This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We propose here complementary multi-edge X-ray absorption spectroscopic investigations of site-differentiated cubane-type Mo/Fe/S clusters for developing synthetic targets for homogeneous dinitrogen reduction processes. These targets can be considered as biomimetic compounds of the iron-molybdenum cofactor (FeMo-co) of nitrogenase. We plan to accomplish this in three stages. First, we will characterize cubane type Mo/Fe/S cluster with systematically varied S and Se ligand environments in oxidized and reduced states using sulfide, selenide, diethyldithio carbamate (DTC), and diethyldiseleno carbamate (DSC) ligands. Second, we will modify the terminal ligands around Fe targeting to stabilize low valent states of Fe that likely be poised toward activating dinitrogen because of the increased nucleophilicity of the cluster core. Third we perturb the coordination environments around Mo to thoroughly assess the ligand field effect on the electronic and geometric structures of the clusters. We plan to take advantage of complementarity of XAS data at the Fe K-, Fe L-, Mo L-, Cl K-, Se K- and S K-edges to generate desirable structural properties of future clusters. Recent beamline development at BL4-3 now allows for collection of S and Cl EXAFS, which in combination with Fe/Mo/S EXAFS can provide more reliable fits of geometric structure. XANES analysis of pre-edge and rising-edge features of Fe L-edge and S K-edge spectra provides electronic structural information about effective nuclear charges, oxidation and spin states of metal/ligand centers, orbital compositions, ligand-field splitting of metal centers, and possible non-innocent behavior of ligands. Separation of overlapping S K- and Mo L-edges will be achieved by utilizing Se derivatives. In addition to the potential of making new biomimetic compounds insights from the proposed experiments will have impact on the understanding of the controversial role of the Mo-site in activation of dinitrogen.
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