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Lacunary polyoxometalates (POMs), featuring highly reactive vacant sites, serve as valuable building blocks and precursors for the rational design of functional materials with widespread applications in catalysis, analytical chemistry, energy conversion and storage, medicine, and optical materials. While diverse Keggin-type polyoxotungstates, including both plenary and lacunary species, have been synthesized through dehydration condensation reactions and equilibrium displacement in aqueous solvents, the isolation and use of lacunary polyoxomolybdates remain challenging. To address this, the current study proposes a "ligand-directed top-down synthetic approach" for producing lacunary polyoxomolybdates from plenary Keggin-type species in organic solvents. By reacting plenary Keggin-type polyoxomolybdates [α-XMo 12 O 40 ] 3- (X = P, As, V) with 4-methoxypyridine (pyOMe) in acetonitrile, we successfully synthesized the corresponding trivacant lacunary polyoxomolybdates ([XMo 9 O 31 (pyOMe) 3 ] 3- ), where the vacant sites are stabilized by three pyOMe ligands. Remarkably, this approach enables the synthesis of a lacunary vanadomolybdate, a species previously unattainable through equilibrium control in aqueous systems. Furthermore, the reversible coordination of pyOMe ligands to molybdenum atoms at the vacant sites makes these lacunary polyoxomolybdates highly versatile precursors for assembling POM-organic hybrids. Overall, this study introduces an innovative synthetic methodology for POMs, demonstrating notable potential for advancing the development of functional materials.

The adsorption behaviors of H 2 and CO molecules in crown-motif [PtAu 8 (PPh 3 ) 8 ]-H[PMo 12 O 40 ] (PtAu8-PMo12) solids were investigated by in situ quick-scan X-ray absorption fine structure (QXAFS) measurements with a time resolution of 0.1 s. The electronic state of Pt in PtAu8-PMo12 was drastically changed by the adsorption of H 2 and CO molecules because of the formation of Pt-H 2 /Pt-CO interactions. H 2 was adsorbed more rapidly (<0.5 s) on Pt than CO (∼2.5 s) and showed reversible adsorption/desorption behavior on Pt atoms in PtAu8-PMo12. The rapid adsorption of H 2 is due to the fast diffusion of H 2 , which has a smaller kinetic diameter than CO, in the narrow channels between the closed voids in PtAu8-PMo12. Meanwhile, CO was irreversibly adsorbed on Pt, resulting in structural isomerization to the stable "chalice-motif" PtAu8, which was determined by XAFS analysis and density functional theory calculations. Structural isomerization was involved by pushing ligands aside to make space for CO adsorption as the void size near Pt in the crown-motif PtAu8-PMo12 was narrower than the kinetic diameter of CO.

Sulfur, a group 16 element, can substitute the oxygen sites of metal oxides, potentially providing them with unique properties and enabling new applications. Polyoxometalates (POMs) are anionic metal oxide clusters with wide structural diversity owing to arbitrary selection of their constituting metal atoms. However, substitution of the oxygen sites of POMs with sulfur atoms has been rarely explored. Recently, we reacted a Keggin-type POM [XW 12 O 40 ] 4- ( I X ; X = Si and Ge) with a sulfurizing reagent (Lawesson's reagent), synthesizing Keggin-type polyoxothiometalates (POTMs) [XW 12 O 28 S 12 ] 4- ( II X , X = Si and Ge) in which all 12 terminal oxygen atoms of I X were site-selectively substituted with sulfur atoms. Although the selected heteroatoms (X) and anion charges substantially influence the properties and reactivities of POMs, oxygen-sulfur substitution of Keggin-type POMs with other heteroatoms and anion charges has not been attempted. Herein, we report the oxygen-sulfur substitution reaction of Keggin-type POMs [XW 12 O 40 ] n - ( I X ; X = Al, Si, and P; n = 5, 4, and 3, respectively) with various heteroatoms, yielding the corresponding Keggin-type POTMs [XW 12 O 28 S 12 ] n - ( II X ). The oxygen-sulfur substitution reaction depended on the heteroatom: POMs with higher anion charges reacted more strongly with the sulfurizing reagents. We also investigated the reaction mechanism of POMs and sulfurizing reagents.

