Thermodynamic and Kinetic Analysis of the Fe ^III/IV Decamethylferrocene Process in Ionic Liquids Using Background-Excluded Fourier Transformed AC Voltammetry

The thermodynamics and electrode kinetics for the FeIII/IV oxidation ([Me10Fc]+/2+, [Me10Fc] = decamethylferrocene process) that occurs at very positive potentials have been determined at glassy carbon (GC) and boron-doped diamond (BDD) electrodes in room temperature ionic liquids (RTILs) containing bis(trifluoromethanesulfonyl)azanide ([TFSA]−) and [PF6]− anions and a range of cations. Use of Fourier-Transformed AC (FT-AC) voltammetry overcomes limitations in quantitative studies with DC voltammetry by isolating the [Me10Fc]+/2+ process from overlapping background RTIL oxidation, double layer current contributions, and unexpectedly enhanced background current between the FeII/III and the FeIII/IV processes. The FT-AC voltammetric data have been modeled as an EE mechanism and data optimization employed to determine the reversible potentials (E0), heterogeneous charge transfer rate constants (k0), and charge transfer coefficients (α) for both processes. The FeII/III process is close to reversible with a significantly larger k0 value than the FeIII/IV process. The electrode material significantly impacts the electrode kinetics, with generally larger k0 values found with GC compared to BDD electrodes. For the outer sphere FeII/III process, a plot of D versus the reciprocal of viscosity (1/η) for the [TFSA]−- and [PF6]−-based RTILs follows the Stokes–Einstein relationship and the dependence of k0 on η is consistent with adiabatic electron transfer theory. In contrast, the FeIII/IV process shows a more complex RTIL-dependent behavior. In particular, significant interactions of the [Me10Fc]2+ dication and RTIL anion are assumed to be the origin of the substantial dependence of E0 for the FeIII/IV process on the RTIL anion. By comparison, the E0 value for the FeII/III process is regarded as relatively medium insensitive and hence often recommended for reference scale calibration purposes. The FeIII/IV process in RTILs with other anions such as [BF4]− and [B(CN)4]− is chemically irreversible but is chemically reversible in hydrophobic RTILs containing the (tris(pentafluoroethyl)trifluorophosphate anion.