Hydrogen peroxide disproportionation by the [TPA(2)Mn(2)(mu-Cl)(2)](2+) complex

Abstract

The dichloride-bridged (TPA(2)Mn(2)(mu-Cl)(2)](2+) complex (I) was synthesized as a structural and functional model complex of the chloride-inhibited manganese catalase, and its catalytic properties in MeCN have been studied. Complex I shows sigmoidal kinetics and the activity is significantly inhibited in the presence of water. The kinetic parameters of the hydrogen peroxide disproportionation by complex I have been successfully fitted with non-Michaelis-Menten kinetics of Hill's equation, which implies a multiple-step substrate activation of complex I. After termination of the catalysis, the mononuclear [TPA(2)Mn](ClO4)(2) was isolated from the solution. During the catalysis, a new penta-coordinate [TPAMnCl](ClO4) complex (IV), tending to accumulate at a lower ratio of H2O2, was isolated and its X-ray crystallographic structure, as well as physical properties, was determined. Transformation of complex I in the presence of different molar ratios of H2O2 was studied by UV-Vis, EPR and ESI-MS spectroscopy. Upon addition of H2O2, the catalytic solution turned dark green, with instant evolution of oxygen gas, and the electronic spectra obtained were identical to that of the dark green dioxo bridged [TPA(2)Mn(2)(mu-O)(2)](ClO4)(3) complex (III). When the catalytic solution was subjected to EPR measurement, the transient peaks corresponding to the electronically localized Mn(II) species developed in a short time at lower concentrations of H2O2. The signal was more distinctive in the presence of water, and the complex I center dot H2O2 adduct was suggested as the intermediate species based on ESI-MS measurements. The EPR signal corresponding to complex III was detected at higher concentrations (>800 equiv.) of H2O2. As possible catalytic intermediates, the [TPA(2)Mn(2)(III)(mu-O)(2)](2+) and [TPA(2)Mn(2)(III)(mu-O)(mu-OH)](+) species were suggested. A possible catalytic mechanism of H2O2 disproportionation by complex I, including the formation of active species and termination of the catalysis, has also been suggested. (C) 2010 Elsevier Ltd. All rights reserved.