electrochemical and electron transfer behavior of o-chloranil with the presence of mg2+ in...

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Electrochemical and Electron Transfer Behavior of o-Chloranilwith the Presence of Mg2� in AcetonitrileHyun Park,a Munetaka Oyama*b

a Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japanb Division of Creative Research, International Innovation Center, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan; e-mail:

[email protected]

Received: November 2, 2001Final version: January 15, 2002

AbstractThe reduction process of o-chloranil (o-CA) with the presence of Mg2� was observed in acetonitrile on a platinumelectrode to compare that of p-chloranil (p-CA) with Mg2�. Although the reduction potentials of o-CA and p-CAwere not so different without Mg2�, apparent differences were recognized for both reduction processes in the presenceof Mg2�. While the adsorption of a Mg2� salt of p-CA dianion, Mg2�p-CA2�, was reported, no adsorption of thereduced forms of o-CA was observed. Alternatively, significant changes in the electrochemical responses wereobserved implying a rigid complexation between Mg2� and o-CA2� in solution. In addition , using a stopped-flowtechnique, the occurrence of the electron transfer between ferrocene, whose formal oxidation potential is �0.26 V (vs.I�3 , I�), and o-CA, whose formal reduction potential is 0.00 V, was observed in the presence of Mg2�, though thespontaneous electron transfer did not occur without Mg2�. This indicates the effect of Mg2� to make the redoxpotential of o-CA positive by complexing, while this is not the case of p-CA.

Keywords: o-Chloranil, Complexation with Mg2�, Electron transfer reactions, Redox behavior, Stopped-flow method

1. Introduction

Hydrogen-bonding and protonation are fundamental fac-tors for elucidating the reduction behavior of quinonederivatives, so that electrochemical analysis on suchinteractions has been carried out by measuring thevoltammetric responses in aprotic solvents with H� donors[1, 2]. Because the interaction is mainly based on theelectrostatic interaction between H� and reduced quinones,the effects of metal cations are also worthwhile analyzing astypes of the cation-anion interactions. In some cases, thereduction processes of organic compounds are quitesensitive to the small amount of metal cations, which arecoexisting in aprotic solvents and whose concentration is assmall as that of the substrate, as summarized previously [3].Thus, the electrochemical analysis is powerful and necessaryto understand such interactions that change the thermody-namic properties of molecules by complexing with metalcations.

In the electrochemical reduction of p-chloranil (p-CA) inacetonitrile (AN) on a platinum electrode surface, we havebeen observing unusual behavior in the presence of Mg2�,which promotes the formation of the reduced states of p-CAby making the ion paring complexes [4 ± 7]. The adsorptionof theMg2� salt of p-CA2�,Mg2�p-CA2�, has been confirmedusing Raman spectroscopy [4] and an EQCM measurement[7]. For this adsorption process, it is considered that theinteraction between the p-CA�� and Mg2� is the initialprocess, and then Mg2�p-CA2� is formed in the furtherreduction.

Over the case of p-CA, o-chloranil (o-CA) is expected tohave significant interactions with metal cations, in partic-ular, with the divalent cations. It is because the directinteraction toward two oxygen atoms is considered for o-chloranil with, e.g., Mg2�, as below.

Additionally, o-quinones are important molecules inbiological systems as well as p-quinones. For an instance,pyrroloquinoline quinone is a cofactor that has an o-quinone moiety, for which the interaction with divalentmetal cations, such as Mg2� and Ca2�, has been reported tostabilize the semiquinone radical [8, 9]. Thus, the electro-chemical analysis of o-quinone in aprotic solvents isvaluable for pursuing, in particular with the clarification ofthe elects of metal cations.

However, such fundamental studies are scarce. On theinteractions of o-CA�� with metal cations, up to now, theelectrochemical analysis of the complexation only with Na�

has been carried out from the viewpoint of the formation ofelectrochromic materials [10, 11].

