. 2Sn(OR)n-x (R CO2 )x −→ [(OR)n-x-1 Sn–O–Sn(R CO2 )x-1 ] + RCO2 R

(2.12.3)

The ease of formation of various stable Sn(II) heterometallic species follow the order: Sn(OR)n > 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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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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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. M¨ossbauer 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 M¨ossbauer 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 M¨ossbauer 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

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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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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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

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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 M¨ossbauer–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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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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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

Tin Compounds For CVD

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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19. (a) A. L. Dawar and J. C. Joshi, J. Mater. Sci., 19, 1 (1984); (b) H. Hiramatsu, W. S. Seo, and K. Koumoto, Chem. Mater., 10, 3033 (1998). 20. R. J. Francis, S. J. Price, J. S. O. Evans, S. O’Brien, D. O’Hare, and S. M. Clark, Chem. Mater., 8, 2102 (1996). 21. G. A. Shaw and I. P. Parkin, Main Group Met. Chem., 19, 499 (1996). 22. 10 C. Y. Tan, Y. Y. Xia, Y. P. Chen, S. Y. Li, J. T. Liu, X. D. Liu, B. Z. Xu, J. H. Li, and W. J. Cao, J. Appl. Phys., 73, 4266 (1993). 23. P. Balaz, T. Ohtani, Z. Bastl, and E. Boldizanova, J. Sol. State Chem., 144, 1 (1999). 24. R. Larciprete, E. Borsella, P. De Padova, P. Perfetti, and C. Crotti, J. Vac. Sci. Technol. A, 15, 2492 (1997). 25. (a) P. Boudjouk, M. P. Remington, D. G. Grier, W. Triebold, and B. R. Jarabek, Organometallics, 18, 4534 (1999); (b) P. Boudjouk, D. G. Grier, D. J. Seidler, J. Dean, and G. J. McCarthy, Chem. Mater., 8, 1189 (1996); (c) S. A. Papargyri, D. N. Tsipas, D. A. Papargyris, A. I. Botis, and A. D. Papargyris, Solid State Phenom., 106, 57 (2005); (d) B. Subramanian, C. Sanjeeviraja, and M. Jayachandran, Bull. Electrochem., 18, 349 (2002). 26. A. G. Pereira, A. O. Porto, G. G. Silva, G. M. de Lima, H. G. L. Siebald, and J. L. Neto, Phys. Chem. Chem. Phys., 4, 4528 (2002). 27. A. G. Pereira, L. A. R. Batalha, A. O. Porto, G. M. de Lima, G. G. Silva, J. D. Ardisson, and H. G. L. Siebald, Mater. Res. Bull., 38, 1805 (2003). 28. R. F. Bartholo and M. H. Garfinke, J. Electrochem. Soc., 116, 1205 (1969). 29. A. K. Saxena, R. Thangaraj, S. P. Singh, and O. P. Agnihotri, Thin Solid Films, 131, 121 (1985). 30. (a) B. J. Baliga and S. K. Ghandhi, J. Electrochem. Soc., 123, 941 (1976); Y. S. Hsu and S. K. Ghandhi, J. Electrochem. Soc., 126, 1434 (1979); (b) K. S. Chen, M. S. Li, H. M. Wu, M. R. Yang, J. Y. Tian, F. Y. Huang, and H. Y. Hung, Surf. Coat. Tech., 200, 3270 (2006); (c) J. Lancok, T. A. Santoni, M. Penza, S. Loreti, I. Menicucci, C. Minarini, and M. Jelinek, Surf. Coat. Tech., 200, 1057 (2005); (d) S. Tamura, T. Ishida, H. Magara, T. Mihara, S. Mochizuki, and T. Tatsuta, Appl. Surf. Sci., 169, 425 (2001); (e) S. Tamura, T. Ishida, H. Magara, T. Mihara, O. Tabata, and T. Tatsuta, Thin Solid Films, 142, 343 (1999). 31. N. Bertrand, P. Duverneuil, M. Amjoud, and F. Maury, J. Phys. IV, 9, 651 (1999). 32. J. Kane, H. P. Schweitzer, and W. Kern, J. Electrochem. Soc., 123, 270 (1976). 33. T. Yamazaki, U. Mizutani, and Y. Iwama, Jpn. J. Appl. Phys., 22, 454 (1983). 34. R. Y. Korotkov, P. Ricou, and A. J. E. Farran, Thin Solid Films, 502, 79 (2006). 35. L. M. Atagi, D. M. Hoffman, J. R. Liu, Z. Zheng, W. K. Chu, R. R. Rubiano, R. W. Springer, and D. C. Smith, Chem. Mater., 6, 360 (1994). 36. J. Sundqvist, J. Lu, M. Ottosson, and A. Harsta, Thin Solid Films, 514, 63 (2006). 37. A. M. B. van Mol, Y. Chae, A.H. McDaniel, and M.D. Allendorf, Thin Solid Films, 502, 72 (2006). 38. A. M. B. van Mol and M.D. Allendorf, Top. Organomet. Chem., 9, 1 (2005). 39. (a) S. Suh and D. M. Hoffman, Inorg. Chem., 35, 6164 (1996); (b) S. Mathur, V. Sivakov, H. Shen, S. Barth, C. Cavelius, A. Nilsson, and P. Kuhn, Thin Solid Films, 502, 88 (2006); (c) M. Veith, J. Freres, P. Konig, O. Schutt, V. Huch, and J. Blin, Eur. J. Inorg.Chem., 18, 3699 (2005) 40. N. Hollingsworth, G. A. Horley, M. Mazhar, M. F. Mahon, K. C. Molloy, P. W. Haycock, C. P. Meyers, and G. W. Critchlow, Appl. Organomet. Chem., 20, 687 (2006). 41. N. Bertrand, F. Maury, and P. Duverneuil, Surf. Coat. Tech., 200, 6733 (2006). 42. Q. Kuang, S. F. Li, Z. X. Xie, S. C. Lin, X. H. Zhang, S. Y. Xie, R. B. Huang, and L. S. Zheng, Carbon, 44, 1166 (2006). 43. M. Veith, S. J. Kneip, A. Jungmann, and S. Hufner, Z. Anorg. Allgem. Chem., 623, 1507 (1997). 44. M. Amjoud and F. Maury, J. Phys. IV, 9, 643 (1999). 45. D. Belanger, J. P. Dodelet, B. A. Lombos, and J. I. Dickson, J. Electrochem. Soc., 132, 1398 (1985). 46. (a) J. Kane, H. P. Schweizer, and W. Kern, Thin solid Films, 29, 155 (1975) (b) O. O. Akinwunmi, M. A. Eleruja, J. O. Olowolafe, G. A. Adegboyega, and E. O. B. Ajayi, Optical Mater., 13, 255 (1999). 47. J. Ni, L. Wang, Y. Yang, H. Yan, S. Jin, T. J. Marks, J. R. Ireland, and C. R. Kannewurf, Inorg. Chem., 44, 6071 (2005). 48. J. Ni, H. Yan, A. Wang, Y. Yang, C. L. Stern, A. W. Metz, S. Jin, L. Wang, T. J. Marks, J. R. Ireland, and C. R. Kannewurf, J. Am. Chem. Soc., 127, 5613 (2005).

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49. A. Ortiz, J. Vac. Sci. Technol., 20, 7 (1982). 50. A. W. Metz, M. A. Lane, C. R. Kannewurt, K. R. Poeppelmeier, and T. J. Marks, Chem. Vapor Depos., 10, 297 (2004). 51. Y. Senzaki, G. B. Alers, A. K. Hochberg, D. A. Roberts, J. A. T. Norman, R. M. Fleming, and H. Krautter, Electrochem. Solid St., 3, 435 (2000). 52. E. Mays, D. W. Hess, and W. S. Rees, Jr., J. Crystal Growth, 261, 309 (2004). 53. J. F. Eichler, O. Just, and W. S. Rees, Jr., J. Mater. Chem., 14, 3139 (2004). 54. A. Sanchez-Juarez and A. Ortiz J. Electrochem. Soc 147, 3708 (2000). 55. T. Jiang and G. A. Ozin, J. Mater. Chem., 8, 1099 (1998). 56. K. Deraman, S. Sakrani, B. B. Ismail, Y. Wahab, and R. D. Gould, Int. J. Electronics, 76, 917 (1996). 57. M. Parenteau and C. Carlone, Phys. Rev. B, 41, 5227 (1990). 58. L. I. Berger,Semiconductor Materials, CRC Press, New York, 1997. 59. (a) A. Mondal, T. K. Chaudari, and P. Pramanik, Sol. Energ. Mater., 7, 431 (1983); (b) J. B. Johnson, H. Jones, B. S. Lathan, J. D. Parker, R. D., Engelken, and C. Barber, Semicond. Sci. Technol., 14, 501 (1999); (c) S. C. Ray, M. K. Karanjai, and D. Dasgupta, Thin Sol. Films, 350, 72 (1999). 60. P. Boudjouk, D. J. Seidler, S. R. Bahr, and G. J. MacCarthey, Chem. Mater., 6, 2108 (1994). 61. B. Palosz, W. Steurer, and H. Schultz, Acta Crystllogr. Sect. B., 46, 449 (1990). 62. D. O’Hare, W. Jaegermann, D. L. Williamson, F. S. Ohuchi, and B. A. Parkinson, Inorg. Chem., 27, 1537 (1988); C. A. Formstone, E. T. Fitsgerald, D. O’Hare, P. A. Cox, M. Kurmoo, J. W. Hodby, D. Lillicrap, and M. Gosscustard, J. Chem. Soc. Chem. Comm., 501 (1990). 63. U. V. Alpen, J. Fenner, and E. Gmelin, Mat. Res. Bull., 10, 175 (1975). 64. R. Kniep, D. Mootz, U. Severin, and H. Wunderlich, Acta Crystallogr., Section B, 38, 2022 (1982). 65. D. A. Dixon, J. Phys. Chem., 89, 5334 (1985). 66. G. M. de Lima, G. A. A. Costa, M. C. Silva, A. C. B. Silva, M. T. C. Sansiviero, and R. M. Lago, Phys. Chem. Chem. Phys., 2, 5708 (2001). 67. A. O. Porto, G. M. de Lima, A. G. Pereira, L. A. R. Batalha, and J. D. Ardisson, Appl. Organomet. Chem., 18, 39 (2004). 68. D. C. Menezes, G. M. de Lima, A. O. Porto, C. L. Donnici, J. D. Ardisson, A. C. Doriguetto, and J. Ellena, Polyhedron, 23, 2103 (2004). 69. D. C. Menezes, G. M. de Lima, A. O. Porto, and J. D. Ardisson, Phys. Chem. Chem. Phys., (2007), submitted. 70. G. Barone, T. Chaplin, T. G. Hibbert, A. T. Kana, M. F. Mahon, K. C. Molloy, I. D. Worsley, I. P. Parkin, and L. S. Price, J. Chem. Soc. Dalton, 6, 1085 (2002). 71. A. Sanchez-Juarez and A. Ortiz, Semicond. Sci. Technol., 17, 931 (2002). 72. A. Sanchez-Juarez, A. Tiburcio-Silver, and A. Ortiz, Thin Solid Films, 480, 452 (2005). 73. S. Lopez, S. Granados, and A. Ortiz, Semicond. Sci. Technol., 11, 433 (1996). 74. C. L. Bowes and G. A. Ozin, J. Mater. Chem., 8, 1281 (1998). 75. A. T. Kana, T. G. Hibbert, M. F. Mahon, K. C. Molloy, I. P. Parkin, and L. S. Price, Polyhedron, 20, 2989 (2001). 76. (a) S. O. Ferreira, P. H. O. Rappl, H. Closs, and I. N. Bandeira J. Appl. Phys., 82, 2405, (1997); (b) N. Suzuki and S. Adachi, Jpn. J. Appl. Phys., 34, 5977-5983 (1995); (c) J. R. Singh and R. K. Bedi, Thin Solid Films, 199, 9 (1991); (d) K. Fukui, J. Phys. Soc. Jpn., 61, 2018 (1992). 77. Y. Cheng, T. J. Emge,, and J. G. Brennan, Inorg. Chem., 35, 342 (1996). 78. S. I. Chuprakov, D. Klaus-Hermann, J. J. Schneider, and J. Hagen, Chem. Mater., 10, 3467 (1998). 79. M. Huang and Y. P. Feng, Phys. Rev. B, 70, 184116 (2004). 80. P. Clark, W. Li and S. T. Oyama, J. Catal. 200, 140 (2001). 81. R. Binions, C. S. Blackman, C. J. Carmalt, S. A. O’Neill, I. P. Parkin, K. Molloy, and L. Apostilco, Polyhedron, 21, 1943 (2002). 82. S. Recchia, C. Dossi, A. Fusi, L. Sordelli, and R. Psaro, Appl. Catal. A: General, 182, 41 (1999). 83. J. Kouvetakis, J. Menendez, and A.V.G. Chizmeshya, Annu. Rev. Mater. Res., 36, 497 (2006). 84. A. Onda, T. Komatsu, and T. Yashima, J. Catal., 201, 13 (2001).

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3.2

Class II Tin-Based Hybrid Materials Prepared From Alkynyltin Precursors

Thierry Toupance University de Bordeaux, Institut des Sciences Mol´eculaires, Groupe Mat´eriaux, 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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Chain loading (mmol.g )

OTE/SPH

b

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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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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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

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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. Labrug`ere, Dr. V. Vivier, Dr. H. Cachet and Dr. J. Br¨otz 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 Calv´e, 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. Dieudonn´e, 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. Dieudonn´e, 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. Labrug`ere, 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. L´eonardelli, L. Facchini, C. Fr´etigny, P. Tougne, and A.P. Legrand, J. Am. Chem. Soc., 114, 6412 (1992). 23. F. Banse, F. Ribot, P. Tol´edano, 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. Gr¨atzel, 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. Br¨otz, Chem. Mater, 18, 6364 (2006). 32. H. Elhamzaoui, B. Jousseaume, H. Riague, T. Toupance, P. Dieudonn´e, 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 M¨ossbauer 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, %

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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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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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Stabilizer conversion, %

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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

Yellowness Index

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40

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Figure 3.3.8 Elsevier.)

20

40

60 80 Time, minutes

100

120

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)

OTE/SPH

60

1

2

50 40 30 20 10

4

0 0

5

10

15

20

25

30

35

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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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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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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10 9 8 7 6 5 4 3 2 1 0 0

2

1

4

6 Time, Min.

8

10

0 0

20

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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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

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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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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) (Begr¨undung 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. Z¨und, 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. Z¨und, 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. Kr¨uttli, 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. R¨uhlemann, 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

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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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 <0.1% Cl <1% free H2 O 3.40 2–3 >200 <0.01% 1.9 Very low*

ZnSnO3 232.07 12036-37-2 405-290-6 Yes White powder 51% Sn 28% Zn <0.1% Cl <1% free H2 O 4.25 2–3 >570 <0.01% 1.9 Very low*

Specific gravity Median particle size (μm) Decomposition temperature (◦ C) Aqueous solubility (at 20◦ C) Refractive index (at 20◦ C) Acute oral toxicity * LD50 (rats) > 5000 mg/kg

Despite the clear technical advantages of ZHS and ZS over many competitive flame retardants, their relatively high price compared to, for example, the widely used antimony(III) oxide (Sb2 O3 ), has somewhat limited their industrial usage to date. Recent market surveys have indicated that worldwide ZHS/ZS consumption is currently in the range 1000–1500 tonnes per annum, with a medium-term target of 5000 tonnes per annum being considered realistic if environmental and toxicological concerns about Sb2 O3 and certain other flame retardants persist.3 Comprehensive fire test data for ZHS and ZS in a range of polymers have been published by ITRI.24 Halogen-Containing Polymer Formulations

Although Sb2 O3 has long been established as the most widely used flame-retardant synergist for use in PVC and other halogen-containing polymeric materials, concerns about its toxicity25 combined with fluctuations in price have led to an increasing demand for alternative synergists. Inorganic tin compounds, particularly ZHS and ZS, and to a lesser extent, SnO2 , have been widely studied at ITRI and elsewhere, in a range of materials that includes rigid and flexible PVC, unsaturated polyester resins, chlorinated elastomers, paints and coatings, and in various thermoplastics where halogen is incorporated as a chlorinated or brominated flame retardant additive (Table 3.5.2). Typical incorporation levels of ZHS and ZS range from 1–10 parts per hundred of resin (phr). The relative fire-retardant efficiency of tin additives, including ZHS and ZS, compared to that of Sb2 O3 depends on a number of factors: r Physical form of the additive (e.g. particle size, surface area). r Nature of the host polymer. r Chemical nature of the halogen source (e.g. aliphatic vs. aromatic). r Ratio of halogen to synergist in the formulation. r Presence of other additives in the formulation. r Test method used to evaluate fire-retardant performance.

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ZHS and ZS in halogen-containing polymer formulations

Polymer

FR additives

References

Rigid PVC Flexible PVC Polychloroprene (‘Neoprene’) Chlorosulphonated polyethylene (‘Hypalon’) Polyester resins Nylons Polystyrene (e.g. HIPS) Alkyd resin paints

ZHS, ZS (& SnO2 ) ZHS & ZS ZHS ZHS ZHS ZS ZS ZHS &ZS

26–28 27, 29, 30 29, 31 32 33–39 27, 40 41 42

Comprehensive studies at ITRI and elsewhere have indicated that, whereas Sb2 O3 is generally more effective than ZHS/ZS when used with aromatic halogen compounds, the tin synergists are better when used in conjunction with aliphatic or alicyclic halogen compounds.43 It has been demonstrated that the optimum atomic ratio of halogen to tin for flame-retardant efficiency in the acrylonitrile-butadienestyrene (ABS)/decabromobiphenyl/hydrous SnO2 system is much higher (ca. 9:1) than that expected on a stoichiometric basis (4:1).44 Similarly, optimum flame retardancy for ZHS and ZS often occurs at relatively high halogen to metal ratios, although the precise ratio appears to be dependent on the nature of the halogen source and that of the host polymer itself.43 Probably the most important performance benefit of ZHS and ZS over Sb2 O3 relates to their outstanding smoke-suppressant properties. Hence, in a commercial flexible PVC cable formulation, whereas 7 phr addition level of Sb2 O3 was found to markedly increase smoke emission, similar incorporation levels of either ZHS or ZS gave significant decreases in smoke production compared to the control formulation.23 Measured maximum smoke densities for the ZHS/ZS systems were 55–60% lower than for the corresponding Sb2 O3 system. In terms of practical usage, PVC wire and cable insulation has been one of the key application areas for zinc stannates, particularly in plenum systems (these being concealed spaces above rooms which contain electrical wires and cables), where very severe fire standards must be met.45 The grades of ZHS and ZS used in PVC electrical cables must have very low electrolyte levels and must not contain any free zinc oxide, since presence of ZnO can result in a phenomenon known as ‘zinc burning’ in which the PVC undergoes dehydrochlorination during processing. This problem has long been overcome and current commercial ZHS and ZS powders can be processed into PVC and other halogenated polymers without any thermal degradation drawbacks.43 Halogen-Free Polymer Formulations

In recent years, concerns have been widely expressed about the inherent toxicity of certain halogenated flame retardants and their persistence in the environment.6 Furthermore, during incineration processes, polymers containing certain brominated and chlorinated additives are believed to contribute to highly toxic dioxin emissions46 and these factors have led to an increasing demand for ‘low smoke, zero halogen’ flame-retardant formulations in transportation, construction, and, particularly, the electronics industry.47 Halogen-free flame retardancy is commonly achieved by the incorporation of inorganic fillers, typically alumina trihydrate (ATH) or magnesium hydroxide (MH), into the host polymer.48 Although these fillers are essentially non-toxic and relatively inexpensive, the high levels required for adequate flame retardancy often lead to processing difficulties and a marked deterioration in other critical polymer characteristics, including mechanical, physical, and electrical properties.

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ZHS and ZS in halogen-free polymer formulations

Polymer

FR additives

References

Natural & synthetic rubbers Polyester resins & GRP Epoxy resins Epoxy resins Polyolefins Polyolefins EVA & EEA copolymers EVA copolymer EEA & polyethylene (LDPE) EVA & polyethylene (LLDPE) PE – EVA blend PC – ABS blend ABS & PBT (glass-reinforced) PET & PBT

ZHS or ZS + ATH ZHS, ZS or other metal stannates (+ ATH) ZHS or ZS + MH + mica ZS + ATH ZHS or ZS + MH + ammonium polyphosphate ZS + MH + nanoclay ZHS or ZS + ATH or MH ZS + MH ZS + MH ZS + MH (+ ammonium octamolybdate + silicone) ZHS + ATH ZS + triphenyl phosphate + Sb2 O3 + ‘ultracarb’ ZS + aluminium diethylphosphinate ZS + red phosphorus

49,50 43, 51 52 53 54 55, 56 5, 57 58 59 60 5 61 62 63

Although commercial applications of ZHS and ZS fire retardants have primarily focused on halogenated polymers, recent work at ITRI and elsewhere has demonstrated flame-retardant and smoke-suppressant activity in halogen-free formulations (Table 3.5.3). There has been considerable activity in the electronics industry to develop halogen-free alternatives to brominated flame retardants, certain of which have been phased out under the EU Restriction of Hazardous Substances (RoHS) Directive, 2002/95/EC. Although many bromine-containing flame retardants are not restricted by RoHS, the related Waste Electrical and Electronic Equipment (WEEE) Directive, 2002/96/EC, will require separation and special handling of plastics containing any brominated flame retardants, and electronic companies are therefore developing alternatives to avoid the extra costs of separation. Since 2000, major Japanese companies have been developing halogen-free polymer systems for electronic applications and many of these utilize zinc stannates (see Table 3.5.3). Apart from ZHS and ZS, little work has generally been undertaken on tin-based fire retardants in nonhalogen polymer systems. However, certain tin(II) compounds have shown excellent flame-retardant and smoke-suppressant properties when incorporated at levels of 20–30% into aromatic polyesters, specifically polybutylene terephthalate (PBT).64 Hence, tin(II) oxide, tin(II) oxalate, and tin(II) phosphate have been shown to markedly increase flame retardancy in PBT, whereas, interestingly, tin(IV) oxide is almost totally ineffective in the same polymeric substrate. 3.5.4

Recent Developments

Inorganic Tin Colloids

ITRI has developed and patented processes for preparing and utilizing stable aqueous colloidal sols of tin(IV) oxide and related materials.65,66 These products, which contain nanometer-scale particulate tin oxide, are useful precursors for the synthesis of ceramic bodies, powders, and coatings, and may find application in encapsulated pigments, electroconductive materials, catalysts, and transparent tin oxide films on glass and other substrates. In the context of fire retardancy, aqueous tin colloids are particularly suitable for treatment of hydrophilic natural fibers. In addition to earlier studies using soluble tin salts,67 ITRI has developed processes

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based on colloidal suspensions of tin(IV) oxide, tin(IV) borate, tin(IV) phosphate, and tin(IV) tartrate for flame-resistant treatments of paper,65,66 cotton,68 and wool,17 the latter substrate being proteinaceous rather than cellulosic in nature. Further work is being undertaken to commercialize these processes as environmentally acceptable alternatives to current flame-retardant formulations that are facing increasing scrutiny. Ultrafine Powders

Despite their clear technical benefits, including non-toxicity and excellent smoke-suppressant activity, markets for ZHS and ZS have been somewhat limited to date, primarily because of their relatively high price compared with certain traditional flame retardants, particularly Sb2 O3 . Consequently, recent studies have focused on the development of more cost effective tin-based systems. Initial investigations at ITRI resulted in the development of processes for producing ultrafine ZHS and ZS powders.69 These powders, with typical particle sizes of 0.1–0.3 microns (Figure 3.5.1 right), were shown to exhibit a number of performance benefits compared with commercial grades of ZHS and ZS, which are typically 2–3 microns in particle size (Figure 3.5.1 left). In particular, the ultrafine powders do not settle out in thermosetting resins, they can be used in formulations where translucency is required, and, perhaps most importantly, their fire-retardant efficiency is markedly improved compared with conventional ZHS and ZS powders. This enhanced activity allows reductions to be made in additive incorporation levels for any given fire-retardant performance.50 Coated Fillers

Although ultrafine and colloidal additives have been shown to exhibit significant performance benefits in systems where good dispersion can be achieved, agglomeration of ultrafine particles and the

Figure 3.5.1 Commercial grade (left) and ultrafine (right) ZHS and ZS powders. (Reproduced from Paper 6, Proceedings from ‘High Performance Fillers 2005’ Conference (Cologne, Germany, 8–9th March 2005), with permission from Rapra Technology.)

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Figure 3.5.2 Ultrafine ZHS coated on an inert filler. (Reproduced from Paper 6, Proceedings from ‘High Performance Fillers 2005’ Conference (Cologne, Germany, 8–9th March 2005), with permission from Rapra Technology.)

incompatibility of aqueous colloids with many polymeric substrates have proved to be major drawbacks to their widespread applicability. However, these problems have been largely overcome by coating the active tin species onto the surface of low cost inorganic fillers, which effectively act as carriers and prevent agglomeration when these coated fillers are incorporated into a polymeric matrix. Apart from their well-established synergistic action in halogen-containing systems (Table 3.5.2), tin additives have also been shown to exhibit synergism when used in conjunction with hydrated fillers, particularly alumina trihydrate (ATH) and magnesium hydroxide (MH), (Table 3.5.3). In order to maximize these synergistic interactions, ITRI has developed processes for coating ultrafine ZHS/ZS70 or nano-particulate tin species71 onto a range of inorganic fillers, including ATH, MH, and ‘inert’ fillers such as calcium carbonate, silica, alumina, titanium dioxide, and talc (Figure 3.5.2). Extensive studies in thermoplastic, thermosetting and elastomeric polymers have shown that these coated fillers outperform the fillers themselves or equivalent composition physical mixtures of tin additive plus filler.5,24 Consequently, lower addition levels of the coated grades, compared with uncoated fillers, are required for a given fire-retardant performance and this reduction in filler loading is expected to lead to better polymer processing and improved physical, mechanical, and electrical properties, as appropriate. ZHS-coated fillers have been produced with compositions ranging from 1% ZHS:99% filler, to 50% ZHS:50% filler, by weight, although typical commercial products usually contain 5–10% ZHS, which appears to be the optimum range, at least as far as cost efficiency is concerned. X-ray photoelectron spectroscopy (XPS) and diffuse reflectance infrared Fourier-transform spectroscopy have been used to investigate the interaction between the ZHS coating and the hydrated filler substrate; although interaction is confirmed, no evidence has been found for condensation reactions occurring between ZHS and ATH (MH) or the formation of Sn–O–Al (Mg) bonds.72,73 Coated fillers have been evaluated in a range of halogen-containing polymer systems, including rigid and flexible PVC,74,75 polychloroprene (Neoprene),75 and in thermoplastic and thermosetting materials containing additive or reactive halogenated flame retardants.75 In general, it is found that the coated filler outperforms the filler itself or an equivalent weight mixture of the tin compound plus filler, presumably because of the markedly improved dispersion of the active tin component within the polymeric substrate.

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Recent studies of coated fillers have focused on their activity in halogen-free polymer formulations and encouraging results have been obtained in ethylene-vinyl acetate (EVA),5,57 ethylene-ethyl acrylate (EEA)5 and epoxy resins,76 particularly for applications in the electronics industry. Nanoclays

Recent years have seen increasing interest in nano-composite materials, comprising polymer-layered silicate clay intercalated structures, and their flame-retardant properties.77 More specifically, it has recently been claimed that the use of a combination of a conventional hydrated filler (preferably ATH) with a nanoclay, as a fire-retardant system for polymeric materials, gives a more coherent char during combustion than using either the hydrated filler or the nanoclay alone.78 Subsequently, ITRI work on a halogen-free EVA cable formulation has demonstrated that, in addition to improving the performance of the hydrated filler itself, the incorporation of a montmorillonite-type nanoclay along with ZHS (either as an additive or as a coated filler) leads to further synergistic effects.57,79,80 3.5.5

Fire-Retardant Mechanism

Halogen-Containing Systems

Although much work has been carried out on the mode of action of flame retardants generally, the mechanisms associated with tin additives are only partially understood. It is clear that tin-based fire retardants can exert their action in both the condensed and vapor phases, and that the precise action in any particular system depends on a number of factors, including incorporation level, the amount and chemical nature of other additives present, and the nature of the polymer itself. Thermal analysis experiments have clearly shown that tin-based fire retardants markedly alter both the initial pyrolysis and the oxidative burn off stages that occur during polymer breakdown.33,35 These changes have been interpreted as being indicative of an extensive condensed phase action for the tin additive, in which the thermal breakdown of the polymer is altered to give increased formation of a thermally stable carbonaceous char at the expense of volatile, flammable products. The consequent reduction in the amount of fuel supplied to the flame largely accounts for the beneficial smoke-suppressant properties associated with zinc stannates and other tin-based fire retardants. Elemental analyses of char residues have shown that tin and, to a lesser extent, zinc are partially volatilized during combustion of ZHS/ZS-containing halogenated polymers, suggesting that these metals may also exhibit vapor phase flame-inhibiting activity.33,35 Antimony, which undergoes almost complete volatilization in the same systems, shows little char-enhancing behavior and operates primarily in the vapor phase by forming highly volatile antimony halides or oxyhalides. Boiling points of metal halides are given in Table 3.5.4. Table 3.5.4

Boiling points of metal halides Chlorides

Bromides ◦

Compound

Boiling point ( C)

Compound

Boiling point (◦ C)

SbCl3 SnCl4 SnCl2 ZnCl2

283 114 652 732

SbBr3 SnBr4 SnBr2 ZnBr2

280 202 620 650

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In line with these observations, earlier 119 Sn M¨ossbauer spectroscopic studies of rigid PVC samples containing SnO2 as a fire-retardant additive have shown that Sn(IV) is partially reduced to Sn(II) species (SnCl2 and SnO) and even to metallic tin during thermal degradation and combustion processes.26 In this case, the relatively involatile SnCl2 is detected in the char residue, whereas highly volatile SnCl4 is not. Halogen-Free Systems

The mode of action of tin additives in halogen-free polymers has not been studied in detail, but the near quantitative retention of tin in the char residue is indicative of condensed phase activity.49 Thermal analysis of halogen-free polyester resin containing ZHS and ATH indicates that the tin compound exhibits significant char enhancing properties (Table 3.5.5).43 Hence, ZHS is found to markedly increase the weight loss associated with char oxidation at the expense of the initial pyrolysis loss, when compared with the resin containing ATH alone. Further evidence of the char-promoting activity of ZHS is provided by the observed residual yield at 600 ◦ C, which is significantly greater than that expected on the basis of involatile inorganic material (i.e. ZnSnO3 + Al2 O3 ) which remains in the char. In accord with this finding, certain metal oxides are believed to act as dehydrogenation catalysts in halogen-free polymer systems, and proprietary grades of ATH and MH containing small amounts of char-promoting metal oxides81 or nanoclays82 have been developed. In addition to the above processes, the highly endothermic dehydration of ZHS at temperatures above ca. 200 ◦ C may partially account for its fire-retardant activity when used in halogen-free polymer formulations. In the case of coated fillers, the char-promoting effect of the tin component supplements the endothermic activity of the hydrated filler, which itself involves: (a) reduction in heat feedback from the burning gases in the flame to the decomposing polymer beneath, (b) the formation of an insulating char layer above the unburnt polymer, and (c) the absorption of volatile species and fragments on the very high surface area anhydrous metal oxide residue.83 Furthermore, the ultrafine or nano-particulate tin species on the coated filler is thought to thermally decompose to form a highly active catalytic surface, thereby maximizing any synergistic effect with the filler. The fire-retardant mechanism associated with nanoclays has recently been studied and is likely to involve the formation of a ceramic skin which catalyzes char formation by thermal dehydrogenation of the host polymer to produce a conjugated polyene structure.84 The nanocomposite structure present in the resulting char appears to enhance the performance of the char through reinforcement of the char layer.85 These effects would explain the apparent fire-retardant synergy observed when nanoclays are incorporated into polymer formulations containing condensed phase fire-retardant systems, including coated fillers.

Table 3.5.5

Thermal analysis data for halogen-free polyester resin samples∗ Pyrolysis stage∗∗

Char oxidation stage∗∗∗

Residue at 600◦ C

ZHS (phr)

ATH (phr)

Weight loss (%)

DTGmax (◦ C)

Weight loss (%)

DTGmax (◦ C)

Observed (%)

Calculated (%)

None None 5

None 25 25

88.6 77.6 67.8

363 342 336

11.3 9.9 14.1

544 534 497

0.1 12.5 18.1

0 13.1 15.7

∗

Heating rate = 10◦ C / minute in flowing air Temperature range = ca. 260–450◦ C ∗∗∗ Temperature range = ca. 450–570◦ C ∗∗

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Summary

Certain tin compounds, particularly zinc hydroxystannate (ZHS) and zinc stannate (ZS), are effective flame retardants in a wide range of polymeric substrates and offer technical advantages over many competitive additives: r r r r r

Very low toxicity. Dual phase action. Combined flame retardancy and smoke suppression. Effective at low incorporation levels. Synergistic effects when used in conjunction with other additives and fillers.

Beneficial effects are observed both in halogen-containing and halogen-free polymeric formulations, and commercial products are available for use in plastics, rubbers, paints, coatings, foams, fibers, and composites. Recent developments have focused on more cost effective tin-based fire retardants, including colloids, ultrafine powders, and coated fillers. Although current worldwide usage of tin-based fire retardants is probably little over 1000 tonnes per annum,3 growing concerns about the toxic nature and detrimental environmental impact of many conventional flame retardants are expected to result in significant market growth for ZHS, ZS, and other tin additives in the years ahead. References 1. Anon, Flame Retardants – Frequently Asked Questions, The European Flame Retardants Association, Brussels, January 2004. 2. Anon, Flame Retardants, DG Environment, European Commission, April 2000. 3. Anon, Market Study – Flame Retardants (UC 405E), CERESANA Research, Konstanz, Germany, July 2006. 4. F. Versmann and A. Oppenheim, Eng. Pat. 2077 (1859). 5. P.A. Cusack, Proceedings of High Performance Fillers, Rapra Technology, Cologne, Germany, March 2005, Paper 6. 6. G. Lyons, Brominated Flame Retardants, WWF Toxics Programme, May 1999. 7. Anon, European Food Safety Authority Journal, 254, 1 (2005). 8. W.H. Perkin, Eng. Pat. 9620 (1902). 9. W.H. Perkin, Text. Manuf., 40, 27 (1914). 10. J.E. Ramsbottom and A.W. Snoad, The Fireproofing of Fabrics, 2nd Report Fabrics Coord. Res. Comm., Dept. Sci. Ind. Res. (GB), 1930. 11. S. Coppick and W.P. Hall, in Flameproofing Textile Fabrics, R.W. Little (Ed.), Am. Chem. Soc. Monogr. Ser. No. 104, Reinhold, New York, 1947, p.221. 12. G.P. Nair, Colourage, 47, 21 (2000). 13. G.P. Nair, Colourage, 47, 27 (2000). 14. P.E. Ingham, Tin and Its Uses, 105, 5 (1975). 15. L. Benisek, Br. Pat. 1,385,399 (1975). 16. P.A. Cusack, P.J. Smith, J.S. Brooks, and R. Smith, J. Text. Inst., 70, 308 (1979). 17. P.A. Cusack, Tin Intern., 75, 11 (2002). 18. S.J. Blunden, P.A. Cusack, and R. Hill, The Industrial Uses of Tin Chemicals, Roy. Soc. Chem., London, 1985, p.202. 19. Toray Industries Inc., Brit. Pat. 1382659 (1975). 20. F. Kuegler and H. Schneider, Aust. Pat. 362866 (1981). 21. K. Takeya, H. Suzuki, and T. Ichimaru, Can. Pat. 1052517 (1979). 22. H.W. Finck and G. Tscheulin, Kunststoffe, 71, 320 (1981). 23. P.A. Cusack, Tin Intern., 72, 4 (1999).

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24. Anon, ITRI Technical Bulletins Nos. 1–4, (1999). 25. Anon, Antimony and its Compounds: Health Hazards and Precautionary Measures, Guidance Note EH 19 (Revised), HSE Solutions Ltd., Glasgow, 1997. 26. P.A. Cusack, P.J. Smith, and W.J. Kroenke, Polym. Degrad. and Stab., 14, 307 (1986). 27. D. Chaplin, Eur. Pat. 541618 (1993). 28. N.L. Thomas, Plastics, Rubber and Composites, 32, 413 (2003). 29. D. Chaplin and L.R. Tingley, Eur. Pat. 482093 (1991). 30. M.J. Herbert, US Pat. 5891571 (1999). 31. P.R. Hornsby, P.A. Mitchell, and P.A. Cusack, Polym. Degrad. and Stab., 32, 299 (1991). 32. P.R. Hornsby, P. Winter, and P.A. Cusack, Polym. Degrad. and Stab., 44, 177 (1994). 33. P.A. Cusack, A.W. Monk, J.A. Pearce, and S.J. Reynolds, Fire and Materials, 14, 23 (1989). 34. R.S. Bains, P.A. Cusack, and A.W. Monk, Eur. Polym. J., 26, 1221 (1990). 35. P.A. Cusack, M.S. Heer, and A.W. Monk, Polym. Degrad. and Stab., 32, 177 (1991). 36. P.A. Cusack, Fire and Materials, 17, 1 (1993). 37. F. Andre, P.A. Cusack, A.W. Monk, and R. Seangprasertkij, Polym. Degrad. and Stab., 40, 267 (1993). 38. P.A. Cusack, Polimery, 40, 650 (1995). 39. P.A. Cusack, M.S. Heer, and A.W. Monk, Polym. Degrad. and Stab., 58, 229 (1997). 40. R. L. Markezich and R.F. Mundhenke, Proceedings of Flame Retardants 96, Interscience Communications, London, January 1996, p.173. 41. C.J. Nalepa, Proceedings of 20th Intern. Conf. Fire Safety, Product Safety Corporation, Sunnyvale, California, January 1995. 42. R.S. Bains and P.A. Cusack, J. Oil and Colour Chemists Assoc., 73, 340 (1990). 43. P.A. Cusack, in Plastics Additives: An A–Z Reference, G. Pritchard (Ed.), Chapman and Hall, London, 1998, p.339. 44. J.D. Donaldson, J. Donbavand, and M.M. Hirschler, Eur. Polym. J., 19, 33 (1983). 45. M.J. Herbert, Proceedings of Flame Retardants 96, Interscience Communications, London, January 1996, p.157. 46. Anon, Proceedings of Dioxin 98, Swedish Environmental Protection Agency, Stockholm, August 1998. 47. C.G. Bergendahl, Alternatives to Halogenated Flame Retardants in Electronic and Electrical Products, IVF Research Publication 99824, Molndal, Sweden, 1999. 48. W.E. Horn Jr., in Fire Retardancy of Polymeric Materials, A.F. Grand and C.A. Wilkie (Eds), Marcel Dekker, New York, 2000, p.285. 49. P.A. Cusack and P.I. Fontaine, Speciality Chemicals, 9, 194 (1989). 50. P.A. Cusack and S. Karpel, Tin and Its Uses, 165, 1 (1991). 51. P.A. Cusack, P.J. Smith, and L.T. Arthur, J. Fire Retardant Chem., 7, 9 (1980). 52. K. Ikegai and H. Suwabe, Jap. Pat. 98/146917 (1998). 53. S. Osawa, Jap. Pat. 02/60592 (2002). 54. K. Ishida and A. Suzuki, Jap. Pat. 02/332384 (2002). 55. S. Ebrahimian and M.A. Jozokos, US Pat. 6492453 (2002). 56. S. Ebrahimian and M.A. Jozokos, US Pat. 7078452 (2006). 57. M.S. Cross, P.A. Cusack, and P.R. Hornsby, Polym. Degrad. and Stab., 79, 309 (2003). 58. H. Takahashi and K. Mizuno, Jap. Pat. 01/72824 (2001). 59. Y. Namiki, Y. Kato, M. Hanai, Y. Kitano, and H. Kurisu, US Pat. 5726231 (1998). 60. I.H. Kim, G.J. Nam, and G.J. Lee, Proceedings of Antec 2006, 64th SPE Ann. Conf., Charlotte, North Carolina, May 2006, p.1819. 61. H.C. Jung, W.N. Kim, C.R. Lee, K.S. Suh, and S.R. Kim, J. Polym. Eng., 18, 115 (1998). 62. E. Schlosser, B. Nass and W. Wanzke, US Pat. 6547992 (2003). 63. K. Hironaka and M. Suzuki, US Pat. 6248814 (2001). 64. S.L. Tondre, A.S. Yeung and V. Jansons, US Pat. 5908887 (1999). 65. S.J. Blunden, P.A. Cusack, and A.J. Wallace, UK Pat. Appl. 96/13073 (1996). 66. S.J. Blunden, P.A. Cusack, and A.J. Wallace, Eur. Pat. 896649 (2001).

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Tin Chemistry: Fundamentals, Frontiers and Applications P.A. Cusack, L.A. Hobbs, P.J. Smith, and J.S. Brooks, ITRI Publ. No. 641 (1984). P.A. Cusack, D.R. Clack, and A.W. Monk, ITRI Internal Report (2001). P.A. Cusack and J.A. Pearce, Intern. Pat. Appl. PCT WO 90/9962 (1990). P.A. Cusack, B. Patel, M.S. Heer, and R.G. Baggaley, US Pat. 6150447 (2000). D.R. Clack and P.A. Cusack, Intern. Pat. Appl. PCT WO 03/97735 (2003). M. Mohai, A. Toth, P.R. Hornsby, P.A. Cusack, M.S. Cross, and G. Marosi, Surf. Interface Anal., 34, 735 (2002). P.R. Hornsby, P.A. Cusack, M.S. Cross, A. Toth, B. Zelei, and G. Marosi, J. Mater. Sci., 38, 2893 (2003). R.G. Baggaley, P.R. Hornsby, R. Yahya, P.A. Cusack, and A.W. Monk, Fire and Materials, 21, 179 (1997). P.A. Cusack and P.R. Hornsby, J. Vinyl Additive Technol., 5, 21 (1999). Anon, Safer Flame Retardants for the Electronics Industry (HALFREE), 2006. Available online at: http://www.ohlsti.co.uk/ohl/stipdfs/ohl sti38.pdf B.K. Kandola and A.R. Horrocks, in Fire Retardant Materials, A.R. Horrocks and D. Price (Eds), Woodhead Publishing, Cambridge, UK, 2001, p.182. S.C. Brown, M.L. David, K.A. Evans, and J.P. Garcia, Intern. Pat. Appl. PCT WO 00/66657 (2000). M.S. Cross and P.A. Cusack, UK Pat. Appl. 02/12306 (2002). M.S. Cross, The Development and Application of Halogen-Free Tin-Based Fire Retardant Additives in EVA, Eng. D. Thesis, Brunel University, Uxbridge, UK, 2004. S. Miyata, US Pat. 5571526 (1996). D.A. Lee and R. Herbiet, Proceedings of Polyolefins 2006, SPE Conf., Houston, Texas, February 2006, Paper 33. S.C. Brown, in Plastics Additives: An A–Z Reference, G. Pritchard (Ed), Chapman and Hall, London, 1998, p.287. G. Camino, F. Rossetti, and C. Manferti, Proceedings of Flame Retardants for Electrical Applications, European Plastics News/Plastics and Rubber Weekly, Brussels, March 2003. T. Kashiwagi and J.W. Gilman, in Fire Retardancy of Polymeric Materials, A.F. Grand and C.A. Wilkie (Eds), Marcel Dekker, New York, 2000, p.353.

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351

Quadratic Non-Linear Optical Properties of Tin-Based Coordination Compounds

Pascal G. Lacroixa and Norberto Farf´anb a b

Laboratoire de Chimie de Coordination du CNRS, Toulouse, France Facultad de Qu´ımica, Departamento de Qu´ımica Org´anica, Universidad Nacional Aut´onoma de M´exico, M´exico

3.6.1

Introduction

Coordination chemistry is a multi-disciplinary research area, which has led to the design of various new organic–inorganic hybrid species, with intriguing new chemical and physical behavior. These results have been boosted by the tremendous capabilities of organic synthesis, applied in the context of non-traditional electronic behavior of the metal centers. Traditionally, coordination compounds have found applications in catalysis, bioinorganics, and material science. Surprisingly, while tin-based compounds are widely reported in catalysis and medicinal chemistry,1 only a few investigations have been devoted to tin species from the perspective of molecular materials. For instance, although tin is used in niobium alloys to construct superconducting magnets,2 there is no entry for tin-based coordination compounds in the important reference book of Kahn, devoted to molecular magnetism.3 Indeed, tin, having the [Kr]4d10 5s2 5p2 electronic configuration, provides derivatives in the MII and MIV oxidation states,4 with electronic structures in which all electrons are paired. More surprisingly, only few investigations of non-linear optical (NLO) properties in tin derivatives have been performed, although optical non-linearities have been reported since the late eighties in molecules containing metallic5 and organometallic6 fragments. One reason that may have hampered this development is the fact that tin is not parametrized in the ZINDO (Zerner intermediate neglect of differential overlap) formalism.7 Indeed, since the early nineties, the science of chromophore design has been deeply influenced by computational chemistry procedures. In particular, the proven ZINDO-SOS approach8,9 has been used successfully to predict, and moreover to provide a precise description of, the NLO response at the microscopic level. This has oriented synthetic chemists towards the most promising candidates, for which chemical intuition could have led, at best, to qualitative prediction of the NLO response. Studying molecules containing heavy metals (e.g. tin) is still accessible by a derivative approach (see below), which, however, does not provide the chemical insights required for the ultimate understanding of the microscopic origin of the property. The present chapter will summarize the few recent investigations that have been devoted to tin-based derivatives as molecular materials with a quadratic NLO response. After a short introduction to the basic concepts of non-linear optics, and computational methods currently available, the different tin-based materials will be reviewed. 3.6.2

Basic Concepts of Quadratic Non-Linear Optics

Comprehensive introductions to the NLO properties of molecules have been reported in many reviews and textbooks,10,11 and therefore will not be presented here. Nevertheless, it may be helpful to remind the reader that the molecular NLO response is expressed from the polarization (μ) of a molecule subjected to an intense laser light: μ(E) = μ0 + αE + βE2 + . . .

(3.6.1)

In this expression, μ0 is the permanent dipole moment, α the linear polarizability and β the quadratic hyperpolarizability (origin of the NLO behavior), E being the electric field component of the light. The

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polarization of a macroscopic material is again given by an expression analogous to Equation (3.6.1), where the macroscopic polarization (P) is expressed as follows: P = χ (1) E + χ (2) E 2 + . . .

(3.6.2)

χ (2) is the quadratic susceptibility, related to the underlying β. An important point to note is that, for χ (2) to be non-zero, the material needs to be non-centrosymmetric, otherwise P(−E) = −P(E), then χ (2) = 0. The oldest strategy in solid-state engineering for χ (2) NLO materials is based on non-centrosymmetric crystal growth. Accordingly, chirality provides the only way to guarantee that a pure enantiomer will necessarily crystallize in a non-centrosymmetric space group. The most traditional experimental determination of β is the electric field-induced second harmonic (EFISH) method, which requires the molecules to be aligned in solution by an electric field, by means of their static dipole moment (μ0 ).12 The EFISH signal is therefore proportional to μ0 and to βvec (projection of β on μ0 ), which is assumed to be equal to β in most cases. The bulk NLO properties are frequently evaluated as the efficiency of a powdered sample in second-harmonic generation (SHG),13 or as the d components of the χ (2) tensor. Besides the ultimate experimental determination of the properties, it is important to point out that the development of computational quantum procedures has also significantly boosted the search for NLO chromophores. There are basically two different computational approaches towards β: (i) the sum over states (SOS) perturbation theory, and (ii) the derivative method, using the finite field (FF) procedure. The ZINDO method,7 in connection with the SOS procedure,8 relates the NLO properties to all electronic transitions having charge transfer character. Therefore, this approach provides important chemical insights and efficient guidelines for synthetic chemists. Unfortunately, it is not operative for tin derivatives, because heavy metals are not parameterized in ZINDO. The hyperpolarizability of tin derivatives can alternatively be computed within the framework of the density functional theory (DFT) approach (e.g. at the B3PW91/6-31+G*/LANL2DZ(Sn) level), using the time-consuming finite field procedure.14 The use of a pseudo-potential is required to allow the description of relativistic effects for tin. In this approach, β is obtained as the numerical partial derivative of the energy (W ) with respect to the electric field (E), evaluated at zero field, according to the following equation: ∂3W βi jk = − (3.6.3) ∂ E i ∂ E j ∂ E k E=0 This expression is only valid for the static field limit, where β (so-called β0 ) is independent of 9 the laser frequency. Following this approach, β is the magnitude of the vectorial hyperpolarizability (β = (βx )2 + (β y )2 + (βz )2 with βi = βi x x + βi yy + βi zz , after assumption of the Kleinman symmetry conditions.15 The frequency-dependent β value is now accessible with the time-dependent density functional theory (TD-DFT). However, and although considerable improvement of this method has been achieved in recent years,16 the use of TD-DFT for β calculations remains not fully reliable in many cases.17 3.6.3

Tin-Based Materials in Quadratic Non-Linear Optics

Tridimensional Ferroelectrics

The purpose of the present contribution is to focus on molecular materials. Nevertheless, it may be interesting to start this review with M2 P2 S6 (M is a metal in the 2+ oxidation state), which forms a family of hypodithiophosphate derivatives first described by Hahn and Klingen.18 These materials are built up from MII cations, linked together by (P2 S6 )4– anions, and must therefore be considered as coordination

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compounds. As far as NLO properties are involved, Mn2 P2 S6 , and Cd2 P2 S6 , have been used by Cl´ement et al.19 as guest structures for the design of various hybrid organic–inorganic materials, with SHG efficiencies several hundreds times that of urea in some cases. Interestingly, while most of the pristine M2 P2 S6 (MII = MnII , CdII , FeII , ZnII , . . . ) possess lamellar centrosymmetric structures, and therefore are SHG silent, Sn2 P2 S6 is a three-dimensional compound which crystallizes in the monoclinic space group P21 /n at high temperature, but undergoes a ferroelectric phase transition to the non-centrosymmetric Pc space group on cooling down to 60 ◦ C.20 This phase transition leads to symmetry lowering and the appearance of potential quadratic NLO capabilities. Indeed, SHG efficiencies in Sn2 P2 S6 were reported by Cleary et al. in 1993.21 Furthermore, Sn2 P2 S6 , can be grown as large phase-matchable crystals, and its d21 NLO coefficient has been reported to be 3.6 times the d33 value of KTiOPO4 (KTP),22 one of the main NLO materials that is commercially available. While Sn2 P2 S6 , is typically synthesised at high temperature by the techniques of solid-state chemistry, it is also accessible from soft chemistry by metathesis reaction in aqueous media, starting from an SnCl4 precursor and Li4 P2 S6 . The resulting materials have revealed non-zero SHG efficiency, however more reduced than that of the compound synthesized at high temperature.23 Molecular Materials

Traditional inorganic materials are usually submitted to very high temperatures during the synthetic process, which ensures thermodynamic stability, but leads to limited chemical flexibility. By contrast, coordination chemistry has the potential to provide intriguing molecular materials combining the attractive properties and synthetic tailorability of organic molecules with extra capabilities of metal complexes (e.g. mechanical hardness, access to non-standard geometries, or additional electronic behavior). Additionally, molecular NLO materials possess very large and ultra-fast responses, and high optical damage thresholds.10,24 The first report of NLO properties in tin-based coordination compounds appeared in 1991 and is due to Mingos et al.25 The authors investigated a wide range of tris(pyrocatecholato)stannate(IV) complexes and have recorded some SHG effects in several cases. In particular, bis(triethylammonium) tris(4nitropyrocatecholato-O,O)stannate(IV) (1, Figure 3.6.1) exhibits an efficiency equal to 1.33 times that of urea at 1.064 μm. However, the origin of the NLO response has not been thoroughly discussed for this material, as the only crystal structure available was solved in the centrosymmetric space group Pbca, which implies a cancellation of any quadratic (∝ E 2 ) NLO properties. Subsequently, various investigations were NO2 O O2N

O

(--) O

Sn O

O

O

1 NO2

Figure 3.6.1 Tris(4-nitropyrocatecholato-O,O)stannate(IV) anion with a non-crystallographic three-fold symmetry axis

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Sn

β xyz (at 1.064 μm)

R

13 x 10 –30 cm5 esu–1

2

Sn

N 4 24 x 10 –30 cm5 esu–1

3

Sn

N 4

4

Sn

159 x 10 –30 cm5 esu–1

N N

N 4

Figure 3.6.2

Tetrahedral organotin derivatives with large molecular hyperpolarizabilities

conducted on tin-containing species, which will now be reviewed. For clarity, we will summarize first the reports of molecular (β) NLO responses, and finally the reports of solid-state NLO properties. (1) Molecular NLO response: β values of tin-based compounds have been approached experimentally, from the perspective of NLO materials with non-traditional geometry. Indeed, while most NLO chromophores are one-dimensional ‘push-pull’ molecules, Zyss et al. have proposed that three-dimensional molecules would exhibit an intense NLO response in non-centrosymmetric (e.g. Td ) point groups.26 While such symmetries are not traditional in conjugated organic molecules, they are accessible in organometallic chemistry. Following this work, Simon et al. have reported on the first sizeable NLO response within a strict Td symmetry in the case of a series of tetrahedral organotin compounds 2–4 (Figure 3.6.2).27 A β value as large as 159 × 10−30 cm5 esu−1 is observed in 4, strongly enhanced by resonance between the 532 nm second harmonic and the 421 nm absorption maximum. Nevertheless, it leads to a sizeable static β0 equal to 47.5 × 10−30 cm5 esu−1 . Another application of tin coordination chemistry has been proposed, with a comparison of sets of ‘push-pull’ boron (5,6) and tin (7,8) analogs.28 In these derivatives, the origin of the NLO response is mostly related to the ‘push-pull’ π -electronic structure of the ligands. Following a previous investigation of boron derivatives,29 the tin analogs have provided more planar molecular structures, leading to an optimization of the charge transfer behavior, and finally to enhanced β values in all instances (Figure 3.6.3). Finally, computed β values have also been recently reported for trisorganotin-substituted β-Keggin polyoxotungstate derivatives within the framework of the TD-DFT approach, by Su et al.30 In particular, β0 (β value at infinite wavelength) was predicted to scale as high as 1570 × 10−30 cm−5 esu−1 , in the case of the [SiW9 O37 (SnPhC≡CPh)3 ]7– anion (9 Figure 3.6.4). The origin of the NLO response in 9 is due to intense charge transfers from the electron-rich polyoxometalate inorganic fragment to the three organic segments, resulting in a ‘push-pull’ effect in the

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NO2 NO2

N

N R

O

compound 5 (R = MeO–) 6 (R = Et2N–)

Figure 3.6.3

B

R

O

O

O

Sn Bu Bu

β (1.907 μm) 27.4 x 10 –30 cm5 esu–1 46.4 x 10 –30 cm5 esu–1

compound

β (1.907 μm)

7 (R = MeO–) 48.9 x 10 –30 cm5 esu–1 8 (R = Et2N–) 63.5 x 10 –30 cm5 esu–1

Comparison of the NLO responses in sets of tin and boron-based derivatives

direction of the molecular three-fold axis. However, the β magnitude may be somewhat surprising, and must be partially related to the observation that large chromophores are invariably found to exhibit a much larger NLO response than those of smaller analogs (e.g. the general trend for β enhancement on passing from short to long conjugation length).31 This effect is further exemplified here, where anion 9 is built up from 92 non-hydrogen atoms (M.W. = 3162). This intriguing computational prediction would certainly deserve an experimental confirmation. (2) Solid-state NLO response: Although the origin of the NLO response of molecular materials is ultimately related to the behavior of a single molecule, single molecule non-linear optics is a tedious challenge, which has only been approached in a few cases.32 This implies that, from the materials science perspective, the property of interest must be that of the bulk level (e.g. crystal state or poled polymer approach).

C3 Sn Sn Sn

"SiW9O37"

9

Figure 3.6.4

Polymetallic chromophore with giant (β0 = 1570 esu−1 ) NLO response

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*

CH3 NH NO2

O2N

Figure 3.6.5

10

MAP, the first chiral molecule investigated in non-linear optics

In contrast to the organic poled polymer approach, non-linear optics in coordination polymers is a far less investigated topic, which, however has led to various interesting reports.33 Nevertheless, these contributions have focused on non-centrosymmetric crystalline polymers and furthermore have never been dedicated to the NLO properties of tin derivatives. The more traditional organic poled polymer strategy is based on the possibility of poling amorphous polymeric structures above their glass transition temperature, by means of strong static electric fields. The poled polymer is then frozen at room temperature, where the polymer retains some non-centrosymmetry (and hence SHG efficiency) over a long period of time.34 This route has been approached only once with tin-based chromophores, by Fiorini et al. who reported on the tetrahedral organotin azobenzene derivative 4, embedded in poly(methylmethacrylate) (PMMA).35 Due to the non-polar geometry of the chromophore, the centrosymmetry of the polymer was broken by a non-traditional ‘all-optical poling’ process.35,36 Unfortunately, the bulk NLO response is modest, which probably reflects the deleterious effect of molecular relaxation dynamics in this system. Before the development of poled polymer materials in non-linear optics in the eighties,34 the oldest strategy in crystal engineering for SHG efficient materials was that of chirality, which provides a means of guaranteeing that crystallization of a pure enantiomer will necessarily occur in a non-centrosymmetric space group, thus providing some SHG efficiencies. Methyl 2-(2,4-dinitrophenyl)-aminopropanoate (MAP, 10 Figure 3.6.5) was the first thoroughly studied NLO material based on this strategy, reported in 1977.37 The first example of a chiral tin-containing material was reported by Qu, Xiong et al. with the investigation of the hybrid compound (S)-4-(4’-ammoniumphenyl)-2-ammoniumbutanoic acid trichlorostannite (11, Figure 3.6.6).3 This compound was the first crystallographically characterized trichlorostannite salt of an amino acid. It crystallizes in the chiral I 2 space group and exhibits ferroelectric behavior. Although 11 does not possess the ‘push-pull’ electronic character required for large NLO efficiency, a powdered sample has provided an SHG response about 0.8 times that of urea at 1.064 μm. Although the origin of the optical

H H2 C C H2

+ NH3 COOH

SnCl3-

2

+ H3N 11

Figure 3.6.6

The first chiral tin-containing material with NLO properties

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Optical Properties of Tin-Based Coordination Compounds R1

R2

R3

R4 R1

12

CH 3 H

Ph

H N

13

H

Ph

H

H

14

H

H

CH 3

H

15

H

Ph

H

Ph

Figure 3.6.7

357

Sn O O

N

R2 R3 R4

Chiral organotin(IV) Schiff base complexes with solid state NLO properties

non-linearity was not thoroughly investigated in 11, it probably arises from the organic moieties of the hybrid material, SnCl–3 being most likely spectator counterion. Recently, we have reported on SHG activity in a series of chiral organotin(IV) Schiff base complexes (12–15, Figure 3.6.7).39 A computational DFT study suggests that the origin of the optical non-linearity is localised on the amine→imine conjugated pathway in all instances, leading to the assumption that β is grossly the same in the 12–15 series. Nevertheless, the NLO responses measured by the EFISH technique (∝ μ0 × βvec ) are very different in relation to a possible rotation between μ0 and β induced by the different nature of the R1−4 substituents. Although, the R1−4 groups are innocent in terms of charge transfer behavior, they play a major role in the crystal state engineering of 12–15. While 12 and 15 exhibit vanishing SHG signals in relation to the pseudo-centrosymmetry of the macroscopic charge transfer processes, an SHG efficiency of 11 times that of urea is observed at 1.907 μm in the case of 13. The present example illustrates the fact that although chirality ensures crystallization in noncentrosymmetric space groups, it does not guarantee that the molecular packing will be optimized for SHG effects. Nevertheless, it seems natural to expect that ‘more chiral’ molecules should lead to ‘more chiral crystal structures’ and hence to more efficient SHG materials. The report by Farf´an et al.40 that Schiff base organotin complexes containing various chiral amino acid fragments are easily accessible as single crystals and exhibit a tendency for crystallizing in the same space group, encouraged us to study the possibility of a quantification of chirality by means of SHG efficiencies. From a reference achiral model (16), we have investigated a series of six chiral organotin Schiff base derivatives (17–21), which crystallize in the same space group P21 21 21 , and to a large extent can be assumed to be isostructural (Figure 3.6.8).41 R

R N N

O Sn

* O

O

16 17 18 19 20 21

-

–H –CH(CH3)CH2CH3 –CH2CH(CH3)2 –CH2CH2SCH3 –CH(CH3)2 –C6H5

Figure 3.6.8 Series of tin-based complexes investigated in an attempt to establish a potential chirality-NLO property relationship

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Figure 3.6.9 chirality’ dχ

Solid-state NLO efficiencies observed for tin-based complexes of various ‘degrees of

All of them exhibit the same optical spectra, dominated by the ‘push-pull’ Et2 N→C=N based charge transfer transition and hence have the same β value. In this series, a so-called ‘degree of chirality’ (dχ ) has been defined as: di × m i i dχ = (3.6.4) MW The summation runs over all atoms i of the substituents H and R linked to the asymmetric carbon noted * in Figure 3.6.8. The distance, di , is that between atom i and the mean plane defined by the 15 atoms building the rigid charge-transfer moieties of the chromophores, m i is the atomic mass of atom i, and MW the molecular weight of the chromophore. As dχ has the dimension of a distance, it therefore quantifies ‘how far from being achiral the chromophore is.’41 Interestingly, the SHG intensities recorded for 17–21 appear largely correlated with the ‘degree of chirality,’ an observation which up to now has never been reported (Figure 3.6.9). 3.6.4

Concluding Remarks

Second harmonic generation in tin-based coordination compounds is a rather new topic, which has mostly been reported in the case of organotin(IV) complexes. Although SHG efficiencies more than 10 times that of urea have been observed in some cases, the main interest in these materials has been observed in the domain of the structure–property relationships: (i) access to new tri-dimensional (e.g. Td ) geometries, (ii) enhanced planarities and hence NLO responses versus those of their boron-based analogs, (iii) modulation of the orientation of μ0 along the β direction for a better use of the potential NLO properties in techniques requiring electric fields (e.g. the poled polymer approach), and finally (iv) an approach towards the intriguing concept of ‘quantifying chirality.’ Non-linear optics in molecular materials has long been restricted to the investigation of organic chromophores. Most of the tin-based coordination compounds have been proposed only recently. Additionally, heavy atoms, such as tin, have also been used to enhance the optical-limiting response of metal phthalocyanines.42 Although this property derives from the cubic (∝ E 3 ) NLO response, and falls outside the scope of the present review, it provides additional insights suggesting that tin-based coordination compounds could deserve more attention in the search for future NLO chromophores.

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359

Acknowledgments The authors wish to acknowledge Consejo Nacional de Ciencia y Tecnolog´ıa (CONACYT), and CNRS (France) for financial support. They are gratefull to Prof. K. Nakatani (E.N.S. Cachan, France) for the laser studies, and the hospitality of his laboratory, to Dr R. Santillan (CINVESTAV, Mexico City), and to C. Lepetit (LCC, Toulouse). References 1. Tin Chemistry – Fundamentals, Frontiers and Applications, A. G. Davies, M. Gielen, K. Pannell, and E.R.T. Tiekink (Eds.), John Wiley & Sons, Ltd, Chichester, 2008, Chapters 3 and 5. 2. Handbook of Chemistry and Physics, CRC Press, Boca Raton, 1997, pp. 4–30. 3. O. Kahn, Molecular Magnetism, VCH, Weinheim, 1993. 4. (a) J. Parr, in Comprehensive Coordination Chemistry II, J.A. Mc Cleverty and T.J. Meyer (Eds.), Elsevier, Oxford, 2004, Vol. 3, p.545; (b) A.G. Davies, in Comprehensive Organometallic Chemistry II, E.W. Abel, F.G.A. Stone, and G. Wilkinson (Eds.), Elsevier, Oxford, 1995, Vol. 2, p. 217. 5. For an example of NLO investigation in inorganic chemistry, see: T. Thami, P. Bassoul, M. A. Petit, J. Simon, A. Fort, M. Barzoukas, and A. Villaeys, J. Am. Chem. Soc. 114, 915 (1992) (and references therein). 6. For an early review on NLO properties in organometallic chemistry, see : S. R. Marder, in Inorganic Materials, D. W. Bruce, and D. O’Hare (Eds.), John Wiley & Sons, Inc., New York, 1992. 7. (a) M.C Zerner, G. Loew, R. Kirchner, and U. Mueller-Westerhoff, J. Am. Chem. Soc. 102, 589 (1980); (b) W. P. Anderson, D. Edwards, and M. C. Zerner, Inorg. Chem. 25, 2728 (1986). 8. J. F. Ward, Rev. Mod. Phys. 37, 1 (1965). 9. For a review on various computational approaches towards the NLO response, with a highlight on the INDO-SOS method, see: D. R. Kanis, M. A. Ratner, and T. J. Marks, Chem. Rev. 94, 195 (1994). 10. For an introduction of non-linear optics with chemical perspectives, see: H.S. Nalwa and S. Miyata (Eds.), Nonlinear Optics of Organic Molecules and Polymers, CRC Press, Boca Raton, 1997. 11. For a more rigorous introduction of the theory of non-linear optics, see: Y.R. Shen, The Principles of Nonlinear Optics, John Wiley & Sons, Inc., New York, 1984. 12. B. F. Levine and C. G. Betha, J. Chem. Phys. 63, 2666 (1975) 13. (a) S.K. Kurtz and T.T. Perry J. Appl. Phys. 39, 3798 (1968); (b) J.P. Dougherty and S.K. Kurtz, J. Appl. Crystallogr. 9, 145 (1976). 14. (a) A.D. Becke, J. Chem. Phys. 98, 1372 (1993); (b) J.P. Perdew and Y. Wang, Phys. Rev. B 45, 13244 (1992). 15. D. A. Kleinman, Phys. Rev. 126, 1977 (1962). 16. For an example of good agreement between TD-DFT computation and experimental data, see: B.J. Coe, J.A. Harris, B.S. Brunschwig, J. Garin, J. Orduna, S.J. Coles, and M.B. Hursthouse, J. Am. Chem. Soc. 126, 10418 (2004). 17. For critical review on β calculations by TD-DFT, see: (a) V. Monev, Ann. Univ. Sofia Fac. Chim. 91, 69 and 85 (2001); (b) N.N. Matsuzawa and D.A. Dixon, Mol. Cryst. Liq. Cryst. Sci. Technol. Sect. B 26, 17 (2000). 18. H. Hahn and W. Klingen, Naturwissenschaften, 52, 494 (1965). 19. (a) P.G. Lacroix, R. Cl´ement, K. Nakatani, J. Zyss, and I. Ledoux, Science 263, 658 (1994); (b) S. B´enard, P. Yu, J-P. Audi`ere, E. Rivi`ere, R. Cl´ement, J. Guilhem, L. Tchertanov and K. Nakatani, J. Am. Chem. Soc. 122, 9444 (2000). 20. B. Scott, M. Presspricht, R.D. Willet, and D.A. Cleary, J. Solid State Chem. 96, 294 (1992). 21. D.A. Cleary, R.D. Willet, F. Ghebremichael, and M.G. Kuzyk, Solid State Commun. 88, 39 (1993). 22. A. Anema and A. Grabar, Th. Rasing, Ferroelectrics 183, 181 (1996). 23. X. Bourdon and V.B. Cajipe, J. Solid State Chem. 141, 290 (1998). 24. (a) Nonlinear Optical Properties of Organic Molecules and Crystals, D.S. Chemla and J. Zyss (Eds.), Academic Press, Orlando, 1987, Vol. 1 and 2; (b) J. Zyss, Molecular Nonlinear Optics: Materials, Physics, and Devices, Academic Press, Boston, 1994.

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34. 35. 36. 37. 38. 39. 40. 41. 42.

