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

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.