State-of-the-art advances in the structural diversities and catalytic applications of polyoxoniobate-based materials

Organic-inorganic hybrid Co-containing polyoxoniobates as hydrogen evolution catalysts in alkaline media

Herein, cobalt ammine complexes are combined with alkali-resistant polyoxoniobates (PONbs) at the molecular level to obtain two new organic-inorganic hybrid Co-containing PONbs with the same polyanions but different Co contents. The Co-containing PONb with higher Co content exhibits superior hydrogen evolution reaction (HER) activity under highly alkaline conditions, which is better than the results for the similar Cu-containing PONb, highlighting the critical role of cobalt in improving HER performance. This study provides a new insight into the relationship between the structural properties of metal complexes during electrocatalysis.

A Highly Biocompatible Polyoxotungstate with Fenton-like Reaction Activity for Potent Chemodynamic Therapy of Tumors

Integrating Fenton chemistry and nanomedicine into cancer therapy has significantly promoted the development of chemodynamic therapy (CDT). Nanoscale polyoxometalates (POMs), with their reversible redox properties, exhibit promising potential in developing outstanding CDT drugs by exploring their Fenton-like catalytic reactivity in tumor environments. However, such research is still in its infancy due to the challenges of acquiring POMs that are both easily prepared and possess ideal therapeutic effects, physiological solubility, biocompatibility and safety. In this work, we report the synthesis of a new crystalline antimonotungstate {Dy2Sb2W7O23(OH)(DMF)2(SbW9O33)2} (1, DMF=N, N-dimethylformamide) with gram-scale high yield via a facile "one-pot" solvothermal reaction. 1 exhibits not only a soluble and water-stable POM nanocluster, but also excellent catalytic activity for hydroxyl radical-generating Fenton-like reactions. Further biomedical studies reveal that 1 can trigger cell apoptosis and promote lipid peroxidation, exhibiting high cytotoxicity and selectivity towards B16-F10 mouse melanoma cancer cells with an IC50 value of 4.75 μM. Especially, 1 can inhibit melanoma growth in vivo with favorable biosafety, achieving a 5.2-fold reduction in tumor volume and a weight loss of 76.0 % at the dose of 70 μg/kg. This research not only demonstrates the immense potential of antimonotungstates in CDT drug development for the first time but also provides new insights and directions for the development of novel anticancer drugs.

Application of Hard and Soft Acid-base Theory to Construct Heterometallic Materials with Metal-oxo Clusters

Heterometallic cluster-based materials offer the potential to incorporate multiple functionalities, leveraging the aggregation effects of clusters and translating this heterogeneity and complexity into unexpected properties that are more than just the sum of their components. However, the rational construction of heterometallic cluster-based materials remains challenging due to the complexity of metal cation coordination and structural unpredictability. This minireview provides insights into a general synthetic strategy based on Hard and Soft Acids and Bases (HSAB) theory, summarizing its advantages in the designed synthesis of discrete heterometallic clusters (intracluster assembly) and infinite heterometallic cluster-based materials (intercluster assembly). Furthermore, it emphasizes the potential to exploit the intrinsic properties of mixed components to achieve breakthroughs across a broad range of applications. The insights from this review are expected to drive the progress of heterometallic cluster-based materials in a controllable and predictable manner.

A Two-Dimensional Layered Heteropolyoxoniobate Based on Cubic Sn(IV)-Containing {Sn12Nb56O200} Cages with Good Proton Conductivity Property

The first example of a Sn(IV)-containing heteropolyoxoniobate K6H18[Cu(en)2]8{[Sn(OH)2]12 (HNb7O22)8}·2en·88H2O (1) is built from nanoscale high-nuclearity cubic {[Sn(OH)2]12(HNb7O22)8} cluster and [Cu(en)2]2+ complexes. The cubic {[Sn(OH)2]12(HNb7O22)8} cage is composed of eight {Nb7O22} clusters and 12 SnO6 octahedrons. The eight {Nb7O22} fragments are situated at the vertices of the cubic cage, while the 12 SnO6 octahedrons are positioned along the edges of the cubic cage. The [Cu(en)2]2+ complexes link the {[Sn(OH)2]12(HNb7O22)8} clusters into two-dimensional (2D) (4,4) grid-like layers. The N-H···O hydrogen bonds between the [Cu(en)2]2+ complexes and the {[Sn(OH)2]12(HNb7O22)8} clusters link the layers to form a 3D supramolecular structure. Compound 1 exhibits a good proton conductivity of 3.1 × 10-2 S cm-1 at 85 °C and 98% relative humidity.

