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

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.

A porous Anderson-type polyoxometalate-based metal-organic framework as a multifunctional platform for selective oxidative coupling with amines

Incorporating catalytic units into a crystalline porous matrix represents a facile way to build high-efficiency heterogeneous catalysts, and by rational design of the porous skeleton with appropriate building blocks the catalytic performance can be significantly enhanced for a series of organic transformations owing to the synergistic effect from the multicomponent and confined porous microenvironment around catalytically active sites. Herein, we demonstrate that the design and synthesis of a porous polyoxometalate-based metal-organic framework YL2(H2O)2[CrMo6O18(PET)2]·4H2O (POMOF-1) constructed from Anderson-type [CrMo6O18(PET)2] (PET = pentaerythritol), which can be employed as a multifunctional platform for synthesis of N-containing compounds via selective oxidative coupling with amines. POMOF-1 features microporous 1D channels defined by Y3+ and L, with [CrMo6O18(PET)2] arranged orderly between adjacent Lvia electrostatic interactions. Upon using POMOF-1 as a catalyst and H2O2 as an oxidant, a variety of amines could be effectively converted to value-added amides, imines and azobenzenes via the oxidative cross-coupling with alcohols or homo-coupling. In particular, POMOF-1 showed dramatically improved activity for the N-formylation reaction owing to the synergistic and confinement effect, with the yield of amides up to 95% and 4 times higher than that of homogeneous [CrMo6O18(PET)2]. Meanwhile, the oxidative homo-coupling of arylmethylamines and arylamines can be facilely tuned by adjustment of the amount of oxidant, solvent and additive, affording imines and azobenzenes in high selectivity and yield, respectively. POMOF-1 is robust and can be reused for 5 cycles with little loss of catalytic activity and structural integrity. The work demonstrates that the combination of catalytically active POMs with crystalline porous MOFs holds great potential to build robust and recyclable heterogeneous systems with enhanced activity and selectivity for multifunctional catalysis.

Periodate-Mediated Aerobic Oxidation of Sulfides over a Bifunctional Porphyrin-polyoxometalate Catalyst: Photosensitized Singlet Oxygen Oxidation of Iodate to Periodate

Regeneration of terminal oxidants by molecular oxygen in metal-catalyzed oxidations of organic substrates has the advantage of avoiding the use of stoichiometric amounts of hazardous and/or expensive reagents to meet (some of) the green chemistry requirements. In the present study, photosensitized singlet oxygen oxidation of iodate to periodate has been used to regenerate the oxidant in polyoxometalate (POM)-catalyzed oxidation of sulfides to sulfoxides with periodate in water. To the best of our knowledge, it is the first report on singlet oxygen oxidation of iodate to periodate. In order to determine the contribution of photooxidation and oxidation pathways in the formation of sulfoxide, the oxidation of diphenyl sulfide with a very low reactivity toward aerobic photooxidation was studied; a sevenfold increase in the conversion of the sulfide to the diphenyl sulfoxide was observed for the reaction conducted in the presence of H2TMPyP-PW12O40/IO3-/O2/hν compared to that in the presence of H2TMPyP-PW12O40/O2/hν. Also, under the same conditions, a ca. 1.5-fold increase was observed in the case of methyl phenyl sulfide, which shows high reactivity toward both the oxidation and photooxidation reactions. A porphyrin-POM nanocomposite formed by the electrostatic immobilization of meso-tetra(N-methylpyridinium-4-yl)porphyrin (H2TMPyP) on PW12O40 was employed for the one-pot oxidation and photooxidation reactions. Field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), diffuse-reflectance UV-vis spectroscopy, thermal gravimetric analysis, and Fourier transform infrared were used to characterize the formation of the hybrid compound. An average particle size of 42 nm was estimated for H2TMPyP-PW12O40 from XRD peak broadening using the Scherrer equation. Also, FESEM images showed the formation of nearly spherical nanoparticles with a size of ca. 200 nm. The redshift of the Soret band of H2TMPyP upon immobilization on POM was attributed to strong N-H···O hydrogen-bond interactions between POM and porphyrin.

