Anderson-type polyoxometalate-based metal-organic framework as an efficient heterogeneous catalyst for selective oxidation of benzylic C-H bonds

Oxidative transformation of benzylic C-H bonds into functional carbonyl groups under mild conditions represents an efficient method for the synthesis of aromatic carboxylic acids and ketones. Here we report a high-efficiency catalyst system constructed from an Anderson-type polyoxometalate-based metal-Organic framework (POMOF-1) and N-hydroxyphthalimide (NHPI) for selective oxidation of methylarenes and alkylarenes under 1 atm O2 atmosphere. POMOF-1 exerted a synergistic effect originating from the well-aligned Anderson {CrMo6} clusters and Cu centers within the framework, and this entailed good cooperation with NHPI to catalyze the selective oxidation. Accordingly, the reactions exhibit good tolerance and chemical selectivity for a wide range of substrates bearing diverse substituent groups, and the corresponding carboxylic acids and ketones were harvested in good yields under mild conditions. Mechanism study reveals that POMOF-1 worked synergistically with NPHI to activate the benzylic C-H bonds of substrates, which are sequentially oxidized by oxygen and HOO˙ to give rise to the products. This work may pave a way to design high-efficiency catalysts by integration of polyoxometalate-based materials with NPHI for challenging C-H activation.

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.

Four octamolybdate compounds with properties of organic dye adsorption and photocatalytic reduction of Cr(VI)

Hydrothermal synthesis was used to create four different POM-based compounds, namely {[Co(Hptpm)2(β-Mo8O26)0.5(ξ-Mo8O26)0.5]}·H2O (1), [Co(Hptpm)2(δ-Mo8O26)]·H2O (2), [Co(Hptpm)2(β-Mo8O26)] (3) and [Zn(Hptpm)2(β-Mo8O26)] (4) (ptpm = 4-[3-(3-pyridine-2-yl-[1,2,4]triazol-4-yl)-propyl]-morpholine). 1-4 containing different octamolybdate isomers were characterized. Compounds 1-4 showed good electrochemical performance and can be utilized as bifunctional sensors for NO2-, H2O2, Cr(VI) and Fe(III). Taking compound 1 as an example, the detection limits are 0.081 μM for NO2-, 0.072 μM for H2O2, 0.054 μM for Cr(VI) and 0.063 μM for Fe(III), respectively. Compounds 1-4 have good capacitance. Moreover, compounds 1-4 also show good adsorption properties for organic cationic dyes. The cationic dyes include methylene blue (MB), crystal violet (CV) and neutral red (NR). In addition, 1-4 have excellent characteristics that can reduce Cr(VI) to Cr(III) by photocatalytic technology. Within 30 min, the reduction rates were 95.85% for 1, 93.99% for 2, 90.29% for 3 and 88.18% for 4.

Adsorption and Photocatalytic Degradation of Methylene Blue on TiO2 Thin Films Impregnated with Anderson-Evans Al-Polyoxometalates: Experimental and DFT Study

In this work, we fabricated a TiO2 thin film, and the same film was modified with an Anderson aluminum polyoxometalate (TiO2-AlPOM). Physical-chemical characterization of the catalysts showed a significant change in morphological and optical properties of the TiO2 thin films after surface modification. We applied the kinetic and isothermal models to the methylene blue (MB) adsorption process on both catalysts. The pseudo-second order model was the best fitting model for the kinetic results; qe (mg/g) was 11.9 for TiO2 thin films and 14.6 for TiO2-AlPOM thin films, and k2 (g mg-1 min-1) was 16.3 × 10-2 for TiO2 thin films and 28.2 × 10-2 for TiO2-AlPOM thin films. Furthermore, the Freundlich model was suitable to describe the isothermal behavior of TiO2, KF (5.42 mg/g), and 1/n (0.312). The kinetics of photocatalytic degradation was fitted using the Langmuir-Hinshelwood model; kap was 7 × 10-4 min-1 for TiO2 and 13 × 10-4 min-1 for TiO2-AlPOM. The comparative study showed that TiO2 thin films reach a 19.6% MB degradation under UV irradiation and 9.1% MB adsorption, while the TiO2-AlPOM thin films reach a 32.6% MB degradation and 12.2% MB adsorption on their surface. The surface modification improves the morphological, optical, and photocatalytic properties of the thin films. Finally, the DFT study supports all the previously shown results.

