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 ¹H-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 SiF ₆^2- encapsulated complex, [L]₂·[SiF ₆(H₂O)₂]·[TBA]₂ (3). Crystallographic result shows the formation of [F₂(H ₂O)₄]^2- and [Cl₂(H ₂O)₄]^2- 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 ¹H-NMR studies in D₂O-acetone-d₆ (1 : 19, v/v) support 1 : 4 (host-guest) binding stoichiometry of F^-, Cl^-, Br^-, NO ₃^-, HSO₄^- and H₂PO ₄^- 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^- > H₂PO ₄^-. Further, solution state ¹⁹F-NMR studies are also carried out to establish the F^- binding with L in DMSO-d ₆.