Nickel(II) Complexes Containing Bidentate Diarylamido Phosphine Chelates: Kumada Couplings Kinetically Preferred to β-Hydrogen Elimination

Abstract

A series of divalent nickel complexes containing diarylamido phosphine ligands of the type (o-ArNC6H4PR2)− (1a, Ar = 2,6-C6H3Me2, R = Ph; 1b, Ar = 2,6-C6H3iPr2, R = Ph; 1c, Ar = 2,6-C6H3Me2, R = iPr; 1d, Ar = 2,6-C6H3iPr2, R = iPr) have been prepared and structurally characterized. The dimeric nickel chloride derivatives {[1b–d]Ni(μ-Cl)}2 (2b–d) were isolated as brick red microcrystals in high yields from the reactions of NiCl2(DME) with either Li[1b–d](solv)x or H[1b–d] in the presence of NEt3. Similar reactions employing [1a]−, however, generated homoleptic Ni[1a]2 (3a) as paramagnetic, dark red prisms in high yield. Addition of trimethylphosphine to red solutions of 2b,c in THF at room temperature afforded emerald crystals of [1b,c]NiCl(PMe3) (4b,c). Interestingly, solution NMR spectroscopic and X-ray crystallographic data of these PMe3 adducts reveal the exclusive formation of cis-4b and trans-4c, as defined by the mutual orientation of the two phosphorus donors incorporated. Metathetical reactions of 4b,c with RMgCl (R = Me, CH2SiMe3, Ph) in THF at −35 °C produced high yields of red or brownish red crystalline [1b,c]NiR(PMe3) (R = Me (5b,c), CH2SiMe3 (6b,c), Ph (7b,c)). Analogous reactions of 4c with EtMgCl or nBuMgCl, however, led instead to the isolation of the hydrido species [1c]NiH(PMe3) (8c) in quantitative yield. Solution NMR data of the methyl complexes 5b,c indicate the presence of both cis and trans isomers; the major component of 5b is cis whereas that of 5c is trans. In contrast, complexes 6b,c, 7b,c, and 8c all exist exclusively in the trans form. The chloro complexes 2b–d are all active catalyst precursors for Kumada couplings under mild conditions. In particular, this catalysis is compatible with alkyl nucleophiles that contain β-hydrogen atoms, even in reactions with chlorobenzene. The X-ray structures of 2d, 3a, 4c, 5c, 6b,c, 7c, and 8c are presented.