A dimeric titanium-containing polyoxometalate. Synthesis, characterization, and catalysis of H2O2-based thioether oxidation

Two Tetra-Nuclear Ln-Substituted Prazine Dicarboxylic Acid-Functionalized Selenotungstates with Catalytic Oxidation of Thioether Properties

Two two-dimensional Ln-substituted prazine dicarboxylic acid-functionalized selenotungstates Na3H9[(H2N(CH3)2]2{(Se4W27O100)[Ln4(H2O)8(Hpzdc)2(pzdc)]}·26H2O [Ln = Nd (1) and Ce (2)]; H2pzdc = 2,3-pyrazine dicarboxylic acid) have been synthesized by one-pot self-assembly strategy, in which the basic polyanion [Se4W27O100]22-was composed of two [SeW8O31]10- fragments, a [SeW9O33]8- segment and an intriguing {SeO} group, simultaneously tetra-nuclear Ln3+ ions with H2pzdc pendants were embedded. Compounds 1 and 2 showed excellent catalytic oxidation of thioether properties within a short time (20 min) with high 100% conversion and 98.9% selectivity. In addition, the pioneering Ln-substituted selenotungstates were used as catalysts to degrade sulfur mustard simulant 2-chloroethyl ethyl sulfide at room temperature with 99% conversion and 100% selectivity. The chemical kinetic experiment studies revealed that the catalytic reaction was in compliance with the first-order reaction, and the kinetic half-life (t1/2) values were 3.814 and 3.849 min, respectively.

Recent Advances of Ti/Zr-Substituted Polyoxometalates: From Structural Diversity to Functional Applications

Polyoxometalates (POMs), a large family of anionic polynuclear metal-oxo clusters, have received considerable research attention due to their structural versatility and diverse physicochemical properties. Lacunary POMs are key building blocks for the syntheses of functional POMs due to their highly active multidentate O-donor sites. In this review, we have addressed the structural diversities of Ti/Zr-substituted POMs based on the polymerization number of POM building blocks and the number of Ti and Zr centers. The synthetic strategies and relevant catalytic applications of some representative Ti/Zr-substituted POMs have been discussed in detail. Finally, the outlook on the future development of this area is also prospected.

Recent Studies on Hydrogels Based on H2O2-Responsive Moieties: Mechanism, Preparation and Application

H₂O₂ is essential for cellular processes and plays a vital role in the regulation of cell signaling pathways, which can be viewed as a warning signal for many kinds of disease including cancer, cardiovascular disease, reproductive abnormalities, diabetes, and renal failure. A H₂O₂-responsive hydrogel (H₂O₂-Gel) is a promising candidate for biomedical applications because of its good biocompatibility, similarity to soft biological tissues, ease of preparation, and its ability to respond to H₂O₂. In this study, the H₂O₂-responsive moieties used to fabricate H₂O₂-Gels were reviewed, including thioethers, disulfide bonds, selenides, diselenium bonds, diketones, boronic, and others. Next, the preparation method of H₂O₂-Gel was divided into two major categories according to their reaction mechanisms: either self-crosslinking or mechanisms entailing the addition of difunctional crosslinkers. Last, the applications of H₂O₂-Gels were emphasized, which have been viewed as desirable candidates in the fields of drug delivery, the detection of H₂O₂, glucose-responsive systems, ROS scavengers, tissue engineering, and cell-encapsulation.

Three Lanthanide-Functionalized Antimonotungstate Clusters with a {Sb4O4Ln3(H2O)8} Core: Syntheses, Structures, and Properties

Three lanthanide (Ln)-functionalized antimonotungstate (AT) clusters with a {Sb₄O₄Ln₃(H₂O)₈} core [H₂N(CH₃)₂]₁₄Na₈H₁₆[Sb₄O₄Ln₃(H₂O)₈W₂O₄(H₂O)₂(B-α-SbW₉O₃₃)₄]₂·87H₂O [Ln = Dy³⁺ (1), Ho³⁺ (2), Y³⁺ (3)] were synthesized in an acidic aqueous solution. Their molecular structural unit comprises two {Sb₄O₄Ln₃(H₂O)₈}-core-incorporated tetrameric [Sb₄O₄Ln₃(H₂O)₈W₂O₄(H₂O)₂ (B-α-SbW₉O₃₃)₄]^19- polyanionic units, each of which is assembled from an unprecedented [Sb₄O₄Ln₃(H₂O)₈W₂O₄(H₂O)₂]¹⁷⁺ heteroatom cluster surrounded by four trivacant [B-α-SbW₉O₃₃]^9- subunits. What is noteworthy is that a tetrahedral {Sb₄O₄} cluster is located at the center of the polyanionic unit, as far as we know, which is very infrequent in multi-Ln-functionalized polyoxometalate chemistry. Solid-state luminescent properties and energy migration of AT ligands to Dy³⁺ and Ho³⁺ cations in 1 and 2 have been intensively probed at ambient temperature. By varying the exciting wavelength from 250 to 450 nm, the emitting color could vary from blue to yellow for 1 and blue to green-yellow for 2, separately. In addition, high catalytic activities and good reusability of 2 as a heterogeneous catalyst for oxygenation reaction of sulfides have been systematically performed.

