The chemistry of fullerenes

THE

CHEMISTRY OF FULLE S

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Advanced Series in

~ ~ ~ 1 E R E ~ E ~ Vol. 4

THE

CHEMISTRY FULLERENS

edited by

R Taylor University of Sussex, UK

World Scientific Singapore. NewJersey*London*HongKong

Published by World Scientific PublishingCo. Re. Ltd. P 0 Box 128, F m r Road, Singapore9128 USA oflce: Suite IB, 1060Main Street, River Edge,NJ 07661 UK o%ce: 57 Shelton Street, Covent Garden, London WC2H 9HB

THE CHEMISTRY OF nnLERENES Copyright 8 1995 by World Scientific Publishing Co. Re. Ltd. AN rights reserved. This book, orparfsthereof; may not be reproduced in anyform or by m y mem, electronic or mechanical, including photocopying, recording or any informationstorage and retrieval system now known or to be invented, without wriftenp e ~ i s s ~from o n the Publ~her.

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ISBN: 981-02-2304-8

Printed in Singapore.

Foreword

Carbon must surely have been the first pure element (in the form of charcoal p r o d u d by natural fires) that early man would have been aware of. It is therefore astoniehieg that it should be possibly the last element to reveal that it has a new (in this case third) form. The story of how it was recorplised as a result stimulated by an interest in the chemistry of the cosmos is now so well known as to need no firther elaboration here. The latcan#ls of its discovtry by no means reflects incompetence or lack of observation and a w a m m by earlier scientists, but rather the need for sophisticated instmmentation that hu only recently become available (and not widely availabIe at that). In this book you will read about some of the great technical ingenuity that is being used by some of the thoussnds of scientists worldwide in the race (for it is no less than that) to uncover the seotsts that this third form may reveal. Thc discovery of the fullerenes with their geodesic structures (the name comes from tb fbct that the American architect, R. BuckminsteAller was pre-cminent in publidsing the geodesic architectural form) has been widely described as one of the scieasifio discoveries of the XXth century. It has certainly captured the imagination of sciontisrs in a manner possibly without precedent. At the time of writing, and only four years since the first pure materials were isolated, over 3,500 papers on fullerenes have been poblihed (three new papers appear every day) and hundreds of patents filed. A whole new field of science, and especially of organic/organometallic chemistry (the growtharea to come) has been born It is really too early to be confident of predicting uses for this material, but superconductors, optical limiters, polymers, additives for photowxlucting polymers, transfer reagents, electrical storage devices, and inhibitors of enzymss in relation to the HIV virus feature amongst the possibilities so far considered or investigeted. This book is not intended to give an exhaustive coverage, nor could it given the hectic pace of discovery. But the main principles of the chemical behaviour of the fillemaea (at least of c60, or [bO]fbllerene as it will in hture be known) are now sufl8cicatly established to make publication of this book timely The book begins with a brief iutmduction, followed by a description of nomenclature, something that will becorn of increasing importance as the chemistry develops This is followed by a survey of the methods of making fidlerenes, description of how to isolate and purify them, and their b d c properties. Chapters then describe each of the main types of reaction that the fullemnee undergo. It is the intention that anyone wishing to enter this exciting field will find all the key information within this book and will then be sufficiently equipped to enter the fray. In conclusion I would like to thank all of the contributors for producing their manuhpta when there are so many other demands upon their time in this intensely competitive field. I feel sure this book will be an important reference source for some time to come. R. Taylor Sussex, January, 1995

V

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CONTENTS Foreword

Chupter 2

Chapter 2

V

Introduction and Nomenclature 1. Addition Patterns I. 1 Groups with no Eclipsing Steric Hindrance 1.2 Groups with Small Eclipsing Interactions - Addition of Hydrogen 1.3 Groups with Moderate Eclipsing Interactions 1.4 Groups with Large Steric Interactions 2. Numbering of Fullerenes

Fullerane produetion (LL). Lumb) 1. 2. 3. 4. 5.

Introduction Soot production Extraction Separation Endohedrai Fullerenes 6. Nanombes and Nanoparticles 7. Other Production and Separation Techniques 8. Health and Safety Issues soot-making Soot harvesting and handling Solvents Heavy metals Fullerene-containing materials

Chaptur 3

Properties of Fulleren- (R. Taylor) 1. Appearance and Purity Determination 1.1 13C NMR Spectroscopy 1.2 IR spectroscopy 1.3 Raman Spectroscopy 1.4 U V M S Spectroscopy 1.5 Differential Scanning Calorimetry 1.6 X-ray Powder Diffraction 2. Solubility 3. Thermal and Oxidative Stability 4. Oxidising Properties and Toxicity 5. Miscellaneous Properties

vii

1 3 3 5

6 6 7

20 20 20 23 24 25 26 28 29 29 30 30 30 30

35 35 37 41 42 43 44 45 45 47 48 49

viii

Chupter 4

Chupter 5

Chapter 6

Hydrogenated Fullerenes (P. A. Cahill) 1. Introduction 2. CaHz and C7& 2.1 Theoretical Investigations 2.2 Experimental Investigations 3. CaH4 3.1 Theoretical Investigations 3.2 Experimental Investigations 4. CmHz,,, n > 2 5. C70Hznrn 2 2 6. C,Hzn, m > 70, n 2 1 7. summary Chemical Transformation of Ca into Water-Soluble Polyhydroxylated Fullerene Derivatives (L.Y. Chiang) 1. Introduction 2. Reaction Strategy for Introducing Polar Functions onto Fullerenes 3. Synthesis of Fullerenols by the Aqueous Acid Chemistry Route 4. Synthesis of Fullerenols by Nitronium Ion Chemistry 5. Synthesis of Fullerenols by Hydrolysis of Polycyclosulfated Fullerene Precursors 5.1 Characterization of the Cyclosulfatic Structure in 9 5.2 Determination of Hydroxyl Content in Fullerenol 3 by XPS 5.3 Thermal Analysis of Polycyclosulfated Derivatives 9 5.4 Consideration of the Reaction Mechanism for Cyclosulfonation 6. Synthesis of Fullerenols from Hydroborated Fullerene Derivatives 7. Structural Differences in Fullerenols Prepared by Different Methods Fluorination (J. H. Holloway and E. G. Hope) 1. Introduction 2. Theoretical Considerations 3. Preparations 3.1 Fluorination of Cm and C ~ Mixtures O 3.2 Hyperfluorination of C a Fullerene 3.3 Fluorination of C ~ Fullerene O 4. Chemistry of Fluorinated Cm and C7,

53 53 53 53 58 59 61 62 63 63

64

67

67 69 73 78 85

86 90 92 93 96 98 109

109 110 111 111 115 116 117

ix

Chapter 7

Iodination, Bromination and Chlorination (R. Taylor) 1. lodination 2. Bromination 3. Chlorination

123 124 125 130

Criaptsr 8

Aryl Derivatives of F’ullerenes (R. Taylor)

135 135 141 146

1. Arylation Using the Parent Fullerene 2. Arylation Using Preformed Halofullerenes 3. Arenylation

Chapter 9

Fulleroids, Methanofhllerenes and Oxa, Aza-, and 151 Sila-Homologues (M. Pruto and F. Wudl) 152 1. Fulleroids and Methanofullerenes 153 1.1 Diazomethane 154 1.2 Substituted Dimmethanes 156 1.3 Applications 160 1.4 Other Approaches to Methanofullerenes 1.5 Electronic Properties of Fulleroids and Methanofullerenes 163 164 1.6 Electrochemical Properties 1.7 Magnetic Properties of Fulleroids and Methanofullerenes 166 167 1.8 Multiple Additions and Relative Regiochemistry 168 2. Fullerene Oxides 169 3. Azaffulleroids and Fullerene Aziridines 170 4. Fullerene Siliranes 171 5. Conclusions

chaptar 10

Cycloadditions to Ca (M.S. Meier) 1,3-Dienes Cyclopentadiene Anthracene Quinodimethanes and Quinomethides Acyclic 1.3-Dienes Oxygenated Dienes 1,3-Dipolar Addition Diazoalkanes Nitrile Oxides Methylenecyclopropene Carbene Adducts Azomethien Ylides Disilolanes [2 + 21 Thermal Cycloadditions [2 + 21 Photocycloadditions

174 174 174 176 177 179 180 181 181 182 183 184 185 186 187 188

X

Chapter 11

Chapter 12

Cfuzpter 13

Photopolymerization Ynamines Enones Conclusions

188 189 190 191

Formation of Anions and Electrophilic Addition (R.Taylor) 1. Electron Addition 2. Reaction with Electrophiles

195

Nucleophilic Addition and Substitution (R.Taylor) 1. Addition of Neutral Nucleophiles 2. Addition of Charged Nucleophiles 3. NucleophiIic Substitution

202

202 205

208

Radical Additions to Fullerenes: Fullerenyl Radicals

(R.Taylor) Chapter 14

195 198

Reactions of Fullerenes with Inorganic and Organometallfc Compounds (A. L. Bulch) 1. Introduction 2. Additions to Cm 2.1 Osmylation 2.2 Addition of Pt(PR3)Z and Related Palladium and Nickel Complexes 2.3 Addition of Vaska-Type Complexes, Ir(CO)CI(PR3)2 2.4 Addition of Ir&~-C1)2(q~-CnH& 2.5 Reaction with the Hydrogenation Catalyst RhH(CO)(PPh& 2.6 Reaction with (r15-C9H7)Ir(CO)(t12-CnH,z) 2.7 Formation of a Pd(0) Polymer 2.8 Other Addition Reactions 3. Reactions that Result in Reduction of Cm 4. Cocrystallization of C, with Organometallic and Inorganic Reagents 5. Additions to C70 5.1 Addition of Ir(CO)Cl(PR& 5.2 Osymlation 6. Addition to C76, Osymlation 7. Addition to c84 8. Isolation of Endohedral Compounds

211

220

220 22 1 222 225 23 1 238 238 239 24 I 24 1 242 244 247 247 25 1 252 252 256

xi

Chapter IS

Fullerene-containingPolymers 1. ‘Pearl-Necklace’ Polymers 1.1 Directly-linked Fullerenes 1.2 Indirectly-linked Fullerenes 1.3 Copolymers 1.3.1 Attachement of a Fullerene to a Preformed Polymer Chain 1.3.2 Incorporation of a Fullerene into a Forming Polymer Chain 2. Dendrimers 3. Star Polymers 4. Electroactive Polymers

263 253 263 265 267 267 270 270 27 1 272

Chapter 1 Introduction and Nomenclature The description 'third form of carbon' is a little misleading: third forms would be more accurate since there are, in principle, an infinite number of Wlerenes possible. A general understanding of the term fillerene is that it describes a closed cage molecule comprised entirely of s&t-hybridised carbons arranged in hexagons and pentagons, and of the latter there are always twelve as a consequence of Euler's theorem. The fillerene comprised of twelve pentagonal rings is C20 (or [20]fillerene). There can be any number of hexagons (except one)' and for each hexagon present two extra carbon atoms are needed. Thus C6o ([60]fillerene), the most acoessible (but not necessarily the most stable fillerene), contains twelve pentagons and twenty hexagons. The definition can be widened to include molecules with heptagonal rings, and in this case to close the cage requires an additional pentagon €or each heptagon present.2 Since heptagons in the structure produce a concave region in the surfhe, to close the cage without undue strain requires a large cage. So if heptagoncontaining Werenes are found, they will probably be rather large, possibly containing more than 100 carbon atoms. It is also possible to have a closed cage containing a c o m b d o n of octagons, hexagons, and squares, derived fiom a regular fbllerene by replacing two pentagons by squares, and two adjacent hexagons by an octagon, as for example in octahedral C48.3Closed cages containing other than 5- and 6-membered rings are known as qw+lllerenes. Hitherto, the fillerenes have been referred to generally by their molecular formula, e.g. C60, C70 etc. However, now that derivatisationis becoming commonplace, as will be evident fiom reading the chapters in this book, it is necessary to introduce a proper nomenclature term, since one cannot refer to methylc60 etc. A system similar to that uetd with the annulenes is being adopted by IUPAC, and in this C6o is written as [6O]firtlerene. As in the case of the annulenes, the number in square brackets indicates the number of atoms and dso the number of n-electrons. (Note that the overall number of bonds in a hllerene C, is 3n/2.) This method has been introduced too late for it to have been incorporated in all of the chapters of this book, but it appears in some. For quasifillerenes, the sizes of rings present are indicated in curved brackets in front of the name. Thus octahedral C48 is described as (4,6,8)[48-oh]qumi-f%llerene. It is just over four years since pure [60]fillerene (Fig. 1) and [70]fUerene (Fig. 2) were first isolated and characterised.4.5At that time it was widely assumed that these 1

2

molecules, and [60]fbllerene in particular would be very unreactive. This was based upon the fact that this molecule has 12,500 resonance structures.6 However, all but one of these involves the presence of a double bond in at least one pentagonal ring. This is a destabiiising feature because strained five-membered rings adjacent to benzenoid rings tend to avoid having a double bond in the pentagonal ring, for to do so would increase the main in an already-strained ring; this latter is the basis of the long-known MillsNixon effect.7 The heats of formation of [60]-and [7O]fbllerene are respectively 10.15 and 9.34 kcal/moIe* (which makes then thermodynamically much less stable than either diamond or graphite), and this is consistent with the expectation that stability will increase as the fullerene surface becomes more planar, which it will do with incrersing size (at least for near spherical cages). Approximately 80% of this heat of formation is due to strain, and to add a double bond to a pentagon increases the strain by cu. 8 kdrnole.

Figure 1

[M)]Fullerene

Figure 2

[7O]Fullerene

Thus there is considerable bond-fixation in [60]fidlerene (and in the other fillerenes as well9), making the molecules giant three-dimensional electron-deficient alkenes, possessing a relatively small amount of delocalisation (certainly in comparison with typical aromatic compounds). Hence in [60]fbllerene the length of the bond between two hexagons (commonly referred to as the 6-6 bond) is 1.38 whereas that of the 5-6 bond (between a pentagon and a hexagon) is 1.45 A. The characteristic reactions are therefore addition; the molecules cannot undergo substitittion and cannot therefore possess substituents. Only fbllerene derivatives, possessing addends; can undorgo substitution. The mechanism of the addition processes is in most cases not yet clear. What is certain is that the electron deficiency, which arises from a combination of

3

strain,10 and the presence of such a large number of -I inductively electronwithdrawing*1 s$-hyb~dised carbon atoms, makes the molecules very powerhl electrophiles.Consequently the fbllerenes readily undergo nucleophilic addition and their derivatives undergo nucleophilic substitution(in some cases more readily). At the time of w r i tiq it is becoming apparent that the electrophilicity decreases with increasing size of the iltllcrcne, and that the reactivity decreases in the same direction. It would of course be most surprising if this were not the case, because an infinitely large fbllerene is effectivdy a sheet of graphite, which is rather unreactive. Addition tends to take place across the 1,2-bond and equivalents in both [60]- and [70]hllerene. Further addition in [70]fidlarme can also take place across the 5,6-bond and across the 7,23-positions and equivalents. (For numbering system see below.) 1. Addition Patterns

Some of the major addition patterns for [60]fillerene are now apparent; patterns for [70]fidlerene are just emerging. The patterns can be categorised according to the extent of eclipsing steric hindrance that exists,betweenthe addends: 1.1 Groups with no Eclipsing Steric Hindrance. These groups bridge two adjacent carbon atoms. They may be small e.g. 0, CH2, medium sized, e.g. benzyne and cyclopentadiene, or large, e.g. CPh2, Pt(PEt3)2. The latter group has been shown to undergo six-fold addition to [60]fUllerene, the addends being disposed octahedrally.12 The octahedral array not only avoids steric interactions between the components of the addend, but creates a product which has eight benzenoid rings in which the restriction on delocaliation, present in [60]fullerene, is partly removed (Fig. 3); the structure is thereby greatly stabilised.13The greater delocalisation in these benzenoid rings is shown by the bond lengths of 1.42 and 1.39 4 i.e. bond alternation is greatly reduced compared to that in the original [60]hllerene. (It is probably generally the case that provided no severe steric interactions are introduced by a given addition, and a substantial number of free hexagons remain, then the derivatives will be more stable than the parent fullerene.) Six-fold addition has been shown to take place with CPh2.14 benzyne,l5 cycloptadiene,16 morpholine, 17 and methylene,l 8 and octahedral dispositions of these groups, though not yet confirmed, is highly probable. Octahedral addition is also produced by two identical trios of (mutually orthogonal) additions, whereby the first trio conserves the decacyclene moiety which is a common feature of a number of stable Illerenes. 13

4

Figure 3

Schlegel diagram for (O]fiiflerenewith octahedral addition sites encircled. Addition increases aromatisationof the hexagonalrings (s).

The smaller of these groups may also add at fixther sites in the molecule, and evidence that for this is provided by the formation of epoxides derived fiom fluorornerenes, with up to eighteen oxygens attached to the cage . l9

(a)

(b)

Ngure 4 Products that can in principle arise from reaction of a group X at a 6:6-dngjunction

(4 Ngure 5

(b)

Products that can in principle arise from reactionof a group X at a 5:6-ringJunction

5

Addition of such groups e.g. CH2 commonly takes place across a 1,2-bond (6:6ring junction) to give a bridged (methanofuUerene) structure (Fig 4a), rather than an inserted one (homofkUerene or fulleroid, Fig. 4b) because the former maintains the Conversely favourablearrangement of double bonds being exocyclic to ~entagons.2072~ however, insertion into the lp-bond (5:6-ringjunction) is favoured (Fig 5a) because this not only gives the preferred benzenoid structure in one of the hexagons, but the alternativebridged structure(Fig. 5b) would have a double bond in a pentagon.2*>21 1.2. Gtoups with Small &lipsing inteructiom-AaZition of Hydrogen.Hydrogen adds across the 1,2-bond in [6O]fullerene and the 1,2- and 5,6-bonds in [70]fi~llerene(new

numbering - see Fig. 8), to &ve the corresponding dihydrolllerenes. Further addition can then take place across the equivalent bonds throughout the molecule, giving first a variety of tetrahydro derivatives (see Chapter 4), and then higher hydrogenated derivatives.22Although the low steric interactions suggest that in principle it should be possible to produce the filly hydrogenated derivatives, evidence for these has not yet been obtained, possibly because all of the C-Hbonds will be eclipsed, and the eclipsing can be relieved only slightly by twisting of the C-C bonds of the cage. Moreover, addition tends to stop at C6$36 (see Chap. 4), this compound being particularly stable because the T isomer (Fig. 4) contains four benzenoid rings;13720323 further addition to these highly delocalised rings is then difficult. It is also significant that octahedral addition is on the pathway to the Tisomer of C6$36.

Figure 6

Schlegel diagram for the T isomer of C60H-36; (l3) denotes benzenoid rings.

6

Addition to [7O]fullerene also stops largely at the C70H36 stage, though here, significant amounts of C7oH3g and C7OH40 are also produced.24 This can be explained by the formation of an equatorialbelt of either two phenanthrenoid Mgs and a bewenoid one @is. 7) or of one phenanthrenoid ring and two naphthalenoid ones.25 Further reduction of the bonds corresponding to the 9,lO-bonds in phenanthrene (encirded in Fig. 7) accounts for the two higher reduction levels obtained. These structures may in due course be found to have importance in other additions of [7O]lllerene.

Figare 7 Schiegel diagram for a proposed structure of C70H36; aromatic regions outlined,and ‘9,IO’-bondof phenanthrene moiety encircled

1.3. Groups with Moderate Eclipsing Interactions. Fluorine falls into this category, and

there is general agreement amongst the various reports that ca forty-eight fluorines can be added without much difficulty (Chap. 6). Thereafter it becomes progressively harder to add the remaining twelve fluorines to produce the hlly saturated derivative but has been achieved. The question as to why fluorination does not stop at the thirty-six hydrogen stage, as hydrogenation laregly does, can be attributabed to the much lower activation energy for fluorination compared to hydrogenation. 1.4. Groups with h r g e Steric Znteractions. The situation here awaits further

clarification. Radicals formed by the addition of R groups to (6O]fhllerene adopt a structure such that the groups are beta to a pentagonal ring, this having the advantages

7

that the groups do not steridy interact and the radical is delocalised around the p e n t q p d ring (Chap. 13).25 It should also be possible to produce a diradid with each radical delocalisad around diametrically opposed pentagons. There are also five other (equivalent) pentagonal rings that could be the centm for radical addition, but such additioa would introduce a measure of steric interactionwhich is probably unfavourable. Up to twenty-four chlorine or bromine atoms can be added to [6O]fIdlerene, and in the redant derivativeno two addends are adjacent to each other, which follows &om the e x p t e d stcric interactions. In the C6& compounds, (the structures of which r e w n b h that for the radicals) one pair of halogens are adjacent, and this is true also for C70Cl10 (we Chap. 7), but this is probably as much eclipsing that can be tolerated. As in the case of the radicals one further pentagon could be a probable centre for addition, so that compsund C6oX12 should be isolable. In the compound C60Br8 the bromines are also nolladjacent, and in principle it should be possible to produce C M r 1 6 with the same sieht-fold disposition ofthe bromine atoms, but this derivativewould not be on the pathway to CMr24; C&rZo with benzenoid rings at opposite poles also seems a feasiblederivative. The bulkiest groups that have been attached to the [6O]fullerene are pertluodkyls and phenyl (Chaps. 13 and 8), with evidence for the addition of up to 16 groups, though the locations are unknown as yet.26 In summary a correlation is emerging between the sue of the addend and the number that can be attached to the cage. In due course one can expect to see a further correlation between the number of addends for a particular reaction, and the size of the tiJllerem. 2. Numbering of Fullereneta

In this section the hllerene numbering schemes that seem likely to be recommended by IUPAC are presented in the form of Schlegel diagrams. To obtain these dhgams, one pole of the fullerene is expanded whilst the other one is shrunk until a t w d i s i o n a l representationis obtained. There is no doubt that the speed with which chemists began to make derivatives of fidhnes caught nomenclaturists unawares. Consequently a numbering system for [6O]lllsrene involving numbering around a pentagon was provisionally intr0duced,2~ and seemed logical enough since there is a similar pentagonal cap in [70]lllerene. However, it transpired that contiguous numbering cannot be achieved in [70]fillereneby numbating from the pentagonal cap (Fig. 8), and moreover it quickly became clear that

8

one bond in [6O]fullerene is the site of nearly all reactions. It made sense therefore to number this as the 1,2-bond, and hence to commence numbering around a hexagon (Fig. 9).28 Further consideration has revealed that contiguous numbering can be achieved in [7O]lUerene by numbering around a hexagon adjacent to the pentagonal cap (Fig. lo), and this has the advantage that the most reactive bond in [70]fhllerene is also the 1,2bond, and carbons 1-38 have the same environments in both molecules. The system given by Figure 8 has been used in a number of papers hitherto, and in some parts of this book. However, it is recommended that henceforth the more convenient system given in Figure 10 should be used. Before giving examples of the numbering schemes it is necessary to elaborate fhrther on the description of the fhllerene. For [60]fullerene and [70]fullerene only two isomers concern us (at least in relation to the chemistry), and so it is unnecessary to give the fid descriptions viz. [60-Zh]fullerene and [70-D5h]fullerene respectively. Where, due to degeneracy, subdivision is necessary, then this is indicated by capital Roman numerals. These are assigned priority on the basis of the lowest numeral sequence at the first point of difference, when the incidence of the 12 pentagonal faces in a ring spiral is stated as a list of numbers in the range 1 to n/2 + 2 in ascending order, using the sequence given in the 'Atlas of Fullerenes' by P.W. Fowler and D. E. Manolopoulos, O.U. Press, 1994. These degeneracies have been indicated previously by various other methods some of which have been directly contradictory. Henceforth, only the designations given in Figures 11-22, which show some isolated pentagon isomers of [76-, 78-, 82-, and 84]fidlerenes, should be used. Not all of these fdlerenes have yet been isolated and characterised

9

Pipllm 8 Non-contiguous (former) numbering scheme for [7O]fullerene

Figure 9 Recommendednumbering scheme for [60]fuIlerene

10 58

""

OY

Figure 10 Recommended (new)numbering scheme for [7O]fullerene I5

Figure 11 Numbering scheme for [76-D2]fuIIerene

12

Figure 14 Numbering scheme for [784'2v(II)]fullerene

ie

Figure 15 Nunibcnng scheme for [78-D3h(f)lfullerene

13

13

Figure 16 Numbering scheme. for [78-D3h(II)]fullercne

79

Eptsure 17 Numkering scheme for [82€~v(I)ffullerene

14

Figure 18 Numbering scheme for [82-C-j$I)]fullerene

8*

Figure 19 Numbering scheme for [84-D2(Iv)]hllereneene

15

Figure 21 Numbering scheme for [Srl-Djd]fUllercnc

16

Figure 22 Numbering scheme for [84-D6hJhllerene

The term hlleroids has been used provisionally to describe some carbon-bridged derivatives of [6O]hllerenes, but these can now be replaced by the existing organic nomenclature, as exemplified in Figures 23 and 24.

Figure 23 1,2-Mcthano[60]Nlerene (X= CHZ); 6 1,61-Diphenyl-l,2-rnethano[60]hrllerene (X = CPh2)

17

Figwe 24 1(6)a-Hom0[60]fi1llercne(X= CH2); la, la-Diphenyl-1(6)a-homoI60]hillereneQ = CPh2)

The tam endohedrai which has been introduced to describe fbllerenes with entities inside the cage is as yet not a problem since no derivatives of them have been made. However,inevitably this situation will change, and conflict will arise with Diels-Alder adducts possessing endo and em substituentson the addend. To forestall this problem it is recommended by IUPAC that since the term incurceranes already exists as an approved nomenclature term, the term incur (symbol i) be used henceforth to describe non-empty Illerenes. Lastly, though strictly not nomenclature, the notations for the carbon atoms in Fig. 25 has been fbund useful by many workers e d c b a

Figure 25. Notation for the five different types of carbon atoms in [7O]futlerene

18

References

1. P. W. Fowler and J. I. Steer, J. Chem. Soc.. Chem. Commun. (1987)1403. 2. R. Taylor, Interdisciplinury Sci. Rev. 17 (1 992) 1 61. 3. R.Taylor, Fullerenes Chemistry, Physics, and New Directions VI, (185th Meeting of the Electrochemical Society, San Francisco, May 1994); B. I. Dunlap and R. Taylor, J. Phys. Chem.,98 (1994)11018. 4. W. Kratschmer, L. D. Lamb, K. Fostiropoulos, and D. R. Huffinan, NarUre, 347 (1990)354. 5 . R.Taylor, J. P.Hare, A. K. Abdul-Sada, and H. W. Kroto, J. Chem. Soc., Chem. Commun. (1990)1423. 6. D. J. Klein, T. G. Schmalz, T. G.Hite, and W. A. Seitz, J. Am. Chem. Soc.,108 (1986)130. 7. W. H. Mills and I. G. Nixon, J. Chem. Soc. (1930)2510; R. Taylor Electrophilic Aromatic Substitution, Wiley, Chichester (1989)p. 53. 8. H.-D. Beckhaus, S. Verevkin, C. Riichardt, F. Diederich, C. Thilgen, H,-U. ter Meer, H. Mohn, and W. Miiller, Angew. Chem. Intl. E&. Engl., 33, (1994)996. 9. R.Taylor, J. Chem. Soc.,Perkin Truns. 2 (1 993)813. 10. A Streitwieser, G. R. Ziegler, P. C. Mowery, A Lewis, and R. G. Lawler, A Am. Chem. Soc. 90 (1968)1357. 11. H. A. Bent, Chem. Rev. 61 (1961)275. 12. P.J. Fagan, J. C. Calabrese, and B. Malone, J. Am. Chem. Soc. 113 (1991)9408. 13. (a) R.Taylor, J. Chem. Soc., Perkin Trum. 2 (1992)1667;(b) P.W. Fowler, D. J. Collins, and S. J. Austin, J. Chem. Soc., Perkin Truns. 2 (1993)275. 14. T. Suzuki, K.C. Khemani, F. Wudl, and 0. Almarsson, Science 254 (1991) 1186; J. Am. Chem. Soc.114 (1992)7300. 15. M. Tsuda, T.Ishida, T. Nogami, S. Kurono, and M. Ohashi, Chem. Lett. (1992) 2333. 16. M. F. Meidine, R. Roers, G. J. Langley, A. G. Avent, A. D. Darwish, S. Firth, H. W. Kroto, R. Taylor, and D. R. M.Walton, J. Chem. Soc.,Chem. Commun. (1993) 1342. 17. A.Hirsch, Q.Li, and F. Wudl, Angew. Chem. Intl. &in. Engl. 30 (1991)1309. 18. P. R. Birkett A. D. Darwish, H. W. Kroto, G. J. Langley, R. Taylor, and D. R.M. Walton, J. Chem. Soc.,Perhn Truns. 2, (1995)51 1.

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19

19. R. Taylor, G. J. Langley, A. K. Brisdon, J. H. Holloway, E. G. Hope, H. W. Kroto,

and D.R. M. Walton, J Chem. Soc.,Chem. Commun., (1993) 875; J Chem. Soc. Perkin Zhans.2, (1995) 181. 20. R. Tsylor, Phil. Z h m .Roy. Soc.b h n , Ser. A, 343 (1993) 87; F. Diederich, L. Irwacs, and D. Philp, J. Chem. Soc,Perkin Zhans. 2 (1994) 391; Chem. Soc.Rev., (1994) 243. 21. R Taylor and D. R M. Walton, Nulure, 363 (1993) 685. 22. A.D.DarWish, A.K.Abdd-Sada, G. J. Langlq, H.W. Kroto, R. Taylor, and D. 23. 24.

25. 26.

27 28.

R M.Walton, unpublished work. S. J. Austin, S.C. Batten, P. W. Fowler, D. B. Redmond, and R. Taylor, J. Chem. Soc., Perkin Dans. 2, (1993) 1383. M. D.Gerst. H.-D. Beckhaus, C. RUchardt, E. E. B. Campbell,and R. Tellgmann, Tebdm#vn Len., 34 (1993) 7729; K. Shigematsu and K. Abe, Chem. Kkpress,7 (1992) 905; K.Shigematsu, K.Abe, M.Mitani, and K. Tanaka, ibid., 8 (1993) 483; Fullerene Science and Technology, 1 (1993) 309. R Taylor, J. Chem. Soc., Perkin fians.2., (1994) 2497. P.I. Fagan, P. J. Krusic, C. N. McEwg J. Lazor, D. H. Parker,N. Herroq, and E. Wasserman, Science, 262 (1993) 404; J. D. Crane,G. J. Langley, H. W. Kroto, R. Taylor, and D. R. M. Walton, unpublished work. P. R Birkett, P. B. Hitchcock, H. W. Kroto, R. Taylor, and D. R. M. Walton, N m e , 357 (1992) 479. R. Taylor, J Chem. Soc., Perkin Trans. 2 ( 1 9 3 ) 813.

chapter 2 FuUerene Production LowellD. Lamb Department of Physics, University of Arizona, Tucson, Arizona 85721, USA

1 Introduction Fullerene production is a simple process (see Figure 1). A large electric current is paased through graphite rods in a quenching atmosphere of inert gas, thus evaporating the rods to produce a light, fluffy condensate called “fullerene soot.” Under the right conditions a substantial fraction is comprised of fullerenes, and they can be extracted either by sublimation or, more commonly, by solvation. In spite of the use of the term “soot,” this is not a combustion product; pure carbon is vaporized in aa inert atmosphere, and the vapor condenses into a variety of forms including the fullerenes. Besides Ceo and C70, hundreds of larger fullerenes, endohedral fullerenes, single- and multi-walled nanotubes and nanoparticles, and nanotubes and nanoparticles containing nanocrystals of “foreign” materials can be made simply and in abundance. This chapter will give an overview of the Kratschmer-Huffman process [2] and its variants, with special emphasis on the practical aspects of obtaining Cm and CT0.Concerning the published literature, an attempt will be made to cite recent and comprehensive articles instead of trying to give a complete history of production. 2

Soot Production

The fullerene-soot generators currently in use can be divided into two broad classes: small “bench-top” chambers intended for the production of limited quantities of soot (on the order of 10-20 grams per day) and large, high-output chambers capable of producing hundreds of grams of soot per day. A small chamber, if adequate, offers a number of advantages including simplicity and low cost. Good descriptions of bench-top soot chambers have been published by Koch et al. [3] and by Scrivens and Tour [4]. In both cases the reaction vessel was made from commercially available glassware, out-of-pocket expenses were several hundred dollars, and construction required a few hours of machine work and a few days of labor. If a large soot generator is needed, there are several issues that have added importance in the design. These include turn-around time (i.e., the time between “burns” required to cool the chamber, break vacuum, install a new rod (or rods), and to pump and purge the system), the amount of soot generated per burn (i.e., the rod diameter and the maximum length of rod that can be burned), soot harvesting, and, probably most important, the generation of large amounts of heat. To deal with the last problem virtually all of

20

21

I

to-

&%%!!

a Figure 1: Basic steps in fullerene production [l]. the large chambers are water cooled in some way. For example, in various fullerenesoot generators at the University of Arizona both electrodes are water-cooled and a water-cooled shroud shields the chamber from direct radiation from the arc. Good discussions of large chambers (with drawings) can be found in articles by Haufler et al. [5] and Parker et al. IS]. Excluding the vacuum pump and power supply (which may be larger and more expensive than those needed for a small chamber), the conversion of an existing vacuum chamber into a large fullerene-soot generator may cost $1,000-2,000 and require several weeks of labor.

The optimum conditions for soot production are extremely chamber- and techniquedependent. Indeed, all of the influential variables in the process probably have not been identified, let alone had their effects characterized. Variables that appear to be universally important include quenching-gas species and purity, chamber pressure, and rod size. Variables whose importance is less clearly understood, or that seem to be more important in some chambers than in others, include voltage, current,.burn rate, the use of an ax. versus a d.c. power supply, chamber dimensions and geometry (including the presence of cooled collection-surfaces),type of burn, static versus flowthrough quenching atmosphere, convection, and the effects of arc-produced radiation on the soot. Brief discussions of some of these variables follow. Although fullerenes will form in a variety of inert atmospheres (e. g., argon and moleculaz nitrogen 1511, a t present helium is the universal choice because the yields are substantially higher [5],[7].In this context the yield is the fraction of the soot that

22

consists of soluble fullerenes. The large range of optimum quenching-gas pressures that has been reported is perhaps the best evidence of the chamber-dependent nature of soot production. Although fullerenes will form with a wide range of pressures (at the University of Arizona fullerenes have been made with pressures ranging from 25 Torr of helium to 3 atmospheres of argon), most workers report optimum pressures in the range of 100-200 Torr. R. E. Haufler has been involved in the design and construction of 10 fullerene-soot generators at several different facilities, and as part of the characterization of each chamber he has conducted a yield versus pressure study (81. He reports that the yield maxima for the various chambers ranged from 50 Torr to slightly less than one atmosphere, with a possible weak trend of increasing optimum pressure with increasing chamber volume. Further, for all chambers Haufler was able to find soot-generation conditions that reliably resulted in yields of lo+%. Another report of an optimum pressure significantly outside of the 100-200 Torr range is by Scrivens and Tour, who found that a pressure of 450 Torr gives the highest yield for their chamber [4]. Additionally, they report a local maximum in the yield at e l 5 0 Torr. This wide and apparently unpredictable variation in the optimum pressure from one chamber to another makes a yield versw pressure study an essential step in the characterization of any fullerene-soot generator. High-purity (99.99+%) graphite rods are available from a variety of manufacturers, and to date there have been no reports of differences in fullerene yield due to the brand of rod used. The rod diameter, however, does dramatically influence the process, with the yield falling as the rod diameter increases [9]. Specifically, 6 mm ($")-diameter rods are used most frequently, and under properly optimized conditions lo+% yields are common. By contrast, yields for 3 mm (:")-diameter rods can be 20+% [lo], and for 12.5 mm ( $")-rods yields are typically 4-7% [9]. In selecting the rod size the competing considerations are, of course, the larger volume of soot generated by vaporizing large rods versus the higher yield that comes from using smaller rods. At present there does not seem to be a case for the superiority of either ax. or d.c. power in terms of fullerene yield. Equally unclear is the issue of how the yield is effected by the size of the electrode gap and the rate of evaporation. For example, Parker et al. [S]and Scrivens and Tour [4] both report finding maximumyield conditions for their respective chambers using relatively large electrode gaps and relatively long burn times (4 mm gap and 0.5 cm/min rod-consumption rate, and 2-4 mm gap and 0.2 cm/min rod-consumption rate, respectively). On the other hand, Haufler reports that for all of the fullerene generators he has built and characterized, maximum-yield conditions invariably include burning at the highest rate allowed by the power supply, which implies that the electrode gap is quite small [8]. Besides the obvious time savings, a rapid burn with a small electrode gap limits the arc-generated radiation and reduces the amount of heat generated. The typical fullerene-soot generator in use today is a converted vacuum chamber, hence as a rule the volume and the details of the chamber geometry are largely

23 accidental. Although the effects of a chamber’s design on fullerene production are much harder to determine than are, for instance, the effects of varying the helium pressure, there are indications that chamber geometry can be quite influential. While some objectives in chamber design are reasonably well defined, such as minimization of heat production, other potentially important considerations, such as convection, are poorly understood. For instance, in one of the Rice University chambers the placement of a water-cooled “chimney” directly above the arc significantly increased the fulletene yield [5]. Also, Parker et al. report that the convection patterns within the Argonne soot-production chamber are markedly different depending on whether or not a set of collection surfaces (“shims”) are in place, and that the fullerene yield is a factor of two higher when the shims are used [6]. The presence of contaminants, especially hydrogen-containing compounds, in the rods, in the gas supply, or due to chamber leaks, has two undesirable consequences. First, as the level of impurities increases the fullerene yield decreases. Laser-ablation [ll] and arc-discharge [12]studies involving the systematic addition of hydrogencontaining gases (Hz [ll],methane and cyclopentadiene vapor [12]) to the quenching atmosphere have shown that a relatively small concentration of hydrogen can have a large impact on fullerene production, In one of the studies a 10% fraction of Hz completely poisoned the reaction and no fullerenes were formed Ill]. Similarly, the presence of trace amounts of oxygen can lead to the production of fullerene oxides [13]. “Curing” the rods before a burn by heating them in a vacuum to drive off adsorbed volatilea can have a positive impact on the yield [8]. The second and perhaps more serioue consequence of the presence of impurities during fullerene synthesis is the likely formation of significant quantities of polycyclic aromatic hydrocarbons (PAHs), many of which are known carcinogens. In addition to the health and safety issues discussed in Section 8, the incredible messiness of fullerene soot makes it imperative that harvesting be done as carefully and aa cleanly as possible. This is especially important for larger chambers, which are heavily used as a rule and generate large quantities of soot. Greatly complicating the task of harvesting is the extreme flocculence of fullerene soot. A substantial fraction of the soot is composed of very small, fluffy particles that become airborne with the slightest agitation, and once airborne can travel surprising distances before settling (or being inhaled). It is strongly recommended that the soot chamber reside in a hood so that rod replacement and soot collection take place in a well-ventilated environment. Also, proper gloves and dust masks should be worn whenever handling soot, and skin contact with the soot should be avoided, 3 Extraction

.Although the fullerenes axe soluble in a wide variety of suhents [la], toluene is most commonly used because of its low cost, relative safety, low boiling point, and

24

reasonably large capacity for carrying fullerenes in solution (at room temperature, saturation of C a in toluene occurs at z2.8 mg/ml [14]). As a usual first step, mixed C ~ O%20% , Cto, and 31% larger fullerenes) fullerenes (typically consisting of ~ 8 0 % are extracted from the soot with toluene, either in a Soxhlet apparatus or simply by dispersing the soot in tofuene and filtering (“cold extraction”). In a cold extraction it may be helpful to sonicate the soot/tohene slurry before filtering. Depending upon the soot/solvent ratio of the initial wash, it may be useful to repeat the procedure more than once. At the University of Arizona excellent results have been obtained by: (1) adding 40 grams of fullerene soot to 4 liters of toluene in a glass bottle and vigorously shaking the bottle to mix thoroughly; (2) leaving the bottle to stand overnight, thus allowing the vast majority of the insoluble component of the soot to settle; (3) carefully decanting the reddish-brown liquor from the bottle, disturbing the undissolved soot as little as possible; (4) filtering the liquor with a coarse and then a fine paper filter. Because a substantial fraction of fullerene soot is composed of graphitic particles 0.1 pm or less in diameter, considerable care must be taken in this step to insure that all particulate matter indeed is removed from the liquor. Using a short chromatography column as a find filtration step may be more effective than a paper filter [8],I15j; ( 5 ) removing the toluene with a rotary evaporator, leaving the mixed fullerenes in the form of a crystalline powder (“extract”); and (6) collecting the depleted fullerene soot from several first washes and repeating steps (1)-(5)2 or 3 times, depending upon the color of the third wash. The wide range in reported typical yields of extract (=6-20%) almost certainly is due in part to the lack of a standardized assaying method. Besides the most common methods of mass balancing and HPLC (an HPLC protocol described in a paper by Diederich and Whetten [16] has been used with excellent results at the University of Arizona), mass spectrometry and uv-visible absorption spectroacopy have also been used. It is not likely, however, that differences in analytical methods constitute the entire explanation for the large variation in the reported yields. One important possibility is contamination. Any soluble contaminants origindly present in the soot (PAHs, vacuum grease, pump oil, etc.) will be in the extract, and they should be removed before the final yield determination. At the University of Arizona contaminants are routinely removed by first thoroughly washing the extract with hexane or ether and then gently baking the sample under vacuum. 4

Separation

Straightforward methods for the chromatographic preparation of relatively large quantities of high-purity Cso and have been developed and are widely used within the fuilerene community. The first efficient method that was reported uses recycling, hot hexane in a modified Kauffman chromatography column of alumina 1171. Such a system runs largely unattended, and can provide large quantities of very pure Ca in

25

2-3 days and large quantities of c70 in 7-10 days. More recently, several activatedcharcoal-based systems for the purification of Cm and C70 have been described 11811211. This approach is especially well suited for preparation of c60; with the proper choice of activated charcoal several grams of very pure Cm may be obtained in =30 minutes via straightforward plug-filtration of fullerene/toluene solutions 1191, [21]. High-purity C70 also may be obtained using either flash chromatography 1201 or the recycling apparatus used for the hexanelalumina system [Zl]. Although flash chromatography is faster (a few hours versus 2 days for the recycling system), it should 0 of 1,2be noted that for each gram of pure c70 prepared in this fashion ~ 1 liters dichlorobenzene (boiling point: 178” C) must be removed with a rotary evaporator. It is now well known that fullerene soot contains hundreds of larger, soluble, allcarbon clusters besides C ~ and O (270. Fullerenes as large as have been detected with mass spectrometry in solvent extracts [22], and it appears that in these mixtures of larger fullerenes every even-numbered carbon cluster beyond Css is present in some concentration. In addition to the large, even-numbered’clusters present in the soot several stable, toluene-soluble, odd-numbered clusters (C,,,, C ~ Z B and , C139) have been found [23].Using high-boiling-pointsolvents such as l12,4-trichlorobenzene appreciable quantities of mixed, larger fullerenes can be extracted from depleted fullerene soot (i.e., soot that first has been washed thoroughly with toluene to reO c70 as possible) [13]. Excellent discussionsof various extraction move as much C ~ and schemes can be‘ found in papers by Parker et al. [6], [24], by Smart et af. [25], and by Diederich and Whetten [l6]. Of this extensive family of molecules only a few of the smaller, even-numbered species have been isolated. After Cm and (270, the next most abundant species are c76, C78, and C S ~and , tens of milligrams of these molecules have been isolated using normal column chromatography and HPLC [13], [16], [26], [27]. Beyond these, however, the problems of separation are formidable [26]-[28]. 5

Endohedral Fullerenes

Besides the “empty,” all-carbon fullerenes discussed so far, macroscopic quantities of soluble fullerenes with various metal atoms or small clusters of metal atoms trapped inside the cage (endohedruf fullerenes or metallojullerenes) can be synthesized, either by laser-ablation of metal-impregnated graphite [29], or more commonly by using doped-graphite electrodes in a standard fullerene-soot generator. So far, soots containing endohedral species with “captive” La, Y , Sc, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, or Er ions or clusters have been made. The interior volume of a C60 molecule, the smallest of the fullerenes, will accommodate an atom of any element of the periodic table up to and including uranium, which raises the possibility of someday being able to “tune” the chemical and electrical properties of a given fullerene by choosing properly the imprisoned atom or molecule. Articles by Giilan et al. [30] and by Bethune et ul. 1311 contain excellent introductions to the field.

