Solvent Effects on the Electrochemistry of C60. Thermodynamics of Solvation of C60 and Fullerides

The previously published solvent dependence of the electrochemistry of C 60 has been extended and reanalyzed. The potentials have been measured against the decamethylferrocenium/decamethylferrocene (Me 10 Fc +/0 ) couple, which is much less solvent-dependent than the ferrocenium/ferrocene (Fc +/0 ) couple, and hence gives a more accurate reflection of the solvent dependence of the C 60 n -/( n +1)- couples. The formal redox potentials of the C 60 n -/( n +1)- couples exhibit substantial dependences on the solvent and become more solvent-dependent as the charge increases. This dependence arises from two main interactions with the solvents: hydrogen-bonding interactions with acidic functionality and electrostatic interactions with solvent dipoles. The strength of the hydrogen-bonding interaction increases as the square of the solute charge due to the increase in basicity of the fullerides. In addition to the contribution of the hydrogen-bonding acidity and the solvent dipoles, the potentials of the C 60 0/1- couple are influenced by the Lewis basicities of the solvents. The latter is likely to be due to ion-pair formation between the fullerides and the Bu 4 N + ion and/or interactions between the Lewis basicity of the solvent and the Me 10 Fc + ion of the reference couple. This interaction is much smaller compared to the solvent/solute interaction. π-Stacking interactions between C 60 and aromatic solvents results in a significant contribution of polarizability to the first reduction. The solvent dependences of the fullerenes can be used to model electrode/solvent interactions in which the surface (albeit a distorted one) goes from the point-of-zero-charge (C 60 ) to a variety of discrete negative surface charges (fullerides).