Earth Abundant Transition Metal Molecular Complexes for Water Splitting

Abstract

Further development and understanding of the chemistry of earth-abundant transition-metal molecular catalysts for water splitting will aid development of a sustainable hydrogen economy. If dihydrogen can be produced sustainably, such as by water splitting, dihydrogen can assist in our transition to a sustainable society, as it is a promising as a clean fuel, for energy storage, and for ammonia and other chemical syntheses. On the reductive side of water splitting, Chapters 2 and 3 of this thesis study cobalt bis(benzenedithiolate) type catalysts for the hydrogen evolution reaction (HER). This family of catalysts is highly active for HER and our group has previously studied their immobilization through physisorption to graphitic electrode surfaces. However, in these studies the catalytic activity declined over time, and this was thought to be due to the catalyst dissociating from the electrode surface. Chapter 2 describes the synthesis and characterization of a new amine modified cobalt bis(benzenedithiolate) catalyst, which is then covalently attached to graphene oxide. The drop-cast thin films of this material are shown to be highly active for HER and more durable than the physisorbed analog, confirming that dephysisorption is a decay method for the physisorbed systems. Detailed electrocatalysis investigations expand our understanding of thin film electrocatalysis. Controlled potential electrolysis experiments show slow decline in catalytic activity over 8 hours, primarily due to demetallation. To further understand how this highly active catalyst family performs HER and how they decay, their mechanism was further investigated in Chapter 3. It was previously proposed that the starting Co(III) catalyst, when reduced by one electron distorted from square planar to tetrahedral which would promote de-physisorption during catalysis. Instead, this Co(II) complex is shown to be a St = 1/2 square planar complex. This suggests that dephysisorption of this intermediate is not the primary reason for deactivation of the physisorbed catalyst systems. Reactivity studies suggest that Co(II) bis(benzenedichlorodithiolate) reacts with acid, but under the explored conditions does not make hydrogen, suggesting Co bis(benzenedithiolate)s could be useful catalysts beyond HER. On the oxidative side of water splitting, in collaborative studies, Chapters 4, 5, and 6 spectroscopically and theoretically investigate transition metal complexes with reactive metal-oxygen moieties. First, with the Nam Group in Seoul Korea, in Chapter 4, a reactive formal Ni(IV)-oxo species is shown to be best described as a St = 1 Ni(III)-oxyl species. The radical nature of this species may contribute to its high reactivity, while simultaneously avoiding the infamous “oxo-wall”. Second, in Chapter 5, with the Ray Group in Berlin Germany, a Fe(II)-phenoxyl radical species is characterized with in-depth spectroscopy and theoretical calculations. Interestingly, this unusual species maintains an Fe(II) oxidation state in the presence of an oxidizing phenoxy radical ligand, due to electronically tuning of its co-ligand. This shows that careful ligand tuning can temporarily stabilize reactive radicals at low-valent metal centers for further reactivity. Finally, in Chapter 6, with the Nam group, a series of Fe(III)-peroxide complexes with drastically different reactivity are characterized spectroscopically and theoretically. Electronic structure calculations allow the assignment of the main characteristic electronic transition, which shows the smaller co-ligand ring size correlates to a stronger Fe-peroxide bond. These investigations on reactive intermediates provide spectroscopic markers and chemical information for future studies and inform our understanding of the oxidative side of water splitting, hydrogen fuel cell reactions, and oxidative reactions in bioinorganic chemistry.