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Synthetic manganese catalysts that activate hydrogen peroxide perform a variety of hydrocarbon oxidation reactions. The most commonly proposed mechanism for these catalysts involves the generation of a manganese(iii)-hydroperoxo intermediate that decays via heterolytic O–O bond cleavage to generate a Mn( v)-oxo species that initiates substrate oxidation. Due to the paucity of well-defined Mn(III)-hydroperoxo complexes, Mn(III)-alkylperoxo complexes are often employed to understand the factors that affect the O–O cleavage reaction. Herein, we examine the decay pathways of the Mn(III)-alkylperoxo complexes [Mn(III)(OOtBu)(6Me dpaq)]+ and [Mn(III)(OOtBu)(N4S)]+, which have distinct coordination environments (N5− and N4S− , respectively). Through the use of density functional theory (DFT) calculations and comparisons with published experimental data, we are able to rationalize the differences in the decay pathways of these complexes. For the [Mn(III)(OOtBu)(N4S)]+ system, O–O homolysis proceeds via a two-state mechanism that involves a crossing from the quintet reactant to a triplet state. A high energy singlet state discourages O–O heterolysis for this complex. In contrast, while quintet–triplet crossing is unfavorable for [Mn(III)(OOtBu)(6Medpaq)]+, a relatively low-energy single state accounts for the observation of both O–O homolysis and heterolysis products for this complex. The origins of these differences in decay pathways are linked to variations in the electronic structures of the Mn(III)-alkylperoxo complexes.
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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 of2is based on electronic absorption,1H NMR, IR, and ESI‐MS data. The thermal decay of2follows a first order process, and variable‐temperature kinetic studies of the decay of2yielded activation parameters that could be compared with those of a MnIII‐alkylperoxo analogue. Complex2reacts with the hydrogen‐atom donor TEMPOH two‐fold faster than the MnIII‐hydroxo complex1. Complex2also 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 of2with similar MnIII‐hydroperoxo and MnIII‐alkylperoxo complexes.
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- Award ID(s):
- 2154955
- PAR ID:
- 10642344
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Chemistry – A European Journal
- Volume:
- 30
- Issue:
- 70
- ISSN:
- 0947-6539
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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