Date of Award

Fall 10-1-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Holland, Patrick

Abstract

The large-scale industrial fixation of N2 to NH3 through the Haber-Bosch process has cemented itself as the primary means to provide N for fertilizer and commodity chemicals globally. However, our dependence on this process is unsustainable in the long term due to its reliance on fossil fuels to generate H2 and to provide the substantial energy input for the reaction, paired with high infrastructure requirements that necessitate centralized synthesis plants and sophisticated transportation networks. As an alternative, electrochemical fixation of N2, coupling water oxidation to provide proton (H+) and electron (e–) equivalents with the N2 reduction reaction (NRR) to achieve the 6H+/6e– reduction of N2 to 2 NH3, could operate on a smaller, localized scale while utilizing renewable sources to generate electrical energy to drive the reaction. A key challenge in achieving electrochemical N2 fixation is the development of catalysts for electrochemical NRR. Existing heterogeneous catalysts for NRR suffer from poor activity, selectivity, and robustness. Insights that aid the development of better NRR catalysts may be found by studying molecular systems that can reduce N2. This thesis probes potential N2 functionalization pathways that could be involved in electrochemical NRR by studying molecular model systems in which N2 binds to, or is cleaved by, reduced metal-pincer complexes. Chapter 1 describes electrochemical N2 fixation as an alternative to the Haber-Bosch process. A molecular approach towards understanding electrochemical NRR is proposed, especially through bimetallic N2 cleavage to form metal nitrides. Strategies for the subsequent functionalization of the metal nitride are discussed, primarily via proton-coupled electron transfer (PCET) reduction of the nitride into NH3. Challenges involved in PCET nitride reduction, as well as opportunities inspired by molecular N2 reduction catalysts and recent discoveries of potent PCET reagents, are identified and applied to a hypothetical system for electrochemical NRR. Chapter 2 describes the protonation and electrochemical reduction of Ir- and Rh-pincer complexes that can strongly bind N2. The potential utility of these complexes in an electrochemical NRR system are assessed by complimentary electrochemical and spectroscopic studies exploring their stepwise protonation and electrochemical reduction. Protonation was found to be a prerequisite for electrochemical reduction of the N2 complexes, with protonation occurring at the metal center to form metal hydrides. Protonation triggers release of the N2 ligand, preventing reductive N2 functionalization with these complexes. Chapter 3 investigates the possibility of oxidative functionalization of an N2-derived Re nitride in order to form NOx species. Although no N–O bond formation was achieved at the nitride, a series of Re nitrides was synthesized and characterized in which the metal center is oxidized by 1e– and/or the supporting pincer ligand is oxidized to a nitroxide. The Re-nitride interaction was monitored over the series using NMR and IR spectroscopies, X-ray crystallography, and computational methods. Cooperative oxidation of both the metal center and the supporting ligand results in the weakest Re-nitride interaction, more localization of the LUMO at the nitride ligand, and an umpolung in nitride reactivity. Chapter 4 applies PCET methods to N2-derived Re nitrides in an attempt to reduce the nitride to NH3, thus closing the cycle of N2 to NH3. Stepwise PCET mechanisms were prohibited by high-energy intermediates in both systems; however, the combination of SmI2 and H2O to generate a strong concerted PCET reagent resulted in formation of 74% yield of NH4+ in one system, but exclusive production of H2 in the other. Other PCET methods, such as pairing organic H-atom transfer reagents with SmI2, are also assessed for PCET nitride reduction. Chapter 5 studies the conversion of NH3 to a nitride in a Re system that can also cleave N2. Re-ammine and Re-amide intermediates were isolated, and the mechanisms of H atom removal from these to form the nitride were identified. Experimental determination of the N–H bond enthalpies in the Re-amide were used to benchmark computational studies elucidating the thermodynamics of N–H bond cleavage (and formation, the microscopic reverse). The putative Re-imide intermediate in the PCET reduction pathway was found to feature a particularly weak N–H bond, representing a thermodynamic bottleneck to PCET nitride reduction in this system.

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