The binding and selectivity of halides (spherical) and oxyanions (tetrahedral) toward a recently reported pentafluorophenyl-substituted tripodal urea-based receptor L¹ are examined thoroughly in the solid state by single-crystal X-ray crystallography as well as in solution by multinuclear NMR techniques. Crystallographic results show proof of a fluoride encapsulation in the cavity of L¹ in complex [L¹(F)][Bu₄N], 1. Fluoride encapsulation inside the C_3v symmetric cleft is observed via six hydrogen bonds to all six urea protons of the receptor. In case of complex 2 crystallographic results show encapsulation of sulfate ion inside a supramolecular cage formed upon 1 : 2 (guest–host) complex formation between sulfate and L¹. Sulfate encapsulation is observed via fourteen hydrogen bonding interactions from all six urea moieties of two L¹ units. Our effort to isolate single crystal of halides/oxyanions complexes of L² always yield single crystals of free L² though literature shows anion binding with this receptor in solution. Solution state binding studies of L¹ are carried out by ¹H-NMR titration to calculate binding constants, which show the following anion binding sequence H₂PO₄ ⁻ > SO₄ ²⁻> CH₃COO⁻ > F ⁻ > Cl ⁻ >> Br ⁻ whereas there is no binding with I⁻, NO₃ ⁻ and ClO₄ ⁻ guests. Comparison of phosphate and sulfate binding in L¹ and L², show higher binding with the pentafluorophenyl substituted receptor, L¹. Further ¹⁹F and ³¹P-NMR experiments in solution are also carried out to probe the binding of F ⁻ and H₂PO₄ ⁻ with L¹, respectively. Extensive ¹H-NMR experiments in solution and crystallization in the presence of multiple anions are also undertaken to evaluate the selectivity of H₂PO₄ ⁻ over other anions.

An arene based tripodal amide receptor shows selective dimeric capsule formation templated by fluoride–water cluster, [F₂(H₂O)₆]²⁻, and non-capsular aggregation for chloride, nitrate and acetate anions.

Researchers world-wide have employed diverse strategies to achieve various anion binding hosts and anion induced supramolecular architectures due to the increasing appreciation of anion receptor chemistry. Intellectual discovery of molecular capsules for the recognition of different guest species has opened up a new field of research in the area of supramolecular chemistry. This feature article aims to provide an overview of the current status and recent achievements made by us and others in the last decade in the area of anion induced construction of supramolecular capsules and anion binding in molecular capsules.