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

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 L^1(2-) (H₂L¹ = 2-(N,N-bis(2-(pyridylmethyl)aminomethyl)-6-(N-(2-hydroxybenzyl)benzylaminomethyl)-4-methylphenol) and N₄O₃-heptadentate L^2(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 k_cat/K_M criterion.