- Email: [email protected]
Preparation and voltammetric characterization of Keggin-type tungstovanadate [VW12O40]3− and [V(VW11)O40]4− complexes
Inorganica Chimica Acta 348 (2003) 57 /62 www.elsevier.com/locate/ica
Preparation and voltammetric characterization of Keggin-type tungstovanadate [VW12O40]3 and [V(VW11)O40]4 complexes Sadayuki Himeno a,*, Masayo Takamoto a, Ayumi Higuchi a, Masahiko Maekawa b a
b
Department of Chemistry, Faculty of Science, Kobe University, Kobe 657-8501, Japan Research Institute for Science and Technology, Kinki University, Kowakae, Higashi-Osaka 577-8502, Japan Received 25 July 2002; accepted 4 November 2002
Abstract A 12-tungstovanadate complex, [VW12O40]3 was prepared as the tetrabutylammonium (n-Bu4N ) salt by heating a 50 mM (M /mol dm 3) W(VI) /50 mM V(V) /1.5 M HCl /80% (v/v) CH3CN system at 70 8C for 24 h. The a-Keggin structure was confirmed by 183W NMR spectroscopy. Besides, the n-Bu4N salt of its monovanadium-substituted derivative, [V(VW11)O40]4 was also prepared from a 50 mM W(VI) /50 mM V(V) /0.5 M HClO4 /50% (v/v) CH3CN system. Both Keggin-type complexes were characterized voltammetrically. A reversible one-electron redox wave due to the central V atom was obtained at potentials more positive than the reduction wave due to the peripheral W atom. Their formation and conversion processes were also elucidated with a combined Raman and voltammetric study. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Keggin complexes; Tungstovanadates; Voltammetry
1. Introduction The voltammetric properties of the so-called Keggin complexes have been a topic of continuing interest, owing to their potential use in electroanalysis and electrocatalysis. In general, the voltammetric waves are due to the reduction of the peripheral atoms in the Keggin structure, although the central hetero-atoms in [CuIIW12O40]6 , [CoIIIW12O40]5 , [FeIIIW12O40]5 and [VVMo12O40]3 are known to be electroreducible [1 /4]. However, the voltammetric properties involving the redox process of the central hetero-atom have not been fully elucidated. Various attempts have been made to prepare a Keggin-type [VVW12O40]3 complex, because the central V atom is expected to be electrochemically reducible. In acidified aqueous media, the reaction of W(VI) with V(V) has produced several tungstovanadate complexes such as [Vx W6 x O19](2x ) (x /1 /3) and [(H2)(VW11)O40]7 [5 /10]. Vanadium-substituted deri-
* Corresponding author. Tel./fax: /81-78-803 5680. E-mail address: [email protected] (S. Himeno).
vatives of the Keggin complex, [V(Vx W12 x )O40](3x ) (x /2, 3) have been prepared by heating an acidified aqueous solution of [V2W4O19]4 [6,11 /14], although their voltammetric properties have not been reported. To our knowledge, neither of the Keggin complex, [VW12O40]3 and the monovanadium-substituted derivative, [V(VW11)O40]4 has been prepared so far, although the hydrothermal reaction of W(VI) with vanadium metal and (CH3)4NCl at 160 8C produced the reduced form of the Keggin-type tungstovanadate complex, [(CH3)4N]7[VIV(WV 3 W9)O40] in the solid state [15]. The use of CH3CN as an auxiliary solvent offers a promising possibility to prepare new polyoxometalate complexes. As far as Keggin-type molybdovanadates are concerned, we have prepared a yellow Keggin complex, [VMo12O40]3 , and subsequently its vanadium-substituted derivatives, [V(Vx Mo12 x )O40](3x ) (x /1 /3) from a Mo(VI) /V(V) /HCl/CH3CN system [4,16]. Later, the crystallographic structure of the Keggin complex, Na3[VMo12O40]×/19H2O was determined [17]. The present study revealed that W(VI) reacted directly with V(V) in such acidified aqueous-CH3CN media to form the Keggin complex, [VW12O40]3 and the monovanadium-substituted derivative, [V(VW11)-
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0020-1693(02)01481-0
58
S. Himeno et al. / Inorganica Chimica Acta 348 (2003) 57 /62
O40]4 . The [V(VW11)O40]4 anion once formed was transformed spontaneously into the [VW12O40]3 anion in acidified aqueous-CH3CN media. This study was undertaken to characterize these Keggin-type tungstovanadate complexes and to elucidate the role of the central and peripheral V atoms in the voltammetric properties.
