Polyoxometalate chemistry at volcanoes: discovery of a novel class of polyoxocuprate nanoclusters in fumarolic minerals

Polyoxometalate (POM) chemistry is an important avenue of comprehensive chemical research, due to the broad chemical, topological and structural variations of multinuclear polyoxoanions that result in advanced functionality of their derivatives. The majority of compounds in the polyoxometalate kingdom are synthesized under laboratory conditions. However, Nature has its own labs with the conditions often unconceivable to the mankind. The striking example of such a unique environment is volcanic fumaroles – the natural factories of gas-transport synthesis. We herein report on the discovery of a novel class of complex polyoxocuprates grown in the hot active fumaroles of the Tolbachik volcano at the Kamchatka Peninsula, Russia. The cuboctahedral nanoclusters {[MCu₁₂O₈](AsO₄)₈} are stabilized by the core Fe(III) or Ti(IV) cations residing in the unique cubic coordination. The nanoclusters are uniformly dispersed over the anion- and cation-deficient NaCl matrix. Our discovery might have promising implications for synthetic chemistry, indicating the possibility of preparation of complex polyoxocuprates by chemical vapor transport (CVT) techniques that emulate formation of minerals in high-temperature volcanic fumaroles.

Discrete Hf18 Metal-oxo Cluster as a Heterogeneous Nanozyme for Site-Specific Proteolysis

The selective hydrolysis of proteins by non-enzymatic catalysis is difficult to achieve, yet it is crucial for applications in biotechnology and proteomics. Herein, we report that discrete hafnium metal-oxo cluster [Hf₁₈ O₁₀ (OH)₂₆ (SO₄ )₁₃ ⋅(H₂ O)₃₃ ] (Hf₁₈ ), which is centred by the same hexamer motif found in many MOFs, acts as a heterogeneous catalyst for the efficient hydrolysis of horse heart myoglobin (HHM) in low buffer concentrations. Among 154 amino acids present in the sequence of HHM, strictly selective cleavage at only 6 solvent accessible aspartate residues was observed. Mechanistic experiments suggest that the hydrolytic activity is likely derived from the actuation of Hf^IV Lewis acidic sites and the Brønsted acidic surface of Hf₁₈ . X-ray scattering and ESI-MS revealed that Hf₁₈ is completely insoluble in these conditions, confirming the HHM hydrolysis is caused by a heterogeneous reaction of the solid Hf₁₈ cluster, and not from smaller, soluble Hf species that could leach into solution.

Proton-conducting layered structures based on transition metal oxo-clusters supported by Sb(iii) tartrate scaffolds

Two transition metal-antimony oxo-cluster based compounds, H₅{MCd(H₂O)₆[M(H₂O)₃Co₃Sb^VSb(μ₃-O)₈(l-tta)₆]}·7H₂O (1) (M = Cd_0.5 + Co_0.5) and H₃K₅(H₂O)₁₁{Cd(H₂O)₄[Cd(H₂O)Fe₄Cd₂Sb₆(μ₄-O)₅(μ₃-O)₃(l-tta)₆][Cd(H₂O)₂Fe₄Cd₂Sb₆(μ₄-O)₄(μ₃-O)₄(l-tta)₆][Cd(H₂O)₂Fe₄Cd₂Sb₆(μ₄-O)₄(μ₃-O)₄(l-tta)₆Cd(H₂O)₅]}·17H₂O (2) (L-H₄tta = l-tartaric acid) were hydrothermally synthesized and characterized. Compound 1 features a [MCo₃Sb^VSb(μ₃-O)₈(l-tta)₆(H₂O)₃]^9- cluster, while compound 2 contains three types of clusters, namely, [Cd(H₂O)Fe₄Cd₂Sb₆(μ₄-O)₅(μ₃-O)₃(l-tta)₆]^4-, [Cd(H₂O)₂Fe₄Cd₂Sb₆(μ₄-O)₄(μ₃-O)₄(l-tta)₆]^4- and [Cd(H₂O)₂Fe₄Cd₂Sb₆(μ₄-O)₄(μ₃-O)₄(l-tta)₆Cd(H₂O)₅]^2-. All the clusters are of sandwich-type with {Sb₃(μ₃-O)(l-tta)} scaffolds on the top and bottom. The Cd (and M in 1) ions interconnect the clusters into layered structures in both compounds. To the best of our knowledge, this is the first report of transition metal-antimony oxo-clusters that simultaneously contain the first-row and second-raw transition metal ions, and compound 1 represents the first example of such type of clusters that contain Sb(v). The two compounds exhibit proton conductivity with the values of 2.43 × 10^-3 and 2.95 × 10^-3 S cm^-1 at 85 °C under 98% relative humidity, respectively.