26 The doped rods used in endohedral production typically are prepared by: (1) boring a coaxial hole in a graphite rod; (2) packing the resulting cavity with a mixture of graphite cement and metal powder, metal oxide, or metal carbide, with the metal content being a few percent of the overall carbon content; and (3) baking the rod in a vacuum or inert atmosphere to graphitize the cement and drive off any volatile species. The total and relative yields of the metallofullerenes can vary considerably with the level of doping. In general, a smaller metal fraction results in a higher overall yield of metallofullerenes with single-metal-atom species dominating, and a higher metal fraction results in a lower overall yield with multiple-metal-atom species dominating [32]. Although endohedral fullerena can be synthesized with reasonable yields using such rods, they always occur together with a mixture of empty fullerenes, and isolating even small amounts of individual endohedral species requires multi-step HPLC protocols [33], [34]. Greatly complicating the problem is the fact that many endohedral species are air sensitive; for example, LaQCal is quite stable and has been extracted, isolated, and characterized without special handling [34], while the extraction of LaQC76 has been possible only under anaerobic conditions [35]. Bethune et al. have recently reported the isolation of milligram quantities of SczQC8.l and tmme characterization of its crystalline phase [32]. Using high pressure and temperature, Saunders et al. have recently produced macroscopic quantities of MQC60 and MQC70, with M = He, Ne, Ar, Kr, or Xe [36]. By heating a solid c60/c70 mixture at a temperature of ~ 6 0 0 ”C for several hours in the presence of a noble gas with a pressure in the 20,000-40,000 psi range, they obtained yields as high as a few tenths of a percent of the endohedral product relative to the empty fullerene precursor. These extraordinary materials are soluble in toluene and apparently include the first stable compounds involving helium and neon. Sufficiently large samples of 3HeQCm and 3HeQC,0 have been made to allow NMR characterization [37]. 6 N a n o t u b e s a n d Nanoparticles

In addition to the fullerenes and endohedral fullerenes, we now know that other classes of carbon-based materials can be produced by vaporizing graphite (or doped graphite) in an appropriate atmosphere, namely carbon nanotubes (“buckytubes”) and nanoparticles. These fascinating structures can be synthesized in gram quantities, and it has been speculated that applications may include electronics, magnetic storage of information, gas storage, catalysis, chemical separation, and composite materials. Below are brief discussions of the production and separation methods now in we. Following Iijima’s initial report [38] of finding carbon nanotubes in a deposit on the cathode of a fullerene-soot generator, Ebbesen & Ajayan published a recipe for making large quantities of this novel class of carbon structures [39]. Using a typical fullerene-production apparatus and a higher pressure than normal (500 Torr instead

27 of the usual 100-200 Torr), they found conditions under which a particular type of carbonaceous deposit would form on the cathode. In electron-microscopy studies of these Structures it waa discovered that certain interior regions contained copious quantitiee of carbon nanotubes mixed with carbon nanoparticles. Tubes that are produced in this fashion typically consist of two or more concentric shells, with diameters ranging from 2 to 20 nanometers and lengths sometimes as great as several micrometers. The nanoparticles are typically tens of nanometers in diameter, and consist of many concentric graphitic layers. Systematic studies of the effects of varying the production parameters on the yield and morphology of the nanotubes have been published by Ando and Iijima [40] and Seraphin et al. [41], and an overview of the field may be found in a review article by Ebbesen [42]. It is worth noting that although fullerenes, nanotubes and nanoparticles can form in the same environment, and that their formation processes are clearly related, nanotubes and nanoparticles form in significant yield under a broader range of conditions than do fullerenes. For example, it is possible to make nanotubes and nanoparticles, but not fullerenes, in a quenching atmosphere of CHI [40]. Although the tangled mixture of nanotubes and nanoparticlea has not (to the author’s knowledge) proved separable via any standard technique, Ebbeatn and coworkers have reported the isolation of pure nanotubes by heating mixtures of nanotubes and nanoparticlee in pure oxygen [43]. The nanoparticles burn of the starting material is more quickly than do the nanotubes, and although ~ 9 9 % consumed in the process a sample composed solely of nanotubes is obtained.

Making up a separate claas of fullerene-related structures are the single-walled buckytubes, which can be made copiously under the right conditions [44]. In this p r o m the presence of a transition metal (Fe, Co, Ni) as a catalyst is an important requirement. Unlike the multi-walled tubes, these tubes are not found in the cathode deposit. Rather, they are mixed with carbon nanoparticles in web-like structures and rubbery sheets in the soot, and are made easily in gram quantities. The small diameters of these tubes (-1 nm) and their location in the soot suggests that the single-walled tubes are closer relatives of the fullerenes than are the multi-walled tubes and particles discussed above. The yields, lengths, and distribution of radii of the tubes are greatly affected by the choice of catalyst and other parameters [45]. The covaporiaation of metals and carbon has been exploited by various groups with two objectives: to influence the yields and morphologies of pure-carbon nanotubea and nanocrystals, and to produce nanocrystals of various compounds that are completely within nested carbon structures. By doping the electrodes with metals, carbides, or metal oxides (in the same manner as for the production of endohedral fullerenes) and by properly choosing the production conditions, nanotubes and nanoparticles have been formed that encapsulate nanocrystals containing Y, La, Ce Pr, Nd,Gd, Tb, Dy, Ho, Er, Lu, Co, Fe, or Ni. An excellent overview of the field may be found in an article by Saito et al. [46]. Particularly interesting has been the observation by the Carnegie Mellon University group (Pittsburgh, Pennsylvania USA) of

28

ferromagnetism, paramagnetism, and even “textbook” superparamagnetism in certain carbon-coated nanocrystals found in the soot [47]. The extremely small size of these particles (typically 1-10 nm in diameter), and the fact that the magnetic material is protected from chemical attack by the carbon coating, suggest possible utility in the magnetic storage of information. It is also worth noting that the Carnegie Mellon group has had some success in extracting the magnetic nanoparticles from the raw soot with a magnetic-field-gradient separator [47]. 7 Other P r o d u c t i o n and Separation Techniques The fullerene production techniques involving the laser ablation of graphite [29], sputtering or electron-beam evaporation of graphite [48], inductive heating of graphite [49], vaporization of graphite powder in an Ar plasma jet [50], and the use of concentrated solar radiation to vaporize graphite [9], [51], appear to be close relatives of the original Kratschmer-Huffman method [2]. In all cases pure graphite is vaporized in the presence of an inert, quenching atmosphere, and the resulting vapor condenses into a fullerene-bearing soot. It is not unreasonable to speculate that the fullerene-formation mechanisms for these processes are very similar. However, it is not clear what the relationships are among “evaporative” fullerene formation, formation of fullerenes in a special class of sooting flames [52], and formation of fullerenes via pyrolysis of naphthalene 153). In the first of these non-evaporative processes, special, subatmospheric-pressure benzene/oxygen flames have readily produced gram quantities of soot with 20% fullerene yields. Besides possessing the potentially important advantage of being an exothermic reaction, it is possible to “tune” the Cm/Cm ratio over a wide range by adjusting the flame conditions. An excellent review of this method is found in an article by Howard et al. (541. In the other non-evaporative technique, Taylor et al. have shown that pyrolytic decomposition of naphthalene in a low-temperature (1000” C) argon atmosphere can result in the formation of macroscopic quantities of c60 and C,O via combination of Clo fragments [53]. As they point out, this process in principle could be operated in a continuous fashion, and it may prove possible to form selectively a particular fullerene or even endohedral fullerene by choosing judiciously the hydrocarbon precursor and other reactants. The major problems with the chromatographic techniques discussed in Section 4 are stated easily: besides c60 and C70, only a few of the hundreds of other fullerenes and endohedral fullerenes can be isolated in any quantity, and then only with an enormous investment of effort and other resources; a significant quantity of the starting material is lost irretrievably with any of the methods currently in use; and the final product invariably contains trace amounts of solvent trapped in the solid, Of the other approaches that have been tried because of these difficulties, a few deserve brief mention. Darwish et al. report that the careful concentration of an already saturated C60/C70 solution results in the precipitation of a solid that is richer in C70

29

than the starting solution, and by repeating the process several times high-purity C70 may b obtained [21]. As they point out, this method is suitable for large scale procesaing, and there are no ~ O S W with repeated cycling. In a similar vein, Ruoff et al. have made progress in enriching the higher-fullerenecontent of fullerene mixtures using selective-extraction techniques [55]. Another widely discussed approach involves chemical modification of the fullerenes to facilitate separation. For example, Atwood et at. [as] and Suzuki et al. 1571 have reported the purification of Cm and c70 by reversible complexation with calixarenes, which are bowl-shaped molecules that selectively bind to Cm in a toluene solution. Briefly, the calixarene/Cso complex forms a precipitate in toluene that is filtered out and redissolved in chloroform, whereupon the complex decomposes and the Cm forms a precipitate while the calixarene stays in solution. Although it is not clear that this approach is superior to the activated, idea of charcoal-based schemes for the large-scale preparation of CSOand C T ~the someday being able to design a complexation 'partner" for a specific fullerene or endohedral fullerene is very appealing, given the current unsatisfactory state of the art. Lastly, the convenience and effectiveness of solvent-based extraction and separation are not without price; residual amounts of the solvent invariably are trapped in the interstices of any fullerene crystal that has been grown out of solution. Prolonged baking under vacuum does not remove fully this stubborn form of contamination, and sublimation or even repeated sublimation in a vacuum may be the only certain way of purifying completely fullerenes that have been in solution. Accordingly, there has been some work on the extraction and Separation of the fullerenes via sublimation and trmsport in an inert gas [58]. The total fullerene yields can be comparable to those obtained by toluene extraction, and high-purity Cm and C70 samples have been made. 8 Health and Safety Issues

After four years of intensive, world-wide fullerene research, the health-impact picture is encouraging, if admittedly very incomplete. To the author's knowledge there have been no reports of any adverse health effects due to exposure to fullerenecontaining materials. Even if the fullerenes themselves ultimately prove to be safe, however, there are a number of clearly identifiable hazards associated with fullerene research. Although some of the issues discussed below are quite serious, the majority of them are routinely encountered in many research and industrial facilities, and it is clear that fullerene production and research can be safe if standard laboratory practices are followed. Soot-making: When using a glass reaction vessel, thought must be given to the heating of the glass during soot production, as heat-induced stress may lead to implosive failure. If the arc is monitored visually, appropriate, uv-blocking eye-protection should be used. It is important to note that not all dark glasses offer sufficient uv-

30 protection, and the use of uv-transparent dark glasses may actually increase the risk of eye damage, as the eye will dilate in response to the attenuation of the visible output of the arc. As discussed above, impurities in the rods, quenching gas, or those present due to chamber leaks should be minimized to reduce the chances of forming PAHs.

Soot harvesting a n d handling: There is a very strong sentiment in the fullerene community that inadvertent, long-term inhalation of airborne fullerene-soot particles is the most serious potential health problem commonly encountered in fullerene production, Once these particles are airborne they are very difficult to filter out of the air, and the use of adequate hoods with high air-flow rates is strongly recommended, It is well known that inhaling of any type of particulate contamination over an extended period can result in serious lung damage. Given that very little is known about the health impact of exposure to fullerene soot, and that it is common for diseases caused by inhalation of particles to take years to develop, there is a very real and compelling need to prevent inhalation of and skin contact with the soot. Solvents: All of the solvents commonly used in fullerene production are toxic, inflammable or explosive, and many are carcinogenic, mutagenic, or teratogenic. When using these chemicals standard laboratory procedures should be carefully followed. All solvents should be safely stored, gloves should be worn to avoid skin contact, open containers of solvent should be handled only in well-ventilated hoods, and used solvents should be disposed of properly. H e a v y metals: Many of the materials used in endohedral-fullerene production are known toxins (e. g., heavy metals), and should be handled using standard laboratory procedures. Fullerene-containing materials: Although there has been no indication to date , that the most common fullerenes and fullerene-containing materials ( C ~ OC, ~ Oetc.) are dangerous in any way, until the actual health impact has been determined prudence dictates that these substances be treated with the same respect accorded any other unknown chemical. Specifically, care should be taken to prevent ingestion, inhalation, or contact with the skin.

Acknowledgments The author thanks M. C. Zumwalt for assistance in preparing the figure, and also thanks the Division of Advanced Energy Projects of the Office of Basic Energy Sciences, Department on Energy for kind support of this work.

31

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K.C. Khemani, and F. Wudl, J. Org. Chem. 56 (1991) 4543-4545.

[4] W. A. Scrivens and J. M. Tour, J . Org. Chem. 57 (1992) 6932-6936. [5] R. E. Haufler, Y. Chai, L. P. F. Chibante, J. Conceicao, C. Jin, L.-S. Wang, S. Maruyama, and R. E. Smalley, Mat. Res. SOC.Syrnp. Proc. 208 (1991) 627-637.

[6] D. H. Parker, K. Chatterjee, P. Wurz, K. R. Lykke, and J. C. Hemminger, Carbon 30 (1992) 1167-1182.

M. J. Pellin, L. M. Stock,

[7] Y. Saito, M. Inagaki, H. Shinohara, H. Nagashima, M. Ohkohchi, and Y. Ando, Chem. Phys. Lett. 200 (1992) 643-648. [8] R. E. Haufler, private communication.

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[9] L.

[lo] R. L. Whetten, M. M. Alvarez, S. J. Anz, K. E. Schriver, R. D. Beck, F. N. Diederich, Y.Rubin, R. Ettl, C. S. Foote, A. P. Darmanyan, and J. W. Arbogast, Mat. Res. SOC.Symp. Proc. 208 (1991) 639-650. Ill] M. S.de Vries, H. R. Wendt, H. Hunziker, E. Peterson, and S. Chang, Proceedings of the X X I l LzLnar and Planetary Sciences Conference (1991) 315-316.

[I21 M. A. Wilson, L. S. K. Pang, R. A. Quezada, K. J. Fisher, I. G. Dance, and G. D. Willett, Carbon 31 (1993) 393-397. (131 F. Diederich, R. Ettl, Y. Rubin, R. L. Whetten, R. Beck, M. Alvarez, S. Anz, D. Sensharma, F. Wudl, K. C. Khemani, and A. Koch, Science 252 (1991) 548-551. [14] R. S. Ruoff, D. S. Tse, R. Malhotra, and D. C. Lorents, J . Phys. Chem., 97,33793383 (1993); W. A. Scrivens and J. M. Tour, 1. Chem. SOC.,Chem. Commun. 1995, 1207-1209. [15] F. A. Tinker and D. R. Huffman, unpublished results. 1161 F. Diederich and R. L. Whetten, Acc. Chem. Res. 25 (1992) 119-126.

32 [17] K. Chatterjee, D. H. Parker, P. Wurz, K. R. Lykke, D. M. Gruen, and L. M. Stock, J . Org. Chem. 57 (1992) 3253-3254; K. C. Khemani, M. Prato, and F. I Wudl, J. Org. Chem. 57 (1992) 3254-3256. (181 W. A. Scrivens, P. V. Bedworth, and J. M. Tour, J. Am. Chem. Soc. 114 (1992) 7917-79 19. I191 L. Isaccs, A. Wehrsig, and F. Diederich, Helu. Chim. Acta 76 (1993) 1231-1250. [20] W. A. Scrivens, A. M. Cassell, B. North, and J. M. Tour, J. Am. Chem. SOC., Communication, in press (1994).

(21) A. D. Darwish, H. W. Kroto, R. Taylor, and D. R. M. Walton, J . Chem. SOC., Chem. Commun. 1994,15-16. [22]

H.Shinohara, H. Sato, Y. Saito, A. Izuoka, T. Sugawara, H. Ito, T. Sakurai, and T. Matsuo, Rapid Comm. Mass. Spec. 6 (1992) 413-416.

[23] S. W. McElvany, J. H. Callahan, M. M. Ross, L. D. Lamb, and D. R. Huffman, Science 260 (1993) 1632-1634. [24] I). H. Parker, P. Wurz, K. Chatterjee, K. R. Lykke, J. E. Hunt, M. J. Pellin, J. C . Hemminger, D. M. Gruen, and L. M. Stock, J. Am. Chem. SOC.113 (1991) 7499-7503. [25] C . Smart, B. Eldrige, W. Reuter, J. A. Zimmerman, W. R. Creasy, N. Rivera, and R. S. Ruoff, Chem. Phys. Lett. 188 (1992) 171-176.

I261 K. Kikuchi, N. Nakahara, M. Honda, S; Suzuki, K. Saito, H. Shiromaru, K. Yamauchi, I. Ikemoto, T. Kuramochi, S. Hino, and Y. Achiba, Chem. Lett. (Japan) 1991 1607-1610; K. Kikuchi, N. Nakahara, T. Wakabayashi, M. Honda, H. Matsumiya, T. Moriwaki, s. Suzuki, H. Shiromaru, K. Saito, K. Yamauchi, I. Ikemoto, and Y . Achiba, Chem. Phys. Lett. 188 (1992) 177-180. [27] R. Taylor, C. J. Langley, A. G. Avent, T. J. S. Dennis, H. W. Kroto, and D. R. M. Walton, J . Chem. Soc., Perkin Trans. 2 1993,1029-1036. [28] Y . Achiba, K. Kikuchi, T. Wakabayashi, N. Nakahara, and S. Suzuki, “Higher Fullerenes: Isolation, Characterization, and Growth Mechanisms,” in Proceedings of the First Italian Workshop on Fullerenes: Status and Perspectives, Bologna, 6,7February, 1992,C. Taliani, G. Ruani, and R. Zamboni, Eds. (World Scientific, Singapore, 1992) 13-19. [291 Y . Chai, T. Guo, C. Jin, R. E. Haufler, L. P. F. Chibante, J. Fure, L. Wang, J. M. Alford, and R. E. Smalley, J. Phys. Chem. 95 (1991) 7564-7568.

33 [30]E. G.Gillan, C. Yeretzian, K. S. Min, M. M. Alvarez, R. L. Whetten, and R. B. Kaner, J. Phys. Chem. 96 (1992)6869-6871. [31]D. S. Bethune, R. D. Johnson, J. R. Salem, M. S. de Vries, and C. S. Yannoni, Nature 366 (1994)123-128. [32]D.S. Bethune, C.-H. Kiang, R. Beyers, P. H. M. van Loosdrecht, M. S. de Vries, J. R.Salem, C. S. Yannoni, and R. D. Johnson, Proc. ZWEPNM94, Kirchberg/Tirol, Awtria, 5-12 March 1994. [33]H. Shinohara, H.Yamaguchi, N. Hayashi, H. Sato, M. Ohkohchi, Y. Ando, and Y . Saito, J. Phys. Chem. 97 (1993)4259-4261. [34]K. Kikuchi, S. Suzuki, Y. N a b , N. Nakahara, T. Wakabayashi, H. Shiromaru, K. Saito, I. Ikemoto, and Y. Achiba, Chem. Phys. Lett. 216 (1993)67-71. [35]S. Bandow, H. Kitagawa, T. Mitani, H. Inokuchi, Y. Saito, H. Yamaguchi, N. Hayashi, H. Sato, and H. Shinohara, J. Phys. Chem. 96 (1992) 9609-9612. [36]M.Saunders, H. A. Jimknez-Vkquez, R. J. Cross, S. Mroczkowski, M. L. Gross, D. E. Giblin, and R. J. Poreda, J. Am. Chem. SOC.116 (1994)2193-2194. [37]M. Saunders, H. A. Jimbnez-Vbquez, R. J. Cross, S. Mroczkowski, D. I. Freedberg, and F. A. L. Anet, Nature 367 (1994)256-258. [38]S. Iijima, Nature 354 (1991)56-58. [39]T.W. Ebbesen and P. M. Ajayan, Nature 358 (1992)220-222. [40]T. Ando and S. Iijima, Jpn. J. Appl. Phys. 32 (1993)L107-Ll09. [41]S. Seraphin, D. Zhou, J. Jiao, J. C. Withers, and R. Loutfy, Carbon 31 (1993) 685-689;S. Seraphin and D. Zhou, Carbon, in press (1994). [42]T. W. Ebbesen, Annu. Rev. Mater. Sci. 24 (1994)235-264. [43] T. W.Ebbesen, P. M. Ajayan, H. Hiura, and K. Tanigaki, Nature 367 (1994) 519. [44]S. Iijima and T. Ichinashi, Nature 363 (1993) 603-605;D. S. Bethune, C.-H. Kiang, M. S. de Vries, G. Gorman, R. Savoy, J. Vazquez, and R. Beyers, Nature 363 (1993)605-607. [45]P. M. Ajayan, J. M. Larnbert, P. Bernier, L. Barbedette, C. Colliex, and J. M. Planeix, Chem. Phys. Lett. 215 (1993)509-517;S. Seraphin and D. Zhao, Appl. Phys. Lett., in press (1994);C.-H. Kiang, W. A. Goddard, R. Beyers, J. R. Salem, and D. S. Bethune, J. Phys. Chem., in press (1994).

34 [46] Y. Saito, T.Yoshikawa, M. Okuda, N. Fujimoto, K. Sumiyama, K. Suzuki, A. Kasuya, Y. Nishina, J . Phys. Chem. Solids 54 (1993) 1849-1860. (471 S. A. Majetich, J. 0. Artman, M. E. McHenry, N. T. Nuhfer, and S. W. Staley, Phys. Rev. B, Rapid Comm. 48 (1993) 16845-16848;B. Diggs, C. Silva, B. Brunett, S. Kirkpatrick, A. Zhou, D. Petasis, N. T. Nuhfer, S. A. Majetich, M. E. McHenry, J. 0. Artman, and S. W. Staley, J . AppI. Phys. 7 5 (1994)5879-5881; E. M. Brunsman, R. Sutton, E. Bortz, S. Kirkpatrick, K. Midelfort, J. Williams, P. Smith, M. E. McHenry, S. A. Majetich, J. 0. Artman, M. De Graef, and S.W. Staley, J . Appl. Phys. 7 5 (1994)5882-5884;M. E. McHenry, S. A. Majetich, J. 0. Artman, M. De Graef, and S. W. Staley, Phys. Rev. B49 (1994)11358-11363. [48]R. F. Bunshah, S. Jou, S. Prakash, H. J. Doerr, L. Issilcs, A. Wehrsig, C. Yeretzian, H. Cynn, and F. Diederich, J . Phys. Chem. 96 (1992) 6866-6869. [49] G. Peters and M. Jansen, Angew. Chem. Int. Ed. Engl. 31 (1992) 223-224. (501 K.-I. Yoshi, S. Kasuya, K. Eguchi, and T. Yoshida, Appl. Phys. Lett. 01 (1992) 2782-2783. [51]C . L. Fields, J. R. Pitts, M. J. Hale, C. Bingham, A. Lewandowski, and D. E. King, J . Phys. Chem. 97 (1993) 8701-8702. [52] J. B. Howard, J . T. McKinnon, Y. Makarovsky, A. L. Lafleur, and M. E. Johnson, Nature 3 5 2 (1991)139-141. [53] R. Taylor, G.J. Langley, H. W. Kroto, and D. R. M. Walton, Nature 366 (1993) 729-731. [54] J. B. Howard, A. L. Lafleur, Y. Makarovsky, S. Mitra, C. J. Pope, and T. K. Yadav, Carbon 30 (1992)1183-1201. (551 R. S. Ruoff, private communication. (561 J. L. Atwood, G. A. Koutsantonis, and C. L. Raston, Nature 368 (1994)229-231. [57] T. Suzuki, K. Nakashima, and S. Shinkai, Chem. Lett. (Japan) 1994,699-702. [58]D. M.Cox, R. D. Sherwood, P. Tindall, K. M. Creegan, W. Anderson, and D. J. Martella, “Mass Spectrometric, Thermal, and Separation Studies of Fuilerenes,” Chapter 8 in Fullerenes: Synthesis, Properties, and Chemistry of Large Carbon Clusters, ACS Symposium Series No. 481, G. S. Hammond and V. J. Kuck, Eds. (American Chemical Society, Washington, D. C., 1992); R. N. Compton, private communication.

Chapter 3 Properties of Fullerenes Roger Taylor

Investigationof the physical properties of lllerenes has been most intensive, and has r d t e i in the pubfiation of many thousands of papers. However, much of the published information is of less direct interest to the synthetic chemist, and only the most salient htures are considered in this chapter. For practical purposes, description is

limited to [60]-, [70]-, [76]-, [78]-, [82]-, and [84]lllerenes. Given that [7684]Mermes comprise only about 1% of the soot extract, and isolation of 1 mg of the pure mmpounds is hugely expensive in both time (ca.250 h are required) and solvents, and requires state-of-theart High Pressure Liquid Chromatography, it is not surprising therefire that to date, little or no chemistry has been performed on these compounds. For even higher fillerenes the problem becomes an order of magnitude more difficult, and bianing some technological breakthrough, the chemistry of these is untikely to concern us during the next decade. Even when significant amounts of these componds becomcr available, there will be difficulties due to the decreasing solubility and reactivity that p d d s increasing molecular weight of fkllerenes

1. Appearance and Purity Determination

The appearance of lllerenes differs according to the thickness of the material, whether or not they are in solution, and the nature of the solvent. Very thin films of [6O]iwlerene are mustard yellow, the colour becoming brown, and iinally black as the 6lm thickness increases.1 [7O]Futlerene is red in thin films, then reddish-brown and ultimately black as the fiIm becomes thicker. 1 Solutions of [60]fullerene have a beautiful magenta d o u r in hydrocarbon solvents,* but appear brown in some pi-donor solvents, due probably to some charge-transfer interactions (and possibly some electrophilic addition catalysed by Lewis acid sites in the glass container). Solutions of [70]fiAlerene in hydrocarbon solvents are port-wine red.* In the same solvents (and also chloroalkanes) [76]fillerene gives yellow-green solution2 as does [84]fillerene? whilst [82]fuUerene has a less greenish tinge;4 [78]fiIlerene solutions are golden chestnutbrown c o l o ~ r . ~ 7 ~ 35

36 The extinction coefficient of [70]firllerene in the visible is higher than that for [6O]Olfullerene. A consequence is that mere traces of the former in solutions of the latter impart a d o u r detectable to the eye. This enables experienced workers to judge the purity of chromatographically purified samples of [6O]fbllerene without recourse to timeconsuming 13C NMR analysis. Unfortunately the reverse is not the case and solutions of [70]fUllerene containing as much as 70% of [60]hllerene are visibly indistinguishable from pure [70]hUerene. This led to some embarassment by fledgling lllerene manufacturers who lacked (as is usually the case) the necessary analytical equipment. [6O]Fullerene is the material that most workers wilI become familiar with. Although it is black in the solid, very finely divided material can appear brown. However, material in the latter form is preferably to be avoided since the tendency towards atmospheric oxidation increases with surface area. Crystals of various shapes and S i z e s can be grown easily e.g. from benzene, needles appear which are actuaUy a series of overlapping plates rather like a cart spring.1 If however the recrystallisation fiom benzene is very slow, deep magenta crystals of [SO]&llerene.4C& are obtained.6 Most material is supplied fiom manufacturers as granules ranging in size up to cu. 1 mm3, Unless suppliers have been meticulous in removing occluded solvents (and dioctyl phthdate, the plasticiser present in all solvents including those claimed to be of HPLC grade), and even ~ulphur,~ it will be necessary for rigorous work to sublime the fbllerene in vacuum (at cu. 400 "C and 0.01 W g ) . In this way, ciystds in excess of 100 mg in weight have been grown. For preparative chemistry (as opposed to physical measurements) it will in most cases be unnecessary to purifjr by sublimation. The purification regime devised by the writer consists of removal of solvent after chromatographic separation (this is most Simply, rapidly, and economically carried out using a column packed with Norit 'Elorit' grade carbon).8 The resultant solid is then digested repeatedly with AR grade acetone until the supernatant liquid (removed each time with a Pasteur pipette becomes colourless (this removes non-fullerene impurities), and after removal of residual acetone (under nitrogen), heating the solid at cu. 170 *C at 0 5 mmHg for 2 h.8 Because the crystal packing improves with increasing purity, pure [60]fullerene is significantly harder to dissolve than less pure material. The same purification regime and solubility aspects apply to [7O]hllerene. The next problem to be faced is the determination of the purity of the material that is available. This is difficult, and little notice should be taken of early papers claiming quite unmeasurable purities, and not too much should be taken of some manufacturers

37

claims. For example, purity said to be 'checked by HPLC' can be misleading unless great carc hu been taken to compensate for the differences in extinction coefficients of the other W m n e 'impurities'. This is particularly true of [70]fillerene because the lower extiuchn wefiicient of [6O]fullerene, the usual impurity, means that estimates of its mnCdlltration may be too low. There are problems too with mass spectrometry, because the voktility of lllerenes decreases markedly with increasing cage size, and unless the temperakrre of the probe is ramped quickly up to that at which the ions appear for the higher fbllerenes, they may appear to be absent; their concentrationswill not in any case be proportional to the ion intensities. 1.1 13CN M R Spctrosco~.The most reliable indicator of purity is 1%

but here there are dif€idties because of the differing numbers of identical carbons in a given fUma~e.Thus for example, [7O]hllerene, having five carbon types gives ca.one-fifth of the si@ intensity for each carbon atom compared to [6O]fillerene. Thus the presence of [70]iUame in [6O]fullerene can go undetected unless a sufficiently long accumulation time for the spectrum is used. The problem is of course exacerbated with the higher (and less symmetrical) hllerenes. The shorten the accumulation time, chromium (acac)3 is usually employed as a relaxation agent, and provided the relaxation times are sufficiently long (a total of 6 s for relaxation plus acquisition is adequate), each hllerene will give signals proportional to their concentrations. The chromium compound can be removed and the pure fullerene recovered, by repeated digestion with acetone (in which the hllercne is insoluble), and removal of the Supernatant liquid by means of a Pasteur pipette. The chemical shifts obtained will depend slightly on the solvent used for obtainiag the spectrum, and also on the deuteriated compound used for the lock signal. Figures 1-3 show 13C N M R spectra for [60-Ih]-,14 [70-Lk,h]-,14 and [7602]Werenes.2 Figure 4 shows the spectrum for the C2v(I) isomer of [78]fUllerene,S Figures 5 and 6 are the spectra for mixtures (differing proportions) of the C2V(I)-, CzV@)-, and D3-isomers of [78]111erene~;~31~ lines for [76]fUllerene and traces of [60]and [70]fullerenes10 have been deleted for clarity in Figure 6. The proportions of the various [78]hllerene isomers obtained are very dependent upon reactor conditions.1 Figure 7 shows the spectrum for a mixture of C2-, QV, and C3,,-isomers of [82]flrlleiene? and Figure 8 shows two spectra for a mixture of the Dz(1V)- and &&I) isomers of [84]f~lIerene;~~10 the latter shows how reproducible the spectra can be, provided that the stabilities of the component isomers are similar, and are thus produced in comparable quantitiesin different reactors.12

38 I

-

-

w

i47

Figure 3

i42 ppm i37

132

I3C NMR spectrum for [76-D?]fullerene(deuteriated acetone lock signal)

39

1

1

148

1

146

1

144

I I II i i i i i i

1

1

142

i

1

140

~

1

138

1

1

136

~

1

*

134

I II I I Ill I I I I

Ill I

F i p m 5 (above) and 6 I3C NMR spectra in CS2 for mixtures (Merent proposrtions)of [78]fullerene isomers (starred lines in Fig. 6 are half-intensity)

~

1

II

.

. .

150

145

140

I

I

135

130

Chemical shin (p.p.m.)

Figure 7 I3C NMR specmm in CS2 for a mixture of [82]fullerene isomers

-

144

142

140

138

136

134

6

Figure 8 13C NMR spectra for a mixure of D2 and D2d isomers of [84]fullerenes

41

For fillerene adducts, 13C N M R spectra give information as to the location of the addend. For example, in most cases, addition to [6O]fillerene takes place across the 1,Z-bond so that two resonances due to C1 and C2 appear in the sp3 region of the spectrum. Of the 58 remaining carbons, two (C55 and C60) lie on the C, symmetry axh, and generally give peaks between 6 146.5 and 147.5 if the addend has no plane of symmetry orthogonal to the C, axis. If the addend has such a plane of symmetry, (e.g. in the case of addition of a XJ3R group), then there will be one line in the sp3 region, and twenty-seven l i e s (intensity 2) and four lines (intensity 1) in the sp2 region. If the addend is symmetrical across both axes (e.g. in the case of addition of >cR2), then one line (iimsity 2) in the sp3 region, and thirteen lines (intensity 4) together with three lines of intensity 2 (C55/60; C22/32; C23/31) in the sp2 region will result. Similar reasoning can ascertain the positions of addition to [7O]lllerene and higher fillerenes. 1.2. IR Spectroscopy.

The IR spectra of [60]- and [70]Mlerenes have been published.13,14 Due to the high symmetry of the molecule, the former (Fig. 9a) consists of four lines only, in agreement with theoretical calculations,*5 at 527, 576.5, 1182.5, and 1429 cm-l.13 For [7O]llIerene, the spectrum (Fig 9b) shows twelve lines at 458.2,

Figure8 Ja,b IR spectra for [60]fbllerene (above) and [70]hllerene (below)

42

535.5, 565.7, 578.0, 642.4, 674.2, 795.1, 1087.0, 1133.2, 1414.0, 1430.7, and 1461.0 cm-1.13 Before running IR spectra, it is important that the fullerene should be baked as described above, otherwisebands due to occluded solvent will appear. Even thin films of fhUerene, deposited from solvents onto KBr plates, required prolonged heating under vacuum at 200 "C before traces of solvent are completely removed.

No IR spectra have yet been published for higher fullerenes. 1.3.

Raman Spcfroscopy. Raman spectroscopy is another means of charactmising

[a]and

[70]fullerenes, though the equipment needed for accurate work (1 cm-1 resolution) is not widely available. Spectra have been obtained for both Mlerenes p i p r e 10% b)16 (6also ref 17). The vibrational frequencies for [6O]fullerene appear at 272, 429, 495, 570, 772, 1099, 1250, 1422, 1467, and 1573.5 cm-1, with minor featwes at 266, 485, 711, and 1479 cm-1. The peak at 1479 cm-1 may be due to C60O. For [7O]lllerene, the vibrational frequencies are at 226, 252, 259, 394,410, 417,430,456, 566, 700, 736, 1061, 1180, 1221, 1226, 1294, 1423, 1465, 1509, and 1562 cm-1, with additional very weak features at 489,520,534,547, 1329, 1348, and 1365 cm-*. No Raman spectra have been reported yet for higher fullerenes.

Figures lOa, b Raman spectra for [60]fullerene(above) and (70]!hllerene(below)

43 1.4. W N S Spectroscopy.Whilst these spectra are relatively easy to 0btain,1811~ the information is of less value as a criterion of purity. For [60]Mlerene there are sharp bands at 213, 257, and 329 tun (with E= 135,000, 175,000, and 51,000, respedvdy), and weak features at cu. 500, 540, 570, 600, and 625 nm (Fig. lla). Derivath bnner by addition across the I ,2-bond show a characteristicband at 435 m. For [70lfWlcrene there are sharp bands at 214, 246, 331, 360, 378, and 468 nm, with weak features at 550,600,620,645, and 665 nm (Fig. 1lb). The W spectra have been published for [76-D2]f%ferend and for the C 2 4 ) and r>3 isomers of [78]fUereness (Figures 12a,b, and c, respectively).

3

400

6oo

nm

800

Figurn 11%b WMS spectra for [a]and [7O]fullerenes recorded in heme, inset spectra are recorded in benzene at higher concentration

44

0

5I .1 0"

L

n m

210

400

580

720

880

L

Wavelength (nm)

W m w h g m (nm)

Figures 1 2 a . b ~ WMS spectra obtained in dichlommethanefor [76-&]-,[78+,.(Ill[78-Dj]fullerenes, respectively

1

and

1.5. DlfJerential Scanning Calorimetry. This is a reasonably sensitive means of determining the purity of [60J&lIerene, and involves detecting the change in phase with temperature as [60]lllerene changes from a single cubic lattice to disorder at around 260 K.20. Ifthe material is pure, a sharp transition line such as that shown in Figure 13 will be obtained.

210

2s.3

TIK

Figure 13 Differential scanning Calorimetry curve for [6O]fullerene. a: cooling. b: heating

45

1.6. X-Ray Pow& Difiaction. Pure material gives the spectrum shown in the lower trace of Figure 14.21Impure material always shows a shoulder to the left of the left-hand peak, aa shown in the upper trace of Figure 14.

