HYPERCARBON CHEMISTRY

HYPERCARBON CHEMISTRY

Second Edition

GEORGE A. OLAHG. K. SURYA PRAKASHKENNETH WADEÁRPÁD MOLNÁRROBERT E. WILLIAMS

A JOHN WILEY & SONS, INC., PUBLICATION

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Library of Congress Cataloging-in-Publication Data:Hypercarbon chemistry / by George A. Olah . . . [et al.]. – 2nd ed. p. cm. Includes index. ISBN 978-0-470-93568-2 (cloth) 1. Carbonium ions. 2. Organometallic chemistry. I. Olah, George A. (George Andrew), 1927- QD305.C3H97 2011 547.01–dc22 2010044306

Printed in the United States of America

ePDF ISBN 9781118016442ePub ISBN 9781118016459oBook ISBN 9781118016466

10 9 8 7 6 5 4 3 2 1

In Memory of the Late Professor William N. Lipscomb

CONTENTS

Foreword to the First Edition xiii

Preface to the Second Edition xv

Preface to the First Edition xvii

1. Introduction: General Aspects 1

1.1. Aims and Objectives 11.2. Some Defi nitions 21.3. Structures of Some Typical Hypercarbon Systems 51.4. The Three-Center Bond Concept: Types of

Three-Center Bonds 101.5. The Bonding in More Highly Delocalized Systems 211.6. Reactions Involving Hypercarbon Intermediates 26References 31

2. Carbon-Bridged (Associated) Metal Alkyls 37

2.1. Introduction 372.2. Bridged Organoaluminum Compounds 412.3. Beryllium and Magnesium Compounds 502.4. Organolithium Compounds 532.5. Organocopper, Silver, and Gold Compounds 582.6. Scandium, Yttrium, and Lanthanide Compounds 622.7. Titanium, Zirconium, and Hafnium Compounds 642.8. Manganese Compounds 662.9. Other Metal Compounds with Bridging Alkyl Groups 68

vii

viii CONTENTS

2.10. Agostic Systems Containing Carbon–Hydrogen–Metal 3c–2e Bonds 70

2.11. Conclusions 76References 77

3. Carboranes and Metallacarboranes 85

3.1. Introduction 853.2. Carborane Structures and Skeletal Electron Numbers 87

3.2.1. Closo Carboranes 883.2.2. Nido and Arachno Carboranes 893.2.3. Carbon Sites in Carboranes; Skeletal Connectivities k 973.2.4. Skeletal Bond Orders in Boranes and Carboranes 98

3.3. Localized Bond Schemes for Closo Boranes and Carboranes 983.3.1. Lipscomb’s Styx Rules and Williams’ Stx Rules 983.3.2. Bond Orders and Skeletal Connectivities 1003.3.3. Bond Networks and Skeletal Connectivities 1013.3.4. Calculated Charge Distributions and Edge

Bond Orders 1023.4. MO Treatments of Closo Boranes and Carboranes 1043.5. The Bonding in Nido and Arachno Carboranes 107

3.5.1. Localized Bond Schemes 1073.5.2. MO Treatments of Nido and Arachno Boranes and

Carboranes 1083.5.3. Some Boron-Free Nido and Arachno Systems 110

3.6. Methods of Synthesis and Interconversion Reactions 1113.7. Some Carbon-Derivatized Carboranes 114

3.7.1. Carboranyl C–H---X Hydrogen-Bonded Systems 1143.7.2. Carboranyl–Metal Systems 1143.7.3. Some Aryl-Carboranes 116

3.8. Boron-Derivatized Carboranes: Weakly Basic Anions [CB11H6X6]− 122

3.9. Metallacarboranes 1233.9.1. Structural Types, Electron Counts, and Isolobal Units 1233.9.2. Predicting Structures from Formulae 1263.9.3. Metal Complexes of CxBy Ring Systems 130

3.10. Supraicosahedral Carborane Systems 1333.11. Conclusions 137References 137

4. Mixed Metal–Carbon Clusters and Metal Carbides 149

4.1. Introduction 1494.2. Complexes of CnHn Ring Systems with a Metal Atom:

Nido-Shaped MCn Clusters 1504.3. Metal Complexes of Acyclic Unsaturated Ligands, CnHn+2 157

CONTENTS ix

4.4. Complexes of Unsaturated Organic Ligands with Two or More Metal Atoms: Mixed Metal–Carbon Clusters 160

4.5. Metal Clusters Incorporating Core Hypercarbon Atoms 1624.6. Bulk Metal Carbides 1734.7. Metallated Carbocations 1764.8. Conclusions 176References 177

