Acid–base equilibria of various oxidation states of aqua–ruthenium complexes with 1,10-phenanthroline-5,6-dione in aqueous media

Abstract

Syntheses and pH dependent electrochemical properties of aqua–ruthenium(II) complexes, [Ru(trpy)(PDA-N,N′)(OH₂)](ClO₄)₂ ([1](ClO₄)₂) and [Ru(trpy)(PD-N,N′)(OH₂)](ClO₄)₂ ([2](ClO₄)₂) (trpy = 2,2′:6′,2″-terpyridine, PDA = 6-acetonyl-6-hydroxy-1,10-phenanthroline-5-one, PD = 1,10-phenanthroline-5,6-dione) are presented. Treatment of [Ru(trpy)(PD-N,N′)Cl](PF₆) with AgClO₄ in a mixed solvent of acetone and H₂O selectively produced the acetonyl-PD complex [1](ClO₄)₂, and the similar treatment in a mixed solvent of 2-methoxyethanol and H₂O gave the PD complex [2](ClO₄)₂. The molecular structures of both complexes were determined by X-ray structural analysis. The proton dissociation constants of various oxidations state of [1]²⁺ and [2]²⁺ were evaluated by simulation of E_1/2 values of those redox potentials depending on pH. The simulation revealed that the acetonyl-PD complex [1]²⁺ underwent successive Ru(II)/Ru(III) and Ru(III)/Ru(IV) redox couples though the two redox reactions were not separated in the cyclic voltammograms. The redox behavior of [2]²⁺ in H₂O is reasonably explained by not only the similar successive metal-centered redox reactions but also simultaneous two-electron quinone/catechol redox couple of the PD ligand including the contribution of hydration on a carbonyl carbon.