Date of Award
Fall 2023
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Chemistry
First Advisor
Brudvig, Gary
Abstract
In order to meet rising global energy and material demands while mitigating negative effects on our climate, renewable energy sources must offer a competitive alternative to fossil fuels. Understanding chemical activation of small molecules such as water can provide a basis for leveraging electrocatalytic reactions using electricity from photovoltaics paired with catalysts for chemical transformations. For an efficient and widely applicable system, catalysts should be robust and use cheap, earth-abundant elements. This thesis investigates Cu-based complexes for electrocatalytic transformations, structural studies, and bond activation. First, we investigate the influence of ligand structure on water oxidation electrocatalysis. In comparison to the heterogeneous counterparts, molecular water oxidation catalysts (WOCs) enable tunability, mechanistic understanding, and high turnover frequencies. Though ligand modification to influence catalytic properties is a key advantage of molecular systems, there remain few examples of systematic studies of these effects. PyalkH (2-pyridyl-2-propanol) is an oxidatively resistant ligand that has been shown to complex with copper(II) and catalyze water oxidation, providing an attractive scaffold for systematic ligand tuning. Here, we report copper complexes with electron-donating (methoxy) and -withdrawing (ester) groups at the para-position of the pyalk ligand. While the modified complexes show activity for water oxidation, lowered Faradaic efficiency in comparison to the parent complex highlights the importance of stability considerations for catalyst tuning. Investigations of copper complexes for water oxidation led to the discovery of a Cu(II)-(L-CF3)2 complex (L-CF3 = 2,2,2-trifluoro-N-[2-(pyridin-2-yl)propan-2-yl]acetamide) with a distorted “seesaw” geometry. It has the shortest crystallographic CF···Cu distances yet reported, to the best of our knowledge (<2.6 Å), for which computational and experimental data indicate a secondary bonding interaction. Comparisons with a CCl3 version and one without ligand backbone gem-dimethyl groups suggest a steric origin for the distorted geometry. This close CF···M interaction is of interest for understanding the activation of inert C–F bonds, and CF···M interactions have been reported to play a role in catalytic conversions. Most energy-relevant chemical transformations, including water oxidation by the family of Cu(pyalk)2 catalysts, involve the transfer of protons and electrons. Understanding how these move, whether in a stepwise or concerted fashion, can provide a more detailed mechanistic understanding of energy-related reactions. To this end, a high-valent formally copper(III) complex, [Cu(pyalk)2]+, was isolated and was found to undergo fast proton-coupled electron transfer with phenol and hydrocarbon substrates. Analysis of kinetic data for reactions with both types of substrates suggests that [Cu(pyalk)2]+ reacts through a concerted proton-electron transfer (CPET) pathway. The copper complex was compared to its isostructural nickel(III) analogue, [Ni(pyalk)2]+, which we propose may undergo CPET through a more basic asynchronous pathway than the copper analogue, which may explain their difference in reactivity. Finally, we present a new binuclear copper complex featuring a bridging pyridine-N-oxide motif, Cu2(panoap)(OAc)2 (panoap = 2,6-bis((2-(pyridin-2-yl)propan-2-yl)carbamoyl)pyridine 1-oxide). Incorporating multiple metal centers into one ligand scaffold is an attractive approach for developing new WOCs with lower overpotentials that operate under mild conditions such as neutral pH. While the complex as synthesized is not competent for electrocatalytic water oxidation, comparisons with mononuclear complexes with similar structural characteristics provide insight on ligand design principles for future development of efficient binuclear copper catalysts.
Recommended Citation
Cody, Claire C., "Copper Complexes for Water Oxidation Electrocatalysis and Strong Bond Activation" (2023). Yale Graduate School of Arts and Sciences Dissertations. 1235.
https://elischolar.library.yale.edu/gsas_dissertations/1235
