Zr-monosubstituted Lindqvist-type polyoxometalates (Zr-POMs), (Bu 4 N) 2 [W 5 O 18 Zr(H 2 O) 3 ] (1) and (Bu 4 N) 6 [{W 5 O 18 Zr(μ-OH)} 2 ] (2), have been employed as molecular models to unravel the mechanism of hydrogen peroxide activation over Zr(IV) sites. Compounds 1 and 2 are hydrolytically stable and catalyze the epoxidation of CC bonds in unfunctionalized alkenes and α,β-unsaturated ketones, as well as sulfoxidation of thioethers. Monomer 1 is more active than dimer 2. Acid additives greatly accelerate the oxygenation reactions and increase oxidant utilization efficiency up to >99%. Product distributions are indicative of a heterolytic oxygen transfer mechanism that involves electrophilic oxidizing species formed upon the interaction of Zr-POM and H 2 O 2 . The interaction of 1 and 2 with H 2 O 2 and the resulting peroxo derivatives have been investigated by UV−vis, FTIR, Raman spectroscopy, HR-ESI-MS, and combined HPLC-ICPatomic emission spectroscopy techniques. The interaction between an 17 O-enriched dimer, (Bu 4 N) 6 [{W 5 O 18 Zr(μ-OCH 3 )} 2 ] (2′), and H 2 O 2 was also analyzed by 17 O NMR spectroscopy. Combining these experimental studies with DFT calculations suggested the existence of dimeric peroxo species [(μ-η 2 :η 2 -O 2 ){ZrW 5 O 18 } 2 ] 6− as well as monomeric Zr-hydroperoxo [W 5 O 18 Zr(η 2 -OOH)] 3− and Zr-peroxo [HW 5 O 18 Zr(η 2 - O 2 )] 3− species. Reactivity studies revealed that the dimeric peroxo is inert toward alkenes but is able to transfer oxygen atoms to thioethers, while the monomeric peroxo intermediate is capable of epoxidizing CC bonds. DFT analysis of the reaction mechanism identifies the monomeric Zr-hydroperoxo intermediate as the real epoxidizing species and the corresponding α-oxygen transfer to the substrate as the rate-determining step. The calculations also showed that protonation of Zr-POM significantly reduces the free-energy barrier of the key oxygen-transfer step because of the greater electrophilicity of the catalyst and that dimeric species hampers the approach of alkene substrates due to steric repulsions reducing its reactivity. The improved performance of the Zr(IV) catalyst relative to Ti(IV) and Nb(V) catalysts is respectively due to a flexible coordination environment and a low tendency to form energy deep-well and low-reactive Zr-peroxo intermediates.

Nb-monosubstituted Lindqvist-type polyoxometalates (POM), (Bu4N)4[(NbW5O18)2O] (1) and (Bu4N)3[Nb­(O)­W5O18] (2), catalyze epoxidation of alkenes with hydrogen peroxide and mimic the catalytic performance of heterogeneous Nb-silicate catalysts. Dimer 1 is more active than monomer 2, but the catalytic activity of the latter increases in the presence of acid. Kinetic and spectroscopic studies suggest a mechanism that involves generation of monomer (Bu4N)2[Nb­(OH)­W5O18] (3), interaction of 3 with H2O2 leading to a protonated peroxo niobium species, (Bu4N)2[HNb­(O2)­W5O18] (4), followed by oxygen transfer to a CC bond in alkene. The previously unknown peroxo complex 4 has been isolated and characterized by elemental analysis; UV–vis, FT-IR, Raman, 93Nb, 17O and 183W NMR spectroscopy; cyclic voltammetry; and potentiometric titration. The physicochemical techniques support a monomeric Lindqvist structure of 4 bearing one peroxo ligand attached to Nb­(V) in a η2-coordination mode. While the unprotonated peroxo complex (Bu4N)3[Nb­(O2)­W5O18] (5) is inert toward alkenes under stoichiometric conditions, 4 readily reacts with cyclohexene to afford epoxide and 1,2-trans-cyclohexane diol, which proves the key role of protons for heterolytic activation of H2O2 over Nb­(V). The IR, Raman, UV–vis, and 17O NMR spectroscopic studies along with DFT calculations showed that the activating proton in 4 is predominantly located at a Nb–O–W bridging oxygen. However, DFT calculations revealed that the protonated peroxo species “HNb­(O2)” is present in equilibrium with a hydroperoxo species “Nb­(η2-OOH),” which has a lower activation barrier for the oxygen transfer to cyclohexene and is, therefore, the main epoxidizing species. The calculations indicate that protonation is crucial to generating the active species and to increasing POM electrophilicity.

