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

Three new polyoxometalate(POM)-based polynuclear nickel clusters, Na-24[Ni-12(OH)(9)(CO3)(3)(PO4)(SiW9O34)(3)] 56H(2)O (1), Na-25[Ni-13(H2O)(3)(OH)(9)(PO4)(4)(SiW9O34)(3)] 50H(2)O (2), and Na-50[Ni-25(H2O)(2)OH)(18)(CO3)(2)(PO4)(6)(SiW9O34)(6)] 85H(2)O (3) were synthesized and structurally characterized. Compounds 1-3 contain {Ni-12}, {Ni-13} and {Ni-25} core, respectively, connected by the inorganic {OH}, {PO4} and/or {CO3} linkers and encapsulated by the lacunary A-alpha-{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 (THpANO(3)) 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.