Combined Solution‐Phase, Solid‐Phase and Phase‐Interface Anion Binding and Extraction Studies by a Simple Tripodal Thiourea Receptor

Abstract

AbstractTris(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.