22 Fullerene chemistry P. R. Birkett Fullerene Science Centre, School of Chemistry, Physics and Environmental Science, University of Sussex, Brighton, UK BN1 9QJ 1 Introduction The following are highlights of the large number of publications covering fullerene and carbon nanotube chemistry.A major emphasis has been placed on the preparation and chemistry of new fullerene derivatives and related nanotube materials. 2 Synthesis, separation and biological properties of fullerenes Reviews of the combustion of hydrocarbons producing fullerenes, polyaromatic hydrocarbons and soot1 and of the potential uses of fullerenes for the synthesis of diamonds, diamond films, thin fullerene films for microelectronics and as lubricants are available.2 In situ electron microscopy has led to the direct observation of fullerene formation from graphene sheets3 and the same authors have observed the formation of fullerenes from a methane precursor using a radio frequency plasma enhanced chemical vapour deposition technique.4 Approaches towards the synthesis of fullerenes and related bowl-like structures using traditional organic chemistry techniques have also been reviewed.5 A thirty-two carbon atom cage has been suggested, using theoretical considerations, 6 to be the most stable fullerene-like cluster below [60]fullerene.A new, small fullerene-like cage with D 6) symmetry consisting of 36 carbon atoms, C 36 1, has been prepared using a modified arc-discharge method.7 The electronic and structural properties of the foregoing C 36 have been investigated theoretically using the pseudopotential density functional approach.8 An independent search of 598 closed thirty-six carbon atom cage structures revealed that a cage with D 2$ symmetry would have the lowest energy.9 A second type of fullerene, represented, for example, by C 74 , which are either free radicals or have small HOMO–LUMO gaps (in contrast to the well known, stable closed cage fullerenes which have large HOMO–LUMOgaps such as C 60 ), have been isolated after the electrochemical reduction of their insoluble polymerised solids obtained from fullerene containing soot and subsequent reduction to soluble anions.10 The high yield synthesis of C 74 using the arc-discharge method at high He pressures has also been reported.11 Similarly, an increased yield of higher fullerenes has been achieved by vaporising a variety of doped graphite anodes.12 The Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 431–451 431Fig. 1 Computer generated structures of the D 6) symmetrical isomer of C 36 1, the [2]2] cycloadduct C 120 2 and the dimer (C 60 ) 2 C 2 3. two major isomers of C 84 , i.e. D 2 and D 2$, have been separated for the first time using a recycling HPLC technique.13 In addition, high resolution synchrotron X-ray powder di§raction shows that a mixture of the foregoing two isomers adopts a face-centredcubic (fcc) structure at all temperatures.14 Two approaches to the synthesis of the [2]2] type fullerene dimer, C 120 2 (Fig. 1), have been reported. The first uses a mechanomechanical synthesis procedure by reacting C 60 with KCN in a ‘high-speed’ vibrating mill.15 A modified mechanomechanical method was subsequently used to produce the dimer in similar yield using metallic Li as the reaction catalyst and in addition the Raman spectra of the dimer was reported.16 The second technique involves squeezing the molecular crystal of (ET) 2 C 60 at 200 °C and 5 GPa.17 Theoretical calculations predict that the formation of fullerene dimers of this type may play an important role in the mechanism during the incorporation of helium into C 60 .18 Reaction of C 60 with diazotetrazole results in the formation of a di§erent type of fullerene dimer, (C 60 ) 2 C 2 3, in which the fullerene moieties are linked by a C––C function.19 The selective separation of the fullerenes C 60 and C 70 with unmatched selectivity using a stationary phase consisting of hydroxyphenyltriphenylporphyrin on silica has been reported.20 In contrast, C 60 based stationary phases have proved to be useful in independent studies for the separation of high energy nitroaromatics21 or polychlorinated biphenyls.22 A fullerene/lipid device capable of stable and reversible electron transfer reactions has been prepared.23 Similarly, photoinduced electron transfer in porous polymer membranes modified by the incorporation of fullerene and phospholipids has been found to depend on the nature of the polymer used.24 An EPR study of aqueous Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 431–451 432solutions of C 60 and C 70 reveals thatOH· andO 2 ·~ may be the primary factors for the biological damage caused by fullerenes under photoirradiation conditions.25 Singlet oxygen has also been implicated in the oxidative damage which is induced by fullerenes on photosentisization.26 Fullerene-specific antibodies have been produced after immunization of mice with a C 60 derivative covalently attached to bovine thyroglobulin.27 Moreover, C 60 has been shown to inhibit glutathione S-transferase activity towards ethacrynic acid.28 There is also evidence that C 60 does not remain unchanged in the liver, as previously believed, following the identification of both retinol and retinyl palmitate C 60 adducts which are formed by in vitro cycloaddition reactions.29 3 Endohedral fullerenes A simple formula has been developed that permits an estimation of the oxidation state of a fullerene encapsulated atom based on the positive shift