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 F₄₃₀ 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_N2 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.