New dimanganese(III) complexes of pentadentate (N 2O 3) Schiff base ligands with the [Mn 2(μ-OAc)(μ-OR) 2] 3+ core: Synthesis, characterization and mechanistic studies of H 2O 2 disproportionation

Abstract

Two new diMn III complexes [ Mn 2 III L 1 ( μ - AcO ) ( μ - MeO ) ( methanol ) 2 ] Br ( 1) and [ Mn 2 III L 2 ( μ - AcO ) ( μ - MeO ) ( methanol ) ( ClO 4 ) ] ( 2) (L 1H 3 = 1,5-bis(2-hydroxybenzophenylideneamino)pentan-3-ol; L 2H 3 = 1,5-bis(2-hydroxynaphtylideneamino)pentan-3-ol) were synthesized and structurally characterized. Structural studies evidence that these complexes have a bis(μ-alkoxo)(μ-carboxylato) triply bridged diMn III core in the solid state and in solution, with two substitution-labile sites – one on each Mn ion – in cis-position. The two complexes show catalytic activity toward disproportionation of H 2O 2, with saturation kinetics on [H 2O 2], in methanol and dimethyl formamide at 25 °C. Spectroscopic monitoring of the H 2O 2 disproportionation reaction suggests that (i) complexes 1 and 2 dismutate H 2O 2 by a mechanism involving redox cycling between Mn 2 III and Mn 2 IV , (ii) the complexes retain the dinuclearity during catalysis, (iii) the active form of the catalyst contains bound acetate, and (iv) protons favors the formation of inactive Mn II species. Comparison to other dimanganese complexes of the same family shows that the rate of catalase reaction is not critically dependent on the redox potential of the catalyst, that substitution of phenolate by naphtolate in the Schiff base ligand favors formation of the catalyst-substrate adduct, and that, in the non-protic solvent, the bulkier substituent at the imine proton position hampers the binding to the substrate.