On the nature of solvent effects on redox properties

The one-electron reduction potentials of six radical cations, four cations, and four neutral radicals in tetrahydrofuran, dichloromethane, dimethyl sulfoxide, N-methyl-2-pyrrolidinone, N,N-dimethylformamide, acetonitrile, methanol, ethanol, 2-propanol, acetone, formamide, and 1,1,1,3,3,3-hexafluoropropan-2-ol have been measured by cyclic voltammetry. For 10 of the redox couples, the redox process was reversible in all solvents. These results have been used to evaluate solvent effects by means of the Kamlet-Taft relationship. The relative importance of the solvent parameters alpha, beta, pi*, and delta(H) is 54.9, 9.6, 15.5 and 20.0%, respectively, for the radical cation displaying the strongest solvent dependence. In addition, we have studied the entropy contribution to some of the observed solvent effects by measuring the redox potentials as a function of temperature. The absolute value of the entropy appears to increase with increasing hydrogen bond donor ability of the solvent. The variation in entropy indicates that specific solvation is of main importance when considering solvent effects on redox properties.