Amido pincer complexes of nickel(II): Synthesis, structure, and reactivity

A series of diamagnetic divalent nickel complexes supported by a tridentate amido diphosphine ligand [N(o-C6H4PR2)(2)](-) ([R-PNP](-); R = Ph, Pr-i, Cy) have been prepared and characterized. Deprotonation of H[Ph-PNP], H[Pr-i-PNP], and H[Cy-PNP] with n-BuLi in ethereal solutions at -35 degrees C produced the lithium complexes [Ph-PNP]Li(THF)(2), [Pr-i-PNP]Li(solv) (solv = THF, Et2O), and [Cy-PNP]Li(solv) (solv = THF, Et2O), respectively. The reactions of [R-PNP]Li(solv)(n) with NiCl2(DME) in THF at -35 degrees C generated [R-PNP]NiCl, which was then reacted with a variety of Grignard reagents to afford the corresponding hydrocarbyl complexes [R-PNP]NiR' (R = Ph, R' = Me, Et, n-Bu, i-Bu, n-hexyl, CH2-SiMe3, Ph; R = Pr-i, R' = Me, Et, n-Bu; R = Cy, R' = Me, Et, n-Bu). Of particular interest among the compounds isolated are alkyl complexes that contain beta-hydrogen atoms. Treatment of the hydrocarbyl complexes with halogenated hydrocarbons such as dichloromethane, benzyl bromide, and phenyl iodide produced the corresponding nickel halide derivatives. The chloride complexes [Ph-PNP]NiCl, [Pr-i-PNP]NiCl. and [Cy-PNP]NiCl are all active catalyst precursors for Kumada coupling reactions, including those of alkyls containing beta-hydrogen atoms. In addition to spectroscopic data for all new compounds, X-ray structures of [Cy-PNP]Li(OEt2), [Ph-PNP]NiCl, [Ph-PNP]NiBr, [Ph-PNP]NiMe, [Ph-PNP]Ni(n-Bu), [Ph-PNP]NiCH2SiMe3, [Pr-i-PNP]NiCl, [Pr-i-PNP]NiMe, [Pr-i-PNP]Ni(n-Bu), and [Cy-PNP]NiMe are presented.