Alkoxides of Trivacant Lacunary Polyoxometalates

Self-assembled molecular hybrids comprising lacunary polyoxometalates and multidentate imidazole ligands

Self-assembly via coordination bonding facilitates the creation of diverse inorganic-organic molecular hybrids with distinct structures and properties. Recent advances in this field have been driven by the versatility of organic ligands and inorganic units. Lacunary polyoxometalates are a class of well-defined metal-oxide clusters with a customizable number of reactive sites and bond directions, which make them promising inorganic units for self-assembled molecular hybrids. Herein, we report a novel synthesis method for self-assembled molecular hybrids utilizing the reversible coordination of multidentate imidazole ligands to the vacant sites of lacunary polyoxometalates. We synthesized self-assembled molecular hybrids including monomer, dimers, and tetramer, demonstrating the potential of our method for constructing intricate hybrids with tailored properties and functionalities.

Ultra-stable and highly reactive colloidal gold nanoparticle catalysts protected using multi-dentate metal oxide nanoclusters

Owing to their remarkable properties, gold nanoparticles are applied in diverse fields, including catalysis, electronics, energy conversion and sensors. However, for catalytic applications of colloidal gold nanoparticles, the trade-off between their reactivity and stability is a significant concern. Here we report a universal approach for preparing stable and reactive colloidal small (~3 nm) gold nanoparticles by using multi-dentate polyoxometalates as protecting agents in non-polar solvents. These nanoparticles exhibit exceptional stability even under conditions of high concentration, long-term storage, heating and addition of bases. Moreover, they display excellent catalytic performance in various oxidation reactions of organic substrates using molecular oxygen as the sole oxidant. Our findings highlight the ability of inorganic multi-dentate ligands with structural stability and robust steric and electronic effects to confer stability and reactivity upon gold nanoparticles. This approach can be extended to prepare metal nanoparticles other than gold, enabling the design of novel nanomaterials with promising applications.

Molecular hybrids of trivacant lacunary polyoxomolybdate and multidentate organic ligands

Functional molecular inorganic-organic hybrids of lacunary polyoxometalates and organic ligands attract much attention for advanced material applications. However, the inherent instability of lacunary polyoxomolybdates hinders the synthesis of hybrids and their utilization. Herein, we present a viable approach for the synthesis of molecular hybrids of trivacant lacunary Keggin-type polyoxomolybdates and multidentate organic ligands including carboxylates and phosphonates, which is based on the use of a lacunary structure stabilized by removable pyridyl ligands as a starting material.

Ligand-Protecting Strategy for the Controlled Construction of Multinuclear Copper Cores within a Ring-Shaped Polyoxometalate

The ring-shaped polyoxometalate (POM) [P₈W₄₈O₁₈₄]^40- contains a large cavity and is an attractive inorganic multidentate ligand for accumulating metal cations. Until now, several multinuclear metal cores are constructed within the {P₈W₄₈} framework in aqueous solvents. However, it is still challenging to control the number and arrangement of introduced metal cations because of the numerous coordination sites inside the {P₈W₄₈} framework. In this study, we developed a novel approach for the selective synthesis of several multinuclear copper-containing ring-shaped POMs in organic solvents using methoxy groups as organic protecting ligands for the reactive coordination sites. Reacting a tetra-n-butylammonium (TBA) salt of [P₈W₄₈O₁₈₄]^40- (P8W48) with 4 and 8 equiv of copper(II) acetate in the presence of methanol (MeOH), tetra- and octacopper cores were incorporated into the cavity to form TBA₁₁H₁₃[Cu₄(H₂O)₄P₈W₄₈O₁₇₆(OCH₃)₈]·28H₂O·3CH₃NO₂ (Cu4) and TBA₁₄H₂[Cu₈(H₂O)₁₂P₈W₄₈O₁₇₆(OCH₃)₈]·24H₂O·CH₃CN (Cu8), respectively. For both structures, methoxy groups served as protecting ligands and temporarily inactivated vacant coordination sites. Without MeOH, dodeca- and hexadecacopper cores were constructed inside the cavity to form TBA₁₄H₂[Cu₁₂(H₂O)₁₆P₈W₄₈O₁₈₄]·4H₂O (Cu12) and TBA₁₆H₈[Cu₁₆(OH)₁₆(H₂O)₄P₈W₄₈O₁₈₄]·12H₂O·C₃H₆O (Cu16), respectively. The arrangement of copper ions on the same {P₂W₁₂} units could be controlled by the input number of copper ions. Moreover, all four POMs could be synthesized from P8W48 by the stepwise addition of 4 equiv of copper(II) acetate, clarifying the introduction process.

