Uranyl–organic assemblies with the macrocyclic ligand 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetate (TETA)

Abstract

Three uranyl ion complexes obtained from the reaction of uranyl nitrate hexahydrate and 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid (H₄TETA) were characterized by their crystal structure. The centrosymmetric macrocycle, in the [3434] conformation with either carbon or nitrogen atoms as corners, is zwitterionic, with four carboxylate groups and two or four protonated nitrogen atoms. The complexes [(UO₂)₂(H₂TETA)(C₂O₄)(H₂O)₂] (1) and [UO₂(H₄TETA)(H₂O)]·NO₃·Cl (2) were synthesized under hydrothermal conditions and 1 includes additional oxalate ligands generated in situ. Both compounds are two-dimensional polymers in which the four-fold monodentate macrocyclic ligand unites either four uranyl oxalate dinuclear species (1) or isolated uranyl ions (2). Complex 3, [UO₂(H₂TETA)(H₂O)]·6H₂O, was obtained at room temperature and, although it displays the same stoichiometry as 2, it differs from it by being a three-dimensional framework. Both 2 and 3 present channels containing either counter-ions or water molecules. As often observed, the water content of the low-temperature species, 3, is higher than that of the high-temperature one, 2.