Rare sandwich-type polyoxomolybdates constructed from Di-/tetra-nuclear transition-metal clusters and trivacant keggin germanomolybdate fragments

Self-Assembled 8-Ti-Containing Polyoxomolybdate for the Photocatalytic Hydrogen Evolution

An S-shaped 8-Ti-containing polyoxomolybdate (NH4)Cs2Na6H3[Ti8(GeMo9O34)2(GeMo5O23)2]·44H2O (1) has been prepared under the one-pot hydrothermal method and further characterized. According to single-crystal X-ray, compound 1 consists of two {GeMo9O34} and two {GeMo5O23} segments, which are linked with a {Ti8O34} cluster. Moreover, the {GeMo5O23} fragment is rare and the first discovery, which enriches the lacunary Keggin-type family and offers the possibility of obtaining new structures. Among all of the reported polyoxomolybdates, compound 1 has the highest nuclearity of Ti centers. From the photocatalytic hydrogen evolution studies, compound 1 can be used as the heterogeneous catalyst with an H2 evolution rate of 2392.6 μmol g-1 h-1 under minimally optimized conditions.

Synthesis and Characterization of 6-Ti-Substituted Polyoxomolybdate with High Catalytic Activity for Sulfide Oxidation

A 6-Ti-substituted polyoxometalate, (NH4)5Cs7Na3H2[Cs@(Ti2GeMo10O39)3]·34H2O (1), was synthesized by reacting (NH4)6Mo7O24·4H2O, GeO2, and TiOSO4 through the conventional aqueous method. Polyanion 1a is composed of three {Ti2GeMo10} segments linked by Ti-O-Ti linkages and shows a trefoil-shaped structure. Furthermore, one Cs+ cation is encapsulated in the cavity of 1a. Notably, it possesses the highest number of Ti centers among the reported polyoxomolybdates. In addition, serving as a high-efficiency heterogeneous catalyst, 1 enables the conversion of methyl phenyl sulfide within 20 min, yielding 96.4% of the corresponding sulfoxide with good recyclability.

Gold Nanoparticles Capped with a Novel Titanium(IV)-Containing Polyoxomolybdate Cluster: Selective and Enhanced Bactericidal Effect Against Escherichia coli

Bacterial infections are a public health threat of increasing concern in medical care systems; hence, the search for novel strategies to lower the use of antibiotics and their harmful effects becomes imperative. Herein, the antimicrobial performance of four polyoxometalate (POM)-stabilized gold nanoparticles (Au@POM) against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) as Gram-negative and Gram-positive bacteria models, respectively, is studied. The bactericidal studies performed, both in planktonic and sessile forms, evidence the antimicrobial potential of these hybrid nanostructures with selectivity toward Gram-negative species. In particular, the Au@GeMoTi composite with the novel [Ti2 (HGeMo7 O28 )2 ]10- POM capping ligand exhibits outstanding bactericidal efficiency with a minimum inhibitory concentration of just 3.12 µm for the E. coli strain, thus outperforming the other three Au@POM counterparts. GeMoTi represents the fourth example of a water-soluble TiIV -containing polyoxomolybdate, and among them, the first sandwich-type structure having heteroatoms in high-oxidation state. The evaluation of the bactericidal mechanisms of action points to the cell membrane hyperpolarization, disruption, and subsequent nucleotide leakage and the low cytotoxicity exerted on five different cell lines at antimicrobial doses demonstrates the antibiotic-like character. These studies highlight the successful design and development of a new POM-based nanomaterial able to eradicate Gram-negative bacteria without damaging mammalian cells.

