Bis(ether-functionalized NHC) Nickel(II) Complexes, Trans to Cis Isomerization Triggered by Water Coordination, and Catalytic Ethylene Oligomerization

The new nickel­(II) complexes containing NHC ligands N-substituted by a CH2CH2OR ether group (R = Me or Ph) [NiCl2{ImMes­(C2OMe)}2] (6), [NiCl2{Imn-Bu­(C2OMe)}2] (7), [NiBr2{ImDiPP­(C2OMe)}2] (8), [NiBr2{ImMes­(C2OMe)}2] (9), [NiBr2{Imn-Bu­(C2OMe)}2] (10), NiBr2{ImMes­(C2OPh)}2] (18), [NiI2{ImDiPP­(C2OMe)}2] (21), [NiI2{ImMes­(C2OMe)}2] (22), and [NiI2{Imn-Bu­(C2OMe)}2] (23) were synthesized in good yields and fully characterized by NMR spectroscopy and X-ray diffraction analysis. The reaction conditions were optimized and further applied to thioether or nonfunctionalized NHC ligands, affording [NiBr2{ImDiPP­(C2SPh)}2] (19) and [NiBr2{ImDiPP­(n-Bu)}2] (20), respectively. Equilibria involving syn/anti isomers were unveiled for complexes [NiCl2{ImDiPP­(C2OMe)}2] (5), 6–10, and 18–23. Reactions of 6 and 20 with a halide abstractor afforded the dicationic aquo complexes cis-[Ni­{ImMes­(C2OMe)}2(H2O)2]­[PF6]2 (27) and cis-[Ni­{ImDiPP­(n-Bu)}2(H2O)2]­[PF6]2 (28), in which a cis arrangement of the carbene ligands is evidenced, which contrasts with that in their precursors. These molecules represent rare examples of nickel aquo NHC complexes and of complexes with two cis monodentate NHC ligands. The new complexes reported in this work (15 crystal structures) displayed moderate activities as precatalysts for ethylene oligomerization and favored dimerization.