Polyoxometalate Research Articles

Capped Polyoxometalate-Based Metal-Organic Complex with Mixed-Valence CuI/CuII for Synergistic Catalytic Synthesis of p-Benzoquinone.

The design and synthesis of efficient and environmentally friendly heterogeneous catalysts for the oxidation of phenols to benzoquinones are of great significance. Considering the highly catalytic activity of mixed-valence metals and polyoxometalates (POMs), a three-dimensional polyoxometalate-based metal-organic complex with mixed-valence CuI/CuII, [Cu2ICu2II(pyca)4(OH)3(PMo12O40Cu2II)(H2O)4]·3.5H2O (1, Hpyca = 2-pyrazinecarboxylic acid) has been successfully synthesized under solvothermal conditions. Surprisingly, complex 1 not only combines the desired mixed-valent CuI/CuII and POM sites but also contains a rare double Cu-capped POM structure. Due to the ternary synergistic effect of CuI, CuII, and POM, complex 1 exhibits excellent catalytic activity in the oxidation reaction of 2,3,6-trimethylphenol to the corresponding p-benzoquinone. The conversion and selectivity can reach 98 and 99% within 5 min, respectively, and the turnover frequency (TOF) value is as high as 8167 h-1, which is superior to most catalysts.

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.

Boosting cesium retention efficiency with polyoxometalate-ionic liquid composites: Insights into {WO6} octahedral affinity

Commonly used polyoxometalate (POM) adsorbents exhibit high cesium (Cs) adsorption capacity. However, its impotent ability to sequester adsorbates poses a risk of Cs migration in radioactive waste decontamination and disposal. By leveraging the cesium affinity of POMs and the regulatory properties of ionic liquids (ILs), POM-IL composites hold promise for mitigating the risk of cesium migration through efficient adsorption and retention. Herein, we designed two POM-ILs composed of the phosphotungstate anion (PW) and tetrabutylammonium cation (TBA), i.e., the {WO6}-saturated TBA-PW12 and mono-lacunary TBA-PW11, for the capture and retention of aqueous Cs. The composites were then characterized, and the performance and mechanism of Cs uptake were investigated. TBA-PW12 exhibited ∼300 nm aggregate of particles around 20–60 nm. TBA-PW12 retained the Keggin structural features of PW12O403−, which was proven essential for the binding of Cs+. With a retention efficiency of 97.3% in an NH4Cl eluent and a maximum adsorption capacity of 400.9 mg/g at 338 K, TBA-PW12 exhibited superior Cs capture and retention compared to the POM-IL derived from common Cs adsorbent phosphomolybdate. The Cs adsorption onto TBA-PWs achieved equilibrium within 180 min. Cs uptake by TBA-PWs was monolayer chemisorption of Cs+ accompanied by H+ release, and the removal efficiencies stabilized around initial pH 5–10. Due to intense affinity between {WO6} and Cs+, TBA-PW12 efficiently sequestered Cs + through {WO6} octahedra so that the {WO6}-saturated TBA-PW12 presented better Cs uptake performance than the mono-lacunary TBA-PW11. The IL anion [N(C4H9)4]+ provided a composite-water biphasic interface for aqueous Cs adsorption, while retaining the ability of POM cation PW12O403− to form a stable structure with Cs via its affinity to {WO6} octahedra. These findings support the design of POM-IL composites as efficient adsorbents for capturing and retaining Cs, aiding in the decontamination and safe disposal of radioactive waste, thereby minimizing the risk of nuclide leakage into the environment.

Co4-polyoxometalates in aqueous solutions. Stability, ion pairing and electron transfer: Some insights from molecular modeling

Some qualitatively important features of the electrochemical behavior of Co(II)-based polyoxometalates (POM) are addressed by means of density functional theory and classical molecular dynamics. According to the results obtained, a subsequent oxidation of four Co(II) centers does not result in а chemical bond break, although the innersphere reorganization energy is significant. Standard electrode potentials are estimated; only the first Co(II)/Co(III) redox process seems to be feasible at usual electrochemical conditions and has a fast rate. Ion pairing in aqueous solution leads to the formation of Co4-POM/nNa+ associates (existing in both contact and solvent shared forms), which favours electron-transfer steps. The diffusion of water molecules and Na cations in two different solution regions is also investigated. Some equilibria describing initial steps of the Co4-POM degradation are modeled as well. It is shown that the polyoxometalate is the most stable in neutral aqueous solutions; the degradation becomes remarkably feasible in alkali media and a clear trend of decreasing the Co4-POM stability is observed in acid solutions. These findings are in qualitatively agreement with experiment.

