Synthesis and Electrochemical Properties of 2,5-Disubstituted 1,4-Bis(4,5-diphenyl-1H-imidazol-2-yl)benzene Derivatives

Article abstract of DOI:10.1134/S1070428020070167

Abstract: 2,5-Disubstituted 1,4-bis(4,5-diphenyl-1H-imidazol-2-yl)benzenes have been synthesized, and their electrochemical properties have been studied by cyclic voltammetry, in particular the effect of substituents on the redox characteristics has been examined. The reversible electrochemical redox transformation quinone–biradical of the title compounds occurs at positive potentials.

Full text of DOI:10.1134/S1070428020070167

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2020, Vol. 56, No. 7, pp. 1222–1227. © Pleiades Publishing, Ltd., 2020.
Russian Text © The Author(s), 2020, published in Zhurnal Organicheskoi Khimii, 2020, Vol. 56, No. 7, pp. 1115–1122.
Synthesis and Electrochemical Properties of 2,5-Disubstituted
1,4-Bis(4,5-diphenyl-1H-imidazol-2-yl)benzene Derivatives
D. B. Chugunova,*, E. V. Okinaa, A. S. Timoninaa, L. A. Klimaevaa, and Yu. M. Selivanovaa
a Ogarev Mordovian National Research State University, Saransk, 430005 Russia
*e-mail: iman081@gmail.com
Received February 7, 2020; revised April 17, 2020; accepted April 21, 2020
Abstract—2,5-Disubstituted 1,4-bis(4,5-diphenyl-1H-imidazol-2-yl)benzenes have been synthesized, and their
electrochemical properties have been studied by cyclic voltammetry, in particular the effect of substituents on
the redox characteristics has been examined. The reversible electrochemical redox transformation quinone–
biradical of the title compounds occurs at positive potentials.
Keywords: biradicals, monoradicals, cyclic voltammetry, 1,4-bis(4,5-diphenyl-1H-imidazol-2-yl)benzene
DOI: 10.1134/S1070428020070167
Due to their high stability toward atmospheric
oxygen, triarylimidazolyl radicals are fairly convenient
and widely used models for studying structure–reac-
tivity relationships in the series of stable radicals [1–3].
However, there are almost no published data on bi-
radicals derived from these compounds.
metry. It was found that steric strain created by two
bromine atoms in the central benzene ring is respon-
sible for unusual redox properties of these compounds,
in particular for the shift of their reduction potentials to
the positive region. The reduction potentials of 2,5-di-
substituted 1,4-bis(4,5-diphenyl-1H-imidazol-2-yl)-
benzenes are unprecedented among substituted cyclo-
hexa-1,4-dienes.
We previously synthesized 1,4-bis(imidazolyl)ben-
zene systems whose oxidation gave the corresponding
quinoid structures, and the latter showed paramagnetic
susceptibility on heating as a result of rupture of
quinoid bonds. It was presumed that the presence of
a substituent in the ortho position of the central ben-
zene ring with respect to the imidazolyl substituent
should hamper formation of quinoid structures and that
biradicals derived therefrom should possess unusual
redox properties.
2,5-Disubstituted 1,4-bis(4,5-diphenyl-1H-imid-
azol-2-yl)benzenes 2a–2c were synthesized as outlined
in Scheme 1. Initial 2-bromo- and 2,5-dibromotere-
phthalic acids necessary for the synthesis of dialde-
hydes 1b and 1c were prepared according to the proce-
dures described in [4, 5]. Dialdehydes 1a–1c were
reacted with 2 equiv of benzil and 4 equiv of ammo-
nium acetate to obtain bis-imidazoles 2a–2c, and the
latter were oxidized to quinones with potassium hexa-
cyanoferrate(III) in the two-phase system benzene–
water in the presence of KOH (Scheme 2). Quinones
3a–3c separated from the reaction mixtures as solids
partially soluble in benzene. According to the TLC data
Herein, we report the synthesis of 2-mono- and
2,5-dibromo-1,4-bis(4,5-diphenyl-1H-imidazolyl-2)-
benzenes as the simplest bisimidazoles. Their electro-
chemical properties were studied by cyclic voltam-
Scheme 1.
