2.B.27 The Tris-Urea Anion Transporter (TUAT) Family 

A symmetric urea-based heteroditopic tripodal receptor capable of recognizing both anions and ion pairs was designed, synthesized, and characterized (A. Basu and D Gopal, J Org Chem., 2014, 79, 2647). The protonated receptor forms a sulfate complex which encapsulates a single DMF in the tripodal cavity of the receptor. However, the SO₄^2– anion is located outside the tripodal cavity and is stabilized by N–H···O hydrogen bonds from the urea functions of four receptor cations. With TBA-HSO₄ the receptor forms a contact ion pair complex, where both the TBA⁺ and SO₄^2– groups are pseudoencapsulated in the tripodal cavity of the protonated receptor. The receptor forms a charge-separated polymeric ion pair complex with K⁺ and HPO₄^2– via formation of a dimeric capsular assembly of the receptor, in which three K⁺ encapsulated dimeric capsular assemblies interdigitate to form a precise cavity that further encapsulates HPO₄^2–. The receptor also forms an anion complex with CO₃^2– via formation of dimeric capsular self-assembly of the receptor. Solution-state binding studies of the receptor with oxyanions have also been carried out by ¹H NMR titration experiments, which show the oxyanion binding trend HCO₃^– > H₂PO₄^– > HSO₄^–, whereas no binding with NO₃^– and ClO₄^– anions was observed.

Subsequently, nine tris-urea receptors were synthesised and shown to coordinate to a range of anionic guests both by ¹H-NMR titration techniques and single crystal X-ray structural analyses. The compounds mediate exchange of chloride and nitrate and also chloride and bicarbonate across POPC or POPC : cholesterol 7 : 3 vesicle bilayer membranes at low transporter loadings. A dependency of anion transport on the nature of the cation provided evidence to suggest that a M⁺/Cl^- cotransport process may also contribute to the release of chloride from the vesicles (Olivari et al. 2016).

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