Distinct Reactivity Differences of Metal Oxo and Its Corresponding Hydroxo Moieties in Oxidations: Implications from a Manganese(IV) Complex Having Dihydroxide Ligand

摘要

Transition-metal ions play the critical roles in a series of enzymatic electron-transfer, hydrogen-abstraction, and oxygenation events in nature, and the active metal oxo functional groups (i.e., M~(n+)=O) have been long recognized as the key active moieties in these biological and chemical oxidation processes. However, recent discoveries revealed significant exceptions. Green found that the compound II intermediate in chloroperoxidase is in an iron(IV) hydroxo form (i.e., (Porp)Fe~(IV)—OH; Porp = porphyrin ligand), rather than the generally expected iron(IV) oxo form, while the active forms of the corresponding compound II in other peroxidases are still controversial. In xanthine oxidases, a molybdenum(VI) hydroxo group (i.e., Mo~(VI)—OH) was suggested to play the key role in hydroxylation of hypoxanthine even though an Mo~(VI)=O group exists in the same molybdenum(VI) center. In lipoxygenases, iron(III) hydroxo or manganese-(III) hydroxo, rather than their corresponding oxo groups, were also proposed to perform hydrogen abstraction from the unsaturated fatty acid, and a few inorganic models with the M~(n+)-OH moiety confirmed its hydrogen-abstraction ability.