CHO
R1
O
(1) AcOH, Δ
(2) Aq. NH3
H
R1
Ph
Ph
Ph
Ph
N
N
Ph
+ 2
+ 4NH4OAc
+ 4AcOH + 6H2O
Ph
R2
N
N
H
O
R2
2a–2c
CHO
1a–1c
R1 = R2 = H (a); R1 = H, R2 = Br (b); R1 = R2 = Br (c).
1222

SYNTHESIS AND ELECTROCHEMICAL PROPERTIES OF 2,5-DISUBSTITUTED ...
1223
Scheme 2.
R1
K3[Fe(CN)6]
H2O, PhH
Ph
Ph
Ph
Ph
N
N
N
N
2a–2c
R2
3a–3c
R1 = R2 = H (a); R1 = H, R2 = Br (b); R1 = R2 = Br (c).
(Silufol, eluent benzene), the oxidation of bis-imidaz-
oles 2a–2c was not complete under the given con-
ditions, and the oxidation products contained unreacted
initial compounds 2a–2c and unidentified impurities.
Quinones 3a–3c were purified by dissolution in boiling
benzene, followed by filtration from the undissolved
material, evaporation of the filtrate under reduced
pressure, and precipitation of the pure product with
n-hexane.
at a single potential value are rare cases. For example,
if the second electron is transferred at a more positive
potential than the first one, this may look as a two-
electron process. Another case is that where the first
electron transfer is immediately followed by a very fast
chemical reaction (as in the reduction of quinones at
certain pH values) [6]. However, in this case, the shape
of the voltammograms changes: it becomes narrower,
and the difference between the forward and reverse
sweep potentials reaches 28 mV; simultaneously, the
peak current increases [6]. If two similar redox centers
are distant from each other, the peak currents are
simply summed up, and the shape of the CV curve
almost does not change. This is observed in Fig. 1.
Therefore, the two-electron character of the reduction
wave can be rationalized as follows: radical anion gen-
erated in the first step rapidly abstracts a proton from
the solvent and is simultaneously reduced at the same
potential value to another radical anion which is capa-
ble of undergoing various chemical transformations,
e.g., protonation (Scheme 3).
The electrochemical behavior of compounds 3a–3c
was studied by cyclic voltammetry. The measurements
were carried out in acetonitrile using a platinum elec-
trode. Figure 1 shows the cyclic voltammogram (CV)
of 3a. One two-electron wave (E = 0.17 V, relative to
Ag/AgCl/KCl) with a diffusion-limited current (Fig. 2)
was observed in the potential range from 1 to –1. The
reverse potential sweep showed an oxidation peak,
indicating reversibility of the reduction process. This
peak did not disappear even at low potential sweep
rates, which suggests high stability of the reduced
species. Thus, species generated by electrochemical
reduction are stable, and they do not undergo any
chemical transformations.
Such sequence of electrochemical and chemical
processes is referred to as ECEC mechanism. However,
the reduction waves in an ECEC process should be
irreversible since the electrochemical stage is followed
by chemical reaction, whereas in our case, a reversible
It should be noted that electrochemical reduction
processes with simultaneous transfer of two electrons
E, mV
ν1/2, mV1/2/s1/2
Fig. 1. Cyclic voltammogram of compound 3a (1 mM, Pt,
Fig. 2. Plot of reduction peak current of compound 3a versus
CH2Cl2, 0.1 M Bu4NPF6, 100 mV/s).
square root of potential sweep rate.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 56 No. 7 2020

CHUGUNOV et al.
1224
Scheme 3.
Ph
Ph
Ph
Ph
Ph
Ph
Ph
e–
N
N
N
N
N
N
N
N
Ph
H
N
H
N
Ph
Ph
Ph
Ph
Ph
Ph
N
N
N
e–
Solvent
N
N
Ph
Ph
N
Ph
Ph
N
H
N
Ph
Ph
Solvent
N
N
H
Scheme 4.
Ph
Ph
e–
N
N
N
3a
N
Ph
Ph
Ph
Ph
N
N
N
N
Disproportionation
+
3a
Ph
Ph
wave is observed on the reverse CV (Fig. 1). Therefore,
ECEC mechanism is not applicable to the observed
experimental pattern, and the transformation of 3a into
dianion whose oxidation is observed in the reverse scan
should follow a different mechanism. Presumably,
radical anion generated in the first stage undergoes
very fast disproportionation to give dianion and initial
compound, and the latter is reduced again (Scheme 4).
