Structure, Bonding, and Reactivity of Ti and Zr Amidate Complexes: DFT and X-Ray Crystallographic Studies

Easily prepared and highly modular organic amide proligands have been used to synthesize a series of new
bis(amidate)-bis(amido) Ti and Zr complexes via protonolysis. These complexes have been structurally characterized
by NMR spectroscopy and X-ray crystallography. The solid-state molecular structures of these complexes indicate
that the amidate ligands bind to the metal centers in an exclusively bidentate fashion, resulting in discrete monomeric
species. Geometric isomerism in these species is highly dependent upon the steric characteristics of the proligands
utilized in the synthesis. In solution, these complexes are observed to isomerize on the NMR time scale, with one
isomer being predominant. Bonding in the bis(amidate)-bis(amido) complexes was investigated by DFT calculations.
The geometric isomers predicted by theory matched the experimentally observed results, within experimental error.
The orbitals associated with amidate−metal bonding are energetically well below the frontier orbitals. The HOMO
in these complexes is a π orbital associated with amido ligand-to-metal bonding character, while the LUMO in all
cases is a vacant d orbital on the metal center.

制备简便且高度模块化的有机酰胺前配体，经质子解反应合成了一系列新型双(酰胺酸根)-双(酰胺基)钛(Ti)与锆(Zr)配合物。本研究采用核磁共振波谱法（NMR）与X射线晶体衍射技术对上述配合物完成结构表征。固态分子结构分析显示，酰胺酸根配体仅以双齿配位模式与金属中心结合，形成离散的单核物种。此类配合物的几何异构现象高度依赖于合成过程中所用前配体的空间位阻特征。在溶液中，这些配合物可在核磁共振时间尺度内发生异构化，且其中一种异构体占主导地位。本研究通过密度泛函理论（DFT）计算，对双(酰胺酸根)-双(酰胺基)配合物的成键作用展开了探究。理论预测的几何异构体与实验观测结果在实验误差范围内吻合良好。与酰胺酸根-金属成键相关的轨道在能量上远低于前线轨道。该类配合物的最高占据分子轨道（HOMO）为与酰胺基配体向金属中心成键相关的π轨道，而所有体系的最低未占据分子轨道（LUMO）均为金属中心的空d轨道。