Mononuclear nickel(ii)-superoxo and nickel(iii)-peroxo complexes bearing a common macrocyclic TMC ligand

Mononuclear metal-dioxygen adducts, such as metal-superoxo and -peroxo species, are generated as key intermediates in the catalytic cycles of dioxygen activation by heme and non-heme metalloenzymes. We have shown recently that the geometric and electronic structure of the Ni–O₂core in [Ni(n-TMC)(O₂)]⁺(n = 12 and 14) varies depending on the ring size of the supporting TMC ligand. In this study, mononuclear Ni(ii)-superoxo and Ni(iii)-peroxo complexes bearing a common macrocylic 13-TMC ligand, such as [Ni^II(13-TMC)(O₂)]⁺and [Ni^III(13-TMC)(O₂)]⁺, were synthesized in the reaction of [Ni^II(13-TMC)(CH₃CN)]²⁺and H₂O₂in the presence of tetramethylammonium hydroxide (TMAH) and triethylamine (TEA), respectively. The Ni(ii)-superoxo and Ni(iii)-peroxo complexes bearing the common 13-TMC ligand were successfully characterized by various spectroscopic methods, X-ray crystallography and DFT calculations. Based on the combined experimental and theoretical studies, we conclude that the superoxo ligand in [Ni^II(13-TMC)(O₂)]⁺is bound in an end-on fashion to the nickel(ii) center, whereas the peroxo ligand in [Ni^III(13-TMC)(O₂)]⁺is bound in a side-on fashion to the nickel(iii) center. Reactivity studies performed with the Ni(ii)-superoxo and Ni(iii)-peroxo complexes toward organic substrates reveal that the former possesses an electrophilic character, whereas the latter is an active oxidant in nucleophilic reaction.