Beyond Charge Balance: Counter-Cations in Polyoxometalate Chemistry

Polyoxometalates and their composites for antimicrobial applications: Advances, mechanisms and future prospects

The overuse of antibiotics can lead to the development of antibiotic-resistant bacteria, which can be even more difficult to treat and pose an even greater threat to public health. In order to address the issue of antibiotic-resistant bacteria, researchers currently are exploring alternative methods of sterilization that are both effective and sustainable. Polyoxometalates (POMs), as emerging transition metal oxide compounds, exhibit significant potential in various applications due to their remarkable tunable physical and chemical performance, especially in antibacterial fields. They constitute a diverse family of inorganic clusters, characterized by a wide array of composition, structures and charges. Presently, several studies indicated that POM-based composites have garnered extensive attention in the realms of the antibacterial field and may become promising materials for future medical applications. Moreover, this review will focus on exploring the antibacterial properties and mechanisms of different kinds of organic-inorganic hybrid POMs, POM-based composites, films and hydrogels with substantial bioactivity, while POM-based composites have the dual advantages of POMs and other materials. Additionally, the potential antimicrobial mechanisms have also been discussed, mainly encompassing cell wall/membrane disruption, intracellular material leakage, heightened intracellular reactive oxygen species (ROS) levels, and depletion of glutathione (GSH). These findings open up exciting possibilities for POMs as exemplary materials in the antibacterial arena and expand their prospective applications.

Enhanced Proton Conduction via Proton-Coupled Electron Transfer Reaction by a Keplerate-Type Polyoxometalate Capsule

Polyoxometalates (POMs), anionic metal-oxide clusters, are actively studied for their versatile structural designs and element selectivity. A series of Keplerate-type POMs with core-shell structures, known as POM capsules, that feature a Keggin-type POM core, has been reported. These POM capsules, with their neutral to negative charge and large molecular surface area, can serve as platforms for proton (H+) conduction. In this study, we aimed to introduce proton carriers into the POM capsule through a proton-coupled electron transfer (PCET) reaction. It is crucial to avoid excessive reduction, as it increases electrostatic interaction between the proton and the POM capsule, hindering proton conduction. The number of incorporated electrons was varied from 0 to 11, and the POM capsule containing 3 electrons exhibited proton conductivity an order of magnitude higher than that of pristine or highly reduced POM capsules. This study serves as a proof of concept, demonstrating that PCET, combined with the core-shell structure, can effectively enhance the proton conductivity without being limited by the choice of constituent elements.

Giant {Mo132} polyoxometalate isolated with diverse organic cations: a systematic proton conductivity study

The development of efficient and stable proton conductors is a pivotal area of research due to their transformative potential in alternative energy technologies. Recently, there has been a surge of interest in synthesizing proton conductors based on polyoxometalate (POM) materials, attributed to their highly negatively charged and oxygen-rich surfaces. In this study, we report on a highly water-soluble giant POM, (NH4)42[Mo132O372(CH3COO)30(H2O)72]·ca.300H2O·ca.10CH3COONH4 (designated as {Mo132}), which was rendered insoluble in water by exchanging its ammonium cations with larger organic cations, specifically histidinium, pyridinium, bipyridinium, and methyl viologen, resulting in His-Mo132, Py-Mo132, Bpy-Mo132 and MV-Mo132, respectively. These ion-exchanged compounds were thoroughly characterized through comprehensive spectral analyses, elemental analyses and microscopic studies. The substitution with organic cations containing nitrogen centres not only rendered {Mo132} insoluble, but also increased the number of proton hopping sites, thereby enhancing proton transport. Consequently, His-Mo132, Py-Mo132, Bpy-Mo132 and MV-Mo132 demonstrated impressive proton conductivity. Among these, Py-Mo132 stood out with a proton conductivity of 1.07 × 10-2 S cm-1 under 98% relative humidity at 80 °C. All four compounds exhibited proton conduction predominantly via the Grotthuss mechanism. Furthermore, stability assessments of these Mo132-based proton conductors were conducted under operational conditions to evaluate their performance in practical applications.

Isolation and characterization of a new polyoxometalate ligand, H3SbW14O5010-, and its interactions with f-elements

We report the synthesis and characterization (Raman, FTIR, DLS, single crystal XRD) of Cs10H3SbW14O50·6H2O, which contains a new polyoxometalate building block: H3SbW14O5010- (SbW14). Solution-state fluorescence, DLS, and UV-vis absorbance results with Nd3+, Eu3+, Am3+, and Cm3+ confirm that SbW14 acts as an efficient complexant and fluorescence sensitizer for f-elements.

Ammoniated-driven green synthesis of charged polyoxometalate supported ionic liquids for exceptional heavy metal remediation in actual industrial wastewater

Reducing toxic metal concentrations to extremely low levels has long posed a challenge. Polyoxometalate supported ionic liquids (POM-SILs) offer significant potential for advanced water remediation, but their application is limited by complex preparation, toxic solvents, and poor stability due to leaching, compromising sustainability. We introduced a sustainable approach for selectively removing Pb(II) in complex electroplating wastewater using charged POM-SILs composite, synthesized by directly grafting lacunary Keggin ions ([α-SiW11O39]8-, SiW11) onto charged ammoniated polystyrene via a straightforward, solvent-free process. These POM-SILs featured monodisperse nanoclusters (<5 nm) in a cross-linked polymer matrix, ensuring optimal site accessibility and enhanced stability with negligible leaching. They achieved exceptional Pb(II) selectivity, boasting a distribution coefficient (Kd) of 23,605 mL g-1-over 120 times greater than conventional ion-exchange resins-and a Pb(II) removal efficiency exceeding 97.6%, even in high-salinity, diverse heavy metal environments. They reached a large Q0.01 value of 0.371 mg g-1, effectively treating up to 2,200 liters of wastewater per kg composite, reducing Pb(II) concentrations to below 0.01 mg L-1, meeting drinking water standards. This method marks a substantial advancement in heavy metal remediation, offering an efficient and sustainable solution for industrial wastewater treatment.

Iso- and hetero-polyoxovanadium glycolates [VIV6O6(glyc)6MIII(OH)6]3- (M = V, Cr, Fe, and Al) encapsulating metal hydroxides

Polyoxovanadium glycolates (NH4)3[V6O6(glyc)6V(μ-OH)6]·10H2O (1-V7) (H2glyc = glycolic acid) and its derivatives added with ammonium sulfates (NH4)3[V6O6(glyc)6M(μ-OH)6][(NH4)2SO4]2·xH2O (M = Cr, x = 6, 2-CrV6; M = Fe, x = 7, 3-FeV6; M = Al, x = 6, 4-AlV6) were obtained through self-assembly and fully characterized. Compounds 1-4 are composed of the same fully reduced cyclic {VIV6O6} unit bridged by six glycolate ligands. The framework encapsulated an octahedral metal(III) hydroxide {M(OH)6} (M = V3+, Cr3+, Fe3+, and Al3+) in the center, forming an iso- or hetero-heptanuclear Anderson-type structure. Moreover, two sulfate anions capped the two sides of cyclic {V6O6} units in compounds 2-4. Each sulfate formed a strong triple hydrogen bond (2.657-2.829 Å) with the μ3-OH group of the hydroxide. The sulfate anions played important roles in the formation of layered structures in 2-4, while the clusters were stacked compactly through strong hydrogen bonds in 1-V7. Because of the different central metals, these heptanuclear clusters exhibited distinct electronic structures as well as redox and magnetic properties. Magnetic studies showed that compounds 1-3 exhibited weak antiferromagnetic interactions in decreasing order with Fe3+, V3+ and Cr3+, while 4-AlV6 displayed weak ferromagnetic interactions. Their relationships with the local environment of the FeV-cofactor in V-nitrogenase are also discussed.

