Department of Chemistry, Oregon State University, Corvallis, Oregon 97331, United States.
Inorg Chem. 2020 Dec 7;59(23):17049-17057. doi: 10.1021/acs.inorgchem.0c02267. Epub 2020 Nov 19.
Isolating isomorphic compounds of tetravalent actinides (i.e., Th, U, Np, and Pu) improve our understanding of the bonding behavior across the series, in addition to their relationship with tetravalent transition metals (Zr and Hf) and lanthanides (Ce). Similarities between these tetravalent metals are particularly illuminated in their hydrolysis and condensation behavior in aqueous systems, leading to polynuclear clusters typified by the hexamer [MO(OH)] building block. Prior studies have shown the predominance and coexistence of smaller species for Th (monomers, dimers, and hexamers) and larger species for U, Np, and Pu (including 38-mers and 70-mers). We show here that aqueous uranium(IV) sulfate also displays behavior similar to that of Th (and Zr) in its isolated solid-phase and solution speciation. Two single-crystal X-ray structures are described: a dihydroxide-bridged dimer () formulated as U(OH)(SO)(HO) and a monomer-linked hexamer framework () as (U(HO))UO(OH)(SO)(HO). These structures are similar to those previously described for Th. Moreover, cocrystallization of monomer and dimer and of dimer and monomer-hexamer phases for both Th (prior) and U (current) indicates the coexistence of these species in solution. Because it was not possible to effectively study the sulfate-rich solutions via X-ray scattering from which and crystallized, we provide a parallel solution speciation study in low sulfate conditions, as a function of the pH. Raman spectroscopy, UV-vis spectroscopy, and small-angle X-ray scattering of these show decreasing sulfate binding, increased hydrolysis, increased species size, and increased complexity, with increasing pH. This study describes a bridge across the first half the actinide series, highlighting U similarities to Th, in addition to the previously known similarities to the transuranic elements.
分离四价锕系元素（即 Th、U、Np 和 Pu）的同构化合物可增进我们对该系列中键合行为的了解，此外还可增进它们与四价过渡金属（Zr 和 Hf）和镧系元素（Ce）的关系。这些四价金属在水系统中的水解和缩合行为非常相似，由此形成了多核簇，典型的结构单元是六聚体[MO(OH)]。先前的研究表明，Th（单体、二聚体和六聚体）的较小物种占优势且共存，而 U、Np 和 Pu（包括 38 聚体和 70 聚体）的较大物种占优势。我们在这里表明，水合铀(IV)硫酸盐在其孤立的固相和溶液形态中也表现出与 Th（和 Zr）相似的行为。描述了两个单晶 X 射线结构：一个是二羟基桥接的二聚体（），化学式为 U(OH)(SO)(HO)，另一个是单体连接的六聚体骨架（），化学式为(U(HO))UO(OH)(SO)(HO)。这些结构与之前描述的 Th 结构相似。此外，Th（之前）和 U（当前）的单体和二聚体以及二聚体和单体-六聚体相的共结晶表明这些物种在溶液中共存。由于无法通过 X 射线散射有效研究富含硫酸盐的溶液，而 和 就是从这些溶液中结晶出来的，因此我们在低硫酸盐条件下进行了平行的溶液形态研究，作为 pH 的函数。这些溶液的拉曼光谱、紫外-可见光谱和小角 X 射线散射表明，随着 pH 值的增加，硫酸盐结合减少，水解增加，物种尺寸增大，复杂性增加。这项研究跨越了锕系元素的前半部分，突出了 U 与 Th 的相似性，此外还与先前已知的与超铀元素的相似性。
