Photolysis Secondary Products of Cobaloximes and Imino/Oxime Compounds Controlled by Steric Hindrance Imposed by the Lewis Base

Abstract

Anaerobic photolysis of alkylpyridinecobaloximes and alkylpyridine and alkylphosphine imino/oxime compounds with visible light was studied by electron paramagnetic resonance spectroscopy in aprotic solvents, in frozen (77 K) and in fluid solution (250 K and 300 K). Irradiation of samples in a frozen matrix allows identification of photolysis primary products, which are, for all the compounds studied, a Co(II) five-coordinate species and an organic radical, thus implying homolytic cleavage of the cobalt−carbon bond. The behavior of imino/oxime compounds is identical for all the alkyl radicals, contrasting with that observed for methyl and benzyl pyridine cobaloximes, for which no cleavage of the Co−C bond was observed when irradiation was performed with visible light at 250 and 77 K. Relaxation of the frozen samples to room temperature, 300 K, results in the formation of Co(II) secondary products, namely, six-coordinate species containing two molecules of base, with the exception of ortho-substituted pyridines or triphenylphosphine, for which the five-coordinate cobalt(II) species is not involved in further reactions. These results are rationalized in terms of structural deformations induced by the Lewis base in the equatorial moiety. The results obtained for imino/oxime compounds constitute spectroscopic evidence that formation of six-coordinate species with two axially bound bases results from abstraction of base molecules from the five-coordinate Co(II) species as a Co(II) four-coordinate species is observed simultaneously, and its EPR signal is lost if photolysis is performed in the presence of an excess of base. The results suggest that formation of diamagnetic entities after abstraction of base molecules occurs for cobaloximes but not for imino/oxime compounds. The results observed for samples irradiated at 250 and 300 K are identical to those obtained for samples irradiated at 77 K after relaxation to room temperature, thus indicating that the same mechanism is operative at both irradiation temperatures.