Samperisi Laura, Jaworski Aleksander, Kaur Gurpreet, Lillerud Karl Petter, Zou Xiaodong, Huang Zhehao
Department of Materials and Environmental Chemistry, Stockholm University, Stockholm SE-106 91, Sweden.
Department of Organic Chemistry, Stockholm University, Stockholm SE-106 91, Sweden.
J Am Chem Soc. 2021 Nov 3;143(43):17947-17952. doi: 10.1021/jacs.1c08354. Epub 2021 Oct 25.
Flexible metal-organic frameworks (MOFs) are known for their vast functional diversities and variable pore architectures. Dynamic motions or perturbations are among the highly desired flexibilities, which are key to guest diffusion processes. Therefore, probing such motions, especially at an atomic level, is crucial for revealing the unique properties and identifying the applications of MOFs. Nuclear magnetic resonance (NMR) and single-crystal X-ray diffraction (SCXRD) are the most important techniques to characterize molecular motions but require pure samples or large single crystals (>5 × 5 × 5 μm), which are often inaccessible for MOF synthesis. Recent developments of three-dimensional electron diffraction (3D ED) have pushed the limits of single-crystal structural analysis. Accurate atomic information can be obtained by 3D ED from nanometer- and submicrometer-sized crystals and samples containing multiple phases. Here, we report the study of molecular motions by using the 3D ED method in MIL-140C and UiO-67, which are obtained as nanosized crystals coexisting in a mixture. In addition to an determination of their framework structures, we discovered that motions of the linker molecules could be revealed by observing the thermal ellipsoid models and analyzing the atomic anisotropic displacement parameters (ADPs) at room temperature (298 K) and cryogenic temperature (98 K). Interestingly, despite the same type of linker molecule occupying two symmetry-independent positions in MIL-140C, we observed significantly larger motions for the isolated linkers in comparison to those reinforced by π-π stacking. With an accuracy comparable to that of SCXRD, we show for the first time that 3D ED can be a powerful tool to investigate dynamics at an atomic level, which is particularly beneficial for nanocrystalline materials and/or phase mixtures.
柔性金属有机框架材料（MOFs）以其丰富的功能多样性和可变的孔结构而闻名。动态运动或扰动是其高度期望的灵活性之一，这是客体扩散过程的关键。因此，探测此类运动，尤其是在原子水平上，对于揭示MOFs的独特性质和确定其应用至关重要。核磁共振（NMR）和单晶X射线衍射（SCXRD）是表征分子运动的最重要技术，但需要纯样品或大单晶（>5×5×5μm），而这对于MOF合成来说往往难以获得。三维电子衍射（3D ED）的最新进展突破了单晶结构分析的极限。通过3D ED可以从纳米级和亚微米级晶体以及包含多相的样品中获得准确的原子信息。在此，我们报告了使用3D ED方法对MIL-140C和UiO-67中分子运动的研究，它们是以纳米晶体形式共存于混合物中获得的。除了确定其骨架结构外，我们发现通过观察热椭球模型并分析室温（298 K）和低温（98 K）下的原子各向异性位移参数（ADPs），可以揭示连接体分子的运动。有趣的是，尽管在MIL-140C中相同类型的连接体分子占据两个对称独立的位置，但我们观察到孤立连接体的运动比通过π-π堆积增强的连接体的运动要大得多。我们首次表明，3D ED能够以与SCXRD相当的精度成为研究原子水平动力学的有力工具，这对于纳米晶体材料和/或相混合物尤其有益。
