Mononculear hydroxomanganese(III) centers are common active site motifs for a variety of manganese-dependent enzymes, including manganese superoxide dismutase (MnSOD) and manganese lipoxygenase (MnLOX). In the latter enzyme, the hydroxomanganese(III) unit initiates the dioxygenation of a fatty acid substrate by performing a proton-coupled electron-transfer (PCET) reaction. In synthetic inorganic chemistry, hydroxomanganese(III) adducts are rather unusual, and the number of such complexes known to participate in PCET reactions is quite limited. We have generated hydroxomanganese(III) adducts supported by an N5, amide-containing ligand (dpaq) and dpaq derivatives with altered steric and electronic properties. Kinetic investigations show that derivatization of the dpaq ligand can have dramatic effects on PCET reactions with substrates with weak O-H (<i>e.g.</i>, TEMPOH and phenols) and C-H (<i>e.g.</i>, xanthene) bonds. For example, a modest increase in the steric bulk of the dpaq ligand can increase the rate of TEMPOH oxidation by more than 100-fold, but this same complex shows reduced rates of oxidation of bulky phenols. The basis for these perturbations in PCET reactivity in terms of the geometric and electronic structure of the Mn(III)-hydroxo unit will be discussed.

Steric and electronic control of proton-coupled electron-transfer reactions of mononuclear hydroxomanganese(III) complexes

251st National Meeting (2016)

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