Journal of

www.elsevier.com/locate/jelechem

Journal of Electroanalytical Chemistry 603 (2007) 155–165

ElectroanalyticalChemistry

Effects of the molecular structure on the electrochemical propertiesof naturally occurring a-hydroxyquinones. An electrochemical

and ESR study

Carlos Frontana *, Ignacio Gonzalez *

UAM-Iztapalapa, Departamento de Quımica, Apartado postal 55-534, 09340, Mexico D.F., Mexico

Received 22 October 2006; received in revised form 7 January 2007; accepted 23 January 2007Available online 12 February 2007

Abstract

The electrochemical study of a series of naturally occurring a-hydroxyquinones (2-hydroxy-1,4-naphthoquinone, Perezone, Hormi-none and 7a-O-methyl-Conacytone), revealed that the reduction mechanism occurring at the first voltammetric reduction signal, is deter-mined by the presence of selfprotonation reactions. Furthermore, the detailed voltammetric analysis of this signal, reveals that there is adependence of the structure of the chemical compound bearing the a-hydroxy-moiety in the reduction mechanism. These differences aredetermined by the stability and basicity of the first electrogenerated anion radical for each compound. However, since this species dis-proportionates readily in solution, it was not possible to analyze its structural properties by coupled electrochemical – ESR experiments.As during the selfprotonation process an important amount of deprotonated quinone is produced, the spectroelectrochemical study at apotential region at the second reduction process for this type of compounds showed well resolved ESR spectra, corresponding to thedianion radical structures originated from the reduction of the deprotonated quinone. The differences in chemical behavior showed thatthe abietanic skeleton of naturally occurring quinones Horminone and 7a-O-methyl-Conacytone induces changes in the spin structurewhich prove valuable in determining the stability and reactivity of the electrogenerated dianion radical.� 2007 Elsevier B.V. All rights reserved.

Keywords: Horminone; Perezone; Conacytone; Radical dianion; ESR; Selfprotonation; Deoxygenation; Cyclic voltammetry

1. Introduction

The study of a-hydroxyquinones (a-OH–Q) is a subjectof most importance, since many of the compounds wherethis functionality is present possess important biologicalactivities [1–3]. Also, these compounds are present inimportant amounts in several species of plants in Mexico,such as varieties of Salvia and Perezia, which have beenused in traditional medicine since prehispanic times [4,5].Since the biological activity seems to be related to the pres-ence of reactive radical species – characteristic of quinone

0022-0728/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.jelechem.2007.01.024

* Corresponding authors. Tel.: +52 55 58044671x12; fax: +52 5558044666.

E-mail addresses: ultrabuho@yahoo.com.mx (C. Frontana), igm@xanum.uam.mx (I. Gonzalez).

compounds – the use of electrochemical methods to gener-ate and study this type of intermediates has proven to beuseful [6,7].

During the electrochemical study of this type of com-pounds, it has been shown that the experimental behaviordoes not follow the typical two monoelectronic reversiblecharge transfer processes, occurring for quinones in aproticmedium [8]. The observed electrochemical behavior for a-hydroxyquinones is consistent with a reduction mechanisminvolving both electron transfer processes and coupledchemical reactions, where selfprotonation processes [9]occur. This occurs due to the higher acidity of the hydroxylfunction at the a position in front of the first electrogener-ated anion radical [6,7,10,11].

It is interesting to note that the presence of selfprotona-tion processes consumes quickly any stable radical species

156 C. Frontana, I. Gonzalez / Journal of Electroanalytical Chemistry 603 (2007) 155–165

which could be generated at these energetic conditions.Therefore, it is difficult to explain any biological mecha-nism involving the generation of stable radicals at thereduction potential of the first reduction peak. This is rele-vant, considering that most of the biological activity of qui-nonoid systems is related to their capacity of carrying outfree radical generation via redox reactions, such as the gen-eration of Reactive Oxygen Species (ROS, e.g. superoxideanion radical and H2O2) in biological systems [12,13].

However, during the electrochemical study of the secondreduction signal occurring for a-hydroxyquinones, it hasbeen found that this signal is consistent with the reductionof the deprotonated quinone (Q–O�) formed during theselfprotonation sequence [14–16], leading to the formationof a radical dianion structure. This type of intermediatescan be characterized by employing coupled Electrochemi-cal-Electron Spin Resonance (EC-ESR) experiments [17].As these intermediates are the only stable radical speciespresent during the reduction of a-hydroxyquinones, theyare promising candidates for understanding the biologicalactivity of this kind of compounds.

Even though the reactivity of several types of a-hydroxy-quinones have been characterized by electrochemical meth-ods, it is unclear how does the chemical structure affects theproperties of the dianion radical species formed during thereduction of natural products. In this context, it is useful toperform these experiments under aprotic conditions, sincethe quinones and their corresponding anions are solvatedfar less efficiently [18,19] than in water. Moreover, aproticsolvents mimic nonpolar environments of the cell wheremuch of the biological electron transfer occurs [20–25].

In this work, an electrochemical and spectroelectro-chemical-ESR study in aprotic media of a series ofnaturally occurring a-OH–Q’s (Perezone, Horminone and7a-O-methyl-Conacytone, Fig. 1) is presented. Cyclic vol-

Fig. 1. Chemical structures of the studied a-hydroxyquinones. 1: 2-hydroxy-1,4-naphthoquinone; 2: Perezone; 3: Horminone; 4: 7a-O-methyl-Conacytone.

tammetry, double potential step chronoamperometry andin situ spectroelectrochemical-ESR studies, were used inorder to analyze the influence of the molecular structureon the reactivity of the corresponding radical dianions.For comparative purposes, the results of these studies werealso performed for 2-hydroxy-1,4-naphthoquinone (Fig. 1).