Synergetic Enzyme-Incorporated Metal-Organic Framework and Polyoxometalate Nanozyme: Achieving Stable Tandem Catalysis for Organic Photoelectrochemical Transistor Bioanalysis.

Organic photoelectrochemical transistor (OPECT) has emerged as a promising technique for biomolecule detection, yet its operational rationale remains limited due to its short development time. This study introduces a stable tandem catalysis protocol by synergizing the enzyme-incorporated metal-organic frameworks (E-MOFs) with polyoxometalate (POM) nanozyme for sensitive OPECT bioanalysis. The zeolitic imidazolate framework-8 (ZIF-8) acts as the skeleton to protect the encapsulated glucose oxidase (GOx), allowing the stable catalytic generation of H2O2. With peroxidase-like activity, a phosphotungstic acid hydrate (PW12) is then able to utilize the H2O2 to induce the biomimetic precipitation on the photogate, ultimately resulting in the altered device characteristics for quantitative detection. This work reveals the potential and versatility of an engineered enzymatic system as a key enabler to achieve novel OPECT bioanalysis, which is believed to offer a feasible framework to explore new operational rationale in optoelectronic and bioelectronic detection.

Trivacant Keggin-type polyoxometalate/TiO2 composite for enhanced sonocatalytic degradation of AR88 from aqueous solutions

Polyoxometalates (POMs) have been limited in their widespread use in catalysis in previous studies due to their strong carrier complexation and limited solubility. To overcome this limitation, in this study, Z-type heterojunction composites constructed with trivacant Keggin-type POMs K9SbW9O33·19.5H2O and TiO2 were synthesized by the sol–gel method and used for the sonocatalytic removal of the organic pollutant AR88. The results showed that, compared with pure K9SbW9O33·19.5H2O, the optimum K9SbW9O33·19.5H2O/TiO2 showed a significantly higher sonocatalytic degradation efficiency of 87.2 % for AR88, and the mineralization rate reached 51.8 %. Parameters such as dye pH and sonication power were optimized to determine the best conditions for sonocatalysis. In addition, the K9SbW9O33·19.5H2O/TiO2 Z-type heterojunction composites showed good stability and reusability. The excellent catalytic performance is attributed to the effective separation of charges and holes by the Z-type heterojunction, which enables them to produce active substances for AR88 decomposition. This study demonstrates the high potential of POMs for the application of sonocatalysis in the treatment of organic pollutants.

Fabrication of a Multi-Functional Separator Incorporating Crown ether- Polyoxometalate Supramolecular Compound for Lithium-Sulfur Batteries.

Recently, research on polyoxometalates (POMs) has gained significant momentum. Owing to their properties as electronic sponges, POMs catalyst harbor substantial potential in lithium-sulfur battery research. However, POMs undergo a transformation into reduced heteropoly blue (HPB) during electrochemical reactions, which then dissolve into the electrolyte, resulting in catalyst loss. In this research, we amalgamated 18-crown-6 (CR6) with K3PW12O40, (KPW), synthesized a novel POM-based supramolecular compound, and integrated it with graphene oxide (GO) to fabricate a multi-functional composite polypropylene (PP) separator KPW-CR6/GO/PP. The crown ether array was employed to immobilize POM and construct ion transport channels, thereby enhancing the Li+ migration rate and capturing polysulfides. Subsequently, leveraging the stable structure and redox properties of POM, the polysulfide is catalyzed to transform and inhibit the shuttle effect, thereby protecting the Li anode. The lithium-sulfur batteries with the Crown ether-POM supramolecular compound separators, exhibit enhanced capacity and stability (1073.3 mAh g-1 at 1.0 C, and 81.5 % retention rate after 250 cycles). The battery (S loading: 3.2 mg cm-2) presents an initial specific discharge capacity of 543.4 mAh g-1 at 0.5 C, with 89.8 % of the capacity retained after 160 cycles. This underlines the practical application potential of Crown ether-POM supramolecular materials in Li-S batteries.

