Normal and abnormal electron spin resonance spectra of low-spin cobalt(II)[N4]-macrocyclic complexes. A means of breaking the Co—C bond in B12 Co-enzyme

Abstract

Co(cyen)† normally has an e.s.r. spectrum characteristic of a low-spin cobalt-(II) complex in a ²B₂ ground state. However, its BF₃ adduct, Co(cyen)· BF₃ and its bis(nitro) derivative, Co(5-O₂N-cyen) have ²A₁ ground states. It is proposed that the ground state of this sort of system can be changed from A₁ to B₂ in principle by increasing the ligand field. This idea is applied to the mechanism of Co—C homolytic fission in vitamin B12 coenzyme, and it is suggested that the approach of an amine group of the 5′-deoxyadenosyl group to the corrin ligand may stabilize an intermediate of B₂ symmetry. Fission of the Co—C bond may occur by a ‘staircase lock’ mechanism.