2. Experimental section

2.1. Substances

Naturally occurring a-OH–Q’s Perezone (2), Hormi-none (3) and 7a-O-methyl-Conacytone (4) (Fig. 1) wereemployed. 2-hydroxy-1,4-naphthoquinone (1, A. R. grade,Aldrich�), was also used. Natural occurring quinones 2and 4 were kindly supplied by Dr. Bernardo A. Fron-tana-Uribe from Instituto de Quımica, UNAM, Mexico.Horminone was supplied by Dr. Juan Manuel Aceves fromFES Cuautitlan, UNAM, Mexico. Recrystallized naturaloccurring quinones from ethyl acetate/hexane mixtureswere used for the purposes of this work, while 2-hydroxy-1,4-naphthoquinone was used without furtherpurification.

2.2. Solvent and supporting electrolyte

Anhydrous acetonitrile (CH3CN, Aldrich 98%) wasdried overnight with P2O5, and distilled prior to use. Thedistillate was received over oven-activated 3 A molecularsieves (Merck) and kept in a desiccator. The method is use-ful to obtain dry acetonitrile, characterized by the absenceof OH bands in IR spectra. Tetraethylammonium tetra-fluoroborate (Fluka Chemika, Electrochemical grade,Et4NBF4) was used as a supporting electrolyte. The saltwas dried the night before use, at 90 �C; and a 0.1 mol L�1

supporting electrolyte solutions were prepared from it.

2.3. Electrochemical determinations

Cyclic voltammetry and double potential step chrono-amperometry experiments were performed with an AUTO-LAB PGSTAT 30 potentiostat/galvanostat. IR Dropcorrection was performed during all the experiments, usingRu values (82 Ohms) obtained with the positive feedbacktechnique and employing the ‘‘High Speed’’ mode of thepotentiostat. The Ru values obtained represent approxi-mately 90% of the value where potentiostat instability isreached. A conventional three electrode cell was used tocarry out these experiments. The working electrode was aplatinum microelectrode (BAS, Surface: 0.025 cm2); priorto its use, it was polished with 0.05 lm alumina (Buehler),sonicated in distilled water for 10 min and rinsed with ace-tone. For each compound, the polishing process was per-formed while the electrode was rinsed with acetonebetween each voltammetric and chronoamperometric runduring the electrochemical study. Also, five consecutivedouble potential step experiments were performed in order

C. Frontana, I. Gonzalez / Journal of Electroanalytical Chemistry 603 (2007) 155–165 157

to account for the residual background current, as recom-mended by Amatore and Saveant [26]. A platinum meshwas used as the counterelectrode (Surface: 0.6 cm2). Thepotential values were obtained against the Ag/0.01 mol L�1

AgNO3 + 0.1 M tetrabutylammonium perchlorate (TBAP)reference (Bioanalytical Systems, BAS) in acetonitrile, sep-arated from the medium by a Vycor membrane. Potentialvalues are reported vs the ferricinium/ferrocene couple(Fc+/Fc), according to the IUPAC recommendation [27].The Fc+/Fc potential couple was measured for each com-pound independently in order to avoid potential changesthat might be caused by the Vycor aging (due to modifica-tion in the reference potential).

Electrochemical experiments were carried out as follows:quinones solutions (1: 6 mM; 2: 4 mM; 3: 2.9 mM; 4:2.2 mM), were dissolved in 0.1 mol L�1 tetraethylammo-nium tetrafluoroborate (Et4NBF4) in acetonitrile solution.With these prepared solutions, cyclic voltammetry experi-ments at several scan rates in the interval of 0.01–2 V s�1

were performed. Double potential step experiments wereperformed in the interval of 0.001–1 s for the direct poten-tial pulse (h), while the inverse pulse lasted 10 times thevalue of h and employing the same solutions used for thevoltammetric study of each compound. All the obtainedpeak potential values are referred to the Fc+/Fc coupleas recommended by the IUPAC [27].

For the experiments performed under basic conditions,an equivalent of 1 M tetrabutylammonium hydroxide (n-But4NOH) solution in water (Aldrich�) was added to thequinone solutions.

2.4. Spectroelectrochemical-ESR spectroscopy experiments

ESR spectra were recorded in the X band (9.85 GHz),using a Brucker ELEXSYS 500 instrument with a rectan-gular TE102 cavity. A commercially available spectroelect-rochemical cell (Wilmad) was used. A platinum mesh(�0.2 cm2) working electrode was introduced in the flatpath of the cell. Another platinum wire was used as counterelectrode (2.5 cm2). The Ag/0.01 mol L�1 AgNO3 + 0.1mol L�1 TBAP reference in acetonitrile (BAS) wasemployed as reference electrode. Potential sweep controlwas performed in a 100 B/W Voltammetric Analyzer ofBioanalytical Systems (BAS) with a personal computerinterface.

Quinones solutions of 1 mM concentration were usedfor spectroelectrochemical experiments. These solutionswere prepared by dissolving the desired compound in0.1 mol L�1 Et4NBF4/CH3CN. The prepared solutionswere deoxygenated for 30 min prior to each experimentand the cell was kept under a nitrogen atmosphere (grade5, Praxair) throughout the experiment.

2.5. ESR simulations

PEST WinSim free software Version 0.96 (NationalInstitute of Environmental Health Sciences) was use to per-

form ESR spectra simulation, from the hyperfine couplingconstant values (a) measured, to compare with the experi-mental ones. This program was also useful to evaluate a

values in the case where a direct measurement would be dif-ficult under the spectra acquisition conditions.

2.6. Theoretical calculations

PM3 [28–30] calculations were performed with Hyper-Chem (HyperCube Inc.) Ver. 7.51. The geometry optimiza-tion for both, the neutral and semiquinone structures, weredone employing UHF (Unrestricted Hartree–Fock) at thePM3 level, and they were used as inputs for single pointenergy calculations. Vibrational analysis was performedto check that the obtained structures were indeed the min-imum energy conformers.

