Mechanistic Aspects of the Kharasch Addition Reaction Catalyzed by Organonickel(II) Complexes Containing the Monoanionic Terdentate Aryldiamine Ligand System [C₆H₂(CH₂NMe₂)₂-2,6-R-4]^-†

The addition reaction of polyhalogenated alkanes to alkenes (Kharasch addition reaction) is homogeneously catalyzed in the absence of O₂ under mild reaction conditions (25 °C) by the arylnickel complexes [Ni^II{C₆H₂(CH₂NMe₂)₂-2,6-R-4}Br] (R = H, MeC(O), Cl, MeO, NH₂) and shows a high selectivity for the 1:1 adduct. Kinetic data on the catalytic system with [Ni{C₆H₃(CH₂NMe₂)₂-2,6}Br] (R = H; abbreviated as [Ni(NCN)Br]), methyl methacrylate, and CCl₄ reveal a rate of reaction that is first order in nickel complex and in alkene. In our series of para-substituted arylnickel catalysts, the rate of catalysis increases with the electron donating character of the para substituents on the aryl ligand and this rate correlates directly with the Ni^II/Ni^III redox potential. These data, together with separate spectroscopic studies and results from individual experiments employing other solvents, other polyhalogenated alkanes such as CBr₄ and CF₃CCl₃ and other alkene substrates such as styrene, 1-octene, and cyclohexene, lead to the proposal of a catalytic cycle based on a nonchain mechanism with a mononuclear nickel species. Before or in the rate-determining step oxidation of the Ni(II) center to a d⁷ arylnickel(III) species occurs by a single electron transfer and halide transfer from the polyhalogenated alkane in an inner-sphere activated complex [Ni(NCN)(μ-Cl)CCl₄]. This step generates an organic radical intermediate which is proposed to stay in the coordination sphere of the metal where it reacts with the alkene. The reaction rate decreases with an increase in the steric congestion at the N-donor centers in derivatives of the [Ni(NCN)Br] catalyst (i.e., NMe₂ > NEt₂ > NMe(i-Pr) > NMe(t-Bu)). This behavior is consistent with the characteristics for an inner-sphere electron-transfer process. Selective 1:1 Kharasch product formation then results from a chain transfer in the Ni(III) coordination sphere by the reverse process, i.e., single electron transfer with concomitant halide transfer. Important conclusions of this study are that the initially active site of the catalyst is the Ni−X unit (X = halide) and that activation of CCl₄ occurs in the absence of a free coordination site.