The [Mn2(2-OHsalpn)2]2-,1-,0 System: An Efficient Functional Model for the Reactivity and Inactivation of the Manganese Catalases

Abstract

The complexes [Mn(2-OH(X-sal)pn)]2n- (where X = 5-OCH3, H, 5-Cl, 3,5-diCl, or 5-NO2 and where n = 0 or 2) are shown to be excellent hydrogen peroxide disproportionation catalysts in acetonitrile. When carried out in an open vessel, the reaction can occur for over 5000 turnovers without an indication of catalyst decomposition. The disproportionation reaction cycles between the [MnIII(2-OH(X-sal)pn)]2 and the [MnII(2-OH(X-sal)pn)]22- oxidation levels. All derivatives show saturation kinetics with the highest kcat (21.9 ± 0.2 s-1) observed for the [MnIII(2-OH(5-Clsal)pn)]2 dimer and the optimal kcat/KM (990 ± 60 s-1·M-1) observed for the [MnIII(2-OHsalpn)]2. The first step of the reaction is proposed to be the binding of peroxide to the [MnIII(2-OH(X-sal)pn)]2 through an alkoxide shift to form a ternary intermediate {[MnIII(2-OH(X-sal)pn)]2(H2O2)}. We propose that the turnover-limiting step is the oxidation of peroxide from this intermediate. The binding efficiency of the peroxide is dependent on the phenyl-ring substitution with the derivatives donating the most electrons having the highest affinity for the substrate. Studies with isotopically labeled H2O2 indicate that protons are important in the turnover-limiting step of the reaction and that the O−O bond is not cleaved during peroxide oxidation. In a closed vessel, the product dioxygen will oxidize [MnII(2-OH(5-NO2sal)pn)]22- to [MnII/III(2-OH(5-NO2sal)pn)2]-, and this species can then be stoichiometrically oxidized by hydrogen peroxide to give [MnIII/IV(2-OH(5-NO2sal)pn)2(μ2-O)2]-. This di-μ2-oxobridged species is catalytically incompetent; however, addition of hydroxylamine hydrochloride restores catalytic activity. The relationship of this catalytic disproportionation of hydrogen peroxide and inactivation of the catalyst will be used to define a model for similar reactions observed for the Lactobacillus plantarum Mn catalase.