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.

A wheel-shaped Zr-substituted phosphotungstate [{Zr(C2O4)2}3 (PO4)(P6W39O150)]39- with tunable proton conduction properties

A wheel-shaped Zr-substituted phosphotungstate, [N(CH3)4]2K16Na10.5H10.5[{Zr(C2O4)2}3(PO4)(P6W39O150)]·45H2O (1), was synthesised from a hexavacant Dawson-type precursor [H2P2W12O48]12-via a conventional solution method. Compound 1 features a wheel-shaped polyanion comprising an annular [P6W39O150]36- cluster supported by a turbine-shaped [{Zr(C2O4)2}3(PO4)]3- fragment, with three oxalate groups covalently anchored to W atoms. Compound 1 was systematically characterized by IR, UV, PXRD, TGA and 31P NMR spectra. The 31P NMR spectra over time were monitored to verify the stability of 1 in aqueous solution. This compound possesses remarkable proton conductive behavior with a high conductivity of 1.18 × 10-2 S cm-1 at 368 K.

A Zr-added Dawson-type poly(polyoxometalate)

A novel Zr-added trimer, [H₂N(CH₃)₂]₁₀H₁₄[(Zr₂P₂W₁₆O₆₁)₃]·7H₂O (1), has been made under hydrothermal conditions, and contains the highest number of Zr centers in known Dawson-type poly(POM)s. A remarkable feature of this study is the first discovery of a new type of divacant [α-5,10-P₂W₁₆O₆₀]^14- fragment, which assembles with Zr⁴⁺ ions to form a cyclic trimer. Furthermore, 1 as a heterogeneous catalyst exhibits high activity for the selective oxidative degradation of a sulfur mustard simulant CEES.

Space-Confined Nucleation of Semimetal-Oxo Clusters within a [H7P8W48O184]33- Macrocycle: Synthesis, Structure, and Enhanced Proton Conductivity

Spatially confined assembly of semimetallic oxyanions (AsO₃^3- and SbO₃^3-) within a [H₇P₈W₄₈O₁₈₄]^33- (P₈W₄₈) macrocycle has afforded three nanoscale polyanions, [{As^III₅O₄(OH)₃}₂(P₈W₄₈O₁₈₄)]^32- (As₁₀), [(Sb^IIIOH)₄(P₈W₄₈O₁₈₄)]^32- (Sb₄), and [(Sb^IIIOH)₈(P₈W₄₈O₁₈₄)]^24- (Sb₈), which were crystallized as the hydrated mixed-cation salts (Me₂NH₂)₁₃K₇Na₂Li₁₀[{As^III₅O₄(OH)₃}₂(P₈W₄₈O₁₈₄)]·32H₂O (DMA-KNaLi-As₁₀), K₂₀Li₁₂[(Sb^IIIOH)₄(P₈W₄₈O₁₈₄)]·52H₂O (KLi-Sb₄), and (Me₂NH₂)₈K₆Na₅Li₅[(Sb^IIIOH)₈(P₈W₄₈O₁₈₄)]·65H₂O (DMA-KNaLi-Sb₈), respectively. A multitude of solid- and solution-state physicochemical techniques were employed to systematically characterize the structure and composition of the as-made compounds. The polyanion of As₁₀ represents the first example of a semimetal-oxo cluster-substituted P₈W₄₈ and accommodates the largest As^III-oxo cluster in polyoxometalates (POMs) reported to date. The number of incorporated SbO₃^3- groups in Sb₄ and Sb₈ could be customized by a simple variation of Sb^III-containing precursors. Encapsulation of semimetallic oxyanions inside P₈W₄₈ sets out a valid strategy not only for the development of host-guest assemblies in POM chemistry but also for their function expansion in emerging applications such as proton-conducting materials, for which DMA-KNaLi-As₁₀ showcases an outstanding conductivity of 1.2 × 10^-2 S cm^-1 at 85 °C and 70% RH.

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.

{Ti6}/{Ti10} Wheel Cluster Substituted Silicotungstate Aggregates

Two novel Ti-oxo wheel cluster substituted silicotungstates (STs) [H₂N(CH₃)₂]₉H₉[Ti₆O₆(SiW₁₀O₃₇)₃]·11H₂O (1) and [H₂N(CH₃)₂]₁₆H₁₀[Ti₁₀O₁₁(SiW₁₀O₃₇)₂(SiW₉O₃₅)₂]·14H₂O (2) have been made by hydrothermal reactions. The polyoxoanion of 1 is a ring-shaped trimer where a Ti₆O₆ ({Ti₆}) wheel cluster is encapsulated by three divacant [SiW₁₀O₃₇]^10- (SiW₁₀O₃₇) fragments. However, 2 is built by two divacant SiW₁₀O₃₇ units and two rare trivacant [SiW₉O₃₅]^12- (SiW₉O₃₅) fragments and further installs an unprecedented Ti₁₀O₁₁ ({Ti₁₀}) double-wheel cluster. To the best of our knowledge, 2 is rare in POM chemistry. Studies on the catalytic oxidation properties reveal that 1 exhibits high catalytic activity toward the oxidation of various sulfides using H₂O₂ as an oxidant. Furthermore, 1 can be facilely recycled and reused for at least five cycles without obvious loss of catalytic activity.

