Voltammetric and Spectroscopic Studies of Î±- and Î²-[PW12O40]3

Article pubs.acs.org/IC

Voltammetric and Spectroscopic Studies of α- and β‑[PW12O40]3− Polyoxometalates in Neutral and Acidic Media: Structural Characterization as Their [(n‑Bu4N)3][PW12O40] Salts Tadaharu Ueda,*,† Keisuke Kodani,† Hiromi Ota,‡ Motoo Shiro,§ Si-Xuan Guo,∥,# John F. Boas,⊥ and Alan M. Bond*,∥ †

Department of Applied Science, Faculty of Science, Kochi University, Kochi 780-8520, Japan Division of Instrumental Analysis, Department of Instrumental Analysis and Cryogenics, Advanced Science Research Center, Okayama University, Okayama 700-8530, Japan § X-ray Research Laboratory, Rigaku Corporation, Akishima, Tokyo 196-8666, Japan ∥ School of Chemistry, #ARC Centre of Excellence for Electromaterials Science, and ⊥School of Physics and Astronomy, Monash University, Clayton, Victoria 3800, Australia ‡

S Supporting Information *

ABSTRACT: The structure of the Keggin-type β-[PW12O40]3− (PW12) polyoxometalate, with n-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 [n-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 βPW12 imply that the bridging oxygens between the WI and WII units are preferentially protonated in acidic media. EPR data on frozen solutions of one-electron reduced β-[PWVWVI11O40]4− indicate that either the WI or the WIII unit in β-PW12 is reduced in the β-[PWVI12O40]3−/β-[PWVWVI11O40]4− process. In the absence of acid, reversible potentials obtained from the α- and βisomers of PW12 and [SiW12O40]4− exhibit a linear relationship with solvent properties such as Lewis acidity, acceptor number, and polarity index.

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[XM12O40]n− and [X2M18O62]n−, respectively, exhibit a series of well-resolved one-electron transfer reactions in neutral or slightly acidic aqueous media that merge into two or more electron transfer reactions in strongly acidic aqueous solutions.6 Recently, new functional materials based on POMs and carbon materials have been developed for energy conversion, energy storage, and electrocatalysis, using the rich electrochemical

INTRODUCTION The chemical and physical properties of polyoxometalates (POMs) have been studied extensively.1 POMs exhibit a wide range of structures, and many have been used in catalysis, analytical chemistry, biochemistry, and materials chemistry.2−4 POMs also exhibit extensive and often complicated redox behavior. The ability to exhibit multielectron and protontransfer reactions facilitates their applications as oxidation catalysts in organic synthesis and water splitting.2a,b,e,5 In general, Keggin and Wells−Dawson-type POMs, such as © 2017 American Chemical Society

Received: December 22, 2016 Published: March 14, 2017 3990

DOI: 10.1021/acs.inorgchem.6b03046 Inorg. Chem. 2017, 56, 3990−4001

Article

Inorganic Chemistry properties and catalytic abilities of POMs.7 In the case of [XW12O40]n− (X = Co, Zn, H2, B, Al, Ga, Si, Ge, P, As, S), the reversible potentials have been related to the total charge of POMs and the mean bond length between tungsten and oxygens, which are bonded to hetero atoms.8,9 The thermodynamics (E0′ values) of the primary electron transfer steps and coupled acid base chemistry of many POMs are available.8d,f,9−12 In the case of α-[PW12O40]3−, quantitative studies involving simulation−experiment comparisons have been reported in acid media by Himeno et al.12b and in a study by Eda et al.10e For the β-isomer, no quantitative solution phase data are available, so the relative acidities are unknown, although the protonation site of the 4e-reduced β[PMo12O40]7− has been proposed on the basis of a crystal structure.1g Recently, we explained the acid dependence of the vanadium(V/IV) component of the reduction in Keggin-type vanadium-substituted POMs, [XVM11O40]n− (X = P, As, S; M = Mo, W), in CH3CN (0.1 M [n-Bu4N][PF6]) by combining detailed simulations of cyclic voltammograms with NMR (the V(V) component) and EPR (the V(IV) component) data.11 The acid−base chemistry associated with the reduction of [SiW12O40]4− also has been investigated in buffered aqueous media (pH 2.1−6.8).8f With Keggin-type POMs, five isomers (α, β, γ, δ, and ε), generated by π/3 rotations of the M3O13 unit, are known. In the case of [PW12O40]3−, it has been established that the E0′ values of the β-form are more positive than those of the α-form,8d,12a as was also reported in other POM studies.12c In this study, the X-ray crystallographic structures of Bu4N+ salts of α- and β-PW12 are reported. Structures of the α-isomer containing a range of counter cations are known,13 and indeed the first one for this isomer was undertaken by Keggin1f in his early pioneering work in this field of chemistry. A study of the α-isomer with Bu4N+ as the countercation was reported many years ago,13a but no cif files are available. The authors also reported a γ-structure13a,b with coordinates,13b which confusingly has the structural characteristics of an α-isomer. Thus, we have undertaken a new determination of the α-structures (maybe a different polymorph), and details of our structure are compared to those for the β-isomer, which is now reported for the first time. Solution-phase cyclic voltammetric studies on both isomers have been undertaken in CH3CN (0.1 M [nBu4N][PF6]) in the absence and presence of acid. Simulations in acid media that mimic the voltammetry associated with the conversion of two one-electron transfers into a single twoelectron transfer by the protonation of α- and β-PW12 are consistent with NMR (oxidized forms) and EPR (one-electron reduced forms generated by bulk electrolysis) spectroscopic data. In combination, the electrochemical and spectroscopic data provide some insights into the nature of the one-electron [PWVI12O40]3−/[PWVWVI11O40]4− charge transfer step for both isomers including relative basicities. In addition, the effects of organic solvents6 on the reversible potentials for these processes are compared to those for the α- and β[SiW12O40]4− (α- and β-SiW12).14a