Apart from change of the solvent, the stability of
primary radical anion can be enhanced by introducing
an electron-withdrawing substituent into the central
benzene ring. For this purpose, we examined the elec-
trochemical behavior of compounds 3b and 3c.
Like compound 3a, the CV curve of 3b showed one
reversible wave at E = 0.195 V. The reduction potential
appears at more positive potentials, which is consistent
with the electronic properties of the substituent.
However, introduction of one bromine atom did not
prevent disproportionation of the radical anion
generated in the first reduction step. When the potential
sweep rate was increased to 1 V/s (Fig. 3), no radical
anion was detected. Thus, the electrochemical behavior
of 3b conforms to Scheme 4.
If the proposed scheme is valid, the CV curve
should display a wave corresponding to a formal two-
electron process, which is indeed the case. The radical
anion formed in the first stage is likely to be unstable,
which favors its disproportionation. However, this
somewhat contradicts published data on the reduction
of quinoid compounds [6] which are known to be
reduced in two successive one-electron steps. Most
published data on the reduction of quinoid compounds
were obtained for only one, so that it is impossible to
estimate solvent effect on the stability of intermediate
species. Meanwhile, solvent effect is a very important
factor responsible for the stability of mixed-valence
states. Study of electrochemical properties of com-
pounds 3a–3c in other solvents will be performed in
the future.
Introduction of the second bromine atom into the
benzene ring qualitatively changed the electrochemical
behavior (Fig. 4). The CV curves showed two revers-
ible readuction peaks at E = 0.400 and 0.275 V even at
relatively low potential sweep rates. Unlike 3a and 3b,
the first reduction wave of 3c is strongly shifted to the
anodic region. The observed potential shift is so large
that it cannot by explained by electronic effect of
an additional bromine atom, since the reduction
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 56 No. 7 2020

SYNTHESIS AND ELECTROCHEMICAL PROPERTIES OF 2,5-DISUBSTITUTED ...
1225
E, mV
E, mV
Fig. 3. Cyclic voltammogram of compound 3b (1 mM, Pt,
Fig. 4. Cyclic voltammogram of 3c (1 mM, Pt, CH2Cl2,
CH2Cl2, 0.1 M Bu4NPF6, 100 mV/s).
0.1 M Bu4NPF6, 100 mV/s).
potential of 3b differs only slightly from the reduction
potential of 3a. Furthermore, the second reduction
potential of 3c was also more positive than the first
potential of 3a and 3b, indicating that even radical
anion derived from 3c is a stronger acceptor. Presum-
ably, compound 3c in solution tends to form a biradical
species, where the two radical centers do not interact
with each other or this interaction is weak, and the
reduction process involves these radical centers. This
may be due to distorted planar structure of the quinoid
system in 3c as a result of introduction of bromine
atoms into positions 2 and 5 of the central benzene
ring.
second stage, the radical anion is reduced to stable
dianion A which does not undergo further trans-
formations.
The obtained CV data suggest fairly facile forma-
tion of biradical from compound 3c in solution. The
biradical species is characterized by strong electron-
acceptor properties both in the neutral form and as
radical anion. This was attained via introduction of
substituents which distorted planar structure of the
quinoid system of 3c, so that the radical centers were
localized on the imidazole fragments. As a result, the
biradical system behaved formally as a monoradical.
The reduction potential of 3c is significantly shifted to
the anodic region, which is well consistent with pub-
lished data on the reduction of monoradicals.
In summary, we have synthesized 2,5-disubstituted
1,4-bis(4,5-diphenyl-1H-imidazol-2-yl)benzenes and
studied their electrochemical properties by cyclic volt-
ammetry. Unexpectedly, the reversible redox trans-
formation quinone–biradical of 2,5-disubstituted
1,4-bis(4,5-diphenyl-1H-imidazol-2-yl)benzenes has
been observed at positive potential values.
