Unveiling the Active Surface Sites in Heterogeneous Titanium-Based Silicalite Epoxidation Catalysts: Input of Silanol-Functionalized Polyoxotungstates as Soluble Analogues

Effect of the Polyanion Structure on the Mechanism of Alcohol Oxidation with H2O2 Catalyzed by Zr-Substituted Polyoxotungstates

Zr-monosubstituted polyoxometalates (Zr-POMs) of the Lindqvist (Bu4N)6[{W5O18Zr(μ-OH)}2] (1), Keggin (Bu4N)8[{PW11O39Zr(μ-OH)}2] (2), and Wells-Dawson (Bu4N)11.3K2.5H0.2[{P2W17O61Zr}2(μ-OH)2] (3) structures catalyze oxidation of alcohols using aqueous hydrogen peroxide as an oxidant. With 1 equiv of H2O2 and 1 mol % of Zr-POM, selectivity toward aldehydes and ketones varied from good to excellent, depending on the alcohol nature. Catalytic activity and attainable substrate conversions strongly depended on the Zr-POM structure and most often decreased in the order 1 > 2 ≫ 3. The reaction mechanism was probed using a test substrate, cyclobutanol, radical and 1O2 scavengers, and kinetic and spectroscopic (attenuated total reflectance-Fourier transform infrared (ATR-FT-IR), 31P NMR and electrospray ionization-mass spectrometry (ESI-MS)) tools. The results point to heterolytic alcohol oxidation in the presence of 1 and 2 and homolytic alcohol oxidation in the presence of 3. Kinetic and spectroscopic studies implicated an oxidation mechanism that involves both alcohol and peroxide binding to 2 followed by an inner-sphere heterolytic H-abstraction from the α-C-H bond by the Zr-hydroperoxo group, leading to a carbonyl compound. The unique capability of 1 to generate 1O2 upon interaction with H2O2 complicates the reaction kinetics and improves the product yield. Spectroscopic studies coupled with stoichiometric experiments unveiled that dimeric monoperoxo {Zr2(μ-η2:η2-O2)} and monomeric hydroperoxo {Zr(η2-OOH)} species accomplish the transformation of alcohols to carbonyl compounds.

Covalent organic-inorganic polyoxometalate hybrids in catalysis

Polyoxometalates (POMs) are a class of compounds known for the vast range of tunable structures and properties available, leading to applications in areas such as catalysis, energy, and advanced medicine. The ability to covalently functionalize POMs with organic components has been investigated extensively to tune the physical and chemical properties of the resulting hybrid materials. These hybrids, where the organic entity is covalently attached to the POM-core ( Class II hybrid POMs) result in a vast library of promising customizable catalytic systems, displaying tunable properties with a high level of synergy between the polyanion and the organic component. A number of Class II hybrids have been investigated for a wide range of catalytic applications, and here, we give a brief overview of Class II hybrids of the p-block elements and their applications in catalysis.

Effective oxygen activation on polyoxometalate-based hybrids for epoxidation of alkenes

Two polyoxometalate-based hybrids, [M(btap)3(H2O)3(HPW12O40)]·xH2O (M-PW, M = Co/Mn, btap = 3,5-bis(1',2',4'-triazol-1'-yl)pyridine) were synthesized. Co-PW exhibited higher activity and selectivity towards olefin epoxidation than Mn-PW due to the synergistic effect between CoII and PW, in which the Co centers activate O2 to ˙O2- and further binding of free H+ from PW affords the active peroxyacid.

Halide-free CO2 cycloaddition onto styrene oxide catalysed by first row transition-metal derivatives of polyoxotungstates

Quaternary ammonium salts of metal derivatives of polyoxometalates [XW11O39M(H2O)]n- (X = P, Si; M = Cr, Mn, Co, Ni, Zn) were successfully tested instead of quaternary ammonium halides as catalysts in the cycloaddition of CO2 to styrene oxide. Remarkably, they gave very satisfactory yields of styrene carbonate at moderate temperature (80 °C).

