Ion-Pair Complexation with a Cavitand Receptor

The capability of resorcinarenes to bind anions within the alkyl feet at the lower rim has been exploited as the starting point for developing a new cavitand able to engulf contact ion pairs of primary ammonium salts in chlorinated solvents with association constants (K-ass) in the range of 10(3)-10(4) M-1. Methylene bridges were introduced into the upper rim to freeze the resorcinarene in the cone conformation with the four H-down protons converging in the lower pocket, thereby maximizing the CH-anion interactions responsible for the anion binding. Four additional phosphate moieties were introduced into the lower rim in close proximity to the anionic site to provide hydrogen-bonding-acceptor P=O groups and promote cation complexation at the bottom of the cavitand. The binding ability of the synthesized ligands was analyzed by H-1 NMR spectroscopy and, when possible, by isothermal titration calorimetry (ITC); the data were in agreement when complementary techniques were used.