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

Crystal structures of dioxouranium(VI) complexes of three homooxacalix[n]ar enes (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 sin gle 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 bi nds a unique tetrauranate cluster containing both double and single oxygen bridges between uranium atoms and involving only phenoxide-oxygen coordinat ion to the macrocycle. An additional crystal structure determination on the triethylammonium complex of the singly deprotonated homooxacalix[6]arene e nables some comments to be made on the effects of coordination on the macro cycle conformation. The relation between the nuclearity of the uranyl compl exes with ligands in the calixarene/homooxacalixarene family and the size o f the ligands is discussed.