Synthesis, spectroscopic and electrochemical characterization of secnidazole esters

Article abstract of DOI:10.1016/j.molstruc.2017.08.036

We report a low-cost, less toxic to environment and simple method for the esterification of secnidazole. This is first comprehensive structural characterization of novel secnidazole esters by the spectroscopic and electrochemical methods. The important EI

Full text of DOI:10.1016/j.molstruc.2017.08.036

Accepted Manuscript
Synthesis, spectroscopic and electrochemical characterization of secnidazole esters
Hafiz Abdullah Shahid, Sajid Jahangir, Muddasir Hanif, Tianrou Xiong, Haji
Muhammad, Sana Wahid, Sammer Yousuf, Naseem Qureshi
PII:
S0022-2860(17)31104-3
10.1016/j.molstruc.2017.08.036
MOLSTR 24174
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 25 June 2017
Revised Date: 9 August 2017
Accepted Date: 10 August 2017
Please cite this article as: H.A. Shahid, S. Jahangir, M. Hanif, T. Xiong, H. Muhammad, S. Wahid, S.
Yousuf, N. Qureshi, Synthesis, spectroscopic and electrochemical characterization of secnidazole
esters, Journal of Molecular Structure (2017), doi: 10.1016/j.molstruc.2017.08.036.
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ACCEPTED MANUSCRIPT
Graphical Abstract:

ACCEPTED MANUSCRIPT
Synthesis, spectroscopic and electrochemical characterization of
secnidazole esters
a
a
b
b
Hafiz Abdullah Shahid, Sajid Jahangir, * Muddasir Hanif, * Tianrou Xiong ,
a
c
d
Haji Muhammad, Sana Wahid, Sammer Yousuf, Naseem Qureshi, *
a
Department of Chemistry, Faculty of Science, Federal Urdu University of Arts, Science and Technology, Gulshan-e-Iqbal, Karachi 75300, Pakistan
b
Department of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang, Jiangxi 330022, People’s Republic of China
c
HEJ Research Institute of Chemistry, International Center for Chemical and Biological Sciences, University of Karachi, Karachi 75270, Pakistan
d
Department of Chemistry, Karakoram International University, Gilgit-Baltistan,15100 Gilgit, Pakistan
*Corresponding Authors; E-mail: sajid.jahangir@fuuast.edu.pk (S. J.), muddasirhanif@yahoo.com (M. H.), ngq18@hotmail.com (N.Q.)
Abstract: We report a low-cost, less toxic to environment and simple method for the
esterification of secnidazole. This is first comprehensive structural characterization of novel
secnidazole esters by the spectroscopic and electrochemical methods. The important EIMS
fragmentation analysis showed unique contribution of heteroatom bonds explained by the
fragmentation patterns. These peaks originate from the loss of single electron, loss of HCN,
1
3
M-O, M-NO, M-NO , M-C H N O , and M-C H N O . The comparison of C NMR
2
7
10
3
3
8
10
3
4
predicted values with the experimental values showed that ChemBioDraw Ultra 14.0 has
2
3
advantage of predicting aromatic (sp ) carbons, while MestReNova 6.1 predicts sp hybrid
carbons more accurately. The electrochemical properties indicated an irreversible oxidation
process and reversible reduction process in these ester molecules similar to the parent
secnidazole.
Keywords: Synthesis, EIMS fragmentation, Hetero Atom, Spectroscopic Characterization,
NMR prediction, Electrochemistry
1

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1
. Introduction
Esterification is a well-known and very important reaction in chemistry. The German
chemist Leopold Gmelin (1788-1853) born in Göttingen into a distinguished family of
chemists, coined the word “ester”. [1] Esters are widespread in nature (eg. triesters)
[
2-4] found in many types of food and responsible for the characteristic aroma and
medicinal effects.[5-8] Polyesters, polyethylene terephthalate, acrylate esters, and
cellulose acetate are some examples of the industrially produced esters in very large
quantities.[9-13]
Esterification can be regarded as the transformation of the carboxylic acids or their
derivatives into esters. Similarly, the counterpart esterification reaction: the
transformation of alcohols into esters (acylation of hydroxy groups) is equally
important. [14]
Fig. 1 Chemical structures of common 5-nitroimidazole and clioquinol
The Fig. 1 shows some 5-nitroimidazoles useful as antibiotics for the treatment of
bacterial and parasitic infections. [15,16] Among these Secnidazole (Comp. 4, Fig. 1)
has been explored for the treatment of amoebiasis, giardiasis, urogenital
trichomoniasis and nonspecific bacterial vaginosis. A previous study proved that
secnidazole is significantly faster and more effective treatment than clioquinol (Comp.
5
, Fig. 1) for the patients suffering from acute intestinal amoebiasis.[17] The ester
derivatives of secnidazole and other similar compounds have been synthesized by
different synthetic methods to obtain novel compounds with enhanced biological
2