of the atomic orbital energies.30 The production and isolation of the ‘missing’ endohedral fullerenes such as Ca@C 72 ,31 Ca@C 74 ,31 La 2 @C 72 ,32 Eu@C 74 ,33 Ca@C 80 ,34 and Ba@C 80 34 have all been reported. A two step extraction process using first xylene to remove empty fullerenes and second N,N-dimethylformamide has been developed for the isolation of endometallofullerenes.35 A high temperature and pressure protocol also results in the e¶cient extraction of endometallofullerenes.36 Nucleus-independent chemical shifts have been calculated for a range of fullerenes.37 The 3He NMR spectra of He 2 @C 70 38 and Ne 2 @C 70 39 have been reported.The most shielded and deshielded signals in 3He NMR spectroscopy of 3He inside a fullerene result from the reduction of C 60 to C 60 6~ and C 70 to C 70 6~ respectively.40 The former providing compelling evidence for the ability of electrons to move freely about the surface of a spheroidal p-system. 4 Organic chemistry Reviews of electroactive fullerenes, from intercalated systems to covalently linked fullerene–donor dyads,41 and of the family of fulleropyrrolidines are available.42 Two boronic acid groups have been regioselectively introduced into the fullerene cage using saccharides as template molecules.43 The previously reported addition of diethylaminopropyne to C 60 results in a [2]2] cycloadduct which can be ring-opened under acidic conditions, and when followed by oxidative cyclisation in the presence of activated carbon, yields novel fullerene lactones.44 Further work on the reaction of C 60 with propiolates in the presence of triphenylphosphine has resulted in the synthesis of bismethanofullerenes and a [2]2] cycloadduct consisting of a cyclobutene ring fused to the carbon atoms of a 6,6-ring junction.45 A novel ring opening of a methano[60]fullerene under reducing conditions, NaBH 3 CN, resulted in the formation of a 1,2-dihydro[60]fullerylglycine derivative.46 The photoreaction of C 60 with tetramethyl ethylenediaminetetraacetate results in the formation of a number of mono-adducts with mutliple methyl carboxylate groups which, it is suggested by the authors, may have useful biological activity.47 A fullerene modified protein has been Annu. Rep.Prog. Chem., Sect. A, 1999, 95, 431–451 433Fig. 2 Computer generated structures of diisopropylcyclohexyl [60]fullerene alcohol 4 and the diphenyl [60]fullerene alcohol 5 targeted as fullerene inhibitors of HIV-1 protease synthesised by combination of a surface cysteine-containing redox protein with a [60]fullerene derivative labelled with a thiol selective reagent; significant electrochemical interaction between the fullerene and protein redox centre was also reported.48 A dimethylpyrrolidinium [60]fullerene derivative has been synthesised and complexed with DNA resulting in the formation of hybrid DNA/fullerene-based architectures which were imaged using transmission electron microscopy (TEM).49 The successful enzymatic modification of fullerene derivatives by a restricted number of lipases, which were selected from the range of enzymes tested, has been reported for the first time.50 The highly diastereoselective formation of stable cluster opened fulleroids has been reported with the bulkier groups preferentially located above a five-membered ring of the fullerene cage.51 High a¶nity [60]fullerene ligands 4 and 5 (Fig. 2) of the HIV-1 protease, which have an increase of 50-fold binding over previously tested fullerene compounds, have been designed and synthesised using Bingel addition to [60]- fullerene.52 Highly water soluble dendro[60]fullerenes which have 18 peripheral carboxylic acid groups have also been synthesised by the nucleophilic cyclopropanation method.53 Cycloaddition of per-O-acetyl glycosyl azides to [60]fullerene result in the formation of two unseparable stereoisomers, which the authors suggest may have important biological activity.54 The addition of benzyne to C 70 has been repeated and the four main isomeric mono-adducts isolated by HPLC; the unusual addition of a benzyne group across a closed 5,6-ring fusion results in the formation of the third most abundant mono-adduct isomer.55 Several groups have synthesised and examined the properties of fullerene–porphyrin diads with a variety of structural architectures.For example, a zinc porphyrin and C 60 diad was constructed via the cyclopropanation route; almost no electronic Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 431–451 434Fig. 3 A tetraphenylporphyrin with four [60]fullerene substituents 6.(Reproduced by permission from Angew. Chem., Int. Ed., 1998, 37, 1934.) interaction between the fullerene and porphyrin moieties was observed.56 In contrast, two-fold regioselective cyclopropanation leads to a [60]fullerene-porphyrin diad which has significant interaction between the two p–p stacked chromophores owing to the restricted conformational freedom of the covalent linkages.57 A porphyrin with two fullerene substituents has been prepared and a variable temperature NMR study used to show that there is a mixture of two conformers present which are in slow equilibrium.57b,58 Similarly, two [60]fullerene units have been covalently linked to a porphyrin unit using the pyrrolidine methodology; in this example, considerable interaction of the chromophores in the cis-configuration owing to the close proximity of the two [60]fullerene moieties has been reported.59 Four [60]fullerene groups have also been appended to a central porphyrin