Electrochemical Reaction of Tris(1,10-phenanthroline)Iron Complexes in Some Amide-Type Ionic Liquids

Electrochemical behavior of a charged species in ionic liquids have been known to depend on the physicochemical properties of the ionic liquids[1-4]. Diffusion of the charged species is affected not only by the viscosity of the ionic liquids but also by the coulombic interaction between the charged species and the ions composing the ionic liquids. Thus, Stokes-Einstein relation is generally inapplicable to diffusion of charge species in ionic liquids. The electron transfer rates are also influenced by the viscosity and the coulombic interaction, which is considered related to the outer component of reorganization energy. Furthermore, the reaction entropy of the redox couples without change of their coordination environment has been found to relate to the coulombic interaction[5,6]. We have reported these results with tris(2,2'-bipyridine) complexes of iron and ruthenium in some ionic liquids. In the present study, electrochemical reactions of tris(1,10-phenanthroline)iron and tris(2,2'-bipyridine)iron complexes were investigated in 1-butyl-1-methylpyrrolidinium (BMP⁺) and 1-ethyl-3-methylimidazolium (EMI⁺) bis(trifluoromethylsulfonyl)amide (TFSA^–) ionic liquids in order to elucidate the effects of the charge density of the redox species on electrode kinetics and reaction entropy. The diffusion coefficient of [Fe(phen)₃]ⁿ⁺ was smaller than that of [Fe(bpy)₃]ⁿ⁺ (n = 2 and 3) in BMPTFSA and EMITFSA. The ratios of diffusion coefficients of [Fe(phen)₃]³⁺ to [Fe(phen)₃]²⁺ were larger than those of [Fe(bpy)₃]³⁺ to [Fe(bpy)₃]²⁺, indicating the coulombic interaction of TFSA^– with [Fe(phen)₃]ⁿ⁺ is weaker than that with [Fe(bpy)₃]ⁿ⁺ since the size of [Fe(phen)₃]ⁿ⁺ is larger than that of [Fe(bpy)₃]ⁿ⁺. The rate constants for [Fe(phen)₃]^3+/2+ were close to those for [Fe(bpy)₃]^3+/2+. On the other hand, the reaction entropy for [Fe(phen)₃]^3+/2+ was smaller than that for [Fe(bpy)₃]^3+/2+, reflecting the difference in the charge densities.

References 

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