Effect of Electron (De)localization and Pairing in the Electrochemistryof Polyoxometalates: Study of Wells−DawsonMolybdotungstophosphate DerivativesLoïc Parent,† Pablo A. Aparicio,‡ Pedro de Oliveira,§ Anne-Lucie Teillout,§ Josep M. Poblet,‡

Xavier Lopez,*,‡ and Israel M. Mbomekalle*,†,§

†Universite de Versailles St. Quentin, Institut Lavoisier de Versailles, UMR 8180 CNRS, Versailles, F-78035, France‡Departament de Química Física i Inorganica, Universitat Rovira i Virgili, Marcel·li Domingo s/n, 43007 Tarragona, Spain§Universite Paris-Sud, Laboratoire de Chimie-Physique, Equipe d’Electrochimie et de Photoelectrochimie, UMR 8000 CNRS, Orsay,F-91405, France

*S Supporting Information

ABSTRACT: Polyoxometalates (POMs) are inorganic entities featuring extensive and sometimes unusual redox properties. Inthis work, several experimental techniques as well as density functional theory (DFT) calculations have been applied to identifyand assess the relevance of factors influencing the redox potentials of POMs. First, the position of the Mo substituent atom in theWells−Dawson structure, α1- or α2-P2W17Mo, determines the potential of the first 1e− reduction wave. For P2W18−xMox systemscontaining more than one Mo atom, reduction takes place at successively more positive potentials. We attribute this fact to thehigher electron delocalization when some Mo oxidizing atoms are connected. After having analyzed the experimental andtheoretical data for the monosubstituted α1- and α2-P2W17Mo anions, some relevant facts arise that may help to rationalize theredox behavior of POMs in general. Three aspects concern the stability of systems: (i) the favorable electron delocalization, (ii)the unfavorable e−−e− electrostatic repulsion, and (iii) the favorable electron pairing. They explain trends such as the secondreduction wave occurring at more positive potentials in α1- than in α2-P2W17Mo, and also the third electron reduction takingplace at a less negative potential in the case of α2, reversing the observed behavior for the first and the second waves. In P2W17Vderivatives, the nature of the first “d” electron is more localized because of the stronger oxidant character of VV. Thus, thereduction potentials as well as the computed reduction energies (REs) for the second reduction of either isomer are closer toeach other than in Mo-substituted POMs. This may be explained by the lack of electron delocalization in monoreduced P2W17V

IV

systems.

■ INTRODUCTION

Polyoxometalates (POMs) are a family of discrete molecularentities often considered as analogues of soluble molecularoxides.1−4 These species have enormous potential, sincefeatures such as size, shape, and chemical composition maybe tailored for a particular purpose.5,6 For example, in thefamily of tungsten-containing Wells−Dawson-type POMs,Contant et al. have shown that through stereoselective,multistep syntheses it is possible to replace 1 up to 6 WVI

centers with either MoVI or VV centers in a controlled way.7−10

By and large, POMs are seen as electron sponges and are oftenimplicated in reversible electron transfer processes, either at anelectrode/solution interface or between species in solution,which makes them excellent models to study reaction

mechanisms in electrochemistry.11−23 The above-mentionedmolybdenum-containing Wells−Dawson-type POMs, and inparticular the behavior of electrons transferred to them, hasattracted the attention of several authors. The aim of theirstudies was to determine, in a first step, if the electrons arepreferentially transferred into a defined site (atom or group ofatoms) and, in a second step, to check if the electrons remainlocated in that site or delocalize over neighboring sites or overthe whole molecule, as is the case for tungsten-containingKeggin-type POMs. Other works may be cited, like the seminalpapers by Livage and co-workers,24,25 Contant et al.,26,27 and

Received: January 14, 2014

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© XXXX American Chemical Society A dx.doi.org/10.1021/ic500087t | Inorg. Chem. XXXX, XXX, XXX−XXX

Baker et al.28−30 These authors used either spectroscopicmethods (NMR, EPR) or magnetic susceptibility measurementapproaches on samples containing reduced POMs. Electro-chemical results have seldom been compared to theoreticalcalculations, and in the majority of cases experimenters do notgo beyond the description of the electrochemical features of thecompounds.11−19

In this work, we combine standard electrochemicaltechniques (cyclic voltammetry (CV) and controlled potentialcoulometry (CPC)), UV−visible spectrophotometry, and DFTcalculations in order to endeavor to unravel the mechanismsruling electron transfer and electron distribution within themolecular orbitals of molybdo-tungstic Wells−Dawson-typePOMs. The structures selected for the present study are shownin Figure 1: α1- and α2-[P2W17MoO62]

6−, α-[P2W15Mo3O62]6−,

and α-[P2W12Mo6O62]6−. They contain one MoVI center, three

equivalent MoVI centers, and six MoVI centers, equivalent in a2(cap):4(belt) fashion, respectively. We resort to bothexperimental and theoretical methods in an effort to under-stand and rationalize the electron distribution between themolybdenum and the tungsten orbitals within these species.