2. Experimental 2.1. Apparatus and materials Voltammetric measurements were performed with a Hokuto Denko Model HA1010mM1A potentiostat interfaced to a microcomputer-controlled system. The working electrode was a Tokai glassy carbon rod (GC30S) with a surface area of 0.20 cm2. The GC electrode was polished manually with 0.25 mm diamond slurry before each measurement. A three-electrode system was employed with a platinum wire as the counter electrode. The reference electrode was an Ag/Ag (0.01 M; CH3CN) electrode; the redox potential of a ferrocene/ ferricinium couple was found to be 0.09 V. The voltammetric measurements were made at 259/0.1 8C. Raman spectra were obtained with a Jobin Yvon Model Ramanor U-1000 spectrophotometer equipped with a liquid nitrogen-cooled CCD detector. For Raman excitation, an argon ion laser at 514.5 nm was used. For quantitative measurements, the Raman intensities were normalized with the intensity of the CH3CN band at 922 cm 1 or the ClO4 band at 936 cm 1. The 183W NMR spectra were obtained with a JEOL Model JNMGX500 spectrometer at 20.837 MHz. Chemical shifts were referenced to 1 M Na2WO4 ×/2H2O in D2O. A Thermo Nicolet Model Avatar 360 spectrophotometer was used to record IR spectra in the range of 1600 /450 cm 1 in the KBr pellets. UV /Vis spectra were recorded on a Hitachi Model U-3000 spectrophotometer. 2.2. Preparation of the [V(VW11)O40] [VW12O40]3 complexes
4
and
2.2.1. Preparation of (n-Bu4N)4[V(VW11)O40] To a solution of 8.3 g Na2WO4 ×/2H2O and 2.9 g NH4VO3 in 225 ml of warm water was added 250 ml of CH3CN. To a clear solution was added dropwise 27 ml of 9.2 M HClO4 with vigorous stirring. The resultant orange solution was heated at 70 8C for 24 h; during the time, an excess of V(V) was precipitated as a reddish brown oxide. After cooling to room temperature (r.t.), the precipitate was filtered off. To the filtrate was added 5 g of n-Bu4NBr to precipitate an orange salt. The salt was collected by filtration, and washed with water and ethanol, and air-dried (yield; 4.7 g). The salt was further purified by recrystallization from a 5:2 (v/v) CH3CN /
C2H5OH solution. Anal . Found: C, 20.8; H, 3.98; N, 1.50; V, 2.85; W, 53.8. Calc. for (n-Bu4N)4[V(VW11)O40]: C, 20.6; H, 3.89; N, 1.50; V, 2.73; W, 54.2%. 2.2.2. Preparation of (n-Bu4N)3[VW12O40] To a solution of 8.3 g Na2WO4 ×/2H2O and 2.9 g NH4VO3 in 35 ml of warm water was added 400 ml of CH3CN. To the mixture consisting of two liquid-layers was added dropwise 65 ml of conc. HCl with continuous stirring. The resultant turbid solution was heated at 70 8C for 24 h. After cooling to room temperature (r.t.), the precipitate was filtered off. To the filtrate was added 15 g of n-Bu4NBr to precipitate a white salt. The salt was collected by filtration, and washed with water and ethanol, and air-dried (yield; 2.6 g). The salt was recrystallized twice from a 4:1 (v/v) CH3CN /C2H5OH solution. Anal . Found: C, 16.0; H, 3.01; N, 1.17; V, 1.49; W, 61.5. Calc. for (n -Bu4N)3[VW12O40]: C, 15.9; H, 3.00; N, 1.16; V, 1.41; W, 60.9%. The [VW12O40]3 complex was also produced by the spontaneous transformation of [V(VW11)O40]4 in a 70% (v/v) CH3CN /2 M HCl system. After a 1.9 g quantity of (n-Bu4N)4[V(VW11)O40] was dissolved in 500 ml of the CH3CN /HCl media, the resultant orange solution was heated at 70 8C for 24 h. The colorless solution, which consisted of two liquid-layers, resulted after cooling to r.t. After the lower layer was discarded, 5 g of n-Bu4NBr was added to the precipitate, a white salt. The salt was isolated by filtration, washed with water and ethanol, and air-dried (yield; 1.4 g). The salt was further purified by recrystallization from a 4:1 (v/v) CH3CN /C2H5OH solution. For comparative purposes, a-(n-Bu4N)3[PW12O40] and (n-Bu4N)4[P(VW11)O40] were prepared according to the literature method [18,19].
3. Results and discussion 3.1. Characterization of [VW12O40]3 and [V(VW11)O40]4 3.1.1. 183W NMR In order to characterize structurally the [VW12O40]3 complex, the 183W NMR spectrum was recorded for the n-Bu4N salt dissolved in propylene carbonate. The [VW12O40]3 complex showed a single 183W NMR line at /78.8 ppm (Fig. 1). This result indicates that the complex possesses the a-Keggin structure, because the W atoms are equivalent in the framework of the tungstovanadate structure. 3.1.2. IR and Raman spectra Fig. 2 shows IR spectra of [VW12O40]3 and [V(VW11)O40]4 in the KBr pellets. The IR spectrum of [VW12O40]3 is characterized by prominent bands at
S. Himeno et al. / Inorganica Chimica Acta 348 (2003) 57 /62
59
3.1.3. UV/Vis spectra Fig. 3 shows UV /Vis spectra for 1.0 /10 4 M solutions of [VW12O40]3 and [V(VW11)O40]4 in CH3CN. The spectrum of [VW12O40]3 is characterized by a rather sharp absorption maximum at 270 nm (o / 4.72 /104 M 1 cm 1) (Fig. 3(a)), similar to that of a[PW12O40]3 [20]. The [V(VW11)O40]4 anion shows a round maximum at 265 nm (o /3.78 /104 M 1 cm 1) (Fig. 3(b)). Both spectra conform to Beer’s law in the spectra region studied. 3.2. Voltammetric behavior Fig. 1. A 183W NMR spectrum for the (n-Bu4N)3[VW12O40] salt dissolved in propylene carbonate.
Fig. 2. IR spectra of (a) (n-Bu4N)3[VW12O40] and (b) (n-Bu4N)4[V(VW11)O40] in the KBr pellets. Numerical data are given in the text.
981, 905, 780 and 522 cm 1 (Fig. 2(a)). The [V(VW11)O40]4 complex showed the corresponding bands at 967, 897, 775 and 520 cm 1 (Fig. 2(b)). The IR spectra of the same pattern have already been observed for the Keggin-type Mo-analogues, [VMo12O40]3 (958, 887 and 776 cm 1) and [V(VMo11)O40]4 (946, 880 and 774 cm 1) [4,16]. When the n-Bu4N salts of the tungstovanadate complexes were dissolved in CH3CN, [VW12O40]3 exhibited a strong Raman band at 1002 cm 1, along with a small band at 986 cm 1, while for [V(VW11)O40]4 , a major Raman band was obtained at 996 cm 1. Thus, the [VW12O40]3 and [V(VW11)O40]4 complexes can be easily distinguished by their IR and Raman spectra.
3.2.1. Voltammetric behavior of [VW12O40]3 and [PW12O40]3 Fig. 4(a) shows a cyclic voltammogram of 0.50 mM [VW12O40]3 in CH3CN containing 0.1 M n-Bu4NClO4. Four one-electron redox waves were obtained with midpoint potentials (Emid) of /0.02, /0.85, /1.45 and / 1.93 V where Emid /(Epc /Epa)/2; Epc and Epa are the cathodic and anodic peak-potentials, respectively. The separation of the Epc and Epa values for each redox couple averaged 57 mV, and the peak-potentials (Ep’s) are independent of the voltage scan rate (50 /200 mV s 1), indicating the reversible nature of each wave. For comparison, Fig. 5(a) shows a cyclic voltammogram of [PW12O40]3 in CH3CN containing 0.1 M nBu4NClO4. Similarly, [PW12O40]3 exhibited four reversible one-electron redox waves with Emid values of / 0.58, /1.10, /1.80 and /2.29 V. It is well known that the Emid values of the first one-electron waves are almost identical for the Keggin-type [XM12O40]n (M /W or Mo) anions with identical ionic charge, and show a linear dependence on their ionic charge of [XMo12O40]n or [XW12O40]n [3,21/24]. It should be noted that the first one-electron wave of [VW12O40]3 is situated at ca. 0.56 V more positive than the corresponding wave of [PW12O40]3 , suggesting that the first oneelectron wave of [VW12O40]3 cannot be ascribed to the reduction of W in the peripheral position.