The electrochemical behavior of compound 3c can
be illustrated by Scheme 5. In the first stage, the
reduction of 3c gives stable radical anion. Taking into
account that the presence of two bromine atoms in the
central benzene ring strongly reduces the energy of the
highest occupied molecular orbital (HOMO) which is
involved in the reduction process, the reduction poten-
tial significantly shifts toward the anodic region. In the
Scheme 5.
Br
Br
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
N
N
N
N
N
N
N
N
e–
–e–
Br
Br
Br
Ph
Ph
N
N
N
e–
–e–
N
Ph
Ph
Br
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 56 No. 7 2020

CHUGUNOV et al.
1226
EXPERIMENTAL
ethanol, 60 mL of water, and 8 mL of concentrated
sulfuric acid. The mixture was refluxed for 1.5 h,
cooled to room temperature, and diluted with 200 mL
of water. The light yellow solid was filtered off and
purified by recrystallization from chloroform–petro-
leum ether.
The solvents used were preliminarily dried and
distilled. All commercially available reagents were
purchased from Sigma–Aldrich and were used as
received. The IR spectra were recorded in KBr on
an InfraLYuM FT-02 spectrometer with Fourier trans-
2-Bromoterephthalaldehyde (1b). Yield 2.56 g
(80.1%). mp 75–78°C. IR spectrum, ν, cm–1: 1696 s
1
form (Russia). The H and 13C NMR spectra were
recorded on a Jeol JNM-ECX400 spectrometer (Japan)
at 400.1 and 100.6 MHz, respectively, using CDCl3 or
DMSO-d6 as solvent and reference. Elemental analyses
were carried out with a Vario MICRO CHNS analyzer
(Germany). Analytical thin-layer chromatography was
performed on Silufol UV-254 plates (methylene
chloride–ethyl acetate, 2:1); spots were detected by
treatment with iodine vapor. Silica gel L (40–100 μm)
was used for column chromatography (eluent benzene).
1
(C=O), 1199 m, 817 m, 694 m. H NMR spectrum
(CDCl3), δ, ppm: 7.72–7.81 m (2H), 8.15 d.d (1H, J =
1.8, 0.4 Hz), 10.33 s (1H), 10.37 s (1H). 13C NMR
spectrum (CDCl3), δC, ppm: 125.3, 126.1, 130.5, 131.7,
135.4, 137.6, 189.5, 190.2 (S=O). Found, %: C 45.18;
H 2.36. C8H5BrO2. Calculated, %: C 45.11; H 2.37.
Compounds 2a–2c (general procedure). A 50-mL
round-bottom flask equipped with a reflux condenser
was charged with 0.7 mmol of dialdehyde 1a–1c,
300 mg (1.4 mmol) of benzil, 700 mg (1.9 mmol) of
ammonium acetate, and 20 mL of glacial acetic acid,
and the mixture was refluxed for 2 h. The mixture
became homogeneous and turned goldish. The mixture
was cooled, 50 mL of water and 10 mL of aqueous
ammonia were added, and the precipitate was filtered
off and dried in air. Compounds 2a–2c were isolated as
yellow finely crystalline powders.
The reduction potentials were measured with
a Gamry digital potentiostat–galvanostat (Canada)
equipped with a 5-mL electrochemical cell and coupled
with a PC. The voltammograms were recorded at dif-
ferent potential sweep rates against 0.05 M n-Bu4NBF4
(as supporting electrolyte) in acetonitrile at 20°C using
a special 10-mL electrochemical cell which was pre-
liminarily freed from atmospheric oxygen by purging
with dry argon. The cell was equipped with a stationary
platinum electrode with an area S of 0.6 cm2) and
a saturated silver chloride electrode as reference one
(0.55 V relative to Fc/Fc+ in acetonitrile). The meas-
ured potential values were corrected for ohmic losses.
1,4-Bis(4,5-diphenyl-1H-imidazol-2-yl)benzene
(2a). Yield 270 mg (75%), mp 300°C, decomposition
point 305°C. IR spectrum, ν, cm–1: 3375 m (NH),
1659 m (C=Carom), 1509 s (C=Carom), 1339 m (C=N),
1
1126 m, 895 s. H NMR spectrum, δ, ppm: 7.29–
Terephthalaldehyde (1a) and 2,5-dibromoterephthal-
aldehyde (1c) [7], imidazoles 2a–2c, and quinones 3a–
3c were synthesized according to modified procedures
described in [8, 9].
