Studies on electron transfer reactions: reduction of heteropoly 11-tungstophosphovanadate(V) by <Emphasis Type="SmallCaps">l</Emphasis>-cysteine and thioglycolic acid in aqueous acid medium

The rates of the electron transfer reaction of l-cysteine and thioglycolic acid with the polyoxometalate, [PV^VW₁₁O₄₀]⁴⁻, have been measured spectrophotometrically in aqueous acid medium. The polyoxometalate oxidizes cysteine to cystine and thioglycolic acid to dithioglycolic acid and gets reduced to heteropoly blue, [PV^IVW₁₁O₄₀]⁵⁻. The order of the reaction with respect to oxidant is one, whereas the reaction shows second order dependence on the substrates. The rate–pH profile shows that both the unionized and ionized thiol groups of the substrates are active species involved in electron transfer. A suitable mechanism has been proposed for the title reaction based on the results of kinetic studies.     

Liquid phase benzylation of toluene using benzyl alcohol over iron promoted sulfated zirconia

Concentrated (0.2 M) aqueous solutions of HP-acids, such as H_3+x+mPV^IV _mV^V _x-mMo_12-xO₄₀ and their analogues with an excess VO²⁺ cation, are oxidized by dioxygen at 343 K and atmospheric pressure through intermediate active complexes (IAC) [H_x+m-1PV^IV _mV^V _x-mMo_12-xO₄₀ ^{4 -}] · [VO²⁺]_y · O₂, where m + y ≥ 3. The electron transfer to the coordinated O₂ molecule inside AC is the limiting stage at high m. At low m, the formation of IAC becomes the limiting stage that results in a sharp decrease in the oxidation rate. This revised version was published online in August 2006 with corrections to the Cover Date.     

Activation-controlled outer-sphere electron transfer reactions: reduction of heteropoly 11-tungstovanadophosphate and heteropoly 10-tungstodivanado phosphate by thiourea in aqueous acid medium

The kinetics of reduction of heteropoly 11-tungstovanadophosphate, [PV^VW₁₁O₄₀]⁴⁻, (HPA1) and heteropoly 10-tungstodivanadophosphate, [PV^VV^VW₁₀O₄₀]⁵⁻, (HPA2) by thiourea has been investigated in HClO₄/phthalate/acetate buffer solutions spectrophotometrically at 25 °C in aqueous medium. The stoichiometry of the reaction is 1:1 in both cases. The HPAs are converted into the corresponding one-electron reduced heteropoly blues, namely, [PV^IVW₁₁O₄₀]⁵⁻ and [PV^IVV^VW₁₀O₄₀]⁶⁻, and thiourea is oxidised to formamidine disulphide. The reaction shows first-order dependence in both [HPA] and [thiourea] at constant pH. The rate–pH profile shows the participation of both the neutral and deprotonated forms of thiourea in the reaction. The reaction proceeds through an outer sphere electron transfer mechanism in which activation-controlled electron transfer is the rate-determining step. Self-exchange rate constants for the couples [PV^VW₁₁O₄₀]⁴⁻/[PV^IVW₁₁O₄₀]⁵⁻, [PV^VV^VW₁₀O₄₀]⁵⁻/[PV^IVV^VW₁₀O₄₀]⁶⁻ and H₂NCSNH₂/H₂NCS^·+NH₂ have been evaluated by Marcus theory.     

Regioselective synthesis and characterization of monovanadium‐substituted β‐octamolybdate [VMo7O26]5−

The monovanadium‐substituted polyoxometalate anion [VMo₇O₂₆]^5?, exhibiting a β‐octamolybdate archetype structure, was selectively prepared as pentapotassium [hexaikosaoxido(heptamolybdenumvanadium)]ate hexahydrate, K₅[VMo₇O₂₆]·6H₂O ( VMo₇ ), by oxidation of a reduced vanadomolybdate solution with hydrogen peroxide in a fast one‐pot approach. X‐ray structure analysis revealed that the V atom occupies a single position in the cluster that differs from the other positions by the presence of one doubly‐bonded O atom instead of two terminal oxide ligands in all other positions. The composition and structure of VMo₇ was also confirmed by elemental analyses and IR spectroscopy. The selectivity of the synthesis was inspected by a ⁵¹V NMR investigation which showed that this species bound about 95% of V^V in the crystallization solution. Upon dissolution of VMo₇ in aqueous solution, the [VMo₇O₂₆]^5? anion is substantially decomposed, mostly into [VMo₅O₁₉]^3?, α‐[VMo₇O₂₆]^4? and [V₂Mo₄O₁₉]^4?, depending on the pH.     

