Cation-pi interactions between neutral calix[5]arene hosts and cationic organic guests

The binding properties of the 1,3-bridged calix[5]crowns 1-3 towards a number of quaternary ammonium, phosphonium, and iminium ions have been investigated by H-1 NMR in CDCl3 solution, where the sole driving force for association is provided by cation-pi interactions. We have found that the cavity of a calix[5]arene fixed in a cone-like conformation provides a fairly efficient, but rather unselective, receptor site for quaternary salts. The conformationally mobile p-tert-butylcalix[5]arene (4) is in general a much less efficient binder than the more preorganized calixcrowns, but displays a remarkable selectivity towards N-methylquinuclidinium ion that is believed to arise from a good complementarity between the globe-shaped guest and the highly adaptable host. The adverse effect on complexation of the p-tert-butyl groups at the upper rim has been assessed by comparing the binding properties of 1 vs. its de-tert-butylated analogue 3. Furthermore, some information on the importance of counterion and solvent effects have been obtained.