Selective Oxidation of Benzylic C-H Bonds Catalyzed by Cu(II)/{PMo12}

Precise catalytic regulation of carbon radical generation by a highly active oxygen radical to abstract the H atom in a C-H bond is an effective method for the selective activation of C-H synthetic chemistry. Herein, we report a facile catalyst system with commercially available copper(II)/{PMo₁₂} to form a tert-butanol radical intermediate for the selective oxidation of benzylic C-H bonds. The reaction shows a broad range of substrates (benzyl methylene, benzyl alcohols) with good functional group tolerance and chemical selectivity. The corresponding carbonyl compounds were synthesized with good yields under mild conditions. DFT calculations and experimental analysis further demonstrated a reasonable carbon radical mechanism for this type of organic transformation reaction.

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.

Hybrid assemblies of a symmetric designer protein and polyoxometalates with matching symmetry

A symmetric designer protein forms hybrid complexes with different polyoxometalates and may serve as a building block for porous frameworks.

Intrinsically porous molecular building blocks for metal organic frameworks tailored by the bridging effect of counter cations

Intrinsically porous molecular building blocks are used for the rational design and construction of molecular-level controlled porous materials.

Rational regulation of transition metals in polyoxometalate hybrids without noble metal assistance for efficient light-driven H2 production

Four new noble metal-free hybrids are reported with a maximum photocatalytic hydrogen evolution rate of 12245.59 μmol g⁻¹ h⁻¹, which outperform all known polyoxometalate (POM)-based metal–organic photocatalysts.

A novel sandwich shaped {CoIII2CoII12MoV24} cluster with a CoII4 triangle encapsulated in two capped CoIIICoII4MoV12O40 fragments

A novel discrete {Co₁₄Mo₂₄} nanoscaled cluster, with a triangle Co₄ core encapsulated in two novel capped Co-substituted Keggin-type Co₅Mo₁₂O₄₀ anions, was isolated from alkaline methanol solution. And the HRESI-MS of {Co₁₄Mo₂₄} cluster was recorded.

Self-assembled tetramethyl cucurbit[6]uril–polyoxometalate nanocubes as efficient and recyclable catalysts for the preparation of propyl gallate

A novel class of cucurbit[n]urils–polyoxometalate (Q[n]–POM) hybrids and tetramethyl cucurbit[6]uril–phosphomolybdic acid (TMeQ[6]–PMA) nanocubes (NCs), are synthesized via a one-step self-assembly method.

Hydrogen bonding assisted formation of sandwich-type titanium-containing heteropolymolybdates: water-soluble and photoelectroactive

Two water-soluble titanium-containing polyoxomolybdates have been synthesized by means of a hydrogen bonding assisted strategy. Their applications in the photoelectrochemical sensing of bisphenol A in aqueous solution were also investigated.

Giant iron polyoxometalate that works as a catalyst for water oxidation

A polyoxometalate (POM) cluster [Mo₇₂Fe₃₀O₂₅₂(CH₃COO)₁₂{Mo₂O₇(H₂O)}₂ {H₂Mo₂O₈(H₂O)} (H₂O)₉₁]. ca. 150 H₂O (catalyst I) has been explored as a light-driven water oxidation catalyst. The catalyst is stable and could be reused/recycled several times.

Selenotungstates incorporating organophosphonate ligands and metal ions: synthesis, characterization, magnetism and catalytic efficiency in the Knoevenagel condensation reaction

Three new sandwich-type selenotungstate anion structures and Co1 show superior catalytic performance in the Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate under mild conditions.

Mechanistic insights into polyoxometalate self-assembly in organic solvent: conversion of a cyclic polyoxovanadate-ethoxide to its Lindqvist congener

We report the synthesis of a cyclic hexavanadate polyoxovanadate-alkoxide cluster, [VO(OC2H5)2]6, and its conversion, under solvothermal conditions, to an oxygen-deficient Lindqvist assembly, [V₆O₆(OC₂H₅)₁₂]ⁿ (n = 1−, 0).