Redox Self-Equilibration in Molecular Vanadium Oxide Mixtures Enables Multi-Electron Storage

Polyoxometalates (POMs) are ideal components for reversible multi-electron storage in energy technologies. To-date, most redox-applications employ only single, individual POM species, which limits the number of electrons that can be stored within a given potential window. Here, we report that spontaneous redox self-equilibration during cluster synthesis leads to the formation of two structurally related polyoxovanadates which subsequently aggregate into co-crystals. This results in systems with significantly increased redox reactivity. The mixed POM system was formed by non-aqueous self-assembly of a vanadate precursor in the presence of Mg2+, resulting in two mixed-valent (VIV/V) species, [(MgOH)V13O33Cl]4- (={MgV13}) and the di-vanadium-functionalized species [V14O34Cl]4- (={V14}), which co-crystallize in a 1 : 1 molar stoichiometry. Experimental data indicate that in the native state, {MgV13} is reduced by three electrons, and {V14} is reduced by five electrons. Electrochemical studies in solution show, that the system can reversibly undergo up to fourteen redox transitions (tentatively assigned to twelve 1-electron processes and two 2-electron processes) in the potential range between -2.15 V to +1.35 V (vs Fc+/Fc). The study demonstrates how highly redox-active, well-defined molecular mixtures of mixed-valent molecular metal oxides can be accessed by redox-equilibration during synthesis, opening new avenues for molecular energy storage.

Cluster-Cation Pairs Mediated Assembly of Subnanometer Polyoxometalates Superstructures

Superstructures assembled by subnanometer polyoxometalate (POM) clusters are interesting for their attractive structures and excellent properties. However, the complex interactions between clusters and cations make it challenging to control the assembly of POM clusters at the subnanometer scale. Here, 20 cluster-assembled superstructures built by two types of MP2W17O61 (M = La-Lu) clusters are successfully synthesized. The precise structures and configurations of the subnanostructures, including nanowires, tetragonal nanosheets, and rectangular nanosheets, are characterized and presented. Molecular dynamics (MD) simulations reveal that the difference in interactions of POM clusters and cations leads to the formation of distinct superstructures. Two mechanisms of superstructure formation are proposed. Furthermore, the EuP2W17 nanosheet behaves with a high Faradaic efficiency of 90.2% and selectivity of 87.3% for glycolic acid in the electrocatalytic ethylene glycol oxidation reaction, which is much higher than that of isolated cluster components. This work connects the cluster topologies and cluster-cation pairs to the superstructures of cluster assemblies, providing general guidelines for the supramolecular self-assembly of POM clusters.

Keplerate polyoxometalate compounds: a multifunctional nano-platform for advanced materials

Polyoxometalates (POMs) are robust, discrete, and structurally well-defined metal-oxide cluster anions that have stimulated research in broad fields of science. Keplerates, as porous giant POMs, serve as a multifunctional nano-platform exhibiting fascinating chemical properties stemming from the porous molecular structure, substantial interior space, delocalization of d-electrons over the large molecular surface, etc. Consequently, Keplerates have attracted significant attention from scientists in the fields of chemistry, physics, biology, and materials sciences. This work reviews recent research progress on Keplerates as nanocontainers, catalysts, and battery materials. Furthermore, current challenges and potential future research directions are discussed, providing a reference for the development and effective application of Keplerates and Keplerate-based materials.

Electronic state modulation of Ag30 nanoclusters within a ring-shaped polyoxometalate

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 {Ag30}22+ nanocluster encapsulated within a ring-shaped polyoxometalate (POM) [P8W48O184]40- by reacting 16 Ag+-containing [P8W48O184]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 {Ag30}16+ nanocluster. In this study, we demonstrated the modulation of the electronic states of Ag30 nanoclusters within the ring-shaped POM through two different approaches. A face-centered cubic {Ag30}18+ nanocluster, featuring distinct oxidation states compared to previously reported {Ag30}22+ and {Ag30}16+ nanoclusters, was synthesized using tetra-n-butylammonium borohydride, a stronger reducing agent than DMF, in the reaction of 16 Ag+-containing [P8W48O184]40- and Ag+. Additionally, by leveraging the acid-base properties of POMs, we demonstrated the reversible, stepwise modulation of the charge distribution in the Ag30 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.

Metal-ion-selective chiral gelation of a mannose-grafted polyoxometalate cluster

A series of supramolecular hydrogels are constructed through selective metal coordination to a mannose-grafted Anderson-type polyoxometalate under sonication. The crosslinked helical nanofibers in the hydrogels are characterized by the synthesized hybrid clusters adopting a half-overlapped head-to-tail packing while the chirality of the mannose group directs the preferential assembly.

Assembled Structures and Electrical Conductivities of Salts Comprised of Cationic Sandwich Complexes and Polyoxometalates

Polyoxometalates (POMs) are recognized for their diverse structures and catalytic properties, yet their organometallic salts have not been thoroughly investigated. In this study, we synthesized salts of a Keggin-type POM with cationic sandwich complexes featuring various substituents, [X]2[SMo12O40] [X = Ru(C5H5)(C6H5R); R = H (1a), Me (1b), Bu (1c)], [Co(C5H5)2 (2), and Co(C5Me5)2 (3)]. These salts exhibited similar structural characteristics, with each anion surrounded by 10 cations, forming two-dimensional sheet arrangements through O···O anion-anion contacts, except for the salt of 1c, which exhibited one-dimensional contact owing to the larger cation substituent. Additionally, [2](THA)[Mo6O19], [3]2[Mo6O19], and [3](THA)[SMo12O40], containing the Lindqvist-type POM [Mo6O19] and/or tetrahexylammonium (THA) cation, were obtained, with the packing structure of [3]2[Mo6O19] closely resembling that of [X]2[SMo12O40]. Solvate crystals of [1a]3[PMo12O40] were also prepared. [1a]2[SMo12O40] exhibits an electrical conductivity σ of 1.4 × 10-7 S cm-1 at 373 K, which is 2 orders of magnitude higher than those of (THA)2[SMo12O40] and [1a]PF6.

