Dual activity of electrocatalytic activated CO2 toward pyridine for synthesis of isonicotinic acid: An EC′C′C mechanism

Article abstract of DOI:10.1016/j.elecom.2014.11.025

The present study demonstrates the indirect electrocatalytic synthesis of isonicotinic acid using a Ni complex, [Ni^II(Me₄-(NO₂Bzo)₂[14]tetraeneN₄)], in an acetonitrile solution at room temperature. The complex was used as an excellent electrocatalyst for the reduction of carbon dioxide. The results indicate that the electrocatalytic reduction product of CO₂ (CO₂·^-) has a dual role in the electrosynthesis of isonicotinic acid. The dual activity of CO₂·^- involved indirect electrocatalytic reduction of pyridine as well as its radical reaction with pyridine radical anion to form isonicotinic acid. Finally, EC′C′C mechanism was proposed for the synthesis of isonicotinic acid. In contrast, the reaction of pyridine with CO₂ in the absence of the complex follows an EC′C mechanism, and the final product is 4,4′-bipyridine.

Full text of DOI:10.1016/j.elecom.2014.11.025

Dual activity of electrocatalytic activated CO2 toward pyridine for synthesis
of isonicotinic acid: An EC’C’C mechanism
Hossein Khoshro, Hamid R. Zare, Abbas A. Jafari, Alireza Gorji
PII:
DOI:
Reference:
S1388-2481(14)00371-3
doi: 10.1016/j.elecom.2014.11.025
ELECOM 5332
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Electrochemistry Communications
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Revised date:
Accepted date:
22 October 2014
16 November 2014
27 November 2014
Please cite this article as: Hossein Khoshro, Hamid R. Zare, Abbas A. Jafari, Alireza
Gorji, Dual activity of electrocatalytic activated CO2 toward pyridine for synthesis of
isonicotinic acid: An EC’C’C mechanism, Electrochemistry Communications (2014), doi:
10.1016/j.elecom.2014.11.025
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ACCEPTED MANUSCRIPT
Dual activity of electrocatalytic activated CO2 toward pyridine for synthesis of
isonicotinic acid: An EC'C'C mechanism
Hossein Khoshro, Hamid R. Zare*, Abbas A. Jafari, Alireza Gorji
Department of Chemistry, Yazd University, Yazd, 89195-741, Iran
Fax number: +98 351 8210991 Telephone number: +98 351 8122669
E-mail address: hrzare@yazd.ac.ir
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Abstract
The present study demonstrates the indirect electrocatalytic synthesis of isonicotinic acid using a
Ni complex, [NiII(Me4-(NO2Bzo)2[14]tetraeneN4)], in an acetonitrile solution at room
temperature. The complex was used as an excellent electrocatalyst for the reduction of carbon
•−
dioxide. The results indicate that the electrocatalytic reduction product of CO2 (CO2 ) has a dual
•−
role in the electrosynthesis of isonicotinic acid. The dual activity of CO2 involved indirect
electrocatalytic reduction of pyridine as well as its radical reaction with pyridine radical anion to
form isonicotinic acid. Finally, EC'C'C mechanism was proposed for the synthesis of isonicotinic
acid. In contrast, the reaction of pyridine with CO2 in the absence of the complex follows an
EC'C mechanism, and the final product is 4,4'-bipyridine.