3. Results and discussion

3.1. Electrochemical study of a series of a-hydroxyquinones

The typical voltammetric signals of a-OH–Q’s 2-hydroxy-1,4-naphthoquinone 1, Perezone 2, Horminone 3and 7a-O-methyl-Conacytone 4 are depicted in Fig. 2.The choice of the employed concentrations for each qui-none was performed on an experimental basis, since itwas preferred the highest possible faradaic contributionof the current with the minimum interference from capaci-tive effects. This is particularly important for Horminoneand 7a-O-methyl-Conacytone, which have a very low diffu-sion coefficient value. However, these last compounds arenot quite soluble in acetonitrile and therefore the concen-tration values are near their solubility limit, which arecomprised within the same order of magnitude as for 2-hydroxy-1,4-naphthoquinone and Perezone. Despite ofthese differences, the comparison of the voltammetricbehaviour is not affected. As a general result, the studiedquinones present a voltammogram consisting of two reduc-tion signals (peaks Ic and IIc). The first reduction peakpotential (EpIc) for the quinones 1–4 shifts slightly towardsmore negative potential values as the number of fused ringsincreases in the chemical structure of the a-hydroxy qui-none (Table 1). Upon inversion of the scan sweep at apotential value more negative than EpIc, it can be seen thatsignal Ic does not present a reversible behavior, which dif-fers from the typical two reversible electron transfer pro-cess for the electroreduction of quinones in aprotic media[8]. This indicates that the semiquinone generated at thefirst reduction signal is not stable and undergoes a chemicalsequence following its generation, by which it is consumed.However, two characteristic anodic signals (Ia, Ia 0) appear,indicating the oxidation of intermediates or products gen-erated during reduction process Ic.

As it was mentioned above, the reduction mechanismoccurring at the peak Ic for a-OH–Q’s has been describedas a sequence of reactions involving a selfprotonation pro-cess which generates as major products hydroquinone and

Fig. 2. Typical cyclic voltammograms of (A) 6 mM 2-hydroxy-1,4-naphthoquinone 1; (B) 4 mM Perezone 2; (C) 2.9 mM Horminone 3 and (D) 2.2 mM7a-O-methyl-Conacytone 4, obtained with a platinum microelectrode (0.025 cm2) in 0.1 M Et4NBF4/CH3CN. The potential scan was initiated from450 mV vs Fc+/Fc, in the negative direction. Two different inversion potential (Ek) conditions are shown for each compound. v: 100 mV s�1. Both cathodicand anodic peaks are indicated. The chemical structures for each studied quinone are indicated in the corresponding figure.

Table 1Data from the voltammetric analysis of the studied a-hydroxyquinones

Compound EpIc (V) EpIa (V) EpIa0 (V) EpIIc (V) EpIIa (V) EpIIc0b (V) EpIIa0

b (V)

2-Hydroxy-1,4-naphthoquinone 1 �1.11 �0.49 0.25 �1.96a �1.8 �1.87a �1.79Perezone 2 �1.15 �0.51 �0.06 �1.91a �1.74 �1.98a �1.60Horminone 3 �1.17 �0.45 0.01 �1.89a �1.71 �1.78a �1.387a-O-methyl-Conacytone 4 �1.18 �0.33 0.04 �1.89a �1.48 �1.87a �1.43

Data recorded from voltammetric experiments at 0.1 V s�1. Potential values are reported vs Fc+/Fc.a Data from signals corresponding to the generation of a stable radical species.b Data obtained upon addition of 1 equiv. of tetrabutylammonium hydroxide.

158 C. Frontana, I. Gonzalez / Journal of Electroanalytical Chemistry 603 (2007) 155–165

deprotonated quinone, by the next set of chemicalequations:

Q–OHþ e� � ðQ–OHÞ�� ð1ÞðQ–OHÞ�� þQ–OH� ! ðHQ–OHÞ þQ–O� ð2ÞðHQ–OHÞ� þ e� � ðHQ–OHÞ� ð3ÞðHQ–OHÞ� þQ–OH! H2Q–OHþQ–O� ð4Þ3Q–OHþ 2e� ! H2Q–OHþ 2Q–O� ð5Þ

The second electron transfer uptake involved in thissequence (Eq. (3)) can also occur homogenously [31], wherethe protonated semiquinone (HQ–OH) undergoes a solu-tion electron transfer via a disproportionation reaction.The chemical equation describing this behavior can be rep-resented as:

ðQ–OHÞ�� þ ðHQ–OHÞ� ! Q–OHþ ðHQ–OHÞ� ð6Þ

Considering the above presented sequence, the observedoxidation signals (Ia, Ia 0) should be associated to the oxi-dation of the electrogenerated hydroquinone forms((HQ–OH)�, H2Q–OH) or the deprotonated quinone (Q–O�). The corresponding peak potential values appear atdifferent potentials for each studied compound, which indi-cates that the involved intermediates present chemical dif-ferences for each structure.

As signals Ia and Ia 0 are related to the oxidation of theproducts electrogenerated during the reduction step Ic, it isnecessary to consider the chemical nature of the speciesbeing oxidized. For this purpose, it is important to recallthe similarities in the electrochemical behavior that thesecompounds present between each other. In the previous

Fig. 3. Variation of the quotient I(2h)/I(h), for different values of the pulsetime (h) for (s) 6 mM 2-hydroxy-1,4-naphthoquinone 1: Edirect pulse (Edp):�1.28 V, Einverse pulse (Eip): �0.9 V vs Fc+/Fc. (e) 4 mM Perezone 2:Edp:�1.3 V, Eip: �0.95 V vs Fc+/Fc; (h) 2.9 mM Horminone 3: Edp:�1.3 V; Eip: �0.9 V vs Fc+/Fc. (m) 2.2 mM 7a-O-methyl-Conacytone 4.Edp: �1.35 V, Eip: �0.9 V vs Fc+/Fc. Dotted lined on top represents theexperimental limit obtained for the reversible monoelectronic system (1,4-benzoquinone: 0.290).

C. Frontana, I. Gonzalez / Journal of Electroanalytical Chemistry 603 (2007) 155–165 159

studies [7,17], the analysis of the oxidation signal Ia for 2-hydroxy-1,4-naphthoquinone 1 and Perezone 2, showedthat this signal is related to the oxidation of the electrogen-erated hydroquinone ((HQ–OH)� or H2Q–OH, the prod-ucts from Eqs. (3) or (4), respectively). With thisconsideration, the experimental differences DEpIc–Ia canbe related to the final chemical structure of the hydroqui-none electrogenerated, being the protonated hydroquinoneH2Q–OH more difficult to oxidize than the correspondingdeprotonated hydroquinone H2Q–O� [11]. The presenceof peak Ia 0 is related to the oxidation of the correspondingdeprotonated quinone (Q–O�).

