Triarylpyridinium salts: Synthesis and electrochemical properties

Article abstract of DOI:10.1002/poc.3930

The electrochemical properties of the triarylpyridinium salts were studied. The effect of functional substituents on redox properties has been investigated in detail. Electrochemical reduction of salts to the anion in acetonitrile is the result of two one

Full text of DOI:10.1002/poc.3930

Received: 3 April 2018
DOI: 10.1002/poc.3930
Revised: 8 October 2018
Accepted: 5 December 2018
R E S E A R C H A R T I C L E
Triarylpyridinium salts: Synthesis and electrochemical
properties
Alexandr V. Dolganov
| Boris S. Tanaseichuk | Margarita K. Pryanichnikova |
Alina Y. Ivleva | Oksana Y. Chernyaeva | Karina A. Grigoryan | Ekaterina A. Chugunova |
Veronika Y. Yurova
Department of Chemistry, Mordovian
Abstract
Ogarev State University, Saransk, Russia
The electrochemical properties of the triarylpyridinium salts were studied. The
Correspondence
A.V. Dolganov, Department of Chemistry,
effect of functional substituents on redox properties has been investigated in
detail. Electrochemical reduction of salts to the anion in acetonitrile is the
Mordovian Ogarev State University,
Saransk 430005, Russia.
result of two one‐electron‐transfer steps through the stage of the radical forma-
tion and its subsequent reduction (EE electrochemical mechanism). A correla-
tion between the reduction potentials and the structure of the salts was
established.
Email: dolganov_sasha@mail.ru
Funding information
Russian Foundation for Basic Research,
Grant/Award Number: 18‐03‐00211;
Ministry of Education and Science of the
Russian Federation within the framework
of government task in the field of scientific
activities, Grant/Award Number:
4.4566.2017/8.9
KEYWORDS
electrochemical reduction, metal‐free electrocatalyst, redox property, triarylpyridinium salt
1 | INTRODUCTION
pH, and oxygen availability).[6] Thus, the development of
new‐generation catalysts for the electrochemical process
Despite the significant progress made in recent years in
the field of hydrogen energy, the practical use of the
developed technologies is still very limited due to the
low efficiency of synthetic catalysts.[1,2] Work in this area
aimed to obtaining of catalysts based on transition metals
such as cobalt, iron, or copper, as well as biomimetic
models of hydrogenases.[3–5] In present, many complexes
of transition metals have a high activity in the electrocat-
alytic generation of hydrogen including those that exceed
the hydrogenases by catalytic activity and the magnitude
of the overpotential.[6–11] Despite this, none of the large
variety of molecular complexes has combined low
overpotential with availability, high activity, and stability,
which are a prerequisite for an economically viable cata-
lyst.[12] Although the hydrogenases possess high catalytic
activity, in the absence of overpotential, their use on an
industrial scale is not possible due to the complexity of
their production in large quantities and their extremely
high sensitivity to environmental conditions (temperature,
of obtaining molecular hydrogen is an important and
actual task.
Previously, we first described a new type metal‐free
electrocatalyst for the hydrogen evolution reaction
on base of the 9‐phenyl‐N‐phenylacridinium iodide
(PhAcrI).[13,14] The proposed catalytic system equally
effectively catalyzed the process of electrochemical hydro-
gen production from both aprotic solvents in the presence
of perchloric acid and from water at different pH values.
Here, we report the synthesis and electrochemical proper-
ties of new heterocyclic salts based on 2,4,6‐triaryl
pyridinyl derivatives, close analogues of acridine deriva-
tives (Scheme 1). The presence of substituents in the 2‐,
4‐, and 6‐ positions is necessary for shielding the reactive
intermediates formed during the electrocatalytic process.
When reducing unsubstituted pyridium salts, the radicals
formed in the first stage have high chemical activity and
rapidly are reacted, particular are easily dimerized.[15]
Thus, the absence of substituents in the presented
J Phys Org Chem. 2019;e3930.
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© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/poc.3930

