Abstract
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 μm3), 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 ab initio 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.
Highlights
Flexible metal−organic frameworks (MOFs) are known for their vast functional diversities and variable pore architectures
Structural flexibility is a unique characteristic of metal− organic frameworks (MOFs)/porous coordination polymers (PCPs).[1−6] It plays a central role in the host−guest chemistry and is attractive for a wide range of applications, including gas storage,[7−10] catalysis,[11−13] and separation.[14−17] In addition to the well-known phenomenon of breathing and swelling, local flexibilities, such as linker rotation and swing, are not accompanied by phase transitions yet play an important role in controlling the pore accessibility and diffusion rates of the guest molecules in the framework.[18−21] As the dynamic properties are closely associated with structural features, a deep understanding of MOFs, down to an atomic level, is essential for further development of rational design strategies and exploitation of their applications
MOFs are often only obtained as nanosized crystals, which are too small to be studied by single-crystal Xray diffraction (SCXRD)
Summary
Flexible metal−organic frameworks (MOFs) are known for their vast functional diversities and variable pore architectures. Communication collection protocol of continuous rotation electron diffraction (cRED) developed in our group.[46] Using Zr as the metal source and 4,4′-biphenyldicarboxylate (bpdc) as the organic linker, UiO-67 and MIL-140C were synthesized in a phase mixture, with a ZrCl4:H2bpdc:benzoic acid:N,N-dimethylformamide (DMF):water molar ratio of 1:1:3:50:3 (Figures S1 and S2; see the Supporting Information for more details).
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