Synthesis and Characterization of Novel Wells–Dawson-Type Mono Vanadium(V)-Substituted Tungsto-polyoxometalate Isomers: 1- and 4‑[S₂VW₁₇O₆₂]^5–

Two vanadium­(V)-substituted tungsto-polyoxometalate isomers, 1- and 4-[S₂VW₁₇O₆₂]^5–, were prepared as their tetra-alkyl ammonium salts from a W^VI–H₂SO₄–V^V reaction mixture in aqueous CH₃CN solution. X-ray crystallographic structural analysis revealed that both isomers have a Wells–Dawson-type structure with a higher occupancy of vanadium at polar sites and belt sites for 1- and 4-[S₂VW₁₇O₆₂]^5–, respectively. The isomers were also characterized by elemental analysis, infrared, Raman, UV–vis, and ⁵¹V NMR spectroscopies as well as voltammetry, and the data obtained were compared with that derived from [S₂W₁₈O₆₂]^4–. Significantly, the reversible potentials for the vanadium­(V/IV) couple for both 1- and 4-[S₂VW₁₇O₆₂]^5– in CH₃CN (0.1 M <i>n</i>-Bu₄NPF₆) are considerably more positive than the tungstate reduction process exhibited by the [S₂W₁₈O₆₂]^4– framework, implying that the presence of vanadium should be useful in catalytic reactions. The one-electron-reduced [S₂V^IVW₁₇O₆₂]^6– forms of both isomers were prepared in solution by controlled potential bulk electrolysis and characterized by voltammetry and EPR spectroscopy

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

The structure of the Keggin-type β-[PW₁₂O₄₀]^3– (PW₁₂) polyoxometalate, with <i>n</i>-Bu₄N⁺ as the countercation, has been determined for the first time by single-crystal X-ray analysis and compared to data obtained from a new determination of the structure of the α-PW₁₂ isomer, having the same countercation. Analysis of cyclic voltammograms obtained in CH₃CN (0.1 M [<i>n</i>-Bu₄N]­[PF₆]) reveals that the reversible potential for the β-PW₁₂ isomer always remains ca. 100 mV more positive than that of the α-PW₁₂ isomer on addition of the acid CF₃SO₃H. Simulations of the cyclic voltammetry as a function of acid concentration over the range 0–5 mM mimic experimental data exceptionally well. These simulation–experiment comparisons provide access to reversible potentials and acidity constants associated with α and β fully oxidized and one- and two-electron reduced systems and also explain how the two well-resolved one-electron W­(VI)/W­(V) processes converge into a single two-electron process if sufficient acid is present. ¹⁸³W NMR spectra of the oxidized forms of the PW₁₂ isomers are acid dependent and in the case of β-PW₁₂ imply that the bridging oxygens between the W_I and W_II units are preferentially protonated in acidic media. EPR data on frozen solutions of one-electron reduced β-[PW^VW^VI₁₁O₄₀]^4– indicate that either the W_I or the W_III unit in β-PW₁₂ is reduced in the β-[PW^VI₁₂O₄₀]^3–/β-[PW^VW^VI₁₁O₄₀]^4– process. In the absence of acid, reversible potentials obtained from the α- and β-isomers of PW₁₂ and [SiW₁₂O₄₀]^4– exhibit a linear relationship with solvent properties such as Lewis acidity, acceptor number, and polarity index

Synthesis and Characterization of Novel Wells–Dawson-Type Mono Vanadium(V)-Substituted Tungsto-polyoxometalate Isomers: 1- and 4‑[S₂VW₁₇O₆₂]^5–

Two vanadium­(V)-substituted tungsto-polyoxometalate isomers, 1- and 4-[S₂VW₁₇O₆₂]^5–, were prepared as their tetra-alkyl ammonium salts from a W^VI–H₂SO₄–V^V reaction mixture in aqueous CH₃CN solution. X-ray crystallographic structural analysis revealed that both isomers have a Wells–Dawson-type structure with a higher occupancy of vanadium at polar sites and belt sites for 1- and 4-[S₂VW₁₇O₆₂]^5–, respectively. The isomers were also characterized by elemental analysis, infrared, Raman, UV–vis, and ⁵¹V NMR spectroscopies as well as voltammetry, and the data obtained were compared with that derived from [S₂W₁₈O₆₂]^4–. Significantly, the reversible potentials for the vanadium­(V/IV) couple for both 1- and 4-[S₂VW₁₇O₆₂]^5– in CH₃CN (0.1 M <i>n</i>-Bu₄NPF₆) are considerably more positive than the tungstate reduction process exhibited by the [S₂W₁₈O₆₂]^4– framework, implying that the presence of vanadium should be useful in catalytic reactions. The one-electron-reduced [S₂V^IVW₁₇O₆₂]^6– forms of both isomers were prepared in solution by controlled potential bulk electrolysis and characterized by voltammetry and EPR spectroscopy