A Bicapped α‐Keggin Unit‐Supported Transition Metal Complex: Hydrothermal Synthesis and Characterization of [Cu(en)2(H2O)]2[Cu(en)2]0.5[Mo8V7O42{Cu(en)2}]

A new metal‐oxo cluster supported transition metal complex, [Cu(en)₂(H₂O)]₂[Cu(en)₂]_0.5[Mo^VI₈V^IV₆V^VO₄₂{Cu(en)₂}], has been synthesized under hydrothermal conditions. Its structure was determined by single‐crystal X‐ray diffraction. The compound crystallizes in the triclinic system, space group (No. 2), a = 12.245(5), b = 12.669(5), c = 20.949(8) Å, α = 77.120(13), β = 78.107(17), γ = 65.560(14)°, V = 2860(2) ų, Z = 2. The metal‐oxo cluster contains a novel bicapped a‐Keggin structure unit and a [Cu(en)₂]²⁺ unit covalently bonded to the [Mo₈V₇O₄₂]^7? cluster.     

Diamond‐ and Graphite‐Like Octacyanometalate‐Based Polymers Induced by Metal Ions

By using cyclohexane‐1,2‐diamine (chxn), Ni(ClO₄)₂ ? 6H₂O and Na₃[Mo(CN)₈] ? 4H₂O, a 3D diamond‐like polymer {[Ni^II(chxn)₂]₂[Mo^IV(CN)₈] ? 8H₂O}_n ( 1 ) was synthesised, whereas the reaction of chxn and Cu(ClO₄)₂ ? 6H₂O with Na₃[M^V(CN)₈] ? 4H₂O (M=Mo, W) afforded two isomorphous graphite‐like complexes {[Cu^II(chxn)₂]₃[Mo^V(CN)₈]₂ ? 2H₂O}_n ( 2 ) and {[Cu^II(chxn)₂]₃[W^V(CN)₈]₂ ? 2H₂O}_n ( 3 ). When the same synthetic procedure was employed, but replacing Na₃[Mo(CN)₈] ? 4H₂O by (Bu₃NH)₃[Mo(CN)₈] ? 4H₂O (Bu₃N=tributylamine), {[Cu^II(chxn)₂Mo^IV(CN)₈][Cu^II(chxn)₂] ? 2H₂O}_n ( 4 ) was obtained. Single‐crystal X‐ray diffraction analyses showed that the framework of 4 is similar to 2 and 3 , except that a discrete [Cu(chxn)₂]²⁺ moiety in 4 possesses large channels of parallel adjacent layers. The experimental results showed that in this system, the diamond‐ or graphite‐like framework was strongly influenced by the inducement of metal ions. The magnetic properties illustrate that the diamagnetic [Mo^IV(CN)₈] bridges mediate very weak antiferromagnetic coupling between the Ni^II ions in 1 , but lead to the paramagnetic behaviour in 4 because [Mo^IV(CN)₈] weakly coordinates to the Cu^II ions. The magnetic investigations of 2 and 3 indicate the presence of ferromagnetic coupling between the Cu^II and W^V/Mo^V ions, and the more diffuse 5d orbitals lead to a stronger magnetic coupling interaction between the W^V and Cu^II ions than between the Mo^V and Cu^II ions.     

