Aerial CO 2 Trapped as CO 3 2– Ions in a Dimeric Capsule That Efficiently Extracts Chromate, Sulfate, and Thiosulfate from Water by Anion‐Exchange Metathesis

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 CO 3 2– ions as the carbonate capsule [( L1 ) 2 · (CO 3 ) · (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 CO 3 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 CHCl 3 and CH 2 Cl 2 solubility of 1 has been exploited for the liquid–liquid (L–L) extraction of CrO 4 2– , SO 4 2– , and S 2 O 3 2– ions from water by anion‐exchange metathesis. The extraction of these anions from water was unambiguously confirmed by 1 H NMR spectroscopy, IR spectroscopy, powder XRD (PXRD), and single‐crystal X‐ray diffraction analysis. The 1 H NMR spectroscopic analysis of the bulk extracts supports the formation of 2:1 (host–guest) complexes. For the CrO 4 2– ion, the 53 Cr NMR spectrum of the bulk extract shows a characteristic peak at δ = –99.98 ppm. The complexes of CrO 4 2– , S 2 O 3 2– , and SO 4 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 · (CrO 4 ) · (TBA) 2 ] ( 2 ), 9.61 Å for [( L1 ) 2 · (S 2 O 3 ) · (TBA) 2 ] ( 3 ), and 9.71 Å for [( L1 ) 2 · (SO 4 ) · (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 SO 4 2– and CrO 4 2– ions were further verified by gravimetric analysis by BaSO 4 and BaCrO 4 precipitation techniques, respectively. The extraction of SO 4 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 CrO 4 2– extraction was established by solution‐state UV/Vis studies.