Calixarenes as scaffolds: Introduction of tridentate rare earth metal binding units into calix[4]arene

The reactions of various derivatives of chelidamic acid (4-hydroxypyridine- 2,6-dicarboxylic acid) with p-tert-butylcalix[4]arene have produced several new mono- and difunctionalized derivatives of the calixarene in which trid entate functional groups suitable for the binding of rare earth metal catio ns have been incorporated. Single-crystal X-ray structure determinations ha ve been performed on two difunctionalized calixarenes found to adopt differ ent "cone" and "1,2-alternate" conformations, as well as on a complex of eu ropium(III) with the phenoxide form of a monofunctionalized ligand, this st ructure confirming that charge factors are dominant in determining the site of lanthanide metal binding in these ligands.