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

• Published in: Inorganic chemistry 2024-07, Vol.63 (30), p.14103-14115
• Main Authors: Gonzalvez, Miguel A., Gundry, Luke, Garcia-Quintana, Laura, Guo, Si-Xuan, Bond, Alan M., Zhang, Jie
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X; 1520-510X
• DOI: 10.1021/acs.inorgchem.4c01932

Under voltammetric conditions, the neutral decamethylferrocene ([Me10Fc]) to cationic ([Me10Fc]+) FeII/III process is a well-known reversible outer-sphere reaction. A companion cationic [Me10Fc]+ to dicationic [Me10Fc]2+ FeIII/IV process has been reported under direct current (DC) cyclic voltammetric conditions at highly positive potentials in liquid SO2 at low temperatures and in a 1.5:1.0 AlCl3/1-butylpyridinium chloride melt. This study demonstrates that in room-temperature ionic liquids containing the hard to oxidize and hydrophobic tris­(pentafluoroethyl)­trifluorophosphate anion, the [Me10Fc]+/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 FeIII/IV process in the second and higher-order harmonics. This enables a straightforward determination of the thermodynamics and kinetics for both the FeII/III and FeIII/IV processes. Unlike the ideal outer-sphere FeII/III process, the parameters of the FeIII/IV process may be impacted by ion-interaction effects. For the faster FeII/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 FeIII/IV process. The slower BDD kinetics may be attributed in part to a density of states lower than that at GC.