三价超铀元素镅冠醚包合物的合成与表征

Synthesis and characterization of americium(Ⅲ)crown ether inclusion complexes

本文报道了95号元素镅(Am)的冠醚包合物[Am(NO_(3))_(2)(18-crown-6)]_(3)·[Am(NO_(3))_(6)](简写为Am-18C6)合成、单晶结构和固体紫外-可见-近红外光谱.X射线单晶衍射表明,Am-18C6晶体属于单斜晶系中的C2/m空间群,非对称单元由[Am(NO_(3))_(6)]^(3-)、[Am(NO_(3))_(2)(18C6)]^(+)和二重无序的[Am(NO_(3))_(2)(18C6)]^(+)孤立离子对组装而成.我们系统比较了Am-O键长与镧系同构晶体中Nd-O键长的差异,并将其与密度泛函理论(density function theory,DFT)计算的理论键长进行了对比.结果表明,Am^(3+)和Nd^(3+)的电子密度变化截然不同,Am^(3+)表现出明显地失去5f电子密度,这意味着Am^(3+)的5f电子在向着O–Am配位键中心延展和极化,而Nd^(3+)并未表现出此特性.本工作对加深三价超铀元素和大环配体之间主客体相互作用理解提供了帮助,为设计新型镧锕分离萃取剂提供了结构模型与理论基础.

The coordination chemistry of americium holds significant importance in the fields of spent fuel reprocessing and nuclearwaste disposal. However, owing to its scarcity and inherent high radioactivity, only a limited number of radiochemicallaboratories worldwide possess the ability to handle americium, making the study of its coordination chemistryexceptionally challenging. Compared with research on uranium, neptunium, and plutonium, studies on the coordinationchemistry of americium are relatively scarce. The number of reported crystal structures for high-valent uranium reaches7000, while those for are 399 and 254, respectively. In contrast, there are only 113 entries for americium crystal structuresin the Cambridge Structural Database. This has resulted in limited knowledge about the chemistry of americium elements.Crown ethers are a class of macrocyclic ligands with adjustable cavity sizes, molecular architectures, and donor atoms thatexhibit selective complexation abilities for metal ions. Studies on solid-state chemistry involving crown ethers andtransuranic elements are limited;thus far, only a few complexes comprising transuranic elements and crown ethers havebeen synthesized and characterized. To the best of our knowledge, there have been limited thermodynamic studies onamericium and crown ether ligands, with no synthesis or structural determination of the americium crown ether complex.Herein, we report the synthesis, single-crystal structure, and solid-state ultraviolet–visible–near-infrared spectroscopyanalysis of the first americium crown ether inclusion complex, [Am(NO_(3))_(2)(18-crown-6)]_(3)·[Am(NO_(3))_(6)] (denoted as Am18C6).Single-crystal X-ray diffraction reveals that the Am-18C6 structure is isomorphic to the synthesized Nd-18C6, andboth structures crystallize in the monoclinic space group of C2/m, which is assembled by independent ionic pairs including[M(NO_(3))_(6)]^(3−) (M1), [M(NO_(3))_(2)(18C6)]^(+) (M2) and disordered [M(NO_(3))_(2)(18C6)]^(+) (M3). The analysis of the structures revealthat M1 is coordinated with 12 oxygen atoms provided by six NO^(3−) ions, with Am1−O bond lengths of 2.577 to 2.658 Åand Nd1−O bond lengths of 2.580 to 2.653 Å. M2 is coordinated with 10 oxygen atoms provided by one 18-crown-6 ligandand two NO^(3−) ions, with Am2−O bond lengths of 2.427 to 2.563 Å and Nd2−O bond lengths of 2.463 to 2.607 Å.Similarly, the M3 atom exhibits a coordination configuration identical to that of M2, with Am−O bond lengths of 2.484 to2.564 Å and Nd−O bond lengths of 2.490 to 2.570 Å.We systematically compared the bond lengths of Am−O and Nd−O with those of isomorphic structures, and furthercompared them with theoretically calculated bond lengths using density functional theory (DFT). The results demonstratethat the cavity of 18C6 is more compatible with Nd^(3+) ions. The calculated average M−O bond distances are in agreementwith the interaction energy findings. Furthermore, a notable variation in electron density is observed between Am^(3+) andNd^(3+), wherein Am^(3+) significantly reduces the in 5f electron density. This observation suggests that the 5f electrons of Am^(3+)are extended and polarized toward the O–Am coordination bond center, whereas Nd^(3+) does not exhibit this characteristic.The spectrum exhibited characteristic f-f transition absorption peaks at 507 and 817 nm, which are diagnostic for Am^(3+).The findings presented in this study contribute to an enhanced understanding of host-guest interactions involving trivalenttransuranic and macrocyclic ligands, thereby providing a theoretical foundation for designing lanthanide and actinideseparation extractants.