N-substituted 2,5-Di(2-thienyl)pyrroles. Synthesis and electrochemical properties

Article abstract of DOI:10.1134/S1070363213040208

2,5-Dithienylpyrroles containing p-substituted benzene ring at the nitrogen atom were synthesized. The formylation and subsequent crotonic condensation of N-(4-nitrophenyl)-2,5-di (2-thienyl)pyrrole was performed. Electrochemical behavior of the compounds

Full text of DOI:10.1134/S1070363213040208

ISSN 1070-3632, Russian Journal of General Chemistry, 2013, Vol. 83, No. 4, pp. 726–730. © Pleiades Publishing, Ltd., 2013.
Original Russian Text © T.S. El’shina, E.A. Sosnin, E.V. Shklyaeva, G.G. Abashev, 2013, published in Zhurnal Obshchei Khimii, 2013, Vol. 83, No. 4,
pp. 644–648.
N-Substituted 2,5-Di(2-thienyl)pyrroles.
Synthesis and Electrochemical Properties
T. S. El’shinaa, E. A. Sosninb, E. V. Shklyaevac, and G. G. Abashevb,c
a Perm State National Research University, ul. Bukireva 15, Perm, Russia
b Institute of Technical Chemistry, Ural Branch, Russian Academy of Sciences, Perm, Russia
e-mail: gabashev@psu.ru
c Institute of Natural Sciences, Perm, Russia
Received March 1, 2012
Abstract―2,5-Dithienylpyrroles containing p-substituted benzene ring at the nitrogen atom were synthesized.
The formylation and subsequent crotonic condensation of N-(4-nitrophenyl)-2,5-di (2-thienyl)pyrrole was
performed. Electrochemical behavior of the compounds and electrochromic properties of 2,5-di(2-thienyl)-
pyrrole containing p-semidine fragment at its nitrogen atom were studied.
DOI: 10.1134/S1070363213040208
Among the conducting polymers polypyrrole and
polythiophene are of particular interest due to their
high conductivity, stability in the oxidized state, inte-
resting redox properties, and simplicity and acces-
sibility of the preparation of the starting monomers,
pyrroles and thiophenes [1–3]. The polypyrrole
conductivity is 10–10 to 10–5 Cm cm–1, the bandgap is
4 eV [4]. The introduction into its molecule of
counterions (doping) leads to a manyfold increase in
the conductivity, and the polymer can behave as a
semiconductor (bandgap ≤ 2.5 eV) or metal [4, 5]. The
polythiophenes conductivity is also high, bandgap 1.0–
1.2 to 0.58 eV, and they are more stable. From this
point of view it is interesting to obtain conjugated
polymers that include both heterocycles simultane-
ously. This can be achieved in three ways [2]:
preparation of thiophene and pyrrole copolymers from
a mixture of the corresponding monomers, copoly-
merization of pyrrole and bi- or α-terthiophenes, and
synthesis and further polymerization of substituted
monomers incorporating in their structure both thio-
phene and pyrrole heterocycles, like substituted 2-(2-
thienyl)pyrrole and 2,5-di(2-thienyl)pyrrole. The synthesis
of unsubstituted and substituted 2,5-di(2-thienyl)pyrro-
les and the study of their electrochemical behavior
were carried out by many authors. A review [2] com-
prises the methods of preparation, electrochemical
behavior, and electrochemical and chemical polymeriza-
tion of N-substituted 2,5-di(2-thienyl)pyrroles.
This paper describes the synthesis and properties of
2,5-di(2-thienyl)pyrroles containing substituted
a
benzene ring at the nitrogen atom. For the preparation
we used the Paal–Knorr method. The p-nitrophenyl
derivative II was synthesized and electrochemically
investigated earlier [6, 7], but the chemical
transformations of 2,5-di(2-thienyl)pyrrole were not
described. By reduction of the nitro group in II with
sodium sulfide we synthesized the amino derivative
SNS-IV, previously synthesized by heating diketone I
with 1,4-diaminobenzene in the medium of propionic
acid and toluene [11]. By formylation of II in
dichloroethane along the Vilsmeier–Haack method we
isolated aldehyde V in a good yield.
In turn, by heating aldehyde V with 2 acetyl-
thiophene in 10% NaOH solution in alcohol we syn-
thesized new chalcone VI, a red-orange crystalline
solid. The preparation and study of dithienylpyrrole III
is of particular interest because its structure contains
two polymerizable fragment, 2,5-di(2-thienyl)pyrrole
and N-semidine, the latter being actually a dimer of
aniline (>NC6H4NHPh). The structure of the com-
1
pounds obtained was confirmed by H NMR spec-
troscopy and the gas chromatography-mass spectrometry.