The differences in peak potential values for signals Icand Ia (Table 1) increase as the number of fused rings con-stituting the corresponding a-hydroxyquinone alsoincreases (DEpIc–Ia: (1) 0.60 V; (2) 0.64 V; (3) 0.73 V; (4)0.85 V). With the considerations presented above, the pro-gressive increase in the experimental DEpIc–Ia values forquinones 1–4 suggests that the generation of the proton-ated hydroquinone moiety is favored by the increase ofthe number of rings constituting the a-hydroxyquinone.This behavior can be related either to the basicity of theintermediates generated during the selfprotonationsequence or the acidity of the a-hydroxyquinone beingreduced. A quantitative analysis of the selfprotonation pro-cess can help to understand how does the reduction mech-anism is affected by the molecular structure of eachcompound.

The analysis of the behavior of the function EpIc vs log v

for the quinones under study, can help to devise if the reduc-tion process is determined by the presence of two heteroge-neous steps (ECE mechanism, Eqs. (1)–(3)), or thehomogenous electron transfer marked as Eq. (6) (DISPmechanisms, Eqs. (1), (2) and (6); DISP1: Rate determiningstep is Eq. (2); DISP 2: Rate determining step is Eq. (6))[9,26,32]. This can be evaluated by the changes in the slopesof the corresponding EpIc vs log v functions, which wereexperimentally different for each compound. For instance,2-hydroxy-1,4-naphthoquinone 1 presents a slope value of�28.0 ± 2.7 mV dec�1 (n = 8, r2 = 0.9949), Perezone 2 pre-sents a �30.5 ± 1.4 mV dec�1 slope (n = 16, r2 = 0.9713),while Horminone 3 has a slope value of �22.7 ± 0.9 mVdec�1 (n = 17, r2 = 0.9758) and 7a-O-methyl-Conacytone4 shows a �48.5 mV ± 1.7 dec�1 value (n = 14, r2 =0.9848), where n is the number of experimental points andr2 is the determination coefficient for each linear regression.From this data, it can be proposed that an ECE/DISP1sequence operates for 2-hydroxy-1,4-naphthoquinone 1

and Perezone 2, while a DISP2 mechanism occurs forHorminone 3 (Ep vs log v for ECE/DISP1 type mecha-nisms: �29.6 mV dec�1; DISP2 mechanism: �19.7 mVdec�1) [9,26,32]. 7a-O-methyl-Conacytone 4 slope value ismuch higher and therefore cannot be directly classified asone of the proposed mechanisms. This behavior indicatesthat the first electron uptake involved as Eq. (1) for quinone4 shows a lower rate than those occurring for the other com-pounds and proves to be the rate determining step.

The presence of a DISP type sequence for Horminoneand 7a-O-methyl-Conacytone was proven by performingdouble potential step chronoamperometry experiments atthe region of the first reduction potential for both com-pounds, as presented in Fig. 3. These experiments were per-formed in a similar way as it was done for 1 and 2 [17],defining the direct pulse at a potential value where reduc-tion process Ic is diffusion limited, and the inverse pulseis set at a potential value in the base of the Ic peak. Thelow value of the quotient I(2h)/I(h) shows that, at theemployed pulse times, the semiquinone species presents alow stability in solution. This accounts for the fast rateof the selfprotonation reactions (Eqs. (2)–(4) and (6)), asit is not possible to detect the semiquinone intermediateeven at the shortest possible pulse time (0.001 s). ForHorminone 3, there is a slight recovery, but it never reachesthe expected 0.293 value for the case where the semiqui-none can be detected in a pure monoelectronic electrontransfer.

It has also been reported that oxygen can indeed alterthe electron transfer pathway [33], but the evaluation ofthis effect is out of the scope of this manuscript. Therefore,the previous and forthcoming discussion deals with thestudy in the absence of oxygen to try to analyze the viabil-ity of the radical anions electrogenerated.

In the case of the charge transfer processes IIc and IIa,the experimental peak potential differences (DEpIIc–IIa)showed variations between each compound: (1) 0.16 V;(2) 0.20 V; (3) 0.17 V; (4) 0.41 V. It is interesting to recallthat the presence of signals IIc and IIa are associated tothe electroreduction of the deprotonated quinone, gener-ated during the selfprotonation sequence (Eq. (5)) [17].

Q–O� þ e��Q–O�¼ ð7Þ

For the studied compounds, these reduction processesdo not appear as reversible charge transfers, due to thehigh value of the experimental peak potential differences

160 C. Frontana, I. Gonzalez / Journal of Electroanalytical Chemistry 603 (2007) 155–165

DEpIIc–IIa. However, as the electrochemical properties ofthe observed signals are dependent on both the precedingchemical sequence for each compound and on the chemicalproperties of the radical dianions electrogenerated, it isnecessary to employ conditions where the preceding self-protonation sequence does not affect the behavior of thedeprotonated quinone. In order to avoid these complica-tions, the voltammograms were repeated in the presenceof a basic compound (TBAOH) which deprotonates thea-hydroxy quinones [14,17]. For all the studied quinones,the addition of 1 equiv. of the basic compound to the qui-none solution, provokes the disapearance of the signal Ic(Fig. 4), and a new pair of signals (IIc 0 and IIa 0) appears,from which the peak potential difference can be estimated.For quinone 1 the peak potential difference DEpIIc 0–IIa 0 iscloser to the value of a monoelectronic charge transfer,(0.08 V, Table 1) meanwhile for Perezone, DEpIIc 0–IIa0 =0.38, higher than the corresponding DEpIIc–IIa difference.The obtained differences for quinones 3 and 4 also increaseas a result of the base addition (DEpIIc 0–IIa 0 = 3: 0.40 V; 4:

Fig. 4. Typical cyclic voltammograms of (A) 6 mM 2-hydroxy-1,4-naphthoqu7a-O-methyl-Conacytone 4, in the presence of 1 equiv. of tetrabutylammoniumin 0.1 M Et4NBF4/CH3CN. v: 100 mV s�1. Both cathodic and anodic peaks arebase.

