Vanadium ( V )-Substitution Reactions of Wells – Dawson-Type Polyoxometalates : From [ X 2 M 18 O 62 ]

The formation processes of V(V)-substituted polyoxometalates with the Wells–Dawson-type structure were studied by cyclic voltammetry and by 31P NMR and Raman spectroscopy. Generally, the vanadium-substituted heteropolytungstates, [P2VW17O62]7− and [As2VW17O62]7−, were prepared by mixing equimolar amounts of the corresponding lacunary species—[P2W17O61]10− and [As2W17O61]10−—and vanadate. According to the results of various measurements in the present study, the tungsten site in the framework of [P2W18O62]6− and [As2W18O62]6− without defect sites could be substituted with V(V) to form the [P2VW17O62]7− and [As2VW17O62]7−, respectively. The order in which the reagents were mixed was observed to be the key factor for the formation of Dawson-type V(V)-substituted polyoxometalates. Even when the concentration of each reagent was identical, the final products differed depending on the order of their addition to the reaction mixture. Unlike Wells–Dawson-type heteropolytungstates, the molybdenum sites in the framework of [P2Mo18O62]6− and [As2Mo18O62]6− were substituted with V(V), but formed Keggin-type [PVMo11O40]4− and [AsVMo11O40]4− instead of [P2VMo17O62]7− and [As2VMo17O62]7−, respectively, even though a variety of reaction conditions were used. The formation constant of the [PVMo11O40]4− and [AsVMo11O40]4− was hypothesized to be substantially greater than that of the [P2VMo17O62]7− and [As2VMo17O62]7−. OPEN ACCESS Inorganics 2015, 3 356 Keyword: polyoxometalate; Wells–Dawson; vanadium; substitution


Characterization of POMs Related to the Present Study
Solid-state Raman bands of all of the investigated POMs and the 31 P NMR signals of P2W18, P2VW17, P2Mo18 and related phosphorus-containing POMs are listed in Tables 1 and 2, respectively.Characteristic bands and signals were used to assign peaks observed in the spectra of the products of the vanadium-substitution reaction in the solution phase.The major Raman lines of the As2W18 and As2VW17 complexes were observed at 995 and 982 cm −1 , respectively.The Raman band at approximately 990 cm −1 is due to the stretching mode of W-Od and is affected by the cation in the solid state [52].The Raman bands of the n-Bu4N + salts of As2W18 and P2W18, in which a small number of H + are included as counter cations, occur at lower wavenumbers than those of small cation salts, such as potassium.These Raman bands were used as reference signals to investigate the formation and conversion processes of the As2W18 and As2VW17 complexes in the reaction mixtures, while taking into consideration the effect of the proton.
Cyclic voltammograms of 5.0 × 10 −4 M X2VaW18−a (X = P, As; a = 0, 1) were measured in 95% (v/v) CH3CN containing 0.1 M HClO4 (Figure A1).Details are provided in the Supporting Information.The voltammetric behaviors of X2VW17 were used to determine whether the vanadium-substitution reaction occurred on the basis of the appearance of a new redox wave at a more positive potential than that of the corresponding parent non-substituted species.

Figure 2. Cyclic voltammograms (A) and
Raman spectra (B) of a 100 mM W(VI)-200 mM As(V)-10 mM V(V)-pH 2.0 system collected (a) after a solution without V(V) was heated at 80 °C for one week and (b) after 10 mM V(V) was added and the solution was heated again at 80 °C for one day.

