Strained Hydrocarbons: Beyond the van't Hoff and Le Bel Hypothesis

Strained Hydrocarbons Edited by Helena Dodziuk 1410vch00.indd I 16.02.2009 13:05:07 Further Reading A. V. Demchenk...

Author: Helena Dodziuk | Roald Hoffmann (Editors)

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Strained Hydrocarbons Edited by Helena Dodziuk

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Further Reading

A. V. Demchenko (Ed.)

Handbook of Chemical Glycosylation 2008 ISBN: 978-3-527-31780-6

P. G. Andersson, I. J. Munslow (Eds.)

Modern Reduction Methods 2008 ISBN: 978-3-527-31862-9

L. Kollár (Ed.)

Modern Carbonylation Methods 2008 ISBN: 978-3-527-31896-4

E. M. Carreira, L. Kvaerno (Eds.)

Classics in Stereoselective Synthesis 2008 ISBN: 978-3-527-29966-9

M. M. Haley, R. R. Tykwinski (Eds.)

Carbon-Rich Compounds From Molecules to Materials 2006 ISBN: 978-3-527-31224-5

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Strained Hydrocarbons Beyond the van’t Hoff and Le Bel Hypothesis

Edited by Helena Dodziuk

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The Editor Prof. Dr. Helena Dodziuk Institute of Physical Chemistry Polish Academy of Sciences Kasprzaka 44 01-224 Warsaw Poland

All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate. Library of Congress Card No.: applied for

Cover illustration Cover picture kindly provided by Dr. K. S. Nowinski

British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. Bibliographic information published by the Deutsche Nationalbibliothek Die Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available in the Internet at http://dnb.d-nb.de. ¤ 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Printed in the Federal Republic of Germany Printed on acid-free paper Typesetting Manuela Treindl, Laaber Printing betz-druck GmbH Darmstadt Bookbinding Litges & Dopf GmbH, Heppenheim ISBN: 978-3-527-31767-7

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V

Foreword Chemistry is the truly anthropic science. The molecules we make can heal us, and they can hurt us, because they are on the scale of the molecules that make up our bodies. And our synthetic creations interact, even react, with the molecules that nature – our enzymes, the environment – put into us. Molecular science is also anthropic (male and female, of course) because it presents a challenge to human intelligence that is just right, commensurate with our intellect. The exciting story this book develops bears testimony on every page to that anthropic cognitive nature of organic chemistry. Let me explain: our remarkable neural system is steered by a complex brain. That brain has prejudices for sure; it tends to simplify things, falling at every proffered opportunity for beautiful equations, simple mechanisms, Platonic solids and the honeyed simplicities of politicians. But when challenged, we can deal with substantial complexity. Indeed, the brain relishes being stretched: by rich sensual inputs, by patterns, by puzzles. Along comes a science, our chemistry. It offers in its molecular structures, a game that is at first sight deceptively simple. Take hydrocarbons (most of the molecules in this book are in this category) – what could be simpler? Two elements, C and H, that by a transparent rule of intercombination form four bonds, and one bond, respectively. You are well aware of the manifestation of these rules and combinatorics – a chemical universe of incredible diversity. These molecules can not only be thought up, they can also be synthesized in a human span – roughly the time it takes for a graduate student to get a Ph.D. We are not making a ladybug, nor a spiral galaxy; we are making a paracyclophane. The complexity of the challenge is on the human scale. And so are the possibilities: What can I do to string eight carbons across the para positions of a benzene? Can I reduce the bridging carbons to seven? Will I make it easier if the eight carbons are partially in a benzene ring themselves? The questions just flow one after the other; it takes no talent to ask them, just a normal curious human being, privy to the structural codes of chemistry. So the game itself, the game of chemical structure, is exciting. Chess pales by comparison. Add to that ludic challenge potential utility, and also the natural human desire to probe limits (just how far can I distort that double bond out of its planar normalcy?), and you have all the makings of intense interaction, part Strained Hydrocarbons: Beyond the van’t Hoff and Le Bel Hypothesis. Edited by Helena Dodziuk Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31767-7

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VI

Foreword

intellectual, part emotional, between a human being and an object of his or her creation. The object of our intense contemplation – a compound macroscopically, a molecule microscopically – is complex enough not to be boring, yet not unpredictably chaotic. The strained molecule is just right for some of us to exercise our creativity in thinking up these strange beasts, others in coming up with ingenious ways of making them (for molecules are real!), all of us admiring the complexity, simplicity and function all rolled into one. Enjoy reading this book!

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Roald Hoffmann

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VII

Preface Strained hydrocarbons represent an amazing domain. About 80 years after the formulation of van’t Hoff and Le Bel’s hypothesis new, exciting molecules representing in Hoffmann and Hopf formulation (R. Hoffmann, H. Hopf, Angewandte Chemie, submitted), what is probably too much of anthropomorphization, ‘molecular sadism’ were synthesized. Paraphrasing D. J. Cram, one could say that such molecules elicit wonder, stimulate the imagination and challenge both synthetic talents and interpretive instincts. Up to the early 1990s the field of strained hydrocarbons was a kind of elitist area in which only the best synthetic and theoretical chemists were active. It was a playground of few, characterized by vivid interactions between synthetic and theoretical chemistry allowing one to propose plausible synthetic targets on the basis of model calculations. On the other hand, it allowed Bader, Wiberg and their followers to refine the definition of the chemical bond. The situation in the domain of distorted molecules changed after the discovery of fullerenes and nanotubes which attracted numerous researchers. These molecules, having nonplanar systems of conjugated bonds, are not hydrocarbons but their derivatives are numerous. Thus, they have been included into this volume in view of the rapid development of these areas and, still largely unfulfilled, prospects of their applications. Several researchers helped me in this project. First of all, I would like to thank all contributors to this volume. I would like also to acknowledge the support I have obtained from Professors T. Marek Krygowski, Jay S. Siegel and Henning Hopf in the initial stage. Finding contributors was sometimes a difficult task. The help of Professors A. de Meijere, E. Osawa, F. Diederich, C. Thilgen, W. T. Borden, J. Cioslowski and H. Kuzmany in the search for coauthors is gratefully acknowledged. I am deeply obliged to my colleague, Dr. K. S. Nowinski, for designing the cover picture. On the other hand, I owe a deep apology to the authors of many interesting papers on strained hydrocarbons which could not be presented in this book or were insufficiently covered due to space limitations. The question: ‘To what extent can a bond be distorted without being broken?’ is fascinating. This book is devoted to the presentation of distorted hydrocarbons. It is an effort to counteract, in this limited volume, overspecialization by showing not only syntheses, physicochemical studies and theoretical calculations of these molecules, but also the prospects of their applications. Strained molecules are Strained Hydrocarbons: Beyond the van’t Hoff and Le Bel Hypothesis. Edited by Helena Dodziuk Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31767-7

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Preface

exciting objects for studies per se. With several novel hypothetical molecules waiting to be synthesized on the one hand, and with the possibility of obtaining fascinating supramolecular complexes with distorted hydrocarbons as building blocks on the other, this domain will remain enthralling. Warsaw, January 2009

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Helena Dodziuk

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IX

Contents Foreword Preface

V VII

List of Contributors XVII 1

Introduction 1

1.1

Initial Remarks 1 Helena Dodziuk Hydrocarbons with Unusual Spatial Structure: the Need to Finance Basic Research 5 Helena Dodziuk Computations on Strained Hydrocarbons 12 Andrey A. Fokin and Peter R. Schreiner Gallery of Molecules That Could Have Been Included in This Book 18 Helena Dodziuk Introductory Remarks 18 Saturated Hydrocarbons 18 Distorted Double Bonds 21 Benzene Rings with Nontypical Spatial Structures 22 Cumulenes 25 Acetylenes 26 References 27

1.2

1.3 1.4

1.4.1 1.4.2 1.4.3 1.4.4 1.4.5 1.4.6

2

Distorted Saturated Hydrocarbons 33

2.1

Molecules with Inverted Carbon Atoms 33 Kata Mlinariü-Majerski 2.1.1 Introduction 33 2.1.2 Small-ring Propellanes: Computational and Physicochemical Studies 35 2.1.3 Small-ring Propellanes: Experimental Results 38 2.1.3.1 Preparation and Reactivity of [1.1.1]Propellane 38 Strained Hydrocarbons: Beyond the van’t Hoff and Le Bel Hypothesis. Edited by Helena Dodziuk Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31767-7

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2.1.3.2 Preparation and Reactivity of [2.1.1]Propellane and [2.2.1]Propellane 41 2.1.3.3 [1.1.1]Propellane as the Precursor for the Synthesis of Other Unusual Molecules 42 2.1.4 New Hypothetical Molecules with Inverted Carbon Atoms 43 2.2 Molecules with Planar and Pyramidal Carbon Atoms 44 Helena Dodziuk 2.3 A Theoretical Approach to the Study and Design of Prismane Systems 49 Tatyana N. Gribanova, Vladimir I. Minkin and Ruslan M. Minyaev 2.3.1 Introduction 49 2.3.2 Prismanes 49 2.3.3 Expanded Prismanes 52 2.3.3.1 Asteranes 52 2.3.3.2 Ethynyl-expanded Prismanes 54 2.3.4 Dehydroprismanes 55 2.3.5 Polyprismanes 56 2.3.5.1 Cubane Oligomers 56 2.3.5.2 Fused Prismanes 57 2.3.6 Conclusions 58 2.4 (CH)2n Cage Structures, ‘in’-‘out’ Isomerism in Perhydrogenated Fullerenes and Planar Cyclohexane Rings 59 Helena Dodziuk 2.4.1 (CH)2n Cage Structures 59 2.4.1.1 Tetrahedrane 61 2.4.1.2 Triprismane 62 2.4.1.3 Cubane 61, Cuneane 100 and Octabisvalene 101 C8H8 62 2.4.1.4 C10H10 Saturated Cages 63 2.4.1.5 C12H12 Saturated Cages 63 2.4.1.6 Higher [n]Prismanes, Dodecahedrane 64 2.4.1.7 ‘In’-‘out’ Isomerism in Perhydrogenated Fullerenes C60H60 64 2.4.1.8 Summary 67 2.4.2 Planar Cyclohexane Rings 67 2.5 Ultralong C–C Bonds 70 Takanori Suzuki, Takashi Takeda, Hidetoshi Kawai and Kenshu Fujiwara 2.5.1 Introduction 70 2.5.2 Ultralong C–C Bonds Conﬁned in a Stiff Molecular Frame 72 2.5.3 Tetraphenylnaphthocyclobutene as a Scaffold to Produce Ultralong C–C Bonds 73 2.5.4 ‘Clumped’ Hexaphenylethane Derivatives with Elongated and Ultralong C–C Bonds 74 2.5.5 HPE Derivatives with a Super-ultralong C–C Bond 78 2.5.6 ‘Expandability’ of the Ultralong C–C Bond: Conformational Isomorphs with Different Bond Lengths 79 2.5.7 Future Outlook 82

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Contents

2.6 2.6.1 2.6.2 2.6.3 2.6.4 2.6.5

Ultrashort C–C Bonds 82 Vladimir Y. Lee and Akira Sekiguchi Introduction 82 Tricyclo[2.1.0.02,5]pentanes: Ultrashort Endocyclic Bridging C–C Bonds 83 Coupled Cage Compounds: Ultrashort Exocyclic Intercage C–C Bonds 86 Sterically Congested in-Methylcyclophanes: Ultrashort C–C(Me) Bonds 91 Conclusions 91 References 92

3

Distorted Alkenes 103

3.1

Nonplanar Alkenes 103 Dieter Lenoir, Paul J. Smith and Joel F. Liebman Introduction and Context 103 Bridgehead Alkenes 103 t-Butyl-substituted Ethylenes 104 Investigations of t-Butylated Ethylenes and Other Acyclic Alkenes 106 Cyclo and Bicycloalkenes … and on to Polycyclic Analogs 107 Adamantylideneadamantane and its Derivatives 108 t-Butyl-substituted and Cyclic Stilbenes 108 Multiply Unsaturated Bicycloalkenes, Homoaromaticity and Cyclophanes 109 The Most Distorted Ethylenes and Seemingly Simple Analogs 111 Small Ring and Cage Structures Involving Nonplanar C=C Bonds 112 Athanassios Nicolaides Pyramidalized Alkenes 112 Tricyclo[3.3.11.03,7]undec-3(7)-ene 38 115 Tricyclo[3.3.10.03,7]dec-3(7)-ene 39 and tricyclo[3.3.9.03,7]non-3(7)-ene 40 117 Tricyclo[3.3.0.03,7]oct-1(5)-ene 41 119 (Ph3P)2Pt Complexes 119 Conclusions 121 Strained Cyclic Allenes and Cumulenes 122 Richard P. Johnson and Kaleen M. Konrad Introduction 122 Allene S Bond Deformations and Strain Estimates 123 Four- and Five-membered Ring Allenes 124 1,2-Cyclohexadienes 125 1,2,4-Cyclohexatrienes 127 6-Methylene-1,2,4-Cyclohexatrienes and Related Structures 131 Seven-membered Ring Allenes 131

3.1.1 3.1.2 3.1.2.1 3.1.2.2 3.1.2.3 3.1.2.4 3.1.2.5 3.1.3 3.1.3.1 3.2

3.2.1 3.2.1.1 3.2.1.2 3.2.1.3 3.2.1.4 3.2.2 3.3 3.3.1 3.3.2 3.3.3 3.3.4 3.3.5 3.3.5.1 3.3.6

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Contents

3.3.6.1 3.3.7 3.3.8 3.3.9 3.3.10 3.3.10.1 3.3.10.2 3.3.11

Cycloheptatetraenes 132 Eight-membered Ring Allenes 134 Polycyclic Allenes 135 Cyclic Bisallenes 136 Cyclic Butatrienes 136 Butatriene S Bond Deformations and Strain Estimates 137 Five- to Nine-membered Ring Cyclic Butatrienes 137 Conclusions 139 References 140

Strained Aromatic Molecules 147 Nonstandard Benzenes 147 Paul J. Smith and Joel F. Liebman 4.1.1 Introduction and Context 147 4.1.2 Alkylated Aromatics 148 4.1.3 Helicenes 148 4.1.4 [n]Circulenes 149 4.1.5 Cyclophanes 150 4.2 Distorted Cyclophanes 153 Henning Hopf 4.2.1 Introduction 153 4.2.2 The [n]Cyclophanes 154 4.2.2.1 [n]Paracyclophanes 154 4.2.2.2 [n]Metacyclophanes 160 4.2.3 The [m.n]Paracyclophanes 161 4.2.4 Distorted Aromatic Rings and ‘Aromatic Character’ 164 4.2.5 NMR Characteristics of Cyclophanes 165 4.3 Helicenes 166 Ivo Starý and Irena G. Stará 4.3.1 Introduction 166 4.3.2 Synthesis of Helicenes 166 4.3.3 Nonracemic Helicenes 171 4.3.4 Intriguing Helicene Structures 172 4.3.5 Physicochemical Properties and Applications 173 4.3.6 Theoretical Studies 175 4.3.7 Outlook 176 4.4 Cycloproparenes 176 Brian Halton 4.4.1 Introduction 176 4.4.2 Synthetic Considerations 177 4.4.3 Chemical Considerations 183 4.4.4 Heteroatom Derivatives 187 4.4.5 Physicochemical and Theoretical Considerations 188 References 193 4

4.1

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Contents

5

5.1 5.2

5.2.1 5.2.2 5.2.3 5.2.3.1 5.2.3.2 5.2.3.3 5.2.4 5.3 5.3.1

5.3.1.1 5.3.1.2 5.3.1.3 5.3.1.4 5.3.1.5 5.3.1.6 5.3.1.7 5.3.2 5.3.2.1 5.3.2.2 5.3.2.3 5.3.2.4 5.3.3 5.3.3.1 5.3.3.2 5.3.3.3 5.3.4 5.3.4.1 5.3.4.2 5.3.4.3 5.3.4.4 5.4

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205 Introduction 205 Helena Dodziuk Chemistry Inﬂuenced by the Nontypical Structure: Modiﬁcation of [60]Fullerene 208 Takuma Hara, Takashi Konno, Yosuke Nakamura and Jun Nishimura Introduction 208 General Overview 209 Modiﬁcation Reactions 215 Reduction and Oxidation 215 Alkylation 217 Cycloadditions 218 Conclusions 224 Physicochemical Properties and the Unusual Structure of Fullerenes 225 Single-crystal X-ray Structures of Fullerenes and Their Derivatives 225 Olga V. Boltalina, Alexey A. Popov and Steven H. Strauss Introduction 225 Disorder 226 Nonplanar Steric Strain 226 Nonplanar Steric Strain Parameters 229 Are Non-IPR Fullerenes Sterically Unstable? 232 Long and Short C(sp2)–C(sp2) Bonds in Fullerene Cages 232 Steric Strain in C60(X)n Isomers 236 Vibrational and Electronic Spectra 238 Alexey A. Popov Introduction 238 Vibrational Spectra of Fullerenes 239 The Orbital Picture of Fullerenes: High-energy Electronic Spectra 243 Electronic Excitations. UPS, UV/Vis/NIR Absorption and Fluorescence Spectroscopy 246 Nuclear Magnetic Resonance 250 Toni Shiroka Introduction 250 NMR of Fullerenes 251 Concluding Remarks 259 Electrochemistry 259 Renata Bilewicz and Kazimierz Chmurski Electronic Properties of Fullerenes 259 Electrochemical Properties of Soluble Fullerene Derivatives 263 Electrocatalytic Activity of Fullerenes 270 Conclusions and Outlook 272 Fullerene Aggregates 273 Fullerenes

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Contents

5.4.1 5.4.2 5.4.3 5.5 5.5.1 5.5.2 5.5.2.1 5.5.2.2 5.5.2.3 5.5.3 5.5.3.1 5.5.3.2 5.5.4 5.5.5 5.6 5.6.1 5.6.2 5.6.3 5.6.4 5.7 5.7.1 5.7.2 5.7.2.1 5.7.2.2 5.7.3 5.7.4

6

Carbon Nanotubes 335

6.1

The Structure and Properties of Carbon Nanotubes 335 Anke Krueger Introduction 335 The Structure of Single-walled Carbon Nanotubes 335 The Structure of Multi-walled Carbon Nanotubes 342 The Aromaticity of Carbon Nanotubes 345 Conclusions 347 The Functionalization of Carbon Nanotubes 347 Anke Krueger Introduction 347 Functionalization of the Nanotube Tips 348

6.1.1 6.1.2 6.1.3 6.1.4 6.1.5 6.2 6.2.1 6.2.2

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Tommi Vuorinen Film Preparation Methods 274 Fullerene Film Properties 277 Conclusions 282 Endohedral Fullerenes with Neutral Atoms and Molecules 282 Sho-ichi Iwamatsu Introduction 282 Preparation 282 Direct Approach Using an Existing Fullerene 282 Molecular Surgery Approach via an Open-cage Fullerene 284 Open-cage Fullerenes, Reversible Molecular Incorporations and Ejections 285 Properties 287 Host Fullerenes 287 Guest Substrates 288 Binding Energies, Theoretical Investigations 290 Summary 291 Hydrogenated Fullerenes 291 Mark S. Meier Synthesis and Structure 291 C70 Chemistry 295 Higher Fullerenes 297 Reactivity of Hydrogenated Fullerenes 297 Applications of Fullerenes 299 Rossimiriam Pereira de Freitas and Jean-François Nierengarten Introduction 299 Applications in Materials Science 299 C60 Derivatives for Optical Limiting Applications 299 C60 Derivatives for Photovoltaic Applications 304 Biological Applications 310 Conclusions 314 References 315

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Contents

6.2.3 6.2.4 6.2.5 6.2.6 6.3 6.3.1 6.3.2 6.3.2.1 6.3.2.2 6.3.2.3 6.3.3 6.3.3.1 6.3.3.2 6.3.3.3 6.3.4

7

7.1 7.2 7.2.1 7.2.2 7.3 7.3.1 7.3.2 7.3.3 7.4 7.4.1 7.4.2 7.5 7.6

8

8.1 8.2 8.2.1 8.3

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Non-covalent Functionalization of Carbon Nanotubes 349 Covalent Side-wall Functionalization of Carbon Nanotubes 352 Endohedral Functionalization of Carbon Nanotubes 355 Conclusions 356 Applications of Carbon Nanotubes 356 Marc Monthioux Introduction 356 Properties of CNTs 357 Which CNT for Which Application? 357 Why is ‘Nano’ Beautiful? 358 Potential Problems Related to the Use of CNTs 360 Applications of CNTs 362 Prospective Applications 362 Applications Under Development 364 Applications on the Market 366 Conclusions 367 References 368 Angle-strained Cycloalkynes 375

Henning Hopf and Jörg Grunenberg Introduction 375 Cyclopropyne and Cyclobutyne: Speculations and Calculations on Non-isolable Cycloalkynes 376 Cyclopropyne and Related Systems 376 Cyclobutyne 378 Cyclopentyne, Cyclohexyne, Cycloheptyne: from Reactive Intermediates to Isolable Compounds 379 Cyclopentyne and its Derivatives 379 Cyclohexyne and its Derivatives 382 Cycloheptyne and its Derivatives 384 The Isolable Angle-strained Cycloalkynes: Cyclooctyne, Cyclononyne, and Beyond 385 Cyclooctyne and its Derivatives 385 Cyclononyne and Cyclodecyne 386 Cyclic Polyacetylenes 387 Spectroscopic Properties of Angle-strained Cycloalkynes 392 References 393 Molecules with Labile Bonds: Selected Annulenes and Bridged Homotropilidenes 399

Richard V. Williams Introduction 399 Annulenes 399 Cyclobutadiene 399 Cyclooctatetraene 403

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Contents

8.4 8.5 8.6 8.7 8.8 8.9 8.9

Bond Shifting, Ring Inversion and Antiaromaticity 405 Valence Isomerization 409 Ions Derived from COT 410 The Higher Annulenes 411 Bridged Homotropilidenes 413 Recent Developments 415 Conclusions 419 References 420

9

Molecules with Nonstandard Topological Properties: Centrohexaindane, Kuratowski’s Cyclophane and Other Graph-theoretically Nonplanar Molecules 425

9.1 9.1.1 9.2 9.2.1 9.2.2 9.3 9.3.1 9.3.2 9.3.3 9.4 9.4.1 9.5 9.5.1 9.5.2 9.6

10

Dietmar Kuck Introduction 425 Is All This Trivial? 425 Topologically Nonplanar Graphs and Molecular Motifs 427 The Centrohexaquinacene Core 427 The Nonplanar Graphs K5 and K3,3 and Some Molecular Representatives 428 Centrohexaindane 430 Centrohexaindane and Structural Regularities of the Centropolyindane Family 431 Syntheses of Centrohexaindane 433 Multiply-functionalized Centrohexaindanes 436 K5 versus K3,3 Molecules 438 Topologically Nonplanar Polyethers and Other K3,3 Compounds 438 Kuratowski’s Cyclophane 441 Synthesis of Kuratowski’s Cyclophane 441 The Structure of Kuratowski’s Cyclophane 443 Conclusions 444 References 445 Short-lived Species Stabilized in ‘Molecular’ or ‘Supramolecular Flasks’ 449

Helena Dodziuk References 456 11

Concluding Remarks 459

Helena Dodziuk 461

References Index

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463

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XVII

List of Contributors Renata Bilewicz University of Warsaw Department of Chemistry Pasteura 1 02093 Warsaw Poland Olga V. Boltalina Colorado State University Department of Chemistry Fort Collins, CO 80523 USA Kazimierz Chmurski University of Warsaw Department of Chemistry Pasteura 1 02-093 Warsaw Poland Helena Dodziuk Polish Academy of Sciences Institute of Physical Chemistry Kasprzaka 44/52 01-224 Warsaw Poland Andrey A. Fokin Kiev Polytechnic Institute Department of Organic Chemistry Spr. Pobedy 37 03056 Kiev Ukraine

Kenshu Fujiwara Hokkaido Univeresity Faculty of Science Department of Chemistry N10W8, North-ward Sapporo 060-0810 Japan Tatyana N. Gribanova Southern Federal University Institute of Physical and Organic Chemistry 194/2 Stachka Ave 344090 Rostov on Don Russia Jörg Grunenberg Carolo Wilhelmina Technical University of Braunschweig Institute of Organic Chemistry Hagenring 30 38106 Braunschweig Germany Brian Halton Victoria University of Wellington School of Chemical & Physical Sciences PO Box 600 Wellington 6140 New Zealand

Strained Hydrocarbons: Beyond the van’t Hoff and Le Bel Hypothesis. Edited by Helena Dodziuk Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31767-7

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List of Contributors

Takuma Hara Evonik Degussa Japan Co. Ltd. Business Line Catalysts Tsukuba Minami Daiichi Kogyo Danchi 21 Kasuminosato Ami-machi Inashiki-gun Ibaraki-ken 300-0315 Japan

Takashi Konno Gunma University Graduate School of Engineering Department of Chemistry and Chemical Biology Tenjincho 1-5-1 Kiryu, Gunma 376-8515 Japan

Henning Hopf Technical University of Braunschweig Carolo Wilhelmina Institute of Organic Chemistry Hagenring 30 38106 Braunschweig Germany

Kaleen M. Konrad Merck Research Laboratories 33 Avenue Louis Pasteur Boston, MA 02115 USA

Sho-ichi Iwamatsu Nagoya University Graduate School of Environmental Studies Chikusa Ku Nagoya, Aichi 464-8601 Japan Richard P. Johnson University of New Hampshire Department of Chemistry Durham, NH 03824 USA Hidetoshi Kawai Hokkaido University Faculty of Science Department of Chemistry N10W8, North-ward Sapporo 060-0810 Japan

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Anke Krüger University of Kiel Otto-Diehls-Institute of Organic Chemistry Otto-Hahn-Platz 3 24098 Kiel Germany Dietmar Kuck Universität Bielefeld Fakultät für Chemie Postfach 100131 33501 Bielefeld Germany Vladimir Y. Lee University of Tsukuba Graduate School of Pure and Applied Science Department of Chemistry Tsukuba, Ibaraki 305-8571 Japan Dieter Lenoir GSF-Research Center Neuherberg Institute of Ecological Chemistry Postfach 1129 85758 Neuherberg Germany

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List of Contributors

Joel F. Liebman University of Maryland, Baltimore County Department of Chemistry & Biochemistry 1000 Hilltop Circle Baltimore, MD 21250 USA Mark S. Meier University of Kentucky Department of Chemistry Lexington, KY 40506 USA Vladimir I. Minkin Southern Federal University Institute of Physical and Organic Chemistry 194/2 Stachka Ave 344090 Rostov on Don Russia Ruslan M. Minyaev Southern Federal University Institute of Physical and Organic Chemistry 194/2 Stachka Ave 344090 Rostov on Don Russia Kata Mlinaric´-Majerski Rudjer Boškoviü Institute Department of Organic Chemistry and Biochemistry Bijeniþka 54, P.O. Box 180 10002 Zagreb Croatia

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Marc Monthioux Université Toulouse III CEMES, UPR-8011 CNRS BP 94347 29 rue Jeanne Marvik 31055 Toulouse Cedex 4 France Yosuke Nakamura Gunma University Graduate School of Engineering Department of Chemistry and Chemical Biology Tenjincho 1-5-1 Kiryu, Gunma 376-8515 Japan Jean-François Nierengarten Université de Strasbourg Laboratoire de Chemie des Matériaux Moléculaires (UMR 7509) 25 rue Becquerel 67087 Strasbourg Cedex 2 France Athanassios Nikolaides University of Cyprus Department of Chemistry P.O. Box 20537 Nicosia 1678 Cyprus Jun Nishimura Gunma University Graduate School of Engineering Department of Nano-Material Systems Tenjincho 1-5-1 Kiryu, Gunma 376-8515 Japan

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List of Contributors

Rossimiriam Pereira de Freitas Universidade Federal de Minas Gerais Departamento de Química Av. Antônio Carlos 6627 Belo Horizonte 312-70901, MG Brazil

Paul J. Smith University of Maryland Baltimore County Department of Chemistry & Biochemistry 1000 Hilltop Circle Baltimore, MD 21250 USA

Alexey A. Popov Leibniz-Institute for Solid State and Materials Research IFW Group of Electrochemistry and Conducting Polymers Helmholtzstrasse 20 01069 Dresden Germany

Irena G. Stará Academy of Sciences of the Czech Republic Institute of Organic Chemistry & Biochemistry Center for Biomolecules & Complex Molecular Systems Flemigovo nám. 2 16610 Prague 6 Czech Republic

Peter R. Schreiner Justus-Liebig University Institut für Organische Chemie Heinrich-Buff-Ring 58 35392 Giessen Germany Akira Sekiguchi University of Tsukuba Department of Chemistry Graduate School of Pure and Applied Sciences Tsukuba, Ibaraki 305-8571 Japan Toni Shiroka Paul Scherrer Institut Laboratory for Muon-Spin Spectroscopy CH-5232 Villigen PSI Switzerland

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Ivo Starý Academy of Sciences of the Czech Republic Institute of Organic Chemistry & Biochemistry Center for Biomolecules & Complex Molecular Systems Flemigovo nám. 2 16610 Prague 6 Czech Republic Steven H. Strauss Colorado State University Department of Chemistry Fort Collins, CO 80523 USA Takanori Suzuki Hokkaido University Faculty of Science Department of Chemistry N10W8, North-ward Sapporo 060-0810 Japan

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List of Contributors

Takashi Takeda Hokkaido University Faculty of Science Departmeht of Chemistry N10W8, North-ward Sapporo 060-0810 Japan

XXI

Richard V. Williams University of Idaho Department of Chemistry P.O. Box 442343 Moscow, ID 83844-2343 USA

Tommi Vuorinen Tampere University of Technology Institute of Materials Chemistry P.O. Box 541 33101 Tampere Finland

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1 Introduction 1.1 Initial Remarks

Helena Dodziuk Let us start with a bit of history. Today, it is hard to imagine how difficult it was to develop basic concepts and ideas of chemistry in the second half of the 19th century. The story about Kekulé’s fight for his benzene structure shows that not all the arguments he used in its favor are valid today [1]. His idea could not be supported by the poor experimental instrumentation of that time. There was no X-ray analysis, no modern spectroscopic techniques and no calorimetry. The idea of the constitution of molecules, that is building them from a certain number of different types of atoms, was established, as well as several experimental findings which demanded rationalization. Among them were optical activity and the existence of a number of different molecules with the same constitution. Pasteur foresaw that the former phenomenon could be related to the positioning of atoms in space but only the van’t Hoff [2] and Le Bel [3] hypotheses on the tetrahedral arrangement of substituents on the tetravalent carbon atom explained most observations known at that time. Interestingly, the independently proposed models differed slightly: that of van’t Hoff was based on a regular tetrahedron, while in the second one used an irregular tetrahedron to represent the carbon atom. This difference was not significant but, remarkably, the more idealized van’t Hoff approach was generally accepted. An illustration from the 1908 German edition of van’t Hoff’s book, showing two stereoisomers of the tetrasubstituted ethane molecule CR1CR2rCR3CR4r shows the way in which molecules were depicted at that time (Figure 1.1). The van’t Hoff and Le Bel hypothesis was met with strong criticism, not always expressed in impartial scientific language. The renowned chemist and editor of the German Journal für praktische Chemie, Prof. Adolf Kolbe wrote: ‘A Dr. H. van ’t Hoff of the Veterinary School at Utrecht has no liking, apparently, for exact chemical investigation. He has considered it more comfortable to mount Pegasus (apparently borrowed from the Veterinary School) and to proclaim in his ‘La chimie dans l’éspace’ how the atoms appear to him to be arranged in space, when he is on the chemical Mt. Parnassus which he has reached by bold fly’. Strained Hydrocarbons: Beyond the van’t Hoff and Le Bel Hypothesis. Edited by Helena Dodziuk Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31767-7

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1 Introduction

Figure 1.1 The representation of two stereoisomers of a tetrasubstituted ethane molecule CR1CR2rCR3CR4r, as published 100 years ago in van’t Hoff ’s book [197].

In spite of such a strong attack and the difficulties associated with the lack of modern physicochemical methods, the idea of the tetrahedral arrangement of substituents around a tetravalent carbon atom was generally accepted and van’t Hoff became a first recipient of the Nobel prize for Chemistry in 1901, interestingly not for his stereochemical ideas but ‘in recognition of the extraordinary services he has rendered by the discovery of the laws of chemical dynamics and osmotic pressure in solutions’. Remarkably, an early idea of Sachse on the cyclohexane conformations (today known under the names chair and twist-boat) [4, 5] could not be proved at that time and was not accepted. Then, it took almost 80 years to understand all the consequences of the van’t Hoff and Le Bel concepts which not always were based on justified assumptions. For instance, the van’t Hoff understanding of the C–C bond implied free rotation around it. This assumption was only shown to be invalid by Pitzer’s work [6, 7] on the hindering of the rotation and preferred orientations of substituents on the C–C bond, started in 1936, which marked an important step in development of stereochemistry [8]. The combination of the ideas on tetrahedral orientation of substituents on a tetravalent carbon atom and of the hindered rotation around the C–C bond resulted in rationalization of the cyclohexane conformations and the number of isomers of its derivatives summarized in the Hassel [9, 10] and Barton [11] studies which were also honored by a Nobel Prize ‘for their contributions to the development of the concept of conformation and its application in chemistry’. Analogous studies of the spatial structure of alkenes, alkynes and aromatic compounds followed. With these achievements, the basis of the organic stereochemistry seemed to be laid, and models could be built, of spatial structures of molecules from welldefined rigid fragments. Eaton’s report on the synthesis of cubane 1 in 1964 [12] and especially the Wiberg synthesis of [1.1.1]propellane 2 [13] have shown that, in addition to the small-ring cycloalkanes, well-known since the second half of the 19th century, that exhibit Bayer strain [14], hydrocarbons having structures strongly departing from that suggested by van’t Hoff and Le Bel can exist. This book is devoted to such nonstandard structures. Let us first define what the standard hydrocarbons are: first, these are saturated hydrocarbons with the arrangement of substituents on the carbon atoms close to tetrahedral; then, double bonds and aromatic rings lying in a plane with its substituents and, last but not least, linear

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acetylenes. Also of interest are bond lengths that depart far from the standard value of 154 pm. Of course, fullerenes and carbon nanotubes in their idealized form are not hydrocarbons, but these conjugated aromatic systems are nonplanar and they are definitely the most widely studied distorted aromatic systems today. They also offer a unique possibility of investigating the effect of nonplanarity on structure, physicochemical properties and reactivity. Actually, this book should also be understood as part of my private campaign against too deep specialization, from which we all suffer. Thus, showing the influence exerted by molecular distortions on various physicochemical properties using fullerenes as an example seemed to be of importance. Highly distorted hydrocarbons are sometimes considered to be of no importance in view of their lack of practical applications. The significance of investigating such systems is discussed in Section 1.2. They are studied by both experimental and theoretical methods that, as discussed in Section 1.3, are of special significance in this domain. As shown by several examples (among other 1 [15] and heptacyclo[6.4.0.02,4.03,7.05,12.06,10.09,11]dodecane 3 [16, 17] which have first been studied theoretically then synthesized [13, 18]), to propose novel plausible synthetic targets on the basis of molecular modeling is a reliable aim of calculations.

We are experiencing such a rapid development of this science that it is not possible to discuss all the unusual hydrocarbons. Therefore, a selection, by no means considered to be exhaustive, of interesting molecules which did not find a place in other chapters is presented in Section 1.4. Simple strained saturated hydrocarbons are presented in Chapter 2. Both known, such as [1.1.1]propellane 2, and hypothetical molecules having inverted carbon atoms are discussed in Section 2.1. The importance of these molecules is emphasized by the discussion of the existence of the central bond in 2 which permitted precise definition of a bond in quantum chemistry [19]. The fascinating Hoffmann idea of planar carbon atom lying in a plane with its four substituents [20] was only realized in silico [21] and, as described in Section 2.2, still awaits realization. Prismanes (one of which is cubane 1) and asteranes are discussed mainly from the theoretical point of view in Section 2.3 but the influence of molecular distortions on the properties of the known systems is presented there, too. Saturated hydrocarbon cages and planar cyclohexanes (Section 2.4) as well as molecules with ultralong (Section 2.5) and ultrashort (Section 2.6) C–C bonds are also discussed in Chapter 2. In Chapter 3 which is devoted to alkenes, energetic aspects of distorted double bonds are presented in Section 3.1, small cage alkenes are discussed in Section 3.2 while Section 3.3 is devoted to strained cumulenes.

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1 Introduction

Analogously, in Chapter 4 energetical considerations concerning strained aromatic systems are discussed in Section 4.1, while bridged aromatic rings (i.e. cyclophanes), helicenes like 4, and cycloproparenes like 5 are presented in Sections 4.2 to 4.4, respectively.

As discussed earlier fullerenes, to which the longest Chapter 5 is devoted, are actually not hydrocarbons but their extended closed nonplanar conjugated aromatic systems deserve to be discussed in this book. After a short introduction in Section 5.1 their chemistry is presented in Section 5.2, and physicochemical properties reflecting their distorted structure (X-ray in Section 5.3.1, UV/Vis spectra in Section 5.3.2, NMR spectra in Section 5.3.3 and electrochemistry in Section 5.3.4) are shown. Next, fullerene films (Section 5.4), endohedral fullerene complexes (Section 5.5), an exciting application of NMR to study the structure of hydrogenated fullerenes (Section 5.6) and, mostly prospective, fullerene applications (Section 5.7). Unfortunately, I did not succeed in finding a specialist willing to present theoretical fullerene studies and their limitations due to the size of these huge cage molecules. This a serious drawback in spite of the inclusion of some fullerene calculations in other chapters. The inclusion of nanotubes into this monograph was based on similar arguments as those advocating the inclusion of fullerenes. In Chapter 6, structure, chemistry and also mostly prospective applications are shown (Sections 6.1–6.3, respectively). There has been a fascinating development in the financing of carbon nanotubes: people expected to get really big money from investing in CNTs about five years ago and then realized that the returns do not come immediately. Some of the applications introduced or expected to be introduced soon in large-scale manufacturing have not been successful. For instance, using polymer nanocomposites containing small amount of CNTs for electropainting of cars has been abandoned by General Motors, and Korean plans to build a factory for displays involving CNTs few years ago have not been fulfilled. Moreover, a recent observation on the carcenogenicity of multiwalled nanotubes may further slow down the development of CNT applications [22]. Even if we cannot see their rapid introduction, I am sure they will be important in the longer run. Cyclic alkynes with nonlinear triple bonds are discussed in Chapter 7, while molecules with labile bonds are presented in Chapter 8. They include rigid cyclobutadiene 6, highly mobile molecules like cyclooctatetraene 7 and shortlived species that could only be trapped at very low temperature in matrices. The fascinating discussion of interconversions between some of these species is

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presented in some detail from both theoretical and experimental (mainly NMR, IR) points of view. Hydrocarbons with nonplanar graphs, discussed in Chapter 9, conform to van’t Hoff and Le Bel stereochemistry but they are so unusual that they were considered to fit into this monograph. Such molecules having distinct topological properties represent an exciting border area between chemistry and mathematics. The molecules presented in this chapter have been obtained using elegant methods of traditional organic chemistry, while other systems exhibiting nontrivial topological properties (catenanes, rotaxanes, knots, etc.) could mostly be obtained by taking advantage of methods typical of supramolecular chemistry [23]. In the last Chapter 10, novel ways of obtaining and stabilizing unstable highly strained species in ‘molecular flasks’ are presented. The latter method makes use of supramolecular chemistry enabling, for instance, storage of the highly unstable cyclobutadiene 6 for a month at room temperature [24].

1.2 Hydrocarbons with Unusual Spatial Structure: the Need to Finance Basic Research

Helena Dodziuk By reporting the results on hypothetical hexahydrosuperphane 8 [25], a highly strained molecule with a planar cyclohexane ring, we have been confronted with the question as to whether this molecule had been chosen for study because of its future practical applications. The question was posed by a computer scientist who had no knowledge of stereochemistry, but it corresponds to a general attitude governed by grant funding for scientific research. Namely, any grant application has to show its immediate usefulness. However, in reality hardly any grants that use such justification will bring marketable results at all. For the applications of others we will probably wait 10 or more years and only a few of such grants will find their way to industry soon. Let’s inspect some examples in some detail. Liquid crystals today are commonly applied in displays but several other uses (as surface thermometers showing temperature distribution over a body, in optical

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1 Introduction

imaging, etc., http://plc.cwru.edu/tutorial/enhanced/files/textbook.htm) are foreseen for them. Discovered in 1888 by a botanist, Reinitzer [26], they were for almost 100 years considered to be the physicists’ toys. In 1966, an article entitled ‘Liquid crystals – an area of research of little use?’ appeared in the German journal Nachrichten für Chemie, Technik und Labor [27]. Then in 1972 the first liquid crystal display was built giving birth to a thriving branch of industry. Exciting results reported in a series of works carried out by the Stoddart group on so-called molecular machines show that there is a long path from a concept to a marketable device [28]. By using rotaxane systems like 9a [29] a family of versatile systems has been created which can be used as sensor, switches, ‘molecular abacus’ or nanomotor. In addition to the spacer 9e, the axle consists of the S-electron acceptor groups 9c, 9d which, upon the imposed conditions, can selectively bind the S-electron-donating macrocycle 9e shuttling it between the position shown in the formula 9a and that around the 3,3c-dimethyl-4,4cbipydidinium unit 9c. These studies, combining sophisticated syntheses with physicochemical methods, certainly provide an example of an important direction for nanotechnology research in the next few years. However, it is questionable whether they will bring marketable results within this time. This does not mean that such studies are not worth pursuing. Impractical studies elucidating the wave or corpuscular character of light and matter have been carried out since the famous dispute between Newton and Huygens for almost 300 years and, as the diffraction experiments on fullerene C60 10 (this group of molecules is discussed in detail in Chapter 5) from the Zeilinger group show [30], this topic is still vivid. Of course, choosing objects for a project out of more than 27 500 000 known molecules is a hard task, but its expeditious applicability should not be decisive. Most basic

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research is driven by curiosity not practicality but somehow in the long run it pays off. No-one thought about chemical applications when the foundations of a new branch of mathematics – topology – were formulated in the 1820s, long before such abstract concepts as links (catenanes) like olympiadane 11, Möbius strip, and knots, like double knot 12, were shown to be of use in chemistry [31] (some hydrocarbons with distinct topological properties are presented in Chapter 9).

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1 Introduction

Moreover, the discovery in the 1990s that circular DNAs in the living organisms [32–34] form links, knots and other systems with nontrivial topological properties, will have consequences which cannot be anticipated today. We begin to understand the mechanism of their formation but the role they play in nature is still unclear. Similarly, Einstein’s work on the photoelectric effect published in 1905, for which he was awarded the Nobel prize, certainly did not seem to have any practical significance at that time. Nor were cosmic studies carried out to develop kevlar and teflon! These materials were spin-offs of the space journeys. For examples closer to chemistry let us look at the fullerene 10 applications, much praised in the 1990s [35]. The Krätschmer method [36] which produces a significant amount of the substances spurred numerous proposals for their application. They were thought to exhibit superconductivity, serve as a drug carrier, its derivative C60F60 was anticipated to be an ideal lubricant, etc [35]. None of these promises was fulfilled. Superconductivity of fullerene derivatives is exhibited only at very low temperatures [37]; C60F60 was synthesized but it turned out to decompose in air with the HF formation [38]; and, in spite of promising reports, to our best knowledge no drug involving fullerene is on market. One of few fullerene applications today consists in their use as AFM tips (http://www.foresight.org/Updates/Update27/Update27.3.html). Few other are presented in Section 5.0 while those discussed in Section 5.7 still await marketable applications. This does not mean that fullerenes should not be intensively studied and that they will not finally be of practical use. At present these elegant-looking, highly symmetrical molecules are exciting and worth studying simply because of their unusual properties. They (1) form the nonplanar system of conjugated bonds; (2) have a hollow space inside that can accommodate other, smaller molecules, ions or even an elementary particle [39]; (3) with cations inside they form unusual salts since the fullerene cage assumes the negative charge, thus the salt can be dissociated only by its destruction; and, last but not least, (4) their formula merely look beautiful or, in other words, they are aesthetically appealing. The last point, that is the beauty of molecular formulae as the driving force for studying a molecule, was strongly denied by Jansen and Schön in their essay provocatively entitled ‘Design in chemical synthesis – an illusion?’ [40]. Their argument, which deserves much longer comment, contraposes purely aesthetical Gropius’ teapot designs (apparently not constrained by the properties of materials from which the objects were to be made) to the design of molecules whose structure is unequivocally defined by the energy hypersurface. No objection: molecular structure must obey the basic rules of chemistry and physics. However, within these limits there is plenty of room for designing molecules with predefined desirable properties. In addition to the complicated and not always successful field of drug design, hydrocarbons with unusual spatial structure present numerous examples of molecular design which was not usually aimed at marketability. For instance, let us look at cubane 1 (discussed in Sections 2.3 and 2.4) synthesized almost 50 years ago. The molecule was obtained by Eaton [41] not because of its immediate applicability. Its synthesis presented a considerable challenge and 1 was aesthetically pleasing (the aspect denied by Jansen and Schön [40]). The

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molecule turned out to have untypical properties, due to its nonstandard structure, e.g. 1 exhibits unusual rearrangement reactions such as the rearrangement of cubane to cuneane 13. In addition, NMR spectra of cubane allow one to explore the Karplus dependence of the 3J coupling constants [42]. And as for the cubane applications not looked for by Eaton? For more than 30 years secret studies were carried out by the American Army on nitro-derivatives of 1 because the high energy content of the cubane core magnified by the nitro-substituents suggested that these materials might have extraordinary explosive properties. (The synthesis of octanitrocubane 14 was eventually reported a few years ago [43].) There were also attempts to use cubane derivatives as therapeutic agents [44, 45].

However, the emerging applications of some hydrocarbons with unusual spatial structure should not deceive us. The main goal of studying them is not their marketing but to deepen our understanding of the chemical bond. Interestingly, until recently this very fruitful concept, on which all chemistry is based, was not anchored in quantum chemistry. We could carry out calculations on the molecule as a whole but, without using artificial approximate constructs, were unable to analyze properties of specific bonds within it. In particular, studying molecules with bonds which are very different from the standard is indispensable to understand the limits of the very concept of the chemical bond. The question as to what extent a chemical bond can be distorted without breaking is thought provoking. Moreover, in certain cases even the mere existence of a bond between two carbon atoms has been questioned. This was the case encountered in [1.1.1]propellane 2 (discussed in some detail in Sections 1.3 and 2.1) [46]. The synthesis of this exciting molecule, preceded by the calculations supporting its feasibility and predicting the propellane properties, serves as a fascinating example of a mutually fruitful interaction of theoretical and experimental studies [47]. 2 represents one of the most amazing examples from the point of view of organic stereochemistry since, contrary to van’t Hoff [48] and Le Bel [49] hypothesis, all four substituents on its bridgehead atoms lie in one hemisphere. Such atoms bearing the name ‘inverted carbons’, are also present in other small-ring propellanes (discussed in detail in Section 2.1) such as a derivative of [4.1.1]propellane 15 and that of [1.1.1]propellane 16 [50]. The discussion of the existence of the bridgehead–bridgehead bond in small-ring propellanes is remarkable [51]. The distance between the atoms in these molecules is about 1.6 Å [52], which is significantly longer than the typical C–C bond of 1.54 Å (some authors [50] consider this difference small but the energy required for such bond lengthening is considerable). However, 1.6 Å or even longer bonds have been encountered in several hydrocarbons [53]. In spite

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1 Introduction

of the reliable bond length of the former bond in small ring propellanes, the differential electron density maps for 15 [54] and 16 [50] measured in X-ray studies have not shown any build-up of the differential electron density between the bridgehead propellane atoms which should accompany the bond between them. The former finding and the possibility of a biradical structure for 14 without the central bond triggered a discussion on the reliability of the maps as the criterion of the bonding. On the one hand, the formulation of the limitations of this criterion resulted, stating that the lack of the build-up of the differential electron density is an artifact of the promolecule density distribution not reflecting the relative properties of the charge distributions [47]. The quantum calculations for 2 carried by Wiberg, Bader and Lau [47] showed that there is the bond-critical point along the line connecting bridgehead atoms in this molecule, thus proving the existence of the bond between the atoms. These calculations also revealed that the exceptional stability of this molecule is not due to the typical two-center integrals describing chemical bonds but is the result of the operation of the three-center integrals. In addition, other criteria for the existence of the central bond in 2 modeling [1.1.1] propellane have been checked [50].

Helvetane 17 and israelane 18 appeared as a joke in a 1st April issue of Nouveau Journal de Chimie [55]. These highly strained hypothetical molecules belong to a very interesting class of (CH)2n cage compounds to which cubane 1, dodecahedrane C20H20 19 and hypothetical perhydrogenated fullerene C60H60 [56] (discussed in Sections 2.3 and 2.4) belong. These molecules were shown to be of much higher energies than other members of the C20H20 family [57] which should be much more easy to synthesize. Nevertheless, they have been calculated by several theoreticians who pointed out than removing symmetry constraints would significantly lower the energy of 17 and 18. Then such molecules being members of a large group of isomers without interesting properties would seem to be of no specific interest.

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It should be stressed that a molecule dismissed as purely hypothetical today can be a plausible synthetic target tomorrow. Herzberg, later awarded Nobel Prize, stated in his seminal ‘Infrared and Raman Spectra of Polyatomic Molecules’ in 1945 that it is not likely that molecules of Ih symmetry will ever be found [58]. It took several years of hard work for the Paquette group to synthesize the first molecule of such a high symmetry, aforementioned dodecahedrane 19, about 40 years later [59]. (In our opinion this synthesis deserves the name molecular design vigorously discredited by Jansen and Schön [40].) Today, the best known such molecules are fullerene 10 (discussed in Chapter 10), parent fullerene C20 20 [60] and perfluorinated fullerane C60F60 (which, as discussed in Chapter 10, similar to other short-lived species could be stabilized in molecular flasks) as well as the most symmetrical isomers of their higher homologs, like C240, C540, C960, etc and some nested fullerenes formed by carbon cage compounds [61] belonging to the latter group.

A kind of laborious play, that seems not to promise serious consequences but bears all the attributes of a standard synthetic work, has been reported by Chanteau and Tour [62]. They described the syntheses of nanoputanes, like 21, the anthropomorphic molecules named after the Jonathan Swift lilliputanes. With meticulously described syntheses, the authors showed not only the way to obtain the nanokid 21 but also got a ‘dancing’ nanoputanes layer on a surface 22.

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To summarize, the choice of a molecule for studies is not simple. Most standard systems are trivial and unworthy of serious consideration in basic research. Other untypical molecules can also have their pitfalls. Keeping in mind that what is impossible to synthesize today can be realizable tomorrow, one should nevertheless exhibit caution when choosing an object to study which should be of a serious scientific interest. However, it should be stressed once more that immediate applications should not be the reason for financing basic research.

1.3 Computations on Strained Hydrocarbons

Andrey A. Fokin and Peter R. Schreiner Despite some potential applications as high-energy materials and specialty polymers, highly strained compounds mostly play a conceptual and educational role. Over 10 000 chemical papers contain the key words ‘strained hydrocarbon’ and more than 18 000 ACS papers alone the terms ‘strain energy.’ Despite the fact that our rationalistic and thrifty ages leave lesser space for exotic molecules, the aesthetic beauty of the cages such as cubane 1, tetrahedrane 23, octahedrane 24 or dodecahedrane 25 still fascinate organic chemists and represent the artistry of organic synthesis. Nature also uses highly strained compounds: The recent discovery of ladderanes 26 and 27 as membrane lipids of certain anaerobic bacteria [63] and natural antifungal oligocyclopropane antibiotic 28 [64] underline the importance of such structures (Scheme 1.1).

It is generally considered that highly strained compounds are difficult and expensive to make and, sometimes, also to store. However, once a challenging molecule has been prepared, the development of a simpler way for its synthesis is impending. The most recent example is highly strained octahedrane 24, which was first prepared in 1993 by an expensive and elaborate procedure [65, 66]. Now some octahedrane derivatives such as 30 can be prepared by one-step photochemical dimerization of readily available aromatic cyclophane 29 [67]. A simple preparation of octacyclopropylcubane 32 by an effective two-step condensation of four dicyclopropylacetylenes 31 is another remarkable example (Scheme 1.2) [68]. The stability of strained compounds is not necessarily a concern: cubane and its derivatives are stable even at high temperatures because the strain is uniformly distributed throughout the molecule and orbital symmetry forbids the cleavage of two C–C bonds at the same time. Some unstable and highly strained hydro-

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Scheme 1.1 Highly strained hydrocarbons and some natural compounds containing strained moieties.

Scheme 1.2 Highly strained derivatives of octahedrane and cubane prepared recently through short and simple procedures.

carbons have been successfully encapsulated and stored at room temperature as guest molecules in hemicarceplexes [69]. The chemistry of strained organic molecules probably began with the realization of cyclopropane derivatives by Perkin [70] that was almost immediately followed by the development of strain theory [71]. It was soon recognized that Baeyer’s angular strain is the main contributor to the potential energies of organic molecules. The quantitative description of strain was first proposed in the mid

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1940s by Hill [72] and was developed further by Westheimer into molecular mechanics [73]. As with many other useful chemistry concepts like conjugation, aromaticity, chemical bonding, etc., strain itself is not defined exactly, but can be expressed well quantitatively by ‘strain energy,’ which is, however, not measurable experimentally. Its value is calculated as the difference between the experimental enthalpy of formation of the molecule of interest and that of a hypothetically strainfree structure. The enthalpies of formation of strain-free reference compounds are calculated through group additivity schemes [74] based on the ‘averaged’ contributions of groups (CH3, CH2, CH, etc.) from the straight-chain hydrocarbons. The group’s contributions are derived from thermochemical measurements for which, however, equilibrium conformer distributions are difficult to take into account. Group equivalent schemes also require experimental thermochemical data on the molecule of interest but these are equally error-prone. The most recent example is the heat of formation of cubane that was reinterpreted based on the corrected value of its sublimation enthalpy [75]. Unluckily, cubane has already been used for the parameterization of some molecular mechanical methods [76] that now require re-parameterization. New additivity schemes [77, 78] demonstrate excellent accuracy, but only for moderately strained hydrocarbons. The computations of 'Hf° through atomization energies and bond/group separation reactions are more trustworthy; atomization energies are more useful for computation of the enthalpies of formation of small molecules [79–81]. Bond separation (isodesmic) equations proposed by Pople [82] for which the bonds between the non-hydrogen atoms are separated into strain-free reference molecules, give the strain energy directly (cf. Equation 1.1 for the evaluation of the strain energy of cubane). Alternatively, homodesmotic [83] equations (such as Equation 1.2), that contain an equal number of groups and bonds on both sides of the same type, largely cancel systematic computational errors. These two approaches lead to different strain energies (Scheme 1.3). The choice of strain-free reference compounds is problematic. The generally accepted strain energy of cubane (164.8 kcal mol–1, without the newest correction for its enthalpy of sublimation) was computed through homodesmotic Equation 1.2. However, the strain energy evaluation in Equation 1.2 is not properly balanced because the eight isobutane molecules are stabilized by twelve additional 1,3-interactions (protobranching) [84] relative to cubane. Thus, isoalkanes have ‘negative strain’ relative to n-alkanes, and cannot be used as references for strain energy evaluations: using branched alkanes artificially increases the strain

Scheme 1.3 Isodesmic (1.1) and homodesmotic (1.2) equations to determine the strain energy (Est) of cubane.

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of the molecules of interest. On the other hand, Equation 1.1 is properly balanced because it is based only on strain-free molecules – methane and ethane. The strain energy of cubane calculated via Equation 1.1 is 102.9 kcal mol–1 [85] and this value should be used. Equally, other strained hydrocarbons appear to be less strained than originally assumed. Concerning the validity of isodesmic Equation 1.1, one can argue that ethane is also not a strain-free molecule because of van der Waals’ contacts while methane is destabilized by Pauli repulsions. As these effects are present in all organic molecules these two hydrocarbons are still the best candidates as strain-free reference hydrocarbons; there is no conformational problem either. Computations are the only way to evaluate the strain energies of molecules for which the experimental thermochemistry is not available. Chemically accurate computations for small molecules nowadays are inexpensive, fast, and they can be used with ease. However, there are many sources of systematic as well as nonsystematic errors in computational chemistry modeling. Some of them are of general character, others are typical only for strain energy evaluations. For instance, due to the large number of reference molecules used in the above equations, the errors in zero-point vibrational energy (ZPVE) corrections, which are usually derived from a crude harmonic approximation model, do not effectively cancel. The choice of a proper computational method especially for ‘unusual’ strained molecules is critical. Computational chemistry estimates the contributions of angular strain well, even at the level of molecular mechanics. Another source of strain, nonbonding attractions/repulsions, is more challenging to compute correctly as only very expensive state-of-the-art computational methods are able to describe them accurately. Popular density functional theory (DFT) methods offer numerous functionals with different empiric exchange-correlation terms. While some of them are especially designed to describe certain types of interactions properly, virtually all of them systematically underestimate or completely neglect weak interactions [86]. DFT methods, such as the most popular B3LYP functional, give rise to various unsystematic errors and, worse, these increase dramatically with the size of the molecules [87, 88]. DFT methods may also exhibit some artifacts like electron self-exchange, which affects the electron energies considerably. Medium-range electron correlation, which contributes to the energies of saturated systems significantly, is poorly described both by local and hybrid functionals [89]. All of the above problems lead to unacceptable DFT errors for unstrained [86, 89] and, especially, strained [88, 90] molecules with more than ten heavy (= non-hydrogen) atoms – the ones for which DFT methods currently are used most often [91]. The use of new DFT formulations [92, 93] or a posteriori corrections (MP2 or coupled cluster) together with accurate thermochemical methods (Gn [94], Wn [95], or CBS [96]) significantly improves the quality of strain energy evaluations [79]. Nevertheless, most of the computational errors in strain energy evaluations are smaller than the discrepancies resulting from the arbitrary choice of strain-free reference states. Chemists targeting the preparation of a potentially strained compound are faced with the problem of predicting its stability and reactivity. The ‘strain energy per

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heavy atom’ or ‘per bond’ is a good starting point but it reveals little about the kinetic stability of a molecule. If an appreciable reaction pathway for lowering the potential energy does not exist, the molecule may be stable despite being highly strained. The extraordinary thermal stabilities of prismane and cubane are associated with the feature that breaking just one C–C bond causes only minimal changes in the remaining part of the molecular structure. Stabilization of tetrahedrane with bulky groups, which hinder rearrangements due to the ‘corset effect,’ is another example [97, 98]. Computational chemistry can help predict the behavior of strained compounds, and there are many inspiring examples (for selection see Scheme 1.4). The unusual stability of highly strained [1.1.1]propellane 2 [99–101] (discussed in Section 2.1) protonated pyramidane derivatives 33 [102, 103], and the Td-1,2-dehydro-5,7-adamantanediyl dication 34 [104] first predicted computationally, initiated successful attempts to prepare these highly strained systems.

Scheme 1.4 Some highly strained molecules, whose anomalous stability was predicted computationally before preparation.

The prediction of the thermal stabilities of strained compounds is a routine, albeit elaborate, procedure now and involves computations on the barriers of the crucial bond breaking pathway (see, for instance, a recent study on the kinetic stability of tetrahedrane) [98]. As the kinetic stability is a relative value, i.e. it depends on the reaction partner and the conditions, the barriers for the attack of radicals on strained compounds not only allow one to analyze their potential stability, but also to choose a proper reagent for their derivatization. For instance, cubane [105] and octahedrane [66] were found to be highly sensitive to the nature of the attacking radicals and they follow either C–C addition or C–H substitution paths. Computations on the reactions of strained compounds with electrophiles are more difficult, because the carbocationic species that form after primary electrophilic attack are largely prone to rearrangements to release the strain. Even reproducing experimental proton affinities is difficult, especially for a system as strained as cubane [106]. Cyclopropane is an exception because the edge-protonated form is a minimum and the downhill ring-opening path has a relatively high barrier [107]. Highly strained compounds quite often represent intriguing bonding situations, which modern computational methods are able to describe well. They offer not only accurate energies and geometries of strained compounds, but also provide information about electron density distributions, molecular orbitals, bond critical points and so forth [108]. One of the examples is the unusual bonding situation between inverted carbons of [1.1.1]propellane 2. Twenty years ago theory predicted

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[109] that there is a bond critical point between the central carbon atoms of 2 despite the fact that the electron density does not accumulate in this region. Recent synchrotron experiments on derivatives of 2 confirmed that this remarkable computational description indeed is correct [110]. The examples of m-benzyne 35 and m-dehydrocubane 36 demonstrate the borderline between proper C–C bonding and open-shell singlet biradical states (Scheme 1.5). The latter is favored for m-benzyne if dynamic electron correlation is included exhaustively: the fundamental frequencies of the m-benzyne singlet biradical computed at CCSD(T) [111] perfectly agree with the experimental IR spectra [112]. m-Dehydrocubane forms a singlet state and is predicted to exhibit an extremely long C–C bond (1.844 Å from REKS-B3LYP/6-31G* data) [113]. Unusually short bonds were found for the dimers of strained compounds. In 1989 the shortest single C–C bond (1.438 Å) was computed [114] for bis-tetrahedrane 37; recently this value was confirmed experimentally [115]. The properties of highly pyramidalized alkenes are difficult to study experimentally as only some matrix IR-spectra are available [116] and computational results are difficult to validate. A real breakthrough in this area was achieved recently when it was found that the computed proton affinities and heats of hydrogenation of 1,5-dehydroquadricyclane 38 agreed well with experiment [117].

Scheme 1.5 Selected strained molecules that represent unusual C–C bonding situations.

Most importantly, computational chemistry can not only predict the properties of molecules, but also help to discover new classes of strained compounds that may challenge experimentalists (Scheme 1.6). Molecules with planar tetracoordinated carbon: fenestranes 39 [118], tricyclo[2.1.0.01,3]pentane 40 [119, 120], and tetracyclo[3.1.0.01,3.03,5]hexane 41 [121]; with inverted geometries around the carbon atoms: pyramidane 42 [122] and bowlane 43 [123]; as well as with highly twisted double bonds: orthogonene 44 [124] and tetra-t-butyl ethylene 45 [125],

Scheme 1.6 Highly strained molecules computationally predicted to be isolable.

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were computationally predicted to be isolable and thus represent challenging yet realistic synthetic targets. The development of the chemistry of strained compounds has been closely connected to the progress of computational chemistry for the last three decades. We have finally reached a situation where geometries, electronic and thermodynamic properties as well as the reactivity of highly strained and ‘unusual’ small molecules may be estimated computationally with chemical accuracy.

1.4 Gallery of Molecules That Could Have Been Included in This Book

Helena Dodziuk 1.4.1 Introductory Remarks

The aim of this monograph is to present the richness of the domain of hydrocarbons with unusual spatial structure, not only in chemistry but also in their physicochemical properties, and not only in experimental studies but also in model calculations that play an increasingly important role in this domain. Clearly, such a broad scope, to be understood as a protest against the narrow specialization from which we all suffer, could not be fully covered in this limited volume. Thus, for various reasons, not all molecules deserving incorporation in this book could even be mentioned. To counteract this situation, in this chapter several fascinating molecules that have not been presented in other chapters will simply be listed with short notes showing why they are of interest. This is of particular importance since at least some of them merit further, more detailed studies. 1.4.2 Saturated Hydrocarbons

Of the family of bridged spiropentanes 46, the known [4.1.0.01,6]tricycloheptane (n = 2) 46a is stable and exhibits a considerable widening of the C2C1C7 angle up to about 160° [126, 127]. There is NMR evidence of [2.1.0.01,3]tricyclopentane 46b for which ab initio calculations yielded a pyramidal configuration on the central carbon atom [128, 129]. On the basis of ab initio quantum chemical calculations, QC, exciting tricyclo[3.1.01,3]hexane 46c has been found to exhibit almost linear arrangement of the formally Csp3–Csp3 bonds with the
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178° [130]. Wiberg and Snoonian [131] reported the synthesis of the derivative 47 observed at 10 K having the highly reactive ketene group. Thus, until now the highly unusual spatial structure of 46c could not be proven. One of the largest member of the trangulane family is branched C2v-[15]triangulene 48. A shortening of the central C–C bond in this molecule has been interpreted in terms of a considerable change in hybridization of the two central spirocarbon atoms due to severe steric strain [132]. Smaller, but also overcrowded, triangulanes also studied by the de Meijere group exhibited some unusual reactivity [133].

As shown by boat–twist conformation of the central C6 ring in trispirocyclopropanated cyclohexane 49 [134–136] and by the boat conformation of these rings in tetraasterane 50 [137, 138] discussed in Section 2.3, the cyclohexane ring does not necessarily have to assume the chair conformation. Recently synthesized octacyclopropylcubane 51 is not very stable: it has a half-life of 3 h at 250 °C and has ‘tremendous overall strain’ of 390 kcal mol–1 [139]. In the crystal it exhibits quite rare C4h symmetry. The average length of C–C bonds in the cubane core of 158.3 pm have been found to be slightly but distinctly longer than that in cubane (156.5 pm in the gas phase and 155.1 in the crystal).

Three examples of interesting stereochemical and/or structural phenomena will be given at the end of this subchapter.

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Figure 1.2 Bicyclic hydrocarbons.

A rare but interesting observation, i.e. in, out isomerism in bicyclic hydrocarbons (Figure 1.2) [140–142], has also been noticed in several natural products [143]. For larger values of k, l and m the molecules exhibit dynamic equilibrium. Steric strain may also cause ‘squeezing’ a molecule leading to a very close distance between the nonbonded atoms. Since X-ray analysis is not the most reliable tool for the determination of H atoms’ position, indirect arguments are sometimes used for their estimation as was done for 52 [144].

As found for cubane 1 [145] and C60 53, highly symmetrical structures may exhibit interesting dynamic behavior in the solid state [146, 147]. In the latter case, it also leads to the structural diversity of host–guest and intercalation complexes of the fullerene as studied by X-ray technique [148]. It took more than 20 years to synthesize dodecahedron 54 [149] which, due to its strain, exhibits quite unusual rearrangement reactions. Remarkably, both 53 and 54 are of Ih symmetry, that is every carbon atom (and hydrogen, respectively) in these molecules is identical with the other.

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1.4.3 Distorted Double Bonds

A detailed discussion of several routes that were expected to lead to highly strained tetrakis-t-butylethene 55 but were unsuccessful is given in detail in Ref. [150].

Hypothetical bicyclo[1.1.0]-1(4)-pentene 56a and bicyclo[1.1.0]-1(3)-butene 56b remain unknown but according to ab initio calculations such molecules should have considerably pyramidalized formally Csp2 carbon atoms [151, 152].

Pyramidalized carbon atoms should be exhibited in known bridged bicyclobutane 57 (n = 3) [153–156] and 58 which has probably been observed [157].

The smallest synthesized [m][n]betweenanene 59 has m = n = 8 [158, 159]. To the best of our knowledge, no X-ray structure determination exists but simple MM modeling indicates significant distortions from standard geometry [160]. The larger (m = 22, n = 10) not highly strained betweenanenes have been expected to exhibit interesting dynamic effects involving ‘a jumping of the longer chain’ around [161].

Tricyclo[4.2.2.22,5]dodeca-1,5-diene 60 [162] and its tetraaryl-substituted derivative 61 [163, 164], both with strongly pyramidalized Csp2 carbon atoms, are known, while diene 62, also with very close distance between double bonds, is still unknown [165]. Noteworthy, the aromatic rings in 61 are planar.

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The name Dewar benzene of 63 is thought to have no sound basis [166]. The system is stable in the form of its tri-t-butyl [167, 168] or hexamethyl [169, 170] derivatives. Although the highly reactive 63 was obtained more than 40 years ago and attracted the attention of several theoreticians [171], the reactivity of this molecule and/or its derivatives is still the subject of studies today [172].

Out of two possible 64a and 64b diastereomers of [2.2]cyclooctatetraenophane, which are present as a (d,l) mixture, the former was synthesized and found to isomerize to the latter [173].

1.4.4 Benzene Rings with Nontypical Spatial Structures

Typical structure of aromatic systems consists in the planarity of the aromatic ring and its substituents and in close to 120° value of all bond angles. Steric hindrance can force another spatial structure. To the best of our knowledge, highly strained hexa-t-butylbenzene 65 (X = C) is not known. However, a less strained derivative (due to longer C–Si than C–C bonds), hexakis(trimethylsilyl) derivative 65, exhibits an unusual distorted chair conformation [174]. Another, less symmetrical type of nonplanar distortion of the aromatic ring is provided by 1,2,3-tri-t-butylnaphthalene 66 [175]. Considerable strain in the latter molecule allowed for the freezing of internal rotations of the methyl groups in 1- and 2-positions at 193 K. Interestingly, in disagreement with molecular mechanics [176], modeling the barrier for the rotation of the groups at the 3-position was the smallest. It is also noteworthy that due to its large size, 66 did not form the inclusion complex with J-cyclodextrin but rather ‘sat’ on top of the macrocyclic sugar.

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Twisted acenes are schematically presented in formula 67 [177]. Additional phenyl groups as in 68 stabilize the molecule and have allowed Pascal to achieve a formidable twist of 144° between the planes of terminal aromatic rings [178]. Interestingly, 68 has been resolved into enantiomers. Such highly twisted aromatic systems can find an application as porous solids. They are also expected to have chiroptical properties and have been incorporated into light emitting diodes (www. cnsi.ucla.edu/arr/paper?paper_id=193298). According to DFT calculations, 68 is a disjointed radical exhibiting exciting electronic structure [179].

Latos–Grazynski group reported the synthesis of di-p-benzhexaphyrin that is in dynamic equilibrium of two forms: ‘standard’ 69a and 69b representing a topologically nontrivial Möbius strip [180]. (Other topological nontrivial molecules are presented in Chapter 9.) In the solid state, only the latter was found to exist.

Cyclophanes [181] (covered in Section 4.2) have been studied mainly because of their non-standard structure and a strong S–S interaction between close-lying aromatic rings [182] manifesting itself in UV/Vis [183] and NMR spectra [184].

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Syntheses of impressive layered para-cyclophanes called cochins having up to six aromatic rings as in 70 and 71 were reported by Otsubo and coworkers [185–187] while those of three isomers of four-layered [2.2]metacyclophanes 72–74 were published by Umemoto [188, 189]. In analogy with the smaller [2.2]metacyclophane 75 discussed in detail in Section 4.2, protons of C–H bonds situated between two bridges exhibit very unusual values of chemical shifts since they lie above (or below) the plane of the neighboring aromatic ring.

In spite of its high strain, superphane 76 [190, 191] is relatively stable, even [26] (1,2,3,4,5,6)cyclophane-1-ene 77 with an additional double bond has been reported [191]. The benzene Csp2 carbon atoms in 76 all lie in the respective planes but its spatial structure is untypical since not all substituents on the aromatic rings lie in the plane of the rings [192]. 78 and 79 still await their syntheses [190].

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Corannulene 80 has the shape of a bowl because it includes a five-membered ring, and is known to invert rapidly [193]. In addition to its nonstandard geometry and dynamic behavior, the molecule attracted a lot of interest since it has been considered as an important building block that should enable the organic chemistry synthesis of C60 53. Corrannulene derivatives also exhibit interesting packing behavior in the solid state [193]. As discussed in detail in Kawase and Kurata review [194] not only bowl-shaped but also ball- and belt-shaped aromatic systems provide an exciting opportunity to explore the concave–convex S–S interactions by studying their complexation.

Another type of revealing distortion in aromatic rings consists in differentiation of their bond lengths achieved by fusing cyclobutane or cyclopentane rings to them, resulting in the remarkable differences in the C–C bond lengths for 81 [195] and 82 [196]. Such systems are indispensable for studying the limits of aromaticity.

1.4.5 Cumulenes

Interestingly, the cumulenes’ structure was predicted by van’t Hoff [197] who stated that in cumulenes with an even number of double bonds the four substituents must be placed in two perpendicular planes while for the odd-numbered series the substituents must lie in one plane with the double bonds. The cumulenes discussed in Section 3.3 are distorted from such arrangements. No X-ray structure of bicyclic allene 83 [198] and triene 84 [199] have been published but, according to MM modeling, the planes of respective bonds are at angles different from zero and 180° [160]. Similarly, to the best of our knowledge no structural data for hexaene 85 [200] and pentaene 86 [201] have been published but the molecules, with t-butyl groups added to increase stability, definitely do not have the standard structure.

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1.4.6 Acetylenes

Permethylated [5]pericyclyne 87 (R = Me) and larger analogs are known [202, 203]. Interestingly, in analogy with cyclopentane the central ring in 87 adopts the envelop conformation even in the solid state while model calculations indicate that in permethylated [6]pericyclyne the central ring [204] can adopt either the most stable chair conformation or boat or twist-boat ones. Aromaticity and the role of conjugation in 88 and other analogous carbocycles have been studied by Lepetit [205]. X-ray spectra of an octaphenyl derivative of 89 [206] reveal the planar structure of the bicyclic core with considerable bond angle distortions.

Only a few of many exciting distorted hydrocarbons could be mentioned in this chapter. It should be stressed, however, that with a new domain of macrocyclic host molecules rapidly developing this area will expand further since not all large macrocycles are strain-free. For instance, 90 can host C60 53 into its cavity assuming C6v symmetry [194, 207].

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133 de Meijere, A.; von Seebach, M.; Zollner, S.; Kozhushkov, S. I.; Belov, V. N.; Boese, R.; Haumann, T.; Yufit, D. S.; Howard, A. J. K. Chem. Eur. J. 2001, 7, 4021. 134 Fitjer, L.; Scheuermann, H.-J.; Klages, U.; Wehle, D.; Stephenson, D. S.; Binsch, G. Chem. Ber. 1986, 119, 1144. 135 Wulf, K.; Klages, U.; Rissom, B.; Fitjer, L. Tetrahedron 1997, 53, 6011. 136 Weiser, J.; Golan, O.; Fitjer, L.; Biali, S. E. J. Org. Chem. 1996, 61, 8277. 137 Hutmacher, H. M.; Fritz, H.-G.; Musso, H. Angew. Chem. Int. Ed. 1975, 14, 180. 138 Fritz, H.-G.; Hutmacher, H. M.; Musso, H.; Åhlgren, G.; Akermark, B.; Karlsson, R. Chem. Ber. 1976, 3781, 180. 139 de Meijere, A.; Redlich, S.; Frank, D.; Magull, J.; Hofmeister, A.; Menzel, H.; Kenig, B.; Svoboda, J. Angew. Chem. Int. Ed. 2007, 46, 4574. 140 McMurry, J. E.; Hodge, C. N. J. Am. Chem. Soc. 1984, 106, 6450. 141 McMurry, J. E.; Lectka, T.; Hodge, C. N. J. Am. Chem. Soc. 1989, 111, 8867. 142 McMurry, J. E.; Lectka, T. J. Am. Chem. Soc. 1993, 115, 10167. 143 Alder, R. W.; East, S. P. Chem. Rev. 1996, 96, 2097. 144 Bodige, S. G.; Sun, D. L.; Marchand, A. P.; Namboothiri, N. N.; Shukla, R.; Watson, W. H. J. Chem. Crystall. 1999, 29, 523. 145 Yildirim, T.; Gehring, P. M.; Neumann, D. A.; Eaton, P. E.; Emrick, T. Carbon 1998, 36, 809. 146 Moret, R. Acta Crystallogr. A 2005, 61, 62. 147 Blank, V. D.; Buga, S. G.; Dubitsky, G. A.; Serebryanaya, N. R.; Denisov, V. N.; Marvin, A. V. I. B. N.; Popov, M. Y. Mol. Cryst. Liquid Cryst. Sci. Technol. C 1996, 7, 251. 148 Makha, M.; Purich, A.; Raston, C. L.; Sobolev, A. N. Eur. J. Inorg. Chem. 2006, 3, 507. 149 Ternansky, R. J.; Balogh, D. W.; Paquette, L. A. J. Am. Chem. Soc. 1982, 104, 4503. 150 Hopf, H. Classics in Hydrocarbon Chemistry; Wiley-VCH, 2000, p. 138. 151 Hehre, W. J.; Pople, J. A. JACS 1975, 97, 6941.

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173 Paquette, L. A.; Kesselmayer, M. A. J. Am. Chem. Soc. 1990, 112, 1258. 174 Sakurai, H.; Ebata, K.; Kabuto, C.; Sekiguchi, A. J. Am. Chem. Soc. 1990, 112, 1799. 175 Dodziuk, H.; Sybilska, D.; Miki, S.; Yoshida, Z.; Sitkowski, J.; Asztemborska, M.; Bielejewska, A.; Kowalczyk, J.; Duszczyk, K.; Stefaniak, L. Tetrahedron 1994, 50, 3619. 176 Osawa, E.; Musso, H. Angew. Chem, Int. Ed. Engl. 1983, 22, 1. 177 Pascal, R. A. J. Chem. Rev. 2006, 106, 4809. 178 Lu, J.; Ho, D. M.; Vogelaar, N. J.; Kraml, C. M.; Pascal, R. A. J. J. Am. Chem. Soc. 2004, 126, 11168. 179 Norton, J. E.; Houk, K. N. J. Am. Chem. Soc. 2004, 127, 4162. 180 Stepien, L.; Latos-Grazynski, L.; Sprutta, N.; Chwalisz, P.; Szterenberg, L. Angew. Chem, Int. Ed. 2007, 46, 7869. 181 In: Modern cyclophane chemistry; Gleiter, R.; Hopf, H., eds.; Wiley-VCH: Weinheim, 2004. 182 Misumi, S.; Otsubo, T. Acc. Chem. Res. 1978, 11, 251. 183 Rademacher, P. In: Modern cyclophane chemistry; Gleiter, R., Hopf, H., Eds.; Wiley-VCH: Weinheim, 2004, p. 275. 184 Ernst, L.; Ibrom, K. In: Modern cyclophane chemistry; Gleiter, R., Hopf, H., Eds.; Wiley-VCH: Weinheim, 2004, p. 381. 185 Otsubo, T.; Horita, H.; Misumi, S. Synth. Commun. 1976, 6, 591. 186 Otsubo, T.; Tozuka, Z.; Mizogami, S.; Sakata, Y.; Misumi, S. Tetrahedron Lett. 1972, 297. 187 Otsubo, T.; Mizogami, S.; Otsubo, I.; Tozuka, Z.; Sakagami, A.; Sakata, Y.; Misumi, S. Bull. Chem. Soc. Japan 1973, 46, 3519. 188 T. Umemoto; Otsubo, T.; Misumi, S. Tetrahedron Lett. 1974, 1573. 189 T. Umemoto; Otsubo, T.; Sakata, Y.; Misumi, S. Tetrahedron Lett. 1973, 593. 190 Gleiter, R.; Roers, R. In: Modern cyclophane chemistry; Gleiter, R., Hopf, H., Eds.; Wiley-VCH: Weinheim, 2004, p. 105. 191 Sekine, Y.; Boeckelheide, V. J. Am. Chem. Soc. 1981, 103, 1777.

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201 Negi, T.; Kaneda, T.; Sakata, Y.; Misumi, S. Chem. Lett. 1972, 703. 202 Utimoto, K.; Tanaka, M.; Kitai, M.; Nozaki, H. Tetrahedron Lett. 1978, 2301. 203 Scott, L. T.; DeCicco, D. J.; Hyun, J. L.; Reinhardt, G. J. Am. Chem. Soc. 1985, 107, 6546. 204 Houk, K. N.; Scott, L. T.; Rondan, N. G.; Spellmeyer, D. C.; Reinhardt, G.; Hyun, J. L.; DeCicco, D. J.; Weiss, R.; Chen, M. H. M.; Bass, L. S.; Clardy, J.; Jorgensen, F. S.; Eaton, T. A.; Sarkozi, V.; Petit, C. M.; Ng, L.; Jordan, K. D. J. Am. Chem. Soc. 1985, 107, 6556. 205 Lepetit, C.; Godard, C.; Chauvin, R. New J. Chem. 2001, 25, 572. 206 Gholami, M.; Melin, F.; McDonald, R.; Ferguson, M. J.; Echegoyen, L.; Tykwinski, R. R. Angew. Chem, Int. Ed. 2007, 46, 9081. 207 Kawasa, T.; Oda, M. Pure and Appl. Chem. 2006, 78, 831.

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2 Distorted Saturated Hydrocarbons 2.1 Molecules with Inverted Carbon Atoms

Kata Mlinariü-Majerski 2.1.1 Introduction

The tetrahedral geometry at a saturated carbon atom has been known as the only possible geometry for almost a century [1]. However, in the late 1960s theoretical predictions and some experimental results suggested other possibilities such as planar, pyramidal or even inverted geometry at the carbon [2]. The interest in nontetrahedral saturated carbon atoms has been growing very rapidly. Nontetrahedral geometries at a tetravalent carbon atom are defined by the four interatomic vectors emanating from the saturated carbons. Deformation of the conventional tetrahedral arrangement (Figure 2.1a) via an umbrella motion leads to the inverted geometry in which all of the four substituents lie in the same hemisphere (Figure 2.1b) [3a]. As established by microwave spectra [4a], the simplest molecule having the carbon atoms with inverted geometry is bicyclobutane (Figure 2.1c) but the existence of this unusual feature was only recognized later by Paddon-Row and coworkers [4b]. The possible existence of this unusual geometry was explored by Wiberg who carried out ab initio calculations at the 6–31G* level, first predicted

Figure 2.1 (a) Tetrahedral geometry at carbon; (b) Inverted geometry at carbon; (c) bicyclobutane; (d) [k.l.m]propellanes. Strained Hydrocarbons: Beyond the van’t Hoff and Le Bel Hypothesis. Edited by Helena Dodziuk Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31767-7

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properties of small-ring propellanes, synthesized the molecules and confirmed the predictions [3, 5a]. In particular, the ultimate member of the small-ring propellanes, extensively studied by Wiberg [3, 5], [1.1.1]propellane 1 was found to be much more stable than higher members of the series [k.l.m]propellanes with k = 2, d; l, m = 1, 2. This remarkable molecule represents a landmark in the quest for highly strained small-ring organic molecules. Propellanes are defined as the tricyclic systems (Figure 2.1d) in which three rings are fused together by a common central, bridgehead–bridgehead C–C bond [6]. Large-ring propellanes (Figure 2.1d; k, l, m t 3) behave chemically like ‘normal’ polycyclic hydrocarbons and their bridgehead carbon atoms have tetrahedral configuration [7]. However, reduction of the ring size as in 1–7 leads to the small-ring propellanes possessing two inverted carbon atoms in the central bond [3]. Such a nonstandard structure leads to several peculiarities. For instance, the small-ring propellanes 1–7 are remarkably reactive toward electrophiles and free radicals; 2, 3 and 5 have remarkable thermal lability while 1 is stable [3].

Oxa[3.2.1]propellane 8, is the first reported small-ring propellane [8], and the synthesis of the parent hydrocarbon 7 soon followed [9, 10]. The X-ray structure analysis of dichloro[3.2.1]propellane 7a proved the inverted configuration on the bridgehead carbon atoms [11]. Interestingly, in spite of the distance between the bridgehead atoms of 1.60 Å in 1 [5a], the mere existence of the central bond has been questioned for long time [2a] and the small-ring propellanes have become the subject of many theoretical studies and a great challenge to synthetic chemists. Until discovery of the molecules having inverted carbon atoms, in quantum chemistry there was no definition of such a useful concept as the chemical bond.

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35

One could only carry out calculations for a molecule as a whole. As discussed below, introduction of bond critical points allowed the researchers to refine the definition [34b, 34c]. This topic is discussed in Section 1.3. Several other small-ring propellanes 5, 6 and 7 containing a total of six atoms in their bridges as in [2.2.2]propellane [12], [3.2.1]propellane [9–11, 13, 14], and [4.1.1] propellane [15–17], respectively, have been prepared. Of these, particular interest was paid to [2.2.2]propellane 5 and many theoretical studies have been undertaken [18–21]. Prior to the synthesis of 5, Stohrer and Hoffmann had predicted a facile cleavage of its central C–C bond [20]. Davidson [21] calculated the rearrangement of 5 by applying a variety of theoretical methods (CASSCF, PUHF, MUMP2, DFT, UDFT, CI) suggested as methods for obtaining a reliable potential energy surface for diradicals. The results obtained suggested that [2.2.2]propellane 5 can exist and would have a substantial barrier for opening to the bicyclic diradical or rearrangement to dimethylenecyclohexane. However, monosubstituted [2.2.2]propellane proved highly unstable [12a]. Recently, the extremely stable perfluoro[2.2.2] propellane has been prepared [12c]. A more strained geometry than that in 5 was expected in [3.1.1]-, [2.2.1]-, [2.1.1]-, and particularly in [1.1.1]propellanes (4–1, respectively). As mentioned earlier, even the possibility of their existence was questioned [2a, 18]. However, for the [1.1.1]propellane 1 the calculations led to the conclusion that 1 should be more stable than the corresponding diradical [19, 20]. Wiberg and Walker found that high energy of 65 kcal mol–1 is needed to break the central bond of 1 to obtain the bicyclo[1.1.1]pentyl diradical [5a]. Amazingly, that calculation result also suggested that 1 should be the most easily prepared and the most stable small-ring propellane. This prediction proved correct [5]. Indeed, the small-ring propellanes, [3.1.1]propellane 4 [15c, 15d, 22, 23] and [1.1.1]propellane 1 [5] or their derivatives [24–26] have been synthesized. On the other hand, their homologs [2.2.1]propellane 3 [27–29] and [2.1.1]propellane 2 [30, 31] were observed either at low temperature, captured in the argon matrix, or as intermediates which were subsequently trapped by reagents to give stable products. 2.1.2 Small-ring Propellanes: Computational and Physicochemical Studies

In view of the exceptional role that calculations have played for these compounds, this Section will begin with a discussion of theoretical results. As mentioned earlier, the unusual properties of the inverted carbon atoms and the mere existence and nature of the central bond that connects the two inverted bridgehead atoms in the small-ring propellanes have been the subject of many theoretical investigations [19, 20, 27, 30, 32–41], X-ray [11, 15b, 42] and electron-diffraction analyses [15b, 43, 44], vibrational [45], photoelectron [46] and electron impact [47] spectroscopic and NMR studies [23b, 48–52]. The experimental and theoretical results of many different research groups have led to the apparently opposite conclusions on the small-ring propellane structure.

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Some researchers even questioned the very existence of the central bond assuming that the molecules have biradical structure with lone-pairs directed outside the cage [19a, 20, 32]. For instance, according to the early low-level ab initio molecular orbital studies on [1.1.1]propellane 1 [19a] there is no evidence for a central bond in terms of charge distribution since the sp4 hybrid orbitals forming the bond are directed away from each other and with the zero overlap population in this bond. Jackson and Allen [32a] pointed out that 1 is electron-deficient because its HOMO is nonbonding or slightly antibonding and there is low electronic density between the bridgehead carbon atoms C1 and C3. The C1–C3 bond is considered to be formed by three-center two-electron molecular orbitals and termed ‘V-bridged S bond’. These theoretical results are supported by an experimental investigation of the differential electron density of two [1.1.1]propellane derivatives [44]. On the basis of ab initio quantum chemical calculations, Feller and Davidson [35] came to the conclusion that 1 is just a strained cage with negligible bridgehead to bridgehead through-space covalent bonding. Wiberg, Bader and Lau [34b, c] came to a different conclusion by analyzing the bond critical points. Their reasoning was based on the assumption that the electronic charge density is a physical property of the system and as such it should be model-independent. On the basis of an analysis of the second derivative of the electron density determined by all occupied orbitals, the authors concluded (1) that the exceptional stability of 1 is due to specific three-center integrals and (2) that bonding was found between the bridgehead atoms since there is an appreciable accumulation of charge between the bridgehead atoms. The results of quasi ab initio PRDDO calculation additionally support the conclusion that the bridgehead atoms in 1 are weakly bonded in the ground state [38]. Recently, the electron density and bonding between the inverted carbon atoms have been studied experimentally on the [1.1.1]propellane derivative 9 by Luger [39] who discussed different criteria for the existence of a bond.

The authors discussed the quantitative results obtained by a synchrotron experiment and model calculations concluding that ‘the bond critical point was found between the bridgehead atoms (C1–C3) in 9. This bond is unusual according to topological analysis: it has a bond path with a bond critical point of significant density, as is characteristic for covalent bond, but no charge accumulation is evident at the bond critical point where the Laplacian is positive’. A bond order n was derived from the electron density at the critical point and was found to be 0.71 which is close to 0.73 as determined by Wiberg [34]. A similar low bond order was determined for [2.1.1]propellane 2, while in [2.2.1]propellane 3 bond order was 1.0 and in [2.2.2] propellane 5 it was 1.3 [34b]. The nature of bonding in 1 is also discussed on the basis of localized MOs and bond indices [40]. LMO and bond indices analyses indicate that there is a significant interaction between the bridgehead carbon

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37

Figure 2.2 The strain energy (SE, kcal mol–1) of small-ring propellanes 1–3, 5 and bicyclo[1.1.1] pentane (10) at the 6-31G*/6-31G* level [34a].

atoms in 1 and that about 80% of the total contribution of the central bond comes from the HOMO. The authors have also found out that in the series of the small-ring propellanes the total bond index increases in the order [1.1.1] < [2.1.1] < [2.2.1] < [2.2.2]propellane whereas the HOMO contribution remains almost constant [40]. Therefore, the difference is in the electron density of the outer envelope of the central propellane bond which most probably influences their reactivity. However, the strain energies of these propellanes (of 103 kcal mol–1, 106 kcal mol–1, 109 kcal mol–1, 97 kcal mol–1 and 67 kcal mol–1 for 1, 2, 3, 5, 10, respectively) are very similar (Figure 2.2) [34a]. Although they are all sufficiently strained to permit facile reaction, some are quite reactive and other quite stable, as is [1.1.1]propellane 1. The high stability of 1 is due to the low exothermicity of the reactions of breaking the C1–C3 bond to give bicyclo[1.1.1]pentane 10, still highly strained structure. The strain release is less than a third of the strain energy and breaking the side bond is forbidden by symmetry [34]. Several studies of the strain energy of small-ring propellanes have been reported [34, 37]. They point to the charge withdrawal from the neighboring groups to the bridgehead region with the increasing strain [34c]. A photoelectron (PE) spectroscopic study of [1.1.1]propellane 1 revealed that there should be only minute change in the geometry between 1 and its radical cation [46a]. This was attributed to the nonbonding or slightly antibonding character of the HOMO of 1. PE investigation on the less strained [3.1.1]propellane 4 [46b] was indicative of a slightly bonding HOMO, while the PE spectra of several [n.1.1]propellanes [46c] showed that the energy of the first band depends very strongly on n. For n = 1 the ionization energy is around 9 eV, while for n = 3 and 4 it is around 8 eV. The central C1–C3 bond in small-ring propellanes are longer (e.g. 159.6 ± 0.05 pm in 1) than normal C–C bond (154 pm) whereas the length of the bridge– bridgehead C–C bonds of 152.5 pm were found to be close to the bond length in a cyclopropane ring (151 pm) [43–45b]. The results of natural bond orbital (NBO) analysis of the central bond in [1.1.1]propellane and some [1.1.1]heteropropellanes

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suggested that in these compounds the C1–C3 bond lengths are closely controlled by p character of the hybrid orbitals [53]. While Jarret and Cusumano [49], on the basis of NMR measurements, suggested a low p character of the central propellane bond other authors assigned a very high p character to this central bond on the basis of coupling constants [50]. Recently, the high-level ab initio calculations yielded the values of all NMR shielding and spin–spin coupling constants in 1 [51]. The computed NMR parameters agree with the experimental values, except for the C1–C3 coupling constants. The ab initio studies using the equations of motion approach have also been reported J(CC) of the bridgehead bond in other small-ring propellanes [52]. The controversy regarding the central bond and the nature of bonding interactions in the small-ring propellanes still revolves around different explanations of the similar experimental and theoretical results. Recently, the comprehensive work concerning [1.1.1]propellane and the other compounds containing the bicyclo[1.1.1]pentane skeleton was reviewed by Michl [54]. The work on the propellane addresses the very important issue of the nature of central (C1–C3) bond. The results recently reported by Luger [39] represent a culmination of the experimental studies of charge density and, as pointed out by Coppens [55], the characterization of this chemical bond is not a closed subject. 2.1.3 Small-ring Propellanes: Experimental Results 2.1.3.1

Preparation and Reactivity of [1.1.1]Propellane

As predicted by Wiberg [5a], the synthesis of [1.1.1]propellane 1 was successfully carried out by reduction of dibromide 11 with butyllithium. However, 1 had remained rather inaccessible because of the long and tedious synthesis of precursor 11, until Szeimies [24a] synthesized 1 in a single step (Scheme 2.1), starting from the readily available 1,1-dibromo-2,2-bis(chloromethyl)cyclopropane 12. The latter procedure made 1 the most easily prepared small-ring propellane.

Scheme 2.1

The method of ring closure with 2 equiv. of alkyllithium was also used to prepare the bridged [1.1.1]propellanes 13 and the related propellane 14 [24a, 24b, 25b]. In addition, the propellane 13 was also prepared by intramolecular addition of carbene to a double bond [25a].

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39

Wiberg and McMurdie [56] reported the formation of 1 by nucleophilic attack on 1,3-diiodobicyclo[1.1.1]pentane 15, as shown on Scheme 2.2.

Scheme 2.2

The availability of 1 enabled the study of properties and reactivity of that remarkable molecule. Most of the properties of 1, first predicted by calculation, have been confirmed experimentally. For example, the reaction of 1 with acetic acid led to ring opening and formation of 3-methylenecyclobutyl acetate (16, Scheme 2.3). The enthalpy of that reaction was measured [45a] and the enthalpy of formation of 16 was determined to be 85 kcal mol–1 in good agreement with the theoretically estimated value of 89 kcal mol–1 [34a, 57].

Scheme 2.3

[1.1.1]Propellane 1 is known to be stable at the room temperature but it rearranges to methylenecyclobutene 17 [5a] at 114 °C or to 1,2-dimethylenecyclopropane 18 at 370 °C (Scheme 2.4) [24b]. At first, the reason for this difference

Scheme 2.4

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was not understood. Recently, Jarosch, Walsh and Szeimies [58] investigated the kinetics of thermal rearrangement of 1 by gas-phase pyrolysis in the stationary system and found that the unimolecular reaction leads to dimethylenecyclopropane 18 and its thermal isomerization product ethenylidenecyclopropane 20. The ab initio and DFT calculations of the potential energy surface indicated that the isomerization follows an asynchronous reaction path in which two side bonds of 1 are broken with activation barrier of 40.0 kcal mol–1. Furthermore, the authors showed that the minor product methylenecyclobutene 17 and its thermal isomerization product 1,2,4-pentatriene 19 resulted from the side reaction catalyzed by the reaction vessel surface (Scheme 2.4). Wiberg [59] studied a wide variety of the free radical reactions of 1 and compared its reactivity to those of bicyclo[1.1.0]butane 21 and bicyclo[2.1.0]pentane 22. Having observed that the reactions of 1 with free radicals were faster then that of 21, whereas 22 was relatively inert, the authors concluded that the reactivity was not determined by the strain energy relief or the HOMO energy of these compounds, but rather by the local charge distribution that could be a more important factor.

A free radical addition represents the most effective route of preparing various 1,3-disubstituted bicyclo[1.1.1]pentane derivatives 23 (Scheme 2.5) [5b, 24a, 59–61].

