Oxidation and Reactions with Electrophiles

Recently, a rather stable Cgo salt was isolated and the redox potentials of the fullerene 

Whereas Cgg and C g are easy to reduce, their oxidation occius at comparatively high anodic potentials [1, 2]. Theoretical investigations predict the first oxidation potential ofCgg to be comparable to that of naphthalene [3]. Anodic electrochemistry with fuUerenes has been carried out with Cgg films [4] as well as in solution [5-7]. Cyclic voltammetry of Cgg in a 0.1 M solution of Bu4NPFg in trichloroethylene (TCE) at room temperatare exhibits a chemically reversible, one-electron oxidation wave at -tl.26 V vs Fc/Fc (Figiue 8.1) [7]. Under identical conditions, a one-electron, chemically reversible oxidation is also observed for Cyg. The oxidation of Cyg occurs 60 mV more negative than that of Cgg at -tl.20 V vs Fc/Fc. The energy difference between the first oxidation and the first reduction potential is a measiue of the 

CVs were run at a scan rate of 100 mV s. CV with Cgo (A) and Cjq (B), no ferrocene added. Cathodic and anodic cyclic scans were performed separately [7]. 

The stability of the Cgg -cation can be increased by adding trifluoromethane-sulfonic acid CF3SO3H to the TCE/(TBA)PFg solutions [8]. Under these conditions the life-time of the cation is estimated to be at least some hours. Cgg is probably stabilized by CF3SO3H because of acid s ability to scavenge water and nucleophiles that would react readily with the cation. The elevated lifetime of the cation made it 

UV/VIS/NIR spectroscopy and ESR spectroscopy. The UV/VIS/NIR spectrum shows a sharp peak at 983 nm and a broad peak at 846 nm. These two absorbances are attributed to allowed NIR-transitions and these values are consistent with spectra of the cation obtained with other methods [2]. EPR spectroscopy of Cgg-cations, produced by different methods, leads to a broad distribution of measured g-values. These differences are caused by the short lifetime of the cation, the usually low signal-noise ratio and the uncertainty of the purity. The most reliable value imtil now is probably the one obtained by Reed and co-workers for the salt Cgg (CBiiHgClg)-(g= 2.0022) [2,9] (see also Section 8.5). Ex situ ESR spectroscopy of above-mentioned bulk electrolysis solutions led to a g-value of2.0027 [8], which is very close to that of the salt, whereas the ESR spectra of this electro lyticaUy formed cation shows features not observed earlier. The observed splitting of the ESR signal at lower modulation amplitudes was assigned to a rhombic symmetry of the cation radical at lower temperatures (5-200 K). 

The fluorination of Cgg was achieved either by the treatment of dichloro-methane solutions with Xep2 [154] or by allowing fluorine gas at low pressure to 

A polychlorination of Cgg was carried out by allowing a slow stream of chlorine gas to react with Cgg in a hot class tube at temperatures between 250 and 400 °C [161] or by the treatment of sohdCgo with liquid chlorine at-35 °C [162]. The chlorofuUerene CggClg (see Sect. 3.2.4) as an isomerically pure single product was synthesized by the reaction of Cgg with an excess of ICl in benzene or toluene at RT [163]. 

The treatment of Cgg with Hquid bromine afforded a T ,-symmetric C60 r24 (7) as yellow orange crystalline compound (Fig. 14) [164]. Bromination in CS2 gave CggBrg (8) as dark brown crystals in 80% yield [165]. The bromide CggBrg, iso-structural to CggClg, was formed by bromination in benzene or CCI4 [165,166]. 

A review on halogenated fuUerenes was provided by Taylor [167]. Here their properties are also compared and summarized. The stability order of the halo-genofullerenes is comparable to that generally of other organic haUdes, since it increases along the series iodo- bromo- chlorofullerene( fluorofullerene). 

Persistent radical cations of Cgg have been observed in super acidic media, for example in magic acid (FSO3H, SbFj) [168]. Multiply charged fuUerene radical cations were also stabifized in a mixture of fuming sulfuric acid and SO2CIF at low temperatures [169]. The reaction of Cgo with sulfuric acid and nitric acid and the subsequent hydrolysis of the intermediates with aqueous base resulted in the formation of fuUerenols [170,171]. 

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