A C_3v-Symmetric Tripodal Urea Receptor for Anions and Ion Pairs: Formation of Dimeric Capsular Assemblies of the Receptor during Anion and Ion Pair Coordination

A new C_3v-symmetric urea-based heteroditopic tripodal receptor capable of recognizing both anions and ion pairs was designed, synthesized, and characterized. 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 TBAHSO₄ 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. Significantly, 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 is observed.