by the reaction of a bis-adduct of C 60 bearing an aldehyde group with pyrrole under acidic conditions (6, Fig. 3); electrochemical studies of the product indicate that there is electronic interaction between the molecular components in both the ground and excited states.60 An alternative approach to the synthesis of diads of this type was achieved by the reaction of bis(octaethylporphyrinato)ruthenium(II) with C 60 and has resulted in the first example of a porphyrin and [60]fullerene linked directly by a metal bond; the fullerene is g2-coordinated to the the Ru metal.61 Further spectroscopic evidence for photoinduced electron transfer in triads, which mimic similar processes observed in photosynthetic reaction centres, and which consist of a porphyrin covalently linked to a [60]fullerene group by a carotenoid polyene has been obtained at 20 K.62 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 431–451 435Other approaches to the construction of fullerene diads have been developed.A series of methano[60]fullerene based ligands in which a tridentate 2,2@: 6@,2A-terpyridine group is attached to the fullerene have been synthesised and interaction between the fullerene and metal centre observed when they are spatially close.63 The synthesis of other methanofullerenes which incorporate terdentate diamino aryl ligands which can be converted to the nickel(II) compound via an oxidative addition reaction has been achieved.64 The self-assembly of a dipyridyl substituted methano[ 60]fullerene is directed by Pt(II) centers.65 The 5,6-open 2-aminomethylpyridineappended [60]fullerene interacts more strongly with Ag` than the comparable 6,6 closed isomer.66 Other [60]fullerene diads containing cyclohexene-fused tetrathiafulvalene67 and anthraquinone-based dienes have also been synthesised.68 The electron accepting properties of a family of N-methylpyrrolidinium fullerene iodide salts are enhanced when compared to those of the parent fulleropyrrolidines and [60]- fullerene.69 A number of fullerene–crown ether/calixarene hybrids have also now been synthesised.The regioselective bis-addition of a crown ether using the Bingel reaction has resulted in a [60]fullerene conjugate which is capable of binding metal cations.70 A di§erent approach based on azido addition to fullerenes produced a calix[4]arene linked to the fullerene core via ionophoric chains which enables e¶cient complexing of metal cations.71 Other hybrids include [60]fulleropyrrolidines which have calix[4]- arenes72 and crown ether groups.73 Even a C 60 functionalised with a calix[4]-spiro-bis crown has been produced.74 The Bingel reaction of [60]fullerene has been used to synthesise a thermotropic liquid crystalline material which contains ferrocene moieties.75 Regioselective bisaddition to C 60 has been achieved by reaction with bis(b-keto esters) in the presence of DBUand iodine.76 As an extenson of this work, amphiphilic [60]fullerene bis-adducts with a polar head group and long pendant chains have been prepared and found to form stable Langmuir films at the air–water interface.77 It has been found that a bisazafulleroid 8 is converted to the aza-aziridinofullerene 7 when exposed to ambient light whilst the retro-reaction is achieved by thermolysis in refluxing toluene (Fig. 4).78 Isomerization of bis-adducts of C 60 can also be achieved under electrochemical conditions by migration of di(alkoxycarbonyl)methano bridges.79 Similarly, the retro- Bingel reaction under electrochemical conditions has been used to obtain enantiomerically pure samples of [76]fullerene.80 The synthesis and properties of a range of enantiomerically pure bis- and tris-adducts of [60]fullerene, which may find use as chiral auxiliaries in enantioselective synthesis, with inherent chiral addition patterns have been reported.81 The absolute configuration of some chiral fullerene derivatives have been assigned by comparison of experimental circular dichroism (CD) spectra with theoretically predicted spectra.82 The CD spectra of some other enantiomeric [60]fullerene derivatives have also been recorded83 whilst the enantiomers of a cageopened keto lactam [60]fullerene derivative have been separated by chiral HPLC and their CD spectra recorded.84 Reviews of the elecrochemistry of fullerenes and their derivatives85 and the addition of free radicals to [60]fullerene are available.86 The electrochemical properties of a range of covalent derivatives of higher fullerenes, C 70 , C 76 and two isomers of C 78 have been investigated by cyclic voltammetry; in most of the derivatives reduction requires more energy than for the parent fullerene whereas the oxidation becomes progressively Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 431–451 436Fig. 4 A possible mechanism for the conversion of the aza-aziridinofulleroid 7 to the bisazafulleroid 8.The substituent on nitrogen has been omitted for clarity. (Reproduced by permission from Chem. Eur. J., 1998, 4, 2037.) easier with increasing carbon number.87 The selective generation of [70]fullerene anions has been achieved using a variety of reduction conditions and an aqueous colloidal solution of C 70 found to have excellent optical limiting properties.88 The photochemical reduction of a water-soluble N-methylpyrrolidinium [60]fullerene iodide in a titanium dioxide suspension is observed to proceed analagously to that of pure [60]fullerene.89 The formation of organofullerenes from fullerene dianions is reported to result from a