5. Hypercoordinate Carbocations and Their Borane Analogs 185

5.1. General Concept of Carbocations: Carbenium Versus Carbonium Ions 1855.1.1. Trivalent–Tricoordinate (Classical) Carbenium Ions 1865.1.2. Hypercoordinate (Nonclassical) Carbonium Ions 187

5.2. Methods of Generating Hypercoordinate Carbocations 1885.3. Methods Used to Study Hypercoordinate Carbocations 189

5.3.1. NMR Spectroscopy in Solution 1895.3.2. 13C NMR Chemical Shift Additivity 1925.3.3. Isotopic Perturbation Method 1925.3.4. Solid-State 13C NMR at Extremely Low Temperature 1935.3.5. X-Ray Diffraction 1935.3.6. Tool of Increasing Electron Demand 1945.3.7. ESCA 1945.3.8. Low Temperature Solution Calorimetry 1955.3.9. Quantum Mechanical Calculations 195

5.4. Methonium Ion (CH5+) and Its Analogs 195

5.4.1. Alkonium Ions Incorporating Bridging Hydrogens (Protonated Alkanes, CnH2n+3

+) 1955.4.1.1. The Methonium Ion (CH5

+) 1965.4.1.2. Multiply-Protonated Methane Ions and their

Analogs 2025.4.1.3. Varied Methane Cations 2055.4.1.4. Ethonium Ion (C2H7

+) and Analogs 2085.4.1.5. Proponium Ions and Analogs 2105.4.1.6. Higher Alkonium Ions 2115.4.1.7. Adamantonium Ions 2175.4.1.8. Hydrogen-Bridged Cycloalkonium Ions 2175.4.1.9. Hydrogen-Bridged Acyclic Ions 221

5.4.1.10. Five-Center, Four-Electron Bonding Structures 2235.4.2. Hypercoordinate Carbocations Containing 3c–2e

C---C---C Bonds 2235.4.2.1. Cyclopropylmethyl and Cyclobutyl Cations 2235.4.2.2. The 2-Norbornyl Cation 2295.4.2.3. The 7-Norbornyl Cation 2435.4.2.4. The 2-Bicyclo[2.1.1]hexyl Cation 2435.4.2.5. The Polymethyl 2-Adamantyl Cations 245

x CONTENTS

5.5. Homoaromatic Cations 2475.5.1. Monohomoaromatic Cations 2475.5.2. Bishomoaromatic Cations 2495.5.3. Trishomoaromatic Cations 2565.5.4. Three-Dimensional Homoaromaticity 2585.5.5. Möbius Homoaromaticity 259

5.6. Hypercoordinate (Nonclassical) Pyramidal Carbocations 2605.6.1. (CH)5

+-Type Cations 2605.6.2. (CH)6

2+-Type Dications 2645.7. Hypercoordinate Heterocations 266

5.7.1. Introduction 2665.7.2. Hydrogen-Bridged Silyl Cations 2665.7.3. Homoaromatic Heterocations 267

5.8. Carbocation–Borane Analogs 2685.8.1. Introduction 2685.8.2. Hypercoordinate Methonium and Boronium Ions 2725.8.3. Cage Systems 2725.8.4. Hypercoordinate Onium–Carbonium Dications and

Isoelectronic Onium–Boronium Cations 2745.9. Conclusions 276References 277

6. Reactions Involving Hypercarbon Intermediates 295

6.1. Introduction 2956.2. Reactions of Electrophiles with C–H and C–C Single

Bonds 2986.2.1. Acid-Catalyzed Reactions and Rearrangements of

Alkanes, Cycloalkanes, and Related Compounds 2986.2.1.1. Carbon–Hydrogen and Carbon–Carbon

Bond Protolysis 2986.2.1.2. Isomerization and Rearrangement 3076.2.1.3. Alkylation 320

6.2.2. Nitration and Nitrosation 3256.2.3. Halogenation 3286.2.4. Carbonylation 3316.2.5. Oxyfunctionalization 332

6.2.5.1. Oxygenation with Hydrogen Peroxide 3326.2.5.2. Oxygenation with Ozone 3346.2.5.3. Oxygenation with Other Reagents 337

6.2.6. Sulfuration 3396.2.7. Reactions of Coordinatively Unsaturated Metal

Compounds and Fragments with C–H and C–C σ Bonds 340

CONTENTS xi

6.2.7.1. Carbon–Hydrogen Bond Insertion 3426.2.7.2. Carbon–Carbon Bond Insertion 362

6.2.8. Reactions of Singlet Carbenes, Nitrenes, and Heavy Carbene Analogs with C–H and C–C Bonds 371

6.2.9. Rearrangement to Electron-Defi cient Metal, Nitrogen, and Oxygen Centers 3776.2.9.1. Isomerization, Rearrangement, and

