Selective inclusion of PO43− within persistent dimeric capsules of a tris(thiourea) receptor and evidence of cation/solvent sealed unimolecular capsules

Abstract

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 PO₄³⁻ within persistent and rigid dimeric capsules, assembled by aromatic π-stacking interactions between the receptor side-arms. A quaternary ammonium salt of PO₄³⁻ 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 HPO₄²⁻ anion of complex 1a (HPO₄²⁻ complex of protonated L, 2 : 1 host–guest), induced by the presence of a large excess of anions such as HCO₃⁻, CH₃CO₂⁻, and F⁻. Qualitative as well as quantitative ¹H and ³¹P NMR experiments (DMSO-d₆) have been carried out in detail to demonstrate the selective and preferential inclusion of PO₄³⁻ by L in solution-states. Competitive crystallization experiments performed in the presence of an excess of anions such as HCO₃⁻, HSO₄⁻, CH₃CO₂⁻, NO₃⁻ and halides (F⁻ and Cl⁻) further establish the phenomenon of selective PO₄³⁻ encapsulation as confirmed by ¹H NMR, ³¹P NMR, FT-IR and powder X-ray diffraction patterns of the isolated crystals. X-ray structural analyses and ³¹P 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 CO₃²⁻ encapsulated centrosymmetric molecular capsule in 3 (2 : 1 host–guest) and a cation (tetrabutylammonium) sealed SO₄²⁻ 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 PO₄³⁻-encapsulated complex (1b) which showed a 1 : 1 host–guest stoichiometry in solution.