combination of (i) electron transfer which initially forms C 60 R~ (R\alkyl group such as Bu5) and is then followed by (ii) formation of C 60 R 2 by an S N 2 reaction; a range of fullerene derivatives with two di§erent alkyl groups have been prepared using this methodology.90 The reaction of [60]fullerene with methoxide anion results in the formation of the C 60 ·~ which then reacts to produce the dimethoxy[60]fullerene; the reaction is found to be highly dependent on the concentration of methoxide anion present.91 The same [60]fullerene radical anion, C 60 ·~, is also produced during the reaction of C 60 with 1-benzyl-1,4-dihydronicotinamide via a photoinduced electron transfer reaction.92 The nucleophilic addition of a variety of lithium acetylides to [60]fullerene followed by quenching under acidic conditions has resulted in the formation of a range of derivatives which have polar functional groups.93 Similarly, addition of Grignard reagents containing protected mercaptoalkyl groups have been successfully employed to produce [60]fullerene derivatives with thiol moieties in the addended group.94 During the addition of propiolates in the presence of trialkylphosphines described earlier,45 the authors also observed the reaction of trialkylphosphine oxides with [60]fullerene, which results in the formation of 1,2-addended [60]fullerenes.95 C 60 F 36 consists of two isomers which have T and C 3 symmetry.96 An epoxide, C 60 F 18 O, other oxides and hydroxides have been isolated as either by-products formed during fluorination of [60]fullerene or as decomposition products of the fluorinated fullerene.97 The reaction of C 60 F 18 with benzene under Friedel–Crafts Annu. Rep.Prog. Chem., Sect. A, 1999, 95, 431–451 437Fig. 5 Computer generated structure of the trisphenylated adduct of [70]fullerene 9 conditions produces a single isomer of C 60 F 15 Ph 3 , where the most accessible fluorines have been replaced.98 Fluorinated [60]fullerene has also been reacted with methyland phenyllithium resulting in mixtures of poly-methylated and -phenylated [60]- fullerene.99 The Friedel–Crafts methodology has been extended to include the reaction of C 60 Cl 6 with allyltrimethylsilane and results in the substitution of all six chlorine atoms by allyl groups.100 Moreover, the first stable [60]fullerene carbocation, C 60 Ph 5 `, has been characterised by in situ NMR experiments and confirms the reaction mechanism to be S N 1 via frontside attack.101 Electrochemical studies of a dihydroxyanthraquinone [60]fullerene derivative suggests that this species may also form a stablised fullerene cation.102 Reaction of the monoarylated azafullerene, ArC 59 N, with iodine monochloride mirrors the reactivity observed for C 60 and produces Cl 4 ArC 59 N which has a pyrrole moiety embedded in the surface of the fullerene cage.103 The foregoing arylated fullerene is produced by the thermal treatment of the dimer, (C 59 N) 2 , with an aromatic species under acidic conditions.104 [70]fullerene reacts with organocopper reagents producing the tris-adduct, C 70 Ar 3 H (9, Fig. 5) which has an indene-like substructure flanked by three sp3 carbons embedded within the fullerene cage; the foregoing derivatives were used to synthesise metal complexes.105 Hydrogenation of C 70 using a Zn(Cu) couple has allowed the separation of C 70 H 2 , C 70 H 4 and C 70 H 8 ; interestingly the authors have suggested that three di§erent reduction mechanisms are in action at any one time as, for example, upon further reduction of C 70 H 2 the same isomer of C 70 H 8 is not produced.106 Hexanitro[60]fullerene, C 60 (NO 2 ) 6 , has been produced by the reaction of [60] fullerene with NO 2 (no information was provided regarding addition sites of the nitro groups) and subsequently reacted with aniline.107 Oxidative sulfation was used to produce a mixture of isomers of hexacyclosulfated fullerene which upon hydrolysis is Annu.Rep.Prog. Chem., Sect. A, 1999, 95, 431–451 438converted to the dodecahydroxyfullerene (again no information was provided regarding the addition sites).108 Electron reduction of [60]fullerene followed by quenching with 1,4-butane sultone produced highly water soluble hexa(sulfobutyl)fullerenes as mixtures of isomers which behave as potent free-radical scavengers in physiologic media.109 Oleum induced oxidative sulfation of [70]fullerene followed by hydrolysis has resulted in the production of a mixture of isomers of octadecahydroxylated [70]fullerene, C 70 (OH) 18 .110 The anti-proliferative e§ect of polyhydroxylated [60] fullerene on smooth muscle cells has been ascribed, in part, to be mediated through the inhibition of tyrosine kinase.111 Significant membrane damage was observed upon the photosensitisation of rat liver microsomes in the presence of polyhydroxylated fullerenes.112 Singlet oxygen does not react with ground state [60]fullerene but does react with the lowest triplet exited state to produce the fullerene epoxide, C 60 O.113 C 60 has been found to oxidise to C 120 O, which has a furanoid bridge between the two fullerene moieties, at ambient temperature and pressure.114 A series of dimeric fullerene oxides have been extracted from fullerene soot using a novel hydrothermally initiated dynamic extraction method.115 Additional oxides of [60]fullerene have also been isolated as by-products of the reaction between C 60 and C 60 O which produces the aforementioned C 120 O.116 Matrix assisted laser desorption-ionization Fourier transform