a. 黑龙江大学功能无机材料化学教育部重点实验室 化学化工与材料学院 哈尔滨 150080;
b. 中国科学院高能物理研究所核能放射化学实验室 北京 100049;
c. 吉林大学理论化学研究所 长春 130023
Structures and Uranium-Uranium Multiple Bond of Binuclear Divalent Uranium Complex of Pyrrolic Schiff-base Macrocycle: a Relativistic DFT Probe
Chen Fangyuana, Qu Ninga, Wu Qunyanb, Zhang Hongxingc, Shi Weiqunb, Pan Qingjianga
a. Key Laboratory of Functional Inorganic Material Chemistry of Education Ministry, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080;
b. Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China;
c. Institute of Theoretical Chemistry, Jilin University, Changchun 130023
经三十余年努力，二价铀溶液化学于近年取得突破性进展，Evans和Meyer等成功获得两个可通过X-ray晶体衍射表征结构的有机金属铀（II）配合物.为进一步拓展UII配合物化学和探索可能存在的金属多重键，设计双核配合物[（UII）2（L）]（L为八齿氮供体低聚吡咯大环四价阴离子），并使用相对论密度泛函理论优化其可能的电子自旋态异构体结构和计算相关性质.结果表明，[（UII）2（L）]具有三重态基态，其电子组态为π4σ2δ2；U（5f）原子轨道对高占据分子轨道有重要贡献；它的U—U键长为2.33Å、Mayer键级为3.89和对应的伸缩振动频率为259 cm-1，被指认为U—U弱四重键.这一结论与QTAIM （quantum theory of atoms in molecule）的U—U键临界点处的电子/能量密度拓扑分析结果相一致.与不同氧化态铀同类物[（Um）2（L）]n+ （m=III，n=2；m=IV，n=4）比较显示，随着铀氧化态增大，U—U距离增长、键级变小、伸缩振动频率变小，金属铀电子自旋密度与常规预期值的差值（?SU）增大；结合分子轨道和QTAIM参数分析，发现金属氧化态可以调控配体和金属轨道能级匹配程度和改变金属-金属多重键.
Although attempts to synthesize divalent uranium molecules were begun three decades ago, molecular U(II) species isolable in solution have been not achieved until recent years. In 2013, Evans and co-workers synthesized the first U(II) complex, [U(Cp')3]·[K(2,2,2-cryptand)] (Cp'=C5H4SiMe3) via flash reduction, that was suitable for X-ray crystal diffraction characterization. A year later, the group of Meyer obtained another divalent uranium complex, [U((Ad,MeArO)3mes)]·[K(2,2,2-cryptand)] employing their particularly interesting tris(aryloxide) arene ligand. The 5f36d1 and 5f4 ground states were assigned to these two complexes, respectively, by the jointed experimental/theoretical studies. It was demonstrated that the ligand significantly affect the nature of the ground state of divalent uranium complex by tuning the energetic separation of the 5f and 6d orbitals. Therefore, careful selection of ligand makes it possible to have access to +II oxidation state of uranium and prepare new UII complex. A flexible octadentate polypyrrollic Schiff-base macrocycle (H4L) has been developed to complex a variety of metals such as actinides, rare earth and transition metals that show a wide range of size and diverse oxidation states. Both mono- and bimetallic complexes featured with an intriguing “Pacman-like” structure were obtained. For example, the reaction of H4L with a trivalent uranium precursor [(UIII)I3(THF)4] yielded a neutral [(UIV)(L)] complex, where the uranium ion was determined by the single crystal X-ray diffraction to be situated inside the ligand mouth and held by eight nitrogen atoms together. The +IV oxidation state was assigned to the uranium by presuming dihydrogen elimination. Considering the flexibility, tetravalent-anion nature as well as capability of accommodating bimetallic ions and stabilizing various oxidation states of uranium (e.g. III~VI complexes have been found so far) that the polypyrrolic ligand has exhibited in previously synthesized complexes, two divalent uranium ions would be likely complexated by the ligand to generate a complex, [(UII)2(L)]. In addition to enriching the coordination chemistry of U(II), it is also a good example to explore electronic structures of the low-valent uranium complex and unravel the uranium-uranium multiple bonding nature. Although many theoretical studies have explored uranium complexes, the study focusing on the divalent diuranium complex of a single macrocyclic ligand remains rare. In the work, a relativistic density functional theory has been employed to investigate [(UII)2(L)]. The structures in electron spin states (singlet, triplet, quintet, septet and nonet) were optimized. Short distances of U—U (2.32~2.67 Å), large bond order (2.95~3.90) and high stretching vibrational frequencies (180~263 cm-1) were calculated. Energetic calculations find that its triplet state is the ground state. It has the electronic configuration of π4σ2δ2, primarily contributed by U(5f) character. Structural and molecular-orbital analyses suggest a slightly weak uranium-uranium quadruple bond, which is confirmed by the quantum theory of atoms in molecule (QTAIM) calculations. Further comparison with analogues [(UIII)2(L)]2+ and [(UIV)2(L)]4+ was also addressed. It is found that the uranium oxidation state is able to tune the energetic matching between the highest-energy occupied orbital of ligand and the adjacent low-energy metal-based orbital, as well as correlates with the electron transfer between metal and ligand and the diuranium multiple bond number.