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)( ^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.
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