SPECTROSCOPIC STUDIES OF OXIDIZED MANGANESE CATALASE AND MU-OXO-BRIDGED DIMANGANESE(III) MODEL COMPLEXES - ELECTRONIC-STRUCTURE OF THE ACTIVE-SITE AND ITS RELATION TO CATALYSIS

Spectroscopic and electronic structure studies of oxidized manganese c atalase (OxMnCAT) and mu-oxo-di-mu-carboxylato-bridged dimanganese(III ) model complexes are presented and discussed, providing, for the firs t time, a detailed experimental description of the catalytically relev ant Mn(III)/Mn(III) binuclear active site of MnCAT. The absorption and MCD spectra of the synthetic complexes are similar to one another, in dicating that the transitions below 30 000 cm(-1) are inherent to the [(Mn2O)-O-III(OAc)(2)](2+) core. A combination of Raman excitation pro file and polarized single-crystal absorption measurements on [Mn2O(OAc )(2)(Me(3)tacn)(2)]-(ClO4)(2) . H2O and density functional electronic structure calculations are used to identify the key spectral features of the model complexes and their relation to the dominant Mn-O(oxo) bo nding interaction in these dimers. On the basis of these results, anal ysis of the spectroscopic data of OxMnCAT leads to the proposal that t he active site consists of two ferromagnetically coupled five-coordina te Mn(In) ions linked by a hydroxo bridge, contrasting previous propos als of an oxo bridge at this site. The interaction of OxMnCAT with azi de, an electronic structural mimic of H2O2, has been studied using abs orption and MCD spectroscopies. It appears that azide binds to both Mn (III) ions of the active site by displacement of one ligand on each me tal, indicating that both ions are available for reaction with H2O2 Th e significance of our results with respect to the catalytic peroxide d isproportionation reaction accomplished by MnCAT is discussed.