Synthesis and Characterization of Novel Wells–Dawson-Type Mono Vanadium(V)-Substituted Tungsto-polyoxometalate Isomers: 1- and 4‑[S₂VW₁₇O₆₂]^5–

Two vanadium­(V)-substituted tungsto-polyoxometalate isomers, 1- and 4-[S₂VW₁₇O₆₂]^5–, were prepared as their tetra-alkyl ammonium salts from a W^VI–H₂SO₄–V^V reaction mixture in aqueous CH₃CN solution. X-ray crystallographic structural analysis revealed that both isomers have a Wells–Dawson-type structure with a higher occupancy of vanadium at polar sites and belt sites for 1- and 4-[S₂VW₁₇O₆₂]^5–, respectively. The isomers were also characterized by elemental analysis, infrared, Raman, UV–vis, and ⁵¹V NMR spectroscopies as well as voltammetry, and the data obtained were compared with that derived from [S₂W₁₈O₆₂]^4–. Significantly, the reversible potentials for the vanadium­(V/IV) couple for both 1- and 4-[S₂VW₁₇O₆₂]^5– in CH₃CN (0.1 M <i>n</i>-Bu₄NPF₆) are considerably more positive than the tungstate reduction process exhibited by the [S₂W₁₈O₆₂]^4– framework, implying that the presence of vanadium should be useful in catalytic reactions. The one-electron-reduced [S₂V^IVW₁₇O₆₂]^6– forms of both isomers were prepared in solution by controlled potential bulk electrolysis and characterized by voltammetry and EPR spectroscopy

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

The structure of the Keggin-type β-[PW₁₂O₄₀]^3– (PW₁₂) polyoxometalate, with <i>n</i>-Bu₄N⁺ as the countercation, has been determined for the first time by single-crystal X-ray analysis and compared to data obtained from a new determination of the structure of the α-PW₁₂ isomer, having the same countercation. Analysis of cyclic voltammograms obtained in CH₃CN (0.1 M [<i>n</i>-Bu₄N]­[PF₆]) reveals that the reversible potential for the β-PW₁₂ isomer always remains ca. 100 mV more positive than that of the α-PW₁₂ isomer on addition of the acid CF₃SO₃H. Simulations of the cyclic voltammetry as a function of acid concentration over the range 0–5 mM mimic experimental data exceptionally well. These simulation–experiment comparisons provide access to reversible potentials and acidity constants associated with α and β fully oxidized and one- and two-electron reduced systems and also explain how the two well-resolved one-electron W­(VI)/W­(V) processes converge into a single two-electron process if sufficient acid is present. ¹⁸³W NMR spectra of the oxidized forms of the PW₁₂ isomers are acid dependent and in the case of β-PW₁₂ imply that the bridging oxygens between the W_I and W_II units are preferentially protonated in acidic media. EPR data on frozen solutions of one-electron reduced β-[PW^VW^VI₁₁O₄₀]^4– indicate that either the W_I or the W_III unit in β-PW₁₂ is reduced in the β-[PW^VI₁₂O₄₀]^3–/β-[PW^VW^VI₁₁O₄₀]^4– process. In the absence of acid, reversible potentials obtained from the α- and β-isomers of PW₁₂ and [SiW₁₂O₄₀]^4– exhibit a linear relationship with solvent properties such as Lewis acidity, acceptor number, and polarity index

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

The structure of the Keggin-type β-[PW₁₂O₄₀]^3– (PW₁₂) polyoxometalate, with <i>n</i>-Bu₄N⁺ as the countercation, has been determined for the first time by single-crystal X-ray analysis and compared to data obtained from a new determination of the structure of the α-PW₁₂ isomer, having the same countercation. Analysis of cyclic voltammograms obtained in CH₃CN (0.1 M [<i>n</i>-Bu₄N]­[PF₆]) reveals that the reversible potential for the β-PW₁₂ isomer always remains ca. 100 mV more positive than that of the α-PW₁₂ isomer on addition of the acid CF₃SO₃H. Simulations of the cyclic voltammetry as a function of acid concentration over the range 0–5 mM mimic experimental data exceptionally well. These simulation–experiment comparisons provide access to reversible potentials and acidity constants associated with α and β fully oxidized and one- and two-electron reduced systems and also explain how the two well-resolved one-electron W­(VI)/W­(V) processes converge into a single two-electron process if sufficient acid is present. ¹⁸³W NMR spectra of the oxidized forms of the PW₁₂ isomers are acid dependent and in the case of β-PW₁₂ imply that the bridging oxygens between the W_I and W_II units are preferentially protonated in acidic media. EPR data on frozen solutions of one-electron reduced β-[PW^VW^VI₁₁O₄₀]^4– indicate that either the W_I or the W_III unit in β-PW₁₂ is reduced in the β-[PW^VI₁₂O₄₀]^3–/β-[PW^VW^VI₁₁O₄₀]^4– process. In the absence of acid, reversible potentials obtained from the α- and β-isomers of PW₁₂ and [SiW₁₂O₄₀]^4– exhibit a linear relationship with solvent properties such as Lewis acidity, acceptor number, and polarity index