Scheme 2.5

In some cases these reactions lead to oligomers [5b]. The free radicals were found to react more readily with 1 than with styrene [62]. Furthermore, it was found that the electronic energy transfer to 1 occurs with a rate constant significantly below the diffusion-controlled limit (for instance, triplet benzophenone was quenched by 1 with a bimolecular rate constant of 9.9 u 106 M–1s–1) [62b]. In addition, the reactions of 1 with electron deficient alkenes and alkynes have been studied and compared with the corresponding reactions of 21 and 22 [5b, 63]. In these reactions relative reactivity of 1 and 21 varied considerably with the used reagent while 22 was again unreactive. The bridged [1.1.1]propellanes (for example 13 and 14) behave in a similar fashion as the parent propellane 1 [24a, 24b, 25].

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2.1.3.2

41

Preparation and Reactivity of [2.1.1]Propellane and [2.2.1]Propellane

There are few reports on experimental observation of [2.1.1]propellane 2 [30–31] and [2.2.1]propellane 3 [27–29]. These two propellanes are the only members of the small-ring propellanes that could not be isolated as the stable molecules but were observed in the argon matrix at low temperature (~30 K) by IR spectroscopy. The propellanes 2 [30] and 3 [27] were obtained by gas-phase dehalogenation of 24 and 25, respectively, with potassium. When the matrix was warmed up to about 50 K the compounds polymerized. When bromine was introduced to the matrix of 3, 1,4-dibromonorbornane 26 was obtained (Scheme 2.6).

Scheme 2.6

It is also suggested that [2.2.1]propellane 3 is an intermediate in the electrolytic reduction of the corresponding 1,4-dihalonorbornanes 25 and 26 [28]. Experimental results for 2 and 3 and their derivatives [29, 31b, 31c] showed their high reactivity toward free radicals and some organometalic reagents. Recently, Jarosch and Szeimies [64] studied thermal rearrangement of 2 by using density functional and the ab initio molecular orbital calculations and found that the low energy isomerization path proceeds via a retro-carbene reaction to give carbene 27 (Scheme 2.7).

Scheme 2.7

The most favorable consecutive reactions were found to be 1,2-hydrogen shifts leading to dienes 28 and 29. A few higher propellanes are discussed in Section 3.3.

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2.1.3.3

[1.1.1]Propellane as the Precursor for the Synthesis of Other Unusual Molecules

The feasibility of radical addition across the central bond of [1.1.1]propellane 1 enables polymerization of 1 into functionalized [n]staffanes [n]1. Michl [65] reported the synthesis of [n]staffanes (Scheme 2.8) as a new family of the endfunctionalized, inert, transparent and straight rods with a van der Waals radius of 2.3 Å and a length increment of 3.35 Å, which could be used as a construction element [54, 65]. Schlüter reported the formation of the same structures by spontaneous polymerization of neat 1 [66]. Also, he reported the first anionicallyinduced polymerization of the bridged [1.1.1]propellane 13 [67]. The [n]staffane skeleton is expected to be of importance for prospective applications as an electrical insulator [54, 68].

Scheme 2.8

[1.1.1]Propellane 1 also served as the precursor for the preparation of an unnatural amino acid, 3-aminobicyclo[1.1.1]pentane-1-carboxylic acid which has been incorporated into linear and cyclic peptides [69]. Next, the truly remarkable molecule of high symmetry, [1.1.1.1]paddlane 30 that should have two pyramidal carbon atoms, was reported to be one of the products (although only in 7% of yield) in the direct photolysis of 1 in the presence of diazomethane [70] (Scheme 2.9).

Scheme 2.9

The authors reported the NMR spectrum to be compatible with the proposed structure. However, in view of high strain in this molecule, which was estimated to be 456 kcal mol–1 [71], the claim that they obtained 30 is extremely surprising.

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2.1.4 New Hypothetical Molecules with Inverted Carbon Atoms

On the basis of simple model arguments an infinite family of [k.1.1]propellanes was found to possess inverted carbon atoms independent of the k value [36a]. This is not surprising in view of the bicyclobutane structure. Dodziuk and coworkers [36, 72], on the basis of molecular mechanics [36a] and ab initio quantum chemical calculations [72], proposed small ring geminanes 31–33 as another group of molecules that should have inverted carbon atoms.

The latter molecules have not yet been obtained but they seem to be plausible synthetic targets and challenge for the synthetic chemist.

Acknowledgments

I am sincerely grateful to my late husband Professor Zdenko Majerski who triggered my interest and love for chemistry of the strained ring compounds. I gratefully acknowledge financial support of the Ministry of Science, Education and Sport of the Republic of Croatia (grant 098-0982933-2911).

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2.2 Molecules with Planar and Pyramidal Carbon Atoms

Helena Dodziuk About 90 years after its formulation, van’t Hoff and Le Bel’s hypothesis [73, 74] on the tetrahedral orientation of substituents on a tetravalent carbon atom has proven its validity and predictive power. In particular, it revealed the molecular foundations of chirality and the number of isomers of cyclohexane derivatives. The idea was commonly accepted in the 1960s. Therefore, it took the great intellectual courage of Roald Hoffmann and his collaborators to propose ‘planar methane’, that is a tetravalent carbon atom lying in a plane with its four substituents [75, 76] as a part of an plausible organic structure. On the basis of very simple model quantum chemical calculations, QC, the authors analyzed prospective hydrocarbons that could eventually have an atom exhibiting this configuration. Several reviews on the planar and pyramidal carbon atoms have been published [77–83]. Most studies in this area dealt with molecules in which heteroatoms took part in forcing the planar configuration on a carbon atom [81, 84, 85]. Since this monograph is devoted to hydrocarbons, the studies dealing with this mechanism of planarization will not be discussed here. Interestingly, QC have shown that the planar methane configuration does not correspond to a minimum on the potential energy surface [86, 87]. However, the latter observation did not hamper the development and the idea of the so called planar carbon turned out to be very fruitful resulting, in particular, in vigorous research in small ring fenestranes 34 [88, 89] and paddlanes 2 [90, 91] which were thought to have carbon atom(s) with the planar configuration. Concerning the former group of compounds, QC have shown that [4.4.4.4]fenestrane 34a (k = l = m = n = 1), also called windowpane, assumes a nonplanar arrangement of substituents on the central atom that is called half-planar (bisphenoidal) configuration [92]. This configuration was close to that determined by X-ray for the smallest fenestrane synthesized [4.4.4.5]fenestrane 34 (k = 2, l = m = n = 1) [93]. As concerns paddlanes 35 [89], using model calculations Wiberg showed that the smallest, [1.1.1.1]- and [2.2.2.2]-paddlanes 35 (k = l = m = n = 1 and 2, respectively), were too strained to exist [91] (a published synthesis of the former molecule briefly discussed in the former section seems unreliable [94]) while the smallest synthesized paddlane 36 (k = 10, l = m = n = 2] understandably had a close to tetrahedral configuration on the ternary carbon atoms [90].

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When analyzing the possibility of planar configuration at a tetravalent carbon atom Hoffmann group used extended Hückel, EHT, and CNDO/2 calculations [75, 76] to estimate the energy difference between the planar and tetrahedral configurations of methane

'E = Eplanar – Etetrahedral

(2.1)

equal to 5.5 eV and 10.8 eV, respectively, and discussed two ways in which the value can be lowered. The former configuration could be forced either by destabilizing the tetrahedral arrangement (for instance by steric hindrance in the system as in 34 with k, l, m, n smaller than 3) or by stabilizing the planar configuration on the carbon atom. With this purpose in mind he discussed the bonding in square planar methane in the valence bond framework. Within this model two sp2 hybrids at the carbon are engaged in normal two-electron two-center bonds with two hydrogen atoms making use of two out of the four carbon valence electrons. The third of the sp2 hybrids forms a two-electron three-center bond with the remaining two hydrogens using only the hydrogen electrons, while the remaining two valence electrons of carbon are placed in the 2p orbital perpendicular to the molecular plane. Equivalence of all CH bonds results from resonance among equivalent structures with different positioning of three-center and two-center CH bonds. Such a procedure or analysis of the planar methane orbitals lead to the following conclusions: 1. All CH bonds in planar methane are weaker than in tetrahedral one with the average bond order of 3/4 in the former case. In terms of MO, there are only six bonding electrons in the planar configuration while there are eight in the tetrahedral one leading to a significant V bond weakening. 2. There is a considerable electron transfer from hydrogens to carbon locating considerable electron density on the latter atom. 3. There is a pure 2p electron lone pair perpendicular to molecular plane. 4. Two deformations of the tetrahedral methane (Td o D2 o D4h and Td o D2d o D4h) which represent a symmetry allowed process can transform it to the planar one. Conclusions 2 and 3 formed the basis of the strategy that should lead to a carbon atom lying in a plane with its four substituents. The substitution of hydrogen atoms by good electron acceptors, like C{N, should cause delocalization of the lone pair reducing 'E value (EHT method) to 3.4 eV. An alternative procedure consisted in

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incorporation of the lone pair to form (4n + 2) S-electron system as in the V-cation of an aromatic anion 37 or benzonium ion 38 resulting in 'E (EHT) values of 4.2 and 2.9 eV, respectively. Substitution of hydrogen atoms by less electronegative groups yielded further 'E lowering to 1.8 eV in C(BH2)4 and 2.9 eV for C(SiH3)4 when 3d orbitals on Si were included. In the latter case Si simultaneously acted as a V-donor and S-acceptor. Both factors acting in the direction of favoring the planar configuration at the central carbon operate in hypothetical tetrasila[5.5.5.5] fenestrane 39. The effect of electronegativity differential should be even stronger for Li substitution or replacement of carbon by N+.

Purely qualitative criteria were invoked to reject unsaturated fenestranes 40–42 as candidates for an effective S-stabilization while 43–45 were found to be especially promising.

As mentioned before, vigorous activity in the domains of fenestranes and paddlanes did not produce hydrocarbons having either pyramidal or planar carbon atom. Even if those attempts were unsuccessful, nevertheless they show that a incorrect prediction can lead to exciting chemistry. By playing with molecular Dreiding models Dodziuk has found that cyclooctane in the crown conformation could accommodate a carbon atom in the planar configuration. The resulting molecule was dubbed bowlane [95]. Subsequent molecular mechanics, MM, [96, 97] calculations [95] yielded a structure 46 of C4v symmetry with the pyramidal carbon atom. A whole family of such molecules

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47

46–51 was then studied using MM and semiempirical quantum chemical calculations [98] (yielding intermediate configurations between the distorted pyramidal and tetrahedral ones) and together with two ‘dimers’ 52 and 53. The former was expected to have the planar configuration on the central carbon atom since the upper and lower ‘halves’ were thought to force the desired bonds configuration. However, these expectations were not fulfilled, resulting in C4v symmetry and both 52 and 53 were found to be highly strained.

On the other hand, ab initio quantum chemical calculations for the parent 46 using STO-3G and 6-31G* basis sets carried out by McGrath et al. [99] yielded a structure of a lower C2v symmetry as an energy minimum with the bonds connecting the apical carbon atom with its neighbors departing only slightly from the planar arrangement. Similarly, HF/6-31G* calculations for 52, named octaplane, carried out by the latter group [100] showed that the S4 structure corresponding to a local minimum represents ‘the closest approach to planarity for a tetracoordinated carbon atom in a neutral saturated hydrocarbon reported today’. The most interesting features of the calculated minimum structure of 52 were considerable bond length distortions (up to 161.5 pm), moderate energy cost for achieving planarity of ca. 70 kJ mol–1 calculated at MP2/6-31G*//HF/6-31G* level and the fact that the highest occupied molecular orbital is essentially a pure lone pair orbital localized on the quaternary carbon atom. By capping the central fragment with two cyclobutanes 55, cyclohexanes in the boat conformation 56, or cyclooctane

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in the crown conformation 52 (that is earlier proposed bowlane ‘dimer’ [98]) the authors arrived at the family of alkaplanes [100, 101].

The aim of designing theoretically a neutral hydrocarbon with a planar tetracoordinated carbon atom was achieved by Radom and Rasmussen [101] when dimethanospiro[2.2]octaplane 54 was obtained by linking two opposite pairs of adjacent carbon atoms D to the central one and adding methylene bridges connecting the top and bottom caps. The calculations at the MP2 level using 6-31G(d) basis set resulted in D2h symmetrical minimum structure having the planar configuration on the central carbon atom with the longest C–C bond of 159.1 pm and the shortest one in cyclopropane rings of 144.5 pm. By preliminary exploration of a number of decomposition pathways corresponding to the lowest vibrational modes, the authors checked that, in spite of high degree of strain, the structure found for 54 appeared to lie in a relatively deep potential well. Similarly to 46, the HOMO in 54 has predominantly p-type lone pair character localized at the central quaternary carbon atom and surrounded by the hydrocarbon cage. Interestingly, the latter molecule has an extremely low ionization energy of ca. 5 eV comparable to those of alkali metals Li and Na. Bowlane 46 also formed the basis of a hypothetical hemispiroalkaplanes family 57–59 consisting, among other of cyclohexane in the boat conformation, norbornane or cyclooctane in the crown conformation fragments capped with bent spiro[3.3]pentane unit [102]which also contained a pyramidal carbon atom.

To summarize, up to the present theoretical studies of molecules that should have planar carbon atom(s) have been carried out mostly at the HF level using

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rather small basis sets. The MP2 calculations have been carried out at best. It should be stressed that DFT calculations seem not to be fully reliable, in particular when all the rearrangement/dissociation pathways are analyzed. Until now no neutral hydrocarbon with a planar carbon atom has been synthesized. Hoffmann’s bright idea of molecules having such unusual spatial structure still awaits its realization.

2.3 A Theoretical Approach to the Study and Design of Prismane Systems

Tatyana N. Gribanova, Vladimir I. Minkin and Ruslan M. Minyaev 2.3.1 Introduction

The aesthetically attractive highly symmetrical molecular structure and unusual properties of prismanes ensure that they continue to draw the attention of both experimentalists and theoreticians [103–105]. The starting point for the extensive studies of these compounds was the synthesis of cubane [106, 107] which initiated the progress of a specific area of these highly strained, albeit kinetically stable compounds [107–111]. In contrast with cubane, the experimental study of two other currently synthesized members of this class, triprismane and pentaprismane, is much scarcer. Theoretical investigations of prismane systems come to the fore more and more actively. Whereas in the first years of the development of prismane chemistry the theoretical work was mostly limited to the analysis of geometries and steric strain of the synthesized compounds [104, 109], the modern quantum chemical calculations of prismanes based on more sophisticated and reliable methods, and more powerful computers are focused on the design of new compounds with interesting properties. This review concentrates on the most important and interesting results of the computational chemistry of prismane systems. In some cases the calculations performed served as essential additions to the experimental data and their explanation. In other cases, they led to the formulation of new experiments and opened the way to new structural motifs. Special attention is given to the studies related to the theoretical design of new hypothetical prismanes. 2.3.2 Prismanes

The molecules of C2nH2n prismanes, formed by two parallel regular n-gons connected by rectangular faces, are highly strained cage systems in which the bond angles differ considerably from the tetrahedral angle of sp3-hybridized carbon. The first three members of the prismane series have so far been synthesized: triprismane 60 [112], cubane 61 [106, 107] and pentaprismane 62 [113]. Attempts

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to synthesize hexaprismane 63 have not yet been successful although certain progress in this area has been reported [104, 114]. At the same time, according to ab initio [115], DFT [116] and molecular mechanics [117] calculations, hexaprismane is one of the least stable members of C12H12 family with low probability of being synthesized [117]. Prismane systems provoked the interest of researchers by their aesthetically attractive highly symmetrical structure and extraordinary kinetic stability. The structural and electronic properties of cubane were investigated using various experimental techniques and quantum chemical calculations and the results achieved in both directions have been reviewed [103–105, 108–111]. A novel focus of the recent work is the insight into structural and dynamic properties of solid cubane [118]. The structural parameters of substituted triprismanes and substituted pentaprismane were determined by X-ray crystallography and for the parent hydrocarbons with help of quantum chemical calculations (see [119–122] and references cited therein). There is a good agreement between the calculated and experimental results (Figure 2.3). The higher prismanes remain a subject of only theoretical study [119, 122, 127, 128]. The primary challenges are elucidation of the stability limits of the prismatic structures with Dnh-symmetry and the trends in the geometry and energy characteristics with increase in n. According to higher level B3LYP/6-311G(2df,p) computational results [122], the highest member of the [n]prismane family is represented by decaprismane C20H20. The principal components of strain energy of prismanes are angular strain in the n-membered cycles and in the four-membered faces. The calculated SE of cubane (Figure 2.3) is close to the experimental value [129] of 157 kcal mol–1 (see, however, discussion in Section 1.3). Pentaprismane 62 is characterized by the lowest strain energy and heat of formation ('Hf) and is the most stable molecule within the family [105, 119, 122, 128]. For the other members of the series, an increase in n is accompanied by an increase in the SE and 'Hf values and correlates with an increase in the number of strained tetragonal faces. Destabilization of the higher prismanes is caused by influence of the lateral four-membered cycles as well as by the eclipse of vicinal hydrogens [119, 122]. A significant contribution to the strain energy of prismanes is their V-antiaromaticity [130]. Analysis of V-aromaticity of 60–64 by NICS indices shows that cubane 61 may be considered to be a ‘super V-antiaromatic’ system [130]. The high strain of prismanes determines their low thermodynamic stability: all members of the family are strongly destabilized relative to other isomeric forms. The main reason for the high kinetic stability of prismanes is that their rearrangement and decomposition reactions are forbidden by the rules of conservation of orbital symmetry. Stability of the Dnh-symmetrical prismane structures is determined by the stabilized orbirtal SV–SV interactions between the parallel n-membered fragments [131]. The CH bonds in prismanes are shortened because the carbon AO has a high percent of s-character; this is caused by rehybridization of the sterically strained carbon center [113]. The increase in ‘n’ leads to weakening of the CH bonds.

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Figure 2.3 Structural characteristics and strain energies (SE) of prismanes calculated [122] by the B3LYP/6-311G(2df,p) method. The experimental data obtained by X-ray crystallography (italic) for the derivatives of 60 [123] and 62 [124] and by electron diffraction (italic) [125] and microwave spectroscopy (bold italic) [126] for 61. Here and in other figures bond lengths are given in angström, angles in degrees, strain energies in kcal mol–1.

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Figure 2.4 Calculated [136] and determined experimentally [134, 138] structural characteristics of nitrocubane 68 and octanitrocubane 69.

According to quantum-chemical calculations [132, 133], substitution of hydrogen atoms in prismanes by substituents possessing V-donor and S-acceptor properties (Li, BeH, BH2) provide, as a rule, for the lowering of the strain energy. The S-donating substituents (such as NH2) exert a similar effect. As discussed in Section 1.2 nitrocubanes are of great interest as high-energy systems suitable for the use as explosives and fuels [108]. Many nitrocubanes including hepta- and octanitroderivatives have been synthesized (see [134]). The important factor governing the structure and relative stability of nitrocubanes is the repulsive interaction of the nitro-groups, which resuls in a decrease in stability upon increase in the degree of replacement [135, 136]. According to RHF and B3LYP calculations [135], the highest nitrocubanes are characterized by highest values of SE and 'Hf . The calculations of octanitrocubane lead to the conclusion on very low rotation barrier (0.006 kcal mol–1) of nitro groups, which can be described as cooperative disrotatory motion of two subgroups located in the apexes of the carbon tetrahedrons [136, 137]. The calculated structural parameters of nitrocubanes agree well with those determined by an X-ray study (Figure 2.4). 2.3.3 Expanded Prismanes 2.3.3.1

Asteranes

The steric strain of prismanes can be lowered considerably by expanding their cages by introducing additional atomic groups. Examples of such expanded prismanes are asterane molecules formed by inclusion of CH2-groups in the lateral edges of prismanes.

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Only [3]-asterane 70 [139] and [3]-asterane 71 [140] have been synthesized. Also bi[4]asterane representing the class of fused compounds (see Section 5.2) has been preparatively isolated [141]. Like prismanes, asteranes are characterized by high kinetic stability: for example, 4-asterane is stable at temperature higher than 300 °C [141]. According to the B3LYP/6-311G(2df,p) calculations [122], only asterane with n = 3–7 are characterized by stable structure of Dnh-symmetry (Figure 2.5). The calculated geometric parameters of 70 agree well with the data obtained by the electron diffraction study [142]. The calculated SE values of 70–74 (Figure 2.5) are notably lower than those for the prismane analogs 60–64. Changes of the strain energies in the asterane family are similar to the trends which have been found for the prismane systems. In contrast to the prismanes which experience the destabilizing influence of the sterically strained lateral four-membered cycles, the main factor destabilizing higher asteranes is repulsive interaction between hydrogen atoms of the adjacent methylene groups [122,143].

Figure 2.5 Structural parameters and strain energies of asteranes 70–74 calculated [122] by the B3LYP/6-311G(2df, p) method. Experimental data for 70 (italic) taken from Ref. [142].

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The dehydrogenated asterane frameworks may be used for the formation of hypothetical hydrocarbon cages like 75, 76 containing hypercoordinated centers [144–146]. The stability of such systems is governed by 8e rule [131]. The regulation of charge may occur by variation of basal and bridged atoms and allow the design of various non-classical structures [145, 146].

2.3.3.2

Ethynyl-expanded Prismanes

Another interesting type of expanded prismane is ethynyl (77, 78) and diethynylexpanded prismanes (79, 80) of which only a derivative of 80 has been synthesized [147]. These molecules fall into the category of nonlinear acetylenes discussed in detail in Chapter 7. Unlike cubane, synthesized diethynyl-expanded cubane is very unstable. Preparative yield of this compound is extremely low, which made measurement of its physical properties and calorimetric study difficult or even impracticable. However, many important characteristics of the expanded prismanes have been obtained by quantum chemical calculations [148–150].

According to DFT and MP2 calculations [148, 150] of 77 and 78, the inclusion of ethynyl groups into the C–C bonds of triprismane and cubane provides for expansion of the valence angles at tetravalent carbon atoms, which values are close to that of a tetrahedral angle. The flexibility of the diine fragments leads to the formation of the structures with bent bonds. The strain energies of 77 and 78, calculated at the MP2/6-31+G* level, are 58.5 and 33.3 kcal mol–1, respectively, and are much lower than those of the parent hydrocarbons. Another interesting feature of the expanded systems is the increase in their acidic properties compared with parent prismanes; this is caused by the presence of triple bonds enhancing the acidity of adjacent protons. The further expansion of the prismane framework (systems 79, 80) results in lowering of free energies of the deprotonation and steric strain. The structural distortion of the diethynyl fragments leads to a significant rise of their reactivity and can be considered as one of the factors of the kinetic instability of preparatively accessible diethynyl-expanded cubane [149]. Expanded

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prismanes offer a great opportunity to create systems with specific absorption and interesting optoelectronic properties [148–150]. In contrast to prismanes, whose small size and high steric strain make the formation of endohedral complexes difficult [151], the inner cavity of the expanded prismanes allows them to form complexes with alkaline and alkaline-earth metals and even with small neutral molecules [148–150]. 2.3.4 Dehydroprismanes

An interesting type of extraordinary cage compounds, whose structure is based on prismanes is prismenes – the compounds of highly pyramidalized olefin class. The possibility of the existence of 1,2–dehydrocubane (cubene) 81 was discussed on the basis of ab initio calculations [152, 153] which showed that, in spite of the presence of the highly pyramidalized double bond, preparative isolation of this compound is possible. This conclusion was corroborated by the experimental detection of cubene [154].

The calculated [153] characteristics of 81 (olefin strain energy (OSE) = 58.9 kcal mol–1; heat of hydrogenation = 82.5 kcal mol–1) are in a good agreement with experimental [155] data (OSE = 63±4 kcal mol–1; heat of hydrogenation = 90±4 kcal mol–1). The presence of the strongly distorted double bond in 81 leads to the increase in the strain energy (227±4 kcal mol–1) compared with cubane. The calculations indicate that there is sufficient overlap of the p-orbitals to consider cubene as an olefin and not as a biradical. According to experimental data, exceptionally reactive cubene is capable of reactions typical of olefins [156]. In agreement with expectations, ab initio and DFT calculations [157, 158] testify for the highest stability of ortho-cubene 81 compared with meta- and para-isomers, 82 and 83. In all three cases the ground singlet states are well separated from the corresponding lowest lying triplet states. Like 81, 83 was generated upon treatment of dihalocubanes with organolithium compounds [159, 160] and it has been suggested that 82 might be preparable by the same type of reaction [157]. According to theoretical and experimental data, 83 does not contain a diagonal C–C bond, the most probable state of 83 is a singlet biradical [157, 159, 160]. The highly pyramidalized and even more strained dehydrotriprismane (triprismene) was studied by ab initio methods [161]. Of two possible triprismene isomers the most stable is the structure 84, which can be considered as an olefin. A tentshaped isomer 85 is slightly destabilized relative to 84. The calculations indicate kinetic stability of triprismene and the possibility of its experimental detection. However, attempts [162] at such detection have so far been unsuccessful.

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2.3.5 Polyprismanes 2.3.5.1

Cubane Oligomers

An essential property of cubane derivatives is their tendency to form multidimensional structures [163–175]. The series of polycubanes have been obtained by nucleophilic addition to 1,2-dehydrocubane and 1,4-dehydrocubane [164–166]. The simplest system of such type is cubylcubane 86 (Figure 2.6). According to experimental and theoretical data [165], a remarkable structural feature of 86 is the unusually short C–C bond linking two cubane fragments; such systems are discussed later in Section 2.6. The high s-character of the exocyclic orbitals of cubane leads to the formation of more compact bonds compared with those formed by sp3-hybridized carbon atoms. The p-[n]cubyl oligomers are sufficiently rigid rod-type systems (analogous to staffands presented in Section 2.1.3.3) retaining geometrical parameters of the cubane fragments since stretching or angle distortions of the intercage bonds require significant energy costs [164]. According to PWSCF [168] and B3LYP/6-311G** [169] calculations on the polycubane one-dimensional chain and two-dimensional network, such properties as ionization potential and electron affinity are only slightly dependent on the number of building blocks involved. Cubane chains maintain the electronic properties of insulator; at the same time they have very interesting elastic qualities and are of interest in the development of electronic nanodevices [168]. As shown by DFT/PW91 calculations, inclusion of electron-enriched functional groups (NO, NS) in cubane chains favors electronic conduction [170]. An example of a three-dimensional polymeric cubane derivative is supercubane (C8)n, proposed as an allotrope of carbon [171–175]. The simplest non-periodic supercubane model is percubylcubane 87 (Figure 2.6) which was studied by

Figure 2.6 Calculated structural parameters of cubylcubane 86 and percubylcubane 87. Experimental data for 86 taken from Ref. [165].

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HF/STO-3G [174] and PM5 [175] calculations. New structures termed cubanoids were computationally designed on the basis of percubylcubane [175]. 2.3.5.2

Fused Prismanes

The formation of fused systems joined at n-membered faces serves as another way for the formation of polymeric prismane compounds, which may be termed poly[n]prismanes [176–179] (such as bi[n]prismanes 88–91). The unusual structural feature of poly[n]prismanes is the presence of carbon centers with nonclassic bisphenoidal configuration of bonds.

Bi[n] and tri[n]prismanes (n = 3–6) have been studied using DFT calculations [177]. Although poly[n]prismanes are strongly destabilized with respect to their ‘classic’ isomeric forms, calculations indicate their kinetic stability and possible experimental detection. The origin of their relatively high stability is to be found in the strong SV–SV orbital interaction between the symmetry adapted frontier MOs of the external annulene (CH)n and the inner Cn rings. Increase in the number of fused cages leads to increase in strain compared with prismanes. Maximal steric strain is observed for the first members (n = 3, 4) of each family and for each n the strain increases with increase in the size of the structure. The decrease in stability of poly[n]prismanes on passing from prismanes to their bi, and tri-congeners also manifests itself in progressive decrease of the energy gap between frontier orbitals along this set of compounds. The increase in the number of fused cages is accompanied by the further lowering of the stability [178]. The interesting characteristic of poly[n]prismanes is their auxetic property, i.e. the ability of these molecules to become thicker by longitudinally stretching and thinner by compression. According to quantum-chemical calculations, poly[n]-prismanes are the first systems that manifest auxetic behavior at the molecular level [179]. ‘Mixed’ polycage prismane derivatives 92–94 containing pentaprismane and dodecahedron cages fused by five-membered faces were proposed on the basis

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of B3LYP/6-31G* calculations [180–182]. Increase in the number of fused cages leads to increase in strain and decrease in stability of these systems compared with pentaprismane and dodecahedrane. The possible experimental preparation of fused prismanes is supported by successful synthesis of bi[4]asterane 95 and its substituted derivatives [141, 183]. In contrast with [4]asterane 71 which is stable at temperatures over 300 °C, the transformation of 95 occurs at a lower temperature (270 °C) [141] confirming the theoretical conclusions about the decrease of stability of fused systems upon increase in the number of building blocks. It is also possible to form fused polyprismane structures by fusing fourmembered faces of the cage systems. In contrast with cape-fused prismanes containing nonclassical bisphenoidal carbon centers, the face-fused prismanes are characterized by the presence of carbon atoms with nonclassic inverted (umbrella) configuration of bonds. Such molecules called propellanes are discussed in detail in Section 2.1. The existence of bipentaprismane 96 was predicted using ab initio and DFT calculations [184, 185]. Even more unusual example of face-fused poly[5] prismanes 97 is a hypothetical product of multiple pentaprismane splicing, in which the [2+2] cycloreversions of cyclobutane rings, leading to the series of isomers with pairs of parallel S-bonds interconverting by Cope rearrangement may be realized [186]. By analogy with ‘mixed’ cape-fused polyprismanes the ‘mixed’ face-fused systems can be formed. The stable system 98 investigated [187] by B3LYP/6-31G* method is derived from cubane and two pentaprismane cages fused by two four-membered faces. Based on the results of the computational studies of the fused prismanes, one can conclude that an increase in the number of fused cages is accompanied by increase in steric strain, decrease in lowest vibration frequencies and HOMO-LUMO energy gap. All these effects lead to lowering in the stability of designed complicated systems.

2.3.6 Conclusions

The comparison of theoretical and experimental data indicates that theoretical methods can be a reliable tool for comprehensive investigation and design of various molecules constructed on the basis of prismanes. Prismane systems, which in themselves are unique molecules with nonstandard stereochemistry and properties, can form series of novel unusual systems such as dehydroprismanes representing a class of pyramidalized olefins; supercubane as an allotrope of carbon; fused prismanes, including nonclassical bisphenoidal and inverted carbon

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centers, as well as asterane derivatives, containing hypercoordinated centers, etc. Various modifications of prismanes lead to the appearance of new intriguing properties. Poly[n]prismanes are expected to show auxetic behavior; ethynyl-expanded prismanes can reveal useful optoelectronic characteristics; nitro-prismanes are good candidates for powerful, high-density explosives; prismanes networks are of interest in the development of nanoarchitectures. The variety of the systems generated on the basis of prismanes is not exhausted (for example, an extensive class of heteroprismanes [128]) and requires further studies, which open the way to create new materials with unusual properties. Although the majority of reported studies correspond to cubane derivatives, available theoretical data reveal similar tendencies in behavior and properties of other members of the family. The principles of formation of the lowest prismane derivatives can be extrapolated to the area of more complex compounds. Synthesis of the molecules considered in this chapter seems to be a great challenge for chemists and the success in the field of preparation of numerous cubane derivatives (in particular, of octanitrocubane [138]) is an impressive manifestation of progress and rich opportunities of modern experimental techniques. The data presented show that many hypothetical systems can be suitable objects for the synthetic quest and we believe that results of quantum-chemical research help to invigorate new experimental efforts in this field.

Acknowledgments

This work was supported by Russian Foundation for Basic Research (grant 07-03-00223), RF Ministry of Industry and Science (grant 4849.2006.3).

2.4 (CH)2n Cage Structures, ‘in’-‘out’ Isomerism in Perhydrogenated Fullerenes and Planar Cyclohexane Rings

Helena Dodziuk 2.4.1 (CH)2n Cage Structures

Studies pertaining to saturated cage compounds of a general formula (CH2)n can be traced back to the ancient Greeks since the ideal polyhedra: the tetrahedron, the cube and the dodecahedron, which represent the carbon skeletons of the molecules with n = 2, 4 and 10, were discussed by Plato and Pythagoras [188]. The molecules are formed by 2n carbon atoms each of which is connected to three carbon and one hydrogen atoms. For n = 2 and 3 there is only one possible isomer: tetrahedrane 99a and triprismane 60, respectively, while for larger n the number of isomers increases. Cubane 61 (discussed in detail in Section 2.3), cuneane 100 and octabis-

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valene 101 are the three isomers for n = 4 and the number of possible structures increases rapidly with the increase of n; for instance, it is equal to 9 for n = 5, 32 for n = 6, etc. Of the higher members of the series only pentaprismane 62, and diademane 102 with n = 5, heptacyclo[6.4.0.02,4.03,7.05,12.06,10.09,11]dodecane 103 (n = 6), dodecahedrane 104 (n = 10) and a derivative of perhydrogenated fullerene C60X60 105 (n = 30, X = F) have been synthesized. These molecules are highly strained hydrocarbons exhibiting, among other, nontypical rearrangement [189, 190] (Figure 2.7) and cyclization reactions [191]. The rigid structure of saturated cage compounds means that they can serve as ideal models in studies both of the angular dependence of the coupling constants, and of dependence of J(13C–H) on the hybridization (see Section 2.6).

Figure 2.7 Untypical rearrangement reactions of cubane into cuneane (a) and pagodane into dodecahedrane (b).

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Tetrahedrane

The elegant molecule tetrahedrane, 99a C4H4 (n = 2), has attracted the attention of synthetic and theoretical chemists for almost 100 years [192, 193] and the first review on this molecule [194] was published almost at the same time as the synthesis of its tetra-t-butyl derivative 99b [195]. Then for a long time, the latter molecule was the only tetrahedrane derivative known [196]. A tetrasiladerivative 99c and other siladerivatives with much smaller substituents 106 (X = Li, CH3, H, C(SiMe3)3) as well as tetrahedryltetrahedrane 107 (X = SiMe3) [197] followed. Syntheses of these molecules have recently been reviewed by de Meijere and coworkers [198]. Physicochemical studies of 99b included X-ray [199], spectral [200] and NMR [201] measurements. Its high kinetic stability was partly ascribed by Minkin [202] to unfavorable steric repulsions in its rearrangement product 108b.

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Other theoretical studies ranged from molecular mechanics, MM, [203] prediction of T symmetry of 99b [204] to high level quantum chemical, QC, calculations indicating that 99a should be isolable [205]. However, unlike cyclobutadiene 108a [206], discussed in Chapter 10, unsubstituted tetrahedrane remains unknown. The NMR coupling constants in 99a, 60, 61, 62, 102 and hexaprismane 63 were calculated by Krivdin [207], while of several calculations analyzing reasons which cause shortening of the linking C–C bond in tetrahedranyltetrahedrane 107 (Y = H), recent work by Mo should be mentioned [208].

2.4.1.2

Triprismane

Triprismane 60 C6H6 (n = 3) [209], also called [3]prismane, is discussed in Section 2.3 together with other prismanes. 2.4.1.3

Cubane 61, Cuneane 100 and Octabisvalene 101 C8H8

Cubane 61 is discussed mainly from theoretical point of view in Section 2.3, so here only few aspects of its structure and properties will be covered. This highly strained molecule is remarkably stable since it corresponds to a very deep narrow minimum on the potential energy surface. Its chemistry, physicochemical properties and theoretical studies have been reviewed by Griffin and Marchand [210] and by Hassenrück and coworkers [211]. Its high symmetry allowed the Hedberg group [212] to compare the accuracy of electron diffraction [213], microwave [214] and X-ray [215] determinations of C–C bond length. The unusual skeletal rearrangement of 61 into 100 (Figure 2.7) was mentioned above. An interesting model of cubane hydrogenolysis was discussed by Stober et al. [216]. Several interesting cubane derivatives are discussed in Section 2.3. They include known propella[34]prismane 109 [217] and cubene 81 [218] (which lies outside the scope of this chapter but was presented in the former section). The trimethylene bridges in the former molecule are very flexible leading to only three signals in 13C NMR spectrum [217].

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Cuneane 100 and octabisvalene 101 have been much less studied [189, 211, 219, 220] and the existence of inverted carbon atoms (discussed in Section 2.1) in the latter molecule, which can be considered as a bicyclobutane dimer, has been mostly overlooked [219]. The X-ray structures of octamethylcubane and octamethylcuneane have been determined by Irngartinger [221]. A large group recently carried out a calorimetric, crystallographic and computational study of 61, 100 and their carboxylates [222]. 2.4.1.4

C10H10 Saturated Cages

Pentaprismane 62 [223] and diademane 102 [224, 225] are the only known members of this family. For the former molecule, only photoelectron spectra [226] and X-ray analysis [227] are known. For the latter, several centrally bridgehead-substituted derivatives have been obtained recently [228]. According to semiempirical [229] and ab initio [230] calculations, the cyclohexane ring in 102 should be planar, as are such rings in other molecules having the cis-tris-V-homobenzene fragment 110 presented later in this chapter. Chemistry of 102 and some of its derivatives and heteroanalogs has recently been discussed by de Meijere and coworkers [198].

2.4.1.5

C12H12 Saturated Cages

The highly symmetrical hexaprismane 63 and truncated tetrahedrane 111 have been long pursued but, contrary to the common practice not to publish unsuccessful attempts, only failed syntheses of them or larger such molecules have been reported [231–234]. The synthesis of a derivative of heptacyclo[6.4.0.02,4.03,7.05,12.06,10.09,11] dodecane 103 went unnoticed [235] as there was a considerable puckering of the central cyclohexane ring in the parent compound when it was synthesized by de Meijere’s group [236]. A more detailed study of the reactivity of 103 was recently published by the same group [237] and its chemistry reviewed [198]. Several members of this family have been analyzed using MM [203] model calculations yielding 103 as the most stable and hexaprismane 63 as the least stable molecules among those studied. Both the MM study and QC calculations [238] for 111 yielded a highly symmetrical Td structure and close values of heat of formation (ca 91 kcal mol–1 by QC [239] and ca 87 kcal mol–1 by the MM method [240]).

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According to MM modeling [240] (in the QC studies symmetry constraints were imposed), 63 and 111 are characterized by planar C6 rings [240] while the central cyclohexane ring in 103 was found to be considerably more puckered than in the standard cyclohexane structure, first on the basis of modeling [219, 240] and then experimentally [237]. Of several theoretical and experimental studies discussing preparation of hexaprismanes, papers from the Mehta group aiming at the synthesis of [6]- and [7]prismanes [241–243] and the paper by Chou [244] should be mentioned. In spite of this massive effort, obtaining hexaprismane 63 remains a great unsolved problem in synthetic organic chemistry [245]. Maybe reactions in molecular flasks, as presented in Chapter 10, will enable us to obtain 63. 2.4.1.6

Higher [n]Prismanes, Dodecahedrane

Hypothetical higher [n]prismanes were discussed in Section 2.3. Of still higher prismanes, israelane 112 and helvetane 113 can be mentioned. Following a humorous idea of Eschenmoser, these C24H24 molecules were discussed by Ginsburg in the 1st April issue of 1982 of the Nouveau Journal de Chimie [246]. The family to which they belong consists of numerous molecules and 112 and 113 would definitely be of very high energy, precluding their existence. Thus, some calculations of their structure and energy discussed in [247] seem insignificant.

It took the Paquette group almost 20 years to synthesize dodecahedrane C20H20 104 [248]. It was the first molecule of the Ih symmetry synthesized in a laboratory, contrary to the Herzberg prediction that such molecules would be unlikely to exist [249]. It is the only known member of the family under scrutiny. Dodecahedrane chemistry has been discussed in detail [250] but its physicochemical properties received little attention. 2.4.1.7

‘In’-‘out’ Isomerism in Perhydrogenated Fullerenes C60H60

As fullerenes are discussed at length in Chapter 5, only the exciting ‘in’-‘out’ isomerism in hypothetical fully hydrogenated fullerenes C60H60 (named fullerane) proposed by Saunders [251] will be considered here. On the basis of MM calculations, he found that of all possible ‘in’ and ‘out’ isomers, a nonsymmetrical one with 10 C–H bonds pointing inside should be the most stable. ‘In’-‘out’ isomerism in hydrogenated fullerenes has also been studied using the MM [252] and/or QC [252–254] methods. Two isomers with two C–H bonds pointing inside (other C–H bonds are omitted for the sake of clarity) are shown in Figure 2.8. Let us note that,

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Figure 2.8 Two geometrical isomers of C60H60 with two CH bonds pointing inside.

as shown in the latter figure, the carbon cage in the in-isomers undergoes considerable distortions and such isomers have a lower symmetry. It should be stressed that for higher number n of bonds pointing inside a real challenge in the calculations was to generate sufficiently large set of the ‘in’ structures to be sure that the one of the lowest energy for the given n represents the lowest energy structure. Dodziuk and Nowinski noticed that two isomers with a C–H bond pointing inside and outside the cage are topological isomers [255] since in order to come from one of them to another, one has to break C–C bonds of the C60 cage. Dodziuk also studied the ‘in’-‘out’isomerism in C60H60 using a different procedure for generation of isomers than that applied by Saunders [251] and different parameterization and found n = 10 CH bonds pointing inside the C60 cage for the lowest energy structure. The latter result agreed with that of Saunders, however, as shown on the right of Figure 2.9, the optimum structure for n = 10 was of high symmetry. Comparison of the results by Saunders and those by Dodziuk group shows the limitations of the applicability of the simple MM model and only the results that do not depend on the force field used are reliable. Dodziuk group has also shown that not only several C–H bonds pointing inside the cage lower its energy but there is sufficient space in the cage for a methyl, ethyl or n-propyl substituent inserted inside and these isomers are considerably more stable than their fully ‘out’ counterparts. For iso-propyl and n-butyl substituents the energy of ‘in’ and ‘out’ isomers assumed comparable values while for more bulky substituents the ‘all-out’ isomer was more stable. Moreover, two methyl groups situated on the opposite ends of the cage in 1,51-dimethyl fullerane (see Figure 2.10 for computer-generated atom numbering) were also calculated to be more stable than their isomers with one or both methyl groups pointing out [256]. Interestingly, not only was a methyl group ‘in’ more stable than that with the group ‘out’ but also its energy, independently of the force field applied, was very close to the energy of the C60H60 with one hydrogen ‘in’ [256]. Saunders [251] and Yoshida [252] groups claimed that one could obtain ‘in’ isomers of hydrogenated fullerenes by heating or inversion on a protonated carbon atom while Dodziuk group postulated that, in view of high barrier that must be overcome to move a hydrogen atom from an outside to an inside position, the aimed synthesis must be carried out to obtain the ‘in’ isomers.

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Figure 2.9 Two projections of the calculated structures of ‘all-out’ C60H60 (left) and the most stable ‘ten-in’ isomer (right). For clarity only carbon and ‘in’ hydrogen atoms are shown. The projections along a C5 symmetry axis are shown in the upper part while the lower ones are along one of the C6 axes.

Figure 2.10 Atom numbering in C60 determined by Rose (as cited in Kroto [258]).

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As discussed in Chapter 5, in the early days of fullerene chemistry numerous, sometimes peculiar, applications of this molecule were proposed [257]. The use of C60F60 as an ideal lubricant was one of them. The molecule was synthesized in all-‘out’ form but proved highly unstable when exposed to air, producing HF as a decomposition product [258]. The instability of C60F60 can be, at least partly, related to the high strain found in its hydrogenated analog [251, 259]. 2.4.1.8

Summary

As discussed in Chapter 1, it has become fashionable today to show practical applications of the molecules under scrutiny. Let us repeat once more, Eaton tried to obtain cubane because synthesis of this attractive highly symmetrical molecule was a great challenge. However, in agreement with the above-mentioned trend, two interesting applications will be mentioned here. Cage compounds discussed in this chapter are too small to host other molecules. However, the possibility that they could play the role of host for atomic hydrogen or ions has been explored theoretically [260] and the use of a cubane derivative has been proposed in a drug delivery system [261]. 2.4.2 Planar Cyclohexane Rings

Let us recognize that the statement ‘a cyclohexane ring can be planar’ can cost a student a failure of examination [247]. However, molecules 114–116 and some complexes of 117 with cis-fusion of C3 rings have been proven to exhibit such structures [262–265]. As discussed in the former chapter, diademane 102 [224, 225], hypothetical hexaprismane 63 and truncated tetrahedrane 111 are predicted to have planar rings [240]. Cyclophanes 118 and 119 synthesized by the de Meijere group [266, 267] should also possess the planar C6 ring. Similarly, according to model calculations a hypothetical hexahydrosuperphane 120 [268] should also have such a ring. As will be briefly discussed below, planarization can result either from fusion of at least three three-membered rings to the saturated sixmembered one [269] or it can be forced by the cage structure in which such a ring is built [240, 268]. As discussed in detail in Stereochemistry of Organic Compounds [270] the first notion of chair and boat conformations of the ring stemming from the tetrahedral arrangement of substituents on a tetravalent carbon atom were expressed by Sachse as early as in 1890 [271]. For long time physicochemical methods were too crude to enable the observation of nonplanarity of these conformations. Although several experimental results pointed to the chair shape of the six-membered ring, only a pioneering paper by Barton [272] brought a full comprehension of the physical and chemical consequences of such a structure. For this and subsequent studies he was awarded a Nobel Prize for Chemistry in 1969. The ring inversion leading to axial–equatorial equilibrium of substituents in monosubstituted cyclohexanes is a rapid process (of ca. 2 u 105 s–1) the observation of which depends on the timescale of experimental technique applied. For

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instance, at room temperature two bands corresponding to the stretching C–Br vibrations of bromocyclohexane are observed in the IR spectra [273] while only one, average signal can be seen in the corresponding spectra of the hydrogen attached to the bromine substituted carbon in the NMR spectrum. This signal is split into two at about –50 °C when the ring inversion is frozen [274, 275]. Thus, in many cases only a combination of different experimental techniques can give a real insight into the molecular structure. Cyclohexanes with flattened rings have been known for long time [276–278] but, as mentioned earlier, the existence of molecules with a planar saturated C6 ring is not fully recognized even today. As concerns hydrocarbons, according to Engelhardt and Lüttke [279] trans-tris-V-homobenzene 110a should have such a ring, although no sound argument in favor of the planarity has been given. On the other hand, a close to planar C6 ring in the hexamethyl derivative of 117 [280] substantiated this claim. A historical account of the studies of tris-V-homobenzene has been published by Rücker [281]. A planar cyclohexane ring was proposed for known [224, 225] diademane 102 on the basis of computer modeling [229, 230] and, as discussed in Section 2.3, for hypothetical [6]asterane 121 [203]. A planar central ring was also found in the X-ray study of the tribenzoderivative of tetracyclo[8.2.0.02,506,9]dodecane 122a [282] but on the basis of model calculations [269] discussed below, the planarity cannot be due to the effect of fusion of fourmembered rings to a cyclohexane ring. It should rather be ascribed to crystal forces, in particular those operating among the stacked aromatic rings. A recent determination of the nonplanarity of the central ring in 122b supports the latter conclusion. On the other hand, the central ring in 123 and 124 has been proven to be planar [283]. The influence of a cis fusion of with a smaller ring on the planarization of the C6 ring was systematically studied by Dodziuk [269] using the MM method by changing the number from 1 to 6 and size of the fused ring(s) 110, 125–136 (some of them, like 132, 134 and 136 purely hypothetical). A detailed comparison of the calculated geometry with the experimental one proved difficult since X-ray analysis is mainly carried out, often for a derivative of the molecule under

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69

scruting, in the solid state while the calculations refer to the isolated molecule. Moreover, the crystals sometimes contain solvent molecules influencing the results obtained for the molecule under investigation. However, the comparison of the calculated and available experimental data showed that the primitive MM model used in 1987 reproduced semiquantitatively torsional angles allowing one to analyze the planarity of the cyclohexane ring. The calculations led to the conclusion that fusion of smaller and/or more rings causes larger distortion of the C6 ring toward planarity. In consequence, cis- and trans- tris-V-homobenzenes 110 should have a planar central ring. A comparison of the results of calculations for 130 [269, 284], that is the saturated core of 122a and 122b, with the experimental data for the latter molecules indicate that, as mentioned earlier, the planarity of the central ring in the 122a is forced by intermolecular forces present in the crystalline state. Similarly, on the basis of the same type of modeling, planar rings should be present in 110a [269], 63, and 111 [240]. For 102, the same conclusion was drawn from semiempirical [229] and ab initio [230] QC. The planarity of the C6 ring built into a cyclophane cage in 120 was obtained on the basis of both ab initio and DFT calculations [268]. Interestingly, both factors forcing planarity are present in cyclophanes 118 and 119 synthesized by the de Meijere group [228]. Thus, the molecules should have a planar cyclohexane ring although no proof of this has been reported.

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2.5 Ultralong C–C Bonds

Takanori Suzuki, Takashi Takeda, Hidetoshi Kawai and Kenshu Fujiwara 2.5.1 Introduction

The bond length between the two sp3 carbons is one of the fundamental parameters in chemistry. However, there are cases [285] where the bond length d significantly exceeds the standard value of 1.54 Å [286], as in sterically congested molecules such as t-Bu3CH [1.611(5) Å] [287]. Furthermore, it has been experimentally verified that a small number of compounds possesses the ultralong C–C bond (d > 1.70 Å) [288–290]. This chapter describes how the Csp3–Csp3 bond can be elongated to such an extreme length and predicts a limit for the ultralong C–C bond. The most popular method to precisely determine the values of the length in crystalline materials is single-crystal X-ray analysis, although Raman [289] and 13C NMR spectra [291] were also found effective in some cases. Diffraction measurement is often carried out at low temperature to attain high accuracy by suppressing thermal motion of atoms in crystal. However, the X-ray methodology dose not always give the right answer. It has been pointed out [292] that some of the ultralong C–C bonds in the earlier reports [293] were obtained as the result of unintentional overestimation by crystallographic artifacts, caused by incorporation of the valence isomer with a large separation of atoms in the crystal (Scheme 2.10a). In another case, the length was determined to be much smaller

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Scheme 2.10

than the actual d value due to positional disorder [294] or internal motion in the crystal [295] (Scheme 2.10b). Despite the opposite outcome, the thermal ellipsoids for the atoms in question are mis-shaped to indicate the presence of anomalies in both cases. Another common feature is the large estimated standard deviation (esd) for the bond length and other structural parameters. To avoid being misled by these crystallographic artifacts, only X-ray structures with a suitable accuracy (esd for d < 0.01 Å) are considered in this chapter [296]. The representative ultralong bonds can be found in Toda’s naphthocyclobutene derivatives 137 [288] [1.712(5)–1.734(5) Å] and the benzannulated caged hydrocarbon 138 [1.713(2) Å] reported by Herges [289]. Before recent results by the authors were reported [290], diiodonaphthocyclobutene 137-I [1.734(5) Å] had been considered as the world record holder in terms of the length of a conventional C–C bond [297]. According to the linear correlation between bond dissociation energy (BDE) and d proposed by Zavitsas (d/Å = 1.748–0.002371 u BDE/kcal mol–1) [298], the experimentally determined values for 137 and 138 are just at the edge of a maximum bond length limit of 1.75 Å. In the next section, the pioneering work on these compounds is described to show how the syntheses of these elegant molecules with such a ultralong C–C bond were successfully realized.

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2.5.2 Ultralong C–C Bonds Confined in a Stiff Molecular Frame

Tricyclo[4.2.2.22,5]dodeca-1,3,5,7,9,11-hexaene 139 is the S-bond shift isomer of [0.0]paracyclophane 139c. This hydrocarbon is highly reactive because it has a strained molecular framework with the bridgehead unsaturated bonds [299]. Its tetrabenzannulated derivative 140 is much more stable than 139 [300], yet it still can undergo thermal [2+2] cycloaddition with benzyne. In this way, Herges and co-workers synthesized the caged hydrocarbon 138 [289], in which the ultralong bond in question faces the strongly pyramidalized double bond (Scheme 2.11). X-ray analysis at –130 °C revealed that 138 has one of the longest C–C bonds of 1.713(2) Å among hydrocarbons. The extremely large value of d is not related to a contribution from the bond-dissociated isomer 141 because the stiff molecular frame prevents symmetry-allowed conrotatory ring opening into the o-quinodimethane derivative. The presence of an ultralong C–C bond in 138 was also predicted theoretically (B3LYP/6-31G*), where the calculated value of d = 1.721 Å is comparable to the experimental value [289]. Even when the calculation started with the geometry of ring-opened isomer 141, the high-level optimization converged into the structure of ring-closed form 138, since the structure of 141 has been not found to be a stationary point at the DFT level.

Scheme 2.11

In the Raman spectrum of 138, the C–C stretching vibration was observed at a lower wavelength (687 cm–1) than the standard value (995 cm–1 for ethane). This observation is in accordance with the extraordinary bond expansion that causes considerable decrease in strength of the C–C bond. Its force constant (133.1 N m–1) was calculated as less than one-third of that for ethane (455.7 N m–1). Successful isolation and characterization of this caged hydrocarbon show that the stiff molecular framework effectively maintains the weakened ultralong C–C bond in 138.

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2.5.3 Tetraphenylnaphthocyclobutene as a Scaffold to Produce Ultralong C–C Bonds

Based on their own discovery that thermal cyclization of diallenes proceeds smoothly in the solid state [301], Toda, Tanaka, and co-workers prepared a variety of cyclobutene-fused aromatic compounds from 1,2-diallenylarenes [288d], some of which have an ultralong C–C bonds. Among them, 3,8-dichloro-1,1,2,2tetraphenylcyclobuta[b]naphthalene 137-Cl [1.720(4) Å at 25 °C] [288a] is the most well-studied species, whose structural analyses [1.710(2) Å at –183 °C] [288b] and high-level calculations [302] were also carried out by several research groups. All the results support the intrinsic nature of the ultralong bond of 137-Cl. The origin of the bond elongation is primarily explained by a classical steric argument without the need to resort to orbital interaction. The (S-V*)-type through-bond orbital interaction had previously been proposed as a plausible reason to account for the bond elongation [303], yet several recent experimental studies did not support its importance [304]. The through-bond orbital interaction [305] is mainly caused by (S-V*)-type filled–filled coupling [306], and perturbation through the (S-V*)-type coupling was proven to be negligible according to highlevel theoretical calculation [307]. Especially indicative that the (S-V*)-type coupling is unimportant are the values of d experimentally determined in a series of cyclobutaarenes [288c, 288d]. For example, the X-ray structural analyses were carried out on two isomeric compounds 142, in both of which the two t-butyl groups are substituted for the phenyls in 137-Cl. The d value in gem-di(t-butyl) derivative gem-142 [1.729(2) Å] is much larger than the corresponding value in trans-1,2-di(t-butyl) derivative trans-142 [1.686(5) Å], although the latter must be quite suitable for the (S-V*)-type coupling. The larger d in the former molecule is accounted for mainly by the more severe steric repulsion in gem-142 than in trans-142, which also induces outward deviation of bonding electron-density on the molecular plane around the carbon with the two t-Bu groups attached.

Compounds 137 and 138 with an ultralong C–C bond have a 1,1,2,2-tetraaryl benzocyclobutene skeleton in common, which can be regarded as a sort of ‘condensed’ derivative [308] of hexaphenylethane (HPE). The parent hydrocarbon 143 was shown to undergo easy interconversion into 7,7,8,8,-tetraphenylo-quinodimethane 144, which is too labile to be isolated, as it undergoes further

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Scheme 2.12

pericyclic reaction to dihydroanthracene (Scheme 2.12a) [309]. The successfully isolated dibenzo analog of o-quinodimethane 146 [310] exhibits no signs of conversion into the cyclobutene-type isomer 145 (Scheme 2.12b) probably because of the easy cleavage of the weakened bond in 145, if formed. Thus, the successful observation of ultralong C–C bond in 137 and 138 relies on the suppression of their ring-opening pericyclic reaction to o-quinodimethanes: its formation is disfavored by the delocalization energy loss for the 2,3-naphthoquinodimethane moiety in 137 and by the stiff molecular framework in 138. In the following sections, other members of HPE-type molecules are described which have a longer ‘ethane’ bond than the standard. 2.5.4 ‘Clumped’ Hexaphenylethane Derivatives with Elongated and Ultralong C–C Bonds

Since the pioneering work by Mislow [291, 311], HPEs with three bulky aryl groups at both ends of ethane bond have been used as excellent models to observe the elongated C–C bond because the steric repulsion between substituents is the most important and powerful factor to expand the C–C bond. One of the major obstacles in studying HPEs is the bond-dissociation equilibrium generating trityl radicals, which is also related to D,p-dimer formation or high reactivity toward oxygen [312].When the two trityls are connected by a suitable bridge, intramolecular bond-formation process giving D,D-dimer (HPE) becomes much more facile (Scheme 2.13a), thus endowing the ‘cross-clumped’ HPEs such as 147-H with high

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Scheme 2.13

stability despite the elongated C–C bond (calc. 1.64 Å) [311, 313]. Another type of clump (‘back-clump’), by which two trityl parts are not connected as in the case of 9,9c-diphenyl-9,9c-bifluorenyl, does not prevent cleavage of the weakened bond so effectively (Scheme 2.13b). Since BDEs decrease with the increase in C–C bond length [298], the bridging modification is essential to obtain thermodynamicallystabilized HPEs that can allow investigation on the weakened and easily-dissociated long C–C bond. ‘Cross-clumped’ HPE 147-H with the dihydrophenanthrene skeleton was prepared from the bis(triphenylmethanol) 148-H via the dianionic species (Scheme 2.14a) [313]. When the electron-donating substituents are attached to the aryl groups, acid treatment of diols 148-NMe2 and 148-OMe gave stable dications, [biphenyl-2,2c-diyl bis(diarylmethylium)s], which were transformed to HPEs 147-NMe2 and 147-OMe smoothly upon reduction [314]. The reductive generation of HPEs via the dicationic species has synthetic merit for generating highly congested HPEs 147 from the less-hindered precursors through short-step transformation. Dispiro-HPEs (147-spiro-N and 147-spiro-O) were prepared by this protocol (Scheme 2.14b), whose ‘ethane’ bond lengths are as long as 1.635(2) Å [314c] and 1.656(5) Å [314d], respectively. Not only the dihydrophenanthrene-type compounds, but also HPEs with an acenaphthene skeleton were available by this method. Thus, naphthalene-1,8diyl bis(diarylmethylium)s could be converted into 1,1,2,2-tetraarylacenaphthene derivatives 149 (Scheme 2.15) [315], whose ‘ethane’ bond is as long as 1.70 Å [1.701(3) Å for 149-H [315b] and 1.707(2), 1.705(2) Å for 149-F (two independent

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Scheme 2.14

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Scheme 2.15

molecules in crystal) [315c]. Torsional fixation in the five-membered ring is the key feature for producing the larger ‘front strain’ [311a] resulting in elongation of the C–C bonds in 149 compared with that in 147, since the latter can relieve the strain by adopting a chair-like conformation of the central six-membered ring. Although the estimated BDE for the long C–C bond of 1.70 Å is only 20 kcal mol–1, the acenaphthene-type HPEs 149 could be isolated with no signs of decomposition under ambient conditions. The remarkable stability of 149 can be rationalized by considering that the bond-dissociated species with the non-Kekulé structure (naphthalene-1,8-diyl), if formed, only undergo bond-reformation to give 149. This is in sharp contrast to the case of tetraarylbenzocyclobutene 143 [309], which is transformed into the dihydroanthracene derivative via the bond-dissociated isomer, tetraaryl-o-quinodimethanes 144, as discussed previously (Scheme 2.12a). It is clear from the above results that the fate of the bond-dissociated species is the major determinant factor for successful isolation and characterization of a compound with an ultralong C–C bond (d > 1.70 Å). By rational design to prevent the bond-dissociated species from intramolecular reaction other than bond-reformation, and from intermolecular reactions to give the adduct [316], the molecules with a super-ultralong C–C bond (d > 1.75 Å) would be obtained, whose bond length comes closer to the shortest nonbonded 1,3 C–C contact (1.80–1.90 Å) [317].

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2.5.5 HPE Derivatives with a Super-ultralong C–C Bond

The acenaphthene scaffold in HPEs 149 is one of the ideal skeletons for further exploration, yet the drawback is that the ‘rigid’ naphthalene-fused five-membered ring is not so rigid as expected. The experimentally determined d values for the ‘ethane’ bond in HPEs 149 exhibit considerable variation [1.633(3) Å for 149-Cl [315b] and 1.670(3) Å for 149-MeO [315a]], and the smaller values were observed when the acenaphthene skeleton adopted the skewed conformation with the larger torsion angle for C8a-C1-C2-C2a as in 149-Cl [27.9(3)°] [315b] and 149-MeO [20.6(2)°]. Apparently, the HPEs 149-H [0.0(2)°] and 149-F [3.2(2)°, 3.6(2)°] with the eclipsed conformation suffer from the larger front strain, which expands the polyarylated C1–C2 bond. Based on our recent results, the electronic effects of the substituents on the aryl groups do not determine the preference for the eclipsed/ skewed conformation [315c]. Thus, the torsion angle would be much affected by crystal packing force. When the acenaphthene skeleton is modified to increase its rigidity, skewing deformation would become more costly in energy than expanding the long C–C bond. Thus, the pyracene-type HPEs 150 with the extra five-membered ring are better candidates to exhibit an ultralong C–C bond. Furthermore, the additional bridge induces expansion of the C1–C2 bond [318] in the parent hydrocarbons [calculated by B3LYP/6-31G* [319]: 1.586 Å for pyracene and 1.569 Å for acenaphthene, respectively]. Since the bond-elongation effect is strongly enhanced for the prestrained bond [303a, 320], the steric repulsion in the pyracene skeleton must enlarge the d value for the ‘ethane’ bond more effectively in 150 (Scheme 2.16). This is also the heart of the authors’ molecular design to realize the super-ultralong C–C bond, whose d is larger than 1.75 Å. Various diarylketones were reacted with 5,6-dilithioacenaphthene generated in situ from 5,6-dibromide. Upon treatment with acid under dehydrating conditions, the resultant diols are smoothly converted to a series of acenaphthene-5,6-diyl bis(diarylmethylium)s, from which the desired HPEs 150 were generated upon reduction [321]. X-ray analyses have shown that they all adopt the eclipsed conformation with the torsion angle of C8a-C1-C2-C2a less than 3°, and consequently, all of

Scheme 2.16

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Figure 2.11 X-ray structure of 1,1,2,2-tetraphenylpyracene 150-H with a super-ultralong C1-C2 bond length [1.754(2) Å] (C2/c, Z = 4, R = 5.5%, T = 123 K). The molecule is located on the crystallographic 2-fold axis.

the ‘ethane’ bonds are longer than 1.7 Å. The super-ultralong bond (d > 1.75 Å) is found in several of them, such as 150-H [1.754(2) Å] and 150-F [1.761(4) Å] [321]. Though largely separated, these carbon atoms must form ordinary covalent bonds. The two carbons are hybridized in the sp3 manner, as judged by the tetrahedral coordination determined by X-ray analysis (Figure 2.11). Despite the marginal down-field shift, the 13C signal (78.73 ppm for 150-H in CDCl3) stays within the region typical of the sp3 carbons. Theoretical calculation (B3LYP/6-31G*) gave the d values of 1.758 Å for 150-H and 1.762 Å for 150-F, respectively, which correspond well with the observed values [321]. Through this achievement, the authors have proposed a general scheme to design the HPE derivatives with a super-ultralong C–C bond by taking advantage of: (1) increase in front strain by torsional fixation and skeletal rigidity; (2) non-Kekulé structure for the bond-dissociated species that can only undergo bond-reforming without giving any by-products. The latter condition is essential to isolate and characterize the HPE containing a weakened ethane bond. 2.5.6 ‘Expandability’ of the Ultralong C–C Bond: Conformational Isomorphs with Different Bond Lengths

Based on the proposal for a linear relationship between bond lengths and BDEs [298], the ‘ethane’ bond in 150-H and 150-F would have ‘negative’ BDEs, which is unrealistic. There should be a nonlinear relationship or some special effects in the region of the ultralong and the super-ultralong C–C bonds. During further examination of the extremely long C–C bond in the pyracene-type HPEs, the authors have encountered a special case of a ‘soft’ long C–C bond. That is, the

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same C–C bond in a certain HPE exhibits several different values (1.71–1.77 Å) depending on the requisites from crystal packing. Compared with the conformational polymorphs where different conformers crystallize separately in different crystal forms, conformational isomorphs have seldom been observed, in which the different conformers are packed orderly in the same crystal. The latter is the case for the pyracene-type HPE 151 having two spiro(10-methylacridan) units. According to the X-ray analysis at –180 °C, there are four crystallographically independent molecules in the crystal of 151. Two adopt the eclipsed conformation [torsion angle: 3.4(2), 9.4(2)°] and other two are skewedshaped [23.4(1), 24.7(1)°] (Figure 2.12). Adoption of two different conformations is similar to the case of acenaphthene-type HPEs (149-H, 149-F: eclipsed; 149-Cl, 149-MeO: skewed), yet both conformers of 151 do coexist in the same crystal. As expected, the eclipsed conformers possess much longer ‘ethane’ bond [1.771(3) and 1.758(3) Å] than the skewed molecules [1.712(2) and 1.707(2) Å]. By measuring the diffraction data at the elevated temperature, the longest bond length becomes as large as 1.781(6) Å at –40 °C. This value of d is one of the largest ever reported for the C–C bond length determined with sufficient accuracy [290]. The molecular structures in conformational isomorphs often differ significantly in torsional angles but not in bond angles; changes in bond lengths are found less frequently. The last cases are usually related to the presence of differently charged molecules [322] or tautomerism in the crystal [323]. Even when the molecule crystallizes in several forms (polymorph), the chemically equivalent bonds in general exhibit the same bond length within experimental error. Thus, the crystallographically determined value of d has been considered as ‘intrinsic’ or an ‘eigenvalue’ for a compound of the same charge and the same atom connectivity. This provides the basis for the validity of comparison of the bond lengths determined by X-ray analyses and to discuss the relationship between bond lengths and chemical structures. What happens when the same molecule possesses different bond lengths in polymorphs? What happens when the chemically equivalent but crystallographically independent molecules exhibit considerably different bond lengths in the same single crystal? This is the case for HPE 151 described above. So, none of the d values precisely determined [1.771(3), 1.758(3), 1.712(2), 1.707(2) Å] is the eigenvalue of compound 151, but X-ray analysis just demonstrates that the bond in question can be expanded, if it likes, to 1.771(3) Å under the given circumstances, and that the bond can also remain much shorter than that. This is not a unique and special phenomenon for 151: structurally related compound 152, which has the etheno-bridge instead of ethano-bridge in 151, also crystallizes as a conformational isomorph [319]. Its crystal packing is quite different from that in 151, and there are three crystallographically independent molecules of 152: one eclipsed [bond length and torsion angle at –130 °C: 1.749(2) Å and 5.5(2)°] and two skewed forms [1.726(2) Å and 21.3(2)°; 1.721(2) Å and 22.8(2)°]. This dihydropyracylene-type HPE 152 is another example that shows the large ‘expandability’ of the ultralong C–C bond. In contrast, HPE 153 with a less rigid acenaphthene skeleton only adopts the skewed conformation with a much smaller d value [1.696(3) Å and 18.1(3)°] [315d] than 151 or 152.

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Figure 2.12 X-ray structures of two of four crystallographically independent molecules of bis(10-methylspiroacridine)-type HPE 151 with an eclipsed (a) and skewed (b) conformation (P1bar, Z = 8, R = 5.2%, T = 93 K).

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2.5.7 Future Outlook

Ultralong C–C bonds present intriguing facets of organic covalent bonds. For the super-ultralong bond with d > 1.75 Å, there is no longer the linear relationship between d and BDE. The super-ultralong bond can exhibit a range of d values in crystal depending on the packing arrangement since only a small energy is necessary to expand the ‘expandable’ covalent bond. Thus, the precisely determined d value for the ‘soft’ bond is not necessarily the ‘eigenvalue’ of the bond in a certain compound, but it just points out to the length it can exhibit. That value can be the higher limit in one case or the lower limit in another case. In any case, the experimentally determined large d value will attract attention. It surely shows that the bond can be longer than the standard value. Chemists can continue the game to find the further elongated C–C bond, and the outstanding stability of compounds 150 has prompted the authors to prepare the derivative that shows even larger value of d in crystal (1.79 Å) [321], which is nearly equal to the shortest nonbonded 1,3 C–C distance (1.80 Å) [317]. The authors appreciate Herges’ view [289] that ‘the stable C–C bonds even longer than the smallest nonbonding distance are conceivable’.

2.6 Ultrashort C–C Bonds

Vladimir Y. Lee and Akira Sekiguchi 2.6.1 Introduction

The generally accepted normal value for the single C–C bond length between two sp3-hybridized tetracoordinate carbon atoms, compiled from a large number of X-ray structures, is 1.54 Å [324, 325], the value close to that of the C–C bond in diamond. In a vast number of crystallographically characterized organic compounds, the length of ordinary C–C bonds may deviate slightly from this average value, albeit being still very close to it. However, the two limiting cases of nonclassical C–C bonds are of particular importance: extremely long (ultralong) and extremely short (ultrashort) C–C bonds. The first topic, ultralong C–C bonds, is covered by Suzuki (Section 2.5), whereas our goal is to discuss the second topic, ultrashort C–C bonds, bringing together the most important experimental accomplishments and theoretical considerations in this field.Basically, one can distinguish four fundamental classes of compounds featuring ultrashort C–C single bonds, which are typically markedly shorter than the border value of 1.50 Å. The first class includes those compounds in which the C–C bond is framed on both sides by either C=C or C{C multiple bonds: the most common examples are 1,3-butadiene, H2C=CH–CH=CH2, and 1,3-butadiyne, HC{C–C{CH, where the central C–C bonds are considerably shortened to 1.463(3) Å [326] and 1.384(2) Å

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[327], respectively, being just intermediate between those of the normal C–C single (1.54 Å) and C=C double (1.34 Å) bond lengths. This phenomenon is typically interpreted in terms of hybridization (increased s-character of the sp2- or sp-hybrids used to form the C–C V-bonds) and, more importantly, conjugation (multiple bonds conjugation leading to the delocalization of S-electrons over the entire molecule and shortening of the central C–C bond due to its partial double bond character). In contrast to this first class of ultrashort C–C bond compounds, well known from standard textbooks of organic chemistry, the other three classes are much less familiar to the general chemical community. The second class of compounds, exhibiting extraordinarily short C–C bonds between the tetracoordinated C atoms, includes the derivatives of tricyclo[2.1.0.02,5]pentane, featuring exceptionally short endocyclic bridging bonds. The third class of compounds, for which ultrashort C–C bonds were experimentally found, is represented by the dimeric polyhedral compounds (e.g., bi(tetrahedranyl), bi(bicyclo[1.1.0]butyl), bi(bicyclo[1.1.1]pentyl), bi(cubyl)), exhibiting remarkably short central intercage exocyclic bonds. The fourth and newest class of compounds featuring the ultrashort C–C bonds is known more computationally than experimentally at present. In such derivatives the C–C bond, force to point inside the inert cage (like cyclophane), is compressed by the evident steric congestions. In this section, we will deal with the last three classes of compounds featuring ultrashort C–C bonds, briefly describing their structural peculiarities and particularly emphasizing the origin of the exceptional bond shortening. 2.6.2 Tricyclo[2.1.0.02,5]pentanes: Ultrashort Endocyclic Bridging C–C Bonds

The derivatives of tricyclo[2.1.0.02,5]pentane 154a–h, known since 1964 [328, 329], particularly those of tricyclo[2.1.0.02,5]pentan-3-one 154 (R2, R2 = C=O), represent the record shortening of the single bond between two tetracoordinate C atoms (Table 2.1). Indeed, the most remarkable structural feature of all tricyclo[2.1.0.02,5]pentane derivatives 154 is the exceedingly short bond between the bridgehead carbon atoms C1–C5, ranging from 1.408 to 1.509 Å (av. 1.45 Å) [330–340]. The only exceptional case is represented by the heavy group 14 elements containing 2,4-disila-1-germatricyclo[2.1.0.02,5]pentane derivative 155, in which the bridging Ge–C bond of 2.242(3) Å was extraordinarily stretched, being 15% longer than

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2 Distorted Saturated Hydrocarbons Table 2.1 Endocyclic C1–C5 bridging bonds in tricycle[2.1.0.02,5]pentane derivatives 154.

Tricyclo[2.1.0.02,5]pentane derivative

R1, R2

r (C1–C5), Å

Ref.

154a

R1 = Ph; R2 = OCO(p-Br-C6H4)

1.44(5)

[330] [331]

154b

R1 = Ph; R2,R2 = C=O

1.444(2)

[332]

154c

R1 = Me; R2,R2 = C=O

1.408(3) at 298 K (1.417(1) at 118 K)

[332] [(334)]

154d

R1 = CH2OCOCH3; R2,R2 = C=O

1.416(2)

[333] [335]

154e

R1 = CH2OCOCH3; R2,R2 = –OCH2CH2O–

1.455(3)

[336]

154f

R1,R1 = –CH2N(COOMe)N(COOMe)CH2–; R2 = OEt

1.509(2)

[337]

154g

R1 = COOCH3; R2,R2 = –OCH2CH2O–

1.485(2)

[338]

154h

R1 = COOCH3; R2,R2 = C=O

1.453(2)

[338]

the average value, the fact interpreted in terms of the appreciable singlet biradical contribution to the nature of the Ge–C bridging bond [341]. The shortest known endocyclic C–C single bonds were measured for 1,5-dimethyltricyclo[2.1.0.02,5]pentan-3-one 154c (1.408(3) Å at 298 K [332] and 1.417(1) Å at 118 K [334]) and 1,5-bis(acetoxymethyl)tricyclo[2.1.0.02,5]pentan-3-one 154d (1.416(2) Å) [333, 335]. The recent DFT calculations fairly well reproduced the structural parameters of 154c: bridging bond length of 1.425 Å (1.417(1) Å by X-ray diffraction) and interplanar angle 94.96° (95.0° by X-ray diffraction) [342]. In sharp contrast, bicyclo[1.1.0]butane derivatives 156, which represent a part of the tricyclo[2.1.0.02,5]pentane skeleton, exhibit either normal (short bond isomers) or very long (long bond isomers) bridging C–C distances (Scheme 2.17). Such a striking difference between these two classes of polycyclic derivatives is due to the evident geometrical constraints in tricyclo[2.1.0.02,5]pentanes 154, in which the methylene C3-bridge between the C2 and C4 atoms ‘tightens’ the two three-membered rings, thus providing a fixed interplanar angle T, much smaller than that of the ‘nonfixed’ bicyclo[1.1.0]butanes 156: 94–99° vs. 113–130° [339].

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85

Scheme 2.17

Consequently, such a small interplanar angle T forces the hybrids at C1 and C5 atoms in 154 to form an extraordinarily short highly bent C1–C5 bond [339, 340, 343]. However, this bridgehead C1–C5 bond is not a classical V-bond formed by the linear overlap of two sp3-hybrids. Thus, the electron density distribution study of 1,5-dimethyltricyclo[2.1.0.02,5]pentan-3-one 154c by X-ray analysis at 118 K revealed that the electron density maximum of the bridging C1–C5 bond is displaced by ~0.40 Å outwards from the bond axis, which amounts to an extreme bending by 28° [334]. Consequently, the C1–C5 bridging bond is best described as a highly bent V-bond. The early calculations considered the bridging C–C bond in 154 to be a result of the S-type overlap of the two pz-orbitals from each bridgehead carbon atom, this suggestion being based on the general reactivity of this bond [344]; however, later DFT calculations objected such a viewpoint, showing that the bridging C–C bond has no S character and is to be described as a classical two-center bent V-bond [342]. The hybridization of the bridgehead carbon atoms C1 and C5 in 154 sharply deviates from the normal sp3-state: the hybrid orbitals of the exocyclic bonds are high in s-character (the coupling constant 1 JCH for exocyclic C1–H and C5–H bonds of 2,4-dimethyltricyclo[2.1.0.02,5]pentane were measured to be as large as 212 Hz) [345], whereas the hybrids used for the formation of the endocyclic C1–C5 bond have substantial p-character (the value of sp4.21 was calculated for the unsubstituted tricyclo[2.1.0.02,5]pentan-3-one 154 (R1 = R2 = H)342) [334, 339]. The extent of the shortening of the bridging C1–C5 bond in 154 is greatly affected by two principal factors: geometrical (interplanar angle T) and electronic (influence of the S-accepting substituents). Accordingly, the bridging bond becomes longer when the interplanar angle widens, and vice versa–the bridging bond shortens upon decreasing the interplanar angle [332]. Another geometrical factor, greatly influencing the length of the bridging bond, is the external bond angle I. In complete accordance with the theoretical prediction [343], the relationship between these two geometrical parameters is inverse: the larger the bond angle I, the shorter the bridging bond, and vice versa [337]. Electronically, S-accepting substituents (like carbonyl, phenyl groups) may effectively interact with the bridging C1–C5 bond in 154, because it is made of hybrids high in p-character (vide supra); however, the direction of such influence depends on the position of the S-accepting group [339]. For example, the carbonyl groups at the bridgehead atoms C1 and C5 give rise

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to a lengthening of the C1–C5 bridging bond in 154h by ~0.03 Å because of the optimal orientation of the carbonyl groups, allowing for the effective interaction of their C=O S-bonds with the bridging C1–C5 V-bond [338, 339]. In contrast, the carbonyl group at the bridging position C3 (C3=O), tightening the bicyclo[1.1.0]butane fragment at the C2 and C4 positions, shows the opposite effect, shortening the bridging C1–C5 bond in 154h by the same extent of ~0.03 Å [338, 339,346]. 2.6.3 Coupled Cage Compounds: Ultrashort Exocyclic Intercage C–C Bonds

This class of compounds featuring ultrashort C–C bonds best of all fits the original definition of such bonds, lacking the highly perturbing factors of either conjugation to multiple bonds (as the representatives of the first class of compounds with ultrashort C–C bonds, 1,3-butadiene and 1,3-butadiyne) or incorporation into the highly strained tricyclic framework (as the representatives of the second class of compounds with ultrashort C–C bonds, derivatives of tricyclo[2.1.0.02,5]pentane). Indeed, in striking contrast to the highly bent nature of the endocyclic bridging C–C V-bonds of tricyclo[2.1.0.02,5]pentane derivatives, which makes the definition of the bond length somewhat ambiguous, the intercage C–C V-bonds are exocyclic and nonbent, thus experiencing minimal geometrical constraints.

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87

The first experimental accomplishments in this field were preceded by the prediction that the exocyclic intercage C–C bonds in the hypothetical bi(cubyl) 157 and 1,1c-bi(bicyclo[1.1.1]pentyl) 159 could be markedly shortened by up to 0.08 Å compared with the normal values [347].

This hypothesis was based on the general idea that the C–C bonds are expected to be appreciably shortened if they were involved in widened bond angles (and vice versa: the C–C bonds adjacent to compressed bond angles should be stretched). This was the case of compounds 157 and 159, in which the intercage C–C bonds participate in six significantly widened C–C–C bond angles [347]. A year later this fruitful idea found spectacular experimental confirmation in the synthesis and crystal structure determination of bicubyl 157 and (2-t-butylcubyl)cubane 158 [348]: the intercage C–C bonds in both 157 and 158 were significantly contracted, 1.458(8) and 1.464(5) Å, respectively, being in very good agreement with the predicted value of 1.46 Å [347] and very close to the value of 1.463(3) Å for the central C–C bond in 1,3-butadiene [326]. Such appreciable shortening was simply explained in hybridization terms: endocyclic C–C bonds of bicubyl derivatives are richer in p-character than the normal sp3-hybrids, whereas the exocyclic C–C bonds are higher in s-character, which allows them to form shorter bonds to substituents than their sp3-counterparts do. The specific geometrical constraints of bicubyl molecules create less steric crowding than, for example, in hexasubstituted ethane, the latter factor also favoring shortening of the intercage C–C bond. In contrast to 157 and 158, in 1,1c-biadamantyl 163 the intercage C–C bond is formed by nearly pure sp3-hybrids; consequently, this bond is not ultrashort but stretched to 1.578(2) Å [349], being 0.12 Å longer than that in bicubyl 157.

Other examples of coupled cage compounds featuring ultrashort intercage C–C bonds include: bi- and tri(bicyclo[1.1.1]pentyl) derivatives 160–162 and 164,

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1,1c-bihomocubyl 165 [350–353]. Thus, in keeping with expectations [347], the bi(bicyclo[1.1.1]pentyl) derivative 160 showed a short intercage bond distance of 1.480(4) Å [350], whereas the tri(bicyclo[1.1.1]pentyl) derivative 164 manifested even shorter distances of 1.464(7) and 1.476(7) Å [351].

The other two representatives of bi(bicyclo[1.1.1]pentyl) derivatives, 161 and 162, also displayed diagnostically short intercage C–C bonds of 1.480(3) and 1.469(6) Å, respectively [352]. An MNDO calculation showed that the nature of the substituents at the bridgehead 3,3c-positions (H, NH2, or CN) has almost no influence on the length of the intercage C–C bond, being nearly invariable (1.470–1.472 Å) in all calculated structures. Quite similar to the above cases of bicubyl 157 and (2-t-butylcubyl)cubane 158 [348], and in complete accord with the prediction [347], 1,1c-bihomocubyl 165 exhibited the short intercage C–C bond length of 1.460(1) Å, this value is very close to the corresponding values in 157 (1.458(8) Å) and 158 (1.464(5) Å) [353].

The 1,1c-coupled derivatives of bicyclo[1.1.0]butane were expected to manifest a further shortening of the intercage C–C bonds, because their adjacent bond angles are very wide, being larger than those in bicubyl 157 and bi(bicyclo[1.1.1]pentyl) 159. Indeed, in the two structurally characterized examples of such compounds, namely, 1,1c-bi(tricyclo[4.1.0.02,7]heptyl) 166 and 1,1c-bi(tricyclo[3.1.0.02,6]hexyl) 167 derivatives the exocyclic intercage C–C bonds were exceptionally short, 1.445(3) and 1.440(2) Å, respectively, being ~0.10 Å shorter than the average value of 1.54 Å [354].

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89

Extrapolating the experimental results on the lengths of the intercage C–C bonds in coupled cage compounds, the authors deduced the shortening of ~0.10 Å for the central bond in the hypothetical bitetrahedranyl 169 [354]. Such impressive experimental accomplishments raised a great deal of interest among theoreticians about the problem of extreme shortening of the intercage C–C bonds. Thus, the length of this bond in bicubyl 157 was calculated at the HF/DZ+d SCF level as 1.484 Å, this value being 0.026 Å longer than the experimental value of 1.458(8) Å [355]. Definitely, the Hartree–Fock method markedly overestimated the length of the central C–C bond in the coupled cage compounds. Likewise, the early calculations on the parent unsubstituted 1,1c-bi(tricyclo[3.1.0.02,6]hexyl) 168 produced the overestimated values of 1.477 Å at the STO-3G SCF level and 1.453 Å at the DZ SCF level [356] (cf. 1.440(2) Å experimentally observed in 167 [354]). On the other hand, the semiempirical AM1 calculations gave underestimated values of the intercage C–C bond in the 1,1c-bi(tricyclo[3.1.0.02,6]hexyl) derivatives: 1.409 Å for the unsubstituted model 168 and 1.411 Å for the real compound 167 [357]. The recent DFT B3LYP/6-31G(d,p) calculations on the real molecules 166 and 167 provided results most closely approaching the experimental X-ray data: 1.456 and 1.452 Å vs. 1.445(3) and 1.440(2) Å [342]. Consequently, the hybridization of the intercage C–C bond in 166 and 167 was calculated as sp1.31 and sp1.26, which amounts to an s-contribution to the bonding orbitals of 43.2% and 44.3%, respectively. The extreme shortening of the intercage C–C bonds in both 166 and 167 was further manifested in the extraordinarily large values of the 1J(C, C) coupling constant of 99.8 and 85.9 Hz, respectively, undoubtedly reflecting the great s-character of these bonds [342]. The most intriguing molecule to study computationally was bi(tetrahedranyl) 169, whose structure was calculated by several research groups. (Other tetrahedrane derivatives were discussed in Section 2.4.1.1.) The earliest calculations agreed well with the above prediction of ~1.44 Å [354], giving the estimates of the intercage C–C bond in 169 as 1.438 Å at the 6-31G* [358] and 1.444 Å at the DZ+P SCF [356] levels of theory. In the latter study, the six equivalent bond angles at the central C–C bond were calculated to be 144.6°, which corresponded to a bond angle widening of 35.1° compared with a normal tetrahedral angle value of 109.5° [356]. Consequently, the intercage C–C bond length in 169 was expected to be intermediate between the 1.440(2) Å of 1,1c-bi(tricyclo[3.1.0.02,6]hexyl) derivative

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167 (widening angle 130.9° – 109.5° = 21.4°) and the 1.384(2) Å of 1,3-butadiyne, H–C{C–C{C–H (widening angle 180.0° – 109.5° = 71.5°). A subsequent study by the same authors performed at the MP2 level with the same DZ+d basis set showed a slightly shorter intercage C–C bond in 169 of 1.434 Å [355]. A larger shortening of the central C–C bond in 169 was found by the semiempirical AM1 calculations: 1.386 Å Å [357]. It is well-known that the NMR coupling constant 1 J(C, C) between the two carbon atoms forming the intercage C–C bond is another diagnostic measure of the electronic distribution in alkane bonds, effectively indicating the alterations in the hybridization of carbon atoms and, consequently, in the C–C bond length. Thus, the extreme shortening of the C–C bond linking the two tetrahedrane units in 169 (1.444 Å) was manifested in an extraordinarily large 1 J(C, C) coupling constant of 151 Hz [359]. This 1J(C, C) value is much greater than the values of 35.0 Hz measured for isobutane (CH3)2CHCH3 [360], and 53.7 Hz for the central C–C bond of 1,3-butadiene, H2C=CH–CH=CH2 [361], closely approaching the value of 154.8 Hz measured for 1,3-butadiyne, HC{C–C{CH [362]. Consequently, the calculated 1J(C, C) coupling constant of the intercage C–C bond in 169 is very similar to that of a bond between the two sp-hybridized carbon atoms, and it possibly represents the highest 1J(C, C) value ever reported for a C–C bond between two saturated tetracoordinated carbon atoms. The experimental realization of the theoretical predictions discussed above was achieved a couple of years ago, when the structure of the stable hexakis(trimethylsilyl)bi(tetrahedranyl) 170 was established [363]. The intercage C–C bond in 170 was exceedingly short, 1.436(3) Å, this value being very close to those predicted by calculations (vide supra) of 1.434–1.444 Å, being the shortest acyclic nonbent C–C single bond between two saturated tetracoordinated carbon atoms known thus far. The principal reason for such a great bond shortening was ascribed to the high s-character of this exocyclic C–C bond in 170: the hybridization of this bond was calculated (NBO analysis) to be sp1.53 at the B3LYP/6-31G(d) level [363]. However, the latest computations at the HF/6-311+G(d,p) level pointed out that apart from this evident cause, there is another reason behind the appreciable intercage bond shortening in bi(tetrahedranyl) 169, namely, vicinal hyperconjugative orbital interactions between the two tetrahedranyl units over the central C–C bond [364]. This hyperconjugation, facilitated by six electron donating silyl substituents, was calculated to be even more important than conjugation in 1,3-butadiene: the hyperconjugation energy in the staggered conformation of bi(tetrahedranyl) 169 of –15.2 kcal/mol vs. conjugation energy in 1,3-butadiene of –9.9 kcal mol–1. Such an interaction of the two tetrahedranyl units in 170 raised the HOMO energy level and lowered the LUMO energy level, resulting in the decrease in the HOMO–LUMO energy gap [365]. This was reflected in the red shift of the longest wavelength absorption of 170, compared with that of tetrakis(trimethylsilyl)tetrahedrane [365–367]. Thus, the two comparable causes responsible for the extreme shortening of the intercage C–C bond in 170 were suggested: the high s-character of this exocyclic bond in bi(tetrahedranyl) (58% contribution) and vicinal hyperconjugative interactions between the tetrahedranyl groups (42% contribution) [363, 364].

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91

2.6.4 Sterically Congested in-Methylcyclophanes: Ultrashort C–C(Me) Bonds

Recently, other examples of compounds featuring ultrashort C–C bonds were computationally studied [368], however, it should be noted that at present such chemistry seems to be difficult to realize experimentally. The first class of such derivatives includes those compounds in which a particular C–C bond is pressed by encapsulation in an inert cage, either inter- (neopentane inside C60) or intramolecularly (cyclophanes in which a methyl group is pressed towards the aromatic ring similarly to the recently reviewed molecular ‘iron maidens’ [369]. Until now only two examples of the compounds of such type were reported by Pascal et al., namely, sterically congested in-methylcyclophane derivatives featuring very short C–Me bond distances of 1.475(6) and 1.495(6) Å [370]. However, it seems possible to shorten the C–C bond without pushing on it, just by covalent means (for example, C–C bonds inside the small covalent cage) [368]. Other examples of ultrashort C–C bonds are represented by the stiffened and strapped intracage bonds or tied-up bicycloalkanes (for example, interior propellanes formed by strapping bicyclo[n.n.n]alkane units together) [368]. All of these calculated molecules feature very short C–C bonds; however, the reason for such marked bond shortening is not as simple as just their increased s-character. Thus, the calculated C–C bond shortening at least by 0.1 Å exceeds the value that would be expected from hybridization alone. An additional bond shortening originates from the strain caused by the threefold symmetric geometry constraints: the computed ultrashort C–C bonds in the above systems are simply unable to achieve normal bond lengths without causing notable perturbation in the whole molecule. 2.6.5 Conclusions

The general definition of the ‘normal’ C–C single bond length is somewhat arbitrary: the value of 1.54 Å is just an average of the vast number of available crystallographic data. However, depending on the particular case, the ‘normal’ C–C bond can be significantly stretched or shortened; consequently the two extremes, the ultralong C–C bonds and the ultrashort C–C bonds, may deviate from the average value by ca. 0.10 Å (6.5%) each, thus giving rise to the overall variation of ca. 0.20 Å (13%) with respect to the average value of 1.54 Å. In this section we have discussed in detail the two most important contributions to the field of ultrashort C–C bonds: derivatives of tricyclo[2.1.0.02,5]pentanes with squeezed endocyclic bridging C–C bonds and coupled cage compounds featuring contracted exocyclic intercage C–C bonds. The origin of the bond shortening in these two cases is clearly different, because in tricyclo[2.1.0.02,5]pentanes the short C–C bond is endocyclic and highly bent, whereas in the coupled cage compounds the short C–C bond is exocyclic and nonbent. After the characterization of bi(tetrahedranyl) 170, manifesting the shortest acyclic nonbent C–C single bond between two saturated tetracoordinate carbon atoms, the next prominent candidates for molecules featuring ultrashort

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C–C bonds would be polycyclic compounds, in which the particular C–C bond is inter- or intramolecularly involved in the cage fragment. This job has already been approached computationally–now it is the experimentalists’ turn.

Acknowledgments

We are greatly indebted to all our coworkers, who have made an invaluable experimental contribution to this work and whose names are listed in the references. This work was supported by a Grant-in-Aid for Scientific Research (Nos. 17550029, 19105001, 19020012, 19022004, 19029006) from the Ministry of Education, Science, Sports, and Culture of Japan.

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3 Distorted Alkenes 3.1 Nonplanar Alkenes

Dieter Lenoir, Paul J. Smith and Joel F. Liebman 3.1.1 Introduction and Context

Sterically-strained alkenes are important species in the study of physical organic chemistry [1]. Steric strain can result in torsion (twisting) and/or bending of the double bond [2]. Bending can occur in a syn- or anti-mode fashion as the two pairs of geminal groups on the double bond crinkle towards each other or apart. Strain diversely affects alkenes: (a) energy, measured by standard heat (enthalpy) of formation (e.g. calorimetric measurement of heat of combustion and hydrogenation); (b) kinetics, e.g. as measured by thermal (Z)/(E)-rotational barriers; (c) reaction mechanisms and reactive intermediates. While energetics and kinetics dominate our interest, examples of such special behavior are sprinkled throughout this chapter. 3.1.2 Bridgehead Alkenes

Bridgehead alkenes (also discussed in Section 3.2) have been widely investigated [1–3]. For example, consider adamantene 1 [4] shown below. According to Bredt’s rule, which is the absolute proscription against bridgehead double bonds, this species should not exist. Highly strained bridgehead alkenes such as 1 cannot be isolated, since they tend to dimerize spontaneously after their formation. Like some other bridgehead alkenes, 1 has been trapped after its formation in solution by cycloaddition with butadiene [5]. Wiseman [6] has developed a rule to predict the stability of bridgehead alkenes by comparing the stability of these species with that of the corresponding E-cycloalkene that serves as the substructure thereof. 1 is a derivative of E-cyclohexene, which does not exist under normal conditions, while alkene 2 is a derivative of the stable E-cyclooctene and does exist. There is another Strained Hydrocarbons: Beyond the van’t Hoff and Le Bel Hypothesis. Edited by Helena Dodziuk Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31767-7

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general rule developed by Maier and Schleyer [7] to predict whether a cyclic alkene can be isolated as a monomer: if the calculated strain energy difference of alkene and the corresponding saturated hydrocarbon (OS, the ‘olefin strain’) exceeds a value of 21 kcal mol–1, then the olefin tends to dimerize at room temperature.

Several highly strained alkenes have been isolated in a cryogenic matrix [8a], e.g. 1 and the related aza [8b] and chloro [8c] derivatives. However, 2-(1-adamantyl) adamantene cannot be isolated because of fast rearrangement to 3-(1-adamantyl)4-protoadamantylidene [8d]. E-cycloheptene is the smallest E-cycloalkene that can be isolated at –40 °C, rapidly transforming to its Z-isomer at room temperature [9] through a low barrier biradical Z/E transition state. 3.1.2.1

t-Butyl-substituted Ethylenes

t-Butyl is sterically an extremely bulky alkyl group. Sequential addition of this group to a C=C double bond leads to a significant increase in strain. The five alkenes 3–7, as both the E and Z-isomers, have been prepared. The E-isomers can be prepared by McMurry coupling [10] from the corresponding ketones and their Z-isomers by photochemical Z/E isomerization and subsequent separation using column chromatography. The heats of formation have been determined from the measurement of the corresponding heats of hydrogenation [11]. Strain energies have been calculated from these values; the highest strain energy value of 31.5 kcal mol–1 is found for the Z-isomer of 3,4-diethyl-2,2,5,5-tetramethylhex3-ene 7 [11]. The gas phase Z/E barriers of these alkenes have been measured using high temperature, low pressure, shock tubes. The values for these alkenes vary greatly with a range of ca. 30 kcal mol–1. The so-called inherent value, the barrier of ethylene, has been calculated to be 65.9(0.9) kcal mol–1 [11] (Parentheses convey uncertainties in measured values). Some of their isomers, the geminal 1,1-dit-butyl-2,2-dialkyl ethylenes, 8 and 9, have been synthesized, and their electrochemical oxidation potentials leading to radical cations investigated [12a]. 9 is more strained by 9.2 kcal mol–1 than its regioisomer E-7, an effect of geminal vs vicinal t-butyl groups in compound 9 [12b]. Strain energies of these highly crowded Z-ethylenes can be correlated with rotational Z/E barriers [13] using an exponential function, see Figure 3.1 combining thermodynamic and kinetic data. Other functional forms relating rotational barriers and strain energies are of lower accuracy [13]. As recently reviewed [14], tetra-t-butylethylene 10 remains unprepared despite numerous attempts by different research groups. However, many tied-back derivatives are known [15] such as 11 and its isomer, syn-fenchylidenefenchane.