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Other reagents were of analytical grade and used as received. The nBu4N+ salts of α- and β-forms of PW12 and SiW12 were prepared according to literature methods.8d,14 The α-isomer is readily obtained in pure form. Isomerically pure polycrystalline samples of the β isomer (>99% β-form) were derived from recrystallization from acetone and used for all electrochemical and spectroscopic measurements. However, recrystallization from acetone did not give single crystals of the pure β isomer suitable for X-ray structural analysis, while recrystallization of the n-Bu4N+ salt of β-PW12 from acetonitrile always contained some of the α-form. Nevertheless, a crystal containing about 11% of the α-isomer obtained by this procedure was found to be suitable for X-ray analysis.8d A CH3CN solvent of crystallization also was included in this structure. The dimensions of β-PW12 were obtained after correction for the contribution from the α-PW12 component. X-ray single-crystal crystallographic data were collected with a Rigaku Saturn 724 diffractometer using multilayer mirror monochromated Mo Kα radiation. The structures were solved by direct methods (SHELXL-9715 for α-PW12 and SIR9216 for β-PW12) and expanded using Fourier techniques. Some non-hydrogen atoms were refined anisotropically, while the rest were refined isotropically. Hydrogen atoms were refined using the riding model. The final cycle of full-matrix least-squares refinement17 on F2 was based on 15 263 and 961 observed reflections and 18 999 and 1069 variable parameters for α- and β-PW12, respectively, and converged (largest parameter shift was 0.01 times its esd) with unweighted and weighted agreement factors of: R1 = ∑||Fo| − |Fc||/∑|Fo| (I > 2.00σ(I)); wR2 = [∑(w(Fo2 − Fc2)2)/∑w(Fo2)2]1/2. The goodness of fit (GOF)18 with unit weighting was 1.025 for αPW12 and 1.057 for β-PW12. Plots of ∑w(|Fo| − |Fc|)2 versus |Fo|, reflection order in data collection, sin θ/λ, and various classes of indices showed no unusual trends. The maximum and minimum peaks on the final difference Fourier map correspond to 7.66 and −3.91 e−/ Å3 for α-PW12, and 8.06 and −3.88 e−/Å3 for β-PW12, respectively. The absolute structure of β-PW12 was deduced on the basis of a Flack parameter19 of 0.060(5). Further comments on the refinement of the structures are contained in the footnotes to Table 1 and the cif files. Neutral atom scattering factors were taken from Cromer and Waber20 for α-PW12 and International Tables for Crystallography (IT), Vol. C, Table 6.1.1.421 for β-PW12. Anomalous dispersion effects were included in Fcalc;22 the values for Δf′ and Δf ″, as well as for the mass attenuation coefficients, are those of Creagh and McAuley.23,24 All calculations were performed using the CrystalStructure ver. 4.1 crystallographic software package (Rigaku) except for refinement, which was performed using SHELXL-97 for α-PW12 and SHELXL2013 for β-PW12.25 Voltammetric and bulk electrolysis measurements were carried out with a BAS 50W (Bioanalytical Systems, BAS) electrochemical workstation. A standard three-electrode arrangement was employed. For voltammetric studies, a glassy carbon disk electrode with a surface area of 0.071 cm2 was used as the working electrode, platinum wire as the counter electrode, and Ag/Ag+ (0.01 M AgNO3 in CH3CN) as the reference electrode (silver wire in CH3CN containing 0.01 M AgNO3 in a double jacket and separated from the test solution by a sintered glass frit). For bulk electrolysis, a carbon cloth with a large surface area was used as the working electrode along with the same reference and auxiliary electrodes as used in the voltammetry. However, in this case, the working and auxiliary electrode compartments were also separated by sintered glass frits. Unless otherwise stated, the scan rate in voltammetric experiments was 100 mV s−1. The measured potential versus Ag/Ag+ was converted to the Fc/Fc+ (Fc = ferrocene) scale using data derived from voltammograms for oxidation of ferrocene. Prior to each measurement, the glassy carbon electrode (GCE) was polished with aqueous 0.1 μm diamond slurry and washed with distilled water and dried under nitrogen gas. The sample solution was always purged with nitrogen or argon gas to remove dissolved oxygen. All electrochemical measurements were made at 25.0 ± 0.2 °C. Simulations of cyclic voltammograms were undertaken with DigiSim version 3.03b software (BAS).

EXPERIMENTAL SECTION

Tetrabutylammonium hexafluorophosphate, [n-Bu4N][PF6], recrystallized at least twice from ethanol, was used as the supporting electrolyte in electrochemical experiments. Acetonitrile (CH3CN) of LC−MS grade and spectroscopic grade acetone (AC), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and nitromethane (NM) were purchased from WAKO and used as received from the manufacturer. 3991


Article

Inorganic Chemistry

be indistinguishable from the γ one reported earlier.13a We have also now structurally characterized the n-Bu4N+ salt of the βisomer of PW12 for the first time. Our new structures are provided in Figure 1, and the crystallographic and structural refinement data are summarized

Table 1. Crystallographic and Structure Refinement Data for α-PW12 and β-PW12 empirical formula FW (g mol−1) T (K) radiation (λ, Å) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z dcalcd (g/cm3) no. obs. (all data) GOF final R indices (I > 2.00σ(I)) final R indices (all data)

α-PW12a,b

β-PW12c

C48H108N3O40PW12 3604.55 123(1) 0.71075 triclinic P1̅ (2) 13.7108(19) 15.384(2) 19.356(3) 95.385(3) 93.074(3) 90.727(3) 4058.2(10) 2 2.950 15179 1.025 R1 = 0.0791 wR2 = 0.1970 R1 = 0.0940 wR2 = 0.2078

C50H111N4O40PW12 3645.60 120(1) 0.71075 orthorhombic P212121 (19) 14.650(3) 22.345(4) 25.449(4) 90 90 90 8313(3) 4 2.906 112412 1.057 R1 = 0.0371 wR2 = 0.0801 R1 = 0.0379 wR2 = 0.0806

a The W−O cage in W12O36 incorporates the phosphate anion in its center, which lies on an inversion center of the crystal. The phosphate anion is disordered around the inversion center. Only one of the three tetrabutylammonium cations could be resolved with the other two disordered (occupancy ratio = 0.417:0.587). The C atoms in the disordered model are heavily superimposed, so that they were refined isotropically. bAppears to be indistinguishable from the γ-isomer reported in refs 13a and 13b with respect to space groups, bond lengths, bond angles, and other parameters. The α-isomer in ref 13a is therefore assumed to be a polymorph of our α-isomer. cThis crystal contains 11% of the α-isomer. Both isomers have approximately the same atomic coordinates except for the rotated “cap” unit in the beta isomer. In refining the structure for the beta isomer, the rotated “cap” unit was treated as disordered as was one of the “caps” in the alpha component. Predominantly, the beta structure was observed in the refinement. By treating the positive densities as arising from the alpha isomer, the refinement converged to provide the parameter values given in this table.

