Four Alkoxohexavanadate-Based Pd-Polyoxovanadates as Robust Heterogeneous Catalysts for Oxidation of Benzyl-Alkanes

Cavity-Directed Synthesis of Labile Polyoxometalates for Catalysis in Confined Spaces

The artificial microenvironments inside coordination cages have gained significant attention for performing enzyme-like catalytic reactions by facilitating the formation of labile and complex molecules through a "ship-in-a-bottle" approach. Despite many fascinating examples, this approach remains scarcely explored in the context of synthesizing metallic clusters such as polyoxometalates (POMs). The development of innovative approaches to control and influence the speciation of POMs in aqueous solutions would greatly advance their applicability and could ultimately lead to the formation of elusive clusters that cannot be synthesized by using traditional methods. In this study, we employ host-guest stabilization within a coordination cage to enable a novel cavity-directed synthesis of labile POMs in aqueous solutions under mild conditions. The elusive Lindqvist [M6O19]2- (M=Mo or W) POMs were successfully synthesized at room temperature via the condensation of molybdate or tungstate building blocks within the confined cavity of a robust and water-soluble Pt6L4(NO3)12 coordination cage. Importantly, the encapsulation of these POMs enhances their stability in water, rendering them efficient catalysts for environmentally friendly and selective sulfoxidation reactions using H2O2 as a green oxidant in a pure aqueous medium. The approach developed in this paper offers a means to synthesize and stabilize the otherwise unstable metal-oxo clusters in water, which can broaden the scope of their applications.

Polyoxometalate-based ionic liquids: efficient reversible phase transformation-type catalysts for thiolation of alcohols to construct C-S bonds

As important building blocks in natural products and organic synthesis, thioethers have a wide range of potential applications. Herein, polyoxometalate-based ionic liquids (POM-ILs-SO3H) derived from N-alkyl imidazole were synthesized and used for the first time for the thiolation of alcohols to construct C-S bonds in a series of benzyl thioethers. This type of POM-ILs-SO3H catalyst exhibited high catalytic activity, providing up to 98% yield of thioether within 1 h at 70 °C. The alkyl chain length of the imidazole had a certain effect on the solubility of the POM-ILs-SO3H catalysts in the reaction solvent, and then affected their catalytic activity. The catalytic system had a wide substrate scope and was suitable for the reaction of tertiary and secondary benzyl alcohols with thiophenols or cycloalkyl thiols. In particular, [PIMPS]3PW12O40 (PIM = 1-propylimidazole, PS = propane sulfonate) as a reversible phase transformation-type catalyst, combining the advantages of homogeneous and heterogeneous catalysts, exhibited high activity and good recyclability with only a slight decrease in the yield after five runs. Additionally, a carbocation mechanism was proposed for the thiolation reaction of alcohols.

Improved Catalytic Performance toward Selective Oxidation of Benzyl Alcohols Originated from New Open-Framework Copper Molybdovanadate with a Unique V/Mo Ratio

A new organic-inorganic hybrid open-framework molybdovanadate with mixed-valences of vanadium (V4+ /V5+ =4/3) and molybdenum (Mo5+ /Mo6+ =8/2) cations has been synthesized. The complex possesses the unique V/Mo ratio (7/10), fascinating 8-C topological network and 1D 4-MR channels (7.793 Å×6.699 Å). Importantly, its catalytic activities for the selective oxidation of benzyl alcohol to benzaldehyde (oxidant: H2 O2 , 30 wt %) have been well evaluated. The results indicated that it exhibited improved catalytic activities (conv.: 96.8 %) compared with the catalyst (Cpyr)5 PV2 Mo5 W5 O40 [conv.: 88.51 %, Cpyr=(C16 H32 C5 H4 N)+ )], high recyclability and structural stability. Moreover, the conversions and selectivities (conv.: 82.4-92.5 %; sele.: 91.5-95.7 %) of the substrates containing electron donating groups (-OH, -CH3 , -OCH3 and -Cl) were significantly higher than those of the substrate containing electron withdrawing group (-NO2 ) (conv. 67.4 %; sele.: 80.8 %). This is due to the fact that the -NO2 with a large Hammett substituent constant is not conducive to the generation of transition state products. The studies revealed the complex could act as a highly efficient heterogeneous catalyst in selective oxidation of benzyl alcohols.