Synthesis, Single Crystal X-Ray Structure, Theoretical Studies of Triple-{MnIII-Schiff-Base}-Decorated Molybdate

The design and synthesis of magnets with low dimensionality is currently an attractive research topic. The reaction of a Schiff base metal complex with simple molybdate has led to the formation of a new {MnIII-Schiff-base}-decorated molybdate heterometallic cluster, [{Mn(salpn)(H2O)}3MoO4](CH3COO)·2H2O (1) (salpn = N,N’-(1,3-propylene)bis(salicylideneiminate)). The hybrid aggregate was characterized using a range of methods including elemental analysis, single crystal x-ray diffraction, Infrared spectra (IR) spectroscopy, and x-ray structural analysis. The crystals of 1 are hexagonal: P63/m, a = 14.027(5) Å, c = 17.487(5) Å, V = 1142.6(3) Å3. Structural analyses indicate that we report on the preparation of the first triple metallic-oligomer held by orthomolybdate. Density functional theoretical calculation (DFT) studies have been performed to calculate electronic structure and potential energy landscapes. Additionally, the magnetic property of 1 indicates an antiferromagnetic coupling between the metal centers in the complex.

Controlled Assembly Synthesis of Atomically Precise Ultrastable Silver Nanoclusters with Polyoxometalates

Silver nanoclusters have attracted scientific interest due to their properties and applications. However, practical synthetic methods to access these materials are still limited mainly due to the low stability. Here, we report a controlled assembly strategy for fabricating atomically precise silver nanoclusters using polyoxometalates (POMs) as structure-directing as well as functionalizing units. A trefoil-propeller-shaped {Ag₂₇}¹⁷⁺ nanocluster was synthesized by assembling reactive nanoclusters supported by open-Dawson-type POMs [Si₂W₁₈O₆₆]^16-. The {Ag₂₇}¹⁷⁺ nanocluster possessed 10 delocalized valence electrons and showed unprecedented ultrastability in solutions. The cluster showed unique {Ag₂₇}-to-POM charge transfer bands in the visible light region.

Highly efficient and selective antimicrobial isonicotinylhydrazide-coated polyoxometalate-functionalized silver nanoparticles

With the rapidly approaching post-antibiotic era, new and effective combinations of antibiotics are imperative to combat multiple drug resistance (MDR). We have synthesized multimodal antimicrobials that integrate the antibiotic isonicotinylhydrazide (INH), silver nanoparticles (AgNPs^INH), and two different polyoxometalates (POMs) namely, phosphotungstic acid (PTA) and phosphomolybdic acid (PMA) to prepare AgNPs^INH@PTA and AgNPs^INH@PMA, respectively. AgNPs^INH have peroxidase-like (nanozyme) activity and very high antibacterial potential toward S. aureus, which was further enhanced upon modification with POMs. The selectivity of these functional nanozymes was evaluated with m5S mouse fibroblasts using WST-8, LDH viability, in vitro reactive oxygen species (ROS) generation assays, and crystal violet morphological studies. These investigations showed very low cytotoxicity for the nanoparticles compared to free metal ions (Ag⁺), pristine POMs and INH, demonstrating the ability of multifunctional materials to provide efficient and selective antimicrobials.

A CsCl-type inorganic cluster-based high-symmetry crystal built from {Mo4.55V7.45PO40}10.45- with a high ratio of vanadium to molybdenum and {(H2O)0.3@K6(H2O)12}6+ clusters exhibiting proton conduction below the freezing point of water

As a class of anionic oxoclusters of early transition metals in their highest oxidation states, polyoxometalates (POMs) show considerable structural versatility and unique chemical and physical properties, making them promising multifunctional materials. In this study, a Keggin-type POM has been achieved, with a formula of [(H2O)0.3@K6(H2O)12]H4.45[PV7.45Mo4.55O40]·11H2O (1), and its microcrystals and nanocrystals have been obtained, respectively. This POM was characterized by elemental analysis for C, H and N, ICP-MS, TG, PXRD, SEM, X-band EPR and XPS techniques. Single crystal X-ray diffraction analysis demonstrated that 1 shows a rare extended structure with a high-connected three-dimensional (3D) all inorganic network of a Keggin-type POM, built from {Mo4.55V7.45PO40}10.45- polyoxoanions and {(H2O)0.3@K6(H2O)12}6+ clusters with CsCl-type crystal structure. In addition, to the best of our knowledge, 1 shows the highest ratio of vanadium to molybdenum among Keggin-type POMs reported thus far. Most interestingly, 1 exhibits intrinsic proton conduction below the freezing point of water, with a proton conductivity of 6.90 × 10-7 S cm-1 at 249 K and further reaching 3.36 × 10-6 S cm-1 at 272 K and Ea = 0.44 eV at 249-272 K.

