Mechanistic insights into dioxygen activation by a manganese corrole complex: a broken-symmetry DFT study

The Mn-oxygen species have been implicated as key intermediates in various Mn-mediated oxidation reactions. However, artificial oxidants were often used for the synthesis of the Mn-oxygen intermediates. Remarkably, the Mn(v)-oxo and Mn(iv)-peroxo species have been observed in the activation of O₂ by Mn(iii) corroles in the presence of base (OH^-) and hydrogen donors. In this work, density functional theory methods were used to get insight into the mechanism of dioxygen activation and formation of Mn(v)-oxo. The results demonstrated that the dioxygen cannot bind to Mn without the axial OH^- ligand. Upon the addition of the axial OH^- ligand, the dioxygen can bind to Mn in an end-on fashion to give the Mn(iv)-superoxo species. The hydrogen atom transfer from the hydrogen donor (substrate) to the Mn(iv)-superoxo species is the rate-limiting step, having a high reaction barrier and a large endothermicity. Subsequently, the O-C bond formation is concerted with an electron transfer from the substrate radical to the Mn and a proton transfer from the hydroperoxo moiety to the nearby N atom of the corrole ring, generating an alkylperoxo Mn(iii) complex. The alkylperoxo O-O bond cleavage affords a Mn(v)-oxo complex and a hydroxylated substrate. This novel mechanism for the Mn(v)-oxo formation via an alkylperoxo Mn(iii) intermediate gives insight into the O-O bond activation by manganese complexes.