Formation of Dawson-Type Vanadium-Substituted Tungstoarsenate and Tungstophosphate Complexes
Figures 2A(a) and 2B(a) show a cyclic voltammogram and a Raman spectrum, respectively, of a 100 mM W(VI)-200 mM As(V)-pH 2.5 system heated at 80 °C for one week.Four-step reduction waves were observed at 95, −83, −416 and −676 mV, which corresponded to one-, one-, two-and two-electron transfers, respectively.In addition, a Raman peak appeared at 994 cm −1 .These voltammetric waves and the Raman peak are due to the formation of the As2W18.After 10 mM V(V) was added, the pH was re-adjusted to 2.5 with conc.HCl and the solution was heated at 80 °C for one day; the original reduction waves and the Raman peak at 994 cm Raman intensity B vanadate to form the As2VW17.The vanadium substitution of P2W18 leads to 1-P2VW17, whose vanadium is located at a polar site, as described below.In addition, the Wells-Dawson-type 1-S2VW17 was prepared by addition of V(V) to the reaction mixture, where the parent S2W18 is dominant [46].The vanadium unit of the observed As2VW17 was located at the polar position, although Raman spectroscopy, cyclic voltammetry and other measurements provided no information on this perspective.
Figure 3. 31 P NMR spectrum of a 100 mM W(VI)-500 mM P(V)-V(V)-pH 2.0 system collected (a) after a solution without V(V) was heated at 80 °C for one week, (b) after 50 mM V(V) was added and the solution was heated again at 80 °C for one day and (c) after a solution of 100 mM W(VI)-500 mM P(V)-50 mM V(V)-pH 2 was heated at 80 °C for one week.
The formation of P2VW17 by the substitution reaction of P2W18 with V(V) was investigated using 31 P NMR.After a solution of 100 mM W(VI) and 500 mM P(V) was heated at pH 2 at 80 °C for one week, four main 31 P NMR peaks at −10.9, −11.2, −11.7 and −12.4 ppm were observed; these peaks were attributed to the formation of α-P2W18 (−12.4 ppm), β-P2W18 (−10.9, −11.7 ppm) and PW11 (−11.2 ppm) (Figure 3a).Upon further heating at 80 °C for one day following the addition of 50 mM V(V) and adjustment of the pH to 2.0, the three peaks at -10.9, −11.2 and −11.7 ppm disappeared, whereas two new peaks appeared at -10.8 and -12.9 ppm; these peaks were ascribed to the formation of the 1-P2VW17.However, the peak at -12.4 ppm did not completely disappear despite the addition of 100 mM V(V).A similar result was obtained in the case of the substitution reaction of the Keggin-type XM12 (X = P, As; M = Mo, W) with V(V).The PW12 and PMo12 were not fully converted into PVaW12-a and PVaMo12-a, respectively, in aqueous CH3CN solution, although both AsW12 and AsMo12 were fully converted into the corresponding vanadium-substituted species [44,45].The relatively high stability of P2W18, compared to that of As2W18 would lead to incomplete conversion of the P2W18 into the P2VW17, similar to the conversion of the PM12 (M = Mo, W) into the PVM11.To investigate the direct formation of X2VW17 (X = As, P), but not through X2W18 as an intermediate species, Raman spectra and 31 P NMR spectra were collected after a solution of W(VI)-V(V)-As(V) or -P(V) was heated at an appropriate pH. Figure 4 shows the Raman spectra collected after a solution of 100 mM W(VI), 200 mM As(V) and 0-20 mM V(V) were heated at pH 2 at 80 °C for one week.In the presence of 5-15 mM V(V), the Raman peak shifted from 993 cm −1 to 987 cm −1 when the concentration of V(V) was increased, thereby indicating the substitution of a tungsten unit in As2W18 with V(V) to form As2VW17.However, a broad, new Raman peak appeared at 1005 cm −1 , and its intensity increased depending on the concentration of V(V) (7-20 mM) (Figure 4, (d) and (e)).The new peak at approximately 1005 cm −1 was due to the formation of Keggin-type AsVW11 [45].Indeed, AsVW11 was isolated along with As2VW17 when prepared in accordance with the literature [48], where the solution conditions used in the synthesis were similar to those used in our experiments, as previously described.
Figure 3c shows the 31 P NMR spectrum collected for a solution of 100 mM W(VI), 500 mM P(V) and 50 mM V(V) at pH 2 heated at 80 °C for one week.Only one peak (at -14.3 ppm) was observed that was attributable to the formation of the Keggin-type PVW11.Even when the concentration of V(V) was increased or decreased, the two peaks at -10.8 and -12.9 ppm associated with the formation of P2VW17 were not observed.Notably, the occurrence of As2W18 or P2W18 in the reaction mixture appears to be essential for the formation of the corresponding V(V)-substituted species, As2VW17 and P2VW17.Figure 5. Cyclic voltammograms of a 100 mM Mo(VI)-10 mM As(V)-10 mM V(V)-0.5 M HCl system collected (a) after a solution without V(V) was heated at 80 °C for 7 h, (b) immediately after 10 mM V(V) was added to solution (a) and subsequently heated at 80 °C for 2 h and (c) after solution (b) was heated at 80 °C for one day.

