Catalase activity of diMnIII complexes with the [Mn2(m-O2C2H3)(m-OL)(m-OX)]3+ core (L = polydentate ligand; X = CH3 or OC2H3): Structural features that control catalysis

Abstract

Mn catalases catalyze disproportionation of H2O2 into H2O and O2 by using a Mn2(m-O2CR)(m-O/OH/H2O)2 structural unit that cycles between the MnII2 and MnIII2 oxidation states. Because of the fast kinetics of this enzymatic reaction each independent step of the catalytic cycle has not yet been characterized. In this context, biomimetic compounds provide a unique way for testing mechanisms in these enzymes. The fine-tuning of Mn redox states is a critical feature when using artificial compounds to model the enzymatic activity. We have evidenced some of the key structural factors that control the oxidation states of Mn during H2O2 decomposition by comparing the catalase activity of diMn complexes of 1,3-bis[(2-hydroxybenzyl)(2-pyridylmethyl)amino]propan-2-ol (hbpmpnOH) with that of diMn compounds including other dinucleating ligands. New complexes, [Mn2(hbpmpnO)(m-O2C2H3)(m-OCH3)]BPh4 and [Mn2(X-hbpmpnO)(m-O2C2H3)2]BPh4 (X = Cl, OMe), were synthesized and characterized, and their catalase-like activity was evaluated. These compounds possess a triply bridged diMnIII core, including either bis(m-alkoxo)(m-carboxylato) or (m-alkoxo)bis(m-carboxylato) bridges, with the remaining coordination sites of the two Mn’s occupied by the six donor atoms of the polydentate ligand. These complexes show catalytic activity toward disproportionation of H2O2, without a lag phase and first-order kinetic on [catalyst]. Spectroscopic monitoring of the H2O2 disproportionation reaction showed that these diMn compounds dismutate H2O2 by a mechanism involving redox cycling between Mn2II/Mn2III levels with retention of dinuclearity during catalysis. A correlation between the electronic properties of the different ring substituents, the redox potentials of the dinuclear complexes and their catalase activity was evidenced. Comparison of the structure, kinetic parameters and redox potentials of the present diMn compounds with those of other diMn complexes also including polydentate ligands with a central bridging alkoxo revealed that the oxidation states of Mn during the catalytic cycle are not critically dependent on the redox potentials of the catalyst but strongly depend on the peroxide binding mode to the metal centre(s).