NMR study of the leaving ligand and solvent effects on the solvolysis of (μ-oxo)bis(μ-acetato)diruthenium(III) complexes

A mechanistic 1H NMR kinetic study on the solvolysis of complexes [Ru2(μ-O)(μ-O2CCH3)2(4-picoline)6](PF6)2 (1), [Ru2(μ-O)(μ-O2CCH3)2(pyridine)6](PF6)2 (2), and [Ru2(μ-O)(μ-O2CCH3)2(4-picoline)4(pyridine)2](PF6)2 (3) in DMSO-d6, CD3OD and CD3CN was carried out. All rate constants were calculated from the kinetic data, which fit in a series of reversible pseudo first order and second order reactions. The comparative kinetic study and the large ΔH‡ and positive ΔS‡ values for all solvolysis support the dissociative nature of the ligand exchange mechanism. The leaving ligand effect, determined by comparison of the forward reactions for complexes 1 and 3, showed that the rate constants for 4-picoline substitution are smaller than for pyridine. An apparent contradiction with the proposed mechanism for the reverse reaction in DMSO-d6 where the less basic ligand coordinates faster, was explained by a ligand–solvent interaction, which was demonstrated by our kinetics results, molecular modeling and through the determination of diffusion coefficients in DMSO-d6 and CD3OD using DOSY.