{CoO} and {CoNiO} 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 {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.