Specific smart sensing of electron-rich antibiotics or histidine improves the antenna effect, luminescence, and photodynamic sterilization capabilities of lanthanide polyoxometalates

Excessive discharge of antibiotics seriously threatens human health and is thus a global public health problem. This highlights the urgent need to develop intelligent sensing materials for specific antibiotics that are highly visual, fast, convenient, and inexpensive. Herein, two reverse α-octamolybdate polyoxometalates (POMs; Mo8) were used to chelate lanthanide ions to obtain lanthanide POMs (LnPOMs; LnMo16; Ln = Eu, Sm, Tb, Gd) with highly sensitive smart photoresponses to specific antibiotics (ofloxacin [OFN], norfloxacin [NOR], enrofloxacin [ENR], and oxytetracycline [OTC]) and histidine (His) with luminescence turn-on. Specific antibiotics and His, which has an electron-rich structure, can efficiently enhance the antenna effect, thereby greatly improving the luminescence of EuMo16. Surprisingly, OFN and NOR both enhanced the luminescence of Eu(III) ions and Mo8, whereas ENR and OTC only enhanced the luminescence of Eu(III) ions, showing a differentiated sensitization effect. More notably, the combination of POMs and Ln(III) ions enhanced the ability of LnPOMs to produce reactive oxygen species under light irradiation, and these LnPOMs showed significant sterilization effects on Escherichia coli and Staphylococcus aureus. To our knowledge, this is the first time electron-rich antibiotics or amino acids were used to enhance the luminescence of LnPOMs, achieving luminescence-enhanced photoresponse to specific antibiotics and amino acids.

Integrating Polyoxometalate into Dy(III)-based Single-molecule Magnets with Pentagonal Bipyramidal Symmetry

Polyoxometalates (POMs) with various coordination fashions are versatile ligands for constructing single-ion magnets (SIMs), but enforcing POM-SIMs with a specific geometry remains a synthetic challenge. Herein, we synthesized a POM-cocrystallized DyIII-SIM [Dy(OPPh3)4(H2O)3][PW12O40]·4EtOH (1Dy) and a POM-ligated DyIII-SIM [{Dy(OPPh3)3(H2O)3}{PW12O40}]·Ph3PO·H2O (2Dy) with pentagonal bipyramidal local coordination geometry. Magnetic measurements indicate that 1Dy displays field-induced single-molecule magnet (SMM) behavior and the relaxation is dominated by under-barrier processes. 2Dy exhibits spin-lattice relaxation at a broader temperature region with a reversal barrier over 300 K. Magneto-structural analysis reveals that the enhancement of SMM behavior originated from the equatorial replacement of Ph3PO by POM, which strengthens the axial anisotropy in 2Dy. Luminescent experiments indicate that the characteristic DyIII emissions of 1Dy are covered up by the strong π-π* emission of Ph3PO at low-temperature regions. As for 2Dy, partial DyIII emission persists thanks to the antenna effect between DyIII and POM.

A water-soluble mixed-valent {Mn11} cluster embedded heteropolyoxoniobate with magnetic properties

A rare all-inorganic high-nuclearity mixed-valent {Mn11} cluster embedded polyoxoniobate, K25H43{(Te4Nb9O33)3(Nb6O19)5(TeVINb5O14)[(TeIVO3)2(MnII7MnIII4O19)]}·97H2O (1), has been synthesized by a one-pot reaction. Compound 1 contains the largest manganese cluster {Mn11} core among polyoxoniobates reported to date. {Mn11} consists of three quasi-cubane {Mn3O4} units and is simultaneously encapsulated by lacunary α-Keggin {Te4Nb9O36} and Lindqvist {Nb6O19} units. Compound 1 exhibits significant magnetic anisotropy and excellent water solubility and stability. The findings suggest a new, all-inorganic polynulear Mn-based structural paradigm for aqueous solution chemistry and magnetic materials.

