Oxa- and azacalixarenes as ligands for uranyl ions - Evidence for two different complexation modes

The first crystal structures are reported for uncomplexed parent tetrahomod ioxa- and tetrahomosiazacalix[4]arene compounds and their 1:1 uranyl comple xes. A 1:I uranyl complex is also reported for a hexahomotriazacalix[3]aren e. While analogous features are observed for the uncomplexed dioxa- and dia zacalixarenes, completely different coordination modes are observed in thei r uranyl complexes. The complex of p-tert-butyltetrahomodioxacalix[4]arene is obtained in the presence of triethylamine and consists of an anionic cor e in which the cation is located at the centre of the lower rim, as is freq uently observed. Both p-chloro-N-benzyl-hexahomo-triazacalix [3]arene and p -methyl-N-benzyl-tetrahomodiazacalix[4]arene react with uranyl nitrate with out adding a basic agent. The resulting 1:1 complexes are neutral, the cati on being located outside the macrocycle and bound to two phenoxide groups o f its zwitterionic form and to the nitrate counter-ions. The difference in coordination mode between homooxa- and homoazacalixarene complexes is attri buted to the electrostatic repulsion between the uranyl ions and the ammoni um groups located around the lower rim in the latter case, which prevents a close approach of the complexed cation.