Recent advances in oxidative catalytic applications of polyoxovanadate-based inorganic-organic hybrids

Polyoxovanadates (POVs) have received widespread attention in catalytic applications due to their various structures and remarkable redox properties. By introducing a second transition metal, POV-based inorganic-organic hybrid (POVH) catalysts show increasing stability and more catalytic active sites compared with pure POVs. In this perspective article, POVH materials as oxidative catalysts have been classified into two main categories according to the interactions between transition metal-complex units and POV clusters: (i) hybrids with metal-organic units act as isolated cations and (ii) hybrids with an organic ligand coordinate to the second transition metal, which is further linked to a POV cluster via oxygen bridges directly or indirectly to give zero-, one-, two- or three-dimensional supramolecular structures. The oxidative conversion of organic compounds, including thiophene derivatives, thioethers, alkanes, alcohols, and alkenes, and oxidative detoxification of a sulfur mustard simulant or degradation of lignin, along with the oxidative photo/electrocatalytic transformation of organic compounds catalyzed by POVH materials, are discussed in detail. Furthermore, the challenges and prospects toward the development of POVH catalysts are explored briefly from our perspectives.

A Keggin-type polyoxometalate-based metal-organic complex as a highly efficient heterogeneous catalyst for the selective oxidation of alkylbenzenes

The direct oxidation of C-H bonds in organic materials into necessary oxygen-containing compounds under mild conditions has attracted increasing attention. A Keggin-type polyoxometalate-based metal-organic complex (MOC), [CuII4Cu^I(H₂trz)₄(C₂O₄)(H₂O)₄(H₃PW_11.18CuII0.82O₄₀)]·8H₂O (1) (H₃trz = 1,2,4-triazole), was designed and synthesized under hydrothermal conditions, and was structurally characterized by single crystal X-ray diffraction, PXRD, IR spectroscopy, TGA, and XPS. Complex 1 is a 3D 4,6,8-connected architecture derived from [Cu₅(H₂trz)₄(C₂O₄)₂(H₂O)₂]_n units and [PW_11.18Cu_0.82O₄₀]^6- anions, which can catalytically oxidize various types of alkylbenzenes. Gas chromatographic analysis showed that complex 1 as a heterogeneous catalyst could effectively catalyze the oxidation of diphenylmethane with 93% conversion and 99% selectivity within 6 h. In addition, the conversion for the catalytic oxidation of ethylbenzene was 96% with 99% selectivity. Compared with some reported catalysts, complex 1 exhibited a better catalytic effect and lower reaction time. Meanwhile, the catalytic oxidation of other benzyl derivatives with complex 1 was also investigated, which indicated that complex 1 possessed excellent catalytic performance.

A self-assembly solvothermal synthesis of SiMoVn@[Cu6O(TZI)3(H2O)6]4·nH2O for the efficient selective oxidation of various alkylbenzenes

A series of SiMoVn@rht-MOF-1 were isolated via a one-pot self-assembly solvothermal synthesis, exhibiting effective catalytic activity and excellent recyclability.

Polyoxometalate-based CuII/CoII complexes tuned using various metal–pyrazole loops: design, diverse architectures and catalytic activity toward the oxidation of thioether derivatives

Three new polyoxometalate-based complexes with unique metal–pyrazole loops were synthesized, showing various architectures and finally resulting in different catalysis performance.

Polyoxometalate-Incorporated Framework as a Heterogeneous Catalyst for Selective Oxidation of C-H Bonds of Alkylbenzenes

Developing new catalysts for highly efficient and selective oxidation of saturated C-H bonds is significant due to their thermodynamic strength. Via incorporation of PW₁₂O₄₀^3-, pyridine-2,5-dicarboxylic acids (pydc), and Fe(III) ions into one framework, a new polyoxometalate-based metal-organic framework, [HFe₄O₂(H₂O)₄(pydc)₃PW₁₂O₄₀]·10.5H₂O (FeW-PYDC), was successfully prepared by a hydrothermal method. Interestingly, FeW-PYDC features a three-dimensional porous structure with {Fe₄O₂} interconnecting with PW₁₂O₄₀^3- units. FeW-PYDC displayed excellent performance in the selective oxidation of C-H bonds of alkylbenzenes with high conversion (95.7%) and selectivity (96.6%). As an effective heterogeneous catalyst, FeW-PYDC demonstrates good reusability and structural stability.