Polyoxometalates as chemically and structurally versatile components in self-assembled materials

Polyoxometalates (POMs) are anionic molecular metal oxides with expansive diversity in terms of their composition, structure, nuclearity and charge. Within this vast collection of compounds are dominant structural motifs (POM platforms), that are amenable to significant chemical tuning with minimal perturbation of the inorganic oxide molecular structure. Consequently, this enables the systematic investigation of these compounds as inorganic additives within materials whereby structure and charge can be tuned independently i.e. [PW12O40]3- vs. [SiW12O40]4- while also investigating the impact of varying the charge balancing cations on self-assembly. The rich surface chemistry of POMs also supports their functionalisation by organic components to yield so-called inorganic-organic hybrids which will be the key focus of this perspective. We will introduce the modifications possible for each POM platform, as well as discussing the range of nanoparticles, microparticles and surfaces that have been developed using both surfactant and polymer building blocks. We will also illustrate important examples of POM-hybrids alongside their potential utility in applications such as imaging, therapeutic delivery and energy storage.

Ru(III) -based polyoxometalate tetramers as highly efficient heterogeneous catalysts for alcohol oxidation reactions at room temperature

A novel ruthenium-containing polyoxometalate-based organic-inorganic hybrid, K₄Na₉H_7.4[(AsW₉O₃₃)₄(WO₂)₄{Ru_3.2(C₃H₃N₂)₂}]·42H₂O (1), was successfully synthesized by a one-step hydrothermal method under acidic conditions, which applied a self-assembly strategy between inorganic polyoxometalate based on trivacant [B-α-AsW₉O₃₃]^9- {AsW₉} fragments and an organic ligand, imidazole (C₃H₄N₂). Compound 1 was further characterized by single-crystal X-ray diffraction, PXRD, IR spectroscopy, UV-Vis spectroscopy, ESI-MS, elemental analysis and TGA. Single-crystal X-ray diffraction data reveal that the polyanion consists of four trivacant Keggin-type polyanion {AsW₉} building blocks bridged by four {WO₆} units, leading to a crown-shaped tetrameric structure [(AsW₉O₃₃)₄(WO₂)₄{Ru_3.2(C₃H₃N₂)₂}]^20.4-. The ESI-MS result reveals that the polyanion unit has excellent structural integrity in water. Moreover, the catalysis study of 1 was also further investigated, and the experimental results indicate heterogeneous catalyst 1 presents high efficiency (yield = 98%), excellent selectivity (>99%), and good recyclability for the oxidation of 1-(4-chlorophenyl)ethanol to 4'-chloroacetophenone with commercially available 70% aqueous tert-butyl hydroperoxide {TBHP (aq.)} as the oxidant at room temperature.

Two Ni-Substituted Trilacunary Keggin-Type Polyoxometalates: Syntheses, Crystal Structures, NLO Studies, and Magnetic Properties

Two Ni-substituted polyoxometalates (NiSPs), [Ni₆(Py)₆(H₂O)₅(μ₃-OH)₃(PW₉O₃₄)]₂·10H₂O (1), [Ni₇(Py)₆(Im)(H₂O)₅O(WO₄)(μ₃-OH)₃(H₂PW₉O₃₄)]·3H₂O (2) (Py = pyridine, Im = imidazole), were successfully hydrothermally synthesized. Compounds 1 and 2 have significantly different configurations by introducing different amounts of imidazole ligands. For compound 1, two malposed {Ni₆(Py)₆PW₉} units that are face to face are bridged by two Ni-O-W bonds to constitute an isolated dimeric structure. Differently, the {Ni₇(Py)₆(Im)PW₉}₂ dimer in compound 2 connects with four adjacent dimers by four {WO₄} groups in an interesting two-dimensional (2-D) arrangement. The magnetism of compounds 1 and 2 was studied, and magnetic test results demonstrated that both compounds have ferromagnetic interactions between the nickel centers. Meanwhile, the third-order nonlinear optical (NLO) measurements indicated that compound 1 can serve as potential nonlinear optical materials.

