Influence of the Net Charge on the Reactivity of a Manganese(IV) Species: Leading to the Correlation of Its Physicochemical Properties with Reactivity

Clarifying how versatile physicochemical parameters of an active metal intermediate affect its reactivity would help to understand its roles in chemical and enzymatic oxidations. The influence of the net charge on electron transfer and hydrogen abstraction reactions of a manganese(IV) species having hydroxide ligand has been investigated here. It was found that increasing one unit of the positive net charge from 2+ to 3+ would accelerate its electron-transfer rate by 10–20 fold in oxygenation of tris(4-methoxyphenyl)phosphine. In contrast, the hydrogen abstraction rate is insensitive to its net charge change, and the insensitivity has been attributed to the compensation effect between the redox potential and pK a, which determine the hydrogen abstraction capability of a metal ion. Similar net-charge-promoted electron transfer but not hydrogen abstraction has also been observed in intramolecular electron transfer and hydrogen abstraction reactions when using thioxanthene as substrate. Together with the previous understanding of the reactivity of the identical manganese(IV) species having MnIV–OH or MnIV=O functional groups, the relationships of the oxidative reactivity of an active metal intermediate with its physicochemical parameters such as the net charge, the redox potential and the metal–oxygen bond order (M–O versus MO) have been discussed with this manganese(IV) model.