Organonickel chemistry in the catalytic hydrodechlorination of polychlorobiphenyls (PCBs): Ligand steric effects and molecular structure of reaction intermediates

Article abstract of DOI:10.1016/S0022-328X(00)00031-0

Soluble homogeneous organophosphorus - nickel complexes have been used to detoxify polychlorinated biphenyls (PCBs) by catalyzed hydrodechlorination using NaBH₂(OCH₂CH₂OCH₃)₂ as the hydrogen source. The reactions appear to proceed by NiL₃ oxidative addition into C-Cl bonds followed by hydrogenolysis of the metal-carbon bond. In model experiments with decachlorobiphenyl, the cone angle of the organophosphorus ligand L was shown to be a key factor controlling the magnitude and position of chlorine displacement. Hence, ligands leading to para displacement (e.g. (o-MeC₆H₄O)₃P), meta - para displacement (e.g. (EtO)₃P and (PhO)₃P), and ortho - meta - para displacement (e.g. Me₃P and Et₃P) were found. Significantly, the highly toxic, coplanar dioxin precursor 3,3′,4,4′-tetrachlorobiphenyl, a meta - para chlorine-substituted congener, was dechlorinated quantitatively with the Et₃P catalyst system. Evidence for the presence of organonickel intermediates in the reaction mixtures was obtained by mass spectroscopic and X-ray diffraction studies. Of particular interest is the isolation of square planar complexes p-C₆Cl₅C₆Cl₄Ni(PEt₃) ₂Cl from the reaction of decachlorobiphenyl with NaBH₂(OCH₂CH₂OCH₃)₂ - (Et₃P)₂NiCl₂ as the catalyst precursor and m-C₆Cl₅C₆Cl₄Ni(PEt₃) ₂Cl from decachlorobiphenyl - Ni(1,5-C₈H₁₂)₂ - PEt₃ at room temperature. All are oxidative addition intermediates and precursors for decachlorobiphenyl hydrodechlorination.