Redox Levels of a <i>closo</i>-Osmaborane: A Density Functional Theory, Electron Paramagnetic Resonance and Electrochemical Study

A <i>closo</i>-type 11-vertex osmaborane [1-(η⁶-pcym)-1-OsB₁₀H₁₀] (pcym = <i>para</i>-cymene) has been synthesized and characterized by single-crystal X-ray diffraction and elemental analysis, as well as by ¹¹B and ¹H NMR, UV–visible, and mass spectrometry. The redox chemistry has been probed by dc and Fourier transformed ac voltammetry and bulk reductive electrolysis in CH₃CN (0.10 M (<i>n</i>-Bu)₄NPF₆) and by voltammetry in the ionic liquid <i>N</i>-butyl-<i>N-</i>methylpyrrolidinium bis­(trifluoromethylsulfonyl)­amide (Pyrr_1,4-NTf₂), which allows the oxidative chemistry of the osmaborane to be studied. A single-crystal X-ray diffraction analysis has shown that [1-(η⁶-pcym)-1-OsB₁₀H₁₀] is isostructural with other metallaborane compounds of this type. In CH₃CN (0.10 M (<i>n</i>-Bu)₄NPF₆), [1-(η⁶-pcym)-1-OsB₁₀H₁₀] undergoes two well-resolved one-electron reduction processes with reversible potentials separated by ca. 0.63–0.64 V. Analysis based on a comparison of experimental and simulated ac voltammetric data shows that the heterogeneous electron transfer rate constant (<i>k</i>⁰) for the first reduction process is larger than that for the second step at GC, Pt, and Au electrodes. <i>k</i>⁰ values for both processes are also larger at GC than metal electrodes and depend on the electrode pretreatment, implying that reductions involve specific interaction with the electrode surface. EPR spectra derived from the product formed by one-electron reduction of [1-(η⁶-pcym)-1-OsB₁₀H₁₀] in CH₃CN (0.10 M (<i>n</i>-Bu)₄NPF₆) and electron orbital data derived from the DFT calculations are used to establish that the formal oxidation state of the metal center of the original unreduced compound is Os^II. On this basis it is concluded that the metal atom in [1-(η⁶-pcym)-1-OsB₁₀H₁₀] and related metallaboranes makes a 3-orbital 2-electron contribution to the borane cluster. Oxidation of [1-(η⁶-pcym)-1-OsB₁₀H₁₀] coupled to fast chemical transformation was observed at 1.6 V vs ferrocene^0/+ in Pyrr_1,4-NTf₂. A reaction scheme for the oxidation involving formation of [1-(η⁶-pcym)-1-OsB₁₀H₁₀]⁺, which rearranges to an unknown electroactive derivative, is proposed, and simulations of the voltammograms are provided

Studies on the Nuances of the Electrochemically Induced Room Temperature Isomerization of <i>cis</i>-Stilbene in Acetonitrile and Ionic Liquids

Electrochemical reduction of <i>cis</i>-stilbene occurs by two well-resolved one-electron reduction steps in acetonitrile with (<i>n</i>-Bu)₄NPF₆ as the supporting electrolyte and in <i>N</i>-butyl-<i>N</i>-methylpyrrolidinium (Pyrr_1,4⁺) and (trimethylamine)­(dimethylethylamine)-dihydroborate bis­(trifluoromethylsulfonyl)­amide (NTf₂^–) ionic liquids (ILs). Mechanistic details of the electroreduction have been probed by dc and Fourier transformed ac voltammetry, simulation of the voltammetry, bulk electrolysis, and EPR spectroscopy. The first one-electron reduction induces fast <i>cis</i> to <i>trans</i> isomerization in CH₃CN and ILs, most likely occurring via disproportionation of <i>cis</i>-stilbene radical anions and fast transformation of the <i>cis</i>-dianion to the <i>trans</i>-configuration. The second reduction process is chemically irreversible in CH₃CN due to protonation of the dianion but chemically reversible in highly aprotic ILs under high <i>cis</i>-stilbene concentration conditions. Increase of the (<i>n</i>-Bu)₄NPF₆ supporting electrolyte concentration (0.01–1.0 M) in CH₃CN induces substantial positive shifts in the potentials for reduction of <i>cis</i>-stilbene, consistent with strong ion pairing of the anion radical and dianion with (<i>n</i>-Bu)₄N⁺. However, protection by ion pairing against protonation of the stilbene dianions or electrochemically induced <i>cis</i>–<i>trans</i>-stilbene isomerization is not achieved. Differences in electrode kinetics and reversible potentials for <i>cis</i>-stilbene^0/•– and <i>trans</i>-stilbene^0/•– processes are less pronounced in the Pyrr_1,4–NTf₂ ionic liquid than in the molecular solvent acetonitrile

