Microwave-assisted synthesis of 2-(2-pyridyl)azoles. Study of their corrosion inhibiting properties

Article abstract of DOI:10.1002/jhet.5570440123

(Chemical Equation Presented) An efficient and fast procedure for the synthesis of 2-(2-pyridyl)azoles is described using ionic liquids as catalysts under microwave irradiation. The X-ray crystallographic analyses for three of the four synthesized compounds are presented. Potentiodynamic polarization studies were carried out to analyze the electrochemical behavior of the compounds in corrosive acidic media. Of the four derivatives, one compound was detected to be an effective corrosion inhibitor prototype for oil refinery environments.

Full text of DOI:10.1002/jhet.5570440123

Jan-Feb 2007
145
Microwave-assisted Synthesis of 2-(2-Pyridyl)azoles. Study of their
Corrosion Inhibiting Properties
Natalya V. Likhanova,A M. Aurora Veloz,B Herbert Höpfl,C Diana J. Matías,B Victor E.
Reyes-Cruz,B Octavio Olivares,D and Rafael Martínez-PalouA,*
A Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas No. 152, 07730 México, D.F., México.
E-mail: rpalou@imp.mx
B Universidad Autónoma del Estado de Hidalgo, Instituto de Ciencias Básicas e Ingeniería, Área de Materiales
y Metalurgia, Carr. Pachuca-Tulancingo KM 4.5, 42184, Pachuca, Hgo. México
C Universidad Autónoma del Estado de Morelos, Centro de Investigaciones Químicas, Av. Universidad 1001,
C.P. 62210, Cuernavaca, México.
D Centro Universitario de Vinculación de la Benemérita Universidad Autónoma de Puebla, Av. San Claudio,
Ciudad Universitaria Col. San Manuel. Puebla, Pue. CP 72570, México.
Received April 17, 2006
X
N
XH
[BMIM]Cl
MW
+
O
N
N
Y
Y
NH2
OH
1, X = NH, Y = CH
2, X = O, Y = CH
3, X = NH, Y = N
4, X = O, Y = N
5a-5d
6
1
2
3
An efficient and fast procedure for the synthesis of 2-(2-pyridyl)azoles is described using ionic liquids as catalysts
under microwave irradiation. The X-ray crystallographic analyses for three of the four synthesized compounds are
presented. Potentiodynamic polarization studies were carried out to analyze the electrochemical behavior of the
compounds in corrosive acidic media. Of the four derivatives, one compound was detected to be an effective corrosion
inhibitor prototype for oil refinery environments
J. Heterocyclic Chem., 44, 145 (2007).
INTRODUCTION
catalyst: 2-(2-pyridyl)-1H-benzimidazole (1), 2-(2-pyridyl)-
benzooxazole (2), 2-(2-pyridyl)-1H-imidazo[4,5-b]pyridine
(3) and 2-(2-pyridyl)oxazole[4,5-b]pyridine (4). The
electrochemical polarization technique was used to test
the inhibitory properties of these compounds against SAE
1018 C-steel corrosion in acidic media. Furthermore, the
X-ray structures of compounds 1, 2 and 3 are presented.
Benzimidazole [1], benzooxazole [2], imidazo[4,5-
b]pyridine [3] and oxazolo[4,5-b]pyridine derivatives [4]
are important heterocyclic compounds with well-known
biological activity. In addition, these compounds are
useful intermediates in organic [5] and polymer synthesis
[6], and several pyridine [7], benzimidazole [8] and
benzooxazole [9] derivatives have been previously
reported as effective corrosion inhibitors.
In recent years, microwave-assisted organic synthesis
(MAOS) has attracted considerable interest, due to the
generally short reaction times and the circumstance that the
resulting products are obtained with high purity and yields
[10]. A recent trend in this area is to use ionic liquids (ILs) as
solvent, co-solvent and/or catalyst, since they are
“ecofriendly” [11] and since their ionic nature allows a very
effective coupling with microwave energy [12].
RESULTS AND DISCUSSION
Synthesis: Formerly, the most straightforward methods
for the preparation of benzoazoles involved the
cyclodehydration reaction between ortho-substituted
aromatic amines and carboxylic acids, which proceeds
only under harsh reaction conditions [13].
