Sarkar Sounak, Grønbech Thomas Bjørn Egede, Mamakhel Aref, Bondesgaard Martin, Sugimoto Kunihisa, Nishibori Eiji, Iversen Bo Brummerstedt
Center for Materials Crystallography, Department of Chemistry and Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Langelandsgade 140, 8000, Aarhus, Denmark.
Japan Synchrotron Radiation Research Institute (JASRI), 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo, 679-5198, Japan.
Angew Chem Int Ed Engl. 2022 May 23;61(22):e202202742. doi: 10.1002/anie.202202742. Epub 2022 Mar 29.
Glass-forming metal-organic frameworks (MOFs) have novel applications, but the origin of their peculiar melting behavior is unclear. Here, we report synchrotron X-ray diffraction electron densities of two zeolitic imidazolate frameworks (ZIFs), the glass-forming Zn-ZIF-zni and the isostructural thermally decomposing Co-ZIF-zni. Electron density analysis shows that the Zn-N bonds are more ionic than the Co-N bonds, which have distinct covalent features. Variable-temperature Raman spectra reveal the onset of significant imidazolate bond weakening in Co-ZIF-zni above 673 K. Melting can be controlled by tuning the metal-ligand and imidazole bonding strength as shown from thermal analysis of nine solid-solution Co Zn -ZIF-zni (x=0.3 to 0.003) MOFs, and a mere 4 % Co-doping into Zn-ZIF-zni results in thermal decomposition instead of melting. The present findings demonstrate the key role of the metal-ligand bonds and imidazolate bonds in controlling the delicate balance between melting and decomposition processes in this class of ZIF compounds.
玻璃态形成金属有机框架材料（MOFs）具有新颖的应用，但它们独特熔化行为的起源尚不清楚。在此，我们报告了两种沸石咪唑酯框架材料（ZIFs）的同步加速器X射线衍射电子密度，即玻璃态形成的Zn-ZIF-zni和同结构热分解的Co-ZIF-zni。电子密度分析表明，Zn-N键比具有明显共价特征的Co-N键更具离子性。变温拉曼光谱揭示了在673 K以上Co-ZIF-zni中咪唑酯键显著减弱的起始温度。如对九种固溶体Co Zn -ZIF-zni（x = 0.3至0.003）MOF材料的热分析所示，通过调节金属-配体和咪唑键合强度可以控制熔化过程，并且仅向Zn-ZIF-zni中掺入4 %的Co就会导致热分解而非熔化。本研究结果证明了金属-配体键和咪唑酯键在控制这类ZIF化合物熔化和分解过程之间微妙平衡中的关键作用。