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105

These species have crystallographically-determined twist angles of 17.0° and 11.8°, respectively, and, as their direct equilibration with CH2Cl2/AlCl3 shows, their free energies differ by only 0.2 kcal mol–1 at 250 °C. However, their reactivity with elemental bromine is markedly different, the former rapidly reacting and the other very slowly and both yielding ill-defined, but different sets of products.

Figure 3.1 Correlation of strain energy of Z-ethylenes with Z/E barriers, Ea,rot = 64.024exp(-0.0176SE), R2 = 0.9986.

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Some ‘monomeric’ carbenes could be dimerized to the desired alkenes. However, di-t-butylcarbene does not dimerize to the hoped for 10 since the carbene kinetically prefers intramolecular insertion into the methyl group leading to 1,1-dimethyl-2-t-butylcycloprop-ane calculated to be of lower thermodynamic stability than the desired olefin [16]. 3.1.2.2

Investigations of t-Butylated Ethylenes and Other Acyclic Alkenes

We now turn our attention to other nonplanar alkenes for which thermochemical measurements, enthalpies of formation, combustion and/or hydrogenation have been reported. The most significant example of this is (Z)-1,2-di-t-butylethylene (2,2,5,5-tetramethylhex-3-ene) 3. As cited in a recent monograph on heats of hydrogenation [17], there are two reported measurements, both in glacial acetic acid, as opposed to the recommended [18] medium of heptane or other nonpolar, inert gas mimicking media. The two values are 36.2(0.2) and 37.7(0.1) kcal mol–1 from which we derive an average or consensus value of 37.0(0.8) kcal mol–1. This value may be compared with those of a variety of strain-free analogs. Perhaps the most direct analogy is with its own (E)-isomer for which the corresponding two primary or directly measured values are 26.9(0.1) and 28.1(0.2) kcal mol–1 resulting in a consensus value of 27.5(0.6) kcal mol–1. This suggests some 9.5 kcal mol–1 strain energy associated with the vicinal t-butyl groups. Indeed, the difference in hydrogenation enthalpies between (Z)- and (E)-alkenes, where no such destabilization for the (Z)-isomer is expected beyond about one kcal mol–1. Two other comparisons may naturally be made. The first is with the 1,2-dineopentylethylenes 12 for which the hydrogenation enthalpies of the (Z) and (E)isomers are 26.9(0.1) and 26.0(0.1) kcal mol–1, respectively, with a difference of 0.8(0.2) kcal mol–1. 12 in both the (Z)- and (E)-forms is essentially unstrained. The other comparison is with 6, the 1,2-dimethyl derivative of 1,2-di-t-butylethylene. The relevant hydrogenation enthalpies for the (Z)- and (E)-isomers are 43.7(0.2) and 37.4(0.2) kcal mol–1, respectively, suggesting that the former is only 6.3(0.3) kcal mol–1 more strained than the latter one. This difference is smaller than for the aforementioned difference of the parent di-t-butylethylenes. This does not mean that there is less strain in the dimethyl species. Rather, the hydrogenation enthalpy is higher for the dimethyl compounds than the corresponding parent olefins. This reminds us of the importance of carefully defining reference species and strain energies, when making comparisons even for seemingly highly related species. Reactivity considerations accompany discussions of the large strain energy of (Z)-di-t-butylethylene. For example, despite its considerable destabilization, acidcatalyzed isomerization to the much more stable (E)-species does not proceed [19]. Both isomers of 3 are readily chlorinated: while the (Z)-isomer cleanly yields the classical vicinal species, the (E)-isomer results in an isomerized 1,3-dichloride [19, 20]. Photochemically, the isomers of 3 interconvert, also forming 1,1-dimethyl2-neopentylcyclopropane [21]. Upon irradiation, tri-t-butylethylene, with strain presumably interpolating species 3 and 10, results in 1,1-dimethyl-2,3-bis-(t-butyl) cyclobutane and 1,2-dimethyl-3-bis(t-butyl)methylcyclopropane.

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3.1.2.3

107

Cyclo and Bicycloalkenes … and on to Polycyclic Analogs

Let us now consider monocyclic species, and start with cycloalkenes [22]. Solution phase hydrogenation enthalpy measurements have also been useful here for the understanding of strain and stability of these species. Under comparable conditions of an inert media, or otherwise correcting for solvent effects, the difference of hydrogenation enthalpies for (Z)- and the distorted (E)-cyclooctene 13 is about 11.5(0.5) kcal mol–1; the latter value is reduced to 9.8(0.4) kcal mol–1 in acetic acid solution, suggestive of enhanced solvation of the strained S bond in the (E)-species over that of its more stable (Z)-isomer. For the corresponding cyclononenes, cyclodecenes and cyclododecenes in acetic acid, hydrogenation of the (E)-isomers is more exothermic than those of the (Z)-isomers by 2.9(0.1), 3.3(0.1) and 0.5 kcal mol–1, respectively, and the (Z)-isomers are somewhat more stable than their (E)-counterparts. By contrast, the related acyclic 3-hexene shows the (E)-isomer to be more stable by 1.5 kcal mol–1 as determined by hydrogenation in hydrocarbon solution. Summarizing, the additional strain in these higher cycloalkenes, beyond that of their medium size rings, is minimal. Introducing additional unsaturation brings us to the corresponding cycloalkadienes [23], in particular the cycloctadienes. Changing one (Z)- linkage to (E)- in the 1,3-isomer 14 increases the hydrogenation enthalpy to 15 kcal mol–1 (no experimentally determined data for the (E,E)-isomer is available). For the 1,5-isomer 15, the hydrogenation enthalpies increase in the order (Z,Z) < (Z,E) < (E,E) with sequential differences of 12 and 9 kcal mol–1. The difference between the values for the (Z)- and (E)-cyclooctene is about 12 kcal mol–1, suggesting little thermochemical consequence of the additional unsaturation. Consider now [22] singly unsaturated bicyclic species. Table 3.1 gives the hydrogenation enthalpies of the derivatives of bicyclo[m.n.0] alkenes 16 wherein the double bond spans the 0 bridge, always m = n, and some related polycyclic species. For comparison, the corresponding data for the acyclic 2,3-dimethyl-2-butene Table 3.1 Hydrogenation enthalpies (kcal mol–1) of some bicycloalkenes and related species. Compound

'HH2

Method and comments [Ref.]

Tetramethylethylene

26.2(0.1)

Classical calorimetry

16, m = n = 1

79(10)

Gas phase ion, radical, radical ion cycle [24]

16, m = n = 2

43(2)

Classical hydrogenation calorimetry of bicyclo[2.2.0]hexene and -hexane

17, '1,5-dehydroquadricyclane

91(5)

Gas phase ion, radical, radical ion cycle [25]

18, cubene

88(5)

Gas phase ion, radical, radical ion cycle [26]

16, m = n = 3

27(2)

Quantum chem calculation for olefin [25, 27]

19, dodecahedrene

64(4)

Gas phase ion comparison [28]

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(TME) are also included there. Cubene for which the values are also given in Table 3.1 is briefly discussed in Chapter 2.3. Enthalpies of formation of many of the aforementioned olefins of interest are unavailable because we lack the necessary data for many ancillary species such as the saturated cubane [29] (see, however, the discussion of the cubane heat of formation in Chapter 1.2) and dodecahedrane [30]. We thus defer making any thermochemical comparisons with singly, and a fortiori, the multiply-unsaturated derivatives of dodecahedrane [31] and diverse hydrogenated fullerenes (presented in Chapter 5.6) and their derivatives [32], which are fascinating, clearly strained and worthy of further thermochemical study. For the latter we also need reliable heat of formation information [33] for the plausibly strain-free bicyclo[4.4.0]dec1(9)-ene (16, m = n = 4). Other data are absent: for example, the reader might wish to use gas phase ion reaction results of the acyclic 2,3-dimethyl-2-butene. This is not achievable because the stability of the radical anion of the bicyclobutene arises precisely from its nonplanarity [34] as ‘simple’ alkenes fail to bind the defining extra electron [35]. 3.1.2.4

Adamantylideneadamantane and its Derivatives

While the double bond of adamantylideneadamantane 20 is nearly planar [36] when R = H, the three derivatives substituted at the trans allylic bridgehead positions show severe twisting and out-of-plane bending of the double bond. [37] The calculated strain energy from Allinger’s MM2 method, increases in the order; 1,1c-dimethyl, 21.6 kcal mol–1, 1,1c-diethyl, 28.1 kcal mol–1 compared to the parent alkene. The X-ray structure of the ethyl derivative shows a torsion angle of 12.3° with a pyramidalization angle of 8.9° accompanied by significant elongation of the C=C double bond and the vinylic bonds. This highly crowded structure of the ethyl derivative has been used to test, and affirm [37] the validity of Allinger’s MMP2 method for highly crowded ethylenes. The corresponding 1,1c-diphenyl derivative of adamantylideneadamantane (singled out to presage phenylated olefins in the following section) has also been prepared recently and likewise shows [38] the greatest deformation of the double bond of all species 20. In no case, however, is there any measured enthalpy of formation or of hydrogenation to test any thermochemical conclusion. Adamantylideneadamantanes 20, especially the parent hydrocarbon, have greatly contributed to our understanding of diverse reactive intermediates. For example, the unsubstituted olefin forms solid bromonium ion salts [39] determined crystallographically as the triflate salt [40] while all other derivatives of 20 lead only to products of addition/elimination when reacting with bromine [41]. An (asymmetric) chloronium salt is known as well [42]. Parent 20 readily exhibits radical cation derived chemistry as part of reactions involving excited state uranyl ion [43a], strong Brønsted acids [43b], and neutral and cationic NOx species [43c, d]. 3.1.2.5

t-Butyl-substituted and Cyclic Stilbenes

When both of the vinylic hydrogens in 1,2-di-t-butylethylene are substituted by phenyl groups or equivalently those of stilbene are substituted by t-butyl groups,

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to form olefins 21, significant geometric change occurs. The increase of strain is calculated to be about 18.0 kcal mol–1. The central double bonds do not show the common distortions like torsion or bending, but the vinylic bonds with the phenyl groups are rotated into a perpendicular conformation to avoid steric repulsion of the t-butyl groups [44]. The UV spectrum shows two isolated phenyl and olefinic chromophores [45]. While the resonance stabilization of the stilbene has presumably vanished, calorimetric corroboration remains absent. The thermal Z/E barrier for these isomers is significantly reduced from the value of 42.8 kcal mol–1 for the parent stilbenes [45]. The Ea of the solid E- and Z-21 have been determined in the condensed phase [46] in contrast to the gas phase values measured for ethylenes 3–7 [11]. DSC measurements for the temperature range of 50–250 °C resulted in a value of Ea = 35.3 (1.0) kcal mol–1 as determined using an Ozawa-Flynn-Wall analysis [47], in close agreement with 32.0(1.3) kcal mol–1 obtained by NMR in solution. Various physicochemical properties of stilbenes E- and Z-21 have been studied [48]. Their chemical activity includes formation of the first stable S complex found for a bromination reaction [49]. E-1-((2,2-dimethyl-1-tetralinylidene)-2,2-dimethyltetralin, 22, is the most strained of these species with its central double bond twisted by 36.7° [50]. The Z-isomer of 22 has evaded isolation. However, its fleeting existence has been documented by its NMR and UV spectra, when the E-isomer, dissolved in THF was irradiated with UV (235 nm) in a NMR quartz tube at –70° leading to the Z-isomer of 22. The Z-isomer rearranges back to the E-isomer at room temperature with a low thermal barrier of Ea = 21.0 kcal mol–1 [50]. A torsion angle of 73° has been calculated for the Z-isomer by MMP2 [50], but recent DFT and ab initio calculations give a smaller value of about 42° [51]. 3.1.3 Multiply Unsaturated Bicycloalkenes, Homoaromaticity and Cyclophanes

We now turn to the parent and hydrogenated derivatives of bicyclo[4.4.1]undecapentaene and bicyclo[5.3.1]undecapentaene [52]. These hydrocarbons are also known as 1,6-methano[10]annulene 23 and 1,5-methano[10]annulene 24, respectively, and may be recognized as strained species with distorted aromatic rings. The hydrogenated counterparts of the former species and those of the related bicyclo[4.4.2]dodecapentaene 25 series with an ethano bridge, show [53] an interesting balance between these compounds and the tricyclic species with a 0 bridge, a full C–C bond, spanning the bridgeheads to form corresponding propellane derivatives (e.g. 26). We recognize the bicyclic species as having severely twisted double bonds and the possibility of homoaromaticity [54]; the tricyclic species look quite classical – at least when compared with small-ring propellanes discussed in Chapter 2.1. To our knowledge, there are no measured enthalpies of hydrogenation, nor any of formation, for the mostly saturated bicycloundecenes and dodecenes with but one double bond for which we may disentangle the effect of twisting, independent of any aromaticity or homoaromaticity.

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For the same reason we will ignore here the literature calorimetric discussion [55] of the hydrogenation of bicyclo[5.3.1]undeca-1,3,5(11)-triene, also known as [5]metacyclophane as well as its [6]metacyclophane (a bicyclo[6.3.1]dodecatriene) homolog (species 27, n = 5 and 6, respectively) (cyclophanes are discussed in Section 4.2). We merely note that these last species hydrogenate readily, as befits their distorted benzene derivatives, to form bridgehead monoenes and then the hydrogenation stops, or more properly slows below calorimetric detectability.

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Model calculations show these monoenes to be ‘hyperstable’ [56], i.e. their hydrogenation enthalpy is less than that of standard olefinic species and so they are not really within the purview of the current volume on strained species. Perhaps it is ‘sour grapes’ to avoid them here because of the lack of data. Interestingly, and perversely, the hydrogenation of [2.2]paracyclophane 28 to a mixture of bridgehead dienes has been interpreted in terms of hyperstable olefins [57]. However, while the totally saturated polycycle has been calorimetrically investigated [58] but the hydrogenation was not, we are thwarted in this case as well. 3.1.3.1

The Most Distorted Ethylenes and Seemingly Simple Analogs

Contenders for the most distorted olefins are the fulvalene 29 with a distortion angle of 66° [59] and the hydrindanylidenehydrindane 30 with an angle of 49° [60]. No heat of formation data is known for either compound. We conclude this chapter by acknowledging the diverse fulvalene derivatives as examples of distorted olefins (and not just 29 and 30), and note thermochemical data are essentially absent for this class of compounds. For the parent fulvalene 31, we find a measurement [61] for the enthalpy of hydrogenation of its (Cr-Cr) bis chromium tricarbonyl derivative to the dihydride, [K5-C5H4Cr(CO)3H]2 32 but no enthalpy of formation data to even estimate the enthalpy of formation of the parent hydrocarbon (as opposed to its bimetallic complex). For heptafulvalene 33, the enthalpy of hydrogenation to form cycloheptylidenecycloheptane 34 has been

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known for 50 years [62] but as with the above hydrogenated cyclophanes, the last step to form bis(cycloheptyl) has evaded calorimetric determination. So, where are the synthetic chemists to make the key compounds and calorimetrists to make the key measurements? Quantitative understanding of the so much of the chemistry and thermochemistry of strained alkenes is still awaited [63]. Acknowledgment

We thank Dr. C. Wattenbach for his help and technical assistance.

3.2 Small Ring and Cage Structures Involving Nonplanar C=C Bonds

Athanassios Nicolaides According to introductory textbooks the olefinic carbons of idealized alkenes are sp2 hybridized and the overall geometry is planar. At this geometry the lateral overlap of the two p atomic orbitals of the olefinic carbons is maximal and gives rise to a strong S bond (Figure 3.2a). Within this model one can think of two ways to distort the system and introduce strain. One is a torsional distortion around the C–C V bond, giving rise to torsionally strained olefins, also known as twisted or anti-Bredt olefins (Figure 3.2b), which are discussed in the preceding section. The second is by moving the olefinic carbons away from the plane defined by their four substituents, giving rise to pyramidalized olefins. While twisted olefins have received considerable attention [64] rather less is known about pyramidalized olefins [65–68].

Figure 3.2 Planar and distorted geometries of alkenes.

In torsionally strained olefins the loss of S bonding due to twisting is partially recovered by rehybridization of the olefinic carbons, the consequence of which is pyramidalization [64c]. On the other hand it is possible to design purely pyramidalized alkenes without torsionally strained S bonds as in 38 and 40. 3.2.1 Pyramidalized Alkenes

In a strict sense pyramidalization of alkenes is very common and is expected to take place whenever the two faces of the double bond are not symmetrically

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113

equivalent [66]. However, in the majority of the cases the pyramidalization is quite small and trivial. Often the term ‘pyramidalized alkene’ implies that pyramidalization is large enough to affect the physical and chemical properties of the system in a significant way. Several ways to quantify pyramidalization have been proposed [68]. The most widely used is that by Borden [69], which defines the pyramidalization angle M as the angle between the plane containing one of the doubly-bonded carbons and its two substituents and the extension of the double bond (Figure 3.2c). In terms of the bond angles D and E, M can be obtained from the formula: cos M = –cos D/cos (E/2) [66]. 9,9c-Didehydroanthracene (DDA, 35) was the first pyramidalized alkene to be isolated almost 40 years ago by Weinshenker and Greene [70]. The addition of base (t-butoxide) to the pyramidalized C=C double bond provided an early demonstration that pyramidalized alkenes are unusually reactive with nucleophiles. The most cited pyramidalized polyene is fullerene C60 [71] and its derivatives, discussed in Chapter 5. Alkenes with pyramidalized double bonds include cubene [72], bicyclo[1.1.0]but-1(3)-ene [73] and its 2,4-bridged derivatives [74], dodecahedrene and related compounds [75] and others which have been reviewed recently in an authoritative manner [68].

This chapter focuses on tricyclo[3.3.n.03,7]alk-3(7)-enes 36, a family of pyramidalized alkenes that has been studied extensively and systematically both experimentally and theoretically [66, 68].

In a formal sense, bicyclo[3.3.0]oct-1(5)-ene 37 can be thought of as the generator of the homologous series 36 by connecting carbons 3 and 7 with a bridge of n methylene groups. The first member of this series is 41 in which carbons 3 and 7 are directly bonded (n = 0). The five-membered rings of 37 are puckered in the same direction [76, 77], making the two faces of the double bond non-equivalent. As a result the olefinic carbons are trivially pyramidalized with a computed pyramidalization angle of M = 5.8° [77]. The small value of M implies that 36 is a normal tetraalkyl-substituted alkene.

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3 Distorted Alkenes Table 3.2 Computational data for alkenes 37– 41. Alkene

na)

Mb,h)

ELUMOc,i)

EHOMOd,i)

'Ee,i)

C=Cf,h)

OSEg,h)

41

0

61.9

–0.50

0.61

10.14

1.380

72.8

40

1

53.7

–2.51

0.65

11.09

1.362

52.3

39

2

42.2

–1.38

0.33

12.54

1.349

37.4

38

3

27.9

–0.79

0.24

13.22

1.342

17.7

37

–

5.8

0

14.25

1.337

0

a) b) c) d) e) f) g) h) i)

0

Number of methylene groups in the bridge. Pyramidalization angle (°) [68]. LUMO energies relative to that of 37 (5.65 eV) [76]. HOMO energies relative to that of 37 (–8.60 eV) [76]. Energy difference LUMO–HOMO (eV) [76]. Bond-length (Å) [73c]. Olefin strain energy (kcal mol–1) [68]. Computed with the B3LYP/6-31G(d) method. Computed with the SCF/3-21G method.

No special properties are expected to arise for olefins 36 as long as the length of the methylene bridge is sufficiently long (i.e. for a sufficiently large n). However, as the bridge becomes shorter it is expected that at some point it will cause considerable puckering of the bicyclic moiety and pyramidalization of the double bond. Indeed, the computed data (Table 3.2) show that the pyramidalization angle increases from about 28° for 38, having a tri-methylene bridge, to about 62° for 41, where carbons 3 and 5 are directly bonded. Thus, homologous series 36 provides a nice way of studying systematically the effect of pyramidalization on the properties of the C=C double bond as a function of n. A qualitative model based on simple molecular orbital theory has been proposed as a guide to understanding the properties of these compounds and of pyramidalized alkenes in general [66, 76]. The pyramidalization effect on the electronic properties of the C=C double bond can be understood on the basis of two factors (Figure 3.3). As pyramidalization (M) increases, the overlap between the p atomic orbitals forming the S bond decreases and this raises the energy of the bonding S MO (HOMO) but lowers the energy of the anti-bonding S* (LUMO). Pyramidalization also causes mixing of the V and S MOs, which in terms of atomic orbitals is a form of rehybridization. This increases the s character of the atomic orbitals and therefore energetically stabilizes both the HOMO and the LUMO. Overall, both factors stabilize the LUMO, but have opposite effects on the HOMO and therefore they tend to cancel out. Thus, in the case of the LUMO strong stabilization is expected, but the energy of the HOMO is expected to stay roughly the same. Indeed computations (Table 3.2) find that the energy of the LUMO in pyramidalized alkenes 36 is lower with respect to the reference compound 37 and decreases further as pyramidalization of the alkene increases. On the other hand,

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Figure 3.3 Effect of pyramidalization on the relative energies of the HOMO (S) and LUMO (S*) orbitals of an alkene.

the energy of the HOMO increases, but this is considerably less in magnitude than the corresponding decrease of the HOMO, in agreement with the qualitative model. Overall a decrease of the HOMO–LUMO gap is predicted with increased pyramidalization. Olefin strain energy (OSE) [76, 78] is a quantitative measure of the strain of the alkene. With alkene 37 serving as the ‘strain-free’ reference point of homologous series 36, the computed OSEs show that there is a considerable increase in the strain of the alkene with pyramidalization. Based on empirical rules and the computational results (Table 3.2) the most pyramidal alkenes of this series 39–41 are not predicted to be isolable under normal conditions, while for 38 it is difficult to make a secure prediction. As the strain of the S bond increases, one expects the S-bond order to decrease and therefore the C=C bond length to increase. Indeed, the C=C bond length of 41 is the longest, but it is still sufficiently short to suggest that the residual S bond is still strong despite the large OSE. This simple qualitative model rationalizes the computational data shown in Table 3.2 in a satisfactory way. Additionally, research has sought to confirm this model by generating the pyramidalized alkenes 37–41 and studying their physicochemical properties [66, 68]. In general, pyramidalized alkenes, like other strained molecules, require the development of special synthetic methods for their formation. Apart from the synthetic challenge, isolation of the most reactive ones is impossible under normal conditions and special techniques are needed. Some of the most reactive pyramidalized alkenes have been studied directly by spectroscopy in the gas phase or after matrix isolation at low temperatures. More often the intermediacy of pyramidalized alkenes is deduced from the product analysis and/or from chemical trapping of the alkene [68]. 3.2.1.1

Tricyclo[3.3.11.03,7]undec-3(7)-ene 38

Alkene 38 has been synthesized by the reduction of its dimesylate precursor 38-OMs. Even though 38 is an isolable solid, X-ray crystallographic data for it have

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not been reported. Upon exposure to air it oxidizes readily, forming among other things its epoxide [79] a reaction characteristic of pyramidalized alkenes [80, 81]. Alkene 38 has been characterized spectroscopically by IR, UV, NMR and electron transmission (ET) spectroscopy.

As expected of symmetrically substituted alkenes, the C=C bond stretching vibration of reference compound 37 is forbidden by IR spectroscopy but is observable by Raman spectroscopy. In contrast, the C=C bond stretch of 38 appears both in the IR and Raman spectra. In the IR spectrum of 38 it appears as a very weak band and this can be attributed to the fact that pyramidalization of the double bond causes the C=C stretching mode to have a transition dipole, which is oriented perpendicular to the C–C bond. It is noteworthy that the C=C stretching frequency of 38 is 70 cm–1 lower than that in 37, in qualitative agreement with the weaker S bond of the former [82, 83]. ET spectroscopy has been employed to measure the vertical electron affinities (EA) of 37 and 38. Both are negative indicating that in both cases the radical anion is not bound. However, the important result is that 38 has a greater EA than 37, compatible with the former having a lower-energy LUMO than the latter [79]. The well-known property of C60 to accept electrons forming anions and polyanions can be attributed to the pyramidalization of its C=C double bonds. The adiabatic ionization energy (IE) as given by photoelectron spectroscopy is found to be lower in 38 than in 37 by 0.31 eV. This difference can be related via Koopmans’ theorem [84] to a higher-energy HOMO in 38 than in 37, in agreement with the computational data of Table 3.2. The UV spectrum of 37 does not show a maximum above 200 nm. According to electron energy loss (EEL) spectroscopy, the S o S* singlet excitation requires 6.54 eV corresponding to photons of 190 nm wavelength. This is very close to the S o S* transition of tetramethylethylene (6.61 eV, 188 nm) demonstrating that 37 is a rather standard alkene. Olefin 38 exhibits a UV absorption with Omax = 217 nm (5.73 eV). This red shift of 27 nm (or 0.81 eV) is in qualitative agreement with the smaller HOMO–LUMO gap of 38 as compared to 37 [79]. Protonation of a pyramidalized olefin is expected to relieve olefin strain and therefore 38 should have a substantially greater proton affinity, PA, than 37. The experiment was carried out in the gas phase using the kinetic method and established that the PA of 38 is indeed greater than that of 37. The apparent PA of 38 was measured at 219 kcal mol–1 or 22.5 kcal mol–1 higher than that of 37. On the other hand, the calculations predicted that the difference in PA between 37 and

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Table 3.3 Available spectroscopic data for alkenes 37–40. Alkene

UVa)

40

IRb)

EAc)

IPd)

PAe)

1496 [88]

39

245 [82]

1557 [82]

38

217 [79]

1615 [79]

–2.27 [79]

7.80 [79]

208 [85]f)

37

190 [68]

1685 [68]

–2.44 [79]

8.11 [79]

196.5 [85]

a) b) c) d) e) f)

nm. cm–1. Vertical electron affinities (eV). Adiabatic ionization potentials (eV). Proton affinities (kcal/mol). Based on the experimental PA of 37 and the computed difference of 11.7 kcal/mol between the PAs of 37 and 38.

38 should be only 11.7 kcal mol–1. Further experimentation and careful analysis of the experimental data with the help of calculations revealed that under the experimental conditions, skeletal rearrangements are possible and that what was measured was not the PA of 38, but rather of a diene isomer of 38. The skeletal rearrangement seems to take place in the protonated form of 38 and involves a retrograde vinylcyclopropane rearrangement. This type of rearrangement is endothermic for alkene 38, but it is known for the lower homologs of 38 [85]. 3.2.1.2

Tricyclo[3.3.10.03,7]dec-3(7)-ene 39 and tricyclo[3.3.9.03,7]non-3(7)-ene 40

Alkene 39 was first formed by decarboxylation of lactone 39-lac both in solution and in the gas phase (Figure 3.4) [86]. Evidence for the formation of 39 in solution was given by trapping the alkene as the Diels–Alder 39-DA. Pyrolysis at 530 °C led to the isolation of alkene 43, an isomer of 39, which can be formed via a retrograde vinylcyclopropane rearrangement. From experimental data it was estimated that the strain in 39 must be at least 21 kcal mol–1 in order for the rearrangement to be energetically favorable [87].

Figure 3.4 Pyrolysis of lactones 39-lac and 40-lac.

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When the pyrolysis was carried out at lower temperature (410 °C) and the pyrolysate trapped in an Ar matrix at 10 K, alkene 39 was detected by its IR absorption at 1557 cm–1, 58 cm–1 lower than the C=C stretching frequency of 38 [83]. Warming up of the matrix led to the isolation of the dimer of 39, cyclobutane 39-dim, providing more evidence for the formation of 39. The UV spectrum from the matrix isolated 39, exhibited a Omax red-shifted to that of alkene 38.

Lactone 40-lac proved more difficult to pyrolyze than 39-lac. Even at 550 °C only 50% of the lactone reacted, but not by loss of CO2 but by isomerizing to ketoketene 47, which was identified by IR spectroscopy and by chemical trapping with methanol. From this remarkable rearrangement it was estimated that the strain in 40 exceeds 39 kcal mol–1 [88]. At temperatures higher than 550 °C decarboxylation did take place, but only traces of the dimer of 40 (40-dim) were isolated, indicating that this is not a good route for the formation of 40. The major product of the pyrolysis was diene 46, presumably formed via alkene 45, which is the product of the retrograde vinylcyclopropane rearrangement of 40. Dimesylate 36-OMs (n = 1, 40-OMs) can be easily synthesized from the corresponding diol 40-OH. However reduction with Na/Hg does not yield alkene 40. A better precursor for alkene 40 is diiodide 40-I (36-I, n = 1). Treatment of 40-I in THF at –78 °C with n-butyllithium gave quantitatively the dimer 40-dim [89]. When the reaction was repeated in the presence of 1,3-diphenylisobenzofuran (1,3-DPIBF, 42), the expected Diels–Alder adduct (40-DA) was isolated. The reduction can also be carried out at room temperature with Na/Hg again giving products that imply the intermediate formation of 40. Gas-phase dehalogenation made possible the matrix isolation of alkene 40 and its IR spectroscopic characterization. Like its higher homologs, alkene 40 exhibits a C=C stretch as a weak band in the IR spectrum, with a frequency 61 cm–1 lower than that in the spectrum of 39. Unfortunately, the presence of metal atoms in the matrix prevented the UV spectrum from being obtained, although indirect experimental evidence for the existence of a long wavelength absorption in 40 was provided.

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3.2.1.3

119

Tricyclo[3.3.0.03,7]oct-1(5)-ene 41

Alkene 41 being the consummate member of the homologous series 36 has attracted considerable attention. Neither dimesylate 41-OMs, n = 0 nor the bistriflate 41-OTf, n = 0 (Tf = CF3SO2– ), give the desired alkene by reduction by Na/Hg, yielding instead the starting material [68, 90]. In 1996 the first synthesis of 41 was reported [91]. Reduction of diiodide 36-I, n = 0 (41-I) afforded a mixture of the dimer 41-dim and the diene 41-dn, with the latter thermodynamically more stable than the former. In the presence of 1,3-DPIBF the Diels–Alder product 41-DA (36-DA, n = 0) was formed. Unfortunately, no spectroscopic information is available for alkene 41.

Some derivatives of 41 (41c [92], 43 [93] and 44 [94]) have been successfully generated, but like 41 they have not been characterized spectroscopically. Evidence for their formation is based on product analysis. The least perturbed homolog of 41 that has been studied is 41c, with two Me groups on the bridge, which can be formed from its diiodide precursor (41c-I). Evidence for the generation of 41c is the isolation of the dimer 41c-dm, when 41c-I is reduced in solution, and the isolation of the expected Diels–Alder products when the reduction is carried out in the presence of 1,3-DPIBF (42) or of 11,12-dimethylene-9,10-dihydro-9,10-ethanoanthracene (DDE, 48). Some extra experimental evidence for formation of 41c is the detection and spectroscopic characterization of its (Ph3P)2Pt complex 41c-Pt [95]. 3.2.1.4

(Ph3P)2Pt Complexes

Organic reactive intermediates can be stabilized by complexation to organometallic fragments. A number of such complexes containing highly reactive S bonds, have been isolated as (Ph3P)2Pt complexes. For example, the (Ph3P)2Pt complexes of cycloalkynes that have bent triple bonds: cyclopentyne, cyclohexyne, and cycloheptyne are known. (Small-ring cycloalkynes are discussed in Chapter 7.) The (Ph3P)2Pt complexes of the pyramidalized alkenes 36, n = 1–3 (36-Pt, n = 1–3) have been isolated and studied spectroscopically in detail [95]. Recently, the complex of the 41c (41c-Pt) has also been reported.

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Tetrasubstituted alkenes do not ordinarily form stable complexes with (Ph3P)2Pt. Indeed, the (Ph3P)2Pt complex of 37, the unbridged generator of alkenes 36, was not formed [96]. This implies that pyramidalization of the double bond is responsible for the formation of complexes 36-Pt, n = 1–3 and 41c-Pt. As mentioned above, the major electronic effect of double bond pyramidalization is to lower the energy of the S* LUMO, making it a better acceptor of electron density from the metal fragment [79, 97]. This simple picture is capable of rationalizing most of the NMR data of Table 3.4. Thus, with increased pyramidalization, 13C-NMR resonances move to higher field and 195Pt ones to lower field although the latter not in a monotonic way. The trend of the 31P chemical shifts is not that predicted, but it is compatible with transfer of electron density from the (Ph3P)2Pt fragment to the complexed olefin as judged by other experimental data [96, 98, 99]. Table 3.4 13C (of the olefinic carbons), 195Pt and 31P chemical shifts (ppm) and 1JPt–P and 1JPt–C coupling constants (Hz). GC 41c-Pta) 40-Pt

a)

66.9

a)

74.9

39-Pt

a)

1

–4956.6

30.7b)

2850

–5058.5

30.5

2960

407

–5105.5

31.1

3115

343

JPt–P

1

JPt–C

78.8,79.2

–5092.5

32.2

3332

296d)

(Ph3P)2PtC2H4

(39.2)

–5146.5

34.1

3740

194

b)

c) d)

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GP

38-Pt

a)

c)

GPt

Benzene-d6, 298 K. Actual measured value (G 32.4 ppm) adjusted so that the value for the 31P chemical shift of (Ph3P)2PtC2H4 of Ref. [95] (G 35.8 ppm) becomes the same as the one reported in Ref. [96] (G 34.1 ppm). Toluene-d8, 229 K. Toluene-d8, 338 K.

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The 1JPt–P and 1JPt–C coupling constants follow the expected trend. In particular, the former decreases as the latter increases in a linear fashion, implying that electron density shifts away from the metal fragment and the Pt–C bonding strengthens as pyramidalization of the olefin increases. Thus, complex 41c-Pt is expected to have the largest alkene-Pt(PPh3)2 binding energy in this series, an expectation that is in agreement with computational findings [95]. Despite the stronger C–Pt bonds in 41c-Pt, this complex was found to react with ethanol to give 45. This is interesting because under the same conditions complexes 36-Pt, n = 1–3 are recrystallized from ethanol. The alcoholysis of 41c-Pt with ethanol is a rather unusual reaction [97] and paves the way for studying the reactivity of such complexes with the aim of using them as synthetic equivalents of reactive pyramidalized olefins. 3.2.2 Conclusions

In conclusion, the above synthetic data for alkenes 36, n = 0–3 are summarized in Figure 3.5. The general way for the formation of these alkenes is by reduction of the corresponding diiodide (for the first two members of the homologous series 36) or dimesylate precursor (for the higher homologs 38 and 39). Alkene 38 is stable enough to be isolated under normal conditions provided oxygen is excluded [79]. The more pyramidalized alkenes 39–41, cannot be isolated under the same conditions. Instead, in the absence of any trapping reagent their formal

Figure 3.5 Generation and trapping of tricyclo[3.3.n.03,7]alk-3(7)-enes (36).

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3 Distorted Alkenes

[2+2] dimers (36-dim) are isolated. The strained 36-dim, n = 0 (41-dim) easily isomerizes to the diene 41-dn. When the reduction is carried out in the presence of 1,3-DPIBF 42, the expected Diels–Alder product is isolated. In the presence of (Ph3P)2PtC2H4 the corresponding organoplatinum complexes 36-Pt of 38–40 have been isolated. Although, the experimental structure of 38 has not been determined yet, this alkene has been characterized extensively spectroscopically. Properties of the alkenes 38–40 are also quite well understood, even though the UV spectrum of 40 has not been recorded yet. Although, synthetic routes to 41 have been developed, direct spectroscopic observation of 41 is still lacking, and its (Ph3P)2Pt complex (41-Pt) is still not known. Acknowledgement

This work has been supported significantly by the U.S. National Science Foundation and also by the Research Promotion Foundation of Cyprus.

3.3 Strained Cyclic Allenes and Cumulenes

Richard P. Johnson and Kaleen M. Konrad 3.3.1 Introduction

Cycloalkenes are well known to any student of chemistry but the more exotic homologous series of cycloallenes 49–54 and cyclobutatrienes 55–60 rarely find their way into textbooks. Decreasing ring size in these two series leads to a rapid increase in strain and reactivity caused by deformation of the normally linear allene or butatriene.

Among milestones in the history of cyclic cumulenes, Ball and Landor first described the chemistry of 51–53 in 1961 [101], but the synthesis of a derivative of 1,2-cyclopentadiene 50 awaited the efforts of Balci and co-workers which were reported in 2002 [102]. Evidence for derivatives of 49 was found in studies on enyne photochemistry described by Meier and König in 1986 [103]. In the butatriene se-

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ries, a stable 10-membered ring homolog was described by Moore and Ozretich in 1967 [104]. Synthesis of 57 as a reactive intermediate was reported by Shakespeare and Johnson in 1990 [105]. The parent hydrocarbon 56 remains unknown, but heterocyclic analogs have been reported by Wong and co-workers [106, 107]. Essential features of this structural map are thus known. Experiment and theory support the existence of cyclic allenes in all ring sizes 49 through 54 (and larger), but structures smaller than an eight-membered ring are not isolable without special features. The smallest cyclic butatriene 55 is predicted not to be an energy minimum but instead is a transition state on the C4H2 energy surface. There is evidence for cyclic butatrienes in ring sizes 56 through 60 but only 60 and larger ring homologs have been isolated. This chapter presents the structure, syntheses and chemical behavior of four- to nine-membered ring cyclic allenes and butatrienes, illustrating the status of this field with selections from recent chemistry. Readers are referred to earlier reviews for more comprehensive treatments [108–111]. 3.3.2 Allene S Bond Deformations and Strain Estimates

In a ring of fewer than ten carbons, the allene group is bent and twisted toward planarity. Johnson and co-workers recently estimated strain by applying isodesmic and homodesmic relationships [112]. As one example, allene functional group strain in 51 may be estimated from the following isodesmic reaction, using B3LYP/6-311+G(d,p) energies. This equation exchanges strained and unstrained functional groups.

Figure 3.6 summarizes estimates for allene strain, i.e. strain primarily localized in that functional group, as well as total molecular strain, and observed chemical behavior for cyclic allenes. Only modest levels of strain result in kinetic instability for allenes of eight carbons or fewer; allene 53 and most derivatives dimerize quickly at ambient temperature [113, 114].

Figure 3.6 Strain estimates and chemical behavior of cyclic allenes.

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3 Distorted Alkenes

Ring constraints alter the barrier to allene chiral inversion. For parent allene 63, the transition state is best described as planar diradical 64, with one S electron in plane and three out of plane [115, 116]. Brudzynski and Hudson [117] measured a torsional barrier for allene of 43 kcal mol–1; this is easily reproduced by DFT calculations [116]. With cyclic allenes, the predicted inversion barrier decreases in proportion to strain. For 1,2-cyclohexadiene 51, the predicted inversion barrier 65 is 14.1 kcal mol–1 [118]. This interconverts the M and P enantiomers (helicity nomenclature), consistent with experiments by Balci and Jones that demonstrated the chirality of 51 [119]. Smaller homolog 50 is predicted to be chiral but with an inversion barrier of < 1 kcal mol–1 [112], while 1,2-cyclobutadiene 49, should be a nearly planar diradical [120].

3.3.3 Four- and Five-membered Ring Allenes

Can a remarkable substance such as 1,2-cyclobutadiene 49 exist? In a 1986 paper, Meier and König reported the photorearrangement of 66 to 68 (Scheme 3.1) and made the courageous suggestion of 67 as a likely intermediate [103]. Seven years later, Johnson and co-workers showed this to be a general singlet photorearrangement [120]. Photoreaction of 69 is typical; irradiation results in the interconversion of 69 and 71. The most straightforward mechanism is excited state closure to afford ground state 70, which can open thermally in either direction. MP2, CASSCF and DFT calculations all support the existence of a 1,2-cyclobutadiene intermediate.

Scheme 3.1 Photorearrangements of enynes.

Balci’s group reported preparation of the first 1,2-cyclopentadiene derivative in 2002. Endo fluoro isomer 72 reacts with methyllithium (Scheme 3.2) to give 73, which was trapped with furan to give 74 [102]. This achievement followed many unsuccessful attempts by the same authors [110]. The same group later reported evidence for 1-phenyl-1,2-cyclopentadiene [121].

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Scheme 3.2 Synthesis of a 1,2-cyclopentadiene.

3.3.4 1,2-Cyclohexadienes

1,2-Cyclohexadienes are surprisingly common reactive intermediates. The parent structure 3 is now securely characterized as a chiral allene [119]. Early synthetic routes to 51 included treatment of 79 with strong base (Scheme 3.3) and reaction of 77 with methyllithium [101, 108, 122]. This latter reaction presumably proceeds through carbene 76 and/or the related carbenoid. This was an early example of the Doering-Moore-Skattebøl rearrangement [123–126]. Schleyer and co-workers have predicted a barrier of only 0.5 kcal mol–1 for disrotatory ring opening of 76 [127]. Shakespeare and Johnson have reported that 51 is easily generated by treatment of 78 with fluoride ion [105]. Diels–Alder reaction of enynes is described in a later section. Werstiuk and co-workers have reported the photoelectron spectrum of 51 which was generated from 75 by retro-cycloaddition [128]. Although there have been several earlier reports of matrix isolated 51, agreement with the computed allene vibrational frequency remains uncertain [129–131]; a definitive route to matrix isolated 51 is needed.

Scheme 3.3 Synthetic routes to 1,2-cyclohexadiene.

Scheme 3.4 summarizes common reactions of 51 and its derivatives. Dimerization and alkene or diene additions proceed rapidly through diradicals 80 or 81 to give [2+2] and [2+4] cycloadducts [111]. Caubere reported that nucleophiles such as enolates add to 3, affording a wide range of products [132]; of course this is facilitated by strain relief. Christl has presented a detailed review of [2+2] and [2+4] cycloadditions to 51 and derivatives, summarizing the evidence for stepwise reactions [111]. Computational studies by Tolbert, Houk and co-workers support the conclusions of a stepwise mechanism [133].

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3 Distorted Alkenes

Scheme 3.4 Reactions of 1,2-cyclohexadiene.

It has long been recognized that Diels–Alder cycloaddition of enynes with alkenes should give 1,2-cyclohexadienes. Computational studies by several groups have yielded activation energies only ca. 6 kcal mol–1 above a conventional Diels– Alder reaction [134, 135]. Recent examples of this surprising reaction are shown in Scheme 3.5. Miller and co-workers showed that dienyne 82 undergoes double addition of fumarate to give 84, presumably through allene 83 [136]. Johnson and co-workers studied the first intramolecular version of an enyne plus ene cycloaddition [134]. Flash vapor pyrolysis (FVP) of enyne 85 cleanly afforded 87; the most straightforward mechanism is cycloaddition to give 86, followed by cycloreversion. Maas and co-workers have reported preparation of 90 by solution thermolysis of pyridinium triflate 88 [137]. Many heterocyclic analogs of 1,2-cyclohexadiene 91 have been prepared as intermediates and these provide routes to complex ring systems [111, 138–141].

Scheme 3.5 Examples of enyne Diels–Alder reactions.

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127

Several different strategies have led to isolable derivatives of 51. Longer carbon– heteroatom bonds and steric protection arising from bulky substituents explain the isolability of 92 [142] and 93 [143]. Metal complexation as in 94 provides an additional approach to kinetic stabilization [144, 145]. 3.3.5 1,2,4-Cyclohexatrienes

1,2,4-Cyclohexatrienes (‘isobenzenes’) and their benzannelated derivatives are reactive intermediates that lie at the center of an impressive range of chemistry. For 61, present results support a chiral structure, and a diradical transition state 61a for inversion, much as described for 51. Replacement of one or more ring carbon can lead to a large variety of heterocyclic analogs of 1,2,4-cyclohexatrience. As first noted by Shevlin, these may exist as either a chiral cyclic allene 95 or planar ylide 96 [146–148]. Engels has presented a comprehensive analysis of related structures containing second and third row elements [118, 149]. Sheridan and co-workers have provided the clearest confirmation of allenic structure by generating a variety of heterocyclic 1,2,4-cyclohexatriences in cryogenic matrices [150–152].

Scheme 3.6 summarizes synthetic routes to 1,2,4-cyclohexatrienes. The two pericyclic reactions have been most widely applied. Christl and co-workers have reported in great detail on other routes to 1,2,4-cyclohexatriene and its benzannelated derivative [153, 154]. The Doering-Moore-Skattebøl reaction of 98 and dehydrohalogenation of 100 provide excellent routes to 61. Common reactions of 1,2,4-cyclohexatrienes are summarized in Scheme 3.7. The allene is easily trapped with furan. With styrene, both [2+2] and [2+4] products are isolated, presumably through diradical intermediate 101. In 1969, Hopf and Musso reported that cis-1,3-hexadiene-5-yne 102 thermally isomerizes to benzene [155]. This reaction, now known as the Hopf cyclization, provides explanation for diverse high-temperature chemistry [156]. Experimental and theoretical results support two general pathways (Scheme 3.8). The

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3 Distorted Alkenes

Scheme 3.6 Common routes to 1,2,4-cyclohexatrienes.

Scheme 3.7 Reactions of 1,2,4-cyclohexatriene.

Scheme 3.8 Hopf cyclization chemistry.

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129

Type A mechanism proceeds by electrocyclization to 61, followed by sequential 1,2-hydrogen shifts passing through carbene 104. An alternative Type B route begins with Brown rearrangement (1,2-shift) to vinylidene 105, followed by C–H insertion. Support for the Type A cyclic allene mechanism comes from trapping of 61 with styrene and inhibition of rearrangement in the presence of O2 [157]. Hopf and co-workers studied the homologous series 106 [158]. In this case, smaller rings cyclize readily, while no reaction is observed for the largest homologs. These results correlated nicely with computations [156, 158]. One chemical process may generate 61 in great abundance. Among the wide array of intermediates in alkane combustion, Miller and Melius have proposed that 61 may be one stop (Scheme 3.9) on the road to aromatic rings and soot formation. [159]. Head to tail dimerization of propargyl radical 108, a common species in flames, might be followed by Brown rearrangement and C–H insertion to give 61. This can aromatize by the route described earlier. Alternatively, we note that 61 should have a low barrier for C–H dissociation to give phenyl radical, another flame intermediate.

Scheme 3.9 1,2,4-Cyclohexatriene in combustion chemistry.

Photochemical Hopf-type cyclizations are known from the earlier work of Laarhovens (Scheme 3.10) [160] who showed that 109 undergoes photocyclization, presumably through cyclic allene 110. Uncertainty remains about the aromatization steps, since sequential 1,2-H shifts have barriers too high to occur under these conditions. Lewis and co-workers have studied several chromophores that should photogenerate cyclic allenes [161, 162]. Experiments with 111 lead to the conclusion that products arise from intermolecular protonation, rather than hydrogen shifts in strained allene 112.

Scheme 3.10 Photochemical Hopf cyclizations.

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3 Distorted Alkenes

The enyne + alkyne cycloaddition has increasingly been applied in the synthesis of polycyclic structures. Computational studies on enyne + yne cycloadditions support the existence of cyclic allene intermediates [134, 135]. Scheme 3.11 presents recent examples. In 1994, Danheiser and co-workers reported that ynone + enyne cycloadditions of 113 proceed either by thermal activation or with AlCl3 catalysis [163]. One advantage of this reaction is that it proceeds directly to an aromatic ring. Johnson and co-workers applied flash vapor pyrolysis to a simpler structure 114 [134]. The product distribution was consistent with competitive electrocyclic opening and aromatization of the allene intermediate. Ananikov has reported computational models for this reaction that agree well with experiment [135]. Saa’s group has reported extensive studies on intramolecular enyne + yne cycloadditions.[164]. As one example, solution-phase thermolysis of 115 unexpectedly afforded a mixture of benzo[b] and benzo[c] fluorenones 116 and 117 [165]. The authors proposed that the initially formed allene opens to a 10-membered ring, which can then close in two directions. This novel result suggests that at high temperature other isonaphthalenes might undergo complex atom scrambling. In the final example, Danheiser’s group applied a clever intramolecular benzyne + enyne addition to prepare 119 and a variety of related heterocycles [166].

Scheme 3.11 Dehydro-Diels-Alder routes to 1,2,4-cyclohexatrienes.

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3.3.5.1

131

6-Methylene-1,2,4-Cyclohexatrienes and Related Structures

Thermal cyclization of structures 120 affords (Scheme 3.12) an intermediate that might be characterized as a chiral allene, diradical or zwitterion. These are not resonance structures since each implies a different geometry and electronic state. With X = CH2, this is known as a Myers-Saito cyclization [167, 168]. At present, there is neither experimental nor computational support for the allene structure 121. Computations with a closed-shell configuration optimize to chiral structure 121 but the wavefunction is unstable, with a lower energy open-shell solution. Thus, diradical 122 would appear to be the best description for these structures. and not allene 121 [169].

Scheme 3.12 Myers-Saito type cyclizations.

3.3.6 Seven-membered Ring Allenes

Strain has been estimated at 14 kcal mol–1 in 1,2-cycloheptadiene 52. The principal routes to 52 (Scheme 3.13) have come from dehydrohalogenation [108, 110, 111]. The carbenoid route to 52 unexpectedly fails, giving 126 as the major product. This longstanding mystery has now been explained by Schleyer and co-workers who showed by DFT calculations that the barrier to ring opening of 125 is unusually large because of a conformational change leading to the transition state [129]. One new route reported to the parent allene is from treatment of E-bromosilane 124 with fluoride [170].

Scheme 3.13 Chemistry of 1,2,-cycloheptadiene.

Selected examples of other novel 1,2-cycloheptadiene derivatives are summarized in Scheme 3.14. Huisgen has reported a [4+3] cycloaddition strategy that leads to the isolation of seven-membered ring ketenimines such as 127; this work has been reviewed [171]. Agosta and co-workers have reported that the dimer of 129 is isolated by photolysis of ketone 128 [172]. This type of radical combination might offer a more general route to cyclic allenes. A number of sila-substituted

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3 Distorted Alkenes

Scheme 3.14 Examples of 1,2-cycloheptadiene chemistry.

1,2-cycloheptadienes have been reported. In these cases, the longer C–Si and Si–Si bonds decrease bending and strain in the allene functional group. In the most recent example, Baines and co-workers reacted tetramesityldisylene with an alkynylcyclopropane to produce a mixture of allene 133 and other isomers [173]. 3.3.6.1

Cycloheptatetraenes

1,2,4,6-Cycloheptatetraene 134 is one of the best studied cyclic allenes, with sustained interest in related chemistry over nearly four decades. Interest has focused on the structure of 134 and its relationship with carbene 135. Initially viewed as highly improbable, the cyclic allene now is securely characterized as an important intermediate on the C7H6 potential energy surface. Chemistry of benzannelated derivatives is also well known [174]. The most common routes to 134 are shown in Scheme 3.15. Nearly four decades ago, Wentrup showed that 139 could undergo ring expansion [175]; without the presence of a trapping agent, dimer 137 was isolated. In 1982, Chapman and co-workers generated 134 in an argon matrix by photolysis of 140 [176]. The IR spectrum showed bands at 1824 and 1816 cm–1 which supported the chiral cyclic allene. This result was a revelation at a time when strained cyclic allenes were poorly understood; however, the role of carbene 135 remained uncertain. In 1996, three theoretical studies converged on the currently accepted view of this energy surface [177–179]. Essential features are summarized in Figure 3.7. The chiral allene was predicted to be the lowest energy structure on this portion of the energy surface, in agreement with experiment and earlier computations. More interestingly, it was shown that the closed shell (135, state symmetry 1 A1) is not an energy minimum. The open shell singlet state 141 (1A2) was found to be the most stable planar structure. This is the transition state for allene

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133

Scheme 3.15 Chemistry of cycloheptatetraene.

enantiomerization. Matzinger and Bally later subjected 134 and 139 to a complete characterization by UV/Vis and IR spectroscopy, where the spectra were assigned based on high-level ab initio calculations [180]. The route from singlet carbene 139 to 134 is a two-step process, passing through 133. This bicyclic intermediate has not been observed, probably because it sits in a shallow energy well. Effects of aryl substituents on the interconversion of phenyl carbene, bicycloheptatriene and cycloheptatetraene were examined by Geise and Hadad [181]. Reaction of benzene with atomic carbon offers another entry into the C7H6 surface. Shevlin has provided evidence that this reaction proceeds by insertion into a C–H bond to generate phenylcarbene [182, 183]. Rapid dimerization precludes observation of 134 under ordinary solution-phase conditions. However, with the use of Cram’s hemicarcerand [184], Warmuth and Marvel reported the generation (Scheme 3.16) of this strained cycloallene

Figure 3.7 Energetics of 1,2,4,5-cycloheptatrienes.

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134

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Scheme 3.16 Generation of “incarcerated” cycloheptatetraene.

at room temperature inside the carcerand [185, 186]. This and other reactions carried out in molecular flasks stabilizing the short-lived guest are discussed in Chapter 10. Hemicarceplexed phenylcarbene was found to competitively insert in the C–H bonds of the hemicarcerand, resulting in low yields of 134. The yield was increased by incarcerating phenyldiazirine in a partially deuterated hemicarcerand, resulting in a 67% yield of 134. Similar results were found for the photolysis of p-tolyldiazirine in this molecular container [187]. Small molecules can diffuse into the hemicarcerand to characterize reactions of the allene. Reaction with HCl gave 142, while O2 lead to a novel carbon loss. In spite of this intense scrutiny, we can find no strain estimates reported for 134. The isodesmic reaction scheme below, calculated at the B3LYP/6-311+G(d,p) + ZPVE level of theory, predicts 13.8 kcal mol–1 in allene strain. This is very similar to 1,2-cycloheptadiene in the estimate of allene strain.

Wentrup’s group has reported extensive studies (Scheme 3.17) on the aza analogs of 134. In this case, carbene 143 and nitrene 146 interconvert through 145 [188, 189].

Scheme 3.17 Azacycloheptatetraene chemistry.

3.3.7 Eight-membered Ring Allenes

Doering-Moore-Skattebøl chemistry (Scheme 3.18) provides the most common route to 1,2-cyclooctadienes 147. Strain in this ring size has diminished to ca. 5 kcal mol–1, but the parent structure can only be briefly isolated at ambient temperature and most examples should be considered as reactive intermediates [113,

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114, 190]. Price and Johnson found that 1-t-butyl-1-2-cyclooctadiene is isolable and could even be purified by gas chromatography [191]. More complex 1,2-cyclooctadienes have been utilized as intermediates. For example, Moore and co-workers designed an alkoxy-Cope rearrangement route to eight-membered ring allene 148 which provides a versatile triquinane synthesis [192].

Scheme 3.18 1,2-Cyclooctadiene chemistry.

3.3.8 Polycyclic Allenes

Only a few bicyclic allenes have been reported. Allene 149 (Scheme 3.19) has been of interest for some years; early reports [193] on its preparation now seem to be in doubt but Zen and Balci has recently shown that 149 can be successfully

Scheme 3.19 Polycyclic allenes.

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generated by a more reliable carbenoid route [194]. Balci’s group has also reported on the allene generated from D-pinene [195]. Okazaki and co-workers recently reported evidence for 3,4-homoadamantadiene 151, the first allene built on such a complex tricyclic framework [196]. Dehalogenation of 150 led to isolation of dimers of allene 151. In the presence of diphenylisobenzofuran (DIBF), adduct 152 was isolated. DFT calculations on 151 predicted a pure bending deformation of the allene, with an angle of 135.9°. Barton and co-workers reported the synthesis and isolation of 153. In this structure, the allene is linear but twisted to a dihedral angle of 72.4º [197]. 3.3.9 Cyclic Bisallenes

A modest collection of bis-allenes has been reported. Mitchell and Sondheimer first described the interesting pericyclic cascade of 154 through bis-allene 155 (Scheme 3.20) to dimers of 156 [198]. More recently, Wang has developed this as a general route to more complex polycyclics [199]. Cyclic bis-allenes can exist as diastereomers. Dehmlow and co-workers have shown that dibromides such as 157 open stereospecifically, in this case giving the dl bisallene [200]. Barton’s group reported the synthesis of bis-allenes with silicon in the ring [201]. Kamigata and co-workers have reported a detailed ab initio study on meso and dl stereoisomers of 7–10-membered ring bisallenes [202]. For 7–9-membered rings, the meso isomers were predicted to be of slightly lower energy. Strain in this series was estimated by comparison to deformation in 2,3-pentadiene. In the 9–10-membered ring allenes, strain was predicted to be < 2 kcal mol–1 per allene unit; these structures have allenes that are nearly linear. The value increased to 3.6 or 5.9 and 13.7 or 31.0 kcal mol–1 in dl or meso isomers of the eight- and seven-membered rings, respectively.

Scheme 3.20 Bis-allene chemistry.

3.3.10 Cyclic Butatrienes

Cyclic butatrienes have four contiguous S-bonded carbons. Spectacular examples of this functional group include 159 (Scheme 3.21), an intermediate in neo-

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Scheme 3.21 Cyclization of cyclic cumulenes to diradicals.

carzinostatin chemistry [203, 104] and 160, a 10 S aromatic substance first prepared by Myers and Finney [205, 206]. These structures have minimal strain but still cyclize readily to diradicals with modest thermal activation. 3.3.10.1 Butatriene S Bond Deformations and Strain Estimates

Isodesmic and homodesmic estimates for butatriene strain and total strain in cyclic butatrienes are summarized in Figure 3. Strain increases rapidly with decreasing ring size [112]. In the smallest ring, Mabry and Johnson characterized 1,2,3-cyclobutatriene 55 as a transition state rather than an energy minimum [207]. DFT optimized structures for 56–60 show that the butatriene group remains close to planarity. For a comparable ring size, cyclic butatrienes have ca. twice the strain of cyclic allenes. Yavari and co-workers have studied the conformational properties of 56–60 using HF and MP2 methods [208].

Figure 3.8 Strain estimates and chemical behavior of cyclic butatrienes.

3.3.10.2 Five- to Nine-membered Ring Cyclic Butatrienes

The parent cumulene 56 remains unreported, in spite of a number of attempts, presumably because of its 85 kcal mol–1 of estimated strain. However, Wong and

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Scheme 3.22 Generation and trapping of a 1,2,3-cyclopentatriene.

co-workers have reported routes to thia and azacyclopentatrienes (Scheme 3.22) [106, 107]. Treatment of iodonium triflate 161 with fluoride at ambient temperature in the presence of various dienes afforded modest yields of substances assigned as cycloadducts of 162. Very similar chemistry was utilized to trap the aza analog of 162. An earlier study by Shakespeare and Johnson had developed this methodology (Scheme 3.23) for the first synthesis of 1,2,3-cyclohexatriene 57 [105]. This new benzene isomer was easily trapped. Hickey and Paquette later described the efficient addition of 57 to cyclopentadiene [209].

Scheme 3.23 1,2,3-Cyclohexatriene chemistry.

Dehydro-Diels–Alder chemistry provides a more novel approach to derivatives of 57. Johnson and co-workers found that flash pyrolysis of diyne 164 afforded a high yield of dienyne 166 [134]. Intramolecular cycloaddition, followed by electrocyclic opening of 117 provide the most logical mechanism. Ring opening of 165 is expected to be substantially exothermic and it seems unlikely that this process can be run in reverse. Lu and co-workers have predicted that 1,3-diynes may add to Si(100) and Ge(100) surfaces to produce layers of reactive 1,2,3-cyclohexatrienes [210].

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Cyclic medium-ring 1,2,3-butatrienes are highly reactive (Scheme 3.24). Szeimies reported the first synthesis of 58 through rearrangement of bicyclobutene 167 and later described Ni catalyzed dimerization to 168 [211, 212]. The eight-membered ring butatriene 59 prepared from 169 has only a brief lifetime in solution [213] while the next homolog 60 is isolable [214].

Scheme 3.24 Cyclic butatriene chemistry.

Some of the most spectacular examples of cyclic cumulenes come from the domain of organometallic chemistry. In 1993, Buchwald and co-workers reported a serendipitous synthesis of zirconacyclohepta-2,4,5,6-tetraene 170 [215]. The crystal structure showed a planar cumulene with an unusual zigzag geometry. Rosenthal and co-workers have reported in great detail on the chemistry of metallacyclopenta2,3,4-trienes 171 which can be readily prepared by transfer of Cp2M to 1,3-diynes [216, 217]. Internal bond angles in these structures range from 142–150°. The molecular geometry and computations indicate strong stabilizing interactions between the metal and in-plane cumulene S bond. Steric stabilization of these complexes was also suggested.

3.3.11 Conclusions

Strained cyclic allenes and cumulenes present an enormous diversity of structure and a complete range of chemical behavior, from shelf-stable substances through increasingly fragile reactive intermediates. These simple hydrocarbons and their derivatives are now recognized as intermediates in a wide array of chemical reactions. Ease of preparation and high chemical reactivity have encouraged a growing number of sophisticated applications in synthesis.

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Acknowledgments

Support from the National Science Foundation for our earlier work on strained cumulenes is gratefully acknowledged. Thanks to Catherine Johnson for typing a semi-legible manuscript.

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4 Strained Aromatic Molecules 4.1 Nonstandard Benzenes

Paul J. Smith and Joel F. Liebman 4.1.1 Introduction and Context

It was a tenet of organic chemistry that aromatic species are planar. A lengthy section of the now classic book [1] on strained organic molecules was devoted to species with distorted rings, and indeed, the current volume elsewhere discusses such compounds. Interpolating these past and present monographs on strained species, recent topical issues of Chemical Reviews [2–4] document that ‘bent and battered’ rings [5] are increasingly part of the chemical landscape. Terms such as ‘fullerenes’ and ‘single wall nanotubes’ attest to increasing comfort with manifestly nonplanar aromatic species. We limit our attention to cases where we can compare the energetics of these species with essentially planar species. We prefer experimental data over that derived from calculations. While it was recently found that many ‘popular theoretical methods predict benzene and other arenes to be nonplanar’ [6], it was also noted that more sophisticated approaches show this failure while more common, cheap and conventional approaches do not. As such, this finding should not threaten the increasingly strong coalition of experimentalists and theorists, nor prejudice for planar arenes. We focus on hydrocarbons, preferably gaseous or in some innocuous solvent to minimize the effects of intermolecular interactions. While relative reactivity generally runs inverse to stability, we primarily discuss enthalpies of formation ('Hf) and reaction, and not of activation.

Strained Hydrocarbons: Beyond the van’t Hoff and Le Bel Hypothesis. Edited by Helena Dodziuk Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31767-7

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4.1.2 Alkylated Aromatics

Starting with derivatives of benzene itself, the three isomeric xylenes have essentially indistinguishable gas phase 'Hf [7], o-, 19.1(1.0); m-, 17.3(0.7); p-, 18.1(1.0) kJ mol–1 (4.184 kJ { 1 kcal): methyl–methyl repulsions are small. The three isomeric t-butyltoluenes show the o-isomer to be less stable by 20 kJ mol–1 than its m- and p-isomers [8]. For the more strained di-t-butylbenzenes, condensed phase studies show that the o-isomer is some 80(10) kJ mol–1 less stable than the nearly isoenergetic m- and p-isomers [9]. Encouragingly, this difference is very much the same as for 1,2,4- and 1,3,5-tri-t-butylbenzene, 75(15) kJ mol–1 in the condensed phase [9a,10]. No experimental data allow comparison of 1- and 2-t-butylnaphthalene and determination of the destabilization from a t-butyl group in a peri (1,8) position. Computational chemistry shows the 2-isomer to be ca. 22 kJ mol–1 more stable than its 1-counterpart [11]. The 2-isomer behaves as unstrained hydrocarbon: the difference of its 'Hf [12] and that of naphthalene is ca. 110 kJ mol–1 [12], vs. 106 kJ mol–1 for related benzenes. The presence of two t-butyl groups on naphthalene allows for the possibility of vicinal (1,2- or 2,3-) and peri- (1,8-) situated groups. Such peri species interconvert with their Dewar isomers thermally [13a], photochemically [13b], and via 1-electron reduction [13c]. The aromatic isomer is energetically and kinetically disfavored by repulsion of the t-butyl groups. Racemization of seriously nonplanar 1,8-dit-butylnaphthalene requires ca. 95 kJ mol–1 [14]. 9-t-butylanthracene 1 is nonplanar [15], readily interconverting with its 9,10Dewar isomer 2 [16]. Equilibration studies show 1- and 9-t-butylanthracene are less stable than the 2-isomer [17]: the former are absent under reaction conditions. Likewise, the undetected 1- and 4-t-butyl derivatives of phenanthrene 3 must be more strained than the 2- or 3-isomers [18]. 4-t-butyl-5-methylphenanthrene (studied as its 1,8-dimethylated derivative) is nonplanar [19] and has a strain energy (SE) of 142 kJ mol–1; by contrast, 4,5-di-t-butylphenanthrene has a SE of 208 kJ mol–1. The nonplanar [20] 4,5-dimethyl species has a SE of ca. 50 kJ mol–1, defined as the 'Hf difference of this species and its 2,7-dimethyl isomer [21]. The strain in 4,5-dimethylphenanthrene is also shown by polarography [22a], calculation-guided spectroscopy [22b] and mass spectrometry [22c]. Were only some of the above simpler alkylated benzenes, naphthalenes and anthracenes so thoroughly studied! 4.1.3 Helicenes

As of [4]helicene (benzo[a]phenanthrene, 4), these species are significantly nonplanar as shown by X-ray crystallography [23a], NMR [23b], photoelectron [23c] and UV spectroscopy [23d]. (A detailed presentation of helicenes is given in Section 4.3.) The racemization free energy of the parent [6]helicene 5 and some

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larger counterparts [24] have been measured. However, to determine their strain energies requires 'Hf measurements which are absent except for [4]helicene itself: this species is ca. 17 kJ mol–1 less stable than its triphenylene isomer 6. Thus, we cannot deduce how homologous is this series – when is the 'Hf difference of [n+1] helicene and [n]helicene constant? We recognize these helicenes are catacondensed (ortho-fused), and note for the simpler class of homologous catacondensed species, the acenes, our thermochemical knowledge tantalizingly ends at n = 4, naphthacene. We now discuss substituted derivatives. Methyl derivatives of [6]helicene show positional effects on racemization rates [25]: unsubstituted ~ 3 ~ 4 ~ 13 ~ 14 < 2 ~ 15 . 1 ~ 16. The bis-indeno derivatives of 4,5-diphenylphenanthrene[26a], 1,12-diphenyl[4]helicene and 1,14-diphenyl[5]helicene [26b] show large twist angles by crystallography. These species arise from mild base-catalyzed rearrangements of unstrained PhC{CC6H4C{CCH2-derivatives of benzene, naphthalene and phenanthrene. It is a testimony to the high energy of the C{C triple bond in the precursors and the resonance stabilization of newly formed benzene rings in the products that such twisted polynuclear aromatic hydrocarbons are readily formed. Indeed, the photochemical isomerization of diphenylacetylene to phenanthrene [27] is ca. 180 kJ mol–1 exothermic using contemporary 'Hf [28]. This quantity is large enough to accommodate considerable strain in highly distorted aromatic products [29]. 4.1.4 [n]Circulenes

Two circulenes with known 'Hf are corannulene (n = 5, 7) and coronene (n = 6, 8). The significantly nonplanar corannulene and planar coronene have gas phase 'Hf [30] of 460.6(6.5) kJ mol–1 and 307.5(9.8) kJ mol–1, respectively. Lacking 4,5 H,H repulsions we take coronene to be unstrained, neglecting SE as found in phenanthrene. Naively, the 'Hf of corannulene would be 5/6 that of coronene, but is more positive by 204 kJ mol–1, a situation exacerbated by another 40 kJ mol–1 for planar corannulene [31]. To put this strain and aromaticity difference into context, the isomeric naphthalene and azulene show the former to be more stable by almost 140 kJ mol–1: in both cases the totally benzenoid species is more stable by some 12 kJ mol–1 per carbon. On the other hand, the parent (C60) and C70-fullerenes (for which corannulene may be understood as en route to) have 'Hf of ca. 42 and 39 kJ mol–1 per carbon [32], while ‘gaseous’ benzenoid graphite is much more stable, with 'Hf of 6 kJ mol–1 per carbon [33]. While a ‘convenient synthesis … of [7]circulene’ 9 has been reported [34], its enthalpy of formation is unknown. Interestingly, bowl-shaped corannulene and saddle-shaped [7]circulene have comparable planarization energies [35]: how does that relate to their strain energies? We note one synthetic detail [36] for [7] circulene: dehydrogenation of 1,16-didehydro-2,15-dimethyl[6]helicene 10 with a ‘preformed’ 7-membered ring core did not result in 9 or its 1,2-dihydro derivative, but instead, the fluorene derivative 11.

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4.1.5 Cyclophanes

Cyclophanes with small bridges, discussed in detail in Section 4.2, represent classic examples [5] of distorted benzene rings, most typically reminiscent of boat cyclohexane. Commencing with [n]cyclophanes, the quintessential question is how small a polymethylene chain can be used to affix two carbons of a benzene ring. Starting with planar orthocyclophanes, perhaps we are jaded but the [1]-derivative 12 looks normal. More commonly known as cycloproparenes discussed in Section 4.4, the SE of this species was indirectly determined by reaction calorimetry [37] to be 284 kJ mol–1. However, orthocyclophanes do not exhibit nonplanar distortions and so will not be further discussed. What about [1]metacyclophane 13 and/or [1]paracyclophane 14? The 'Hf of the corresponding ‘ring-opened’ species, D,3- and D,4-didehydrotoluene (e.g. 15a/15b), are 431(13) kJ mol–1 for both isomers[38]. However, should we preclude the existence of [1]-paracyclophane, perchance recognized as norbornadiene-1,7-diyl 16 the ‘opened’ [2.2.1]propelladiene wherein aromaticity is sacrificed to accommodate bonding of the methylene group to ring carbons? We remember the high kinetic stability of [1.1.1]propellane presented in Chapter 2.1 and diverse derivatives despite exceptional SE relative to ring-opened mono- and bicyclic products [39]. Regardless, we know of no evidence for [1]paracyclophane, nor its isomer [1]metacyclophane, nor of their presumably less strained [2] and [3] counterparts. How small can we go? [3]Paracyclophane 17 and its meta isomer remain unknown. [4]Metacyclophane 18 remains but a reaction intermediate [40] while its Dewar isomer 19 is isolable [41], and [4]orthocyclophane is tetralin. [4]Paracyclophanes return us to precariously isolable species. Discussed in [42], NMR spectra of [4]paracyclophane and even the crystal structure of some derivatives can be obtained. These species show both considerable geometric distortion and significant remnants of the aromaticity of p-disubstituted benzenes. Interconversion of [4]paracyclophane and its more stable Dewar isomer has been observed in both directions. Two unsaturated derivatives are particularly interesting: polyene 20, 1,2,3,4-tetradehydro[4]paracyclophane, has been observed [43] and shown by reaction chemistry, spectroscopy and model calculations to be energetically preferred over its S-bond isomer 21, bicyclo[4.2.2]deca-1,3,5,7,9-pentaene, a formally unsaturated analog of [2]-1,4-cyclooctatetraenophane. There are seemingly no cyclooctatetraenophanes with such a short bridge, although longer, saturated derivatives are known, as are longer 1,3- and 1,5-cyclooctatetraenophanes [44]. A less relevant nonhydrocarbon example [43] has a (NC)2C=C-CH=CH-C=C(CN)2 group para-bridging a benzene ring. Powerfully demonstrating the role of substituents in kinetically stabilizing small cyclophanes, we have ‘ almost a limiting case of experimentally characterizable strained paracyclophanes’ [43]. What about [5]cyclophanes? We don’t know 'Hf and/or reaction enthalpy for [5] paracyclophane and its Dewar isomer. [5]Metacyclophane is an isolable species [45], but attempts to determine its 'Hf were thwarted due to formation of a hyperstable

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olefin during calorimetric hydrogenation [46]. The recently developed alternate approach to aromaticity, the so-called ‘isomerization method’ [47], contrasts a methylated putative aromatic (e.g., 22) with its ‘isotoluene’ analog (e.g., 23). The 'Hf difference of these isomers (the ISE – isomerization stabilization energy) from quantum chemical calculations [47] is 139 kJ mol–1, reduced to 82 kJ mol–1 for [5]paracyclophane, suggesting that most of the aromaticity remains for this species. We welcome corresponding analysis of other cyclophanes. For [6]paracyclophane and its derivatives, Dewar isomers were experimentally shown [48] to be less stable than the benzene form by some 130 kJ mol–1 – however distorted the benzene ring, aromaticity prevails over strain. [6]metacyclophane is isolable, but, as with its smaller [5] counterpart, 'Hf determination by measurement [46] of its enthalpy of hydrogenation was thwarted. We now turn to cyclophanes containing ring systems other than benzene. Recalling earlier discussion of species containing cyclooctatetraene, we jump to those with a 9,10-bridged central anthracene ring, with –CH2S(CH2)n-4SCH2– (n = 8, 9, 10, 12). As shown in basic solution by NMR, the methylenedihydroanthracene isomer with the tautomerized =CHS(CH2)n-4SCH2- bridge is thermodynamically preferred [49]. Similar isomerism is seen for undistorted ‘acyclic’ anthracene with two –CH2SC3H7 groups, unlike methylanthracene [50]. The all hydrocarbon parent and 1,4,5,8-tetrasubstituted [6] (9,10)anthracenophanes [51] undergo acidcatalyzed rearrangement to the corresponding methylenedihydroanthracene. Yet aromaticity prevails: the Dewar isomers of these anthracenophanes are less stable than their benzenoid counterparts. Returning to [1]bridged cyclophanes, stable 1,5- and 1,6-methano[10]annulene 24, 25 were studied by hydrogen calorimetry [52]. The calculated ISE [47] for the latter is 106 kJ mol–1 vs. 215 kJ mol–1 for naphthalene. No numbers are available for the former species – because different SEs accompany bridgehead double bonds in bicyclo[5.3.1] and [4.4.1]undecenes, we hesitate to use the difference for 'Hf of the two methanoannulenes, ca. 80 kJ mol–1. We are even more hesitant to suggest a related difference of 1,5-methano[10]annulene and its unknown tautomer bicyclo[5.3.1]undeca-1,3,5(11),6,9-pentaene 26, a [5]metacyclophane derivative. This brings us to [m,n]cyclophanes, which we limit to meta 27, metapara 28 or para 29. As reviewed in [42], [1,1]paracyclophanes are characterized in terms of structure and activation parameters for isomerization with their Dewar counterparts. We also note the authors of [45] are optimistic about the eventual formation and characterization of [1,1]metacyclophane. [1,1]metaparacyclophanes are likewise unknown – photolysis of diketo derivatives of [2,2]metaparacyclophane do not produce the desired species by double elimination of CO. Rather, a different fragmentation process [53] results in m-benzyne, another relatively exotic species [54]. The relative energies of [1,1]meta, metapara and paracyclophane remain unknown. For the larger considerably less strained [2,2]cyclophanes (cf. ref. [5]), 'Hf of these species, studied as gases, increase in the same order: meta, 170.5(6.5); metapara, 218.4(1.6), para, 244.1(2.6) kJ mol–1. For the corresponding dienes, the enthalpies of formation are ca. 407 kJ mol–1 [32], unknown, and 493.0(5.0) [55].

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We note that the presence of the two double bonds affects changes in 'Hf: meta, 237 and para, 249 kJ mol–1. By contrast, twice the difference of orbitally conjugated [56] (E)-stilbene and 1,2-diphenylethane is 186.4(3.6) kJ mol–1, from which it may be deduced that the two double bonds increase the SE of [2,2]meta- and paracyclophane. Relative reactivity, rather than calorimetry, has been used to derive the same conclusion for [2,2]metaparacyclophane [57].

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We close this discussion of cyclophanes and this chapter with the conclusion that [6,6]paracyclophane has negative SE [58] – while this species is therefore outside the scope of the current chapter because of its lack of strain, this observation nonetheless intrigues [59].

4.2 Distorted Cyclophanes

Henning Hopf 4.2.1 Introduction

Since Kekulé proposed the benzene ring structure, the image of this hydrocarbon as a rigid, flat, hexagonal molecular object has burned itself into the minds of chemists. The ideal of the rigid hexagonal ‘tile’ has not only been reinforced by the structures of the polycondensed aromatic systems but also by the Hückel theory of aromaticity which proposes that the ‘aromatic character’ of benzene 30 and its derivatives is most pronounced when the axes of the p-orbitals are parallel allowing optimal p,p-overlap. Intuitively, one might expect that any disturbance of this overlap, 31, would reduce the aromaticity. This expectation has been the main driving force for the attempt to prepare deformed aromatic systems. As we will see, the planarity of aromatic compounds can be deformed considerably before there is any disturbance in their chemical and spectroscopic behavior (Figure 4.1) [60]. In principle, there are many ways to deform a benzene ring and a number of these distortions are encountered in the different normal modes. To translate this dynamic behavior into a stable distorted structure the simplest approach uses the incorporation of a molecular bridge, usually a polymethylene chain, causing the generation of a cyclophane. For a simple benzene ring as the aromatic moiety to be spanned, the bridge may be anchored in 1,4- ([n]paracyclophane 32) or in the

Figure 4.1 Bridging the benzene ring.

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1,3-position ([n]metacyclophane 33). The benzene ring in ortho-arrangement, 34 is usually planar. Employing the ‘phane concept’ numerous bridged aromatic compounds has been prepared during the last decades [61], although the first cyclophanes, Lüttringhaus’ ansa compounds [62], were already prepared in the early 1940s. A particular attraction is exhibited by cyclophanes possessing a second aromatic ‘deck’ as in 35, since in these molecules the distance and orientation of the aromatic ‘layers’ may be adjusted by the lengths of the bridges (m, n in 35). Because of space limitations this chapter will concentrate on representative examples derived from 32, 33, and 35, and the main questions to be answered will be the preparation of these compounds and the relationship between bridge length, ring distortion, and ‘aromatic character’, the latter being judged mainly be the NMR properties of the cyclophanes (see Section 4.2.5). Available calorimetry data for cyclophanes are discussed in Section 4.1.5. 4.2.2 The [n]Cyclophanes 4.2.2.1

[n]Paracyclophanes

Within the class of cyclophanes containing a single aromatic moiety only, this group has received the highest attention [61, 63]. Calculations by different methods support the expected trend; the shorter the bridge in 32 the more distorted is its benzene ring. Table 4.1 summarizes the relationship between bridge length, distortion angle D, and heat of formation of the respective [n]paracyclophane according to AM1 calculations [64]. Table 4.1 Relationship between bridge length, distortion angle D, and heat of formation of respective [n]paracyclophane according to AM1 calculations.

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'Hf (kcal mol–1]

n

D [°]

14

0.5

–62.3

13

1.3

–54.9

12

1.9

–48.9

11

2.2

–40.5

10

3.4

–29.5

9

9.6

–18.9

8

13.4

–8.7

7

16.4

5.1

6

22.3

26.2

5

28.6

53.8

4

35.6

87.8

3

79.4

117.8

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(a) ' (23%); (b) Zn/Hg, HOAc, HCl (54%); (c) Br2, MeOH, –30 °C, H2O (47%); (d) LiAlH4/AlCl3 (65%) Scheme 4.1

To prepare [n]paracyclophanes 32 with n down to about 9 should pose no particular synthetic problems. The typical methods used to make most of the smaller [n]paracyclophanes (n d 8) can all be demonstrated by the preparation of [8]paracyclophane 39. By employing a mixture of the two Hofmann bases 36 and 40 and subjecting it to a 1,6-elimination (Scheme 4.1) [65] the p-xylylene 37 and its furanoid analog 41 are generated which cycloadd to produce the furanophane 38, that subsequently can be reduced directly to 39 or via the diketone 42. The gain in aromatic resonance is also made use of when the bis-spiro diene 43 is pyrolyzed in the presence of trapping agents such as 1,3-butadiene (46, Scheme 4.2). The reaction begins with pyrolysis of a three-membered ring to provide the diradical 44 l 45 first. Trapping then provides the monoene 47 that is hydrogenated to the saturated hydrocarbon 39 [66]. An important general approach (Scheme 4.2) to the smaller [n]paracyclophanes employs spirocyclic ketones such as 48, which is first converted to a cross-conjugated triene 49 followed by flashvacuum pyrolysis (FVP) to provide the diradical 50 that recloses to 39 with the remarkable yield of 70% [63]. In another approach strain-free precursors, like 51, are first built-up and subsequently subjected to ring-contraction processes as shown in Scheme 4.3 for 4-carboxy[8]paracyclophane 54 [67]. 51 was prepared by an acyloin condensation/oxidation protocol from the appropriate open-chain starting materials, and then ring-contracted by a Wolff rearrangement of 52 via ketene 53 to the acid 54. After conversion of 54 into 4-keto[8] paracyclophane the sequence could be repeated, and 3-carboxy[7]paracyclophane be obtained [68]. The approach beginning with D,Dc-dichloro-p-xylene 55 and 1,6-hexandithiol 56 illustrates the principle of the uncoupling of the two critical steps in [n]paracyclophane chemistry: the construction of an intermediate that

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Scheme 4.2

Scheme 4.3

Scheme 4.4

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Scheme 4.5

already has the correct topology of the target molecule (here [10]paracyclophane 57) and the built-up of strain in the last step [69]. Ring contraction is accomplished by sulfone pyrolysis [70], using the bis sulfone 58, obtained readily from the bis thioether 57. In the acid 54 the experimentally determined deformation angle D (see 32) amounts to 9.1° [71], whereas in the lower homolog [68], it has increased to 16.5°, in excellent agreement with the calculated value (see above). The parent system of the latter was obtained using the approach via reactive carbene 60 (Scheme 4.4, Jones route) [72]. Here the spirocyclohexadienone tosylhydrazone 59 was metalated with n-butyl lithium and the resulting salt thermally decomposed under FVP conditions to 61 [73]. A conceptually interesting route to [n]paracyclophanes involves the conversion of a heterophane into a benzophane (Tochtermann route) [74]. As shown in Scheme 4.5 the bridged furan 62 undergoes the expected Diels-Alder addition with dimethyl acetylene dicarboxylate (DMDA) to the adduct 63, that is converted via the oxaquadricyclane 64 into the bridged oxepin 65 by a sequence of pericyclic reactions. When the latter was brominated the dibromide 66 is obtained, which on debromination/deoxygenation yielded the diester 67. Various derivatives of 67 show deformation angles D around 15° [74]. Moving down to the parent system [6]paracyclophane 71, both the Jones route (Scheme 4.4) [75] as well as the Tochtermann route to the dimethyl diester were successful. Apparently, both methods reached their limits here, since they failed in the preparation of [5]paracyclophane or its derivatives. Several derivatives of these cyclophanes could be prepared, and their solid state structures be determined: for [6]paracyclophane-8-carboxylic acid the bending angles D (see 3) amount to 20.3 and 21.1°, respectively [76], and for the dimethyl 8,9-dicarboxylate this angle is 19.5° [77]. The 3,3c-disubstutited bicyclopropenyl derivative 68 can be rearranged into their Dewar benzene isomers, followed by valence isomerization as shown in Scheme 4.6. The metal-catalyzed step afforded a mixture of the two isomeric Dewar benzenes 69 and 70, which after separation were thermally opened to their aromatic isomers 71 and 72, respectively. On irradiation 71 recloses to 69, indicating the high strain of this cyclophane [78].

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Scheme 4.6

A longer, but superior route was developed to prepare [6]paracyclophane (Scheme 4.7) [79]. The bicyclic ketone 73 was first converted into the propellane 74 by addition of acetylene to the double bond of the substrate 73. Wolff rearrangement of the D-diazoketone 75, prepared from 74 by azo group transfer, next furnished the ester 78. The Dewar benzene intermediate of 78 was opened thermally to the [6]paracyclophane ester 77, which on hydrolysis provided 76 in 10% yield. Alternatively, the parent system was prepared from 78 by first saponifying it and subsequently oxidizing the formed acid. The 1H NMR spectrum of 71 does not differ significantly from that of an undistorted aromatic compound [75, 78]. [5]Paracyclophane 80 could be only generated by irradiation of the 1,4-pentamethylene Dewar benzene analogue of 69 in THF-d8 at –80 °C [80]. Hydrocarbon 80 decomposes rapidly above 0 °C, but it was identified unequivocally by its UV

Scheme 4.7

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159

and 1H NMR spectra [81]. Its aromatic protons absorb as an AAcXXc-multiplet at 7.38 and 7.44 ppm, respectively, whereas the benzylic protons are registered at 2.77 ppm, both shifted downfield from the respective protons of p-xylene. Esters such as 81 [82] and 82 [83], obtained by photoisomerization of their respective Dewar benzene isomers have half-lives of several hours at room temperature.

The smallest [n]paracyclophane, identified by trapping experiments, is the tetramethylene bridged system 84 (Scheme 4.8). Irradiation of the butano-bridged substrate 83 at –20 °C in THF in the presence of trifluoroacetic acid, provided the addition products 86 and 87 [84]. If the experiment is performed in methanol the ether 88 is produced. It is rationalized that the precursor 83 indeed photoisomerizes to 84, which is protonated to 85, that subsequently yields the trapping products 86–88. In a second trapping experiment the Dewar benzene ester 89 on photolysis in the presence of ethanol yielded the regioisomeric 1,4-trapping products 91 and 92, for which the [4]paracyclophane ester 90 is the most reasonable precursor. When 83 was irradiated in a matrix at 77 K, the UV spectrum of 84 could be recorded [85]. Although computational studies concerning [3]paracyclophane have been published [86], no preparative efforts have been undertaken.

Scheme 4.8

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4.2.2.2

[n]Metacyclophanes

A significant ring strain arises in [n]metacyclophanes only when the bridge length is seven or smaller [63, 87]. Three basic approaches have been developed to synthesize [n]metacyclophanes as summarized in Scheme 4.9. The first route begins with cyclic precursors such as dodecanone 93, i.e. substrates that contain the future molecular bridge from the very beginning. In case of 93 this is accomplished via intermediates such as 94 [88]. In the second route the aromatic unit is present from the start, e.g. as the dithiol 97, and the rest of the synthesis involves bridging it; a typical example is given by the sulfone pyrolysis of 98 yielding [9] metacyclophane 99 [89]. In the last approach the metacyclophane is obtained from its para-isomer by acid-catalyzed isomerisation. All three approaches have been employed to synthesize e.g. [7]metacyclophane [90–92]. For the synthesis of lower homologues such as [5]metacyclophane, methods similar to those employed in the para-series have been developed [93]. Again, the [4]cyclophane represents the smallest hydrocarbon (generated as an intermediate) in this series also [94]. The parent [n]metacyclophanes have low melting points, so only a few derivatives have been subjected to X-ray structural analysis. 8,11-dichloro[5]metacyclophane possesses a pronounced boat-structure, with the functionalised carbon atoms bent out of the plane by 12° (unbridged) and 27° (bridged side). Its 1H-NMR spectrum that displays a ring current equal to that of analogous planar benzene derivatives [95].

Scheme 4.9

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4.2.3 The [m.n]Paracyclophanes

Among the [m.n]cyclophanes, those in which the two alkano bridges are anchored in the para-positions of both benzene ‘decks’, have received greatest attention. We will begin with [0.0]paracyclophane 103 (Scheme 4.10) and end with the [4.4] homolog for this discussion.

Scheme 4.10

The extremely strained 103 should better be regarded as a valence isomer of tricyclo[4.2.2.22,5]dodeca-1,3,5,7,9,11-hexaene 104 of which recently the dicyano derivative 76 has been generated as a reaction intermediate [96]. If one of the bridges is formally ‘opened’ the para-bridged biphenyl derivatives 106 result [97]. The [1.n]paracyclophane series begins with [1.1]paracyclophane 114 (Scheme 4.11); its synthesis employs many of the steps that have been discussed above for the preparation of the smaller [n]paracyclophanes [98]. Tricyclic diketone 107 on photoaddition of acetylene afforded bis-adduct 108, which was converted to the bis-diazo diketone 109. Wolff rearrangement in methanol then furnished 110, in which the double Dewar benzene structure is clearly discernible already. The missing double bonds were introduced by double Hofmann elimination of diamine 111, obtained from 110 by routine methods. Irradiation of 112 at 77 K in an EPA matrix then furnished 114 via ‘half-opened’ 113. That 114 had been generated was inferred from UV/Vis and NMR spectra. However, on warming to room temperature, 114 was destroyed within 4 h [98]. On extended irradiation 114 was converted into the interesting hydrocarbon 115. Calculations revealed that its benzene rings are approximately as distorted as the one in [5]paracyclophane 80, the bending angle by which the bridgehead carbon atoms are ‘removed’ from the plane amounting to 23°, compared with the 12.6° of [2.2]paracyclophane (exp., see below) and 23.5° (calc.) for 80. The intraannular distance between the atoms of the benzene rings is ca. 236–240 pm, i.e. roughly 100 pm shorter than the inter-layer distance in graphite. These calculations indicate

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4 Strained Aromatic Molecules

Scheme 4.11

the delocalisation of the electrons despite severe bending of the aromatic moieties. The calculated strain energy of 114 is in the range of 93–128 kcal/mol depending on the level of theory. Although the parent hydrocarbon [1.2]paracyclophane is unknown and presumably would not differ very much in strain and benzene ring deformation from the [1.1]compound 114 [99], the [1.n]phanes 116 [100] and the analogous ketones 117 [101] have been prepared by FVP of the corresponding sulfones. In the pentamethylene bridged parent hydrocarbon 118 [102] the bridgehead atoms are not only displaced out of the plane by about 10°, but with only 235 pm the distance between the substituted carbon atoms at the ‘methylene side’, distinctly shorter than the analogous distance in [2.2]paracyclophane (278 pm, see below). This observed intraannular distance agrees remarkably well with the calculated one for [1.1]paracyclophane 114 (see above) and for different highly functionalized derivatives of 114 [103].

The by far most thoroughly studied phanes are [2.2]paracyclophane 119 (Scheme 4.12) and its derivatives. Numerous routes to prepare these compounds have been described, and their chemical and structural properties have been reviewed [60, 61, 63], that an additional summary is unnecessary here. The basic strategies to prepare [2.2]paracylophanes are summarized in highly condensed retrosynthetic form in Scheme 4.12.

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163

Scheme 4.12

Cleavage of one of the ethano bridges in 119 results in the formation of the diradical 120 for whose generation either other cyclophanes such as 124 and 125 or open-chain precursors such as the dibromide 126 may be used. All that remains to be done in the last steps is a ring-contraction, for which the photochemical or high temperature extrusion of small molecules (e.q. CO, CO2 or SO2) are the methods of choice. In the case of 126 only Wurtz coupling is required; however, since in this process intermolecular coupling competes with intramolecular C-C bond formation, yields are low [104]. If the other bridge in 120 is also broken, the diradical 121 results, which is nothing else than p-xylylene or p-quinodimethane 37. For this reactive tetraene numerous derivatives of p-xylene 127 can serve as a precursor, beginning with p-xylene itself (127, X = Y = H) [105], and ending e.g. with the Hofmann base derived from D-bromo-p-xylene 127 (X = H, Y = N(CH3)3OH) [106]. Continuing our retro-synthetic deconstruction, we next decompose 37 in retro-Diels–Alder fashion and arrive at 1,2,4,5-hexatetraene 122 as the diene and acetylene 123 as the dienophile component. Bisallene 122 is prepared by dimerization of propargyl bromide 128 and since simple alkynes are not reactive enough to undergo Diels-Alder additions with it, 123 has to be replaced by an activated acetylene 129 with various electron-electron-withdrawing substituents [107]. The structure of 119 has been determined several times [108, 109]. Only some important structural data resulting from the latest study (recorded at 19 K) are given in Figure 4.2. As expected, extension of the alkano bridges leads to a flattening of the benzene rings and an increase in the distance between them. In [3.3] paracyclophane 130 the above deformation angle has been halved to 6.4° (intraannular distance at the bridgeheads 314 pm and distance between the planes passing through the remaining aromatic carbon atoms: 320 pm) [110] and [4.4]paracyclophane 131 has flat aromatic rings [111]. In the hybrid case of [2.4]paracyclophane 132 the two benzene rings are inclined at an angle of 21°, the distortion angle at the bridgeheads is slightly above 5° (at all positions), and the two non-bonded distances at the bridgeheads amount to 279 pm (ethano side) and 386 pm (butano

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Figure 4.2 Some structural parameters of [2.2]paracyclophane.

side) [112]. A fascinating known completely bridged cyclophane called superphane is mentioned in Chapter 9, its hypothetical hexahydrogenated analogue with a planar cyclohexane ring is briefly discussed in Section 2.4 while known but insufficiently studied layered cyclophanes are introduced in Section 1.4. 4.2.4 Distorted Aromatic Rings and ‘Aromatic Character’

The planarity of classical aromatic systems (usually benzene rings) may be distorted effectively by the introduction of molecular bridges. Using this device highly bent benzene rings with deformation angles D (see structure 32) up to 25° may be generated readily. As judged from their X-ray structures and NMRproperties (see Section 5) these strongly bent benzene ring systems are delocalized to the same extent as benzene and its simple derivatives. On the other hand, many cyclophanes with short bridges display a reactivity unprecedented for normal aromatics, for example they undergo Diels-Alder additions readily [113] or can readily be hydrogenated [114]. For example, it has been calculated that [5] metacyclophane has a strain energy of 43 kcal/mol [115]; for [2.2]paracyclophane 119 a strain energy of 30.1 kcal/mol has been determined [116]. Strain release would be effective not only for the products of the reaction, in which the formal sp2 hybridization changed to sp3, which makes bridge formation easier. As an additional factor the deformation of the S-electron cloud of the benzene ring could play a role, since this raises the HOMO and lowers the LUMO energy. Localization of S-electrons into a cyclohexatriene moiety would increase the violation of Bredt’s rule even further, and is hence avoided. And it may finally play a role that it appears to be the V-frame that dominates the delocalization in benzene rather than the S-system [117].

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4.2.5 NMR Characteristics of Cyclophanes 1

H NMR spectroscopy can furnish detailed information with respect to structures, conformations and conformational dynamics of this class of compounds and a number of NMR reviews on cyclophanes is available [118–122]. The most important feature in the 1H NMR spectra are the unusual chemical shifts of protons lying above the plane of the aromatic component of the cyclophane. Shortly after Pople suggested [123] that protons connected to aromatic rings are deshielded by a ring current induced by the external magnetic field, Waugh and Fessenden [124] reported the increased shielding, by 0.7 ppm, of the central methylene protons in [10]paracyclophane 32 relative to those in cyclohexane. These protons are held in a position over the center of the ring. Shortening the methylene bridge decreases the average distance between these protons and the region of maximum shielding. The most highly shielded proton in [5]paracyclophane 80 has G = 0.01 ppm [81] while in [6]paracyclophane 71 G = –0.62 ppm [125].

In [2.2]paracyclophane 119 the aromatic rings cause mutual shielding of their aryl protons by –0.62 ppm [G(119) = 6.48 ppm [126], G(1,4-diethylbenzene) = 7.10 ppm)]. This is only a moderate effect because the rings are eclipsed. The effect in [2.2.2](1,3,5)cyclophane 133 is somewhat larger because the triple bridging decreases the average distance between the rings [G(133) = 5.71 ppm [126], G(1,3,5-triethylbenzene) = 6.86 ppm)]. In anti-[2.2]metacyclophane 134, however, the protons experience strong shielding [G(134) = –2.75, 4.27 ppm [126], G(1,3-diethylbenzene) = 7.02 ppm)] as they are situated much closer to the region of maximum shielding of the opposite aromatic ring. Finally, Pascal’s in-cyclophanes show extreme shielding effects, for example G = –4.03 ppm for Hi in 135 [127], because the geometry of the molecular cage forces the internal proton to point toward the center of the aromatic ring at a very small distance (172 pm, calc.).