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Tin Chemistry: Fundamentals, Frontiers and Applications C. Lamberth, J.C. Machell, D.M.P. Mingos, and T.L. Stolberg, J. Mater. Chem. 1, 775 (1991). J. Zyss and I. Ledoux, Chem. Rev. 94, 77 (1994). M. Lequan, C. Branger, J. Simon, T. Thami, E. Chauchard, and A. Persoons, Adv. Mater. 6, 851 (1994). H. Reyes, C. Garc´ıa, N. Farf´an, R. Santillan, P.G. Lacroix, C. Lepetit, and K. Nakatani, J. Organomet. Chem. 6, 2303 (2004). H. Reyes, B.M. Mu˜noz, N. Farf´an, R. Santillan, S. Rojas-Lima, P.G. Lacroix, and K. Nakatani, J. Mater. Chem. 12, 2898 (2002). W. Guan, G. Yang, L. Yan, and Z. Su, Inorg. Chem. 45, 7864 (2006). H.S. Nalwa, T. Watanabe, and S. Miyata, in ref. 10, pp. 89–350. For a review on NLO response in single molecules, see for example: Ph. Tamarat, F. Jelezko, Ch. Brunel, A. Maali, B. Lounis, and M. Orris, Chem. Phys. 245, 121 (1999). For recent reports on solid state SGH efficiencies in coordination polymers see for example: (a) S.L. Li, J.Y. Wu, Y.P. Tian, H. Ming, Pei, Wang, M.H. Jiang, and H.K. Fun, Eur. J. Inorg. Chem. 14, 2900 (2006); (b) S. Zang, Y.L. Su, Y.N. Li, Z. Ni, and Q. Meng, Inorg. Chem. 45, 174 (2006); (c) Q. Ye, Y.H. Li, Q. Wu, Y.M. Song, J.X. Wang, H. Zhao, R.G. Xiong, and Z. Xue, Chem. Eur. J. 11, 988 (2005). P. N. Prasad and D. J. Williams, Introduction to Nonlinear Optical Effects in Molecules and Polymers, John Wiley & Sons, Inc., New York, 1990. C. Fiorini, J.M. Nunzi, P. Raimond, C. Branger, M. Lequan, and R.M. Lequan, Syn. Met. 115, 127 (2000). C. Fiorini, F. Charra, J.M. Nunzi, and P. Raimond, J. Opt. Soc. Am. B14 1984 (1997). J. Oudar and R. Hierle, J. Appl. Phys. 48, 2699 (1977). Y.H. Li, Z.R. Qu, H. Zhao, Q. Ye, L.X. Xing, X.S. Wang, R.G. Xiong, and X.Z. You, Inorg. Chem. 43, 3768 (2004). J.M. Rivera, D. Guzm´an, M. Rodr´ıguez, J.F. Lam`ere, K. Nakatani, R. Santillan, P.G. Lacroix, and N. Farf´an, J. Organomet. Chem. 691, 1722 (2006). H.I. Beltran, L.S. Zamudio-Rivera, T. Mancilla, R. Santillan, and N. Farf´an, Chem. Eur. J. 9, 2291 (2003). J.M. Rivera, H. Reyes, A. Cort´es, R. Santillan, P.G. Lacroix, C. Lepetit, K. Nakatani, and N. Farf´an, Chem. Mater. 18, 1174 (2006). J.W. Perry, K. Mansour, S.R. Marder, K.J. Perry, D. Alvarez Jr., and I. Choong, Opt. Lett. 19, 625 (1994).

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3.7

361

Monoorganotin Precursors For Hybrid Materials

Bernard Jousseaume Universit´e de Bordeaux, Institut des Sciences Mol´eculaires, Groupe Mat´eriaux, Talence, France

3.7.1

Introduction

Nanotechnology, the engineering of functional systems at the molecular scale, allows the construction of items from the bottom up, using techniques and tools to make complex, high-performance products. Chemistry is heavily involved in this process, as it allows the building of materials from defined nanoobjects with a fine control of their organization and architecture, from which result specific physical and chemical properties. The sol-gel route, which allows the preparation of materials under very mild conditions, makes it possible to prepare hybrid materials where an organic and an inorganic component are intimately mixed at the nanometer scale. The incorporation of an organic component reduces some disadvantages of pure inorganic sol-gel materials, such as shrinkage and cracking upon solvent removal during synthesis, and also adds functionalities. The organic component can be linked to the inorganic one through either weak (electrostatic, hydrogen, van der Waals etc.) or strong (covalent) bonds. Unlike inorganic sol-gel materials, which can be prepared using almost all metals,1 hybrid materials, where both components are bound through strong covalent bonds have been mainly developed from organosilicon compounds, which form hydrolytically stable metal–carbon bonds.2 Besides silicon, tin also forms metal–carbon bonds strong enough not to be destroyed under the experimental conditions of the sol-gel process. However, the chemistry of tin-based hybrid materials is not as developed as the chemistry of silicon-based hybrid materials because of the lack of an easy access to their precursors. Herein are reported recently developed synthetic routes towards these organotin compounds. The natural tin analogs of organosilicon precursors of hybrid materials would have been functional organotin trialkoxides. However, since these compounds are difficult to prepare and do not show a high stability,3 another type of precursor was targeted: the trialkynylorganotins. The alkynyl group was selected because, first, the tin–alkynyl bond is easily cleaved by water, leading to the inorganic network of the hybrid materials; second, the alkynyl group is easy to introduce onto tin through a lithium alkynide; and third, trialkynylorganotins are often solid compounds relatively easy to purify. The other group linked to tin bears either a functionality able to build the organic network of the hybrid material by polymerization, or a part of the organic network. The synthesis of these functional organotins is presented first followed by the preparation of bridged ditins. 3.7.2

Functional Trialkynylorganotins

To obtain the requisite functional trialkynylorganotins it was first necessary to obtain the corresponding trichlorides. Only a few general methods are available to prepare organotin trihalides.4 They involve the electrophilic cleavage of organic group–tin bonds by hydrogen halides, halogens, or tin tetrahalides. However, when they are applied to functional organotins, such as tetrakis(4-vinylphenyl)tin or triaryl(3butenyl)tins, they do not lead to the corresponding trichlorides. We experienced either a competitive addition of the electrophile on the double bond of the organic group or the polymerization of the double bond. The electrophilic cleavage of one organic group is usually easy and occurs under mild conditions, while it is far more difficult to cleave three organic groups linked to the tin. Thus, we turned to another method, where only one organic group, instead of three, is cleaved to reach the targeted organotin compounds. To be able to prepare organotins with a primary alkyl organic group attached to the tin atom, we had to use an auxiliary organic group, which would be more strongly linked than a primary one.

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As the selectivity of the electrophilic cleavage of tin–carbon bonds is known as being very high, it was expected that tricyclohexylorganotins would be interesting precursors of the corresponding trihalides, the tin–cyclohexyl bond being assumed to be unreactive during the cleavage of the other functional organic groups linked to the tin. To test this hypothesis, it first had to be demonstrated that functional tricyclohexylorganotins were easy to prepare through the well-known routes to unsymmetrical substituted organotins. Some examples of coupling of lithium or Grignard reagents with tricyclohexyltin chloride have already been described and, when they were applied to unsaturated halides, they were revealed to be very efficient for the preparation of compounds 12, 16, 19, 21, and 26 (Scheme 3.7.1).∗ RMgX

Scheme 3.7.1

+

Cy3SnCl

– MgXCl

Cy3SnR

Coupling of Grignard reagents with tricyclohexyltin chloride

The coupling of tricyclohexyltin with an organic halide or the hydrostannation with tricyclohexyltin hydride had not been reported before. This important entry to unsymmetrical organotin compounds had thus to be checked. Tricyclohexyltin hydride was first prepared directly from the reduction of tricyclohexyltin hydroxide with sodium borohydride, which avoids the preparation of a chloride intermediate. It was then metalated with lithium diisopropylamide and reacted with various halides. The couplings proceeded smoothly and were more successful with primary organic bromides. They gave compounds 1, 3, and 11 in good yields (Scheme 3.7.2). Tricyclohexyltin lithium reacts in a similar way as tri-n-butyltin lithium and is thus an interesting intermediate for the preparation of tricyclohexylorganotins. Cy3SnLi

Scheme 3.7.2

+

RX

- LiX

Cy3SnR

Coupling of tricyclohexyltin lithium with organic halides

Hydrostannation with tricyclohexyltin hydride is only successful with unactivated unsaturated compounds when the reaction is conducted at high temperature (110 ◦ C) with azobis(isobutyronitrile) added portion-wise, as an initiator. Under these conditions, it gives the expected adducts 29, 32, 34, 36, 38, 40, 43, 45, 48, and 50 in good yields with total regiospecificity, the tin being linked to the terminal carbon atom of the unsaturated compound (Scheme 3.7.3).5 Hydrostannation of unactivated alkenes with tricyclohexyltin hydride has been reported up to now as being unsuccessful, even under high pressure.6

Cy3SnH

Cy3SnH

+

+

AIBN

Cy3Sn

R

R

R

AIBN

Cy3Sn R

Scheme 3.7.3

∗

Hydrostannation with tricyclohexyltin hydride

The structural formulae of the prepared functional monoorganotins are presented in (Table 3.7.1)

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363

Functional tricyclohexyl-, trichloro- and trialkynylorganotins Y = Cy (% Yield)

MeSnY3 n-PrSnY3 n-BuSnY3 n-BuSnY3 n-BuSnY3 i-PrSnY3 t-BuSnY3 n-Bu2 SnY2

Y = Cl (% Yield)

1 (63) 3 (72)

Y = C ≡ CR2 (% Yield), (R2 )

Y = OR3 (% Yield), (R3 )

2 (71) 4 (75) (Ph)

5 (80) (CH2 Ph) 6 (55) (s-Bu) 7 (76) (i-Bu)

8 (89) (Ph) 9 (82) (Ph) 10 (93) (Me) 11 (67)

SnY3

SnY 3

SnY3

SnY3

12 (78)

13 (92)

16 (81)

17 (87)

19 (74)

20 (83)

21 (75)

22 (73)

14 (50) (n-Bu)

15 (70) (t-Amyl) 18 (75) (t-Amyl)

Cl

SnY3

25 (60) (Ph)

SnY3

SnY3

23 (76) (Me)

26 (85)

27 (63)

28 (62) (n-Bu)

29 (85)

30 (58)

31 (77) (n-Bu)

32 (88)

33 (85)

34 (60)

35 (74)

F F 3C

HO

HO

SnY 3

SnY3

SnY3

24 (72) (t-Amyl)

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Table 3.7.1

(Continued)

SnY3

HO

HO

SnY3 SnY3

AcO

AcO

SnY3

SnY 3

AcO

SnY3

PhCH 2O

O O

Y = Cy (% Yield)

Y = Cl (% Yield)

Y = C ≡ CR2 (% Yield), (R2 )

36 (79)

37 (63)

38 (65)

39 (90)

40 (78)

41 (90)

43 (72)

44 (89)

45 (71)

46 (67)

48 (68)

49 (75)

50 (39)

51 (73)

52 (46) (n-Bu)

53 (49)

54 (96)

55 (30) (Me)

56 (54)

57 (95)

(58 (95) (Me)

59 (52)

60 (98)

61 (90) (Me)

42 (45) (n-Bu)

47 (42) (n-Bu)

SnY3

SnY3

SnY3

SnY3

Y = OR3 (% Yield), (R3 )

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Hydrostannation is also very efficient in the case of functional alkynes. In this case, the syn addition of tricyclohexyltin hydride gives stereospecifically the (E) adduct 53.7 The previous reactions described approaches based on the creation of a tin–carbon bond. It is also possible to start from a functional tricyclohexylorganotin and to modify its functionality. For instance, (ωhydroxyalkyl)tricyclohexyltins are successively converted into the corresponding tosylates and iodides, which are subjected to the action of perylenyllithium. This entry to tricyclohexylorganotins furnishes the expected compounds 56 and 59 in fair yields (Scheme 3.7.4).7 ( )n

Cy3Sn

+ RLi

I

- LiI

Cy3Sn

( )n

R

n = 2 (56), 4 (59) R=

Scheme 3.7.4

Functionalization of tricyclohexylorganotins

These tricyclohexylorganotins were treated with tin tetrachloride in order to furnish the corresponding functional organotin trichlorides. This reaction usually works well, at room temperature or with moderate heating (Scheme 3.7.5). The trichlorides are separated from the tricyclohexyltin chloride by acetonitrile/pentane partition, the polar trichlorides migrating into the polar phase. When acetonitrile is difficult to remove by distillation under reduced pressure from the trichlorides, because of too strong a coordination to the metal, the simple washing of the crude reaction mixture with pentane is sufficient to separate the polar coordinating solvent. Alternatively, two successive washings of the acetonitrile adduct with tetrahydrofuran, followed by the evaporation of this last solvent, less associated than acetonitrile to the electrophilic tin center, leads to pure trichlorides.5 Cy3SnR

+

Scheme 3.7.5

SnCl 4

– Cy3SnCl

RSnCl 3

Preparation of trichloroorganotins

When the organotin trichlorides are substituted with a functional group capable of coordinating to the tin center, such as methoxycarbonyl or hydroxyl groups, upfield shifts of the 119 Sn NMR signals with respect to the corresponding unsubstituted organotin trichlorides are recorded, as is the case with some other functional tetraorganotins or organotin halides.8 With the esters 41, 44 and 46, this shift depends on the concentration of the solutions. This behavior was explained through gradient-assisted 2D 1 H-119 Sn HMQC investigations by the existence of a fast equilibrium between coordinated and uncoordinated species. In the case of 41 (n = 1), tin coordination occurs through the intra-molecular coordination of the alkoxyl oxygen of the carboxyl group, while in 44 and 46 an inter-molecular coordination through the oxygen of the carbonyl takes place (Figure 3.7.1).9 The coordination scheme of 41 represents a very rare example of coordination of a metal by the alkoxyl oxygen of a carboxyl group, the coordination through the oxygen atom of the carbonyl being by far the more common rule in organometallic chemistry. In the solid state, the crystal structure of 46 reveals the presence of dimers where the tin centers are coordinated through the oxygen atom of the carbonyl (Figure 3.7.2).10 The behavior of alcohols 33 and 35 is different, as their chemical shifts in solution are moved upfield with respect to the unfunctional analogs, more than for the corresponding esters, and are insensitive

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Figure 3.7.1

Coordination scheme of methoxycarbonyl-substituted trichloroorganotins

Figure 3.7.2

Figure 3.7.3

Supramolecular structure of 44

Coordination scheme of hydroxyl-substituted trichloroorganotins

to concentration. Gradient-assisted 2D 1 H-119 Sn HMQC and 119 Sn NMR spectroscopy investigations reveals that in solution the alcohols 33 and 35 display almost exclusively an intra-molecular coordination of the hydroxide group to the metal center forming five- and six-membered ring structures, respectively. Alcohol 37 does not show the same coordinating scheme (Figure 3.7.3). It is not soluble enough to be studied by any NMR technique because inter-molecular coordination of the tin center by the hydroxyl group leads to an insoluble polymer. This hypothesis was confirmed by X-ray crystallography, where its crystal structure shows a polymeric arrangement (Figure 3.7.4).11 These trichlorides are then transformed into the corresponding trialkynyl derivatives under treatment with a stoichiometric amount of alkynyllithium,12 prepared by metalation of the corresponding alkyne with butyllithium according to the well-known route to alkynyltins (Scheme 3.7.6). Propyne, hexyne and phenylacetylene can be used in this reaction with equal success. The functional trialkynylorganotins are stable enough to be handled in air for a short period of time without extensive decomposition, and can

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O CI 235.5 pm Sn O CI CI

Sn

Figure 3.7.4

Molecular structure of 37

be chromatographed on Florisil. They are solids, which can also be purified by recrystallization, unlike some trialkoxy analogs, which are liquid at room temperature.13 The presence of electron-withdrawing alkynyl substituents on tin could make the metal electrophilic enough to undergo an intra- or intermolecular nucleophilic coordination by a functional group in the chain. Such a phenomenon was searched for in methyl 3-tris(phenylethynyl)tin propanoate, where the tin and the carboxyl oxygen are in the most favourable positions to show intra-molecular coordination by formation of a five-membered ring. As expected, NMR investigations indicated the presence of an interaction characterized by an upfield displacement of the chemical shift of the tin atom, weaker than in the corresponding trichloride, as would be expected from the weaker electron-withdrawing effect of the alkynyl substituents.14 R1SnCl 3 + 3 R

2

Li

Scheme 3.7.6

– 3 LiCl

1

2

R Sn(

R )3

Alkynylation of trichloroorganotins

This route to functional trialkynylorganotins is straightforward for the alkynyltins, for which the corresponding trichlorides are easily available. When the trichloride cannot be prepared, as, for instance, when the tin is substituted with an ω-stryrylalkyl group, it is not useful. In this case, the styryl group of the corresponding trichloride, activated by the presence of the alkyl chain in the position para to the vinyl group, undergoes an autocatalyzed electrophilic polymerization, which prevents its isolation. It is worth noting that when the trichlorotin group is linked directly to the ring, the corresponding styrenyl derivative is stable. So, another, more direct route, to functional trialkynylorganotins was desired. It was anticipated that a selective transmetallation15 reaction would give unsymmetrical substituted trialkynylorganotins. When tetralkynyltins are treated with one equivalent of Grignard reagent, a smooth transmetallation takes place and leads to the expected organotin compound in good yields (Scheme 3.7.7). This is the first example of transmetallation conducted with a Grignard reagent in organotin chemistry. Its selectivity, defined as the presence or not of a disubstituted compound, depends on the

2

(R

– R2 )4Sn

+

R1MgX

MgX

1

R Sn(

2

R )3

R1 = Me, Bu, i-Pr, t-Bu, R2 = Me, n-Bu, Ph

Scheme 3.7.7

Selective alkylation of tetraalkynyltins with Grignard reagents

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nature of the alkynyl group linked to the tin. The best results (100%) are obtained with the phenylalkynyl group rather than with the propynyl or hexynyl groups (85–90%). Primary, secondary, or tertiary Grignard reagents can be successfully used (4–9). This new route is very convenient for the preparation of (4-styrylbutyl)tris(phenylethynyl)tin (25). With two equivalents of the Grignard reagent, this reaction can be extended to a double transmetallation, which corresponds to a new entry to dialkynyldiorganotins, as for 10 (Scheme 3.7.8).16 2

–2 R

2

(R

)4Sn

MgX

1

R 2Sn(

+ 2 R1MgX

2

R )2

R1 = n-Bu, i-Pr, R2 = Me, n-Bu, Ph

Scheme 3.7.8

Selective dialkylation of tetraalkynyltins with Grignard reagents

These trialkynylorganotins can be successfully converted into the corresponding trialkoxides by reaction with the corresponding alcohols (Scheme 3.7.9). The best results are obtained with primary and secondary alcohols (5–7). Tertiary alcohols are not acidic enough to cleave the alkynyl–tin bonds to a sufficient extent to obtain the t-alkoxides in good yields. 2

1

R Sn(

2

R )3 + 3 R3OH

–3 R

H

R1Sn(OR3)3

R1 = n-Bu R2 = Me R3 = i-Bu, s-Bu, –CH2Ph

Scheme 3.7.9

Trialkoxyorganotins from trialkynylorganotins

Experiments were also conducted to see whether that the alkynyl groups are perfect substitutes for alkoxyl groups towards hydrolysis. Indeed, when 4 is hydrolyzed, it leads to the same butyltin oxidehydroxide cluster with 12 tin atoms as when butyltin triisopropoxide is used. Moreover, with the more bulky triisopropylphenyl group linked to the tin, two unusual oxide-hydroxide clusters, one with 10 tin atoms and the other with six, are produced, depending on the experimental conditions.17 3.7.3

Bridged Ditins

The trialkynylorganotins described above are the precursors of hybrid materials, where the organic network is obtained by polymerization of an organic functionality. This network is thus rather undefined and made of longer and shorter chains. To have a better control of this network it is advantageous to start from precursors capable of furnishing a defined network, such as bridged ditins, where the two tin atoms are separated by an organic chain. It is thus easy to modulate the properties of the resulting hybrid material by tuning the properties of the spacer, such as flexibility, length, self-assembly properties, etc. This topic leads to rather interesting results in organosilicon chemistry.18 Hexaalkynylorganoditins, where the tin atoms are bound by a spacer were thus prepared, following the same chemical scheme involving tricyclohexyltin and trichlorotin groups as for the functional trialkynylorganotins, which would allow the use of alkyl-, benzyl-, and aryl-type spacers. This study was limited to symmetrical derivatives. In this case, the method of simultaneous introduction of tin atoms can be easily used. Known routes involve the coupling of a triorganotin species with an organic dihalide,19 the coupling of a di-Grignard or

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a dilithium reagent with a triorganotin halide,20 or the palladium-catalyzed21 addition of hexaorganoditins to unsaturated compounds. The addition of a triorganotin hydride to α,ω-unsaturated compounds is also disclosed here. The reaction of α,ω-dibromomagnesioalkanes with tricyclohexyltin chloride leads to the expected ditins† 62, 65, 67, and 70, separated by up to 10 methylene groups in high yields (Scheme 3.7.10).22 M-R-M

+

2 Cy3 SnCl

– 2 MCl

Cy3 SnRSnCy3 M = Li, MgX

Scheme 3.7.10

Coupling of dimetal species with tricyclohexyltin chloride

This access to α,ω-bis(tricyclohexyltin)alkanes proves to be very convenient when the corresponding organic dihalides are available. When this is not the case, and when α,ω-dienes are more easily accessible, a double hydrostannation of these unsaturated compounds can be used.23 As indicated above, this radical addition has to be conducted in less mild conditions than for the addition of tri-n-butyltin hydride, for instance, because of the bulkiness of the organotin center and also of the more nucleophilic character of the tricyclohexyltin radical. Nevertheless, the treatment of 4,4 -di(butenyl)biphenyl or 4,4 bis(but-3-enyloxymethyl)biphenyl with tricyclohexyltin hydride at 130 ◦ C over seven days affords the corresponding adducts 72 and 75 in good yields (Scheme 3.7.11). Due to its rapid decomposition at this temperature, the AIBN initiator has to be added in small portions for the duration of the reaction. 2 Cy3SnH

AIBN

R

+

Cy3Sn

R

SnCy3

72, 75

Scheme 3.7.11

Double hydrostannation of α,ω-dienes

It is facile to prepare dibenzyl derivatives, such as 78, when the corresponding Grignard reagents are stable. However, when they are not, because of the occurrence of an intensive polycondensation of the dihalide in spite of high dilution conditions, Barbier conditions prove to be very efficient (Scheme 3.7.12). The simultaneous reaction of 1,4-bis(chloromethyl)benzene or 4,4 -bis(chloromethyl)biphenyl with tricyclohexyltin chloride and magnesium allows the preparation of p-substituted dibenzyl derivatives 81 and 84, with one or two phenyl rings. Cl + Cy3SnCl n

Cl

Mg

Cy3Sn

- MgCl 2

SnCy3 n = 1 (81), 2 (84)

Scheme 3.7.12

Barbier coupling of bis(chloromethylated) derivatives

Hexacyclohexylditins 87 and 90, where the tins are separated by phenyl rings are prepared from the corresponding Grignard reagent when it is a convenient route, i.e. in more simple cases, when the corresponding di-Grignard is soluble enough to be useful. The dilithium reagent, obtained by exchange from t-butyllithium, can be used to afford 93 or 96, when two or three phenyl rings separate the tins, respectively (Scheme 3.7.13). †

The structural formulae of the prepared organoditins are presented in Table 3.7.2.

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Table 3.7.2 α,ω-Bis(tricyclohexyltin), α,ω-bis(trichlorotin) and α,ω -bis(tripropynyltin) bridged compounds

Y3Sn

SnY 3

Y3Sn

SnY3

Y3Sn

SnY 3

Y3Sn

SnY3

Y3Sn (CH2)4

Y3Sn

Y (% Yield)

Y = Cl (% Yield)

Y = C≡CMe (% Yield)

62 (71) (Cy)

63 (94

64 (59)

65 (61) (Cy)

66 (93)

67 (58) (Cy)

68 (97)

70 (61) (Cy)

71 (98)

72 (60) (Cy)

73 (97)

74 (62)

75 (60) (Cy)

76 (96)

77 (63)

78 (69) (Cy)

79 (90)

80 (61)

81 (78) (Cy)

82 (89

83 (62)

84 (80) (Cy)

85 (80)

86 (70)

87 (65) (Cy)

88 (95)

89 (47)

90 (69) (Cy)

91 (90)

92 (61)

93 (50) (Cy)

94 (85)

95 (60)

69 (83)

(CH2)4 SnY3

(CH2)4O O(CH2)4

SnY3

SnY3

SnY3

Y3Sn SnY3

SnY3 Y3Sn Y3Sn

SnY3

SnY3 SnY3

Y3Sn

SnY3

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(Continued) Y (% Yield)

Y = Cl (% Yield)

Y = C≡CMe (% Yield)

96 (25) (Cy)

97 (87)

98 (60)

99 (72) (Me)

100 (94)

101 (94)

105 (90)

106 (90)

SnY3

Y3Sn

OC8H17 Y3Sn

371

SnY3

C8H17O

102 (50) (Cy)

OC8H17 Y3Sn

SnY3

C8H17O

103 Me (50)

OC16H33 Y3Sn

SnY3

C16H33O

104 Me (87)

OC8H17 Y3Sn

SnY3 C8H17O

MgBr BrMg

– [2 MgClBr] + 2 Cy3SnCl

– [2 MgClBr]

Li

Li

+ 2 Cy3SnCl

SnCy3 Cy3Sn

SnCy3

Cy3Sn n

n

n = 2, 3

Scheme 3.7.13

Preparation of bis(tricyclohexyltin)-substituted aromatic compounds

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The use of cyclohexyl substituents on the tin proves to be very useful for the preparation of the corresponding trichlorides, because of the large difference in solubility between tricyclohexyltin chloride and the organotin trichlorides or the organoditin hexachlorides that are necessary for this study. However, with cross-shaped derivatives with long lipophilic organic chains linked to the spacers, this difference in solubility is not high enough to lead to an easy recovery of the corresponding organoditin hexachlorides.24 The corresponding hexamethyl derivatives 99, 103, and 104 can thus be prepared. It is expected that the secondary product, trimethyltin chloride, coming from their treatment with tin tetrachloride is easy to separate from the organoditin hexachloride by distillation, under conditions mild enough to avoid any dismutation of the ditin hexachlorides. These hexamethylorganoditins can be prepared from the corresponding dilithium reagents in cases where one phenyl ring separates the tin atoms (Scheme 3.7.14). The corresponding di-Grignard reagents are found to be more convenient than the dilithium reagents when three phenyl rings separate the tin atom, probably due to solubility problems (Scheme 3.7.15). OR Li

1

OR Li

+ 2 R23SnCl

- 2 LiCl

1

2

2 3

R 3Sn

1

SnR 1

RO

RO

Scheme 3.7.14

Cross-shaped bis(triorganotin) compounds from dilithium derivatives

OC8H17

OC8H17 – 2 [MgClBr] MgBr + 2 Me3SnCl

BrMg

Me3Sn

SnMe3 C8H17O

C8H17O

Scheme 3.7.15

104

Cross-shaped bis(triorganotin) compounds from di-Grignard derivatives

Then, when the hexacyclohexylorganoditins are treated with one equivalent of tin tetrachloride, the corresponding trichlorides are recovered in high yield, after extraction of tricyclohexyltin chloride from the reaction mixture with pentane (Scheme 3.7.16). In the case of hexamethylorganoditins, an excess of tin tetrachloride is necessary to obtain the corresponding hexachlorides, 100 and 105, and no decomposition of the expected organoditin hexachlorides occurs during the distillation of the mixture of methyltin chlorides formed. R13Sn-R 2 -SnR13

+

n SnCl 4

Cl3Sn-R 2-SnCl 3 R1 = Me, n = 9 R1 = Cy, n = 2

Scheme 3.7.16

Preparation of hexachloroorganoditins

When conducted under mild conditions, or with only one equivalent of tin tetrachloride, the dealkylation process can be stopped after reaction of one tin center, in the case of 87. The presence of the electron-withdrawing trichlorotin group in the para-position of the ring deactivates the metalated aryl carbon bond to such extent that the intermediate bearing a trichlorotin and a tricyclohexyltin group can

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be isolated. It can be further characterized by alkylation with methylmagnesium chloride which leads to 1-tricyclohexylstannyl-4-trimethylstannylbenzene (Scheme 3.7.17). This desymmetrization reaction constitutes an interesting entry to previously unreported ditins, where the tin atoms bear different organic groups. It could be extended to polycyclic ditins, where a strong electronic relationship exists between the tin centers. SnCy3

Cy3Sn

SnCl4

SnCl3

Cy3Sn

MeMgCl Cy3Sn

SnMe3

87

Scheme 3.7.17

Unsymmetrically substituted arylditins

A stoichiometric amount of propynyllithium is then used to perform the hexaalkynylation of the organoditin hexachlorides at low temperature (Scheme 3.7.18). The reaction leads to solid compounds, which can be easily purified by recrystallization. – 6 LiCl Cl3Sn

R SnCl3 + 6 Me

Li

Scheme 3.7.18

(Me

)3Sn

R

Sn(

Me)3

Preparation of hexaalkynylorganoditins

The presence in some of the organoditin hexachlorides of oxygen atoms, either in the chain of the spacer or in the adjacent substituents, and electrophilic tin atoms induces interactions between these atoms. For instance, the molecular structure of 76 (Figure 3.7.5) reveals the formation of two sixmembered [1,2]oxastanninane rings by coordination of the tin centers by the oxygen atoms of the spacer. This internal coordination is characterized by an oxygen bond length of 247 pm, about equal to that in ester 44 (246 pm), but longer than in polymeric 36 (236 pm). Compounds 99–102, where the tin and the oxygen atoms are in favourable positions to form a fourmembered ring by intra-molecular interaction were studied by X-ray crystallography. For these derivatives, a theoretical bond distance of 323 pm between the tin and the oxygen atoms, based on measured tin–carbon and oxygen–carbon bond lengths and an ideal angle of 60◦ between these bonds, was calculated. In 102, the distance measures 321 pm, which shows that almost no coordination exists in this molecule because of the presence of bulky electron-donating groups on the tin making access difficult and its electrophilicity low. In 99, the measured distance diminishes to 309 pm, which indicates a rather weak, but positive, interaction between the tin and the oxygen toms. This effect increases strongly in 100, where the presence of electron-withdrawing chloride atoms enhances the nucleophilicity of the metal, since a tin–oxygen distance of 286 pm is measured. With respect to the distance between the tin and the oxygen

Figure 3.7.5

Molecular structure of 76

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atoms in 102, this value represents an 11% decrease. The electron-withdrawing effect of alkynyl groups is strong enough to induce a marked effect on the tin–oxygen interaction as the tin–oxygen distance in 101 is 296 pm, 10 pm less than with methyl substituents on the tin.

3.7.4

Conclusion

It has been shown that functional trialkynylorganotins can be easily prepared from the alkynylation of the corresponding trichlorides. A new access to these halides had to be developed, as known methods of preparation do not give the expected results. The new procedure involves the use of a cyclohexyl group, rather unusual in organotin chemistry, as an auxiliary group, which leads to the expected functional organotin trichlorides in convenient conditions. This study could be successfully extended to the preparation of hexaalkynylorganotins, where the tin atoms are linked by an organic spacer, which could be of various types, alkylene, arylene, or dimethylenearyl. These organotins were used successfully as precursors of organic–inorganic materials (see Chapter 3.2) with various applications.25 Acknowledgments I wish to thank Professor T. Toupance for helpful discussions and Dr. M. Lahcini, Dr. P. Jaumier, Dr. H. Riague, Dr. G. Vila¸ca, Dr. H. Elhamzaoui, Dr. G. Prabusankar, and M-C. Rascle for their crucial contributions to various aspects of this work, as well as Professor M. Biesemans, Professor E.R.T. Tiekink, Professor R. Willem, and Dr. H. Allouchi for their fruitful collaboration. References 1. 2. 3. 4. 5. 6. 7. 8.

9. 10. 11. 12. 13. 14. 15. 16. 17.

C. J. Brinker and G. W. Scherer, Sol-Gel Science, Academic Press, London, 1990. P. Gomez-Romero and C. Sanchez, Functional Hybrid Materials, Wiley-VCH, Weinheim, 2003. J. D. Kennedy, W. McFarlane, P.J. Smith, R.F.M. White, and L. Smith, J. Chem. Soc., Perkin 2, 1785 (1973). H. Schumann and I. Schumann, Organotin Compounds in Gmelin Handbook of Inorganic Chemistry, H. Bitterer, (Ed), Springer-Verlag, Berlin, 1979, p 210. B. Jousseaume, M. Lahcini, M-C. Rascle, F. Ribot, and C. Sanchez, Organometallics, 14, 685 (1995). A. Rahm, F. Ferkous, M. Degueil-Castaing, J. Jurczak, and A. Golebiowski, Synth. React. Inorg. Met.-Org. Chem., 17, 937 (1987). G. Vilaca, K. Barathieu, B. Jousseaume, T. Toupance, and H. Allouchi, Organometallics, 22, 4584 (2003). U. Kolb, M. Dr¨ager, and B. Jousseaume, Organometallics, 10, 2737 (1991); H. Pan, R. Willem, J. Meunier-Piret, and M. Gielen, Organometallics, 9, 2199 (1990); J. W. Burley, P. Hope, and A. G. Mack, J. Organomet. Chem., 277, 37 (1984); B. Jousseaume, P. Villeneuve, M. Dr¨ager, S. Roller, and J. M. Chezeau, J. Organomet. Chem., 349, C1 (1988). M. Biesemans, R. Willem, S. Damoun, P. Geerlings, M. Lahcini, P. Jaumier, and B. Jousseaume, Organometallics, 15, 2237 (1996). P. Jaumier, B. Jousseaume, E. R. T. Tiekink, M. Biesemans, and R. Willem, Organometallics, 16, 5124 (1997). M. Biesemans, R. Willem, S. Damoun, P. Geerlings, E. R. T. Tiekink, P. Jaumier, M. Lahcini, and B. Jousseaume, Organometallics, 17, 90 (1998). A. G. Davies, Organotin Chemistry, Wiley-VCH, Weinheim, 2004, p.114. P. Jaumier, B. Jousseaume, M. Lahcini, F. Ribot, and C. Sanchez, Chem. Commun., 1998, 369. R. Willem, M. Biesemans, P. Jaumier, and B. Jousseaume, J. Organomet. Chem., 572, 233 (1999). D. Seyferth and M. A. Weiner, J. Am. Chem. Soc., 84, 361 (1962). P. Jaumier, B. Jousseaume, and M. Lahcini, Angew. Chem., Int. Ed., 38, 402 (1999). G. Prabusankar, B. Jousseaume, T. Toupance, and H. Allouchi, Angew. Chem., Int. Ed., 45, 1255 (2006).

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18. D. A. Loy and K. J. Shea, Chem. Rev., 95, 1431 (1999); R. J. P. Corriu, Angew. Chem., Int. Ed., 39, 1376 (2000). 19. T. G. Traylor and G. S. Koermer, J. Org. Chem., 46, 3651 (1981); K. R. Wursthorn, H. G. Kuivila, and G. F. Smith, J. Am. Chem. Soc., 100, 2779 (1978). 20. E. J. Bulten and H. A. Budding, J. Organomet. Chem., 110, 167 (1976); S. Bou´e, M. Gielen, and J. Nasielski, Bull. Soc. Chim. Belg., 76, 559 (1967). 21. T. N. Mitchell, A. Amamria, H. Killing, and D. Rutschow, J. Organomet. Chem., 304, 257 (1986). 22. B. Jousseaume, H. Riague, T. Toupance, M. Lahcini, P. Mountford, and B. R. Tyrrell, Organometallics, 21, 4590 (2002). 23. H. Elhamzaoui, B. Jousseaume, T. Toupance, C. Zakri, M. Biesemans, R. Willem, and H. Allouchi, Chem. Commun., 2006, 1304. 24. H. Elhamzaoui, B. Jousseaume, T. Toupance, and H. Allouchi, Organometallics, 26, 3908 (2007). 25. H. Elhamzaoui, B. Jousseaume, H. Riague, T. Toupance, P. Dieudonne, C. Zakri, M. Maugey, and H. Allouchi, J. Am. Chem. Soc., 126, 8130 (2004); H. Elhamzaoui, T. Toupance, M. Maugey, C. Zakri, and B. Jousseaume, Langmuir, 23, 785 (2007); T. Toupance, H. Elhamzaoui, B. Jousseaume, H. Riague, I. Saadeddin, G. Campet, and J. Brotz, Chem. Mater., 18, 6364 (2006); S. Boutet, B. Jousseaume, T. Toupance, M. Biesemans, R. Willem, C. Labrugere, and L. Delattre, Chem. Mater., 17, 1803 (2005); C. Franc, B. Jousseaume, M. Linker, and T. Toupance, Chem. Mater., 12, 3100 (2000); G. Vila¸ca, B. Jousseaume, C. Mahieux, C. Belin, H. Cachet, M.-C. Bernard, V. Vivier, and T. Toupance, Adv. Mater., 18, 1073 (2006).

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3.8

Organotin Polymers and Related Materials

Hemant K. Sharma and Keith H. Pannell Department of Chemistry, University of Texas at El Paso, USA

3.8.1

Introduction

The group 14 elements have an almost unique capacity to catenate; however, this tendency decreases markedly down the group from carbon to lead. The polymers of group 14 elements (E = Si, Ge, Sn, Pb), of the general formula (R2 E)n , are formally structural analogs of saturated polymeric hydrocarbons, (R2 C)n , but their properties are quite dissimilar and the electronic properties of (R2 E)n exhibit more similarities with conjugated polyenes. Such polymetallates exhibit an intense low-energy absorption, associated with σ –σ * transitions, which undergo red shifts with increasing chain lengths. In this general class of polymers, the polysilanes are well studied due to their remarkable electronic, optical, and chemical properties, and this has stimulated interest in the heavier element congeners, i.e. E = Ge, Sn. Such materials might be expected to give greater σ -delocalization due to more diffuse bonding orbitals, lower band gaps, and more metallic character. Furthermore, such properties could be tailored for applications in chargetransport, photoresists in microlithiography, non-linear optical materials, semiconductors in doping, and electronic devices.1 The relationship between structure and the electronic properties of polymetallates involves the degree of σ -conjugation, which depends upon the changes in solid-state structures, backbone conformation, chain-length of the polymer and the substituents attached to the main chains. The purpose of this chapter is to review the synthesis and properties of linear oligo- and polystannanes and other organometallics in which tin is in the polymer/oligomer backbone.2,3 3.8.2

Synthesis of Linear Oligostannanes

Linear oligostannanes can serve as models for the systematic study of the properties and reactivity of higher molecular weight polystannanes. Simple salt elimination reactions have been employed to construct chains up to six tin atoms, Equation (3.8.1). However, these reactions are not clean and give a mixture of products, due to the cleavage of tin–tin bonds by the organotin lithium reagents, and, in general, the oligostannanes are isolated in poor yields.4 2 Ph3 SnLi + I(t-Bu2 Sn)n I

−2 Lil −→ Ph3 Sn(t-Bu2 Sn)n SnPh3

(n = 1, 2)

(3.8.1)

The amine elimination reaction between tin amides and organotin hydrides is an attractive reaction to form Sn–Sn bonds and has been used to construct linear tri- and tetrastannanes, Equation (3.8.2).5 2 R3 SnNMe2 + H(R2 Sn)n H

−2 Me2 NH −→ R3 Sn(R2 Sn)n SnR3

(R = Ph, t-Bu, n = 1, 2)

(3.8.2)

Sita modified the above reaction and used a stepwise approach consisting of two reactions to construct the linear chain of six tin atoms, by introducing a β-alkoxy group as a protecting reagent on tin6 , Scheme 3.8.1. The β-alkoxy(di-n-butyl)tin amide underwent amine elimination with tri-n-butyltin hydride to form a distannane in the first reaction. The β-alkoxy group was subsequently transformed to the Sn-H functionality using diisobutylaluminum hydride (iso-Bu2 Al-H) thus facilitating chain growth.

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Bu2SnNMe 2 + R

-Me2NH

Bu3SnH

Bu2SnSnBu 3 R

iso-Bu2Al-H -C2H4 -Bu2AlOEt

377

Bu2Sn SnBu3 H -Me2NH

Bu2Sn NMe2 R

Bu2Sn

Bu2Sn(SnBu2)2SnBu3 R -C2H4 -Bu2AlOEt

NMe2

R -Me2NH

Bu2SnSnBu2SnBu3 H

iso-Bu2Al-H -C2H4 -Bu2AlOEt

Bu2SnSnBu2SnBu3 R

iso-Bu2Al-H

Bu2Sn(SnBu2)2SnBu3 H

Scheme 3.8.1

Synthesis of linear tetrastannane, (R = (CH2 )2 OEt)

Despite the elegant nature of this chemistry, the sequence is cumbersome, since two steps are required in the chain growth step and the synthesis and purification of air-sensitive materials is time-consuming and lowers the overall yield of oligomeric products. A simple modification of the above synthetic strategy leads to the systematic construction of oligostannanes up to 15 tin atoms in the linear chain as shown in Scheme 3.8.2.7 The mechanism of this process is not clear, but it is presumed that the di-n-butylstannylene, Bu2 Sn: generated from the decomposition of β-ethoxyethyl(di-n-butyl)stannane with lithium diisopropylamide (LDA), inserts into the tin–hydrogen bond in the chain growth step. The air-sensitive colorless oligomers are separated by reverse-phase HPLC using dichloromethane/acetonitrile solvent mixture. Short chain oligomers, n = 3, 4, and 5, dominate in the mixture of oligomers, n = 1–15. Bu2SnH

Bu2SnH R

LDA

Bu2SnLi R

THF

-LiOEt -C2H4

: Bu2Sn

R

Bu2SnSnBu2H R n x Bu2Sn

Bu2SnNMe2

Bu2Sn(SnBu2)n+1SnBu2H R

R -Me2NH

Bu2Sn(SnBu2)nSnBu2H R

(n = 0-12) Scheme 3.8.2

Synthesis of oligostannanes containing up to 15 tin atoms, (R = (CH2 )2 OEt)

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Synthesis of Polystannanes

Wurtz-Like Coupling

In general, Wurtz-like coupling reactions are highly sensitive to the substituents on the substrate, solvents, concentrations, alkali metals, reaction period, and temperature. Thus, it is often difficult to optimize and reproduce the conditions for the synthesis of high molecular weight polymers. Polysilanes and polygermanes have been successfully synthesized by high-temperature reductive coupling of the R2 ECl2 with alkali metals in hydrocarbon or ethereal solvent and this approach has been applied to polystannane synthesis, Equation (3.8.3).8,9 R2 ECl2

alkali metal −→ [−R2 E−]n high temp (E = Si, Ge, Sn)

(3.8.3)

Despite this promising model, the relatively harsh conditions generally produce low molecular weight oligomeric stannanes or cyclic oligomers (Table 3.8.1) in relatively poor yields, due to the lability of the tin–tin bond.10 The average bond dissociation energy of 163 kJ mol−1 for the tin–tin bond can be compared to the analogous values of 285 kJ mol−1 and 248 kJ mol−1 for Si–Si and Ge–Ge bonds, respectively,11 and accounts for the results. A promising early report12 concerning the synthesis of the high molecular weight poly(di-nbutyl)stannane, (Bu2 Sn)n , from the Wurtz coupling of n-Bu2 SnCl2 with Na dispersion in toluene, in the presence of a crown ether, often gives variable results. In general, only low molecular weight oligomers are obtained under these conditions.13 However, a high molecular weight poly(di-n-butyl)stannane, Mw = 1.09 × 106 , was obtained at 60 ◦ C in toluene using a shorter reaction period of 4 h, suggesting that the longer reaction times result in the degradation of the polymer chain.14 Overall, Wurtz coupling permits little control over the stereoselectivity of the chain growth and only simple alkyl- or aryl-substituted polymers can be obtained.15 The functionalities which are necessary for the modification of polymers for various applications cannot be introduced into the polymer chain. A distinctive coupling approach for diorganotin dihalides involves the use of SmI2 as a reducing agent under mild homogeneous conditions at room temperature. However, despite the narrow polydispersities only low molecular weight polystannanes have been obtained thus far, (R2 Sn)n , (R = Me, Et, n-Hex).16,17 Electrochemical

The electrochemical reduction of diorganotin dihalides seems a promising route for the synthesis of moderate molecular weight polystannanes in good and reproducible yields.18−20 Dimethyl-, di-n-butyl- and di-n-octyltin polymers have been synthesized in DME as solvent, with tetrabutylammonium perchlorate as a supporting electrolyte, in a single compartment cell with a Pt cathode and Ag anode, Equation (3.8.4). The mild conditions in this approach prevent the scission of Sn–Sn bonds in the chain growth step that complicates the Wurtz coupling. The method suffers from the limitations associated with the attrition of the electrodes. nR2 SnC12

− +2ne−→ [−R2 Sn−]n −

−2 C1

(R = n-Bu, n-Oct)

(3.8.4)

Dehydropolymerization

The dehydrogenative coupling of hydrosilanes and hydrogermanes catalyzed by the early transition metal complexes is an attractive route for the synthesis of derivative polymers.21,22 A major advantage of this

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approach is that a number of functionalized high molecular weight polymers with narrow polydispersities can be synthesized under relatively mild conditions. This approach can be applied successfully to the synthesis of polystannanes (Table 3.8.1) obtained from secondary stannanes, R2 SnH2 . A variety of transition metals catalysts involving complexes of Ti, Zr, Hf, Cr, Mo, W, Rh, Pt and heterobimetallic Fe–Pd13,23−32 have been used, Equation (3.8.5). metal catalyst −→ H[R2 Sn]n H + cyclo − [−R2 Sn−]m −H2 (n = number of monomer units in linear polymer chain,

R2 SnH2

m = number of monomer units in cyclic oligomers)

(3.8.5)

Early studies on dehydropolymerization suggested that metallocene dichlorides Cp2 MCl2 , (M = Ti, Zr, Hf) could be used as catalysts, but they were not very effective; only low molecular weight polymers were Table 3.8.1

Representative examples for the synthesis of polystannanes

No. Monomer

Method Polymerization Conditionsa

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

3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.2 3.2 3.2 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3

a

n-Bu2 SnCl2 n-Bu2 SnCl2 n-Bu2 SnCl2 Me2 SnCl2 Et2 SnCl2 n-Bu2 SnCl2 n-Hex2 SnCl2 Me2 SnCl2 n-Bu2 SnCl2 n-Oct2 SnCl2 n-Bu2 SnH2 n-Bu2 SnH2 n-Bu2 SnH2 n-Bu2 SnH2 n-Bu2 SnH2 n-Bu2 SnH2 n-Bu2 SnH2 n-Hex2 SnH2 n-Hex2 SnH2 n-Oct2 SnH2 n-Oct2 SnH2 Ph2 SnH2 ( p-But -C6 H4 )2 SnH2 ( p-Hex-C6 H4 )2 SnH2 (o-Et-C6 H4 )2 SnH2 ( p-BuO-C6 H4 )2 SnH2 (o-Et- p-BuO-C6 H3 )2 SnH2 ( p-(Me3 Si)2 N-C6 H4 )2 SnH2

Na/15-crown-5/ toluene/60 ◦ C/14h Na/15-crown-5/ toluene/60 ◦ C/14h Na/15-crown-5/ toluene/60 ◦ C/4h SmI2 /HMPA-THF/ RT/24 h SmI2 /HMPA-THF/ RT/24 h SmI2 /HMPA-THF/ RT/24 h SmI2 /HMPA-THF/ RT/120 h DME/Bu4 NClO4 DME/Bu4 NClO4 DME/Bu4 NClO4 neat/CpCp*Zr [Si(SiMe3 )3 ]Me/24h neat /CpCp*ZrMe2 / 5h neat /Cp2 ZrMe2 /24h neat/Me2 C(η5 C5 H4 )2 Zr[Si(SiMe3 )3 ]Me/11h C6 D6 /Me2 C(η5 C5 H4 )2 Hf SnHMes2 ]NMe2 CH2 Cl2 /ClRh (PPh3 )3 /2h toluene/HRh(CO) (PPh3 )3 /13h neat/CpCp*Zr [Si(SiMe3 )3 ]Me/24h neat/Me2 C(η5 C5 H4 )2 Zr[Si(SiMe3 )3 ]Me/10min neat /CpCp*Zr [Si(SiMe3 )3 ]Me/15h neat /Cp2 ZrMe2 /15h neat/CpCp*Zr [Si(SiMe3 )3 ]Me/24h 0.09M pentane/ Cp2 ZrMe2 /24h 0.11M pentane/ Cp2 ZrMe2 /7h neat/Cp2 ZrMe2 /12h/ 90 ◦ C 0.11M pentane/ Cp2 ZrMe2 /47h neat/Cp2 ZrMe2 /3h/90 ◦ C neat/Cp2 ZrMe2 /3h/90 ◦ C

Mw 103 PDIb Ref. 14 2.4 1090 1.12 4.8 2.1 2.7 2.65 10.9 5.9 17.5 74 46 66.9 20.12 20 50.2 36.8 34.6 95.7 92.6 6.7 54 48.2 22 10 4.4 4.2

2 2.6 1.5 1.2 1.0 1.2 – 2.6 1.7 2.2 6.9 3.3 3.3 2.2 3.3 1.4 2.4 2.4 6.7 4.3 1.3 3.6 2.4 1.8 2.0 1.1 1.1

Catalyst (2–4 mol%) was used in dehydropolymerizations; b PDI = Polydispersity, Mw = weight-average molecular weight

12 13 14 16 17 17 17 20 18 18 13,23 13 13 13 24 27 32 13 13 13 13 13 25 25 25 25 25 25

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obtained along with significant amounts of cyclic oligomers. The mixed catalyst system Cp2 MCl2 /BuLi, which has been effectively used in the polymerization of hydrosilanes, seemed to be more active in the synthesis of poly(di-n-butylstannane).31 Mixing red-Al (sodium bis(2-methoxyethoxy)aluminium hydride) with the metallocene dichlorides, Cp2 MCl2 /red-Al, and metal carbonyls, [M(CO)6 ], (M = Cr, Mo, W) activated the catalysts towards the dehydrocoupling of Bu2 SnH2 . Insoluble cross-linked poly(din-butylstannanes) were obtained.26 The dehydropolymerization of Bu2 SnH2 with RhH(CO)(PPh3 )3 as catalyst in toluene yielded a highly branched, high molecular weight, polymer with a Mw /Mn ratio (Mw = weight-average molecular weight, Mn = number average molecular weight) of 50 240/35 130. Interestingly, when the same reaction was carried out in the absence of a solvent, only cyclic oligomers were obtained.32 Use of the related Wilkinson’s catalyst produced only linear poly(di-n-butylstannane)27 (Mw /Mn 20 000/6000). Clearly, such reactions are very susceptible to major variations depending upon reaction conditions and the catalyst used. Zirconocene catalysts13 are more active than the hafnocene analogs and the ansa-bridged Me2 C(η5 C5 H4 )2 Zr[Si(SiMe3 )3 ]Me was found be the most active catalyst, which produced (n-Bu2 Sn)n with a Mw /Mn ratio of 66 900/20 300 along with 18% cyclic oligomers, mainly cyclo-(n-Bu2 Sn)5 and -(n-Bu2 Sn)6 . The dehydropolymerization of secondary stannanes, i.e. Me2 SnH2 , PhMeSnH2 , and Ph2 SnH2 with Zr catalysts produced only low molecular weight polymers attributable to the low solubility of the oligomeric polystannanes resulting in low conversions of the Sn–H bonds into the polymer chains. Since poly(diphenylstannane) has a low solubility13,25,33 , the addition of alkyl chains at the paraposition of the phenyl ring was studied in an attempt to increase solubility and hence obtain greater molecular weights for polydiarylstannanes, (Ar2 Sn)n (Ar = p-But -C6 H4 , p-Hex-C6 H4 , p-BuO-C6 H4 , o-Et- p-BuO-C6 H3 and p-(Me3 Si)2 N-C6 H4 ).25 The results were in accord with expectation and soluble poly(diarylstannanes) with significantly higher molecular weights were obtained from these reactions, Tables 3.8.1 and 3.8.3. For example, the reaction of ( p-Hex-C6 H4 )2 SnH2 with Cp2 ZrMe2 catalyst (2–4 mol%) in pentane gave a homogeneous reaction mixture and a high molecular weight fraction with Mw /Mn ratio of 54 000/15 000 was isolated.25 Further research is still required in this area to discover more effective and reproducible reaction conditions/catalyst systems. Such an approach could optimize formation of only the high molecular weight linear polystannananes with better structural control and suppression of cyclic oligomer formation. Mechanistic studies on the dehydropolymerization of secondary stannanes are limited. The σ -bond metathesis mechanism, involving a four-center transition state, proposed for dehydropolymerization of hydrosilanes has been ruled out as a significant mechanism for polystannane formation.21 The recent isolation of Cp2 Hf(Cl)(SnMes2 H)34 lends support to the α-H elimination process, producing free stannylene.24 The reactive stannylene could undergo insertion either into an M–Sn or an Sn–H bond. It was suggested that stannylene was preferentially inserted into the M–Sn bond in the chain growth step rather than the Sn–H bond to produce polystannane, as depicted in Scheme 3.8.3. If the stannylenes had inserted into the –H2

LnMH + R2SnH2

–L nM-H

LnMSnR2H

.

n x SnR2 LnM(SnR2)H

H(SnR2)n+1H

Scheme 3.8.3

H2 –LnM-H

LnM(SnR2)n+1H

Probable mechanism for the dehydropolymerization of stannanes

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Sn–H bond, as shown by Sita7 , see above, then lower molecular weight oligomers and/or cycles would be expected. Polystannasilanes and -Germanes

Intra-group 14 copolymers have received limited attention,35−37 despite the fact that the physical, electronic, and thermal properties, and the reactivity of the Sn–E bond (E = Si, Ge) in the copolymers are worth examining. In theory, the properties of such materials can be tuned by varying the ratios of monomer units and the molecular weights of the resulting copolymers. A low molecular weight stannane-silane copolymer was obtained by Wurtz coupling via sonication.36 The coupling of Bu2 SnCl2 /PhMeSiCl2 in different molar ratios varying from 1:2 to 1:4, with a sodium dispersion, with or without a solvent, yielded poly(di-n-butylstannane-co-methylphenylsilane) copolymers, with Sn:Si ratios from 1:6 to 1:9. The average molecular weights ranged from 5740 to 22 700 in 10–22% yields,35 Equation (3.8.6). Interestingly, the copolymerization reactions without a solvent gave higher molecular weight copolymers. Bu2SnCl2

+

PhMeSiCl2

Na -NaCl

SnBu2

SiPhMe x

y

(3.8.6)

The structure of the copolymer, poly(di-n-butylstannane-co-methylphenylsilane) with a Sn:Si ratio of 1:3 was determined with X-ray absorption fine-structure spectroscopy (EXAFS) and X-ray absorption ˚ Sn–Si (2.58 A), ˚ and Sn–C near-edge spectroscopy (XANES).35 The bond distances of Sn–Sn (2.82 A), 38 ˚ (2.15 A) in the copolymer were comparable with the cyclooligostannylsilane, illustrated in Figure 3.8.1. The electrochemical reduction method was also applied to synthesize organotin-germanium, and organotin-silicon copolymers from the mixtures of Bu2 SnCl2 -Bu2 GeCl2 and Bu2 SnCl2 -Bu2 SiCl2 monomers.37 Both the molecular weight and the solubility of the copolymers were increased if the ratio of Bu2 SnCl2 was increased in the monomer mixture. 3.8.4

Properties of Polystannanes

Chemical and Thermal Properties

Polystannanes are yellow/orange viscous oils or solids, stable at room temperature, but sensitive to light and moisture. They are relatively more stable in the presence of oxygen and even bubbling of oxygen in a pentane solution of poly(di-n-butylstannane) does not lead to significant decomposition.18 They are photochemically labile and even in the presence of ambient light undergo deoligomerizations with the scission of Sn–Sn bonds to yield cyclic oligomers, mainly cyclo-(R2 Sn)5 and cyclo-(R2 Sn)6 ,

Figure 3.8.1

A cyclooligostannylsilane

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384 nm

Absorption

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200

250

300 350 400 Wavelength (nm)

450

500

Figure 3.8.2 The electronic spectrum for (n-Bu2 Sn)n showing photochemical deoligomerization. The final spectrum is shown by the ----- dotted line. (Adapted from Reference 13.)

Figure 3.8.2. This property is similar to their polysilane and -germane analogs. The photobleaching of the polystannanes13 is solvent-dependent and occurs more rapidly in pentane than in THF. Thermogravimetric analysis (TGA) of the dialkyltin polymers,13 (R2 Sn)n (R= n-Bu, n-Hex, n-Oct) under nitrogen has shown that decomposition begins in the temperature range 255–270 ◦ C with yields of metallic tin in the range of 18–34% at 400 ◦ C. Under an oxygen atmosphere, the decomposition of the poly(dialkylstannanes) onsets at lower temperatures with higher ceramic yields of pure SnO2 , which has potential applications in thin-film semiconductors. The poly(diarylstannanes)25 are more stable than poly(dialkylstannanes) and they undergo thermal decompositions at a slightly higher temperature of ∼300 ◦ C. Conductivity and Non-Linear Optical Properties

Thin films of the poly(dialkyl)stannanes, doped with SbF5 , exhibit electroconductivities as high as 0.3 S cm–1 , i.e. at the level of a semiconductor.13 The third-order non-linear optical susceptibility (χ 3 ) of poly(di-n-butylstannane) was measured at 532 nm with a single beam Z-scan, using degenerate four-wave mixing at different concentrations in chloroform. The χ 3 value of 10−10 esu was two orders of magnitude greater than that of polysilanes.39 The charge mobility along the sigma-bonded backbone of tin atoms in poly(di-n-butylstannane) was found to be 0.1 cm2 V−1 s−1 in the crystalline-phase.40 The high mobility and alignment of the polymer chains make them potential candidates for charge-transport in electronic devices.1 Developments are also underway to use polystannanes in electroconducting films, color filters for liquid crystal displays, and electroluminescent devices.41,42 Electronic and Thermochromic Properties

The electronic spectra of the linear oligostannanes2−7,43 e.g. for the homologous series Et3 Sn-(SnEt2 )n SnEt3 , (n = 0–4) are detailed in Tables 3.8.2 and 3.8.3. They exhibit a systematic red shift of the lowest energy transition, which is associated with a σ –σ * transition, upon increasing the chain length. For example, the λmax for perethylated distannane is red shifted from 232 nm to 325 nm for hexastannane. Similar trends were also observed in other series, as illustrated for n-Bu3 Sn(Sn-n-Bu2 )n Sn(n-Bu2 )R (n = 0–4, R = (CH2 )2 OEt)6,7 in Figure 3.8.3. The degree of the red shift and the magnitudes of these absorptions are dependent upon the substituents on tin, the solvent, and the temperature of the measurements.

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Table 3.8.2 Electronic spectral data for linear perethylated oligostannanes,43 Et3 Sn-(SnEt2 )n -SnEt3 , (n = 0-4)

a

Table 3.8.3

Stannane

λmax (nm)a

Et3 Sn-SnEt3 Et3 Sn-SnEt2 -SnEt3 Et3 Sn-SnEt2 -SnEt2 -SnEt3 Et3 Sn-SnEt2 -SnEt2 -SnEt2 -SnEt3 Et3 Sn-SnEt2 -SnEt2 -SnEt2 -SnEt2 -SnEt3

232 250 290 310 325

In cyclohexane

UV and 119 Sn NMR spectral data for selected linear oligostannanes

Stannane

UV (nm)

119

Me3 Sna Sna Me3 n-Bu3 Sna Sna n-Bu3 t-Bu3 Sna Sna t-Bu3 Ph3 Sna Sna Ph3 Me3 Sna Snb Me2 Sna Me3 n-Bu3 Sna Snb n-Bu2 Sna n-Bu3 t-Bu3 Sna Snb t-Bu2 Sna t-Bu3 Ph3 Sna Snb Ph2 Sna Ph3 (n-Bu3 Sna Snb n-Bu2 )2 (Brt-Bu2 Sna Snb t-Bu2 )2 (Ph3 Sna Snb Ph2 )2 (n-Bu3 Sna Snb n-Bu2 )2 (Snc n-Bu2 ) (Ph3 Sna But2 Snb )2 (Snc t-Bu2 )2

210

–109 –83 –6 –145 –100.8 Sna , –263 Snb –75.6Sna , –226.6 Snb 20.4Sna , 17.0 Snb –139Sna , –230 Snb –74.3Sna , –212.6 Snb 102.8Sna , 25.2 Snb –138Sna , –219 Snb –75.6Sna , –199 Snb , –210 Snc –156Sna , –7Snb , 68Snc

a

222 247

298 268

1 119

Sn NMR

J(

Sna -119 Snb )(Hz)

4404 2748 57 4480 2900 1596 810 2874 1596 1991 2676 1368 170

Ref. 43 44,45 45,49 45,46 45,47 48 2a 2a 48 49 2a 48 3

Terminal Sn; b,c Internal Sn

8

Absorbance

6

4

2

0 210

n=0

230

250

n=1

270

n=2

290

310

n=3 n=4

330

350

Wavelength

Figure 3.8.3 erence 6.)

The electronic spectra of n-Bu3 Sn(n-Bu2 Sn)n SnBu2 -n(CH2 )2 OEt, (n =0–4). (Adapted from Ref-

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Tin Chemistry: Fundamentals, Frontiers and Applications Table 3.8.4

UV/Visa and 119 Sn NMR spectral data for selected polystannanes

Entry

Polymer

λmax (nm)a

1 2 3 4 5 6 7 8 9 10 11 12 13

(Et2 Sn)n (n-Bu2 Sn)n (n-Bu2 Sn)n (n-Bu2 Sn)n (n-Hex2 Sn)n (n-Oct2 Sn)n (Ph2 Sn)n [(o-Et-C6 H4 )2 Sn)]n [( p-But -C6 H4 )2 Sn)]n [( p-Hex-C6 H4 )2 Sn)]n [( p-BuO-C6 H4 )2 Sn)]n [(o-Et- p-BuO-C6 H3 )2 Sn]n [( p-(Me3 Si)2 N-C6 H4 )2 Sn)]n

368 390 382 394 384 388 402 468b 432 436 448 506 450

a

119

Sn NMR (ppm)

–189.6 –190.6 –189 –190.9 –190.7

–197.0 –196.0 –183.5 –125

Ref. 17 13 27 32 13 13 13 25 25 25 25 25 25

In THF; b As film

The electronic spectra of the hydro-terminated poly(dialkyl)stannanes, H-(SnR2 )n -H (Table 3.8.4) exhibit λmax values in the range 380–480 nm.13 These absorptions are sensitive to the polymer conformation, solvent, temperature, and the molecular weights, and the degree to which there are non-H-terminated cycles present. For example, a sample of H-{Sn(n-Bu2 )}n -H with an Mw /Mn of 41 200/13 900 prepared from dehydropolymerization with a Zr catalyst,13 exhibits an absorption at 390 nm, whereas another sample of poly(di-n-butyl)stannane synthesized from Wurtz-like coupling14 displays a transition at 365 nm. For comparison, the λmax values for poly(di-n-butylsilane) (Bu2 Si)n and poly(di-n-butylgermane) (Bu2 Ge)n are 314 and 333 nm, respectively.21,22 Therefore, the polystannanes are red-shifted by ca. 70 and ca. 50 nm from the related poly-silanes and -germanes. The λmax values increase with the molecular weight, reaching a limiting value with chain lengths of about 30 tin atoms.7 The λmax values of poly(diarylstannanes)25 in the range 430–506 nm are significantly red-shifted by 30–40 nm in comparison with poly(dialkylstannanes). The poly(diarylstannanes), (Ph2 Sn)n , [( p-But -C6 H4 )2 Sn]n , [ p-HexC6 H4 )2 Sn]n , and [o-Et-p-BuO-C6 H3 )2 Sn]n , exhibit transitions at 402, 432, 436, and 506 nm, respectively, suggesting some degree of σ –π conjugation between the tin backbone and aryl substituents on tin. The λmax value of poly(diaryl)stannane, [o-Et- p-BuO-C6 H3 )2 Sn]n at 506 nm exhibits the largest shift for any polystannane, even though the molecular weight is relatively low, Mw /Mn of 4400/4000. It also exhibits the smallest band gap (2.3 eV) observed for conjugated group 14 polymers so far reported.25 The poly(stannane-co-germane) and poly(stannane-co-silane) materials exhibit λmax between those of the respective homopolymers. However, increasing the stannane contents in the copolymers results in a red shift of the UV absorption probably due to the effective overlap of orbitals ascribed to the increased atomic radius of tin over Ge(Si). As an example, for poly(di-n-butylstannane-co-di-n-butylsilane) with an Sn:Si ratio of 43:57, λmax = 375 nm, as compared to 314 and 384 nm for poly(di-n-butylsilane) and poly(di-n-butylstannane), respectively. The poly(dialkyl) stannanes, (Hex2 Sn)n and (Oct2 Sn)n exhibit a reversible thermochromic behavior upon warming above room temperature.13 The λmax values of these polymers undergo a blue-shift from 384 to 369 nm for poly(di-n-octyl)stannane between 30–40 ◦ C in toluene and from 392 to 382 nm in

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402 nm

1.70

0.72 350

400

450

Wavelength

Figure 3.8.4 The electronic spectra of (n-Oct2 Sn)n polymer showing thermochromism at two temperatures, at ca. 45 ◦ C (λmax 402 nm), and at ca. 20 ◦ C (λmax 378 nm). (Adapted from Reference 13.)

the same temperature range for poly(di-n-hexyl)stannane solid films, Figure 3.8.4. The thermochromic changes are associated with the phase change, as shown by endotherms in DSC measurements. Poly(din-butyl)- and poly(diaryl)tin polymers do not show the thermochromic behavior over the temperature range of –10 to 90 ◦ C. Interestingly, the phase transitions for poly(di-n-butylstannane)50 were observed at a slightly lower temperature of ∼ –15 ◦ C. The phase changes were detected by variable-temperature UV-Vis coupled with Raman spectroscopy over a temperature range of –15 to –73 ◦ C and exhibited two phases: one of which was highly disordered, (UV absorption at λmax 390 nm and ν(Sn . . . Sn) broad Raman absorption at 120 cm–1 ). The other phase was more ordered (anti or transoidal) and red-shifted to 420 nm, and showed a sharp line in the Raman spectrum shifted to higher frequency at 145 cm–1 . A second sample of a poly(di-n-butylstannane), obtained from the dehydropolymerization of Bu2 SnH2 using Wilkinson’s catalyst, exhibited a reversible phase transition at a higher temperature, ∼1 ◦ C. The polymer was in a crystalline-liquid phase at room temperature, as indicated by VT WAXS, from which highly ordered films could be drawn.27 The distinction between the material properties of the samples of poly(di-n-butylstannane) synthesized by differing routes is probably the purity of the materials. The latter polymer, obtained from the use of Wilkinson’s catalyst, was free from cyclic oligomers. Obviously, the presence of a small amount of impurities can influence the material properties, particularly the glass transition temperatures, and this presents a significant difficulty in the potential use of these polystannane materials. 119

Sn NMR

Polystannanes are routinely characterized by 1 H and 13 C spectroscopy, and (GPC) against polystyrene standards of different molecular weights in THF. However, it is often 119 Sn NMR in particular that is a valuable tool to determine the structure of the linear oligomeric and polymeric chains and provides useful information regarding purity of polymer chains and ratios and types of cyclic oligomers. The 119 Sn NMR data and 1 J(119 Sn–119 Sn) coupling constants for selected oligomeric stannanes are provided in Table 3.8.3. The alkyl substituents (R = Me, n-Bu, t-Bu) in distannanes have a pronounced effect on the chemical shift values ranging from –109 ppm for permethylated disilane, –83 ppm (hexan-butyldistannane) to –6 ppm for hexa-t-butyldistannane. The internal tin atoms of tri-, tetra- and pentastannanes exhibit significant high-field shifts, as shown in Table 3.8.3, and, in general, cyclic oligomers exhibit resonances at a higher field compared to linear polymers. For example, the linear poly(di-nbutyl)stannane13 exhibits a resonance at –189.6 and (n-Bu2 Sn)5 and (n-Bu2 Sn)6 cycles show resonances

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at –200.9 and –202.1 ppm respectively. The 1 J(119 Sn–119 Sn) coupling constants continuously decrease from di- to pentastannanes. The 119 Sn NMR spectra of the diaryltin polymers (Table 3.8.4) exhibit resonances in the range of –183 to –197 ppm, except for the [(o-Et- p-BuO-C6 H3 )2 Sn]n polymer, which shows a signal at lower field –125 ppm.25 1 H and 13 C NMR are particularly useful to observe the end groups in low molecular weight polymers and give information about the fine structure, stereochemistry, and the ratios of various groups present in the polymer chain, as well as the percentages of cycles vs. polymers. 3.8.5

Polymers with Tin in the Backbone

Polyferrocenylenestannanes

High molecular weight polymetallocenes with group 14 atoms in the backbone have attracted considerable attention, with interesting thermal, optical, electrochemical, and magnetic properties with potential applications as electrochemical sensors, self-assembled materials, and precursors for magnetic ceramics and/or doped conducting materials.51 Thermal, anionic, cationic, and transition metal-catalyzed ring-opening polymerization of strained, ring-tilted (∼20◦ ) of sila- or -germa-[1]-ferrocenophanes have produced high molecular weight (Mn 104−6 ) polymers.52 Tin-bridged ferrocenophanes are expected to be less strained than the corresponding sila(germa)-ferrocenophanes due to the larger covalent radius of the tin, but they are also expected to be less stable due to the weaker cyclopentadienyl Cipso –Sn bond. Therefore, earlier attempts to synthesize tin-bridged [1]-ferrocenophanes by the reaction of 1,1-dilithioferrocene with dichloro-diorganotins, R2 SnCl2 (R =Me, Et, n-Bu, Ph) resulted in the isolation of low molecular weight oligomers and cyclic dimers.53,54 The introduction of sterically demanding substituents (t-Bu, Mes, 2,4,6-triisopropylphenyl) on tin resulted in the stabilization of stanna-[1]-ferrocenophanes and their isolation as orange crystalline solids.55,56 As expected, the ring tilt angle in stanna-[1]-ferrocenophanes was found to be ∼15◦ , significantly lower than sila- and germa-[1]-ferrocenophanes. The strain energy estimated for (t-Bu)2 Sn-[1]-ferrocenophane55 of ca. 151 = ± 38 kJ mol−1 was significantly lower than the silicon-bridged ferrocenophanes. Despite the lower strain energy, the tin-bridged ferrocenophanes underwent thermal ring-opening polymerization in the melt at 150–180 ◦ C due to the cleavage of the weaker cyclopentadienyl Cipso –Sn bond, Equation (3.8.7).

