Electrochemical and Computational Study of Ion Association in the 3− Electroreduction of PW12O40 J

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Cite This: J. Phys. Chem. C XXXX, XXX, XXX-XXX pubs.acs.org/JPCC

Electrochemical and Computational Study of Ion Association in the 3− Electroreduction of PW12O40 J. M. Gomez-Gil,́ † E. Laborda,† J. Gonzalez,† A. Molina,*,† and R. G. Compton‡

† Departamento de Química Física, Facultad de Química, Regional Campus of International Excellence “Campus Mare Nostrum”, Universidad de Murcia, 30100 Murcia, Spain ‡ Department of Chemistry, Physical & Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, United Kingdom

*S Supporting Information

ABSTRACT: Insights into ion pairing effects on the redox 3− properties of the Keggin-type polyoxotungstate PW12O40 are gained by combining electrochemical experiments and density functional theory (DFT) calculations. Such effects have been reported to affect the performance of these species as molecular electrocatalysts. Experimental square wave voltam- 3− metry (SWV) of the two-electron reduction of PW12O40 in 4− acetonitrile evidences that the reduced forms PW12O40 and 5− fi PW12O40 can be signi cantly stabilized by ion association. The strength and stoichiometry of the corresponding aggregates are estimated as a function of the nature of the cation (lithium, sodium, and tetramethylammonium) and the oxidation state of the polyoxometalate. The results obtained in combination with DFT enable us to examine the roles of the cation solvation and the charge number and distribution of the polyanions.

1. INTRODUCTION electrochemical and quantum-chemical approach. As will be discussed, the “apparent” relative stability in solution of PW3− Polyoxometalates (POMs) are anionic metal-oxide clusters that 4− 5− show particular electronic properties of great interest in a large and the reduced forms PW and PW can be elucidated from − number of areas,1 5 specifically in electrocatalysis (water voltammetric measurements in an aprotic medium (acetoni- oxidation,6,7 epoxidation of alkenes,8 bromate reduction,9 trile). For this, the use of square wave voltammetry (SWV) in hydrogen evolution reaction,10 etc.).11,12 POMs can undergo combination with microelectrodes provides important advan- 1,13,14 tages for accurate quantitative analyses, mainly well-defined, multiple electron transfers where the stability and 34,35 ff fi peak-shaped signals with reduced ohmic drop and reactivity of the di erent oxidation states are de ned by their 36−38 intrinsic electronic properties15 (structure16,17 and elemental capacitive distortions. As will be discussed, the variation 18−20 “ ” 21 of the position of the experimental SWV voltammograms composition )andalsoby environmental factors 3− 22−24 corresponding to the first two electroreductions of PW upon (solvation, protonation, ... ). Among the latter, ion pairing ff hasbeenreportedtoaffect the efficiency of molecular the addition of di erent monovalent cations (lithium, sodium, ff and tetramethylammonium) clearly reflects the occurrence of electrocatalysis of di erent systems even in aqueous fi media25,26 by decreasing the reactivity of the catalyst upon ion association, which depends signi cantly on the nature of the ion association;15 also, the catalytic pathway and turnover the cation and on the oxidation state of the polyoxometalate. 27 ff Through the theory developed in a recent work for frequency can be a ected as a result of the change in the 39 “apparent” formal potential9 and the electron density multielectron transfers coupled with chemical equilibria and ff with the assistance of density functional theory (DFT) distribution (see below). For the investigation of these e ects, 40−46 electrochemical methods are very valuable, since they enable calculations, a consistent picture is gained about the ion fi direct access to the redox behavior of species under operational pairing of polyoxometalates, including the identi cation of the conditions, either dissolved in solution or surface-immobilized. chief physicochemical factors (ion size, charge number and POMs have been reported to undergo ion association distribution, solvation, and steric hindrance), the determination − ffi processes28 30 such that their redox behavior and electro- of the anion:cation stoichiometries (which can be a di cult catalytic activity will be affected by the ionic composition of the − medium.31 33 In this work, ion pairing effects on the Received: July 18, 2017 electrochemical properties of the Keggin-type polyoxotungstate Revised: November 2, 2017 3− 3− PW12O40 (PW ) will be investigated in detail via a joint Published: November 2, 2017

