Titanium-Substituted Heteropolytungstates as Model Catalysts for Studying the Mechanisms of Selective Oxidation by Hydrogen Peroxide

The ³¹P NMR method shows that four forms of titanium(IV)-monosubstituted Keggin-type heteropolytungstate (Ti–HPA) exist in MeCN: the dimer (Bu₄N)₇[{PTiW₁₁O₃₉}₂OH] (in the abbreviated form, (PW₁₁Ti)₂OH or H1), its conjugate base (PW₁₁Ti)₂O (1), and two monomers, PW₁₁TiO (2) and PW₁₁TiOH (H2). The ratio between the forms depends on the concentrations of H⁺and H₂O. Dimer H1is produced from 2in MeCN when H⁺(1.5 mol) is added, and monomer H2is the key intermediate in this process. The catalytic activity of Ti–HPA in the oxidation of thioethers by H₂O₂correlates with their activity in peroxo complex formation and decreases in the order H2> H1> 2. The reaction of 2with H₂O₂in MeCN occurs slowly to form the inactive peroxo complex PW₁₁TiO₂(A). The addition of H₂O₂to H1and H2most likely results in the formation of the active hydroperoxo complex PW₁₁TiOOH (B). Complexes Aand Btransform into each other when H⁺or OH^–(1 mol) is added per 1 mol of Aor B, respectively. The activity of Btoward thioethers in the stoichiometric reaction is proven by ³¹PNMR and optical spectroscopy.     

Physicochemical Properties of Mesoporous Mesophase Silicate Materials Formed via the Electrostatic S+I– Mechanism

The formation of the structure of a highly organized silicate mesoporous mesophase material (MMM) with hexagonal packing via the S⁺I^–reaction pathway and MMM-based aluminosilicates (Al,Si)-MMM and titanosilicates (Ti,Si)-MMM with different concentrations of the elements are considered. The structural, textural, and catalytic properties of the materials are studied.     

Catalytic properties of the polyoxometalate [Ti2(OH)2As2W19O67(H2O)]8? in selective oxidations with hydrogen peroxide

The catalytic properties of the sandwich polyoxometalate [Ti₂(OH)₂As₂W₁₉O₆₇(H₂O)]⁸⁻, which contains two (B-α-As^IIIW₉O₃₃) fragments linked together by a “belt” consisting of one octahedral WO(H₂O)⁴⁺ and two square-pyramidal Ti(OH)³⁺ groups, have been investigated in the selective liquid-phase oxidation of organic compounds by aqueous hydrogen peroxide. The polyoxometalate shows high catalytic activity and selectivity in the oxidation of alkenes, alcohols, diols, and thioethers. The composition of the reaction products indicates that hydrogen peroxide is activated via a heterolytic mechanism.     

Catalytic Performance of Zr-Based Metal–Organic Frameworks Zr-abtc and MIP-200 in Selective Oxidations with H2O2

The catalytic performance of Zr-abtc and MIP-200 metal–organic frameworks consisting of 8-connected Zr₆ clusters and tetratopic linkers was investigated in H₂O₂-based selective oxidations and compared with that of 12-coordinated UiO-66 and UiO-67. Zr-abtc demonstrated advantages in both substrate conversion and product selectivity for epoxidation of electron-deficient C=C bonds in α,β-unsaturated ketones. The significant predominance of 1,2-epoxide in carvone epoxidation, coupled with high sulfone selectivity in thioether oxidation, points to a nucleophilic oxidation mechanism over Zr-abtc. The superior catalytic performance in the epoxidation of unsaturated ketones correlates with a larger amount of weak basic sites in Zr-abtc. Electrophilic activation of H₂O₂ can also be realized, as evidenced by the high activity of Zr-abtc in epoxidation of the electron-rich C=C bond in caryophyllene. XRD and FTIR studies confirmed the retention of the Zr-abtc structure after the catalysis. The low activity of MIP-200 in H₂O₂-based oxidations is most likely related to its specific hydrophilicity, which disfavors adsorption of organic substrates and H₂O₂.     

Guests Like Gear Levers: Donor Binding to Coordinatively Unsaturated Metal Sites in MIL-101 Controls the Linker′s Rotation

We present investigation of the effect of electron-donor guests on framework mobility in the metal–organic framework (MOF) MIL-101(Cr) monitored by solid state ²H NMR spectroscopy. In a guest-free material, the mobile phenylene fragments of the terephthalate (TP) linkers populate two fractions with notably different kinetic parameters for torsional motion. Two fractions of rotational motion are indicative of non-equivalence of TP linker binding to the Cr₃O trimer, the primary building unit of the MIL-101 framework. It is established that the interaction of the guest molecules with coordinatively unsaturated metal sites (CUS) of the MOF dramatically decreases torsional barriers for the linker motions, enhancing the rotation rate. This result is opposite to a more conventional slowing down effect on the linker rotation of the guests not selectively interacting with the adsorption sites inside the framework of the MOFs. The effect of coordination on both the torsional barrier and the rotation rate depends notably on the particular guest interacting with the CUS. The found effects of the guest on the rotational motion represent a basis for developing the strategy for ruling and controlling the linker rotation in MOFs with CUS. It is shown that if water occupies CUS, another guest (tert-butanol, cyclohexanone) fails to competitively coordinate to the site.     

