Synthesis, Characterization, and Catalase Activity of a Water-Soluble diMnIII Complex of a Sulphonato-Substituted Schiff Base Ligand: An Efficient Catalyst for H2O2 Disproportionation

A new diMnIII complex, Na[Mn2(3-Me-5-SO3-salpentO)(μ-MeO)(μ-AcO)(H2O)]·4H2O (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 diMnIII 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 MnIII2 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 H2O2 + 1 + Bu4NOH mixtures in DMF suggest that the catalytic cycle involves MnIII2/MnIV2 oxidation levels. At pH 10.6 in an Et3N/Et3NH+ buffer, complex 1 catalyzes dismutation of H2O2 with saturation kinetics on the substrate, first order dependence on the catalyst, and kcat/KM = 16(1) × 102 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 KM), rendering 1 a highly efficient catalyst for H2O2 disproportionation.