© XXXX American Chemical Society A DOI: 10.1021/acs.jpcc.7b07073 J. Phys. Chem. C XXXX, XXX, XXX−XXX The Journal of Physical Chemistry C Article task30), and the value of the association constants. The results concentrations of 0.05−5 mM with the Fuoss−Hsia− can assist the optimization of operating conditions for the use Fernandez−Prini equation51,52 (see section S.2 of the of polyoxometalates as electrocatalysts. Supporting Information). The experimental values obtained for the association constant and the limiting molar conductivity 2. EXPERIMENTAL SECTION Λ are given in Table 1; note that in all cases the 0-value agrees 2.1. Chemical Reagents. Anhydrous acetonitrile (MeCN, satisfactorily with the data reported in the literature. Sigma-Aldrich, 99.8%), ferrocene (Fe(C5H5)2, Aldrich, 97%), K tungstophosphoric acid sodium salt (Na3[PW12O40], Riedel-de- Table 1. Values of the Ion Association Constant ( A) and Haen,̈ analytical reagent grade), tetrahexylammonium hexa- Limiting Molar Conductivity (Λ ) between the Ions of the fl 0 uorophosphate (THAPF6, Sigma-Aldrich, 97%), tetramethy- Supporting Electrolytes Obtained via Conductivity with the fl 51,52 a lammonium hexa uorophosphate (TMAPF6, Sigma-Aldrich, Fuoss−Hsia−Fernandez−Prini Equation 98%), sodium hexafluorophosphate (NaPF , Sigma-Aldrich, 6 c Λ Λ 48,53−55 fl KA (this work) 0 (this work) 0 98%), and lithium hexa uorophosphate (LiPF6, Sigma-Aldrich, −1 2 −1 2 −1 fi electrolyte (M ) (S cm mol ) (S cm mol ) 98%) were all used as received without further puri cation. ± ± ± 2.2. Instrumentation. LiPF6 21 4 169.4 0.2 169.75 0.05 All electrochemical measurements ± ± ± NaPF6 29 3 174.3 0.2 178.6 0.02 were performed with a home-built potentiostat. A Pt wire was ± ± used as the counter electrode, a silver wire as the quasi- TMAPF6 33 4 204.9 0.2 196.75 fi aT = 298 ± 2 K. Error bars correspond to the standard deviation reference electrode, and a carbon ber (CF) microdisc ff electrode of 33 μm diameter (ALS Co.) or a glassy carbon obtained from three di erent sets of measurements. (GC) disc of 3 mm diameter (CH Instruments) as the working electrode. The electrodes were polished prior to the experiments using 1.0, 0.3, and 0.05 μm alumina−water slurry on soft 2.5. Computational Details. The Gaussian 09, revision lapping pads (Buehler, Illinois), and the electrode radius was 56 calibrated via chronoamperometry.37,38,47 D.01, package program was employed in all of the quantum- The conductivity measurements were performed with a chemical computations performed. All of the density functional theory (DFT) calculations were carried out with the B3LYP conductimeter BASIC 30 (Crison) with built-in temperature 57,58 correction. functional and the 6-31+G(d) basis set. In the case of the PW species, quasi-relativistic pseudopotentials of the W atoms 2.3. Electrochemical Measurements. The study of the 59 3− 3− proposed by Hay and Wadt were employed and the electroreduction of [PW12O40] (PW ) was performed at different concentrations of hexafluorophosphate salts of the LANL2DZ basis sets associated with the pseudopotential were adopted. For the rest of the elements, 6-31+G(d) was cations under study: LiPF6, NaPF6, or TMAPF6. Acetonitrile solutions were deaerated prior to experiments, and a nitrogen employed as the basis set. For optimizations, the SCF convergence criteria was set to atmosphere was maintained in the cell meanwhile. A silver wire −7 fi was employed to avoid any water contamination and 10 . An ultra ne integration grid was considered for the density functional theory (DFT) calculations and a fine one to uncertainties related to junction potentials, with the ferro- − cene−ferrocenium (Fc/Fc+) redox couple as the internal solve the coupled perturbed Hartree Fock (CPHF) equations. − reference.48 50 Frequency calculations were performed at the same level of In order to fix the ionic strength in all solutions at 0.1 M, theory as the geometry optimizations to characterize the fl stationary points as local minima (equilibrium structures). No tetrahexylammonium hexa uorophosphate (THAPF6)was ff employed, which can be expected to be fully dissociated scaling procedures were considered. Also, the e ect of the + solvent was taken into account by using the CPCM solvation given the large size of the THA cation. For the same reason, 60−62 ion association between the PW anions and THA+ can be model (conductor-like polarizable continuum model). disregarded (see sections S.3 and S.4 of the Supporting Information). This was further verified with SWV experiments 3. RESULTS AND DISCUSSION under different concentrations of THA+ where the experimental 3.1. Theoretical Treatment of the Electrochemical SWV curves did not show any dependence on the THA+ SWV Response. Given that species PW3− and the reduced concentration beyond that associated with the variation of the forms PW4− and PW5− are bulky and highly charged, the ionic strength. possibility of association with multiple cations can be envisaged. 2.4. Supporting Electrolyte Ion Pairing. Possible Hence, the general reaction Scheme 1 will be considered for the association between the cations under study X+ and their study of the first two electroreductions of PW3− in the presence − + 3− counterion in the supporting electrolyte (PF6 ) will act as a of the cation X . According to Scheme 1,PW undergoes two competing chemical equilibrium reducing the actual concen- electroreductions and, in principle, all of the redox species can “ ” * + + ≡ + + + tration of free cations in solution, cX+. In order to take this associate with X (X TMA ,Li,orNa), forming ion pairs, into account in the quantitative study (section 3), the including aggregates of three (two cations and a single anion), c + + association constants (KA) between the cations (Li ,Na, and four, or even more ions. + − TMA ) and the anion PF6 were determined by conductivity: Attending to the large size of the polyoxometalate with respect to cations X+, we will assume that the diffusivity of the c* XPF6 ion associates {X(PW)} is similar to that of species PW such XPF{XPF},+−+−+⇄K c = 66A** that they all have the same value of the diffusion coefficient, D. ccXPF+− (1) 6 Also, as ion pairing processes generally show very fast 63,64 The analysis of the decrease of the molar conductivity of LiPF6, kinetics, it can be assumed that chemical equilibrium NaPF6, and TMAPF6 in acetonitrile due to the formation of conditions hold at any point in solution (q) and time of the + − 65,66 electroneutral ion pairs {X PF6 } was analyzed in the range of experiment (t)

