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 det oxify polychlorinated biphenyls (PCBs) by catalyzed hydrodechlorination usi ng NaBH2(OCH2CH2OCH3)(2) as the hydrogen source. The reactions appear to pr oceed by NiL3 oxidative addition into C-Cl bonds followed by hydrogenolysis of the metal-carbon bond. In model experiments with decachlorobiphenyl, th e cone angle of the organophosphorus ligand L was shown to be a key factor controlling the magnitude and position of chlorine displacement. Hence, lig ands leading to para displacement (e.g. (o-MeC6H4O)(3)P), meta-para displac ement (e.g. (EtO)(3)P and (PhO)(3)P), and ortho-meta-para displacement (e.g . Me3P and Et3P) were found. Significantly, the highly toxic, coplanar diox in precursor 3,3',4,4'-tetrachlorobiphenyl, a meta-para chlorine-substitute d congener, was dechlorinated quantitatively with the Et3P catalyst system. Evidence for the presence of organonickel intermediates in the reaction mi xtures was obtained by mass spectroscopic and X-ray diffraction studies. Of particular interest is the isolation of square planar complexes p-C6Cl5C6C l4Ni(PEt3)(2)Cl from the reaction of decachlorobiphenyl with NaBH2(OCH2CH2O CH3)(2)-(Et3P)(2)NiCl2 as the catalyst precursor and m-C6Cl5C6Cl4Ni(PEt3)(2 )Cl from decachlorobiphenyl - Ni(1,5-C8H12)(2)-PEt3 at room temperature. Al l are oxidative addition intermediates and precursors for decachlorobipheny l hydrodechlorination. (C) 2000 Elsevier Science S.A. All rights reserved.