Artificial Enzymes: Development of Biomimetic Catalysts in Metal-Organic Frameworks

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PROJECT SUMMARY/ABSTRACT This research aims to create new metal-organic frameworks with FeS clusters modeled from metalloproteins. Using earth-abundant, biocompatible metals such as Fe in the biomimetic catalysts will enhance health by contributing to the development of materials that may be used on human samples for applications such as sensing biological markers. More specifically, successful synthesis of the proposed materials will allow for the catalysis of more complex and wide-ranging reactions in artificial enzymes. Two related projects are proposed. First, a biomimetic MOF containing [2Fe:2S] clusters and photosensitizers that can efficiently catalyzes the reduction of protons to molecular hydrogen will be constructed. A few examples of [2Fe:2S]-MOFs are reported; however, they have not been optimized for efficient catalysis. Using a recently presented MOF called PCN-700, a linker containing a [2Fe:2S] cluster and another containing a photosensitizer will be postsynthetically installed in the PCN-700 framework to yield a catalytic MOF. These [2Fe:2S]-MOFs can then be easily tuned for catalytic efficiency by installing a third linker that is functionalized to control polar contacts within the MOF. Second, MOFs containing [4Fe:4S] clusters will be synthesized and assessed for catalytic activity. Successful inclusion of [4Fe:4S] clusters in a MOF would expand the catalytic and/or biomimetic potential of MOFs by utilizing the additional available oxidation states afforded by the four Fe sites compared to smaller FeS clusters. For example, more complex reactions could be carried out, such as those requiring four or more electrons. Also, since [4Fe:4S] clusters can bind reaction intermediates (which has not been demonstrated with [2Fe:2S] clusters in a protein), the [4Fe:4S]-MOFs may be used to study reaction intermediates that have not been successfully characterized using the biological system or small molecules. Two approaches are proposed for synthesizing [4Fe:4S]-MOFs. First, free [4Fe:4S] clusters will be synthesized with -SCH3 groups on each Fe. These SCH3 ligands will be exchanged with the bifunctional linker 4,4’-biphenyldithiol to yield a framework, in the presence of reductant and acid. The second synthetic approach involves assembling the FeS clusters and the framework simultaneously by using 4,4’-biphenyldithiol instead of HSCH3 in the scheme originally designed for synthesizing free [4Fe:4S] clusters. Overall, the results of this work will advance the field of biomimetic catalysts using FeS clusters. The proposed [2Fe:2S]-MOFs will be optimized for catalytic activity, and the proposed [4Fe:4S]-MOFs would be the first of their kind, thereby introducing a new class of biomimetic catalysts.