B DOI: 10.1021/acs.jpcc.7b07073 J. Phys. Chem. C XXXX, XXX, XXX−XXX The Journal of Physical Chemistry C Article

Scheme 1. Extended Rectangular Scheme of 3 × (n +1)− Under the conditions discussed above, the superposition a Members principle can be applied and the expression of the current− potential response is given by the sum of products of a potential-dependent function and a function dependent on time and on the electrode geometry,67,68 whatever the electrode geometry (G) and the voltammetric technique considered. Thus, for the yth cycle of the SWV perturbation (with y =1, [y],F 2, ..., N/2), the corresponding forward (IG ) and backward [y],B 39 (IG ) components of the SWV signal are

⎧21y− ⎫ ⎪ ⎪ I[],Fy =−−FA D⎨ ∑ [( Wms−1, W ms , ) f ( q , (2 y m )τ )]⎬ G G ⎪ G G ⎪ ⎩ m=1 ⎭ ⎧ 2y ⎫ ′ ⎪ ⎪ a 0 [],By ms−1, ms , Eh (V) are the formal potentials of the (free-ligand) redox couple IFADW=−−+⎨∑ [( Wfqym ) ( , (2 1)τ )]⎬ G G ⎩⎪ G G ⎭⎪ involved in the hth electron transfer (h = 1 or 2), whereas Ki,j m=1 correspond to the apparent chemical equilibrium constants of the jth yN= 1, 2, ..., ( /2) (4) association (j = 1, 2, ..., n) of the ith oxidation state (i =3−,4−,or − 5 ). All of the chemical and electrochemical steps are assumed to be Δ [y],SWV and the net square wave current of the yth cycle ( IG )is in equilibrium. ff Δ [y],SWV [y],F − [y],B given by the di erence IG = IG IG , which can be cqteq (, ) ⎧ expressed in a normalized form as follows XPWi i =−3,4,or5 − − ∀=qt,: Kc j ⎨ ij, cqtcqteq (, )eq (, ) ⎩ jn= 1, 2, ..., [],SWVy XPWi X ΔI τ j−1 Ψ[],SWVy = G G * (2) FAGT c D (5) c where Ki,j is the ion association constant based on concentrations. The modeling of the electrochemical response where AG is the electrode area. The function f G(qG, t) depends of Scheme 1 greatly simplifies by working under conditions on time and on the shape and size of the electrode, being given where the concentration of species X+ can be assumed as exactly for a macroelectrode (f P(x, t)) and approximately to * within 0.6% for all times for a disc-microelectrode (fd(rd, t)) by constant: cX(q, t)=cX . Thus, in the voltammetric experiments, ff + the e ective concentration of species X is in excess (at least 20 1 times) with respect to the polyoxotungstate and the following fxt(,)= P πDt apparent (or conditional) equilibrium constant Ki,j can be fi de ned: 41⎛ r frt(,)=+⎜0.7854 0.44315d + 0.2146 eq d d ⎝ cqti(, ) ⎧i =−3,4,or5 − − π rd Dt c * XPWj ⎨ ⎛ ⎞⎞ ∀=qt,: Kij,,X Kc ij = eq r cqt(, ) ⎩ jn= 1, 2, ..., ⎜ d ⎟ XPWi exp− 0.39115 ⎟ j−1 ⎝ Dt ⎠⎠ (3) (6) * ff + m−1,s − m,s cX is the e ective bulk concentration of species X (in this case The potential-dependent function (W W ) for the after taking into account the weak ion pairing between X+ and reaction Scheme 1 when the chemical and electrochemical − the salt counterion PF6 (see section 2.4)). processes are at equilibrium takes the form

Figure 1. Inﬂuence of the concentration of TMA+ cation on the SWV curves corresponding to the two ﬁrst electroreductions of the PW3− at the CF microdisc electrode of 33 μm radius (a) and at a GC macroelectrode (b) with c* − = 50μ M. Baseline-corrected experimental square wave PW3 voltammograms obtained with ESW = 25 mV (CF microdisc electrode, a) and ESW = 10 mV (GC macroelectrode, b). For both electrodes: f SW =10 Hz, |ΔE| = 5 mV, ionic strength set at 0.1 M; T = 298 ± 2K.

