Bao Zhe, Zhao Hong-Bo, Qu Ning, Schreckenbach Georg, Pan Qing-Jiang
Key Laboratory of Functional Inorganic Material Chemistry of Education Ministry, School of Chemistry and Materials Science, Heilongjiang University, Harbin, China 150080.
Department of Chemistry, University of Manitoba, Winnipeg, MB, CanadaR3T 2N2.
Dalton Trans. 2016 Oct 12;45(40):15970-15982. doi: 10.1039/c6dt01930g.
The flexible small-cavity macrocycle, trans-calix[2]benzene[2]pyrrolide (HL), has been found experimentally to complexate low-valent U and U with binding pockets (BP) of bis(arene) (Ar) and bis(pyrrolide) (Pl), respectively. This switchable coordination of the uranium center has been explored using relativistic density functional theory (DFT) in this work. Systematic calculations of [(BP-L)An(η-H)BH)] (BP = Ar and Pl; An = U, Np and Pu; m = III and IV; n = 2 and 3; and z = 0 and 1), labeled as BP-An-nH, were carried out. Energetics and geometrical/electronic-structure analyses reveal that the size matching between actinide ions and the binding pocket plays a significant role in determining the energetic ordering of isomers. The relatively large-size U and Np ions are selectively preferred by the large bis(arene) pocket, yielding the most stable isomer of Ar-An-2H; simultaneously formed δ(An-Ar) bonding helps stabilizing the system. In contrast, the small-size Pu and An are held by the smaller bis(pyrrolide) to show the energetically favored Pl-An-3H isomer. This size argument is further supported by calculations on the related Th and Pa compounds. The formation reactions of BP-An-nH demonstrate an endothermic process when using the HL ligand reactant. Applying a more basic alkali ligand (AL; A = Li, Na and K) as the reactant significantly reduces the reaction energy and presents thermodynamic possibility to prepare the low-valent actinide complexes. This is in agreement with the experimental synthesis where KL was utilized. The redox potentials (E) from tri- to tetravalent actinides were calculated while including both solvation and spin-orbit coupling effects. The highly reductive nature of the U complex was manifested by the calculated E of over 1.1 V.
实验发现，柔性小腔大环化合物反式杯[2]苯并[2]吡咯（HL）能分别与低价铀以及具有双（芳烃）（Ar）和双（吡咯）（Pl）结合口袋（BP）的铀络合。本工作利用相对论密度泛函理论（DFT）对铀中心的这种可切换配位进行了研究。对标记为BP-An-nH的[(BP-L)An(η-H)BH)]（BP = Ar和Pl；An = U、Np和Pu；m = III和IV；n = 2和3；z = 0和1）进行了系统计算。能量学以及几何/电子结构分析表明，锕系离子与结合口袋之间的尺寸匹配在决定异构体的能量排序方面起着重要作用。相对较大尺寸的U和Np离子被较大的双（芳烃）口袋选择性优先结合，生成最稳定的Ar-An-2H异构体；同时形成的δ(An-Ar)键有助于稳定体系。相比之下，较小尺寸的Pu和An则被较小的双（吡咯）口袋容纳，呈现出能量上更有利的Pl-An-3H异构体。对相关钍和镤化合物的计算进一步支持了这一尺寸观点。当使用HL配体反应物时，BP-An-nH的形成反应表现为吸热过程。使用碱性更强的碱金属配体（AL；A = Li、Na和K）作为反应物可显著降低反应能量，并为制备低价锕系配合物提供了热力学可能性。这与使用KL的实验合成结果一致。在考虑溶剂化和自旋轨道耦合效应的同时，计算了三价到四价锕系元素的氧化还原电位（E）。计算得到的E超过1.1 V，表明U配合物具有高度还原性。
