Metallocenes as references for the determination of redox potentials by cyclic voltammetry - Permethylated iron and cobalt sandwich complexes, inhibition by polyamine dendrimers, and the role of hydroxy-containing ferrocenes

Measurement of the redox potentials (E-1/2) of ferrocene (1), decamethylferrocene (2), decamethylcobaltocene (3), and [(FeCp)-Cp-I*(eta(6)-C6Me6)] (4) (Cp* = eta(5)-C5Me5) in DMF, CH3CN, THF, DME, CH2Cl2, and DMSO were recorded vs. the saturated calomel electrode (SCE), showing that the E-1/2 value of 1 was more solvent dependent than those of 2-4, and that the difference in E-1/2 values among 2-4 was solvent independent, unlike those found between 1 and 2-4. This shows that 2-4 are much more reliable internal references for determining redox potentials than 1, but using data from this article, one can convert the literature values reported vs. 1 into values vs. 2, 3, or 4. The electrochemistry of 1 and 2 is all the more inhibited by polyamine dendrimers as the dendrimer generation is higher. On the other hand, ferrocenes or other iron sandwich compounds containing hydroxy groups can reestablish reversible cyclic voltammograms. It is shown, using a tetra(ferrocenylmethylamine), that this phenomenon is due to hydrogen bonding between the amido and phenol groups rather than bonding of hydroxy groups to the electrode surface.