Photolysis Primary Products of Alkylcobaloximes Controlled by the Cobalt−Carbon Bond Strength

Abstract

Anaerobic photolysis of several methyl- and benzylpyridinecobaloximes was studied by electron spin resonance spectroscopy in aprotic solvents, at three temperatures of irradiation and with use of visible and UV light sources. Homolysis of the cobalt−carbon bond is photochemically induced when irradiation with visible light is performed at room temperature, giving rise to Co(II) five-coordinate species and organic radicals. Irradiation with visible light of fluid (250 K) or frozen (77 K) solutions does not cause homolysis of the cobalt−carbon bond, in contrast with the results reported for other alkylpyridinecobaloximes. Homolysis of the cobalt−carbon bond is achieved at all temperatures if a UV light source is used, and the results are rationalized in terms of the values of the cobalt−carbon bond dissociation energies. The results obtained at 250 K for a series of isotopically substituted methylpyridinecobaloximes allowed the definitive assignment of the spectrum obtained upon photolysis at 250 K to a cobalt(III) nitroxide radical and not to a Co(II) center as has been proposed earlier.