(3.8.7)

The molecular weight of the poly(ferrocenylene-di-t-butylstannane) obtained from thermal ringopening polymerization (ROP) was found to be very high; Mw /Mn = 133 000/83 000. The (di-tbutyl)stanna-[1]-ferrocenophane also underwent slow (four weeks) ROP in the solid state at room temperature and a bimodal molecular weight distribution was obtained (high molecular weight fraction Mw /Mn = 231 000/130 000). In toluene solution a quantitative ROP occurred at room temperature to form high molecular weight polymer, Mw /Mn = 900 000/560 000.55 The (diaryl)stanna-[1]-ferrocenophanes are relatively more stable than the (di-t-butyl)tin analog.55,56 The bis(triisopropylphenyl)tin-ferrocenophane is stable in the presence of both air and moisture, and does not undergo anionic or transition-metal catalyzed ROP. However, at elevated temperature it undergoes

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ROP at 180 ◦ C to an insoluble polymer56 . The (dimesityl)tin-[1]-ferrocenophane, on the other hand, produces a soluble polymer with Mw /Mn = 155 000/82 000. It also undergoes slow ROP in benzene, (50% conversion) and a high molecular weight fraction, Mw /Mn = 1 350 000/1 050 000 is formed along with cyclic dimers. Poly(ferrocenylalkylstannane) is thermally less stable than poly(ferrocenylalkylsilanes). TGA measurements of poly(ferrocenyldi-t-butylstannane) exhibit thermal decomposition at 210 ◦ C, whereas the poly(ferrocenylalkylsilanes) are stable even at 350 ◦ C under N2 atmosphere. Cyclic voltammetric studies of poly(ferrocenylene-di-t-butylstannane) exhibit two reversible oxidation waves, [E 1/2 = 0.00 and 0.14 V], the second oxidation occurring at a slightly higher potential consistent with the poly(ferrocenyl)silicon and -germanium polymers, indicating significant Fe · · · Fe interactions through the tin spacers.55 Two new mechanisms in ROP of poly(ferrocenylstannanes)57 have been proposed in which both basic and acidic impurities seemed to play a role in ROP. A mechanism involving nucleophile-assisted ringopening in which traces of amines or other nucleophile impurities are involved, while another mechanism proceeds via traces of cationic species, such as H+ or Bu3 Sn+ . Polycarbostannanes

Only two reports have appeared on the formation and properties of linear unsaturated polycarbostannanes and this is a fruitful area for continued study. Such materials have been synthesized by acylic diene metathesis (ACDMET) of bis(4-pentenyl)dibutyltin either with Schrock’s Moalkylidene, Mo(=CHMe2 Ph)(N -2,6-C6 H3 − i-Pr2 )(OCMe(CF3 )2 )2 , or with the aryloxo-tungsten catalyst W(O)Cl2 (O-2,6-C6 H3 -Br2 )2 , Equation 3.8.8).58,59

(3.8.8)

End-group analysis via quantitative 13 C NMR spectroscopy gave a number-average molecular weight of 17 000 and 9 300 g mol−1 using the Mo and W catalysts, respectively. However, as is often the case in MW determinations, GPC analysis of the polymers showed higher molecular weights than those obtained from the end group analysis. 119 Sn NMR showed three resonances at –12.8, –13.0 and –13.2 in a 65:35:5 ratio, respectively, indicating trans–trans, trans–cis and cis–cis stereochemistry, relative to the double bond in the polymer chain. Polystannasiloxanes

Given the wide commercial applications associated with polysiloxanes, it is surprising that related polystannasiloxanes, i.e. systematic substitution of R2 Si by R2 Sn groups, have not been extensively studied. Such polystannasiloxane chains have been obtained from the reaction of di-t-butyltin dichloride with [Ph2 (OH)Si]2 O, as shown in Equation (3.8.9).60

(3.8.9)

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The X-ray analysis indicated a zig-zag chain in the solid state with an Sn–O–Si bond angle of 156 ◦ and perfectly linear Si–O–Si linkage. Interestingly, the polymeric solid-state structure transforms to the six-membered cyclic structure in solution, Equation (3.8.10).

[t-Bu2SnO(Ph2SiO)2]n

O

t-Bu2 Sn O

1/n

(3.8.10)

Ph2Si

SiPh2 O

The different structures in the solid and solution states were also confirmed by 119 Sn NMR; the solid state exhibiting a resonance at –167.1 ppm, while in solution the resonance was at –119.5 ppm. An alternative approach to such materials involves acid-catalyzed ring-opening polymerization of macrocyclic stannasiloxane rings to produce waxy solids.61 The molecular weight of these materials could not be determined. 3.8.6

Polymers with Tin as a Pendant Group

A large range of organic polymers with dangling organotin functionalities have been studied. In general, these materials are outside the range of this review; however, a few pertinent points can be addressed. Poly(trimethyltin) alkylmethylacrylates and poly(trimethyltin)styrenes represent a class of organotin polymers that are useful imaging materials for microlithiography.62 The trimethylstannyl groups are sensitive to electron-beam radiation, but conveniently resistant to plasma degradation.63 Polymethacrylates with pendant trimethyltin chains, where the tin is substituted at C1 of the ester side chain, were successfully synthesized by AIBN-initiated radical polymerization of organotin methacrylates, Equation (3.8.11).64

(3.8.11)

Copolymerization with methyl methacrylate was also performed and produced random copolymers. The introduction of the Me3 Sn group into the poly(methylmethacrylate) depressed the Tg by 75 ◦ C cf. poly(methylmethacrylate) (PMMA). In contrast to these organotin methacrylate polymers, a number of polyorganotin methacrylates and copolymers with styrene65−68 were synthesized in which the tin is bonded to the oxygen of the ester chain, as shown in Figure 3.8.5. These polymers were used as anti-fouling coatings in paints and slowly leached R3 Sn+ fragments (R = n-Bu or Ph) by the hydrolysis of the Sn–O bonds in the polymer.

Figure 3.8.5

Copolymer of styrene (R) and organotin methacrylate

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A number of organotin polyesters, polyamides, polythiols, polyethers, and polyaminoesters have been synthesized from dialkyltin halides with dicarboxylic acids, diols, diamines, dithiols, urea, thiourea, amino acids, and hydroxyl acids involving polycondensation reactions.69 These polymers have potential applications as thermostabilizers for PVC. Organotin polyesters derived from 1,1’-ferrocenedicarboxylic acid were tested for their biological activities against normal Balb/3T3 cells and they inhibited cell growth at concentrations below that of cisplatin.70 In other studies it has been demonstrated that, whereas the monomers ampicillin and norfloxacin have little ability to inhibit selected viruses, their diorganotin polymeric/oligomeric derivatives exhibit a good ability to do so.71 Furthermore these organotin polymers also show good inhibition of a series of cancer cell lines. However, it is not clear that the polymeric materials are sufficiently superior to the simple diorganotin dichlorides to warrant their usage. Polymers containing the potentially bioactive Bu2 SnX (X = OAc or Cl) groups have been synthesized by n-Bu2 SnHCl hydrostannylation of the vinyl group in such polymers as polybutadiene, poly(allylmethacrylate) or poly(tetraethyleneglycol-mono-allyl ether methacrylate). The resulting materials, linked to the polymer via Sn–C bonds were found to be inactive as anti-fouling materials in marine environments. This inactivity illustrates the need for mobility of organotin biocides and the Sn–C anchor linkage clearly stops such mobility until this bond can be cleaved.72 Acknowledgements We thank NIH (MARC program) and Welch Foundation (Grant # AH-546) for support of this work. References 1. (a) T. A. Skotheim, Handbook of Conductive Polymers, Marcel Dekker, New York, Vols. 1 & 2, 1986; (b) J. L. Bredas and R. Silbey, Conjugated Polymers, Kluwer Academic Publishers, Dordrecht, Netherlands, 1991; (c) C. U. Pittman, C. E. Carraher, M. Zeldin, J. E. Sheats, and B. M. Culbertson, in Metal Containing Polymeric Materials, Plenum, New York, 1996. 2. For reviews on tin polymers and oligomers, see (a) S. Adams and M. Dr¨ager, Main Group Met.Chem., 11, 151 (1988); (b) K. Jurkschat and M. Mehring, in The Chemistry of Organic Germanium, Tin and Lead Compounds, Vol 2, Z. Rappoport (Ed.), John Wiley & Sons, Ltd, Chichester, 1543–1651 2002; (c) C. E. Carraher, in Macromolecules containing Metal and Metal-Like Elements, Vol 4, A. S. Abd-El-Aziz, C. E. Carraher Jr, C. U. Pittman, and M. Zeldin, (Eds), John Wiley & Sons, Ltd, Chichester, 263, 2005; (d) I. Manners, Angew. Chem. Int. Ed. Engl., 35, 1602 (1996). 3. (a) L. R. Sita, Adv. Organomet. Chem., 38, 189 (1995); (b) L.R. Sita, Acc. Chem. Res., 27, 191 (1994). 4. S. Adams and M. Dr¨ager, Angew. Chem. Int. Ed. Engl., 26, 1255 (1987). 5. R. Sommers, B. Schneider, and W. P. Neumann, Justus Liebigs Ann. Chem., 692, 12 (1966). 6. L. R. Sita, Organometallics, 11, 1442 (1992). 7. L. R. Sita, K. W. Terry, and K. Shibata, J. Am. Chem. Soc., 117, 8049 (1995). 8. (a) R. West, J. Organomet. Chem., 300, 327 (1986); (b) R. D. Miller and J. Michl, Chem. Rev., 89, 1359 (1989); (c) R. West, in Comprehensive Organometallic Chemistry, Vol. 2, G. Wilkinson, F. G. A. Stone, and E. W. Abel, (Eds), Pergamon Press, Oxford, 365, 1982. 9. (a) P. Trefonas and R. West, J. Polym. Sci., Polym. Chem. Ed., 23, 2099 (1985); (b) R. D. Miller and R. Sooriyakumaran, J. Polym. Sci. Part A: Polymer Chem., 25, 111 (1987). 10. (a) W. P. Neumann, The Organic Chemistry of Tin, Wiley & Sons, Inc., New York, 1970; (b) C. Grugel, W. P. Neumann, and P. Seifert, Tetrahedron Lett., 2205 (1977); (c) R. C. Poller, The Chemistry of Organotin Compounds, Academic Press, New York, Ch. 9, 145, 1970; (d) A. K. Sawyer, Organotin Compounds, Vols. 1–3, Marcel Dekker, New York, 1972. 11. B. C. Gilbert and A.F. Parsons, J. Chem. Soc. Perkin Trans. II, 367 (2002).

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3.9

Intermolecular Tin . . . π-Aryl Interactions: fact or artifact?∗ A New Bonding Motif For Supramolecular Self-Assembly in Organotin Compounds

Ionel Haiduc,a Edward R. T. Tiekink,b and Julio Zukerman-Schpector c a

Department of Chemistry, Babes-Bolyai University, Romania Department of Chemistry, The University of Texas at San Antonio, Texas, USA c Department of Chemistry, Universidade Federal de S˜ao Carlos, Brazil b

3.9.1

Introduction

Supramolecular chemistry, one of the most flourishing branches of contemporary science, is the discipline dealing with ‘the chemistry of molecular assemblies and of the intermolecular bond’ and deals with ‘organized entities that result from the association of two or more chemical species held together by intermolecular forces.’ It deals with two types of ‘objects:’ supermolecules, i.e. ‘well-defined oligomolecular species that result from the intermolecular association of a few components,’ and molecular assemblies or supramolecular arrays, which are ‘polymolecular systems that result from the spontaneous association of a non-defined number of components.’1,2 There is a broad variety of non-covalent intermolecular forces3 able to hold molecular building units (also called ‘tectons’) together.4,5 Examples of non-covalent interactions include hydrogen bonds,6,7 dative coordinate bonds,8 π-π stacking,9 secondary bonds (or ‘soft–soft’ interactions),10 or cation–π interactions,11 and other types of interactions (electrostatic forces, etc.). An interesting bonding motif includes metal–π interactions.12 Cyclopentadienyl–metal π -complexes (metallocenes and related compounds) are typical for transition metals, but some main group derivatives are also known.13 As a bonding motif for supramolecular self-assembly the metal . . . π -cyclopentadienyl bonds can be illustrated in the structures of bis(cyclopentadienyl)lead(II) or ‘plumbocene,’ [Pb(η5 C5 H5 )2 ]n (n = 6 or ∞), which forms both cyclic hexameric supermolecules and helical supramolecular chains14 or with the supramolecular structures of [(η5 -C5 Me5 )SbCl2 ]n .15 In both types of compound, the metal atoms alternate with five-membered cyclopentadienyl rings . . . M . . . (π -Cp) . . . M . . . (π -Cp) . . . . The metal–π bonds are weaker than those in transition metal cyclopentadienyls, as illustrated by the ˚ and inter-atomic distances from the metal to the C5 ring centroid: Pb . . . centroid(Cp) (2.59–3.54 A) 16 ˚ ˚ Sb . . . centroid(Cp) (3.41–3.58 A), compared to 1.90–2.70 A, being metal–centroid distances in metallocenes. Benzene rings form numerous η6 -aryl–metal complexes with transition metals (e.g. dibenzenechromium). Metal . . . π-arene complexes of main group metals are less common and are known as ‘Menschutkin complexes,’17 e.g. C6 H6 SbCl3 ,18 C6 Me6 .2SbX3 (X = Cl, Br),19 C6 H3 Me3 .SbBr3 ,20 ˚ 22 C6 H6 .BiCl3 ,21 and [(1,2-Me2 C6 H4 )2 Pb(AlCl4 )2 ] (Pb . . . centroid(C6 ) distance 2.88 A). ˚ The metal–benzene ring centroid distances of approximately 3.5 A indicate weak bonding. In organotin chemistry both metal–cyclopentadienyl complexes, e.g. [η5 -C5 H5 SnCl]23 n and metal–π -arene complexes, e.g. [η6 -C6 H6 SnCl(AlCl4 )]n ,24,25 [η6 -C6 H6 Sn(AlCl4 )2 ]2 ,26 or [η6 -C6 Me6 SnCl(AlCl4 )]4 ,27 are known. All are supramolecular aggregates, in which the tectons are associated through secondary Sn . . . Cl interactions.12, p.232−235 The Sn . . . centroid(C6 H6 ) distances in these compounds are in the ˚ In these benzene complexes the aromatic molecules are attached to the metal as side range 2.88–3.26 A. groups.

∗

Artifact = a structure or feature, visible only as a result of external action or experimental error. See http://en.wikipedia.org/wiki/Artifact

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We are interested in the use of metal . . . π -arene interactions as bonding motifs for supramolecular self-assembly, as we have illustrated previously for a significant number of Te . . . π -aryl complexes, most of them identified by data mining of the Cambridge Crystallographic Database.28 In this work the crystal structures of organotin compounds containing aromatic groups were reexamined, to check for short contacts between the metal atoms and the aromatic rings and to assess whether such contacts reflect real π-aryl interactions or are simply artifacts. The Cambridge Crystallographic Database (version 5.28),29a with the aid of ConQuest (version 1.9),29b was searched to identify the compounds in which the distance between the tin atom and the centroid of a facing phenyl ring was shorter than the van der Waals distances. The van der Waals radius of tin was taken ˚ 30 and the half-thickness of a phenyl ring as 1.80 A. ˚ 28 The distance and angle constraints used for as 2.17 A the search are shown in Scheme 3.9.1. The search revealed 22 crystal structures in which Sn . . . π -arene intermolecular interactions are suggested by Sn . . . ring centroid distances that are shorter than the sum of van der Waals radius and ring half-thickness. Of the 22 structures only a small fraction, i.e. just over 25%, have been described in the published papers as containing Sn . . . π-arene interactions. Diagrams were drawn with the DIAMOND program.31

˚ 0◦ < α < 20 ◦ . V is the normal to the Scheme 3.9.1 Parameters used for the CSD search: 3.0 < |v1 | < 4.1 A; 2 plane of the aromatic ring

3.9.2

Discussion

Chemical diagrams for the molecular structures included in this review are shown in Schemes 3.9.2– 3.9.4. Initially, structures in which Sn . . . π contacts are found in solvates are described. Following these is a discussion of Sn . . . π interactions occurring in two ionic complexes. The remaining structures are discussed in terms of increasing complexity guided by tin atom nuclearity and the number of interactions found in their supramolecular structures. It is stressed here that this review is not intended to be an exhaustive evaluation of crystal packing for the individual structures, but is designed to ascertain the presence of Sn . . . π interactions and determine, when present, their influence on the global crystal packing. Accordingly, emphasis is given to describing Sn . . . π interactions when there are no other obvious complementary/dominating supramolecular associations in force. Solvates

Chemical diagrams for the molecules described in this and the next section are shown in Scheme 3.9.2. The first crystal structure to be described is that of 2,3-bis(trimethylsilyl)-1-stanna-2,3-dicarba-closoheptaborane(4)32 (1), isolated as a benzene hemisolvate. The immediate coordination geometry about the tin atom is defined by three boron and two carbon atoms, all of which lie on one side of the tin atom, leaving a large gap in the environment of the tin atom. In the solid-state supramolecular structure, two tin atoms associate with a solvent benzene molecule that is disposed about a crystallographic center of

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Scheme 3.9.2

Chemical structures for 1–7, 19

inversion, and in the crystal structure the solvent benzene molecules occupy channels surrounded by four molecules of 1. Details of the Sn . . . π interactions are collected in Table 3.9.1 for this and all other structures described herein. In 1.0.5(C6 H6 ), the Sn . . . C distances span a relatively wide range, i.e. 3.69– ˚ and this disparity is reflected in the relatively wide α angle of 13.6◦ . The Sn . . . C interactions 4.22 A, play a pivotal role in the stabilization of the structure, and when combined with intermolecular Sn . . . B ˚ interactions (the closest of these are a pair of contacts, shown in Figure 3.9.1 as dashed lines, of 3.35 A ˚ that occur between centrosymmetrically related pairs of 1, lead to the formation of chains and 3.57 A) with a step-ladder topology, Figure 3.9.1. In the solvated structure of (μ2 -1,8-bis(trimethylsilylamido)naphthalene-N,N,N’N’)-bis(chlorotin)33 (2), the ratio of the dinuclear molecule to toluene solvent is 2:1. While the toluene molecule is disordered about a center of inversion, the dinuclear species does not have crystallographically imposed symmetry so that there are two independent tin atoms, Sn1 and Sn2, which differ in terms of the Sn . . . π interactions

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˚ ◦) Table 3.9.1 Selected geometric parameters (A, describing Sn . . . π interactions Compound

Sn . . . C(g)

Sn . . . C(g) . . . plane, α a

Ref.

1 2 3 4 5 6

3.73 3.53 3.32 3.72 3.92 3.42 3.74 3.78 3.97 3.82 3.56 3.46 3.25 3.76 3.76 3.66 3.58 3.66 3.53 3.83 3.51 3.96 3.74 3.91

13.6 16.5 6.1 10.5 17.9 5.5 7.3 11.8 10.6 12.0 5.7 4.9 5.8 11.2 1.2 5.1 4.4 5.9 5.6 14.9 9.4 6.9 12.8 15.6

32 33 34 35 36 37

7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

38 39 40 41 42 43 44 45 46 47 48 49 35 50 51 52

a

The angle between the Sn . . . C(g) vector and the least-squares plane of the aromatic ring, α, see Scheme 3.9.1

Figure 3.9.1

Chain-like supramolecular arrays in 1.0.5(C6 H6 ) mediated by tin . . . π and Sn . . . B interactions

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Figure 3.9.2 Supramolecular chain-like arrays with a flattened step-ladder topology in 2. C7 H7 mediated by Sn . . . π interactions. Note that the toluene molecule is disordered about a center of inversion so that the methyl groups have 50% site occupancy factors only

they form. Each tin atom is three-coordinate, existing in pyramidal geometries defined by ClN2 donor sets. As can be seen from Figure 3.9.2, the toluene molecule is closely associated with two tin, Sn1, atoms, in much the same way as the benzene molecule in 1. However, the Sn . . . π interactions involving the Sn1 atom and toluene molecule span a large range of distances with one Sn . . . C separation being ˚ and the remaining being greater than 4.00 A. ˚ Reflecting the weak nature of these contacts is the 3.85 A, Sn . . . C(g) . . . plane(C7 ), α, angle of 26.4◦ . More significant are Sn . . . π interactions occurring between the Sn2 atom and one of the aromatic rings of the naphthalene residue, Figure 3.9.2 and Table 3.9.1, which occur cooperatively between centrosymmetrically related molecules. Further, this arrangement places the second aromatic ring of the naphthalene residue in relatively close proximity to the Sn2 atom, ˚ and the α angle to the C6 plane is 27.6◦ , not shown in so that the Sn2 . . . Cg(C6 ) distance is 3.86 A Figure 3.9.2. The Sn2 . . . π interactions coupled with the aforementioned Sn1 . . . π interactions lead to a chain-like supramolecular array, with a flattened step-ladder topology, mediated exclusively by Sn . . . π interactions. These chains are connected to orthogonal chains, primarily via C–H . . . Cl interactions, to consolidate the crystal packing. The tetranuclear species bis(μ3 -oxo)-octakis(μ-trifluoroacetato)-di-tin(II)-di-tin(IV)34 (3), was crystallized as a mono-benzene solvate. The structure of 3 is constructed about a centrosymmetric, central Sn2 O2 square core; the tin atoms forming this core are endocyclic and exist in the +IV oxidation state. Each tin atom of the core is linked to two exocyclic tin atoms via two bridging carboxylate ligands in each case. The endo- and exo-cyclic tin atoms are six- and four-coordinate, respectively. In terms of supramolecular organization, chains mediated by Sn . . . π interactions are again found, Figure 3.9.3. The Sn . . . π interactions involve the exocyclic tin(II) atoms, rather than the endocyclic tin(IV) atoms, and the topology of the chain is a somewhat flattened step-ladder. Interactions in alternative directions in the crystal are of the type C–H . . . F and are fluorophilic interactions. The molecules of the tetranuclear tin/cubane compound, tetrakis[(μ3 -4-methoxy-2methylphenylimido)tin(II)]35 (4), crystallizes as a 1:1.5 toluene solvate; one of the toluene molecules is disordered about a center of inversion. In 4, an Sn4 N4 cube with alternating tin and nitrogen atoms comprises the core. Each tin atom is therefore tri-coordinated by three nitrogen atoms in an approximately pyramidal fashion. In this example, there is no Sn . . . π interaction of note involving the solvent

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Figure 3.9.3 interactions

397

Supramolecular chains with flattened step-ladder topology in 3. C6 H6 mediated by tin . . . π

molecules. One of the tin atoms of the Sn4 N4 cluster forms an Sn . . . π interaction and these combine cooperatively with C–H . . . O interactions to form a supramolecular chain, as shown in Figure 3.9.4. In the structure of a closely related compound to that just described, i.e. tetrakis[(μ3 -2,5dimethoxyphenylimido)tin(II)]36 (5), characterized as a hemi-toluene solvate, dimeric pairs of Sn4 N4 clusters associate via Sn7 . . . π interactions as well as a series of C–H . . . O contacts. By contrast to that observed for 4.1.5(C7 H7 ), no further aggregation occurs between molecules via Sn . . . π interactions. Ionic Complexes

There are two examples, Scheme 3.9.2, in the literature where intermolecular Sn . . . π interactions are implied, in accord with the criteria established above. It is debatable whether the associations, via Sn . . . π

Figure 3.9.4

Supramolecular chains in 4.1.5(C7 H7 ) mediated by Sn . . . π and C–H . . . O interactions

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Figure 3.9.5

Sn . . . π interactions between ionic components in the structures of: (a) 6 and (b) 7

interactions, between the components of the ionic complexes are truly representative of inter- or intramolecular Sn . . . π interactions, or are better described as simply charge-assisted contacts.11 Nevertheless, the description of the structures of 6 and 7 are included for the sake of completeness. In the first example, i.e. benzophenone-N -methyliminium trichlorostannate37 (6), there are three crystallographically independent ionic pairs, and a representative pair is shown in Figure 3.9.5(a). Evident from this figure is the presence of an Sn . . . π interaction and a key difference between the three independent ionic pairs is the magnitude of the Sn . . . Cg distance, see Table 3.9.1. The coordination geometry for each tin atom is conveniently described as 3+1 tetra-coordinated with the immediate geometry about the tin atom being pyramidal N3 , but with a capping aromatic ring. Indeed, there is evidence for a trend between the Sn . . . C(g) distance and the magnitude of α, with longer distances being associated with wider angles. While the crystal packing in 6 appears to be dominated by Sn . . . Cl interactions, there is ˚ and with α angles > 24◦ ; these are some evidence of weaker Sn . . . π interactions with Sn . . . C(g) > 4 A not discussed here. Two independent sets of bis(tetraphenylphosphonium) bis(trithiocarbonato-S, S’)tin(II) comprise the crystallographic asymmetric unit of 7.38 The immediate coordination geometry for each tin atom is defined by an S4 donor set, derived from two asymmetrically chelating trithiocarbonato ligands. In one of the ionic pairs, Sn . . . π interactions are found that fall within the established criteria, see Figure 3.9.5(b) and Table 3.9.1, but not for the other. For the second ionic pair, close Sn . . . π interactions are observed, but these occur between the tin atom and two pairs of carbon atoms belonging to two different aromatic rings that approach the tin atom more side-on.

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Mononuclear Tin Species

There were 11 mononuclear structures extracted from the CSD with geometric parameters matching the established criteria for significant Sn . . . π interactions; their chemical structures are shown in Schemes 3.9.3 and 3.9.4. In the structure of (S,S)-(2,6-bis(4-phenyloxazolin-2-yl)pyridyl-N ,N ,N )-bis(trifluoromethanesulfonate)tin(II)39 (8), the tin atom exists within a five-coordinate N3 O2 coordination geometry, which

Scheme 3.9.3

Chemical structures for 8–14

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Figure 3.9.6 and (d) 11

Dimeric aggregates mediated by Sn . . . π interactions in the structures of: (a) 8, (b) 9, (c) 10,

is best described as being based on a square pyramidal geometry leaving an open coordination site. In 8, molecules associate around a crystallographic two-fold axis via Sn . . . π interactions to form dimeric aggregates, as shown in Figure 3.9.6(a). These associate in turn via C–H . . . O interactions to form a chain motif. Isolated dimeric aggregates mediated by cooperative Sn . . . π interactions between centrosymmetric pairs are also found in the crystal structure of N ,N ’-bis(neopentyl)-1,2-phenylene-bis(amido)tin(II)40 (9). In this case, the coordination geometry is formally two-coordinate, defined by two nitrogen atoms of the chelating ligand. With this in mind, it is perhaps not surprising that the Sn . . . π interactions in 9, shown in Figure 3.9.6(b), represent the shortest, and presumably strongest, amongst the structures surveyed herein. A dimeric aggregate facilitated by Sn . . . π interactions is also found in the crystal structure

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of chloro(N ,N ’-diphenyl-2,4-pentanediiminato)tin(II)41 (10); see Figure 3.9.6(c). In this case, the tin atom is in a pyramidal geometry defined by a ClN2 donor set. In the crystal structure, molecules are consolidated into a three-dimensional array via a relatively large number of weak C–H . . . Cl interactions. A four-coordinate tin atom, defined by an N4 donor set, derived from two symmetrically chelating ligands, is found in the structure of 11, i.e. (dibenzotetramethyltetra-aza(14)annulene-N ,N ’,N ”,N ”’)tin(II).42 Here, centrosymmetrically related molecules aggregate into dimers via Sn . . . π interactions, Figure 3.9.6(d). There is evidence for additional C–H . . . π interactions in the structure that occur between the methyl H atoms and the ring centroids of an SnC3 N2 six-membered chelate ring. Each molecule of 11 participates in four such interactions, i.e. two donor and two acceptor, and these appear to be the primary interactions that serve to consolidate the crystal packing. A similar tetra-coordination geometry to that found in 11 is found in the molecular structure of bis(1,3diphenylpropane-1,3-dionato)tin(II)43 (12), but defined by an O4 donor set. Centrosymmetric dimers mediated by Sn . . . π interactions are found in the crystal structure of 12, Figure 3.9.7. In contrast to the earlier structures described above, the dimers are linked into a polymeric chain via well-defined ˚ inter-molecular Sn . . . O interactions of 2.983(4) A. In the crystal structure of (2,6-di-t-butyl-4-methylphenoxy) bis(trimethylsilyl)amine-tin(II)44 (13), a two-coordinate tin atom is found, defined by an NO donor set. In the crystal structure, Sn . . . π interactions are formed on either side of the molecule leading to the formation of a zig-zag supramolecular chain that is propagated by crystallographic glide symmetry, as shown in Figure 3.9.8. In the structure of (N ,N ’-bis(salicylidene)ethane-1,2-diamine-N ,N ’,O,O’)tin(II) (14), a fourcoordinate donor set, akin to those found in the structures of 11 and 12, is found, but in this case the donor set is N2 O2 , derived from two symmetrically chelating N,O-bidentate ligands.45 As with previous examples, Sn . . . π interactions connect molecules into pairs around a crystallographic two-fold axis of symmetry. The dimeric aggregates are linked into a linear chain, with the topology of a supramolecular ˚ and 2.66 A), ˚ as shown in ladder, via well-defined pairs of bifurcated C–H . . . O interactions (2.62 A Figure 3.9.9.

Figure 3.9.7 The supramolecular double-chain array in 12 mediated between intermolecular Sn . . . π and Sn . . . O interactions

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Figure 3.9.8

A zig-zag supramolecular chain mediated by Sn . . . π interactions in the structure of 13

Centrosymmetric dimers in the structures of bis(2,6-di-t-butylphenolato-O)tin(II) (15), with twocoordinated tin atoms within O2 donor sets, are linked into loosely associated pairs via comparatively strong Sn . . . π interactions, Table 3.9.1.46 These are connected with four other dimeric pairs via C–H . . . π interactions so as to generate a two-dimensional array, as shown in Figure 3.9.10. Two formula units associate via an inversion center in the structure of bis(μ2 triphenylphosphoraneiminato)-di-iodo-ditin(II) (16).47 Here, the pyramidal coordination geometry

Figure 3.9.9

Supramolecular chains mediated by bifurcated C-H . . . O interactions in the structure of 14

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Scheme 3.9.4

Chemical diagrams for 15–18, 20–22

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Figure 3.9.10 Layered supramolecular structure mediated by Sn . . . π and C H . . . π interactions in the structure of 15

for tin is defined by an IN2 donor set. Complimentary Sn . . . π and C–H . . . π interactions occur between molecules in the structure of 16, as can be seen from Figure 3.9.11, each dimer forming four Sn . . . π contacts as well as four C–H . . . π contacts so as to lead to chain-like arrays. The molecular structure of bis(μ2 − N -(2-oxidophenyl)salicylideneiminato-N ,O,O,O’)-ditin(II) (17), comprises two tin atoms mutually bridged by a pair of phenoxide ligands, with the resulting dimer having crystallographically imposed two-fold symmetry.48 The remaining positions in the tin atom geometry are occupied by nitrogen and oxygen atoms derived from a chelating salicylideneiminate ligand,

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Figure 3.9.11 Supramolecular chain-like arrays mediated by complimentary Sn . . . π and C-H . . . π interactions in the structure of 16

so that the coordination geometry is based on an NO3 donor set. As seen from Figure 3.9.12, centrosymmetrically related pairs associate via Sn . . . π interactions that lead to a supramolecular chain. Within these chains, the six-membered chelate rings of adjacent dimers also face each other, with the separation ˚ between them indicative of weak π . . . π interactions; these are not shown in Figure 3.9.12. of 3.68 A The only other obvious inter-molecular interactions operating in the crystal structure are of the type C–H . . . O. These lead to connections that radiate in four directions normal to the axis of the chain, and in this way create a stable crystal structure. Arguably one of the more complicated mononuclear tin structures, at least in terms of supramolecular aggregation, is found in the heterometallic structure of [(trichlorotin(II)]-Cl)(2,11bis[(diphenylphosphino)methyl)-benzo(c)phenanthrene-P,P’]silver(I) (18).49 The tin atom in 18 is tricoordinated in a pyramidal geometry by three chloride ions, one of which bridges to the silver atom, which has its three-coordinate, approximately trigonal planar, coordination geometry completed by two phosphorus atoms derived from a bidentate bi-phosphine ligand. Inter-molecular Sn . . . π interactions in 18 lead to a zig-zag chain, as represented in Figure 3.9.13. There are a considerable number of C–H . . . Cl interactions operating in the structure of 18 and the strongest of these contributes to the stabilization of the zig-zag chain (not shown in Figure 3.9.13); other C–H . . . Cl interactions link chains into a consolidated crystal structure. In addition to the aforementioned Sn . . . π interactions, there are other clearly identifiable Sn . . . π interactions in the structure of 18. A weaker intermolecular Sn . . . π interaction, beyond the

Figure 3.9.12

Supramolecular chain-like arrays mediated by Sn . . . π interactions in the structure of 17

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Figure 3.9.13

Supramolecular zig-zag chains mediated by Sn . . . π interactions in the structure of 18

˚ and the α angle is 37.5◦ , reinforces the zig-zag chain shown established criteria, i.e. Sn . . . Cg is 3.97 A in Figure 3.9.13, and, in addition, there is an intra-molecular Sn . . . π interaction with Sn . . . Cg of 4.04 ˚ and an α angle of 17.1◦ . A Polynuclear Tin Species

A total of four polynuclear tin species were found in the CSD29 to contain indications of significant intermolecular Sn . . . π interactions in their crystal structures; their chemical diagrams are shown in Schemes 3.9.2 and 3.9.4.

Figure 3.9.14

Dimeric supermolecules mediated by Sn . . . π interactions in the structure of 20

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The tetranuclear structure of tetrakis[(μ3 -2,4-dimethoxyphenylimido)tin(II)] (19), is related to the Sn4 N4 cubes described above as (4)35 and (5),36 each of which was characterized as a solvate. Although this structure came out as a ‘hit’ in the described search of the CSD, disorder in the aromatic rings preclude a detailed analysis of the crystal packing. However, it is apparent that a linear chain is found, mediated by the formation of a pair of Sn . . . π interactions on either side of the cluster, as described for (5). The trinuclear structure of tris(μ2 -chloro)-tris(N -mesityl-N -(trimethylsilyl)amido-N )-tri-tin(II) (20), features a highly puckered Sn3 Cl3 ring, with each tin atom further coordinated by an amido ligand so as to define a pyramidal geometry within a Cl2 N donor set.50 The trimers associate about a center of inversion via a pair of Sn . . . π interactions, Figure 3.9.14 and Table 3.9.1. There is a clear intramolecular ˚ and an α angle of 7.9◦ . The crystal structure is stabilized by Sn . . . π interaction with Sn . . . Cg of 3.20 A a network of C-H . . . π interactions; the remaining tin atom is embedded in the middle of the hexameric aggregate. A dimeric aggregate is also found in the trinuclear tin structure of bis(μ2 -dimethylamino)-bis(μ2 -2,6di-isopropylanilino)-tri-tin(II)51 (21). In this case, two pairs of tin atoms in the Sn3 N3 ring are linked by two 2,6-di-isopropylanilino molecules and the ring is closed by a pair of dimethylamino bridges. In this scenario, one might expect the di-coordinate tin (N2 donor set) atom, rather than one of the tri-coordinated tin atoms (pyramidal N3 donor sets) to form intermolecular Sn . . . π interactions, but it seems that the

Figure 3.9.15 Layered supramolecular structure mediated by Sn . . . π and C-H . . . π interactions in the structure of 21

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aromatic-bound alkyl substituents preclude close approach of neighboring molecules. Instead, one of the tri-coordinate tin atoms forms an Sn . . . π contact with a centrosymmetrically related molecule to form a dimeric, hexanuclear tin aggregate. Unprecedented amongst the structures described herein, is the observation that the aromatic ring involved in the formation of the Sn . . . π contact further participates in supramolecular association, this time via C–H . . . π interactions. These radiate out from the hexanuclear aggregate in four directions, approximately in the same plane, so that a two-dimensional array results, as shown in Figure 3.9.15. In the final structure to be described herein, namely that of catena-(μ2 -phenylphosphinato-O,O’)chloro-tin(II) (22), a polymeric array is found through the agency of Sn–O bonds.52 The immediate coordination geometry about the tin atom is defined by a ClO2 donor set, but, as described below, there are a number of additional intermolecular contacts to consider. Here, two polymeric strands of

Figure 3.9.16 of 22

Two views of the layer structure mediated by Sn . . . π and Sn . . . Cl interactions in the structure

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Figure 3.9.16

409

(Continued)

–O–Sn–O–Sn–O– links face each other so as to allow for the formation of a series of inter-chain Sn . . . Cl ˚ and 3.376(1) A. ˚ This interactions; the two independent inter-chain Sn . . . Cl separations are 3.316(1) A arrangement places the phosphorus-bound phenyl groups in close proximity to tin atoms belonging to neighboring double chains and hence, the formation of Sn . . . π interactions. Two views of the resultant two-dimensional array are shown in Figure 3.9.16. 3.9.3

Conclusions and Outlook

The foregoing discussion plainly indicates that Sn . . . π interactions can and do exist in the structures described herein. At a minimum, molecules aggregate to form a dimer and more often than not, these form the supramolecular tecton that associates via other intermolecular interactions, sometimes of the type Sn . . . π , into one-, two- and three-dimensional supramolecular architectures. Without exception, Sn . . . π interactions involve tin atoms present in the formal oxidation state +II. This leads to the conclusion that one possible explanation for Sn . . . π interactions lies in a charge transfer from the tin-bound lone pair of electrons into the LUMO of the aromatic system. Acknowledgments It is with pleasure that E.R.T.T. and J.Z.-S. thank Babes-Bolyai University for generous hospitality to allow the preparation of this review. References J.-M. Lehn, Angew. Chem., Int. Ed. Engl., 29, 1304 (1990). J.-M. Lehn, Supramolecular Chemistry: Concepts and Perspectives, VCH, Weinheim, 1995. K. M¨uller-Dethlefs and P. Hobza, Chem. Rev., 100, 143 (2000). S. Simard, D. Su and J.D. Wuest, J. Am. Chem. Soc., 113, 4696 (1991). M.C.T. Fyfe and J.F. Stoddart, Acc. Chem. Res., 30, 393 (1997). C.B. A¨akeroy, Acta Crystallogr., B53, 569 (1997); C.B. A¨akeroy, A.M. Beatty, and D.S. Leinen, Angew. Chem., Int. Ed., 1999, 38, 1815–1819; A.M. Beatty, CrystEngComm, 3, 1 (2001). 7. D. Braga, F. Grepioni, and G.R. Desiraju, Chem. Rev., 98, 1375 (1998); D. Braga and F. Grepioni, Coord. Chem. Rev., 183, 19 (1999). 8. S. Leininger, B. Olenyuk, and P.J. Stang, Chem. Rev., 100, 853 (2000); M. Fujita, Chem. Soc. Rev., 27, 417 (1998); M.J. Zaworotko, Chem. Commun., 1 (2001); A.J. Blake, N.R. Champness, P. Hubberstey, W.S. Li, A. Withersby, and M. Schr¨oder, Coord. Chem. Rev., 183, 117 (1999); J.A.R. Navarro and B. Lippert, Coord. Chem. Rev., 185/186, 653 (1999); R. Robson, J. Chem. Soc., Dalton Trans., 3735 (2000). 1. 2. 3. 4. 5. 6.

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9. C. Janiak, J. Chem. Soc., Dalton Trans., 3885 (2000); D.B. Amabilino and J.F. Stoddart, Chem. Rev. 95, 2725 (1995). 10. N.W. Alcock, Adv. Inorg. Chem. Radiochem., 15, 1 (1972); I. Haiduc I. in vol. Encyclopedia of Supramolecular Chemistry, J. Steed and J. Atwood (Eds), M. Dekker, Inc., New York, 1224 (2004). 11. J.C. Ma and D.A. Dougherty, Chem. Rev., 97, 1303 (1997). 12. I. Haiduc and F.T. Edelmann, Supramolecular Organometallic Chemistry, Wiley-VCH, Weinheim, New York, 1999. 13. I. Haiduc and J.J. Zuckerman, Basic Organometallic Chemistry, Walter de Gruyter & Co., Berlin, 1985. 14. M.A. Beswick, C. Lopez-Casideo, M.A. Parver, P.R. Raithby, C.A. Russell, A. Steiner, and D.S. Wright, Chem. Commun., 109 (1997); J.S. Overby, T.P. Hanusa, and V.G. Young, Inorg. Chem., 37, 163 (1998). 15. P. Jutzi, U. Meyer, S. Opiela, M.M. Olmstead, and H.G. Stammler, Organometallics, 9, 1459 (1990); R.A. Bartlett, A. Cowley, P. Jutzi, M.M. Olmstead, and H.G. Stammler, Organometallics, 11, 2837 (1992). 16. P. Jutzi and N. Burford, Chem. Rev., 99, 969 (1999). 17. A. Schier, J.M. Wallis, G. M¨uller, and H. Schmidbaur, Angew. Chem., Int. Ed. Engl., 25, 757 (1986). 18. D. Mootz and V. H¨andler, Z. Anorg. Allg. Chem. 533, 23 (1986). 19. H. Schmidbaur, R. Novak, A. Schier, J.M. Wallis, B. Huber, and G. M¨uller, Chem. Ber., 120, 1829 (1987); H. Schmidbaur, R. Novak, O. Steigelmann, and G. M¨uller, Chem. Ber., 123, 1221 (1990). 20. H. Schmidbaur, J.M. Wallis, R. Novak, B. Huber, and G. M¨uller, Chem. Ber., 120, 1837 (1987). 21. W. Frank, J. Schneider, and S. Muller-Becker, J. Chem. Soc., Chem. Commun., 799 (1993). 22. W. Frank and F.G. Wittmer, Chem. Ber. 130, 1731 (1997). 23. K.D. Boss, E.J. Bulten, J.G. Noltes, and A.L. Spek, J. Organomet. Chem., 99, 71 (1975). 24. M.S. Weininger, P.F. Rodesiler, A.G. Gash, and E.L. Amma, J. Am. Chem. Soc., 94, 2135 (1972). 25. M.S. Weininger, P.F. Rodesiler and E.L. Amma, Inorg. Chem., 18, 751 (1979). 26. H. Schmidbaur, T. Probst, O. Steigelmann, and G. M¨uller, Z. Naturforsch., 44b, 1175 (1989). 27. H. Schmidbaur, T. Probst, B. Huber, G. M¨uller, and C. Kr¨uger, J. Organomet. Chem., 365, 53 (1989). 28. J. Zukerman-Schpector and I. Haiduc, Cryst. Eng. Comm., 4, 178 (2002). 29. (a) F.H. Allen, Acta Cryst., B58, 380–388 (2002); (b) I.J. Bruno, J.C. Cole, P.R. Edington, M. Kessler, C.F. Macrae, P. McCabe, J. Pearson, and R. Taylor, Acta Cryst., B58, 389 (2002) 30. J.E. Huheey, E.A. Keiter, and R.L. Keiter, Inorganic Chemistry: Principles of Structure and Reactivity, 4th Edn, HarperCollins, New York, USA, 1993. 31. DIAMOND, Visual Crystal Structure Information System, Version 3.1, CRYSTAL IMPACT, Postfach 1251, D-53002 Bonn, Germany, 2006. 32. N.S. Hosmane, H. Zhang, J.A. Maguire, T. Demissie, A.R. Oki, A. Saxena, and W.N. Lipscomb, Main Group Met. Chem., 24, 589 (2001). 33. A.G. Avent, C. Drost, B. Gehrhus, P.B. Hitchcock, and M.F. Lappert, Z. Anorg. Allg. Chem., 630, 2090 (2004). 34. T. Birchall and J.P. Johnson, J. Chem. Soc., Dalton Trans., 69 (1981). 35. A. Bashall, N. Feeder, E.A. Harron, M. McPartlin, M.E.G. Mosquera, D. S´aez, and D.S. Wright, J. Chem. Soc., Dalton Trans., 4104 (2000). 36. 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). 37. W. Clegg. Private Communication. CSD refcode: VECVIJ. 38. A. M¨uller, E. Krickemeyer, F. El-Katri, D. Rehder, A. Stammler, H. B¨ogge, and F. Hellweg, Z. Anorg. Allg. Chem., 621, 1160 (1995). 39. D.A. Evans, D.W.C. MacMillan, and K.R. Campos, J. Am. Chem. Soc., 119, 10859 (1997). 40. H. Braunschweig, B. Gehrhus, P.B. Hitchcock, and M.F. Lappert, Z. Anorg. Allg. Chem., 621, 1922 (1995). 41. A. Akkari, J.J. Byrne, I. Saur, G. Rima, H. Gornitzka, and J. Barrau, J. Organomet. Chem., 622, 190 (2001). 42. D.A. Atwood, V.O. Atwood, A.H. Cowley, J.L. Atwood, and E. Rom´an, Inorg. Chem., 31, 3871 (1992). 43. T. Uchida, K. Kozawa and H. Obara, Acta Cryst., B33, 3227 (1977). 44. H. Braunschweig, R.W. Chorley, P.B. Hitchcock, and M.F. Lappert, Chem. Commun., 1311 (1992). 45. D. Agustin, G. Rima, H. Gomitzka, and J. Barrau, Main Group Metal Chem., 22, 703 (1999).

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D.M. Barnhart, D.L. Clark, and J.G. Watkin, Acta Cryst., C50, 702 (1994). S. Chitsaz, B. Neum¨uller, and K. Dehnicke, Z. Anorg. Allg. Chem., 625, 1670 (1999). A.M. van den Bergen, J.D. Cashion, G.D. Fallon, and B.O. West, Aust. J. Chem., 43, 1559 (1990). M. Barrow, H.-B. B¨urgi, M. Camalli, F. Caruso, E. Fischer, L.M. Venanzi, and L. Zambonelli, Inorg. Chem., 22, 2356 (1983). 50. Y. Tang, A.M. Felix, L.N. Zakharov, A.L. Rheingold, and R.A. Kemp, Inorg. Chem., 43, 7239 (2004). 51. 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). 52. B.A. Adair, S. Neeraj, and A.K. Cheetham, Chem. Mater. 15, 1518 (2003). 46. 47. 48. 49.

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4 Medicinal/Biocidal Applications of Tin Compounds and Environmental Aspects 4.1

The Cardiovascular Activity of Organotin Compounds

Mala Nath Department of Chemistry, Indian Institute of Technology, Roorkee, India

4.1.1

Introduction

Organotins belong to the most widely used organometallic compounds, with an estimated annual production of approximately 60 000 tons. The chemistry of organotin(IV) compounds has emerged as one of the strongest areas in the inter-disciplinary field owing to their wide spectrum of industrial, synthetic, agricultural, and biological applications.1−4 Organotin compounds have found diversified commercial applications, such as polyvinyl chloride (PVC) stabilizers, industrial biocides, industrial catalysts, surface curing agents,1−4 scintillation detectors for γ- and X- rays,4 ballistic additives for solid-rocket engine fuels,4 ionophores in liquid membrane ion-selective electrodes,5,6 and so forth. Diorganotin compounds, particularly di-n-butyltin, diphenyltin and di-n-octyltin compounds, are used in industry as stabilizers for PVC, as catalysts in various products and for the curing of room-temperature vulcanized (RTV) silicone elastomers to produce flexible silicone rubber,3 whereas triorganotin compounds, including tri-n-butyltin, tricyclohexyltin and triphenyltin compounds, are used in agriculture as fungicides, molluscicides, acaricides, and pesticides, and in industry as biocides, and were used as marine anti-fouling agents in paints for large ships.1,2,7−9 Besides the aforementioned applications, organotin compounds have also been used as surface-modifying agents, as surface disinfectants, as bacteriostats, as anti-microbials and slimicides, as laundry sanitizers and mildewcides, as rodent-repellents, and as hospital and veterinary disinfectants.1,3,4 Along with these numerous applications, and the rapid growth in the production and consumption of organotin compounds, the concern about possible environmental and health effects has increased. Organotin compounds are ubiquitous environmental pollutants especially relevant for water ecosystems. Because of their wide agricultural and industrial applications, several studies have focused on the increasing amounts of both organic and inorganic tin present in the environment, having been evaluated as

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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the third most important elemental pollutant in the ecosystem. This has naturally raised the concern that tin may enter and accumulate in the environment,10,11 and therefore, the human food chain.12 In higher species, including mammals, organotin compounds tend to accumulate in certain organs, namely the liver, r (triphenyltin acetate), kidneys, and brain.13 However, some of triorganotin compounds, viz. Brestan r r Duter (triphenyltin hydroxide), and Plictran (tricyclohexyltin hydroxide), are commercially used as fungicides for the control of phytophthora (late blight) on potatoes, sugar beets, pecans, and peanuts, and for control of phytophagous (plant-feeding) mites on apples, pears, citrus fruits, and hops.1 The organotin compounds have the advantage over the very effective mercury compounds of being degraded to relatively harmless inorganic tin compounds. Though their use is decreasing, since their biodegradation is very slow, organotin compounds are very persistent in the environment, thus causing toxicological problems to humans and animals,2,12,14 and induce imposex (imposition of male sex characters onto the female) in several marine species15,16 as well as neurotoxic17,18 and immunotoxic effects19−21 in higher animals. Many ‘in vitro’ studies have been performed in order to explain the mechanism that is responsible for the toxicity of organotin compounds in whole organisms. In addition to their neurotoxic effects, organotin compounds have also been shown to interfere with heme metabolism as well as the cardiovascular system.22

4.1.2

Cardiovascular Activity of Organotin Compounds

Cardiovascular Activity of Organotin Halides/ Hydroxides/Oxides

Stoner et al.23 observed vasodilatation in rabbits following alkyltin treatment. In acute and chronic experiments, they observed that triethyltin (Et3 Sn) compounds (most toxic) produced muscular weakness followed by some recovery, then muscular tremors, convulsions, and death. The main site of action of these alkyltin compounds was the central nervous system, and it was unassociated with concentration of tin at any particular site. The experimental studies on effects of n-butyltin salts on hemolysis in vitro indicated that the hemolytic action of Bu3 SnCl on red blood cells of rabbits was greater than Bu2 SnCl2 by a factor of 100, whereas Bu4 Sn scarcely showed any action.24 The pharmacological study on bis-(trin-butyltin) oxide (TBTO) on rats, rabbits, and dogs indicated that TBTO caused a fall in blood pressure that resulted from a depression of the vascular smooth muscle.25 The death of the animal in a sub-chronic study of TBTO was due to respiratory arrest by a central mechanism, but its action on the ortho- and parasympathetic system is slight and non-specific. On the other hand, Tauberger observed that intravenous administration of triphenyltin acetate (TPTA) in cats at a dose of 1 mg kg−1 produced an increase in blood pressure and a short interruption of respiration, followed by stimulation of respiration and clonic contractions of the limb muscles.26 The repeated administration of 1–2 mg kg−1 at 20–60 minute intervals led to arterial hypotension. A decrease in the effect of noradrenaline on blood pressure was also reported. Death took place after 4–14 mg kg−1 of TPTA, from paralysis of the respiratory centers. When TPTA was given orally to groups of young rats, 10 of each gender, at dose levels of 0, 5, 10, 25, and 50 ppm for 12 weeks, a decrease in the number of leucocytes in the blood was reported at 10 ppm and above, and at 50 ppm the hemoglobin was reduced.27 At the highest level there was a decrease of the organ/heart ratio for the thyroid, pituitary, and pancreas in all animals, and the uterus and ovaries in the females. Similarly, the oral administration of triphenyltin hydroxide (TPTH) resulted in a decrease in the number of leucocytes at 25 ppm in females and a decrease of hemoglobin and leucocytes in each sex at 50 ppm.27,28 Similar results of decreasing leucocytes, as well as erythrocytes and hemoglobin content of the blood, were also observed when TPTA (5–20 ppm)/TPTH (5–20 ppm) was given orally to guinea pigs for 12 weeks.27 No histopathological details of these studies are yet available.

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BEEF HEART MITOCHONDRIAL ATPases Ca++ ATPase OS Mg++ ATPase

100 PERCENT INHIBITION

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BEEF HEART MITOCHONDRIAL Ca++ ATPase Km CONTROL 1.82 5 nM PLICTRAN 1.82 50 nM PLICTRAN 2.50

Vmax 15.4 10.5 6.4

1.0 0.8

60

1/V 0.6

40

0.4 20

0.2 20

40

80 60 PLICTRAN {nM} (a)

100

−20

−10

10

20

1/ATP {mM} (b)

Figure 4.1.1 (a) % Inhibition of beef heart mitochondrial Ca2+ -ATPase (• − − − •) and oligomycin-sensitive Mg2+ -ATPase (o − − − −o) activities by Plictran. ∗ Denotes a significant difference (P < 0.05) between absence and presence of Plictran in reaction; (b) Effect of Plictran on ATP activation kinetics of Ca2+ -ATPase in beef heart mitochondria. ATP concentration was varied as shown from 0.5–4 mM, all other conditions remaining constant. Control, (• − − − •), 5 nM Plictran (• − − − •), 50 nM Plictran ((o − − − −o). Each point in the graphs represents mean ± SE, three different preparations, each assayed in triplicate. (Reproduced from Reference 29, copyright 1985, Elsevier.)

r ) inhibited both oligomycin-sensitive (o.s.) Mg2+ -ATPase and Tricyclohexyltin hydroxide (Plictran 2+ Ca -ATPase activities in beef heart mitochondria at nanomolar concentrations (Figure 4.1.1a) at pH r did not affect the oligomycin-insensitive (o.i.) Mg2+ -ATPase activity at any 7.5. However, Plictran 29 concentration studied. It is well established that mitochondrial membrane contains o.s. Mg2+ -ATPase and Ca2+ -ATPase, which are involved in the terminal step of oxidative phosphorylation, resulting in the synthesis of ATP30 and in the maintenance of cytoplasmic Ca2+ levels,31 respectively. The significant r may be interfering with inhibition of o.s. Mg2+ -ATPase and Ca2+ -ATPase suggests that Plictran oxidative phosphorylation and thereby reducing ATP synthesis, and calcium ion transport, respectively, r inhibited o.s. Mg2+ in beef heart mitochondria. Substrate activation kinetics revealed that Plictran 2+ 29 ATPase uncompetitively and Ca -ATPase non-competitively (Figure 4.1.1b). The most sensitive indices of triorganotin toxicity appear to be those associated with changes in the r ), and Cy SnOH lymphatic tissues and blood composition of mice. Ph3 SnCl, Ph3 Sn(OAc) (Brestan 3 r 32 (Plictran ) (the most potent) altered the blood composition: reduction in lymphocytes and total leucocytes and an increase in erythrocytes, hemoglobin level, and hematocrit value; and resulted in a loss in heart, liver, spleen and body weight (Table 4.1.1) with mature mice feeding for four days on diets containing 260 μ equivalent (μ equiv.) organotin kg−1 , whereas (Bu3 Sn)2 O (TBTO) and [(PhCMe2 CH2 )3 Sn]2 O r ) were less potent, requiring dietary levels of 780 and 2340 μ equiv. kg−1 for pronounced (Vendex effects. The organotin compounds PhSnCl3 , Ph2 SnCl2 , and Ph4 Sn had little or no effect even at a 2340 μ equiv. kg−1 diet.32 The R3 Sn+ -induced reduction in heart and liver weights of young mice is generally proportional to the reduction in body weight (Table 4.1.1). The large amount of lymphatic tissues in the spleen and its

–

–

–

104 ± 4 97 ± 13 63 ± 5 – 67 ± 4 – – 79 ± 4 – 82 ± 3 – – 93 ± 5 – – 92 ± 5 – 92 ± 3 – 85 ± 5 – 0.11 ± 0.01

99 ± 3 86 ± 2 69 ± 2 – 64 ± 3 – – 76 ± 2 – 72±2 – – 101± 3 – – 91 ± 3 – 91 ± 3 – 86 ± 5 – 20.1 ± 0.6 9

88 63 61 73 63 51 110 91 58 – 86 63 – 80 73 – 122 – 106 – 113 –

Spleen weight relative to control (%)b.c

3

112 108 118 100 104 115 105 105 116 – 104 98 – 100 122 – 97 – 101 – 92 –

Hbe

2

107 100 113 99 102 109 103 108 113 – 101 94 – 102 119 – 98 – 97 – 93 –

Hec f

3

106 104 113 – – 110 102 104 107 – 102 96 – 100 109 – 96 – 95 – 90 –

Erythrocytes

9

98 69 66 72 67 66 108 90 82 – 106 69 – 89 81 – 91 – 80 – 84 –

Leucocytes

Data are the mean and SE (standard error) values based on 10 young mice (initial weight 13.6 ± 0.3 g) for each treatment after 7 days on treated diets; b Data are the mean and SE values based on four mature mice (initial weight 28.3 ± 0.5 g) for each treatment after 4 days on treated diets; c Control spleen weight, 196 ± 7 mg; d Control blood composition: 15.9 ± 0.2 g Hb/100 ml; 48.9 ± 0.7 % Hec; 7.1 ± 0.3 × 106 erythrocytes/mm3 ; 23 ± 2 × 103 total leucocytes/mm3 ; e Hb, Hemoglobin content; f Hec, Hematocrit value

a

Average SE values

Control value (g)

Ph4 Sn

Ph2 SnCl2

PhSnCl3

[(PhCMe2 CH2 )3 Sn]2 O

(Bu3 Sn)2 O

Cy3 SnOH

Heart

Body

23:15

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78 260 780 78 260 780 78 260 780 260 780 2340 260 780 2340 780 2340 780 2340 780 2340 –

(Dietary level μ equiv kg−1 )

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Ph3 SnCl

Organotin

Blood composition relative to control (%)b,d

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Table 4.1.1 Effects of dietary organotins on body, heart, and spleen weights and blood composition of male mice

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Table 4.1.2 Hematological parameters in male and female Wistar rats exposed to TBTO a Dietary concentration(mg kg−1 ) Wistar Rat Males Body weight Heart Hemoglobin (mmol l−1 ) Hematocrit value (l l−1 ) Erythrocytes (1012 l−1 ) MCV (fl) MCH (amol) MCHC (mmol l−1 ) Leukocytes (106 l−1 ) Lymphocytes Females Body weight Heart Hemoglobin (mmol l−1 ) Hematocrit value (l l−1 ) Erythrocytes (1012 l−1 ) MCV (fl) MCH (amol) MCHC (mmol l−1 ) Leukocytes (106 l−1 ) Lymphocytes

Control

5

20

80

320

245 0.76 9.7 0.42 7.7 55 1260 23 12 360 11 186

240 0.75 9.6 0.42 7.7 54 1240 23 12 480 11 057

241 0.76 9.8 0.42 7.8 54 1250 23 11 420 10 109

226b 0.73 8.9c 0.39 7.6 51c 1180c 23 10 500c 9 045c

113d 0.47d 8.2d 0.35d 7.1b 50d 1160d 23 7080d 4 753d

162 0.60 9.3 0.41 7.5 55 1240 23 9840 8761

162 0.59 9.1 0.40 7.5 53b 1220 23 8780 7521

167 0.58 9.1 0.40 7.5 54 1230 23 9140 7871

161 0.57 8.8b 0.39b 7.5 52d 1180c 23 9380 7837

100d 0.46d 8.2d 0.36d 6.9c 52c 1190b 23 5960d 4349d

a

Rats were exposed for 4 weeks; MCV, mean corpuscular volume; MCH, mean corpuscular hemoglobin mass; MCHC, mean corpuscular hemoglobin concentration; values are mean of 10 animals/group; b p < 0.05; c p < 0.01; d p < 0.001

importance in formation and destruction of blood cells suggest that action at this site may impair the immunity mechanisms of the animal, reduce the ability to disintegrate aged erythrocytes, and possibly produce a stress condition. These changes could also be a response to stress resulting from other causes, such as inhibition of oxidative phosphorylation by the triorganotin compounds. Further, Krajnc et al. reported a significant loss of heart weight, following TBTO treatment33 of male and female Wistar rats. In the groups of rats receiving 80 or 320 mg TBTO kg−1 , microcyctic anemia was found. The white blood cell counts were decreased, due to reduction in the number of lymphocytes in the 80 (males) and 320 mg TBTO kg−1 groups. Changes in hemocytometric values for both sexes (Table 4.1.2) comprised a decreased hemoglobin concentration and hematocrit value from 80 mg kg−1 onward, while at 320 mg kg−1 the erythrocyte concentration also was reduced. The mean corpuscular volume (MCV) and the mean corpuscular hemoglobin mass (MCH) were lowered in the same dose groups, whereas the mean corpuscular hemoglobin concentration (MCHC) remained unchanged.33 The sarcoplasmic reticulum (SR) calcium pump together with phosphorylation of specific proteins, has an important role in myocardial contraction and relaxation.34 Phospholamban, a 20–24 kDa proteolipid,35 is phosphorylated by cAMP-dependent protein kinase36 as well as calmodulin-dependent protein kinase.37 Phosphorylation of this protein results in stimulation of Ca2+ -ATPase and Ca2+ transport by the SR (Scheme 4.1.1).36,38

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Tin Chemistry: Fundamentals, Frontiers and Applications Ca 2+ movements

cAMP-dependent phosphorylation Phosphorylation of phospholamban Sarcoplasmic reticulum

release

Enhanced contraction

and Phospholamban induced augmentation of ATPase

Scheme 4.1.1

Ca

2+

Mechanical effects

Ca

2+

uptake

Accelerated rate of relaxation

Biochemical events involved in contraction-relaxation cycle of the heart36

Therefore, active calcium transport by cardiac SR has a key role in the excitation-contraction coupling of the myocardium, where Ca2+ release from the SR induces contraction and re-accumulation of Ca2+ by the SR leads to relaxation34,36 (Scheme 4.1.1). If any compound alters SR calcium transport, it obviously affects the normal functioning of the heart. A similar potent concentration-dependent inhibition of both basal- and isoproterenol (a beta-adrenergic agonist)-stimulated cardiac SR 45 Ca2+ uptake and Ca2+ r , an organotin acaricide, with an estimated IC of 2.5 ×10−8 M has been reported39 ATPase by Plictran 50 by Sahib and Desaiah in vivo as well as in vitro in rat heart ventricular membrane vesicles. Since cardiac relaxation is mediated by beta-adrenergic stimulation via Ca2+ uptake by the SR, the inhibition of calcium r may result in alteration in cardiac Ca2+ fluxes leading to cardiac dysfunction. pump activity by Plictran r , the effects of Plictran r In order to evaluate the mechanism of inhibition of Ca2+ -ATPase by Plictran 2+ on substrate and cationic activation kinetics of beta-adrenergic-stimulated cardiac Ca -ATPase have r (1 and also been investigated by Desaiah et al.40 Data indicated alteration of Vmax and K m by Plictran −8 5 × 10 M), suggesting a mixed type of inhibition. The beta-adrenergic agonist isoproterenol increased Vmax of both ATP- and Ca2+ -dependent enzyme activities. However, the K m of enzyme was decreased r -inhibited isoproterenol-stimulated Ca2+ -ATPase activity by altering both V only for Ca2+ Plictran max and K m of ATP as well as Ca2+ -dependent enzyme activities, suggesting that after binding to a single r inhibits enzyme catalysis by decreasing the affinity of the enzyme for ATP independent site, Plictran 2+ as well as for Ca . Preincubation of the enzyme with 15 μM cAMP or the addition of 2mM ATP to the r -inhibited enzyme. Pretreatment of SR with reaction mixture, resulted in slight activation of Plictran r −7 −8 5 × 10 M propranolol and 5 × 10 M Plictran resulted in inhibition of basal activity in addition to the loss of stimulated activity. These studies suggest that some critical sites common to both basal- and r , and the resultant beta-adrenergic-stimulated cardiac Ca2+ -ATPase are sensitive to binding by Plictran 40 conformational change may lead to inhibition of beta-adrenergic stimulation. Tri-n-butyltin bromide (TBT), triethyltin bromide (TET), and trimethyltin chloride (TMT) inhibited the cardiac SR 45 Ca uptake (Figure 4.1.2) and Ca2+ -ATPase (Figure 4.1.3) in vitro in rat heart in a concentration-dependent manner.41 The order of potency for Ca2+ -ATPase as determined by IC50 , is TBT (2 μM) > 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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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

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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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TET

TBT

100 % of Control

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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

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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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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

Immediate (Mean ± SE)

Delayed (Mean ± SE)

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

– – – – – ↓↑ ↓↑ ↓↑

Delayed (Mean ± SE)

Duration (min) (Mean ± SE)

Control (Mean ± SE)

Immediate (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. Sch¨onfelder 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 Lascourr`eges 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, Sch¨uu¨ rmann and R¨oderer95 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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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. K¨ummel, and M. Hardtman (for I. G. Farbenindustrie AG), German Patent, 485 646, (1929). E. Hartmann, M. Hardtmann, and P. K¨ummel (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. Jir´asko, 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. Sch¨onfelder 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. Lind´en, 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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Tin Chemistry: Fundamentals, Frontiers and Applications R. Boopathy and L. Daniels, Appl. Environ. Microbiol, 57, 1189 (1991). J. F. Lascourr`eges, 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. Sch¨uu¨ rmann and G. R¨oderer, 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

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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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.

(a) E. Frankland, Liebigs Ann. Chem. 71, 171–213 (1849); (b) E. Frankland, Liebigs Ann. Chem. 71, 213 (1849). J.S. White, J.M. Tobin, and J.J. Cooney, Can. J. Microbiol., 45, 541 (1999). K. Fent, Sci. Total Environ., 185, 151 (1996). 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), and references therein. M. Gielen, Appl. Organomet. Chem., 16, 481 (2002). D.V. Dick, R. Willem, M. Gielen, K. E. van Wingerden, and K. Nooter, Metal Based Drugs, 5, 179 (1998). Y. Zhou, T. Jiang, S. Ren, J. Yu, and Z. Xia, J. Organomet. Chem., 690, 2186 (2005). S. Tabassum and C. Pettinari, J. Organomet. Chem., 691, 8, 1761 (2006). A. Perez-Rebolledo, J. D. Ayala, G. M. de Lima, N. Marchini, G. Bombieri, C. L. Zani, E. M. Souza-Fagundes, and H. Beraldo, Eur. J. Med. Chem., 40, 467 (2005). H. I. Beltr´an, C. Damian-Zea, S. Hern´andez-Ortega, A. Nieto-Camacho, and M.T. Ram´ırez-Apan, J. Inorg. Biochem. 101, 1070 (2007). M. Nath, S. Pokharia, G. Eng, X. Song, and A. Kumar, Spectrochim. Acta A, 63, 66 (2006), and references therein. L. Pellerito and L. Nagy, Coord. Chem. Rev., 224, 11 (2002), and references therein. K.E. Appel, Drug Metab. Rev. 36 763 (2004). J.S. White and J.M. Tobin, Environ. Sci. Technol, 38, 3877 (2004). J.S. White and J.M. Tobin, Appl. Microbiol. Biotechnol., 63, 445 (2004). C.J. Evans and R. Hill, J. Oil Colour Chem. Assoc., 64, 215 (1981). D. Liu, R.J. Maguire, Y.L. Lau, G.J. Pacepavicius, H. Okamaru, and I. Aoyama, Water Res., 31, 2363 (1997). N. Voulvoulis, M.D. Scrimshaw, and J.N. Lester, Appl. Organomet. Chem. 13, 135 (1999). R. J. Maguire and S. P. Batchelor, Water Qual. Res. J. Can. 40, 431 (2005). J. G. Vos, E. Dybing, H. A. Greim, O. Ladefoged, C. Lambre, J. V. Tarazona, I. Brandt, and A. D. Vethaak, Crit. Rev. Toxicol. 30, 71 (2000). M. Hoch and D. Schwesig, Appl. Geochem. 19, 323 (2004). S. Shahzadi, S. Ali, M. H. Bhatti, M. Fettouhi, and M Athar, J. Organomet. Chem. 691, 1797 (2006). M. A. Girasolo, D. Schillaci, C. Di Salvo, G. Barone, A. Silvestri, and G. Ruisi, J. Organomet. Chem., 691, 693 (2006). M. A.Girasolo, C. Di Salvo, D. Schillaci, G. Barone, A. Silvestri, and G. Ruisi, J. Organomet. Chem. 690, 4773 (2005).

