The Decamethylferrocenium/Decamethylferrocene Redox Couple: A Superior Redox Standard to the Ferrocenium/Ferrocene Redox Couple for Studying Solvent Effects on the Thermodynamics of Electron Transfer

Abstract

The solvent dependence of the formal redox potentials of the ferrocenium/ferrocene (Fc+/0) and 1,2,3,4,5-pentamethylferrocenium/1,2,3,4,5-pentamethylferrocene (Me5Fc+/0) couples versus the decamethylferrocenium/decamethylferrocene (Me10Fc+/0) couple indicates that the latter is a superior redox standard for studying solvent effects on the thermodynamics of electron transfer. The couples were studied in 29 solvents and the differences in formal redox potentials between the MenFc+/0 (n = 5, 10) and Fc+/0 couples are surprisingly solvent dependent. In the case of the Fc+/0 couple versus the Me10Fc+/0 couple, the potential difference ranges from +583 mV in 2,2,2-trifluoroethanol to +293 mV in water. The positive shifts for the Me5Fc+/0 couple versus the Me10Fc+/0 couple were about half of these values. The Me10Fc+/0 redox couple can also be used in easily oxidized solvents, such as N-methylaniline and N,N-dimethylaniline, or in conjunction with a Hg working electrode. Statistical multiparameter analysis of the differences in potential versus empirical solvent parameters indicate that the redox potential of the Fc+/0 couple is more solvent dependent than that of the Me5Fc+/0 couple. The latter, in turn, is notably more solvent dependent than that of the Me10Fc+/0 couple. These results contradict the widely used “ferrocene assumption” that the redox potential of the Fc+/0 couple is not very solvent dependent. The data show that the Me10Fc+/0 couple is better suited than the Fc+/0 couple as a redox standard for studies of the thermodynamics of solvation of other redox couples. The data also enable previous measurements using the Fc+/0 couple to be corrected to values referenced against the Me10Fc+/0 couple.