Encapsulation of [X2(H2O)4]2- (X = F/Cl) clusters by pyridyl terminated tripodal amide receptor in aqueous medium: single crystal X-ray structural evidence.

A new tris-amide receptor L based on 1,3,5-methyl substituted benzene platform and pyridyl as an attached unit is synthesized and explored towards anion recognition in aqueous environment. The presence of pyridyl terminal in L facilitates its aqueous solubility. The binding of halides and oxyanions towards L are examined by (1)H-NMR technique in solution and by single crystal X-ray crystallography in solid state studies. Crystallization of fluoride and chloride with L is carried out in acetone-water (1 : 1, v/v) binary solvent mixture that yields crystals for respective host-guest complexes, [L]2·[F2(H2O)4]·[TBA]2 (1) and [L]2·[Cl2(H2O)4]·[TBA]2 (2) suitable for single crystal X-ray diffraction studies. On the other hand, complexation of L with fluoride in dioxane-acetone (1 : 1, v/v) solvent mixture, results the formation of SiF6(2-) encapsulated complex, [L]2·[SiF6(H2O)2]·[TBA]2 (3). Crystallographic result shows the formation of F2(H2O)4 and Cl2(H2O)4 zipped 1D-polymeric tweezer-like assemblies of L in acetone-water (1 : 1, v/v) binary solvent mixture in complexes 1 and 2 respectively. Solution state (1)H-NMR studies in D2O-acetone-d6 (1 : 19, v/v) support 1 : 4 (host-guest) binding stoichiometry of F(-), Cl(-), Br(-), NO3(-), HSO4(-) and H2PO4(-) with L. Binding constants of these investigated anions with L by 1 : 1 binding model are calculated which show the following binding order: NO3(-) ≈ HSO4(-) > F(-) ≈ Cl(-) ≈ Br(-) > H2PO4(-). Further, solution state (19)F-NMR studies are also carried out to establish the F(-) binding with L in DMSO-d6.