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

Abstract

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-O₂ core in [Ni(<i>n</i>-TMC)(O₂)]⁺ (<i>n</i> = 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 H₂O₂ in 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.