Electrochemical behavior of the compounds
obtained in the work was studied using cyclic volt-
ammetry, at the same time we obtained the
electrochemical characteristics of the already described
726

N-SUBSTITUTED 2,5-DI(2-THIENYL)PYRROLES.
727
EtOH, Na2S,
boiling,
H2N
NO2
H2N
NH2
N
N
24 h
S
S
S
S
I
S
toluene/xelene,
24 h, boiling,
Ar
C2H5COOH
toluene, 24 h
2
O
NO2
NO2
I
II
IV
1,2-dichloroethane, DMF,
POCl3, 5 h, 80oC
HN
toluene: AcOH (1:1),
72 h, boiling, Ar
2-ThCOCH3, EtOH,
NaOH, Δ, 5_12 h
NH2
N
N
S
S
S
S
O
H
S
O
N
S
S
NO2
NO2
V
VI
NH
III
icantly. All compounds were polymerized, and on the
working electrodes a dark green or blue film formed.
compound II under the conditions used in this study.
For the measurements potentiostats P8 and IPC
Compact, three-electrode cell with the working
electrode, reference silver chloride electrode, and
auxiliary platinum electrode were used. The monomer
concentration was 1×10–3 M, the concentration of
supporting electrolyte 0.1 M, solvent acetonitrile. To
determine the best conditions for the electrochemical
polymerization of the compounds (monomers) we
performed initial oxidation using a glassy carbon
electrode changing the rate of the scanning potential
(Vscan) in the range of 50–500 mV s–1, as well as the
composition of supporting electrolyte (Et4NClO4 or
Bu4NPF6). After selecting optimum conditions using
the glassy carbon electrode, Et4NClO4 as supporting
electrolyte, and Vscan of 100 mV s–1 the further
measurements were carried out with an ITO electrode
(a glass plate covered with a layer of indium–tin
oxide). Figures 1 and 2 are examples of cyclovolt-
The electrochemical polymerization proceeds most
typically with dithienylpyrrole III containing at a
nitrogen atom the p-semidine residue.
I, μA
Е, mV
ammogramms obtained under these conditions, at Vscan
=
Fig. 1. Cyclovoltammogram of compound III (glassy carbon,
100 mV s–1. The comparison of Figs. 4a and 4b shows
that the nature of the redox curves can vary signif-
1 cycle, (1) Et4NClO4, (2) Bu4NClO4, Vscan 100 mV s–1
,
MeCN).
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728
EL’SHINA et al.
The curves obtained in the electrospectrophoto-
I, μV
metric studies are shown in Fig. 4. We used a
spectrophotometer SF2000, P8 potentiostat, a three-
electrode cell (photometric cell) with a ITO working
electrode, silver-silver chloride reference electrode,
and platinum auxiliary electrode, monomer concentra-
tion [c] = 3×10–4 mol. A film of the polymer was pre-
deposited electrochemically on the ITO-electrode
before the registration of the photometric curves. The
potential applied to the plate, increased abruptly as
shown in Fig. 4. At high values of potential (Eox
>
1000 mV), the film became almost black and its color
did not change (irreversible oxidation). At low limiting
potential (Eox < 1000 mV) a change in the film color
was observed from colorless to green and back
(reversible redox-transformation).
Е, mV
Fig. 2. Cyclovoltammogram of compound III (ITO, 10 cycles,
Bu4NClO4, Vscan 100 mV s–1, MeCN).
Figure 1 shows the cyclic voltammograms of III
obtained with the glassy carbon electrode with
different supporting electrolytes, Et4NClO4 (1) and
Bu4NPF6 (2). Figure 2 shows the cyclovoltammograms
obtained with the ITO-electrode (supporting
electrolyte Bu4NPF6). As seen from Fig. 2, with
increasing number of cycles the current density grows,
which is due to the rapid growth of the polymer film
on the electrode surface. In the voltammograms the
Figure 4 shows that with the growth of the potential
the absorption of the film in the region of 700–950 nm
increases dramatically. Also the absorption of film in
the regions of 500–700 nm and 950–1000 nm slightly
increases, which is probably due to an increase in the
film thickness at the electropolymerization. The film of
poly-III was studied by cyclic voltammetry in a
solution of supporting electrolyte (Fig. 5). The arrows
indicate the values of potentials Eox onset and Ered onset
.
