Sadayuki Himeno

A voltammetric study of Keggin-type heteropolymolybdate anions

Abstract The voltammetric behavior of several Keggin-type heteropolymolybdate anions, [XMo12O40]n− (XS, P, As, V, Si, Ge; n = 2–4) at a glassy carbon (GC) electrode was investigated in acetone or acetonitrile. The one-electron reduction waves for the Keggin anions were converted into two-electron waves as a function of acid concentration. The effect of protons on the successive reduction steps was verified experimentally, and the reduction mechanism of the conversion process was elucidated.

Synthesis, characterisation and voltammetric study of a β-Keggin-type [PW12O40]3− complex

Abstract The Keggin-type β-[PW 12 O 40 ] 3− complex was prepared, and the voltammetric behavior was compared with that of the Keggin-type α-[PW 12 O 40 ] 3− complex. Both complexes underwent a four-step one-electron reduction in CH 3 CN where no protonation accompanied reduction. With the addition of CF 3 SO 3 H, the one-electron reduction waves were converted into two two-electron waves, followed by an ill-defined wave. The addition of water to the CH 3 CN+CF 3 SO 3 H system caused the first two-electron wave to split into two one-electron waves. The effect of acid or water on the voltammetric behavior is discussed also.

Charge dependence of one-electron redox potentials of Keggin-type heteropolyoxometalate anions

Abstract The two-step one-electron reduction processes of Keggin-type heteropolymolybdate and heteropolytungstate anions were investigated in several solvents using cyclic voltammetry. The slope of a plot of standard redox potentials E° against the ionic charge ΔE° was evaluated. It was found by theoretical considerations that ΔE° consists of a constant term independent of the solvent and another term comprising the charge-dependent component of the solvation energy. The difference between ΔE° for water and for 1,2-dichloroethane (1,2-DCE) was in good agreement with the value estimated from the transfer energies across the 1,2-DCE|water interface which were measured by ion transfer voltammetry. The values of ΔE° in various solvents cannot be explained by the Born-type electrostatic solvation energy, suggesting that the short-range interaction is important for the interaction of a Keggin anion with solvent molecules.

Formation of voltammetrically-active isopolyoxotungstate complexes in aqueous CH3CN media

Abstract A new route to the preparation of voltammetrically-active isopolyoxotungstate complexes was developed by the presence of CH 3 CN in aqueous W(VI) solutions. In the presence of 40% (v/v) CH 3 CN as an auxiliary solvent, the [W 6 O 19 ] 2− complex was formed in equilibrium with the [W 10 O 32 ] 4− complex, which was subsequently transformed into the previously unknown α-Keggin-type [H 4 W 12 O 40 ] 4− complex at weaker acidities. The α-[H 4 W 12 O 40 ] 4− complex was characterized by voltammetric and spectroscopic behaviors.

The structure of 18-molybdodisulphate(VI)(4–) ion in (NEt4)4S2Mo18O62·CH3CN

The structure of (NEt4)4S2Mo18,O62·CH3CN has been determined by the single-crystal X-ray method. The crystals are triclinic, space group P, with cell parameters a= 15.062(4), b= 23.296(4), c= 13.813(4)A, and α= 97.66(4), β= 115.98(2), γ= 84.63(4)°, and Z= 2. The structure model was refined by full-matrix least squares to R′= 0.063 for 1 1 700 reflections with Fo > 3σ(F). The existence of the [S2Mo18O62]4– heteropolyanion in the crystals has been confirmed. It is composed of two SO4 tetrahedra and eighteen MoO6 octahedra linked to give the Dawson α-[P2W18O62]6– structure. The anions are separated by NEt4+ cations and CH3CN solvate molecules.

Difference in voltammetric properties between the Keggin-type [XW12O40]n− and [XMo12O40]n− complexes

Abstract The voltammetric properties of the Keggin-type [XW12O40]n− complexes were compared with those of the corresponding Mo-analogues, [XMo12O40]n−, where X=P, Ge; n=3, 4. The one-electron waves for the Keggin anions are converted into two-electron waves by the presence of small cations such as H+, Li+ and Na+, and the conversion occurs much more easily for [XMo12O40]n−, as compared with [XW12O40]n− with an identical ionic charge. A comparison of the redox potentials for the V(V)/V(IV) couple of the mono-vanadium derivatives made it possible to estimate the ionic radius of [XMo12O40]n− as 5.4 A, with the ionic radius of [XW12O40]n−=5.6 A as reference. The big difference in voltammetric behaviors between the Mo- and W-complexes was interpreted in terms of the smaller ionic radius and consequently greater basicity of [XMo12O40]3/4−, as compared with [XW12O40]3/4−.

