Combined solution-phase, solid-phase and phase-interface anion binding and extraction studies by a simple tripodal thiourea receptor

Abstract

Tris(2-aminoethyl)amine (tren) based 4-cyanophenyl-substituted tripodal L, tris{[(4-cyanophenyl)amino]ethyl}thiourea receptor, was synthesized and explored thoroughly for anion recognition in solution by NMR spectroscopy and isothermal titration calorimetry (ITC) as well as in the solid state by single-crystal X-ray diffraction studies. Anion recognition properties of L were further exploited toward the extraction of sulfate as well as fluoride from aqueous media using a liquid–liquid extraction technique. A solution-state anion binding study using NMR spectroscopy in [D₆]DMSO and ITC measurements in dry acetonitrile show a relatively higher association constant of L with halides (F^– and Cl^–) over oxyanions (H₂PO₄^– and HSO₄^–). The single-crystal X-ray structural analysis of complex 1 reveals a monotopic encapsulation of fluoride in L through six N–H···F^– interactions with a distorted trigonal-prismatic geometry, whereas sulfate and carbonate induce dimeric assemblies of L in complexes 2 and 3, respectively. In the case of sulfate, a tight dimeric capsular assembly of ca. 9.5 Å is observed through 15 N–H···O interactions, whereas carbonate forms a sandwich-like dimeric molecular aggregation through 14 N–H···O interactions. In the presence of tetrabutylammonium iodide as the phase transfer agent, L has shown ca. 70 % extraction of fluoride (based on L) and ca. 40 % extraction of sulfate (based on L) from aqueous solutions using an anion-exchange-based liquid–liquid extraction strategy. Extraction of these anions is unambiguously demonstrated by ¹H NMR, ¹⁹F NMR and FTIR spectroscopy, PXRD and single-crystal X-ray diffraction studies.