Enhanced Electrical Conductivity of Polyoxometalates by Bridging with Mixed-Valent Multinuclear Platinum Complexes

Polyoxometalates (POMs) are nanosized molecular metal oxide anion clusters with tuneable structures and functionalities, and they exhibit a redox chemistry and catalytic activity in multielectron redox processes. These are typically poor electrical conductors (<10-10 Scm-1), which is attributed to negligible electronic interactions among anions in the solid state. Since the reduced electrons on the d0 metals in POMs are delocalized, electrical conductivity was improved when judicious pathways for the electrons were created by bridging the POMs. Utilized with the electronic interactions between bridging oxygen atoms with the highest occupied molecular orbital in the POMs and the metal dz2 orbitals in the multinuclear platinum complexes, and three mixed-valent assemblies were synthesized and characterized. Simply mixing Keggin-type or Dawson-type POMs with tetranuclear or trinuclear platinum complexes in solution afforded three single crystals, and all three compounds were paramagnetic with mixed oxidation states and better conductivities at room temperature than the parent compounds.

POMSimulator: An open-source tool for predicting the aqueous speciation and self-assembly mechanisms of polyoxometalates

Elucidating the speciation (in terms of concentration versus pH) and understanding the formation mechanisms of polyoxometalates remains a significant challenge, both in experimental and computational domains. POMSimulator is a new methodology that tackles this problem from a purely computational perspective. The methodology uses results from quantum mechanics based methods to automatically set up the chemical reaction network, and to build speciation models. As a result, it becomes possible to predict speciation and phase diagrams, as well as to derive new insights into the formation mechanisms of large molecular clusters. In this work we present the main features of the first open-source version of the software. Since the first report [Chem. Sci. 2020, 11, 8448-8456], POMSimulator has undergone several improvements to keep up with the growing challenges that were tackled. After four years of research, we recognize that the source code is sufficiently stable to share a polished and user-friendly version. The Python code, manual, examples, and install instructions can be found at https://github.com/petrusen/pomsimulator.

Tuning Proton Conduction by Staggered Arrays of Polar Preyssler-Type Oxoclusters

Polyoxometalates (POMs), anionic nanosized oxoclusters that can be considered as fragments of metal oxides, have been extensively studied for their diverse composition and structure, showing promise in various fields such as catalysis and electronics. Proton conduction, relevant to catalysis and electronics, has attracted interest in materials chemistry, and POM anions are advantageous in terms of their proton carrier density and mobility. Recently, polar POMs have attracted attention for their unique ferroelectric behaviors, yet they have been little studied with regard to proton conduction, as their polarity has generally been believed to have a negative impact. Here, we propose that polar POMs can be used to align polar proton carriers, such as H2O and polymers, to construct efficient proton-conducting pathways. In this study, we present ionic crystals composed of polar Preyssler-type POMs ([Xn+(H2O)P5W30O110](15-n)-, Xn+ = Ca2+, Eu3+) and K+ exhibiting ultrahigh proton conductivity surpassing 10-2 S cm-1, which is required for practical applications. In contrast, ionic crystals with nonpolar Preyssler-type POMs show an order of magnitude lower proton conductivity. Structural and spectroscopic studies combined with theoretical calculations reveal that proton carriers align with the aid of staggered arrays of polar POMs, forming a hydrogen-bonding network favorable for proton conduction. This study integrates molecular chemistry by the design of POMs and solid-state chemistry by exploring long-range proton conduction mechanisms, offering novel insights for future materials design.

Self-assembly of hard anions around cationic gold nanorods: potential structures for SERS

The placement of polyoxometalates next to the surface of noble metallic nanoparticles has been found to enhance the surface-enhanced Raman scattering (SERS) effect. The enhancement is believed to stem from either charge (electrostatic attraction) or chemical effects. Anisotropic gold nanorods are recognized as useful nanostructures for SERS, mainly due to the high electric field enhancement at their ends. The presented work examines the use of a polyoxometalate encapsulated gold nanorod for SERS, to assess whether the two enhancement pathways would be synergetic. For this, a gold nanorod-polyoxometalate composite was synthesized by coating cetyltrimethylammonium bromide-stabilized gold nanorods with a silicotungstic Keggin anion through electrostatic attraction. The structure was characterized, confirming that the nanorods have been fully encapsulated by the polyoxometalate. The SERS performance of the composite was assessed in solution using crystal violet as a SERS indicator, finding an analytical enhancement factor of 1.8 × 104 in colloidal solution. The enhancement mechanism was examined first by comparison to gold nanorods stabilized by a cetyltriethylammonium bromide bilayer, cationic thiol bound polyoxometalate, and polyelectrolyte coating. Next, composites made using polyoxometalates of different atomic composition and charge were examined. It was concluded that the polyoxometalate charge had a noticeable effect on the enhancement while the atomic composition did not. Furthermore, high enhancement is observed mainly in cases where the nanorod monolayer allows the sequestration of the dye molecule into the nanoparticle's ligand layer. The proposed mechanism therefore involves the negative charge of the polyoxometalate attracting the positively charged dye, and facilitating the sequestration of the dye within the ligand bilayer, closer to the nanorod's surface.

The Hybridization of Polymers with Metal Oxide Clusters for the Design of Non-Fluorinated Proton Exchange Membranes

As the key component of various energy storage and conversion devices, proton exchange membranes (PEMs) have been attracting significant interest. However, their further development is limited by the high cost of perfluorosulfonic acid polymers and the poor stability of acid-dopped non-fluorinated polymers. Recently, a new group of PEMs has been developed by hybridizing polyoxometalates (POMs), a group of super acidic sub-nanoscale metal oxide clusters, with polymers. POMs can serve simultaneously as both proton sponges and stabilizing agents, and their complexation with polymers can further improve polymers' mechanical performance and processability. Enormous efforts have been focused on studying supramolecular complexation or covalent grafting of POMs with various polymers to optimize PEMs in terms of cost, mechanical properties and stabilities. This concept summarizes recent advances in this emerging field and outlines the design strategies and application perspectives employed for using POM-polymer hybrid materials as PEMs.

Nanoconfinement of polyoxometalates in cyclodextrin: computational inspections of the binding affinity and experimental demonstrations of reactivity modulation

Chaotropic polyoxometalates (POMs) form robust host-guest complexes with γ-cyclodextrin (γ-CD), offering promising applications in catalysis, electrochemical energy storage, and nanotechnology. In this article, we provide the first computational insights on the supramolecular binding mechanisms using density-functional theory and classical molecular dynamics simulations. Focusing on the encapsulation of archetypal Keggin-type POMs (PW12O40 3-, SiW12O40 4- and BW12O40 5-), our findings reveal that the lowest-charged POM, namely PW12O40 3- spontaneously confines within the wider rim of γ-CD, but BW12O40 5- does not exhibit this behaviour. This striking affinity for the hydrophobic pocket of γ-CD originates from the structural characteristics of water molecules surrounding PW12O40 3-. Moreover, through validation using 31P NMR spectroscopy, we demonstrate that this nanoconfinement regulates drastically the POM reactivity, including its capability to undergo electron transfer and intermolecular metalate Mo/W exchanges. Finally, we exploit this nanoconfinement strategy to isolate the elusive mixed addenda POM PW11MoO40 3-.

