Solid-state NMR studies of alkali metal ion complexes of p-tertbutyl-calixarenes

Calixarenes of different molecular sizes form complexes with alkali metal cations but have different affinities for small cations (Li+ and Na+) and large cations (Rb+ and Cs+). p-tert-Butyl-calix[4]arene easily forms complexes with the small cations by virtue of steric recognition. In p-tert-butyl-calix[8]arene, the sites next to the oxygens are more accessible to large cations because of the more extensive hydrogen-bonding network. The characteristic behaviour of p-tert-butyl-calix[6]arene in the complexation of Rb+ and Cs+ is caused by its `pinchedʹ conformation.