Regulating the catalytic activity of multi-Ru-bridged polyoxometalates based on differential active site environments with six-coordinate geometry and five-coordinate geometry transitions

Five-coordinate geometry around ruthenium with highly exposed active sites has attracted intensive scientific interest due to its superior properties and extensive applications. Herein, we report a series of structurally controllable multi-Ru-bridged polyoxometalates, K5NaH10[{Ru4(H2O)n}(WO2)4(AsW9O33)4]·mH2O {1, 1-dehyd-373K, 1-dehyd-473K, 1-dehyd-573K; n = 4, m = 36; n = 4, m = 6; n = 4, m = 0; n = 0, m = 0} fabricated through a feasible assembly strategy using arsenotungstate {2, KNa12H17Cl2(As4W40O140)·29H2O} as a structure-directing unit. Systematic characterization methods identified that the six-coordinate geometry can successfully transform into five-coordinate geometry about active sites (Ru) by removing aqua ligands under high reaction temperatures. All the multi-Ru-bridged polyoxometalates demonstrated strong stability and catalytic effectiveness in the transformation of 1-(4-chlorophenyl)ethanol to 4'-chloroacetophenone under very mild conditions. 1-dehyd-573K, specifically, achieves the best catalytic effectiveness with a turnover frequency (TOF) = 25 100·h-1 owing to its unique five-coordinate geometry on the Ru sites. To our knowledge, 1-dehyd-573K outperforms other POM-based catalysts in the oxidative catalysis of 1-(4-chlorophenyl)ethanol. The heterogeneous polyoxometalates were also proven to be strongly reusable, with their structural integrities well maintained after multiple-cycle catalytic reactions.

A Copper-Containing Polyoxometalate-Based Metal-Organic Framework as an Efficient Catalyst for Selective Catalytic Oxidation of Alkylbenzenes

A copper-containing polyoxometalate-based metal-organic framework (POMOF), Cu^I₁₂Cl₂(trz)₈[HPW₁₂O₄₀] (HENU-7, HENU = Henan University; trz = 1,2,4-triazole), has been successfully synthesized and well-characterized. In addition, the excellent catalytic ability of HENU-7 has been proved by the selective oxidation of diphenylmethane. Under the optimal conditions, the diphenylmethane conversion obtained over HENU-7 is 96%, while the selectivity to benzophenone is 99%, which outperforms most noble-metal-free POM-based catalysts. Moreover, HENU-7 is stable to reuse for five runs without an obvious loss in activity and also can catalyze the oxidation of different benzylic C-H with satisfactory conversions and selectivities, which implied the significant catalytic activity and recyclability.

α-γ-Type [Mo8O26]4--Containing Metal-Organic Complex Possessing Efficient Catalytic Activity toward the Oxidation of Thioether Derivatives

In this work, a new α-γ-type [Mo₈O₂₆]^4- anion was first synthesized and characterized by single-crystal X-ray diffraction analysis and was obtained by introducing molybdate to the synthesis of metal-organic complex (MOC) under hydrothermal conditions. An octamolybdate-based MOC, namely, {[Cu₈(H₂O)₆](dpyh)₄(α-γ-Mo₈O₂₆) }·(β-Mo₈O₂₆)·8.5H₂O (H₂dpyh = N,N-bis(3-pyrazolamide)-1,2-hexahydrobenzene), was obtained. The α-γ-type [Mo₈O₂₆]^4- anion was composed of four MoO₆ octahedra and four MoO₅ trigonal bipyramids by sharing their edges and corners. The title complex exhibited a 1D structure in which an α-γ-type [Mo₈O₂₆]^4- anion was connected with [Cu₄(dpyh)₂] units in a staggered manner. Under optimized conditions, complex 1 as the catalyst can achieve a highly efficient conversion (more than 99%) of thioanisole within 30 min and above 99% selectivity toward sulfoxide. Furthermore, efficient catalytic oxidation of thioether derivatives was also performed with 1 as the catalyst. In addition, the stable electrochemical sensing performance and adsorption capacity toward organic dyes were tested.

