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

NiII complexes of the new pincer amidobis(amine) ligand are described. The Ni chloride complex catalyzes Kumada-Corriu-Tamao coupling of unactivated alkyl halides with alkyl Grignard reagents, as well as double C-C coupling of CH2Cl2 with alkyl Grignard reagents (see schemes). The synthesis, properties, and reactivity of nickel(II) complexes of a newly developed pincer amidobis(amine) ligand (MeNN2) are described. Neutral or cationic complexes [(MeNN2)NiX] (X=OTf (6), OC(O)CH3 (7), CH3CN (8), OMe (9)) were prepared by salt metathesis or chloride abstraction from the previously reported [(MeNN2)NiCl] (1). The Lewis acidity of the {(MeNN2)Ni} fragment was measured by the 1H 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 [(MeNN2)NiEt] (3). In all complexes, the MeNN2 ligand coordinates to the NiII ion in a mer fashion, and the square-planar coordination sphere of the metal is completed by an additional donor. The coordination chemistry of MeNN2 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 CC 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 CH2Cl2 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 NiII alkyl intermediates.