NHC Metal (Silver, Mercury, and Nickel) Complexes Based on Quinoxaline–Dibenzimidazolium Salts: Synthesis, Structural Studies, and Fluorescent Chemosensors for Cu2+ by Charge Transfer

The two dibenzimidazolium salts 2,3-bis­[(1-nPr-benzimidazoliumyl)­methyl]­quinoxaline hexafluorophosphate (H2-1a) and 2,3-bis­[(1-Pi-benzimidazoliumyl)­methyl]­quinoxaline hexafluorophosphate (H2-1b) (nPr = n-propyl and Pi = picolyl) and their four NHC metal complexes [Ag2(1a)2]­(PF6)2 (2a), [Hg­(1a)­(OAc)­(H2O)]2(PF6)2 (2b), [Ag2(1b)2]­(PF6)2 (2c) and [Ni­(1b)]­(NiCl4) (2d) have been synthesized and characterized. Each cation in 2a,c possesses a boxlike structure with different sizes. In 2b, one 18-membered macrometallacycle is formed by two bidentate NHC ligands and two mercury­(II) atoms, in which an inversion center is observed. In 2d, one nickel­(II) atom is surrounded by two carbene carbon atoms and two nitrogen atoms from two pyridine rings to adopt a square geometry. The recognition of Cu2+ using 2a,c as chemosensors by charge transfer was investigated on the basis of fluorescence and UV/vis spectroscopic titrations. The results show that complexes 2a,c are effective chemosensors for Cu2+.