Reductive Samarium Catalysis Enabled By A Thermochemical Roadmap

Citation

Boyd, Emily A. (2025) Reductive Samarium Catalysis Enabled By A Thermochemical Roadmap. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/7f8x-qg41. https://resolver.caltech.edu/CaltechTHESIS:05222025-205440915

Abstract

Samarium diiodide is a versatile single-electron reductant. Its reactivity is modulated by recruitment of a wide range of additives to its large coordination sphere. Binding of strong Lewis bases produces more potent Sm(II) reductants, while polar protic donors promote net proton-coupled electron transfer to a variety of unsaturated substrates including intermediates of molybdenum-catalyzed nitrogen reduction. However, samarium(II) reagents are used (super)stoichiometrically in all but a few select cases because mild, tunable methods for selective reduction of oxidized samarium(III) products back to the active samarium(III) state were unavailable at the outset of the following studies. Chapter 1 frames the challenge of catalytic samarium turnover in the context of nitrogen fixation. Proton-coupled electron transfer and inner-sphere electron transfer are introduced as two potential catalytic roles for samarium(II), and a strategy for proton-coupled reduction of problematic samarium(III)-alkoxide intermediates to achieve turnover is outlined. Chapter 2 describes a well-defined model system used to construct extended quantitative thermochemical cycles mapping proton transfer, electron transfer, and ligand association at samarium. The samarium(II) complex binds a secondary amide to generate a remarkably potent net hydrogen atom donor. In Chapter 2, this driving force is leveraged in iron-catalyzed nitrogen reduction; the strongly reducing, weakly acidic nature of the samarium reagent leads to selective generation of hydrazine over ammonia (99:1). In Chapter 3, the benchmarked samarium(III)-alkoxide protonolysis thermodynamics inform selection of Brønsted acids that can be coupled with a mild reductant (zinc powder or an applied electrochemical potential) to achieve catalytic samarium turnover in reductive coupling of ketones and acrylates to form γ-lactones. Photodriven methods for this samarium-catalyzed transformation are reported in Chapter 5. Finally, in Chapter 6, the hypothesis that samarium(II) might serve as an inner-sphere reductant in nitrogen reduction with transition metal catalysts guides design of conditions for tandem samarium/molybdenum catalysis in electrocatalytic nitrogen reduction to ammonia with the lowest driving force and highest Faradaic efficiency (82%) reported to date for a nonaqueous system at atmospheric pressure.

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