Figure 1. Stick and ball and polyhedral representation of the structures of (a) α-PW12 and (b) β-PW12. Red, gray, and sky-blue colored balls represent phosphorus, tungsten, and oxygen, respectively. The goldcolored balls indicate the π/3 rotated tungsten unit.

in Table 1. The mean bond lengths of the tungsten−oxygen and phosphorus−oxygen bonds were calculated and compared to those published for α-SW12, α-PW12, α-SiW12, β-SiW12, αAlW12, and β-AlW12 (Table 2).12c,14a,29−33 The uncertainties in the data for many of these POMs have been carefully considered in ref 14a. In the case of the β-SiW12 structure, these are substantial. This important consideration has limited the extent to which reliable comparisons of bond lengths can be made. Nevertheless, according to these data, the nature of the countercation has no effect within experimental uncertainty on the structure of the α-PW12. It is assumed that the same situation applies to the other POMs. Furthermore, the α- and β-bond lengths do not differ significantly. Voltammetry of α- and β-PW12. Cyclic voltammograms for reduction of 0.4 mM α- and β-PW12 were initially obtained in CH3CN (0.1 M [n-Bu4N][PF6]) in the absence of acid. For α-PW12, four chemically reversible couples were observed in the absence of acid with Em values of −670, −1190, −1890, and −2360 mV vs Fc0/+ (Figures 2A(a) and S1), where the midpoint potential Em = (Epc + Epa)/2 and Epc and Epa are reduction and oxidation peak potentials, respectively. These Em values, which are assumed to be equal to the formal potential (E0′), agree well with published data.8f,9,12b For β-PW12, in the absence of acid, the Em values are −560, −1100, −1770, and −2230 mV (Figures 2B(a) and S2). Bulk electrolysis applied to the first process for both isomers, with coulometric analysis of current−time data and analysis of the relevant characteristics of normal pulse, differential pulse, and cyclic voltammetry (scan rate range of 20−500 mV s−1), confirmed that each is a oneelectron charge transfer process. Diffusion-control was confirmed at the peak potential of the first process via the linear dependence of the peak current on the square root of scan rate.

183 W NMR spectra were obtained at 20.70 MHz with a JEOL model JNM-LA500 spectrometer. Chemical shifts are referenced to 2 M Na2WO4 in D2O. For the EPR measurements, bulk electrolysis was used to generate the paramagnetic one-electron reduced polyoxometalate and was performed under a nitrogen atmosphere. The reduced solution was then pipetted into an EPR tube and frozen immediately by insertion into liquid nitrogen. The X-band (∼9.5 GHz) spectra of the frozen solutions were recorded at 3.6 K with a Bruker EMX-plus spectrometer using the standard TE102 rectangular cavity. Spectrum simulations were performed using the Bruker XSophe-Xepr software suite26 as described elsewhere27,28 and in the Supporting Information.

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RESULTS AND DISCUSSION X-ray Structural Analysis of α-PW12 and β-PW12. The structural characterization of α-PW12 as the protonated form was described many years ago by J. F. Keggin.1f The structures of several other salts of α-PW12 are also available.7d Two structures with Bu4N+ as the countercation13a,b have been reported, which are labeled as α and γ, with coordinates available only for the so-called γ isomer. We have redetermined the structure of the α-isomer, which confusingly turned out to 3992


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Inorganic Chemistry Table 2. Selected Mean Bond Length of α- and β-Forms of XW12 (X = P, Si, Al)a POMs

cation

W−Oa

W−Ob

W−Oc

W−Od

X−Oa

ref

α-PW12 α-PW12 α-PW12 β-PW12c

n-Bu4N+ TMUb H+ n-Bu4N+ n-Bu4N+ Me4N+ H+ K+ n-Bu4N+

β-AlW12

K+

1.89 1.91 1.90 1.89 1.89d 1.89 1.89 1.90 1.91 1.94 2.01d 1.91 1.92d

1.89 1.92 1.91 1.91 1.90d 1.90 1.92 1.90 1.94 1.95 1.94d 1.93 1.94d

1.68 1.69 1.70 1.68 1.67d 1.66 1.71 1.68 1.71 1.74 1.68d 1.72 1.71d

1.52 1.53 1.52 1.53 1.53e 1.43 1.63 1.60 1.74 1.73 1.63e 1.73 1.75e

this study 13c 29 this study

α-SW12 α-SiW12 α-SiW12 α-AlW12 β-SiW12f

2.48 2.45 2.44 2.46 2.48d 2.57 2.35 2.36 2.27 2.29 2.35d 2.26 2.25d

30 31 32 12c 31 14a

a

Oa, oxygen bonded with heteroatom; Ob, octahedral corner-sharing oxygen; Oc, octahedral edge-sharing oxygen; Od, terminal oxygen. bTMU, hydrogen-bis(1,1,3,3-tetramethylurea). cα-PW12 component removed from the X-ray analysis data. dTungsten−oxygen bond lengths in the π/3rotated W3O13 unit. eHeteroatom bonded to the oxygen atom in the π/3-rotated W3O13 unit. fIn the Commentary in Supporting Information to ref 14a, a detailed analysis of the large uncertainties in the bond lengths is provided.

simulations of the voltammetry are presented, spectroscopic studies were undertaken to obtain some of the data needed to assist in the modeling of the acid−base chemistry that accompanies the charge transfer processes. NMR Spectroscopy of α- and β-PW12 in CH3CN in the Absence and Presence of Acid. Qualitatively, the positive shift in potential with increasing acid concentration observed in our cyclic voltammetric studies implies that the reduced forms in each couple in CH3CN are more basic than oxidized forms. Previously, Himeno et al. assumed that the oxidized form of αPW12 is not protonated in CH3CN,12b whereas Eda et al. considered this possibility.10e Simulations of the pH behavior of the cyclic voltammetry of [SiW12O40]4− in aqueous media employed protonation of the reduced polyoxometalate, but did not include protonation of oxidized [SiW12O40]4−.8f However, the interaction of protons with POMs in CH3CN is likely to be stronger than that in an aqueous medium because the extent of solvation of the proton is diminished. The impact of the solvent is evident in the changed voltammetric behavior of [(P2O7)Mo18O54]4− in CH3CN containing CF3SO3H in the presence of a small amount of water.34 To ascertain if protonation of the oxidized forms of α- and βPW12 occurs in CH3CN, 183W NMR spectra were obtained with 8 mM (saturated solution) α- and β-PW12 in CH3CN in the absence and presence of 25 mM acid (Figure 3). In neutral CH3CN, a single 183W resonance was observed at −88.3 ppm

Figure 2. Cyclic voltammograms of 0.4 mM (A) α-PW12 and (B) βPW12 in CH3CN (0.1 M [n-Bu4N][PF6]) in the presence of designated concentrations of CF3SO3H. [CF3SO3H]/mM = (a, −) 0.0; (b, − − −) 0.1; (c, − · − · −) 0.2; (d, · · · ·) 1.0; and (e, − ·· − ·· −) 5.0.