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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. Cort´es, 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. Cort´es, 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. Cort´es, 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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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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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

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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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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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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. L`opez-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 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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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
The effects of TMTC exposures on the lytic function of NK cells are summarized in Table 4.5.4. Exposure of NK cells to TMTC for 1 h caused no significant decreases in cytotoxic function at any concentration. After 24 h, 10 μM TMTC caused a very significant decrease (76%) in the capacity of NK cells to lyse tumor cells. A 48 h exposure decreased the ability of NK cells to lyse tumor cells at concentrations as low as 2.5 μM (23%). A 6 d exposure to 5 μM TMTC caused a very significant loss of cytotoxic function (96%).81 Effects on Binding Function

The ability of TMTC to interfere with the ability of NK cells to bind to tumor target cells was examined at those concentrations of TMTC that caused decreased cytotoxic function. NK cells exposed to 10 and 5 μM TMTC for 24 h showed no decrease in their ability to bind K562 cells (Table 4.5.4). There was also no effect on binding after a 48 h exposure to TMTC. However, after a 6 d exposure to TMTC there was a greater than 90% decrease in binding function at 5 μM (Table 4.5.4).81

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Table 4.5.4 Effects of trimethyltin chloride on the lytic function and binding function of human NK cellsa [TBTC] μM

Length of exposure

10

1h 24 h 48 h 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 24 h 48 h 6 days

5

2.5 1 0.5 0.25

% Decrease Lysis

Binding

NS 76 85 NS 31 85 49 12 23 49 NS NS 9 NS NS 15 NS NS NS

– NS NS – NS NS NS – – NS – – NS – – – – – –

a

Data summarized from reference 81 NS, not significant; –, not determined

4.5.5

Effects of Dimethylphenyltin Chloride (DMPTC) on Human NK Cell Function

Effects on Lytic Function

Exposure of NK cells to DMPTC for 1 h caused no significant decreases in cytotoxic function at any concentration (1–10 μM) (Table 4.5.5). However, a 24 h exposure to 10 μM DMPTC caused a 98% decrease in the capacity of NK cells to lyse tumor cells and 2.5 μM DMPTC for 24 h decreased NK lytic function by 52%. A 48 h exposure significantly decreased the ability of NK cells to lyse tumor cells at concentrations as low as 1 μM (45%) and a 6 d exposure to 1 μM decreased lytic function by 84% (Table 4.5.5).81 Effects on Binding Function

Exposures of NK cells to 10, 5, or 2.5 μM DMPTC for 24 h caused no change in binding function. Likewise, 48 h exposures caused no decrease in binding function at 5 or 2.5 μM DMPTC. However, a 6 d exposure to DMPTC caused significant decreases in binding function (80%) at the 2.5 μM concentration (Table 4.5.5).81 4.5.6

Effects of Methyldiphenyltin Chloride (MDPTC) on Human NK Cell Function

Effects on Lytic Function

In contrast to TMTC and DMPTC, exposure of NK cells to MDPTC for 1 h caused very significant decreases in NK lytic function at both 5 and 2.5 μM (Table 4.5.6). Exposure to 200 and 100 nM MDPTC

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Tin Chemistry: Fundamentals, Frontiers and Applications Table 4.5.5 Effects of dimethylphenyltin chloride on the lytic function and binding function of human NK cellsa [DMPTC] μM 10 5 2.5

1

0.2

Length of exposure 1h 24h 1h 24h 48 h 1h 24h 48 h 6 days 1h 24h 48 h 6 days 24h 48 h 6 days 48 h 6 days

% Decrease Lysis

Binding

NS 98 NS 86 91 NS 52 71 95 NS 34 45 84 NS NS NS NS NS

– NS – NS NS – NS NS 80 – – NS NS – – NS – –

a

Data summarized from reference 81 NS, not significant; –, not determined

for 24 h caused 83% and 62% decreases, respectively, in the capacity of NK cells to lyse tumor cells. A 6 d exposure to 100 nM MDPTC decreased lytic function by 84% and 50 nM MDPTC decreased function by 37% (Table 4.5.6).81 Effects on Binding Function

The ability of NK cells to bind to K562 tumor cells was unaffected by exposure to MDPTC for 24 and 48 h. However, a 6 d exposure to as low as 100 nM MDPTC decreased the binding capacity of NK cells by about 40% (Table 4.5.6).81

4.5.7

Effects of Triphenyltin Chloride (TPTC) on Human NK Cell Function

Effects on Lytic Function

Exposure of NK cells for 1 h to 1, 0.5 and 0.2 μM TPTC caused decreases in NK lytic function of 95%, 84%, and 31% (Table 4.5.7). After a 48 h exposure to 0.2 μM TPTC, lysis was inhibited by 94%. A 6 d exposure to TPTC caused a decrease in lytic function of 38 ± 27% at 0.05 μM (Table 4.5.7).80,81 In addition we have shown that a 1 h exposure to TPTC caused a persistent loss of lytic function, as was seen with TBTC and DBTC (Table 4.5.7).100

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Table 4.5.6 Effects of methyldiphenyltin chloride on the lytic function and binding function of human NK cellsa [MDPTC] μM 5 2.5 1 0.2 0.1 0.05 0.025 0.01

% Decrease

Length of exposure

Lysis

Binding

1h 1h 1h 1h 24 h 48 h 24 h 48 h 6 days 24 h 48 h 6 days 24 h 48 h 6 days 6 days

93 66 18 NS 83 76 62 64 84 NS 42 37 NS NS 15 NS

– – – – NS NS NS NS 40 – – – – – – –

a

Data summarized from reference 81 NS, not significant; –, not determined

Effects on Binding Function

NK cells exposed to TPTC for 24 h showed no significant change in their ability to bind to tumor cells. However, after a 48 h exposure to 200 nM TPTC binding of NK cells to tumor cells was decreased by about 40% (Table 4.5.7). Lower concentrations of TPTC had no significant affect on binding at 48 h. A 6 d exposure to 200 nM TPTC caused a significant decrease in binding function (75%). NK cells exposed to 100 nM TPTC for 6 d also exhibited decreased binding function (60%) (Table 4.5.7).81 4.5.8

Summary

We have established that in vitro exposures to n-butyltins, methyltins, and phenyltins very significantly reduce the ability of human NK cells to lyse tumor target cells (lytic function). This effect is most pronounced with triorganotins, with TBTC being the most effective of the triorganotins at blocking NK lytic function. TPTC and MDPTC are also very effective at blocking lytic function. DBTC is approximately 5–10 times less potent than TBTC in its capacity to block NK function. Diphenyltin chloride (DPTC) is also able to decrease NK function, but as with DBTC it is on the order of five-fold less effective than TPTC.80,100 In order for NK cells to lyse their targets, including tumor cells, the NK cell must bind to the target utilizing cell surface proteins present on the NK cell, as well as on the target. We found that each of the OTs is able to decrease the ability of NK cells to bind to their targets. However, this decrease is a slower consequence of OT exposure than is the decrease in lytic function.

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[TPTC] μM

Length of exposure

Lysis

Binding

1 0.5 0.2

1h 1h 1h 24 h 48 h 6 days 1h 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

95 84 31 86 94 95 NS 30 56 84 NS 26 38 NS NS 15 90 89 95 90 96 95 NS NS NS

– – – NS 40 75 – NS NS 60 – – NS – – NS NS 32 – – – – – – –

0.1

0.05 0.025 0.75 0.5 0.25

a

Data summarized from references 80, 81, 100 NK cells were exposed to TPTC for 1 h after which the TPTC was removed and the cells were washed twice in TPTC-free media and then suspended for 24 h, 48 h, or 6 days in TPTC-free media; NS, not significant; –, not determined b

ATP is needed for cells to carry out their functions, and is thought to be necessary for granule exocytosis,101 as seen when NK cells release the contents of their cytolytic granules onto the surface of targets cells. For many years it has been known that TBTC is able to decrease the function of the enzyme, ATP synthase, which is responsible for mitochondrial ATP production. Due to the importance of ATP for many cellular functions, we assessed whether OT-induced decreases in ATP might correlate with the OT-induced decreases in lytic function. We saw OT-induced decreases in ATP levels in NK cells but they did not correlate with the initial loss of lytic function. As with binding capacity, the decreases in ATP levels took longer to occur than the loss of lytic function. The decreases in ATP have been seen with both DBTC and TBTC. We have not yet completed studies with the methyltins and phenyltins. However, preliminary studies indicate that they, like the butyltins, can cause decreases in intra-cellular ATP levels. Like TBTC and DBTC, the decreases in ATP induced by the methyltins and phenyltins appear to

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correlate with loss of lytic function only after longer periods of exposure, but do not account for loss of lytic function within 24 to 48 h of exposure. One consequence of NK cell exposure to OT that occurs quickly (within 10 minutes) is an increase in the activation of mitogen-activated protein kinases (MAPK). These enzymes are critical to the function of the NK cells, as well as nearly all other cell types.72 TBTC can significantly activate MAPKs within 10 min of exposure and this activation can be accounted for by TBTC-induced activation of the immediate upstream activators of MAPKs,93 referred to as MAPK kinases (MAP2Ks).72 Activation of MAPKs by TBTC would leave the NK cell unable to respond to target cells when they were encountered. The activation of the MAPK pathway by OT exposure occurs within a timeframe that could account for the loss of lytic function seen at exposures of less than 48 h, while the other alterations such as ATP decreases and binding decreases take longer to occur. We have unpublished data indicating that DBTC is also able to increase MAPK activity in NK cells within 10 min. Thus, it will be important to examine the other OTs to determine if this is a common mechanism in the OT-induced inhibition of NK lytic function. Finally, a series of OTs that differed in their lipophilicity (TMTC, DMPTC, MDPTC, and TPTC) has been shown to decrease the lytic function of NK cells in the order TMTC
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4.6

Biological Aspects of Organotins: Perspectives in Structural and Molecular Biology

Hiram I. Beltr´an,a Rosa Santillanb and Norberto Farf´anc a

Departamento de Ciencias Naturales, Divisi´on de Ciencias Naturales e Ingenier´ıa Universidad Aut´onoma Metropolitana, Cuajimalpa, M´exico b Departamento de Qu´ımica, Centro de Investigaci´on y de Estudios Avanzados del Instituto Polit´ecnico Nacional, M´exico c Facultad de Qu´ımica, Departamento de Qu´ımica Org´anica, Universidad Nacional Aut´onoma de M´exico, M´exico

4.6.1

Introduction

The present review surveys mono-, di-, tri- and tetraorganotin derivatives used in biological systems to ascertain their mechanism of action. Important details emerging during the compilation of this information are organized according to whether they stem from procaryotic or eukaryotic cells, as well as differentiation between their biological origins. Synthetic methods describing the reaction between biological system mimics and organotin compounds, including their physicochemical properties, spectroscopic measurements, biological activities, and their effect at different levels of cells are covered. This reveals, in a concise manner, the nano-scale effects of these chemicals in higher organisms. Moreover, the interaction of organotin compounds with biological systems permits the tracking of single variables. These include steric1 and synergistic effects with other molecules,2−6 in biomotors, 7−10 evaluation against different types of lipid monolayers and vesicles,11−16 cell walls,17 cell signaling,18−23 gene transcription,24−29 mitochondria,19,30−38 red blood cells,39−42 bioenergetics,43 interactions with glutamaergic, GABAergic, RXR, and other receptors.11,20,22,23,27,29,30,44−55 4.6.2

Interaction of Organotin Compounds with Biological Systems and Mimic Xenobiotics

An essential aspect of the interaction of organotin compounds with biological molecules concerns the stability of organotins under the conditions where the structure and functionality of these biomolecules are found. It is likely that hydrolysis to give organotin oxides could be partially prevented by phosphate buffers or by interaction with other biomolecules, as has been stated e.g. for diethylenetin chloride.56 The triggering of alternative biological pathways caused mainly by interaction of organotin compounds with histidine and cysteine residues in proteins was systematically studied by Chang in 1990.57 Surprisingly, compared to other organometallic reagents, organotin compounds are more specific towards their biological receptors.38 It has been proven that organotin compounds specifically target vicinal dithiols,58 as shown for peptidomimetic systems resembling an important part of the membrane protein (stannin), responsible for toxicity caused by organotins. The experiments were carried out employing (CH3 )3 SnCl, (CH3 )2 SnCl2 , and the linear peptide ILGCWCYLR, with two vicinal cysteine fragments. This peptide was capable of dealkylating (CH3 )3 SnCl, as well as binding the dimethyltin fragment by formation of two S Sn bonds with tetracoordination of tin. In the case of (CH3 )2 SnCl2 , the results were very similar, but did not require the dealkylation step. When the peptide is linked to the tin moiety, the covalent species presents a stable type-I β-turn conformation. This survey is a chemical milestone in the toxicity of organotin compounds in biological systems for the dealkylation process from trialkyltin to dialkyltin, which diminishes the observed toxicity.59 An example of the investigation of organotin compounds with deoxyribonucleic acid (DNA), the interaction of two moderately water-soluble anti-tumor organotins,60 bis[(di-n-butyl-3,6-dioxaheptanoato)tin]

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O

O

O O

Bu Sn

O

O

O O

Bu 2

BDBDOHT

O

O

483

Bu Sn Bu

Bu

2

TBTTOD

Figure 4.6.1 Structure of water-soluble organotin compounds: bis[(di-n-butyl 3,6-dioxaheptanoato)tin] (BDBDOTH) and tri-n-butyltin 3,6,9-trioxadecanoate (TBTTOD)

and tri-n-butyltin 3,6,9-trioxadecanoate (TBTTOD), with calf thymus DNA, was reported by Casini and coworkers in 2001 (Figure 4.6.1).61 It was found that the two organotin derivatives caused only slight modifications in the B-type circular dichroism spectroscopy of the free DNA molecule. DNA gel mobility tests showed a preferred interaction of these organotins with the phosphate groups of DNA, rather than intercalation between base pairs. In addition, both compounds affected the thermally induced helix-tocoil transition and finally, the melted DNA samples were not denatured upon the addition of these two organotin compounds. It has been found that certain cells synthesize a family of proteins to mediate toxicity caused by organotins. Stannin is an example of such proteins, identified as the specific marker for neuronal cell apoptosis and certainly mediates the toxicity of (CH3 )3 SnCl and (CH3 )2 SnCl2 . It is a mitochondrial membrane protein found primarily in vertebrates constituted by 88 amino acid residues.62,63 The structure, dynamics, and membrane topology of stannin were determined recently.62 It turned out to be a monotopic transmembrane protein, since just one α-helix moiety is embedded (residues 10–33) into the membrane, according to the NMR data analysis in solution and in the solid state at 800 and 400 MHz. The determination of the three-dimensional structure of stannin was resolved by using different small unilamellar vesicles of 1-palmitoyl-2-hydroxy-Sn-glycero-3-[phospho-RAC-(1-glycerol)], dodecylphosphocholine, and sodium dodecyl sulfate surfactants as cell membrane models at pH 4 and 6. Another 28 amino acid residue, denominated an unstructured linker, possesses a conserved cysteine-X-cysteine (C-X-C) metal binding motif (X = other amino acid) and a 14-3-3ζ binding domain. Finally, a distorted cytoplasmic helix was determined, comprising residues 61–79, partially adsorbed into the lipid bilayer surface. These motifs remain partially absorbed into the plane of the lipid bilayer practically perpendicular to the membrane normal (tilt angle ≈ 80◦ ). Other proteic systems have been tested for the inhibitory effect of organotin compounds, for example the investigation of (C6 H5 )3 SnCl on the activity of membrane-bound pyrophosphatase of Rhodospirillum rubrum and comparison with ATPase systems.64 The results show that inhibition of the hydrolysis of chromophore membrane bound pyrophosphatase by (C6 H5 )3 SnCl is pH dependent and maximal at pH 9–10. The (C6 H5 )3 SnCl inhibition of membrane-bound pyrophosphatase is similar to the inhibition of chromatophore H+ATPase. The data shows that (C6 H5 )3 SnCl is a potent inhibitor of the membranebound, but not of cytoplasmic pyrophosphatase, thus indicating that specificity probably depends on the conformation adopted by the enzyme embedded in the bilayer. An interesting finding in molecular biology is that histone acetylation is crucial for the regulation of gene expression, and in a correlated fashion for the regulation of the cell cycle. This acetylation process is catalyzed, for example by histone deacetylase and histone acetyltransferase (HAT),27 thus providing an important target for the design of anti-neoplastic agents. It is well known that organotin compounds

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promote green and mild transesterification and transamidation reactions of carboxylic acids and their esters.65,66 Based on this knowledge, it seemed interesting to ascertain whether this type of reaction proceeds in biochemical systems. Osada and coworkers27 have demonstrated that the activity of HAT obtained from rat liver nuclear extracts, is enhanced by certain organotin compounds, including (C6 H5 )3 SnCl, (CH3 CH2 CH2 CH2 )3 SnCl as major catalysts, and their metabolites, as well as (CH3 CH2 CH2 CH2 )2 SnCl2 and (C6 H5 )2 SnCl2 , but to a lesser extent. However, mono-organotin compounds do not cause significant changes. In particular, these results indicated that: (i) triorganotin compounds are better enhancers of the activity of HAT, (ii) organotin compounds with short alkyl chains showed no effect, compared to other biological activities that depend on the phylogenesis of superior and inferior organisms,67−70 and (iii) the presence of the tin atom is important for the enhancement of HAT activity since other xenobiotics do not produce a similar response, even when specific coordination is plausible. Thus, the absence of a metallic center, or the use of silicon as the isosteric metal, decreases activity. It is likely that the acetyl CoA (involved in the Kreb’s cycle) binding pocket or the substrate-binding site of HAT can tolerate a small compound, such as (CH3 )3 SnCl or (CH3 CH2 )3 SnCl. To ascertain a plausible mechanism for enhancement of the activity of HAT caused by organotin compounds, deoxyribonucleic acid (cDNA) microarray analysis revealed that expression of about 130 genes were induced by treatment of the ascidian Ciona intestinalis with (CH3 CH2 CH2 CH2 )3 SnCl.71 Another important contribution from the group of Nakanishi72 reported that the level of the mRNA for aromatase/CYP19 (essential for converting androgenic to estrogenic steroids) was increased by treatment of human choriocarcinoma JAR cells with (CH3 CH2 CH2 CH2 )3 SnCl. It is also worth mentioning that the mechanism of the induction of these mRNAs by (CH3 CH2 CH2 CH2 )3 SnCl is still an open field of research. Aberrant HAT activity induced by (CH3 CH2 CH2 CH2 )3 SnCl treatment might cause unusual expression of these genes. The reported data27 indicate that organotin compounds have unique effects on HAT independent of their endocrine-disrupting chemical activities, and suggest that the varied toxicities of organotin compounds may be caused by aberrant gene expression following altered histone acetylation. It has been reported that (CH3 CH2 CH2 CH2 )3 SnCl exposure in the uterus of rats causes a dramatic increase in low weights of fetuses, due to hypothyroidism in the carrier.73 Other xenobiotics, such as the RXR agonist bexarotene, have been reported to result in similar symptoms and effects in human patients with lymphoma.74 Very recently, Nakanishi reported an important piece of work related to nuclear receptor signaling effects caused by organotin compounds, particularly for the RXR and the peroxisome proliferator-activated receptor, identifying (CH3 CH2 CH2 CH2 )3 SnCl and (C6 H5 )3 SnCl as nanomolar active agonists in these two cases.23 4.6.3

Interaction of Organotin Compounds in Real and Model Membranes

Lipids are building blocks of model and real membranes, which can be combined with proteins and some other important biomolecules to simulate real membranes. The simplest model is hence the selfassembly of only one component of the complex membrane, in this case the lipids. These mono-component lipidic models are often employed in studies as their interaction with small molecules mimics the actual relationship between the cell membrane and a substrate. A commonly employed amphiphatic lipid, dipalmitoyl phosphatidylcholine (DPPC)75 (Figure 4.6.2), has been widely used to construct these cell membrane motifs, due to its high content in animal cells, and thus its tendency to mimic a valid animal cell. The supramolecular organization of these (a) DPPC amphiphatic molecules lead to a (b) Langmuir monolayer, (c) bilayer, (d) micelle, and (e) vesicle, which are the available levels of modeling to mimic the cellular membrane.

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Air

N+ O

O

O OP O O O

Polar head

Water b) Langmuir monolayer

O

c) Bilayer DPPC

Hydrophobic tails

a) Amphiphatic lipid

d) Micelle e) Vesicle

Figure 4.6.2 The chemical structure of a dipalmitoyl phosphatidylcholine (DPPC) amphiphatic lipid and its organization in model membranes

According to the chemical structure of organotin compounds, they interact with real and model cell membranes locating them in specific regions depending on their physicochemical properties. The interaction of organotin compounds with real cells and their plasmatic vesicles (proteins and lipids), has been investigated using erythrocytes as a source of these two levels of membrane target.76 Experiments performed to determine hemolytic activity, by measuring the optical density at 540 nm, provide hemoglobin concentration as soluble species in the supernatant solution due to rupture or fissure of the cell membrane and thus hemolytic activity. Even though this hemolytic process is due to the action of different types of biochemical damage, the observation of soluble hemoglobin species with UV-vis absorption has been widely used to evaluate the modifications induced in the physicochemical properties of the phospholipids by organotin compounds; 1,6-diphenyl-1,3,5-hexatriene (DPH) and its charged derivative 1-(4-trimethylamino-phenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) were measured as molecular probes embedded in the membrane for steady-state fluorescence studies.76 The organotin compounds assayed were (CH3 CH2 CH2 CH2 )3 SnCl, (CH3 CH2 CH2 CH2 )2 SnCl2 , CH3 CH2 CH2 CH2 SnCl3 , (C6 H5 )3 SnCl, and di-n-butyltin-3-hydroxyflavone bromide (DBT-3-HFBr), the latter being an antioxidant moiety par excellence. The (CH3 CH2 CH2 CH2 )3 SnCl increased hemolysis, starting at concentrations of 10 μM, while (CH3 CH2 CH2 CH2 )2 SnCl2 had a scant protective effect in concentrations between 10 and 90 μM, and CH3 CH2 CH2 CH2 SnCl3 did not present any measurable effect within the concentrations tested. Also, experiments were performed in the absence and presence of CO, since the formation of met-hemoglobin in red blood cells is associated with oxidative hemolysis77 when glutathione peroxidase is inhibited. Therefore, in the presence of CO, formation of the hemoglobin-CO species, which is more stable than hemoglobin-O2 , inhibits the peroxidase mechanism. Under these conditions, (CH3 CH2 CH2 CH2 )3 SnCl, (CH3 CH2 CH2 CH2 )2 SnCl2 , CH3 CH2 CH2 CH2 SnCl3 , (C6 H5 )3 SnCl, and

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DBT-3-HFBr induced an increase in the rate of hemolysis. By contrast, in the absence of atmospheric CO, all organotin compounds tested displayed varying effects on hemolysis, indicating a structure–activity relationship. The controls for the hemolysis experiments were t1/2(CO) = 303 min (in the presence of CO) and t1/2 = 242 min (in the absence of CO). The CH3 CH2 CH2 CH2 SnCl3 presented an almost negligible effect, with t1/2(CO) = 284 and t1/2 = 252. Again, the DBTC presented a small protective action, with t1/2 = 274 and a slight increase in hemolysis with t1/2(CO) = 280. For (CH3 CH2 CH2 CH2 )3 SnCl, a greater effect was observed, with t1/2(CO) = 239 and t1/2 = 162, compared with (C6 H5 )3 SnCl (t1/2(CO) = 275 and t1/2 = 208), where the former is more hemolytic than the latter under CO conditions. Finally, DBT-3HFBr, with t1/2(CO) = 260 and t1/2 = 225, is the only species that was more active in the absence of CO, which can be attributed to the presence of the flavone moiety in the molecular structure. Fluorescence measurements were determined with DPH located in the hydrocarbon core, and TMA-DPH, located close to the bilayer surface because of its charged amino group; the (DBT-3-HFBr) derivative was not used for this experiment since its chromophoric nature could interfere with the absorption and emission of the probes. Therefore, the DPH anisotropy was decreased by (CH3 CH2 CH2 CH2 )3 SnCl in 70% (50 μM), by (CH3 CH2 CH2 CH2 )2 SnCl2 in 12% (50 μM), and by CH3 CH2 CH2 CH2 SnCl3 in 11% (50 μM). In the case of the TMA-DPH probe, no significant changes were observed for any of the organotin compounds tested, with the exception of (C6 H5 )3 SnCl (30 μM), which showed a significant increase in anisotropy of ≈11%. Other types of experiments using real and model membranes with three different fractions of trout erythrocytes, differing in density, a property directly related to cell aging, indicated that the denser cells were the younger.12 In this case, (CH3 CH2 CH2 CH2 )3 SnCl and (C6 H5 )3 SnCl were selected for the measurement of hemolysis and organotin positioning (selectivity) in the lipid bilayer.12 The optical density at 540 nm was measured in the case of the hemolytic activity, while the steady-state fluorescence was determined with lipids extracted from the three different density types of erythrocytes for positioning of the organotin compounds. The effectiveness of (CH3 CH2 CH2 CH2 )3 SnCl in hemolytic tests was more pronounced than that of (C6 H5 )3 SnCl. The (CH3 CH2 CH2 CH2 )3 SnCl significantly increased hemolysis at concentrations over 5 μM, and at 20 μM it was dependent on the density of the fraction in the order: high density (B, bottom) > middle density (M, middle) > low density (T, top) with half life of 113, 75, and 22 min, respectively. The use of specific molecular probes located at the hydrophilic–hydrophobic interface of the bilayer, such as DPH and 6-dodecanoyl-N,N-dimethyl-2-naphthylamine (LAURDAN), coupled with steady-state fluorescence, made it possible to account for the effect of (CH3 CH2 CH2 CH2 )3 SnCl on the polarity of the membrane. These experiments were conducted on liposomes formed by the lipids extracted from the different density fractions, B, M, and T. The control related the decrease in DPH anisotropy as M > 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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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 M¨ossbauer 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 M¨ossbauer 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. Farf´an thanks UNAM (PAPIIT IN203207) and H. I. Beltr´an 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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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

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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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 Kov´ac 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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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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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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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 (<26 ppm), in comparison to the shifts observed when the temperature is lowered to –80 ◦ C (<175 ppm) or when three equivalents are added at room temperature (<59 ppm).62 The rate of change in chemical shift with temperature had lessened at the lowest temperatures studied, but it did not appear that limiting values had been reached.60 These results indicate that a rapid equilibrium occurs between the uncoordinated tributylstannyl ethers and one or more species in which the tin atoms have higher than four coordination. The relatively small chemical shift changes observed on addition of one equivalent of NMI at room temperature indicates that the uncoordinated species are much more highly populated than the coordinated species at room temperature and above.

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Reactions

Dialkylstannylene Acetals

Dialkylstannylene acetals react with a wide range of electrophiles, including acyl halides and anhydrides, sulfonyl halides, alkyl halides or sulfonates, triorganosilyl chlorides, sulfur trioxide-amine complexes, phosphorylating agents, halogens, and N -halosuccinimides.6 The normal reaction path involves formation of a single oxygen–electrophile bond per organotin unit. The solvents used range widely from polar solvents like methanol, N,N-dimethylformamide, or acetonitrile to somewhat polar solvents like tetrahydrofuran or dichloromethane, to non-polar solvents like benzene, toluene, or benzyl bromide. Reactions with acylating, sulfonating, sulfating, and silylating reagents are normally performed at room temperature or below.6,5 However, as will be discussed later, acylation reactions can be performed to advantage under conditions under which product equilibration takes place at >80 ◦ C.63,64 Martinelli et al. have discovered that p-toluenesulfonation reactions directly on diols proceed rapidly using catalytic dibutyltin oxide in chloroform at room temperature, only in the presence of a stoichiometric amount of triethylamine or another base. Under conditions where the base is present in less than a stoichiometric amount, the reaction proceeds until the base is consumed and no further.21,65 119 Sn NMR has been used to show that the tin-containing intermediates are still present as dimers under these conditions.21 The base must remove by-product HCl, which destroys the stannylene acetal. Alkylation requires more vigorous conditions. These reactions were originally performed on the stannylene acetal with the alkylating reagent in DMF at elevated temperatures, 45 ◦ C for methyl iodide or 100 ◦ C for benzyl bromide.66 It was then discovered that the presence of added nucleophiles markedly accelerates the reactions so that alkylation of dibutylstannylene acetals in benzene, which were very slow at reflux with benzyl bromide alone, occur at a reasonable speed at reflux in the presence of added tetrabutylammonium halides.52 Many other nucleophiles are also effective, including N -methylimidazole30 and cesium fluoride.53,67 Cesium fluoride has been used mainly in DMF and the combination of an added nucleophile with a polar aprotic solvent allows benzylation with benzyl bromide to occur efficiently at room temperature. If the stannylene acetal is a 1,2-O-stannylene acetal derivative of a carbohydrate and the electrophile is a carbohydrate-derived triflate, oligosaccharide synthesis can be achieved.68,69 More recently, both formation of the stannylene acetal and alkylation in the presence of tetrabutylammonium iodide have been performed under microwave irradiation; excellent yields have been obtained in <0.5 h and the short reaction times allow the use of more sensitive reagents (Scheme 5.1.10).20 OH

OH O

1. Bu2 SnO, benzene/CH 3 CN (5:1), MW,150 °C, 5 min

HO OH O

OH O

HO OH

Scheme 5.1.10

2. Bu 4NI, BnBr, O(CH2 )3 NHBoc MW,150 °C, 10 min

O BnO OH O

OH O

HO

80%

O(CH 2) 3NHBoc OH

Stannylene acetal formation and reaction under microwave conditions20

Some reactions follow paths different from the single bond per electrophile sequence. Dibutylstannylene acetals derived from all-trans-diols react with phenoxythiocarbonyl chloride at room temperature in dioxane to give non-cyclic phenylthionocarbonates, but when cis-diols are present, cyclic

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thionocarbonates spanning the oxygen atoms of the cis-diol are obtained both for pyranosides70 and furanosides.71 Reaction with thiophosgene gives thionocarbonates for some substrates containing trans-diols.72 Dibutylstannylene acetals have yielded propenylidene acetals on reaction with tetrakis (triphenylphosphine) palladium and acrolein diacetate, in some cases with excellent regioselectivity.73 For instance, d-glucal, methyl α-d-glucopyranoside, methyl α-d-mannopyranoside, and methyl β-d-glucopyranoside yield the 4,6-O-propenylidene acetals in 89, 85, 83, and 80% yields, respectively, as mixtures of diastereomers at the acetal center. Galactose derivatives gave mixtures of 3,4- and 4,6-O-propenylidene acetals.73 Before workup of the reaction of the dibutylstannylene acetal of a diol with an electrophile such as an acyl, alkyl, or sulfonyl halide, the product present in non-polar solvents has a halodibutylstannyl group attached to the non-reacted oxygen atom. If no free hydroxyl groups are present, these halodibutylstannyl ethers dimerize inter-molecularly.74 This organotin derivative can be cleaved with water or mild acid, but chromatography on silica gel is usually sufficient to remove it. Some research groups make use of the strong Sn–F bond to remove tin-containing by-products by washing with fluoride ions. Partition between hexane and acetonitrile has been used for the same purpose.75 Reactions of Stannylene Acetals Not Having Additional Hydroxyl Groups

The reactions of stannylene acetals formed from diols are inherently simpler, because the complication of equilibration between stannylene acetals is removed. These reactions may broadly be classified into three types: (i) kinetically controlled reactions from dimers, (ii) reactions in which products equilibrate, and (iii) reactions that proceed via kinetic control in the presence of added nucleophiles or bases. Only the first two types of reactions yield different outcomes for stannylene acetals with and without hydroxyl groups. (i) Kinetically controlled reactions from dimers. Kinetically controlled reactions from dimers include oxidation with bromine or NBS and p-toluenesulfonation, if conducted in the absence of added nucleophiles. Oxidation with bromine appears to be the fastest reaction, but acylation reactions are also very fast.76 The faster the reaction, the more reactant-like the transition state and the more likely the populations of the reactant dimers are to influence regioselectivity. In any dimer, it seems likely that the dicoordinate oxygen atoms are much more reactive than the tricoordinate oxygen atoms for three reasons. They are sterically less hindered because they are flanked by one tin atom rather than two, each tin atom bearing two butyl or other alkyl groups that are orthogonal to the Sn O plane. They are probably more nucleophilic because they are not involved in an additional dative bond to a second tin atom. The dicoordinate tin atoms are apical in the trigonal bipyramidal geometry, which also conveys reactivity.2 The reactivity of the individual dicoordinate oxygen atoms is critical for regioselectivity. Clearly, primary oxygen atoms are more reactive than secondary oxygen atoms.77 David and Malleron have demonstrated, by competition experiments, that the oxygen atom in the stannylene acetal of a trans-diol on a pyranose ring that is flanked by an axial group is much more reactive than oxygen atoms in stannylene acetals derived from cis-diols.76 In addition, the structure of the most populated dimer and the extent to which it is preferred in solution can be important.41,46 Dimer populations appear to be influenced mainly by steric effects. Stannylene acetals derived from trans-diols on pyranose rings flanked by one axial group and one equatorial group are present in the symmetrical dimer having the dicoordinate oxygens adjacent to the axial group.45 Stannylene acetals derived from trans-diols on pyranose rings flanked by two equatorial groups exist

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as mixtures of dimers that do not inter-convert rapidly on the NMR timescale at 110 ◦ C in toluene. Stannylene acetals derived from trans-diols on pyranose rings flanked by two axial groups exist as rapidly inter-converting mixtures at room temperature.45 These observations are consistent with steric interactions between the alkyl groups on tin, and the axial groups destabilizing dimers having tricoordinate oxygen atoms adjacent to the axial groups. For stannylene acetals derived from trans-diols flanked by one axial group, both reactivity and preferred dimer structure favor reaction on the oxygen atom adjacent to the axial group. Consistent regioselectivity for reaction on oxygen atoms in these types of environments is observed.6,20 For stannylene acetals derived from terminal 1,2-diols, steric effects favor dimers that have the secondary oxygen atoms dicoordinate and reactive, whereas oxygen atom reactivity favors reaction at the primary oxygen. The size of the group on the secondary carbon will determine the extent to which one dimer is preferred so that stannylene acetals derived from terminal diols on the side chains of hexofuranosides are likely to exist in the symmetric 2,2dimer to a much greater extent than the stannylene acetal derived from less substituted terminal diols, such as 1,2-propanediol. Oxidation reactions with bromine or N -bromosuccinide consistently yield products of oxidation at the secondary oxygen (Scheme 5.1.11).78−80 HO

HO

HO

O OH

O

1. Bu2SnO

O OH

2. NBS/CHCl3

O

O

O

O 84%

Scheme 5.1.11

Oxidation of the dibutylstannylene acetal of 1,2-O-isopropylidene-α-D-glucofuranose79

p-Toluenesulfonates do not migrate under the conditions used for their formation from stannylene acetals and hence provide a measure of selectivity for a reaction of stannylene acetals that is slower than oxidation. When reactions are conducted with p-toluenesulfonyl chloride on dibutystannylene acetals derived from terminal 1,2-diols in the absence of added nucleophiles, variable regioselectivity is obtained that depends on the size of the group on the secondary carbon atom as well as the size and shape of the group on tin (Schemes 5.1.12 and 5.1.13).21,41 These results are only consistent with competition HO HO

TsO O OBn

1. R2SnO O

2.TsCl

HO

HO TsO

O OBn

+

CHCl3

O

O OBn

O

O

O R = Bu R = i-Bu 2 R = (CH2)6

43% 30% 5%

O 52% 67% 91%

Scheme 5.1.12 The influence of the structure of the alkyl group on the regioselectivity of ptoluenesulfonation reactions on dialkylstannylene acetals of a hindered terminal-1,2-diol 41

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O

O

O

O

O

1. R2SnO

BnO OBn

BnO

BnO

+

OBn

2. TsCl/CHCl3

OBn OTs

OH

OH

CH2OH

CH2OTs

CH2OH R = Bu 2 R = (CH2)6

507

80% 17%

12% 60%

Scheme 5.1.13 The influence of a cyclic alkyl group on tin on the regioselectivity of p-toluenesulfonation reactions on stannylene acetals from a terminal-1,2-diol 41

between dimer structure and oxygen atom reactivity for control of regioselectivity. Increasing the size of the groups on tin (e.g., isobutyl) or restricting their ability to avoid steric interactions (e.g., making them into a ring 2R = (CH2 )6 ) increases their steric interactions with the remainder of the molecule, favoring the 6,6-dimer, which has both inherently less reactive secondary oxygen atoms dicoordinate, over the 5,6-dimer which has one more reactive primary oxygen atom dicoordinate (see Scheme 5.1.14).41,46 The rate of the reaction at each oxygen atom will be the product of the rate constant for the reaction at that oxygen and the population of the dimer in which that oxygen atom is dicoordinate (× 2 if the structure is symmetric). The rate constant for reaction at the primary oxygen atom will be much bigger that that for reaction at the secondary oxygen atom, but the only way the observed effects of tin substitutents on structure can be explained is if the increase in steric effects of these substituents decreases the populations of dimers having primary oxygen atoms dicoordinate substantially. O

O

R

R R

O

O Sn O

O Bn

6,6-dimer

R

O

R

O BnO

OBn O O

Scheme 5.1.14 glucofuranose

O R

5,6-dimer

O

R

O

O

Sn

O

O Sn

O

O

Sn

O

BnO

R

O O

The populated dimers for stannylene acetals of 3-O-benzyl-1,2-O-isopropylidene-α-D-

Acylation reactions are also very fast, being complete in 5 min at room temperature for reactive substrates.76,77 For 1-phenyl-1,2-ethanediol, an unhindered terminal 1,2-diol, the initially formed products rearrange to a thermodynamic mixture over one hour at –40 ◦ C77 and evidence of rearrangements of initial product has also been obtained for reactions from more hindered carbohydrate-derived terminal 1,2-diols.41 The initial product ratio for the acylation reaction of the dibutylstannylene acetal of 1-phenyl1,2-ethanediol at –40 ◦ C appears to be about 2:1 in favor of acylation on the primary oxygen atom, from published NMR spectra,77 in agreement with the observations on p-toluenesulfonation reactions

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above. The products of benzoylation of the dibutylstannylene acetal of methyl 4,6-O-benzylidene-α-dglucopyranoside only rearrange at temperatures above 85 ◦ C.63 Stannylene acetals obtained from cis-1,2-diols on pyranose rings appear to adopt structures in solution, both dimers and higher oligomers, that have the axial oxygens dicoordinate. The sole crystal structure of this type was a pentamer with the end units having the axial oxygen atoms dicoordinate.40 Oxidation of this type of acetal with bromine results in highly regioselective oxidation at the center bearing the axial oxygen atom (Scheme 5.1.15).78,81 OH 1. Bu 2 SnO

O HO

OBn OBn

2. Br2 / CH2Cl2

O

O OBn

HO OBn 76%

Scheme 5.1.15

Oxidation of the dibutylstannylene acetal derived from a cis-diol on a pyranose ring81

As noted above, the products of acylation reactions rearrange in reactions of stannylene acetals derived from terminal 1,2-diols, but do not rearrange at room temperature for those derived from trans-diols on pyranose rings. In the absence of added nucleophiles, the dibutylstannylene acetal of methyl 4,6O-benzylidene-α-d-mannopyranoside yields an 85:15 ratio of the axial benzoate over the equatorial benzoate, only consistent with kinetic acylation on the dicoordinate oxygen atom.30 Silylation reactions with trimethylsilyl or t-butyldimethylsilyl chloride in chloroform in the absence of base are slower than oxidation or acylation, but faster than p-toluenesulfonation reactions.77,75 In all reactions, products having silyl groups on primary oxygen atoms are strongly preferred. The steric effects exerted by this tertiary group in the transition state are much larger than those of the other electrophiles considered thus far and this may be sufficient to cause the product of substitution on the primary oxygen atom to be obtained with high regioselectivity.75,77,82−84 Alternatively, this could be the result of product equilibration, where the bulk of the tertiary silyl group, combined with the Si O bond being shorter than the Sn–O bond, make these products more stable. Moreover, in contrast to all other reactions of stannylene acetals, the results of competition reactions and the qualitative evaluation of rates of reaction have shown that t-butyldimethylsilyl chloride prefers to react with the primary oxygen atom of stannylene acetals derived from 1,3-diols rather than 1,2-diols (see Scheme 5.1.16).75 This could be due either to lesser steric hindrance in the transition state with six-membered ring dimers or perhaps because oligomers of six-membered rings are less stable than their five-membered ring counterparts36 and hence are more reactive. OH

1. Bu2SnO (1 eq)

>99%

OH

OH OH

OH

2. t -BuMe2 SiCl (1.2 eq) / CHCl3, rt, 20 min

Scheme 5.1.16

Si O

High regioselectivity in a silylation reaction75

(ii) Reactions in Which Products Equilibrate. Acylation reactions can proceed via product equilibration under conditions that vary with the substrate as detailed above. Two studies have appeared in which

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polyols are reacted with excess dibutyltin oxide and then acylation is conducted at temperatures above that required for rearrangement.64,85 In benzoylation reactions, the regioselectivity observed is consistent with initial benzoylation at the expected sites, followed by rearrangement and additional benzoylation at the expected sites (Scheme 5.1.17).64 HO

OMe 1. 3.0 eq Bu2SnO /benzene, toluene

OH O O

HO

HO

O O

BzO

2. 3.3 eq BzCl, 22 oC, 5 h

OH

OMe

OBz

OH

1. 3.0 eq Bu2SnO /benzene, toluene

BzO

93%

OMe

OBz O O

BzO

2. 3.3 eq BzCl, 100 oC, 30 min

OH

92%

Scheme 5.1.17 64 Benzoylation of the dibutylstannylene acetal of p-methoxylphenyl β-D-galactopyranoside under conditions where rearrangement does not occur (top) and where it does occur (bottom)64

In acetylation reactions with acetyl chloride, the products rearrange at room temperature, but higher temperatures are required for rearrangement if acetic anhydride is the acylating reagent.85 When additional hydroxyl groups are present in the molecule containing the stannylene acetal, equilibration of all possible stannylene acetals occurs.7,76 Mechanisms have been proposed for intra-molecular equilibration,77 and these may occur, but inter-molecular equilibration has been demonstrated to occur rapidly.76 (iii) Reactions That Proceed Via Kinetic Control in the Presence of Added Nucleophiles or Bases. The slowest reactions of stannylene acetals are alkylation reactions. In the absence of added nucleophiles, these reactions proceed at elevated temperatures in low yields with the most active alkylating reagents, methyl iodide or benzyl bromide. The discovery that added nucleophiles accelerated these reactions made the use of alkylation reactions of stannylene acetals practical.52−54,67,86 It seems likely that the added nucleophile reacts with the stannylene acetal dimer to release monomer complexes with pentacoordinate tin atoms as intermediates. The rate acceleration probably arises from reduced steric hindrance to approach of the electrophile. Some spectacularly regioselective examples have been obtained (Schemes 5.1.18 and 5.1.19). HO

OH

HO

OH

O

HO OH

O HO

O OMe

1. Bu2SnO

OH

RO

OH

O OH HO

AllBr, Bu4NBr / Benzene, 80 °C p-MeOBnCl, Bu4NI / Benzene, 80 °C MeI, Bu4NBr / CH3CN,45 °C

Scheme 5.1.18

OH

O

O OMe OH

R = All 70% R = p-MeOBn 70% R = Me 54%

Regioselectivity in reactions of the monostannylene acetal of methyl β-lactoside87,88,89

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OHOH

O O O

HO

OH HO

OH

OHOH

a) Bu2SnO (1.1 eq) / MeOH reflux 3 h

O O O

HO

OH

OH HO

OH

b) CH3(CH2)13Br (6.5eq), CsF (3.4eq) / DMF 72 h, 65 °C

O

(CH2)13CH3

25%

High selectivity in alkylation of a dibutylstannylene acetal 90

Scheme 5.1.19

The regioselectivity observed in these reactions is consistent with reactions from monomeric stannylene acetals coordinated to the base or nucleophile, on the understanding that 1,3,2-dioxastannolanes are more stable than 1,3,2-dioxastanninanes. The most reactive oxygen atom is the primary oxygen of a 1,2-diol.90 The next most reactive oxygen atom is the equatorial oxygen of a cis-1,2-diol on a pyranose ring.87 The observations of David and Malleron76 suggest that this latter observation is due to the greater reactivity of the oxygen adjacent to the axial group in a trans-1,2-stannylene acetal rather than the larger population of the more stable cis-1,2-stanylene acetal. In competition experiments, David and Malleron demonstrated unambiguously that the equatorial oxygen atom of a dibutylstannylene acetal from an isolated cis-diol on a pyranose ring is much less reactive than the equatorial oxygen atom in an isolated trans-diol that was adjacent to a protected axial oxygen atom (see Scheme 5.1.20).76 Consistent with this conclusion are the many observations that in reactions conducted in the presence of added nucleophiles on trans1,2-stannylene acetals adjacent to protected axial oxygen atoms, the preferred reaction site is next to the axial oxygen.91,52,45 Ph HO

OBn O

HO

Bn O

O

O

+

5 OBn (1 mmole)

O

OBn

HO

6 (1 mmole)

BzCl Bu 2SnO (1 mmole) (1 mmole)

OH

benzene (reflux until clear) HO

rt Ph OBn

O

O

+

BzO

Bn O 5%

O

OBn

O OBn

BzO OH

75%

Scheme 5.1.20 In competition reactions, stannylene acetals derived from isolated cis-diols are less reactive than those from trans-diols76

Other reactions in which added nucleophiles or bases are present, notably p-toluenesulfonation, give the same regioselectivity observed for alkylation.21,92,65,71,93,16 Dialkylations can also be achieved on formation of dialkylstannylene acetals, usually with the regioselectivity expected on the basis of reactions of monostannylene acetals (Schemes 5.1.21 and 5.1.22).94,95 Interestingly, poylols can be alkylated until only one oxygen atom is left unsubstituted, if the monostannylene acetal is reacted under forcing conditions with base present to remove byproduct hydrogen bromide (Scheme 5.1.23).96 This reaction presumably occurs via initial reaction to give a benzyl ether

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OH

/toluene, reflux

O

HO HO

HO

OBn O

HO HO

BnO

2. BnBr, 85 °C, 24 h

OMe

511

OMe

82%

Di-O-benzylation using benzyl bromide as the reagent and the solvent 94

Scheme 5.1.21

1. 3 eq Bu 2SnO /MeOH, reflux

OH

HO

OH

HO

2. BnBr (4 eq), Bu4NBr /toluene, reflux, 4 h

OH

HO BnO

OBn 70%

Di-O-benzylation of pentaerythritol 94

Scheme 5.1.22

plus a bromodibutylstannyl ether, which can undergo an intra-molecular cyclization reaction with a free hyroxyl group with loss of HBr. The reaction only proceeds if the HBr is removed, but stops when cyclization is no longer possible.96 1. 1 eq Bu 2 SnO /MeOH, reflux, 2h

OH

HO HO

OH

OH

BnO BnO

2. BnBr (16 eq), Bu4NBr /xylene, reflux, 2 h

OBn 73%

3. (iPr)2EtN, reflux, 12 h

Scheme 5.1.23

Polyalkylation of pentaerythritol 96

Tributylstannyl Ethers

The reactions of tributylstannyl ethers have attracted less attention than those of stannylene acetals. The trends in regioselectivity observed have many similarities to those of the stannylene acetals.6 For instance, with methyl 4,6-O-benzylidene-α-d-glucopyranoside, both organotin intermediates show preferences for reaction with the oxygen atom O-2 adjacent to the axial glycoside.97,98 When tributyltin ethers are formed from compounds having a number of hydroxyls free, including the primary hydroxyl of a pyranose ring, reaction occurs on the primary hydroxyl,99,100 in contrast to the results with stannylene acetals (Schemes 5.1.24 and 5.1.25).100,101,102

OH HO HO

O HO OMe

Scheme 5.1.24

1. 1.5 eq (Bu3Sn)2O / toluene, reflux 2. BzCl (4 eq), toluene, —10 °C, 1.5 h

OBz HO HO

O HO OMe

73% (+ 20% 2,6)

Benzoylation via a tributylstannyl ether99

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OH HO HO

1. Bu2SnO (1 eq) /MeOH

O HO OMe

Scheme 5.1.25

2. BzCl (1.1 eq) /dioxane

OH O

HO HO

BzO OMe

76%

Benzoylation via a dibutystannylene acetal100

Tsuda and his coworkers have investigated oxidation reactions of stannyl ethers comprehensively.103 Treatment of methyl hexopyranosides with two equivalents of bis(tributyltin) oxide in chloroform, followed by reflux for 3 hours gave clear solutions that were cooled to 0 ◦ C and stirred with two equivalents of bromine until the color disappeared. Oxidation of secondary carbons bearing axial oxygen atoms of cis-diols is preferred over reaction either at secondary carbons bearing equatorial oxygen atoms or at primary carbon atoms. An additional, but lesser, preference is for oxidation at C-3 for equatorial glycosides and C-4 for axial glycosides.104,103 References 1. M.J. Selwyn, Biological Chemistry of Tin, in Chemistry of Tin, P.G. Harrison (Ed.), Blackie, Glasgow, 1989. 2. S. David and S. Hanessian, Tetrahedron, 41, 643 (1985). 3. M. Pereyre, J.P. Quintard, and A. Rahm, Organotin Alkoxides, in Tin in Organic Synthesis, Butterworths, London, 1987. 4. T.B. Grindley, Applications of Stannyl Ethers and Stannylene Acetals to Oligosaccharide Synthesis, in Synthetic Oligosaccharides: Indispensable Probes for the Life Sciences, P. Kov´ac (Ed.), ACS, Washington, 1994. 5. S. David, Selective O-Substitution and Oxidation Using Stannylene Acetals and Stannyl Ethers, in Preparative Carbohydrate Chemistry, S. Hanessian (Ed.), Marcel Dekker, New York, 1996. 6. T.B. Grindley, Adv. Carbohydr. Chem. Biochem., 53, 17 (1998). 7. M.W. Bredenkamp, S. Afr. J. Chem., 52, 56 (1999). 8. IUPAC. Commission on the Nomenclature of Organic Compounds, R. Panico, W.H. Powell, and J.-C. Richer, A Guide to IUPAC Nomenclature of Organic Compounds, Blackwell Scientific, Oxford, UK, 1993. 9. D. Wagner, J.P.H. Verheyden, and J.G. Moffatt, J. Org. Chem., 39, 24 (1974). 10. S. David, C. R. Acad. Sci., Ser. C, 278, 1051 (1974). 11. O. Kjølberg and K. Neumann, Acta Chem. Scand., 47, 721 (1993). 12. P. Kov´ac and K.J. Edgar, J. Org. Chem., 57, 2455 (1992). 13. G. Yang, F. Kong, and S. Zhou, Carbohydr. Res., 211, 179 (1991). 14. G.-J. Boons, G.H. Castle, J.A. Clase, P. Grice, S.V. Ley, and C. Pinel, Synlett, 913 (1993). 15. G.-J. Boons, G.H. Castle, J.A. Clase, P. Grice, S.V. Ley, and C. Pinel, Synlett, 764 (1994). 16. M. Kawana, M. Tsujimoto, and S. Takahashi, J. Carbohydr. Chem., 19, 67 (2000). 17. A. Morcuende, S. Valverde, and B. Herrad´on, Synlett, 89 (1994). 18. B. Herrad´on, A. Morcuende, and S. Valverde, Synlett, 455 (1995). 19. A. Morcuende, M. Ors, S. Valverde, and B. Herrad´on, J. Org. Chem., 61, 5264 (1996). 20. L. Ballell, J.A.F. Joosten, F.A. el Maate, R.M.J. Liskamp, and R.J. Pieters, Tetrahedron Lett., 45, 6685 (2004). 21. M.J. Martinelli, R. Vaidyanathan, J.M. Pawlak, N.K. Nayyar, U.P. Dhokte, C.W. Doecke, L.M.H. Zollars, E.D. Moher, V. van Khau, and B. Kosmrlj, J. Am. Chem. Soc., 124, 3578 (2002). 22. E. Fasoli, A. Caligiuri, S. Servi, and D. Tessaro, J. Mol. Catalysis A, 244, 41 (2006). 23. E. Fasoli, A. Arnone, A. Caligiuri, P. D’Arrigo, L. de Ferra, and S. Servi, Org. Biomol. Chem., 4, 2974 (2006). 24. B. Bucher and D.P. Curran, Tetrahedron Lett., 41, 9617 (2000). 25. J. Otera, Acc. Chem. Res., 37, 288 (2004).

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Tin Chemistry: Fundamentals, Frontiers and Applications C.W. Holzapfel, J.J. Huyser, T.L. van der Merwe, and F.R. van Heerden, Heterocycles, 32, 1445 (1991). M.W. Bredenkamp, H.S.C. Spies, and M.J. van der Merwe, Tetrahedron Lett., 41, 547 (2000). D.A. Leigh, R.P. Martin, J.S. Smart, and A.M. Truscello, J. Chem. Soc., Chem. Commun., 1373 (1994). S. David and A. Malleron, Carbohydr. Res., 329, 215 (2000). S. Roelens, J. Org. Chem., 61, 5257 (1996). S. David and A. Thi´effry, J. Chem. Soc., Perkin Trans., 1, 1568 (1979). X. Kong and T.B. Grindley, J. Carbohydr. Chem., 12, 557 (1993). A.B. Reitz and E.W. Baxter, Tetrahedron Lett., 31, 6777 (1990). R.D. Groneberg, T. Miyazaki, N.A. Stylianides, T.J. Schulze, W. Stahl, E.P. Schreiner, T. Suzuki, Y. Iwabuchi, A.L. Smith, and K.C. Nicolaou, J. Am. Chem. Soc., 115, 7593 (1993). A. Glen, D.A. Leigh, R.P. Martin, J.S. Smart, and A.M. Truscello, Carbohydr. Res., 248, 365 (1993). G. Reginato, A. Ricci, S. Roelens, and S. Scapecchi, J. Org. Chem., 55, 5132 (1990). M.W. Bredenkamp, S. Afr. J. Chem., 48, 154 (1995). H. Dong, Z.C. Pei, S. Bystrom, and O. Ramstrom, J. Org. Chem., 72, 1499 (2007). S.J. Danishefsky, R. Hungate, and G. Schulte, J. Am. Chem. Soc., 110, 7434 (1988). J. Alais, A. Maranduba, and A. Veyri`eres, Tetrahedron Lett., 24, 2383 (1983). F.A.W. Koeman, J.W.G. Meissner, H.R.P. van Ritter, J.P. Kamerling, and J.F.G. Vliegenthart, J. Carbohydr. Chem., 13, 1 (1994). P. Fern´andez, J. Jim´enez-Barbero, and M. Mart´ın-Lomas, Carbohydr. Res., 254, 61 (1994). A.G. Gon¸calves, M.D. Noseda, M.E.R. Duarte, and T.B. Grindley, J. Org. Chem., 72, 9896 (2007). R.M. Munavu and H.H. Szmant, J. Org. Chem., 41, 1832 (1976). M.J. Martinelli, R. Vaidyanathan, and V. van Khau, Tetrahedron Lett., 41, 3773 (2000). R.L. Halcomb, S.H. Boyer, M.D. Wittman, S.H. Olson, D.J. Denhart, K.K.C. Liu, and S.J. Danishefsky, J. Am. Chem. Soc., 117, 5720 (1995). H. Qin and T.B. Grindley, J. Carbohydr. Chem., 13, 475 (1994). S. David, Carbohydr. Res., 331, 327 (2001). H. Al-Mughaid and T.B. Grindley, Carbohydr. Res., 339, 2607 (2004). T. Ogawa and T. Kaburagi, Carbohydr. Res., 103, 53 (1982). F. Dasgupta and P.J. Garegg, Synthesis, 1121 (1994). T. Ogawa and M. Matsui, Tetrahedron, 37, 2363 (1981). Y. Tsuda, M.E. Haque, and K. Yoshimoto, Chem. Pharm. Bull., 31, 1612 (1983). S. Langston, B. Bernet, and A. Vasella, Helv. Chim. Acta, 77, 2341 (1994). M.E. Haque, T. Kikuchi, K. Yoshimoto, and Y. Tsuda, Chem. Pharm. Bull., 33, 2243 (1985). Y. Tsuda, M. Hanajima, N. Matsuhira, Y. Okuno, and K. Kanemitsu, Chem. Pharm. Bull., 37, 2344 (1989). H.-M. Liu, Y. Sato, and Y. Tsuda, Chem. Pharm. Bull., 41, 491 (1993).

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515

Reactions of SE Substitution for Organostannanes in Organic Synthesis

David R. Williams, and Partha P. Nag Department of Chemistry, Indiana University, Bloomington, IN, USA

5.2.1

Introduction

Among categories of reactions that are used to classify the electrophilic substitution processes1 of organotin compounds,2 the SE reactions of allylstannanes are particularly noteworthy.3 In this transformation, initial electrophilic π-complexation of a carbon–carbon double bond leads to electron deficiency at the carbon site which is vicinal to the C Sn bond. Stabilization of the developing β-carbocation occurs with alignment of the C Sn single bond, and orbital overlap provides a description of a non-classical cation in which the stannyl substituent carries a substantial fraction of the positive charge. Formation of a carbon–carbon double bond proceeds with loss of the stannyl cation, and thus, electrophilic substitution of the tin substituent has taken place with allylic (SE ) transposition. The mechanistic rationale suggests that the departing substituent must readily abandon its bonding electron pair. In fact, organostannanes are generally well suited to perform electrophilic substitution reactions because of the highly polarized C Sn bond and the ability to accommodate an electropositive metal center. Allylic transposition processes have been extensively reviewed for a variety of allyl metal and allylic non-metal species. The most rudimentary example of allylic transposition is the Alder-ene reaction.4 On the other hand, the SE reactions of allylic organostannanes are most often compared with a significant body of information for the reactions of allylic silanes, and this latter category has been the subject of several important reviews.5 In this chapter, an overview of the electrophilic substitution chemistry of allyltin compounds leading to the formation of a new C C bond is presented. Mechanistic considerations and a synopsis of information derived from detailed studies of allyltin reactions develop a perspective, which serves as a guide for planning new reactions. The major emphasis of the chapter is devoted to Lewis acid-promoted allylation reactions of aldehydes and related species. An important section of the chapter has been dedicated to the SE reactions of allenylstannanes, and the SE reactions of propargylic organostannanes are briefly reviewed as a transformation, which efficiently enables the preparation of allenes.6 The chapter concludes with a summary of promising opportunities for enantiocontrolled processes. General comments regarding practical aspects of the applications of laboratory techniques in this area are appropriate. Many organostannane reagents are liquids and are easily transferred under anhydrous conditions, without the need of special techniques or apparatus. Allylic organostannanes are usually lipophilic in nature and mixtures of these compounds often pose problems for chromatographic separation techniques. Flash silica gel chromatography is commonly practiced as a means for preparative purification, but may lead to protodestannylation in some cases. Destannylation can usually be suppressed by prior treatment of silica gel with eluent containing small amounts (0.5 to 1.0% by volume) of triethylamine. Allylic stannanes are characterized by high reactivity at −78 ◦ C with many common electrophilic components, and, in some cases, facile reactions have been noted at −90 ◦ C. Reactions with aldehydes are promoted by the presence of Lewis acids. Thus practitioners may take advantage of the high intrinsic reactivity of allylstannanes in order to select among several Lewis acids to optimize the SE process. Mild conditions and low reaction temperatures provide features of functional group compatibility and high stereoselectivity. These characteristics suggest that SE reactions of allylstannanes are among the most synthetically useful of carbon–carbon bond-forming techniques. Indeed, the versatility of allylic stannanes has led to widespread applications in the synthesis of complex molecular constructions.3 However,

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issues that relate to the toxicity, particularly in the case of volatile organostannanes, and the disposal of these substances, are important concerns, which currently hinder the deployment of reactions in chemical process applications.7 5.2.2

Mechanistic Considerations and a Predictive Model for Reactions with Aldehydes

The most widely utilized SE reaction of allylic stannanes 1 is an alkylation process featuring aldehyde 2 as the electrophilic component (Scheme 5.2.1). Lewis-acid activation leading to the alcohol 3 was first described in 1979 by Naruta,8 and by Sakurai and Hosomi.9 Subsequent reports by Yamamoto investigated the stereoselectivity of BF3 rOEt2 -promoted reactions of 2-butenylstannanes with achiral aldehydes.10 High diastereoselectivity was observed with formation of syn-homoallylic alcohols regardless of the starting alkene geometry. Yamamoto postulated that an acyclic transition state aligned the carbonyl and alkenyl double bonds in an antiperiplanar arrangement, with the minimization of non-bonded interactions. Pre-complexation to provide an oxocarbenium ion was a significant activation event, and the structure of the Lewis acid complex would become an important consideration.

Scheme 5.2.1

Reactions of allylic stannanes with aldehydes

The addition processes are characterized as Type II allylation reactions and feature open transition states.3 Denmark11 and Keck12 have described mechanistic studies, which have provided basic information shaping current views regarding the Type II allylations of organostannanes. Denmark’s models are briefly summarized by the cyclization of deuterium-labeled 4 (Scheme 5.2.2). Intra-molecular allylation proceeds with Lewis and Brønsted acids to yield 6 with high selectivity. The alcohol 6 is rationalized through the synclinal transition state 5 via anti-SE substitution. To a lesser degree, various Lewis acids also produce the

Scheme 5.2.2

Denmark’s mechanistic studies of SE reactions with aldehydes

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diastereomeric alcohol 8 from 4, and this product is believed to arise from the antiperiplanar arrangement in 7 with anti-SE destannylation. Keck examined allylations with enriched mixtures of E- and Z -2-butenylstannanes 9 and several simple aldehydes 10.12 a Reactions were generally selective for production of the syn-homoallylic alcohols 11, and diastereoselectivity was enhanced by higher proportions of the E-2-butenylstannanes (Scheme 5.2.3). The analysis recognizes three staggered rearrangements 13, 14 and 15, which may contribute to the formation of syn-11. The preference of one transition state over another is dependent on non-bonded interactions and especially the interactions introduced by Lewis acid complexation.

Scheme 5.2.3

Allylation of aldehydes with enriched E/Z-mixtures of 2-butenylstannanes

Keck has also described a mechanistic study of the intra-molecular allylations of Z -16 and E-19 under a variety of conditions.12 b These results are summarized in Scheme 5.2.4, as the Z -16 consistently gave the major product 18 upon treatment with Brønsted and Lewis acids. The observed stereoselectivity is rationalized through the synclinal arrangement 17.

Scheme 5.2.4

Keck’s mechanistic studies of intramolecular SE allylations

However, E-19 produces all four possible cyclohexanol diastereomers in ratios which vary based on the choice of Brønsted or Lewis acid. Major products in the latter reactions are 21 and 23 (dr ∼ = 2:1 with BF3 • OEt2 and MgBr2 • OEt2 ), which also arise via synclinal 20 and 22, respectively. The studies by

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Denmark and Keck provide important evidence of the synclinal model as the preferred open transition state, and it has been proposed that attractive forces of secondary orbital interactions of the HOMO at the site of the stannyl methylene with the LUMO of the carbonyl oxygen may introduce a stabilizing contribution in this arrangement.12 b Further evidence of inherent advantages in the synclinal transition state is acquired from the efforts of Nishigaichi.13 These results demonstrate stereospecificity in the allylation reactions of (E)- and (Z )-3,3-dialkyl-substituted allylic stannanes with a number of simple aldehydes. A brief description of the mechanistic rationale for thermal reactions of aldehydes and allylic stannanes is distinguished from Lewis acid-promoted additions by the supposition of closed cyclic transition states.14 The Lewis acidity of tin provides for pre-organization and the E- or Z -geometry of the starting allylic stannane is reflected in the formation of anti- and syn-allylation products 26 and 29 (Scheme 5.2.5).

Scheme 5.2.5

Thermal reactions of E/Z-2-butenylstannanes with aldehydes

Elegant labeling studies by Denmark, summarized in Scheme 5.2.1, showed that the intra-molecular thermal allylation from 4 exclusively provides the corresponding E-isomer of 6 via a syn-SE reaction, indicative of the internal C→O transfer of the stannyl residue. Keck also reported the formation of 18 by thermolysis of Z -16 (Scheme 5.2.3). However, in this study the thermal cyclization of isomeric E-19 yielded largely axial alcohol 21, which cannot be rationalized through an internal migration of tin to the carbonyl oxygen. Numerous investigations of reactions of allylic stannanes with aldehydes have demonstrated that changes in steric interactions and electronic effects are introduced by the coordination of Lewis acids. In fact, the oxocarbenium species serves as the reactive partner in the allylation process, and deficiencies of detailed knowledge regarding the structure, reactivity, and steric requirements of oxocarbenium complexes leads to some uncertainties in providing a simple model to forecast stereoselectivity. Based upon a body of results accumulated from a variety of investigators, the pre-complexation with a specific Lewis acid and aldehyde may sustain the inherent attributes of the synclinal transition state, or the coordination complex may present features that override this tendency in favor of antiperiplanar arrangements. A general model has evolved to serve as a guide of predictive value for assessing stereoselectivity and for designing improved methodology. This information is summarized in Scheme 5.2.6. Oxonium ions A and B illustrate precomplexation of aldehyde in which a sterically demanding substituent (MLn ) is located syn, with respect to the adjacent aldehydic hydrogen. This arrangement features the most stable disposition for complexation of Lewis acids with simple aldehydes or the preferred geometry of oxocarbenium

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A general model for assessing stereoselectivity in allylation reactions of aldehydes

ion formation upon O-alkylation with highly reactive electrophiles. Staggered open transition states for the bimolecular reaction are considered by minimizing potential non-bonded interactions. The small vinylic substituent of the allylic stannane, usually hydrogen, is placed in the region occupied by the large MLn . While the synclinal arrangement B is favored, the nature of the R1 and R2 substituents may impose steric interactions which destabilize B versus the antiperiplanar arrangement A. In this regard, the prospects for a Felkin–Anh addition via A or B are considered for aldehydes possessing α-chirality as a result of non-chelating substituents. Alternatively, the oxocarbenium species may be represented by geometrical arrangements in C and D. These situations frequently occur when bidentate Lewis acids are internally coordinated by heteroatoms located within the R1 substituent as in the established models for α- and β-chelation control in carbonyl addition reactions. However, the model is also relevant for oxocarbenium species derived from solvolysis of cyclic acetals or the generation of iminium intermediates. In these cases, the small vinylic substituent (hydrogen) occupies a placement over the tethered ring system, and the antiperiplanar arrangement in C is preferred when R2 is the more sterically demanding substituent. 5.2.3

Allylation Reactions of Substrate Control

Lewis acid-promoted reactions of C-1 substituted allylic stannanes with simple aldehydes predominantly give rise to syn condensation products, as predicted from synclinal arrangements and supported by the studies of Denmark and Keck. In this section, examples of substrate control will be discussed in which the reacting aldehyde does not contain proximate sites for internal chelation of the Lewis acid. Monodentate coordination of Lewis acids will lead to an oxocarbenium ion of E-geometry by complexation of the metal adjacent to the aldehydic hydrogen. Boron trifluoride etherate is frequently employed for activation, however, other Lewis acids used in this manner may include TiCl4 , SnCl4 , ZnBr2 , and MgBr2 •OEt2 . Facial selectivity is generally determined by a Felkin–Anh approach. In this regard, it is important to recognize that silyl ethers rarely participate in α- or β-chelation.15 Thus, the reaction of aldehyde 30 with a mixture of E/Z -crotyltri-n-butylstannanes (E/Z = 55:45) yields predominantly the syn-product 31 via the arrangement in 32.16 However, the choice of Lewis acid in the case of 33 also dictates formation of Felkin product 34 (Scheme 5.2.7).17

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Scheme 5.2.7

Reaction of aldehydes 30 and 33 with a mixture of E/Z-crotyltri-n-butylstannanes

Felkin–Anh addition in the case of the α-silyloxy aldehyde 35 leads to the anti stereochemistry in the homoallylic alcohol 36 as depicted in the antiperiplanar arrangement 37 (Scheme 5.2.8).18

Scheme 5.2.8

Felkin–Anh addition of allyltributyltin to the α-silyloxy aldehyde 35

Similarly, non-racemic 2,3-epoxyaldehyde 38 displays a preference for Felkin–Anh addition with the bis-stannylated 39 yielding the highly functionalized epoxy-alcohol 40 (Scheme 5.2.9).19

Scheme 5.2.9

Felkin–Anh addtion of bis-stannylated 39 to the 2,3-epoxyaldehyde 38

Reetz and coworkers reported the stereoselective formation of 1,3-anti-alcohol [dr 85:15], resulting from BF3 •OEt2 catalysis in the allylation of (R)-3-benzyloxybutanal with allyltri-n-butylstannane.20 Subsequently, Evans and coworkers explained the observed 1,3-asymmetric induction by the consideration of conformations which minimize dipole interactions of the β-alkoxy substituent and the oxocarbenium ion.21 In fact, reactions of aldehyde 41 with allyl and methallyltri-n-butylstannane demonstrate outstanding selectivity owing to reinforcing stereogenicity. The features of merged 1,2- and 1,3-asymmetric induction, as shown in 43 provide for Felkin–Anh addition via the antiperiplanar transition state in the case of the 2-(R)-methyl diastereomer 41 (Scheme 5.2.10). The corresponding 2,3-syn diastereomer of aldehyde 41 reacted with anti-Felkin stereocontrol. The reaction of (E/Z )-2-butenyltri-n-butylstannane with aldehyde 44, as reported by Keck and coworkers22 occurred with 1,3-asymmetric induction to yield 45 via the synclinal arrangement, illustrated from the Evans model 46 (Scheme 5.2.11).