reversible oxidation and reduction peaks (E1 = 725
Based on these values we calculated ionization
potential IP, electron affinity EA, and the difference of
ox
mV, E1 = 525 mV) and an irreversible reduction
red
peak, E2 = 912 mV can be seen. We found that
the HOMO–LUMO energy levels in the resulting film
red
onset
compound III exhibits electrochromic properties: in
the process of electrochemical oxidation in the
conditions of cyclic voltammetry (CVA) the color of
the film at the working electrode changes (light →
dark → light → dark). Figure 3 shows a view of the
film formed on the ITO electrode, its reduced and
oxidized states, as well as the photo of the oxidized
film obtained with a Metam PB 21 microscope (6c).
by the method described in [15, 16]. E , 505 mV,
ox
onset
red
E
480 mV, IP 4.86 eV, EA 3.87 eV; HOMO–
LUMO 0.99 eV.
EXPERIMENTAL
1H NMR spectra were obtained on a Mercury
plus300 instrument, internal reference HMDS. Mass
spectra were obtained on an Agilent GC 6890N MSD
(а)
(b)
(c)
Fig. 3. Film of compound III grown electrochemiclly: (a) grown on ITO electrode, reduced state, (b) grown on ITO electrode,
oxidized state, and (c) photo of oxidized film.
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N-SUBSTITUTED 2,5-DI(2-THIENYL)PYRROLES.
729
D
I, μA
Е, mV
λ, nm
Fig. 4. Electron absorption spectrum of poly-III film at Eox
500–1000 mV: (1) 500, (2) 700, (3) 900, (4) 1000 mV.
Fig. 5. Cyclovoltammogram of poly-III film.
5975B gas chromatography–mass spectrometer (EI,
70 eV). The monitoring of reactions was performed
and purity of the obtained compounds was checked by
TLC (Silufol, Cavalier). The separation of mixtures
and purification of target products was performed on a
column filled with silica gel (Lancaster, Silica gel 60,
0.060–0.2mm). Electrochemical studies were per-
formed on potentiostat P8 (Elins) and IPC Compact
(Volta), an electrochemical sensor module EM-04. The
data were processed with the IPC-compact 8.60F software.
thienyl)pyrrole II (7.6 mmol) and 1.84 g of sodium
sulfate (5 mmol) in 75 ml of ethanol was heated on a
boiling water bath for 24 h. The residue obtained after
evaporation of ethanol was purified by column
chromatography (SiO2, eluent, methylene chloride).
Yield 30%, white crystalline solid, mp 160–161°C. 1H
NMR spectrum (CDCl3, δ, ppm, J, Hz): 3.87 br.s (2H,
NH2), 6.51 s (2H, pyrrole), 6.62 d (2H, thiophene, J =
3.6). 6.70, d (2H, phenyl, J = 8.4), 7.02, d (2H,
thiophene, J = 5.1). 7.07, d (2H, phenyl, J = 8.4). Mass
spectrum, m/z, (I, %): 324 (11), 323 (22) [M+ + H], 322
(100), [M+], 212 (22.5), 65 (13).
Succinyl chloride was obtained according to [12],
1,4-di(2-thienyl)butane-1,4-dione according to [13], 2-
acetylthiophene was synthesized according to [14].
1-(4-Nitrophenyl)-2-(2-thienyl)-5-(5-formylthi-
enyl)pyrrole (V). To a solution of 2.18 g of 1-(4-
nitrophenyl)-2,5-di(2-thienyl)pyrrole (6 mmol) in 60
ml of water-free 1,2-dichloroethane at room tem-
perature were added sequentially 0.47 ml of water-free
DMF (0.44 g, 6 mmol) and 0.55 ml of POCl3 (0.9 g,
6 mmol), and the reaction mixture was heated for 5 h
at a temperature not exceeding 80◦C, cooled, poured
into 300 ml of ice-cold water, carefully acidified with
10% HCl, extracted with CH2Cl2, then solvent was
distilled off. The residue was separated by column
chromatography (SiO2, eluent methylene chloride) to
give the V in the form of yellow crystals. Yield 44%,
1-[4-(4-Phenyl)aminophenyl]-2,5-di(2-thienyl)-
pyrrole (III). To a mixture of 0.5 g of 1,4-di(2-
thienyl)butane-1,4-dione (2 mmol) and 1.47 g of p-
semidine (8 mmol) was added a solution of 20 ml of
toluene and acetic acid (1:1). The reaction mixture was
refluxed for 72 h under argon, cooled, solvents were
evaporated, the residue was purified by column
chromatography (SiO2, eluent methylene chloride).