0.44 V). This reveals a significant difference between theanalyzed structures, as the a-hydroxy-naphthoquinone(1), behaves in a dissimilar way compared to the a-hydroxy-benzoquinones (2–4). In order to have moreinsights of the nature of these differences, it is necessaryto evaluate by an independent method, the chemical prop-erties of the electrogenerated radical dianion structures,which can be studied performing spectroelectrochemical-Electron Spin Resonance (ESR) experiments.

3.2. Spectroelectrochemical-ESR study of the

a-hydroxyquinones

A spectroelectrochemical-ESR study was carried out forquinones 1–4. As a general result, a lack of stable radical sig-nals was found by studying the spectroscopic response atapplied potential values in the potential region between sig-nals Ic and IIc. This result confirms that the electrogener-ated semiquinone species, are readily consumed during theselfprotonation sequence and therefore cannot be detected

inone 1; (B) 4 mM Perezone 2; (C) 2.9 mM Horminone 3 and (D) 2.2 mMhydroxide (TBAOH), obtained with a platinum microelectrode (0.025 cm2)indicated. Dashed lines represent the cyclic voltammograms without added

C. Frontana, I. Gonzalez / Journal of Electroanalytical Chemistry 603 (2007) 155–165 161

at the acquisition times of the ESR spectrometer, in agree-ment with the results presented above obtained by voltam-metry and double potential step chronoamperometry.

However, upon electrochemical reduction at potentialvalues more negative than peak IIc, the presence of radicalspecies was characterized for the studied compounds.These radical species were the same for both, the directreduction of the compounds at potential values more neg-ative than peak IIc, and by reduction in the presence of oneequivalent of a TBAOH solution at the potential regionmore negative than peak IIc 0 (Fig. 4). For 2-hydroxy-1,4-naphthoquinone 2, the ESR experimental spectrumobtained by reduction at these potential values is presentedin Fig. 5A.

The experimental spectrum for quinone 1 obtained at�1.7 V vs Fc+/Fc (Fig. 5A), shows the presence of fivehyperfine coupling constants (a, Table 2), which is consis-tent with coupling of the unpaired electron spin with thehydrogen atoms at positions C-3 and C-5 to C-8. This rad-ical structure also appears upon reduction of the solutionwith 1 equiv. of TBAOH added and confirms that this spe-cies is generated upon reduction of the deprotonated qui-

Fig. 5. ESR spectrum of the radical dianion of 2-hydroxy-1,4-naphthoquinondashed line (B): isotropic simulation of the experimental spectrum. The spinshown in the figure.

Table 2Hyperfine coupling constants and linewidths for the observed radical dianion

Compound a1 (G) a2 (G)

2-Hydroxy-1,4-naphthoquinone 1 2.46D (H-3) 0.54D (H-5)Perezone 2 3.9D (H-5) 0.32D (H-7)Horminone 3 1.92D (H-5) 2.47D (H-7)7a-O-methyl-Conacytone 4 7.73D 0.5D

Data obtained upon the reduction of the deprotonated quinones by addition osolution.

a Assignment for each hyperfine coupling constant with the corresponding hThe subscript indicates the multiplicity for each signal (D: Doublet; Q: Quart

b Data associated to a second radical structure observed experimentally.c This data is not certain, since the linewidth (C) has a similar value.

none compound. The hyperfine coupling constant valueswere confirmed by plotting the simulated ESR spectrum(Fig. 5B). The experimental resolution presented in thisspectrum results in an improvement compared to experi-mental data previously presented [17] and allows to identifyclearly the electrogenerated species as a radical dianionstructure, as it lacks of a sixth hyperfine coupling constant.However, the assignment of a values for the correspondinghydrogen atoms is not trivial and requires a complete cal-culation of the electronic structure of the radical dianionspecies. The employment of semiempirical PM3 calcula-tions [28–30,34], suggests that the highest spin densityresides at the hydrogen atom at the C-3 position, whichindicates that the carbonyl function located at C-1 is thefirst carbonyl group to be reduced. This result is consistentwith other results presented in the literature for the reduc-tion of a-hydroxyquinones [11]. It should be noticed thateven though the employment of semiempirical methods isnot recommended to calculate the electronic structure ofthis type of species, the calculated spin densities with thismethod share a relationship with the experimental hyper-fine coupling constants, as expected from the McConnell

e 1 at �2.1 V vs Fc+/Fc. Upper solid line (A): experimental data; lowerdensity structure evaluated from the analysis of the experimental data is

structures for the studied a-hydroxyquinonesa

a3 (G) a4 (G) a5 (G) C (G)

1.42D (H-6) 0.11D (H-7) 0.26D (H-8) 0.051.26Q (H-14) ND NA 0.11.92D (H-15) 1.86D (H-20) 0.06D (OH-7) 0.10.35D(?)c 3.01D

b 0.23Q(?)b,c 0.2/0.23b

f 1 equiv. of tetrabutylammonium hydroxide (TBAOH), from a 1 M water

ydrogen atom is indicated for each compound (see Fig. 1 for numbering).et).

Fig. 6. ESR spectrum of the radical dianion of Perezone 2 at �2.1 V vs Fc+/Fc. Upper solid line (A): experimental data; lower dashed line (B): isotropicsimulation of the experimental spectrum. The spin density structure evaluated from the analysis of the experimental data is shown in the figure.

Fig. 7. ESR spectrum of the radical dianion of Horminone 3 at �2 V vsFc+/Fc. Upper solid line (A): experimental data; lower dashed line (B):isotropic simulation of the experimental spectrum. Arrows indicate theposition of spectral lines corresponding to a second radical structuredetected in this spectrum.

162 C. Frontana, I. Gonzalez / Journal of Electroanalytical Chemistry 603 (2007) 155–165

relationship [35,36]. For the purposes of this work, thisresult is good enough to identify the spin density distribu-tion in the radical dianion.

