Reductions of Fullerenes

Reduction of fullerenes to fullerides — Reversible electrochemical reduction of Ceo in anhydrous dimethylformamide/toluene mixtures at low temperatures leads to the air-sensitive coloured anions Qo" , ( = 1-6). The successive mid-point reduction potentials, 1/2, at -60°C are -0.82, -1.26, -1.82, -2.33, —2.89 and —3.34 V, respectively. Liquid NH3 solutions can also be used. " Ceo is thus a very strong oxidizing agent, its first reduction potential being at least 1 V more positive than those of polycyclic aromatic hydrocarbons. C70 can also be reversibly reduced and various ions up to... 

Nakanishi, T, Ohwaki, H., Tanaka, H., Murakami, H., Sagara, T. and Nakashima, N. (2004) Electrochemical and chemical reduction of fullerenes C o and C70 embedded in cast films of artificial lipids in aqueous media, f Phys. Chem. B, 108, 7754-7762. 

Considering the large variety of fullerene adducts, only a few investigations on photochemical processes of fullerene adducts have been reported. These studies deal mainly with reductive processes due to the easy reduction of fullerenes and its adducts. Studies on electron transfer processes of several pyrrolidino fullerenes (Fig. 20) with donors such as dimethylaniline (DMA) or tetrakis(dimethy-lamino)ethylene (TDAE) show that the electron transfer rate constants decrease compared to that of C6o (3.5 X 109 M 1 s 1 for C6o, 0.5 X 109 M 1 s 1 for the derivatives (Table 6, Fig. 21) [179], This can be interpreted in terms of decreasing u-conjugation of the C60 moiety, which causes an increase in LUMO energy level and a decrease in the lowest triplet energy. Therefore, the substituents influ-... 

A parabolic driving force dependence of logket is also observed for electron-transfer reduction of fullerenes in PhCN, as shown in Fig. 13.10 [18, 28-32]. 

In addition to quinone reduction and hydroquinone oxidation, electrode reactions of many organic compounds are also inner-sphere. In these charge transfer is accompanied by profound transformation of the organic molecules. Some reactions are complicated by reactant and/or product adsorption. Anodic oxidation of chlorpro-mazine [54], ascorbic acid [127], anthraquinone-2,6-disulfonate [128], amines [129], phenol, and isopropanol [130] have been investigated. The latter reaction can be used for purification of wastewater. The cyclic voltammogram for cathodic reduction of fullerene Cm in acetonitrile solution exhibits 5 current peaks corresponding to different redox steps [131]. 

VII. LITHIUM REDUCTION OF FULLERENES-REDUCED POLYCYCLIC BALLS... 

FIGURE 19. Corannulene as an electron shuttle in the reduction of fullerenes... 

Intermolecular electron transfer between radiolytically-reduced metalloporphyrins and [60]fullerene (51,52) Metalloporhyrins were reduced in a radiolytical experiment carried out in a similar solvent mixture described above for the reduction of fullerenes, e.g., containing toluene, acetone and 2-propanol in a 8 1 1 v/v ratio. Again, the solely reducing species in this solvent mixture is the radical derived from 2-propanol by hydrogen-abstraction, i.e. (CH3)2 COH radical. This radical is known to reduce a number of metalloporphyrins (MP) quite rapidly. 

Reduction of Fullerenes. In agreement with the existence of low-lying un occupied molecular orbitals described above, fullerenes are mild oxidation agents. Reduction potentials for buckminsterfullerene can be appreciated in the cyclic voltammetry studies illustrated in Fig. 4.39 in which the electrochemical behavior of 50 with the diphylderivative Ph2C6i is compared (vide infra). Interestingly C70 shows a similar electrochemical behavior. 

The new carbon allotropes, fullerenes [14] and nanotubes [15], can accept a large number of electrons to their -system however, the neutral compounds are hardly soluble, especially in THF. Because of the low solubility of fullerenes, their complete reduction to hexaanions with Li is possible only under extreme conditions, such as the use of an ultrasound bath [16], When the reduction of fullerenes is performed in the presence of a small amount of 2, moderate conditions are required, e.g., low temperature and without sonication [17]. Corannulene, which is a fragment of fullerenes, is highly soluble in THF and is easily reduced. The resulting corannulene anion serves as an efficient electron shuttle, which assists in complete reduction of the practically insoluble fullerenes. In a similar way, single walled nanotubes were reduced by charge transfer by the use of small aromatic compounds, such as naphthalene, fluorenone and anthraquinone [18]. 

One important characteristic of fullerenes is their ability to accept a large number of electrons. This ability was predicted in the 1980s when the first calculations were performed on these systems. It was found that fullerenes have low HOMO-LUMO gaps and multiple degenerate energy levels, as is the case in the triply degenerate LUMO of Cgo [131] (Fig. 13.5). The reduction of fullerenes thus became one of their most studied facets. These studies focused on the ability of fullerenes to accept a large number of electrons (six), the unique aromatic properties of the anions formed, and the use of reduction as a synthetic route for fullerene derivatives. 

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). 

The aromaticity of fullerenes has been investigated theoretically and substantiated experimentally by NMR [154]. The best experimental method for studying the aromaticity of fullerenes was devised by Saunders and Cross, who encapsulated helium atoms inside fullerenes and measured their NMR spectra [155]. He NMR measurements show that C o exhibits the least aromatic character whereas C70 has the highest, and that the aromaticity of higher fullerenes lies between these two extremes. This is in contradiction to the initial suggestion in 1985 that C o is covered by a sea of n electrons that make it aromatic [14a, 156], and to the assumption of high aromaticity due to the large number of possible Kekule structures [157]. The reduction of fullerenes is not expected to modify their shape or symme-... 

Reduction of fullerenes is a method of choice for preparing fullerene adducts, since the fullerene anions exhibit nucleophilic behavior. Fullerene derivatives, such as alkyl fullerenes, were prepared by chemical reduction of the fullerene [169] or via organic electrosynthesis [38], mostly via [147c],... 

Promising research was carried out on the reduction of fullerene derivatives, which is expected to tune the sphere s electronic properties and investigate how the nature, geometry, structure, and number of adducts influence the electrochemical behavior of this fidlerene [33]. The nature and the number of the adduct groups could both increase and decrease the electron affinity, as observed in CV experiments. Hydrofidlerene [172] and epoxyfullerene [173] are two examples in... 

For the reduction of fullerenes a small amount (0.5-1 mg) of corannulene is added to the sample. 

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