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

The solvent dependence of the formal redox potentials of the ferrocenium/ferrocene (Fc(+/0)) and 1,2,3,4,5-wpentamethylferrocenium/1,2,3,4,5-pentamethylferrocene (Me(5)Fc(+/0)) couples versus the decamethylferrocenium/ decamethylferrocene (Me(10)Fc(+/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 Me(n)Fc(+/0) (n = 5, 10) and Fc(+/0) couples are surprisingly solvent dependent. In the case of the Fc(+/0) couple versus the Me(10)Fc(+/0) couple, the potential difference ranges from +583 mV in 2,2,2-trifluoroethanol to +293 mV in water. The positive shifts for the Me(5)Fc(+/0) couple versus the Me(10)Fc(+/0) couple were about half of these values. The Me(10)Fc(+/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 Me(5)Fc(+/0) couple. The latter, in turn, is notably more solvent dependent than that of the Me(10)Fc(+/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 Me(10)Fc(+/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 Me(10)Fc(+/0) couple.