Porous {P6Mo18O73}-type Poly(oxometalate) Metal-Organic Frameworks for Improved Pseudocapacitance and Electrochemical Sensing Performance

{P₆Mo₁₈} poly(oxometalate) (POM) clusters have huge steric hindrance and limited active oxygen atoms, which make them difficult to combine with metal-organic units to form three-dimensional (3D) porous structures. Therefore, functionalization of such POMs has always been a bottleneck that is difficult to break through. In this study, {P₆Mo₁₈} POM was successfully grafted on a lock-like metal-organic chain to generate a multiporous coordination polymer, [{Na(H₂O)(H₂btb)}{Cu₄^I(H₂O)(pz)₅Cl}{H₂Sr⊂P₆Mo₂^VMo₁₆^VIO₇₃}]·3H₂O (1) (pz = pyrazine; btb = 1,4-bis(1,2,4-triazole) butane). Meanwhile, a zero-dimensional (0-D) control compound with only btb ligands as counterions, (H₄btb)[H₄Sr⊂P₆Mo₂^VMo₁₆^VIO₇₃]·3H₂O (2), was also obtained via a hydrothermal reaction. Compound 1 represents the first basket-type 3D poly(oxometalate) metal-organic framework (POMOF) assembly, which possesses interpenetrating pores and complex topology. 1-GO-CPE displays improved supercapacitor (SC) performance (the specific capacitance of 929.4 F g^-1 at a current density of 3 A g^-1 with 94.1% of cycle efficiency after 5000 cycles) compared with 2-GO-CPE and most reported POMOF electrode materials, which may be due to the outstanding redox capability of basket-POM, introduction of metal-organic chains, intersecting pores, and excellent conductivity of graphene. An asymmetric SC device with 1-GO-CPE as the negative electrode exhibits an energy density of 29.7 Wh kg^-1 with a power density of 3148.2 W kg^-1 and long-lasting cycling life. In addition, 1-GO-GCE as an electrochemical sensor responds to dopamine (DA) at a voltage of 0.40 V and shows lower detection limits (0.19 μM (signal-to-noise ratio (SNR) = 3)), higher selectivity, and good reproducibility in the linear range of 0.56 μM to 0.24 mM. The ability to accurately detect the content of DA in biological samples further proves the feasibility of the sensor in practical applications.

Designed Syntheses of Three {Ni6PW9}-Based Polyoxometalates, from Isolated Cluster to Cluster-Organic Helical Chain

Three new hexa-Ni-substituted Keggin-type polyoxometalates (POMs), [Ni6(OH)3- (DACH)3(H2O)6(PW9O34)]·31H2O (1), [Ni(DACH)2][Ni6(OH)3(DACH)3(HMIP)2(H2O)2(PW9O34)]·56 H2O (2), and [Ni(DACH)2][Ni6(OH)3(DACH)2(AP)(H2O)5(PW9O34)]·2H2O (3) (DACH = 1,2-Diami- nocyclohexane, MIP = 5-Methylisophthalate, AP = Adipate) were successfully made in the presence of DACH under hydrothermal conditions. 1 is an isolated hexa-Ni-substituted Keggin unit decorated by DACH. In order to further construct POM cluster-organic frameworks (POMCOFs) on the basis of 1, by analyzing the steric hindrances and orientations of the POM units, the rigid HMIP and flexible AP ligands were successively incorporated, and another anionic monomeric POM 2 and the new 1D POM cluster organic chain (POMCOC) 3 were obtained. HMIP ligand still acts as a decorating group on the Ni6 core of 2 but results in the different spatial arrangement of the {Ni6PW9} units. AP ligands in 3 successfully bridge adjacent isolated POM cluster units to 1D POMCOC with left-hand helices. The AP in 3 is the longest aliphatic carboxylic acid ligand in POMs, and the 1D POM cluster-AP helical chain represents the first 1D POMCOC with a helical feature.

