Voltammetric and Spectroscopic Studies of α- and β-[PW12O40]3– Polyoxometalates in Neutral and Acidic Media: Structural Characterization as Their [(n-Bu4N)3][PW12O40] Salts

Factors governing the protonation of Keggin-type polyoxometalates: influence of the core structure in clusters

Atomic substitution is a promising approach for controlling structures and properties for developing clusters with desired responses. Although many possible coordination candidates could be deduced for substitution, not all can be prepared. Therefore, predicting the correlation between structures and physical properties is important prior to synthesis. In this study, regarding Keggin-type polyoxometalates (POMs) as a model cluster, the dominant factors affecting the protonation were investigated by atomic substitutions and geometry changes. The valence of Keggin-type POMs and the constituent elements of the cluster shell structure affect the charge and potential distribution, which change the protonation sites. Furthermore, the valence of Keggin-type POMs and the bond length between the core and shell structure determine the protonation energy. These factors also affect the HOMO-LUMO gap, which governs photochemical and redox reactions. These governing factors derived from actual parameters of the α-isomer of Keggin-type POMs enabled us to deduce the protonation energy of the β-isomer, which is more difficult to prepare and isolate than the α-isomer.

Noncovalent Dualism in Perylene-Diimide-Based Keggin Anion Complexes: Theoretical and Experimental studies

We report the synthesis and comprehensive characterization of organic-inorganic hybrid salts formed by bis-cationic N,N'-bis(2-(trimethylammonium)ethylene)perylene-3,4,9,10-tetracarboxylic acid bisimide (PTCD2+) and Keggin-type [XW12O40]n- (X = Si, n = 4; X = P, n = 3) polyoxometalates. (PTCD)3[PW12O40]2·3DMSO·2H2O (2) and (PTCD)2[SiW12O40]·DMSO·2H2O (3) were structurally characterized by single crystal X-ray diffraction. The cations in both structures exhibited infinite chainlike arrangements through π-π interactions, contrasting with the previously reported cation-anion stacking observed in naphthalene diimide derivatives. A detailed theoretical study employing topological analysis of the electron density distribution within the quantum theory of atoms in molecules approach provided further insights into this structural dualism. Atomic force microscopy analyses revealed the formation of self-assembled supramolecular structures on graphite from molecular monolayers (3 nm of thick) to submicrometer aggregates for 2. Hyperspectral Raman spectroscopy imaging revealed that such heterostructures are likely formed by an enhanced π-π interactions. Both complexes demonstrated interesting electrochemical behavior, photoluminescence and X-ray-induced luminescence. Electron spin resonance analysis confirmed charge separation in both compounds, with enhanced efficiency observed in compound 2. Our findings of these perylene-based organic-inorganic hybrid salts offer the potential for their application in optoelectronic devices and functional materials.

H2 Evolution at a Reduced Hybrid Polyoxometalate and Its Vanadium-Oxo Derivative Used as Molecular Models for Reducible Metal Oxides

We herein report our investigations on the use of a tris-silanol-decorated polyoxotungstate, [SbW₉O₃₃(tBuSiOH)₃]^3-, as a molecular support model to describe the coordination of an isolated vanadium atom at metal oxides, focusing on the reactivity of the reduced derivatives in the presence of protons. Accumulation of electrons and protons at an active site is a main feature associated with heterogeneous catalysts able to conduct the (oxy)dehydrogenation of alkanes or alcohols. Our results indicate that two-electron reduced derivatives release H₂ upon protonation, a reaction that probably takes place at the polyoxotungstic framework rather than at the vanadium center.

Cation effect on the electrochemical reduction of polyoxometalates in room temperature ionic liquids

Polyoxometalates (POMs) are compounds that undergo multiple successive one-electron redox transitions, making them convenient model reactants to study ion solvation effects. Room temperature ionic liquids (RTILs) are solvents made entirely of ions, and are expected to have interactions with the highly negatively charged POM reduction products. In this work, 12 RTILs with a range of different anions ([FSI]−=bis(fluorosulfonyl)imide, [TFSI]−=bis(trifluoromethylsulfonyl)imide, [BETI]−=bis(pentafluoroethylsulfonyl)imide, [BF4]−, [PF6]−) and cations (imidazolium, pyrrolidinium, sulfonium, ammonium, phosphonium) were employed as solvents to study the kinetics and thermodynamics of [S2W18O62]4− reduction, to shed light on solvation effects and ion-pairing effects caused by different RTIL structures. Up to six reversible reduction processes (producing highly negatively charged [S2W18O62]10−) were observed. For the RTILs that showed multiple processes, a clear trend in both the thermodynamics (inferred from the reduction peak potentials) and kinetics (inferred from the peak-to-peak separation) was observed, in the order: imidazolium < sulfonium ≈ ammonium < pyrrolidinium < phosphonium, supporting strong interactions of the negatively charged POM reduction products with the cation. Two related POMs, [P2W18O62]6− and [PW12O40]3−, were also studied in the optimum RTIL found for [S2W18O62]4− ([C2mim][FSI]=1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide), revealing fast kinetics and asymmetric peaks for [PW12O40]3−. This work demonstrates the importance of understanding the solvation effects of RTIL ions for highly charged electrogenerated products, allowing tuning of the RTIL structure to achieve the optimum kinetics and thermodynamics for an electrochemical process.

