This PhD thesis is focused on the development of molecular water oxidation catalysts (WOCs) based on first row transition metals, which are attractive because of their abundancy and low price. However, earth-abundant metals are labile in solvents as water and their electronic structure doesn’t allow low energy access to multiple oxidation states, necessary to trigger the oxidation of water to dioxygen. In order to overcome these challenges an option is to rationally design ligands that strongly attached to the metal even in aqueous media and that allow and also stabilize sufficiently higher oxidation states.
The third Chapter of this thesis, shows how a tetra-amidate macrocyclic ligand influences the stability and activity of its copper complex as a WOC. The catalyst has been studied using several techniques such as electrochemistry, UV-Vis, EPR, XAS and DFT calculations, which made possible to associate the high stability and activity at neutral pH, to the non-innocent character of the ligand, which participates actively in the formation of ligand based radical cations as intermediates in the oxidation of water to dioxygen.
In the fourth Chapter, the highly efficient tetra-amidate ligand reported in Chapter 3 is modified adding two pyrenes moieties with the aim of immobilize the catalyst on conducting surface generating new molecular anodes. Taking advantage of the capacity of pyrene group to electropolymerize at oxidative potentials, I successfully manage to anchor the corresponding Cu-complexes on ITO electrodes. A detailed electrochemical analysis proved the capability of the new molecular anodes to perform water oxidation at neutral and basic pH.
The last project presented in Chapter 5, focuses on the determination of the nature of the water oxidation active species in a pentanuclear Fe complex, Fe5. The stability of a this Fe5 was carefully studied under catalytic conditions. The electrochemical data collected indicated the formation of FeOx on surface of the working electrode under turnover conditions. The analysis of the electrode surface by microscopic and spectroscopic techniques such as EDX, SEM, XPS and XAS confirmed that hematite (Fe2O3) is the active species responsible for the water oxidation activity.
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