Ionic γ-FeO(OH) Nanocrystal Stabilized by Small Isopolymolybdate Clusters as Reactive Core for Water Oxidation

At near neutral to basic pH, hydrolysis-induced aggregation to insoluble bulk iron-oxide is often regarded as the pitfalls of molecular iron clusters. Iron-oxide nanocrystals are encouragingly active over the molecular clusters and/or bulk oxides albeit, stabilizing such nanostructures in aqueous pH and under turnover condition remain a perdurable challenge. Herein, an Anderson-type [Mo₇ O₂₄ ]^6- isopolyanion, a small (dimension ca. 0.85 nm) isolable polyoxometalate (POM) possessing only {31} atoms, has been introduced for the first time as a covalent linker to stabilize an infinitely stable and aqueous-soluble γ-FeO(OH) nanocore. During the hydrothermal isolation of the material, a partial dissociation of the parent [Mo₇ O₂₄ ]^6- may lead to the in situ generation of few analogous [Mo_x O_y ]^n- clusters, proved by Raman study, which can also participate in stabilizing the γ-FeO(OH) nanocore, Mo_x O_y @FeO(OH). However, due to high ionic charge on {Mo=O} terminals of the [Mo_x O_y ]^n- , they are covalently linked via Mo^VI -μ₂ O-Fe^III bridging to γ-FeO(OH) core in Mo_x O_y @FeO(OH), established by numerous spectroscopic and microscopic evidence. Such bonding mode is more likely as precedent from the coordination motif documented in the transition metal clusters stabilized by this POM. The γ-FeO(OH) nanocore of Mo_x O_y @FeO(OH) behaves as potent active center for electrochemical water oxidation with a overpotential, 263 mV @ 10 mA cm^-2 , lower than that observed for bare γ-FeO(OH). Despite of some molybdenum dissolution from the POM ligands to the electrolyte, residual anionic POM fragments covalently bound to the OER active γ-FeO(OH) core of the Mo_x O_y @FeO(OH) makes the surface predominantly ionic that results in an ordered electrical double layer to promote a better charge transport across the electrode-electrolyte junction, less likely in bulk γ-FeO(OH).

MOF/POM hybrids as catalysts for organic transformations

Insertion of molecular metal oxides, e.g. polyoxometalates (POMs), into metal-organic frameworks (MOFs) opens up new research opportunities in various fields, particularly in catalysis. POM/MOF composites have strong acidity, oxygen-rich surface, and redox capacity due to typical characteristics of POMs and the large surface area, highly organized structures, tunable pore size, and shape are due to MOFs. Such hybrid materials have gained a lot of attention due to astonishing structural features, and hence have potential applications in organic catalysis, sorption and separation, proton conduction, magnetism, lithium-ion batteries, supercapacitors, electrochemistry, medicine, bio-fuel, and so on. The exceptional chemical and physical characteristics of POMOFs make them useful as catalysts in simple organic transformations with high capacity and selectivity. Here, the thorough catalytic study starts with a brief introduction related to POMs and MOFs, and is followed by the synthetic strategies and applications of these materials in several catalytic organic transformations. Furthermore, catalytic conversions like oxidation, condensation, esterification, and some other types of catalytic reactions including photocatalytic reactions are discussed in length with their plausible catalytic mechanisms. The disadvantages of the POMOFs and difficulties faced in the field have also been explored briefly from our perspectives.

POM-based porous supramolecular framework for the efficient sulfide-sulfoxide transformation with low molar O/S ratio

The selective oxidation of organic sulfides is a pivotal avenue to prepare sulfoxides that can act as synthetic intermediates of fine chemicals, bioactive molecules, and asymmetric catalysis ligands. To construct...

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.

Anderson-type polyoxometalates: from structures to functions

Anderson-type polyoxometalates (POMs) are one of the most important groups of the POM family. In the past decade, the functionalization of Anderson-type POMs has achieved significant progress and these materials have already shown unique charm in catalysis, molecular devices, energy materials, and inorganic biochemical drugs. In particular, their highly flexible topological structure and diverse functionalization methods make them the most convenient and universal platforms for rational design and controllable synthesis. This review provides a deep discussion on the recent progress in the synthetic methodology, structural exploration, and promising applications of Anderson-type POMs. It also summarizes the latest research directions and provides future prospects.

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.

Polyoxometalate-based metal–organic frameworks for heterogeneous catalysis

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

A series of A- and B-type Anderson compounds with Al, Te and Cr as centers by tuning different ligands: syntheses, electrochemical, photocatalytic and CO2RR properties

A series of N- and N/O-containing ligands were used, inducing the 2D structures of compounds 1, 3, 4 and 6, a discrete unit containing the tri-nuclear [Cu^II₃(tybm)₃(H₂O)₄]⁶⁺ cluster of 2, and a 3D framework of 5.

Copper(ii)-containing tungstotellurates(vi): syntheses, structures and their catalytic performances in selective oxidation of thioethers

We report the syntheses and structures of two new copper(ii)-containing tungstotellurates(vi) Na₁₂[Te^VI₂W₈O₃₈Cu₂(H₂O)₂]·7H₂O (Te₂W₈Cu₂) and Na₆[Te^VIW₆O₂₄Cu(NH₂CH₂CO₂)₂]·6H₂O (TeW₆Cu). The two compounds were synthesized by a simple one-pot method and characterized by single-crystal X-ray diffraction (XRD), powder XRD, FT-IR spectroscopy, elemental analysis, and thermogravimetric analysis in the solid state. Furthermore, their catalytic properties for the selective oxidation of thioethers were also studied systematically. The catalytic experiment results indicate that the tungstotellurate(vi) Te₂W₈Cu₂ is an effective heterogeneous catalyst for the selective oxidation of thioethers to sulfoxides or sulfones by an H₂O₂ oxidant at room temperature. Under the ambient conditions, Te₂W₈Cu₂ can convert 99% of methyl(phenyl)sulfane to sulfoxides or sulfones with 96% or 99% selectivity, respectively, and the utilization rate of H₂O₂ is up to 80%. Furthermore, Te₂W₈Cu₂ as a heterogeneous catalyst is stable in the reaction and could be reused at least five cycles with conserved activity.

