Series of Mixed Uranyl–Lanthanide (Ce, Nd) Organic Coordination Polymers with Aromatic Polycarboxylates Linkers

Three series of mixed uranyl-lanthanide (Ce or Nd) carboxylate coordination polymers have been successfully synthesized by means of a hydrothermal route using either conventional or microwave heating methods. These compounds have been prepared from mixtures of uranyl nitrate, lanthanide nitrate together with phthalic acid (1,2), pyromellitic acid (3,4), or mellitic acid (5,6) in aqueous solution. The X-ray diffraction (XRD) single-crystal revealed that the phthalate complex (UO2)4O2Ln­(H2O)7(1,2-bdc)4·NH4·xH2O (Ln = Ce­(1), Nd­(2); x = 1 for 1, x = 0 for 2), is based on the connection of tetranuclear uranyl-centered building blocks linked to discrete monomeric units LnO2(H2O)7 via the organic species to generate infinite chains, intercalated by free ammonium cations. The pyromellitate phase (UO2)3Ln2(H2O)12(btec)3·5H2O (Ce­(3), Nd­(4)) contains layers of monomeric uranyl-centered hexagonal and pentagonal bipyramids linked via the carboxylate arms of the organic molecules. The three-dimensionality of the structure is ensured by the connection of remaining free carboxylate groups with isolated monomeric units LnO2(H2O)7. The network of the third series (UO2)2(OH)­Ln­(H2O)7(mel)·5H2O (Ce­(5), Nd­(6)) is built up from dinuclear uranyl units forming layers through connection with the mellitate ligands, which are further linked to each other through discrete monomers LnO3(H2O)6. The thermal decomposition of the various coordination complexes led to the formation of mixed uranium-lanthanide oxide, with the fluorite-type structure at 1500 °C (for 1, 2) or 1400 °C for 3–6. Expected U/Ln ratio from the crystal structures were observed for compounds 1–6.

本研究通过常规加热或微波辅助加热的水热合成法，成功制备了三系列铀酰-镧系（uranyl-lanthanide，铈Ce或钕Nd）混合羧酸盐配位聚合物。该类化合物以硝酸铀酰、硝酸镧系金属盐与邻苯二甲酸（1,2位取代）、均苯四甲酸（3,4位取代）或苯六甲酸（5,6位取代）的水溶液混合物为原料合成。
X射线衍射（X-ray diffraction，XRD）单晶结构分析表明，邻苯二甲酸根配合物(UO₂)₄O₂Ln(H₂O)₇(1,2-bdc)₄·NH₄·xH₂O（Ln=Ce(1)、Nd(2)；x=1对应化合物1，x=0对应化合物2）以四核铀酰基结构基元为核心，通过有机配体将其与离散单核LnO₂(H₂O)₇单元相连，形成无限链状结构，游离铵阳离子插层于链间。
均苯四甲酸根相(UO₂)₃Ln₂(H₂O)₁₂(btec)₃·5H₂O（Ce(3)、Nd(4)）由以铀酰为中心的单核六方与五角双锥结构单元通过有机分子的羧酸根臂相互连接，形成层状结构；剩余游离羧酸根基团与孤立单核LnO₂(H₂O)₇单元进一步连接，赋予该结构三维连通性。
第三系列配合物(UO₂)₂(OH)Ln(H₂O)₇(mel)·5H₂O（Ce(5)、Nd(6)）的骨架由双核铀酰单元构成，通过蜜勒酸配体连接形成层状结构，层间再通过离散单核LnO₃(H₂O)₆单元相互交联。
对上述各类配位聚合物进行热分解实验，可在1500℃（对应化合物1、2）或1400℃（对应化合物3~6）下得到具有萤石型结构的混合铀-镧系金属氧化物；化合物1~6的实测铀/镧系金属比例与晶体结构预测的理论值一致。