Thus, in the present work, we compared the electro-chemical and redox behaviors of o-CA in the presence of

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Electroanalysis 2002, 14, No. 18 ¹ 2002 WILEY-VCH Verlag GmbH& Co. KGaA, Weinheim 1040-0397/02/1809-1269 $ 17.50+.50/0

Mg2� with those of p-CA on the basis of the previous results[4 ± 7]. It is known that the reduction potentials shift topositive due to the chloro-substituents for both p-CA and o-CA, but that the difference in the formal reduction potentialis not so significant in AN [12, 13]. However, if there aresome special interactions that can be inspired from themolecular structure, the electrochemical and the redoxbehaviors would be different significantly in the presence ofMg2�.

In addition to the cyclic voltammetric measurements, inthe present work, a stopped-flow method has been utilizedfor the evaluation of the redox behavior. By mixing an ANsolution of p-CA or o-CA in the presence or absence ofMg2� with an AN solution of the typical oxidized species,such as ferrocene (Fc) and decamethyl ferrocene (DMFc),we could observe the specific interaction between o-CAand Mg2�.

2. Experimental

Cyclic voltammograms were obtained using a computer-controlled PAR 263 potentiostat. The working electrodeused was a 1.6 mm diameter platinum disk electrode (BASCo. Ltd) and the reference electrode used was a Pt � I�3

�, I�)

electrode in AN. The measurements were carried out afterN2 gas was bubbled into the solutions at room temperature.

For the electrochemical quartz crystal microbalance(EQCM) measurements, a quartz crystal microbalanceanalyzer (QC917, Seiko EG & G) was used with the PAR263 potentiostat. Pt sputterd AT-cut quartz crystals (9 MHz,Seiko EG&G) were used as the working electrode.

For the stopped-flow measurements, a rapid-scan stop-ped-flow spectroscopic system, RSP-601 (Unisoku Co. Ltd.,Hirakata, Japan), was used [14].

p-CA, o-CA, Fc, DMFc and Mg(ClO4)2, which is thesource of Mg2� in AN, were all purchased from Aldrich; thereagents with highest purities available were used asreceived. Although Mg(ClO4)2 is hygroscopic, it was usedin the dry state. For a solvent, acetonitrile dehydrated(Wako Chem., H2O is less than 50 ppm) was used asreceived. As a supporting electrolyte for the electrochem-ical measurements, tetrabutylammonium hexafluorophos-phate (TBAPF6, Fluka, puriss, electrochemical grade) wasused as received. For the stopped-flow measurements toevaluate the electron-transfer reactions, the AN solution oftris(p-bromophenyl)amine cation radical (TBPA�

�) wasprepared electrochemically, whose method was previouslydescribed in detail [14].

3. Results and Discussion

3.1. Cyclic Voltammetry of p-CA in the Presence of Mg2�

Figure 1 showed the cyclic voltammograms (CVs) of1.0 mM Fc, DMFc, and o-CA in AN containing 0.1 MTBAPF6. All the redox processes are reversible at the scan

rate of 100 mV s�1, and the redox potentials were deter-mined to be �0.26 V, �0.23 V, and 0.00 V (vs. I�3 , I�),respectively. Because the formal redox potential p-CA was�0.10 V (vs. I�3 , I�) [4], the potential difference in thereduction between p-CA and o-CA is as small as 100 mV.

Next, we measured the CVs of 1.0 mM o-CA in thepresence of Mg2� in AN. Consequently, with the increase ofthe Mg2� concentrations in solutions, remarkable changes inthe redox behavior on the Pt electrode were recognized asshown in Figure 2.

As is apparent from the changes in the CVs (Fig. 2), a pre-reduction wave appeared at the potential region positive tothe reduction wave of o-CA, and concurrently, a broadoxidation wave appeared in the positive going scan. Whilethe peak potentials of the pre-reduction wave was almostidentical in Figure 2B± D, those of the oxidation wave in thereversed scan shifted to positive with the increase of theMg2� concentration. When the molar ratio of o-CA:Mg2�

was 1 :1 (Fig. 2D), the reversible redox peaks of free o-CAhas totally disappeared, and only the pre-reduction and itsoxidation peaks were only observed at �0.16 and �0.80 V,respectively.