Tandem-like vanadium cluster chains in a polyoxovanadate-based metal–organic framework for efficient catalytic oxidation of sulfides

The vanadium cluster chain in V-Ni-MOF can efficiently catalyze the oxidation of sulfides with hydrogen peroxide as the oxidant, achieving the complete conversion from sulfides to sulfones within 1 hour at 40 °C.

A nanohybrid self-assembled from exfoliated layered vanadium oxide nanosheets and Keggin Al13 for selective catalytic oxidation of alcohols

A porous V₂O₅–Al₁₃ nanohybrid based on the self-assembly of Keggin Al₁₃ and exfoliated V₂O₅ nanosheets for selective oxidation of alcohols.

Inorganic–organic hybrid polyoxovanadates based on [V4O12]4− or [VO3]22− clusters: controllable synthesis, crystal structures and catalytic properties in selective oxidation of sulfides

Three inorganic–organic hybrid polyoxovanadates have been synthesized. Among them, complex [Ni₂(1-vIM)₇H₂O][V₄O₁₂]·H₂O (1) exhibits extraordinary heterogeneous catalytic performance in the selective oxidation of sulfides using H₂O₂ as the oxidant.

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.

One-Step Template-Free Fabrication of Ultrathin Mixed-Valence Polyoxovanadate-Incorporated Metal-Organic Framework Nanosheets for Highly Efficient Selective Oxidation Catalysis in Air

Polyoxometalate (POM)-based metal-organic frameworks (MOFs) with nanostructure represent a class of promising heterogeneous nanocatalysts. As yet, direct one-step controllable synthesis of pure nanoscale POM-MOFs catalysts is an extremely huge challenge owing to highly complicated synthetic conditions. Herein, for the first time, we fabricated ultrathin (∼5 nm) mixed-valence {V₁₆} clusters-incorporated metal-organic framework nanosheets [Ni(4,4'-bpy)₂]₂ [V₇^IVV₉^VO₃₈Cl]·(4,4'-bpy)·6H₂O (NENU-MV-1a) via one-step template-free strategy and successfully achieved one-step removal of all impurities from the multicomponent complex system. The obtained NENU-MV-1a nanosheets possess dramatically different physiochemical properties from bulk crystal, including larger lateral area, and more active sites originated from their nanostructures. As a proof-of-concept application, NENU-MV-1a was applied in olefin epoxidation in air and exhibited more excellent catalytic activity (95% conversion) than the bulk crystal (35%). In addition, detailed catalytic mechanism studies revealed the structure-property correlations of NENU-MV-1a and proposed V^IV-V^V synergistic catalytic effect. Our investigations are of great significance for the development of more active and/or selective mixed-valence metal-oxygen cluster-based MOF nanocatalysts.

Enhancing catalytic aerobic oxidation performance of cyclohexaneviasize regulation of mixed-valence {V16} cluster-based metal–organic frameworks

Size-controllable synthesis of mixed-valence {V₁₆} cluster-based metal–organic frameworks used for highly efficient catalytic oxidation of cyclohexane.

Two organic–inorganic hybrid polyoxovanadates as reusable catalysts for Knoevenagel condensation

Two novel polyoxovanadates as heterogeneous catalysts have exhibited excellent catalytic properties in the Knoevenagel condensation, especially compound 1's activity is basically maintained after three cycles.

A polyoxometalate-based metal–organic polyhedron constructed from a {V5O9Cl} building unit with rhombicuboctahedral geometry

The design and construction of metal–organic polyhedra has received much attention by chemists due to the intriguing diversity of architectures and topologies that can be achieved. There are several crucial factors which should be considered for the construction of metal–organic polyhedra, such as the starting materials, reaction time and temperature, solvent and suitable organic ligands. Recently, polyoxometalates (POMs), serving as secondary building units to construct POM-based metal–organic polyhedra, have been the subject of much interest. The title compound, dodecakis(dimethylammonium) octakis(μ-benzene-1,3,5-tricarboxylato)hexa-μ-chlorido-tetracosa-μ-oxido-triacontaoxidotriacontavanadium, (NH2Me2)12[(V5O9Cl)6(C9H3O6)8], was synthesized successfully by self-assembly of VOCl3 and benzene-1,3,5-tricarboxylic acid under solvothermal conditions. The title polyhedron has an rdo topology when the {V5O9Cl} building unit and the benzene-1,3,5-tricarboxylate (BTC3−) ligand were simplified into 4-connected and 3-connected vertices. Interestingly, when the {V5O9Cl} building unit and the BTC3− ligand are considered as quadrangular and triangular faces, the structure displays rhombicuboctahedral geometry with an outer diameter of 21.88 Å. The packing of the polyhedra produces a circular channel with a diameter of 8.2 Å. The title compound was characterized by single-crystal X-ray diffraction, elemental analysis, IR spectroscopy, thermogravimetric analysis and powder X-ray diffraction.

