Deciphering structural heterogeneity in metal-organic frameworks towards an understanding of structure-property relationships

The work presented herein describes the relevance and study of structural heterogeneity in the porous crystalline metal-organic framework (MOF) materials. These frameworks, which consist of metal clusters bridged by organic linking units, frequently contain defects or other components that are not described simply by the unit cell of the framework. Intriguingly, even if the features make up a small part of the overall structure, these can dominate a given material’s properties. This is evidenced in Chapters 2 and 3, which focus on characterizing the structural heterogeneity on a molecular level, specifically of the ‘missing linker’ defects in UiO-66 and the origin of superacidity in sulfated MOF-808 respectively. The former displays Lewis acidic sites and different protonation states while the latter has superacidic Brønsted acid sites following exchange of sulfuric acid into the pristine MOF-808 framework. These sites are dependent on the post-synthetic treatment of the material, and are a direct result of the heterogeneous chemical features that are not necessarily ordered in a long-range manner. Chapter 4 extends the characterization of sulfated MOF-808 by investigating other oxides that may be incorporated into the framework, focusing on their structural characterization. The concept of structural heterogeneity is not limited to defects and other features that are not ordered over long-range. MOFs can be developed into smart solid-state materials by incorporating multiple types of functional groups with the capacity for different reactivity and chemistry within the same material. This is the focus of Chapter 5, which discusses the synthesis and characterization of MOFs containing phosphonate groups with multiple binding modes and protonation states, carboxylate-based MOFs with disulfide units, and the incorporation of peptide units into the MOF linker to stabilize alkali metal-based clusters.Finally, the full potential of these materials with complex structural landscapes can only be realized through careful structural characterization and analysis. Chapter 6 highlights the case study of fifteen mixed-linker zeolitic imidazolate frameworks (ZIFs) and how their single crystal structures are characterized from low resolution data. While the information at low resolution is clearly less than what can be uncovered at high resolution, there is still much structural information to be gained. The procedures to obtain the data and model the structures, the limitations and methods used, are described