On addition of acid, all processes shifted to more positive potentials (Figures 2A, B(b−e), S1, and S2). In the presence of 5 mM CF3SO3H, pairs of one-electron waves merged into overall two-electron transfer processes (shown for the first pair in Figure 2A(e) and B(e)) with Em values of 560 and 710 mV for α-PW12 (in good agreement with ref 10e), and −420 and −550 mV for β-PW12. Even though the voltammetry is relatively simple in the presence of 5 mM acid, the voltammograms at intermediate acidities are a complex function of acid concentration (Figures 2, S1, and S2). With 0.4 mM α-PW12, as acid is added up to 0.3 mM, the current magnitude for the first process increased but the peak potential remained constant. In contrast, the current magnitude of the second one-electron process with Epc = −1230 mV decreased on addition of acid, and a broad new process appeared between the two original processes. In the presence of 0.3−5.0 mM CF3SO3H, the first process shifted to more positive potentials with the current magnitude continuing to increase. Upon addition of >5.0 mM acid, Em continued to become more positive, but with the current magnitude now constant. The voltammetric behavior of β-PW12 is similar to that of α-PW12 (compare Figure 2A and B) with the shift in Em being slightly larger for a given acid concentration. The simulation of voltammograms that mimic the acid dependence requires a multiparameter model. Thus, before the

Figure 3. 183W NMR spectra of solutions containing 8 mM of (A) αPW12 and (B) β-PW12 in CH3CN in the (a, −) absence and (b, − − −) presence of 25 mM CF3SO3H. 3993


Article

Inorganic Chemistry for α-PW12, with three resonances found at −94.5, −102.6, and −112.0 ppm (1:2:1) for β-PW12. On the basis of the 1:2:1 intensity ratio, the three resonances in the β-isomer are assigned to tungsten atoms in regions I, II, and III, respectively, of the β-isomer (Figure 1b),35 because the 183W NMR spectrum is similar to those of β-[SiW12O40]4− (β-SiW12) and β-[AlW12O40]5− (β-AlW12).12c After addition of excess acid (ca. 25 mM), the 183W resonance for α-PW12 shifted from −88.3 to −90.7 ppm, whereas those for β-PW12 at −94.5 and −102.6 ppm shifted to −96.1 and −105.1 ppm, but the resonance at −112.0 ppm remained unchanged. The 183W NMR data for αand β-PW12 confirm that the oxidized forms of both isomers are protonated in CH3CN. DFT calculations predict that the protonation should preferentially occur at the bridging oxygens between the tungsten atoms.36,37 The shift of the 183W signals for regions I and II of β-PW12 on addition of acid indicates that the protons in this isomer are attached to the bridging oxygens between WI and WII. However, an analogous preferential site location(s) cannot be deduced in the case of α-PW12 because all bridging oxygens are equivalent. In case of the four-electron reduced β-[PMo12O40]7−, the protonation site was reported to be the edge-shared oxygen of the WII region.1g Proton ion pairing with the α- and β-PW12 anions also could contribute to the chemical shifts. Importantly, the NMR data confirm that protonation of the oxidized forms of α- and β-PW12 need to be considered in simulations of cyclic voltammograms in acidic CH3CN solutions. EPR Spectroscopy of α- and β-PWVWVI11 in CH3CN in the Absence of Acid. The EPR spectrum of a 0.4 mM solution of α-PWVWVI11 in CH3CN (0.1 M [n-Bu4N][PF6]) after one-electron reduction and freezing to 3.6 K is shown in Figure 4a. The spectrum exhibits the three g-values character-

magnitude similar to those of [W6O19]3− are commonly observed for other compounds where the unpaired electron is clearly localized on a single W site.39 Spectrum simulations were performed for each of n = 1−6 where n is the number of equivalent tungsten sites over which the unpaired electron is delocalized. Progressively better fits to the experimental spectrum were obtained as n was increased from one to six, with the closest approach to the experimental spectrum of Figure 4a, shown as Figure 4b, being obtained with n = 6 and the g-values, hyperfine interaction, and line width parameters listed in Table 3. Simulations involving more than six Table 3. Spin Hamiltonian and Line Width Parameters for α[PWVWVI11O40]4− Generated from Electrochemical Reduction of α-[PWVI12O40]3− in CH3CN (0.1 M [nBu4N][PF6]) and Freezing the Solution to 3.6 Ka gi Ai σi δgi/gi

x

y

z

1.854 6.0 17 0.008

1.815 4.0 20 0.003

1.777 4.0 20 0.003

a

The simulations were performed assuming that the unpaired electron is delocalized over six equivalent W sites (see text). Hyperfine interactions (Ai) and line widths (σi) are in units of 10−4 cm−1. The average hyperfine interaction, Aav, is 4.7 × 10−4 cm−1.

equivalent sites required a prohibitive amount of computer time and were not performed. The experimental spectrum was better matched by a Lorentzian line shape at lower magnetic fields and a more Gaussian shape at higher fields, but these changes in line shape across the spectrum could not be accommodated in a single simulation. The labeling of the g-values so that gz < gy < gx is based on the similarity of the present spectra to those of the reduced W12 polyoxotungstates reported by Sanchez et al.38 where this ordering was suggested by the temperature dependence of the spectra below about 60 K. This order of the g-values is consistent with the unpaired electron being located in orbitals with 5dxy character. Because the deviation of the g-values from the free-electron ge (2.0023) is due to spin−orbit coupling, which only operates on the electronic component of the wave functions, the g-values themselves are essentially independent of the degree of delocalization. However, the g-values can be used in a crystal field calculation40 to estimate the hyperfine interactions and thus the unpaired electron density at a single nucleus. The number of equivalent centers, N, over which the unpaired electron is delocalized can then be estimated because the electron density at a single nucleus is reduced proportionately to 1/N. In the present case, the calculation using the g-values given in Table 3 leads to an average 183W hyperfine interaction of magnitude Aav ≈ 68 × 10−4 cm−1. The average of the hyperfine interaction parameters, as obtained through the spectrum simulation and listed in Table 3, is 4.7 × 10−4 cm−1. Although this is less than one-twelfth of the value of 68 × 10−4 cm−1 obtained by calculation from the g-values, the electron density will not be located exclusively on the W ion sites but will be partially delocalized over the POM framework. This suggests that the electron is delocalized over all 12 equivalent tungsten sites in α-PW12 at 3.6 K, even though the comparison of experimental and simulated spectra only allows us to conclude that the unpaired electron is delocalized over at least six