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DOLGANOV ET AL.
2.2.4 | N‐Phenyl(p‐bromophenyl)‐2,4,6‐
triphenylpyridinium perchlorate (2c)
1
Yield 0.6 (43.5%). H NMR (CD2Cl2): δ 7.37‐7.66 (17H);
8.32‐8.34 m (2H); 8.65 s (2H). 13C NMR (CD2Cl2): δ
125.52, 127.47, 128.90, 132.35, 133.96, 155.86, 128.91.
SCHEME 1 General view heterocyclic salts based on 2,4,6‐triaryl
pyridinyl perchloroate
2.2.5 | N‐Phenyl(p‐tolyl)‐2,4,6‐
triphenylpyridinium perchlorate (2d)
1
positions leads to a rapid loss of catalytic аctivity of the
сatalyst duе to the оccurrence of side сhemical processеs.
Yield 0.78 g (65.53%). H NMR (CD2Cl2): δ 2.08 s (3H);
6.97, 7.3, 7.44, 7.66 m (17H); 8.33 d (2H); 8.61 s (2H).
13C NMR (CD2Cl2): δ 21.06, 125.76, 128.62, 130.23,
130.46, 133.78, 134.04, 139.98, 155.99, 156.96.
2 | MATERIAL AND METHODS
2.1 | Materials
2.2.6 | N‐Aryl‐2,6‐diphenyl‐4‐(p‐
methoxyphenyl)pyridinium perchlorates
(3a‐d)
Reagents and solvents were purchased from Aldrich and
used as received unless otherwise noted.
N‐Phenyl‐2,6‐diphenyl‐4‐(p‐methoxyphenyl)
pyridinium perchlorate (3a)
2.2 | Synthetic procedures
A mixture of 0.44 g (1 mmol) of 2,6‐diphenyl‐4‐(p‐
methoxyphenyl)pyrilium perchlorate (1a) and 7 mL of
ethanol was boiled for 20 minutes, and then a solution
of 0.19 g (2 mmol) of aniline in 3 mL of ethanol was
added. The reaction mixture was boiled for 1 hour;
0.4 mL of triethylamine was added, and the mixture
was boiled for an additional 3 hours until complete disso-
lution (1a). At the end of the reaction, the mixture was
cooled to room temperature. The precipitate that formed
was filtered off, dried in air, and purified by recrystalliza-
tion from isobutanol. We obtained shiny crystals (3a) of
2.2.1 | N‐Phenyl‐2,6‐diphenyl‐4‐
arylpyridinium perchlorates (2a‐d)
A mixture of 2.4 mmol of 2,6‐diphenyl‐4‐arylpyridium
perchlorate (1a‐d), 2.4 mmol of arylamine, 0.33 mL
(2.4 mmol) of triethylamine, and 10 mL of chloroform
was kept at room temperature and with constant stirring
for 10 to 15 minutes then 0.4 M (7.5 mmol) of glacial acetic
acid and stirring was continued for an additional 40 to
60 minutes during which time the discoloration of the solu-
tion was observed. After that, approximately 10 to 15 mL
of ethanol was added to the reaction mixture. The precipi-
tate that formed was filtered off. We dried it in the air.
1
white color. Yield: 0.38 g (74%). H NMR (CD2Cl2): δ
3.49 s (3H); 6.76‐7.04 m (17H); 7.96‐8.00 d (2H); 8.18 s
(2H). 13C NMR (CD2Cl2): δ 55.91, 115.41, 123.88, 125.43,
128.19, 129.84, 131.05, 133.42, 139.18, 155.01, 156.05,
163.32.
2.2.2 | N ‐Phenyl‐2,4,6‐triphenylpyridinium
perchlorate (2a)
N‐(p‐Methoxyphenyl‐2,6‐diphenyl‐4‐(p‐
methoxyphenyl)pyridinium perchlorate (3b)
1
Yield 0.82 g (70.7%). H NMR (CD2Cl2): 6.97, 7.3, 7.44,
A mixture of 0.44 g (1 mmol) of 2,6‐diphenyl‐4‐(p‐
methoxyphenyl)pyrilium perchlorate and 7 mL of ethanol
was boiled for 20 minutes, and then a solution of 0.12 g
(1 mmol) of p‐anisidine in 3 mL of ethanol was added.
The reaction mixture was boiled for 1 hour; 0.4 mL of
triethylamine was added, and the mixture was boiled for
another 2 hours until complete dissolution (1a). After
completion of the reaction, the mixture was cooled to
room temperature. The precipitate was filtered off, dried
in air, and purified by recrystallization from isobutanol,
7.66 m (20H); 8.33 d (2H); 8.61 s (2H). 13C NMR (CD2Cl2):
δ 21.06, 125.76, 128.62, 130.23, 130.46, 133.78, 134.04,
139.98, 155.99, 156.96.
2.2.3 | N‐Phenyl(p‐methoxyphenyl)‐2,4,6‐
triphenylpyridinium perchlorate (2b)
1
Yield 0.84 g (61.6%). H NMR (CD2Cl2): δ 3.57 s (3H);
6.69, 6.71, 7.3, 7.46, 7.66 m (15H); 8.31, 8.60 d (4H). 13C
NMR (CD2Cl2): δ 55.92, 114.06, 125.72, 128.66, 130.23,
130.48, 133.87, 155.96, 157.24, 159.96.
1
crystals (3b) green color. Yield: 0.35 g (64%). H NMR
(CD2Cl2): δ 3.17 (3H), 3.48 s (3H), 6.27‐6.29 d (2H),

