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

Abstract

Two mixed-valence Mn(II)Mn(III) complexes, [Mn2L 1 (OAc)2(H2O)]BPh4•2.5H2O and [Mn2L 2 (OAc)2]•4H2O, obtained with unsymmetrical N4O2-hexadentate L 1(2-) (H2L 1 = 2-(N,Nbis(2-(pyridylmethyl)aminomethyl)-6-(N-(2-hydroxybenzyl)benzylaminomethyl)-4methylphenol) and N4O3-heptadentate L 2(3-) (NaH2L 2 = 2-(N,N-bis(2-(pyridylmethyl)aminomethyl)-6-(N'-(2-hydroxybenzyl)(carboxymethyl)aminomethyl)-4methylphenol 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 nonprotic 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 [Mn2L 2 (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 L 1 by a carboxylate arm in L 2 which facilitates the formation of the catalyst-H2O2 adduct, placing [Mn2L 2 (OAc)2] as the most efficient among the phenolate-bridged diMn catalysts based on the kcat/KM criterion.

Mechanistic Insights into Superoxide Dismutation Driven by Dinuclear Manganese Complexes: The Role of the Mn2-Core

“…To further prove the redox state of the Mn centers in peroxo adduct ( 1 ), Mn 2 L 2 Ac was reacted with KO 2 at −40 °C in acetonitrile, and the frozen solution was analyzed by CW X-band EPR spectroscopy (Figures b and S42). The EPR spectrum of Mn 2 L 2 Ac at 95 K in the presence of 1 equiv of KO 2 shows a broad signal consistent with a weakly coupled Mn­(II)­Mn­(III)-core with a six-line signal at g ≈ 2 and S = 1/2 ground state . The weak coupling between the Mn centers is ascribable to the elongation and weakening of the phenolate bridges after the peroxo binds to the Mn­(III) ion in the dinuclear unit.…”

“…This peak overlaps with that of the superoxo Mn 2 (III,II)–(O 2 •– ) ( m / z = 375.0774; Scheme ) and the hydroxo Mn 2 (III,III)–(OH) 2 species ( m / z = 377.0882; Scheme ). The EPR analysis upon reaction with KO 2 also shows the six-line signal ( g ≈ 2, Figures b and S41) with S = 1/2 ground state, as obtained for ( 1 ) in the case of Mn 2 L 2 Ac …”