Figure 4. EPR spectra at 3.6 K derived from frozen solutions prepared by one-electron reduction of 0.4 mM α-PW12 in CH3CN (0.1 M [nBu4N][PF6]). (a) No added acid; (b) simulation of (a) as described in the text and using spin Hamiltonian and line width parameters as listed in Table 3. Gaussian line shape cutoff at 10σ. Experimental conditions: microwave frequency 9.504 GHz, microwave power 0.1002 mW, 100 kHz modulation amplitude 0.10 mT, field scan rate 1.25 mT/s, time constant 2.56 ms.

istic of a system with orthorhombic symmetry but shows no evidence for features attributable to the hyperfine interactions of the unpaired electron with the 14.31% abundant 183W (I = 1/2) nucleus. The absence of 183W hyperfine features is an indication that the unpaired electron is delocalized over a number of tungsten sites and contrasts with the spectrum of the one-electron reduced polyoxotungstate [W6O19]3− where the unpaired electron is presumed to be localized on a single WV site and the features due to 183W hyperfine interactions are clearly observed.38 183W hyperfine interactions of an order of 3994


Article

Inorganic Chemistry

from the measured g-values. This is also consistent with the unpaired electron being delocalized over the three W sites of either the WI or WIII regions and is also consistent with the NMR spectrum of the nonreduced β-PW12 described above and shown in Figure 3. The localization of the unpaired electron to one of the cap regions at 3.6 K would appear to be a consequence of the existence of a potential barrier between the regions arising from the rotation of the cap WIII by π/3 relative to the cap WI. The existence of similar, symmetry related, potential barriers between different regions of a cluster has been invoked to explain the localization of unpaired electrons to different regions of the cluster in some Wells−Dawson-type polyoxomtalates.27,28 However, the EPR spectrum does not allow us to decide whether the unpaired electron is delocalized over regions WI or WIII, as these are magnetically equivalent although being structurally inequivalent. As was the case for the α-isomer, the spectrum at 3.6 K does not enable a determination of whether gz < gy < gx or gz > gy > gx. The two features in close proximity at ∼381 mT, well separated from the feature at ∼367 mT, suggest the latter combination. This is not consistent with the g-value assignments of other W12 polyoxometalate systems,38 the expectation from crystal field theory,40−42 or with the expectation that the largest hyperfine interaction should be associated with the zdirection. However, the simulation assumes that the axes of the g- and hyperfine matrices of the individual tungsten ions are coincident. In the present case, the g-matrix axes of the individual ions will not necessarily be coincident with each other or with those of the hyperfine interaction, where the latter reflects the principal directions of the electron distribution over the relevant region of the cluster. A noncoincidence of g- and hyperfine matrix axes for β-PW12 is presumably a reflection of localization of the unpaired electron to one of the cap regions at 3.6 K. Simulations of Cyclic Voltammograms of α- and βPW12 in CH3CN in the Presence of Acid. To simulate cyclic voltammograms that closely resemble the experimental data, all electron transfer reactions had to be fully reversible (heterogeneous electron transfer rate constants being very fast (k0′ ≥ 0.3 cm s−1)). CF3SO3H was assumed to be a fully dissociated strong acid. Additionally, all second-order acid association rate constants are assumed to be diffusion controlled with a value of 1.0 × 1010 M−1 s−1. To assist in determining which protonation reactions should be included in the simulations over the full range of acidity examined, the reversible potential was determined in the presence of 5−20 mM of CF3SO3H. In principle, in this acid concentration range, the voltammetry of the now overall reversible two-electron reduction process can be described by eq 1:

equivalent W sites. Delocalization over all 12 tungsten sites is also consistent with the NMR data, which show that all W sites are equivalent in nonreduced α-PW12. The EPR spectrum of a 0.4 mM solution of β-PWVWVI11 in CH3CN (0.1 M [n-Bu4N][PF6]) after one-electron reduction and freezing to 3.6 K is shown in Figure 5a. The features of the spectra are readily associated with the g-values of an orthorhombic system. The barely resolved shoulder at around 389 mT is attributable to one of the components of the resonances arising from the hyperfine splitting due to interaction of the unpaired electron with the nucleus 183W (I = 1/2; 14.31% abundance). No other features identifiable as arising from hyperfine interactions are observed. The appearance of this spectrum and the relative intensity of the feature at ∼389 mT are best simulated by assuming that the unpaired electron is equally delocalized over three tungsten sites and using the g-values and hyperfine interaction parameters listed in Table 4. Delocalization over three W sites places the unpaired electron on tungsten ions located in one of the cap regions WI or WIII. Table 4. Spin Hamiltonian and Line Width Parameters for β[PWVWVI11O40]4− Generated from Electrochemical Reduction of β-[PWVI12O40]3− in CH3CN (0.1 M [nBu4N][PF6]) and Freezing the Solution to 3.6 Ka gi Ai σi δgi/gi

x

y

z

1.850 15 8 0.0025

1.780 15 8 0.0025

1.760 35 8 0.001

a

The simulations were performed assuming that the unpaired electron is delocalized over three equivalent W sites. Hyperfine interactions (Ai) and line widths (σi) are in units of 10−4 cm−1. The average hyperfine interaction, Aav, is 21.3 × 10−4 cm−1.

