ELECTROREDUCTION OF BUCKMINSTERFULLERENE, C-60, IN APROTIC-SOLVENTS - SOLVENT, SUPPORTING ELECTROLYTE, AND TEMPERATURE EFFECTS

The electroreductions of Buckminsterfullerene (C60) in aprotic solvents were examined as a function of solvent, supporting electrolyte, and temperature. Altogether, 11 different solvents and 17 different supporting electrolytes were utilized in measurements made between 223 and 348 K. The cations of the supporting electrolytes were Li+ and Na+ as well as quaternary ammonium and quaternary phosphonium cations. The anions of the supporting electrolytes were ClO4-, BF4-, PF6-, and Br-. Cyclic voltammograms, rotating disk electrode voltammograms, and controlled potential coulometry revealed up to five reversible one-electron reductions. A qualitative approach is used to elucidate the effects of solvent, supporting electrolyte, and temperature on the half-wave potentials, E1/2, of the reductions of C60. E1/2 for the first reduction correlates well with the Gutmann donor number of the solvent with a positive slope, but it also shows a linear correlation with the Gutmann acceptor number of the solvent with a negative slope. In contrast, the third reduction E1/2 correlates fairly with the Gutmann acceptor number with a positive slope. The first three reductions also correlate with the normalized Dimroth-Reichardt solvent parameter. The inorganic anions of the supporting electrolytes do not significantly affect the half-wave potentials, but these values vary substantially with the type and size of the supporting electrolyte cations. The relative magnitudes of the solvent and supporting electrolyte effects on E1/2 differ for each redox process of C60, and values of E1/2 shift over a range of 280-600 mV for a given redox couple. The shifts in reduction potentials were rationalized in terms of the following: (i) charge density on the fulleride anions, (ii) solvophobic effects involving C60 (aggregation), (iii) solvophobic type interactions involving C60 anions and the larger cations of the supporting electrolytes in polar solvents, (iv) ion pairing of C60 anions with smaller cations in nonpolar solvents, and (v) the specific acceptor or donor properties of the solvents. The reversible half-wave potentials were also measured as a function of temperature in eight different solvent/supporting electrolyte systems, and the measured values of DELTA-E1/2/DELTA-T were used to calculate the change of entropy associated with each electron-transfer step. The shifts in E1/2 with temperature are relatively large and indicate that an unusually large change of entropy accompanies each electroreduction step. Diffusion coefficients, Stokes radii, and apparent solvation numbers of neutral C60 were also determined in different solvent systems, and these values are discussed with respect to the nature of the solvent-solute interaction.