Elucidating the Mechanism of Dinitrogen Reduction to Ammonia: Novel Intermediates in the Protonation of Fe(DMeOPrPE)2N2
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The reduction of dinitrogen (N2) to ammonia (NH3) will continue to play a vital role in society as the population of the world grows and maintains its dependence on artificial fertilizers. This energy-intensive transformation is achieved industrially by the Haber-Bosch process and naturally via nitrogenase enzymes. Recent synthetic systems attempt to produce NH3 artificially but with lower energy costs than Haber-Bosch by modeling their designs after nitrogenase. This dissertation describes the progress made in one iron-phosphine system, the water-soluble Fe(DMeOPrPE)2N2, capable of producing NH3 at room temperature and pressure. Chapter I describes the history of the coordination chemistry of N2 to a variety of metals, with a focus on iron complexes. In addition to exploring the range of coordination geometries and supporting ligands of such complexes, the application of N2 coordination complexes towards NH3 formation is analyzed. Chapter II discusses the various methods for quantifying yields of ammonia. Along with a historical perspective on the popular indophenol method, the challenges and best conditions for measuring NH3 in the Fe-DMeOPrPE system are defined. Chapter III explores a series of trans-hydrido intermediates along a potential protonation pathway of Fe(DMeOPrPE)2N2. The complete series of reduced dinitrogen ligands (N2, N2H2, N2H4, and NH3) on the Fe(DMeOPrPE)2H+ scaffold is described. Chapter IV highlights the discovery and characterization of a unique bridged Fe(I) dimer, observed during the protonation of Fe(DMeOPrPE)2N2 as a dark purple intermediate. Chapter V describes the electrochemistry of certain intermediates in the Fe-DMeOPrPE system. This insight should open new avenues for future investigations. By altering the electronics of the system, more NH3 may eventually be produced. Chapter VI provides a summary of this work. This dissertation includes previously published and unpublished co-authored material.