Tuning Reactivity of Zr-Substituted Keggin Phosphotungstate in Alkene Epoxidation through Balancing H2O2 Activation Pathways: Unusual Effect of Base

The Zr-monosubstituted Keggin-type dimeric phosphotungstate (Bu4N)8[{PW11O39Zr(μ-OH)(H2O)}2] (1) efficiently catalyzes epoxidation of C═C bonds in various kinds of alkenes, including terminal ones, with aqueous H2O2 as oxidant. Less sterically hindered double bonds are preferably epoxidized despite their lower nucleophilicity. Basic additives (Bu4NOH) in the amount of 1 equiv per dimer 1 suppress H2O2 unproductive decomposition, increase substrate conversion, improve yield of heterolytic oxidation products and oxidant utilization efficiency, and also affect regioselectivity of epoxidation, enhancing oxygen transfer to sterically hindered electron-rich C═C bonds. Acid additives produce a reverse effect on the substrate conversion and H2O2 efficiency. The reaction mechanism was explored using a range of test substrates, kinetic, and spectroscopic tools. The opposite effects of acid and base additives on alkene epoxidation and H2O2 degradation have been rationalized in terms of their impact on hydrolysis of 1 to form monomeric species, [PW11O39Zr(OH)(H2O)x]4- (1-M, x = 1 or 2), which favors H2O2 homolytic decomposition. The interaction of 1 with H2O2 has been investigated by HR-ESI-MS, ATR-FT-IR, and 31P NMR spectroscopic techniques. The combination of spectroscopic studies and kinetic modeling implicated the existence of two types of dimeric peroxo complexes, [Zr2(μ-η2:η2-O2){PW11O39}2(H2O)x]]8- and [{Zr(μ-η2-O2)}2(PW11O39)2(H2O)y]10-, along with monomeric Zr (hydro)peroxo species that begin to dominate at a high excess of H2O2. Both dimeric μ-η2-peroxo intermediates are inert toward alkenes under stoichiometric conditions. V-shape Hammett plots obtained for epoxidation of p-substituted styrenes suggested a biphilic nature of the active oxidizing species, which are monomeric Zr-hydroperoxo and peroxo species. Small basic additives increase the electrophilicity of the catalyst and decrease its nucleophilicity. HR-ESI-MS has identified a dimeric, most likely, bridging hydroperoxo species [{PW11O39Zr}2(μ-O)(μ-OOH)]9-, which may account for the improved epoxidation selectivity and regioselectivity toward sterically hindered C═C bonds.

Recent Advances of Ti/Zr-Substituted Polyoxometalates: From Structural Diversity to Functional Applications

Polyoxometalates (POMs), a large family of anionic polynuclear metal-oxo clusters, have received considerable research attention due to their structural versatility and diverse physicochemical properties. Lacunary POMs are key building blocks for the syntheses of functional POMs due to their highly active multidentate O-donor sites. In this review, we have addressed the structural diversities of Ti/Zr-substituted POMs based on the polymerization number of POM building blocks and the number of Ti and Zr centers. The synthetic strategies and relevant catalytic applications of some representative Ti/Zr-substituted POMs have been discussed in detail. Finally, the outlook on the future development of this area is also prospected.

Hetero-bimetallic alkali titanosilicates [MOTi{OSi(OtBu)3}3]2 (M = Li-Cs) with terminal Ti-O- groups

The molecular titanosilicate [(^tBuO₃)₃SiO]₃TiNEt₂ (1) was obtained from the reaction between silanol (^tBuO₃)₃SiOH and titanium amide Ti(NEt₂)₄. The reaction of 1 with alkali metal hydroxides MOH (M = Li, Na, K, Rb, Cs) offers a straightforward route to the alkaline salts of titanosilicates [MOTi{OSi(O^tBu)₃}₃]₂ with a terminal Ti-O^- moiety. All compounds were characterised by single-crystal X-ray diffraction studies. Hirshfeld atom refinement and QTAIM analysis of the electron density in 1 and in the Rb salt 5 revealed the D-A nature of the Ti-O and Ti-N bonds and the presence of agostic C-H⋯Rb interactions.