Redistribution of Alkylmetal Compounds 3776.2.9.2. Rearrangements to Electron-Defi cient

Nitrogen and Oxygen Centers 3816.3. Electrophilic Reactions of π-Donor Systems 3836.4. Bridging Hypercoordinate Species with Donor Atom

Participation 3886.4.1. Carbocations with 3c–2e Bond 3886.4.2. Five-Coordinate SN2 Reaction Transition States and

Claimed Intermediates 3896.4.3. Six-Coordinate Hypervalent Compounds 393

6.5. Conclusions 394References 394

Conclusions and Outlook 417

Index 419

xiii

FOREWORD TO THE FIRST EDITION

The periodic nature of the properties of atoms and the nature and chemistry of molecules are based on the wave property of matter and the associated energetics. Concepts including the electron - pair bond between two atoms and the associated three - dimensional properties of molecules and reactions have served the chemist well, and will continue to do so in the future.

The completely delocalized bonds of π - aromatic molecules, introduced by W. H ü ckel, also provided a basis for a rational description of molecular orbitals in these systems. An extended H ü ckel theory allowed a study of molecular orbitals throughout chemistry at a certain level of approximation.

The localized multicenter orbital holds a certain intermediate ground, and is particularly useful when there are more valence orbitals then electrons in a molecule or transition state. First widely used in the boron hydrides and car-boranes, these three - center and multicenter orbitals provide a coherent and consistent description of much of the structure and chemistry of the upper left side of the periodic table, and of the interactions of metallic ions with other atoms or molecules.

Skeletal electron counts (the sum of the styx numbers), fi rst proposed by Wade, Mingos, and Rudolph, have also provided a guide for synthesis, and have given a basis for fi lled bonding description of polyhedral species and their fragments. Together with the isolobal concept, diverse areas of chemistry have thereby been unifi ed.

In this book, one sees the remarkable way in which these ideas bring together structure and reactivity in a great diversity of novel carbon chemistry and its relationship with that of boron, lithium, hydrogen, the metals, and others. The authors are to be congratulated.

xiv FOREWORD TO THE FIRST EDITION

Rather than ask why it has taken some 30 years for these concepts to become widely known, one can be amazed that the background for this fi ne book developed at all. It is due in no small part to the reluctance of chemists to adapt to the dynamic changes of chemistry. One can also hope that chem-istry will recover from the recent neglect of support of research in mechanistic organic chemistry and synthesis of compounds of the main group elements. In addition, much of the molecular structure determination that is so central to these arguments had to await the newer methods of X - ray diffraction and nuclear magnetic resonance, and the theory had to await the modern develop-ment in methods and computers. Thus, the emergence of the depth and breadth of these concepts in this book is a tribute to the dedication of the authors and to the vitality of the ideas themselves.

W illiam N. L ipscomb

May 1986

xv

PREFACE TO THE SECOND EDITION

More than 20 years have passed since the publication of our book on hyper-carbon chemistry. The book became out of print and much progress has since been made in the fi eld. Hypercarbon chemistry has continued to grow, and indeed has become an integral part of the chemistry of carbon compounds usually referred to as high coordination compounds. Hence, it seems war-ranted to provide a comprehensively updated review and discussion of the fi eld with literature coverage until mid - 2009. Les Field was no longer available to help revise our book. However, our friend and colleague Á rp á d Moln á r joined us as a coauthor during a sabbatical year in Los Angeles, and should be credited for his outstanding effort to make the new edition possible, which we hope will be of use to the chemical community. Our publisher is thanked for arranging the new updated edition.

G eorge A. O lah G. K. S urya P rakash

K enneth W ade Á rp á d M oln á r

November 2009 R obert E. W illiams

xvii

PREFACE TO THE FIRST EDITION

Organic chemistry is concerned with carbon compounds. Over 6 million such compounds are now known, and their number is increasing rapidly. They range from the simplest compound methane, the major component of natural gas, to the marvelously intricate macromolecules that nature uses in life processes.

Within such a rich and diverse subject, it is diffi cult for someone deeply familiar with one area to keep abreast of developments in others. This can hinder progress if discoveries in one fi eld that can have signifi cant impact on others are not recognized in a timely fashion. For example, developments in the chemistry of carbohydrates, proteins, or nucleotides are traditionally exploited by biochemists and biologists more than by organic chemists. Developments in organometallic chemistry, while increasingly attracting the attention of inorganic chemists, are not as well appreciated by mainstream organic chemists.