Synergistic reductive catalytic effects of an organic and inorganic hybrid covalent organic framework for hydrogen fuel production

Electrocatalytic hydrogen generation in alkaline medium has become widely used in a variety of sectors. However, the possibility for additional performance improvement is hampered by slow kinetics. Because of this restriction, careful control over processes such as water dissociation, hydroxyl desorption and hydrogen recombination is required. Covalent organic frameworks (COFs) based on porphyrin and polyoxometalates (POMs) show encouraging electrocatalytic performance, offering a viable route for effective and sustainable hydrogen generation. Their specific architectures lead to increased electrocatalytic activity, which makes them excellent choices for developing water electrolysis as a clean energy conversion method in the alkaline medium. In this regard, TTris@ZnPor and Lindqvist POM were coordinated to create a new eco-friendly and highly active covalent organic framework (TP@VL-COF). In order to describe TP@VL-COF, extensive structural and morphological investigations were carried out through FTIR, 1H NMR, elemental analysis, SEM, fluorescence, UV-visible, PXRD, CV, N2-adsorption isotherm, TGA and DSC analyses. In an alkaline medium, the electrocatalytic capability of 20%C/Pt, TTris@ZnPor, Lindqvist POM and TP@VL-COF was explored and compared for the hydrogen evolution reaction (HER). The TP@VL-COF showed the best catalytic efficiency for HER in an alkaline electrolyte, requiring just a 75 mV overpotential to drive 10 mA cm-2 and outperforming 20%C/Pt, TTris@ZnPor, Lindqvist POM and other reported catalysts. The Tafel slope value also indicates faster kinetics for TP@VL-COF (114 mV dec-1) than for 20%C/Pt (182 mV dec-1) TTris@ZnPor (116 mV dec-1) and Lindqvist POM (125 mV dec-1).

An Organodiphosphate-Containing Polyoxoniobate Ring and Its Assembly into a Three-Dimensional Framework through Hydrogen Bonding

In this work, a novel organodiphosphate-containing inorganic-organic hybrid polyoxoniobate (PONb) ring {(PO3CH2CH2PO3H)4Nb8O16}4- (Nb8P8) has been achieved by a one-pot hydrothermal method. The ring is constructed from a tetragonal {Nb8O36} motif and four {PO3CH2CH2PO3H} ligands. Interestingly, Nb8P8 can be joined together via K-H2O clusters {K2(H2O)4(OH)2} to form one-dimensional chains {[K2(H2O)4(OH)2]Nb8P8}n and further linked by {Cu(en)2}2+ (en = ethylenediamine) complexes, resulting in a three-dimensional supramolecular framework {[Cu(en)2]2[K2(H2O)4(OH)2]Nb8P8}·3en·H2O (1). 1 exhibits good chemical and thermal stability and has a high water vapor adsorption capacity of ≤224 cm3 g-1 (22.71 mol·mol-1) at 298 K, outperforming most of the known polyoxometalate-based materials. Impedance measurements prove that 1 can transfer protons with moderate conductivity. This study not only contributes to the structural diversity of organodiphosphate-containing PONbs and PONb rings but also provides a reference for the development of PONb-based materials with unique performance.

A 3D heteropolyoxoniobate framework based on heart-shaped {Te2Nb19O60} clusters with proton conductivity property

A 3D tellurium-substituted heteropolyoxoniobate framework H5K3Na[Cu(en)2]2[Cu(en)0.75(H2O)2.5]{[(Te2Nb19O58)(μ3-OH)2]}·24H2O (1, en = ethylenediamine) with a 6-connected pcu topology is built from heart-shaped {Te2Nb19O60} clusters and copper complexes. The {Te2Nb19O60} cluster represents the new tellurniobate structure type with a 19-nuclearity Nb cluster. It consists of two new monovacant Lindqvist {Nb5O19} clusters, one boat-shaped {Nb9O32} cluster and two TeO32- anions. The {Te2Nb19O60} polyanions are interlinked by [Cu(en)2]2+ complexes into a 2D (4, 4) grid-like layer containing rhombic sheets. The Cu2+ supports the adjacent layers through Te-O-Cu-O-Te- bonds to form a three-dimensional heteropolyoxoniobate framework with 1D channels. This compound exhibits good chemical and solvent stability and proton conductivity, with a conductivity of 7.9 × 10-3 S cm-1 at 85 °C under 98% RH.

