Advances in Dynamic in Situ Single Crystal X-ray Diffraction to Understand the Structural Dynamics in Flexible Metal-Organic Frameworks and Develop Insight into Crystal Engineering
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Metal-organic frameworks (MOFs) are a class of permanently porous coordination networks that form by metal ions coordinating to one another via organic linker molecules. These crystalline systems are primarily discussed in the context of their void spaces, areas of empty space that often contain guest molecules. Serving as unique sites for guest binding and/or chemical transformations, MOF research largely investigates how changes in lattice structure influence the properties of the material, including guest selectivity, binging affinity, and storage capacity. An exciting new development in the field of MOFs involves frameworks that undergo dramatic lattice distortions, broadly expansion or contraction, in response to the local chemical environment of the material. These flexible frameworks can have significant utility because they increase the interactions between host framework and guest molecule, making it easier to control and fine tune the properties of these frameworks, as well as making it easier to resolve the guest molecules, thereby revealing the structure of all components involved in the transformation.In order to create flexible crystalline frameworks ‘by design’, one must understand the underlying mechanisms of how these materials transform in order to establish the principles by which new materials may be devised and ultimately constructed. To that end, this body of work presents advances in both the instrumentation and application of dynamic in situ single crystal X-ray diffraction (DIX) methods to nanoporous crystalline systems. Modifications to an environmental control cell previously developed in the Benedict group have greatly improved the reliability and reproducibility of DIX measurement and have expanded the library of solvents and gases that may be used with the system. A reaction in which a hydrated flexible crystalline framework (CoMOF) was exposed to methanol was monitored in situ using the environmental control cell. The in situ results revealed the presence of new structures and provided direct insight into the guest exchange mechanism – something not possible based upon previous studies using ex situ methods. The dehydration reaction of the same framework (CoMOF) was used to assess potential differences between DIX on powders versus single crystal samples. These measurements revealed the two techniques to be highly complementary. Powder samples provide faster time resolution potentially leading to greater insight regarding the kinetics of the guest exchange process. However, powder measurements come at a critical cost: They are unable to provide the facile and reliable structures that may be obtained when DIX measurements are performed on single crystals. Finally, DIX was used to examine the thermal expansion properties of a framework material known to undergo a highly unusual process in which the volume of the material decreases with temperature: Negative thermal expansion. These measurements revealed how these properties might be affected by the identity of the solvent present in the void spaces.