DOLGANOV ET AL.
3 of 6
6.76‐7.04 m (14H), 7.94‐7.97 m (2H), 8.14 s (2H). 13C
NMR (CD2Cl2): δ 55.50, 55.88, 113.61, 115.38, 123.87,
125.48, 128.22, 129.79, 130.99, 133.59, 154.82, 156.45,
159.48, 163.25.
2.4 | Spectroscopy
NMR spectrums 1Н and 13С solutions of bonds in СDCl3‐
d6 and DMSO‐d6 are written down on spectrometer JNM‐
ECX400 of firm JEOL (400 MHz). Intrinsic standard
hexamethyldisiloxane (GMDS). Conditions of thin‐layer
chromatography : adsorbent ‐ Silufol UV‐254, eluent ‐ ethyl
ether and ethyl acetate (1:1).
N‐(p‐Bromophenyl‐2,6‐diphenyl‐4‐(p‐methoxyphenyl)
pyridinium perchlorate (3c)
A mixture of 0.44 g (1 mmol) of 2,6‐diphenyl‐4‐(p‐
methoxyphenyl)pyrylium perchlorate and 7 mL of etha-
nol was boiled for 20 minutes, and then a solution of
0.17 g (1 mmol) of p‐bromoaniline in 5 mL of ethanol
was added. The reaction mixture was boiled for 1 hour;
0.4 mL of triethylamine was added, and the mixture
was boiled for a further 2 hours before the complete dis-
solution (1b). After the reaction was completed, the mix-
ture was cooled to room temperature. The precipitate was
filtered off, dried in air, and purified by recrystallization
from isobutanol, crystals (3c). Yield: 0.34 g (57%). 13C
1H NMR (CD2Cl2): δ 123.71, 125.77, 128.78, 130.28,
132.14, 133.48, 133.98, 156.43, 156.73.
3 | RESULTS AND DISCUSSION
Synthesis of 2,4,6‐triarylpyridinyl salts was carried out
according to the following scheme. Triarylpyridinium salt
(2a‐d and 2a‐d) was obtained from the corresponding
pyrilium salts (1a and 1b) (Scheme 2).
The resulting pyridinium salts 2a‐d and 3a‐d in con-
trast to the intensely stained pyridium salts 1a‐d are usu-
ally colorless (2a‐d, 3d, 3d) or slightly colored (3b, 3c)
crystalline compounds melting at high temperature
(approximately 250°C) with decomposition. Compared
with the initial pyrilium salts, the salts 2a‐d and 3a‐d
are better soluble in organic solvents.
N‐(p‐Tolyl‐2,6‐diphenyl‐4‐(p‐methoxyphenyl)
pyridinium perchlorate (3d)
A mixture of 0.44 g (1 mmol) of 2,6‐diphenyl‐4‐(p‐
methoxyphenyl)pyrilium perchlorate and 7 mL of ethanol
was boiled for 20 minutes, and then a solution of 0.1 g
(1 mmol) of p‐methylaniline in 3 mL of ethanol was
added. The reaction mixture was boiled for 1 hour;
0.4 mL of triethylamine was added, and the mixture
was boiled for another 2 hours until complete dissolution
(1b). After the reaction was completed, the mixture was
cooled to room temperature. The precipitate was filtered
off, dried in air, and purified by recrystallization from
isobutanol. Yield: 0.39 g (74%). 1H NMR (CD2Cl2): δ
3.39, 4.47, 6.58, 7.13, 7.18, 7.29, 7.63, 7.66, 7.67. 13C
NMR (CD2Cl2): δ 55.48, 114.24, 120.67, 126.22, 128.85,
133.31, 136.98, 138.94, 152.37, 160.74, 190.07, 196.23.
All the compounds obtained are capable of oxidation
and a reduction in a wide range of potentials (Table 1).
All the waves observed on voltammograms corre-
spond to processes with diffusion limitation in the sweep
2.3 | Cyclic voltammetry
The CV experiments were carried out in acetonitrile with
0.1M [n‐Bu4N][BF4] as a supporting electrolyte using a
Gamry Instruments Reference 3000 (10 mL of solution).
Glass carbon electrode (S = 0.125 cm2) was used as a
working electrode. The sweep rate of the potential was
200 mV/s. The electrode was carefully polished and
washed before and between measurements. A platinum
counter electrode and a standard Ag/AgCl reference elec-
trode were used. Ferrocene was used as an internal stan-
dard: the E0(Fc+/Fc) = 0.31 V versus Ag/AgCl in
acetonitrile.
SCHEME 2 General route of synthesis of 2,4,6‐triarylpyridinyl
salts