Fig. 3. UV /Vis spectra for 1.0 /10 4 M solutions of (a) [VW12O40]3 and (b) [V(VW11)O40]4 in CH3CN. Path length; 2.0 mm.
60
S. Himeno et al. / Inorganica Chimica Acta 348 (2003) 57 /62
into a two-electron wave, indicating that the reduction of the central V atom is responsible for the first wave. The positive shift of the first wave is ascribed to the protonation of the one-electron-reduced form, [VIVW12O40]4 at the electrode surface. The existence 7 of the reduced Keggin anion, [VIV(WV has 3 W9)O40] already been ascertained in the solid state [15]. The electrochemical reduction at the first wave produces a colorless [VIVW12O40]4 anion, unlike the so-called m heteropoly-blue anions, [X(WV . x W12 x )O40]
Fig. 4. Cyclic voltammograms of 0.50 mM [VW12O40]3 in CH3CN containing (a) 0.1 M n-Bu4NClO4; (b) (a) /10 mM CF3SO3H.
Fig. 5. Cyclic voltammograms of 0.50 mM [PW12O40]3 in CH3CN containing (a) 0.1 M n-Bu4NClO4; (b) (a) /10 mM CF3SO3H.
For the Keggin and Dawson complexes, the presence of H caused the one-electron waves to be converted into two-electron waves in CH3CN [21,24/26]. This behavior has been accounted for in terms of the protonation of the reduced species at the electrode surface. With the addition of 10 mM CF3SO3H, the one-electron waves for [PW12O40]3 were transformed into a two-step two-electron redox wave (Fig. 5(b)) [27]. As shown in Fig. 4(b), entirely different behavior was observed for [VW12O40]3 . The addition of CF3SO3H caused the first one-electron wave to move slightly to more positive potentials, and new waves grew at more negative potentials as the CF3SO3H concentration was increased. Ultimately, three redox waves with Emid values of 0.17, /0.13 and /0.20 V were obtained in the presence of 10 mM CF3SO3H. Their current ratios in the normal pulse voltammogram were 1:4:2, and the coulometric study confirmed that the respective reduction waves involved one-, four- and two-electron transfers. In contrast to the behavior for [PW12O40]3 (Fig. 5), the first one-electron wave of [VW12O40]3 was shifted to more positive potentials without conversion
3.2.2. Voltammetric behavior of [V(VW11)O40]4 and [P(VW11)O40]4 In order to study further the role of the V atom in the peripheral position, the voltammetric behavior of [V(VW11)O40]4 was compared with that of [P(VW11)O40]4 . As shown in Fig. 6(a), [P(VW11)O40]4 exhibited three reversible one-electron redox waves with Emid values of /0.21, /1.72 and /2.22 V in CH3CN containing 0.1 M n-Bu4NClO4. The first wave can be ascribed to the reduction of V(V) in the peripheral position, and the remaining two to the reduction of W(VI) [22]. With the addition of CF3SO3H, the initial first wave was replaced by a one-electron reduction wave at more positive potentials, followed by a two-step wave. Ultimately, [P(VW11)O40]4 gave a three-step redox wave with Emid values of /0.48, /0.36 and / 0.44 V at a CF3SO3H concentration of 10 mM (Fig. 6(b)). Each reduction current corresponded to one-, four- and two-electron transfers. Fig. 7(a) shows a cyclic voltammogram of 0.50 mM [V(VW11)O40]4 in CH3CN containing 0.1 M n-Bu4NClO4. Like [P(VW11)O40]4 , [V(VW11)O40]4 exhibited a three-step one-electron redox wave with Emid values of /0.14, /0.96 and /1.72 V. Each one-electron wave was reversible. The first and third waves may correspond to the first and second waves of [P(VW11)O40]4 (Fig. 6(a)), which means that they can be ascribed to the
Fig. 6. Cyclic voltammograms of 0.50 mM [P(VW11)O40]4 in CH3CN containing (a) 0.1 M n-Bu4NClO4; (b) (a) /10 mM CF3SO3H.
S. Himeno et al. / Inorganica Chimica Acta 348 (2003) 57 /62
Fig. 7. Cyclic voltammograms of 0.50 mM [V(VW11)O40]4 in CH3CN containing (a) 0.1 M n-Bu4NClO4; (b) (a) /10 mM CF3SO3H.
redox couples of V(V)/V(IV) and W(VI)/W(V) in the peripheral positions, respectively. The remaining wave at /0.96 V corresponds to the reduction of the central V atom. In the presence of 10 mM CF3SO3H, as shown in Fig. 7(b), [V(VW11)O40]4 exhibited consecutive one-, one-, two-, two- and two-electron reduction waves with Emid values of /0.61, /0.20, /0.07, /0.19 and /0.32 V. The appearance of the first two one-electron waves demonstrates the validity of the above assignment. 3.3. Raman study for the formation and transformation of [VW12O40]3 and [V(VW11)O40]4 3.3.1. Formation of [VW12O40]3 and [V(VW11)O40]4 With given W(VI) and V(V) concentrations of 50 mM each, the test solutions became turbid after heating at 70 8C for 24 h. After the precipitates were filtered off, the Raman measurements were made for the clear filtrates. The formation and transformation processes are strongly dependent on the solution conditions including the CH3CN concentration, the kind and concentration of acid, temperature and reaction time. It turned out that [V(VW11)O40]4 once formed was transformed spontaneously into [VW12O40]3 at higher concentrations of HCl and CH3CN, while [V(VW11)O40]4 was kinetically stable only at lower concentrations of HClO4 and CH3CN. As shown in Fig. 8(a), a pair of Raman bands appeared at 1003 and 987 cm 1 for a 50 mM W(VI) / 0.5 M HClO4 /50% (v/v) CH3CN system; the 1003 cm 1 band is assigned to [W6O19]2 , and the 987 cm 1 band to [W10O32]4 [28]. In the presence of 50 mM V(V), a new Raman band appeared at 997 cm 1 with a disappearance of the 1003 and 987 cm 1 bands (Fig. 8(b)). No isopolyoxovanadates are responsible for the 997 cm 1 band (Fig. 8(c)). By comparison with the Raman frequency of the reference compounds dissolved
61
Fig. 8. Raman spectra for 50 mM W(VI) /0.5 M HClO4 /50% (v/v) CH3CN systems (a) without V(V) and (b) with 50 mM V(V); (c) for a 50 mM V(V) /0.5 M HClO4 /50% (v/v) CH3CN system. Recorded after heating the solutions at 70 8C for 24 h.