An estimate of the number of sites over which the unpaired electron is delocalized may also be made using the measured gvalues and crystal field theory as described above and in ref 38. The value of Aav of 21.3 × 10−4 cm−1 determined from the simulation of the envelope of the experimental spectrum is approximately one-third of the Aav ≈ 69 × 10−4 cm−1 calculated

α/β ‐[ HxPW12](3 − x) − + y H+ + 2e− ⇌ α/β ‐[ Hx + yPWV 2W10](5 − x − y) −

(1)

An estimate of the difference (y) in the number of protons associated with the PWVI12 and PWV2WVI10 redox levels can be gained from the slope ( yRT ) of a plot of reversible potential nF versus the logarithm of the acid concentration.43 The slopes obtained were 60.4 and 61.4 mV/log[H+] for α-PW12 and βPW12, respectively, indicating that y = 2 for both isomers, given that n = 2. The most likely interpretation of this finding is that minimal protonation occurs in the oxidized form and two protons are added to the polyoxometalate following the two-

Figure 5. EPR spectra at 3.6 K derived from frozen solutions prepared by one-electron reduction of 0.4 mM β-PW12 in CH3CN (0.1 M [nBu4N][PF6]). (a) No added acid; (b) simulation of (a) as described in the text and using spin Hamiltonian and line width parameters as listed in Table 4. Gaussian line shape, cutoff 10σ. Experimental conditions: microwave frequency 9.498 GHz, microwave power 0.1002 mW, 100 kHz modulation amplitude 0.10, field scan rate 1.25 mT/s, time constant 2.56 ms. 3995


Article

Inorganic Chemistry

Figure 6. Comparison of simulated (red ○) and experimental (−) cyclic voltammograms for reduction of α-PW12 in CH3CN (0.1 M [nBu4N][PF6]) in the presence of designated concentrations of CF3SO3H. [CF3SO3H]/mM = (a) 0.0; (b) 0.1; (c) 0.2; (d) 0.3; (e) 0.6; (f) 1.0; (g) 1.4; (h) 3.0; (i) 5.0. Scan rate: 100 mV s−1.

electron reduction. The implication also is that equilibria of the type 2PWVWVI11− + 2H+ ⇌ PWVI12 + H2PWV2WVI10 lie strongly to the right. On the basis of the above analysis, simulations need to accommodate zero, mono, and diprotonated forms of polyoxometalates at all redox levels. The diffusion coefficients (D values) of unprotonated POMs were calculated from cyclic voltammograms obtained in the absence of acid over the scan rate range of 20−500 mV s−1 and from use of the Randles−Sevcik equation.44 D values of protonated species were estimated analogously from the scan rate dependence found in the presence of sufficient excess acid when only a single two-electron process is found. D values calculated via these procedures are 5.0 and 4.7 (×10−6 cm2 s−1) for nonprotonated α-PW12 and β-PW12 species, respectively, and 4.7 and 4.6 (×10−6 cm2 s−1) for their protonated counterparts. The D value for the alpha isomer is similar to that reported in ref 8f, but lower than the unexpectedly high values used for this high molecular weight POM in simulations by Eda10e and Himeno.12b A D value for H+ in CH3CN of 3.1× 10−5 cm2 s−1, reported under similar conditions, was used for this species in the simulations.45 The k0 values for all α- and βPW12 reduction processes were set at 0.3 cm s−1, which corresponds to the reversible limit at a scan rate of 100 mV s−1. On the basis of the NMR data and the dependence of the reversible potential for high concentrations of H+, simulations of cyclic voltammetry based on reaction Scheme 1 should mimic the first and second one-electron reduction processes over the entire acid concentration range studied. Furthermore, simulations should predict the convergence of the resolved oneelectron processes in the absence of acid into an overall twoelectron reduction process for α-PW12 and β-PW12 in sufficiently acidic media. Comparisons of simulated and experimental cyclic voltammograms for the initial two WVI/ WV processes associated with the reduction of α-PW12 and βPW12 are shown in Figures 6 and 7, respectively. The simulated and experimental cyclic voltammograms are in very good agreement over all concentrations of CF3SO3H studied. The parameters listed in Tables 5 and 6 were used in the simulations and imply that protonation of PWV2W10 is very strong, while those of PW12 and PWVW11 are relatively weak. On this basis, the reaction 2PWVWVI11− + 2H+ ⇌ PWVI12 + H2PWV2WVI10 is highly favored in the presence of acid. However, the excellent

Scheme 1. Coupled Electron Transfer and Proton Reaction Scheme Used for Simulations of the Cyclic Voltammetry Associated with the First Two Reduction Steps for α-PW12 and β-PW12 in Acidic Solutions of CH3CN (0.1 M [nBu4N][PF6])

agreement between experimental and simulated data does not mean that the set of parameters contained in Tables 5 and 6 uniquely mimics the voltammetry as the cross redox and disproportionation reactions have not been specifically included in the simulation. Himeno et al.12b and Eda and Osakai10e simulated data with zero and a large excess of acid concentration where the voltammetry corresponds to apparently two-electron steps. Further, only one acid concentration was considered. The reported protonation association constants K differ substantially from ours. Our comprehensive study encompasses a range of acid concentrations where the first two one-electron processes converge to an apparently two-electron step, and excellent agreement of simulated and experimental data is obtained for all acid concentrations in a region where the voltammetry is sensitive to K values. Simulations using K values reported in refs 10e and 12b employing just one high acid concentration did not give good agreement with our experimental data over the low acid concentration range. Values deduced from our more comprehensive study should be more reliable. On this basis, we conclude that the one-electron and two-electron reduced forms of the β-isomer are more basic than the α form (compare data for protonation constants for the one-electron and two-electron reduced forms in Tables 5 and 6). This is consistent with the slightly larger shift in reversible 3996


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Inorganic Chemistry

Figure 7. Comparison of simulated (red ○) and experimental (−) cyclic voltammograms for reduction of β-PW12 in CH3CN (0.1 M [nBu4N][PF6]) in the presence of designated concentrations of CF3SO3H. [CF3SO3H]/mM = (a) 0.0; (b) 0.1; (c) 0.2; (d) 0.3; (e) 0.6; (f) 1.0; (g) 1.4; (h) 3.0; (i) 5.0. Scan rate: 100 mV s−1.