Figure 14 X-Ray powder diffraction spectra for [60]lllerene

2. Solubility

This is a factor of particular interest to the synthetic chemist. Accurate determination of solubilities is difficult, partly because saturated solutions are dark and opaque, hence it is not easy to see if all the solid hllerene has dissolved. Also,a curious feature is that the solubility passes through a maximum with increasing temperature.22 However, there is some evidence that the behaviour of (6O]lllerene in this respect depends upon whether or not the sample has been baked free of solvent prior to carrying out the temperaturdsolubility measurements.23 This aspect is still under investigation, but it seems possible for example, that clustering (there is evidence that this exists in soluti0n),2~may differ appreciably for baked- and unbaked samples. The close packing in bakd samples could cause clusters to be solvated as such, the clusters then gradually breaking up as the temperature of the solution is raised. The solubilities of [60]fullerene have been determined in two studies and the results (which are reasonably consistent) are shown in Table l.22,25 Data for

46

Solvent

Table 1 Solub~tyof [60]fuUerene in mg/mL Solvent

pentane hexane octane isooctane decane dodecane tetradecane cyclopentane cyclohexane decalins (mix) cis-decane trans-decane dichloromethane chloroform carbon tetrachloride trichloroethylene tetrachloroethylene dichlorodifluoroethane 1,1,2-trichlorofluoroethane

1,1,2,2-tetrachloroethane tetrahydrothiophene 2-methylthiophene carbon disulphide

0.005,0.004 I 0.043,0.040 1 0.025 I 0.026 I 0.071,0.070 0.091 I 0.126 I 0.0020 I 0.036,O.OSl I 4.6 I 2.2 I 1.3 I 0.26 I 0.16 I 0.32,0.45 I 1.4 I 1.2 1 0.020 1 0.014 I 5.3 1 0.030 I 6.8 I 7.9, 5.16 1

I

I I I

methanol ethanol acetone nitromethane nitroethane acetonitrile N-methyl-2-pyrroiidone dioxane pyridine tetrahydrofuran benzene toluene xylenes mesityiene tetralin o-cresol benzonitrile fluorobenzene chlorobenzene bromobenzene 1,Zdichlorobenzene 1,2,4-trichiorobenzene 1-methylnaphthalene dimethylnaphthalenes 1-phenylnaphtfialene 1-chloronaphthalene

0.000 0.001 0.001 0.000 0.002 0.000 0.89 0.041 0.89 0.000

1.7, 1.44 2.8,2.15 5.2 1.5,0.997 16 0.014 0.4 I 0.59 7.0 3.3 27 8.5 33 36 50 51 ~~

~

Table 2 Solubility of f70Jlllerene in m g / d Solvent pentane hexane heptane octane decane dodecane cyclohexane acetone isopropanoi

Solvent 0.002 0.013 0.047 0.042 0.053 0.098 0.080

0.019 0.021

J

I I I I I I

1

carbon tetrachloride benzene toluene p-xylene mesitylene 1,2-dichlorobenzene dichloromethane carbon tetrachloride

0.121 1.30 1.406 3.985 1.472 36.2 0.080

0.121

47

[7O]fuIlem1$~ are gken in Table 2. It is evident that there is no solvent parameter which univerdy explains the solubility of [6O]fullerene. There does seem to be an increate k! rolubility with increase in solvent molecular size,22 but any correlation with (say) decmndonating abiities of the solvent is lacking. Thus whilst addition of electrowupplying aikyi groups to benzene favours solvation of the fiUerene7 the behavim ofbromo- and chlorobenzenes is difFcult to explain. Ona note of caution is appropriate here. There is evidence that the Werenes may react with some solvents in a manner as yet not properly understood. For example, a slow rdaction takes place with pyridine? and also te$rahydrohan,27 90 tests should be carried ~ ubefore t employing a particular solvent. In general the solubilities of Werene derivatives are considerably greater than the parent ibllarene, and this may reflect the greater Mculty in achieving close packing. Some ddvatives, e.g. fluorolltaerenes are readily solubie in a wide range of polar s o l v t ~ dMost reactions of lllerenes do not go to completion, and this solubdity d e e n c e provides a convenient means of separatingout most of the u~eactedfullerene, i.e. by adding (say) acetone to dissolve the derivative, and then atering from the U N & ~ Illerene. This can greatly ease subsequent purification of the product by

HPLC. 3. Thermal and Oxidative Stability

The melting points of the parent lltaerenes have not been determined. However, they are estimated to be >lo00 "C. There are as yet no reports of the melting points of charactaid derivatives, but it is likely (polymers apart) that they will be somewhat lower. [#]FuUerene is unstable towards high temperatures and measurable decomposition a n be observed above a.750 OC?* but this wiU generally not be a problem for the syntheticorganic chemist. Decomposition is much more rapid in the presence of oxygen,29 which can be absorbed to the extent of 4% 0 2 per [6O]fuUerene.3* Thus for example, at ca. 250 OC oxidation to an anhydridetype species takes place which in turn is oxidised to an acid; at high temperatures, oxidation to CO and C02 occurs.3~Oxidative degradation in the presence of strong W li@t (discovered during chromatography on neutral alumina)32 is very rapid and produces both the epoxid23 and water-soluble products;32 titanium dioxide also enhanws the W-catalysed oxidative degradation of [70]fullerene.34 The reactioa with oxygen under irradiation is accompanied by an intensification of the

48

normally weak ESR signal @-value of ca. 2.0) that [60]Mlerene gives rise to, indicating that this signal (which should be absent) is due to oxygenated i r n ~ u r i t i e sThe . ~ ~probable involvement of ozone and epoxide intermediates32 is supported by studies of direct ozonolysis. One of these gave derivatives showing ketone, ester, and epoxide pr0perties.3~A second study of ozonolysis of toluene solutions of the hllerenes showed that both [60]- and [7O]lllerenes were rapidly oxidised at all temperaturesbetween -78 and +lo0 OC,giving monoxides, dioxides (and possibly higher oxides) with [60]fUerene reacting faster than [70]fi~Uerene.~~ A similar study of [60]-, [70]-, [76]-, 1781-, and [MJWerenes at 23.5 OC, showed that all were degraded by ozone ([7O]lllerene the least rapidly) but oxides derived from [76]-,. [78]-, or [M]fullerenes were not detected.37 Because both [60]- and [70]kllerenes have low-lying HOMOS, electrochemical oxidation is quite dacult, but has been achieved. The chemically reversible oneelectron oxidation waves (giving cation radicals with lifetimes >30 s) occur at 1.26and 1.20 v, respectively, and for [70]fUllerene a second one-electron oxidation takes place at 1,75 v, giving a di~ation.~*

4. Oridising Properties and Toxicity

[60]Fullerene is a potent producer of singlet oxygen39 and therefore precautions should be taken to avoid contact with the skin. However, in this connection it should be noted that whilst a water-soluble lllerene carboxylic acid showed biological activitycytoxicity and G-selective DNA cleaving ability (but only under the action of low-level visible light)>o and that hllerenes are potentially excellent photoinduced electrontransport moietiespl [60]hUerene has been found not to affect the proliferation of human keratinocytes or human fibroplasts$2 indicating that its accumulation in human cells does not result in acute toxicity Under visible or UV irradiation in the presence of molecular oxygen, alkenes and dienes undergo reaction with singlet oxygen if a trace of hllerene is present to give photo-oxygenation products There is however disagreement as to the relative effectiveness of [70]fbllerene and [60]hllerene, some worked3 finding the former to be the more efficientcatalyst, whilst others find it to be the latter.44 Because [60]hllerene has a low-lying LUMO, it is readily reduced and can acquire, reversibly, up to six electrons under electrochemical conditions. The potentials at which the electrons are

49 added depends on the extent to which the solvent either donates electrons to the cage, or accepts them from it.45 Thus [60]fullerene has good oxidising properties and may oxidise d i y other materials in which it is contact. For example, it will oxidise hydrogen eulphide to sulphur.46 Hydrogenated fillerenes are readily oxidised, but this tendency is markedly reduced ifthey are free fiom the parent fillerene (Chap. 4), which again points to the oxidising behaviour of the parent. 5. Mbedianeous Propertits

Some useful data for [6O]fillerene are the following: Mean diameter, 6.83 4 outer diameter, 10.18 A; inner diameter, 3.48 A; average C-Cdistance 1.44 A FCC (fbce-centred cubic) lattice constant 14.17 A.

Mass density, 1.72 g cm-1; molecular density, 1.44 x 1021 cm-3;; thermal conductivity (at 300 K), 0.4 W mK-1; phase transition temperatures, 90 K, 260 K. Binding energy per atom, 7.4 eV; electron affinity, 2.65 eV, 1st ionisation potdid, 7.58 eV; 2nd ionisation potential, 1l.S eV; HOMO-LUMO band gap, 1.7 eV. Rotational rate at room temperature, c4. 1 x. 1O1O s-l. Although this has prevented a single cxystal X-ray structure being obtained for the free molecule, structures have been obtained by using various complexes (which thereby reduce the rotational rate), r.g. with benzene$7, ferrocene$* Sg,49 and i o d i n d t ~ l u e n e . ~ ~ [7O]Fullerene is also rotationally disordered at romm temperature.

References 1. 2. 3. 4. 5.

6.

7.

R.Taylor, J. P. Hare, A. K. Abdul-Sada, and H. W. Kroto, J. Chem. Soc.,Chem. Commzin., (1990) 1423. R. Ettl, 1. Chao, F. Diederich, and R. L. Whetten, Nature, 353 (1991) 149. F. Diederich, R. Ettl, Y.Rubm, R. L. Whetten, R.Beck, M. Alvarez, S. h,D. Sensharma, F. Wudl, K. C. Khemani, and A. Koch. Science, 252 (1991) 548. R.Taylor, unpublished work. F. Diederich, R. L. Whetten, C. Thilgen, R. Ettl, I. Chao, and M. M. Alvarez, Science, 254 (1991) 1768. M.F. Meidine, P. B. Hitchcock, H. W. Kroto, R. Taylor, and D. R. M. Walton, J. Chem.Soc.,Chem. Commun.,(1992) 1534. N. Manalova, H. Pilliere, S. Delpeux, I. Rashkov, and F.Beguin, Fullerene Science

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and Technology, 2 (1994) 195. 8. A. D.D h s h , H. W. Kroto, R. Taylor, and D. R. M. Walton, J Chem. Soc., Chem. Commun.,(1 994) 15. 9. K.Kikuchi, N.Nakahara, T. Wakabayashi, S. Suzuki,, H. Shirornaru, Y.Miyake, K. Saito, I. Ikemoto, M. Kainosho, and Y. Achiba, Nature, 357 (1992) 142. 10. R. Taylor, G. J. Langley, T.J. S. Dennis, H. W. Kroto, and D. R. M. Walton, J. Chem. Soc.,Chem. Commun.,(1992) 1043;R.Taylor, G.J. Langley, A. 0.Avmt, T. J. S. Dennis, H. W. Kroto, and D. R.M. Walton, J. Chem. Soc.,Perkin Z h s . 2, (1993) 1029. 11. T. Wakabayashi, K. Kikuchi, S. Suzuki, H. Shiromaru, and Y. Achiba, J. P h s . Chem.,98(1994)3090. 12.D.E. Manolopoulos, P. W. Fowler, R. Taylor, H. W. Kroto, and D. R.M . Walton, J. Chem. Soc.,Farad- Trans.,88 (1992)3117. 13.J. P. Hare, T. J. Dennis, H. W. Kroto, R. Taylor, A. W. Allaf, S. Balm, andD. R. M. Walton, J ; Chem. Soc.,Chem. Commun.,(1991)412. 14.W.Krtltschmer, L. D. Lamb, K.Fostiropoulos, and D. R.HufFman, Nature, 347 (1991)354. 15.D. Bakowies and W. Thiel, Chem. Phys., 151 (1991)309. 16.T. J. Dennis, J. P. Hare, H. W. Kroto, R.Taylor, D. R.M. Walton, and P. J. Hendra, Spectrochim. Acfa,47A (1991) 1289. 17.D. S.Bethune, G. Meijer, W. C. Tang, andH. J. Rosen, Chem. Phys. Lett., 174 (1990)219;Y.Wang, J. M. Holden, X.Bi, and P. C. EMund, Chem. Phys. Lett.,217 (1994)413. 18.J. P. Hare, H. W. Kroto, and R. Taylor, Chem. Phys. Lett., 177 (1991)394. 19.H.Ajie, M.M. Alvarez, S. J. Anz, R. D. Beck, F. Diederich, K. Fostiropoulos, D. R. Huffman, W. Krtltschmer, Y. Rubin, K. E. Schriver, D. Sensharma, and R. L. Whetten, J. Phys. Chem.,94 (1990)8630. 20.A.Dworkin, H. Szwarc, S. Leach, J. P. Hare, T.J. Dennis, H. W. Kroto, R. Taylor, and D. R. M. Wdton, Compt. Rend. A d Sci., Series 11, 312 (1991) 979; A Dworkin, C. Fabre, D. Schutz, G. Kriza, R. Ceolin, H. Szwarc, P. Bernier, D. Jerome, S.Leach, A. Rassat, J. P.Hare, T.J. Dennis, H. W. Kroto, R. Taylor, and D. R.M. Walton, Compt. Rend. Acua! Sci., SerksU, 313 (1991) 1017. 21.H.W.Kroto, K.Prassides, R. Taylor, and D. R. M. Walton, Physicu Scrpa, T45 (1992)314. 22.R. S.Ruoff, D. S. Tse, R. Malhotra, and D. C . Lorents, J Phys. Chem., 97 (1993)

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3379. 23. A. L. Smith. personal communication. 24. V. N. Bezmelnitsin, A. V. Eletskii, and E. G. Stepanov, J. Phys. Chem., 98 (1994)

6665. 25. N. Sivaraman, R. Dhamodaran, I. Kaliappan, T. G. Srinivasan, P. R.Vasudeva Rao,

and C. K.Matthews,J. Org. Chem., 57 (1992) 6077. 26. N. Swaman, R.Dhamodaran, I. Kaliappan, T. G. Srinivasan, P. R. Vasudeva Rao, and C . K . Matthews, Fullerene Science and Technology, 2 (1994) 233. 27. P. R Bukett, A. D. Darwish, H. W. Kroto, G. J. Langley, R. Taylor, and D. R.M. Walton, J. Chem. Soc.,Perkin Trans. 2, in press. 28. C. J. H.Crowley and R Taylor, unpublished work, C. S. Sundar, A. Bharathi, Y. Hariharan, J. Janaki, V. S.Sastry, and T. S . Radhakrishnan, Solid State Comm., 84 (1992) 823. 29. I. -our,

J. P.Hare, H. W. Kroto, and R. Taylor, Lunar Plunet. Sci. Con$ XUZ,

(1991) 445; M.Gevaert and P. M. Kamat, J. Chem. Soc.,Chem. Commun., (1992) 1470; H.S. Chen, A. R.Kortan, R.C. Haddon, and D. A. Fleming, J. Phys. Chem.,

96 (1992) 1016. 30. T.

Ark, Y.Murakami, H. Suematsu, K. Kikuchi, Y.Achiba and I. Ikemoto, Solid

Stute Comm., 84 (1992) 827. 31. A. Datta, R.Y.Kelkar, P. Borojerdian, S. K. Kulkami, and M. Datta, Bull. Chem.

Soc.Jpn., 67 ((1994) 1517. 32. R.Taylor, J. P. Parsons, A. G. Avent, S.P. Rannard, T. J. Dennis, J. P. Hare, H. W. Kroto, and D. R. M. Walton, Nature, 351 (1991) 277. 33. J. M. Wood, B. Kahr, S.H. Hoke, H. L. Dejatme, R.G. Cooks, and D. Ben Amotz,

J. Am. Chem. Soc., 113 (1991)5907; K. M. Creegan, J. L. Robbins, W. K. Robbins, J. M. Millar, R.D. Sherwood, P. J. Tindall, D. M. Cox, A. B. Smith, J. P. McCauley, D. R Jones, and R.T. Gallagher, J. Am. Chem. Soc.,114 (1992) 1103. 34. M.Gevaert and P. N.. Kamat, J. Chem. Soc., Chem. Commun., (1992) 1470. 35. S. Kawata, K. Yamauchi, S. Suzuki, K. Kikuchi, H. Shiromaru, M. Katad, K. Saito, I. Ikemoto, and Y.Achiba, Chem. Letf.,(1992) 1659. 36. R.Malhotra, S. Kumar, and A. Satyam, J. Chem. Soc.,Chem. Commun., (1994) 1339. 37. D. Heyman and L. P. F. Chibante, Red. Trav. Chim. Pays-Bas, 112 (1993) 531,639. 38. D. Dubois, K. M. Kadish, S. Flanagan, and L. J. Wilson, J. Am. Chem. Soc., 113

(1991) 7773; Q. Xie, F. Arias, and L. Echegoyen, J. Am. Chem.

Soc.,115 (1993)

52

9818. 39. J. W. Arbogast, A. P. Dmanyan, C. S. Foote,

Y. Rubin, F. Diedench, M.M.

Alvarez, S. J. Am,and R. L. Whetten, J. Phys. Chem., 94 (1991) 11; J. W. A h g u t and C. S. Foote, J. Am. Chem. Soc., 113 (1991) 8886; R. R. Hung and J. J. Grabowski, J. Phys. Chem.,95 (1991) 6073; Chem. Phys. Left., 192 (1992) 249. 40. H. Tokuyama, S. Yamago, and E. Nakamura, J. Am. Chem. Sue., 115 (1993) 7918; A. S. Boutorhe, H. Tokuyama,M. Takasugi, H. Isobe, E. Nakamura, and C . Hblhe, Angew. Chem. Intl. Edn. Engl., 33 (1994) 2462. 41. K. C. Hwang and D. Mauzerall, Nurure, 361 (1993) 138. 42. W. A. Scnvens. J. M. Tour, K. E. Creek, and L. Pirisi, J. Am. Chem. SOC.,116 (1994) 4517. 43. H.Tokuyama and E. Nakamura, J. Org. Chem., 59 (1994) 1135. 44. M.Orfanopoulos and S Kambourakis, TetrahedronLett.,35 (1994) 1945.

S. Flanagan, R. E. Haufler, L. P. F. Chibante, and L. J. Wilson, J. Am. Chem. Sue., 113 (1991) 4364; Q. Xie, E. Perez-Cordero, and L.

45. D. Dubois, K.M. Kadish,

Echegoyen, ibid., 114 (1992) 3978; D. Dubois, G. Moninot, W. Kutnet, M.T. Jones, and K.M. Kadish, J. Phys. Chem., 96 (1992) 7137. 46. A. D. Darwish, H. W. Kroto, R. Taylor, and D. R. M. Walton, Fulferene Science and Technology, 1 (1993) 571, 47. M.F. Meidine, P. B. Hitchcock, H. W. Kroto, R. Taylor, and D. R. M. Walton, J. Chem. Soc.,Chem. Commun., (1992) 1534; A. L.Balch, J. W. Noll, and M.M. Olmstead, ibid., (1993) 56. 48. J. D. Crane, P. W. Hitchcock, H. W.Kroto, R. Taylor, and D. R. M. Walton, J. Chem. Soc.,Chem. Commun., (1992) 1534. 49. G. Roth, P. Adelamnn J. Phys. i France, 2 (1993) 1541, 50. P. R. Birkett, C. Christides, P. B. Hitchcock, H. W. Kroto, K.Prassides, R.Taylor, and D. R. M. Walton, J. Chem. Soc.,Perkin Trans. 2 (1993) 1407.

Chapter 4 Hydrogenated Fullerenes Paul A. Cahill Chemistry of Organic Materials Department Sandia National Laboratories Albuquerque, NM 87185-0368 USA

1. Introduction Pulletme hydrogenation yields fundamental information about the kinetics and themdynamics of addition to these soluble, strained, aromatic molecules. Steric effects in hydmfidlerenes are minimal, and their IH nuclei are excellent NMR probes for rapid structutaf analyses of product mixtures. In addition, aspects of the chemistry of fuIletene derivative8 that would only be available through extensive experimentation, if at all, have been revealed through ab initio Hartree-Fock calculations. This chapter is a critical review of both experimental and theoretical reports 'on hydrogenated fullerenes and has the goal of revealing the nature of chemoselective hydrogen addition. Many reagents have been reported to reduce fullerenes to yield mixtures of products with stoichiometries up to at least h H 3 6 . Separations of mixtures of C a b are often tedious even on specially designed high resolution HPLC columns. These derivatives are typically stable toward at least brief exposure to air. Several significant gaps in our knowledge exist, e.g., structure and energy data for C,=j0Hzn and C70Ha isomers (n 2 3). and heats of hydrogenation for hydrofullerenes generally. and c7d2 These simple hydrogenated fullerenes have been the most intensely studied fullerene derivatives as they lend themselves to both experimental kinetic and thermodynamic192 studies as well as to quantum chemical calculations.2-8 Hydrogen has been observed to add 1,2 to the shorter, or double, bonds that fuse 6,6 rings and to add 1,4 to cyclohexatriene rings. 2. C&2

2.1 nisoretical Investigations

Both semiempirical and ab initio calculations indicate that, of the 23 possible (exo) isomers, only 1,2-%H2 would be present at equilibrium at room temperature

54

Figure 1. The structure of 12-CaH2. Shaded bonds lead to the 8 unique C d 4 isomers that may result from a second H2 to a 6 6 ring fusion. (Table 1). [Many nomenclature systems for addended fullerenes have been proposed. The term hydrofullerene is preferred over the commonly used term “fullerene hydride”. The generally accepted numbering is used throughout.f2 See Figures 1 and 2.1 No qualitative difference between the semiempirical and ab initio results exists for CmH2, but the isomer energy differences are striking. Semiempirical calculations require relatively little computing power, but are unreliable for predictions of isomer energy ordering (see below). Therefore, more expensive fully geometry optimized ab initio H a w - F o c k (HF) calculations and 3-21G and 6-31G* basis sets (20-200 Cray Y CPU hourdgeometry optimization) have been used to study the most interesting isomers. In order of increasing energy, the favored addition patterns can be described as: (1) 1.2-addition to a 6,6-ring fusion (also known as an interpentagonal bond) (2) 1,Caddition to a 6-ring (1,4-CmH2). and (3) 2.6-addition to a “naphthalene subunit” (1,16 -C&H2).

55

a

b C

d

e

d' (4)

b' (di) a' (I/)

Figure 2. Addition to the 1,9-, 7,B- and 21.42 carbons of C70 (shaded) lead to the three lowest energy isomers of C7oH2.

Three conclusions can be drawn from theoretical results on the basis of excellent agnemMlt between HF/6-31G* calculations and experiment (Table 1): (1) the difference between the single and double bond character of the 5,6 and 6,6 ring fusions is 24 kcaYmol (HF/6-31G*), that is, all ring fusions are intermediate between single and double bonds, (2) the lowest energy patterns of addition to Cm are those local to one region of the s p k e and follow from addition to double bonds located at the 6.6 ring fusions, and relative isomer energies do not appear to correlate with the number of double bonds localiusd by addition to 5.6 fusions, and (3) semiempirical calculations and ab initio calculations are qualitatively, but not quantitatively, consistent. That is, where large isomer energy differences are observed (e.g. > 8 kcal/mol), semiempiricalcalculations reveal trends only.

56 Table 1. Relative energies of dihydrofullerenes at various levels of theory (kcaVmo1).

QH2

;::

1'16 1'6 c7d2

~9 7'8 2 1.42 1.7 69 1.6

21,22

MNDO-PM33 MNDO-MI7

30:;

15.5 18.4

0.0 -1.1

0.3 1.4 5.4

5.4 6.0

0.0 -1.1 0.6 2.2

HF-3/21G3

HF/6-31G*3

23.1 26.4

20.9 24.0

0.0 0.2

0.0

::: 2.1

5.8

Expt.2

:;

1.3 4.5 6.4

1.4ffl.2

5.8 8.7 5.7

The most favorable patterns of addition to C70 are more complex than those for

CaO because of its lower symmetry which results in 143 possible C70H2 isomers. AU

143

isomers have been evaluated with semiempirical methods and were found to be minima on the potential energy surface, but only the four lowest energy isomers were geometry optimized with ab initio methods. In order of increasing ab initio (where known) and then MNDO-PM3 calculated energies (Table l), the lowest energy patterns of addition to C70 are: (1) 1,2-addition to the 6.6-ring fusion at the poles, closely followed by (2) 1,2-addition to the 6.6-ring fusion at mid-latitudes ("c" layer), (3) 1,4-addition (to "d,d " not "e" layer) the 6-ring that spans the equator (4) two 1.4-additions to the 6-ring near the poles ( 5 ) .t3-addltlon ' ' to the 6-ring at the pole, and (6) 1,Zaddition to the ' at low-latitudes ("d" layer). Based on the structural similarity between Ccjo and C70, patterns (1). (2) and (4) were expected, but the three underlined findings were unanticipated. Addition to the 21,42 carbons is strongly favored and would yield products functionalimd across (not dong) the C70 equator. This 1,4 addition to what appears to be the most aromatic part of C70 was not suggested by bond order analyses and was totally unexpected. Bond order arguments are consistent with pattern (6) as low energy K6kulC structures place double bonds at this unique 5,6 ring fusion. Overail, qualitative simple arguments about the patterns of addition to C a do not carry over uniformly to C70. Unlie for CmH2, predicted isomer energy orderings are a function of the method applied, with MNDO methods preferring the 7.8-isomer and ab initio HF/6-31G* methods preferring the 19-isomer. Equilibration of 7,8- and 1,9-C7oH2 has yielded a free energy of formation difference (MG) that is in quantitative agreement only with HF/631G+ calculations (Table l)? and therefore the results of such calculations are proposed

57

as essentially quantitative estimates of the free energy of formation differences of hydmfullerenes. These results indicate that relative semiempirical heats of formation are accurate to only 4-8 kcal/mol, and that such energetics may only be used with great care. These limitations ate independent of the parameterization ( M I , PM3). Calculated bond lengths for Cm and C70 dong with the relevant sp3-sp3 bond lengths in and C7oH2 are listed in Table 2. The most reactive 6.6 ring fusions (1.2 bonds for & and 1,9 and 7.8 bonds for C70) are calculated and observed to be much shorter than any of the other bonds, but as shown for C70H2. the shortest bond, i.e.. the 7,8-bond with the greatest double bond character, is not the lowest free energy C7oH2 isomer. Overall good agreement between calculated and experimentalvalues is observed for the parent fullerenes. but crystallographic data is not yet available for the hydrofullerenes. Table 2. Calculated and measured bond lengths (A).

c60

1.2 19

MNDO-PM3315 HF13-21G3 HFl6-31G*3 EXDL 1.362 1.367 1.373 1.4019 1.457 1.453 1.449 1.458

C70

1,2(=1,5) 1.9 6.7 7.8 7.22 21.22 21.40 23.24

1.457 1.386 1.453 1.374 1.463 1.426 1.412 1.463

1.452 1.370 1.447 1.356 1.458 1.414 1.403 1.475

1.446 1.375 1.442 1.363 1.453 1.413 1.406 1.472

C&2

SP~-SP~

1.562

1.586

1.582

C 7 d 2 1 9 s p 3 - s ~1.565 ~

1.588 1.567

1.583 1.565

7.8 sp3-sp3 1.546

1.4610;1.46411 1.38; 1.37 1.45; 1.47 1.37; 1.37 1.43; 1.46 1.W. 1.47 1.42; 1.39 1.46; 1.41

Lastly, the calculations suggest that chemistry unique to C70 remains undiscovered. For example, products that would result from 1,4-addition to carbons 21 and 42 (spanning the C70 equatorial region) are predicted to be particularly stable, 3.1 kcaYmol more favorable than a 1.4-addition to C a . Furthermore, the isomer that would result from 1,Zaddition to the 5.6 ring fusion (a very unfavorably pattern for Go) at carbons 21 and 22 in C70 is not unfavorable. And finally, although the poles (&o and C70 at first appear similar. the low energy 13-addition pattern to carbons 1 and 6 of C70 is

58 (even at the semiempirical level) m l v 50 kcaVmol more stable than the corresponding addition pattern to C a . 2.2 Experimental Investigations

Hydrogenation via (1) hydroboration.l,2,14115 (2) hydrozirconation)3*16 (3) zincacid reduction,17 (4) high pressure hydrogenation.18 ( 5 ) heterogeneous14 and homogeneous19 catalytic hydrogenation, (6) diimide reduction.1420 (7)hydrogen sulfide reduction,*4 (8) transfer hydrogenation.21 (9) chromium acetate.14 (10) dissolving metal (Birch) red~ction,22-~4 and (1 1) free radicals25 all lead to mixtures of hydrogeaated products. Of these methods, the highest conversions and yields of C&I2 and C7& are obtained with diimide (diazine), HN=NH.141m Detailed experimentai work on and C70 diimide hydrogenation was published in early 1994.14 1,2-CaH2 is the major product observed from the hydroboration or diimide reduction of Ca. The key to its structure assignment was the synthesis of CmHD by deuterolysis of the proposed Qo(H)(BH2) intermediate. The 1H N M R s p m m of Q H D is a triplet due to 35 coupling constant of 2.4 Hz which is consistent only with vicinal addition. That addition occurred to a 6.6 ring fusion and not a 5.6 ring fusion was fust proposed on the basis of calculations, and has been further supported by subsequent results on G H 4 and other compounds. The 1H NMR chemical shift of 1,2-C&2 and other hydrogenated fullerenes are extremely solvent sensitive (Table 3). A second w 2 product, proposed as the 1,4-isomer, is also formed in both borane and d h i d e reductions. This product's protons resonate at a lower field (6 6.13 vs. 5.89 in C&)14 and the structure assignment has been made on the basis that this is the next higher energy isomer. No other data is available on this compound because it either isomerizes or decomposes on a Buckyclutcher I column.26 The pKa of 1,2-C6oH2 has been measured by electrochemical methods to be 4.8.27 1.2-CaH2 can be isolated as a brown solid, but it does not completely redissolve if exposed to air and light for more than a few minutes. In toluene solution at room temperature and in the absence of Cm, the brown compound is stable indefinitely. In the presence of C6o and light and air. C6oH2(0), apparently an epoxide, is formed.' Furthermore, in the presence of trace active metals -- even the zinc-containing solder in an HPLC injector is sufficient -- reversion (by an unknown mechanism) to Q occurs. The UV-vis spectrum of this compound tails monotonically from high energy with the exception of a small peak at 430 nm. This absorption has been proposed to be characteristic of 1.2-addended C a derivatives. Both 1.9 and 7,8-C70H2 are formed upon reduction of C70 by diimide 01 hydroboration. C70 is much less reactive toward hydroboration than is C6o: at O' C, conditions that lead to smooth conversion of c60 to G o H 2 , reduction of C70 is not observed. At room temperature, C70 is smoothly hydroborated, but under these conditions, Q yields insoluble products as well as small amounts of 1,2-CaH2. The 1H NMR chemical shifts (Table 3) for the C70 derivatives are near 6 4.0 ppm in toluene, 2 ppm upfield from the 6 5.93 of CmH2 in the same solvent. The large difference in

chemical shifts suggests a large pKa difference (not measured) and is consistent with the calculations that suggest a substantial difference in chemistry between Cm and C70Hydroboration yields a 1.9 :7,8 product ratio of 21, wheFeas diimide reduction yields an approximate 8 1 ratio, neither of which represent the thermodynamic ratio of approximately 11:1. This was an early indication that additions to fullerenes are under kinetic, not thermodynamic control. Further evidence for kinetic over thermodynamic control has been observed for h H 4 and c6o(CRR')2.3

3. c *

is a model for the chemoselectivity of multiple addition to C&o. The same experimental and theoretical techniques that have been applied successfully to Ctj13I32and C7oH2 have recently been applied to C&4. and as a result, the lowest energy patterns of tetra-addition to Qo a~ now understood.15 These calculations have been extended to Gj13(C!H2)2mwith similar results, and may also be applicable to derivatives of C70 and higher fulferenes.

Figure 3. Alternate view of C ~ for Q analysis of CmH46 3.I Theoretical Investigations

Ab initio methods (vide supra) clearly indicate the strong preference (7.6 kcdrnol) of 1.2- (vicinal) over 1,saddition in c60 in the absence of steric effects.Therefore. it is reasonable to examine products resulting from 1,2 addition to two 6.6 ring fusions as was first done by Matsuzawa, et al.? with semiempirical methods. The eight possible products are shown in Figure 3 and their relative energies at several levels of theory are listed in Table 4. Experimentally. only C d 4 - l (1,2,3,4-C~Ha; Figures 1 and 3) is present at equilibrium. This is in qualitative agreement with ab initio calculations, but indicates that the semiempirical results a x again misleading.

Tabfe 3.1H NMR data for assignable isomers of C6oH2, C7oH2, C&4, Compoundd 1.2-CaH2

1,4-C@H2 1.9C7OH2

7.8-C70H2

1,2.3,4-%& c6oH4 - 1

1,2,18,36-%H4'

c6oH4-4 1,2.55.6O-~H4 CaH4-8 1,5,6,9-C70H4

1,7.8,9-C70H4

and C7&.

6 (ppm)a

"J (Hz)

5.93 (s), 6.0313a (C7D8) 5.85 (s),13b 5.88.5.89 (c6D6). 6.97 (s) (CS2) 6-13(s) (c6D6) 7.23 (s) (CS2) 4.00 (ABquartet) (C7D8) (VA - VB) = 204 HZ 3.97 ( v 6 ) 5.08 (CSz) 3-91 (s) (C7D8) 3.97 ( W 6 ) 4.96 (CS2) 5.03 (AA'BB') (C7D8)l5 A8 = 300 Hz 5.369.4.562 (c&j) 6.323.5.726 (CS 1 5.36 (2H. s) (C7D8) 5.05 (1H. AB quartet) (VA - VB) = 120 HZ 5.34 6)(c@6) 6.27 (s) (CS2) 6.33 (s) (C7D@ 4.475,4.127 (AA'BB'), (CS2)

35 HD = 2.41

5.414.4.618 4.417.2.283 (ABCD). (CS2)

~ J H =H 15.714 33 HC = 141.214

3

5 = 16.1 ~ ~

33 HD = 2.3 f 0.2

35 AB = 14.1 f 0.515.14.114

35 BB'= 9.8 f 0.515,9.814 43AB' = -0.2 f o.215, -o.214 5J AA' = 1.7 f 0.515, 1.614 6J~~ 0.5 min. Apparently, the high electron-ionization

70 threshold (> 7.61 v24of neutral C a molecules greatly hindered the attempt to generate a higher oxidation state of C60. The observed high electron aftinity of C6o compared to its electron-donating ability leads to the prediction that this class of materials is more susceptible to nucleophilic additionsz5 than to electrophilic additions (Figure 2). Electron Oxidation

/

1 h

Electron

Reduction

EQ

Electrophilic Additions

Nucleophilic Additions

0 bffu

Nucleophilic Replacement

U

Nu

Nu

Nu

M p r e 2. The synthetic strategy for GOfunctionatizationby introducing polar functiom

In fact, certain nudeophiles such as organoamines?6 and organolithium or Grignard reagent^?^^^^ were found to be effective for nucleophilic additions on fullerenes. The lack of detection of oxidative fullerene intermediates was presumabty due to the low chemical stability of the resulting radical-cation. Stability of C,jo radical-oation can be enhanced significantly in strongly acidic media. A respectable intensity of spins, attributed to the C6o radical-cation, was detected in EPR spectroscopic studies of c60 molecules dissolved in fuming sulfuric acid (H2S04-S03)28-30 and magic acid FS03HS ~ F S )Despite . ~ ~ the resistance of C6o to electronic oxidation in organic solvents, it was found to be susceptible to the attack of certain electrophilic reagents (though radicals m o t be &counted in many cases). Early examples of apparent electrophilic attack were given by the halogenation of c60 yielding highly fluorinated ,3 brominated,33~34

71

and chlorinated35J6 derivatives, and the addition of benzene and toluene in the presence of aluminium chloride.3' The high reactivity of hllerenes towards attack of other electrophiles in the presence of nucleophilic reagents, such as water or carboxylic acids under mild reaction conditions, has been demonstrated re~ently.38~1 The observed nucleophilicity of fullerenes in terms of their vulnerability to electrophilic additions compensates for difficulties that have been encountered in achieving hrther hnctionalization of C a molwuh via electronic oxidation. This enables the use of olefinic moieties of C6o as the reactive electron-rich sites for reactions. This chemistry enriches the scope of synthetic methodoloey for fbllerene derivatization, and provides practical routes for the preparation of c60 derivatives with multiple polar functional groups such as hydroxyl and wboxylate groups, leading to an enormous variety of novel fullerene derivatives. '"his type of electrophilic approach involves the addition of strong electrophiles acros~the oletinic moieties of fillerene molecules, forming cationic intermediates as shown in Figure 2. These intermediates are decomposed in situ by weak nucleophiles to afford Werene derivatives containing multiple hnctional groups. Utilization of this synthetic skategy has been exemplified in the reaction of C6o with either a mixture of sulfuric kid and nitric acid in aqueous solution,38~39or nitronium tetrdu0roborate4~in ordcr to synthesize water-soluble polyhydroxylated hllerene derivatives (fullerenols, 1). Both the acid- and the nitronium chemistry transform the instability of the resulting cationic intermediates into highly reactive intermediates suitable for derivatization. In the former case, these intermediates can be converted, in the presence of water, directly into polyhydroxylated fullerene derivatives. This type of reaction is best carried out at 85-95 "C in a solution of sulhric acid, potassium nitrate and water without any hydrocarbon-containing reagents. Potassium nitrate was utilized as the precursor for generating Ntric acid in situ. The resulting polyhydroxylated hllerene derivatives are highly soluble in an aqueous acidic medium, but precipitate fiom basic media when the pH ofthe solution higher than ca. 9.0 This unusual solubility property of hllerenols facilitated isolation and purification of the nearly salt-free products. The lllerenols were charaeteriaed as containing an average of 14-15 hydroxyl groups per molecule. In the latter case, nitronium tetrafluoroborate was used as an electrophilic reagent to activate olefinic bonds of the lllerene prior to the in situ nucleophilic addition of mild nucleophiles, such as arene- or alkylcarboxylic acids, in a non-aqueous medium. The

72

hydrolysis of the ester moieties of polyhydroxyorganocarboxylated lllerene derivatives provides an efficient route for the preparation of water-soluble lllerenols. These fillerene derivatives were characterized as possessing an average of 13-15 hydroxyl groups and 4-5 pbromobenzoate groups per C6o molecule. Hydrolysis of these derivatives in aqueous alkaline solution affords the corresponding water-soluble lllerenols having an average of 18-20 hydroxyl groups per c60 molecule. The newly developed method for the preparation of polyhydroxylated fuUerene derivatives involved a hydrolytic reaction of polycyclosulfated fillerene derivativeswhich were synthesized through lIlerene cyclosulfation ~hemistry.4~ Cyclosulfation of c 6 0 was performed in neat l m h g sulfuric acid at 55-60 OC under N2.Hydrolysis o f these derivatives by water at 85-90 "C or by aqueous NaOH at ambient temperature gave the corresponding polyhydroxytated fuUerenes (fullerenols, 3), devoid of the hemiketal structure 2 that is found in llterenols 1. Fullerenols 3 were found to contain an average of 10-12 hydroxyl addends, and this can be correlated to a polycyclosulfated precursor, containing 5-6 cyclosulfate units. Fullerenols 3 exhibit enhanced stability in acidic media at ambient temperature, and remarkable solubility in organic solvents such as dimethylformamide, dimethyl sulfoxide, and tetrahydrofiran.

-

2 (The representativehemiketal structureof fitllerenols 1)

The structure of fillerenol 1 prepared either from the aqueous acid chemistry or from the nhronium chemistry was characterized and found to consist of several hemiketal carbons (Z), each linked with enol-ether and incorporated as a part of the expanded lllerene cage, in addition to the tertiary hydroxyl gr0ups.~3These particular fiuictional moieties can be viewed as hydroxylated fullerene oxides.& Evidence for the hemiketal structure was given by the observation of chemical shifts of vinyl ether carbons and ketal carbons centered at S 170.3 and 100.0, respectively, in the 13C NMR spectrum of fbllerenols 1. Conclusive verification of the presence of hemiketal knctions in fullerenols

73

corresponding ketone upon variation of pH in aqueous solution, and this is contrary to the case of fililerenoi 3. 1 was provided by the reversible interconversionof these moieties to the

3. Synt&tuisof Fulleren& by the Aqueous Acid Chemistry Route The aqueous acid chemistry route was found to be an etficient method for introdwing multiple hydroxy groups onto fullerenes.38,39 The chemistry produces fullerenole in a neatly quantitative yield. The synthetic study was n o d y carried out using either pure C m or a mixture of C a (84%) and C70 (16%) in an aqueous medium in the p-~.e of sulfuric acid and nitric acid at different temperatures. Without nitric acid, only a low yield of Illermol-telated product was obtained. The reaction was found to proceed most readily at temperatures between 85 and 115 "C using potassium nitrate as the prscursor for nitric acid. It was proposed that the reaction involved an situ generation of nirronium ion NO^+) as the eiectrophilic reagent initiating oiefimc addition aa shown in Scheme 1. The elevated temperature is therefore necessary to etFkct the uaUa dehydrative conversion of nitric acid to nitronium ion. Completion of r d o n was easily determined when the reaction mixture became a clear yellowish brown solution, indicating high solubility of polyhydroxylated hllerenols 1 in the aqueous acid medium.After the reaction was completed, the acid solution was diluted with water and

(Fullerenols, t) I

& b e I, Electrophitic addition of nitronium ions to oleiinic moietiesof C a followed by nucleophiiic nplaoement by H20

74

filtered through Celite to remove any unreacted fullerene particles. Neutralization with NaOH to precipitate products followed. Full precipitation was achieved at pH > 9.0. The resulting fine amorphous brown solid was moderately soluble in water and very soluble in aqueous acids at pH < 5.0 . In general, the thorough removal of water from partidy hydrated products in vacuum at 50 OC affords a brittle brown solid with a slightly lower solubilii in water. However, its solubility in acidic water remains unchanged. The product yield of this hydroxylation reaction was estimated to be greattr than 95%, with only a trace amount of starting C6o recovered. This is attributable to the partially hydroxyl-added lllerene intermediate having a higher solubility in acidic water than the unreacted hllerene itself Thus, the reaction tends to continue on the intermediate until completion. The elemental analysis of samples from several batches showed a high oxygen to carbon ratio of 0.4 to 0.5 with only a trace quantity of nitrogen (< 0.6% by weight). Because of the high compatibility of polhydroxylated lllerene derivatives 1 with sodium sulfate in aqueous solution, the product often contained one to several sulfate molecules per fillerene derivative, in addition to water fiom the workup procedure. Further purification by redissolving them in dilute hydrochloric acid (0. lN), followed by reprecipitation after neutralization with NaOH, teduced the sodium sulfate level to a minumum level of less than 0.1 molecule per fullerenol. Complete removal of sodium sulfate can be achieved by repeating the above procedure or by washing the product with dilute NaOH solution. The IR spectrum of hllerenols clearly indicates the disappearance of the characteristic C a peaks at 1428, 1183, 578, and 528 cm-1. A very strong hydroxyl absorption band at 3424 cm-1 and three absorption bands centered at 1595, 1392, and 1084 cm-l are evident as shown in Figure 3a. The broad absorption band is due presumably to the heterogenous nature of the product. Several conventional soft ionization techniques have been used for the mass spectrometric study of fidlerenols. No molecular ions of products were captured. For example, when the desorption electroncapture ionization technique was used, only C6o and C70 anions were observed in the spectrum. The fullerenol sample was dissolved in H20 and filtered through Celite prior to measurement. Since the fbllerenes are themselves not water soluble, the presence of C6o and C70 anions in the negative ion spectrum is apparently a result of thermal decomposition of fullerenols occurring during the desorption process. The spectrum is also indicative of the products being C6o derivatives.