Atomically precise Ag nanoclusters display distinctive properties that are dictated by their structures and electronic states. However, manipulating the electronic states of Ag nanoclusters is challenging owing to their inherent instability and susceptibility to undesired structural changes, decomposition, and aggregation. Recently, we reported the synthesis of a body-centered cubic {Ag 30 } 22+ nanocluster encapsulated within a ring-shaped polyoxometalate (POM) [P 8 W 48 O 184 ] 40- by reacting 16 Ag + -containing [P 8 W 48 O 184 ] 40- with Ag + using N , N -dimethylformamide (DMF) as a mild reducing agent. This led to a redox-induced structural transformation into a face-centered cubic {Ag 30 } 16+ nanocluster. In this study, we demonstrated the modulation of the electronic states of Ag 30 nanoclusters within the ring-shaped POM through two different approaches. A face-centered cubic {Ag 30 } 18+ nanocluster, featuring distinct oxidation states compared to previously reported {Ag 30 } 22+ and {Ag 30 } 16+ nanoclusters, was synthesized using tetra- n -butylammonium borohydride, a stronger reducing agent than DMF, in the reaction of 16 Ag + -containing [P 8 W 48 O 184 ] 40- and Ag + . Additionally, by leveraging the acid-base properties of POMs, we demonstrated the reversible, stepwise modulation of the charge distribution in the Ag 30 nanocluster through controlling protonation states of the ring-shaped POM ligand. These results highlight the potential of engineering POM-stabilized Ag nanoclusters with diverse structures and electronic states, thereby facilitating the exploration of novel properties and applications utilizing the unique characteristics of the POM ligands.

Alloying is an effective method for modulating metal nanoclusters to enrich their structural diversity and physicochemical properties. Recent investigations have demonstrated that polyoxometalates (POMs) can act as effective multidentate ligands for silver (Ag) nanoclusters to endow them with synergistic properties, reactivity, catalytic properties, and stability. However, the application of POMs as ligands has been confined predominantly to monometallic nanoclusters. Herein, we report a synthetic method for fabricating surface-exposed gold (Au)-Ag alloy nanoclusters within a ring-shaped POM ([P 8 W 48 O 184 ] 40- ). Reacting an Ag nanocluster stabilized by the ring-shaped POM with Au ions (Au + ) was found to substitute several Ag atoms at the core of the nanocluster with Au atoms. The resultant {Au 8 Ag 26 } alloy nanocluster demonstrated superior photocatalytic activity and stability compared to the pristine Ag nanocluster in the aerobic oxidation of α-terpinene under visible-light irradiation. These findings provide fundamental insights into the formation and catalytic properties of POM-stabilized alloy nanoclusters and advance exploration into the synthesis and applications of diverse metal nanoclusters., (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Polyoxometalates (POMs) function as platforms for synthesizing structurally well-defined inorganic molecules with diverse structures, metals, compositions, and arrangements. Although post-editing of the oxygen sites of POMs has great potential for development of unprecedented structures, electronic states, properties, and applications, facile methods for site-selective substitution of the oxygen sites with other atoms remain limited. Herein, we report a direct site-selective oxygen-sulfur substitution method that enables transforming POMs [XW 12 O 40 ] 4- (X = Si, Ge) to Keggin-type polyoxothiometalates (POTMs) [XW 12 O 28 S 12 ] 4- using sulfurizing reagents in an organic solvent. The resulting POTMs retain the original Keggin-type structure, with all 12 surface W[double bond, length as m-dash]O groups selectively converted to W[double bond, length as m-dash]S without sulfurization of other oxygen sites. These POTMs show high stability against water and O 2 in organic solvents and a drastic change in the electronic states and redox properties. The findings of this study represent a facile method for converting POMs to POTMs, leading to the development of their unique properties and applications in diverse fields, including (photo)catalysis, sensing, optics, electronics, energy conversion, and batteries., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Cu nanoclusters exhibit distinctive physicochemical properties and hold significant potential for multifaceted applications. Although Cu nanoclusters are synthesized by reacting Cu ions and reducing agents by covering their surfaces using organic protecting ligands or supporting them inside porous materials, the synthesis of surface-exposed Cu nanoclusters with a controlled number of Cu atoms remains challenging. This study presents a solid-state reduction method for the synthesis of Cu nanoclusters employing a ring-shaped polyoxometalate (POM) as a structurally defined and rigid molecular nanoreactor. Through the reduction of Cu 2+ incorporated within the cavity of a ring-shaped POM using H 2 at 140 °C, spectroscopic studies and single-crystal X-ray diffraction analysis revealed the formation of surface-exposed Cu nanoclusters with a defined number of Cu atoms within the cavities of POMs. Furthermore, the Cu nanoclusters underwent a reversible redox transformation within the cavity upon alternating the gas atmosphere (i.e., H 2 or O 2 ). These Cu nanoclusters produced active hydrogen species that can efficiently hydrogenate various functional groups such as alkenes, alkynes, carbonyls, and nitro groups using H 2 as a reductant. We expect that this synthesis approach will facilitate the development of a wide variety of metal nanoclusters with high reactivity and unexplored properties.

