Synthesis, structural, and spectroscopic characterization and reactivities of mononuclear cobalt(III)-peroxo complexes

Metal-dioxygen adducts are key intermediates detected in the catalytic cycles of dioxygen activation by metalloenzymes and biomimetic compounds. In this study, mononuclear cobalt(III)-peroxo complexes bearing tetraazamacrocyclic ligands, [Co(12-TMC)(O₂)]⁺ and [Co(13-TMC)(O ₂)]⁺, were synthesized by reacting [Co(12-TMC)(CH ₃CN)]²⁺ and [Co(13-TMC)(CH₃CN)]²⁺, respectively, with H₂O₂ in the presence of triethylamine. The mononuclear cobalt(III)-peroxo intermediates were isolated and characterized by various spectroscopic techniques and X-ray crystallography, and the structural and spectroscopic characterization demonstrated unambiguously that the peroxo ligand is bound in a side-on η² fashion. The O-O bond stretching frequency of [Co(12-TMC)(O₂)]⁺ and [Co(13-TMC)(O₂)]⁺ was determined to be 902 cm^-1 by resonance Raman spectroscopy. The structural properties of the CoO ₂ core in both complexes are nearly identical; the O-O bond distances of [Co(12-TMC)(O₂)]⁺ and [Co(13-TMC)(O₂)] ⁺ were 1.4389(17) Å and 1.438(6) Å, respectively. The cobalt(III)-peroxo complexes showed reactivities in the oxidation of aldehydes and O₂-transfer reactions. In the aldehyde oxidation reactions, the nucleophilic reactivity of the cobalt-peroxo complexes was significantly dependent on the ring size of the macrocyclic ligands, with the reactivity of [Co(13-TMC)(O₂)]⁺ > [Co(12-TMC)(O₂)] ⁺. In the O₂-transfer reactions, the cobalt(III)-peroxo complexes transferred the bound peroxo group to a manganese(II) complex, affording the corresponding cobalt(II) and manganese(III)-peroxo complexes. The reactivity of the cobalt-peroxo complexes in O₂-transfer was also significantly dependent on the ring size of tetraazamacrocycles, and the reactivity order in the O₂-transfer reactions was the same as that observed in the aldehyde oxidation reactions.