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-O-2 core in [Ni(n-TMC)(O-2)](+) (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)(O-2)](+) and [Ni-III(13-TMC)(O-2)](+), were synthesized in the reaction of [Ni-II(13TMC)( 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)(O-2)](+) is bound in an end-on fashion to the nickel(II) center, whereas the peroxo ligand in [Ni-III(13-TMC)(O-2)](+) 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.