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., Competing Interests: There are no conflicts of interest to declare., (This journal is © The Royal Society of Chemistry.)

Organic-polyoxometalate (POM) hybrids have recently attracted considerable interest because of their distinctive properties and wide-ranging applications. For the construction of organic-POM hybrids, porphyrins are promising building units owing to their optical properties and reactivity, including strong visible-light absorption and subsequent singlet-oxygen ( 1 O 2 *) generation. However, the practical utilization of porphyrins as photocatalysts and photosensitizers is often hindered by their own degradation by 1 O 2 *. Therefore, there is a substantial demand for the development of porphyrin-derived photocatalysts with both high efficiency and durability. Herein, we present a porphyrin-polyoxotungstate molecular hybrid featuring a face-to-face stacked porphyrin dimer ( I ) fastened by four lacunary polyoxotungstates. Hybrid I exhibited remarkable efficiency and durability in photocatalytic aerobic oxidation reactions, and the selective oxidation of various dienes, alkenes, sulfides, and amines proceeded using just 0.003 mol % of the catalyst. Mechanistic investigations suggested that the high activity of I stems from the efficient generation of 1 O 2 *, resulting from the heavy-atom effect of POMs. Furthermore, despite its high efficiency in 1 O 2 * generation compared to free porphyrins, I exhibited superior durability against 1 O 2 *-induced degradation under photoirradiation.

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., (© 2024. The Author(s).)

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., Competing Interests: There are no conflicts of interest to declare., (This journal is © The Royal Society of Chemistry.)

The surfaces of metal nanoclusters, including their interface with metal oxides, exhibit a high reactivity that is attractive for practical purposes. This high reactivity, however, has also hindered the synthesis of structurally well-defined hybrids of metal nanoclusters and metal oxides with exposed surfaces and/or interfaces. Here we report the sequential synthesis of structurally well-defined {Ag 30 } nanoclusters in the cavity of ring-shaped molecular metal oxides known as polyoxometalates. The {Ag 30 } nanoclusters possess exposed silver surfaces yet are stabilized both in solution and the solid state by the surrounding ring-shaped polyoxometalate species. The clusters underwent a redox-induced structural transformation without undesirable agglomeration or decomposition. Furthermore, {Ag 30 } nanoclusters showed high catalytic activity for the selective reduction of several organic functional groups using H 2 under mild reaction conditions. We believe that these findings will serve for the discrete synthesis of surface-exposed metal nanoclusters stabilized by molecular metal oxides, which may in turn find applications in, for example, the fields of catalysis and energy conversion., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)