Similarly, most of the synthetic methods to prepare
azole[4,5-b]pyridines start from properly substituted
pyridines to generate the imidazole or oxazole ring [14],
or from conveniently substituted azoles to generate the
pyridine ring [15], involving generally more than one
reaction step, complicated work-up and modest yields.
Herein, a successful microwave-promoted synthesis of 2-
(2-pyridyl)azoles is reported employing an ionic liquid as

146
N. V. Likhanova, M. A. Veloz, H. Höpfl, D. J. Matías, V. E. Reyes, O. Olivares, R. Martínez-Palou
Vol 44
XH
X
N
Microwave-assisted methodologies have been described
for the synthesis of aryl- and 2-methylbenzoazoles, either
in the presence [16] or absence of a solvent [17].
Microwave synthesis has also been widely used for the
synthesis of many other heterocyclic compounds [18].
Very recently we reported on the microwave-assisted
synthesis of 2-long alkyl benzoazoles and azole[4,5-b]-
pyridines in solvent-free conditions [19], however, this
procedure failed when it is applied to the synthesis of 2-
pyridyl-substituted analogues, presumably due to the
descarboxylation of the picolinic acid (6).
After some experimentation we have now discovered that
using an ionic liquid as catalyst in the reactions, the
descarboxylation of the picolinic acid was inhibited,
presumably as a consequence of a C-HꢀꢀꢀO hydrogen-bonding
interaction between the ionic liquid and the oxygen of the
hydroxyl group of the picolinic acid. Thus, 2-(2-pyridyl)-
benzoazoles (1 and 2) and 2-(2-pyridyl)-azole[4,5-b]pyridines
(3 and 4) were prepared very quick and efficiently.
[BMIM]Cl
MW
+
O
N
N
Y
Y
NH2
OH
1, X = NH, Y = CH
2, X = O, Y = CH
3, X = NH, Y = N
4, X = O, Y = N
5a-5d
6
Scheme 1. Synthesis of 2-(2-pyridyl)benzoazoles (1 and 2) and 2-(2-
pyridyl)azole[4,5-b] pyridines (3 and 4).
The course of the reactions was followed by thin-layer
chromatography using as reference samples prepared by
conventional heating (polyphosphoric acid (PPA) as
solvent and catalyst, 200°C, 8 h).
Table 1
Reaction conditions and yields for the cyclocondensation reactions of 5
and 6 under solvent-free MW at 600 W (T = 160 ± 5°C) employing
[BMIM]Cl, as catalyst.
Compound Reagent [a] Time (min) [b] Yield(%) [c], [d]
Having evaluated three ionic liquids, ([BMIM]Cl,
[BMIM]BF4 and [BMPy]PF6), optimized the microwave
reaction conditions such as temperature and power level
of irradiation and tested the influence of solvent or solid
supports (data not shown), we could establish as optimum
parameters for the synthesis of 1-4 (Scheme 1), using the
conditions shown in Table 1 (catalyst: 1-butyl-3-methyl-
imidazolium chloride, [BMIM]Cl).
(1)
(2)
(3)
(4)
5a/6
5b/6
5c/6
5d/6
7
8
6
7
84(85)
79(83)
82(80)
80(78)
[a] Molar ratio of reagents 5 and 6, 1:1.8; [b] Total reaction time of
irradiation; [c] Isolated yield; [d] In bracket: isolated yield of the
compounds prepared under conventional thermal conditions: 8 hours in
PPA at 200°C (see Experimental Section).
Table 2
Crystallographic data for compounds 1, 2 and 3.
