Photooxidation of some nickel(II) macrocyclic complexes in non-aqueous solvents

Abstract

The complexes [Ni(tet-a)](ClO₄)₂, 1, [Ni(htcd)](ClO₄)₂, 2 and [Ni(AT)]ClO₄, 3 (where tet-a = 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, htcd = 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene and AT = 11,13-dimethyl-1,4,7,10-tetraazacyclotrideca-10,12-dienato) were photolysed in the charge transfer to solvent band in non-aqueous solvents using 254 nm radiation. The complex [Ni(BG-opdn)], 4 (where BG-opdn = 4,9-dihydroxy-3,4,9,10-tetraphenyl-6,7-benzo-1,2,5,8,11,12-hexaazacyclotetradeca-2,6,10,12,14-pentaene) was photolysed using sunlight in dichloromethane and the solid product was isolated and characterised. Flash photolysis of the complex 1 or 2 in pure acetonitrile produces transients which could be assigned to the nickel(III)-carbon bonded species whereas continuous photolysis in presence of oxygen produces stable five coordinated nickel(III) complexes. Formation of nickel(III) complex is seen as a growth in absorbance at 370 nm in the case of complexes 1 and 2 on flash photolysis in oxygenated acetonitrile containing dichloromethane. Rate constants for the formation of nickel(III) by the reaction of alkylperoxyl radical with nickel(II) complex were determined as a function of the concentration of the complex. Formation of stable nickel(III)-complex was confirmed in the case of complexes 1 and 2 by absorption and ESR spectra on continuous photolysis of complexes in acetonitrile solutions containing 1 M dichloromethane. In the case of complex 3 which contains a negatively charged macrocyclic ligand the oxidation of the metal centre is followed presumably by another intramolecular electron transfer to produce a nickel(II) radical centred on nitrogen which decays subsequently. In the case of complex 4 continuous photolysis using sunlight in air-equilibrated dichloromethane shows absorption spectral change and the photoproduct was isolated and characterised as nickel(II) stabilised ligand radical. Flash photolysis of 4 does not show any transient species in the microsecond time scale.