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 CO₂ 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 Nb‐μ‐O‐Nb linkages that bind and convert CO₂ 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 solar‐light driven activation and reduction of small molecules.

A distinguishing feature of MAX phases and their MXene derivatives is their remarkable chemical diversity. This diversity, coupled with the 2D nature of MXenes, positions them as outstanding candidates for a wide range of electrochemical applications. Chemical disorder introduced by a solid solution can improve electrochemical behavior. Up to now, adding considerable amount of tungsten (W) in MAX phase and MXenes solid solutions, which can enhance electrochemical performance, proved challenging. In this study, the synthesis of M site Ti_1‐xW_x solid solution MAX phases are reported. The 211‐type (Ti_1‐xW_x)₂AlC exhibits a disordered solid solution, whereas the 312‐type (Ti_1‐xW_x)₃AlC₂ displays a near‐ordered structure, resembling o‐MAX, with W atoms preferentially occupying the outer planes. Solid‐solution MXenes, Ti_2.4W_0.6C₂T_z, and Ti_1.6W_0.4CT_z, are synthesized via selective etching of high‐purity MAX powder precursors containing 20% W. These MXenes are evaluated as sodium‐ion battery anodes, with Ti_1.6W_0.4CT_z showing exceptional capacity, outperforming existing multilayer MXene chemistries. This work not only demonstrates the successful integration of W in meaningful quantities into a double transition metal solid solution MAX phase, but also paves the way for the development of cost‐effective MXenes containing W. Such advancements significantly widen their application spectrum by fine‐tuning their physical, electronic, mechanical, electrochemical, and catalytic properties.

The formation of small 1 to 3 nm organic‐ligand 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, [Nb₆O₁₉]⁸⁻, 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 size‐dependent properties of very small semiconductor NCs as the cores of versatile, entirely‐inorganic complexes.

Dissolution of the polyoxometalate (POM) cluster anion H₅[PV₂Mo₁₀O₄₀] (1; a mixture of positional isomers) in 50% aq H₂SO₄ 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) H₂SO₄. Data from ⁵¹V NMR spectroscopy and cyclic voltammetry reveal that as the volume of H₂SO₄ in water is incrementally increased to 50%, V(V) ions are stoichiometrically released from 1, generating two reactive pervanadyl, VO₂⁺, 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 H₂SO₄. Phosphorus-31 NMR spectra obtained in parallel reveal the presence of PO₄^3–, which at 50% H₂SO₄ accounts for all the P(V) initially present in 1. Addition of (NH₄)₂SO₄ leads to the formation of crystalline [NH₄]₆[Mo₂O₅(SO₄)₄] (34% yield based on Mo), whose structure (from single-crystal X-ray diffraction) features a corner-shared, permolybdenyl [Mo₂O₅]²⁺ core, conceptually derived by acid condensation of two MoO₃ moieties. While 1 in 50% aq H₂SO₄ oxidizes p-xylene to p-methylbenzaldehyde with conversion and selectivity both greater than 90%, reaction with VO₂⁺ alone gives the same high conversion, but at a significantly lower selectivity. Importantly, selectivity is fully restored by adding [NH₄]₆[Mo₂O₅(SO₄)₄], suggesting a central role for Mo(VI) in attenuating the (generally) poor selectivity achievable using VO₂⁺ alone. Finally, ³¹P and ⁵¹V NMR spectra show that intact 1 is fully restored upon dilution to 1 M H₂SO₄.