Recent Achievements in the Analysis of the Electrochemical ...

Recent Achievements in the Analysis of the Electrochemical Properties of Polyoxometalates

Tadaharu Ueda Department of Applied Science, Faculty of Science, Kochi University, Kochi, 780-8520, Japan

E-mail: [email protected]

Received April 13, 2015

Polyoxometalates (POMs) have been studied for ca. 200 years since the first report on the synthesis of (NH4)3PMo12O40 in 1826. Even now, novel POMs are still being prepared,

characterized and applied to various fields. Multi-step, multi-electron transfers reversibly occur for many redox active POMs, which is interesting to many electrochemists. Recently, the

electrocatalytic behaviour of POMs has been focussed on due to the potential as a solution for energy problems. However, unclear aspects remain in terms of the fundamental electrochemical properties of POMs. This article reviews recent achievements on the electrochemistry of POMs in

the solution phase.

Keywords: Polyoxometalate, Keggin, Wells–Dawson, simulation, redox mechanism Introduction

Polyoxometalates (POMs) are a class of metal-oxide clusters. They consist of addenda atoms,

such as tungsten and molybdenum, hetero atoms, such as phosphorus and silicon, and oxygen atoms,

which are bonded with the addenda atoms and hetero atoms.1) They are discrete anions with similar structures to the surface of metal-oxides. The

structure of POMs is a three-dimensional molecular architecture. Recently, many researchers have

focussed on the synthesis of structure-controlled hybrid materials of POMs with organic molecules. The Keggin-type structure, [XM12O40]n–, and the

Wells-Dawson-type structure, [X2M18O62]n–, are typical among all POMs (Fig. 1). Most POMs based

on the Keggin-type and the Wells-Dawson-type

structures are redox active. Multi-electron transfers reversibly occur in multi-steps, which are unique

chemical properties, rather than metal complexes with various ligands. In addition, reduced species or

mixed-valence species of POMs exhibit intense blue colour. The terms ‘heteropolyblue’ or ‘molybdenum blue’ are used to describe reduced POMs. Based on

the redox properties with colour-changing, POMs have been applied to analytical chemistry and

materials chemistry. Trace amounts of phosphorus in sample solution, e.g., sea water, are determined by the molybdenum blue method based on the

formation reaction of [PMo12O40]3– by mixing Mo(VI) and P(V) under an acidic condition and

2014年志方メダル受賞記念総説 Award Review Article, 2014 Shikata Medal

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Review of Polarography, Vol.61, No.1, (2015)

reduction by the addition of reductants, such as

L-ascorbic acid.2) In addition, POMs have been applied to biochemistry and catalytic chemistry.

The redox potentials of POMs are generally more positive than those of metal complexes. In addition, the protonated POMs exhibit stronger acidity and

less corrosivity than the mineral acid: H2SO4 and HCl.3) POMs have been used as environmentally

friendly acid and oxidation catalysts for a variety of organic syntheses. POMs have been fundamentally

and practically studied for a long time. Although the electrochemical behaviour of POMs has also been extensively investigated, unclear aspects remain. In

terms of the electrochemistry of POMs, the reduced sites in POMs, when two and more electrons are

incorporated into anion, and the detailed redox mechanism coupled with protons, are still ambiguous.

This review article describes the recent achievements of the studies on the electrochemistry

of POMs reported mostly after the previous review published by Chemical Reviews in 1998.4) In

addition, this review focuses on the electro-

chemistry of POMs in the solution phase.

General electrochemical properties of POMs The electrochemical behaviour of Keggin-type

POMs has been extensively investigated in aqueous and non-aqueous media. Because the redox

potentials of POMs are sensitive to the concentra-tions of acid (pH) and protons contained as a counter cation, the solution conditions in which the

voltammograms of POMs are measured should be checked very carefully. The redox potentials in

non-aqueous media and ionic liquids under neutral conditions are listed in Tables 1 and 2.5) For brevity, oxygen is omitted in this paper.