■ EXPERIMENTAL SECTIONGeneral Methods and Materials. Pure water was obtained with a

RiOs 8 unit followed by a Millipore-Q Academic purification set. Allreagents were of high-purity grade and were used as purchased withoutfurther purification: H2SO4 (Sigma-Aldrich), H3PO4 (Sigma-Aldrich),HCl (VWR), anhydrous Na2SO4 (Riedel-de Haen), Na2MoO4·2H2O(Prolabo), Na2WO4·2H2O (Chempur), and KCl (Fluka). Puresamples of K6·α1-[P2W17MoO62]·19H2O (α1-P2W17Mo), K6·α2-[P2W17MoO62]·19H2O (α2-P2W17Mo), K6·α-[P2W15Mo3O62]·19H2O(α-P2W15Mo3), and K6·α-[P2W12Mo6O62]·19H2O (α-P2W12Mo6)were obtained as previously described31 from K10·α1-[P2W17O61]·20H2O (α1-P2W17), K10·α2-[P2W17O61]·20H2O (α2-P2W17), Na12·α-[P2W15O56]·24H2O (α-P2W15), and K12·α-[H2P2W12O48]·24H2O (α-P2W12), respectively, after sequential degradation of K6·α-[P2W18O62]·14H2O (α-P2W18).

32 Purity was confirmed by IR, 31P NMR, and cyclicvoltammetry.The stability of the polyanions α1-P2W17Mo, α2-P2W17Mo, α-

P2W15Mo3, and α-P2W12Mo6 in solution as a function of the pH andtime was studied by monitoring the evolution of their UV−visiblespectra at least over 6 h. Such duration is long enough for theelectrochemical characterization of each compound. All compoundswere found to be stable between pH 0 and pH 3.The IR spectra were recorded on a Nicolet 6700 FT Spectrometer

driven by a PC with the OMNIC E.S.P. 5.2 software.The 31P NMR spectra were recorded on a Bruker AC-300

spectrometer operating at 121.5 MHz in 5 mm tubes with 1Hdecoupling.The UV−visible spectra were recorded on a PerkinElmer Lambda

19 spectrophotometer with 2.5 × 10−5 M solutions of the relevant

polyanion. Matched 10.000 mm optical path quartz cuvettes wereused.

Electrochemical data were obtained using an EG & G 273 Apotentiostat driven by a PC with the M270 software. A one-compartment cell with a standard three-electrode configuration wasused for cyclic voltammetry experiments. The reference electrode wasa saturated calomel electrode (SCE) and the counter electrode, aplatinum gauze of large surface area; both electrodes were separatedfrom the bulk electrolyte solution via fritted compartments filled withthe same electrolyte. The working electrode was a 3 mm OD glassycarbon disc or a ca. 120 × 10 × 2 mm3 stick (GC, Le Carbone-Lorraine, France). The pretreatment of this electrode before eachexperiment has been described elsewhere.33 The polyanion concen-tration was 5.0 × 10−4 M. Prior to each experiment, solutions werethoroughly deaerated for at least 30 min with pure Ar. A positivepressure of this gas was maintained during subsequent work. All cyclicvoltammograms were recorded at a scan rate of 10 mV s−1, andpotentials are quoted against the saturated calomel electrode (SCE)unless otherwise stated. All experiments were performed at roomtemperature, which is controlled and fixed for the laboratory at 20 °C.Results were very reproducible from one experiment to the other, andslight variations observed over successive runs are rather attributed tothe uncertainty associated with the detection limit of our equipment(potentiostat, hardware and software) and not to the workingelectrode pretreatment nor to possible temperature fluctuations.

Computational Details. Density functional theory (DFT)calculations were performed on a series of compounds using theGaussian 09 suite of programs.34 The geometries of all the structureswere optimized in the oxidized and reduced states for the sake ofaccuracy in the calculation of reduction energies. Especially whenlocalized metal electrons are present, local structural changes from theoxidized geometry may become relevant. We applied the B3LYPhybrid functional35 with double-ζ quality basis set supplemented withpolarization functions (d for oxygen, f for transition metals). For heavyelements, we used the LANL pseudopotentials of Hay and Wadt.36

The calculations include the polarizable continuum model (PCM)37 toaccount for the stabilizing effects of an aqueous solution with adielectric constant ε = 78.4. The solute cavity was created using ascaled van der Waals surface and a grid of 5 points per Å2. The atomicradii correspond to the Universal Force Field parameters. We appliedthe spin-unrestricted formalism to compute the electron density ofopen-shell molecules. Atomic spin densities and charges were obtainedby means of the Mulliken procedure.

In an electrochemical process, the redox potential (E) and thereaction Gibbs free energy (ΔG) are formally linked by the number ofelectrons exchanged in the process and the Faraday constant:

Δ = −G nFE

We present reduction energies, defined as RE = E(POMn‑red) −E(POMox), for the processes POMox + ne− → POMn‑red. For thispurpose, we computed electronic energies for (n-fold) reduced andoxidized forms with the energy of the free electron taken as zero.Assuming that the electronic energy change during the reductionprocess is practically equal to the Gibbs free energy change (entropic

Figure 1. Idealized structure of α1-[P2W17MoO62]6−, α2-[P2W17MoO62]

6−, α-[P2W15Mo3O62]6−, and α-[P2W12Mo6O62]

6− derivatives. White andgray octahedra contain W and Mo atoms in the center, respectively.