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activities, improved physiochemical properties with lower toxicity values than their
parent drugs. [18-21] Therefore esterification of drug molecules is highly desirable.
Scheme1General reaction and mechanism of the Steglich esterification
Table 1 Safety information of DMAP, DCU and Pyridine
Standard
Material
CAS
Symbol
Safety Information
Safety Code
a
a
Acute toxicity
DMAP
1122-58-3
GHS06
(
oral, dermal, inhalation, Categ: 1,2,3)
a
a
Acute toxicity(oral, dermal, inhalation, Categ: 4)
Toxic to aquatic life with long lasting effects
GHS07
H411
b
DCU
2387-23-7
110-86-1
b
a
a
Acute toxicity
Pyridine
GHS07
(
oral, dermal, inhalation, Categ: 4)
Source: www.sigmaaldrich.com, www.caymaneurope.com
a
b
There are many catalysts or reagents used for the esterification (acids, lewis acids).
22,23] For an example, the Steglich esterification reaction is a versatile (Scheme 1)
method which uses the combination of DCC (dicyclohexylcarbodiimide) and DMAP
Dimethlyaminopyridine).[24-27] Previously this method has been utilized [28] for
[
(
the esterification of secnidazole with the following molar ratios, Secnidazole:
Bromobenzoic acid:DCC:DMAP = 3:3.3:3.3:0.2 (mmol) in CH Cl2 at room
2
temperature, for 12 h and resulting product (ester) was purified through column
chromatography (ethylacetate:petroleum ether = 1:1) in 88% yield. Similar reactions
reported by other researchers indicate that yields can vary between 53-73%. [18,19]
The mechanism of Steglich reaction [29,30] shows that there can be two possible
materials left (Scheme 1) after the product (ester) formation is complete. The two
byproducts DMAP and 1,3-dicyclohexylurea (DCU) possess acute toxicity (Table 1).
This shows threat to the environment will be much higher if we apply Steglich
3

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reaction on an industrial scale. Therefore less toxic esterification methods are highly
desirable.
The easier availability of carboxylic acids allows easier excess to the corresponding
acid-chlorides. Therefore we have utilized the carboxylic acid-chlorides to esterify
(one-pot reaction) the Secnidazole (alcohol component) in presence of pyridine
present in toluene (solvent/catalyst system). The HCl released during the reaction can
be easily neutralized by a glass-u-tube connected with a conical flask containing
aqueous NaOH. This leaves pyridine in the reaction mixture, which is much less toxic
when compared to the DMAP and 1,3-dicyclohexylurea. There are other two
advantages, firstly the toluene/pyridine mixed solvent system is easier to recycle when
compared with the DMAP, 1,3-dicyclohexylurea. Secondly, simple recrystallization
gives pure product which eliminates the solvent consumption in column
chromatography thereby decreases the production cost. This esterification procedure
has the potential for application to other molecules.
In this manuscript we present environmentally safer synthesis of four new secnidazole
ester molecules spectroscopic properties and electrochemical properties. To best of
our knowledge (SCI Finder) this is first report describing a detailed synthesis,
1
13
structural characterization of secnidazole esters, by the EIMS, NMR ( H and
C
NMR), UV-vis absorption spectroscopy and cyclic voltammetry. The details of
structural properties are very important to characterize, analyze the stability and
metabolism of these molecules for present and future researchers.
. Experimental Details
2
2
.1 General Methods and Characterization
All chemicals and solvents were of reagent grade and purchased from commercial
sources and used without further purification. Secnidazole and other APIs were
4