Crystal data
Formula
1
2
3
C12H9N3
C12H N2O,
4(C12 H8 N2 O)
0.38 x 0.41 x 0.50
981.02
C11H8N4
Crystal size (mm)
MW (g mol-1)
Space group
Cell parameters
a (Å)
0.09 x 0.17 x 0.25
195.22
Pbca
0.39 x 0.42 x 0.45
196.21
P21/n
P21/c
10.7720(17)
10.1628(16)
18.602(3)
90
2036.5(6)
8
6.4390(7)
21.216(2)
17.8046(19)
99.759(2)
2397.1(4)
2
7.4600(13)
13.497(2)
9.4972(16)
102.654(3)
933.0(3)
4
b (Å)
c (Å)
ꢀ (°)
V (Å3)
Z
μ (mm-1)
0.080
1.273
0.090
1.359
0.090
1.397
ꢁ
calcd
(g cm-3)
Data collection
ꢂ limits (°)
No. collected refl.
No. ind. refl. (Rint)
No. observed refl.[a]
Refinement
2 < ꢂ < 27
21252
2226 (0.068)
1359
2 < ꢂ < 25
22836
4230 (0.037)
2950
2 < ꢂ < 26
9504
1832 (0.028)
1589
RA,B
0.049
0.137
136
1.001
-0.19
0.16
0.065
0.203
374
0.970
-0.45
0.52
0.036
0.095
139
1.065
-0.17
0.12
C,D
Rw
No. of variables
GOF
ꢃꢁ
min
ꢃꢁ
max
(e Å-3)
(e Å-3)
2
2
2
2
A I > 2ꢁ(I), B R = ꢂ(Fo -Fc2)/ꢂFo , C all data, D Rw = [ꢂw(Fo -Fc2)2/ꢂw(Fo )2]1/2

Jan-Feb 2007
Synthesis of 2-(2-Pyridyl)azoles. Study of their Corrosion Inhibiting Properties
147
After purification by flash column chromatography, the
products were recrystallized from ethanol. In the case of
compounds 1, 2 and 3 the monocrystals obtained
permitted the determination of their molecular structures
by X-ray crystallography. Although these studies give
only information on the molecular structure and
intermolecular interactions present in the solid state, these
data are helpful for a more profound future understanding
of the inhibitory effects present in solution and on metal
surfaces (vide infra).
X-ray diffraction analysis: The most relevant
crystallographic data for compounds 1, 2 and 3 are shown
in Table 2. Selected geometric parameters are summarized
in Table 3.
With respect to the molecular structures of compounds
1-3 (Figure 2-4) the two heterocyclic rings are almost
coplanar between each other (N1-C1-C2-N3 = -13.0° for
the 2-(2-pyridyl)benzimidazole 1 polymeric hydrogen-
bonded chains are formed, in which each molecule
participates simultaneously in two intermolecular N1-
HꢁꢁꢁN2 (0.860 Å, 2.107 Å, 2.917(2) Å and 156.6º) and C3-
HꢁꢁꢁN3i (0.930 Å, 2.585 Å, 3.487(3) Å, 163.6º; -
x+1/2+1,+y+1/2,+z) interactions. Thereby, neighboring
molecules are twisted against each other by an angle of
49.9º (Figure 1).
For the 2-(2-pyridyl)benzoxazole 2 there are three
independent molecules in the asymmetric unit, each
giving an independent polymeric chain with six hydrogen
bonds per molecule. Since the hydrogen bonding
geometries are very similar for the three chains, for
illustrative purposes only the parameters for that
described in Figure 2 are indicated: C7-HꢁꢁꢁN1i (0.930 Å,
2.567 Å, 3.464(4) Å, 162.0º; x+1,+y,+z), C9-HꢁꢁꢁO1ii
(0.930 Å, 2.925 Å, 3.765(4) Å, 150.9º; x-1,+y,+z) and
C10-HꢁꢁꢁN3ii (0.930 Å, 2.940 Å, 3.783(5) Å, 151.4º; x-
1,+y,+z).
1, O1ꢀC1ꢀC8ꢀN3
=
2.4(4) and 0.3(3)° for 2,
N1ꢀC1ꢀC7ꢀN4 = 13.4(2)° for 3) and do not present any
anomalous features in their planarities, bond distances and
aromaticity indices. The bond lengths and bond angles
shown in Table 3 agree well with the values observed for
related structures [20].
Figure 2. Perspective of the molecular structure of compound 2 and
illustration of the most relevant hydrogen-bonding interactions within
the crystal lattice.