Three Ways to (Pseudo)cubic Structure Models: Phase Transition and Pseudosymmetry in Orthorhombic Cs3MO4 (M = V, Nb, or Ta)

The new oxometalates Cs3NbO4 and Cs3TaO4 together with Cs3VO4 crystallize with the K3NO4 structure type [Pnma, a = 12.495(2) Å, b = 9.0183(14) Å, and c = 6.6529(10) Å for the V compound; a = 12.928(2) Å, b = 9.177(3) Å, and c = 6.739(4) Å for the Nb; and a = 12.963(4) Å, b = 9.122(2) Å, and c = 6.774(1) Å for the Ta compound]. Their crystal structures were evaluated on the basis of single-crystal and powder X-ray diffractometry, assisted by vibrational spectroscopy, thermoanalysis, and DFT calculations. The crystal structures contain tetrahedral [M5+O4]3- anions, representing the first occurrence of Nb and Ta in a tetrahedral oxidic environment. Many representatives of the orthorhombic K3NO4 structure type have been described in the literature with a cubic structure model with disordered O atomic positions. Based on studies on Cs3MO4 (M = P, V, Nb, or Ta), we show here three different effects which can lead to (pseudo)cubic data sets. Two of them are problems of crystallographic nature (overlooked twinning or adverse atomic form factor ratios), but the third one, phase transformation into a plastic crystalline high-temperature modification, leads to a "truly" cubic structure with dynamically disordered (freely rotating) oxometalate anions. This might be of interest with respect to a large and growing number of sulfido- and selenidometalate materials which are today in discussion as solid-state electrolytes and to the mechanism of the unusually efficient ion transport therein.

Enhanced Catalytic Activity of a Copper(II) Metal-Organic Framework Constructed via Semireversible Single-Crystal-to-Single-Crystal Dehydration

Herein, we present a copper(II) metal-organic framework, [Cu2(btec)(OH2)4]·2H2O (1) [(btec)4- = 1,2,4,5-benzenetetracarboxylate], that undergoes single-crystal-to-single-crystal transformations into two anhydrous phases 2' and 2″ with the chemical formula [Cu2(btec)], triggered by two-step dehydration at 403 and 433 K, respectively. After immersion in water for 3 days at room temperature, 2' transformed into [Cu2(btec)(OH2)] (3), while both 2' and 2″ took 1 week to revert to 1. Dynamic vapor sorption studies validated water-induced reversible structural transformations at 70% relative humidity (RH). According to single-crystal X-ray diffraction (SC-XRD), the local coordination geometry of the Cu2+ ion in 2' changed from a saturated octahedron to a coordinatively unsaturated square-based pyramid in 3, manifested by changes in color and dimensionality. From a topological point of view, all of the scaffolds show a binodal (3,6)-connected kgd topology with the point symbol {43}2{46}. In addition, the materials were thoroughly characterized using routine spectroscopic data and various analytical techniques. The catalytic activity of the microporous materials in the liquid-phase oxidation of styrene in acetonitrile, using 30% (wt) H2O2 as the oxidant, was investigated. The excellent performance of the monohydrous phase 3 was shown to be superior to the pristine framework and the anhydrous counterparts, as evidenced by a good turnover number (TON) and turnover frequency (TOF) = 82.6 and 21.0 h-1, respectively. Within 4 h, the substrates were catalytically oxidized to the desired products with up to 67% conversion and 100% benzaldehyde selectivity. It is worth noting that the accessible active metal sites and higher surface area enhanced the catalytic properties of 3. Furthermore, the maintenance of catalytic efficiency over five cycles and reusability are illustrated and discussed in terms of the structural differences of the microporous frameworks. Thus, a preliminary reaction mechanism for the selective oxidation of styrene is proposed. This study not only provides a fascinating example of MOF chromism achieved by thermal activation and rehydration but also sheds some light on the relationship between pore-surface- or metal-engineered sites in MOFs and their heterogeneous catalytic performances.