mass spectrometry (MALDI-FTMS) has been shown to be a fast and e¶cient method for the analysis of fullerene oxides.117 5 Polymers A range of fullerene polymers have been synthesised using a variety of techniques.Fullerene functionalised polycarbonates have been produced by the reaction of [60]- fullerene with polycarbonates in the presence of AlCl 3 ; the resultant polymer limits strong 532nm optical pulses more e§ectively than the parent fullerene.118 An alternative approach allows the synthesis of fullerene-containing polymers by the reaction of functional groups, such as carboxylic acid moieties, within a fullerene derivative with, for example, a diamine resulting in the formation of polyamides.119 Fullerene functionalised polydienes have been prepared by grafting C 60 onto a lithiated polymer chain and the fullerene moieties attached to the polymer chain have subsequently been converted to polyhydroxylated derivatives.120 Nucleophilic addition of poly(vinyl alcohol) to [60]fullerene results in polymers with reduced viscosity as the concentration of C 60 within the polymer increases.121 [60]Fullerene end-capped triarm poly(ethylene oxide) stars produce highly expanded fluid Langmuir monolayers.122 Novel copolymers of [60]fullerene/styrene with di§ering contents of [60]fullerene have been synthesised by anionic polymerization.123 Methacrylate containing ‘charmbracelet’ fullerene polymers have been produced by the reaction of [60]fullerene with azide-containing methacrylate monomers; the polymers obtained were subsequently copolymerised under anionic conditions with a Grignard reagent.124 C 60 has also been covalently incorporated into amphiphilic polysulfonates where the fullerene moieties show extreme blue shifted fluorescence due to the highly constrained hydrophobic microenvironment in which they are trapped.125 Star-like polymers of polyoxyethylenes which have C 60 molecules at their core have herbicidal activity.126 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 431–451 439Palm-tree like architectures where five polystyrene branches of equal length are connected to a trunk of polystyrene through a single [60]fullerene molecule have been prepared.127 Two routes have been used to graft [60]fullerene to polythiophene producing novel intramolecular donor–acceptor polymers.128 First, the fullerene was covalently attached to a bithiophene derivative which was electropolymerised and second, polythiophene was prepared and then functionalized with [60]fullerene.Poly(N-vinylcarbazole) has been modified by anionic addition to [70]fullerene and the resulting polymer found to have enhanced stability when compared with the parent polymer.129 Polyaniline emeraldine base, a conducting polymer, has been doped with polysulfonated [60]fullerene which acts as a protonic acid dopant; the resultant polymers have conductivities of about six orders of magnitude higher than that of typical fullerene-doped conducting polymers.130 Conjugated polymer/fullerene blends produce photo-induced changes in the complex index of refraction through charge transfer. 131 Multilayers of sexithienyl polymer and [60]fullerene do not have a stable charge-transfer state at the interfaces of the two layers but strong photoluminesence reduction and a much faster radiative lifetime do provide indirect evidence for the formation of a metastable charge transfer state.132 The dominant long-lived photoexcitations in [60]fullerene–poly(p-phenylene vinylene) polymers consist of interchain spin-correlated polaron pairs which were characterised using transient spectroscopy. 133 Conjugated polymer:fullerene blends have been embedded in a conventional polymer, polystyrene, and the photoexcitations of the composite materials investigated. 134 6 Organometallic chemistry A comprehensive review of fullerene organometallic chemistry is available covering all aspects of the field.135 A computational study explains the ready formation of g2- fullerene complexes due to the orientation of the p-orbitals and the large release of strain energy upon bonding a metal atom.Similarly the p-orbitals have been shown to be poorly oriented for overlap with an exohedral metal atom centred over five- or six-membered rings whereas an endohedral metal atom is well positioned for orbital overlap under a five- or six-membered ring.136 Photolysis of M(CO) 4 (dppe) (M\W or Mo) with C 60 or C 70 results in the formation of g2-complexes of the fullerenes, M(CO) 3 (dppe)(g2-C 60 ) and two isomers of Mo(CO) 3 (dppe)(g2-C 70 ) where the metal centres have distorted octahedral geometries in all cases.137A series of transition metal derivatives, including M(NO)(PPh 3 ) 2 (g2-C 60 ) (M\Co or Rh), RuX(NO)(PPh 3 ) 2 (g2- C 60 ) (X\Cl or H), and TaH(g-C 5 H 5 ) 2 (g2-C 60 ) have been prepared and an additional example of complexation involving adjacent fullerene double bonds, Re 2 H 8 (PMe 3 ) 4 (g2,g2-C 60 ), reported.138 Reaction of [60]fullerene with iridium cycloocta- 1,5-diene dimers, Ir 2 (OR) 2 (cod) 2 (R\Me, Et, Pr/, Ph) containing bridged alkoxide groups results in complexes, Ir 2 (OR) 2 (cod) 2 (g2,g2-C 60 ), which again involve coordination at adjacent double bonds.139 Examples of coordination in which the [60]- fullerene acts as a cyclohexatriene-like ligand have also been produced.Reaction of [60]fullerene with Ru 5 C(CO) 15 with C 60 followed by PPh 3 , dppe or dppf forms Ru 5 C(CO) 11 (PPh 3 )(g2,g2,g2-C 60 ), Ru 5 C(CO) 10 (dppe)(g2,g2,g2-C 60 ), and Ru 5 