A {Ti6W4}-Cluster-Substituted Polyoxotungstate: Synthesis, Structure, and Catalytic Oxidation Properties

A novel Ti-W-O-cluster-substituted tungstoantimonate (TA), [H₂N(CH₃)₂]₃Na₄H₉[{Ti₆W₄O₁₈(OH)(H₂O)₃}(B-α-SbW₉O₃₃)₃]·20H₂O (1), has been made by hydrothermal reactions of trivacant [B-α-SbW₉O₃₃]^9- units, Ti⁴⁺ cations, and WO₄^2- anions in the presence of [H₂N(CH₃)₂]·Cl and structurally characterized. Intriguingly, the polyoxoanion of 1 is constructed from three [B-α-SbW₉O₃₃]^9- units and a previously unobserved decanuclear heterometallic Ti-W-O cluster [Ti₆W₄O₁₈(OH)(H₂O)₃]¹¹⁺ ({Ti₆W₄}) that is comprised of an octahedral [Ti₆WO₆(H₂O)₃]¹⁸⁺ cluster and an edge-sharing [W₃O₁₂(OH)]^7- fragment via six W-O-Ti/W linkers. Furthermore, studies on the catalytic oxidation properties reveal that 1 possesses good catalytic activity toward the oxidation reactions of various sulfides and cyclooctene based on the environmentally friendly oxidant H₂O₂.

A μ-AsO4-Bridging Hexadecanuclear Ni-Substituted Polyoxotungstate

A novel tetrahedral μ-AsO₄-bridging hexadecanuclear Ni-substituted silicotungstate (ST) Na₂₁H₁₀[(AsO₄){Ni₈(OH)₆(H₂O)₂(CO₃)₂(A-α-SiW₉O₃₄)₂}₂]·60H₂O (1) was made by the reactions of trivacant [A-α-SiW₉O₃₄]^10- ({SiW₉}) units with Ni²⁺ cations and Na₃AsO₄·12H₂O and characterized by IR spectrometry, elemental analysis, thermogravimetric analysis (TGA), and powder X-ray diffraction (PXRD). 1 contains a novel polyoxoanion [(AsO₄){Ni₈(OH)₆(H₂O)₂(CO₃)₂(A-α-SiW₉O₃₄)₂}₂]^31- built by four trivacant Keggin [A-α-SiW₉O₃₄]^10- fragments linked through an unprecedented [(AsO₄){Ni₈(OH)₆(H₂O)₂(CO₃)₂}₂]⁹⁺ cluster, where the tetrahedral AsO₄ acts as an exclusively μ₂-bridging unit to link multiple Ni centers; such a connection mode appears for the first time in polyoxometalate chemistry. Furthermore, the electrochemical and catalytic oxidation properties of compound 1 have been investigated.

Heterolytic alkene oxidation with H2O2 catalyzed by Nb-substituted Lindqvist tungstates immobilized on carbon nanotubes

Immobilization of Nb-substituted Lindqvist tungstates on carbon nanotubes stabilizes a monomeric form, Nb(OH)W₅, and leads to efficient and recyclable catalysts for heterolytic alkene oxidation with H₂O₂.

Two unusual nanosized Nd3+-substituted selenotungstate aggregates simultaneously comprising lacunary Keggin and Dawson polyoxotungstate segments