Generally, the redox potentials of molybdenum- POMs, heteropolymolybdates, under neutral condi-

tions are more positive than those of the corresponding tungsten-POMs, heteropoly-

tungstates, e.g., PMo12 > PW12, SiMo12 > SiW12. In addition, the 1st redox potentials of POMs with the same framework are linearly related to the anion

charge of POMs in any solvent, e.g., SMo12 > PMo12 > SiMo12.5b) Keita et al., found that the redox

potentials of SiW12 and P2W18, respectively, were linearly dependent on the acceptor number of the organic solvent.6) Himeno et al., found the first

redox potentials of PW12, PMo12, and GeMo12 in various organic solvents are related to both the

donor number and permittivity.7) Keggin-type POMs have several isomers generated by the π/3

rotation of each M3O13 unit. Redox waves of the β-form appear at more positive potentials than those of the corresponding α-form, e.g., β-PMo12 >

α-PMo12. The ion-transfer voltammetric behaviour of POMs has been investigated to find the

relationship between the ion-transfer potentials and the size and charge of POMs.8)

Heteropolytungstates with Keggin and Wells-

Dawson-type structures can be partially decom-posed by the addition of a weak base, such as

KHCO3, or sophisticated pH control to form

Figure 1 Polyhedral (a) and stick and ball (b) expressions of Keggin-type (A) and Wells-Dawson- type (B) POMs.

12


lacunary species, such as [XW11O39]n– and [X2W17O61]n–.9) Other metal ions can be

incorporated into the defect sites of lacunary POMs to form metal-substituted POMs, which exhibit

fascinating chemical properties depending on the incorporated metal ions. The electrochemical properties of metal-substituted POMs are also

changed from those of the corresponding parent POMs. In the case of most redox active metal

ion-incorporated POMs, redox waves due to redox of the incorporated ions were observed at more

positive potentials than those due to the reduction of W(VI/V).10) Ru-substituted POMs exhibit excellent electrochemical and photochemical properties.11)

Especially, in the case of [SiW11O39Ru(bipy)]5–, reversible three redox waves were observed at more

positive values than the redox of the framework parts in CH3CN (0.1 M n-Bu4NPF6), corresponding

to the redox couples of Ru(II/III), Ru(III/IV), and Ru(IV/V).11b)

In the case of Wells-Dawson-type POMs, two

types of isomers exist, depending on whether the polar or belt position metals are substituted, which

are described as α2 (or 1)-[X2MW17O61]n– or α1 (or 4)-[X2MW17O61]n– (X = S, P, As; M = substituted

Table 1 Potentials (E1, E2, mV vs. Fc/Fc+) due to the 1st and 2nd redox of MoVI/V in Keggin-type V(V)-POMs

AC: acetone; ACN: acetonitrile; 1,2-DCE: 1,2- dichloroethane; DMSO: dimethyl sulfoxide; NB: nitrobenzene; PC: propylene carbonate a: Measured after the addition of n-Bu4NOH to neutralize the protons present as a counter cation.

Table 2 Potentials (E1, E2, mV vs. Fc/Fc+) due to the 1st and 2nd redox of WVI/V in Keggin-type V(V)-POMs

a: Measured after the addition of n-Bu4NOH to neutralize the protons present as a counter cation. b: vs. Ag/Ag+ c: vs. CoCp2/ CoCp2

+

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metals), respectively. The potential due to the redox of the vanadium component for the α1-isomer of

X2VW17 is more positive than that for the α2-isomer (Table 3).12)

Small cations and protons can affect the voltammetric behaviour of POMs in organic

solvents. The effects of Li+ and Na+ on the voltammetric behaviour of Keggin-type POMs were investigated in various solvents, leading to selective

solvation of Li+ in binary solvents with the help of 7Li NMR.13) The electrochemical properties of the

Wells-Dawson-type POM S2Mo18 were investigated in CH3CN in the presence of LiClO4 and H2O to elucidate the Li+-coupled redox process and

solvation of Li with H2O.14)

Theoretical analysis of the voltammetric behaviour

As X-ray analysis of POMs has progressed,

structural information has become available for many POMs. DFT calculations based on structural information may give us reasonable explanations

for the stability, reduced position, and protonation sites of POMs. Because the α-form of Keggin-type

POMs has high symmetry and each of the twelve addenda atoms is equivalent, one electron can be

accommodated into addenda atoms with the same probability. However, two types of addenda atoms exist in the polar and belt sites in the case of

Wells-Dawson-type POMs, indicating that the reduced probability should be different. DFT

calculations indicate the first two electrons should be accommodated into addenda atoms in the belt

position, which is in good agreement with the experimental data.15a)

