Synthesis, characterization and catalase activity of a water soluble diMnIII complex of a sulphonato-substituted Schiff base ligand: An efficient catalyst for H2O2 disproportionation

Abstract

A new diMn III complex, Na[Mn 2(3-Me-5-SO 3-salpentO)(μ-MeO)(μ-AcO)(H 2O)]•4H 2O(1), where salpentOH = 1,5-bis(salicylidenamino) pentan-3-ol, was synthesized and structurally characterized. The complex possesses a bis(μ-alkoxo)(μ-acetato) triply bridged diMn III core, the structure of which is retained upon dissolution. Complex 1 is highly efficient to disproportionate H2O2 in an aqueous solution of pH ≥ 8.5 or in DMF, with only a slight decrease of activity. Electrospray ionization mass spectrometry, EPR, and UV-vis spectroscopy used to monitor the H2O2 disproportionation in buffered basic medium, suggest that the major active form of the catalyst during cycling occurs in the Mn III 2 oxidation state and that the starting complex retains the dinuclearity and composition during catalysis, with the acetate that moves from bridging to terminal ligand. UV-vis and Raman spectroscopy of H 2O 2 + 1 + Bu 4NOH mixtures in DMF suggest that the catalytic cycle involves Mn III 2/Mn IV 2 oxidation levels. At pH 10.6 in an Et 3N/Et 3NH + buffer, complex 1 catalyzes dismutation of H2O2 with saturation kinetics on the substrate, first order dependence on the catalyst, and k cat/K M = 16(1) ×10 2 s -1 M -1. During catalysis, the exogenous base contributes to retain the integrity of the bis(μ-alkoxo) doubly bridged diMn core and favors the formation of the catalyst-peroxide adduct (low value of K M), rendering 1 a highly efficient catalyst for H2O2 disproportionation.