Binary Pd–Polyoxometalates and Isolation of a Ternary Pd–V–Polyoxomolybdate Active Species for Selective Aerobic Oxidation of Alcohols

Binary Pd–polyoxometalates [Pd(dpa)₂]₃[PW₁₂O₄₀]₂ ? 12 DMSO ( 2 ), [Pd(dpa)₂]₃[PMo₁₂O₄₀]₂ ? 12 DMSO ? 2 H₂O ( 3 ), and [Pd(dpa)(DMSO)₂]₂[HPMo₁₀V₂O₄₀] ? 4 DMSO ( 4 ) were synthesized by reaction of [Pd(dpa)(OAc)₂] ? 2 H₂O ( 1 ; dpa=2,2′‐dipyridylamine) with three Keggin‐type polyoxometalates and fully characterized by single‐crystal and powder XRD analyses, IR spectroscopy, and elemental analyses. The synthesis is facile and straightforward, and the complicated ligand‐modification procedure often used in the traditional charge‐transfer method can be omitted. In 2 – 4 , Pd complexes and polyoxometalate anions are coupled through electrostatic interaction. Compound 4 is more active than the other three compounds in the selective aerobic oxidation of alcohols at ambient pressure. Interestingly, during catalytic recycling of compound 4 , unprecedented ternary Pd–V–polyoxometalate [Pd(dpa)₂{VO(DMSO)₅}₂][PMo₁₂O₄₀]₂ ? 4 DMSO ( 5 ), which was captured and characterized by single‐crystal XRD, proved to be the true active species and showed high catalytic activity for the selective aerobic oxidation of aromatic alcohols (98.1–99.8 % conversion, 91.5–99.1 % selectivity). Moreover, on the basis of control experiments and EPR and UV/Vis spectra, a plausible reaction mechanism for the oxidation of alcohols catalyzed by 5 was proposed.     

Controlling the Self‐Assembly of a Mixed‐Metal Mo/V–Selenite Family of Polyoxometalates

Five mixed‐metal mixed‐valence Mo/V polyoxoanions, templated by the pyramidal SeO₃^2? heteroanion have been isolated: K₁₀[Mo^VI₁₂V^V₁₀O₅₈(SeO₃)₈]?18 H₂O ( 1 ), K₇[Mo^VI₁₁V^V₅V^IV₂O₅₂(SeO₃)]?31 H₂O ( 2 ), (NH₄)₇K₃[Mo^VI₁₁V^V₅V^IV₂O₅₂(SeO₃)(Mo^V₆V^V‐ O₂₂)]?40 H₂O ( 3 ), (NH₄)₁₉K₃[Mo^VI₂₀V^V₁₂V^IV₄O₉₉(SeO₃)₁₀]?36 H₂O ( 4 ) and [Na₃(H₂O)₅{Mo_18?xV_xO₅₂(SeO₃)} {Mo_9?yV_yO₂₄(SeO₃)₄}] ( 5 ). All five compounds were characterised by single‐crystal X‐ray structure analysis, TGA, UV/Vis and FT‐IR spectroscopy, redox titrations, and elemental and flame atomic absorption spectroscopy (FAAS) analysis. X‐ray studies revealed two novel coordination modes for the selenite anion in compounds 1 and 4 showing η,μ and μ,μ coordination motifs. Compounds 1 and 2 were characterised in solution by using high‐resolution ESI‐MS. The ESI‐MS spectra of these compounds revealed characteristic patterns showing distribution envelopes corresponding to 2? and 3? anionic charge states. Also, the isolation of these compounds shows that it may be possible to direct the self‐assembly process of the mixed‐metal systems by controlling the interplay between the cation “shrink‐wrapping” effect, the non‐conventional geometry of the selenite anion and fine adjustment of the experimental variables. Also a detailed IR spectroscopic analysis unveiled a simple way to identify the type of coordination mode of the selenite anions present in POM‐based architectures.     

Unprecedented High‐Nuclear Transition‐Metal‐Cluster‐Substituted Heteropolyoxoniobates: Synthesis by {V8} Ring Insertion into the POM Matrix and Antitumor Activities

