Anion binding by a tetradipicolylamine-substituted resorcinarene cavitand

Anion binding has been achieved with a resorcinarene substituted with four 2,2′-dipicolylamine moieties on the upper rim. The four dipicolylamine groups reside in proximity on one rim of the cavitand. The dipicolylamine groups were protonated with triflic acid to provide the cationic ammonium sites for anion binding. This anion receptor binds strongly to anions of different geometries, such as H ₂PO ₄ ^-, Cl ^-, F ^-, CH ₃CO ₂ ^-, HSO ₄ ^-, and NO ₃ ^-. The association constants for binding these anions are large, on the order of log K = 5 in CD ₃CN, a solvent of intermediate dielectric constant. These values represent significant binding compared to other cavitands with nitrogen pendant groups. Evidence suggests that the cavitand provides two identical receptor sites formed by two dipicolylamine groups, facilitating the simultaneous binding of two anions. Intramolecular binding of anions between two protonated dipicolylamine groups is indicated on the basis of the comparison to a structurally similar monomeric analogue and by semiempirical PM3 molecular modeling. Titrations with the analogue result in much weaker anion association, even at high concentrations, indicating the importance of proximity and preorganization of sites on the cavitand upper rim.