Three-dimensional electron diffraction for studying order, disorder and flexibility in metal-organic frameworks

Abstract: Metal-organic frameworks (MOFs) represent a class of 3D crystalline porous materials composed of organic linkers and metal nodes. Over the years, tens of thousands of MOF architectures have been developed, addressing various applications such as gas storage and separation, catalysis, chemical sensing, ion exchange and drug delivery. One of the most fascinating properties of MOFs lies in their flexible character responsible, for example, for the rotational dynamics of linkers. Three-dimensional electron diffraction (3D ED) has shown to be a powerful tool to solve the structure of nano- or submicrometer-sized crystals coexisting in mixtures, overcoming the limitations of x-ray diffraction.  In this thesis the great potential of one continuous 3D ED protocol, namely continuous rotation electron diffraction (cRED), for the investigation of MOFs is described. cRED has routinely been used in the past decade for obtaining accurate atomic coordinates and perform structure determination of MOFs. In this thesis it is introduced how the limits of the classical approaches for structure determination by cRED can be tackled by individually adjusting the strategies to the requirements of the structures. Thanks to these approaches, full determination of complex structures and fine structural features previously considered impossible to retrieve by 3D ED data, can now be achieved. The complete structure determination of MOFs with highly complex structures, low crystallinity, sensitivity to electron beam and high-vacuum, displacive disorder and long-range structural dynamics is presented. Specifically, in this thesis it is shown how it was possible to achieve the ab initio full determination of MIL-100, an architecture with a unit cell of several hundred thousand cubic Ångstroms, and the discovery of a new class of materials (M-HAF-2), with a connectivity between those of MOFs and hydrogen-bonded organic frameworks. Additionally, the displacive disorder and dynamics in UiO-67 and MIL-140C were investigated showing for the first time that 3D ED can be applied for probing displacive disorder and molecular motion by analyzing the anisotropic displacement parameters. Methods to obtain maximum structure information from anisotropic atomic displacement parameters are also provided through careful investigations of the refinement of ZIF-EC1, MIL-140C and Ga(OH)(1,4-ndc).