In this book we have attempted to alleviate this problem by pooling our diverse experience and backgrounds but centering on a common interest in the fascinating topic of hypercarbon chemistry. The book centers on the theme that carbon, despite its fi rmly established tetravalency, can still bond simulta-neously to fi ve or more other atoms. We refer to such atoms as hypercarbon atoms (short for hypercoordinated atoms), since four valency [hence four coordination, using normal two - center, two - electron type bonds] is the upper limit for carbon (being a fi rst - row element, it can accommodate no more than eight electrons in its valence shell). Since their early detection in bridged metal alkyls, where they helped advance the concept of the three - center, two - electron bond (and later, the four - center, two - electron bond), hypercarbon atoms have now become a signifi cant feature of organometallics, carborane,

xviii PREFACE TO THE FIRST EDITION

and cluster (carbide) chemistry, as well as acid - catalyzed hydrocarbon chem-istry and the diverse chemistry of carbocations.

First, we survey the major types of compounds that contain hypercarbon. The relationships that link these apparently disparate species are demon-strated by showing how the bonding problems they pose can be solved by the use of three - or multicenter electron - pair bond descriptions or simple MO treatments. We also show the role played by hypercoordinated carbon inter-mediates in many familiar reactions (carbocationic or otherwise). Our aim here is to demonstrate that carbon atoms in general can increase their coor-dination numbers in a whole range in reactions.

In our original plans for the book, we were privileged to have our friend and colleague Paul v. R. Schleyer participate, and we regret that other obliga-tions have made it impossible for him to continue. We gratefully acknowledge his many suggestions and thank him for his continued encouragement. We have mainly focused our attention on experimentally known hypercarbon systems and are not discussing only computationally studied ones (these are reviewed by Paul Schleyer elsewhere).

Most chemists ’ familiarity with chemical bonding evolved in electron - suffi cient systems, where there are enough electrons not only for (2 c – 2 e ) bonds but also for nonbonded electron pairs. Hypercarbon atoms are generally found in electron - defi cient systems where electrons are in short supply and thus have to be spread relatively thinly to hold molecules or ions together. A relative defi ciency of electrons is not uncommon in chemistry, particularly in the chem-istry of the metallic elements. The (3 c – 2 e ) and multicenter bonding concept of boranes and carboranes, pioneered by Lipscomb, further emphasizes this point. Thus, it is not surprising that the concept of hypercarbon bonding was accepted by inorganic and organometallic chemists earlier than by their organic colleagues. The well - publicized spirited debate over the classical – nonclassical nature of some carbocationic systems preceded their preparation and their spectroscopic study under long - lived stable ion conditions, which unequivocally established their structures. Debate, and even controversy, is frequently an essential part of the “ growing pains ” of a maturing fi eld, and they should be welcomed as they help progress in fi nding answers. The impor-tance of hypercoordination in carbocations and related hydrocarbon is now fi rmly established. At the same time, hypercoordinate carbocations are but one aspect of the much wider fi eld of hypercarbon chemistry.

It is signifi cant to note that almost all carbocations have known isoelec-tronic and isostructural neutral boron analogs. Boron compounds also provide useful models for many types of intermediates (transition states) of electro-philic organic reactions.

The fi eld of hypercarbon chemistry is already so extensive that it is impos-sible to give an encyclopedic coverage of the topic. Instead, we have taken the liberty of organizing our discussion around selected topics with representative examples to emphasize major aspects. Our choices were arbitrary and we apologize for inevitably omitting much signifi cant work.

PREFACE TO THE FIRST EDITION xix

Multiauthor books frequently lack the uniformity that a single - author book is able to convey. Our close cooperation, made possible by the Loker Hydrocarbon Research Institute, has helped us give a homogeneous presenta-tion that merges our individual viewpoints to refl ect our common goal. If we had succeeded in calling attention to the ubiquitous presence of hypercarbon compounds, breaching the conventional boundaries of chemistry, and arousing the interest of our readers, then we shall have achieved our purpose.

We thank Ms. Cheri Gilmour for typing the manuscript and our editor, Dr. Theodore P. Hoffman, for helping along the project in his always friendly and effi cient way. Many friends and colleagues offered helpful comments and sug-gestions and we are grateful to them all.

G eorge A. O lah G.K. S urya P rakash R obert E. W illiams

L eslie D. F ield K enneth W ade

October 1986

1

1 INTRODUCTION: GENERAL ASPECTS

1.1. AIMS AND OBJECTIVES

This book is concerned with an important area of organic (i.e., carbon) chem-istry that has developed enormously over the past half - century, yet is still neglected in many organic textbooks. This is the chemistry of compounds in which carbon atoms are covalently bonded to more neighboring atoms than can be explained in terms of classical two - center, electron - pair bonds. Such carbon atoms are referred to as hypercarbon atoms 1 (short for hypercoordi-nated carbon atoms ) because when fi rst discovered, their coordination numbers seemed unexpectedly high .

