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 SiF₆²⁻ 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.