Understanding the Decamethylferrocene Fe III/IV Oxidation Process in Tris(pentafluoroethyl)trifluorophosphate-Containing Ionic Liquids at Glassy Carbon and Boron-Doped Diamond Electrodes.

Under voltammetric conditions, the neutral decamethylferrocene ([Me 10 Fc]) to cationic ([Me 10 Fc] + ) Fe II/III process is a well-known reversible outer-sphere reaction. A companion cationic [Me 10 Fc] + to dicationic [Me 10 Fc] 2+ Fe III/IV process has been reported under direct current (DC) cyclic voltammetric conditions at highly positive potentials in liquid SO 2 at low temperatures and in a 1.5:1.0 AlCl 3 /1-butylpyridinium chloride melt. This study demonstrates that in room-temperature ionic liquids containing the hard to oxidize and hydrophobic tris(pentafluoroethyl)trifluorophosphate anion, the [Me 10 Fc] +/2+ process can be detected as a quasi-reversible reaction at glassy carbon (GC) and boron-doped diamond (BDD) electrodes. Large amplitude Fourier-transformed alternating current (FT-AC) voltammetry minimizes background current contributions occurring at potentials similar to those of the Fe III/IV process in the second and higher-order harmonics. This enables a straightforward determination of the thermodynamics and kinetics for both the Fe II/III and Fe III/IV processes. Unlike the ideal outer-sphere Fe II/III process, the parameters of the Fe III/IV process may be impacted by ion-interaction effects. For the faster Fe II/III process, heterogeneous rate constants are approximately 10 times smaller at BDD than those at GC electrodes. This electrode dependence is less pronounced for the slower Fe III/IV process. The slower BDD kinetics may be attributed in part to a density of states lower than that at GC.