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 CO₃^2– 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 CO₃^2– 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 CrO₄^2–, SO₄^2–, and S₂O₃^2– 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 CrO₄^2– ion, the ⁵³Cr NMR spectrum of the bulk extract shows a characteristic peak at δ = –99.98 ppm. The complexes of CrO₄^2–, S₂O₃^2–, and SO₄^2– 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 SO₄^2– and CrO₄^2– ions were further verified by gravimetric analysis by BaSO₄ and BaCrO₄ precipitation techniques, respectively. The extraction of SO₄^2– 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 CrO₄^2– extraction was established by solution-state UV/Vis studies.