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OPMB

SnBu3

CH3

H CH3

OH

CH3

CH3

41

H

δ+ O

CH3

BF3•OEt2, CH2Cl2 95%, dr > 99:1

SnBu3

H

R

OPMB

521

H

F3B

CH3

H

42

H3 C

OPMB CH3

43 R = CH3

Scheme 5.2.10

Felkin–Ahn addition of allyltri-n-butylstannane to the 2-(R)-methyl diastereomer 41 Bu3Sn

TBSO

TBSO

CH3 H 3C

H3C

CHO CH3O

OPMB

SnBu3

BF3•OEt2, CH2Cl2 –78 °C, 64%

44

CH3

CH3

CH3O

OPMB OH 45 (major)

CH3

H

H

H3 C

H

δ+ O F3B

H OPMB

H

R 46

Scheme 5.2.11

Reaction of (E/Z)-2-butenyltri-n-butylstannane with aldehyde 44

These characteristics of reinforcing features for 1,2- and 1,3-asymmetric induction are applied in the case of complex aldehyde 47, in combination with the steric preference for a synclinal arrangement in the C-1 substituted stannane 48, to afford high diastereoselection in the synthesis of alcohol 49, a segment of spongistatin 1 (Scheme 5.2.12).23

Scheme 5.2.12

5.2.4

Reinforcing features for asymmetric induction leading to 49

Allylation Reactions Exhibiting α-Chelation Control

Aldehydes that contain a heteroatom substituent at the α-carbon often display high stereoselectivity in reactions with allylic stannanes. This behavior is particularly the case for heteroatom substituents permitting effective chelation with a Lewis acid. Internal activation of the carbonyl oxygen provides a five-membered chelation complex with Lewis acids, which minimally offer two coordination sites. The stability of the metallocycle may account for high diastereoselection, as nucleophilic approach of the stannane occurs to the less hindered face of the carbonyl. Keck16 described studies of chelation-controlled additions of enriched mixtures of E- and Z -crotyl tri-n-butylstannanes. Complexation of the α-benzyloxyaldehyde 51 with MgBr2 •OEt2 led to selective

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formation of the syn,syn-adduct 52 via the antiperiplanar transition state 54. Similarly, the reaction of stannane 55 with aldehyde 56 yielded the syn,syn-adduct 57 via the antiperiplanar transition state 58 in which the OTBS group is presumably the more sterically demanding substituent (Scheme 5.2.13).24

Scheme 5.2.13 aldehyde 51

Scheme 5.2.14

Chelation-controlled addition of E- and Z-crotyltri-n-butylstannanes to α-benzyloxy-

Chelation-controlled reaction of γ -alkoxy-Z-stannane with α-benzyloxyaldehyde 56

Keck25 also examined the behavior of four Lewis acids in one of the first examples of allylations with α-benzyloxy aldehyde 59. The chelation-controlled pathway is most selective with MgBr2 •OEt2 producing the expected syn,syn-product 61 via 63. The use of ZnI2 or TiCl4 led to mixtures of 61 and 62, owing to the competing pathway proceeding via the synclinal transition state 64 (Scheme 5.2.15).

Scheme 5.2.15

A study of Lewis acids for α-chelation controlled reactions

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Similarly the reaction of (E)-2-butenyl-tri-n-butylstannane with 65 afforded the products of a chelationcontrolled addition in the presence of MgBr2 •OEt2 , favoring the syn-adduct 66 (Scheme 5.2.16).26

Scheme 5.2.16

Chelation-controlled diastereoselectivity in the reaction of aldehyde 65

In addition, recent studies towards the synthesis of bryostatin 1 have described high diastereoselectivity for the chelation-controlled condensation of the substituted stannane 68 with ent-33 leading to the desired syn-product 69 via 70 (Scheme 5.2.17).27 While ketones have rarely been explored as reactive electrophiles for stereoselective allylations, proximate coordination of a Lewis acid through α-chelation can produce favorable results, as illustrated in the exclusive formation of the tertiary homoallylic alcohol 72 from 71 (Scheme 5.2.17).28

Scheme 5.2.17

High diastereoselectivity for the chelation-controlled allylations

Cyclic ethers also serve as excellent ligands for chelation-controlled reactions, and this technique is especially useful for the synthesis of carbohydrate derivatives and polyether antibiotics. Studies toward ciguatoxin (Scheme 5.2.18) illustrate the oxidative cleavage of alkene 73 to provide the α-alkoxy aldehyde 74 for subsequent chelation-controlled allylation to yield 75.29

Scheme 5.2.18

Chelation-based stereocontrol in the allylation of tetrahydropyranyl carboxaldehyde 74

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Efforts toward antascomicin B have incorporated the allylation of complex aldehyde 76 with a mixture of E- and Z -allylic stannanes 77, as a key convergent operation in the successful total synthesis.30 High stereocontrol is observed in an SE process dominated by the features of α-chelation in 79 (Scheme 5.2.19).

Scheme 5.2.19

5.2.5

Crotylation studies toward antascomicin B

Allylation Reactions Exhibiting β-Chelation Control

Allylation reactions can be designed to effect high stereoselectivity in the case of chiral β-alkoxy aldehydes, in which the ether oxygen provides for effective coordination with a Lewis acid. Multi-valent, oxophilic Lewis acids serve to pre-organize the aldehyde substrate in a six-membered chelation complex. As in the examples of α-chelation control, an open transition state is deployed with synclinal or antiperiplanar orientations based upon the consideration of steric interactions with placement of the small (hydrogen) vinyl substituent of the allylic stannane over the preformed metallocycle. Several examples are illustrated in Scheme 5.2.20.31 Although β-chelation-controlled reactions have not been as widely studied as α-chelation processes,25 c this technique has considerable potential, with optimization of solvent polarity, Lewis acidity, and coordination geometry of the metal cation. Results suggest that β-silyl ethers, which are generally employed as protecting groups in organic synthesis, are uncommon contributors for stable βchelation. While MgBr2 , TiCl4 , and SnCl4 are among the most utilized Lewis acids for this purpose, practitioners must be aware of the opportunity for facile transmetalation of the allylic stannane in the latter two examples. The presence of an alkyl (methyl) substituent at the α-carbon of the starting aldehyde is sufficient to direct 4,5-anti-stereoselectivity as shown in 81, 89, and 92 of Scheme 5.2.20. Aldehydes lacking substitution at the α-position react efficiently with somewhat diminished stereocontrol as shown in the allylation of 84 (Scheme 5.2.20). On the other hand, aldehydes exhibiting 2,3-syn-substitution, such as 91, provide for a chelation complex 93, which reinforces high facial selectivity as shown in the generation of the 4,5-anti alcohol 92.22 Studies directed toward the synthesis of bryostatin 1 have described the optimization of the coupling of key fragments 94 and 95.32 A dramatic improvement in diastereoselectivity is obtained in formation of 96 with the use of toluene to maximize the efficiency of β-chelation (Scheme 5.2.21). Excess Me2 AlCl (5.0 equiv.) is used due to the disproportionation of this Lewis acid following complexation, as well as the possibilities of sequestration at other sites. Yamamoto has reported the desymmetrizing intra-molecular cyclization of the 1,3-diketone 97 for diastereoselective formation of cis–cis 98 vs. cis–trans 99. Optimized conditions with SnCl4 led to the β-chelation in the synclinal arrangement 100 to give 98, whereas the use of TiCl4 facilitated rapid transmetalation of the organostannane yielding 99 (Scheme 5.2.22).33

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Scheme 5.2.20

Scheme 5.2.21

Examples of β-chelation control

The use of β-Chelation control in the formation of 96

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Scheme 5.2.22

5.2.6

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Desymmetrization of 1,3-diketone 97 via intramolecular allylation

Reactions of γ-(Alkoxy)allylstannanes

A number of investigations have explored the reactions of allylic stannanes containing a γ -alkoxy substituent. A direct preparation of these substances utilizes the kinetic deprotonation of an allyl ether followed by alkylation with tri-n-butylstannyl chloride. In a typical experiment, the deprotonation of 101 with S-butyllithium leads to internal coordination of lithium cation and provides formation of the Z -allylstannane 102. The behavior of γ -alkoxyallylstannanes is similar to the corresponding Z -alkylstannanes, and as a result, the reaction provides a stereoselective route for the synthesis of complex diol derivatives. In the allylation of chiral aldehyde 80 with stannane 102, β-chelation dictates face selectivity. The expected syn,anti-product 104 is obtained with high diastereoselection via the antiperiplanar 103, which accommodates the sterically demanding silyl (TBS) ether (Scheme 5.2.23).23

Scheme 5.2.23

β-Chelation-control in the utilization of substituted stannane 102

The palladium-catalyzed stannylation of allenes is an efficient approach for the preparation of γ -substituted allylic stannanes.34 Williams has recently described the bis-stannylation of 1methoxymethyleneoxyallene yielding (E)-105, as well as the use of this functionalized stannane in allylation reactions.35 Pre-organization of the aldehyde by α-chelation provides for the synclinal transition state 108 leading to anti,syn-adduct 107 (Scheme 5.2.24). The mild conditions of the reaction retain the alkenylstannane of the product for further elaboration via cross-coupling reactions.

Scheme 5.2.24

Allylation using bis-stannylated (E)-105

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Additional examples, shown in Scheme 5.2.25, illustrate α-chelation control in the formation of 111 and 114,36 , 37 as well as the inclusion of chirality in the γ -alkoxy substituent in 115 leading to 116,38 and a role for coordination in the case of the α-amido substituent of 118, which yields selective formation of diol derivative 119 via 121.39

Scheme 5.2.25

Examples of reactions of γ -(alkoxy)allylstannanes

Yamamoto has investigated the intra-molecular condensation of γ -(alkoxy)allylstannanes with aldehydes as a general approach for the stereocontrolled synthesis of polycyclic ethers (Scheme 5.2.26).40 The strategy has been iteratively applied to the total synthesis of hemibrevetoxin B.41 In this fashion, the Lewis acid-promoted reaction of 122 leads to formation of seven-membered ether 123, which is transformed into 124 for intra-molecular allylation to the fused-oxepane 125. 5.2.7

Reactions of Oxocarbenium Cations

Cyclic oxocarbenium cations approximate the α- and β-chelation models for allylations of substituted aldehydes. In this analogy, the nature and the degree of substitution of the carbon bound to the oxonium site replaces the role of coordinated bidentate Lewis acids. Aside from the obvious concerns about the relative reactivity of 126 (Scheme 5.2.27), as compared with metal cation activation of precomplexed ˚ than distances associated with the aldehyde 127, the C O bond of 126 is much shorter (1.514 A)

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Scheme 5.2.26

Scheme 5.2.27

Yamamoto’s studies toward the synthesis of hemibrevetoxin

A model for the allylation reactions of oxocarbenium systems

metal–oxygen coordination. Furthermore expectations for reactions of 126 would include an analysis of contributing ring conformations and stereoelectronic control that delivers axial C C bond formation via a developing chair transition state, as illustrated in 128. Finally, oxocarbenium cations 126 are generated in a number of different ways, most of which employ Lewis acids. In these events, little is known about the nature and structure of the ion pair. In spite of a large number of variables, it is reasonable to consider formation of 129 via models for synclinal addition 130 and antiperiplanar addition 131, with the caveat that substituents located on the oxacycle will encourage diastereofacial selectivity via an axial alkylation. As in chelation models, the small (hydrogen) atom of the allylic stannane is positioned over the ring system, and 130 and 131 are evaluated based on non-bonded interactions. An early study of the allylation reactions of oxocarbenium systems has been described by Danishefsky for synthesis efforts directed towards indanomycin.42 The reactions of (E)-and (Z)-2-butenylsilanes silanes are the focus for this report. However, the study also examined reactions of the (E,Z)-mixture of 2butenyl-tri-n-butylstannanes with tri-o-acetyl-d-galactal 132 at −30 ◦ C, in the presence of BF3 •OEt2

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(Scheme 5.2.28). Anticipated production of oxonium 133 leads to the C-1 alkylation products 134 and 135 in a 3:1 ratio, which can be rationalized via synclinal 136 and antiperiplanar 137, respectively. The lack of facial basis in the oxocarbenium 133 leads equally to attack from above and below the plane of the conjugated cation, yielding an additional pair of syn and anti adducts analogous to 134/135 in similar ratio (dr 4:1). In related fashion, Rychnovsky has described the allylation reactions of 4-acetoxy-1,3dioxanes.43 The presence of the 5-methyl substituent in 138 requires treatment with reactive E-2-butenyl tri-n-butylstannane, affording 139 as the major product (dr 4:1) via the antiperiplanar transition state arrangement analogous to 137.

Scheme 5.2.28

Crotylation reactions of oxocarbenium systems

Marked differences in the yields and diastereoselectivity of reactions of allyl tri-n-butylstannane with various Lewis acids have been recorded. Selective C-glycosylation of 140 is attributed to neighboring group stabilization of the oxocarbenium ion 141, giving rise to the 2,5-cis-disubstituted tetrahydrofuran 142 (Scheme 5.2.29).44

Scheme 5.2.29

C-glycosylation via oxocarbenium ion 141

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The conjugate addition of allyl tri-n-butylstannane to pyranose 144 occurs in high yield under mild conditions, and exhibits impressive stereofacial selectivity corresponding to axial addition of the nucleophile in 145 to produce the 2,6-trans-pyran 146 (Scheme 5.2.30).84

Scheme 5.2.30

The conjugate addition of allyl tri-n-butylstannane to pyranose 144

Significant improvement is generally seen in face selectivity, when glycal epoxides are used to generate the reactive oxocarbenium intermediate. Evans and coworkers46 have advantageously utilized this method in studies for the total synthesis of spongistatin 2 (Scheme 5.2.31). Glycal 147 undergoes a highly selective epoxidation and treatment of 148 with tri-n-butylstannyl triflate and methallyltri-n-butylstannane affords 150. Effective formation of the oxonium species 149 retains the Lewis acid (Bu3 Sn) as a ligand, which may sterically impede a syn alkylation. The Evans study meets the demanding situation imposed by the total synthesis with the incorporation of the functionalized stannane 152 for stereoselective formation of the fully substituted pyran 153 (Scheme 5.2.31). These promising results warrant further consideration in related studies of E- and Z -crotylstannanes.

Scheme 5.2.31

Studies of allylation reactions via glycal epoxides

Brevetoxin B and related marine natural products present remarkable molecular architectures of transfused polyether skeletons. Kadota and Yamamoto have reviewed a substantial body of results for linear and convergent approaches for the synthesis of these substances based on intra-molecular allylations.40 b An attractive convergent concept illustrates the intra-molecular allylation of the oxocarbenium intermediate generated from α-acetoxyether 154 leading to stereocontrolled production of the divinyl ether 155. Ringclosing metathesis of 155 directly yields polycyclic ether 156 (Scheme 5.2.32).57

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Scheme 5.2.32

5.2.8

531

Intra-molecular allylations for the synthesis of polyether natural products

Reactions of N-Acyliminium Cations

N -Acyliminium ions are suitable partners for electrophilic addition reactions of allylstannanes. Although the corresponding silanes have been more widely studied in these cases,48 allylic stannanes are fully competent reaction participants, as illustrated by formation of the 1-azabicyclo[3.1.0]pentane 159 via intra-molecular cyclization of the E-allylic stannane 157 (Scheme 5.2.33). The reaction produces the vinyl cyclopropane under protic acid conditions with complete stereocontrol.49

Scheme 5.2.33

Intramolecular allylation of E-stannane 157 via the N-acyliminium species 158

In spite of the success of the intra-molecular cyclization, the generation of the reactive N -acyliminium ion under aprotic conditions is desirable, to avoid competing protodestannylation. Yoshida and coworkers have demonstrated a three-component coupling in which the enamide 160 reacts with the N -acyliminium ion 161 yielding a stabilized N -acyliminium species 162, which is captured by allyl-tri-n-butylstannane to give 163 (Scheme 5.2.34).50

Scheme 5.2.34

Yoshida’s three-component coupling

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Lewis acids are commonly used to initiate solvolysis, as in the production of N -acyliminium species 165, and diastereoselectivity is observed based on the minimization of steric factors in 166. In the N Boc derivative 165, the vicinal methyl substituent of the half-chair conformer is placed axially to avoid in-plane A1,2 strain, suggesting the development of 1,3-diaxial interactions in 166 leading to 167.51

Scheme 5.2.35

Stereocontrol in the allylation of cyclic, substituted N-acyliminium 165

Coleman and coworkers52 have described a convergent synthesic route toward the mitomycin antibiotics, which features an effective allylation strategy from the ketal 168 and 169. Solvolysis generates the N -acyliminium 170, and the reaction proceeds via synclinal 171, smoothly providing excellent stereocontrol in the coupled product 172 (75% yield) (Scheme 5.2.36).

Scheme 5.2.36

5.2.9

Coleman’s convergent synthesis route toward the mitomycin antibiotics

Reactions with α, β-Unsaturated Carbonyl Compounds

The 1,4-conjugate SE addition of allylic stannanes is a successful variant of the Michael reaction, particularly in examples which provide stable coordination complexes with strong Lewis acids. For the most part, simple allylstannanes have been utilized in these reactions. The addition of (E)-2-butenyltrin-butylstannane to diethyl ethylidenemalonate (173) proceeds in modest yield with high stereoselectivity to give the anti-isomer 174 via the antiperiplanar arrangement 175 (Scheme 5.2.37).53

Scheme 5.2.37

The addition of (E)-2-butenyltri-n-butylstannane to 173

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Williams54 has reported an enantiocontrolled approach by low temperature formation of a sixmembered coordination complex of chiral N -enoyl-1,3-oxazolidin-2-ones with ZrCl4 . Conjugate SE additions of a variety of substituted allylic stannanes occur with moderate to high stereoselectivity based on the oxazolidinone auxiliary, as shown by the conversion of 176 to 177 (Scheme 5.2.38). The precise role of the chiral auxiliary for determining the facial selectivity in the reaction is not fully understood.

Scheme 5.2.38

Diastereoselective conjugate SE addition to 176

Intra-molecular examples provide an annulation process which is also feasible in cases of less reactive β,β-dialkyl enone substitution, as shown by the transformation of 178 to the cis-fused ketone 179 (Scheme 5.2.39).55

Scheme 5.2.39

5.2.10

Annulation via intramolecular allylation from 178

Reactions of Allylic Stannanes with Imines

Under thermal conditions, imines do not react with allyl-tri-n-butylstannane. However, complexation of imines with strong Lewis acids leads to iminium cations, which are reactive electrophiles with allylic

Scheme 5.2.40

Mechanistic considerations for reactions of E/Z-2-butenylstannanes with imines

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stannanes.56 These allylation processes have not been widely studied. In the case of acyclic aldimines, one challenging aspect is illustrated by the facile isomerization of E- and Z-isomers 182ab (Scheme 5.3.40). Thus, the syn-amine 183 is the major product of the addition of (E,Z)-2-butenylstannane with 181.57 However, the product ratio is dependent on the time allowed for pre-complexation with TiCl4 . The initial complex 182a may provide for the staggered open transition states 185 and 186 leading to syn and anti products, respectively. On the other hand, isomerization to the more stable E-iminium ion 182b, based on steric requirements of the Lewis acid, significantly favors the synclinal arrangement 187 leading to syn-183. As a predictive model, the considerations of stereochemical features and internal chelation, as applied to the activation of aldehydes are also valid for assessing facial selectivity in these allylation reactions. A starting point for the evaluation of synclinal and antiperiplanar transition states lies in recognizing the steric requirements for Lewis acid coordination. In the case of imines, factors that dictate the geometry of the reactive iminium electrophile are significant, and these experiments may be complicated by competing transmetalation of the allylic stannane. The presence of a coordinating ether can effect α- or β-chelation, as shown for imine 188. The stereochemical features of the stable six-membered metallocycle from 188 provide for diastereofacial selectivity, as illustrated in the formation of 189 (Scheme 5.2.41).58

Scheme 5.2.41

Diastereocontrol in the allylation of imine 188

Benzaldimines have been reported to give high yields of condensation products with allylic stannanes, in the presence of palladium catalysts, under neutral conditions.59 Studies support transmetalation of the stannane to yield a bis-π-allylpalladium complex, which binds imine 190 for allyl transfer. In the case of the catalyst 191, the chiral allyl ligand is not transferable, but determines facial selectivity in the optically enriched amine 192 (80% ee) (Scheme 5.2.42).

Scheme 5.2.42

Enantioselective allylations using catalyst 191

Yamamoto and coworkers60 have demonstrated intra-molecular examples of the addition of allylic stannanes to iminium ions with good to excellent yields. The incorporation of chirality using (R)-(+)-1phenylethylamine provides for facial selectivity in the Lewis acid-promoted ring closure of 193. Transtetrahydropyranyl amine 195 is obtained as the major product using a variety of Lewis acids resulting from the synclinal arrangement 194, which leads to diequatorial substitution (Scheme 5.2.43).

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Scheme 5.2.43

5.2.11

535

Intramolecular reaction of an allylic stannane via iminium 194

Transmetalation Reactions of Allylic Stannanes

One of the most challenging aspects for understanding C C bond-forming reactions of allylic stannanes lies in the potential for competing SE transmetalations with Lewis acids, which may be intended for the primary role of increasing the electrophilicity of the reacting partner. Disproportionation processes readily take place with tetraallyltin and common Lewis acids such as BCl3 , AlCl3 , SnCl4 , TiCl4 , or InCl3 . Based on stoichiometry, reactivity, and a host of other factors, it may become difficult to know the precise identity of the reacting species. Allyl tributylstannane and tetraallylstannane undergo instantaneous SE metatheses with SnCl4 at −80 ◦ C.61 A general hypothesis would suggest that transmetalations, as illustrated for 196 (Scheme 5.2.44) with SnCl4 , are driven by thermodynamic considerations to the allyltrichloro species 198, and reactions with aldehydes occur via the closed, six-membered transition state 199, owing to the highly oxophilic nature of the trichlorotin to give anti-alcohols 200.61,62 Spectroscopic studies of SnCl4 -promoted reactions of β-benzyloxyaldehydes with allyltributylstannane have concluded that transmetalations are facile, and reaction pathways may be extremely sensitive to the stoichiometry of SnCl4 and the aldehyde structure.61

Scheme 5.2.44

Transmetalation with SnCl4 and allylation via closed transition states

The transmetalation process essentially leads to a new allylic nucleophile, which may exhibit very different properties compared to the starting stannane. A change in diastereofacial selectivity is often observed. For example, stannane 201 reacts with 3-methylbutanal to yield primarily the 1,4-syn product 202 via an internally coordinated synclinal arrangement 203 (Scheme 5.2.45).

Scheme 5.2.45

Lewis acid coordination in the allylation reactions of 201

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However, the 1,4-anti adduct 205 is optimized as the major isomer by initial transmetalation with SnCl4 in the case of the acetate derivative 204 which may provide a ligand in the cyclic six-membered transition state 206 (Scheme 5.2.46).63

Scheme 5.2.46

Transmetalation of 204 with SnCl4 provides internal coordination via 206

Thomas has reported elegant studies of transmetalations in the course of allylation reactions with aldehydes. As illustrated in Scheme 5.2.47, chiral, non-racemic 207 undergoes transmetalation with SnCl4 to provide stabilization of the trichlorostannyl species 208 by vicinal coordination with the benzyl ether.64 Activation of aldehyde and addition proceeds via the closed, chair-like transition state 209. The features of the four-membered metallocycle 209 account for the exclusive formation of the Z -homoallylic alcohol and the observed 1,5-syn-diastereoselection in 210. Similar results are obtained with ethers that have coordinating capabilities (PMB and MOM).

Scheme 5.2.47

Transmetalation and SE allylation with 1,5-asymmetric induction

These studies have been extended to the homolog 211 (Scheme 5.2.48), which demonstrates similar reactivity, leading to highly selective formation of Z -syn 212.65 The methodology has been utilized to effectively demonstrate 1,5-, 1,6- and 1,7-stereocontrol,66 and the application of this approach has led

Scheme 5.2.48

Transmetalation and allylation with stannane 211

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to total syntheses of epothidones B and D.67 Thomas and coworkers have also examined the use of indium(III) and bismuth(III) halides as agents for transmetalation of these allylic stannanes, and report a reversal in stereoselectivity with simple aldehydes.68 Recent reports of the rapid transmetalation of the highly functionalized allylic stannane 216 (Scheme 5.2.49) leads to the trichloro species 217, which provides for greater Lewis acidity and increased reactivity compared to the parent trialkyl reagent.69 Although transmetalation assumes a dynamic equilibrium of 217 and 218, optimized allylation conditions using SnCl4 (1.0 equivalent) give the syn,anti-product 219 in 96% yield.

Scheme 5.2.49

Stereoselectivity in the SE reaction of stannane 216

The authors surmise that the trichlorostannyl intermediate 218 directs a chelation-controlled addition via 220, which may involve pseudo-axial complexation of the carbamate carbonyl. The stereoselectivity of the allylation is significantly altered by the use of 2.0 equivalents of SnCl4 to produce the corresponding syn,syn-isomer of 219 via the antiperiplanar transition state derived from the α-chelation model for addition of γ -alkoxyallylstannanes. Thomas has extended the application of remote asymmetric induction to include conjugated imines (Scheme 5.2.50).70 For example, the formation of the 1,5-anti-product 224 demonstrates a matched facial

Scheme 5.2.50

Chiral imines for diastereoselective allylation with stannane 223

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preference for the reaction of the (S)-imine 221, with the trichlorostannyl intermediate derived from transmetalation of 223. The corresponding (R)-imine 222 displays slightly reduced stereoselectivity. (E)-Alkenes are produced in these reactions, as compared with the (Z )-alkenols obtained in analogous reactions of stannane 223 with aldehydes (Scheme 5.2.47). A mechanistic rationale, which accounts for the major product 224, illustrates formation of the transmetalation intermediate and nucleophilic delivery via the antiperiplanar transition state 226 with activation via internal Lewis acid coordination of the imine and neighboring ester. In fact, the closed arrangement 227 is also consistent with the observed stereoselectivity. The efficient transmetalation of allylic stannanes to allylboron reagents has generated an attractive methodology for asymmetric allylation. Corey and coworkers first described the use of enantiomers of bromoborane 228 (Scheme 5.2.51) for mild and quantitative transmetalation of allylstannane to yield the allylboron reagent 229.71 The asymmetry in the bis-toluenesulfonamide of 228 is derived from 1,2-diamino-1,2-diphenylethane, and both antipodes are readily available in high optical purity, by resolution of the starting diamines producing (R,R)- and (S,S)-‘Stein’ chiral auxiliaries in transmetalation product 229.

Scheme 5.2.51

Use of the ‘Stein’ chiral auxiliary in transmetalations of allylic stannanes

Diastereofacial selectivity of the allylation is induced, upon coordination and activation of the aldehyde, syn with respect to the aldehydic hydrogen. The conformation of the heterocyclic, five-membered auxiliary in 230 is dictated by the phenyl substituents. Each toluenesulfonyl group occupies a trans-disposition with respect to the adjacent phenyl. Reorientation of 230 by a 90◦ rotation illustrates the favored Zimmerman– Traxler, closed transition state 231 leading to the (S)-homoallylic alcohol 233. The alternative transition state in 232 introduces non-bonded interactions of the tolyl substituent with the allylic methylene, and a destabilizing interaction of the aldehydic (axial) hydrogen and sulfonyl oxygen. Corey has also discussed the possible influence of hydrogen bonding of HA with the nitrogen lone pair in 231.72 Williams and coworkers have expanded the utility of this methodology via the application to highly functionalized substrates.73 Mild transmetalation with (R,R)- or (S,S)-228 is quantitative and dependable, with a variety of allylic stannanes with C-2 substitution. Carbon chains at C-2 of the allyl component may contain additional functionality, including benzyl and allyl ethers, silyl ethers, esters, alkenes, halogens, or thioacetals, as well as stereogenicity. Asymmetric induction in the condensation with aldehydes is dominated by the chiral auxiliary. In this fashion, the allylation reaction may be designed as a convergent

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operation of a synthesis strategy, which condenses fragments of significant complexity. The method has been successfully applied for several total syntheses, including (–)-hennoxazole A,74 amphidinolide K,19 leucascandrolide A,75 and phorboxazole A.76 In the synthesis of phorboxazole A (234, Scheme 5.2.52), the asymmetric allylation reaction was employed iteratively from stannane 235 to construct the C3 –C19 bis-pyran segment 239. The chirality of the 4,5-diphenyl-1,3-diaza-2-borolidine auxiliaries (R,R)-228 and (S,S)-228 provided for high diastereofacial selectivity based on transition state model 231 (Scheme 5.2.51).76

Scheme 5.2.52

Iterative allylations for the synthesis of 239 in the synthesis of phorboxazole A

In addition, the asymmetric allylation was utilized to address the issues of stereocontrol at C37 and C38 in the construction of the acyclic carbon chain of 234 (Scheme 5.2.53). Reactions of the sensitive β,γ -unsaturated aldehyde 241 were effected upon transmetalation of stannane 240 using (R,R)−228 ,

Scheme 5.2.53

The use of R,R-228 for construction of ketal 243

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which provided the anti-Felkin product 242 as the major component of a mixture of C37 diastereomers [dr 7.2:1]. Further transformations of 242 gave the C28 –C41 aldehyde 243 as a key component for the total synthesis.76 The asymmetric allylation protocol has been featured in convergent operations towards the synthesis of leucascandrolide A, as well as a second-generation synthesis of the 18-membered macrolactone 244.75 The broader context of these studies have examined reinforcing and non-reinforcing relationships of allylic chirality in the stannane component with the auxiliary as presented for 245 (Scheme 5.2.54).73 High diastereocontrol is observed following the transmetalation of 245 with (R,R)-228 and low temperature reaction with aldehyde 246 yields 247. The reinforcing model 248 displays the favorable features of the earlier transition state and minimizes A(1,3) interactions while directing the allylic hydrogen into the region of the sulfonyl unit of the auxiliary. This technique permits complex constructions of 1,4-syn stereochemistry by condensing a chiral, non-racemic allylic stannane with a functionalized aldehyde. Overall diastereoselection is dominated by the reinforcing stereochemical features found in the transmetalation to the chiral β-allyl-1,3-diaza-2-borolidine intermediate.

Scheme 5.2.54

Allylation studies for synthesis of macrolide 244

The use of InCl3 for Lewis acid activation of aldehyde substrates leads to rapid transmetalation of the allylic stannane, followed by carbonyl addition reactions of an allyl indium reagent. Premixing of InCl3 and the allylic stannane in the absence of aldehyde often produces precipitation and poor results. On the other hand, allyl indium reagents have been independently prepared by several procedures, including reductive metalations. Several important reviews describe the methods of preparation and the reactivity of allyl indium compounds.77 This discussion will be limited to key factors regarding the transmetalation of allylic stannanes in the presence of aldehydes. Stereochemical events leading to the production of anti adducts as major products are illustrated in Scheme 5.2.55. The initial transmetalation of 249 leads to 250 via anti-SE substitution and additional isomerizations provide 251 and 252. The closed, six-membered transition state 253 from 250 is used to rationalize formation of the E-anti-alcohol 254 as the major product, whereas the minor E-syn-adduct 256 is generated from 252 via the closed arrangement 255.78 The stereochemical consequences for InCl3 transmetalations are complimentary to the syn outcome of reactions for γ -(alkoxy)allylstannanes. As a result, the technique has been utilized in organic synthesis, with the illustration of several applications in Scheme 5.2.56.79

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Scheme 5.2.55

Scheme 5.2.56

5.2.12

541

Mechanistic considerations in transmetalations with InCl3

Examples of reactions involving transmetalations with InCl3

Reactions of Chiral Allylic Stannanes

The preparation of enantioenriched α-(alkoxy)allylstannanes has been advanced by the asymmetric Noyori reduction of the stannyl ketone 264 with 2,2 -dihydroxy-1,1 -binapthyl lithium aluminum hydride (BINAL-H) reagents affording the non-racemic (S)-265 with ≥ 95% ee, upon O-alkylation with common protecting groups (Scheme 5.2.57, top).80

Scheme 5.2.57 Noyori reductions for preparation of enantioenriched α-(alkoxy)allylstannanes and γ (alkoxy)allylstannanes

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Upon treatment with BF3 •OEt2 , stannane 265 is isomerized via an inter-molecular pathway, resulting in allylic transposition and stereochemical inversion of configuration to yield 266 (Scheme 5.2.57, bottom), and this process provides an efficient route to non-racemic γ -(alkoxy)allylic stannanes.81 The reaction of stannane 267 is advanced with unsaturated aldehydes and achieves facial selectivity by the anti-SE mechanism giving mainly the syn product 268, containing an E-alkenyl ether. The anti-SE arrangement shown in 270 minimizes non-bonded interactions leading to the major product. Similarly, the chiral stannane 271 adds to aliphatic aldehydes to produce the E-syn-alcohol 272 (Scheme 5.2.58).82

Scheme 5.2.58

Examples of facial selectivity via the anti-SE mechanism

Intra-molecular allylations are documented, as shown by the cyclization of enantioenriched stannane 273 to yield the 14-membered carbocycle 274.83 Constraints of the carbon tether may determine a preferred transition state geometry to minimize ring strain, delivering the Z -alkene.

Scheme 5.2.59

Intramolecular allylation of 273

Chiral non-racemic γ -[alkoxy]allylic stannanes, such as 275, react with simple aldehydes to yield syn-1,2-diol derivative 276 in high enantiomeric excess. The anti-SE arrangement in 277 minimizes electronic and steric interactions. Matched and mismatched characteristics have been observed in reactions with non-racemic αbenzyloxypropionaldehyde. The matched asymmetric allylation of (R)-stannane 278 with (S)-aldehyde, initiated by complexation with BF3 •OEt2 , exclusively provides the E-4,5-syn-5,6-anti compound 27984 as the expected Felkin–Anh adduct (Scheme 5.2.61, top). On the other hand, the α-chelation-controlled process can also be achieved via a matched case of double diastereoselection using the (S)-stannane 280 and pre-complexation with MgBr2 •OEt2 .84 The syn product 281 is rationalized via the antiperiplanar transition state 282 (Scheme 5.2.61, bottom).

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Scheme 5.2.60

Scheme 5.2.61 and 280

5.2.13

543

Stereocontrol in the allylation reactions of γ -(alkoxy)allylstannanes

Examples of matched asymmetric allylation reactions of chiral, nonracemic stannanes 278

Reactions of Allenylstannanes

Lewis acid-mediated additions of allenylstannanes to aldehydes and other activated species are synthetically useful reactions which often proceed via the SE pathway with high regio- and stereocontrol. Early studies by Lequan and Guillerm,85 Mukaiyama and Harada,86 and Ruitenberg87 led to the substantial efforts of Marshall and coworkers,3 , 6 who advanced the scope and utility of these processes. The potential of the reaction is illustrated in the synthesis of rubifolide,88 where the intra-molecular SE addition of allenylstannane 283 was strategically designed to serve as the key cyclization reaction leading to a 14-membered homopropargylic alcohol. Subsequent isomerization and oxidation gave the allenic ketone 284 for facile conversion to the desired furan 285.

Scheme 5.2.62

Studies toward rubifolide

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Throughout this section, the P and M nomenclature89 (Scheme 5.2.63) will be used to describe absolute configurations of chiral allenylstannanes.

Scheme 5.2.63

Nomenclature for chiral allenylstannanes

Reactions of chiral allenylstannane (P)-286 demonstrate high syn selectivity in BF3 •OEt2 and MgBr2 catalyzed additions to simple aldehydes (Scheme 5.2.64).90 A stereoelectronic preference for the antiperiplanar transition state 287 places the stannyl substituent anti with respect to the newly forming C C bond, giving 288. Lewis acids, such as SnCl4 and BuSnCl3 , trigger a facile anti-SE transmetalation of (P)-286 leading to a less stable propargylic metal species 289, which undergoes in situ isomerization to (M)-290 following a syn SE mechanism, with overall inversion of configuration.91 Transmetalations of (P)-286 with InI3 occur via the anti-SE transformation to 291, and anti-SE isomerization delivers the (P)-292 with net retention of the allene configuration.92 Both 290 and 292 react with aldehydes through closed transition states 293 and 294, respectively, generating enantiomeric anti-stereoisomers 295 and 296.

Scheme 5.2.64

General reactivity profile of allenylstannane (P)-286 in the presence of Lewis acids

Reactions of enantiomeric allenylstannanes (P)-286 and (M)-286 with (S)-2-benzyloxypropionaldehyde, in the presence of MgBr2 •OEt2 , are highly substrate-controlled processes,93 giving the syn,syn297 and anti,syn-298 diastereomers, respectively (Scheme 5.2.65).

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Scheme 5.2.65

545

Reactions of chiral 286 with (S)-2-benzyloxypropionaldehyde

Bidentate Lewis acids result in α-chelation, and the (P)- and (M)-stannanes 286 approach from the less hindered face of the carbonyl, with the small substituent (H) over the metallocycle in the transition states 299 and 300, respectively. In the case of BF3 •OEt2 , reactions of allenylstannanes (P)-286 and (M)-286 create a matched/mismatched scenario with the (S)-aldehyde (Scheme 5.2.66).93 Addition of (M)-286 gives the syn,anti diastereomer 301 via the Cornforth or polar Felkin–Anh transition state 303, whereas allenylstannane (P)-286 results in a diastereomeric mixture of syn,syn-304 (via 306) and anti,anti-305.

Scheme 5.2.66

Reactions of enantiomeric allenylstannanes 286 in the presence of BF3 •OEt2

Condensation reactions of (P)- and (M)-286 with optically pure (2R)-3-benzyloxy-2-methylpropanal in the presence of BF3 •OEt2 are useful syn-selective processes (Scheme 5.2.67, top).94 However (M)-286 provides the matched case for the Felkin–Anh transition state in 310 leading to alcohol 309 (Scheme 5.2.67, middle). Similarly, the reactions promoted by complexation of MgBr2 •OEt2 with this chiral aldehyde are also syn-selective (Scheme 5.2.67, bottom). In the case of (P)-286, the β-chelation model 312 accounts for formation of 311, whereas the corresponding reaction of (M)-286 with this aldehyde unexpectedly gives ent-301, which is rationalized via the analogous Felkin–Anh arrangement of 303. In this latter reaction, it is surprising that β-chelation does not give way to a synclinal addition of (M)-286 to produce the 4,5-anti-5,6-anti-adduct.

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Scheme 5.2.67

Reactions of enantiomeric allenylstannanes 286 with (R)-3-benzyloxy-2-methylpropanal

Reactions of allenylstannanes with oxocarbenium cations are also facile processes.95 Williams,96 and subsequently Nelson,97 have demonstrated the use of glycal acetate 313 in the course of the total synthesis of laulimalide, to generate a reactive oxocarbenium species in the presence of Lewis acids. Axial addition of allenylstannanes 314 and 316 resulted in trans-2,6-disubstituted dihydropyrans 315 and 317, respectively (Scheme 5.2.68). Similarly, the propargylation of lactol 318 is affected with allenylstannane 319 in the presence of BF3 •OEt2 (Eq. 24).98

Scheme 5.2.68

Reactions of allenylstannanes with oxocarbenium species

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Acylation of quinoline with methylchloroformate (Scheme 5.2.69) generates the N -acyliminium ion in situ, which is efficiently trapped by allenyltri-n-butylstannane to give the 1,2-dihydroquinoline 322 in excellent yield.99 A potentially important SE substitution is illustrated by the reaction of 4-acetoxy-2azetidinone 323 with the mixture of diastereomers 324 (Scheme 5.2.69) in the presence of BF3 •OEt2 . Generation of the reactive N -acyliminium species leads to formation of β-lactam 325 with moderate diastereoselection.100

Scheme 5.2.69

Reactions of allenylstannanes with N-acyliminium ions

Finally, the Lewis acid activation of α,β-unsaturated carbonyl compounds and α-nitroalkenes is sufficient to induce productive SE reactions with allenylstannanes (Scheme 5.2.70). Haruta and Kita101 have successfully achieved condensation reactions with 326 and 328 in the presence of TiCl4 , and cyclohexenones 330 and 332 also serve as synthetically effective substrates for the 1,4-conjugate addition.102 In the case of enone 332, activation with TBSOTf led to the isolation of silyl enol ether 333.103

Scheme 5.2.70

5.2.14

Examples of conjugate addition reactions of allenylstannanes

Transmetalation Reactions of Allenylstannanes

Transmetalations of allenylstannanes represent an important conceptual development, which substantially increases the potential utility of these reagents.6 , 104 Corey established an early example via the transmetalation of 3-triphenylstannyl-1-propyne with (R,R)-B-bromo-4,5-diphenyl-1,3-diaza-2-borolidine (R,R-228) yielding the intermediate formation of allenylborane 334. Low temperature condensations of 334 with aldehydes occurred via closed transition states in which the chirality of the ‘Stein’ auxiliary determined the stereogenicity of the enantioenriched R-alcohol 335 (Scheme 5.2.70).71 , 105 The ramifications of SE transmetalations for chiral allenylstannanes are briefly summarized in Scheme 5.2.72 in which the common electrophile, (2R)-3-benzyloxy-2-methylpropanal, serves to selectively generate two distinct diastereoisomers 336 and 337. The reaction with (M)-286 and SnCl4 proceeds via anti-SE transmetalation followed by stereospecific isomerization to give the (P)-trichloroallenylstannane

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Scheme 5.2.71 228

The production of enantioenriched homo-propargylic alcohols via transmetalation with

Scheme 5.2.72

Access to anti,syn and anti,anti diastereomers by transmetalation processes

(as previously described in Scheme 5.2.64). This oxophilic tin species provides 336 by β-chelation control and internal delivery of the allylation nucleophile, as proposed in the cyclic arrangement 338.91 Similarly, preparation of the 4,5-anti-5,6-syn product 337 has been reported by transmetalation of (P)-286 with n-BuSnCl3 , which provides in situ stereospecific generation of the corresponding chiral (M)-allenylic stannane. In this case, the reversible coordination of tin with aldehyde favors the closed transition state for Felkin–Anh addition in 339, whereas the β-chelation model of 340 introduces destabilizing steric interactions owing to placement of the methyl substituent of the chiral allene. Transmetalations of chiral allenylstannanes with InBr3 occur with net retention of allene geometry (Scheme 5.2.64). Thus, the starting (P)-286 can also be utilized for a stereoselective reaction with the corresponding (S)-aldehyde (Scheme 5.2.72, bottom). The enantiomeric alcohol ent-337 is produced via the closed transition state

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of 341, with Felkin–Anh features, as compared to the open transition state of the BF3 •OEt2 reaction (Scheme 5.2.67, middle).92 Marshall has impressively developed the chemistry of chiral allenylstannanes for the stereocontrolled preparation of all eight stereoisomers of 337, as a key stereotriad building block prevalent in polyketide natural products.91 b As a result, the methodology has been featured in the syntheses of (+)-discodermolide,106 leptofuranin D,107 (–)-callystatin A,108 (–)-membrenone C (343) via the key intermediate alkyne 342,109 and tautomycin (344) via the stereotriad component 346 (Scheme 5.2.73).110

Scheme 5.2.73

Representative natural product syntheses featuring allenylstannane methodology

Yamamoto has reported impressive results for the intra-molecular cyclizations of γ alkoxyallenylstannanes leading to five, six, and seven-membered ethers.111 Oxidation of 347 provided an intermediate aldehyde 348 for in situ cyclization to the trans-tetrahydrofuran 350 (Scheme 5.2.74, top). However, transmetalation of the α-alkoxystannane 351 with SnCl4 produced the SE trichlorostannane 352 which afforded the cis-tetrahydrofuran 354 via the closed transition state 353 (Scheme 5.2.74, bottom). Roush has explored this strategy to affect an interesting kinetic resolution of racemic γ alkoxyallenylstannanes derived by transmetalation of 355 in the presence of non-racemic aldehyde 357, yielding 358 (Scheme 5.2.75).112 In a related example, Hegedus has incorporated chirality into the functionalization of the allenylstannane. Lewis acid-promoted additions of stannylallenamide 359 to simple aldehydes and imines are high syn-selective processes (Scheme 5.2.76).113 The reaction is presumed to occur via antiperiplanar 362, illustrating the anti-SE characteristics of stannyl substitution with a minimization of non-bonding interactions.

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Scheme 5.2.74

Scheme 5.2.75

Intra-molecular cyclizations of γ -alkoxyallenylstannanes

Kinetic resolution of racemic 356 in reactions with nonracemic aldehyde 357

Scheme 5.2.76

5.2.15

Reactions of chiral allene 359

Reactions of Propargylic Stannanes

Several methods are available for the synthesis of achiral and chiral propargylic stannanes.91 , 114 The tendency to isomerize to the corresponding allenylstannane limits direct applications in organic synthesis. The presence of a Lewis acid initiates facile transmetalations and mixtures of allenyl and alkynyl products are often obtained. Marshall and coworkers91 have exercised some control of this reaction manifold by replacing SnCl4 with n-BuSnCl3 in reactions of the non-racemic allene 363. While transmetalation produced the (R)-propargylic stannane 364 by anti-SE substitution, the rate of further transmetalation and isomerization is reduced permitting reaction with isobutyraldehyde to give allenylic alcohol 365 with excellent diastereoselection (Scheme 5.2.77).

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Scheme 5.2.77 cohols

551

Diastereoselection in reactions of nonracemic allenylstannanes leading to chiral allenylal-

Matched and mismatched characteristics are observed in reactions of 364 with (S)- and (R)-3benzyloxy-2-methylpropanals. Reaction with (R)-aldehyde generates the syn,syn-diastereomer 366 as the major product [dr 9:1], via the proposed Felkin–Anh mechanism 367, whereas the (S)-aldehyde yields exclusively the anti,anti-isomer 368, which may be explained from the coordination arrangement 369 (Scheme 5.2.78).

Scheme 5.2.78

Reactions of 364 with (S)- and (R)-3-benzyloxy-2-methylpropanals

Hegedus has reported the transmetalation of 370 with n-BuSnCl3 , which presumably generates the coordinated stannane 371 in situ for a stereoselective anti-SE reaction leading to the allenic alcohol 372 (Scheme 5.2.79).113

Scheme 5.2.79

The transmetalation of 370 with n-BuSnCl3 leading to the allenic alcohol 372

In analogous fashion, Corey has described the transmetalation of allenylstannane with (R,R)bromoborane (R,R)-228 for in situ formation of the non-racemic propargylic boron reagent 373, which provides for the preparation of allenic alcohol 374 with high enantioselectivity (Scheme 5.2.80).71

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Scheme 5.2.80

5.2.16

12:56

Enantioselective preparation of 374 from transmetalation of allenylstannane with (R, R)-228

Enantioselective Reactions with Chiral Lewis Acids

The development of chiral Lewis acids as reactive catalysts for enantioselective allylation reactions is a significant advancement of synthesis methodology. Recent reviews have summarized new discoveries and general strategies for enantioselective allylations with various organometallic reagents.3 Several distinct catalyst systems have emerged thus far. Yamamoto and coworkers first described the chiral (acyloxy)borane (CAB) catalysts in 1991.115 In the presence of CAB catalyst 375 and trifluoroacetic anhydride, Marshall demonstrated the addition of stannane 376 to benzaldehyde producing syn-adduct 377 with high enantiocontrol (Scheme 5.2.81).

Scheme 5.2.81

Enantiocontrol in reactions of 376 using CAB catalyst 375

In a study of the allylation of chiral aldehydes (R)-378 and (S)-378, the CAB catalyst 375 improved upon the observed internal stereoselectivity to give the syn-alcohol 379 [dr 98:2], whereas the intrinsic facial selectivity for the (S)-378 was overridden by the chiral Lewis acid yielding the 3,4-syn-4,5-antiisomer 380 [dr 90:10] (Scheme 5.2.82).116

Scheme 5.2.82

Use of the CAB catalyst 375 for stereocontrol in the allylation reactions of (R)- and (S)-378

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The 1,1 -binaphthalene-2,2 -diol complexes of Lewis acids have received considerably more attention. Mikami first reported the application of BINOL/Ti(IV) complexes for enantioselective allylations of glyoxylates.117 Keck118 and Umani-Ronchi/Tagliavini119 independently devised allylation procedures with (R)-BINOL/Ti(O-iPr)4 and (S)-BINOL/Ti(O-iPr)2 Cl2 , respectively (Scheme 5.2.83). The use of molecular sieves is essential for high reactivity and stereoselectivity.

Scheme 5.2.83

Allylation reactions with (S)- and (R)-BINOL catalysts

Enantiomeric ratios are generally excellent, ranging approximately from 90:10 to > 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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Scheme 5.2.87

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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.

References 1. (a) M.B. Smith and J. March, March’s Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 6th Edn., JohnWiley & Sons, Hoboken, New Jersey, 2007, Chapter 12; (b) N. Isaacs, Physical Organic Chemistry, 2nd Edn., Longman, Essex, England, 1995, Chapter 10; (c) T.H. Lowry and K.S. Richardson, Mechanism and Theory in Organic Chemistry, 3rd Edn., Harper Collins Publishers, New York, 1987, Chapter 6. 2. (a) A.G. Davies, Organotin Chemistry, 2nd Edn.,Wiley-VCHVerlagGmbH&Co. KGaA,Weinheim, Germany, 2004; (b) The Chemistry of Organic Germanium, Tin and Lead Compounds, Z. Rappoport (Ed.), John Wiley & Sons Ltd, Chichester, UK, 2002; (c) Chemistry of Tin, P.G. Harrison (Ed.), Chapman and Hall, New York, 1989; (d) M. Pereyre, J.-P. Quintard, and A. Rahm, Tin in Organic Synthesis, Butterworths, London, 1987. 3. (a) J.A. Marshall, J. Org. Chem., 72, 8153 (2007); (b) B.W. Gung, Organic Reactions, 64, 1 (2004); (c) S.E. Denmark and J. Fu, Chem. Rev., 103, 2763 (2003); (d) E.J. Thomas, 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, 195–204; (e) S.R. Chemler and W.R. Roush, in Modern Carbonyl Chemistry; J. Otera (Ed.), Wiley-VCH, Weinheim, 2000, Chapter 11; (f) S.E. Denmark and N.G. Almstead, in Modern Carbonyl Chemistry; J. Otera (Ed.),Wiley-VCH,Weinheim, 2000, Chapter 10; (g) J.A. Marshall, in Lewis Acids in Organic Synthesis, Y. Hisashi (Ed.), Wiley-VCH Verlag GmbH, Weinheim, 2000, Vol. 1, 453–522; (h) J.A. Marshall, Chem. Rev., 96, 31 (1996); (i) Y. Yamamoto and N. Asao, Chem. Rev., 93, 2207 (1993); (j) W.R. Roush, in Comprehensive Organic Synthesis, B.A. Trost and I. Fleming (Eds), Pergamon Press, Oxford, 1991, Vol. 2, 1–53. 4. (a) K. Mikami and T. Nakai, in Catalytic Asymmetric Synthesis, I. Ojima (Ed.), Wiley-VCH, New York, 2000, 543–568; (b) L.C. Dias, Curr. Org. Chem., 4, 305 (2000); (c) K. Mikami and M. Terada, in Comprehensive Asymmetric Catalysis III; E.N. Jacobsen, A. Pfaltz, and H.Yamamoto (Eds), Springer, Berlin, 1999, Vol. III, 1143–1174; (d) K. Mikami and M. Shimizu, Chem. Rev., 92, 1021 (1992); (e) H.M.R. Hoffman, Angew. Chem., Int. Ed., 8, 556 (1969). 5. (a) I. Marek, and G. Sklute, Chem. Comm., 17, 1683 (2007); (b) J.W.J. Kennedy and D.G. Hall, Angew. Chem., Int. Ed., 42, 4732 (2003); (c) C. Masse and J.S. Panek, Chem. Rev., 95, 1293 (1995). 6. J.A. Marshall, B.W. Gung, and M.L. Grachan, in Modern Allene Chemistry, N. Krause and A. S. K. Hashmi (Eds), Wiley-VCH: Weinheim, Germany, Vol. 1, 2004 Chapter 9; (b) K. M. Brummond and J.E. DeForrest, Synthesis, 795 (2007). 7. M.J. Selwyn, in Chemistry of Tin, P.G. Harrison (Ed.), Chapman and Hall, New York, 1989, 362–367. 8. Y. Naruta, S. Ushida, and K. Maruyama, Chem. Lett., 8, 919 (1979). 9. A. Hosomi, H. Iguchi, M. Endo, and H. Sakurai, Chem. Lett., 8, 977 (1979). 10. (a) Y. Yamamoto, H. Yatagai, Y. Naruta, and K. Maruyama, J. Am. Chem. Soc., 102, 7107 (1980); (b) Y. Yamamoto, H. Yatagai, Y. Ishihara, N. Maeda, and K. Maruyama, Tetrahedron, 40, 2239 (1984). 11. S.E. Denmark, E.J. Weber, T.M. Wilson, and T.M. Wilson, Tetrahedron, 45, 1053 (1989); (b) S.E. Denmark and S. Hosoi, J. Org. Chem., 59, 5133 (1994); see also (c) Y. Nishigaichi and A. Takuwa, Tetrahedron Lett., 43, 3045 (2002). 12. (a) G.E. Keck, K.A. Savin, E.N.K. Cressman, and D.E. Abbott, J. Org. Chem., 59, 7889 (1994); (b) G.E. Keck, S.M. Dougherty, and K.A. Savin, J. Am. Chem. Soc., 117, 6210 (1995); see also (c) Y. Nishigaichi and A. Takuwa, Tetrahedron Lett., 44, 1405 (2003). 13. Y. Nishigaichi and A. Takuwa, Tetrahedron Lett., 40, 109 (1999).

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5.3

561

Cross-Coupling of Organotin Compounds for Carbon Carbon Bond Formation

Pablo Espinet and Miroslav Genov IU CINQUIMA/Qu´ımica Inorg´anica, 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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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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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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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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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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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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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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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. P´erez, 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. F¨urstner and A. Leitner, Angew. Chem. 2002,114, 632; Angew. Chem. Int. Ed. 2002, 41, 609; (b) A. F¨urstner, A. Leitner, M. M´endez, 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. Lled´os, 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. C´ardenas, Angew. Chem. 1999, 111, 3201–3203; Angew. Chem. Int. Ed. 1999, 38, 3018; (b) D. J. C´ardenas, 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 (n´ee 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. Qu´eron and R. Lett, Tetrahedron Lett. 2004, 45, 4539. 55. J.-M. Chr´etien, 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. Lof`u, 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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5.4

579

Stille Cross-Coupling for the Synthesis of Natural Products

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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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 Br¨uckner 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

+ 5´E-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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Stille Cross-Coupling for the Synthesis of Natural Products

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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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

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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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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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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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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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57. (a) I. Kadota, H. Takamura, K. Sato, A. Ohno, K. Matsuda, and Y. Yamamoto, J. Am. Chem. Soc., 125, 46 (2003); (b) I. Kadota, H. Takamura, K. Sato, A. Ohno, K. Matsuda, M. Satake, and Y. Yamamoto, J. Am. Chem. Soc., 125, 11893 (2003). 58. Conditions developed originally by Corey for similar alkenyl–alkenyl couplings: X. Han, B. M. Stolz, and E. J. Corey, J. Am. Chem. Soc., 121, 7600 (1999). 59. (a) H. Fuwa, M. Ebine, and M. Sasaki, J. Am. Chem. Soc., 128, 9648 (2006); (b) H. Fuwa, M. Ebine, A. J. Bourdelais, D. G. Baden, and M. Sasaki, J. Am. Chem. Soc., 128, 16989 (2006). 60. (a) A. M. E. Richecoeur and J. B. Sweeney, Tetrahedron Lett., 39, 8901 (1998); (b) A. M. E. Richecoeur and J. B. Sweeney, Tetrahedron, 56, 389 (2000). 61. T. A. Dineen and W. R. Roush, Org. Lett., 6, 2043 (2004). 62. S. Kroiss and W. Steglich, Tetrahedron, 60, 4921 (2004). 63. G. Liang, A. K. Miller, and D. Trauner, Org. Lett., 7, 819 (2005). 64. J. E. Barbarow, A. K. Miller, and D. Trauner, Org. Lett., 7, 2901 (2005). 65. (a) C. M. Beaudry and D. Trauner, Org. Lett., 4, 2221 (2002); (b) C. M. Beaudry and D. Trauner, Org. Lett., 7, 4475 (2005). 66. See also: (a) J. E. Moses, J. E. Baldwin, R. Marquez, R. M. Adlington, and A. R. Cowley, Org. Lett., 4, 3731 (2002); (b) M. F. Jacobsen, J. E. Moses, R. M. Adlington, and J. E. Baldwin, Org. Lett., 7, 2473 (2005). 67. (a) J. E. Moses, L. Commeiras, J. E. Baldwin, and R. M. Adlington, Org. Lett., 5, 2987 (2003); (b) L. Commeiras, J. E. Moses, R. M. Adlington, J. E. Baldwin, A. R. Cowley, C. M. Baker, B. Albrecht, and G. H. Grant, Tetrahedron, 62, 9892 (2006). 68. S. F. Martin, J. M. Humphrey, A. Ali, and M. C. Hillier, J. Am. Chem. Soc., 121, 866 (1999); (b) J. M. Humphrey, Y. Liao, A. Ali, T. Rein, Y.-L. Wong, H.-J. Chen, A. K. Courtney, and S. F. Martin, J. Am. Chem. Soc., 124, 8584 (2002). 69. M. Juhl, T. E. Nielsen, S. Le Quement, and D. Tanner, J. Org. Chem., 71, 265 (2006); (b) M. Juhl, R. Monrad, I. Sotofte, and D. Tanner, J. Org. Chem., 72, 4644 (2007). 70. (a) M. Asano, M. Inoue, and T. Katoh, Synlett, 1539 (2005); (b) M. Asano, M. Inoue, K. Watanabe, H. Abe, and T. Katoh, J. Org. Chem., 71, 6942 (2006). 71. C. J. Sinz and S. D. Rychnovsky, Angew. Chem. Int. Ed., 40, 3224 (2001). 72. T. K. Chakraborty and D. Thippeswamy, Synlett, 150 (1999). 73. H. W. Lam and G. Pattenden, Angew. Chem. Int. Ed., 41, 508 (2002). 74. D. R. Williams, B. J. Myers, and L. Mi, Org. Lett., 2, 945 (2000). 75. R. E. Maleczka, L. R. Terrell, F. Geng, and J. S. Ward, Org. Lett., 4, 2841 (2002). 76. M. K. Gurjar, S. Mohapatra, U. D. Phalgune, V. G. Puranik, and D. K. Mohapatra, Tetrahedron Lett., 45, 7899 (2004). 77. B. M. Trost, O. Dirat, and J. L. Gunzner, Angew. Chem. Int. Ed., 41, 841 (2002). 78. H. F. Olivo, F. Velazquez, and H. C. Trevisan, Org. Lett., 2, 4055 (2000). 79. (a) J. T. Feutrill, M. J. Lilly, and M. A. Rizzacasa, Org. Lett., 2, 3365 (2000); (b) J. T. Feutrill, M. J. Lilly, and M. A. Rizzacasa, Org. Lett., 4, 525 (2002). 80. (a) L. C. Dias and L. G. de Oliveira, Org. Lett., 3, 3951 (2001); (b) L. C. Dias, L. G. de Oliveira, J. D. Vilcachagua, and F. Nigsch, J. Org. Chem., 70, 2225 (2005). 81. S. Hanessian, J. Ma, and W. Wang, J. Am. Chem. Soc., 123, 10200 (2001). 82. Bafilomycin V1: J. A. Marshall, and N. D. Adams, J. Org. Chem., 67, 733 (2002). 83. (a) D. A. Longbottom, A. J. Morrison, D. J. Dixon, and S. V. Ley, Angew. Chem. Int. Ed., 41, 2786 (2002); (b) D. A. Longbottom, A. J. Morrison, D. J. Dixon, and S. V. Ley, Tetrahedron, 59, 6955 (2003). 84. I. Paterson, R. Britton, O. Delgado, A. Meyer, and K. G. Poullennec, Angew. Chem. Int. Ed., 43, 4629 (2004). 85. T. Motozaki, K. Sawamura, A. Suzuki, K. Yoshida, T. Ueki, A. Ohara, R. Munakata, K. Takao, and K. Tadano, Org. Lett., 7, 2265 (2005). 86. (a) K. Maki, R. Motoki, K. Fujii, M. Kanai, T. Kobayashi, S. Tamura, and M. Shibasaki, J. Am. Chem. Soc., 127, 17111 (2005); see also: (b) T. Takeuchi, K. Kuramochi, S. Kobayashi, and F. Sugawara, Org. Lett., 8, 5307 (2006).

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87. Synthesis of the related leustroducsin B: K. Miyashita, T. Tsunemi, T. Hosokawa, M. Ikejiri, and T. Imanishi, Tetrahedron Lett., 48, 3829 (2007). 88. P. A. Roethle, I. T. Chen, and D. Trauner, J. Am. Chem. Soc., 129, 8960 (2007). 89. K. C. Nicolaou, Y. Li, K. Sugita, H. Monenschein, P. Guntupalli, H. J. Mitchell, K. C. Fylaktakidou, D. Vourloumis, P. Giannakakou, and A. O’Brate, J. Am. Chem. Soc., 125, 15443 (2003). 90. Coupling promoted by copper thiophene-2-carboxylate (CuTC): I. Paterson, H.-G. Lombart, and C. Allerton, Org. Lett., 1, 19 (1999). 91. D. A. Entwistle, S. I. Jordan, J. Montgomery, and G. Pattenden, Synthesis, 603 (1998). 92. A. B. Smith III and G. R. Ott, J. Am. Chem. Soc., 120, 3935 (1998). 93. (a) K. C. Nicolaou, J. Xu, F. Murphy, S. Barluenga, O. Baudoin, H.-X. Wei, D.L.F. Gray, and T. Ohshima, Angew. Chem. Int. Ed., 38, 2447 (1999); (b) K. C. Nicolaou, F. Murphy, S. Barluenga, T. Ohshima, H. Wei, J. Xu, D.L.F. Gray, and O. Baudoin, J. Am. Chem. Soc., 122, 3830 (2000). 94. See also: (a) M. Duan, and L. A. Paquette, Angew. Chem. Int. Ed., 40, 3632 (2001); (b) L. A. Paquette, M. Duan, I. Konetzki, and C. Kempmann, J. Am. Chem. Soc., 124, 4257 (2002). 95. I. Paterson, V. A. Doughty, M. D. McLeod, and T. Trieselmann, Angew. Chem. Int. Ed., 39, 1308 (2000). 96. K. Toshima, T. Jyojima, N. Miyamoto, M. Katohno, M. Nakata, and S. Matsumura, J. Org. Chem., 66, 1708 (2001). 97. N. K. Garg, S. Hiebert, and L. E. Overman, Angew. Chem. Int. Ed., 45, 2912 (2006). 98. (a) J. S. Panek and C. E. Masse, J. Org. Chem., 62, 8290 (1997); (b) C. E. Masse, M. Yang, J. Solomon, and J. S. Panek, J. Am. Chem. Soc., 120, 4123 (1998). 99. T. J. Greshock and R. L. Funk, J. Am. Chem. Soc., 128, 4946 (2006). 100. R. J. Huntley and R. L. Funk, Org. Lett., 8, 3403 (2006). 101. (a) B. Salem and J. Suffert, Angew. Chem. Int. Ed., 43, 2826 (2004); (b) B. Salem, E. Delort, P. Klotz and J. Suffert, Org. Lett., 5, 2307 (2003); (c) B. Salem, P. Klotz and J. Suffert, Org. Lett., 5, 845 (2003); (d) J. Suffert, B. Salem, and P. Klotz, J. Am. Chem. Soc., 123, 12107 (2001); see also: (e) A. J. Mota, A. Dedieu, C. Bour, and J. Suffert, J. Am. Chem. Soc., 127, 7171 (2005); (f) C. Bour and J. Suffert, Org. Lett., 7, 653 (2005). 102. M. T. Barros, C. D. Maycock, and M. R. Ventura, Chem. Eur. J., 6, 3991 (2000). 103. L. Commeiras, M. Santelli, and J.-L. Parrain, Org. Lett., 3, 1713 (2001). 104. C. Mukai, N. Miyakoshi, and M. Hanaoka, J. Org. Chem., 66, 5875 (2001). 105. (a) L. Commeiras, M. Santelli, and J.-L. Parrain, Synlett, 743 (2002); (b) L. Commeiras and J.-L. Parrain Tetrahedron: Asymm., 15, 509 (2004). 106. (a) A. B. Smith III, P. R. Verhoest, K. P. Minbiole, and M. Schelhaas, J. Am. Chem. Soc., 123, 4834 (2001); (b) A. B. Smith III, P. Minbiole, P. R. Verhoest, and M. Schelhaas, J. Am. Chem. Soc., 123, 10942 (2001); (c) A. B. Smith III, T. M. Razler, J. P. Ciavarri, T. Hirose, and T. Ishikawa, Org. Lett., 7, 4399 (2005). 107. A. D. Lebsack, J. T. Link, L. E. Overman, and B. A. Stearns, J. Am. Chem. Soc., 124, 9008 (2002). 108. J. J. Kodanko and L. E. Overman, Angew. Chem. Int. Ed., 42, 2528 (2003). 109. X. Gao, Y. Matsuo, and B. B. Snider, Org. Lett., 8, 2123 (2006). 110. X. Lei and J. A. Porco, J. Am. Chem. Soc., 128, 14790 (2006). 111. (a) A. M. Echavarren, N. Tamayo, O. de Frutos, and A. Garcia, Tetrahedron, 53, 16835 (1997); (b) O. de Frutos; C. Atienza, and A. M. Echavarren, Eur. J. Org. Chem, 163 (2001). 112. (a) G. Bringmann and C. Guenther, Synlett 1999, 216; (b) G. Bringmann, C. Gunther, E.-M. Peters, and K. Peters, Tetrahedron, 57, 1253 (2001). 113. (a) T. M. Kamenecka and S. J. Danishefsky, Angew. Chem. Int. Ed., 37, 2993 (1998); (b) T. M. Kamenecka and S. J. Danishefsky, Angew. Chem. Int. Ed., 37, 2995 (1998); (c) T. M. Kamenecka and S. J. Danishefsky, Chem. Eur. J., 7, 41 (2001). 114. J. Shao and J. S. Panek, Org. Lett., 6, 3083 (2004). 115. R. J. Huntley and R. L. Funk, Org. Lett., 8, 4775 (2006). 116. H. M. Mueller, O. Delgado, and T. Bach, Angew. Chem. Int. Ed., 46, 4771 (2007). 117. See, inter alia: (a) S. Amano, N. Ogawa, M. Ohtsuka, and N. Chida, Tetrahedron, 55, 2205 (1999); (b) J. D. White, R. G. Carter, K. F. Sundermann, and M. Wartmann, J. Am. Chem. Soc., 123, 5407 (2001); (c) Y. Gu

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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. H¨ugel, 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. F¨urstner, 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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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 Chr´etien,a,b Jeremy D. Kilburn,b Franc¸oise Zammattio,a Erwan Le Grognec,a and Jean-Paul Quintard a a b

University de Nantes, CNRS “Laboratoire de Synth`ese 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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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 <10 ppm in favorable cases.18 In cases where weakly polar compounds are synthesized, a reverse strategy must be used and polar organotin reagents have been proposed. For example organotin reagents incorporating both PEG and carboxylate functionalities have been described,19,20 as have organotin reagents incorporating basic nitrogen functionality, which can be protonated and partitioned into aqueous acidic solution.21−24 The use of pyrenyltin reagents and subsequent elimination of the by-products by adsorption on activated charcoal,25 or of fluoroalkyltin reagents removable in a fluorinated phase26−29 have also been reported. Monoorganotins able to give non-toxic inorganic tin residues after reaction have also been proposed.30 In this case, the possibility of using penta-coordinate organotin intermediates has allowed otherwise difficult reactions, such as alkyl transfer in Stille cross-coupling reactions.31,32 The use of tetraallyltin as a reagent in highly polar media33–35 also leads to inorganic tin derivatives and to the efficient separation of the by-products on silica gel.36 Finally, ionic liquids have been used in conjunction with organotin reagents, for example in the allylation of aldehydes37 and in Stille cross-coupling reactions.38 A recent example of a Stille coupling used an organotin reagent chemically anchored to the ionic liquid, providing a method for easy purification and potential recycling of the tin reagent.39a In a related area, triarylphosphonium-supported tin reagents were very recently used for Stille crosscoupling reactions, dehalogenation reactions, cyclization reactions, and allylation reactions affording the desired products with very low tin pollution (5–15 ppm).39b

5.5.3

Solid-Supported Organotin Reagents

The general concept of a supported organotin reagent contained in the above example (using an ionic liquid as the support39 ) is not new, and several other approaches have been reported previously; a review article has already been published on this topic.40 For instance, C-stannylated alkoxysilanes have been grafted onto an inorganic matrix such as Al2 O3 , SiO2 , TiO2 , or ZrO2 through siloxanic bridges and used mainly for reduction reactions (Scheme 5.5.1).41,42 In these examples, the supported reagents were prepared as hydrides (X = H) or generated in situ from the exchange reaction between hydrogenosilanes and organotin alkoxides. However, these types of reagents have not been extensively developed due to the low loadings generally obtained on this type of matrix (5 to 800 μmol g−1 ), and most of the efforts have been devoted to the use of polystyrene-supported reagents.