The experimental spectrum of the radical dianion ofPerezone 2 is presented in Fig. 6A. The spectrum shows ahyperfine coupling structure compatible with the interac-tion between the unpaired electron and the hydrogen atomsin the vicinity of the quinone moiety (at C-5, C-7 and C-14). The structure is comprised of three hyperfine couplingconstants, the first being related to the doublet main signal(a = 3.9 G), the second gives rise to a quartet splitting(a = 1.26 G) and the third doublets again each spectral sig-nal with a lower coupling constant (a = 0.32 G). Theseinteractions arise from hyperfine coupling with the hydro-gen atoms at C-5, those present in the methyl moiety atC-14 and the sole hydrogen atom at C-7, respectively(Fig. 1). Therefore, the hyperfine coupling constants canbe assigned directly in terms of the experimental results,performing the appropriate spectrum simulation(Fig. 6B). In a similar way as also occurred for the radicaldianion of quinone 1, the high spin density located at C-5accounts for the reduction of the carbonyl group located atC-1. However, the experimental linewidth (C) is higher(0.1 G), than the one observed by the radical dianion struc-ture of 2-hydroxy-1,4-naphthoquinone (0.05 G, Table 2).These results indicate that the variations in linewidthscan be generated by a chemical process occurring in solu-tion, such as the homogeneous electron transfer occurringbetween the radical dianion species and the deprotonatedquinone [37]. This is in agreement with experimental differ-ences for the electrochemical behavior of the deprotonatedquinones DEpIIc 0–IIa0 (1: 0.08 V; 2: 0.46 V), which are indic-ative of differences in the electron transfer kinetics for bothcompounds.

For Horminone 3, the ESR spectrum shows an experi-mental coupling pattern presented in Fig. 7A.

The hyperfine coupling structure shows the presence offive hyperfine coupling constant values upon interactionbetween the unpaired electron spin and the hydrogen

atoms (Table 2): 2.68 G (doublet); 2.47 G (doublet),1.92 G (doublet), 1.86 G (quartet) and 0.06 G (doublet),with these values the hyperfine structure was simulated(Fig. 7B). The lack of a multiplet signal arising from inter-action with the 6 equivalent hydrogen atoms at positionsC-16 and C-17 shows that the spin density is not locatedin an important amount near that position. Therefore,and differing from the case of the radical dianion structureof Perezone 2, the assignment of the spin coupling structurecannot be performed by directly comparing the chemicalstructure of the radical dianion with the spectral patternsabove referred. Therefore, it was necessary again to evalu-ate the electronic structure of the corresponding radicaldianion employing the semiempirical method PM3 [28–30], leading to the spin density trace shown in Fig. 8.

The calculated spin density pattern (Fig. 8) shows a highspin density at position C-8, similarly to the pattern shownby Perezone 2, and a low spin density at position C-13.This last result is consistent with the experimental lack ofthe multiplet signal arising from spin–nucleus interactionswith the hydrogen atoms located at C-16 and C-17. Both

Fig. 8. Calculated spin density isosurface for the radical dianion ofHorminone 3, employing the PM3 semiempirical method. Isosurface isplotted at a constant value of 0.005.

Fig. 9. ESR spectrum of the radical dianion of 7a-O-methyl-Conacytone4 at �2 V vs Fc+/Fc. (A) Experimental spectrum; (B) central linessimulation; (C) side lines simulation; (D) superposition of spectra B and C.

C. Frontana, I. Gonzalez / Journal of Electroanalytical Chemistry 603 (2007) 155–165 163

results suggest that the first carbonyl moiety being reducedis the one near the deprotonated hydroxyl function, as alsooccurred for the previous hydroxyquinones studied (1 and2). However, the spin density pattern shows that the methylgroup located at position C-20 has a high spin density. Thisgroup can be therefore responsible of the experimental cou-pling constant of 1.86 G. This result suggests that the spindensity in this radical dianion structure is highly located atthe ring adjacent to the quinone moiety. Another interest-ing feature is that the distance between the hydroxyl func-tion located at position C-7 and the Oxygen atom at C-14 is2.4 A, which is lower that the sum of the Van der Waalsradii of the involved atoms (1.2–1.45 A for H and 1.5 Afor O [38]), and suggests the formation of an intramolecu-lar hydrogen bond [39,40]. Due to the distance betweeneach participating atoms, the interaction with this hydro-gen atom can be related to the lowest hyperfine couplingconstant observed experimentally (0.06 G, Table 2). Themagnitude of the experimental hyperfine coupling constantfor this hydrogen bond is lower than the observed forhydrogen atoms participating in such interactions for theradical anions electrogenerated from b-hydroxynaphtho-quinones (of about 0.5 G [40]), which indicates the lowenergy of this interaction. Based on the spin density isosur-face (Fig. 8), the remaining hyperfine coupling constantscan be associated to the interactions between the unpairedelectron spin with the hydrogen atoms at the positions C-5,C-7 and C-15.

Although the observed spectrum for the radical dianionof Horminone 3 appears to be associated with the presenceof a single radical species, some experimental signals arenot related to the described hyperfine structure (indicatedwith the arrows in Fig. 7A). However, upon variation ofthe irradiated microwave power intensity, it was not possi-ble to decouple these lines with the remaining signals.These results suggest that the electrogenerated radical dian-ion structure evolves slowly into another radical structure.The formation of another species can account for theexperimental DEp differences associated to the peaks IIc–

IIa (0.17 V) and IIc 0–IIa 0 (0.40 V) presented previously.The understanding of the chemical reaction occurring atthis level of potential is currently under study, although aproposal can be made by analyzing the reactivity of theradical dianion of 7a-O-methyl-Conacytone, as presentedbelow.

The experimental spectrum for the radical dianion elec-trogenerated upon reduction of 7a-O-methyl-Conacytoneat peak IIc is shown in Fig. 9A.

