Solvent-Dependent Redox Thermodynamics of Metal Amine Complexes. The Delineation of Specific Solvation Effects

Solvent-dependent formal potentials, Ef, and reaction entropies, for four reversible Co(III)/(II) couples featuring macrobicyclic cage ligands have been obtained. These are utilized, together with corresponding data for other Co(III)/(II) and Ru(III)/(II) amine couples and for M(III)/(II) polypyridine couples, to unravel effects on the redox thermodynamics arising from specific ligand-solvent and other interactions. The solvent dependencies of Ef (vs. ferrocenium-ferrocene) for each redox couple are fitted by using a multiparameter approach, incorporating various well-known empirical quantities describing solvent basicity, acidity, polarity, and internal ordering. For most amine couples, solvent basicity appears to play a prevailing role, associated with interactions involving the amine hydrogens, although solvent polarity also exerts a substantial, albeit less solvent-dependent, influence upon EfD.