A high-spin nickel(ii) borohydride complex in dehalogenation

Abstract

A nickel(II)–borohydride complex bearing a macrocyclic tridentate N-donor ligand, [Ni(Me₃-TACN)(BH₄)(CH₃CN)]⁺ (Me₃-TACN = 1,4,7-trimethyl-1,4,7-triazacyclononane), was prepared, isolated, and characterized by various physicochemical methods, including UV-vis, ESI-MS, IR and X-ray analyses. The structural and spectroscopic characterization clearly shows that the borohydride ligand is bound to the high-spin nickel(II) center in an η²-manner. Density functional theory calculations provided geometric information of 2, showing that the η²-binding of borohydride to the nickel center is more favorable than the η³-binding mode in CH₃CN. The complex is paramagnetic with an effective magnetic moment of 2.9μ_B consistent with a d⁸ high-spin system. The reactivity of the high-spin nickel(II)–borohydride complex was examined in dehalogenation with numerous halocarbons. A kinetic isotope effect value of 1.7 was observed in the dehalogenation of CHCl₃ by the nickel(II)–borohydride complex. Kinetic studies and isotopic labeling experiments implicate that hydride ion or hydrogen atom transfer from the borohydride group is the rate determining step. The positive Hammett ρ value of 1.2, obtained in the reactions of [Ni(Me₃-TACN)(BH₄)(CH₃CN)]⁺ and para-substituted benzoyl chloride, indicates that the dehalogenation by the nickel(II)–borohydride species occurs via a nucleophilic reaction.