Uranyl ion complexation by medium and large homooxacalixarenes:: from mono- to poly-uranate complexes

Crystal structures of dioxouranium(VI) complexes of three homooxacalix[n]arenes (n=4, 6 or 8) have shown the possibility to complex metal moieties of variable nuclearity, depending upon the number of phenolic groups and ether links they contain. p-tert-Butyltetrahomodioxacalix[6]arene embraces a single uranyl unit through tetraphenoxide coordination as commonly observed in calixarene complexes, whereas p-methyloctahomotetraoxacalix[4]arene binds a doubly bridged binuclear species coordinated to both phenoxide and ether oxygen donors, while the large p-tert-butyloctahomotetraoxacalix[8]arene binds a unique tetrauranate cluster containing both double and single oxygen bridges between uranium atoms and involving only phenoxide-oxygen coordination to the macrocycle. An additional crystal structure determination on the triethylammonium complex of the singly deprotonated homooxacalix[6]arene enables some comments to be made on the effects of coordination on the macrocycle conformation. The relation between the nuclearity of the uranyl complexes with ligands in the calixarene/homooxacalixarene family and the size of the ligands is discussed.