Electroreduction of Buckminsterfullerene, C60, in aprotic solvents. Solvent, supporting electrolyte, and temperature effects

Abstract

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 C104-, 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, E l / z , of the reductions of c 6 0 . E l l 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 negatiue 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 E l I z differ for each redox process of Cdo, and values of 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 Cso (aggregation), (iii) solvophobic type interactions involving Cm anions and the larger cations of the supporting electrolytes in polar solvents, (iv) ion pairing of Cdo 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 AElI2/AT were used to calculate the change of entropy associated with each electron-transfer step. The shifts in E l I z 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 Cdo were also determined in different solvent systems, and these values are discussed with respect to the nature of the solventsolute interaction.

Discrete Fulleride Anions and Fullerenium Cations

“…57 The effect of the medium on the redox potentials of C 60 has been studied by a number of groups. Independent studies by Fawcett, 68 Kadish,69,70 Bard, 71 and Compton 72 reveal a significant influence of the cation on the kinetics and, to a lesser extent, the thermodynamics of the first electron transfer to C 60 .…”

“…A correlation of potential with R 4 N + cation size has been reported, 69,77 but its statistical validity has been questioned. 78 Cation variation for the more highly charged redox couples is much less significant.…”

“…69,78 The effect of solvent on the redox potentials of C 60 is more substantial than the effect of electrolyte. Early indications of a correlation of the first reduction potential with solvent donicity 51,69 (e.g., Gutmann donor number) have held up to more detailed analysis, 78 but the interpretation of experimental data has turned out to be less straightforward than originally believed. The basic problem lies in obtaining statistically significant data that are not compromised by the solvent dependence of the reference couple used.…”

“…The basic problem lies in obtaining statistically significant data that are not compromised by the solvent dependence of the reference couple used. For this reason, Lay and co-workers 78 have used the decamethylferrocene/decamethylferrocenium couple rather than the ferrocene/ferrocenium (Fc/Fc + ) couple as a reference, reinterpreting the data of Kadish and co-workers 69 to produce statistically valid analyses. Data are listed in Table 2.…”

Absolute Standard Redox Potential of Monolayer-Protected Gold Nanoclusters

“…4,[7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] This ensemble behavior is equivalent to a series of classical Coulomb staircase charging events or sequential electrochemical redox reactions, which occur as MPCs encompass an intermediate dimension between small molecules and bulk materials. Previously, the sequential electrontransferhasbeenobservedforfullerene(andderivatives) [22][23][24][25][26] and Pt-carbonyl nanoclusters [Pt n (CO) m ], [27][28][29] which led Weaver et al 30 to develop an electrostatic model to relate the electron transfer energetics of molecular capacitances in gas-and solution-phase systems. On the basis of this model, Chen et al 8 ascribed the occurrence of the successive electron transfer for MPCs to the extremely small (sub-attoFarad, aF) molecular capacitances of MPCs associated with a combination of a small metallic core and a dielectric protecting layer.…”