Thus, the significant affect of coexisted Mg2�, which is lessthan 1.0 mM in AN, could be recognized from thevoltammetric responses in Figure 2. Because the reductionpotential of Mg2� is much more negative to that of o-CA inAN, it is apparent that the presence of Mg2� has changed thereduction property of o-CA. This is similar to the results ofthe electrode reduction of p-CA with Mg2� [4, 5, 7].

In the present experimental conditions, the content ofH2O in AN is less than 10 mM, which is even higher than theconcentration of Mg2�. However, because the reversible

Fig. 1. Cyclic voltammograms of 1.0 mM A) ferrocene (Fc), B)decamethylferrocene (DMFc), and C) o-chloranil (o-CA) inacetonitrile containing 0.1 M TBAPF6. Scan rate; 100 mVs�1.

1270 H. Park, M. Oyama et al.

Electroanalysis 2002, 14, No. 18

response of o-CA was observed in the absence of Mg2� inFigure 2A, the interaction of o-CAwith H� is expected to bevery minor, if at all. In the electrochemical reduction of p-benzoquinone derivatives in the presence of H� donors suchas alcohols, it has been reported that the first reduction is notsensitive to the presence of alcohols, but that the secondreduction changes at the concentrations higher than thesubstrate [1, 2].

Comparing the present electrochemical results of o-CAwith Mg2� with those of p-CA with Mg2� [4, 5, 7], it can benoted that the shift of the pre-reduction peak is significant.While the pre-wave with Mg2� and the reduction wavewithout Mg2� were merged together in the case of p-CA [4,5], the two peaks are easily distinguishable in the presentcase of o-CA. In contrast, the oxidation peaks appeared inthe reversed scan due to the presence of Mg2�, the potentialregion in the case of o-CAwith Mg2� was negative to that ofthe desorption of Mg2�p-CA2� [7].

Thus, to observe the adsorption and desorption behaviorsof o-CA, next an EQCM measurement was carried out tocompare with the previous result of the adsorption and thedesorption of Mg2�p-CA2� [7]. However, in the present caseof o-CA with Mg2�, completely no increases of the mass onthe electrode surface were observed in the conditions ofFigure 2 in spite of the significant changes in the electro-chemical responses. Therefore, as a large difference in theMg2� pair formation between p-CA and o-CA, it can beconcluded that o-CA is reduced with Mg2� to form thecomplex in solution as in Equation 1, while p-CA is reduced

with Mg2� to form the adsorbed complex as in Equation 2[4, 7].

o-CA�Mg2�� 2e��Mg2�o-CA2�in solution (1)

p-CA�Mg2�� 2e��Mg2�p-CA2�adsorption (2)

For the present case of o-CA with Mg2�, the changes in theCVs with the o-CA:Mg2� ratio indicate the formation of theo-CA2� salt, which is similar to the case of p-CA. In addition,when the potential was scanned to �1.2 V for the solutionsin Figures 2A ± D, the decrease in the second reduction peakwas observed with the increase of Mg2�, and finally itdisappeared when o-CA:Mg2�� 1 :1. Thus, the differencebetween p-CA and o-CA is summarized as in Equations 1and 2. Judging from the decrease of the reduction peakcurrent of free o-CA at �0.03 V and the increase of thecurrent of the pre-reduction peak at �0.16 V with theincrease of Mg2� (Fig. 2), it is expected that the formedcomplex is very stable.

Now that the above different adsorption property hasapparent through the EQCM measurement and the CVs,some minor differences in the electrochemical responsescan be noted further. That is, i) the shape of the pre-reduction wave in the case of o-CAwith Mg2� was peak-like,while a steep increase in the reduction current was observedin the case of p-CAwithMg2� [4, 5, 7]; ii) the oxidation peaksof Mg2�o-CA2� in the reversed scan is peak-like rather thanthat of Mg2�p-CA2� [7].

Hence, to discuss the present case of o-CA with Mg2� toform Mg2�o-CA2�

in solution in detail, we observed the changesin the CVs for the AN solution both containing 1.0 mM o-CA and 1.0 mM Mg2� depending on the scan rates. Thechanges in the shapes of the CVs are shown in Figure 3togetherwith the plots of peak currents vs. the square root ofthe scan rate. From the linear relationship for the latter(Fig. 3B), the redox behavior involving the species insolution can be confirmed also using the electrochemicalmeasurement, though the redox peak shape is not thatusually observed for a simple reversible system (Fig. 3A).