Table 5. Parameters Used in the Simulation of Cyclic Voltammograms for Reduction of α-[PWVI12O40]3− to α[PWVWVI11O40]4− and α-[PWV2WVI10O40]5− in Acidified Acetonitrile (0.1 M [n-Bu4N][PF6]) Solutionsa

Table 6. Parameters Used in the Simulation of Cyclic Voltammograms for Reduction of β-[PWVI12O40]3− to β[PWVWVI11O40]4− and β-[PWV2WVI10O40]5− in Acidified Acetonitrile (0.1 M [n-Bu4N][PF6]) Solutionsa

Heterogeneous Reactions process E0′/mV process E0′/mV

Heterogeneous Reactions

B + e− = C A + e− = B −670 −1190 HB + e− = HC H2A + e− = H2B −890 −450 Homogeneous Reactions

HA + e− = HB −590 H2B + e− = H2C −450

process E0′/mV process E0′/mV

A + H+ ⇄ HA KHA 25 kfHA 1.00 × 1010 kbHA 4.00 × 108 B + H+ ⇄ HB KHB 562 kfHB 1.00 × 1010 kbHB 1.78 × 107 C + H+ ⇄ HC KHC 6.61 × 107 kfHC 1.00 × 1010 kbHC 151

HA + H+ ⇄ H2A KH2A 0.03 kfH2A 3.00 × 108 kbH2A 1.00 × 1010 HB + H+ ⇄ H2B KH2B 6.97 kfH2B 1.00 × 1010 kbH2B 1.44 × 109 HC + H+ ⇄ H2C KH2C 1.90 × 108 kfH2C 1.00 × 1010 kbH2C 52.6

process protonation const. K (M−1)

process protonation const. K (M−1) forward rate const. kf (M−1 s−1) back rate const. kb (s−1) process protonation const. K (M−1) forward rate const. kf (M−1 s−1) back rate const. kb (s−1) process protonation const. K (M−1) forward rate const. kf (M−1 s−1) back rate const. kb (s−1)

B + e− = C A + e− = B −560 −1100 HB + e− = HC H2A + e− = H2B −840 −340 Homogeneous Reactions

HA + e− = HB −390 H2B + e− = H2C −340

A + H+ ⇄ HA KHA 15 kfHA 1.00 × 1010 kbHA 6.67 × 108 B + H+ ⇄ HB KHB 1.65 × 104 kfHB 1.00 × 1010 kbHB 6.05 × 105 C + H+ ⇄ HC KHC 7.34 × 108 kfHC 1.00 × 1010 kbHC 13.6

HA + H+ ⇄ H2A KH2A 2.0 kfH2A 3.00 × 108 kbH2A 5.00 × 109 HB + H+ ⇄ H2B KH2B 14.0 kfH2B 1.00 × 1010 kbH2B 7.14 × 108 HC + H+ ⇄ H2C KH2C 3.95 × 109 kfH2C 1.00 × 1010 kbH2C 2.53

forward rate const. kf (M−1 s−1) back rate const. kb (s−1) process protonation const. K (M−1) forward rate const. kf (M−1 s−1) back rate const. kb (s−1) process protonation const. K (M−1) forward rate const. kf (M−1 s−1) back rate const. kb (s−1)

a A, α-[PWVI12O40]3−; B, α-[PWVWVI11O40]4−; C, α[PWV2WVI10O40]5−.

a A, β-[PWVI12O40]3−; B, β-[PWVWVI11O40]4−; C, β[PWV2WVI10O40]5−.

potential per unit concentration of acid for the β-isomer (compare Figures 6 and 7) as noted above. It is also evident that the two-electron reduced forms of both isomers are much more basic than the one-electron reduced forms, which in turn are considerably more basic than the fully oxidized forms. Finally, it is noted that in a study on the vanadium substituted [XVW11O40]n− (X = P, As (n = 4), S (n = 3)), the addition of

acid was considered with respect to the VV/IV couple.9 In the current case, the WVI/V couple is examined. The reduced tungstate form is significantly more basic than the reduced VIV. Solvent Effect on the Voltammetry of α- and β-PW12. The voltammetric behavior of many Keggin-type polyoxometalates has been extensively investigated in organic solvents in the absence of acid. Em values for the first reduction process are 3997


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Inorganic Chemistry

Table 7. Em Values for the First and Second Reduction Processes for α- and β-PW12 and SiW12 in Organic Solvents (0.1 M [nBu4N][PF6]) and AN and ETN Values for the Organic Solventsa + 2nd E1st m /Em (mV vs Fc/Fc )

a b

POM

AC

α-PW12 β-PW12 α-SiW12 β-SiW12 A Nd ETNd

−870/−1404 −756/−1333 −1274/−1772c −1218/−1749 12.5 0.355 c

DMF

CH3CN

−798/−1396 −670/−1290 −1193/−1769 −1088/−1623 16.0 0.386

−670/−1190 −560/−1100 −1135/−1636c −1011/−1542 18.9 0.460

NMb

DMSO c

−654/−1204 −540/−1130 −1115/−1648c −1014/−1581 19.3 0.444 c

−617 −523 −1114 −1006 20.5 0.481

AC = acetone, DMF = dimethylformamide, DMSO = dimethyl sulfoxide, NM = nitromethane, AN = acceptor number, ETN = polarity index. Second process not well-resolved. cSlightly different values reported in refs 8f and 46. dData taken from ref 47.

absence and presence of acid confirm that the oxidized form interacts very weakly with protons. Protonation of β-PW12 is indicated to occur at bridging oxygen between WI and WII on the basis of analysis of NMR data. Low-temperature (3.6 K) EPR studies on frozen solutions of the one-electron reduced isomers have been undertaken. The results suggest that the electron is delocalized over all 12 equivalent tungsten sites in αPW12, even though the comparison of experimental and simulated spectra only allows us to conclude that the unpaired electron is delocalized over at least six equivalent W sites. In the case of β-PW12, the comparison of experimental and simulated spectra suggests that the electron is delocalized over only three equivalent W sites, most likely over one of the cap regions WI or WIII. The noncoincidence of g- and hyperfine matrix axes for β-PW12 is presumably a consequence of this localization of the unpaired electron on one of the cap regions. On the basis of the details deduced from spectroscopic and electrochemical data, a mechanism was proposed to describe the acid dependence and used for simulations of cyclic voltammograms. For all concentrations of CF3SO3H, excellent agreement of simulated experimental data was obtained. Consequently, the simulations provided an estimate of acid association constants, which confirms that PWV2WVI102− is strongly protonated with the order of the strength of protonation being PWV2WVI102− > PWVWVI11− > PWVI12. The mechanisms proposed for reduction of α-PW12 and β-PW12 in the presence of acid are the same, although acid association constants for the oxidized and reduced β-PW12 are larger than for their α-PW12 counterparts. Reversible potentials are a function of the characteritics of the oxidized and reduced forms of the POMs. Unfortunately, structural characterization of the reduced forms is not available, so the origins of the differences in potentials and protonation constants are not understood in a structural context. The first and second reversible potentials for α- and β-PW12 in the absence of acid are linearly related to the Lewis acidity, acceptor number, and ETN value of organic solvents, as also applies to α- and β-SiW12. In addition, the reversible potentials of the β-form are more positive than those of the corresponding α-form in all organic solvents.