It is important to know which part of the POMs will be protonated. The basicity of POMs with

several types of structures was calculated by DFT, and the results indicate that the basicity of oxygen decreases in the order: edge-shared oxygen >

corner-shared oxygen > terminal oxygen.15b) In addition, it was reported that the edge-shared

oxygen around substituted metals exhibit the highest basicity in metal-substituted POMs. These results indicate that reduced POMs will be

protonated at edge-shared oxygen while measuring the voltammograms of POMs in the presence of

acid. The α-form of Keggin-type POMs is more

stable than the β-form in the oxidized form, and vice versa in the reduced form, which was confirmed by the DFT calculation.15c)

Recently, it was reported that the first redox potential obtained under neutral conditions was

related to the mean bond valence of W-µ4O, of which the oxygen is linked with hetero atoms, as well as the anion charge of Keggin-type POMs.5e)

Moreover, based on the relationship between the redox potential and the mean bond valence, the

introduction of the energy of Coulomb interaction can theoretically lead to the redox potentials of four

one-electron processes. In addition, the redox potentials of two two-electron processes coupled with protons can be calculated by taking Gibbs

energy of protonation into consideration.16)

Detailed analysis of the voltammetric behaviour of POMs

Protons greatly affect the voltammetric

behaviour and the formation of POMs. Generally, one electron transfers occur in a stepwise fashion

under neutral conditions, whereas two electron

Table 3 Potentials (E1, mV) due to the redox of the VV/IV component in Wells-Dawson-type V(V)-POMs

a: Measured in CH3CN containing 0.1 M n-Bu4NPF6. b: Measured in an aqueous solution at pH 7.0 (0.4 M NaH2PO4 + NaOH). c: vs. Fc/Fc+ d: vs. SCE

14


transfers occur under highly acidic conditions. Qualitative analysis of the voltammetric behaviour

of POMs has been performed in aqueous and non-aqueous media in the presence of designated

concentrations of acid. Fewer reports on the quantitative analysis of the redox mechanisms of

POMs have been published, although in some cases, it was proposed from the analysis of complicated voltammograms and redox potential changes.17)

Simulation software is a powerful tool for the analysis of voltammetric behaviour. Digisim

(Bioanalytical Systems Inc.) and DigiElch (GAMRY) are commercially available for the simulation of cyclic voltammograms. As many

electrochemists know, simulation software provides some parameters to fit observed voltammograms,

regardless of the chemically correct answer. If the redox mechanism is proposed from the

experimental results from various electrochemical measurements, as well as the other measurements, e.g., spectroscopy, and simulation is conducted

based on this redox mechanism, the parameters obtained from the simulation may be close to

chemically correct. The Bond research group investigated the

detailed voltammetric behaviour of the FeIII/FeII

component of α2-Fe(OH2)P2W17, SiW12, and sandwich-type [{Ru4O4(OH)2(H2O)4}(γ-SiW10-

O36)2]10– in aqueous solution and S2W18 in CH3CN in the presence of designated concentrations of acid

with the help of hydrodynamic voltammetry, FT-AC voltammetry and simulation.18) The Himeno group investigated the proton-coupled two-electron

reduction process of XM12 in CH3CN and the Li-coupled reduction processes of PMo12 in acetone

to obtain protonation constants and Li-association constants, respectively.7,13a)

Among metal-substituted POMs, the

vanadium-substituted POMs(V(V)-POMs), [XVx-M12–xO40]n– and [X2VxM18–xO61]n–, have been widely

used as oxidation catalysts because the redox

potentials of V(V)-POMs are more positive than those of the corresponding parent POMs, indicating

V(V)-POMs should be stronger oxidants. However, the detailed quantitative analysis of the

proton-coupled voltammetric behaviour of V(V)- POMs has not been conducted. The parameters

obtained from this analysis are important to strategically develop new catalytic oxidation reactions. Taking the reaction stage of the catalytic

oxidation reaction with V(V)-POMs and the stability of reduced V(V)-POMs into consideration,

the voltammetric behaviour of the vanadium component of Keggin-type V(V)-POMs, [XV-M11O40]n- (X = P, As, S; M = Mo, W) in CH3CN,

was extensively investigated in the presence of designated concentrations of acid.19) For example,

Fig. 2 provides details of the changes in the VV/IV component for the reduction of SVMo11 as a

function of the acid concentration.

The diffusion coefficients of unprotonated POMs were calculated from cyclic voltammograms

obtained over the scan rate range of 20 to 500 mV s–1 and the Randles-Sevcik equation in the absence

of acid. The diffusion coefficients of the protonated

Figure 2 Cyclic voltammograms of 0.5 mM AsVW11 in CH3CN (0.1 M n-Bu4NPF6) in the presence of acid. [CF3SO3H] = (a) 0; (d) 0.5; (e) 1.0; (f) 5.0 mM. Scan rate: 100 mV s–1.

