Exploring the effects of multi-iron site cooperativity and second sphere ligand interactions on NN bond cleavage in high-spin iron complexes

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 DESCRIPTION (provided by applicant): Nitrogenase is an enzyme that is able to cleave the seemingly inert NN triple bond to form biologically available ammonia. The enzyme employs a structurally unprecedented Fe-S cluster, the iron-molybdenum cofactor, "FeMoco," to reduce N2 to NH3. Examination of fundamental Fe-N2 chemistry and how changes in coordination at Fe as well as the surrounding environment influence the ability of complexes to reduce N2 in will offer insight to the mechanism of nitrogenase. The effects of these features will be tested by synthesis of complexes that divide and simplify the FeMoco allowing for the study each feature has on Fe-N2 chemistry. The FeMoco contains the only example of a carbide in biology, however, how the carbide effects the Fe centers and their ability to reduce N2 remains speculative. Interactions of Fe-S and Fe-carbide clusters with N2 is unknown in synthetic complexes, and development of Fe-N2 complexes that do possess these ligands will provide a chemical basis for the proposed mechanisms of nitrogenase. Our guiding hypothesis is that N2 binds to the cluster through cleavage of either a Fe-C bond or Fe-S bond leading to a N2 complex that can interact with residues nearby the active site during enzyme turnover. In the proposed research, we plan to synthesize Fe-N2 complexes with novel functionalities: (1) second-sphere protic groups, which we hypothesize will allow for use of milder reagents for N2 reduction to ammonia compared with the systems reported previously that required strong acids, (2) introduction of electron-rich, anionic sulfur ligands should increase the donating abiliy of Fe making the bound N2 easier to reduce, and (3) formation of an Fe-carbide complex that also binds N2 to explore the role of the electron-rich carbide in nitrogenase through a synthetic analogue. Each new set of complexes will be examined through reactivity, kinetic, and X-ray crystallographic studies to understand how each impacts the interaction between Fe and dinitrogen. Study of these complexes through spectroscopic techniques such as EPR, ENDOR, and EXAFS will offer support for the chemical and structural interpretation of the data from nitrogenase.