Iridium Complexes with Novel Ligand Architectures for Efficient and Robust Water Oxidation Catalysis

The constantly growing global energy demand and the negative environmental and social impacts from extensive fossil fuels uses have triggered intensive research to
discover and utilize abundant and sustainable solar energy. One attractive approach is the use of solar energy in chemical bonds as fuels through artificial photosynthetic redox processes. While great attention has been placed on the fuel producing half-reaction, the oxidative half-reaction is arguably more challenging due to the high energy barrier
required to overcome. A desirable oxidative process currently under investigation is the oxidation of water, an abundant and benign feedstock. The direct use of O2 released from water oxidation in fuel cells and combustion applications makes water oxidation superior to many other currently studied oxidative processes. Significant efforts have been devoted to the development of water oxidation catalysts, but great challenges still remain due to the difficulty in accessing and stabilizing the required high energetic intermediates as well as the complexity of the sequential transfer of four electrons and four protons
necessary for the O-O bond formation. This thesis documents studies involving various iridium(III) water oxidation catalysts (WOCs) with novel ligand architectures that are robust and tunable and investigates their catalytic performance under various conditions in depth. These
complexes exhibit efficient and durable catalytic activities that are comparable with many benchmark molecular WOCs. Moreover, the electronic properties of these tunable WOCs were explored through electrochemical studies and theoretical modeling. These results aid to illustrate the effects of different substituents on the reactivity and electronic stability of these water oxidation catalysts. The mechanistic exploration of these WOCs provides valuable insights into the catalytic pathways and offers practical guidance for future catalyst design and structural optimization. Chapter 1 provides an introduction to the development of efficient and robust water oxidation catalysts and their utilization in solar energy schemes to harness chemical fuels. Chapter 2 reports a new family of tetradentate iridium(III) bis-pyridine-2-sulfonamide
(bpsa) complexes with a resilient wrap-around ligand sphere. These iridium complexes catalyzed water oxidation reaction efficiently and demonstrated electronic tunability on a
molecular level. The robustness of these WOCs was investigated by electrochemical quartz crystal microbalance (EQCM) and dynamic light scattering (DLS), confirming that
molecular species rather than IrOx particles formed by ligand degradation are responsible for the observed water oxidation activity. Building on the work on the iridium(III) bpsa complexes presented in chapter 2, the subsequent chapter investigates systematic modification of the sulfonamide linker moiety within the ligand scaffold through substitution with electron-donating or electronwithdrawing
substituents, allowing the synthetic tuning of the electronic properties of these iridium WOCs. The differences in the electronic structures of these complexes lead to significantly different kinetics and robustness in water oxidation catalysis. Factors that were found to affect the catalytic performance of these WOCs by cyclic voltammetry (CV) and DFT calculations include the oxidative driving force provided by the oxidized catalyst, the accessibility of highly oxidized species, and the electronic stability of the
active species involved in the catalytic process. Chapter 4 exploits a series of efficient iridium Cp* WOCs with the newly designed donor-flexible pyridylideneamide (PYA) ligands which can coordinate to the metal center
as a -acidic imine or as a -basic zwitterionic pyridinium amide. The dynamic ligands exhibiting two resonance forms facilitate the stabilization of a variety of different iridium
oxidation states and as a consequence, water oxidation catalysis. Such properties also minimize catalyst deactivation, which is likely to be responsible for producing high turnover numbers (TONs) in the iridium PYA systems with the catalysts being active for over 28 days, reaching up to 86,000 turnovers. Further modifications on these catalysts, such as the incorporation of the electron-donating methoxy substituents on the aryl ring and the alteration of the donor strength of the PYA ligands, allowed significant
enhancement of the catalytic activity in water oxidation catalysis. Chapter 5 summarizes the projects and provides an outlook of water oxidation catalysis for energy applications.