Transition Metals as Catalysts for Cross-Coupling and Dinitrogen Fixation

Citation

Buscagan, Trixia Marie (2018) Transition Metals as Catalysts for Cross-Coupling and Dinitrogen Fixation. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z98P5XPM. https://resolver.caltech.edu/CaltechTHESIS:09202017-130540216

Abstract

Transition metals are used as catalysts in the laboratory and by nature to facilitate difficult chemical transformations. Herein, three different metal containing catalysts are discussed: Pd and Ni catalysts towards the formation of carbon-carbon (C-C) bonds and Fe catalysts towards the reduction of N₂ to NH₃.

In Chapter 2, mechanistic studies of Pd- and Ni-catalyzed cross-coupling reactions are discussed. The mechanism of transmetalation of a Pd-catalyzed Suzuki cross-coupling reaction is studied using a stereochemical probe, revealing that transmetalation occurs with retention of configuration, consistent with transmetalation occurring through a frontside-attack mechanism. Next, to explore the viability of a transmetalation first pathway in an asymmetric Negishi cross-coupling reaction, S = 1/2 Ni^IBr and Ni^I–alkyl complexes were synthesized, crystallographically characterized, and their reactivities explored. Based on these reactivity studies, evidence against a transmetalation first pathway is provided using a variety of spectroscopic methods.

In Chapter 3, new Fe(N₂)(H)_x complexes are synthesized. These complexes catalyze the reduction of N₂ to NH₃ and the yields for NH₃ are improved if the reactions are performed in the presence of Hg lamp photolysis. Preliminary mechanistic studies exploring the role of light are discussed. In the final chapter, new ligand scaffolds are developed that can bind a Lewis acidic and Lewis basic metal center. These ligand frameworks support one- and two-atom bridges between the two metal sites. Finally, we discovered that some of the new complexes are catalysts for N₂ to NH₃ reduction and olefin hydrogenation.

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