Carbon contains four atomic orbitals (AOs) in its valence shell (the 2 s , 2 p x , 2 p y , and 2 p z AOs) and thus can accommodate at most four electron pairs (the “ octet rule ” ). 2 Commonly, these electron pairs are used to form four single bonds (as in alkanes), two single bonds and one double bond (as in alkenes), one single bond and one triple bond (as in alkynes), or two double bonds (as in cumulenes). With only four bond pairs, carbon atoms cannot bond simultaneously to more than four neighboring atoms using only two - center electron - pair bonds. If attached to more than four neighboring atoms, they must resort to some form of multicenter σ bonding , in which the bonding power of a pair of electrons is spread over more than two atoms. All carbon atoms with coordination numbers greater than four are therefore necessarily hypercoordinated, and compounds containing such atoms (of which there are

Hypercarbon Chemistry, Second Edition. George A. Olah, G. K. Surya Prakash, Kenneth Wade, Árpád Molnár, Robert E. Williams.© 2011 John Wiley & Sons, Inc. Published by John Wiley & Sons, Inc.

2 INTRODUCTION: GENERAL ASPECTS

now a very large number) will be the main concern of this book. However, there are circumstances in which carbon atoms with only three or four neigh-bors may participate in multicenter σ bonding to two or even three of these neighbors, and we shall include them in our discussion where appropriate.

We have four main objectives:

1. To illustrate the wide and developing scope of hypercarbon chemistry by illustrating the variety of compounds now known to contain hypercarbon atoms (carbocations, 3 – 6 organometallics, 7 – 9 carboranes, 10 metal – carbon cluster compounds, 11,12 and metal carbides 13 ). They include bridged metal alkyls such as alkyl - lithium reagents (LiR) n 14 – 17 in which the hypercoor-dinated nature of the metal - attached carbon atoms, and the roles that the metal atoms play in their chemistry, are often overlooked.

2. To discuss the ways in which the bonding in such systems can be described , notably using three - center – two - electron (3 c – 2 e ) bonds as well as classi-cal two - center – two - electron (2 c – 2 e ) bonds, but also by simple molecular orbital (MO) treatments that shed useful light on the more symmetrical systems.

3. To show how hypercarbon compounds are closely related to many clas-sically bonded systems and aromatic systems , and are not exotic species remote from mainstream organic chemistry.

4. To show how the study of hypercarbon compounds helps us to understand the mechanisms of many organic reactions , reactions in which carbon atoms become temporarily hypercoordinated in intermediates or transi-tion states even though the reagents and products contain only normally coordinated carbon atoms.

In introducing the subject in Section 1.2 , we defi ne some of the terms we shall be using. In Section 1.3 , we illustrate the various types of hypercarbon com-pounds now known. Since we shall rely heavily on the 3c – 2e bond concept in their bonding, and since its usefulness is perhaps less widely appreciated in organic chemistry than in inorganic or organometallic chemistry, we devote Section 1.4 of this introductory chapter to discussion of that concept and illus-trate its value for selected systems. We also demonstrate the relevance and value of some simple MO arguments applied to hypercarbon systems (Sections 1.4 and 1.5 ), and conclude this introductory chapter by indicating the types of reactions thought to involve hypercarbon systems. More detailed discussion of particular categories of hypercarbon compounds, including structural, bonding, thermochemical, and reactivity aspects, follow in subsequent chapters.

1.2. SOME DEFINITIONS

Throughout this book, we shall be concerned with the twin issues of coordina-tion and bonding. The terminology by which chemists refer to these issues

SOME DEFINITIONS 3

varies considerably from area to area. It is important, therefore, to defi ne and to illustrate the sense in which certain terms will be used here.

We defi ne the coordination number of an atom as the number of neighboring atoms by which that atom is directly surrounded, to each of which it is attached by the direct sharing of electronic charge. The coordinating atoms may not all be at the same distance (some may be bonded more strongly than others, and so may be closer to the atom under consideration), but all will be located in direc-tions and at distances that indicate sharing of electronic charge with the central atom, rather than linkage to the central atom via a second neighboring atom.

On occasions, the term “ valence ” is used as if it were synonymous with “ coordination number. ” We shall not use it in that sense here. We defi ne the valence of an atom as the number of bonding electron pairs used by that atom . Normally, carbon is tetravalent (i.e., the octet rule is obeyed), and hypercarbon compounds are no exception. (See also discussions about hypervalency by Akiba 18 and the octet rule and hypervalency by Gillespie and Silvi. 19 ) A hyper-carbon atom uses four electron pairs to bond to whatever number of atoms there are in its coordination sphere. The carbon atom in methane is tetravalent and four coordinate, forming four 2 c – 2 e bonds to its neighboring hydrogen atoms. It remains tetravalent but becomes pentacoordinate when methane is protonated to form the methonium ion (CH 5 + ), an energetic, highly reactive species 20 – 22 with a structure in which three hydrogen atoms remain at a normal, single - bond distance while the other two are at a greater distance. 20,23 – 26 However, the methyl cation CH 3 + into which CH 5 + decomposes contains a triply coordinated trivalent carbon atom [Eq. (1.1) : The lines from carbon in that equation represent links to the coordinating hydrogen atoms, not neces-sarily bonds in the classical electron - pair sense].

