A terpyridyl-imidazole (tpy-HImzPh3) based bifunctional receptor for multichannel detection ofFe2+ and F− ions

Abstract

The X-ray crystal structures of the tridentate ligand, 4′-[4-(4,5-diphenyl-1H-imidazol-2-yl)-phenyl]-[2,2′:6′,2′′]terpyridine (tpy-HImzPh₃) and its bis-homoleptic iron(II) complex of composition [Fe(tpy-HImzPh₃)₂]²⁺ have been determined, showing that the ligand crystallized in a monoclinic form with the space group P2₁/c while its Fe(II) complex crystallizes in an orthorhombic form with space group Fddd. Both the anion and cation binding properties of the receptor were thoroughly investigated in dimethylformamide-acetonitrile (1 : 9) solution using absorption, emission, and ¹H NMR spectral studies which revealed that the receptor acts as a sensor for both F⁻ and Fe²⁺. In the presence of excess F⁻ ion, deprotonation of the imidazole N–H fragment of the receptor occurs, an event which is signaled by the development of a yellow color visible with the naked eye. The estimated value of the equilibrium constant of the receptor with F⁻ is 1.9 × 10⁴ M⁻¹. Deprotonation is also observed in the presence of hydroxide. The receptor also shows colorimetric and fluorimetric sensing ability towards Fe²⁺ ions. The binding site for the metal ion in the system has been unambiguously established by single-crystal X-ray diffraction studies of the Fe(II) complex of the receptor. The influence of solvents on the absorption and fluorescence spectra of the receptor has been investigated in detail. Cyclic voltammetric (CV) and square wave voltammetric (SWV) measurements carried out in dimethylformamide-acetonitrile (2 : 3) provided evidence in favor of cation (Fe²⁺) and anion (F⁻) concentration dependent electrochemical responses, enabling the ligand to act as a suitable electrochemical sensor for F⁻ and Fe²⁺ ions.