Multiunit Catalysts with Synergistic Reactivity: Three-Dimensional Polyoxometalate-Based Coordination Polymers for Highly Efficient Synthesis of Functionalized p-Benzoquinones

A Stable Polyoxovanadate-based Metal-organic Framework Containing Dual Catalytic Sites for Efficient Synthesis of p-benzoquinones

A novel polyoxovanadate-based metal-organic framework (POV-MOF), [Cu(bix)]{V2O6} (V-Cu-MOF, bix=1,4-bis(imidazol-1-ylmethyl)benzene), has been successfully synthesized and characterized. The V-Cu-MOF demonstrates exceptional performance in the selective oxidation of 2,3,6-trimethylphenol to 2,3,5-trimethyl-1,4-benzoquinone within just 10 minutes under mild conditions using hydrogen peroxide as the oxidant. The V-Cu-MOF exhibits remarkable versatility, efficiently oxidizing various substituted phenols while maintaining high catalytic activity. Additionally, it shows excellent recyclability, with conversion and selectivity consistently exceeding 97 % after multiple cycles. Mechanistic studies reveal that the reaction benefits from dual-active centers {V2O6}2- clusters and binuclear copper units. The weak substrate adsorption by binuclear copper prevents catalyst poisoning, ensuring sustained activity throughout the process.

Polyoxovanadate-Based Metal-Organic Frameworks with Dual Active Sites for the Synthesis of p-Benzoquinones

p-Benzoquinones are important organic intermediates in the synthesis of biopharmaceuticals and fine chemicals. In this study, two crystalline 3D polyoxovanadate-based metal-organic frameworks, H[Cu(tpi)2]{Cu2V7O21}·H2O (1, tpi = C18N5H13) and [Co(Htpi)2]{V4O12} (2, Htpi = C18N5H14), were synthesized, which as heterogeneous catalysts showed excellent catalytic activities for the synthesis of p-benzoquinones. Both compounds were characterized by IR, UV-vis diffuse reflectance spectroscopy, TG, XPS, X-ray diffraction, etc. In 1, {Cu2V7} clusters are connected together by copper cations and 1D Cu-organic coordination chains to yield a 3D polyoxometalate-based metal-organic framework (POMOF); in 2, adjacent 2D bimetallic oxide layers, constructed from 1D polyoxovanadate chains and cobalt ions, are further connected by 1D Co-organic coordination chains to form a 3D POMOF. Noteworthily, in the synthesis of trimethyl-p-benzoquinone, the key intermediate of vitamin E, using 2,3,6-trimethylphenol as the model substrate, the turnover frequency values for compounds 1 and 2 can, respectively, reach 607 and 380 h-1 in 8 min. Furthermore, both compounds demonstrated excellent recyclability and structural stability, characterized by PXRD and IR. The catalytic mechanism reveals that both the homolytic radical mechanism and heterolytic oxygen atom transfer mechanism are involved.

Encapsulation of H4SiW12O40 into an Amide-Functionalized MOF: A Highly Efficient Nanocomposite Catalyst for Oxidative Desulfurization of Diesel Fuel

Production of hydrocarbon fuels containing sulfur in ultralow levels is in high demand and requires the development of novel catalytic systems for oxidative desulfurization (ODS). Herein, a new nanocomposite SiW12@ZSTU-10 catalyst containing H4SiW12O40 (SiW12) encapsulated into a zinc(II) 3D metal-organic framework (MOF) (ZSTU-10) was assembled and characterized. The intricate structure and porosity of ZSTU-10 permit efficient encapsulation of the catalytically active SiW12 cages. The impact of different experimental parameters on the ODS of model oil containing dibenzothiophene as a typical S-based contaminant was evaluated. The SiW12@ZSTU-10 catalyst exhibits remarkable activity with up to 99.8% sulfur removal in 30 min. Kinetic features, trapping tests, and mechanistic studies were also performed. Furthermore, the catalyst offered an outstanding thermal and chemical stability, without apparent leaching and decline in the activity after six cycles. Such an improved catalytic efficiency of SiW12@ZSTU-10 can be assigned to (i) size-matched occupation of the ZSTU-10 pores by SiW12-active species, (ii) prevention of polyoxometalate (POM) leaching from the MOF matrix, (iii) facilitation of the access of S-based substrates to the active sites of SiW12, and (iv) excellent stability and recyclability of the obtained nanocomposite. The preset work widens a family of promising nanocomposite catalysts for improving the desulfurization performance of hybrid POM-MOF catalytic systems.

Building Porous Ni(Salen)-Based Catalysts from Waste Styrofoam via Autocatalytic Coupling Chemistry for Heterogeneous Oxidation with Molecular Oxygen

The development of robust and industrially viable catalysts from plastic waste is of great significance, and the facile construction of high performance heterogeneous catalyst systems for phenol-quinone conversions remains a grand challenge. Herein, a feasible strategy is demonstrated to reclaim Styrofoam into hierarchically porous nickel-salen-loaded hypercrosslinked polystyrene (PS@Ni-salen) catalysts with high activities through an unusual autocatalytic coupling route. The salen is immobilized onto PS chain by Friedel-Crafts alkylation of benzyl chloride derivatives, and the generated hydrogen chloride coordinately promotes the simultaneous crosslinking and bridge formation between aromatic rings via a Scholl coupling route, leading to hierarchically porous networks. After the metallization with Ni, the resultant networks exhibit high catalytic activity for the oxidation of 2,3,6-trimethylphenol to 2,3,5-trimethyl-1,4-benzoquinone under mild conditions (303 K, 1 bar of O2 ). This catalyst also demonstrates attractive recycling performance without an obvious loss of catalytic efficiency over five consecutive cycles. This methodology might provide a potential sustainable alternative to construct environmentally benign and cost-effective catalysts for specific organic transformation.