From +I to +IV, Alkalis to Actinides: Capturing Cations across the Periodic Table with Keggin Polyoxometalate Ligands

Coordination chemistry trends across the periodic table are often difficult to probe experimentally due to limitations in finding a versatile but consistent chelating platform that can accommodate various elements without changing its coordination mode. Herein, we present new metal/ligand systems covering a wide range of ionic radii, charges, and elements. Five different ligands derived from the Keggin structure (HBW11O398-, PW11O397-, SiW11O398-, GeW11O398-, and GaW11O399-) were successfully crystallized with six different cations (Na+, Sr2+, Ba2+, La3+, Ce4+, and Th4+) and characterized by single-crystal X-ray diffraction. Twenty-five new compounds were obtained by using Cs+ as the counterion, yielding a consistent base formula of Csx[M(XW11O39)2]·nH2O. Despite having a similar first-coordination sphere geometry (i.e., 8-coordinated), the nature of the central cation was found to impact the long-range geometry of the complexes. This unique crystallographic data set shows that, despite the traditional consensus, the local geometry of the cation (i.e., metal-oxygen bond distance) is not enough to depict the full impact of the complexed metal ion. The bending and twisting of the complexes, as well as ligand-ligand distances, were all impacted by the nature of the central cation. We also observed that counterions play a critical role by stabilizing the geometry of the M(XW11)2 complex and directing complex-complex interactions in the lattice. We also define certain structural limits for this type of complex, with the large Ba2+ ion seemingly approaching those limits. This study thus lays the foundation for capturing the coordination chemistry of other rarer elements across the periodic table such as Ra2+, Ac3+, Bk4+, Cf3+, etc.

Polyoxometalate Clusters Confined in Reduced Graphene Oxide Membranes for Effective Ion Sieving and Desalination

Efficient 2D membranes play a critical role in water purification and desalination. However, most 2D membranes, such as graphene oxide (GO) membranes, tend to swell or disintegrate in liquid, making precise ionic sieving a tough challenge. Herein, the fabrication of the polyoxometalate clusters (PW12) intercalated reduced graphene oxide (rGO) membrane (rGO-PW12) is reported through a polyoxometalate-assisted in situ photoreduction strategy. The intercalated PW12 result in the interlayer spacing in the sub-nanometer scale and induce a nanoconfinement effect to repel the ions in various salt solutions. The permeation rate of rGO-PW12 membranes are about two orders of magnitude lower than those through the GO membrane. The confinement of nanochannels also generate the excellent non-swelling stability of rGO-PW12 membranes in aqueous solutions up to 400 h. Moreover, when applied in forward osmosis, the rGO-PW12 membranes with a thickness of 90 nm not only exhibit a high-water permeance of up to 0.11790 L m-2 h-1 bar-1 and high NaCl rejection (98.3%), but also reveal an ultrahigh water/salt selectivity of 4740. Such significantly improved ion-exclusion ability and high-water flux benefit from the multi-interactions and nanoconfinement effect between PW12 and rGO nanosheets, which afford a well-interlinked lamellar structure via hydrogen bonding and van der Waals interactions.

Pd-incorporated polyoxometalate catalysts for electrochemical CO2 reduction

Polyoxometalates (POMs), representing anionic metal-oxo clusters, display diverse properties depending on their structures, constituent elements, and countercations. These characteristics position them as promising catalysts or catalyst precursors for electrochemical carbon dioxide reduction reaction (CO2RR). This study synthesized various salts-TBA+ (tetra-n-butylammonium), Cs+, Sr2+, and Ba2+-of a dipalladium-incorporated POM (Pd2, [γ-H2SiW10O36Pd2(OAc)2]4-) immobilized on a carbon support (Pd2/C). The synthesized catalysts-TBAPd2/C, CsPd2/C, SrPd2/C, and BaPd2/C-were deposited on a gas-diffusion carbon electrode, and the CO2RR performance was subsequently evaluated using a gas-diffusion flow electrolysis cell. Among the catalysts tested, BaPd2/C exhibited high selectivity toward carbon monoxide (CO) production (ca. 90%), while TBAPd2/C produced CO and hydrogen (H2) with moderate selectivity (ca. 40% for CO and ca. 60% for H2). Moreover, BaPd2/C exhibited high selectivity toward CO production over 12 h, while palladium acetate, a precursor of Pd2, showed a significant decline in CO selectivity during the CO2RR. Although both BaPd2/C and TBAPd2/C transformed into Pd nanoparticles and WO x nanospecies during the CO2RR, the influence of countercations on their product selectivity was significant. These results highlight that POMs and their countercations can effectively modulate the catalytic performance of POM-based electrocatalysts in CO2RR.

Coacervate Formation in Dilute Aqueous Solutions of Inorganic Molecular Clusters with Simple Divalent Countercations

We report a complex coacervate formed by a 2.5 nm-diameter, rigid uranyl peroxide molecular cluster (Li68K12(OH)20)[UO2(O2)OH]60, U6060-) and SrCl2 salt in dilute aqueous solutions, including its location in the phase diagram, composition, rheological features, and critical conditions for phase transitions. In this coacervate, the Sr2+ cations are a major building component, and the coacervate phase covers a substantial region of the phase diagram. This coacervate demonstrates features that differ from traditional coacervates formed by oppositely charged long-chain polyelectrolytes, especially in its formation mechanism, dehydration, enhancement of mechanical strength with increasing ionic strength, and the change of salt partition preference into the coacervate and supernatant phases with ionic strength.

Reversible Optical Switching of Polyoxovanadates and Their Communication via Photoexcited States

The 2-bit Lindqvist-type polyoxometalate (POM) [V6O13((OCH2)3CCH2N3)2]2- with a diamagnetic {V6O19} core and azide termini shows six fully oxidized VV centers in solution as well as the solid state, according to 51V NMR spectroscopy. Under UV irradiation, it exhibits reversible switching between its ground S0 state and the energetically higher lying states in acetonitrile and water solutions. TD-DFT calculations demonstrate that this process is mainly initialized by excitation from the S0 to S9 state. Pulse radiolysis transient absorption spectroscopy experiments with a solvated electron point out photochemically induced charge disproportionation of VV into VIV and electron communication between the POM molecules via their excited states. The existence of this unique POM-to-POM electron communication is also indicated by X-ray photoelectron spectroscopy (XPS) studies on gold-metalized silicon wafers (Au//SiO2//Si) under ambient conditions. The amount of reduced vanadium centers in the "confined" environment increases substantially after beam irradiation with soft X-rays compared to non-irradiated samples. The excited state of one POM anion seems to give rise to subsequent electron transfer from another POM anion. However, this reaction is prohibited as soon as the relaxed T1 state of the POM is reached.

