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

Abstract

Soluble homogeneous organophosphorus—nickel complexes have been used to detoxify polychlorinated biphenyls (PCBs) by catalyzed hydrodechlorination using NaBH 2(OCH 2CH 2OCH 3) 2 as the hydrogen source. The reactions appear to proceed by NiL 3 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-MeC 6H 4O) 3P), meta—para displacement (e.g. (EtO) 3P and (PhO) 3P), and ortho—meta—para displacement (e.g. Me 3P and Et 3P) 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 Et 3P 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-C 6Cl 5C 6Cl 4Ni(PEt 3) 2Cl from the reaction of decachlorobiphenyl with NaBH 2(OCH 2CH 2OCH 3) 2—(Et 3P) 2NiCl 2 as the catalyst precursor and m-C 6Cl 5C 6Cl 4Ni(PEt 3) 2Cl from decachlorobiphenyl—Ni(1,5-C 8H 12) 2—PEt 3 at room temperature. All are oxidative addition intermediates and precursors for decachlorobiphenyl hydrodechlorination.