Uranium Pyrophosphate/Methylenediphosphonate Polyoxometalate Cage Clusters

Despite potential applications in advanced nuclear energy systems, nanoscale control of uranium materials is in its infancy. In its hexavalent state, U occurs as (UO2)2+ uranyl ions that are coordinated by various ligands to give square, pentagonal, or hexagonal bipyramids. Creation and design of nanostructured uranyl materials requires interruption of the tendency of uranyl bipyramids to share equatorial edges to form infinite sheets that occur in extended structures. Where a bidentate peroxide group bridges uranyl bipyramids, the configuration is inherently bent, fostering formation of cage clusters. Here the bent configurations of four- and five-membered rings of uranyl peroxide hexagonal bipyramids are bridged by pyrophosphate or methylenediphosphonate, creating eight chemically complex cage clusters with specific topologies. Chemical complexity in such clusters provides opportunities for the tuning of cage sizes, pore sizes, and properties such as aqueous solubility. Several of these are topological derivatives of simpler clusters that contain only uranyl bipyramids, whereas others exhibit new topologies.

尽管铀材料的纳米级调控在先进核能系统中具备潜在应用前景，但目前该领域尚处于起步阶段。铀的六价态以（UO₂）²+铀酰离子（uranyl ions）形式存在，该离子可与多种配体配位，形成四方、五角或六角双锥结构。构建与设计纳米结构铀酰材料，需要阻断铀酰双锥通过共享赤道边缘形成扩展结构中无限层状骨架的趋势。当双齿过氧基团桥连铀酰双锥时，其构型固有地呈现弯曲特征，可促进笼状簇（cage clusters）的形成。本研究中，由铀酰过氧六角双锥（uranyl peroxide hexagonal bipyramids）构成的四元环与五元环，通过焦磷酸根（pyrophosphate）或亚甲基二膦酸根（methylenediphosphonate）进行桥连，得到8个具备特定拓扑结构的化学复杂笼状簇。这类簇合物的化学复杂性为调控笼体尺寸、孔径大小以及水溶性等性能提供了可行途径。其中部分簇为仅含铀酰双锥的简单簇合物的拓扑衍生结构，其余则展现出全新的拓扑构型。