Electrochemical and optical recognition studies of anions using a multi-site ferrocene-based benzimidazolium receptor

A novel multi-site receptor 1 bearing four ferrocenyl arms comprising benzimidazolium moieties was synthesized and structurally characterized by IR, NMR, elemental analysis, mass spectra. Moreover, the structure of receptor 1 was confirmed by X-ray crystallography, it was clear that the cationic heterocyclic was interacted with PF₆⁻ through C–H···F hydrogen bonds. Its electrochemical properties of sensing the various anions were investigated by cyclic voltammograms (CV) and differential pulse voltammetry (DPV), the receptor 1 displayed a significant cathodic shift for F⁻and OH⁻. The addition of F⁻ and OH⁻to the solution of receptor 1 resulted in obvious absorption changes in the UV–Vis spectrum, showing that receptor 1 was an excellent sensor for these analytes (F⁻: LOD = 3.11 × 10⁻⁶ M, LOQ = 1.03 × 10⁻⁵ M, K_a = 5.83 × 10⁵ M⁻¹; and OH^–: LOD = 1.36 × 10⁻⁶ M, LOQ = 4.11 × 10⁻⁵ M, K_a = 9.03 × 10⁵ M⁻¹). ¹H NMR titrations demonstrated that anion was recognized by receptor 1 through (C–H)⁺···X⁻ and C–H···X hydrogen bonds, the binding process involve initial formation of hydrogen bond and followed by deprotonation.