3D electron diffraction of nanocrystalline MOFs obtained by nonconventional synthetic methods

MOFs are being synthesized and investigated at a fast pace due to their potential for numerous applications, most notably in relation to gas adsorption, energy conversion and storage and even medicine. To increase the sustainability of the solvothermal processes typically used for MOF synthesis, the development of more environmentally friendly approaches is desirable. Moreover, exploring different synthetic conditions allow to obtain products, polymorphs and topologies different from those obtained with conventional methods. Alternative synthetic methods involving solventless- or liquidassisted mechanochemistry as well as the use of supercritical fluids are becoming more and more popular and an object of research themselves.1 The products obtained by non-conventional methods are often polycrystalline and the control and optimization of the crystal growth is not easy to achieve. Conventional single-crystal XRD is therefore usually not applicable in such cases and also powder methods are extremely challenging, especially in case of large unit cells, low symmetry, severe peak broadening and, not least, because of the difficulty of obtaining pure phases. Further complications can be induced by the grinding process or rapid nucleation in non-equilibrium conditions. Eventually, a proper structural characterization of these materials can only be addressed with single-crystal crystallographic techniques capable of high spatial resolution. 3D electron diffraction has nowadays evolved to become the method of choice to solve complex crystallographic puzzles at the nanoscale. 2 In this contribution we will show examples of successful indexing and structure solution of new metal-organic materials obtained via mechanochemistry or synthesis in supercritical CO2. 3,4 New structures with different dimensionalities, porosity and properties have been determined, overcoming their typical beam sensitivity.