75

I

I

I

I

I

I

I

1

wo a n - 1 Fvrpvc 3. The IR spectra of (a) fullaenols, (b) 4, the products of reaction of Nlerenols with 5bromovaleryl chloride, and (c) the products of reaction of Nlerenols with 4 - b r o ~ acid ~ c 4 a a o ~ 3 w o ~ 2 0 0 0 lsoo

loo0

The x-ray photoelectron spectroscopic ( X P S ) measurement on a hllerene film cast fiom an aqueous solution of polyhydroxylated fullerene derivatives 1, gave mainly Cis (58%) and Ols (35%) peaks with other binding energies for Nals (3.6%), CazP (1.7%), SiZp (0.8%), and AIzp (1 .O%). There were no detectable peaks for Nls or SzP binding energy in the specttum, indicating no incorporation of nitro (from nitric acid) or sulfonyl (fiom sulfuric acid) hnctional groups in the hllerenol structure. The spectrum also suggests contamination with small amounts of sodium hydroxide, metal oxides, and water, resulting from the work-up procedure. Since there were no carbon-containing chemical reagents or solvents used in the reaction, the carbon peak in the spectrum should correspond solely to hllerene derivatives. Therefore the analysis of the carbon peak provides an important insight into the structural nature of carbon atoms in fbllerenol 1. Figure 4 shows the overlay of C 1 binding energy of hllerenals (curve a and C a ( m e b), which peaks at 285.9 eV with a full width at half maximum (FHWM width) of 1.49eV, assuming the bindmg energy of trace silicone impurities in these two samples is identical. The slight shift of peak position to the higher energy from the normal carbon (1 s) binding energy at about 285 eV for hydrocarbons, reveals that these samples were homogeneous. Interestingly a clear deviation of the C1 peak of fullerenol from that of

76 c60 was observed. The large shift indicates that in hllerenols, a higher percentage of

cazrbons are in a high oxidation state compared to the parent fillerene. Simplecurvefitting analysis of the hilerenol carbon peak (curbve c of Fig. 4) indicates the presance of more than two different carbon oxidation states, assuming that the first peak matches with the lower binding energy region of the experimental curve. 10

9 8

ul

7

..6 - 5

$ 4

3 2 1 298 296 294 292 290 288 286 204 282 280

Binding energy f eV Figure 4. The x-ray photoelectron (XPS) spectrum of the Ctsbinding energy of hllerenols ( m u ) , Cm fuilerene (curve b), and the curve-fitting anatysis for non-oxygenatedhrllerenes in 6 (curve c) in fuilerenols showing higher oxidation statesof &XIS

With fillerenol solids of a heterogeneouscomposition, a slightly higher FWHM width of about 1.85 ev for the peak was expected. Based upon this procedure of analysis, the absorption with a binding energy at 286.1 eV (68%) was assigned to non-oxygenated carbons, and the remaining absorptions (32%) to carbons with a higher oxidation state. These higher oxidation state carbons spread over a range of about 4 eV in the spectrum, which is too broad to account alone for mono-oxygenated carbons such as ethereal or hydroxylated ones. It indicates the existence in the hllerenol structure of a certain quantity of dioxygenated carbons such as carbonyl (C=O), ketal (RO-C-OR) or hemiketal (RO-C-OH). Due to the lack of carbonyl absorption bands in the IR spectruum of fillerenols (Fig. 3a) and the low probability of ketal formation on C ~ Q we attribute the carbon atoms with the highest oxidation state in fidlerenols 1 to hemiketal hnctions. One conventional chemical method for substantiating the hydroxyl fhction in fillerenols is to esterifj with alkanoyl chlorides, and verifjl the carbonyl absorption band in the IR spectrum of the resulting products. In this case, the reaction was performed at

77

85 "C either in dimethylfomamidesolution using 5-bromovaleryl chloride as reagent, or in THP solution with 4-bromobutyric acid in the presence of carbodiimide to afford partidy esterified lllerenols 4 and 5, respectively. Both compounds 4 and 5 show a significsnt solubility change fkom water to organic solvents, and a clear carbonyl absorption at 1720-1740 cm-l in the IR spectrum of the solids.45 Both elemental analysis and x-ray photoelectron spectroscopic measurement of compound 4 gave a carbon:bromine ratio of 29, indicating an average of 2.5 5-bromovaleryl ester substitutions per c60 molecule. The hydroxyl content of ester 4, characterized by 1H NMR (acetone-d6), showed three groups of carbon proton peaks centered at 1.9, 2.9, and 3.7 ppm, corresponding to protons on carbons C3 and C4, C2 (acarbon), and Cg of the S-bromovaleryl ester group, respectively. The hydroxyl peak centered at 3.3 ppm was verified by removal through deuterium exchange with D20.Even though three different types of protons between 2.5 and 4.0 ppm displayed a certain degree of overlap, a rougbly 1.2 to 1 intensity ratio of hydroxyl protons YS. combined protons at C2 and Cg carbons was estimated from the proton integration. That corresponds to a ratio of the number of hydroxyl groups vs. 5-bromovaleryl ester groups, of 4.8. In combination with data obtained from the elemental. analysis, an average chemical structure for 4 was calculated consistent with the addition of 12 hydroxyl groups and 2.5 bromovaleryl ester goups per C6o molecule. This process of compositional analysis indicated the fUllerenols to posse%san average of 14-15 hydroxyl addends.

600 940 1280 1620 1960 2300 2640 2980 3320 3660 4000

ml

I

Fignre5. The mass spectmm of 4, obtained using the laserdemqxiontechnique and a 4-hydroxyucyanocinnamic acid matrix

Partially esterified fUUerenols 4 were also investigated by matrix-assisted laser desorption mass spectrometry using 4-hydroxy-u-cyanocinamic acid as a matrix. The

78

distinct peaks at m/z 720 and 840 respectively represent the c60 and C70 frasment ions arising from laser ablation of 4. A broad unresolved distribution of ions ranging f?om d z 940 to 1900, centered at m/z 1425 was observed in Figure 5. The spectrum is comktent with an average molecular weight for 4 since C~~(-~CO-CH~CH~CH~CH~B~)~(O 2 requires m h = 1464. The higher end o p h e ion distribution ( d z 1840-1875) represents the upper limit of the molecular weight distribution of 4, and corresponds to 8 C6o molecule possessing 18-20 hydroxyl groups and five 5-bromovaleryl ester group4 and thus an initial total of 23-25hydroxyl addends per molecule. In summary, fillerenols 1, synthesized by the aqueous acid method, have been characterized to contain, on average, 14-15 hydroxyl addends per C6o molecult. The chemistry has demonstrated high reactivity of lllerenes towards electrophilic attaok in a reaction medium containing both an electrophilicreagent and a mild nucleophilic reagent. Hypothetically the reaction involves the electrophilic addition of nitronium ion to the olefin moieties of the Mlerene, followed by nucleophilic substitution with water to introduce the hydroxyl groups onto the fidlerene as shown in Scheme 1. 4. Synthesis of Fullercnols by Nitroaium Ion Chemistry

The proposed electrophilic addition mechanism of nitronium ions above, prompted an extended study utilizing the high electrophilicity of the nitronium ion for fillerene fbnctionaiization under non-aqueous conditions.40 In this case, &ronium tetrafluoroborate was used as a versatile reagent. Experimentally, C60, partially dissoIved in methylene chloride (distilled from CaH2 and stored over molecular Sieved) was allowed to react with an excess of nitronium tetratluoroborate (20 equiv) in the presence of either an aromatic or an aliphatic carboxylic acid (20 equiv) at ambient temperature under an atmospheric pressure of N2.Within 30 min, the brown suspension changed to a clear yellow or reddish solution (depending upon the acid used), indicating a fast addition of organocarboxylic acid to the lllerene After completion of reaction, methylene chloride was removed by evaporation, and the resulting mixtures stirred in water for 3 h to afford yellow to orange solids, which were then repeatedly washed with both water and a mixture of diethylether and hexane to remove unreacted reagents. Most of the reaction products exhibited good solubility in THF, acetone, and many organic solvents. Upon treatment of nitronium tetrafluoroborate with organocarboxylic acid, the corresponding nitronium carboxylate salt 6 was immediately formed, as shown in Scheme 2. The initial electrophilic attack of nitronium ion on fbllerenes gave,

79

presumably, intermediate C~O(NO~+)X adducts 7. The subsequent nucleophilic 1,3- or 1,5-substitutionof organocarboxylates on adduct 7, followed by treatment with water, afforded the correspondingpolyhydroxyorganocarboxylate firllerene derivatives, 8. Even though there is evidence that C6o can form an adduct with multiple nitronium ions, the mechanism of most addition and substitution reactions is stepwise. Surprisingly, both the elemental analysis and x-ray photoelectron spectroscopic (XPS) measurement of dexivatives 8 indicated consistently, a chemical composition of less than one nitrogen atom per C6o molecule. The result revealed that most nitro groups in derivatives 8 have been replaced. It is understandable that the tertiary nitro group, behaving as a good leaving group, can easily be substitutedby nucleophiles in acidic media. IR spectra of polyhydroxylorganocarboxylate hllerene derivatives8a or 8b (Fig. 6b), synthesized from benzoic acid or p-bromobenzoic acid, respectively, showed the absence of the characteristic c60 peaks, but a strong broad absorption band centered at 3430-3440 cm-l, correspondingto hydroxyl moieties of 8. A carbonyl absorption band at 1725-1730 cm-1 in 8a and 8b clearly indicates the incorporation of benzoate ester

I

!

I

I

I

4000 3500 3000 2500 2000

I

I

1So0

1000

I

1

500 Cml

Figure 6. IR spectra of (a) thty1p-bromobenzoate,(b)the products8b from the reaction of Cm with pbromobeIlzoicacid and nitronium te.~uoroborate,and (c) fidlerenofs prepand fbm the hydrolysis of 8b

groups. To correlate peaks in Figure 6b to the characteristic absorption bands of benzoate moieties of Sb, a model compound, tert-butyl p-bromobenzoate was synthesized; its IR ~pectrum4~ (shown in Fig. 6a) evidently supports the StrUcNral characterization of Sb. Addition of hydroxyl and ester functional groups onto hUerene carbons was also substantiatedby the 13C "MR spectrum of 8b in "€IF48 (Figure 7b, 6 relative to TMS). Due to the heterogeneous molecular distribution of Sb, the spectrum shows a weak, broad band centered at 6 82.0 and a broad peak centered at 6 149.5, correspondingto hydroxylated (C-OH) or carboxylated C-OCO) carbons, and unrwcted olefinic carbons of fullerenes, respectively.Thespectrum also exhibited three groups of carbon peaks centered at 6 129.5, 133.0, and 166.0, corresponding to carbons ofpbromobenzoate moieties in 8b. The chemical shifts of these benzoate carbons were compared with those in p-bromobenzoic acid (Fig. 7a). The distinct peaks at 6 128.0, 131.2, 132.2, 132.4, and 167.3 in Figure 7b are apperently derived &om the residualpbromobenzoic acid physically absorbed in 8b, and were utilized as internal standards for the spectral comparison. In the expanded spectrum of Figure 7b, the chemical shifts of ester d o n s (WO) centered 8t 6 166.0 and non-hydrogenated carbons (C1 and C4 of the benzene ring in p-bromobenzoate ester moieties at 6 128.5-13 1 are only ca.1.O ppm upfield- or downfield shifted &om those of the correspondingcarbons in p-bromobenzoic acid. Moreover, the chemid shifts of hydrogenated carbons (C2, C3, Cg, and c 6 of the benzene ring in p-bromobenzoate ester moieties) at 6 132.2 and 132 4 are identical to

81

lb

200

NO

160

tis

lio

140

Ib

la

111

t i

14a

10

z-7G

do

;o

115 ppm

w h ;D ppn

Figure 7. 1% NMR spcaxa of (a) pbromdbcnzoic acid and (b)the lllerene derivative 8b in THF-dg showing both hydmxylatedandp-bromobenzoatecarbon peaks

those of the corresponding carbons in p-bromobenzoic acid. The spectrum provides direct evidencefor additions ofp-bromobenzoate on fbllerenes. The lH NMR spectrum of 8b in THF-dg, after the complete removal of residual p-bromobenzoicacid (Fig. 8a, 6 relative to TMS) shows a broad proton peak centered at 6 3.20 and two broad peaks centered at 6 7.72 and 8.10, corresponding to hydroxyl protons and two adjacent protons in p-bromobenzoate moieties of 8b, respectively. Hydroxyl protons in the spectrum were substantiatedby deuterium exchange with D20 as shown in Figures 8b and 8c (deuteriated DMF is here a better solvent for the experiment). A downfield solvent shift (about 1 ppm) of hydroxyl proton peak position compared to that in Figure 8a was detected. Interestingly, in DMF47, the chemical shifts of two aromatic protons (doublets at 6 7.62 and 7.93) are well resolved, with less than

a2

-

IR

b.0

S.0

1J

U

L4 Ppn

Figure 8. IH "MR spectra of the fbllerene derivative 8b in (a) THF-43, (b) DMF-d7, and (c) DMFd7-DzO

0 1 ppm downfield shift From the corresponding protons (two sharp doublets at 6 7.56 and 7.83)in tert-butyl p-bromobenzoate. Upon treatment with D20, the broad peak of hydroxyl protons ( G O B centered at 6 4.25 disappeared (Figure 8c) and a new sharp peak of water protons at 6 3.90 was obtained. From the proton integration in FigUtes 8a and 8b, the ratio between the number of hydroxyl protons and the total number of aromatic p-bromobenzoate protons was determined as 0.65 and 0.78, respectively. Elemental analysis of compound 8b indicated a carbon:bromine ratio of 20, which corresponds to an average chemical composition for 8b of 4-5 p-bromobenzoate

83

addends par CfjO molecule Overall, this indicates in the structure of 8b, an average of 13-15 hydroxyl addends and 5 p-bromobenzoate addends per C6o molecule. Thus the hydrolysis of 8b in aqueous alkaline solution should give the corresponding watersolublefwiarenols 1, having an average of 18-20 hydroxyl groups per c60 molecule. The IR spectrum of Mlerenols 1 (Fig. 6c) showed a strong broad hydroxyl absorption band centered at 3432 cm-1 and three broad absorption bands centered at 1590, 1380, and 1045 cm-1, closely resembling those in the IR spectrum (Fig. 3a) of lllerenols prepared by the aqueous acid chemistry. Mass spectroscopic study of 8b was perfonntd using the fast caesium-ion bombardment [or liquid secondary ion mass spectromq (LSIMS)] technique. A VG ZAB-T four sector instrument was used for the positive LSIMS experiments. Prior to the FABILSIMS analysis, fullereme derivative 8b were 6ltertd through Celite, and deposited on a matrix of (3:l) dithiothreitol and dithioeaythritol. The matrix was bombarded by a Cs+ ion beam (35 kev). The positive ion FABILSIMS spectrum of 8b (Figure 9) shows that the most abundant ions are 720 separated by multiples of centered at m/z 720. There are many ion clusters above d. 16 (0),17 (OH), or 18 (OH2) mass units, providing evidence of oxygen entities in the molecule. Ion clusters of higher fbllerenes, presumably generated by the carbon recombmtion of fragments under the MS conditions, were observed above m/z 900. The much lower intensity of ion fragments above nu'z 1500 indicates the fast cleavage of p-bromobenzoate moieties upon caesium ion bombardment. However, two groups of ion fiagments with the highest mass peak at d z 1457, or 1473 were detected, in good agreement with the mass of an ion of a C6o derivative with 2 pbromobenzoate and 20 hydroxyl addends (m/z 1458) or 21 hydroxyl addends (m/z 1475), respectively. A c o e v e weight loss of 17 mass units from an ion at m/Z 1458 (A+) was observed and gave groups of ion fragmentations centered at m/z 1441 (A+-OH), 1424 (A+-20H), 1407 (A+-30H), 1390 (A+-40H), 1373 (A+-SOH), and 1356 (A+-60H). Interestingly,the spectrum showed a secondary band with the highest mass peak at d z 1239 indicating a fragment derived fiom the dehydration of the ion at d z 1257, which corresponds to a C60 derivative with one p-bromobenzoate, 18 hydroxyl, and two epoxide (&om 1,Z-diol dehydration) additions. Further sequential loss of 17 mass units from the ion at d z 1239 (B+) was detected and gave groups of ion fragmentations centerad at m/z 1222 @+-OH), 1205 (B+-20H), and 1188 @+-3OH) These results provide strong evidence supporting the characterized chemical composition of 8b.

84

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9. The mass spechum (positive ion FAF3LSIMS) of the fullerene derivative 8b, prcparad from pure c60. The mass of the highest ion peak in the fragmentation group is indicated. Ion clusters of higher fullerenes, 24 amu (C2) apart, and generated presumably by the carbon recombination of fragments under the mass spectrometryconditions, are marked by (c)

In summary, fullerene molecules exhibit high reactivity toward electrophilic attack by the nitronium ion in the presence of nucleophilic reagents under mild reaction conditions. the method exemplifies an alternative versatile synthetic methodology for introducing multiple polar functions, especially hydroxyl and fbctionalized carboxylic ester groups, onto fullerene molecules. The chemicaI composition of these fuSkrene derivatives was characterized as having an average of 13-15 hydroxyl addends and 4-5 pbromobenzoate addends per C6o molecule. Hydrolysis of these derivatives in aqueous

85

alkaline solution gives the corresponding water-soluble fbllerenols 1, possessing an average of 18-20 hydroxyl groups per C6o molecule. 5. Syntheris of FuUerenols by Hydrolysis of Polyeyelosulfated Fullerene Precursors Oleum [H2S04-S03(28%)] is one of the most widely used reagents for sulfating or sullbnating unsaturated hydrocarbons. The high reactivity of suffir trioxide towards olefins often results in sulfonation across C=C double bonds, thus converting olefins into a mixture of sultones and alkylsulfonic acids.47 In the presence of concentrated sulfuric acid the direct sulfation of alkene, catalyzed by the electrophilic proton, to form a COSOJH moiety, is also possible. Therefore it is reasonable to predict that the reaction of C6o with oleum may proceed with either the sulfonation or sulfation of Werenk C-C bonds. Experimentally, the reaction of C6o with oleum was carried out at 55-65 "C under an inert atmosphere.Upon exposure of C6o to fuming sulfivic acid, a partially

..

9 Polycyclosulfated C,

c60

Pyncylene moiety of Cso

10

(x = 6-5 and y = 61)

Y1ysi -

Scheme 3. Synthesis of fullerenols by the hydrolysis of polycyclosulfated fullerene derivatives 9 under mild conditions

86

suspended green solution was obtained immediately, indicating the existence of organic radicals. Detection of these radicals in the EPR spectrum 28-30 provided clear evidence of a reaction mechanism involving the electronic oxidation of C6o via initial formation of its cation-radicals. Within 5 h of reaction, a visible change from a dark green to a brown suspension was observed, revealing the limited stability of C6o cation-radicals in the reaction medium. This instability enhances the reactivity of them towards addition of suhric acid. At the end of reaction, the acid suspension was added dropwise to anhydrous diethyl ether to precipitate polycyclosulfated fullerene products 9, as orange soIids, as shown in Scheme 3. Hydrolysis of 9 in water at 85 "C or in aqueous & d i e solution at ambient temperature afforded hllerenols 3 in an overall yield of cu. 80% (from c60). The reaction proceeds with an efficient hllerene cyclosulfation followedby partial hydrolysis, to give the corresponding hydroxy hydrogen-sulfate intermediate 11. 5. I Characterization of the Cyclosu~aticStructure in 9

The IR spectrum of 3 (Fig. 10e) displayed a strong hydroxyl absorption band at 3317 cm-1 and three additional bands at 1623,1381, and 1049 cm-* in a similar range to those of 1 (Figs. 3a and 6c) Interestingly, both the elemental analysis and the X-ray photoelectron spectroscopic (XPS)measurements of fbllerenol 3 gave only a low sulhr content of less than 0.3% by weight and 0.1 % by atomic ratio. Furthermore, no absorption in the C-S region e.g 500-700 cm-l) of the Raman spectrum of 9 was observed. These results evidently exclude, in the structure of 9, direct bonding of sulfur to fullerenic carbons, hence sulfonyl or sulfonic acid functional groups with C-SO2linkages. This narrows the possible polar hnctional moieties in 9 to the sulfate (COSO2-0-) or sulfinate (C-OSO-0-) groups. Compound 9 exhibited relatively weak and broad alkanoic hydroxyl and hydrogen-sulfatic hydroxyl absorptions centered at 2920 and 2400 cm-1, respectively in its IR spectrum (Fig. 1Oc) It showed two strong absorptions centered at 1426 and 1233 cm-l corresponding to asymmetric RO-Sg2-OR stretch- and symmetric RO-SO2-OR stretch bands, respectively. Both the absorption intensities and positions of these two bands were in good agreement with those of -SOT absorptions (1386 and 1195 cm-1) in the spectrum of diethyl sulfate, indicating prominent sulfate (polycyclosulfate) entities in the structure of 9. The close resemblance of the overall spectrum of 9 (Fig. 1Oc) to that of diethyl sulfate (Fig. lob) in the 5001500 cm-1 region, and the deviation for the spectrum of 2-hydroxyethyi hydrogen sulfate (Fig. lOa), also provides additional support for this argument 2-Hydroxyethyl hydrogen

87

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sulfate was synthesized from the reaction of sulfur trioxide-DMF complex with an excess of ethylene glycol in MeCN at 65 "C. A weak asymmetric, and a strong, broad symmetric C - O s 2 0 H absorption band of 2-hydroxyethyl hydrogen sulfate were found to center at 1454 and 1227 cm-1, respectively (Fig. lOa). The 1H NMR spectrum of 9 in DMF47 showed a weak and broad peak centered at roughly8 14.5 correspondhg to acidic protons, thus indicating incorporationof a small amount of hydrogen sulfate O-SO2-0H groups as a partial structure of 9. The 13C NMR spectrum of 9 in D W - 4 (Fig. 1Ic) exhibited a broad peak centered at 6 148.0and weak broad peaks between 670.080.0 corresponding to olefinic carbons and mono-oxygenated carbons, respectively. The spectrum was nearly reproduciblefor fullereno13in the same solvent, as showing a slight shift of hydroxylated carbon peaks to 6 7085 (Fig. 1Id). The lack of carbon

*

lsoo 1wo sOoan-1 Figure 10. IR spectra of (a) 2-hydroxyethyl hydrogen sulfate; peaks in the re@on Of 160-175, 0 diethy1 mhw, (c) polycyclosulfated CfjO deriv. 9; (d) correspondingto the chemical shift partially hydrolyzed plycyclosulfatcd c60 deriv. 11; (e.0 in spectra ether fidlerenols prepared at 85 "C in H$) or NaOH&O, Of respectively; (g) 10 fromthe reaction of 3 with H2SO4-SO3 1 Ic and 1Id strongly suggested the 4ooQ3wQ3000zYJo2000

88

exclusion of hemiketal functional moieties 2 in fuflerenol 3. That differentiates the chemical structures of fullerenols 3 and 1. As expected, the simple polyhydroxylated lllerene structure of 3 showed a sole strong peak at 6 4.85, corresponding to hydroxyl. protons in the l H NMR spectrum (Fig. Ila). These were substantiated by deuterium exchange with D20 as shown in Fig. 1 lb, whereupon the hydroxyl proton peaks (C-OH) disappeared and were replaced by a new sharp peak of water protons at 6 4.0.

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The lH NMR spectrum of (a) fullerenols (3). derived from the hydrolysis of plycyclosulfated C60,in DMF-dy, (b) the solution (a) treated with 90,showing disappearance of the peak at 6 4.85 due to the deuterium exchange of hydroxyl protons, (c) the I3C NMR spectrum of plysulfated c60 derivatives 9 and (d) fullerenols 3

89 Polycyclosulfated C6o derivatives 9 readily underwent partial hydrolysis in wet diethyl ether solution in the presence of residual acids to afford the corresponding hydroxyl hydrogen sulfated derivative 11. To minimize the hydrolysis of fbllerenic cyclosulfate, complete removal of residual acids and the use of vigorously dried solvent in the isolation and purification procedure under an inert atmosphere was necessary. Predictably, the IR spectrum of partially hydrolyzed derivatives 11 (Fig. 10d) showed a sharp decrease of the RO-SQ2-OR absorption band at 1426 cm-l. To further substantiate and differentiate the characteristicIR absorptions of the fbllerenic hydroxyl hydrogen sulfate from that of the lllerenic diakyl sulfate, fillereno1 3 was allowed to react with H2S0&303 or SO3-DMF complex at ambient temperature to afford polyhydroxy hydrogen sulfated fullerene derivatives 10 as shown in Scheme 3. Compound 10 gave an IR spectrum (Fig. lob) with absorption bands relatively close to those of 11, showing a strong symmetric O-Xl2-OH absorption at 1225 cm-l and a weak asymmetric O-Sp2-OH absorption at 1413 cm-l. That confums the hydrogen sulfate moieties in the structure of 11. The sulfur (S2p) peaks with a maximum absorption centered at the binding energy of 169.8and 169.9eV in the XPS spectrum of 9 and 10, respectively, agreed well with that of the s&r fiurction in diethyl sulfate (169.4 eV). The data firther proved that the sulfur atoms in 9 and 10 are in the same oxidation state as that of sulfur in diethyl sulfate. By utilizing the two resolved S2p peaks of diethyl sulfate (Fig. 12bl) obtained from the curve-fittinganalysis as a reference, two sets of sulfbr peaks (S1 and S2) could be fitted into the experimental S2,, absorption band of compound 9 as depicted in Fig. 12b2. These two sets of sulfur peaks S1 and S2, corresponding to the absorption of different sulfbr atoms in d i m 1 sulfate and hydrogen sulfate fbnction, respectively, showed an intensity ratio of roughly 4:l This indicated a much higher content of cyclosulthtemoieties than hydrogen sulfate moieties in the structure of 9, consistent with the IR data. Both the positive and negative ion mass spectra [liquid secondary ion mass spectrometry (LSIMS) technique] of polyhydroxy hydrogen sulfated C6o derivatives, casted from DMF solution, showed a maximum relative peak intensity at m/z 721 and 720, respectively, correspondingto c60 ion fragmentation. Interestingly, it was followed by groups with a consecutive weight increase of mostly 16, 18, or 32 mass units apart, Corresponding to the gain of an oxygen atom, a water molecule, or a sulfur atom to the preceding ion fiagrnent. This observation agress well with the structure of partially hydroyzed polycyclosulfated C6o derivatives. The highest detectable mass peak in the

90 fragmentation group was found at m/z 1129 and 1056 in positive and negative ion FAl3 mass spectra, respectively.

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Figurn 12. The X-ray photoelectron (XPS spectrum of the SzP binding energy of (a) polycyclorulrated fullerene derivatives9 (solid curve), Et2SO4 (dottedw e ) , and partially hydrogen sulfated fullereno1 derivatives 10 (dashed curve); (bl) Et2S04; (b2) polycyclosulfated hllerene derivatives 9. In Figures bl and b2, the curve-fittinganalysis (dottedlines is indicated inside the experimental curves (solid lines). The binding energy in the spectra is not calibnted

5.2 Determination of the H y & q l Content in Fullereno1 3 by XPS Addition of oxygen-containing functions onto fullerenes to create carboaoxygen covalent bondings can be validated readily by X-ray photoelectron spectroscopic measurements. Both compounds 9 and fillerenol 3 showed a similar peak profile of carbon absorptions. Absorptions with a binding energy higher than 285.5 eV, corresponding to oxygenated carbons, were well separated fiom that of the nonoxygenated carbons, thereby allowing a reasonably accurate curve-fitting analysis of the carbon peak (Fig. 13). Since the reaction involved no hydrocarbon-containing reagent or solvents, the carbon peak in the spectrum should correspond solely to the absorption of carbon elements in the fillerene derivatives. The analysis of carbon peaks therefore

91

provides insight into the structural nature of 3 and 9, and allows a fair determination of the average number of hydroxyl addends on each hlierene molecule. In the analysis it was assumed that the first fitted carbon peak matches hlly with the lower binding energy region ofthe experimentalcurve, and then the second carbon peak was inserted to fit the rest of the absorptions in the higher binding energy region. As a result, the spectrum in Figure 138 for the cyclosulfated derivative 9 fitted in good agreement with a strong absorption band of Cis bindmg energy, corresponding to the non-oxygenated carbons (C-C), 04Iltered at 284.2 eV (76.6%) with a 111 width at half maximum (FWHM width) of 1.6 eV, and the second absorption band, correspondingto mono-oxygenated carbons (GO), centered at 286.3 eV (19.4%) with a FWHMwidth of 2.0 eV. 10

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Fipn 13. The X-ray photoelectron (nS) specrnun of the ClSbinding energy of (a) polycyclosulfated iblkrene derivatives9; (b)Mlerenols 3; (c) partially esteritled trinuoracety~lerenol derivatives 12. The we-fitting analysis is indicated inside the experimental curves

With solid hllerenols of heterogeneous composition, a slightly larger FWHM width of 1.6-2.0 eV was expected, compared to 2.1 eV above. The atomic ratio between non-oxygenated carbons vs. mono-oxygenated carbons was thus 48 : 12 indicating that 12 carbonsbear oxygen in the polysulfated C(j0 derivative9. The spectrum of hllerenols 3 in Fig. 13b showed a non-oxygenated carbon band, and a mono-oxygenated carbon

92

band centered at 284.2 (74.6%) and 286.1 (20.2%)) with a FWHM width of 1.6 eV and 1.9 eV, respectively. The atomic ratio between non-oxygenated carbons of 3 was found to be 47 : 13, similar to that of 9. Further esterificationof 3,using trifluoroacetic anhydride as reagent, afforded fbllerenol trifluoroacetates 12. The XPS data of 12 (Fig. 13c) showed a clear four-band spectrum, with binding energy maxima centered at 284.2 (68.7%, FWHM of 1.7 eV), 286.3 (19.9%,FWHM of 1.9 ev), 288.8 eV (67.7%, FWHM of 2.4 eV, ester carbons), and 292.0 (3.7%, FWHM of 1.7 eV, fluorinated carbons). The atomic ratio of non-oxygenated carbons : mono-oxygenated carbons : fluorinated carbons of 12 is thus 47 : 13 : 2.5, which agrees reasonably with a partially esterifed fbllerenol structure containing nine hydroxyl and three trifluoroacetoxy groups on average. The results consistently support an average structure of fhllerenols 3 having a maximum of twelve hydroxyl addends. In combination with the data obtained from elemental analysis and spectroscopic measurements, it was conciuded that the structure of 9 can be described as a poIysulfated hllerene derivative, containing a maximum of 5-6 cyclosulfate units. Partial hydrolysis of 9 readiiy occurred during the work-up procedure to give chemical characteristicsof hydroxyl and hydrogen sulfate functions. Since the hydrogen sulfate moieties of the partially hydrolyzed compound 11 contains acidic protons, 11 was complexed with an excess of p-dimethylaminopyridine @MAP) in DMF solution. Elemental analysis of the precipitated 11-DMAP adduct revealed a C:S:N ratio of 39:1:2 corresponding to 1.9 DMAP units per C6o molecule. The 1H NMR spectrum of the adduct in DMSOd6-DMFd7 solution (dried over molecular sieves) showed distinguishable peaks at 6 8.13, 6.62, 3.34, and 2.98, corresponding to the aromatic a- and J3-protons, hydroxyl, and methyl protons, respectively. The hydroxyl peak was fbrther identified by deuterium exchange with D20. From the integration ratio between aromatic and hydroxyl protons, more than 10 hydroxyl groups were estimated to be in the 11-DMAP adduct. These results also support the above structural assignment of fhllerenol3. 5.3 Thermal Analysis of Polycyclosu&zied Derivatives 9 The quantity of sulfate moieties per c60 molecule and .their chemical characteristics were hrther confirmed by a thermogravimetric analysis (TGA)-mass spectrometry study. In the TGA-MS spectrum of polycyclosulfated derivatives 9 (Fig. 14), a clear desorption (4% weight loss) of the physically absorbed water molecules

93 below 180 "C was detected. It was followed by a strong aand sharp thermal evolution of SO2 molecules at an onset temperature of 187 "C that extended to a maximum at 245 "C. The observed SO2 evolution fiom thermal treatment of compound 9 confirmed the presence of cyclic sulfate 0-SO2-0- entities, cleavage of S - 0 bonds evidently occuring. Other gases evolved in this temperature range included CO and CO2; the latter may rev& the occurrence of structural rearrangement of radical intermediates in conjunctionwith the SO2 elimination. Since the intensity of C 0 2 was significantly lower than that of SO2 aand the intensity of CO was trivial, the major portion of material weight loss (28%) at temperatures between 190 "C and 3 10 OC can be correlated to the quantity of SO2 molecules eliminated. That allowed an estimate of slightly less than six cyclosu&tc units per C6o molecule in 9,consistent with the X P S data.

0

100

200

300

400

500

600

Temperature ( O C ) Figure 14. TOA II~&FSspectrascopic data showing the percentage weight change (TG), differential weight change (DTG), and the intensity of SO, evolved upon heating the polycyclosulfated derivative9 at a rate of 6 "C/min 5.4 Considerationof the Reaction Mechanismfor Cycbsurfonation

One plausible reaction mechanism involves the attack of sulfur trioxide on the hexagon moiety of C6o forming the 1,2(u) or 1,4(b)-sultone derivative 13, as unsaturated hydrocarbons do normally in reaction with SO3. Hydrolysis of compound 13 should then give hydroxysulfonic acid 14, containing a covalent C-S bond. Therefore, the lack of detection of sulfur atoms in both the elemental analysis and X P S measurements of the hydrolyzed products led to the conclusion that treatment of C6o with &sOq-s03

94

13

L4

%heme 4. Exclusion of cfjosultonation in the reactionof fullerenes with fuming sulfuric acid

did not lead to formation of sultone 13, as shown in Scheme 4. Rather, the primary fbnction of sulhr trioxide molecules in this case is to promote one-electron oxidation of hllerene molecules, forming a C6o cation-radical, as depicted in Scheme 5. Trapping of the c60 cation 15 by anionic HSO4- or HS2O7- pISOq'(SO3)] species affords the corresponding hydrogen sulphated C6o radical 16. Since the open allylic carbocationic C6o radicals 15 are prone to rearrange, the formation of various isomeric products is feasible. Further electron oxidation of the radical intermediate 16 yields the corresponding cationic hydrogen-sulfated c60, 17. Intramolecular cyclition of intermediate 17 via the elimination of one proton produces the desired product of 1,2or 1,4-cycIosulfated C6o derivatives 9. Unlike organic sulfonates which exhibit remarkable hydrolytic stability even after prolonged heating, cyclosulfated C6o derivatives 9 with a -C-OSOz- linkage are readily susceptible towards acid hydrolysis. It is noteworthy that the hydroysis rate for cyclic sulfates can be 20 times as fast as that of open-chain dialkyl sulfates.49 Hydrolysis proceeds mainly by the C-0 bond cleavage via the attack of water molecules at the conjugated P-carbon, as depicted in Scheme 3, to yield fkllerenols 3 as the final product. In summary, the novel cyclosulfation chemistry for the functionalization of fbllerenemolecules, resulting in the DMF-soluble polycyclosulfated fkllerene derivatives 9, has been demonstrated.42 The cyclosulfation reaction of c60 was performed in neat fuming sulfixric acid at 55-65 OC under N2. Hydrolysis of 9 either in the presence of

HzSO4-SO3

I@{

*

Electronic Oxidation

95

/

\

(Corannulene Cation Radlcal)

- e-

s

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Scheme 5. Hypothetical c60 cyclosulfationreaction mechanism involving stepwise electronic oxidation

water at 85-90 "C or in aqueous NaOH solution at ambient temperature gave a high yield of polyhydroxylated fbllerene 3, containing no hemiketal structure. The facile hydrolysis of the cyclosulfated C60, 9, or the hydrogen-sulfated C6o during the purification procedure often prevented accurate quantitative determination of sulfhr atoms. The functional characteristics and quantity of sulfate moieties per C6o molecule in polycyclosulfated fillerene precursors 9 and the structure of fbllerenols 3 were determined mainly through analysis of the TGA-mass spectrum, and of two absorption bands, correspondingto the binding energy of sulfbr (Szp) and carbon (C 1s), in the X P S spectrum. The XPS spectrum was used to determine the oxidation state of sulfbr and the quantity of carbons possessing a higher oxidation state than that of the unreacted lllerenic carbons. As a result, an average of 10 to 12 hydroxyl addends in the structure of lllereno13 was correlated to the structure of polycyclosulfated fullerene 9 containing 5 to 6 cyclosulfate units. Mechanistically the cyclosulfation reaction is apparently initiated by a one-electron oxidation of C6o molecules, followed by the attack of anionic sulfate species on the resulting c60 cation-radical intermediates, to give the corresponding hydrogen-sulfated C6o radicals. Further electron oxidation and

96 intramolecular cyclization of this hydrogen-sulfated c60 yielded the desired polycyclosulfated C6o products. Fullerenols 3, consisting of a simple chemical composition of 60 carbons and multiple hydroxyl groups, are significantly different from hllerenols 1. Interestingly, if the hydrolysis of 9 was carried out in aqueous NaOH solution at higher temperature of 85-90 "C, the derived fullerenol products gave an IR spectrum (Fig. 100 closely resembling fbllerenols 1 (Fig. 6c). A secondary reaction, other than simple hydrolysis, probably occurs under alkaline conditions at elevated temperature. 6. Synthesis of Fullerenols from Hydroborated Fullerene Derivatives Most of the chemistry described in this section is based upon the work af the Sussex gr0~p.50 The hydroboration reaction has been studied as a part of C6o fbnctionaLization with the object of obtaining multiple additions to fullerenes. When hydroboration was carried out with 2 equivalents of BH3 in THF followed by decomposition with acetic acid, a hydrogenated product, C6OH2, was obtained.51 However, the use of an excess quantity of BH3 in THF in a similar reaction gave polyhydroxylated fullerene products instead of polyhydrogenated ones.50 The implication is that C-H bonds on the fullerene cage are readily oxidized to C-OH bonds. Experimentally, an excess of 1M BHyTHF complex was added to a solution of fullerene dissolved in dry toluene. A rapid colour change from magenta to brown occurred. During 2-3 h of stimng, the reaction mixture became increasingly brown due to precipitation of a solid, leaving the supernatant toluene colourless. The solid precipitates were assumed to be the c6O@H2)n. Intermediates with more than one hllerene C6o molecule attached to a single boron were reasonably excluded on the grounds of steric hindrance. The IR spectrum of a portion of the intermediate, removed by filtration and recorded after only brief contact with air, showed a strong absorption at 3212 cm-I (superimposed upon a broader band at higher wave number), an intense broad band at 1451 cm-l, and less intense bands at 1197 cm-l (sharp) and 831 cm-l (broad). Only a very weak C-H stretching band at 2956 cm-* is evident. This band disappeared after prolonged standing of the sample in air, the other bands remaining effectively unaltered. Since phenylboric acid (benzeneboronic acid) showed intense broad bands at 3275 and 1360 cm -1 (the latter due to B-0 stretchsz), the results suggest that the intermediate is of the form C~O(OH)B(OH)~, arising from oxygen insertions. This chemistly may be

97

correlated to the allylic oxidation of C-H bonds, catalyzed by singlet oxygen,53 of which lllerene molecules are potent producers.54 The intermediate was then reacted with a solution ofhydrogen peroxide followed by sodium hydroxide. The resulting mixture was stirred for 3h and allowed to settle overnight. The toluene layer was removed to leave a yellow-orange aqueous layer, together with a brown precipitate, which was separated by filtration. The precipitate gave IR absorption bands at 3430, 1631, 1385, cu. 1090, aand 450-550 cm-I as shown in Figure 15, and these are commensurate with those of lllerenol 1 and 3. The relative band intensities at 1632, 1385, and 1090 cm-* were somewhat batch-dependent, but the similarity to the spectra shown in Figs. 3a and 6c is most strilring. Other notable features include the absence of stretching bands.

4000

2000 1500 1000 500 cm-1 Figure 15. The IR specirm of the reaction product resultingfrom treating c i ; with ~ an excess of diborane followed by H202/NaOH 3000

A slight variation of reaction conditions was used for the synthesis of polyhydroxylated lllerene derivatives. For example, C6o in toluene was allowed to react with 30 equiv. of BHyTHF for 19 h with stirring, followed by hydrolysis of redting mixtures by acetic acid for 3 h to afford products showing very strong -OH bands with only very weak -CH stretching bands in the IR spectrum. The IR spectrum of the residual solid that had been washed with dichloromethane and toluene was virtually identical to that in Figure 6c of hllerenols 3. In order to confirm the presence of hydroxyl groups, the esterification method was adopted by reacting the hydrolyzed hydroboration products with benzoyl chloride in the presence of triethylamine in dichloromethane. The isolated ester derivatives gave IR bands at cu 1730 cm-1 correspondingto the absorption of carbonyl (C=O) and 1600 cm-1,typical of benzoates.

98 The spectrum also showed a peak at 1788 cm-l which disappeared slowly upon heating under vacuum. Therefore, this peak was believed to arise from acid chloride trapped in the lattice of the fbllerene derivative. The heated materials gave a multiplet in the l H NMR spectrum at 6 162.3, corresponding to the chemical shift of ester carbons. These spectroscopicdate confirmed benzoate formation in the reaction. Further experients were carried out in order to confirm the reactive role of oxygen during hydroboration of fbllerenes. Two runs were performed in parallel, using an excess of BH3-THF: one under nitrogen, the other in air. Only a small amount of precipitate was obtained in the reaction carried out under nitrogen. The products of both reactions were quenched with acetic acid, followed by sequential washing with water and sodium carbonate. The isolated products from the reaction carried out in air showed an IR spectrum similar to those of hllerenols. However, the IR spectrum of products synthesized from the reaction carried out under nitrogen was quite different, and showed the expected strong C-H stretching bands. These results substantiated the contribution of oxygen in the reaction mechanism of C6o hydroboration, leading to the formation of polyhydroxylated fbllerene derivatives. 7. Structural Differences in Fullerenols Prepared by Different Methods

Fullerenols synthesized by different methods exhibit variations in the number of hydroxyl addends, and the cage structure. The most significant feature, the incorporation of the hemiketal functional moieties in the structure of fullerenol 1 as well as hydroxyl groups, has been studied extensively 43 In the X-ray photoelectronic spectroscopic (XPS) study of fbllerenols 1, analysis of the C i s binding energy peak in the spectrum (Fig. 4) showed a large absorption deviation from that of C6o fbllerene, indicating both a high percentage of carbons in a higher oxidation state than parent fbllerene carbons, and carbons in a higher oxidation state than mono-oxygenated carbons. These latter spread over a range of more than 4 eV in the spectrum, which is too broad to be caused only by mono-oxygenated (e.g ethereal or hydroxylated) carbons, and indicated di-oxygenated carbons in the structure Curve-fitting analysis of the fullerenol peak fitted reasonably with three different oxidation states of carbon, assuming that the peak separation between each oxidation state has a similar range of 1 8 eV In Fig 4 the fitted peaks with a binding energy of 286 1 eV (68%) were assigned to non-oxygenated carbons, 287.9 eV (23%) to mono-oxygenated e.g ethereal or hydroxylated carbons, and 289 7 (YA)to dioxygenated carbons such as carbonyl (C=O), ketal (RO-C-OR), or hemiketal

99

(RO-C-OH). Since the lR spectrum of hllerenols contain no carbonyl absorption bands, carbon atoms with the highest oxidation state were attributed to hemiketal carbons. Even though this arbitrary curve-fitting provides no absolute quantity of carbons in different oxidation states, it provided a valuable estimate of the average number of hydroxyl addends (14-16, consistent with that obtained from the chemical method), and approximately 6-7 hemiketal moieties, per fbllerene molecule. The chemical method determines the number of hydroxyl groups per kfferene molecule, &om the proton integration ratio of hydroxyl protons : predefined ester C-H protons in the 1H NMR spectnun of partistty estetified fidierenol derivatives. Solid state *%XMR study of hllerenols 1, using the cross polarizatiodmagic angle Spinning (CP/MAS) technique, was performed at different spinning speeds of 3.8 lcHz and 3.0kHz as shown in Figures 16a and 16b, respectively.Both spectra showed

I " " l ' " ' I " " l " " 1 " " 1 ' " ' 1 ' 200 150 1W 50

250

0

-50 ppm

Figure 16. The I3C NMR spectrum of fulierenols1 obtained using: (a) solid-state CPlMAS at 3.8 Wiz spinning speed,@) solid-state C P M S at 3.0 kHz spinning speed, and (c) in QO s1ution

100

three distinct carbon peaks with chemical shifts centered at roughly 6 80, 140, and 170180, and one weak carbon peak centered at 6 103.2. These spectra were reproduced in D20 solution, showing two major bands with chemical shifts centered at 6 79.0 and 140.3, corresponding to hydroxylated carbons (C,) and unreacted olefinic carbon, respectively, Due to the lack of carbonyl absorptions in the IR spectrum of fidlerenols (Fig. 3a), which excluded ester or related enctions in 1, the third downfield band at 6 170.3 was assigned to the vinyl ether carbons (C,). The fourth weak band centered at 6 100.0 corresponds clearly to di-oxygenated carbons such as ketal (RO-C-OR) or hemiketal (RO-C-OH) carbons. From the low probability of ketal formation on C6o it is suggested that the di-oxygenated carbons in fillerenols 1 arise from hemiketal fbnction. These spectra thus provide consistent evidence to support the structural characterization of fullerenolsin terms of the presence of hemiketal functions with vinyl ether linkages.