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DOLGANOV ET AL.
TABLE 1 Electrochemical characteristics (mV) for the
triarylpyridinium salt (CH3CN, GC electrode, 200 mV/s, rel. Ag/
AgCl/KCl)
Compound Reduction, E1red ΔE Reduction, E2red ΔE
2a
2b
2c
2d
3a
3b
3c
3d
–0.91
–0.91
–0.90
–0.92
–0.93
–0.93
–0.90
–0.92
–0.80
–0.78
–0.81
–0.78
–0.86
–0.8
0.11 –1.01
0.13 –1.06
0.11 –1.07
0.12 –1.03
0.17 –1.15
0.13 –1.27
0.1 –1.20
0.1 –1.18
–0.94
–0.98
–0.97
–0.92
–1.06
–1.18
–1.12
–1.10
0.07
0.08
0.1
SCHEME 3 EE mechanism reduction triarylpyridinium salts
0.09
0.09
0.09
0.09
0.08
–0.80
–0.82
speed range from 50 to 1000 mV/s. The method of the ref-
erence additive (Fc) was used to determine the number of
electrons participating in each stage of the electrochemi-
cal processes.
As can be seen from Table 1, the electrochemical
properties strongly depend on the nature of the substitu-
ents located both at the nitrogen atom and on the fourth
carbon atom. The cyclic voltammograms (cva) for the
pyrilium salts 2a‐d and 3a‐d on the anodic region have
similar shapes and contain only one irreversible wave to
oxidation of the perchlorate‐ion (E = 1.85 V): The typical
cyclic voltammogram obtained for the compound 2a is
shown in Figure 1.
(A)
In the cathode region, two quasi‐reversible one‐
electron waves are observed for the 2a‐3d compounds in
the CV in the corresponding sequential electrochemical
reduction of the salts to the anion through the radical
formation stage (Scheme 3), which corresponds to the
EE mechanism[16] (Figure 2).
(B)
Cyclic voltammograms for 1mM acetonitrile
FIGURE
2
solutions of (A) 2a‐d and of (B) 3a‐d on a glassy carbon electrode
As was shown earlier, in the case of one‐electron elec-
trochemical reduction of the pyridinium ion PyH+, a rad-
ical (PyH˙) is formed, and the nature of which is
confirmed by EPR spectroscopy.[17] By analogous mecha-
nism (EE), an electrochemical reduction of 9‐phenyl‐N‐
phenylacridinium chloride occurs.[18]
As can be seen from Table 1 and Figure 1, the substit-
uents located both in the phenyl ring at the nitrogen
atom and in the phenyl ring at fourth position have a
slight effect on the values of the first potentials of the
FIGURE 1 Cyclic voltammogram for 1mM acetonitrile solutions
of pyridinium salt 2a on a glassy carbon electrode