Heteroatom-Modulated Assembly of Hexalanthanoid-Containing Polyoxometalate-Based Coordination Networks

Two series of lanthanoid (Ln)-containing polyoxometalates (POMs) {[Ln₆(ampH)₄(H₂O)_24-n(ampH₂)_n(PW₁₁O₃₉)₂]·21H₂O (Ln = Tb, n = 0 (1), Ln = Er, n = 1 (2)) and K₂[Ln₆(ampH)₄(H₂O)₂₂(SiW₁₁O₃₉)₂]·23H₂O (Ln = Tb (3), Er (4)) (ampH₂ = (aminomethyl) phosphonic acid)} have been synthesized with tri-lacunary Keggin-type POMs containing different types of heteroatoms. Compounds 1 and 2 display neutral organic-inorganic hybrid POM molecules containing {Ln₆(ampH)₄} ({Ln₆}) cores sandwiched by two {PW₁₁O₃₉} units. By changing the heteroatoms from P^V to Si^IV, the extended 2D networks of 3 and 4 were successfully isolated where the adjacent {Ln₆} clusters were connected by {SiW₁₁O₃₉} moieties. Luminescence performances and magnetic properties of 1-4 have been systematically surveyed. The solid-state fluorescence spectra of 1-4 display characteristic emissions of Ln components resulting from the 4f-4f transitions, and energy transfer from the POM segments to Ln³⁺ centers in 1 and 3 has been observed based on the lifetime decay behaviors. Furthermore, all compounds can be utilized as electrocatalysts toward reduction of nitrite with high stability.

A {Co9}-Added Polyoxometalate for Efficient Visible-Light-Driven Hydrogen Evolution

A polyanion cluster H₆Na₈Cs₃[Co₉(μ₃-OH)₃(H₂O)₆(HPO₄)₂(B-α-PW₉O₃₄)₃]Cl·40H₂O (1) was made with the guidance of the lacunary directing strategy under hydrothermal conditions. Compound 1 was characterized by single-crystal X-ray diffraction, powder X-ray diffraction, and thermogravimetric analysis, respectively. Single-crystal X-ray diffraction analyses showed that 1 consists of three anions [B-α-PW₉O₃₄]^9- and a cyclic cationic [Co₉(μ₃-OH)₃(H₂O)₆]¹⁵⁺ and two anions HPO₄^2-. Variable-magnetic properties indicate antiferromagnetic interactions in 1. Visible-light-driven hydrogen evolution tests demonstrated that 1 was an efficient water reduction catalyst with an H₂ evolution rate of 1217.6 μmol h^-1 g^-1.

Synthesis and Structure of a Copper-Based Functional Network for Efficient Organic Dye Adsorption

In the work, by incorporating polyoxometalates (POMs) into a copper(II)-based network, a novel three-dimensional (3D) porous framework, [Cu₁₇Cl₃(trz)₁₂]H₃[GaW₁₂O₄₀]·9H₂O (Cu-GaW-TRZ), was successfully prepared and explored for the adsorption of dyes. The adsorption capacity of Cu-GaW-TRZ was calculated as 13.11 mg/g, and the dye adsorption rate equaled 96.2% for the adsorption of methylene blue (MB). Furthermore, this recyclable adsorbent is stable enough without obvious loss of adsorption capacity for at least five runs. Meanwhile, the structure of the macropores is suitable for the entry of large molecular dyes, and [GaW₁₂O₄₀]^5- also can achieve efficient adsorption for cationic dyes. The results displayed a pseudo-second-order kinetic model and were well matched for MB adsorption onto Cu-GaW-TRZ. The free energy, entropy, and enthalpy of the thermodynamic parameters for the adsorption of MB were calculated, which revealed that the adsorption process was befitting for the adsorption of MB.

Facile synthesis of polyoxometalate supported on magnetic graphene oxide as a hybrid catalyst for efficient oxidation of aldehydes

In the present study, Anderson-type polyoxometalate [N(C₄H₉)₄] [FeMo₆O₁₈(OH)₆] (FeMo₆) was immobilized on amino-modified magnetic graphene oxide and employed as a new hybrid catalyst in oxidation of aldehydes to carboxylic acids. The synthesized hybrid catalyst Fe₃O₄/GO/[FeMo₆] was characterized using thermogravimetric analysis (TGA), scanning electron microscopies (SEM), Fourier transform infrared (FT-IR), vibrating sample magnetometry (VSM), energy-dispersive X-ray analysis (EDX), Raman spectroscopy and inductively coupled plasma atomic emission spectroscopy (ICP-OES). The results indicated that our catalyst was quite active in oxidizing the aldehydes to their corresponding carboxylic acids in the presence of hydrogen peroxide. The synthesized catalyst can be easily separated from the reaction medium and reused for six consecutive runs without a significant reduction in reaction efficiency.