However, the present peak shape and the linear responsesin Figure 3 are quite similar to those of the electrochemicalreduction process of p-benzoquinone (p-BQ) to formhydroquinone (H2BQ) in buffered aqueous solutions asexpressed in Equation 3.

p-BQ� 2H�� 2e��H2BQin solution (3)

This similarity indicates that, even in the AN solution, theinteraction between Mg2� and o-CA is strong enough toform the Mg2�o-CA2� complex effectively. While the exactreason of the difference between in Equations 1 and 2 isunclear, it is inferred that the contact and rigid interaction ofMg2� toward two oxygen atoms of o-CA prevents theaggregation of the complexes to form the adsorption.

Fig. 2. Changes in cyclic voltammograms of 1.0 mM o-CA withthe concentration of Mg2� in acetonitrile containing 0.1 MTBAPF6. [Mg2�]; A) 0 mM, B) 0.25 mM, C) 0.5 mM and D)1.0 mM. Scan rate; 100 mVs�1.

1271Electron Transfer Behavior of o-Chloranil


3.2. Redox Behavior of o-CA with Mg2� Observed Using aStopped-Flow Method

Next, for analyzing the complexation between o-CA withMg2� in detail, we observed some electron transfer reactionsinvolving typical oxidized molecules in AN, i.e., Fc andDMFc,whose CVs are shown in Figure 1, using the stopped-flow method.

Because the formal redox potentials of Fc, o-CA, p-CAandDMFcare�0.26 V, 0.00 V,�0.10 Vand�0.23 V (vs. I�3 ,I�),respectively, the spontaneous electron transfer reactionsare expected between o-CA and DMFc (Eq. 4), andbetween p-CA and DMFc (Eq. 5), though those with Fcdo not occur thermodynamically.

o-CA�DMFc�o-CA��DMFc� (4)

p-CA�DMFc�p-CA��DMFc� (5)

Actually, by mixing AN solutions of p-CA and DMFc, thequantitative formation of p-CA�

� was spectrophotometri-cally confirmed from the observed absorption spectrum ofp-CA�

� having an absorption maximum at 450 nm, whichwas identical to that obtained previously via the electro-chemical preparation [5]. In the same reaction, the

formation of DMFc� was confirmed by the appearance ofthe absorption of Figure 4A in the longer wavelengthregion. Being not observed in the electrochemical gener-ation of p-CA�� [5], the absorption peak in Figure 4Acan beassigned to that of DMFc� formed via Equation 5. This wasalso confirmed by observing the electron transfer reactionbetween a stable cation radical, TBPA�� (whose formalredox potential is 0.97 V), with the equimolar of DMFc toform DMFc� quantitatively.

In the reaction between o-CA and DMFc in Equation 4, aremarkable change was observed in the longer wavelengthregion as shown in Figure 4B. The increase in the absorptionwas attributed to the generation of o-CA�� in Equation 4.The wavelength region is in agreement with the reportedresults [11, 15].

Because the absorption of p-CA�� was varied by thecomplexation with Mg2� [5], the changes in the spectrum ofo-CA�� were observed after the mixing of the AN solutionboth containing o-CA and Mg2� with the AN solution ofDMFc. As a result, however, a remarkable change inabsorption spectrum was only observed in the longerwavelength region of Figure 4. That is, with the presenceof Mg2�, the change from Figure 4B to Figure 4A was onlyobserved from just after the mixing. This means that theabsorption of free o-CA�� disappeared after the mixing, andthat only the absorption ofDMFc� remained. Thus, from thetransformation of the absorption of o-CA��, it is expectedthat the same ion pair, Mg2�o-CA2�, is formed in solution,via Equation 6 following Equation 4.

2 o-CA��Mg2�� Mg2�o-CA2�� o-CA (6)

However, unfortunately, the increase of the absorption dueto Mg2�o-CA2� was not observed in the wavelength region

Fig. 3. A) Changes in cyclic voltammograms of 1.0 mM o-CAwith 1.0 mM Mg2� with the scan rates. With the increase of thescan rates (20, 50, 100, 200 and 400 mVs�1), the increase in theredox currents were observed. B) Plot of the redox currents versusthe square root of the scan rate. (�) Reduction and (�) oxidationcurrent.

