Electrochimica Acta 53 (2008) 5968–5976

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Electrochimica Acta

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Understanding specific effects on the standard potential shifts ofelectrogenerated species in 1-butyl-3-methylimidazolium ionic liquids

∗

ona, S

thatargeose

kis(dietrathnediaoniumnion

ons an6

− catener

Hugo Cruz, Iluminada Gallardo ∗, Gonzalo GuiradoDepartament de Quımica, Universitat Autonoma de Barcelona, 08193-Bellaterra, Barcel

a r t i c l e i n f o

Article history:Received 5 March 2008Accepted 20 March 2008Available online 1 April 2008

Keywords:Ionic liquidsElectrochemistryIon-pairing effectsSolvation effectsRedox probes

a b s t r a c t

It has been establishedmedia selected for a ldation. For such a purp1,3-dinitrobenzene, tetraor oxidation (ferrocene, ttetramethyl-para-phenylecations (quaternary ammnating inorganic anion (aimidazolium ions and aniferences between BF4

−/PFfollowed by these electrog

1. Introduction

Current green chemistry research has focused on the use ofenvironmentally friendly substitutes for organic solvents [1]. Theuse of ionic liquids (ILs) has been recently very popular in severalimportant fields, such as organic–inorganic synthetic procedures,polymerization [2–3], catalysis [4] and gas separation [5]. The factthat ILs are in a liquid state over a wide range of temperatures(<100 ◦C), which implies lower viscosity, is a major advantage overthe traditional molten salts used previously [6]. From an electro-chemical point of view, since an ILs is a liquid consisting of onlyions, a high ion content is guaranteed, which obviously assuresmoderate-high conductivities [7]. In addition, ILs possess severalother attractive chemical properties, such as non-flammability,thermal stability and non-volatility [8]. Finally, their electrochemi-cal stability (a wide electrochemical window, from −2.20 to 1.70 V)[9–10] makes them a potentially attractive “green” alternative toreplace classical electrolyte solutions (obtained by dissolution ofsalts in molecular solvents) in a large number of electrochemicalapplications [11].

Room-temperature ionic liquids (RTILs) are ILs compounds com-posed entirely of ions that exist in liquid state around 298 K.

∗ Corresponding authors. Tel.: +34 581 48 82; fax: +34 93 581 29 20.E-mail addresses: Iluminada.Gallardo@uab.es (I. Gallardo),

Gonzalo.Guirado@uab.es (G. Guirado).

0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.electacta.2008.03.062

pain

the dependence of the E◦ values in function of the electrochemicalamount of reversible redox probes in reduction and also in oxi-several electroactive substances either in reduction (4-nitrotoluene,methylamino)ethylene, 1,3,5-trinitrobenzene, and 2,4,6-trinitroanisole)iofulvalene, tris-4-bromophenylamine, tris-4-tolylamine, and N,N,N′,N′-mine) have been studied in aprotic RTILs based on unsymmetrical organic

cations, such as 1-butyl-3-methyl imidazolium) and a weakly coordi-s with low Lewis basicities, e.g., BF4, PF6). Ion-pairing effects betweend dianions for the electrochemically generated species, the solvation dif-ions and dications as well as some different reaction mechanism pathwaysated species in function of the solvent have also been carefully examined.

© 2008 Elsevier Ltd. All rights reserved.

Particularly interesting for electrochemical investigations is theuse of aprotic RTILs based on unsymmetrical organic cations (qua-ternary ammonium cations, such as imidazolium, pyridinium,ammonium, phosphonium. . .) and a weakly coordinating inorganicanion (anions with low Lewis basicities, e.g., BF4

−, PF6−,. . .) [12].

These RTILs are a credible alternative reaction media to organic

volatile solvents, in which several important chemical and elec-trochemical processes are performed. Thus, a rapid increase ininvestigations of 1-butyl-3-methylimidazolium tetrafluoroborateor hexafluorophosphate ([BMIM]BF4 and [BMIM]PF6) type RTILs asthe most effective and promising of substitutes for classical solventsis underway (Scheme 1) [13,14].