A protecting group strategy to access stable lacunary polyoxomolybdates for introducing multinuclear metal clusters

Although metal-containing polyoxomolybdates (molybdenum oxide clusters) exhibit outstanding catalytic properties, their precise synthetic method has not yet been developed. This is mainly because the very low stability of the multivacant lacunary polyoxomolybdates limited their use as synthetic precursors. Here, we present a "protecting group strategy" in polyoxometalate synthesis and successfully develop an efficient method for synthesising multinuclear metal-containing polyoxomolybdates using pyridine as a protecting group for unstable trivacant lacunary Keggin-type polyoxomolybdate [PMo₉O₃₄]^9-. Specifically, tetranuclear cubane- and planar-type manganese clusters were selectively synthesised in the polyoxomolybdates using the present method. The importance of this work is that, in addition to being the first practical way of utilizing multivacant lacunary polyoxomolybdates as precursors, this new "protecting group strategy" will make it possible to produce polyoxometalates with unexplored structures and properties.

In Vitro Anti-tumoral and Anti-bacterial Activity of an Octamolybdate Cluster-Based Hybrid Solid Incorporated with a Copper Picolinate Complex

Inorganic drugs, especially polyoxometalate-based hybrids, are expected to be developed as promising future metallodrugs. Herein, an organic-inorganic hybrid solid based on pyridine-2-carboxylic acid or picolinic acid (pic), [(Cu(pic)₂)₂(Mo₈O₂₆)]·8H₂O (1), was synthesized. A single-crystal structure of a solid possesses a discrete β-type octamolybdate cluster that supramolecularly aggregates with a {Cu₂(pic)₄}^4- complex and eight lattice water molecules. The study indicates that the solid is stable in aqueous medium and less toxic toward normal cell lines. The in vitro anti-bacterial and anti-tumor properties of the solid 1 were investigated. The results of the anti-tumor action against various human cancer cell lines, namely, lung (A549), breast (MCF-7), and liver (HepG2) cancer cells suggest that this β-octamolybdate-based solid yielded the lowest IC₅₀ value reported so far among octamolybdate anion-based hybrid solids, i.e., 24.24 μM for MCF-7, 21.56 μM for HepG2, and 25 μM for A549, indicating significant anti-cancer activity. The cell cycle analysis further reveals the observed anti-tumor effect to be governed by the arrest of breast cancer cells in the G2/M phase while that of lung and liver cancer cells in the S phase of the cell cycle. A fluorescence quenching study suggests the binding interaction between solid and ctDNA, which in turn induces apoptosis and necrosis pathways leading to cancer cell death. This is also the first study of {Mo₈O₂₆}^4- cluster-based solids as an anti-bacterial agent against Escherichia coli, and it was found to be very effective with a minimal inhibitory concentration value of ∼135 μg/mL, which is the lowest so far reported for any octamolybdate-based solid.

An unprecedented polyhydroxycarboxylic acid ligand bridged multi-EuIII incorporated tellurotungstate and its luminescence properties

The first polyhydroxycarboxylic acid ligand bridged multi-EuIII-incorporated tellurotungstate K14H10[Eu4(H2O)4W6(H2glu)4O12(B-α-TeW9O33)4]·60H2O (H6glu = d-gluconic acid) (1) was synthesized via an organic ligand-driven self-assembly strategy. The polyhydroxycarboxylic acid ligand bridged tetrameric polyoxoanion [Eu4(H2O)4W6(H2glu)4O12(B-α-TeW9O33)4]24- in 1 can be viewed as an aggregation of four trivacant Keggin [B-α-TeW9O33]8- fragments and an innovative heterometallic [Eu4(H2O)4W6(H2glu)4O12]8+ cluster, in which four high-coordinate polyhydroxy flexible H2glu4- ligands chelate W and Eu centers through carboxyl and hydroxyl groups, giving rise to a heterometallic cluster. The hexagonal packing of the tetrameric polyoxoanions in 1 along the c axis provides excellent porous channels, which greatly increases the specific surface area of the whole framework and may be of benefit for fluorescence sensing in aqueous solution. 1 can function as a "turn-off" luminescence sensor to detect Cu2+ ions in aqueous solution. The limit of detection (LOD) of the 1-sensor is 8.82 × 10-6 mM, which is the lowest among the reported polyoxometalate-based fluorescence sensors. As for the Cu2+-quenching system, it can function as an "off-on" sensor to detect cysteine in an aqueous system, affording a LOD of 1.75 × 10-4 mM. This work opens up an avenue to broaden the applications of polyoxometalate-based materials in the optical intelligence detection field.

