Radical oxidation of nickel(II) complexes of tetra-azamacrocyclic ligands and the reactions of the resulting nickel(III) complexes: a pulse-radiolysis and flash-photolysis study

Abstract

The nickel(II) complexes [NiL]²⁺[L = 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetra-azacyclotetradecane (L¹), 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetra-azacyclotetradeca-4,11-diene (L²), or 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetra-azacyclotetradeca-1,4,8,11-tetraene (L³)] are oxidised to the nickel(III) complexes in aqueous solutions by the radical ions [Cl₂]^–, [Br₂]^–, and [(SCN)₂]^–, generated by pulse radiolysis and flash photolysis. The outer-sphere electron-transfer reactions are diffusion controlled. Nickel(III) complexes of the ligands which contain NH groups (L¹ and L²) are converted into nickel(II) complexes of radical forms of the ligands at pH >3. Hydroxyl radicals oxidise the nickel(II) complexes by initial H-atom abstraction from the ligand, and the nickel(III) complexes are then formed by proton-assisted intramolecular electron transfer. The nickel(II) ligand-radical complexes are more stable than the nickel(II) complexes and are intermediates in the decay of the nickel(III) complexes in water. The nickel(III) complexes are powerful oxidants and the rates of oxidation of Cl^–, Br^–, [SCN]^–, and [N₃]^– are dependent on the reduction potentials of the complexes. Hydrogen peroxide is oxidised by the nickel(III) and nickel(II) ligand-radical complexes at similar rates. Oxidation of [O₂]^– radical ion by the nickel(III) complexes is diffusion controlled; the rates of oxidation of HO₂ radicals are much smaller and are the same for all three complexes. Perhydroxyl radicals oxidise the nickel(II) complex of the saturated ligand, L¹, to the nickel(III) complex.