Redox Reactions of Ag(I)/Ag and Ferrocenium/Ferrocene in 1-Butyl-1-methylpyrrolidinium Bis(fluorosulfonyl)amide Ionic Liquid

Abstract

The electrode reactions of Ag(I)/Ag and ferrocenium/ferrocene (Fc+/Fc) were investigated in an ionic liquid, 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)amide (BMPFSA). The potential of Ag(I)/Ag depended on the logarithm of the concentration of Ag(I), as predicted by the Nernst equation, indicating the Ag(I)/Ag can be used as a reference electrode reaction in BMPFSA. The reversible electrode reaction of Fc+/Fc was observed in BMPFSA by cyclic voltammetry. The donor number of BMPFSA was estimated to be 13 from the difference in the formal potentials of Ag(I)/Ag and Fc+/Fc, indicating the coordination ability of FSA– was slightly stronger than that of bis(trifluoromethylsulfonyl)amide (TFSA–). The diffusion coefficients (D) of Fc and Fc+ were (5.7 ± 0.7) and (3.3 ± 0.2) × 10–7 cm2 s–1, respectively. The ratio of D of Fc+ against that of Fc was smaller than those in TFSA–-type ionic liquids, reflecting the higher charge density of FSA–. The standard rate constant (k 0) of Fc+/Fc was estimated to be (5.4 ± 1.1) × 10–3 cm s–1. The apparent activation energy for k 0 was close to the activation energy for D, suggesting the electrode reaction of Fc+/Fc can be regarded as the outer sphere electron transfer reaction with a very small reorganization energy.