Ionic Ni(ii) complexes containing a triphenylphosphineligand and an imidazolium cation: synthesis, structures and catalysis for aryl Grignard cross-coupling of aryl halides

Abstract

Novel ionic Ni(II) complexes of general formula [R₂im][Ni(PPh₃)Cl₃] (R₂im = 1,3-bis(2,6-diisopropylphenyl)imidazolium cation, IPrim, 1a; R₂im = 1,3-diisopropylimidazolium cation, ⁱPrim, 2a) were easily prepared in high yields by the direct reaction of Ni(PPh₃)₂Cl₂ with 1 equiv. of N,N′-dihydrocarbylimidazolium salt, [R₂im]Cl. Their bromide analogs [R₂im][Ni(PPh₃)Br₃] (R₂im = IPrim, 1b; R₂im = ⁱPrim, 2b) were synthesized by the same reaction in the presence of excess NaBr. The reaction of Ni(DME)Cl₂ (DME = 1,2-dimethoxyethane) with 2 equiv. of [IPrim]Cl led to the formation of the complex [IPrim]₂[NiCl₄] (3) in an almost quantitative yield. All these complexes were characterized by elemental analysis, electrospray ionization mass spectrometry (ESI-MS), and ¹H NMR spectroscopy; X-ray crystallography was performed for 1a, 1b, 2a, and 2b. A catalytic study of the cross-coupling reactions of aryl Grignard reagents with aryl halides revealed that complexes 1a and 1b possessed the highest activities. In comparison, complexes 2a, 2b, 3, and the related biscarbene Ni(II) complex Ni(IPr)₂Cl₂ [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene] exhibited moderate activities; the least active complexes were Ni(PPh₃)₂Cl₂ and [NEt₄][Ni(PPh₃)Cl₃].