Nickel complexes incorporating an amido phosphine chelate with a pendant amine arm: Synthesis, structure, and catalytic Kumada coupling

Abstract

A series of organonickel(II) complexes incorporating an amido phosphine ligand tethered with an amino pendant have been prepared and characterized. Deprotonation of N-(dimethylaminoethyl)-2-diphenylphosphinoaniline (H[PNN]) with one equivalent of n-BuLi in ethereal or hydrocarbon solutions at −35 °C generates cleanly dimeric {Li[PNN]}₂ as yellow crystals. The reaction of NiCl₂(DME) with {Li[PNN]}₂ in THF at −35 °C affords green crystalline [PNN]NiCl. Treating [PNN]NiCl with NaX in acetone solutions gives [PNN]NiX (X = Br, I). Alkylation or arylation of [PNN]NiCl with appropriate Grignard reagents in THF at −35 °C produces red crystalline [PNN]NiR (R = Me, Et, i-Bu, n-hexyl, CH₂Ph, Ph). The chloride complex [PNN]NiCl was found to be an active catalyst precursor for Kumada coupling reactions of PhX (X = I, Br, Cl) with aryl or alkyl Grignard reagents, including those containing β-hydrogen atoms. The X-ray structures of {Li[PNN]}₂ and [PNN]NiX (X = Cl, Br, Me, Et, n-hexyl) are reported.