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The tris(thiourea) receptor, L, in the presence of excess H(2)PO(4)(-) has been found to encapsulate a trivalent phosphate ion within a π-stacked dimeric capsular assembly of the receptor with twelve strong H-bonds via deprotonation, as evident from the origin of a new set of signals in the (1)H NMR titration experiments.

A tris(2-aminoethyl)amine-based tris(urea) receptor, L, with electron-withdrawing m-nitrophenyl terminals has been established as a potential system that can efficiently capture and fix atmospheric CO(2) as air-stable crystals of a CO(3)(2-)-encapsulated molecular capsule (complex 1), triggered by the presence of n-tetrabutylammonium hydroxide/fluoride in a dimethyl sulfoxide solution of L. Additionally, L in the presence of excess HSO(4)(-) has been found to encapsulate a divalent sulfate anion (SO(4)(2-)) within a dimeric capsular assembly of the receptor (complex 2) via hydrogen-bonding-activated proton transfer between the free and bound HSO(4)(-) anions. Crystallographic results show proof of oxyanion encapsulation within the centrosymmetric cage of L via multiple N-H···O hydrogen bonds to the six urea functions of two inversion-symmetric molecules. The solution-state binding and encapsulation of oxyanions by N-H···O hydrogen bonding has also been confirmed by quantitative (1)H NMR titration experiments, 2D NOESY NMR experiments, and Fourier transform IR analyses of the isolated crystals of the complexes that show huge spectral changes relative to the free receptor.

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A set of three positional isomers of nitro-phenyl functionalized tris(urea) scaffold has been extensively studied as the anion coordinating host. The para-isomer, L 1 , has been structurally authenticated to self-assemble into a dimeric (pseudo)molecular capsule in the presence of oxoanions of varied dimensionality (carbonate and terephthalate anions). Whereas the meta-isomer, L 2 , has been found to self-assemble into a dimeric molecular capsule in presence of inorganic oxoanions such as hydrogen phosphate and adopts a flat trigonal planar like geometry in the presence of the terephthalate anion where each receptor side arm is in interaction with a carboxylate group of the anion. However, successful crystallization of the ortho-isomer, L 3 , in the presence of different oxoanions was not fruitful presumably due to the steric effect provided by the nitro group at the ortho-position, which hinders the facile inclusion and coordination of an anion due to electrostatic factor, as confirmed by 2D NOESY NMR analysis of the free receptor. Qualitative and quantitative 1 H NMR experimental results showed that the ortho-isomer binds with oxoanions rather feebly as reflected from the apparent binding constant values of HCO 3 − and H 2 PO 4 2− as compared to the meta-and para-isomers which can coordinate to these anions very strongly via encapsulation, as confirmed by 2D NOESY NMR analysis.

A tren-based tris(thiourea) receptor, L with electron-withdrawing p-nitrophenyl terminals has been established as a competent hydrogen-bonding scaffold that can selectively encapsulate PO(4)(3-) within persistent and rigid dimeric capsules, assembled by aromatic π-stacking interactions between the receptor side-arms. A quaternary ammonium salt of PO(4)(3-) capsules (complexes 1 and 1b, 2:1 host-guest) can reproducibly be obtained in quantitative yields by a solution-state deprotonation of [HL](+) moieties and a bound HPO(4)(2-) anion of complex 1a (HPO(4)(2-) complex of protonated L, 2:1 host-guest), induced by the presence of a large excess of anions such as HCO(3)(-), CH(3)CO(2)(-), and F(-). Qualitative as well as quantitative (1)H and (31)P NMR experiments (DMSO-d(6)) have been carried out in detail to demonstrate the selective and preferential inclusion of PO(4)(3-) by L in solution-states. Competitive crystallization experiments performed in the presence of an excess of anions such as HCO(3)(-), HSO(4)(-), CH(3)CO(2)(-), NO(3)(-) and halides (F(-) and Cl(-)) further establish the phenomenon of selective PO(4)(3-) encapsulation as confirmed by (1)H NMR, (31)P NMR, FT-IR and powder X-ray diffraction patterns of the isolated crystals. X-ray structural analyses and (31)P NMR studies of the isolated crystals of phosphate complexes (1, 1a and 1b) provide evidence of the binding discrepancy of inorganic phosphates with protonated and neutral form of L. Furthermore, extensive studies have been carried out with other anions of different sizes and dimensions in solid- and solution-states (complexes 2a, 3, 4 and 5). Crystal structure elucidation revealed the formation of a solvent (DMSO) sealed unimolecular capsule in the F(-) encapsulated complex, 2a (1:1 host-guest), a CO(3)(2-) encapsulated centrosymmetric molecular capsule in 3 (2:1 host-guest) and a cation (tetrabutylammonium) sealed SO(4)(2-) encapsulated unimolecular capsule in 4 (1:1 host-guest). 2D-NOESY NMR experiments carried out on these capsule complexes further confirm the relevant binding stoichiometry of complexes (2a-4) except for the PO(4)(3-)-encapsulated complex (1b) which showed a 1:1 host-guest stoichiometry in solution.

A tris(2-aminoethyl)amine-based tris(urea) receptor L, in the presence of excess TBA(H 2 PO 4 ) self-assembled into a tetrahedral molecular cage by encapsulation of a tetrameric tetrahedral mixed phosphate cluster (H 2 PO 4¯· HPO 4 2¯) 2 via 24 -NH hydrogen bonds [complex (n-TBA) 6 {4L(H 2 PO 4¯· HPO 4 2¯) 2 }] (1). Further, in presence of orthophosphoric acid the receptor L has been observed to be self-assembled with side-cleft bonded tetrameric anion-acid cluster (H 2 PO 4 − ·H 3 PO 4 ) 2 [complex {(LH) + (H 2 PO 4¯· H 3 PO 4 )DMSO·H 2 O}] (2). In a proof of concept experiment, transformation of salt complex 2 into neutral phosphate encapsulated complex has been demonstrated in solution-state by quantitative 1 H NMR titration with (TBA)OH in DMSO-d 6 (298 K). Extensive studies with receptor L have also been carried out with other halides in solid-state. Crystal structure elucidation revealed the formation of a pseudodimeric (2+2) capsular self-assembly with 1:1 host: guest ratio in fluoride encapsulated complex, (TEA)[L(F)](3). 2D-NOESY NMR (DMSO-d 6 ) experiments were carried out on the phosphate complexes (complexes 1 and 2) to study the binding discrepancies of phosphate anion with the neutral and charged host.

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