Shape recognition of alkylammonium ions by 1,3-bridged calix[5]arene crown-6 ethers:: Endo- vs exo-cavity complexation

A series of tri-O-substituted 1,3-bridged calix[5]arene crown-6 ethers bearing alkyl, arylalkyl, alkoxyalkyl, and alkoxycarbonylmethyl residues at the lower rim and either Bu-t or H substituents at the upper rim have been synthesized. H-1 NMR studies have shown that p-tert-butylcalix[5]crowns, irrespective of the size and nature of their lower rim pendant groups, adopt preorganized conelike conformations, whereas p-H-calix[51 crowns with bulky substituents preferentially exist in solution as partial cone conformers (C-1 symmetry). Calix[5]crown derivatives behave as mono-or ditopic receptors for isomeric butylammonium ions, forming endo-cavity (inside the calixarene cup) and/or exo-cavity (at the crown ether moiety) 1:1 complexes according to the shape of the guest. These two binding modes can be clearly distinguished and monitored by H-1 NMR titration experiments.