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

Abstract

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 MnIII‐hydroperoxo intermediates. Examples of well‐defined MnIII‐hydroperoxo complexes are rare, and the properties of these species are often inferred from MnIII‐alkylperoxo analogues. In this study, we show that the reaction of the MnIII‐hydroxo complex [MnIII(OH)(6Medpaq)]+ (1) with hydrogen peroxide and acid results in the formation of a dark‐green MnIII‐hydroperoxo species [MnIII(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 MnIII‐alkylperoxo analogue. Complex 2 reacts with the hydrogen‐atom donor TEMPOH two‐fold faster than the MnIII‐hydroxo complex 1. Complex 2 also oxidizes PPh3, and this MnIII‐hydroperoxo species is 600‐fold more reactive with this substrate than its MnIII‐alkylperoxo analogue [MnIII(OOtBu)(6Medpaq)]+. DFT and time‐dependent (TD) DFT computations are used to compare the electronic structure of 2 with similar MnIII‐hydroperoxo and MnIII‐alkylperoxo complexes.