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 SO42– 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 SO42– groups are pseudoencapsulated in the tripodal cavity of the protonated receptor. The receptor forms a charge-separated polymeric ion pair complex with K+ and HPO42– 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 HPO42–. The receptor also forms an anion complex with CO32– 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).