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What influences the water oxidation activity of a bioinspired molecular CoII4O4Cubane? An in-depth exploration of catalytic pathways


Hodel, Florian H; Luber, Sandra (2016). What influences the water oxidation activity of a bioinspired molecular CoII4O4Cubane? An in-depth exploration of catalytic pathways. ACS Catalysis, 6(3):1505-1517.

Abstract

Solar light to chemical energy conversion is an important topic of research due to global climate change and an increasing shortage of fossil fuels. Artificial photosynthesis, as a possible solution to these issues, is strongly dependent on efficient water oxidation. The exact way in which molecular water oxidation catalysts (WOCs), in particular biomimetic cubanes, perform the task of splitting water into oxygen, protons, and electrons still remains unclear to a large extent. We investigated the reaction mechanism of the recently presented first Co(II)-based WOC, [CoII4(hmp)4(μ-OAc)2(μ2-OAc)2(H2O)2] (hmp = 2-(hydroxymethyl)pyridine) [Evangelisti, F.; Güttinger, R.; Moré, R.; Luber, S.; Patzke, G. R. J. Am. Chem. Soc. 2013, 135, 18734−18737], which is one of the rare stable homogeneous cubane-type WOCs and the design of which has been inspired by nature’s oxygen evolving complex of photosystem II (PSII). Two possible different catalytic cycles have been envisioned: A single-site pathway involving one cobalt center and a water attack on an oxo ligand or, alternatively, an oxo–oxo coupling pathway where, after the replacement of an acetate ligand by water, two terminal oxo ligands of the cubane couple and are released as O2. Using density functional theory and an explicit solvation shell, we compare relative free energies of all states of the catalytic pathways, also with different ligand environments, and analyze the stability and reactivity of each catalytic state in detail. Furthermore, we compute barriers and reaction paths for water attack and O2 release steps. With this knowledge at hand, we propose possibilities to tune catalytic activity paving the way to informed design of high-performance PSII mimics.

Abstract

Solar light to chemical energy conversion is an important topic of research due to global climate change and an increasing shortage of fossil fuels. Artificial photosynthesis, as a possible solution to these issues, is strongly dependent on efficient water oxidation. The exact way in which molecular water oxidation catalysts (WOCs), in particular biomimetic cubanes, perform the task of splitting water into oxygen, protons, and electrons still remains unclear to a large extent. We investigated the reaction mechanism of the recently presented first Co(II)-based WOC, [CoII4(hmp)4(μ-OAc)2(μ2-OAc)2(H2O)2] (hmp = 2-(hydroxymethyl)pyridine) [Evangelisti, F.; Güttinger, R.; Moré, R.; Luber, S.; Patzke, G. R. J. Am. Chem. Soc. 2013, 135, 18734−18737], which is one of the rare stable homogeneous cubane-type WOCs and the design of which has been inspired by nature’s oxygen evolving complex of photosystem II (PSII). Two possible different catalytic cycles have been envisioned: A single-site pathway involving one cobalt center and a water attack on an oxo ligand or, alternatively, an oxo–oxo coupling pathway where, after the replacement of an acetate ligand by water, two terminal oxo ligands of the cubane couple and are released as O2. Using density functional theory and an explicit solvation shell, we compare relative free energies of all states of the catalytic pathways, also with different ligand environments, and analyze the stability and reactivity of each catalytic state in detail. Furthermore, we compute barriers and reaction paths for water attack and O2 release steps. With this knowledge at hand, we propose possibilities to tune catalytic activity paving the way to informed design of high-performance PSII mimics.

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Item Type:Journal Article, not refereed, original work
Communities & Collections:07 Faculty of Science > Department of Chemistry
Dewey Decimal Classification:540 Chemistry
Language:English
Date:2016
Deposited On:16 Dec 2016 10:06
Last Modified:08 Dec 2017 21:17
Publisher:American Chemical Society (ACS)
ISSN:2155-5435
Additional Information:This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Catalysis, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acscatal.5b02507
Publisher DOI:https://doi.org/10.1021/acscatal.5b02507

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