Reactions of hexaniobate with vanadate in the presence of Ni²⁺, Zn²⁺, or Cu²⁺ have furnished three high‐nuclear vanadium cluster‐substituted heteropolyoxoniobates (HPNs): {Ni(en)₃}₅H{V^VNb₈V^IV₈O₄₄} ? 9 H₂O ( 1 ), (H₂en)Na₂[{Zn(en)₂(Hen)}{Zn(en)₂(H₂O)}₂{PNb₈V^IV₈O₄₄}] ? 11 H₂O ( 2 ), and Na{Cu(en)₂}₃{[Cu(en)₂]₂[PNb₈V^IV₈O₄₄]} ? 11 H₂O ( 3 ) (en=1,2‐diaminoethane). Their structures have been determined and characterized by single‐crystal X‐ray diffraction analysis, thermogravimetric analysis (TGA), and elemental analysis. Structural analysis has revealed that compounds 1 – 3 contain similar {V₈}‐substituted [X^VNb₈V^IV₈O₄₄]^11? (X=P, V) clusters, obtained by inserting a {V₈} ring into tetravacant HPN [XNb₈O₃₆]^27?. To the best of our knowledge, compounds 1 – 3 represent the first high‐nuclear vanadium cluster‐substituted HPNs, and compound 1 is the largest vanadoniobate cluster yet obtained in HPN chemistry. Nickel and zinc cations have been introduced into HPNs for the first time, which might promise a more diverse set of structures in this family. Antitumor studies have indicated that compounds 1 and 2 exhibit high activity against human gastric cancer SGC‐7901 cells, SC‐1680 cells, and MG‐63 cells.     

1D Polyoxometalate‐based Inorganic‐Organic Hybrid Derived from ß‐Octamolybdate — Synthesis and Crystal Structure of [CoII(2, 2′‐bipy)2]2[Mo8O26]

A polyoxometalate‐based inorganic–organic hybrid compound [Co^II(2, 2′‐bpy)₂]₂[Mo₈O₂₆] ( 1 ) was synthesized by hydrothermal methods and structurally characterized by IR spectrum, TG analysis and X‐ray diffraction. The compound crystallizes in the monoclinic system, space group P2₁/n, a = 10.0681(2), b = 16.4467(2), c = 15.7838(3) Å, β = 100.046(1)°, V = 2573.52(8) ų, Z = 2. The structure of 1 is built up from β‐[Mo₈O₂₆]^4? subunits covalently linked via [Co^II(2, 2′‐bpy)₂]²⁺ fragments into a infinite 1D {[Co^II(2, 2′‐bpy)₂]₂[Mo₈O₂₆]}_∞ polymer.     

Electrochemical and Spectroscopic Behaviour of Bis(2‐mercaptopyridine‐N‐oxide)oxovanadium(IV)

The complex [VO(MPO)₂] (MPO = deprotonated 2‐mercaptopyridine N‐oxide) was synthesized and characterized by IR spectroscopy. Its electrochemical behaviour was investigated by cyclic voltammetry in different organic solvents. The V^IV/V^V and V^IV/V^III couples could be identified. The nature of the electroactive species is strongly dependent on the solvent. The results are discussed in terms of a reaction mechanism describing the characteristics of the electron transfer processes and the involved chemical reactions, and the stability of the complex in each solvent was also determined. The electronic spectra of the investigated solutions gave additional support to the proposed mechanisms.     

A New Organic‐Inorganic Charge‐transfer Salt [C15H17N4]4[Mo8O26] — Synthesis,Properties and Crystal Structure

A new organic donor 3‐amino‐6‐dimethylamino‐2‐methyl‐phenazine was introduced to charge‐transfer complex with polyoxometalate. The complex [C₁₅H₁₇N₄]₄[Mo₈O₂₆] ( 1 ) was synthesized by hydrothermal reaction of neutral red chloride (3‐amino‐6‐dimethylamino‐2‐methyl‐phenazine hydrochloride) and (NH₄)₆[Mo₇O₂₄] · 4H₂O and was characterized by EPR, element analysis and single crystal x‐ray diffraction.     