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4.3 Helicenes

Ivo Starý and Irena G. Stará 4.3.1 Introduction

Helicenes as unique, inherently chiral three-dimensional aromatics have been attracting continuous attention for decades. They are usually chemically stable and soluble in common organic solvents, which makes a difference from many other large S-conjugated systems. The chemistry of helicenes has been reviewed several times [128–135]. However, since an enormous amount of progress in their synthesis and use took place in the early 1990s, the recent period has not been covered by a comprehensive review. Thus, this chapter is mostly focused on the important achievements in helicene chemistry within the last two decades. Helicenes are polycyclic aromatic systems consisting of all-ortho-fused aromatic rings. The prefix before the name expresses the number of fused cycles as exemplified by hexahelicene or simply [6]helicene. Provided all these rings are benzenes, such compounds are called carbohelicenes 136. If one (or more) benzene unit is formally replaced by a heterocycle, such a skeletal modification leads to heterohelicenes 137. Finally, helicene-like compounds represent the third family of helicenes, which can differ significantly from fully aromatic parent helicenes but having a similar molecular shape 138. As helicenes can exist in two enantiomeric forms regardless of their configurational stability, the handedness of the helix is specified by adding the (M) (minus) or (P) (plus) prefix (Scheme 4.13). This chapter deals only with carbohelicenes, which are simply called helicenes within the following text.

4.3.2 Synthesis of Helicenes

[6]Helicene was the first helicene representative prepared intentionally by Newman and Lednicer in 1956 [136], who also pioneered resolution of the racemate employing a S-S donor-acceptor interaction between helicene and optically pure (–)or (+)-2-(2,4,5,7-tetranitro-9-fluorenylideneamino-oxy)propionic acid (TAPA).

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167

Scheme 4.13

The real breakthrough in the synthesis of helicenes came in the late sixties when the Martin group introduced photodehydrocyclisation of stilbene-type precursors [137–139] as the first general method for their preparation [140]. It was based on UV-light induced cis/trans isomerisation of 1,2-diarylethylenes followed by conrotatory electrocyclisation of the cis isomer to generate a primary dihydroaromatic product with trans configuration (Scheme 4.14).

Scheme 4.14

Then, in the presence of air and a catalytic amount of iodine, it was immediately converted to a fully aromatic system [135]. However, photocyclisation leading to an all-ortho-fused system may often be disfavoured for steric or other reasons and, therefore, the process may exhibit low regioselectivity. To overcome this problem,

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Liu and Katz developed synthetic methodology utilising a bromine substituent on a benzene ring, which directs photocyclisation away from its ortho position [141]. A remarkable step forward was made in 1991 when Katz and colleagues published an updated version of photodehydrocyclisation of stilbene-type precursors (Scheme 4.15) [142]. They found that iodine in a stoichiometric amount is superior oxidant and propylene oxide is an effective scavenger of hydrogen iodide. Since then, this improved methodology has become a standard tool for the synthesis of helicenes. Numerous examples of successful use of photodehydrocyclisation in its original or innovated version were published to prepare, inter alia, [5]helicene [141], [6]helicene [143–145], [7]helicene [143], [8]helicene [145–147], [9]helicene [145, 146, 148], [10]helicene [145, 148], [11]helicene [145, 148, 149], [12]helicene [145, 149], [13]helicene [145, 148, 150], and [14]helicene [149]. Despite the synthetic accessibility of stilbene-type precursors and wide applicability and simplicity of photodehydrocyclisation, this methodology suffers from several drawbacks. To prevent photodimerisation, the irradiation must be performed under high dilution conditions which limits any scale-up [151, 152]. Furthermore, acetyl, dimethylamino and nitro groups that can perturb or depopulate the reactive excited state should be avoided [152, 153].

Scheme 4.15

Since the early 1990s, various non photochemical approaches have emerged to circumvent disadvantages of photodehydrocyclisation. The most important method eliminating the irradiation was published by the Katz group [152]. It was based on thermal Diels-Alder reaction of aromatic bisvinylethers with p-benzoquinone in excess to afford helicenes having terminal quinone rings (Scheme 4.16 [161]). It evolved into a robust and versatile method, which has also been employed

Scheme 4.16

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by other authors [154, 155] to prepare derivatives of [5]helicene [152, 156, 157], [6]helicene [152, 157], and [7]helicene [158–160]. The importance of the Katz group contribution basically consists of developing the simple methodology, which for the first time allows the synthesis of a wide series of functionalised helicenes on a multigramme scale and in providing highly attractive compounds for other applications [161]. The simplicity of Katz’ Diels-Alder methodology represents, on the other hand, its drawback. Variations of functionalities on the terminal (hydro)quinone rings of a helicene skeleton are rather limited. Therefore, other new methods for the preparation of helicenes have recently emerged on the stage. Stará and colleagues have developed a new strategy based on intramolecular [2+2+2] cycloisomerisation of aromatic triynes catalysed by Ni0 or CoI complexes (Scheme 4.17 [164]) [162–165]. Using this organometallic approach, various helicene derivatives were synthesised ranging from penta- to heptacyclic systems. This methodology exhibits considerable potential due to its modularity, chemo- and regioselectivity and allows for the formation of three new cycles of the helical scaffold all at once.

Scheme 4.17

Axially chiral biaryls can be viewed as truncated helicenes. Indeed, there is a group of diverse synthetic methods transforming biaryls to helicenes. Bestmann and Both [166] and Stará and colleagues [167] described the transformation of 1,1cbinaphtyl derived bisphosphinealkylene or dihydroazepinium salt, respectively, to [5]helicene (Scheme 4.18 [167]). Gingras and Dubois developed syntheses of [5]- and [7]helicene based on carbenoid coupling, which started in the case of [7] helicene from 4,4c-biphenanthrene-3,3c-diol (Scheme 4.19) [168]. Alternatively, they

Scheme 4.18

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4 Strained Aromatic Molecules

Scheme 4.19

applied McMurry coupling to the synthesis of [5]helicene from 1,1c-binaphthalene2,2c-dicarbaldehyde [169]. So far the latest contribution to this field was published by Collins et al., who employed ring-closing olefin metathesis to construct [5]-, [6]- and [7]helicene from divinyl biaryls (Scheme 4.20) [170]. The synthesis of helicenes from biaryls has significant potential but such an approach leads to the construction of only one new benzene ring in the helicene scaffold.

Scheme 4.20

Photocyclodehydrogenation of stilbenes to prepare helicenes has inspired other authors to develop its nonphotochemical alternative. Harrowven and colleagues used homolytic aromatic substitution in the case of diiodo cis,cis-distilbenes, which under treatment with tributyltin hydride and the VAZO radical initiator provided functionalised [5]- and [7]helicenes (Scheme 4.21) [171].

Scheme 4.21

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4.3.3 Nonracemic Helicenes

Since Newman’s preparation of optically pure [6]helicene [136], different routes based on the resolution of racemate have been explored to produce enantiomerically enriched or pure helicenes. The resolution of helicene racemates by HPLC on a chiral stationary phase is general and can be employed for analytical as well as preparative purposes [172–174]. There is only one practical method allowing for resolution of racemic helicenes on a multigramme scale so far. Katz and colleagues developed a robust procedure using chromatography of diastereomeric helicene pairs on silica gel (Scheme 4.22) [175]. This is suitable exclusively for helicen-1-ols, which are converted to (1S)-camphanates.

Scheme 4.22

Asymmetric synthesis of helicenes and their congeners is envisaged to be the most straightforward and efficient route to single enantiomers. Various concepts have emerged demonstrating basic principles rather than generally useful methodologies. Nevertheless, some of them might be highly promising. Classical photodehydrocyclisation of stilbene-type precursors can be carried out in an astonishingly stereoselective fashion. This was well demonstrated by the pioneering works by Vanest and Martin [176] and Katz and colleagues [177] who used stereocentre(s) external or internal to the helix to control stereoselectivity of helicene cyclisations. Carreño et al. developed an asymmetric version of the Diels–Alder approach providing helicene quinones with excellent optical purities (Scheme 4.23) [155, 178–180]. In addition to that, the last decade has witnessed other attempts at asymmetric synthesis of helicenes but stereocontrol observed has been moderate as published by Stará and colleagues (enantioselective Nicatalysed [2+2+2] cycloisomerisation of aromatic triynes) [162, 164, 174]. In spite of the above mentioned achievements, practical asymmetric synthesis of helical aromatics has so far remained a challenging task.

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Scheme 4.23

4.3.4 Intriguing Helicene Structures

Regarding the highest helicene homologues so far synthesised, the world record among carbohelicenes belongs to Martin and Baes, who synthesised [14]helicene (Scheme 4.24) [149]. In order to prepare much longer helicene structures, the Katz group explored the ways of polymerising bifunctional helicene units. Placing a salicylaldehyde functionality at each end of optically pure [6]helicene, reaction with o-phenylenediamine and nickel(II) salt led to the first polymer with an unbroken network of double bonds that winds in one direction along a helix [181, 182]. The degree of delocalisation was higher than in optically active [7]helicene-based cobaltocenium oligomers [183, 184]. Structural diversity of helicene molecules is broad, encompassing helical metallocenes by Katz and Pesti (the Fe or Co atom spans the ends of a helicene backbone, Scheme 4.25) [185], helical metal phthalocyanine derivatives by the Katz group [186], conjugated helical acetylene-bridged cyclophanes by Fox et al. [187], 2,2c-bis[6]helicyl by Laarhoven and Veldhuis [188], helicenes containing a cyclophane unit by Nakazaki et al. [189], Martin and colleagues [190], and S-shaped [191] or 3-shaped [192] double helicenes by the Laarhoven group, to mention but a few.

Scheme 4.24

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Scheme 4.25

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4.3.5 Physicochemical Properties and Applications

As helicenes have robust, inherently chiral scaffolds, it is not surprising that they were soon applied by the Martin group to asymmetric synthesis as chiral auxiliaries (in diastereoselective reduction of D-keto esters [193], ene reaction [194] and atrolactic synthesis [195]) or chiral reagents (in hydroxyamination [196] or epoxidation [197] of olefins, Scheme 4.26) with remarkable success. After certain delays, attention has turned to enantioselective catalysis and so pioneering exploitations of helicene ligands have recently emerged. Highly stimulating results were obtained in asymmetric hydrogenation (Reetz et al. [198], Nakano and Yamaguchi [199]), allylic substitution (Reetz and Sostmann [200], Scheme 4.27) and diorganozinc addition to aldehydes (Katz et al. [156], Soai et al. [201]).

Scheme 4.26

Scheme 4.27

Regarding self-assembly, one of the most astonishing attributes of helicene assemblies was described by the Katz group [204] (Scheme 4.28) [130]. Properly substituted nonracemic helicenes, possessing both electron-rich inside and electron-deficient outside regions, can aggregate spontaneously to create columnar structures exhibiting enormous optical rotation values and NLO properties.

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Scheme 4.28

In Langmuir-Blodgett films, an array of parallel columns can be observed directly by AFM [202]. Moreover, these columns are further organised into long micrometre-wide lamellar fibres visible under an optical microscope [203]. The chiroptical properties of such assemblies are so remarkable that CD spectra could be measured for a monolayer [204]. The Moore group showed that a [6]helicenecontaining foldamer provided highly solvent dependent CD spectra [205]. There are remarkable examples of helicene use in molecular recognition. The [7]helicene-based helicopodand was used by Diederich and colleagues in molecular recognition of dicarboxylic acids with high diastereoselectivity [206]. Nonracemic helicene diol was successfully used by Reetz and Sostmann reporting on enantioselective fluorescence quenching by chiral amines [173]. A helicene derivatising reagent developed by the Katz group can serve as a remote chirality sensor for chiral alcohols, amines and phenols when detected by NMR measurements [207]. Helicenes were deposited on metal surfaces and studied by various techniques. Ernst et al. used near-edge X-ray absorption spectroscopy with linearly polarised synchrotron radiation (NEXAFS) to study the orientation of (P)-[7]helicene on a Ni(100) surface under ultrahigh vacuum (UHV) conditions [208]. This group also studied chirality transfer from (M)-[7]helicene into handed supramolecular structures on a Cu(111) surface by STM [209] and the orientation and the intramolecular relaxation due to adsorption of nonracemic [7]helicene on Cu(111) and Cu(332) surfaces by means of angle-scanned full-hemispherical X-ray photoelectron diffraction [210]. Various spectral and physicochemical properties of helicenes have been investigated. The fluorescence spectra, emission lifetimes, quantum yields of fluorescence and triplet state formation of a series of helicenes were studied by Vander Donckt and colleagues [211, 212]. They found the photophysical properties of the helicenes evolved steadily as a function of the number of ortho-fused benzene rings. Experimental photoelectron spectra of helicenes were analysed by Obenland and Schmidt to conclude that interaction between the S-orbitals of overlapping benzene rings is much smaller than in cyclophanes [213]. The comparison between experimental and calculated VCD spectra allowed Bürgi et al. to make the unequivocal assignment of the absolute configuration of [7]helicene [172]. An electrochemical study on helicenes was performed by Laarhoven and Brus who measured values of polarographic half-wave potentials [214]. Experimental

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Scheme 4.29

estimation of the reduction and oxidation potential of [7]helicene was done by Rulíšek et al. by measuring current-voltage curves on inert electrodes [215]. Focussing on molecular machinery, there is a fascinating application of a helicene structure by the Kelly group. In an artificial molecular motor, a helicene ratchet ensured unidirectional rotary motion fuelled by the chemical energy of periodic bond making/bond breaking processes (Scheme 4.29) [216]. 4.3.6 Theoretical Studies

The unique helicene structure has steadily attracted the attention of theoretical chemists. Grimme and Peyerimhoff studied the relationship between the structure and racemisation barrier in the series of helicenes by means of semiempirical AM-1 and ab initio SCF methods [217]. Similarly, Haufe and colleagues recalculated barriers to racemisation for helicenes finding an excellent agreement with the experimental results [218]. They confirmed the fact that the barriers for carbohelicenes converge to the value of about 45 kcal/mol. Ab initio calculations were carried out by Schulman and Disch in the series of helicenes and their closest topological isomers, planar phenacenes [219]. Their comparison revealed only slight loss of aromatic character in the former molecules. Most interestingly, the current-voltage characteristics of helicenes were calculated and their intriguing conductance was foreseen by Treboux et al. [220]. Helicenes can also be viewed as tiny mechanical objects since their structure resembles a molecular spring. Hartree–Fock calculations by Lipkowitz and colleagues using PM3 Hamiltonian revealed that the nanospring stiffness could be modulated by increasing/decreasing the electron density and length of helicenes [221]. Optical and chiroptical properties of helicenes were studied in detail by computational methods. The electronic CD spectra were calculated by the Ahlrichs group for helicenes, exploiting the adiabatic time-dependent DFT method as a prime tool for chiroptical property investigations. Thus agreement between the most important spectral features and theory was found [222]. There are also other theoretical approaches to chiroptical spectroscopy published by Hansen and Bak [223]. The hyper-Rayleigh scattering second-order NLO responses of helicenes were investigated by the Botek group employing the time-dependent Hartree-Fock

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approach and AM1 semi-empirical Hamiltonian [224]. The results of a series of DFT and DFT-D (the empirical dispersion energy terms included) calculations were reported by Rulíšek et al. with the aim to predict the physicochemical properties (equilibrium structures, stabilisation energies, redox potentials, excitation and CD spectra, electronic conductivity and elasticity) of elongating helicene structures [215]. It was shown that many of them are converged on [14]helicene. 4.3.7 Outlook

After five decades of helicene chemistry, interest in these unique aromatic compounds will certainly continue in the forthcoming years and there are many good reasons for it. As the synthesis of helicenes has recently witnessed significant progress, various helical aromatics have become more easily available than before. Important achievements might be expected, for instance in developing general asymmetric synthesis of helicenes, which remains still unsolved. However, applications of helicenes in various branches of chemistry, material science and nanoscience will be central to further efforts.

4.4 Cycloproparenes

Brian Halton 4.4.1 Introduction

The most highly strained ring-fused aromatics, the cyclopropa- and cyclobutabenzenes, have provided distinct fields of study for more than 50 years [225, 226]. Cyclopropabenzene 139 made its debut in an 1888 paper by Perkin [227] where its synthesis was noted as yet to be done; cyclobutabenzene 140 appeared some 20 years later in a 1909 thesis footnote [228]. Each parent has provided classes of compounds that continue to be the subjects of detailed scrutiny with numerous reviews covering various aspects of cycloproparene [226, 229–232] and cyclobutarene [225, 233] chemistry. The present cycloproparene synopsis summarizes developments in cycloproparene chemistry over the past five years [229].

The first cycloproparene claim [234] made in 1930, was inconclusive [235] and the 1953 addition [236] of Ph2CN2 to imide 141 did not give 143 as thought but the sulfonamides 142 [237] as now confirmed from X-ray data [238].

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4.4.2 Synthetic Considerations

That 3H-indazoles, analogs of 143, are precursors to cycloproparenes was confirmed by dinitrogen loss from 144 that gave the first stable derivative [239], but the route is not general. Wege found [240] that debromosilylation of 145 led to quinone 146 as a transient trapped by furan as exo/endo adducts. Attempts to obtain a kinetically more stable quinone targeted [241] C1 dialkylation via diazopropane addition to 1,4-naphthoquinone and subsequent deazetation.

The addition gave 147 that was easily oxidized to 148 (37%) (Scheme 4.30) but irradiation (350 nm) then gave a complex mixture [241]. Ultimately, primary adduct 147 was diverted to 149 (30%) and this lost nitrogen affording 150 (50%) and 151 (38%) as expected [232, 239, 242]. Attempts to dideacetylate 149 to hydroquinone and/or quinone were unsuccessful. Modification of the acetate groups of 149 gave further six derivatives of which only the 2-acetoxy-7-methoxy affords a cycloproparene on photolysis (Table 4.2, see p. 179). The Collis–Wege results [241] indicate limitations in the 3H-indazole route. Sylvania 350 nm lamps are ideal for excitation of C9 acetoxy compounds absorbing close to 350 nm (Table 4.2). Substrates that absorb at longer wavelength do not react because of insufficient energy absorption or enhanced mesomerism to N1 that strengthens the C9a–N1 bond; impact of the C9 substituent on the excited state also needs consideration [241]. The planarization of annulenes by annelating small rings across strategic sites has been recognized for some time. Dürr [243] synthesized 152 in 1983 and demonstrated close planarity by X-ray analysis. Schleyer’s group subsequently predicted highly aromatic planar all-cis-[10]annulenes from cyclopropa fusions [244], and Sastry’s group now have extended this to the rim of corannulene 153 (B3LYP/6-31G+G* calculations) where the inversion barrier is reduced by > 50% if all rim bonds are cyclopropannelated [245]. Now, the first parent non-benzenoid

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4 Strained Aromatic Molecules

Scheme 4.30

cyclopropannulene has been isolated by Stevenson [246]. Reaction of CH2Cl2 and cycloC8H7Br with t-BuOK in HMPA generates anion radical 154, but in THF 155 is formed from I2 oxidation of the dianion. Unlike the foul smell of 139, 155 has a sweetly olefinic odor. NMR spectra have the CH2 protons at G 4.61 (139: G 3.11) and those of the eight-membered ring at G 3.6–3.7; C1 (G 90.4) is deshielded 72 ppm relative to 139 and 13C labeling gives JC1–H 162 Hz (139: 170 Hz [247]; cycloC3H4: 167 Hz [248]). Clearly 155 has C1a–C2 and C7–C7a S bonded and a paratropic ring current. Calculations (B3LPY/6-31G*) confirm this with C1a–C7a 141.7 and C1a–C2 132.9 pm [246]. The formation of benzdiynes by loss of CO and CO2 from designed precursors continues with Sato confirming [249] 157 as a primary product from 156, and that loss of CO then gives diyne 158. Coupled with low temperature IR studies, energy diagrams (B3LPY/6-31G*) place the derivatives in the order of stability 157 > 159 > 160 > 158 with energy differences as shown in Scheme 4.31. Likewise, benzocyclopropenone 161 is 138 kJ mol–1 more stable than 162, but only 42 kJ mol–1 below ring contracted 163.

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Table 4.2 Photochemical behavior of 4,9-disubstituted 3,3-dimethyl-3H-benz[f]indazoles.a)

R1

R2

¼max log ¶

Outcomeb)

Ac

Ac

354 (3.49)

CPN (50%) + STY (38%)

Me

Ac

357 (3.57)

CPN (32%) + STY (43%)

H

TBDMSc)

390 (3.68)

decomposition

Ac

TBDMS

368 (3.65)

decomposition

Me

TBDMS

375 (3.73)

decomposition

Ac

H

372 (3.67)

no reaction

Ac

Me

366 (2.99)

no reaction

Me

H

379 (3.74)

no reaction

Me

Me

372 (3.71)

no reaction

a)

b) c)

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Data reproduced with permission from the Australian Journal of Chemistry: htpp://www.publish.csiro.au/journals/ajc; see Ref. [17]. CPN = cyclopropanaphthalene; STY = styrene. TBDMS = tBuMe2Si-

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Scheme 4.31

Scheme 4.32

The most general route to the cycloproparenes is from use of 1-bromo-2-chlorocyclopropene in Diels–Alder cycloaddition and subsequent didehydrohalogenation as illustrated for 164 [229, 250, 251]. Addition of :CCl2 to cycloC6H8 and

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Scheme 4.33

subsequent dehydrochlorination (the Billups synthesis; Scheme 4.32) remains the method of choice for parents 139 and 165 [229, 252]. Detailed procedures for preparation of alkylidenes 167 from 165 via anion 166 and silyl-Wittig olefination (Scheme 4.33) [229, 253] are now formalized into four protocols that gave ~30 new 3,6-dimethoxycyclopropa[b]naphthalenes [254]. Applications are largely to S-extended, conjugated and cross-conjugated derivatives with ‘push–pull’ character that include dithioles 168/169 [255], trienes 170–172 [256], and ‘spaced’ [257] 173–175; p-HCO-C6H4-CHO and 166 give 174 with 173 as a trace product. Not all reactions go to plan [258].

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Scheme 4.34

Reactive bicyclopropenylidenes–novel C6H4 valence bond isomers–are formed from anion 166 with bulkily substituted cyclopropenones 176 (Scheme 4.34) [259]. While complex mixtures ensue, those from 176b/176c afford diones 178b/178c in low yield (~ 8%) via triafulalvenes 177, as established from the mass spectrum of isolated 177b and its subsequent transformation to 178b in air. An inverse electron demand route to alkylidenecycloproparenes has yet to receive the detailed assessment it deserves [260]. Condensation of cation 179 [261] with anion 180 results in coupling and in situ HCl loss to 181 that co-crystallizes with water as a hemihydrate.

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4.4.3 Chemical Considerations

Cycloproparene functional group modification can provide access to difficultly accessible derivatives. Dilithiation of 182, reaction with PhCONMe2, and condensation of the ensuing dione with LiC5H5 gives S extended 183 that packs in sheets in the solid state (Scheme 4.35) [255]; by analogy 184 gives 185 that provides 186 and 187. Horner–Wittig reaction of cyclopropaquinone 188 provides 189 and 190 but the reaction fails with lower homolog 191, likely due to its enhanced enedione character [255]. Bis(dithiole) 190, sensitive to acid and light, decomposes on purification. In fact, many S extended alkylidenes are only available in poor yields with their ‘push–pull’ ability for the new materials arena untested. The HOMO of 139 is located at the bridge and the C3–C4 bond [262] so that inverse electron demand [2+4] cycloadditions are to the bridge as illustrated by

Scheme 4.35

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192/193 [263]. Strain is manifested by these reactions and those that use the lateral V bond as a two-electron component [229]. Such [3+2] cycloadditions, usually catalyzed by AgBF4 or Eu(fod)3, involve ionic intermediates and proceed best in polar media to a wide range of heterocycles as summarized for 165 in Scheme 4.36 [264]. Notable is the cyclization to 195 via C–S bond formation in 194 in a yield that almost doubles under Eu(fod)3 catalysis; the alternative C–N closure is not seen [265]. As for formation of 200, thiotropone adds to 165 but the initial spirocycle rearranges via [1.7] C shift as shown to give 201 in analogy to tropone; the yield is but 5% [266]. Exocyclic alkene 181, explicitly prepared for flash vacuum thermolysis (FVT) study, ejects Me2CO and CO2 as expected and gives 202 (Ar matrix, 20 K) characterized by its IR spectrum matching that calculated (B3LPY/6-31G*) [260]. The existence of such strained and bent ethylideneone derivatives of cyclopropabenzene, first proposed some 17 years ago from bis(diazoketone) photolyses [229, 267] (cf. 163, Scheme 4.31) is now settled. FVT details for methylidenes 203 and 204 alluded to [268] in 1990 have appeared [269]; they do not parallel parents 139 and 165 in their behavior [270]. Diphenyl 203 cyclodehydrogenates to 204 and both proceed to a range of C24H14 polycyclic aromatics (Schemes 4.37 and 4.38). FVT of 203 gives [e,k]acephen-206 [271] (7%) and [a,e]ace-anthrylene 209 (12%). With 204, [e,l]acephenanthrylene 212 (47%) is obtained via 210; 206 is the minor polycyclic aromatic hydrocarbon (< 4%). Traces of dimer 215 were also detected. Automerization of the products in analogy to acephenanthrylene [272] could lead from 206 to 208, and from 212 to 214 [269].

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185

Scheme 4.36

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Scheme 4.37

Scheme 4.38

The well used [250, 273] Ag(I) catalyzed dimerization of cycloproparenes fails [274] with alkylidenes 167 (Scheme 4.39). Conditions effective for 139 and 165 do not apply to 216, the simplest compound tested; ethyne 217 and ethanone 218 (3 : 1) are obtained without 219. While metal ion is involved, none of the aryl derivatives studied dimerizes due to steric constraints at C1; simpler alkylidene derivatives such as C1 ethylidene (=CHMe) remain unknown.

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Scheme 4.39

4.4.4 Heteroatom Derivatives

Cycloproparenes carrying a heteroatom other than in the 6–3 fusion are unexceptional [229], those with the atom in the fused six-membered ring date to 1987 [275], and those involving N [276], S or Se [277] at C1 have been covered [229]. Recent efforts are aimed at incorporating group III/IV atoms at C1. Benzoborirenes 220 have been characterized [278]. Computed structures of 220 (R1 = R2 = H) and the cyclopropabenzenyl cation [279] show marked reverse Mills–Nixon deformation within the aromatic unit due to strong S delocalization over the three-membered ring and rehybridization of the fusion sites.

Kinetically stabilized metallacyclopropabenzenes are obtained as stable compounds when the very bulky Dip and Tbt substituents (Scheme 4.40) are present at C1 [280]. Dibromides 221 transform to 222 that react with o-dibromobenzene to give not only 223 (34%) [281, 282] and 224 (40%) [283], but also the bisheterocycles 225 [281, 284] and 226 [285], as separable cis/trans-isomers. Structures are confirmed from X-ray and spectroscopic data (Section 4.4.5) and delineate the impact of strain in the cycloproparenes.

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Scheme 4.40

4.4.5 Physicochemical and Theoretical Considerations

The cycloproparenes are ‘push–pull’ S systems, the direction of which depends on the nature of the attached substituents [231, 286] as confirmed by 3,6-dimethoxynaphthalene derivatives [254, 287]. Cycloheptatrienylidenes 227–231 with electron donation from the cycloproparene to the seven-membered ring best illustrate this [287]. The dipole moments of 227–231 range from 1.4(5) D to 1.8(5) D with those of parents, 233 and 234 directed away from the cycloproparene core and that of 232 directed towards it (HF/6-31G(d,p) calculations) [288]. That diethers 229 and 231 are each more polar than parents 228 and 230 verify this for 227, 228 and 234. Furthermore, X-ray analyses of 227 and 229 show the seven-membered rings markedly distorted from planarity. The cycloheptatrienylidene –CH=CH– moiety of 227 is bent out of the cycloproparene plane by ~28° and that of more polar 229 by ~45° (Figure 4.3). This represents physical resistance of the seven-membered ring to developing 8S antiaromatic character enforced by the more powerful electron donating cycloproparene. In contrast, the 6S 5C cyclopentadienylidenes 235 and 204 are essentially planar [288].

Figure 4.3 Superimposed side perspectives of cycloheptatrienylidenes 227 (dashed) and 229 (solid). (Reproduced from [63] with permission from Elsevier).

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4.4 Cycloproparenes

189

The linearly dependence of cycloproparene 13C NMR shifts on p-aryl- and p,pcdiarylmethylidene substituents [289] extends to the range of 3,6-dimethoxy derivatives [254] as shown in Figure 4.4. The correlations apply also to the S-extended derivatives 168–175. Cycloproparene charge transfer (CT) complexation is demonstrated by admixture of dithiole 190 and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in MeCN with an instantaneous green that has a new broad CT absorption in the 540–620 nm range; the complex was too unstable for isolation [255]. A more systematic study [290] with p-substituted mono- and diaryl derivatives of 167 showed only the bis(dimethylanilino) of the diaryls to complex, and it did so with, DDQ, 7,7,8,8-tetracyano-1,4-benzoquinodimethane (TCNQ), 2,3,5,6-tetrafluoro-7,7,8,8tetracyano-1,4-benzoquinodimethane (TCNQF4), tetracyanoethene (TCNE), and o-chloranil. In contrast, the mono anilino complexes with DDQ only and in MeCN, while the p-OMe, p-SMe, and p-Ph homologs complex with TCNQF4, but only in CH2Cl2. HF/6-31G**-derived HOMO-LUMO interactions account for the complexation as illustrated by 236 and 237. That of the diaryl is via a pendant aromatic while the essentially planar mono-aryls facilitate face-to-face interaction. A more detailed study is needed.

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4 Strained Aromatic Molecules

Figure 4.4 Plots of Vp+ vs GC for (a) 1-(arylmethylidene)-1H-cyclopropa[b]naphthalenes and (b) 3,6-dimethoxy analogs. (Reproduced from [254[ with permission from the Royal Society of Chemistry).

Much impetus for cycloproparene study comes from the juxtaposition of aromaticity and strain with the concepts of aromatic bond localization (Mills–Nixon effect) and S delocalization central. Noted earlier [245] was the flattening of cyclopropa-fused conjugated polyenes (B3LPY/6-31G*), and now we know that the bond localization imposed on benzene by tris-fusion as in 238 [291] and 239 [292] extends to the pyridazines 240 and 241 [293]. Crystallographic analyses of the essential cycloproparenes have been discussed [229] and current interest lies

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191

with heteroatom derivatives. The cycloproparenes are fully delocalized aromatics whose V frames are distorted primarily about the fusion sites [139: C1a–C5a, 133.4(4) pm, C1a–C2, 136.3(3) pm] [294] as expected from strain induced bond localization (SIBL) [295]. Optimized structures (B3LYP/aug-cc-pVDZ) give negative nucleus independent chemical shift (NICS) values fully commensurate with aromaticity [296]. Maksiü identifies positive and negative Mills–Nixon effects with differing substituents [279, 297] (cf. 220 and cyclopropabenzenyl cations [279]). Importantly, clear distinction between crystallographic V frame measurements and strain effects that impact on the S orbitals is vital; cycloproparenes have no meaningful bond alternation. The diatropic ring current can now be visualized using the distributed-origin, coupled Hartree–Fock method [298]. Structures of heteroatom derivatives 223–226 have appeared [280, 285]. Table 4.3 shows that a large atom at the 1-position alleviates steric constraints and reduces bond length deviations; comparative data for 139 and (unknown) biscycloproparene 242 are included. Bridge bonds are lengthened from 133.4 pm in 139 to between 139.0 pm (223) and 141.5 pm (cis-225), and there is essentially no aromatic bond length variation in bis-225 and 226. This is in stark contrast to unknown 242 where marked variations are predicted. Despite larger atom sizes, the heteroatom derivatives retain internal six-membered ring angles E less (111.9–117.3°) and D/J greater (121.2–124.7°) than the 120° of the ideal sp2-hybrid with deviations greatest in the bis-derivatives. Three-membered ring fusion continues to manifest itself even with heteroatoms present, but it is the parent hydrocarbons that are forced to maximize the distortions and provide unique structures. Cyclopropabenzenyl anion (139A), generated by hydroxide ion deprotonation in the gas phase and computed at MP2/6-31+G(d)//HF/6-31+G(d), is more stable [299] than its C2 and C3 isomers by 28 and 42 kJ mol–1. According to MP2/6-31+G(d) computation, 4S interaction in the three-membered ring is minimized when bonds about the bridge sites (C1a–C5a, 139 pm; C1a–C2, 137.9 pm) are lengthened and C1 pyramidalized (C1–H is bent 50.6° from the cycloproparene plane) [300]. Homolog 165A comes from fluoride ion deprotonation (Fourier transform mass spectrometry) [300, 301]. The proton affinities (PA) are 139A: 1614.2; 165A: 1535.5; 243A (unknown) 1607.5 kJ mol–1, respectively [300]. While the calculated energies of 165 and 243 differ by only 4 kJ mol–1, the PAs show the former to be 72 kJ mol–1 more acidic consistent with resonance structures and avoidance of 8S antiaromaticity. Notable is the lower acidity of 139 than that of toluene by ~12.5 ± 13 kJ mol–1 while 165A is more acidic than 2-methylnaphthalene by 32 ± 13 kJ mol–1 due to increased ring size and reduced importance of the 4S interaction; 165A is an aromatic anion [301]. Because 243A is ~40 kJ mol–1 less stable

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192

4 Strained Aromatic Molecules Table 4.3 Observed and calculated structure parameters for cyclopropabenzene and its heteroatom derivatives.a)

139b)

224c)

cis-225c) trans225c,d) Mol A

trans225c,d) Mol B

cis-226c) trans226c)

242e)

a

133.4(4) 139.0(4) 139.1(3) 141.5(6) 140.5(11) 139.8(11) 139.4(6) 139.4(6) 134.6

b

136.3(3) 138.8(4) 138.5(3) 140.9(5) 138.8(11) 138.9(11) 138.1(8) 139.4(6) 139.2 139.4(4) 138.8(3) 141.4(6) 141.4(11) 137.6(11) 140.3(9) 140.3(6)

c

138.7(4) 138.1(4) 139.2(3) 139.7(6) 139.6(9) 140.5(11) 139.6(9) 139.7(6) 139.2 138.3(4) 139.1(2) 139.4(6) 138.9(11) 140.1(11) 139.1(6)

d

139.0(5) 140.3(3) 139.7(3) 141.2(6) 1.420(10) 138.0(11) 136.4(9) 138.9(6) 134.6

e

149.8(3) 182.6(2) 198.2(2) – 182.8(3) 194.0(2)

D

124.5(2) 121.3(3) 121.9(2) 123.2(4) 124.7(7) 123.0(7) 123.1(6) 123.8(4) 126.1 121.5(2) 122.0(2) 122.9(3) 122.7(4) 123.2(8) 123.5(6) 123.6(4)

E

113.2(2) 117.3(3) 116.5(2) 113.7(7) 113.7(7) 113.0(7) 113.3(6) 112.5(4) 107.8 117.3(3) 116.3(2) 114.3(4) 114.8(7) 114.6(7) 111.9(6) 112.3(4)

J

122.4(2) 121.2(3) 121.6(2) 122.9(4) 122.9(4) 124.2(7) 124.2(6) 123.7(4) 126.1 121.4(3) 121.7(2) 123.1(4) 122.7(4) 121.8(7) 122.7(4) 124.2(4)

G

52.8(2)

a) b) c) d) e)

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223c)

44.7(1)

42.11(8) –

–

–

–

–

–

–

–

–

–

–

Data taken from Refs. [280] and [285]; bond lengths in picometres (pm), angles in degrees (°). X-ray data at –150 °C taken from Ref. [294]. X-ray at –170 °C. Independent molecules A and B are in the unit cell. Optimized at B3LYP/6-311G(2d,p).

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References

193

than its acyclic counterparts, it is less able to alleviate the unfavorable 4S interaction in the three-membered ring. It is best regarded as antiaromatic. Calculated proton acidities of homologs 244A–246A are 1483.6, 1611.7, and 1531.3 kJ mol–1 respectively [300]. Computational study of mono-anionic dicycloproparenes 247A and 248A (PA: 1579.0/1605.4 kJ mol–1) shows that the second three-membered ring enhances acidity. The proton affinity of 139A is 35.2 kJ mol–1 above that of 247A but only 8.8 kJ mol–1 above that of 248A [300]. The impact of mesomeric CN groups and inductive F substituents on the stability of 139A has been assessed [301]. The CN group, able to conjugate to the C1 anion, increases acidity more at C2(5) than C3(4). Difluorination inductively raises acidity by 43.1 kJ mol–1 at C2(5) but reduces it by 43.9 kJ mol–1 at C3(4); replacement of all aromatic ring protons in 139A by F decreases acidity by 83.7 kJ mol–1 [300]. A range of C1 substituents enhance acidity; Me has least (< 1 kJ mol–1) and SO2H most (141 kJ mol–1) impact [302], but their impact is less than on the cyclopropenyl anion.

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4 Strained Aromatic Molecules summing experimental measurements of the enthalpy of combustion (and hence of formation) with those of sublimation or vaporization (and hence of solid or liquid o gas interconversion). 8 Brown, H. C.; Domash, L. J. Am. Chem. Soc. 1956, 78, 5384, reporting otherwise unpublished results by E. J. Prosen. 9 (a) Arnett, E. M.; Sanda, J. C.; Bollinger, J. M.; Barber, M. J. Am. Chem. Soc. 1967, 89, 5389; (b) Nesterova, T. N.; Verevkin, S. P.; Karaseva, S. Y.; Rozhnov, A. M.; Tsvetkov, V. F. Russ. J. Phys. Chem. (Engl. Transl.) 1984, 58, 297. Also see, Verevkin, S. P. J. Chem. Thermodyn. 1998, 30, 1029. 10 Kruerke, U.; Hoogzand, C.; Hubel, W. Chem. Ber., 1961, 94, 2817. 11 Tasi, G.; Mizukami, F.; Toba, M.; Niwa, S.; Palinko, I. J. Phys. Chem. A 2000, 104, 1337. 12 The value is the sum of the archival enthalpy of formation of the liquid and an estimated enthalpy of vaporization from the simple approach in Chickos, J. S.; Hyman, A. S.; Ladon, L. H.; Liebman, J. F. J. Org. Chem. 1981, 46, 4294. 13 (a) Miki, S.; Katayama, T.; Yoshida, Z. Chem. Lett. 1992, 41; (b) Mandella, W. L.; Franck, R. W. J. Am. Chem. Soc. 1973, 95, 971; (c) Goldberg, I. B.; Crowe, H. R.; Franck, R. W. J. Am. Chem. Soc. 1976, 98, 7641. 14 (a) Anderson, J. E.; Franck, R. W.; Mandella, W. L. J. Am. Chem. Soc., 1972 94, 4608; Anderson, J. E.; Franck, R. W. J. Chem. Soc. Perkin Trans. 2, 1984, 1581; (b) Handal, J.; White, J. G.; Franck, R. W.; Yuh, Y. H.; Allinger, N. L. J. Am. Chem. Soc. 1977, 99, 3345. 15 Penner, G. H.; Chang, Y.-C. P.; Nechala, P.; Froese, R. J. Org. Chem. 1999, 64, 447. 16 (a) Kraljic, I.; Mintas, M.; Klasinc, L.; Ranogajec, F.; Guesten, H. Nouv. J. Chim. 1983, 7, 239; (b) Abdul-Ghani, A. J.; Bashi, N. O. T.; Maree, S. N. J. Solar Energ. Res. 1987, 5, 53. 17 Fu, P. P.; Harvey, R. G. J. Org. Chem. 1977, 42, 2407. 18 Pozdnyakovich, Y. V. Zhur. Org. Khim 1988, 24, 1076.

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19 Grimme, S.; Pischel, I.; Nieger, M.; Voegtle, F. J. Chem. Soc., Perkin Trans. 2 (Phys. Org. Chem.) 1996, 2771. 20 Armstrong, R. N.; Ammon, H. L.; Darnow, J. N. Am. Chem. Soc. 1987, 109, 2077. 21 Frisch, M. A.; Barker, C.; Margrave, J. L.; Newman, M. S. J. Am. Chem. Soc. 1963, 85, 2356. Actually, we should have said negligibly strained in that the parent hydrocarbon phenanthrene itself has been deduced to be strained by 4.9(2.8) kJ mol–1, Nagano, Y.; Nakano, M. J. Chem. Thermodyn. 2003, 35, 1403. 22 (a) Mark, H. B., Jr.; Jezorek, J. R. J. Org. Chem.1971, 36, 666; (b) Grimme, S.; Loehmannsroeben, H. G. J. Phys. Chem. 1992, 96, 7005; (c) Dougherty, R. C.; Bertorello, H. E.; Martinez de Bertorello, M. Org. Mass Spectry. 1971, 5, 1321. 23 (a) Herbstein, F. H.; Schmidt, G. M. J.; J. Chem. Soc. 1954, 3302; (b) Haigh, C. W.; Mallion, R. B. Mol. Phys. 1971, 22, 945; Martin, R. H.; Defay, N.; Zimmermann, D. Tetrahedron Lett. 1971, 1871; (c) Schmidt, W. J. Chem. Phys. 1977, 66, 828; (d) Wynberg, H.; Nieuwpoort, W. C.; Jonkman, H. T. Tetrahedron Lett. 1973, 4623. 24 Martin, R. H.; Marchant, M. J. Tetrahedron Lett. 1972, 3707. 25 Borkent, J. H.; Laarhoven, W. H. Tetrahedron 1978, 34, 2565. 26 (a) Li, H.; Petersen, J. L.; Wang, K. K. J. Org. Chem. 2001, 66, 7804; (b) Zhang, Y.; Petersen, J. L.; Wang, K. K. Org. Lett. 2007, 9, 1025 27 (a) Templeton, W. J. Chem. Soc. D: Chem. Commun. 1970, 21, 1412; (b) Roberts, T. D. J. Chem. Soc. D: Chem. Commun. 1971, 362. 28 (a) Phenanthrene, Nagano and Nakano, op. cit. ref. [21] and Steele, W. V.; Chirico, R. D.; Nguyen, A.; Hossenlopp, I. A.; Smith, N. K. Am. Inst. Chem. Eng. Symp. Ser. (AIChE Symp. Ser.), 1990, 138; (b) Diphenylacetylene, Davis, H. E.; Allinger, N. L.; Rogers, D. W. J. Org. Chem. 1985, 50, 3601. 29 However, stability for such strained species as the bisindenoarenes is clearly precarious in that a not altogether

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42 Tsuji, T.; Ohkita, M.; Kawai, H. Bull. Chem. Soc. Jpn 2002, 75, 415. 43 Tsuji, T.; Nishida, S. O. M.; Osawa, E. J. Am. Chem. Soc. 1995, 117, 9804. 44 (a) 1,4-, Paquette, L. A.; Trova, M. P. J. Am. Chem. Soc. 1988, 110, 8197; (b) 1,3-, Wang, T. Z.; Paquette, L. A. Tetrahedron Lett. 1988, 29, 41; (c) 1,5-, Paquette, L. A.; Trova, M. P.; Luo, J.; Clough, A. E.; Anderson, L. B. J. Am. Chem. Soc. 1990, 112, 228. 45 Van Eis, M. J.; de Wolf, W. H.; Bickelhaupt, F.; Boese, R. J. Chem. Soc., Perkin Trans. 2 (Phys. Org. Chem.) 2000, 793. 46 Van Eis, M. J.; Wijsman, G. W.; De Wolf, W. H.; Bickelhaupt, F.; Rogers, D. W.; Kooijman, H.; Spek, A. L. Chem. Eur. J. 2000, 6, 1537. 47 v. Schleyer, P. R.; Puehlhofer, F. Org. Lett. 2002, 4, 2873. 48 Dreeskamp, H.; Sarge, S. M.; Tochtermann, W. Tetrahedron 1995, 51, 3137. 49 Rosenfeld, S.; Shedlow, A. J. Org. Chem. 1991, 56, 2247. 50 Bartmess, J. E.; Griffith, S. S. J. Am. Chem. Soc. 1990, 112, 2931. This reference shows using both gas and solution phase acidity measurements (a laboratory realization of the above ISE analysis, see [47]) that the enthalpy of formation difference of 9-methylanthracene and 9-methylenedihydroanthracene is some 20 kJ mol–1 favoring the former. 51 Tobe, Y.; Saiki, S.; Utsumi, N.; Kusumoto, T.; Ishii, H.; Kakiuchi, K.; Kobiro, K.; Naemura, K. J. Am. Chem. Soc. 1996, 118, 9488. 52 Roth, W. R.; Boehm, M.; Lennartz, H.-W.; Vogel, E. Angew. Chem. 1983, 95, 1011. 53 Sander, W.; Exner, M. J. Chem. Soc., Perkin 2 1999, 2285. 54 JFL once asked pioneering fluorine chemist Harry J. Emeléus how he devised the syntheses of so many exotic species. He replied (within the limits of memory): “When I ask myself how to make a given compound, I think of logical syntheses. If the compounds react as I hope, that’s fine. If they don’t, the compounds are reactive enough to produce something else of interest. So, how can I lose?”

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4 Strained Aromatic Molecules 55 de Meijere, A.; Kozhushkov, S. I.; Rauch, K.; Schill, H.; Verevkin, S. P.; Kuemmerlin, M.; Beckhaus, H.-D.; Ruechardt, C.; Yufit, D. S. J. Am. Chem. Soc. 2003, 125, 15110. 56 (a) Cram, D. J.; Dewhirst, K. C. J. Am. Chem. Soc. 1959, 81, 5963; (b) Hopf, H.; Laue, T.; Zander, M. Angew. Chem. Int. Ed. Engl. 1991, 30, 432. 57 Yamato, T.; Noda, K.; Tsuzuki, H.; New J. Chem. 2001, 25, 721. 58 Shieh, C.-F.; McNally, D.; Boyd, R. H. Tetrahedron 1969, 25, 3653. 59 The following story told by Irvin Greenberg to JFL may be evocative. A talk to a major professional organization by Carl Rogers, a premier humanistic psychologist, and roughly entitled “What it Means to be a Psychologist” resulted in a listener’s question “What do you do when you find a client boring?”, also admitting concern because it seemed unprofessional. Rogers acknowledged having similar feelings and said “Whenever I find myself bored with a patient, I ask myself why I am bored. Attempting to answer this, now that I have a research problem, I am no longer bored.” Translating to chemistry, who would have thought that cyclophanes are normal looking? 60 The title of this chapter was first used in a review on his cyclophane work by Donald Cram, one of the founding fathers of cyclophane chemistry: Cram, D. J.; Cram, J. M. Acc. Chem. Res. 1971, 4, 204. 61 Hopf, H. Classics in Hydrocarbon Chemistry; Wiley-VCH, Weinheim, 2000; R. Gleiter, H. Hopf (eds.) Modern Cyclophane Chemistry; Wiley-VCH, Weinheim, 2005. 62 Lüttringhaus, A.; Gralheer, H. Justus Liebigs Ann. Chem. 1942, 550, 67. 63 Bickelhaupt, F.; de Wolf, W. H. Advances in Strain in Organic Chemistry (Halton, B., ed.), JAI Press, London, 1993, Vol. 3, pp. 185; Kane, V. V.; de Wolf, W. H.; Bickelhaupt, F. Tetrahedron, 1994, 50, 4575; Bickelhaupt, F.; de Wolf, W. H. J. Phys. Org. Chem. 1998, 11, 362. 64 Hopf, H.; Goldberg, N., unpublished results; for high level ab inbitio

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268 Halton, B. Pure Appl. Chem. 1990, 62, 541–546. 269 Halton, B. Tetrahedron Lett. 2006, 47, 1077–1079. 270 Wentrup, C.; Wentrup-Byrne, E.; Müller, P. J. Chem. Soc., Chem. Commun. 1977, 210–211; Wentrup, C.; Wentrup-Byrne, E.; Müller, P.; Becker, J. Tetrahedron Lett. 1979, 4249–4252. 271 Campbell, N.; Wang, H. J. Chem. Soc. 1949, 1513. 272 Scott, L. T.; Roelofs, N. H. J. Am. Chem. Soc. 1987, 109, 5461–5465. 273 Billups, W. E.; McCord, D. J.; Maughon, B. R. Tetrahedron Lett. 1994, 35, 4493–4496, J. Am. Chem. Soc. 1994, 116, 8831–8832. 274 Halton, B.; Dixon, G. M.; Forman, G. S. ARKIVOC 2006, 2006, 38–45. 275 Bambal, R.; Fritz, H.; Rihs, G.; Tschamber, T.; Streith, J. Angew. Chem., Int. Ed. Engl. 1987, 26, 668–669. 276 Yranzo, G. I.; Elguero, J. E.; Flammang, R.; Wentrup, C. Eur. J. Org. Chem. 2001, 2209–2220 and refs. cited. 277 Schulz, R.; Schweig, A. Tetrahedron Lett. 1979, 59–62, Tetrahedron Lett. 1984, 25, 2337–2340. 278 Kaiser, R. I.; Bettinger, H. F. Angew. Chem., Int. Ed. 2002, 41, 2350–2353; Bettinger, H. J. Am. Chem. Soc. 2006, 128, 2534–2535, Chem. Comm. 2005, 2756–2757. 279 Maksic, Z. B.; Eckert-Maksic, M.; Pfeifer, K. H. J. Mol. Struct. 1993, 300, 445–453. 280 Sasamori, S.; Tokitoh, N. Organometallics 2006, 25, 3522–3532. 281 Tajima, T.; Hatano, H.; Sasaki, T.; Sasamori, T.; Takeda, N.; Tokitoh, N.; Takagi, N.; Nagase, S. J. Organometal. Chem. 2003, 686, 118–126. 282 Tokitoh, N. Phosphorus, Sulfur Silicon Relat. Elem. 2001, 168–169, 31–40; Hatano, K.; Tokitoh, N.; Takagi, N.; Nagase, S. J. Am. Chem. Soc. 2000, 122, 4829–4830. 283 Tokitoh, N.; Hatano, K.; Sasaki, T.; Sasamori, T.; Takeda, N.; Takagi, N.; Nagase, S. Organometallics 2002, 21, 4309–4311. 284 Tajima, T.; Hatano, K.; Sasaki, T.; Sasamori, T.; Takeda, N.; Tokitoh, N. Chem. Lett. 2003, 32, 220–221.

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5 Fullerenes 5.1 Introduction

Helena Dodziuk At the beginning of this chapter the inclusion of fullerenes and nanotubes in this book has to be explained once more. Of course, speaking formally these compounds are not hydrocarbons, since in the pure state they consist only of the carbon atoms. However, their extended systems of nonplanar aromatic rings provide a model for the analysis of the effect of distortions on physicochemical properties. The serendipitous discovery of C60 , 1, resembles a detective story [1]. The molecule is remarkable for its high symmetry that determines the equivalence of all carbon atoms. However, the possibility of the mere existence of molecules of this point group was denied by Herzberg [2], later awarded Nobel Prize in chemistry for his spectroscopic studies. In his seminal monograph he stated: ‘ The regular icosahedron and the regular dodecahedron belong to point group Ih. It is not likely that molecules of such a symmetry will ever be found.’ Then C60 was independently proposed by two groups of theoreticians [3, 4]. However, the suggestion was totally forgotten. Interestingly, Rohlfing and coworkers observed a strong signal corresponding to C60 in their study of small carbon clusters in 1984 [5]. They even reported the whole mass spectrum with the remarkable C60 signal but, focused on small structures, they limited their discussion exclusively to ions smaller than 40. The following year, an observation by Curl, Kroto and Smalley of a strong C60 peak (accompanied by a smaller C70 one) in a carbon soot, produced by discharges in vacuum that should mimic conditions in the cosmos, marked the beginning of the fullerene era. Intriguingly, the authors, not knowing the earlier proposals, had trouble assigning the structure and did not know what shape it represented. Kroto’s hobby interest in graphics and his fascination for the geodesic domes of the American architect Buckminster Fuller provided clues, helped clear up the problem and brought the molecule its name. Then, a phone call to the Mathematical Department brought the answer: soccer ball.

Strained Hydrocarbons: Beyond the van’t Hoff and Le Bel Hypothesis. Edited by Helena Dodziuk Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31767-7

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To begin with, the available amount of fullerene was very small, preventing proof of the structure. A purification procedure developed by the Krätschmer group solved the problem of obtaining bulk amounts of C60 [6], thus enabling further studies. Four sharp bands in the IR spectrum of the enriched soot were compatible with the Ih symmetry of the molecule. Kroto learned about the purification procedure by refereeing the paper. After writing a positive review, he purified a sample and let it be measured by 13C NMR [1]. The spectrum consisted of only one line providing the first proof of the equivalence of all 60 carbon atoms, thus proving the spatial fullerene structure. Interestingly, the first X-ray measurements yielded the C60 cage diameter and the distance between the molecules in the crystal only since the near-spherical shape allowed practically undisturbed rotation of the molecules at room temperature. As a result, two different C–C bond lengths were first, although quite inaccurately, determined by NMR technique [7]. Structural aspects of fullerene chemistry and chirality were recently summarized by Thilgen and Diederich [8]. The highly symmetrical structure of 1 seems quite artificial but the molecule has been found in some minerals (http://webmineral.com/data/Fullerite.shtml) and in the cosmos [9, 10] and its presence in Nature precluded its patenting. A few C60 molecules are even produced by burning dinner candles [11]. All fullerenes are composed of 12 cyclopentane rings and a certain number of six-membered ones. Most stable fullerenes are known to conform to so called Isolated Pentagon Rule, IPR, stating that their cyclopentane rings do not have common atoms or bonds. The smallest carbon cage satisfying this condition is that of 1 and it represents the most frequently encountered fullerene structure. The next one, discovered together with C60, is 2. In addition, numerous smaller (not complying with IPR) and higher fullerenes have been reported. Contrary to 1 and 2, for which only one isomer is possible, the higher members of the family can assume several structures of, sometimes different, symmetry. An Atlas of Fullerenes [12] collects all possible IPR structures and their symmetries from C20 to C100. The Fullerene Gallery at the address: http://cochem2.tutkie.tut.ac.jp:8000/ Fuller/fsl/fsl.html presents many of them including non-IPR ones. Seven possible IPR isomers of C80 are shown in Figure 5.1.

C20 can be considered as the archetypal fullerene, built of fully unsaturated five-membered rings, since it is a highly unstable fulleroid structure without any six-membered rings. Its synthesis was accomplished by the Prinzbach group [13–15].

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Figure 5.1 Seven possible isomers of C80 with their symmetry according to An Atlas of Fullerenes [12].

Scheme 5.1

As noticed by J.-M. Lehn [16], the synthesis of C60 represents a unique example of the covalent self-assembly in which 90 bonds are formed simultaneously. Efforts of several groups trying to synthesize the molecule in a series of ‘true chemical reactions’ resulted in rich corannulene chemistry (Scheme 5.1) [17–19]. The interest in fullerenes was triggered by the observation that other molecules or ions can be hosted inside their cages. Such supramolecular complexes, which bear the name endohedral fullerene complexes, were thought to allow one to propose exciting applications. The first suggestions seemed to be too good to be true. For instance, Stoddart wrote about a fullerene cage with ‘a door’ which could serve as a drug carrier releasing the content at a desired place [20]. He also predicted that extending the superconductivity of 1 to room temperature or using perfluorinated C60F60 as a lubricant would revolutionize our all lives since we would avoid losses due to electrical resistance or friction. Later we have learned

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that these hopes (discussed in some details in Chapter 2.4) cannot be fulfilled [21, 22]. Model calculations have shown that production and purification of much larger fullerenes must be mastered to allow much larger guests of practical interest to be hosted [23]. Fullerenes are relatively large objects, difficult to study both experimentally and computationally. Highly symmetrical C60 is a kind of exception although it also poses several problems. Other fullerenes are available in small quantities and their lower symmetry complicates their study. Only the most important experimental methods, often combined with discussions of theoretical results, will be presented in this chapter and several exciting aspects of fullerene research will not be covered. For instance, endohedral fullerene complexes with metal ions inside forming untypical salts that can be dissociated only by destroying the cage, will practically not be discussed here. It should be stressed, however, that the formation of endohedral complexes with these or other guests can stabilize otherwise unstable non-IPR fullerenes [24]. Fullerene studies are often triggered by practical considerations. However, these highly symmetrical systems are of critical importance for basic research. As shown by thought-inspiring experiments in the Zeilinger group on the obtaining of diffraction patterns of C60 and C70 [25, 26], such purely scientific studies could eventually lead to marketable devices. Applications of fullerene will be in detail discussed in Section 5.7. It suffices to mention here that at present only few have reached the market. Interestingly, the hopes for the applications were high in the late 1980s and early 1990s. Then carbon nanotubes took the lead and electronic devices built from one nanotube were thought to revolutionize our whole life. For instance, nanotube transistor [27] or flat displays (http://news.com.com/Carbon+nanotube+TV+trials+ on+horizon/2100-1041_3-6051476.htm) were expected to be sold soon. However, they seem to pose numerous technical problems, resulting in new interest in fullerene applications. In any case, in spite of very few practical applications of fullerenes and nanotubes at present, these molecules have given a strong boost to the development of nanotechnology which sooner or later is destined to change our everyday life.

5.2 Chemistry Influenced by the Nontypical Structure: Modification of [60]Fullerene

Takuma Hara, Takashi Konno, Yosuke Nakamura and Jun Nishimura 5.2.1 Introduction

As is well known, Kroto, Smally, and Curl worked together with their associates and during their study on interstellar carbon clusters, serendipitously discovered a soccer ball compound or C60 which originally was named buckminsterfullerene

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Figure 5.2 Number of papers reported on fullerenes and CNTs.

dedicated to genius architect Richard Buckminster Fuller [28]. Since its large-scale preparation was developed by Krätschmer and Huffman [29], the chemical world has been caught by the fullerene fever for many years [30]. Only recently, because of the prospects of their applications, the fever has passed most of its heat to carbon nanotubes (CNTs), as shown in Figure 5.2. However, the unusual properties of C60 and other fullerenes continue to make them fascinating objects for study. During the hot years, many useful C60 derivatives were obtained. Most modifications are of three general types: i.e. those with nucleophilic, radical, and electrophilic reagents. Since C60 is an electron acceptor itself, most of the modifications have been carried out by using nucleophilic, electron-donating species or radicals. In this chapter we first overview the fullerene reactions and then summarize the modifications, dividing them into six subclasses according to the addend structures. Since, at least theoretically, almost all reactions are applicable to all types of fullerenes, the reactions of C60 which is the best studied, most easily available and also interesting for applications, will mainly be discussed here. Moreover it should be stressed that within the last 15 years the circumstances around the fullerene chemistry have been well established except for the safety issues [31]; i.e. the supply of various fullerenes [32], the IUPAC nomenclature [33], and many useful HPLC columns for their separations [34], etc. A variety of findings in C60 chemistry have surprised a number of audiences not only in organic, theoretical and general chemistry but even in natural philosophy. 5.2.2 General Overview

Fullerenes have a system of conjugated double bonds and receive various additions at the sites, although these are not regular double bonds. The bonds are classified into (5,6) junction (single bond) and (6,6) junction (double bond), which mean the edges connecting a five-membered ring and a six-membered ring, and two five-membered rings, respectively [35]. These bonds have different properties, as shown in Figure 5.3. Many reactions do not usually occur at (5,6) junctions, but at (6,6) junctions to give 1,2-adducts [36].

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Figure 5.3 Two different bonds: (5,6) junction with 1.45 Å of bond length and (6,6) one with 1.38 Å [9].

A variety of reaction mechanisms have been suggested for the novel electronaccepting, large S-conjugating fullerenes, which can be roughly classified as depicted in Figure 5.4.

Figure 5.4 Reactions classified.

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Species 3 are generated in many ways by the reactions of organolithium [37] including acetylides [37d], Grignard reagents with [30] or without copper salt [37b,c, 38a–c], and typical nucleophiles [39] such as CN– [40]. It was reported that the negative charge of a typical t-BuC60– anion 3 is not fully delocalized but rather localized at the adjacent carbon (2-position) of the t-Bu addend (1-position) [37b]. Grignard reagents react with C60 more slowly than organolithium, and the products are 1,4-adducts in contrast with the organolithium cases providing 1,2adducts [37b,c]. Grignard reagents like the phenyl one together in the presence of CuBr-Me2S complex react with the fullerene to afford interesting adducts having five addends at the 2,5,10,21,24-positions, which have been extensively applied to various fields, not only organic chemistry but also materials science [38d–f ]. Sodium cyanide and fullerene dissolved in DMF-o-dichlorobenzene mixed solvent generated carbanion 3, which was trapped by various electrophiles like p-t-butyl(bromomethyl)benzene to give the appropriate 1,2-adducts [40]. Diethyl bromomalonate gives 3 through the addition of its ester enolate, which rapidly eliminates bromide anion to end up with the formation of the corresponding methanofullerene. This reaction is called the Bingel reaction, dedicated to the inventor [39]. Dianion C602– formed by the two-electron reduction of C60 can act as a nucleophile, which reacts with, for example, benzyl bromide to afford 1,4-dibenzylfullerene [41]. Since fullerene reacts with radicals so rapidly, it is often called a radical sponge [42]. Generally, a radical species adds at the fullerene double bonds and generates a fullerene monoadduct radical species, which accepts another radical at the 4-position toward the 1-addend position. Such reactivity is explained by acknowledging that fullerene does not have a fully conjugated S-system but rather [5]-radialene substructure which becomes the stable conjugated cyclopentadienyl radical 10 after the addition of five radicals as shown in Scheme 5.2.

Scheme 5.2 Radical addition to the fullerene with [5]radialene substructure.

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The fullerene does not usually react well with Brønsted acids, but does somehow with sulfuric acid–nitric acid mixture to afford polyhydroxyfullerenes [43]. Lewisacid catalyzed transformation was reported to proceed through arenium cation [44]. The double bond of fullerene generally behaves as an electron-accepting site (7) such as dienophile or dipolarophile. Therefore, C60 exhibits many pericyclic reactions; for example, Diels–Alder reaction, 1,3-dipolar cycloaddition (Huisgen reaction) such as Prato reaction, and so forth. There are many synthetic data accumulated so far on several reactions. These reactions proceed with the interaction of fullerene (acceptor) LUMO and reagent (donor) HOMO (7) or the reverse combination (8). The bisadduct distribution seems to provide a method that can qualitatively define whether the reagents behave as acceptors or donors. Several popular reactions like Bingel [45], Prato [46], and Diels–Alder [47], which give 3-membered, 5-membered, and 6-membered rings as the junctions between the fullerene and addends, were reported to have nucleophilic character, while benzyne [48] and nitrene [49] cycloadditions seem to be electrophilic. The multi-addition, especially bisaddition, is one of the interesting fields in fullerene chemistry. Yet even the bisaddition of a symmetric reagent to C60 can give at least 8 isomers: cis-1, cis-2, cis-3, e, trans-4, trans-3, trans-2, and trans-1 (see Figure 5.5 and Table 5.1). The regioselectivities of bisaddition were thoroughly examined on the three major fullerene modification reactions mentioned above. Moreover, several C2-symmetric regioisomers, cis-3, trans-3, and trans-2, are chiral, and their absolute configurations have been determined by CD spectroscopy after optical resolution by using some elaborate chiral HPLC columns [34]. Generally speaking, the regioisomer distribution of bisadducts was qualitatively correlated with the frontier orbital coefficients of the corresponding monoadducts; e and trans-3 bisadducts were formed preferentially among eight isomers [45–49]. The isomer ratios in Bingel, Prato, and Diels–Alder bisadditions are depicted in Figure 5.6a. The reactive species in these reactions, malonate anions, Huisgen ylides, and dienes, are nucleophiles in their nature, so that similar regioselectivities are obtained. The isomer ratios in benzyne and nitrene bisadditions are shown in Figure 5.6b. The most remarkable difference between these bis-additions and those shown in Figure 5.6a is the high ratio of cis-1 in the former. Generally, the cis-1 position is most stressed by the first addition so that this double bond is apt to be more reactive and to accept the addition of species more than the other sites. However, the reactions except for benzyne and nitrene additions suffer from steric hindrance between the first addend moiety and incoming species, and usually result in a Table 5.1 Number of isomers with symmetrical achiral addends [50].

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Number of additions

Mono

Bis

Tris

Tetrakis

Number of isomers

1

8

46

262

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Figure 5.5 Eight possible isomers of bisadducts.

negligible amount of cis-1 product. Actually benzyne and nitrene can give flat addends, while other reactions afford more bulky addends. The second apparent difference of benzyne addition from others is that it has less total content of the southern hemisphere products. The same tendency is observed for the nitrene bisaddition, since the methoxycarbonylnitrene is regarded as an electrophilic species, although there is some subtle difference between the two electrophilic additions. For selective modification, especially for multiadditions, the use of tethers has been proposed. For example, Diederich’s and Hirsch’s groups developed the selective Bingel addition toward bisadducts and trisadducts [45c, 51]. Nakamura’s group successfully obtained a chiral bisadduct having a cyclopentane ring at the junction [52]. Recently a selective Prato reaction was also achieved using the tether manipulation [53]. Diels–Alder reaction was made selective at the bisaddition using the same procedure [54].

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Figure 5.6 Distribution of regioisomeric bisadducts.

Another method to make multi-additions selective was developed by Hirsch’s group, using a reversible template [55]. Using the reversible Diels–Alder reaction of 9,10-dimethylanthracene as a template, they obtained Oh-symmetrically modified hexakisadducts by Bingel addition, although a recent paper reported that similar selectivities were obtained without using the anthracene derivative [56]. Returning to Figure 5.4, finally the thermal or photochemical electron transfer reactions from a donor like alkylamine to the fullerene must be mentioned. At the very beginning of the fullerene chemistry, the addition of alkylamines was discovered [57]. It is suggested that this reaction involves the formation of fullerene radical anion and amine radical cation through single electron transfer (SET) and

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the radical coupling of both species to afford the corresponding amine adduct through the proton transfer [57a]. On the other hand, under photoirradiation an electron-rich ketene silylacetal 11 reacts with C60 through SET to give an ene reaction product as shown in Scheme 5.3 [58]. In this reaction, first the fullerene absorbs light to excite itself to 1C60*, which rapidly and efficiently renders intersystem crossing to 3C60*. Then 3C60* gets an electron from the acetal to generate its anion radical and acetal cation radical, which combine with each other finally giving product 12.

Scheme 5.3 SET under photoirradiation.

Many reactions follow one of the above reaction types. The stability of adducts is sometimes an important issue, and depends largely on the reversibility of modification reactions. Since pristine fullerene itself is thermodynamically stable enough, most fullerene derivatives reproduce fullerene under harsh conditions, even though the reactions are not reversible. Bingel adducts are decomposed by electrolysis [59]. Prato adducts give pristine fullerene when they are treated with a strong dipolarophile such as maleic anhydride in the presence of a catalyst such as Wilkinson’s complex or copper triflate in o-dichlorobenzene at reflux for 8 to 18 h [60]. Diels–Alder adducts easily decompose or usually undergo retro-Diels–Alder reaction at high temperatures or sometimes even at r.t. (see below) [61]. 5.2.3 Modification Reactions

Showing typical products with their structures, a variety of modifications are introduced in this section, focusing on the joint structures between the fullerene and addends. 5.2.3.1

Reduction and Oxidation

Many investigations related to the reduction have been reported [62], using direct hydrogenation at high temperatures, electrochemical reduction, BH3-THF, NH=NH, LiBEt3H then treated with HBF4, Birch reduction, Zn-HCl, Zn(Cu), hydrozirconation, H2Rh(0)-Al2O3, Pd/C-catalyzed reduction, and so forth. Theoretically all 30 double bonds ((6,6) junctions) can accept hydrogen atoms leading to C60H60. After receiving a certain number of hydrogen atoms, however, the remaining double bonds are buried inside the C60 surface too deeply to react or the products are too much deformed to stay stable. The theoretical studies indicate that C60H60 with all CH bonds pointing outwards is much less stable than the

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Figure 5.7 Hydrogenation and oxidation of C60.

isomers with the bonds pointing inside the C60 cage [63] (see Section 2.4.1.7). Until now the structures of C60H2, C60H4, C60H6, and C60H18 have been well examined and reported (Figure 5.7). Among the series of C60H24 to C60H36 various reducing agents under various conditions selectively gave C60H36, which is revealed as a mixture of isomers. The structure of hydrogenated fullerenes mainly studied by NMR spectra is discussed in Section 5.6. The fullerene is easily oxidized, and even its commercial samples usually contain the oxide. The reactivity to oxidation was anticipated at the very beginning of C60 studies. It is oxidized to form fullerene oxide or epoxide 13. The oxidation is carried out by electrochemical reaction [64a], irradiation with oxygen [64b], direct oxidation at high temperatures [64c], oxygen with a radical initiator like AIBN [64d], ozone [64e], dimethyldioxolane [64f ], MCPBA [64g], P450 model system [64h], and so forth. Recently Tajima and his co-workers reported an intriguing transformation of the oxide shown in Scheme 5.4 [65], which may be useful in the materials science.

Scheme 5.4 Transformation of fullerene oxide.

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The nitration of C60 with NO2 gave mixtures of tetra- and hexanitro derivatives in the ratio depending on the reaction time [66]. Then a selective synthetic method toward hexanitro[60]fullerenes was developed [43c]. The nitroallylic moieties in hexanitro[60]fullerenes were found to be excellent leaving groups for the nucleophilic substitution by amines, such as anilines, leading to the formation of hexaanilino[60]fullerenes. Moreover, hexanitro[60]fullerene was reported to be a better electron acceptor and form nitrofullerene complexes with aniline, o-, m- and p-phenylenediamine, pyridine, tacrine, triethylamine, 1-aminoanthraquinone, N,N-dimethylaniline, TTF, and BEDT-TTF (ET) [67]. The nitrated products partly isomerize to the nitrite esters, which, with subsequent hydrolysis by atmospheric moisture, give nitrofullerols containing 6–8 nitro and 7–12 hydroxy groups per fullerene [68]. Hydrolysis of polynitrofullerenes in aqueous NaOH gave the corresponding polyhydroxylated fullerene derivatives or fullerols in moderate overall yields. Halogenated fullerenes were used as reagents to prepare fullerols. Halogens were selectively substituted when fullerene bromides and chlorides were treated with silver trifluoroacetate [69]. 5.2.3.2

Alkylation

Here the alkylations of fullerene are summarized. These addends are formed via intermediates 3, 4, 5, and 9. Some alkyl- and ethynyl-fullerenes were produced by the reaction with t-butyl-, 9-fluorenyl-, and phenylethynyl-lithium reagents. Typical reaction products are listed in Figure 5.8 [37c, 38c, 70].

Figure 5.8 Some derivatives obtained from intermediate 3; 15a [37c], b [70a,b], c [70a], d [70c], e [70e], f [70d], g [70f ], h [70f ], i [70f ], j [37c], k [37c], l [37c], m [37c], n [37c], o [37c], p [38c].

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Grignard reagents react with fullerene to afford some intriguing products as shown in Figure 5.8 [38c]. It was reported that the reaction is influenced by solvents and presence of oxygen in the reaction media. Nakamura and his co-workers discovered so-called Nakamura reaction that, using Grignard reagent combined with copper reagents and dimethyl sulfide, affords several penta-adducts around the corannulene subunit in excellent yields [71]. Recently they were successful in the first synthesis of beltane derivative or 40S-[10]cyclophenacene 16, which can be regarded as the smallest unit of (5,5)CNT [72].

5.2.3.3

Cycloadditions

Wudl once suggested that some cycloadditions are the most useful in the modifications of fullerenes [73]. The following four subsections summarize these useful cycloaddition reactions. Three-membered Ring-fused C60. A three-membered ring as fullerene-addend linkage is formed through intermediates, 3, 7, and 8. Among the reactions through 3, Bingel reaction is the most popular [39], which becomes now one of the most useful fullerene modification reactions (Scheme 5.5) [73, 74]. Using this reaction, many fascinating and intriguing derivatives have been synthesized as shown in Figure 5.9. For example, Nakamura, Nishimura, and their co-workers carried out Bingel reaction at –78 °C to r.t. and for the first time successfully obtained some catenanes possessing C60 surface as the ring component [75]. Other interesting Bingel products are also depicted in Figure 5.9 [36, 76].

Scheme 5.5 Bingel reaction [39].

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Figure 5.9 Fascinating Bingel products 18 [75], 19 [36], and 20 [76].

The Bingel products are apt to render electrochemically induced isomerization by the migration of cyclopropane rings on the C60 surface, if the controlled potential electrolysis is not exhaustive [77]. The isomerization was expressed as ‘electrochemical walk on the sphere’. Many related reactions have been reported. Monoalkyl malonates react with iodine in the presence of DBU to afford iodomethanofullerenes 21 [78]. BrCH2CN and bromoform also give corresponding methanofullerenes by the treatment of strong base LDA [79]. The reaction with a special enolate ion gives a derivative 22 [80]. Phosphonium [81] and sulfonium [82] ylides afford a variety of methanofullerenes. Recently Saigo and his co-workers developed fullerene-acid chloride 23 as a building block of the synthetic chemistry via the sulfonium ylide transformation [83].

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Scheme 5.6 Stepwise and direct formation of fulleroid and/or methanofullerene.

Diazomethane and related diazo compounds react with fullerene double bonds and form corresponding pyrazolines, which decompose to give fulleroids with the loss of nitrogen (see Scheme 5.6) [84]. Often fulleroids readily rearrange into methanofullerenes under heating or photoirradiation [85]. The reactions of diazo compounds with the fullerene do not always provide pyrazolines. In some cases, the corresponding carbenes are first generated and then add to fullerene to give methanofullerenes. Recently Akasaka and his coworkers examined thoroughly whether the reactions with diazoadamantane and alkyldiazirines pass through carbenes or pyrazolines. They successfully partitioned reactions after the analysis of methanofullerene and fulleroid contents [86]. As other three-membered ring formations, aziridine framework is used as the junction between the fullerene and a functional group. RO-CO-N3 forms aziridine ring between fullerene and alkoxy- or aryloxy-carbonyl group (see 27) [87]. On the other hand, RCH2N3 adds to the fullerene to afford azafulleroid 28 [88], one of which is derived to aza[60]fullerene as its dimer form 29, only accessible hetero-

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fullerene. Recently the chemistry of heterofullerene has been developed by Hirsch’s group [89]. Silylenes generated in situ add to the fullerene to afford corresponding silacylopropane product 30 [90]. Four-membered Ring-fused C60. The most striking four-membered ring between

fullerene and a partner was disclosed as the form of fullerene-dimer 31 by Komatsu and his co-workers [91]. This dumbbell type structure is often used as a symbol of chemical achievement. The dumbbell is formed through 3. Naturally a kind of [2+2] cycloaddition was expected to occur under photoirradiation. Using supramolecular and photochemical technique, McClenaghan and his co-workers first successfully obtained corresponding fullerene dimer [92]. Some 1,3-butadienes [93a], styrenes [93b], and enones [93c] also gave cyclobutane-annulated fullerenes under photoirradiation through radical species 5. Under thermal conditions some reactive acetylenes [94a], dimethyleneketene acetals [94b], ketenes [94c], and allenamides [94d] made cyclobutene-ring-joints for the fullerene. Besides the examples mentioned above, there are a few ones, mostly from the reaction of benzyne [47, 95] as shown in Scheme 5.7.

Scheme 5.7 Four membered-ring formation; 31 [91], 32 [94a], and 33 [47].

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Five-membered Ring-fused C60. The five-membered rings can be formed by [3+2]

cycloaddition, where three sp2 atoms come from reagents and two sp2 atoms derive from the fullerene double bond at 1,2-positions. Reagents possessing three-atom active site are 1,3-dipoles or Huisgen ylides, trimethylene methane diradical, disilylane, and so forth (Scheme 5.8). The reaction path usually is considered to be through 5 for photoreactions or 7 for pericyclic reactions of electron-rich ylides. Here in this section only stable products are dealt; unstable pyrazolines mentioned above are not included. Moreover, some cases treated above are not touched below such as dioxacyclopentane system 14.

Scheme 5.8 Prato reaction.

Figure 5.10 Fascinating Prato products; 35 [98], 36 [99], and 37 [100].

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Figure 5.11 Some five-membered ring-fused fullerenes with reagents or reactive species; 38 [101], 39 [102], 40 [103], 41 [104], 42 [105], 43 [106], 44 [107], 45 [107], and 46 [107].

The most useful convenient method among the kind of modifications is Prato reaction, one of pericyclic reactions [96]. It is one of convergent syntheses in which two high molecular weight substrates can be joined by one step. Actually fullerene, N-alkylglycine, and an aldehyde, such as formaldehyde or a high molecular weight aldehyde prepared, are dissolved in toluene and the mixture is heated to reflux. The procedure is just simple and the yield is usually high to excellent. Therefore it is so popular in materials science field as well as the fullerene chemistry. Recently this reaction was applied to make CNTs soluble in conventional organic solvents such as acetone and chloroform [97]. Fascinating Prato products 35–37 [98–100] are depicted in Figure 5.10. Other five-membered-ring formations are summarized in Figure 5.11 with reagents or reactive species [101–107]. Six-membered Ring-fused C60. There are some stepwise and concerted [4+2] cycliza-

tion reactions toward six-membered ring-fused fullerenes. The most popular one of the latter category is Diels–Alder reaction. As discussed above, the Diels–Alder adducts of fullerene are not always stable enough [61], so that unstable dienes such

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Figure 5.12 Some Diels–Alder examples together with a stepwise addition of amine; 47 [108e], 48 [111], 49 [112], and 50 [113].

as o-quinodimethanes are used to prevent the retro-Diels–Alder reaction [108] or the adducts are further transformed into more stable products [109]. Moreover the instability of Diels–Alder adducts is applied to the regioselective Bingel reaction mentioned above [54, 55], as well as the isolation of a higher fullerene such as C84 from the mixture with C76 and C78 [110]. Nevertheless making precursors for quinodimethanes, for example, is time consuming so that the Diels–Alder modification is not applied frequently to the materials science fields. Some Diels–Alder examples together with a stepwise addition of amine are depicted in Figure 5.12 [108e, 111–113]. 5.2.4 Conclusions

In the last one-and-a-half decades, a variety of modification reactions have been developed using the most easily available C60 in the chemistry. Yet still some methods are being searched especially in the transition metal catalyzed reactions. Since the application to materials science is now booming, the fullerene properties have attracted much attention, such as the high coagulation tendency, the poor solubility in usual organic solvents, the nanometer-scale volume, as well as the electron-accepting nature. When its electronic properties are important at the applications, however, the tendency must be kept in mind that C60 gradually loses its properties by the modification. For example, its quantum yield of the singlet oxygen generation under photoirradiation decreases from 1.0 for the pristine one as a sensitizer to 0.9 for monoadduct, and then to 0.5 for some bisadduct isomers [114]. And now the isolation of unstable fullerenes seems to be focused on, which is mentioned briefly above [110]. In the next decade we will see a lot of exotic

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fullerenes isolated from the soot obtained by a variety of methods. Those will not only be limited to the higher fullerenes but also found in lower fullerenes, which will give some impacts in organic chemistry itself at the same time.

5.3 Physicochemical Properties and the Unusual Structure of Fullerenes 5.3.1 Single-crystal X-ray Structures of Fullerenes and Their Derivatives

Olga V. Boltalina, Alexey A. Popov and Steven H. Strauss 5.3.1.1

Introduction

Fullerenes are closed-carbon-cage molecules containing 12 pentagons and (except for C20) one or more hexagons of three-connected C atoms that are nominally sp2 hybridized [115]. All isolated-pentagon-rule (IPR) [116] C60+x fullerenes have 60 C atoms that are at the junction of two hexagons and one pentagon (HHPJs); the other x C atoms are at the junctions of three hexagons (triple-hexagon junctions, or THJs). The curvature of fullerene cages requires that fullerene C atoms cannot be coplanar with their three nearest neighbors, as shown in Figure 5.13. This is a significant departure from the usual planar geometry of C(sp2) atoms, and the nonplanarity introduces steric strain in all fullerenes relative to planar graphite. The exohedral derivatization of fullerenes is driven, in large measure, by the relief of this strain as substituents [117] are added to some cage C atoms, changing the hybridization of those C atoms from (approximately) sp2 to sp3 [118]. The drawings in Figure 5.13, made with the coordinates of DFT-optimized C60 and the known D2-symmetry isomer of C76 (which is denoted C76-D2(1)) [119], show that HHPJs are more pyramidal with respect to their neighbors than THJs, and consequently no hollow fullerene derivative with fewer than 38 substituents has been found to have a substituent on a THJ [120–122]. In other words, HHPJs are

Figure 5.13 One of the 60 hexagon–hexagon–pentagon junctions (HHPJs) in the DFT-optimized structure of C60 (top) and one of the 16 triple-hexagon junctions (THJs) in the DFT-optimized structure of C76-D2(1) (bottom). Both views are parallel to the plane of the three darkly-shaded C atoms to which the junction carbon atom is attached. The darkly-shaded C atoms are connected with dashed lines.

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more reactive with respect to addition reactions than THJs because of the difference in pyramidalization [117]. There are two excellent reviews of fullerene X-ray structures, one by Neretin and Slovokhotov [123], which lists nearly 400 fullerene structures, and the more focused review by Balch [124]. The focus of this chapter is the inherent steric strain in fullerenes, as measured by S-orbital axis vector (POAV) analysis [125]. Only leading references will be cited. The examples of fullerene derivative X-ray structures we have chosen for analysis have F and CF3 substituents because these structures are more numerous [126, 127] than structures with other substituents. This will allow a comparison to be made of structures with identical compositions. X-ray structures of endohedral metallofullerenes (EMFs) will not be discussed, and space limitations further prevent us from reviewing the many structures of fullerene cycloadducts [128] and the growing class of norfullerene derivatives [128, 129]. Finally, the DFT-predicted results (PBE functional) in this chapter that are not explicitly attributed to previous publications were determined for this chapter using methods previously described [126]. 5.3.1.2

Disorder

The first attempts to determine precise C–C distances in crystalline C60 by X-ray diffraction revealed that the icosahedral molecule exhibits rotational disorder, and only the cage radius and intercage distance could be determined [123]. Consequently, as discussed in Section 5.3.3, the first determination of C60 bond distances was carried out using solid-state NMR spectroscopy [130], and only later by X-ray [131] and neutron diffraction [132] (in the latter two cases the fullerene was disordered even at low temperature). Although the solid-state disorder is severe, it is not completely isotropic [133]. To varying extents, higher fullerenes and endohedral fullerenes without exohedral substituents also exhibit disorder of cage C atoms and hence generally afford only imprecise diffraction-derived positional parameters [123, 124]. This problem has been overcome in favorable cases by cocrystallization with other molecules or by lowering the temperature of the crystal [123, 124, 134, 135]. The most precise structure of C60 is in C60·Pt(OEP)·2 C6H6; the fullerene is on a general position and the estimated standard deviations (esd) for the 90 cage C–C distances are 0.003 Å [135]. In most endohedral fullerene structures, some or all of the endohedral atoms exhibit positional disorder, even when exohedral substituents or cocrystallization with metalloporphyrins has minimized or eliminated cage disorder. However, the first completely ordered EMF derivative was recently reported [136]. 5.3.1.3

Nonplanar Steric Strain

The structures, Schlegel diagrams, and IUPAC numbering schemes for C60 and C70 are shown in Figure 5.14. Every fullerene larger than C60 has more than 20 hexagons (C70 has 25 hexagons and ten THJs). The nonplanar strain energy is considerable. It is believed to be ca. 2 000 kJ mol–1 in C60 (33 kJ mol–1 C-atom–1), which is 80% of the excess energy of the C atoms relative to graphite [125]. The POAV angle TVS, defined in Figure 5.15 [125], is 101.64° for DFT-optimized C60

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Figure 5.14 DFT-optimized structures and Schlegel diagrams of C60-Ih(1) and C70-D5h(1). The 10 triple hexagon junctions around the equator of C70 are C2, C5, C12, C20, C30, C40, C50, C59, C65, and C70.

Figure 5.15 In this version of the S-orbital axis vector approximation, the POAV is defined as the vector that makes equal angles to the three V bonds at a conjugated C atom. The common angle is denoted TVS. In C60, the Tp angle (i.e. TVS – 90°) is 11.64°.

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(the reduced POAV angles, defined as (TVS – 90°) and hereinafter denoted Tp, range from 11.4° to 11.9° for the C60 molecule in C60·Pt(OEP)·2 C6H6 [135], and are 8.6°, 10.2°, 11.5°, 11.8°, and 12.0° for C2, C1, C7, C24, and C8, respectively, in the DFT-optimized structure of C70-D5h(1). DFT average values of Tp are 10.9° for C70, 10.3° for the known D5d(1) isomer of C80, and 9.9° for the known D2(22) isomer of C84. It is sensible that the average value of Tp decreases as the number of C atoms in the cage increases because, in a C60+x fullerene, x C atoms are THJs and are therefore in a more planar environment than the 60 C atoms that form the 12 pentagons. The strain energy in fullerenes varies from isomer to isomer in higher fullerenes, being lowest for isomers in which the pentagon-induced curvature is distributed as uniformly as possible over the fullerene surface (i.e. for isomers with pentagons as far apart as possible) [115]. This is an extension of the isolated-pentagon rule (IPR) [116]. The lower stability of fullerenes with adjacent pentagons is generally understood to be due to two factors, (1) resonance destablization and (2) an increase in steric strain [118]. The larger steric strain in some of the cage C atoms of a fullerene with adjacent pentagons is clearly seen in Figure 5.16, which shows the DFT-optimized structure of the hypothetical non-IPR fullerene C84-Cs(51365) (an actual derivative of it, the EMF Tb3N@C84-Cs(51365), has been studied by X-ray diffraction [137]). The two pent-pent junction C atoms have Tp angles of 16.1° (note that the corresponding angle for a C(sp3) atom would be 19.5°). However, the evidence discussed below, based on DFT-optimized structures [138], indicates that the overall nonplanar steric strain in C84-Cs(51365) is not unusually high relative to many stable IPR fullerenes.

Figure 5.16 The DFT-optimized structure of the hypothetical non-IPR fullerene C84-Cs(51365). Note the unusually prominent non-planarity of the cage C atoms that form the pentagon– pentagon junction at the top of the drawing. The Tp angle for these symmetry-related C atoms is 16.1° (cf. 11.6° for C60 and 10.5° for the two C atoms the form the pentagon–hexagon junction at the bottom of the structure.

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229

Nonplanar Steric Strain Parameters

A measure of steric strain in IPR fullerenes known as Vh was described in the Atlas of Fullerenes [115]. It is ‘the standard deviation … of the hexagon neighbor index distribution’ (each hexagon in a fullerene has an index, 3, 4, 5, or 6, which is the number of hexagons that surround it) [115]. For IPR fullerenes up to the 46 isomers of C90, Vh varies from 0.000 (for the ‘least strained’ fullerenes according to this concept, which include C60, C80-Ih, and C80-D5h) to ca. 2.400, but nearly all are between 0.000 and 1.000. This follows Raghavachari’s suggestion [139] that nonplanar steric strain in IPR fullerenes is minimized when ‘the pentagon-induced curvature is distributed as uniformly as possible over the fullerene surface’ [115]. Some relevant Vh values are listed in Table 5.2 [119, 126, 135, 138, 140–151]. For example, Vh for C70 is 0.490. The Ih and D5d isomers of C80, which differ substantially in shape and therefore in the distribution of strain energy, are shown in Figure 5.17. Their Vh values are 0.000 and 0.817, respectively. It has proven difficult to apportion the energy difference between fullerene isomers to nonplanar steric strain and to S electronic effects that are not due to nonplanarity [115]. What is known is that, except for C84 isomers, IPR fullerenes with maximum electronic stability generally have high steric strain and vice versa [115]. For example, despite the greater steric strain implied by its high Vh value, the D5d isomer of C80 has a DFT energy that is 72 kJ mol–1 lower than for the Ih isomer [142]. Several authors have proposed using either (1) the parameter Tp (earlier defined as TVS – 90°) [116] or (2) the fractional s character for a fullerene C atom S atomic orbital as a measure of the nonplanar strain energy for that C atom, and the sum over all C atoms as an index of the nonplanar strain energy for the entire fullerene [115, 152]. Since Vh is only defined for IPR fullerenes with no substituents, we propose that a similar parameter be used to compare overall nonplanar steric strain in all fullerenes and their derivatives. That dimensionless parameter, V(Tp2), is the standard deviation of the Tp2 distribution for all C(sp2) atoms in the cage.

Figure 5.17 The DFT-optimized structures of two of the seven IPR isomers of C80 that have D5d and Ih symmetry. The steric parameter Vh is 0.817 for C80-D5d(1) and 0.000 for C80-Ih(7). The dimensionless steric parameters V(Tp2) is 36.0 for C80-D5d(1) and 10.1 for C80-Ih(7).

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Figure 5.18 The correlation of the steric parameters Vh and V(Tp2) for the 24 DFT-optimized IPR isomers of C84 (the parameters are defined in the text). The line is a linear least-squares fit to the data. The DFT structure of C84-D2(1) is shown with its 84 POAV vectors. It is the least stable of the IPR C84 cages as well as the least uniform in surface curvature. The Tp angles for the vectors labelled a and b are 12.3° and 4.6°, respectively.

To demonstrate that Vh and V(Tp2) can be used interchangeably for IPR fullerenes, and therefore that V(Tp2) reliably measures nonplanar steric strain, we determined V(Tp2) values of the 24 IPR isomers of C84 and compared them with their Vh values (Figure 5.18). A similar plot for IPR isomers of C80 (not shown) is also essentially linear with similar slope and intercept. This suggests that it may be meaningful to compare V(Tp2) values for cages with different numbers of C atoms such as those listed in Table 5.2. Table 5.2 Structural parameters and DFT-predicted relative 'Hf values for fullerenes and fullerene(X)n derivatives.a) Ähb) [115]

Ä(¹p2)c)

Average ¹p, degg)

DFT or X-ray

C60

0.000

0.0

11.6

DFT

C60

0.000

2.5

11.6

X-ray [135]

C70

0.490

25.0

10.9

DFT

49.3

9.0

X-ray [140]

23.2

10.5

DFT [119]

35.7

8.4

X-ray [141]

Compound

'Hf for isomers, kJ mol–1

C70(CF3)10-1 C74-D3h(1) (C74-D3h(1))(CF3)12

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0.415

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Table 5.2 (continued) Compound

'Hf for isomers, kJ mol–1

Ähb) [115]

Ä(¹p2)c)

Average ¹p, degg)

DFT or X-ray

C80-Ih(7)

72d)

0.000

10.1

10.1

DFT [138]

d)

0.365

23.4

10.1

DFT [138]

0.817

36.0

10.3

DFT [138]

C80-C2v(5)

18

C80-D5d(1)

d)

0

e)

C84-D2(22)

0.0

C84-D2(21)

0.331

22.8

9.9

DFT [138]

e)

0.331

23.2

9.9

DFT [138]

e)

0.485

28.7

9.9

DFT [138]

0.927

46.0

10.0

DFT [138]

36.1

9.9

DFT [138]

0.331

28.7

9.9

DFT [138]

0.331

27.9

9.9

DFT [138]

0.485

33.1

9.9

DFT [138]

54.0

C84-C2v(7)

88.3

e)

C84-D2(1)

200.8

e)

C84-Cs(51365)

258.5

C84-D2(22)6–

38.5e)

6–

e)

C84-D2(21)

0.0

6–

e)

C84-C2v(7)

48.6 6–

e)

C84-Cs(51365)

1.8

37.5

9.9

DFT [138]

C60(CF3)10-2

0.1

31.5

9.5

X-ray [143]

C60(CF3)10-3

4.7

35.9

9.4

X-ray [144]

C60(CF3)10-4

7.6

36.4

9.5

X-ray [142]

C60(CF3)10-5

1.2

34.3

9.5

X-ray [126]

C60(CF3)10-6

0.0

28.7

9.4

X-ray [146]

C60(CF3)12-1

0.0f)

29.7

9.0

X-ray [147]

33.3

8.9

X-ray [148]

C1-C60(CF3)18

28.8

7.0

X-ray [150]

C3v-C60F18

53.9

8.6

X-ray [149]

T-C60F36

3.3

1.7

X-ray [151]

C60(CF3)12-2

a) b)

c)

d) e) f) g)

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f)

39.8

All data from this work unless otherwise noted. Reference numbers are shown in square brackets. The Vh values are standard deviations of the hexagon neighbor index distributions. The Vh parameter is only defined for IPR fullerenes, not for non-IPR cages (e.g. C84-Cs(51365)) or for exohedral fullerene derivatives. These dimensionless steric parameters, defined in the text, were calculated for this chapter using the DFT-optimized coordinates for all of the three-connected carbon atoms in a fullerene or fullerene derivative. Ref. [142]; these relative energies were calculated for singlet ground states. Ref. [138]. Ref. [148]. This average is over all cage C atoms that are nominally sp2 hybridized and does not include sp3 hybridized C atoms bearing substituents.

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5.3.1.5

Are Non-IPR Fullerenes Sterically Unstable?

The relative 'Hf and V(Tp2) values in Table 5.2 reveal an interesting aspect about the structure of non-IPR C84-Cs(51365). It is predicted to be 58 kJ mol–1 less stable than the least stable IPR isomer, C84-D2(1), and 258 kJ mol–1 less stable than the most stable IPR isomer, C84-D2(22). However, its V(Tp2) value is significantly lower than that for C84-D2(1) (it is also lower than V(Tp2) for two other IPR C84 cages). In addition, its V(Tp2) value is the same as for IPR C80-D5d(1), the most stable of the IPR C80 cages [142]. The average values of Tp for C84-D2(22) and C84-Cs(51365) are also the same. Based on these results, it does not appear that C84-Cs(51365) has an unusual amount of nonplanar strain energy relative to some of the most stable IPR fullerenes that are known to exist.The DFT relative energies of several 6– cages were recently reported [128]. They were calculated to predict the most C84 stable cages that should be present in isolable iM3N@C84 compounds. The data in Table 5.2 show that V(Tp2) values for the neutral cages and the 6– ions are only marginally different. Significantly, the C84-Cs(51365)6– cage is energetically as stable as the most stable hexaanionic IPR cage, C84-D2(21)6–. With the right electron count, the non-IPR isomer can be as stable as any IPR isomer. This can only be true if the non-IPR cage is not sterically very unstable. Therefore, it appears that the instability of neutral C84-Cs(51365) is predominantly electronic in origin. This may also be true for other non-IPR fullerenes, and the idea that non-IPR fullerenes are sterically very unstable should be carefully reexamined. 5.3.1.6

Long and Short C(sp2)–C(sp2) Bonds in Fullerene Cages

With the exception of three non-IPR EMF X-ray structures, Tb3N@C84-Cs(51365) [137]. La@C72(C6H3Cl2) [153], and Sc3N@C68-D3(6140) [151], all other fullerene X-ray structures reported to date are of IPR fullerenes. The first X-ray structures of non-IPR hollow fullerenes were reported in late 2008 [591]. The smallest IPR fullerene is C60, and its X-ray structure [135] revealed that it has 30 short bonds (1.379(3)–1.396(3) Å), which are hexagon-hexagon junctions, and 60 long bonds (1.440(3)–1.461(3) Å), which are hexagon-pentagon junctions, as shown in Figure 5.19 (the DFT-optimized distances are 1.399 and 1.453 Å, respectively, and the NMR-determined distances are 1.40 and 1.45 Å, respectively [130]). The short hex-hex junctions and long hex-pent junctions are usually referred to as ‘double’ bonds and ‘single’ bonds, respectively, although this is clearly an oversimplification. These and other results [155, 156] have led to the oft-quoted ‘rule’ that double bonds in pentagons (DBIPs) are destabilizing in fullerenes and exohedral fullerene derivatives, although the underlying steric and/or electronic basis for this rule has not been convincingly explained. Higher fullerenes have more than two types of cage C–C bonds. The structures of C70·6 S8 [157, 158] and C76·6 S8 [159] were reported in the 1990s, but they did not have a precision that allowed the C–C bond distances of the various bond types to be distinguished from one another. For C70, this was still true at the end of 2006 [160]. In order to get a solution to the structure in some cases, noncrystallographic symmetry had to be imposed (e.g. D5h symmetry for C70 [158]) or a rigid-body DFT-optimized cage has been assumed and used in the refinement (e.g.

