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, C 2 v -C78, and D 3-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 C 2 v -C78 and D 3-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 C 2 v -C78, not detected in the regular synthetic mixture, was prepared via electrolysis of a trisadduct. This C 2-symmetrical bisadduct seems to be present as an intermediate during every electrolysis of other bis- and trisadducts of C 2 v -C78.

Discrete Fulleride Anions and Fullerenium Cations

“…It is a particular feature of fullerenes as a class of molecules, reflecting their delocalized LUMOs, and is observed in C 70 , C 76 , C 78 , and C 84 . 9,51,[55][56][57][58][59] It has also been calculated using density functional theory. 60 Table 1 lists representative redox data for the more commonly known fullerenes.…”

“…The electrochemistry of derivatized fullerenes is part of a recent brief review, 83 and derivatized higher fullerenes are now receiving attention. 59 As discussed in section X.A, electrochemical formation of derivatized fullerides is being used to synthetic advantage. A number of reports on the electrochemistry of endohedral fullerenes have also appeared.…”

“…Although initially believed to form at an unusually low potential, 87 C 76 + is now known to form reversibly at 0.81 V. 57 The C 2v and D 3 isomers of C 78 oxidize at 0.95 and 0.70 V, respectively. 57 C 84 has a number of isomers, all of which oxidize around 0.9 V. 57,59 A short review on the redox potentials of the higher fullerenes has appeared. 88 In keeping with their delocalized charges, the first oxidation potentials of C 60 , C 70 , C 76 , and C 78 follow the same trend as their first ionization potentials (7.61, 7.58, 7.10, and 7.05 eV, respectively).…”

“…88 In keeping with their delocalized charges, the first oxidation potentials of C 60 , C 70 , C 76 , and C 78 follow the same trend as their first ionization potentials (7.61, 7.58, 7.10, and 7.05 eV, respectively). 89,90 In contrast to reduction, the oxidation of fullerenes typically becomes easier upon derivatization, 59,80,91,92 particularly with bis(silyl) derivatization, 93 but no cations have been characterized.…”

Fullerenes as Novel Acceptors in Photosynthetic Electron Transfer

“…[90] Rotational freedom around the methylene spacers in 8 and 9 would not affect the electron transfer rates, a situation that is similar to the previously reported porphyrinϪquinones [14] and porphyrinϪC 60 dyads. [18] [19] Since the first reduction potentials of C 60 and C 70 are reported to be quite similar, [91] the inherent shape and size effect of C 70 in electron transfer is evaluated accurately by comparing the electron transfer rates of 8 and 9. Actually, the first reduction potential (Ϫ1.10 V vs. Fc/Fc ϩ ) of the C 70 moiety in 8 is the same as that of the C 60 moiety in 9.…”