In comparison to other areas where some of the more importantreactions have been carried out and investigated in [BMIM]BF4 and[BMIM]PF6 [2–5] in electrochemistry fundamental studies [15–22]based on the study of the determination of the diffusion coefficients(D) [8,15–22] the heterogeneous electron transfer constant (kap

s )[19–22] the electrochemical potential window of the melt of theseionic liquids [8–10] and a few electrosynthetic processes [12–14]have been reported. The fact that RTILs provide a reaction environ-ment that is completely different from that offered by conventionalsolvents, a modification of the mechanism or even a change to theoutcome of a reaction is possible, meaning such fundamentallymechanistic studies are mandatory prior to more ambitious goals.In all the E◦ investigations, when changing the RTILs, the choice ofthe reference electrode remains a problem. The large uncertaintiesin the determination of the standard potentials even for well-

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H. Cruz et al. / Electrochim

Scheme 1. Structures of [BMIM]BF4 and [BMIM]PF6.

known redox probes, such as ferrocene or nitrobenzenes, probablydue to the use of different types of reference electrodes makesnot possible to accurate determine at which potential the electro-chemical process would take place [19–22]. Although, this point hasbeen addressed in the first papers related to the electrochemistryin ionic liquid there is still some controversy in determining themost appropriate system for referring the electrochemical poten-tial of the investigated substance. Hence, redox potentials havebeen mainly measured using either a reference electrode that isrelative quoted to an internal reference compound (e.g., ferroceneor cobaltocene) or a pseudo-reference electrode which is simply aplatinum or silver wire immersed in the solution [15–22]. The sec-ond one has the advantage that there can be no contamination ofthe electroactive substance, but the disadvantage is that the refer-ence potential is unknown, as it is dependent on the composition

of the electroactive solution. Therefore, redox potentials measuredusing a pseudo-reference electrode are also mainly be quoted rela-tive to an internal redox probe, which is obviously a problem whensynthetic electrochemical processes are the greatest application ofRTIL as a Green Chemistry solvent.

Thus, the aim of this paper is to understand the specific effectson the standard potential shifts and/or the changing shapes of thecyclic voltammogram of electrogenerated species in RTILs. For sucha purpose it is mandatory to establish firstly a suitable easy repro-ducible electrochemical set-up capable of providing concrete standardpotential values for later analyzing different electroactive substanceseither in reduction (such as 4-nitrotoluene, 1, 1,3-dinitrobenzene,2, tetrakis(dimethylamino)ethylene, 3, 1,3,5-trinitrobenzene, 4,and 2,4,6-trinitroanisole, 5) or oxidation (ferrocene, 6, tetrathio-fulvalene, 7, tris-4-tolylamine, 8, tris-4-bromophenylamine, 9and N,N,N′,N′-tetramethyl-para-phenylenediamine, 10) have beenstudied in two different media, aprotic solvent and RTILs. TheseRTILs are comprised of a 1-butyl-3-methylimidazolium cation andtwo different anions BF4

−, PF6−. The ion-pairing effects between

imidazolium ions and anion radicals and dianions for the elec-trochemically generated species as well as the ion-paring effects

Scheme 2. Chart of structures of

ta 53 (2008) 5968–5976 5969

between BF4−/PF6

− cation radicals and dications are also carefullyexamined (Scheme 2).

2. Experimental

2.1. Materials and methods

2.1.1. A. ChemicalsAnhydrous acetonitrile (ACN) and N,N-dimethylformamide

(DMF) stored in an inert atmosphere and molecular sieves werepurchased from Across. nBu4NBF4 (Fluka, puriss.) were used with-out further purification. All the commercially available reactantswere of high purity and were used without purification. RTILs([BMIM]BF4 and [BMIM]PF6) were firstly purchased from Solvionicwhich guarantee that the amount of water present is always lessthan 500 ppm. Later both RTILs ([BMIM]BF4 and [BMIM]PF6) wereprepared from BMIMCl (Fluka) aqueous solutions according to pre-viously published procedures [23,24]. The product was purified byrepeated washing with water and purified by column chromatog-raphy over silica under continuous nitrogen-flow (Scheme 3). Theliquid ionic was conveniently dry until the theoretically expectedelectrochemical response is obtained for a water sensible redoxprobes, such as 2,4-dinitroaniline, when either the commercial orthe prepared RTILs is used.

2.1.2. B. Cyclic voltammetry experimentsAn electrochemical conical cell equipped with a methanol

jacket, which makes it possible to fix the temperature at 20 ◦Cby means of a thermostat, was used for the set-up of the three-electrode system. For cyclic voltammetry experiments, the workingelectrode was, in all cases, a glassy carbon disk of a diameter of0.5 mm. It was polished using a 1 �m diamond paste. The counterelectrode was a Pt disk of a diameter of 1 mm. All of the poten-tials are reported vs. an aqueous saturated calomel electrode (SCE)isolated from the working electrode compartment by a salt bridge.The salt solution of the reference calomel electrode is separatedfrom the electrochemical solution by a salt-bridge ended with afrit, which is made of a ceramic material, allowing ionic conductionbetween the two solutions and avoiding appreciable contamina-tion. Ideally, the electrolyte solution present in the bridge (organicsolvent +0.1 M of supporting electrolyte or pure RTIL in the ionicliquid experiments) is the same as that used for the electrochemi-cal solution so as to minimize junction potentials. To final checkingof the system we replace the SCE-salt-bridge frit reference systemeither for a silver wire or for ferrocene observing the expected shiftof the E◦ values. It is important to remark that no differences weredetected in the shape of the cyclic voltammograms. Thus, since theabove-described electrochemical set-up do not involve the used of

the studied redox probes.