Two Unprecedented Germanoniobate Frameworks Based on High-Nuclearity Peanut-Shaped {Ge12Nb38} Clusters

By variation of the amount of GeO2, two organic-inorganic hybrid germanoniobate frameworks with 6-connected pcu and 10-connected bct topologies were constructed from peanut-shaped {α-Ge12Nb38} and {β-Ge12Nb38} clusters, respectively. The {α-Ge12Nb38} and {β-Ge12Nb38} clusters contain the most Ge centers of germanoniobates reported so far. The compounds exhibit proton conduction properties with a conductivity of 3.04 × 10-4 S·cm-3 for 1 and 1.62 × 10-4 S·cm-3 for 2 at 85 °C and 98% RH. The water vapor adsorption capacities for 1 and 2 are 5.86 and 4.40 mmol·g-1, respectively.

A Rhombus-Like Tetrameric Vanadoniobate Containing Pseudo-Sandwich-Type {Li ⊂ V2O8(Nb5O14)2} and Its Electrocatalytic Activity for the Selective Oxidation of Benzyl Alcohol

Ongoing research on V-containing polyoxoniobates (PONbs) is driven by their diverse structures and potential applications. Although Lindqvist-type {Nb6O19} is a widely used building block in PONbs, vanadoniobates based on {Nb6O19} and/or its derivatives are still very limited. Herein, a discrete vanadoniobate, LiNa14K11[Li2 ⊂ VIV8Nb32O110]·45H2O (1), has been synthesized by a hydrothermal method, which shows a rhombus-like tetrameric structure composed of two {V2O6(Nb6O19)} and two {Li ⊂ V2O8(Nb5O14)2} subunits derived from {Nb6O19}. Notably, the {Li ⊂ V2O8(Nb5O14)2} subunit has an interesting pseudo-sandwich-type structure, where a {LiV2O8} belt is coordinated by two monolacunary {Nb5O14} molecules and the central site of the cluster is occupied by Li+. Considering that 1 has both basic hexaniobates and redox active V centers, 1 was used as a noble metal-free electrocatalyst for the selective oxidation of benzyl alcohol to benzaldehyde, achieving complete conversion of benzyl alcohol with 94% selectivity for benzaldehyde in 3 h under ambient conditions without using any alkaline additives. Moreover, the catalytic performance of 1 remained largely unchanged after four cycles.

Oxalate-assisted assembly of two polyoxotantalate supramolecular frameworks with proton conduction properties

An oxalate-assisted strategy was first developed for creating new polyoxotantalates (POTas). With this strategy, two brand-new POTa supramolecular frameworks based on uncommon dimeric POTa secondary building units (SBUs) were constructed and characterized. Interestingly, the oxalate ligand can not only serve as a coordination ligand to form unique POTa SBUs but also act as a key hydrogen bond acceptor to construct supramolecular architectures. Besides, the architectures show outstanding proton conductivity. The strategy opens up new opportunities for developing new POTa materials.

Decoration of the [Nb6O19]8– cluster shell with six Cu2+-centred complexes generates the [(Cu(cyclen))6Nb6O19]4+ moiety: room temperature synthesis, crystal structure and selected properties

Mixing an aqueous solution of K8[Nb6O19]⋅16H2O with a DMSO/H2O solution of Cu(ClO4)2 · 6 H2O and cyclen at room temperature afforded crystallization of blue crystals of [(Cu(cyclen))6Nb6O19]⋅[ClO4]4·≈4H2O after slow evaporation of the solvents. The crystal structure contains the Lindqvist anion [Nb6O19]8– which is covalently expanded by six symmetry-related [Cu(cyclen)]2+ complexes via Nb-μ 2-O-Cu bridges yielding the positively charged [(Cu(cyclen))6Nb6O19]4+ cluster shell. The ClO4 − anions and crystal water molecules reside in the empty spaces of the packed clusters. The compound shows two electronic d-d transitions at energetic positions explaining the blue color.