in CH3CN, the 997 cm 1 band can be assigned to [V(VW11)O40]4 . When Raman spectra were measured after heating a 50 mM W(VI) /1.5 M HCl/80% (v/v) CH3CN system at 70 8C for 24 h, no Raman bands were observed in the W /O stretching region (Fig. 9(a)). With 50 mM V(V) present, a Raman band appeared at 1004 cm 1, indicating the formation of [VW12O40]3 (Fig. 9(b)). A Raman band at 970 cm 1 is due to some isopolyoxovanadate (Fig. 9(c)). As described in Section 2, the [V(VW11)O40]4 and [VW12O40]3 complexes were isolated as the n-Bu4N salts from the 0.5 M HClO4 /50% (v/v) CH3CN and 1.5 M HCl/80% (v/v) CH3CN systems, respectively. 3.3.2. Transformation of [V(VW11)O40]4 into [VW12O40]3 In order to study the transformation process of [V(VW11)O40]4 into [VW12O40]3 , Raman spectra
Fig. 9. Raman spectra for 50 mM W(VI) /1.5 M HCl /80% (v/v) CH3CN systems (a) without V(V) and (b) with 50 mM V(V); (c) for a 50 mM V(V) /1.5 M HC /80% (v/v) CH3CN system. Recorded after heating the solutions at 70 8C for 24 h.
62
S. Himeno et al. / Inorganica Chimica Acta 348 (2003) 57 /62
Fig. 10. Raman spectra for 1.0 mM [V(VW11)O40]4 in a 70% (v/v) CH3CN /2 M HCl system. Recorded (a) immediately after the preparation; (b) after heating at 70 8C for 24 h.
were recorded for 1.0 mM (n-Bu4N)4[V(VW11)O40] in a 70% (v/v) CH3CN /2 M HCl system and are shown in Fig. 10. Immediately after the preparation of the solution, a Raman band due to [V(VW11)O40]4 appeared at 998 cm 1. When the measurement was made again after heating the solution at 70 8C, a new Raman band grew at 1004 cm 1 at the expense of the 998 cm 1 band. The 1004 cm 1 band replaced the 998 cm 1 band after 24 h of heating at 70 8C (Fig. 10(b)). These results confirm the spontaneous transformation of [V(VW11)O40]4 into [VW12O40]3 in such acidified aqueous-CH3CN media. The [VW12O40]3 complex is kinetically stable under these conditions, as judged by no change in the Raman spectrum.
References [1] D.R. Wexell, M. T. Pope, J. Chem. Soc., Chem. Commun. (1971) 886.
[2] L.C.W. Baker, V.E. Simmons, J. Am. Chem. Soc. 81 (1959) 4744. [3] M.T. Pope, G.M. Varga, Jr., Inorg. Chem. 5 (1966) 1249. [4] S. Himeno, A. Saito, Inorg. Chim. Acta 171 (1990) 135. [5] C.M. Flynn, Jr., M.T. Pope, Inorg. Chem. 10 (1971) 2524. [6] C.M. Flynn, Jr., M.T. Pope, Inorg. Chem. 10 (1971) 2745. [7] W.G. Klemperer, W. Shum, J. Am. Chem. Soc. 100 (1978) 4891. [8] I. Andersson, J.J. Hastings, O.W. Howarth, L. Pettersson, J. Chem. Soc., Dalton Trans. (1996) 2705. [9] A. Botar, J. Fuchs, Z. Naturforsch., T. B 37 (1982) 806. [10] K. Nishikawa, A. Kobayashi, Y. Sasaki, Bull. Chem. Soc. Jpn. 48 (1975) 889. [11] S.E. O’Donnell, M.T. Pope, J. Chem. Soc., Dalton Trans. (1976) 2290. [12] M.A. Leparulo-Loftus, M.T. Pope, Inorg. Chem. 26 (1987) 2112. [13] C.M. Flynn, Jr., M.T. Pope, Inorg. Chem. 11 (1972) 1950. [14] K. Nishikawa, A. Kobayashi, Y. Sasaki, Bull. Chem. Soc. Jpn. 48 (1975) 3152. [15] M.I. Khan, S. Cevik, R. Hayashi, J. Chem. Soc., Dalton Trans. (1999) 1651. [16] S. Himeno, T. Osakai, A. Saito, Bull. Chem. Soc. Jpn. 64 (1991) 21. [17] S.W. Zhang, G.Q. Huang, Y.G. Wei, C. Shao, Y.Q. Tang, Acta Crystallogr., C 49 (1993) 1446. [18] C. Rocchiccioli-Deltcheff, M. Fournier, R. Franck, R. Thouvenot, Inorg. Chem. 22 (1983) 207. [19] E. Radkov, R.H. Beer, Polyhedron 14 (1995) 2139. [20] S. Himeno, M. Takamoto, T. Ueda, J. Electroanal. Chem. 465 (1999) 129. [21] M. Sadakane, E. Steckhan, Chem. Rev. 98 (1998) 219. [22] J.J. Altenau, M.T. Pope, R.A. Prados, H. So, Inorg. Chem. 14 (1975) 417. [23] K. Maeda, H. Katano, T. Osakai, S. Himeno, A. Saito, J. Electroanal. Chem. 389 (1995) 167. [24] K. Maeda, S. Himeno, T. Osakai, A. Saito, T. Hori, J. Electroanal. Chem. 364 (1994) 149. [25] S. Himeno, K. Maeda, T. Osakai, A. Saito, T. Hori, Bull. Chem. Soc. Jpn. 66 (1993) 109. [26] S. Himeno, M. Takamoto, T. Ueda, J. Electroanal. Chem. 485 (2000) 49. [27] S. Himeno, M. Takamoto, J. Electroanal. Chem. 528 (2002) 170. [28] S. Himeno, M. Yoshihara, M. Maekawa, Inorg. Chim. Acta 298 (2000) 165.
