Presentation Title

Energetic and intramolecular characterization of water oxidation catalyzed by mononuclear metal complexes

Start Date

November 2016

End Date

November 2016

Location

HUB 302-#124

Type of Presentation

Poster

Abstract

Water oxidation produces protons that can be reduced for use as a source of clean and renewable carbon-free fuel. Ruthenium complexes can be effective water oxidation catalysts but are too costly for large-scale application. Iron displays similarities to ruthenium and is much more abundant but has not been shown to be effective as a catalyst in the water oxidation mechanism as suggested for ruthenium. Accordingly, we employ density functional theory (DFT) calculations of eleven mononuclear ruthenium catalysts and compare these with analogous iron catalysts to investigate the energetic bottlenecks along the water oxidation mechanism. From these results, four novel catalysts with varying electron-donating and electron-withdrawing character are proposed and characterized. The energetic features along the mechanism vary across the set of ruthenium and iron catalysts, in some instances even changing which step becomes the primary barrier to catalyst performance. In the final mechanistic step where O2 is released and replaced by water, further intermolecular characterizations were performed to determine the most stable coordination of the O2 along with appropriate spin assignment for the catalyst complex. The original 11 mononuclear iron catalysts showed a 12% higher potential barrier on average than the analogous ruthenium catalysts, which could render them ineffective as water oxidation catalysts. However, the novel catalysts display kinetic and thermodynamic performance similar to ruthenium catalysts, suggesting that these may be suitable catalysts. Our results suggest that different ligands subtly influence the intramolecular features of the catalyst but can lead to dramatic impact on the energetics. With the appropriate ligand design, our results indicate that mononuclear iron catalysts designed with a bidentate and a tridentate ligand could serve as successful water oxidation catalysts.

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Energetic and intramolecular characterization of water oxidation catalyzed by mononuclear metal complexes

HUB 302-#124

Water oxidation produces protons that can be reduced for use as a source of clean and renewable carbon-free fuel. Ruthenium complexes can be effective water oxidation catalysts but are too costly for large-scale application. Iron displays similarities to ruthenium and is much more abundant but has not been shown to be effective as a catalyst in the water oxidation mechanism as suggested for ruthenium. Accordingly, we employ density functional theory (DFT) calculations of eleven mononuclear ruthenium catalysts and compare these with analogous iron catalysts to investigate the energetic bottlenecks along the water oxidation mechanism. From these results, four novel catalysts with varying electron-donating and electron-withdrawing character are proposed and characterized. The energetic features along the mechanism vary across the set of ruthenium and iron catalysts, in some instances even changing which step becomes the primary barrier to catalyst performance. In the final mechanistic step where O2 is released and replaced by water, further intermolecular characterizations were performed to determine the most stable coordination of the O2 along with appropriate spin assignment for the catalyst complex. The original 11 mononuclear iron catalysts showed a 12% higher potential barrier on average than the analogous ruthenium catalysts, which could render them ineffective as water oxidation catalysts. However, the novel catalysts display kinetic and thermodynamic performance similar to ruthenium catalysts, suggesting that these may be suitable catalysts. Our results suggest that different ligands subtly influence the intramolecular features of the catalyst but can lead to dramatic impact on the energetics. With the appropriate ligand design, our results indicate that mononuclear iron catalysts designed with a bidentate and a tridentate ligand could serve as successful water oxidation catalysts.