Polyoxometalate-Based Nickel Clusters as Visible Light-Driven Water Oxidation Catalysts

Three new polyoxometalate­(POM)-based polynuclear nickel clusters, Na24[Ni12(OH)9(CO3)3(PO4)­(SiW9O34)3]·56H2O (1), Na25­[Ni13­(H2O)3­(OH)9­(PO4)4­(SiW9­O34)3]·50H2O (2), and Na50[Ni25­(H2O)2­OH)18­(CO3)2­(PO4)6­(SiW9O34)6]·85H2­O (3) were synthesized and structurally characterized. Compounds 1–3 contain {Ni12}, {Ni13} and {Ni25} core, respectively, connected by the inorganic {OH}, {PO4} and/or {CO3} linkers and encapsulated by the lacunary A-α-{SiW9O34} POM units. Compound 3 represents the currently largest POM-based Ni clusters. All three compounds contain {Ni3O3} quasi-cubane or {Ni4O4} cubane units, which are similar to the natural oxygen-evolving center {Mn4O5Ca} in photosystem II (PSII). Visible light-driven water oxidation experiments with compounds 1–3 as the homogeneous catalysts indicate that all three compounds show good photocatalytic activities. The O2 evolution amount corresponds to a high TON of 128.2 for 1, 147.6 for 2, and 204.5 for 3, respectively. Multiple experiments including dynamic light-scattering, UV–vis absorption, catalysts aged experiments, tetra-n-heptylammonium nitrate (THpANO3) toluene extraction, and capillary electrophoretic measurements results confirm that compounds 1–3 are dominant active catalysts but not Ni2+ ions­(aq) or nickel oxide under the photocatalytic conditions. The above research results indicate a new and all-inorganic polynuclear Ni-based structural model as the visible light-driven water oxidation catalysts.