Recent investigations reveal that by providing active sites for O−O bond formation, Fe(III) oxyhydroxides (FeOOH) dramatically enhance the oxygen evolution activities of iron-containing abundant-earth CoO x H y and NiO x H y electrocatalysts. In contrast to α-Fe 2 O 3 (hematite), however, little detailed information is available concerning fundamental reactivities of the Fe(III) oxyhydroxides themselves. We here report a macroanion-like polyoxometalate cluster-anion complex of 2.6 nm γ-FeOOH nanocrystals, 1, that not only catalyzes visible light-driven water oxidation with no need for added photosensitizers but also whose unique stability and solubility facilitate investigation of oxygen evolution using the toolbox of solution-state methods typically reserved for molecular catalysis. The γ-FeOOH active centers of 1 are comprised of ca. 250 Fe atoms and coordinated by an average of six oxo-donor ligands, [α-PW 11 O 39 Fe III ] 4− -μ-O − , each with a formal charge of 5−, giving freely diffusing macroanion-like hexacoordinate complexes readily observed in their native, vitreous water solution state by cryogenic TEM. With a bandgap energy of 2.3 eV and valence-and conduction-band (VB and CB) energies of 2.34 and 0.04 V vs NHE, 1 catalyzes visible light-driven water oxidation by orthoperiodate {H 3 I VII O 6 } 2− at pH 8, at a rate similar to that documented for hematite nanocrystals. Kinetic data show the reaction to be one-half order in concentrations of both 1 and {H 3 I VII O 6 } 2− , indicative of a chain mechanism. A solvent kinetic isotope effect (KIE), k H /k D , of 1.32 was assigned to the rate-limiting trapping of photoexcited electrons by {H 3 I VII O 6 } 2− , which initiates a radical-chain process inhibited by added iodate [I V O 3 ] − . In contrast to the rate-determining O−O bond formation typical of metal-oxide electrocatalysts and of many molecular catalysts, chain mechanisms initiated by the rate-limiting trapping of excited-state electrons may prove a general feature of water oxidation by freely diffusing photoactive nanocrystals.

Among molecular building blocks, metal oxide cluster anions and their countercations provide multiple options for the self-assembly of functional materials. Currently, however, rational design concepts are limited to electrostatic interactions with metal or organic countercations or to the attachment and subsequent reactions of functionalized organic ligands. We now demonstrate that bridging μ-oxo linkages can be used to string together a bifunctional Keggin anion building block, [PNb 2 Mo 10 O 40 ] 5– ( 1 ), the diniobium(V) analogue of [PV 2 Mo 10 O 40 ] 5– ( 2 ). Induction of μ-oxo ligation between the Nb V =O moieties of 1 in acetonitrile via step-growth polymerization gives linear polymers with entirely inorganic backbones, some comprising over 140 000 repeating units, each with a 3– charge, exceeding that of previously reported organic or inorganic polyelectrolytes. As the chain grows, its flexible μ-oxo-linked backbone, with associated countercations, coils into a compact 270 nm diameter spherical secondary structure as a result of electrostatic interactions not unlike those within ionic lattices. More generally, the findings point to new options for the rational design of multidimensional structures based on μ-oxo linkages between Nb V =O-functionalized building blocks.

Although pure and functionalized solid-state polyniobates such as layered perovskites and niobate nanosheets are photocatalysts for renewable-energy processes, analogous reactions by molecular polyoxoniobate cluster-anions are nearly absent from the literature. We now report that under simulated solar light, hexaniobate cluster-anion encapsulated 30-Ni II -ion "fragments" of surface-protonated cubic-phase-like NiO cores activate the hexaniobate ligands towards CO 2 reduction by water. Photoexcitation of the NiO cores promotes charge-transfer reduction of Nb V to Nb IV , increasing electron density at bridging oxo atoms of Nbμ-O-Nb linkages that bind and convert CO 2 to CO. Photogenerated NiO "holes" simultaneously oxidize water to dioxygen. The findings point to molecular complexation of suitable semiconductor "fragments" as a general method for utilizing electron-dense polyoxoniobate anions as nucleophilic photocatalysts for solarlight driven activation and reduction of small molecules.

While sophisticated computational methods can predict 31P NMR spectra of phosphorus atoms encapsulated within Keggin-derived heteropoly tungstate and molybdate cluster anions, calculated and experimental chemical shift values typically deviate considerably from one another. Motivated by the observation that experimentally determined 31P chemical shift values within a series of water-soluble plenary and metal-cation substituted lacunary Keggin anions, [PM n W11O39](7–n)– (M n = Ag+, Zn2+, Nb5+, W6+) and [(PW11O39)2M n ](14–n)– (M n = Y3+, Zr4+), varied as a linear function of the oxidation states, n, of the complexed M n cations, a linear correlation was sought between observed chemical shift values and the net Coulombic forces experienced by the encapsulated phosphorus atoms. The Coulombic model based on Shannon radii, published electronegativity values, and bond angles from X-ray crystallographic data remarkably accounted for the relative 31P chemical shift values of phosphorus atoms in over 50 metal-oxide cluster anions, including large structures comprised of up to four Keggin-derived fragments with an overall R 2 value of 0.974. With the model being applied here to three cluster anions whose 31P chemical shift values are reported here for the first time, predicted and experimental values differed by only ±0.4 ppm.

Although pure and functionalized solid-state polyniobates such as layered perovskites and niobate nanosheets are photocatalysts for renewable-energy processes, analogous reactions by molecular polyoxoniobate cluster-anions are nearly absent from the literature. We now report that under simulated solar light, hexaniobate cluster-anion encapsulated 30-Ni II -ion "fragments" of surface-protonated cubic-phase-like NiO cores activate the hexaniobate ligands towards CO 2 reduction by water. Photoexcitation of the NiO cores promotes charge-transfer reduction of Nb V to Nb IV , increasing electron density at bridging oxo atoms of Nbμ-O-Nb linkages that bind and convert CO 2 to CO. Photogenerated NiO "holes" simultaneously oxidize water to dioxygen. The findings point to molecular complexation of suitable semiconductor "fragments" as a general method for utilizing electron-dense polyoxoniobate anions as nucleophilic photocatalysts for solarlight driven activation and reduction of small molecules.