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Figure 5.19 The 90 fullerene cage C–C bond distances in the structure of C60 · Pt(OEP) · 2 C6H6 [135]. The error bars shown are ±3V. The 30 shorter distances (‘double bonds’) are hexagon-hexagon junctions and the 60 longer distances (‘single bonds’) are hexagon-pentagon junctions.

Figure 5.20 The shortest and longest distance for the eight types of cage C–C bonds in two of the three C70 molecules in the X-ray structure of C9H3Cl6N · 3 (C70) · 3 (C6H5Cl) (±3V error bars) [163]. Each of the eight diamond-shaped points is the DFT-optimized distance for that C–C bond type (this work).

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for BaC74-D3h [161] and Sc3N@C78-D5h(5) [162]). A precise structure of C70 was finally determined in 2007 using crystals of C9H3Cl6N·3(C70)·3 (C6H5Cl), with esd for cage C–C distances of 0.004 Å for two of the three independent C70 molecules [163]. The ranges for the eight types of C–C distances are shown in Figure 5.20 and are compared with the corresponding DFT predicted distances. The addition of substituents to the fullerene cage can also lead to ordered and precise structures, albeit at the expense of altering the fullerene cage from its original state as a pure carbon allotrope. The esd for cage C–C distances can be as low as 0.003 [140], 0.002 [141], or 0.001 Å [147]. For example, the X-ray structure of C70(CF3)10 [140] is shown in Figure 5.21a along with a plot of its 75 C(sp2)–C(sp2) distances. The esd for these distances are 0.003 Å. A plot of the 60 C(sp2)–C(sp2) distances in an isomer of C60(CF3)10 is also shown in Figure 5.21b. These plots show that the sharp distinction between cage C–C ‘single’ and ‘double’ bonds, which was meaningful when applied to C60, is much less distinct for higher fullerenes and fullerene(X)n derivatives (including C60Xn derivatives).

Figure 5.21 Plots of the 75 (a) and 60 (b) cage C(sp2)–C(sp2) distances in the X-ray structures of one of the isomers of C70(CF3)10 [140] and C60(CF3)10 [143]. The insets are (upper left in both cases) a 50% probability thermal ellipsoid plot of the molecule and (lower right in both cases) a Schlegel diagram showing the ribbon of edge-sharing C6(CF3)2 hexagons (each black circle represents a cage C atom bearing a CF3 group; the letter ‘m’ denotes the meta-C6(CF3)2 hexagon).

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There are now nearly 30 X-ray structures of fullerene(CF3)n compounds (and a growing number of fullerene(C2F5)n structures) [126, 127], and many of these are among the most precise X-ray structures of any fullerene or fullerene derivative. For the CF3 compounds, the value of n varies from 2 to 18. The CF3 substituent is sterically bulky (it is larger than an iodine atom) [144]. In most of the structures with n d 12, the CF3 groups are distributed one per pentagon (there are only two exceptions), and CF3 groups are rarely found on adjacent cage C atoms (two different exceptions). In nearly every case, the CF3 groups are found on isolated para-C6(CF3)2 hexagons and/or on one or more ribbons or loops of edge-sharing meta- and para-C6(CF3)2 hexagons [126, 127]. In two cases, C2-p11-(C74-D3h) (CF3)12 and C2-p11-(C78-D3h(5))(CF3)12 [141] the X-ray structures were the first unambiguous proof of the existence of the hollow C74-D3h and C78-D3h(5) cages (‘p11’ in the formula indicates that the 12 CF3 groups are arranged on a ribbon of 11 edge-sharing para-C6(CF3)2 hexagons). The structure of the C74 compound was sufficiently precise (esd = 0.002 Å) that the cage C–C distances could be compared with the DFT-predicted distances (Figure 5.22). For all but three of the C–C bonds, the X-ray distance matched the DFT distance to within ±3V, validating the PBE functional used in that study (and in our other publications, including this chapter) [125].

Figure 5.22 X-ray vs DFT-predicted cage C–C distances for the structure of C2-(C74-D3h)(CF3)12 (the molecule has crystallographic C2 symmetry) [141]. The error bars shown are ±3V. The insets are (upper left) a 50% probability thermal ellipsoid plot of the molecule and (lower right) a Schlegel diagram showing the p11 [125] ribbon of edge-sharing C6(CF3)2 hexagons (each black circle represents a cage C atom bearing a CF3 group).

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5.3.1.7

Steric Strain in C60(X)n Isomers

One question that has not been addressed very often in the literature is whether or not different C60(X)n isomers might have different amounts of nonplanar steric strain that can affect their relative energies [164–166]. (In this part of our analysis, we do not consider steric strain induced by proximate X substituents [165] or potential angle strain in cage C(sp3) atoms). Listed in Table 5.2 are the DFT relative energies and X-ray derived V(Tp2) values for five isomers of C60(CF3)10. Schlegel diagrams of the isomers are shown in Figure 5.23 (the X-ray structure of isomer 1 has not yet been determined). Not listed in Table 5.2 are the range, average, and standard deviation for the bond angles for C(sp2) atoms within the pentagons for each isomer. All three of these angle parameters are virtually the same for the five isomers as well as for C60, indicating negligible differences in angle strain due to C(sp2) atoms having C–C–C angles of ca. 108° [167]. For example, the sets of {range, average, standard deviation of the average} for these angles are {107.4–108.6°, 108.0°, and 0.2°} for C60, and {106.9–110.8°, 109.0°, and 0.9°} for C60(CF3)10-2, and {106.9–110.7°, 109.0°, and 0.9°} for C60(CF3)10-6 (all three sets from X-ray structures). The relative energies for the five isomers of C60(CF3)10 range from 0.0 to 7.6 kJ mol–1 and the V(Tp2) values range from 28.7 to 36.4. Since these are both small ranges, we conclude that this series of nearly equienergetic isomers have comparable values of total steric strain (since differences in angle strain are also negligible) and therefore have similar S electron energies. Two isomers of C60(CF3)12 are also listed in Table 5.2 (see Figure 5.23 for Schlegel diagrams). In this case the relative energies differ by 40 kJ mol–1, a significant amount. However, the V(Tp2) values are similar, so it would seem that the energy difference can be attributed to differences in S electron energy. This tentative conclusion should be tested in the future by a computational study. Next, compare the two X-ray structures with very different addition patterns, C3v-C60F18 and C1-C60(CF3)18 (see Table 5.2 and Figure 5.23). In this case, the V(Tp2) values are quite different, 53.9 for C3v-C60F18 but only 28.8 for C1-C60(CF3)18. This shows that different addition patterns for a given number of substituents can lead to significantly different amounts of nonplanar steric strain, although this was not true for the five structurally similar isomers of C60(CF3)10. The DFT energies of C3v-C60F18 and the isomer of C60F18 with the same addition pattern of C1-C60(CF3)18 revealed that C3v-C60F18 was 290 kJ mol–1 more stable. How much of that difference is due to differences in steric strain and how much is due to differences in S electron energy remains to be seen. Finally, consider the concept that relative planarity for one part of a C2n fullerene is sterically destabilizing because, since the surface must be closed, it introduces ‘high curvature in another part of the fullerene surface’ [115, 152, 168]. This is a valid concept for fullerenes without exohedral substituents, but is it also valid for fullerene(X)n derivatives? The answer may be ‘No.’ In C2n fullerenes, all C atoms are nominally sp2 hybridized, but in fullerene derivatives there are cage C(sp3) atoms, and these atoms, which do not experience the nonplanar steric strain that the C(sp2) atoms experience, can contribute to closing the surface of the fullerene.

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Figure 5.23 Schlegel diagrams of some of the C60Xn derivatives listed in Table 5.2 (X = F, CF3). Each black circle is a cage carbon atom to which a substituent is attached. For the C60(CF3)n compounds with n = 10 and 12, the ribbons or loops of edge-sharing meta- and para-C6(CF3)2 hexagons are highlighted (the letter ‘m’ denotes the meta-C6(CF3)2 hexagon) and the IUPAC lowest locants of the cage C(sp3) atoms are indicated.

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In fact, the average Tp for the cage C(sp2) atoms in the X-ray structures of C60, C60(CF3)10-6, C60(CF3)12-2, C3v-C60F18, C1-C60(CF3)18, and T-C60F36 (Figure 5.23) are 11.6°, 9.5°, 8.9°, 8.6°, 7.0°, and 1.7°, respectively. The introduction of exohedral substituents clearly reduces the average nonplanar steric strain in cage C(sp2) atoms, to the point where the 24 remaining C(sp2) atoms in T-C60F36 are in nearly planar environments. In reviewing the X-ray structures (and some relevant DFT-optimized structures) of fullerenes and their derivatives, we have raised several questions about how to determine steric strain in these compounds, especially in exohedral derivatives. We do not believe that we have answered these questions fully, because our analysis has been simple. Instead, we have highlighted some aspects of steric strain in closed-curved-surface carbon compounds that should be studied with more sophisticated computational methods in the future, especially as more and more precise X-ray structures become available.

Acknowledgment

We thank Prof. Marilyn Olmstead for her assistance in preparing this chapter, and we acknowledge the financial support of the Civilian Research and Development Foundation (RUC2-2830-MO-06 to AAP and SHS) and the U.S. National Science Foundation (CHE-0707223 to OVB and SHS). 5.3.2 Vibrational and Electronic Spectra

Alexey A. Popov 5.3.2.1

Introduction

Spectroscopic studies of the fullerenes played an important role from the very beginning of the fullerene era. Historically, when Wolfgang Krätschmer and coauthors discovered that the soot prepared by the arc-discharge method under certain conditions was enriched with fullerenes, a characteristic IR spectrum with four sharp lines emerging over the broad band background was the first proof that C60 was present in the sample, because exactly four IR active modes are expected for the icosahedral C60 molecule from group theory analysis [169]. Subsequent discovery of the method of extracting the fullerenes from carbon soot [170] as well as progress in HPLC separation of fullerene mixtures, boosted numerous studies of spectroscopic and photophysical properties of these molecules. In this chapter, a survey of vibrational and electronic spectroscopic studies of fullerenes is presented. We should note that the number of published papers dedicated to the given fullerene molecule scales in accordance with the fraction of this fullerene in the arc-discharge soot. Hence, the ‘archetypical fullerene’ C60, which constitutes ca 85% of the crude fullerene extract, is by far the best studied member of the fullerene family, and it is inevitable that any review like this is to a large extent a review of the numerous studies on C60. The properties

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revealed for C60 are to some extent inherited by all other fullerenes; however, the high symmetry of the C60 molecule makes this fullerene very special in terms of spectroscopic properties, since the majority of its energy levels are degenerated, and many vibrational and electronic transitions are forbidden by symmetry selection rules. 5.3.2.2

Vibrational Spectra of Fullerenes

It is natural to expect the vibrational spectra of fullerenes to be quite complex: even for C60, the smallest classical fullerene, the number of modes is as large as 174. However, vibrational representation of the free C60 molecule spans 2 Ag + 3 T1g + 4 T2g+ 6 Gg + 8 Hg + Au + 4 T1u + 5 T2u + 6 Gu + 7 Hu symmetry types of Ih group, and due to the high symmetry, 174 vibrational degrees of freedom are substantially degenerated and grouped into 46 normal modes, of which only 14 are optically active (Ag and Hg symmetry types in Raman and T1u type in IR spectra). In accordance with the symmetry considerations, four primary IR and ten Raman lines are observed in the experimental spectra of crystalline C60. As the highest site symmetry of C60 in the room-temperature lattice is Th (and is further reduced at lower temperature), the fact that the spectra of solid C60 samples can be described in terms of the isolated fullerene molecule with unperturbed Ih symmetry show that crystalline fullerenes can be treated as molecular crystals with weak intermolecular interactions. The same conclusion can be drawn from the X-ray data discussed in the former section. Contrary to the optically-active modes, assignment of the many of 32 inactive fundamentals of C60 is still ambiguous (see, for instance, detailed analysis given in Refs. [171] and [172]). The most detailed, but also the most complicated, information can be obtained from the second-order IR and Raman spectra [173, 174], i.e. the spectra of thick films or single crystals, which exhibit multiple bands with intensities at least an order of magnitude lower than those of the symmetry-allowed modes (Figure 5.24). The spectra are very rich and can be attributed to the silent modes as well as overtones and combination modes. The vibrational spectrum of C60 was also studied by fluorescence spectroscopy (the measurements of the matrix-isolated C60 at helium temperatures are particularly informative) [175, 176], inelastic neutron scattering (INS) spectroscopy [177, 178], photoluminescence spectroscopy of the singlet oxygen entrapped in C60 lattice (SOPL) [179], and high resolution electron energy loss spectroscopy (HREELS) [180] (see Figure 5.24). While fluorescence spectroscopy provides information on some optically-forbidden ungerade modes, INS, SOPL and HREELS in principle provide complete vibrational density of states (VDOS) irrespective of the symmetry types (however, selection rules for SOPL spectrum are not yet clear). Unfortunately, the spectral resolution of these methods is rather low, especially in the high frequency range. In 1998 Heid [178] reported assignment of all vibrational modes in the 250–600 cm–1 range based on the INS study of a large single crystal of C60. Besides C60, detailed vibrational studies by means of IR, Raman, fluorescence, HREELS, INS and SOPL spectroscopies were reported only for D5h–C70. As can be expected from the lower symmetry of the molecule, the spectra of C70 are consider-

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Figure 5.24 (a) IR and Raman spectra of C60. Raman spectra are shown for two excitation wavelengths. Note strong resonance enhancement of Ag(2) modes in the spectrum excited in the visible range. (b) ‘Second-order’ IR and Raman spectra of C60. (c) Vibrational density of states in C60 measured by INS [177], HREELS [181], and SOPL [179] and calculated with DFT (PBE/TZ2P) [182]. Stretching O-O modes in SOPL spectra of 18O2 and 16O2 are denoted by asterisks.

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ably richer than those of C60. Due to the much higher number of fundamentals (46 in C60 vs 122 in C70, including 53 Raman active and 31 IR active vibrations), assignments of C70 normal modes are less reliable than those for C60, and some ambiguities exist even in the assignment of several IR and Raman active modes. The use of INS or SOPL for C70 appears less advantageous than for C60 because the lower degeneracy of C70 normal modes and their much higher total number result in the smeared VDOS in the whole frequency range [183, 184]. The difficulties with the isolation of the isomerically pure samples of higher fullerenes result in the lack of vibrational spectroscopic data for many of them. To our knowledge, IR and Raman spectra have been reported for D2(1)-C76 [185], C2v(2)-C78 [186], C2v(3)-C78 [186], D2(22)-C84 [187, 188], and D2d(23)-C84 [187, 188]; for D3(1)-C78 and D2(2)-C80 only Raman spectra are available [189]. As anticipated, complexity of the spectra increases with the number of atoms in the fullerene molecule and with the decrease of its symmetry. At the same time, vibrational spectra appear to be very structure sensitive (for instance, IR and Raman spectra of two C2v isomers of C78 are substantially different, Figure 5.25), a property that can be used for structure determination of the new fullerene isomers, especially if experimental studies are supported by theoretical calculations. A systematic study of the Raman spectra of the group of fullerenes were performed by Eisler et al. [189, 190] in 2000. The authors studied isomerically pure fullerenes C60, C70, C76 (D2), C78 (D3, C2v, Cc2v), and C80 (D2) and isomeric mixtures of C82 (two C2 isomers in 1 : 1 ratio) and C84 (D2 and D2d isomers in 2 : 1 ratio). Room-temperature spectra of all fullerenes were measured with 514, 693, 794, 1064 nm excitation wavelengths (see Figure 5.25 for the spectra obtained at 693 nm wavelength). To establish the common vibrational phenomena for all fullerenes, the authors employed Lamb theory of oscillations of isotropic spherical and spheroidal shells. Two types of vibration, referred to as monopolar and quadrupolar, were found to be especially characteristic in the analysis of the Raman spectra of fullerenes. Monopolar modes can be easily recognized as they sustain medium to strong Raman intensity over all excitation wavelength (the example is ‘breathing’ Ag(1) vibration of C60). Their frequencies scale inversely with the square root of mass and are shape-insensitive, that is, different isomers with the same mass (e.g. C78) exhibit nearly the same frequency. Quadrupolar mode (the example is ‘squashing’ Hg(1) vibration of C60) is five-fold degenerate for a sphere and C60, but is split into several components for all other fullerenes. Importantly, the magnitude of splitting correlates with the deviation of the fullerene shape from the sphere (viz. 15 cm–1 for nearly spherical D2–C84 versus 45 cm–1 for elongated D2-C80), and thus can be used to determine the isomeric structures of newly isolated fullerenes.

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Figure 5.25 (a) IR spectra of higher fullerenes [185–188]; (b) Raman spectra of higher fullerenes [185–188] (Oex = 1064 nm for C70–C78, 514 nm for C84); (c) Raman spectra of some fullerenes, excitation 693 nm [189]. Asterisks in (c) denote breathing mode.

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243

The Orbital Picture of Fullerenes: High-energy Electronic Spectra

Though non-planarity of the fullerene molecules results in the partial mixing of V and S states (see Section 5.3.3.3), high-lying occupied and low-lying unoccupied molecular orbitals (MOs) of fullerenes are still of the essentially S-nature and may be adequately described by the theoretical methods developed for the S-systems. Due to the high symmetry of C60, it is straightforward to treat this molecule as a spherical shell whose electronic states can be classified in terms of the orbital momentum l. In such a ‘superatomic’ model, all levels up to l = 4 are occupied, while the level with l = 5 is partially filled with 10 electrons (Figure 5.26) [180]. Though very simple, the model gives a reasonable description of the electronic structure of C60 which can be correlated with the orbital picture obtained by using more sophisticated theoretical approaches (Figure 5.26), especially for the occupied states. Under icosahedral perturbation, the electronic levels are split but still retain a substantial extent of degeneracy. The majority of C60 MOs have either t(u,g), g(u,g) or h(u,g) symmetry, including five-fold degenerated HOMO (hu) and three-fold degenerated LUMO (t1u). LDA calculations of the band structure in crystalline C60 have shown that the widths of the bands derived from the frontier orbitals do not exceed 0.5 eV [191], and hence discrete density of states (DOS) characteristic for C60 in the gas should be preserved in the solid phase. Experimentally electronic states of C60 were probed by a variety of high-energy spectroscopic techniques (see Refs. [180, 192, 193] for reviews). Valence occupied states were extensively studied by ultraviolet photoemission spectroscopy (UPS) [180, 194] providing a direct measure of orbital ionization energies and DOS. A complementary technique, providing information about density of unoccupied states (DUOS) is inversed photoemission spectroscopy (IPES) [195], in which the surplus electron is immersed into unoccupied orbitals. Other methods extensively used to analyze DUOS of fullerenes are core-level electron energy loss spectroscopy (EELS) [180, 192, 193] and X-ray absorption spectroscopy (XAS) [196, 197]. In core-level EELS and XAS, electrons from C1s levels are excited to unoccupied C2p-derived states. Formally this provides a measure of DUOS, however the energy levels are affected by the interactions with the core hole, resulting in the possible difference from the IPES spectra (which are free from the excitonic effect). Figure 5.27 shows representative UPS, IPES, XAS and EELS spectra of C60 compared with the DOS and DOUS of the isolated molecule computed at the PBE/ TZ2P level. The spectra in the valence region are characterized by sharp distinct peaks, which reflect the extensively degenerated MO levels of C60. UPS and IPES spectra show good agreement with theoretically computed DOS and DUOS in the energy range of S and S* states, respectively, and the features derived from the highest-energy occupied orbitals can be assigned straightforwardly (Figure 5.26). The patterns of EELS and XAS spectra are virtually identical and, though they also resemble IPES spectra, the shift of the peak positions is obvious, pointing to the importance of the excitonic effect in the fullerene. UPS spectra measured in the gas and the solid state are very similar proving weak intermolecular interaction in the latter, however the fine vibronic structure

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Figure 5.26 Upper panel: Electronic energy levels in C60 as obtained with the model: electron on a sphere [180] (left), Hückel approximation (middle) and DFT PBE/TZ2P [182] (right). Note that the energy scales for the three models are different. Middle and lower panels: DOS and DUOS in C60 as measured by different methods [180, 194–197] and calculated by DFT [182]. The asterisks in the gas-phase spectra denote CO bands.

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resolved in the spectrum of the gaseous C60 is absent in the solid state spectra [194]. Ionization potential (IP) and electron affinity (EA) of C60, estimated from the gas-phase UPS spectra of neutral C60 and its anion [198], are 7.3 and 2.67 eV, respectively, which formally gives the energy gap between the HOMO and LUMO levels of 4.7 eV, while in the solid state the energy difference of the HOMO and LUMO-derived peaks of UPS and IPEs spectra is reduced to 3.75 eV due to the screening effect of the surrounding lattice molecules. Significantly, both gas and solid state values are considerably larger than the band gap ' = 1.8–2.2 eV measured by optical absorption spectroscopy and low-energy EELS. The reason for such a discrepancy is the different nature of the final states. While the ' value corresponds to the formation of the exciton (hole-electron pair) localized on one molecule, in the UPS and IPES techniques the final states are ionic. That is, (IP-EA) difference or the peak-to-peak distance in UPS/IPES spectra is the formation energy of the hole and the electron localized on two different molecules. The difference (IP–EA)–' defines the energy of on-site Coulomb interaction (so called charge correlation energy), Hubbard U, which therefore has the order of 3.0 eV for the free molecule and 1.6 eV for C60 in the solid state. As can be expected on the basis of their common molecular architecture and all-carbon S systems, DOS and DUOS of C70 and the higher fullerenes resemble those of C60 (Figures 5.26 and 5.27). However, the lower symmetry and larger number of atoms result in an increase of the number of states with lower degeneracy, and the spectral patterns are less resolved than those in C60 exhibiting many overlapping bands. Meanwhile, the spectra still exhibit individual characteristic

Figure 5.27 UPS and C1s EELS spectra of selected fullerenes [193] (middle panels) compared to the DOS (left) and DUOS (right) computed at the PBE/TZ2P level [182]. C84 is a mixture of D2(22) and D2d(23) isomers in 2 : 1 ratio.

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features, resembling peculiarities of the electronic structure of each fullerene, which can be best demonstrated by significant difference observed in the DOS and UPS spectra of two C2v isomers of C78 [180, 192, 193]. 5.3.2.4

Electronic Excitations. UPS, UV/Vis/NIR Absorption and Fluorescence Spectroscopy

Excitation spectra of the fullerenes in the near-infrared, visible and near-UV range are attributed to the S–S* transitions and are very structure sensitive. The lowest energy transition, which can be ascribed to the excitation through the HOMO-LUMO gap, is an important fundamental property determining the kinetic stability of the given fullerene. Only molecules with a considerably large HOMO-LUMO gap can be isolated from the soot by conventional organic solvents, while structures with a small gap, albeit they may be formed in considerable amount and be thermodynamically stable, cannot be extracted from the soot, presumably because they have polymerized. C74, D3h(5)-C78 or Td-C76 are examples of such ‘insoluble’ fullerenes, whose formation in the arc-discharge synthesis was proven by electrochemical or chemical transformation to the soluble forms [199]. Excitation spectra of fullerenes have been studied by many techniques, including, but not limited to, low-energy EELS, core-level XPS (satellite structure of C1s peak), UPS of the anionic species, and optical absorption and fluorescence spectroscopy. Description of the results obtained by EELS and XPS are beyond the scope of this review (for a review, see Refs. [180, 192, 193]), and here we mainly focus on UPS and optical spectroscopy of fullerenes. The UPS of the gas-phase anions is an important information source for two kinds of fundamental properties of fullerenes. First, the lowest energy band in the UPS spectrum corresponds to the detachment of the electron from the singleoccupied orbital of the anion and therefore corresponds exactly to the electron affinity. Secondly, photodetachment of the electron from the doubly-occupied orbital results in singlet or triplet excited state of the neutral molecule. Thus, UPS spectrum of the anion provides information on the excited states of the fullerene and is especially useful in the determination of the lowest energy excitations. As UPS measurements of the gas-phase anions require very small amounts of the sample, while the mass-selection technique allows the mixtures of fullerenes to be studied without their preliminary separation, it is not surprising that the first study of the anions of C60 and C70 fullerenes by UPS was done in 1987 [200], before the method of their bulk-production was discovered. Likewise, UPS is the only method of choice for the studies of the electronic structure of exotic small fullerenes C20–C50 (Figure 5.28) [201, 202]. Thus, C32 has been shown to have a large HOMO-LUMO gap and is therefore expected to be stable [201]. Vibrationally resolved UPS spectra of C –60 [198], and C –70 [203], cooled in the ion trap are shown in Figure 5.28. The second and the third lowest energy bands in the spectrum of C –60 are attributed to the first triplet and singlet excited states of C60. The energy of the former is estimated as 1.62 ± 0.01 eV, while the multiplet splitting (i.e. the energy gap between triplet and singlet states derived from the same excitation) is 0.2–0.3 eV.

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Figure 5.28 UPS spectra of anions of some fullerenes; vibrationally resolved UPS spectra of C –60 [198], and C –70 [203]. ‘A. D.’ in the spectrum of C –60 marks an auto-detachment band.

Though UPS remains an invaluable source of information on the structures whose isolation in bulk amounts is not possible, the method requires rather complex equipment, while the information it gives is actually limited to the lowest-energy excitation, because at the higher energies the density of excited states of fullerenes is usually very high. For this reason, the UV/Vis/NIR absorption spectroscopy is much more advantageous now, when at least > 1 mg amounts of the fullerenes have become available. Simple relatively inexpensive instrumentation and sampling techniques make this method accessible to virtually all researchers worldwide, and the richness and uniqueness of S–S* excitation spectrum for each fullerene has made it almost a standard now that the characterization of any new fullerene cannot be complete without the UV/Vis/NIR absorption spectrum. Room-temperature UV/Vis absorption spectrum of C60 in n-hexane is shown in Figure 5.29. Three regions with the intensities covering three orders of magnitude can be roughly distinguished in the spectrum: low-intensity highly structural bands in the visible range (450–700 nm), several more intense sharp peaks around 380–420 nm, and finally three very strong broad absorptions at 328 nm (log(Hmax) = 5.20), 256 nm (5.24) and 211 nm (5.20) in the UV range [204]. As only the excitation to the singlet states of T1u symmetry are optically allowed for C60, its HOMO o LUMO (hu o t1u) transition is optically forbidden, giving rise to the excited states of T1g, T2g, Gg, and Hg symmetry. The lowest energy 1T1u state can be attributed to the sharp feature at 408 nm (3.04 eV) in the experimental

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Figure 5.29 Left panel: Room-temperature UV/Vis absorption spectrum of C60 in n-hexane. Insets show low temperature ‘absorption’ spectra: resonant two-photon ionization spectrum in ultrasonic beam [210] (R2PI); fluorescence-excitation spectrum in Ne matrix at 4 K [175], absorption spectrum in helium droplets at T~0.4 K [207]. Vertical bars show excitation energies and oscillator strengths predicted by TD-DFT [205]. Right panel: Low temperature fluorescence spectra of C60 in different matrices [175, 208].

spectrum. Interpretation of the absorption spectrum of C60 in the UV range was given in 1992 by Leach [204]. A new assignment for some bands was proposed by Bauernschmitt [205] in 1998 with the use of more refined time-dependent (TD) DFT calculations (see Figure 5.29 for the comparison of TD-DFT computed and experimental spectra of C60). Though HOMO o LUMO transitions in C60 are symmetry forbidden, they may be activated through a Herzberg–Teller (HT) coupling to the appropriate molecular vibrations. In due turn, each HT-activated transition is an origin for the Franck–Condon (FC) and Jahn–Teller (JT) progressions via coupling to the Ag and Hg modes, respectively. Altogether, these multiple vibronic excitations form a complex pattern observed in the experimental spectrum of C60 in the visible range. There have been numerous efforts, both theoretical and experimental, to interpret the low-energy excitation spectrum of C60 (see Ref. [176] for a detailed review). Vibrationally resolved data on cold C60 molecules were obtained by twophoton ionization spectroscopy in supersonic beam [206], absorption spectra of C60 in He droplets (T | 0.4 K) [207], or by measuring the fluorescence and fluorescence excitation spectra in various matrices or single-crystal C60 at low temperatures (T = 1.2–5 K) [175, 176, 208]. The survey of experimental spectra is presented in Figure 5.29. Semiempirical (CNDO/S, QCFF/PI) and TD-DFT calculations predict that three lowest energy excited states, 1T1g , 1T2g and 1Gg at ca 1.7–2.2 eV are quasi-degenerated within 0.05 eV. Excitations to 1T1g state are activated by Au, Hu, and T1u vibrational modes, to 1T2g state – by Gu and Hu modes, and to 1Gg state – by T2u, Gu, and Hu modes, and therefore vibrationally resolved

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excitation and especially fluorescence spectra provide additional information on the silent fundamentals of C60. Detailed analysis of the vibronic structure based on the CNDO/S calculations was reported by Negri [175, 209]. It was shown that the lowest excited state is 1T1g with the energy of 1.93–1.94 eV depending on the matrix and temperature, while 1T2g and 1Gg states are ca 10 and 50 cm–1 higher in energy. The survey of available absorption spectra for the major isomers of the higher fullerenes C70-C84 is given in Figure 5.30. It can be seen that the spectra are highly structural and specific for each fullerene. Significant variation of the spectra with the isomeric structures of the same molecular size observed in C78 [186, 211], C80 [212, 213] and C84 [212, 213] is especially remarkable as it shows that UV/Vis/ NIR absorption spectroscopy may be used to distinguish the fullerene isomers. In 1998 Bauernschmitt [205] reported that TD-DFT calculations gave very good agreement with the experimental absorption spectra and thus can be used to determine the isomeric structures of newly isolated fullerenes when assignment based on 13C NMR is ambiguous. IR and Raman spectra of isomerically pure C2(3)-C82 fullerene have been recently reported [590].

Figure 5.30 Room-temperature UV/Vis/NIR absorption spectra of isomerically pure fullerenes [186, 205, 211–213] (for the isomers of a given C2n, the spectra are given in the order of their yield; the ratio of C80-D2(2) to C80-D2d(1) is ca 30 : 1 [213]; for C84 isomers, the ratio of D2(22) : D2d(23) : C2(11) : Cs(16) : Cs(14) : D2d(4) : D2(5) is 1 : 0.5 : 0.2 : 0.1 : 0.1 : 0.05 : 0.03 [214]). Enhanced spectra in the low-energy range are given as additional curves.

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5.3.3 Nuclear Magnetic Resonance

Toni Shiroka 5.3.3.1

Introduction

Nuclear magnetic resonance (NMR) represents the selective absorption of electromagnetic radiation by nuclei with nonzero spin placed in an external magnetic field. The dependence of its main parameters (such as the resonance frequency Q and width 'Q, the spin–lattice T1 and spin–spin T2 relaxation times) from the nucleus surroundings, makes it a highly sensitive probe to the immediate environment and, therefore, an extremely versatile tool of spectroscopic investigation. The magnetic resonance spectra can broadly be classified into two categories: high-resolution solution NMR and solid-state NMR [215, 216]. In the first case, most appropriate for structural analysis of single molecules, the material is dissolved in suitable solvents, where the interactions that cause the broadening of spectral lines are generally averaged to zero as a result of the random molecular motion. The relative differences in resonance frequency with respect to a reference nucleus, known as chemical (orbital) shifts, reflect the different electronic screening of the external field in various chemical environments. The same principles are also applicable to solid-state NMR. However, the restricted molecular motion in solids can result in very broad spectra. For metallic samples moreover, there is an additional contribution to the frequency shift, arising from the spin polarization of the conduction electrons, known as Knight (spin) shift [217]. The latter implicitly includes a contact hyperfine interaction term, due to electrons in s orbitals, and a dipolar contribution arising from non-s electron spins, respectively related with the isotropic and anisotropic part of the shift. Advanced techniques, including magic-angle spinning, cross polarization, spectral editing, etc., allow nevertheless to recover a wealth of information even from the overcrowded solid-state spectra, thus successfully complementing or supporting other data. Two other techniques, closely related to NMR, are the nuclear quadrupole resonance (NQR) [218] and the muon spin rotation (µSR) [219, 220], both of which have also contributed to our knowledge of doped fullerenes. NQR relies on the fact that in many solids, the spectra of nonspherical nuclei (spin I > 1/2) are dominated by quadrupole effects, arising from the interaction of the nuclear electric quadrupole moment with the nonspherically symmetrical field gradient generated by the surrounding electrons. The NQR spectra can be collected even in zero magnetic field and often provide decisive information about structural properties and dynamics. µSR on the other hand makes use of 100% polarized muons, elementary particles similar to electrons, but positively charged, hence chemically behaving as light isotopes of hydrogen. Once implanted in matter, muons precess in the local magnetic field and information on both precession and relaxation is delivered by the decay positrons, emitted preferentially along the muon spin direction. The main µSR advantages with respect to conventional

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NMR consist in the higher sensitivity, the complementary (interstitial) ‘point of view’, and the possibility to perform measurements even in the absence of an applied magnetic field. 5.3.3.2

NMR of Fullerenes

To illustrate the NMR contribution in the study of fullerene properties we resort to some selected representative examples which, however, are not intended to be either definitive, or exhaustive. For a detailed account of the field the interested reader should refer to the many excellent reviews [221–224]. Depending on the unit being investigated: individual molecules, or ensembles of them, the reported examples are conveniently grouped accordingly, roughly corresponding to solution NMR and solid-state NMR experiments, respectively. Individual Fullerene Property Studies. The first data to support the highly symmetric

structure of C60 were due to NMR [225]. The exact equivalence of all the carbon atoms, arising from the icosahedral Ih symmetry, implies a single narrow line for the 13C solution NMR spectrum. This is indeed the case, as shown in Figure 5.31a, where a remarkable number of atoms gives rise to a single line resonance, an

Figure 5.31 (a) 13C NMR spectra of pure C60, (b) of a mixture of C60 with C70 (b), and (c) of pure C70. Notice the presence of five different lines in the symmetry reduced C70, whose intensity ratio 10 : 10 : 20 : 20 : 10 reflects the five sets of nonequivalent carbons. (Reprinted with permission from [225]).

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unprecedented feature for such a large molecule. Despite its simplicity, this early spectrum is even more revealing. Indeed, the observed chemical shift, 142.7 ppm from a tetramethyl silane (TMS) reference, is significantly lower than the peaks for the corresponding positions in aromatic compounds such as benzene (120 ppm) or naphthalene (133.7 ppm). On the other hand, it is well known that the strain-induced hybridization produces a downfield shift, as seen for example in the 13C peaks for the bridgehead carbons in indane C9H10 (143.9 ppm) and benzocyclobutene C8H6 (146.3 ppm). The observation of a similar shift also in C60 represented a beautiful demonstration of its curved geometry. This first, simple NMR experiment contributed decisively to the elucidation of the fullerene structure, consisting of pentagonal and hexagonal carbon rings. Even more interesting is the case of C70, where the icosahedral symmetry is reduced to a lower D5h symmetry. Accordingly, its solution NMR spectrum consists of five different lines [225], reflecting the five nonequivalent carbon atom sites, as shown in Figure 5.31c. More complex, two-dimensional NMR experiments, making use of double-quantum coherence in 13C enriched samples, were able to examine also the nature of the bonding and to determine the bonding topology [226]. The observation of an isotropic Knight shift, Kiso, in the NMR spectra of conducting fullerenes provides another, independent confirmation of the curved geometry of the single fullerene units [227–230]. Indeed, since the probability density of p-electrons at the nucleus site vanishes, they cannot contribute to the isotropic Knight shift. In the case of a perfect sp2 hybridization, where neither the sp2 orbitals, nor the residual (non hybridized) pz orbitals overlap with the nucleus, Kiso should be exactly zero. In the presence of curvature, though, this simple picture will change. On curved carbon surfaces, the normally planar sp2 orbitals (orthogonal to the axis defined by the pz orbital) must tilt somewhat to match the neighboring carbons (see Figure 5.32a). This distortion is achieved by a small admixture of the 2s wave function into the 2pz orbital, producing a state whose hybridization is intermediate between the planar sp2 and the tetrahedral sp3. The same process can be described also as a small S–V hybridization, if the overlap of neighboring carbon atom orbitals is considered. The rehybridization process on curved surfaces will therefore always retain a small 2s character, resulting in a nonzero direct Fermi contact interaction and hence in a small isotropic Knight shift, whose magnitude depends on the degree of curvature. NMR measurements in a metallic compound, such as Rb3C60 [228], fully confirm this picture by providing an isotropic Knight shift value of ~44 ppm, less than half the anisotropic shift value. Much smaller shifts, ~10 ppm, inversely proportional to the tube diameter, are foreseen for the conducting nanotubes, where the curvature is present only along one direction [231] (carbon nanotubes will be discussed in Chapter 6). Quantitative predictions about the degree of orbital mixing, based on the so-called ‘orbital following’ assumption, initially assigned a ~9% 2s character to the fullerene molecule, with a pyramidalization angle of ~12% [232, 233]. The resulting hyperfine coupling, together with the measured susceptibility values, predicted nevertheless an isotropic Knight shift value (K = A · F) 10 times higher than that actually measured! The solution to this puzzle came again from NMR

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Figure 5.32 Two possible carbon–carbon bonding schemes in C60, depicting one of the three 2sp2 orbitals, together with the non hybridized 2pz orbital. (a) Diagram showing the ‘orbital following’ bonding scheme, with the hybridized orbitals inclined with respect to the radially oriented 2pz orbitals. (b) In the more realistic ‘bent’ V bonding, the hybridized orbitals remain nearly perpendicular to the respective 2pz orbitals, resulting in a smaller 2s admixture, thus accounting for the anomalously small isotropic Knight shift observed.

experiments. From the 13C coupling constants in a 2D experiment, Hawkins and co-workers [234] could estimate a fractional rehybridization of 3% in neutral C60 and, more importantly, established that the correct carbon bonding model is that of ‘bent’ V-bonding and not the stiff orbital following (see Figure 5.32b). This model, which is the currently accepted one [224], resulted also in the correct Kiso values. Because it has a nearly spherical shape, at room temperature C60 maintains a high reorientation rate, even in the solid state (vide infra discussed also in the preceding sections). The resulting static and dynamic disorder makes diffraction investigations problematic, giving access only to the average charge distribution and precluding the details of the internal molecular structure. Carr-Purcell solidstate NMR measurements of 13C–13C magnetic dipolar coupling, on the other hand, provided the first experimental determination of bond lengths in C60 [235]. This particular pulse sequence can selectively remove the broadening effects due to chemical shift anisotropy (differences in local magnetic field shielding in a powder sample), but retains the dipole–dipole coupling. Since the dipolar interaction strength depends on the inverse cube of the C–C distance, the method not only can distinguish between the 1.40 and 1.45 Å bond lengths, but can assess them with better than 1% precision! Only at a later time, as described in Section 5.3.1, could low-temperature X-ray studies determine the bond lengths in C60 even more accurately. Figure 5.33 shows both the measured and the simulated powder spectra at a temperature sufficiently low, so that molecular rotation does not average the ~4 kHz dipolar coupling between directly bonded carbons. The very large central peak (cropped for clarity) arises from 13C nuclei with no 13C nearest neighbors, and is flanked by two weak doublets with different intensity and frequency, arising from the directly bonded 13C atoms. As expected, the inner doublet, due to 60 pentagon bonds, is twice as intense as the external one, due to 30 bonds between pentagons. This fully confirms the truncated icosahedron geometry, since an early proposed structure, consisting in a truncated dodecahedron, yields a rather poor agreement with the experimental data (Figure 5.33, panel c).

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Figure 5.33 (a) Fourier transform of the 13C signal obtained in a Carr–Purcell sequence on 13 C enriched fullerene sample at 77 K. The intense center line has been cropped for clarity (see text). (b) Simulation of two Pake doublets with carbon–carbon bond lengths of 1.45 and 1.40 Å; (c) as in (b), but with bond lengths of 1.451 and 1.345 Å. (Reprinted with permission from [235]).

To conclude this section on individual fullerene properties, we present an ingenious way to follow the fullerene reactions by monitoring the NMR signal of their endohedrally labeled versions [236]. Although 13C NMR spectra of C60 and C70 are simple (see above), the spectra of the reaction products are rather complex, since the attachment of groups to the fullerene skeleton reduces the initially high symmetry. The study of these spectra is not only time consuming but, especially when many products are involved, it presents serious difficulties in assigning the carbon peaks. The 1H NMR signal of the ubiquitous protons can in principle help. However, the tendency of solvents to be trapped in the fullerene lattice, and the hydrogen presence also in the reagents and byproducts makes it unpractical. On the other hand, (as discussed also in Section 5.6 on hydrogenated fullerenes) the use of endohedral fullerene complexes of noble gases, as e.g. 3He, does not have any of the previous drawbacks. Each helium-labeled fullerene gives a single sharp peak with a typical line width below 1 Hz and a relatively large negative chemical shift (–6.3 ppm and –28.8 ppm from a 3He reference in C60 and C70 respectively). The diamagnetic chemical shifts of the centrally located helium nucleus, not only prove the substantial aromatic ring currents present in all the fullerenes, but their modifications can be used as a reaction indicator. Indeed, the conjecture that alterations of the S-bonding structure of the fullerene through reaction would produce substantial shifts in the 3He peak, has been successfully verified experimentally. A simple addition reaction, with just one of the 30 double bonds being modified, produced a strikingly large –3 ppm shift of the helium signal. Considering also that in this case the peak areas are accurate indicators of the relative amounts, and that to date no two different products have been found to have the same helium shift, the potentialities of the method as a reaction monitoring tool are clear.

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Collective Fullerene Property Studies. While solution NMR measurements can

reveal properties of single molecules, solid-state NMR is an excellent probe of the collective behavior of fullerenes. This, together with the fact that both pure and doped fullerenes are available mainly as powders, explains the extremely vast and diversified range of experiments belonging to this class. The investigation of C60 rotational dynamics in the solid state, represents perhaps one of the best known examples of this kind of studies. Figure 5.34 shows the evolution of 13C NMR line shape of fullerene, as the temperature is lowered [237, 238]. Surprisingly, at room temperature, instead of a broad and featureless spectrum, typical of a powder sample, C60 shows an extremely narrow line, very similar to those observed in a solution. This extreme ‘motional narrowing’ effect implies a rapid and isotropic molecular reorientation which, successive relaxation rate studies have shown to be even more effective than in the liquid phase. At the lowest temperature the molecules are stationary but randomly oriented, and

Figure 5.34 13C NMR spectra of solid fullerene taken at different temperatures. The surprisingly narrow line at room temperature gradually broadens and changes its shape, as the isotropic rotational motion slows down. (Reprinted with permission from [237]).

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therefore subject to different magnetic field shieldings (technically, to a chemical shift anisotropy). A fit of the powder pattern yields an asymmetric chemical shift tensor (220, 186, 25) [237], quite similar to those of aromatic carbon compounds. The coexistence, at intermediate temperatures, of sharp features superimposed on a broad spectrum indicates the simultaneous presence of two different phases: a fast rotating phase with another one where the rotations are blocked. To find more information about them one can resort to the spin–lattice NMR relaxation rates. The motional dynamics affects the intensity of magnetic field fluctuations which, in turn, will be reflected in the measured relaxation rates (1/T1). The numerical analysis of room temperature data provides an astonishingly fast rotational correlation time, W = 9.2 ps, only three times longer than that measured in the gas phase! Some fascinating, recent attempts towards applications of this finding, although somewhat speculative, include the use of C60 as a molecular ball bearing for nanoscale machine design [239]. Temperature dependent X-ray and neutron diffraction studies [240] hinted at a structural fcc–sc phase transition at 249 K, but little was known about the details. An NMR examination of the transition [241, 242], could find a sharp drop in the T1 relaxation times, despite no appreciable change in the spectrum shape. These measurements suggested a high-temperature phase, characterized by free rotation (‘rotator’ phase), and a low-temperature (‘ratchet’) phase, characterized by rotational jump diffusion among symmetry-equivalent orientations. Due to the hampered rotations, the ‘ratchet’ phase shows also much higher relaxation rates (i.e. shorter relaxation times), and its slower dynamics seems to arise from an orientational ordering, such that electron-rich interpentagon bonds are facing electron-poor pentagon centers of neighboring C60 molecules. This also explains the lowered structural symmetry (sc instead of fcc) of this phase. The strong tendency toward polymerization of fullerene molecules into one-, two-, and even three-dimensionally connected polymers, either by photoexcitation, or by high-temperature and high-pressure treatments, has attracted a continuous scientific interest. From the early studies of relatively simple monodimensional chains in AC60 (A = K, Rb, Cs) [243], to RbC60 dimers [244], up to the most recent elucidation of a particularly complex polymerization in Li4C60 [245], NMR has always played a key role. This class of experiments has in common the successful use of magic angle spinning (MAS) [246] to simplify the extremely broad powder pattern arising from carbon polymerization [247]. The sample spinning produces an effect very similar to the motional narrowing, by suppressing all the anisotropic contributions from the relevant interactions. Figure 5.35 shows the 13C NMR MAS spectrum of Li4C60, together with its structure, consisting in a 2D planar polymerization pattern, characterized by the coexistence of single and ‘double’ C–C bonds propagating along two orthogonal directions. The spectrum is broadly divided into two regions, whose frequency domains are commonly attributed to sp2 (~150 ppm) and to sp3 (~60 ppm) hybridized carbon atoms. The fullerene distortion induced by the polymerization removes the equivalence of the sp2 atoms, giving rise to a multitude of isotropic chemical shifts. The two distinct peaks in the sp3 region, instead, are reminiscent

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Figure 5.35 Room temperature 13C MAS (8 kHz) spectrum of Li4C60 showing the different intensities for sp2 and sp3 carbon atoms. The two peaks in the sp3 region arise from two different types of C60 polymerization along a and b directions (see structure in inset). (Reprinted with permission from [245]).

of the two different kinds of covalent bonds, as confirmed both by the dissimilar shifts and by the 4 : 2 integrated intensity ratio. Despite the abundance of bonds, conventional NMR measurements (not shown) did not find any trace of Knight shifts (either isotropic, or anisotropic), thus demonstrating the insulating character of the polymer. We dedicate the last example to the A3C60 (A = K, Rb, Cs) fulleride superconductors, because their NMR investigation represents also a remarkable example of electronic property studies [224, 248, 249]. Contrary to endohedral fullerene complexes involving He discussed above, the solid state fullerides consist of empty fullerenes, with the alkali ions residing in the voids of the fullerene lattice. Since the superconductivity in fullerides is widely accepted to be of phononic nature, the most important NMR method in their studies has naturally been the measurement of the spin-lattice relaxation times T1. Differently from the insulating undoped C60, where the main relaxation mechanisms are the molecular reorientation and the chemical shift anisotropy (see above), in the metallic A3C60 family the most important mechanism is the so-called Korringa relaxation [250]. It stems from the significant electron-nucleus hyperfine interaction in metals which, therefore, provides also the dominant relaxation channel, with the nuclei being restored to their ground state level through spin-flip exchanges with electrons. Since the only electronic states available for the spin-flip transitions are those within kT of the Fermi energy level, the resulting relaxation rate is proportional to temperature, i.e. [216]:

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SkT 1 = ⋅ A 2 N (E F )2 , = T1

(5.1)

with A the hyperfine coupling constant, and N(EF) the electronic density of states at Fermi level. Taking into account the expression for the (temperature independent) Knight shift, K = A F, it can easily be shown that (Korringa relation): T1 T =

J2 1 = ⋅ 2e ⋅ 2 , 4 S k Jn K

(5.2)

with Je and Jn the gyromagnetic ratios of the electron and the nucleus. Since the spin-lattice relaxation in alkali fullerides is dominated by the dipolar, rather than the contact hyperfine interaction [251], the Korringa relation should include a scaling factor of ~3 [229]. However, provided powder averages are considered [252], the above relation is still valid. In the normal metallic state, we would therefore expect a temperature independent 1/(T1 T) factor and, to a very good extent, this is what one really observes in many alkali-metal fullerides [228]. More interesting, but also more challenging, were the experiments performed in the superconducting state. Here, the gradual formation of the Cooper pairs implies an exponential increase of the relaxation times. This behavior not only was observed, but it could be also used to evaluate the low-temperature superconducting gap '0 [228]. The final, and for a long time puzzling, issue was the absence of a Hebel–Slichter peak [253] in the transition to the superconducting state. This peak, which appears as a slight upsurge in the 1/(T1 T) data vs. temperature, is due both to the electronic state ‘pile up’ during the opening of the superconducting gap and to an electronic coherence factor. Since it is universally regarded as a strong indicator of BCS superconductivity, its first observation in Rb3C60 [254] (see Figure 5.36) using the muon-spin relaxation technique (µSR), and, successively, also in other compounds [255], definitely

Figure 5.36 Muon-spin relaxation (µSR) measurement of 1/(T1 T ) in Rb3C60 at 1.5 T. The solid line is a fit to the Hebel–Slichter theory, with a broadened density of states. The almost constant 1/(T1 T ) value above the critical temperature corresponds to normal metallic behavior. (Reprinted with permission from [254]).

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confirmed the BCS nature of alkali-fulleride superconductors. Subsequent NMR measurements [256], performed at relatively low fields, could reproduce these results and explain the absence of a peak in early data by its suppression by the high field. 5.3.3.3

Concluding Remarks

Magnetic resonance spectroscopy, both in its solution- and solid-state NMR forms, has proved an extremely useful tool for the investigation of the structural, dynamical and electronic properties of fullerenes, which rivals in importance and successfully complements techniques such as X-ray and neutron diffraction. Even simple, one-dimensional NMR experiments, carried out on static samples, can provide useful and often decisive information, otherwise difficult or impossible to obtain by other methods. The elucidation of fullerene structure, the study of its rotational dynamics in the solid state, the investigation of the different electronic properties of doped fullerenes, and the study of carbon polymerization are just a few of the most important contributions of NMR to the fullerene field. Even though current research interests have gradually shifted to nanotubes and similar classes of materials, the investigative role of NMR remains central in our continuously advancing understanding of these unusual carbon forms. 5.3.4 Electrochemistry

Renata Bilewicz and Kazimierz Chmurski 5.3.4.1

Electronic Properties of Fullerenes

Fullerenes and their derivatives are unique in the richness of their redox behavior and electrochemical reactions, and every year new publications including reviews and monographs appear highlighting various electrochemical aspects of these fascinating molecules [257–262]. Since early research on the electrochemistry of C60 to C84, their isomers and their derivatives has been summarized in excellent chapters of Echegoyen et al. [261, 263], in the following chapter we focus mainly on more recent work devoted to the relationship between the fullerene structure and their electrode behavior. Fullerenes exhibit a triply degenerated LUMO (t1u) of low energy and, therefore, behave as an electronegative molecule reversibly accepting up to 6 e– [264–265]. Electron affinities, EA, and ionization potentials, IP, calculated for C60 to C84 [263] are shown in Table 5.3. In early reports, only two or three reduction steps were recorded for C60 using cyclic voltammetry [264], however, in mixed solvents and lowered temperatures six reversible 1 e– reductions can be easily resolved (Figure 5.37) [266]. For C70, 3– the reduction is all six waves are seen at room temperature and starting from C70 easier than that of corresponding C60 anions. This was explained by Cox in terms of larger size of C70 and charge separation delocalization model [267].

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5 Fullerenes Table 5.3 Estimated electron affinities and ionization potentials of selected fullerenes [263]. (isomer)

EA (eV)

IP (eV)

C60

2.7

7.8

C70

2.8

7.3

C76 (D2)

3.2

6.7

C78 (C2v)

3.4

6.8

C82 (C2)

3.5

6.6

C84 (D2)

3.5

7.0

C84 (D2d)

3.3

7.0

Figure 5.37 Reduction of C60 in CH3CN/toluene at –10 °C using cyclic voltammetry at a 0.1 V s–1 scan rate and differential pulse voltammetry (0.050 V pulse width, 0.3 s period, 0.025 V s–1) upper and lower curves, respectively. (Adapted from [266]).

The interactions of solvent molecules with fullerenes and fullerides are significant mainly due to the size of the fullerene molecule compared to those of the solvent, hence, large solvent–fullerene interaction surface [268–274]. The surface exposes a strained S-orbital system consisting of sp2 orbitals with enhanced s n–/(n + 1)– couples become more character. The formal redox potentials of the C60 solvent–dependent as the charge increases due to stronger electrostatic interactions with solvent dipoles and the hydrogen bonding interactions with the fullerides of increasing basicity (Table 5.4).

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n–/(n + 1)– Table 5.4 Formal reduction potentials of C60 couples referenced to the reduction +/0 potential of the Me10Fc couple in 0.1 M tetra-n-butyl ammoniumperchlorate (TBAClO4) [273].

Solvent

C600/1–

acetonitrile

C601–/2–

C602–/3–

C603–/4–

–735

–1225

–1685

anilineb)

–396c)

–693c)

–1158c)

–1626d)

benzonitrile

–397

–817

–1297

–1807

benzyl alcoholb)

–443c)

–817c)

bromobenzeneb)

–587d)

chlorobenzene

–573

–953

chloroformb)

–554d)

–908d)

1,2-dichlorobenzeneb)

–535c)

–907c)

–1360c)

1,2-dichloroethane

–448

–848

–1298

dichloromethane

–468

–858

–1308

–1758

N,N-dimethylanilineb)

–547c)

dimethylformamide

–312

–772

–1362

–1902

N-methylanilineb)

–442d)

–782d)

nitrobenzene

–406

pyridine

–343

–763

–1283

–1813

tetrahydrofuran

–473

–1063

–1633

–2133

a) b) c) d)

–1438

–1841d)

From Ref. [268]. Data recorrected to the Me10Fc+/0 couple. Determined in the present study. CV data. DPV data.

Large interactions of the charged fullerides with the solvent are explained in terms of solvent dependent Jahn–Teller distortions [273, 275] and large quadrupole moments [276]. S-stacking interactions between C60 and aromatic solvents result in a substantial contribution of the polarizability correction term to the first reduction. Formation of ion pairs between the more electron-rich fullerides and the cations of the supporting electrolyte add to the complex solvent/solute interactions influencing also the values of reduction potentials. Interactions with cations are especially important in halogenated and nitrile solvents for which slowing diffusion rates, slower kinetics and changes of reduction potentials were reported

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[274, 277–279]. Dimerization of C –60 may occur in more concentrated solutions, and in the presence of NBu4+ cations, the salts of [NBu4+][C60][C –60] stoichiometry can be formed [277, 280]. Fairly large solvent/solute interactions observed were shown to arise from contributions of different, relatively weak interactions [273 and refs therein]. Attempts to compensate for the effects of counterions included the use of microelectrodes instead of conventional size electrodes which allowed one to work without added supporting electrolyte [281]. This useful alternative approach should be used with caution since unknown residual impurities in the solution may play the role of counter ions, and affect the experimentally obtained values of potentials. In the analysis of solvent and counter ions effects on the electroreduction potential of fullerenes, the quality of the reference electrode or of the internal redox standard are of major importance. Decamethylferrocenium/ decamethylferrocene couple was proposed as favorable internal standard for the electrochemical measurements compared to the ferrocene (Fc/Fc+) system due to its negligible solvent dependence and interactions with counterions [273]. Low value of HOMO of C60 and high value of IP (Table 5.3) indicate that fullerene should not be prone to oxidation [263]. When a low-nucleophilicity electrolyte (Bu4NAsF6) was used, three 1 e– oxidation peaks were reported [282]. Contrary to stable multianions formed upon reduction [266], the cationic species produced 0/+1 upon electrooxidation of C60 or C70 are rather unstable [283]. Reversible C60 + oxidation at 1.26 V vs. Fc/Fc was found in 1,1,2,2-tetrachloroethane (TCE) as 0/+1 process the potential is 1.21 V. The addition solvent [284]. For the analogous C70 of CF3SO3H to the TCE solutions increases longevity of the cation radicals. C+60 radicals formed by constant potential electrolysis in a thin layer electrochemical cell are stable for 3–4 h at 233 K. The characteristic IR bands appear at 10 170 cm–1, 11 820 and less intense at 8950 cm–1. Upon transfer to the EPR tubes, the X-band EPR spectra of frozen solutions of C+60 contain one rhombic shaped signal at 133 K consistent with the cation radical having rhombic symmetry (hence distorted from the Ih symmetry of the neutral species) [285]. The electrochemical data obtained upon long term oxidation of C70 are more complicated and indicated instability of the cation radical even at low temperatures and in the TCE/CF3SO3H medium in comparison to those in C60. With increasing fullerene size and the number of carbon atoms in the cage, fullerenes become more readily reduced. Half-wave reduction and oxidation potentials of higher fullerenes (C70, C76, C78, C84) and of their isomeric forms were discussed by Echegoyen et al. [263] (see Chapter 1 and Tables 5.3 to 5.6). The higher fullerenes are easier to oxidize, e.g. C+76 is formed in a reversible process at 0.8 V and isomers of C84 are oxidized at ca. 0.9 V [284, 286, 287]. The low HOMO to LUMO gap (2.34 eV) resulting from unusual hybridization and the extended conjugated S-electronic structure leads to special electronic and electrochemical properties of C60. Both oxidation and reduction peaks can be recorded for it at 1.27 V and –1.06 V vs. Fc/Fc+ reference potential, respectively [282]. The difference between these values agrees well with the optical gap. The energy gap decreases as already mentioned with the increasing number of carbon atoms in the cage.

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Figure 5.38 Potassium ion induced switching of intra- to intermolecular electron transfer (forward reaction) and 18-crown-6 induced reversible switching of the inter- to intramolecular process (backward reaction) [289].

Fullerenes act as moderate electron acceptors and form weak charge transfer complexes with common electron donors both inorganic and organic e.g. TTF [259, 288]. In the donor–acceptor complexes fullerenes accelerate forward electron transfer and slow backward electron transfer resulting in the formation of longlived charge separated states. Porphyrin and fullerene entities show weak S–S type interactions which can be strengthened by covalent linking the porphyrin to a crown ether in such a way that both functionalities contribute to the stability of the complex [288]. Due to two modes of binding depending on the location of the crown ether either intra or intermolecular electron transfer can be realized. Interestingly, shifting of the electron-transfer pathway from intra to intermolecular route was achieved by complexing potassium ions in the crown ether cavity (Figure 5.38), thus potassium ion induced a reversible switching phenomenon in this system. With strong electron donors such as alkali metals, radical anion salts are generated in the solution and often reveal interesting superconducting properties [260, 289]. 5.3.4.2

Electrochemical Properties of Soluble Fullerene Derivatives

In order to tune the electronic properties of fullerenes for specific electronic and photophysical devices additional units are introduced into the fullerene structure. Schematic representation of such building blocks linked to the C60 moiety with spacer units is shown in Figure 5.39. Functionalization of fullerene in most cases reduces its electron affinity due to disruption of the conjugated system. The reduction potentials of fullerene derivatives classified conveniently as: singly bonded functionalized derivatives, cyclopropanated and cycloaddition products are collected in [263] while those of several C60-acceptor dyads are listed in Table 5.4 of Ref. [259]. Saturation of a double bond of the fullerene structure leads to a higher LUMO energy [292, 293]. The loss of conjugation may be, however, compensated by the introduction of electron withdrawing groups directly connected to the C60 cage. Such functionalization may result in even stronger electron accepting molecule than the parent

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Figure 5.39 Schematic representation of dyads and triads involving the C60 electrophore: (a) C60 donor, (b) C60 acceptor, (c) C60 donor1-donor2 (monoadduct with respect to C60) and (d) C60 donor-acceptor (bisadduct with respect to C60) [291].

one. (Fullerene functionalization is discussed in Section 5.2.) The reduction potentials of [1,2]dicyanofullerenes [294] or dicyano-[6,6]methanofullerene [295] are ca. 0.1 V more positive than that of the parent fullerene. The electron affinity of fluorofullerenes increases by ca. 0.05 eV per each fluorine atom added, thus, highly fluorinated fullerenes become exceptionally strong electron acceptors [296, 297]. Other strategies to improve electron acceptor properties of a fullerene derivative include insertion of electron-withdrawing substituents connected to the fullerene cage through a methano bridge, and formation of pyrrolidinium salts [288, 298]. The electroreduction potential depends on whether the first reduction is fullerene or addend based. Efficient electron acceptor organofullerenes are those exhibiting the periconjugative effect. In case of quinone type methanofullerenes, the intramolecular electronic interaction can occur between pz–S orbitals of the olefinic carbons of the quinone moiety and the adjacent carbon atoms of C60 separated by a spiro carbon atom. The molecule with more extended conjugation has better acceptor properties [288]. Five reversible electroreduction waves were observed for fullerene linked to the malonate group containing flexible long alkyl chains to two biphenyl-phenyl units [299]. For some of the derivatives, following first electron transfer a hemolytic cleavage of one of the bonds connecting the addend to C60 may take place leading to irreversible electrochemical behavior [300]. The voltammetric experiment may even show reversible behavior of the compound, but the constant potential electrolysis (CPE) performed at a longer time-scale, often reveals the presence of several side products of chemical reactions in the reaction mixture which can be identified by HPLC and MALDI-TOF (Figure 5.40) [288]. In the case of CPE of methanofullerenes with nitrophenyl groups shown in Figure 5.40 after reoxidation, small amounts of C60, bisadducts and ca. 40% of starting material were found. Voltammetry performed during CPE may lead to valuable information concerning the mechanism of reactions and is often very helpful for understanding of the

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Figure 5.40 Cyclic voltammetry of methanofullerenes terminated with an ESR active nitrophenyl group [288] in THF vs. Fc.

pathways involved in the process. For example, in case of bis(ethoxycarbonyl) derivative of methano C70 fullerene, Echegoyen and coworkers [301] obtained evidence of a new stable intermediate exhibiting reversible electrochemical behavior. The malonate group with its electron withdrawing affinity allowed stabilization of the negative charge in the adduct and then formation of an intercage bond between two fullerene core radicals. Dimerization was found here to inhibit the expected electrochemical retro-cylopropanation reaction. The proposed mechanism of the retro-Bingel reaction taking place during reductive electrochemistry is presented in Figure 5.41. Addition of K+ ion to the solution can cause a positive shift of reduction potential by ca. 90 mV as shown for a C60-dibenzo-18-crown6-conjugate (Figure 5.42) [302, 303]. This shift can be explained by the electrostatic effect of K+ bound in the cavities of the crown ether which decreases the influence of electronic interaction of the C60 moiety with the substituent. However, the reduction potential is still more negative than that of pristine C60, showing that raising the LUMO level by saturation of few double bonds of the fullerene cage is the main factor determining the value of the reduction potential in this case. Finding new and improved C60-based acceptors, that would show less negative reduction potentials but at the same time, high stabilities of the anionic radicals formed, is a challenging goal important for designing fullerene based photovoltaic devices and artificial photosynthetic systems.

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Figure 5.41 Proposed mechanism for the formation of dimeric C70 fullerene derivatives from the electroreduced form (radical position and connecting bond between the C70 units are chosen arbitrarily with respect to possible regioisomers) [301].

Figure 5.42 Representative examples of fullerene intramolecular complexes with better reduction potentials than pristine C60 fullerene. (Adapted from [288]).

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Effect of Addition Pattern on the Electrochemical Properties of Fullerenes. The addition – pattern is an important factor influencing the value of E1/2 potential of the C60/C 60 step of the exohedral fullerene derivatives. For 18 C60(CF3)n derivatives (which had been discussed in Section 5.3) this potential has been shown to be a linear function of the DFT-predicted E(LUMO) values [304]. The reduction potential can be changed to a great extent by altering the addition pattern of C60(X)n and C70(X)n as shown for C60(CF3)n, C60(C(CO2Et)2)3 [305] and C70(Ph)i [306]. The effect is especially strong when X is large enough so that 1,4 additions are favored over 1,2 ones. Moving e.g. one CF3 group by only two cage carbon atoms on its own pentagon, would change the reduction potential by 0.45 V. The effect of particular substituent on shifting E(LUMO) values is a function of the substituent ability to withdraw electrons from the fullerene cage [307, 308]. As shown, it depends on the addition patterns of the compounds in question. Different addition patterns result in different numbers of non-terminal double bonds in pentagons (defined as short pent-hex junctions that have two C(sp2) nearest neighbors). When the LUMO fragments associated with them overlap, E(LUMO) becomes very low and the E1/2 values are relatively high. This shows, that the addition pattern of a fullerene derivative may be even a more important factor than the number of subtituents for the electron acceptor efficiency of the molecule (Figure 5.43) [309].

Figure 5.43 E1/2 values of C70(CF3)n derivatives from electrochemical and DFT study [309].

Addition patterns leading to non-terminal double bonds in pentagons and extensive LUMO delocalization allow one to obtain controlled electron–acceptor properties. Organofullerenes with Electron Donor Moieties. Due to the degree of electron de-

localization within the S-system and large size, the molecules under discussion are convenient building blocks for the construction of covalent donor–acceptor

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dyads, triads and more complicated systems in molecular devices for electronics, molecular switches, and photoconductors. Absorption of fullerenes in the visible spectrum region and their ability of rapid photoinduced charge separation make fullerene derivatives fascinating materials for photovoltaic applications [310]. The intramolecular transfer dynamics in molecules with fullerenes linked to an electroactive or photoactive species is function of excited state of the antenna molecule, donor–acceptor distance and the solvent used [311]. The energy levels of the charge-separated states ('GRIP) can be evaluated using the Weller type approach utilizing the redox potentials, center-to-center distance, and dielectric permittivity of the solvent [312]: 'GRIP = E ox − E red + 'Gs , where 'Gs = –e2/4 S H0 HR RCt–Ct and H0 and HR refer to vacuum and solvent permittivity. By comparing these energy levels with the energy levels of the excited states, the driving forces are evaluated. Electrochemical gap as small as 1 eV has been reported for some fullerene–ferrocene dyads [259]. Ferrocene (Fc, E1/2 = 0.50 V vs. SCE), N,Nc-dimethylaniline (E1/2 = 0.81 V vs. SCE), N-methylphenothiazine (E1/2 = 0.70 V vs. SCE) and tetrathiafulvalene (TTF, E1/2 = 0.37 V vs. SCE) are common donor components of these dyads [313]. This type of hybrids were often classified as electroactive but, due to the insignificant VIS absorption of the added group, photoinactive. The electrochemical data for the TTF–C60 dyads are reviewed by Bendikov et al. [310]. Due to very different structures such dyads exhibit different solvation effects and dependence of the electrochemical gap on the solvent used [314]. A thermodynamically stable supramolecular donor–acceptor system with C60 and TTF assembled through guanidinium–carboxylate ion pair stabilized by two hydrogen bonds has been proposed [315]. Multiple substitution of the fullerene core leading from dyads (Figure 5.44) to triads [315] tetrads and quintads [316] enlarged the HOMO–LUMO gap and lead to a decrease of the C60 reduction potential. Because of the moderate fullerene absorption in the VIS region, functionalization of C60 with a chromophoric addend improves the light harvesting efficiency of the fullerene dyad. In such a system fullerenes operate as electron or energy acceptor moieties. Donors such as metalloporphyrins, zinc phthalocyanines, ruthenium (II) polypyridyl complexes with strong metal-to-ligand charge transfer (MLCT) are powerful units in the excited state. The radical pair for the latter case involves oxidized complex and anionic fullerene radical with a lifetime of 304 ns in deoxygenated CH2Cl2. However, in the presence of traces of oxygen the fullerene triplet excited site is quenched to produce cytotoxic singlet oxygen species [311]. In the C60-ferrocene conjugates the intimate contacts between the donor and acceptor result in large ground-state interactions suggesting a substantial charge density shift from donor to acceptor. In the excited states, the processes in these conjugates are dominated by rapid charge separation reactions (0.8 ps) to yield metastable ion pairs with the radical pair lifetime of ca. 30 ps. As discussed by Guldi and coworkers [317], no prominent charge-transfer features were observed

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Figure 5.44 Cyclic voltammogram of TTF-C60 diad [R=S(CH2CH2O)4Me] recorded in 1,2-dichlorobenzene. (Adapted from [310]).

for the bucky ruthenocene conjugates and only an intrinsically faster excitedstate deactivation (ca. 200 ps) evolved. The authors attribute this difference to the unfavorably shifted oxidation potential of about 0.61 V in ruthenocene. For the fullerene derivative –'GET is 0.35 eV, while for ruthenocene conjugate it is –0.26 eV rendering the charge separation thermodynamically unfeasible. Modulation of the electronic coupling between C60 acceptor and various donors for the construction of new optoelectronic devices requires also searching for new type of connecting units between the donor and acceptor. Longer-distance charge separation was achieved for oligothiophene–fullerene dyads (nT–C60) and triads 8T–4T–C60 [318]. Making use of coordination of fullerene derivatives to transition metal ions is in general a promising approach. Fullerene coordination ligands in which one bipirydine or terpyridine unit was directly attached to the nitrogen atom of a fulleropyrrolidine ensure a linear communication pathway between fullerene and coordinated ruthenium (II). This formed a linear rod-like donor–acceptor system (Figure 5.45) [319]. The reduction of the fullerene unit in the complex is easier than in the free ligand while Ru-based reductions are shifted by 50 mV more negative relative to the simple [Ru(bpy)3]Cl2 complex. These electrochemical results indicate electronic coupling existing between the fullerene core and the ruthenium ion center and are in line with the MLCT transition band, which is subjected to a red shift for the dyad. Ferrocene–porphyrin–fullerene constructs with crown ether appended porphyrins were self-assembled through host–guest interactions with alkylammonium cation functionalized fullerenes. Interactions between the porphyrin and fullerene entities were followed by voltammetry. The crown ether–alkylammonium cation complexation binding strategy lead to well-defined stable triads with charge-recombination in two steps and direct charge recombination from the porphyrin cation radical. This system fulfilled the condition of long-lived charge-separated state.

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Figure 5.45 Cyclic voltammograms of [Ru(bpy)3]Cl2 (black), bpy-C60 (red) and complex (blue) in CH2Cl2 with 0.1 M TBA[PF6] (tetrabutylammonium hexafluorophosphate) [319].

An interesting design strategy is based on dendritic molecules appended with multiple zinc porphyrin units, to trap pyridine compounds carrying multiple (n = 1–3) fullerene units (Figure 5.46) [320]. The complexes presented in the scheme have a photoactive layer of spatially segregated donor and acceptor arrays. Voltammetry experiments revealed electron donation from ligating pyridine to the zinc porphyrin units. Photoinduced electron transfer was confirmed by flash photolysis measurements. High ratio of charge-separation rate constant to charge– recombination rate constant (3400) was obtained. 5.3.4.3

Electrocatalytic Activity of Fullerenes

Early attempts to use fullerenes as catalysts involved reduction of organic halides [321]. Regeneration of the oxidized form of C60 catalyst at the electrode surface was responsible for the electrocatalytic current. Electrocatalytic reduction of dihalogenated alkanes was investigated using C60 and C70 in solution and using 2–/3– 1–/2– and C60 were the catalytically active couples. coated electrodes [322–324]. C60 In Figure 5.47, the cyclic voltammograms reveal the catalytic effects observed for 2–/3– electrode dihaloethanes in the range of potentials corresponding to the C60 process. The peaks appearing at positive potentials are due to the oxidation of halogenide released in the catalytic reduction process. Anions of higher fullerenes C76, C78, C84 are relatively more stable with respect to derivatization and can serve as better electrocatalysts than anions of C60 [323]. The rate constants for the pseudo-first-order conditions can be calculated from the rotating disk electrode measurements [325]. Alkanes and monoiodoalkanes were products of the C60 reaction with D,Z-diiodoalkanes except for reaction 3– adduct formation was with 1,3-diiodopropane and 1,5-diiodopentane where C60 reported [322]. For each fullerene anion kcat increased in the order Cl < Br < I in

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Figure 5.46 Molecular structure of zinc complex of multiporphyrin dendrimer DP24 and schematic representation of the complexation of DP24 with fullerene appended bipyridine ligand Py2F3. (Adapted from [320]).

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Figure 5.47 Cyclic voltammograms for 0.0001 M C70 (curve 1), 0.0001 M C70 and 0.4 M 1,2-dichloroetane (curve 2), 0.0001 mM C70 and 0.4 M 1,2-dibromoethane (curve 3), as well as 0.0001 M C70 and 0.0014 M 1,2-diiodoethane (curve 4) in 0.00001 M (TBA)PF6 in benzonitrile, at Pt electrode. Potential scan rate is 0.1 V s–1.

agreement with a dissociative mechanism of reduction [326] and increased in the order C84 < C78 < C76 < C70 < C60 for each 1,2-dihaloethane [327]. Although, lower rate constants were obtained for the higher fullerenes, less negative potentials and the lack of alkyl adduct formation were of advantage. Electrochemically generated anions of C60 were used also in deprotonation of the organic acids. A weekly basic monoanion of fullerene deprotonates a relatively strong acid e.g. ethyl nitroacetate. C –60 anion catalyzed nitroacetate reaction with ethyl acrylate and acetonitrile to form double addition products [328]. 5.3.4.4

Conclusions and Outlook

Fullerene based films, discussed in the next section, attract considerable interest as materials possessing the unique properties of fullerenes, but at the same time more suitable for practical applications in nanotechnological, electrochemical sensing and photophysical devices. Since C60 molecules exhibit electron-mediating properties, upon immobilization in mono- and multilayer films on electrodes they can efficiently promote reduction and oxidation processes. In these exciting applications, albeit outside the scope of this chapter, their poor solubility in many solvents and water becomes an advantage, contrary to the case of diffusion charge mediation. In general, more uniform coverage of the electrode substrates and higher reversibility of the electrode processes were reported for C60 embedded in different type of matrices than for the polycrystalline C60 films deposited on electrodes [262]. C60 modified electrodes can be easily prepared by placing a C60 drop onto the electrode which is then overlaid with nafion protecting coat, as shown first by Compton et al. [280]. Two to four quasireversible 1 e– reductions were usually observed in aqueous or mixed solvent and electrocatalytic application of surface immobilized C60 molecules were demonstrated e.g. for oxygen and nitrobenzene reductions [262].

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Figure 5.48 Proposed mechanism of cytochrome c immobilization, and electrochemical reduction by C60-Pd polymer film. (Adapted from [331]).

Several attempts concern biocatalytic systems important for the field of bioelectronics [329], e.g. C60 molecules assembled in monolayers were used to mediate electrons between enzymes and the electrodes [330]. Two different fullerenemodified electrodes were used: electrical contacting an enzyme and cytochrome c with the electrode (Figure 5.48) [331]. Fast development of applications of the C60 films as mediating units in sensing and catalytic devices may be expected in the near future.

5.4 Fullerene Aggregates

Tommi Vuorinen Many extraordinary properties of fullerenes have alerted researchers to find and study the properties of aggregated fullerene structures. The fullerenes have also drawn much interest as construction materials for thin films and other ordered solid nanostructures. One can prepare photovoltaic devices which use photogenerated charge separation as a result of the excellent ability of fullerenes to accept electrons from electron-donating compounds. A well-known example is the plastic solar cell with bulk hetero junction structure, where an electronegative fullerene derivative is incorporated into conjugated electron-rich polymer [332]. Many interesting features of fullerene and fullerene derivative films have been discovered, for example, superconductivity [333, 334], unique redox properties [335], photo-electrochemical response [336, 337] among others. The most common fullerenes studied in all respects are C60 and C70, so this chapter will concentrate on the research based on these two carbon balls.

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5.4.1 Film Preparation Methods

Numerous supramolecular assemblies containing fullerene as a functional moiety have been synthesized [338–340]. In order to use the functions of the supramolecular structures created, the assemblies should in most cases be bound as solid well-ordered films [340]. With ordered nanostructured films one may be able to scale the functions taking place at the molecular level up to macroscopic level. This may happen when billions of supramolecular assemblies work coherently in an ordered film and the sum of all actions at the nanoscale is seen as the resulting macroscopic function. For this reason thin fullerene films are also in focus when, for example, molecular electronics are developed. A variety of film preparation methods have been used in fullerene thin film fabrication and will be briefly reviewed here. Drop casting and spin-coating are based on solvent evaporation and precipitation of the solute. In general, the film morphology can be varied by changing the solvent evaporation rate. The type of the solvent can also influence how the precipitate will crystallize. The drop cast fullerene films are used, for example, as starting material for electrochemical polymerization of fullerene copolymers [341]. Compared with drop casting, spin-coating can provide better control over the film formation process. In spin-coating the film is formed by precipitation onto a substrate which is rotated while evaporation takes place. Thus in addition to the solvent evaporation the film morphology is affected by the centrifugal forces caused by the substrate rotation. The film morphology and thickness can be varied, for example, by changing the rotation speed, the solvent, and the concentration of active material in the solution. Spin-coating is usually used when a fullerene derivative is mixed with a conjugated polymer. This way one can get a bulk hetero junction between the electron-donating conjugated polymer and electron-accepting fullerene. The bulk hetero junction structure has been proved to act as an efficient organic photodiode when placed between two electrodes with different work functions [332]. The Langmuir–Blodgett (LB) and Langmuir–Schäffer (LS) methods are sophisticated techniques for the preparation of well-ordered one molecule thick films from surface active molecules [342, 343]. The first step in both techniques is formation of the Langmuir film that is a stable molecular monolayer at the gas–liquid interface. Water is definitely the most commonly used liquid as the subphase for monolayer formation. Water can easily provide very strong anisotropic interaction between the molecules and the interface. In practice, the subphase is either pure water or water including a small amount of some salt. Naturally, it is easiest to use ambient air as the gas phase. The need of anisotropic interaction sets a prerequisite for the molecules which are used for the monolayer formation. The molecules should be amphiphilic, having one part of the molecule hydrophilic, i.e. soluble in water, and the other part hydrophobic. The strong attraction to water forces molecules to be as close to the water surface as possible. On the other hand the water insoluble part stops the molecules from dissolving in the subphase. This

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kind of anisotropic interaction makes it possible to form stable monomolecular films at the air–water interface. The attraction to water prevents the molecules at the interface from forming three-dimensional crystals. The Langmuir film preparation starts by dissolving the surfactants in a waterimmiscible volatile solvent, such as chloroform or hexane. The solution is then carefully spread onto the clean water surface so that the total area of all the molecules at the air–water interface is much smaller than the available area of the water surface. One can get a simplified, but descriptive, picture of the phase transitions of the molecular film at the interface when one considers two-dimensional phases analogous to the three-dimensional ones. Thus in the beginning, when the molecules have space to move freely in two dimensions they behave like a two-dimensional gas (a in Figure 5.49). In this phase the surfactants have very small, if any, effect on the surface tension of water. As three-dimensional gas, the two-dimensional gas can also be condensed by increasing the pressure. In order to increase the two-dimensional pressure of the molecules at the interface the surface area should be decreased. Usually, the LB system consists of a trough with computer controlled barriers. By moving these barriers the area of the water surface can be controlled. When the surface area is decreased, the free space around the molecules is reduced and molecules start to influence each other. At some point of the compression, when the surfactants are close enough to each other, they start to decrease the surface tension of water. The observed decrease in the surface tension is referred to as the surface pressure (3) which is the difference in the surface tensions of the pure water surface (J*) and the water surface with the surfactants (J): 3 = J* – J. The surface pressure is plotted as a function of available area per molecule, i.e. mean molecular area. The resulted graph is referred to as surface pressure–area isotherm. For an example, an isotherm of an imaginary surfactant is presented in Figure 5.49. The phase transitions cause changes in the gradient of the area–surface pressure isotherm. The most common method of obtaining information about the phase transitions in the monolayer is to measure the surface pressure online with the Wilhelmy method while the molecules are compressed; the more densely the molecules are packed

Figure 5.49 Schematic illustration of a surface pressure–area isotherm of a surfactant. The monolayer phases are indicated and illustrated: (a) two-dimensional gas, (b) two-dimensional liquid, (c) two-dimensional solid, and (d) collapse of the monolayer.

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in the monolayer, the more they lower the surface tension of water. With decreasing surface tension the surface pressure increases. When the gaseous molecules are compressed enough a two-dimensional liquid, or expanded condensed phase (b in Figure 5.49), is formed. Further compression results in a faster rising in the surface pressure when compared with that observed for the gas phase. In order to transfer the monolayer from the interface onto a solid substrate, the monolayer should be stable. An essential requirement to get a stable monolayer is that the surfactants are able to form a solid phase at the ambient temperature. When the monolayer has reached the two-dimensional solid phase, or condensed phase (c in Figure 5.49), the available surface area per molecule corresponds the molecular cross section which is approximately 0.2 nm2 for saturated fatty acids. Usually, the first Langmuir–Blodgett deposition takes place by passing the hydrophilic substrate through the monolayer at the interface from water to air, so that the water soluble ends are attached to the substrate. For the layers following the first deposition, depending on the dipping direction, either from air to water or from water to air, the molecules in the monolayer may have different orientation in respect to the substrate. Concurrently with passing the substrate through the monolayer the surface pressure is kept constant by moving the barriers with proper speed. The transfer ratio is the area the barriers moved during deposition divided by the substrate area passed though the film. The transfer ratio describes the quality of the film deposition, and for the ideal case it is unity. The Langmuir– Schäffer method is sometimes referred to as horizontal lifting. In the LS method the substrate is kept parallel to the water surface when the substrate is brought in contact with the Langmuir film. The Langmuir monolayer adheres onto the substrate and is lifted with the substrate from the interface. It might seem that the LB technique is strictly limited by the requirement of amphiphilicity of the molecules but actually it is possible to prepare, for example, films where totally hydrophobic compounds are mixed in a surfactant matrix. Formation of the Langmuir and Langmuir–Blodgett films of pristine fullerenes is somewhat controversial, but this will be discussed later in the text. Physical vapor deposition in high vacuum is a very well known technique to prepare thin metal, semiconductor, and organic films. Among other organic materials, also fullerene thin films have been prepared by thermal vapor deposition in vacuum [344–347]. The fullerene vaporized is produced in the high vacuum by heating electrically the resistive source at temperature of 300 to 550 qC. With organic materials the temperature control of the source has high importance in order to avoid destruction of the material. Nanostructured fullerene films can be fabricated by the method referred to as electrodeposition [348, 349]. In this method one uses the ability of C60 and C70 to form clusters in solution of mixed solvents at room temperature. A usual diameter for such a fullerene cluster is 100–300 nm. The driving force for cluster formation is the three-dimensional hydrophobic interaction between fullerene units in the solution of polar and non-polar solvents. One method of preparing clusters is to inject a toluene solution of fullerene into acetonitrile. In the final cluster suspension the ratio of mixed solvents is between 3 : 1 and 9 : 1 of acetonitrile–toluene. The

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fullerene cluster formation process and the stability of the clusters are affected by the ratio between the solvents and by the fullerene concentration in the original toluene solution. Especially, C60 can precipitate, for example, when the fullerene concentration exceeds 50 µM and when volume fraction of acetonitrile is more than half [350]. The cluster formation is confirmed by solvatochromic changes when acetonitrile is added to the toluene solution of fullerene. The color changes from the purple of C60 dissolved in toluene to yellow-green of the C60 cluster solution. By electrodeposition one can transfer these fullerene aggregates onto a conductive solid substrate, for example indium-tin-oxide (ITO) coated glass. The electrodeposition takes place in a two-electrode cell where one of the electrodes is the substrate. The spacing between the electrodes is few millimeters and a DC voltage from 20 to 400 V is applied between the substrate and the other electrode. Under the influence of the DC electric field, the fullerene clusters can become charged. The charged clusters are transferred by the electric field onto the substrate with positive potential. Pristine fullerenes in solution could not be charged by the electric field in a similar way as the clusters, thus fullerene aggregation must first take place before electrodeposition. By this method relatively thick fullerene films can be deposited easily and fast. For example, when the cell has been filled with approximately 0.1 mM fullerene cluster solution and electric field is applied for one minute, the solution becomes colorless and a brown film forms onto the substrate. With these conditions the resulted film has a thickness of approximately 1 µm which corresponds to surface concentration of 0.2 µmol cm–2 of fullerene molecules [348]. The film thickness can be controlled by varying the fullerene cluster concentration, the deposition time, and applied DC voltage. In order to increase the film thickness one should use higher cluster concentration, longer deposition time, and/or apply higher DC voltage. 5.4.2 Fullerene Film Properties

Pristine C60 and C70 consist solely of carbon atoms and thus are purely hydrophobic. The formation of a stable fullerene Langmuir monolayer at the air–water interface is somewhat controversial. There are reports about successfully formed monolayers which have been transferred onto the solid substrate from the interface [351, 352]. When fullerene forms a real monolayer on the water surface the limiting area, i.e. the molecular area obtained by extrapolating the isotherm to zero pressure from its steepest part, is approximately 1 nm2. Most of the results indicate that fullerenes tend to form an aggregated multilayer structure rather than a stable monolayer [353]. The mean molecular area obtained decreases dramatically when the fullerenes do not form a monolayer at the air–water interface. Usually, the molecular area is less than half of that of the perfect monolayer. When the surface pressure–area isotherm results in a molecular area lower than one could expect from the physical size of the molecule, the conclusion is that the molecules do not form a perfect monolayer but they are placed on top of each other as a double layer or a multilayer. The Langmuir monolayer measurements have shown that a

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stable fullerene monolayer can be formed on the water surface in very narrow range of experimental conditions [351, 352]. For example, with concentrations higher than 10–4 M, a perfect monolayer cannot be obtained [351]. The simulations of the fullerene monolayers on the water surface help to understand the reason for the observed irregular monolayer formation [354]. The pristine fullerene does not have anisotropic interaction with the water to keep a stable monolayer structure. For Langmuir film preparation a dilute fullerene solution is spread onto the water surface. After solvent evaporation fullerenes form a two-dimensional gas phase. The single molecule can move away from this phase in two ways: it may attach itself to other fullerenes and form a monolayer raft at the interface, or it may escape from the water surface onto the formed raft. A stable monolayer is formed if every fullerene in the raft has six neighboring fullerenes. Six fullerene–fullerene bonds keep the surrounded fullerene tightly in place on the water surface [354]. Naturally, the raft is not infinite but it has boundaries where there are also fullerenes which have less than three neighboring fullerenes. The fullerenes on the edge can easily be promoted on top of the fullerene raft. For example, if a fullerene has only two neighboring fullerenes no fullerene–fullerene bonds have to be broken when this fullerene turns onto the fullerene raft. When conditions are such that this promotion takes place it is impossible to obtain a fullerene monolayer at the air–water interface. On the water surface the monolayers of pristine fullerene are extremely rigid and incompressible having high collapse pressure [351]. In order to overcome the problematic monolayer formation of pristine fullerenes, caused by the fullerene–fullerene and fullerene–water interactions, one can use two different strategies. The first is to use some matrix compound which prevents direct contact, and thus interaction, between fullerenes. The pristine fullerene can be mixed with long chain alcohols or fatty acids [351]. The surfactants and fullerenes are supposed to spread homogenously onto the water surface and thus fullerenes tend to aggregate less than when only pristine fullerene is spread onto the water surface. In addition, some special matrices with cavities where fullerene can fit in, such as azocrowns [355], calixarenes [356], and resorcarenes [357] have been used to enclose fullerenes in the LB films. These cavitants offer a hydrophobic cavity where fullerene can be incorporated and so isolated from the water surface when a one-to-one mixture of the cavitant and fullerene is spread on the water. The cavitant has a hydrophilic part which enables formation of stable Langmuir monolayer at the air–water interface. When any matrix is used the resulting fullerene film does not solely consist of fullerene. A rather exciting way to make a uniform fullerene LB film is to use specially selected matrix molecules which can be selectively dissolved away from the film deposited onto the solid substrate [358]. One can enhance miscibility of fullerene in the matrix by replacing fatty acids with surfactant with, for example, a fulvalene moiety. The electron-accepting fullerene and the electron-donating matrix molecule may have ground state interactions which can increase mixing compared with that of fatty acids and fullerene [358]. The second option to decrease the tendency of fullerenes to form multilayer stacks at the air–water interface is to use fullerenes substituted with some hydro-

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philic group in order to increase surface–molecule interaction. The amphiphilic fullerene derivatives are capable of forming the Langmuir monolayers themselves [337, 359–361]. An amphiphilic fullerene can be also mixed with fatty acid matrix to ease film formation and deposition [336]. The hydrophilic adducts on fullerenes will keep the molecules as a monolayer at the air–water interface even during compression. In other words, the interaction between the water surface and the adducts will defeat the fullerene–fullerene attraction, which usually causes formation of fullerene multilayers at the interface during compression. Addition of hydrophilic side group to the fullerene core does not cancel attraction between the fullerenes. The attraction between the fullerene cores causes irreversible clustering, which is seen as unequal isotherms for the first compression and for the subsequent expansion of the molecular film [362]. This is observed when the monolayer of the hydrophilic fullerene derivative on the water surface is compressed until it collapses and after that the surface area is increased. The expansion isotherm resembles the compression isotherm observed for pristine fullerene forming a multilayer on the water surface. This means that after the monolayer collapse the fullerene derivatives are no longer spread uniformly onto the water surface but the fullerene aggregates are held together by the attraction between the fullerene cores. In order to improve the spreading behavior one can combine the shielding effect of matrix and the properties of hydrophilic adducts by chemical engineering [362]. Fullerene derivatives with several long hydrocarbon chains and hydrophilic groups as adducts have been designed and the LB properties of these molecules have been tested [363]. Already four alkyl chains added onto the fullerene core reduced the attraction between the fullerene cores at the air–water interface. This is seen as better reversibility in isotherms measured with successive compression/expansion cycles. Still the alkyl chains do not provide proper shielding of fullerene cores but, for example, the film absorption spectra show broadening as a result of fullerene aggregation. By placing the fullerene core in the middle of an amphiphilic dendritic structure, one can achieve a fullerene derivative with excellent spreading properties on the water surface. The dendritic adducts make it possible to prepare films where fullerenes are isolated from each other [364, 365]. The LB monolayers of such fullerene derivatives have shown similar absorption spectra to those measured for dilute dichloromethane solutions of the same compounds. Similar absorption spectra for film and solution indicate that the fullerene cores are isolated even in the LB film. Films consisting of isolated fullerenes are interesting in point of view of nonlinear optics, especially, optical limiting applications [362]. Electronegative fullerene can be also modified by linking it covalently with an electron donating chromophore, such as, porphyrin [338, 366], phthalocyanine [367], or phytochlorin [368]. The resulting compound is capable of the photoinduced intramolecular electron transfer from the donor moiety to the fullerene moiety [366]. The donor–fullerene dyads can be synthesized to have amphiphilic character and thus they are suitable for the LB film preparation [369–372]. By the LB technique the electron donor–acceptor dyads can be deposited as monolayers with specified orientation of all the molecules in the film. In other words, in the

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dyad monolayer the electron transfer takes place perpendicular to the substrate and in the same direction in all dyad molecules coherently [369, 371]. When the vectorial photoinduced intramolecular electron transfer takes place from donor to fullerene in billions of dyads simultaneously, a potential of several millivolts is produced [371]. Fullerenes are electrochemically interesting. In solutions, pristine C60 and C70 6– 6– and C70 anions, are able to reversibly accept up to six electrons resulting in C60 respectively [373]. Fullerenes electrochemistry is discussed in more detail in the former section. In contrast, the oxidation of fullerene is more difficult and is usually limited to the formation of dication as the highest oxidation state [374]. Similar step-wise reduction is also seen in fullerene films, but the behavior has been found to be more complicated than for dissolved molecules [335]. The voltammograms for drop-cast fullerene films showed larger peak splitting between the measured reduction and re-oxidation peaks than for dissolved fullerenes. The observed hysteresis for the films suggests large structural changes in the film [335, 375]. Upon reduction, most of fullerenes are reduced to mono-anions. The charge in the film is balanced by diffusion of cations from the supporting electrolyte. The bigger the diffused cations are in size, the larger are the structural changes seen in the film. The changes are reversible when fullerenes are re-oxidized back to the neutral form. With the electrodeposition one can easily prepare micrometer thick fullerene films on optically transparent electrodes. Absorption spectra for electrodeposited C60 films on three different substrates are presented in Figure 5.50. The inset

Figure 5.50 Absorption spectra of C60 clusters deposited on different substrates (a) OTE/(C60)n, (b)OTE/TiO2/(C60)n, and (c) OTE/SnO2/(C60)n film. The absorption spectra of native electrodes, (d) OTE/TiO2 and (e) OTE/SnO2, recorded prior to electrodeposition are also shown. The inset shows solution absorption spectra of (f) 19 µM C60 in toluene and (g) same amount of C60 in the form of clusters in 1:3 (v/v) toluene–acetonitrile. (Reproduced with permission from J. Phys. Chem. B 2000, 104, 4014–4017; Copyright 2000 America Chemical Society).

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in Figure 5.50 shows the difference in the absorption spectra of the C60 toluene solution and the cluster suspension. The characteristic absorption band of C60 at approximately 350 nm is red-shifted and broadened when clusters are formed. In addition, the very weak absorption band of C60 monomer in the 500–600 nm region gets stronger and shifts slightly to shorter wavelengths when aggregation takes place [350]. Similar spectral shifts and broadening of the absorption bands, compared with the fullerene solution spectrum, can be seen as an indication of fullerene–fullerene interaction in films prepared by various methods. The fullerene films electrodeposited onto the transparent electrode coated with tin oxide nanoparticles show photocurrent generation in electrochemical photocurrent measurements. In the experiments, the fullerene film is immersed in an acetonitrile electrolyte with iodine/iodide redox-pair [348]. The current–voltage characteristics for this system established a photodiode behavior. The photocurrent generation in this system is most likely due to photogalvanic type of behavior. This involves electron injection from the iodine/iodide pair to photoexcited fullerene. The generated fullerene anion transfers electron to SnO2 nanoparticle. For nonlinear optics, fullerenes and fullerene derivatives have shown good responses, for example, as optical limiting materials. Usually, for this purpose fullerenes are dispersed into a solid polymer [376] or phosphate glass [377] matrix. Also, sol-gels of fullerene derivatives can be prepared [378]. By this manner one may have films with high concentration of isolated fullerenes. The optical power limiting is a nonlinear optical phenomenon where the optical absorption increases while incident light intensity increases [379]. As seen in the inset in Figure 5.50, isolated C60 molecules in solution have low steady-state absorption in the visible range. C60 at its excited singlet or triplet state has considerably higher molar absorption coefficient in the 300–700 nm range than at the ground state [380]. The absorption properties result in reverse saturable absorption characteristics for fullerene, and thus make it a suitable material for optical limiting applications. Since reversible changes in optical transparency are based on fullerene excited state absorption, a requirement for an efficient operation is the long lifetime of the excited state. Actually, fullerene aggregation causes self-quenching of the excited states and thus restricts the optical limiting time to sub-nanosecond level. The desire for longer working times directs the interest towards solid films where fullerenes are not aggregated but are isolated from each other. High temperature superconductivity has been observed for alkali metal fullerides [334]. The critical temperature of 33 K can be observed for alkali fulleride salts with stoichiometries A3C60, discussed in Section 5.3.3.2. The salt is formed by refluxing fullerene and alkali metal under an oxygen free atmosphere, and electronegative fullerenes oxidize alkali metal atoms. The superconductive phase of the resulted salt adopts, in general, face-centered-cubic or primitive cubic structure. Essentially complete electron transfer from electropositive alkali metal to electronegative fullerene fills the fullerene conduction band, the lowest unoccupied molecular orbital, halfway. Fullerene films with superconductive character have been prepared also by doping fullerene LB films with potassium [335]. After a fullerene multilayer LB film has been deposited it has been doped

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with potassium in high vacuum at elevated temperature. The potassium doped film had superconducting transition at approximately 8 K. 5.4.3 Conclusions

A vast amount of research has been reported in scientific journals, and review articles in journals and books, about fullerene films, their functions, and future prospects. The fullerenes have not yet been – and probably never will be – used as tiny bearings for nanosized machines, although it was seen as a future application a long time ago. But the fullerenes have shown to be excellent material for organic solar cells and other applications related to producing and storing electrical energy. In addition, some applications can be found from the area of optical devices. Thus far no commercial application of fullerene has been introduced. There have been, and there are, many potential trials to find a fullerene application useful for the human race. The enthusiastic research on fullerenes has opened already markets for mass production of these tiny and fascinating carbon balls.