7.52 m (12H), 7.62–7.71 m (8H), 8.07 d.d (4H, J = 8.7,
1.9 Hz), 12.92 br.s (2H, NH). 13C NMR spectrum, δC,
ppm: 124.2, 126.9, 127.1, 128.6, 128.9, 130.4, 133.3,
137.7, 148.2. Found, %: C 84.05; H 5.07; N 10.86.
C36H26N4. Calculated, %: C 84.02; H 5.09; N 10.89.
2-Bromoterephthalaldehyde (1b). A 200-mL
three-necked flask equipped with a refluxed condenser,
mechanical stirrer, and thermometer, was charged with
3.7 g (0.02 mol) of 2-bromoxylene, 60 mL (0.95 mol)
of glacial acetic acid, and 56.5 mL (0.1 mol) of acetic
anhydride, and 8.5 mL (0.15 mol) of sulfuric acid was
added in small portions with continuous stirring and
cooling, maintaining the temperature at 6–12°C.
Chromium(VI) oxide, 10 g (0.1 mol), was then added
in portions at 5–10°C, and the mixture was stirred for
1.5 h at 6–12°C. The mixture was then poured into ice
water and left to stand at room temperature for 24 h,
and the precipitate was filtered off, dried in air, and
recrystallized from ethanol.
2-Bromo-1,4-bis(4,5-diphenyl-1H-imidazol-2-yl)-
benzene (2b). Yield 350 mg (84%), mp 300°C,
decomposition point 315°C. IR spectrum, ν, cm–1:
3360 m (NH), 1662 m (C=Carom), 1504 s (C=Carom),
1343 m (C=N), 1130 m, 894 s, 698 s (C–Br). 1H NMR
spectrum, δ, ppm: 7.45–7.64 m (12H), 7.78 d.d (1H,
J = 8.5, 1.9 Hz), 7.83–7.92 m (4H), 7.95–8.02 m (3H),
8.08 d.d (1H, J = 1.9, 0.4 Hz), 12.71 br.s (2H, NH).
13C NMR spectrum, δC, ppm: 122.7, 124.2, 126.3,
126.8, 126.9, 127.1, 127.8, 128.6, 129.0, 130.4, 130.7,
133.3, 137.7, 148.2. Found, %: C 72.66; H 4.24;
N 9.46. C36H25BrN4. Calculated, %: C 72.85; H 4.25;
N 9.44.
2,5-Dibromo-1,4-bis(4,5-diphenyl-1H-imidazol-
2-yl)benzene (2c). Yield 330 mg (70%), mp 300°C,
decomposition point 325°C. IR spectrum (KBr), ν,
A 250-mL round-bottom flask equipped with a re-
flux condenser was charged with 6.26 g (0.015 mol) of
the corresponding acetoxy derivative, 60 mL of
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SYNTHESIS AND ELECTROCHEMICAL PROPERTIES OF 2,5-DISUBSTITUTED ...
1227
cm–1: 3325 m (NH), 1672 s (C=Carom), 1594 m
(C=Carom), 1344 m (C=N), 1284 s, 694 s (C–Br).
1H NMR spectrum (DMSO-d6), δ, ppm: 7.45–7.63 m
(12H), 7.87 d.d.d (4H, J = 7.1, 2.0, 1.5 Hz), 7.93 d.d.d
(4H, J = 7.7, 1.7, 1.5 Hz), 8.08 d (2H, J = 0.4 Hz),
12.75 br.s (2H, NH). 13C NMR spectrum (CDCl3), δC,
ppm: 120.7, 124.2, 126.3, 126.9, 128.6, 128.9, 129.2,
130.4, 130.7, 133.3, 137.7, 148.2. Found, %: C 64.32;
H 3.61; N 8.36. C36H24Br2N4. Calculated, %: C 64.30;
H 3.60; N 8.33.
13C NMR spectrum (DMSO-d6), δC, ppm: 108.1, 114.2,
121.4, 127.2, 128.7, 128.9, 129.2, 132.2, 133.2, 134.9,
150.6, 158.9. Found, %: C 73.12; H 3.91; N 9.46.