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change term negligible), RE ≈ ΔG, the computed REs may be seen astheoretical measures of the experimental reduction potentials:

≈ −nFERE

The last expression shows that a species with a more negative REthan another will consequently have a more positive E, and vice versa.We will discuss the computational results mostly as dif ferences betweenREs (in eV) or E (in V).

■ RESULTS AND DISCUSSIONExperimental Part. α2-[P2W17MoO62]

6− Redox Behavior ina pH 3.0 Solution: Comparison with the Parent Compoundsα-[P2W18O62]

6− and α2-[P2W17O61]10−. The loss of a WO4+

moiety during the formation of α2-P2W17 starting from α-P2W18is accompanied by the increase of the overall absolute electricalcharge of the polyanion, from −6 to −10, and a concomitantstrengthening of its basicity. Several changes are thereforeobserved when the CVs of both compounds are compared, asshown in Figure 2. The first two one-electron waves of theplenary structure α-P2W18 (black curve) merge into a firstsingle two-electron wave in the case of the lacunary structureα2-P2W17 (red curve). The latter is located at more negativepotentials and is pH dependent, revealing the influence of amore pronounced alkaline character.22

Results gathered from different pertinent articles show thatupon replacing the missing W atom with a “d” metal cation, theredox behavior of α-P2W18 is not regenerated; i.e. theobservation of two one-electron redox processes that are pHindependent is not recovered.38 Indeed, it appears that in termsof the redox behavior of the tungstic framework, the newsubstituted compound α2-P2W17M (with M = MnII, FeIII, CoII,NiII, CuII, or ZnII) is closer to its lacunary parent, α2-P2W17 thanto its plenary parent α-P2W18. The first three redox processesattributed to the tungsten framework remain bielectronic andpH dependent. Even in the case of α2-P2W17V where thesubstituent VIV cation is in the same coordination environmentas W, i.e. with a terminal oxo group, VO, the redox signatureof α2-P2W17 remains: three successive pH dependent two-electron reversible processes (see Figure S1).39 W reductionwaves usually stand clearly apart from the redox wave of thesubstituent cation M, when the latter is easier to reduce thanW.3,9 The first redox process, which is monoelectronic and pH

independent, is easily attributed to the reduction of the VV

center, the following redox processes which may be assigned tothe reduction of the W framework being almost the same as theones observed on the CV of α2-P2W17.Observations made on the CV of α2-P2W17MoVI have clearly

shown a different behavior, thus suggesting another inter-pretation. The presence of a MoVI substituent cation results inthe appearance of two one-electron waves located at lessnegative potentials compared to the W waves of the parent α2-P2W17. The most positive of these waves is easily attributed tothe reduction of the MoVI center, and the second one isattributed to the reduction of WVI.40 Compared to the CV of α-P2W18 (Figure 3), the first W reduction wave in α2-P2W17MoVI

cathodically shifts about 200 mV but remains monoelectronic.Reminiscence of the redox behavior of the lacunary parentspecies, α2-P2W17, observed on the CVs of all the othersubstituted compounds α2-P2W17M (with M = VV, MnII, FeIII,CoII, NiII, CuII or ZnII),8,9 has almost disappeared here. Thefirst W reduction wave (wave II′ of the red curve in Figure 3)

Figure 2. CVs of α-[P2W18O62]6− (black line) and α2-[P2W17O61]

10− (red line) in 0.5 M Na2SO4 + H2SO4, pH = 3.0. Polyoxometalate concentration0.5 mM; scan rate 10 mV s−1; working electrode: glassy carbon; reference electrode: SCE. (A) All the reversible waves. (B) The processescorresponding to the transfer of the first two electrons.

Figure 3. CVs of α-[P2W18O62]6− (black line) and α2-

[P2W17MoVIO62]6− (red line) in 0.5 M Na2SO4 + H2SO4, pH = 3.0.

Polyoxometalate concentration: 0.5 mM; scan rate: 10 mV s−1;working electrode: glassy carbon; reference electrode: SCE.

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has turned single-electronic again and pH-independent, as isthe case for α-P2W18. However, the following one (wave III′ ofthe red curve in Figure 3) is a two-electron, pH-dependentwave, whose behavior resembles that of α2-P2W17. In fact, whenthe first three waves of α2-P2W17MoVI are compared with thoseof α-P2W18 obtained in the same experimental conditions, afeature stands out: the addition of a MoVI center leads to apartial regeneration of the behavior of the saturated compoundα-P2W18. Also, it is important to note that the two species, α2-P2W17MoVI and α-P2W18, have the same formal charge. Thepositive shift of wave III′ in α2-P2W17MoVI with respect to waveIII for α-P2W18 will be discussed in the DFT section.The electrochemical behavior of α2-P2W17MoVI is clearly

different from those of the parent compounds, which have thesaturated structure α-P2W18 and the lacunary structure α2-P2W17, and from those of the monosubstituted analogues ofglobal formula α2-P2W17M (with M = VV, MnII, FeIII, CoII, NiII,CuII, or ZnII). In an effort to understand and rationalize thisbehavior, several comparisons taking into account the presenceof MoVI (α2-P2W17 and α2-P2W17MoVI), the position and/or thenumber of substituent cations (α2-P2W17MoVI, α1-P2W17MoVI,α-P2W15Mo3 and α-P2W12Mo6), or the nature of thesubstituted cation (α-P2W15V