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provided by NabiQasim Pvt. Thin layer chromatography was performed on Merck
silica gel 60F254 plates and observed under UV. Melting point was recorded in a
glass capillary tube on Gallenkamp (sonyo) apparatus. FTIR spectra were recorded
-
directly on Bruker FT-IR spectrophotometer (Vector 22) in the range of 4000-400 cm
1
1
.
EIMS and HR-MS spectra were recorded on JeolMS JMS-HX-110. H-NMR and
1
3
1
C-NMR were recorded on BrukerAvance500 spectrophotometers at 500 MHz for H
1
3
and 125 MHz for C NMR. The CV spectra were recorded on CHI660 instrument.
2
.2 Synthesis of 1-(2-methyl-5-nitro-1H-imidazol-1-yl)propan-2-yl-4-methylbenzoate (M1)
In a round bottom flask (fitted with dropping funnel, condenser, and a glass u-tube
connected to a flask containing NaOH to neutralize the HCl) containing Secnidazole (2
g, 10.8 mmol) we added toluene (20 mL), and pyridine (2 mL). The flask was heated
o
with stirring for 30 minutes at 70 C. Later 4-methylbenzoylchloride (CAS: 212-864-8,
Aldrich, 2 g, 12.9 mmol) was slowly added (through dropping funnel) to the flask and
o
stirring continued at 70 C for 4.5 h. The reaction mixture was allowed to cool and
treated with saturated aqueous NaHCO solution and water. The organic layer was
3
separated (separatory funnel) and solvent was removed (rotary/vacuum) to obtain the
product. This was recrystallized from chloroform/toluene to obtain pure white crystals
o
1
in 77% yield. M.P.: 112-114 C. H NMR (500 MHz, DMSO-d ): δ 7.964 (s, 1 H,
6
imidazole H), 7.719-7.702 (d, J = 8.5 Hz, 2 H, ArH), 7.303-7.287 (d, J=8.5 Hz, 2 H,
ArH), 5.461-5.398 (m, 1H, CH), 4.666-4.525 (m, 2 H, CH ), 2.407 (s, 3 H, CH ), 2.354
2
3
1
3
(
s, 3 H, CH ), 1.408-1.395 (d, J=6.5 Hz, 3 H, CH ). C NMR (125 MHz, DMSO-d ): δ
3 3 6
1
64.68 (C=O), 151.34 (N=C), 143.95 (aromatic carbon), 138.52 (C-NO ), 133.14,
2
1
29.31, 129.00, 126.26 (aromatic carbons), 69.09 (O-CH), 49.46 (N-CH ), 21.10
2
-1
(
CH ), 17.3 (CH ), 13.95 (CH ). FTIR (neat, ῡ, cm ): 3127, 3019, 2992, 2969, 2929,
3 3 3
+
2
876, 1718, 1430, 1532, 1366, 1270, 1191. EIMS m/z (R.A.%): 303 (M , 5), 257 (82),
5