Self-assembly of Dawson-type H6P2W18O62@[Cu6O(TZI)3(H2O)6]4 for high-performance aerobic oxidation desulfurization of fuel

Because global sulfur emission has escalated, the development of high-efficiency deep desulfurization techniques has become imperative. Herein, to design a high-activity heterogeneous catalyst for the aerobic oxidation desulfurization (AODS) of fuel, Dawson-type polyoxometalate (H6P2W18O62 abbreviated as D-P2W18), characterized by its high activity and strong oxidative capacity, was applied to react with CuCl2·2H2O and H3TZI via a one-pot hydrothermal method. Consequently, blue crystalline H6P2W18O62@[Cu6O(TZI)3(H2O)6]4 (abbreviated as D-P2W18@rht-MOF-1; rht-MOF-1 = [Cu6O(TZI)3(H2O)6]4·nH2O) was afforded. X-ray diffraction analysis indicated that D-P2W18 was successfully encapsulated in two different cages of rht-MOF-1, which is distinct from the crystal structure of Keggin-type POMs@rht-MOF-1. It represents the first crystal structure of Dawson-type POMs@rht-MOF-1. When D-P2W18@rht-MOF-1 was employed as a catalyst for AODS under ambient oxygen pressure with the assistance of surfactant dioctadecyl dimethyl ammonium chloride (DODMAC), it demonstrated remarkable catalytic capability and recyclability for both model fuel and commercial diesel. Further, the AODS reaction mechanism, identified as a free radical oxidation-reduction process, was verified by way of radical quenching experiments, EPR and XPS analysis. This approach offers a feasible route for the synthesis of new Dawson-type POMs@MOFs of heterogeneous catalysts for highly active AODS of fuel.

Transition metal-substituted polyoxometalate-ionic liquids with remarkable flame retardancy performance

The development of effective and novel flame retardants has been attracting considerable attention in extenuating the fire threat of flammable polymer materials including the widely-used epoxy resins. In this work, we pioneeringly report the construction of transition-metal-substituted polyoxometalate-ionic liquids (tmsPOM-ILs) as effective flame retardants, which consist of tetra-metal-containing POMs ([M4(H2O)2(PW9O34)2]10-, M4P2, M = Ni, Cu) anions and tetra-n-heptylammonium [(n-C7H15)4N+, THPA] cations. The resulting tmsPOM-ILs exhibited remarkably improved fire-safety of the epoxy resin (EP) matrix and even at a loading amount of as low as 3 wt%, the flame retardancy efficiency was even higher than that of commercial flame retardants (aluminum hydroxide (ATH), triphenyl phosphate (TPP), and decabromodiphenyl ethane (DBDPE)). Physicochemical and mechanistic studies revealed that the remarkable flame retardancy performance of the tmsPOM-ILs reported is due to their excellent epoxy matrix compatibility and remarkable catalytic charring ability. This work opens up a brand-new research direction of developing next-generation compatible and effective tmsPOM-based molecular flame retardants at the molecular level.

The effect of ionic versus covalent functionalization of polyoxometalate hybrid materials with coordinating subunits on their stability and interaction with DNA

Inorganic-organic hybrid materials that combine both Polyoxometalates (POMs) and metal ion coordinating subunits (CSUs) represent promising multifunctional materials. Though their individual components are often biologically active, utilization of hybrid materials in bioassays significantly depends on the functionalization method and thus resulting stability of the system. Quite intriguingly, these aspects were very scarcely studied in hybrid materials based on the Wells-Dawson POM (WD POM) scaffold and remain unknown. We chose two model WD POM hybrid systems to establish how the functionalization mode (ionic vs. covalent) affects their stability in biological medium and interaction with nucleic acids. The synthetic scope and limitations of the covalent POM-terpyridine hybrids were demonstrated and compared with the ionic Complex-Decorated Surfactant Encapsulated-Clusters (CD-SECs) hybrids. The nature of POM and CSU binding can be utilized to modulate the stability of the hybrid and the extent of DNA binding. The above systems show potential to behave as model cargo-platforms for potential utilization in medicine and pharmacy.

pH-Driven Rotational Configuration of Keggin-Fe13 Clusters and Their Transformations

Keggin-Fe13 clusters are considered foundational building blocks or prenucleation precursors of ferrihydrite. Understanding the factors that influence the rotational configuration of these clusters, and their transformations in water, is vital for comprehending the formation mechanism of ferrihydrite. Here, we report syntheses and crystal structures of four lanthanide-iron-oxo clusters, namely, [Dy6Fe13(Gly)12(μ2-OH)6(μ3-OH)18(μ4-O)4(H2O)17]·13ClO4·19H2O (1), [Dy6Fe13(Gly)12(μ3-OH)24(μ4-O)4(H2O)18]·13ClO4·14H2O (2), [Pr8Fe34(Gly)24(μ3-OH)28(μ3-O)30(μ4-O)4(H2O)30]·6ClO4·20H2O (3), and [Pr6Fe13(Gly)12(μ3-OH)24(μ4-O)4(H2O)18]·13ClO4·22H2O (4, Gly = glycine). Single-crystal analyses reveal that 1 has a β-Keggin-Fe13 cluster, marking the first documented instance of such a cluster to date. Conversely, both 2 and 4 contain an α-Keggin-Fe13 cluster, while 3 is characterized by four hexavacant ε-Keggin-Fe13 clusters. Magnetic property investigations of 1 and 2 show that 2 exhibits ferromagnetic interactions, while 1 exhibits antiferromagnetic interactions. An exploration of the synthetic conditions for 1 and 2 indicates that a higher pH promotes the formation of α-Keggin-Fe13 clusters, while a lower pH favors β-Keggin-Fe13 clusters. A detailed analysis of the transition from 3 to 4 emphasizes that lacunary Keggin-Fe13 clusters can morph into Keggin-Fe13 clusters with a decrease in pH, accompanied by a significant change in their rotational configuration.

Recent Advances on the Functionalities of Polyoxometalate-Based Ionic Liquids

Polyoxometalate (POM)-based ionic liquids (POM-ILs) are gaining increasing attention due to their diverse structures and functionalities. POMs in POM-ILs not only act as essential structural building blocks but also play a crucial role in their functional performance. With the incorporation of POMs, POM-ILs find applications in various fields such as chemical catalysis, energy science, materials science, sensors, and more. The abundant availability of POMs and other building blocks in POM-ILs, along with their versatile combination possibilities, present promising opportunities for the future. Rather than focusing solely on discovering new structures of POM-ILs, current developments in this field emphasize exploring their functions, leading to the emergence of numerous new applications. Summarizing these advancements aids in understanding the latest trends and facilitates rapid evolution. This review examines the recent five years' worth of results to analyze the new functions of POM-ILs, categorizing them based on their unique characteristics.

A stable hybrid catalyst (POM-PPPh3/L/Ni) for the reduction of toxic nitroarene compounds in water

Reduction of nitroarenes to aminoarenes using novel and selective catalysts is an important and desirable approach in green chemistry. In this work, a new heterogeneous nanocatalyst, POM-PPPh3/L/Ni, was designed and prepared via functionalizing a Keggin-type polyoxometalate (H3PMo12O40) with (3-bromopropyl)triphenylphosphonium bromide (BPPPh3Br) through strong electrostatic interactions to prepare [PMo12O40][PPPh3]3 (denoted as POM-PPPh3). The obtained compound was modified via nucleophilic attack of the nitrogen donor of a multidentate Schiff base ligand (L) on its propyl chain to produce [PMo12O40][PPPh3/L]2 (denoted as POM-PPPh3/L), which was finally metallated with nickel cations to achieve [PMo12O40][PPPh3/L/Ni]2 (denoted as POM-PPPh3/L/Ni). After full characterization of the prepared material with various physicochemical methods, its catalytic behavior was investigated in the catalytic nitroarene reduction. The influence of various factors on catalytic conversion and selectivity was considered. The synthesized nanocatalyst showed excellent performance in the reduction of nitroarenes in aqueous media in the presence of NaBH4 as a reducing agent. Mild reaction conditions and a short reaction time (10 min) are the prominent features of this new nanocatalyst. In addition, the catalyst was recovered and reused for up to four cycles of catalytic reduction without any significant loss of conversion.

