Role of protonation and of axial ligands in the reductive dechlorination of alkyl chlorides by vitamin B12 complexes. Reductive cleavage of chloroacetonitrile by Co(I) cobalamins and cobinamides

Cobalt(I) cobalamin and cobinamide are efficient catalysts of the hydrogenolysis of aliphatic chloro compounds. Taking chloroacetonitrile as example, the first requirement for high catalytic efficiency is fulfilled by the high reactivity of the Co(I) complex toward the substrate, leading to the alkylcobalt(III) complex. This is further reduced into the alkylcobalt(II) complex. However, the fact that these two reactions are fast is not enough to ensure an efficient catalysis: in DMF catalysis is very poor, while it is high in water. The experiments carried out in DMF with addition of an acid show that a crucial step in the catalytic process is the proton transfer decomposition of the alkylcobalt(II) complex, leading to the product, and closing the catalytic loop by regeneration of the cobalt(I) complex. Another important feature of these catalytic reactions is the role played by axial ligands present in the solution, particularly those that are produced by the catalytic reaction itself, namely, chloride ions and the counteranion of the added acid. The stronger these ligands, the more negative the potential required for the reduction of the alkylcobalt(III) complex. This amounts to a self-moderation effect: the more efficient catalysis, the slower its second step, i.e., the conversion of the alkylcobalt(III) complex into the alkylcobalt(II).