Applications of Synthesis in Copolymer Upcycling, Reduction of CO₂, Ligand Non-Innocence, and New Weakly-Coordinating Anions

Citation

Bruening, Meaghan Ann (2024) Applications of Synthesis in Copolymer Upcycling, Reduction of CO₂, Ligand Non-Innocence, and New Weakly-Coordinating Anions. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/dm7p-vv68. https://resolver.caltech.edu/CaltechTHESIS:06032024-222310291

Abstract

This dissertation focuses on a very diverse series of studies with synthetic applications to organometallic systems supported by a redox non-innocent ligand architecture, α,ω diene generation, ionomers for tuning performance of electrochemical CO₂ reduction, and design of new weakly coordinating anions.

Chapter 2 discusses the reactivity of a 9,10-anthracenediyl bis(phenoxide) titanium complex, where the polyaromatic anthracene motif functions as a non-innocent ligand. This enables access to a reduced Ti complex which is competent for oxidative coupling of alkyne and nitrile substrates, and pyrimidines and substituted benzenes can be accessed catalytically. Additional reactivity with oximes and oxidative coupling of alkynes and CO₂ is explored.

Chapter 3 reports a two-step method for generation of a distribution of α,ω-dienes from ethylene and butadiene. Copolymers are prepared with variable butadiene content, where 1,4-butadiene incorporation ensures the double bonds are in the polymer backbone. Subsequent ethenolysis of the copolymers produces α,ω-dienes in the C₁₀-C₂₀ range. The conditions can be modified to control product selectivity.

Chapter 4 presents a series of polystyrene-based ionomers to probe the impact of local [K⁺] in the Cu electrode microenvironment on CO₂R performance (Part A). Partial current density towards C₂₊ products (|j_C2+|) increases with [K⁺] in ionomer, up to 225 mA cm^–2. When K⁺ is replaced with [Me₄N]⁺, performance lowers to the level of bare Cu, highlighting the crucial role of K⁺ in improving C₂₊ product selectivity. Molecular dynamics simulations and partial pressure CO₂ experiments support enhanced CO₂ mass transport with the ionomers. An expanded series of ionomers is presented (Part B), where the incorporation of different neutral comonomers (vinyl biphenyl) and cross-linking monomers (biphenyl and terphenyl) dramatically boosts performance. Direct tends between K⁺ content and CO₂R performance are not observed with the expanded series, highlighting the non-innocent role of the neutral monomer and polymer structure.

Chapter 5 presents a new, Si-based weakly-coordinating anion. A library of anions bearing a variety of R groups is prepared, enabling facile tuning of sterics and solubility. A range of cations employed in chemical reactivity is supported by these anions, including ether-free alkali cations, Ph₃C⁺, and an ethylene/CO copolymerization catalyst [Pd(dppe)(NCMe)Me]⁺ (generated by salt metathesis or protonation of a metal-alkyl bond). Electrochemical studies on the [Bu₄N]⁺ variant show an exceptionally wide stability window for the [^MeSiF]^- anion of 7.5 V in MeCN. The [^C6F5SiF]^- variant can be readily modified to access additional diverse and/or dianionic variants.

Chapter 6 discusses the electrochemical performance of a new class of magnesium electrolytes for next-generation batteries. [Mg(DME)₃][^MeSiF]₂ demonstrates remarkable performance with good stability, moderate conditioning, high coulombic efficiency ( > 96%), and high current density ( ~100 mA cm^-2). However, the sensitivity of the experiments requires careful study of many parameters including impact of MgR₂ additives (identity and concentration), cycling protocol, synthetic route, and the Pt working electrode to understand how the electrochemical performance is impacted. While addition of MgMe₂ improves electrochemical performance, it is not inherently required for reversible Mg deposition and stripping. Following an extensive investigation of the cyclic voltammetry (CV) performance of [Mg(DME)₃][^MeSiF]₂, preliminary screening of additional [Mg(DME)₃][^RSiF]₂ variants is discussed.

Appendix I discusses the characterization of ethylene/α-olefin copolymers generated from monometallic and bimetallic Zr catalysts. The decreased polar monomer incorporation for the bimetallic vs. monometallic catalysts is attributed to the steric clash of larger comonomers with the distal metal site in bimetallic catalysts.

Appendix II discusses the preparation of bulky anthracene phenoxide ligands for Ti and Ta.

Appendix III discusses the preparation of mixed aryl/hydride borates as electrolytes for next-generation Mg batteries.