Coordination polymers of uranium(iv) terephthalates

Abstract

A series of tetravalent uranium terephthalates has been solvothermally synthesized in the solvent N,N-dimethylformamide (DMF) at temperature 100–150 °C with different water amounts. Composition diagrams have been determined for the U⁴⁺ metallic cation in the presence of terephthalic acid, and their crystal structures revealed the occurrence of two- or three-dimensional coordination polymers. In the absence of water, a mixture of two polytypes T-U₂Cl₂(bdc)₃(DMF)₄ (1) and M-U₂Cl₂(bdc)₃(DMF)₄ (2) has been identified at low temperature (100–110 °C) for bdc/U = 1–4 (bdc = terephthalate linker). Their structures are built up from isolated uranium centers in nine-fold coordination, surrounded by 6 carboxyl oxygen atoms, 2 oxygen atoms coming from DMF molecules and one chlorine atom. The uranium cations are linked to each other through the bdc ligand in order to generate a 3D framework. By increasing the temperature (130–150 °C), a layered like compound has been isolated, U₂(bdc)₄(DMF)₄ (3). It is composed of discrete actinide centers in ten-fold coordination, with 8 carboxyl oxygen atoms and 2 oxygen atoms from DMF molecules. The connection of the UO₁₀ units with the bdc linkers generates 2D sheets. When a controlled amount of water is added to the reaction medium, the crystallization of the UiO-66-like U₆O₄(OH)₄(H₂O)₆(bdc)₆·10DMF solid (containing a hexanuclear sub-unit) is observed for temperature 110–120 °C and the H₂O/U molar ratio in the range of 2–10. At higher temperature (140–150 °C), a distinct phase appeared, U₂O₂(bdc)₂(DMF) (4), which consists of infinite chains of uranium centers, linked to each other via the bdc ligands. Higher water contents led to the formation of urania UO₂.