5.5 Endohedral Fullerenes with Neutral Atoms and Molecules

Sho-ichi Iwamatsu 5.5.1 Introduction

The inside cavity of a fullerene (3.5 Å in diameter for C60) is large enough to enclose an atom or a very small molecule. This section describes the endohedral fullerenes enclosing nonmetal substrates inside the cages. Unlike metallofullerenes, in which an electron transfer takes place from the metal to the fullerene cage, these guests are bound to the cage only by weak van der Waals’ forces and by the structural impossibility of escaping. Consequently, there are few changes in properties of the host fullerenes. Such inert guests have served as valuable probes for exploring both the interior and exterior of fullerenes. Also, fullerene containers can keep highly reactive chemical species that are unable to exist in an ordinary system. Synthetic procedures and properties of the products will be presented along with theoretical studies. 5.5.2 Preparation 5.5.2.1

Direct Approach Using an Existing Fullerene

Since He@C60 (C60 with a helium atom inside) was discovered in a fullerene soot [381], continuous efforts have been paid to the syntheses of such gas complexes

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Scheme 5.9 Direct approach to the endohedral nonmetal complexes of fullerenes.

with high yields. The first successful methodology was the forced penetration of a guest into an existing fullerene. This is different from the case of metallofullerene in which a metal fragment is captured in a cage during the formation of a fullerene framework. (a) High-pressure and high-temperature encapsulations; (b) ion beam implantations; and (c) nuclear reactions have been developed to achieve such incorporations (Scheme 5.9). The high-pressure and high-temperature incorporations are generally carried out by heating a pristine fullerene at 650 °C under 300 MPa of a desired gas using a hydrostatic high-pressure vessel [382, 383]. A series of noble gas complexes, from He@C60 to Xe@C60, have been synthesized by this method [383–389]. Fractions of the endohedral complexes in recovered fullerenes were 0.03–0.1%. Trace amounts of complexes containing two helium or neon atoms inside (He2@Cn and Ne2@Cn, n = 60, 70) were also detected by 3He NMR and/or mass spectrometry [382, 389]. Chemical bonding between two atoms confined in a fullerene cage have been investigated in theory [390]. Similarly, formation of some diatomic molecule complexes with diatomic molecule quests, such as 13CO@C60, N2@C60, and N2@ C70, was confirmed by mass spectrometry [391]. Fractions of these products were estimated to be in the range of 0.02–0.05%. The reaction is applicable to higher or mixed fullerenes. Ion beam implantations have been mainly employed for the syntheses of group V atomic complexes, such as N@C60 [382, 392]. Commonly, low energetic ions are provided to a continuously growing fullerene film. The amounts of fullerenes recovered were 10–20%, because the fullerenes employed decomposed. Fractions of the desired endohedral complexes in these portions were in the order of 0.01%. At present, successful examples are limited to N@C60, N@C70, P@C60, and N2@C60 [393–396]. It has been suggested that an atomic complex with a higher fullerene cage would be unstable because decreased curvature of the cage might increase its reactivity toward a substrate inside [392, 394]. Also, the incorporation of arsenic has been attempted without success [392, 394]. Though the smallest 1H@C60 has not been synthesized so far by the above mentioned methods, its isotopic and radioactive homolog, 3H@C60 (3H = T, tritium), has been synthesized by the nuclear reaction of 6Li with C60 or neutron irradiation on the 3He atom in 3He@C60 [17, 382, 383]. Counting a soluble portion in a scintillation counter showed the presence of tritium. Other radioactive complexes, such as 41Ar, 85Kr, and 133Xe@Cn (n = 60 or 70), were synthesized by neutron irradiations or ion implantation [398–400].

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5 Fullerenes Table 5.5 Separation factors in HPLC and 13C NMR chemical shifts (in ppm) of X@C60.

Ar@C60 Kr@C60

13

µb)

Refs.

143.2c)

+0.2

[384, 385]

d)

+0.4

[386]

c)

[388]

C NMR, µ

Separation factor in HPLCa)

X@C60

C60

1.04–1.05

143.4 143.6

1.09

143.2

Xe@C60

1.08

144.5

143.5

+1.0

N@C60

1.06–1.08

–

–

–

[393, 395]

N2@C60

1.10–1.13

–

–

–

[396]

H2@C60 a) b) c) d) e)

1.01

142.84

142.76

e)

+0.08

[407, 408]

Retention time relative to that of C60. 'G G(X@C60)–G(C60). in C6D6. in C6D6/C6H6. in o-dichlorobenzene-d4.

In all cases, fractions of the desired complexes in recovered fullerenes are less than 0.5%. Despite such extremely low content, Ar@C60 [384, 385], Kr@C60 [386], Xe@C60 [388], N@C60 [393, 395], and N2@C60 [396] were enriched to the analytical level by high performance liquid chromatography (HPLC). Multistage purification is essential because of poor separation factors (see retention times relative to that of C60 collected in Table 5.5) in addition to low fractions. 5.5.2.2

Molecular Surgery Approach via an Open-cage Fullerene

Since the carbon–carbon bond of C60 was cleaved to give a hole-opened, so-called an open-cage fullerene 51 (Scheme 5.10) [401], an alternative strategy has been proposed as a rational approach to obtain the desired endohedral complex at a high incorporation level [402]. It consists of the following three stages: (1) chemical cage scission to make an opening, (2) insertion of a guest through the opening, and then (3) closing the cage with the guest inside. He@51 prepared from He@ C60 revealed that an opening formed by one bond scission is too small to permit passage even of the smallest helium atom. Then, larger openings in 52–56 were created by regioselective and multiple bond scissions (Scheme 5.10) [403–413]. Among successful molecular incorporations into these derivatives (described in Section 5.5.2.3), a pure and macroscopic quantity of H2@C60 was recently synthesized from C60 through 53 (Scheme 5.11) [406–408]. First, the 100% encapsulation of a hydrogen molecule into 53 was achieved at 80 MPa of H2 and at 200 °C [406]. Then, closure of the opening in H2@53 was carried out with a trapped H2 in four steps to afford H2@C60 [407, 408]. Although slight escape of the trapped H2 had taken place during the closure, a rich fraction of H2@C60, higher than 90%, permitted its complete separation by using the repeated HPLC system, despite an extremely low separation factor (Table 5.5).

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Scheme 5.10 Molecular structures of open-cage C60 derivatives 51–56.

Scheme 5.11 Synthesis of H2@C60 following the molecular surgery approach.

5.5.2.3

Open-cage Fullerenes, Reversible Molecular Incorporations and Ejections

Though successful restoration of the opening in open-cage fullerenes has been limited to H2@53, several molecular encapsulations have been performed using 52–56. The first successful examples were incorporations of helium and hydrogen gas (H2) into 52 [404]. Encapsulation of helium was achieved by heating solid 52 at 305 °C under 48 MPa of helium (Table 5.6). The fraction of He@52 was 1.5%. Compound 53 has a larger opening than that of 52, and the same encapsulation could be done below 2 MPa [405]. Despite easy encapsulation, however, the fraction of He@53 was nearly the same as that of He@52. This is because molecular incorporation is a reversible and equilibrium process, and a larger opening allows

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5 Fullerenes Table 5.6 Endohedral He and H2 complexes of open-cage C60 derivatives: reaction conditions, fractions, energy barriers to escape (kcal mol–1), and NMR chemical shifts of the guests (in ppm). Condition

Fraction (%)

Barrier to escape exp.

(theory)

3

He/1H NMRa)

Refs.

3

–

–

–

–

–6.40

[38]

3

48 MPa, ~305 °C

1.5

24.6

(24.3)

–10.10

[404]

3

He@3

2 MPa, 80 °C

1.5

22.8

(18.6)

–11.86

[405]

H2@C60

–

–

–

–

–1.44

[408]

H2@2

10 MPa, 400 °C

5

–

(40.0)

–5.43

[404]

H2@3

81 MPa, 200 °C

100

34.3

(28.7)

–7.25

[406]

H2@4

13.5 MPa, 100 °C

83

21.7

(19.8)

–7.34

[410]

He@C60 He@2

a) 3

He signals NMR relative to dissolved 3He gas, 1H signals NMR relative to TMS.

not only for encapsulation but also rapid escape. Actually, the trapped helium atom can be released from the cage by heating. The activation energies for the helium escape from He@52 and He@53 were estimated to be 24.6 kcal mol–1 and 22.8 kcal mol–1, respectively, in accordance with the size of the openings in 52 and 53 (Table 5.6) [404, 405]. Easy escape also suggests that there is negligible binding energy between the trapped helium and the fullerene cage. Encapsulation of molecular hydrogen has been achieved using 52–54 [404, 406, 409, 410]. H2@52 was obtained in 5% fraction by heating solid 52 at 400 °C under 10 MPa of H2 (Table 5.5) [404]. Compound 53 allowed for 100% H2 encapsulation at 80 MPa of H2 and at 200 °C as mentioned earlier [406]. Compound 54 has a larger opening than those of 52 and 53, and allowed entry of H2 under mild conditions [410]. The trapped H2 can be released by heating, as in the case of helium complexes. Energy barriers of the H2 escape from H2@53 and H2@54 were estimated to be 34.3 kcal mol–1 and 21.7 kcal mol–1, respectively (Table 5.6) [406, 410]. Recently, an useful index was reported for the evaluation of effective areas of these openings bearing different shapes and functional groups [409]. Among the open-cage derivatives of fullerenes, compound 55 has the largest opening at present. Although endohedral fullerene complexes bearing a di- or tri-atomic molecule inside are still limited [391, 396, 407, 408], compound 55 can hold one water or a carbon monoxide molecule inside the cage [411–413]. Unlike other entries, a water molecule is incorporated into 55 spontaneously [411]. H2O@55 was obtained as a mixture with the empty 55 in the product. In solution, the trapped water molecule in H2O@55 is in rapid equilibrium with residual water in the solvent. The fraction of H2O@55 was about 75% in commercial CDCl3, but decreased with decline of the water content in the solvent.

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Also, in the 1H NMR spectrum the signal of H2O in H2O@55 disappeared rapidly after treatment with D2O. The inclusion property can be refined by the size and structure of the opening. Compound 56 has a similar but smaller opening than that in 55. The fraction of H2O@56 was less than 10% under conditions identical to those in which H2O@55 was obtained, but could be improved to 85% by refluxing in a mixture of toluene and water, indicating higher activation energies of both incorporation and escape [412]. Functional groups around the opening also affected the inclusion property of a water molecule. Carbon monoxide was incorporated into 55 by heating a solution of a mixture of H2O@55 and 55 in the presence of CO [413]. The fraction of CO@55 reached 84% under 9.0 MPa of CO and at 100 °C. CO@55 gradually released the trapped CO and reverted to a mixture of 55 and H2O@55 under ambient conditions. This is in contrast to the spontaneous H2O encapsulation into 55, indicating that H2O binds more tightly than does CO. 5.5.3 Properties 5.5.3.1

Host Fullerenes

Basically, complete endohedral fullerenes are stable under ambient conditions. No decomposition was observed on H2@C60 at 500 °C [407, 408]. Thermal escape of helium and neon from He@C60 or Ne@C60 only took place above 600 °C [381–383, 414]. Since the energy required to escape from the pristine cage is too high to produce it thermally, it has been proposed that one or two fullerene bonds are reversibly cleaved to form a temporary window to allow the guest release. On the other hand, escape of the nitrogen atom in N@C60 took place at a relatively low temperature (260 °C) [392, 394]. A different pathway through the insertion of the trapped nitrogen atom into a fullerene bond has been proposed to account for the difference. Endohedral open-cage complexes release the guests by heating, as described earlier. There are only weak van der Waals’ interactions between the guests and the fullerene cages. The 13C NMR chemical shifts of the X@C60 cages (X = guest) are representatives as well as poor separation factors in HPLC (Table 5.5). In all cases, the signals appeared downfield relative to that of the empty C60 cage. The shift increased with the size of the guest but was less than 1 ppm even for the maximum case of Xe@C60. Slight changes were observed in the IR and UV/Vis spectra of X@C60 as shown below, but further study is necessary for systematic considerations. In the IR spectrum of Kr@C60, three of the four well-known C60 bands (528, 1183, and 1429 cm–1) shifted by 8–16 cm–1 to higher frequencies [386]. The remaining absorption band at 577 cm–1 became significantly weak, but the reason for this observation is not clear. The IR spectrum of Ar@C60 also showed small (2–4 cm–1) shifts below 600 cm–1 in the same direction, but opposite shifts were observed on other higher frequency bands [385]. For H2@C60, only a small shift was observed on the absorption band at 577 cm–1 [408]. The UV/Vis spectrum of Kr@C60

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showed red shifts by 1–3 nm in the range of 580–640 nm [386], but no detectable change was observed on either Ar@C60 or H2@C60 [385, 408]. N@C60 displays a yellowish-brown color in solution, unlike the magenta color of C60 [395]. Accordingly, broad absorption bands of C60 in the range of 440–640 nm are notably suppressed in the case of N@C60. The chemical reactivity of the fullerene cage is not affected by such a guest, as expected. Organic reactions developed for fullerenes are reproducible under the same conditions [383, 415, 416]. The availability of the products will be shown in the next paragraph. 5.5.3.2

Guest Substrates

In contrast to the insensitivity of host fullerenes, guests are quite sensitive to the cage size, electronic state, and organic addends of the hosts. This property has enabled their valuable applications as chemical probes and markers. The smallest helium atom has played a major role in NMR studies [382, 338]. The abundant 4He is not an NMR active nucleus, but isotopic 3He has a spin of I = 1/2 and exhibits a good NMR sensitivity. Generally, signals of the trapped atoms and molecules shift upfield due to magnetic shielding by the fullerene cage. Helium-3 in 3He@ C60 and 3He@C70 appeared at G = –6.4 and –28.8 ppm, respectively, relative to the dissolved 3He gas [383, 417]. Chemical shifts of the 3He atoms trapped in higher fullerenes vary in the range between those of 3He@C60 and 3He@C70. Higher fullerenes often have several isomeric structures with different symmetries, and each shows multiple signals in the 13C NMR spectra. Also, multiple addition reactions on C60 generally produce an inseparable mixture of up to 8 regioisomers, and each adduct shows multiple signals in 13C NMR due to a lowering of molecular symmetry. In contrast, a trapped 3He atom in any structure shows only one single line in the 3He NMR spectrum. Thus, 3He complexes can be used as markers in the analyses of mixtures of higher fullerenes and multiple adducts of fullerenes [383, 417, 418]. Their use for studying structures of hydrogenated fullerenes will be discussed in Section 5.6. Organic addends on 3He@C60 generally produce additional upfield shifts [383]. A [5,6]-opened homofullerene structure retains the original 60 S elelectronic structure, and the internal 3He atom shows only a small shift ('G = –0.2 ppm) (Table 5.7). On the other hand, a [6,6]-closed structure accompanies a loss of S-electrons, and the trapped He atoms show much larger shifts ('G = –1.8 – –3.2 ppm) than those in the former [5,6]-opened structure. Adding electrons to 3H@C60 produces a drastic 6– showed the signal at G = –48.7 ppm upfield shift [417]. The hexa-anion, He@C60 corresponding to 'G = –42.4 ppm relative to that of He@C60. It should be noted that He@C70 shows opposite downfield shifts by both organic addends and elec6– showed the signal at G = +8.3 ppm, tronic reductions. For example, 3He@C70 which corresponded to 'G = +37.1 ppm relative to He@C70. Recent success in the macroscopic synthesis of H2@C60 has allowed measurement of the most familiar 1H NMR spectra. The trends observed were the same as in the case of 3He@C60. The trapped H2 molecule in H2@C60 appeared at G = –1.44 ppm, which corresponded to 'G = –5.97 ppm relative to the dissolved H2

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Table 5.7 3He and 1H NMR chemical shifts (in ppm) of the functionalized endohedral fullerenes with 3He atom or H2 molecule inside [383, 408].

3

–6.40

–6.63

–8.11

–8.11

–9.45

1

–5.97

–

–

–7.80

–9.17

He NMR H NMRa)

a)

Relative to dissolved H2 gas (not TMS).

gas (Table 5.7) [407, 408]. Organic addends induced additional upfield shifts [407, 408, 416]. The relaxation time (T1) of H2 in H2@C60 was in the range of 0.04–0.12 s, 10–20 times shorter than that of the dissolved H2 (0.84–1.44 s) [419]. With respect to the guests in open-cage C60 derivatives, creating an opening on the C60 surface induces an upfield shift similar to those observed in organic addends. Shifts become larger with increase in the size of the openings in both 3He and 1H NMR. 3He@52 and 3He@53 showed upfield shifts of 'G = –3.70 ppm and 'G = –5.46 ppm, respectively, relative to 3He@C60 [404, 405] (Table 5.6). Similarly, H2@52–54 showed upfield shifts of 'G = –3.99 ppm (H2@52), 'G = –5.81 ppm (H2@53), and 'G = –5.90 ppm (H2@54) relative to H2@C60 [404, 406, 408, 410]. The proton signals of water molecule in H2O@55 appeared at G = –11.4 ppm in the 1H NMR spectrum [411]. Although H2O@C60 has not still been synthesized, it corresponds to the upfield shift by 'G = –13 ppm relative to the residual water in the solvent (G = 1.6 ppm). The signal was a sharp singlet indicating free rotation of the trapped H2O in the C60 cage. The carbon signal of the trapped CO in CO@55 appeared at G = 174.6 ppm shifted by 'G = –10 ppm relative to the dissolved free CO gas (G = 184.6 ppm) [413]. The signal appeared as a singlet line even at –80 °C, indicating that CO rotates rapidly on the NMR time scale. In the IR spectrum, in contrast, two CO absorption bands were observed at Q = 2125 and 2112 cm–1. These are shifted by –18 and –31 cm–1 to lower frequencies, respectively, from the CO gas frequency (Q = 2143 cm–1). The observed difference between NMR and IR can be interpreted in terms of the difference in the time scale of these measurements. As for N@C60, atomic nitrogen has unpaired electrons and is paramagnetic. The excellent sensitivity of EPR (electron paramagnetic resonance) spectroscopy has allowed the detection and analysis of N@C60 even at an extremely low fraction.

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The ESR spectrum of N@C60 shows three lines originating from the hyperfine interaction of the electron spin with the nuclear spin of 14N (I = 1) [392–395], indicating that the trapped nitrogen atom keeps its quartet spin ground state configuration without an electron transfer to the cage. Regarding cage size, electronic state, and organic addends, trends observed were the same as in the cases of 3He and 1H NMR. Compared with N@C60, the central line of the triplet of N@C70 shifted upfield by 0.0065 mT in the magnetic field corresponding to –19 ppm [392]. The hexa-anion N@C606– showed an upfield shift by –35 ppm 6– did a downfield shift by +41 ppm relative to N@ relative to N@C60, but N@C70 C70. Organic addends also caused an upfield shift for N@C60 but a downfield shift for N@C70 [392]. 5.5.4 Binding Energies, Theoretical Investigations

To maximize yield of the endohedral fullerenes, and to construct an efficient host system using open-cage fullerene derivatives, it is important to understand the interaction between the guest and the fullerene cage. Generally, trapped guests have been believed to have attractive interactions with the fullerene cage, because molecular incorporation is an entropically disfavored process. Various theoretical studies have been carried out along with synthetic experiments [420–427]. The binding energies of noble gas complexes of C60 were estimated by ab initio and DFT (density functional theory) calculations. At the MP2 (secondorder Møller–Plesset perturbation) theory, a series of noble gas complexes, from He@C60 to Xe@C60, were computed to have stabilization energies of –0.3 to –7.5 kcal mol–1, indicating attractive interactions in all cases [421, 422]. In contrast, the frequently used B3LYP (Becke three-parameter exchange functional coupled with the Lee-Yang-Parr correction) functional in DFT studies generally underestimates weak molecular interaction and predicts repulsive destabilization [421, 423]. For example, H2@C60 was estimated to be unstable by +1.6 kcal mol–1 at the B3LYP/6-31G(d,p) level, whereas it was stabilized by –4.0 kcal mol–1 at the MP2/6-31G(d,p) [423, 424]. Recently, the MPWB1K functional (modified PerdewWang and Becke functionals) of DFT was reported to give results consistent with the MP2 calculations (–7.9 kcal mol–1 for H2@C60) [424]. In a real experiment, however, the DSC (differential scanning calorimetry) measurement for the H2 escape from the open-cage H2@54 suggested that the trapped H2 destabilizes the cage [410]. Also, simple molecular mechanics and semiempirical quantum calculations have been discussed over a variety of endohedral complexes including multiple guest systems [425–427]. The possibility of housing more than one H2 molecule in the C60 and higher fullerene cages has been examined by Dodziuk [426]. Further experiments with a variety of guests are needed to reach reliable and systematic conclusions.

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5.5.5 Summary

Endohedral nonmetal complexes of fullerenes allow us to study the inside of the fullerene sphere. The properties of host fullerenes scarcely change in response to the neutral guest substrates. In contrast, guests are quite sensitive to the host size, structure, electronic state, and organic addends, including chemical openings. Along with first generations produced by forced gas penetrations, new endohedral complexes are beginning to be synthesized by the molecular surgery method, which will continue to expand the library of endohedral fullerenes in macroscopic quantities and at high incorporation levels, leading to practical applications.

5.6 Hydrogenated Fullerenes

Mark S. Meier Fullerenes are among the most highly strained aromatic compounds known, approaching 500 kcal mol–1 in strain energy. Pyramidalization of sp2 carbon atoms, by ~8° – 12° in fullerenes, is required to form the closed shells, but also results in a degree of rehybridization [428] that diminishes overlap in the S-system. The pyramidalization induces strain, and in turn that strain introduces reactivity [429] not normally seen in other aromatic systems. The interplay between strain, aromaticity [430], thermodynamics, and kinetics leads to a very rich chemistry, producing some beautiful structures in situations where one might expect intractable mixtures of products. Fullerene reactions are also discussed in more general terms in Section 5.2. The hydrogenation of fullerenes [431] provides a clear context for the study of fullerene reactivity. The continuous series of C60H2n, with n ranging (at least in principle) from 1 to 30, covers a molecular formula range from C60 to C60H60. Hydrogen (followed closely by fluorine) is the least bulky covalent addend that may be bonded to a fullerene, and hence provides an excellent probe for revealing the reactivity of the fullerene core without interference from steric effects. In practice, only a handful of the 30 different C60H2n molecular formulae are formed in the course of most hydrogenations of C60. This result is striking in its own right, but the selectivity for a limited number of isomers, when vast numbers are possible in most cases, is truly amazing. 5.6.1 Synthesis and Structure

The simplest of the hydrogenated fullerenes is C60H2. This compound has been prepared by a long list of methods, including methods as diverse as hydroboration [432], electrochemical reduction/protonation [433–435], dissolving metal

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Figure 5.51 C60H2 (57), shown in a 3-dimensional view and in a planar projection (Schlegel diagram).

reduction/protonation [436], borohydride reduction [437], hydrozirconation [438], thermal [437] and sonochemical [439] transfer hydrogenation, direct hydrogenation with metal catalysts [440–444], and with diimide [440]. This compound has been made indirectly through decomposition of a Si-bridged C60 dimer [445], as well as by hydrolysis of acylfullerenes [446], and it has been detected in fullereneproducing flames [447]. While a considerable number of isomers of C60H2 are possible [448, 449], only one has been prepared, purified, and fully characterized. The 1,2-isomer (57, Figure 5.51) appears to be the most thermodynamically stable isomer, and given the high acidity of fullerene C–H bonds [450, 451], it seems likely that any unstable isomers that may be formed would be transient and easily undergo isomerization. C60H2 forms a brown solution in toluene, strikingly different from the magenta color of C60 solutions. The 1H NMR spectrum is quite simple – a singlet at 5.12 ppm (CS2 solution). The significant downfield shift of this resonance is likely caused by large ring currents in the remaining S-electron system [452]. The 13C NMR spectrum is consistent with the Cs-symmetry, with a single sp3 resonance at 54.2 ppm [453]. Notably, the 1H-coupled 13C spectrum reveals the 2-bond H-C coupling constant (6.7 Hz), which is a useful indicator of a 1,2 arrangement of hydrogen atoms on the fullerene surface. Essentially any reagent that is capable of hydrogenating C60 is likely to be capable of hydrogenating C60H2 to C60H4 or farther. An elegant study by Cahill produced and characterized several isomers of C60H4 [454]. Hydroboration–protonolysis of C60 produces a mixture of six isomers (Figure 5.52) of which two could be isolated and definitively identified, with 58a being the major product. A mixture of C60H4 compounds could be isomerized on platinum to the 1,2,3,4-isomer (cis-1) 58a, indicating that this not only the kinetic isomer but is likely the thermodynamically most stable isomer as well. This crucial experiment validates the computational prediction that the cis-1 isomer (58a) is one of the lowest energy structures studied. Dissolving metal reduction also produces C60H4, with 58b and 58c being identified in the Zn(Cu) reduction [455]. Dissolving metal reduction of C60 using Zn(Cu) results in the formation of a major isomer 59 (trans-3, trans-3) and 2 minor isomers [455]. The remarkably simple 1H (singlet) and 13C NMR spectra (10 resonances) indicates a structure

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Figure 5.52 Isomers of C60H4.

Figure 5.53 C60H6 (59).

with D3-symmetry, and the measured 6.8 Hz 2-bond H–C–C coupling constant (indicating a 1,2-arrangement of hydrogens in 3 equivalent sets) permits assignment of the structure (Figure 5.53). Chromatography suggests that a second, minor isomer is formed along with 59, and investigation of the 3He NMR spectra of 3HeC60H4 proves that there are at least two minor isomers present in the Zn(Cu) reduction [456], although the structures have not been determined. Clearly the major isomer 59 does not result from hydrogenation of the most stable isomer of C60H4 (58a) but from one of the less stable ones (58b). Computational work [457] has suggested that most stable C60H6 structures include one related to the thermodynamic isomer of C60H4 (58a) and one that is analogous to the C60Cl6 structure [458]. This result highlights the fact that fullerene chemistry is a kinetic world not always ruled by thermodynamics. Beyond C60H6 lies a homologous series of compounds (C60H8, C60H10, C60H12, etc.) in which no one species emerges as significantly more stable than others in the immediate neighborhood. Accordingly, to date none of these species has been isolated in good yield in pure form. The remarkable exceptions to this rule are C60H18 and C60H36. The fact that specific isomers of these two molecular formulae appear as islands in a vast sea of possible formulae and isomers is one of the most startling results in fullerene chemistry.

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Figure 5.54 C3v-C60H18 60. Left and middle: bottom and side views; right: planar projection.

C60H18, like C60H36 (below), is more stable than the nearby homologs (C60H16, C60H20, etc), appearing as a significant product in the Birch reduction [459], transfer hydrogenation [460], and in the Zn/HCl reduction [461] of C60. These reactions could produce every adjacent oxidation state, but C60H18 is formed to the near exclusion of the adjacent formulae. This compound is formed as a C3v ‘turtle shell’ isomer 60 [462]. This structure contains a nearly planar benzene ring at the base (Figure 5.54). A wide array of reaction conditions lead to this same C3v isomer of C60H18. These methods include reduction of C60 with amines [463], hydrogenation [464, 465], and dehydrogenation of C60H36 [466]. Reduction of C60 with lithium in ammonia containing t-BuOH (Birch reduction) results in the formation of C60H36 [459]. This result is striking, as there are numerous adjacent oxidation states (… C60H34, C60H38, etc) that might be formed, but C60H36 is formed as a major product to the near exclusion of other products. Other reagents also lead to C60H36, including transfer hydrogenation [467, 468], reduction by Zn metal in acidic solution [469], and catalytic hydrogenation [441]. The number of possible isomers for C60H36 is truly staggering, exceeding 1014 by one report [448]. A number of methods for preparation of this compound have been reported [441, 459, 460, 467, 469, 470], but most result in complex mixtures of species from which C60H36 must be isolated. Transfer hydrogenation [460, 467, 468, 471] using 9,10-dihydroanthracene produces a mixture of three isomers, with a C1 isomer 61a being strongly dominant, although a smaller amount of a C3 isomer 61b and a trace of T isomer 61c are observed [466]. Detailed NMR work on this mixture has led to the structural assignments shown (Figure 5.55). It is believed that the formation of this limited array of structures, out of the huge number possible, is the result of rapid isomerization under the conditions of transfer hydrogenation. Further heating of this mixture results in conversion of the T and C1 isomers to the C3 isomer, supporting both the notion of thermal isomerization as well as supporting calculations that suggest that the C3 isomer is the most stable. Reduction of C60H36 with lithium in diethylamine leads to a mixture of highly hydrogenated products, composed of C60H38 though C60H44 [472]. These compounds were detected by MS, but despite extensive HPLC work no one compound could be isolated. 3He NMR spectra of the Li/diethylamine reduction of He@C60H36

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Figure 5.55 C60H36 structures.

confirms that numerous different compounds are present. A small amount of C60H48 was observed in the Zn/HCl reduction of C60, but again no structural assignment has been made [469]. Isolation and definitive structural assignment of any member of this class of compounds is a major challenge, as they are relatively insoluble (even among fullerenes) as well as fragile, decomposing in solution in air and light. The endpoint of hydrogenation of C60, C60H60, has not been isolated or characterized. This is not surprising, as the large number of eclipsing interactions would make an all-exo structure impossibly high in energy. Structures with some of the hydrogens on the interior would be significantly more stable [473, 474], but synthesis is a challenge. These structures are discussed in Section 2.4.7.1. 5.6.2 C70 Chemistry

The reactivity of C70 is similar to that of C60 and as expected, the less pyramidalized [429] carbons along the equator are typically less reactive than the more pyramidalized carbon at the poles. As is the case with the reduction of C60 to C60H2, reduction of C70 to C70H2 can be accomplished by a variety of methods and has been demonstrated with many of the same reagents used with C60 [440]. The first step in the hydrogenation of C70 results in the formation of 1,2-C70H2 (62a, Figure 5.56), resulting from hydrogenation of the most highly strained part of the molecule [475, 476]. This regiochemistry is the common result for addition to one C-C bond in C70, being observed as the major product in Diels–Alder, Hirsch-Bingel, Prato, and other additions. The 2-bond H-C-C coupling constants (4.7–5.2 Hz) in 62a are somewhat smaller than the analogous coupling in C60H2. A smaller amount of the 5,6-isomer 62b is formed along with 62a, resulting from reduction of the second most strained site in C70.

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Figure 5.56 1,2-C70H2 (6a), 5,6-C70H2 (62b), and C70H4 (63a).

The second hydrogenation step is an interesting one, because if strain determines the regiochemistry of addition then at least five different isomers of C70H4 could result from the reduction of C70H2 (62a), resulting from addition to any one of the remaining 6–6 ring fusions at the poles. In practice, the Zn(Cu) reduction of C70 results in the formation of C70H4 (63a) from C70H2 (62a) [476]. The Zn(Cu) reduction of C70 produces C70H8 (64) as a major product (Figure 5.57) [476]. This compound does not form by reduction of C70H4 (63a), but instead from an ‘unseen’ pathway starting at C70 and proceeding through a series of intermediates (presumably C70H2-6 species) that are reactive and rapidly reduce again, until this relatively stable isomer is formed. At that point, 64 is relatively slow to reduce again, and hence it builds in concentration. Structure 64 is analogous to the structure of C70Cl10 [477], but is unusual for the 1,4-pattern of hydrogenation. A 1,2-pattern is found is virtually all other examples. The crowded 1H NMR resonances are more upfield than seen in 62a and 63a, consistent with hydrogenation away from the poles. The 1,4-pattern is suggested from the absence of the familiar 2-bond C–H coupling constant, replaced instead by smaller coupling constants.

Figure 5.57 C70H8 (64).

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Figure 5.58 Polar views of C70H8 (64) and C70H10 (65).

Further reduction of 64 leads primarily to C70H10 (65), although two minor and unidentified isomers are also formed (Figure 5.58). The 13C spectrum of 65 indicates a plane of symmetry, and a typical 2-bond C–H coupling constant (5.6 Hz) appears again. This structure results from reduction of 64 in such a manner as to cause minimal disruption of the remaining aromatic system. As with C60, powerful hydrogenating conditions result in highly hydrogenated compounds. With extended reaction times, the Zn(Cu) reduction produces material beyond C70H10, but numerous compounds result. The Zn/HCl reduction can also produce highly hydrogenated materials, Strongly reducing conditions (catalytic hydrogenation [441], Zn/HCl reduction, transfer hydrogenation [460, 468]) leads to the formation of C70H36, in a close parallel to the fate of C60 under the same conditions. 5.6.3 Higher Fullerenes

Reduction of C76 (as a mixture of isomers) with Zn in HCl leads to a mixture of products, ranging from C76H46 to C76H50. Likewise, treatment of C78 and C84 (again, both as mixtures of isomers) under these conditions produces complex mixtures of products. The reduction of C78 produces a spectrum of products, with C78H36 being a major product accompanied by C78H48. Reduction of the higher fullerenes has been reported to be accompanied by a measure of breakdown of the fullerene cages, as evidenced by the observation C60H36 and/or C70H36 species [478]. 5.6.4 Reactivity of Hydrogenated Fullerenes

Hydrogenation of fullerenes is an easily reversible process, with dehydrogenation back to the parent fullerene being brought about by reagents such as DDQ [459] and metal hydrogenation catalysts [479]. The most remarkable aspect of the chemistry of the higher fullerenes is the pronounced acidity of the C-H bonds. It is possible to deprotonate t-BuC60H with weak bases such as acetate, and the pKa of the fullerene C-H bond was estimated to be 5.7 [450]. In DMSO, the pKa of C60H2 was determined to be 4.6, quite acidic for a CnHn compound [451].

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The acidity of other hydrogenated fullerenes is pronounced [480, 481], and a prediction of 8.0 has been made for C60H6 [480]. In practice, deprotonation of hydrogenated fullerenes is a facile method for formation of fullerene anions. These anions react readily with electron deficient alkenes such as acrylonitriles to form aminocyclopentenes [482]. Treatment of C60H2 with weak bases and alkyl halides produces 1,2-monoalkyl- or 1,4-dialkyl fullerenes [483] although yields are mediocre, presumably because the anions undergo rapid oxidation by adventitious oxidants. Alkylation with alkyl tosylates is not effective, because fullerene anions typically react by electron transfer [484] rather than by a standard Sn2 mechanism. Dialkylation of C60H2 with CH3I results in a mixture of 1,2- and 1,4-dialkylation, which is consistent with 1,2-addition being nominally the most stable arrangement, but the crowding of addends destabilizes this arrangement, leading to the second alkylation step proceeding at an allylic position. Treatment of C70H2 (63a) with one equivalent of base and ethyl bromoacetate results in a 37 : 1 ratio of C-1 alkylation 66a to C-2 alkylation 66b (Figure 5.59) [485]. The selectivity here probably results from more rapid deprotonation at C-1, a reaction that probably produces a more stable, delocalized anion than would be produced by deprotonation at C-2.

Figure 5.59 Monobenzyl C70 from alkylation at C-1 (66a) and from alkylation at C-2 (66b).

Treatment of C70H2 (62a) with two equivalents of base and alkyl halide produces a mixture of products (Figure 5.60). While a trace amount of the 1,2-bibenzyl product 67a is obtained the dominant product, resulting from alkylation near the equator 67b, is obtained in only 10% yield [483].

2– Figure 5.60 Products of dibenzylation of C70 .

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5.7 Applications of Fullerenes

Rossimiriam Pereira de Freitas and Jean-François Nierengarten 5.7.1 Introduction

When fullerenes were first discovered more than two decades ago [486], there was much excitement about possible applications for this new molecular material. In spite of initial speculation, the fullerenes could not be extensively studied until 1990 when Krätschmer et al. [29] made C60 available in macroscopic quantities. Since then, the unusual properties of this fascinating allotropic carbon form have been intensively investigated. Among the most spectacular findings, C60 was found to behave like an electronegative molecule able to reversibly accept up to six electrons [487], to become a supraconductor in M3C60 species (M = alkali metals) [488, 489] or to be an interesting material with nonlinear optical properties [490]. Although possessing exceptional properties, C60 is difficult to handle because it forms aggregates and is insoluble or only sparingly soluble in most solvents [491]. This serious obstacle for practical applications can be overcome, at least in part, with the help of organic modification. Effectively, the recent developments in the functionalization of fullerenes allow the preparation of highly soluble C60 derivatives which are easier to handle, and whose electronic properties, such as facile multiple reducibility, optical nonlinearity or efficient photosensitization that are characteristic of the parent fullerene, are maintained for most of the C60 derivatives [492–497]. After several years of research, fullerene chemistry is now a well-established field and the knowledge acquired has revealed both potentials and limitations of this class of compounds. The present chapter illustrates some of the most promising applications for chemically modified fullerenes. The objective is not to present an exhaustive review but to describe some of the most illustrative examples in materials science and biology. 5.7.2 Applications in Materials Science 5.7.2.1

C60 Derivatives for Optical Limiting Applications

Optical Limiting (OL) is a nonlinear phenomenon in which the absorption of a material increases when the incident radiation intensity increases. Materials or devices with transmission that decreases with light level are called optical limiters and can potentially be used to protect optical sensors, including the human eye, from dangerous laser beams. Recently much effort has been invested in the research of organic materials that can behave as nonlinear absorbers because they are, in general, easily integrated into optical devices. C60 itself has been widely investigated for potential application in the field of optical limiting [490, 498, 499]. Effectively, the transmission of fullerene solutions

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decreases by increasing the light intensity. For short pulses (ps), the limiting action is ascribed to pure reverse saturable absorption (RSA), whereas for longer pulses (ns-µs) thermal effects are also invoked [500–505]. Even if fullerene solutions are efficient optical limiters, the use of solid devices is largely preferred for practical applications because they are easier to handle. Therefore, crystalline films of C60 have been studied, but found to be inefficient against pulses longer than tens of ps. This result is ascribed to a fast de-excitation of the laser-created excited state due to the interactions of neighboring C60 molecules in the solid phase [501]. In contrast, it has been shown that C60 keeps its limiting properties after inclusion in solid matrices such as sol–gel glasses [506–508], polymethylmethacrylate (PMMA) matrices [509] and glass–polymer composite samples [510]. As far as sol–gel glasses are concerned, special procedures have to be employed since good solvents for fullerenes are incompatible with the sol–gel process [500, 511–513]. Actually, incorporation of fullerenes is typically achieved by soaking mesoporous silica glasses with a solution of C60 [506–508]. Another efficient approach is to incorporate water-soluble C –60 derivatives, compatible with the sol–gel process, directly into

Figure 5.61 Transmission versus incident fluence at 532 nm of a sol-gel sample containing compound 68.

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the sol [514]. For example, the water soluble methanofullerene 68 (Figure 5.61) has been synthesized [514] and successfully included in a sol–gel during the gelation process. The transmission of the sample at 532 nm as a function of the incoming laser fluence is shown in Figure 5.61. With increasing pulse energy, the transmission of the sample clearly decreases. The threshold for the onset of the limiting action is located at about 3 mJ cm–2, a value comparable or even slightly lower than that obtained with inclusions of C60 in sol–gel matrices [501]. The damage threshold of samples is about 200 mJ cm–2. Up to this fluence, the effect is fully reversible. For higher values, cumulative damage of the C60 molecules was observed in their glass environment. With the same pulse lengths, the threshold value is considerably smaller than that of materials showing simultaneous twophoton-absorption. Even among materials showing RSA, this value is quite good [501] and similar to that found in C60 solutions, confirming the fact that C60 keeps its favorable limiting properties even after chemical modification. However, for sol–gel glasses doped with plain C60 or methanofullerene 68, faster de-excitation dynamics and reduced triplet yields were observed when compared with the solutions. The latter observations have been mainly explained by two factors: (1) perturbation of the molecular energy levels by the interactions with the sol–gel matrix and (2) interactions between neighboring fullerene spheres caused by aggregation [500, 514]. To prevent such undesirable effects, fullerodendrimers 69–70 (Figure 5.62) in which the C60 core is buried in the middle of a dendritic structure [514–519] were prepared and incorporated in sol–gel glasses by soaking mesoporous silica glasses with a solution of 69 and 70 [516].

Figure 5.62 Fullerodendrimers 69 and 70.

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Figure 5.63 Transmission versus incident fluence at 532 nm of a sol-gel sample containing compound 70.

The resulting samples contain only well-dispersed fullerodendrimer molecules. Measurements on the resulting doped samples have revealed efficient optical limiting properties [516]. The transmission as a function of the fluence of the laser pulses is shown in Figure 5.63. It remains nearly constant for fluences lower than 5 mJ cm–2. When the intensity increases above this threshold, the effect of induced absorption appears, and the transmission diminishes rapidly, thus showing the potential of these materials for optical limiting applications. Stable sol–gel glasses have been also prepared from several fullerene derivatives containing both solubilizing chains and siloxane groups (Figure 5.64) [520, 521]. The optical-limiting properties of these derivatives have been investigated both in solution and in sol–gel glasses. By irradiation at 652 nm compounds 71–75 gave better results than nonfunctionalized fullerene, while at 532 nm C60 gave the best result. The photoinduced optical second-harmonic generation (PISHG) and the transmission versus the intensity of the laser beam have been studied in solution and in the solid state for a series of C60-tetrathiafulvalene (TTF) dyads (Figure 5.65) [522–524]. Compounds 76a–b and 77 revealed improved nonlinear optical (NLO) properties when compared with C60. Using molecular dynamics simulations and quantum chemical calculations, the authors have shown that an asymmetry in the excited states influences the NLO properties of these systems as a result of a photoinduced charge redistribution both on the intra- and intermolecular levels. The synthesis and electronic properties of a series of multiple C60 terminated oligo(p-phenylene ethynylene) (OPE) hybrid compounds obtained by an in situ ethynylation method [525] was reported by Tour and coworkers (Figure 5.66). There was evidence of a synergistic interaction between the conjugated OPE backbone and fullerene. C60-OPE hybrid 77 presented an enhanced NLO performance relative to its OPE precursor; this behavior is presumably due to the occurrence of periconjugation and/or charge transfer effects in the excited state.

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Figure 5.64 Fullerene derivatives containing siloxane groups.

Figure 5.65 Fullerene-TTF dyads studied as optical limiters.

Figure 5.66 Fullerene-OPE hybrid 78.

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Figure 5.67 Combination of phthalocyanine and fullerene moieties for optical limiting.

In the optical limiting field, combination of phtalocyanine and fullerene moieties has been made by different researchers [526, 527] in order to provide improved performances through a synergistic effect. The optical limiting behavior of copper phthalocyanine-fullerene dyad 78 [526] (Figure 5.67) has been investigated in solution and as a nanoparticle dispersion using nanosecond laser pulses at 532 nm. An enhanced optical limiting performance of the nanoparticle sample compared with that of the solution sample has been observed. The formation of ordered aggregates with a well-defined ‘face-to-face’ packing fashion is proposed to be responsible for the enhancement of the optical limiting performance of the nanoparticle sample. 5.7.2.2

C60 Derivatives for Photovoltaic Applications

The interaction of C60 with light has attracted considerable interest in the exploration of applications related to photophysical, photochemical and photoinduced charge transfer properties of [60]fullerene derivatives. Following the observation of ultrafast photoinduced electron transfer from S-conjugated polymers to C60 core [528] giving rise to long-lived charge-separated states [529], intensive research programs are focused on the use of fullerene derivatives acting as electron acceptors in organic solar cells. The development of these devices was stimulated by the inherent advantages of organic materials such as their low weight and cost, and by the possibility of fabricating large active surfaces thanks to their processability. After only ten years of studies, it is now clearly established that [60]fullerenebased materials are among the most important candidates for the expansion of plastic solar cells and for renewable sources of electrical energy [530]. Similarly to the photosynthesis process in plants, organic solar cells are based on absorption of light followed by cascades of energy and electron transfer events. Typically, organic solar cells are constituted of at least four distinct layers [531], not counting the substrate, which may be glass or some flexible, transparent polymer (Figure 5.68). On top of the substrate the cathode is laid. Indium tin oxide (ITO), is a popular cathodic material because it is transparent, and glass substrate coated with ITO is commercially available. The cathode is very often aluminum (calcium, magnesium, gold are also used). Inserted (or sandwiched) between the

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Figure 5.68 Typical structure of an organic solar cell (a) and production of photocurrent (b).

two electrodes, the photoactive layer is responsible for light absorption, exciton generation/dissociation and charge carrier diffusion. These heterojunctions are typically fabricated using p-type donor (D) and n-type acceptor (A) semiconductors. A layer of the conductive polymer mixture poly(3,4-ethylenedioxythiophene)/ poly(styrenesulfonate) (PEDOT-PSS) may be applied between the cathode and the active layer. The PEDOT-PSS layer serves several functions. Not only does it serve as a hole transporter and exciton blocker, but it also smooths out the ITO surface, seals the active layer from oxygen, and keeps cathode material from diffusing into the active layer, which can lead to unwanted trap sites. Under illumination, electron transfer from the donor to the acceptor and generation of excitons followed by charge separation and transport of carriers to the electrodes induces a photocurrent (see Figure 5.69). At present, thanks to the invention of N.S. Sariciftci and A.J. Heeger [532], one of the most used acceptors in heterojunction photovoltaic cells is plain C60 or fullerene derivatives. From the theoretical point of view, the principal feature of the p–n heterojunction is the built-in potential at the interface between both materials presenting a difference of electronegativities [532]. In fact, the absorption of light induces the promotion of an electron from the highest occupied molecular orbital (HOMO) [or the valence band (VB)] of the donor to the lowest unoccupied molecular orbital (LUMO) [or the conducting band (CB)] of the acceptor, generating an exciton at the interface of the junction. Then the built-in potential and the associated difference in electronegativities of materials allow the exciton dissociation [533]. The charge separation occurs at D/A interfaces and free charge carriers are transported through semiconducting materials with the electron reaching the cathode (Al) and the hole reaching the anode (ITO). The first heterojunction with a conjugated polymer and C60 was reported in 1993 [534]. In this work, the device ITO/MEH-PPV C60/Au (or Al) was fabricated by sublimation of fullerene onto a MEH-PPV (poly[2-methoxy-5-(2’-ethylhexyloxy)1,4-phenylenevinylene]) layer spin-coated on ITO-covered glass. This solar cell showed a relative high FF of 0.48 and a power conversion efficiency (PCE) of 0.04% under monochromatic illumination.

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In such devices, the interaction between the electron donor and the electron acceptor materials is only effective at the interface, and is limited by the diffusion length of the exciton (near 20 nm maximum). As a consequence, low short-circuit photocurrent (Isc) values and conversion efficiency were obtained. In order to overcome these deficiencies, interpenetrating networks were developed as ideal photovoltaic materials for a high-efficiency photovoltaic conversion. A crucial and major breakthrough towards efficient organic devices was realized by Heeger, Wudl and coworkers with the development of the ‘bulk-heterojunction’ concept [535]; that is an interpenetrating network of a (p-type) donor conjugated polymer and C60 or another fullerene derivative as (n-type) acceptor material. Consequently, the photoactive layer of these solar cells consists of blending the conjugated polymer and the fullerene derivative. The effective interaction between the donor and the acceptor compounds within these so-called ‘bulk-heterojunction’ solar cells can take place in the volume of the entire device. Subsequently, the separated charge carriers are transported to the electrodes via an interpenetrating network. A major shortcoming of this kind of device is the tendency, especially for pristine C60, to phase separate and then to crystallize. This aggregation phenomenon imposes important consequences on the solubility of C60 within a conjugated polymer matrix. For that reason, intensive efforts on organic solar cells rapidly focused on interpenetrated networks of conjugated polymers with the 1-(3-methoxycarbonyl)propyl-1-1-phenyl-[6,6]methanofullerene 80 ([60]PCBM) (Figure 5.69). This compound, initially synthesized by Wudl and coworkers [536], is more soluble in organic solvents than pristine C60. The first example of blend between MEH-PPV and [60]PCBM 80 exhibited a PCE of 2.9% [530], but under monochromatic low intensity light [535].

Figure 5.69 Structure of [60]PCBM 80 and some conjugated polymers commonly used for the preparation of organic solar cells.

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Remarkably, the efficiency of the bulk heterojunction devices consisting of poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-p-phenylene vinylene (MDMO-PPV) and [60]PCBM was increased to 3% by the group of Sariciftci [537]. In the recent years, regioregular polyalkylthiophenes (PAT) which combine the potential advantages of a better photostability and a smaller bandgap than PPV derivatives, were the most widely used S-donor polymers associated with [60]PCBM. Bulk-heterojunction solar cells consisting of poly(3-hexylthiophene), P3HT and [60]PCBM have reached a PCE around 5% [538, 539]. However, for commercial use the efficiency and the stability of the organic photodiodes have to be improved dramatically. For these purposes, S-conjugated polymers with a strong absorption in all visible range and a good stability towards light are needed. Additionally, the initially formed phases between the donor and acceptor have to be fixed by either crosslinking the two compounds or using polymer/polymer mixtures with high Tg since the two phases tend to separate as a result of the the operational heat through illumination, thus reducing progressively the performances of the device. Hence, in parallel to the development of the polymer/fullerene derivative bulk-heterojunctions, C60 functionalized macromolecules [540–544] have been investigated for the preparation of all-polymer solar cells (Figure 5.70) [540, 541]. The controlled incorporation of fullerenes into well-defined linear polymers was achieved by polycondensation of a bifunctional fullerene with diols affording the C60-based polymer 81 with an average polymerization degree of 25 [540]. Photovoltaic cells were prepared by blending this soluble C60-polymer 81 with MDMO-PPV. The device ITO/PEDOT-PSS/MDMOPPV:C60-polymer(1:5)/LiF/Al showed clear photovoltaic behavior (Voc = 0.36 V,

Figure 5.70 C60-functionalized macromolecules for the preparation of all-polymer solar cells.

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Jsc = 0.7 µA cm–2, FF = 0.36) but these weak values of short- and open-circuit currents might be ascribed to the low conductivity of the fullerene polymer. The presence of large solubilizing groups may inhibit interactions between fullerenes, thus decreasing the charge transport [545]. The performances of bulk heterojunction devices obtained from S-conjugated polymers and C60 derivatives are very sensitive to the morphology of the blend [546]. Ideally (to ensure efficient exciton dissociation), an acceptor species should be within the exciton diffusion range from any donor species and vice versa. Moreover, both the donor and the acceptor phases should form a bi-continuous microphase separated network to allow bipolar charge transport. However, the donor and acceptor molecules are usually incompatible and tend to undergo uncontrolled macrophase separation. In particular, phase separation and clustering of the fullerene can occur caused by the operational heat through illumination, thus reducing the effective donor/acceptor interfacial area and the efficiency of the devices [546]. In order to prevent such undesirable effects, it was proposed that the bicontinuous network could be simply obtained by chemically linking a hole-conducting moiety to the electron-conducting fullerene subunit [547]. Based on these considerations, compound 82 in which an oligophenylenevinylene (OPV) moiety is covalently linked to the fullerene sphere (Figure 5.71) was prepared [547]. The use of a fulleropyrrolidine derivative attached to an oligophenylenevinylene can be considered as the first example of a molecular heterojunction specifically designed for photovoltaic conversion. The fulleropyrrolidine OPV-C60 82 was incorporated in a photovoltaic device by spin-casting between aluminum and ITO electrodes. The device ITO/OPV-C60 82/Al delivered a low PCE of 0.01% (Voc = 0.46 V, Jsc = 10 µA cm–2, FF = 0.3) under monochromatic irradiation (400 nm, 12 mW cm–2 ). This study showed that plastic solar cells can be obtained by chemically linking the hole-conducting and the electron-conducting units. The length of the OPV was increased from three to four units and the performances of the ITO//OPV-C60 83/Al were significantly improved with a monochromatic PCE of 0.03%. The limited efficiency of these devices was attributed to the competition between energy transfer and electron transfer [547]. The OPV-C60 hybrid 84 was tested as an active material in photovoltaic cells and the device ITO/PEDOT-PSS/84/Al presented enhanced I/V characteristics (Voc = 0.65 V, Jsc = 235 µA cm–2 under white light illumination at 65 mW cm–2)

Figure 5.71 OPV-C60 hybrids as molecular heterojunctions for photovoltaic conversion.

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(Figure 5.72). Even if these solar cells are not prepared under similar conditions used for OPV-C60 derivatives 82 and 83, the increased donating ability of the OPV moiety is an important argument for the improvement as that of the device performance [548]. Photovoltaic devices were also prepared from oligophenyleneethynylene – C60 (OPE-C60) oligomers 85 and 86. Under light, both devices show clear photovoltaic behavior. Interestingly, the performances of the devices prepared from the N,N-dialkylaniline terminated derivative 86 are significantly improved when compared with those obtained with 85. The latter observation can be related to the differences in their first oxidation potentials. Effectively, due to the increased donating ability of the OPE moiety in 86 when compared with 85, the energy level of the charge separated states resulting from a photoinduced electron transfer is significantly lower in energy. Therefore, the thermodynamic driving force is more favorable, thus electron transfer which is one of the key step for the photocurrent production must be more efficient for 86. As a result, the power conversion efficiency of the devices is increased by one order of magnitude [549]. This clearly demonstrated the interest of the molecular approach, which allows to establish structure/activity relationships.

Figure 5.72 OPV- and OPE-C60 hybrids tested as active material in photovoltaic cells.

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5.7.3 Biological Applications

[60]fullerene derivatives have attracted attention regarding their pharmacological properties since their discovery, but the low solubility of this material in aqueous media was initially an obvious problem for biological studies. Over the past few years, several strategies have been developed to overcome the natural repulsion of fullerenes for water and render them biocompatible. Generally, these strategies including chemical covalent modification of their surface with polar groups as terminal amines, alcohols, carboxylic acids, amino acids [550, 551] and sugars [552], or preparation of water soluble supramolecular complexes with macrocyclic host systems such as cyclodextrin, cyclotriveratrylene or calixerene derivatives [553–556]. Once in solution, fullerenes and derivatives exhibit a wide range of biological activity [557–564]. A potential application of fullerene derivatives is related to the easy photoexcitation of C60 by visible light. The resulting singlet excited-state 1C60 is readily converted to the long-lived triplet 3C60 via intersystem crossing. In the presence of molecular oxygen, the fullerene can decay from its triplet to the ground state, transferring its energy to O2, generating singlet oxygen 1O2, known to be a highly cytotoxic species. Therefore fullerenes constitute an excellent photosensitizer to be used in photodynamic therapy (PDT) [565]. There are two different pathways of DNA photocleavage acting mainly at guanine sites. The generation of singlet oxygen (type II photosensitization) as well as the energy transfer from the triplet excited state of fullerene to bases (type I photosensitization) can be responsible of the oxidation of guanosines and these modifications increase the instability of the phosphodiesteric bond that becomes easily susceptible of alkaline hydrolysis. The first report on the DNA photocleaving activity of fullerenes was made in 1993 by Nakamura and coworkers [566]. The brief light exposure of a mixture of compound 87 (Figure 5.73) and a plasmid DNA resulted in single-strand nicking, and longer exposure led to double-strand cleavage, largely at guanine sites. No cleavage was observed in the dark. Since then, numerous approaches [557–564] have been developed to obtain fullerenes derivatives capable of cleaving the DNA under photoirradiation. Among recent reports, there is the preparation of derivatives 88–91 a–e containing monoand disaccharides (Figure 5.74) [552]. The introduction of sugar moieties improves the biological properties of fullerene because they increase its solubility and they play an important role in cell–cell interaction. The production of singlet oxygen was analyzed by measuring near IR emission at 1270 nm. The bisadducts were

Figure 5.73 Example of a fullerene derivative used for DNA photocleavage.

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Figure 5.74 Water soluble derivatives containing mono- and disaccharides substituents.

less effective in generating singlet oxygen. The treatment of HeLa cells with these derivatives was almost not effective in dark condition, while phototoxicity was more efficient upon incubation with monoadducts. Fullerene derivative 92 presenting a water-soluble E-cyclodextrin as appendage was able to act as DNA-cleavage agent after photoirradiation (Figure 5.75) [567]. The authors studied the mechanism of action and demonstrated the necessity of oxygen in the system. Moreover, EPR studies evidenced the presence of 1O2 as active species in the cleavage process.

Figure 5.75 Fullerene derivative substituted with a E-cyclodextrin subunit.

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Figure 5.76 Fullerene-phorphyrin hybrids.

Fullerene-phorphyrin hybrids possess attractive photoabsorption properties and application of this material in PDT has been realized by some researchers. Comparison between porphyrin, porphyrin-fullerene dyad (P-C60) 93 and metalated porphyrin-fullerene dyad (ZnP-C60) 94 (Figure 5.76) showed that the phototoxic activity decreased from P-C60 to ZnP-C60 and to porphyrin alone. This behavior was found also in anaerobic conditions, demonstrating that in this case both Type I and Type II mechanisms were involved [568]. Fullerene is an excellent acceptor in the ground state and can accept, reversibly, up to six electrons in solution. This property renders C60 an excellent radical scavenger and provides a possible therapeutic approach for some neurodegenerative disorders such as Parkinson’s, Alzheimer’s and Lou Gehrig’s diseases. There is evidence to suggest that these diseases are due to hyper-production of reactive oxygen species (ROS). An important class of fullerene derivatives studied mainly as neuroprotective agents and radical scavengers comprises carboxyfullerenes such as the tris-malonic acid derivatives [569] with C3 or D3 symmetry 95 and 96 (Figure 5.77). Many researchers have investigated the antioxidant mechanism of these two regioisomers

Figure 5.77 Regioisomers of tris-malonic acid derivatives with C3 and D3 symmetry.

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[570, 571]. For the compound C3 the neuroprotective effect was not only related to its ability of scavenging free radicals but also to the nitric oxide synthase inhibition and the possible suppression of toxic cytokines. Polyhydroxylated fullerenes named fullerenols [C60(OH)n] have been shown to be excellent antioxidants. Djordjevic and coworkers analyzed the mechanism of action of C60(OH)2–26 by ESR in presence of 2,2-diphenyl-1-picryhydrazyl (DPPH) free radical and OH radicals generated by Fenton reaction [572]. The addition of fullerenol to the DPPH solution decreased the radical concentration as demonstrated by reduction of EPR signal of DPPH. The same behavior, with better results, was found in the case of OH radicals produced by Fenton reaction. Electron or hydrogen atom donation from fullerenol to free radicals was the proposed mechanism. Fullerenols C60(OH)22, C60(OH)7±2 and C60(OH)24 have been also tested as scavenger of ROS and nitric oxide [573–575]. The ability of E-alanine fullerene derivatives 97–99 in scavenging OH radicals has been studied by chemiluminescence (Figure 5.78) [576]. The radical sponge action is dose-dependent and the three adducts presented a variable scavenger activity. Compound 98 was the best, followed by 99, while 97 was the less effective in the series.

Figure 5.78 E-alanine fullerene derivatives with scavenger activity.

The antibacterial activity of fullerene derivatives was first reported in 1996 for fulleropyrrolidinium salts 100a–c (Figure 5.79) [577]. The hypothesized mechanism of action was attributed to the cell membrane disruption by the bulky carbon cage, which seemed not really adaptable to planar cellular surface. Since this first report, numerous papers have related the potential antimicrobial effects of C60 and derivatives. More recently, Mashino and coworkers [578] studied the antibacterial activity of C60-bis(N,N-dimethylpyrrolidinium salts) regioisomers 101–103 against Escherichia coli and Gram-positive bacteria such as Enterococcus faecalis. The three compounds demonstrated excellent and not significantly different antibacterial activity, which was comparable with that of vancomycin. The mechanism of action seemed to be inhibition of the respiratory chain. In 1993 Wuld and coworkers [579] reported for the first time the inhibition of HIV-protease probably by interaction of fullerene with the hydrophobic active site of the enzyme. Since then, many studies on different fullerene derivatives have been performed [580–584]. Recently Marchesan and coworkers obtained good results against HIV on CEM cells infected by HIV-1(IIIB) or HIV-2 (ROD) using a series of N,N-dimethyl bis fulleropyrrolidinium salts. However, the mechanism involved in the antiviral activity was not elucidated. Analyses in this sense have been performed by Mashino and coworkers. They identified efficient inhibitors of

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Figure 5.79 Fulleropyrrolidinium salts with antibacterial activity.

the HIV reverse transcriptase, more active than nevirapine. The same compounds were found to be active also against hepatitis C virus RNA polymerases [585]. Recent studies have shown that chemically modified fullerenes can still be considered potentially interesting systems for drug delivery [586] and gene therapy [587]. Finally, radiolabeled fullerenes [588, 589] can became nano-vehicles for imaging, diagnosis, therapy and microsurgery because they can be used as contrast agents or radiotracers. 5.7.4 Conclusions

Recent progress in the chemistry of C60 allowed the synthesis of a large variety of fullerene derivatives for various applications in materials science and biology. In the first part of this chapter we have shown that fullerenes are efficient optical limiters and interesting acceptors for photovoltaic applications. In the second part, the biological properties of fullerene derivatives have been summarized. Despite some remarkable recent achievements, it is clear that the examples discussed herein represent only the first steps towards the design of commercial drugs or fullerene-based materials which can display functionality at the macroscopic level. More research in these areas is clearly needed to fully explore the possibilities offered by these compounds.

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6 Carbon Nanotubes 6.1 The Structure and Properties of Carbon Nanotubes

Anke Krueger 6.1.1 Introduction

Another class of strained (hydro)carbon structures consists of only carbon in its pristine state. Tubular objects with a graphitic structure can be considered an extremely unsaturated form of an aromatic hydrocarbon. These so-called carbon nanotubes (CNTs) had been discovered long before they became a major research object for chemists and physicists alike. M. Endo described tubular carbon structures with several walls and coined the name ‘carbon nanotubes’ for these interesting objects (Figure 6.1a) and S. Iijima recognized the concentrically rolled shape of these objects [1]. Originally, they were thought to be just another type of carbon fiber before their unique properties were discovered. It was only in the 1990s that another type of CNT, the single-walled parent systems of the more complex MWNT (multi-walled nanotubes), were described by Iijima (Figure 6.1b) [2]. Their structure was unambiguously determined by electron microscopy (HRTEM, STEM; STM, AFM) and their spectroscopic properties investigated [3]. To understand these materials the knowledge of their structure is of utmost importance. This chapter will discuss the structure of single-walled nanotubes (SWNT) as well as their multi-walled counterparts. Additionally, the aspect of aromaticity will be reviewed. 6.1.2 The Structure of Single-walled Carbon Nanotubes

The basic structural element of SWNT is the graphene plane. This is the basic unit of graphite, which can be formally rolled up to yield a tubular structure in the case of CNTs (Figure 6.1). Of course, the real production of CNTs is carried out in different ways. Various methods including CVD techniques, laser ablation, Strained Hydrocarbons: Beyond the van’t Hoff and Le Bel Hypothesis. Edited by Helena Dodziuk Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31767-7

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Figure 6.1 HRTEM images of (a) MWNT (from Nature 1991, 354, 56), (b) SWNT (courtesy of F. Banhart) and (c) the formal roll-up of graphene to form tubes.

arc discharge, the HiPCo process etc. have been developed for different demands of quality, size and purity of CNT samples [3]. The seamless tubular objects obtained are nevertheless directly related to their planar counterpart (graphene or graphite) concerning their symmetry, spectroscopic and chemical properties, but their unique structure causes various peculiarities as well. In order to understand these features the structure of the different types of CNTs has to be known in detail. Dresselhaus defined a logical system for all types of CNT in order to approach the structural description systematically [4]. The basis of this method is the graphene layer. Depending on the way of seamless roll-up, one obtains different structure types of CNT. There are two privileged directions on the graphene sheet. They are represented by the unit vectors of its unit cell and form an angle of 60° (Figure 6.2). When a nanotube is formed from a graphene sheet, the roll-up is defined by a vector C that corresponds to its circumference after the tube formation (Figure 6.2). The direction of this vector defines the roll-up direction of the graphene sheet with the tube axis being perpendicular to C. The vector C can be described as the vector sum of multiples of the unit vectors a1 and a2 of the graphene lattice with C = n ⋅ a1 + m ⋅ a 2

with n ≥ m and m ≥ 0

(6.1)

A CNT is then unambiguously described by the set of descriptors n and m. Only its length needs to be defined, as all tubes with the same diameter and orientation but different lengths possess the same pair (n, m). It turns out that the length does not have a major importance for most of the real CNTs as they are very long compared to their diameter (up to several microns compared to a few nano-

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Figure 6.2 Construction of CNTs from graphene (top: the vector for a (6,3) tube is inscribed), and chiral angle T and length of the translational unit cell of a (6,3) nanotube (bottom).

meters) [5]. Only in the case of very short tubes do the effects of the ends, such as wall bending, have to be taken into account [6]. The diameter and the length of CNTs are quantified according to the quantification of the underlying graphene lattice. The diameter needs to yield a seamless tube; the length is determined by the numbers of hexagons along the tube axis. Some rules exist for the unambiguous description of CNT by this system. Firstly, m and n are real numbers; secondly, m must be greater than or equal to n in order to avoid redundancy in the description, e.g. a (6,0) nanotube is identical to a (0,6) nanotube. In this case only the orientation of the rolled-up graphene sheet is changed by 60°. Three different types of SWNT can be distinguished (Figure 6.3): When n = 0, the orientation of the vector C is parallel to a1. This type is called zigzag nanotube because it has a zigzag structure at either end. The other archetype nanotube with m = n is called armchair nanotube. Its ends show a pattern that resembles the armchair conformation of cyclic hydrocarbons.

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Figure 6.3 Zigzag, armchair and chiral carbon nanotubes.

In between these two extremes, all sets of m and n shape chiral nanotubes. The chirality originates from a helical roll-up of the graphene sheet and the chiral angle T is defined as   3m T = sin −1  2 2 n + n m + m 

(6.2)

These chiral tubes exist in two enantiomeric forms (left-handed and right-handed roll-up), which have been separated after noncovalent functionalization [7]. The diameter of the nanotube is also defined by the structure parameter set: dCNT =

1 ⋅ a ⋅ n2 + n m + m2 S

(6.3)

According to Equation (6.3), a (5,5) nanotube, a (7,3) nanotube and a (9,0) tube have a very similar diameter of ~0.7 nm, close to the diameter of the archetypical fullerene C60. Although indistinguishable by their size (e.g. in a HRTEM image), these tubes exhibit completely different properties. Whereas the (5,5) armchair tube is electrically conducting, the zigzag (9,0) tube is a semiconductor at absolute zero but conducting at room temperature, whereas the (7,3) nanotube is a semiconductor. The different symmetry of chiral, armchair and zigzag tubes is also responsible for their different Raman spectra [8]. In general, all properties depend on the respective nanotube symmetry. The density of states (see Figure 6.4 for examples) and consequently the electrical conductivity depend on the following conditions. The conductivity is governed by the existence of a gap at the Fermi level. All armchair nanotubes are metallic. In the case of semiconducting tubes a gap is opening between the valence and the conduction band [9]. Those nanotubes with n – m = 3 q (q z 0) possess a small gap at the absolute zero; at room temperature they behave like metallic tubes (Table 6.1) [9].

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Figure 6.4 Density of states (DOS) of a (9,0) and a (10,0) carbon nanotube [12]. In the case of the (9,0) tube the density of states is not zero at the Fermi level. Table 6.1 Electrical conductivity as a function of structure parameters m and n [9]. Nanotube

Conditions

Electrically conducting metallic nanotubes

n=m

Semiconducting nanotubes with a small band gap (conducting at room temperature)

n – m = 3 q (q z 0)

Semiconducting nanotubes

n – m z 3q

In real samples of CNTs these types coexist, and so far it has not been possible to produce a certain type or even tubes with a defined set (m,n) selectively [10]. Only the enrichment was achieved by different methods including electrophoresis [11] and selective functionalization (see next section). Another important aspect of the CNT structure is the construction of the tips. The formal roll-up of a graphene sheet would lead to unsaturated bonds at the

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Figure 6.5 Short carbon nanotubes with functionalized tips; (a) hydrogenated, (b) oxidized tip. In the case of the short hydrogenated tube the widening of the ends is highlighted.

terminal carbon atoms at both rims and therefore to an elevated and unfavorable energetic state. In reality, the ends of CNTs show many different ways to overcome the dangling bonds and to produce a valence-saturated structure. The simplest would be the saturation of all dangling bonds with hydrogen (Figure 6.5a). Calculations by Schleyer et al. showed that short fragments of CNTs with hydrogenated ends are widened at their tips and no longer exhibit a perfect tubular shape [6]. Other functional groups can be also present at the tips of CNTs (Figure 6.5b). They are usually introduced by oxidative reagents, e.g. mineral acids, and are often used to introduce more complex moieties at the CNT surface (see Section 6.2). After production, the tips of SWNTs are usually closed by caps of different shape (see Figure 6.6). In principle, it is possible to postulate partial fullerene structures that fit exactly to the rim to be covered with a cap. Fujita and Dresselhaus have developed a formalism for the construction of CNT tips by arranging five-membered rings on the graphene sheet at the ends of the tube shell (Figure 6.6b) [13]. The five-membered rings are responsible for the curvature of the cap and the formation of a partial fullerene structure. The opening angle of the cap depends linearly on the number of five-membered rings in the fullerenic cap (Table 6.2) [14]. In the case of the archetypical zigzag (9,0) and armchair (5,5) carbon nanotubes it is theoretically possible to cover their ends with cut C60 molecules. The fullerene hemisphere has to be selected according to the rim structure of the nanotube (Figure 6.7). Although highly symmetrical, those structures are rarely formed. Most often somewhat irregular nanotube caps are observed, often having sharp ends [15]. Real CNTs are usually not defect free (Figure 6.8). In addition to the imperfect structures at their ends they have also defects in the sidewalls. Several types of these defects are known. Missing carbon atoms create holes in the graphene network of the wall, disturbing the electronic properties as well as the S-conjugation Table 6.2 Opening angle of the nanotube cap as a function of five-membered rings in the cap. Number of five-membered ring Opening angle (deg.)

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1

2

3

4

5

112.9

83.6

60.0

38.9

19.2

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Figure 6.6 (a) HRTEM image of open (arrows) and closed SWNTs (from Chem. Phys. Lett. 2000, 316, 349). (b) An example for the construction of a theoretically possible cap according to the method of Fujita and Dresselhaus. Highlighted hexagons represent the positions of pentagons in the cap (following the scheme in the lower right corner a segment from these hexagons is removed). Superimposing the fields with equal numbers results in the formation of the curved cap.

Figure 6.7 Capping of a (5,5) and a (9,0) carbon nanotube by appropriate half-spheres of C60. The rims of the tubes are marked grey.

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Figure 6.8 Defects in the side wall of CNTs. (a) Holes; (b) Stone–Wales defect (side and top view) with the highest reactivity at the borders of the defect (highlighted in grey); (c) the migration of Stone–Wales defects (top) and Stone–Wales rearrangement (bottom).

(see below) [16]. Another defect consists in the substitution of six-membered rings by pairs of five-membered and seven-membered rings. This so-called Stone–Wales defect is usually built by two sets of these rings (Figure 6.8) [17]. It can be produced either by a rearrangement where a bond between two six-membered rings is turned by 90° or by the addition of a C2 unit to the graphene network [18]. In summary, the effect of a Stone–Wales defect on the curvature is zero as the convex contribution of the pentagons is compensated by the concave contribution of the heptagons. These rings do not necessarily occupy neighboring positions. In a rearrangement reaction including a four-membered Hückel transition state they migrate away from each other leading to bent or even coiled nanotubes, if a certain number of these defects is positioned in a regular manner [19]. For example, a pentagon on one (convex) side of the nanotube and a heptagon on the other (concave) side leads to a kink in the respective nanotube. In the case of a complete Stone–Wales defect the curvature is not evenly distributed over the whole defect structure. Thus the reactivity of the different bonds depends on the prehybridization and strain exerted by the additional curvature (see Figure 6.8). 6.1.3 The Structure of Multi-walled Carbon Nanotubes

The structure of MWNTs is closely related to that of SWNTs. Each wall of the MWNT represents a SWNT, only their diameters differ. The whole object is a

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combination of these walls and the main question is whether there are interactions between the single layers of a MWNT. The inter-wall distance in MWNT is mostly slightly larger than the distance between the graphene sheets in graphite [20]. This indicates that the interactions between the walls should be somewhat weaker than in graphite. Inspection of the possible interactions between the walls in different types of MWNT reveals a fundamental difficulty: The interlayer distance of 0.34 nm defines a certain increase of 'u = 2.14 nm in circumference (because of the increase of 'd = 0.68 nm in diameter). Depending on the type of CNT this can be expressed in multiples of the basic unit of the circumferential vector C. In the case of armchair nanotubes this corresponds well to 'u = 5 · 0.426 nm. On the other hand, it is difficult to produce a good fit for zigzag tubes as the nearest multiple is 'u = 9 · 0.246 nm = 2.214 nm. With this increase in diameter the interwall distance increases to 0.352 nm. A real MWNT will rarely be formed of just one type of CNT, making the situation even more complex. Chiral tubes are very unlikely to fit with their neighboring tubes and a mixture of zigzag, armchair and chiral tubes is usually present in real MWNTs [21]. As the interaction between the walls is rather weak, the mobility of the inner walls is only slightly hindered and rotation is possible [22]. This is also responsible for the ‘sword in a sheath’ phenomenon which is observed when a MWNT filled composite is mechanically strained in the direction of the MWNT [23]. Multi-walled carbon nanotubes show mixed electronic behavior [24]. Some authors state that the outermost wall determines whether a tube is metallic or semiconducting [25]. In similarity to single-walled CNTs, MWNTs exhibit even more complex structures at the ends of the cylindrical body. Especially, the presence of sevenmembered rings in addition to the usual five-membered rings, opens many possibilities for tip structures. Mostly, slightly unsymmetrical tips are observed, beak-like structures as well as so-called ‘acute’ tips with sharp ends can also be found (Figure 6.9) [26]. Another feature of real MWNTs is the occurrence of internal structures. These include inner caps as well as bamboo-like structures (Figure 6.10). These features can be formed by one or more internal walls of the MWNT [27]. In some cases one observes toroidal structures at the tips of opened MWNT. The neighboring walls are joined by C–C bonds, diminishing the number of dangling bonds [28].

Figure 6.9 Cap structures of real MWNTs.

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Figure 6.10 Internal structures of multi-walled carbon nanotubes. Internal caps at a bend (left) and HRTEM image of bamboo-like carbon structures (right). (From Chem. Phys. Lett. 2000, 323, 560).

Figure 6.11 Electron-microscopic images of coiled multi-walled carbon nanotubes (from J. Mater. Res. 2000, 15, 808 (top) and Appl. Phys. Lett. 2002, 81, 3567 (bottom)).

Coiled and bent MWNTs have been also found in nanotube samples. These defects are formed by the existence of five- and seven-membered rings in the graphene network (see above) [19]. With appropriate conditions, strongly coiled MWNTs can be produced in macroscopic amounts (Figure 6.11) [29]. In comparison to SWNTs the defectiveness of the outer walls of a MWNT does not affect the mechanical and chemical properties in the same dramatic way. Even if the outermost walls have holes, the tube shape remains still closed as the inner walls are still intact. On the other hand, the electronic properties are

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affected more seriously because of the major contribution of the outer shell to the electrical transport [30]. 6.1.4 The Aromaticity of Carbon Nanotubes

When we regard carbon nanotubes as a type of highly unsaturated form of hydrocarbon the issue of aromaticity needs to be discussed. The concept of aromaticity is difficult to apply to carbon nanotubes as the conjugated system shows a significant curvature along its circumference. On the other hand there is no curvature in the direction of the tube axis. Is a nanotube aromatic or not? The question has to be answered whether a sufficient delocalization of the S-electrons is realized or not. According to the bond lengths, conjugation reaches a high level as there is no significant alternation of single and double bonds [31]. For the discussion it is useful to neglect the effects of the tips in the first instance as they represent defects in the perfect structure of a straight, defect-free CNT. The endless graphene layer, from which a hypothetical perfect nanotube is formally derived, possesses a planar, fully delocalized S-electron cloud and therefore can be considered to be aromatic [32]. In the case of the rolled-up nanotube the participating electrons and their orbitals are still the same. Only the orientation and to some extent the shape of the S-orbitals is different. A 3D structure is formed and the conjugation is weakened by the radial distribution of the S-orbitals (Figure 6.12). Along the tube axis the extent of conjugation remains the same but in the circumferential direction it changes the more, the smaller the diameter of the nanotube [31, 33].

Figure 6.12 Orientation and conjugation of S-orbitals in graphene and CNTs.

It has been shown that the form of possible resonance structures depends on the set of structure parameters (m, n). Metallic tubes with m – n mod 3 = R and R = 0 (n mod x yields the remainder of the division of n by x) can be described with a Clar resonance structure consisting only of complete benzene rings. These are fully conjugated [34]. All other tubes with R = 1 or R = 2 (m – n not divisible by 3) have at least one seam of isolated double bonds in their resonance structure (Figure 6.13) [34]. These tubes are semiconducting. A special case are the tubes with (m, 0) and R = 1. Here it would be possible to construct a chiral Clar resonance structure with the double bond seam as the chiral element. To avoid this, a structure is formulated that shows an achiral quinoidal seam (Figure 6.13, right).

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Figure 6.13 Resonance structures for a (12,9), a (12,8), a (12,7) and a (19,0) nanotube (from J. Org. Chem. 2004, 69, 4287).

The calculation of NICS (nucleus independent chemical shift) values by Ormsby and King has shown the validity of the model and the dependence of the electronic properties on the tube diameter and the graphene sheet orientation [35]. In reality, the nanotubes are not endless and therefore some restrictions apply to the number of their S-electrons and the options for fully delocalized systems. To assess the S-electron delocalization, the resonance structures should be inspected for finite pieces of CNTs. Nakamura showed that the length of the tube fragment plays an important role in the full delocalization of the S-electrons in the case of armchair CNTs [36]. Only in the case of a length that accommodates full aromatic sextets can a complete conjugation be achieved. Otherwise, complete and incomplete Clar structures best represent the situation in finite CNTs. Interestingly, the HOMO-LUMO gap and the frontier orbital energy oscillate as well (Figure 6.14).

Figure 6.14 Resonance structures in finite armchair CNTs.

Functionalization has an important influence on the conjugation of the S-electrons [37]. Depending on the addition mode of reactants, the conjugation can be diminished or even completely destroyed. In all cases this has an influence on the electronic properties of the tube as well. Metallic CNTs become semiconducting when they are functionalized on the side walls. The conjugation is interrupted at the position of an addition reaction.

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Similar effects are observed for defects in the side wall. The conjugation is broken at places where carbon atoms are missing or where the graphene network is interrupted by the existence of ring defects or functional groups [38]. 6.1.5 Conclusions

In summary, the tubular structure of CNTs is unique in the field of highly unsaturated hydrocarbons. Depending on the structure parameters quite different properties are observed. Fully conjugated S-systems show metallic conductivity whereas semiconductivity occurs with isolated double bonds in the resonance structure of the respective tube. The curvature induces an increased reactivity of the carbon atoms. In the next section the reactivity of these highly inspiring molecules will be discussed. The three-dimensional S-system allows for a variety of useful reactions and the application of CNTs in various field, e.g electronics, composites, and biomedical applications. In order to achieve these high expectations, it will be necessary to produce homogeneous samples of CNTs with defined electronic properties, i.e. defined structure parameters. So far only mixtures are available, but significant progress has been made towards the separation of different types of nanotubes.

6.2 The Functionalization of Carbon Nanotubes

Anke Krueger 6.2.1 Introduction

Functionalization of materials is one of the most important research areas. It is only after appropriate modification of the pristine material’s properties that many applications open up for the new compounds or composites. Especially for biological applications such as drug delivery or sensing applications, a suitable and stable functionalization is of importance. It is only natural that for carbon nanotubes (CNT) the research on their surface modification began just shortly after their production in sufficient amounts. The work has continued ever since then, as a vast range of reactions has been tested and various new materials have been developed. In the following sections we will discuss the different types of surface modification for CNTs. In this, MWNTs will only be separately mentioned if a significant difference is observed compared with SWNTs. Normally, the behavior of these two classes of CNTs is rather similar. Only large MWNTs with decreased curvature show lower reactivity because there is a smaller prehybridization of the carbon atoms, i.e. a smaller deviation from sp2 towards sp3 hybridization caused by curvature.

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Figure 6.15 Functionalization types for carbon nanotubes.

The topology of CNTs permits different kinds of modification: reaction at the tips, at the outer or inner wall or the inclusion of objects in the internal space (Figure 6.15). Functionalization can be realized by covalent bonds or by noncovalent interactions, e.g. electrostatic forces [39]. Compared with the fullerenes, there are some structural differences. In fullerenes the six-membered rings are partially connected to five-membered rings inducing three-dimensional curvature and strain, whereas CNTs are not curved along their tube axis. Additionally, the diameter of the tubes is often (especially in the case of MWNTs) larger than for the archetypical fullerene C60. Therefore, the reactivity of CNTs should be reduced compared with C60, which is actually the case [40]. Many of the reactions described for fullerenes can be carried out with CNTs as well but need harsher conditions and/or longer reaction times [41]. Only the tips of carbon nanotubes exhibit a comparable reactivity because of the existence of pentagons [42]. A similar effect is observed for the so-called Stone–Wales defect. They consist of two pentagons and two heptagons (see former Section) and some of their bonds are much more strained than the usual bonds in CNT [43]. 6.2.2 Functionalization of the Nanotube Tips

The first procedures for the functionalization of CNTs were closely related to their purification. It was observed that reaction with oxidizing acids or piranha water (mixture of sulfuric acid and hydrogen peroxide) not only opened the nanotube tips but introduced carboxylic groups at the rims of the cylindrical structures [44]. It is obvious why this reaction preferably takes place at the ends of the tubes. The higher strain and prehybridization induced by the five-membered rings in the caps cause a higher reactivity. The carbon is oxidized and the caps removed. The resulting dangling bonds are saturated with carboxylic groups, which can further react with, for example, amines or alcohols to form amides or esters [45]. Using long-chain alkyl alcohols or amines for this purpose results in solubility of the modified tubes in organic solvents (Figure 6.16) [46]. Solubility of the CNTs in reaction or physiological media (important for future bioapplications) is one of the most challenging and important issues of CNT research [47]. Certain reactions only take place in solution; homogeneous distribution is possible and applications in blood or serum depend on stable solutions of the nanotubes. For nanotubes there are several approaches to this problem: Addition of surface active compounds [48] or coating of the tube with alkyl chains [49] or hydrophilic groups [50] result in improved dispersibility.

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Figure 6.16 Functionalization of the tips of a CNT.

Under the reaction conditions that are necessary to remove the caps of CNTs, a large number of defects is created in the side walls of the tubes [51]. Especially at the position of already existing small holes, larger defects are produced by oxidative removal of carbon atoms. The rims of these holes are also covered with carboxylic groups (Figure 6.17).

Figure 6.17 Defects in the side wall carry carboxylic groups at the rims after oxidative treatment. Further grafting of e.g. alkyl chains via amidation or esterification leads to improved dispersibility.

6.2.3 Non-covalent Functionalization of Carbon Nanotubes

As fully unsaturated hydrocarbons, carbon nanotubes are strongly hydrophobic. They are therefore not dispersible in any polar solvent or water. But they can undergo strong interactions with other hydrophobic compounds, especially when these are able to form S–S interactions [52]. Benzene and other small aromatic compounds do not establish very strong interactions with the nanotube surface.

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Figure 6.18 Arrangement of pyrene molecules along the nanotube axis (left); a larger planar aromatic system does not improve the interaction with the nanotube (right).

But condensed aromatic compounds such as pyrene are highly suitable for this kind of surface modification [53]. Pyrene with its elongated shape is able to establish S–S interactions throughout its S-system. It can arrange itself with its long axis parallel to the tube axis in order to maximize the binding force (Figure 6.18). Bigger planar aromatics are not necessarily more strongly bound because of the increasing distance between the S-system of the nanotube and the ends of the aromatic compound (Figure 6.18) [54]. A perfect fit would be achieved with a curved structure that imitates the curvature of the respective nanotube. In this case it would be possible to observe selective interactions with the nanotubes of the best-fitting diameter. Depending on the functionalization of the pyrenes that are interacting with the nanotubes it is possible to tune the hydrophilicity of the resulting composite resulting ultimately in water solubility (Figure 6.19) [55], debundling [56] or solubility in organic solvents if the pyrenes carry long side chains [57]. At the same time they influence the band structure of the functionalized nanotube as can be seen from the resulting absorption spectra (Figure 6.20) [58]. Polymers bearing pyrenes in their side chains can be mixed with nanotubes to yield a non-covalently bound composite material that unites the properties of the nanotubes with those of the polymer [59].

Figure 6.19 Functionalization of a CNT with pyrene derivatives. The pyrene moieties are arranged along the tube axis. Depending on the side chain the conjugates are dispersible in different media.

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Figure 6.20 Absorption spectra of CNTs before and after functionalization with DomP (from J. Am. Chem. Soc. 2004, 126, 10234).

Similar to pyrenes, phthalocyanins and porphyrins interact with the CNT surface via van der Waals interactions and S-stacking [60]. They can be used to study the energy transfer from the porphyrin skeleton to the nanotube. In 2004 Sun et al. reported on the selective interaction of semiconducting nanotubes while the metallic ones remained unchanged and insoluble [61]. Not only organic molecules can be deposited on the nanotube surface but also metallic clusters. Usually, this requires the addition of a reducing agent (except for electrodeposition) and a thorough surface cleaning of the nanotube surface [62]. Deposited clusters include gold, platinum and palladium nanoparticles, zinc and magnesium oxide [63]. These composites are interesting for catalytic applications. Another topologically completely different approach is taken with long, chainlike molecules, in general polymeric materials. These can be wrapped around single nanotubes or small bundles [64]. Usually the interaction is based on electrostatic forces or S-stacking. Examples (Figure 6.21) for this functionalization method include the wrapping of CNTs with amylose, peptides, polyaniline and polymers

Figure 6.21 Non-covalent wrapping of a CNT with hydrophilic amylose (lower left) or hydrophobic PmPV (lower right).

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such as PVP (polyvinylpyrrolidone) or PmPV (poly-m-phenylenevinylidene) [65]. The latter produces interesting composites with CNTs where the electrical conductivity of the composite is significantly increased whereas the luminescence properties of PmPV remain basically unchanged [66]. The electrical conductivity is also increased in polyaniline (PANI) composites [67]. The circular and sizeselective complexation of CNTs is achieved with cyclodextrins [68, 69]. 6.2.4 Covalent Side-wall Functionalization of Carbon Nanotubes

Instead of using electrostatic interactions and S-stacking, CNTs can also be functionalized covalently at their side-walls. The conceptually easiest way of modifying the surface structure of a CNT consists in reducing the number of unsaturated bonds, and therefore its hydrogenation (Figure 6.22). So far, a completely hydrogenated CNT has not been reported. Only partial reduction was observed after reaction in a hydrogen plasma or with lithium in liquid ammonia (Birch type reaction), after high temperature treatment in a hydrogen atmosphere or electrochemical hydrogenation [70, 71]. According to calculations a completely hydrogenated CNT should be stable up to a diameter of 1.25 nm [72].

Figure 6.22 Hydrogenation of CNTs.

In close resemblance to the hydrogenation, nanotubes can also be halogenated. Especially the reaction with fluorine yields samples with a high halogen content of up to 100% [73]. This does not mean that each carbon is carrying exactly one fluorine atom, but there are positions like the tips and defects where the fluorination grade is elevated. The reaction is reversible; treatment with hydrazine yields the clean nanotubes after annealing [74]. The fluorination reaction was used to study the addition mechanism of the addition to CNTs. Usually, the fluorine atoms are positioned close to each other and further fluorination continues along the circumference, not the axis of the tube [75]. This results in a nonregular distribution of highly fluorinated areas and some with almost no fluorine. One explanation for this behavior is the preferred 1,4-addition (over 1,2-addition), which is also corroborated by computational results (Figure 6.23) [76]. The electronic properties of fluorinated CNT as very good insulators strengthen the argumentation, too [77]. The 1,4-addition destroys the conjugation of the S-bonds over the whole diameter. Fluorinated CNTs can be used in further reactions such as nucleophilic substitutions with amines or carbanions [78]. Other halogens are much less reactive towards CNTs. Only an electrochemical method for the halogenation has been described [79].

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Figure 6.23 Fluorination of CNTs (1,4- vs. 1,2 reaction).

Figure 6.24 Bingel reaction on CNTs and further modification of the ester groups.

Another important reaction of CNTs is the nucleophilic attack by bromomalonates in the presence of a base (Figure 6.24). This so-called Bingel reaction is one of the most versatile reactions in fullerene chemistry and can be applied to CNTs, although only under harsher conditions [80]. The ester groups of the malonate can be modified in various ways, enabling a vast range of further CNT derivatives and improved solubility [81, 82]. In general, the reaction with nucleophiles as described for fullerenes can be transferred to CNTs (e.g. addition of carbenes and nitrenes [83]), but the lower reactivity in some cases remains an obstacle to efficient functionalization. Radical reactions are another option for the surface grafting of organic moieties. Perfluoroalkyl radicals as well as acyloyls radical have been used for the modification of the nanotubes surface [84]. The reaction with diazonium salts is a very important reaction of CNTs. The intermediate phenyl radicals directly bind to the tube by C–C bonds [85]. Strano et al. reported on the selective functionalization of metallic tubes with this reaction [86]. CNTs are good candidates for cycloaddition reactions because they have a system of conjugated double bonds (Figure 6.25). They always react as the -ene component, even in [4+2] cycloadditions [87]. It is rather unlikely that a reaction as the diene component would be observed, because the nanotube has unfavorable geometry and also is electron deficient [88]. The most common cycloaddition on CNTs is

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Figure 6.25 Cycloaddition reactions of CNTs.

the Prato reaction, that has also been extensively studied in the case of fullerenes [89]. This 1,3-dipolar cyclodaddition of azomethineylides yields functionalized pyrrolidine rings on the nanotube surface. This reaction accepts a broad range of functional groups in the sidechains. That is why this reaction is often used to immobilize biological structures like DNA, antibodies or viruses on CNTs for applications such as drug or antibody delivery [90]. Solubility in physiological media is achieved with bridging oligoethylenglycol moieties [91]. Additionally, the reactions with nitrile imines and ozone are [3+2] cycloadditions. The latter can be used to establish carboxylic, keto or hydroxylic groups at the nanotube surface depending on the selected work-up [92]. Compared to [3+2] cycloadditions, the [4+2] addition has rarely been reported. Only a few examples using o-chinodimethanes have been described [93]. Fluorinated CNTs are better dienophiles because of the strain induced by the sp3 carbon atoms that exist in the neighborhood of the remaining double bonds and the electron-withdrawing properties of the fluorine substituents [94]. [2+2] cycloadditions to CNTs have not been reported so far.

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Figure 6.26 Transition metal complexes with CNTs have so far been reported only via functional groups (right). Dihapto and hexahapto complexes are difficult to achieve due to the weak donor character of the tubes and the unfavorable geometry.

Altogether the cycloaddition chemistry of CNTs is less pronounced than for fullerenes. The lower curvature and the lower electrophilicity play the key roles for the lower reactivity. An interesting question is whether CNTs are able to form coordination complexes with transition metals. Their double bonds could act as S-donors in these compounds. So far, only complexes via functional groups of the nanotube have been described, such as the CNT–Wilkinson complex conjugate (Figure 6.26) [95]. These derivatives are interesting objects for catalyst research. The reasons for the inability to complex the metal centers directly at the nanotube are the better conjugation of the double bonds, the absence of five-membered rings and the larger HOMO-LUMO gap compared with C60 [96]. 6.2.5 Endohedral Functionalization of Carbon Nanotubes

After the removal of the caps from the nanotube tips the inner space becomes accessible. Many reports on the endohedral functionalization of carbon nanotubes have been published [97]. Most of these results have been obtained for SWNTs. Small molecules as well as metals [98] or metal oxides [99] can be included in the tube. Besides, organic molecules can be filled into CNTs. These include large molecules such as polypyrrole or carotene [100] and small structures like metallocenes [101] and aromatic compounds [102, 103]. A very special example for the endohedral functionalization of SWNTs is the inclusion of fullerenes inside CNTs [104]. The resulting material is a ‘carbon-only’ compound with a tube-like shell and dot-shaped inclusions. For a long time it was believed that covalent functionalization of the nanotube wall is prevented by the small orbital lobes inside the tube. However, Gao et al. reported on the grafting of amino groups inside CNTs [105].

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6.2.6 Conclusions

In summary, the chemistry of carbon nanotubes is a vast research area. There are many options for the modification of both SWNTs and MWNTs. These include the removal of the caps with subsequent formation of carboxylic groups, the endohedral inclusion of small clusters and molecules, and the side-wall functionalization at defects or regular double bonds. This can be done with surface active agents leading to non-covalent composites of nanotubes with organic molecules. Metal clusters can be deposited on the CNT surface and wrapping with macromolecular compounds results in better dispersibility of the tubes. In the case of SWNTs this can even break up the bundles of tubes. Covalent surface modification can make use of existing functional groups or directly use the conjugated double bonds of the graphene lattice. Cycloadditions, radical additions of diazonium salt etc. and the Bingel reaction are examples for the efficient modification of the carbon nanotube surface. All these reactions lead to new materials with interesting properties, e.g. the selective functionalization of metallic or semiconducting tubes. For new applications in composites or electronics these features are very useful.

6.3 Applications of Carbon Nanotubes

Marc Monthioux 6.3.1 Introduction

Carbon nanofilaments – or nanotubes – were actually very probably the core of the much larger vapor-grown carbon filaments used by Edison to operate the early version of his light-bulb. However, although carbon nanotubes (CNTs) were unambiguously revealed as early as 1952 [1], and shown to exhibit diameters as low as 3–4 nm as early as 1976 [1], they have only been thought to be useful for a wide range of applications from the 1990s, i.e. from the time of the well-known paper by Iijima [1]. In this regard, CNTs are a good example of availability being not the only condition for a material to be considered of some interest from the technological point of view. Both technology and the minds (of scientists and engineers) have to be prepared, specifically when downsizing is involved [106]. Fifty years ago, vapor-grown carbon (nano)filaments were mostly undesirable by-products of industrial processing that were studied to be avoided. Nanotubes are nowadays the most popular nanomaterials, as measured from the number of scientific articles published yearly, and from the amount of investment dedicated to related R&D. Consequently, applications involving CNTs are now coming to the market although they are probably still too few with respect to the expectations. This section will briefly address these aspects, along with providing examples of applications.