Tuning the redox profiles of polyoxovanadate-alkoxide clusters via heterometal installation: toward designer redox Reagents

Herein, we describe a rational synthetic approach for tuning the electrochemical profiles of a series of Lindqvist polyoxovanadate-alkoxide clusters through heterometal functionalization. Synthetic procedures for group(IV) functionalization of the mixed-valent POV-alkoxide cluster, [V₆O₇(OCH₃)₁₂], are established, resulting in the heterometallic species, [NBu₄][V₅O₆(OCH₃)₁₂MOCH₃] (M = Ti, Zr, Hf). We demonstrate that these d⁰, heterometallic dopants anodically shift the potential of the electrochemical processes associated with the cluster, making the molecule more resistant to oxidation. Conversely, incorporation of electron rich heterometals yields a more readily oxidized molecule, with redox processes shifted cathodically. The predictable tuning and remarkable electrochemical profiles of this family of heterometal-functionalized polyoxometalates highlights their potential use as designer redox agents.

Synthesis, structures, and magnetic properties of metal-organic polyhedra based on unprecedented {V7} isopolyoxometalate clusters

Two isostructural vanadium-based metal-organic polyhedra (denoted as VMOP-16 and VMOP-17) were synthesized by a solvothermal method, which are built from unprecedented {V₇} isopolyoxometalate clusters and dicarboxylate ligands. To our knowledge, the {V₇} second building unit is reported for the first time and features the highest nuclearity of vanadium-oxygen clusters compared with reported vanadium-based MOPs.

Diversified polyoxovanadate derivatives obtained by copper(i)-catalysed azide–alkyne cycloaddition reaction: their synthesis and structural characterization

Four novel 1,2,3-triazole functionalized polyoxovanadate (POV) organic–inorganic hybrids were successfully prepared through the copper-catalyzed azide–alkyne cycloaddition (CuAAC) procedure for the first time.

Three Pd-decavanadates with a controllable molar ratio of Pd to decavanadate and their heterogeneous aerobic oxidation of benzylic C–H bonds

Three Pd-decavanadates have been synthesized and they are active in the heterogeneous aerobic oxidation of benzylic hydrocarbons.

Crystal structure of bis(3-(3-ethylureido)-N,N-dimethylpropan-1-aminium) bis (μ3-2-(hydroxymethyl)-2-(oxidomethyl)propane-1,3-bis(olato))-(μ6-oxo)-hexakis(μ2-oxo)-hexaoxo-hexavanadium(V) – dichloromethane (1/1), C27H60Cl2N6O23V6

C27H60Cl2N6O23V6, C2/c (no. 15), a = 26.1482(12) Å, b = 10.6096(4) Å, c = 17.4426(6) Å, β = 90.932(4)°, V = 4838.3(3) Å3, Z = 4, Rgt(F) = 0.0565, wRref(F2) = 0.1477, T = 106 K.

The cumene/O2 system: a very simple tool for the radical chain oxidation of some functional groups

Due to the relative stability of the cumyl radical, cumenes and α-methyl-styrenes are ideally structured to directly harvest the oxidizing reactivity of O2 and initiate radical chain reactions in catalyst-free conditions. In the absence of additional substrates, these processes can lead to acetophenones. In the presence of substrates, the cumene oxidation process can be intercepted in various chain reactions, affording very simple protocols for functional group oxidation.

Two 2-D multifunctional cobalt(ii) compounds: field-induced single-ion magnetism and catalytic oxidation of benzylic C–H bonds

Two 2-D multifunctional cobalt(ii) compounds were reported with both field-induced single-ion magnetism and catalytic oxidation of benzylic C–H bonds.