Development of sterically hindered siloxide-functionalized polyoxotungstates for the complexation of 5d-metals

In this study, we extend the family of organosilyl-functionalized trivacant Keggin polyoxotungstates, [PW₉O₃₄(RSiOH)₃]^3- (R = ⁿPr, ⁱPr, ^tBu), through the introduction of bulky aryl and aliphatic silanol substituents, namely phenyl, cyclohexyl and biphenyl. This work was performed in order to study the impact of these large functional groups on the accessibility of the well-defined tridentate coordination site. Coordination of hafnium to these type II hybrid polyoxotungstates was conducted in order to study the ability of the bulkier ligand pockets to support larger cations in comparison to those previously reported (e.g. Ti⁴⁺, V³⁺, V⁵⁺, Ge⁴⁺). Increased steric hindrance around the coordination site from the biphenyl groups resulted in much longer reaction times for the complexation reaction compared to the other functional groups used, but the impact of our design toward stabilizing reactive species proved limited, as all complexes easily undergo hydrolysis of the Hf-O^tBu bond in the presence of water. Electrochemical investigations of the ligands and hafnium complexes reveal that the redox events centered on the polyoxotungstate core can be tuned by varying the substituents on the silyl fragment, and exhibit a cathodic shift after coordination of the redox inactive tetravalent cation.

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.

Synthesis of a phosphomolybdate with a tetranuclear vanadium core by installing vanadium atoms in a lacunary template using the protecting group strategy

Vanadium-containing phosphomolybdates have outstanding catalytic and electrochemical properties. However, their traditional one-pot synthesis in aqueous media generates a mixture of randomly distributed isomers and undesirable impurities, which has hampered the in-depth study of the catalysis and reaction mechanism of these compounds. Here, the selective synthesis of a tetranuclear vanadium-containing phosphomolybdate, [PV3Mo9O40(VO)]3-, was achieved for the first time by using a pyridine-protected [A-α-PMo9O31(py)3]3- (py = pyridine) as the template.

The Surface Chemistry of Metal Oxide Clusters: From Metal-Organic Frameworks to Minerals

Many metal-organic frameworks (MOFs) incorporate nodes that are small metal oxide clusters. Some of these MOFs are stable at high temperatures, offering good prospects as catalysts-prospects that focus attention on their defect sites and reactivities-all part of a broader subject: the surface chemistry of metal oxide clusters, illustrated here for MOF nodes and for polyoxocations and polyoxoanions. Ligands on MOF defect sites form during synthesis and are central to the understanding and control of MOF reactivity. Reactions of alcohols are illustrative probes of Zr₆O₈ node defects in UiO-66, characterized by the interconversions of formate, methoxy, hydroxy, and linker carboxylate ligands and by catalysis of alcohol dehydration reactions. We posit that new reactivities of MOF nodes will emerge from incorporation of a wide range of groups on their surfaces and from targeted substitutions of metals within them.

Oxygen-atom vacancy formation and reactivity in polyoxovanadate clusters

Overview of recent work detailing oxygen-deficient polyoxovanadate clusters as models for reducible metal oxides: toward gaining a fundamental understanding the consequences of vacancy formation on metal oxide surfaces during catalysis.

{VMo9O31[RC(CH2O)3]}6−: the first class of triol ligand covalently-decorated Keggin-type polyoxomolybdates

The first class of triol-ligand covalently-decorated Keggin-type polyoxomolybdates through C–O–Mo bonds were obtained and characterized.

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.

Hybrid polyoxometalates as post-functionalization platforms: from fundamentals to emerging applications

