Tetrapalladium-Containing Polyoxotungstate [Pd^II₄(α‑P₂W₁₅O₅₆)₂]^16–: A Comparative Study

The novel tetrapalladium­(II)-containing polyoxometalate [Pd^II₄(<i>α-</i>P₂W₁₅O₅₆)₂]^16– has been prepared in aqueous medium and characterized as its hydrated sodium salt Na₁₆[Pd₄(α-P₂W₁₅O₅₆)₂]·71H₂O by single-crystal XRD, elemental analysis, IR, Raman, multinuclear NMR, and UV–vis spectroscopy. The complex exists in anti and syn conformations, which form in a 2:1 ratio, and possesses unique structural characteristics in comparison with known {M₄(P₂W₁₅)₂} species. ³¹P and ¹⁸³W NMR spectroscopy are consistent with the long-term stability of the both isomers in aqueous solutions

Highly Selective Oxidation of Alkylphenols to <i>p</i>‑Benzoquinones with Aqueous Hydrogen Peroxide Catalyzed by Divanadium-Substituted Polyoxotungstates

The catalytic performance of divanadium- and dititanium-substituted γ-Keggin polyoxotungstates, TBA₄H­[γ-PW₁₀V₂O₄₀] (<b>I</b>, TBA = tetra-<i>n</i>-butylammonium), TBA₄H₂[γ-SiW₁₀V₂O₄₀] (<b>II</b>), and TBA₈[{γ-SiW₁₀Ti₂O₃₆(OH)₂}₂(μ-O)₂] (<b>III</b>) has been assessed in the selective oxidation of industrially important alkylphenols/naphthols with the green oxidant 35% aqueous H₂O₂. Phosphotungstate <b>I</b> revealed a superior catalytic performance in terms of activity and selectivity and produced alkylsubstituted <i>p</i>-benzo- and naphthoquinones with good to excellent yields. By applying the optimized reaction conditions, 2,3,5-trimethyl-<i>p</i>-benzoquinone (TMBQ, vitamin E key intermediate) was obtained in a nearly quantitative yield via oxidation of 2,3,6-trimethylphenol (TMP). The efficiency of H₂O₂ utilization reached 90%. The catalyst retained its structure under turnover conditions and could be recycled and reused. An active peroxo vanadium complex responsible for the oxidation of TMP to TMBQ has been identified using ⁵¹V and ³¹P NMR spectroscopy

Understanding the Regioselectivity of Aromatic Hydroxylation over Divanadium-Substituted γ‑Keggin Polyoxotungstate

The aromatic hydroxylation of pseudocumene (PC) with aqueous hydrogen peroxide catalyzed by the divanadium-substituted γ-Keggin polyoxotungstate TBA₄[γ-PW₁₀O₃₈V₂(μ-O)­(μ-OH)] (TBA-<b>1H</b>, TBA = tetrabutylammonium) has been studied using kinetic modeling and DFT calculations. This reaction features high chemoselectivity and unusual regioselectivity, affording 2,4,5-trimethylphenol (TMP) as the main product. Then the computational study was extended to the analysis of the regioselectivity for other alkoxy- and alkylarene substrates. The protonation/deprotonation of TBA-<b>1H</b> in MeCN/<i>t</i>BuOH (1:1) was investigated by ³¹P NMR spectroscopy. Forms with different protonation states, [γ-PV₂W₁₀O₄₀]^5– (<b>1</b>), [γ-HPV₂W₁₀O₄₀]^4– (<b>1H</b>), and [γ-H₂PV₂W₁₀O₄₀]^3– (<b>1H</b>_<b>2</b>), have been identified, and the protonation equilibrium constants were estimated on the basis of the ³¹P NMR data. DFT calculations were used to investigate the oxygen transfer process from hydroperoxo species, [γ-PW₁₀O₃₈V₂(μ-O)­(μ-OOH)]^4– (<b>2</b>) and [γ-PW₁₀O₃₈V₂(μ-OH)­(μ-OOH)]^3– (<b>2H</b>), and peroxo complex [γ-PW₁₀O₃₈V₂(μ-η²:η²-O₂)]^3– (<b>3</b>) toward the different positions in the aromatic ring of PC, anisole, and toluene substrates. Product, kinetic, and computational studies on the PC hydroxylation strongly support a mechanism of electrophilic oxygen atom transfer from peroxo complex <b>3</b> to the aromatic ring of PC. The kinetic modeling revealed that the contribution of <b>3</b> into the initial reaction rate is, on average, about 70%, but it may depend on the reaction conditions. DFT calculations showed that the steric hindrance exerted by peroxo complex <b>3</b> is responsible for the origin of the unusual regioselectivity observed in PC hydroxylation, while for anisole and toluene the regioselective <i>para</i>-hydroxylation is due to electronic preference during the oxygen transfer from the active peroxo species <b>3</b>