The electrochemistry of arylated anthraquinones in room temperature ionic liquids

Article abstract of DOI:10.1002/poc.3098

Arylated anthraquinone derivatives of different sizes and different π-basicities have been prepared, and the electrochemical behaviour of these substances has been studied on screen printed graphite electrodes in the three room temperature ionic liquids (RTILs), 1-butyl-3-methylimidazolium hexafluorophosphate ([C₄MIM][PF₆]), 1-hexyl-3- methylimidazolium hexafluorophosphate ([C₆MIM][PF₆]) and 1-octyl-3-methylimidazolium hexafluorophosphate ([C₈MIM][PF ₆]). Half redox potentials for the first and second one electron reduction waves were identified, and the diffusion coefficient values were estimated from cyclic voltammetry measurements. The influence of the nature of the RTIL and of the substitution pattern of the anthraquinone on the solvodynamic radii were studied. A correlation of the reductive potentials with the corresponding Hammett constants of the substituents was tested. Copyright

Full text of DOI:10.1002/poc.3098

Research Article
Received: 11 October 2012,
Revised: 15 January 2013,
Accepted: 21 January 2013,
Published online in Wiley Online Library: 15 March 2013
(wileyonlinelibrary.com) DOI: 10.1002/poc.3098
The electrochemistry of arylated
anthraquinones in room temperature ionic
liquids
Alicia Gomis-Berenguera, Maria Gómez-Mingota, Leticia García-Cruza,
Thies Thiemannb**, Craig E. Banksc, Vicente Montiela and Jesús Iniestaa*
Arylated anthraquinone derivatives of different sizes and different p-basicities have been prepared, and the
electrochemical behaviour of these substances has been studied on screen printed graphite electrodes in the three
room temperature ionic liquids (RTILs), 1-butyl-3-methylimidazolium hexafluorophosphate ([C4MIM][PF6]), 1-hexyl-
3-methylimidazolium hexafluorophosphate ([C6MIM][PF6]) and 1-octyl-3-methylimidazolium hexafluorophosphate
([C8MIM][PF6]). Half redox potentials for the first and second one electron reduction waves were identified, and
the diffusion coefficient values were estimated from cyclic voltammetry measurements. The influence of the nature
of the RTIL and of the substitution pattern of the anthraquinone on the solvodynamic radii were studied. A
correlation of the reductive potentials with the corresponding Hammett constants of the substituents was tested.
Copyright © 2013 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this paper
Keywords: arylated anthraquinones; electron transfer; quinone; room temperature ionic liquid; screen printed graphite
electrodes; solvodynamic radii
bis(trifluoromethylsulfonyl)imide ([C2MIM][NTf2]), 1-butyl-1-
methylpyrrolidinium bis(trifluoromethylsulfonyl) imide ([C4mpyrr]
INTRODUCTION
Room temperature ionic liquids (RTILs) have become interesting
media in electrochemistry.[1–7] The use of RTILs in batteries,
electrochemical synthesis, electrocatalysis, electrochemical
capacitors, electrochemical sensing and biosensing, and in
modified electrode preparation, among others, has elicited
attention. The advantages of RTILs as chemical media are
deemed to be their relatively low toxicity, low vapor pressure
and tunable solvent properties. Among the many available RTILs
for electrochemical applications, they are chosen according to
their viscosity, conductivity and accessible electrochemical
potential windows; their increased use in electrochemical
sensors and biosensors can be therefore be foreseen.[8]
[NTf2]) and trihexyltetradecylphosphonium trifluorotris(pentafluor-
oethyl)phosphate ([P6,6,6,14][FAP]).[14] While Nikitina and col. have
reported on two well-resolved, one-electron reduction steps from
the quinone to the radical anion and hence to the dianion,[13] as
found for quinones under most conditions,[15] Compton and
co-workers have found that the size and nature of the ionic
liquid influenced the electrochemical behaviour of the bound
anthraquinone, as in larger RTILs, steric hindrance inhibits
charge compensation near the electrode, leading in certain cases
to incomplete reduction to the dianion.[14] Also, Nikitina and
co-workers showed an appreciable, specific solvation effect of
The electrochemistry of quinones, especially of benzoquinone,
in aqueous and organic medium is well-established.[9] Thus, the
quinones constitute a class of molecules that can be used as
model redox systems to study the behaviour of electrodes under
diverse conditions. Additionally, quinone structures and their
reduced forms can be found in biomolecules of importance.[10]
In the 9,10-anthraquinone series, Osteryoung and co-workers
were the first to study their electrochemical behaviour in room
temperature chloroaluminate molten salts, namely in mixtures
of AlCl3 and 1-ethyl-3-methylimidazolium chloride (ImCl)[11] or
n-butylpyridinium chloride (BuPyCl).[12] Subsequently, Nikitina’s
group have studied the electrochemical behaviour of 9,
10-anthraquinone in 1-butyl-3-methylimidazolium tetrafluoro-
borate ([C4MIM][BF4]) and in 1-butyl-3-methylimidazolium hex-
afluorophosphate ([C4MIM][PF6]) at a platinum electrode,[13] and
Compton’s group have looked at the electrochemistry of glassy car-
bon microelectrodes modified with a multilayer film of 2-anthranyl
groups in three different RTILs 1-ethyl-3-methylimidazolium
* Correspondence to: Jesús Iniesta, Department of Physical Chemistry and
Institute of Electrochemistry, University of Alicante, E-03080 Alicante, Spain.
