Spectroscopic Properties and Reactivity of a Mn^III-Hydroperoxo Complex that is Stable at Room Temperature

Manganese catalysts that activate hydrogen peroxide have seen increased use in organic transformations, such as olefin epoxidation and alkane C-H bond oxidation. Proposed mechanisms for these catalysts involve the formation and activation of Mn^III-hydroperoxo intermediates. Examples of well-defined Mn^III-hydroperoxo complexes are rare, and the properties of these species are often inferred from Mn^III-alkylperoxo analogues. In this study, we show that the reaction of the Mn^III-hydroxo complex [Mn^III(OH)(^6Medpaq)]⁺ (1) with hydrogen peroxide and acid results in the formation of a dark-green Mn^III-hydroperoxo species [Mn^III(OOH)(^6Medpaq)]⁺ (2). The formulation of 2 is based on electronic absorption, ¹H NMR, IR, and ESI-MS data. The thermal decay of 2 follows a first order process, and variable-temperature kinetic studies of the decay of 2 yielded activation parameters that could be compared with those of a Mn^III-alkylperoxo analogue. Complex 2 reacts with the hydrogen-atom donor TEMPOH two-fold faster than the Mn^III-hydroxo complex 1. Complex 2 also oxidizes PPh₃, and this Mn^III-hydroperoxo species is 600-fold more reactive with this substrate than its Mn^III-alkylperoxo analogue [Mn^III(OO^tBu)(^6Medpaq)]⁺. DFT and time-dependent (TD) DFT computations are used to compare the electronic structure of 2 with similar Mn^III-hydroperoxo and Mn^III-alkylperoxo complexes.