C36H23BrN4. Calculated, %: C 73.10; H 3.92; N 9.47.
2,5-Dibromo-3,6-bis(4,5-diphenyl-2,3-dihydro-
1H-imidazol-2-ylidene)cyclohexa-1,4-diene (3c).
Yield 270 mg (30.2%), mp 172–174°C (from PhH–
C8H18). IR spectrum (KBr), ν, cm–1: 1609 m (C=Carom),
1570 m (C=Carom), 1333 m (C=N), 1282 m, 896 s,
1
699 s (C–Br). H NMR spectrum (DMSO-d6), δ, ppm:
7.55–7.67 m (12H), 7.78–7.85 m (8H), 8.02 s (2H).
13C NMR spectrum (DMSO-d6), δC, ppm: 115.2, 120.4,
128.2, 129.2, 129.8, 131.9, 133.0, 133.2, 158.9.
Found, %: C 64.47; H 3.32; N 8.39. C36H22Br2N4. Cal-
culated, %: C 64.50; H 3.31; N 8.36.
Quinones 3a–3c (general procedure). A 100-mL
four-necked flask equipped with a mechanical stirrer,
dropping funnel, and gas inlet system was charged with
12.5 mL of 6% aqueous potassium hydroxide and
a suspension of 1 mmol of compound 2a–2c in 25 mL
of dioxane. A 20% solution of K3[Fe(CN)6], 100 mL,
was added dropwise over a period of 2 h with con-
tinuous stirring under argon, maintaining the tempera-
ture at 5–7°C. After addition of first drops of
K3[Fe(CN)6] solution, the mixture turned purple and
then changed to green. When the addition of
K3[Fe(CN)6] was complete, the mixture was stirred
under argon at 5–7°C for 30 min, and the precipitate
was filtered off, washed on a filter with a large amount
of water, and dried in air. The products were violet
finely crystalline powders. To remove the impurity of
unreacted initial compound, the product was dissolved
in 50 mL of benzene, the undissolved material was
separated, the benzene solution was evaporated to
a volume of 10 mL under reduced pressure, 5 mL of
octane was added to the residue, and the precipitate
was filtered off and dried in air. Quinones 3a–3c were
isolated as green finely crystalline powders.
FUNDING
This study was performed under financial support by the
Ministry of Science and Higher Education of the Russian
Federation in the framework of state assignment in the field
of research activity (project no. 4.4566.2017/8.9).
CONFLICT OF INTEREST
The authors declare no conflict of interest.
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1
1333 m (C=N), 1290 m, 895 s. H NMR spectrum
(DMSO-d6), δ, ppm: 6.95 d (4H, J = 10.2 Hz), 7.52–
7.62 m (12H), 8.07 d.d.d (8H, J = 7.9, 1.7, 1.5 Hz).
13C NMR spectrum (DMSO-d6), δC, ppm: 107.1, 128.2,
129.1, 131.2, 132.1, 133.2, 134.9, 140.1, 159.8.
Found, %: C 84.32; H 4.71; N 10.96. C36H24N4. Cal-
culated, %: C 84.35; H 4.72; N 10.93.
2-Bromo-3,6-bis(4,5-diphenyl-2,3-dihydro-1H-
imidazol-2-ylidene)cyclohexa-1,4-diene (3b). Yield
140 mg (23.6%), mp 157–158°C (from PhH–C8H18).
IR spectrum (KBr), ν, cm–1: 1604 m (C=Carom), 1566 m
(C=Carom), 1326 m (C=N), 1280 m, 894 s, 694 s
(C–Br). 1H NMR spectrum (DMSO-d6), δ, ppm: 7.51–
7.58 m (4H), 7.58–7.68 m (8H), 7.78–7.85 m (4H),
7.93–8.04 m (2H), 8.12 s (1H), 8.20–8.26 m (4H).
https://doi.org/10.1246/nikkashi.1972.100
9. Sakaino, Y., Kakisawa, H., Kusumi, T., and Maeda, K.,
J. Org. Chem., 1979, vol. 44, p. 1241.
https://doi.org/10.1021/jo01322a010
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