Redox Characteristics and Anion Association Behaviour of Stereoisomeric Forms of Mono- and Oligonuclear Metal Complexes Using High Pressure Electrochemistry

Abstract

The effect of the presence of the anions PF6−, BF4−, ClO4−, and tosylate− on the potentials of the RuIII/II redox processes in a series of mono-, di-, and trinuclear complexes involving polypyridyl ligands are reported. The anions gives rise to a cathodic shift in the respective redox potentials in the sequence PF6− < BF4− ≈︁ ClO4− < tos−, with the magnitude of the shifts being largest for the tri-nuclear and smallest for the mononuclear species. In one dinuclear complex, a variation of anion was also observed to affect the difference in potentials between the [6+/5+] and [5+/4+] couples. These effects are consistent with known specific association of the anions with metal complexes of this type. In the presence of PF6−, high-pressure electrochemical studies revealed the molar volume changes for successive redox couples in di- and tri-nuclear complexes were almost invariant (ΔVcomplex = 24 ± 2 cm3 mol−1) and consistent with the value for mononuclear species. This constancy is interpreted in terms of isolated RuIII/II processes in all complexes and implies that the electrostrictive effects on solvent are limited to a very short range.