Although supported anionic gold nanoparticle catalysts have been theoretically investigated for their efficacy in activating O 2 in aerobic oxidation reactions, limited studies have been reported due to the difficulty of designing these catalysts. Herein, we developed a feasible method for preparing supported anionic gold nanoparticle catalysts using multivacant lacunary polyoxometalates with high negative charges. We confirmed the strong and robust electronic interaction between gold nanoparticles and multivacant lacunary polyoxometalates, and the electronic states of the supported gold nanoparticle catalysts can be sequentially modulated. Particularly, the catalyst prepared using [SiW 9 O 34 ] 10- acted as an efficient reusable heterogeneous catalyst, showing superior catalytic performance for the oxidative dehydrogenation of piperidone derivatives to the corresponding enaminones and remarkably higher stability than supported gold nanoparticle catalysts without this modification., (© 2022 Wiley-VCH GmbH.)

The ring-shaped polyoxometalate (POM) [P 8 W 48 O 184 ] 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 {P 8 W 48 } 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 {P 8 W 48 } 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 [P 8 W 48 O 184 ] 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 TBA 11 H 13 [Cu 4 (H 2 O) 4 P 8 W 48 O 176 (OCH 3 ) 8 ]·28H 2 O·3CH 3 NO 2 ( Cu4 ) and TBA 14 H 2 [Cu 8 (H 2 O) 12 P 8 W 48 O 176 (OCH 3 ) 8 ]·24H 2 O·CH 3 CN ( 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 TBA 14 H 2 [Cu 12 (H 2 O) 16 P 8 W 48 O 184 ]·4H 2 O ( Cu12 ) and TBA 16 H 8 [Cu 16 (OH) 16 (H 2 O) 4 P 8 W 48 O 184 ]·12H 2 O·C 3 H 6 O ( Cu16 ), respectively. The arrangement of copper ions on the same {P 2 W 12 } 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.

The properties of metal nanoclusters depend on both their structures and electronic states. However, in contrast to the significant advances achieved in the synthesis of structurally well-defined metal nanoclusters, systematic control of their electronic states is still challenging. In particular, stimuli-responsive and reversible control of the electronic states of metal nanoclusters is attractive from the viewpoint of their practical applications. Recently, we developed a synthesis method for atomically precise Ag nanoclusters using polyoxometalates (POMs) as inorganic ligands. Herein, we exploited the acid/base nature of POMs to reversibly change the electronic states of an atomically precise {Ag 27 } nanocluster via protonation/deprotonation of the surrounding POM ligands. We succeeded in systematically controlling the electronic states of the {Ag 27 } nanocluster by adding an acid or a base (0-6 equivalents), which was accompanied by drastic changes in the ultraviolet-visible absorption spectra of the nanocluster solutions. These results demonstrate the great potential of Ag nanoclusters for unprecedented applications in various fields such as sensing, biolabeling, electronics, and catalysis., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Nanosized manganese oxides have recently received considerable attention for their synthesis, structures, and potential applications. Although various synthetic methods have been developed, precise synthesis of novel nanostructured manganese oxides are still challenging. In this study, using a structurally defined nanosized cavity inside a ring-shaped polyoxometalate, we succeeded in synthesizing two types of discrete 18 and 20 nuclear nanostructured manganese oxides, Mn18 and Mn20, respectively. In particular, Mn18 showed much higher catalytic activity than other manganese oxides for the oxygenation of alkylarenes including electron-deficient ones, and the reaction proceeded through a unique reaction mechanism due to its unusual manganese oxide structure., (© 2021 Wiley-VCH GmbH.)

