Synthesis, characterization, and reactivity studies of a water-soluble bis(alkoxo)(carboxylato)-bridged diMnIII complex modeling the active site in catalase

Abstract

A new diMn^III complex, Na[Mn₂(5-SO₃-salpentO)(μ-OAc)(μ-OMe)(H₂O)]·4H₂O, where 5-SO₃-salpentOH = 1,5-bis(5-sulphonatosalicylidenamino)pentan-3-ol, has been prepared and characterized. ESI-mass spectrometry, paramagnetic ¹H NMR, EPR and UV-visible spectroscopic studies on freshly prepared solutions of the complex in methanol and 9 : 1 methanol–water mixtures showed that the compound retains the triply bridged bis(μ-alkoxo)(μ-acetato)Mn₂³⁺ core in solution. In the 9 : 1 methanol–water mixture, slow substitution of acetate by water molecules took place, and after one month, the doubly bridged diMn^III complex, [Mn₂(5-SO₃-salpentO)(μ-OMe)(H₂O)₃]·5H₂O, formed and could be characterized by X-ray diffraction analysis. In methanolic or aqueous basic media, acetate shifts from a bridging to a terminal coordination mode, affording the highly stable [Mn₂(5-SO₃-salpentO)(μ-OMe)(OAc)]⁻ anion. The efficiency of the complex in disproportionating H₂O₂ depends on the solvent and correlates with the stability of the complex (towards metal dissociation) in each medium: basic buffer > aqueous base > water. The buffer preserves the integrity of the catalyst and the rate of O₂ evolution remains essentially constant after successive additions of excess of H₂O₂. Turnovers as high as 3000 mol H₂O₂ per mol of catalyst, without significant decomposition and with an efficiency of k_cat/K_M = 1028 M⁻¹ s⁻¹, were measured for the complex in aqueous buffers of pH 11. Kinetic and spectroscopic results suggest a catalytic cycle that runs between Mn^III₂ and Mn^IV₂ oxidation states, which is consistent with the low redox potential observed for the Mn^III₂/Mn^IIIMn^IV couple of the catalyst in basic medium.