Cations/Anions Effects of Imidazolium-based Ionic Liquids on the Diffusion Properties of Iron- and Ruthenium-bipyridine/phenanthroline Complexes

- Diffusion coefficients of bulky metal complexes are not depend on viscosity only.
- Anions of ionic liquids plays a strong part in controlling diffusion coefficients.
- Hydrodynamic radius of ion complex is independent of ILs viscosity and temperature.

The diffusion coefficients of both iron(II)-bipyridine ([Fe(bpy)3]2+), ruthenium(II)-bipyridine ([Ru(bpy)3]2+) are reported in five of different imidazolium-based ionic liquids (ILs) and iron(II)-phenanthroline ([Fe(phen)3]2+), ruthenium(II)-phenanthroline ([Ru(phen)3]2+) in one of imidazolium-based ionic liquid, using cyclic voltammetry and chronoamperometry at microdisc electrodes. Activation energies of diffusion, ED, are calculated for each system. Diffusion coefficient ratios between oxidation and reduction species for both complexes were found to be close to 1. Plots of diffusion coefficients versus inverse viscosity deviate from expected linearity, especially for ILs with different kind of anions, suggesting that anions of the ILs contribute to the values of diffusion coefficients. ED for both complexes in ILs are found slightly larger compared to the activation energies of viscous flow (Eη) for the corresponding IL, indicating that the viscosity is not the only factor determining the diffusion of the complexes. Furthermore, the hydrodynamic radius of complexes in ILs were estimated based on the diffusion coefficient values. They are found to be independent of temperature and ILs viscosity.