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ILs ([B

media [25–26]. In this sense the comparison of those E◦ values ina DMF containing bulky organic counter-cations with E◦ values inRTILs would make it possible to evaluate solvation in these ionicliquids. Finally, in order to generalize the study to other reduc-tive redox substances, tetrakis(dimethylamino)ethylene, 3, has alsobeen chosen.

The electrochemical behavior of 4-nitrotoluene 1, which waspreviously described in aprotic solvents [25] is similar to othernitroaromatic compounds. It is reported that 1 shows two wavesin a cathodic scan. The first of these (at −1.222 V) is a one-electronreversible electron transfer, which leads to its anion radical. In turn,the second (at −2.240 V vs. SCE) is a chemically irreversible multi-electron transfer wave. It is well known for different nitroaromaticcompounds that nitroso or related derivatives, such as anilines, areproduced at the same or more negative potentials than the reduc-tion value of this second wave.

The electrochemical behavior of 1, at the first wave level, inDMF and in both RTILs is fairly similar (Fig. 1). A well-defined one-

5970 H. Cruz et al. / Electrochim

Scheme 3. Synthesis of RT

external probes and allow to determine accurately the redox poten-tial in which the electrosynthetic processes should be performed.

Solutions were prepared using acetonitrile (ACN) or N,N-dimethylformamide (DMF) as a solvent and they were purged withnitrogen before the measurements, and nitrogen was allowed toflow under the solution during the measurements. The concentra-tion of the amines was ∼10−3 M; while the supporting electrolyteconcentration was 0.1 M of tetrabutylammonium tetrafluoroborate.

Because of the low ionic conductivity of [BMIM]BF4 and[BMIM]PF6 (7 × 10−3 and 1.8 × 10−3 S cm−1, respectively) [7]induced by the high viscosity and low diffusion coefficients; thecyclic voltammetry experiments were performed at ca. 5 mM elec-troactive substance concentration. The electrochemical cell usedis the same as the above-described for the case of aprotic polarsolvents. Experiments were performed at 20.0 ± 0.5 ◦C under N2atmosphere. At the end of the process the RTILs were suitablyrecovered and recycled to start a new set of experiments bywater/dichloromethane washings.

Cyclic voltammetry experiments of the redox probes in DMF andin RTILs were performed at different scan rates from 0.1 to 1 V/s. Atthis scan rate range the neither the peak potential value nor the�Ep/2 was scan rate dependent, so it is fairly to think that all theredox systems are under thermodynamic control. Thus, the E◦ valueis calculated as the half-sum between forward and reversed peakpotentials from a single voltammogram. The E◦ values shown inall the tables are the average of three independent sets of exper-iments. The error associated to these standard potential values isless than 5 mV. The ohmic drop can be one of the main sourcesof error where RTILs are used as a solvent since there are a moreresistive media than apropic polar solvents with 0.1 M concentra-tion of supporting electrolyte. In order to minimize the ohmic dropeffects the electrode used as a working electron is a small elec-trode, 0.5 mm diameter glassy carbon electrode and the scan ratesinvestigated were in the range 0.1–1 V/s. Positive feed-back iR com-pensation was used throughout. Typically compensated resistances

were 0.5–1.0 k�, depending on electrode position.

Due to the low conductivity and larger viscosity of RTILs in orderto facilitate the direct comparison between the cyclic voltammo-grams of redox probes in the different media, the current valuesobtained were first normalized by the concentration and the scanrate (Ip/cv1/2). The compounds studied in the current work are well-known fully reversible redox couple with one- or two-electronreversible waves allowing the quick comparison in the three sol-vents studies by determining their diffusion coefficients. Later,those values were also normalized considering the relationshipbetween their diffusion coefficients, which are about two ordersof magnitude smaller than in aprotic solvents. The relationshipbetween the three solvents can be expressed by a total normaliza-tion coefficient. This coefficient can be expressed as a ratio betweenDMF/[BMIM]BF4 and DMF/[BMIM]PF6 being 8.10 and 12.14, respec-tively.

2.1.3. C. Graphical representationsGraphical Representations are performed using MOPAC com-

putations directly on the model in the Chem3D model window,which provides a graphical user interface (GUI). It is able to view the

ta 53 (2008) 5968–5976

MIM]BF4 and [BMIM]PF6).

model change appearance, which reflects the computed result asa consequence of the geometry optimization (minimization of thegeometry energy) and single-point computations. Hence, MOPAC isused as a molecular computation application using semi-empiricalmethods (AM1) and unrestricted open shell functions.