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.

Pentadecanuclear Fe-Containing Polyoxometalate Catalyst for Visible-Light-Driven Generation of Hydrogen

The structurally new, carbon-free pentadecanuclear Fe-containing polyoxometalate, Na₂₁[NaFe₁₅(OH)₁₂(PO₄)₄(A-α-SiW₉O₃₄)₄]·85H₂O (Na₂₁-Fe₁₅P₄(SiW₉)₄), was synthesized using a facile one-pot, solution-based synthetic approach and systematically characterized by various spectroscopic techniques. Single-crystal X-ray diffraction reveals that the title complex is composed of two [Fe₄(A-α-SiW₉O₃₄)] fragments and two [Fe_3.5(A-α-SiW₉O₃₄)] fragments stabilized by four PO₄ linkers in a tetrameric style with idealized T_d point group symmetry. When coupling with (4,4'-ditert-butyl-2,2'-dipyridyl)-bis(coumarin)-iridium(III) hexafluorophosphate ([Ir(coumarin)₂(dtbbpy)][PF₆]) photosensitizer and triethanolamine (TEOA) sacrificial electron donor, polyoxoanion Fe₁₅P₄(SiW₉)₄ effectively catalyzed hydrogen production with a minimally optimized TON of 986, which represents, to our knowledge, one of the highest values among known Fe-substituted POM-catalyzed hydrogen production systems. Both a mercury-poisoning test and FT-IR characterizations proved the structural stability of Fe₁₅P₄(SiW₉)₄ catalyst under photocatalytic conditions. The photocatalytic mechanism of the present hydrogen-evolving system was investigated by time-solved luminescence and static emission quenching measurements.

Three ring-shaped Zr(iv)-substituted silicotungstates: syntheses, structures and their properties

Three Zr(iv)-substituted polyoxometalate nanoclusters, H₁₆(H₂dap)₄[{Zr₂(OH)₂(α-SiW₁₀O₃₈)}₂{Zr₂(OH)₂(β-SiW₁₀O₃₈)}₂]·8H₂O (1, dap = 1,3-diaminopropane), H₁₇Na₉[(Zr₂(OH)₂)₂(Zr₂BO(OH)₄)₂(β-SiW₁₀O₃₈)₄]·50H₂O (2) and H₂₀Na₈[(Zr₂BO(OH)₄)₂(Zr₂B₂O₂(OH)₅)₂(β-SiW₁₀O₃₈)₄]·40H₂O (3), were made by a hydrothermal reaction of trivacant [A-α-SiW₉O₃₄]^10-, Zr⁴⁺ ions, and H₃BO₃ under different alkaline conditions. Single-crystal X-ray structure analysis revealed that they are ring-shaped polyoxotungstate aggregates built by Zr₂(OH)₂ ({Zr₂}) clusters and two types of dilacunary α-/β-SiW₁₀O₃₈ ({α-/β-SiW₁₀}) fragments for 1, {Zr₂}, {β-SiW₁₀}, and Zr₂BO(OH)₄ ({Zr₂B}) clusters for 2, and {β-SiW₁₀}, {Zr₂B}, and Zr₂B₂O₂(OH)₅ ({Zr₂B₂}) clusters for 3, respectively, showing that the Zr-B-O clusters, {Zr₂B} and {Zr₂B₂}, have been introduced to the lacunary sites of polyoxometalates for the first time. The electrochemistry and electrocatalytic properties of 2 were studied, and it exhibited effective catalytic activities for the reduction of H₂O₂, NO₂^-, and BrO₃^-. Moreover, the catalysis of 2 for the oxidation reactions of various thioethers was investigated, and it was found that 2 possessed high conversion and remarkable selectivity.

