Structural and Luminescent Properties of Homoleptic Silver(I), Gold(I), and Palladium(II) Complexes with nNHC-tzNHC 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 (nNHC) with a 1,2,3-triazol-5-ylidene (tzNHC) connected by a propylene bridge. The silver­(I) and gold­(I) complexes were dinuclear species, [M2(nNHC-tzNHC)2]­(PF6)2 (M = Ag or Au), with the two bidentate ligands bridging the metal centers, whereas in the palladium­(II) complex [Pd­(nNHC-tzNHC)2]­(PF6)2, 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-syn 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 T1 level (phosphorescence).