Organic-inorganic hybrid 1-D double chain heteropolymolybdates constructed from plenary Keggin germanomolybdate anions and hepta-nuclear Cu-RE-pic heterometallic clusters

Two unprecedented organic-inorganic hybrid 1-D double chain germanomolybdates containing hepta-nuclear Cu-RE-pic heterometallic clusters [NH₄]₂[RE(H₂O)₅]₂[Cu(pic)₂]₂[Cu(pic)₂(H₂O)₂]₃[α-GeMo₁₂O₄₀]₂·22H₂O [RE = La³⁺ (1), Ce³⁺ (2), and Hpic = 2-picolinic acid] were successfully obtained by the stepwise self-assembly strategy via the conventional solution method. The most striking structural feature of 1 and 2 is that the two plenary Keggin [α-GeMo₁₂O₄₀]^4- polyoxoanions are joined by an organic-inorganic hybrid hepta-nuclear Cu-RE-pic {[RE(H₂O)₅]₂[Cu(pic)₂(H₂O)₂]₃[Cu(pic)₂]₂}⁶⁺ heterometallic moiety. What is more interesting is that their adjacent structural units are connected together by {Cu(pic)₂} bridges, forming a 1-D extended double chain architecture. Furthermore, the adsorption capacity of 1 toward dyes in aqueous solutions was deeply investigated. It is fascinating that 1 shows a good adsorption capacity toward basic violet 3 (BV 3) in aqueous solutions and the adsorption kinetics conforms to the second-order kinetic model.

Self-Assembly of a Phosphate-Centered Polyoxo-Titanium Cluster: Discovery of the Heteroatom Keggin Family

Over the past 200 years, the most famous and important heteroatom Keggin architecture in polyoxometalates has only been synthesized with Mo, W, V, or Nb. Now, the self-assembly of two phosphate (PO₄ ^3- )-centered polyoxo-titanium clusters (PTCs) is presented, PTi₁₆ and PTi₁₂ , which display classic heteroatom Keggin and its trivacant structures, respectively. Because Ti^IV has lower oxidate state and larger ionic radius than Mo^VI , W^VI , V^V , and Nb^V , additional Ti^IV centres in these PTCs are used to stabilize the resultant heteroatom Keggin structures, as demonstrated by the cooresponding theoretical calculation results. These photoactive PTCs can be utilized as efficient photocatalysts for highly selective CO₂ -to-HCOOH conversion. This new discovery indicates that the classic heteroatom Keggin family can be assembled with Ti, thus opening a research avenue for the development of PTC chemistry.

Novel Ti4+-Chelated Polyoxometalate/Polydopamine Composite Microspheres for Highly Selective Isolation and Enrichment of Phosphoproteins

Selective isolation and enrichment of phosphoproteins play critical roles for identification of biomarkers in biological applications. Herein, a kind of polyoxometalate (P₅W₃₀)/polydopamine (PDA) composite microspheres is readily synthesized via an in situ polymerization way, followed by immobilization of Ti⁴⁺ on the surface of the microspheres to obtain P₅W₃₀/PDA-Ti⁴⁺. Due to metal affinity and π stacking interaction, this novel material exhibits high selectivity to β-casein (β-ca), and the theoretical maximum adsorption capacity is as high as 1250 mg g^-1, fitting well with the Langmuir model. The captured β-ca can be collected by using Britton-Robinson (B-R) buffer at pH 7.0, and a recovery of 81.5% is acquired. The enrichment factor is over 150 at a mass ratio of BSA/β-ca = 100:1, indicating that phosphoproteins can be purified by P₅W₃₀/PDA-Ti⁴⁺. Moreover, the application of P₅W₃₀/PDA-Ti⁴⁺ as sorbent in real biological samples has been investigated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis, and the consequences show that this kind of material is able to selectively isolate phosphoproteins from complex samples such as drinking milk and chicken egg white.

Combining autoclave and LCWM reactor studies to shed light on the kinetics of glucose oxidation catalyzed by doped molybdenum-based heteropoly acids

In this work we combined kinetic studies for aqueous-phase glucose oxidation in a high-pressure autoclave setup with catalyst reoxidation studies in a liquid-core waveguide membrane reactor. Hereby, we investigated the influence of Nb- and Ta-doping on Mo-based Keggin-polyoxometalates for both reaction steps independently. Most importantly, we could demonstrate a significant increase of glucose oxidation kinetics by Ta- and especially Nb-doping by factors of 1.1 and 1.5 compared to the classical HPA-Mo. Moreover, activation energies for the substrate oxidation step could be significantly reduced from around 80 kJ mol⁻¹ for the classical HPA-Mo to 61 kJ mol⁻¹ for the Ta- and 55 kJ mol⁻¹ for the Nb-doped species, respectively. Regarding catalyst reoxidation kinetics, the doping did not show significant differences between the different catalysts.