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+
1
3
77 (4), 119 (100), 91 (20), 80 (2), 65 (5), 53 (5). HRMS [M ] calcd. for C H N O
15 17 3 4
03.1219 (found 303.1216).
2
.3 Synthesis of 1-(2-methyl-5-nitro-1H-imidazol-1-yl)propan-2-yl-3,4-dimethoxybenzoate (M2)
Similar to the above procedure we added 3,4-dimethoxybenzoyl chloride (CAS: 3535-
o
3
7-3, Aldrich, 5.7 mmol) to the hot solution (at 70 C) of secnidazole (5.5 mmol), in
toluene (20 mL) and pyridine (2 mL). The reaction mixture was allowed to cool and
treated with saturated aqueous NaHCO solution and water. The organic layer was
3
separated (separatory funnel) and solvent was removed (rotary/vacuum) to obtain the
product. This was recrystallized from chloroform/toluene to obtain pure product in
o
1
6
1% yield. M.P.: 115-117 C. H NMR (500MHz, DMSO-d ): δ 7.969 (s, 1 H,
6
imidazole H), 7.467-7.029 (m, 3 H, Ar H), 5.427-5.349 (m, 1 H, CH), 4.662-4.558 (m,
H, CH ), 3.815 (s, 3 H, CH ), 3.777 (s, 3H, CH ), 2.446 (s, 3 H, CH ), 1.405-1.389
2
2
3
3
3
1
3
(
d, J=6.4 Hz, 3 H, CH ). C NMR (125 MHz, DMSO-d ): δ 164.47 (C=O), 153.15 (O-
3
6
Ar), 151.42 (O-Ar), 148.38 (N=C), 138 (C-NO ), 133.19, 128.62, 123.09, 111.42,
2
1
11.15 (aromatic carbons), 69.08 (O-CH), 55.7, 55.45 (O-CH ), 49.36 (N-CH ), 17.37
3 2
-1
(
CH ), 13.99 (CH ). FTIR (neat, ῡ, cm ): 3122, 3079, 2960, 2934, 2874, 2860, 2841,
3 3
+
1
703, 1531, 1437, 1357, 1269, 1188. EIMS m/z (R.A.%): 349 (M , 16), 303 (5), 223
(2), 182 (15), 165 (100), 151 (4), 137 (6), 111 (4), 83 (7), 80 (3), 53 (3.0), 44 (5).
+
HRMS [M ] calcd. for C H N O 349.1268 (found 349.1281).
1
6
19
3
6
2
.4 Synthesis of 1-(2-methyl-5-nitro-1H-imidazol-1-yl)propan-2-yl-2-(4-
isobutylphenyl)propanoate (M3)
For M3, we needed the 2-(4-isobutylphenyl)propanoyl chloride which is commercially
unavailable therefore, In a separate round bottom flask (fitted with distillation set up),
we transformed 2-(4-isobutylphenyl)propanoic acid (commercial name ibuprofen,
CAS: 15687-27-1, 1.1085 g, 5.37 mmol) to 2-(4-isobutylphenyl)propanoyl chloride by
6

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reaction with oxalyl chloride (CAS: 79-37-8, 0.53 mL, 6.05 mmol) in 20 mL of
CH Cl and catalytic anhydrous DMF (3 mL) by stirring at room temperature for 12 h.
2
2
o
The solvents were removed and the remaining liquid was added to a warm (70 C),
well stirred solution (toluene 20 mL and pyridine 2 mL, round bottom flask) of
secnidazole (1 g, 5.4 mmol). After complete addition, the reaction mixture was heated
for 5 h. The reaction mixture was allowed to cool and treated with saturated aqueous
NaHCO solution and water. The organic layer was separated (separatory funnel) and
3
solvent was removed (rotary/vacuum) to obtain the product. The product was
o
1
recrystallized from toluene/hexane to obtain M3 in 67 % yield. M.P.: 93-96 C. H
NMR (500 MHz, DMSO-d ): δ 8.021 (s, 1 H, imidazole H), 7.080-7.028 (m, 4 H, Ar
6
H), 5.162-5.115 (m, 1 H, CH), 4.552-4.316 (m, 2 H, CH ), 3.575-3.522 (m,1 H, CH),
2
2
7
2
.401 (s, 2 H, CH ), 2.382 (s, 3 H, CH ), 1.838-1.737 (m, 1 H, CH), 1.205-1.187 (d, J=
2 3
.2 Hz, 3 H, CH ), 1.161-1.146 (d, J= 6 Hz, 3 H, CH ), 0.84-0.824 (d, J=6.4 Hz, 6 H,
3
3
13
CH3). C NMR (125 MHz, DMSO-d ): δ 173.14 (C=O), 151.59 (N=C), 139.79
6
(
Aromatic carbon), 138 (C-NO ), 137.32, 133.08, 129.05, 126.78 (Aromatic carbons),
2
6
2
3
9.25 (O-CH), 49.22 (N-CH ), 44.12 (CH), 44.00 (CH ), 29.50 (CH), 22.10 (CH ),
2 2 3
-1
2.08 (CH ), 18.15 (CH ), 16.90 (CH ), 13.97 (CH ). FTIR (neat, ῡ, cm ): 3130, 3051,
3
3
3
3
011, 2984, 2956, 2928, 2867, 2845, 1736, 1531, 1429, 1377, 1264, 1195. EIMS m/z
+
(
(
(
(
R.A.%): 373 (M , 5), 356 (47), 357(19), 330 (100), 327 (2), 247 (17), 245(34), 231
46), 232 (8), 219 (86), 188 (43), 170 (22), 168 (3), 161 (97), 145 (64), 118 (66), 105
+
15), 91 (22), 80 (15), 53 (15), 42 (10). HRMS [M ] calcd. for C H N O 373.2002
2
0
27
3
4
found 373.1982).
2
.5 Synthesis of Bis(1-(2-methyl-5-nitro-1H-imidazol-1-yl)propan-2-yl) oxalate (M4)
In a round bottom flask (fitted with dropping funnel, condenser, and a glass u-tube
connected to a flask containing Na CO to neutralize the HCl) containing Secnidazole
2
3
7