CAnnu.Rep. Prog. Chem., Sect. A, 1999, 95, 431–451 440Fig. 6 Side view of the crystal structure of [60]fullerene and a tribromohexahomooxacalix[ 3]arene. (Reproduced by permission from Chem. Commun., 1998, 896.) (CO) 10 (g1,g1-dppf)(g2,g2,g2-C 60 ).140 Trisosmium complexes of [60]fullerene, Os 3 (CO) 9 (g2,g2,g2-C 60 ), Os 3 (CO) 8 (PMe 3 )(g2,g2,g2-C 60 ), Os 3 (CO) 8 (PPh 3 ) (g2,g2,g2-C 60 ) and Os 3 (CO) 7 (PMe 3 ) 2 (g2,g2,g2-C 60 ) in addition to some osmium g2-C 60 complexes have also been synthesised and characterised.141 A short review of the use of fullerene-based (including nanotubes) materials for use as new carbon-support media in heterogeneous catalysis by supported metals is available.142 The electrochemical reduction of [60]fullerene in the presence of (PhCN) 2 PdCl 2 , Ir(CO) 2 Cl(C 7 H 9 N) (C 7 H 9 N\p-toluidine) or (CF 3 CO 2 ) 4 Rh 2 produces three di§erent redox active black films which coat the electrode; analysis suggests that the metal atoms within the films are g2-bonded to the fullerene itself.143 A Pt-based catalyst supported on silica was grafted with [60]fullerene and the liquid phase hydrogenation of cinnamaldehyde evaluated; a fairly good selectivity to cinnamyl alcohol was observed.144 New charge-transfer complexes of [60]fullerene derivatives, 1,2-dicyanodihydro[60]fullerene [C 60 (CN) 2 ]145 and 3-aminophenylmethano[ 60]fullerene [C 60 (CHPhNH 2 )],146 with cobaltacene have been prepared; the fullerene derivative in the former complex is reduced to the anion. 7 Inclusion complexes In addition to the many covalent crown ether–calixarene fullerene derivatives described earlier there have been a considerable number of non-covalent supramolecular complexes synthesised during the last year.Single crystal analysis of C 60 and a hexahomooxacalix[3]arene, substituted with bromine atoms on the upper rim, indicates that van der Waals attractive interaction is the main driving force which locates the six-membered rings of the [60]fullerene close to the aromatic rings and the dibenzyl ether oxygen (Fig. 6).147 Crystal structure analysis of the isostructural com- Annu.Rep. Prog.Chem., Sect. A, 1999, 95, 431–451 441Fig. 7 Plot of the cationic and anionic units in [Ni(NH 3 ) 6 ]C 60 ·6NH 3 . (Reproduced by permission from Chem. Commun., 1998, 1205.) plexes, [(calix[6]arene)(C 60 ) 2 ] and [(calix[6]arene)(C 70 ) 2 ] shows that the calixarenes are in the double-cone conformation and that each cavity is occupied by a fullerene molecule.148 Cyclotriveratrylene (CTV) derivatives with pendant aromatic groups have been synthesised and complexed with [60]- and [70]-fullerenes; the CTV derivatives with N-methylpyrrole substituents were found to have the highest association constant with C 60 .149 Similar bis-CTV molecules with rigid acetylenic spacers have also been synthesised and found to complex [60]fullerene e¶ciently.150 Bis-calix[5]- arenes singly bridged at the upper rim having a but-2-enyl linker and carrying allyl groups at all of the other available p-positions are particularly e§ective at complexing [60]fullerene.151A series of bridged calix[5]arenes with rigid alkyne containing spacer groups preferentially bind [70]fullerene.152 During a study to examine which calix[n]- arenes e¶ciently bind fullerenes it was found that calix[n]aryl esters do not interact with [60]fullerene however they become excellent fullerene acceptors upon the addition of specific metal cations and this fact is attributed to a change of conformation in the calix[n]arene ester induced by the metal cation.153 Amphiphilic poly(phenylquinoline) –block-polystyrene rod–coil diblock copolymers self-organise into micrometre scale spherical aggregates from solution, which have large hollow cavities in which it is possible to encapsulate 1010 fullerene molecules per aggregate.154 8 Metal fullerides The optical properties155 and recent developments in the chemistry and physics of metal fullerides have been reviewed.156 A new and versatile method has been developed for the synthesis of fulleride materials based on ion exchange in liquid ammonia.For example, the ammoniate [Ni(NH 3 ) 6 ]C 60 ·6NH 3 was prepared from K 2 C 60 via an ionic exchange resin loaded with Ni2` in liquid ammonia (Fig. 7).157 The same authors have thus far used the same technique to produce [Ba(NH 3 ) 6 ]C 60 ·NH 3 ,158 [Mn(NH 3 ) 6 ]C 60 ·NH 3 ,159 [Cd(NH 3 ) 6 ]C 60 ·NH 3 159 and even organic equivalents, (PhCH 2 NMe 3 ) 2 C 60 ·3NH 3 .160 Using a low-temperature synthesis protocol Cs 4 C 60 has been obtained as a pure single phase for the first time and is found to have an Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 431–451 442orthorhombic distortion of the body-centred tetragonal structure observed for K 4 C 60 .161 A mechanochemical methodology has been used to synthesise alkali and alkaline earth fullerides in organic solvents.162 Coevaporation of [60]fullerene and titanium under ultra high vacuum conditions results in the formation of titanium fulleride films, (TixC 60 ).163 Europium fullerides, EuxC 60 (x ca. 3 and 6) have been prepared and the structures found to have structures analagous to those of other metal fullerides with similar compositions.164 Stable fulleride films with two di§erent structures are believed to have been formed during the examination of the redox behaviour of [60]fullerene films in the presence of 18-crown-6 complexes of alkali metals.165 The crystalline compound [Ph 4 P`] 2 [C 60 ]~[I~]x contains