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 degrees C, successively followed by the crystallization of the uranium oxide U3O8.