Supramolecular Modifying Nafion with Fluoroalkyl‐Functionalized Polyoxometalate Nanoclusters for High‐Selective Proton Conduction

Fluoroalkyl-grafted polyoxometalate nanoclusters are used as supramolecular additives to precisely modify the ionic domains of Nafion, which can increase the proton conductivity and selectivity simultaneously. The resulting hybrid membranes show significantly enhanced power density in fuel cells and improved energy efficiency in vanadium flow batteries.

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.

A solvent-free processed low-temperature tolerant adhesive

Ultra-low temperature resistant adhesive is highly desired yet scarce for material adhesion for the potential usage in Arctic/Antarctic or outer space exploration. Here we develop a solvent-free processed low-temperature tolerant adhesive with excellent adhesion strength and organic solvent stability, wide tolerable temperature range (i.e. -196 to 55 °C), long-lasting adhesion effect ( > 60 days, -196 °C) that exceeds the classic commercial hot melt adhesives. Furthermore, combine experimental results with theoretical calculations, the strong interaction energy between polyoxometalate and polymer is the main factor for the low-temperature tolerant adhesive, possessing enhanced cohesion strength, suppressed polymer crystallization and volumetric contraction. Notably, manufacturing at scale can be easily achieved by the facile scale-up solvent-free processing, showing much potential towards practical application in Arctic/Antarctic or planetary exploration.

Role of Metal-Organic Framework Topology on Thermodynamics of Polyoxometalate Encapsulation

Polyoxometalates (POMs) are discrete anionic clusters whose rich redox properties, strong Bro̷nsted acidity, and high availability of active sites make them potent catalysts for oxidation reactions. Metal-organic frameworks (MOFs) have emerged as tunable, porous platforms to immobilize POMs, thus increasing their solution stability and catalytic activity. While POM@MOF composite materials have been widely used for a variety of applications, little is known about the thermodynamics of the encapsulation process. Here, we utilize an up-and-coming technique in the field of heterogeneous materials, isothermal titration calorimetry (ITC), to obtain full thermodynamic profiles (ΔH, ΔS, ΔG, and Ka) of POM binding. Six different 8-connected hexanuclear Zr-MOFs were investigated to determine the impact of MOF topology (csq, scu, and the) on POM encapsulation thermodynamics.

Multifunctional Polyoxometalates-Based Ionohydrogels toward Flexible Electronics

Multifunctional flexible electronics present tremendous opportunities in the rapidly evolving digital age. One potential avenue to realize this goal is the integration of polyoxometalates (POMs) and ionic liquid-based gels (ILGs), but the challenge of macrophase separation due to poor compatibility, especially caused by repulsion between like-charged units, poses a significant hurdle. Herein, the possibilities of producing diverse and homogenous POMs-containing ionohydrogels by nanoconfining POMs and ionic liquids (ILs) within an elastomer-like polyzwitterionic hydrogel using a simple one-step random copolymerization method, are expanded vastly. The incorporation of polyzwitterions provides a nanoconfined microenvironment and effectively modulates excessive electrostatic interactions in POMs/ILs/H2O blending system, facilitating a phase transition from macrophase separation to a submillimeter scale worm-like microphase-separation system. Moreover, combining POMs-reinforced ionohydrogels with a developed integrated self-powered sensing system utilizing strain sensors and Zn-ion hybrid supercapacitors has enabled efficient energy storage and detection of external strain changes with high precision. This work not only provides guidelines for manipulating morphology within phase-separation gelation systems, but also paves the way for developing versatile POMs-based ionohydrogels for state-of-the-art smart flexible electronics.

Tuning the Chirality Evolution in Achiral Subnanometer Systems by Judicious Control of Molecule Interactions

Chirality evolution from molecule levels to the nanoscale in an achiral system is a fundamental issue that remains undiscovered. Here, we report the assembly of polyoxometalate (POM) clusters into chiral subnanostructures in achiral systems by programmable single-molecule interactions. Driven by the competing binding of Ca2+ and surface ligands, POM assemblies would twist into helical nanobelts, nanorings, and nanotubes with tunable helicity. Chiral molecules can be used to differentiate the formation energies of chiral isomers and immobilize the homochiral isomer, where strong circular dichroism (CD) signals are obtained in both solutions and films. Chiral helical nanobelts can be used as circularly polarized light (CPL) photodetectors due to their distinct chiroptic responsivity for right and left CPL. By the fine-tuning of interactions at single-molecule levels, the morphology and CD spectra of helical assemblies can be precisely controlled, providing an atomic precision model for investigation of the structure-chirality relationship and chirality manipulation at the nanoscale.

Photocatalytic aerobic α-oxygenation of amides to imides using a highly durable decatungstate tetraphenylphosphonium salt

Decatungstate is a potent photocatalyst for hydrogen atom transfer (HAT) but faces degradation issues when using a typical tetra-n-butylammonium salt. Herein, we employed tetraphenylphosphonium as a countercation to yield a highly durable and efficient HAT photocatalyst, enabling α-oxygenation of amides to their corresponding imides using O2 as an oxidant.

Critical Conditions Regulating the Gelation in Macroionic Cluster Solutions

The critical gelation conditions observed in dilute aqueous solutions of multiple nanoscale uranyl peroxide molecular clusters are reported, in the presence of multivalent cations. This gelation is dominantly driven by counterion-mediated attraction. The gelation areas in the corresponding phase diagrams all appear in similar locations, with a characteristic triangle shape outlining three critical boundary conditions, corresponding to the critical cluster concentration, cation/cluster ratio, and the degree of counterion association with increasing cluster concentration. These interesting phrasal observations reveal general conditions for gelation driven by electrostatic interactions in hydrophilic macroionic solutions.

Synthesis and structural characterization of a hydrated sodium-caesium tetra-cosa-tungstate(VI), Na5Cs19[W24O84]·21H2O

Crystal formation of penta-sodium nona-deca-cesium tetra-cosa-tungstate(VI) heneikosahydrate, Na5Cs19[W24O84]·21H2O, was successfully achieved by the conversion of [H2W12O42]10- through the addition of excess Cs+. The crystal structure comprising the toroidal isopolyoxidometalate is presented, as well as its Raman spectrum. Na5Cs19(H2O)21W24O84 crystallizes in the rhombohedral space group R with an obverse centering. The title compound represents the addition of a new member to the isopolytungstate family with mixed alkali counter-ions and contains rarely observed five-coordinate tungsten(VI) atoms in the [W24O84]24- anion (site symmetry C 3i ) arising from the conversion mediated by Cs+ counter-ions.