Polyoxometalate-Incorporated Metal-Organic Network as a Heterogeneous Catalyst for Selective Oxidation of Aryl Alkenes

Selective oxidation of aryl alkenes is important for chemical synthesis reactions, in which the key lies in the rational design of efficient catalysts. Herein, four polyoxometalate (POM)-incorporated metal-organic networks, with the formulas of [Co(ttb)(H₂O)₃]₂[SiMo₁₂O₄₀]·2H₂O (1), [Co(ttb)(H₂O)₂]₂[SiW₁₂O₄₀]·8H₂O (2), [Zn(Httb)(H₂ttb)][BW₁₂O₄₀]·9H₂O (3) and {[Zn(H₂O)₃(ttb)]₄[Zn₃(H₂O)₆]}[H₃SiW_10.5Zn_1.5O₄₀]₂·24H₂O (4) (ttb = 1,3,5-tri(1,2,4-triazol-1-ylmethyl)-2,4,6-trimethylbenzene), were hydrothermally synthesized and structurally characterized. Structural analysis showed that compound 1 consists of a POM-encapsulated three-dimensional (3-D) supramolecular framework; compound 2 is composed of a POM-supported 3-D coordination network; and compounds 3-4 show POM-incorporated 3-D supramolecular networks. Using selective catalytic oxidation of styrene as the model reaction, compounds 1-4 as heterogeneous catalysts display excellent performance with the double advantages of high styrene conversion and benzaldehyde selectivity owing to the synergistic effect among POM anions and transition metal (TM) centers. Among them, compound 1 exhibits the highest performance with ca. 96% styrene conversion and ca. 99% benzaldehyde selectivity in 3 h. In addition, compound 1 also displays excellent substrate compatibility, good reusability, and structural stability. Thus, a plausible reaction pathway for the selective oxidation of styrene is proposed. This study on the structure-function relationship paves a way for the rational design of POM-based heterogeneous catalysts for important catalysis applications.

Copper-Containing Polyoxometalate-Based Metal-Organic Frameworks as Heterogeneous Catalysts for the Synthesis of N-Heterocycles

Three new polyoxometalate-based metal-organic frameworks (POMOFs) [Cu₄(μ₃-OH)₂(tba)₃(H₂O)₅(SiW₁₂O₄₀)_0.5](H₂SiW₁₂O₄₀)_0.5·2.5H₂O (CuSiW), [Cu₃(μ₃-OH)(tba)₃(Htba)(H₂O)₂(HPMo₁₂O₄₀)]·7H₂O (CuPMo), and [Cu₄(μ₃-OH)₂(tba)₃(H₂O)₃(PW₁₂O₄₀)_0.5]₂(PW₁₂O₄₀)·0.5H₂O (CuPW) were constructed using multinuclear copper clusters, 3-(4H-1,2,4-triazol-4-yl)benzoic acid (Htba), and Keggin polyoxometalates (POMs). Different POMs regulate the formation of different multinuclear copper clusters ("boat" tetranuclear clusters in CuSiW, trinuclear clusters in CuPMo, and "chair" tetranuclear clusters in CuPW) and different topological structures of CuSiW, CuPMo, and CuPW (3-connected two-dimensional (2D) network for CuSiW, 4-connected 2D network for CuPMo, and (4,6)-connected three-dimensional network for CuPW). CuSiW, CuPMo, and CuPW as heterogeneous catalysts combine the high stability of MOFs in polar solvents and excellent catalytic activity of POMs and could be used for the synthesis of nitrogen-heterocycle compounds. The condensation cyclization reactions of 2-aminophenols/benzenesulfonyl hydrazines with 1,3-diketones produce benzoazoles and pyrazoles in good to excellent yields under the catalysis of CuPMo. Moreover, the catalyst could be reused at least for 7 runs, and this protocol was suitable for gram-scale reactions.

Chiral {Ni6PW9} Cluster-Organic Framework: Synthesis, Structure, and Properties

A chiral three-dimensional polyoxometalate cluster-organic framework (POMCOF) H₃[(btc)Ni₆(μ₃-OH)₃(H₂O)₅(B-α-PW₉O₃₄)]·17H₂O (1, btc = 1,2,4-benzenetricarboxylate) has been made under hydrothermal conditions in the absence of amine or chiral starting reagents. 1 shows high stability in CH₃CN/DMF (1:3), acidic, and basic solutions with the pH ranging from 2 to 12 for 5 days. The UV-vis reflectance spectra and Mott-Schottky measurements reveal that 1 could be a suitable catalyst for photocatalysis. Visible-light-driven H₂ evolution studies have demonstrated that 1 is an ecofriendly, efficient, and recyclable catalyst with a H₂ evolution rate of 1058.24 μmol h^-1g^-1. Nonlinear optical (NLO) measurement reveals that 1 exhibits a second-harmonic generation (SHG) response of about 1.4 times that of KH₂PO₄ (KDP), indicating that 1 is a potential NLO material as well.

