Formation of superoxide anion on aerial oxidation of Cu(II)-porphyrinogen in the synthesis of tetrakis(cyclohexyl)porphyrinogenCu(III) anion

Tetraethylammonium-tetrakis(cyclohexyl)porphyrinogenCu(II) (1) is spontaneously oxidized by aerial oxygen to the corresponding Cu(III) (2) species, producing 1 equiv of O2(-). Steric crowding of the peripheral hydrogens in 1 prevented any direct Cu-O2 bond formation in the oxidation process, which suggests an outer-sphere electron transfer reaction.