V3 and α-P2W12V

V6) are made in

the following paragraphs.Compared behavior of the α1 and α2 Isomers. Figure

4A shows the CVs of the two isomers α1- and α2-[P2W17MoVIO62]

6− obtained at pH 3.0. As expected, MoVI iseasier to reduce in the α1 position than in the α2 position. Thefirst electron taken by the α1 isomer partially delocalizes overthe belt region of the molecule, while it is trapped in one of thecaps in the α2 isomer. In the following electron transfer step,expected to be the first reduction of the W centers, the α1isomer is still easier to reduce than the α2 isomer (Table 1).Interestingly, for this electron transfer, the theoreticalcalculations for the α1 and α2 isomers show that the electronpreferentially goes into the metal centers situated in a beltposition of the Wells−Dawson structure.41 After this secondredox process, and if we concentrate on the belt region of thesemolecules, which is strongly implicated in electron transfer, werealize that the electron density is higher in the case of the α1isomer than in the case of the α2 isomer. As a consequence, thethird reduction wave is found at a more negative potential for

isomer α1 since the belt region is more electron populated (twobelt electrons) than in the α2 isomer (one belt electron) at thisstage. Indeed, an inversion in the precedence of the wavesoccurs; that is, the third wave occurs now at a more negativepotential for α1 than for α2 (Figure 4A and B). This observationconstitutes a supplementary proof of the fact that the beltregion of the Wells−Dawson-type structure is the electrontransfer preferential zone when these molecules undergoreduction processes. This point will also be discussed in theDFT section.

Nature and Number of Substituents: Comparisonwith P2W15Mo3 and P2W15V3. The electrochemical behaviorof tri-Mo-substituted polytungstate, α-[P2W15Mo3

VIO62]6− (see

Figure S2), has been described before.42

Redox steps associated with the oxidation of the Mo centersin the three-electron reduced species α-[P2W15Mo3O62]

9−

confirmed that the three electrons remained delocalized inthe Mo3O13 cap, which gives rise to three identical MoVI centers(see Table 2). The same behavior is observed with thetrivanadium derivative, α-[P2W15V3

VO62]9−, the three VV

Figure 4. (A) CVs of α1-[P2W17MoVIO62]6− (black line) and α2-[P2W17MoVIO62]

6− (red line) in 0.5 M Na2SO4 + H2SO4, pH = 3.0. Polyoxometalateconcentration: 0.5 mM; scan rate: 10 mV s−1; working electrode: glassy carbon; reference electrode: SCE. (B) Evolution between the midpointredox potential values, E1

0′, E20′, and E30′ for α1-[P2W17MoVIO62]

6− (black line) and α2-[P2W17MoVIO62]6− (red line).

Table 1. Midpoint Redox Potential Values, E10′, E2

0′, andE3

0′, for the First Three Redox Processes of α1-[P2W17MoVIO62]

6− and α2-[P2W17MoVIO62]6− in 0.5 M

Na2SO4 + H2SO4, pH 3.0

V vs SCE E10′ E2

0′ E30′

Mo(1e) W(1e) W(2e)α1-P2W17Mo 0.42 −0.03 −0.50α2-P2W17Mo 0.25 −0.18 −0.31ΔE(α1- α2) 0.17 0.15 −0.19

Table 2. Number of Electrons Transferred at Each RedoxProcess for α-[P2W15O56]

12−, α-[P2W15V3VO62]

9−, and α-[P2W15Mo3

VIO62]6− Evaluated upon Comparing the Peak

Currents Measured on CVs Obtained under the SameConditions (0.5 M Na2SO4 + H2SO4, pH 3.0)

(V/Mo)I (V/Mo)II (V/Mo)III WI WII

α-[P2W15O56]12‑ 4e 2e

α-[P2W15V3VO62]

9− 2e 1e 4e 2eα-[P2W15Mo3

VIO62]6− 1e 1e 1e 6e 2e

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centers being reduced to the +IV oxidation state in two veryclose, successive steps, a two-electron one followed by a single-electron one (see Table 2 and Figure S3). The reduction of theV centers takes place at more positive redox potentials.Finally, whatever the position of the substituted MoVI (or

VV) cation on the belt or on the cap, and whatever theirnumber (1 or 3), they are always preferentially reduced whencompared to the W framework and either for one or threeadded electrons, respectively. It is interesting to check if in thecases of α-[P2W12Mo6

VIO62]6− and α-[P2W12V6

VO62]12−, the

first six added electrons are preferentially transferred anddelocalized on the six MoVI or VV centers.The CV of α-[P2W12Mo6

VIO62]6− in 0.5 M Na2SO4 +

H2SO4/pH 3.0 begins with three well-behaved reversible wavesthat feature the reduction of Mo centers (see Figure S4). Thefirst two-electron wave is largely separated from the twosubsequent ones, which are also two-electron. Controlledpotential coulometry experiments conducted on a solution ofα-[P2W12Mo6

VIO62]6− in 0.1 M HCl/pH 1 with the potential

set at −0.35 V vs SCE (i.e., after the first three waves attributedto the reduction of Mo centers) consumed an electrical chargethat corresponds to six moles of electrons per mole of α-[P2W12Mo6

