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.

本研究通过水热合成法，以四种不同的双齿芳香二羧酸盐为配体，成功制备了四类铀酰有机框架（uranyl-organic framework）化合物。所采用的配体分别为对苯二甲酸根（1,4-benzenedicarboxylate，简称1,4-bdc）、4,4′-联苯二羧酸根（4,4′-biphenyldicarboxylate，简称4,4′-bpdc）、4,4′-偶氮苯二羧酸根（4,4′-azobenzenedicarboxylate，简称4,4′-adc）以及间苯二甲酸根（1,3-benzenedicarboxylate，简称1,3-bdc），最终得到系列晶相产物UO₂(L)（其中L分别对应1,4-bdc（化合物1）、4,4′-bpdc（化合物2）、4,4′-adc（化合物3）或1,3-bdc（化合物4））。该系列产物中的铀酰中心均为单分子四方双锥结构，配位数为6。晶体结构分析结果表明，羧基对位的配体（即化合物1~3所使用的配体）倾向于形成层状扩展组装结构；而间位异构体间苯二甲酸根则诱导生成三维框架结构。热行为原位X射线衍射测试显示，上述不同网络结构的化合物在360~420 ℃范围内依次发生分解，随后结晶生成八氧化三铀（U₃O₈）。