Nickel Complexes of a Pincer Amidobis(amine) Ligand: Synthesis, Structure, and Activity in Stoichiometric and Catalytic CC Bond‐Forming Reactions of Alkyl Halides

Abstract

The synthesis, properties, and reactivity of nickel(II) complexes of a newly developed pincer amidobis(amine) ligand ((Me)NN(2)) are described. Neutral or cationic complexes [((Me)NN(2))NiX] (X = OTf (6), OC(O)CH(3) (7), CH(3)CN (8), OMe (9)) were prepared by salt metathesis or chloride abstraction from the previously reported [((Me)NN(2))NiCl] (1). The Lewis acidity of the {((Me)NN(2))Ni} fragment was measured by the (1)H NMR chemical shift of the coordinated CH(3)CN molecule in 8. Electrochemical measurements on 1 and 8 indicate that the electron-donating properties of NN(2) are similar to those of the analogous amidobis(phosphine) (pnp) ligands. The solid-state structures of 6-8 were determined and compared to those of 1 and [((Me)NN(2))NiEt] (3). In all complexes, the (Me)NN(2) ligand coordinates to the Ni(II) ion in a mer fashion, and the square-planar coordination sphere of the metal is completed by an additional donor. The coordination chemistry of (Me)NN(2) thus resembles that of other three-dentate pincer ligands, for example, pnp and arylbis(amine) (ncn). Reactions of 2 with alkyl monohalides, dichlorides, and trichlorides were investigated. Selective C-C bond formation was observed in many cases. Based on these reactions, efficient Kumada-Corriu-Tamao coupling of unactivated alkyl halides and alkyl Grignard reagents with 1 as the precatalyst was developed. Good yields were obtained for the coupling of primary and secondary iodides and bromides. Double C-C coupling of CH(2)Cl(2) with alkyl Grignard reagents by 1 was also realized. The scope and limitations of these transformations were studied. Evidence was found for a radical pathway in Ni-catalyzed C-C cross-coupling reactions, which involves Ni(II) alkyl intermediates.