Metalloenzymes, such as cytochrome P450 and sulphite oxidase, acts as oxidants to perform essentially challenging reactions by activating dioxygen. These reactions commonly feature high-valent metal oxo species as key intermediates responsible for transfer of an oxygen atom from the metal-center to the substrate under favourable conditions. To understand the mechanistic aspects of such oxygen-atom transfer reactions, a series of biomimetic manganese(IV)-oxo systems supported by neutral, pentadentate N<sub>5</sub> ligands have been studied extensively. A relationship between equatorial ligand field strength of the Mn(IV)-oxo complexes and the reaction rate of thioanisole oxidation was established revealing large rate enhancement for Mn(IV)-oxo adducts with weakened equatorial field. As large rate variations are often associated with a change in mechanism from OAT to outer-sphere electron transfer (ET), we performed a detailed study with thioanisole derivatives to gain insight into the factors influencing this structure-reactivity correlation. A Hammett analysis of kinetic data yields similar slopes for the sulfoxidation reactions with the separate Mn(IV)-oxo adducts, suggesting that these complexes share a common mechanism. This conclusion is further supported by an analysis of the data using the Marcus theory of outer-sphere electron transfer, where rates predicted for an ET mechanism are much slower than experimental rates. This rate enhancement is attributed to a concerted OAT mechanism that avoids high barriers associated with the step-wise ET mechanism. The oxidative reactivity of Mn(IV)-oxo adducts have also been probed in olefin oxidation reactions. Interestingly, the identity of the Mn(IV)-oxo adduct governs the preference for C – H bond oxidation or C=C bond epoxidation during oxidation of olefin substrates.

Exploring the reactivity of manganese(IV)-oxo complexes in oxygen atom transfer reactions

ACS Spring 2022

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