3. Results and discussion

3.1. Reactivity of anion radicals and dianions in RTILs

A large family of nitroaromatic compounds (1, 2, 4, 5) has beenselected as redox probes to evaluate the dependence on the E◦

(standard potential) values of those molecules. It is well knownthat the reductive electrochemical behavior of nitro-, dinitro- andtrinitro-aromatic compounds is highly sensitive to ion-pairingeffects depending on the solvent and the cations present in the

electron reversible reduction wave is observed, thus the generalnature of the electrogenerated anion radical is not affected in thosemedia (Table 1). Since the first reduction wave is not irreversible at

Fig. 1. Cyclic voltammograms of 1 showing the reduction 5 mM in DMF ( ),[BMIM]BF4 (—) and [BMIM]PF6 (- -) on a glassy carbon disk (0.5 mm diameter), scanrate of 1.0 V s−1.

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dazolium moiety, is the most sensitive to those solvation effects.

H. Cruz et al. / Electrochim

Table 1Standard potential, anodic and cathodic peak potential (in V vs. SCE) and �Ep for 1in different solvents at 20 ◦C

Solvent E◦ (V) Epc (V) Epa (V) �Ep (mV)a

DMFb −1.222 −1.267 −1.176 91[BMIM]BF4 −1.103 −1.142 −1.064 78[BMIM]PF6 −1.303 −1.367 −1.239 128

a �Ep = |Epc − Epa| in mV.b The DMF solution contains 0.1 M of n-tetrabutylammonium tetrafluoroborate

(TBABF4).

this potential as well as there is not appearance of oxidation wavecorresponding to nitroso anion radical, it is fairly to think that thereis no protonation effect [25]. It has been also confirmed, as was pre-viously pointed out by other authors, that the peak current valuesare much smaller in RTILs than in aprotic organic solvents [15].Since we are not interested in either the nature of those variationsor the determination of diffusion coefficient values of the electroac-tive substrates, the cyclic voltammograms presented in the presentwork show a normalized current axis scale (Ip/cv1/2, see Section 2).

The cyclic voltammetry experiment reveals some importantdifferences concerning the thermodynamic value, E◦. The nitro-toluene anion radical is 119 mV positively shifted in [BMIM]BF4while [BMIM]PF6 is 81 mV negatively shifted. Previously, the smalldifferences ca. 50 mV in the E◦ values compared to the “reference”value obtained in DMF have also been indicated for nitroaromaticradical anions; they have been attributed to differences in the liquidunion potentials between the reference and the ionic liquid and theDMF [26]. Since we have carefully addressed this point using a saltbridge, which contains pure RTILs in which the E◦s differences are

higher, it may be reasonable to consider other alternatives. The ionicliquid is a pure ion solvent, the concentration of counter-cations isconsiderably higher in RTIL than in the DMF solution where “only”a 0.1 M concentration of tetrabutylammonium cation is present.Thus, an ion-pairing effect appears to be a plausible explanation forthose differences but not the only one. Previous studies performedby Saveant in related systems [27] showed the shift of the poten-tial to less negative values, ca. 60 mV at 25 ◦C per 10-fold excess ofion concentration. This effect is exclusively attributed to the excessof alkali cations since strong ion-pairs are formed, whereas in thiscase the anion acts as a mere spectator.

The BMIM cations are able to form more effective ion-pairs thanthe tetrabutylammonium cations [26]. This may be due to the factthat the imidazolium ions are mainly planar, so they can get closerto the aromatic system than tetrabutylammonium cations, whichis a more bulky ion. Moreover, in the case of BMIM ions it may bepossible to form face-to-face ion-pairs between the �-system ofthe nitroaromatic anion radical and imidazolium cation which willlead to stronger interactions [26]. This could be the reason why theE◦ of the nitrotoluene anion radical is 119 mV positively shifted in

Scheme 4. Graphical representation of the ion-pairs forme

ta 53 (2008) 5968–5976 5971

Fig. 2. Cyclic voltammograms of 2 showing the reduction 5 mM in DMF ( ),[BMIM]BF4 (—) and [BMIM]PF6 (- -) on a glassy carbon disk (0.5 mm diameter), scanrate of 1.0 Vs−1.

[BMIM]BF4. However, it does not explain why there is a 200 mVshift between both RTIL taking into account the fact that the cationconcentrations are exactly the same.

