Low-Coordinate Synthetic Models for Nitrogenase Activity

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Bacterial nitrogenases catalyze the reduction ("fixation") of N/2 to ammonia, an amazing process that gives the precursor to all nitrogen-containing biomolecules. Because it is the ultimate multielectron reduction, discovery of the detailed mechanism of nitrogenase is a great challenge in biochemistry. N2 reduction occurs at a metal cluster called the FeMoco. After addition of electrons and protons, the FeMoco somehow binds, breaks, and protonates N2 to NH4*. Mutations near the FeMoco show that N2 and other substrates bind near the iron-rich center of the cluster. Based on literature crystallographic, spectroscopic, and mechanistic studies on nitrogenase, we have formulated a reasonable iron-based mechanism for reduction and N2 binding. This mechanism, based on low-coordinate iron, serves as our guiding hypothesis. Key intermediates have iron with only 3 or 4 attachments, and others with Fe-H bonds. However, the literature has few synthetic 3-coordinate iron compounds, and no Fe-H complexes with a coordination number of four or less. This proposal describes the systematic study of synthetic low-coordinate iron compounds: their properties, spectroscopic signatures, reactivity toward nitrogenase substrates, and characteristic reaction patterns. Experiments are chosen to address several steps of the hypothetical mechanism, including reductive activation, N2 binding, N2 reactions and cleavage, and N-H bond formation. Because isolated Fe-H complexes completely break multiple bonds, they will be studied as reactivity models for nitrogenase intermediates. Multimetallic complexes with constrained geometry will show the arrangement most conducive to N2 cleavage. Mechanistic studies on key reactions will clarify allowed and forbidden reactions of iron atoms like those in the hypotheticaL FeMoco mechanism. These studies will elucidate the fundamentals of low-coordinate iron chemistry, which is necessary to understand the workings of the FeMoco and large synthetic clusters.