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Keywords:

  • Uranyl groups;
  • Zinc;
  • Carb­oxylate ligands;
  • Hydrothermal synthesis;
  • Heterometallic complexes

Abstract

A new heterometallic uranyl zinc carboxylate, (UO2)Zn(H2O)4(H2mel)·2H2O, has been hydrothermally prepared (150 °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 °C) analyses and in situ infrared spectroscopy (up to 210 °C) indicated that both free and bound water species are evacuated from the structure in one step between 80 and 170 °C, followed by its transformation into an unknown, anhydrous, poorly crystalline phase [UO2Zn(mel)] up to 320 °C. After the formation of an amorphous phase, the re-crystallization of oxides α-ZnU3O10 and ZnO was observed from 460 °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.