Cation effects on the voltammetric behavior of α-Keggin-type [SiMo12O40]4− and [PMo12O40]3− complexes in CH3COCH3 and CH3CN

The effect of small cations such as H+, Li+ and Na+ on the voltammetric behavior of α-Keggin-type [SiMo12O40]4− and [PMo12O40]3− complexes was investigated in CH3COCH3 and CH3CN. For the [SiMo12O40]4− complex, the presence of Li+ or Na+ caused the one-electron waves to be converted into a two-electron wave at ca. 0.3 V more positive than the first one-electron wave. In the presence of Li+ or Na+, the [PMo12O40]3− complex underwent a two-electron reduction at the same potential as the original first one-electron wave in CH3COCH3, whereas it exhibited only successive one-electron waves in CH3CN. The addition of a trace amount of H+ produced new two-electron waves at more positive potentials. These findings give a clue to the understanding of the reactivity of polyoxometalates as redox catalysts.

A voltammetric study on the formation of V(V)- and V(IV)-substituted molybdophosphate(V) complexes in aqueous solution

Abstract The formation processes of V(V)- and V(IV)-substituted molybdophosphate complexes with the Keggin structure were studied by a combined redox and ion-transfer voltammetric study. There was a big difference in the formation processes between V(V)- and V(IV)-substituted mixed complexes in aqueous solution. The Mo(VI)+P(V)+V(IV) system produced a blue mixed complex in which the peripheral Mo atoms possessed the oxidation number of VI; only a mono-substituted complex was formed even in the presence of excess of V(IV). In the Mo(VI)+P(V)+V(V) system, on the other hand, evidence was obtained for the spontaneous transformation of a mono-substituted complex into mixed complexes with higher V(V) contents.

Solvation effect of Li+ on the voltammetric properties of [PMo12O40]3− in binary solvent mixtures

Abstract The conversion process between one- and two-electron waves for [PMo 12 O 40 ] 3− was investigated in a binary mixture of acetonitrile and solvent S (S= N , N -dimethylformamide (DMF), dimethylsulfoxide (DMSO), H 2 O, propylene carbonate (PC), or 1,4-dioxane) containing 0.10 M (M=mol dm −3 ) LiClO 4 . A reversible two-electron wave was obtained in a binary mixture of acetonitrile and 1,4-dioxane. On the other hand, the first two-electron wave was converted into two one-electron waves in a mixture of acetonitrile and DMF, DMSO, H 2 O, or PC. The 7 Li-NMR studies demonstrated preferential solvation of Li + by one of the solvents with stronger Lewis basicity. The electrode process was explained on the basis of the relative permittivity ( e r ) of the mixed solvent and the relative donicity of acetonitrile and solvent S. It turned out that the number of transferred electrons at the electrode could be controlled by a proper choice of binary solvent mixtures.

Simultaneous electrophoretic determination of vanadium(V) and vanadium(IV) based on the complex formation with a Mo(VI)–P(V) reagent

A capillary electrophoretic (CE) method was developed for the sensitive determination of V(V) and V(IV). A Mo(VI)-P(V) reagent reacted with a mixture of trace amounts of V(V) and V(IV) to form the [P(V(V)Mo(11))O40]4- and [P(V(IV)Mo11)O40]5- complexes in 0.1 M monochloroacetate buffer (pH 2.2). Since the V-substituted Keggin anions possessed high molar absorptivities in the UV region and the peaks due to their migrations were well separated in the electropherogram, the pre-column complex formation reaction was applied to the simultaneous CE determination of V(V) and V(IV) with direct UV detection at 220 nm. The calibration curves were linear over two orders of magnitude with detection limits of 5 x 10(-7) M for V(V) and 2 x 10(-7) M for V(IV).