VIO62]6− (see Figure S5). This confirms the

observations and assertions made by Contant et al. whoestablished a direct relation between the number of substitutedMo centers on the Wells−Dawson framework and the numberof electrons involved in the first reduction steps attributed tothem.10a Furthermore, the comparison with the CV of thelacunary species α-P2W12 recorded under the same conditions(see Figure S4) is in total agreement with the assertion that thefirst six added electrons are transferred to the six MoVI centers.The same observation is made with the vanadium-containinganalogue α-P2W12V6 (see Figure S6). This may now beestablished as a general rule that, during the reduction of Mo-or V-substituted Wells−Dawson polytungstates, added elec-trons are first transferred to the Mo or V centers, whatever theirnumber and position. When all of the Mo or V centers are one-electron reduced each, the subsequent added electrons are nowtransferred to the W framework. Mo and V centers are theinitial sites of electron transfer in Wells−Dawson substitutedpolytungstates. Obviously, it is hard to imagine that theseelectrons will be trapped in Mo or V orbitals.11a

DFT Calculations. DFT is recognized as an optimaltechnique to mimic, understand, and predict many phys-icochemical properties of systems as large as POMs.43 Some ofus recently published a theoretical work showing that DFT/B3LYP calculations including solvent effects are able toreproduce accurately the electrochemical trends for mixed-metal POMs, notably the redox potential differences betweencompounds.44 This firm ground allows us to make use of theDFT to rationalize the present experimental CV data. The maincomputational results are the REs listed in Table 3 which willbe referred to the parent compound α-P2W18 (RE = −4.234

eV) during the following discussion. It is clear that most REsare more negative than −4.234 eV, indicating the presence ofstronger oxidant species, in line with the reduction potentialsdiscussed above. We want to stress that the systems have beenmodeled in conditions of no protonation, which is not alwaysthe most realistic approach. This fact is taken into account inthe analysis whenever required. In the present section, we makea theoretical analysis of the distribution of the extra electronsamong the metal centers and how this is related withelectrochemical measurements, making a special emphasis inthe different oxidant power of the α1/α2 isomers of P2MoW17.

Calculations on α-P2W18, α2-P2W17, α2-P2W15Mo3, andP2W12Mo6. The plenary α-P2W18 system is an oxidant asstrong as, for instance, the Keggin anion, [PW12O40]

3−, despitecarrying a higher negative charge owing to the fact that thecharge −6 is distributed over a larger structure. For α-P2W18,the first electron(s) occupy the belt region, which is moreelectron attracting than the cap regions. Compared to it, thelacunary α2-P2W17 system is more difficult to reduce, with a REbeing 1.6 eV less favorable (in nonprotonated form) than for α-P2W18, a fact arising from the large negative charge of −10.However, the electrochemical measurement gives a smallerdifference between the reduction waves of these twocompounds. In the conditions of measurement, α2-P2W17 isprotonated so its total absolute charge is less negative than −10,explaining the theoretically predicted value for α2-P2W17.Inspection of the molecular orbital occupied by the firstincoming electron shows that it is also delocalized over theequatorial (belt) region.As shown in Table 3, α-P2W15Mo3 and the monosubstituted

α2-P2W17Mo compounds have similar REs, the former being 70meV more negative. The presence of the Mo3 unit in one of thecaps allows for some degree of electron delocalization afterreduction and, consequently, a more favorable process than theextra electron being more localized in a single MoV site. TheCV measurements give a difference of 35 mV at pH 3 betweenthe mentioned compounds. Our DFT data show that each Moin the cap retains the same amount of the extra electron, withsome participation of the nearest W neighbors.In α-P2W12Mo6, the ellipsoidal Mo6 framework can favor

delocalization of extra electron(s) even more than in the above-mentioned α-P2W12Mo3 system. For the DFT calculations wehave taken into consideration the experimental fact that the firstreduction wave is a 2e− process. To obtain computationally aRE (or E) comparable with the position of the first reductionCV wave, a 2e− wave, we computed the 2e−-reduced and theoxidized forms and therefore obtained −4.610 eV as the valueto be compared with the first midpoint potential of 0.465 V vsSCE. The theoretical value is in good agreement with themeasurements since it is the most negative RE of the series,slightly more negative than the RE obtained for the 1e−

reduction of α1-P2W17Mo. The more advantageous reductionin the hexamolybdate derivative is a consequence of electron

Table 3. Computed REsa and Eb for the Wells−Dawson Compounds Discussed in This Section

α-P2W18 α2-P2W17 α2-P2W17Mo α1-P2W17Mo α-P2W15Mo3 α-P2W12Mo6

first reduction −4.234 (0.0) −2.590 (−1.644) −4.426 (+0.192) −4.594 (+0.360) −4.495 (+0.261) −4.610 (2e)c

(+0.376)second reduction −3.586 (−0.648) −3.767 (−0.467)α2-P2W17V α1-P2W17V

first reduction −4.576 (+0.342) −4.673 (+0.439)second reduction −3.255 (−0.979) −3.298 (−0.936)

aValues in eV obtained under conditions of no protonation. bIn parentheses, values in eV with respect to α-P2W18.cTwo-electron process.