At this point one may reasonably ask why a counter-anionappears to affect the ion-pair formation despite both inorganicanions being weakly coordinating. Nuclear magnetic resonanceexperiments revealed a chemical shift of the aromatic hydrogenatoms in [BMIM]BF4 and [BMIM]PF6 in nine different deuteratedsolvents. This was exclusively related with solvation effects [28].For both salts, the interaction involving the counter-anion and theH located in position 2, between the two nitrogen atoms of the imi-

Hence, it is possible that the interaction of the imidazolium cationwith the relatively small and basic anion, BF4

−, is more intimatethan the interaction with the fairly large, polarizable and less basicPF6

− anion. On the basis of the above-exposed arguments, it is fairto assume that, in the presence of BF4

−, the distance between theimidazolium and benzene rings to form cation–anion radical face-to-face complex by a planar approach should be smaller than inthe presence of PF6

− (Scheme 4). This diminishment of the ion-pair stabilization energy is reflected in an E◦ shift of 200 mV vs. thevalue in [BMIM]BF4. The fact that the E◦ value is negatively shiftedin [BMIM]PF6 in 81 mV vs. the DMF value also means that evena bulky tetraalkylammonium cation has “tighter” interaction withthe nitroaromatic anion radical.

The electrochemical behavior of 1,3-dinitrobenzene, 2, showstwo successive one-electron reversible reduction waves, at −0.905and −1.328 V, respectively, in a cathodic scan [25]. Moreover,a third chemically irreversible multi-electron transfer wave isobserved at −2.450 V). Nitroso or related compounds, such asanilines, are produced at the same or more negative potentialsthan the reduction value of this wave. The same general trend is

d by 1•− in [BMIM]BF4 (left) and [BMIM]PF6 (right).

5972 H. Cruz et al. / Electrochimica Acta 53 (2008) 5968–5976

Table 2Standard potential, anodic and cathodic peak potential (in V vs. SCE) and �Ep (mV) for 2 in different solvents at 20 ◦C

Solvent E◦1 E

◦2 Epc,1 Epc,2 Epa,1 Epa,2 �Ep,1 �Ep,2

DMFa −0.905 −1.328 −0.944 −1.385 −0.866 −1.271 78 114[BMIM]BF4 −0.815 −1.043 −0.845 −1.077 −0.785 −1.008 60 69[BMIM]PF6 −0.969 −1.175 −1.009 −1.215 −0.928 −1.135 81 80

a The DMF solution contains 0.1 M of n-tetrabutylammonium tetrafluoroborate (TBABF4).

MIM]

Scheme 5. Graphical representation of the ion-pairs formed by 22− in [B

observed when the electrochemical study of 2 is performed in RTILs(Fig. 2).

Focusing on the standard potential value (E◦) of the first reduc-

tion waves, a solvent dependence is also observed. As in the caseof the previously studied compound, 1, there is a shift of the firstvoltammetric peak of 90 mV positive and more than 64 mV negativefrom their corresponding values in DMF (Table 2).

This effect on the shift of the E◦ values dramatically increasesat the second reduction wave level. Note that in this case, the E◦

of the second reduction wave is positively shifted in both cases([BMIM]BF4 and [BMIM]PF6) from the “reference” value obtainedin DMF. As a consequence the E◦ values corresponding to the anionradical (E

◦1) and the dianion (E

◦2) are more than 228 and 206 mV

closer, respectively. The fact that the species formed is a stable dian-ion favors the formation of stronger ion-pairs which enhances theshift effect. For electrostatic reasons, the presence of [BMIM] ionsin higher concentrations when RTIL are used as a solvent comparedto the tetraalkylammonium ions in DMF makes it possible that theE◦ could be less negative, even [BMIM]PF6, where the formation ofion-pairs are less favorable due to the size and polarizability of thePF6

− anions (Scheme 5, top) [25].In order to confirm the dependence of the E◦ values with the

ion-pairing effects, we decided to try to minimize those effects byavoiding the use of steric hindrance electroactive substances, where

Table 3Standard potential, anodic and cathodic peak potential (in V vs. SCE) and �Ep (mV) for 3

Solvent E◦1 (3)a E

◦2 (3) Epc,1 (3) Epc,2 (

DMFa −0.690 (2e−) −0.714[BMIM]BF4 −0.946 (2e−) −0.996[BMIM]PF6 −0.821 −0.977 −0.868 −1.011

a The DMF solution contains 0.1 M of n-tetrabutylammonium tetrafluoroborate (TBABF

BF4 and [BMIM]PF6 (top); and 3 in [BMIM]BF4 and [BMIM]PF6 (bottom).

