三明治型四聚吡咯低价铀配合物结构、成键和氧化还原性质的相对论密度泛函理论研究

Structural,actinide-ligand bonding and redox properties of Sandwich-type tetrapyrrolic complexes of uranium in low oxidation state: a relativistic DFT study

使用密度泛函理论(B3LYP)方法,结合准相对论有效核势(ECP)和二分量ZORA相对论Hamiltonian,探索三明治型四聚吡咯铀配合物LUmL'(配体为porphyrizine或porphyrine;m=III和IV)的结构、成键和反应性质。优化多种空间和"电子自旋态"异构体结构,发现铀配合物具有交叉式、高电子自旋态的基态。QTAIM(Quantum theory of atoms in molecule)的U-N键临界点处的电子/能量密度拓扑分析以及counterpoise计算U-N作用能,揭示该化学键为弱极性共价键。计算显示在采用适当的三价和四价铀源和强碱性配体时,可经过放热反应生成配合物。考虑溶剂(苯、THF和水)和旋-轨耦合影响,计算从四价铀配合物到三价的还原电势,得到数值均小于1 V,这与四价配合物相对稳定、三价铀配合物具有强还原性的实验结果相符合。研究发现降低溶剂极性,配合物的还原电势向负方向移动,这可为实验中采用溶剂极性来调控配合物的氧化还原性质提供理论指导。

A series of sandwich-type tetrapyrrolic uranium complexes, LUmL＇ （ L = Pz = porphyrizine and L^s = Por = porphyrine; m = Ⅲ or Ⅳ） , were examined for their structure, U -N bonding and en- ergetic properties with relativistic density functional theory （DFT）. Optimization on the isomers were car- ried out with two configurations （eclipsed and staggered） and all possible electron-spin states. It was found that the triplet state with s-configuration was the energetically lowest for the uranium （IV） com- plex, while the quartet state with s-configuration for the uranium （III） counterparts. QTAIM （quantum theory of atoms in molecule） and counterpoise BSSE （Basis set superposition error） calculations revealed that interaction between the uranium and nitrogen of ligand was a strong covalent polar bond. Formation reaction as an exothermic process was feasible using suitable uranium sources and strong basic tetrapyrrol- ic anions. The single-electron reduction potentials from the tetravalent to trivalent uranium complexes were computed to show a negative shift as the solvent polarity decreased from water （dielectric constant 78.36） to THF （7.43） and to benzene （2.27）. This theoretical calculations supported experimentally tuning redox chemistry of low-valent uranium complexes.