Two unique nanosized Nd³⁺-substituted selenotungstates Na₉K₈{[W₃Nd₂(H₂O)₃(NO₃)O₆](B-α-SeW₉O₃₃)₂(α-Se₂W₁₄O₅₂)}·35H₂O (1) and [H₂N(CH₃)₂]₇H₉Na₄{[W₂Nd₂(H₂O)₈O₆(OH)₂(β-Se₂W₁₄O₅₂)][W₃Nd₂(H₂O)₆O₇(B-α-SeW₉O₃₃)₂]₂}·84H₂O (2) were prepared by reacting NaSeO₃, Na₂WO₄·2H₂O with Nd(NO₃)₃·6H₂O in aqueous solution by controlling different cations and pH values. 1 was synthesized at pH = 4.3 in the presence of KCl, whereas 2 was synthesized at pH = 3.0 in the presence of [H₂N(CH₃)₂]·Cl. The most striking structural feature of 1 and 2 is the coexistence of vacant Keggin and Dawson segments in the polyoxoanion, which is extremely rare in the field of polyoxometalate chemistry. The trimeric polyoxoanion of 1 can be perceived as a fusion of one α-type tetravacant Dawson [α-Se₂W₁₄O₅₂]^14- unit and two trivacant Keggin [B-α-SeW₉O₃₃]^8- segments sealing a trigonal bipyramid pentanuclear [W₃Nd₂(H₂O)₃(NO₃)O₆]¹¹⁺ cluster, while the pentameric polyoxoanion of 2 can be described as one β-type tetravacant Dawson [β-Se₂W₁₄O₅₂]^14- fragment and four trivacant Keggin [B-α-SeW₉O₃₃]^8- segments anchoring a saddle-shaped [W₈Nd₆(H₂O)₂₀O₂₀(OH)₂]²⁴⁺ cluster. In addition, the measurements of catalytic oxidation of aromatic thioethers show that 2 as a catalyst possesses extremely outstanding catalytic performance under mild reaction conditions.

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.

Wheel-Like Icosanuclear Peroxotitanate-A Stable Water-Soluble Catalyst for Oxygen Transfer Reactions

Water-soluble wheel-like icosanuclear peroxotitanate K₁₆[Ti₂₀(μ-O)₈(HO₂)₈(O₂)₁₂( R, R-tart)₁₂]·52H₂O (1) chelated by tartrate has been successfully isolated. As the largest peroxotitanate reported, {Ti₂₀} features 20 (hydro)peroxo groups with three kinds of coordination modes in μ-η¹:η², μ-η²:η², and η² fashions. The cluster is stable in solution and solid states. It has been tested for the catalytic oxidations of methyl phenyl sulfide and pyridine with hydrogen peroxide, respectively, which shows reversible elimination and the addition of peroxo groups. This provides a rare example of well-characterized titanium peroxide for homogeneous catalysis and mechanism research.

Hybrid dimers based on metal-substituted Keggin polyoxometalates (metal = Ti, Ln) for cyanosilylation catalysis

Six hybrid dimers 1–6 constructed from [PW₁₀Ti₂O₄₀]⁷⁻/[PW₁₁LnO₃₉]⁴⁻ and lanthanide–organic units are reported, which as heterogeneous catalysts show high activity for cyanosilylation under solvent-free conditions.

DFT characterization on the mechanism of sulfoxidation with H2O2 catalyzed by tetranuclear peroxotungstates [XO4{WO(O2)2}4](n-) (X = Si(IV), P(V), S(VI), As(V), and Se(VI))

A thorough theoretical analysis was carried out on the sulfoxidation with H2O2 catalyzed by a tetranuclear peroxotungstate [SiO4{WO(O2)2}4](4-). The active species is the [SiO4{WO(O2)2}4(H2O2)](4-) (SiW4(H2O2)) complex rather than [SiO4{WO(O2)2}4](4-) (SiW4). The catalytic cycle consists of three elementary processes: oxygen transfer, sulfoxide dissociation, and catalyst regeneration. The oxygen transfer occurs from the peroxo oxygen atom O1 of SiW4(H2O2) to the sulfur center of dimethyl sulfide with a moderate Gibbs activation energy (ΔG°(‡)) of 17.1 kcal mol(-1). By comparing potential energy surfaces and condensed Fukui functions (ƒ(+)), the electrophilicity of the outer peroxo atoms in SiW4(H2O2) determines which oxygen transfers to the dimethyl sulfide. Then, the sulfoxide dissociation proceeds with a small ΔG°(‡) value of 2.3 kcal mol(-1) by elongation of the peroxo O1-O4 distance and elimination of the product dimethylsulfoxide. Finally, the catalyst regeneration is found to occur via two successive proton transfers from H2O2 to the oxygen atoms of peroxotungstates with the ΔG°(‡) values of 15.9 and 15.3 kcal mol(-1), which has been firstly examined in the present study. All of these steps occur easily with moderate ΔG°(‡) values, but the oxygen transfer is the rate-determining step of this catalytic reaction. In addition, the catalytic activity of peroxotungstates can be effectively tuned by changing the heteroatom X of [XO4{WO(O2)2}4(H2O2)](n-) in the order: Se(VI) ≈ S(VI) > As(V) ≈ P(V) > Si(IV).