Fig. 4. A) Absorption spectrum of 0.50 mM DMFc� in acetoni-trile quantitatively formed via the electron transfer reactionbetween p-CA and DMFc. B) Absorption spectrum obtained aftermixing of the AN solutions of 0.50 mM DMFc and 0.50 mM o-CA.



of 350 ± 900 nm, presumably because it is in the wavelengthregion shorter than 350 nm and overlapping with those ofDMFc and o-CA.

In addition to the complete decrease of the absorption offree o-CA�� observed previously, the higher reactivity of o-CA�� could be recognized as a remarkable difference in thereactions between p-CA�� and o-CA�� toward Mg2�. It isbecause only the constant spectrum of Figure 4A wasobserved after the mixing, while the kinetic process wasanalyzed successfully following the decrease of free p-CA��

with Mg2� [5, 16]. Thus, the strong interaction of o-CA��

toward Mg2� was kinetically confirmed.On the other hand, the electron transfer reactions with Fc

cannot be expected for both p-CA and o-CA on the basis ofthe formal redox potentials. Actually, in the stopped-flowmeasurements, such electron transfer reactions were notobserved. And next, the effect of Mg2� was analyzed for thecase of p-CA. However, no electron transfer reactions couldbe observed (Eq. 7).

Fc�Mg2�� p-CA �� (7)

In contrast, interestingly, the spectral changes with time canbe detected when the AN solution of Fc was mixed with theAN solution both containing o-CA and Mg2�. Figures 5 and6 show the changes in absorption spectra due to the presenceof Mg2� in the reaction between Fc and o-CA. In the resultsobtained after mixing of 0.50 mM Fc with 0.50 mM o-CA�0.50 mM Mg2�, the increase of absorption maximum can beassigned to the formation of Fc� (Figs. 5B and 6B), and thedecrease to the consumption of o-CA (Fig. 6B). Therefore,

thepromotionof the electron transfer reactions involvingFccould be confirmed in the presence of Mg2� from theseresults.

For this electron transfer, if the formation of Mg2�o-CA2�

is presumed, the overall reaction can be written asEquation 8.

2 Fc�Mg2�� o-CA� 2 Fc��Mg2�o-CA2� (8)

As the initial step, it is expected that Mg2� promotes theelectron trasfer of Equation 9.

Fc�Mg2� ¥ ¥ ¥ o-CA�Fc��Mg2�o-CA�� (9)

Following the electron transfer step of Equation 9, twopossibilities are considered as follows to complete thereaction of Equation 8.

Fc�Mg2�o-CA��Fc��Mg2�o-CA2� (10)

2 Mg2�o-CA��Mg2�o-CA2��Mg2�� o-CA (11)

To inspect the mechanism, we observed the changes in theincrease process of Fc� depending on the mixing ratio of Fc,o-CA and Mg2�, whose results are shown in Figure 7.Because the reactions are not necessarily completedquantitatively in these conditions as recognized from thefinal absorbance values in Figure 7, the exact determinationof the rate law is not easy. However, two conclusions can bedrawn. First, the increase of Fc�, i.e., the decrease of Fc, is asecond order reaction, not a first-order, in all the cases.Secondly, the increase rate of Fc� depends on [o-CA] ratherthan [Mg2�]. This result rules out a simple mechanism ofEquations 9 and 10, in which the two-electron oxidation ofMg2� ¥ ¥ ¥ o-CA is assumed. Thus, more complex interactions,

Fig. 5. Changes in absorption spectra due to the presence ofMg2� in the reaction between Fc and o-CA. A) Absorptionspectrum obtained after mixing the AN solutions of 0.50 mM Fcand 0.50 mM o-CA. In this case, no electron transfer reactionoccurred; thus the observed spectrum is that of o-CA. B)Absorption spectrum obtained after mixing of the AN solutionof 0.50 mM Fc with the AN solution both containing 0.50 mM o-CA and 0.50 mM Mg2�. The increase of Fc� was observed with thetime intervals of 10 ms.

