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

Abstract

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.