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delocalization observed in the calculations. DFT results alsosuggest that the first electrochemically injected electrons areconfined to the four belt molybdenum atoms with theparticipation of some neighboring belt W centers. We alsocomputed the hypothetical 1e−-reduction process (α-P2W15Mo6

6− + e− → α-P2W15Mo67−), obtaining atomic spin

populations of 0.25 electrons per Mo and therefore reinforcingthe idea that the first electron(s) is (are) delocalized over thebelt positions only, leaving the two cap Mo centers fullyoxidized. These data put in evidence the importance ofdelocalization on the electrochemical properties of POMs.Calculations on α1-P2W17Mo and α2-P2W17Mo. First

Reduction Process. The molybdenum monosubstitutedWells−Dawson anions deserve a detailed analysis since theylead to interesting conclusions. Besides the well-known fact thatWells−Dawson compounds containing Mo are more oxidantthan the parent species α-P2W18, the position of Mo within thestructure plays a crucial role in the overall oxidizing power, notonly with respect to the first reduction process but also in thesecond and third ones. In the cap-substituted α2 isomer there isa “competition” for the first incoming electron between theMoVI atom, in a polar position, and the belt W atoms. Suchcompetition derives from two opposing facts: (i) the emptyorbitals of MoVI have lower energy compared to the WVI ones,and (ii) the empty belt orbitals are lower in energy than theempty cap orbitals. In the end, DFT results show that the cap-MoVI/V process is 440 mV more favorable than the belt-Wreduction for α2-P2W17Mo. Thus, the first metal electron islocalized in the cap. The other positional isomer, α1-P2W17Mo,behaves similarly although a larger degree of electrondelocalization can be observed in the 1e-reduced form basedon atomic population analysis. When Mo is in the cap positionit retains about 82% of the extra electron, whereas it decreasesto 77% when Mo is in the belt site. Since electrondelocalization usually gives extra stabilization to reducedforms in POMs, the computed 1e−-reduction process(P2W17Mo6− + e− → P2W17Mo7−) is more favorable by ∼170mV in the α1 form, in good agreement with the experimentaldifference of 170 mV (see Table 1). Thermodynamically, thefirst 1e−-reduction process is more favorable for the belt-substituted compound, where the chemical and structuraleffects add up to favor reduction.The oxidizing power of α1/α2-P2W17Mo must also be

compared with that of α-P2W15Mo3. DFT calculations, inagreement with CV measurements, show that α1-P2W17Mo is astronger oxidant than α-P2W15Mo3 by about 100 mV (seeTable 3). The advantageous delocalization in the Mo3 polargroup experienced by the metal electron in the 1e−-reduced α-P2W15Mo3 system cannot be on a par with the extrastabilization produced in the Mo belt position of α1-P2W17Mo. The fact that α2-P2W17Mo is slightly less oxidantthan α-P2W15Mo3, both being cap-substituted compounds, iseasily explained by the enhanced electron delocalizationoccurring in the latter compound.To end with the discussion on the first reduction processes,

we add a comment on the monosubstituted vanadate, P2W17V,since it helps to rationalize the previously discussed facts. Therelative shift between the first 1e− wave for α1- and α2-P2W17Vis ΔRE = 97 meV (measured ΔE = 89 mV). This smalldifference compared with P2W17Mo is attributed to the morelocalized nature of the extra electron in reduced V-containingsystems. In other words, V preserves its nature more than Mowhen placed in the Wells−Dawson structure, and therefore, its

position (cap or belt) is electrochemically less relevant. Thecomputed atomic spin populations for the 1e−-reduced α1 andα2 tungstovanadates are ∼1.0 on the V center, a value to becompared with 0.82 and 0.77 per Mo atom in the homologousmolybdate compounds.The above discussion allows us to establish a difference of

about 90 meV as the energy change of belt vs cap metalposition, which we estimate from the one-electron REdifference for α1/α2-P2W17V. Extra RE difference betweenboth isomers, like in Mo-substituted anions, comes from themore delocalized nature of the involved orbitals (which is morepronounced in the belt region). In other words, the ability of anelectron to hop from one center to another, larger in Mo thanin V, stabilizes the molecular orbital containing that electronand favors reduction. This explains that the RE difference forα1- and α2-P2W17V is smaller than that for α1- and α2-P2W17Mo.Therefore, we infer that the extra stabilization of a belt-localizedelectron compared to the cap-localized case is intimately relatedwith the different degree of electron delocalization in the beltregion.We carried out a complementary calculation to evaluate

further the effect of electron delocalization upon the reductionpotential. We compare two systems: α1-P2W17Mo and thehypothetical α-P2W12Mo6 model with six neighboring Moatoms in a single belt ring (W3:Mo6:W6:W3; see Figure S7).Both molecules are equally charged and contain Mo atoms inthe equatorial positions, the difference being the number of Moatoms. If we consider the first reduction as a 1e−-process, wefind a reduction potential difference of 290 mV in favor of α-P2W12Mo6. Such a difference can only be attributed to the effectof electron delocalization. A very similar value of 265 mV wasrecently computed for the Keggin structure.44 As a matter offact, the energies of the LUMOs of the oxidized form for eachcompound are progressively deeper in energy as the number ofimplicated Mo atoms increases, namely, the LUMO for α-P2W12Mo6 is 120 meV lower in energy than that of α1-P2W17Mo. If we look at the atomic spin populations of the 1e−-reduced forms, we find that in α1-P2W17Mo the extra electron isdelocalized among the Mo atom and two or three vicinal Watoms. In the case of α-P2W12Mo6, 80% of the extra electron isdelocalized over the Mo6 ring, and the other 20% among theother W6 in the belt. The larger the number of metal centersaccepting a fraction of the incoming electron, the morefavorable the reduction process is. This phenomenon isapplicable when comparing α2-P2W17Mo and α-P2W15Mo3,for instance, or α1-P2W17Mo and α-P2W12Mo6.

