Investigations into the biosynthesis of the nitrogenase cofactor, and the donor and magnetic properties of novel group 5 anions coordinated to a paramagnetic metal center




Shupp, John Patrick

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The reduction of dinitrogen to ammonia is a critical transformation to modern society and is responsible for much of the agricultural advancement over the past century. Industrially, this is accomplished by the Haber-Bosch process, where N₂ and H₂ are converted to NH₃ with the use of high temperatures and pressures; a process which consumes over 1% of the world’s energy annually. This is further exacerbated by the need to generate H₂ from natural gas leading to additional production of greenhouse gasses. The same reactivity is accomplished by nitrogenase at ambient temperatures and pressures, and further understanding of the enzyme could lend insight towards more efficient industrial processes. The cofactor of nitrogenase is highly unique, featuring a carbide (C⁴⁻) ligated to 6 Fe centers. To understand the biogenesis of the unique active site, a set of alkali supported, reduced FeS clusters were synthesized. A closed loop of cluster interconversions was documented, and it was demonstrated that Fe-coordinated PhS⁻ can be converted to S²⁻ under strongly reducing conditions. The all-inorganic FeS clusters further offered the unique opportunity to study radical hydrogen abstraction of a bound methyl thiolate; a process which seeks to model the S-adenosyl methionine mediated carbide formation of the nitrogenase cofactor in NifB. Separately, novel anionic ligands utilizing Sb and P chelated to a stabilizing biphenyl unit were synthesized and coordinated to a paramagnetic Co(I) metal center. The donor properties and extent of translation spin orbit coupling of heavy atoms were investigated utilizing magnetization and near-IR absorption measurements. It was determined that the Sb analogue indeed showed a greater extent of axial zero field splitting (D), which is known to increase with increasing spin orbit coupling. A tentative value for the spin orbit coupling constant [lambda] was also found by utilizing values obtained for μ [subscript eff] and 10 dq.



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