Exploring secondary interactions in Metal-Organic Frameworks

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Metal-organic frameworks (MOFs) are porous materials formed from inorganic cations, dimers, or clusters (nodes), and organic bridging ligands (linkers). Each pore of the MOF can be viewed as a reaction chamber built for an application. With judicious choice of node and linker, the pore aperture, size, and functional group decorating the pore can be tuned. Barring defects/surface effects, the porous and crystalline nature of MOFs results in nodes and linkers existing in isolated and well-defined environments that can be accessed by guest molecules. For adsorption/catalysis, building an active site into a MOF removes deactivation pathways such as aggregation. Researchers have explored this by building MOFs with linkers/nodes that have strongly adsorbing/reactive sites. Secondary interactions are interactions that improve or modify the existing adsorption/catalysis process. They can interact with the guest, the catalyst, or modify the sterics/electronics around the active site. These secondary interactions can improve adsorption, transition state stabilization, or desorption. Despite this, introducing secondary sites into MOFs has been poorly explored. The proposal will explore secondary interactions in MOFs using three linker systems. Each system enables us to study a different aspect of secondary interactions. Linker System A is based on a known porphyrin linker. The linker contains two locations on the porphyrin where secondary interactions will be incorporated in such a way as to participate with chemistry around the metal core of the porphyrin. This will give us the opportunity to explore secondary interactions early on. Linker System B is a new porphyrin linker where the four meso carbons will be modified with secondary interaction groups. With the linker, the active site can be congested to examine sterics. Additionally, complimentary secondary interactions (e.g., donor and acceptor) will be explored. Linker System C is a phthalocyanine linker. These linkers are poorly explored due to the symmetry of the phthalocyanine and the associated isomers. We have identified a method to prepare a phthalocyanine linker that doesn't have the symmetry limitation while also enabling up to eight locations to introduce secondary interactions. Two types of secondary interactions will be explored. Rigid secondary interactions force functional groups to remain in the same environment around the active site. Flexible secondary interactions are dynamic and functional groups can enter/exit the active site. The new MOFs will be explored using single component gas adsorption to determine thermodynamics, and single/multicomponent breakthrough experiments to study kinetics and competitive binding. We will couple this with in-situ IR experiments to determine the mode of action of secondary interactions. We will utilize this information to explore applications in hemoglobin mimics, methane to methanol conversion, chiral catalysis, and explore singlet oxygen chemistry.