Aerial CO2 trapped as CO3 2- ions in a dimeric capsule that efficiently extracts chromate, sulfate, and thiosulfate from water by anion-exchange metathesis

Abstract

The tris(2-aminoethyl)amine-based (tren-based) 3-cyanophenyl-substituted tripodal urea L1, one of the familiar urea-based anion receptors, has shown encapsulation of CO32– ions as the carbonate capsule [(L1)2·(CO3)·(TBA)2] (1, TBA = tetrabutylammonium) by the fixation of aerial carbon dioxide from basic dimethyl sulfoxide (DMSO) solution. Single-crystal X-ray structural analysis confirmed the encapsulation of CO32– ions in the cavity of a dimeric capsular assembly of L1 (9.62 Å) through the formation of twelve strong N–H···O hydrogen-bonding interactions. The excellent CHCl3 and CH2Cl2 solubility of 1 has been exploited for the liquid–liquid (L–L) extraction of CrO42–, SO42–, and S2O32– ions from water by anion-exchange metathesis. The extraction of these anions from water was unambiguously confirmed by 1H NMR spectroscopy, IR spectroscopy, powder XRD (PXRD), and single-crystal X-ray diffraction analysis. The 1H NMR spectroscopic analysis of the bulk extracts supports the formation of 2:1 (host–guest) complexes. For the CrO42– ion, the 53Cr NMR spectrum of the bulk extract shows a characteristic peak at δ = –99.98 ppm. The complexes of CrO42–, S2O32–, and SO42– ions with L1 (i.e., 2–4, respectively) were obtained from crystallization of the bulk extracts and show anion-assisted dimeric capsular assemblies of L1 through multiple N–H···X (X = O, S) interactions. The dimensions of the anion-encapsulated capsular assemblies are quite similar to that of the carbonate capsule and are 9.70 Å for [(L1)2·(CrO4)·(TBA)2] (2), 9.61 Å for [(L1)2·(S2O3)·(TBA)2] (3), and 9.71 Å for [(L1)2·(SO4)·(TBA)2] (4). Quantification by weighing the bulk extract shows that 1 can separately extract ca. 90 % of the above three anions from water by anion-exchange metathesis. The quantitative estimations of the extractions of SO42– and CrO42– ions were further verified by gravimetric analysis by BaSO4 and BaCrO4 precipitation techniques, respectively. The extraction of SO42– ions from water was also demonstrated under alkaline conditions (pH 12.5) and in the presence of an excess of nitrate ions. Further, the quantification of CrO42– extraction was established by solution-state UV/Vis studies.