E-mail: jesus.iniesta@ua.es
** Correspondence to: Thies Thiemann, Department of Chemistry, Faculty of
Science, United Arab Emirates University, PO Box 17551, Al Ain, United Arab
Emirates.
E-mail: thiesthiemann@yahoo.de, thies@uaeu.ac.ae
a A. Gomis-Berenguer, M. Gómez-Mingot, L. García-Cruz, V. Montiel, J. Iniesta
Department of Physical Chemistry and Institute of Electrochemistry, University
of Alicante, E-03080, Alicante, Spain
b T. Thiemann
Department of Chemistry, Faculty of Science, United Arab Emirates University,
Al Ain, United Arab Emirates
c C. E. Banks
Faculty of Science and Engineering, School of Science and the Environment,
Division of Chemistry and Environmental Science, Manchester Metropolitan
University, Chester Street, Manchester M1 5GD, Lancs, United Kingdom
J. Phys. Org. Chem. 2013, 26 367–375
Copyright © 2013 John Wiley & Sons, Ltd.

A. GOMIS-BERENGUER ET AL.
the different RTILs used on the anthraquinone-derived radical an-
ion and the dihydroxyanthracene dianion.[13] This has established
redox potential data of 9,10-anthraquinone in the RTILs ([C4MIM]
[BF4]) and ([C4MIM][PF6])[13] as an addition to the redox potential
data of quinones previously known in a larger variety of organic
solvents. Nikitina’s group have added to their investigation with
computational data that has led to the formulation of solvent-de-
pendent redox-potential scales[16,17] for different quinones that
also incorporate RTILs as solvents.[13]
5a:
5b:
Recently, the authors have developed a simple synthetic
strategy to aryl-substituted 9,10-anthraquinones, based on an
oxidative cycloaddition of halogenated thiophenes to benzoqui-
nones with a subsequent Suzuki-type arylation of the haloan-
thraquinone intermediates.[18,19] As a continuation of the
characterization of screen printed graphite electrodes (SPGEs) in
ionic liquids,[7] this allowed us to investigate the electrochemical
response at SPGEs of a number of arylated 9,10-anthraquinones
of different sizes and different p-basicity (Fig. 1A) in the three
RTILs, 1-butyl-3-methylimidazolium ([C4MIM][PF6]), 1-hexyl-3-
methylimidazolium hexafluorophosphate ([C6MIM][PF6]) and 1-
octyl-3-methylimidazolium hexafluorophosphate ([C8MIM][PF6])
(Fig. 1B). The dependence of the half wave redox potential of
the anthraquinone on the viscosity of the RTIL and therefore on
the size of the cation have been tested. Diffusion coefficients are
deduced for all tested arylated anthraquinones, together with
their solvodynamic radii. Finally, the effect of different substituted
groups on the aryl rings was considered in order to investigate
how the polarity of the cation of RTIL influences the half wave
redox potential.
1,4-diphenyl-9,10-anthraquinone
1, 4-bis-(p-methoxyphenyl)anthraquinone
5c:
5d:
1,4-di-(p-tolyl)anthraquinone
1,4-diphenyl-6,7-dimethylanthraquinone
5e:
5f:
1,4-bis-(o-methoxyphenyl)anthraquinone 1-chloro-4-(p-tolyl)anthraquinone
1A
RTIL
CATION
ANION
EXPERIMENTAL SECTION
[C4MIM][PF6]
N
N
F
All chemicals were of analytical grade and were used as received.
RTILs [C4MIM][PF6], [C6MIM][PF6] and [C8MIM][PF6] were obtained
from Merck (synthesis reagent). Anthraquinone derivatives were
synthesized via halogenated thiophenes as precursors.[18] 1,4-
diphenyl-9,10-anthraquinone (5a),[19] 1,4-bis-(p–methoxyphenyl)
anthraquinone (5b),[19] 1,4-(bis-p-methylphenyl)anthraquinone
(5c),[19] 6,7-dimethyl-1,4-diphenyl-anthraquinone (5d),[19] 1,4-
bis-(o-methoxyphenyl)anthraquinone (5e)[19] and 1-chloro-4–
(p-tolyl)anthraquinone (5f),[18] were prepared according to the
literature.
F
F
F
N
N
[C6MIM][PF6]
[C8MIM][PF6]
P
F
F
N
N
1B
Figure 1. 1A. The structures of the range of anthraquinones studied. 1B.
Structures of the ionic liquids (RTILs) utilized in this work
Acetonitrile (HPLC grade, from Scharlau), chloroform (from
AnalaR NORMAPUR) and tetrabutylammonium hexafluoropho-
sphate (Bu4NPF6) (from Fluka) were used without further
purification.
All anthraquinones examined in this work were weighted
using microbalance and then dissolved in chloroform (25 mL),
to which ionic liquid (100 mL) was added to obtain an established
concentration. Thereafter, the solution was concentrated in
vacuo overnight at 45 ꢀC in order to remove the organic solvent.
Water contents were determined, prior to electrochemical
measurements, by coulometric Karl–Fischer titration, and were
found to be 0.7 % for [C4MIM][PF6] and 0.6 % for both [C6MIM]
[PF6] and [C8MIM][PF6].
