A kinetic study on pyridine exchange reactions at a bis(μ-acetato)(μ-oxo)diruthenium(III) center: effects of monodentate and bidentate N-heterocyclic ligands cis to the oxo bridge

A series of (μ-oxo)bis(μ-acetato)diruthenium(III) complexes containing two pyridine (py) ligands and varied N-heterocyclic ligands in the positions trans and cis to μ-O, respectively, have been prepared to study py/py-d5 exchange reactions using 1H NMR spectroscopy. The diruthenium(III) complexes under investigation are [Ru2(μ-O)(μ-CH3COO)2(py)6](PF6)2 (1), [Ru2(μ-O)(μ-CH3COO)2(bpy)2(py)2](PF6)2 (2), [Ru2(μ-O)(μ-CH3COO)2(acpy)4(py)2](FF6)2 (3), and [Ru2(μ-O)(μ-CH3COO)2(dmbpy)2(py)2](PF6)2 (4), where bpy=2,2′-bipyridine, acpy=4-acetylpyridine, and dmbpy=4,4′-dimethyl-2,2′-bipyridine. Pseudo-first order rate constants for the ligand-exchange reactions are 10−6−10−5 s−1 for 1–4 in CD3CN at 298 K. It is found that the rate of the py/py-d5 exchange reactions is controlled by the electronic nature of the cis-oriented ancillary ligands, while the exchange mechanisms are tuned principally by the ligand steric factors. The activation parameters (ΔH‡ and ΔS‡) indicate that exchange reactions proceed through the dissociative (D) or the interchange dissociative (Id) mechanism for 1 and 3. Negative ΔS‡ values observed for 2 and 4 suggest a significant contribution of incoming ligands to the exchange pathway. The kinetic and thermodynamic parameters for the diruthenium series and the corresponding data for Ru-based oxo bridged trinuclear complexes established previously are compared and discussed.