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 (1)H NMR titration experiments, which show the oxyanion binding trend HCO3(-) > H2PO4(-) > HSO4(-), whereas no binding with NO3(-) and ClO4(-) anions is observed.