Preparation and voltammetric characterization of Keggin-type tungstovanadate [VW12O40]3 and [V(VW11)O40]4 complexes Sadayuki Himeno a,*, Masayo Takamoto a, Ayumi Higuchi a, Masahiko Maekawa b a
b
Department of Chemistry, Faculty of Science, Kobe University, Kobe 657-8501, Japan Research Institute for Science and Technology, Kinki University, Kowakae, Higashi-Osaka 577-8502, Japan Received 25 July 2002; accepted 4 November 2002
Abstract A 12-tungstovanadate complex, [VW12O40]3 was prepared as the tetrabutylammonium (n-Bu4N ) salt by heating a 50 mM (M /mol dm 3) W(VI) /50 mM V(V) /1.5 M HCl /80% (v/v) CH3CN system at 70 8C for 24 h. The a-Keggin structure was confirmed by 183W NMR spectroscopy. Besides, the n-Bu4N salt of its monovanadium-substituted derivative, [V(VW11)O40]4 was also prepared from a 50 mM W(VI) /50 mM V(V) /0.5 M HClO4 /50% (v/v) CH3CN system. Both Keggin-type complexes were characterized voltammetrically. A reversible one-electron redox wave due to the central V atom was obtained at potentials more positive than the reduction wave due to the peripheral W atom. Their formation and conversion processes were also elucidated with a combined Raman and voltammetric study. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Keggin complexes; Tungstovanadates; Voltammetry
1. Introduction The voltammetric properties of the so-called Keggin complexes have been a topic of continuing interest, owing to their potential use in electroanalysis and electrocatalysis. In general, the voltammetric waves are due to the reduction of the peripheral atoms in the Keggin structure, although the central hetero-atoms in [CuIIW12O40]6 , [CoIIIW12O40]5 , [FeIIIW12O40]5 and [VVMo12O40]3 are known to be electroreducible [1 /4]. However, the voltammetric properties involving the redox process of the central hetero-atom have not been fully elucidated. Various attempts have been made to prepare a Keggin-type [VVW12O40]3 complex, because the central V atom is expected to be electrochemically reducible. In acidified aqueous media, the reaction of W(VI) with V(V) has produced several tungstovanadate complexes such as [Vx W6 x O19](2x ) (x /1 /3) and [(H2)(VW11)O40]7 [5 /10]. Vanadium-substituted deri-
* Corresponding author. Tel./fax: /81-78-803 5680. E-mail address: [email protected] (S. Himeno).
vatives of the Keggin complex, [V(Vx W12 x )O40](3x ) (x /2, 3) have been prepared by heating an acidified aqueous solution of [V2W4O19]4 [6,11 /14], although their voltammetric properties have not been reported. To our knowledge, neither of the Keggin complex, [VW12O40]3 and the monovanadium-substituted derivative, [V(VW11)O40]4 has been prepared so far, although the hydrothermal reaction of W(VI) with vanadium metal and (CH3)4NCl at 160 8C produced the reduced form of the Keggin-type tungstovanadate complex, [(CH3)4N]7[VIV(WV 3 W9)O40] in the solid state [15]. The use of CH3CN as an auxiliary solvent offers a promising possibility to prepare new polyoxometalate complexes. As far as Keggin-type molybdovanadates are concerned, we have prepared a yellow Keggin complex, [VMo12O40]3 , and subsequently its vanadium-substituted derivatives, [V(Vx Mo12 x )O40](3x ) (x /1 /3) from a Mo(VI) /V(V) /HCl/CH3CN system [4,16]. Later, the crystallographic structure of the Keggin complex, Na3[VMo12O40]×/19H2O was determined [17]. The present study revealed that W(VI) reacted directly with V(V) in such acidified aqueous-CH3CN media to form the Keggin complex, [VW12O40]3 and the monovanadium-substituted derivative, [V(VW11)-
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0020-1693(02)01481-0
58
S. Himeno et al. / Inorganica Chimica Acta 348 (2003) 57 /62
O40]4 . The [V(VW11)O40]4 anion once formed was transformed spontaneously into the [VW12O40]3 anion in acidified aqueous-CH3CN media. This study was undertaken to characterize these Keggin-type tungstovanadate complexes and to elucidate the role of the central and peripheral V atoms in the voltammetric properties.
2. Experimental 2.1. Apparatus and materials Voltammetric measurements were performed with a Hokuto Denko Model HA1010mM1A potentiostat interfaced to a microcomputer-controlled system. The working electrode was a Tokai glassy carbon rod (GC30S) with a surface area of 0.20 cm2. The GC electrode was polished manually with 0.25 mm diamond slurry before each measurement. A three-electrode system was employed with a platinum wire as the counter electrode. The reference electrode was an Ag/Ag (0.01 M; CH3CN) electrode; the redox potential of a ferrocene/ ferricinium couple was found to be 0.09 V. The voltammetric measurements were made at 259/0.1 8C. Raman spectra were obtained with a Jobin Yvon Model Ramanor U-1000 spectrophotometer equipped with a liquid nitrogen-cooled CCD detector. For Raman excitation, an argon ion laser at 514.5 nm was used. For quantitative measurements, the Raman intensities were normalized with the intensity of the CH3CN band at 922 cm 1 or the ClO4 band at 936 cm 1. The 183W NMR spectra were obtained with a JEOL Model JNMGX500 spectrometer at 20.837 MHz. Chemical shifts were referenced to 1 M Na2WO4 ×/2H2O in D2O. A Thermo Nicolet Model Avatar 360 spectrophotometer was used to record IR spectra in the range of 1600 /450 cm 1 in the KBr pellets. UV /Vis spectra were recorded on a Hitachi Model U-3000 spectrophotometer. 2.2. Preparation of the [V(VW11)O40] [VW12O40]3 complexes
4
and