In this work we combined kinetic studies for aqueous-phase glucose oxidation in a high-pressure autoclave setup with catalyst reoxidation studies in a liquid-core waveguide membrane reactor.

Core-dependent properties of copper nanoclusters: valence-pure nanoclusters as NIR TADF emitters and mixed-valence ones as semiconductors

While valence-pure copper alkynyl nanoclusters show near-infrared TADF, the mixed-valence ones exhibit semiconductivity.

We report herein that copper alkynyl nanoclusters show metal-core dependent properties via a charge-transfer mechanism, which enables new understanding of their structure–property relationship. Initially, nanoclusters 1 and 2 bearing respective Cu(i)₁₅ (C1) and Cu(i)₂₈ (C2) cores were prepared and revealed to display near-infrared (NIR) photoluminescence mainly from the mixed alkynyl → Cu(i) ligand-to-metal charge transfer (LMCT) and cluster-centered transition, and they further exhibit thermally activated delayed fluorescence (TADF). Subsequently, a vanadate-induced oxidative approach to in situ generate a nucleating Cu(ii) cation led to assembly of 3 and 4 featuring respective [Cu(ii)O₆]@Cu(i)₄₇ (C3) and {[Cu(ii)O₄]·[VO₄]₂}@Cu(i)₄₆ (C4) cores. While interstitial occupancy of Cu(ii) triggers inter-valence charge-transfer (IVCT) from Cu(i) to Cu(ii) to quench the photoluminescence of 3 and 4, such a process facilitates charge mobility to render them semiconductive. Overall, metal-core modification results in an interplay between charge-transfer processes to switch TADF to semiconductivity, which underpins an unusual structure–property correlation for designed synthesis of metal nanoclusters with unique properties and functions.

Tuning Redox Active Polyoxometalates for Efficient Electron-Coupled Proton-Buffer-Mediated Water Splitting

We present strategies to tune the redox properties of polyoxometalate clusters to enhance the electron-coupled proton-buffer-mediated water splitting process, in which the evolution of hydrogen and oxygen can occur in different forms and is separated in time and space. By substituting the heteroatom template in the Keggin-type polyoxometalate cluster, H6 ZnW12 O40 , it is possible to double the number of electrons and protonation in the redox reactions (from two to four). This increase can be achieved with better matching of the energy levels as indicated by the redox potentials, compared to the ones of well-studied H3 PW12 O40 and H4 SiW12 O40 . This means that H6 ZnW12 O40 can act as a high-performance redox mediator in an electrolytic cell for the on-demand generation of hydrogen with a high decoupling efficiency of 95.5 % and an electrochemical energy efficiency of 83.3 %. Furthermore, the H6 ZnW12 O40 cluster also exhibits an excellent cycling behaviour and redox reversibility with almost 100 % H2 -mediated capacity retention during 200 cycles and a high coulombic efficiency >92 % each cycle at 30 mA cm-2 .

Composition-defined nanosized assemblies that contain heterometallic early 4d/5d-transition-metals

The controlled assembly of early transition metals remains a challenging research target, especially with respect to the generation of heterometallic molecules and nanomaterials. In this study, metal chlorides of the early 4d/5d-transition-metals, i.e., ZrCl₄, NbCl₅, MoCl₅, HfCl₄, TaCl₅, and WCl₆, were stoichiometrically introduced into a tetraphenylmethane-core dendritic-phenylazomethine generation 4 dendrimer in the presence of an optimal amount of organic ligands such as pyridine and 3-chloropyridine. The coordinative interactions between the metal chlorides and the imines in the dendrimers indicated a positive correlation for the Lewis acidity of the metals. Moreover, it was clearly demonstrated for the first time that heterometallic assemblies of defined composition contain four kinds of early 4d/5d-transition-metals, such as Ta^V, Nb^V, Mo^V, and Zr^IV, which was confirmed by UV-vis titration, XPS, and HAADF-STEM/EDS measurements. The results of this study should provide access to new routes to produce nanomaterials composed of heterometallic early 4d/5d-transition metals.