Keywords: Carbon dioxide, Pyridine, Isonicotinic acid, Bipyridine, Nikel (II) complex
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1. Introduction
The transformation of CO2, as an ideal C1 source, to useful organic compounds is of great
interest and importance [1, 2]. The ability of carbon dioxide to promote the reduction of organic
substrates has been reported in the literature [3, 4]. In the case of pyridine, which is reduced at
•−
potentials more negative than CO2, the radical anion CO2 promotes the conversion of pyridine
into 4,4'-bipyridine [4]. Owing to the thermodynamically and kinetically stability of CO2 [5], a
number of transition metal complexes have been extensively employed as homogeneous
catalysts for the electrochemical reduction of CO2 [6-9]. As this paper is concerned, in the field
of carbon-carbon bond formation using carbon dioxide, a Ni complex, [NiII(Me4-
(NO2Bzo)2[14]tetraeneN4)], was used for indirect electrocatalytic synthesis of isonicotinic acid
from CO2 and pyridine in an acetonitrile solution. The complex was used as an excellent
•−
electrocatalyst for reduction of CO2. The electrocatalytic reduction product of CO2 (CO2 ) has a
•−
dual role in the electrosynthesis of isonicotinic acid. The dual activity of CO2 involved at first
pyridine indirect electrocatalytic reduction and then its radical reaction with pyridine radical
anion to form isonicotinic acid. Finally, EC'C'C mechanism was proposed for synthesis of
isonicotinic acid. Isonicotinic acid is used in the synthesis of some pharmaceutically important
drugs such as isoniazid (an antituberculostatic drug), terefenadine (an antihistamine) and
nialamide (an antidepressant) [10, 11]. Isonicotinic acid is also used for the synthesis of
inabenfide (a plant growth regulator). Furthermore, isonicotinic acid has emerged as an
anticorrosion reagent, an electroplating additive and a photosensitive resin stabilizer [12].
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2. Experimental
2.1. Chemicals and apparatus
A Ni (II) complex, [NiII(Me4-(NO2Bzo)2[14]tetraeneN4)], was prepared as previously described
[13]. All the other chemicals were of analytical reagent grades, purchased from Merck Company
and used without any further purification. Argon (Ar) and CO2 had a purity of 99.995% and were
used directly.
Cyclic voltammetry was carried out using an EG&G PARSTAT 2273 equipped with a Power
Suite software in a conventional three-electrode electrochemical cell containing a glassy carbon
electrode (GCE) as a working electrode, a Pt wire as a counter electrode, and Ag/Ag+ (0.01 M
AgNO3 in a 0.1 M TBAP acetonitrile solution) as a reference electrode. Controlled potential
coulometry was performed using a SAMA 500 electroanalyzer system in an undivided glass cell
equipped with a gas inlet and an outlet with a graphite rod as the cathode, a platinum plate (ca.6
cm2) as the anode and Ag/Ag+ (0.01 M AgNO3 in 0.1 M TBAP acetonitrile solution) as a
reference electrode. 1H and 13C NMR were measured on a DRX-400 (Bruker) spectrometer with
CDCl3 as a solvent in the presence of SiMe4 as an internal standard. Fourier transform infrared
(FTIR) spectrum analysis was performed on an EQUINOX55 spectrometer.
2.2. Typical electrolysis procedure
The electrolysis was carried out in 50.0 mL ACN containing 0.1 mol L-1 TBAP, 2.0 mmol of
pyridine, and CO2 in the absence and the presence of 0.1 mmol L-1 Ni (II) complex as a catalyst.
Prior to every experiment, the solution was bubbled with Ar gas for 30 minutes. In controlled
potential coulometry, constant potentials of -1.75 V and -2.4 V were applied to the working
electrode in the presence and the absence of the Ni complex, respectively. The results showed
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that the pyridine conversions were about 100% after passing 2.5 and 3.0 F mol-1 of the starting
substrate (pyridine) in the presence and the absence of the Ni complex at room temperature,
respectively. At the end of the electrolysis, the solvent was removed under reduced pressure, and
the residue was extracted with diethyl ether (4×25 mL). After evaporation of the ether, the crude
product was characterized by FTIR, 1H, and 13C NMR.
The spectral characteristics of the products were determined as follows (see Figs. S1 – S6 of
1
Supplementary Information): isonicotinic acid: H NMR (CDCl3, 400 MHz): δ 7.47 (d.d, 2H,
j=3.2), δ 7.66 (d.d, 2H, j=3.2), δ 7.95 (s, 1H, CO2H),13C NMR (CDCl3, 100.6 MHz): δ 128.72,
130.83, 132.41, 167.63 ppm, IR: 3379 cm-1(O-H), 1714, 1665 (C=O), 1590 (C=C), 1095 (C-O);
4,4'-bipyridine: 1H NMR (CDCl3, 400 MHz): 1H NMR: 7.48 (d, 4H, j= 6), 8.68 (d, 4H, j= 5), 13C
NMR (CDCl3, 100.6 MHz): δ 121.44, 145.57, 150.67 ppm; FT-IR: 1696, 1653 cm-1(C=N), 1578,
1555 cm-1 (C=C), 1453, 1248, 1085 and 756 cm-1.