Comparing the hydrogen bonding interactions between
the 2-(2-pyridyl)benzimidazole derivatives 1 and 3, the
pattern changes completely, since the presence of the
additional pyridine nitrogen atom in 3 (Figure 3), allows
for the formation of a homodimeric motif containing two
N-HꢁꢁꢁN interactions instead of one: N1-HꢁꢁꢁN3i (0.860 Å,
2.054 Å, 2.889(1) Å and 163.4º; -x+2,-y+1,-z).
Figure 1. Perspective of the molecular structure of compound 1 and
illustration of the most relevant hydrogen-bonding interactions within
the crystal lattice.
In all three crystal lattices the 2-(2-pyridyl)benz-
imidazole and benzoxazole moieties are involved in
hydrogen-bonding interactions (Figure 1-3). In the case of

148
N. V. Likhanova, M. A. Veloz, H. Höpfl, D. J. Matías, V. E. Reyes, O. Olivares, R. Martínez-Palou
Vol 44
Figure 3. Perspective of the molecular structure of compound 3 and illustration of the most relevant hydrogen-bonding
interactions within the crystal lattice.
Table 3
Selected bond lengths, bond angles and torsion angles for compounds 1, 2 and 3.
1
2
3
Bond lengths [Å]
C1ꢀN1
1.355(2)
--
1.285(3)
1.292(3)
1.350(3)
1.359(3)
--
1.360(2)
--
C1ꢀO1
C1ꢀN2
1.317(2)
1.314(2)
Bond angles [°]
N1ꢀC1ꢀN2
O1ꢀC1ꢀN1
113.07(16)
--
--
113.7(1)
--
115.5(3)
115.2(2)
Torsion angles [°]
N1ꢀC1ꢀC2ꢀN3
O1ꢀC1ꢀC8ꢀN3
-13.0(3)
--
--
--
--
2.4(4)
0.3(3)
--
N1ꢀC1ꢀC7ꢀN4
--
13.4(2)
Potentiodynamic polarization: The potentiodynamic
polarization method was used to determine the inhibition
properties of the synthesized compounds. To carry out the
test a typical three-electrode cell was used, in which the
working electrode interacts with the corrosive media,
without and with each compound, during different
immersion times (10 min, 3 h, 24 h). The corrosive
environment employed in this study is one of the most
severe used to evaluate metals and alloys submitted to
corrosion stress or hydrogen induced cracking.
time, both currents have increased again, indicating that
the film has dissolved, such as it has been reported in the
literature [21].
The modifications of the currents shown by the systems
in function of the immersion time indicate that the
corrosion behavior changes over the time and, at present,
these results provide an efficient way to compare the
system responses with the effects of inhibitors on it.
In order to determine the concentration, at which the
best inhibitory effect of each compound could be
observed, a polarization study was carried out. Several
concentrations of each compound were tested. Figure 5
shows only the polarization curves for compound 1, since
these are representative for all four compound responses.
It can be observed that the cathodic process is very
sensitive to the variation of the concentration, while the
anodic process shows only slight changes.
Figure 4 shows the electrochemical response for the
system without additives during different immersion
times. This figure indicates that after three hours of
exposition, the anodic and cathodic currents diminished,
when compared to the system that was immersed only for
10 minutes, probably due to the presence of a film of
corrosion products. However, after 24 hours of exposition

Jan-Feb 2007
Synthesis of 2-(2-Pyridyl)azoles. Study of their Corrosion Inhibiting Properties
149
Figure 4. Polarization curves obtained on the system SAE 1018 carbon steel in acidic media for different immersion
times (10 min, 3 h, 24 h). Scan speed 0.1 mV/seg.
On the other hand, the sample with a concentration of
100 ppm presents a visible change in comparison to the
system without additives. Since this concentration is
considered the maximum usable, in most of the laboratory
studies, it was decided to use this concentration to test the
immersion time effect on the systems with each of the
synthesized compounds.
For this purpose, each compound was added to the
system and left to act on the steel electrode surface during
10 minutes (time 0), 3 and 24 hours. The results showed
that every compound has a main effect on the cathodic
the currents. On the other hand, the variations of the
oxidation currents were small.
