Water-Soluble Calixarene Hosts that Specifically Recognize the Trimethylammonium Group or the Benzene Ring of Aromatic Ammonium Cations: A Combined1H NMR, Calorimetric, and Molecular Mechanics Investigation

A selective dual binding mode towards aromatic ammonium ions is shown by two water-soluble calix[4]arene tetracarboxylate hosts 1 and 2, fixed in the cone conformation. The presence of sulfonate groups at the upper rim (2) induces inclusion of the aromatic ring of trimethylanilinium ion into the apolar calixarene cavity, whereas receptor 1 specifically recognizes the polar trimethylammonium group of this guest. Intermediate behavior is observed in other cases.