Designed assembly of heterometallic zeolite-like framework materials from two different supertetrahedral metal clusters

This work demonstrates a feasible strategy for the synthesis of new zeolite-like framework materials. With the strategy of using two different supertetrahedral clusters as secondary building units, two new heterometallic zeolite-like frameworks with isomeric structures but tunable topologies were first made. Besides, the porous nature and the proton conduction performance of the new materials were further studied.

Ultrastable Polyoxometalate-Encapsulated Supramolecular Metal-Organic Nanotubes for Single-Crystal Proton Conduction

Two unique polyoxometalate (POM)-encapsulated tubular materials with the formula K(H₂O)₆[M₆(btp)₆(H₂O)₂₂](P₂W₁₈O₆₂)₃(Hbtp)₅(btp)₃·52H₂O [M = Mn (1) and Co (2); btp = 2,6-bis(1,2,4-triazol-1-yl)pyridine] were designed and synthesized based on the Dawson POM and V-type btp ligand, as confirmed by IR, X-ray diffraction (XRD), and element analysis. Single-crystal XRD analyses of compound 1 show that two kinds of remarkable metal-organic supramolecular nanotubes, including trigonal and hexagonal nanotubes, are constructed along the c-axis direction via π···π-packing interactions between {Mn₃(btp)₃} rings and the btp ligands, of which [α-P₂W₁₈O₆₂]^6- anions are confined in channels, making the entire structure extraordinarily stable. Meanwhile, the coordinated [α-P₂W₁₈O₆₂]^6- anion within the hexagonal channel makes the channel highly hydrophilic and attracts a number of guest water molecules to fill in the free space, conducive to proton transport. Therefore, the single-crystal sample of 1 exhibits a high proton conductivity of 6.39 × 10^-3 S cm^-1 along one-dimensional channels, 30 times higher than that of a pellet sample at 358 K and 98% relative humidity.

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.

Two POM-based compounds containing Zn-capped Keggin anions as decent heterogeneous catalysts for sulfur oxidation and cycloaddition of CO2 reactions

Carbon dioxide (CO2) and the combustion of sulfide in gasoline are the main causes of air pollution. Great deal of attention has been committed to solving the problem and the...

Prediction of Solution Behavior via Calorimetric Measurements Allows for Detailed Elucidation of Polyoxometalate Transformation

The solution behavior of a polyoxometalate cluster, LiNa-U₂₄Pp₁₂ (Li₂₄Na₂₄[(UO₂O₂)₂₄(P₂O₇)₁₂]) that consists of 24 uranyl ions, peroxide groups, and 12 pyrophosphate linkers, was successfully predicted based on new thermodynamic results using a calorimetric method recently described for uranyl peroxide nanoclusters (UPCs), molybdenum blues, and molybdenum browns. The breakdown of LiNa-U₂₄Pp₁₂ and formation of U₂₄ (Li₂₄[UO₂O₂OH]₂₄) was monitored in situ via Raman spectroscopy using a custom heating apparatus. A combination of analytical techniques confirmed the simultaneous existence of U₂₄Pp₁₂ and U₂₄ midway through the conversion process and U₂₄ as the single end product. The application of a molecular weight filter resulted in a complete and successful separation of UPCs from solution and, in conjunction with DOSY results, confirmed the presence of large intermediate cluster building blocks.

Recent progress and perspectives on the structural design on metal-organic zeolite (MOZ) frameworks

As a typical group of coordination polymers, metal-organic zeolite (MOZs) frameworks inherit the topological and structural advantages of inorganic zeolites and display great application potential in many areas, including gas adsorption/separation, catalysis, luminescence and chemical sensing. In this review, we outline the recent progress in the synthesis, functionalization and application of metal-organic zeolite frameworks, mainly focusing on the basic structural design principle and synthesis strategy on 4-connect inorganic nodes and 2-connect organic linkers. Employing different valent metals, small inorganic TO₄^2- units and high-nuclear clusters as 4-connect nodes, we derived multi-types of MOZs with a modified framework charge, improved stability and enhanced photo-/eletrocatalytic activity. Besides, the selection, functionalization and defect-engineering on the 2-connect ligands generated different topological and functional MOZs. Finally, the future trends and some perspectives in this area are outlined.

