Low-Coordinate Synthetic Models for Nitrogenase Activity

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Project Summary In nitrogenases, iron­sulfur clusters transcend their usual role as electron transfer sites, by performing the multielectron reduction of N2 to NH3. This enzyme thus shows the amazing catalytic potential of iron­sulfur clusters in biological systems. In addition to its unique ability to reduce N2, the FeMoco active site of nitrogenase has a carbide (C4­), a feature that is new in biological chemistry. Intermediates in the biosynthesis and catalytic mechanism are likely to have hydride, carbene, N2, and hydrazine moieties, which are unknown in other enzymes. Learning the relationship between the structure and function of nitrogenase is aided by synthetic molecules that have specific similarities to the FeMoco. Though they are simplified, they make it possible to test structural features one at a time without the complication of the other cofactors and protein. Our guiding hypothesis is that carbide holds and releases low­coordinate iron, which can form Fe­N2 and Fe­H intermediates. In this hypothesis, sulfide donors in the FeMoco give reactive high­spin electronic configurations. We will test these ideas using synthetic iron clusters with combinations of sulfide, nitride, carbene and carbide bridges. Synthetic compounds with these features will show the feasibility of the proposed functional groups on iron­sulfur clusters, establish the spectroscopic signatures of these functional groups, and show whether their behavior is consistent with the models for FeMoco biosynthesis and mechanism. In the proposed research, we will create synthetic iron­containing compounds with each of the following novel functionalities: unsaturated iron­sulfur clusters, iron­sulfide­hydride clusters, high­ spin iron­carbene and carbide clusters, and N2­cleaving iron complexes. The isolation and characterization of these compounds is made possible by the use of bulky supporting groups. The bulky groups also facilitate crystallization, and enhance solubility in solvents that can be used at low temperature. Crystallography, kinetic studies, electrochemistry, and reactivity will be used to elucidate the atomic­level detail of the elementary steps of small­molecule binding and reduction. The synthetic complexes will be evaluated by ENDOR, infrared, Raman, Mössbauer, and X­ray absorption spectroscopies to provide a link between the structures of novel model compounds and the known data for nitrogenases. We anticipate that the proposed work will lead to valuable precedents for reaction pathways in nitrogenases. Although much is known about the mechanisms of multielectron oxidation reactions in bioinorganic chemistry, the knowledge about multielectron biological reductions lags far behind, and there is particular need for research on small­molecule reactions of iron­sulfur clusters. Therefore, there is fundamental importance in learning how the iron­sulfide cluster in nitrogenase binds and transforms small molecules that are essential for life. In the long run, understanding the mechanisms of small­molecule reduction in biological systems may also lead to new catalysts for use in chemical synthesis, giving an even broader impact.