TY - JOUR

T1 - Mobility of the 2-Methylimidazolate Linkers in ZIF-8 Probed by 2H NMR

T2 - Saloon Doors for the Guests

AU - Kolokolov, Daniil I.

AU - Stepanov, Alexander G.

AU - Jobic, Hervé

PY - 2015/12/10

Y1 - 2015/12/10

N2 - 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, ∼3.4 Å, 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 2φf = 34° with very low activation barrier (1.5 kJ mol-1) and high rates (∼1012 s-1). Above 380 K the linkers begin to demonstrate additional fast librations with gradually increasing amplitudes γlib comparable with the two-site flips (γlib = ±17° 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.

AB - 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, ∼3.4 Å, 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 2φf = 34° with very low activation barrier (1.5 kJ mol-1) and high rates (∼1012 s-1). Above 380 K the linkers begin to demonstrate additional fast librations with gradually increasing amplitudes γlib comparable with the two-site flips (γlib = ±17° 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.

UR - http://www.scopus.com/inward/record.url?scp=84949486557&partnerID=8YFLogxK

U2 - 10.1021/acs.jpcc.5b09312

DO - 10.1021/acs.jpcc.5b09312

M3 - Article

AN - SCOPUS:84949486557

VL - 119

SP - 27512

EP - 27520

JO - Journal of Physical Chemistry C

JF - Journal of Physical Chemistry C

SN - 1932-7447

IS - 49

ER -