In order to analyze the results quantitatively, the
corrosion parameters were calculated from the
polarization curves. Corrosion potential, polarization
resistance and corrosion current are shown in Table 4.
The corrosion potentials of the systems containing 100
ppm of the corresponding compound display very little
variation with respect to the additive-free system, so that
the surface energetic conditions are not greatly affected by
the presence of the compounds. On the other hand, the
Figure 5. Polarization curves obtained on the system SAE 1018 carbon steel in acidic media for different concentrations of compound
1 after 24 hours of inmertion time. Scan speed 0.1 mV/seg.
process: the longer the immersion time, the lower were

150
N. V. Likhanova, M. A. Veloz, H. Höpfl, D. J. Matías, V. E. Reyes, O. Olivares, R. Martínez-Palou
Vol 44
Table 4
Corrosion parameters obtained from the polarization curves for the SAE 1018 carbon steel system immersed in an
acidic environment of the type NACE 0177.
Compound
Immersion
time(h)
Ecorr
(V)
Rp
(ohm)
Icorr
(A)
0
3
24
0
3
24
0
3
24
0
3
-0.616
-0.638
-0.630
-0.616
-0.609
-0.614
-0.618
-0.627
-0.644
-0.636
-0.637
-0.666
-0.625
-0.623
-0.645
209.12
865.41
484.72
485.93
641.37
1282.1
9.56 x 10-5
2.30 x 10-5
4.10 x 10-5
4.12 x 10-5
3.12 x 10-5
1.56 x 10-5
blank
1
2
3
4
146.66
505.2
1.36 x 10-4
3.96 x 10-5
2.63 x 10-5
1.50 x 10-4
3.66 x 10-5
2.55 x 10-5
1.20 x 10-4
4.15 x 10-5
2.34 x 10-5
761.25
132.95
546.37
784.59
166.04
482.15
856.48
24
0
3
24
polarization resistance is more sensitive and a quantitative
comparison can be made.
Figure 6 shows the micrographs of the carbon steel
specimens exposed during 48 hours the corrosive
environment used in this study, with and without the
addition of 50 ppm of compounds 1-4. The protective
effect of the compounds 1 is clearly observed.
The corrosion currents were calculated from the
polarization curves and the polarization resistances obtained
in the range of Ecorr ± 0.02 V. This analysis showed that the
corrosion currents were always higher than that of the
additive-free system, with exception of compound 1, and
after 3 hours all values were higher than that of the blank.
These results also indicate that the evaluated
compounds present a type of competence with corrosive
agents such that the double layer could be modified
because of the strong interaction between the compounds
and the metallic surface.
After 24 hours of immersion time, a significant decrease
of the corrosion currents was observed, being compound 1
that, which presented the lowest corrosion current (see
Table 4). This fact indicates that compound 1 could be an
effective corrosion inhibitor prototype (efficiency: 86% at
100 ppm) in oil refinery environments.
An explanation for the electrochemical behavior and
the inhibitory effects of these compounds is not straight-
forward, since there are several possibilities for the metal-
inhibitor interaction, considering that the atoms at the
metal surface may be neutral or oxidized: (i) the
formation of coordinative bonds with the donating atoms
of the ligand, (ii) the occurrence of an interaction with the
ꢀ–electron density of the ligands and (iii) the presence of
hydrogen-bonding and other electrostatic interactions,
considering that some of the metal centers at the metal
surface may carry already substituents such as oxygen or
sulfur form the corrosive medium. According to the
crystallographic data and theoretical studies we have
carried out on the present ligands [22], each of the three
inhibition mechanisms is feasible for the compounds
studied herein. The molecules are planar or almost planar
(extensive ꢀ–electron density), positively charged in the
corrosive medium (protonated form) and capable of
strong hydrogen-bonding interactions. The theoretical
calculations above have shown that there is a correlation
between the hardness of the compounds tested as inhibitors
and the inhibitory efficiency, assuming that the adsorption
to the metal surface results from an interactions with the
ꢀ–electrons (including backdonation) [23].
All compounds presented a very thin film on the
electrode surface that was, visible with an optical
microscope, but this film was homogeneous and without
fissures only for compound 1.
