The structure of a 1,3-alternate-calixarene·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-calixarene cavity.
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