METHANOFULLERENES AND METHANOFULLEROIDS HAVE DIFFERENT ELECTROCHEMICAL-BEHAVIOR AT NEGATIVE POTENTIALS

Cathodic electrochemistry was performed for two methanofullerene derivatives, 3 and 4. Since it was possible to isolate the [6,6] closed isomeric forms of these derivatives (called simply methanofullerenes), the electrochemistry of the pure isomers was recorded. The electrochemical behavior of the corresponding mixtures, which also contained the [5,6] open forms (simply referred to as methanofulleroids), was also obtained. In the solvent mixture used, acetonitrile:toluene (1:5), no difference was observed for the potentials of the first four electrochemical waves of the two isomers. However, the methanofulleroid exhibited an anodically-shifted fifth reduction wave, relative to that corresponding to the methanofullerene. This difference in potential was 0.20 V for the fullerene-fulleroid isomers of both 3 and for those of 4. On the basis of the voltammetric data, the fulleroids behave more as triply degenerate systems, very much like the behavior for C-60. This was anticipated, since they are, as is the parent C-60, 60 pi electron systems. However, the methanofullerenes have 58 pi electrons, and part of their conjugated network is destroyed. The fullerenes exhibit what appear to be doubly degenerate LUMOs with a LUMO(+) that is some 4-5 kcal/mol higher in energy. The presence of the methanofullerene was confirmed by simultaneous W-vis spectrophotometry in the case of 3. Thermalization of isomeric mixtures of 3 and 4 resulted in the quantitative formation of the methanofullerene and the total disappearance of the methanofulleroid, consistent with previous results.