Multi-stimuli-responsive polymer degradation by polyoxometalate photocatalysis and chloride ions

Photocatalytic polymer degradation based on harnessing the abundant light energy present in the environment is one of the promising approaches to address the issue of plastic waste. In this study, we developed a multi-stimuli-responsive photocatalytic polymer degradation system facilitated by the photocatalysis of a polyoxometalate [γ-PV2W10O40]5- in conjunction with chloride ions (Cl-) as harmless and abundant stimuli. The degradation of various polymers was significantly accelerated in the presence of Cl-, which was attributed to the oxidation of Cl- by the polyoxometalate photocatalysis into a highly reactive chlorine radical that can efficiently generate a carbon-centered radical for subsequent polymer degradation. Although organic and organometallic photocatalysts decomposed under the conditions for photocatalytic polymer degradation in the presence of Cl-, [γ-PV2W10O40]5- retained its structure even under these highly oxidative conditions.

Reactivity of Organoiridium Tungsten Oxide Clusters with Transition Metal Aquo Cations

Organometallic-polyoxometalate (POM) complexes form a unique class of molecular organometallic oxides characterized by the dynamic behavior of the organometallic cations. Herein, we investigated the reactivity of Cp*Ir-octatungstate clusters (where Cp* represents pentamethylcyclopentadienyl, C5Me5-) with Werner-type transition-metal aquo cations. The addition of Ag+, Co2+, Ni2+, and M3+ (M = Cr, Fe, or In) cations to the aqueous solution of Cp*Ir-octatungstate clusters resulted in the formation of [{Ag(OH2)2}2{Cp*Ir(OH2)}2{Cp*IrW3O12(OH)}2(WO2)2] (1), Co1.5K0.8Na0.2[{trans-Co(OH2)2}{Cp*IrW3O12(OH)}2(WO2)1.3{cis-Co(OH2)2}0.7] (2-Co), Ni0.2K1.4Na0.2[{Ni(OH2)4}2{Cp*IrW3O12(OH)}2(WO2)1.1{cis-Ni(OH2)2}0.9] (2-Ni), and [{M(OH2)4}2{Cp*IrW3O12(OH)}2{cis-M(OH2)2}2](NO3)2 (M = Cr, 3-Cr; Fe, 3-Fe; or In, 3-In), respectively. All clusters share the same cubane-type {Cp*IrW3O12(OH)}5- building block, representing the first examples of organoiridium-POMs functionalized by transition-metal aquo cations. These compounds are insoluble in water, facilitating the evaluation of their heterogeneous water-oxidation properties. Notably, 2-Co generates the highest catalytic water oxidation current. This work provides a new synthetic method to introduce metal-aquo complexes on an organometallic oxide cluster, producing multimetallic molecules that model the catalytic sites of complex oxides.

Supramolecular Complexation of Metal Oxide Cluster and Non-Fluorinated Polymer for Large-Scale Fabrication of Proton Exchange Membranes for High-Power-Density Fuel Cells

Cost-effective, non-fluorinated polymer proton exchange membranes (PEMs) are highly desirable in emerging hydrogen fuel cells (FCs) technology; however, their low proton conductivities and poor chemical and dimension stabilities hinder their further development as alternatives to commercial Nafion®. Here, we report the inorganic-organic hybridization strategy by facilely complexing commercial polymers, polyvinyl butyral (PVB), with inorganic molecular nanoparticles, H3 PW12 O40 (PW) via supramolecular interaction. The strong affinity among them endows the obtained nanocomposites amphiphilicity and further lead to phase separation for bi-continuous structures with both inter-connected proton transportation channels and robust polymer scaffold, enabling high proton conductivities, mechanical/dimension stability and barrier performance, and the H2 /O2 FCs equipped with the composite PEM show promising power densities and long-term stability. Interestingly, the hybrid PEM can be fabricated continuously in large scale at challenging ~10 μm thickness via typical tape casting technique originated from their facile complexing strategy and the hybrids' excellent mechanical properties. This work not only provides potential material systems for commercial PEMs, but also raises interest for the research on hybrid composites for PEMs.

Recent advances in lanthanide-based POMs for photoluminescent applications

Since the first formation of the famous "Peacock-Weakley" anions [Ln(W5O18)2]8/9-, a steady stream of breakthroughs have been made in the chemistry of multitalented lanthanide (Ln)-based polyoxometalates (POMs) for their potentially desirable properties. In particular, LnIII ions are generally recognised as the "vitamins of the modern industry" owing to their ability to cover a wide emission range, endowing Ln-based POMs with great potential for versatile and diverse luminescence-related applications. In this frontier, we discuss the synthesis strategies and intramolecular energy transfer in Ln-based POM derivatives. Then, the progressive improvements achieved with Ln-based POMs in photoluminescence applications are highlighted, focusing mainly on luminescent and fluorescent probes. Finally, the challenges for Ln-based POM materials for photoluminescence applications are discussed.

Dynamic and Reversible Blending Interface on Polyoxovanadate Electrode for High-Performance Lithium-Ion Batteries

Solid electrolyte interphase (SEI) plays a critical role in the performance of lithium-ion batteries (LIBs). In contrast to the clear interface between the traditional consecutive electrode materials and SEI, ionic polyoxometalates (POMs) as electrode could bilaterally diffuse with SEI and form a blending interface for superior electrochemical performance. POMs have recently aroused much interest as electrode materials in LIBs due to their structural flexibility, high capacity, and cycling stability. However, the interface evolution between POM-based electrodes and SEI, which is critical for Li+ ion transportation, has rarely been explored. Herein, we choose Li10[V12B18O60H6] (LVB) as an example to investigate the formation and structural evolution of the electrode-electrolyte interface. Time-of-flight secondary ion mass spectrometry together with X-ray photoelectron spectroscopy demonstrates the evolution of a blending layer at the interface containing typical SEI components, a polyanion from LVB and a phosphate anion from decomposition products of LiPF6. In the blending layer, ion migration takes place between the P-related inorganic species and the polyanion during the Li+ insertion/extraction reaction. Such a compatible blending layer favors Li+ transportation and the reversibility of the redox reactions, as supported by a series of electrochemical analyses. This work provides detailed insights into understanding the interface evolution of the LVB electrode and demonstrates the importance of interfacial engineering to induce proper interface layers in the development of high-performance POM-based electrodes for LIBs.

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.

Electron-Sponge Nature of Polyoxometalates for Next-Generation Electrocatalytic Water Splitting and Nonvolatile Neuromorphic Devices

Designing next-generation molecular devices typically necessitates plentiful oxygen-bearing sites to facilitate multiple-electron transfers. However, the theoretical limits of existing materials for energy conversion and information storage devices make it inevitable to hunt for new competitors. Polyoxometalates (POMs), a unique class of metal-oxide clusters, have been investigated exponentially due to their structural diversity and tunable redox properties. POMs behave as electron-sponges owing to their intrinsic ability of reversible uptake-release of multiple electrons. In this review, numerous POM-frameworks together with desired features of a contender material and inherited properties of POMs are systematically discussed to demonstrate how and why the electron-sponge-like nature of POMs is beneficial to design next-generation water oxidation/reduction electrocatalysts, and neuromorphic nonvolatile resistance-switching random-access memory devices. The aim is to converge the attention of scientists who are working separately on electrocatalysts and memory devices, on a point that, although the application types are different, they all hunt for a material that could exhibit electron-sponge-like feature to realize boosted performances and thus, encouraging the scientists of two completely different fields to explore POMs as imperious contenders to design next-generation nanodevices. Finally, challenges and promising prospects in this research field are also highlighted.

