New Mixed-Donor Bidentate Ligands Based on N-Heterocyclic Carbene and Thione Donors

A new family of mixed-donor bidentate ligands containing both N-heterocyclic carbene (NHC) and thione functionalities is reported. The imidazolium salt precursors 1,1'-methylene(3-R-imidazol-2-ylidene)(3-R-2H-imidazole-2-thione) [(CSH)-H-R] halide (where R = methyl, benzyl; halide = bromide, iodide) have been synthesized. Their coordination to copper(I) salts has been explored, providing the complexes [Cu (CSR)X](2) (where R = methyl, benzyl; halide = bromide, iodide). Structural characterization of two of the complexes confirmed the dimeric nature of these complexes in the solid state but revealed different coordination modes for each case (kappa(1)-C,kappa(1)-S, mu-S and kappa(1)-C, kappa(1)-S). Solutions of the copper complexes slowly react with oxygen under aerobic conditions to form uncoordinated 1,1'-methylene(3-R-2H-imidazol-2-one)(3-R-2H-imidazole-2-thione), SOR (where R = methyl, benzyl), where the NHC moiety has been converted into a urea functional group. The complexes were investigated as candidates in copper-catalyzed cross-coupling reactions.