Formation of Wells-Dawson-Type Vanadium-Substituted Molybdoarsenate and Molybdophosphate Complexes
The vanadium substitution reactions of P2Mo18 and As2Mo18 were also investigated using cyclic voltammetry and 31 P NMR spectroscopy.Three reduction peaks appeared at 477, 363 and 194 mV in the cyclic voltammogram collected after a solution of 100 mM Mo(VI), 10 mM As(V) and 0.5 M HCl was heated at 80 °C for 8 h; these peaks were due to the formation of As2Mo18 (Figure 5(a)) [47].After 10 mM V(V) was added and the solution was heated at 80 °C for 2 h, the original three peaks increased in magnitude of current, but the reduction potentials did not change (Figure 5(b)).Moreover, after the solution was heated at 80 °C for 15 h, a new reduction peak appeared at 500 mV in addition to a small peak at 375 mV, whereas the intensities of the original three reduction waves were diminished (Figure 5(c)).This new wave is ascribed to the formation of Keggin-type AsVaMo12-a [44].In fact, only AsVzMo12−z (z = 1,2) could be isolated as a tetra-alkyl ammonium salt from this solution (see the Supporting Information).The occurrence of AsVMo17 has not been confirmed, although we have extensively examined the solution under various conditions.In addition, we have investigated the direct formation of As2VMo17 from a Mo(VI)-As(V)-V(V)-HCl system under various conditions.However, only Keggin-type vanadium-substituted molybdoarsenate was formed in the solution.The substitution reaction of a molybdenum unit in P2Mo18 with V(V) into P2VMo17 was also extensively investigated.The 31 P NMR spectrum for a 100 mM Mo(VI)-100 mM P(V)-1.0 M HCl system showed two peaks at −0.97 and −2.49ppm due to the formation of PMo9 and P2Mo18, respectively (Figure 6a) [33,34,47].After 10 mM V(V) was added and the solution was allowed to stand at room temperature for 1 h, new peaks appeared at −3.57 in addition several peaks at around −3.4 ppm, which are ascribed to the formation of mono-vanadium-substituted and multi-vanadium-substituted Keggin-type molybdophosphate, respectively.Meanwhile, the intensities of the peaks at −0.97 ppm and −2.49ppm decreased in intensity, indicating that a small amount of P2Mo18 remained in the solution.
Although we have investigated the solutions under various conditions by changing the order of addition of reagents and the heating time, only Keggin-type vanadium-substituted molybdophosphates were formed.As with the Mo(VI)-As(V)-V(V) system, no P2VMo17 appeared at any considerable concentration in the Mo(VI)-P(V)-V(V) system.For both the Mo(VI)-V(V)-As(V) and -P(V) systems, no X2VMo17(X = As, P) formed from X2Mo18.These results suggest that Keggin-type XVMo11 should be substantially more stable than Wells-Dawson-type X2VMo17 in the solution. - δ /ppm Scheme 1. Vanadium substitution reaction of Wells-Dawson-type POMs for the M(VI)-X(V)-V(V) (M = W, Mo; X = P, As) systems in aqueous solution.

Experimental Section
Voltammetric measurements were performed using a microcomputer-controlled system.A glassy carbon (GC) electrode (BAS GC-30S) with a surface area of 0.071 cm 2 was used as the working electrode, and a platinum wire served as the counter electrode.The reference electrodes were Ag/AgCl (saturated KCl) for aqueous solutions and Ag/Ag + (0.01 M (M = mol/dm 3 ) AgNO3 in acetonitrile) for acetonitrile solutions.Prior to each measurement, the GC electrode was polished with 0.1-μm diamond slurry and washed with distilled water.The voltammograms were recorded at 25 ± 0.1 °C.Unless otherwise noted, the voltage scan rate was 100 mV/s.Raman spectra were recorded on a Horiba Jobin Yvon model HR-800 spectrophotometer.The argon line at 514.5 nm was used for excitation.The Raman measurements were conducted at 20 °C.For quantitative measurements, the Raman intensities were normalized by the 1048-cm −1 band associated with NO3 − ; 0.1 M NaNO3 was added and used as an internal standard.The 31 P NMR measurements were performed on a JEOL model JNM-LA400 spectrometer at 161.70 MHz.An inner tube containing D2O was used as an instrumental lock.Chemical shifts were referenced to 85% H3PO4.The preparation procedures of the n-Bu4N + salts of the POMs used in the present study are described in the Appendix.

