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

Abstract

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 SiF6^2– encapsulated complex, [L]₂·[SiF6(H₂O)₂]·[TBA]₂ (3). Crystallographic result shows the formation of [F₂(H₂O)₄]^2– and [Cl₂(H₂O)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^–, 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 ¹⁹F-NMR studies are also carried out to establish the F^– binding with L in DMSO-d₆.