Controlled synthesis of nickel(ii) dihalides bearing two different or identical N-heterocyclic carbeneligands and the influence of carbeneligands on their structures and catalysis

Abstract

Ni(II) dihalides bearing two different or identical NHC ligands have been prepared via a controlled indene elimination synthesis, and the former product provides a new route for the design of biscarbene Ni(II)-based catalysts. The indene elimination reaction of the indenynickel(II) complex (1-H-Ind)Ni(NHC)X (Ind = indenyl) with one equiv. of a distinct imidazolium salt at 100 °C afforded the first example of Ni(II) dihalides bearing two different NHC ligands, i.e., Ni(iPr)(IPr)X₂ [iPr = 1,3-diisopropylimidazol-2-ylidene, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), X = Cl, 1; X = Br, 2] and Ni(iPr)(IMes)Br₂ [IMes = 1,3-bis(mesityl)imidazol-2-ylidene, 3]. Alternatively, complexes 1–3 can be synthesized using a bis-indenyl Ni(II) complex (1-H-Ind)₂Ni as starting materials via a step-by-step indene elimination at different reaction temperatures. The direct reaction of (1-R-Ind)₂Ni (R = H or Me) with two equiv. of imidazolium salts at 100 °C afforded Ni(II) dihalides bearing two identical NHC ligands, i.e., Ni(iPr)X₂ (X = Cl, 4; X = Br, 5) and Ni(IPr)Cl₂ (6). All of these complexes were characterized by elemental analysis, NMR spectroscopy and X-ray crystallography for complexes 1–5. The two identical or different NHC ligands in complexes 1–6 changed the coordination sphere of the nickel center from a typical square-planar geometry to a slightly tetrahedral array. A preliminary catalytic study on the cross-coupling reactions of aryl Grignard reagents with aryl halides revealed that complexes 1 and 2 possess the highest activity. In comparison, complexes 3 and 6 exhibited moderate activity and the least active complexes were 4 and 5.