Fig. 3. Cyclic voltammograms of 3 showing the reduction 5 mM in DMF ( ),[BMIM]BF4 (—) and [BMIM]PF6 (- -) on a glassy carbon disk (0.5 mm diameter), scanrate of 1.0 V s−1.

the formation of ion-pairs will be less favorable. For such a pur-pose tetrakis(dimethylamino)ethylene, 3, was chosen. Compound 3undergoes following two simultaneous electron transfer processes,leading to the corresponding dianion at −0.690 V in DMF [29]. In thecase when the organic solvent is replaced by RTILs a shift in the E◦

value is also observed, however, in this case the values are shifted to

in different solvents at 20 ◦C

3) Epa,1 (3) Epa,2 (3) �Ep,1 (3) �Ep,2 (3)

−0.665 49−0.895 101−0.774 −0.942 94 69

4).

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H. Cruz et al. / Electrochim

Fig. 4. (a) Cyclic voltammograms of 4 showing the reduction 5 mM in DMF ( ), [BMIMvoltammograms of 5 showing the reduction 5 mM in DMF ( ), [BMIM]BF4 (—) and [BM

more negative potentials when either [BMIM]BF4 or [BMIM]PF6 areused (Fig. 3, Table 3). The planar configuration of the imidazoliumions, which cannot approach this bulky dianion, makes this dian-ion more stable when it is surrounded by tetraalkylammonium ions(Scheme 5, bottom). Note that there is a significant difference when[BMIM]PF6 is used as a solvent. Two simultaneous electron trans-fers become two successive one-electron transfers. This is clearlyseen from the �Ep shown in Table 3, where this value changes from49 mV (two electron transfer) to 101 mV indicating a close spacingbetween the waves. In the case of [BMIM]PF6, the spacing betweenthe two successive one-electron transfers is 156 mV. This effect maybe used in synthetic processes that are initiated by single electrontransfer processes.

Finally, trinitrobenzene derivates, 4–5, were also studied inRTILs. A careful analysis of the voltammetric waves reveals that

Table 4Standard potential, anodic and cathodic peak potential (in V vs. SCE) and �Ep for 6, 8 and

Solvent E◦ (6)a Epa (6)

DMFb 0.406 0.446[BMIM]BF4 0.324 0.356[BMIM]PF6 0.176 0.211

Solvent E◦ (8)c Epa (8)

DMFb 0.735 0.772[BMIM]BF4 0.612 0.640[BMIM]PF6 0.538 0.566

Solvent E◦ (9)d Epc (9)

DMFb 1.064 1.105[BMIM]BF4 1.049 1.087[BMIM]PF6 0.884 0.914

Standard potential, anodic and cathodic peak potential (in V vs. SCE) and �Ep (mV) for 7

Solvent E◦1 (7)e E

◦2 (7) Epa,1 (7) Epa,2 (7) Epc,1 (7) Epc,2

DMFb 0.277 0.524 0.319 0.568 0.234 0.479[BMIM]BF4 0.136 0.456 0.164 0.490 0.107 0.421[BMIM]PF6 0.120 0.533 0.184 0.610 0.056 0.456

Solvent E◦1 (10)f E

◦2 (10) Epa,1 (10) Epa,2 (1

DMFb 0.052 – 0.100 0.647DMFb + 20% H2O −0.008 0.511 0.044 0.568ACN −0.049 0.555 0.017 0.595[BMIM]BF4 −0.099 0.420 −0.063 0.455[BMIM]PF6 −0.132 0.444 −0.163 0.485

a Data referred to compound 6.b The DMF solution contains 0.1 M of n-tetrabutylammonium tetrafluoroborate (TBABFc Data referred to compound 8.d Data referred to compound 9.e Data referred to compound 7.f Data referred to compound 10.

ta 53 (2008) 5968–5976 5973

]PF6 (- -) on a glassy carbon disk (0.5 mm diameter), scan rate of 1.0 V s−1. (b) CyclicIM]PF6 (- -) on a glassy carbon disk (0.5 mm diameter), scan rate of 1.0 V s−1.

there is also a general trend in the shift of the Ep values for theirfirst electron transfer (Fig. 4b). Thus, the Ep values are less neg-ative in [BMIM]BF4 and more positive in [BMIM]PF6, −20 and+20 mV, respectively, compared to those obtained in DMF. Thefact that the initial trinitrobenzene anion radicals form strongion-pairs with the [BMIM] cations when [BMIM]BF4 is used as asolvent changes the well-known reactivity of those radicals. It hasrecently been established that 1,3,5-trinitrobenzene, 4, undergoesreversible dimerization of two units of trinitrobenzene anion rad-ical, 4•−, after electrochemical reduction [30–32]. However, whenthe DMF is replaced by [BMIM]BF4 the dimerization process isslower and the first reduction wave become partially reversibleeven at low scan rates (Fig. 4a). The increase in the stability of theanion radical can be explained in terms of the formation of strongion-pairs and the higher viscosity of the medium. The same effect