Co-Dissolved Isostructural Polyoxovanadates to Construct Single-Atom-Site Catalysts for Efficient CO2 Photoreduction

We explored a co-dissolved strategy to embed mono-dispersed Pt center into V₂ O₅ support via dissolving [PtV₉ O₂₈ ]^7- into [V₁₀ O₂₈ ]^6- aqueous solution. The uniform dispersion of [PtV₉ O₂₈ ]^7- in [V₁₀ O₂₈ ]^6- solution allows [PtV₉ O₂₈ ]^7- to be surrounded by [V₁₀ O₂₈ ]^6- clusters via a freeze-drying process. The V centers in both [PtV₉ O₂₈ ]^7- and [V₁₀ O₂₈ ]^6- were converted into V₂ O₅ via a calcination process to stabilize Pt center. These double separations can effectively prevent the Pt center agglomeration during the high-temperature conversion process, and achieve 100 % utilization of Pt in [PtV₉ O₂₈ ]^7- . The resulting Pt-V₂ O₅ single-atom-site catalysts exhibit a CH₄ yield of 247.6 μmol g^-1  h^-1 , 25 times higher than that of Pt nanoparticle on the V₂ O₅ support, which was accompanied by the lactic acid photooxidation to form pyruvic acid. Systematical investigations on this unambiguous structure demonstrate an important role of Pt-O atomic pair synergy for highly efficient CO₂ photoreduction.

Platonic and Archimedean solids in discrete metal-containing clusters

Metal-containing clusters have attracted increasing attention over the past 2-3 decades. This intense interest can be attributed to the fact that these discrete metal aggregates, whose atomically precise structures are resolved by single-crystal X-ray diffraction (SCXRD), often possess intriguing geometrical features (high symmetry, aesthetically pleasing shapes and architectures) and fascinating physical properties, providing invaluable opportunities for the intersection of different disciplines including chemistry, physics, mathematical geometry and materials science. In this review, we attempt to reinterpret and connect these fascinating clusters from the perspective of Platonic and Archimedean solid characteristics, focusing on highly symmetrical and complex metal-containing (metal = Al, Ti, V, Mo, W, U, Mn, Fe, Co, Ni, Pd, Pt, Cu, Ag, Au, lanthanoids (Ln), and actinoids) high-nuclearity clusters, including metal-oxo/hydroxide/chalcogenide clusters and metal clusters (with metal-metal binding) protected by surface organic ligands, such as thiolate, phosphine, alkynyl, carbonyl and nitrogen/oxygen donor ligands. Furthermore, we present the symmetrical beauty of metal cluster structures and the geometrical similarity of different types of clusters and provide a large number of examples to show how to accurately describe the metal clusters from the perspective of highly symmetrical polyhedra. Finally, knowledge and further insights into the design and synthesis of unknown metal clusters are put forward by summarizing these "star" molecules.

Assemblies of Increasingly Large Ln-Containing Polyoxoniobates and Intermolecular Aggregation-Disaggregation Interconversions

An oxalate-assisted lanthanide (Ln) incorporation strategy is first demonstrated for creating rare high-nuclearity Ln-containing polyoxoniobates (PONbs). With the strategy, a series of high-nuclearity Ln-containing PONbs of 50-nuclearity Dy₂Nb₄₈, 103-nuclearity Dy₇Nb₉₆, 200-nuclearity Dy₁₀Nb₁₉₀, and 206-nuclearity Dy₁₄Nb₁₉₂ have been made, showing an increasingly large structure evolution from Dy₂Nb₄₈ monomer to Dy₇Nb₉₆ dimer and to distinct Dy₁₀Nb₁₉₀ and Dy₁₄Nb₁₉₂ tetramers. Among them, Dy₁₄Nb₁₉₂ presents the largest heterometallic PONb and also the PONb with the greatest number of Ln ions reported thus far. Interestingly, both giant Dy₁₄Nb₁₉₂ and Dy₁₀Nb₁₉₀ molecules can further undergo single-crystal to single-crystal intermolecular aggregations, forming infinite {Dy₁₄Nb₁₉₂}_∞ and {Dy₁₀Nb₁₉₀}_∞ chains, respectively. The former structural transformation shows a reversible humidity-dependent aggregation-disaggregation process accompanied by a proton conductivity response, while the latter structural transformation is irreversible. These new species largely enrich the very limited members of Ln-containing PONb family and offer rare examples for studying structural transformations between giant molecular aggregates and infinitely extended structures at the atomic level.

