Synthesis and Photophysical Properties of Symmetrical Mononuclear Gold(I)-N-Heterocyclic Carbene Complexes

Seven new mononuclear gold(I)-N-heterocyclic carbene (NHC) complexes, [bis(1-butyl-3-alkylbenzimidazolium)gold(I)] hexafluorophosphate (alkyl = methyl, ethyl, butyl, pentyl, hexyl, heptyl, benzyl), 1-7 were reported. All these complexes were successfully synthesized through transmetalation method of their respective silver(I)-NHC complexes, proved by elemental analyses, FTIR, H-1, C-13 NMR and single crystal X-ray diffraction analyses. All the C-13 NMR spectra of the complexes showed singlet peaks in the range of 189.0-191.2 ppm, belong to the carbene carbon that directly bonded with gold(I) ion, thus, suggesting the successful formation of gold(I)-NHC complexes. The single crystal X-ray diffraction data of 3 shows the presence of bis-NHC mononuclear complex with the chemical formula of [NHC-Au-NHC]center dot PF6 (NHC = 1,3-bisbutylbenzimidazol-2-ylidene ligand). At room temperature, complex 3 emits weak blue emission under UV light radiation. The photoluminescence study of 3 in solid state shows the excitation and emission spectra at 360 and 460 nm at room temperature while 306 and 406 nm at 77 K, respectively. With the aurophilic interaction excluded, these bands are attributed to metal-to-ligand transfer charge (MLCT) and ligand-centered pi-pi* transitions. The luminescence of 3 becomes more intense at 77 K compared to room temperature, with the large blue shift of the emission bands.