C DOI: 10.1021/acs.jpcc.7b07073 J. Phys. Chem. C XXXX, XXX, XXX−XXX The Journal of Physical Chemistry C Article

35,36,38 ⎛ ηη[]mm[] η[] m⎞ port. Nevertheless, these electrode kinetic effects do not [],ms ⎜ 2eapp,1 eapp,2 + e app,2 ⎟ Wc= * ⎜ ⎟; m= 1,2,..., psignificantly affect the peak position of the second peak, as PW ηη[]mm[] η[] m ⎝1ee++app,1 app,2 eapp,2 ⎠ proven from the good agreement between the experimental E2,peak-value obtained at micro- and macroelectrodes (<5 mV, [0],s * * 74 WWc= = 2 PW not shown). + (7) As can be seen in Figure 1, when the concentration of TMA * is increased, the positions of the two peaks shift toward less where cPW is the bulk concentration of electroactive species and negative values, which reveals the occurrence of ion associations F between the polyoxometalate anions and TMA+. The shift is η[]mm=−();1or2EE [] 0′ h = app,h RT app,h (8) more obvious in the second peak such that it can be concluded that the strength of the association follows the order PW5− > 0′ 4− 3− with Eapp,h being the apparent formal potentials that depend on PW >PW . This behavior is that predicted by electrostatic- 39,50,69,70 the values of the ion association constants as follows only considerations, being also in agreement with the results of ⎡ n ⎤ quantum-chemical calculations with DFT methods (see below). 1 + ∑ β 0′′0 RT ⎢ j=1 (4− ),j ⎥ Figure 2 shows the electrostatic potential mapped electron EEapp,1 =+1 ln n (a) ff F ⎣⎢1 + ∑ β ⎦⎥ density surface (ESP) of the di erent PW anions. As could be j=1 (3− ),j expected, the “outer shell” of the PW species is negatively ⎡ n ⎤ charged and it becomes more negative as the PW species gets 1 + ∑ β ′′RT ⎢ j=1 (5− ),j ⎥ EE0 =+0 ln (b) further reduced. The minimum ESP-value is located at the app,2 2 ⎢ n ⎥ holes defined by four octahedrals of two different M O triads F ⎣1 + ∑j=1 β(4− ),j ⎦ 3 13 (9) (see red regions in Figure 2 and section S.5 of the Supporting β Information). where i,j are the conditional overall association constants: Next, as indicated previously (section 3.1) and as detailed in eq j cqt(, ) j ref 39, the elucidation of the ion pairing mechanism and the OXij c * β ==∏∏Kia, eq =Kcia,X determination of the association constants and stoichiometries ij, cqt(, ) a==1 Oi a 1 (10) ff of the di erent PW12 species were performed by the analysis of the variation of the apparent formal potentials with the From the above expressions, it can be inferred that the + association of the polyoxometalate anions with X+ will result in concentration of TMA (eq 9), with the corresponding 0′ 0′ equilibrium constants as adjustable parameters. Given that the the change of the apparent formal potentials Eapp,1 and Eapp,2 fi Δ − ≤ according to eqs 9. By determining their values from the SWV rst and second peaks are well-separated ( Ep−p = Ep,2 Ep,1 ff * −436 mV), the electron transfers can be treated separately and signal as detailed in refs 50 and 71 for di erent values of cX , the association constants and stoichiometries can be investigated as the SW peak potential coincides with the apparent formal ≡ 0′ 39 potential (Epeak,h Eapp,h , see eq 9) whatever the electrode size discussed in the following sections. 75 3.2. Electrochemical Study of the Ion Association and shape considered. between TMA+ and PW Anions. The mechanistic analysis is not straightforward, since Ion pairing between PW ff anions and tetramethylammonium (TMA+) was first inves- multiple ion associates of di erent stoichiometries can be + envisaged given the high negative charge and large size of the tigated. The effective concentration of TMA (c* +) was TMA PW anions. In order to establish realistic and consistent always in excess with respect to the initial concentration of mechanisms and thermodynamic parameters, the fitting 3− * * PW (ccTMA+ ≥ 20 PW ) according to the value of the procedure was carried out under the following considerations: association constant obtained from conductivity measurements • The values of the formal potentials of the redox couples (see section 2.4 and sections S.2 and S.3 of the Supporting − − − − PW3 /PW4 and PW4 /PW5 were obtained from Information52). independent SWV experiments in 0.1 M THAPF6 In Figure 1, representative experimental square wave + 0′ − ± solutions (in the absence of TMA ): E1 = 703 3 voltammograms at the highest and lowest concentrations of 0′ − ± TMA+ assayed are shown, as obtained at a carbon fiber mV and E2 = 1216 2 mV. The values obtained point out that the two SWV peaks correspond to the α- microdisc (Figure 1a) and at a glassy carbon macroelectrode 72 (Figure 1b). The square wave voltammograms show two well- isomer. • defined peaks that correspond to two reversible, one-electron Where necessary, the cation:anion stoichiometry was fi reductions, as can be inferred from the values of the half-peak increased up to achieving satisfactory ts of experimental 2 width (see below). The first peak is ascribed to the reduction of data (R ≥ 0.99). PW3− to PW4− while the second peak to the reduction of PW4− • The values of the equilibrium constants were constrained to PW5−.14,72 For both peaks, the experimental values of the to be positive values, performing several independent fi ff half-peak width (W1/2) are very close to the theoretical value ttings with di erent initial values with a perfect predicted for reversible one-electron transfers for the square coincidence. Exp − • wave amplitudes employed: W1/2 =90 94 mV for ESW =10 The equilibrium constants obtained with a value of the theor Exp − 76 mV (vs W1/2 = 92 mV) and W1/2 =96 105 mV for ESW =25 dependency parameter close to 1 (>0.85) were not theor 34,35,73 mV (vs W1/2 = 99 mV). The second peak at the considered. fi microelectrode is broader and smaller than the rst one In addition to the above, a “purely electrostatic model” for the Exp,2nd‐ micro (W1/2 =−98 115 mV), which is likely related to the ion association was initially considered. Thus, ion desolvation quasi-reversible character of the electron transfer, since the and anion−cation Coulombic interactions are the main forces influence of the electrochemical kinetics is more significant at involved in the ion pair formation. In the case of low-dielectric- microelectrodes due to the enhanced diffusion mass trans- constant media and highly charged species, one could expect

