Activation of Electrophilic Reactivity by Protonation and Oxidation of a Peroxo CoIIICoIII Complex: Reactivity and Molecular Structures of Hydroperoxo, Superoxo, and Peroxo CoIIICoIII Complexes

In nature, oxidizing metalloenzymes usually bind O₂ in the form of peroxo intermediates that frequently requires for activation a further protonation, for which the formation of hydroperoxo species is proposed. In dinuclear metalloenzymes, which typically feature Fe, Mn, and Cu, structures and electrophilic reactivity of peroxo, superoxo, and hydroperoxo intermediates are generally difficult to study due to their reactive and transient nature, which is why more stable peroxo Co^IIICo^III complexes might be helpful models. Here, we present the molecular structures of a series of μ-1,2-peroxo, μ-1,2-superoxo, and μ-1,1-hydroperoxo Co^IIICo^III complexes and the variation of their electrophilic hydrogen-atom-transfer (HAT) and oxygen-atom-transfer (OAT) reactivity. The μ-1,2-peroxo complex exhibits no electrophilic reactivity, whereas oxidation to the μ-1,2-superoxo complex activates electrophilic HAT reactivity, while protonation to the μ-1,1-hydroperoxo complex activates electrophilic OAT reactivity. The latter occurs by an associated mechanism with the μ-1,1-hydroperoxo ligand being the electrophilic OAT agent. Protonation to the μ-1,1-hydroperoxo elongates the O–O bond, while oxidation to the μ-1,2-superoxo shortens it relative to the μ-1,2-peroxo. The protonation of the μ-1,2-peroxo complex could be made accessible by the introduction of remote electron-donating substituents into a recently reported μ-1,2-peroxo Co^IIICo^III complex that increases the μ-1,2-peroxo basicity by more than five pK_a units. Hence, the initially counterintuitive increase of electron donation by the ligand environment increases the μ-1,2-peroxo basicity allowing its protonation to a more electrophilic μ-1,1-hydroperoxo ligand. More generally, HAT and OAT reactivity is correlated with accessibility of the following intermediate.