34

-34 10

150

290

430 rn Temperature (OC)

no

850

heating rate :6oC / min

Figure 17. TGA-mass spectroscopicdata of Illerenois 1, showing the percentage weight change (TG), differential weight change (DTG), and the intensity of H20,C0,aand C02 gases evolved upon heating at a rate of 6 "Urnin

A TGA-mass spectrocopic study of fkllerenol 1 was used to detect the thermal

evolution of H20, CO, and C 0 2 In this case, thermal cleavage of mono-oxygenated

101

carbons is the main source of CO whereas the thermal cleavage of di-oxygenated carbons leads to &a release of C02. As shown in Figure 17, the physically absorbed I420 (6% by weight) can be e l i t e d below 200 OC. The subsequent evolution of water due to the dehydration of polyol moieties of 1 was observed between 280 and 430 "C, indicating the polyhydroxy nature of fullerenol. This chemical dehydration process corresponds to a weight less of 8%. The thermal elimination of CO and C02 occurred in two stages start& &om 450 OC with C02 as the major gas detected, and giving a combined weight loss of 19%. For C02 evolution, peak maxima occurred at 555 and 700 "C, with much higher evolution at the first stage. By contrast, the intensity of CO evolution was relatively weak with peak maxima at 575 and 695 "C. In a TGA-FTIR study of fillerenol 1 (Fig. 18), the intensity of the C02 absorption band in the IR increased to a maximum at 605 OC, whilst the onset temperature for the detection of the CO absorption band was 650 OC with a peak 99.3

-

Ecn

5

s

ss a

%

G

47.0

mOO

lam

1;m

I*

no0

r*m

noo

-8

1

mrouconm,

Figure 18. "he TGA-IWR measurement of fullerenols 1, showing the temperaturedependent weight loss and IR absorptionbands of H20, C02,and CO evolved upon heat treatment at a rate of 6 OC/min

102

maximum at 735 O C . The weight loss of fullerenol in the sample used for this experiment was 14% and 27.5%, respectively, for chemical dehydration, and thermal loss of C 0 2 and CO. Figure 18 shows that the C 0 2 absorption band was detected in the IR spectrum at a temperature as low as 175 "C. Since the thermal evolution of C02 from the structural skeleton of fullereno1 involves cleavage of several sigma-bonds, it would be expected that a temperature higher than 400 "C would be necessary to accomplish the process. A conceivable explanation for the elimination of C 0 2 at low temperatures involves a bond-cleavage initiation via the chemical dehydration of polyols, in conjunction with thermal lability of the hemiketal structure.

1 (hemlketal moieties)

{*} \ /

Y

??

Scheme 6. Mechanisms for acid-catalysedrearrangements in fullerenols

An established chemical method for substantiating a hemiketal function is to convert the hemiketal to the corresponding ketone, by treatment with acid. This transformation can be readily detected through observation of the carbonyl absorption in the IR spectrum of the resulting ketone. However, the reaction of fullerenols containing

103

vic-diols with acid can potentially be complicated by the Pinacol rearrangement, whereby tertiary uic-diol moieties of 1 are transformed into the corresponding ketone 19, as shown in Scheme 6. Differentiation between the hemiketal-ketone equilibriation, and the Pinacol rearrangement of vic-diol under acidic conditions can be made through examination of the full reversibility of the former interconversion, leading conceivably to the formation of several different hydroxyketone moieties, represented as the partial structure 21 in Scheme 6. Not only is the Pinacol rearrangement irreversible, but in the present case it requires both cleavage and formation of carbon-carbon bonds in the protonated structure of 18 to afford ketone f9 (which contains highfy strained tetracyclic 5,5,6,6-mmbered rings sharing a common carbon atom). Fullcrenols with multiple tertiary hydroxyl groups were indeed found to be sensitive to certain acid conditions (aqueous hydrochloric acid, or HBFq-Et20 in methand). For example, lllerenol 1 was treated with dilute aqueous HCI (0.4 M) at 80 "C for 5-16 h. Suspended brown solids were slowly formed fiom an initially dark brown solution. At the end of reaction, solids 2la were collected by centrifugation,and

Figure 19. The IR spectrum of: (a) fullerenols 1; (b,c) water-soluble products 21b and solid precipitates tla from the reaction of fullerenols with &I. HCl(0.4 M ) at 80 "C;(d) recovered fullerenols from the treatment of 21a and 21b with aqueous NaOH

104

the remaining solution was dried in vaccum at 50 "C to give solid products 21b. W spectra of both 21a and 21b showed a new pronounced carbonyl absorption band at 1722 cm-1 and the retention of five characteristic hllerenol absorptions at 3424, 1595, 1392, 1084, and 593 cm-1, the intensities differing fiom that of the initial hllerenol (Fig. 19a). The observed carbonyl absorption confirmed the ketone presence in 21. The spectra indicated that the chemical structures of 21a and 21b are related to each other, presumably in having a different degree of hemiketal to ketone conversion. The driving force for this facile acid-catalyzed conversion is the stabilization, by an oxygen atom, of a tertiary alkyl carbocation derived from the protonation of the hemiketal hnction in 20, and the elimination of a hydroxy proton to give the corresponding ketone. Interestingly, brown solids are precipitated by treatment of 21a or 2lb with aqueous alkaline solution (2M NaOH). After the removal of NaOH by repeated washing with water and methanol, these brown solids exhibited a nearly identical IR spectrum (Fig. 19d) to that of the starting fbllerenols. This pH-dependent reversible interconversion of hemiketal and ketone, clearly excludes the alternative Pinacol rearrangement. To further support the existence of hemiketal structures in hllerenols 1, an ethylation reaction was performed with triethyloxonium tetrafluoroborate (Et30+-BFq-) in I , 1,2-trichloroethane at 85 "C in order to attack the ketal moiety The reaction converted the hemiketal groups, through ring opening, to the corresponding ethoxy and ketone structures. As expected, a carbonyl absorption band at 1726 cm-l was observed in the IR spectrum of the products, indicating ketone formation during the reaction, consistent with the above structural hypothesis. In summary, the chemical structure of hllerenols 1 was shown to consist of hemiketal moieties as well as tertiary hydroxy groups. This interesting observation of hemiketal functions on the fullerene skeleton requires in turn, postulation of a hypothetical mechanism involving hydroxylated fullerene oxide as one of the reaction intermediates. Treatment of hllerenol 3,which lacks hemiketal moieties, with dilute HCI (2M) at ambient temperature, gave products showing a nearly identical IR spectrum to that of the starting material, and no carbonyl absorptions. Moreover, the lack of carbon peaks in the region of 6 160-175 (corresponding to the chemical shift of vinyl ether carbons) in Fig. 1 Ic and 1 Id, strongly suggested the exclusion of hemiketal moieties in fUllerenol3. That differentiates the chemical structure of 3 fiom 1.

105

References 1. The first theoretical prediction: Z. Yoshida and E. Osawa, Kugakudojin, Kyoto (1971) 174. 2. For overviews of fullerene preparation and properties see: H. W. Kroto, A. W. Allaf, and S. P. Balm,Chem. Rev. 91 (1991) 1213 and references therein. 3. A. F. Hebard, M. J. Rosseinsky, R. C. Haddon, D. W. Murphy, S. H. Glarum, T.T. M. Palstra. A. P. Ramirez and A. R. Kortan, Nature 350 (1991) 600. 4. R. C . Haddon. Acc. Chem. Res. 25 (1992) 127. 5 . K. Tanigaki, T.W. Ebbesen, S. Saito, J. Mizuki, J. S. Tsai, Y. Kubo and S. Kuroshima, Nature 352 (1991) 222. 6. For review of fullerene chemistry see: R. Taylor and D. R. M. Walton, Nature 363 (1993) 685.

7. R. D. Johnson, G. Meijer and D. S . Bethune, J. Am. Chem. SOC.112 (1990) 8983. 8. P. M.Allemand, K. C. Khemani, A. Koch, F. Wudl, K. Holczer, S. Donovan, G. Gnmer, J. D. Thompson, Science 253 (1991) 301. 9. Y . Wang, f. Chern. Phys. 96 (1992) 764. 10. N. S.Saricifti, L. Smilowitz, A. J. Heeger and F. Wudl, Science 258 (1992) 1474. 11. C. S. Kuo, F. G. Wakim, S. K. Sengupta and S . K. Tripathy, Solid State commun. 87 (1993) 115. 12. K. Chen, W. B. Caldwell and C. A. Mirkin, J. Am. Chem. Soc. 115 (1993) 1193. 13. W. B. Calwell, K. Chen, C. A. Mirkin and S. J. Babinec, Langmuir 9 (1993) 1945. 14. J. A. Chupa. S. Xu, R. F. Fischetti. R. M. Strongin, J. P. McCauley, A. B. Smith, J. K. Blasie, L. J. Peticolas and J. C. Bean, J. Am. Chem. Soc. 1993, 115, 4383. 15. K. L. Wooley, C. J. Hawker, J. M. J. Frechet, F. Wudl, G. Srdanov, S. Shi, C. Li and M. Kao, J. Am. Chem. SOC.115 (1993) 9836. 16. R. E.Haufler, J. Conceicao, L. P. F. Chibante, Y. Chai, N. E. Byrne, S. Flanagan, M. M. Haley, S. C. OBrien, C. Pan, Z. Xiao, W. E. Billups, M. A. Ciufolini, R. H. Hauge, J. L. Margrave, L. J. Wilson, R. F.Curl and R. E. Smalley, J. Phys. Chem. 94 (1990) 8634. 17. P. M. Allemand, A. Koch, F. Wudl, Y. Rubin, F. Diederich. M. M. Alvarez, S. J. Anz and R. L. Whetten, J. Am. Chem. SOC. 113 (1991) 1050. 18. D. M. Cox, S. Bahal, M. Disko, S. M. Gorun, M. Greaney, C. S. Hsu, E. B. Kollin, J. Millar, J. Robbins, R. D. Shenvood and P. Tindall, J. Am. Chem. SOC. 113 (1991) 2940.

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19. C. Jehoulet, A. J. Bard and F. Wudl, J. Am. Chem. SOC.113 (1991) 5456. 20. D. Dubois, K. M. Kadish, S. Flanagan, R. E. Haufler, L. P. Chibante and L. J. Wilson, J. Am. Chem. SOC.113 (1991) 4364. 21. D. Dubois, K. M. Kadish, S. Flanagan and L. J. Wilson, J . Am. Chem. SOC.113 (1991) 7773. 22. Q. Xie, E. Perez-Corder0 and L. Echegoyen, J. Am. Chem. SOC.114 (1992) 3978. 23. Q. Xie, F. Arias and L. Echegoyen, J. Am. Chem. SOC. 115 (1993) 9818. 24. D. L. Lichtenberger, K. W. Nebesny, C. D. Ray, D. R. Huffman and L. D. Lamb, Chem. Phys. Lett. 176 (1991) 203. 25. F. Wudl, A. Hirsch, K. Khemani, T. Suzuki, P. M.Allemand, A. Koch, H. Eckert, G . Srdanov and H. Webb, ACS Symp. Ser. 481 (1992) 161. 26. A. Hirsch, Q. Li and F. Wudl, Angew. Chem., Int. Ed. Engl. 30 (1991) 1309. 27. A. Hirsch, A. Soi and H. R. Karfunkel, Angew. Chem., Int. Ed. Engl.. 31 (1992) 766. 28. S. G. Kukolich and D. R. Huffman, Chem. Phys. Lett. 182 (1991) 263. 29. H. Thomann, M. Bernardo and G. P. Miller, J. Am. Chem. SOC.114 (1992) 6593. 30. G. P. Miller, C. S. Hsu, H. Thomann, L. Y. Chiang and M. Bernardo, Mat. Res. Symp. Proc. 247 (1992) 293. 31. H. Selig, C. Lifshitz, T. Peres, J. E. Fischer, A. R. McGhie, W. J. Romanoq. J. P. McCauley Jr. and A. B. Smith, J. Am. Chem. SOC.113 (1991) 5475. 32. J. H. Holloway, E. G. Hope, R. Taylor, G. J. Langley, A. G. Avent, J. T. Dennis, J. P. Hare, H. W. Kroto and D. R. M. Walton, J. Chem. SOC.,Chem. Cornmun. (1991) 9666. 33. P. R. Birkett, P. W. Hitchcock, H. W. Kroto, R. Taylor and D. R. M. Walton, Nature 357 (1992) 479. 34. F. N. Tebbe, R. L. Harlow, D. B. Chase, D. L. Thorn, G. C. Campbell, J. C. Calabrese, N. Herron, R. J. Young and E. Wasserrnan, Science 256 (1992) 822. 35. G. A. Olah, I. Bucsi, C. Lambert, R. Aniszfeld, N. J. Trivedi, D. K. Sensharma, G. K. S. Prakash, 1. Am. Chem. SOC.113 (1991) 9385. 36. F. N. Tebbe, J. Y. Becker, D. B. Chase, L. E. Firment, E. R. Holler, B. S. Malone, P. J. Krusic and E. Wasserman, J. Am. Chem. SOC.113 (1991) 9900. 37. G. A. Olah, I. Bucsi, C. Lambert, R. Aniszfeld, N. J. Trivedi, D. K. Sensharma. G. K. S. Prakash, J. Am. Chem. SOC.113 (1991) 9387.

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38. L. Y.Chiang. R. B. Upasani and J. W. Swirczewski, Muter. Res. SOC.Symp. Proc. 247 (1992) 285. 39. L. Y.Chiang, J. W. Swirczewski, C. S. Hsu,S. K. Chowdhury. S. Cameron and K. Creegan, J. Chem. Sac., Chem. Commun. (1992) 1791. 40. L. Y.Chiang,R. B. Upasani and J. W. Swirczewski, J. Am. Chem. SOC.114 (1992) 10154. 41. L. Y.Chiang, R. B. Upasani and J. W. Swirczewski, US.Patent 5,177,248, (1993). 42. L. Y.Chiang,L. Y.Wang, J. W. Swirczewski. G. Miller, S. Soled and S . Cameron. J. Org. Chern. (1994) (in press). 43. L. Y.Chiang, R. B. Upasani, J. W. Swirczewski and S . Soled, J. Am. Chem. SOC. 115 (1993) 5453. 44. Without the stabilization of hemiketal function, fullerene oxide preferentially equilibrates in the epoxide form. K. M. Creegan, J. L. Robbins, W. K. Robbins, J. M. Millar, R. D. Shenvood, P. 3. Tindall, D. M. Cox, A. B. Smith, J. P. McCauley, D. R. Jones and R. T. Gallagher, J. Am. Chen SOC.114 (1992) 1103. 45. IR spectrum of 4 (KBr) U,,, 3434,2938, 1733 (C=O), 1629, 1411, and 1162(br) cm-1. IR spectrum of 5 (KBr) Vmax 3422, 2927, 2855, 1727 (C=O), 1613, 1503(w), 1450, and 1080(br) crn-1. 46. Infrared spectrum of rerr-butylp-bromobenzoate: vmax (KBr) 2973,2917, 1713 (s, C=O), 1586, 1476, 1454, 1393, 1367, 1293, 1163, 1118, 1070, 1011,848,758. 47. D. W.Roberts and D. L. Williams, Tetrahedron 43 (1987) 1027. 48. The sulfur binding energy of diethylsulfate was found to center at 169.3 eV (66.6% and FWHM width = 1.65 eV) and 170.5 eV (33.3% and FWHM width = 1.65 eV) in a ratio of 2: 1. Whereas the curve-fitting analysis of the sulfur peak in the XPS spectrum of 4 gave four peaks (two sets) with a binding energy centered at 169.2 eV (52.9% and FWHM width = 1.65 eV) and 170.3 eV (26.5% and FWHM width = 1.65 eV) as the first set and at 170.5 eV (13.7% and FWHM width = 1.65 eV) and 171.5 eV (6.8% and FWHM width = 1.65 eV) as the second set. 49. T. Deacon, A. Steltner and A. Williams, J. Chem. Soc., Perkin 11 (1975) 1778. 50. N. S , Schneider, A. D. Darwish, H. W. Kroto, R. Taylor and D. R. M. Walton, J. Chem. Soc., Chem. Commun., (1 994) 15. 5 1. C. C. Henderson and P. A. Cahill, Science 259 (1993) 1885.

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52. L. J. Bellamy, W. Gerrard, M. F. Lappert and R. L. Williams, J. Chem. SOC. (1958) 2412. 53. J. March, 'Advanced Organic Chemistry", Wiley, Chichester, (1992) pp. 705-708. 54. J. W. Abogast, A. P. Darmanyan. C. S. Foote, Y. Rubin, F. Diederich, M. M. Alvarez, S. J. Anz and R. L. Whetten, J. Phys. Chem. 95 (1991) 11.

Chapter 6 Fluorination John H. Holloway and Eric G. Hope Chemistry Department, University of Leicester, University Road, Leicester LEI 7RH, U.K. 1. Introduction

The possibility that the products of fluorination of hllerenes might be of value in materials science, and in particular, that fluorination of [60]hllerene might produce a ballbearing-like analogue of Teflon with superlative lubrication properties, coupled with inconclusive predictions as to whether c6@60 would be stable, gave special impetus to early studies in this area. Progress in the field, however, has been hampered significantly by difaculties in controlling the fluorination process such that single compounds are produced and in finding methods of separating single components from the mixtures. What has emerged is that mass spectrometric evidence for fluorinated species up to C6oFm has been detected in fluorinations of [60]hllerene ushg elemental fluorhe1p2 and hyperfluorinated species up to C(joF76 are evident in the products of fluorination using e h e n t a l fluorine in the presence of ultraviolet radiation3 or up to C60F78 using KrF2 in anhydrous HF.4 For [70]fullerene, the most highly fluorinated product detected so far by mass spectrometry is C7oF54.5 Fullerenes containing only a few fluorine atoms have not been isolated, although recent evidence suggests that such entities occur and are not only fiagmentation products arising from mass spectroscopic examination of more extenaivcly fluorinated species.6 The mass spectra of products from the fluorination of [6O]lllerene have maximum intensity around C6@4&@48 1-375,738 which compares with C7Op46 5 for the fluorination of [70]hllerene. The fluorinated fullerenes are highly soluble in polar solvents. Although more stable than either the chloro- or bromo-compounds, the fluoro[6O]fullerenes decompose when stored at room temperature in C@b8 or when heated above 80 "C9 These obsetvations, coupled with the fact that, although the products of fluorination of C60 are stable in air, they hydrolyse rapidly in the presence of water and a cosolvent for the fluorinated species, means that c6@60 and its relatives are unlikely to be able to function as ine-rf superlubricants.10911

109

110

An important feature of fullerene chemistry is that oxygen is often attached to

the hllerene cage. The fluorinated fbllerenes are no exception, oxygens having been found in the reaction products of fluorination of C6o and C6o/c70 mixtures.19**10*12It is also very likely that the oxygen is present as the epoxide.1,3,12,13 The origin of the oxygen atoms is uncertain but it has been demonstrated that the number incorporated can be increased by reaction with aqueous methanol, whereby nucleophilic substitution of F by OH followed by cis elimination of HF appears to 0ccur.%10,1~Since fillerenes are reactive towards nucleophiles the halogeno-derivatives should also be susceptible to nucleophillic substitution. Indeed, the order of reactivity is fluoro->chloro-~bromo-.9 Since halogeno-derivatives have the potential to be usell intermediates in OfGtiniC synthesis and since the fluoro-derivatives are the most stable and the simplest to manipulate, the reactions of fluorofillerenes with nucleophiles should be an important sphere of development for fillerene chemistry if fluorination stoicheiometries can be controlled as they are during br~mination.~ 2. Theoretical Considerations

There was initial disagreement amongst theoreticians as to whether C&so should occur. Whilst ab initio Hartree-Fock calculations using a double-zeta plus potential @ZP) method suggested that it should,l4 a similar independent study produced a more sceptical conclusion, namely, that perfluoro [60]hllerene 'may not be easily isolable'. However, both agreed that steric interactions between adjacent fluorines would result in lengthening of the C-C bonds in the [60] hllerene cage to 1.672 A. Independent calculations employing density function methods have predicted a 15% weakening of the C-Fbonds compared with those in CF4 but suggest stability for C6oF60 l6 while others have indicated that twisting of the carbon skeleton can partially reduce the strain but retain I symmetry.17 A series of papers18319920 using semi-empirical calculations have demonstrated that [60]hllerene is susceptible to equimolar X2 addition of H, F, CI, Br and I, forming the 1,2 and 1,4-additions analogous to the addition to the ethene and butadiene moieties respectively. In considering the electronic structures of 1,2- and 1,4- added C6#2n1s (n = is exothermic. 1, 2, 4, 6, 8, 10, 12, 18, 14 and 30) the reaction C6o + nF2 -+ The exothermicity in the fluorine case is lower than that for the other halogens. A larger destabilization as n increases is predicted for 1,Zaddition whereas this is not the case for 1,Caddition up to n = 12. The destabilization for 1,Z-addition begins at ca. n = 8 and '

*

111

originate8 &om the interaction of substituents which are added at adjacent double bonds

in [6O]lllerene. Preference for 1,Zaddition for F should occur at n < 8.20 Similar calculations by an independent group have confirmed that the favoured isomers involve 1,2- and 1,4-additions with higher energy ones resulting from 1,6- and 48-additions. Consideration of isomers represented by 1,3-, 1,5- and 1,7-additions have led to the conclusion that these are only stable in the case of fluorine addition.2l Pluoro[60]Mlerene has been used as an example to confirm that steric22 and n-electrOnic factors23 are major influences on the stability of regioisomers of oxidized fillerones and, more recently, the energetics and geometries of the local structures of both Q @ X o and C 6 o X e d , and C7oX clusters have been studied. This has led to the conclusion that a structure having even one C-F endohedral bond is less stable and that the differdnce in energy between C60Fendo and c60Fex0 clusters is 223 kl mol-1 . The study has also produced data indicating four stable isomers for C7oF.24 3. Preparations 3.1 Fluorination of C60 and C6OK70 Mixtures

Since only 24 groups can be attached to a spherical [6O]fillerene cage without being dacent, for many molecular groupings this is the maximum number of groups that will be linked to the fillerene cage. Such is the case when CH3 groups11 or large single atoms like chl0rin6~9~7 or b r o m i n e l 3 ~are ~ ~attached. When smaller atoms such as hydrogen are used, larger numbers of attachments can be anticipated. Indeed, hydrogenation is rather easy but the products are difficult to characterize definitively and mass spectrometry indicates only the partially hydrogenated species C6$-€2,29C&18 and w 3 6 , 3 0 all of which are unstable. Fluorine is also small and forms rather strong bonds with most other elements including carbon. Studies in which [60]fillerene was expored to elemental fluorine have shown that, unlike the hydrogen case, comprehensive adtakes place although the extent depends on experimental ~onditions.7~3~ Direct fluoriMion of pure [6O]fUerene to saturation at about 70 OC is slow and mass spectrometric data on the partially fluorinated products suggested particular stability for intermuiiates of composition c6$6 and c6@42. A 1% NMR spectrum ofthe ultimate off-white product after 12 days exposure exhibited a singlet at -150.5 and/or 153 ppm supQimposed upon a double-humped background, the humps being centred at approximately -144.4 and -158.0 ppm, in addition to a peak at -188.5 ppm indicative of the prsrrence of €IF. It was suggested that the singlet might be due to C&6o but that

112

other symmetrical species c6@6, c6OF12 or c6@20 could also be responsible for the spectnun.31 Subsequently, this singlet has been observed independently by other researchers and attributed to a highly-symmetric fluorofbllerene other than C60F60*32 and l% solid state NMR spectra on the products of the fluorination of C6o and C6dc70 mixtures have been shown to exhibit one broad line centred at -70 ppm//CF3COOH which, again, coincides with the values found in the solution studies (i.e. -70 + -78 = -148 ~ p m ) . ~Very 3 recently, the I9F NMR spectrum of the product of the fluorination of [60]hllerene with KrF2 has been found to be similar to that of the products of direct fluorination and, in particular, shows the sharp singlet at -151 ~ p r n . ~ The earliest study on the fluorination of fbllerenes was carried out on C60/C70 mixtures at room temperature.7 The reactions occur much more rapidly than in the [60]fhllerene case,31 probably because of the facile fluorination of [70]hllerene which is usually complete within a few minutes (see Section 3.3). Results varied, but fluorine uptake was estimated to yield products with F:C60 ratio >30 whilst fluorination of one sample of pure [60]fbllerene gave a weight increase corresponding to a ratio of 38.7 It is clear from mass spectrometric measurements of these products that C6oF36 has a particular stability which compares with that of the product of reduction, c6@36, in which it has been postulated that the stability derives from the formation of four highly delocalised benzenoid rings.3o Evidence of compounds with higher fluorine content in which the number of fluorines is always an even number (c6oF38 -k -+ c6@52+) were also apparent.7 The most intense peak in the products of fluorination of 70]fUllerene corresponded to that Of C7@40+ and masses attributable to higher fluorination products up to C7@46+ were also found7 Room-temperature fluorination by the same group using pure [60]fullerene32 has confirmed much of the earlier work?i namely that the fluorination process is slow, a major peak in the mass spectrum is associated with c6oF42, and a sharp peak is observed at -149 ppm in the 1% NMR spectrum as well as the singlet associated with HF. In addition, the presence of traces of C6@60 in unsublimed products was also detected. However, the infrared spectra associated with the final products of fluorination from the two groups differ 31,32 In one case31 a strong broad C-F stretching band at 1148 cm-1, prominent in the products of initial fluorination, diminished with hrther fluorination to yield sharp bands at 1060 and 1035 cm-l, and bands at 1067 and 1027 cm-1 were correlated with the species giving rise to the l% NMR signal at -150.5 ppm. In the second,32 a strong broad absorption at 1165 cm-1 associated with the C-F stretching vibration was observed, which shies to lower frequency 0s fluorine uptake

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increased. The lowest frequency band observed was 1130 cm-l which is ca. 100 cm-l higher than the towest frequency observed by the first group. Efforts to synthesize filly fluorinated [60]fillerene in high yield have continued. Attempts in which extreme physical conditions have been applied have resulted in the break-up of the [60]hllerene sphere with the production of perfluoronaphthalene and perfl~orodecalin.3~ The use of the halogen fluorides, ClF3, BrF5 and IF7 have shown that IF7 does not react in the gaseous state while ClF3 and BrF5 act as both halogenofluorhating agents as well as fluorinating agents. However, the number of halogen atoms in the products are lower than with fluorine alone, possibly because of stereoohemical hindrance. The products also contain significant quantities of oxygencontainiq species.2 Krypton difluoride is a more powerful low-temperature fluorinating agent than fluorine itself and should offer one of the best possibilities of producing perfluoro[60]fUllerene. However,mass spectrometry of the white product shows the most abundant specisr to be C6oF46. Cage-opened species up to c6oF78 are also present.4 This result is clearly similar to those obtained by the firther fluorination of a fluorohllerene with simultaneousU.V. radiation3 (see below and Section 3.2). It is now established that duect fluorination of [60]hllerene with elemental fluorine gives a variety of species up to C6oF60 but no single compounds have been Conolusively characterized and isolated. It is also evident that the nature of the [6O]fullc#ene affects the product. Some samples undergo surface fluorination such that unrG(Lctdd [6O]lllerene remains in the product. Although it has been suggested recently that fwlerenes containing only a few fluorines can exist other than as products of massspectrometric decomposition6 the only experimental evidence of such species has come from e.p.r, examination of CdSF5Cl mixtures in ten-butylbenzene or toluene which indicated the presence of C6oF formed presumably from decomposition of unstable 6&Fj (f.e. C60SF5 + SF4 + So far, chloro- and bromo-fbllerenes have proved to be too unstable to yield mass spectra but the fluorofillerenes have been successfidly extensively studied 1-395~7~8 For axample, the products of fluorination of the C6o and C60/70 (ca.9: 1) mixtures with fluorindhelium mixtures at different temperatures have been studied by both positive and negative ion mass spectrometric techniques. Although no pertluoro-derivatives were detected species of composition C6& and C70Fy x 5 48; y S 56 respectively, were o b m e d . In spite of the application of a range of different ionisation methods, and in partiaular the fact that neither plasma-desorption negative ion time-of-flight mass

114

spectrometry nor synchrotron-radiation photoemission studies provided evidence of no C6o- species with x > 48, suggested that such products are too labile to be detected or that C6#48 might represent the highest limit of fluorination.8 More recent data, however, has provided evidence for C6&0 in small yield,l and fluorination in the presence of U.V. radiation has produced evidence of being the major product with mass spectrometric evidence for C6@60+ at ca. 1.3% of the base peak being also present. Furthermore, the presence of the C60F2n+ series well beyond n = 30 was evident and fragments up to c6#76+ were discernable.3 The suggestions that c6@48 may be the limit of fluorination8and that it might also be the key intermediate in the process of hyperfluorination3 give it a special importance. The observation that it is produced in high yield in the fluorination of [60]fullerene in the presence of NaF has facilitated an interesting shift correlation (COSY) NMR study which suggests that C6@48 exists as an enantiomeric RR and SS pair as shown in Figure 1.36 The fact that b/e, d/e, e/g and f7?1couplings are not observed, however, have given some cause for concernp If the structure is indeed wniirmed, this observation of the production of essentially one enantiomeric pair out of many possible isomers indicates that steric and energy hctors are of major importance in determining what is obtained. Also interesting is the fact that the signals in the NMR for the only fluorines surrounded by three C-F bonds (fluorines e and f i n the Figure 1) are in the same region as the shift of the singlet attributed to either a m e t r i c a l c6@6, c6$12. c6@20, or c6@60 A U.V. photoemission measurement of CIS core level emission from a film of composition c6@46G 37 and X-ray photoelectron spectroscopic ( X P S ) studies of directly fluorinated and that fluorinated in the presence of U.V. lightl,8339 are in reasonable agreement with mass spectrometric investigations. In particular, quantitative measurements of the Fls, CIS and Ols X P S signals provide F:C ratios for different samples ranging from 28:60 to 53:60.39 A combination of ClS X P S and solid-state NMR studies show evidence that C-C bond breaking occurs increasingly as fluorination increases and CF;! and CF3 groups are f0rmed.~9 X-ray powder examination of fluorinated [6O]fiiUerene has shown there to be a mixture consisting of 60% of a fcc phase with u(fcc) = 16.677A and 40% of an hcp phase with ufhcp) = 11.85 A and c = 19.28 A.2 Related but independent studies havs also identified fcc lattices witha(fcc) = 17.1,'$O17.19 and 17.93A.41

115

ss

RR

Figure 1 Structure and labelling ofthe enantiomer pair for %#48

3.2 Hyperjluorination of [60]Fullerene The mass spectrometric identification of c6&n+ (n > 30) molecular ions indicates that the [60]fidlerene skeleton with no x bonds and 60 fluorine addends has given way to 'hyperaddition' in which rupture of skeletal 0-bonds has occurred. It is not evident at what point skeletal disruption takes place although it must be beyond the c6@6() composition and may be via reaction with a c6@48 intermediate species.3 The occurrence of 'hyperfluorination' means that holes are being opened up in the fbllerene cage. Indeed, it has been suggested that the hypeffluorinated compounds 'fill the gap between the presently known fluorocarbons of low molecular weight -C1 to C30- and fluoropofymers -C~OOOto C 10,000' and that the so-called 'buckysponges' and 'buckybowls' produced could have both novel properties and application^.^^ Fluorination of [60]fhllerene with fluorine in anhydrous HF at room temperature has also produced evidence for hypeffluorination.4 In this, the peak intensity maximum in the mass spectrum occurs at c60F46 and species up to C60F78 are evident (c.f. c6@48 and c6oF76 from the U.V. fl~onnation).~In both cases there is a significant

116

decrease in ion intensity above c6oF48. In one case this has been attributed to the more highly fluorinated species being cage opened,4 but in the other, comparison of the fragmentation products with those from the products of simple direct fluorination have shown a greater preponderance of odd-mass species on hyperfluorination, and has been taken to be indicative of closed-cage species being the origin. The argument is that an important pathway for decomposition involves loss of CF3 8s12 as well as F and CF2;12 an open-cage entity must have CF2 groups present initially which may be lost preferentially thus retaining an even mass, but a closed-cage species will always have an even number of fluorines present as C-F, and loss of F or CF3 followed by CFz will give rise to odd-mass fiagmentation.4 Little is Mitten about oxygen-containing species in the products of the U.V. fluorination of (2603 but it is evident in the KrF2-fluorinated products, despite exposure of the samples to air prior to analysis, oxygen derivatives are in only low concentration. This implies that the oxides are not a result of atmospheric oxidation. The possibility that oxygen incorporation may be dependent on the fluorination method3 may be correct; for example, oxygen contamination of the fluorine itself could be the origin. Krypton difluoride will be free of such impurity. However, the fact that the action of methanol with the product gave little change in the mass spectrum is surprising. The reason may be that the hypduorinated material, being largely cage opened, lessens the driving force for nucleophilic substitution which is so marked in the C6oF, (x I60) species4 It is also noteworthy that the NMR spectrum of the product of KrF2 fluorination in THF solution is similar to that found with the products of fluorination by elemental f l ~ o r i n e . ~ 3.3 Fluorination of [7O]Fullerene

Mixtures of [60]-and [7O]fullerenes fluorinate more rapidly than pure [60]fullerene (see Section 3.1)5-7J1,33 and it has also been demonstrated that pure [70]fulIerene fluorinates more rapidly than [60]fullerene, probably because of the better molecular packing in the latter.33,43 Differences in the time taken for completion oEthe reaction and the extent of fluorination are also affected by variation in the crystallinity of the f70]lllerene. Detailed mass-spectrometric examination of the pale pink solid fluoro[7O]fullerenes, produced by reaction of elemental fluorine with [70]fbllerene at ca. 70 "C for up to six weeks, show that C7oF48 is the most highly fluorinated species detected.5 This compares with C70F44,3~ C7(-~F46,~and C70F54~ found when C6dC70 mixtures are used. The most abundant species detected mass-spectrometridy

117

is C7oF385 which again differs from the C7#407 and C70F52~species which are detected in the products emanating from fluorination of mixtures. The 19F NMR spectrum of the products of fluorination of [7O]fullerene exhibits singlets at -152.36 and -153.03 ppm (intensity ratio 2: 1) with smaller singlets at -151.2, 151.79, and -153.65 ppm, However, there is little evidence for the broad background hump which is associated with the spectrum of the products of fluorination of [6O]tirllerene.5 The singlets have been assumed to indicate the presence of one or more products of high symmetry. The infrared spectrum of the product, deposited on a KBr window fiom THF solution, exhibits a broad peak at 11 12 cm-1 associated with a C-F stretching band, along with weaker bands attributable to C=O, OH and C-H which may be due to occluded solvent. Warming of the sample at 80 "C under vacuum, however, failed to remove these.

-

4. Chemlstry of Fluorinated C60 anil C70

The fact that the products of the reaction of [60]fuUerene with elemental fluorine react readily with nucleophiles including water with the evolution of HF43 suggested that, if the fluorination could be controlled to deliver individual species (with prdinaBly few fluorines attached), these would become vaIuabfe precursors in organic synthesis, The reason being that, although bromination provides the most readily controlled halogenation, the products are exceedingly insoluble. The fluoro-derivatives, by contrast, are soluble and more stable and easier, consequently, to manipulate. So far, failure to accomplish controlled or selective fluorination to yield fluorofullerenes of known stoicheiometry has been a serious limiting factor only mitigated by the claim36 that rather pure C6@48 can be produced. The replacement of fluorine on the [60]%llerene cage by a variety of carbon, oxygen and nitrogen nucleophiles, as well as by hydride ion, is certainly possible. lo The rate of nudeophilic substitutiondecreases with the degree of substitution of the fluorines on the cage, which is almost certainly due to the steady reduction in electronegativity of the cage as substitution takes place. Inevitably, strong nucleophiles such as diethylamine react vigorously and rapid reactions also occur with methyl-, vinyl- and ethylmagnesium bromides, with phenyl- and tris(trimethylsilyl)methyl-lithiu~and with lithium aluminium hydride being particularly quick, and resulting in instantaneous loss of fluorine. Substitution by weak nucleophiles such as acetic acid is very much slower and can take several weeks.lo In general, little is known about the stoicheiometriesof the products of such reactions, but chlorinated [6O]%llerene has been shown to react with sodium

118

methoxide to give a number of products up to C6o(oMe)34 Z7 and a similar broad envelope of peaks centred at 3.7 ppm are also observed in the proton NMR of the products of reaction of NaOMeMeOH with fluorinated C60.10 Sharp singlets whiah are indicative of symmetrical products are also found in the products of reaction of this same mixtUre with brominated (260.44 The observation of oxygen attached to fillerene cages is a characteristic of Merene chemistry. First observed in the mass spectrum of C~OO!~this was rapidly ~ o n f i r m e d ~and ~ 4c6002,46,47 ~ C600n (n = 3-5) 47,49 and C700m (m = 1-2) 47,50 have also been detected. Oxygenated species are also clearly evident in the fluorinated Illerenes. The presence of one or tw0,3,5,12~33 five,l and up to eighte.en%5,12 oxygens have been found in the products of the reaction of the fluorination of C m or c6dC70 mixtures. The species with the highest oxygen content, C6&2018, accounting for 58 site occupancies on the [60]hUerene ~ a g e , ~which , 1 ~ is the same as that in the case where up to five oxygens were found (i.e. C6oF4805, C6#5004, C&5,50). Fluorohalo-derivativesof [60]fullerene resulting &om the reaction of CW3 or BrF5 have also been inferred to contain up to 12 oxygen atoms2 The source of these oxygens is not clear but samples left in the open laboratory for some weeks exhibit only small spectral difference^.^^ Possible sources of contamination are oxygen bound to the surfaces of reaction vessels3 or accumulated from solvents used in their manipulation.5 The most recent study of the products of direct fluorination of [60]- and [70]fUorenes show the presence of molecules containing up to 11 or 16 oxygens but, after expoare to aqueous methanol, this rises to 18 in both cases. Assuming that oxygen is present 85 the epoxide, the maximum site occupancies of the species detected are 68 and 70 respectively.57'2 The observation that, although 0x0-species are present in the products of fluorination of [6O]hllerene with KrF2 [in particular C6&O (x = 18-32)] but that the yields are much lowe4 is interesting and was discussed in Section 3.2. The faot that a reduction in the concentration of oxides is observed on heating in vacuum to 70 "C for 3h suggests that either the 0x0-species are more volatile or that they deoxygenate on heating.4 Rather little progress has been made in understanding the reactions of the fluorinated kllerences with other compounds, not least because of the dficulty of knowing the composition of the fluorinated species. Reactions both of [60]fillerenc with fluorosulphuric acid and SbF5,52 and of [60]fiIllerene and fluorinated [6OlfiIUerene with SbF553 result in oxidation of a radical cation, although the nature of this species has not been identified. Similarly, although it is known that both fluoroalkyl iodides and

119

fluorodiacyl peroxides (Rf-C(O))-OC-Rf) react with [60]hllerene to give up to 16 radicals attached to the cage as fluoroalkyl derivatives54 the precise nature is not undenrto0d. Efforts to ascertain the precise stoicheiometries of the CF3 derivatives formed in the r d o n of fluorinated [6O]lllerene with Cd(CF3)2 have also fi1iled.5~ No reaction appears to take place between fluorinated[60]fullerene and either HOTeF5 or OsOq/pyridine mixtures.53 The more highly fluorinated [60] fillerenes have strong oxidizing and fluorinating properties. Thus C@44 - C6oF46 species liberate iodine from NaI solutions, oxidize i s o p r o p l to acetone and fluorinate aromatic compounds in the presence of acid catalysts such as BF3.Et20. Reaction of these fluorohllerenes with organofithiurn reagents gives rise to fluoroaromaticproducts and they also fluorinate ~ y r i d i n e . ~ ~

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53. A.K. Brisdon, J.H. Holloway and E.G. Hope, unpublished work. 54. P.J.Fagan, P.J. Krusic, C.N. McEwan, J. Lazar,D. Holrnes Parker, N. Herron and E. Wasserman, Science, 262 (1993) 404. 55. A.A. Gakh, A.A. Tuinman, J.L.Adcock and R.N. Cornpton, Tetrahedron Lett., 34 (1993) 7167.

