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

The synthesis, properties, and reactivity of nickel(II) complexes of a newly developed pincer amidobis-(amine) ligand ((NN2)-N-Me) are described. Neutral or cationic complexes [((NN2)-N-Me)NiX] (X=OTf (6), OC(O)CH3 (7), CH3CN (8), OMe (9)) were prepared by salt metathesis or chloride abstraction from the previously reported [((NN2)-N-Me)NiCl] (1). The Lewis acidity of the {((NN2)-N-Me)Ni} fragment was measured by the H-1 NMR chemical shift of the coordinated CH3CN molecule in 8. Electrochemical measurements on 1 and 8 indicate that the electron-donating properties of NN2 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 [((NN2)-N-Me)NiEt] (3). In all complexes, the (NN2)-N-Me 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 (NN2)-N-Me 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 I as the precatalyst was developed. Good yields were obtained for the coupling of primary and secondary iodides and bromides. Double C-C coupling of CH2Cl2 with alkyl Grignard reagents by I 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.