2.2.1. Preparation of (n-Bu4N)4[V(VW11)O40] To a solution of 8.3 g Na2WO4 ×/2H2O and 2.9 g NH4VO3 in 225 ml of warm water was added 250 ml of CH3CN. To a clear solution was added dropwise 27 ml of 9.2 M HClO4 with vigorous stirring. The resultant orange solution was heated at 70 8C for 24 h; during the time, an excess of V(V) was precipitated as a reddish brown oxide. After cooling to room temperature (r.t.), the precipitate was filtered off. To the filtrate was added 5 g of n-Bu4NBr to precipitate an orange salt. The salt was collected by filtration, and washed with water and ethanol, and air-dried (yield; 4.7 g). The salt was further purified by recrystallization from a 5:2 (v/v) CH3CN /
C2H5OH solution. Anal . Found: C, 20.8; H, 3.98; N, 1.50; V, 2.85; W, 53.8. Calc. for (n-Bu4N)4[V(VW11)O40]: C, 20.6; H, 3.89; N, 1.50; V, 2.73; W, 54.2%. 2.2.2. Preparation of (n-Bu4N)3[VW12O40] To a solution of 8.3 g Na2WO4 ×/2H2O and 2.9 g NH4VO3 in 35 ml of warm water was added 400 ml of CH3CN. To the mixture consisting of two liquid-layers was added dropwise 65 ml of conc. HCl with continuous stirring. The resultant turbid solution was heated at 70 8C for 24 h. After cooling to room temperature (r.t.), the precipitate was filtered off. To the filtrate was added 15 g of n-Bu4NBr to precipitate a white salt. The salt was collected by filtration, and washed with water and ethanol, and air-dried (yield; 2.6 g). The salt was recrystallized twice from a 4:1 (v/v) CH3CN /C2H5OH solution. Anal . Found: C, 16.0; H, 3.01; N, 1.17; V, 1.49; W, 61.5. Calc. for (n -Bu4N)3[VW12O40]: C, 15.9; H, 3.00; N, 1.16; V, 1.41; W, 60.9%. The [VW12O40]3 complex was also produced by the spontaneous transformation of [V(VW11)O40]4 in a 70% (v/v) CH3CN /2 M HCl system. After a 1.9 g quantity of (n-Bu4N)4[V(VW11)O40] was dissolved in 500 ml of the CH3CN /HCl media, the resultant orange solution was heated at 70 8C for 24 h. The colorless solution, which consisted of two liquid-layers, resulted after cooling to r.t. After the lower layer was discarded, 5 g of n-Bu4NBr was added to the precipitate, a white salt. The salt was isolated by filtration, washed with water and ethanol, and air-dried (yield; 1.4 g). The salt was further purified by recrystallization from a 4:1 (v/v) CH3CN /C2H5OH solution. For comparative purposes, a-(n-Bu4N)3[PW12O40] and (n-Bu4N)4[P(VW11)O40] were prepared according to the literature method [18,19].
3. Results and discussion 3.1. Characterization of [VW12O40]3 and [V(VW11)O40]4 3.1.1. 183W NMR In order to characterize structurally the [VW12O40]3 complex, the 183W NMR spectrum was recorded for the n-Bu4N salt dissolved in propylene carbonate. The [VW12O40]3 complex showed a single 183W NMR line at /78.8 ppm (Fig. 1). This result indicates that the complex possesses the a-Keggin structure, because the W atoms are equivalent in the framework of the tungstovanadate structure. 3.1.2. IR and Raman spectra Fig. 2 shows IR spectra of [VW12O40]3 and [V(VW11)O40]4 in the KBr pellets. The IR spectrum of [VW12O40]3 is characterized by prominent bands at
S. Himeno et al. / Inorganica Chimica Acta 348 (2003) 57 /62
59
3.1.3. UV/Vis spectra Fig. 3 shows UV /Vis spectra for 1.0 /10 4 M solutions of [VW12O40]3 and [V(VW11)O40]4 in CH3CN. The spectrum of [VW12O40]3 is characterized by a rather sharp absorption maximum at 270 nm (o / 4.72 /104 M 1 cm 1) (Fig. 3(a)), similar to that of a[PW12O40]3 [20]. The [V(VW11)O40]4 anion shows a round maximum at 265 nm (o /3.78 /104 M 1 cm 1) (Fig. 3(b)). Both spectra conform to Beer’s law in the spectra region studied. 3.2. Voltammetric behavior Fig. 1. A 183W NMR spectrum for the (n-Bu4N)3[VW12O40] salt dissolved in propylene carbonate.
Fig. 2. IR spectra of (a) (n-Bu4N)3[VW12O40] and (b) (n-Bu4N)4[V(VW11)O40] in the KBr pellets. Numerical data are given in the text.
981, 905, 780 and 522 cm 1 (Fig. 2(a)). The [V(VW11)O40]4 complex showed the corresponding bands at 967, 897, 775 and 520 cm 1 (Fig. 2(b)). The IR spectra of the same pattern have already been observed for the Keggin-type Mo-analogues, [VMo12O40]3 (958, 887 and 776 cm 1) and [V(VMo11)O40]4 (946, 880 and 774 cm 1) [4,16]. When the n-Bu4N salts of the tungstovanadate complexes were dissolved in CH3CN, [VW12O40]3 exhibited a strong Raman band at 1002 cm 1, along with a small band at 986 cm 1, while for [V(VW11)O40]4 , a major Raman band was obtained at 996 cm 1. Thus, the [VW12O40]3 and [V(VW11)O40]4 complexes can be easily distinguished by their IR and Raman spectra.
3.2.1. Voltammetric behavior of [VW12O40]3 and [PW12O40]3 Fig. 4(a) shows a cyclic voltammogram of 0.50 mM [VW12O40]3 in CH3CN containing 0.1 M n-Bu4NClO4. Four one-electron redox waves were obtained with midpoint potentials (Emid) of /0.02, /0.85, /1.45 and / 1.93 V where Emid /(Epc /Epa)/2; Epc and Epa are the cathodic and anodic peak-potentials, respectively. The separation of the Epc and Epa values for each redox couple averaged 57 mV, and the peak-potentials (Ep’s) are independent of the voltage scan rate (50 /200 mV s 1), indicating the reversible nature of each wave. For comparison, Fig. 5(a) shows a cyclic voltammogram of [PW12O40]3 in CH3CN containing 0.1 M nBu4NClO4. Similarly, [PW12O40]3 exhibited four reversible one-electron redox waves with Emid values of / 0.58, /1.10, /1.80 and /2.29 V. It is well known that the Emid values of the first one-electron waves are almost identical for the Keggin-type [XM12O40]n (M /W or Mo) anions with identical ionic charge, and show a linear dependence on their ionic charge of [XMo12O40]n or [XW12O40]n [3,21/24]. It should be noted that the first one-electron wave of [VW12O40]3 is situated at ca. 0.56 V more positive than the corresponding wave of [PW12O40]3 , suggesting that the first oneelectron wave of [VW12O40]3 cannot be ascribed to the reduction of W in the peripheral position.
