Manganese lipoxygenase is proposed to utilize a mid-valent manganese(III)-hydroxo to abstract a hydrogen atom from a poly-unsaturated fatty acid, initiating substrate peroxidation. The hydrogen-atom transfer step occurs rapidly and with a large primary kinetic isotope effect. Such reactivity is unusual for a manganese(III)-hydroxo species, which are typically considered mild oxidants. Understanding how the enzyme achieves such reactivity for a mid-valent oxidant is currently lacking, and relevant model studies using an easily modifiable ligand-system can provide an excellent way to probe structural effects on reactivity. Our group has reported the [Mn(III)(OH)(dpaqR)]+ (R = 2H, 2Me) complexes, which feature Mn(III)-OH moieties supported by a pentadentate N5 ligand. The 2Me derivative contains a methyl-substitution near the reaction center and has been shown to have greatly increased reactivity for the small substrate TEMPOH. In contrast, reduced reactivity was observed when using bulkier substrates. Additional modifications are utilized via inclusion of either electron donating or withdrawing groups capable of tuning the trans-amide donor without imposing steric limitations. The effects of these substitutions are explored using a combination of spectroscopy, kinetics, and theoretical calculations.

Tuning the proton-coupled electron-transfer reactivity of Mn(III)-hydroxo complexes via ligand perturbations

253rd National Meeting (2017)

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