Reaction landscape of a mononuclear Mn^III-hydroxo complex with hydrogen peroxide

Peroxomanganese species have been proposed as key intermediates in the catalytic cycles of both manganese enzymes and synthetic catalysts. However, many of these intermediates have yet to be observed. Here, we report the formation of a series of intermediates, each generated from the reaction of the mononuclear Mn^III-hydroxo complex [Mn^III(OH)(dpaq^2Me)]⁺ with hydrogen peroxide under slightly different conditions. By changing the acidity of the reaction mixture and/or the quantity of hydrogen peroxide added, we are able to control which intermediate forms. Using a combination of electronic absorption, ¹H NMR, EPR, and X-ray absorption spectroscopies, as well as density functional theory (DFT) and complete active space self-consistent field (CASSCF) calculations, we formulate these intermediates as the bis(μ-oxo)dimanganese(III,IV) complex [Mn^IIIMn^IV(μ-O)₂(dpaq^2Me)₂]⁺, the Mn^III-hydroperoxo complex [Mn^III(OOH)(dpaq^2Me)]⁺, and the Mn^III-peroxo complex [Mn^III(O₂)(dpaq^2Me)]. The formation of the Mn^III-hydroperoxo species from the reaction of a Mn^III-hydroxo complex with hydrogen peroxide mimics an elementary reaction proposed for many synthetic manganese catalysts that activate hydrogen peroxide.