DOI: 10.1002/chem.201203748 Structural Observations of Heterometallic Uranyl Copper(II) Carboxylates and Their Solid-State Topotactic Transformation upon Dehydration Jakub Olchowka, ClØment Falaise, Christophe Volkringer, Natacha Henry, and Thierry Loiseau* [a] Introduction Over the last few decades, there has been growing interest in the synthesis and structural characterization of actinide- based compounds by associating metallic centers with O- or N-donor organic ligands to generate multidimensional coor- dination polymers. [1] Most of the contributions reported to date have involved the elaboration of uranium-containing solids, because this particular 5f element finds its main use as a fuel in nuclear power plants. Present in many natural minerals, [2] the chemical reactivity of uranium has been in- tensively investigated in comparison with those of its neigh- boring 5f metals in the periodic table (except for thorium), which are characterized by highly radio-toxicity activity, thus making their handling more difficult. By using hexava- lent uranium (UO 2 2+ ), which represents its most stable oxi- dation state under ambient conditions, a large number of co- ordination polymers have been reported that show different local environments around the cation (tetragonal, pentago- nal, and hexagonal bipyramidal geometries), together with different dimensionalities of the hybrid network. Following the successful synthesis of metal–organic framework (MOF) materials, [3] a wide variety of combinations of multi-topic carboxylate-based organic ligands have been tested with these uranyl species to create new atomic arrangements. [1c] Besides the use of organic molecules with targeted function- alities, an alternative strategy involves the formation of as- semblies of 5f elements with distinct heterometallic ele- ments, such as lanthanides or transition metals. An interest- ing structural feature is observed in divalent copper, which usually possesses a specific environment that is defined by four short Cu  O bonds in a square plane and two long axial Cu O bonds, thereby resulting in an elongated octahedral geometry, owing to the Jahn–Teller effect. In the various re- ported crystal structures (Inorganic Crystal Structure Data- base, FIZ Karlsruhe, Germany) of mixed uranyl copper oxides and their derived solids (phosphates, silicates, etc.), the uranyl  oxo group was often found to belong to the coor- dination sphere of copper through a long apical bond. This Abstract: The hydrothermal reactions of uranyl nitrate and metallic copper with aromatic polycarboxylic acids gave rise to the formation of five heter- ometallic UO 2 2+  Cu 2+ coordination polymers : (UO 2 )CuACHTUNGTRENNUNG(H 2 O) 2 ACHTUNGTRENNUNG(1,2-bdc) 2 (1; 1,2-bdc = phthalate), (UO 2 )Cu- ACHTUNGTRENNUNG(H 2 O) 2 ACHTUNGTRENNUNG(btec)·4H 2 O(2) and (UO 2 )Cu- ACHTUNGTRENNUNG(btec) (2’; btec = pyromellitate), (UO 2 ) 2 CuACHTUNGTRENNUNG(H 2 O) 4 ACHTUNGTRENNUNG(mel) (3 ; mel = melli- tate), and (UO 2 ) 2 O(OH) 2 CuACHTUNGTRENNUNG(H 2 O) 2 - ACHTUNGTRENNUNG(1,3-bdc)·H 2 O (4 ; 1,3-bdc = isophthla- late). Single-crystal X-ray diffraction (XRD) analysis of compound 1 re- vealed 2D layers of chains of UO 8 and CuO 4 ACHTUNGTRENNUNG(H 2 O) 2 units that were connected through the phthalate ligands. In com- pound 2, these sheets were connected to each other through the two addition- al carboxylate arms of the pyromelli- tate, thus resulting in a 3D open-frame- work with 1D channels that trapped water molecules. Upon heating, free and bonded water species (from Cu  OH 2 ) were evacuated from the struc- ture. This thermal transition was fol- lowed by in situ XRD and IR spectros- ACHTUNGTRENNUNGcopy. Heating induced a solid-state top- otactic transformation with the forma- tion of a new set of Cu  O interactions in the crystalline anhydrous structure (2’), in order to keep the square-planar environment around the copper cen- ters. The structure of compound 3 was built up from trinuclear motifs, in which one copper center, CuO 4 ACHTUNGTRENNUNG(OH 2 ) 2 , was linked to two uranium units, UO 5 - ACHTUNGTRENNUNG(H 2 O) 2 . The assembly of this trimer, “U 2 Cu”, with the mellitate generated a 3D network. Complex 4 contained a tetranuclear uranyl core of UO 5 (OH) 2 and UO 6 (OH) units that were linked to two copper centers, CuO(OH) 2 - ACHTUNGTRENNUNG(H 2 O) 2 , which were then connected to each other through isophthalate ligands and U=O  Cu interactions to create a 3D structure. The common structural feature of these different compounds is a bridging oxo group of U=OCu type, which is reflected by apical Cu O dis- tances in the range 2.350(3)– 2.745(5) . In the case of a shorter Cu  O distance, a slight lengthening of the uranyl bond (U=O) is observed (e.g., 1.805(3)  in complex 4). Keywords: copper · hydrothermal synthesis · topochemistry · uranium · X-ray diffraction [a] J. Olchowka, C.Falaise, Dr. C. Volkringer, Dr. N. Henry, Dr. T. Loiseau UnitØ de Catalyse et Chimie du Solide (UCCS) UMR CNRS 8181, UniversitØ de Lille Nord de France USTL-ENSCL, Bat C7, BP 90108, 59652 Villeneuve dAscq (France) Fax: (+ 33) 3-20-43-48-95 E-mail: thierry.loiseau@ensc-lille.fr Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201203748.  2013 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim Chem. Eur. J. 2013, 19, 2012 – 2022 2012