Fig. 6. Changes in absorption spectra in the reaction between Fcand o-CA due to the presence of Mg2�. The conditions wereidentical to those in Figure 5, but only the wavelength region andabsorbance axis are different.

1273Electron Transfer Behavior of o-Chloranil


e.g., the catalytic effect of Mg2� as Equations 9 and 11,should be considered for the present electron transferreaction.

4. Conclusions

In the present work, the Mg2�-complexation between o-CAwas studied using the electrochemical and the stopped-flowmeasurements, and compared to that of p-CA.The results ofthe electrochemical measurements showed that Mg2�o-CA2� is formed in solution in the reduction of o-CA withMg2�, while the adorption of Mg2�p-CA2� was observed inthe reduction of p-CA with Mg2�. The electrochemicalresponses of o-CAwith Mg2�, were similar to those of p-BQin aqueous solution. This means that, even in the ANsolution, the interaction between Mg2� and o-CA is strongenough to form the Mg2�o-CA2� complex effectively.

In addition, the electron transfer between o-CA and Fcwas found to be promoted by the presence of Mg2�, while itdid not occur in the absence of Mg2�. Although the complexaspect of the mechanism was revealed implying the catalyticfunction of Mg2�, more than the mechanistic aspect, it is

interesting that Mg2� promotes such electron transfer for o-CA. While such a catalytic function of Mg2� is known forseveral electron transfer reactions [17], the rigid interactionbetween two oxygen atoms of o-CA2� and Mg2� isconsidered to be a key feature of the promotion in thepresent case.

5. Acknowledgements

This work was supported in part by a Grant-in-Aid forScientific Research from the Ministry of Education,Culture, Science, Sports and Technology, Japan, Nos.01299289 and13640602. H. P. would like to thank the JSPS(Japan Society for the Promotion of Science) Post-DoctoralFellowship for Foreign Researchers. M. O. would like tothank the Mitsubishi Foundation for financial support.

6. References

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65, 1448.[3] K. Miyoshi, M. Oyama, S. Okazaki, Electroanalysis 2001, 13,

917.[4] M. Yamanuki, T. Hoshino, M. Oyama, S. Okazaki, J.

Electroanal. Chem. 1998, 458, 191.[5] T. Hoshino, M. Oyama, S. Okazaki, J. Electroanal. Chem.

1999, 472, 91.[6] T. Hoshino, S. Kamei, D. Yu, M. Oyama, S. Okazaki,

Electroanalysis 2000, 12, 553.[7] M. Oyama, Electroanalysis 2002, 14, 277.[8] D. Iswantini, K. Kano, T. Ikeda, Biochem. J. 2000, 350, 917.[9] A. Sato, K. Takagi, K. Kano, N. Kato, J. A. Duine, T. Ikeda,

Biochem. J. 2001, 357, 893.[10] A. Debene-Monvernaay, A. Cherigui, P. C. Lacaze, J. E. E.

Dubois, J. Electroanal. Chem. 1989, 260, 75.[11] A. Debene-Monvernaay, P. C. Lacaze, A. Cherigui, J.

Electroanal. Chem. 1989, 260, 75.[12] N. Gupta, H. Linschitz, J. Am. Chem. Soc. 1997, 119, 6384.[13] M. W. Lehmann, D. H. Evans, J. Electroanal. Chem. 2001,

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1373.[17] S. Fukuzumi, Bull. Chem. Soc. Jpn. 1997, 70, 1.

Fig. 7. Changes in the increasing rate of Fc� dependingon the mixing ratio of [Fc] : [o-CA] : [Mg2�] in the reactionbetween Fc and o-CA with Mg2�. [Fc] : [o-CA] : [Mg2�]A) 0.50 mM:0.25 mM:0.25 mM; B) 0.50 mM:0.25 mM:0.50 mM;C) 0.50 mM:0.50 mM:0.25 mM; and D) 0.50 mM:0.50 mM:0.50 mM. Wavelength: 618 nm.



electrochemical and electron transfer behavior of o-chloranil with the presence of mg2+ in...

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