A {Ti6W4}-Cluster-Substituted Polyoxotungstate: Synthesis, Structure, and Catalytic Oxidation Properties

A novel Ti-W-O-cluster-substituted tungstoantimonate (TA), [H₂N(CH₃)₂]₃Na₄H₉[{Ti₆W₄O₁₈(OH)(H₂O)₃}(B-α-SbW₉O₃₃)₃]·20H₂O (1), has been made by hydrothermal reactions of trivacant [B-α-SbW₉O₃₃]^9- units, Ti⁴⁺ cations, and WO₄^2- anions in the presence of [H₂N(CH₃)₂]·Cl and structurally characterized. Intriguingly, the polyoxoanion of 1 is constructed from three [B-α-SbW₉O₃₃]^9- units and a previously unobserved decanuclear heterometallic Ti-W-O cluster [Ti₆W₄O₁₈(OH)(H₂O)₃]¹¹⁺ ({Ti₆W₄}) that is comprised of an octahedral [Ti₆WO₆(H₂O)₃]¹⁸⁺ cluster and an edge-sharing [W₃O₁₂(OH)]^7- fragment via six W-O-Ti/W linkers. Furthermore, studies on the catalytic oxidation properties reveal that 1 possesses good catalytic activity toward the oxidation reactions of various sulfides and cyclooctene based on the environmentally friendly oxidant H₂O₂.

Development of sterically hindered siloxide-functionalized polyoxotungstates for the complexation of 5d-metals

In this study, we extend the family of organosilyl-functionalized trivacant Keggin polyoxotungstates, [PW₉O₃₄(RSiOH)₃]^3- (R = ⁿPr, ⁱPr, ^tBu), through the introduction of bulky aryl and aliphatic silanol substituents, namely phenyl, cyclohexyl and biphenyl. This work was performed in order to study the impact of these large functional groups on the accessibility of the well-defined tridentate coordination site. Coordination of hafnium to these type II hybrid polyoxotungstates was conducted in order to study the ability of the bulkier ligand pockets to support larger cations in comparison to those previously reported (e.g. Ti⁴⁺, V³⁺, V⁵⁺, Ge⁴⁺). Increased steric hindrance around the coordination site from the biphenyl groups resulted in much longer reaction times for the complexation reaction compared to the other functional groups used, but the impact of our design toward stabilizing reactive species proved limited, as all complexes easily undergo hydrolysis of the Hf-O^tBu bond in the presence of water. Electrochemical investigations of the ligands and hafnium complexes reveal that the redox events centered on the polyoxotungstate core can be tuned by varying the substituents on the silyl fragment, and exhibit a cathodic shift after coordination of the redox inactive tetravalent cation.

Heterolytic alkene oxidation with H2O2 catalyzed by Nb-substituted Lindqvist tungstates immobilized on carbon nanotubes

Immobilization of Nb-substituted Lindqvist tungstates on carbon nanotubes stabilizes a monomeric form, Nb(OH)W₅, and leads to efficient and recyclable catalysts for heterolytic alkene oxidation with H₂O₂.

Hybrid polyoxometalates as post-functionalization platforms: from fundamentals to emerging applications

