Catalytic Proton-Coupled Reductions of Dinitrogen and Cyanide

Citation

Johansen, Christian Marinelli (2025) Catalytic Proton-Coupled Reductions of Dinitrogen and Cyanide. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/jrh7-2a15. https://resolver.caltech.edu/CaltechTHESIS:05282025-142549821

Abstract

This thesis, directly and indirectly, focuses on mechanisms and strategies for the 6H⁺/6e⁻ reduction of N₂ to NH₃ (nitrogen reduction; N₂R) using well-defined molecular catalysts. In nature, nitrogenases reduce N₂ to NH₃, but nitrogenases can also reduce cyanide to CH₄ and NH₃, making CN⁻ and N₂ reduction interesting to compare. We describe the highly selective catalytic reduction of CN⁻ to NH₃ and CH₄ by a mononuclear Fe-catalyst related to Fe-based N₂R systems. Mechanistic studies suggest several intermediates, including iron isocyanides (FeCNH), aminocarbynes (FeCNH₂), and aminocarbenes (FeC(H)NH₂⁺), allowing a comparison to N₂R. We then show the 2H⁺/2e⁻ equilibration of iron cyanide to the iron aminocarbyne complexes of these early intermediates of catalysis. Such reversible triple bond activations are rare. We show that key to this transformation is the H-bond facilitated multisite proton-coupled electron transfer (MS-PCET).

Next, seeking alternative ways to drive N₂R, a photodriven approach is explored. The Hantzsch ester (HEH₂), a dihydropyridine, is utilized as a 2H⁺/2e⁻ photoreductant, and when partnered with a suitable catalyst (Mo) and an organic buffer (collidine/collidinium; Col/ColH⁺) under blue light irradiation allows for photodriven N₂R. Catalysis is enhanced by addition of a photoredox catalyst (Ir). This photodriven N₂R is thermodynamically comparable to the industrial hydrogenation of N₂, but light is used to drive the reaction. Mechanistic studies of the Ir-free conditions show that Col-buffer is essential for transferring H⁺/e⁻ from HEH₂ to N₂. An H-bonded pre-association can form between [ColH]⁺ and HEH₂, allowing for rapid oxidative quenching of the excited HEH₂. Subsequently, the base deprotonates HEH₂•⁺, circumventing back electron transfer. In net ColH• and HEH•, two potent H-atom donors are generated. This reagent combination is competent for the photoreduction of organic substrates as well. Lessons from this mechanistic study drove the development of photodriven methods for Sm^III-to-Sm^II reduction, an appealing prospect given SmI₂ being a potent and selective reductant, including for N₂R. HEH₂ can serve either as a direct photoreductant or as the reductive quencher for an Ir photoredox catalyst. Both methods for SmI₂ generation translate to proof-of-concept photodriven, Sm-catalyzed reductive cross-coupling reactions.

Thesis Files