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

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 ti me on excitation in the charge transfer to solvent band. The adduct formed in the primary photochemical process with the complex 1 is efficiently scav enged in presence of H+ and O-2 to produce HO2. radical. This radical furth er oxidizes the nickel(II) macrocycIic complex to produce nickel(m) complex in the secondary thermal reaction as seen by the growth in absorbance at 3 70 nm in the conventional flash photolysis setup. Rate constants for the fo rmation of nickel(III) by the reaction of HO2. radical with 1 was determine d as a function of the concentration of the complex and hydrogen ion. Conve ntional flash photolysis of the complex 2 in presence of N2O produces a tra nsient which shows maximum at 610 nm. Stable nickel(III) complex is isolate d on continuous photolysis of the complex 1 and characterized by electronic , IR, ESR spectroscopy and CHN analysis. In deaerated acidic aqueous soluti on 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(m) complex is found to be 0.03 +/- 0.005. (C) 2001 Elsev ier Science B.V. All rights reserved.