Steric Effect on the Nucleophilic Reactivity of Nickel(III) Peroxo Complexes

A set of nickel­(III) peroxo complexes bearing tetraazamacrocyclic ligands, [Ni^III(TBDAP)­(O₂)]⁺ (TBDAP = N,N′-di-tert-butyl-2,11-diaza­[3.3]­(2,6)­pyridinophane) and [Ni^III(CHDAP)­(O₂)]⁺ (CHDAP = N,N′-dicyclohexyl-2,11-diaza­[3.3]­(2,6)­pyridinophane), were prepared by reacting [Ni^II(TBDAP)­(NO₃)­(H₂O)]⁺ and [Ni^II(CHDAP)­(NO₃)]⁺, respectively, with H₂O₂ in the presence of triethylamine. The mononuclear nickel­(III) peroxo complexes were fully characterized by various physicochemical methods, such as UV–vis, electrospray ionization mass spectrometry, resonance Raman, electron paramagnetic resonance, and X-ray analysis. The spectroscopic and structural characterization clearly shows that the NiO₂ cores are almost identical where the peroxo ligand is bound in a side-on fashion. However, the different steric properties of the supporting ligands were confirmed by X-ray crystallography, where the CHDAP ligand gives enough space around the Ni core compared to the TBDAP ligand. The nickel­(III) peroxo complexes showed reactivity in the oxidation of aldehydes. In the aldehyde deformylation reaction, the nucleophilic reactivity of the nickel­(III) peroxo complexes was highly dependent on the steric properties of the macrocyclic ligands, with a reactivity order of [Ni^III(TBDAP)­(O₂)]⁺ < [Ni^III(CHDAP)­(O₂)]⁺. This result provides fundamental insight into the mechanism of the structure (steric)–reactivity relationship of metal peroxo intermediates.