15


species were estimated in the same way from the

scan rate dependence found in the presence of a significant excess of acid when only a single reduction process was found. The diffusion

coefficients calculated via these procedures are listed in Table 4. The reported diffusion coefficient

for H+ in acetonitrile is 3.1 × 10–5 cm2 s–1 for simulation.

In the presence of excess acid, in principle, the voltammetry can be described by the reaction given in equation 1.

[HxXVVM11]x+ + yH+ + e- →←

[Hx+yXVIVM11](x+y-1)+ (X = P, As, S; M = Mo, W) (1) Furthermore, an estimate of the difference (y) in

the number of protons in the VV and VIV redox levels can be gained from the slope of a plot of the

reversible potential versus the logarithm of the concentration of acid, when the acid concentration

is in a large excess. The slopes of the Emid versus –log[H+] plot of ca. 120 and 180 for XVW11 and XVMo11 are indicative of maximum values of y = 2

and 3, respectively. The 51V NMR spectra of XVMo11 and XVW11

in the absence and presence of 5 mM CF3SO3H suggested that all V(V)-POMs were protonated, even in the oxidized form. Analysis of the EPR

spectra of XVIVMo11 and XVIVW11 in the absence and presence of 5 mM CF3SO3H estimated the

respective amount of protonated species of

XVIVMo11 and XVIVW11. From the results of the voltammetry, NMR and EPR measurements, the

redox mechanism with protonation reactions could be proposed as Schemes 1 and 2.

Simulations of the voltammetry were under-taken as a function of the acid concentration

according to Schemes 1 and 2. In addition, disproportionation reactions need to be included in this simulation to mimic the irreversibility observed

under some conditions (The details are described in ref. (19)). A comparison of the simulated and

experimental cyclic voltammograms for the VV/VIV process in the initial reduction of AsVW11 is shown in Fig. 3. The simulated cyclic voltammograms are

generally in good agreement with the observed ones

Table 4 Diffusion coefficients (×10–6) of V(V)-POMs in CH3CN

Scheme 1

Scheme 2

16


over all concentrations of CF3SO3H studied. The detailed parameters used for the simulation can be

seen in ref. (19) Moreover, voltammograms of the Keggin-type

XVM11 (X = Si, Ge, P, As, S; M = Mo, W) were

measured in CH3CN under neutral conditions (Table 5). Similar to the case of non-substituted

Keggin-type POMs, the potential due to the redox of VV/IV components is linearly related to the anion charge, implying that the mean bond valence

estimated from structural information of V(V)- POMs could be related to the redox potentials.

Concluding remarks The quantitative and theoretical analysis of the

electrochemical properties of POMs can be conducted with simulation and DFT calculation by NMR, EPR, UV-Vis and other measurements,

although fine structural information is needed, and no guarantee can be given that the simulation is

unique with a large number of unknown parameters. If the detailed voltammetric behaviour of most

POMs was elucidated, various electrochemical properties could be controlled and tuned by the appropriate solution conditions, and the applicable

range of POMs would be widespread. Acknowledgements

I express sincere thanks to Prof. Sadayuki Himeno,

A/Prof. Toshiyuki Osakai, Prof. Hajime Katano, Prof.

Kohji Maeda, Prof. Hiyoshizo Kotsuki, Prof. Kazumichi

Yanagisawa, Dr. Ayumu Onda, Dr. Shuntaro Tsubaki,

Prof. Alan M. Bond, Dr. Jie Zhang, Dr. Si-Xuan Guo, Dr.

John F. Boas, my laboratory mates, my students and

Alan’s group members for fruitful discussions, kind help

and encouragement.

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Figure 3 Comparison of simulated (○○○) and experimental (–––) cyclic voltammograms for the reduction of 0.5 mM AsVW11 in CH3CN (0.1 M n-Bu4NPF6) in the presence of designated CF3SO3H concentrations of (a) 0; (b) 0.4; and (c) 5.0 mM.

Table 5 Potentials (E1, mV vs. Fc/Fc+) due to the redox of VV/IV component in Keggin-type V(V)- POMs

All samples were measured in CH3CN containing 0.1 M n-Bu4NPF6. a: Measured after the addition of n-Bu4NOH to neutralize the protons present as a counter cation.

17


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