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

A set of nickel­(III) peroxo complexes bearing tetraazamacrocyclic ligands, [NiIII(TBDAP)­(O2)]+ (TBDAP = N,N′-di-tert-butyl-2,11-diaza­[3.3]­(2,6)­pyridinophane) and [NiIII(CHDAP)­(O2)]+ (CHDAP = N,N′-dicyclohexyl-2,11-diaza­[3.3]­(2,6)­pyridinophane), were prepared by reacting [NiII(TBDAP)­(NO3)­(H2O)]+ and [NiII(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 [NiIII(TBDAP)­(O2)]+ < [NiIII(CHDAP)­(O2)]+. This result provides fundamental insight into the mechanism of the structure (steric)–reactivity relationship of metal peroxo intermediates.