Oxa- and Azacalixarenes as Ligands for Uranyl Ions − Evidence for Two Different Complexation Modes

Abstract

The first crystal structures are reported for uncomplexed parent tetrahomodioxa- and tetrahomodiazacalix[4]arene compounds and their 1:1 uranyl complexes. A 1:1 uranyl complex is also reported for a hexahomotriazacalix[3]arene. While analogous features are observed for the uncomplexed dioxa- and diazacalixarenes, completely different coordination modes are observed in their uranyl complexes. The complex of p-tert-butyltetrahomodioxacalix[4]arene is obtained in the presence of triethylamine and consists of an anionic core in which the cation is located at the centre of the lower rim, as is frequently observed. Both p-chloro-N-benzyl-hexahomotriazacalix[3]arene and p-methyl-N-benzyl-tetrahomodiazacalix[4]arene react with uranyl nitrate without adding a basic agent. The resulting 1:1 complexes are neutral, the cation being located outside the macrocycle and bound to two phenoxide groups of its zwitterionic form and to the nitrate counter-ions. The difference in coordination mode between homooxa- and homoazacalixarene complexes is attributed to the electrostatic repulsion between the uranyl ions and the ammonium groups located around the lower rim in the latter case, which prevents a close approach of the complexed cation.