Two organic–inorganic hybrid compounds constructed from [PMoV1MoVI7VIV2VV4O42]4− and different transition metal fragments

Two new organic–inorganic hybrid compounds, [Cu(phen)(prz)]₂[PMo^V₁Mo^VI₇V^IV₂V^V₄O₄₂]·4H₂O (1) and [Ag₂(phen)₄]₂[PMo^V₁Mo^VI₇V^IV₂V^V₄O₄₂] (2) (phen = 1,10-phenanthroline, prz = pyrazine), have been synthesized and characterized by IR, XPS, XRD, UV–vis, fluorescent spectra analyses, elemental analyses, X-ray diffraction analyses, TG analyses, and cyclic voltammetric measurements. Both compounds are formed by Keggin POM cores and transition metal fragments. Compound 1 exhibits an unprecedented 1-D chain structure constructed from [PMo^V₁Mo^VI₇V^IV₂V^V₄O₄₂]^4? and [Cu(phen)(prz)]²⁺ in the –A–B↑–C–B↓– linking mode. Compound 2 shows a supramolecular structure formed by [PMo^V₁Mo^VI₇V^IV₂V^V₄O₄₂]^4? and [Ag₂(phen)₄]²⁺.     

A Soluble Phosphorus‐Centered Keggin Polyoxoniobate with Bicapping Vanadyl Groups

A water‐soluble tetramethylammonium (TMA) salt of a novel Keggin‐type polyoxoniobate has been isolated as TMA₉[PV₂Nb₁₂O₄₂]?19H₂O ( 1 ). This species contains a central phosphorus site and two capping vanadyl sites. Previously only a single example of a phosphorus‐containing polyoxoniobate, [(PO₂)₃PNb₉O₃₄]^15?, was known, which is a lacunary Keggin ion decorated with three PO₂ units. However, that cluster was isolated as an insoluble structure consisting of chains linked by sodium counterions. In contrast, the [PV₂Nb₁₂O₄₂]^9? cluster in 1 is stable over a wide pH range, as evident by ³¹P and ⁵¹V NMR, UV/Vis spectroscopy, and ESI‐MS spectrometry. The ease of substitution of phosphate into the central tetrahedral position suggests that other oxoanions can be similarly substituted, promising a richer set of structures in this class.     

Oxidation of Hypophosphite and Phosphite by Anderson‐Type Hexamolybdocobaltate(III) Anion

The oxidation of hypophosphite and phosphite by the Anderson‐type hexamolybdocobaltate(III), [H₆Co^IIIMo₆O₂₄]^3?, anion was investigated at pH 2 and 1, respectively, in aqueous medium. The reaction is found to occur through an outer‐sphere mechanism with a prior weak complex formation between the reactants. Under the reaction conditions, the oxidant exists in the [H₅Co^IIIMo₅O₂₀]^2?, [H₆Co^IIIMo₆O₂₄]^3?, and [H₄Co^III₂Mo₁₀O₃₈]^6?(dimer) forms, and [H₅Co^IIIMo₅O₂₀]^2? is the active species. Inhibition of the reaction by the oxidant anion and added molybdate ion kinetically indicates existence of prior equilibria between various forms of the oxidant. Both hypophosphite and phosphite exists in their protonated forms. The reaction involves direct electron transfer from the phosphorus center to the anion‐generating free radicals in a rate‐determining step. The effect of ionic strength and change in the solvent polarity did not affect the rate of the reaction. A probable mechanism was proposed leading to a complicated rate law as a result of involvement of prior equilibria between various forms of the oxidant. The activation parameters were also determined and are in support of the proposed mechanism.     

Hexa‐ and Dodecanuclear Polyoxomolybdate Cyclic Compounds: Application toward the Facile Synthesis of Nanoparticles and Film Electrodeposition

