Structural Observations of Heterometallic Uranyl Copper(II) Carboxylates and Their Solid-State Topotactic Transformation upon Dehydration

The hydrothermal reactions of uranyl nitrate and metallic copper with aromatic polycarboxylic acids gave rise to the formation of five heterometallic UO22+Cu2+ coordination polymers: (UO2)Cu(H2O)2(1,2-bdc)2 (1; 1,2-bdc=phthalate), (UO2)Cu(H2O)2(btec)4H2O (2) and (UO2)Cu(btec) (2; btec=pyromellitate), (UO2)2Cu(H2O)4(mel) (3; mel=mellitate), and (UO2)2O(OH)2Cu(H2O)2(1,3-bdc)H2O (4; 1,3-bdc=isophthlalate). Single-crystal X-ray diffraction (XRD) analysis of compound 1 revealed 2D layers of chains of UO8 and CuO4(H2O)2 units that were connected through the phthalate ligands. In compound 2, these sheets were connected to each other through the two additional carboxylate arms of the pyromellitate, thus resulting in a 3D open-framework with 1D channels that trapped water molecules. Upon heating, free and bonded water species (from CuOH2) were evacuated from the structure. This thermal transition was followed by in situ XRD and IR spectroscopy. Heating induced a solid-state topotactic transformation with the formation of a new set of CuO interactions in the crystalline anhydrous structure (2), in order to keep the square-planar environment around the copper centers. The structure of compound 3 was built up from trinuclear motifs, in which one copper center, CuO4(OH2)2, was linked to two uranium units, UO5(H2O)2. The assembly of this trimer, U2Cu, with the mellitate generated a 3D network. Complex 4 contained a tetranuclear uranyl core of UO5(OH)2 and UO6(OH) units that were linked to two copper centers, CuO(OH)2(H2O)2, which were then connected to each other through isophthalate ligands and UOCu interactions to create a 3D structure. The common structural feature of these different compounds is a bridging oxo group of UOCu type, which is reflected by apical CuO distances in the range 2.350(3)2.745(5)angstrom. In the case of a shorter CuO distance, a slight lengthening of the uranyl bond (UO) is observed (e.g., 1.805(3)angstrom in complex 4).