Molecular Structure and Electrochemical Behavior of Uranyl(VI) Complex with Pentadentate Schiff Base Ligand: Prevention of Uranyl(V) Cation−Cation Interaction by Fully Chelating Equatorial Coordination Sites

The UVI complex with a pentadentate Schiff base ligand (N,N′-disalicylidenediethylenetriaminate = saldien2−) was prepared as a starting material of a potentially stable UV complex without any possibility of UVO2+···UVO2+ cation−cation interaction and was found in three different crystal phases. Two of them had the same composition of UVIO2(saldien)·DMSO in orthorhombic and monoclinic systems (DMSO = dimethyl sulfoxide, 1a and 1c, respectively). The DMSO molecule in both 1a and 1c does not show any coordination to UVIO2(saldien), but it is just present as a solvent in the crystal structures. The other isolated crystals consisted only of UVIO2(saldien) without incorporation of solvent molecules (1b, orthorhombic). A different conformation of the coordinated saldien2− in 1c from those in 1a and 1b was observed. The conformers exchange each other in a solution through a flipping motion of the phenyl rings. The pentagonal equatorial coordination of UVIO2(saldien) remains unchanged even in strongly Lewis-basic solvents, DMSO and N,N-dimethylformamide. Cyclic voltammetry of UVIO2(saldien) in DMSO showed a quasireversible redox reaction without any successive reactions. The electron stoichiometry determined by the UV−vis−NIR spectroelectrochemical technique is close to 1, indicating that the reduction product of UVIO2(saldien) is [UVO2(saldien)]−, which is stable in DMSO. The standard redox potential of [UVO2(saldien)]−/UVIO2(saldien) in DMSO is −1.584 V vs Fc/Fc+. This UV complex shows the characteristic absorption bands due to f−f transitions in its 5f1 configuration and charge-transfer from the axial oxygen to U5+.

本研究合成了一种含有五齿希夫碱配体（N,N'-二亚水杨基二乙三胺根，记为saldien²⁻）的铀(VI)配合物。该配合物作为一种潜在稳定的铀配合物前驱体，可避免出现UVO2+···UVO2+阳离子-阳离子相互作用，且存在三种不同晶相。其中两种配合物的组成为UVIO2(saldien)·二甲基亚砜（dimethyl sulfoxide, DMSO），分别结晶于正交晶系（orthorhombic）和单斜晶系（monoclinic），依次记为1a和1c。1a与1c中的DMSO分子均未与UVIO2(saldien)发生配位作用，仅作为结晶溶剂存在于晶体结构中。第三种分离获得的晶体仅由UVIO2(saldien)构成，未结合任何溶剂分子，记为1b，结晶于正交晶系（orthorhombic）。研究发现，1c中配位的saldien²⁻配体的构象与1a和1b存在显著差异；该类构象异构体可在溶液中通过苯环的翻转运动实现相互转化。即便在强路易斯碱性溶剂（如DMSO和N,N-二甲基甲酰胺（N,N-dimethylformamide））中，UVIO2(saldien)的五边形赤道配位环境仍保持稳定。在DMSO中对UVIO2(saldien)进行循环伏安测试，结果显示其存在准可逆氧化还原反应，且无后续副反应发生。通过紫外-可见-近红外（UV−vis−NIR）光谱电化学技术测定的电子计量比接近1，表明UVIO2(saldien)的还原产物为[UVO2(saldien)]⁻，该产物在DMSO溶剂中具有良好稳定性。在DMSO溶剂中，[UVO2(saldien)]⁻/UVIO2(saldien)电对的标准氧化还原电位相对于二茂铁/二茂铁正离子（Fc/Fc⁺）为-1.584 V。该铀配合物在5f¹电子构型下，因f-f跃迁以及轴向氧到U⁵+的电荷转移作用，呈现出特征吸收峰。