Heterogeneous rate constants of the electron-transfer of iron- And ruthenium-bipyridine complexes in imidazolium-based ionic liquids

The heterogeneous rate constants (k_s) for the oxidation of iron- and ruthenium-bipyridine complexes, [Fe(bpy)3]²+/3⁺ and [Ru(bpy)3]²+/3⁺ have been determined using Nicholson’s method in imidazolium-based ionic liquids with five different types of cation/anion over a range of temperature from 298–318 K. The heterogeneous rate constants of these redox reactions range from 10−⁴ to 10−³ cm s⁻¹, depending on the dynamic viscosity and type of cation/anion of the ILs. Marcus-Hush theory is used to explain the activation energies, E_a. Similar activation energies are found for the Fe-bipyridine and Ru-bipyridine complexes indicating that inner-sphere reorganization energy has only a small influence. For the calculation of the solvent-dependent outer sphere reorganization energy a dipole-free expression is used, adopted to the different dielectric properties of ionic liquids compared to organic solvents. i-R_u drops have been compensated using the corresponding experimental uncompensated R_u-values obtained from temperature dependent Bode-plots.