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matrix

Scheme 5.5.1 5.5.3.1

O O O

Si

609

SnRmX(3-m) n n = 1,4

Organotin reagent grafted on an inorganic matrix

Supported Organotin Reagents on Insoluble Polystyrene Matrix

This type of support has been the most commonly used and, depending on the degree of cross-linking, their properties are significantly modified. In practice, with styrene/divinylbenzene (S/DVB) polymers containing 5–20% DVB, macroporous polymers are obtained. Their advantage is the presence of permanent pores on the surface due to the use of an appropriate porogen in their industrial preparation, a parameter which increases their contact area with the reagents. Furthermore, the higher degrees of cross-linking increase the rigidity and resistance to abrasive effects.43 An alternative type of insoluble polystyrene matrix is obtained with lower amounts of DVB (0.5 to 2%), affording microporous polymers or so-called ‘gel-type resins.’ These polymers are characterized by the absence of permanent pores and by a low external area. Accordingly, the swelling of the resin in the solvent is of great importance to achieve chemical reactions in appropriate conditions and for this purpose, solvents such as toluene, dichloromethane or THF are generally the more suitable ones. It is also worth noting that this type of structure increases the sensitivity to mechanical abrasive effects and to osmotic pressure.43 (1) Supported Organotin Reagents on Macroporous Polymers. The synthesis of these supported organotin reagents can be achieved according to two routes: (i) Grafting of a Side Chain onto the Macroporous Polymer. This approach was first developed in pioneering work more than 30 years ago using a sequence starting from Amberlite XE 305.44 Accordingly, an ‘SnBuCl2 ’ functionalized Amberlite XE 305 (polymer A, Scheme 5.5.2) was obtained and converted into the corresponding dihydride. This method allows the grafting of about 2 mmol g−1 of tin hydride functionality on the new polymer, but difficulties were encountered with recycling the reagent when used for reduction of organic halides. Reduction of alkyl halides was improved by preparing a resin functionalized with both a tin dichloride and a crown ether, which could be used in sub-stoichiometric amounts in conjunction with a large excess of sodium borohydride.45 H

MgBr

BuSnCl3

SnBuCl2

LiAlH4

SnBuH2

THF Amberlite XE-305

Polymer A

Scheme 5.5.2

Synthesis of polymer A

Recognizing that the proximity of the resin backbone and the incorporated reactive tin functionality can be a major drawback, several approaches have been developed for the incorporation of a two carbon unit by hydrostannylation of a vinyl group on the S/DVB polymer (Scheme 5.5.3).46

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H

SnBu2Cl

MH

SnBu2H

AIBN Amberlite XE-305

Polymer B

Scheme 5.5.3

Incorporation of an organotin function through a 2C spacer

Applying this approach with Amberlite XE 305 (12% DVB) affords polymer B and hence supported tin hydride with a loading of 1.5 mmol g−1 of Sn H as well as ditin (0.15 mmol g−1 ) and tri-n-butylstannylated species (0.1 mmol g−1 ). Similar experiments conducted with Lewatit OC1018 (7% DVB) gave lower amounts of Sn H (0.9 mmol g−1 ) and higher amounts of ditin compounds (0.3 mmol g−1 ). Furthermore, the authors showed that polymer B could react as a conventional tin halide to afford new supported organotin reagents (PS CH2 CH2 SnBu2 R) via substitution reactions.47 Reductive coupling of polymer B was also possible to give supported ditin reagents.48 In order to provide a modular route compatible with different lengths of the spacer (>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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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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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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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 (<53 ppm), but increased slightly when the polymer was recycled for a second run.75 When a supported tin halide was used as a catalyst (0.01–0.2 eq.), with sodium borohydride for the in situ regeneration of the active tin hydride, the resulting tin contamination was as low as 5–10 ppm.76 Soluble polystyrenes containing side-chain tributyltin carboxylate moieties linked to the aromatic ring through various spacers (PS (CH2 )n COOSnBu3 with n = 1-4, PS CH=CH COOSnBu3 ), have also been prepared, although in this work the polymers have been used in electrochemical applications rather than for synthesis.78 Tin carboxylates anchored to poly(ethyleneglycol) have also been synthesized, again for electrochemical applications.79 5.5.4

Use of Supported Organotins in Organic Synthesis

Generally, most of the aforementioned polymers can be used in most of the usual reactions involving organotin reagents and in a range of temperatures between −30 and 100 ◦ C. Supported organotin reagents have mainly been used to improve purification of organic products at the end of the reaction (Scheme 5.5.11). linker

Sn(Alkyl)2Σ

Insoluble organotin reagent

+

EX soluble reactive substrate

Scheme 5.5.11

linker

Sn(Alkyl)2X

insoluble organotin by-product

+

EΣ soluble organic product

Use of supported organotins: the concept

In principle, if no further complications arise, the organotin residues should remain on the support and can be removed by filtration, so that the organic product can be recovered uncontamined by tin residues. Unfortunately, this expectation is too optimistic. When examined at trace level, variable amounts of tin

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residues are found. This may be due to leaching of tin residues contained into the polymer following its preparation, or because of chemical cleavage of tin residues. The effectiveness of supported organotin reagents, especially when used catalytically, is also strongly dependent on the experimental conditions and, for example, tin contamination may result from mechanical damage to the support because of high temperatures,63,64 abrasion due to stirring,80 or on precipitation, in the case of soluble polymers. 5.5.4.1

Use of Supported Organotin Hydrides

The reduction of alkyl halides is routinely carried out using tin hydride reagents. In practice, all supported tin hydrides have proved to be applicable to this well-known reaction, but vary in the amount of resulting tin contamination and how easily they can be recycled (Scheme 5.5.12).40 AIBN, Δ SnR2-H

+

R'X (X = Br, I)

Scheme 5.5.12

SnR2-X

+

R'H

Reduction of organic halides

The reduction of simple alkyl halides is generally achieved with good yields and low levels of tin contamination when the conversion of the Sn X function into an Sn H function is efficient. Conversion of Sn X to give Sn H is generally best achieved using di-n-butylaluminium hydride or sodium borohydride,40,46,80,81 but lithium aluminium hydride can also be used under appropriate experimental conditions.51 It is worth noting that the recycling of supported hydrides may be problematic because they may be transformed into ditin species. Furthermore, the radical processes that they promote can lead to hydrogen atom abstraction, particularly of benzylic hydrogens, a reaction which can contribute significantly to cleavage (β-elimination) of the organotin from the polymer, when two carbon spacers are used, as, for instance, in polymer B or copolymer E. A few general trends are given in Table 5.5.1 and provide a primary evaluation of the stannylated polymers which can be used either directly as a source of stoichiometric tin hydride or catalytically as the tin chloride, with in situ reduction using a metal hydride. Even allowing disparities due to the variation in reaction conditions used with the different polymers (temperature, stirring mode), this table highlights the following points: r Polymer A cannot be recommended due to significant formation of ditin species, which prohibits efficient recycling. r Polymer B and copolymer E give high levels of contamination, which can reasonably be assigned to β-stannyl fragmentation via a readily formed benzylic radical on the polymer. r Polymer C, with a four-carbon spacer, is an efficient reagent both for recycling and for low levels of tin contamination. Furthermore, contamination decreases generally after the two first cycles of use due to the initial leaching of soluble organotin by-products following the synthesis of the supported reagent. One might assume, even without any more detailed information, that polymers bearing longer spacers should behave similarly. r Copolymer F, of the type PS/DVB/VB–CH2 O(CH2 )3 SnBu2 Cl, appears to be sensitive to temperature and therefore induces some tin contamination in the product. However, soluble polystyrene having the same spacer seems to be better at avoiding tin contamination. In addition to the reduction of organic halides, which is the usual test experiment,40,76,82,83 supported tin hydrides can also be used in the Barton–McCombie deoxygenation of alcohols84 (which can be achieved

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615

Comparison of different supported organotin reagents

a

Polymers are considered as the Sn Cl moiety which can be modified to give Sn H before reaction or during the reaction (catalytic mode); b PS SnBuCl2 obtained by functionalization of a macroporous polystyrene; c PS CH2 CH2 SnBu2 Cl obtained by functionalization of a macroporous polystyrene; d PS (CH2 )4 –SnBu2 Cl obtained by functionalization of a macroporous polystyrene; e PS/DVB/VB–CH2 CH2 SnBu2 Cl; f P maleimide/allyl Z (CH2 )3 SnBu2 Cl; g non-cross-linked PS Z (CH2 )3 SnBu2 Cl; h not determined; i Z= CH2 O .

in a catalytic mode using trimethoxysilane for regeneration of the tin hydride);85 deamination of primary amines via isonitriles;82 addition of alkyl radicals to acrylonitrile (the Giese reaction);86 rearrangement reactions such as ring enlargement of cyclohexadienones;87 or cyclisation of hexen-5-yl radicals and related species.58,88 Although less well developed, the reduction of aldehydes and ketones is also possible with organotin hydrides supported on polystyrene matrix or on silica.42b,44 5.5.4.2

Use of Supported Organotins Containing an Sn Heteroatom Bond

(1) Supported Ditin Reagents. Ditin reagents can be obtained by reduction of organotin halides with arene metal radical anions89 or by dehydrogenation of supported organotin hydrides in the presence of Pd(PPh3 )4 (Scheme 5.5.13)67,68 SnR2Cl

Naphthalene/Li

SnR2

SnR2Cl

or Anthracene/Mg

SnR2

Scheme 5.5.13

Pd(PPh3)4

SnR2H SnR2H

Synthesis of supported ditin reagents

Using supported ditin compounds, it is possible to promote free radical addition of organic iodides to triple bonds90 and atom transfer cyclization of ε-unsaturated iodoacetates or iodoamides68,89 using UV irradiation. In this latter case, tin contamination was found to be 5–34 ppm using 0.1 eq. of ditin reagent, and recycling of the tin reagent was also possible. (2) Supported Organotin Polyhalides, Oxides, Hydroxides or Alkoxides. The first contributions in this field have exploited the activation of sugar hydroxyl groups as organotin alkoxides in order to obtain regioselective acylation of sucrose91 or methyl α-d-manno- and α-d-glucopyranosides,65 the advantage being, once more, easier purification of the products.

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For transesterification of ethyl acetate with various alcohols promoted by mixed organotin oxides, more detailed studies have been carried out on the structure and catalytic properties of supported tin dichloride and trichloride precursors, and on mixed organotin oxides of the type [P (CH2 )n SnBuCl]2 O.54−56,92−94 It was found that ring opening polymerization of ε-caprolactone using the supported mixed organotin oxide, in the presence of propanol, required lengthy spacers (11 carbon atoms was found to be the best) between the polystyrene matrix and tin, in order to avoid the collapse of the resin beads (as observed for a six carbon spacer).55,56 The tin contamination could be reduced to <5 ppm in the final products when these types of catalysts were used in the transesterification of ethyl acetate and various alcohols.54 The same transformation can also be carried out catalytically using polymer-supported tin carboxylates.62 (3) Supported Organotins with Sn N Linkage. While rarely used, the reactivity of the Sn N linkage can be exploited in supported organotins, and it has simultaneously been used as a para-directing group in the halogenation of anilines (Scheme 5.5.14).95 1. BuLi, Et2O, -78 °C R 2. NH2

X 3. Br2 or ICl, -78°C

Scheme 5.5.14

R

(CH2)4SnBu2I

NH2

(X = Br, I)

rt

Use of supported tin halides as para orientating group in halogenation of anilines

In this example, the recycling of the supported organotin halide was possible without loss of activity, and even in an unfavourable case using Br2 or ICl as reagent, which might cause cleavage of the Sn C bond, tin contamination was only ∼30 ppm after the first cycle, and decreased when recycled polymer was used in further reactions. 5.5.4.3

Use of Supported Organotins Containing a Reactive Sn C Bond

These supported reagents, generally aryltins,69 allyltins,52 or vinyltins47,51,66 can be obtained by substitution of the supported tin halide with appropriate organometallic reagents or by hydrostannylation of alkynes and can be used in conventional organotin reactions with a significant reduction of contamination by organotin residues. (1) Halodestannylation Reactions: Although only occasionally used, halodestannylation of vinyl or aryltins provides an efficient route to vinyl or aryl halides,96 and can be especially useful when radiolabelled compounds are desired97−99 due to the much easier purification of the products. (2) Allylation Reactions: In spite of their potential in substitution or in addition reactions, supported allyltins have not been used very often. Examples include radical transfer of an allyl unit to α-bromo ketones or esters, which was described for allyltins grafted on a soluble non-cross-linked polystyrene support.75 In this case, tin contamination for the initial reaction was 7–54 ppm but increased to 15–80 ppm when recycled polymer was used. In another study, an allyltin anchored on a macroporous polystyrene added to the aldehyde of pbromobenzaldehyde under Lewis acid conditions, but underwent a Stille cross-coupling using Pd(0) catalysis, and in both cases the reactions were completely chemoselective.40 The allylation of aldehydes has recently been more extensively studied using a macroporous polystyrene as support and could be achieved at 60 ◦ C in acetonitrile using CeCl3 .7H2 0/10% NaI as promoter when an allyl or methallyl unit were involved. When indium tribromide was used as a promoter, the reaction took

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place at 25 ◦ C in dichloromethane for a range of allyl derivatives CH2 (R)C CH2 (R = H, Me, CO2 Et, CN).52 Due to the mild experimental conditions (room temperature to 60 ◦ C and gentle mechanical shaking), the recycling of the supported reagent could easily be achieved without significant loss of reactivity (yields decreased from 95% to 90% over five cycles) and with a remarkable control of tin contamination <5 ppm.52 (3) Stille Cross-coupling Reactions: Stille couplings using organotin reagents are routinely used for the formation of sp2 C–sp2 C bonds. Examples of Stille couplings using macroporous polymer-supported allyl, vinyl, and alkynyl organotins have been described in pioneering work.47 Microporous polymer-supported vinyltins have been also used in an intra-molecular cyclization leading to (S)-Zearalenone.70 Using microporous polymer-supported tin chlorides, the vinyltin intermediate can be obtained in situ by reduction to a tin hydride (using polymethylhydrosiloxane) and subsequent hydrostannylation of an alkyne. Accordingly, the reaction can be carried out in nearly catalytic conditions when unhindered supported organotin chlorides are used (Scheme 5.5.15).68 This work emphasizes the need to use SnMe2 Cl organotin functionality in order to facilitate the kinetics of the reaction. Low tin contamination was found in the products of the reaction using this strategy (<5–60 ppm). O

OH MeO

I

+

Me

Scheme 5.5.15

HO

H Ph

OMe

SnMe2Cl

1% Pd2Cl2(PPh3)2, 1% Pd2(dba)3 4% (2-furyl)3P, PMHS, Na2CO3 H2O/THF

Me

Ph

One pot Stille cross-coupling using a stannylated microporous polymer

Stille cross-coupling of aryl iodides and bromides with macroporous polymer-supported vinyltins also gave good yields and very low levels of contamination. The worst example gave 16 ppm of tin contamination after simple filtration of the product, but subsequent purification by flash chromatography reduced the tin contamination to <1 ppm.51 A significant decrease of the efficiency was observed when the resin was recycled, together with a coloration of the resin, possibly due to contamination with the palladium catalyst (Pd(PPh3 )4 ). This observation was subsequently verified in the cross-coupling of supported aryltins with 2-halogenopyridines by washing the resin with trithiocyanuric acid, which led to a recovery of the cross-coupling efficiency at least on the first reuse.53 5.5.5

Perspectives and Conclusions

This short overview of supported organotin reagents demonstrates that they can be successfully used in a large range of reactions commonly performed using soluble organotins. The key advantage, that products can be more easily purified, has been demonstrated in many examples and clearly indicates that these reagents are promising tools for the preparation of compounds, acceptable for biological tests in drug discovery programs of pharmaceutical or agrochemical companies. On the basis of the observed results, macroporous polystyrene/DVB matrix stannylated with a polymethylene spacer (CH2 )n or with a CH2 O (CH2 )n spacer (with n > 3) seem to be the most promising. However, it is also clear that gentle mechanical shaking (rather than stirring with magnetic stirrer) and

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mild reaction temperature are required to minimize damage to the polymer and consequent release of tin impurities into solution. For the future, while recycling can be by-passed in discovery research, it nevertheless has to be better understood in order to facilitate catalytic reactions, with, perhaps, in such cases the help of stronger supports like grafted silica. For the future also, outside of the reaction yields and of the contamination measurements, the transformations should be explored within the solid matrix using appropriate techniques (e.g. microscopy, solid-state MAS NMR) in order to identify and to prevent any possible problems. Taking into account these recommendations, one can reasonably expect a rehabilitation of organotin chemistry in discovery research, but elsewhere as well! References 1. (a) M. Pereyre, J.-P. Quintard, and A. Rahm, Tin in Organic Synthesis, Butterworths, London, 1987; (b) A. G. Davies, Organotin Chemistry, VCH, Weinheim, 1997. 2. (a) W. P. Neumann, Synthesis, 665, (1987); (b) D. P. Curran Synthesis, 417, (1988). (c) D. P. Curran, Synthesis, 489, (1988). 3. V. Darmency and P. Renaud, Topics in Current Chemistry, 263, 71, (2006). 4. (a) J. K. Stille, Angew. Chem. Int. Ed., 25, 508, (1986); (b) V. Farina, V. Krishnamurthy, and W. J. Scott, ‘The Stille Reaction’, Organic Reactions, 50, 1, (1997). 5. (a) J. B. Verlhac, E. Chanson, B. Jousseaume, and J.-P. Quintard, Tetrahedron Lett., 26, 6075, (1985); (b) J.-L. Parrain, I. Beaudet, A. Duchˆene, S. Watrelot, and J.-P. Quintard, Tetrahedron Lett., 34, 5445 (1993). 6. (a) K. C. Nicolaou, P. G. Bulger, and D. Sarlah, Angew. Chem. Int. Ed., 44, 4442, (2005). 7. (a) Y. Yamamoto and N. Asao, Chem. Rev., 93, 2207, (1993); (b) J. A. Marshall, Chem. Rev., 96, 31, (1996); (c) E. J. Thomas, Chem. Commun., 411, (1997); (d) S. E. Denmark and J. Fu, Chem. Rev., 103, 2763, (2003); (d) M. Booth, C. Brain, P. Castreno, S. Donnelly, E. K. Dorling, O. Germay, L. Hobson, N. Kumar, N. Martin, C. Moore, D. Negi, E. J. Thomas, and A. Weston, Pure Appl. Chem., 78, 2015, (2006); (e) E. J. Thomas, Chem. Record, 7, 115, (2007); (f) V. Fargeas, F. Zammattio, J.-M. Chr´etien, M.-J. Bertrand, M. Paris, and J.-P. Quintard, Eur. J. Org. Chem., 1681, (2008). 8. S. David and S. Hanessian, Tetrahedron, 41, 643, (1985). 9. K. V. V. Prasada Rao, R. Dandala, V. K. Handa, I. V. Subramanyeswara Rao, A. Rani, S. Shivashankar, and A. Naidu, Synlett, 1289, (2007). ´ 10. Etain et Organostanniques : Mise au point pr´eliminaire – Crit`eres d’Hygi`ene de l’Environnement, Organisation Mondiale de la Sant´e, Gen`eve, vol 15, 1981. 11. (a) C. Alzieu, Y. Thibaud, M. Heral, and B. Boutier, Rev. Trav. Inst. Pˆeches Marit., 44, 301, (1980); (b) R. B. Laughlin Jr., and O. Linden, Ambio, 14, 88, (1985). 12. (a) R.J. Maguire, Environ. Sci. Technol., 18, 291, (1984); (b) R. J. Amouroux, E. Tessier, and O. F. X. Donard, Environ. Sci. Technol., 34, 988, (2000). 13. (a) J. Lipowitz and S. A. Bowman, J. Org. Chem., 38, 162, (1973); (b) E. J. Corey and J. W. Suggs, J. Org. Chem., 40, 2554, (1975); (c) G. Stork and P. M. Sher, J. Am. Chem. Soc., 108, 303, (1986); (d) D. S. Hays and G. C. Fu, Tetrahedron, 55, 8815, (1999); (e) W. P. Gallagher, I. Terstiege, and R. E. Maleczka, Jr., J. Am. Chem. Soc., 123, 3194, (2001); (f) W. P. Gallagher and R. E. Maleczka, Jr., J. Org. Chem., 70, 841, (2005). 14. (a) D. Milstein and J. K. Stille, J. Am. Chem. Soc., 100, 3636, (1978); (b) J. E. Leibner and J. Jacobus, J. Org. Chem., 44, 449, (1979). 15. (a) B. S. Edelson, B. M. Stoltz, and E. J. Corey, Tetrahedron Lett., 40, 6729, (1999); (b) A. F. Littke and G. C. Fu, Angew. Chem. Int. Ed., 38, 2411, (1999). 16. (a) D. C. Harrowven and I. L. Guy, Chem. Commun., 1968, (2004); (b) D. C. Harrowven and I. L. Guy, Brit. Patent, GB2412074, 2005.

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46. (a) U. Gerigk, M. Gerlach, W. P. Neumann, R. Vieler, and V. Weintritt, Synthesis, 448, (1990); (b) J. R. Miller, and J. W. Hershberger, J. Polym. Sci., Polym. Lett., 25, 219, (1987); (c) W. P. Neumann and M. Peterseim, React. Polym., 20, 189, (1993). 47. H. Kuhn and W. P. Neumann, Synlett, 123, (1994). 48. J. Junggebauer and W. P. Neumann, Tetrahedron, 53, 1301, (1997). 49. (a) G. Ruel, N. K. The, G. Dumartin, B. Delmond, and M. Pereyre, J. Organometal. Chem., 444, C18, (1993); (b) G. Dumartin, J. Kharboutli, B. Delmond, M. Pereyre, M. Biesemans, M. Gielen, and R. Willem, Organometallics, 15, 19, (1996). 50. G. Dumartin, G. Ruel, J. Kharboutli, B. Delmond, M.-F. Connil, B. Jousseaume, and M. Pereyre, Synlett, 952, (1994). 51. J.-M. Chr´etien, A. Mallinger, F. Zammattio, E. Le Grognec, M. Paris, G. Montavon, and J.-P. Quintard, Tetrahedron Lett., 48, 1781, (2007). 52. J.-M. Chr´etien, F. Zammattio, D. Gauthier, E. Le Grognec, M. Paris, and J.-P. Quintard, Chem. Eur. J., 12, 6816, (2006). 53. G. Kerric, J.-M. Chr´etien, A. Tabatchnik-Rebillon, E. Le Grognec, F. Zammattio, and J.-P. Quintard, Abst 12th ICCOC-GTL, Galway, Ireland, p. 28, (2007). 54. C. Camacho-Camacho, M. Biesemans, M. Van Poeck, F. A. G. Mercier, R. Willem, K. Darriet-Jambert, B. Jousseaume, T. Toupance, U. Schneider, and U. Gerigk, Chem. Eur. J., 11, 2455, (2005). 55. G. Deshayes, K. Poelmans, I. Verbruggen, C. Camacho-Camacho, P. Deg´ee, V. Pinoie, J. C. Martins, M. Piotto, M. Biesemans, R. Willem, and P. Dubois, Chem. Eur. J., 11, 4552, (2005). 56. K. Poelmans, V. Pinoie, I. Verbruggen, M. Biesemans, G. Van Assche, G. Deshayes, P. Deg´ee, P. Dubois, and R. Willem, Appl. Organometal. Chem., 21, 504, (2007). 57. G. Ruel, G. Dumartin, B. Delmond, B. Lal`ere, O. F. X. Donard, and M. Pereyre, Appl. Organometal. Chem., 9, 591, (1995). 58. Y. Ueno, K. Chino, M. Watanabe, O. Moriya, and M. Okawara, J. Am. Chem. Soc., 104, 5564, (1982). 59. Q. Jiang, C. McDade, and A. W. Gross, US Patent 5436357 (1995) and US Patent 5561205 (1996). 60. (a) A. Chemin, H. Deleuze, and B. Maillard, Eur. Polym. J., 34, 1395, (1998); (b) A. Mercier, H. Deleuze, B. Maillard, and O. Mondain-Monval, Adv. Synth. Cat., 344, 33, (2002). 61. D. H. Hunter and C. McRoberts, Organometallics, 18, 5577, (1999). 62. (a) L. Angiolini, D. Caretti, L. Mazzocchetti, E. Salatelli, R. Willem, and M. Biesemans, Appl. Organometal. Chem., 19, 841, (2005); (b) L. Angiolini, D. Caretti, L. Mazzocchetti, E. Salatelli, R. Willem, and M. Biesemans, J. Organometal. Chem., 691, 1965, (2006); (c) L. Angiolini, D. Caretti, L. Mazzocchetti, E. Salatelli, R. Willem, and M. Biesemans, J. Organometal. Chem., 691, 3043, (2006). 63. A. Chemin, H. Deleuze, and B. Maillard, J. Chem. Soc., Perkin Trans. 1, 137, (1999). 64. A. Chemin, H. Deleuze, and B. Maillard, J. Appl. Polym. Sci., 79, 1297, (2001). 65. D. M. Whitfield and T. Ogawa, Glycoconjugate J., 15, 75, (1998). 66. A. G. Hern´an, V. Guillot, A. Kuvshinov, and J. D. Kilburn, Tetrahedron Lett., 44, 8601, (2003). 67. A. G. Hern´an and J. D. Kilburn, Tetrahedron Lett., 45, 831, (2004). 68. A. G. Hern´an, P. N. Horton, M. B. Hursthouse, and J. D. Kilburn, J. Organometal. Chem., 691, 1466, (2006). 69. X. Zhu, B. E. Blough, and F. I. Carroll, Tetrahedron Lett., 41, 9219, (2000). 70. K. C. Nicolaou, N. Winssinger, J. Pastor, and F. Murphy, Angew. Chem. Int. Ed., 37, 2534, (1998). 71. J. Cossy, C. Rasamison, and D. Gomez Pardo, J. Org. Chem., 66, 7195, (2001). 72. J. Cossy, C. Rasamison, D. Gomez Pardo, and J. A. Marshall, Synlett, 629, (2001). 73. J. Cossy, M. Defosseux, and C. Meyer, Synlett, 815, (2001). 74. S. Br¨ase, J. H. Kirchhoff, and J. K¨obberling, Tetrahedron, 59, 885, (2003). 75. E. J. Enholm, M. E. Gallagher, K. M. Moran, J. S. Lombardi, and J. P. Schulte, II., Org. Lett., 1, 689, (1999). 76. E. J. Enholm and J. P. Schulte, II., Org. Lett., 1, 1275, (1999). 77. S. Thibaud, L. Moine, M. Degueil, and B. Maillard, Eur. Polym. J., 42, 1273, (2006).

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78. (a) L. Angiolini, M. Biesemans, D. Caretti, E. Salatelli, and R. Willem, Polymer, 42, 3913, (2000); (b) H. Dalil, M. Biesemans, R. Willem, L. Angiolini, E. Salatelli, D. Caretti, N. A. Chaniotakis, and K. Perdikaki, Helv. Chim. Acta, 85, 852, (2002); (c) 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); (d) L. Angiolini, D. Caretti, L. Mazzocchetti, E. Salatelli, R. Willem, and M. Biesemans, J. Polym. Sci. Part A: Polym. Chem., 42, 5372, (2004). 79. (a) L. Angiolini, D. Caretti, L. Mazzocchetti, E. Salatelli, R. Willem, and M. Biesemans, J. Polym. Sci. Part A: Polym. Chem., 43, 3091, (2005); (b) D. Tonelli, I. Carpani, L. Mazzocchetti, L. Angiolini, D. Caretti, E. Salatelli, and F. Tarterini, Electronalysis, 18, 1055, (2006). 80. G. Dumartin, M. Pourcel, B. Delmond, O. Donard, and M. Pereyre, Tetrahedron Lett., 39, 4663, (1998). 81. M. Pourcel, PhD. Thesis, University of Bordeaux 1 (France), 1997. 82. M. Gerlach, F. J¨ordens, H. Kuhn, W. P. Neumann, and M. Peterseim, J. Org. Chem., 56, 5971, (1991). 83. W. P. Neumann, J. Organometal. Chem., 437, 23, (1992). 84. W. P. Neumann and M. Peterseim, Synlett, 801, (1992). 85. P. Boussaget, B. Delmond, G. Dumartin, and M. Pereyre, Tetrahedron Lett., 41, 3377, (2000). 86. C. Bokelmann, W. P. Neumann, and M. Peterseim, J. Chem. Soc., Perkin Trans. 1, 3165, (1992). 87. D. P. Dygutsch, W. P. Neumann, and M. Peterseim, Synlett, 363, (1994). 88. A. Chemin, A. Mercier, H. Deleuze, B. Maillard, and O. Mondain-Monval, J. Chem. Soc., Perkin Trans. 1, 366, (2001). 89. J. Junggebauer and W. P. Neumann, Tetrahedron, 53, 1301, (1997). 90. M. Harendza, K. Leßmann, and W. P. Neumann, Synlett, 283, (1993). 91. W. M. Macindoe, A. Williams, and R. Khan, Carbohydr. Res., 283, 17, (1996). 92. F. A. G. Mercier, M. Biesemans, R. Altmann, R. Willem, R. Pintelon, J. Schoukens, B. Delmond, and G. Dumartin, Organometallics, 20, 958, (2001). 93. J. C. Martins, F. A. G. Mercier, A. Vandervelden, M. Biesemans, J.-M. Wieruszeski, E. Humpfer, R. Willem, and G. Lippens, Chem. Eur. J., 8, 3431, (2002). 94. M. Biesemans, F. A. G. Mercier, M. Van Poeck, J. C. Martins, G. Dumartin, and R. Willem, Eur. J. Inorg. Chem., 2908, (2004). 95. J.-M. Chr´etien, F. Zammattio, E. Le Grognec, M. Paris, B. Cahingt, G. Montavon, and J.-P. Quintard, J. Org. Chem., 70, 2870, (2005). 96. G. Dumartin, J. Kharboutli, B. Delmond, Y. Frangin, and M. Pereyre, Eur. J. Org. Chem., 781, (1999). 97. G. W. Kabalka, M. M. Goodman, R. S. Srivastava, K. R. Bowers, and R. C. Marks, J. Label. Compd. Radiopharm., 35, 220, (1994). 98. E. Berthommier, S. Chalon, B. Delmond, G. Dumartin, F. Marchi, L. Mauclaire, and M. Pereyre, J. Label. Compd. Radiopharm., 40, 96, (1997). 99. (a) P. A. Culbert and D. H. Hunter, React. Polym., 19, 247, (1993); (b) D. H. Hunter, A. M. Marinesch, C. Loc’h, and B. Mazi`ere, J. Label. Compd. Radiopharm., 37, 144, (1995).

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5.6

Palladium-Catalyzed Cascade Cyclization-Anion Capture Processes Employing Pre- and In Situ-Formed Organostannanes

Ron Grigg and Visuvanathar Sridharan Molecular Innovation, Diversity and Automated Synthesis (MIDAS) Centre, School of Chemistry, Leeds University, UK

5.6.1

Introduction

Catalytic cascade ring forming processes offer wide ranging opportunities for substantially leveraging molecular complexity and diversity via multiple insertions, with concomitant introduction of functionally by replacing the β-hydride elimination step of a Heck reaction with group or atom transfer. This concept has been developed into powerful, widely applicable, highly chemo- regio-, and stereo-selective palladium-catalyzed cyclization–anion capture methodologies.1,2 The use of the word ‘anion’ for the group or atom transfer/capture embraces both ionic and covalent sources of Y (Table 5.6.1) and is felt to be more appropriate than cross-coupling. Table 4.6.1 summarizes the basic process, which comprises four segments (i.e starter species, relay species, terminating species, and anion capture agent). The starter species is usually an appropriate halide (Cl, Br, I) or triflate, in which case the cascade begins with an oxidative addition reaction between the starter or ‘zipper’ species and Pd(0) to generate an organopalladium(II) species. Alternative starter strategies include hydropalladation of alkynes using a combination of Pd(0) and HX (CH3 COOH or [NHR3 ]+ X– ) to generate HPdXL2 species.3 In monocyclizations, the organopalladium(II) species cyclizes onto the terminating species (T). Exchange of halide or triflate with the anion capture agent Y followed by reductive elimination generates a regiospecifically functionalized monocyclic product and regenerates Pd(0). In polycyclization processes the initial Pd(II) intermediate engages one or more relay species (R) before passing to the terminating phase and anion capture. Thus, the potential exists to intersperse the relay species summarized in Table 5.6.1 with additional components, which would offer the potential to switch the cascade between inter- and intra-molecular processes, whilst incorporating valuable additional functionality. We have called such components relay switches because of their ability to extend the relay phase and their potential to switch the cascade between intra- and inter-molecular processes (See Section 5.6.4 for relay switches). This methodology is dependent on cyclization rates and relay switch capture rates being significantly faster than anion capture. Our experience thus far indicates that 3–7 membered rings can be constructed, according to Table 5.6.1 with little or no competition from direct capture. Organotin (IV) reagents are particularly attractive anion capture reagents due to their ease of preparation and tolerance for air and moisture. Moreover, in situ generation of organostannanes is readily achieved by palladium-catalyzed hydrostannylation of terminal Table 5.6.1

Potential combinations for (poly) cyclization–anion capture processes

Starter species

Relay species (R)

Terminating species (T)

Allyl Alkyl Vinyl Allenyl Carbamyl Oxycarbamyl

Alkene Alkyne 1,2-Diene 1,3-Diene

Alkene Alkyne 1,2-Diene 1,3-Diene

Y Anionic [H, OAc, CN, N3 , TsNR, SO2 Ph, HCH(CO2 R)2 , enolates] Neutral (amines, MeOH/CO, acrylates, allenes) Organometallics RM [M = Sn(IV), B(III), Zn(II)]

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alkynes bearing a β-heteroatom. This chapter describes cyclization–anion capture processes using preand in situ-formed organostannanes.

5.6.2

Mono-Cyclization–Anion Capture Processes

Carbamyl and Oxycarbonyl Starter Species

Carbamyl 1, 2 and oxycarbonyl 5 chlorides have proven effective starter species although their cyclization– anion capture processes have been only briefly explored (Scheme 5.6.1).4,5 The terminating species in Scheme 5.6.1 are alkynyl (1, 5) and alkenyl (3) with pre-formed Sn(IV) capture agents. These processes, as expected, are regio-and stereoselective and their full potential remains to be explored, but reference to Table 5.6.1 indicates the substantial leverage available from alternative terminating and anion capture species.

S PdCl Cl

O

Pd(0)

NPh SnBu3

1

O

O

NPh

NPh

Cl N 3

O

N

SnBu3 S toluene, 100 °C

Ph

S

PdCl O

Pd(0) O

87%

2

S toluene, 100 °C

N Ph

Ph

4

50%

S PdCl

Cl O

O

Pd(0) O

O

O

O

THF, rt 5 S

SnBu3

Scheme 5.6.1

6

51%

Carbamyl and oxycarbonyl initiated cascades

Allyl and Benzyl Halides, and Acetates as Starter Species

Palladium-catalyzed cyclization–anion capture involving 1,6-en-yne 7 proceeds via vinylpalladium species 8 (Scheme 5.6.2) to give 9 in good yield.6 Similar processes using allyl acetate or halide starter species and phenylboronic acid or lithium acetate as anion capture agents have been reported.7,8

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OAc H Pd

SnBu3

Pd(0) / THF 60 oC

H

H

H

H

N

N PhO2S

H

N

SO2Ph

7

SO2Ph

8

Scheme 5.6.2

43%

9

π-Allyl initiated cascade

Vinyl Halide Starter Species

Suffert et al.9,10 reported the first examples of palladium-catalyzed 4-exo-dig cyclization–anion capture processes (Scheme 5.6.3). In an ingenious sequence, which incorporates two separate electrocyclic steps, the vinyl starter species 10, in the presence of Pd(0) and trans-vinyl distannane 11 in benzene at 90 ◦ C for 30 min, affords 12, which on further heating undergoes disrotatory electrocyclization to the strained tricyclic diol 13 (Scheme 5.6.3). The electrocyclization process establishes the expected antistereochemistry of the 6-H and tri-n-butylstannyl group. Intermediate 13 undergoes elimination-

HO

SiMe3

OH

HO HO

HO Br

SnBu3

SiMe3

Pd(PPh3)4

HO SiMe3

benzene, 90 °C

+ benzene,

Bu3Sn 10

SnBu3

90 °C 0.5 h

11

2h

6

13

12

H

SnBu3

-

O

O HO

O

H

H

SiMe3

62%

SiMe3

H

15

HO

14 OH

SiMe3 HO

HO Br +

OH

SiMe3

H H

-

OH

Bu3Sn

CO2Et CO2Et 16

10

Scheme 5.6.3

SiMe3

Pd(PPh3)4 benzene, 90 °C 2 h

H 17

CO2Et 30% CO2Et

Vinyl halide initiated endo-dig cyclization-rearrangement cascades

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fragmentation to14 followed by hemiketalization to 15. An analogous reaction of 10 and 16 under essentially the same conditions affords 17 via thermal 8π -electrocyclization of a similar intermediate to 12. Examples of palladium-catalyzed 5-exo-trig cyclization–anion capture processes are shown in Scheme 5.6.4.11 Vinyl starter species 18 and 20 in the presence of Pd(0) and vinyltri-n-butyltin afford both 19 and 21 in 60% yield. Cyclization of the initial vinyl Pd(II) bromides onto the conjugated dienes (terminating species) generates π-allylpalladium (II) intermediates which, on coupling to a vinyl stannane, generate skipped diene products 19 and 21. This general strategy provides a facile route to decorating ring systems with skipped dienes. Suffert et al. have also reported a 5-exo-dig cyclization–anion capture process using alkynylstannanes as anion capture agents.12

Pd(0) CH2=CHSnBu3

Br

60%

18

19

LPd Pd(0)

EtO2C

Br

EtO2C

CH2=CHSnBu3

EtO2C

EtO2C

EtO2C

EtO2C 60% PdLn 21

20

Scheme 5.6.4

Facile routes to skipped dienes

When an alkene is used as a relay or as a terminating species, it may be necessary to incorporate a blocking substitutent to prevent an unwanted β-hydride elimination superseding further cyclization– anion capture. An alternative strategy is to position a proximate alkene or other coordinating moiety to suppress the undesirable syn-β-H elimination. This latter strategy is especially useful in five-membered ring formation. A recent example of this is shown in Scheme 5.6.5.13a Thus vinyl bromide 22 reacts with aryl boronic acids in the presence of Pd(0) in THF/H2 O at 80 ◦ C to afford the cyclization–anion capture products 24 in 69–92% yield. The authors suggested that one of the N -sulphonyl oxygen atoms coordinatively stabilizes the alkyl Pd intermediate 23a, protecting it from the usual β-hydride elimination. H PdBr Br N Ts

Pd(PPh3)4

N S

ArB(OH)2, Na2CO3

22

23a

Scheme 5.6.5

Ar

O or

THF/H2O (6:1), 80 °C

BrPd

O

N Ts

N Ts 23b

24 Ar = phenyl 92% Ar = 1-naphthyl 88% Ar = p-methoxyphenyl 69%

Circumventing β-H elimination-alternative rationalizations

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However, there is ample literature13b to suggest that the π -complex 23b is the more likely source of suppression of β-hydride elimination. Aryl Iodide Starter Species

Aryl/heteroaryl iodides constitute the most widely used starter species to date.1a (1) In situ generation of aryl halide starter species. Cascade in situ generation of starter species is a challenging and novel area whose successful realization would greatly expand the synthetic flexibility of Table 5.6.1, whilst dramatically accelerating access to a wide variety of starter species with concomitant increase in molecular diversity of the products. We have recently reported palladium-catalyzed in situ generation of starter species using CO and allene as relay switches (see below) and their subsequent cyclization–anion capture processes (Scheme 5.6.6). Because of their ability to extend the relay phase and switch the cascade between intra-and inter-molecular processes, CO and allene are called relay switches (see Section 5.6.4). Thus Pd(0) reacts selectively with 2-iodothiophene 25 or vinyl triflate 30. The resultant aryl or vinyl palladium(II) species then react with allene or CO to give π -allyl palladium(II) or acyl palladium(II) species that are captured inter-molecularly by the nitrogen/oxygen nucleophiles to generate the zippers 27 and 31. These in situ-generated zippers undergo palladium-catalyzed cyclization– anion capture with boronic acids or stannanes to afford 28 and 32 in good yield (Scheme 5.6.6).14,15

S S

I

+

.

(i) Pd(PPh3)4

I

Ph

toluene, 70°C

+ NH

25

26

(ii) PhB(OH)2

N

110 °C

Ts

Ts

70%

28 (14 examples)

S I N 27

(i) Pd(0), K2CO3

I + OH

Ts

toluene, 70 °C

CO +

(ii) Bu3SnC

TfO

29

O CH

30

O 32

I O O 31

Scheme 5.6.6

Strategies for in situ generation of zippers

81% (2 examples)

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627

(2) In situ generation of anion-capture agents. The basic strategy makes use of known regioselective palladium-catalyzed processes such as hydrostannylation of terminal alkynes bearing a β- or γ-heteroatom (Scheme 5.6.7).16 This strategy has proved extremely powerful in generating complex organotin(IV) anion capture species in situ, via regioselective hydrostannylation of appropriate alkynes as part of a temperature-controlled cyclization–anion capture cascade. This extraordinarily versatile protocol gives access to bridged and spirocyclic small, as well as 11–17-membered macrocyclic, heterocycles.17,18 Additionally, this protocol facilitates the incorporation of a wide variety of biomolecules: α-amino esters, N -heterocycles, β-aryl/heteroaryl ethylamines, sugars, nucleosides, purines, benzodiazepines and βlactams, into the cascade via in situ formation of their vinyl stannanes.19,20 A typical nucleoside example is shown in Scheme 5.6.8 and representative examples of the 57 compounds synthesized are collected in Figure 5.6.1.

Pd(0) + XR

Bu3SnH 0 C - rt

X = O, NR1

major

Scheme 5.6.7

+

Bu3Sn

o

Bu3Sn

XR

XR

minor

Strategy for in situ generation of anion-capture agents

O O THPO

I

N

O

Bu3SnH

N

O

N

O

X

toluene, 0 °C - rt

+ X

N

(i) Pd2(dba)3 / TFP

O

O

(ii) 110 °C

O

O 34 a. X = O 60% O b. X = NTs 65% OTHP

33 a. X = O b. X = NTs

Scheme 5.6.8

Cascade employing an in situ generated complex vinyl stannane

I

N CO2 Me SO2 Ph

N CO2Me SO2 Ph

65%

O

I N Me

O

O O

O

O

O

O

O N O Me

Figure 5.6.1

O

O

O

Cascade products from in situ hydrostannylation of alkynes

O

O 65%

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We have transferred the in situ generation of anion capture agents/cyclization–anion capture processes to solid supports.21 Thus 35, attached to Wang resin, was reacted with alkyne 36 in the presence of Pd(0) and Bu3 SnH in toluene initially at 0 ◦ C for 1 h, then 110 ◦ C for 16 h. The expected product 37 was cleaved from the resin by transesterification with MeOH in good yield (Scheme 5.6.9). (i) Pd2(dba)3 /

O I

O

Bu3SnH

N +

N N

N

P

O

O

3

toluene, 0 °C-rt

N

MeO

N

(ii) 110 °C

N

(iii) MeOH / THF

CO2Me

Scheme 5.6.9

CO2Me

NaCN, Et3N

36

35

N

37

79%

Solid-phase synthesis with an in situ generated organostannane

Sequential decarboxylative azomethine ylide cycloaddition–palladium catalyzed hydrostannylation– cyclization–anion capture processes have also been achieved (Scheme 5.6.10).22 (i) Pd2(dba)3 /

O I

O

N +

N N

N

P

O

toluene, 0 °C-rt

O

3

N

MeO

N

(ii) 110 °C

N

(iii) MeOH / THF

CO2Me 35

Bu3SnH

36

NaCN, Et3N

N

CO2Me 37

79%

Scheme 5.6.10 Two-step protocol for interfacing azomethine ylide cycloadditions with catalytic cyclization anion–capture by an in situ generated vinylstannane

Thus, 38 reacts with benzaldehyde and sarcosine (toluene, 110 ◦ C, 20 h) to give a mixture of cycloadducts 39 and 40. The endo-cycloadduct 39 is hydrostannylated in situ, in the presence of zipper molecule 33a to afford 41 in 66% yield as a 1:1 mixture of diastereoisomers (Scheme 5.6.10). Overall, the two-step protocol combines six reactants and delivers 41, in which two rings, four C C bonds, one C N bond, and four stereocentres and one tetra-substituted C-centre have been formed. (3) Preformed Aryl Iodide Starter Species. Aryl/heteroaryl iodide zippers undergo monocyclization– anion capture processes with alkenes, alkynes, 1,3-dienes, or 1,2-dienes as terminating species and organostannanes as anion capture agents to generate fused, bridged, and spirocyclic compounds.23 Others have also reported similar processes using aryl halide starter species.24 Table 5.6.2 illustrates how complex spiroisoindolones are readily accessible by this cascade approach. Typical examples of fused and spirocyclic ring systems are shown in Table 5.6.2.23,24 The utilization of vinyl tri-n-butylstannanes as anion capture agents opens up many opportunities for interfacing palladium chemistry with ruthenium-catalyzed olefin metathesis. Thus, 42 reacts with tri-nbutylvinyltin (toluene, 110 ◦ C) to afford 43 in 77% yield. Diene 43 undergoes ring-closing metathesis (RCM) with Grubb’s first generation catalyst (DCM, rt) to give the corresponding spirocycle 44 in 85% yield (Scheme 5.6.11).25

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Palladium-Catalyzed Cascade Cyclization-Anion Capture Processes Employing Table 5.6.2

Stereoselective 5-exo-trig and -dig cyclization–anion capture

I

O

N Bn

O 23

SnBu 3

O

N Bn

SnBu 3

90%

I 23 N SO2 Ph

O

O 86%

Me

N SO2Ph

Me

I

23

SnBu3 N Ac

N Ac

60%

H

I

24

SnBu3

43%

Ph N

O

23 SnBu3

I

N H

I N Bn MeO 2C I O

O

60%

Ph

23

SnBu3

O

60%

BnN MeO 2C

O Ph

PhSnBu 3 O

50%

24

629

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Tin Chemistry: Fundamentals, Frontiers and Applications PCy3 Cl Cl I

Pd2dba3 /

N Ts 42

O

P 3

N Ts

toluene, 110 oC Bu3SnCH=CH2

77%

Ph Ru 5 mol% PCy3

DCM, rt

N 85% Ts 44

43

dba = dibenzyldine acetone

Scheme 5.6.11

5.6.3

Two-step protocol for interfacing cyclization-organostannane capture with olefin metathesis

Bis-Cyclization–Anion Capture Processes

Allenyl Starter Species

Propargylic carbonates are valuable substrates in Pd(0)-catalyzed processes.26 Their chemistry differs mechanistically from that of simple alkynes in that they form allenyl palladium species. We27 and others28 have shown these versatile intermediates can serve a double function as both starter and terminating species in bis-cyclization–anion capture cascades A typical example is the process 45 → 46, which generates azabicyclo [3.1.0] hexanes (Scheme 5.6.12) in good yield.27 OCO2Me Pd(OAc)2 / PPh3 N SO2Ph 45

LiCl, THF, 60 oC

.

PdX

N SO2Ph

5-exo-trig

.

R

XPd

H

3-exo-trig RSnBu3 or SO2Ph NaBPh4

N

N

SO2Ph 46

R = Ph 73% R = 2- thienyl 80% (4 examples)

Scheme 5.6.12 Propargylic carbonates, as precursors to allenyl palladium(II) species, provide both starter and terminating species

We have provided numerous examples that show that these bis-cyclization–anion capture processes are diastereoselective. For example, 47a, b in the presence of Pd(0) and 2-thienyl tri-n-butylstannane in THF at 60 ◦ C for 2–4 h afford 48 and 49 as single diastereomers (Scheme 5.6.13).29 No products resulting from β-hydride elimination–readdition processes are observed, showing that 3-exo-trig cyclization is faster than β-hydride elimination, even in the presence of two further β-hydrogen atoms. The origin of the diastereoselectivity can be accounted for by examining the pre-transition state conformers (Scheme 5.6.14). A pseudo 1,3-diaxial interaction which exists between the protons and the Z -olefin substituent in the allenyl Pd(II) species A, increases in both B and in the vinyl Pd(II) species C, and in the transition state leading to C. This steric effect is reflected in slower reaction rates for the Z -isomers compared to the E-isomers, which proceed via D, E, and F. The clean diastereomer formation attests to the configurational stability of intermediates B and E.29

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Palladium-Catalyzed Cascade Cyclization-Anion Capture Processes Employing

S H

OCO2Me

47a

Et

R

o

Ts 81%

48

Scheme 5.6.13 H

THF, 60 °C

N

SnBu3

S

Ts 47 a. R1 = Et, R2 = H b. R1 = H, R2 = Et

H

R

.

N

H PdL n

R

49

H

H

.

N Ts

H

H H

H

.

R PdL n

E

Scheme 5.6.14

H

N

PdL n

H

R

.

H

PdLn

N

R

C

H

Ts D

Ts 53%

Ts

B

H N

SnBu3

PdLn

A

Ts

S

N

The stereoselectivity of the cyclopropanation cascade

H

Ts

H

R2

THF, 60 C

N

H

Pd(OAc)2 / PPh3

Pd(OAc)2 / PPh3

H

S Et

47b

1

631

H

N

R PdL n

Ts F

Proposed mechanism for the cyclopropanation cascades

Oppolzer et al. have utilized the above methodology to synthesize enantiomerically pure (-)α–thujone, using dimethylzinc as the anion capture agent.28 Aryl Iodide Starter Species

Bis-cyclization processes can be engineered to deliver linear or angularly fused systems (Scheme 5.6.15) or spirocycles.

I

Pd(0) RSnBu3

R

R I

Pd(0) RSnBu3

Scheme 5.6.15

Strategies for bis-cyclization–anion capture with aryl halide starter species

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Tin Chemistry: Fundamentals, Frontiers and Applications

Typical examples of bis-cyclization anion capture processes forming fused and spirocyclic rings, triggered by aryliodide species, are shown in Scheme 5.6.16. Thus, enamide 50 reacts with N -SEMindolyl stannane in the presence of Pd(0) to afford a 5:1 diastereomer mixture of 51 and 52 in 74% yield, whilst 53 reacts with 2-pyridyltri-n-butyltin in the presence of Pd(0) to afford 54a as a single diastereoisomer.23

I

N SEM

Pd(0) N

N SnBu 3

O 50

N

+

O

N SEM

O 52

51 5

:

PhO 2S

SO 2Ph N

1

N

PdI

N SO2 Ph

N SO2 Ph

PdI

N SO2 Ph

53 a

53

74%

PhO2 S

N Pd(0)

I

N SEM

53 b

Bu 3SnCH=CH 2 PhO 2S

N

SnBu 3

N

PhO2 S N

N N SO2Ph

N SO2 Ph 54a

54b

Scheme 5.6.16

Stereochemical outcome of bis-cyclization-organnostannane-capture cascades

The formation of a 5:1 mixture of 51 and 52 suggests that steric effects play a role in the diastereoselectivity, as illustrated in Scheme 5.6.17. The second cyclization requires an eclipsed alignment of the Pd C and olefin C C bonds. This arrangement creates a pseudo seven-membered ring, which can L

L

L

Pd

Pd Me

L Me Me

51 Ar N

Scheme 5.6.17

Me Ar

52

N

Mechanistic rationalization for the stereoselectivity in the formation of 51 and 52

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Palladium-Catalyzed Cascade Cyclization-Anion Capture Processes Employing

633

adopt a chair or boat-like conformation (Scheme 5.6.17). The developing Me/Me steric interaction in the boat-like transition state is destabilizing with respect to the trans Me/Me arrangement in the chair-like transition state. In the case of 53, a chair-like pre-transition state conformer 53a is thought to be involved, in which the bulky aryl and palladium groups are pseudo equatorial. This gives rise to the energetically most favourable equatorial conformer 53b, which is transformed into 54a (Scheme 5.6.16). Under identical conditions, in the presence of vinyltri-n-butyltin, the intermediate 53a is intercepted by vinyl transfer (Scheme 5.6.16) showing that this inter-molecular anion capture 54b is significantly faster than the 6-exo-trig cyclization–anion capture. Tris-Cyclization–Anion Capture Processes

Only one example of tris-cyclization–anion capture has been reported to date. The cascade employs an allenyl Pd(II) species generated from an alkynyl carbonate (Scheme 5.6.18). Thus, 55 undergoes triscyclization to 57 (THF, reflux, 24 h) with organotin(IV) anion capture reagents, using a catalyst system comprising 10 mol% Pd(OAc)2 , 20 mol% PPh3 , and LiCl (2 mol equiv.).30 This process generates four C C bonds and three rings.

OCO2Me

O

XPd

PdX

LiCl, THF, 60 °C RSnBu3

CH2

.

O

Pd(OAc)2/PPh3

CH2

N

.

CH2

N

CH2

N CO2Me

CO2Me

CO2Me

O

CH2 Pd-shuttle

55 R

A O

N

N

CO2Me

CO2Me 56 a. A = PdX b. A = OCO2Me

57 Ph

R

Ph S

Yield (%)

82

Scheme 5.6.18 forming 57

80

77

O

C C

O 74

62

Mechanistic interpretation of the tris-cyclization-organostannane capture cascade

An alternative mechanism for Scheme 5.6.18 involves a palladium-catalyzed formal [2+2+2] cycloaddition 55 → 56b, followed by oxidative addition of Pd(0) to give 56a as an intermediate common to both cascades. Apparently, both mechanisms may operate, since mixtures of 56b and 57 are obtained under certain conditions. Significantly, under the conditions of Scheme 5.6.18, 56b is not converted to 57.

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Tin Chemistry: Fundamentals, Frontiers and Applications

CO

-

+ RN

C

RC

CR

O O O

Figure 5.6.2

5.6.4

R

. R = H, alkyl, aryl, R = CO2 R, SO2 R R = OR

X

Y Z

X

X = O, NR, Y = CO, CH2 Z = O, NR, S

Some potential relay switch reactants

Cyclization–Anion Capture Processes Involving Relay Switches

The standard cyclization–anion capture processes suffer from the constraint that they are two component processes, i.e. the ‘zipper’ precursor and Y. This constraint would be circumvented if polycomponent processes could be achieved by extension of the relay phase by incorporation of further reactants that facilitate subsequent additional inter- and intra-molecular cascade segments. Thus, the potential exists to interface the relay species summarized in Figure 5.6.2 with additional components, which would offer the potential to switch the cascade between inter-and intra-molecular processes, whilst incorporating valuable additional functionality. Such components might intercept the cyclization–anion capture cascade only at the relay phase, only at the terminating phase, or participate in both relay and terminating phases. We have called such components relay switches (Figure 5.6.2) because of their ability to extend the relay phase and switch the cascade between intra- and inter-molecular processes. Such components impart a major increase in the scope of the original cyclization–anion capture scheme, whilst offering tremendous increases in the diversity and complexity of the products. At present, the most fully developed relay switches are CO and allenes.1,2 CO as Relay Switch

(1) Monocarbonylation processes. A significant advantage of carbon monoxide as a relay switch is the wide range of anion capture agents that can be employed to capture the resultant acylpalladium species, together with the potential for multiple CO insertions. Higher-level polycyclization–anion capture processes that employ the relay switch concept (see below) allow the assembly of polycyclic lactones.31 Initially, we showed that a series of three component processes could be readily achieved by employing CO (1 atm) in combination with anionic, neutral, and MR groups of capture agents Y.32 Nitrogen nucleophiles33 oxygen nucleophiles,34−38 and hydride39 have been successfully used as anion capture agent in monocyclization process using CO as a relay switch, as have processes employing organostannanes as anion capture agents.40 Some typical examples of the latter are shown in Table 5.6.3, which illustrates the flexibility of the cascade with respect to ring size and type, together with the ability to generate spirocyclic products. Cyclization–carbonylation–anion capture processes involving in situ generated vinylstannanes as terminating agents (as in Section 5.6.2) afford a variety of complex heterocyclic α, β-unsaturated ketones in good yield. Typical examples are shown in Table 5.6.4.40 Enhancement of diversity and complexity is available by coupling the cyclization–anion capture cascades with core organic reactions in extended cascades. Equally powerful is the use of high atom economy core reactions to generate zippers for the cyclization–anion capture cascade. Thus Grigg et al.41 have utilized Katritzky’s oxypyridinium betaine cycloaddition reaction42 to generate bridged bicyclic starter species 58 and carried out palladium-catalyzed cyclization–carbonylation–anion capture processes with organostannanes to generate polycyclic heterocycles 59 a–d (Scheme 5.6.19). Moreover, the cycloaddition–cyclization can be carried out as a one-pot process.

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Termolecular queuing cascades with (CO 1 atm) and anion transfer from Sn (IV)a and Bb

Table 5.6.3

I Bu3 Sn N Bn

N SEM

O

Bn N

N SEM

O O N Bn

(61)

O

I

SnBu3

O

O

N Bn

O

H O (82) Ph I

CH 2

NaBPh4

O

NBn

NBn

O a b

(60)

O

Reactions carried out in toluene at 110 ◦ C, using a catalyst system comprising 10 mol% Pd(OAc2 , 20 mol% TFP Reactions carried out in anisole at 110 ◦ C, using a catalyst system comprising 10 mol% PD (OAC)2 , 20 mol% PPh3 , ET4 NCl (I mol equiv.)

Table 5.6.4

Cascade hydrostannylation–cyclization–carbonylation–anion-capture processesa

Substrate SO2Ph N

SO2Ph N CO2Me

I

O

N

N

SO2Ph

SO2Ph

CO2Me (61) O

O I

N

N O

N

N

O

O

O (58)

N Bn

O N

I O

N

N O

N Bn

N

HN

O

O O

(45)

Reactions were carried out using 2.5 mol% Pd2 dba3 , 10 mol% TFP, tri-n-butyltin hydride (1 mmol) in toluene at 0 ◦ C for 1 h, at which point the zipper (1 mmol) was added, a CO-filled balloon was attached, and the mixture stirred and heated at 100 ◦ C for 6–24 h

a

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Tin Chemistry: Fundamentals, Frontiers and Applications I OH

+ N

O I

R N

O

O O R

N

N

N Pd(OAc)2 , TFP toluene, CO (1 atm)O 85 o C

H

O

Br

H 58

N

R

Bu3 SnY

O

a. R = 4-EtC 6 H 4 b. R = 2,6-( iPr) 2C 6H 3

O H H

O

H

Y

59 a. Y = 3-quinolyl, R = 4-EtC 6 H4 b. Y = 2-thiazolyl, R = 4-EtC 6 H4 c. Y = 2-thienyl, R = 2,6-(iPr)2 C6H3 d. Y = 2-furyl, R = 2,6-(iPr)2 C6 H 3

75% 54% 84% 51%

Scheme 5.6.19 One-pot sequential process interfacing oxypyridinium betaine cycloaddition with cyclization-carbonylation-organostannane capture

(2) Multiple CO Insertions. The cyclization–anion capture methodology allows incorporation of two or more non-contiguous carbonyl groups. One structural strategy to achieve multiple CO insertion–anion capture processes utilizes the fact that carbopalladation, forming a four-membered ring, is slower than acylpalladation, forming a five–membered ring, under 1 atm of CO. Typical examples are shown in Scheme 5.6.20, in which the first CO insertion occurs faster than a slow 4-exo-trig carbopalladation. The acylpalladium species then undergoes a facile 5-exo-trig acylpalladation, followed by insertion of a further CO and anion capture.43,44 O I CO (1 atm) +

+

Bu3 SnPh

Ph

Pd(PPh 3) 2/PPh3 O

Et3N, toluene, 110 °C

86% O

I +

CO (1 atm) +

MeOH

Et3N, toluene, 80 °C

Scheme 5.6.20

OMe

Pd(PPh 3) 2/PPh3 H O 90%

Cascade strategy for assembling functionalized indanones

Benzyne as Relay Switch

The recently disclosed facile generation of benzyne 61 from 2-(trimethylsilyl)phenyltriflate 60, by fluoride ion-initiated elimination has generated renewed interest in this reactive species. For example, Cheng et al.45 have reported a palladium-catalyzed three-component reaction involving benzyne, allyl chloride, and an allenylstannane to afford 1-allyl-2-allenyl benzene 62 (Scheme 5.6.21). In unpublished work, shown in Scheme 5.6.22, we have shown that 33b, 2-(trimethylsilyl) phenyl triflate and 2-thienyl tri-n-butylstannane, in the presence of Pd(0) in MeCN at 80 ◦ C for 16 h, afford 63 in 40% yield.46 Work is in hand to optimize the process.

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Palladium-Catalyzed Cascade Cyclization-Anion Capture Processes Employing SiMe3

60 OTf

OTf

61

.

+ Bu3Sn

SiMe3

Scheme 5.6.21 capture

CsF

Cl

+

637

.

Pd(dba) 2/dppe,CsF MeCN, rt, 10 h

93%

62

Three-component cascade with in situ benzyne formation terminating with allenylstannane

S

I

SiMe3 +

N 33b Ts

Pd(OAc)2 /(o-Tol)3 P

+ OTf

S

SnBu 3

N Ts

CsF, MeCN,80 °C 63

Scheme 5.6.22 Three-component cyclization cascade with in situ benzyne formation terminating in thienylstannane capture

Allene as Relay Switch

The use of allene as a relay switch introduces the benefits/challenges of regio- and stereo-selectivity. Our initial studies of cyclization–anion capture with allene as a relay switch generated allylic amines in excellent yield. Reactions were conducted in toluene (90 ◦ C, 20 h) in a Schlenk tube, and employed 0.5–1 atm of allene and various amines, using alkenes/alkynes as relay species (Scheme 5.6.23).47 The use of organostannanes as anion-capture agents in this cascade has yet to be explored. 5.6.5

Summary

Palladium-catalyzed cyclization–anion capture processes terminating in a Stille coupling to sp3 or sp2 C-centres can be achieved with both in situ-generated or pre-formed organostannanes. In the former case, the organostannane is readily, and cleanly, generated at 0 ◦ C by the reaction of tri-n-butyltin hydride with a strategically located terminal alkyne, activated by a β-heteroatom (O or N), in the presence of a Pd(0) catalyst and all the other reagents. The relative rates of 3,5- and 6-exo-trig processes and 5- and 6-exo-dig cyclizations of organopalladium(II) species are normally significantly faster than direct capture of the organostannanes. The (poly)cyclization phase of the cascade delivers fused, bridged, and spirocyclic ring systems with exquisite regiochemical control, allowing access to large rings, e.g. 12–17-membered bridged rings. These cascades are readily interfaced with carbonylation processes employing CO (1 atm), allowing ready access to ketones, α, β-unsaturated ketones, and processes delivering incorporation of two non-contiguous CO moieties.

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Tin Chemistry: Fundamentals, Frontiers and Applications R N 5-6

Pd(0) I +

.

R1

NHR1 R2 Pd(0)

R 5-6

RSnBu 3

R N R1

I +

.

Pd(0)

5-7

NHR1 R2

R

Pd(0) RSnBu 3 5-7

Scheme 5.6.23

Some strategies for monocyclization–allene incorporation–anion capture

References 1. R. Grigg and V. Sridharan, J. Organomet. Chem., 576, 65 (1999). 2. R. Grigg and V. Sridharan, Pure and Appl.Chem., 70, 1047 (1998). 3. (a) B. M. Trost, F. J. Fleitz, and W. J. Watkins, J. Am. Chem. Soc., 118, 5146 (1996); (b) S. Torii, H. Okumoto, M. Sadakane, and L. He Xu, Chem. Lett.,10, 1673 (1991) 4. M. R. Fielding, R. Grigg, and C. J. Urch, Chem. Commun., 2239 (2000) 5. R. Grigg and V. Savic, Chem. Commun., 2381 (2000). 6. (a) R. Grigg, S. Sukirthalingam, and V. Sridharan, Tetrahedron Lett., 32, 2545 (1991); (b) P. K. Bhatt, D. S. Shin, and J. R. Falck, Tetrahedron Lett., 33, 4885 (1992). 7. G. Zhu and Z. Zhang, Org. Lett., 5, 3645 (2003) 8. (a) W. Oppolzer, Angew.Chem. Int. Ed., 28, 38 (1989); (b) B. M. Trost and J. I. Luengo, J. Am. Chem. Soc., 110, 8239 (1988). 9. (a) J. Suffert, B. Salem, and P. Klotz, J. Am. Chem. Soc., 123, 12107 (2001); (b) B. Salem, P. Klotz, and J. Suffert, Org. Lett., 5, 845 (2003); (c) J. Suffert, B. Salem, and P. Koltz, Synthesis, 298 (2004). 10. (a) B. Salem and J. Suffert, Angew. Chem. Int. Ed., 43, 2826 (2004); (b) C. Bour, G. Blond, B. Salem, and J. Suffert, Tetrahedron, 62, 10567 (2006); (c) C. Bour and J. Suffert, Eur. J. Org. Chem., 1390 (2006). 11. B. Burns, R. Grigg, P. Ratananukul, V. Sridharan, P. Stevenson, S. Sukirthalingam, and T. Worakun, Tetrahedron Lett., 29, 5565 (1988) 12. C. Bour and J. Suffert, Org. Lett., 7, 653 (2005). 13. (a) C. W. Lee, K. S. Oh, K. S. Kim, and K. H. Ahn, Org. Lett., 2, 1213 (2000); (b) C. M. Andersson, J. Larsson, and A. Hallberg, J. Org. Chem., 46, 5757 (1990); M. Larthed, C. M. Andersson, and A. Hallberg, Tetrahedron, 50, 285 (1994) 14. R. Grigg, E. Mariani, and V. Sridharan, Tetrahedron Lett., 42, 8677 (2001). 15. R. Grigg, P. Bogdan, and C. J. Urch, Tetrahedron Lett., 37, 695 (1996).

