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

著者 Song F , Moré R , Schilling M , Smolentsev G , Azzaroli N , Fox T , Luber S , Patzke GR
J Am Chem Soc.2017 Sep 27 ; ():.
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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(II)4O4} cubane [Co(II)4(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 (1)H/(2)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 {Co(II)4O4} 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)xNi4-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.
PMID: 28953394 [PubMed - as supplied by publisher]
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