Mechanistic Study of Cu-Mediated, Photoinduced C–S Bond Formation and Demonstration of Electrochemical Ammonia Production by a Surface-Attached Iron Complex

Citation

Hannoun, Kareem Imad (2019) Mechanistic Study of Cu-Mediated, Photoinduced C–S Bond Formation and Demonstration of Electrochemical Ammonia Production by a Surface-Attached Iron Complex. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/R9EJ-KX83. https://resolver.caltech.edu/CaltechTHESIS:05302019-070503222

Abstract

The worldwide reliance on fossil fuels for energy and petrochemicals poses a massive environmental hazard. Furthermore, many chemical processes rely on precious metals that have low abundance on Earth and are threatened. As the world population grows rapidly, these factors pose an increasing threat to our planet and new chemical processes are needed that employ earth-abundant catalysts and alternative chemical currencies such as light and electricity derived from renewable sources.

Chapter 2 discusses an in-depth mechanistic study of the photoinduced, copper-mediated cross-coupling of aryl thiols with aryl halides. This reaction employs light energy and an earth-abundant metal to achieve bond formation through a pathway distinct from that of thermal reactions. In particular, I focus on the stoichiometric photochemistry and subsequent reactivity of a [Cu^I(SAr)₂]^– complex (Ar = 2,6-dimethylphenyl). A broad array of experimental techniques furnish data consistent with a pathway in which a photoexcited [Cu^I(SAr)₂]^-* complex undergoes SET to generate a Cu^II species and an aryl radical, which then couple through an in-cage radical recombination.

Chapter 3 discusses the surface attachment of a P₃^BFe complex to a carbon electrode, and the electrochemical generation of ammonia from N₂ by the surface-appended species (P₃^BFe = tris-phosphinoborane). Ammonia production is achieved industrially by the combination of N₂ and H₂, the latter of which is derived from methane with concomitant production of CO₂. Alternative chemical processes, such as the use of energy derived from electricity, are vital for the decreasing the carbon footprint of ammonia production. Synthetic modification of a previously-reported P₃^BFe complex by addition of three pyrene substituents onto the catalyst backbone allows non-covalent attachment onto a graphite surface. The resulting functionalized electrode shows good stability towards iron desorption under highly reducing conditions, and produces 1.4 equiv NH₃ per iron site. The data presented provide the first demonstration of electrochemical nitrogen fixation by a molecular complex appended to an electrode.