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.

Synthesis and anion binding studies of tris(3-aminopropyl)amine-based tripodal urea and thiourea receptors: proton transfer-induced selectivity for hydrogen sulfate over sulfate

“…20,21 For example, a m -cyanophenyl-based tripodal urea reported by Custelcean et al was shown to form a silver-based MOF that encapsulated sulfate by a total of twelve hydrogen bonds. 20a Wu et al reported a 3-pyridyl-based tripodal urea that also showed strong affinity for sulfate.…”

“…22 Gale et al reported a phenyl-based thiourea tripodal receptor that formed a carbonate complex from a mixture of the host with [Et 4 N][HCO 3 ]. 21a The compound was able to transport bicarbonate across lipid membranes. While fluorinated tripodal ureas and thioureas were shown to transport chloride anions through a lipid bilayer.…”

“…While fluorinated tripodal ureas and thioureas were shown to transport chloride anions through a lipid bilayer. 21b In the case of p -fluorophenyl tripodal thiourea, an encapsulated chloride complex and a sulfate capsular complex were structurally characterized. 21b A tren-based tris(thiourea) receptor substituted with p -nitrophenyl groups was shown to form a rigid dimeric capsule with trivalent phosphate.…”

“…21b In the case of p -fluorophenyl tripodal thiourea, an encapsulated chloride complex and a sulfate capsular complex were structurally characterized. 21b A tren-based tris(thiourea) receptor substituted with p -nitrophenyl groups was shown to form a rigid dimeric capsule with trivalent phosphate. 21c Our group has recently reported a p -cyanophenyl tripodal urea for sulfate forming a seven coordinate sulfate complex.…”

“…21b A tren-based tris(thiourea) receptor substituted with p -nitrophenyl groups was shown to form a rigid dimeric capsule with trivalent phosphate. 21c Our group has recently reported a p -cyanophenyl tripodal urea for sulfate forming a seven coordinate sulfate complex. 23a Further work on this receptor for halides has demonstrated the binding trend in the order of fluoride > chloride > bromide > iodide in solution.…”