Synthesis, Structural Characterization, and Dehydration Analysis of Uranyl Zinc Mellitate, (UO 2 )Zn(H 2 O) 4 (H 2 mel)·2H 2 O

A new heterometallic uranyl zinc carboxylate, (UO 2 )Zn(H 2 O) 4 (H 2 mel) · 2H 2 O, 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, UO 8 ) or sixfold [octahedron, ZnO 2 (H 2 O) 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 [UO 2 Zn(mel)] up to 320 °C. After the formation of an amorphous phase, the re‐crystallization of oxides α‐ZnU 3 O 10 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.