Synthesis, Assembly And Water Splitting Study Of The POM-Based High-Nuclearity Transition Metal-Oxygen Clusters

The solar-driven production of H₂ and O₂ from water based-on earth-abundant materials represents a promising and desirable approach to sustainably provide clean and renewable fuel. Recently, exploring visible light-driven molecular photocatalysts with well-defined structure for water splitting are currently a major focus for solar energy harvesting,transformation and storage due to the ease of their structure, component and property tuning.Polyoxometalates（POMs）, a typical class of molecular metal-oxo clusters with oxygen-rich surfaces, could undergo fast reversible and stepwise multielectron-transfer reactions without changing their structures, which have made them as the efficient individual functional components for constructing the molecule photocatalysts. POMs, composed of such as W, Mo,V, Nb, Ta centers with their highest oxidation states, could be used as the oxidatively resistant inorganic multi-dentate ligands instead of organic ligands for constructing well-defined model systems of solar photoredox processes in complex architectures. In this paper, we have synthesized a series of POM-based novel high-nuclear clusters with excellent performances by the introduction of transition metal, which based on the point of molecular design,self-assembly. We have studied the synthesis, structure and photocatalytic water oxidation properties of these compounds, which provide not only an unprecedented opportunity for further design and synthesis of more efficient and stable POM-based high-nuclearity clusters but also a significant opportunity to further broaden the application of high-nuclearity clusters.With a conventional synthesis procedure, seven POM-based high-nuclear clusters have been synthesized, and characterized by element analysis, IR, UV-Vis, TG, and single-crystal X-ray analysis etc.. And the photocatalytic water oxidation properties of these compounds have been investigated.1. Trivacant Keggin POMs and CoCl₂ followed by the addition of Na₃PO₄ led to a series of Si-centered, Ge-centered, P-centered, and As-centered POM-based Co-Pi clusters. All these compounds contain the {Co₄O₄} cubane core which is structurally analogous to the[Mn₃CaO₄] core of the oxygen-evolving center（OEC） in photosystem II（PSII）. Variables of photocatalytic reaction conditions, including catalyst concentrations, sacrificial electron acceptor concentrations, p Hs and photosensitizers concentrations were systemically investigated. Multiple experiments（DLS,31 P NMR, UV-vis, ICP-MS, XPS and catalyst aged experiments etc.） results collectively confirmed that compounds 1-4 maintain their structural integrity under the photocatalytic conditions. They are genuine, effective, and stablemolecular water oxidation catalyst.[{Co₄（OH）₃PO₄}₄（SiW₉O₃4）₄]^32–（1）[{Co₄（OH）₃PO₄}₄（GeW₉O₃4）₄]^32–（2）[{Co₄（OH）₃PO₄}₄（PW₉O₃4）₄]^28–（3）[{Co₄（OH）₃PO₄}₄（AsW9O₃4）₄]^28–（4）2. Compounds 5-7 was synthesized in a simple one-pot reaction of trivacant{A-α-SiW₉O₃4} POM, NiCl₂ and Na₃PO₄. All three compounds contain the {Ni₃O₄}quasi-cubane or {Ni-₄O₄} cubane cores, which not only are structurally similar to the natural OEC {Mn₄O₅Ca} cluster in PSII, but also show good catalytic activities in the visible light-driven water oxidation process. Multiple experiments（DLS, UV-vis, ICP-MS,capillary electrophoretic measurements and catalyst aged experiment etc.） confirmed that compounds 5-7 were efficient, stable water oxidation catalyst.[Ni₁2（OH）₉（CO₃）₃（PO₄）（SiW₉O₃4）₄)₃]^24–（5）[Ni₁3（H₂O）₃（OH）₉（PO₄）₄（SiW₉O₃4）₄)₃]^25–（6）[Ni₂5（H₂O）₂（OH）₁8（CO₃）₂（PO₄）₆（SiW₉O₃4）₄)₆]^50–（7）...