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

Abstract

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+.