Two new compounds based on O₃PCH₂PO₃^4? ligands and {Mo^V₂O₄} dimeric units have been synthesized and structurally characterized. The dodecanuclear Mo^V polyoxomolybdate species in (NH₄)₁₈[(Mo^V₂O₄)₆(OH)₆(O₃PCH₂PO₃)₆]?33 H₂O ( 1 ) is a cyclohexane‐like ring in a chair conformation with pseudo S₆ symmetry. In the solid state, the wheels align side by side, thus delimiting large rectangular voids. The hexanuclear anion in Na₈[(Mo^V₂O₄)₃(O₃PCH₂PO₃)₃(CH₃AsO₃)]? 19 H₂O ( 2 ) has a triangular framework and encapsulates a methylarsenato ligand. ³¹P NMR spectroscopic analysis revealed the stability of 2 in various aqueous media, whereas the stability of 1 depends on the nature of the cations present in solution. It has been evidenced that the transformation of 1 into 2 occurs in the presence of CH₃AsO₃^2? ions. This behavior shows that 1 can be used as a new precursor for the synthesis of Mo^V/diphosphonate systems. The two complexes were very efficient both as reductants of Pt and Pd metallic salts and as capping agents for the resulting Pt⁰ and Pd⁰ nanoparticles. The size of the obtained nanoparticles depends both on the nature of the polyoxometalate (POM; i.e., 1 or 2 ) and on the [metallic salt]/[POM] ratio. In all cases, X‐ray photoelectron spectroscopy (XPS) measurements have revealed the presence of Mo^VI species that stabilize the nanoparticles and the absence of Mo^V moieties. Diffuse‐reflectance FTIR spectra of the Pt nanoparticles show that the capping Mo^VI POMs are identical for both systems and contain the diphosphonato ligand. The colloidal solutions do not show any precipitate and the nanoparticles remain well‐dispersed for several months. The electrochemical reduction of Mo^V species was studied for 2 . Cyclic voltammetry alone and electrochemical quartz crystal microbalance coupled with cyclic voltammetry show the deposition of a film on the electrode surface during this reduction.     

The Electron Transfer Series [MoIII(bpy)3]n (n=3+, 2+, 1+, 0, 1−), and the Dinuclear Species [{MoIIICl(Mebpy)2}2(μ2‐O)]Cl2 and [{MoIV(tpy.)2}2(μ2‐MoO4)](PF6)2⋅4 MeCN

The electronic structures of the five members of the electron transfer series [Mo(bpy)₃]ⁿ (n=3+, 2+, 1+, 0, 1?) are determined through a combination of techniques: electro‐ and magnetochemistry, UV/Vis and EPR spectroscopies, and X‐ray crystallography. The mono‐ and dication are prepared and isolated as PF₆ salts for the first time. It is shown that all species contain a central Mo^III ion (4d³). The successive one‐electron reductions/oxidations within the series are all ligand‐based, involving neutral (bpy⁰), the π‐radical anion (bpy^.)^1?, and the diamagnetic dianion (bpy^2?)^2?: [Mo^III(bpy⁰)₃]³⁺ (S=3/2), [Mo^III(bpy^.)(bpy⁰)₂]²⁺ (S=1), [Mo^III(bpy^.)₂(bpy⁰)]¹⁺ (S=1/2), [Mo^III(bpy^.)₃] (S=0), and [Mo^III(bpy^.)₂(bpy^2?)]^1? (S=1/2). The previously described diamagnetic dication “[Mo^II(bpy⁰)₃](BF₄)₂” is proposed to be a diamagnetic dinuclear species [{Mo(bpy)₃}₂(μ₂‐O)](BF₄)₄. Two new polynuclear complexes are prepared and structurally characterized: [{Mo^IIICl(^Mebpy⁰)₂}₂(μ₂‐O)]Cl₂ and [{Mo^IV(tpy^.)₂}₂(μ₂‐Mo^VIO₄)](PF₆)₂?4 MeCN.     

Polyoxoanion with Octahedral Germanium(IV) Hetero Atom: Synthesis, Structure, Magnetism, EPR, Electrochemistry and XPS Studies on the Mixed-Valence 14-Vanadogermanate [GeVV 12VIV 2O40]8−