Polyoxometalate-based metal–organic frameworks for heterogeneous catalysis

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

Proton conductive polyoxoniobate frameworks constructed from nanoscale {Nb68O200} cages

Nanoscale {Nb₆₈O₂₀₀} cages have been successfully employed as flexible and stable secondary building units to combine with bridging copper–amine complexes to construct two proton conductive polyoxoniobate frameworks, demonstrating a promising strategy for making new porous materials.

A Rare 3D Porous Inorganic-Organic Hybrid Polyoxometalate Framework Based on a Cubic Polyoxoniobate-Cupric-Complex Cage with a High Water Vapor Adsorption Capacity

A rare 3D porous inorganic-organic polyoxoniobate framework based on the cubic polyoxoniobate-cupric-complex cage {[Cu(en)₂]@{[Cu₂(en)₂(trz)₂]₆(Nb₆₈O₁₈₈)}} (1a), has been successfully synthesized by a hydrothermal method. The cubic cages 1a are connected with 4-(tetrazol-5-yl)pyridine to form a 1D pillar-like chain structure, and every 1D pillar-like chain is further linked with four adjacent pillar-like chains by the [Cu(en)₂]²⁺ complex to form a 3D porous inorganic-organic polyoxoniobate framework with 4-connected CdSO₄-type topology. To our knowledge, it is the first time that three different types of organic ligands are simultaneously introduced into one polyoxoniobate. This material also exhibits a high vapor adsorption capacity and good ionic conductivity properties.

Water and Metal-Organic Frameworks: From Interaction toward Utilization

The steep stepwise uptake of water vapor and easy release at low relative pressures and moderate temperatures together with high working capacities make metal-organic frameworks (MOFs) attractive, promising materials for energy efficient applications in adsorption devices for humidity control (evaporation and condensation processes) and heat reallocation (heating and cooling) by utilizing water as benign sorptive and low-grade renewable or waste heat. Emerging MOF-based process applications covered are desiccation, heat pumps/chillers, water harvesting, air conditioning, and desalination. Governing parameters of the intrinsic sorption properties and stability under humid conditions and cyclic operation are identified. Transport of mass and heat in MOF structures, at least as important, is still an underexposed topic. Essential engineering elements of operation and implementation are presented. An update on stability of MOFs in water vapor and liquid systems is provided, and a suite of 18 MOFs are identified for selective use in heat pumps and chillers, while several can be used for air conditioning, water harvesting, and desalination. Most applications with MOFs are still in an exploratory state. An outlook is given for further R&D to realize these applications, providing essential kinetic parameters, performing smart engineering in the design of systems, and conceptual process designs to benchmark them against existing technologies. A concerted effort bridging chemistry, materials science, and engineering is required.

All-inorganic open frameworks based on gigantic four-shell Ln@W8@Ln8@(SiW12)6 clusters

A series of POM-based all-inorganic open frameworks, H₉[Ln₉W₈(μ₄-O)₁₂(μ₂-O)₂₄(H₂O)₂₄](SiW₁₂O₄₀)₃·60H₂O (1-Ln, Ln = La, Pr and Nd), have been fabricated and structurally characterized for the first time. The unique architectures are constructed from nanoscale four-shell Ln@W₈@Ln₈@(SiW₁₂)₆ clusters. Additionally, the porous nature of the frameworks has also been investigated.

