Redox-Inert Cations Enhancing Water Oxidation Activity: The Crucial Role of Flexibility

The devastating effects of global climate change, which is in part caused by anthropogenic CO2 emissions from fossil fuels, force us to find clean fuels produced by environmentally friendly methods. Splitting water into oxygen and hydrogen using solar light is one possible solution, the successful implementation of which depends not least on the development of efficient water oxidation catalysts (WOCs). With the water splitting reaction of photosystem II, specifically the oxygen evolving complex, which features a cubane structure with a redox-inert metal center, nature provides us with clues for the construction of such WOCs. Any approach more sophisticated than a simple trial-and-error method will rely on knowledge of mechanistic details of biomimetic catalysts. Recently, a step in that direction has been made with computational investigations of the different possible catalytic pathways of a {Co­(II)4O4} cubane-based WOC. The present study, which focuses on the {Co­(II)3LnO4} (Ln = Er, Tm) cubane family, is complementary to the previous one and sheds light on the importance of redox inert Ln3+ cations for the mechanism of water oxidation. Using density functional theory and an explicit solvation shell, as well as a solvent continuum model, the WOCs are compared in terms of relative free energies of their catalytic states, and the reaction barriers of water attack and oxygen release. Furthermore, in-depth investigations into the electronic and molecular structures of the catalysts are carried out, resulting in the discovery of a flexibility of the cubane cage during the catalytic cycle.