Dehalogenation of chloroalkanes by nickel(I) porphyrin derivatives, a computational study

The nickel( I ) octaethylisobacteriochlorin anion ([ OEiBCh -Ni (I) ] − ) is commonly used as a synthetic model of cofactor F 430  from Methyl-Coenzyme M Reductase. In this regard, experimental studies show that [ OEiBCh -Ni (I) ] −  can catalyze dehalogenation of aliphatic halides in DMF solution by a highly efficient S N 2 reaction. To better understand this process, we constructed theoretical models of the dehalogenation of chloromethane by a simple nickel( I ) isobacteriochlorin anion and compared its reactivity with that of similar Ni (I)  complexes with other porphyrin-derived ligands: porphyrin, chlorin, bactreriochlorin, hexahydroporphyrin and octahydroporphyrin. Our calculations predict that all of the porphyrin derivative's model reactions proceed through low-spin complexes. Relative to the energy of the separate reactants the theoretical activation energies (free-energy barriers with solvation corrections) for the dehalogenation of chloromethane are similar for all of the porphyrin derivatives and range for the different functionals from 10–15 kcal mol −1  for B3LYP to 5–10 kcal mol −1  for M06-L and to 13–18 kcal mol −1  for ωB97X-D. The relative free energies of the products of the dehalogenation step,  L -Ni–Me adducts, have a range from −5 to −40 kcal mol −1  for all functionals; generally becoming more negative with increasing saturation of the porphyrin ligand. Moreover, no significant differences in the theoretical chlorine kinetic isotope effect were discernable with change of porphyrin ligand.