Three Zr(IV)-Substituted Polyoxotungstate Aggregates: Structural Transformation from Tungstoantimonate to Tungstophosphate Induced by pH

Three novel Zr-substituted polyoxotungstate aggregates [H₂N(CH₃)₂]₇NaH₂[Zr₂Sb₂O₃(A-α-PW₉O₃₄)₂]·16H₂O (1), [H₂N(CH₃)₂]₆H₁₂[ZrSb₄(OH)O₂(A-α-PW₈O₃₂)(A-α-PW₉O₃₄)]₂·33H₂O (2), and [H₂N(CH₃)₂]₄Na_11.5H_4.5[Zr₄W₈Sb₄P₅O₄₉(OH)₅(B-α-SbW₉O₃₃)₂]·53H₂O (3) have been made in hydrothermal reactions of the [B-α-SbW₉O₃₃]^9- precursor with Zr⁴⁺ cations and PO₄^3- anions in the presence of dimethylamine hydrochloride and sodium acetate buffer (pH = 4.8) and structurally characterized. Different pH values induce structural transformation from tungstoantimonate (TA) to tungstophosphate (TP). 1 is a di-Zr-substituted sandwich-type TP, the tetranuclear heterometallic [Zr₂Sb₂O₃]⁸⁺ entity sandwiched by two [A-α-PW₉O₃₄]^9- moieties. 2 is a double sandwich-type structure, which can be perceived as two equivalent sandwiched [Sb₃(PW₈O₃₂)(PW₉O₃₄)]^11- further sandwiching one [Sb₂Zr₂(OH)₂O₄]⁴⁺ core to form a novel large-size sandwich-type architecture. Different from 1 and 2, 3 is a tetra-Zr-substituted sandwiched configuration, in which two [B-α-SbW₉O₃₃]^9- fragments sandwich a unique 21-core Sb-P-W-Zr oxo cluster ({Zr₄W₈Sb₄P₅}). Furthermore, the catalytic oxidation of aromatic thioethers by 3 as the heterogeneous catalyst has been investigated, showing high conversion and remarkable selectivity as well as excellent recyclability.

Syntheses, Structures, and Electrochemical Properties of Three New Acetate-Functionalized Zirconium-Substituted Germanotungstates: From Dimer to Tetramer

Under hydrothermal conditions, three new acetate-functionalized zirconium-substituted polyoxometalates, H₄Na₂[Na₆(H₂O)₂₂][Zr₄(μ₃-O)₂(OH)₂(OAc)₂(α-GeW₁₀O₃₇)₂]·32H₂O (1), H₈K₃Na₅[Zr₆(μ₃-O)₃(OH)₃(OAc)(H₂O)(β-GeW₁₀O₃₇)₃]·20H₂O (2), H₆K₄Na₁₂[{Zr₅(μ₃-OH)₄(OH)₂}@{Zr₂(OAc)₂(α-GeW₁₀O₃₈)₂}₂]·22H₂O (3), were synthesized and characterized. In 1, the sandwiched dimer [Zr₄(μ₃-O)₂(OH)₂(OAc)₂(α-GeW₁₀O₃₇)₂]^12- was linked to a hexameric [Na₆(H₂O)₂₂]⁶⁺ cluster to form a 1D chain. In 2, the trimer was constructed from three [β₂-GeW₁₀O₃₇]^10- units and a new [Zr₆O₃(OH)₃(OAc)(H₂O)]¹⁴⁺ cluster. In 3, the tetramer was built by two novel sandwich-type dimers [Zr₂(OAc)₂(α₂-GeW₁₀O₃₈)₂]^18- and one unique [Zr₅(μ₃-OH)₄(OH)₂]¹⁴⁺ core in an approximately orthogonal fashion, showing a staggered tetrahedral polyanion. Also, the electrochemistry and electrocatalytic properties of 3 were studied, exhibiting good catalytic activities toward the reduction of H₂O₂, BrO₃^-, and NO₂^-.

A new octa-Mn-substituted poly(polyoxotungstate)

A novel octa-Mn-substituted silicotungstate (ST) tetramer [H₂N(CH₃)₂]₈H₁₉Na₄{[K(H₂O)₄WO₄] [SiMnII2W₁₀O₃₈]₃}[SiMnIII2W₁₀O₃₈]·27H₂O (1) was prepared, and its electrochemistry, thermal stability and magnetic properties were investigated.

Octahedron-shaped three-shell Ln14-substituted polyoxotungstogermanates encapsulating a W4O15 cluster: luminescence and frequency dependent magnetic properties

The octahedron-shaped three-shell structure of the [(Ln₁₄(H₂O)W₄(OH)O₁₄)(WO₄)₄(GeW₁₀O₃₈)₆]⁴³⁻ polyanion contains a W₄ tetrahedron, a Ln₁₄W₄ tetrahedron and a Ge₆W₆₀ octahedron.