Reduction Stages of Fullerenes

Electron affinity measurement [132] and reduction experiments [133] showed that fullerenes can accept a large number of electrons. The anions formed were charac- 

Ceo in CHjCN/toluene at -10 °C showing six successive reversible reductions to Cso - Reproduced with permission from J. Am. Chem. Soc. 1992, 114, 3978-3980. Copyright 1992 Am. Chem. Soc. 

Electrochemistry holds a central place in the study of fullerene anions. After several experiments, the optimized electrochemical conditions made it possible to observe the fifth and sixth reduction waves on the cyclic voltammetry (CV) time scale (Fig. 13.6) [34]. It was found that the medium highly influences the redox potential [133, 134]. The counter cations influence the kinetics, whereas the solvent affects the redox potentials substantially. For a number of fullerenes, a good correlation was found between the calculated HOMO-LUMO gap and the difference between the first reduction potential and the first oxidation potential in the CV [135]. 

Fullerenes have been reduced in the solid state, using the alkali metals Li, Na, K, Rb and Cs, which have the ability to intercalate in the vapor phase into solid fullerenes [133]. The discovery of the superconductivity of metal intercalated C o [136] attracted much attention to such materials therefore, the metal intercalation of 

In solution, the reduction of fullerenes is typically performed in etheral solvents (e.g., tetrahydrofuran, dimethoxyethane) [138] or liquid ammonia [139]. Using Li as a reducing agent it is possible to reach the highest reduction step, the hexa-anion. With the other alkali metals this was observed only when naphthalide salt was added [140]. The reduction of C o and all the higher fullerenes to their hexa-anions was first made possible by sonication with excess Li [16] and later by adding a small amount of 2 as an electron shuttle (vide supra). 

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