This tutorial review focuses on some recent aspects in the development of synthetic receptors for selective sulfate anion recognition and separation, with a special emphasis to: (i) receptors for selective recognition of sulfate in organic and aqueous media and (ii) receptors for separation of sulfate from water via liquid–liquid extraction and crystallization.

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.

Single crystal X-ray crystallographic signature of the pentafluorophenyl substituted tripodal urea-based receptor shows formation of a pseudo dimeric cage which also encapsulates a phosphate dimer via numerous hydrogen bonding and anion⋯π interactions.

Two thiourea and urea based indole conjugated ligands show selective color changes with fluoride along with an absorbance band at 936 nm and 904 nm respectively.

Efficient fixation of aerial carbon dioxide as carbonate by a simple tripodal urea receptor is demonstrated by crystallizing the carbonate encapsulated molecular capsule in almost quantitative yield, followed by regeneration of free receptor from the capsule under mild conditions.