Dehalogenation of chloroalkenes at cobalt centers. A model density functional study

Reaction of a model cobaloxime, MeCo(Hgly)2(NHCH2) (Hgly = glyoximato), with chloro ethenes ClX1CCX2X3 (X = Cl or H) under reductive conditions has been investigated at the BP86 level of density functional theory. Initial one-electron reduction of the metal complex affords either [MeCo(Hgly)2]•- or [Co(Hgly)2(NHCH2)]-. Both can react with the substrates under chloride elimination to form the corresponding vinylcobalt complexes with a Co−CX1CX2X3 moiety, which is indicated to be the rate-limiting step in a potential catalytic cycle. Free energies of activation on the order of 20 kcal/mol are predicted for these processes, suggesting that such an inner-sphere reduction (i.e., one that takes place in the coordination sphere of the metal) could be a viable pathway. Implications for the mechanism with functional biological systems based on vitamin B12 are discussed.