D DOI: 10.1021/acs.jpcc.7b07073 J. Phys. Chem. C XXXX, XXX, XXX−XXX The Journal of Physical Chemistry C Article

that electrostatic interactions will prevail over desolvation. Accordingly, the value of the association constants can be presumed to be determined by the charge number of the cation and anion taking part in the ion pairing process, for example, the following equilibria

PW332−++⇄ X {X + (PW −− )} +−−+43 +−−32 {X (PW )}+⇄ X {X2 (PW )} +−−+53 +−−42 {X2 (PW )}+⇄ X {X3 (PW )} (11)

where all of the starting anions that have the same charge number (z =3−) would have the same value of the association constant K : KKKK==≡. Assuming this, eq za 3,1−−−− 4,2 5,3 3 9 becomes ⎛ ⎞ RT 1+ Kccc* + KKc ( c* )2 + ... EE=+0′ ln⎜ 4X−−− 3 4X ⎟ peak,1 1 c * F ⎝ 1+ Kc3X− + ... ⎠ RT EE=+0′ peak,2 2 F ⎛ ⎞ 1+ Kcccc* + KK ( c* )2 + KKKc cc ( c* )3 ... ln⎜ 5X−−−−−− 4 5 X 3 45 X ⎟ ⎝ ccc* * 2 ⎠ 1+ Kc4X−−−+ KK 3 4 ( c X )+ ... (12) The fittings of the experimental data of the peak potentials with the “electrostatic model” (eq 12) were not satisfactory in any case (R2 ≤ 0.90). To shed some light on this issue, ESP surfaces of PW3− and [Li+− PW43 ] − (i.e., two PW species with the same charge number) were calculated, and they are shown in Figure 3. Note that the localized sites of electron density differ between both of them, with the species [Li+− PW43 ] − having a region with higher electron density at the “outer shell” than PW3−. Hence, both the electrochemical and computational results support the idea that the overall charge of the anion species is not the only key parameter that determines the strength of ion pairing in this system. It is also important to notice that the association of a cation with PW breaks down the symmetry of the electron density distribution, as can be inferred from the ESP surfaces shown in Figure 2. Electrostatic potential on the 0.001 au molecular surface of Figure 2 (and in Figure 3a). Thus, the association of a single PW3− (top), PW4− (center), and PW5− (bottom) species, computed lithium cation at one of the sites of highest electron density with the B3LYP functional as indicated in section 2.5. Optimized (Figure 3b) leads to a higher density at the opposite one. structures were obtained with Td symmetry constraint (α-isomer) and Hence, a statistical theoretical treatment as frequently used for considering the CPCM approach to take into account solvent effects. macromolecules77 is not suitable in this case.

Figure 3. Electrostatic potential mapped surface of PW3− (a, left side) and [Li(PW4−)]3− species (b, right side) under the same conditions as in Figure 2. Position of the lithium cation indicated with a yellow arrow.

E DOI: 10.1021/acs.jpcc.7b07073 J. Phys. Chem. C XXXX, XXX, XXX−XXX The Journal of Physical Chemistry C Article

Figure 4. Experimental variation of the peak positions with the concentration of TMA+ (points). Best-ﬁt theoretical curve (black solid line, eq 9). Error bars of the peak potentials correspond to the standard deviation calculated from three independent SWV experiments and those of the estimated equilibrium constants to the asymptotic standard error.

Scheme 2. Ion Pairing between the PW Species and TMA+ as Elucidated from SWV Experiments

In view of the above results, the fitting of the experimental high acid character of this form of the polyoxometalate reported 0′ * 15,78 Eapp,h vs cX data was carried out with arbitrary, adjustable values in the literature. for all of the association constants. As shown in Figure 4, for 3.3. Effect of the Size of the Ions: Cation Desolvation the satisfactory description of the experimental variation of vs Coulombic Attraction. The electroreduction of PW3− in both peak potentials (R2 > 0.99), it is necessary to include the acetonitrile was also studied in the presence of two different − formation of triple ions for PW4 and quadruple ions in the case alkaline cations (lithium (Li+) and sodium (Na+)) in order to − of PW5 (Scheme 2). The formation of multiple ion associates gain insight into the effects of the nature and size of the cations with the TMA+ cations can be explained by the large size and on the ion association. As in the case of TMA+, the effective − − high negative charge of the PW4 and PW5 anions such that a * 12 12 concentration of these cations (cLi+ /Na+, as calculated from the very high Coulombic attraction toward the cations is expected fi ff value of the association constant obtained in section 2) was to take place without signi cant steric e ects between the always in excess with respect to the bulk concentration of PW3− associated cations. (i.e., cc*++≥ 20 * 3−). The values obtained for the chemical equilibrium constants Li /Na PW c c c follow the order K − > K − > K − , which is consistent with In Figure 5, representative experimental SWV curves at the 5 ,j 4 ,j 3 ,j fi the ESP surfaces (Figure 2). Also, it is quite remarkable that the carbon ber microelectrode (Figure 5a,c) and the glassy carbon most oxidized species PW3− undergoes no ion association in macroelectrode (Figure 5b,d) are shown for the highest and * spite of its high negative charge. This is in line with the very lowest cLi+ /Na+-values assayed. As in the experiments with

