Mobility of the 2-Methylimidazolate Linkers in ZIF-8 Probed by 2H NMR: Saloon Doors for the Guests

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• 作者：Daniil I. Kolokolov ; Alexander G. Stepanov ; Herv茅 Jobic
• 刊名：Journal of Physical Chemistry C
• 出版年：2015
• 出版时间：December 10, 2015
• 年：2015
• 卷：119
• 期：49
• 页码：27512-27520
• 全文大小：436K
• ISSN：1932-7455

文摘

ZIF-8 is one of the most interesting metal鈥搊rganic frameworks due to its high stability and unique capabilities for hydrocarbons separation. Its porous network is formed by large almost spherical cavities interconnected by very narrow windows, 鈭?.4 脜, which should be too small even for methane. At the same time the direct experimental observations show that ZIF-8 cavities are able to host even such large molecules as benzene. This effect is associated with the flexibility of the cavity widows, arising from dynamical freedom on the 2-methylimidazole (2-mIM) linkers that form the framework. In this work, by means of ²H NMR we show that the 2-mIM linkers of the ZIF-8 are very mobile and their mobility is sensitive to the presence of benzene guest. In contrast with other known MOFs based on linearly bonded carboxylates, in guest-free ZIF-8 the plane of 2-mIM linker exhibits two-site flips within a sector of 2蠁_f = 34掳 with very low activation barrier (1.5 kJ mol^鈥?) and high rates (鈭?0¹² s^鈥?). Above 380 K the linkers begin to demonstrate additional fast librations with gradually increasing amplitudes 纬_lib comparable with the two-site flips (纬_lib = 卤17掳 above 560 K). This is direct evidence that the ZIF-8 linkers twist notably, thus increasing the aperture of the windows sufficiently to fit very large molecules. Upon benzene loading, the geometry of the observed motions remains similar but the potential barrier of the linkers flipping rises up to 9 kJ mol^鈥?.