Alkyl-substituted benzoquinones serve as versatile building blocks for a variety of biologically active compounds. In this work, we present an approach for the environmentally benign synthesis of alkyl-p-benzoquinones, in particular trimethyl-p-benzoquinone (TMBQ, vitamin E precursor), which employs aqueous hydrogen peroxide as oxidant and a divanadium-substituted γ-Keggin polyoxotungstate, [γ-PW10O38V2(μ-O)­(μ–OH)]4– (V2-POM), immobilized on nitrogen-doped carbon nanotubes (N-CNTs) as heterogeneous catalyst. A series of supported catalysts V2-POM/N-CNTs containing 5–25 wt % of V2-POM and 0–4.8 atom % of N has been prepared and characterized by elemental analysis, N2 adsorption, SEM, TEM, XPS, and FTIR techniques. The catalytic performance of V2-POM/N-CNTs was assessed in the selective oxidation of 2,3,6-trimethylphenol (TMP) with H2O2 under mild reaction conditions (60 °C, MeCN). The presence of nitrogen in the support ensures strong adsorption and molecular dispersion of V2-POM on the carbon surface, leading to highly active and selective heterogeneous catalysts, which do not suffer from metal leaching and can be used repeatedly without loss of the catalytic performance. By application of the optimal catalyst V2-POM/N-CNTs enclosing 15 wt % of V2-POM and 1.8 atom % of N, TMBQ could be obtained with 99% yield and 80% oxidant utilization efficiency. The catalyst demonstrated the truly heterogeneous nature of the catalysis and high turnover frequencies (500 h–1) and space–time yield (450 g L–1 h–1). FTIR and XPS techniques confirmed the stability of V2-POM and N-CNT support under the turnover conditions.

Zr-based metal–organic frameworks (Zr-MOF) UiO-66 and UiO-67 catalyze thioether oxidation in nonprotic solvents with unprecedentedly high selectivity toward corresponding sulfones (96–99% at ca. 50% sulfide conversion with only 1 equiv of H2O2). The reaction mechanism has been investigated using test substrates, kinetic, adsorption, isotopic (18O) labeling, and spectroscopic tools. The following facts point out a nucleophilic character of the peroxo species responsible for the superior formation of sulfones: (1) nucleophilic parameter XNu = 0.92 in the oxidation of thianthrene 5-oxide and its decrease upon addition of acid; (2) sulfone to sulfoxide ratio of 24 in the competitive oxidation of methyl phenyl sulfoxide and p-Br-methyl phenyl sulfide; (3) significantly lower initial rates of methyl phenyl sulfide oxidation relative to methyl phenyl sulfoxide (k S/k SO = 0.05); and (4) positive slope ρ = +0.42 of the Hammett plot for competitive oxidation of p-substituted aryl methyl sulfoxides. Nucleophilic activation of H2O2 on Zr-MOF is also manifested by their capability of catalyzing epoxidation of electron-deficient CC bonds in α,β-unsaturated ketones accompanied by oxidation of acetonitrile solvent. Kinetic modeling on methyl phenyl sulfoxide oxidation coupled with adsorption studies supports a mechanism that involves the interaction of H2O2 with Zr sites with the formation of a nucleophilic oxidizing species and release of water followed by oxygen atom transfer from the nucleophilic oxidant to sulfoxide that competes with water for Zr sites. The nucleophilic peroxo species coexists with an electrophilic one, ZrOOH, capable of oxygen atom transfer to nucleophilic sulfides. The predominance of nucleophilic activation of H2O2 over electrophilic one is, most likely, ensured by the presence of weak basic sites in Zr-MOFs identified by FTIR spectroscopy of adsorbed CDCl3 and quantified by adsorption of isobutyric acid.