9 in different solvents at 20 ◦C

Epc (6) �Ep (6, mV)

0.366 800.292 640.141 70

Epc (8) �Ep (8, mV)

0.698 740.584 560.510 56

Epa (9) �Ep (9, mV)

1.022 831.011 760.853 61

and 10 in different solvents at 20 ◦C

(7) �Ep,1 (7) �Ep,2 (7)

85 8957 69

128 154

0) Epc,1 (10) Epc,2 (10) �Ep,1 (10) �Ep,2 (10)

0.003 – 97 –−0.059 0.454 103 114−0.066 0.515 83 80−0.135 0.385 72 70−0.100 0.403 63 82

4).

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Fig. 6. Cyclic voltammograms of 7 showing the oxidation 5 mM in DMF ( ),[BMIM]BF4 (—) and [BMIM]PF6 (- -) on a glassy carbon disk (0.5 mm diameter), scanrate of 1.0 V s−1.

ruthenium (II) complexes where the compounds containing thePF6

− counter-anion are also more stable than others containingBF4

− as the counter part [31].This general trend was also observed when other stable cation

radicals were investigated. In the case of 8 and 9 the same behavioris mainly observed, a displacement of the oxidation wave to a lesspositive potential is observed when the electroactive substancesare studied in RTILs rather than DMF (Table 4).

5974 H. Cruz et al. / Electrochim

Fig. 5. Cyclic voltammograms of 6 showing the oxidation 5 mM in DMF ( ),[BMIM]BF4 (—) and [BMIM]PF6 (- -) on a glassy carbon disk (0.5 mm diameter), scanrate of 1.0 V s−1.

is also observed when [BMIM]PF6 is used as a solvent, although thedimerization process is faster than in [BMIM]BF4.

3.2. Reactivity of cation radicals and dications in RTILs

Analogously, electrochemical studies of well-known oxidativeredox probes have been performed. In these it is worth highlight-ing the fact that previous studies devoted to the determination oftheir diffusion coefficients and their comparison with the obtainedvalues in aprotic polar solvents have been performed either for fer-rocene, 6, or N,N,N′,N′-tetramethyl-para-phenylenediamine, 10, inRTILs [18,20]. Since the redox properties of 6 and 10 could be used ina relatively straightforward fashion to determine and also comparewith bibliographic data, we decided to include those molecules inthe present study. As far as we are aware no studies of compounds7, 8 and 9 have been previously reported in the literature. Notethat the electrochemical studies of 7 in RTILs would be particularlyattractive since those derivatives are able to charge-transfer com-plexes that may be applied to various fields of scientific researchand practical applications, especially the field of molecular elec-tronics research. The fact that RTILs are a promising media for thedevelopment of new electrolyte media in advanced electrochemi-cal devices makes it highly desirable for a fundamental study to bemade of these compounds.

The shape of the ferrocene voltammograms is fairly similar to

those obtained in DMF (Fig. 5). However, a negative shift in theE◦ values in function of the solvent is clearly seen, those valuesare shifted 82 mV in [BMIM]BF4 and 230 mV in [BMIM]PF6 com-pared to the “reference” value obtained in DMF (Table 4). Thisshift could be explained in terms of solvation, the electrochemi-cally generated ferrocinium cation is better solvated in RTILs thanin DMF since the concentration of anions is greater. The differencebetween [BMIM]BF4 and DMF, which contains 0.1 M of TBABF4 issmaller than [BMIM]PF6 because in this case the only differenceis the concentration of anion, BF4

−, not its nature. Moreover, onecan, in principle, expect a better solvation of BF4

− than PF6− due

to the anion size, the BF4− anion is smaller than that of the PF6

−,which is the opposite of what is experimentally observed. How-ever, we should also take into account the fact that BF4

− is a lesspolarizable anion and has a harder base than PF6

−, and it is, there-fore, fair to assume that it has a greater ability to form a tighterinteraction with the hydrogen atoms of the imidazolium cation. Asa result of a stronger interaction between [BMIM]+ BF4

−, there is a“less free negative” charge of the anion. Thus, PF6

− acts as a “free”anion which is able to solvatize those electrogenerated cations con-siderably better. This result agrees with studies of the stability of

ta 53 (2008) 5968–5976

Fig. 7. Cyclic voltammograms of 10 on a glassy carbon disk (0.5 mm diameter), scanrate of 1.0 V s−1 showing the oxidation: (a) 5 mM in ACN ( ), [BMIM]BF4 (—) and[BMIM]PF6 (- -). (b) 5 mM in DMF ( ). (c) 5 mM in DMF + 20% H2O (—).

