Key Laboratory of Bio-based Material Science & Technology of Education Ministry, College of Material Science and Engineering, Northeast Forestry University, Harbin 150040, China.
Inorg Chem. 2013 Aug 5;52(15):9143-52. doi: 10.1021/ic401440w. Epub 2013 Jul 8.
To advance the understanding of the chemical behavior of actinides, a series of trans-bis(imido) uranium(VI) complexes, U(NR)2(THF)2(cis-I2) (2R; R = H, Me, (t)Bu, Cy, and Ph), U(NR)2(THF)3(trans-I2) (3R; R = H, Me, (t)Bu, Cy, and Ph) and U(N(t)Bu)2(THF)3(cis-I2) (3(t)Bu'), were investigated using relativistic density functional theory. The axial U═N bonds in these complexes have partial triple bonding character. The calculated bond lengths, bond orders, and stretching vibrational frequencies reveal that the U═N bonds of the bis-imido complexes can be tuned by the variation of their axial substituents. This has been evidenced by the analysis of electronic structures. 2H, for instance, was calculated to show iodine-based high-lying occupied orbitals and U(f)-type low-lying unoccupied orbitals. Its U═N bonding orbitals, formed by U(f) and N(p), occur in a region of the relatively low energy. Upon varying the axial substituent from H to (t)Bu and Ph, the U═N bonding orbitals of 2(t)Bu and 2Ph are greatly destabilized. We further compared the U═E (E = N and O) bonds of 2H with 3H and their uranyl analogues, to address effects of the equatorial tetrahydrofuran (THF) ligand and the E group. It is found that the U═N bonds are slightly weaker than the U═O bonds of their uranyl analogues. This is in line with the finding that cis-UNR2 isomers, although energetically unfavorable, are more accessible than cis-UO2 would be. It is also evident that 2H and 3H display lower U═(NH) stretching vibrations at 740 cm(-1) than the U═O at 820 cm(-1) of uranyl complexes. With the inclusion of both solvation and spin-orbit coupling, the free energies of the formation reactions of the bis-imido uranium complexes were calculated. The formation of the experimentally synthesized 3Me, 3Ph, and 2(t)Bu are found to be thermodynamically favorable. Finally, the absorption bands previously obtained from experimental studies were well reproduced by time-dependent density functional theory calculations.
为了深入了解锕系元素的化学行为，我们研究了一系列trans-双（亚氨基）铀（VI）配合物，U(NR)2(THF)2(cis-I2) (2R; R = H, Me, (t)Bu, Cy, 和 Ph)、U(NR)2(THF)3(trans-I2) (3R; R = H, Me, (t)Bu, Cy, 和 Ph) 和 U(N(t)Bu)2(THF)3(cis-I2) (3(t)Bu')，使用相对论密度泛函理论进行了研究。这些配合物中的轴向 U═N 键具有部分三键特征。计算得到的键长、键序和伸缩振动频率表明，双亚氨基配合物的 U═N 键可以通过其轴向取代基的变化来调节。这一点可以通过电子结构分析得到证实。例如，2H 被计算出具有基于碘的高能占据轨道和 U(f)-型低能未占据轨道。其 U═N 成键轨道由 U(f)和 N(p)组成，位于相对较低的能量区域。随着轴向取代基从 H 变为 (t)Bu 和 Ph，2(t)Bu 和 2Ph 的 U═N 成键轨道大大失稳。我们进一步比较了 2H 的 U═E (E = N 和 O) 键与 3H 及其铀酰类似物的 U═E 键，以研究四氢呋喃（THF）配体和 E 基团的影响。结果发现，U═N 键比其铀酰类似物的 U═O 键略弱。这与 cis-UNR2 异构体尽管在能量上不利，但比 cis-UO2 更容易获得的发现是一致的。还明显的是，2H 和 3H 的 U═(NH)伸缩振动在 740 cm(-1)处低于铀酰配合物的 U═O 在 820 cm(-1)处。在包括溶剂化和自旋轨道耦合的情况下，计算了双亚氨基铀配合物形成反应的自由能。实验合成的 3Me、3Ph 和 2(t)Bu 的形成被发现是热力学有利的。最后，通过时间相关密度泛函理论计算很好地再现了先前从实验研究中获得的吸收带。