Polyoxometalate-embedded copper-organic network as dual-site synergetic catalyst for the selective oxidation of benzylic C−H bond

Selective activation and oxidization of inert C−H bonds into value-added chemicals affords attractively economic and ecological benefits, as well as central challenge in modern synthetic chemistry. Herein, by integrating the redox polyoxometalate (POM) centers and multinuclear copper sites in one system, four compounds were constructed with formulas of [Cu2(μ2-Cl)(H2O)2L3][PM12O40]·4H2O (M = Mo for 1, W for 2), and [Cu4(μ2-OH)4L2] [H3PM11CuO40]·12H2O (M = Mo for 3, W for 4, L = 1,4-di[4H-1,2,4-triazol-4-yl]-benzene). Both compounds 1–2 exhibited 3-D POM-embedded supramolecular networks constructed by discrete [PM12O40]3− clusters and binuclear {Cu2(μ2-Cl)(N–N)2} unit-based metal-organic sheets. While compounds 3–4 displayed 3-D POM-supported supramolecular networks composed of monosubstituted Keggin-type anions [H3PM11CuO40]4− and {Cu2(μ2-OH)(N–N)2} chain-based metal-organic layers. Four crystalline structures with distinct dual-active site distribution showed significant peroxidase-like activities owing to the collaborative catalysis of multinuclear copper units and polyanionic clusters, which can smoothly catalyze the C−H bond oxidation of diphenylmethane to benzophenone with ca. 97 % conversion and >98 % selectivity within 24 h. Notably, the monosubstituted [H3PM11CuO40]4−-based compounds 3−4 showed higher catalytic activities than [PM12O40]3−-based compounds 1−2 (88 % conversion and >97 % selectivity within 12 h). This kind of crystalline structures also showed good recoverability. The synergetic reaction mechanism was also explored. This work provides a new strategy to design robust heterogeneous POM-based synergetic catalysts for C−H bond functionalization.

Polyoxometalates Ameliorate Metabolic Dysfunction-Associated Steatotic Liver Disease by Activating the AMPK Signaling Pathway.

Metabolic dysfunction-associated steatotic liver disease (MASLD), the most prevalent chronic liver disorder, has garnered increasing attention globally owing to its associated health complications. However, the lack of available therapeutic medications and inadequate management of complications in metabolic dysfunction-associated steatohepatitis (MASH) present significant challenges. There are little studies evaluating the effectiveness of POM in treating MASLD. In this study, we synthesized polyoxometalates (POM) for potential treatment of MASLD. We induced liver disease in mice using two approaches: feeding a high-fat diet (HFD) to establish MASLD or feeding a methionine-choline deficient (MCD) diet to induce hepatic lipotoxicity and MASH. Various metabolic parameters were detected, and biochemical and histological evaluations were conducted on MASLD. Western blotting, qRT-PCR and immunofluorescence assays were used to elucidate the molecular mechanism of POM in the treatment of MASLD. POM therapy resulted in significant improvements in weight gain, dyslipidemia, liver injury, and hepatic steatosis in mice fed a HFD. Notably, in a more severe dietary-induced MASH model with MCD diet, POM significantly attenuated hepatic lipid accumulation, inflammation, and fibrosis. POM treatment effectively attenuated palmitic acid and oleic acid-induced lipid accumulation in HepG2 and Huh7 cells by targeting the AMPK pathway to regulate lipid metabolism, which was confirmed by AMPK inhibitor. Additionally, the activation of AMPK signaling by POM suppressed the expression of lipid synthesis genes, including sterol regulatory element-binding protein 1c (SREBP1c) and SREBP2, while concurrently upregulating the expression of sirtuin 1 (SIRT1) to promote fatty acid oxidation. These findings suggest that POM is a promising therapeutic strategy with high efficacy in multiple MASLD models.

Electron-withdrawing effect of polyoxometalates in Cu(I)-based metal–organic frameworks for enhanced azide-alkyne “click” reaction

Boosting the nucleophilicity of Cu(I) sites is an essential strategy to enhance the efficiency of Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. In this work, a Lindquist-type polyoxometalate (POM)-based metal–organic framework, [CuI4(W6O19)2(L)]·2H2O (NEAU-1), was synthesized via an in-situ solvothermal method. Single-crystal X-ray diffraction results reveal that NEAU-1 exhibits a sandwich structure, with POMs intercalated between the two-dimensional layers formed by resorcin[4]arene ligands and Cu(I) ions. NEAU-1 possesses abundant Cu(I) active sites and high chemical stability, making it an effective heterogeneous catalyst for the CuAAC reaction. More importantly, the presence of POMs effectively reduces the electron cloud density around Cu(I) sites, significantly lowering the energy barrier for the formation of copper-acetylide compounds and facilitating subsequent nucleophilic reactions. The synergistic catalytic effect of POMs and Cu(I) can achieve a conversion rate of over 99 % for benzyl azide and phenylacetylene within 40 min. This work presents a sustainable molecular-level strategy to enhance the activity of the CuAAC reaction.

