Synthesis and structural characterization of nickel(II) complexes of 20-membered macrocyclic rings bearing chelating bis(N-heterocyclic carbene) ligands

Abstract

20-Membered macrocyclic rings (Lbenz and Limid) containing two amine moieties and two N-heterocyclic carbene (NHC) moieties coordinate in a κ2 coordination mode to Ni(II) through the NHC moieties. Complexes 3a and 3b were synthesized by generating free carbenes in situ from bis-benzimidazolium bromide salts and subsequent addition of one equivalent of [Ni(DME)X2] (X = Cl, Br) at 25 °C. Free carbenes of 20-membered macrocyclic ligands Lbenz and Limid can be generated in situ in THF-d8 and characterized by 1H and 13C NMR spectroscopy. 3b can also be synthesized by treatment of silver(I)-bis(NHC) complex 2a with one equivalent of [NiBr2(DME)] in refluxing methylene chloride for 20 h affording yields of up to70%. Complexes 3a and 3b were characterized by X-ray diffraction. [NiI2(Lbenz)] 5c was prepared by halogen exchange of 3a or 3b with NaI at 25 °C. Ni complexes of Limid can be prepared and characterized in situ but cannot be isolated cleanly from the reaction mixtures. The cis and trans isomers of [NiCl2(Limid)] (4a and 4b) and the bromide derivative 4c were characterized by X-ray crystallography by judicious selection of individual, high-quality crystals obtained from crude reaction mixtures.