Two copper(ii)-containing tungstotellurates(vi) were synthesized and present very high catalytic activity for the selective oxidation of thioethers under ambient conditions.

Highly selective and efficient olefin epoxidation with pure inorganic-ligand supported iron catalysts

Over the past two decades, there have been major developments in the transition iron-catalyzed selective oxidation of alkenes to epoxides; a common structure found in drug, isolated natural products, and fine chemicals. Many of these approaches have enabled highly efficient and selective epoxidation of alkenes via the design of specialized ligands, which facilitates to control the activity and selectivity of the reactions catalyzed by iron atom. Herein, we report the development of the olefin epoxidation with inorganic-ligand supported iron-catalysts using 30% H₂O₂ as an oxidant, and the mechanism is similar to iron-porphyrin type. With the catalyst 1, (NH₄)₃[FeMo₆O₁₈(OH)₆], various aromatic and aliphatic alkenes were successfully transformed into the corresponding epoxides with excellent yields as well as chemo- and stereo-selectivity. This catalytic system possesses the advantages of being able to avoid the use of expensive, toxic, air/moisture sensitive and commercially unavailable organic ligands. The generality of this methodology is simple to operate and exhibits high catalytic activity as well as excellent stability, which gives it the potential to be used on an industrial scale, and maybe opens a way for the catalytic oxidation reaction via inorganic-ligand coordinated iron catalysis.

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.

Organometallic functionalized non-classical polyoxometalates: synthesis, characterization and electrochemical properties

Two monomeric non-classical tellurium-containing heteropolymolybdate-supported metal carbonyl derivatives (NH4)5H3{[Te2Mo12(OH)O44][Mn(CO)3]}·18H2O (1) and (NH4)8{[Te2Mo12(OH)O44][Re(CO)3]}·13H2O (2) have been successfully isolated in good yields. Compounds 1 and 2 are the first two examples of metal carbonyl derivatives of heteromolybdate with tellurium as the central atom. These two compounds have been thoroughly characterized by single-crystal X-ray diffraction, elemental analyses, X-ray powder diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, and UV-Vis spectroscopy. The electrochemical catalytic activities of 1 and 2 including cyclic voltammetry and catalytic oxidation of nitrite have also been investigated.

Heterogeneous catalysis by tungsten-based heteropoly compounds

In this review, the recent works on heterogeneous catalytic applications of polyoxotungstates in liquid-phase organic reactions are reviewed.

Hybrid compounds assembled from copper-triazole complexes and phosphomolybdic acid as advanced catalysts for the oxidation of olefins with oxygen

Hybrid compounds of [CuI4(3atrz)₄][PMoVI11Mo^VO₄₀] (1) and [CuI6(3atrz)₆][PMo₁₂O₄₀]₂ (2) are active catalysts for olefin oxidation.

A survey of the different roles of polyoxometalates in their interaction with amino acids, peptides and proteins

This perspective provides a comprehensive description of the different roles of POMs in their interaction with relevant biological molecules.

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.

Heterogeneous Catalysis of Polyoxometalate Based Organic-Inorganic Hybrids

Organic–inorganic hybrid polyoxometalate (POM) compounds are a subset of materials with unique structures and physical/chemical properties. The combination of metal-organic coordination complexes with classical POMs not only provides a powerful way to gain multifarious new compounds but also affords a new method to modify and functionalize POMs. In parallel with the many reports on the synthesis and structure of new hybrid POM compounds, the application of these compounds for heterogeneous catalysis has also attracted considerable attention. The hybrid POM compounds show noteworthy catalytic performance in acid, oxidation, and even in asymmetric catalytic reactions. This review summarizes the design and synthesis of organic–inorganic hybrid POM compounds and particularly highlights their recent progress in heterogeneous catalysis.

Two novel Anderson-type polyoxometalate-based metal–organic complexes with high-efficiency photocatalysis towards degradation of organic dyes under UV and visible light irradiation

Two novel Anderson-type polyoxometalate-based metal–organic complexes constructed from two bis-pyridyl-bis-amide ligands have been hydrothermally synthesized, showing excellent photocatalytic activities toward the degradation of dyes.

Polyoxometalate based open-frameworks (POM-OFs)

Polyoxometalate-based open frameworks (POM-OFs) are extended architectures incorporating metal-oxide cluster units and comprise an emergent family of materials with a large diversity of topologies, structural flexibility and functionality at the nanoscale.

The synthesis and characterization of three organic–inorganic hybrids based on different transition metal complexes and {As8V14O42(H2O)} clusters

Three new arsenic–vanadium compounds have been prepared and the influence of structures on their catalytic properties has been discussed.

Assembly and photocatalysis of two novel 3D Anderson-type polyoxometalate-based metal–organic frameworks constructed from isomeric bis(pyridylformyl)piperazine ligands

Two 3D Anderson-based MOFs constructed from two isomeric bis(pyridylformyl)piperazine have been synthesized. The isomeric ligands and Anderson-type polyanions play key roles in the construction of the final frameworks.