Crystallographic studies of gas sorption in metal–organic frameworks

Department of Chemistry, University of Stellenbosch, Private Bax X1, Matieland, Western Cape Province 7602, South Africa Conventional crystal structure analysis is typically carried out with the sample exposed to ambient conditions, or immersed in the temperature-controlled gas stream (N2 or He at approximately atmospheric pressure) of a cryostat. This approach is usually sufficient for the vast majority of studies involving the determination of molecular structure and packing interactions. However, the recent explosion of interest in porous materials has triggered a renewed demand for protocols that allow the analysis of crystals under controlled environments, and particularly those involving gas pressure. A variety of commercial instruments are now available for recording gas sorption isotherms, i.e. plots of the amount of gas absorbed as a function of equilibrium pressure. Although the shapes of these plots provide limited information about the mechanisms that control the sorption/desorption process, they often imply that interesting structural phenomena might be taking place within the porous host material (e.g. stepped sorption profiles, hysteresis and induction periods). In order to understand the structure–property relationships behind these phenomena, it is necessary to carry out structural analyses under conditions that closely mimic those of the sorption experiment. Only then can one answer important questions such as ‘where is the gas-binding site?’, ‘which interactions control guest retention and selectivity?’ and ‘how does the host structure respond to the presence of the guest at a given pressure?’. The in situ structural analysis of gas-loaded crystals poses several technical challenges, including the development of a suitable environmental gas cell that can withstand the desired pressure and still allow the collection of intensity data without undue absorption of the incident and diffracted beams. A conventional laboratory diffractometer for singlecrystal diffraction possesses limited space for an environmental device that would avoid collisions with the instrument during normal operation – laboratory powder diffractometers allow more freedom from such space constraints but provide less useful data. Higher quality data can be obtained at large beamline facilities such as synchrotrons and nuclear reactors, which allow even more spatial freedom but are not as easily accessible as in-house instruments. Commercial environmental sample chambers are currently available for powder diffractometers, but not for single-crystal systems. Over the past several decades researchers have therefore devised a wide variety of highly ingenious approaches to in situ diffraction analysis of gas-loaded crystals. Early studies of this nature were carried out on zeolites and gas hydrates while macromolecular crystallographers have long been infusing heavy-element gases such as xenon into macromolecules in order to facilitate phasing. In recent years similar methods have been employed extensively and to great effect for metal–organic frameworks (MOFs) and the results of these studies are discussed in the timely review by Carrington et al. (2014). Indeed, this comprehensive review should be considered as essential reading for any researcher who is interested in the important insights that can be obtained from in situ gas-loaded crystallographic studies.