Electrode kinetics of tris(2,2′-bipyridine)ruthenium complexes in 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid

The electrode reaction of tris(2,2′-bipyridine)ruthenium complexes ([Ru(bpy)₃]ⁿ⁺, n = 2 and 3) has been investigated in 1-ethyl-3- methylimidazolium tetrafluoroborate (EMI[BF₄]) ionic liquid. The formal potential of [Ru(bpy)₃]^3+/2+ couple was 0.85 V vs. ferrocene/ferrocenium at 25°C. The diffusion coefficients of [Ru(bpy)₃]²⁺ and [Ru(bpy)₃]³⁺ were (7.8 ± 0.1) × 10^-8 and (3.3 ± 0.1) × 10^-8 cm² s^-1, respectively, at 25°C. The diffusion coefficients in EMI[BF₄] were smaller than those reported in EMITFSA (TFSA^- = bis(trifluoromethylsulfonyl)amide), of which the viscosity is close to that of EMI[BF₄]. In addition, the ratio of the diffusion coefficient of [Ru(bpy)₃]²⁺ to that of [Ru(bpy)₃]³⁺ in EMI[BF₄] was smaller than that in EMITFSA, indicating the diffusivity of the cationic species is strongly affected by the charge density of the anion of ionic liquids. The rate constant for [Ru(bpy)₃]^3+/2+ couple at 25°C was (6.7 ± 0.2) × 10^-5 cm s^-1, which was smaller by two orders of magnitude than that in EMITFSA, suggesting the difference in the charge densities between [BF₄]^- and TFSA^- also affects the outer-sphere component of reorganizing energy of the complexes.