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(2 g, 10.8 mmol) we added toluene (20 mL), and pyridine (2 mL). The flask was
o
heated with stirring for 30 minutes at 40 C. Later Oxalyl chloride (CAS: 79-37-8, 5.6
mmol) was carefully added (through dropping funnel) to the flask and stirring
o
continued at 40 C for 2.5 h. The reaction mixture was allowed to cool and treated with
saturated aqueous NaHCO solution and water. The organic layer was separated and
3
solvent was removed to obtain the product. The product was recrystallized by using
DMF/CH Cl to obtain 91% product.
2
2
o
1
M.P.: 201-203 C. H NMR (500 MHz, DMSO-d ): δ 7.997 (s, 2 H, 2imidazole H),
6
5
1
1
4
2
.242-5.182 (m, 2 H, 2CH), 4.636-4.404 (m, 4 H, 2CH ), 2.434 (s, 6 H, 2CH ), 1.342-
2 3
1
3
.320 (d, J=6.6 Hz, 6 H, 2CH ). C NMR (125 MHz, DMSO-d ): δ 155.376 (C=O),
3
6
51.778 (N=C), 138.452 (C-NO ), 133.141(aromatic carbons), 72.563 (O-CH),
2
-1
8.954 (N-CH ), 16.58 (CH ), 13.913 (CH ). FTIR (neat, ῡ, cm ): 3129, 3020, 2990,
2
3
3
+
945, 1741, 1528, 1429, 1372, 1267, 1191. EI-MS m/z (R.A.%): 424 (M , 6), 407
(43), 408 (9),378 (68), 210(2), 184(8), 168 (100), 151 (27), 137 (19), 128 (25), 111
+
(
14), 95 (43), 80 (75), 67 (27), 53 (75), 42 (53). HRMS [M ] calcd.for C H N O
16
20
6
8
4
24.1337 (found 424.1347).
8

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3
. Results and Discussion
3
.1 Synthesis
Scheme 2 Synthesis route for the secnidazole esters (M1, M2, M3 and M4)
The Scheme 2 shows the synthetic route to the secnidazole esters (M1, M2, M3 and
M4). The reaction has the advantage of shorter time, when compared to the Steglich
reaction applied to the secnidazole likely due to higher reactivity of carboxylic acid-
chlorides.
Scheme 3 Proposed esterification reaction-mechanism for the secnidazole
The products were purified by simple workup, washing with aqueous NaHCO3,
vacuum evaporation of solvent, and recrystallization which eliminates the solvent
consumption in column chromatography thereby decreases the production cost. The
other advantage is easier recycling of mixed catalytic solvent system
(toluene/pyridine). The M4 shows an example in which oxalyl chloride was used
instead of aromatic carboxylic acid-chloride. This reaction happens at relatively lower
9

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temperature because of higher reactivity of oxalyl chloride. The scheme 3 shows a
proposed mechanism for the esterification under the given reaction conditions. A
comparison of Table 1, scheme 1 and scheme 3 shows that the reaction is relatively
simple. In addition, the byproducts (pyridine and HCl) of this reaction are less toxic
therefore, relatively safer for the environment when compared to the Steglich
esterification reaction.
3
.2 Spectroscopic Properties
(
a)
1
EIMS of M1
(b)
EIMS of M2
00
1
00
8
6
4
0
0
0
80
60
40
20
0
20
0
60
90
120
150
180
210
240
270
300
330
60
90
120
150
180
210
240
270
300
330
360
390
m/z
m/z
(
c)1
(d)100 EIMS of M4
00
EIMS of M3
80
60
40
20
0
80
60
40
20
0
60
90
120
150
180
210
240
270
300
330
360
390
30
60
90
120
150
180
210
240
270
300
330
360
390
420
m/z
m/z
Fig. 2 EIMS of M1(a), M2(b), M3(c) and M4(d).
1
0