Cyclic voltammetric (CV) measurements were carried out
using a m-Autolab III (Eco Chemie, The Netherlands) potentio-
stat/galvanostat, controlled by Autolab, GPES software version
4.9 for Windows XP. All measurements were conducted using a
three-electrode configuration, where the working and the
counter electrode consisted of graphite ink with a pseudo
silver/silver chloride reference electrode and were fabricated
in-house.[20] The fabricated SPGEs have a 3.1 mm diameter
with a carbon counter electrode and a silver/silver chloride
pseudo-reference electrode on a flexible plastic base.[21] The
type of carbon ink used in the SPGE preparation was described
previously.[22] All electrochemical experiments were carried out
under a high purity argon flow (from Air-liquide) in order to keep
an inert atmosphere and avoid the electrochemical reduction of
oxygen. A reference of ferrocinium/ferrocene (Fc+/Fc) versus Ag/
AgCl pseudo-reference electrode was used for all three RTILs.
Connectors for the electrochemical connection of the SPGEs
were purchased from Kanichi Research Services Ltd. (UK).[23]
SPGEs were used as manufactured without further conditioning
of the working electrode surface. CVs were carried out at 293 ꢁ 2
K. Ultraviolet–Visible (UV–Vis) spectroscopy analysis was carried
out using a UV–Vis spectrometer Shimadzu 2401 PC.
The cyclic voltammetry behaviour of distinct anthraquinones
in conventional aprotic solvents such as acetonitrile was
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Copyright © 2013 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2013, 26 367–375

ELECTROCHEMISTRY OF ARYLATED ANTHRAQUINONES
compared using boron-doped diamond (BDD, Windsor
a
4 directly (Fig. 2A). These lend themselves to Suzuki–Miyaura cross-
coupling reactions with arylboronic acids, using Pd(0) catalysts, to
give the arylated 9,10-anthraquinones 5 (Fig. 2B). It was found that
also the chloro-9,10-anthraquinones such as 4c undergo the cross-
coupling reactions effectively, even with the commercial catalyst
Pd(PPh3)4.[19]
Diamonds, U.K.) bare electrode with a 3.0 mm diameter. The
counter electrode was a platinum wire, and the pseudo-
reference electrode was a silver wire. The BDD electrode’s surface
was pre-treated before experiments by abrasive erosion using
alumina (successively of 1.00, 0.30 and 0.05 mm in diameter) for
5 min for each alumina type used. Thereafter, the electrode was
sonicated for 1 min in ultra pure water using an ultrasonic
cleaning bath. Then, it was rinsed with acetonitrile and finally
dried under an argon atmosphere. All measurements were
carried out under an argon atmosphere. A reference Fc+/Fc
versus Ag electrode was used for all these measurements.
UV–Vis spectroscopy analysis
The UV–Visible spectra of chloroform solutions of most of the
arylated anthraquinones show at least three distinct bands,
usually associated with p–p* transitions.[24,25] For a particular
study, Fig. SM-1 shows the comparison between UV–Vis
spectrums of AQ 5a and AQ with a C6/C7-methylated anthraqui-
none core (5d) leading to a shift of Δl = 10 nm and l = 265 –
270 nm, in the benzoid band and in ‘quinoid band’, respectively
(Fig. SM-1).
RESULTS AND DISCUSSION
Synthesis of arylated and halogenated anthraquinones
Shown in Figs. 2A and 2B are details of the synthesis of
anthraquinone derivatives used in this study. The preparation
of the arylated anthraquinones 5 was carried out following a
sequence, developed earlier[18,19] using 2,5-dibromothiophene
(1a) and 2,5-dichlorothiophene (1b) as starting materials. These
are treated with meta chloroperoxybenzoic acid (m-CPBA) in
the presence of benzoquinones. The peracid oxidizes the
thiophenes 1 to the corresponding thiophene-S-oxides, which
are very good dienes in Diels–Alder reactions, in contrast to their
thiophene parents. In situ cycloaddition with the benzoquinones
2 as dienophiles gives the 15-thiatetracyclo[10.2.1.02,11.04,9]-
pentadecatetraene-3,10-dione S-oxides 3 as intermediates. These
can be isolated, but can be made to undergo an oxidative SO-
extrusion reaction at slightly elevated temperatures in the
presence of m-CPBA to give the halogenated 9,10-anthraquinones
Cyclic voltammetry of anthraquinones 5a and 5d
First, we investigated the cyclic voltammetric response of the
redox couple Fc+/Fc, used for the calibration of the pseudo-
reference Ag/AgCl electrode for the three RTILs under study.
CV revealed half wave potential (E1/2) for the couple Fc+/Fc
of 0.278; 0.444 and 0.322 V versus Ag/AgCl for ferrocene in
[C4MIM][PF6], [C6MIM][PF6] and [C8MIM][PF6], respectively
(Fig. 3). These values are reliable with the number of experi-
ments and the disposable SPGEs. E1/2 data for the Fc+/Fc redox
couple do not obey any trend. We have found in the literature
how the ionic liquid used might affect the value of formal
potential against a pseudo-reference electrode. For example,
Nikitina and col.[13] have observed that formal potentials of
the Fc+/Fc couple in [C4MIM][BF4] and [C4MIM][PF6] RTILs were
equal to 0.19 and 0.15 V, respectively, versus a platinum
pseudo-reference electrode. This difference was found to be
40 mV; here, E1/2 difference was 44 mV between [C4MIM] and
[C8MIM] cations. However, E1/2 for the RTIL [C6MIM] is about
166 mV and 122 mV higher than those presented for [C4MIM]
and [C8MIM] cations. A plausible explanation for this shifting in
E1/2 may be ascribed to the viscosity (371, 690 and 866 cP for the
[CnMIM], n = 4, 6, 8) and the intrinsic amount of water present in
the RTIL.
