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

Abstract

A set of nickel(III) peroxo complexes bearing tetraazamacrocyclic ligands, [NiIII(TBDAP)(O<inf>2</inf>)]+ (TBDAP = N,N′-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane) and [NiIII(CHDAP)(O<inf>2</inf>)]+ (CHDAP = N,N′-dicyclohexyl-2,11-diaza[3.3](2,6)pyridinophane), were prepared by reacting [NiII(TBDAP)(NO<inf>3</inf>)(H<inf>2</inf>O)]+ and [NiII(CHDAP)(NO<inf>3</inf>)]+, respectively, with H<inf>2</inf>O<inf>2</inf> 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 NiO<inf>2</inf> 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)(O<inf>2</inf>)]+ < [NiIII(CHDAP)(O<inf>2</inf>)]+. This result provides fundamental insight into the mechanism of the structure (steric)-reactivity relationship of metal peroxo intermediates. (Figure Presented). © 2015 American Chemical Society.