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{Co4O4} and {CoxNi4–xO4} Cubane Water Oxidation Catalysts as Surface Cut-Outs of Cobalt Oxides

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journal contribution
posted on 27.09.2017, 17:57 by Fangyuan Song, René Moré, Mauro Schilling, Grigory Smolentsev, Nicolo Azzaroli, Thomas Fox, Sandra Luber, Greta R. Patzke
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.