Ohmic resistances (Rohm) have been calculated from CVs of
Fig. 3. In order to compensate the ohmic drop, we have assumed
Fe+/Fe redox couple behaves reversibly in all RTILs tested in this
study. For a theoretical CV trace with a value of 59 mV of ΔE
between cathodic and anodic peak potentials. Table SM-1
summarizes the solution resistances for all three RTILs tested
for experiments carried out upon the first and third scans.
Moreover, E1/2 data were also compared after ohmic drop
correction. Thus, ohmic drop resulted to be almost constant for
a wide range of scan rates (10–800 mV sꢂ1) denoting that
deviation from reversibility is not associated to any kinetic
component, but just related to the ohmic drop. Furthermore,
Table SM-1 points out that E1/2 values for the first scan for the
three RTILs resulted to be similar, whereas after the reproducible
CV traces on the second and third scans, E1/2 values were found
to be 0.250, 0.417 and 0.269 V for [C4MIM][PF6], [C6MIM][PF6] and
[C8MIM][PF6], respectively. However, E1/2 value for the [C6MIM]
[PF6] still deviates from the other E1/2 values obtained for
[C4MIM][PF6] and [C8MIM][PF6] RTILs.
Figure 2. 2A. Reaction mechanism of the synthesis of halogenated
anthraquinones. 2B. Reaction mechanism of the synthesis of arylated
anthraquinones 5a – 5 f
J. Phys. Org. Chem. 2013, 26 367–375
Copyright © 2013 John Wiley & Sons, Ltd.
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A. GOMIS-BERENGUER ET AL.
Figure 3. Cyclic voltammograms of ferrocene (A) 5.8 mM in [C4MIM]
[PF6], (B) 6.7 mM in [C6MIM][PF6] and (C) 7.6 mM in [C8MIM][PF6] obtained
at SPGE using 35 mL of sample solution. Scan rate 50 mV sꢂ1. First scan
(dashed line) and third scan (solid line)
Figure 4 shows the CVs for the electrochemical behaviour of
7.81, 5.36 and 8.43 mM AQ 5a in all three RTILs tested in this
study, leading to two continuous one electron processes, leading
to the quinone radical anion and subsequently to the dianion, as
described previously for the electrochemistry of quinones in
RTILs.[26,27] The peak-to-peak separation for the first redox couple
(1) at slow scan rate (20–50 mV sꢂ1) has an average value of
100 mV, typical of a quasi-reversible electrochemical process
while the second redox couple (2) has a peak-to-peak separation
close to 107 mV.
Figure 4. Cyclic voltammograms of (a) 7.81 mM AQ 5a in [C4MIM][PF6], (b)
5.36 mM AQ 5a in [C6MIM][PF6] and (c) 8.43 mM AQ 5a in [C8MIM][PF6]
obtained at a SPGE. 35 mL of sample solution. Scan rate 50mVsꢂ1. The third
scan is shown
the temperature and F is the Faraday´s constꢃant. Equilibrium
constant for the reaction AQ2- + AQ ↔ 2 AQ - was provided
assuming that formal potentials for the processes (1) and (2)
approximate E1/2 data obtained in this study. For an average
E1/2 separation between processes (1) and (2) of 0.31 V, the
The same percentage of the radical anions of AQ 5a was
reduced in the three ionic liquids, as evidenced by the constant
equilibrium constant in all three RTILs tested was 8.4 ꢄ 10ꢂ5
,
Ip c(2)/Ip
ratio, where Ip
and Ip
correspond to the peak
with a value of ΔG0 of ꢂ29.9 kJ molꢂ1, which is significant.
Previous reports by Compton’s group studied the comproportio-
nation of benzoquinone in [C4MIM][PF6], providing an
equilibrium constant with similar values.[28]
c(1)
c(2)
c(1)
currents of the AQ.-/AQ2- and AQ/AQ.- reductions, respectively.
In [C4MIM][PF6], Ipc(2)/Ip was found to be 0.69, in [C6MIM][PF6]
c(1)
0.78 and in [C8MIM][PF6] 0.62, after base line corrections for a
scan rate of 50 mV sꢂ1. For the second redox couple (2) of AQ
5a, the ratio of cathodic to anodic currents was close to unity
The plots of the first cathodic peak current for AQ 5a in all
three RTILs tested as a function of the square root of the scan
rate are linear from 0.02 to 1 V sꢂ1 (R2 = 0.99), which indicates
that the process is purely diffusion controlled. These findings
were confirmed by Cottrell plots, where the slopes are ꢂ0.483,
ꢂ0.497 and ꢂ0.510 for [C4MIM][PF6], [C6MIM][PF6] and [C8MIM]
[PF6], respectively, values very close to the theoretical value of
ꢂ0.5. The diffusion coefficient of neutral AQ 5a species in the
RTILs under study is presented in Table 2. Diffusion coefficient
c
a
for all three RTILs used, whereas the Ip /Ip ratio deviated from
unity for the first redox couple (1) with a value close to 1.92 for
[C4MIM][PF6] and 1.76 [C6MIM][PF6] and 1.21 for [C8MIM][PF6],
possibly denoting a lower diffusion coefficient for the radical
anion species in RTILs.