A Mn(iii) polyoxotungstate in the oxidation of organosulfur compounds by H2O2 at room temperature: an environmentally safe catalytic approach

A manganese monosubstituted Keggin-type polyoxometalate was used as a catalyst in the oxidation of recalcitrant organosulfur compounds by hydrogen peroxide at room temperature.

The use of polyoxometalates in protein crystallography - An attempt to widen a well-known bottleneck

Polyoxometalates (POMs) are discrete polynuclear metal-oxo anions with a fascinating variety of structures and unique chemical and physical properties. Their application in various fields is well covered in the literature, however little information about their usage in protein crystallization is available. This review summarizes the impact of the vast class of POMs on the formation of protein crystals, a well-known (frustrating) bottleneck in macromolecular crystallography, with the associated structure elucidation and a particular emphasis focused on POM's potential as a powerful crystallization additive for future research. The Protein Data Bank (PDB) was scanned for protein structures with incorporated POMs which were assigned a PDB ligand ID resulting in 30 PDB entries. These structures have been analyzed with regard to (i) the structure of POM itself in the immediate protein environment, (ii) the kind of interaction and position of the POM within the protein structure and (iii) the beneficial effects of POM on protein crystallography apparent so far.

Synthesis and structural characterization of BINOL-modified chiral polyoxometalates

Chiral ligand-modified polyoxometalates (POMs) R-I and S-I were successfully synthesized by the introduction of BINOL into the dititanium-substituted POM.

Synergistic combination of multi-Zr(IV) cations and lacunary Keggin germanotungstates leading to a gigantic Zr24-cluster-substituted polyoxometalate

Synergistic directing roles of six lacunary fragments resulted in an unprecedented Zr24-cluster substituted poly(polyoxotungstate) Na10K22[Zr24O22(OH)10(H2O)2(W2O10H)2(GeW9O34)4(GeW8O31)2]·85H2O (Na10K22·1·85H2O), which contains the largest [Zr24O22(OH)10(H2O)2] (Zr24) cluster in all the Zr-based poly(polyoxometalate)s to date. The most remarkable feature is that the centrosymmetric Zr24-cluster-based hexamer contains two symmetry-related [Zr12O11(OH)5(H2O)(W2O10H)(GeW9O34)2(GeW8O31)](16-) trimers via six μ3-oxo bridges and was simultaneously trapped by three types of different segments of B-α-GeW9O34, B-α-GeW8O31, and W2O10. The other interesting characteristic is that there are two pairs of intriguing triangular atom alignments: one is composed of the Zr(2,4,6,8,11) and W21 atoms and the other contains the Ge(1-3), Zr(3,5,7,9,10,12) and W26 atoms, and the Zr5 atom is inside the triangle; a linking mode is unobserved. The oxygenation reactions of thioethers by H2O2 were evaluated when Na10K22·1·85H2O served as a catalyst. Results show that it is an effective catalyst for oxygenation of thioethers by H2O2. The unique redox property of oxygen-enriched polyoxotungstate fragments and Lewis acidity of the Zr cluster imbedded in Na10K22·1·85H2O provide a sufficient driving force for the catalytic conversion from thioethers to sulfoxides/sulfones.

Ti7-containing, tetrahedral 36-tungsto-4-arsenate(iii) [Ti6(TiO6)(AsW9O33)4]20−

Ti₇-containing [Ti₆(TiO₆)(AsW₉O₃₃)₄]²⁰⁻ (1) comprises a central, octahedrally coordinated Ti⁴⁺ ion bridged to six square-pyramidally coordinated Ti⁴⁺ ions, and this titanium-oxo core is surrounded by four {AsW₉O₃₃} capping units, resulting in an overall tetrahedral assembly.

Oxidation of SCN- with air and micellar polyoxoperoxometalates

The nanocatalysts were fabricated by reacting amphiphilic quaternary ammoniums with polyoxoperoxometalates (POPMs) K(n)[PW(12-)(x)Ti(x)O(40-)(x)(O(2))(x)](x=1, 2 and 3; n=5, 7 and 9). Fourier transform infrared spectroscopy (IR), transmission electron microscopy (TEM), and UV-vis-NIR spectrophotometer were used to characterize the resulting samples. This kind of nanocatalysts could provide a kind of nanoreactors to promote the degradation of NH(4)SCN into simple inorganic compounds such as SO(4)(2-), HCO(3)(-) and NO(3)(-) using oxygen as an oxidant under room conditions. The leaching test showed that the POPMs micellar catalyst had excellent stability and could be used as a heterogeneous one at least five times.