A rare 4-connected neb-type 3D chiral polyoxometalate framework based on {KNb24O72} clusters

A rare 3D porous chiral polyoxoniobate framework has been constructed from two types of isomeric {Cu(en)₂KNb₂₄O₇₂} clusters with a 4-connected neb-type topology. This compound exhibits proton conduction and high water vapor adsorption properties.

Graphite-like polyoxometalate-based metal–organic framework as an efficient anode for lithium ion batteries

A new porous POM supported graphite-like MOF (Cu-POM) as a LIB anode material was designed and synthesized, and its lithium storage mechanism was explored using impedance spectra.

A heterometallic D–A hybrid heterostructural framework with enhanced visible-light photocatalytic properties

A heterometallic D–A hybrid heterostructural framework from the ternary combination of CuI, lanthanide cations and viologen tectons exhibited enhanced photocatalytic activities for degradation of organic dyes.

An unprecedented cobalt(ii)-containing Wells–Dawson-type tungstovanadate-based metal–organic framework as an efficient catalyst for ring-opening polymerization of ε-caprolactone

An unprecedented organic–inorganic hybrid material based on Wells–Dawson-type tungstovanadate building blocks and cobalt(ii)–organic framework with a bis(triazole) ligand was prepared and employed to catalyze solvent-free ROP of caprolactone.

Multifunctional Polymolybdate-Based Metal-Organic Framework as an Efficient Catalyst for the CO2 Cycloaddition and as the Anode of a Lithium-Ion Battery

A three-dimensional polymolybdate-based metal-organic framework (POMOF) consisting of Zn-ε-Keggin unit and organic linker, {[PMo₈^VMo₄^VIO₃₇(OH)₃Zn₄][BPE]₂}·[BPE] (1), was successfully obtained by the hydrothermal method. Compound 1 is composed of Zn-ε-Keggin units and BPE ligands, featuring a fascinating 5-fold interpenetrating framework with dia topology. The catalytic performance of compound 1 was investigated, and experiments showed that 1 could effectively facilitate the cycloaddition reaction of CO₂ with epoxides as Lewis acid heterogeneous catalyst. Moreover, compound 1 also was studied as LIBs anode material, and it showed reversible capacity of 546 mA h g^-1 at 100th cycle.

Polyoxometalate-Cyclodextrin Metal-Organic Frameworks: From Tunable Structure to Customized Storage Functionality

Self-assembly allows structures to organize themselves into regular patterns by using local forces to find the lowest-energy configuration. However, assembling organic and inorganic building blocks in an ordered framework remains challenging due to  difficulties in rationally interfacing two dissimilar materials. Herein, the ensemble of polyoxometalates (POMs) and cyclodextrins (CDs) as molecular building blocks (MBBs) has yielded two unprecedented POM-CD-MOFs, namely [PW₁₂O₄₀]^3- and α-CD MOF (POT-CD) as well as [P₁₀Pd_15.5O₅₀]^19- and γ-CD MOF (POP-CD), with distinct properties not shared by their isolated parent MBBs. Markedly, the POT-CD features a nontraditional enhanced Li storage behavior by virtue of a unique "amorphization and pulverization" process. This opens the door to a new generation of hybrid materials with tuned structures and customized functionalities.

Syntheses, structures and properties of three organic–inorganic hybrid polyoxotungstates constructed from {Ni6PW9} building blocks: from isolated clusters to 2-D layers

Three organic–inorganic hybrid POMs were prepared and characterized, exhibiting 0-D to 2-D assemblies made by {Ni₆PW₉} SBUs and organic ligands.

Inorganic–organic hybrid high-dimensional polyoxotantalates and their structural transformations triggered by water

In this work, novel dimeric polyoxotantalate (POTa) clusters {Cu(en)(Ta₆O₁₉)}₂/{Cu(enMe)(Ta₆O₁₉)}₂ were introduced as SBUs to construct a new family of extended POTa materials, including the first two 3D POTa frameworks and two 2D POTa layers.