Two novel 3D polyoxometalate-based metal–organic frameworks for structure-directed selective adsorption and photodegradation of organic dyes

Degradation of organic dyes from industrial wastewater is crucial for human health and ecological balance. Synergy adsorption and photodegradation is one of the effective methods to degrade organic dyes at present. The former is usually limited by the challenge of its the recovery and the poor adsorption capacity of the adsorbent, whereas the latter frequently suffers from sluggish reaction kinetics. Herein, the introduction of the unique electron-rich Keggin polyoxometalates (CuW12 and SiW12) into electron-deficient metal photosensitive viologen framework to construct two polyoxometalate(POM)-based metal organic frameworks (POMOFs) of 3D porous layered structures, namely [Cu2(ipbp)2(H2O)2][CuW12O40]·3H2O (1) and [Co2(H2ipbp)3(H2O)2(CoO6)2][SiW12O40]·H2O (2); (H2ipbp·Cl = 1-(3,5-dicarboxyphenyl)-4,4′-bipyridinium), which enable selective adsorption of methylene blue (MB) and effectively photocatalytic degradation cationic dyes. In addition, POMs and metal-organic frameworks act in synergy to result in much improved adsorption of certain cationic dyes as well as enhanced photocatalytic activity that allow the catalyst to show better excellent reusability and stability. The adsorption degradation rate of MB and rhodamine B (RhB) of 2 is faster than that of 1 because of the lower Zeta potential of 2 suggesting that there are electrostatic interactions in the adsorption process. This work adopted a novel view to comprehening the technology.

Supercapacitance and Photocatalytic Properties of Keggin Polyoxomolybdated-based 3D Supramolecular Compounds

Constructing efficient electrode materials and photocatalysts to tackle energy scarcity and water pollution is still extremely challenging. Here, four isomorphic Keggin compounds, namely {[M(Tetp)6][H3XMo12O40]2}·6H2O (M=Co/Ni, X=Si/Ge, Co-SiMo12, Ni-SiMo12, Co-GeMo12, Ni-GeMo12, Tetp = 4-[4-(2-Thiophen-2-yl-ethyl)-4H-[1,2,4]triazole-3-yl]-pyridine) were synthesized and directly used as electrode materials for supercapacitors and photocatalysts to reduce Cr (VI). In particular, the combination of polyoxometalates (POMs) that have abundant redox active sites with transition metal-organic compounds (TMCs) improves the internal ion/electron transport efficiency and photocatalytic activity. Specifically, Co-SiMo12 exhibits a higher specific capacitance value (588.46 F g−1 at 0.5 A g−1) and the photocatalytic reduction efficiency (over 90%) of Cr (VI) in 25 min. This work provides a feasible scheme for supercapacitor energy storage and photocatalytic degradation of water pollutants.

Polymetallic MnW/Co3O4 porous composites derived from polyoxometalate-induced “cage-within-cage” zeolitic imidazolate frameworks with high efficiency towards NOx reduction

Exploitation of highly active, economical and environmentally friendly catalysts is the primary imperative for addressing the pivotal concerns confronting NOx pollution. Transition metal oxides have shown good application prospects in NOx purification due to their plentiful availability and affordable pricing. Herein, a confinement-pyrolysis strategy for preparation of MnW/Co3O4 composites with porous structures and high dispersion of active components transformed from polyoxometalate (POM)-encapsulated zeolitic imidazolate frameworks (ZIFs) was showcased. The strong interaction of Mn and Co contributes to the production of rich Mn3+ and Co3+ content (Mn4+ + Co2+ → Mn3+ + Co3+; Mn2+ + Co3+ → Mn3+ + Co2+). Compared with W/Co3O4, the Mn addition can increase the accessibility of active sites, induce more oxygen vacancies, acid sites and defects formation at the interfacial regions, promote the adsorption and activation of NH3 and NO, resulting in a 10–50 % higher NOx conversion at the 100−300 °C under 40,000 h−1. More encouragingly, experimental results suggested that the Mn-containing catalyst also exhibits much better tolerance against SO2, H2O, and HC than the Mn-free catalyst. This work demonstrates that guest@host POM@ZIF-functionalized derivative materials possess promising application perspective for low-temperature NOx reduction.

