C–Cl bond activation and catalytic hydrodechlorination of hexachlorobenzene by cobalt and nickel complexes with sodium formate as a reducing agent

Abstract

A benzyne cobalt complex, Co(η²-C₆Cl₄)(PMe₃)₃ (2), was generated from the reaction of hexachlorobenzene with 2 equiv. of Co(PMe₃)₄ through selective activation of two C–Cl bonds of hexachlorobenzene. Meanwhile, the byproduct CoCl₂(PMe₃)₃ was also confirmed by IR spectra. The cobalt(II) complex, CoCl(C₆Cl₅)(PMe₃)₃ (1), as an intermediate in the formation of aryne complex 2, was also isolated by the reaction of hexachlorobenzene with the stoichiometric amount of Co(PMe₃)₄. Complex 2 could be obtained by the reaction of 1 with Co(PMe₃)₄. Under similar reaction conditions, the reaction of Ni(PMe₃)₄ with hexachlorobenzene afforded only a mono-(C–Cl) bond activation nickel(II) complex, NiCl(C₆H₅)(PMe₃)₂ (5). The expected benzyne nickel complex was not formed. The structures of complexes 2 and 5 were determined by X-ray single crystal diffraction. Successful selective hydrodechlorinations of hexachlorobenzene were studied and in the presence of Co(PMe₃)₄ or Ni(PMe₃)₄ as catalysts and sodium formate as a reducing agent pentachlorobenzene and 1,2,4,5-tetrachlorobenzene were obtained. The catalytic hydrodechlorination mechanism is proposed and discussed.