Polyoxometalates (POMs) represent an important group of metal-oxo nanoclusters, typically comprised of early transition metals in high oxidation states (mainly V, Mo and W). Many plenary POMs exhibit good pH, solvent, thermal and redox stability, which makes them attractive components for the design of covalently integrated hybrid organic-inorganic molecules, herein referred to as hybrid-POMs. Until now, thousands of organic hybrid-POMs have been reported; however, only a small fraction can be further functionalized using other organic molecules or metal cations. This emerging class of 'post-functionalizable' hybrid-POMs constitute a valuable modular platform that permits coupling of POM properties with different organic and metal cation functionalities, thereby expanding the key physicochemical properties that are relevant for application in (photo)catalysis, bioinorganic chemistry and materials science. The post-functionalizable hybrid-POM platforms offer an opportunity to covalently link multi-electron redox responsive POM cores with virtually any (bio)organic molecule or metal cation, generating a wide range of materials with tailored properties. Over the past few years, these materials have been showcased in the preparation of framework materials, functional surfaces, surfactants, homogeneous and heterogeneous catalysts and light harvesting materials, among others. This review article provides an overview on the state of the art in POM post-functionalization and highlights the key design and structural features that permit the discovery of new hybrid-POM platforms. In doing so, we aim to make the subject more comprehensible, both for chemists and for scientists with different materials science backgrounds interested in the applications of hybrid (POM) materials. The review article goes beyond the realms of polyoxometalate chemistry and encompasses emerging research domains such as reticular materials, surfactants, surface functionalization, light harvesting materials, non-linear optics, charge storing materials, and homogeneous acid-base catalysis among others.

Oxygen transfer in electrophilic epoxidation probed by 17O NMR: differentiating between oxidants and role of spectator metal oxo

Peroxide compounds are used both in laboratory and industrial processes for the electrophilic epoxidation of olefins. Using NMR-spectroscopy, we investigate why certain peroxides engage in this type of reaction while others require activation by metal catalysts, e.g. methyltrioxorhenium (MTO). More precisely, an analysis of ¹⁷O NMR chemical shift and quadrupolar coupling parameters provides insights into the relative energy of specific frontier molecular orbitals relevant for reactivity. For organic peroxides or H₂O₂ a large deshielding is indicative of an energetically high-lying lone-pair on oxygen in combination with a low-lying σ*(O-O) orbital. This feature is particularly pronounced in species that engage in electrophilic epoxidation, such as peracids or dimethyldioxirane (DMDO), and much less pronounced in unreactive peroxides such as H₂O₂ and ROOH, which can however be activated by transition-metal catalysts. In fact, for the proposed active peroxo species in MTO-catalyzed electrophilic epoxidation with H₂O₂ an analysis of the ¹⁷O NMR chemical shift highlights specific π- and δ-type orbital interactions between the so-called metal spectator oxo and the peroxo moieties that raise the energy of the high-lying lone-pair on oxygen, thus increasing the reactivity of the peroxo species.

Wheel-Like Icosanuclear Peroxotitanate-A Stable Water-Soluble Catalyst for Oxygen Transfer Reactions

Water-soluble wheel-like icosanuclear peroxotitanate K₁₆[Ti₂₀(μ-O)₈(HO₂)₈(O₂)₁₂( R, R-tart)₁₂]·52H₂O (1) chelated by tartrate has been successfully isolated. As the largest peroxotitanate reported, {Ti₂₀} features 20 (hydro)peroxo groups with three kinds of coordination modes in μ-η¹:η², μ-η²:η², and η² fashions. The cluster is stable in solution and solid states. It has been tested for the catalytic oxidations of methyl phenyl sulfide and pyridine with hydrogen peroxide, respectively, which shows reversible elimination and the addition of peroxo groups. This provides a rare example of well-characterized titanium peroxide for homogeneous catalysis and mechanism research.

Modeling the Oxygen Vacancy at a Molecular Vanadium(III) Silica-Supported Catalyst

