What Influences the Water Oxidation Activity of a Bioinspired Molecular Co^II₄O₄ Cubane? An In-Depth Exploration of Catalytic Pathways

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( -OAc)2( 2-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.

Recent Advances in First‐Row Transition Metal Clusters for Photocatalytic Water Splitting

“…Furthermore, Luber and the co‐workers compute the barriers and reaction paths for water boding and O 2 release under theoretical calculation method. These theoretical works show the possibility to tune and design the high performance PSII mimic catalysts …”

“…These theoretical works show the possibility to tune and design the high performance PSII mimic catalysts. [54]…”

Investigating the Structure and Dynamics of Apo‐Photosystem II

“…have done . Another idea is to mimic the cubane structure for example with {Co II 4 O 4 } and {Co III 4 O 4 } cubanes, which have been object of experimental and computational research.…”

“…One possibility to mimic the inorganic catalyst is to use the same components it is built up from like Zhang et al have done. [12] Another idea is to mimic the cubane structure for example with {Co II 4 O 4 } [13][14][15][16][17][18] and {Co III 4 O 4 } [19][20][21] cubanes, which have been object of experimental and computational research.…”

Impact of the Ligand Flexibility and Solvent on the O–O Bond Formation Step in a Highly Active Ruthenium Water Oxidation Catalyst

“…13−15 Additionally, some recent modeling studies of cobalt- and iron-based molecular catalysts for water oxidation have incorporated an explicit solvent for a detailed understanding of the reaction mechanism. 16−18 …”