Pore design of POM@MOF hybrids for enhanced methylene blue capture

Abstract Adsorption of methylene blue from aqueous effluents is a key technique to treat wastewater discharge from chemical industries. Toward methylene blue adsorption, porous metal–organic frameworks have been promising owing to their high surface areas and tunable porosity. The incorporation of polyoxometalates (POMs) into metal–organic frameworks has been found to facilitate methylene blue adsorption due to electrostatic interactions between the anionic polyoxometalate and the cationic methylene blue. However, it remains unclear how the customizable pore of POM@MOF hybrids affects the capture of methylene blue. In this work, we study the effect of the size and environment of metal–organic frameworks on the property of methylene blue capture for [Zr6O4(OH)4(1,4-benzenedicarboxylate)6] (UiO-66) embedding an α-Keggin-type polyoxometalate [α-SiW12O40]4− (SiW12). Tuning pore size and environment based on ligand engineering of the metal–organic frameworks suggests that (i) lengthening the organic linkers and (ii) functionalization of the linker with −NO2 groups could enhance the methylene blue uptakes. Notably, an increase in the loading amounts of SiW12 on UiO-66 functionalized with −NO2 groups led to outstanding methylene blue capture, comparable to that of representative zeolites such as ZSM-5 and zeolite Y.

H2O2 Self‐Supplying CaO2/POM@MOF Bimodal Nanogeneration Materials for Photothermal and Chemodynamic Synergistic Antimicrobials

ABSTRACTThe emergence of bacterial resistance has a negative impact on the conventional antimicrobial treatments, and research into the development of the new antimicrobial materials is of great significance. Multimodal synergistic antimicrobial materials exert better antimicrobial effects compared with a single modality. In recent years, Polyoxometalate (POM) has shown great potential in the biomedical field due to its high catalytic activity and high photothermal conversion ability. However, owing to its small surface area, its applications have been greatly limited. Herein, we designed a H2O2 self‐supplied CDT and PTT bimodal nanogeneration material (i.e., CaO2/POM@MOF) possessing a larger surface area for the treatment of the bacterial infections, in which CaO2 could release O2 and H2O2 in the weakly acidic microenvironment of bacterial infection and the dual catalytic site of POM@MOF could enhance the CDT reaction to generate ROS, resulting in the bacterial oxidative stress and the leakage of the bacterial contents, and the exposure to the NIR light generates localized high temperature that cause the rupture of bacterial cell membranes and the denaturation of their proteins. Meanwhile, the superior stability of POM@MOF can release fewer metal ions to improve its biocompatibility. The in vitro antimicrobial experiments demonstrated that CaO2/POM@MOF possessed the combined CDT and PTT effect and exhibited excellent antimicrobial efficacy. This work presents a promising strategy to combat the bacterial infections.

Environment-friendly heterogeneous poly(ionic liquid)s immobilized polyoxometalate catalysts towards the efficient oxidation of alkenes

A novel quaternization reagent of (1E,1′E)-1,1′-(1,4-phenylene)bis(N-(3-(1H-imidazol-1-yl)propyl)methanimine) (PP) was synthesized and used for the preparation of a series of poly(ionic liquid)s (PILs) via the quaternization reaction. These PILs were subsequently loaded with polyoxometalates (POMs) to create innovative catalysts for the oxidation of alkenes. Comprehensive structural analysis of the catalysts showed that these PILs exhibited a high loading amount ratio of up to 542 % for POMs, which were well-dispersed on the surface of PILs. The PILs loaded POMs catalysts, especially for PW-PIL-TB-2PP with H3PW12O40 as the POMs, displayed remarkable activity and stability in oxidizing cyclooctene, achieving a conversion rate of 97 % and a selectivity of 99 % for 1,2-epoxycyclooctane over six consecutive runs without notable loss. Furthermore, the PW-PIL-TB-2PP catalyst could also be used for the oxidation of a wide range of cyclic, linear, and aromatic alkenes.

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.

