X-ray crystallographic studies of a 1,3-alternate-calix[4]arene·Na⁺ complex. Is the cation-π interaction operative between the benzene rings and Na⁺?

The structure of a 1,3-alternate-calix[4]arene·Na⁺ complex was determined by X-ray crystallography for the first time. It showed that two well-preorganized oxygen bases (OCH₂CO groups) contribute to the Na⁺-binding whereas two π-basic benzene rings, unlike those for the K⁺ complex, are not involved. The complex adopts a highly symmetrical structure, which is the origin of the facile "Na⁺-tunneling" across the 1,3-alternate-calix[4]arene cavity.