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N. D. Smith, J. Mancusco, and M. Lautens, Chem. Rev., 100, 3257 (2000) A. Casachi, R. Grigg, J. M. Sansano, D. Wilson, and J. Redpath, Tetrahedron Lett., 37, 4413 (1996) A. Casaschi, R. Grigg, J. M. Sanasano, D. Wilson, and J. Redpath, Tetrahedron, 56, 7541 (2000). A. Casachi, R. Grigg, and J. M. Sansano, Tetrahedron, 57, 607 (2001). A. Casaschi, R. Grigg, and J. M. Sansano, Tetrahedron, 56, 7553 (2000). R. Grigg, W. S. MacLachlan, D. T. MacPherson, V. Sridharan, and S. Suganthan, Tetrahedron, 57, 10335 (2001). R. Grigg, M. F. Jones, M. McTiernan, and V. Sridharan, Tetrahedron, 57, 7979 (2001). P. Fretwell, R. Grigg, J. M. Sansano, V. Sridharan, S. Sukirthalingam, D. Wilson, and J. Redpath, Tetrahedron, 56, 7525 (2000). (a) F. T. Luo and R. T. Wang, Tetrahedron Lett., 52, 7703 (1991); (b) M. S. Pinaud, P. Diaz, J. Martinez, and F. Lamaty, Tetrahedron, 63, 3340 (2007). (a) R. Grigg and M. I. Ramzan, unpublished work; (b) R. Grigg, V. Sridharan, and J. Zhang, Tetrahedron Lett., 40, 8277 (1999). J. Tsuji and T. Mandai, Angew. Chem. Int. Ed. Engl., 34, 2589 (1995). R. Grigg, R. Rasul, and D. Wilson, Tetrahedron Lett., 37, 4609 (1996). (a) W. Oppolzer, A. Pimm, B. Stammen, and W. Hulme, Helv. Chim. Acta., 1997, 80, 623 (b) A. G. Steining and A. de Meijere, Eur. J. Org. Chem., 1333 (1999). J. Bohmer, R. Grigg, and J. D. Marchbank, Chem. Commun., 768 (2002). R. Grigg, R. Rasul, and V. Savic, Tetrahedron Lett., 38, 1825–1828 (1997). T. Sugihara, C. Coperet, Z. Owczarczyk, L. S. Harring, and E. I. Negishi, J. Am. Chem. Soc., 116, 7923 (1994) R. Grigg, P. Kennewell, and A. J. Teasdale, Tetrahedron Lett., 33, 7789 (1992); R. Grigg and V. Sridharan, Tetrahedron Lett., 34, 7471 (1993). R. Grigg, J. P. Major, F. M. Martin, and M. Whittaker, Tetrahedron Lett., 40, 7709 (1999) C. Coperet and E. I.Negishi, Angew. Chem. Int. Ed. 1, 165 (1999) D. H. Hua, K. Takasu, X. Huang, G. S. Millward, Y. Chen, and J. Fan, Tetrahedron, 56, 7389 (2000) G. D. Artman and S. M. Weinreb, Org. Lett., 5, 1523 (2003) V. K. Aggarwal, P. W. Davies, and W. O. Moss, Chem. Commun., 972 (2002) V. K. Aggarwal, P. W. Davies, and A. T. Schmidt, Chem. Commun.,1232 (2004) S. Brown, S. Clarkson, R. Grigg, W. A. Thomas, V. Sridharan, and D. M. Wilson, Tetrahedron, 57, 1347 (2001) U. Anwar, A. Casachi, R. Grigg, and J. M. Sansno, Tetrahedron, 57, 1361 (2001) M. R. Fielding, R. Grigg, V. Sridharan, M. Thornton-Pett, and C. Urch, Tetrahedron, 57, 7737 (2001) N. Dennis, A. R. Katritzky, and Y. Takeuchi, Angew. Chem. Int. Ed. Engl., 15 1–60 (1976) R. Grigg, J. Redpath, V. Sridharan, and D. Wilson, Tetrahedron Lett., 35, 7661 (1994) C. Cop´eret, S. Ma, and E. I. Negishi, Angew. Chem. Int. Ed. Engl., 35, 2125 (1996) M. Jeganmohan and C. Cheng, Synthesis, 10, 1693 (2005) R. Grigg, L. Byrne, and V. Sridharan unpublished results. R. Grigg, V. Savic, V. Sridharan, and C. Terrier, Tetrahedron, 58, 8613 (2002)

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5.7

Carbostannylation of Carbon–Carbon Unsaturated Bonds

Eiji Shirakawa Department of Chemistry, Graduate School of Science, Kyoto University, Japan

5.7.1

Introduction

Addition of carbon–tin bonds to carbon–carbon unsaturated bonds is called carbostannylation, a process where organostannanes are transformed to more complicated ones through the insertion of two or more carbon atoms into a tin–carbon bond (Scheme 5.7.1). Among the organometallic compounds used in carbometallations, organostannanes are the least nucleophilic, and this character makes carbostannylation particularly advantageous from the standpoint of handling and chemoselectivity. Carbon–tin bonds add to diverse types of unsaturated hydrocarbons, such as alkynes, alkenes, and dienes to give alkenyl-, allyl-, and alkylstannanes. All these organostannanes are widely used as synthetic precursors and react with various organic electrophiles, in the presence or absence of an appropriate activator. However, addition to alkynes is the most important among the carbostannylations because regio- and stereoselective carbostannylation of alkynes gives isomerically pure alkenylstannanes, which otherwise are not readily available. The resulting alkenylstannanes are easily transformed to multisubstituted alkenes through the palladium-catalyzed coupling with organic electrophiles. The scope of substituents on the metal is exceptionally wide compared with other carbometallation reactions. Allyl-, alkynyl-, alkenyl-, aryl-, acyl-, and acylmethylstannanes add to various carbon–carbon unsaturated bonds. Although carbostannylation products may also be obtained through the three-component coupling of organic electrophiles, unsaturated compounds, and stannyl nucleophiles such as distannanes, this review does not discuss such types of reactions. R

Sn

R R

Sn

R Sn

•

Sn

Sn R R = allyl, alkynyl, alkenyl, aryl, acyl, acylmethyl Sn = Sn-n-Bu3, SnMe3, SnCl3

Scheme 5.7.1

5.7.2

Carbostannylation reactions

Methods for Activation of Carbon–Tin Bonds Leading to Carbostannylation

Carbostannylation sometimes proceeds with no activators, but the applicability of such reactions is severely limited to a few types of special substrates. The development of activation methods in the mid 1990s made carbostannylation a synthetically significant tool. Three major activators are now available. Thus, radical initiators, Lewis acids, and late transition metals were found to be effective as activators by Hosomi, Yamamoto, and Shirakawa–Hiyama, respectively. These activators are required only in a catalytic amount to promote the reaction. Scheme 5.7.2 summarizes the mechanism of the activation of carbon–tin bonds by these activators. The cleavage of carbon–tin bonds by a radical initiator gives a stannyl radical, which adds to an unsaturated bond. The resulting alkenyl or alkyl radical abstracts an R

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radical from the organostannane to give the corresponding carbostannylation product, and to regenerate the stannyl radical. By contrast, a Lewis acid first activates an unsaturated bond, which accepts the addition of R from an organostannane with the cleavage of the carbon–tin bond. The Lewis acidic metal of the resulting alkenylmetal is replaced by the stannyl cation to give the carbostannylation product, with regeneration of the Lewis acid. Transition metals have several ways to activate carbon–tin bonds. In this context, oxidative addition of carbon–tin bonds to low-valent transition metals is the most important method. The insertion of a carbon–carbon unsaturated bond into the carbon–metal bond or the tin–metal bond of the oxidative adduct, followed by reductive elimination, gives the carbostannylation product and regenerates the low-valent transition metal. Other activation methods with transition metals will be introduced in the following sections in the context of specific reaction types. (a) Radical Initiator Sn

R

Radical Initiator (R'•) – R–R'

Sn

• Sn

R

Sn

Sn R

(b) Lewis Acid

Lewis Acid (M)

M

R

M

Sn

+

Sn

Sn+ R

R (c) Transition Metal

R

Sn

Transition Metal (M)

Scheme 5.7.2

5.7.3

Sn

R

R

M Sn

R

Sn

M

Mechanism of activation of carbon–tin bonds by activators

Carbostannylation of Alkynes

Allylstannylation

In certain cases using highly electron-deficient alkynes, allylstannylation sometimes proceeds without any activators through a six-membered transition state similar to the allylation of carbonyl compounds.1 However, allylstannylation did not gain significant synthetic utility until activators were introduced. Any of the three types of activators described in Scheme 5.7.2 promotes the allylstannylation of alkynes. Hosomi and coworkers reported in 1996 that allylstannanes add to alkynes with anti-selectivity in the presence of a catalytic amount of 2,2 -azobisisobutyronitrile (AIBN) as a radical initiator (Scheme 5.7.3).2 The allylstannylation proceeds in high yields, in particular with allylstannanes having an electron-withdrawing group such as ester and cyano groups at their β-position. In addition to terminal alkynes, internal alkynoates accept the addition of allylstannanes. The intra-molecular version of the allylstannylation in the presence of catalytic amounts of AIBN and n-Bu3 SnH has also been reported.3

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CO2Me

AIBN (5 mol%)

Sn-n-Bu3 + Ph

C6H6, 80 °C, 1 h

Ph

Sn-n-Bu3

94%

Scheme 5.7.3

Allylstannylation of alkynes promoted by AIBN as a radical initiator

Trans-allylstannylation is possible with ZrCl4 as a Lewis acid catalyst, reported by Yamamoto and coworkers, also in 1996 (Scheme 5.7.4).4 Various terminal alkynes such as aryl-, alkyl-, alkenyl-, or nonsubstituted acetylenes undergo the reaction with non- or methyl-substituted allylstannanes. Recently, silver complexes were also found to be effective as catalysts for the intra-molecular allylstannylation of alkynes (Scheme 5.7.5).5 The silver complex is considered to activate the triple bond, giving a cyclopropylmethylidene or an alkenyl complex as an intermediate. Similar intra-molecular allylstannylations, but with syn-selectivity are known to proceed in the presence of a Pd(0)6 or an InCl3 7 catalyst. ZrCl4 (20 mol%)

Sn-n-Bu3 +

Sn-n-Bu3

toluene, –78 to 0 °C, 3 h 99%

Scheme 5.7.4

Zirconium-catalyzed allylstannylation of alkynes

n-Bu3Sn

MeO2C

[AgOTf(PPh3)]3 (10 mol% of Ag)

MeO2C

toluene, 70 °C, 30 min

MeO2C

MeO2C

90%

Scheme 5.7.5

Sn-n-Bu3

Silver-catalyzed intra-molecular allylstannylation of alkynes

Shirakawa and Hiyama found that transition metals such as palladium and nickel are efficient catalysts for a series of carbostannylations, the first one of which is the palladium-catalyzed alkynylstannylation of alkynes described in the following section.8 Both palladium9 and nickel10 catalyze the allylstannylation of alkynes (Schemes 5.7.6 and 5.7.7). In contrast to the methods using a radical initiator or a Lewis acid catalyst, these additions proceed with syn-selectivity. Both catalysts complement each other in terms of the range of alkynes that can be employed: palladium prefers electron-deficient alkynes, Sn-n-Bu3

Ph

1,4-dioxane, 50 °C, 43 h CO2Et

Scheme 5.7.6

100%

n-Bu3Sn

Sn-n-Bu3

Pd2(dba)3 (5 mol% of Pd)

+

+ Ph

CO2Et

Ph 79 : 21

Palladium-catalyzed allylstannylation of alkynes

CO2Et

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+

Sn-n-Bu3

n -Pr

n -Pr

643

Sn-n-Bu3

Ni(cod)2 (5 mol%) toluene, 50 °C, 3 h

n -Pr

n -Pr

87%

Scheme 5.7.7

Nickel-catalyzed allylstannylation of alkynes

whereas nickel favors electron-rich ones. Most of these allylstannylations are considered to follow the reaction mechanism shown in Scheme 5.7.2c starting with oxidative addition of a carbon–tin bond to a palladium(0) or nickel(0) complex. However, the palladium-catalyzed addition of allylstannanes, having no γ-substituents, is thought to be different, as shown in Scheme 5.7.8. Here, the first step is oxidative cyclization of an allylstannane and an alkyne to a palladium(0) complex to give a palladacyclopentene. The carbon–tin bond cleavage takes place at the next β-tin elimination step, which is followed by reductive elimination of the allylstannylation product, regenerating the palladium(0) complex. n-Bu3Sn

Sn-n-Bu3 Pd0

Z

+ Z Z (Z = CO2Me)

Z Sn-n-Bu3 Pd

n-Bu3Sn

Z

Z Pd0 Z

Z n-Bu3Sn

Pd

Z

Z

Scheme 5.7.8

Proposed mechanism of the Palladium-catalyzed allylstannylation of alkynes

The allylstannylation of aryl(perfluoroalkyl)acetylenes with β-(methoxycarbonyl)allylstannanes does not require any activator (Scheme 5.7.9), where addition of a catalytic amount of AIBN shortens the reaction time without a decrease in yield.11 MeO2C CO2Me Sn-n-Bu3 +

Ph

Scheme 5.7.9

CF3

CF3 C6H6, refllux, 12 h under an atmosphere of air 90%

Ph

Sn-n-Bu3

Allylstannylation of highly electron-deficient alkynes

Alkynylstannylation

The first carbostannylation reaction was reported by Himbert in 1979.12 Although the alkynylstannylation of alkynes takes place without any activators, the scope of the reaction is severely limited to

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the combination of highly electron-rich aminoethynylstannanes and highly electron-deficient dimethyl acetylenedicarboxylate. The alkynylstannylation, having certain generality, has only been accomplished by the aid of transition metal catalysts. Palladium13 and nickel10 complexes catalyze the reaction with perfect syn-selectivity and high regioselectivity, whereas these catalysts again compliment each other in the range of alkynes that can be utilized (Schemes 5.7.10 and 5.7.11). Relatively electron-deficient alkynes accept the addition of alkynylstannanes in the presence of a palladium catalyst, whereas electron-rich aliphatic alkynes are good substrates for the nickel catalyst. [PdCl(η3-C3H5)]2 (5 mol% of Pd) n-Bu

N n-Bu

PPh2

Sn-n-Bu3 + Ph

Sn-n-Bu3

(5 mol%)

THF, 50 °C, 12 h

Ph

86%

Scheme 5.7.10

Palladium-catalyzed allylstannylation of alkynes

CF3 CF3

Ni(acac)2 (5 mol%) i-Bu2AlH (10 mol%)

Sn-n-Bu3 + n-Hex

Sn-n-Bu3

toluene, 80 °C, 4 h n-Hex

82%

Scheme 5.7.11

Nickel-catalyzed allylstannylation of alkynes

Acylstannylation

In the presence of a nickel catalyst, aroyl-, alkanoyl, and aminocarbonyl-stannanes add to alkynes to give β-acylvinylstannanes (Scheme 5.7.12).10,14 Compared with other nickel-catalyzed carbostannylations of alkynes, the regioselectivity of the acylstannylation is not sufficient. The aminocarbonylstannylation of alkynes is also catalyzed by a rhodium complex.15 O O Ph

SnMe3 +

Ni(cod)2 (5 mol%)

Ph

CO2Et

Scheme 5.7.12

Me3Si 85%

Ph

CO2Et Me3Si 98 : 2

CO2Et

+

toluene, 30 °C, 3 h

Me3Si

O Me3Sn

SnMe3

Nickel-catalyzed acylstannylation of alkynes

Acylmethylstannylation

Stannyl enolates derived from aromatic ketones were found to add to alkynes in the presence of AIBN (20 mol%), giving β-(acylmethyl)alkenylstannanes (Scheme 5.7.13).16 Acylmethylstannylation products

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are obtained in high yields when the starting alkynes have a substituent such as an ester, aryl, or alkenyl group, which is likely to stabilize alkenyl radical intermediates.

+

AIBN (20 mol%) Ph

OSn-n-Bu3

Scheme 5.7.13

Sn-n-Bu3

C6H6, 80 °C, 4 h

O

70%

Ph

Acylmethylstannylation of alkynes promoted by AIBN as a radical initiator

Intra-Molecular Alkenylstannylation

AIBN as a radical initiator, in combination with a catalytic amount of n-Bu3 SnH, is effective for the alkenylstannylation of alkynes, though only the intra-molecular reaction is available (Scheme 5.7.14).

O

Sn-n-Bu3

C 6H6, 80 °C, 3 h Z : E = 96 : 4

Scheme 5.7.14 n-Bu3 SnH

Sn-n-Bu3

AIBN (20 mol%) n-Bu3SnH (40 mol%)

48%

O Z : E or E : Z = 96 : 4

Intra-molecular alkenylstannylation of alkynes promoted by catalytic amounts of AIBN and

Alkynyl- and Alkenylstannylation of Arynes

In a similar manner to alkynes, arynes accept the addition of alkynylstannanes in the presence of a catalytic amount of a palladium–iminophosphine complex (Scheme 5.7.15).17 Here, a characteristic feature is the participation of a vinylstannane. The usual catalytic cycle (Scheme 5.7.16, left), starting with oxidative addition of a carbon–tin bond, is possible, but rather improbable, because there are no signs of oxidative addition of alkenylstannanes that can participate in the present carbostannylation. Consequently, the palladium(0) complex is likely to undergo oxidative cyclization with an aryne to form a palladacyclopropene (Scheme 5.7.16, right), which then reacts with an organostannane. Dimerization–Carbostannylation of Alkynes

In the presence of a palladium complex coordinated with a diimine ligand, organostannanes react with electron-deficient alkynes, such as dimethyl acetylenedicarboxylate and ethyl propiolate to give the corresponding dimerization–carbostannylation products of the alkynes (Scheme 5.7.17).18 Various organostannanes such as alkynyl-, alkenyl-, aryl-, and heteroarylstannanes undergo dimerization–carbostannylation with perfect syn-selectivity. Complete regioselectivities were also observed in the reaction of ethyl propiolate. Some experiments revealed that the reaction proceeds in a different mechanism to the general one shown in Scheme 5.7.2c. As depicted in Scheme 5.7.18, the palladium(0) complex first reacts, not with an organostannane, but rather with two molecules of an alkyne to give a palladacyclopentadiene, which undergoes reaction with an organostannane to give the dimerization–carbostannylation product and regenerate the palladium(0) complex.

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Catalyst N

3

Ph

[PdCl(η -C3H5)]2 (5 mol% of Pd)

(5 mol%)

PPh2 SiMe3 Ph

Ph

A

Sn-n-Bu3 +

OTf SiMe3

Catalyst CsF MeCN, 50 °C, 3 h

+ Sn-n-Bu3

Ph

from A: 33% (51:49) from B: 35% (52:48)

OTf B SiMe3 Sn-n-Bu3

Sn-n-Bu3

+

Catalyst CsF MeCN, 50 °C, 25 h Sn-n-Bu3

OTf 47%

Scheme 5.7.15

Palladium-catalyzed alkynyl- and vinyl-stannylation of arynes

R–Sn-n-Bu3 R

N

N Pd0

Pd P

Sn-n-Bu3

N

N

P

P R

=

Pd P

N

Ph

PPh2

R–Sn-n-Bu3

Sn-n-Bu3

Scheme 5.7.16

Proposed mechanism of the palladium-catalyzed carbostannylation of arynes

Other Carbostannylations Involving Alkynes

Alkynes and 1,2-dienes successively insert into the carbon–tin bond of alkynylstannanes in the presence of a nickel catalyst to give alkenylstannanes having a dienyne structure that is otherwise hard to access (Scheme 5.7.19).19 Although nickel complexes are known to catalyze the addition of alkynylstannanes both to alkynes (see above) and 1,2-dienes (see below), single alkynylstannylation products of alkynes or 1,2-dienes are produced only in small amounts. Besides this, chemo-, stereo- and regioselectivity are also high, so as to give one isomer predominantly out of 48 possible isomers from the choice of the ligand. Use of an aminophosphine as a ligand gives alkenylstannanes with (Z )-configuration in high stereoselectivity, whereas (E)-alkenylstannanes are mainly obtained in the presence of a nickel–tris[ p(trifluoromethyl)phenyl]phosphine. As described above, acylstannylation products of alkynes are obtained in the presence of a nickel catalyst. By contrast, the addition of acylstannanes to alkynes accompanied by decarbonylation takes

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ligand 3

S

NN(i-Pr) (5 mol%) toluene, 50 °C, 21 h

+ CO2Et

Sn-n-Bu3 EtO2C

80%

Ar = 2,6-(i-Pr)C6H3 (NN(i-Pr)) Ph (NN)

[PdCl(η3-C3H5)]2 (5 mol% of Pd)

n-Bu3Sn

+ Z

Z

NN (5 mol%) toluene, 50 °C, 41 h

Z

Z

n-Bu3Sn Z

Sn-n-Bu3

Z

2 Z

Sn-n-Bu3 Z

Z

Palladium-catalyzed dimerization–carbostannylation of alkynes

Z

Ar N

Z

Ar N Pd0

Z

Ar N

N Ar

NAr =

Pd

N Ar

Z

Z

Z

64%

Scheme 5.7.17

NAr

CO2Et

[PdCl(η -C3H5)]2 (5 mol% of Pd)

Sn-n-Bu3

S

ArN

647

Z Z

N Ar

NAr

Z R–Sn-n-Bu3

R Sn-n-Bu3 Z

Z

Scheme 5.7.18 alkynes

Proposed mechanism of the palladium-catalyzed dimerization–carbostannylation of

place when propargyl esters and Pd/C are used as alkynes and a catalyst, respectively (Scheme 5.7.20).20 The reaction is applicable to aroyl-, alkenoyl-, and alkanoylstannanes, which respectively give aryl-, alkenyl-, and alkyl-stannylation products of propargyl esters. This demonstrates the first aryl- and alkylstannylations of alkynes, albeit in an indirect manner. 5.7.4

Carbostannylation of Alkenes

Allylstannylation

AIBN is effective as a radical initiator for the allylstannylation of alkenes (Scheme 5.7.21).21,2b,3 Methyl acrylate, which is a representative substrate, accepts the addition of allylstannanes having an electronwithdrawing substituent, such as ester and cyano groups, at their β-position. The reaction of allylstannanes

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Ph

SnMe3

+

n-Hex

+

Ni(acac)2 (5 mol%) ligand (5 or 10 mol%)

• n-Bu

50 °C, 24 h

Ph

Ph

SnMe3

SnMe3 + n-Bu n-Hex

n-Hex

n-Bu

ligand NMe2 5 mol% in n -Bu2O

56% (98:2)

10 mol% in toluene

73% (4:96)

PPh2 F 3C

3

Scheme 5.7.19

P

Nickel-catalyzed tandem alkynylstannylation of alkynes and 1,2-dienes

O Sn-n-Bu3

O

+ O

Sn-n-Bu3

Pd/C (0.2 mol%) n-Bu2O, 50 °C, 40 h

O

O

82% O

Scheme 5.7.20

O

Palladium-catalyzed decarbonylative carbostannylation of alkynes

lacking an electron-withdrawing group gives 1:2 adducts of allylstannanes and alkenes in addition to 1:1 allylstannylation products. MeO2C

CO2Me Sn-n-Bu3 + MeO C 2

AIBN (5 mol%) C6H6, 80 °C, 1 h 88%

Scheme 5.7.21

MeO2C

Sn-n-Bu3

Allylstannylation of alkenes promoted by AIBN as a radical initiator

Acylmethylstannylation

As shown in Scheme 5.7.22, electron-deficient alkenes, such as acrylates and acrylonitrile are acylmethylstannylated in the presence of a catalytic amount of AIBN as a radical initiator in a similar manner to alkynes.16

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Carbostannylation of C–C Unsaturated Bonds

O

+

Sn-n-Bu3

AIBN (20 mol%) O

O

C6H6, 80 °C, 5 h

OSn-n-Bu3

649

O O

76%

diastereomer ratio 66:34

Scheme 5.7.22

Acylmethylstannylation of alkenes promoted by AIBN as a radical initiator

Intra-Molecular Alkenylstannylation

The method using a radical initiator is also applicable to intra-molecular alkenylstannylation, though the scope of the reaction is not well known (Scheme 5.7.23).3

O

AIBN (5 mol%) n-Bu3SnH (10 mol%) C6H6, 80 °C, 3 h

Sn-n-Bu3 Ph

Scheme 5.7.23 n-Bu3 SnH

Sn-n-Bu3

O

Ph

73%

Intra-molecular alkenylstannylation of alkenes promoted by catalytic amounts of AIBN and

Arylstannylation of Norbornene

As trialkyl(aryl)stannanes have never been reported to undergo oxidative addition with a palladium(0) or nickel(0) complex, the corresponding arylstannylation is not known. As shown in Scheme 5.7.24, Kosugi and coworkers found that the use of aryl(trichloro)stannanes instead of the trialkyl derivatives enables the palladium-catalyzed arylstannylation of norbornene.22 The reaction using other unsaturated hydrocarbons including ordinary alkenes has not been reported, but the observation that aryl–tin bonds can be activated by palladium(0) complexes has certain significance, and will lead to future development of other arylstannylations.

Ph

SnCl3 +

1) PdCl2(PhCN)2 (1 mol%) C6H6, 55 °C, 2 h 2) MeMgI Et2O, rt, 1 h 86%

Scheme 5.7.24

Ph

SnMe3 +

Ph

83 : 17

Palladium-catalyzed arylstannylation of norbornene

SnMe3

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Tin Chemistry: Fundamentals, Frontiers and Applications

Carbostannylation of Dienes

Acylstannylation of 1,3-Dienes

Acylstannylation of 1,3-dienes proceeded under the conditions employed for the acylstannylation of alkynes (see above), but at lower temperatures. Carbon–tin bonds in aromatic and aliphatic acylstannanes, in addition to an aminocarbonylstannane, react with 1,3-dienes in the presence of a catalytic amount of Ni(cod)2 exclusively in a 1,4-manner with complete syn-selectivity (Scheme 5.7.25), except for the reaction of the aminocarbonylstannane for which an anti-addition product is observed.23 The reaction is applicable to various 2-substituted and 2,3-disubstituted 1,3-dienes, but not for 1-substituted derivatives. The regioselectivity in the reaction of unsymmetrical 1,3-dienes is moderate. O O Ph

SnMe3

Ni(cod)2 (5 mol%)

+

toluene, 50 °C, 2 h

SnMe3

Ph

73%

Scheme 5.7.25

Nickel-catalyzed acylstannylation of 1,3-dienes

Acylstannylation of 1,2-Dienes

The methodology was extended to acylstannylation of 1,2-dienes, which gives α-(acylmethyl)vinylstannanes as the major products through the addition of carbon–tin bonds to internal double bonds of 1,2-dienes (Scheme 5.7.26).24 This type of alkenylstannanes, having an exo-methylene group, are inaccessible by the standard carbostannylation of alkynes. Under conditions similar to those in the acylstannylation of 1,3dienes, various alkyl-, aryl-, and methoxy-allenes undergo acylstannylation with aromatic and aliphatic acylstannanes. Disubstituted allenes can also participate in acylstannylation reactions. O + Ph

SnMe3

• n-Bu

toluene, 50 °C, 1.5 h 79%

Scheme 5.7.26

O

Ni(cod)2 (5 mol%)

SnMe3

Ph n-Bu

Nickel-catalyzed acylstannylation of 1,2-dienes

Alkynylstannylation of 1,2-Dienes

Although the reaction conditions of the acylstannylation of 1,3- and 1,2-dienes cannot be applied to the corresponding reaction of alkynylstannanes as they stand, the alkynylstannylation of 1,2-dienes takes place in the presence of a catalytic amount of a nickel(0) complex coordinated by a 1,3-diphosphinopropane ligand (Scheme 5.7.27).25,24b The substituents on the phosphorus atom of the diphosphine ligands markedly affect the selection of allene double bonds: a Ni–DPPP catalyst favors the internal double bond of allenes, whereas 1,3-bis(dimethylphosphino)propane (DMPP) prefers to react at the terminal double bonds to give (Z )-alkenylstannanes selectively. Various aryl- and silyl-ethynylstannanes add to mono-, di- and non-substituted allenes.

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Carbostannylation of C–C Unsaturated Bonds Ph

SnMe3 +

Ni(cod)2 (5 mol%) ligand (5 mol%) toluene, 50 °C

• t-Bu

SnMe3

Ph

+ t-Bu t-Bu

ligand R2P

Scheme 5.7.27

5.7.6

SnMe3

Ph

651

PR2

R = Ph: DPPP Me: DMPP

DPPP: 57%, 98:2 (70 h) DMPP: 67%, 14:86 (49 h)

Nickel-catalyzed alkynylstannylation of 1,2-dienes

Conclusion

The carbostannylation of carbon–carbon unsaturated bonds has become one of the most important methods for synthesis of organostannanes in the last decade, and now it is possible to make allyl-, alkynyl-, alkenyl-, aryl-, acyl-, and acylmethylstannanes add to alkynes, alkenes, and dienes. The utility of carbostannylation has been brought about by three types of activators: radical initiators, Lewis acid catalysts, and transition metal catalysts. They selectively activate the carbon–tin bond of organostannanes, which otherwise are stable and thus convenient for chemoselective transformations. By using these activators, further development of carbostannylations is expected. In particular, the diversity in activation modes by transition metals will lead to new types of carbostannylations. Palladium, nickel, and rhodium have been exclusively used for carbostannylation thus far, and introduction of other transition metals should be a promising choice. References 1. A. G. Davies, Organotin Chemistry, 2nd Edn., Wiley-VCH, Weinheim, 2004, pp. 140. 2. (a) K. Miura, D. Itoh, T. Hondo, H. Saito, H. Ito, and A. Hosomi, Tetrahedron Lett., 37, 8539 (1996); (b) K. Miura, H. Saito, D. Itoh, T. Matsuda, N. Fujisawa, D. Wang, and A. Hosomi, J. Org. Chem., 66, 3348 (2001). 3. K. Miura, N. Fujisawa, H. Saito, H. Nishikori, and A. Hosomi, Chem. Lett., 32 (2002). 4. (a) N. Asao, Y. Matsukawa, and Y. Yamamoto, Chem. Commun., 1513 (1996); (b) Y. Matsukawa, N. Asao, H. Kitahara, and Y. Yamamoto, Tetrahedron, 55, 3779 (1999); (c) N. Asao and Y. Yamamoto, Bull. Chem. Soc. Jpn., 73, 1071 (2000). 5. S. Porcel and A. M. Echavarren, Angew. Chem. Int. Ed., 46, 2672 (2007). 6. S. Shin and T. V. RajanBabu, J. Am. Chem. Soc., 123, 8416 (2001). 7. K. Miura, N. Fujisawa, and A. Hosomi, J. Org. Chem., 69, 2427 (2004). 8. (a) E. Shirakawa and T. Hiyama, J. Organomet. Chem., 576, 169 (1999); (b) E. Shirakawa and T. Hiyama, in Perspectives in Organopalladium Chemistry for the XXI Century, J. Tsuji (Ed.), Elsevier, Amsterdam, 1999, 169; (c) E. Shirakawa and T. Hiyama, Bull. Chem. Soc. Jpn., 75, 1435 (2002). 9. E. Shirakawa, H. Yoshida, Y. Nakao, and T. Hiyama, Org. Lett., 2, 2209 (2000). 10. E. Shirakawa, K. Yamasaki, H. Yoshida, and T. Hiyama, J. Am. Chem. Soc., 121, 10221 (1999). 11. T. Konno, T. Takehana, J. Chae, T. Ishihara, and H. Yamanaka, J. Org. Chem., 69, 2188 (2004). 12. G. Himbert, J. Chem. Res. S, 88 (1979). 13. (a) E. Shirakawa, H. Yoshida, T. Kurahashi, Y. Nakao, and T. Hiyama, J. Am. Chem. Soc., 120, 2975 (1998); (b) H. Yoshida, E. Shirakawa, T. Kurahashi, Y. Nakao, and T. Hiyama, Organometallics, 19, 5671 (2000); (c) M. Shimizu, G. Jiang, M. Murai, Y. Takeda, Y. Nakao, T. Hiyama, and E. Shirakawa, Chem. Lett., 34, 1700 (2005). 14. E. Shirakawa, Y. Yamamoto, Y. Nakao, T. Tsuchimoto, and T. Hiyama, Chem. Commun., 1926 (2001).

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23. 24. 25.

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Tin Chemistry: Fundamentals, Frontiers and Applications R. Hua, S. Onozawa, and M. Tanaka, Organometallics, 19, 3269 (2000). K. Miura, H. Saito, N. Fujisawa, D. Wang, H. Nishikori, and A. Hosomi, Org. Lett., 3, 4055 (2001). H. Yoshida, Y. Honda, E. Shirakawa, and T. Hiyama, Chem. Commun., 1880 (2001). (a) E. Shirakawa, H. Yoshida, Y. Nakao, and T. Hiyama, J. Am. Chem. Soc., 121, 4290 (1999); (b) H. Yoshida, E. Shirakawa, Y. Nakao, Y. Honda, and T. Hiyama, Bull. Chem. Soc. Jpn., 74, 637 (2001). E. Shirakawa, Y. Yamamoto, Y. Nakao, S. Oda, T. Tsuchimoto, and T. Hiyama, Angew. Chem. Int. Ed., 43, 3448 (2004). Y. Nakao, J. Sato, E. Shirakawa, and T. Hiyama, Angew. Chem. Int. Ed., 45, 2271 (2006). K. Miura, T. Matsuda, T. Hondo, H. Ito, and A. Hosomi, Synlett, 555 (1996). (a) K. Fugami, Y. Mishiba, S. Hagiwara, D. Koyama, M. Kameyama, and M. Kosugi, Synlett, 553 (2000); (b) K. Fugami, K. Kawata, T. Enokido, Y. Mishiba, S. Hagiwara, Y. Hirunuma, D. Koyama, M. Kameyama, and M. Kosugi, J. Organomet. Chem., 611, 433 (2000). E. Shirakawa, Y. Nakao, H. Yoshida, and T. Hiyama, J. Am. Chem. Soc., 122, 9030 (2000). (a) E. Shirakawa, Y. Nakao, and T. Hiyama, Chem. Commun., 263 (2001); (b) Y. Nakao, E. Shirakawa, T. Tsuchimoto, and T. Hiyama, J. Organomet. Chem., 689, 3701 (2004). E. Shirakawa, Y. Nakao, T. Tsuchimoto, and T. Hiyama, Chem. Commun., 1962 (2002).

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Green Organotin Chemistry

5.8

653

Green Organotin Chemistry: an Oxymoron?

David Young Eskitis Institute of Cell and Molecular Therapies, Griffith University, Queensland, Australia

5.8.1

Introduction

An ideal, ‘green synthesis’ would be short, start with renewable feedstocks and selective, recyclable catalysts, be performed in a benign, recoverable solvent (or solventless), incorporate as many atoms as possible into the final product, and proceed in high yield to give non-hazardous, biodegradable material that did not require purifying. It would not generate waste and especially not toxic waste. It could be readily monitored to minimize by-products and wasted energy, and would not involve hazardous reagents or processes.1 Against these criteria, how does a typical organotin-mediated reaction, for example a classical Stille coupling2 (Scheme 5.8.1) measure up?

Scheme 5.8.1

‘Classical’ Stille coupling

This palladium-catalysed reaction proceeds in good yield (91%) and is relatively ‘clean,’ requiring only a simple bulb-to-bulb distillation to provide the purified diene. The major shortcomings from a green chemistry perspective, however, are the low ‘atom economy’3 predominantly due to loss of the heavy tri-n-butyltin moiety as toxic by-products, and the even lower ‘effective mass yield’, defined as the mass of desired product relative to all non-benign materials used in its synthesis.4 This latter measure is dominated by the mass of reaction and purification solvents and could be substantially improved if these solvents were recovered. Nevertheless, while this reaction can be described as ‘efficient’ from a classical synthetic chemistry perspective, it cannot be described as ‘green’, primarily due to the relatively large amount of hazardous organotin waste generated relative to the amount of useful product. Even more troublesome are reactions involving the ubiquitous tri-n-butyltin hydride. This lipophilic, toxic molecule is arguably the pre-eminent organotin reagent in organic synthesis and dominates free radical substitution reactions of hydrogen in place of halogen, hydroxyl, amino, nitro, thiol, selenide, carboxylate, and other functional groups5 (Scheme 5.8.2). The finely tuned reactivity of tri-n-butyltin hydride and of the resulting tin radical allows it to mediate radical additions to alkenes and alkynes in reductive carbon–carbon bond formation and for radical ring closures6 . It is a highly versatile and efficient reagent, but not a very green one delivering one ‘useful’ atom (hydrogen, MW = 1) for every 40 atoms (MW = 290) lost as waste.

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Scheme 5.8.2

5.8.2

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Tri-n-butyltin hydride-mediated homolytic reduction

How Hazardous Are Organostannanes?

The selective toxicity of tri-n-butyltin and triphenyltin species towards certain organisms has been widely exploited in anti-fouling paints, preservatives, fungicides,7 and as potential anticancer agents.8 The toxicity of organostannanes is a function of the number of organic groups attached to the tin atom and the nature of these groups, and is largely independent of the inorganic counterion.9 Triorganotin compounds have the highest biocidal activities and alkyl groups on tin are, in general, more toxic than aryl groups. Trialkyltin compounds with short carbon chains are the most toxic10 and trimethyltin compounds are particularly poisonous to insects and mammals while triphenyltin derivatives demonstrate high toxicity towards fish, fungi, and molluscs. Trimethyltin chloride and triethyltin chloride are neurotoxins affecting specific sub-regions of the brain and spinal cord respectively.11 Di- and tri-n-butyltin compounds appear to be less potent neurotoxins,12 but produce bile duct damage in rate, mice, and hamsters.13 Tri-n-butyltin salts also induce apoptosis in T-cells.14 Exactly how organostannanes exert their toxicity is complex and not well understood. It has been known since 1958 that organostannanes disrupt oxidative phosphorylation in mitochondria15 and this action was subsequently demonstrated to arise from inhibition of ATP synthases.16 A tritium labelling study17 indicated that tri-n-butyltin chloride interacts with a specific ion channel sub-unit in ATP synthase. A second mitochondrial protein, stannin (Snn) has also been identified as a triorganotin receptor responsible for tissue-specific toxicity.18 While different organotin compounds exhibit varying degrees of toxicity towards different organisms, it is unfortunately the case that the most synthetically useful trimethyl- and tri-n-butyltin reagents are also the most hazardous towards human health and to the environment. Risk is a function of hazard and exposure, and subsequent sections of this chapter examine different strategies for ameliorating one or the other of these parameters to allow for the possibility of green organotin chemistry. 5.8.3

Removing Organotin By-Products

The Stille coupling above (Scheme 5.8.1) generates one equivalent of tri-n-butyltin salts that could be efficiently separated from the relatively volatile product by distillation. Separation of triorganotin waste from the product is not usually so straightforward, however, because of the solubility of common organotin contaminants such as tri-n-butyltin halide or hexa-n-butylditin oxide in most organic solvents. The traditional approach to this problem has been to wash the organic extract with an aqueous solution of potassium fluoride.19 Any organotin halide formed is converted to involatile, insoluble organotin fluoride (e.g. Scheme 5.8.3). In this example, reaction solvent is evaporated and the tetradeutero-adamantane and tri-n-butyltin bromide dissolved in ether and washed with saturated KF in water (ca. 10 g in 100 ml). The resulting polymeric tri-n-butyltin fluoride is insoluble in both water and organic solvent and is removed by filtration under vacuum. The desired product is then purified by sublimation. Anyone who has tried this method will know that it is not as straightforward as it appears. The fluoride wash is biphasic and rarely removes all organotin residues from the organic extract. The organotin fluoride

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Green Organotin Chemistry

Scheme 5.8.3

655

‘Classical’ deuteriodehalogenation with tri-n-butyltin deuteride

is also quite intractable, clogging filtration apparatus and coating glassware. From a green chemistry perspective, the use of toxic potassium fluoride lowers the ‘effective mass yield’ of the reaction and generates a new waste stream (aqueous fluoride) that is costly to dispose of. Chang20 has developed a more efficient method for removing tin by-products from hexa-n-butylditin and tri-n-butyltin hydride-mediated radical reactions. This method involves: (1) dilution of the reaction mixture with reagent grade diethylether, (2) addition of a slight excess of diazabicycloundecene (DBU), (3) dropwise titration with an ethereal solution of iodine, and (4) rapid filtration through silica. The crude product is usually >90% free of tin by-products. This procedure relies on conversion of the unreacted hexaorganoditin and triorganotin hydride to trialkyltin iodide, which is hydrolyzed to tin hydroxide with DBU (Scheme 5.8.4).

Scheme 5.8.4

‘DBU-method’ for removing R3 SnX waste

Trialkyltin hydroxide is in equilibrium with hexaalkylditin oxide and both are retained by column chromatography on silica, together with excess DBU and DBU hydroiodide. Harrowven and Guy have recently compared various methods for the removal of tri-n-butyltin byproducts to their own method involving liquid chromatography on a mixture of KF and silica as the stationary phase21 (Scheme 5.8.5). Four methods were used to purify the product from a representative tri-n-butyltin hydride-mediated hydrodehalogenation of an aryl bromide. Chromatography on silica alone gave ca. 50 mol% of tri-n-butyltin impurities. Prior partitioning between acetonitrile and pentane followed by silica chromatography of the acetonitrile fraction still resulted in >25 mol% tin contamination. Prolonged stirring with aqueous potassium fluoride, followed by silica chromatography provided product contaminated with ca. 5 mol% tin impurities. A second column gave a pure sample by combustion analysis, but trace contamination was still evident by 1 H NMR spectroscopy. By comparison, concentrating the reaction mixture and column chromatography through a stationary phase comprising 10% w/w finely ground KF in silica gave product with no tin impurity, as determined by 1 H NMR spectroscopy and less than 30 ppm tin by elemental analysis. The authors claim this method is also less time consuming than either an

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Scheme 5.8.5

Comparison of purification methods for removing R3 SnX waste

aqueous KF extraction or pre-treatment with DBU. Management of the waste stream is also simplified, as organotin impurities are not split between aqueous and organic phases. The British patent application22 for this method reveals that it is less effective for removing hexaalkylditin impurities, although these elute in discrete, early fractions. This certainly appears to be the most effective and simplest of the general methods for removing organotin by-products, although column chromatography is a ‘second-best option’ for a green synthesis, unless the eluting solvent is benign and both the solvent and stationary phase can be readily recycled. 5.8.4

Modified Organotin Reagent for Easy Separation

An alternative strategy to removing traditional organotin reagents is the development of modified reagents that can be easily recovered and (ideally) regenerated and reused. Some organotin reagents of this type (Figure 5.8.1) and other ‘tin-free’ reagents used to mediate radical reactions have been reviewed23 . The diphenyltin hydride derived from 4-vinylpyridine has the advantage that the resulting tin halide sticks to the baseline during column chromatography on silica (R f of ca. 0) eluted with 1:3 ethyl acetate:hexane, allowing easy separation from non-polar products.24 Residual reagent has an R f of ca. 0.2 compared to an R f of ca. 0.9 for triphenyltin hydride. Water-soluble polyether tin hydride25 and the bis(2-cardoxyethyl)tin hydroxide26 allowed reductions and cyclizations to be carried out in water with easy separation from organic soluble products. The latter tin reagent is generated in situ and has not been characterized, but may be a multiplicity of tin oxides and hydroxides. It is soluble in dilute alkali and catalyzes the reduction of alkyl and aryl bromides in the presence of NaBH4 and the water-soluble initiator 4,4 -azobis(4-cyanovaleric acid) (AVCA). A more recent example27 highlights the major shortcoming of elaborately modified organotin reagents from a green chemistry perspective. This glyoxylic acid-derived reagent (Scheme 5.8.6) gives good yields in standard radical reductions (e.g. of aryl halides) and organotin by-products are very largely removed

Figure 5.8.1

Some alternative organotin reagents for homolytic transformations.

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by mild base or acid hydrolysis to the carboxylate or carboxylic acid respectively, followed by washing with aqueous bicarbonate. It is made, however, by a five-step synthesis involving some toxic reagents and solvents in 43% overall yield. Assessing the ‘green-credentials’ of a reagent should also include its synthesis.

Scheme 5.8.6

Synthesis of elaborate triorganotin hydride for easy removal

A more successful alternative in this respect makes use of the affinity of activated carbon for polyaromatic hydrocarbons (PAHs)28 (Scheme 5.8.7).

Scheme 5.8.7

Catalytic reduction with pyrene tethered dimethyltin hydride

The pyrene-tethered dimethyltin hydride is made in six steps and 29% overall yield from commercially available pyrene carboxyaldehyde, but can be used catalytically with a stoichiometric source of hydride. Workup with activated carbon provides product in yields comparable to tri-n-butyltin hydride and with organotin contamination below 2 mol% and undetectable by 1 H NMR spectroscopy. Unfortunately, reagent recovery from the activated carbon is low-yielding. Allyl and phenyl pyrene-tethered tin reagents have also proved effective for use in allylation and Stille coupling respectively and, again, tin contamination of the product are undetectable by 1 H and 13 C NMR spectroscopy. Another approach is the use of a perfluorinated reagent.29 This method takes advantage of the solubility of fluorous reagents bearing greater than ca. 60% fluoride content in fluorocarbon solvents allowing immobilization of fluorinated tin catalysts in a fluorous phase and/or the removal of the resulting perfluorinated tin by-products by fluorous–organic, liquid–liquid extraction or with fluorous reverse-phase silica. Fluorocarbon liquids are poor solvents for organic compounds allowing the tin catalyst and by-products to be readily recovered and recycled.

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A large number of fluorous tin reagents for use in palladium-catalyzed Stille cross-coupling reactions with organic halides or triflates have been developed.30 Reaction times can be reduced to just a few minutes when conducted with microwave heating.31 Three-phase extraction between water (top), dichloromethane (middle), and perfluorohexanes (FC-72) (bottom) allow the coupled product to be isolated from the dichloromethane and the resulting fluorinated tin chloride to be recovered from the FC-72 phase and recycled (Scheme 5.8.8).

Scheme 5.8.8

Stille coupling with a fluorous tin reagent

The ability to recycle perfluoronated tin reagents is crucial to their application. While their synthesis is not particularly long or arduous, the atom economy of their application is poor and this is particularly true for subsequently developed fluorous tin hydrides32 with a molecular weight of >1100 which are used to deliver a hydrogen atom. These reagents can be recycled and reused up to six times in radical reductions (Scheme 5.8.9) requiring as little as 1.6 mol% tin per reduction and with >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 <5–60 ppm compared with >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 (<5–34 ppm) of the isolated product.

Scheme 5.8.13

Catalytic atom transfer cyclization with resin-bound reagent

Drying and reusing the same resin gives a modest 25% conversion, but after regeneration (LiAlH4 and oxidation with Pd(PPh3 )4 ), the recycled resin gives an improved 79% yield.

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More recently resin bound organotin reagents have been used as transesterification catalysts42 and for the allylation of aldehydes43 (e.g. Scheme 5.8.14).

Scheme 5.8.14

Allylation with resin-bound reagent

ICP-MS analysis of the homoallyl alcohols formed in these reactions indicates negligible tin and cerium contamination. The resin can be recovered and regenerated with allyl bromide and zinc or allylmagnesium bromide and reused several times without notable loss of activity. 5.8.6

Less Toxic Organotin Reagents

A simple way to avoid toxic and intractable triorganotin waste is to use monoorganotin reagents that are much less toxic and less lipophilic. Unactivated secondary alkyl halides can be coupled with aryltrichlorotin in a nickel-catalyzed Stille cross-coupling reaction44 (e.g. Scheme 5.8.15).

Scheme 5.8.15

Stille coupling with a monoorganotin reagent

This reaction unfortunately requires stoichiometric monoorganotin trihalide and suffers from poor atom economy and effective mass yield. Reduced toxicity and better atom economy is possible with reagents involving either no organic groups on tin or a tin atom saturated with organic groups. The former class of reactions refer to reagents generated in situ, usually from Sn(II) salts or Sn metal. These reactions presumably still generate organotin waste, but only in trace amounts. Most success with tetraorganostannanes has been achieved with commercially available tetraallyltin used for the allylation of aldehydes and ketones. All four allyl groups are used, giving relatively good atom efficiency (e.g. 83% for the allylation of benzaldehyde) and leaving only inorganic Sn(IV) salts as waste. Allylation can also be achieved enantioselectively45 (Scheme 5.8.16). The achiral allylation of aldehydes has also been achieved in recyclable ionic liquids46 and in water.47 Greener still is the corresponding ‘Barbier’ allylation48 of aldehydes and ketones with allylic bromides in water mediated by tin metal. The atom efficiency of this reaction is actually less than the corresponding tetraallyltin allylation of (say) benzaldehyde (65% and 83%, respectively) because of the loss of the heavy bromine atom, but this neglects the synthesis of tetraallyltin, which is prepared from allyl bromide or chloride. A particularly intriguing recent advance with this thoroughly studied reaction is the use of ‘nano-tin’49 (Scheme 5.8.17). Nano-tin is prepared by reduction of SnCl2 with isopropanol and irradiation under a γ-ray source (60 Co) and has a particle size of 20–30 nm. Allylation of aromatic aldehydes in water are almost quantitative and reaction times are reduced from 8–24 hours for regular tin powder to 1.5–8 hours. Aliphatic aldehydes

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Scheme 5.8.16

Chiral allylation with tetraallyltin

Scheme 5.8.17

Allylation with ‘nano-tin’

and ketones perform a little less well, but yields are still higher for ‘nano-tin’ than for tin powder. In the example shown (Scheme 5.8.17), allylation with nanometer tin proceeds in almost three times the yield achieved with tin powder, and the reaction does not proceed at all with the common allylating metals Zn, Bi, and Al. Tin-mediated allylations are not catalytic and excess metal is generally required with the corresponding metal salt lost as waste. This metal salt can be recycled electrochemically,50 however (Scheme 5.8.18).

Scheme 5.8.18

Tin-mediated allylation with electrochemical recycling

The reaction involves stirring aldehyde, allyl bromide, and stannous chloride in water and applying an electrolytic potential of 2.0 V with graphite electrodes. The cathode reduces Sn(II or IV) salts to tin metal, which react with allyl bromide to generate allyltin(II) bromide and diallyltin dibromide. These organotin species then allylate the aldehyde and the resulting homoallyl alcohol is extracted with diethyl

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ether. Recycling of the aqueous stannous chloride solution up to five times results in only minimal loss of activity. Using the intermediate homoallyloxytin(IV) in a subsequent reaction rather than simply quenching it extends the atom efficiency of Barbier chemistry51 (Scheme 5.8.19).

Scheme 5.8.19

‘Multi-component cascade coupling’

It is an aim of green chemistry (and good synthetic chemistry) to introduce as much useful functionality as possible in the minimum number of steps. Unfortunately, this ‘multi-component cascade coupling’ requires the use of anhydrous organic solvents. Nevertheless, it can be envisaged that the ‘greener’ nanotin and electrochemical approaches described above could possibly be extended to this type of cascade sequence. 5.8.7

No-Tin Reagents

A variety of silicon and germanium hydrides have been investigated as less toxic hydrogen transfer reagents.23 The silicon–hydrogen bond in simple triorganosilanes is much stronger than the corresponding bond in triorganotin hydrides and high temperatures are required to maintain free radical chain reactions with only a limited range of substrates. The most widely studied tri-n-butyltin hydride substitute is tris(trimethylsilylsilane)52 (Scheme 5.8.20) in which the Si–H bond is about 5 kcal mol−1 higher in energy. This slightly higher bond dissociation energy decreases the likelihood of premature reduction of intermediate radicals, which can be a problem with tin hydrides.

Scheme 5.8.20

Tris(trimethylsilyl)silane mediated reduction

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Major drawbacks of tris(trimethylsilyl)silane relative to tri-n-butyltin hydride, however, are the cost of the reagent, the need to handle the reagent under argon and the propensity of the tris(trimethysilyl)silyl radical to add to multiple bonds. Many other silicon and germanium hydrides, as well as sulfur, selenium, and phosphorous radical transfer reagents have been investigated as tin hydride alternatives and while many have niche applications, none so far have the flexibility or range of applications of tin hydrides.23 5.8.8

Conclusion

Is ‘green organotin chemistry’ an oxymoron? Organotin compounds, particularly triorganotin species, are unacceptably hazardous to the environment and to human health. Considerable effort has been devoted to reducing organotin contamination of products and waste generated in these reactions. This endeavor has been driven by a desire to continue exploiting the unique reactivity and stability of inexpensive organotin reagents, such as tri-n-butyltin hydride. This research is bearing fruit and there are now organotinmediated reactions that can be achieved catalytically with recycling of the catalyst, or at least with reduction of tin contamination to ppm levels. Moreover, there has been considerable success at achieving these transformations in benign solvents, such as water and super-critical carbon dioxide. Not only are these new reactions more environmentally sustainable, they are also better synthetic methodology. The inherent toxicity and intractable nature of organostannanes has surprisingly led to this development of elegant and green organotin-mediated synthetic chemistry. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

P.T. Anastas and J.C. Warner, Green Chemistry: Theory and Practice, OUP, Oxford, 1998. W.J. Scott and J.K. Stille, J. Am. Chem. Soc., 108, 3033 (1986). B.M. Trost, Angew. Chem. Int. Ed. Engl., 34, 259 (1995). T. Hudlicky, D.A. Frey, L. Koroniak, C.D. Claeboe, and L.E. Brammer Jr., Green Chem., 57 (1999). W.B. Motherwell and D. Crich, Free Radical Chain Reactions in Organic Synthesis, Academic Press, London, 1992, Chapter 3. (a) K.A. Parker and D. Fokas, J. Am. Chem. Soc., 114, 9688 (1992); (b) S. Handa, G. Pattenden, and W. -S. Li, J. Chem. Soc., Chem. Commun., 311 (1998). J.S. White and J.M. Tobin, Environ. Sci. Technol. 38, 3877 (2004). M. Gielen, M. Biesemans, D. De Vos, and R. Willem, J. Inorg. Biochem., 79, 139 (2000). X. Song, A. Zapata, and G. Eng, J. Organomet. Chem. 691, 1756 (2006). Y. Arakawa, O. Wada, and T.H. Yu, Toxicol. Appl. Pharmacol. 60, 1 (1981). B. Buck, A. Mascioni, L. Que, Jr., and G. Veglia, J. Am. Chem. Soc., 125, 13316 (2003), and references therein. Although dialkyltin coumpounds are reported to have a neurotoxic effect in brain cells at levels of 30 ppb; S.D. Richardson, Anal. Chem., 76, 3337 (2004). I. J. Boyer, Toxicology 55, 253 (1989). Y. Arakawa and O. Wada in Metal ions in Biological Systems, H. Sigel and A. Sigel (Eds), Marcel Dekker, Inc., New York, 101, 1993. W.N. Aldridge, Biochem. J. 69, 367 (1958). (a) M.J. Selwyn, Adv. Chem. 157, 204 (1976); (b) K. Cain and D.E. Griffiths, Biochem. J. 162, 575 (1977). C. von Ballmoos, J. Brunner, and P. Dimroth, Proc. Nat. Acad. Sci. 101, 11239 (2004). (a) B. Buck, A. Mascioni, L. Que, Jr., and G. Veglia, J. Am. Chem. Soc., 125, 13316 (2003); (b) M.L. Billingsley, J. Yun, B.E. Reese, C.E. Davidson, B.A. Buck-Koehntop, and G. Veglia, J. Cell. Biochem. 98, 243 (2006). J.E. Leibner and J. Jacobus, J. Org. Chem. 44, 449 (1979). D.P. Curran and C.-T. Chang, J. Org. Chem. 54, 3140 (1989). D.C. Harrowven and I.L. Guy, J. Chem. Soc., Chem. Commun., 1968 (2004). D.C. Harrowven and I.L. Guy, Brit. UK Pat. Appl. (2005) GB 20050921.

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38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52.

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P.A. Baguley and J.C. Walton, Angew. Chem. Int. Ed., 37, 3072 (1998). D.L.J. Clive and W. Yang, J. Org. Chem., 60, 2607 (1995). J. Light and R. Breslow, Tetrahedron Lett., 31, 2957 (1990). R. Rai and D. B. Collum, Tetrahedron Lett., 35, 6221 (1994). D.L.J. Clive and J. Wang, J. Org. Chem. 67, 1192 (2002). D. Stein and S. Gastaldi, J. Org. Chem. 69, 4464 (2004). (a) D.P. Curran and S. Hadida, J. Am. Chem. Soc., 118, 2531 (1996); (b) J.H. Horner, F.N. Martinez, M. Newcomb, S. Hadida, and D.P. Curran, Tetrahedron Lett., 38, 2783 (1997). M. Hoshino, P. Degenkolb, and D.P. Curran, J. Org. Chem. 62, 8341 (1997). M. Larhed, M. Hoshino, S. Hadida, D.P. Curran, and A. Hallberg, J. Org. Chem. 62, 5583 (1997). D.P. Curran, S. Hadida, S.-Y. Kim, and Z. Luo, J. Am. Chem. Soc. 121, 6607 (1999). S. Hadida, M.S. Super, E.J. Beckman, and D.P. Curran, J. Am. Chem. Soc., 119, 7406 (1997). B. Cornils, Angew. Chem. Int. Ed. Engl. 36, 2057 (1997). P.M. Jenkins, A.M. Steele, and S. C. Tsang, Catal. Commun. 4, 45 (2003). A. Yoshida, X. Hao, O. Yamazaki, and J. Nishikido, QSAR Comb. Sci. 25, 697 (2006). (a) U. Gerigke, M. Gerlach, W.P. Neumann, R. Vieler, and V. Weintritt, Synthesis, 448 (1990); (b) J. Junggebauer and W.P. Neumann, Tetrahedron Lett., 53, 1301 (1997); (c) Q.J. Fu, A.M. Steele, and S.C. Tsang Green Chem., 3, 71 (2001). A.G. Hernan, P.N. Horton, M.B. Hursthouse, and J.D. Kilburn, J. Organomet. Chem., 691, 1466 (2006). W.P. Gallagher, I. Terstiege, and R.E. Maleczka Jr., J. Am. Chem. Soc., 123, 3194 (2001). R.E. Maleczka Jr. and W.P. Gallagher, Org. Lett., 3, 4173 (2001). B.C. Gilbert and A.F. Parsons, J. Chem. Soc., Perkin Trans. 2, 367 (2002). L. Angiolini, D. Caretti, L. Mazzocchetti, E. Salatelli, R. Willem, and M. Biesemans, J. Organomet. Chem. 691, 3043 (2006). J.-M. Chr´etien, F. Zammattio, D. Gauthier, E. Le Grognec, M. Paris, and J.-P. Quintard, Chem. Eur. J. 12, 6816 (2006). D.A. Powell, T. Maki, and G.C. Fu, J. Am. Chem. Soc. 127, 510 (2005). O. Prieto and S. Woodward, J. Organomet. Chem. 691, 1515 (2006). C.M. Gordon and C. Ritchie, Green Chem. 4, 124 (2002). A. McCluskey, Green Chem. 1, 167 (1999). C. Blomberg and F.A. Hartog, Synthesis 18 (1977). Z. Wang, Z. Zha, and C. Zhou, Organic Lett. 4 1683 (2002). Z. Zha, A. Hui, Y. Zhou, Q. Miao, Z. Wang, and H. Zhang, Organic Lett. 7 1903 (2005). U.K. Roy, P.K. Jana, and S. Roy Tetrahedron Lett 48, 1183 (2007). M. Kizil and J.A. Murphy, J. Chem. Soc., Chem. Commun. 1409 (1995).

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6 Tin in Catalysis 6.1

Green Organotin Catalysts

Junzo Otera,a Monique Biesemansb , Vanja Pinoieb , Kevin Poelmansb , and Rudolph Willemb a b

Department of Applied Chemistry, Okayama University of Science, Japan High Resolution NMR Centre (HNMR), Vrije Universiteit Brussel, Brussels, Belgium

6.1.1

Introduction

In numerous natural products and synthetic compounds, ester moieties constitute major organic functional groups. Consequently, (trans)esterification reactions are widely applied in industry,1 e.g. in the synthesis of fatty acid esters,2,3 of polyesters4−9 and of macrolides.10 More specifically, polylactones and polylactides are multipurpose, bio-compatible and -degradable polyesters, which are suitable for biomedical and pharmaceutical applications.11−14 Tin-based Lewis acids like mono- and dialkyltin compounds4,15−19 and tetraalkyldistannoxane derivatives8,20−22 are very efficient catalysts for transesterification reactions under mild conditions. A considerable drawback in the use of these organotin compounds in organic synthesis is the difficulty of their quantitative removal from the reaction mixture, because some organotin compounds are toxic. These limitations to the exploitation of such reagents and/or catalysts in biomedical and pharmaceutical synthesis applications23,24 have been recently overcome by exploring alternative and less toxic catalysts, and by the improvement of workup procedures of several types of homogeneous organotin catalysts. Otera et al. have described the synthesis and catalytic activity of perfluoroalkyl distannoxanes, which combine the advantages of efficient catalysis based on organotins with fluorous biphasic technology.21,25 These types of organotin catalysts, being highly soluble in fluorocarbon solvents, provide solutions which become perfectly miscible with the organic reaction phase upon heating. After cooling the reaction mixture, phase separation is observed and the catalyst returns into the fluorous phase, making it easily and repeatedly recyclable. Another strategy to alleviate the aforementioned toxicity issue, involves grafting the organotin reagent onto an insoluble solid support.26−28 In this way, other types of environmentally friendly organotin catalysts, which can easily be removed from the desired reaction products by simple filtration of the insoluble support to which they are grafted, are being designed. The forecasted deliverables of such systems include improved catalyst recycling ability and a better control

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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over the environmental issues related to the toxicity of the non-supported species, i.e. reduced metal leaching into the final targeted chemical compositions.

6.1.2

Fluorous Distannoxane Catalysts

The basic idea of fluorous technology stems from the facile separation of organic products and fluorous catalysts, staying in the organic and fluorous phases, respectively, owing to incompatibility of the two reaction media.29 The catalysts can be easily recovered from the fluorous phase, or preferably the fluorous solution can be used repeatedly without isolating the catalysts. It is reasonable to assume that such unique solubility characteristics might be reflected in the equilibration as well. Based on this simple expectation, fluorous biphasic (trans)esterification has been pursued by use of fluorous distannoxane catalysts. Despite bearing a large inorganic metalloxane core, 1,3-disubstituted tetraalkyldistannoxanes are soluble in most organic solvents. This solubility is ascribed to a dimeric formulation (A, Figure 6.1.1) in which the metalloxane core is surrounded by eight alkyl groups, making the surface of the molecule lipophilic.30 In addition to the unique solubility, the dimeric structure gives rise to high catalytic activity for transesterification,31 thanks to a template effect induced by the two kinds of tin atoms, which are located close to each other, but in different coordination environments. Mildness is another notable feature of the distannoxane catalysts. Since the distannoxane-catalyzed reactions proceed under almost neutral conditions, various functional groups can survive the reaction conditions, and these merits have found a wide range of synthetic applications. It was therefore reasonable for us to postulate that replacement of surface alkyl groups on the distannoxanes with fluoroalkyl groups should render the molecules fluorophilic, and that mild catalysts would be accessible and workable under fluorous biphasic conditions. Synthesis of Fluorous Distannoxanes

Two fluoroalkyl groups, C6 F13 C2 H4 and C4 F9 C2 H4 , were employed, in which the Cn F2n+1 moiety endows the molecules with fluorophilicity, while the C2 H4 spacer insulates the electronic influence of the fluoroalkyl moieties. The synthesis of fluoroalkyldistannoxanes 1–4 is readily feasible by conventional methods, as shown in Scheme 6.1.1.25d A more convenient method for 1 was developed starting from commercially available Ph2 SnCl2 .25f All fluoroalkyldistannoxanes thus prepared were isolated as crystalline materials, and 119 Sn NMR spectra gave rise to two distinct resonances diagnostic of the characteristic dimeric distannoxane formulation.

Figure 6.1.1

Dimeric 1,3-disubstituted tetraalkyldistannoxanes

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The synthesis of fluoroalkyldistannoxanes 1–4

Solubility of Fluorous Distannoxanes

As expected, these compounds are sparingly soluble in common organic solvents except acetone and ethyl acetate, but well soluble in fluorocarbon solvents such as FC-72 (perfluorohexanes), FC-40 (perfluorododecanes), and octafluorocyclopentene. Partition of 1 between FC-72 and conventional organic solvents was determined (Table 6.1.1).25f All exhibited high preference for FC-72. The perfect bias into the FC-72 phase against hydrocarbons like toluene, benzene, and hexane is remarkable. It should be noted that dibromide 2 exhibits similar solubility to that of 1. Nonafluorohexyl derivatives 3 and 4 are also essentially fluorophilic, albeit to a somewhat lower degree than 1 and 2: for example, partition coefficients (FC-72/toluene) are 32 for 3 and 24 for 4, respectively.25d This is consistent with the well-accepted notion that the solubility in fluorocarbon solvents increases as the fluorine content of the molecule increases. Single Fluorous Solvent System

Upon confirmation of the highly fluorophilic character of fluorous distannoxanes, these compounds were utilized as catalysts for fluorous biphasic transesterification.21,25h First the simplest system, where FC-72 was the sole solvent, was employed. The reactants, in a 1:1 ratio, were heated in the presence of the catalyst at 150 ◦ C in FC-72 in a pressure bottle for 16 hours and the reaction mixture was washed with toluene. GLC analysis of the toluene layer exhibited a single peak assignable to the desired ester Table 6.1.1 Partition of 1 between FC-72 and organic solvents Organic solvent toluene benzene hexane CH2 Cl2 MeOH acetone THF

Partition (FC-72/organic solvent) ∼100:0 ∼100:0 ∼100:0 99:1 98:2 97:3 96:4

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Table 6.1.2

Transesterification in a single fluorous systema RCOOR + R OH

cat RCOOR + R OH FC-72

−−−−−−→

Yield % Entry

RCOOR’

R”OH

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Ph(CH2 )2 COOEt

C8 H17 OH PhCH CHCH2 OH PhCH CHCH2 OH PhCH CHCH2 OH geraniol geraniol PhC CCH2 OH THPO(CH2 )8 OH TBSO(CH2 )8 OH 2-octanol cyclohexanol menthol borneol PhCH CHCH2 OH PhCH CHCH2 OH

Ph(CH2 )2 COOMe Ph(CH2 )2 COOEt

PhCH CHCOOEt PhCOOEt

Cat.

GLC

Isolated

1 1 1 2 1 2 1 1 1 1 1 1 1 1 1

>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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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. T¨uter, H. A. Aksoy, E. E. Gilbaz, E. Kursun, Eur. J. Lipid Sci. Technol. 2004, 106, 513. J. Aburto, I. Alric, E. Borredon, Starch/St¨arke 2005, 57, 145. G. Deshayes, F. A. G. Mercier, P. Deg´ee, 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. Deg´ee, 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. Furl´an, 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. Hern´an, 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. Horv´ath (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. Deg´ee, 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. Deg´ee, P. Dubois, R. Willem, Organometallics 2008, 27, 1841.

