Six-Fold Coordinated Uranyl Cations in Extended Coordination Polymers

Four uranyl-organic framework-type compounds have been hydrothermally synthesized with different ditopic aromatic dicarboxylates. The use of 1,4-benzenedicarboxylate (noted 1,4-bdc), 4,4′-biphenyldicarboxylate (noted 4,4′-bpdc), 4,4′-azobenzenedicarboxylate (noted 4,4′-adc), and 1,3-benzenedicarboxylate (noted 1,3-bdc) gave rise to the crystallization of a series of phases UO₂(L) (L = 1,4-bdc (1), 4,4′-bpdc (2), 4,4′-adc (3), or 1,3-bdc (4)), which exhibited 6-fold coordinated uranyl centers as monomeric square bipyramidal unit. The crystal structures showed that the para position of the carboxylate groups favored the formation of layered extended assemblies in 1–3, whereas the meta isomer isophthalate induced the formation of a three-dimensional framework. In situ X-ray diffraction of the thermal behavior indicated the decomposition of the different networks in the range 360–420 °C, successively followed by the crystallization of the uranium oxide U₃O₈.