Mechanisms for the photolysis and radiolysis of alkylcobaloximes and related compounds. An electron spin resonance study

Abstract

Exposure of a range of alkyl(pyridine)cobaloximes and related compounds in dilute solutions in methanol or 2-methyltetrahydrofuran at 77 K to ⁶⁰Co γ-rays resulted in electron capture. For the methyl derivative the pyridine ligand was lost leaving a novel Co^II derivative whose e.s.r. parameters suggests a (d_z²)¹ configuration (z is normal to the oxime plane). The ¹³CH₃ derivative gave well defined hyperfine coupling ¹³C in accord with the proposed structure. Other alkyl derivatives gave similar similar results, the pyridine ligand being lost in preference to the alkyl ligand. These results make an interesting contrast with previous results for alkylcobalamins, for which initial elctron addition was into a corrin π^* orbital. The chloro derivative lost Cl^– in preference to pyridine on electron addition. The chlorotriphenylphosphine derivative gave evidence for electron addition into the d_z² orbital, chloride ions being lost on annealing above 77 K. The hydridotriphenylphosphine derivative lost hydrogen on electron addition: possible mechanisms for this reaction are discussed.

On photolysis at 77 K in CD₃OD solution poorly defined e.s.r. features for Co^II complexes were obtained together with well defined features for the alkyl radicals [·CH₃, ·CH₂CH₃, ·CH(CH₃)₂ and ·C(CH₃)₃, the last being formed from (CH₃)₂CHĊH₂ radicals]. At high microwave powers broad features tentatively assigned to Co^II···R · pairs were detected, and the marked broadening of the Co^II features is also interpreted in terms of such triplet-state pairs. Similar pair trapping was postulated for the photolysis of alkyl cobalamins. These e.s.r. results are compared with the e.s.r. doublet detected when reacting mixtures of substrates and the enzyme–coenzyme B₁₂ are cooled to 77 K. These results suggest that the primary act of photolysis is homolysis of the Co—C bond. This conclusion is discussed in the light of other e.s.r. studies of these systems.