Reduced Higher Fullerenes

There are fewer reports concerning the reduction of higher fullerenes, since they are less widely available in preparative quantities. Although they have lower symmetries, calculations predict high degenerate energy levels for most of them [151]. CV experiments performed on C76, C78, Cg2, Cg4 and Cs(, showed that they all exhibit multiple reduction waves (four or six) [35,135). Some of the higher fullerenes are easier to reduce to the monoanionic state, but there is no strict correlation between the reduction potential and the fullerene size. 

Radish, Ruoff and Jones succeeded in observing the three reduction stages (1-3) 

An interesting phenomenon was observed in the reduction of the two isomers of C78. NMR measurements at different temperatures distinguished between the two isomers. The room-temperature spectrum contains only 13 major absorptions, which were assigned to the D3 isomer 54. Reduction of the temperature resulted in the appearance of another set of lines, and at 170 K it was possible to see an extra 21 signals, which were assigned to the C2v isomer 53. It was suggested that this behavior is due to some triplet character and the smaller HOMO-LUMO gap in the multiply charged anion of 53 (calculated to be 1.05 and 2.02 eV, for the hexa-naions of 53 and 54, respectively) [153]. 

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Separation of the higher fullerenes is extremely difficult owing to the existence of multiple isomers. is reasonably well known, and is similar to with an extra band of 10 carbons added in the middle, making a rugby ball shape. Other materials such as Cg2 are known. The symmetry has been reduced to in these rugby ball-type materials, and intercalation experiments have shown the existence of No supercon-... 

The separation of C o may further be attained by absorptive filtration on a mixture of charcoal powder and siUca gel. Pure toluene can be used here which distinctly increases the solubiUty, thus accelerating the process and markedly reducing the solvent consumption. Apart from Q,q, it has by now become possible to obtain C70 on a multigram scale as well as to make higher fullerenes available at least in amounts sufficient for physical examination. Highly purified materials can be produced by sublimation of the above substances. Chiral fullerenes (refer to Section 2.2.3) may be split into enantiomers by suitable methods, for example. 

The higher fullerenes are more easily reduced and oxidized than Cgo. The first reduction potential of Cys, for example, is about 100-200 mV (depending on the solvent) more positive than for Cso- Altogether the first one-electron reduction is facilitated with an increasing number of carbon atoms and the reduction potential shifts toward more positive values. Furthermore, with the LUMO degeneration removed, a wider variation is observed for the potentials of different reduction steps. Finally, the HOMO-LUMO-gap, which can be estimated from the difference between the first oxidative and the first reductive step, decreases with a growing number of carbon atoms. 

Recently, the electrochemistry of fullerenes and their derivatives has gained much attention [33]. Cgo, C70 and higher fullerenes were reduced electrochemi-cally, and six reduction waves were observed for both Cgo and C70 [34], as well as for most of the higher fullerenes [35]. The energy levels that were obtained from these experiments were mostly in line with MO calculations. The electrochemistry of numerous fullerene derivatives was studied to compare their electron affinities and energy levels with their parent fullerenes. Electrochemically induced isomer-izations can be observed in CV, as is the case in the rearrangement of fulleroids to methanofullerenes [36]. 

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). 

Electrochemical studies have shown that Ceo is easily reduced (E1/2 = —0.21 and 0.33 V vs Ag/AgCl in tetrahydrofuran and benzonitrile, respectively [52] and —0.42 V vs SCE in benzonitrile [53, 54]). Up to six electrons can be added reversibly [55]. Several authors have shown that the fullerenes form charge-transfer complexes with amines [33, 56-59]. Wudl et al. have shown that Cgo reacts chemically with amines, giving various substitution products [60, 61]. Since the reduction potential of Ceo should be higher than that of the ground state by the amount of the triplet energy [62,63], its first reduction potential should be near 1.14 V vs SCE in benzonitrile [64]. The triplet should therefore be easily reduced by electron transfer from electron donors of lower oxidation potential. 

This result can be explained with the pyramidalization of the C-atoms of Cgg and the reduced p-character of the rr-orbitals. Due to electron pair repulsion the charge density on the outside of the fullerenes is higher than on the inside. This implies that, in contrast to the reactive exterior, the orbital overlap with an N-atom is essentially unfavorable inside Cgo and at the same time there is a repulsion of the valence electron pairs (Fig. 37). 

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