H

H

H

HH

C

H

H

HH

CC

H

H

H

+H+

+

–H2 (1.1)

The carbon atom in CH 3 + is said to be coordinatively unsaturated , a term we shall use in connection with any atom that can readily expand its coordina-tion number, either (as in the case of the carbon atom of CH 3 + ) by bonding to another ligand (a coordinating atom or group) , which supplies electrons for the purpose (e.g., CH 3 + + X − → CH 3 X), or by using electrons that were previ-ously nonbonding , for example, as occurs when coordinatively unsaturated carbon atoms in carbanions R 3 C − are protonated, that is, when nonbonding lone - pair electrons are converted into bond pairs [Eq. (1.2) ]:

R

RR

C : + H+

R

RR

C H_

(1.2)

4 INTRODUCTION: GENERAL ASPECTS

When discussing bonding, we shall fi nd it convenient to retain wherever practicable the concept of single, double, and triple bonds, that is, links between pairs of atoms that involve the sharing between those atoms of two, four, or six electrons, respectively. We shall refer to them as 2 c – 2 e , two - center – four - electron (2 c – 4 e ), and two - center – six - electron (2 c – 6 e ) bonds. However, as already indicated, we shall fi nd it necessary, in discussing hypercarbon com-pounds, to use the concept of multicenter σ bonds , bonds in which the bonding power of a pair of electrons is considered to extend over three or occasionally four atoms. In CH 5 + , for example, a 3c – 2e bond can account for the bonding between the carbon atom and the two hydrogen atoms furthest from the carbon atom, represented as in Scheme 1.1 .

Such a 3 c – 2 e bond is envisaged as resulting from the mutual overlap of a suitable AO from each of the atoms involved, a 1 s AO from each hydrogen atom, and an sp 3 hybrid AO from carbon. The 3 c – 2 e bond can be represented by broken lines from the atoms that meet at the center of that triangle, where the AOs of the three atoms will overlap (Scheme 1.1 ). It must be remembered, however, that there is no atom at the point at which the broken lines meet.

It should be stressed that although such a 3 c – 2 e bond shares the bonding pair of electrons between three atoms instead of two as in classical bonds, and therefore is sometimes referred to as delocalized , the description of the bonding in CH 5 + by three 2 c – 2 e bonds and one 3 c – 2 e bond is nevertheless a description in terms of localized bonds . It is a valence bond description of this cation that attempts to account for the distribution of the atoms and the inter-nuclear distances by allocating pairs of electrons to localized regions between pairs of atoms or within triangular arrays of three atoms. A delocalized descrip-tion of the bonding in this cation would allocate the four pairs of electrons to four MOs embracing all six atoms, each or most making some contribution to all of the pairwise interactions, bonded or nonbonded, in CH 5 + , but generating overall much the same electron density in particular regions as the localized bond model. Thus, electron density corresponding to essentially one pair of electrons would be found in each of the “ normal ” C – H bonds, but the electron

Scheme 1.1

H

H

H

H

H

C

H

H

H

H

H

C

H

HH

H

C

1ssp3

or

H+

+

+

STRUCTURES OF SOME TYPICAL HYPERCARBON SYSTEMS 5

density associated with each long C – H bond, and also in the H - - - H link between the two anomalous (hypercoordinated) hydrogen atoms, would approximate to two - thirds of an electron apiece (for electron bookkeeping purposes, the sharing of a pair of electrons between the three atoms linked by a 3 c – 2 e bond corresponds to the allocation of two - thirds of an electron to each edge of the triangle defi ned by those three atoms.).

An additional term we may fi nd occasionally useful, though we shall restrict its use to avoid ambiguity, is electron defi cient . This term has at least three dif-ferent senses in which it has found use in connection with organic systems. It is often applied as meaning “ center for nucleophilic attack ” to refer to carbon atoms bearing electron - withdrawing substituents. Second, it is also used in referring to compounds with coordinatively unsaturated carbon atoms like those of carbenium ions, R 3 C + , which can accommodate an extra pair of elec-trons. The third usage, 27,28 is as a label for molecules, or sections thereof, that contain too few electrons to allow their bonding to be described exclusively in terms of two - center, electron - pair bonds. In this book we prefer to restrict our discussion to compounds wherein molecules or sets of atoms are held together by multicenter bonding (i.e., by electron - defi cient bonding). Similarly, electron precise 28 is a term that can be used as a label for systems in which there are exactly the right number of electrons to give each pair a two - center - bonding role , as in CH 4 . Electron - rich systems are those containing nonbonding (lone - pair) electrons , as in CH 3 − , NH 3 , or H 2 O.