F DOI: 10.1021/acs.jpcc.7b07073 J. Phys. Chem. C XXXX, XXX, XXX−XXX The Journal of Physical Chemistry C Article

Figure 5. Inﬂuence of the concentration of Li+ and Na+ cations on the SWV curves corresponding to the two ﬁrst electroreductions of the PW3− at the CF microelectrode of 33 μm radius (a−c) and at the GC macroelectrode of 3 mm radius (b−d) with c* − = 50μ M. SWV experimental PW3 parameters and other conditions as in Figure 1.

TMA+, two well-defined and separate peaks are observed that the shift of the second peak in the case of TMA+ is of ca. 120 shift toward less negative potentials when the concentration of mV at 50 mM, whereas in the case of lithium it is of ca. 290 mV Li+ or Na+ is increased. At the highest concentration of Li+ at 30 mM. This simple preliminary analysis suggests that the (Figure 5a and b), a third peak is also observed that Coulombic attraction prevails over cation desolvation for PW5− corresponds to the two-electron reduction of species PW5− to ion associates. PW7−. Both for Na+ and Li+, the first and second peaks are well- fi fi Δ Exp − ≤− The shift of the rst peak is signi cantly smaller than in the separated ( Ep−p = Ep,2 Ep,1 240 mV) for the experiments with TMA+, which can be explained either by the concentration range considered (in the case of the highest ion association of species PW3− and PW4− being of similar concentration of Li+, the difference between the second and strength (see eq 8a) or by the absence of ion pairing with third peaks is also sufficiently negative for the square wave species PW3− and PW4−. The latter is consistent with results amplitudes employed so as to enable the assumption of 22,23,28 Δ exp obtained by Himeno et al. that indicated the negligible addressing both electron transfers as independent ( Ep−p = Ep,3 + − ≤ formation of ion associates between Li and the phosphopo- Ep,2 170 mV, see ref 75)) so that the peak potentials 3−/4− lyoxomolibdates [PMo12O40] by combined cyclic voltam- coincide with the apparent formal potentials given by eq 2. − metry and 7Li NMR. With respect to PW5 , the experimental data are consistent According to the above, only the shift of the second peak was with the formation of quadruple ions (as in the case of TMA+) considered in the quantitative analysis (Figure 6). The shift is with the overall ion pairing constant increasing as the size of the clearly more significant when the size of the cation decreases in cation decreases (see Schemes 2 and 3): Li+ >Na+ > TMA+. spite of the larger desolvation energy (see below); for example, This contrasts with the ordering observed for the strength of

G DOI: 10.1021/acs.jpcc.7b07073 J. Phys. Chem. C XXXX, XXX, XXX−XXX The Journal of Physical Chemistry C Article

Figure 6. Experimental variation of the peak positions with the concentration of Li+ (a, points) and Na+ (b, points) and best-ﬁt theoretical curves (black solid lines, eq 9). Error bars of the peak potentials correspond to the standard deviation calculated from three independent SWV experiments and those of the estimated equilibrium constants to the asymptotic standard error.

a Scheme 3. Ion Pairing between the PW Species and Li+/Na+ as Elucidated via SWV

aAssociation between species PW3− and PW4− with Li+ and Na+ was initially considered in the model (grey color), although the analysis of the experimental SWV results suggests that such ion pairing is negligible under the present conditions. the association between hexafluorophosphate and the same energy calculated according to the thermodynamic cycle shown cations (Li+,Na+, TMA+) via conductimetry (section 2.4), in Scheme 4. pointing out that there is a significant competition between ion For the three cations under study, a tetrahedral four- solvation and electrostatics in this medium. coordinated solvation shell is predicted (Figure 7) and the In order to shed some light on this, as well as to corroborate value of the solvation energy is significant (see Table 2), in the experimental results, a DFT-based computational study was agreement with that reported in refs 45 and 79. As expected, performed to estimate (1) the “strength” of the solvation of cations X+ in acetonitrile as well as the “strength” of the ion fi + − Scheme 4. First Step Corresponding to the Speci c pairing of cations X (2) with PF6 and (3) with the PW Solvation of the Cation X+ Which Has Been Employed anions. + Previously to Measure the Interaction Strength between the (1) In Table 2, the solvation of cations X in acetonitrile is Acetonitrile Groups and the Cation X+45 and Second Step studied through the acetonitrile nitrogen−cation distance and fi fi Δ 0,gas Δ 0 Representing the Nonspeci c Solvation of the Cluster Which the speci c( GSp. Solv.) and global ( GX,T Solv.) solvation Gibbs Is Given by the Difference between the Electronic Energy Obtained with the CPCM Model (s) and in a Vacuum (g) Table 2. Summary of the Parameters of Interest of the Solvation of the X Cations in Acetonitrile Media at the B3LYP/6-31+G(d) Level of Theory

speciﬁc nonspeciﬁc solvation total solvation (g) (s, CPCM) solvation + 0,gas 0 X d − ΔG ΔG N X Sp. Solv. Δ X,T Solv. cations (pm) (kJ/mol) EN−Sp. Solv. (kJ/mol) (kJ/mol) Li+ 205 −369 −154 −523 Na+ 239 −277 −158 −434 TMA+ 385 −60 −157 −216

H DOI: 10.1021/acs.jpcc.7b07073 J. Phys. Chem. C XXXX, XXX, XXX−XXX The Journal of Physical Chemistry C Article

+ + Figure 7. Optimized structure for [Na(ACN)4] (left side) and [TMA(ACN)4] (right side) computed at the B3LYP/6-31+G(d) level as indicated in section 2.5. Optimized structures were obtained with Td symmetry constraint.