The experimental radical dianion structure of 7a-O-methyl-Conacytone shows a lower intensity than the pre-ceding spectral structures as a noisy signal is obtained(Fig. 9A). Moreover, the simulated radical structure isnot consistent with the presence of a single radical species,as the experimental spectrum shows the presence of tworadical species. The first one (Fig. 9B) is associated to thecentral signals of the ESR spectrum, while the side linesarise from the hyperfine coupling occurring with a secondradical structure (Fig. 9C), being the experimental spec-trum the superposition of both structures with relativeintensities 1:8 (Fig. 9D). Even though some values ofhyperfine coupling constants were obtained for each spe-cies, this information does not provide a clear identificationof the structure of the radical species present in the mix-ture, as only two main hyperfine coupling constants areobtained for each radical structure (Table 2). This suggeststhat the electrogenerated radical dianion for 7a-O-methyl-Conacytone undergoes a reaction in solution which gener-ates another radical species as a result, in a similar fashionas the behavior presented by the radical dianion of Hormi-none (Fig. 7). It should be noticed that the electrochemicalbehavior at the second reduction signal for both com-pounds, under basic conditions, is very similar (DEpIIc 0�IIa0:3: 0.40 V; 4: 0.44 V). Therefore and in order to analyze howthe process would be related to the electronic structure ofthe corresponding radical dianion, the calculated spin

Fig. 10. Calculated spin density isosurface for the radical dianion of 7a-O-methyl-Conacytone 4, employing the PM3 semiempirical method. Isosur-face is plotted at a constant value of 0.005.

164 C. Frontana, I. Gonzalez / Journal of Electroanalytical Chemistry 603 (2007) 155–165

density isosurface employing the PM3 method, proved tobe useful to gain some insights in this direction (Fig. 10).

The analysis of the spin density isosurface presented inFig. 10, suggests that the spin density of the correspondingradical dianion structure of 7a-O-methyl-Conacytone islocated in a high proportion near the C-7 and the corre-sponding O-7 position. As the main structure of this com-pound (abietanic skeleton) is the same as for Horminone 3,the same numbering applies and is presented in Fig. 1. Thissuggests that this group can suffer an elimination reactionto generate a stable radical species. This reaction was pro-ven to occur during the electrochemical analysis of themethylated derivative of 7a-O-methyl-Conacytone (7a,12, 20-O-trimethyl-Conacytone [41]). The experimentsreported in this reference demonstrated that the apparentfirst monoelectronic transfer process for such derivative,occurs actually as a two electron uptake, where a coupledchemical reaction occurs. This coupled reaction was consis-tent with the elimination of the methoxy group at positionC-7, in a deoxygenation reaction during the continuousreduction of the methylated derivative. Therefore, it canbe suggested that the generation of a stable radical speciesin this type of quinone moiety, is followed by a chemicalreaction by which another radical structure is generated(i.e. the methoxy radical upon elimination of the group atC-7). This reaction can also occur for the Horminone rad-ical anion to generate a hydroxyl type radical. However, inthe latter case, the elimination of the corresponding hydro-xyl radical would be limited by the presence of the stabiliz-ing intramolecular hydrogen bond referred above. Thiswould account for the low proportion of the second radicalsignal observed for the Horminone radical dianion spectra(Fig. 7A) compared to the spectrum for the same specieselectrogenerated from 7a-O-methyl-Conacytone. It is inter-esting to note that both Horminone 3 and 7a-O-methyl-Conacytone 4 share a common carbon skeleton (Fig. 1),as they both are abietane compounds. Therefore, and fromthe experimental results, it can be proposed that this struc-

ture is related to the particular behavior of both. Sincethere are many natural occurring abietanic quinone struc-tures [5], these results prove to be a starting point toaccount for the behavior of this type of compounds.

4. Conclusions

The electrochemical study of a series of a-hydroxyqui-nones (2-hydroxy-1,4-naphthoquinone, Perezone, Hormi-none and 7a-O-methyl-Conacytone), revealed that thereduction mechanism occurring at the first voltammetricreduction signal, is determined by the presence of selfproto-nation reactions. Furthermore, the detailed voltammetricanalysis for this signal reveals that there is a dependenceof the structure of the chemical compound bearing the a-hydroxy-moiety in the reduction mechanism. These differ-ences are determined by the stability and basicity of thefirst electrogenerated radical anion for each compound,as some changes occur, evidenced during the analysis ofthe DEp values for the peak potentials Ic–Ia, and also forthe slopes of the EpIc vs log v plots. These differences sug-gest variations between the rate determining step on thereduction pathways involved during the selfprotonationprocess. Since the first electrogenerated radical anion dis-proportionates readily in solution, it was not possible toanalyze the properties of this radical species by spectro-electrochemical – ESR experiments. However, since duringthe selfprotonation process an important amount of depro-tonated quinone is produced, the spectroelectrochemicalstudy at a potential region at the second reduction processfor this type of compounds showed well resolved ESR spec-tra, corresponding to the radical dianion structure origi-nated from the reduction of the deprotonated species.The analysis of the obtained ESR spectra showed thatthe abietanic skeleton of natural quinones Horminoneand 7a-O-methyl-Conacytone induces changes in the spinstructure which prove valuable in determining the stabilityand reactivity of the electrogenerated radical dianion. Forboth compounds, the spectral structures showed the pres-ence of mixtures of radical species, differently from thebehavior presented by the radical dianions of diterpenicnatural quinone Perezone and arene derivative 2-hydroxy-1,4-naphthoquinone. This result indicates thatthe abietanic structure alters the electron transfer behaviorof natural a-hydroxyquinones by destabilizing the radicalanionic species electrogenerated.

Acknowledgements

The authors thank Dr. B.A. Frontana-Uribe and J.M.Aceves (UNAM) for supplying the quinone samples stud-ied in this work and to Dr. A. Solano-Peralta (USAI-UNAM) for his help in the ESR spectra acquisition. C.Frontana thanks CONACyT-Mexico for economical sup-port for his Ph.D. studies.

C. Frontana, I. Gonzalez / Journal of Electroanalytical Chemistry 603 (2007) 155–165 165

References

[1] J.J. Inbaraj, R. Ganghidasan, R. Murugesen, Free Rad. Biol. Med. 26(1999) 1072.