a high symmetry two-dimensional network of cations, which function as an e§ective host completely encapsualting the fulleride ion with both face-to-face and face-to-edge fullerene–phenyl interactions being observed.166 The insertion of an excess of potassium into pure D 2 and D 2$ isomers of [84]fullerene produces the cubic phases with composition, K 8`xC 84 ; there is multiple occupancy of the octahedral sites by potassium.167 9 Nanotubes Owing to the very large number of publications concerning the synthesis, properties, applications and theoretical considerations in the carbon nanotube related area only a representative collection of papers are reviewed here.Where possible reference to key reviews of areas less well covered are included for the readers convenience.168 The synthesis of nanotubes has been a major area of research with a number of major breakthroughs in the control of their preparation.Well-aligned clusters of multi-walled nanotubes (MWNTs) have been grown on nickel coated glass at temperatures below 666 °C using acetylene gas as the carbon source and ammonia gas as both carrier and catalyst gas.169 In contrast, individual high quality single-walled nanotubes (SWNTs) are synthesised, and can subsequently be manipulated individually, by chemical vapour deposition of methane on silicon wafers patterned with micrometre scale islands of catalytic material.170 Aligned bundles of nanotubes result from the pyrolysis of ferrocene or ferrocene–acetylene mixtures using a simple two furnace procedure with Ar as carrier gas (Fig. 8).171 Pyrolysis of organic materials over laser etched cobalt thin films produces aligned nanotubes of uniform length and diameter; the tubes grow perpendicularly to the catalytic substrate and only in the etched regions.172 Very long carbon nanotubes, 2mm in length, are produced by the pyrolysis of acetylene over iron/silica substrates.173 The pyrolysis of tripropylamine in the 0.73nm pores of microporous aluminophosphate crystallites results in the formation of parallel-aligned SWNTs.174 Graphitic multi-walled carbon cube-like nanocages result under arc conditions when graphite is evaporated in the presence of calcium or strontium; the cages are deposited on the cathode surface and range in size from 20 to 100 nm.175 Some SWNTs which were produced using pulsed laser vaporization have been found to contain encapsulated [60]fullerene molecules.176 Carbon nanotubes and polyhedra can be formed spontaneously at temperatures as low as 50 °C by the interaction of nano-porous carbon [produced by pyrolysis of a mixture of poly(furfuryl alcohol) and poly(ethylene glycol] with elemental caesium.177 Several approaches to the synthesis of nanotubes based on metal catalysis in Annu.Rep. Prog. Chem., Sect.A, 1999, 95, 431–451 443Fig. 8 SEM images of aligned nanotubes generated by pyrolysis of ferrocene. (Reproduced by permission from Chem. Commun., 1998, 1525.) addition to those described above have been published.178–182 SWNTs are produced at a rate of 1mg h~1, and without termination of tube growth occurring, provided that the reactant gas can di§use through the product nanotube mat that covers the catalyst, by the decomposition of CO or ethylene over a supported metal catalyst.178 Unusual highly graphitic needle-like tubes, which contain encapsulated Ni, are produced when a sandwich of alternating films of [60]fullerene and Ni deposited on a silica plate is heated.179 The production of magnetic-material free SWNTs, which should make them more suitable for electrical and magnetic studies, is remarkably increased when a Rh–Pt catalyst is used during the arc-discharge process.180 Long and wide ropes, larger than those obtained by arc-discharge or laser vaporisation, of approximately aligned SWNT bundles result from the catalytic decomposition of hydrocarbons.181 Purification of carbon nanotubes remains a major challenge and a number of strategies have now been developed to achieve this.Size exclusion chromatography using a stationary phase with a defined pore size of a dispersion of MWNTs results in the isolation of chemically unmodified, almost impurity free and size separated MWNTs.183 The process also works well for SWNTs.184 A similar process uses Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 431–451 444ultrasonically assisted filtration to remove amorphous and crystalline carbon impurities from SWNTs; sonication prevents filter contamination and provides a fine suspension throughout the process, however there is some sonication induced cutting of the tubes.185 Boiling of SWNTs with concentrated nitric acid removes the majority of impurities, however some tube damage is induced during this technique.186 An unusual method for the purification of nanotubes employs ac electrophoresis in isopropyl alcohol, the nanotubes are separated from other material by their movement towards the electrodes for all applied fields.187 The chemistry of the nanotubes is now beginning to develop.SWNTs are selectively opened at their tips by concentrated HCl and may be filled with, for example, crystals of Ru metal using wet chemistry techniques.188 Similarly SWNTs can be cut into smaller sections using sonication in a mixture of concentrated sulfuric and nitric acids.189 Once opened the tubes can be filled with a variety of materials for example enzymes, proteins190 and DNA.191 The biomolecules appear to be encapsulated within an environment which o§ers some protection and subsequent