Old dog, new tricks : repurposing iron-carbide-carbonyl clusters as precursors for structural modeling of the nitrogenase cofactor
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Nitrogenases are the only known biological enzyme capable of catalyzing the transformation of dinitrogen (N₂) into ammonia (NH₃). The active site of nitrogenase is comprised of a double-cuboidal iron-sulfur cluster featuring an interstitial carbide as the shared vertex, three ‘belt’ sulfides bridging the cuboidal components, and either a homocitrate-bearing heterometal (Mo, V) or an Fe at one of the distal capping metal sites. Out of the three nitrogenases, the Mo-dependent variant demonstrates the highest activity for N₂ conversion. The active-site cofactor of Mo-dependent nitrogenase (FeMoco) was first isolated in 1977; however, after decades of kinetic, structural, and spectroscopic research, many questions surrounding the mechanism of substrate reduction and the electronic structure of reaction intermediates remain unanswered. In this regard, the synthetic modelling community has contributed significantly towards directing mechanistic discussions with N₂-reducing functional model compounds. Furthermore, structural model compounds have played a pivotal role in deciphering the structural and electronic properties of FeMoco, including the identification of the central carbide and assignment of metal-site valence and spin states. Despite this remarkable progress, a synthetic model featuring a paramagnetic iron cluster with sulfides, interstitial carbide, and heterometal Mo has yet to be reported. The work relayed in this dissertation outlines our efforts towards pursuing this synthetic goal. As such, we utilize a family of carbonyl-supported iron clusters — first reported in the 1960s — featuring iron-coordinated inorganic carbide. However, the highly symmetric packing structures have made heterometal-containing carbidocarbonyl iron clusters difficult to unambiguously characterize by X-ray crystallography. Moreover, the strongly π-acidic ligation sphere enforces low metal-valance states and overall diamagnetism, and ligand substitution of COs is difficult to control. Here, we demonstrate a strategy to disrupt the symmetry in molybdenum-containing heteroclusters to unambiguously characterize the Mo site in XRD. Additionally, CO→S ligand substitution is achieved with the utilization of electrophilic sulfur sources, leading to progressively higher oxidation state Fe sites. These synthetic approaches for heterometal incorporation and oxidative sulfur insertion will serve as fundamental stepping-stones towards future endeavors in utilizing and functionalizing carbidocarbonyl iron clusters as synthetic precursors and ultimately, in biomimetically modeling the nitrogenase active site cluster.