Masayo Takamoto

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.

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.

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 capillary electrophoretic determination of Sc(III) and Y(III) based on the complex-formation with a lacunary Keggin-type [PW11O39]7− complex

Trace amounts of Sc(III) and Y(III) can react with [PW(11)O(39)](7-) to form the ternary Keggin-type complexes: [P(Sc(III)W(11))O(40)](6-) and [P(Y(III)W(11))O(40)](6-) having high molar absorptivities in the UV region. Since the rate of the complex-formation was very rapid and the kinetically stable ternary anions migrated in the capillary with different electrophoretic mobilities, the complex-formation reaction was applied to the simultaneous CE determination of Sc(III) and Y(III) with direct UV detection at 250nm. For both Sc(III) and Y(III), the pre-column method provided linear calibration curves in the range of 2x10(-7) to 1x10(-5)M; the respective detection limits were 1x10(-7)M (the signal-to-noise ratio=3). The proposed method was successfully applied to the determination of Sc(III) and Y(III) in river water.

Simultaneous capillary electrophoretic determination of Sb(III) and Bi(III) based on the complex-formation with a W(VI)-P(V) reagent

A capillary electrophoretic method was developed for the simultaneous determination of Sb(III) and Bi(III). A 1.0 mM W(VI)-0.10 mM P(V) complexing reagent readily reacted with a mixture of trace amounts of Sb(III) and Bi(III) to form the corresponding ternary Keggin-type complexes; [P(Sb(III)W(11))O(40)](6-) and [P(Bi(III)W(11))O(40)](6-) in 0.01 M malonate buffer (pH 2.4). Since the peaks due to the migrations of the ternary complex anions were well separated in the electropherogram, the pre-column complex-formation reaction was applied to the simultaneous CE determination of Sb(III) and Bi(III) with direct UV detection at 255 nm. The calibration curves were linear in the range of 2x10(-7)-5x10(-5) M; a detection limit of 1x10(-7) M was achieved for Sb(III) or Bi(III) (the signal-to-noise ratio=3).

An approach to the synthesis of polyoxometalate encapsulating different kinds of oxoanions as heteroions: bisphosphitopyrophosphatotriacontamolybdate [(HPO3)2(P2O7)Mo30O90]8-.

A yellow [(HPO(3))(2)(P(2)O(7))Mo(30)O(90)](8-) anion was prepared as a tetrapropylammonium (Pr(4)N(+)) salt from a 50 mM Mo(VI)-2 mM P(2)O(7)(4-)-4 mM HPO(3)(2-)-0.95 M HCl-60% (v/v) CH(3)CN system at ambient temperature. The (Pr(4)N)(8)[(HPO(3))(2)(P(2)O(7))Mo(30)O(90)] salt crystallized in the orthorhombic space group P(nma) (No. 62), with a = 30.827(2) A, b = 22.8060(15) A, c = 30.928(2) A, V = 21743(3) A(3), and Z = 4. The structure contained a (P(2)O(7))Mo(12)O(42) fragment derived from the removal of each corner-shared Mo(3)O(13) unit in a polar position from a [(P(2)O(7))Mo(18)O(54)](4-) structure, and each side of the (P(2)O(7))Mo(12)O(42) fragment was capped by a B-type (HPO(3))Mo(9)O(24) unit. The [(HPO(3))(2)(P(2)O(7))Mo(30)O(90)](8-) anion was characterized by voltammetry and IR, UV-vis, and (31)P NMR spectroscopy. Unlike the Keggin and Dawson anions and the parent [(P(2)O(7))Mo(18)O(54)](4-) anion, the [(HPO(3))(2)(P(2)O(7))Mo(30)O(90)](8-) anion exhibited two-electron redox waves in CH(3)CN with and without acid.

Solvent-effect on the interconversion among one-, two- and four-electron reduction waves for the 18-molybdodiphosphate complex, [(P2O7)Mo18O54]4−

Abstract For the [(P 2 O 7 )Mo 18 O 54 ] 4− complex, the presence of small cations such as H + , Li + and Na + caused one-electron waves to be converted into four- and two-electron waves in a complex manner. With the addition of a trace amount of H + , a four-electron reduction wave was obtained in solvents of weaker basicity like acetone, acetonitrile and propylene carbonate (PC); the relative permittivity did not affect the appearance of the four-electron wave. On the other hand, two-electron waves were obtained in solvents of stronger basicity like N,N -dimethylformamide (DMF), N,N -dimethylacetamide (DMA), and N -methylpyrrolidinone (NMP). With the addition of Li + or Na + , the one-electron waves were converted into two-electron waves only in acetone, indicating that the conversion can occur in solvents of both weak basicity and low relative permittivity.

Differential pulse voltammetric determination of P(V) following adsorptive accumulation of α-[PMo12O40]3− on a polypyrrole-modified glassy carbon electrode

On the basis of the formation and pre-concentration of an alpha-Keggin-type [PMo(12)O(40)](3-) complex, a novel voltammetric method was developed for the determination of trace levels of P(V). The alpha-[PMo(12)O(40)](3-) complex was formed by heating a 5x10(-4) M Mo(VI)-0.2 M HCl-40% (v/v) CH(3)CN system containing a trace amount of P(V) at 70 degrees C for 30 min. During the electrochemical polymerization of pyrrole in the alpha-[PMo(12)O(40)](3-) solution, the alpha-[PMo(12)O(40)](3-) complex was accumulated into the polypyrrole film on a glassy carbon electrode. The differential pulse voltammetric peak current due to the alpha-[PMo(12)O(40)](3-) complex incorporated in the polypyrrole film was linearly dependent on the P(V) concentration in the range of 5x10(-9)-5x10(-7) M; a detection limit of 2x10(-9) M was achieved.