Conclusions
In the present study, the V(V)-substitution reactions of Wells-Dawson-type POMs were investigated in aqueous solution using 31 P NMR and Raman spectroscopy and cyclic voltammetry.The elucidated vanadium substitution reaction processes are summarized in Scheme 1.The addition of V(V) ions to a solution containing As2W18 or P2W18 led to the corresponding As2VW17 or 1-P2VW17, respectively, although not all P2W18 in the solution was completely transformed into 1-P2VW17, even after a large excess of V(V) was added.These results suggest that P2W18 is more stable than As2W18.As2VW17 and 1-P2VW17 can be prepared directly from As2W18 and P2W18, respectively, via the addition of V(V), a synthetic procedure that is simpler than those previously reported [48][49][50][51].This simplified procedure enables the preparation of a large amount of X2VW17 at industrial levels as oxidant catalysts at low cost.Interestingly, tungsten in the one-position of X2W18 could be substituted with V(V) to form X2W17, while X2W18 decomposed with a weak base to form 4-X2W17.The order of addition of the reagents to the reaction mixture was very important.Even when the concentrations of W(VI), V(V), H + and P(V) or As(V) were the same, mixing and then heating the reagents led to the Keggin-type XVW11, although As2VW17 was observed as a mixture in the case of the As(V) system.This result implies that the occurrence of X2W18 in the reaction mixtures was essential for the formation of the corresponding V(V)-substituted species, X2VW17.In contrast, only XVMo11 (X = As, P) formed, rather than X2VMo17, in the Mo(VI)-V(V)-P(V) and -As(V) systems, even when various concentrations of V(V), P(V), As(V) and acid were used and the order of the addition of the reagents was varied.The results suggested that the formation constants of XVMo11 are likely much greater than those of X2VMo17.To the best of our knowledge, the literature contains few reports of the synthesis of Wells-Dawson-type vanadium-substituted phosphorus-and arsenic-centered heteropolymolybdates, X2VMo17 (X = P, As).According to the results of the present study, X2VMo17 cannot be synthesized using simple procedures.split into two one-electron transfer waves at +0.04 and −0.14 V vs. SCE for P2W18, and +0.08 and -0.10 V for As2W18 [53,54].Because the redox potential of the vanadium components of vanadium-substituted POMs XVM11 and S2VW17 was more positive than those of the molybdenum or tungsten components in these complexes, the reduction waves at approximately 200 mV should be ascribed to the reduction of V(V) to V(IV) in the X2VW17 complexes [46,[55][56][57][58].Each of the Emid for PM12 (M = Mo, W) occurred at a more negative potential than that of the corresponding AsM12.Similarly, each Emid for P2W18 and P2VW17 also occurred at a more negative potential than that of the corresponding As2W18 and As2VW17, although we could not elucidate the origin of the oxidation wave at 0 mV in the voltammograms of As2VW17 and P2VW17.No oxidation wave was observed when the switch potential was set at a more positive potential than the tungsten reduction potential.On the basis of these voltammetry results, the occurrence of the vanadium substitution reaction can be determined from the appearance of a new peak at more positive potentials than the redox potentials of the Mo or W component in the POMs after the addition of V(V) to the reaction mixtures.POMs were isolated as a tetra-butylammonium salt from a 100 mM Mo(VI)-10 mM As(V)-0.5 M HCl-5-20 mM V(V) system after measuring cyclic voltammograms as shown in Figure 5.The IR spectra of the isolated salts were measured using a Jasco 460-plus IR spectrometer; IR bands were observed at 950, 890, 847, and 786 cm −1 .These bands were compared with the reported data to identify the mixture of n-Bu4N + salts of [AsVMo11O40]
Figures2A(a) and 2B(a) show a cyclic voltammogram and a Raman spectrum, respectively, of a 100 mM W(VI)-200 mM As(V)-pH 2.5 system heated at 80 °C for one week.Four-step reduction waves were observed at 95, −83, −416 and −676 mV, which corresponded to one-, one-, two-and two-electron transfers, respectively.In addition, a Raman peak appeared at 994 cm −1 .These voltammetric waves and the Raman peak are due to the formation of the As2W18.After 10 mM V(V) was added, the pH was re-adjusted to 2.5 with conc.HCl and the solution was heated at 80 °C for one day; the original reduction waves and the Raman peak at 994 cm −1 disappeared, and new reduction waves at 437, −263 and −448 (Figure2A(b)) and a new Raman band at 988 cm −1 appeared (Figure 2B(b)).The voltammetric and Raman spectral behaviors indicate that one of the tungsten units in the As2W18 was substituted with

Figure 6 .
Figure6.31 P NMR spectra of a 100 mM Mo(VI)-100 mM P(V)-25 mM V(V)-1.0 M HCl system collected (a) after a solution without V(V) was heated at 80 °C for 30 h and (b) after V(V) was added and the solution was heated again at 80 °C for one day.