Variable control of the electronic states of a silver nanocluster via protonation/deprotonation of polyoxometalate ligands

The properties of metal nanoclusters depend on both their structures and electronic states. However, in contrast to the significant advances achieved in the synthesis of structurally well-defined metal nanoclusters, systematic control of their electronic states is still challenging. In particular, stimuli-responsive and reversible control of the electronic states of metal nanoclusters is attractive from the viewpoint of their practical applications. Recently, we developed a synthesis method for atomically precise Ag nanoclusters using polyoxometalates (POMs) as inorganic ligands. Herein, we exploited the acid/base nature of POMs to reversibly change the electronic states of an atomically precise {Ag27} nanocluster via protonation/deprotonation of the surrounding POM ligands. We succeeded in systematically controlling the electronic states of the {Ag27} nanocluster by adding an acid or a base (0-6 equivalents), which was accompanied by drastic changes in the ultraviolet-visible absorption spectra of the nanocluster solutions. These results demonstrate the great potential of Ag nanoclusters for unprecedented applications in various fields such as sensing, biolabeling, electronics, and catalysis.

Charge-State Dependence of Proton Uptake in Polyoxovanadate-alkoxide Clusters

Here, we present an investigation of the thermochemistry of proton uptake in acetonitrile across three charge states of a polyoxovanadate-alkoxide (POV-alkoxide) cluster, [V6O7(OMe)12]n (n = 2-, 1-, and 0). The vanadium oxide assembly studied features bridging sites saturated by methoxide ligands, isolating protonation to terminal vanadyl moieties. Exposure of [V6O7(OMe)12]n to organic acids of appropriate strength results in the protonation of a terminal V═O bond, generating the transient hydroxide-substituted POV-alkoxide cluster [V6O6(OH)(OMe)12]n+1. Evidence for this intermediate proved elusive in our initial report, but here we present the isolation of a divalent anionic cluster that features hydrogen bonding to dimethylammonium at the terminal oxo site. Degradation of the protonated species results in the formation of equimolar quantities of one-electron-oxidized and oxygen-atom-efficient complexes, [V6O7(OMe)12]n+1 and [V6O6(OMe)12]n+1. While analogous reactivity was observed across the three charge states of the cluster, a dependence on the acid strength was observed, suggesting that the oxidation state of the vanadium oxide assembly influences the basicity of the cluster surface. Spectroscopic investigations reveal sigmoidal relationships between the acid strength and cluster conversion across the redox series, allowing for determination of the proton affinity of the surface of the cluster in all three charge states. The fully reduced cluster is found to be the most basic, with higher oxidation states of the assembly possessing substantially reduced proton affinities (∼7 pKa units per electron). These results further our understanding of the site-specific reactivity of terminal M═O bonds with protons in an organic solvent, revealing design criteria for engineering functional surfaces of metal oxide materials of relevance to energy storage and conversion.

One-Pot Tandem Catalytic Epoxidation—CO2 Insertion of Monounsaturated Methyl Oleate to the Corresponding Cyclic Organic Carbonate

Conversion of unsaturated fatty acids, FAMEs or triglycerides into the corresponding cyclic organic carbonates involves two reaction steps—double-bond epoxidation and CO2 insertion into the epoxide—that are generally conducted separately. We describe an assisted-tandem catalytic protocol able to carry out carbonation of unsaturated methyl oleate in one-pot without isolating the epoxide intermediate. Methyl oleate carbonate was obtained in 99% yield and high retention of cis-configuration starting from methyl oleate using hydrogen peroxide and CO2 as green reagents, in a biphasic system and in the presence of an ammonium tungstate ionic liquid catalyst with KBr as co-catalyst.

