The utilization of metal-ligand cooperativity for electrocatalytic reduction and catalytic hydration.
Date on Master's Thesis/Doctoral Dissertation
catalysis; small-molecule reduction; electrochemistry; inorganic chemistry
Small molecules are building blocks for developing larger materials. These small molecules could be extremely small, such as hydrogen, or larger such as a nitrile, but their impact on the global economy is massive. This dissertation describes a catalyst for three reactions involving small molecules; 1) the hydrogen evolution reaction, 2) the carbon dioxide reduction reaction, 3) nitrile hydration. The catalyst Zn(DMTH) (DMTH = diacetyl-2-(4-methyl-3-thiosemicarbazonate)-3-(2-pyridinehydrazonato)) use “metal-ligand cooperativity” between the Lewis acid Zn(II) metal ion and an uncoordinated Lewis base nitrogen in the ligand framework to activate substrates. The complex has been analyzed via NMR, UV/Vis, single crystal X-ray crystallography, electrochemical methods, XPS and computationally. Chapter three discusses the ability of Zn(DMTH) and the methylated form Zn(DMTMH) to utilize solution protons to generate H2. Zn(DMTH) and Zn(DMTMH) have similar turnover frequencies (TOF) of ~7000 s-1, but their overpotential differs by ~700 mV. The overpotential difference is determined to be from both electronic effects, and proton rearrangement. These differences are supported by control experiments, computational work (DFT), and isolation of valuable intermediates. Chapter four focuses on the ability of Zn(DMTH) to activate CO2 for reduction to formate. The catalyst has a TOF of ~70 s-1 albeit at a large overpotential (0.9V). The catalyst is also shown to work at lower operating potentials by using an electrochemical hydride (Pt) and a chemical hydride (NaBH4). Zn(DMTH) operates in an unprecedented mechanism where it utilizes metal-ligand cooperativity to deprotonate a methanol where CO2 insertion occurs. This ability allows Zn(DMTH) to bind CO2 directly from air, making it the first CO2 reduction catalyst that can do this. Chapter five expands upon this idea of metal-ligand cooperativity to activate methanol, by using the same process to activate water. This activation of water is shown by the generation of amides from nitriles. Zn(DMTH) behaves as a nitrile hydration catalyst with a large quantity of nitriles including difficult to hydrate nitriles. Optimization reactions occurred with acetonitrile with a TOF of almost 50 h-1 at 0°C. Analysis of thermodynamic parameters indicate a mechanism with contributions from multiple steps.
Cronin, Steven P., "The utilization of metal-ligand cooperativity for electrocatalytic reduction and catalytic hydration." (2020). Electronic Theses and Dissertations. Paper 4040.
Retrieved from https://ir.library.louisville.edu/etd/4040