analysis of the catalytic activity of the immobilised enzymes showed that a significant amount of the enzyme retains its activity.190 Alternatively materials such as UCl 4 can be deposited inside MWNTs via the capillary action of eutectic and non-eutectic mixtures of UCl 4 and KCl.192 By expressing the wetting conditions as a function of polarizabilities it is possible to predict the threshold diameter for capillary filling of di§erent tubes.193 Alternatively, material can be incorporated within the tubes during preparation of the tubes.Using the arc discharge methodology, tubes have been filled with, for example, transition metals such as Cr, rare earth metals like Yb or covalent solids such as S; when graphite rods which contain no sulfur were used in the process there was no generation of tubes which contained encapsulated material leading the authors to propose a growth mechanism which incorporates carbon, metal and sulfur.194 Microcrystals of refactory carbides such as TaC may also be encapsulated and moreover novel metallic b-Sn nanowires can be produced by electrolyis of graphite electrodes immersed in molten LiCl, which contains SnCl 2 .195 A simple technique of high temperature treatment of a microporous carbon, which has been impregnated with a compound or material to be encapsulated such as molybdenum, uranium or cobalt, results in the formation of metal-filled carbon manoparticles.196 The opening of the tube tips has enabled the resultantOHand CO 2 Hgroups (which are assumed to be present) to be functionalised using standard chemical reaction conditions. For example SWNTs have been functionalised at the opened tips by reaction with thionyl chloride and octadecylamine; the derivatised tubes are more soluble in organic solvents than their parent compounds and can subsequently be reacted further along the waists of the molecules.197 Dichlorocarbene, fluorination, Birch reduction, and photolabelling experiments are also reported to successfully functionalise SWNTs.198 The Young’s modulus for a number of tubes has been estimated and found to be larger than that of the in-plane modulus of graphite by observing their free-standing room temperature vibrations in a transmission electron microscope.199 The buckling and collapse of SWNTs has been observed as a result of their incorporation into polymeric films; the tubes are calculated to have compressive strengths approximately two orders of magnitude higher than any known fibre.200 It is suggested that nanotube tips may be used in nanolithography and a cleaving method has been developed to Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 431–451 445reproducibly obtain dome-closed MWNT tips of suitable length and size.201 Covalently functionalised nanotube tips, which are produced by reaction of the carboxylic acid groups present to form amide moieties, have been used as probes in scanning tunneling microscopy (STM) and are sensitive to specific molecular interactions, a fact which should prove especially useful in the study of biological systems.202 Non-functionalisedMWNTshave also been used as high resolution probes for atomic force microscopy (AFM) by the same authors.203 Dry etching pattern transfer in GaAs using carbon nanotubes as tips in AFMhas been investigated independently.204AFM has also been used to manipulate their shapes, for example tubes can be bent or straightened, and the position of individual MWNTs dispersed on a surface.205 The semimetal-like temperature dependence of the electrical conductivity and a finite gap in the IR absorption spectrum is predicted by first principle calculations for a (10,10) tube with broken symmetry.206 Atomically resolved STM images have allowed the examination of the electronic properties of SWNTs as a function of tube diameter and wrapping angle; both metallic and semiconducting tubes were observed.207 Similar atomic resolution images of individual SWNTs reveal that a triangular arrangement of carbon atoms is seen rather than the expected hexagonal arrays.208 The conductance, which is found to be quantized, of MWNTs was measured by dipping a scanning probe microscope mounted tube into a liquid metal.209 One semiconducting SWNT connected to two metal electrodes has been used to construct a field-e§ect transistor which operates at room temperature.210 Calculations show that nanotubes should have ballistic transport properties.211 There has been considerable interest in the production of field-emission devices from nanotubes.Luminescence during electron field emission on SWNTs and MWNTs has been observed and is believed to be due to electron transitions between di§erent electronic levels participating in the field emission.212 Lighting elements with the structure of a triode-type vacuum tube having the conventional thermionic cathodes replaced with MWNT field emitters have been fabricated and stable electron emission, adequate luminance and long life of the tubes demonstrated.213 Similarly nanotubes are reported to compare favourably with other field emission sources owing to their low operating voltages and high current densities.214 Aligned nanotubes have also been filled with catalyst materials and used to electrocatalyse O 2 reduction and methanol oxidation.215 Addition of nanotubes, which act as heat sinks, to conducting polymers increases their electrical conduction by up to eight orders of magnitude.216 References 1 K.-H.Homann, Angew. 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