A molecule or polyatomic ion containing n atoms can often be identifi ed as electron defi cient from its formula, if it contains fewer than ( n − 1) valence shell electron pairs. This is because at least ( n − 1) two - center covalent links will be needed to hold n atoms together, whatever the structure may be. Thus, the methonium ion, CH 5 + , with six atoms held together by only four valence shell electron pairs, is clearly electron defi cient in this sense. The dication CH 6 2 + , 29 with seven atoms, is even more so.

1.3. STRUCTURES OF SOME TYPICAL HYPERCARBON SYSTEMS

Before exploring the various bonding situations that occur in hypercarbon systems, we illustrate the structures of some representative examples, grouped according to type in Figures 1.1 – 1.6 .

Figure 1.1 shows the structures, determined in pioneering X - ray crystallo-graphic studies, of some bridged metal alkyls , aryls , alkenyls , and alkyn-yls . 9,14 – 17,27,30 – 36 Compounds of these types fi rst showed how the carbon atoms of typical monovalent organic groups could participate in multicenter σ bond-ing . Note that the hypercarbon atoms in all of these compounds bond to either two or three metal atoms, and that, although the coordination numbers of the bridging carbon atoms in (AlPh 3 ) 2 34 (isoBu 2 AlCH = CH tert - Bu) 2 , 35 and (MeBeC ≡ CMeNMe 3 ) 2 36 are not unusual (4, 4, and 3, respectively) the (MC) 2 rings in these compounds (M represents the metal atom), like those in (AlMe 3 ) 2 30

6 INTRODUCTION: GENERAL ASPECTS

Figure 1.1. Representative bridged metal alkyls, aryls, alkenyls, and alkynyls.

CH3

CH3

H3C

H3C CH3

H3C

Al Al

H3C

CH3

CH3

H3C

Mg Mg

n

Li

Li

Li

Li

H3C

C

CH

CH3

Al2Me6 2)n (LiEt)6 4

PhPh

PhPh

Al Al

Al2Ph6

(MgMe (LiMe)

(isoBu2AlCH=CH-tert-Bu)2

isoBuisoBuisoBu

Al Al

tert-Bu-CH=CH

HC=CH-tert-Bu

MeNMe3

Me3N

MeBe Be

C

C

Li

MeC

Li

LiLi

Li

LiCH2Me

CH2Me

CMe

MeH2C

H2

H2

3

H3

MeH2C

isoBu

C

C

[MeBe(Me3N)C CMe]2

Me

Me

and (MgMe 2 ) n 31 are held together by fewer electron pairs than two - center M – C links.

Figure 1.2 shows the structures of various types of carbocations, C x H y n + , including the highly reactive, unstable methonium cation (CH 5 + ), 20,23 the hydrogen - bridged 1,6 - dimethylcyclodecyl cation (1,6 - Me 2 C 10 H 17 + ), 37 the pyra-midal ions (1,2 - Me 2 C 5 H 3 + ) 38,39 and (Me 6 C 6 2 + ), 40 the homoaromatic cation (C 6 H 9 + ), 41 and the 2 - norbornyl cation (C 7 H 11 + ) 42 – 44 the structures of all of which were once the subjects of much debate. Although none of these structures has been determined by X - ray diffraction, good evidence for them was obtained from spectroscopic studies in solutions, 45,46 and the structures have subse-quently been supported by reliable calculations. 47 – 49 (See further discussion in Chapter 5 , Sections 5.4 , 5.5 , and 5.6 ) There was never any doubt about the structures of the two metalla - carbocations also shown in Figure 1.2 , [C(AuPPh 3 ) 5 ] + 50,51 and [C(AuPPh 3 ) 6 ] 2 + , 51,52 which may be regarded as per - metallated derivatives of the elusive cations CH 5 + and CH 6 2 + , in which the hydrogen atoms have been replaced by AuPPh 3 units. Also shown in Figure 1.2 (b) are the structures of the carbocationic transition states through which the classically bonded carbocations isoPrCMe 2 + and tert - BuCMe 2 + can undergo degenerate rearrangement, that is, rearrangement in which migration of an atom or group from one atom to another generates a product equivalent but not identical to the original.