Chapter 7 Idination, Brominrtion and Chlorination Roger Taylor

The introduction of iodine, bromine, or chlorine into firllerenes has been an important target because of the role of organic halides in synthetic organic chemistry. Some aieaiflcant results with [(iolfullerene have been achieved to date, but as in many other studies of fiUerene reactions, there are substantial problems to be overcome. SuBBciant work has been canied out so far, for it to be possible to reach the following concluim: i) The stabiity of the halogenofullerenes is the same as that generally found for organichalides, i.e. increases along the series: iodo- < bromo- < chlorolllerene (< fluorofbllerene). However, the halogenofidlerenes are significantly less stable than other -1 Wdes (and also aryl halides). One reason for this is the tendency to refonn a double bond which will be at least partly conjugated. More important however would seem to be that in order to avoid the bulky halogens being next to each other, halogenation bequently results in double bonds being introduced into pentagonal rings thereby increasing strain and destabilisig the structure. The stability does however increase with increasingn u m k of addends, a feature also found (in the absence of light) with hydrogenated hllerenes, and this may be attributed to the increased bond reorganisationrequired to regain the parent fullerene structure. ii) The solubitity increase in the same order as in (i). iii) The maximum numbez of halogens (excluding fluorine here) that can be attach& to the cage varies according to the size of the halogen. For bromination it is twenty4m,and in these perbrominated derivatives, no halogen atoms are adjacent; they are never closer than 1,3- to each other. For chlorination the maximum number is approximutely forty. What is not known is the mechanism of addition, though it is a reasonable asumption that radicals are involved, especially as the cage itself is such a strong electrophileand therefore reluctant to react with electrophiles.Considerablefurther work will be d e d to establish the halogenation mechanism (or mechanisms), and this is likely to be an important focus of research since it is possible that the mechanism may give clua as to how to control the addition pattern. 123

124

1. Iodination

Iodination of hllerenes has not been achieved. This is not surprising in view of the relative stabilities of halogenohllerenes compared to halogeno-organics in general, and the tendency of both iodoalkanes and iodoarenes to e l i n a t e iodine. Thus although no derivative with fbllerene-iodine bonding has been prepared (nor seems likely to be) intercalation of iodine into the fbllerene lattice has been a~hieved.l-~ For example, heating [60]hllerene with up to twelve equivalents of iodine under vacuum at 250 "C produced a product of approximate stoicheiometry C60.212 with iodine molecules intercalated into a face centred cubic array of [60]hllerene molecules, each llierene layer consisting of a simple hexagonal lattice.1 A similar experiment carried out at 130 "C using a large excess of iodine showed no reaction to occur during 20 h. However, W irradiation (Hg lamp) under the same conditions produced a product of approximate stoicheiometry C60.12. X-ray diffraction studies showed that the closepacked face-centred cubic structure of [60]hllerene is retained, with iodine atoms inserted between the hexagonal basal planes, producing increased spacing between the layers5 These intercalates slowly lose iodine upon standing, and tested down to 4 K, none of them show superconductingproperties. This may not be too surprising, given that iodine is an x-electron acceptor and [60]hllerene is highly electrophilic.

Figure 1 The staggered network of both [6O]fullereneand iodine molecules in the Ca.12.tolueneintercalate

125

An intercalate of composition C60.12.toluene (Fig. 1) is obtained on

recrystallising [60]fillerene from toluene in the presence of iodine.6 In this, the shortest [60]fullcrene-iodme distance is 3.09 A compared to a value of between 3.6 and 4.0 A calculated7 for the intercalate c60.212. A reasonable rationale for this is that toluene, being a xdonor, partly satisfies the electron demand of the iodine, thereby facilitating a closer aproach between iodine and the fillerene cage. 2. Bromiortion

Bromination of [60]fillerene by neat bromine at 20-50 "C was reported initially to result in the uptake of between two and four bromine atoms7 (based on weight uptake). Although this was subsequently found to be incorrect, the bromofillerene product was found to be thermally decomposed (at 150 "C) back into [60]fillerene, and to do so under mass spectrometric conditions. Therein lies one of the problems attendant upon research in this (and indeed many other lllerene areas), naqely the inability to detect the products by mass spectrometry. Circumstantial evidence can be obtained by replacing the bromines by other, less thermally labile groups, but this turns out to be difficult too because the bromo derivativestend to be very insoluble. This also rules out the use of 1% except solid state, though this latter has been precluded at present by the small amounts of material available. More recent studies9.10 showed that bromination of [60]fillerene with neat bromine produces a yellow-orange microcrystalline derivative having the approximate stoicheiometric composition C60Br25-2g. Since twenty-eight bromines cannot be attached to [60]fillerene in a symmetrical way, this suggested (especially in view of the intercalation of iodine) that the derivative was in fact C60Br24 with additional bromine trapped in the interstices of the crystals. This was confirmed by single crystal X-ray studies9 which revealed the Th structure shown in Fig. 2; the corresponding Schlegel diagram is shown in Fig. 3. The high symmetry of this derivative is evident in the simplicity of the IR spectrum (Fig. 4a).9 The particular sample of the derivativethat was used for the X-ray structure contained approximately one molecule of bromine for each C6oBr24 molecule, but with a disordered c o r ~ f i g u r a t i o n The . ~ ~ ~structure ~ of this derivative can be envisaged as consisting either of twelve hexagons, each with a 1,4arrangement of two bromine atoms (in a boat conformation), or of eight hexagons each with a 1,3,5-arrangement of three bromine atoms (in a chair conformation).Although the

126

Figure 2 Crystal StrUcNe for C@r24

Figure 3 Schlegel diagram for W r 2 4 ; w r g has same arrangement as emirledatoms

1.600 1,400 1.200 1,000 800 800 450 an-1

127

molecule has a double bond in each pentagonal ring, these latter also contain two $hybridised carbon atoms which thereby diminishes the strain that would otherwise resu1t.R studies indicated that at any time during heating at 150 "C for 24 h, the product consisted of [60]Werene and undecomposed C6oBr24, suggestingthat the bromine loss is a cascade process.11 The W A C name for C60Br24 is 1,4,7,10,12,14,16,19,22,24,27,29,31,33,36,38,41,43,46,49,52,54,57,60tetracosabromo-1,4,7,10,12,14,16,19,22,24,27,29,31,33,36, 38,41,43,46,49,52,54,57,60-tetraco~ydro-[6O]lllerene,the second half of the name being necessary simply because 'bromo' is a replacement term, and therefore is regarded as having replaced hydrogens (even though these were not there in the 6rst place). It would be unrealistic to imagine that practising lllerene chemists will not dispense with the second half of the name in general discussion and description at the very least. It is perhaps fortunate therefore that c60Bf24 is so insoluble that it may not feature much in lllcrene chemistry. Bromination of [70]lllerene under the same conditions also gives a product of approximatecomposition C70Br28, but the structure is not known.11 It may be relevant ther&re that C7oBr24 is predicted to be the product of highest addition level.12 The bromo adduct decomposes on heating in the manner of the [60]lllerene derivative.11 Brotnination of [60]lllerene by bromine in either carbon disulfide or chloroform gives dark brown prisms of C6gBrg in 80 and 58% yields, respectively. The structure, determined by single crystal X-ray diffraction is shown in Fig. 5 , and the dispositions of the bromines is also encircled in the Schlegel diagram in Fig. 3. There are many ways in which the Brg pattern can be fitted into the Br24 pattern, because in both C60Br24 and C6gBrg, the relative configurationsof the bromines is the same. (NB. The set in Fig. 5 of ref. 11 has been outlined inCOKeCtly, but is shown correctly in Fig. 19 of ref. 13.). It is probable therefore that C(jgBr8 is an intermediate on the pathway to formation of C6oBr24. From inspection of Fig. 3 one can anticipate that there may be other stable intermediates in which the integrity of the Bt-8 pattern is maintained, such as C6@14 and C6oBr20 but none have so far been isolated. As in the case of C6oBr24, molecules of bromine are occluded in the C@r8 lattice, in this case two molecules for each molecule of C@rg, and the bromine is partially ordered. Fig. 6 shows the channels parallel to the c-axis, into which the bromine molecules fit. On heating, C6gBrg loses bromine and does so more readily than C6@r24 This relative stability can be attributed to the lower bond reorganisationneeded to re-acquire

128

Figure 5 Crystal structure for c6OBrg

Figure 6 Crystal structure OfC6@rf~showing the channels into which the bromines fit

the [60]fullerene structure. Like C60B1-24,C6oBrg is very insoluble in organic solvents which makes its use in synthesis less attractive than might otherwise be the case. Because C6oBrg is less symmetrical than C60Bt-24, its IR spectrum is more detailed (Fig. 4b). The IUPAC name for C6gBrg is 1,4,7,10,16,19,24,36-octabromo-1,4,7,10,16,19,24,36octahydro[60]fUllerene, but as in the case of c6oBr24, convenience is likety to dictate the deletion of the second part of the name.

Q

129

Bromination of [60]lllerene in either carbon tetrachloride or benzene yields magenta plates of c6oBf6 in 92 and 54% yields respectively. The structure of this derivative, determined by single crystal X-ray analysis, is shown in Figure 7, and the corresponding Schlegel diagram is shown in Fig. 8. Because of the eclipsing interaction between the two adjacent bromine atoms, the C(2)-Br bond length is greater (2.032A) than the average value (1.963 A) for the other C B r bonds. Here there is one molecule of bromine per C6oBr6 molecule, occluded in the lattice. Unlike the other bromo derivatives, C6@r6, the W A C name for which is 1,2,4,11,15,30-hexabromo-l,2,4,11,15,30-hexahydro[60]lUerene,is useklly soluble in organic solvents. Being the least symmetrical of the bromo derivatives it has the more complicated IR spectnun (Fig. 4c). It is also the least stable of the bromo[60]lllerenes and the crystals degrade on long standing at room temperature. On heating it loses all of the bromine and reverts to [60]lllerene, but in doing so it first rearranges to C6gBrg. A reasonable rationale for this rearrangement is that it is triggered by the eclipsing interaction between the cis bromines, which produce a succession of 1,3 bromine shifts, with additionalbromine being acquired from the solvate, as conjectured in Fig. 9.

Figurn 9 Possible mode of rearrangement of W r 6 into C&g, initiated by the 1,2-bromo eclipsing interaction,followed by addition of lattice-occludedbromine

The structure Of C60&6 is important in relation to both the structure Of c6@r(j, various aryl derivatives, and the stabilities of radicals derived from lllerenes (Chap. 13). The latter in particular differ only in that there is no group attached to C(2), the consequent radical being delocalised around the pentagonal ring. Such radicals will be strongly stabilised by C-C-hyperconjugation,14 a phenomenon which will be extraordinarily important for kllerenes because the appropriate adjacent orbitals are enforced into coplanarity, with no possibility of deviating from that conformation, Fig. 10. However, C-Br hyperconjugation is insignificant, and thus if C60Bry is formed

130

along the pathway to C60Br6 it cannot be stabilised and therefore necessarily acquires an additional bromine, even though this results in unfavourable 1,2-eclipsing interactions.

Figure 10

Steric facilitationof C-C hyperconjugation in lllerenes

The stabilities of the brominated [60]fbllerenes have also been calculated by the AM1 method which indicates that some C6gBr4, C60Br10, C60Eirl2, and C6@r18 derivatives should also be isolable.

*

3. Chlorination There is every indication that chlorination is Likely to be, at least in the short term, the most important of the halogenation reactions of hllerenes. Success has already been achieved in controlling the addition in the manner of bromination, but with the allimportant benefit of solubility of the derivatives. An early report of chlorination of a mixture of (601-and [70]&llerene by chlorine gas at 250 "C indicated the uptake of twenty-four chlorine atoms per molecule of fbllerene mixture, giving a light orange product, soluble in many organic solvents.7 No mass spectrum could be obtained because of the ease with which the chlorine is lost, and separate experiments showed that under argon at 400 "C all the chlorine is lost. This is however a higher temperature than that needed for debromination and demonstrates the higher stability of the chloro derivatives. Other workers reported that the reaction of liquid chlorine at cu. -35 O C with [60]&1lerene produced a brown solid of approximate composition C6oC112 and that this dechiorinated on heating 200-350 OC.1O This thus shows a parallel with the results for the bromo compounds, whereby the derivative with the higher degree of addition is the more stable one. No stmctural characterisationof the compound was possible, and a mixture of product may have been formed. Nevertheless. in view of the results described below it seems quite probable that a major component has two sets of six chlorines arranged around polar pentagons, in the manner found for CLAClL Chdnwl

131

Photocatalysed chlorination by chlorine, of [60]fbllerenein CC4 during 1 h gave a crystaIiine yellow product that gave elemental d y s i s corresponding to ~ 6 0 ~ 4 0 . 1 6 Given that the stability of the structure C6oC136 should parallel that for C6oH36 (see Chapter l), and c6Oc42 should also be stable since it can have three isolated and highly delocalised benzenoid rings, the product may be a mixture of these two isomers. The lack of h e detail in the IR of the product16 confirms this belief. [7O]Fullerene was also chlorimted under these conditions, but no characterisation of the product is yet available.16 At the time of writing, the C6oCk derivative is the first to have been isolated and char-. It is readily made by addmg a solution of iodine monochloride in benzene to [6Olfidlerene in benzene, and then allowing to stand at room temperature for 3 days. Removal of solvent and evolved iodine under vacuum then leaves an orange microcrystalline solid in quantitative yield. The 3-day reaction time is required to allow all of the iodine monochloride to decompose, this being necessary since it interfers with the work-up procedure which consists of washing with pentane and then heating to 50 "C under 0.1 mmHg vacuum. If toluene is used as a solvent, the same product is obtained, but reaction is slower, indicating that a radical mechanism is involved, the retardation resulting fiom toluene acting as a radical scavenger.17

132

c6Oc16 is very soluble in benzene, carbon disulfide, and carbon tetrachloride, moderately soluble in chloroform, dichloromethane, and toluene, and slightly soluble in acetone, diethyl ether, hexane and pentane. The IR spectrum (Fig. 11) showed strong similarities to that for C 6 @ ’ 6 (Fig. 4c), indicating a similar structure. The crystals are too small for X-ray crystallographic analysis, but the compensating solubility has allowed the 13C NMR spectrum to be obtained. The Schlegel diagram for c6@6 shows the molecule to have a plane of symmetry, with two &-hybridised carbon atoms, C(55) and C(60), and two sp3-hybridised carbons, C(l) and C(2) on the symmetry plane. All ofthe other carbons are paired due to symmetry and so for the anticipated c60c16 structure, the spectrum should consist of four peaks in the q 3 region with relative intensities, 2:2:1:1, and twenty-eight lines in the sp2 region of intensities 26 x 2, and 2 x 1. The spectrum obtained agreed precisely, and Fig. 12 which shows the sp3 region of the spectrum and an expanded part of the sp2 region, showing the two peaks of half intensity. The evidence thus very strongly indicates that C60Cl6 is isostructurd with C6oBr6, and its W A C name is thus 1,2,4,11,15,30-hexachloro-l,2,4,11,15,30hexahydro[60]fhllerene

w 148.4 148.2 1480 147.8 147.61147.4 147.2 147.0 148.8 148.6 146.4 146.2

6

I

I

80

70 6

60

Figure 12 Part of the 13C N M R spectrum for C60Cl6, and the proposed structure

133

If chlorination of [6O]lllereneis continued firther, then replacement of chlorines by phenyls (which permits mass spectroscopic analysis) gives [email protected] It is reasonably certain that C6oC112 is thus produced, but the location of the chlorine$ is not yet k n m . Given that the pattern of addition of the first six chlorines is known, it seems most likely that there are two sets of six chlorines in the same pattern as shown in Fig. 12, with one of each group on opposite sides of the cage. This will give three different isomers in which the eclipsed chlorine pairs are either parallel, or at angles either of 72' or 1 4 4 O to each other. Chlorination of [70]fUllerene under the same conditions that produce C60Cb gives C7&l10, the structure of which is believed to be that shown in Fig. 13, in which two chlorine atoms are on adjacent carbon atoms.lg A most interesting feature of this result is that the chlorines straddle the equator of the molecule, and occupy sites predicted by theoretical calculations (see Chap. 4). The unique advantage of this addition pattern is that there are double bonds in any pentagon, thereby making the derivative particularly stable. An interesting feature of this molecule is that it is possible to selectively replace the chlorine atoms (in symmetrical pairs) by aryl groups (see Chap. 8) and this thus constitutesthe first example of selective reaction of multiple addends on a lllerene cage. Chlorination of [60]fbllerene in chlorosulphonic acid gives evidence for the initial formation of the fillerene-chlorine chloronium ion (C6oC1+).20 Further chlorination gives a product which contains (as judged by the IR spectrum), a substantial proportion of

Figure 13 Schlegel diagram for qoCl10

134

References 1. Q. Zhu, D. E. Cox, J. E. Fischer, K. Kniaz, A. R McGhie, and 0.Zhou, Nafure,355 (1992) 712. 2. M. Kobayashi, Y. Akahama, H. Kawamura, H. Shinohara, H. Sato, and Y . Saito,

Solid Stale Commun.,81 (1 992) 93. 3. Y. Maniwa, T. Shibata, K. Mizoguchi, K. Kume, K. Kikuchi, I. Ikemoto, S. Suzuki,

and Y. Achiba, J. Phys. Soc.Jpn., 61 (1992) 22 12 4. 0.Zhou and D. E. Cox, J. Phys. Chem. Soliak, 53 (1992) 1373. 5. T. Zenner and H. Zabel, J. Phys. Chem., 97 (I 993) 8690. 6. P. R. Birkett, C. Christides, P. B. Hitchcock, H. W. Kroto, K.Prassides, R. Taylor, and D. R. M. Walton, J. Chem. Soc.,Perkin Trans. 2, (1993) 1407. 7. G. A. Olah, I. Bucsi, C. Lambert, R. Aniszfeld, N. P.Trivedi, D. K. Sensharma, and G. K. S . Prakash, J. Am. Chem. Soc., 113 (1991) 9385. 8. R. C. Haddon, Acc. Chem. Res., 25 (1992) 127. 9. F. N. Tebbe, R. L. Harlow, D. B. Chase, D. L. Thorn, G. C. Campbell, J. C. Calabrese, N. Herron, R. J. Young, and E. Wassennan, Science, 256 (1992) 822. 10. F. N. Tebbe, J. Y.Becker, D. B. Chase, L.E. Firment, E. R. Holler, B. S. Malone, P. J. Krusic, and E. Wasserman, J. Am. Chem. SOC., 113 (1991) 9900. 11. P.R. Birkett, P. B. Hitchcock, H. W. Kroto, R. Taylor, and D. R. M. Walton, Nature, 357 (1992) 479. 12. R. Taylor, J. Chem. Soc.,Perkin Trans 2., (1993) 8 13, 13. R,Taylor, Phil. Trans. R. SOC.Lo& A , 343 (1993) 87. 14. R. Taylor, Electrophilic Aromatic Substitution, (Wiley, Chichester, 1989), pp. 16-18. 15. J. B. Peel and R. G. Rothwell, Austr. J. Chem.,47 (1994) 13 1. 16. F. Cataldo, Carbon, 32 (1994) 437. 17. P. R. Birkett, A. G. Avent, A. D. Darwish, H. W. Kroto, R. Taylor, and D. R. M. Wdton,J. Chem. Soc., Chem. Commun., (1993) 1230. 18. P. R. Birkett, A. D. Darwish, H. W. Kroto, R. Taylor, and D. R. M. Walton, unpublished work. 19. P. R. Birkett, A. G. Avent, A. D. Darwish,H. W. Kroto, R. Taylor, and D. R. M. Walton, J. Chem. Soc.,Chem. Commun., in press, 20. G. P.Miller, M. A. Buretea, J, W. Swircewski, and J M. Mcconnackie, unpublished work (personal communication fiom G. P. Miller)

Chapter 8

Aryl Derivatives of Fullerenes Roger Taylor

There are various ways in which aryl groups may be attached to fbllerenes: directly or by means of l i g atoms. Direct linkage may involve either a single bond (arylation), or two bonds as a result of reaction with benzyne (arenylation). Linking atoms may be carbon (see Chap. 9) and also in principle 0, S, or N, though no derivativesof this type have yet been isolated and characterised. In the context of this book, reactions are classified in terms of additions to fbllerenes. However, the reactions of fullerenes with aryl groups f d also into one of the most important and best known classes of organic reactions, namely electrophilic aromatic substitutions. Fullerenes are very powefil electrophilesand are able to replace the hydrogens in aromatics, the reactions falling thus into the general category of alkylation by alkenes.1 This form of alkylation has been described asfullerenation,2 but since saturation of the fullerene cage necessarily occurs across the addition site filkrenylution is more consistent with standard nomenclature. Arylation may also be accomplished by using fbllerene halides, and here reaction is faster because the electropbilicity of the fullerene is increased.. 1. Arylation using the Parent Fullerene

The first description of the reaction of [6O]fullerenewith aromaticswas by Cooks and coworkers. They found that heating the hllerene with aromatics such as toluene,pxylena, or anisole in the presence of ferric chloride gave products which, when analysed (without further purification) by mass spectrometry indicated that up to six aromatic rings had become attached to the cage, apparently in the form of adducts.3 In the light of subsequent work (below) it seems likely that aryl and hydrogen groups had become 0bonded to the fbllerene. The correspondingreaction with benzene would only take place if bromine was present in the reaction mixture, no reason for this being then evident, but can now be recognised as being due to the need to increase the electrophilicity of the fullereneby halide formation, since benzene is a less reactive aromatic. Subsequently, the reaction of a mixture of [60]- and [70]fbllerenes with either benzene or toluene in the presence of aluminium chloride was described, mass 135

136

spectrometry of the reaction products indicating the addition of up twelve and sixteen aryl groups to the fUllerene.2 It was suggested that this process takes place via protonation of the hllerene by residual protons in the aluminium chloride, giving a fullerene cation, which as electrophile then attacks the aromatic ring. However, no protonation of [6O]fullerene could be achieved with the superacid, trifluoromethanesulphonicacid? and although PAICl4- (present in AICl3 that contains traces of water) is a stronger super acid, reaction also takes place in the presence of ferric chloride. A mechanism which involves direct coordination of the x-electrons of the fUUerene with the Lewis acid seems more probable. Moreover, [60]fillerene in oleum gives rise to radical cations,S so these may also be involved as electrophiles. Sufficient information is can be gleaned from the above work and that of the Sussex group for the following general conclusionsto be drawn: 1. The Lewis acid forms a complex with the fUllerene which then carries out dkyiation of the aromatic. 2. The bulk of the fillerene is such that substitution at puru sites of the aromatic will be favoured, probably exclusively. 3. The reactions follow the classic Lewis acid catalyst activation order i.e. AICl3 > FeCl3. For example, [60]fUllerene does not react with benzene in the presence of FeCl3, but does so in the presence of AIC13.2 4. The reaction is faster with [60]fullerene than with [70]fullerene.2,6 This is because [60]fUllerene is more electrophilic due to the greater strain in the molecule. This greater electrophilicity shows up in other reactions e.g. the reaction with cyclopentadiene,7 benzyne;6 reduction of [60]fullerene is also faster than that of [701i~erene. 8 5. The reaction is faster with more reactive ar~matics.~ This needs to be taken into account if hllerene soot is soxhlet-extracted with e.g. xylene, mesitylene etc. for here there is the possibility that Lewis acid sites on the glass surface may act as the catalysts, so degrading the quality of the extract (and also incidentally, giving erroneously high extract yields). A more detailed investigation of the reaction of [60]hllerene with bromine, benzene and ferric chloride (in this case the products being subjected to normal work-up procedures after heating under reflux for 24 h) showed (FAB mass spectrum) that a number of products were ~ b t a i n e d Derivatives .~ containing between five and twelve phenyl groups attached to the cage were evident in the mass spectrum, the relative intensities being: 2 : 11 ; 2 : 11 : 1 : 3 : 1 : 2. It is significant that peaks corresponding to

137

the species c6oph6 and C6oPh8 dominate in the spectrum in view of the results obtained in bromination (see Chap. 7). Metastableion studies showed also that each peak contains a contribution (at least) from fragmentation ions of higher phenylated species. Laser desorption mass spectrometry also showed that species up to C6Ophl6 are present, and that fragmmtationwith loss of a single phenyl group is a major process, so that species in this caae containing odd numbers of phenyl groups tend to predominate. The presence of oxygenated species were also detected for the first time, with up to two mygens being attached to each [60]fhllerene molecule. Further work involving HPLC separation of the products revealed the HPLC trace shown in Fig. 1.10 At the time of writing this has not been fully analysed, but some of the peaks have been characterised by EI mass spectrometry, and include C6Oph4 (Peak 7), C6@5H (Peak 81, C6$h5H3 (Pe* 9)s C6@k (Pe* lo), C6@h8 (Peak Il), C6Ophl2 (peak 12), and C60Phl6 (tail material) together with some derivatives containing up to two oxygens, or an oxygen and a methylene group, the latter believed to be abstracted firom solvent during the work-up procedure. Figures 2-5 show the mass Spectra for CfjflhgH, c6@&. C6@hl2, and C6@hSH3. I

9

loll

IIIIII I I

I

II

I 23 4-6

7

12

I

IntcnsiIy

20

40

60

80

tlmin-l

Figure 1 HPLC trace of the product of reaction of [dO]Nlerene with Br2, FeC13 and benzene

138

'I ',' Figure 2 Mass spectrum for C60Ph5H (showingtraces of oxygenated derivatives)

1

I

3

191

1106

k+l$OO ml z

Figure 3 Mass spectrum for c6OPh

d o 0 d o 0 ld00

139 3. Is.

a. 75.

n.

n. a. IS. 54.

6.

*. 35. 3).

18

mlZ

mlZ

Fipm S Mass

of CmPh5H3

The structures of most of these derivatives is not yet known, and analysis of them is an important goal in the search for the various addition patterns that [60]fUllerene is capable of. It would be reasonable to suppose that the structure of C6oPk is similar to

140

that shown in Fig. 6 (the structure of which is discussed fbther below), but with X replaced by a phenyl group, though preliminary evidence suggests that this may not be the case, due to the steric constraints arising from two adjacent phenyl groups.l* CrjoPh12 may then consist of two such assembliesof phenyl groups, but arranged around diametrically opposed pentagons. However, it is also possible that an octahedral pattern could be involved,12 though this is less probable in view of results which have been 40

Figure 6 Structure of the aryl compoundsC60PhsX

Figure 7 Possible structure for C60Ph502H

A1

141

obtained on the fbrther chlorination and arylation of CmCb (see sect. 1.2). The structure for C@b m y be related to that shown in Fig. 6, but without the aryl group A and the group X.A plausible stmcture for the derivativeC60Ph5H02 is that shown in Fig. 7, whwein addition takes place across the highly localised double bonds in the p e n t a g d rings. Though analysis of these products is a major exercise, it will provide a major insight into the intermediates that are formed on bromination and their subsequent substitution. It is not possible to determine the structures of these by mass spectrometry, and t h y are also too insoluble for 13C NMR analysis. These difficulties are circumvented in the phenyl derivatives which are more stable. 2. Arylstion using Preformed Halofuilerenea

Thus the reaction involving bromine, benzene, and ferric chloride is very complicated, at least fiom the analytical viewpoint. Likewise the reaction of polychforinated [6O]fbllerene with benzene in the presence of aluminium chloride p r o d u d a product believed to contain up to twentytwo phenyl groups.13 Much greater sucoess has however been obtained by reaction of C60Ch.j with aromatics.lo,ll Here the reactivity is such that only a trace of ferric chloride is needed to catalyse the reaction. (So much so that reaction can be triggered merely bystirringthereaction

142

mixture with a spatula that has been used previously for dispensing ICI; if a clean spatula is used then no reaction occurs.) With benzene the product is Cfj@hsCI, Fig. 4, X = CI, and this gives the mass spectrum shown in Fig. 8, and is the first that has been obtained for a lllerene with chlorine attached to the cage. Evidently, elimination of PhCl is energetically less favourable than the elimination of CI2 that occurs so readily fiom lllerenes possessing chlorine addends alone. The 13C NMR spectrum of this derivative is similar to that shown in Fig. 12 of Chapter 7 in that there are twenty-eight peaks in the & region of the spectrum (two peaks being of half intensity) and four peaks in the sp3region (peak height ratio, 1 : 1 : 2 : 2). This firmly supports the proposed structure. By reaction with triphenylphosphine in the presence of traces of water, the chlorine is replaced with hydrogen to give C60Ph5H., Fig. 6, X = H. l o Again a similar 13C TWR spectrum is obtained, and the lH NMR spectrum shows a singlet at 6 5.2. This technique has been used to prepare a range of aryl derivatives, the structures of which have been confirmed by a variety of NMR methods.l* For example, the reaction of C6oC16 with toluendfemc chloride followed by reaction with PPh3 gives mainly C60(4-M&&)5H as shown by the mass spectrum in Fig. 9. Some C60(4M&&)6 is also produced together with traces of C60(4-MeCgH&, this latter being obtained also from reaction of [60]lllerene with toluene and bromine in the presence of 1

111 9 0

1000 1100

143

ferric chloride, though here the main product seems to be C60(4-M&6&)4. Reaction of C6oCk with fluorobenzendfemc chloride followed by reaction with PPh3 as above gives C60(4-FC#&H, the product here appearing to be particularly susceptible to oxidation to the di-epoxide c$. Fig. 7. Likewise the analogous reaction with t-butylbenzene produced mainly C60(4-t-BuC&)5Cl (and the hydrodechlorinated derivative), thou@ again small amounts of products with six and seven aryl groups attached to the cage were evident. Reaction also takes place readily with thiophene to give a product with five 2-thiophenyl groups attached to the cage (the first heteroaromatic derivative of a fullerone), but the product is particularly susceptibleto monooxidation. Two-step procedures have also been used. Thus isolation of C60Ph~Cland then reaction with benzene gave C&& the mass spectrum for which was effectively identical to that in Fig. 3. Likewise, reaction with toluene gave C60Ph54-MePh (Fig. 10). If either [60]hUerene, or C60c16 are reacted with ICVCCb followed by reaction with bemadferric chloride, then C6fJPhl2 is obtained, the mass spectrum being similar to that shown in Fig. 4, except that the main component appears to be C6@hgHj, which provides firm evidence that there are two groups of addends present each having the general arrangement shown in Fig. 6. These are, for steric reasons, most probably on opposite sides of the cage, in which case three different isomers are possible, i.e accordingto whether equivalent groups are eclipsed or at angles of either 72' or 144' to each other.

1

144

Since the C-Si bond is much more susceptible to cleavage by electrophiles than the C-Hbond, then this would predict that alkyldesilylation would take place much more readily than alkyldeprotonation.14 Surprisingly therefore, the reaction of trimethylsilylbenzene, MqSiPh, with C6oCldferric chloride as above does not give the expected alkyldesilylation, yielding phenyl groups attached to the cage. Instead, substitution paru to the SiMe3 group occurs giving, after replacement of chlorine by hydrogen, C60(4-SiMe3C&)5H, (Fig. 11) and it is probable that this is a result of steric hindrance. This is an important result since cleavage of the SiMe3 groups by a variety of electrophilesis possible, leading to a potentially large range of derivatives.15

1150

i mlZ

Flgure 12 Mass spectrum of C70Ph 4

I

145

Reaction of [70]Mlerene that has been chlorinated by ICI, with benzendfec chloride gives a mixture of products including c70Phq, C7@hg, and C7@h10 which have been isolated and characterised by NMR and mass spectrometry; Figures 12-13 show the corresponding mass spectra. Figure 14 is the Schlegel diagram for C7Ophio and shows the n.0.e. couplings between adjacent phenyl rings. Like its decachloro precursor, (see Chap. 7, sect. 3) C7@h10 has no double bonds in pentagons.Therings E are prevented fiom rotating at room temperature due to steric interactions, and are the rings believed to be absent in C7@h& which therefore has one double bond in a

m/z

Figure 13 Mass spectra for C70Phg (above) and C7Oph10 (below)

146

Figure 14 Schlegel diagram for C7$'h,o showing n.0.e.couplingsbetween adjacent phenyl groups

pentagon. This bond is very reactive and readily undergoes addition.l6 A further notable feature of C70Ph8 is that it is readily oxidised to a tetra-oxide which undergoes thermal elimination of two molecules of carbon dioxide during EI mass spectrometry to give C&hg. l6 The product of this unique degradation has not yet been fully characterised, but possibly contains a seven-membered ring, arising from elimination of the two carbons at either end of the reactive double bond. Other phenyl derivatives are also present e.g., C70Ph14 but at the time of writing have not been isolated and characterised. It is possible to selectively insert other aryl groups into the positions occupied by the phenyl rings E in C70Ph10.~ 3. Arenylation

The reaction of fbllereneswith benzyne falls properly within the category of 2 + 2 cycloadditions, described in Chapter 12, but is included here also for completeness.The products of such reactions e.g. 1,2-benzo[60]fUllerene (1) are rather stable due to the presence of the benzenoid ring. Consequently there is much less tendency for the retroene reaction to take place during mass spectroscopicanalysis. As in the case of arylation, the ultimate goal must be to introduce substituents into the benzenoid rings so that fbrther derivatisation may be achieved. In the case of the benzyne adducts, electrophilic substitutionat the P-position of the benzenoid ring is to be expected due to strain and so the behaviour should parallel that of ag. benu>cyclobutene. 17 In order to minjmise strain

147

in the 4-membered ring, the x-bonds should be localised at the positions shown in (I), and this will affect the directing effects of the substituents in the benzenoid ring.

Because of the need to avoid having double bonds in the pentagonal rings of fuUerenes,l* transmission of the electronic effects of addends fiom one part of the cages to another is poor. This accounts for the multiple additions so common (and benzyne. [60]Fullmne adds up to six b e n y n e ~ , 1 9 ~the ~ *locations being unknown, but it may reasonably be assumed that they are at the octahedral sites. Addition of one benzyne unit has been shown by N M R analysis19 to take place across the 12-bond (1).

m/Z

Figure 15 Uass spectnun of the product of reaction of ['lO]fullerene with benzyne, showing that ten groups have been added

Under extended reaction times, [7O]tbllerene adds up to ten benzynes (Fig. 151, but the locations of the addends are not yet known.21 Addition across each of the ten equivalent 1,Z-bonds is not possible because of steric hindrance so some other sites must

148

be involved as weli, and it was predicted that the majority of the additions must necessarily across the 5,6-bond (see Chap. 1 for numbering) because of steric constraints. Two isomeric momoadducts of benzyne with [70]fullerene have been isolated. In contrast to all other work on [70]fbllerene (where the 1,Zaddition predominates substantially), addition appears to take place across the 5,6-bond and 1,2-bonds (see Figure 8, Chap. 1 for numbering) in almost equal amounts.2* The difference in the present work can be attributed to the fact that is that it is easier for the rigid benzyne addend to bridge a less curved region of the surface (the 5,6-bond) than it is to bridge a more curved region (the 1,2-bond). This result demonstrates a hitherto unrecognised aspect of fullerene chemistry, and further examples may emerge in due course. 1,I-Naphthyne, produced from the pyrolysis of 1,s-di-iodonaphthalene also adds to [(iO]fuUerene, to give the structure (2) identified by NMR and mass spectrometry Even more intriguing is the fact that ten such naphthyne units have been shown, by mass spectrometry to add to [60]fullerene, and it is difficult at present to see how this can be accomplished in view of the steric hindrance that is anticipated; one possibility could be that naphthynes add to the addends already on the ~ a g e . 2In~this work it was also found that two [60]fbllerene molecules add to a single naphthyne molecule and a structure with fusion across the 1,2- and 7,s-bonds was but this is sterically very crowded. Two alternatives deserve consideration, In one, addition takes place across the 1,2- and 5,6-bonds of naphthalene, and the other is structure (3) which seems especially favourable because strain in the 4-membered ring is reduced through use of the longer bonds of naphthalene, and also the naphthalene canonical is the one of lowest energy.

References 1. R.Taylor, Electrophiiic Aromatic Substitution, (Wiley, Chichester, 1989), pp. 202203, 2. G.A.Olah, I. Bucsi, C. Lambert, R. Aniszfeld, N. J. Trivedi, D. K. Sensharma, and G. K.S . Prakash,J. Am. Chem. Soc.,113 (1991)9387. 3. S.H. Hoke, J. Molstad, G. L. Payne, B. Kahr, D. Ben-Amotz, and R.G. Cooks, RapidCornrn. Mass Spectr. 5 (1991)472. 4. R.Taylor, unpublished work. 5 . G. A. Olah,I. Bucsi, R. Aniszfeld, and G. K. S.Prakash, Carbon, 30 (1992) 1203. 6. A. D. Darwish, unpublished work. 7. L. S.K.Pang and M. A. Wilson, J. Phys. Chem., 97 (1993)6761. 8. A. G.Avmt, A. D. Darwish, D. K. Heimbach, H. W. Kroto, M. F. Meidine, J. P. Parsons,C.Remars, R. Roers, 0. Ohashi, R. Taylor, and D. R. M. Walton, J Chem. Soc..Perkin Trans. 2, (1 994) 15, 9. R. Taylor, G.J. Langley, M. F. Meidine, J. P. Parsons, A. K. Abdul-Sada, T. J. Dennis,J. P. Hare, H. W. Kroto, and D. R. M. Walton, J. Chem. SOC., Chem. Commun.,(1 992)667. 10.A. 0.Avent, P. R. Birkett, J. D. Crane, A. D. Darwish, G. J. Langley, H. W. Kroto, R. Taylor, and D. R. M. Walton, J. Chem. Soc.,Chern. Commun., (1994)1463. 11. P. R. Birkett, A. G. Avent, A. D. Darwish, A. K. Abdul-Sada, H. W. Kroto, R. Taylor, and D. R. M. Walton, unpublished work. 12.R. Taylor, J. Chem. SOC.,Perkin Trans. 2, (1992)1667. 13.G.A. Olah,1. Bucsi, C. Lambert, R. Aniszfeld, N. J. Trivedi, D. K. Sensharma, and G. K.S.Prakash,J. Am. Chem. Soc.,113 (1991)9385. 14.Ref 1,pp. 432-433. 15.Ref. 1,Chapter 10. 16.P.R. Birkett, A. D. Darwish, H. W.Kroto, R. Taylor, and D. R. M. Walton, unpublished work. 17.R d . 1, pp. 77, 145, 150. 18.R. Taylor, Tetrahedron Lett. (1 991) 373 1 ;J. Chem. Soc., Perkin Trans. 2 (1 992)3. 19.S. H. Hoke, J. Molstad, D. Dilettato, M. J. Jay, D. Carlson, B. Kahr and R. G. Cooks, J Org. Chem. 57 (1992)5069. 20.M. Tsuda, T. Ishida, T. Nogami, S. Kurono and M. Ohashi, Chem. Lett. (1992) 2333.

21. A. D. Danvish, A. K. Abdul-Sada, G. J. Langley, H. W. Kroto, R. Taylor, and D. R. M. Walton, J. Chem. Soc., Chem. Commun. (1 994) 2 133. 22. A. D. Darwish, A. G.Avent, H. W. Kroto, R. TayIor, and D. R. M. Wdton, unpublished work. 23. S. H. Hoke,J. Molstad, S. Yang, D. Carlson, and B. Kahr, J. Org. Chem., 59 (1994) 3230.

Chapter 9

Phlleroids, MethanofuUerenes and Oxa-, &a-, and Sifs-Eomologues MauriZio Prato* and Fred Wudl#

*Di&mento

di Scienze Farmaceutiche, Universitri di Trieste, Piazzala Europa 1,34127 Triesta, Italy #Institute for Polymers and Organic Solids, Department of Chemistry and Physics, University of California, Santa Barbara, CA 93 106, USA

'zhir chapter describes the synthesis, characterization and properties of products derived from additiodinsertion of methylene groups (and their 0,N. and Si homologues) acrosahto adjacentcarbons of Qo (Scheme 1).

Scheme 1

2

1

X = CR,, NR,0, SiR, It is interesting to note that, so far, representativesof thii class are the only reported examples of insertion reaction into a fullerene bond (structures 2, Scheme 1). This means that this class of compounds is unique in that it gives isomers with potentially different

151

152 electronic properties (58 K electrons on the spheroid for addition products 1, 60 K electrons for insertion products 2). The reason for this peculiarity can be easily ascribed to the severe geometrical deformation which would occur upon insertion of more than one atom into a double bond of C a . As the vast majority of these studies have focused on the products of additiodinsertion of methylene groups to C a , most of the attention in the present chapter will be devoted to this class of compounds.

