Although the {CaMn4O5} oxygen evolving complex (OEC) of photosystem II is a major paradigm for water oxidation catalyst (WOC) development, the comprehensive translation of its key features into active molecular WOCs remains challenging. The [Co(II)3Ln(hmp)4(OAc)5H2O] ({Co(II)3Ln(OR)4}; Ln = Ho-Yb, hmp = 2-(hydroxymethyl)pyridine) cubane WOC series is introduced as a new springboard to address crucial design parameters, ranging from nuclearity and redox-inactive promoters to operational stability and ligand exchange properties. The {Co(II)3Ln(OR)4} cubanes promote bioinspired WOC design by newly combining Ln(3+) centers as redox-inactive Ca(2+) analogues with flexible aqua-/acetate ligands into active and stable WOCs (max. TON/TOF values of 211/9 s(-1)). Furthermore, they open up the important family of 3d-4f complexes for photocatalytic applications. The stability of the {Co(II)3Ln(OR)4} WOCs under photocatalytic conditions is demonstrated with a comprehensive analytical strategy including trace metal analyses and solution-based X-ray absorption spectroscopy (XAS) investigations. The productive influence of the Ln(3+) centers is linked to favorable ligand mobility, and the experimental trends are substantiated with Born-Oppenheimer molecular dynamics studies.

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.

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.

Bio-mimetic catalysts such as LnCo (OR) (Ln=Er, Tm; OR=alkoxide) cubanes have recently been in the focus of research for artificial water oxidation processes. Previously, the remarkable adaptability with respect to ligand shell, nuclear structure as well as protonation and oxidation states of those catalysts has been shown to be beneficial for the water oxidation process. We further explored the structural flexibility of those catalysts and present here a series of novel structures in which one metal center is pulled out of the cubane cage. This leads to an open cubane core, which is to some extent reminiscent of observed open/closed cubane-core forms of the oxygen-evolving complex in nature's photosystem II. We investigate how those open cubane core models alter the thermodynamics of the water oxidation cycle and how different solvation approaches influence their stability.

We investigate ligand-exchange reactions of a biomimetic Co(II)-based heterocubane complex in aqueous solution by means of various approaches for consideration of solvent effects. Static calculations based on geometry optimizations carried out in vacuum, with solvent continuum models, or with several explicit solvent molecules have been carried out as well as density functional theory (DFT)-based molecular dynamics simulations. In addition, reaction pathways and barriers have been elucidated via nudged elastic band calculations and metadynamics. The results show that static approaches with approximate consideration of the solvent environment lead to reaction energies, which may change drastically depending on the method employed. A more sophisticated approach is DFT-molecular dynamics at ambient conditions with full solvation, i.e. enough solvent molecules to retain bulk water properties far from the solute, which, however, comes with a much higher computational cost. The investigated example of the exchange of an acetate ligand by a hydroxide demonstrates that entropic contributions can be vital and consideration of electronic energies alone may be a rather rough approximation.