Fig. 3. UV /Vis spectra for 1.0 /10 4 M solutions of (a) [VW12O40]3 and (b) [V(VW11)O40]4 in CH3CN. Path length; 2.0 mm.
60
S. Himeno et al. / Inorganica Chimica Acta 348 (2003) 57 /62
into a two-electron wave, indicating that the reduction of the central V atom is responsible for the first wave. The positive shift of the first wave is ascribed to the protonation of the one-electron-reduced form, [VIVW12O40]4 at the electrode surface. The existence 7 of the reduced Keggin anion, [VIV(WV has 3 W9)O40] already been ascertained in the solid state [15]. The electrochemical reduction at the first wave produces a colorless [VIVW12O40]4 anion, unlike the so-called m heteropoly-blue anions, [X(WV . x W12 x )O40]
Fig. 4. Cyclic voltammograms of 0.50 mM [VW12O40]3 in CH3CN containing (a) 0.1 M n-Bu4NClO4; (b) (a) /10 mM CF3SO3H.
Fig. 5. Cyclic voltammograms of 0.50 mM [PW12O40]3 in CH3CN containing (a) 0.1 M n-Bu4NClO4; (b) (a) /10 mM CF3SO3H.
For the Keggin and Dawson complexes, the presence of H caused the one-electron waves to be converted into two-electron waves in CH3CN [21,24/26]. This behavior has been accounted for in terms of the protonation of the reduced species at the electrode surface. With the addition of 10 mM CF3SO3H, the one-electron waves for [PW12O40]3 were transformed into a two-step two-electron redox wave (Fig. 5(b)) [27]. As shown in Fig. 4(b), entirely different behavior was observed for [VW12O40]3 . The addition of CF3SO3H caused the first one-electron wave to move slightly to more positive potentials, and new waves grew at more negative potentials as the CF3SO3H concentration was increased. Ultimately, three redox waves with Emid values of 0.17, /0.13 and /0.20 V were obtained in the presence of 10 mM CF3SO3H. Their current ratios in the normal pulse voltammogram were 1:4:2, and the coulometric study confirmed that the respective reduction waves involved one-, four- and two-electron transfers. In contrast to the behavior for [PW12O40]3 (Fig. 5), the first one-electron wave of [VW12O40]3 was shifted to more positive potentials without conversion
3.2.2. Voltammetric behavior of [V(VW11)O40]4 and [P(VW11)O40]4 In order to study further the role of the V atom in the peripheral position, the voltammetric behavior of [V(VW11)O40]4 was compared with that of [P(VW11)O40]4 . As shown in Fig. 6(a), [P(VW11)O40]4 exhibited three reversible one-electron redox waves with Emid values of /0.21, /1.72 and /2.22 V in CH3CN containing 0.1 M n-Bu4NClO4. The first wave can be ascribed to the reduction of V(V) in the peripheral position, and the remaining two to the reduction of W(VI) [22]. With the addition of CF3SO3H, the initial first wave was replaced by a one-electron reduction wave at more positive potentials, followed by a two-step wave. Ultimately, [P(VW11)O40]4 gave a three-step redox wave with Emid values of /0.48, /0.36 and / 0.44 V at a CF3SO3H concentration of 10 mM (Fig. 6(b)). Each reduction current corresponded to one-, four- and two-electron transfers. Fig. 7(a) shows a cyclic voltammogram of 0.50 mM [V(VW11)O40]4 in CH3CN containing 0.1 M n-Bu4NClO4. Like [P(VW11)O40]4 , [V(VW11)O40]4 exhibited a three-step one-electron redox wave with Emid values of /0.14, /0.96 and /1.72 V. Each one-electron wave was reversible. The first and third waves may correspond to the first and second waves of [P(VW11)O40]4 (Fig. 6(a)), which means that they can be ascribed to the
Fig. 6. Cyclic voltammograms of 0.50 mM [P(VW11)O40]4 in CH3CN containing (a) 0.1 M n-Bu4NClO4; (b) (a) /10 mM CF3SO3H.
S. Himeno et al. / Inorganica Chimica Acta 348 (2003) 57 /62
Fig. 7. Cyclic voltammograms of 0.50 mM [V(VW11)O40]4 in CH3CN containing (a) 0.1 M n-Bu4NClO4; (b) (a) /10 mM CF3SO3H.
redox couples of V(V)/V(IV) and W(VI)/W(V) in the peripheral positions, respectively. The remaining wave at /0.96 V corresponds to the reduction of the central V atom. In the presence of 10 mM CF3SO3H, as shown in Fig. 7(b), [V(VW11)O40]4 exhibited consecutive one-, one-, two-, two- and two-electron reduction waves with Emid values of /0.61, /0.20, /0.07, /0.19 and /0.32 V. The appearance of the first two one-electron waves demonstrates the validity of the above assignment. 3.3. Raman study for the formation and transformation of [VW12O40]3 and [V(VW11)O40]4 3.3.1. Formation of [VW12O40]3 and [V(VW11)O40]4 With given W(VI) and V(V) concentrations of 50 mM each, the test solutions became turbid after heating at 70 8C for 24 h. After the precipitates were filtered off, the Raman measurements were made for the clear filtrates. The formation and transformation processes are strongly dependent on the solution conditions including the CH3CN concentration, the kind and concentration of acid, temperature and reaction time. It turned out that [V(VW11)O40]4 once formed was transformed spontaneously into [VW12O40]3 at higher concentrations of HCl and CH3CN, while [V(VW11)O40]4 was kinetically stable only at lower concentrations of HClO4 and CH3CN. As shown in Fig. 8(a), a pair of Raman bands appeared at 1003 and 987 cm 1 for a 50 mM W(VI) / 0.5 M HClO4 /50% (v/v) CH3CN system; the 1003 cm 1 band is assigned to [W6O19]2 , and the 987 cm 1 band to [W10O32]4 [28]. In the presence of 50 mM V(V), a new Raman band appeared at 997 cm 1 with a disappearance of the 1003 and 987 cm 1 bands (Fig. 8(b)). No isopolyoxovanadates are responsible for the 997 cm 1 band (Fig. 8(c)). By comparison with the Raman frequency of the reference compounds dissolved
61
Fig. 8. Raman spectra for 50 mM W(VI) /0.5 M HClO4 /50% (v/v) CH3CN systems (a) without V(V) and (b) with 50 mM V(V); (c) for a 50 mM V(V) /0.5 M HClO4 /50% (v/v) CH3CN system. Recorded after heating the solutions at 70 8C for 24 h.