Figure 8. Optimized structures of TMA(PF6) (left side) and Li(PF6) (right side), both computed at the B3LYP/6-31+G(d) level as indicated in section 2.5. solvation becomes stronger as the size of the cation decreases Table 3. Summary of Ion Pairing between the Cation X+ with − − a such that the nitrogen X distance and the solvation Gibbs the Anion PF6 at the B3LYP/6-31+G(d) Level energy decrease. cation X+ relative ion pairing energiesb (kJ/mol): Δ(ΔG0 ) (2) The DFT study of the ion pairing strength between the XPF6 − + + PF6 and X was performed according to the following scheme Li 0 Na+ −9.4 +− + [X(ACN)46 ] (s)+ PF (s) TMA −13.6 0 aStructures shown in Figure 8. bGiven that our interest is to compare ΔGXPF (ACN) − 6 − the strength of the interaction between the cations X+ with PF , XYoooooooooooooooo [X(PF63 )(ACN) ](s)+ ACN(s) (13) 6 relative energies have been considered assigning a value of zero to Δ 0 where the possibility of ion pairing including part of the cation G (LiPF6). solvation shell is considered; specifically, the cation is assumed fi to maintain three molecules of acetonitrile of the rst solvation − upon ion association according to ref 46. Given the number of desolvation, as found with the less charged anion PF6 . The − ff fi possibilities of ion pairing between PF6 and the di erent association with the rst cation was considered according to the cations X+ (see refs 46 and 80), the study was restricted to following scheme contact ion pairs involving a single cation and one anion. 0 ⎧ ++ + ΔGXPW XLiorNa= fl X+++(s)+ PWiii (s)XYoooooooooo [X (PW )](1) (s) ⎨ Coordination opposite to a single uorine atom (see Figure ⎩ 8) was found to be the most favorable, and the ordering of the i ≡−3or − 5 ion pairing strength predicted theoretically agrees with that (14) obtained experimentally by conductimetry: TMA+ >Na+ >Li+ where, for the sake of simplicity and given the demand of the (see Table 3). Hence, both DFT and conductivity data indicate DFT calculations, the first solvation shells were not considered. that the ordering of the ion pairing strength between The regions with the minimum values of the ESP mapped hexafluorophosphate and the cations under study is determined surface (“ESP-H” in Table 4) shown in Figure 2 and in section by the cation desolvation. S.3 of the Supporting Information are the most probable sites (3) Finally, a DFT study of the association of the anions for ion pairing, since these regions show the highest “exposed” PW3− and PW5− with the alkaline cations was performed in electron density. Also, the association through the terminal 5− order to examine whether the strength of the ion association is oxygens (OT in Table 4)ofPW was considered attending to predicted to be defined by the Coulombic attraction (as previous computational studies on proton affinity.15,40 In Figure suggested by the experimental data for PW5−) or by the cation 9, one of the optimized structures at the two positions

I DOI: 10.1021/acs.jpcc.7b07073 J. Phys. Chem. C XXXX, XXX, XXX−XXX The Journal of Physical Chemistry C Article

