Ultrasmall Abundant Metal-Based Clusters as Oxygen-Evolving Catalysts.

The oxygen evolution reaction is a crucial step in water electrolysis to develop clean and renewable energy. Although noble metal-based catalysts have demonstrated high activity for the oxygen evolution reaction, their application is limited by their high cost and low availability. Here we report the use of a molecule-to-cluster strategy for preparing ultrasmall trimetallic clusters by using the polyoxometalate molecule as a precursor. Ultrafine (0.8 nm) transition-metal clusters with controllable chemical composition are obtained. The transition-metal clusters enable highly efficient oxygen evolution through water electrolysis in alkaline media, manifested by an overpotential of 192 mV at 10 mA cm-2, a low Tafel slope of 36 mV dec-1, and long-term stability for 30 h of electrolysis. We note, however, that besides the excellent performance as an oxygen evolution catalyst, our molecule-to-cluster strategy provides a means to achieve well-defined transition-metal clusters in the subnanometer regime, which potentially can have an impact on several other applications.

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 Ni(2+) 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.

Polyoxometalate-based cobalt-phosphate molecular catalysts for visible light-driven water oxidation.

A series of all-inorganic, abundant-metal-based, high-nuclearity cobalt-phosphate (Co-Pi) molecular catalysts [{Co4(OH)3(PO4)}4(SiW9O34)4](32-) (1), [{Co4(OH)3(PO4)}4(GeW9O34)4](32-) (2), [{Co4(OH)3(PO4)}4(PW9O34)4](28-) (3), and [{Co4(OH)3(PO4)}4(AsW9O34)4](28-) (4) were synthesized and shown to be highly effective at photocatalytic water oxidation. The {Co16(PO4)4} cluster contains a Co4O4 cubane which is structurally analogous to the [Mn3CaO4] core of the oxygen-evolving complex (OEC) in photosystem II (PSII). Compounds 1-4 were shown to be the first POM-based Co-Pi-cluster molecular catalysts for visible light-driven water oxidation, thus serving as a functional model of the OEC in PSII. The systematic synthesis of four isostructural analogues allowed for investigating the influence of different heteroatoms in the POM ligands on the photocatalytic activities of these Co-Pi cluster WOCs. Further, the POM-based photocatalysts readily recrystallized from the photocatalytic reaction systems with the polyoxoanion structures unchanged, which together with the laser flash photolysis, dynamic light-scattering, (31)P NMR, UV-vis absorption, POM extraction, and ICP-MS analysis results collectively confirmed that compounds 1-4 maintain their structural integrity under the photocatalytic conditions. This study provides not only a valuable molecular model of the "Co-Pi" catalysts with a well-defined structure but also an unprecedented opportunity to fine-tune high-nuclearity POM clusters for visible light-driven water splitting.

Inorganic crown ethers: sulfate-based Preyssler polyoxometalates.

A tungsten crown: The {SO(4)} units were introduced into the {W(30)} crown, resulting in a new Preyssler polyoxoanion {S(5)W(30)} with position-specific, guest, K(+) or Na(+) ions in its geometrical center. The as-prepared polyoxoanion exhibits an interesting ion-selective encapsulation of K(+) and Na(+) (see figure).

Heterometallic appended {MMn(III)4} cubanes encapsulated by lacunary polytungstate ligands.

The heterometallic appended {MMn(III)(4)} (M = Dy(3+) and K(+)) cubanes were firstly trapped by two diamagnetic POM shells, which were robust enough to construct inorganic crystalline tubular materials. Magnetic study reveals the presence of a SMM-like slow magnetic relaxation feature in the heterometallic cluster-containing POM.