Atomically precise silver (Ag) nanoclusters are promising materials as catalysts, photocatalysts, and sensors because of their unique structures and mixed-valence states (Ag + /Ag 0 ). However, their low stability hinders the in-depth study of their intrinsic reactivity and catalytic property accompanying their redox processes. Herein, we demonstrate that a molecular hybrid of an atomically precise {Ag 27 } 17+ nanocluster and polyoxometalates (POMs) can efficiently cleave H 2 into protons and electrons. The Ag nanocluster accommodates electrons through the redox reaction from {Ag 27 } 17+ to {Ag 27 } 13+ , and the POM ligands play the following important roles: (i) a significant stabilization of the typically unstable Ag nanocluster to preserve its structure during the redox reaction with H 2 , (ii) formation of a unique interface between the Ag nanocluster and metal oxides for efficient H 2 cleavage, and (iii) storage of the generated protons on the negatively charged basic surface., (© 2021 Wiley-VCH GmbH.)

Small Ag n nanoclusters (n<10) have been emerging as promising materials as sensing, biolabeling, and catalysis because of their unique electronic states and optical properties. However, studying synthesis, structure determination, and exploration of their properties remain major challenges as a result of the low stability of small Ag nanoclusters. Herein, we synthesized an atomically precise face-centered-cubic-type small {Ag 7 } 5+ nanocluster supported by a novel triangular hollow polyoxometalate (POM) framework [Si 3 W 27 O 96 ] 18- . The cluster showed unique {Ag 7 } 5+ -to-POM charge transfer bands in both visible and UV light regions. Furthermore, this small {Ag 7 } 5+ nanocluster exhibited an unprecedented ultrastability in solution, despite having exposed Ag sites that can be accessed by small molecules, such as O 2 , water, and solvents., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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 {Ag 27 } 17+ nanocluster was synthesized by assembling reactive nanoclusters supported by open-Dawson-type POMs [Si 2 W 18 O 66 ] 16- . The {Ag 27 } 17+ nanocluster possessed 10 delocalized valence electrons and showed unprecedented ultrastability in solutions. The cluster showed unique {Ag 27 }-to-POM charge transfer bands in the visible light region.

A ring-shaped polyoxometalate (POM), [P 8 W 48 O 184 ] 40- (P8W48), formed by condensation of four hexavacant lacunary α-Dawson-type [α-P 2 W 12 O 48 ] 14- units, is an attractive superlacunary compound possessing a large cavity (approximately 10 Å). Although the introduction of metal species into the cavity was attempted in aqueous media using the K + /Li + salt of P8W48, we encountered several challenges: excess metal species are required, and K + and/or Li + remain in the cavity. Therefore, controlling the number and arrangement of the introduced metal species proved difficult. In this study, we overcame the aforementioned problems by developing a novel method for introducing metal species in organic media by employing the organic-solvent-soluble tetra- n-butylammonium (TBA) salt of P8W48 (I, TBA 12 H 28 [P 8 W 48 O 184 ]·30H 2 O). We began by synthesizing compound I via a cation exchange reaction of the K + /Li + salt of P8W48 with TBABr in aqueous nitric acid. Single crystals (I', TBA 14 H 26 [P 8 W 48 O 184 ]·20H 2 O·2CH 3 NO 2 ) were obtained by recrystallizing I from a nitromethane/toluene mixture in the presence of excess TBAOH. The bond valence sum values of I' showed that protons were located on the oxygen atoms inside the ring, and thus highly reactive sites capable of reacting with metal species were selectively generated within the ring. When I was reacted with 8 equiv of 3d metal species (Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ , or Zn 2+ ) in an acetonitrile/1,2-dichloroethane mixture in the presence of TBAOH and methanol, a series of POMs with eight metal sites (II M , TBA 14 H 2 [{M 2 (OH 2 ) 2 } 2 {M(OH 2 ) 2 } 4 P 8 W 48 O 176 (OCH 3 ) 8 ]· nH 2 O· mCH 3 CN, where M = Mn, Co, Ni, Cu, Zn) was synthesized. In II M , two edge-sharing bis(square-pyramidal) {(μ-O) 2 (M-OH 2 )(μ 3 -O) 2 (M-OH 2 )(μ-O) 2 } and four isolated M(OH 2 ) 2 sites were present within the cavity. During the metal introduction, two α-P2W12 units were converted into γ-P2W12 units, and unique α,γ,α,γ-type P8W48 frameworks were obtained.

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.