{Co₄O₄} and {Co_xNi_4–xO₄} Cubane Water Oxidation Catalysts as Surface Cut-Outs of Cobalt Oxides

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 {Co^II₄O₄} cubane [Co^II₄(dpy­{OH}­O)₄­(OAc)₂(H₂O)₂]­(ClO₄)₂ (Co₄O₄-dpk) as the first molecular WOC with the characteristic {H₂O-Co₂(OR)₂-OH₂} 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 Co₄O₄-dpk under photocatalytic conditions ([Ru­(bpy)₃]²⁺/S₂O₈^2–) 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 ¹H/²H 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 {Co^II₄O₄} 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)₃]³⁺ furthermore indicate that the oxidation of all Co­(II) centers of Co₄O₄-dpk to Co­(III) is not a mandatory prerequisite for oxygen evolution. Moreover, we present the [Co^II_xNi_4–x­(dpy­{OH}­O)₄­(OAc)₂(H₂O)₂]­(ClO₄)₂ (Co_xNi_4–xO₄-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.