Effect of coordination dissymmetry on the catalytic activity of manganese catalase mimics

Abstract

Two mixed-valence Mn(II)Mn(III) complexes, [Mn2L1(OAc)2(H2O)]BPh4·2.5H2O and [Mn2L2(OAc)2]·4H2O, obtained with unsymmetrical N4O2-hexadentate L1(2−) (H2L1 = 2-(N,N-bis(2-(pyridylmethyl)aminomethyl)-6-(N-(2-hydroxybenzyl)benzylaminomethyl)-4-methylphenol) and N4O3-heptadentate L2(3−) (NaH2L2 = 2-(N,N-bis(2-(pyridylmethyl)aminomethyl)-6-(N′-(2-hydroxybenzyl)(carboxymethyl)aminomethyl)-4-methylphenol sodium salt) ligands, have been prepared and characterized. Both complexes share a μ-phenolate-bis(μ-acetate)Mn(II)Mn(III) core and N3O3-coordination sphere around the Mn(II) ion, but differ in the donor groups surrounding Mn(III) (NO4(solvent) and NO5). In non-protic solvents, these two complexes are able to disproportionate at least 3600 equiv. of H2O2 without significant decomposition, with first-order dependence on catalyst and saturation kinetics on [H2O2]. Spectroscopic monitoring of the reaction mixtures revealed the two complexes disproportionate H2O2 employing a different redox cycle, with retention of dinuclearity. The higher catalytic efficiency of [Mn2L2(OAc)2] was rationalized in terms of the larger labilizing effect of the heptadentate ligand that favors the acetate-shift and the replacement of the non-coordinating benzyl arm of L1 by a carboxylate arm in L2 which facilitates the formation of the catalyst-H2O2 adduct, placing [Mn2L2(OAc)2] as the most efficient among the phenolate-bridged diMn catalysts based on the kcat/KM criterion.