Synthesis, Characterization and Reactivity Studies of Cobalt(III) Porphyrin-Iodosylarene Adduct and Cobalt(III) Porphyrin π-Cation Radical Species

Biomimetic metalloporphyrin complexes have been employed in a number of catalytic oxidation reactions by utilizing terminal oxidants such as iodosylarenes (ArIO). Although high-valent metal-oxo species have been considered as the reactive intermediates, their precursors, metal-iodosylarene adduct species, also exhibit intriguing oxidation capability under certain conditions. However, late transition metal porphyrin-oxidant adduct species have not been explored in oxidation reactions yet. Herein, we report the synthesis, characterization and reactivity studies of cobalt(III) porphyrin-ArIO adduct complexes. These adduct species exhibit moderate oxidation capability in electron transfer reactions. More interestingly, addition of Brønsted acid or Lewis acid facilitated the O−I bond cleavage, resulted in the formation of cobalt(III) porphyrin π-cation radical species, which is much more reactive than the corresponding adduct species in electron transfer reactions. Kinetic studies and theoretical calculations demonstrate that the O−I bond cleavage is triggered in the presence of acid, affording the porphyrin ligand oxidation while the formation of high-valent cobalt-oxo species is prohibited due to the “oxo-wall” for late transition metals. This study provides a novel model of a late transition metal-iodosylarene adduct species as an active oxidant in oxidation reactions, while in the cases of iron and manganese complexes, high-valent metal-oxo species are generated.