Two Ni/Co-substituted sandwich-type germanomolybdates based on an unprecedented trivacant polyanion [α-GeMo10O36]8

Two Ni/Co-substituted sandwich-type germanomolybdates, {[M₃(NH₂-trz)₆(H₂O)₆][M₄(H₂O)₂(HGeMo₁₀O₃₆)₂]}·nH₂O (M = Ni²⁺ (1) and Co²⁺ (2), n = 10 (1) or 11 (2), NH₂-trz = 4-amino-1,2,4-triazole), have been obtained under hydrothermal conditions. 1 and 2 represent the first trivacant Keggin germanomolybdates involving unprecedented [α-GeMo₁₀O₃₆]^8- fragments and {M₃O₄} quasi-cubane building units. Both of them exhibit electrocatalytic behaviours for H₂O₂ reduction and photocatalytic properties for CO₂ conversion.

Water-soluble titanium-polyoxomolybdate with external μ3 bridging oxygen coordination on a lacunary Keggin structure

The first water-soluble titanium-containing polyoxomolybdate was synthesized and structurally characterized, showing photocatalytic activity in benzaldehyde oxidation.

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.

Polyoxometalates in solution: speciation under spotlight

The review covers stability and transformations of classical polyoxometalates in aqueous solutions and provides their ion-distribution diagrams over a wide pH range.

Utilizing the adaptive precursor [As2W19O67(H2O)]14– to support three hexanuclear lanthanoid-based tungstoarsenate dimers

The photoluminescence properties and polyanionic structures of hexanuclear lanthanoid-based tungstoarsenate dimers.

Redox tuning the Weakley-type polyoxometalate archetype for the oxygen evolution reaction

Water oxidation is a key reaction for the conversion of solar energy into chemical fuels, but effective water-oxidation catalysts are often based on rare and costly precious metals such as Pt, Ir or Ru. Developing strategies based on earth-abundant metals is important to explore critical aspects of this reaction, and to see whether different and more efficient applications are possible for energy systems. Herein, we present an approach to tuning a redox-active electrocatalyst based on the doping of molybdenum into the tungsten framework of [Co₄(H₂O)₂(PW₉O₃₄)₂]^10-, known as the Weakley sandwich. The Mo-doped framework was confirmed by X-ray crystallography, electrospray ionization mass spectrometry and inductively coupled plasma optical emission spectrometry studies. The doping of molybdenum into the robust Weakley sandwich framework leads to the oxidation of water at a low onset potential, and with no catalyst degradation, whereby the overpotential of the oxygen evolution reaction is lowered by 188 mV compared with the pure tungsten framework.

An isotetramolybdate-supported rhenium carbonyl derivative: synthesis, characterization, and use as a catalyst for sulfoxidation

We successfully synthesized an isotetramolybdate-supported rhenium carbonyl derivative, [(CH₃)₄N]₄[{Re(CO)₃}₄(Mo₄O₁₆)]·H₂O, which showed excellent catalytic activity in the selective oxidation of sulfides under comparatively mild reaction conditions.

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.

Synthesis and characterization of a Sb(v)-containing polyoxomolybdate serving as a catalyst for sulfoxidation

A Sb-containing Anderson-based polyoxomolybdate cluster, [(CH₃)₄N]₄H₈[Na₅Sb₃(Sb₂Mo₁₂O₅₇)]·17H₂O [1; (CH₃)₄N⁺ = TMA⁺], has been successfully synthesized by using an aqueous solution method and structurally characterized.

Two unusual 3D honeycomb networks based on Wells–Dawson arsenomolybdates with d10 transition-metal-pyrazole connectors

Two unusual 3D honeycomb networks based on Wells–Dawson arsenomolybdates were first reported, which have highly efficient catalytic ability for the degradation of organic dyes under UV irradiation and obvious electrocatalytic activities for the reduction of H₂O₂.