DOLGANOV ET AL.
5 of 6
reduction waves. It should be noted that the recovery
potentials for the eight compounds are very slightly dif-
ferent. Such unusual electrochemical behavior of salts
containing fragments with various electromeric effects is
difficult to explain within the framework of classical con-
cepts of the effect of substituents on oxidation‐reduction
properties.[16,19] Compounds containing groups with
strong electron‐donating properties (―OCH3) should be
more difficult to reduce compared with compounds con-
taining substituents Br― and ―CH3.[19] One of the rea-
sons for this unusual electrochemical behavior of salts
may be the absence of conjugation between substituents
and the redox active center. Earlier, the same fact was
established in the study of the electrochemical reduction
of N‐substituted pyridines.[20] The conjugation can be
disrupted by the steric repulsion of the phenyl substituent
located at the nitrogen atom and by the substituents at
positions 2 and 6. The plane of the phenyl substituent at
position 4 is broken due to interaction with hydrogen
atoms at positions 3 and 5 (Scheme 4).
It is important to note that the values of ΔE
(ΔE = Ered−Ereox) characterize the reversibility of the
redox process, in all cases, significantly more than 60 mV
(Table 1)[21] that may indicate the quasi‐reversible char-
acter of the reduction process. Indeed, it turned out that
the high values of ΔE are associated with the quasi‐
reversibility of the reduction process, as evidenced by
the CV data obtained at different scan rates (Figure 3).
As can be seen from Figure 3, the parameter ΔE for
both reduction waves increases with increasing the sweep
rate of the potential, which unambiguously (diagnostic
criteria) indicates the quasi‐reversible character of the
reduction process.[15,21] It should be noted that a similar
shape of CV curve is observed for all compounds of the
series. As can be seen from Figure 3, the difference
between direct and backward redox processes increases
with an increase in the scanning rate of the potential.
This fact indicates the occurrence of a quasi‐reversible
process. It should be noted that a similar shape of CV
curve is observed for all compounds of the series. It can
be assumed that the quasi‐reversibility of the reduction
process is associated with small structural deformations
of molecules that occur after one‐electron transfer, for
FIGURE
3
Cyclic voltammograms for 1mM acetonitrile
solutions of 2a at different potential scan rate
example, with partial inversion of substituents at the
nitrogen atom and at the fourth carbon atom. As can be
seen from Table 1, the value of the second reduction
potential, opposite, strongly depends on the nature of
the substituents: In the case of a series of compounds 2,
the difference between the first and second reduction
potential lies in the region of 100 to 150 mV, whereas
for the series of compounds 3, the difference is 220 to
340 mV. The two‐electron triarylpyridinium reduction
product can be considered as a very high‐energy species,
which becomes obvious when the anti‐aromatic (8n)
character of such anion is recognized. Therefore, the
reduction potentials (E2red) are so strongly shifted to the
cathode region. A similar electrochemical behavior was
observed with two electronic reduction of the
unsubstituted pyridinium ion PyH+. The difference
between the reduction potentials was 350 mV.[15] It is
important to note that when a radical is reduced to an
anion, the nature of the substituents on the nitrogen
atom is already beginning to play an important role in
electrochemical behavior. In our opinion, this is due to
the destabilizing effect of the methoxy group on the anion
formed during the electrochemical reduction of the
radical.
4 | CONCLUSIONS
In summary, we have studied the electrochemical proper-
ties of a number of heterocyclic salts based on pyridine.
The main factors influencing the oxidation‐reduction
properties of salts are determined. The potential of the
first reduction wave is mainly influenced by steric factors
and by E2red the electrical factors of the methoxy group.
SCHEME 4 Steric interaction of substituents in the cation
triarylpyridinium

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DOLGANOV ET AL.
[10] V. Thoi, Y. Sun, J. Long, C. Chang, Chem. Soc. Rev. 2013, 42,
ACKNOWLEDGEMENT
2388.
This work was supported by the Russian Foundation for
Basic Research (18‐03‐00211) and Ministry of Education
and Science of the Russian Federation within the frame-
work of government task in the field of scientific activities
(project No. 4.4566.2017/8.9).
[11] A. Dolganov, O. Tarasova, D. Moiseeva, E. Muryumin, Int. J.
Hydrogen Energy 2016, 41, 9312.
[12] V. Artero, M. Chavarot‐Kerlidou, M. Fontecave, Angew. Chem.
Int. Ed. 2011, 50, 7238.
[13] A. Dolganov, B. Tanaseichuk, D. Moiseeva, V. Yurova, J.
Sakanyan, N. Shmelkova, V. Lobanov, Electrochem. Commun.
2016, 68, 59.
ORCID
[14] A. Dolganov, B. Tanaseichuk, P. Ivantsova, Y. Tsebulaeva, S.
Kostrukov, D. Moiseeva, N. Shmelkova, V. Yurova, Int. J.
Electrochem. Sci. 2016, 11, 9559.
Alexandr V. Dolganov
1908
https://orcid.org/0000-0001-7120-
[15] W. T. Bresnuhun, J. Moiroux, Z. Samec, P. J. Elving,
J. Electroanal. Chem. 1980, 116, 125.
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How to cite this article: Dolganov AV,
Tanaseichuk BS, Pryanichnikova MK, et al.
Triarylpyridinium salts: Synthesis and
electrochemical properties. J Phys Org Chem. 2019;
e3930. https://doi.org/10.1002/poc.3930
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