(a)
(b)
(c)
(d)
(e)
(f)
CONCLUSIONS
Figure 6. Micrographs of blistering formed on the 1018 carbon steel
surface in acidic corrosive solution at pH 4.0 after 48 hours of exposure
at 60°C: (a) testing coupon, (b) without additives, (c) 50 ppm of 1, (d)
50 ppm of 2, (e) 50 ppm of 3 and (f) 50 ppm of 4.
A new microwave-assisted methodology, using an ionic
liquid as catalyst, was developed that is quick and
efficient to synthesize 2-pyridyl-azoles. The compounds
synthesized present a potentiodynamic behavior that

Jan-Feb 2007
Synthesis of 2-(2-Pyridyl)azoles. Study of their Corrosion Inhibiting Properties
151
116.3, 119.2, 122.1, 123.2, 124.2, 125.4, 135.0, 138.2, 143.9,
149.2, 150.1, 151.4 ppm.
indicates the formation of a thin film, most probably
consisting also of corrosion products, that protects the
metallic surface from an acidic corrosive environment.
The inhibitory behavior was markedly enhanced for the
system containing 2-(2-pyridyl)-1H-benzimidazole as
inhibitor.
2-(2-Pyridyl)-1H-benzooxazole (2). Colorless crystalline
solid, 79% yield, eluent acetone/hexane gradient, mp 108-109°C
(lit. [14a] 108°C); ir (potassium bromide): 3425, 3060, 2986,
1
1588, 1553, 1456, 1243, 1076, 738 cm-1. H nmr (DMSO-d6):
8.75 (dt, J = 4.8, 0.8 Hz, 1H), 8.28 (dt, J = 8.0, 1.0 Hz, 1H), 8.00
(td, J = 8.0, 1.6 Hz, 1H), 7.84-7.77 (m, 2H), 7.58 (ddd, J = 4.6,
3.0, 1.4, 1.0 Hz, 1H), 7.47-7.35 (m, 2H) ppm; 13C nmr (DMSO-
d6): 111.3, 120.3, 123.5, 125.1, 126.2, 137.61, 141.2, 145.2,
150.2, 150.4, 161.3 ppm.
EXPERIMENTAL
Materials. All reagents (Aldrich) were used without previous
purification except 1-methylimidazole (99%) and pyridine
(99%), which were vacuum-distilled from CaH2 prior to use. 1-
Butyl-3-methylimidazolium chloride ([BMIM]Cl) and 1-Butyl-
3-methylimidazolium tetrafluoro borate ([BMIM]PF6) were
synthesized by a literature procedure [24].
2-(2-Pyridyl)-1H-imidazole[4,5-b]pyridine (3). Colorless
crystalline solid; yield: 82%, eluent: acetone/ethyl acetate
gradient; mp 226-228°C; ir (potassium bromide): 3441, 3049,
1
2963, 1588, 1449, 1270, 1115, 773, 699 cm-1, H nmr (DMSO-
d6): 8.75 (dt, J = 4.8, 0.8 Hz, 1H), 8.39-8.34 (m, 2H), 8.06-7.88
(m, 2H), 7.56 (ddd, J = 4.6, 3.0, 1.4, 1.0 Hz, 1H), 7.26 (dd, J =
4.6, 3.4 Hz, 1H) ppm; 13C nmr (DMSO-d6): 118.6, 120.0, 122.1,
125.2, 135.6, 137.6, 145.5, 148.0, 150.0, 152.3, 156.5 ppm.
Anal. Cacld. for C11H8N4: C 67.34, H, 4.11, N 28.55. Found: C
66.98, H 4.13, N 28.40.
Measurements. Melting points are not corrected and were
measured in a Fisher Scientific apparatus with a 300°C
thermometer. FT-IR spectra were registered on a Nicolet FT-
IR 5DX FT spectrometer as KBr discs. Specific rotation was
1
measured in a Perkin Elmer 241 polarimeter. H NMR (300
2-(2-Pyridyl)-1H-oxazole[4,5-b]pyridine (4).
