Spectroscopic Properties and Reactivity of a MnIII‐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)( 6Me dpaq)] + ( 1 ) with hydrogen peroxide and acid results in the formation of a dark‐green Mn III ‐hydroperoxo species [Mn III (OOH)( 6Me dpaq)] + ( 2 ). The formulation of 2 is based on electronic absorption, 1 H 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 PPh 3 , and this Mn III ‐hydroperoxo species is 600‐fold more reactive with this substrate than its Mn III ‐alkylperoxo analogue [Mn III (OO t Bu)( 6Me dpaq)] + . 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.