Structural and Luminescent Properties of Homoleptic Silver(I), Gold(I), and Palladium(II) Complexes with NHC-NHC Heteroditopic Carbene Ligands.

Novel silver(I), gold(I), and palladium(II) complexes were synthesized with bidentate heteroditopic carbene ligands that combine an imidazol-2-ylidene (NHC) with a 1,2,3-triazol-5-ylidene (NHC) connected by a propylene bridge. The silver(I) and gold(I) complexes were dinuclear species, M(NHC-NHC) (M = Ag or Au), with the two bidentate ligands bridging the metal centers, whereas in the palladium(II) complex Pd(NHC-NHC), the two ligands were chelated on the same metal center. Because of the presence of two different carbene units, isomers were observed for the gold(I) and palladium(II) complexes. The molecular structures of the head-to-tail isomer for gold(I) complexes, with a twisted or folded- conformation of the bridge between the carbene units, were determined by X-ray diffraction analysis. The study was completed with a systematic structural investigation through density functional theory (DFT) calculations. For palladium(II) species, the head-to-head form was structurally characterized. The dinuclear gold(I) complexes were emissive in the solid state in the blue region (PLQY up to 8%); time-dependent density functional theory (abbreviated as TD-DFT) calculations disclosed that the absorption bands have metal-to-ligand-charge-transfer character and evidenced that the emission occurs from the T level (phosphorescence).