Yield 70%, white crystalline solid, mp 193–195°C. 1H
NMR spectrum (CDCl3, δ, ppm, J, Hz): 5.89 br.s (1H,
NH), 6.53 s (2H, pyrrole), 6.65 d.d (2H, thiophene,
J1 = 3.6, J2 = 1.2) , 6.84 m (2H, thiophene, J1 = 3.6),
6.96–7.02 m (2H, thiophene), 7.04–7.8 m (4H, C6H4),
7.13–7.18 m (4H, C6H4), 7.31 m (1H, C6H4, J = 8.1).
Mass spectrum (m/z, %): 400 (12.6) [M+ +2H], 399
(28.5) [M+ + H], 398.1 (100) [M+]. C24H18N2S2. Mass-
spectrum, m/z (I, %): 400 (12.6), [M + H]+, 398.1 (100)
[M]+, 197 (15).
1
mp 1790–180°C. H NMR spectrum (CDCl3, δ, ppm,
J, Hz): 6.34 d.d (1H, thiophene, J1 = 3.6 J2 = 1.2), 6.8 t
(1H, thiophene, J = 3,6), 6.96 d (1H , pyrrole, J = 3.6),
6.98 d (1H, pyrrole, J = 5.1), 7.03 d (2H, thiophene,
J = 7.2), 7.20 d (1H, thiophene, J = 4.5), 7.34 d (2H,
phenyl , J = 8.7), 7.38 d (1H, thiophene, J = 5.1). 8.20
d (2H, phenyl, J = 8.7), 9.84 s (1H, HCO). Mass
spectrum m/z (I, %): 382 (12) [M + 2H]+, 381 (24)
[M + H] +, 380 (100) [M]+, 335 (14), 76 (10).
1-(4-Aminophenyl)-2,5-di(2-thienyl)pyrrole (IV).
A mixture of 2.7 g of 1-(4-nitrophenyl)-2,5-di(2-
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EL’SHINA et al.
(2E)-3-[1-(4-Nitrophenyl)-2,5-di(2-thienyl)-1H-
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solution of NaOH was refluxed for 12 h. After cooling,
the reaction mixture was acidified to pH ~ 3, poured
into cold water, extracted with methylene chloride, the
solvent was removed, the residue was chromato-
graphed on a column (SiO2, eluent methylene chlo-
ride). Yield 70%, orange-red substance, mp (decomp)
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1
222–224°C. H NMR spectrum (CDCl3, δ, ppm, J,
Hz): 6.56 d (2H, pyrrole, J = 3.3), 6.61 m (2H,
thiophene), 6.84 m (2H, thiophene), 6.88 t (1H,
thiophene, J = 4.5),. 6.99 t (1H, thiophene, J = 4.5),
7.21 d.d (1H, thiophene, J1 = 5.4, J2 = 1.5), 7.26–7.32
m (3H, 2H benzene + 1H CH=), 7.48 d.d (1H,
thiophene, J1 = 4.2, J2 = 1.5), 7.51 d (1H, CH=, J =
15.9), 8.16 d (2H, phenyl, J = 9.3). Compound VI
entered in the chromatographic column of the gas
chromatography–mass spectrometer gave a single pek
with the retention time 12.32 min. Mass spectrum did
not contain the molecular ion peak, the observed peaks
were attributable to peaks of molecular ions of the
decomposition products of the chalcone. The presence
of these fragments in the mass spectrum was an
indirect proof of the chalcone structure. Mass
spectrum, m/z, (I, %): 421 (26) [M + H]+, 420 (100)
[M]+, 405 (11.7) 379 (10), 378 (20) [1-(4-nitrophenyl)-
2-(thiophen-2-yl)-5-(5-vinylthiophen-2-yl)pyrrole + H]+,
377 (84) [1-(4-nitrophenyl)-2-(thiophen-2-yl)-5-(5-
vinylthiophen-2-yl)pyrrole – H]+, 359 (21), 332 (10),
331 (36), 330 (18.5), 329 (12.5), 297 (12), 229 (12), 76
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ACKNOWLEDGMENTS
This work was supported by Russian Foundation
for Basic Research (grant no. 10-03-96038-r-ural-a)
and the Ministry of Education and Science Analytical
departmental target program “Development of
Scientific Potential of Higher Education.”
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 83 No. 4 2013