A comparison of the differences in the E1/2 as a function of the
RTIL is summarized in Table 1. E1/2 for the first and second redox
couples are very similar for the three RTILs under study. The
difference in potential between the both half wave potentials
E1/2 for the processes (1) and (2) is ca. - 300 mV (Table 1). This
E1/2 separation indicates that the thermodynamic tendency of
the equilibrium constant K for the reaction (AQ2- + AQ ↔ 2
AQꢃ-) is relatively small in these RTIL media. The equilibrium con-
stant for the above reaction is given by eqn. (1):[28]
values are found to be in the range of 3.7 to 7.9 ꢄ 10ꢂ13 m2 sꢂ1
.
Interestingly, not much differences were found in the D values
for [C4MIM][PF6] and [C6MIM][PF6]. By contrast, the D value of
AQ 5a in an RTIL with a large ionic cation [C8MIM] decreases sig-
nificantly, associated mainly to the higher viscosity of the
[C8MIM][PF6] RTIL. The authors have tested a range of concentra-
tion between 5 and 8 mM for the anthraquinone 5a. For those
concentrations, diffusion coefficients were of the same order
for the [C6mim][PF6]. Any kind of association of the anthraqui-
none 5a will lead to remarkable changes in diffusion coefficient
values. However, the concentration of anthraquinone is limited
to solubility. For example, Nikitina and col.[13] used 8 mM of
RT
ΔE ¼ E10 ꢂ E20 ¼
lnK
(1)
nF
where E01 and E02 are the formal potentials for the first and
second reduction, respectively. R is the ideal gas constant, T
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Copyright © 2013 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2013, 26 367–375

ELECTROCHEMISTRY OF ARYLATED ANTHRAQUINONES
Table 1. Anthraquinones examined along with voltammetric potentials for the first (1) and second (2) reduction waves observed
in [C4MIM][PF6], [C6MIM][PF6] and [C8MIM][PF6] obtained at a SPGE 35 mL of sample solution media. Scan rate 50 mV sꢂ1. Third scan
recorded. All potentials are reported versus Fc+/Fc
AQ/mM
Solvent
Ec(1)/V
Ec(2)/V
❘ΔE❘(1)/V
❘ΔE❘(2)/V
Ec(2)ꢂEc(1)/V
E1/2 (1)/V
E1/2 (2)/V
5a
7.82
5.36
8.43
5b
[C4MIM][PF6]
[C6MIM][PF6]
[C8MIM][PF6]
ꢂ1.21
ꢂ1.25
ꢂ1.26
ꢂ1.53
ꢂ1.57
ꢂ1.59
0.10
0.09
0.11
0.11
0.10
0.11
ꢂ0.31
ꢂ0.32
ꢂ0.32
ꢂ1.17
ꢂ1.21
ꢂ1.21
ꢂ1.47
ꢂ1.52
ꢂ1.53
6.32
5.21
5.51
5c
[C4MIM][PF6]
[C6MIM][PF6]
[C8MIM][PF6]
ꢂ1.21
ꢂ1.24
ꢂ1.26
ꢂ1.53
ꢂ1.54
ꢂ1.60
0.09
0.08
0.09
0.10
0.08
0.10
ꢂ0.32
ꢂ0.30
ꢂ0.30
ꢂ1.17
ꢂ1.20
ꢂ1.22
ꢂ1.48
ꢂ1.50
ꢂ1.50
5.21
6.20
6.31
5d
[C4MIM][PF6]
[C6MIM][PF6]
[C8MIM][PF6]
ꢂ1.22
ꢂ1.25
ꢂ1.25
ꢂ1.54
ꢂ1.57
ꢂ1.61
0.11
0.09
0.11
0.11
0.11
0.12
ꢂ0.32
ꢂ0.32
ꢂ0.35
ꢂ1.16
ꢂ1.20
ꢂ1.20
ꢂ1.48
ꢂ1.52
ꢂ1.54
5.34
5.65
4.75
5e
[C4MIM][PF6]
[C6MIM][PF6]
[C8MIM][PF6]
ꢂ1.23
ꢂ1.29
ꢂ1.28
ꢂ1.56
ꢂ1.62
ꢂ1.58
0.09
0.08
0.08
0.10
0.10
0.09
ꢂ0.33
ꢂ0.33
ꢂ0.30
ꢂ1.19
ꢂ1.25
ꢂ1.25
ꢂ1.51
ꢂ1.57
ꢂ1.54
5.25
5.02
6.20
5f
[C4MIM][PF6]
[C6MIM][PF6]
[C8MIM][PF6]
ꢂ1.29
ꢂ1.34
ꢂ1.34
ꢂ1.54
ꢂ1.54
ꢂ1.63
0.11
0.12
0.11
0.12
0.13
0.12
ꢂ0.26
ꢂ0.21
ꢂ0.29
ꢂ1.23
ꢂ1.28
ꢂ1.28
ꢂ1.48
ꢂ1.48
ꢂ1.56
4.65
4.82
5.81
[C4MIM][PF6]
[C6MIM][PF6]
[C8MIM][PF6]
ꢂ1.21
ꢂ1.23
ꢂ1.30
ꢂ1.47
ꢂ1.51
ꢂ1.54
0.17
0.17
0.20
0.18
0.18
0.20
ꢂ0.27
ꢂ0.28
ꢂ0.24
ꢂ1.12
ꢂ1.15
ꢂ1.19
ꢂ1.38
ꢂ1.42
ꢂ1.43
the 9,10-anthraquinone in [C4mim][BF4]. The same authors did
not mention any possible p–p interaction and association, and
therefore they did not study the effect of concentration on the
electrochemical behaviour.
