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) [CSRH] 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 (κ1-C, κ1-S, μ-S and κ1-C, κ1-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.