Here we report on the use of a silanol-decorated polyoxotungstate, [SbW₉O₃₃( ^tBuSiOH)₃]^3- (1), as a molecular support to describe the coordination of a vanadium atom at a single-site on silica surfaces. By reacting [V(Mes)₃·thf] (Mes = 2,4,6-trimethylphenyl) with 1 in tetrahydrofuran, the vanadium(III) derivative [SbW₉O₃₃( ^tBuSiO)₃V(thf)]^3- (2) was obtained. Compound 2 displays the paramagnetic behavior expected for a d²-V^III high spin complex (SQUID measurements) with a triplet electronic ground state (ca. 30 kcal·mol^-1 more stable than the singlet, from DFT calculations). Compound 2 proves to be a reliable model for reduced isolated-vanadium atom dispersed on silica surfaces [(≡Si-O)₃V^III(OH₂)], an intermediate that is often proposed in a Mars-van Krevelen type mechanism for partial oxidation of light alcohols. Oxidation of 2 under air produced the oxo-derivative [SbW₉O₃₃( ^tBuSiO)₃VO]^3- (3). In compound 2, the d²-electrons are localized in degenerated d(V) orbitals, whereas in the electronically analogous bireduced-[SbW₉O₃₃( tBuSiO)₃VO]^5-, 3·(2e), one electron is localized on d(V) orbital and the second one is delocalized on the polyoxotungstic framework, leading to a unique case of a bireduced heteropolyanion derivative with completely decoupled d¹-V(IV) and d¹-W(V). Our body of experimental results (EPR, magnetic measurements, spectroelectrochemical studies, Raman spectroscopy) and theoretical studies highlights (i) the role of the apical ligand coordination, i.e., thf (σ-donor) vs oxo (π-donor), in destabilizing or stabilizing the d(V) orbitals relative to the d(W) orbitals, and (ii) a geometrical distortion of the O₃VO entity that causes a splitting of the degenerated orbitals and the stabilization of one d(V) orbital in the bireduced compound 3·(2e).

Recent advances of polyoxometalate-catalyzed selective oxidation based on structural classification

The structural diversity and tenability observed in POMs has encouraged extensive investigations into their catalytic activity. Based on the structural classification of POMs, this review summarizes recent advances relating to POM-catalyzed selective oxidation and places most emphasis on dynamic developments from 2015 onwards. Work which contributes to comparing the catalytic performance of POMs with delicate structural differences (e.g. the same type of POM structure with differences of the heteroatom, addenda, protonated state or counter-ion) and in elucidating the origin/distinction of catalytic activity, as well as reasonable mechanisms, are especially highlighted.

Activation mechanism of hydrogen peroxide by a divanadium-substituted polyoxometalate [γ-PV2W10O38(μ-OH)2]3-: A computational study

In the present paper, the reaction mechanism corresponding to activation of hydrogen peroxide (H₂O₂) by a divanadium-substituted polyoxometalate (POM) [γ-PV₂W₁₀O₃₈(μ-OH)₂]^3- (I) to form catalytic active species, peroxo complex [γ-PV₂W₁₀O₃₈(μ-η²,η²-O₂)]^3- (III), was studied by using the density functional theory (DFT) calculations method with B3LYP functional. The results indicate that coordination of H₂O₂ to I proceeds via a vanadium-center-assisted proton transfer pathway to remove the first water molecule and form a hydroperoxy intermediate [γ-PV₂W₁₀O₃₈(μ-OH) (μ-OOH)]^3- (II). And intermediate II occurs through three successive water-assisted proton transfer steps to remove the second water molecule and finally forms catalytic active species. The calculated overall energy profiles show that coordination of H₂O₂ to vanadium center requires a proton transfer barrier of about 24 kcal mol^-1. A detailed comparison of molecular geometries and electronic structure shows that the catalytic active species has a very interesting structural feature, where a superoxide radical (O₂^-) was embedded into two vanadium centers, and may be a potential nucleophile. The unique withdrawing electron properties and flexible bonding ability of the γ-Keggin-type POM ligand contribute to the formation of O₂^- radical. The tunable alternate arrangement of W-O bond series in γ-Keggin-type POM ligand contributes to the flexibility of the γ-Keggin-type POM ligand. Meanwhile, our DFT calculations show a good performance of B3LYP-gauge-independent atomic orbital (IGAIM) method for the calculation of ¹H NMR parameters of divanadium-substituted phosphotungstate.