Photooxidation of nickel(II) azamacrocyclic complexes to trivalent nickel complexes on excitation in the charge-transfer-to-solvent band

Abstract

The nickel(II) azamacroyclic complexes [Ni(II)L₁](ClO₄₎₂ (1), [Ni(II)L₂](ClO₄)₂ (2) and [Ni(II)L₃](ClO₄)₂ (3) produce solvated electron on excitation using 248 nm laser in deaerated aqueous solutions where L₁ = 1,4,8,11-tetraazacyclotetradecane, L₂ = 1,8-diethylhydroxy-1,3,6,8,10,13-hexaazacyclotetradecane and L₃ = 1,3,6,9,11,14-hexaazatricyclo[12.2.1.16.9]octadecane. Formation of stable nickel(III) complexes occurs as a result of the scavenging of the solvated electron in the presence of acids and oxygen and as a result of the thermal reaction between the hydroperoxyl radical and the nickel(II) complexes. Continuous photolysis of the complexes using 254 nm radiation results in the photooxidation of the divalent nickel complexes to trivalent nickel complexes which are identified by ESR spectra. The mechanistic details of the formation of the photoproducts have been investigated in detail. The assignment of CTTS formalism for the reactive excited state was confirmed. Photooxidation of the nickel(II) macrocyclic complexes dissolved in acetonitrile and in a mixture of acetonitrile and dichloromethane has been investigated by flash photolysis of the complexes. Continuous photolysis of the complexes in non-aqueous solutions shows the formation of nickel(III) complexes as stable products, which was confirmed by ESR and optical spectral studies.