Functional modeling of manganese-containing O-2 evolution enzymes with manganese porphyrin dimers

As a functional model of manganese catalyses, manganese porphyrin dimers exhibited high catalytic activity. This modeling system of the enzymatic H2O2 dismutation was supposed to involve the corresponding Mn-2(III)/Mn-2(IV) couple in its catalytic cycle. A Mn-IV porphyrin dimer, [Mn-IV(OR)(2)P](2) (R=OMe or OH), as a candidate of the high-valent Mn intermediate was prepared by the oxidation of the Mn complex with ClO- in order to establish the detailed reaction mechanism of the modeling reaction with a manganese porphyrin dimer. The resultant high-valent Mn complex was characterized by UV-vis, ESR, and IR spectroscopies. Its reduction rate by H2O2 was measured photometrically at low temperatures and compared with that observed by the catalytic dismutation of the corresponding Mn-III complex. The reduction rate of [Mn-IV(OR)(2)P](2) was higher than the overall rate of the catalytic dismutation. Thus, these results suggest that [Mn-IV(OR)(2)P](2) or the related complex could be the intermediate in the dismutation reaction and its reduction by H2O, is a fast process in the overall catalytic cycle. This is in agreement with the previous mechanistic studies and a similar intermediate is likely to be involved in the catalytic cycle of Mn catalyses.