in CH3CN, the 997 cm 1 band can be assigned to [V(VW11)O40]4 . When Raman spectra were measured after heating a 50 mM W(VI) /1.5 M HCl/80% (v/v) CH3CN system at 70 8C for 24 h, no Raman bands were observed in the W /O stretching region (Fig. 9(a)). With 50 mM V(V) present, a Raman band appeared at 1004 cm 1, indicating the formation of [VW12O40]3 (Fig. 9(b)). A Raman band at 970 cm 1 is due to some isopolyoxovanadate (Fig. 9(c)). As described in Section 2, the [V(VW11)O40]4 and [VW12O40]3 complexes were isolated as the n-Bu4N salts from the 0.5 M HClO4 /50% (v/v) CH3CN and 1.5 M HCl/80% (v/v) CH3CN systems, respectively. 3.3.2. Transformation of [V(VW11)O40]4 into [VW12O40]3 In order to study the transformation process of [V(VW11)O40]4 into [VW12O40]3 , Raman spectra
Fig. 9. Raman spectra for 50 mM W(VI) /1.5 M HCl /80% (v/v) CH3CN systems (a) without V(V) and (b) with 50 mM V(V); (c) for a 50 mM V(V) /1.5 M HC /80% (v/v) CH3CN system. Recorded after heating the solutions at 70 8C for 24 h.
62
S. Himeno et al. / Inorganica Chimica Acta 348 (2003) 57 /62
Fig. 10. Raman spectra for 1.0 mM [V(VW11)O40]4 in a 70% (v/v) CH3CN /2 M HCl system. Recorded (a) immediately after the preparation; (b) after heating at 70 8C for 24 h.
were recorded for 1.0 mM (n-Bu4N)4[V(VW11)O40] in a 70% (v/v) CH3CN /2 M HCl system and are shown in Fig. 10. Immediately after the preparation of the solution, a Raman band due to [V(VW11)O40]4 appeared at 998 cm 1. When the measurement was made again after heating the solution at 70 8C, a new Raman band grew at 1004 cm 1 at the expense of the 998 cm 1 band. The 1004 cm 1 band replaced the 998 cm 1 band after 24 h of heating at 70 8C (Fig. 10(b)). These results confirm the spontaneous transformation of [V(VW11)O40]4 into [VW12O40]3 in such acidified aqueous-CH3CN media. The [VW12O40]3 complex is kinetically stable under these conditions, as judged by no change in the Raman spectrum.
References [1] D.R. Wexell, M. T. Pope, J. Chem. Soc., Chem. Commun. (1971) 886.
[2] L.C.W. Baker, V.E. Simmons, J. Am. Chem. Soc. 81 (1959) 4744. [3] M.T. Pope, G.M. Varga, Jr., Inorg. Chem. 5 (1966) 1249. [4] S. Himeno, A. Saito, Inorg. Chim. Acta 171 (1990) 135. [5] C.M. Flynn, Jr., M.T. Pope, Inorg. Chem. 10 (1971) 2524. [6] C.M. Flynn, Jr., M.T. Pope, Inorg. Chem. 10 (1971) 2745. [7] W.G. Klemperer, W. Shum, J. Am. Chem. Soc. 100 (1978) 4891. [8] I. Andersson, J.J. Hastings, O.W. Howarth, L. Pettersson, J. Chem. Soc., Dalton Trans. (1996) 2705. [9] A. Botar, J. Fuchs, Z. Naturforsch., T. B 37 (1982) 806. [10] K. Nishikawa, A. Kobayashi, Y. Sasaki, Bull. Chem. Soc. Jpn. 48 (1975) 889. [11] S.E. O’Donnell, M.T. Pope, J. Chem. Soc., Dalton Trans. (1976) 2290. [12] M.A. Leparulo-Loftus, M.T. Pope, Inorg. Chem. 26 (1987) 2112. [13] C.M. Flynn, Jr., M.T. Pope, Inorg. Chem. 11 (1972) 1950. [14] K. Nishikawa, A. Kobayashi, Y. Sasaki, Bull. Chem. Soc. Jpn. 48 (1975) 3152. [15] M.I. Khan, S. Cevik, R. Hayashi, J. Chem. Soc., Dalton Trans. (1999) 1651. [16] S. Himeno, T. Osakai, A. Saito, Bull. Chem. Soc. Jpn. 64 (1991) 21. [17] S.W. Zhang, G.Q. Huang, Y.G. Wei, C. Shao, Y.Q. Tang, Acta Crystallogr., C 49 (1993) 1446. [18] C. Rocchiccioli-Deltcheff, M. Fournier, R. Franck, R. Thouvenot, Inorg. Chem. 22 (1983) 207. [19] E. Radkov, R.H. Beer, Polyhedron 14 (1995) 2139. [20] S. Himeno, M. Takamoto, T. Ueda, J. Electroanal. Chem. 465 (1999) 129. [21] M. Sadakane, E. Steckhan, Chem. Rev. 98 (1998) 219. [22] J.J. Altenau, M.T. Pope, R.A. Prados, H. So, Inorg. Chem. 14 (1975) 417. [23] K. Maeda, H. Katano, T. Osakai, S. Himeno, A. Saito, J. Electroanal. Chem. 389 (1995) 167. [24] K. Maeda, S. Himeno, T. Osakai, A. Saito, T. Hori, J. Electroanal. Chem. 364 (1994) 149. [25] S. Himeno, K. Maeda, T. Osakai, A. Saito, T. Hori, Bull. Chem. Soc. Jpn. 66 (1993) 109. [26] S. Himeno, M. Takamoto, T. Ueda, J. Electroanal. Chem. 485 (2000) 49. [27] S. Himeno, M. Takamoto, J. Electroanal. Chem. 528 (2002) 170. [28] S. Himeno, M. Yoshihara, M. Maekawa, Inorg. Chim. Acta 298 (2000) 165.