Second and Third Reduction Processes. At this point, let usdiscuss the computational results for the second 1e-reductionprocess in α1/α2-P2W17Mo to complement the CV data. We areespecially interested in unravelling the complete CV (first threereduction waves) of α1/α2-P2W17Mo, notably the tricky relativepositions of the second and third waves, at least at first sight.Experimental data cannot reveal if the second metal electron,going to the belt region, is mostly localized (MoIV character) orpartially delocalized (W17MoV 1e− character). If we assumedthat the first 1e-reduction produces MoV in either isomer, thesecond electron must go to the fully oxidized WVI beltpositions, but at a more negative potential due to the molecularcharge increment that the first reduction entails. But whatcauses the mutual shift of 100 mV of the second wave for eachisomer? The reduction potentials computed for the POM(1e)+ e− → POM(2e) process for both isomers predict that shift tobe around 150 mV, and thus, we may inspect what is the origin

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of this phenomenon. We computed the possible solutions forthe 2e−-reduced systems, namely, the unpaired and the pairedelectron cases represented in Figure 5A.

Interestingly, at the DFT/B3LYP level, each of thesesolutions is the most stable for one of the isomers when M =Mo. In α2-P2W17Mo, the unpaired situation is the most stableby 70 meV, indicating that the second electron prefers todelocalize over the W atoms thus avoiding any MoIV character.On the other hand, the electron-paired solution is 173 meVmore stable than the unpaired one in α1-P2W17Mo. In the twocases (α1 and α2), the second electron goes to the belt regionbut in a different manner and, consequently, with a differentenergy. The pairing process occurring in α1 appears as afavorable one, with some MoIV character as depicted in thescheme, with respect to the nonpaired situation in α2. In α1, thepresence of one electron in the belt MoV does not hinder thesecond one from occupying the same region, but it actuallyfavors it by e−−e− pairing. In α2, provided that the secondelectron is forced to go to the belt region, the second reductionis just favored by the lower e−−e− electrostatic repulsion that,from the present data, appears to be a weaker advantage thane−−e− pairing. The electron pairing argument is reinforced bythe well-known and proved fact that the 2e−-reduced α-P2W18species is strongly diamagnetic.28,45 The character of an electroncan be measured by inspection of the molecular orbital itoccupies, and also by atomic populations. Both of themcoincide in the more delocalized nature of the belt electronswith or without Mo.Present calculations show that, after the second 1e reduction,

the α2-P2W17MoV1e− situation is the most stable by an energydifference of 70 meV. However, things are different in the α1isomer, for which an important MoIV character is acknowl-edged. As shown in Table 3, the RE difference between thesecond 1e− processes (α1 − α2) agrees with the experimentalresults and justifies them by the different character of thesecond electron in either isomer in favor of α1. Thus, thementioned facts suggest a possible competition between twofactors, namely (a) the unfavorable e−−e− electrostaticrepulsion and (b) the favorable electron pairing. Each isomeris characterized by one dominating factor. In α1, the firstelectron already occupies a part of the belt region (MoV andsome WV character of the vicinal atoms). Although the secondelectron experiences the repulsive presence of the first one, theycan pair and thus stabilize the couple (see Figure 6, leftdiagram). On the other hand, the α2 isomer has the first e−

trapped in the cap region, the belt region being free of extraelectron density prior to the second reduction. This being anelectrostatic advantage with respect to the α1 isomer, electronpairing will not be possible. We may deduce that, as long as the

region is sufficiently large for delocalization, the first twoelectrons will be paired and stabilized. This explanation isschematically depicted in Figure 6.It must be pointed out that for P2W17V, the present

discussion stands, but giving a different result. Since the first1e− reduction in the vanadotungstate produces a highlylocalized VIV electron, the second one has hardly a chance ofpairing with it (see Table 3). Thus, the electrostatic repulsionwill be similar irrespective of the position of the initial electron(cap VIV or belt VIV). This results in two second reductionwaves close to each other. The computed values differ by 33mV only.Finally, the third reduction wave, although it is pH

dependent, may be justified using the above arguments. Atthis stage (2e−-reduced anions), the situation favors reductionof α2 at a more positive potential since this isomer contains twounpaired electrons, one in the cap (MoV) and one in the belt(WV character). The third electron can pair with one of these,the one in the belt being the most favorable one. Concerningα1, no advantages with respect to α2 are envisaged since the beltregion is highly electron-populated by two paired electrons.Since the third electron is forced to go to the belt, no electronpairing is possible, and a notable electrostatic repulsion willforce this process to be less exothermic than for its α2 partner.The schematic view of the molecular orbital occupations for3e−-reduced anions is shown in Figure 5B, where the left-handsituation implies some MoIV character, whereas the right-handone corresponds to MoV.