1. Fulleroids and Methanofullerenes

The formal addition or insertion of a carbene unit to a bond of c60 results in the formation of methanofullerenes 1 (Scheme 1, X = CR2) or fulleroids 2 (Scheme 1, X = CR2) respectively.'V2 Methanofullerenes are cyclopropane analogues, whereas fulleroids possess structures similar to Vogel's I , ~ ~ - a n n u l e n eIn s . ~principle, there can be two structurally different methanofullerenes, with the cyclopropane bond fusing a 6.6 or a 5,6 junction of C a (structures 3 and 4 in Scheme 2, respectively). Scheme 2

@ -

\

\ / ' /

3

@ \ / ' /

\

4

Analogously, there will be potentially two different fulleroids, with the bridge carbon spanning over two six-membered rings or over a five- and a six-membered rings of Cm (structures 5 and 6 in Scheme 3, respectively). The structural assignment to the reaction products, i. e., the distinction between 3,4, 5, and 6 is a very intriguing problem, and support from any spectroscopic means is of vital importance. In particular, I3C NMR spectroscopy plays here a fundamental role. The detection of the l3C resonance for the bridgehead carbons in a non-aromatic region can be diagnostic for addition products (3 and 4 vs 5 and 6). On the other hand, the distinction between 6,6 and 5,6 junctions (3 and 5 vs 4 and 6) relies on the correct counting of 13C resonance number. With identical R groups, adducts to 6.6 and 5.6 junctions have

153

different symmetries (CzVand Csrespectively) and thus different number of resonances (17 and 32 respectively).' A relatively simple clue to the structural assignment can be obtained when one R group is H. In this case, the lH-13C coupling constant between the proton and the bridge carbon is diagnostic of an sp2 or an sp3 carbon (160-170Hz for cyclopropanes, 140- 150 Hz for 1,6-methanoannulene~).**~*~ Scheme 3

6

5

The most common way of addinghnserting substituted methylene groups is based on the reaction of Go with diazomethanes (other methods will be reviewed later in this chapter)?V6

1.I. Diatomethane Diazomethane is the simplest diazo compound. It reacts readily with c 6 0 to give the thermally unstable pyrazoline 7 in 44% yield (Scheme 4)? Scheme 4

P

A -

CH2N2 + c60

@

- N2

\

7

/ / 8

/

154

This compound is the result of addition of the 1,3-dipole to a 6.6 ring junction of C a . When a toluene solution of 7 is heated to reflux, the parent fulleroid C61H2(8) is recovered almost quantitatively (Scheme 4). The same product is also formed by thermal decomposition in the solid. The 1H NMR spectrum of the dihydrofulleroid 8 shows two doublets (with coupling constant J = 9.7 Hz) at 2.87 and 6.35 ppm in a ratio 1:l; one hydrogen is significantly deshielded reIative to the other (A6 = 3.48 ppm). The coupling constants between the '3C-labeled bridge carbon and the two methylene protons are: J(13C-Ha) = 145.0 and J('3C-Hb) = 147.8 Hz.The asymmetry of the methylene group, the 13C-H coupling constants, the absence of sp3 carbons in the 13C NMR as well as the number of carbon signals, corroborate the 1.6-fulleroid structure with insertion into a 5,6 ring junction and a Cs symmetry? When 7 is irradiated rather than thermolyzed, a mixture of 8 and another product is obtained, to which the methanofullerene structure 9 has been attributed (in this case the two protons give a singlet at 3.93 ppm, their J('3C-H) = 166.5 Hz,characteristic of cyclopropanes, the number of signals in 13C NMR supports a CzV symmetry, with bridgehead carbons at 7 1.0 ppm).* Scheme 5

N2

7

9

The two compounds 8 and 9 are stable and do not give any interconversion, either by irradiation or thermolysis.8 Accordingly, it was calculated that the interconversion barrier should be relatively high?." 1.2. Substituted Diazomethnes

The addition of substituted diazomethanes to Cm gives more complex mixtures. For example, (4-methoxypheny1)diazoethane reacts with Cm affording a mixture of three isomers. They have not been isolated, but spectroscopic studies on the

155

chromatographically pure mixture (1H NMR, 13C NMR, HETCOR)have allowed the attribution of structures 10-12 to the three isomers (Scheme 6).l In particular, HETCOR analysis shows that the methyl protons at 2.52 ppm (in 10) are connected to the bridge and bridgehead carbons resonating at 46.96 and 80.98 ppm respectively. In the other two isomers, the methyl protons at 1.32 and 3.19 ppm couple with the bridge carbons at 56.06 and 52.72 ppm respectively and the bridgehead carbons at 141.50 and 141.67 in the order. The relative stereochemistry of 11and 12 (methyl on 6or 5-membered ring) has been assigned on the basis of ring current effects (see below). Thermal conversion of 11and 12 to pure 10 can be achieved by heating a toluene solution of the three isomers to reflux.' This is in agreement with teoretical calulations which have shown that methanofullerenes are thermodynamically more stable than f~lleroids.'.~*"A reason for this difference in stability has been attributed to the violation of Bredt's rule, as bridgehead carbons in fulleroids possess a formal sp2 ~haracter.~ Scheme 6

@ @ ' @ ' ' -

/ /

/ /

\

\ /

/

10

\

/

11

\

/

12

Ar = rtmethoxyphenyl

The formation of a mixture of isomers is usually observed in the addition of diazo compounds to C a , including a-diazoketones2 and a-dia~oamides.'~In some cases, the mixture of isomers has been subjected to HPLC and the separated compounds have been characterized. In all cases, the isomers formed possess exclusively the general structures 3 or 6 (Schemes 2 and 3). Structures 4 and 5 (Schemes 2 and 3) have never been observed. Based on a simple valence bond rationale, this energetic preference (3vs 5 and 6 vs 4) has been attributed to the general propensity of fullerenes (and C6o in particular) for structures that avoid the presence of double bonds in 5-membered It has in fact been suggested that double bonds in 5-membered rings increase the strain already pment in 5-membered rings adjacent to aromatic rings.15The contribution of this electronic configuration to the stability is so strong that, contrary to what has been found

156

for 1,6-methanoannulenes,there is no electronic effect in fulleroids and methanofullerenes. In 1,6-methanoannulenes electron-withdrawing groups (e. g., cyan0 groups) favor the cyclopropane form, whereas weak donors (e. g., methyl groups) favor the open On the contrary, in the case of methanofullerenes and fulleroids, even with two strong electron-donating groups, such as two methoxy groups, only the methanofullerene form has been detected (3,R = R' = OMe).'* Conversion to the thermodynamically more stable component (from 6 to 3) can be successfully achieved in most cases by heating the isomeric mixtures. The isomerization process can also be performed electrochemically. Upon accepting the third electron in solution, fulleroid 13 is converted to the corresponding trianion of methanofullerene 14. Bulk electrolysis allows the quantitative conversion of 13 to 14 on a macroscopic scale.16 Scheme 7

C0OCH-j CH30-f&

H J y

@ @ \

\ / ' /

13

\

\ / ' /

14

1.3. Applications

The addition of diazo compounds to Cm has proven very efficient and versatile. For instance,diary1 methanofullerenes 15 have been pEpared with many different substituents on the phenyl rings (Scheme 81.5 Methanofullerene 15h was further elaborated in order to prepare a water-soluble fullerene derivative. On the basis of model building studies, it was indeed found that the fullerene sphere fits the mostly hydrophobic active site of the HIV- 1 protease (HIVP).I7 It was also suggested that a fullerene derivative might inhibit another HIV enzyme, i. e., reverse transcriptase. Design considerations required that the specifically desi ned fullemne derivatives have polar functional groups at one end of the carbon cluster.' For any experimental confirmation of this pEmise, a water soluble Cm derivative was needed.

!

1 57

Scheme 8

15 a, R = H; b, R = CH,; c, R = OCH,; d, R = Br; e, R = N(CH3),; f, R = COOPh; g, R = OH; h. R = (CH,)zNHC(O)CH,

Thus, the amide groups in 1% were hydrolyzed and the resulting amino groups were acylated with succinic anhydride.'* Bis-acid 16 (Scheme 9) was found to inhibit acutely and chronically HIV-1 infected human peripheral blood mononuclear cells (PBMC) with an Ecso of 7 pM,while showing no cytotoxicity in uninfected PBMCt9 as well as whole animals (rnice)?O Scheme 9 HOOCCH2CH2CONHCH2CH2

CH2CH2NHCOCH2CH2COOH

16 The computational work, but especially the experimental results, generated great excitement on the possible biological applications of fullerene derivatives. Methanofullerenes containing amino acids 1712and 1813were soon prepared, and also a

158

pentapeptide was attached to Cm (Scheme lo)?' Fulleropeptide 19 was found to possess both the fullerene and the peptide properties, with a reciprocal weak influence of the two moieties.2' Scheme 10

18

17

CO(L-Ala-Aib)2-L-Ala-OMe

H

19

Ala = alanine Aib = a-aminoisobutyric acid

Dihydroxymethanofullerene 15g (Scheme 8) is a rather versatile intermediate in the preparation of Ca-containing materials. By reaction of 15g with sebacoyl chloride or with hexamethylene diisocyanate, the two charm-bracelet-type polymers 20 and 21 respectively (Scheme 11) were synthesized and characterized.22 Cyclic voltammetry and UV-Vis spectroscopy showed a remarkable retention of the electronic structure of the C6o moiety.22 In a related work, the two phenolic groups in 15g, located at the surface of the functionalized fullerene, were covalently attached, via bromide displacement, to a fourthgeneration d e n d r i m ~ This . ~ ~ resulted in a well-defined macromolecule containing a

159

fullercne unit with a greater control over the formation of polymer-fullerene hybrid structures.

Scheme 11

f

O(CH&X$-n

(-NHC

20

21

A sort of dimeric methanofullerenes (23 and 25) can be obtained by reaction of Go with bis-diazocompounds 22 and 24 (Scheme 12).24

vN2 Scheme 12

c60

__c

N2

/

22

23

c60

N2 24

25

160

Compounds 23 and 25 can be considered basic units of "pearl necklace" polymers, in which, contrary to 20 and 21, the methanofullerene becomes part of the polymer backbone. Reaction of Cm with diary1diazo compound 26, bearing a benzo-18-crown-6 moiety25 (or a benzo-15-crown-5)14led to the preparation of methanofullerene 27.

Scheme 13

c60

@ \

26

\ / ' /

27

This latter product was successfully used to exploit the possibility of analyzing fullerene derivatives by elecaospray ionization mass spectrometry. It is also expected that 2725(and its benzo-15-crown-5 a n a l o g ~ e )may ' ~ form host-guest complexes with selective binding of metal ions. 1.4. Other Approaches to Methanofullerenes It is clear from Sections 1.1 and 1.2 that fulleroids and methanofuIlerenes are formed by nitrogen extrusion from the pyrazoline intermediate, in turn originated from cycloaddition of the diazo compound to the most reactive double bond of Cm, i. e., the 6,6-ring junction. From a literature s e a r ~ h , ' ~it~ is * ~easy to realize that all reactions involving c60 occur at this site, and that 5,6-adducts (fulleroids) are formed only as rearrangement products (see also Section 3, below). Based on this premise, it is also reasonable to assume that, any reactive carbene will attack the same double bond of C a to give addition products (1 + 2 cycloaddition) exclusively at the 6,6-ring junction, i. e., methanofullerenes.

161

Sugar diazirines, considered precursors of nucleophilic or amphiphilic carbenes, have been successfully added to Cm. giving rise to fullerene sugars (Scheme 14).*' These products are considered suitable substrates for the investigation of biochemical and pharmncological properties of fullerenes. Scheme 14

28

R = CH2Ph R = COC(CH&

Dichlorocarbene was produced by pyrolysis of sodium trichloroacetate and added to the 6.6-ring junction of Cm to give the CzVsymmetrical adduct 29 (Scheme

Scheme 15 C b C l

29

The reactive vinyl carbene 30,generated by thermolysis of a cyclopropene precursor, was allowed to react with Q to give methanofullerene 31 (Scheme 16)." Ketene acetal31 was easily hydrolyzed on Amberlyst 15 yielding the stable ester 32.It is inkresting to note that this [ 1 + 21 cycloaddition occurred when R = H or Ph, whereas a [3 + 21 cycloaddition was observed when R = Et, leading to a cyclopentene derivative of C60.1'

162

Scheme 16

30

x yo

4 0 R ’

32

31

The highly nucleophilic dimethoxycarbene can be generated by thermolysis of oxadiazole 33 and easily added to Ca affording the interesting methanofullerenone acetal 34.12.29 Any attempt to hydrolyze 34 to the parent ketone failed under a variety of condition^?^ The only characterizable compound formed in these attempts was the methyl ester 35. This ester was found to be reluctant to further transformations. While showing no reaction under different conditions, 35 gave oxidative decarboxylation to Cm when moce drastic conditions were applied.”

163

Scheme 17

33

34

35

A completely different way of preparing methanofullerenes has been reported by Bingel, via nucleophilic addition of a-stabilized carbanions to C60, followed by intramolecular substitution of the halogen (Scheme 17):'

Scheme 18

B 4 - COOEt

- NaBr, - H2

@ \ / ' \

36

/

37

The reaction, which occurs at the 6.6-ring junction of Ca, proved valid for a variety of nuclaophiles, generated by deprotonation of diethyl bromomalonate (36),methyl 2chloroacetylacetate, obromoacetophenone, and desyl chloride (and DBU as the base).30 The addition of dimethyl bromomalonate to C6o was used by Hirsch to study the regiochemistry of multiple additions to C a (see Section 1.Q3' The same reaction, applied to C70,has represented one of the very few examples of functionalizationof this other fullerene. The addition gave one predominant isomer.3o 1.5. Electronic Properties of FuUeroids and Methanofillerenes Fullemids and C a have 60 '#' electrons. and consequently, their electronic spectra are almost superimposable (Fig. 1). The saturation of a double bond in methanofullerenes

164

changes the original 60 x electron configuration of Cm into a 58 n electron array. A corresponding change in the visible absorption spectrum can be observed (Fig. 2).'+16

0.20

1

I

I

I I

'1 Y

0.20

;

I

I

1

I

I

h

0.161

D

b

'ii 4 !

Wavelength (nm)

Wavelength (nm)

Fig. 1 (left). Vis absorption spectrum of 13 (solid line) and Cs0(dotted line). Fig. 2 (right). Vis absorption spectrum of 14 (solid line) and C, (dotted line.). The maximum in the broad band between 450 and 650 nm in C a is hypsochromically shifted and the fiie structure near 600 nm is lost in methanofullerenes. Other characteristic features are a sharp band near 430 nm and a weak band around 700 nm. These latter two relatively weak absorption bands appear to be typical of all dihydrofullerenes, i. e., C6o derivatives with a saturated double bond.

***

1.6.Electrochemical Properties Diary1 fulleroids and methanofullerenes (15) retain most of the C6o exceptional electrochemical properties. Little difference is observed between the two classes of derivatives when subjected to cyclic voltammetry experiments. Both species experience a shift of the first reduction potential to more negative values of about 100-150 mV with respect to C a , irrespective of the substitutent on the aryl rings. It was also observed that fulleroids may rearrange to the corresponding methanofullerenes upon taking the third electron in solution (see above. Section 1.2).16 A particular case is represented by the spiro-annulated methanofullerenes 38 (Scheme 19). It was found that, contrary to the diary1 methanofullerenes 15, the cyclic voltammetric behavior of 38 is very sensitive to the presence of substituents on the fluorene ~ n o i e t y ? ~With , ~ ~electron-donating groups (38b) the reduction potentials are shifted to more negative values, even relative to 15.

165

Scheme 19

38

39

a,R=H b, R = N(CH,Ph), C. R = NO2 On the other hand, the first reduction potential becomes less negative if strong electronattracting groups are placed in the 9-fluorenyl moiety (38~).The most remarkable case is represented by 9-azafluorenyl-methanofullerene39, whose first potential wave is slightly less negative than C a itself (Table 1). The effect becomes more dramatic going to higher reduction potentials. The third and fourth steps for 38c and 39 are = 0.5 V less negative and thus these adducts are much better acceptors than Q.

Table 1. Cyclic Voltammetry of Cm, 15a, 38a-c, and 39 peak position

(mV)

compds

El

E2

E3

E4

cao

-228

-826

-1418

-1916

15a

-335

-920

-1470

- 1929

38a

-287

-857

-1077

-1559

38b

-376

-927

-1533

-2093

38c

-268

-658

-898

- 1447

-1872

39

-227

-675

-958

- 1460

-1969

E5

-2140

Conditions: Pt working and counter electrodes; scan rate, 100 mVk; AglAgCY3M NaC1, reference electrode; Ferrocene internal reference (+ 620 mV); 0.1 M TEiABF4 in THF.

166

This particular property of spiromethanofullerenes 38 can be attributed to their different geometry from that of diarylmethanofullerenes 15. The fluorenyl plane is held rigidly perpendicular to the surface of the spheroid, whereas in methanofullerenes 15 the phenyl rings are free to rotate, but prefer to lie “parallel” to the ‘‘R” surface.6 There is the possibility, in the case of spiro compounds 38. for a novel type of conjugation, akin to, but not the same as, spiroconjugation, which has been named “ p e r i c ~ n j u g a t i o n ” .The ~~~~ spatial arrangement of the molecular orbital may be responsible for a “through-space” interaction between the fluorenyl moiety and the spheroid, which thus becomes sensitive to electronic changes in the fluorenyl ~ o u n t e r p a r t . ~ ~ . ~ ~

1.7. Magnetic Properties of Fulleroids and Methanofullerenes In general, the functionalizationchemistry of Cm has offered the possibility of placing proton probes (absent in the original Cm molecule) around the surface of the fullerene. In this way, the elusive magnetic properties of Ca can be investigated. In this respect, fulleroids and methanofullerenes are derivatives of choice. As already seen in Section 1.1, fulleroid 8 shows one of the most remarkable examples of difference in chemical shift between two geminal protons (At3 = 3.48 ppm). This effect is clearly due to local ring currents on the fullerene surface. Recent theoretical investigations on Cm have assigned strong paramagnetic currents to 5-membered rings (5-MRs) and weak diamagnetic c m n t s to 6-membered rings ( ~ - M R s ) .Assuming ~ ~ - ~ ~ that the ring currents are also operative in the homorings of 8. the two protons can be assigned: the doublet at 2.87 ppm to the proton over a 6-MR, and the doublet at 6.35 pprn to the proton over a 5-MR.36 The case of methanofullerene 40 is even more suitable for confirming Pasquarello’s computational work.

40 Not only has 40 a proton a which lies exactly on a 5-MR of the spheroid, but also has a reference proton, i. e., a chemically equivalent proton b far from the magnetic influence of the fullerene surface which may serve as a comparison with proton a. The 1H NMR

167

spectrum of 40 fully confirms the paramagnetic properties of 5-MRs in C a . Proton a resonata as a doublet at 6.7 ppm, remarkably deshielded relative to proton b (4.6 p p n ~ ) . ~ ~

1.8. Multiple Additions and Relative Regiochemistry

The addition of more than one carbene unit to Ca can result in the formation of bis-, tris-, tetrakis-, pentakis- and hexakis-adducts. In the case of addition of diphenyl diazomethane.each adduct has been isolated and characterized. Due to the high number of possible regioisomers each chromatographically pure fraction consists of many components which give rather broad signals in IH NMR spectra. The hexakis adduct shows a somewhat sharper NMR pattern, where all the phenyl groups have become equivalent. The most symmetrical arrangment. the octahedral adduct 41. may be responsible for the simplified spectrum.6 Ph,Ph

PhAPh 41

The regiochemistry of addition has been studied in detail in the case of bromomalonate addition in basic ambient (see Section 1.4).31 Seven bis-adducts have been separated by preparative HPLC and characterized by UV-Vis. *Hand 13CNMR, IR spectroscopies. The assignment of the structure is based on a combination of symmetry and polarity reasoning. Surprisingly, the most symmetrical and less polar bis-adduct, with two addends at opposite poles of the sphere, is the minor component (and the least soluble) in the bis-adduct mixture. The low yield of this bis-adduct has been justified as the statistically less favored (one site of attack versus four equivalent sites for the other attacks). The isomerically pure bis-adducts were separately subjected to further reaction to give mixtures of tris-adducts. The major components in the relative mixtures were separated and characterized. Successive reactions and separations led eventually to the isolation of the pure symmetrical hexakis-adduct.

168

2. Fullerene Oxides The fullerene oxide, CcjoO (42), was isolated under two different oxidative conditions. An oxygenated benzene solution of C a was irradiated with UV-Vis light and the epoxide 42 isolated in 7% yield. Addition of benzil as sensitizer improved the yield of 42 and accelerated the oxidation mction?’ Scheme 20

42

43

In a different work, dimethyl dioxirane was employed as the oxidizing agent, and 42 was isolated along with a 13-dioxolane adduct>8 Both research groups discuss the smctute determination of the product and assign the 6.6 ring-closed (epoxide) structure 42 to the fullerene oxide based on the following: (a) the detection of a carbon resonance at 90-91 ppm, attributed to the epoxide carbons; (b) the number of carbon resonances (17), which is in agreement only with a symmetry compound. Recently, the X-ray crystal structure determination of an iridium complex of 42 has been reported. The interatomic C-C bond distance in the epoxide was determined as 1.478 A.39

CcjoO was found to be relatively stable, although it converts back to C a either during chromatography on neutral alumina3’ or in refluxing toluene?* Rather surprisingly, and in contrast to the case of fulleroids and methanofullerenes (Section 1.2) theoretical calculations give the 5,6 open isomer 43 slightly more stable than 42. lo The interconversion barrier between the two isomers was evaluated to about 60-75 kcaYmol, depending on the direction. As 5,6 open compounds (1,6-fulleroids) are probably produced only through nitrogen extrusion from a pyrazoline intermediate (Sections 1.1 and 1.2), it should be possible, at least in principle, to prepare the 5,6 open isomer 43 (so far never isolated) by controlled ozonization. The molozonide intermediate might loose oxygen and then, perhaps, give the more stable 5,6 isomer or, at least, a

169

CmO has been mixture of the two isomers. In a recent paper on the ozonization of indeed detected by mass spectrometry.40 CaOo (42) shows a UV-Vi spectrum very similar to that of the methanofullerenes(see Section 1.5). The cyclic voltammetry of 42 has been reported/* Although CmO gives only bversible reduction waves, differential pulse voltammetry and Osteryoung square wave voltammetry indicate that the first duction peak is shifted to less negative potentials relative to C a . This is unusual, as in most cases monoaddition derivatives of Ctjo are shifted by 100-150 mV to more negative values (cf. Section 1.6). The surprising electrwhmical behavior of CmO has been interpteted in terns of electronegativity of the oxygen atom?* The presence of oxygen has also been found to improve the rather poor amphiphilic properda of Qjo. Good quality Langmuir films of 42 were obtained, probably due to the formation of hydrogen bond interactionswith the water layer."

3. Azahrlleroids and Fullerene Aziridines

Nitrogen homologues of fulleroids and merhanofullerenes have been prepared via addition of aliphatic azides to A very intriguing result has been reported by Wudl and coworker^?^ Azidw 44 add to Qo in refluxing chlorobenzene to give two monoaddition products, A and 45. A is a compound of unsolved structure, that, upon additional heating, converts quantitatively into 45 (Scheme 21)?3 Based on a combination of NMR study (IH,13C and 15N). the azafulleroid structure was assigned to 45. Scheme 21

RCHzN3 44

a, R

= OCH2CH2SiCH3

45

b, R = C,H, R = 4-OCH3-C,H, d, R = 4-Br-C6H4 The major problem in attributing the correct structure to A was the detection (for Aa) of a 1% resonance as low as 159.8 ppm for two equivalent carbons bound to the nitrogen C.

170

bridge. This, of course, necessarily ruled out the possibility of a triazoline, the obvious intermediate in the 1.3-dipolar cycloaddition of azides to olefins. The cyclic voltammetry of 45 shows that the reduction potentials are shifted to more negative values relative to Cm, showing that the electronegative atom plays a minor role in this case (cf. Section 2). Azamethanofullerenes (or fullerene aziridines) have been obtained in the addition of azido formic esters to Cm (Scheme 22).''''-& Scheme 22

@ \

\ / '

R = COOCH2CH3 R = N-phthalimido R = C00-2,4,6-ui-t-butyl-phenyl

/

46

The contrast with the formation of azafulleroids reported above for the addition of alkyl azides is only apparent, as in the present case addition of nitrene species directly to a 6.6 ringjunction of C a may be operative.

4. Eullerene Siliranes

There is only one example, so far, of silylene addition to C ~ OTrisilane . ~ ~ 47, the silylene precursor, was irradiated in a toluene solution of c60. Purification by chromatography afforded 48 in 58% yield. The structural assignement of 48 relied mainly on l3C NMR spectroscopy. The number of signals is diagnostic of a ClV structure and a signal at 71.12 ppm was easily attributed to the sp3 carbons of the silirane 1ing.4~AM1 molecular orbital calculations on the reaction between diphenylsilylene and C a confirmed that the 6,6ring-closed is the preferred product, being much more stable than the 5,6ring-open. The UV-Vis absorption spectrum is typical of most monoaddition derivatives of Qo.

171

Scheme 23

DipzSi(SiCH,),

hv

c

Dip2Si:

c60

47 48

Dip =

The cyclic voltammetry of 48 is impressive in that the reduction potentials m shifted to more negative values even relative to diary1 methanofullerenes15.'" Peak potentials by DPV show that in 48 the presence of the electropositive silicon group facilitate the the electrooxidationprocess.4' 5. Conclusions

The products of additiodinsertion of methylene groups (and their 0, N, Si homologues) to carbon-carbon bonds of C6o represent a very fascinating class of derivatives. The difficulty in the c o m t interpretationof the structure. the possibility of isometization among these molecules as well as their different electronic properties make these compounds uncommonly interesting. Many questions still remain open, such as why the parent C61H2 fulleroid 8 does not isomerize to its methanofullerene analogue 9, or what ia b e structure of compound A in Scheme 21, or how long it will take to prepm all the possible isomers in each class of derivatives (e. g., 5.6 open oxide and 5,6silafulbmid have not yet been experimentally observed). There is currently intense effort directed at answering these and other questions in this very lively research fEld. 6. References

M. Prato. V. Lucchini, M. Maggini, E. Stimpfl, G. Scorrano, M. Eiermann, T. Suzuki and F. Wudl, J. A m Chem. Soc. 115 (1993) 8479. 2. L. Isaacs, A. Wehrsig and F. Diederich, Helv. Chim Acza 76 (1993) 1231. 3. E.Vogel, Pure Appl. Chem 54 (1982) 1015. 4. R. Amz, J. W.M.Carneiro, W. Klug, H. Schmickler, E. Vogel, R. Breuckmann and 1.

F . 4 . Kltlmer. Angew. Chem Inf. Ed Engl. 30 (1991) 683.

172

5 . F. Wudl, Acc. Chem Res. 25 (1992) 157. 6. T. Suzuki, Q. Li, K. C. Khemani, F. Wudl and 6. Almarsson, Science 254 (1991) 1186. 7. T. Suzuki, Q. Li, K. C. Khemani and F. Wudl, J. Am. Chem. SOC.114 (1992) 7301. 8. A. B. Smith 111, R. M. Strongin, L. Brard, G. T. Furst, W. J. Romanow. K. G. Owens and R. C. King, J. Am. Chem SOC. 115 (1993) 5829. 9. F. Diederich, L. Isaacs and D. Philp, J. Chem. Soc., Perkin Trans. 2 (1994) 391. 10. K. Raghavachari and C. Sosa, Chem Phys. Lett. 209 (1993) 223. 11. H. Tokuyama, M. Nakamura and E. Nakamura, Tetrahedron Letr. 34 (193) 7429. 12, L. Isaacs and F. Diederich, Helv. Chim. Acta 76 (1993) 2454. 13. A. Skiebe and A. Hirsch, J. Chem. SOC., Chem. Comm (1994) 335. 14. J. Osterodt, M. Nieger, P.-M. Windscheif and F. V(igtle, Chem. Ber. 126 (1993) 2331. 15. R. Taylor and D. R. M. Walton, Nature363 (1993) 685. 16. M. Eiermann, F. Wudl, M. Prato and M. Maggini, J. A m Chem SOC. 116 (1994) in press. 17. S. H. Friedman, D. L. Decamp, R. P. Sijbesma, G. Srdanov, F. Wudl and G. L. Kenyon, J. Am. Chem SOC. 115 (1993) 6506. 18. R. Sijbesma, G . Srdanov, F. Wudl, J. A. Castoro, C. Wilkins, S. H. Friedman, D. L. Decamp and G. L. Kenyon, J. Am. Chem. SUC.115 (1993) 6510. 19. R. F. Schinazi, R. P. Sijbesma, G. Srdanov, C. L. Hill and F. Wudl, Antimicrob. Agents Chemther. 37 (1993) 1707. 20. R. F. Schinazi, The Electrochemical Society Meeting, San Francisco, CA, May, 1994. 21. M. Prato, A. Bianco, M. Maggini, G. Scorrano, C.Toniolo and F. Wudl, J. Org. Chem. 58 (1993) 5578. 22. S . Shi, Q. Li, K. C. Khemani and F. Wudl, J. Am. Chem SOC. 114 (1992) 10656. 23. K. L. Wooley, C. J. Hawker, J. M. J. Frkhet, F. Wudl, G. Srdanov, S. Shi, C. Li and M. Kao, J. Am. Chem Sac. 115 (1993) 9836. 24. T. Suzuki, Q. Li, K. C. Khemani, F. Wudl and 0. Almarsson, J. Am. Chem SOC. 114 (1992) 7300. 25. S . R. Wilson and Y.Wu, J. Chem Soc., Chem. Comm. (1993) 784. 26. A. Hirsch, Angew. Chem, Int. Ed. Eng. 32 (1993) 1138. 27. A. Vasella, F. Uhlmann, C. A. A. Waldraff, F. Diederich and C. Thilgen, Angew. Chem., Znt. Ed. Engl. 31 (1992) 1388. 28. M. Tsuda, T. Ishida, T. Nogami, S. Kurono and M. Ohashi, Tetrahedron Lett. 34 (1993) 69 1 1. 29. W. W. Win, M. Kao, M. Eiermann, J. J. McNamara and F. Wudl, J. Org. Chem. (1994) in press. 30. K. Bingel, Chem. Ber. 126 (1993) 1957. 31. A. Hirsch, I. Lamparth and H. R. Karfunkel, Angew. Chem., Int. Ed. Eng. 33 (1994) 437. 32. F. Wudl, T. Suzuki and M. Prato, Synthetic Metals 59 (1993) 297.

173

33.

M.Eiennann, M. Prato, T. Suzuki, Q. Li, F.Wudl, R. C. Haddon and M. Maggini, to be published.

34. A. Basquarello. M. SchlUter and R. C. Haddon, Science 257 (1992) 1660. 35. A. Pasquarello, M. SchlUter and R. C. Haddon, Phys. Rev. A 47 (1993) 1783. 36. M.h t o , T. Suzuki. F. Wudl, V. Lucchini and M. Maggini, J. Am. Chem. SOC. 115 (1993) 7876. 37. K. M.Creegan, J. L. Robbins, W. K. Robbins, J. M. Millar, R. D. Sherwood. P. J. Tindall, D. M. Cox, A. B. Smith 111. J. P. McCauley, D. R. Jones and R. T. Gallagher, J. Am. Chem SOC. 114 (1992) 1103. 38. Y.Elemes, S. K. Silverman, C. Sheu, M. Kao, C. S. Foote, M. M.Alvarez and R. R. Whetten, Angew. Chem, Inr. Ed. Engl. 31 (1992) 351. 39. A. L. Balch, The Electrochemical Society Meeting, San Francisco, CA, May, 1994. 40. R. Malhotra, S. Kumar and A. Satyam, J. Chem SOC., Chem C o r n . (1994) 1339. 41. T. Suzuki, Y. Maruyama, T. Akasaka, W. Ando, K. Kobayashi and S. Nagase, J. Am. Chem SOC. 116 (1994) 1359. 42. N. C.Maliszewskyi, P. A. Heiney, D. H. Jones, R. M. Strongin, M. A. Cichy and A. B. Smith 111, Langmuir 9 (1993) 1439. 43. M. Prato, Q. Li, F. Wudl and V. Lucchini, J. Am. Chem SOC. 115 (1993) 1148. 44. T. Ishida, K. Tanaka and T. Nogami, Chem Lett. (1994) 561. 45. S. Kuwashima, M. Kubota, K. Kushida, T. Ishida, M. Ohashi and T. Nogami, Tetrahedron Lett. 35 (1994) 437 1. 46. M. Banks, J. I. G. Cadogan, I. Gosney, P. K. G. Hodgson, P. R. R. LangridgeSmith and D. W.H. Rankin. J. Chem SOC., Chem C o w . (1994) in press. 41. T. Akasaka, W. Ando, K. Kobayashi and S. Nagase, J. Am. Chem. SOC. 115 (1993) 1605.

Chapter 10 Cycloadditions to c60 Mark S.Meier Chemistry Department, University of Kentucky, Lexington 40506-0055,USA This chapter describes the cycloaddition chemistry of C a and C70 and some further chemistry of the cycloadducts. Cycloadditions'-* are widely used in synthesis for the construction of a wide variety of carbocycles and heterocycles. These reactions are particularly attractive for modification of fullerenes, since the two bonds formed between the adduct and the fullerene will be in well-defined positions relative to each other. There are 23 possible isomers for a simple disubstituted fullerene such as C d 2 p yet only two isomers would result from most conceivable cycloaddition reactions. The two most likely such isomers correspond to addition across one of the double bonds of & (the 1,2 positions, the fusion of two six membered rings) or addition across a diene unit (the 1.4 positions). Direct addition across two carbons in the same five membered ring are unlikely (and unobserved), since these bonds are best characterized as single bonds. The superalkene structure of fullerenes (Figure 1) presents a number of opportunities for cycloadditions. There are several questions to be answered here. First, in what types of cycloadditions will Cm participate? Second, will C a participate as a 2n component or a 4n component? The answer to the first question is that fullerenes react in many different types of cycloadditions, the answer to the second seems to be that fullerenes react preferentially as 2n components.

Figure 1. Buckminsterfullerene (c60.1). The back side of the sphere is not shown.

Cyclopentadiene The reaction of Qo with cy~lopentadiene~~~ was among the first reported chemical reactions of a fullerene. Diels-Alder reactioris normally occur between electron deficient dienophiles and electron rich dienes, and low dienophile LUMO energies facilitate the reaction. In practice, the Diels-Alder reaction of C6o with cyclopentadiene is a facile process, taking place rapidly at room temperature. Dissolving C a in neat cyclopentadiene results in the addition of as many as 15 equivalents of the diene.4s5 The monoadduct 2 has been prepared in 74% yield by careful treatment of C6o with a slight excess (1.2 equivalents) of cyclopentadiene in benzene solution at room temperature, and purified by flash chromatography on silica gel using a hexane-dichloromethanemobile phase?.' The Cs symmetry, resulting from [4+2] addition across a double bond (the 1,2 positions) at the fusion of two six membered rings of C& was assigned based on the *3C N M R spectrum. A total of 34 lines are expected in the 13C spectrum, and 33 are observed, with one line presumably obscured by a solvent (benzene) resonance.

174

175

2 Intenstingly, 33 lines are also expected for the product of addition across the 1,2 positions (3). This [6+4] mode of addition can not be distinguished from [4+2] addition across the 1,4 positions by 13C NMR. but is not likely since it forces two double bonds to localized in five membered rings, an arrangement that is not favorable.*v9 Crystal structures of other Diels-Alder adducts (see below) have confirmed a [4+2] mode of addition a m s s the 1,2 positions.

3 The Cgj-cyclopentadieneadduct 2 is found even in crude fullerene extract prepared

from soot produced in oxygen-poor benzene flames.6 This surprising result is due to the natural Occumnce of cyclopentadiene in flames, and the apparent cycloaddition of the two components either in the flame or, more likely, in the soot or tolueqe extract thereof. Cycloreversion of 2 to C a and cyclopentadiene occurs quite easily, proceeding quickly at 95 'C.6 The cycloreversion of 2 complicates the mass spectrometric analysis of this compound and others.6*'0 Reduction of 2 with a slight positive pressure of hydrogen over Pt02 catalyst produces 4 (Eq.21, which is substantially less prone to cycloreversion in the mass spocmmeter than the parent 2.7 Under these reaction conditions the Qo moiety in 2 (and @ itself) is unreactive. Tieatment of 2 with bromine (1 equivalent) produces dibromide 5 in which only the cyclopentene moiety has undergone bromination (Eq. 3). While &reacts with neat bromine," it does not react with dilute solutions of bromine," and the fullcrene moiety in 2 is clearly less reactive than the cyclopentene moiety.

176

H2 (75 rnm) c

PtO*

Br

Tetra- and hexaadducts of cyclopentadiene can be obtained by treatment of Qio with larger amounts of cyclopentadiene, and the remaining non-fullerene double bonds can be reduced as above.7 Pentamethylcyclopentadienereacts with both C a and C70 (at the 1,2- and 19-bonds respectively to give 6 and 7,in each of which the methyl p u p s attached to the single bridge, point away from the cage.12 Rapid addition to the highly pyramidalized, Ca like positions in C70 has been seen in the formation of metal c o m p l e x e ~ ' ~and * ' ~in the addition of Os04l5 and H2.

CH3

HF 6 7 Anthracene Anthracene forms a Diels-Alder adduct with Ca,although higher temperatures are required? The monoadduct (8) can be obtained in 13% yield after heating Ca with 10 equivalents of anthracene in refluxing toluene for three days, and purification with

177

column chromatography on silica using CS2 as the mobile phase (Eq. 4).16 The use of molten naphthalene as solvent increases the yield of this reaction to 6796.” Cyclmversion occurs between 120 ‘C and 190 ‘C in the solid state (by -A).

8

The nversibility of the Diels-Alderreaction of Cm with cyclopentadieneand with anthractne has lead several research groups to investigate methodology that would render the addition irreversible. Reaction of Q,o with diene 9 over 5 hours at 110 ‘C produces 11 in 60%yield. The initial adduct 10 is an unstable 7-oxonorbornadienewhich undergoes rapid, spontaneous loss of CO, producing 11 which is stable toward cyclmersion (Eq. 5).18 Crystallography revealed that 11 adopts a boat conformation in the solid state; the activation barrier for boat-boat interconversionis 14.6 kcaymole as measund by variable temperature NMR. This barrier is 7 kcal/mole higher than the barrier for cyclohexene.

9

/

c

10

11

Quinadlmethanes and Quinomethides A difEerent approach that gives a very similar product is the reaction of Ca with oquinodimethanes(0-xylylene, 13).19 Generation of 13 in the presence of & leads to a 1:l adduct (14)and a 1:2 adduct (15 is one of three possible isomers). These were separated from unreacted %by gel permeation chromatography (GPC) and obtained in 88% and 7% yield, respectively (based on converted Ca)(Eq.6). The structures of 14

178

and 15 were determined by a combination of lH and 13CNMR spectroscopy. The observation of 17 resonances from sp2 carbon atoms, suggesting a CzVproduct, provided evidence for rapid (on the 13C NMR time scale) conformational inversion of the cyclohexene unit in 14.

13

12

14

15

This methodology has been extended to the preparation of fullerene derivatives containing ionophores?O The other isomer of 13, p-xylylene (17). can be prepared by pyrolysis of paracyclophane (16). Cycloadditions of p-xylylene and Ca are not possible and radical polymerization occurs (Eq. 71.2~The resulting Cm-xylylene copolymer contains on between three and four p-xylylene units per C a unit and is thermally stable to 475 'C.

@-=(=+.+-. 650 'C

\ / 16

17

A further extension of this reaction is the addition of 18 to (Equation 8).19 Adduct 19 was isolated from the reaction mixture by GPC,although in this case the retention time was very similar to the retention time of C a , making the separation difficult.

19

(7)

179

The highly reactive diene isobenzofuran (20, essentially a quinodimethane)also undagocs Diels-Alder addition to Ca,resulting in adduct 21 (46% yield) which is stable toward cyclmversion and toward acid (Eq.9>.=

20

21

Thermolysisof 0-hydroxy benzyl alcohol provides highly reactive quinone methide (22). This s ies undergoes a Diels-Alderreaction with CtjO to produce benzopyran 23 in 50% yielg&xi on converted %(Eq. 10).

22

23

Acycllc l,3=Dienes A similar approach has been used to construct fullerene-containingdonor-acceptor systems with a rigid bridge se mating the electron donor group @-dimethoxyphenyl)

from the acceptor (fullerene).% Slow addition of the 1.3-diene 24 to a hot solution of produces 25 in virtually quantitativeyield based on unrecovered C a (63%yield, 99% based on UNtcOvered Ca).% Crystallography revealed that, as in quinodimethane adducts, the cyclohexene fused to the fullerene in 25 adopts a chair conformation in the solid mtc.

24

25

An httresting study, comparing the reactivity of Ca with other dienophiles, has suggested that Ca i s about as reactive as N-phenylmaleirnide in Diels-Alder reactions

180

with acyclic 1-phenyl-1.3 dienes (26).= Yields of 27 in these reactions range from 25% (R = CH3) to 34% (R = H). R

&02>

R

cm PhH, room temp.

r 0 - l

(12)

O C O J

26

27

Oxygenated Dienes Oxygenated 13-dienes add to Qjo under fairly mild conditions. Heating a mixture of and diene 28, followed by silica gel chromatography, produces ketone 29 in 59% yield from Qjo. Further functionalintion of 29 has been achieved through reduction of 29 with diisobutyl aluminum hydride (DIBAL) in toluene, which produces alcohol 30 in 93% yield (Eq. 13). This alcohol has been acylated with protected amino acids using standard DCC methodology to produce peptides as esters with 30?6

c,

YMS

/

0 c

toluene. reflux 53%

28

29

30

The fullerene group exerts a strong influence on the *HNMR chemical shifts of protons on the cyclohexanone ring of 29. A difference of over 2.5 ppm is observed for one of the protons (Ha)relative to protons at the 2-position of cycbhexanone. Variable temperature N M R experiments have shown that the activation bamer for interconversion of cyclohexanonering conformations in 29 is 12 kcalhnole, which is substantially higher than the barrier in cyclohexanone itself, so it is clear that the fullerene group can influence the normal behavior and spectroscopy of substituents. Even more highly oxygenated dienes participate in Diels-Alder reactions with %. Diene 31 (“Danishefsky’s diene”) adds readily to & to produce, after acid hydrolysis of the resulting silyl enol ether, ketone 32 in 39% yield.= Treatment with trifluoroacetic acid results in elimination of methanol, giving enone 33 (Eq.14). Mass spectrometric

181

analysis of 33 only showed the m/e 720 line correspondingto Ca rather than the parent ion canespondin to 33, sug esting that cycloreversionis facile under FAB conditions. Tagging 33 withi6 (R = H) !ollowcd by electrospray MS resulted in a derivative that could be analyzed by elecaospray mass spectrometry,a technique mild enough for the enone to survive.

-

0

TFA

2. H30+

(14)

oc& 31

..

-diton

32

33

reactions almost invariably involve heteroatoms at one or

both terminus of the 1.3 dipole, and as a result these reactions form new carbon-

heteroatom bonds as well as new carbon-carbon bonds. The reaction of Ca with a variety of different 13dipoles have been reported, and one of these, the addition of diazoaliranss, is the most highly developed reaction in fullerene chemistry. This reaction, and the addition of alkyl azides as well, are fully discussed elsewhere and therefore will be discussedin a most cursory fashion here.