Polyoxometalates (POMs) represent an important group of metal-oxo nanoclusters, typically comprised of early transition metals in high oxidation states (mainly V, Mo and W). Many plenary POMs exhibit good pH, solvent, thermal and redox stability, which makes them attractive components for the design of covalently integrated hybrid organic-inorganic molecules, herein referred to as hybrid-POMs. Until now, thousands of organic hybrid-POMs have been reported; however, only a small fraction can be further functionalized using other organic molecules or metal cations. This emerging class of 'post-functionalizable' hybrid-POMs constitute a valuable modular platform that permits coupling of POM properties with different organic and metal cation functionalities, thereby expanding the key physicochemical properties that are relevant for application in (photo)catalysis, bioinorganic chemistry and materials science. The post-functionalizable hybrid-POM platforms offer an opportunity to covalently link multi-electron redox responsive POM cores with virtually any (bio)organic molecule or metal cation, generating a wide range of materials with tailored properties. Over the past few years, these materials have been showcased in the preparation of framework materials, functional surfaces, surfactants, homogeneous and heterogeneous catalysts and light harvesting materials, among others. This review article provides an overview on the state of the art in POM post-functionalization and highlights the key design and structural features that permit the discovery of new hybrid-POM platforms. In doing so, we aim to make the subject more comprehensible, both for chemists and for scientists with different materials science backgrounds interested in the applications of hybrid (POM) materials. The review article goes beyond the realms of polyoxometalate chemistry and encompasses emerging research domains such as reticular materials, surfactants, surface functionalization, light harvesting materials, non-linear optics, charge storing materials, and homogeneous acid-base catalysis among others.

Hexavacant γ-Dawson-type phosphotungstates supporting an edge-sharing bis(square-pyramidal) {O2M(μ3-O)2(μ-OAc)MO2} core (M = Mn2+, Co2+, Ni2+, Cu2+, or Zn2+)

In aqueous media, the introduction of additional metal species into polyoxometalates (POMs) with multiple vacant sites, such as a hexavacant Dawson-type phosphotungstate, which is of interest for the synthesis of novel metal oxide clusters, is generally difficult because they easily undergo self-condensation and/or structural decomposition. In this study, we succeeded in developing a novel synthetic method to obtain metal-substituted γ-Dawson-type phosphotungstate monomers by introducing metal species into an organic solvent-soluble lacunary phosphotungstate, TBA4H10[α-P2W12O48] (I) (TBA = tetra-n-butylammonium), in organic media. The reaction of I, which possessed two types of vacant sites, i.e. middle and edge sites, with divalent metal species such as Mn2+, Co2+, Ni2+, Cu2+, or Zn2+ in acetonitrile afforded a series of isostructural POMs M2 (TBA5[γ-P2W12O44M2(OAc)(CH3CONH)2]·nH2O·mCH3CN; M = Mn2+, Co2+, Ni2+, Cu2+, or Zn2+; OAc = acetate) with an edge-sharing bis(square-pyramidal) {O2M(μ3-O)2(μ-OAc)MO2} core. The bis(square-pyramidal) core was selectively placed at the middle site of the hexavacant lacunary phosphotungstate, and the two metals in the core were bridged by two phosphate units and one acetate species. Meanwhile, the edge sites were capped by acetimidate ligands, which protect the reactive lacunary POM from self-condensation. To the best of our knowledge, this is the first report describing the synthesis and characterization of metal-substituted hexavacant γ-Dawson-type POM monomers.

Direct Single- and Double-Side Triol-Functionalization of the Mixed Type Anderson Polyoxotungstate [Cr(OH)3W6O21]6

Since the first successful triol-functionalization of the Anderson polyoxometalates, the six protons of their central octahedron X(OH)6 (X-heteroatom, p- or d-element) have been considered as a prerequisite for their functionalization with tripodal alcohols, and therefore, the functionalization of Anderson structures from the unprotonated sides have never been reported. Here, we describe the triol-functionalization of [Cr(OH)3W6O21]6- leading to the single-side grafted anions [Cr(OCH2)3CRW6O21]6- (CrW6-tris-R, R = -C2H5, -NH2, -CH2OH) and the unprecedented double-side functionalized anion [Cr((OCH2)3CC2H5)2W6O18]3- (CrW6-(tris-C2H5)2), despite the lack of protons in the parent anion in the solid state. CrW6-(tris-C2H5)2 demonstrates the first example of double-side functionalized Anderson POT with the partially one-side protonated corresponding parent anion. The new heteropolytungstates were characterized by single-crystal X-ray diffraction, elemental analysis, Fourier-transform infrared spectroscopy, thermal gravimetric analysis, cyclic voltammetry, and electrospray ionization mass spectrometry. Density functional theory calculations were performed to investigate and compare the stability among the different isomers of the parent anion [Cr(OH)3W6O21]6-.

Visible-light-responsive catalysis of a zinc-introduced lacunary disilicoicosatungstate for the deoxygenation of pyridine N-oxides

A mononuclear zinc-introduced lacunary disilicoicosatungstate showed efficient photocatalytic activity in the selective deoxygenation of pyridine N-oxides under visible light irradiation.