Synthesis, Structural Characterization, and Dehydration Analysis of Uranyl Zinc Mellitate, (UO2)Zn(H2O)4(H2mel)•2H2O

A new heterometallic uranyl zinc carboxylate, (UO2)Zn(H2O)(4)(H(2)mel)center dot 2H(2)O, has been hydrothermally prepared (150 degrees C, 24 h) by using 1,2,3,4,5,6-benzenehexacarboxylic acid (mellitic acid) as organic linker in order to form a three-dimensional network. Four of the six carboxylate groups of the mellitate ligand interact with mononuclear uranyl or zinc cations, which are eightfold (hexagonal bipyramid, UO8) or sixfold [octahedron, ZnO2(H2O)(4)] coordinated, respectively. The remaining free carboxylate arms of the mellitate species preferentially interact through hydrogen bonds with water molecules trapped within the framework. Thermogravimetric and X-ray thermodiffraction (up to 800 degrees C) analyses and in situ infrared spectroscopy (up to 210 degrees C) indicated that both free and bound water species are evacuated from the structure in one step between 80 and 170 degrees C, followed by its transformation into an unknown, anhydrous, poorly crystalline phase [UO2Zn(mel)] up to 320 degrees C. After the formation of an amorphous phase, the re-crystallization of oxides alpha-ZnU3O10 and ZnO was observed from 460 degrees C. The fluorescence spectrum of the as-synthesized uranyl zinc mellitate shows the six bands that are typical for vibronic couplings of the [O=U=O](2+) moiety.