No abstract

Polyoxotungstates [PW(4)O(24)](3-) (PW(4)) and [PW(12)O(40)](3-) (PW(12)) have been inserted into nanocages of the metal organic framework MIL-101. The hybrid materials PW(x)/MIL-101 (x = 4 or 12) containing 5-14 wt % of polyoxotungstate have been obtained and characterized by elemental analysis, N(2) adsorption, FT-IR, Raman, and (31)P NMR MAS spectroscopic techniques. Their catalytic performance was assessed in the selective oxidation of alkenes with aqueous hydrogen peroxide under mild reaction conditions ([H(2)O(2)] = 0.1-0.2 M, 50 degrees C, MeCN). PW(x)/MIL-101 enclosing 5 wt % of polyoxotungstate demonstrated fairly good catalytic activities in the epoxidation of various alkenes (3-carene, limonene, alpha-pinene, cyclohexene, cyclooctene, 1-octene), the turnover frequencies (TOF) and alkene conversions were close to the corresponding parameters achieved with homogeneous PW(x). For the oxidation of substrates with aromatic groups (styrene, cis- and trans-stilbenes), a higher level of olefin conversion was attained using PW(12)/MIL-101. Moreover, confinement of PW(12) within MIL-101 nanocages allowed us to reach higher epoxide selectivities at higher alkene conversions. The hybrid PW(x)/MIL-101 materials were stable to leaching, behaved as true heterogeneous catalysts, were easily recovered by filtration, and reused several times with the maintenance of the catalytic performance.

Mesoporous metal-organic frameworks Cr- and Fe-MIL-101 are highly efficient, true heterogeneous and recyclable catalysts for solvent-free selective oxidation of cyclohexane with molecular oxygen and/or tert-butyl hydroperoxide under mild conditions.

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.