3. Results and discussion
3.1. Indirect electrochemical reduction of pyridine by the Ni (II) complex
Voltammograms of (a) and (b) of Fig. 1 show the voltammetric responses of the GCE in the
potential range of −0.7 to −2.6 V in an ACN (0.1 M TBAP) solution containing pyridine and
CO2, respectively. As shown, there is no redox peak in the presence of 1.0 mM pyridine
(voltammogram a), which indicates that the reduction potential of pyridine does not take place
even by applying -2.6 V. A weak cathodic current is started at about -2.3 V in the supporting
solution containing CO2 (voltammogram b) and its current remained unchanged during repetitive
scans. Above results indicate that the cathodic charge of GC and its following carboxylation are
not favored under working conditions [14]. This is likely due to using the supporting electrolyte
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with more bulky tetraalkylammonium cation of TBA+ [14]. So, the reduction of carbon dioxide
•−
to the anion radical CO2 is a predominant electrode reaction in comparison with the chemical
carboxylation of GC [14-16]. As previously reported, the cyclic voltammogram of 1.0 mM Ni
(II) complex shows a one-electron redox couple with a formal potential (E0') of -1.65 V
(voltammogram c) [17]. Voltammogram (d) shows the cyclic voltammogram of the Ni (II)
complex solution in the presence of 1.0 mM pyridine. A comparison of voltammograms (c) and
(d) indicates that there is no interaction between pyridine and the complex. Voltammogram (e)
shows the voltammetric response of the supporting electrolyte containing the complex and CO2.
As it can be seen, the reduction peak current of the complex increases significantly in the
presence of CO2 while the anodic peak disappears. These results are due to the electrocatalytic
•−
reduction of CO2 to CO2 by the complex (Eqs. 1 and 2) [17]. Voltammograms (f), (g) and (h)
show the cyclic voltammograms of a 1.0 mM complex solution in the presence of CO2
containing 1.0, 2.0 and 3.0 mM of pyridine, respectively. A comparison of voltammogram (e)
and voltammograms (f)-(h) indicates that the cathodic peak current rises markedly with an
increase of pyridine concentration in the solution containing the complex and CO2. It is noted
that, compounds with reduction potentials both lower and higher than CO2, can enter into the
carboxylation process [4, 17, 18]. It is clear that in both cases a radical-anion is formed in the
initial event either from the substrate or from the CO2 [4, 17, 18]. Considering the lack of
electrocatalytic activity of the complex for reduction of pyridine, it is logical to conclude that the
CO2 activated by the complex promotes the reduction of pyridine. In other words, the initial
•−
process in carboxylation of pyridine is the formation the electroactivated CO2 (CO2 ) in the
presence of the Ni complex. The increase of the reduction peak current in the presence of
pyridine may be explained by an electron transfer from CO2•− to the aromatic ring leading to the
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radical anion of pyridine and regeneration of CO2 according to Eq. (3). The radical anion of
pyridine produced in Eq. (3) may undergo different reactions such as coupling to radical anion
•−
CO2 to form isonicotinic acid (Eq. 4) or coupling to another pyridine radical anion to give a
dimer of 4,4'-bipyridine (Eq. 5) [4]. The spectral characteristics of the electrolysis product
(section 2.2) indicate that the final product is isonicotinic acid. Observation of isonicotinic acid
as the final product during the electrolysis of a solution containing pyridine and CO2 in the
•−
presence of the Ni (II) complex can be rationalized by coupling of CO2 with the radical anion
of pyridine followed by oxidation intermediate (reaction 4). Therefore, an EC'C'C mechanism
can be proposed for the reaction of pyridine with CO2 in the presence of the complex as shown in
reactions (1)-(4). In other words, a dual reactivity of the CO2•− radical anion toward pyridine, i.e.
