{Co4O4} and {CoxNi4–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 {CoII4O4} cubane [CoII4(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+/S2O82–) 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 {CoII4O4} 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 [CoIIxNi4–x­(dpy­{OH}­O)4­(OAc)2(H2O)2]­(ClO4)2 (CoxNi4–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.