Hydrocarbon Oxidation Depth: H2O2/Cu2Cl4·2DMG/CH3CN System

The oxidation of hydrocarbons of different structures under the same conditions is an important stage in the study of the chemical properties of both the hydrocarbons themselves and the oxidation catalysts. In a 50% H2O2/Cu2Cl4·2DMG/CH3CN system, where DMG is dimethylglyoxime (Butane-2,3-dione dioxime), at 50 °C under the same or similar conditions, we oxidized eleven RH hydrocarbons of different structures: mono-, bi- and tri-cyclic, framework and aromatic. To compare the composition of the oxidation products of these hydrocarbons, we introduced a new quantitative characteristic, “distributive oxidation depth D(O), %” and showed the effectiveness of its application. The adiabatic ionization potentials (AIP) and the vertical ionization potentials (VIP) of the molecules of eleven oxidized and related hydrocarbons were calculated using the DFT method in the B3LYP/TZVPP level of theory for comparison with experimental values and correlation with D(O). The same calculations of AIP were made for the molecules of the oxidant, solvent, DMG, related compounds and products. It is shown that component X, which determines the mechanism of oxidation of hydrocarbons RH with AIP(Exp) ≥ AIP(X) = 8.55 ± 0.03 eV, is a trans-DMG molecule. Firstly theoretically estimated experimental values of AIP(trans-DMG) = 8.53 eV and AIP(cis-DMG) = 8.27 eV.

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 series of POM-based compounds by tuning coordination groups and spacers of ligands: electrocatalytic, capacitive and photoelectrocatalytic properties

A series of POM-based compounds can be used as amperometric detectors for NO2− and Cr(vi). They also have capacitive properties and can effect the high-efficiency photoelectrocatalytic degradation of organic dyes.

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.

POM-based compounds modified by mono- and bis-triazole derivatives: photocatalytic, electrochemical, and supercapacitor properties

Using triazole derivatives 5 POM-based compounds with electrocatalytic and capacitive properties were obtained under hydrothermal conditions. The compounds have good photocatalytic activity for the degradation of organic dyes and the reduction of Cr(vi).

Time-Dependent Self-Assembly of Copper(II) Coordination Polymers and Tetranuclear Rings: Catalysts for Oxidative Functionalization of Saturated Hydrocarbons

This study describes a time-dependent self-assembly generation of new copper(II) coordination compounds from an aqueous-medium reaction mixture composed of copper(II) nitrate, H₃bes biobuffer (N,N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid), ammonium hydroxide, and benzenecarboxylic acid, namely, 4-methoxybenzoic (Hfmba) or 4-chlorobenzoic (Hfcba) acid. Two products were isolated from each reaction, namely, 1D coordination polymers [Cu₃(μ₃-OH)₂(μ-fmba)₂(fmba)₂(H₂O)₂]_n (1) or [Cu₂(μ-OH)₂(μ-fcba)₂]_n (2) and discrete tetracopper(II) rings [Cu₄(μ-Hbes)₃(μ-H₂bes)(μ-fmba)]·2H₂O (3) or [Cu₄(μ-Hbes)₃(μ-H₂bes)(μ-fcba)]·4H₂O (4), respectively. These four compounds were obtained as microcrystalline air-stable solids and characterized by standard methods, including the single-crystal X-ray diffraction. The structures of 1 and 2 feature distinct types of metal-organic chains driven by the μ₃- or μ-OH^- ligands along with the μ-benzenecarboxylate linkers. The structures of 3 and 4 disclose the chairlike Cu₄ rings assembled from four μ-bridging and chelating aminoalcoholate ligands along with μ-benzenecarboxylate moieties playing a core-stabilizing role. Catalytic activity of 1-4 was investigated in two model reactions, namely, (a) the mild oxidation of saturated hydrocarbons with hydrogen peroxide to form alcohols and ketones and (b) the mild carboxylation of alkanes with carbon monoxide, water, and peroxodisulfate to generate carboxylic acids. Cyclohexane and propane were used as model cyclic and gaseous alkanes, while the substrate scope also included cyclopentane, cycloheptane, and cyclooctane. Different reaction parameters were investigated, including an effect of the acid cocatalyst and various selectivity parameters. The obtained total product yields (up to 34% based on C₃H₈ or up to 47% based on C₆H₁₂) in the carboxylation of propane and cyclohexane are remarkable taking into account an inertness of these saturated hydrocarbons and low reaction temperatures (50-60 °C). Apart from notable catalytic activity, this study showcases a novel time-dependent synthetic strategy for the self-assembly of two different Cu(II) compounds from the same reaction mixture.

Five compounds based on [TeMo6O24]6− and [β-Mo8O26]4− anions synthesized by using different symmetrical and asymmetric N-donor ligands

A series of POM-based compounds can be used as amperometric detectors and the compounds based on [β-Mo₈O₂₆]⁴⁻ can not only detect the cation Cr(vi), but also can photocatalytically reduce Cr(vi).

Catalytic activity of a microporous–mesoporous composite towards liquid phase oxidation of diphenylmethane

Cobalt-loaded ZSM-5/MCM-41 composites were synthesized and characterized by different characterization techniques and the catalytic activity of the synthesized materials was studied for liquid phase oxidation of diphenylmethane.