The Observation of Dianions Generated by Electrochemical Reduction of <i>trans</i>-Stilbenes in Ionic Liquids at Room Temperature

Three highly aprotic bis­(trifluoromethylsulfonyl)­amide (NTf₂^–) based ionic liquids (ILs) containing the cations trihexyl­(tetradecyl)­phosphonium (P_6,6,6,14⁺), <i>N</i>-butyl-<i>N</i>-methylpyrrolidinium (Pyrr_4,1⁺), and (trimethylamine)­(dimethylethylammine)­dihydroborate ((N₁₁₁)­(N₁₁₂)­BH₂⁺) have been examined as media for room temperature voltammetric detection of highly basic stilbene dianions electrochemically generated by the reduction of <i>trans</i>-stilbene (<i>t</i>-Stb) and its derivatives (4-methoxy-, 2-methoxy-, 4,4′-dimethyl-, and 4-chloromethyl-). Transient and steady-state data in the ILs were compared with results obtained in the molecular solvent acetonitrile. In all media examined, the <i>t</i>-Stb^0/•– process is chemically and electrochemically reversible with a heterogeneous charge transfer rate constant in CH₃CN of 1.5 cm s^–1, as determined by Fourier transformed AC voltammetry. However, further reduction to the dianion was always irreversible in this molecular but weakly acidic solvent. On the other hand, a substantial level of chemical reversibility for the reduction of <i>t</i>-Stb^•– to <i>t</i>-Stb^2– on the time scale of cyclic voltammetry is achieved when the concentration of <i>trans</i>-stilbene, [<i>t</i>-Stb], appreciably exceeds the concentration of adventitious water or other proton sources. In particular, these conditions are met when [<i>t</i>-Stb] ≥ 0.1 M in thoroughly dehydrated and purified ILs, while in the presence of CH₃CN, <i>t</i>-Stb^2– still suffers fast irreversible protonation under these stilbene concentration conditions. The <i>E</i>_0/•–⁰ values (vs Fc^0/+) for substituted <i>trans</i>-stilbenes in acetonitrile and (N₁₁₁)­(N₁₁₂)­BH₂-NTf₂ do not differ substantially, nor do the <i>E</i>_0/•–⁰ and <i>E</i>_•–/2–⁰ differences or other aspects of the voltammetric behavior

Spontaneous Redox Synthesis of the Charge Transfer Material TTF₄[SVMo₁₁O₄₀]

The charge-transfer material TTF-SV^IVMo₁₁O₄₀ (TTF = tetrathiafulvalene) was prepared by a spontaneous redox reaction between TTF and the vanadium-substituted polyoxometalate (n-Bu₄N)₃[SV^VMo₁₁O₄₀] in both solution and solid state phases. Single crystal X-ray diffraction gave the stoichiometry TTF₄[SVMo₁₁O₄₀]·2H₂O·2CH₂Cl₂, with the single V atom positionally disordered with eight Mo atoms over the whole α-Keggin polyanion [SVMo₁₁O₄₀]^4‑. Raman spectra support the 1+ charge assigned to the oxidized TTF deduced from bond lengths, and elemental and voltammetric analysis also are consistent with this formulation. Scanning electron microscopy images showed a rod-type morphology for the new charge-transfer material. The conductivity of the solid at room temperature is in the semiconducting range. The TTF and (n-Bu₄N)₃[SV^VMo₁₁O₄₀] solids also undergo a rapid interfacial reaction, as is the case with TTF and TCNQ (TCNQ = tetracyanoquinodimethane) solids. EPR spectra at temperatures down to 2.6 K confirm the presence of two paramagnetic species, V­(IV) and the oxidized TTF radical. Spectral evidence shows that the TTF-SV^IVMo₁₁O₄₀ materials prepared from either solution or solid state reactions are equivalent. The newly isolated TTF-SV^IVMo₁₁O₄₀ material represents a new class of TTF-polyoxometalate compound having dual electrical and magnetic functionality derived from both the cationic and anionic components