Amphiprotic properties of a bis(μ-hydroxo)divanadium(IV)-substituted γ-Keggin-type silicodecatungstate containing two different kinds of hydroxyl moieties

Charge-State Dependence of Proton Uptake in Polyoxovanadate-alkoxide Clusters

Here, we present an investigation of the thermochemistry of proton uptake in acetonitrile across three charge states of a polyoxovanadate-alkoxide (POV-alkoxide) cluster, [V₆O₇(OMe)₁₂]ⁿ (n = 2–, 1–, and 0). The vanadium oxide assembly studied features bridging sites saturated by methoxide ligands, isolating protonation to terminal vanadyl moieties. Exposure of [V₆O₇(OMe)₁₂]ⁿ to organic acids of appropriate strength results in the protonation of a terminal V=O bond, generating the transient hydroxide-substituted POV-alkoxide cluster [V₆O₆(OH)(OMe)₁₂]ⁿ⁺¹. Evidence for this intermediate proved elusive in our initial report, but here we present the isolation of a divalent anionic cluster that features hydrogen bonding to dimethylammonium at the terminal oxo site. Degradation of the protonated species results in the formation of equimolar quantities of one-electron-oxidized and oxygen-atom-efficient complexes, [V₆O₇(OMe)₁₂]ⁿ⁺¹ and [V₆O₆(OMe)₁₂]ⁿ⁺¹. While analogous reactivity was observed across the three charge states of the cluster, a dependence on the acid strength was observed, suggesting that the oxidation state of the vanadium oxide assembly influences the basicity of the cluster surface. Spectroscopic investigations reveal sigmoidal relationships between the acid strength and cluster conversion across the redox series, allowing for determination of the proton affinity of the surface of the cluster in all three charge states. The fully reduced cluster is found to be the most basic, with higher oxidation states of the assembly possessing substantially reduced proton affinities (∼7 pK_a units per electron). These results further our understanding of the site-specific reactivity of terminal M=O bonds with protons in an organic solvent, revealing design criteria for engineering functional surfaces of metal oxide materials of relevance to energy storage and conversion.

Experimental determination of the surface basicity of polyoxovanadate-alkoxide clusters across three oxidation states reveals a charge-state dependence of proton uptake in molecular, organofunctionalized vanadium oxide assemblies.

Concerted Multiproton-Multielectron Transfer for the Reduction of O2 to H2O with a Polyoxovanadate Cluster

The concerted transfer of protons and electrons enables the activation of small-molecule substrates by bypassing energetically costly intermediates. Here, we present the synthesis and characterization of several hydrogenated forms of an organofunctionalized vanadium oxide assembly, [V₆O₁₃(TRIOL^NO₂)₂]^2-, and their ability to facilitate the concerted transfer of protons and electrons to O₂. Electrochemical analysis reveals that the fully reduced cluster is capable of mediating 2e^-/2H⁺ transfer reactions from surface hydroxide ligands, with an average bond dissociation free energy (BDFE) of 61.6 kcal/mol. Complementary stoichiometric experiments with hydrogen-atom-accepting reagents of established bond strengths confirm that the electrochemically established BDFE predicts the 2H⁺/2e^- transfer reactivity of the assembly. Finally, the reactivity of the reduced polyoxovanadate toward O₂ reduction is summarized; our results indicate a stepwise reduction of the substrate, proceeding through H₂O₂ en route to the formation of H₂O. Kinetic isotope effect experiments confirm the participation of hydrogen transfer in the rate-determining step of both the reduction of O₂ and H₂O₂. This work constitutes the first example of hydrogen atom transfer for small-molecule activation with reduced polyoxometalates, where both electron and proton originate from the cluster.

Modulation of Self-Separating Molecular Catalysts for Highly Efficient Biomass Transformations

The energetically viable fabrication of stable and highly efficient solid acid catalysts is one of the key steps in large‐scale transformation processes of biomass resources. Herein, the covalent modification of the classical Dawson polyoxometalate (POMs) with sulfonic acids (‐SO₃H) is reported by grafting sulfonic acid groups on the POM's surface followed by oxidation of (3‐mercaptopropyl)trimethoxysilane. The acidity of TBA₆‐P₂W₁₇‐SO₃H (TBA=tetrabutyl ammonium) has been demonstrated by using ³¹P NMR spectroscopy, clearly indicating the presence of strong Brønsted acid sites. The presence of TBA counterions renders the solid acid catalyst as a promising candidate for phase transfer catalytic processes. The TBA₆‐P₂W₁₇‐SO₃H shows remarkable activity and selectivity, excellent stability, and great substrate compatibility for the esterification of free fatty acids (FFA) with methanol and conversion into biodiesel at 70 °C with >98 % conversion of oleic acid in 20 min. The excellent catalytic performance can be attributed to the formation of a catalytically active emulsion, which results in a uniform catalytic behavior during the reaction, leading to efficient interaction between the substrate and the active sites of the catalyst. Most importantly, the catalyst can be easily recovered and reused without any loss of its catalytic activity owing to its excellent phase transfer properties. This work offers an efficient and cost‐effective strategy for large‐scale biomass conversion applications.