Colorless
MHz) and 13C NMR (75.4 MHz) spectra were obtained with Jeol
Eclipse-300 equipment using TMS as internal standard. Mass
spectra were recorded on HP 5973 equipment with a selective
mass detector. Elemental analyses were obtained on a CHNO/S
Perkin-Elmer analyzer.
crystalline solid, 80% yield, eluent: acetone, mp 182-184°C (lit.
[4d] 197-198°C); ir (potassium bromide): 3421, 3068, 2990,
1
1584, 1557, 1464, 1406, 1258, 1072, 784, 695 cm-1. H NMR
(DMSO-d6): 8.81 (dt, J1 = 4.8 Hz, J2 = 0.8 Hz, 1H), 8.59 (dd, J1
= 4.6 Hz, J2 = 1.4 Hz, 1H), 8.38 (dt, J1 = 8.0 Hz, J2 = 1.0 Hz,
1H), 8.29 (dd, J1 = 8.0 Hz, J2 = 1.0 Hz, 1H); 8.08 (td, J1 = 1.6
Hz, J2 = 8.0 Hz, 1H), 7.66 (ddd, J1 = 1.0 Hz, J2 = 1.4 Hz, J3 = 3.0
Hz, J4 = 4.6 Hz, 1H), 7.50 (dd, J1 = 4.8 Hz, J2 = 3.6 Hz, 1H)
ppm; 13C nmr (DMSO-d6): 120.4, 122.1, 125.0, 127.6, 138.5,
143.8, 145.4, 147.7, 151.0, 155.8, 164.3 ppm.
Microwave irradiations were carried out utilizing a controlled
single-mode MW equipment (MIC-I, 2450 MHz, power max.
600 W, from SEV, Mexico) [25]. Open reaction tubes (diameter:
5 cm, capacity 50 mL) were used, and temperature was
measured by an on-line IR detector.
General Procedure for the Synthesis of compounds 1-4 by
Thermal Procedure. The corresponding amine (5) (1.0 mmol)
and picolinic acid (6) (0.22 g, 1.8 mmol) were mixed with a
sufficient quantity of polyphosphoric acid (PPA) to give a
stirrable paste. The mixture was heated slowly to 180-200°C and
kept at this temperature for 8 h. The reaction mixture was slowly
cooled to 100°C and added into a large volume of water. The
insoluble residue was collected by filtration, washed with a
solution containing 10% sodium carbonate and then with water.
The crude product was dried at 60°C and recrystallized twice
from an aqueous alcohol mixture subsequent to treatment with a
small amount of activated charcoal.
General Procedure for the Synthesis of Compounds 1-4
under Microwave Irradiation. The corresponding amine
(5) (1.0 mmol), picolinic acid (6) (0.22 g, 1.8 mmol) and 0.1
g of [BMIM]Cl were charged into a large open vial. The vial
was irradiated at 600 W (T = 160 ± 5°C) under stirring
during the time indicated in Table 1. Each 30 seconds the
X-ray Crystallgraphy.
X-ray diffraction data were
collected on a Bruker-AXS diffractometer with a CCD area
detector (ꢀꢀꢁꢁꢂ= 0.71073 Å, monochromator: graphite). Frames
were collected at T = 293 K via ꢃ/ꢄ–rotation at 10 s per frame
[26]. The measured intensities were reduced to F2 and corrected
for absorption with SADABS [27]. Corrections were made for
Lorentz and polarization effects. Structure solution, refinement
and data output were carried out with the SHELXTL-NT
program package [28]. Non hydrogen atoms were refined
anisotropically. C-H hydrogen atoms were placed in
geometrically calculated positions using a riding model. N-H
hydrogen atoms have been localized by difference Fourier maps
and refined fixing the bond length to 0.86 Å; that isotropic
temperature factors have been fixed to a value 1.5 times the one
of the oxygen atoms. Figures were created with SHELXTL-NT
and MERCURY [29]. Hydrogen bonding interactions in the
crystal lattice were calculated by PLATON [30].
One of the three independent molecules in the crystal lattice
of compound 2 is disordered over two positions, and as a
consequence restraints had to be applied in order to obtain a
reasonable structure. Therefore, the corresponding hydrogen
atoms have not been calculated and the R values are higher than
for the other compounds.
