The synthesis of shape-persistent macrocycles towards the rational design of stable large-pore metal-organic frameworks

shayan, Mohsen (2021) The synthesis of shape-persistent macrocycles towards the rational design of stable large-pore metal-organic frameworks. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Metal-organic frameworks (MOFs) are porous materials that have attracted substantial attention due to their exciting features and applications. Synthesis of MOFs involves the self-assembly of organic linkers and metal-containing inorganic nodes through coordination bonds. Given the porous structure of MOFs, it is not surprising that the targeted design of different pore architecture and functionality is of great importance. With that in mind, the pore can be designed and controlled on the molecular level by judicious choice of linkers, nodes, and controlling synthesis conditions. MOFs with large pore sizes can provide porous materials with higher surface areas that are of interest for several applications. For example, in heterogeneous catalysis, larger pores result in faster mass transfer rates of the substrate and products. Moreover, tuning the pore size for larger molecules can accommodate larger reactants inside the pore. In order to design a MOF with larger pore size and surface area, researchers need to utilize larger linkers. However, extension of the organic linker imparts challenges in the resultant MOF structure. The two most common concerns when large linkers are used are robustness (i.e., stability) as well as the formation of interpenetrated networks, which reduces the porosity of the MOF. To make large-pore MOFs that are stable (i.e., do not collapse upon removal of guest molecules from the pore) and non-interpenetrated structures, thereby providing as much pore volume as possible, one approach is to use linkers that are both large and rigid. These large and rigid linkers have very few degrees of conformation freedom that should result in a more stable structure and are sufficiently bulky to prevent interpenetration. When combined with an inorganic node of high connectivity, the linker can result in a stable framework. One of the options is shape-persistent macrocycle scaffolds (e.g., m-phenylene ethynylene macrocycle (PEM)). Another set of rigid and bulky linkers are phthalocyanines (Pc). The aim of this thesis was to synthesize PEM and Pc linkers and explore MOF synthesis with these linkers. PEM is a shape-persistent macrocyclic compound that we were interested in as a linker scaffold due to its rigid structure and large size. I embarked on the synthesis of a tetratopic carboxylate-based PEM through Sonogashira coupling reaction and a final cyclization that is discussed in Chapter 2. The PEM was synthesized with an overall yield of 6%, which is comparable to previously reported compounds; however, our method involves less steps and can be achieved faster. Pcs are aromatic macrocyclic compounds that have structural similarities to porphyrins. What makes Pcs and porphyrins so attractive is that the central ring of these compounds is highly conjugated, bulky, rigid, and many metal ions can be coordinated in the central cavity of these molecule. As such, these linkers are attractive choices for the development of large-pore and robust MOFs for various applications. Porphyrins have received a great deal of attention in MOF chemistry with many different types of porphyrinic linkers synthesized and applied in MOF synthesis. Pcs, on the other hand, have rarely been reported in the MOF field and the existing examples to date are limited to MOFs with a limited number of structures and applications. As such, we opted for the introductions of a new family of Pc linkers. In Chapter 3, I will discuss the synthesis of tetraimidazophthalocyanine linkers that can be used in MOF synthesis. In this thesis, I demonstrate that the strategy for the tunable synthesis of tetraimidazophthalocyaniens through the tetracyclization of imidazophthalonitrile derivatives can make both carboxylate-based and imidazolate-based linkers. Also, the synthesis is flexible to make Pc linkers with other coordination groups. These new linkers provide an opportunity for the synthesis of new PcMOFs, which are lacking in the field. Finally in Chapter 4, I demonstrate the synthesis of PcMOF using the carboxylate-based tetraimidazophthalocyanie linker with zirconium containing nodes. As such, I explored the synthesis of PcMOFs under various condition. The aim was to explore the reaction space associated with this linker to determine if, and how, these MOFs can be formed. These conditions include temperature; different ligands and node precursors, concentrations, and ratios; reaction time; use of different modulators and the modulator concentrations; use of varying concentrations of hydrochloric acid as additive; different solvents and solvents mixture; and sonication of the reaction mixture. Powder X-ray diffraction (PXRD) and N2 gas adsorption of the synthesized samples were evaluated as our metric for the quality of the material. As such, the highest Brunauer-Emmett-Teller (BET) surface area for PcMOF was 1220 m2/g. Pore size distribution (PSD) and PXRD show that the synthesized materials that were obtained under different reaction conditions share similar features in terms of pore size and diffraction pattern. This finding suggests that when a porous material was obtained, the synthesis produced the same material although they have different surface areas. Therefore, the synthesized material needs a better activation method and more efficient removal of the starting materials from the pores.

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