Low-Voltage Nongassing Electroosmotic Pump and Infusion Device with Polyoxometalate-Encapsulated Carbon Nanotubes

A low-voltage nongassing electroosmotic pump was assembled by sandwiching a silica frit between two carbon paper electrodes that were dip-coated with a paste consisting of phosphomolybdic acid/phosphotungstic acid (PMA/PTA)-encapsulated multiwalled carbon nanotubes (MWCNTs) and Nafion. The PMA/PTA encapsulation was a combined effect of their thermomigration and nanocapillary action in MWCNTs. The encapsulated MWCNTs retained desirable redox and charge transfer characteristics of PMA/PTA. The stable voltammogram in 1 M H₂SO₄ solution exhibited 77% charge retention. A total of three different possible pump configurations, namely, PUMP-I = PMA//SiO₂//PMA, PUMP-II = PTA//SiO₂//PTA, and PUMP-III = PMA//SiO₂//PTA were put together. They are in the sequence of the anode, silica frit, and cathode. All pumps showed a linear dependence on the flow rate with a minimum operating voltage of 1 V, which is well below the thermodynamic potential of water splitting. PUMP-I provided an electroosmotic flux of 43.57 μLmin^-1 V^-1 cm^-2 that matched the requirement of an infusion device like an insulin pump. The device was fabricated and its applicability has been demonstrated by delivering ∼1.8 mL of water at a 10 ± 2 μLmin^-1 flow rate at 2 V constant applied voltage over a period of 3 h. Such a wearable device can be programed to deliver model insulin or pain medication drugs for chronic diseases.

Polyoxometalates in Analytical Sciences

Polyoxometalates (POMs) have been used for spectrophotometric determinations of silicon and phosphorus under acidic conditions, referred to as the molybdenum yellow method and molybdenum blue method, respectively. Many POMs are redox active and exhibit fascinating but complicated voltammetric responses. These compounds can reversibly accommodate and release many electrons without exhibiting structural changes, implying that POMs can function as excellent mediators and can be applied to sensitive determination methods based on catalytic electrochemical reactions. In addition, some rare-earth-metal-incorporated POMs exhibit fluorescence, which enables sensitive determination by the enhancement and quenching of fluorescence intensities. In this review, various analytical applications of POMs are introduced, mainly focusing on papers published after 2000, except for the molybdenum yellow method and molybdenum blue method.

Acid-triggering of light-induced charge-separation in hybrid organic/inorganic molecular photoactive dyads for harnessing solar energy

H⁺ modulated charge-transfer in photoexcited covalently linked W and Mo Keggin-bodipy conjugates is demonstrated using transient absorption spectroscopy and photoluminescence. Adding acid switches on (W) or accelerates (Mo) charge separation.

Impact of the Lithium Cation on the Voltammetry and Spectroscopy of [XVM11O40]n- (X = P, As (n = 4), S (n = 3); M = Mo, W): Influence of Charge and Addenda and Hetero Atoms

Polyoxometalates (POMs) have been proposed as electromaterials for lithium-based batteries because they provide access to multiple electron transfer reactions coupled to fast lithium ion transport processes and high stability over many redox cycles. Consequently, knowledge of reversible potentials and Li⁺ cation-POM anion interactions provides a strategic basis for their further development. In this study, detailed cyclic voltammetric studies of a series of [XV^VM₁₁O₄₀]^n- (XVM₁₁^n-) POMs (where X (heteroatom) = P (n = 4), As (n = 4), and S (n = 3) and M (addenda atom) = Mo, W) have been undertaken in CH₃CN in the presence of LiClO₄, with n-Bu₄NPF₆ also present when required to keep the ionic strength close to constant value of 0.1 M. An analysis of the data has allowed the impact of the POM charge, and addenda and hetero atoms on the reversible potentials and the interaction between Li⁺ and the oxidized XV^VM₁₁^n- and reduced XV^IVM₁₁^(n+1)- forms of the V^V/IV redox couple to be determined. The SV^V/IVM₁₁^3-/4- process is independent of the Li⁺ concentration, implying the absence of the association of this cation with either SV^VM₁₁^3- or SV^IVM₁₁^4- redox levels. However, lithium-ion association constants for both V^V and V^IV redox levels were obtained from a comparison of simulated and experimental cyclic voltammograms for the reduction of the more negatively charged XV^VM₁₁^4- (X = P, As; M = Mo, W), since the Li⁺ interaction with these more negatively charged POMs is much stronger. The interaction between Li⁺ and the oxidized, XV^VM₁₁^n-, and reduced, XV^IVM₁₁^(n+1)-, forms was also investigated by ⁵¹V NMR and EPR spectroscopy, respectively, and it was confirmed that, due to their lower charge density, SV^VM₁₁^3- and SV^IVM₁₁^4- interact significantly less strongly with the lithium ion than XV^VM₁₁^4- and XV^IVM₁₁^5- (X = P, As). The lithium-POM association constants are substantially smaller than the corresponding proton association constants reported previously, which is attributed to a smaller surface charge density. The much stronger impact of Li⁺ on the W^VI/V- and Mo^VI/V-based reductions that occur at more negative potentials than the V^V/IV process also has been qualitatively evaluated.

Amperometric sensing and photocatalytic properties under sunlight irradiation of a series of Keggin–AgI compounds through tuning single and mixed ligands

In this work, we synthesized eight compounds based on Keggin anions and Ag^I ions by introducing L^a and L^b.