Redox Characteristics of Covalent Derivatives of the Higher Fullerenes C₇₀, C₇₆, and C₇₈

Abstract

The electrochemical properties of covalent derivatives of the higher fullerenes C70, C76, C2v-C78, and D3-C78, bearing bis(alkoxycarbonyl)methano addends, were investigated by cyclic voltammetry (CV) and/or steady-state voltammetry (SSV). Included in this study were a series of previously prepared mono- to tetrakisadducts of C70, monoadducts of C76, and bis- and trisadducts of C2v-C78 and D3-C78. For all compounds, several (up to five) reduction steps and one or two oxidation steps were observed. The reduction steps usually required more energy than the corresponding processes in the parent fullerenes whereas the oxidation steps became facilitated with increasing degree of addition. A notable exception to this trend was the observation that two C76 monoadducts were more readily reduced than the parent fullerene and one was more difficult to oxidize. The electrochemical properties of C70 derivatives were found to closely resemble those of C60 adducts. Constitutional isomers displayed nearly identical redox characteristics. Only the first reduction process is reversible by CV or SSV in the entire series of C70 adducts whereas the electrochemically generated dianions of some of the derivatives underwent a rapid subsequent chemical reaction. The electrochemistry of covalent adducts of C76 and C78 differs substantially from that of the derivatives of the smaller carbon spheres C60 and C70. Among all compounds studied, monoadducts of C76 are the easiest to be reduced or oxidized and their electrochemically generated multiple anions are much more stable under the conditions of the SSV and CV experiments than those of the adducts of C70 and the C78 isomers. Several of the electrochemically generated trisanions of the adducts of C78 isomers underwent rapid chemical reactions. Indeed, a new bisadduct of C2v-C78, not detected in the regular synthetic mixture, was prepared via electrolysis of a trisadduct. This C2-symmetrical bisadduct seems to be present as an intermediate during every electrolysis of other bis- and trisadducts of C2v-C78.