■ CONCLUSIONSThe four compounds α1- and α2-[P2W17MoO62]

6−, α-[P2W15Mo3O62]

6−, and α-[P2W12Mo6O62]6− have been synthe-

sized and characterized by several physicochemical techniques,namely IR, UV−vis spectrophotometry, NMR, and CV. Their

Figure 5. Schematic view of the most plausible electron distributionsfor (A) the two-electron reduced and (B) the three-electron reducedforms of P2W17Mo. Horizontal lines represent molecular orbitals.When two electrons (circles) occupy the molecular orbital designatedMo/W, some MoIV character is present, whereas one electron in theMo/W-like orbital implies MoV.

Figure 6. Representation of the second and third reduction processestaking place in the α1 and α2 isomers of P2W17Mo (Mo atomsrepresented by gray circles) and the factors favoring them in each case.The four 3:6:6:3 loops of metal atoms in the Wells−Dawson structureare simplified to thin gray lines. Blue arrows are electrochemicallyadded electrons, and yellow curved arrows represent the delocalizednature of the belt electrons.

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electrochemical behavior has been compared to that of theirhomologues containing vanadium, α2-[P2W17V

VO62]7−, α-

[P2W15V3VO62]

9−, and α-[P2W12V6VO62]

12−, and also to thatof their parent compounds, that is the saturated species α-[P2W18O62]

6− and the lacunary derivative α2-[P2W17O61]10−.

This comparative study has allowed, in each case, to identifyclearly the redox processes assignable to the MoVI centers fromthose attributable to the WVI centers. The DFT calculationresults and the experimental observations mentioned abovelead to the same conclusions: the MoVI centers present in themolecules α1- and α2-[P2W17MoO62]

6−, α-[P2W15Mo3O62]6−,

and α-[P2W12Mo6O62]6− are all reduced first, irrespective of

their position or number, and just then the extra electrons aredirected toward the WVI centers. We tried to recognize whichfactors play a role, and what is their relative importance, on theredox potentials of Wells−Dawson anions. DFT calculationscarried out on the title compounds have reproduced the generaltrends of oxidizing power measured by CV. The differencesobserved between the studied compounds may be assigned to afew chemical or structural factors. Taking α-P2W18 as thereference (reduction potential E = 0.0 V for convenience), thecomputed reduction energies of all the Mo-containingcompounds are more negative. In the case of α1- and α2-P2W17Mo, the computed reduction potentials are 360 and 192mV more positive, respectively, than for α-P2W18, following theexperimental observations. Replacing more W atoms with Mo(up to three or six adjacent Mo centers) results in a largerdegree of delocalization of the metal electron(s) and anenhanced exothermic reduction process (a positive shift ofreduction potentials) since the electron-accepting molecularorbitals are more stable as they have more centers involved.This assumption allows one to explain that the cap-substitutedα-P2W15Mo3 anion is more oxidant than α2-P2W17Mo. In asimilar fashion, the α-P2W12Mo6 anion is more oxidant than anyof the other combinations presented here, notably since twofactors add up: the presence of Mo atoms in the belt region andthe synergy of multiple vicinal Mo atoms that boost electrondelocalization. The latter goes in favor of increasing oxidizingpower.Theoretical evidence has been gathered to explain the

relative positions of the first, second, and third reduction wavesin α1/α2-P2W17M isomers with M = Mo or V. The mostrelevant and general conclusion, which might have widerrepercussions than just the redox processes, is the competitionof three factors when one or several “d” metal electrons meet inthe Wells−Dawson structure in what concerns stability: (i) thefavorable electron delocalization, (ii) the unfavorable e−−e−electrostatic repulsion, and (iii) the favorable electron pairing.These factors explain that the second reduction wave occurs atmore positive potentials in α1- than in α2-P2W17Mo, and alsowhy the third electron transfers at a less negative potential inα2, reversing the behavior for the first and second waves. InP2W17V derivatives, the nature of the first “d” electron is morelocalized because of the stronger oxidant character of VV. Thus,the potentials and also the computed REs for the secondreduction of either isomer are more similar to each other thanfor M = Mo. This may be explained by the lack of electrondelocalization in monoreduced P2W17V

IV systems.

■ ASSOCIATED CONTENT*S Supporting InformationAdditional experimental data: CVs (Figures S1−S6); idealizedstructure of the hypothetical α1-[P2W12Mo6O62]

6− derivative

(Figure S7); IR (Figure S8), RMN (Figures S9 and S10), andUV−visible (Figure S11) spectra. This material is available freeof charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Authors*E-mail: javier.lopez@urv.cat.*E-mail: israel.mbomekalle@u-psud.fr.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work was supported by the Universite Paris-Sud, theUniversite of Versailles and the Centre National de laRecherche Scientifique (CNRS; UMR 8000, UMR 8182), theSpanish Ministry of Science and Innovation (Project No.CTQ2011-29054-C02-01/BQU), and the DGR of the General-itat de Catalunya (2009SGR462 and the XRQTC). X.L. isgrateful to the Ramon y Cajal program (grant number RYC-2008-02493). This collaboration takes place in the context ofthe COST PoCheMoN action supported by the EuropeanResearch Area.

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