10-anthraquinone, 1,4-dithien-2’-yl-9,10-anthraquinone and 1,4-bis
(9’,9’-dioctylfluoren-2’-yl)-9,10-anthraquinone in dichloromethane/
0.1 M tetrabutylammonium hexafluorophosphate using a glassy
carbon as working electrode. Though anthraquinones tested in
the former study had a big aromaticity component, Gautrot’s work
did not study the effect of concentration on the CV response,
Furthermore, Gautrot and col.[29] studied the electrochemical
behaviour of several anthraquinones such as 1,4-diphenyl-9,
Table 2. Diffusion coefficients (D, m2 sꢂ1) determined for the studied anthraquinones together with the solvodynamic radius.
AQ
Solvent
D/m2 sꢂ1
Solvodynamic radius (rs)/Å
5a
[C4MIM][PF6]
[C6MIM][PF6]
[C8MIM][PF6]
[C4MIM][PF6]
[C6MIM][PF6]
[C8MIM][PF6]
[C4MIM][PF6]
[C6MIM][PF6]
[C8MIM][PF6]
[C4MIM][PF6]
[C6MIM][PF6]
[C8MIM][PF6]
[C4MIM][PF6]
[C6MIM][PF6]
[C8MIM][PF6]
[C4MIM][PF6]
[C6MIM][PF6]
[C8MIM][PF6]
(6.2 ꢁ 0.6)ꢄ10ꢂ13
(7.9 ꢁ 0.8)ꢄ10ꢂ13
(3.7 ꢁ 0.3)ꢄ10ꢂ13
(8.4 ꢁ 0.7)ꢄ10ꢂ13
(4.8 ꢁ 0.4)ꢄ10ꢂ13
(2.9 ꢁ 0.2)ꢄ10ꢂ13
(13.4 ꢁ 1.2)ꢄ10ꢂ13
(1.5 ꢁ 0.13)ꢄ10ꢂ13
(1.5 ꢁ 0.11)ꢄ10ꢂ13
(6.6 ꢁ 0.6)ꢄ10ꢂ13
(5.5 ꢁ 0.5)ꢄ10ꢂ13
(1.8 ꢁ 0.13)ꢄ10ꢂ13
(2.8 ꢁ 0.2)ꢄ10ꢂ13
(0.9 ꢁ 0.10)ꢄ10ꢂ13
(1.3 ꢁ 0.10)ꢄ10ꢂ13
(7.5 ꢁ 0.5)ꢄ10ꢂ13
(3.0 ꢁ 0.3)ꢄ10ꢂ13
(1.3 ꢁ 0.10)ꢄ10ꢂ13
11.7 ꢁ 1.0
7.1 ꢁ 0.8
13.0 ꢁ 1.03
8.7 ꢁ 0.8
5b
5c
5d
5e
5f
11.7 ꢁ 1.1
16.4 ꢁ 1.3
5.4 ꢁ 0.5
36.3 ꢁ 3.4
31.4 ꢁ 2.5
11.0 ꢁ 1.1
10.2 ꢁ 1.0
27.2 ꢁ 2.2
26.1 ꢁ 2.4
60.8 ꢁ 6.0
37.2 ꢁ 3.0
9.7 ꢁ 0.8
18.8 ꢁ 1.9
37.0 ꢁ 2.9
J. Phys. Org. Chem. 2013, 26 367–375
Copyright © 2013 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

A. GOMIS-BERENGUER ET AL.
and they do not state either any observation on the p–p
interaction between aromatic rings nor association of anthra-
quinone molecules.
ortho positions, respectively. Voltammograms show a very well-
defined two consecutive one electron processes, as shown
previously in Fig. 4 for the anthraquinone 5a. A comparison of
E1/2 to the first and second reductive couple of AQs 5a and 5b
does not show significant differences. The methoxy group at
the para position does not have a significant influence on the
reduction potential. Similar values of E1/2 are seen in all RTILs
used. A comparison of both anthraquinones 5b and 5e reveals
a quasi-reversible behaviour for both redox couples. Nevertheless,
AQ 5b showed a remarkable decrease in current intensity for the
reoxidative process (2) from the butyl to the hexyl immidazolium
Substitution of positions 6 and 7 of the anthraquinone moiety
with methyl groups, as in AQ 5d, leads to a CV pattern similar to
that shown by AQ 5a. However, here, E1/2 values for redox
processes (1) and (2) show a very slight enhancement of ca.
