UCLA

In biology, chemical species (such as oxygen (O2)) and their local concentrations are highly regulated. One example is the reduction of dinitrogen (N2) to ammonia by aerobic bacteria. In this scenario, the O2 sensitive nitrogenase enzyme provides electrons for N2 reduction through aerobic respiration. The tandem reactions of N2 fixation and aerobic respiration are only possible due to the buildup of a chemical O2 concentration gradient where the O2 sensitive nitrogenase is situated in an O2-free region and aerobic respiration in an O2-rich region. Inspired by biological microscopic chemical gradients, my research aims to use electricity and nanomaterials to mimic these natural phenomena and apply them to synthetic organometallic chemistry to offer new pathways in carbon maintenance. Homogeneous organometallic complexes are often utilized for small molecule activation because they offer a delicate synthetic control and can be characterized extensively, but can suffer from air or moisture sensitivity. Therefore, the marriage of traditional organometallic chemistry with electricity and nanoscience can help reconcile this incompatibility by offering new reactivity or unforeseen reaction routes. The projects described below illustrate the spatiotemporal control we can achieve over electroactive chemical species to induce novel catalytic routes and difficult-to-achieve transformations.In my first research project (Chapter 2), I developed an electricity-powered catalytic system mediated by a RhII metalloradical complex for the conversion of methane (CH4) to methanol (CH3OH) with air as the terminal oxidant. The challenge here is that the RhII complex kinetically reacts with O2 faster than with CH4. I reconciled this incompatibility by introducing a nanowire electrode, which, when combined with a reducing potential, created a steep O2 gradient within the wire array while also regenerating the RhII complex. Thus, C‒H activation occurred anaerobically while CH3OH synthesis proceeded aerobically together in one catalytic cycle. Moreover, the reaction rate of CH4 activation increased 220,000-fold within the wire array, achieving a turnover number of 52,000 in 24 hours. Additionally, other light alkanes (such as ethane, propane, and toluene) were used as substrates and oxidation to primary alcohols was acheived in all cases. Building off the work presented in Chapter 2, the work described in Chapter 3 uncovers the identity and the role of the terminal oxidant responsible for generating CH3OH from the methylated Rh complex. While O2 reduction at the silicon nanowire array working electrode creates an anaerobic region within the wire array in air, it simultaneously generates the reactive oxygen species necessary for alcohol formation. The utilization of electron paramagnetic resonance spectroscopy identified the electrogenerated oxidant as superoxide and reactions with a selective chromophore quantified the rate of superoxide generation. While superoxide is the immediately generated species at the electrode, it is presumed that chemical and/or further electrode reactions convert superoxide into a hydroperoxyl or hydroperoxide species, which is ultimately responsible for the oxidation reaction. Chapter 4 investigates the role of the nanowire array’s efficacy in creating a microscopic compartment for the RhII metalloradical-assisted CH4 activation. While quantitative analysis of microscopic compartmentalization is typically applied to biochemical reactions, in this work we translated this quantitative understanding to organometallic cascade reactions in terms of reaction efficiency. We analyzed the reaction efficiency of our nanowire array compartment as the ratio of product molecules leaving the nanowire array to substrate molecules that enter the wire array compartment. In order to properly quantify this reaction metric, we utilized the experimental data obtained in Chapter 3. In conclusion, we found good agreement between our experimental results and the results from our semi-quantitative kinetic model. The final chapter of my dissertation (Chapter 5) aims to translate the O2 gradient observed previously to an electrochemical carbon monoxide (CO) gradient, from carbon dioxide (CO2) reduction, to control the microstructure of synthetic polyketones. The ultimate goal of this project is to integrate, in one pot, the electroreduction of CO2 to CO and the palladium (Pd) assisted copolymerization of CO and ethylene (C2H4). While polyketone synthesis typically follows a perfectly alternating pattern (1:1 stoichiometry between CO and C2H4), recent work has focused on utilizing Pd catalysts with asymmetric ligand frameworks to induce a non-alternating structure. The goal of this effort is to reduce the CO content (through extra C2H4 insertions) in order to improve the degradability of the polymer while maintaining its durability. However, non-alternating copolymerization still requires modulation of both the monomer feed ratio (C2H4 relative to CO) and reaction temperature to control the extent of CO insertion. Based on the previously observed chemical O2 gradient, our hypothesis here is that electricity and a nanowire array will allow us to spatiotemporally control the local concentration of CO and ultimately its incorporation into the polymer. During my graduate career, I have shown that interfacing electrochemistry and nanomaterials with homogeneous catalysis can offer new insight into fundamental chemistry and push the boundaries of what was thought was possible with traditional organometallic chemistry. The utilization of electrochemically generated concentration gradients is a unique phenomenon that can provide an avenue to bypass inherent incompatibilities and enable novel reactivities.