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Organotin Catalysts for Isocyanate Reactions

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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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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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 <1 <1 2 <1 1 1 4 <1 <1 45 30 5 45

180 120 1 5 5 1 4 4 10 1 1 150 90 15 180

Triol:diisocyanate 1:1 NCO:OH, 70% solution in dioxane 70 ◦ C, 1,6-hexamethylene diisocyanate (HDI), 5-isocyanato-1-(isocyanatomethyl)-1,3,3-trimethyl-cyclohexane (IPDI), 1,1’-methylenebis[4-isocyanato-cyclohexane (H12 MDI), 1,1’-methylenebis [isocyanatobenzene (MDI) and 1,3-diisocyanatomethyl-benzene (TDI). Me % = percent metal based on polymer solids

a

This study compares the effect of catalysts on aliphatic and aromatic isocyanates. With the exception of di-n-butyltin dithiocarbonate, all the di-n-butyltin catalysts perform similarly. The DABCO catalyst shows excellent catalysis for aromatic isocyanates and is less effective for aliphatic isocyanates. Combining this amine catalyst with DBTDL gives excellent catalytic activity for both aliphatic and aromatic isocyanates. Stannous, zirconium, and zinc octanoate show reduced activity in comparison to organotin. The reaction rate depends on the structure of the isocyanate and the reactant and also on the catalyst used. Primary hydroxyl groups have been shown to be more than three times faster in their reactions with phenyl isocyanate36 than with secondary hydroxyl groups. Additional rate studies37 with DBTDL, phenyl isocyanate, and a variety of primary and secondary alcohols are shown in Table 6.2.6. This confirmed the higher reaction rate with primary alcohols, but also showed that neighboring groups have an effect on the rate. Table 6.2.6 Second-order rate data for reaction of phenyl isocyanate with hydroxyl compounds a Hydroxyl compounds 1-Butanol 2-Butanol Polyethylene glycol (M.W. 393) 2,3-Butanediol 3-Methoxy-1-propanol 2-Methoxy-1-propanol 1-Methoxy-2-propanol a

k × 104 , 1 equiv−1 s−1 170 15 151 100 (initial) 160 48.3 11.4

Toluene solutions at 30 ◦ C; NCO/OH = 1.1:1; 0.5 × 10−4 mol−1 DBTDL

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Table 6.2.7

Effect of neighboring groups on reaction rate a Reactants

Catalyst None DBTDL Sb(III) octanoate Bi(III) naphthenate Fe(III) octanoate Pb(II) octanoate Mn(II) octanoate Hg(II) naphthenate Sn(II) octanoate

Me, %

BuOH

EG

ETOH

ETSOH

DMAOH

HEAC

NPG

0 0.36 0.44 0.24 0.12 0.48 0.12 0.48 0.58

0 14 2 8 4 2 4 2 2

0 34 31 31 6 34 10 21 31

0 19 25 15 2 23 2 18 20

0 24 24 16 3 28 2 29 24

17 26 26 35 20 35 20 33 –

0 21 16 16 16 22 – 10 16

0 15 31 14 7 14 1 5 22

Temperature rise after 1 min in ◦ C. 1 M solution diisocyanate prepolymer/hydroxyl in dioxane. Start reaction temperature 25–27 ◦ C. Complete reaction 35 ◦ C temperature rise according to FTIR. Catalyst 2 weight % based on solution. Metal content of solution (derived from catalyst) shown in Table

a

BuOH = 1-butanol, EG = ethylene glycol, ETOH = 2-ethoxyethanol, EtSOH = 2-ethylthioethanol, DMAOH = β-dimethylaminoethanol, HEAC = 2-hydroxyethyl acetate, NPG = neopentylglycol. Toluene diisocyanate prepolymer (80% 2,4-TDI; 20% 2,6-TDI) 91 %, trimethylolpropane 9 %

The effect of neighboring groups to the hydroxyl groups is often ignored in the literature, but α-, β-, and γ -activating groups, as shown in Table 6.2.7 can have a significant effect on DBTDL and other metal catalysts.38 2-Tri-n-butylstannylpropanol was found to be an excellent catalyst, superior to DBTDL, in catalyzing the reaction of aromatic isocyanates with tertiary alcohols.39 Generally DBTDL and other dialkyltin catalysts show diminished catalysis with secondary and tertiary hydroxyl groups. Tetraorganostannanes40 of the type of bis-[2-(acyloxy)alkyl]diorganostannanes, are non-catalytic tin compounds, which can be heat-activated by anti β-elimination reaction leading to catalytically active bis(acyloxy)dialkylstannanes.41 R1 2 Sn(CH2 CHR2 OCOR3 )2 −→ R1 2 Sn(OCOR3 )2 + 2H2 C CHR2

(6.2.3)

For many applications, activation at room temperature is required. 1,2-Bis(acyloxy) tetraalkyldistannanes are easily oxidized in air to 1,3-bis(acyloxy)tetraalkyldistannoxanes, which is a proven catalysts for polyurethane formation.42 R1 2 (R2 CO2 )Sn-Sn(O2 CR2 )R1 2 + 1/2 O2 −→ R1 2 (R2 CO2 )Sn-O-Sn(O2 CR2 )R1 2

(6.2.4)

Another example of air-activated catalysts is the use of a blend of α-hydroxybenzylstannane and acylstannane in combination with compounds containing Sn–Sn linkages.43 Side reactions with other H-active sites, such as in urethane or urea can lead to branching, which will have an effect on the elastic properties of linear polyurethanes.44 At higher conversion the concentration of hydroxyl groups diminishes and that of urethane groups increases, and even highly selective catalysts will give increased side reactions at high conversion. Most of the side reactions during the cure of polyurethane systems are with water. Water can readily migrate into coatings, sealants, and adhesives during curing, causing the formation of carbamic acid and in turn amine, carbon dioxide, and eventually urea. The formation of polyureas is a potentially serious problem with coatings. Applications at high moisture can consume isocyanate and lead to polyureas, which are incompatible with the remaining network,

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leading to a loss in gloss and poor network formation. In extreme cases reaction with moisture can lead to blistering at a higher film thickness. In many urethane formulations, a molar excess of isocyanate is used (over-indexing) to compensate for the loss of isocyanate functionality due to side reactions. For linear systems an over-indexing of about 5% is commonly used. In coating applications over-indexing as high as 30% might be employed to compensate for the moisture side reactions and also to achieve networks with improved chemical resistance.45 A catalyst for an over-index formulation has to show high selectivity for the isocyanate–hydroxyl reaction and should show only low activity for the water reaction. The relative reaction rates of aliphatic isocyanate groups with phenolic and hydroxyl groups in the presence of DBTDL, DABCO, triphenylphosphine (TPP), and dinonylnaphthalene disulfonic acid (DNNDSA) have been studied46 . In the presence of DBTDL, the aliphatic hydroxyl groups primarily react with the isocyanate. DABCO and TPP mainly catalyze the reaction of the phenolic group; even when uncatalyzed, the reaction with the phenolic hydroxyl group proceeds first. With DNNDSA both hydroxyl and phenolic groups react with the isocyanate. At higher temperature, the phenolic-urethane deblocks and forms the aliphatic urethane reaction product. n-Butyltin trichloride was found to be an active catalyst47 for the reaction of phenyl isocyanate with 1,1-pentamethylene-3-phenylurea, under conditions where DBTDL, di-n-butyltin diacetate, di-n-butyltin dichloride, tri-n-butyltin chloride, tetramethyltin, trimethyltin bromide, triethyltin fluoride, and 1,2,4trimethylpiperazine were inactive. It was shown that ureas form solid complexes with butyltin trichloride, in which the tin compounds are coordinated through the carbonyl oxygen. The use of organotin catalysts in secondary amine functional polyaspartate-polyisocyanate cross-linked coatings48 has shown to improve the potlife of the formulation. 6.2.9

Applications

The selection of a suitable catalyst for isocyanate reactions is more complicated than simply finding a compound that accelerates the reaction. Many formulations using organotin catalysts are two-component formulations that are mixed just before application. Depending on end-use, application characteristics of a formulation, such as potlife, and the impact of the catalyst on the final product have to be considered. More recently, environmental considerations have played a major role in the selection of a catalyst. In coating applications, urethane chemistry is used to cross-link coatings and also to prepare thermoplastic polyurethanes or isocyanate-terminated pre-polymers. One-component (1K) formulations can be achieved using pre-polymers, which are isocyanate terminated. These prepolymers can be moisture cured. Moisture curing is used in coatings, adhesives, and sealants. With 1K systems it is difficult to achieve adequate long-term stability. All the components have to be carefully dried before blending and the system has to be packaged in the absence of water. Although a catalyst is supposed to accelerate the water reaction, too fast a reaction will lead to carbon dioxide blistering of the film. This is often a problem at a higher film thickness. In coatings applications, cross-linking of polyols with polyisocyanates offers fast, low-temperature curing and produces films with excellent properties. Organic coatings are the largest market for aliphatic isocyanates. Di-n-butyltin compounds are the major catalyst used in the preparation of aliphatic polyurethane polymers and in catalyzing the cross-linking reaction with aliphatic polyisocyanates. The formulation of low-solids content polyisocyanate cross-linked coatings is not too complicated. These low-solids coatings use higher molecular weight pre-polymers and form tack-free films after evaporation of the solvent. Cross-linking with the isocyanate can take place over a longer time period. These low-solids formulations use very low levels of catalyst, and potlife is not affected to a large extent by the catalyst. Further, they contain a large amount of solvents. The desire to develop coatings that emit lower amounts of solvent

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1000 Time, minutes

100

10

1

Figure 6.2.12 Potlife and tack-free time of a high solids polyester/1,6-diisocyanatohexane trimer (isocyanurate). Potlife is defined as doubling of the viscosity. Tack-free time of a coating is measured. -- Potlife, –– Tack free time

(low volatile organic content or low VOC) has generated the need for very low molecular weight resins and also polyisocyanate cross-linker. These polymers do not dry any more by evaporation of the solvent. Figure 6.2.12 illustrates the catalysis of di-n-butyltin dilaurate (DBTDL) in a high-solids polyester49 /HDI trimer system.50 Such a system is typical for a high solids coating formulation used in general industrial or aerospace applications. As illustrated, the usable potlife of such a system at an acceptable curing rate is very low. Figure 6.2.13 shows the relationship between viscosity of the formulation and the amount of isocyanate consumed. A catalyzed formulation shows the same reaction profile; only the scale of the x-axis would be different. For this formulation, this corresponds to a 7% consumption of isocyanate. To achieve a tack-free film, about 65% of the isocyanate would have to be reacted. Ideally, the end-user would like to have a system that has both a long potlife and fast reaction. To increase the application time of polyurethane formulations, delayed catalysts are used, and this delay can be achieved by chelation of the organotin compound with mercaptans or dionate. Table 6.2.8

400

100.0

350

98.0

300

96.0

250

94.0

200

92.0

150

90.0

Unreacted NCO, %

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Figure 6.2.13 Uncatalyzed reaction of a polyester polyol with HDI trimer. –– NCO %, -- Viscosity (cps)

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Table 6.2.8 Potlife of high solids polyester/HDI trimer with DBTDL as a catalyst and 2,4-pentanedione as a stabilizer a 2,4-Pentanedione, % based on solids content Time to double viscosity, minutes a

0 54

0.58 69

1.23 104

1.8 129

NCO/OH ratio 1.05. DBTDL 0.004203 % Sn on binder solids. Solvent: propylene glycol monoethyl ether acetate/xylene

shows that in a high-solids polyester/HDI-trimer formulation the addition of 2,4-pentanedione can more than double the useful potlife of a coating formulation without impairing the cure response.51 We can assume that the chelation of the DBTDL with 2,4-pentanedione reduces the catalysis rate.52 After application, the 2,4-pentanedione evaporates and the normal reaction rate is restored. A benefit to the chelation of DBTDL with 2,4-pentanedione is an improvement in hydrolytic stability of the catalyst. Complexes of organotin with amidines53 have been found to be excellent catalysts for the preparation of polyurethane foams, which do not have the disadvantage of any amine odor and, in addition, delayed onset of the isocyanate–hydroxyl reaction54 . An example of a mercapto-delayed organotin catalyst is 2,2,4,4-tetrakis(alkyl)-1,3,2,4-dithia-stannetane.55 Amine salts of amino acids,56 tertiary amino acids, and tertiary amino acid–nitrile compositions, have been found to be effective as delayed action catalysts for polyurethane synthesis. They are particularly effective when used in combination with an organometallic compound, such as an organotin.57 Novel catalyst compositions comprising complexes of tin(IV) salts and primary amine compounds allow delay of gelation until they dissociate under certain reaction conditions.58 Encapsulated catalysts have been used for delayed reaction in production of explosives;59 this delay permits casting of explosives. To further reduce the emission of solvents from coatings, water-borne two-component coatings crosslinked with isocyanates are being developed. The challenge in formulating these systems is the selection of a catalyst that is hydrolytically stable and catalyzes only the hydroxyl reaction and not the water reaction.60 Di-n-butyltin carboxylates61 or phenoxides62 hydrolyze to the oxide in water. Di-n-butyltin bis(2,3-dihydroxypropylmercaptide) is reported to be stable in water and has been used in two-component water-borne coatings.63 Hydrolytically stable organotin compounds are also being prepared with sulfonic acids.64 These dialkyltin and trialkyltin sulfonates can be used in combination with t-amines as catalysts in the preparation of polyurethane foams. In reaction injection molding compounds65 (RIM), a polyol component and a polyisocyanate are injected into a mold and cured. Essential for this application is fast cure in the mold to reduce cycle time. After demolding, the parts are often post-cured to complete the reaction and improve heat distortion temperature. A high heat distortion temperature (heat-sag) is especially important in automotive applications, where the parts have to go through the paint line and might be exposed to temperatures >150 ◦ C. The catalyst is normally placed in the polyol component, which might also contain other additives, such as filler, reinforcing agents, antioxidants, and blowing agents. The use of blowing agents permits the generation of RIM foams.66 An efficient catalyst is essential for this application. DBTDL is normally combined with amine catalysts and added to the polyol component. It has been found that the addition of the DBTDL catalyst to the polyisocyanate67 not only improves the reactivity of the catalyst system, but also improves the mechanical properties of the part. Because of the slow reaction of aliphatic isocyanates, the choice of the catalysts system is especially critical for light-stable RIMs, such as those used on automotive steering wheels. To achieve the high reaction rate, IPDI and IPDI trimer are used in conjunction with a distannoxane/amine catalyst.68 For complicated or large RIM parts, a longer time is required for filling

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of the mold, therefore, a catalyst with delayed gel-time is required. A solid catalyst prepared by bonding an organotin catalyst to silica gives the desired increase in gel-time and fast reaction in the mold.69 DBTDL was used as a catalyst in the frontal polymerization of 1,6-hexanediisocyanate with ethylene glycol.70 In frontal polymerization the polymerization is locally initiated and the exotherm of the reaction propagates the polymerization throughout the system. Pyrocatechol was used to avoid spontaneous polymerization. Pyrocatechol chelates tin and depresses the catalytic activity at room temperature without affecting catalysis at the higher temperature.71 To achieve a uniform advancing reactive front, and to avoid fingering, the viscosity of the blend was increased with colloidal silica. Polyurethane hydrogels are prepared by reaction of a polyisocyanate with polyethyleneglycol.72 These hydrogels can swell with water and are used in medical applications, such as wound dressings. Although dialkyltin catalysts are claimed in this application, the use of organotin compounds in medical applications with potential contact with blood is questionable, and they are probably not the safest catalysts for this application. Electron transfer polymers were prepared from p-benzoquinone-diols and diisocyanates, in the presence of DBTDL.73 At room temperature, no reaction of the isocyanate with the benzoquinone took place and the polymers were not cross-linked. Di-n-butyltin catalysts are being used in the preparation of polyurethane foams. Most polyurethane foams utilize aromatic isocyanates such as toluene diisocyanate (TDI) or diphenylmethane diisocyanate (MDI) as the isocyanate, and a polyester or polyether polyols as the coreactant. Tertiary amine catalysts are used to accelerate the reaction with water and formation of the carbon dioxide blowing agent. To achieve a controlled rate of reaction with the polyol, an organotin catalyst can be used. Polyurethane foams are not only applied in place, but are also cast in a factory as slabstocks.74 These foam slabs are then cut for use in car seats, mattresses, or home furnishings. DBTDL is an excellent catalyst in high resiliency slabstock foams.75 DBTDL shows an excellent reaction profile for this application; replacement for DBTDL in such an end-use is difficult and requires a substantial reformulation of the foam. 6.2.10

Blocked Isocyanates

The catalyst selection for blocked isocyanates depends on the nature of the blocking agent, the desired cure temperature, the polyol structure, and the application. Mono-, di- and trialkyltin compounds are effective catalysts for many blocked isocyanate reactions with hydroxyl compounds. In most solventborne coatings, DBTDL is used as a catalyst. The formulation of two component one-package polyisocyanate cross-linked coatings is achieved by blocking the isocyanate.19 The most common blocking agents are ketoxime, 3,5-dimethyl-1H-pyrazole, phenol, diethyl malonate, ε-caprolactam, 1,2,4-triazole, alcohol and dialkylamines, or internally blocked uretdiones. Blocked isocyanate cross-linked coatings are used in applications such as chip-resistant automotive primer and clearcoats,76 coil coatings, powder coatings, wire coatings, sealants, and cationic electrocoating.77 The problem of loss of volatile blocking groups during deblocking has been addressed by the use of uretdione and cyclopentanone-2-carboxymethyl esters78 as blocking groups. These blocked isocyanates cross-link by a ring opening mechanism. Table 6.2.9 shows the cure temperature obtainable with a range of blocked isocyanates.79 Co and Mn carboxylates are very effective catalysts, but in most formulations are not acceptable because of the purple or brown color they produce. Some of the lower cure temperatures reported in the literature are onset of cure, or cure in very thin films. Many blocked isocyanate reactions are reversible, and the rate of cure depends on the rate of evaporation of the blocking agent. In most published reports the effectiveness of a catalyst is related to the structure of the blocked isocyanate, and not much attention is paid to the

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Table 6.2.9 Cure temperature of blocked isocyanates, uncatalyzed and catalyzed with various metal carboxylates a Cure Temperature ◦ C Blocking Agent Diethylmalonate 3,5-Dimethyl-1H-pyrazole Ketoxime Phenol ε-Caprolactam Alcohol, glycol ether Cyclopentanone-2-carboxy ester30 IPDI uretdione

Uncatalyzed

Catalyzed

Catalysts

100–120 140 150 150 180 >200 180 >200

100–120 120–140 130–150 130–150 150–180 150–200 120–150 170–200

No effective catalyst DBTDL, Bi, Co Bi, Co, Cr (III), DBTDL, Zn, Ca DBTDL, Bi, Zn, Mn DBTDL, Bi, Zn, Co DBTDL, Bi, Co Zn, Bi, DBTDL DBN, DBTDL, Bi, Zn

a

Aliphatic isocyanates blocked with different blocking agents. HDI trimer or IPDI. DBN = 1,5-diazobicyclo[4.3.0]non-5-ene; Bi, Ca, Co, Cr, Mn, Zn, 2-ethylhexanoic acid salts

polyol. Comparison between cure rates of mono- and dialkyltin catalysts in acrylic and polyester systems leads to the conclusion that the nature of the polymer functional end groups80 has an effect on catalyst performance. The use of polystannoxane catalysts81 with improved cure performance over dibutyltin or other stannoxane compounds has been claimed with a range of blocked isocyanates; iron, zinc, zirconium, nickel, and bismuth salts are claimed as cocatalysts. The improved catalytic activity appears, to some extent, related to increased solubility of the polystannoxanes. Stable one-pack polyurethane compositions containing the oxadiazinetrione ring are catalyzed and cured using 1,3-bis(acetyloxy)-1,1,3,3-tetra-n-butyl-distannoxane.82 The single largest application for organotin catalysts is in blocked isocyanate cross-linked electrocoating.83 Most of the blocked isocyanates used in automotive electrocoating primers are based on toluenediisocyanate or diphenylmethane diisocyanate oligomers blocked with alcohols and glycolethers. Other blocking agents for the isocyanates claimed in patents are oximes, phenol, and dialkylamines.84 The use of β-hydroxyalkyl carbamates prepared from primary amines and cyclic carbonates in electrocoating permits lower cure temperatures in conjunction with di-n-butyltin and bismuth catalysts.85 , 86 Numerous patents claim many different di-n-butyltin salts as catalyst in electrocoating. DBTDL is used in an earlier cationic electrocoating patent.87 Improvement in film appearance of cationic epoxy resin is claimed by the use of dibutyltin dibenzoate and with other aromatic carboxylic acids.88 The literature suggests that these dialkyltin salts are hydrolyzed to di-n-butyltin oxide.89 Di-n-butyltin oxide can be used in these cationic electrocoating systems in pigment form; it codeposits with the resin and is activated during cure. Residual acid in the deposited film assists the activation of the catalyst. In earlier applications, corrosioninhibiting lead salts or pigments were used as catalyst.90 Lower cure temperature is claimed with the use of trialkyltin catalysts.91 The increasing concern about organotin compounds in the environment has led to the replacement of dibutyltin oxide with bismuth compounds.92 Bismuth compounds also offer the advantage of improved corrosion resistance.93 Phenol blocked isocyanates also have been used to prepare hyperbranched polyurethanes by a stepgrowth polycondensation mechanism, using DBTDL as a catalyst.94 Because of the toxicity of isocyanates, there is interest in preparing blocked isocyanates and polyurethanes by a non-isocyanate process.95 Reaction of aliphatic diamines with cyclic carbonates96 (1,3-dioxolan-2-ones) results in the formation of β-hydroxyalkyl carbamates.

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Figure 6.2.14

Reaction of amine with 1,3-dioxolan-2-one

Reaction of a primary amine with a 1,3-dioxolan-2-one substituted in the 4-position can lead to primary or secondary hydroxyalkyl carbamate. In the absence of a catalyst, the ratio of primary to secondary OH was found to be 20:80; in the presence of di-n-butyltin dimethoxide the ratio changed to 74:2697 (Figure 6.2.14). In the presence of an organotin compound, these β-hydroxyalkyl carbamates can be used as crosslinkers or to prepare polyurethane polyols by a transesterification reaction.98 Blocked isocyanates are also used as cross-linking agents for powder coatings. Because of its high melting point, caprolactam blocked, 1-(isocyanatomethyl)-5-isocyanato-1,3,3-trimethyl-cyclohexane (IPDI) is commonly used. Especially suited for this application are isocyanurate trimers.99 Despite catalysis with DBTDL, caprolactam-blocked isocyanates require a cure temperature of 180–200 ◦ C. To achieve a lower cure temperature of about 140 ◦ C, ketoxime-blocked isocyanates are used.100 Blocked isocyanates are being used in coil coatings, depending on cure temperature, and end uses; aliphatic or aromatic isocyanates blocked with 3,5-dimethylpyrazole, methylethyl ketoxime or caprolactam are being employed. Dialkyltin compounds catalyze the preparation of carbamates from aromatic amines and organic carbonates.101 6.2.11

Catalyst Interactions

The activity of a catalyst in a polymer system is not always assured. Dialkyltin catalysts can be deactivated in a formulation, by absorption on a pigment or filler surface, or by ionic impurities. Low pH or strong acids can either inhibit or reduce the reaction rate of DBTDL catalyst systems. In hydroxyl end-capped fluoropolyether polymers, a minimum amount of DBTDL catalyst is required to catalyze the reaction with isocyanates.102 The interaction of a catalyst with impurities can also lead to an increase in catalyst activity. Polyether polyols prepared from ethylene oxide or propylene oxide are often prepared with potassium hydroxide as a catalyst. Residual catalyst in the polyol has been shown to increase the reaction rate with DBTDL.103 6.2.12

Polymer Synthesis

Polyurethane pre-polymer or thermoplastic polyurethanes are prepared by the reaction of polyester or polyether diols with diisocyanates. The reaction can be conducted in bulk, in a solvent, or in an aqueous solution or dispersion. Unless the reaction is carried out at an elevated temperature a catalyst is required. Organotin compounds are the prime catalyst for this reaction. A highly ordered polyurethane with high head-to-tail regularity was prepared by polyaddition reaction of 1-isocyanato-2-[(4-isocyanatophenyl)methyl]-benzene with 1,2-ethylene glycol using 1,3-

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Table 6.2.10 Partial Periodic Table. Elements reported in the literature to be catalytically active in catalyzing isocyanate reactions 1

1A

2

Li

3

Na

4

3

11

19

K

2A

3A

4

5

Be 12

Mg

B 3B

20

Ca Sr

6

Ba

56

5B

22

23

Ti

38

5

4B

40

Zr 57

La

72

Hf

V

6B 24

Cr

7B 25

Mn

42

74

26

Fe 44

Mo W

|—

Ru 75

Re

76

Os

8B 27

Co

—| 28

Ni

1B 29

Cu

2B 30

Zn

45

13

Al

4A 6

O

14

15

16

Si

80

P

7A

8A

8

N

S

31

Ga Sn

Hg

7

6A

C

50

Rh

5A

82

Pb

51

Sb 83

Bi

52

Te 84

Po

bis(isothiocyanate)-1,1,3,3-tetra-n-butyldistannoxane as catalyst104 . Polymerizations were conducted in N , N -dimethylformamide at –40 ◦ C. The ordered polyurethane showed higher melting point and crystallinity, compared to the polymers with lower H-T regularity. For many polyurethane pre-polymer preparations, asymmetry in the reaction with alcohol is desired to achieve polymers with a narrower MW distribution. DBTDL catalysis of the reaction of 3,5-diethyl-4,4’-diisocyanato-diphenylmethane (DEMDI) with both alcohols and aromatic ureas105 increases asymmetric reaction.

Figure 6.2.15 Half-life isocyanate concentration in the reaction of 1,6-hexamethylene diisocyanate trimer with alcohols. (1.12 mol/l NCO) in xylene. Molar ratio NCO/OH 1/1. Catalyst concentration: DBTDL 0.014% Sn; Zr Chelate 0.014% Zr; Bi carboxylate 0.13% Bi; Zn octanoate 0.27 % Zn on total reactants. HE = βhydroxyethylester, HE Carbamate = β-hydroxyethyl carbamate, TMP = methoxytripropylene glycol, Zn oct = Zn 2-ethylhexanoate, Bi carb = Bi 2-ethylhexanoate, Zr CH = Zr dionate complex

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6.2.13

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Tin Chemistry: Fundamentals, Frontiers and Applications

Organotin Replacements

For environmental reasons, organotin catalysts are being replaced in a number of applications with more benign catalysts. The replacement of an organotin catalyst often requires a complete change of formulation. Most alternate catalyst systems offer a different reaction profile. Table 6.2.10 shows a partial periodic table and the elements that, according to the literature and our own screening studies,106 , 107 are active catalysts for the isocyanate reaction. Many potential catalysts are automatically eliminated as unsuitable, because of the color of the ions or salts, because they are strong oxidation catalysts, or because they are potentially even more toxic or of environmental concern. Catalysts that are being used as replacements for organotin compounds in two-component polyisocyanate cross-linked coatings are based on bismuth, zirconium, titanium, and aluminium. Bismuth oxide or carboxylate as catalysts are being used as replacements for organotin compounds in cationic electrodeposition coatings. The reaction rate of isocyanates with hydroxyl compounds is generally considered to be a function of the isocyanate, nucleophilicity of the reaction group, and steric effects. For organotin compounds and many other metal catalysts, primary hydroxyl groups are faster reacting than secondary or sterically hindered groups. For bismuth and zinc carboxylates, a much more complicated picture emerges.108 Bismuth and zinc can undergo interaction with neighboring groups that can substantially enhance the reaction rate. Rate studies of aliphatic isocyanates with n-butanol, isobutanol, 2-butanol, 2-butoxyethanol, 2-hydroxyethyl ester, 2-hydroxyethyl carbamate, and methoxtripropylene glycol are shown in Figure 6.2.15. This fact complicates the job of the formulator, who often lacks the information on the nature of the functional groups on a polymer. References 1. J. Dodge, Polyurethanes and Polyureas: Synthetic Methods in Step-Growth Polymers, 197, John Wiley & Sons, Inc., New York (2003). 2. A. L. Silva and J. C. Bordado, Catalysis Reviews, 46, 31 (2004). 3. L. R. Brecker, Plastics Engineering, 39 (1977). 4. A. G. Davies, Organotin Chemistry, Weinheim: Wiley-VCH (2004), Chapter 14, 218. 5. L. Thiele and R. Becker, Adv. Urethane Sci. Technol., 12, 59 (1993). 6. J. F. Florio, Catalysis of urethane systems, in Handbook of Coatings Additives, 2nd Edn J. Florio and D. Miller (Eds), Marcel Dekker, New York (2004). 7. A. J. Bloodworth and A. G. Davies, J. Chem. Soc., 5238 (1965). 8. R. P. Houghton and A. W. Mulvaney, J. Organomet. Chem., 518, 21 (1995). 9. E. T. Hessell and W. J. Blank, unpublished results. 10. S. L. Reegen and K. C. Frisch, J. Polymer Science, Part A-1, 8, 2883 (1970). 11. L. Thiele, Acta Polymerica, 30, 323 (1979). 12. R. P. Houghton and A. W. Mulvaney, J. Organomet. Chem., 517, 107 (1996). 13. S. Luo, H. Tan, J. Zhan, Y. Wu, F. Pei, and X. Meng, J. Appl. Poly. Sci., 65, 1217 (1997). 14. F. W. Van Der Weij, J. Polymer Science: Polymer Chemistry Edition, 19, 381 (1981). 15. F. W. Van Der Weij, J. Polymer Science: Polymer Chemistry Edition, 19, 3063 (1981). 16. M. Urban and Q. Han, J. Appl. Polymer Sci., 86, 2322 (2002). 17. K. Schwetlick, R. Noack, and F. Stebner, J. Chem. Soc., Perkin Trans II, 599 (1994). 18. I. S. Bechara, ACS Symposium Series, Urethane Chem. Appl. 172, 393 (1981). 19. D. A. Wicks and Z. W. Wicks Jr, Prog. Org. Coatings 36, 148 (1999). 20. H. Kothandaraman, A. Sultan Nasar, and K. R. Suresh, J. Macromolecular Science, Pure and Applied Chemistry, A33 (6) 833 (1996). 21. AICS, DSL, ECL, EINECS, ELINCS, ENCS, NDSL, NLP, PICCS, SWISS, TAIWAN and TSCA Inventories.

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Organotin Catalysts for Isocyanate Reactions 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. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63.

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F. W. Abbate and H. Ulrich, J. Appl. Polym. Sci., 13, 1929 (1969). Y. Nakano, K. Kotsuma, and M. Ito, US Patent 5055543 (1991). K. Strzelec, E. Le´sniak, and G. Janowska, Polymer International, 54, 9, 1337 (2005) A. Schotman. PhD Thesis Technical University Delft (1993). http://repository.tudelft.nl/file/260866/201256 W. Reichmann, K. K¨onig, and M. Scho´´nfelder, US Patent 4340712 (1982). N. V. Seeger and T. G. Mastin, US Patent 2683729 (1954). K. Ashida, K.C. Frisch US Patent 3817938 (1974). T. O. Murdock and B. W. Carlson, US Patent 5138016 (1992). A. Mishra, US Patent 4040992 (1977). I. S. Bechara, F. P. Carroll, D. G. Holland, and R. L. Mascioli, US Patent 4040992 (1997). R. E. Buckles and L. A. McGrew, J. Am. Chem. Soc. 88, 3582 (1966). J. J. Monagle, J. Org. Chem., 27, 3851 (1962). F. Hostettler and E. F. Cox. US Patent 3392128 (1968). J. W. Britain and P. G. Gemeinhardt, J. Appl. Polym. Sci., 4, 207 (1960). E. P. Squiller and J. W. Rosthauser, Prepr. Waterborne and Higher-Solids Coating Symp. New Orleans, LA, 460 (1987). E. Dyer, H. A. Taylor, S. J. Mason, and J. Sampson, J. Am. Chem. Soc., 71, 4106 (1949). L. Rand, B. Thir, S. L. Reegen, and K. C. Frisch, J. Appl. Polym. Sci. 9, 1787 (1965). J. Robins, J. Appl. Polym. SCI., 9, 821 (1965). M. Ratier, D. Khatmi, and J. G. Duboudin, Appl. Organometal. Chem. 6, 293 (1992). B. Jousseaume, N. Noiret, M. Pereyre, and A. Saux, Organometallics, 13, 1034 (1994). B. Jousseaume, N. Noiret, M. Pereyre, J. M. Franc`es, and M. P´etraud, Organometallics, 11, 3910 (1992). M. Yokoo, J. Ogura, J, and T. Kanzawa, Polym. Lett. 5 (1967).; J. Otera, T. Yano and R. Okawara, Chem. Lett., 901 (1986). J. Feldman, S. J. McLain WIPO WO/2006/007407 ˇ K. Duˇsek, M. Spirkovˇ a, and I. Havli´cek, Macromolecules, 23, 1774 (1990). Note by the author: Most Aerospace coatings require resistance to hydraulic liquids; over-indexing permits an increase in polyurea groups in the network that is not soluble in the hydraulic liquid. R. P. Subrayan, S. Zhang, F.N. Jones, V. Swarup, and A. I. Yezrielev, J. Appl. Polym. Sci., 77, 2212 (2000). E. Dyer and R. Bruce Pinkerton, J. Appl. Polym. Sci., 9, 1713 (1965). P. E. Yeske, E. H. Jonsson, and D. A. Wicks, Polym. Mater. Sci. Eng., 70; D. A. Wicks and Philip Yeske, Prog Org. Coatings 30, 265 (1997). Desmophen 651A-65 and Desmophen R-221-75 at a solids weight ratio of 65/35 were used in this formulation. Desmodur N-3300 is a hexamethylene diisocyanate isocyanurate trimer (Bayer Material Science.) W. J. Blank, CHIMIA, 56, 191 (2002). Communication with J. Florio, King Industries. Norwalk, CT USA. M. D. Jayawant, US Patent 3635906 (1972). R. Welte and G. Grogler, US Patent 4006124 (1977). H. A. Green and W. Erner, US Patent 3152094 (1964). J. W. Rosthauser, H. Nefzger, and R. L. Cline, US Patent 6020283 (2000). I. S. Bechara, R. L. Mascioli, and P. J. Zaluska, US Patent 4115634 (1978). I. S. Bechara, P. J. Zaluska, and R. L. Mascioli, US Patent 4086213 (1978). L. A. Grier, P. L. Neill, R. D. Priester, L. W. Mobley, K. W. Skaggs, and R. B. Turner, US Patent 5491174 (1996). L. C. Carlton and J. A. Finnegan, US Statutory Patent Invention Registration H778 (1990). A. Z. He, W. J. Blank, and M. E. Picci, J. Coat. Techn. 74, 930 (2002). T. Yokoyama, N. Kinjo, and J. Mukai, J. Appl. Polym. Sci., 29, 1951 (1984). J. Considine and J. J. Ventura, J. Org. Chem., 28, 221 (1963). K. G. Caldwell, R. H. Fernando, and V. K. Pidugu, US Patent 6316535 (2001).

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700 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 95. 96. 97. 98. 99. 100. 101. 102. 103. 104. 105. 106. 107. 108.

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Tin Chemistry: Fundamentals, Frontiers and Applications C. G. Coe, US Patent 4286073 (1981). http://www.rimmolding.com/rim/index.html W. C. Salisbury, US Patent 4048105 (1977). R. J. G. Dominguez, US Patent 4469657 (1984). E. Du Prez and P. Coppens, US Patent 6242555 (2001). E. R. Falardeau, K. C. Frisch, Jr., and M. R. Lock, US Patent 4507410 (1985). S. Fiori, A. Mariani, L. Ricco, and S. Russo, Macromolecules 36, 2674 (2003). L. G. Dammann and G.M. Carlson, US Patent 4788083 (1988). F. Hostettler, D. Rhum, M. R. Forman, M. N. Helmus, and N. Ding, US Patent 5576072 (1996). G. Wegner, N. Nakabayashi, and H. B. Cassidy, J. Polym. Sci. PART A-1, 6, 3151-3156 (1968). http://www.dow.com/voractiv/slab.htm E. J. Gerard, H. Verstraete, W. Maas, and B. Schlenter, J. Cellular Plastics., 38, 451 (2002). K. Kawazu, H. Iida and R. Asamen, US Patent 5981652 (1999). D. A. Wicks and Z. W. Wicks Jr., Prog. Org. Coatings 41, 1 (2001). M. Schelhaas and C. G¨urtler, US Patent 6827875 (2004). W.J. Blank, Z. A. He, and M. E. Picci, Prep. PMSE Div. ACS Mtg. (1998). H. P. Higginbottom, G. R. Bowers, L. W. Hill, and J. F. Courtier, Prog. Org. Coat., 34, 27 (1998). M. H. Gitlitz and S. R. Seshadri, US Patent 5770672 (1998). M. Tanaka, Y. Kamatani, and K. Nasu, US Patent 4478894 (1984). G. B¯uttner, N. David, and K. Klein, US Patent 5176804 (1993). A. Dobbelstein, M. Geist, G. Ott, and G. Sch´o´n, US Patent 5003025 (1991). W. Jacobs, III, G. G. Parekh, and W. J. Blank, US Patent 4484994 (1984). W. J. Blank, ACS Proc. Polym. Mat. Sci. Eng., 63, 931 (1990). J. R. Marchetti, R. R. Zwack, and R. D. Jerabek, US Patent 4104147 (1978). A. Motohashi, Y. Tsukahara, K. Masuda, H. Haneishi, M. Kume, and H. Hayashi, US Patent 4904361 (1990). W. Paar and J. Gmoser, US Patent 4789696 (1988). R. R. Zwack and R. D. Jerabek, US Patent 4115226 (1978). E. C. Bossert, W. Ranbom, and W. A. Larkin, US Patent 5880178 (1999). Z. He, W. J. Blank, and M. E. Picci, US Patent 6353057 (2002). K. G. Kerlin and P. Hamacher, US Patent 5702581 (1997). R. Spindler and J.M.J. Fr´echet, Macromolecules, 26, 4809 (1993). W. J. Blank, Prog. Org. Coatings, 20, 235 (1992). E. Dyer and H. Scott, J. Am. Chem. Soc., 79, 672 (1956). L. Ubaghs, PhD Thesis Tech. Univ. Aachen, http://darwin.bth.rwth-aachen.de/opus3/volltexte/2005/1056 (2005). W. J. Blank, US Patent 5134205 (1992). R. Gras, F. Schmitt, and E. Wolf, US Patent 4246380 (1981). H. P. M¨uller, K. Wagner, and H. J. Kreuder, US Patent 4150211 (1979). A. E. Gurgiolo, US Patent 4268684 (1981). L. Mashlyakovskiy, E. Khomko, V. Zaiviy, and C. Tonelli, J. Polym. Sci. Part A: Polym. Chem., 38, 2579 (2000). H. G. Wissman, L. Rand, and K. C. Frisch, J. Appl. Polym. Sci. 8, 2971 (1964). T. Hikita, A. Mochizuki, J. Sugiyama, K. Takeuchi, M. Asai, and M. Ueda, Polym. Jour., 33, 547 (2001). L. C. Case, J. Appl. Polym. Sci., 8, 935 (1964). Z. A. He, W. J. Blank, and M. E. Picci, Proc. 26th Int. Waterborne, High-Solids, and Powder Coatings Symp. New Orleans, LA (1999). Communication with J. J. Florio, unpublished King Industries, Inc. Information W. J. Blank, Macromolecular Symp. 187 (Quo Vadis-Coatings), 261 (2002).

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Catalysis of Reactions of Allyltin Compounds and Organotin Phenoxides

6.3

701

Catalysis of Reactions of Allyltin Compounds and Organotin Phenoxides by Lithium Perchlorate

Wojciech J. Kinarta and Cezary M. Kinartb a b

Department of Organic Chemistry, University of Lodz, Poland Department of Chemistry, University of Lodz, Poland

6.3.1

Catalysis of Metalloene Reactions of Allylstannanes by Lithium Perchlorate

6.3.1.1

Reactions of Allylstannanes With 4-Phenyl-1,2,4-Triazoline-3,5-Dione and Diethyl Azodicarboxylate

Reactions of allylstannanes with different enophiles, with shift of the stannyl group, are known where the reagent X Y is O CR2 , RC CR, R2 C CR2 , O O, RN NR, O NR, or O S O. These are metallic equivalents of the ene reactions, in which hydrogen is transferred, and are referred to as metalloene reactions. The reactions involving carbonyl compounds as the enophiles provide an important route to homoallylic alcohols. The reactions with the other enophiles are of more recent recognition, and have, as yet, been used only in a limited way in organic synthesis. The kinetic hydrogen isotope effects, which have been observed in the hydrogen–ene reactions of deuterium-labeled alkenes with singlet oxygen, N -phenyltriazolinedione, nitrosopentafluorobenzene, formaldehyde, methyl chloroacrylate, and ethyl propiolate as the enophiles, and the stereoselectivity of the reactions, imply the initial formation of a complex between the alkene group and the enophile. The various products that have been identified in the metalloene reactions can be rationalized on the basis of a similar mechanism. Allyltin compounds react with singlet oxygen or azo enophiles to give the M–ene, H–ene and cycladdition products, as shown in Scheme 6.3.1. The relative yields depend on the structural and environmental conditions.1,2 A reasonable model is that they involve the prior formation of a complex between the ene and the enophile (X Y),1 which may be represented as an oxirane oxide or an aziridine imine when X Y = O O or RN NR, respectively. Cyclic transfer of the metal or of hydrogen may then lead to the M–ene (Scheme 6.3.1a) or H–ene (Scheme 6.3.1b) products, respectively, and cyclization with migration of the metal (Scheme 6.3.1c) results from β-interaction of the metal with the carbonium ion center2 M X

M X

Y

_

X

Y+

(a)

Y

(c)

(b)

M

M

H

X

X

Y

Y

M-ene

Scheme 6.3.1

H-ene

_

M +

M

X Y

Cycloaddition

Mechanism of metalloene and related reactions

Kinart 3,4 has studied reactions of different allyltin compounds with 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) in diethyl ether in the absence and presence of LiClO4 . The rates of reactions of allylstannanes

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Table 6.3.1 Reactions of allyltin derivatives with PTAD in Et2 O (half-lives of reactions for 0.0046 mol dm−3 solutions) Relative Yields of Products (%) Compound Bu2 ClSnCH2 -CH CH2

Bu2 Sn(CH2 -CH CH2 )2

Ph3 SnCH2 -CH CH2

(cyclohex)3 SnCH2 -CH CH2

Ph3 GeCH2 -CH CH2

[LiClO4 ]

Half-life of

M–ene

H–ene

Cycloaddition

(mol dm−3 )

the reaction (s)

100

–

–

0

15

100

–

–

4

5

79

–

11

0

80

100

–

–

4

<5

69

24

11

0

1140

100

–

–

4

26

62

–

38

0

1805

100

–

–

4

40

53

37

30

0

9000

56

21

23

4

1500

with equimolar amounts of PTAD (0.0046 mol dm−3 ) in Et2 O and 4 mol dm−3 solutions of LiClO4 were followed by UV-vis spectroscopy by measuring half-lives of reactions (times corresponding to the decrease of the initial absorbance by 50%).4 A strong catalytic effect of LiClO4 has been observed in Et2 O. For example, the half-life of the reaction was reduced by a factor of ca. 45 for reactions of allyltriphenyltin and allyltricyclohexyltin. Quite interestingly the analogous effect for allyltriphenylgermane was much smaller. The effect of 4 mol dm−3 LiClO4 on the nature of the products was determined by NMR spectroscopy and is shown in Table 6.3.1. The salt effect has also been studied for the reaction with diethyl azodicarboxylate (DEAD).5,6 The results with DEAD were broadly parallel to these with PTAD. However, the DEAD reacted more slowly (see Table 6.3.2). As before, the studied reactions were faster in 4 M LiClO4 in Et2 O, in comparison to pure diethyl ether. Presumably this is associated with solvation of DEAD by molecules of the solvent. Kinart has compared the half-lives of the addition reaction of diallyldi-n-butyltin with DEAD in the series of 1 M LiClO4 solutions in CH3 CN, Et2 O, ethyl acetate, and acetone (see Table 6.3.3).6 The half-lives, t1/2 , were shortest in Et2 O. Further addition of LiClO4 to all studied solvents resulted in a considerable decrease in the half-lives, particularly in Et2 O and CH3 CN. In order to explain this effect, Kinart carried out comparative studies of both azo compounds in Et2 O and CH3 CN using absorption spectroscopy.6 Acetonitrile was chosen among other studied solvents, where the catalytic activity of LiClO4 was noticeable, due to its transmittance in the UV region. The solution of PTAD in diethyl ether exhibited two absorption maxima λmax at 226 and 258 nm. The increase of the concentration of LiClO4 in Et2 O resulted in a decrease in the molar absorptivity at 226 nm, leading to a decrease in the ratio of molar absorptivities at 226 and 258 nm. For example, this ratio changed from approximately 2.4 to 1.6 with an increase in the concentration of LiClO4 , from 1.6 to 3.8 mol dm−3 in 9 ×

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Table 6.3.2 Reactions of allyltin derivatives with DEAD in Et2 O (half-lives of reactions for 0.0254 mol dm−3 solutions) Compound

[LiClO4 ] (mol dm−3 )

Half-life of the reaction (s)

0

17423

4

26

0

t1/100 >900

4

35

0

155

4

<10

0

t1/100 >1200

4

50

Bu2 Sn(CH2 -CH CH2 )2

Ph3 SnCH2 -CH CH2

Bu2 ClSnCH2 -CH CH2

(cycloHex)3 SnCH2 -CH CH2

10−5 mol dm−3 solution of 4-phenyl-1,2,4-triazoline-3,5-dione in Et2 O. The increase of the concentration of LiClO4 also caused a small decrease in the molar absorptivity at 527 nm. In the case of 6 × 10−5 mol dm−3 solutions of 4-phenyl-1,2,4-triazoline-3,5-dione in acetonitrile, effects associated with the increase in the concentration of LiClO4 were even more intense. In pure solvent, the ratio of molar absorptivities (λmax 215 and 248 nm) was equal to ca. 4.1, whereas in 1 mol dm−3 solution of LiClO4 it achieved a value of ca. 0.6. This change was linked with an decrease in the molar absorptivity at λmax 226 nm and an Table 6.3.3 Reaction of diallyldi-n-butyltin with DEAD (half-lives of the reactions for 0.0254 mol dm−3 solutions)

Solvent CH3 CN

Et2 O

Ethyl acetate

Acetone

[LiClO4 ] (mol dm−3 )

Half-life of the reaction (s)

0

505

1 0

262 226

1 3

109 38

0

386

1

219

0

410

1

234

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increase at λmax 265 nm. A long-wave shift of the two mentioned absorption maxima in comparison to pure CH3 CN was also observed. Additionally, the addition of LiClO4 to CH3 CN resulted in an increase in the molar absorptivity at 530 nm. The absorption spectra of 5.08 × 10−4 mol dm−3 solutions of diethyl azodicarboxylate in Et2 O and CH3 CN also exhibited some differences. Spectra of the studied azo compound in pure Et2 O and 3 mol dm−3 solutions of LiClO4 were similar. In contrast, spectra of 5.08 × 10−4 mol dm−3 solutions of diethyl azodicarboxylate in pure acetonitrile and 1 mol dm−3 solutions of LiClO4 exhibited some differences. The addition of LiClO4 resulted in a small long-wave shift of the absorption maximum (λmax 207 nm) of about 8 nm and a decrease in its molar absorptivity. No effect from the addition of LiClO4 to studied solvents was observed in case of the second absorption maximum (λmax 400 nm). Kinart assumed that the catalytic properties of LiClO4 in diethyl ether are linked with: (1) facilitation of the formation of the polar intermediate by the ionic medium and (2) lowering of the LUMO of the azo compound by association with aggregates of LiClO4 . In the case of 4-phenyl-1,2,4triazoline-3,5-dione, both effects mentioned above may affect its activity. However, the facilitation of the formation of the polar intermediate seems to be responsible for the increased activity of diethyl azodicarboxylate in concentrated solutions of LiClO4 in Et2 O. No effects associated with changes in electron density of this azo compound with the addition of LiClO4 were observed in the absorption spectra. 6.3.1.2

Metalloene Reactions of Allyltin Compounds With Singlet Oxygen

The effect of added LiClO4 was studied with five allyltin derivatives in ether, under standard conditions.4,7 The results are given in Table 6.3.4. In pure ether, the yields of the studied compounds after 3 h were less than 5%. The yield for allyldi-n-butyltin chloride was equal to 30%. Illumination for a longer time led to decomposition. The addition of 4 mol dm−3 of LiClO4 produced a considerable increase in the yield of the reaction; no cycladdition products were detected, and the ratio of the M–ene and H–ene reactions varied from ca. 1:1 for allyltricyclohexyltin to 1:0 for allyltriphenyltin. As observed with the azo enophiles, an increase in the polarity of the medium resulted in an increase in the overall rate of the reaction, and a chemoselectivity favoring the M–ene reaction. Kinart suspected that the mechanism of this catalytic reaction with singlet oxygen is associated with the formation of the polar intermediate during the metalloene reaction being easier. To prove this assumption, photooxygenations of 1.48 × 10−2 mol dm−3 solutions of 1,3-diphenylisobenzofuran in Et2 O and 4 mol dm−3 solutions of LiClO4 in Et2 O were performed.7 The yields of the reactions measured after 90 and 240 s in both solutions were nearly the same (12 and 80%, respectively). It has been shown by other authors that the above reaction is solvent sensitive8 and β values [the ratio of the rate of decay of singlet oxygen (kd ) and the rate of reaction of 1 O2 (krx ) with 1,3-diphenylisobenzofuran] have usually been measured. Table 6.3.4

Reaction of allyltin derivatives with singlet oxygen in 4 mol dm−3 solution of LiClO4 Relative Yields of Products [%]

Compound Bu2 ClSnCH2 -CH CH2 Bu2 Sn(CH2 -CH CH2 )2 Ph3 SnCH2 -CH CH2 (cyclohex)3 SnCH2 -CH CH2 (CH2 -CH CH2 )4 Sn

M–ene

H–ene

Cycloaddition

[LiClO4 ] (mol dm−3 )

Yield of the reaction (%)

100 66 100 47 100

– 34 – 53 –

– – – – –

4 4 4 4 4

100 50 40 40 90

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The solvent may affect either or both values of krx and kd . Therefore, assuming that the yield of this reaction reflects changes in the β value in a quantitative way, it appears that the catalytic role of LiClO4 in case of studied photooxygenations cannot be explained by possible variations in the concentration of 1 O2 under heterogenous conditions, or of its lifetime. 6.3.1.3

Metalloene Reactions of Allylstannanes With Aldehydes

The allylation of carbonyl derivatives, such as aldehydes, is one of the most important carbon–carbon bond-forming reactions, because of the versatility of homoallylic alcohols as synthetic intermediates. Among various allylmetal reagents, allylstannanes are very useful because of their moderate activity, which can be increased by catalyst activation. The addition of allylstannanes to aldehydes has been shown to proceed in the presence of conventional Lewis acids, such as BF3 ·OEt2 , AlCl3 , InCl3 , MeSiCl3 , SnCl2 , SnCl4 , n-BuSnCl3 , n-Bu2 SnCl2 , MgBr2 , ZnCl2 , TiCl4 , Sc(OTf)3 , and PdCl2 (PPh3 )92 In the absence of Lewis acids, allylstannanes do not react with aldehydes at ambient temperature and pressure.10 The reaction of allylstannanes with aldehydes can be regarded as a special group of ene reactions, which have mechanistic features different from those shown in Scheme 6.3.1. Grieco reported11 that the addition of tri-n-butylallylstannane or allyltrimethyltin to α-alkoxy aldehydes proceeds rapidly in a 5.0 M solution of LiClO4 in diethyl ether (LPDE) providing high yields of chelationcontrolled products. In this experiment, a 0.1 M solution of benzyloxy aldehyde (1) (R = CH2 Ph) in 5.0 M LPDE was treated with 1.5 equivalents of tri-n-butylallylstannane (Scheme 6.3.2). After 3.0 h at ambient temperature, an 82% isolated yield of the diastereometric mixture of homoallylic alcohols (2) and (3) (R = CH2 Ph) was obtained in a ratio of 24:1. The use of allyltrimethyltin (1.5 equivalent) in the above reaction gave rise, after only 1.25 h, to a 77% yield of (2) and (3) (R = CH2 Ph), in a ratio of 25:1, whereas exposure of the corresponding silyl ether (1) (R = TBDMS) (0.1 M in 5.0 M LPDE) to 1.5 equiv. of allyltrimethyltin for 5 h gave rise (87%) to a 1.2:1 ratio of (2) and (3) (R = TBDMS), respectively, indicating that the Lewis basicity of the ether oxygen lone pairs is an important factor in controlling the selectivity of the reaction.

OR

OR O H

(1)

Scheme 6.3.2

OR

SnBu3 5.0 M LPDE

OH

(2)

+

OH

(3)

The allylation of benzyloxy aldehyde with tri-n-butylallylstannane

The reactions of allyltrialkylstannanes with dialdose derivatives11 proceed similarly, with a high diastereofacial selectivity in 5.0 M LPDE via chelation-controlled catalysis by the lithium ion. For example, the galactose derivative (4) (1.0 M in ether) reacts with 1.5 equivalents of allyltri-n-butyltin in 5.0 M LPDE, giving rise after 1 h to 96% yield of 5 and 6 in a ratio 25:1, respectively (Scheme 6.3.3). Exposure of (4) to allyltrimethyltin in 5.0 M LPDE proceeded with a high diastereofacial selectivity >25:1. In contrast, treatment of (4) with allyltrimethyltin in 2.0 M LPDE at ambient temperature proceeded with reduced diastereofacial selectivity, giving rise to a 12:1 ratio of (5) and (6), respectively, in only 66% yield.

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O

H

OH

H

HO

H O O

O

TBSA (1.5 equiv) 5.0 M LPDE

+

O

O

O

O

O

O

(4)

O

O

O

O

O

O

(5)

(6)

Scheme 6.3.3 The allylation of 2,2,7,7-tetramethyl-tetrahydro-bis[1,3]dioxolo[4,5-b;4, 5 -d]pyran-5carbaldehyde with allyltri-n-butyltin

The mildness of the lithium perchlorate–diethyl ether medium may be illustrated by the outcome of the reaction of aldehyde (7) in 5.0 M LPDE with 2.3 equivalents of allyltrimethyltin at ambient temperature (Scheme 6.3.4). After 4 h, a 77% yield of the differentially protected triol (8) was obtained as a single diastereomer. The use of conventional Lewis acids (e.g. TiCl4 , SnCl4 ) in methylene chloride to bring about the above transformation leads to an extensive decomposition even at –78 ◦ C. OH CHO

TMAS (2.3 equiv) 5.0 M LPDE

H

H OMOM

TBDMSO

OMOM

TBDMSO

(8)

(7) Scheme 6.3.4 The allylation of acetaldehyde with allyltrimethyltin

methoxymethoxy-[2-(tert-butyldimethylsiloxy)-cyclopent-3-enyl]-

Optically active α,β-epoxyaldehydes are readily available via Sharpless epoxidation of allyl alcohols followed by oxidation. Studies by Heydari12 on the addition of trans-substituted α,β-epoxyaldehydes to tri-n-butylallylstannane provide a general method for the synthesis of the corresponding syn-alcohols (9) with high selectivity (Scheme 6.3.5). R1

O

O + H

SnBu3

LPDE

R1

O

R1

O

+

rt, 1h

OH

OH

(9)

(94:6) R1=TBDMSO Scheme 6.3.5

The allylation of 3-(tert-butyldimethylsiloxy)oxirane-2-carbaldehyde with allyltri-n-butyltin

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Table 6.3.5 Metal perchlorate-promoted reaction of allyltri-n-butyltin towards benzaldehyde Entry

Metal perchlorate (equiv.)

1 2 3 4 5

None LiClO4 (20) Mg(ClO4 )2 (2) Ca(ClO4 )2 (2) Ba(ClO4 )2 (2)

a

Yield (%) a 0 55 80 82 Trace

Isolated yield of 1-phenylbut-3-en-1-ol

Nishigaichi has described how allylic tin compounds, in the presence of various metal perchlorates, undergo efficient regio- and stereochemical isomerization. In this connection, the reaction of allyltri-nbutyltin with benzaldehyde in ether in the presence of LiClO14 4 was examined (Table 6.3.5). With a high concentration of the salt (4 mol dm−3 ; ca. 20 mol equivalent of LiClO4 to the aldehyde), allylstannation proceeded rather well (Table 6.3.5, entry 2). Interestingly, Mg(ClO4 )2 , which is scarcely soluble in ether, was found to be effective (Table 6.3.5, entry 3); in this case, ca. 2 equivalents were enough to increase the yield of the reaction.

RCHO

H+

+ SnBu3

γ

α crotyltin Scheme 6.3.6

R

+ OH

γ-adduct (syn and anti)

R

OH

α-adduct (α)

The allylation of aldehydes with but-2-enyltri-n-butyltin

Nishigaichi14 also studied the reaction of crotyltri-n-butyltin (but-2-enyltri-n-butyltin) with different aldehydes RCHO (R = p-MeC6 H4 , C6 H5 , p-O2 NC6 H4 , C7 H15 (see Scheme 6.3.6 and Table 6.3.6).14 Thus, while (E)-but-2-enyltri-n-butyltin preferentially afforded the corresponding anti-homallyl alcohol (10b) in LiClO4 -promoted reactions with aromatic aldehydes (Table 6.3.6, entries 1–3), the Z -compound afforded the syn-product (10a) with moderate selectivity (Table 6.3.6, entry 4). Similar yields and stereospecificity by (E)- and (Z )-crotyltins were obtained in the presence of 1.5 equivalents of Mg(ClO4 )2 (Table 6.3.6, entries 6–11); Ca(ClO4 )2 also showed a similar result (Table 6.3.6, entry 12). This antiselectivity of (E)-but-2-enyltri-n-butyltin was the reverse of that shown by a typical Lewis acid (BF3 ); syn-selectivity was observed even in diethyl ether (Table 6.3.6, entry 13). With aliphatic aldehydes, a much lower syn-selectivity was observed for both LiClO4 - and Mg(ClO4 )2 -promoted reactions (Table 6.3.6, entries 5 and 11) compared with that for BF3 -promoted reactions. From a competitive experiment, the reactivity toward benzaldehyde of (2-t-butylallyl)tri-n-butyltin was found to be nearly equal to that of allyltri-n-butyltin in the typically Lewis acid-promoted reaction. In contrast, in the metal perchlorate-promoted reactions, the former was found to be less than one tenth (LiClO4 ) and ca. one quarter [Mg(ClO4 )2 ] of the latter; it seems that in a congested cyclic transition state, the steric

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Table 6.3.6

Entry

July 7, 2008

Metal perchlorate-promoted reaction of but-2-enyltri-n-butytina toward aldehydes (RCHO) Metal perchlorate (equiv.) LiClO4 (20)

Mg(ClO4 )2 (1.5)

Ca(ClO4 )2 (2) BF3 ·OEt2 (2)

Product distribution R

10a

10b

11 (α b )

Total yield %

p-MeC6 H4 C6 H5 p-O2 NC6 H4 p-O2 NC6 H4 C7 H15 p-MeC6 H4 C6 H5 C6 H5 p-MeO2 CC6 H4 p-O2 NC6 H4 C7 H15 C6 H5 C6 H5 C7 H15

13 14 34 56 26 26 24 50 22 29 47 22 49 79

57 64 58 39 23 59 68 41 75 71 37 57 18 16

30 22 8 5 51 15 8 9 3 Trace 16 21 33 5

19 15 17 18 22 20 16 13 12 9 18 16 95 75

a

(E)–But-2-enyltri-n-butyltin (E /Z>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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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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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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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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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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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

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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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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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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

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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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

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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

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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

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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

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Index

M¨ossbauer 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 <+2 35 computational studies 277–278 and coordination number 23–27 cyclic structures 27–35 indirect nuclear spin-spin coupling constants 39–45 organotin oxo-clusters 74–76 overview 17–18 polymers 385–386 resonance measurement 18–20 stannylium ions 156–157 tetraorganotin compounds 29–31 tin hydrides 28 tin-chalcogen bond compounds 32–34 tin-silicon, germanium, tin, lead, boron, lithium and potassium bonds 35–36 tin(II) derivatives 256–257 transition metal-tin compounds 37–38 Zintl ions, solution studies 148–150 non-linear optical see NLO norbornene, arylstannylation 649 nuclear magnetic resonance spectroscopy see NMR nuclear spin relaxation 20–21 nuclei quadrupolar 19 spin-1/2 18 oligostannanes 6, 7 linear 376–377 perethylated 383 spectral data 383 tin(II) derivatives 251 ONIOM 271–272 organostannoxanes, with high tin nuclearity 209 organotin carboxylates 93–105

diorganotin 94–103 monorganotin 103–105, 106–107 triorganotin, formation mechanisms 93–94 organotin compounds 7 cardiovascular activity 414–421 with electronegative ligands 9 history 2 low-valence 13 medical applications 413–414 organotin(IV), preparation 10–12, 11 overview 5–8 oxide surfaces, functionalization using alkylorganotins 297–301 oxo-clusters mono-organotin 69–70 formation mechanisms 82–89 molecular structures 70–78 synthesis 78–82, 79, 80 oxocarbenium cations 527–530 oxycarbonyl chlorides 623 p-block elements 239–241 parametized model no.3 271 perflouoroalkanecarboxylates 458 Periodic Table 697 Perna viridis 421 phenyl isocyanate, reaction rates with methanol by catalyst 688 phenyltins 469, 475–476, 487 phosphaalkene 177 phosphorus, clusters with tin 65–66 physiological effects, organotins 416 π-bonds, interactions with metals 392 Plictran 415 plumbylenes, N-heterocyclic 167 polycarbostannanes 387 polycephalin C 585 polyesters 389 polyferrocenylenestannanes 386–387 polymer backbones, ion selective electrodes (ISE) 334 polymers as catalysts 695 di- and triorganotin sulfonate 111 diorganotin carboxylate 104 flame-retardant halogen-free 342–343, 342 mechanism of action 347 with halogens 341–342, 343, 345 mechanism of action 346–347

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Index linear oligostannanes 376–377 macroporous 609–613 physical properties 381–385 polystannasilanes and -germanes 381 as PVC stabilizers 389 as reagent matrix 609–613 with tin in backbone 386–388 with tin as pendant group 388–389 polystannasiloxanes 387–388 polyurethanes 692–693 synthesis 696 poly(vinyl chloride) manufacture 312 stability evaluation 320–322 porphyrin derivatives 461 potassium 234 propylargic stannanes 550–551 proteins anti-organotin toxicity 483 expression effect of n-butyltin chlorides 471–472 effect of di-n-butyltin 473–474 psycholeine 592 PTAD 701–704 quardigemine C 592 radicals 5–6, 7, 8, 660 rats, hematological parameters 417 reaction injection moulding 693–694 reactions allenylstannanes 543–547 allylic stannanes c-(alkoxy)allylstannanes 526–527 chiral 541–542 dialkylstannylene acetals 504–505 distannenes 181 isocyanates 687 mechanisms, computational studies 274–276 stannylene acetals without additional hydroxyl groups 505–511 tributylstannyl ethers 511–512 reactivity, stannenes 186 reagents fluorous 658–659 modification for removal 654–664 solid-phase 659–661 red blood cells 486 reduction, alkyl halides 614–615 relativistic effects 269–270

727

relay switches, in cyclization processes 634–637 resins, gel-type 612–613 ring-opening polymerization (ROP), catalysis using grafted catalysts 677 sarcoplasmic reticulum (SR) 417–418 self-assembly 117–118, 301–306 building blocks 118–119 sensor compounds 324–337 silastannene 193–194 silene 177 silica, functionalization 298–301 silicon, double-bonded with tin 193 silver 236 silyation, stannylene acetals 508–509 silylium ions 152 sodium 233 solid-supported catalysts 672–678 solvates, supramolecular structures 393–397 solvents binary systems 671 for fluorous distannoxanes 669–671 stabilizers for PVC 312–322 mechanism of stabilization 315–317 operational considerations 317–320 types 313–315 mercaptides 315 mechanism of stabilization 315–317 stannacycles 35 stannane radicals 7 stannaromatics 188–193 stannates, of zinc, as fire retardant 340–343 stannenes 183–193 with cumulative double bonds 187–193 physical parameters 184 stable 184 stannin 483 stannocene 7, 8, 160–161 NMR properties 25–26 stannyl radicals 660 stannylenes 7, 8 acetals, silyation 508–509 adducts with Lewis acids 168 benzannulated heterocyclic overview 160 stable 160–166 stannylium ions 26 overview 152–153 cationic 7

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Index

stannylium ions (Contd. ) tricoordinated 156–158 tetracoordinated 155–156 pentacoordinated 154–155 stannynes 195–198 steroids 455 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 and solid-phase reagents 659–660 sp3 -sp2 coupling 594–599 stoichiometry, tin(II) derivatives 258–267 structure carbohydrates 499–503 computational determination 272–274 stannenes 183–186 tributylstannyl ethers 503 triorganotin carboxylates 95 structure-activity relationship 435–439, 491–492 anti-cancer drugs 455–465 fungicides 445–446 insecticides (QSAR) 435–439 substitution reactions electrophilic, organostannanes, overview 515–516 tetraorganodistannoxanes 204–207, 221–229 sulfides, chemical vapor deposition (CVD) 290–291 sulfonates di- and triorganotin 109–113, 110–111 monoorganotin 112–113 organotin 105–113 supermolecules see supramolecular structures supported reagents containing reactive tin-carbon bond 616–617 containing tin-heteroatom bond 615–616 supramolecular structures 392 ionic complexes 397–398 mononuclear tin species 399–406 polynuclear tin species 406–409 solvates 393–397 synthesis carbohydrates, overview 497–498 dialkylstannylene acetals 498–500 diaminostannylenes 161 distannenes 178–181 fluorous distannoxanes 668

functional trialkynylorganotins 361–368 grafted catalysts 672–678 imidotin cubane clusters 53–56 ketones 634 natural products 579–601 oligostannanes, linear 376–377 organotin oxo-clusters 78–82, 79, 80 polycarbostannanes 387 polymers 696–697 polystannasiloxanes 387 polyurethanes 696 propylargic stannanes 550–551 stannylium ions 155 tin(II) derivatives 254–256 tributylstannyl ethers 500 triorganotin carboxylates 93–94 TBT (tri-n-butyltin bromide) 418 tetraorganodistannoxanes 201–229 overview 201–202 spacer-bridged 211–220, 213–214 unsymmetrically substituted 202–207 thermal properties, polystannanes 381–382 thermochemistry, computational studies 276 thiohydrazones, complexed with organotins 449–450 tin (element) 3–5 allotropes 3 history and production 1–2 isotopes 17 physical properties 3 tin(II) dimethoxide 265 tin(II) selenide, chemical vapor deposition (CVD) 291–292 tin(II) telluride, chemical vapor deposition (CVD) 291–292 tin(IV) oxide chemical vapor deposition (CVD) 287–290 as fire retardant 339–340 tin(IV) phosphide 292–293 total surface area (TSA) 437 toxicity 607 anti-cancer compounds 454 fungicidal 445 hepatotoxicity 490–491 organostannanes 654 triorganotins 415, 430 transesterification 670 tin leaching 676 transition metals

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Index complexed with germylenes and stannylenes 166 compounds with tin bonds 237 transmetalation allenylstannanes 547–549, 550 allylic stannanes 535–540 tri-n-butyltin bromide (TBT) 418 tri-n-butyltin fluoride (TBTF) 433 tri-n-butyltin hydride, and green chemistry 653 trialkynylorganotins 361–368 tributylstannyl ethers 503 tributyltin oxide (TBTO) 415–417 tricyclohexyltin trioxide 415 triethyltin bromide (TET) 418 triethyltin lupinylsufide hydrochloride 458 triglycine 424–425, 424 trimethyltin chloride (TMT) 418–419 effects on NK cell function 474–475 trimethyltins environmental impact 469 insecticidal properties 431–432 triorganotin carboxylates 93–94 triorganotins as building block for self-assembly 119 hemolytic activity 487–488 toxicity 430, 654 triphenylarsine 567 triphenyltins, environmental impact 469 tris(trimethylsilyl)methyltin 71–72

729

umbelliferone derivatives, cardiovascular activity 425, 426 urethane, catalysis 681 vanadium, in organotin compounds 73 vicinal dithiols 482–484 Waste Electrical and Electronic Equipment (WEEE) regulations 343 white tin 3 Wurtz coupling, polystannane synthesis 378 X-ray crystallography 256 xerogels 301–306, 303 yellowness index, PVC 320 ytterbium 234–235 zinc hydroxystannate (ZHS) 340–341, 343 zinc stannate 340–341 Zintl, Edward 138–139 Zintl ions 4, 35–39 with caesium 234 capping 146–148, 148 deltahedral 138–150 geometry 140–143 five-atom clusters 141 nine-atom clusters 141–143 history 138–140 NMR spectroscopy studies 148–150 reactions 143–148