Synthesis and Biomedical Applications of Highly Porous Metal-Organic Frameworks

In this review, aspects of the synthesis, framework topologies, and biomedical applications of highly porous metal-organic frameworks are discussed. The term "highly porous metal-organic frameworks" (HPMOFs) is used to denote MOFs with a surface area larger than 4000 m² g^-1. Such compounds are suitable for the encapsulation of a variety of large guest molecules, ranging from organic dyes to drugs and proteins, and hence they can address major contemporary challenges in the environmental and biomedical field. Numerous synthetic approaches towards HPMOFs have been developed and discussed herein. Attempts are made to categorise the most successful synthetic strategies; however, these are often not independent from each other, and a combination of different parameters is required to be thoroughly considered for the synthesis of stable HPMOFs. The majority of the HPMOFs in this review are of special interest not only because of their high porosity and fascinating structures, but also due to their capability to encapsulate and deliver drugs, proteins, enzymes, genes, or cells; hence, they are excellent candidates in biomedical applications that involve drug delivery, enzyme immobilisation, gene targeting, etc. The encapsulation strategies are described, and the MOFs are categorised according to the type of biomolecule they are able to encapsulate. The research field of HPMOFs has witnessed tremendous development recently. Their intriguing features and potential applications attract researchers' interest and promise an auspicious future for this class of highly porous materials.

Application of polyoxometalates in photocatalytic degradation of organic pollutants

Organic pollutants are highly toxic, accumulative, and difficult to degrade or eliminate. As a low-cost, high-efficiency and energy-saving environmental purification technology, photocatalytic technology has shown great advantages in solving increasingly serious environmental pollution problems. The development of efficient and durable photocatalysts for the degradation of organic pollutants is the key to the extensive application of photocatalysis technology. Polyoxometalates (POMs) are a kind of discrete metal-oxide clusters with unique photo/electric properties which have shown promising applications in photocatalytic degradation. This review summarizes the recent advances in the design and synthesis of POM-based photocatalysts, as well as their application in the degradation of organic dyes, pesticides and other pollutants. In-depth perspective views are also proposed in this review.

Deep eutectic solvents as non-traditionally multifunctional media for the desulfurization process of fuel oil

Deep eutectic solvents (DESs) have been intensively pursued in the field of separation processes, catalytic reactions, polymers, nanomaterial science, and sensing technologies due to their unique features such as the low cost of components, ease of preparation, tunable physicochemical properties, negligible vapor pressure, non-toxicity, renewability, and biodegradability in the recent decade. Considering these appealing merits, DESs are widely used as extraction agents, solvents and/or catalysts in the desulfurization process since 2013. This review is focused on summarizing the physicochemical properties of DESs (i.e., freezing point, density, viscosity, ionic conductivity, acidity, hydrophilicity/hydrophobicity, polarity, surface tension, and diffusion) to some extent, and their significant advances in applications related to desulfurization processes such as extraction desulfurization, extraction-oxidation desulfurization, and biomimetic desulfurization. In particular, we systematically compile very recent works concerning the selective aerobic oxidation desulfurization (AODS) under extremely mild conditions (60 °C and ambient pressure) via a biomimetic approach coupling DESs with polyoxometallates (POMs). In this system, DESs act as multifunctional roles such as extraction agents, solvents, and catalysts, while POMs serve as electron transfer mediators. This strategy is inspirational since biomimetic or bioinspired catalysis is the "Holy Grail" of oxidation catalysis, which overcomes the difficulty of O2 activation via introducing electron transfer mediators into this system. It not only can be used for AODS, but also paves a novel way for oxidation catalysis, such as the selective oxyfunctionalization of hydrocarbon. Eventually, the conclusion, current challenges, and future opportunities are discussed. The aim is to provide necessary guidance for precisely designing tailor-made DESs, and to inspire chemists to use DESs as a powerful platform in the field of catalysis science.

Anderson-type polyoxometalate-based complexes constructed from a new ‘V’-like bis-pyridine–bis-amide ligand for selective adsorption of organic dyes and detection of Cr(vi) and Fe(iii) ions

Four isostructural complexes constructed from Anderson-type POM and new ‘V’-like ligands exhibited outstanding selective adsorption capacities for organic dyes, and electrochemical sensing behaviors toward Cr(vi) and Fe(iii) ions.

Aliphatic amine mediated assembly of [M6(mna)6] (M = Cu/Ag) into extended two-dimensional structures: synthesis, structure and Lewis acid catalytic studies

New aliphatic amine directed two-dimensional cadmium coordination polymers were shown to exhibit Lewis-acid catalytic activity for the cyanation of imines.

P5W30/g-C3N4 heterojunction thin film with improved photoelectrochemical performance for solar water splitting

For the first time, to design a new photocatalyst, a Preyssler-type polyoxometalate was composited with g-C₃N₄ to improve its photoelectrochemical performance.

Syntheses, structures and catalytic properties of Evans–Showell-type polyoxometalate-based 3D metal–organic complexes constructed from the semi-rigid bis(pyridylformyl)piperazine ligand and transition metals

Two 3D Evans–Showell-type POM-based different transition metals–bis-pyrazine–bis-amide complexes act as heterogeneous catalysts for the oxidation of benzyl alcohol.

