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

ZIF-8 is one of the most interesting metal-organic 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, similar to 3.4 angstrom, 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 H-2 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 phi(f) = 34 degrees with very low activation barrier (1.5 kJ mol(-1)) and high rates (similar to 10(12) s(-1)) Above 380 K the linkers begin to demonstrate additional fast librations with gradually increasing amplitudes gamma(lib) comparable with the two-site flips (gamma(lib) = +/- 17 degrees 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(-1).