Crystallographic data for the structures reported in this paper
have been deposited with the Cambridge Crystallographic Data
Centre as supplementary publications no. CCDC-292606-
292608. Copies of the data can be obtained free of charge on
application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
(fax: (+44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk,
www: http://www.ccdc.cam.ac.uk).
irradiation was stopped during
5
seconds to avoid
overheating. After cooling, the crude products were purified
through silica-gel chromatography column using the eluent
specified en each case and then recrystallized from ethanol.
2-(2-Pyridyl)-1H-benzimidazole (1). This compounds was
obtained as colorless crystalline solid, 84% yield, eluent:
acetone/hexane gradient, mp 218-220°C (lit. [13f] 218°C); ir
(potassium bromide): 3429, 3056, 2959, 1600, 1449, 1398,
1
1317, 1285, 998, 746, 699 cm-1; H nmr (DMSO-d6): ꢀ 8.70 (dt,
J = 4.8, 0.8 Hz, 1H), 8.34 (dt, J = 8.0, 1.0 Hz, 1H), 7.98 (td, J =
8.0, 1.6 Hz, 1H), 7.68-7.60 (m, 2H), 7.49 (ddd, J = 4.6, 3.0, 1,4,
1.0 Hz, 1H), 7.29-7.18 (m, 2H) ppm; 13C nmr (DMSO-d6):

152
N. V. Likhanova, M. A. Veloz, H. Höpfl, D. J. Matías, V. E. Reyes, O. Olivares, R. Martínez-Palou
Vol 44
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Potentiodynamic Polarization. Potentiodynamic polarization
tests were carried out using a typical three-electrode cell within
the range of Ecorr ± 0.3 V. A calomel electrode was used as
reference, graphite bar as a counter electrode and a carbon steel
(SAE 1018) disc with 0.503 cm2 transversal area as working
electrode. This electrode was polished before each test with SIC
emery paper (600 grid) and rinsed with deionized water.
Corrosive Solution Preparation. A corrosive environment
of the type used in the NACE TM 0177 method was prepared.
The composition of the solution used was: 0.04 M CH3COOH/
NaOOCCH3, pH = 3.5; 30 172 ppm Cl- as NaCl (0.52 M
Chloride). The solution was prepared with deionized water,
deaerated with gaseous nitrogen. Analytical grade reagents were
employed.
Preparation of the Inhibitor Solutions (with 1-4). For each
inhibitor, a 10,000 ppm solution was prepared from 1.00 g of the
compound, which was dissolved in DMSO (10 mL). Depending
on the inhibitor concentration required, a proportional volume of
the compound was calculated, and deposited in a 100 mL
volumetric flask that was filled up with the corrosive solution.
Testing Preparation. The testing consisted of carbon steel
1018 specimens (2.54 x 1.25 x 0.025). Sheet specimens were
ground with silica sand retained in a mesh number 100 in a
universal mixer ERWEKA model AR 400 at 100 rpm for 72 h.
To remove the corrosion products formed on the metallic surface
after grounding, samples were ultrasonically degreased in
hexane, rinsed in double distilled water and acetone, dried in a
forced current of nitrogen and kept in a desiccator before being
exposed to the aqueous test environment.
Experiment for the Specimens Corrosion. The specimens
were immersed into the glass cells containing 100 mL of the
corrosive solution and 0 (blank) and 50 ppm of 1-4, respectively.
Experiments were performed at 60 ºC for 48 h. The cells were
placed in a rotator dynamic evaluator model C5-EDP-020-D.
After of the exposure time, the specimen was taken out and
washed with double distilled water. The corrosion product on
the steel surface was mechanically removed by rubbing it with a
brush. The specimens were washed in an ultrasonic bath with
acetone, dried in a flux of high purity nitrogen, set to dry in a
stove at 100 ºC by 2 h.
Surface Examination. Blistering surfaces were observed and
recording by using a velocity high camera model Kodak Mega
Plus ES310/T; controlled by a PC through the IMAQ Vision
Builder software provided by National Instruments.
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