Figure 1.3 shows the structures of some deltahedral (i.e., triangular - faced polyhedral) carboranes , 8 – 10,53 – 61 mixed hydride clusters of boron and carbon with BBB, BBC, or BCC faces. Each carbon atom in these cluster compounds has a hydrogen atom attached to it by a bond pointing away from the center of the cluster, but otherwise uses its three remaining valences to bond to the

STRUCTURES OF SOME TYPICAL HYPERCARBON SYSTEMS 7

Figure 1.2. Carbocations containing hypercarbon atoms. (a) Carbocations; (b) carbocationic intermediates or transition states ( * denotes hypercarbons).

CH5+ 1,6-Me2C10H17

+ C6H9+

H

H

H

H

H

Me

C C

C

H2C

CH2H2C

H

H

H

C

CCMeC

HC

CMe

H

C C

CC

C

Me

Me

MeMe

Me

MeMeMe

C C C

H

MeMe

Me

Me Me

MeMeMe

HH

C C+ +

C7H11+ 1,2-Me2C5H3

+ Me6C62+

Me4C2H+

Me5C2+

C*

2++

+ + +

+

(a)

(b)

* *

*

* CMe*

* *

MeCMe

MeMe

C C C

Me

MeMe

Me

Me Me

MeMeMe

MeMe

C C+ ++

* *

+

[C(AuPPh3)5]+ [C(AuPPh3)6]2+

* *

CLAu

LAu

Au

Au 2+

AuL

AuL

+Au

LAu

AuLLAu

AuL

C* *

L L

L

H3C

H3C

H**

H

four or fi ve neighboring boron or carbon atoms. The examples chosen include some with fi ve - or six - coordinate carbon atoms (C 2 B 4 H 6 , C 2 B 5 H 7 , C 2 B 10 H 12 ) and others (C 2 B 3 H 5 , C 2 B 5 H 7 ) where the environment (and bonding) of the carbon atoms is similar, although they are only four coordinate. Despite the generally high coordination numbers of their carbon atoms, many carboranes are now known that are highly thermally and oxidatively stable substances, with a vast derivative chemistry and potential for a variety of applications in pure and applied chemistry and in materials and biological sciences.

8 INTRODUCTION: GENERAL ASPECTS

Figure 1.4 shows the structures of some mixed metal – carbon clust-ers . 8,9,11,12,14,27 Their shapes closely resemble those of the carboranes just men-tioned, a resemblance we shall fi nd of considerable signifi cance. It is also apparent that the polyhedral (generally deltahedral) examples chosen [Fe 3 (CO) 9 C 2 Ph 2 , 62 Co 4 (CO) 10 C 2 Et 2 , 63 and Fe 3 (CO) 8 C 4 Ph 4 64 ] have many features in common with the cyclopentadienyl - , cyclobutadiene - , and butadiene - metal complexes (C 5 H 5 )Mn(CO) 3 , (C 4 H 4 )Fe(CO) 3 , and (C 4 H 6 )Fe(CO) 3 also shown. The family relationship that extends from carboranes through mixed metal – carbon clusters to metal complexes of aromatic ring systems like the cyclopen-tadienide anion (C 5 H 5 − ) also extends to aromatic ring systems themselves. 10,65

In Figure 1.5 , we show the structures of some metal carbide clusters , 11,12 compounds in which hypercarbon atoms are embedded in polyhedra (such as square pyramids, 66 octahedra, 67 trigonal prisms, 68 or square antiprisms 69 ) of metal atoms. Although these carbon clusters may appear to be remote from typical organic systems, they illustrate clearly the capacity of carbon atoms to bond simultaneously to fi ve, six, or, even eight neighboring atoms, and provide useful models for what may be the key species in Fischer – Tropsch and related chemistry at metal surfaces. The carbon atoms of carbon monoxide may undergo conversion at metal surfaces into carbide environments such as these, through which loss of carbon to the bulk metal or ultimate conversion into hydrocarbons may take place.

The carbon atoms of most binary metal carbides M x C y have hypercoordi-nated environments like those shown in Figure 1.5 . In particular, octahedral carbon coordination is common in the interstitial carbides formed by many transition metals, materials of variable composition in which carbon atoms

Figure 1.3. Some carboranes.

C

1,5-C2B3H5 1,6-C2B4H6 CB5H7

C

B BB

H H

H

H

H

BBB

C

C

B

H

H

H

HH

H

BBC

C

B

H

H

HH

H

HH

1,2-C2B3H7 2,3-Me2C2B4H5– 2,4-C2B5H7 1,2-C2B10H12

CB

B

B

H

H

H

H

Me

MeH

BC C

B BB

B

H H

H H

H

H

H

HC BB B

BB

B

B

B

CC

B

B

H

H

H

H

H

H

H

HB

H

H

H

BBB

C

B

B

H

H

H

HH

H

H