[2] N.V. Frigaard, S. Tokita, K. Matsuura, Biochem. Biophys. Acta 1413(1999) 108.

[3] M.O.F. Goulart, C.L. Zani, J. Tonholo, L.R. Freitas, F.C. de Abreu,A.B. Oliveira, D.S. Raslan, S. Starling, E. Chiari, Bioorg. Med.Chem. Lett. 7 (1997) 2043.

[4] P. Joseph-Nathan, Rev. Soc. Quim. Mex. 18 (1974) 226.[5] B. Esquivel, J. Calderon, A. Sanchez, T.P. Ramamoorthy, Rev.

Latinoam. Quim. 24 (1996) 44.[6] F.J. Gonzalez, J.M. Aceves, R. Miranda, I. Gonzalez, J. Electroanal.

Chem. 310 (1991) 293.[7] P.A.L. Ferraz, F.C. Abreu, A.V. Pinto, V. Glezer, J. Tonholo,

M.O.F. Goulart, J. Electroanal. Chem. 507 (2001) 275.[8] J.Q. Chambers, in: S. Patai, Z. Rappoport (Eds.), The Chemistry of

Quinonoid Compounds, vol. 1, John Wiley and Sons Ltd., New York,1988 (Chapter 12).

[9] C. Amatore, G. Capobianco, G. Farnia, G. Sandona, J.M. Saveant,M.G. Severin, E. Vianello, J. Am. Chem. Soc. 107 (1985) 1815.

[10] J.L. Ortiz, J. Delgado, A. Baeza, I. Gonzalez, R. Sanabria, R.Miranda, J. Electroanal. Chem. 411 (1996) 103.

[11] M. Aguilar-Martinez, J.A. Bautista-Martinez, N. Macias-Ruvalcaba,I. Gonzalez, E. Tovar, T. Marin del Alizal, O. Collera, G. Cuevas,J. Org. Chem. 66 (2001) 8349.

[12] A. Brunmark, E. Cadenas, Free Rad. Biol. Med. 7 (1989) 435.[13] O.S. Ksenzhek, S.A. Petrova, M.V. Kolodyazhny, S.V. Oleinik,

Bioelectrochem. Bioenerg. 4 (1977) 335.[14] F.J. Gonzalez, Electroanalysis 10 (1998) 638.[15] M.E. Bodini, P.E. Bravo, V. Arancibia, Polyhedron 13 (1994) 497.[16] M.E. Bodini, V. Arancibia, Polyhedron 8 (1989) 1407.[17] C. Frontana, B.A. Frontana-Uribe, I. Gonzalez, J. Electroanal.

Chem. 573 (2004) 307.[18] T.H. Lowry, K.S. Richardson, Mechanism and Theory in Organic

Electrochemistry, Harpenter & Row Publishers, New York, 1987(Chapter 2).

[19] E.M. Kosower, An Introduction to Physical Organic Chemistry, JohnWiley and Sons, New York, 1986 (Part 2).

[20] C.-Y. Li, J. Jenq, Electrochim. Acta 36 (1991) 269.[21] P.W. Crawford, J. Gross, K. Lawson, C.C. Cheng, Q. Dong, D.F.

Liu, Y.L. Luo, B.G. Szczepankiewicz, C.H. Heathcock, J. Electro-chem. Soc. 144 (1997) 3710.

[22] D. Jezziorek, T. Ossowski, A. Liwo, D. Dyl, M. Nowacka, W.Wo’znicki, J. Chem. Soc., Perkin Trans. 2 (1997) 229.

[23] H. Weiss, T. Friedrich, G. Hofhaus, D. Preis, Eur. J. Biochem. 197(1991) 536.

[24] A. Ashnagar, J.M. Bruce, P.L. Dutton, R.C. Prince, Biochim.Biophys. Acta 801 (1984) 351.

[25] P.W. Crawford, E. Carlos, J.C. Ellegood, C.C. Cheng, Q. Dong, D.F.Liu, Y.L. Luo, Electrochim. Acta 41 (1996) 2399.

[26] C. Amatore, J.M. Saveant, J. Electroanal. Chem. 107 (1980) 353.[27] G. Gritzner, J. Kuta, Pure Appl. Chem. 4 (1984) 462.[28] J.P.P. Stewart, J. Comput. Chem. 10 (1989) 209, 221.[29] J.P.P. Stewart, J. Comput. Chem. 11 (1990) 543.[30] J.P.P. Stewart, J. Comput. Chem. 12 (1990) 320.[31] D.H. Evans, Chem. Rev. 90 (1990) 739.[32] C. Amatore, J.M. Saveant, J. Electroanal. Chem. 85 (1977) 27.[33] M.O.F. Goulart, M.F. Nadja, N.M.F. Lima, A.E.G. Sant’Ana,

P.A.L. Ferraz, J.C.M. Cavalcanti, P. Falkowski, T. Ossowski, A.Liwo, J. Electroanal. Chem. 566 (2004) 25.

[34] C. Frontana, Ph.D. Thesis. Universidad Autonoma Metropolitana-Iztapalapa, 2006.

[35] H.M. McConnell, J. Chem. Phys. 24 (1956) 756.[36] H.M. McConnell, D.B. Chesnut, J. Chem. Phys. 28 (1958) 107.[37] J.E. Wertz, J.R. Bolton, Electron Spin Resonance, first ed., Chapman

& Hall, New York, 1972 (Chapter 9).[38] J.E. Huheey, E.A. Keiter, R.L. Keiter, Inorganic Chemistry, Princi-

ples of Structure and Reactivity, fourth ed., Mexico, 1993.[39] J. Gendell, W.R. Miller, G.K. Fraenkel, J. Am. Chem. Soc. 91 (1969)

4369.[40] C. Frontana, I. Gonzalez, J. Braz. Chem. Soc. 16 (2005) 299.[41] C. Frontana, B.A. Frontana-Uribe, I. Gonzalez, Electrochim. Acta

48 (2003) 3593.