Two Compounds Constructed from Bimolybdenum-Capped Sandwich-Type Tetra-Ni-molybdogermanate and N-Donor Multidentate Ligand

Two organic-inorganic hybrid bimolybdenum-capped tetra-Ni(II) sandwich-type molybdogermanates, namely, (H2L)4[Ni4(H2O)2{B-α-GeMo9O34 (MoO2)}2]·6H2O (1), [Ni2(HL)4(H2O)2][Ni4(H2O)2{B-α-GeMo9O34(MoO2)}2]·12H2O (2), L = 2,4,5-tri(4-pyridyl)imidazole, were prepared with hydrothermal synthesis method and characterized by elemental analysis, IR spectroscopy, powder X-ray diffraction, thermogravimetric analysis, and single-crystal X-ray diffraction. Their magnetic properties and electrochemical properties were investigated. The results show that compounds 1 and 2 contain a new bimolybdenum-capped sandwich-type heteropolymolybdate anion, [Ni4(H2O)2{B-α-GeMo9O34(MoO2)}2](8-). The compound 1 is a zero-dimensional supramolecular compound, and the supramolecular architecture of compound 2 is constructed from covalent layers through hydrogen bonds and π-π interaction. The two compounds both present electrocatalytic activities for the reduction of nitrite.

Polyoxometalates-based heterometallic organic–inorganic hybrid materials for rapid adsorption and selective separation of methylene blue from aqueous solutions

A series of POMs-based hybrid materials exhibit rapid adsorption rate, high uptake capacity and excellent selective separation towards cationic dye.

Mixed-solvothermal synthesis, structures, surface photovoltage, luminescence and molecular recognition properties of three new transition metal phosphonates with 3D framework and supramolecular structures

Three new transition metal(ii) phosphonates have been synthesized under mixed-solvothermal conditions. Surface photovoltage and field-induced surface photovoltage spectroscopy of compounds 1 and 2 have been studied. Compound 3 could be used for the sensitive sensing of DMF.

Construction of organic-inorganic hybrid molybdophosphonate clusters with copper-bipyridine

An organic-inorganic hybrid polyoxomolybdate with formula as [Cu(2,2'-bpy)(H2O)]2[Cu(2,2'-bpy)(H2O)3][(O3PCCH3(OH)PO3)2Mo(VI)4O11(H2O)2]·7H2O (1) (2,2'-bpy=2,2'-bipyridine) has been successfully synthesized constructing by diphosphonate H2O3PC(CH3)(OH)PO3H2 (1-hydroxyethylidene-1,1-bisphosphonic acid, noted as HEDP or etidronate) under conventional aqueous solution. 1 was characterized by single-crystal X-ray diffraction, elemental analysis, IR spectroscopy, thermogravimetric (TG) analysis, and X-ray photoelectron spectroscopy (XPS). Each anion cluster of 1 can be regarded as the fusion of two [{Cu(2,2'-bpy)(H2O)}(O3PCCH3(OH)PO3)Mo(VI)2O6](2-) subunits with an additional six-coordinated [Cu(2,2'-bpy)](2+) coordination cation linking on one side and there exist weak π-π interactions among the bipyridine groups. The XPS spectra indicate that the oxidation states of Mo and Cu in 1 are +6 and +2, respectively. Photoluminescence properties of 1 and the free 2,2'-bipyridine ligand have been both analyzed, showing that the presence of ligand-to-metal-charge-transfer (LMCT) transitions and metal-ligand coordination have an important effecting on the photoluminescence.

Synthesis, structures, surface photovoltage and luminescence properties of two new nickel(ii) carboxyphosphonates with a 3D framework structure

Two new 3D nickel(ii) carboxyphosphonates 1 and 2 were synthesized under hydrothermal conditions. The surface photovoltage and luminescent properties were studied.