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6.3.2 Properties of CNTs 6.3.2.1

Which CNT for Which Application?

CNTs were considered very promising because they exhibit extreme properties in almost every aspect. Table 6.3 provides the most important ones, as gathered from literature. As with other kinds of graphene-based carbon materials (otherwise called polyaromatic carbons, including graphite), the properties of CNTs are determined by the level of anisotropy, i.e. the extent to which the graphenes are aligned with respect to the direction at which the property is measured, because graphene and graphene stacks exhibit high intrinsic anisotropy (Table 6.4). Table 6.3 Some extreme properties of CNTs. Property

Values

Aspect ratio

~1000

Comments Diameter > 0.4 nm, length up to cm 2

–1

Specific surface area

~2700 m g

The highest!

Tensile strength

> 45 GPa

The highest ever!

Tensile modulus

1–1.3 TPa

The highest!

Tensile strain

> 40%

Provides the highest toughness ever!

Thermal stability

> 3000 °C 4

7

In oxygen-free atmosphere

Electrical conductivity

10 –10 S cm

Transport regime

Ballistic

–1

Thermal conductivity Electron emission

Better than that of copper Induced superconductivity with Tc < 1 K

–1

~6000 W mK 6

12

–2

10 –10 A cm

Higher than those of diamond or graphite Highest current density!

Table 6.4 Properties for a stack of perfect graphenes (except the tensile strength-to-failure, calculated for a single graphene) as measured parallel or perpendicular to the graphene plane direction. Direction in graphene (stack) Bond energy

kJ mole–1

Perpendicular

524

7

Thermal expansion

°C

1.5 · 10

27 · 10–6

Thermal conductivity

W (m · K)–1

3000

8

Electrical resistivity

: cm

3.8 · 10–5

10–2

Young modulus

GPa

> 1000

~50

Strength-to-failure

GPa

~100

?

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–1

Parallel

–6

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Hence, properties are maximized along the axis of single-wall CNTs (SWNTs) because anisotropy is at maximum (or ‘assumes maximum value’) on the one hand, and SWNT structure is close to perfection on the other hand, with only pentagons or heptagons (see the preceding section) as possible structural defects (vacancies are minor in pristine SWNTs). The fact that SWNTs can be as perfect as a (macro)molecule makes a major difference with regular materials. The situation is quite different for multi-wall CNTs (MWNTs), in which all kinds of orientation of graphene stacks can be found with respect to the nanofilament axis, from fully parallel (as in MWNTs with concentric texture) to fully perpendicular (as in nanofibers with platelet texture), including combinations of both (as in MWNTs with bamboo texture). In addition, they may be affected by a much larger variety of possible structural defects, such as sp3 carbons, heteroatoms, disclinations, in addition to other than six-membered rings as in SWNTs, making graphenes in graphene stacks and the mutual orientation of the latter more or less perfect. Hence, a much wider range of property values can be found for MWNTs. An interesting property of CNTs is their intrinsic biocompatibility, which is of importance not only for medical applications but also for safety issues. Such biocompatibility has long been known for pyrolytic carbon or graphite, because they contain a limited amount of accessible graphene edges, which actually are the reactive sites, as opposed to the graphene plane. In this regard, although the reactivity of SWNTs or concentric MWNTs is low (for being free of graphene edges), using high purity materials is compulsory, otherwise carbon or noncarbon impurities will affect the reactivity, and hence the biocompatibility, of the whole. These brief comments explain why research efforts regarding the synthesis of nanotubes ultimately aim at high-yield production of SWNTs, preferably to MWNTs, although production capacity is still much larger for the latter so far. It also illustrates that, considering carbon nanotube properties (physical as well as chemical, including reactivity), it is of utmost importance to define which kind of nanotubes is involved. 6.3.2.2

Why is ‘Nano’ Beautiful?

As mentioned before, extreme properties of CNTs come from their high anisotropy. But other highly anisotropic polyaromatic carbon materials, carbon fibers, known for long time, also exhibit extreme properties (sometimes close to those of CNTs, e.g. Young’s modulus, thermal conductivity, etc.). They have already found many applications, from daily-life ones, such as sports goods, to high-tech uses in space shuttles. One might then wonder whether it is worth investing millions of dollars worldwide in the R&D of CNTs. Reasons for this are multiple: x CNTs exhibit a much higher specific surface area, thus a much higher surface of interaction, than carbon fibers. This is especially useful not only for composites but also for applications where the specific reactivity of CNTs is exploited (e.g. sensors).

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x CNTs exhibit a much higher aspect ratio than carbon fibers. This feature is very favorable for applications where they are used as a network, e.g. as fillers in electrically or thermally conductive composites built with an insulating matrix (e.g. most of polymers). It is actually well-known that the percolation threshold (i.e. the proportion of filler from which the conductivity suddenly jumps) decreases as the aspect ratio increases. x CNTs have a much better propensity to structural perfection than carbon fibers, especially when considering SWNTs or possibly concentric MWNTs. This provides some of the as-prepared CNTs with ultimate properties, as opposed to carbon fibers which contain numerous and various kinds of defects (Figure 6.27). x Rather simple, mostly single step, fabrication processes can be used to prepare CNTs whereas carbon fibers require complex, multi-step processes. This makes the synthesis of CNTs a much more affordable technology, regarding both the needed equipment and the level of manpower qualification. x Contrary to the reagents used in production of carbon fibers, feedstocks for carbon nanotube synthesis are basic compounds with perfectly defined chemical composition, easy availability, and constant quality (CH4, C2H2, CO, graphite, pure metal catalysts, …). Whereas the first three reasons relate to the intrinsic superiority of CNTs over carbon fibers, the last two relate to the superiority of their synthesis processes, which are likely to ultimately result in much cheaper materials.

Figure 6.27 Tensile mechanical properties of the fibers available on the market, among the most performing ones. Circles and triangles are carbon fiber, polyacrilonitrile-based and pitch-based, respectively. Only carbon nanotubes (SWNT) are able to exhibit both high modulus (1–1.2 TPa) and high strength (45 GPa or more). (Modified from [107]).

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6.3.2.3

Potential Problems Related to the Use of CNTs

However, there are obviously still unsolved problems that prevent CNTs from overwhelming the market: x The most important one is quality. Many marketed CNTs exhibit low purity, since they come along with other carbon nanophases (amorphous carbon, fullerenoids, polyaromatic carbon shells, nano-horns …) and catalyst and/or solvent remnants (Figure 6.28). Enriching in CNT content is a technological challenge because purification processes based on chemicals are obviously more or less harmful to the CNT structure, because the CNTs are chemically similar to some of the various phases to be discarded. Likewise, all marketed nanotube grades exhibit poor selectivity, e.g. regarding the helicity of SWNTs (which controls the metallic vs semi-conductor behavior), CNT diameters, number of walls in MWNTs … (Figure 6.29). x Processing is another major issue because the aspect ratio of nanotubes makes them entangle (except for the carpet-type growth mode, where CNTs are aligned and perpendicular to the substrate). Meanwhile, CNT nanosize makes them sensitive to weak surface forces and clump upon any attempt to mix and disperse them into liquids (e.g. solvents, matrix precursors) using regular mixing procedures (e.g. see Figure 6.30, left). This is highly detrimental to the properties of the resulting composite materials. Various solutions are however being studied (e.g. see Figure 6.30, right). x The low surface reactivity of CNTs causes a problem in some cases, e.g. when strong nanotube/matrix interactions are needed as in composites for structural applications (in which stress transfer from the matrix to the nanotubes would then be minimized), or when there will be a need to interconnect them, (e.g. when used as electronic components). CNT functionalization discussed in the preceding section is a way to overcome this problem. x Cost is still a major issue, as for any recently developed material. Marketed prices are in the range 80–2000 $/g for SWNTs, and higher than 0.05 $/g for MWNTs, depending on the synthesis process, number of post-treatments, and resulting purity, structural quality, etc. Obviously, this is by far too high for incorporating most of nanotube grades in any mass application. But prices tend to get lower every year, and were already divided by 2 to 4 within the past decade. x Health and safety issues are nowadays a major concern for any nanosized material. The toxicity of CNTs is supposed to increase as the CNT aspect ratio, specific surface area, and surface reactivity (when any, e.g. for functionalized nanotubes) increase. But results in literature are still contradictory [110], and there is no certainty yet regarding the actual cyto- and eco-toxicity of CNTs. A significant reason is the very large variety of CNTs on the one hand, and the relatively poor characterization of the nanotubes investigated on the other hand, which make results barely comparable and understandable. Another significant reason is the absence of standard procedures for the evaluation of toxicity. Investigations are in progress worldwide in this regard.

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Figure 6.28 Example of the typical aspect of an as-prepared nanotube material from the arc discharge process (from [108]). Round morphologies with dark contrast are metal catalyst remnants. Every other phase is carbon-based.

Figure 6.29 Example from purified CNTs, formerly prepared via catalystenhanced chemical vapour deposition (CCVD). Even rather clean, nanotubes exhibit discrepancies in diameter, and number of walls (one or two).

Figure 6.30 In spite of the addition of a surfactant such as sodium dodecyl sulfate (SDS), the dispersion of as-prepared SWNTs (from arc discharge) is poor and forms agglomerates visible with an optical microscope (left). If the material is pre-treated with freeze-drying, agglomeration occurs only at a much lower scale, invisible to the optical microscope (right) (from [109]).

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6.3.3 Applications of CNTs

Applications of nanotubes are currently at various levels of development. Considering the variety and excellence of the physical properties of nanotubes, areas in which they can be applied are too numerous to be listed here (Figure 6.31). Hence, only examples will be provided, including (1) prospective applications; (2) applications under development; and (3) applications already on the market.

Figure 6.31 The ‘application-tree’ for CNTs. Not exhaustive (by courtesy of M. Endo, modified from original).

6.3.3.1

Prospective Applications

Applications from this category still have to face one or several drawbacks from those listed in Section 6.3.2.3 (beside cost) to such an extent which severely – yet not definitively – prevents their development. They are therefore still in the research laboratories, although examples of prototypes can be found, mostly fabricated by companies for high-tech advertising purposes rather than for real advances in performance with respect to the existing technology (e.g. sport goods).

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Structural composites. While the benefit of CNTs for tribological applications is still being investigated (for which both the mechanical and thermal performances are likely to be useful), the benefit expected from incorporating nanotubes into various matrices to make composites for structural applications appears obvious from considering Figure 6.27. Difficulty regarding processing (dispersion) is the main problem still to be solved, as well as the low reactivity that prevents good bonding to the matrix. Consequently, even if CNTs (e.g. SWNT, and concentricMWNT types) are actually known to reinforce matrices efficiently [111, 112], the resulting performances still hardly beat those of composites reinforced with regular carbon fibers available on market. Cost and safety are also issues that contribute to delaying the development. Meanwhile, various ways to make nanotube-based fibers are being investigated: (1) as a filler to fiber precursors at soften state (polymer [e.g. 113] or pitch [114]), (2) as a dry-spun [115] or wet-spun [116] nanotube fiber from as-prepared nanotubes. The interest of making micrometer-sized fibers out of nanotubes is to allow weaving fabrics, that can be put in composites or to be used as such. Once the pending problems are solved, nanotube-containing composites will lead to mass applications, starting with sport goods (e.g. golf club shafts, tennis racket frame, fishing rods) then structural parts for transportation vehicles of all kinds, from bicycle to space shuttle, and many other areas. Fabrics prepared with nanotube-based fibers (i.e. with no matrix involved) will be also used for functional clothes (e.g. bullet-proof vest), cables, stain-resistant textiles, etc. Electronic parts. Thanks to the amazing versatility of electronic behavior of SWNTs, they could be components of miniaturized computers, or, in a more distant future, in ultra-fast quantum computers, e.g. as [117]: (1) connectors, using metallic-type SWNTs, with a possibility of building 3D circuitry for better compactness; (2) transistors, using semi-conducting SWNTs (ten times faster than regular metal-oxidesemiconductor transistor); (3) diodes, using combined metallic/semi-conductor SWNT junctions via pentagons and heptagons; (4) superconducting transistors [118]. A common advantage of such carbon-based nano-components for electronics is low energy consumption and their ability to withstand high temperatures, thus not needing cooling systems. Most current obstacles to development are the lack of sources for SWNTs with specific transport behavior, the quality and feasibility of contacts, and more generally the technical difficulty to build electronic circuits with nanosized components. Actuators. Based on the finding that the lattice constants of graphene expand dif-

ferently when doped with anions and when doped with cations, the idea of using doped nanotube-based fibers as current-powered actuators was proposed in 1999 [119]. Making artificial muscles is expected, thanks to both the biocompatibility and the unprecedented mechanical properties of CNTs. Such artificial muscles could be used for repairing surgery (and robotics), if internally implanted, but they also could be used for military purpose and hard structures, if supported by an external frame providing extra strength to anyone wearing it as some kind of

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suit. Problems are many and studies are in progress, in particular at the University of Texas (R. Baughman’s group) and in France (P. Poulin’s group). Voltagepowered nano- or micro-scaled tools, although not necessarily requiring previous ion-doping, such as micro-tweezers [120], have also been proposed. Related applications are mechanical memory elements (as developed by NANTERO), and nanoscale electric motors. Membranes and low-friction surfaces. Filtering membranes can be made which are able to let water molecules go through the inner channels while large ions are blocked. Thanks to the perfect smoothness of the graphene surface, high speed transfer of fluids throughout the inner cavity of nanotubes, five orders of magnitude higher than predicted by theory, was recently demonstrated [121]. Likewise, again thanks to the low friction, but also to the low reactivity and thermal resistance, slick surfaces (slicker than Teflon) can be envisaged. 6.3.3.2

Applications Under Development

Applications from this category are being developed by profit-based companies, which have invested enough to prepare prototypes that exhibit one or several benefits with respect to existing materials and devices. Reaching the market is however still prevented by one or several drawbacks from those listed in Section 6.3.2.3, generally including cost issues. Conductive composites (part 1). They mostly correspond to any application in

which static electricity is a problem [112]. Thanks to the huge aspect ratio of CNTs (in the range of 1000), loading an insulating polymer matrix with them allows reaching the percolation threshold with a CNT content as low as 1 wt% or less (depending on the type of CNTs used), as opposed to ~15 wt% for regular nanosized carbon fillers such as carbon blacks. One of the benefits is that the color of the resulting composite is not affected, as opposed to becoming black, and even photo-transparent polymers remain so. The development of nanotube-loaded conductive composites is less problematic than that of structural composites because there is no need for a specific bonding with the matrix. Consequently, transparent films are prepared as an alternative to regular indium tin oxide films to make flexible polymer-based transistors, touch-screens, and displays (Eiko, Unidym). Good thermal conductivity brought by incorporating nanotubes into materials is also of use. Nanotubes loaded polymer composites are also planned to be applied in tanks dedicated to the storage of spark-sensitive, explosive or inflammable compounds (Arkema). Other related applications are electro-magnetic shielding, radar-absorbing materials, electric motor brushes, heat exchangers and dissipaters, etc. Bio-related materials. Other nanotube-loaded polymer-based manufactured materials are also under development, yet not with the goal to make them electrically conductive but biocompatible. Polymer-based surgery wires (catheters) were successfully tested as an alternative to currently used animal-derived wires (‘catguts’).

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Prosthetic coatings are also considered because cells were shown not to adhere to (some of) nanotubes, on the one hand, while graphene surface has a very low friction coefficient on the other hand. The same properties made them applicable for anti-fouling coatings for boat hulls. Likewise, water and air filtration devices made from nanotubes are supposed to be more resistant to fungus and bacteria colonization. Batteries and supercapacitors. Appropriate features for energy storage device elec-

trodes are both a high amount of mesoporosity (allowing for an easy circulation of the electrolyte) and a high amount of microporosity (presenting a high surface area of charge exchange to the electrolyte which is required for fast current delivery and high speed charging), as well as good electrical conductivity. Appropriate pore size distribution was actually previously achieved using other kinds of carbon materials. However, the preparation of the latter involved chemical treatments (activation) detrimental to the material conductivity. Based on the early work by Niu et al. [122], CNTs were found to be able to intrinsically exhibit all the required features while requiring limited pre-treatments. Development is in progress [123], but nanotubes-based supercapacitors (with capacitance > 300 F/g, exhibiting faster speed charging with respect to other carbon-based supercapacitors) are now built, e.g. by Montena) as well as fuel cell with nanotube electrodes (Nec, Motorola, Intematix). Recently, a foldable, postage-stamp-sized supercapacitor with a voltage of ~2.5 V was developed from a nanotube reinforced cellulose paper [124]. Support for catalysts. Based on a pioneering demonstration by Planeix and

coworkers [125], nanotubes were also found to be good catalyst supports for catalyst particles for heavy chemical industrial process, with promises for needs of mass production (Arkema). High surface area, low chemical reactivity, large mesopore network, and tailorable surface energetics (for retaining catalyst nanoparticles and preventing them from coalescence) are among the suitable properties of nanotubes for this application. With respect to regular ceramic-based catalyst supports (alumina, typically), CNTs also exhibit much higher mechanical strength, which goes with higher durability. Chemical sensors. Based on pioneering works published in 2000 [126, 127] CNTs

can be used as chemical sensors, either under the form of a single SWNT (semiconductor type), or a paper-like SWNT network. Because electron transport in CNTs is a surface phenomenon, the conductance response is affected by the occurrence of molecule(s) adsorbed onto the nanotube surface. More interestingly, the response can be specific since it differs depending on the nature of the molecule adsorbed. Highly sensitive (sometimes in the range of ppm) chemical sensors are therefore currently developed (Nanomix, Motorola) for use in atmosphere control for instance. Among other companies involved are Applied Nanotech, Carbon Nanoprobes, Honeywell International, Nanosensors, etc.

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Electron emitters (part 1). Thanks to their ability to withstand enormous current density (see Table 6.3), it was proposed as early as 1995 [128] that CNTs could be used as powerful field emitters [128, 129]. Alternatively, they are able to carry the same current density as regular emitters (e.g. in Mo or Si) but with a much lower extraction voltage threshold, typically of several orders of magnitude. Depending on the target onto which the emitted electrons are projected, various applications are proposed. If projected onto a phosphor layer, nanotube-based emitter arrays can serve for low consumption displays for TV sets or computers, or high autonomy portable devices (e.g. cell-phones). Samsung was first to develop a prototype of TV, then Motorola, Futaba Corp., Copytele Inc., etc. However, it seems that the fact that no nanotube-based flat screen TV has yet been marketed, while prototypes were shown several years ago, could be because its life-time is still insufficient. On the other hand, instead of considering a periodic array of nanotube emitters, a single nanotube-based tip can be used as a field emission electron source with high coherence and brightness for electron microscopes [129], as currently developed by FEI). 6.3.3.3

Applications on the Market

Applications from this category are those available on market or used by companies to process manufactured products. There are still few, and considering arguments given in Section 6.3.2.3, no wonder if they correspond to applications with low requirements regarding purity and selectivity, and/or with low requirement regarding quantity. Electron emitters (part 2). If the target is a metal cathode (Cu, Mo), the very

high current density carried by a single nanotube tip allows building portable X-ray generators. As opposed to electron sources for electron microscopes (see Section 6.3.3.2), ultimate, expensive technology (e.g. ultra-high vacuum) is not required in that case, which has allowed e.g. Oxford Instruments to put such an application on the market. Near-field microscopy probes. In 1996 Dai et al. [130] were first to propose to use

a single carbon nanotube as a tip for atomic force microscopy, with several advantages with respect to regular ceramic tips (Si, Si3N4), typically a much higher mechanical resistance, providing a longer life-time, a much higher aspect ratio, and a better lateral resolution, and minimizing tip artefacts. Benefits are high enough to compensate still prohibitive prices, in the range of several hundred dollars per tip (Nanoscience Instruments). Conductive composites (part 2). An application of nanotube-loaded conductive polymers that can be considered on the market is electro-painting (in which applying an electric potential difference between the painting nozzle and the piece to be painted prevents electrostatic repulsion effects and allows for a good adhesion of the paint), since General Motors has already applied it to some car parts. Electro-painting of polymer-based parts is also proposed by Hyperion for a good surface finish.

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6.3.4 Conclusions

CNTs dominate R&D in the field of nanotechnology. This is fully justified by their large panel of amazing properties. On the other hand, only a few applications are on the market so far, even though about 15 years have elapsed from the moment when the scientists realized the full potential of nanotubes. This has been enough to inspire recent pessimistic comments regarding the ultimate usefulness of nanotubes that would finally not fulfill the expectations regarding their ubiquitous applicability. Such doubts are also inspired by the previous disappointment related to fullerenes, whose discovery in 1985 was acknowledged with a Nobel Prize in 1996, which have generated millions of dollars investments in research programs worldwide, but have finally resulted in only a few examples of applications. Such an analysis is misleading because it ignores the major difference between fullerenes and nanotubes, which is the high morphological, textural, nanotextural, and structural versatility of the latter with respect to the former. It also ignores the time usually needed to bring the idea to the market in the field of advanced materials. In this regard, the example of carbon fibers teaches us that CNTs are just following the regular path (Figure 6.32). Carbon fibers were actually invented in the 1960s, were first used in sports goods about 12 years later, then started being used in aircraft and space industries after 10 more years. Finally, it took about 40 years in total for the market to become mature and demanding for large quantities, synonymous of large profits. CNTs do not behave differently, and they still have the potential to ultimately dramatically influence our daily life.

Figure 6.32 Evolution of the world market for carbon fibers (by courtesy of M. Endo).

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94 L. Zhang, J. Yang, C. L. Edwards, L. B. Alemany, V. N. Khabashesku, A. R. Barron, Chem. Commun. 2005, 3265. 95 S. Banerjee, M. G. C. Kahn, S. S. Wong, Chem. Eur. J. 2003, 9, 1898. 96 F. Nunzi, F. Mercuri, A. Sgamellotti, N. Re, J. Phys. Chem. B 2002, 106, 10622; F. Nunzi, F. Mercuri, A. Sgamellotti, Mol. Phys. 2003, 101, 2047. 97 A. N. Khlobystov, D. A. Britz, G. A. D. Briggs, Acc. Chem. Res. 2005, 38, 901. 98 D. Ugarte, T. Stöckli, J. M. Bonard, A. Châtelain, W. A. de Heer, Appl. Phys. A 1998, 67, 101. 99 J. Sloan, A. I. Kirkland, J. L. Hutchison, M. L. H. Green, Acc. Chem. Res. 2002, 35, 1054. 100 J. Steinmetz, S. Kwon, H.-J. Lee, E. Abou-Hamad, R. Almairac, C. Goze-Bac, H. Kim, Y.-W. Park, Chem. Phys. Lett. 2006, 431, 139; K. Yanagi, Y. Miyata, H. Kataura, Adv. Mater. 2006, 18, 437. 101 L.-J. Li, A. N. Khlobystov, J. G. Wiltshire, G. A. D. Briggs, R. J. Nicholas, Nature Mater. 2005, 4, 481. 102 F. Stercel, N. M. Nemes, J. E. Fischer, D. E. Luzzi, Mater. Res. Soc. Symp. Proc. 2002, 706, 245; H. Kataura, Y. Maniwa, M. Abe, A. Fujiwara, T. Kodama, K. Kikuchi, H. Imahori, Y. Misaki, S. Suzuki, Y. Achiba, Appl. Phys. A 2002, 74, 349. 103 Y. Fujita, S. Bandow, S. Iijima, Chem. Phys. Lett. 2005, 413, 410. 104 B. W. Smith, M. Monthioux, D. E. Luzzi, Nature 1998, 396, 323. 105 L. Jiang, L. Gao, Carbon 2003, 41, 2923. 106 M. Monthioux, V. L. Kuznetsov, Carbon 2006, 44, 1621. 107 M. Monthioux, P. Serp, E. Flahaut, M. Razafinimanana, C. Laurent, A. Peigney, W. Bacsa, J.-M. Broto, in: Nanotechnology Handbook (ed. B. Bhushan), 2nd Edition (revised), Springer-Verlag, Heidelberg, Germany, 2006, pp. 43. 108 A. Mansour, M. Razafinimanana, M. Monthioux, M. Pacheco, A. Gleizes, Carbon 2007, 45, 1651. 109 M. Maugey, W. Neri, C. Zakri, A. Derré, A. Pénicaud, L. Noé, M. Chorro,

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375

7 Angle-strained Cycloalkynes Henning Hopf and Jörg Grunenberg

7.1 Introduction

According to the sp-hybridization model, the carbon–carbon triple bond possesses a linear structure, and indeed, many alkynes fulfill this prediction. High level ab initio calculations [1] of isolated alkyne molecules, as well as gas phase electron diffraction experiments [2] reveal their linear minimum structure. On the other hand, given that the C{C–R bending force constant becomes intrinsically smaller for substituted alkynes and polyynes, crystal packing effects may lead to deviations from strictly linear geometries [3]. From a dynamic point of view even in the case of acetylene one has to include the – albeit reduced – flexibility of the carbon– carbon triple bond in order to describe, for example, the vinylidene acetylene rearrangement (see below) [4]. Further, the linear sp-hybrid picture only holds true for the electronic ground state. Cis- and trans-bent configurations are known in the case of low-lying electronic states [5]. Nevertheless, many alkynes correspond to the prediction of linearity due to the sp-hybrid model, and we have collected a selection of typical examples, reaching from the parent compound acetylene 1 via a few alkyl 2–4 and aryl acetylenes 5–7 to a functionalized acetylene, dimethyl acetylenedicarboxylate 8, in Scheme 7.1, together with the appropriate references [1, 2, 6–11], all dating from the recent literature. The easiest way to distort a triple bond is to incorporate it into a sufficiently small ring structure. The question, of course, is what this ring size is, and from what ring size on the respective cycloalkynes can we isolate compounds, that can be worked with under normal laboratory conditions [12]. Rather than using the nonspecific term ‘distorted cycloalkyne’ Krebs [12] prefers to speak of ‘angle-strained cycloalkynes’. Since it is easier to deform a C{C–C arrangement than its more hydrogenated olefinic and saturated analogs, relatively large angle deformations are possible in cycloalkynes without significant changes in energy. Krebs arbitrarily considers all cycloalkynes in which the C{C–C angle is deformed by more than 10° as angle-strained [12]. Accepting this definition, the stable cyclononyne (see below) is an angle-strained cycloalkyne, whereas cyclodecyne is not. Strained Hydrocarbons: Beyond the van’t Hoff and Le Bel Hypothesis. Edited by Helena Dodziuk Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31767-7

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Scheme 7.1

7.2 Cyclopropyne and Cyclobutyne: Speculations and Calculations on Non-isolable Cycloalkynes 7.2.1 Cyclopropyne and Related Systems

The smallest conceivable cyclic acetylene is cyclopropynylidene (9, Scheme 7.2), the lowest member of the cyclocarbons (see below).

Scheme 7.2

This structure has been discussed in the chemical literature [13], but only in the context of theoretical investigations. On the other hand, cyclopropyne (general structure 10) has been invoked both in spectroscopic [14] and theoretical studies [15]. According to computational evidence, it appears likely that the tetragonal form of this hydrocarbon, 14, is a transition state in the automerization of propadienylidene 13; whereas planar cyclopropyne 15 could be involved as a transition state in the rearrangement of 13 into cylopropenylidene (11, Scheme 7.3) [15], a C3H2 isomer that has already been prepared in matrix in 1984 [16] and has been discussed in connection with interstellar chemistry [17]. The olefinic system, corresponding to 10, cyclopropene 12, known for a long time, has often been used as an addition partner in preparative chemistry [18].

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377

Scheme 7.3

Replacement of the methylene group in 10 by a SiH2 moiety results in silacyclopropyne 19, a cycloalkyne that has been generated as summarized in Scheme 7.4 [19]. Pulsed flash pyrolysis of the precursor 16 results in the generation of the silylene 17 that undergoes rearrangement to the silacyclopropene 18. Photolysis of either 17 or 18 furnished 19, the first formal cyclopropyne ever to be prepared and characterized experimentally by matrix IR spectroscopy and theoretically by ab initio calculations. The even simpler SiC2 20 has been prepared by laser vaporization of a silicon carbide rod within a pulsed supersonic nozzle [20]. By spectroscopic analysis it was shown that the molecule is triangular in both the ground and excited electronic states. The carbon–carbon length in the ground state is 125 pm and the C–Si–C angle amounts to 40°; i.e. the cycloalkynes possesses C2v symmetry. Didehydrooxirene (21, oxacyclopropyne) has been discussed in connection with mass spectrometric studies of the hydroxypropynal molecular ion [21].

Scheme 7.4

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7.2.2 Cyclobutyne

Experiments to generate and trap cyclobutyne have been scarce until the present day although there were several early attempts to generate this next higher cycloalkyne homolog [22]. According to theoretical studies, cyclobutyne may be the limiting ring size for simple cycloalkynes: in ab initio calculations both singlet and triplet cyclobutyne correspond to energy minima [23]; the structural parameters of a calculated structure, 22, are shown in Scheme 7.5.

Scheme 7.5

Two isomerization processes have been considered likely for the highly reactive 22: thermal isomerization to butatriene 23, and ring contraction to cyclopropylidene methylene 24 [24]. Whereas the (calculated) activation energy for the unimolecular rearrangement of 22 to 23 should be substantial [25], cyclobutyne 22 should isomerize to 24 with little or no barrier making the direct observation of this cycloalkyne difficult or impossible [26]. These calculations are supported by the results summarized in Scheme 7.6 [27].

Scheme 7.6

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7.3 Cyclopentyne, Cyclohexyne, Cycloheptyne: from Reactive Intermediates to Isolable Compounds

379

When the halides 25 are treated with lithium diethylamide (LDA) in THF in the presence of lithium thiophenolate, the two thio ethers 27 and 29 are obtained after work-up. To rationalize these findings it has been proposed that initially a cyclobutyne derivative, bicyclo[3.2.0]-hept-6-yne 26 is generated, that subsequently either adds the trapping agent to form 29 or undergoes the ring-contraction to 28 mentioned, that then reacts with the thiophenolate to give 27. Annelation with an aromatic system has often been employed to stabilize reactive organic molecules; however, in the case of synthesizing benzannulated cyclobutynes this approach failed [28]. Another method of influencing the stability and reactivity of hydrocarbons consists in exchange of all hydrogen substituents for fluorine. However, in the case of cyclobutyne, ab initio calculations suggest that fluorine substitution leads to destabilization with respect to cyclobutene [29]. The ability of metal atoms to complex and stabilize highly reactive molecules is well known [30], and metal complexes containing the ‘cyclobutyne’ ligand such as Os3(CO)9(µ-K2-cyclo-butyne)(µ-SPh)(µ-H) have been described [31].

7.3 Cyclopentyne, Cyclohexyne, Cycloheptyne: from Reactive Intermediates to Isolable Compounds 7.3.1 Cyclopentyne and its Derivatives

Reliable experimental ground is reached with cyclopentyne 34, which has been generated by various routes [32]; three conceptually different ones are summarized in Scheme 7.7. For example, on phenyl lithium treatment, 30 is debrominated to carbene 33, that stabilizes itself by the already discussed vinylidenecarbene o acetylene rearrangement to 34 [26, 33]. That 34 has actually been produced was shown by different trapping experiments: for example, in the presence of trans-2-butene the cyclobutene 36 was obtained [34]. In another route cyclopropenone 31 was prepared by photodecomposition of 2,6-diazocyclohexanone in an argon matrix and was decarbonylated on further irradiation at 8 K to 34 [35]. Originally, only the allene 37 as a secondary product of cyclopentyne was observed; in later experiments the IR spectrum of 34 could be recorded in matrix [36]. Finally, cyclobutanone 32 was converted to 34 via 35 and (presumably) 33. In this case trapping with trans-1-methoxypropene yielded the trans-cycloadduct 38 [37]. Cyclopentyne 34 can be stabilized in various ways (Scheme 7.8). 4-Thia-3,3,5,5-tetramethylcyclopentyne 39 has been obtained by oxidation of the corresponding bis-hydrazone and has been trapped by various addition reagents [38]; the intermediate is presumably less strained because of the longer C–S bonds and consequently larger C{C–C bond angles, and the neighboring gem-dimethyl groups provide further protection (see below).

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Scheme 7.7

Scheme 7.8

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381

Metal complexation provides another means of stabilization. Thus the platinum complex 40 is reduced by sodium amalgam to give a colorless, very reactive solid for which structure 41 has been proposed on the basis of NMR and IR data [39]. On methanol treatment 41 gives instantaneously the dinuclear complex 42, the structure of which was established by X-ray crystallography [39]. Stabilization by condensed aromatic rings is realized in the case of the aromatic hydrocarbon acenaphthyne 45, which has been prepared by several routes as shown in Scheme 7.9. Thus photoextrusion of nitrogen from 43 [35] or nitrogen and carbon monoxide from the precursor 44 in a matrix led to 45 – allowing its UV and IR spectra to be recorded at 15 K – as did the Ramberg–Bäcklund rearrangement of the dibromo sulfone 48 [40]. Chemical proof that 45 has been generated in these transformations was established by its interception with oxygen leading to the quinone 46, and self-trapping furnished the trimer decacyclene (47, see below) [35]. The bicyclic acetylene norbornyne 51 was first prepared [41] by metalating 49 with n-butyllithium; the resulting organolithio intermediate 50 on mild warming lost lithium chloride to provide 51 that trimerizes in poor yield (ca. 10%) to the polycyclic aromatic stereoisomers 52 and 53 (Scheme 7.10) [42].

Scheme 7.9 (a) hQ, Ar, 15 K; (b) hQ, Ar, 15 K; (c) t-BuOK, t-BuOH, room temp.; (d) room temp. (5% from 44); (e) O2.

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Scheme 7.10

The still higher unsaturated cyclopentyne derivative bicyclo[2.2.1]hept-2-en5-yne [43] has also been generated by an elimination reaction [43, 44]. With a calculated strain energy of ca. 97 kcal mol–1 it is approximately 30 kcal mol–1 more strained than cyclopentyne itself [44]. Lower bicyclo[2.1.1]hex-2-yne [45] and higher homologs of 51 have also been produced (see below) [46]; like norbornyne 51 they trimerize to aromatic compounds of type 52/53. 7.3.2 Cyclohexyne and its Derivatives

Among all angle-strained cycloalkynes cyclohexyne 56 has received the greatest attention. Since this classical work has been reviewed several times [47], only the most important approaches to 56 will be discussed here (Scheme 7.11). In the debromination of 1,2-dibromocyclohexene 54 [48] and the dehydrochlorination of 1-chlorocyclohexene 55 [49] the relationship between precursor and the desired hydrocarbon is obvious. The oxidation of the bis-hydrazone 57 to 56 can be rationalized by postulating a bis-carbene intermediate [50]. And the last two approaches [51] could also proceed via a vinylidene carbene, 59. Since cyclohexyne 56 is still too reactive to be isolated as such, the chemical proofs of its intermediate existence again rest on trapping and matrix isolation experiments. For example, it has been reacted with tris(triphenylphosphine)platinum(0) to provide a stable platinum complex [52], and trimerization leads to dodecahydrotriphenylene (see below) [40, 48]. Just as in the case of the lower homolog, matrix isolation has allowed recording of the IR spectrum of 56 (Scheme 7.12) [53, 54]. Flash vacuum pyrolysis of 4-cyclopentylidene-3-methyl-isoxazol-5(4H)-one 61 at 700–800 °C and 10–4 torr causes fragmentation to the carbene 59 that isomerizes to 56. The IR spectrum recorded at 77 K shows absorption bands at 2090 and 2105 cm–1 that were assigned to the stretching vibrations of the deformed triple bond. These signals disappeared on warm-up between –110 and –100 °C, and when room temperature had been reached, 62 had been formed.

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383

Scheme 7.11 (a) Li/Hg, THF; (b) PhLi, ether; (c) HgO, benzene; (d) 480–640 °C; (e) tBuOK.

Scheme 7.12

The tetramethyl flanked cyclohexyne 64 is produced on heating 63 at 370 °C and then quenching the pyrolysate at 12 K in an Ar matrix [55]. Again, the vibrational spectrum could be recorded at low temperature, but, disappointingly, the C{C stretching vibration of 64 could not be identified; very likely because it is hidden under one of the (intense) CO-bands at 2139 and 2149 cm–1. On warming the reaction mixture to 45 K dimerization of 64 to the enyne 65 took place.

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Scheme 7.13

As in the case of the bicyclic cyclopentyne 51 a relatively large number of cyclohexyne derivatives is known [12, 56] in which this strained moiety has been incorporated into a polycyclic framework. The surprisingly stable tetrasila cyclohexyne 68 is obtained in good yield when the tetrasilane 66 is reacted with acetylene bis-Grignard (67, Scheme 7.13). This cyclohexyne is stable at room temperature and has a half life of 8 h at 174 °C in decane; clearly, it is stabilized both by steric protection through the methyl substituents and the long carbon–silicon and silicon–silicon bonds and large C{C–C bond angles, respectively [57]. Still, the acetylene–vinylidenecarbene equilibrium can evidently take place here as well, since in the presence of diphenyldiazomethane, 68 is converted into the allene 70 in quantitative yield, presumably by forming the carbene intermediate 69 first. 7.3.3 Cycloheptyne and its Derivatives

The methods used to prepare cycloheptyne do not differ from those employed to prepare the lower homologs (see above). However, with this hydrocarbon we are beginning to reach the shores of stability. In dilute dichloromethane solution at –25 °C this cycloalkyne (72, Scheme 7.14) has a half-life of less than a minute, but at –78 °C this has already increased to one hour [58]. Irradiation of the cyclopropenone 71 in an Ar matrix at 17 K provided 72 as expected and allowed the measurement of the vibrational spectrum: the triple bond stretching frequency is registered at 2121 cm–1 [59]. The blockage of the immediate vicinity of the triple bond by methyl groups leads to a drastic increase in stability. 3,3,7,7-Tetramethylcycloheptyne 73 is a stable hydrocarbon at room temperature with a half-life for dimer formation of one day at 25 °C [60]. Compared to the parent hydrocarbon 72 the methyl protected derivative

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7.4 The Isolable Angle-strained Cycloalkynes: Cyclooctyne, Cyclononyne, and Beyond

385

Scheme 7.14

dimerizes 107 to 108 times more slowly. Krebs and co-workers have synthesized numerous cycloheptyne derivatives of the general structure 74 [12]; most of them are more stable than 72. For example the thio ether (74, X = S) shows no tendency to di- or oligomerization even at 140 °C [61]. Because of the stability of the sulfur compound it was possible to determine its molecular structure by electron diffraction; with 145.8° for the C{C–C angle the deformation is about 12.7° higher than that observed for cyclooctyne (see below) [62]. As described for cyclohexyne (see above), the platinum complex 75 was obtained from intermediately generated 72 [52]. Its X-ray structural analysis shows that the triple bond deviates from linearity by ca. 40°. The dimeric metal complex 76 is produced if 73 is reacted with CuCl·Me2S. X-ray crystallography reveals that the ‘deformation angle’ at the triple bond amounts to 34.4° in this case [63].

7.4 The Isolable Angle-strained Cycloalkynes: Cyclooctyne, Cyclononyne, and Beyond 7.4.1 Cyclooctyne and its Derivatives

With cyclooctyne (79, Scheme 7.15) we have definitely reached the region of stable acetylenes, and, in fact, the hydrocarbon is so readily available [64], that it has become a versatile building block in organic synthesis [65]. The presently most convenient methods to prepare 79 are summarized in Scheme 7.15.

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Scheme 7.15 (a) Br2,CH2Cl2, –40°C; (b) t-BuOK, ether, THF, 0–15 °C; (c) LDA, hexane, THF, –25 – +15 °C (80–84%); (d) n-BuLi, –70 °C (X = S; 70%. X = Se; 85%).

Starting from cyclooctene 77 bromination first provides the expected dibromide which is dehydrobrominated as shown in the scheme to the vinylbromide 78. After a base switch to LDA a second elimination step furnishes 79 that can be obtained by this route in 30-g lots [66]. Alternatively, the also readily available 1,2,3-thiadiazole (80, X = S) or 1,2,3-selenodiazole (80, X = Se) on treatment with n-butyl lithium at low temperature furnish the cycloalkyne in good yield as well [67]. Yet cyclooctyne still possesses a quite strongly deformed C–C{C–C-unit. Structure determination by electron diffraction shows a reduced C{C–C angle of 154.5° [68]. Whereas many medium-ring cycloalkynes have an overall flat structure (see below), cyclooctyne is twisted. Because of the strained nature of their triple bonds, cyclooctynes form metal complexes readily [69]. A sizeable number of derivatives of cyclooctyne is known, and many of these are summarized in compilations by Krebs and Meier and their coworkers [12, 70–72]. 7.4.2 Cyclononyne and Cyclodecyne

With cyclononyne and cyclodecyne we are ending our journey through the homologous series of cycloalkynes. Both hydrocarbons had been obtained by various elimination reactions from the appropriate precursors but were often contaminated with their allenic and dienic isomers. They were finally obtained in pure form by employing the Curtius route (oxidation of 1,2-bis-hydrazones, see above) to cyclonona- and cyclodeca-1,2-dione [73]. Still, cyclononyne is not free from angle-strain. With a C{C–C angle of 160.2° – determined by electron diffraction [74] – this is only slightly larger than in cycloctyne (see above), while the triple bond has a normal length in cyclononyne. No experimental structural data of cyclodecyne seem to be known. According to molecular mechanics calculations the above angle is 171.6°, i.e. still not yet 180° [75]. It has been pointed out, though, that because of non-bonded interactions, additional deformation and hence additional strain is produced, making it unlikely that cyclodecyne will possess a linear arrangement of the triple bond and the two carbon atoms flanking it [76].

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7.5 Cyclic Polyacetylenes

387

7.5 Cyclic Polyacetylenes

As cyclic polyacetylenes we want to define all cycloalkynes possessing at least two triple bonds, whether these are conjugated or not. Many of these systems possess nonlinear acetylene or diacetylene moieties, and the number of know representatives of this class is growing rapidly. The following compilation does not intend to be comprehensive; it only aims at demonstrating the large variation in structures one is encountering in this sub-group. One the most famous hydrocarbons in this class is 1,5-cyclooctadiyne 81, originally prepared in small yield by the dimerization of butatriene 23 [77], and later with far better success from the dibromide 82 under the conditions shown in Scheme 7.16 [78]. The hydrocarbon is essentially planar, the deviation from the 180° degree geometry of the acyclic model compounds summarized in Scheme 7.1 amounts to 20.7°, and the distance between the parallel double bonds is 259.7 pm. Compared to cyclooctyne 79 (see above) the angle deformation at the triple bond is less pronounced. The two benzannelated hydrocarbons 83 and 84 have also been investigated by X-ray crystallography. In 83 the C{C–C angle is 155.8° and the distance between the triple bonds amounts to 261 pm. The molecule is nearly planar, but the two benzene rings are slightly folded out of the plane of the eightmembered ring by 2°, a distortion that has been related to packing effects [79]. For 84 the corresponding structural data are: acetylene angle distortion 154°, (average) distance between the triple bonds 285 pm [80].

Scheme 7.16

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The interesting tetrasila analog of 81, the octamethyl derivative 86, is obtained, when 85 is subjected to flash vacuum pyrolysis at 680 °C or to irradiation [81]. The compound is practically planar, its deformation from linearity at the triple bonds is smaller than that for 81 (average angle 166°), and the intra-annular distance between the acetylene moieties has increased to 324 pm, a consequence of the longer bonds to silicon. When 86 is pyrolyzed a second time, another diemethylsilylene unit is split off and the seven-membered diacetylene 87 is produced. Gleiter and co-workers have prepared the symmetrical bis homolog of 81, the decadiyne 88 [82], and determined its structural parameters. As indicated in Scheme 7.17 the molecule possesses a chair conformation and the molecular arrangement at the triple bonds (intraannular distance 299 pm) is clearly nonlinear (C{C–C angle 171.7°). In the higher homolog, the dodecadiyne 89, the triple bonds are crossed (crossing angle 24°) and still not quite linear (C{C–C angle 173.8°) [82]. The introduction of heteroatoms into the carbon framework of 89 has no significant influence on the structure. In all systems 90 chair-like conformations prevail, the triple bonds are always slightly bent, the deviation from 180° varying between 6 and 10°, and the distance between the triple bonds lies between 290 and 310 pm [83]. As expected, ring enlargement causes stronger linearization of the C–C{C–C fragment. For example, in the 12-membered hydrocarbon 91 the deviation from 180° amounts to 5.9° only [84], and in the 14-membered bis-ether 94, obtained from the dibromide 92 via the bis-acetylene 93 (Scheme 7.17), the four carbon atoms lie practically on one single line (C{C–C angle 178.7°) [85]. Numerous cyclic and polycyclic systems have been obtained in the meantime that contain more than two triple bonds [86, 87]. A classical example is shown in Scheme 7.18 in which the cyclic polyacetylene 95 is isomerized by base treatment to a fully conjugated dehydroannulene 96. Although many of these intermediates and products are highly instable and could not be studied by X-ray crystallography, in some cases single crystals for X-ray investigations could be obtained, and they showed that these compounds did indeed contain distorted acetylene or diacetylene moieties as subunits. For example, in the tetrayne 97, a hydrocarbon possessing a

Scheme 7.17

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7.5 Cyclic Polyacetylenes

389

Scheme 7.18

chair conformation, the angle {C–C{C amounts to 176.7°, whereas the C{C–CH2 angle is 172.9°; the C1–C11 distance is 309.8 pm, whereas the C2–C10 distance is slightly extended to 339.0 pm [88]. In the even more strained tetrayne 98 the two mentioned distortion angles are 166.6° and 166.3°, respectively, and the C1–C10 distance is 269.6 pm, whereas the C2–C9 separation amounts to 323.6 pm [89]. Overall these molecules adopt an ellipsoidal shape. Under the influence of the discovery of new allotropes of carbon (fullerenes) there has been a veritable renaissance of annulene chemistry and a huge number of new carbon-rich compounds has been prepared. Since there are very recent reviews available in this area, these studies will not be discussed here [90, 91]. A small selection of stable and intermediately produced distorted alkynes are summarized in Scheme 7.19 to illustrate where the development is going. The octaacetylene 99 (Scheme 7.19) has, as expected, a planar structure [92]. That there must be considerable strain in the molecule is indicated by the butadiyne moieties which have bond angles as low as 164.5°. More and more examples are reported in the chemical literature in which distorted alkyne units are incorporated in three-dimensional structures. For example, the vinyl bromide 100 on treatment with potassium tert-butoxide evidently yields the (simplest) cyclophyne, 101. This has not only been inferred from the isolation of its trimer, trifoliaphane 102 [93], but also by a metal complexation experiment similar to the one discussed above for cyclopentyne (cyclophanes are presented in Section 4.2). Thus treatment of dibromide 103 with sodium amalgam in the presence of platinum tris(triphenylphosphine) afforded the complex 104 in good yield. As shown by X-ray structural analysis, the C–Pt–C angle amounts to 38.9°, whereas the C1–C2–C3 angle is only 121°. Very likely the corresponding angle in the precursor acetylene 101 has a similar value [94]. Another phane system with distorted triple bonds is the [2.2]paracyclophane/ dehydroannulene hybrid 105 (Scheme 7.20) [95]. A prominent feature of the [14] annulene core of this hydrocarbon is the inward bending of the monoyne units

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390

7 Angle-strained Cycloalkynes

Scheme 7.19

by 4.5–11.8°, whereas the diyne moiety bends outwards by 8.1–11.9°. A similar structure was observed for the dehydroannulene 106, that corresponds to one deck of 105. Here the monoyne units are bent inwards by 3.9–11.5° and the diyne unit is bent outward by 8.6–11.2° [96]. Finally, the [6.6]naphthalenophane 107 shows interesting structural features in its unsaturated bridges: the angle between the triple bond and the aromatic carbon atom to which it is bound is 173.8°, and that between it and the neighboring sp2-hybridized carbon atom amounts to 175.5°, their four atoms being arranged in a transoid fashion, not linear as for the simple model compounds in Scheme 7.1 [97]. Pericyclynes [98], cyclic compounds in which acetylene units are separated from each other by a methylene group or a heteroatom, are another interesting class of polyacetylenes, since they could possess homoaromatic character. Several of

Scheme 7.20

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7.5 Cyclic Polyacetylenes

391

Scheme 7.21

these contain distorted acetylene units. For example (Scheme 7.21) in the thioether 108 the C{C–C angles lie between 172.5° and 173.3° [99]; and in cyclopentayne 109 the corresponding values for the isolated triple bonds range from 170.8° to 178.8°, whereas in the butadiyne moiety the ‘outer’ angle (to the saturated carbon atom) amounts to 168.2° and the ‘inner’ enclosing three atoms of the triple bonds is 168.5° [99]. Although cyclic polyacetylenes possessing three, 110 (Scheme 7.21) or more consecutive triple bonds apparently have not been prepared, the ultimate molecules in this series are the completely dehydrogenated cyclic polyacetylenes, the cyclo[n] carbons 111. There have been several reports on the preparation – or at least the detection – of this novel allotrope of carbon [100], and the topic has been reviewed several times [101]. Certainly, the lower members must possess (strongly) distorted acetylene units. However, because of the instability of the cyclocarbons no experimental evidence concerning their molecular structures is available. Mass spectrometric techniques are usually employed to detect them after their generation, for example by laser evaporation of graphite or from stable organic precursors [102]. To study this unique form of carbon by electronic and/or vibrational spectroscopy in the gas phase or under matrix isolation conditions would be extremely interesting [103]. It has been estimated that the deformation of the bond angle at each sphybridized carbon atom of cyclo-C18 112 amounts to ca. 160° [104]. With compound 112 we are ending our journey through the interesting world of deformed acetylenes that began with another cyclocarbon, the cyclocarbene 9 (Scheme 7.2). Theoretical studies on ethynyl expanded prismanes are discussed in Section 2.3.3.2.

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392

7 Angle-strained Cycloalkynes

7.6 Spectroscopic Properties of Angle-strained Cycloalkynes

Although spectroscopic data of selected strained cycloalkynes have been discussed several times above it seems reasonable to present these in a separate chapter in comparison to possibly discover any general trends. Needless to say, that this discussion suffers from the fact that the available spectroscopic information is not only incomplete but in the cases of the very reactive cycloalkynes impossible to obtain experimentally. IR and Raman spectra are available for a sizeable number of cyclohexynes to cyclononynes, most of them derivatives [12]; they indicate that a reduction in the C{C–C angle causes a shift to lower wave numbers. A linear correlation between absorption maxima and experimental and/or calculated C{C–C bond angles has been proposed; however, strong deviations from this correlation has also been noted [105]. To rationalize this observation it has been proposed that the hybridization is changed by the deformation from sp towards sp2. This would lead to a lower force constant for the C{C bond stretching vibration and hence a lower frequency of absorption [12]. In several cases two or three bands are registered in the region between 2100 and 2300 cm–1, and this has been attributed to Fermi resonance with overtones or combination bands. Since the triple bond does not carry any hydrogen atom substituents, 1H NMR data are of limited importance to probe the angle strain/deformation in small-ring cycloalkynes. However, 13C NMR data indicate that increasing the ring strain by going to smaller ring sizes or introducing additional double bonds into the rings, shifts the 13C signals to lower fields. A case in point is 3,3,7,7-tetramethylcycloheptyne 73 which with G = 109.8 ppm for the acetylenic carbon atoms has not only the highest value for a simple cyclooctyne but is shifted downfield by 22.8 ppm in comparison to the unstrained reference compound 2,2,5,5-tetramethyl-3-hexyne [12]. Again, the observed shifts due to ring strain are rationalized as resulting from a changed hybridization of the acetylenic carbon atoms in the angle-strained cycloalkynes. In many of the NMR studies reported on these hydrocarbons the emphasis is on the dynamic behavior of these compounds (flexibility of the saturated polymethylene bridge). Hints concerning the orbital structure of angle-strained cycloalkynes may be derived from photoelectron (PE) and electron transmission (ET) spectra of these hydrocarbons. According to ab initio [106] and extended Hückel calculations [107] the energy of the HOMO is hardly effected by cis-bending of a triple bond up to ca. 30°. In contrast, the LUMO energy is decreased considerably in the cis-bent models. Experimental studies on various angle-strained cycloalkynes confirm these predictions. For seven-membered cycloalkynes such as 73 is has been observed that the otherwise degenerate S-orbitals are split into two PE bands [108, 109].

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7 Angle-strained Cycloalkynes 83 R. Gleiter, St. Rittinger, H. Irngartinger, Chem. Ber. 1991, 124, 365; for 1-thia-6azacyclodeca-3,8-diynes see R. Gleiter, J. Ritter, H. Irngartinger, J. Lichtenthaler, Tetrahedron Lett. 1991, 32, 2883; for 1,6-diazacyclodeca-3,8-diynes see J. Ritter, R. Gleiter, H. Irngartinger, Th. Oeser, J. Am. Chem. Soc. 1997, 119, 10599; review: R. Gleiter, R. Merger in Modern Acetylene Chemistry, P. J. Stang, F. Diederich (Eds.), VCH, Weinheim 1995, pp. 285; cf. R. Gleiter, Angew. Chem. 1992, 104, 29; Angew. Chem. Int. Ed. Engl. 1992, 31, 27. 84 R. Gleiter, R. Merger, H. Irngartinger, J. Am. Chem. Soc. 1992, 114, 8927. 85 R. Gleiter, M. Ramming, H. Weigl, V. Wolfart, H. Irngartinger, Th. Oeser, Liebigs Ann./Recueil, 1997, 1545. 86 Review: R. Gleiter, D. B. Werz in Carbon Rich Compounds, M. M. Haley, R. R. Tykwinski (Eds.), Wiley-VCH, Weinheim 2006, pp. 295. 87 Review: F. Sondheimer, Pure Appl. Chem. 1963, 7, 363; cf. M. Nakagawa in The Chemistry of Triple-bonded Functional Groups, S. Patai (ed.), Wiley-Interscience, New York, 1978, Vol. I, pp. 635. 88 R. Gleiter, R. Merger, J. Chavez, T. Oeser, H. Irngartinger, H. Pritzkow, B. Nuber, Eur. J. Org. Chem. 1999, 2841; X-ray of 1-aza-cyclotetradeca-3,5,10,12-tetrayne derivatives: E. M. Schmidt, R. Gleiter, F. Rominger, Chem. Eur. J. 2003, 9, 1814 and E. M. Schmidt, R. Gleiter, F. Rominger, Eur. J. Org. Chem. 2004, 2818. 89 M. Kaftory, I. Agmaon, M. Ladika, P. J. Stang, J. Am. Chem. Soc. 1987, 109, 782; K. N. Houk, L. T. Scott, N. G. Rondan, D. C. Spellmeyer, G. Reinhardt, J. L. Hyun, G. J. DeCicco, R. Weiss, M. H. M. Chen, L. S. Bass, J. Clardy, F. S. Jørgensen, T. A. Eaton, V. Sarkozi, C. M. Petit, L. Ng, K. D. Jordan, J. Am. Chem. Soc. 1985, 107, 6556; A. G. Myers, S. D. Goldberg, Angew. Chem. 2000, 112, 2844; Angew. Chem. Int. Ed. Engl. 2000, 39, 2732. 90 M. M. Haley, R. R. Tykwinski (Eds.), Carbon-Rich Compounds – from Molecules to Materials, Wiley-VCH, Weinheim, 2006; V. Maraval, R. Chauvin, Chem. Rev. 2006, 106, 5317; E. L. Spitler,

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C. A. Johnson, II, M. M. Haley, Chem. Rev. 2006, 106, 5344. 91 M. E. Maier, Synlett, 1995, 13. 92 F. Mitzel, C. Boudon, J.-P. Gisselbrecht, J.-P. Seiler, M. Gross, F. Diederich, Helv. Chim. Acta, 2004, 87, 1130. 93 M. Psiorz, H. Hopf, Angew. Chem. 1982, 94, 639; Angew. Chem. Int. Ed. Engl. 1982, 21, 640; cf. C. W. Chan, H. N. C. Wong, J. Am. Chem. Soc. 1988, 110, 462. 94 K. Albrecht, D. C. R. Hockless, B. König, H. Neumann, M. A. Bennett, A. de Meijere, J. Chem. Soc. Chem. Commun., 1996, 543; cf. B. König, M. A. Bennett, A. de Meijere, Synlett, 1994, 653. 95 H. Hinrichs, A. K. Fischer, P. G. Jones. H. Hopf, M. M. Haley, Org. Lett. 2005, 7, 3793; cf. H. Hinrichs, A. J. Boydston, P. G. Jones, K. Hess, R. Herges, M. M. Haley, H. Hopf, Chem. Eur. J. 2006, 12, 7103. 96 K. P. Baldwin, A. J. Matzger, D. A. Scheiman, C. A. Tessier, K. P. C. Vollhardt, W. J. Youngs, Synlett, 1995, 125. 97 H. Hopf, Chr. Werner, unpublished results. 98 Reviews: L. T. Scott, M. J. Cooney in Modern Acetylene Chemistry (P. J. Stang, F. Diederich, Eds), VCH, Weinheim, 1995, pp. 321; A. de Meijere, S. I. Kozhushkov, Topics in Current Chemistry, Vol. 201, Springer, Berlin, 1999, p. 1. 99 L. T. Scott, private communication. 100 Y. Rubin, M. Kahr, C. B. Knobler, F. Diederich, C. L. Wilkins, J. Am. Chem. Soc. 1991, 113, 495. 101 F. Diederich in Modern Acetylene Chemistry (P. J. Stang, F. Diederich, Eds), VCH, Weinheim 1995, pp. 443; F. Diederich, L. Gobbi, Topics in Current Chemistry, Springer Verlag, Berlin, Vol. 201, 1999, pp. 43; K. Tahara, Y. Tobe, Chem. Rev. 2006, 106, 5274. 102 F. Diederich, Y. Rubin, O. L. Chapman, N. S. Goroff, Helv. Chim. Acta, 1994, 77, 1441; Y. Tobe, T. Fujii, H. Matsumoto, K. Naemura, Y. Achiba, T. Wakabayashi, J. Am. Chem. Soc. 1996, 118, 2758; Y. Tobe, T. Fujii, H. Matsumoto, K. Tsumuraya, D. Naguchi, N. Nakagawa, M. Sonoda, K. Naemura,

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References Y. Achiba, T. Wakabayashi, J. Am. Chem. Soc. 2000, 122, 1762. 103 M. Grutter, M. Wyss, E. Riaplov, J. P. Maier, S. D. Peyerimhoff, M. Hanrath, J. Chem. Phys. 1999, 111, 7397. 104 F. Diederich, Y. Rubin, C. B. Knobler, R. L. Whetten, K. E. Schriver, K. N. Houk, Y. Li, Science, 1989, 245, 1088. Apparently cyclocarbons were first discussed in a publication in Nachr. Chem. Techn. 1984, 32, 379 in a (fictional) contribution by H. Musso and H. Hopf under alias names (S. Housmans, H. P. Honnef).

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105 H. Meier, H. Petersen, H. Kolshorn, Chem. Ber. 1980, 113, 2398. 106 N. G. Rondan, L. N. Domelsmith, K. N. Houk, A. T. Bowne, R. H. Levin, Tetrahedron Lett., 1979, 3237. 107 D. M. Hoffmann, R. Hoffmann; C. R. Fiesel, J. Am. Chem. Soc., 1982, 104, 3858. 108 H. Schmidt, A. Schweig, A. Krebs, Tetrahedron Lett., 1974, 1471. 109 G. Bieri, E. Heilbronner, E. Kloster-Jensen, A. Schmelzer, J. Wirz, Helv. Chim. Acta, 1974, 57, 1265.

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8 Molecules with Labile Bonds: Selected Annulenes and Bridged Homotropilidenes Richard V. Williams

8.1 Introduction

Arguably, molecules with labile bonds are involved in any and every chemical reaction. Therefore, to be tractable, the scope of this chapter must be limited to a highly select range of molecules with especially labile bonds – some annulenes and the bridged homotropilidenes. The emphasis of this review is further refined to concentrate on the properties and consequences of the possession of labile bonds. In-keeping with the title ‘Strained Hydrocarbons’, heteroatoms are only included as modifiers of the parent hydrocarbon and systems in which they are the prime motivators are excluded. In general, metal containing compounds are not considered. The coverage throughout this chapter is limited to material published subsequent to the last major review in that particular area, with a brief overview of prior material sufficient to set the newer work in an appropriate context. Access to the older primary literature is provided, wherever possible, through citations to appropriate reviews.

8.2 Annulenes 8.2.1 Cyclobutadiene

Interest in the simplest annulene, cyclobutadiene or [4]annulene (1), extends back for more than 130 years. Sondheimer introduced the term annulenes to describe ‘the completely conjugated monocarbocyclic polyenes, the ring size being indicated by a number in brackets’ [1]. The salient areas of fascination with cyclobutadiene have been its structure, whether it has a square planar or rectangular, triplet or singlet ground state, and its (anti)aromaticity or lack thereof. Its preparation, chemistry and physical properties are summarized in several excellent reviews Strained Hydrocarbons: Beyond the van’t Hoff and Le Bel Hypothesis. Edited by Helena Dodziuk Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31767-7

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[2–8]. In a very recent overview Bally presents the current state-of-the-art in the cyclobutadiene story as it relates to antiaromaticity [9]. Despite this long history, the first synthesis of a cyclobutadiene was only achieved in 1965. Petit et al. prepared 1 by ceric ion oxidation of the corresponding iron tricarbonyl complex 2 [10]. Subsequently, many alternative syntheses of 1 involving the photolysis or thermolysis of strained precursors such as 3–7 have appeared [11]. 1 is an exceptionally reactive transient intermediate that can only be isolated in low-temperature matrices or even in a hemicarcerand at room temperatures [3–8] (see Chapter 10 for discussion on reactions in “molecular flasks”). It dimerizes by an extremely rapid Diels–Alder self condensation even at cryogenic temperatures, but it can be trapped in situ to form derivatives with suitable reagents. Highly sterically encumbered derivatives, e.g. 8–13, are stable at ambient temperature and several of these compounds have had their X-ray structures determined [3–8].

Cyclobutadienes provide prime examples of the synergy between experiment and theory. Much of the experimental work was and continues to be driven by theory, and theory has played a key role in the interpretation and rationalization of experimental results [3–9]. Coulson recognized that the Hückel molecular orbital theory predicted square planar (D4h) triplet ground state of 1 would be subject to second order Jahn–Teller distortion which should result in a rectangular (D2h) singlet ground state [12]. More advanced calculations incorporating electron correlation favor a rectangular singlet ground state for cyclobutadiene [13]. IR spectra from matrix isolated cyclobutadiene were initially interpreted as indicating a square geometry for 1 [17–19]. However, theory and experiment came into concert when Masamune et al. clearly demonstrated, using a Fourier transform IR spectrometer, that the IR spectrum of 1 does indeed correspond with a rectangular geometry. The earlier investigators were hampered by the low intensity of the signals and were misled by bands due to carbon dioxide (which complexes with the cyclobutadiene) produced in the photolytic preparation of 1 [4, 20]. In addition to the extensive discussion of the IR spectra of cyclobutadienes in the general reviews [4, 6, 7], several detailed analyses of the IR absorptions of cyclobutadienes have appeared [21–23]. Following the first IR investigations on 1, X-ray crystal structures confirming rectangular geometries for the stable substituted cyclobutadienes 8 and 10 became available [24, 25]. Subsequent X-ray structures determinations of 9, 11 [26] and 13 [27] all reveal a rectangular cyclobutadiene

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Figure 8.1 Several isolated cyclobutadiene derivatives with X-ray determined bond lengths.

core. Examination of Figure 8.1, in which the X-ray determined bond lengths for 8–11 and 13 are shown, reveals that in 11 the cyclobutadiene core is almost square. Borden and Davidson rationalized this seeming anomaly by considering the reduction in steric strain (between the tertiary-butyl groups) upon lengthening the double bonds, and the reduction in energy upon concomitant shortening of the formal single bonds [28]. Although the familiar term aromaticity defies a universally acceptable definition [29, 30, 31], it is generally used to describe completely conjugated cyclic systems in which (4n + 2) (where n is an integer) electrons are delocalized in a cyclic array. Such systems enjoy a ‘special stability’ (the aromatic molecule is of lower energy than a suitable non-aromatic model) due to this aromatic delocalization. Conversely, conjugated cyclic systems of 4n electrons do not enjoy significant special stability (non-aromatic) and may even be less stable (antiaromatic) than the appropriate model compound. It should be noted that geometric and magnetic criteria are also used in awarding the designation aromatic or antiaromatic to candidate compounds. It would be preferable if the terms, introduced by Garratt [1], diatropic (for systems having a diamagnetic ring current, equated with 4n + 2 aromatics) and paratropic (for systems having a paramagnetic ring current, equated with 4n species which may or may not be destabilized by cyclic delocalization) were to be universally adopted. Just as benzene is the archetype aromatic molecule, cyclobutadiene was considered to be the prime example of an antiaromatic [5, 8, 32]. Recent work has increasingly tended to call this conclusion into question [9]. It should be noted, Bally and Masamune pointed out in their 1980 review that the high energy of cyclobutadiene could result from several factors, not just its antiaromaticity, and that the reported degree of ‘negative resonance energies’ varied between 0 and more than 30 kcal mol–1 [4]. There is no doubt that 1 exhibits properties associated with antiaromaticity and that it should be classed as antiaromatic [3–9]. However, Mo and Schleyer, in a very detailed analysis of the thermodynamics of (anti)aromaticity, show that the destabilization of cyclobutadiene by S-antiaromaticity is rather small. They attribute the majority of the classical thermodynamic instability of cyclobutadiene to result from strain and V–V and S–S repulsions [33].

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As the ground state of 1 is rectangular, then the two degenerate D2h forms must lie on (at least) a double-minimum potential energy surface (PES). These forms could readily interconvert if the barrier(s) between them is sufficiently low. Balaban and Fărcaúiu introduced the term automerization that they defined as being a particular case of isomerization in which both the molecular and structural formulae are conserved in the ‘reactant’ and ‘product’ [34]. Valence bond isomerization or valence tautomerization equally well describe such processes; however, the interconversions of 1a v 1b and the rectangular forms of other cyclobutadienes are most commonly referred to as automerizations. The first indication that such an automerization is facile came from the NMR spectra of the tri-tertiary-butyl derivatives 10 and 14. In each case the tertiary-butyl substituents at C2 and C4 gave only one 1H resonance. For 14 the C1 1H resonance showed no line broadening down to 133 K and the C2/C4 13C single resonance remained sharp to 88 K giving an estimated 'G‡ of less than 2.5 kcal mol–1 for automerization [35–37]. Interestingly, the ring 1H resonance in 14 occurs at 1.04 ppm higher field than for the C2 1H resonance of cyclopentadiene. This significant upfield shift is taken as indicative of a paramagnetic ring current in 14 and of supporting its designation as antiaromatic [36]. The PES for the automerization of 1 (and some derivatives) has been extensively explored by calculation and there is general agreement that classically (vide infra) 1a automerizes to 1b through the square planar D4h 1c [11]. The calculated barrier heights have varied widely [11]. The current best level of theory (optimized multireference average quadratic coupled cluster, MR-AQCC, employing various extended correlation consistent basis sets) predicts a barrier height of 6.3 kcal mol–1 (electronic and zero-point energies) for the 1a v 1b automerization [16].

Trapping experiments of specifically 1,2-dideuterated cyclobutadiene generated in solution convincingly demonstrate that cyclobutadiene has a rectangular D2h ground state in solution as well as in low temperature matrices [38]. Repeating these trapping experiments at various temperatures and concentrations allowed the estimation of the activation barrier ('H‡ ~1.6–10 kcal mol–1) for the automer-

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ization [39]. This latter study also revealed a surprising large negative entropy of activation. Carpenter proposed that his negative entropy of activation could be rationalized by assuming the automerization of 1 proceeds mostly by heavy-atom tunneling [40]! This controversial proposal stimulated much research resulting in a consensus supporting tunneling in the automerization of 1 [41–43]. In an important experiment, Maier et al. found for the first time in a cyclobutadiene that the 13C NMR spectra of the cyclobutadienes 15 are temperature dependent [44]. Coalescence temperatures were determined for the C2/C4 signals of 15b and 15c and for the quarternary carbons attached to C2/C4 of 15c. From these results 15a was estimated to have 'G‡ ~3.5 kcal mol–1 and for 15b and 15c, 'G‡ was determined to be 4.5 and 5.8 kcal mol–1 respectively. These data demonstrate that the automerization of these substituted cyclobutadienes proceeds by a classical mechanism and does not involve tunneling.

A further reflection of the lability of cyclobutadienes is their photochemical valence isomerization to the most strained cage compounds, tetrahedranes [6, 45–48]. The first tetrahedrane, tetra-tertiary-butyltetrahedrane 16a, to be isolated and fully characterized (including X-ray structure determination) can be prepared by low-temperature irradiation of 11. Similarly, low-temperature irradiation of 12 and 15a yield tetrahedranes 16b and 16c respectively.

8.3 Cyclooctatetraene

Cyclooctatetraene ([8]annulene, COT) (17) was the next annulene to be isolated after benzene [49]. Contrary to the expectations of the time that it should exhibit similar properties (aromaticity) to benzene, it behaves as a simple polyolefin. There are several earlier and extensive reviews [5, 8, 50–54] of cyclooctatetraenes (COTs) and one recent general review of the annulenes which makes brief mention of COTs [55]. In 2001, Klärner reviewed the antiaromaticity of planar COT [56]. Although 17 was originally prepared in 1911 by a low yielding multistep synthesis [49], it remained relatively unavailable until the advent of the Reppe et al. synthesis in 1948 [57]. With practical amounts of material then accessible, progress in the chemistry of COTs was rapid. The infrared and Raman spectra [58–60], and X-ray

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[61, 62] and electron diffraction [63, 64] determinations of COT revealed that it adopts a nonplanar equilibrium conformation. There was initially some dispute as to whether the structure is of D2d (tub-shaped), D4 (crown-shaped with alternating single and double bonds) or D4d (crown-shaped with all equivalent bond lengths). Subsequent electron diffraction studies clearly established a D2d tub geometry 17a for COT [65, 66].

There are 21 isomeric (CH)8 hydrocarbons possible, some of which can be accessed from COT and many of which lead to COT upon thermolysis or photolysis [67, 68]. In the context of this review the low temperature processes of valence isomerization via bond shifting (BS, 17a v 17c), ring inversion (RI, 17a v 17d) and valence isomerizations (VI, 17a v 18) to bicyclo[4.2.0]octa-2,4,7-triene 18 and to semibullvalene 19 via high temperature equilibration are of most importance. It was early recognized that as a consequence of COT’s nonplanar equilibrium geometry, mono- and appropriately polysubstituted derivatives would be chiral and that racemization may ensue should the nonplanar ring undergo inversion with a sufficiently low activation barrier, e.g. 20a v 20d [69, 70]. Similarly, racemization can also result from the higher energy BS [52–54]. In 1962 Mislow and Perlmutter reported the first isolation of an optically active COT, the diacid 21 [71]. From the rate of racemization of 21, they estimated the RI activation energy to be 27 kcal mol–1. The substituents hinder planarization which doubtless results in a higher RI activation energy than that for simpler COTs and permitted the isolation of each enantiomer.

Also in 1962 Anet reported an elegant NMR experiment using the splittings in the 13C satellites of the single 1H resonance of 17 to estimate the barrier for BS ('G‡ ~13.7 kcal mol–1) in COT, 17a v 17c [72]. The BS activation energy was later refined by Luz and Meiboom (10.9 kcal mol–1) from their dynamic NMR studies on partially perdeutero (all hydrogens of some molecules replaced with

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deuterium but not for every molecule in the macroscopic sample) COT in nematic solvents [73], and again by Naor and Luz in a new analysis of Luz and Meiboom’s [73] data obtained for C8H8 COT in nematic solvents ('H‡ = 10.0 kcal mol–1) [74]. Shortly after studying BS in COT, Anet group in another ingenious NMR experiment determined the activation parameters for both BS ('G‡ = 17.1 kcal mol–1 at –2 °C) and RI ('G‡ = 14.7 kcal mol–1 at –2 °C) in the alcohol 22 [75]. Of perhaps greater significance, they postulated that BS involves a planar fully delocalized bond equalized transition state (TS) 23 while RI proceeds through a planar bond alternating TS 24. This notion of planar bond equalized or alternating transition states (TSs) persists today, at least for the parent and simple substituted COTs (vide infra). Oth surveyed his own results and those of others studying the dynamics of COTs [76].

Subsequent to these pioneering studies, many syntheses for specifically functionalized COTs were developed and the BS and RI for a multitude of these COTs were examined [5, 8, 50–54, 68]. In addition to NMR determinations of the BS and RI activation parameters, Paquette, in particular, extracted the RI data from detailed analyses of the complex kinetics for the loss of optical activity in appropriate COTs and their NMR determined BS values [52–54]. It was found that increasing substitution of the COT nucleus tended to slow the rates of both BS and RI.

8.4 Bond Shifting, Ring Inversion and Antiaromaticity

The proposal of Anet group that the TS for COT BS is the planar D8h and for COT RI the planar D4h species 17e and 17f respectively [75], appeared to offer the perfect method of evaluating antiaromaticity in (planar) COT. Assuming BS and RI go through 17e/f, then the difference in activation barriers for BS and RI (''G‡, ''H‡, etc.) directly yields the (classical) resonance energy for the antiaromatic 17e [52]. However, the current view is that both D8h and D4h COT are paratropic and antiaromatic [56, 77]. While 17f appears to be universally accepted as the RI TS, the veracity of 17e as the BS TS has been questioned. Dewar et al. considered the experimental ''G‡s for a range of COTs (typically 2–4 kcal mol–1) to be far too small compared with their calculated differences in the heats of formation for 17e/f (13.9, MINDO/2, and 15.4, S approximation, kcal mol–1) for 17e to be the BS TS [78]. They suggested a crown-shaped species similar to 17b may be the BS TS. Based on symmetry and qualitative energy considerations, Ermer et al. concluded that neither 17e nor a crown TS were likely and that a flattened saddle

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of D2d symmetry 17g was the best choice for the TS [79]. Paquette group initially supported 17e as the BS TS [80], but, following extensive additional studies, later concluded for certain highly substituted or annelated (cyclooctatetraenophanes) COTs that a planar TS is untenable for the observed rapid BSs [54]. He suggested that a lower energy pathway would result from pseudorotation of the COT nucleus to give a flattened saddle TS (analogous to 17g). He left unanswered the question of the BS TS in the parent COT 17.

Hrovat and Borden, using CASSCF calculations, provided an answer to this question [81]. They clearly demonstrated that 17e and 17f are the TSs for the parent COT and that the CASSCF/6-31G* activation BS and RI barriers (including zero-point correction) calculated at the optimized CASSCF/3-21G* geometry (CASSCF/6-31G*//CASSCF/3-21G*) are 14.7 and 10.6 kcal mol–1, respectively. These barriers (and the difference between them, 4.1 kcal mol–1) are in excellent agreement with experiments. The results from several more recent calculations at higher levels of theory all support Borden and Hrovat’s earlier results [82–84]. Garavelli et al. optimized 17e and 17f using CASSCF/6-31G* and found activation barriers of 14.5 and 10.9 kcal mol–1 (including zero-point correction) [84]. Incorporation of dynamic electron correlation (CASPT2) results in 17e and 17f becoming essentially degenerate and after zero-point correction 17e is 1.9 kcal mol–1 lower in energy than 17f results in the authors claim to be within the error bars for CASPT2. It is of interest to note that the singlet D8h TS 17e, just as the singlet D4h TS 1c for cyclobutadiene automerization, violates Hund’s rule [81]. In concurrence with an earlier CASSCF/6-31G* study of the RI and BS processes by Castaño et al. [83], Garavelli et al. also propose that there is a bifurcation point (BP) of D4h symmetry between the two TSs, 17e and 17f and similarly 17e and 17f c, and that BS avoids passage through the lower lying 17f/17f c, instead proceeding directly from the BP to the D2d COT ground state (Figure 8.2). The path followed from 17e toward 17f to yield D2d COT is illustrated by the dashed lines in Figure 8.2.

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Figure 8.2 Bond shifting (BS), ring inversion (RI) showing the proposed bifurcation point (BP) between 17e and 17f.

The COT PES has also been probed by means of transition state spectroscopy [85, 86]. Linberger, Borden and coworkers used photodetachment of an electron from the COT radical anion to generate and study the planar TSs 17e/f by means of photoelectron spectroscopy [87]. They confirmed that 17e and 17f are separated in energy by 3–5 kcal mol–1 and that, in violation of Hund’s rule, the D8h singlet is 12.1 kcal mol–1 lower in energy than the D8h triplet [87, 88]. Zewail et al. also generated the D4h 17f by photodetachment of the COT radical anion and examined the TS dynamics for RI using pump/probe femtosecond laser pulses and timedependent mass spectrometric analysis [89]. It has been suggested that heavy atom tunneling is involved in the COT BS, just as in cyclobutadiene automerization [82, 90]. The small atomic displacements and activation barrier and the negative entropy of activation for BS are supportive of the participation of tunneling in BS. Through suitable substitution the activation barrier for BS can be raised to such a level as to allow the isolation of ‘shelf-stable’ bond shift isomers [52, 54]. For example, the annelated derivatives 25a and 25b were the first bond shift isomers to be isolated and the tetramethyl COTs 26a/26b followed shortly after. Similarly Ermer et al. predicted, from geometric considerations, and Trindle later confirmed, from DFT and ab initio calculations, that annelation of COT with small rings or bicycles would favor a planar conformation of the COT nucleus [79, 91].

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The X-ray structure of perfluorotetracyclobuteno-COT (27a) has a planar COT nucleus with ~D4h symmetry resulting from the small ring annelation increasing the internal COT angle from 120° to 135° [92]. The endocyclic (to the cyclobutene) bonds (en) are on average 0.072 Å shorter than the exocyclic bonds (ex). Baldridge and Siegel calculated (B3PW91/DZ(2d,p)) that 27a is 17.2 kcal mol–1 more stable than 27b, while for the corresponding hydrocarbons 28b is 2.3 kcal mol–1 more stable than 28a [93]. They attribute this reversal in tautomeric stability to the electronic perturbations induced by perfluorination. They also predicted that tetrakisbicyclohexeno-COT should be planar with 29b favored by 32.7 kcal mol–1 over 29a. Matsuura and Komatsu confirmed these predictions by the synthesis of 29b [94]. Considering the 1H chemical shift of the bridgehead bicyclohexeno protons, the nucleus independent chemical shifts (NICS) value and the magnetic susceptibility exaltation of 29, they concluded that 29 is less antiaromatic than the parent D4h COT. They attributed this decreased antiaromaticity to the electronic interaction between the bicyclohexeno-annelating groups and the COT nucleus.

Planar COTs are antiaromatic [56], with the ‘D4h’ conformer 2–4 kcal mol–1 lower in energy than the ‘D8h’ in the parent COT and simple derivatives. However, just as for cyclobutadiene the antiaromatic destabilization is only small and is certainly not responsible for the nonplanar D2d tub ground state [77]. Departure from planarity can be understood in terms of Jahn–Teller distortion from the D8h to the D4h geometry which puckers to the D2d tub principally to relieve angle strain. Pseudo-Jahn–Teller coupling has also been invoked to account for the puckering of the D4h COT [95]. The Hückel rule (4n + 2 = aromatic, 4n = ‘antiaromatic’) strictly only applies to planar monocyclic fully conjugated systems. However, Haddon showed using his S-orbital axis vector (POAV) analysis that conjugation is hardly interrupted even when the S-system is significantly distorted from planarity, as in, for example, the various bridged [10]annulenes which are strongly aromatic despite their considerable nonplanarity [96–98]. Jenneskens et al. determined from the analysis of current density maps that D4h COT has a large paramagnetic ring current and that even as the ring puckers toward the D2d tub a paramagnetic ring current is maintained for ~80% of the geometry change [99, 100].

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8.5 Valence Isomerization

At ambient temperature COT is in rapid equilibrium with bicyclo[4.2.0]octa2,4,7-triene 18 formed by thermally allowed 6-S disrotatory electrocyclization. At 100 °C ~0.01% of 18 is present in the equilibrium mixture [50, 51, 67, 68, 101, 102]. In agreement with Huisgen and Mietzsch’s experimental determination of the 17 v 18 activation barrier (28.1 kcal mol–1) [101], Gravelli et al. calculated (CASPT2, zero point corrected) this barrier to be 26.9 kcal mol–1 (Figure 8.3) [84]. Due to its ground state tub conformation, no diene unit is available in COT to participate in Diels–Alder cycloadditions. However, 18 is an active participant in Diels–Alder chemistry [50, 51]. Polymethylation and bridging of the COT nucleus can increase the thermodynamic stability of the bicyclic tautomers. Such stabilization is apparent for the bridged 30a/b, octamethyl-COT and for tetramethyl 26a and pentamethyl 31a, but not their BS isomers 26b and 31b [50]. Bicyclo[4.2.0] octa-2,4,7-triene 18 and its derivatives are doubtless involved in the rich high temperature chemistry of the COTs [67].

Paquette and coworkers showed that thermolysis of semibullvalene 19 and some of its derivatives yielded COTs [103]. A later kinetic study of the 19 v 17 equilibrium found an activation energy of 39.8 kcal mol–1 with 17 more stable than 19 ('H0 = 2.4 kcal mol–1) and ~2.7% of 19 at 300 °C [104]. Recent CASPT2 calculations are in remarkably good agreement with these experimental data [84], and in accord with earlier CASSCF calculations [105], predict a single step reaction passing through a TS of C2 symmetry and a bifurcation point from which the two semibullvalene tautomers result (Figure 8.3). The position and ease of attaining the semibullvalene v COT thermal equilibrium is highly dependent on substitution [106, 107]. In general, it is more common for semibullvalenes to function as synthetic precursors of specifically substituted COTs than for them to be prepared by this thermal isomerization. The complex photochemistry of COTs is summarized in these referenced reviews and articles [51, 67, 84, 108, 109]. A practical synthesis of semibullvalene 19 results from the gas phase (vibrationally hot) [84] photolysis of COT [109].

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Figure 8.3 Equilibrium between 17, 18 and 19.

Another (CH)8 isomer important in the synthesis of semibullvalene is barrelene 32 [110]. Initial interest in barrelene centered on Hine’s proposal that interaction of its 6-S electrons may lead to aromatic character [111]. Zimmerman, Paufler et al. were the first to prepare 32 and the discovery of semibullvalene resulted from the Zimmerman group’s later investigations of its properties [110, 112, 113]. Zimmerman concluded that barrelene is a delocalized system, but, due to the enforced Möbius S-orbital overlap, it is not stabilized by this delocalization having the same S-energy as three isolated ethylenes and hence is non-aromatic [113, 114]. Photoelectron spectroscopy studies confirmed the delocalization and also suggested a degree of through-bond (hyperconjugative) interaction [115]. The acetone sensitized photolysis of barrelene not only marked the discovery of semibullvalene but also the discovery of the di-S-methane rearrangement [116].

8.6 Ions Derived from COT

Based on the known ease of reduction of COT, Katz carried out a detailed study of the COT dianion 33 [117, 118]. He concluded that 33 is planar and aromatic. The reduction of COTs by both dissolving metals and electrochemically affirms these conclusions [50, 51, 55]. Despite intensive efforts and its anticipated aromaticity, the COT dication 34 remains unknown. A few substituted COT dications, e.g., 35a and 35b, have been prepared at low temperature and deemed to be planar aromatics [119]. Komatsu and coworkers prepared the bicyclo[2.2.2]octeno annelated COT

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411

dication 36 which is stable at room temperature, has a tub conformation and undergoes RI with 'G‡ ~10.8 kcal mol–1 [120, 121]. Protonation of COT or reaction with some other electrophiles yields the homotropylium cation 37 or one of its derivatives [50, 51]. Homotropylium cations are the most extensively studied and best established homoaromatics [122, 123]. The idea of homoaromaticity, first proposed by Roberts [124] and Doering and coworkers [125] in 1956, was generalized 3 years later by Winstein [126] who actually introduced the term ‘homoaromaticity’. If the cyclic conjugation of an aromatic system is interrupted by one or more saturated units (often a CH2 group) and the resulting molecule still enjoys ‘special stability’, then it is termed homoaromatic [122, 123, 127–134]. Thus a molecule will be considered homoaromatic if one (or more) through space interactions complete the cyclic delocalization of 4n + 2 electrons and that this through space interaction is energy lowering. Winstein termed homoaromatic systems with one interruption to the cyclic conjugation monohomoaromatic and those with multiple interruptions, bis-, tris-, and tetrahomoaromatic etc. [127, 128].

8.7 The Higher Annulenes

Two recent reviews [55, 135] and earlier articles [1, 5] provide extensive coverage of the higher annulenes. The focus of interest in these annulenes continues to be the study of their aromaticity, their structures and their complex conformational and configurational isomerism. All annulenes are known between [4]- and [30]annulene except for [26]- and [28]annulenes. Most of the higher ([10]- and larger for the purposes of this review) annulenes are examples of systems with labile bonds and exist in a wide variety of configurations and conformations [55, 135]. In addition to reviewing the dynamics of COTs, Oth also reviewed the dynamics of [12]-, [14]-, [16]- and [18]annulenes as determined by variable-temperature NMR studies [76]. It has long been held that: (1) the 4n + 2 annulenes are diatropic with bond-equalized structures while the singlet planar 4n annulenes are paratropic and bond alternating [30]; (2) as the annulene ring size increases, bond length alternation will set in. The position of onset of bond alternation was debated with a mild consensus in favor of about [20]annulene [55, 135]. Schleyer et al. have challenged this uneasy truce by asserting both [14]- and [18]annulene have significantly bond alternating structures [136]. X-ray structures for [18]annulene determined at 80 K [137] and 111 K [138] indicate a bond equalized D6h geometry.

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Schleyer et al. suggest that these structures are incorrect as a consequence of insoluble disorder problems [136]. In a commentary on the Schleyer et al. paper, Ermer concludes that if there is indeed disorder, it may be possible to solve these problems experimentally [139]. He also pointed out that the Schleyer et al. data apply to the gas and solution phases and postulated that crystal packing forces may favor the D6h geometry. Modeling [10]annulene computationally has proved to be difficult. Castro, Karney et al. summarize previous work in this area and examine the interconversion of various [10]annulene isomers [140]. They support, from their BH&HYLP and coupled cluster calculations, Masamune’s [141] NMR based structural assignments for the two crystalline compounds mono-trans 38 and all-cis 39 isolated by his group and his proposed equilibration of 38 with heart-shaped 40.

As already discussed for cyclobutadiene and COT, the higher annulenes can also undergo bond shift isomerization. This bond shifting along with the conformational and other configurational equilibria [1, 55, 76] of [12]-, [14]- and [16]annulenes were re-examined computationally by Castro and Karney et al. [142–144]. They concluded that the bond shifting, 41 v 42, 43 v 44 and 45 v 46, proceeds through a Möbius transition state (antiaromatic for the [14]- and aromatic for the [12]- and [16]annulenes). The recently revived interest in and study of Möbius systems is reviewed by Herges [145].

Bridged derivatives are known for most of the higher annulenes and these derivatives are discussed in the two most recent reviews on annulenes [55, 135]. In the majority of these annulenes the bridging group is a methano group, e.g., the methano-bridged [10]annulene (47 and 48) and [14]annulene (49 and 50). At ambient temperature, 1,6-methano[10]annulene 47 is in equilibrium with a very small amount of the isomeric norcaradiene 51 [146–152]. Substitution on

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C11 tends to increase the stability of the norcaradienic form and, reaffirming the homoaromatic transannular (C1,C6) interaction [123], it has even been suggested that the dimethyl compound 52 is nonclassical (a barrierless flat PES linking 52a v 52b) [153].

Dimethyldihydropyrene 53, another bridged [14]annulene, has long served as an excellent NMR probe for aromaticity [154, 155] and is now being studied for its potential applications as a photoswitch [156]. The dark green 53 is photo-bleached by visible light to give the colorless cyclophanediene 54 which undergoes ready thermal, although forbidden, electrocyclization to return 53. A recent computational investigation (UB3LYP/6-31G*) revealed the transition state for this 6-S conrotatory electrocyclization is biradical-like accounting for the low thermal activation barrier [157].

8.8 Bridged Homotropilidenes

The trivial name for 1,3,5-cycloheptatriene 55 is tropilidene and hence homotropilidene is bicyclo[5.1.0]octa-2,5-diene 56. Doering reasoned that the Cope rearrangement in homotropilidene, via the cis conformer 56b, would be greatly accelerated compared with the parent 1,5-hexadiene due to the ring strain induced upon cyclopropanation and to the restricted conformational mobility of the vinyl groups [158]. The homotropilidene degenerate Cope rearrangement proved to be exceptionally rapid ('H‡ 21.4 kcal mol–1 less than for 1,5-hexadiene), which lead Doering to introduce a new term ‘fluctional’ now commonly fluxional [159, 160]. He defined fluxional (structure), which refers to a dynamic system and is

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not to be confused with resonance hybrid, as ‘an organic molecule which must be described as the mean between two identical structures’. The next logical development in designing molecules capable of ever more rapid Cope rearrangement was to restrict conformational mobility even further and to more closely mimic the required boat conformation for the Cope TS by imposing a bridge between C4,C8 as in 57 [158–160].

Again expectations were met; the bridged homotropilidenes, bullvalene 58 [161], barbaralane 59 [162], barbaralone 60 [162] and semibullvalene 19 [110], all undergo degenerate Cope rearrangement much more rapidly than 56. Remarkably, by a series of Cope rearrangements every hydrogen and every carbon atom in bullvalene become equivalent. The order of reactivity is 19 > 59 > 60 > 58. Thus began the quest for a neutral bishomoaromatic bridged homotropilidine [123, 132, 163, 164].

It was suggested long ago that the semibullvalene nucleus is the system which most closely approaches the elusive goal of neutral homoaromaticity [165]. Semibullvalene 19 was first prepared by Zimmerman and Grunewald in 1966 [110]. With the low field NMR spectrometer available to them at that time the degenerate Cope rearrangement 19a v 19b could not be frozen out. Later, Anet group using a 251 MHz (1H) NMR spectrometer were able to achieve temperatures lower than the coalescence temperature and consequently determined the activation barrier for this Cope rearrangement ('G‡ 5.5 kcal mol–1, 'H‡ 4.8 kcal mol–1) [166]. The value for the activation barrier was later refined using 13C dynamic NMR data ('G‡ 6.2 kcal mol–1, 'H‡ 5.2 kcal mol–1) [167]. It is generally accepted that the C2v symmetric homoaromatic species 19c is the transition state for this process [163, 164]. Calculations by Dewar and Lo [168] and Hoffmann and Stohrer [169] suggested that substitution of the semibullvalene nucleus with electron withdrawing groups at the 2, 4, 6, and 8 positions and electron donating groups at the 1 and 5 positions would lead to a decrease in the activation energy for the Cope rearrangement through stabilization of the TS. Many syntheses of substituted semibullvalenes and barbaralanes were developed, and in agreement with Dewar’s and Hoffmann’s predictions the activation barrier was lowered [163, 164]. However, no example has yet been prepared in which this ‘barrier’ becomes negative resulting in the delocalized species, analogous to 19c, becoming the ground state [123, 163, 164].

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In an alternative approach to eliminating the barrier to Cope rearrangement in semibullvalenes, small ring annelation was calculated to destabilize the localized forms and to favor the delocalized form, e.g., 61, 62, 63 and 64 [123, 132, 163, 164]. Again no neutral homoaromatic ground state bridged homotropilidene has yet been characterized experimentally.

8.9 Recent Developments Bullvallene. This was used as a starting material in the synthesis of triaza- and trithia-

[3]-peristylanes 65 [170] and 66 [171] and also the bullvalene trisepoxide 67 [172].

Barbaralane. Reiher and Kirchner and Kirchner and Sebastiani computationally

examined the tetraphosphabarbaralane 68, and briefly the corresponding tetraazaanalog and the parent 59 [173, 174]. They conclude that 68 is homoaromatic with a C2v ground state which is not biradicaloid. Using molecular dynamics simulations they demonstrate that at finite temperatures (as opposed to the usual 0 K) 68 still prefers a C2v ground state and that distortion (in a Cope rearrangement sense) to degenerate Cs forms is a vibrational mode (no energy barrier) and not a reaction. Using molecular dynamics modeling they present 3-dimensional NICS analysis confirming the homoaromatic nature of 68. By (computationally) linking together two or more barbaralanes, e.g. 69 and 70, Tantillo et al. arrived at a new class of compounds – V-polyacenes [175]. These extended barbaralanes were computed to have completely delocalized bishomoaromatic singlet ground states and again are predicted not to be biradicaloid. The authors originally envisaged these extended barbaralanes as potential sigmatropic shiftamers in the sense of 69a v 69b [175, 176].

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Semibullvalene. In addition to semibullvalenes, many of the studies considered here also discuss barbaralanes. Some semibullvalenes and barbaralanes, with extremely low activation barriers to the Cope rearrangement, are thermochromic and solvatochromic [177]. The observed solvatochromism is attributed to the changing concentration of the delocalized homoaromatic species. Time-dependent density functional theory (TD-DFT) calculations confirm that the long-wavelength absorptions, responsible for the color of these compounds, arise solely from the delocalized species. For the semibullvalene 71 and barbaralanes 72 and 73, increasing solvent dipolarity (dipolar solvents possess a permanent dipole moment whereas polar solvents are characterized by a significant dielectric constant [178]) increases the concentration and correspondingly the thermodynamic stability of the delocalized forms. The stabilization of the delocalized form by dipolar solvents is so successful with 72 that it goes from a localized (in most solvents) to a delocalized homoaromatic solvate ground state in the highly dipolar solvent N,Nc-dimethylpropylene urea [177]. In complete contrast, the bisanhydride 64 appears to follow the opposite trend with the delocalized form increasing in concentration with decreasing solvent polarity, perhaps supporting the notion that 64 is homoaromatic in the gas phase [164, 179]. Various self-consistent reaction field (SCRF) methods were used to model the effect of solvent on these bridged homotropilidenes [177]. While the results of these calculations indicated small selective solvent stabilizations of the delocalized over localized forms for 71–73, the calculated solvent effects on 64 were negligible. A gas phase electron diffraction structure determination of 64 resulted in three models, a delocalized homoaromatic structure, a localized structure or a 2 : 1 mix of localized:delocalized forms, none of these possibilities could be ruled out as they all fit the diffraction data [179].

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Confirming suggestions in an earlier review [163], calculations on 19, 58, 59 and dihydrobullvalene 74 reveal that semibullvalene rearranges most rapidly of the group as it enjoys the greatest relief of strain in going from the localized form to the homoaromatic TS and that the degree of aromaticity in the TS is greatest for barbaralane (and least for semibullvalene!) [180]. Recently Brown, Henze and Borden used unrestricted DFT, CASSCF and CASPT2 to reinvestigate parent semibullvalene 19 and some derivatives (61 and 75) along with the new system, epoxide 76, with a view to determining if the C2v symmetric species were necessarily homoaromatic [181]. They concluded that the C2v geometries of 19, 75 and 76 enjoy homoaromatic stabilization while that of 61 does not and that these C2v species are the ground states for 61, 75 and 76. Both 61 and 76 were calculated to possess significant triplet character and 61 was predicted to have a triplet ground state.