linearly related to anion charge on the POM and the acceptor number (AN) of the organic solvents.6,46 However, the dependence of Em values on organic solvents for β-PW12 has not been established. Cyclic voltammograms of β-PW12 were therefore obtained in acetone, dimethylformamide, dimethyl sulfoxide, nitromethane, and acetonitrile. The Em values derived from the experiments are compared to those for α-PW12 as well as α-SiW12 and β-SiW12 (Figures 3S and 4S). Table 7 shows the first two Em values for the α- and β-isomers of PW12 and SiW12 used in the plots of Em versus the acceptor number and polarity index (ETN) values of organic solvents (Figures 8 and 5S).47 It is noted that the Em values of β-isomer of PW12 and SiW12 exhibit the same trends as those of the α-form with respect to dependence on physical properties of organic solvents, and both are linearly related to the acceptor number (AN) and polarity index (ETN). That is, both isomers are affected in a similar manner by the Lewis acidity of organic solvents.

Figure 8. Relationship between the first reversible potentials obtained for reduction of POMs in designated organic solvents as a function of AN and ETN. POM = (A, ○) α-PW12, (B, ●) β-PW12, (C, □) α-SiW12, and (D, ■) β-SiW12.

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CONCLUSIONS The X-ray determination of the structures of Bu4N+ salts of αand β-PW12 revealed that the α- and β-bond lengths do not differ significantly. Quantitative aspects of the electron- and proton-transfer reactions associated with reduction of α- and βPW12 have been established in CH3CN (0.1 M [n-Bu4N][PF6]) in the presence of CF3SO3H. In the absence of acid, the βisomer is more easily reduced by about 100 mV. Large positive shifts in reversible potential on addition of acid show that the two-electron reduced form is very basic relative to the fully oxidized PWVI12 and the one-electron reduced PWVWVI11− forms; that is, PWV2WVI102− is more strongly protonated than PWVWVI11−, which in turn is more strongly protonated than PWVI12. 183W NMR measurements on the oxidized form in the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b03046. Details on the simulation of EPR spectra, cyclic voltammograms of α-and β-PW12 in CH3CN over a wide potential range in the presence of designated 3998


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Inorganic Chemistry

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concentrations of acid, cyclic voltammograms for α- and β-PW12 and α- and β-SiW12 in designated organic solvents in the absence of acid, and the relationship between Em for the second redox process in organic solvents, acceptor number, and ETN (PDF) X-ray crystallographic data for α-PW12 (CIF) X-ray crystallographic data for β-PW12 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Tadaharu Ueda: 0000-0001-6797-5716 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-aid for Scientific Research (no. 25410095) from the Ministry of Education, Culture, Sports, Science and Technology of Japan, a Kochi University President’s Discretionary Grant provided by Kochi University, and the Australian Research Council. 183W NMR and EPR spectra were measured at the Instrument Center, the Institute for Molecular Science, the staff of which are thanked for their assistance and also for financial support for travel and accommodation. Valuable discussions on the crystallography with Stuart Batten are gratefully acknowledged.

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Inorganic Chemistry

1983, 105, 6817−6823. For [W6O19]3−, A∥ = 125 × 10−4 cm−1, A⊥ = 61 × 10−4 cm−1, and the average hyperfine interaction is Aav ≈ 82 × 10−4 cm−1. (39) Kon, H.; Sharpless, N. E. Electron Spin Resonance Study of Some Halomolybdenyl, -tungstenyl, and -vanadyl Complexes in Solution. J. Phys. Chem. 1966, 70, 105−111. (40) Wilson, G. L.; Greenwood, R. J.; Pilbrow, J. R.; Spence, J. T.; Wedd, A. G. Molybdenum(V) sites in xanthine oxidase and relevant analog complexes: comparison of molybdenum-95 and sulfur-33 hyperfine coupling. J. Am. Chem. Soc. 1991, 113, 6803−6812. Their eqs 2−7 can be used to estimate the hyperfine interactions from the measured g-values for the case where the unpaired electron is localized over a single W ion site. For α-PW12, we use values of P = 65 × 10−4 cm−1 and κ = 0.85 to derive values of Ax = 44, Ay = 47, Az = 112 × 10−4 cm−1, and Aav = 68 × 10−4 cm−1. For β-PW12, we obtain (using the same values of P and κ) Ax = 44, Ay = 48, Az = 114 × 10−4 cm−1, and Aav = 69 × 10−4 cm−1. (41) Prados, R. A.; Pope, M. T. Low-temperature electron spin resonance spectra of heteropoly blues derived from some 1:12 and 2:18 molybdates and tungstates. Inorg. Chem. 1976, 15, 2547−2553. (42) Yamase, T. Involvement of hydrogen-bonding protons in delocalization of the paramagnetic electron in a single crystal of photoreduced decatungstate. J. Chem. Soc., Dalton Trans. 1987, 1597− 1604. (43) Bond, A. M. Broadening Electrochemical Horizons - Principles and Illustration of Voltammetric and Related Techniques; Oxford: New York, 2002. (44) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley & Sons: New York, 2001. (45) Saotome, M.; Takano, S.; Tokushima, A.; Ito, S.; Nakashima, S.; Nagasawa, Y.; Okada, T.; Miyasaka, H. Picosecond-nanosecond laser photolysis studies of a photoacid generator in solutions: Transient absorption spectroscopy and transient grating measurements. Photochem. Photobiol. Sci. 2005, 4, 83−88. (46) (a) Maeda, K.; Katano, H.; Osakai, T.; Himeno, S.; Saito, A. Charge dependence of one-electron redox potentials of Keggin-type heteropolyoxometalate anions. J. Electroanal. Chem. 1995, 389, 167− 173. (b) Osakai, T.; Maeda, K.; Ebina, K.; Hayamizu, H.; Hoshino, M.; Muto, K.; Himeno, S. Non-Bornian Ion Solvation Energy. An Approach from Redox Potentials of Heteropoly Oxometalate Anions. Bull. Chem. Soc. Jpn. 1997, 70, 2473−2481. (47) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry, 3rd ed.; Wiley-VCH: New York, 2003.

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Voltammetric and Spectroscopic Studies of Î±- and Î²-[PW12O40]3

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