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

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.