The future of artificial photosynthesis depends on economic and robust water oxidation catalysts (WOCs). Cobalt-based WOCs are especially promising for knowledge transfer between homogeneous and heterogeneous catalyst design. We introduce the active and stable {CoO} cubane [Co(dpy{OH}O)(OAc)(HO)](ClO) (CoO-dpk) as the first molecular WOC with the characteristic {HO-Co(OR)-OH} edge-site motif representing the sine qua non moiety of the most efficient heterogeneous Co-oxide WOCs. DFT-MD modelings as well as in situ EXAFS measurements indicate the stability of the cubane cage in solution. The stability of CoO-dpk under photocatalytic conditions ([Ru(bpy)]/SO) was underscored with a wide range of further analytical methods and recycling tests. FT-IR monitoring and HR-ESI-MS spectra point to a stable coordination of the acetate ligands, and DFT-MD simulations along with H/H exchange experiments highlight a favorable intramolecular base functionality of the dpy{OH}O ligands. All three ligand types enhance proton mobility at the edge site through a unique bioinspired environment with multiple hydrogen-bonding interactions. In situ XANES experiments under photocatalytic conditions show that the {CoO} core undergoes oxidation to Co(III) or higher valent states, which recover rather slowly to Co(II). Complementary ex situ chemical oxidation experiments with [Ru(bpy)] furthermore indicate that the oxidation of all Co(II) centers of CoO-dpk to Co(III) is not a mandatory prerequisite for oxygen evolution. Moreover, we present the [CoNi(dpy{OH}O)(OAc)(HO)](ClO) (CoNiO-dpk) series as the first mixed Co/Ni-cubane WOCs. They newly bridge homogeneous and heterogeneous catalyst design through fine-tuned edge-site environments of the Co centers.

Ruthenium complexes containing the pentapyridyl ligand 6,6′′‐(methoxy(pyridin‐2‐yl)methylene)di‐2,2′‐bipyridine (L‐OMe) of general formula trans‐[RuII(X)(L‐OMe‐κ‐N5)]n+ (X=Cl, n=1, trans‐1+; X=H2O, n=2, trans‐22+) have been isolated and characterized in solution (by NMR and UV/Vis spectroscopy) and in the solid state by XRD. Both complexes undergo a series of substitution reactions at oxidation state RuII and RuIII when dissolved in aqueous triflic acid–trifluoroethanol solutions as monitored by UV/Vis spectroscopy, and the corresponding rate constants were determined. In particular, aqueous solutions of the RuIII‐Cl complex trans‐[RuIII(Cl)(L‐OMe‐κ‐N5)]2+ (trans‐12+) generates a family of Ru aquo complexes, namely trans‐[RuIII(H2O)(L‐OMe‐κ‐N5)]3+ (trans‐23+), [RuIII(H2O)2(L‐OMe‐κ‐N4)]3+ (trans‐33+), and [RuIII(Cl)(H2O)(L‐OMe‐κ‐N4)]2+ (trans‐42+). Although complex trans‐42+ is a powerful water oxidation catalyst, complex trans‐23+ has only a moderate activity and trans‐33+ shows no activity. A parallel study with related complexes containing the methyl‐substituted ligand 6,6′′‐(1‐pyridin‐2‐yl)ethane‐1,1‐diyl)di‐2,2′‐bipyridine (L‐Me) was carried out. The behavior of all of these catalysts has been rationalized based on substitution kinetics, oxygen evolution kinetics, electrochemical properties, and density functional theory calculations. The best catalyst, trans‐42+, reaches turnover frequencies of 0.71 s−1 using CeIV as a sacrificial oxidant, with oxidative efficiencies above 95 %.

Mastering artificial water oxidation is a key step on moving away from fossil fuels toward a carbon emission-free society. Unfortunately, the crucial chemical transformation of this reaction, the O-O bond formation, is still not well understood, even though there are various known active water oxidation catalysts, such as Ru-based catalysts bearing a Py5 ligand. Those were recently investigated both experimentally and using a static density functional theory (DFT) approach based on geometry optimizations. In this work, we shed light on the O-O formation catalyzed by those Ru-based complexes, utilizing enhanced sampling techniques such as the Bluemoon ensemble and metadynamics together with high-performance DFT-based molecular dynamics simulations. This allowed unprecedented detailed insights into the process of the oxygen-oxygen bond formation and also extended the view on the reaction network and the flexibility of the product state because of the consideration of the dynamics at ambient conditions. Our model system contained both the catalyst and a large number of explicit water molecules which can participate in the reaction and stabilize intermediates. Moreover, it is demonstrated how crucial the choice of the collective variable is in order to capture relevant features of the studied reaction.

In order to rationally design water oxidation catalysts (WOCs), an in-depth understanding of the reaction mechanism is essential. In this study we showcase the complexity of catalytic water oxidation, by elucidating how modifications of the pentapyridyl (Py5) ligand-framework influence the thermodynamics and kinetics of the process. In the reaction mechanism the pyridine-water exchange was identified as a key reaction which appears to determine the reactivity of the Py5-WOCs. Exploring the capabilities of in silico design we show which modifications of the ligand framework appear promising when attempting to improve the catalytic performance of WOCs derived from Py5.

We study the water oxidation mechanism of the cobalt aqua complex [Co(H 2 O) 6 ] 2+ in a photocatalytic setup by means of density functional theory. Assuming a waternucleophilic-attack or radical coupling mechanism, we investigate how the oxidation state and spin configuration change during the catalytic cycle. In addition, different ligand environments are employed by substituting a water ligand with a halide, pyridine, or derivative thereof. This allows exploration of the effect of such ligands on the frontier orbitals and the thermodynamics of the water oxidation process. Moreover, the thermodynamically most promising water oxidation catalyst can be identified by comparing the computed free energy profiles to the one of an "ideal catalyst". Examination of such simple (hypothetical) water oxidation catalysts provides a basis for the derivation of design guidelines, which are highly sought for the development of efficient homogeneous water oxidation catalysts.