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 (1)H NMR spectroscopy and, when possible, by isothermal titration calorimetry (ITC); the data were in agreement when complementary techniques were used.