Polyoxometalate-induced hydrogel with in situ nano/bioenzyme cascade reaction generating intensive and long-lasting glow-type chemiluminescence

In this study, we introduce a straightforward one-step, in-situ strategy for the generation of long-lasting and intensive glow-type chemiluminescence (CL) for glucose sensing in biological samples. The approach involves the utilization of polyoxometalate (POM)-induced alginate hydrogel, which effectively encapsulates both the catalysts (POM and glucose oxidase (GOx)) and the CL reagent (luminol) in solution. Leveraging the peroxidase-like nanozyme properties of POM and the bioenzyme GOx, embedded within the nano-sized pores of the hydrogel, we achieve intensive and persistent CL emission lasting over 16 h. The POM-hydrogel-based CL system demonstrates remarkable storage stability and reproducibility, and it can be applied to quantitatively determine serum glucose with high accuracy. Our study indicates that the POM-hydrogel platform, supporting and confining nano/bioenzyme cascade reactions, holds great promise for the establishment of glow-type CL systems, which could be of great value in various fields of CL biosensing and imaging.

Redox kinetics of multi-electron transfer reactions of polyoxometalates (POMs) relevant to electrochemical devices

The formal equilibrium potential (E0′) and heterogeneous rate constant (k0) are characteristics of any electron-transfer reaction. The estimation of these characteristics for very fast multiple electron-transfer reactions of polyoxometalates (POMs) relevant to various applications including redox flow batteries, hydrogen storage materials, and electrocatalysis is highly challenging by the conventional electrochemical methods. Therefore, E0′and k0 (k0∼10–2 cm sec‑1) of such reactions (for e.g., that of tungstosilicic acid (SiW12) and phosphotungstic acid (PW12)) separated by small potential windows are estimated from the background current free higher harmonic response of large-amplitude AC voltammetry. For SiW12/PW12 redox species, the E0′ values of processes 1, 2, and 3 estimated from the minima of second harmonic response are -0.326/-0.127, -0.530/-0.380 and -0.691/-0.701 (V vs. Ag/AgCl), respectively. The k0 values of the first redox process (P1) of SiW12 (SiW12/ SiW12-,4.8 × 10–2 cm sec‑1) and PW12 (PW12/PW12-,5.2 × 10–2 cm sec‑1) estimated from the large-amplitude AC voltammetry are further validated through the impedance measurement (2.1 × 10–2 and 2.4 × 10–2 cm sec‑1). The uniqueness of large-amplitude AC voltammetry over conventional DC techniques and impedance spectroscopy is illustrated. The relevance of k0 values in the context of electrocatalytic processes and RFBs system is also presented.

In Situ Ligand-Transformation-Assisted Assembly of a Polyoxometalate and Silver-Phosphine Oxide Cluster for Colorimetric Detection of Phenol Contaminants.

In situ ligand transformation strategies represent an efficient pathway for constructing function-oriented polyoxometalate (POM)-based crystalline materials. Herein, three POM-based hybrid networks were synthesized through in situ transformation of the phosphine ligand, formulated as [Ag(dppeo)6][H2PMo12O40]·5H2O (1), [Ag(dedpo)]4[SiW12O40]·6H2O (2), and [Ag(dppeo)]3[PW12O40]·3H2O (3) (dedpo = (2-(diphenylphosphaneyl)ethyl)diphenylphosphine oxide; dppeo = ethane-1,2-diylbis(diphenylphosphine oxide)). During the synthesis of these compounds, the 1,2-diphenylphosphine ethane molecule underwent in situ oxidation, transforming into dppeo and dedpo ligands, respectively. Compound 1 features a supramolecular architecture assembled from [Ag(dppeo)3]+/[Ag2(dppeo)6]2+ cationic clusters with disordered Ag centers and protonated [H2PMo12O40]- anions. Compound 2 presents a 3-D POM-supported metal-organic framework consisting of binuclear [Ag(dedpo)]22+ units, {-dedpo-Ag-dedpo-} chains, and [SiW12O40]4- polyoxoanions. Compound 3 displays a 2-D layered structure formed by {-dppeo-Ag3-dppeo-} chains and [PW12O40]3- clusters. Pronounced argentophilic interactions are observed in compounds 1 and 3. The three compounds demonstrate satisfactory heterogeneous catalytic activity in the colorimetric detection reactions toward phenol pollutants with detection limits of 1.73, 1.92, and 4.6 μM, respectively. Additionally, compounds 1-3 show high anti-interference capabilities and high sensitivity in differentiating phenol from its halogenated derivatives. This work presents some guidance for designing specific function-oriented POM-based materials via an in situ ligand transformation strategy.