•−
one-electron reduction of pyridine as a result of an electron transfer from the CO2 to pyridine
•−
(reaction 3) and radical addition of CO2 to the radical anion of pyridine (reaction 4), has a
decisive role in the process of electrosynthesis of isonicotinic acid. Voltammograms (i), (j) and
(k) of Fig. 1 show the cyclic voltammograms of the supporting electrolyte solution containing
1.0, 2.0 and 3.0 mM of pyridine, respectively, and CO2 in the absence of Ni (II) complex. The
increase of the reduction current with gradual addition of pyridine concentration and in the
presence of CO2 indicates that CO2 has a direct key role as a promoter in the reduction of
pyridine under these conditions. Moreover, a comparison of voltammogram (i) and (f) shows that
the overpotential for reduction of pyridine in the presence of the complex decreases more than
600 mV. These results suggest that Ni (II) complex has an excellent indirect electrocatalytic
effect for reduction of pyridine.
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3.2. Synthesis of isonicotinic acid by catalytic reduction of pyridine in the presence the Ni (II)
complex
In order to confirm the electrocatalytic behavior of the Ni (II) complex for the synthesis of
isonicotinic acid, controlled potential coulometry was carried out at -1.75 V in a CO2 saturated
solution in the presence of pyridine and the Ni (II) complex as described in section 2.2. Also, to
ascertain the possible synthesis of isonicotinic acid in the absence of the Ni (II) complex,
controlled potential coulometry of ACN-0.1 M TBAP containing pyridine and CO2 was
performed at -2.4 V where reductions of CO2 could take place in the absence of the Ni (II)
complex. At the end of the electrolysis, the solutions were treated as mentioned in the
experimental section. A yield of 90% of isonicotinic acid was achieved in the presence of the
complex while, in its absence, a yield of 80% of 4,4'-bipyridine was reached. Produce of 4,4'-
bipyridine in the absence of the complex is a sign of coupling two radical anions of pyridine
according to Eq. (5) [4]. Consequently, the reaction of pyridine with CO2 in the absence of the
complex follows an EC'C mechanism, and the final product is 4,4'-bipyridine [4]. The
mechanism is proposed to include a one-electron reduction of CO2 (E) followed by reaction (3)
(C') and finally a chemical reaction of two pyridine radical anions and then oxidation of
produced intermediate to give 4,4'-bipyridine (reaction 5) (C) [4]. The significant increase of the
cathodic peak current of the complex in the presence of CO2 (voltammogram e) as compared to
that in the presence of CO2 alone (voltammogram b) indicates that the kinetic of production of
•−
CO2 in the presence of the complex is faster than that in its absent. So, production of
•−
isonicotinic acid instead of 4,4'-bipyridine may be due to more concentration of CO2 in the
presence of the complex. In other words, in the presence of the complex, radical addition of
CO2•− to the radical anion of pyridine occurs preferentially to form isonicotinic acid.
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4. Conclusions
The present study demonstrates the dual reactivity, one-electron transfer and radical addition of
•−
CO2 toward pyridine in the presence of [NiII(Me4-(NO2Bzo)2[14]tetraeneN4)] complex. The
•−
pyridine radical anion is produced from the reaction of CO2 (as an electron transfer) with
•−
pyridine. Also, the radical addition of CO2 to the radical anion of pyridine takes place in the
presence of the Ni complex to form isonicotinic acid. In contrast, 4,4'-bipyridine is produced as a
result of coupling the radical anions of pyridine in the absence of the complex. It is noteworthy
that the reduction potential of CO2 is significantly less negative in the presence of the complex.
Consequently, Ni (II) complex has an excellent indirect electrocatalytic effect for reduction of
pyridine with a diminution of the overpotential about 600 mV. Finally, EC'C'C mechanism is
proposed for synthesis of isonicotinic acid.
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[NiII(Me4-(NO2Bzo)2[14]tetraeneN4)]
Cyclic voltammograms of a glassy carbon electrode in an ACN (1.0 M TEAP) solution
(a) after addition of
Scan rate: 100 mV s-1.
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Scheme 1
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Fig. 1
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Graphical abstract
[Ni(II) complex]
[Ni(I) complex]−
CO2
Py
+e
•−
Py•− + CO2
Isonicotinic
•−
CO2
CO2
Py
Py•− + Py•−
+e
•−
4,4'-bipyridine
CO2
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Highlights:
A Ni complex was used for indirect electrocatalytic synthesis of isonicotinic acid from carbon dioxide and
pyridine.
The results indicate that the electrocatalytic reduction product of CO2 (CO2•−) has a dual role in the
electrosynthesis of isonicotinic acid.
An EC'C'C mechanism was proposed for synthesis of isonicotinic acid.
16