20–40 mV as compared to the E1/2 values of AQ 5a in all RTIL
tested, as depicted in Table 1. Moreover, the D values exhibited
by 5d are more similar in [C4MIM][PF6] and [C6MIM][PF6], with
6.6 ꢄ 10ꢂ13 and 5.5 ꢄ 10ꢂ13 m2 sꢂ1, respectively, and they
were of similar magnitude with those presented by AQ 5a
for the same ionic liquids. Additionally, the D value of AQ 5d
in [C8MIM][PF6] was significantly lower, with a value of
1.8 ꢄ 10ꢂ13 m2 sꢂ1 (see Table 2). The incorporation of two
(2)
cation. Moreover, the Ec ꢂ Ec (1) value for AQ 5b remains similar
to those values presented by AQ 5a and AQ 5d, whereas the
methoxy groups in the ortho position led to a notable decrease
(2)
(1)
in the Ec ꢂ Ec value of about 100 mV for AQ 5e, which is
indicative of a higher comproportionation constant. The position
of the methoxy group is also reflected in the D values. This can
be seen in Table 2, which shows generally small D values for AQ
5e (between 2.8 ꢄ 10ꢂ13 and 0.9 ꢄ 10ꢂ13 m2 sꢂ1), with the
smallest value in [C6MIM][PF6], while D values for AQ 5b were
found to decrease with the size of the RTIL cation. The incorpora-
tion of methoxy groups, both in ortho and para positions, as in AQ
5b and AQ 5e, seems to have a significant influence on the D
values, as can be seen when comparing them to those obtained
from AQ 5a.
(2)
(1)
methyl groups kept unchanged the difference Ec
ꢂ Ec
of ca. ꢂ300 mV in all three RTILs tested, similar to that pre-
vious presented in the literature for anthraquinone in [C4MIM]
[BF4] RTIL with a value for Ec ꢂ Ec (1) of 0.34 V.[30] The same refer-
(2)
ence[30] gives a value for Ec ꢂ Ec (1) of 0.55V for 9,10-anthraqui-
(2)
(2)
(1)
none in acetonitrile. Similarly, our value of Ec ꢂ Ec obtained
from the CV measurements of anthraquinone 5d in acetonitrile,
using a BDD electrode, was 0.53 V. This is shown in the cyclic vol-
tammogram of AQ 5d (0.62 mmolLꢂ1 in acetonitrile, electrolyte:
Bu4NPF6) in Fig. 5. A cyclic voltammogram of AQ 5a under the
same conditions provided a reduction peak-to-peak separation of
0.568 V versus Fc+/Fc. The higher aprotic character of the solvent
as well as the absence of a weak interaction due to ion-pair
formation of the anion radical species with the solvent are
Also, we investigated the influence of the methyl groups
as substituents, namely 1,4-di-p-methylphenylanthraquinone
(AQ 5c). In this case, the substitution has little influence on the
half wave potential shift, and hence there is no effect on
the peak-to-peak separation (see Table 1). Finally, we studied
the electrochemical behaviour of 1-chloro-4-tolylanthraquinone
(AQ 5f) in all three RTILs under study. A comparison between
(2)
(1)
predominantly the causes for this higher Ec ꢂ Ec value, as
shown in Table 3.
(2)
AQ 5c and AQ 5f revealed a slight decrease of Ec ꢂ Ec(1) values
Cyclic voltammetry of substituted alkyl- and
alkoxy-substituted 1,4 diphenylanthraquinone derivatives
(5b, 5c, 5e and 5f)
for AQ 5f, denoted mainly by a reduction of about 100 mV for
the second half wave potential of AQ 5 f. Both anthraquinones
5c and 5f show higher values of D in the [C4MIM][PF6] RTIL.
Strikingly, both AQs showed a notable diminution of D values
for the hexyl and octyl RTILs, especially for the AQ 5c.
Methylation of the aryl groups resulted in lower D values for 5c
in the more highly viscous RTILs as compared to anthraquinone
5a (see Table 2).