H. Cruz et al. / Electrochimica Ac

Scheme 6. Electrochemical oxidation mechanism of 10 in function of the reactionmedia.

In order to confirm this E◦ dependence and evaluate thesolvation effect at the dication level, we decided to use tetrathio-fulvalene, 7. As expected in the three media studied two successiveone-electron oxidation waves are observed at 0.277/0.524 V (DMF),0.136/0.456 V ([BMIM]BF4) and 0.120/0.533 V ([BMIM]PF6) vs. SCE,respectively (Fig. 6, Table 4). Focusing on cation radical level,the general trend due to the above-mentioned solvation effectsis observed, although this first oxidation wave becomes at least160 mV easier to oxidize. The relative differences at the second wavelevel are lower than at the first wave level which seems to indicatethat the solvation effects are not all that important. This argumentis also consistent with the fact that the difference between E◦s interms of the oxidation of the cation radical to the dication are nearlythe same, although both electron transfers are considerably slowerin [BMIM]PF6 than in [BMIM]BF4.

A last electroactive substance, 10, is used as a redox probe. Com-pound 10 undergoes two one-electron reversible successive waves.When the electroactive compound is studied in acetonitrile or inboth RTILs two successive electron waves are found, at −0.049,−0.099 and −0.132 V for the first wave and at 0.555, 0.420 and0.444 V for the second in ACN, [BMIM]BF4 and [BMIM]PF6, respec-tively, which is in clear agreement with the published data [18](Fig. 7a). Note that both general trends previously established for 7are also found in the case of 10, which also demonstrates influence

of the solvation effects on the measured E◦s values.

However, mechanistic differences were found in DMF (Fig. 7b).The second wave, corresponding to the dication formation,becomes irreversible and its intensity peak value is at less thantwo electrons. It is reported that at the second wave level thedication formed can react with a neutral molecule leading toan electroactive dimmer [18,20]. This dimer can be oxidized atthis potential, which is reflected in the voltammogram by anincrease in the second peak current value, leading to a dication(Fig. 7b). This dimer can also be first reduced to the correspond-ing radical cation as can be seen from a cathodic counter scan.Since this process seems highly sensitive to the presence of pro-tons in the solution, TMPD dication only seems to be stablein more acidic solvents like ACN + 0.1 M TBABF4, [BMIM]BF4 and[BMIM]PF6. Thus, it seems that the acetonitrile solvent moleculesand the [BMIM]+ cations do not act as mere spectators sincethey are capable of modifying the chemical process of interest(Scheme 6). The proton dependence of the process was defini-tively established by addition of controlled amounts of water tothe DMF + 0.1 M TBABF4 solution. When 20% of water is containedin a DMF solution, the same voltammetric response as that pre-

[

[

[[[[

ta 53 (2008) 5968–5976 5975

viously described for the other electrochemical media is found(Fig. 7c).

4. Conclusion

We have established the dependence of the E◦ values in the func-tion of the electrochemical media selected for a large amount ofreversible redox probes in reduction and also oxidation. We haveanalyzed the effect on the potential displacement at the first andsecond oxidation and reduction levels establishing the influenceof anionic and cationic moieties not only on the E◦ values but alsoon the reaction pathway of the electrogenerated substances. Themain effects observed at the anion or cation radical level are sol-vation effects, leading in some cases to the formation of strongion-pairs. For instance, in the case of nitroaromatic compounds,the interaction between the corresponding anion radical and theimidazolium cation is clearly observed, although the counter-aniondoes not act as a mere spectator. This interaction, which is mainlyrelated with the ion-pair effect, is stronger in the dianions thanin the anion radicals. However, the cation radicals are also morestable in the presence of PF6

− than BF4− for all the redox probes

tested, which may be related to polarization and base effects. Ithas also been demonstrated that the formation of these strongion-pairs can lead to the coalescence of two one-electron wavessince the spacing between two consecutive cathodic and anodicpeak potentials of the single electron transfers can be consider-ably reduced. The opposite effect can also be observed, when thesolvation effects are dramatically reduced a two-electron electrontransfer may become two suitably spaced single electron transfers.As a result, it is possible to take advantage of those ion-pairingeffects for the amelioration of synthetic routes from the electrodeprocesses. Hence, these effects can potentially be used not only tooptimize the electrode processes, from a wide variety of organiccompounds obtained electrochemically by changing the reactionmechanism or by amelioration of electrosynthetic conditions butalso to design electrochemical devices.

Acknowledgements

We gratefully acknowledge the financial support of the SpanishMinisterio de Educacion y Ciencia through project CTQ2006-01040.

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