Electronic structure contributions to O–O bond cleavage reactions for MnIII-alkylperoxo complexes

Abstract

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(OO^tBu)(^6Medpaq)]⁺ and [Mn^III(OO^tBu)(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(OO^tBu)(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(OO^tBu)(^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.