Photooxidation of nickel(II) macrocyclic complexes on excitation in the charge-transfer-to-solvent band in aqueous solution and in the presence of oxygen

Abstract

Photooxidation of the well known nickel(II)-macrocyclic complexes [Ni(tet-a)] (ClO₄)₂, 1, and [Ni(AT)] ClO₄, 2, (where tet-a=-meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane and AT=11,13-dimethyl-1,4,7,10-tetraazacyclotetradeca-10,12-dienato) has been observed for the first time on excitation in the charge transfer to solvent band. The adduct formed in the primary photochemical process with the complex 1 is efficiently scavenged in presence of H⁺ and O₂ to produce HO₂√ radical. This radical further oxidizes the nickel(II) macrocyclic complex to produce nickel(III) complex in the secondary thermal reaction as seen by the growth in absorbance at 370 nm in the conventional flash photolysis setup. Rate constants for the formation of nickel(III) by the reaction of HO₂√ radical with 1 was determined as a function of the concentration of the complex and hydrogen ion. Conventional flash photolysis of the complex 2 in presence of N₂O produces a transient which shows maximum at 610 nm. Stable nickel(III) complex is isolated on continuous photolysis of the complex 1 and characterized by electronic, IR, ESR spectroscopy and CHN analysis. In deaerated acidic aqueous solution containing alcohols, photolysis of 1 produced hydrogen gas as evidenced by gas chromatographic analysis. Quantum yield for the photooxidation of 1 to give the nickel(III) complex is found to be 0.03±0.005.