Mechanism of Thioether Oxidation over Di‐ and Tetrameric Ti Centres: Kinetic and DFT Studies Based on Model Ti‐Containing Polyoxometalates

The oxidation of thioethers by the green oxidant aqueous H₂O₂ catalysed by the tetratitanium‐substituted Polyoxometalate (POM) (Bu₄N)₈[{γ‐SiTi₂W₁₀O₃₆(OH)₂}₂(μ‐O)₂], as a model catalyst comprising tetrameric titanium centres, was investigated by kinetic modelling and DFT calculations. Several mechanisms of sulfoxidation were evaluated by using methyl phenyl sulfide (PhSMe) as a model substrate in the experiments and dimethyl sulfide in the calculations. The first mechanism assumes that the active hydroperoxo species forms directly through interaction of the Ti₂(μ‐OH)₂ group in [{γ‐SiTi₂W₁₀O₃₆(OH)₂}₂(μ‐O)₂]^8? ( 1 D ) with H₂O₂. The second mechanism includes hydrolysis of Ti‐O‐Ti bonds linking two γ‐Keggin units in structure 1 D to produce the monomer [(γ‐SiW₁₀Ti₂O₃₈H₂)(OH)₂]^4? ( 1 M ), followed by the formation of an active hydroperoxo species upon interaction of the Ti hydroxo group with H₂O₂. Both kinetic modelling and DFT calculations support the mechanism through the monomeric species that involves the hydrolysis step. According to the DFT studies the activation of H₂O₂ by compound 1 M is preferred by 6.5 kcal mol^?1 with respect to anion 1 D due to the more flexible Ti environment of the terminal Ti hydroxo group in 1 M . The calculations also indicate that for the ?monomeric“ mechanism two pathways are operative: the mono‐ and the multinuclear pathway. In the mononuclear mechanism, the active group is the terminal Ti?OH group, whereas in the multinuclear path the active group is the bridging Ti₂(μ‐OH) moiety. Moreover, unlike previous studies, the sulfoxidation is preferred through a β‐oxygen atom transfer from the Ti hydroperoxo group because the α‐oxygen atom transfer leads to an unfavourable seven‐fold coordinated Ti environment in the transition state. Finally, we have generalised these results to other Ti‐containing POMs: the Ti‐monosubstituted α‐Keggin ion [α‐PTi(OH)W₁₁O₃₉]^4? and the dititanium‐substituted sandwich‐type ion [Ti₂(OH)₂As₂W₁₉O₆₇]^8?.     

Alkene oxidation by Ti-containing polyoxometalates. Unambiguous characterization of the role of the protonation state

Kinetic and DFT studies revealed that protonation of Ti-containing polyoxometalates (Ti-POM) lowers significantly the energy barrier for the heterolytic oxygen transfer from the Ti hydroperoxo intermediate to the alkene, increasing the activity and selectivity of alkene oxidation.     

Iron tetrasulfophthalocyanine immobilized on metal organic framework MIL-101: synthesis, characterization and catalytic properties

Iron tetrasulfophthalocyanine (FePcS) has been irreversibly inserted into nanocages of the metal organic framework MIL-101 to give a hybrid material FePcS/MIL-101 which demonstrated a superior catalytic performance in the selective oxidation of aromatic substrates with (t)BuOOH than homogeneous FePcS.     

Allylic oxdation of alkenes with molecular oxygen catalyzed by porous coordination polymers Fe-MIL-101 and Cr-MIL-101

The catalytic performances of Cr-MIL-101 and Fe-MIL-101 porous coordination polymers have been investigated in the allylic oxidation of alkenes, including natural terpenes, with molecular oxygen (1 atm) under mild solvent-free conditions. Both catalysts remain stable under optimal conditions (40°C for Fe-MIL-101 and 60°C for Cr-MIL-101) and can be recycled, at least, four times without loss of the catalytic properties. Fe-MIL-101 favours the formation of unsaturated alcohols, while Cr-MIL-101 mediates the formation of unsaturated ketones. The oxidation process involves the formation of alkene hydroperoxide via conventional radical chain process and its further transformations over the MIL-101 catalysts. The mechanism of the hydroperoxide transformation strongly depends on the metal nature.