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 -C 6 H 4 PR 2 ) 2 ] - ([R-PNP] -; R = Ph, i Pr, Cy) have been prepared and characterized. Deprotonation of H[Ph-PNP], H[ i Pr-PNP], and H[Cy-PNP] with n -BuLi in ethereal solutions at −35 °C produced the lithium complexes [Ph-PNP]Li(THF) 2, [ i Pr-PNP]Li(solv) (solv = THF, Et 2 O), and [Cy-PNP]Li(solv) (solv = THF, Et 2 O), respectively. The reactions of [R-PNP]Li(solv) n with NiCl 2 (DME) in THF at −35 °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, CH 2 SiMe 3, Ph; R = i Pr, R‘ = Me, Et, n -Bu; R = Cy, R‘ = Me, Et, n -Bu). Of particular interest among the compounds isolated are alkyl complexes that contain β-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, [ i Pr-PNP]NiCl, and [Cy-PNP]NiCl are all active catalyst precursors for Kumada coupling reactions, including those of alkyls containing β-hydrogen atoms. In addition to spectroscopic data for all new compounds, X-ray structures of [Cy-PNP]Li(OEt 2 ), [Ph-PNP]NiCl, [Ph-PNP]NiBr, [Ph-PNP]NiMe, [Ph-PNP]Ni( n -Bu), [Ph-PNP]NiCH 2 SiMe 3, [ i Pr-PNP]NiCl, [ i Pr-PNP]NiMe, [ i Pr-PNP]Ni( n -Bu), and [Cy-PNP]NiMe are presented.