Counterion Engineering toward High-Performance and pH-Neutral Polyoxometalates-Based Hole-Transporting Materials for Efficient Organic Optoelectronic Devices

Although protonated polyoxometalates (POMs) are promising hole-transporting layer (HTL) materials for optoelectronic devices owing to their excellent hole collection/injection property, pH neutrality, and noncorrosiveness, POMs are seldom used as high-performance HTL materials. Herein, we designed and synthesized a series of mixed-additive POMs with pH-neutral counterions (NH4+, K+, and Na+) as HTL materials. X-ray photoelectron spectroscopy and single-crystal X-ray analyses indicated that the use of the lacunary heteropolyanion [P2W15O56]12- as an intermediate ensured successful incorporation of the counterions into the mixed-addenda POMs without causing deterioration of the POM frameworks. The hole-transporting layer performance of POM-NH4, which was characterized by a high work function and good conductivity and could be prepared using a low-cost method surpassed those of its protonated counterpart POM-4 and many classic HTL materials. An organic solar cell (OSC) modified with POM-NH4 delivered a power conversion efficiency of 18.0%, which was the highest photovoltaic efficiency achieved by POM-based OSCs to date. Moreover, an HTL material based on POM-NH4 reduced the turn-on voltage of an organic light-emitting diode from 4.2 to 3.2 V. The results of this study suggest that POMs are promising alternatives to the classic HTL materials owing to their excellent hole-collection ability, low costs, neutral nature, and high-chemical stability.

Self-Assembly of Polyoxometalate-Based Nanoparticle Surfactants in Solutions

Nanoparticle surfactants (NPSs) are an emergent class of amphiphiles attractive for their controllable assembly at the liquid-liquid interface. In this work, intriguing self-assembly behavior and stimuli-responsiveness of NPSs in homogeneous solutions are presented. With β-cyclodextrin-grafted polyoxometalates (POMs) and ferrocene (or azobenzene)-terminated polystyrene in water/tetrahydrofuran, POM-based NPSs are formed via host-guest interactions and self-organize to vesicles driven by solvent-phobic effects. The tunable supramolecular interactions allow these assemblies to be responsive to redox or light stimulus, respectively, affording an on-demand assembly/disassembly capacity that shows promise in delivery and release applications.

Computation of 31P NMR chemical shifts in Keggin-based lacunary polyoxotungstates

Density Functional Theory (DFT) calculations were employed to systematically study the accuracy of various exchange-correlation functionals in reproducing experimental 31P NMR chemical shifts, δExp(31P) for Keggin, [PW12O40]3- and corresponding lacunary clusters: [PW11O39]7-, [A-PW9O34]9-, and [B-PW9O34]9-. Initially, computed chemical shifts, δCalc(31P) were obtained with without neutralising their charge in which associated error, δError(31P), decreased as a function of Hartree-Fock (HF) exchange, attributed to constriction of the P-O tetrahedron. By comparison, δCalc(31P) performed with explicitly located counterions to render the system charge neutral, reduced discrepancies, δError(31P) by 1-2 ppm. However, uncertainties in δCalc(31P) remain, particularly for [B-PW9O34]9- anions attributed to direct electrostatic interactions between the counterions and the central tetrahedron. Optimal results were achieved using the PBE/TZP//PBE0/TZP method, achieving a mean absolute error (MAE) and a mean squared error (MSE) of 4.03 ppm. Our results emphasize that understanding the nature of the electrolyte and solvent environment is essential to obtaining reasonable agreement between theoretical and experimental results.

Aryl selenonium vs. aryl sulfonium counterions in polyoxometalate chemistry: the impact of Se+ cationic centers on the photocatalytic reduction of dichromate

A selenonium organic counter ion has been used in polyoxometalate chemistry to develop a new aryl selenonium polyoxometalate (POM) hybrid, and its photocatalytic properties have been explored in comparison with an aryl sulfonium POM-hybrid counterpart for the first time. The chalcogenonium counterions, namely, methyldiphenylsulfonium trifluoromethane sulfonate (MDPST) and methyldiphenylselenonium trifluoromethane sulfonate (MDPSeT), and their octamolybdate ([Mo8O26]4-) hybrids, 1 and 2, with the general formula (C13H13X)4[Mo8O26] (where X = S for 1 and Se for 2) were synthesized and characterized. Hybrids 1 and 2 vary in their chalcogenonium cationic center (S+vs. Se+), which enabled a direct comparison of their photocatalytic properties as a function of the cationic center. The photocatalytic activities of hybrids 1 and 2 were tested using the reduction of dichromate (Cr2O72-) as a model reaction under UV irradiation. A 99% photocatalytic reduction of Cr2O72- with a rate constant of 0.0305 min-1 was achieved with hybrid 2, while only a 67% reduction with a rate constant of 0.0062 min-1 was observed with hybrid 1 in 180 minutes. The better catalytic performance of hybrid 2 may be correlated to the larger atomic radii of Se than S, which helps in better stabilizing the photogenerated electron-hole (e--h+) pair on the POM cluster by polarizing its lone pair more efficiently compared to S. The catalytic recyclability was tested for up to 4 cycles using hybrid 2, and up to 98% reduction was obtained even after the 4th cycle. Recyclability tests and control experiments also indicated the generation of some elemental Se through possible cleavage of some C-Se bonds of MDPSe under prolonged UV exposure during catalysis, and the Se thus generated was found to contribute to the catalytic reduction of dichromate. This study, therefore, opens new avenues for aryl selenonium moieties and their POM hybrids for potential catalytic applications.

Spatiotemporal Studies of Soluble Inorganic Nanostructures with X-rays and Neutrons

This Review addresses the use of X-ray and neutron scattering as well as X-ray absorption to describe how inorganic nanostructured materials assemble, evolve, and function in solution. We first provide an overview of techniques and instrumentation (both large user facilities and benchtop). We review recent studies of soluble inorganic nanostructure assembly, covering the disciplines of materials synthesis, processes in nature, nuclear materials, and the widely applicable fundamental processes of hydrophobic interactions and ion pairing. Reviewed studies cover size regimes and length scales ranging from sub-Ångström (coordination chemistry and ion pairing) to several nanometers (molecular clusters, i.e. polyoxometalates, polyoxocations, and metal-organic polyhedra), to the mesoscale (supramolecular assembly processes). Reviewed studies predominantly exploit 1) SAXS/WAXS/SANS (small- and wide-angle X-ray or neutron scattering), 2) PDF (pair-distribution function analysis of X-ray total scattering), and 3) XANES and EXAFS (X-ray absorption near-edge structure and extended X-ray absorption fine structure, respectively). While the scattering techniques provide structural information, X-ray absorption yields the oxidation state in addition to the local coordination. Our goal for this Review is to provide information and inspiration for the inorganic/materials science communities that may benefit from elucidating the role of solution speciation in natural and synthetic processes.