Spectroscopic Characterization and Reactivity Study of Nickel-Peroxo Species

Abstract

Metal-O2 adducts, metal-superoxo, -peroxo and -hydroperoxo species, are key intermediate in metalloenzymes. Especially, metal-peroxo species plays a crucial role in the catalytic cycle of biological reaction such as aldehyde deformylation. Herein, a couple of Ni(III)-peroxo complexes, [NiIII(Me3-TPADP)(O2)]+ and [NiIII(iPr3-TPADP)(O2)]+, were synthesized by reacting Ni(II) precursor bearing N4-type tetradentate ligand (Me3-TPADP = 3,6,9-trimethyl-3,6,9-triaza-1(2,6)-pyridinacyclodecaphane and iPr3-TPADP = 3,6,9-triisopropyl-3,6,9-triaza-1(2,6)-pyridinacyclodecaphane) with H2O2 (hydrogen peroxide) in the presence of TEA (trimethylamine) and characterized with various physicochemical methods, such as UV-vis, ESI-Master, resonance Raman and EPR. The spectroscopic and electrochemical characterization exhibits that difference of electronic properties by R groups (R = Me and iPr) is negligible. However, different steric effects of the bearing ligand are confirmed by structural analysis. In the aldehyde deformylation, the nucleophilic reactivity of the nickel(III)-peroxo complexes was highly dependent on the steric properties of the macrocyclic ligands with the reactivity order of [NiIII(iPr3-TPADP)(O2)]+ < [NiIII(Me3-TPADP)(O2)]+.