An inorganic-organic hybrid nanomaterial with a core-shell structure constructed by using Mn-BTC and Ag5[BW12O40] for supercapacitors and photocatalytic dye degradation

Creating inorganic-organic hybrids with polyoxometalates (POMs) and metal-organic frameworks (MOFs) as energy storage and dye-degradation materials remains challenging. Here, a new hybrid nanomaterial Mn-BTC@Ag₅[BW₁₂O₄₀] is synthesized by using Ag₅[BW₁₂O₄₀] and Mn₃(BTC)₂(H₂O)₆ (Mn-BTC, BTC = 1,3,5-benzenetricarboxylic acid) through a plain grinding method. The structure and morphology characterization by scanning electron microscopy (SEM), powder X-ray diffraction (XRD), and transmission electron microscopy (TEM) shows that the synthetic products have core-shell construction. Due to its unique structure wherein the core is Mn-BTC and the shell is Ag₅[BW₁₂O₄₀], it exhibits excellent capacitance performance. In a three-electrode system where nickel foam is a collector, at a current density of 1 A g^-1, its specific capacitance is 198.09 F g^-1; after 5000 cycles, the capacitance retention rate is 94.4%. When the power density is 503.1 W kg^-1, the symmetrical supercapacitor reveals a high energy density which is 10.9 W h kg^-1. At the same time, the capacitance retention is 92.9% after 5000 cycles which showed good cycle stability. The photocatalytic degradation efficiencies of rhodamine B (RhB), methyl orange (MO) and methylene blue (MB) dyes exceed 90% after 140 min, and the degradation results remained unchanged after five photocatalytic cycles. The photocatalytic degradation mechanism shows that ˙OH has a major effect. The results show that this research provides a fresh idea for the development of energy storage and dye photocatalytic degradation materials.

Computational Prediction of Speciation Diagrams and Nucleation Mechanisms: Molecular Vanadium, Niobium, and Tantalum Oxide Nanoclusters in Solution

Understanding the aqueous speciation of molecular metal-oxo-clusters plays a key role in different fields such as catalysis, electrochemistry, nuclear waste recycling, and biochemistry. To describe the speciation accurately, it is essential to elucidate the underlying self-assembly processes. Herein, we apply a computational method to predict the speciation and formation mechanisms of polyoxovanadates, -niobates, and -tantalates. While polyoxovanadates have been widely studied, polyoxoniobates and -tantalates lack the same level of understanding. First, we propose a pentavanadate cluster ([V₅O₁₄]^3-) as a key intermediate for the formation of the decavanadate. Our computed phase speciation diagram is in particularly good agreement with the experiments. Second, we report the formation constants of the heptaniobate, [Nb₇O₂₂]^9-, decaniobate, [Nb₁₀O₂₈]^6-, and tetracosaniobate [H₉Nb₂₄O₇₂]^15-. Additionally, we compute the speciation and phase diagram of niobium, which so far was restricted to Lindqvist derivates. Finally, we predict the formation constant of the decatantalate ([Ta₁₀O₂₆]^6-) in water, even though it had only been synthesized in toluene. Furthermore, we also calculate the corresponding speciation and phase diagrams for polyoxotantalates. Overall, we show that our method can be successfully applied to different families of molecular metal oxides without any need for readjustments; therefore, it can be regarded as a trustworthy tool for exploring polyoxometalates' chemistry.