Table 4. Summary of the Ion Pairing between the PW study points out that the ion pairing in aprotic media is Species (PW3− and PW5−) with the Alkaline Cations with the stronger as the polyanion is further reduced and that its charge CPCM Model as Indicated in Section 2.5 distribution plays an important role. Thus, the polyanion PW O 5− has been found to form triple and quadruple ions + Δ Δ 0 3− Δ Δ 0 5− 12 40 X position ( GXPW) (kJ/mol): PW ( GXPW) (kJ/mol): PW with alkaline cations (lithium and sodium) as well as with Li+ tetramethylammonium. − − OT 10.6 25.2 Opposite effects of the size of the cation on the strength of ESP-H 0 −58.1 the ion pairing are observed experimentally between PW5− and Na+ PW3−/4−, being in consistency with DFT calculations. This can − − OT 14.8 24.6 be ascribed to the different magnitude of the Coulombic ESP-H −12.8 −30.5 attraction with respect to the cation desolvation. In the case of 5− PW12O40 , the strength of the ion associations follows the Δ 0 trend established by electrostatic considerations indicating that considered is shown, and the GXPW-values are given in Table the Coulombic attraction prevails; thus, the smaller the cation, 4. 3− the stronger the ion pairing. On the other hand, the cation In the case of species PW , the most favorable position for desolvation predominates in the case of the less charged anions the interaction is the oxygen terminal (as indicated by previous 3−/4− − 15,40 5− (species PW and also PF6 ) so that the association with authors ), whereas in the case of the PW species the TMA+ is more favored than that with Li+ and Na+ that show association is predicted to be more stable at the here-called stronger interaction with their solvation shell. “ESP-H” sites. As expected, the interaction of the alkaline cations in terms of Gibbs energy with the PW5− species is − ■ ASSOCIATED CONTENT 3 always stronger than in the case of PW . * Comparing the data obtained for both cations, the S Supporting Information interaction between lithium and the PW anion is more favored The Supporting Information is available free of charge on the than that for sodium in the case of the most reduced species ACS Publications website at DOI: 10.1021/acs.jpcc.7b07073. (PW5−) which is in accordance with the results obtained in the Glossary; estimation of ion pairing constants of the experimental electrochemical study. The opposite trend is supporting electrolytes considered via conductimetry; observed in the case of PW3− species, with this behavior being experimental verification of the absence of ion pairing − comparable with that obtained for the ion pairing with PF6 . between the polyoxometalate anions and the tetrahexylammonium cation; experimental verification of the 4. CONCLUSIONS negligible influence of the counterion (sodium) of the Ion pairing effects on the electrochemical behavior of the polyoxometalate salt in this study; enlarged electrostatic 3− Keggin-type polyoxometalate PW12O40 have been inves- mapped potential surface of the isolated polyoxometalate tigated through a joint electrochemical and computational (PDF) approach including square wave voltammetry (SWV) experiments and density functional theory (DFT) calculations. The ■ AUTHOR INFORMATION main outcomes provide insights into the magnitude and nature Corresponding Author of such effects that can affect the reactivity and turnover *Phone: +34 868 88 7524. Fax: +34 868 88 4148. E-mail: frequency of these well-known electrocatalysts. [email protected]. The stoichiometry and strength of the ion associations have been evaluated in acetonitrile via SWV for three different ORCID 3− J. Gonzalez: 0000-0001-6848-074X oxidation states of the polyoxotungstate (PW12O40 , 4− 5− ff A. Molina: 0000-0002-9661-1660 PW12O40 ,andPW12O40 )andthreedierent cations (lithium, sodium, and tetramethylammonium). The SWV R. G. Compton: 0000-0001-9841-5041

Figure 9. Optimized structures of Li(PW3−)2− with lithium cation directed toward a region with the highest electron density (ESP-H, left side) and a terminal oxygen (OT, right side), both computed as indicated in section 2.5.

J DOI: 10.1021/acs.jpcc.7b07073 J. Phys. Chem. C XXXX, XXX, XXX−XXX The Journal of Physical Chemistry C Article

γ 4‑ Notes -[SiW12O40] : Isomeric Dependence of Reversible Potentials of The authors declare no competing ﬁnancial interest. Polyoxometalate Anions Using Data Obtained by Novel Dissolution and Conventional Solution-Phase Processes. Inorg. Chem. 2004, 43, − ■ ACKNOWLEDGMENTS 8263 8271. (18) Guo, S.-X.; Feldberg, S. W.; Bond, A. M.; Callahan, D. L.; The authors greatly appreciate the ﬁnancial support provided Richardt,P.J.S.;Wedd,A.G.SystematicApproachtothe by the FundacionSé necá de la Regioń de Murcia (Project Quantitative Voltammetric Analysis of the FeIII/FeII Component of ́ α 7‑/8‑ 19887/GERM/15) as well as by the Ministerio de Economiay the [ 2-Fe(OH2)P2W17O61] Reduction Process in Buffered and Competitividad (Project CTQ-2015-65243-P). J.M.G.-G. Unbuffered Aqueous Media. J. Phys. Chem. B 2005, 109, 20641− thanks the Ministerio de Educacion,́ Cultura y Deporte for 20651. the fellowship “Ayuda de Formacioń de Profesorado Uni- (19) Nakajima, K.; Eda, K.; Himeno, S. Effect of the Central ” ́ Oxoanion Size on the Voltammetric Properties of Keggin-Type versitario 2015 . E.L. thanks the Ministerio de Economiay n‑ − − Competitividad for the fellowship “Juan de la Cierva- [XW12O40] (N = 2 6) Complexes. Inorg. Chem. 2010, 49, 5212 ́ ” 5215. Incorporacion 2015 . (20) Aparicio, P. A.; Poblet, J. M.; Lopez,́ X. Tungsten Redox Waves N‑ in [XMW11O40] (X = P, Si, Al and M = W, Mo, V, Nb, Ti) Keggin ■ REFERENCES Compounds - Effect of Localised/Delocalised Charges. Eur. J. Inorg. (1) Pope, M. T.; Müller, A. Polyoxometalate Chemistry: An Old Chem. 2013, 2013, 1910−1916. Field with New Dimensions in Several Disciplines. Angew. Chem., Int. (21) Evans, D. H. One-Electron and Two-Electron Transfers in Ed. Engl. 1991, 30,34−48. Electrochemistry and Homogeneous Solution Reactions. Chem. Rev. (2) Rhule, J. T.; Hill, C. L.; Judd, D. A.; Schinazi, R. F. 2008, 108, 2113−2144. Polyoxometalates in Medicine. Chem. Rev. 1998, 98, 327−358. 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M DOI: 10.1021/acs.jpcc.7b07073 J. Phys. Chem. C XXXX, XXX, XXX−XXX