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

Two mixed-valence Mn(II)Mn(III) complexes, [MnL(OAc)(HO)]BPh·2.5HO and [MnL(OAc)]·4HO, obtained with unsymmetrical NO-hexadentate L (HL = 2-(N,N-bis(2-(pyridylmethyl)aminomethyl)-6-(N-(2-hydroxybenzyl)benzylaminomethyl)-4-methylphenol) and NO-heptadentate L (NaHL = 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 NO-coordination sphere around the Mn(II) ion, but differ in the donor groups surrounding Mn(III) (NO(solvent) and NO). In non-protic solvents, these two complexes are able to disproportionate at least 3600 equiv. of HO without significant decomposition, with first-order dependence on catalyst and saturation kinetics on [HO]. Spectroscopic monitoring of the reaction mixtures revealed the two complexes disproportionate HO employing a different redox cycle, with retention of dinuclearity. The higher catalytic efficiency of [MnL(OAc)] 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 L by a carboxylate arm in L which facilitates the formation of the catalyst-HO adduct, placing [MnL(OAc)] as the most efficient among the phenolate-bridged diMn catalysts based on the k/K criterion.