Rigid cone calix[4]arenes as pi-donor systems: Complexation of organic molecules and ammonium ions in organic media

One step synthesis of calix[4]arene biscrowns, with a rigid cone structure and alkyl or phenyl groups at the 'upper rim', has been performed. The binding ability of these rigidified cone calix[4]arenes 1-4, 13 has been evaluated, in apolar organic media, towards neutral organic molecules and ammonium cation salts. Comparison with more flexible analogues 5, 6, 11 shows that only rigid cone calix[4]arenes are able to complex organic species. The association constants strongly depend on the type of substituents present at the upper rim. The X-ray crystal structure of the endo complex p-cyclohexyl-25,26-27,28-biscrown-3-calix[4]arene 3 with CH3NO2 has been resolved.