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.

Dissolution of the polyoxometalate (POM) cluster anion H 5 [PV 2 Mo 10 O 40 ] ( 1 ; a mixture of positional isomers) in 50% aq H 2 SO 4 dramatically enhances its ability to oxidize methylarenes, while fully retaining the high selectivities typical of this versatile oxidant. To better understand this impressive reactivity, we now provide new information regarding the nature of 1 (115 mM) in 50% (9.4 M) H 2 SO 4 . Data from 51 V NMR spectroscopy and cyclic voltammetry reveal that as the volume of H 2 SO 4 in water is incrementally increased to 50%, V(V) ions are stoichiometrically released from 1 , generating two reactive pervanadyl, VO 2 + , ions, each with a one-electron reduction potential of ca. 0.95 V (versus Ag/AgCl), compared to 0.46 V for 1 in 1.0 M aq H 2 SO 4 . Phosphorus-31 NMR spectra obtained in parallel reveal the presence of PO 4 3– , which at 50% H 2 SO 4 accounts for all the P(V) initially present in 1 . Addition of (NH 4 ) 2 SO 4 leads to the formation of crystalline [NH 4 ] 6 [Mo 2 O 5 (SO 4 ) 4 ] (34% yield based on Mo), whose structure (from single-crystal X-ray diffraction) features a corner-shared, permolybdenyl [Mo 2 O 5 ] 2+ core, conceptually derived by acid condensation of two MoO 3 moieties. While 1 in 50% aq H 2 SO 4 oxidizes p -xylene to p -methylbenzaldehyde with conversion and selectivity both greater than 90%, reaction with VO 2 + alone gives the same high conversion, but at a significantly lower selectivity. Importantly, selectivity is fully restored by adding [NH 4 ] 6 [Mo 2 O 5 (SO 4 ) 4 ], suggesting a central role for Mo(VI) in attenuating the (generally) poor selectivity achievable using VO 2 + alone. Finally, 31 P and 51 V NMR spectra show that intact 1 is fully restored upon dilution to 1 M H 2 SO 4 .

No abstract

The selective uptake of guests by capsules, cages, and containers, and porous solid-state materials such as zeolites and metal–organic frameworks (MOFs), is generally controlled by pore size and by the dimensions and chemical properties of interior host domains. For soluble and solid-state structures, however, few options are available for modifying their outer pores to impart chemoselectivity to the uptake of similarly sized guests. We now show that by using alkane-coated gold cores as structural building units (SBUs) for the hydrophobic self-assembly of water-soluble suprasphere hosts, ligand exchange can be used to tailor the chemical properties at the pores that provide access to their interiors. For polar polyethylene glycol functionalized ligands, occupancies after equal times increase linearly with the dipole moments of chloro-, nitro- dichloro-, and dinitro- (o-, m-, and p-) benzene guests. Selectivity is reversed, however, upon incorporation of hydrophobic ligands. The findings demonstrate how self-assembled gold-core SBUs, with replaceable ligands, inherently provide for rationally introducing finely tuned and quantitatively predictable chemoselectivity to host–guest chemistry in water.

The formation of small 1 to 3 nm organicligand free metal-oxide nanocrystals (NCs) is essential to utilization of their attractive size-dependent properties in electronic devices and catalysis. We now report that hexaniobate cluster-anions, [Nb 6 O 19 ] 8À , can arrest the growth of metal-oxide NCs and stabilize them as water-soluble complexes. This is exemplified by formation of hexaniobate-complexed 2.4-nm monoclinic-phase CuO NCs (1), whose ca. 350 Cu-atom cores feature quantum-confinement effects that impart an unprecedented ability to catalyze visible-light water oxidation with no added photosensitizers or applied potentials, and at rates exceeding those of hematite NCs. The findings point to polyoxoniobate-ligand entrapment as a potentially general method for harnessing the sizedependent properties of very small semiconductor NCs as the cores of versatile, entirely-inorganic complexes.