Polyoxometalate-Based Metal-Organic Frameworks for Selective Oxidation of Aryl Alkenes to Aldehydes

Polyoxometalates (POMs) show considerable catalytic performance toward the selective oxidation of alkenes to aldehydes, which is commercially valuable for the production of pharmaceuticals, dyes, perfumes, and fine chemicals. However, the low specific surface area of POMs as heterogeneous catalysts and poor recyclability as homogeneous catalysts have hindered their wide application. Dispersing POMs into metal-organic frameworks (MOFs) for the construction of POM-based MOFs (POMOFs) suggests a promising strategy to realize the homogeneity of heterogeneous catalysis. Herein, we report two new POMOFs with chemical formulas of [Co(BBTZ)₂][H₃BW₁₂O₄₀]·10H₂O (1) and [Co₃(H₂O)₆(BBTZ)₄][BW₁₂O₄₀]·NO₃·4H₂O (2) (BBTZ = 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene) for the selective oxidation of alkenes to aldehydes. Compound 1 possesses a non-interpenetrated three-dimensional (3D) cds-type open framework with a 3D channel system. Compound 2 displays a 3D polyrotaxane framework with one-dimensional channels along the [100] direction. In the selective oxidation of styrene into benzaldehyde, compound 1 can achieve a 100% conversion in 4 h with 96% selectivity toward benzaldehyde, which is superior to that of compound 2. A series of control experiments reveal that the co-role of [BW₁₂O₄₀]^5- and Co²⁺ active center as well as a more open framework feature co-promote the catalytic property of the POMOFs in this case. This work may suggest a new option for the development of POMOF catalysts in the selective oxidation of alkenes.

Design and synthesis of multifunctional metal-organic zeolites

Metal-organic zeolites (MOZs) are an important branch of metal-organic frameworks (MOFs) and combine the advantages of zeolites and MOFs, such as high surface area and porosity as well as the exceptional stability of zeolites, which would have a significant impact on catalysis chemistry, inorganic chemistry, coordination chemistry, materials science and other areas. In this review, we focus on the recent advances in MOZs with a brief outline of the most prominent examples. In particular, we highlight the basic principles of the design and synthesis approaches toward the construction of MOZs. Obeying the principle of charge matching, tuning tetrahedral metal centers, using enlarged tetrahedral building units as clusters, introducing functional groups into ligands, and combining traditional inorganic TO₄ sites in MOZs enable the final materials with diverse topological structures to exhibit superior performance for various applications, such as gas sorption/separation, catalysis, enantio-selectivity, luminescence, etc.

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.

A novel metal–organic framework based on hexanuclear Co(ii) clusters as an anode material for lithium-ion batteries

A new metal–organic framework, [Co(L)]·1.5H₂O (1), has been successfully synthesized, where its electrochemical application has been investigated.

Composite cluster-organic frameworks based on polyoxometalates and copper/cobalt–oxygen clusters

The first two cluster organic frameworks based on Anderson-type polyoxometalates and copper/cobalt-oxo cluster secondary building units have been successfully made under solvothermal conditions.

An inorganic–organic hybrid supramolecular framework as a high-performance anode for lithium-ion batteries

We achieved an excellent anode for a lithium-ion battery for the first time based on an inorganic–organic hybrid supramolecular nanotube.

Novel polyoxometalate-based cobalt complexes based on rigid pyridyl-triazole-tetrazole and pyridyl-bis(triazole) ligands

Three novel polyoxometalate (POM)-based cobalt complexes have been synthesized and structurally characterized. The adsorption activities for organic dyes and electrochemical properties have been studied in detail.

A coordination polymer based on dinuclear (pyrazinyl tetrazolate) copper(ii) cations and Wells–Dawson anions for high-performance supercapacitor electrodes

A new POM-based coordination polymer was used as supercapacitor electrode showing excellent performance, which can effectively solve the water solubility and low conductivity of traditional POM electrodes.