The influence of the substitution pattern of the 1,4-diphenyl
groups on the electrochemical behaviour in RTIL was investi-
gated, also. Figure 6 compares the electrochemical behaviour
of 1,4-di-p-methoxyphenylanthraquinone, AQ 5b (Fig. 6i), and
1,4-di-o-methoxyphenylanthraquinone, AQ 5e (Fig. 6ii), in which
methoxy groups of aryl substituent are located in para and
Solvodynamic radius of anthraquinones
Data from the solvodynamic radii (rs) provided in this work is
useful for a comparison of the values of the molecular radii of
isolated p-arylated anthaquinones. The stronger or weaker solva-
tion of our model anthraquinones could be assumed to
differences between the solvodynamic radii and the size of the
isolated molecule. Additionally, some interpretation is obtained
from the solvodynamic radii in terms of number of solvent
molecules. Table 2 also shows solvodynamic radii (rs) of AQs in all
three RTILs. The values were obtained from the Stokes–Einstein
equation (eqn. (2)):
KBT
rs ¼
(2)
4pꢀD
where KB is the Boltzmann constant, T is the absolute
temperature and ꢀ is the viscosity of the medium. The diffusion
coefficients of the different anthraquinones were obtained from
the Randles–Sevcik equation[31] in order to work out the
solvodynamic radii of the diffusing species. For the calculations
Figure 5. Example of a cyclic voltammogram for one of the anthraqui-
none tested, 1,4-diphenyl-6,7-dimethyl anthraquinone (5d) (0.62 mmol Lꢂ1
)
in Bu4NPF6 (0.1molLꢂ1) solution in degassed acetonitrile (scan rate: 50mV
ꢂ1), versus Fc+/Fc. The third scan is shown
s
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ELECTROCHEMISTRY OF ARYLATED ANTHRAQUINONES
Table 3. Potential data from CV measurements of AQ 5a and AQ 5d in degassed acetonitrile solutions (electrolyte: Bu4NPF6, 0.1 M;
scan rate: 50 mV sꢂ1). All potentials are referred to Fc+/Fc redox couple. Mostly, ΔE values exhibit quasi-reversible peaks (1) for the
first reduction peak and (2) for the second reduction peak. Potential data are given ꢁ 10 mV
ΔE(1)/ V
ΔE(2)/ V
Ec ꢂEc (1)/ V
E1/2 (1)/ V
E1/2 (2)/ V
(2)
AQ/mM
Ec (1)/ V
Ec (2)/ V
5a
0.58
5d
ꢂ1.37
ꢂ1.94
0.07
0.14
ꢂ0.57
1.34
1.87
ꢂ1.43
ꢂ1.96
0.09
0.10
ꢂ0.53
1.39
1.91
0.62
Figure 6. Cyclic voltammograms of 5.08, 7.50 and 6.33 mM 1,4-di-o-methoxyphenylanthraquinone (5e) (i) and 6.32, 5.21 and 5.51 mM 1,4-di-p-meth-
oxyphenylanthraquinone (5b) (ii) in (A) [C4MIM][PF6], (B) [C6MIM][PF6] and (C) [C8MIM][PF6] obtained at a SPGE, 35 mL of sample solution. Scan rate
50 mV sꢂ1. The third scan is shown
of rs values, the viscosities of [C4MIM][PF6], [C6MIM][PF6] and
[C8MIM][PF6] were obtained from the literature,[32] with values
of 371, 690 and 866 cP, respectively.
In general, a dependence of the solvodynamic radii of all
anthraquinones upon the cation size of the ionic liquid was
observed. Thus, normally, the solvodynamic radii rs increase with
the length of the alkyl chain in the cation [CnMIM], with n = 4, 6
and 8. A close match in values was found for rs of AQ 5a when
compared to that given in the literature for 9,10-anthraquinone,
when using [C4MIM][BF4] (9 ꢁ 2 Å).[13] Furthermore, a compari-
son of the rs values of substituted arylanthraquinone revealed
that AQ 5e provide the highest rs values. Interestingly, a
comparison between AQs 5b and 5e shows an effect of the
substitution pattern on the phenyl groups (methoxy groups
P
(1)
either in ortho or para position) on the solvodynamic radii.
Similar values of the solvodynamic radii for AQ 5a in all three
RTILs tested indicate that there is not a clear-cut relation
Figure 7. Hammett plots of -Epc versus s for the first reduction of
anthraquinones 5a, 5d and 5e in (
) [C4MIM][PF6], (
) [C6MIM]
[PF6] and (
) [C8MIM][PF6] for the reduction
J. Phys. Org. Chem. 2013, 26 367–375
Copyright © 2013 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

A. GOMIS-BERENGUER ET AL.
between the solvodynamic radius and the size of the cation of
the RTILs for all AQs, although solvation is normally strongly
influenced by the cation orientation and the structure of the
neutral anthraquinone.
to be dependent on the nature of the RTIL cation and, in some
cases, on the position of the substituent on the aryl ring of
the arylated anthraquinones. Only the first redox half poten-
tials of anthraquinones 5a, 5d and 5e were found to be a
direct function of the corresponding Hammett’s substituent
constants, irrespective of the RTIL used. Low values of r
indicate that [C4MIM][PF6], [C6MIM][PF6] and [C8MIM][PF6]
exhibited similar polarity.
Polarity of the RTILs and its effect on the half wave potential
of arylated anthraquinones
We tried to find out whether a correlation between the first
redox potential of the anthraquinones to the anthraquinone
radical anion for all distinct anthraquinones studied and the
Hammett constants of the substituents exists[33,34] and whether
it adheres to the following free-energy relationship (eqn. (3)):
Acknowledgements
M. Gómez-Mingot thanks the University of Alicante for her
fellowship. T. Thiemann thanks funding from Global Centre of
Excellence on New Carbon Resources, Kyushu University, Japan.
J. Iniesta acknowledges Ministerio de Educación y Ciencia MEC
Spain (Project: CTQ2007-62345).
X
E1=2;x ꢂ E1=2;0 ¼ ΔE1=2 ¼ r sx
(3)
where E1/2,0 is the half wave potential for the first reduction of
the unsubstituted anthraquinone, E1/2,x is the half wave potential
P
for the first reduction of the substituted AQ x, sx is the rele-
vant Hammett substituent constant and the slope, r, is the
Hammett solvent-dependent reaction parameter. The r value
varies according to the polarity of the solvent. Figure 7 shows
Hammett plots of –E1/2,x versus sx for the first reduction of
AQs 5a, 5d and 5e in three RTILs under study. The half wave
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P
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J. Phys. Org. Chem. 2013, 26 367–375
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Featured Compounds
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