The mixed-valence 14-vanadogermanate [GeV^V ₁₂V^IV ₂O₄₀]^8? (1) has been synthesized and characterized in solution by ⁵¹V-NMR, UV–vis and electrochemistry and in the solid state by IR, magnetism, EPR, XPS and elemental analysis. Single-crystal X-ray analysis was carried out on K₂Na₆[GeV^V ₁₂V^IV ₂O₄₀]·10H₂O (KNa-1), which crystallizes in the orthorhombic system, space group Immm, with a=10.9623(3) Å, b=11.6205(3) Å, c=20.2658(5) Å, and Z=2. Polyanion 1 is composed of a central Ge^IVO₆ octahedron which is surrounded by a total of 14 VO₆ octahedra. Vanadium-51 NMR in solution results in three peaks with intensity ratio of 8:4:2 which is in complete agreement with the solid state structure. The presence of two V^IV centers was established by UV–vis, electrochemistry, magnetism, EPR, XPS and elemental analysis. Electrochemistry revealed that the two V^IV-centers in 1 are oxidized through a single well-defined step, which does not split with changes in scan rate or pH. Polyanion 1 is also an active two-electron oxidation catalyst for the coenzyme NADH at pH 8, unprecedented in polyoxometalate chemistry. Magnetic susceptibility, magnetization and EPR data on KNa-1 complement the X-ray and electrochemistry results by confirming the presence of two unpaired electrons per molecule of 1. The two V^IV ions possessing the spin are very weakly coupled, essentially acting as two well-isolated S=1/2 ions. The observed g-value of 1.977 from EPR and magnetic susceptibility measurements is consistent with literature reported value for a V^IV ion, suggesting a possible ground state of $3d_{x^{2}-y^{2}}.The mixed-valence 14-vanadogermanate [GeV^V ₁₂V^IV ₂O₄₀]⁸⁻ (1) has been synthesized and characterized in solution by ⁵¹V-NMR, UV–vis and electrochemistry and in the solid state by IR, magnetism, EPR, XPS and elemental analysis. Single-crystal X-ray analysis was carried out on K₂Na₆[GeV^V ₁₂V^IV ₂O₄₀]·10H₂O (KNa-1), which crystallizes in the orthorhombic system, space group Immm, with a=10.9623(3) ?, b=11.6205(3) ?, c=20.2658(5) ?, and Z=2. Polyanion 1 is composed of a central Ge^IVO₆ octahedron which is surrounded by a total of 14 VO₆ octahedra. Vanadium-51 NMR in solution results in three peaks with intensity ratio of 8:4:2 which is in complete agreement with the solid state structure. The presence of two V^IV centers was established by UV–vis, electrochemistry, magnetism, EPR, XPS and elemental analysis. Electrochemistry revealed that the two V^IV-centers in 1 are oxidized through a single well-defined step, which does not split with changes in scan rate or pH. Polyanion 1 is also an active two-electron oxidation catalyst for the coenzyme NADH at pH 8, unprecedented in polyoxometalate chemistry. Magnetic susceptibility, magnetization and EPR data on KNa-1 complement the X-ray and electrochemistry results by confirming the presence of two unpaired electrons per molecule of 1. The two V^IV ions possessing the spin are very weakly coupled, essentially acting as two well-isolated S=1/2 ions. The observed g-value of 1.977 from EPR and magnetic susceptibility measurements is consistent with literature reported value for a V^IV ion, suggesting a possible ground state of XPS measurements on KNa-1 also confirmed the coexistence of V^V and V^IV in 1.Dedicated to Professor Michael T. Pope on the occasion of his retirement.     

[H_2N(C_2H_4)_2NH_2]_4(H_3O)[PMo_2~VMo_6~(VI)V_4~(IV)O_(40)(V~(IV)O)_2]·H_2O: A New Highly Reduced, Bicapped Pseudo-Keggin Vanadylpolymolybdophosphate

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Regenerating Spent Solutions of Vanadium‐containing Heteropoly Acids in the Presence of Additives

Vanadium‐containing heteropoly acid solutions of Keggin H_3+xPMo_12–xV_xO₄₀ and modified H_aP_zMo_yV_xO_b types (P‐Mo‐V HPAs) are promising nanosized inorganic metal‐oxygen cluster compounds with the property of reversible oxidability (V^V ↔ V^IV). The oxidation of reduced P‐Mo‐V HPAs at a temperature of 130–170 °C and an oxygen pressure of 4 atm is a convenient method for their regeneration, but results in regeneration degree of only 75–88 %. Various materials with electron transfer or oxidative properties, such as nitrogen doped carbon nanofibers (N‐CNFs), Sibunit‐4, HNO₃, and MoO₂, were investigated as additives to facilitate and accelerate the regeneration of HPA solutions. Among the studied additives HNO₃ was found to show the best efficiency, resulting in regeneration degree of higher 95 %. Rapid and efficient regeneration of spent HPA catalysts is an important criterion for achieving high productivity and sustainability of oxidative processes on their basis.