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.