A series of 0D to 3D Anderson-type polyoxometalate-based compounds obtained under ambient and hydrothermal conditions

Through using flexible ligands, a series of Anderson POM-based compounds have been obtained under ambient and hydrothermal conditions, respectively.

Two isopolytungstate compounds based on rare [W6O22]8−and [H2W12O42]10−fragments captured by premade copper(ii) complexes

By utilizing the premade copper complexes two rare isopolytungstate [W₆O₂₂]⁸⁻and [H₂W₁₂O₄₂]¹⁰⁻based compounds were constructed under hydrothermal and ambient conditions, respectively.

A new polyoxovanadate-based metal–organic framework: synthesis, structure and photo-/electro-catalytic properties

A new polyoxovanadate-based metal–organic framework has been synthesized, which exhibits high-performance bifunctional photo-/electro-catalytic properties.

Amperometric sensing and photocatalytic properties under sunlight irradiation of a series of Keggin–AgI compounds through tuning single and mixed ligands

In this work, we synthesized eight compounds based on Keggin anions and Ag^I ions by introducing L^a and L^b.

Temperature-controlled formation of Anderson-type compounds and their conversion to [γ-Mo8O26]4−-based variants using pendent ligands

By tuning the reaction temperature, two Anderson- and two [γ-Mo8O26]4−-based compounds decorated by pendent organic ligands, [CuII 9(bpz)2(pz)2(H2O)24][H2(Cr(OH)5Mo6O19)4]·11H2O (1), [CuII(bpz)2(H2O)2(γ-H4Mo8O26)]·2H2O (2), [CuII 2(tea)2(H2O)6(HCr(OH)6Mo6O18)2]· 6H2O (3) and [AgI(bpz)(H2O)(γ-H4Mo8O26)0.5] (4) (bpz=4-butyl-1H-pyrazole, pz=1H-pyrazole, tea=2-[1,2,4]triazol-4-yl-ethylamine), have been hydrothermally synthesized and characterized by single-crystal X-ray diffraction analysis, IR spectra and elemental analyses. In compound 1, there are two kinds of tri-nuclear CuII clusters induced by bpz and pz ligands, respectively. Four Anderson-type anions are linked by these tri-nuclear clusters to form a “W”-type subunit. In compound 2, the [Cu(bpz)2(H2O)2]2+ subunits connect the γ-Mo8 anions to construct a chain. The remaining two non-coordinated N donors in [Cu(bpz)2(H2O)2]2+ further link two adjacent γ-Mo8 anions through Mo–N bonds. In compound 3, there exists a bi-nuclear CuII cluster [Cu2(tea)2(H2O)6]4+. The discrete bi-nuclear CuII clusters and the CrMo6 anions link each other through abundant hydrogen bonding interactions. In compound 4, the [Ag(bpz)(H2O)]+ subunits connect γ-Mo8 anions to build a zigzag chain. The chains are further fused by other [Ag(bpz)(H2O)]+ cations to form a grid-like layer. There still exist Mo–N bonds in 4. We also have investigated the electrochemical and photocatalytic properties of 1–4.

A series of five-coordinated copper coordination polymers for efficient degradation of organic dyes under visible light irradiation

Five-coordinated copper(ii) coordination polymers 1–5 are highly efficient and universal photocatalysts for the degradation of the organic dyes methylene blue (MB), rhodamine B (RhB) and methyl orange (MO) under visible light irradiation.

Evans–Showell-type polyoxometalate constructing novel 3D inorganic architectures with alkaline earth metal linkers: syntheses, structures and catalytic properties

Two 3D frameworks and two 2D networks with an excellent catalytic effect of cyanosilylation were successfully obtained, originating from Evans–Showell-type polyoxoanions [Co₂Mo₁₀H₄O₃₈]⁶⁻ and alkaline earth metal cations (Sr²⁺, Ba²⁺).

Coordination polymers from a flexible alkyldiamine-derived ligand

A traditionally good chelating motif, propanediamine, has been incorporated into a robust coordination framework with vacant amine sites.

An anionic metal–organic framework with ternary building units for rapid and selective adsorption of dyes

JLU-Liu39with ternary building units including a rare hexa-nuclear [Cu₆O₂(SO₄)₆] cluster, a classical paddlewheel and a hetero-N, O donor ligand was synthesized. Owing to the ionic framework and large pore volume,JLU-Liu39shows rapid and selective adsorption of organic dyes.

