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.

Herein we report two ruthenium benzylidene complexes with benzylphosphite ligands for olefin metathesis. Unlike the previously reported benzylidene phosphite complexes, the benzylphosphite complexes adopt a cis-dichloro configuration making them latent at ambient temperatures. Irradiation with visible light (420 nm and blue LED) prompts activation of the complexes and induces catalysis of olefin metathesis reactions. One of the complexes, cis-Ru-1, was found to be especially suitable for 3D printing of multilayered polydicyclopentadiene structures with excellent spatial resolutions. Additionally, complex cis-Ru-2 was designed with a chromatic orthogonal "kill switch" based on the 2-nitrobenzyl chemistry, allowing the destruction of the catalyst upon exposure to UV-C light.

The deposition of metal oxides is essential to the fabrication of numerous multicomponent solid-state devices and catalysts. However, the reproducible formation of homogeneous metal oxide films or of nanoparticle dispersions at solid interfaces remains an ongoing challenge. Here we report that molecular hexaniobate cluster anion complexes of structurally and electronically distinct fragments of cubic-spinel and monoclinic Co 3 O 4 can serve as tractable yet well-defined functional analogues of bulk cobalt oxide. Notably, the energies of the highest-occupied and lowest-unoccupied molecular orbitals (HOMO and LUMO) of the molecular complexes, 1, closely match the valence-and conduction-band (VB and CB) energies of the parent bulk oxides. Use of 1 as a molecular analogue of the parent oxides is demonstrated by its remarkably simple deployment as a cocatalyst for direct Z-scheme reduction of CO 2 by solar light and water. Namely, evaporation of an aqueous solution of 1 on TiO 2 -coated fluorinated tin oxide windows (TiO 2 /FTO), immersion in wet acetonitrile, and irradiation by simulated solar light under an atmosphere of CO 2 give H 2 , CO, and CH 4 in ratios nearly identical to those obtained using 20 nm spinel-Co 3 O 4 nanocrystals, but 15 times more rapidly on a Co basis and more rapidly overall than other reported systems. Detailed investigation of the photocatalytic properties of 1 on TiO 2 /FTO includes confirmation of a direct Z-scheme charge-carrier migration pathway by in situ irradiated X-ray photoelectron spectroscopy. More generally, the findings point to a potentially important new role for coordination chemistry that bridges the conceptual divide between molecular and solid-state science.

Metal hydroxides catalyze organic transformations and photochemical processes and serve as precursors for the oxide layers of functional multicomponent devices. However, no general methods are available for the preparation of stable water-soluble complexes of metal hydroxide nanocrystals (NCs) that might be more effective in catalysis and serve as versatile precursors for the reproducible fabrication of multicomponent devices. We now report that InIII-substituted monodefect Wells–Dawson (WD) polyoxometalate (POM) cluster anions, [α2-P2W17O61InIIIOH)]8–, serve as ligands for stable, water-soluble complexes, 1, of platelike, predominantly cubic-phase (dzhalindite) In­(OH)3 NCs that after optimization contain ca. 10% InOOH. Images from cryogenic tranmsission electron microscopy reveal numerous WD ligands at the surfaces of platelike NCs, with average dimensions of 17 × 28 × 2 nm, each complexed by an average of ca. 450 InIII-substituted WD cluster anions and charge-balanced by 3600 Na+ countercations. Facilitated by the water solubility of 1, countercation exchange is used to stoichiometrically disperse ca. 1800 Cu2+ ions in an atomically homogeneous fashion around the surfaces of each NC core. The utility of this impregnation method is illustrated by using the ion-exchanged material as an electrocatalyst that reduces CO2 to CO 15 times faster per milligram of Cu than does K6Cu­[P2CuII(H2O)­W17O61] (control) alone. More generally, the findings point to POM complexation as a promising method for stabilizing and solubilizing reactive d-, p-, and f-block metal hydroxide NCs and for enabling their utilization as versatile components in the fabrication of functional multicomponent materials.

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.