Calculations on a series of substituted semibullvalenes 71, 77–82 revealed that the B3LYP/6-31G* method tends to overestimate the stability of the delocalized homoaromatic species by up to about 3 kcal mol–1 compared with experimental values determined in solution [182]. In agreement with experiment, two phenyl and two cyano groups 80 were found to be more effective in stabilizing the delocalized form than four phenyl 82 or four cyano 81 substituents. Derivatives 80, 81 and 82 were all predicted to have homoaromatic ground states. Subsequent (TD) B3LYP/6-31G* calculations on semibullvalenes 19, 71, 78, 79, 80 and 83 and barbaralanes 59, 72, 73, 84, 85 and 86 reaffirmed that the delocalized species are responsible for the long wavelength electronic absorptions observed in the visible region for some of these compounds [183]. The position of the calculated and observed IR bands are in good agreement. 80 was predicted to be blue and to be the best target for synthesis of a neutral bishomoaromatic.

Sauer et al. published full experimental details for their ingenious one-pot syntheses of a wide variety of 1,5-dimethyl-3,7-disubstitued semibullvalenes

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Scheme 8.1

(Scheme 8.1) [163, 184], which prompted Zhou and Birney to computationally examine the fragmentation of the proposed intermediate diazo compounds 87 to semibullvalenes 88 [185]. Zhou and Birney concluded that the fragmentation led to a bifurcation point and from there to the localized semibullvalenes and thus avoiding the Cope TS for these semibullvalenes. In another one-pot synthesis, Xi group developed a copper (I) mediated approach to octasubstituted semibullvalenes from 1,4-diiodobutadienes (Scheme 8.2) [186]. Thermolysis of the resulting octasubstituted semibullvalenes provides octasubstituted COTs.

Scheme 8.2

In an extension of Gompper’s modification of the Saunders’ isotopic perturbation method [187, 188], Quast, Williams et al. studied, by variable-temperature 13 C NMR spectroscopy in a variety of solvents, the nondegenerate Cope rearrangement of a series of unsymmetrically substituted semibullvalenes (89–96) [189]. They developed a new treatment in which the effects of the substituents on chemical shift were specifically accounted for and determined the thermodynamic quantities for these skewed equilibria. The Cope equilibrium could only be frozen out for 89 and it was determined that in 89 the preferred valence tautomer is the one with the ethyl group on the cyclopropane unit.

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Jiao and coworkers used the isolobal relationship of a boron carbonyl (BCO) moiety to a CH group in their design of potentially homoaromatic semibullvalenes, barbaralanes and bullvalenes [190]. Their calculations (B3P86/6-311+G**) indicate that 97 and 98 should be homoaromatic. The other BCO ‘replaced’ bridged homotropilidines under investigations showed reduced barriers to their Cope rearrangement compared to their respective hydrocarbons, but still possessed a localized ground state.

8.9 Conclusions

The rapid development and current affordability of computer hardware and software has resulted in an unprecedented ability to carry out ever more sophisticated calculations on increasingly bigger molecules. These developments have had a major impact in the area of molecules with labile bonds. In all the systems discussed in this chapter, theory has played a leading role in identifying synthetic targets and important experimental studies and the rationalization and interpretation of experimental results. Through this synergy between experiment and theory, the complex properties of cyclobutadiene and cyclooctatetraene are now well understood. Enticing new results resurrect old questions about the annulenes: Where does natural bond alternation begin and just how (anti)aromatic are they? The location of Möbius transition states for annulene isomerization nicely addresses the long-standing and vexing question of why these formal cis v trans isomerizations are so facile. The cyclophanediene v dihydropyrene equilibrium is now better understood and predictions regarding substituent effects on the barrier to this electrocyclization are being tested experimentally. Many barbaralanes and semibullvalenes with miniscule experimental barriers to their Cope rearrangement are now known. The thermochromism and solvatochromism of these bridged homotropilidenes has been thoroughly investigated and even a semibullvalene solvate with a homoaromatic ground state found. Again, through calculations, several ground state bishomoaromatic semibullvalenes have been identified. Now all that remains is the synthesis and experimental verification of the first neutral bishomoaromatic!

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9 Molecules with Nonstandard Topological Properties: Centrohexaindane, Kuratowski’s Cyclophane and Other Graph-theoretically Nonplanar Molecules Dietmar Kuck 9.1 Introduction

It may be suspected that students of organic chemistry believe, for probably quite varying periods of time, in the perfect tetrahedral coordination of the saturated carbon atom. Colloquial mentioning that an organic molecule, say, methanol, contains a ‘tetrahedral carbon’ incorrectly implies a sphere of a regular tetrahedron. Van’t Hoff and Le Bel’s model [1] has just been too appealing. The propensity to oversimplify seems to be a major driving force but also a means for our understanding chemical structures. Simplicity comes and goes with symmetry and our individual views of chemical beauty change over the years. What is more beautiful: ideal shape or a (slightly) pertubated appearance [2]? In fact, there are only a few perfectly tetrahedral molecules in organic chemistry. Methane, the tetrahalomethanes and also neopentane belong to this group, albeit only the central carbon atom has the perfectly tetrahedral coordination, that is, its valence orbitals are subject to perfect sp3-hybridization. The central carbons in tetraethylmethane [3] and tetrabenzylmethane [4] do not, nor do many more ‘organic’ carbon atoms [5]. 9.1.1 Is All This Trivial?

Another insight that comes late, or later, in the process of learning about organic chemistry concerns structural topology, rather than three-dimensional (3-D) topography, of organic molecules. It frequently comes as a challenge to students to present the 3-D structural formula of a molecule in the plane of a sheet of paper. Strychnine is a clear example of a complex polycyclic organic structure which can be projected so that we ‘see’ its seven rings put one alongside the other in apparently two dimensions. However, there are naturally occurring and also non-natural organic compounds which cannot be drawn in the plane with one ring beside the other. Strychnine is too simple a structure! This is documented by several reports on topologically nonplanar organic molecules [6–14]. Strained Hydrocarbons: Beyond the van’t Hoff and Le Bel Hypothesis. Edited by Helena Dodziuk Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31767-7

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Ask students for an organic compound the structure of which cannot be drawn in the plane without crossing at least two bonds. It is very likely that they won’t find any but they would spontaneously suggest either C60 , or dodecahedrane (or one of the other two Platonic hydrocarbons) – or diamond, which, notwithstanding the carats involved, may be considered a set of ultimate organic molecules given its hydrogen-terminated surface [8, 15]. In fact, all of the known or otherwise conceivable globular molecules, such as the fullerenes [16] (discussed in Chapter 5), the dodecahedranes [17] (briefly presented in Section 2.4.1.6), superphane [18, 19] (mentioned in Section 4.2), and their synthesis precursors, or the various carcerands [20], are ‘topologically planar’. Even ‘oligomeric’ fullerenes [21] are since their structures can be represented as ‘planar graphs’, that is, as polycyclic arrangements of rings that are multiply fused side-by-side in the plane. It is not possible to draw topologically, or graph-theoretically nonplanar (‘gt-nonplanar’), molecules in such a simple way (Figure 9.1). At least one crossing of a pair of bonds cannot be avoided. The smallest conceivable gt-nonplanar’carbon-based structure bearing, at the same time, a carbon atom with perfect tetrahedral coordination would be the carbon cluster C5 1. However, C5 isomers are known to be linear and, of course, extremely difficult to generate experimentally [22]. A somewhat more realistic analog of 1 is the next-higher homolog bearing a perfectly ‘tetrahedral’ carbon atom, viz. ‘centrohexaquadrane’ (C11H12, 2). This hypothetical hydrocarbon was discussed in 1981 in an inspiring conceptual article concerning the aggregation of rings about a common carbon center [23]. The first experimentally realistic gt-nonplanar hydrocarbon bearing a central carbon atom with ideal tetrahedral symmetry is centrohexaquinane (C17H24, 3). This molecule has been calculated to have overall T molecular symmetry [23, 24] and represents, owing to the almost perfect geometrical fit of the cyclopentane units with the central neopentane core, a low-strain ‘polycyclane’ [23]. More specifically, it may be considered the prototypical member of the unusual family of ‘centropolycyclanes’, that is, polycyclic hydrocarbons bearing several (up to six) rings mutually fused about the four C–C bonds of neopentane [23]. Notwithstanding its geometrically familiar constituents, including the unique, perfectly sp3-hybridized central carbon atom (and 16 nonperfect cases), centrohexaquinane (3) has remained a hypothetical molecule to date [25–29]. The same holds true for its highest unsaturated congener, centrohexaquinacene (4, Figure 9.2), which also has low internal strain but has been calculated to have a perfect Td-symmetrical molecular structure [24].

Figure 9.1 The three simplest topologically nonplanar carbon-based K5 molecules are all hypothetical: tetrahedral cluster C5 1; centrohexaquadrane 2 [23]; centrohexaquinane 3.

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9.2 Topologically Nonplanar Graphs and Molecular Motifs 9.2.1 The Centrohexaquinacene Core

What is the beauty of the centrohexaquinane motif, and of centrohexaquinacene 4, in particular? Most of their properties become evident when regarded with the eyes of an organic chemist and analyzed with a touch of mathematics. 1. It represents a relatively large organic structure centered about a truly sp3hybridized carbon atom. 2. The tetrahedral symmetry of the central carbon atom is extended into the 3-D space. 3. Centrohexaquinacene 4 contains six perfectly planar cyclopentene rings. 4. Each of these six rings is bisected by one of the Cartesian axes and the central carbon atom shared by all rings is placed in the origin of this Cartesian coordinate system. 5. Thus, placed at one of the six tips of an octahedron, each of the six double bonds of centrohexaquinacene 4 points out into one of the six directions of the 3-D space (rx, ry, rz) [28, 29]. 6. The organic substructures of 3 and 4 are manifold: They contain three mutually fused spiro[4.4]nonane units (and thus represent ‘superspiro’ arrangements), four intermingled [3.3.3]propellanes as well as four intermingled triquinacenetype entities, and finally, three partially intercrossing [5.5.5.5]fenestrane units [23, 28]. 7. In these various substructures, the geometrical abnormalities and the corresponding symmetries are largely preserved. In turn, all deviations from perfect geometries and minor symmetries in the substructures are completely cancelled in the complete centrohexacyclic frameworks of 3 and 4, in particular.

Figure 9.2 The hypothetical centrohexaquinacene 4 and an ‘exploded’ view illustrating the orthogonal orientation of the six cyclopentene rings of 4 in the Cartesian coordinate system [28, 29].

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8. Stepwise dismantling of 3 and 4, representing the largest congeners, by removing one C2 (ethano or etheno) bridge after the other leads to all of the lower centropolyquinanes and centropolyquinacenes, respectively. 9. The centrohexacyclic structures of 3 and 4 give rise to another special molecular topography: All surfaces of these C17 molecular cores are concave when regarding the cyclopentene rings of 4 as strictly two-dimensional units. 10. To complete this survey of abnormalities, it is repeated here that the molecular topology of 3 and 4 is nonplanar: The constitution of the 17 carbon atoms corresponds to the complete graph K5, a nonplanar graph comprising five centers all of which are pairwise interconnected. This special abnormality will be discussed in detail below. 9.2.2 The Nonplanar Graphs K5 and K3,3 and Some Molecular Representatives

A key publication on graph theory appeared in 1930 in Fundamenta Mathematicae, written by the Polish mathematician C. Kuratowski [30]. In this article entitled ‘Sur le problème des courbes gauches en topologie’, Kuratowski showed that all topologically nonplanar graphs can be reduced to two graphs, K5 and K3,3, both of which can be sketched by pencil-and-paper drawings of a simple tetrahedron. These two sketches are depicted by a modern computer-aided ‘drawing’ in Figure 9.3. Although the two graphs K5 and K3,3 have a common origin, they are fundamentally different. In the case of the graph K5, the central point of the tetrahedron is added and connected to the four corner points. The whole set of ten lines (edges of the graph) connecting five tetravalent points (the vertices of the graph) comprise the ‘complete graph’ K5. Each of the five vertices is connected to all of the other ones. The graph K3,3, also nonplanar, is somewhat less simple and elegant. In this case, two opposite edges of the tetrahedron are divided by two additional points, which are mutually connected. Coloring these additional points and their neighboring points at the tetrahedron’s tips in opposite ways, as depicted in Figure 9.3, yields two sets of three vertices of the ‘bipartite’ K3,3 graph. Notably, the three trivalent vertices of each set are not interconnected, but each of them is associated with each vertex of the other set. Thus, the graph K3,3 comprises two ‘isolated’ sets of three vertices and a total of nine edges.

Figure 9.3 The sketches of the K5 graph (a) and the K3,3 graph (b), as reproduced from Kuratowski’s publication of 1930 [30] using modern computer-aided graphics.

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Figure 9.4 The complete graph K5 in four different representations (see text).

The graph K5 is often represented as a regular pentagon with ‘exhaustive’ interconnection of the tips (Figure 9.4a). The impossibility of projecting it into the plane without any mutual crossing of edges is most simply illustrated in Figure 9.4b. Slight rearrangement in the plane gives a filled square (Figure 9.4c) with the same topological properties, and distortion into the third dimension generates the tetrahedron with its central vertex being topographically different but topologically equivalent to the other four (Figure 9.4d). This recalls Kuratowski’s original representation of the K5 graph, as depicted in Figure 9.3a. The graph K3,3 is often represented in a rectangular form, as depicted in Figure 9.5a. Maximum efforts to separate the edges again leave one unavoidable crossing, as most simply shown in Figure 9.5b. Rearrangement of points, as discussed for the graph K5, yields the (unusual) rectangular representation of Figure 9.5c, which can easily be converted into the regular hexagonal form of Figure 9.5d. Of course, this representation is still topologically nonplanar, as shown in Figure 9.5e. Finally, out-of-plane distortion generates a spatial impression of the interconnection of the six vertices of the K3,3 graph. Kuratowski’s tetrahedronbased definition (Figure 9.3b) can be most easily recognized from the illustrations in Figure 9.5d and 9.5e, in which the extra diametrical connectivity is represented by the two middle vertices and their common edge.

Figure 9.5 The complete graph K3,3 in six different representations (see text).

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Figure 9.6 Centrohexaindane 5, a Td-symmetrical K5-hydrocarbon and the hexabenzo analog of 3 and 4, and Kuratowski’s cyclophane 6, a formally D2d-symmetrical K3,3-polyether. Note that the front and the back naphthalene-2,7-diyl bridges have been simplified by a two extra long bonds (O–O).

The number and variety of gt-nonplanar organic compounds has increased recently, and the examples are collected and discussed in several reports, including quite recent ones [8–14, 28, 29, 31–34]. Instead of commenting on the various sorts of such nevertheless unusual structures, two of the most representative cases are presented in more detail below. To date, the most versatile among these is centrohexaindane (5, Figure 9.6) [35–37], the hexabenzo analog of hypothetical parent K5 hydrocarbons, centrohexaquinane 3 and centrohexaquinacene 4. This highest member of the centropolyindane family can be made in gram amounts [37] and a variety of derivatives has become accessible [9, 28, 29]. The most representative K3,3 counterpart is considered to be Kuratowski’s cyclophane 6 [38], a macrocyclic polyether, although a number of other topologically nonplanar organic compounds of the K3,3 topology were known previously [8, 10, 11, 39–41]. The synthesis of Kuratowski’s cyclophane was designed as a directed construction of a nonplanar organic compound of the K3,3 type and, as such, it appears to represent a unique example of a non-natural polycyclic structure.

9.3 Centrohexaindane

Centrohexaindane 5 is a colorless, high melting solid (m.p. > 400 °C). Its solubility in various solvents is surprisingly high and single crystals can be grown from some of them, including para-xylene and triethylamine [29]. The fact that this ‘heavy’ C41H24 hydrocarbon does not behave as a stone-like solid is attributed to the peculiarity that it is one of the very organic molecules that, disregarding the rims of the benzene rings, have exclusively concave faces. Thus, solvent molecules like triethylamine tend to fill the four ‘hollow’ tribenzotriquinacene cavities. In fact, a strongly negative electrostatic potential has been calculated for the concave side of the tribenzotriquinacene skeleton [42].

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Figure 9.7 The structure of centrohexaindane 5, as deduced from X-ray structure analysis of a single crystal containing one molecule of triethylamine per 5 (C41H24NEt3) [28, 29, 43]. Views of the tribenzotriquinacene cavity (stick model, left) and of the triptindane (propellane) axis (space-filling model, right).

The molecular structure of centrohexaindane as determined by X-ray diffraction of single crystals grown from triethylamine solutions is shown in Figure 9.7 [43]. All three intermingled [2,2c]spirobiindane units are strictly linear and the two benzene rings of each are mutually orientated at right angles. The four tribenzotriquinacene entities are strictly C3v-symmetrical; not only are the planes of their three indane wings orientated at 120° but the long axes of the indane wings are strictly orthogonal to each other. The three fenestrindane units comprised in 5 are held in a perfect D2d-symmetrical conformation. The six C–C–C bond angles at the central carbon atom of 5 are identical, within the limits of experimental error, with the ideal tetrahedral angle (109° 28c) expected for the four C–C bonds at an undisturbed sp3-hybridized atom. From all this follows the overall Td-symmetry of the ground state of centrohexaindane, which is only very slightly affected by the crystal packing. 9.3.1 Centrohexaindane and Structural Regularities of the Centropolyindane Family

As the highest member of the regular centropolyindanes [28, 29, 44], centrohexaindane (5) contains all of the lower members of this family as substructures. Thus, removal of just one indane unit leads to centropentaindane 7 [45], another very rigid polycyclic system bearing five benzene rings stretching practically orthogonally into five of the six directions of the 3-D space. Removal of another indane unit gives rise to either fenestrindane 8 [46], an interesting congener of the widely studied all-cis-[5.5.5.5]fenestranes [5, 28], or to a trifuso-centrotetraindane 9 [47] which, like 5 and 7, represents a rigid molecular framework. Among the higher centropolyindanes, fenestrindane 8 is particular since it is conformationally flexible, as are two of the three regular centrotriindanes comprised in the framework of centrohexaindane, viz. 11 [46b, 48] and 12 [49, 50]. The only centrotriindanes having a conformationally rigid framework are the tribenzotriquinacenes, e.g. 10 [48, 51]. In fact, the three indane wings of the latter centrotriindane have been shown to stretch into the 3-D space almost perfectly at right angles [28, 29].

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Figure 9.8 Centrohexaindane 5 and the lower centropolyindanes 712. The highly systematic structural and geometrical relationship between the centropolyindanes is reflected by the nearly constant Omax value of their UV/Vis absorption and the systematic incremental 13C chemical shifts of the central carbon atoms. a The Omax value of 12 was taken from [49b].

Centrohexaindane has such a very close structural and geometrical relationship with the various lower centropolyindanes, that a comparison of some spectroscopic properties is informative. The UV/Vis spectrum of centrohexaindane reveals that the six S-electron systems of the mutually fused indane units do not interact, in agreement with the diphenylmethane-type junction of the aromatic rings and the orthogonal arrangement in space [28, 29]. Thus, the absorption at Omax = 276.5 nm (H = 5800) in n-heptane is roughly four times as strong as the corresponding lowenergy band in indane, and there is only a very minor bathochromic shift compared with the latter hydrocarbon (Omax = 273.0 nm, H = 1500) [48]. With the decreasing number of indane units attached to the neopentane core, the extinction coefficient decreases steadily. By contrast, the Omax values of the centropolyindanes were found to reflect a characteristic feature of the indane units annealed to the neopentane

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core, namely their conformational geometry. In centrohexaindane 5 and in all other congeners containing at least one tribenzotriquinacene unit (cf. 10), the indane wings are forced into a planar conformation and their Omax values are identical, viz. 276.0276.5 nm. Of course, this also holds true for the tribenzotriquinacenes themselves, e.g. 10. Fenestrindane 8 and the other two centrotriindanes, 11 and 12, and indane itself have conformationally flexible frameworks [28]. Their cyclopentane rings are allowed to partially move out-of-plane giving an envelope form even in fenestrindane, and the UV/Vis spectra of all of these indane hydrocarbons exhibit Omax values in the range of 273.0274.0 nm (Figure 9.8). In view of the fact that the central carbon atom of centrohexaindane (5) is a perfectly ‘aliphatic’, sp3-hybridzed atom, the resonance of the 13C nucleus at this particular position is also noteworthy. It appears at G(Ccentro) = 95.0 with particularly low relative intensity, as expected for such a nucleus embedded in scaffold of sixteen (!) other quaternary carbon atoms. As measured under standard conditions at 600 MHz, the intensity ratio falls considerably short of the expected value, i.e. [Ccentro] : [CD] . 0.25. The chemical shift of the central carbon nucleus in centrohexaindane decreases systematically when one or more indane wings are removed stepwise from the neopentane core. Thus, the value of G(Ccentro) = 83.2 was found for centropentaindane 7, whereas fenestrindane 8 has G(Ccentro) = 71.9 and the isomeric trifusocentrotetraindane 9 has G(Ccentro) = 70.9. Thus, as illustrated in Figure 9.8, stepwise removal of the ortho-phenylene bridges from centrohexaindane gives rise to a constant upfield shift of 1011.5 ppm. Similarly to the UV/Vis data, this finding reflects the highly systematic aufbau principle of the centropolyindane family, based on largely non-strained indane units and culminating in the Td-symmetrical centrohexaindane. 9.3.2 Syntheses of Centrohexaindane

Centrohexaindane 5 can be prepared along three independent synthesis routes and in gram amounts. The first of these involves 11 steps starting from 1,3-indanedione, a commercially available building block. It involves the synthesis of several benzoannelated [5.5.5.6]- and [5.5.5.5]fenestranes, including fenestrindane 8, and affords centrohexaindane in an overall yield of ca. 12%. One of the variants of the ‘fenestrane route’ to centrohexaindane is shown in Scheme 9.1 [35, 37, 46]. Two-fold Michael addition of 1,3-indanedione 13 to dibenzylideneacetone 14 yields the trans-diphenylspirotriketone 15 [52, 53]. Protection of the sterically accessible cyclohexanone functionality followed by reduction of the other two carbonyl groups gives the dispirane 16. Originally, the trans orientation in 15 and 16 of the phenyl groups appeared to be particularly favorable to construct the all-cis-[5.5.5.6] fenestrane framework of ketone 17 by bicyclodehydration [44b] with concomitant hydrolysis of the acetal group and, in fact, the yield of this conversion is excellent. However, the cis-diphenyl stereoisomers of 15 and 16 were found to undergo the two-fold cyclodehydration as well, giving rise to the strained cis,cis,cis,trans-[5.5.5.6] fenestrane skeleton [54]. Subsequent ring contraction of fenestranone 17 via

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Scheme 9.1 The synthesis of centrohexaindane 5 along the fenestrane route.

the corresponding D,Dc-dibromoketone, followed by Favorskii rearrangement, yields the all-cis-tribenzo[5.5.5.5]fenestrane carboxylic acid 18. Decarboxylation of the latter compound suffers from mediocre efficiency but cleanly affords the tribenzo[5.5.5.5]fenestrene 19, which undergoes benzoannelation at its double bond by use of Raasch’s reagent (tetrachlorothiophene-S,S-dioxide) [55] and subsequent dehalogenation/aromatization. Noteworthily, fenestrindane 8, the all-cis-tetrabenzo[5.5.5.5]fenestrane, is a highly versatile parent centropolyindane which has been functionalized in various ways at both its four bridgehead positions and at the eight outer positions of the arene periphery [56, 57]. In the two final steps, fenestrindane is converted into its tetrabromo derivative 20, a remarkably distorted fenestrane bearing significantly flattened geometry at the central carbon atom [(C–C–C) = 121.5°] [28, 56, 58] and subsequent condensation with two molecules of benzene under Lewis acid catalysis yields centrohexaindane [35]. Although the majority of the steps of this multistep synthesis have good or even very good yields, the fenestrane route to centrohexaindane is rather lengthy and its efficiency is limited. On the other hand, the eventual incorporation of two molecules of benzene across the open angles of the fenestrane framework offers valuable advantages in view of the construction of substituted centrohexaindanes (see below).

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Scheme 9.2 The synthesis of centrohexaindane 5 along the propellane route (via 22 and 23) and along the broken fenestrane route (via 11 and 7).

Two much shorter and more efficient syntheses of centrohexaindane have been developed later and are illustrated in Scheme 9.2. The shortest access to 5 starts again from 1,3-indanedione 13 which, in the first step, is converted into its 2,2-dibenzyl derivative 21 and then to the [3.3.3]propellane ketone 22. Two-step oxidation of the latter compound affords the highly versatile 1,3,3c-triketone 23, called ‘triptindanetrione’ because of its relation to the parent centrotriindane 12, dubbed ‘triptindane’ (Figure 9.8) [49]. Two further steps including three-fold addition of the elements of benzene (from phenyllithium and by hydrolysis) and subsequent three-fold cyclodehydration of the intermediate, highly crowded propellanetriol completes the six-step ‘propellane route’ to centrohexaindane 5 [36, 37]. It is remarkable that a total of ten new C–C bonds are formed by this sequence with an overall yield of 25%. Since three adjacent indane wings are attached in the last two steps, the propellane route represents a valuable alternative to the fenestrane route in case of the synthesis of substituted centrohexaindanes (see below). The third synthesis of centrohexaindane also involves 2,2-dibenzyl-1,3-indanedione 21. This time, however, the diketone is converted into a ‘broken fenestrane’, viz. the C2-symmetrical centrotriindane 11, by reduction to the corresponding trans-1,3-indanediol and subsequent acid-catalyzed two-fold cyclodehydration (Scheme 9.2) [44b]. Careful bromination of the benzhydrylic and benzylic positions of 11 and subsequent incorporation of two further benzene units furnish centropentaindane 7, the second-highest centropolyindane, in strikingly high yield [45]. In the final two steps, the bromination/condensation sequence is repeated,

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involving the highly reactive bridgehead dibromo derivative of 7 and leading to centrohexaindane 5 in an overall yield of 40%. To date, this seven-step ‘broken fenestrane route’ has not been used for the synthesis of derivatives of centrohexaindane but it offers the possibility of incorporating one single arene unit in a directed way [37]. 9.3.3 Multiply-functionalized Centrohexaindanes

In fact, the directed synthesis of centrohexaindanes bearing various patterns of substitution at the six poles of the polycyclic framework appears to be a promising goal. The spatial orientation of the six arene units is well defined, so unusual and interesting supramolecular aggregation, in particular in the solid state, may be envisioned. However, only a few centrohexaindanes bearing a pinpointed set of functional groups have been made to date, and they are shown in Figure 9.9. Nitration of the parent centrohexaindane 5 under optimized conditions gives a mixture of four hexanitro derivatives 24ad, all of which bearing each nitro group at one of the six peripheral C–C edges [29, 59]. Two of these constitutional isomers have C1 molecular symmetry and the other two are C3 symmetrical. Thus, four racemates are formed in ratios (ca. 3 : 3 : 1 : 1) that correspond to the ratios expected for the case of statistical attack of the nitrating reagent. The four racemates can be separated from each other and have been fully characterized [59]. Conversion of the original mixture to the as yet hypothetical twelve-fold nitrated centrohexaindane and use of the latter to generate the corresponding Td-symmetrical dodecaaminocentrohexaindane represents a similarly challenging and promising goal. At variance from the a posteriori functionalization of the parent centrohexaindane, the a priori incorporation of substituents turned out to be useful in several cases. Following the propellane route, the Cs-symmetrical dimethyl and tetramethyl derivatives 25 and 26 were synthesized [60]. Remarkably, these centrohexaindanes contain the substituents at the strongly sterically-shielded ortho positions; in the case of the tetramethyl derivative 26, each of the methyl groups points into one of the four sterically-shielded propellane cavities of the centrohexaindane framework. The tetramethoxycentrohexaindanes 27 and 28 represent two examples of several electron-rich derivatives of 5 that have become accessible along the fenestrane route [61]. The four-fold bridgehead bromination of fenestrindane (8 o 20, cf. Scheme 9.1) can easily be used to prepare the corresponding bridgehead tetraalcohol, which in turn can be condensed under Brønsted acid catalysis with various methoxybenzenes, e.g. with veratrole and hydroquinone dimethyl ether in the cases of 27 and 28, respectively. Again, the facile incorporation of four methoxy groups into the sterically-shielded inner arene positions of 28 is remarkable. Thus, the 1H NMR spectrum of 28 clearly reflects the close neighborhood of the four methoxy groups to the eight ortho protons at the adjacent benzene rings, which resonate at particularly low field (G = 8.22 ppm, compared with G = 7.74 ppm in the case of 27) [59, 61].

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Figure 9.9 Selected derivatives of centrohexaindane synthesized either by a posteriori functionalization of 5 (24ad) or by a priori incorporation of the functional groups (2529, see text).

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The only twelve-fold functionalized derivative of 5 reported so far is the Tdsymmetrical dodecamethoxycentrohexaindane 29 [62]. In this case, the propellane route proved to be viable, albeit with some inevitable restrictions and losses of yields in the last steps. As mentioned above, the construction of centrohexaindanes bearing functional groups at geometrically well-definable points is considered a great challenge.

9.4 K5 versus K3,3 Molecules

Compared with the K3,3 hydrocarbons and the heterocyclic organic molecules with K3,3 topology presented in the next section, most of the gt-nonplanar counterparts of the K5-type are remarkably simple. However, as mentioned above, the synthesis of the smallest prototypical hydrocarbon that conceivably should be accessible, centrohexaquinane 3, has never been achieved. To our knowledge, the singly benzoannelated derivative of 3, benzocentrohexaquinane 30, represents the smallest K5 hydrocarbon synthesized so far, albeit obtained only in minute amounts [27]. The Cs-symmetrical dibenzo and the C3v-symmetrical tribenzo analogs 31 and 32, and other higher benzoannelated congeners have been reported since the first synthesis of centrohexaindane 5 [27, 36].

Figure 9.10 The lowest congener of the K5-hydrocarbons synthesized to date, benzocentrohexaquinane 30 and its next higher Cs-symmetrical and C3v-symmetrical benzo analogs, 31 and 32, respectively. Hydrocarbons 31 and 30 have been synthesized from 32 by partial oxidative degradation of the benzene rings [27].

9.4.1 Topologically Nonplanar Polyethers and Other K3,3 Compounds

The K3,3-cyclophane 6, termed ‘Kuratowski’s cyclophane’, represents a macrocyclic polyether [38]. Remarkably, other representative topologically nonplanar organic compounds are also polyethers. One of them is the trioxacentrohexaquinane 33 (Figure 9.11), a polycyclic polyether of K5 topology published back-to-back in 1981 by Simmons [25], Paquette [26] and coworkers and sometimes just named after both these authors. The other is the first molecular Möbius strip ever known, the twisted and thus gt-nonplanar belt-type structure 34, reported by Walba in 1982 [39], representing a K3,3-type molecule [14, 63, 64]. In contrast to a sizable number of other heterocyclic K3,3 molecules, including several peptides [32–34] and

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Figure 9.11 Two complementary topologically nonplanar polyethers: The ‘Simmons–Paquette’ molecule 33, a molecular K5 graph [25, 26], and Walba’s Möbius ladder 34, a molecular K3,3 graph [39]. Note that two diglycol ether strips of 34 are drawn extra long and that they cross each other in front of the major part of the ladder.

heterobridged pagodanes [41], K3,3-type hydrocarbons that have been studied experimentally and discussed in the literature, are extremely scarce. Thus, the triplelayered naphthalenophane 35 (Figure 9.12) has been synthesized and its electron spectra studied in detail [40], and this compound may be regarded as the first K3,3hydrocarbon ever known. More recently, a hexamantane, viz. ‘cyclohexamantane’ 36, was isolated from petroleum and its structural and spectroscopic properties were reported [65]. It has been discussed previously that at least five adamantane units are required to generate a topologically nonplanar diamondoid framework [8]. However, none of the ten possible members of the pentamantane branch of the polymantane family [66] is known by experiment; rather, anti-tetramantane is the highest congener ever synthesized [67]. Cyclohexamantane 36 undoubtedly represents a gt-nonplanar hydrocarbon, the framework of which contains several K3,3-graphs. While the chemistry of diamondoids beyond adamantane has started to grow impressively [68, 69], the existence and synthesis of other gt-nonplanar diamondiod hydrocarbons in petroleum can be envisioned, just as numerous gt-nonplanar peptides and proteins may exist in biological systems.

Figure 9.12 The first K3,3 hydrocarbon, Otsubo’s cyclophane 35 [40], representing a gt-nonplanar triple-layered [2.2:2.2]naphthalenophane (top view showing one naphthalene2,6-diyl unit on top and another one below the central naphthalene-2,3,6,7-tetrayl unit), and Dahl’s gt-nonplanar K3,3 diamondoid hydrocarbon, cyclohexamantane 36 (side view) [65]. The central naphthalene 35 or decalin 36 units are highlighted in blue.

It is interesting to locate the structural features that render compounds 35 and 36 topologically nonplanar. This is even amusing since, counter-intuitively, these hydrocarbons may be considered closely interrelated, both being naphthalene-based cyclophanes. The K3,3 graph underlying in the triple-layered naphthalenophane 35 is highlighted in Figure 9.13a. Comparison with the simpler benzene-based analog (Figure 9.13b) and the Schlegel diagram of the latter (Figure 9.13c) clearly

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Figure 9.13 (a) The gt-nonplanarity of Otsubo’s cyclophane 35 as evidenced by its K3,3 topology. (b) Simplification to the corresponding benzene-based triple-layered cyclophane. (c) Demonstration of the gt-planarity of the latter structure, implying that all even higher multilayered cyclophanes of this type are gt-planar as well.

Figure 9.14 (a) The gt-nonplanarity of cyclohexamantane 36 as evidenced by its K3,3 topology. (b–d) Gt-planar diamond cuttings derived from 36 by hypothetical removal of the central C–C bond or one or two central CH units to give spherical, bowl-type and belt-type structures, respectively.

demonstrates that multilayered cyclophanes containing only monocyclic units, e.g. benzene rings, are all gt-planar. This also holds true for ‘[6]chochin’, a chiral, six-layered [2.2:2.2:2.2:2.2:2.2]paracyclophane [70–72]. The gt-nonplanarity of cyclohexamantane 36 is illustrated in Figure 9.14a. Similar to the presentation of Otsubo’s cyclophane 35 in Figure 9.13a, it is evident that it is the central C–C bond which renders 36 topologically nonplanar. Thus, the hypothetical removal of that central bond would produce a cage hydrocarbon with a topologically planar structure. (Note that due to the strong transannular Hin…Hin repulsion, the conformation would deviate markedly from that shown in Figure 9.14b). Further removal of the top and bottom methane units would lead to another interesting, belt-like cutting of the diamond lattice (Figure 9.14d). The diagrams clearly show the change from gt-nonplanarity of 36 to gt-planarity in spherical, bowl- and belt-like structures.

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9.5 Kuratowski’s Cyclophane

Kuratowski’s cyclophane 6, described by Siegel et al. in 1995 [38], represents the most recent and certainly also the ‘heaviest’ K3,3-type non-natural topologically nonplanar organic molecule that has been assembled by a designed chemical synthesis. However, it is worth noting that, similarly to the synthesis of Walba’s Möbius-type polyether mentioned above, the construction of 6 notoriously involves the concomitant formation of a topologically planar isomer. Thus, although being a designed synthesis, as is that of centrohexaindane 5, the preparation of 6 is based on a compromise from the very start, as will be shown in the next section. 9.5.1 Synthesis of Kuratowski’s Cyclophane

The synthesis strategy for the construction of 6 can be envisioned easily from the representation of the K3,3 graph shown in Figure 9.5f. One of the ether components is a doubly branched octiphenyl 46 comprising all the six vertices of the K3,3 graph and five of the nine edges required. The remaining four edges are provided by incorporating four naphthalene-2,7-diyl units as the complementary ether components. Hence, the challenge of this synthesis was two-fold: (1) The construction of a suitable octiphenyl derivative, viz. the octabromide 47, and (2) the achievement of the four etherification processes in a one-pot synthesis affording, at least in part, the desired topologically nonplanar octaether 6 (Schemes 9.39.5). The assembly of the octiphenyl 46 is based on the synthesis of several 2c-functionalized meta-terphenyls (Scheme 9.3). Coupling of two equivalents of 3,5-dimethylphenylmagnesium bromide 39 with 1,3-dichloro-2-iodobenzene 38 in the course of a Hart reaction [73] and subsequent quenching of the product mixture with trimethyl borate leads to the terphenylboronate 37a. The corresponding boronic acid 37b can be subjected to a two-fold Suzuki coupling with 4,4c-diiodobiphenyl 45 to furnish the desired octiphenyl (46) in a remarkably short sequence (Scheme 9.4, yields have remained unpublished). Another but relatively lengthy route to 46 involves 2c-iodoterphenyl 40 as the product of the Hart reaction between 38 and 39. Subsequent Ullmann reaction of 40 with 4-iodonitrobenzene (41) to quaterphenyl 42 followed by reduction of the nitro group and Sandmeyer reaction affords the corresponding iodoquaterphenyl 43, and subsequent exchange of the halogen for the boronic acid group gives the quaterphenylboronic acid 44 (Scheme 9.3). The latter two quaterphenyl derivatives both represent suitable moieties of the desired octiphenyl 46, which is accessible (Scheme 9.4, yields have remained unpublished). The remaining section of the synthesis consists of an apparently routine sequence of cyclophane chemistry. Nevertheless, it comprises an eight-fold refunctionalization of the methyl groups of octiphenyl 46 in four subsequent preparative steps to give, in a stated 50% yield, the octiphenyl 47 containing eight benzylic bromomethyl functionalities (Scheme 9.4). In the very last step (Scheme 9.5),

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Scheme 9.3 Synthesis of the building blocks 37b, 43 and 44 for the construction of Kuratowski’s cyclophane 6 [38].

Scheme 9.4 Syntheses of octiphenyl (46) and its octabromo derivative (47) [38].

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Scheme 9.5 Assembly of Kuratowski’s cyclophane 6 and its topologically planar isomer 49 [38].

this intermediate, illustrated in an axial representation corresponding to the ‘inner’ edge of the graph shown in Figure 9.5f, is subjected to an eight-fold Williamson ether synthesis with four equivalents of 2,7-dihydroxynaphthalene 48. Two macrocyclophanes result from this condensation: (1) the desired topologically nonplanar isomer 6 of formal D2d symmetry and (2) the topologically planar isomer 49, of formal D2h symmetry, which can be separated from each other and isolated by flash chromatography and isolated in yields of 15% and 10%, respectively (Scheme 9.5) [38]. 9.5.2 The Structure of Kuratowski’s Cyclophane

X-ray crystal structure analysis [38] revealed that neither of the macrocyclic polyethers 6 and 49 adopts the ideal molecular symmetries mentioned above. This is not surprising in view of the numerous rotational degrees of freedom and the presence of solvent molecules in the crystal. Instead, the topologically nonplanar isomer 6 was found to prefer an approximately S4-symmetrical conformation. Moreover, in both cases, two molecules of benzene per macrocyclophane were

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incorporated in the crystals. The use of Kuratowski’s cyclophane 6 for the design of larger molecules in the 10 kDa range has been emphasized by the authors [38] and, in fact, extension of the molecular poles at the quaterphenyl axis of 6 and at the four lateral naphthalene wings appears tempting. On the other hand, the presence of several benzylic ether linkages may cause limitations in the chemical stability under various reactions conditions.

9.6 Conclusions

It has been demonstrated in this chapter that the use of regular and essentially non-strained organic building blocks, such as the sp3-hybridized carbon centers in the neopentane core and in cyclopentane and cyclopentene rings, as well as the sp2-hybridized carbon atoms in benzene and naphthalene rings, can be used to construct novel polycyclic molecular structures of highly unusual topological properties. Centrohexaindane 5, a large but conceptually simple, possibly ‘elegant’ or even ‘beautiful’, and experimentally easily accessible hydrocarbon, and Kuratowski’s cyclophane 6, an even larger and likewise elegant macrocyclic polyether, represent outstanding examples of such cases. Whereas the former compound is a unique hydrocarbon containing a C17 core that corresponds to the complete and topologically nonplanar graph K5, the latter is a cyclophane designed to contain a bipartite and also topologically nonplanar graph K3,3. However, there are notable differences. Centrohexaindane has a strictly Td-symmetrical and rigid molecular framework consisting of normal (five-membered) rings; it lacks any conformationally dynamic behavior. By contrast, Kuratowski’s cyclophane is a conformationally flexible macrocyclic polyether and exists in a dynamic equilibrium that comprises conformers of lower symmetry than the ideal shape would suggest. Hence, the spatial orientation of the six indane wings of centrohexaindane is perfectly orthogonal to each other and may be accessible to extension into the six directions of the Cartesian coordinate system. The geometrical orthogonality of the centrohexaindane framework is of particular importance in view of the fact that the benzene units can be modified by various functional groups. Kuratowski’s cyclophane, with its central quaterphenyl axis and its lateral naphthalene units may also be extendable by functionalization of the aromatic peripheries, however, the conformational flexibility renders its scaffold less geometrically defined. Whether the conformationally extreme rigidity and exact geometrical shape of 5 and/or the limited flexibility of 6 will turn out to be useful for future development, may depend on the requirements and feasibility of the nanoscale molecular interaction of such unusual organic compounds. In any case, inspiration to create novel molecular architectures based on building blocks that are in conformity with van’t Hoff’s and Le Bel’s hypothesis has brought about untypical organic systems with mathematically interesting, topological properties and aesthetic construction principles [74]. There is no doubt that further such fruitful creativity is needed.

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16 A. Hirsch, M. Brettreich, Fullerenes: Chemistry and Reactions, Wiley-VCH, Weinheim, 2005. 17 (a) L. A. Paquette, Chem. Rev. 1989, 89, 1051–1065; (b) H. Prinzbach, K. Weber, Angew. Chem. Int. Ed. Engl. 1994, 33, 2239–2257. 18 Y. Sekine, M. Brown, V. Boekelheide, J. Am. Chem. Soc. 1979, 101, 3126–3127. 19 (a) H. Hopf, Classics in Hydrocarbon Chemistry, Wiley-VCH, Weinheim, 2000, Chapter 12. (b) See also: R. Gleiter, H. Hopf, Modern Cyclophane Chemistry, Wiley-VCH, Weinheim, 2004. 20 D. J. Cram, J. M. Cram, Container Molecules and Their Guests, Royal Society of Chemistry, Cambridge, 1994. 21 C. Thilgen, F. Diederich, Chem. Rev. 2006, 106, 5049–5135. 22 (a) S. Yang, K. J. Taylor, M. J. Craycraft, J. Conceicao, C. L. Pettiette, O. Cheshnovsky, R. E. Smalley, Chem. Phys. Lett. 1988, 144, 431–436; (b) L. Adamowicz, J. Chem. Phys. 1991, 94, 1241–1246. (c) J. Szczepanski, R. Pellow, M. Vala, Z. Naturforsch. A 1992, 47, 595–604. 23 P. Gund, T. M. Gund, J. Am. Chem. Soc. 1981, 103, 4458–4465. 24 O. Ermer, Aspekte von Kraftfeldrechnungen, Wolfgang-Baur-Verlag, München, 1981; Section 4.6.3. 25 (a) H. E. Simmons III, J. E. Maggio, Tetrahedron Lett. 1981, 22, 287–290; (b) J. E. Maggio, H. E. Simmons III, J. K. Kouba, J. Am. Chem. Soc. 1981, 103, 1579–1581; (c) S. A. Benner, J. E. Maggio, H. E. Simmons III, J. Am. Chem. Soc. 1981, 103, 1581–1582; (d) H. E. Simmons III, PhD Thesis, Harvard University, 1980. 26 (a) L. A. Paquette, M. Vazeux, Tetrahedron Lett. 1981, 22, 291–294; (b) L. A. Paquette, R. V. Williams, M. Vazeux, A. R. Browne, J. Org. Chem. 1984, 49, 2194–2197. 27 D. Gestmann, H. Pritzkow, D. Kuck, Liebigs Ann. 1996, 1349–1359. 28 D. Kuck, Chem. Rev. 2006, 106, 4885–4925. 29 D. Kuck, Pure Appl. Chem. 2006, 78, 749–775.

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9 Molecules with Nonstandard Topological Properties 30 C. Kuratowski, Fund. Math. 1930, 15, 271–283. 31 (a) D. M. Walba, J. D. Armstrong, III, A. E. Perry, R. M. Richards, T. C. Homan, R. C. Haltiwanger, Tetrahedron 1986, 42, 1883–1894; (b) D. M. Walba, T. C. Homan, R. M. Richards, R. C. Haltiwanger, New J. Chem. 1993, 17, 661–681. 32 B. Mao, J. Am. Chem. Soc. 1989, 111, 6132–6136. 33 G. M. Crippen, J. Theor. Biol. 1974, 45, 327–338. 34 (a) C. Liang, K. Mislow, J. Am. Chem. Soc. 1994, 116, 3588–3592; (b) C. Liang, K. Mislow, J. Am. Chem. Soc. 1995, 117, 4201–4213. 35 D. Kuck, A. Schuster, Angew. Chem. Int. Ed. Engl. 1988, 27, 1192–1194. 36 D. Kuck, B. Paisdor, D. Gestmann, Angew. Chem. Int. Ed. Engl. 1994, 33, 1251–1253 37 D. Kuck, A. Schuster, B. Paisdor, D. Gestmann, D. J. Chem. Soc., Perkin Trans. 1 1995, 721–732. 38 C. T. Chao, P. Gantzel, J. S. Siegel, K. K. Baldridge, R. B. English, D. M. Ho, Angew. Chem. Int. Ed. Engl. 1995, 34, 2657–2660. 39 D. M. Walba, R. M. Richards, R. C. Haltiwanger, J. Am. Chem. Soc. 1982, 104, 3219–3221. 40 (a) T. Otsubo, F. Ogura, S. Misumi, Tetrahedron Lett. 1983, 24, 4851–4854; (b) T. Otsubo, Y. Aso, F. Ogura, S. Misumi, A. Kawamoto, J. Tanaka, Bull. Chem. Soc. Jpn. 1989, 62, 164–170. 41 (a) J. P. Melder, R. Pinkos, H. Fritz, J. Wörth, H. Prinzbach, J. Am. Chem. Soc. 1992, 114, 10213–10231; (b) R. Pinkos, J. P. Melder, K. Weber, D. Hunkler, H. Prinzbach, J. Am. Chem. Soc. 1993, 115, 7173–7191. 42 (a) M. Kamieth, F. G. Klärner, F. Diederich, Angew. Chem. Int. Ed. 1998, 37, 3303–3306; (b) F. G. Klärner, J. Panitzky, D. Preda, L. T. Scott, J. Mol. Modeling 2000, 6, 318–327. 43 D. Kuck, J. Tellenbröker, H. Bögge, J. Strübe, B. Neumann, H. G. Stammler, unpublished results. 44 Previous reviews on centropolyindane chemistry: (a) D. Kuck, in I. Hargittai, Ed., Quasicrystals, Networks, and Molecules

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of Fivefold Symmetry, VCH Publishers, New York 1990, Chapter 19; (b) D. Kuck, Synlett 1996, 949–965; (c) D. Kuck, Top. Curr. Chem. 1998, 196, 167–220. 45 (a) D. Kuck, A. Schuster, D. Gestmann, J. Chem. Soc., Chem. Commun. 1994, 609–610; (b) D. Kuck, A. Schuster, D. Gestmann, F. Posteher, H. Pritzkow, Chem. Eur. J. 1996, 2, 58–67. 46 (a) D. Kuck, H. Bögge, J. Am. Chem. Soc. 1986, 108, 8107–8109; (b) D. Kuck, Chem. Ber. 1994, 127, 409–425. 47 D. Kuck, M. Seifert, Chem. Ber. 1992, 125, 1461–1469. 48 D. Kuck, Angew. Chem. Int. Ed. Engl. 1984, 23, 508–509. 49 (a) H. W. Thompson, Tetrahedron Lett. 1966, 6489–6494; (b) H. W. Thompson, J. Org. Chem. 1968, 33, 621–625. 50 B. Paisdor, D. Kuck, J. Org. Chem. 1991, 56, 4753–4759. 51 (a) D. Kuck, T. Lindenthal, A. Schuster, Chem. Ber. 1992, 125, 1449–1460; (b) D. Kuck, E. Neumann, A. Schuster, Chem. Ber. 1994, 127, 151–164. 52 I. Ya. Shternberga, Ya. F. Freimanis, J. Org. Khim. USSR 1968, 4, 1044–1048. 53 W. Ten Hoeve, H. Wynberg, J. Org. Chem. 1979, 44, 1508–1514. 54 B. Bredenkötter, U. Flörke, D. Kuck, Chem. Eur. J. 2001, 7, 3387–3400. 55 M. S. Raasch, J. Org. Chem. 1980, 45, 856–867. 56 D. Kuck, A. Schuster, R. A. Krause, J. Org. Chem. 1991, 56, 3472–3475. 57 (a) J. Tellenbröker, D. Kuck, Eur. J. Org. Chem. 2001, 1483–1489; (b) X. P. Cao, D. Barth, D. Kuck, Eur. J. Org. Chem. 2005, 3482–3488. 58 D. Kuck, in R. P. Thummel, Ed., Advances in Theoretically Interesting Molecules, Vol. 4, JAI Press, Greenwich, London, 1998, pp. 81–155. 59 J. Tellenbröker, D. Barth, D. Kuck, unpublished work. 60 D. Kuck, T. Hackfort, Polish J. Chem. 2007, 81, 875–892. 61 J. Tellenbröker, D. Barth, B. Neumann, H. G. Stammler, D. Kuck, Org. Biomol. Chem. 2005, 3, 570–571. 62 M. Harig, D. Kuck, Eur. J. Org. Chem. 2006, 1647–1655. 63 (a) For the first syntheses of molecules bearing Möbius conjugated S-electron

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References systems, see D. Ajami, O. Oeckler, A. Simon, R. Herges, Nature 2003, 426, 819–821. (b) Note that, in contrast to their S-electron system, the constitution of these hydrocarbons is topologically planar. 64 For related studies on Möbius S-electron systems, see (a) C. Castro, Z. Chen, C. S. Wannere, H. Jiao, W. L. Karney, M. Mauksch, R. Puchta, N. J. R. van Eikema Hommes, P. von Rague Schleyer, J. Am. Chem. Soc. 2005, 127, 2425–2432; (b) H. S. Rzepa, Org. Lett. 2005, 7, 4637–4639. 65 J. E. P. Dahl, J. M. Moldowan, T. M. Peakman, J. C. Clardy, E. Lobkovsky, M. M. Olmstead, P. W. May, T. J. Davies, J. W. Steeds, K. E. Peters, A. Pepper, A. Ekuan, R. M. K. Carlson, Angew. Chem. Int. Ed. 2003, 42, 2040–2044. 66 A. T. Balaban, P. von Ragué Schleyer, Tetrahedron 1978, 34, 3599–3609. 67 (a) W. Burns, M. A. McKervey, T. R. M. Michell, J. J. Rooney, J. Am. Chem. Soc. 1978, 100, 906–911; (b) M. A. McKervey, Chem. Soc. Rev. 1974, 3, 479–512; (c) M. A. McKervey, Tetrahedron 1980, 36, 971–992. 68 (a) P. R. Schreiner, A. A. Fokin, O. Lauenstein, Y. Okamoto, T. Wakita, C. Rinderspacher, G. H. Robinson, J. K. Vohs, C. F. Campana, J. Am. Chem. Soc. 2002, 124, 13348–13349; (b) A. A. Fokin, B. A. Tkachenko, P. A. Gunchenko, D. V. Gusev,

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P. R. Schreiner, Chem. Eur. J. 2005, 11, 7091–7101; (c) P. R. Schreiner, N. A. Fokina, B. A. Tkachenko, H. Hausmann, M. Serafin, J. E. P. Dahl, S. Liu, R. M. K. Carlson, A. A. Fokin, J. Org. Chem. 2006, 71, 6709–6720; (d) A. A. Fokin, P. R. Schreiner, N. A. Fokina, B. A. Tkachenko, H. Hausmann, M. Serafin, J. E. P. Dahl, S. Liu, R. M. K. Carlson, J. Org. Chem. 2006, 71, 8532–8540. 69 Note that, in contrast to cyclohexamantane (36), the globular framework of [1(2,3)4]pentamantane (‘Td-pentamantane’, cf. ref. [68d]) is topologically planar. 70 (a) T. Otsubo, S. Mizogami, I. Otsubo, Z. Tozuka, A. Sakagami, Y. Sakata, S. Misumi, Bull. Chem. Soc. Japan 1973, 46, 3519–3530; (b) T. Otsubo, H. Horita, S. Misumi, Synth. Commun. 1976, 6, 591–596. 71 M. Nakazaki, K. Yamamoto, S. Tanaka, H. Kametani, J. Org. Chem. 1977, 42, 287–291. 72 H. Dodziuk, K. S. NowiĔski, Tetrahedron 1998, 54, 2917–2930. 73 C. J. F. Du, H. Hart, K. K. D. Ng, J. Org. Chem. 1986, 51, 3162–3165. 74 The structure of centrohexaindane (5) was used as ‘The Graph of the Conference’, 19th LL-Seminar on Graph Theory, Vienna, April 25–28, 2002. The author thanks Professor Dr. Peter F. Stadler (Leipzig, Germany) for kindly having communicated this to us.

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10 Short-lived Species Stabilized in ‘Molecular’ or ‘Supramolecular Flasks’ Helena Dodziuk

Highly strained hydrocarbons with unusual spatial structure are often short-lived species observed only in cryogenic matrices [1]. Several exciting systems discussed in this volume are of such character. Recently, it has been shown that encaging a highly reactive species in a larger molecule, i.e. forming an inclusion complex, can in certain cases stabilize it. As will be in detail discussed below, such an approach rooted in supramolecular chemistry – a new interdisciplinary border area situated among chemistry, physics, biology and technology – allows one to observe such species even at room temperature. Although there is no precise definition of this field, it flourishes basing on new concepts of molecular and chiral recognition, self-assembly, self-organization and preorganization as well as cooperativity [2]. Formation of a supramolecular complex is known to change the properties of its constituent parts. The most impressive examples of this phenomenon are represented, probably, by the existence of a single nitrogen atom [3, 4] or noble gas molecule [5, 6] in a fullerene cage and by alkali metal anions [7] created due to the high affinity of crown ethers to alkali metal cations resulting in the formation of alkalides like 1. As exemplified by cyclodextrin complexes, the host molecules in such complexes can either stabilize the guest (as in drugs marketed in form of cyclodextrin complexes) or act as catalyst [8]. The former effect has been exploited in the recent syntheses of short-lived species stabilized inside ‘molecular flasks’.

Strained Hydrocarbons: Beyond the van’t Hoff and Le Bel Hypothesis. Edited by Helena Dodziuk Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31767-7

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The beginning of this exciting development was marked by the synthesis of cyclobutadiene 2 in hemicarcerand 3 (Scheme 10.1) by the Cram group [9] in 1990 few years after he had obtained the Nobel Prize for his participation in the formulation of ideas which formed the basis of supramolecular chemistry. As formulated by these authors, ‘Cyclobutadiene, (CH)4, is the Mona Lisa of organic chemistry in its ability to elicit wonder, stimulate the imagination, and challenge interpretive instincts. No other organic compound combines such a fleeting existence and so many different syntheses, with such a propensity for different chemical reactions, and with the variety of calculations of its structure.’ [9]. Prior to the work by the Cram group the existence of cyclobutadiene was proved but its rapid decomposition precluded detailed experimental studies of its structure and properties [10–13]. Against all expectations, 2 synthetized in the hemicarcerand molecular flask 3 (easily obtained by the guest templated synthesis depicted in Scheme 10.2) was found to be extraordinary stable for months at room temperature. The guest molecule was obtained by the photoisomerization and fragmentation J-pyrone

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Scheme 10.1 Photochemical and thermal reactions of J-pyrone: at 8 K in an argon matrix (---------); in solution and gas phase (–––––); in the inner phase of the hemicarcerand 3 ( ––––– ).

Scheme 10.2 Synthesis of the cage compound templated by guest.

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according to Corey procedure [14, 15]. Cyclobutadiene obtained in this way was proved to be in a singlet ground state and to rotate rapidly in the flask 3. However, in the oxygen-free atmosphere, 2 could be kept at room temperature for months. The ‘inner phase’ of carcerands and hemicarcerands was dubbed in [9] ‘a new phase of matter’ essentially different from the inside phases of clathrates and zeolites since ‘one host molecule provides one discrete molecular inner phase not depending on the bulk phase’. Cram predicted that the chemical reactions carried out in such phases could enable one to obtain and study many highly reactive species. The achievement stimulated vigorous research of cyclobutadiene centered mainly on its antiaromaticity [16], the mechanism of its automerization [17], the Jahn–Teller effect [18] and its exotic analogs [19]. However, only recently the few syntheses of new short-lived species in molecular flasks and systematic studies of such reactions have been reported. In 1997 Warmuth accomplished an exciting synthesis of a compound 4 in the carcerand cage 5 [20]. 4 within 5 was prepared in a solution in deuterated THF by photochemical reaction at 77 K. The highly unstable 4 undergoes [4+2] Diels– Alder reaction with the host during the warming according to Scheme 10.3 [21, 22] yielding cycloaddition product 6. Thought-provokingly, on the basis of NMR spectra Warmuth could not decide whether the compound obtained had the o-benzyne 4a or allene 4b structure [20]. The alternative was solved on the basis of high-level ab initio calculations showing that the calculated structure was closer to that of o-benzyne 4a [23, 24]. Another recent accomplishment in this field is the synthesis of highly unstable 1,3,5,6-cycloheptatetraene 7 [25, 26] in the carcerand flask by Warmuth and Marvel [27, 28] (Scheme 10.4). Obtaining 7 by the latter authors was made possible by taking advantage of a small isotope effect taking place in cage 8 which slowed down a competitive reaction leading to the insertion product 9 analogous to 6.

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Scheme 10.3 [4+2] Diels–Alder reaction of the highly unstable 4 with the host during the warming.

Scheme 10.4 Obtaining 1,3,5,6-cycloheptatetraene 7 in photochemical reactions and its decomposition.

In this case the authors were able to assign the singlet 7 not diradical (with a triplet ground state) structure to the product. These studies were reviewed by Warmuth [29]. The observation that C=C bonds in some terpenes have been found to avoid the bridgehead positions was summarized as Bredt’s rule [30]. So-called anti-Bredt molecules violating this rule have been studied extensively [31, 32]. Roach and Warmuth [33] used the cage of 5 as molecular flask enabling them to obtain two such hydrocarbons 10–12. The latter highly strained olefins containing a transcyclohexane or trans-cycloheptane rings are unexpectedly stable when kept in oxygen-free atmosphere.

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Carcerands 3 and 5 played the role of a molecular flask in the syntheses and stabilization of the short-lived hydrocarbon molecules 2, 4, 7, 10–12. The syntheses of the chiral and somewhat hydrophilic hemicarcerand 13 [34] and of the watersoluble hemicarcerand 14 [35] pave the way for the synthesis of chiral and polar short-lived species. The syntheses of 2, 4a, 7, 10–12 were carried out in ‘molecular flasks’. Another milestone in the domain of the synthesis of unstable species were the syntheses in self-assembled cages playing the ‘flask’ role. Several such syntheses were carried out in Fujita [36–39], Raymond [40] and Warmuth [41–43] groups leading to the stabilization of various types of short-lived species that are not hydrocarbons. They represent supramolecular reactions of a higher complexity than those discussed above since they were carried out in self-assembled, not covalently bound, cages. Some of the most important results of this kind have been reviewed by Schmuck [44]. An exciting recognition by a self-assembled cage of a nine-residue peptide, combined with its D-helical folding described by Fujita group deserves mentioning although it falls outside the scope of this book [45]. Fujita [38, 46, 50], Raymond [51–55] and Warmuth [56–62] groups started systematic studies of chemical reactions carried out inside molecular or supramolecular cages allowing us to hope that several other short-lived species will be possible to obtain.

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Only few reactions in ‘molecular flasks’ have been described in literature but they clearly mark the beginning of an exciting development in the border area among organic, supramolecular and theoretical chemistry. Numerous unusual molecules that have been recently proposed as plausible synthetic targets [63] on the basis of quantum calculations may be only short-lived species. Noble gas compounds like HHeF [64], tetrahedrane 15 (of which only two derivatives are known) [65] and its truncated analog 16 [66], bowlane with a pyramidal carbon atom 17 [67], dimethylspiro[2.2]octaplane 18 with a planar carbon atom developed on its basis [68], tricyclohexane 19 having a bond angle, formed by three tetracoordinated carbon atoms, of ca. 180° [69], [1.1.1]geminane 20 with inverted carbon atoms presented in Chapter 2.1 [70], fused bicubane 21 [71], to name but a few,

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may prove to be not only feasible but stable inside molecular capsules which force close contacts of reagents and may even catalyse the synthesis, on one hand, and protect the product from decomposition, on the other. Such prospects will undoubtedly trigger further computational studies of nonstandard species. Moreover, the enabling of the capture of short-lived molecules like silacyclopropyne 22 known as matrix-isolated species with C{C–Si angle close to 60° [72], fullerene C20 23 [73, 74], or transient intermediates like carbocations [75, 76] and carbenes [77] in (super)molecular cages will beyond question give an impetus to the studies of reaction mechanisms. The molecules 2, 4a, 7 and 23 are symmetrical, aesthetically pleasing systems. So are the hypothetical light noble gas compounds and most of 15–22 awaiting their syntheses. At the advent of a fruitful mutual interaction among organic synthesis and supramolecular and quantum chemistry one can say repeating, with some additions, the words by Goethe’s Faust: You highly strained molecule ‘Stay indeed! You are so beautiful!’

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39 Takaoka, K.; Kawano, M.; Ozeki, T.; Fujita, M. Chem. Commun. 2006, 1625. 40 Ziegler, M.; Brumaghim, J. L.; Raymond, K. N. Angew. Chem. Int. Ed. 2000, 39, 4119. 41 Liu, X. J.; Chu, G. S.; Moss, R. A.; Sauers, R. R.;Warmuth, R. Angew. Chem. Int. Ed. 2005, 44, 1994. 42 Tiedemannm B. E. F.; Raymond, K. N. Angew. Chem. Int. Ed. 2006, 45, 83. 43 Pluth, M. D.; Warmuth, R. Chem. Soc. Rev. 2007, 36, 161. 44 Schmuck, C. Angew. Chem. Int. Ed. 2007, 46, 5830. 45 Tashiro, S.; Tominaga, N.; Tamaguchi, Y.; Kato, T.; Fujita, M. Chem. Eur. J. 2006, 12, 3211. 46 Nishioka, Y.; Yamaguchi, T.; Yoshizawa, M.; Fujita, M. J. Am. Chem. Soc. 2007, 129, 7000. 47 Nakabayashi, K.; Kawano, M.; Kato, T.; Furukawa, K.; Ohkoshi, S.-i.; Fujita, M. Chem. Asian J. 2007, 2, 164. 48 Suzuki, K.; Kawano, M.; Fujita, M. Angew. Chem. Int. Ed. 2007, 46, 2819. 49 Maurizot, V.; Yoshizawa, M.; Kawano, M.; Fujita, M. Dalton Trans. 2006, 2750. 50 Yoshizawa, M.; Tamura, M.; Fujita, M. Science 2006, 312, 251. 51 Tiedemannm B. E. F.; Raymond, K. N. Angew. Chem. Int. Ed. 2007, 46, 4976. 52 Pluth, M. D.; Bergman, R. G.; Warmuth, R. Science 2007, 316, 85. 53 Leung, D. H.; Bergman, R. G.; Warmuth, R. J. Am. Chem. Soc. 2007, 129, 2746. 54 Fiedler, D.; van Halbeek, H.; Bergman, R. G.; Warmuth, R. J. Am. Chem. Soc. 2006, 128, art. no. JA062329B. 55 Fiedler, D.; Bergman, R. G.; Warmuth, R. Angew. Chem. Int. Ed. 2006, 45, 745. 56 Warmuth, R.; Makowiec, S. J. Am. Chem. Soc. 2007, 129, 1233. 57 Liu, X. J.; Warmuth, R. J. Am. Chem. Soc. 2006, 128, 14120. 58 Liu, X. J.; Warmuth, R. Angew. Chem. Int. Ed. 2006, 44, 7107. 59 Carrera, S. S.; Kerdelhue, J. L.; Langenwalter, K. J.; Brown, N.; Warmuth, R. Eur. J. Org. Chem. 2005, 2239.

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10 Short-lived Species Stabilized in ‘Molecular’ or ‘Supramolecular Flasks’ 60 Warmuth, R. Eur. J. Org. Chem. 2001, 423. 61 Warmuth, R.; Kerdelhue, J. L.; Carrera, S. S.; Langenwalter, K. J.; Brown, N. Angew. Chem. Int. Ed. 2002, 41, 96. 62 Kerdelhue, J. L.; Langenwalter, K. J.; Warmuth, R. J. Am. Chem. Soc. 2003, 125, 973. 63 Dodziuk, H. In Modern Conformational Analysis. Elucidating Novel Exciting Molecular Structure; VCH Publishers: New York, 2005. 64 Zefirov, I. S.; Koz’min, A. S.; Abramenkov, A. B. Usp. Khim. 1978, 47, 289, Engl. transl. p. 163. 65 Maier, G.; Pfriem, S.; Schäfer, U.; Matusch, R. Angew. Chem. Int. Ed. 1978, 17, 520. 66 Maier, G.; Born, D. Angew. Chem. Int. Ed. 1989, 28, 1050. 67 Dodziuk, H. J. Mol. Struct. 1990, 239, 167. 68 Rasmussen, D. R.; Radom, L. Angew. Chem. Int. Ed. 1999, 38, 2876.

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69 Dodziuk, H.; Leszczynski, J.; Nowinski, K. S. J. Org. Chem. 1995, 60, 6860. 70 Dodziuk, H.; Leszczynski, J.; Jackowski, K. J. Org. Chem. 1999, 64, 6177. 71 Seidl, E. T.; Schäfer III, H. F. J. Am. Chem. Soc. 1991, 113, 1915. 72 Maier, G.; Pacl, H.; Reisenauer, H. P.; Meudt, M.; Janoschek, R. J. Am. Chem. Soc. 1995, 117, 12712. 73 Prinzbach, H.; Weller, A.; Landenberger, P.; Wahl, F.; Worth, J.; Scott, L. T.; Gelmont, M.; von Olevano, D.; Issendorff, B. Nature 2000, 407, 60. 74 Sackers, E.; Obwald, T.; Weber, K.; Keller, M.; Hunkler, D.; Worth, J.; Knothe, L.; Prinzbach, H. Chem. Eur. J. 2006, 12, 6242. 75 Fernandez, L.; Marti, V.; Garcia, H. Phys. Chem. Chem. Phys. 1999, 1, 3689. 76 Laali, K. K.; Okazaki, T.; Coombs, M. M. J. Org. Chem. 2000, 65, 7379. 77 Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. Rev. 2000, 100, 39.

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11 Concluding Remarks Helena Dodziuk

Our journey through the domain of strained hydrocarbons with unusual spatial structure is coming to its end. It is time to attempt to sketch its future development as it seems today. As described in detail in preceding chapters, several fascinating hydrocarbons with unusual spatial structure still await their syntheses. Geminanes, like 1, with inverted carbon atoms discussed in Chapter 2.1; bowlane 2 and dimethanospiro[2.2]octaplane 3 (Chapter 2.2) with pyramidal and planar carbon atoms, respectively; parent diethynyl expanded prismanes, like 4 (Chapter 2.3); truncated tetrahedrane 5 and hexaprismane 6 (Chapter 2.4); and centrohexaquinane 7, which exhibits little strain but is still unknown (Chapter 9) as well as numerous unusual unsaturated and/or conjugated hydrocarbons that have been predicted to be stable but whose syntheses have so far been unsuccessful. Taking into account the great achievements of the Cram group, that succeeded in obtaining cyclobutadiene 8 [1], and of the Warmuth group involving o-benzyne 9 [2, 3] taking advantage of supramolecular approach (Chapter 10), even the synthesis of the elusive highly strained parent tetrahedrane 10 cannot be excluded. These reactions in ‘molecular flasks’ allow one to obtain and stabilize short-lived molecules, and will certainly enable studies of exciting molecules with the structure strongly departing from the standard one.

Strained Hydrocarbons: Beyond the van’t Hoff and Le Bel Hypothesis. Edited by Helena Dodziuk Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31767-7

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The synthetic chemistry of fullerenes will undoubtedly flourish. One can expect that it will develop in four directions: 1. In addition to a total synthesis of the parent fullerene, those of more specific C60 and higher fullerene derivatives will be vigorously pursued, driven not only by pure interest but also by the prospects of marketable applications. 2. Syntheses of larger fullerene cages will be indispensable for applications of endohedral fullerene complexes. 3. With the same purpose in mind, the syntheses of ‘opened’ fullerenes with a hole and their chemical closing, is discussed in Chapter 5.5. In this respect, the author is very interested to discover whether Murata group [4] will succeed in closing C70 with two hydrogen molecules inside (discussed in Section 5.5), as this had been calculated to be less stable than the complex involving only one H2 molecule inside the cage [5]. 4. Today ‘in’ isomers of hydrogenated fullerenes are hypothetical moieties but, taking into account their unusual structure and predicted interesting properties (for example, the exciting possibility of reactions between substituents inside the cage discussed in Chapter 2.4), they could become a hot topic when the appropriate synthetic methods will be developed. One can also expect numerous synthetic investigations in the field of carbon nanotubes although in this area aggregate formation, called synkinesis by Fuhrhop and Köning [6], will be more actively carried on than the standard synthesis. Physicochemical studies, interesting per se and forming the basis of applications of strained hydrocarbons as well as model calculations of these systems will thrive. As discussed in detail in Chapter 1.2, the applications are not (and should not be) the major driving force for studying strained hydrocarbons. However, eventually many strained hydrocarbons will be used as drug carriers, parts of molecular (or supramolecular) devices, etc. And last, but not least, unusual hydrocarbons or reagents from which they are made will be the subject of rapidly developing supramolecular chemistry. Endohedral fullerene complexes as well as nested fullerenes and multiwalled carbon nanotubes on the one hand and gorgeous supramolecular cube (crystallized from water solution of 11) obtained by the Kuck group in their quest for centropolyindanes [7] illustrate this point.

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References

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I hope that this book has shown that the domain of strained hydrocarbons is an interdisciplinary field in which synthetic, physical and theoretical chemists are actively involved. Driven by the interest of numerous specialists from diversified areas the field will undoubtedly flourish.

References 1 Tanner, M. E.; Knobler, C. B.; Cram, D. J. Angew. Chem., Int. Ed. 1991, 30, 1924. 2 Warmuth, R. Angew. Chem. Int. Ed. 1997, 36, 1347. 3 Jiao, H. J.; Schleyer, P. V.; Beno, B. R.; Houk, K. N.; Warmuth, R. Angew. Chem. Int. Ed. 1998, 36, 2761. 4 Koichi, K.; Murata, M. M. S.; Murata, Y. In: 230th ACS National Meeting 2005 2005, p ORGN-338.

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5 Dodziuk, H. Chem. Phys. Lett. 2005, 410, 39. 6 Fuhrhop, J.-H.; Koening, J. Membranes and Molecular Assemblies. The Synkinetic Approach; The Royal Society: Cambridge, 1994. 7 Kuck, D. Chem. Rev. 2006, 106, 4885.

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Index a acenaphthene skeleton 78 acenaphthene-5,6-diyl bis(diarylmethylium) 78 acene – twisted 23 [e,l] acephenanthrylene 184 acetylene 26 actuator 363 adamantene 103 adamantylideneadamantane 108 – derivative 108 alkaplane 48 alkenes – acyclic 106 – bridgehead 103 f. – computational data 114 – distorted 103 ff., 112 – nonplanar 103 ff. – planar 112 ff. – pyramidalized 112 ff. alkylidenecycloproparene 182 alkynes 75 f. all-cis-tribenzo[5.5.5.5]fenestrane 434 allene – bicyclic 25 – S-bond deformation 123 – cyclic 123 – eight-membered ring 134 – four- and ﬁve-membered ring 124 – polycyclic 134 f. – seven-membered ring 131 – strain estimate 123 – strained cyclic 122 ff. amylose 351 annulene 399 ff. – higher 411 ff. [4]annulene 399 ff. [8]annulene 403 ff. [10]annulene 411 f.

methano-bridged 412 [12]annulene 411 f. [14]annulene 411 f. [16]annulene 411 f. [18]annulene 411 antiaromaticity 399 ff. S-antiaromaticity 401 V-antiaromaticity 50 antibacterial activity 313 armchair nanotube 337 f. aromatic character 164 aromatic ring – aromatic character 153 f. – distorted 164 aromatics – alkylated 148 – strained 147 ff. aromaticity 401 V-aromaticity 50 asterane 52 f. – 3-asterane 53 – 4-asterane 53 automerization 402 f. azacycloheptatetraene 135 aza[60]fullerene 220

b B3LYP (Becke three-parameter exchange functional coupled with Lee-Yang-Parr correction) 290 back-clump 75 barbaralane 414 f. barbaralone 414 barrelene 410 battery 365 benzdiyne 178 benzene – non-standard 147 ff. benzene rings – nontypical spatial structure 22

Strained Hydrocarbons: Beyond the van’t Hoff and Le Bel Hypothesis. Edited by Helena Dodziuk Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31767-7

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Index benzocentrohexaquinane 438 benzophane 157 benzo[a]phenanthrene 148 m-benzyne 17 [m][n]betweenanene 21 1,1c-bi(adamantyl) 87 bi[4]asterane 58 bi(bicyclo[1.1.0]butyl) 83 bi(bicyclo[1.1.1]pentyl) 83 – derivative 88 1,1c-bi(bicyclo[1.1.1]pentyl) 87 bi(cubyl) 83 bicycloalkane 91 bicyclo[n.n.n]alkane 91 bicycloalkene 107 – hydrogenation enthalpy 107 – multiply unsaturated 109 bicyclobutane – inverted geometry 33 bicyclo[2.2.1]hept-2-en-5-yne 382 bicyclo[3.3.0]oct-1(5)-ene 113 bicyclo[4.4.1]undecapentaene 109 bicyclo[4.4.2]dodecapentaene 109 bicyclo[5.1.0]octa-2,5-diene 413 bicyclo[5.3.1]undecapentaene 109 bicyclo[5.3.1]undeca-1,3,5(11)-triene 110 bifurcation point 410 1,1c-bi(homocubyl) 88 Bingel reaction 214 ff. bio-related material 365 bipentaprismane 58 bi[n]prismane 57 bis(acetoxymethyl)tricyclo[2.1.0.02,5]pentan3-one 84 bisaddition – fullerene 213 bisallene – cyclic 136 bishomoaromatic 419 bis(triphenylmethanol) 75 bi(tetrahedranyl) 83 1,1c-bi(tricyclo[3.1.0.02,6]hexyl) 88 f. 1,1c-bi(tricyclo[4.1.0.02,7]heptyl) 88 bond shifting 405 bond – nonplanar C=C 112, 144 – nonplanar C–C 44, 68 f. – shifting 405 – ultralong C=C 70 – ultrashort C=C 82 bowlane 17, 46, 455 – dimer 48 bulk-heterojunction 305 bullvalene 414 f.

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butatriene – S-bond deformation 137 – cyclic 137 ff. – ﬁve- to nine-membered ring – strain estimate 137 9-t-butylanthracene 148 (2-t-butylcubyl)cubane 88

137

c C60, see also fullerene – derivative for optical limiting 299 – derivative for photovoltaic application 304 – ﬁve-membered ring-fused 222 – four-membered ring-fused 221 – six-membered ring-fused 223 – terminated oligo(p-phenylene ethynylene) (OPE) hybrid compound 302 – three-membered ring-fused 218 C60-OPE hybrid 302 C60(X)n isomer – steric strain 236 ff. C70 295 C–C bond – elongated 74 – expandability of ultralong bond 79 – stiff molecular frame 72 – super-ultralong 78 – ultralong 70 ff. – ultrashort 82 ff. – ultrashort endocyclic bridging 83 ff. – ultrashort exocyclic intercage 86 C(sp2)-C(sp2) bond – fullerene cage 232 C=C bond – nonplanar 112 ff. – stretch 116 C–C(Me) bond – ultrashort 91 C2v-[15]triangulene 19 C10H10 saturated cage 63 C12H12 saturated cage 63 C60H6 293 C60H18 294 C60H36 294 f. C60H60 – all-out isomer 66 – atom numbering 66 – in isomers 66 – ten-in isomer 66 C60(OH)n 313 C60-tetrathiafulvalene (TTF) dyad 302 C70H2 298 2– C70 298

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Index C70H8 296 (CH)2n cage structure 59 ff. cage structure – nonplanar C=C bond 112 ff. carbene 456 carbocation 456 carbohelicene 166 carbon atom – inverted 43, 58 – planar 44 ff. – pyramidal 44 ff. carbon nanotube (CNT) 335 ff. – applications 356 ff. – armchair 337 – aromaticity 345 – bamboo-like structure 344 – cap structure 343 – chiral 338 – cycloaddition 353 – electrical conductivity 339 – ﬂuorination 353 – functionalization 347 ff. – HRTEM 341 – hydrogenation 352 – multi-walled (MWNT) 342 ff. – properties 357 – resonance structure 346 – safety 360 – semiconductor-type 365 – single-walled (SWNT) 335 ff. – structure 342 f. – symmetry 338 – toxicity 360 – zigzag 337 carboxyfullerene 312 carcerand cage 452 ff. catalysis – electrocatalytic activity of fullerenes 270 catenane 7 centrohexaindane 430 ff. – multiply functionalized 436 centrohexaquadrane 426 centrohexaquinane 426 centrohexaquinacene 427 centropentaindane 432 ff. centropolycyclane 426 centropolyindane 431 trifuso-centrotetraindane 431 f. centrotriindane 432 chemical sensor 365 chiral nanotube 338 [6]chochin 440 [n]circulene 149

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465

[7]circulene 149 Clar resonance structure 345 f. clathrate 451 clump 75 chochin 24 computational chemistry 15 conductive composite 364 ff. corannulene 25, 149 coronene 149 coupled cage compound 86 – ultrashort exocyclic intercage C–C bond 86 cubane 8 f., 50, 59 ff. – highly strained derivative 13 – oligomer 56 cubene 55, 62 cubylcubane 56 cumulene 25, 122 – cyclic 136 cuneane 59 ff. cyclic system – conjugated 401 cyclization – Hopf 127 – Myers-Saito type 131 cycloaddition 353 f. – carbon nanotube 353 f. cycloalkene 107 cycloalkyne – angle-strained 375 ff. – distorted 375 – spectroscopic property 392 cyclobutadiene 399 ff., 450 – bond length 401 1,2-cyclobutadiene 124 cyclobutyne 378 cyclodecyne 386 cyclodextrin 449 1,2-cycloheptadiene 131 trans-cycloheptane 453 cycloheptatetraene 132 – 1,2,4,6-cycloheptatetraene 132 – 1,3,5,6-cycloheptatetraene 453 – incarterated 134 1,2,4,5- cycloheptatriene 133 1,3,5-cycloheptatriene 413 cycloheptatrienylidene 188 cycloheptyne 384 1,2-cyclohexadiene 125 cyclohexamantane 439 f. cyclohexane – planar ring 67 ff. – trans 454 – trispirocyclopropanated 19

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Index 1,2,3-cyclohexatriene 138 1,2,4-cyclohexatriene 127 f. E-cyclohexene 103 cyclohexyne 382 cyclononyne 386 1,2-cyclooctadiene 135 cyclooctatetraene (COT) 403 ff. – annelated 406 – bond shifting 407 – conformation 404 – ion 410 f. – ring inversion 407 [2.2]cyclooctatetraenophane 22 E-cyclooctene 103 ff. Z-cyclooctene 107 cyclooctyne 385 f. 1,2-cyclopentadiene 124 f. cyclopentyne 379 cyclophane 12, 23, 67, 109, 150 ff. – distorted 153 – K3,3 438 – Kuratowski 430 ff., 441 ff. – naphthalene-based 439 – NMR characteristics 165 [n]cyclophane 154 [m,n]cyclophane 151 [2,2]cyclophane 151 [2.2.2](1,3,5)cyclophane 165 [26](1,2,3,4,5,6)cyclophane-1-ene 24 cyclopropabenzene 192 cyclopropabenzenyl anion 191 cyclopropannulene 178 cycloproparene 176 ff. – charge transfer (CT) complexation 189 – heteroatom 187 cyclopropyne 376 cyclopropynylidene 376

d decaprismane C20H20 50 Td-1,2-dehydro-5,7-adamantanediyl dication 16 1,2-dehydrocubane 55 m-dehydrocubane 17 dehydroprismane 55 1,5-dehydroquadricyclane 17 dehydrotriprismane 55 density functional theory (DFT) method 15 density of state (DOS) 243 f. density of unoccupied state (DUOS) 243 f. diademane 60 ff. di-p-benzhexaphyrin 23 dibenzylation 2– 298 – C70

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di-t-butylbenzene 148 1,2-di-t-butylethylene 108 (Z)-1,2-di-t-butylethylene (2,2,5,5-tetramethylhex-3-ene) 106 dichloro[3.2.1]propellane 34 9,9c-didehydroanthracene (DDA) 113 Diels-Alder reaction – fullerene 213 diiodonaphthocyclobutene 71 5,6-dilithioacenaphthene 78 dimethanospiro[2.2]octaplane 48 N,Nc-dimethylaniline 268 1,1-dimethyl-2,3-bis-(t-butyl)cyclobutane 106 1,2-dimethyl-2,3-bis-(t-butyl) methylcyclopropane 106 2,3-dimethyl-2-butene 108 1,1-dimethyl-2-neopentylcyclopropane 106 dimethylspiro[2.2]octaplane 455 E-1-(2,2-dimethyl-1-tetralinylidene)2,2-dimethyltetralin 109 1,5-dimethyltricyclo[2.1.0.02,5]pentan3-one 84 2,4-dimethyltricyclo[2.1.0.02,5]pentane 85 3,6-dimethoxycyclopropa[b]naphthalene 181 9,9c-diphenyl-9,9c-biﬂuorenyl 75 trans-diphenylspirotriketone 433 DNA photocleavage 310 dodecadiyne 388 dodecahedrane 12, 60 – C20H20 64 dodecahedron 20 Doering-Moore-Skattebøl rearrangement 125 ff. double bond – distorted 21 drop casting method 274

e electrochemistry 259 f. electron emitter 366 electron energy loss spectroscopy (EELS) 243 ff. electron-mediating property 272 electronic part 363 enyne – Diels-Alder reaction 126 – photorearrangement 124 ethylene – most distorted 111 – t-butyl-substituted 104 ff. Z-ethylene 104

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Index

f syn-fenchilydenefenchane 104 fenestrane 17, 44 ff. [4.4.4.4]fenestrane 44 all-cis-[5.5.5.5]fenestrane 431 all-cis-[5.5.5.6]fenestrane 433 fenestranone 434 fenestrindane 431 ferrocene 268 ferrocene (Fc/Fc+) system 262 ferrocene-porphyrin-fullerene construct 269 ﬁlm preparation method 274 fullerene 6 ff., 205 ff. – absorption spectrum 280 – E-alanine derivative 313 – alkylation 217 – application 299 ff. – binding energy 290 – biological application 310 ff. – bond 209, 232 – C20 11, 206 – C60, see also C60 6 f., 205 ff. – C80 207 – cage 232 – cyclic voltammetry 269 f. – cycloaddition 218 – cyclobutane-annulated 221 – E-cyclodextrine subunit 311 – density of state (DOS) 243 f. – density of unoccupied state (DUOS) 243 f. – Diels-Alder adduct 223 – DNA photocleavage 310 – electrocatalytic activity 270 – electron afﬁnity 260 – electronic property 259 – electronic structure 246 – endohedral 282 ff. – ﬁlm 274 ff. – ﬁlm property 277 – fullerene interaction 278 – functionalization 263 – guest substrate 288 – higher 297 – host 287 – hydrogenated 291 – hydrogenation 216 – intra-/intermolecular association 263 – in-out isomerism 64 – ionization potential 260 – isomer 212, 292 ff. – nanostructured ﬁlm 276 – non-IPR stability 232

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– nonplanar steric strain 226 ff. – nonplanar steric strain parameter 229 – nuclear magnetic resonance (NMR) 250 ff. – open cage 284 f. – orbital picture 243 ff. – oxidation 215 – perhydrogenated 60 – physicochemical property 225 – POAV (S-orbital axis vector) analysis 226 ff. – polyhydroxylated 313 – porphyrin hybrid 312 – property 255 – radical addition 211 – reaction 210 – reactivity of hydrogenated fullerene 297 – reduction 215 – reduction potential 261 – reversible molecular incorporation and ejection 285 – Schlegel diagram 226 f. – siloxane group 303 – single crystal X-ray structure 225 – sol-gel glasses 300 ff. – structure 291 – superconductivity 281 – synthesis 291 – tris-malonic acid 312 – UV/Vis absorption spectrum 247 f. – vibrational spectra 239 ff. – water interaction 278 [60]fullerene 208 – photovoltaic application 304 fullerene aggregate 273 fullerene-ferrocene dyad 268 fullerene-OPE hybrid 303 fullerene-porphyrin hybrid 312 fullerene-TTF dyad 303 fullerenol (C60(OH)n) 313 fullerodendrimer 301 fulleroid 220 fulleropyrrolidinium salt 313 fulvalene 111 functionalization 347 ff. – covalent side-wall 352 – endohedral 355 – non-covalent 349 f.

g graph – nonplanar 428 graphene 337

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Index

h H2@C60 284 ff. He@C60 282 ff. helicene 166 – asymmetric synthesis 171 – nonracemic 171 – physicochemical property 173 – structure 172 [4]helicene 148 [5]helicene 168 ff. [6]helicene 166 ff. [7]helicene 168 f. [14]helicene 172 helvetane 64 hemicarcerand 450 ff. heptacyclo[6.4.0.02,4.03,7.05,12.06,10.09,11] dodecane 63 heterophane 157 heterohelicene 166 heterojunction 305 ff. hexaanilino[60]fullerene 217 hexa-t-butylbenzene 22 hexaene 25 hexagon-hexagon-pentagon junction (HHP) 225 hexahelicene 166 hexahydrosuperphane 67 hexakis(trimethylsilyl) derivative 22 hexanitro[60]fullerene 217 hexaphenylethane (HPE) 73 – acenaphthalene-type 80 – bis(10-methylspiroacridine)-type 81 – clumped derivative 74 – cross-clumped 75 – derivative with super-ultralong C–C bond 78 – dihydropyracylene-type 80 – dispiro 75 hexaprismane 67 homoaromaticity 109, 411 homotropilidene 413 bridged 413 ff. homotropylium cation 411 Hopf cyclization chemistry 128 Hückel rule 408 hydrocarbon – bicyclic 20 – distorted saturated 33 ff. – highly strained natural compound 13 – K3,3 438 – K5 438 – saturated 18, 33 ff. – short-lived 449 ff. – strained, see strained hydrocarbon

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– unusual spatial structure hydrogenation enthalpy bicycloalkene 107

5

i in-out isomerism 59 – perhydrogenated fullerene C60H60 64 indium tin oxide (ITO) 304 inversed photoemission spectroscopy (IPES) 243 f. inverted geometry 33 isobenzene 127 isodesmic equation 15 isolated-pentagon rule (IPR) 228 isomorph – conformational 79 israelane 64

k K3,3 molecule 438 K5 molecule 438 Knight shift – isotropic 252 Korringa relation 258 Kuratowski – cyclophane 430 ff., 441 ff. – graph 428

l ladderane 12 Langmuir-Blodgett (LB) method 274 ff. Langmuir-Schäffer (LS) method 274 ff. low-friction surface 364

m magic angle spinning (MAS) 256 membrane 364 [n]metacyclophane 154 ff. [1]metacyclophane 150 [2.2]metacyclophane 24 [2,2]metaparacyclophane 152 [5]metacyclophane 110, 150 f. [6]metacyclophane 110, 151 metallacyclopenta-2,3,4-triene 139 metallacyclopropabenzene 187 methane – planar 45 – tetrahedral 45 1,5-methano[10]annulene 109 1,6-methano[10]annulene 109, 412 methanofullerene 220 methano[70]fullerene 265 1-(3-methoxycarbonyl)propyl-1-1-phenyl-[6,6] methanofullerene ([60]PCBM) 306

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Index in-methylcyclophane 91 6-methylene-1,2,4-cyclohexatriene N-methylphenothiazine 268 Mills-Nixon effect 190 Möbius strip 23 molecular ﬂask 449 ff. molecule – nonplanar 425 ff. monobenzyl C70H2 298 multiporphyrin dendrimer 271

p 131

n N@C60 289 nanokid 11 nanoputane 11 nanotechnology 6 nanotube, see also carbon nanotube 335 ff. nanotube cap 340 naphthalene 148 naphthalene-1,8-diyl bis(diarylmethylium) 75 naphthalenophane 439 [6.6]naphthalenophane 390 naphthocyclobutene derivative 71 Ne@C60 287 near-ﬁeld microscopy probe 366 nitrocubane 52 NMR (Nuclear Magnetic Resonanz) 38 f., 109, 151, 250 ff. norcaradiene 412

o octabisvalene 59 ff. octacyclopropylcubane 12 ff. octahedrane 12 – highly strained derivative 13 octaplane 47 octiphenyl 442 – octabromo derivative 442 oleﬁn strain energy (OSE) 55 olympiadane 7 OPE-C60 hybrid (oligophenyleneethynylene-C60) 309 optical limiting (OL) 299 OPV-C60 hybrid (oligophenylenevinylene-C60) 308 f. organofullerene 267 – electron donor moieties 267 orthogonene 17 oxa[3.2.1]propellane 34

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ʌ–ʌ interaction – carbon nanotube 345 ff. – concave–convex 25 P3HT (poly(3-hexylthiophene)) 307 paddlane 44 ff. [1.1.1.1]paddlane 42 padogane 60 – heterobridged 439 para-cyclophane 24 [n]paracyclophane 154 ff. [m.n]paracyclophane 161 [0.0]paracyclophane 161 [1.n]paracyclophane 161 [1.1]paracyclophane 161 f. [2.2]paracyclophane 110, 161 f. [2.2]paracyclophane/dehydroannulene hybrid 389 [2.2:2.2:2.2:2.2:2.2]paracyclophane 440 [4]paracyclophane 159 [4.4]paracyclophane 163 [6]paracyclophane 158 [10]paracyclophane 155 [60]PCBM 306 pentaene 25 pentamantane 439 pentaprismane 50 ff., 60 ff. percubylcubane 56 perﬂuorotetracyclobutenocyclooctatetraene 408 [5]pericyclyne – permethylated 26 [6]pericyclyne – permethylated 26 (Ph3P)2Pt complex 119 f. phenylcarbene hemicarceplexed 134 photocyclodehydrogenation 169 photodehydrocyclisation 168 photodynamic therapy (PDT) 310 photorearrangement – enyne 124 photovoltaic cell 309 photovoltaic conversion 308 phthalocyanine 304, 351 POAV (S-orbital axis vector) analysis 226 ff., 408 polyacetylene – cyclic 387 f. polyaniline (PANI) 351 polycubane 56 polycyclane 426

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470

Index polyether – topologically nonplanar 438 polyhydroxyfullerene 212 polymantane 439 polyprismane 56 poly[n]prismane 57 porphyrin 351 porphyrin-fullerene-dyad (P-C60) 312 – metalated 312 Prato reaction 213 ff. prismane 16, 49 – C2nH2n prismane 49 – expanded 52 ff. – ethynyl-expanded 54 – fused 57 [n]-prismane 64 [6]prismane 64 [7]prismane 64 pristine fullerene 278 propellane – small-ring 35 f. [k.l.m]propellane 33 f. – inverted geometry 33 [k.1.1]propellane 43 [1.1.1]propellane 9 ff., 34 ff. – precursor 42 – preparation and reactivity 38 ff. [2.1.1]propellane – preparation and reactivity 40 f. [2.2.1]propellane – preparation and reactivity 40 f. [2.2.2]propellane 35 [4.1.1]propellane 35 propella[34]prismane 62 pyramidalization 114 ff. pyramidane 17 – derivative 16 pyrene derivative 350

r ratchet phase 256 reverse saturable absorption (RSA) ring – inversion 405 – nonplanar C=C bond 112 ff. rotator phase 256 rotaxane system 6

300

s scavenger activity 313 semibullvalene 404 ff., 414 ff. – octasubstituted 418 short-lived species – stabilization in molecular ﬂask 449 ff.

1410vch12.indd 470

silacyclopropyne 377, 456 sol-gel glasses 300 f. solar cell 305 ff. – all-polymer 307 spin coating method 274 spiropentane 18 spiro[3.3]pentane 48 [n]staffane 42 stilbene – cyclic 108 f. – t-butyl-substituted 108 f. strain energy 14, 104 f. strain estimate 123, 137 strained hydrocarbon 12 – computation 12 structural composite 363 supercapacity 365 superconductivity 8 – fullerene 281 supercubane 56 supramolecular ﬂask 449 ff. surfactant 275

t 1,1,2,2-tetraarylacenaphthene derivative 75 tetraasterane 19 tetracyclo[3.1.0.01,3.03,5]hexane 17 tetra-tertiary-butyltetrahedrane 403 tetrahedral coordination 425 f. tetrahedrane 12, 61 ff., 403, 455 – bis-tetrahedrane 17 tetrakisbicyclohexenocyclooctatetraene 408 tetrakis-t-butylethene 21 tetralin 150 anti-tetramantane 439 tetramethyl cyclooctatetraene 407 tetraphenylnaphthocyclobutene 73 7,7,8,8-tetraphenyl-o-quinodimethane 73 tetraphosphabarbaralane 415 tetrathiafulvalene (TTF) 268 3,3,7,7-tetramethylcycloheptyne 384 4-thia-3,3,5,5-tetramethylcyclopentyne 379 topography 425 ff. topology 425 ff. triangulane 19 all-cis-tribenzo[5.5.5.5]fenestrane 434 tribenzo[5.5.5.5]fenestrene 434 tri(bicyclo[1.1.1]pentyl) derivative 88 tri-t-butylethylene 106 1,2,3-tri-t-butylnaphthalene 22 tricyclo[2.1.0.01,3]pentane 17

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Index tricyclo[2.1.0.02,5]pentan-3-one 83 ff. tricyclo[2.1.0.02,5]pentane 83 f. – ultrashort endocyclic bridging C–C bond 83 ff. tricyclo[3.1.01,3.03,5]hexane 18 tricyclo[3.3.n.03,7]alk-3(7)-ene 113 ff. tricyclo[3.3.0.03,7]oct-1(7)-ene 119 tricyclo[3.3.9.03,7]non-3(7)-ene 117 tricyclo[3.3.10.03,7]dec-3(7)-ene 117 tricyclo[3.3.11.03,7]undec-3(7)-ene 115 [4.1.0.01,6]tricycloheptane 18 tricyclo[4.2.2.22,5]dodeca-1,5-diene 21 triene 25 trifuso-centrotetraindane 431 f. trioxacentrohexaquinane 438 triprismane 50, 62 tri[n]prismane 55 triprismene 55 tris-V-homobenzene 69 – cis and trans 69 tropolidene 413

1410vch12.indd 471

471

u ultraviolet photoemission spectroscopy (UPS) 243 ff. umbrella conﬁguration 58 UV/Vis spectra 432 UV/Vis spectra of fullerenes 246 ff.

v valence isomerization

404 ff.

w windowpane, see fenestrane

x X-ray of fullerenes 225 ff. X-ray absorption spectroscopy (XAS) xylene 148

243

z zeolite 452 zero-point vibrational energy (ZPVE) 15 zigzag nanotube 337 zirconacyclohepta-2,4,5,6-tetraene 139

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