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.

Both natural enzymatic systems and synthetic porous material catalysts utilize well-defined and uniform channels to dictate reaction selectivities on the basis of size or shape. Mimicry of this design element in homogeneous systems is generally difficult owing to the flexibility inherent in most small molecular species. Herein, we report the synthesis of a tripodal ligand scaffold that orients a narrow and rigid cavity atop accessible metal coordination space. The permanent void is formed through a macrocyclization reaction whereby the 3,5-dihydroxyphenyl arms are covalently linked through methylene bridges. Deprotonative metallation leads to anionic and coordinatively unsaturated complexes of divalent cobalt, nickel, and zinc. An analogous series of trigonal monopyramidal complexes bearing a nonmacrocyclized variant of the tripodal ligand are also reported. Physical characterization of the coordination complexes has been carried out using multiple spectroscopic techniques (NMR, EPR, and UV-vis), cyclic voltammetry, and X-ray diffraction. Complexes of the macrocyclized [L^OCH2O]^3- ligand retain a rigid cavity upon metallation, with this cavity guarding the entrance to the open axial coordination site. Through a combination of spectroscopic and computational studies, it is shown that acetonitrile entry into the void is sterically precluded, disrupting anticipated coordination at the intracavity site.