Diazaelkanes Heating a solution of (trimethylsily1ethoxy)methyl azide (34) and Cm in chlorobenzeneproduces two products, one of which is identified as azafulleroid 35 (30% yield), &e other of which (24% yield) is converted to 35 upon heating. The 1,3-dipolar addition of azides to Ca presumably proceeds through mazole intermediate 36,but 36 itself is not observed. Instead, the reaction produces the equivalent of nitrene insertion, resulting in fullemid type structure 35 (Eq.15).27 A number of other alkyl and aryl azides undergo similar reactions?*28 c, Phcl mflux

ms-ofiN3

-

(15)

34

35

R

L

36

182

There are many examples of the addition of diazoalkanes to C60.29-35The 1.3-dipolar addition of a variety of diazoalkanes takes place across the 1.2 positions (a 6,6 ring fusion), forming Al-pyrazoline 37. In some cases, this intermediate has been isolated and characterized?0 although the common outcome of these reactions is the expulsion of nitrogen and ring closure to afford either a "fulleroid" or a methan~fullerene?~

37

Methanofullerenes have also been prepared by the addition of diethyl bromomalonate anion to ~ 6 0 . 3 6 3 7

Nitrile oxides Nitrile oxides 38 also react as 1,3 dipoles with C a , producing A24soxazolines39 as products3839 in yields that are typically 35% (65% based on recovered Q) after purification by gel permeation chromatography (Eq. 16).38 The cycloaddition has proven to be quite general and has been used to produce a number of compounds with different substituents at C-3.

38

39

R = Me, Et, COzEt, Ph, PhOMe, (CH2)&02Et

The 13C N M R spectra of these compounds indicate that the addition takes place across the 1,2 positions (a 6-6 ring fusion). The correct number of lines are observed in the I3C spectrum, consistent with the Cs symmetry of 39. These similarities support the structural assignment over a fulleroid-type struchire. These compounds are highly resistant to cycloreversion and are stable in solution above 250 'C for extended periods of time. They are also resistant to most of the reagents that are commonly used to reductive13 cleave isoxazoline rings. Nitrile oxides also add to C70 to produce a mixture of isomeric monoadducts in a roughly 1:1:1 Sulfinimide 40 undergoes 1,3-polar addition with excess Cm to produce isothiazole 4 1 in 14% yield after purification by GPC, along with small amounts of diadduct (Eq.

183

17):' The use of a large excess of 40 leads to the formation of a hexaaduct, obtained in high yield (97%) but as a mixture of isomers.

40

41

The 1% NMR spectrum of 41 provided good evidence for addition across the 1.2 positioaa (a 6,6 ring fusion). The appearanceof a single unsplit resonance, different from that of the starting mateiai, is expected from this mode of addition and addition across other positions would product two different 19F signals. Methyhecyclopropene Hating methyknecyclopropene42 provides access to a mmethylene methane (TMM) derivative which undergoes cycloadditionto C a to form two products, ketene acetal43 in 34% yield and ester 44 in 27% yield after purification (Eq. 18).

K O
2.8 is a catalyst for hydrogenation of acetylenes and olefins at room temporam.61 Because polymers with lower Pd/& ratios are inactive as catalysts, it has been suggested that the exposed surface palladium atoms, rather than the buried, cro88 linking palladium atoms, arc involved in this process. Further physical and structural Charactcnzationofthispdymersppearswarranted.

-

2.8

olhrr A&iition Reaclions

Reaction of excess [ ( ~ ~ - C ~ M ~ ~ ) R U ( ~ ~ C N ) ~with + ] Qo ( ~ ~isSreported C F ~ -to) yield brown ( [(~~-C~M~S)RU(CH~CN)~]~C&+) (03SCF3-)3.2122 Unfortunately this complex has not been obtained in a crystalline form suitable for X-ray diffraction,possibly because of the existence of several diffennt regio isomers. However, this result is of particular significance bccause planar, aromatic hydtocarbons bind strongly to the (q5CgMes)Ru+ unit and displace all acetonitrile ligands whereas electron deficient olefins displace only One ~ c c ~ o N U ~ligand.62 ~C T ~ Uthe S rca~tivityof [(~~-C!SMCS)RU(CH~CN)~+J toward &t gave one of the first indicationsthat Cjo would not bind mtaI Centers in the way that aromatic hydrocarbons would, but that electron deficient olefins were better modelsfor its reactivity. A brief report of the formation and spectroscopic characterization of the q2-C& adducts, (qZ-%)Fe(C0)4, (q2-Cso)Mo(CsH5)2, and (qZ-Cm)Ta(q5-CsH5)2H has appeared.55 Photolysis of Ra(C0)10 and Cjo in a benzene solution has been monitored by infrartd spactroscopy.63 A nansient product has been identified as Cjo(Re(CO)5)2,which

242

results from addition of two *Re(CO)sradicals to the fullmne. Numerous examplesof the addition of organic radicals to Qo are known.64 ~ h same c product can dso be formed h m the reaction of with ($-Ph3C)Re(CO)4 in the presence of carbon monoxide. Under these conditions *Re(CO)sis known to be fonmd. C&O(Rc(CO)5)2is unstable in and Re2(C0)10. The rdduct solution and decays in a first order process back to C&O(Re(CO)5)2is believed to have the two organomctallic fhgments attached at opposite ends of a hexagonal face of the fuilerene via 1,4 addition as shown in Figure 20. This bonding mode is novel among organometallics because it is the first apparent example of Q rather than IE attachment of a metal center to

a

Flopre 20. A proposedstnrtun for the invmrtdiateC&(Rc(CO)5)2(from reference 63).

3. Reactions that Result in Reduction of C6o Qo undergoes six reversible d u c t i o n s . 6 ~and ~ it should be possible to select transition-metal based reductantsthat arc capable of selective reductions to form salts of the mono anion the dianion (Qj$-), etc. The f i t thrcc reduction potentials M -0.5, -0.9 and -1.3 V in benmnitrile vs. SCE.65 Treatment of CrlI(TPP) (TPP is the dianion of meso-teaaphenylprphyrin)with C60 in tetrahydrofuran (THF) results in the formation of the salt [CrxII(THF),(TPP)+][c60-]which has been isolated as a purple-black nolid CT~(TPP)+~~(C~O - ) . ~anion * ~ ~apparently does not compete with THP in The ligating the chromium ion. The electronic spcctntm of the complex shows a shift in the Sont band from 421 nm for (TPP)CrII to 451 for the CrIn product, and a shift In the

(w),

243

fullmnc absorption from 330 to 339 which is indicative of one-electron reduction. The magnedc Nsceptibilty of the solid shows Curie-Weiss behavia with a room temperatun moment of 4,2(1)p~.The expected moment for a non-interacting S = 3/2. It2 system is. 4.27 WB. In THF solution the complex behaves as a 1:1 electrolyte. but in toluene solution back elaxma uansfer occul~lto give a so~ution of c r 4 T P P ) and neutral c60. The reduction potentialof cobdtooene ( 4 9 8 V vs. SCE)is such that it is capable of causingeither one or two electron ~eductionof &69*m Addition of cottaltoccne to ~ 6 0 in 1:1 ratio in benmnitrile solution yields brown [(rl5-C5Hs)2Co+](C60-)*PhCN.69 ~ d d i d o nof excess cobaltooene is reported to generate the dianion, (&2-).69 The epr spectrum of [(qs-CsH~hCo+l(Qo-).PhCN in tctrahydmfuran shows a resonance at g = 1.9969 with a temperature dependent linewidth (7 G at 4.5 K. 24 G at 130 K). Additionally, the material shows a sharp spike of low intensity at g = 2.000. Read and coworkerr suggest that the spike arises from t h d population of an excited statc that could result from splitting of the degeneracy of the qustate. Reductionwas suggested to caw a &onion from b to Ds symmetry that would split the qustate into2Ah and2E1, states.@ However, the absence of this spike in other salts raises the possibility that the s p bm t s from Species Other than (*). Ttaatmcnt of Qo with excess cobaltocene in carbon disulfide produces black crystals of [(~~-C~S~CO+](Q~-).CSZ that were bartly suitable for X-ray dithction.70 anion A view of molecule packing of the components is given in R g m 2 1. The

P

244 shows orientational disorder with two different orientations resolved. Unfortunately, the poor crystal quality and the disorder do not allow a meaningfd analysis ofthe effect of reduction on the geometry of the fulleride component. The dimensions of the cobalt cornplex, however, me consistent with the pnsence of the cobaiticiniurn ion rather than neutral cobaltocene. The epr spectrum, with a narrow line at g = 1.998, which broadens on wanning, is indicative of elccrron transfer to give the fulleride ion. The epr spccmm at 4 K does not show the known features of neutral cobaltocene, n a does it show evidence of a namw spike at higher tempratures. The electron-reservoir complex, ((q%!5Hs)FeI(q6-C&qj) 1, is an even stronger

-

reductant (Eln 1.55 V vs. SCE in dimethylfoxmamide) than cobaltocc11e.7~Consequently it should be capable of effwting reduction of %through three steps eventually to form the trianion, &3-. Indeed, titration of Qo with ((~$-CsHs)FeI(q6-QMeg)) produces rhree dark brown, paramagnetic, air sensitive salts: [(qs-C~Hs)Fen(116-CM~)+lo. [(rls-CsHs)Fen(116-CsMe6)+12(C602-), and [(r15-CSHs)Fen(r16-CMe6)+13(C603-).72 A sharp, symmetrical epr line was observed at g = 2.0020 for the salts that contained (&-) (linewidth 2.6 0)and (C&-) (linewidth 4.0 G ) at 300 K. For [(qS-CsH5)Fe~(q6C6Me6)+]3(Ctjo3-)however, , the resonance at g = 2.0040 exhibited a linewidth that dropped markedly with ternpcraturefrom46G at 30 K m 6 G at 4 K.

4 . Cocrystallization of c60 with Organometallic and Inorganic Reagents In addition to forming solids that incorporate significant amounts of solvent rnokcules during crystal growth, as seen in c&4Cfi6,34-36 the fulkrenes have a propensity to cocrystallize with organomctallic and inorganic complexes. Somc degree of charge-transfer. with G jo acting as an electron acceptor, may be present in these mBtcLlials. Consequently, they are related to the salts described in the previous section, and it is anticipated that a range of solids with varying degrees of charge aansfer will be found. Mixing solutions of C+jo and ferrocene results in crystallization of the solid Qj(p((qs-C@s)zFe)2.72 The smcture consists of isolated molecules which make only van der Waals contact with one another. A view of the packing within the solid is shown in Figure 22. One of the cyclopentadienyl rings of the femene molecule is involved in A stacking with QJ However, the oxidation potential for femcene is such that full elcctron transfer is not accomplished. C60 also cocrystallizes from benzene solution with Fe&o)4(q5-cfls)4 to fonn the ternary material C 6 0 . ( k 4 ( C 0 ) 4 ( ~ 5 , C g H 5 ) 4 ) * 3 ~ . 7F3 i p 23 shows a view of the

246

T

246 unit cell down the a axis. The geometrical features of the individual componentsdo not show any significant variation from their n d structurts. The fullerem is ordend in this solid. Three of the four cyclopcntadienylrings of the organomtallic complex BTC involved in face-to-face. IC-IC interactions with adjacent 0molecules. The orientations of these and some dimensions within the triad of Gjrj molecules an also given in Figure 23. The infrared spectrum of the solid shows a slight shift in v(C0) for the iron cluster (from1626 cm for ( F ~ ( C O ) ~ ( ~ S - C ~to H 1640 S ) ~ )cm-l in the ternary cocrystals). This shift is consistent with a weak donor function for the organometailic component Attempts to coordinate PdClz units onto Gjo through reaction with the txakmcly labile complex, (PhCN)2PdC12, in benzene solution have resulted in the formation of another ternary material: Cao.2(P&C112).2.5Qj&74 A view of the unit cell is shown in Figure 24. The P&C112 cluster, which has dimensions similar to that of ( trans Cl-Cl distance, 6.54 A; trans Pd***Pddistance, 4.67 A), forms by a spontaneous, selfassociation process. Benzene rings arc intersperstd between the fullmne and the Pdgc112 clusters. There appears to be a strong attraction between (PQCl12)and conventional,flat aromatic molecules.7S since a variety of binary cocrystals of Pic112 with aromatic molecules (benzene, hexamethylbenzene.and mcsitylene) have been preparcd.74

247

5 . Additions to C70 The structural complexity of the fullennes increases as their size inrreases.76*77 With Qo, the second most abundant fullmne in standard preparations, there are five types of carbon atoms and eight distinct types of bonds.2 These can be visualized by turning to

Figure 25. The five types of carbon atoms are anangd in parallel layers, a feature which is uaiqu~to this fulleme. If addition is confmd to 6 6 ringjunctions. then that arc four possible single addition proaucts that can be anticipated to form upon additions to C70. The mults of H k k d molecular orbital calculationarc included in Figure 25.77 The 1 9 aad 7.8 C-C bonds at the 6 6 ring junctions at the poles of the molecule have the highest x bond &and an expected to be the most lcBctive by that criterion. 5 .I

A&n

of Ir(CO)Cl(PR3)2

The formation Of the adduct ( ~ 2 - c ~ ~ ) ~ ( c o ) ~ ~ ( Pthrough P h ~the )~~2.~c~ xeacdo6 of Ir(CO)Cl(PPhgh with C70 gave a crystalline matcrial that produced the first detailed pmetric picture of any fulleEne larger than Qo.78 Figure 26 shows a view of the m o W that was obtainedfrom a single crystal X-ray study. Despite the possibility of f w g a variety of ngio isomers, only one crystalline compound is formed. It has the iridimtuamc a d h a t 4 to a 1.9 C-C bond type (at a 6:6ringjunction) at one pole of the molecule. The con structure of the fullmne conforms to expectation. It possesses an ellipeoidrl shape with a 7.90 A distance dong the long axis and a 6.82 A diameter. Notice that the a m a m of the mlcculc is highest at the poles, whik the hexagons about the waist of the molecule arc actually concave. The C-C bond distances from that structure arc ComPIped in Table 11to those of C 7 0 - 6 ( S 8 ) , ~to ~ . those ~ of a nlatcd double addition product.81 and to those h m a theoretical calculation.82 These data arc a l l consistent with one another, and they indicate that complex formation causes only a local structural ddonnadon of the carbon cage. The redetermination of the structure in the C70 /S8 cocryst&,0 however, gives the most accurate C C distances. 'IYw hlation of only one isomer of(q2-C7r$rOO)Cl(PPh3)2 can be attributed to the r c v d W t y of the simple addition naction that is involved in its fonnation. The formation of thfr particular isomer ~eflcctsboth the high R b o n d d e r of the 1.9 bond, and the fact that tlm molecule is most curved near the poles while it is flattened about its equator. Because of the curvature, carbon atoms 1 and 9 an the most pyramidalizcd. Binding of ofganomtallic reagents to fullennes results in some degree of local distortion. This distortion arises from the added pyramidalization of the carbon atoms that arc bound

248 Bond

Bond Order

6 6 ring fusion 1.9 7.8 22.23 2334

0.597 0.602 0.545 0.489

6 5 ring fusion 12 0.477 67 0.479 122 0.469 21,22 0.534 77).

Figure 26. The stNCue of (q2-C70)Ir(CO)CI(PPh3)2

nfaencc 78).

as determined by X-ray crystallographv ( b m

249 by the added group. For C70, the most curved, and consequently most pyramidializcd, carbon atoms am thc most thamodynamically reactive atoms in the cluster. Multiple addition of Vaska-type complexes to C70 has also been explored. If a d d i t i d reactivity is confmcd to the 1.9 bonds at the opposite pole of C70 then thne double addition products can result. The possible sites of reactivity arc shown in Figure 27.where site A rcprtscnts the position that places the two added groups closest together, site C is the site &a&positions the two p u p s funhest apart, and site B places them at an intenaediatedistance. With the more reactive complex, Ir(CO)Cl(PMe2Ph)2, crystalline forms of both single and double addition products have been obtained.81 The reaction is controlled by the stoichionwry in the addition reaction. The structure of the singleaddition product rcstmblcs that of the triphenylphosphine analog, and again, addition to a 19 bond occurs 88 secn in Figure 28. The reaction of C70 with a 6- to 12-fold excess of Ir(CO)Cl(PMe2Ph)2 produces a remarkably homogeneous crystalline material, (q2C7o)(Ir(cO)Cl~M~~~)~~3cgH6.~1 The structure of the complex is shown in Figure 28. The product has a structure that cornsponds to addition to site B of Figure 27. That is the site that places the two added groups at an intermediate distance apart. In this double addition product, all of the four phenyl rings hug the C70 unit and arc involved in intratnolccuit~~ z-zcontact.

1.2 Is9 6,7 7.8 7.22 21.22 22.23 23.24

1.451 1.375 1.446 1.361 1.457 1.415 1.407 1.475

1.453 (3) 1.387 (4) 1.445 (3) 1.378 (3) 1.447 (2) 1.426(3) 1.414 (2) 1.462 (4)

1.46(3) 1.38 (I)* 1.45 (3) 1.37 (2) 1.43 (2) 1.44 (1) 1.42 (3) 1.46 (2)

1.440 (6) 1.386 (19p 1.455 (11) 1.374 (13) 1.444(14) 1.427 (8) 1.420 (7) 1.458 (7)

nfarnce 82 80 78 81 a, The bonds that ate coordi~ted to iridium arc excluded h m the avaage.

250

o a

o b @ C

o e

o d @ C

o b

a a

B Figure 27. A diagram that shows thc pmbable sita for ddition of a sccond iridium complex to C70 if first lddirion impedes sddition tothcsamcplc ofthccclrbon clusta(fran ref81).

0

c37

c5

251

5.2 Osmyhtion

Treatmat of C70 with osmium tetroxrde in pyridine produces two isomeric single addition ptOducts.*f These have been separated by chromatography. Structural analysis by 1% NMR spacaoscopy has shown that the two products involve addition to the 1.9 and 7,s bonds (pigwe 25) in the cluster. Drawings of the smctures of these two isomers am shown in Figure 29. The ratio of amounts of the 19 addition product to the 7.8 addition product is 2.1:l. Addition to the 1.9 bond, which is the more prevalent, follows the pattern sccn in the addition of Vaska-type complexes. The pattern of kinetic addition seen for the mylation appears to be mntrolled by two factors; the reactivity as expressed in the 1;bondordcr, and the degree of local curvature within the fullerenene. The 1,9and 7,8 bonds have the highest x bond order, and consequently should be the most reactive However, on this criterion alone, the 7,s bond should be the most reactive, yet the 1,9 isomor is man prevalent. The greater reactivity of the 1.9 site has been attributed to the greater degree of curvature at that site and the degree of pyramidalization of the carbon atoms,w This a n d has also been noted for the thermodynamically controlled addition of vaska-typecomplexes to c7o.n

.

0 PY

1,9 isomer 7,s isomer P I ~ B. U ~trucrure ~ of thc two isolatcd isomers of ~70020~02@y)2 from 1% NMR analysis.

Further addition of osmium tetroxide to these two separate single addition products, results in the formation ofmixtures of double addition products. From the reaction of the 1.9 isomer. six chromatographically separated double-addition products have been found, while with the 7,8 isomer seven double-addition products were obtained. Four of the

252 products in the two sets apparently had similar chromatographic behavior. The formation of these distinct sets of products is a further indication that the osmylation is kinetically ControlIcd 6 . Addition to C76, Osmylation Osmylation of C76 in the presence of the chiral Sharpless cinchona alkaloid l i w d s has been used for the kinetic resolution of this c h i d fullmne into its enantiomas.85 Figure 30 shows a view of C76 which contains 30 different carbon-carbon bonds with fifteen of these at 6:6 ring fusions.851~~ If osmylation occurs at the sites of highest cwaturc or pyramidalization (as it does in C70) then the two bonds designated 1 and 5 in Figure 30 are the likely sites for reaction. The resolution process has involved addition of one equivalent of osmium &@oxideto raccmic C76 in the presence of an excess of the chiral ligands. Unreactcd c76 was chromatographically separated fromthe complexed fullttene. The complexcd fuilerene was raduccd with tin(n)chloride in pyridine to l i b = the c76 enantiomcr. The isolated and purified enantiomers were characterized by their circular dichroism spectla which show a rich array of features in the 300 to 800 nm @on.=

7 . Addition to c g 4 In the conventional contact arc process. C a is one of the wort abundant higher fullaenes,76but its structural charactaisticsare mon complex than the othcr fullcrcnes that have been tnated with organomctallic reagents. The 24 isolated-pentagon smctuces87 that can be drawn for it have &en shown to fall into two disjoint families88 which c ~ l lbe intcrconvmed through the pyracylene of Stone-Wales transformation.@' Within the p u p of 24 isolated pentagon isomers the arc four D2 and two DZd isomers as well as isomefi with different symmetry. Calculations indicate that the specific & (22) and (23) isomers that am shown in Figure 31 an lowest in energy.% 13C NMR studies of Cgq have

concluded that the 32 line pattern can be explained by the presence of a 2: 1 mixture of the D2 (22) and b (23) is0mers>19~but no effective chromatographic separation of these isomers has been found. Treatment of a saturated benzene solution of C84 with an excess of ~04c~6.93 Ir(CO)CI(PPh3)2produced black Crystals Of ( ~ 2 - c ~ ) ~ r ( c o ) ~ ~ ( P P h ~ ) The structtln of the complex as determined by a single crystal X-ray diffraction study is shown in Figure 32. While the complex itself has no crystallographically imposed symmetry, the geometry of the Cw moiety corresponds to that of the (23) isomer. The distance along the axis that bisects the C(32)-C(53) and C(42)-C(43)bonds is 8.61 A, while the distance amom the two axes that an perpendicular to this is 8.34 A. he iridium ion is coordinated to the C(32)-C(53) bond, which is only one of the 19 different types of C-C bonds in Csq. HUckel calculationsindicate that this is the bond in Cw that has the highest x-bond order, and thenfore it should be the most reactive.77 As a consequence of coordination, the C(32)-C(53) bond length (1.455 (6) A) is considerably longer than that of its counterpart, the C(42>c(43) bond (1.332 (1 1) A) at the oppositepole of the fulleme. carefulexamination of the fullerene portion of the structurereveals the presence of a degrec of disorder. During rcfmement, difference maps revealed electron density in the hexagons that were adjacent to three bonds: C(17)-C(18), C(42)-C(43). and C(67)-C(68). Figm 33 shows the location of these additional carbon atoms. In each case the pattern is a supexposition of that given by the Stone-Wales transformation.89 No other orientationof the (23) isomer of Csq would produce such a set of added peaks, but the presence of small amounts of other isomers could produce these features. However, an unambiguous identification of which other isomer (or isomers) was present was not possible. Nevertheless, adduct formation and crystallization has produced a sample that consists pdominantly of the (23) isomer. The selective crystallizationof the Dzd (23) isomer, which is suspected to be less abundant than the & (22) isomer, may result from a stronger tendency of that isomer to form adducu. Hiickel calculations show that the Du (23) isomer has the most localized x-bonding of the fullerenes that have been isolated, and it has been suggested that it will be the most reactive towad addition reactions.7 Since three different fullerenes, C a , C70, and C84. have all been structurally charactaized with the samc organometallic p u p , Ir(CO)Q(PPhjh. aff'iied to the exterior, it is revealing to compare the three. Figure 34 presents a view of this to scale so that the d i f f ~ n c c in s sizes and shapes of the thre fullerenes arc apparent.

254

(22) isomus of c84. (from referem 93). The Figure 31. Ideal s t ~ ~ h ufor c s the (23) and n u m h in pamthesi refer to the tabulation of isomers in nfaence 87.

F C n 32. Ihe strucarnof (q2-€s4)Ir(Co)CI(3~

(from reference 93).

255

256

8 . Isolation of Endohedrai Compounds Then have been a substantial number of reports on the detection of endohednd metal fullercne c l u s t e r s . ~ - 1These ~ have involved mass spectroscopicstudies of species produced in the gas phase or spectroscopic studies that involve the detection of individual species, particular via epr technique, in complex mixtures. Only recently, however, has it proven possible to devise methods that allow for the separation and purification of individualendohedralcomplexes. Thrce discandium fullerenes Scz@c74,sCz@c82. and Scz@Ca have now becn prepared, separated and The synthesis involved the arc burning of rods made from graphite powder, pitch and Sc2O3. The soot that was produced was extracted with carbon disulfide. A toluene solution of the extract was first subjected to HPLC with a polystyrene column which separated out six fractionsthat containedrespectively Cao,C70, c7&78. C a , "t&o", and "C% and higher fullerencs". The fractions containing the two higher fullcrcne materials were then subjected to a sccond chromatographic procedun using a Trident Tri-DNP column108. In this process the ''W fraction was separated into its components Cg4. C86. Scz@C74. (30.Cg2, ( 3 6 . Scz@Cg2, and S c 2 @ 3 ~ 4 . Unfortunately, yields of these products were not reported. The individual components wen identified by lascr-desorption timc-of-flightmass specaoscopythat was performed on the individual isolated materials. Additionally the attempts to obscrvc epr spectral for the samples of the three endohcdral compounds did not produce resonances, and it has been concluded that the products were diamagnetic. The electronic absorption spccaum has been obtained for Sc2@C@in toluene solution, and this is shown in Figure 35. The

C

3

.

2c QJ

c

3 m .L

%

< P

- .

800

ic.

700

.

I

600

.

.

.

I

500

400

300

Wavelength 1 nm

Figure 35. The Uv/vis absorption spectrum of a toluene solution of Sc2@Cu (from reference 107). The arrows show absorptionsthat are absent in the spectrum of Cw.

257

featum at 380,500and 680 nm are characteristic of the endohedral complex since they do not appear in the spectrum of C84 itself. The external shape of sCz@(384 has been examined by scanning-tunneling-microscopy.lw The images show a roughly spherical shape of the molecules with no protrusions. Consequently, it was concluded that the scandium atoms were indeed encapsulated within the cluster. However, some variation in external shape is evident in the images, and this may be due to intrinsic asymmetry of the cluster and or the presence of isomeric species.lw By working under anaerobic conditions, it has been found that a number of newly detected and air-sensitive metallo-fullerenescan be extracted.110 Several new lanthanofullcmncr have been detected by epr spectroscopy and mass spectroscopy in this fashion. Lanthrnumcarbide has been found to give improved yields of these lanthano-fullerenes. 111 hother article reports that contact arc vaporization of graphite in an atmosphere of Fe(C0)s yields an extractable material that may contain The material was subjected to chromatography but not efficiently separated from other fullerenes. It has been examined by M6ssbauer and Fe-K EXAFS measurements. The isolation and purification of these endohedral complexes is an exciting and wclcom advance in this field.1139114 It may be anticipated that this work will lead to more detailed information on the structure and physical properties of these remarkable molecules.

Acknowledgments 1thank the U. S.National Science Foundation for support of fullerene research at Davis, Marilyn Olmstead and Bruce No11 for their crystallographic work, Joong Lee and Bmce Noll for help with the graphics, h a Ginwalla, Dave Costa and Joong Lee for their experimentalcontributions, Joe Calabrese for providing some crystallographiccoordinates, and Eileen Rivera for help in preparing this document.

258

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R. Taylor, J. P. Hare, A. K. Abdul-Sada. H. W. Kroto, J. Chem. SOC.,Chem Commun. (1990) 1423. P. J. Fagan, J. C. Calabrese, B. Malone, Accounts Chem. Res. 25 (1992) 134. H. B. Burgi, E. Blanc, D. Schwarzenbach, S. Liu, Y. Lu, M. M. Kappes, J. A. Ibers, Angew. Chem. Int. Ed. 31 (1992) 640. J. R. Heath, S. C. OBrien, Q. Zhang, Y. Liu, R. F. Curl, H. W. Kroto, F. K. Tittle, R. E. Smalley. J. Am. Chem. SOC.107 (1985) 7779. K. Hedberg, L. Hedberg, D. S. Bethane, C. A. Brown, H. C. Dorn, R. D. Johnson, M.de Vries, Science 254 (1991) 410. M. Haeser, J. Almloef, G. E. Scuseria. Chem. Phys. Lett 181 (1991) 497. M. Schroder, Chem. Rev. 80 (1980) 87. J. M. Wallis, J. K. Kochi, J. Am. Chem. SOC.110 (1988) 8207. J. M. Hawkins, T. A. Lewis, S. D. Loren, A. Meyer, J. R. Heath, Y. Shibato, R. J. Saykally, J. Org. Chem. 55 (1990) 6250. J. M. Hawkins, A. Meyer, T. A. Lewis, S. Loren, F. J. Hollander, Science 252

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Chapter

15

Fullerene-contaioing Polymers Roger Taylor From the time that they were discovered, the possibility of using fullerenes in the formation of polymers has been very much in the minds of chemists and materials scientists. Three main possibilities arise: a) The polymer may consist of linked fullerenes (either directly or by means of a small 'spacer') giving polymers of the 'pearl necklace' type); b) the fullerenes may be attached regularly to a polymeric backbone chain (giving a pendant chain effect); c) the polymer may have the fullerene as a relatively small percentage of the overall composition, the hllerene molecules being located at structurally significant points. The latter group includes the copolymers. Giver the current high cost of fullerenes, only polymers of the latter category have any prospect of being utilised at the present time, and good progress has been made in creating polymers showing promise of having special applications.

1. 'Puui-Neckface' Polymers

1. I Directly-linked Fullerenes. The simplest pattern that can be envisaged here is that shown in Figures 1 and 2. None of the former type are yet known, but the latter can be produced by irradiation of the fullerene in the absence of oxygen (which quenches the first excited triplet state of [60]fullerene and hence reaction) by either W or visible light, resulting in a (2 + 2) cycloaddition of up to twelve [6O]fullerene The product is no longer soluble in toluene, but can be dissolved in boiling isodurene. Given that the number of double bonds in [6O]&llerene is thirty, numerous conformations of the product are possible. However, twelve of these are sterically precluded, and the

n

Fig. 1 Schematic for a 'pearl-necklace' fullerene polymer involving single-bond linkages

263

264 remaining seventeen is reduced to seven by symmetry. Neverthless the number of conformations approaches 7" where n is the number of fillerene molecules involved. (Steric interaction will preclude some of the conformations that involve doubling back of the chain.)

n Fig. 2

Schematic for a 'pearl-necklace'fdlerene polymer involving two-bondlinkages

Polymerisation is accompanied by loss of the'Raman band at 1469 cm-1 and formation of one at 1458 cm-l, consistent with the altered bonding in the product. Polymerisation only occurs to any significant extent above 260 K, the temperature at which [60]fillerene becomes disordered which permits close and parallel approach of the bonds required in the 2 + 2 cycloaddition process, which calculations have indicated to be the most plausible scheme for dimerisation and polymerisation of fillerenes.2 [70]Fullerene also polymerises under the same conditions, but here only four molecules become detectably linked, a result attributed to there being fewer reactive bonds in [70]fullerene compared to [60]fillerene, and fewer conformations available that can result in reaction Moreover, [70]fillerene is less reactive than [60]lllerene towards cycloaddition reactions in general. Linkage of [60]fullerene molecules in this way can also be brought about by the use of very high pressures of 5 gigapascals and temperatures between 500 and 800 OC.4 A toluene-insoluble material is formed, having a 9.22 A intermolecular spacing. A problem arises here (and also in the case of RbC60 which on cooling below 400 K undergoes orthorhombic distortion giving an interhllerene distance of 9.12 A)s in that the interhllerene distance is calculated6 to be 8.5 A for (c60)2, so the interhllerene distance must be exceptionally long, and considerable longer than a normal C-C bond; strong repulsion between the x-clouds could be responsible It is relevant therefore that recent work involving drift-tube experiments has indicated that two main species are involved. One corresponds to structures resulting from 2 + 2 cycloaddition, the other to a more fused (unspecified) isomer.7 Other workers have found that using higher powers for the desorbing laser, species C11g, C116, etc involving C2 loss are obtained, and likewise for species involving fbrther

265

polymerisation. 899 For example, the peak corresponding to 10c60 was 500 amu deficient, correspondingto the loss of 42 carbon atoms. Possible structures for the dimer have been considered, and their energies calculated. These showed that the dispirane (1) could be the structure for C118 and it is relevant therefore that a related spirane stmcture (2) has also been proposed (based on chemical reasoning) for the structure of CI19 produced fiom ~600.10

(1)

(2)

1.2 1.rectIy-linkd Fullerenes. Figure 3 shows the next most simple polymeric fillerem stmcture that can be envisaged, i.e. involving a spacer group.

t.

n

Fig. 3 Schematic for a 'pearl-necklace'fuilerenepolymer, involving a spacer group

Although there is no evidence yet for the presence of the hydrogen shown in such polymers, they (or some other group) are necessary if an exceptional amount of bond reorganidon in the cage (and the unfavourable placement of double bonds in pentagonal rings) is to be avoided, as would be the case if R groups alone are present. However, the latter mmgement has been assumed to exist in the polymer C60Pdn formed from [6O]hIlerene and the complex Pd2(dibenzylideneacetone)3CHC13 (which undergoes ligand diaplacement.If Large values of n were associated with the use of large stoicheiemetricquantities of the complex, but n was never less than one if [6O]fbllerene was in excess, consistent with a chain having one palladium per fbllerene cage. On heating the polymer, some [60]Mlerene is released, and the value of n increases to ca. 3, consistent with the formation of a lattice polymer. For n values > ca. 3, the polymer

266 behaves as a hydrogenation catalyst, indicating the presence of some palladium on the polymer surface. Similar polymers can be formed with platinum. l2 Embryonic ('charm bracelet') structures of the type shown in Figure 3 have been obtained as shown in Figure 4, in which the bonding to the central aromatic ring may be either meta or para, giving an incipient pendant chain. They have been prepared as shown in Scheme 1, and have very low solubility in organic solvents.l3

Fig. 4 Embryonic structure of a 'pearhecklace'fullerene poiymer ( d p linkage to the centrat ring)

Scheme I Methods of formation of poiymer section shown in Fig. 4

The embryonic polymer section shown in Figure 4 was an early demonstration of what might be achieved in terms of multiple addition of hllerene molecules to chains. However, the insolubility of the product showed that the approach would have to be modified if useable products were to be obtained. A number of techniques have been therefore been employed to improve the solubility of the polymers. For those of the type shown in Figure 4, derivatives with hydroxy groups para to the bridging group have been made (Figure 5 ) and then polymerised either with sebacoyl chloride [CICO(CH2)8COCI] to give a polyester, or with hexamethyldiisocyanate to give a polyurethane.14 The latter proved to be insoluble, but the polyester is more tractable.

267

Fig. 5 Routes to the formation of polymkrs and polyurethanes

1.3 Copijmers. In this sub-division of the previous group, the fbllerenes appear within a

polymex chain, but the inter-lllerene length can be considerable. The fullerene can be incorporated either as the other component of the polymer is formed fiom monomer, or attached subsequently to a preformed polymer chain; both techniques have been used. 1.3.1 Atlachment of a Fullerene to a Preformed PoI'er Chain. A polymer with indirect links between the fullcrenes was obtained by reacting non cross-linked polystyrene with fbllerenes in the presence of aluminium trichloride, to give Lewis acid-catalysed akylaticm of the fullerenes and a highly cross-linked product.15 An alternative procedure hss made use of the fact that fbllerenes are highly reactive towards anions and nucleophilca. Polyethylene films, functionalized with diphenylmethyl groups, have acidic hydrogens present as a result of these groups. Deprotonation may be therefore affected with e.g. b u t y l - l i t h i u m - ~ ~ ~ , ~ - t e t r a m e t h y I ~ h y l e n etod i ~give e an anionic polyethylene surface which then attacks [6O]lllerene to give, following quenching with methanol, the fullerene incorporated polymer; properties were not reported.16 In order to try to circumvent the all-important solubility problem, polymers have been made in which [6O]lllerene is added, through cycloaddition, to azido-substituted polystyrmss (44 b, c). After N2 loss, these polystyrenes react with [6O]lllerene to give the corresponding Illerene-containing polymers (5% b, c). Mainly mono(cyc1o)addition occurs and so there is relatively little cross-linking. In these products, the respective amount of [6O]lllerene incorporated is 5.5,21, and 29 wt.-%, and the products are very soluble in organic solvents. The molecular weights range from 27,000 (polydispersity 2.04) to 38,500 (polydispersity 3.12), and the respective glass transition temperatures are 112, 142, and 160 "C compared to 97 "C for azidopoiystyrene.17Note that in these polymers, nitrogen inserts into the bond common to a 5- and a 6-membered ring.

268

(4), a) x = 99, y = 1 b) x = 94, y = 6

c ) x = 8 9 , y = 11

Scheme 2 Cycloaddition of azido-substituted polystyrenes to [60]lllerene, giving soluble polymers

The need to produce solvent-soluble polymers has lead to a number of ingenious solutions, which have in particular concentrated on the incorporation of amino groups. For example, reaction of [60]fullerene with amino-containing poly(oxyethy1ene) or poly(oxypropy1ene) polymers (6, R = I-€, Me, n variable) gave products soluble in both water and toluene. For some of these, water solubility was achieved in acid solution only but the polymer could be completely regenerated upon neutralisation. The formation depends upon the ready reaction of amines (and nucleophiles in general) with [60]hllerene, but the sites of addition are not known; an average of approximately three polymer molecules are joined to each fdlerene molecule. The extent of solubility of the products depended markedly upon the aminopolymer:[60]fullerene ratio. When this was small the opportunity for cross-linking the hllerene was reduced, and so insoluble products were not obtained (though formation of soluble ones was very slow). At higher ratios, soluble products were obtained only during the first 30 h of reaction time, after which cross-linking evidently occurs, 18

Me-qH-CH2-O-[CH-CH2-0ln-CH2-$H-Me NH2 R NH2 (6, R = H, Me, n variable)

Another similar approach (in which an amino-containing polymer goes 'fishing'

269

for [60]hUerene) has been used as shown in Scheme 3.19 The polymers contain ca. 200 mg of fillerene per gram of polymer, and (7a) is soluble in toluene and carbon disulphide.

I I

Scheme 3 'Charmbracelet' soluble polymm formed from [6O]fullerene and aminopolymers

This technique has also been used in the reaction between[60]hllerene and amine-fhctionalised ethylene-propylene terpolymer (Scheme 4).20 Reaction was brought about by stirring the reagents together for 1 week at room temperature. The polymer was precipitated with acetone, redissolved in THF or hexane to give a reddish product, soluble (except for cu. 5% attributed to cross-linking) in xylene.

Scheme 4 R a d o n betwaen I60fhdlereneand amine-fiurctonalisedcthylene-propylene oopolymer

270 1.3.2 Incorporation of a Fullerene into a Forming Pobmer Chain. This technique appears at present to give less control of the polymer product. The sole example concerns pyrolysis of paracyclophane to give para-xylylene diradical as a comonomer, followed by polymerisation of this in the presence of [60]hllerene. This gives the polymer shown in Figure 6,21 in which the d n ratio was about 3.4 but due to extensive cross linking, the polymer was insoluble, and proved also to be unstable in air.

Fig. 6 Structure of a [60]lllerene-xylylenecopolymer

2. Dendrimers

The technique used to prepare the polymers (5) shown in Scheme 2, has also been used to prepare dendritic hllerenes or dendrimers. The polymer shown in Figure 7 is, like the polymers (S), also very soluble in organic solvents, has a glass transition temperature of 325 K (again higher than that, 3 12 K for the initial azide), and is formed with only 5% of diaddition to the hllereneF2 as might be expected in view of the large steric hindrance in the addend.

Fig. 7 Dendritic polymer formed by cycloadditionof an azide to I60jfullerene

271

A soluble dendritic polymer related to the homofbllerene compound shown in Figure 5 has also been made &om the phenolic precursor.23 By suitable choice of conditions to minimise side reactions (which cause increasing lllerene content with increasing molecular weight), the product shown in Figure 8 with each phenolic oxygen coupled to a dendritic unit, was made in 90% yield.

Fig. 8 A soluble dendritic polymer formed from a homofullerene (fulleroid)

3. Starfolymers

Use here has also been made of the fact that fi~llerenesare extremely reactive towards anions.By reacting [60]&llerene with a living poiystyrene anion, polymers with the formula [60]fillerene(p0Iy~tyrene)~, where x = 1 - 10, have been obtained (Scheme 51.24 There was a trend to formation of higher molecular mass species as the

CH3-CHz-CH-Li I

+ styrene

x

Scheme 5 Method of formation of a [60]fullerene-polystyrenecopolymer

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polystyrene:[60)f3lerene ratio increased, and appeared to pass through a maximum for values of x = 4 10, from which it was inferred that this corresponds approximately to the number of chains that can be accommodated on the ikllerene core, which seems reasonable. The evidence is thus that the polymer has star-like characteristics, and other polymers of this type are described below. In contrast to the material obtained fFom polystyrene, the product here was highly soluble and melt processable, making it amenable to spin-coating, solvent casting, and melt extrusion. Star copolymers have also been made by reaction of [60]fillerenenols (see Chap. 5) with diisocyanated urethane polyether prepolymer, followed by quenching with 1dodecanol (Scheme 6).25 The average molecular weight was 18,000 (maximum 26,100) so the polydispersity of the product, 1.45, is extremeiy narrow; it has a glass transition temperature of -67 *C. The molecular weight shows that there are six arms to each fillerene, and possibly these are located at the octahedral sites. Some hydroxy groups must remain unattacked, due to steric hindrance.

-

Scheme 6 Formation of a star polymer by reaction of fulbrenols with urthane polyether prcpolymer

4. Electroactive Polymers Cyclic voltammetry of 6 1,61-bis(trimethylsilylbutadiynyl)-1,2,-dihydro-1,2methano[60]fillerene(8) showed a single well-defined one-electron reduction at 1.OO v. During repeated potential cycling (0.1 vfs), the voltammetric peak current increased with

273

the number of scans, due to the formation of an immobilised electroactive coating on the electrode, believed to be an air-stable polymer. The fact that the film continued to build up even after it had covered the electrode suggested that it is electrically conducting., and can be used as an electrode material.26

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