Synthesis, dynamic NMR characterization and XRD studies of novel N,N'-substituted piperazines for bioorthogonal labeling

Novel, functionalized piperazine derivatives were successfully synthesized and fully characterized by ¹H/¹³C/¹⁹F NMR, MS, elemental analysis and lipophilicity. All piperazine compounds occur as conformers resulting from the partial amide double bond. Furthermore, a second conformational shape was observed for all nitro derivatives due to the limited change of the piperazine chair conformation. Therefore, two coalescence points were determined and their resulting activation energy barriers were calculated using ¹H NMR. To support this result, single crystals of 1-(4-nitrobenzoyl)piperazine (3a, monoclinic, space group C2/c, a = 24.587(2), b = 7.0726(6), c = 14.171(1) Å, β = 119.257(8)°, V = 2149.9(4) ų, Z = 4, D_obs = 1.454 g/cm³) and the alkyne derivative 4-(but-3-yn-1-yl)-1-(4-fluorobenzoyl)piperazine (4b, monoclinic, space group P2₁/n, a = 10.5982(2), b = 8.4705(1), c = 14.8929(3) Å, β = 97.430(1)°, V = 1325.74(4) ų, Z = 4, D_obs = 1.304 g/cm³) were obtained from a saturated ethyl acetate solution. The rotational conformation of these compounds was also verified by XRD. As proof of concept for future labeling purposes, both nitropiperazines were reacted with [¹⁸F]F^–. To test the applicability of these compounds as possible ¹⁸F-building blocks, two biomolecules were modified and chosen for conjugation either using the Huisgen-click reaction or the traceless Staudinger ligation.

Coordination-Driven Self-Assembly of a 2D Graphite-Like Framework Constructed from High-Nuclear Ce10 Cluster Encapsulated Polyoxotungstates

It is challenging to explore and prepare high-nuclear lanthanide (Ln) cluster-encapsulated polyoxometalates (POMs). Herein, we fabricate an unprecedented Ce10-cluster-embedded polyoxotungstate (POT) (TMA)14H2[Ce(III)(H2O)6]{[Ce(IV)7Ce(III)3O6(OH)6(CO3)(H2O)11][(P2W16O59)]3}·41H2O (1) (TMA = tetramethyleneamine) by coordination-driven self-assembly strategy, which contains the largest Ce cluster [Ce(IV)7Ce(III)3O6(OH)6(CO3)(H2O)11] (Ce10) in all the Ln-containing POM chemistry to date. Self-assembly of the in situ dilacunary [P2W16O59](12-) fragments and mixed Ce(3+) and Ce(4+) ions by means of coordination-driven force results in a novel 2D graphite-like framework constructed from mixed-valent cerium(III/IV) cluster {Ce10} encapsulated poly(POT) units and Ce(3+) ions. The most remarkable feature is that the skeleton of the centrosymmetric Ce10-cluster-embedded POT trimer contains three dilacunary [P2W16O59](12-) fragments trapping a novel {Ce10} cluster via 18 terminal-oxo and three μ4-oxo atoms.

Diverse polyoxometalate-based metal–organic complexes constructed by a tetrazole- and pyridyl-containing asymmetric amide ligand or its in situ transformed ligand

Four metal–organic complexes constructed from different polyoxoanions and a tetrazole- and pyridyl-containing asymmetric amide ligand or its in situ transformed ligand have been synthesized and structurally characterized.

Novel Anderson-type [TeMo6O24]6−-based metal–organic complexes tuned by different species and their coordination modes: assembly, various architectures and properties

Four novel Anderson-type polyoxometalate [TeMo₆O₂₄]⁶⁻-based metal–organic complexes were hydrothermally synthesized and characterized, which show fluorescence, electrochemical and photocatalytic properties.

A series of Anderson-type polyoxometalate-based metal–organic complexes: their pH-dependent electrochemical behaviour, and as electrocatalysts and photocatalysts

Seven novel polyoxometalate-based metal–organic complexes are synthesized and structurally characterized, exhibiting high electrochemical sensitivity to pH and photocatalytic activities for the degradation of organic dyes under UV irradiation.

Self-assembled construction of a sheet-type coordination polymer bearing cationic M2L4 cages: creation of channel-like space for removal of ClO4− and NO3− from aqueous solutions

A 2D coordination polymer bearing cationic Cu₂L₄ cages on the sheet showed ClO₄⁻ and NO₃⁻ removal activities from aqueous solutions.

pH-tuned diverse structures and properties: two Anderson-type polyoxometalate-based metal–organic complexes for selective photocatalysis and adsorption of organic dyes

Two novel Anderson-type POM-based metal–organic complexes were synthesized, which exhibit high selective photocatalytic activities for the degradation of organic dyes. Interestingly, 2 also shows selective adsorption capacity of organic dyes.

A series of Keggin-based compounds constructed by conjugate ring-rich pyrazine and quinoxaline derivatives

Ligands L¹ and L² are utilized in POM chemistry for the first time. Different Keggin anions induce distinct 2D structures of compounds 1/2 and 3/4 in Ag–L¹ and Ag–L² systems, respectively. By adding Ag^I and Cu^I to the PMo₁₂–L² system different 3 and 5 were formed.