Synthesis and Full Characterization of One Organometallic Polyoxometalate-Based Copper(I)-Alkene Complex

An alkene-bridged thioether ligand (L) was designed and used for its first study within a polyoxometalate (POM) hybrid system, and a POM-based copper(I)-alkene compound [(Cu^IL)₂(P^VMo^VI₁₂O₄₀)]·(Cu^IL) (1) was isolated and characterized by X-ray crystallography. A unique alkene-coordinating N(η²-C═C)N mode of L is observed, and the Cu centers are captured by σ²,π-L in a pocket fashion, giving birth to discrete [Cu^IL]⁺ cations and [(Cu^IL)₂(P^VMo^VI₁₂)]^- anions. The ionic crystal exhibits solubility in aprotic polar solvents, and the electrospray ionization mass spectrometry is used to explore the nature of species present in the solution. It is found that the whole cluster [P^VMo^VI₁₂]^3- is completely present, and all the main peaks can be assigned to different charged fragments of the same parent cluster.

Menadione: a platform and a target to valuable compounds synthesis

Naphthoquinones are important natural or synthetic compounds belonging to the general class of quinones. Many compounds in this class have become drugs that are on the pharmaceutical market for the treatment of various diseases. A special naphthoquinone derivative is menadione, a synthetic naphthoquinone belonging to the vitamin K group. This compound can be synthesized by different methods and it has a broad range of biological and synthetic applications, which will be highlighted in this review.

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.

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.

Exploring the Coordination Modes of a Keggin-Type [ZnW12O40]6- Anionic Cluster: Bonding Patterns, Crystal Structure, and Semiconducting Properties

Polyoxometalate-based organic-inorganic hybrid compounds (POIHCs) have been greatly developed due to their wide application prospects, but the pursuit of their directed synthesis via molecular design still remains a challenge. Herein, we demonstrate that the coordination modes of the Keggin-type [ZnW₁₂O₄₀]^6- anion can be tuned, which leads to different semiconductor characteristics. Using the same building block, ligand, and metal ion (ZnW₁₂, phen, Cu²⁺), we synthesized three new POIHCs with different bonding patterns by means of different coordination modes of ZnW₁₂. The three POIHCs (H₂phen){ZnW₁₂O₄₀[Cu(phen)₂]₂}·3H₂O (1), {ZnW₁₂O₄₀[Cu(phen)(H₂O)₂]₂[Cu(phen)(H₂O)]}_n·3H₂O (2), and (Me₄N)₂{ZnW₁₂O₄₀[Cu(phen)(H₂O)]₂}_n·5H₂O (3) (phen = 1,10-phenanthroline) have been structurally characterized by single-crystal X-ray diffraction. Compound 1 appears as a zero-dimensional coordination complex cluster, while compounds 2 and 3 are both 1D chain structures with different Cu²⁺ bridge linkages. Although these three POIHCs possess the same chemical components, their semiconductor properties are different, which is demonstrated by measurements of transient photocurrent and band gap (E_g) values. Furthermore, we carried out comparative experiments on the photoconductivity performance of compounds 1-3 and their photocatalytic reduction from O₂ to H₂O₂, indicating the significant influence of the energy level matching on the photocatalytic activity.

Highly Efficient Synthesis of p-Benzoquinones Catalyzed by Robust Two-Dimensional POM-Based Coordination Polymers

Selective oxidation of alkyl-substituted phenols offers efficient access to p-benzoquinones (BQs) that serve as key components for synthesizing biologically active compounds, but rational manufacture of efficient recyclable catalysts for such a reaction remains a severe challenge. Herein, two crystalline 2D polyoxometalate-based coordination polymers (POMCPs), formulated as H₃[Cu^I₃(L)₃]₂[PM₁₂O₄₀]·xH₂O (M = Mo, x = 4 for 1; M = W, x = 6 for 2; and HL = 4-(1H-tetazol-5-yl)pyridine), are prepared by a mineralizer-assisted one-step synthesis strategy and explored as heterogeneous catalysts for p-BQs synthesis. Both compounds have been characterized through elemental analysis, EDS analysis, infrared spectroscopy, UV-vis diffuse reflectance spectrum, EPR, XPS, BET, single-crystal, and powder X-ray diffraction. Single-crystal X-ray diffraction analysis indicates that both 1 and 2 exhibit an interesting 2D sheet structure composed of 2-connected Keggin type anions [PM₁₂O₄₀]^3- and hexa-nuclear {Cu^I₆(HL)₆} cluster-based metal-organic chains via Cu···O interactions. When used as catalysts, POMCPs 1 and 2 have excellent catalytic activities in the selective oxidation of substituted phenols to p-BQs with H₂O₂. Notedly, in the model reaction from 2,3,6-trimethylphenol (TMP) to the vitamin E key intermediate trimethyl-p-benzoquinone (TMBQ), the catalytic activities expressed by turnover frequency (TOF) of 1 and 2 can reach an unprecedented 2400 and 2000 h^-1, respectively, at close to 100% TMBQ yield. The truly heterogeneous nature, stability, and structural integrity of both catalysts were ascertained by FTIR, PXRD techniques, and the following cycles. Mechanism studies reveal that both catalysts can involve a dual reaction pathway through a heterolytic oxygen atom transfer mechanism and homolytic radical mechanism. Moreover, the 2D POMCPs with highly accessible bilateral active sites and efficient mass transfer efficiency possess superior catalytic performance to their analogous 3D species.

Polyoxometalate-based metal–organic frameworks for heterogeneous catalysis

POM-based MOFs simultaneously possessing the virtues of POMs and MOFs exhibit excellent heterogeneous catalytic properties.