Spectroscopic and Structural Characterization of Mn(III)-Alkylperoxo Complexes Supported by Pentadentate...

Manganese-alkylperoxo species have been proposed as important intermediates in certain enzymatic pathways and are presumed to play a key role in catalytic substrate oxidation cycles involving manganese catalysts and peroxide oxidants. However, structural and spectroscopic understanding of these intermediates is very limited, with only one series of synthetic Mn^III-alkylperoxo complexes having been reported. In the present study, we describe the formation and properties of two new Mn^III-alkylperoxo complexes, namely, [Mn^III(OO ^tBu)(dpaq)]⁺ and [Mn^III(OO ^tBu)(dpaq^2Me)]⁺, which utilize the anionic, amide-containing pentadentate dpaq ligand platform. These complexes were generated by reacting the corresponding Mn^II precursors with a large excess of ^tBuOOH at -15 °C in MeCN. In both cases, the corresponding mononuclear Mn^III-hydroxo complexes [Mn^III(OH)(dpaq)]⁺ and [Mn^III(OH)(dpaq^2Me)]⁺ are observed as intermediates en route to the Mn^III-alkylperoxo adducts. These new Mn^III-alkylperoxo complexes were characterized by electronic absorption, infrared, and Mn K-edge X-ray absorption spectroscopies. Complementary density functional theory calculations were also performed to gain insight into their bonding and structural properties. Compared to previously reported Mn^III-alkylperoxo adducts, the Mn^III centers in these complexes exhibit significantly altered primary coordination spheres, with a strongly donating anionic amide nitrogen located trans to the alkylperoxo moiety. This results in Mn^III-alkylperoxo bonding that is dominated by σ-interactions between the alkylperoxo π_ip*(O-O) orbital and the Mn d _z orbital.