Controllable Assembly of Vanadium-Containing Polyoxoniobate-Based Materials and Their Electrocatalytic Activity for Selective Benzyl Alcohol Oxidation

During the controllable synthesis of two vanadium-containing Keggin-type polyoxoniobates (PONbs), [Ni(en)₂]₅[PNb₁₂O₄₀(VO)₅](OH)₅·18H₂O (1) and [Ni(en)₃]₅[PNb₁₂O₄₀(VO)₂]∙17H₂O (2, en = ethylenediamine) are realized by changing the vanadium source and hydrothermal temperature. Compounds 1 and 2 have been thoroughly characterized by single-crystal X-ray diffraction analysis, FT-IR spectra, X-ray photoelectron spectrum (XPS), powder X-ray diffraction (PXRD), etc. Compound 1 contains a penta-capped Keggin-type polyoxoniobate {PNb₁₂O₄₀(VO)₅}, which is connected by adjacent [Ni(en)₂]²⁺ units into a three-dimensional (3D) organic-inorganic framework, representing the first nickel complexes connected vanadoniobate-based 3D material. Compound 2 is a discrete di-capped Keggin-type polyoxoniobate {PNb₁₂O₄₀(VO)₂} with [Ni(en)₃]²⁺ units as counter cations. Compounds 1 and 2 have poor solubility in common solvents and can keep stable in the pH range of 4 to 14. Notably, both 1 and 2 as electrode materials are active for the selective oxidation of benzyl alcohol to benzaldehyde. Under ambient conditions without adding an alkaline additive, compound 1 as a noble metal free electrocatalyst can achieve 92% conversion of benzyl alcohol, giving a Faraday efficiency of 93%; comparatively, 2 converted 79% of the substrate with a Faraday efficiency of 84%. The control experiments indicate that both the alkaline polyoxoniobate cluster and the capped vanadium atoms play an important role during the electrocatalytic oxidation process.

Binary Type-II Heterojunction K7HNb6O19/g-C3N4: An Effective Photocatalyst for Hydrogen Evolution without a Co-Catalyst

The binary type-II heterojunction photocatalyst containing g-C₃N₄ and polyoxoniobate (PONb, K₇HNb₆O₁₉) with excellent H₂ production activity was synthesized by decorating via a facile hydrothermal method for the first time. The as-fabricated Nb–CN-0.4 composite displayed a maximum hydrogen evolution rate of 359.89 µmol g⁻¹ h⁻¹ without a co-catalyst under the irradiation of a 300 W Xenon Lamp, which is the highest among those of the binary PONb-based photocatalytic materials reported. The photophysical and photochemistry analyses indicated that the hydrogen evolution performance could be attributed to the formation of a type-II heterojunction, which could not only accelerate the transfer of photoinduced interfacial charges, but also effectively inhibit the recombination of electrons and holes. This work could provide a useful reference to develop an inexpensive and efficient photocatalytic system based on PONb towards H₂ production.

In situ ball-milling gram-scale preparation of polyoxoniobate-intercalated MgAl-layered double hydroxides for selective aldol and Michael addition cascade reactions

A facile one-step ball-milling strategy to prepare gram-scale Mg3Al-LDH-Nb6 has been demonstrated and the thus-obtained catalyst exhibited efficient selective catalytic activities in the synthesis of biologically active organic molecules in water.

Vanadium O-Centered Selenoiodide Complex: Synthesis and Structure of V4O(Se2)4I6·I2

The first vanadium selenoiodide V₄O(Se₂)₄I₆·I₂ was synthesized at a moderate temperature of 220 °C from V, Se, I₂, and water. Its crystal structure (tetragonal space group P4₂/nbc, a = 11.838(1) Å, c = 18.689(1) Å) contains O-centered vanadium(IV) tetranuclear fragment [V₄(μ₄-O)(μ₂-Se₂)₄(μ₂-I)₂I₄], where the edges of the distorted tetrahedron V₄ are bridged by four diselenide (Se₂)^2- and two iodide ligands; four terminal iodides coordinate V atoms additionally. This type of complex is known for Ti, Nb, and Ta but is new for vanadium. Magnetic susceptibility measurements of V₄O(Se₂)₄I₆·I₂ showed four unpaired electrons on vanadium atoms at room temperature and drop of the effective magnetic moment at cool down, presumably due to partial electron pairing. Probability of this transition to the diamagnetic state is in accord with the calculated electronic structure.