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1
1

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Scheme 4 EIMS fragmentation patterns of 1-methyl-4-nitro-1H-imidazole, M1, M2, M3 and M4
1
We studied the EIMS (Fig. 2, Fig.SA, Scheme 4), FTIR (Fig. 3, Table 2), H-NMR
1
3
(
Fig.SB, Fig.4) and C-NMR (Fig.SC, Fig.4) spectra (supporting information) to
confirm the molecular structures. Although molecular structure of organic molecules
could be confirmed through NMR but many research institutes and pharmaceutical
industries routinely use FTIR and Mass spectrometry analysis therefore these
techniques are very important for routine analysis. To best of our knowledge this is
first report describing a detailed characterization of secnidazole esters, by the EIMS
and NMR spectroscopy. The mass spectrometry (EIMS) provided the molecular
weights (molecular ion peaks or M) of four compounds equal to the calculated
molecular weights. The EIMS spectra indicate characteristic fragmentation patterns
(Scheme 4). To understand the fragmentation patterns of four ester molecules we first
studied the fragmentation pattern of the 1-methyl-4-nitro-1H-imidazole [31-33] and
different ester molecules (Scheme 4).We have observed some similarities (highlighted
in blue color) found in the fragmentation patterns useful to identify these molecules.
The important EIMS peaks showed unique contribution of heteroatom bonds. The important
peaks in the EIMS appear due to the loss of single electron, loss of HCN, M-O, M-
NO, M-NO , M-C H N O , and M-C H N O . The comparison of fragmentation
2
7
10
3
3
8
10
3
4
1
2

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patterns (M1, M2, M3 and M4) clearly indicate that nitroimidazoles and ester linkages
can be easily confirmed through EIMS. The fragmentation patterns will greatly help
in future to fast identify and analyze these molecules.[33]
M4
3129
1741
M3
M2
3130
1736
3122
1703
M1
M4
M3
M2
M1
3127
1718
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm )
Fig. 3 FTIR of M1, M2, M3 and M4
Table 2 FTIR Characterization
Secnidazole
M1
M2
(cm
M3
(cm
M4
(cm )
Functional Groups
Ar-H str.
-1
-1
-1
-1
-1
(
cm
)
(cm
3127, 3019
992, 2969,
929, 2876
1718
)
)
)
3133
3122, 3079,
2960, 2934, 2874,
2860, 2841
1703
3130, 3051, 3011
2984, 2956, 2928,
2867, 2845
1736
3129, 3020
2990, 2945
2
2
Alk-CH str.
2918, 2896
C=O (ester)
-
1741
C=N str.(imidazole)
1430
1430
1437
1429
1429
Ar-NO
2
(sym./asym. str.)
C-N, C-O
1538, 1367
1264, 1184
1532, 1366
1270, 1191
1531,1357
1269, 1188
1531, 1377
1264,1195
1528, 1372
1267, 1191
We have compared the important FTIR peaks of the four products (Table 2) with the
previously known FTIR of secnidazole (starting material) to identify esterification and
-1
presence of other functional groups.[34,35] Similar to the secnidazole (3133 cm ) the
-1
Ar-H stretching vibrations at 3127, 3122, 3130, 3129 cm can be assigned to the
imidazole aromatic ring of the M1, M2, M3 and M4 respectively. Since after
esterification the molecular structure gets additional aromatic rings so we observed
-1
additional weak bands 3079-3011 cm due to the Ar-H stretching. A series of Alkyl-
-1
C-H stretching vibrations (2992-2841 cm ) were observed for the four compounds.
The ester formation can be recognized by the C=O functional group observed at 1718,
-1
1
703, 1736 and 1741 cm for the M1, M2, M3 and M4 respectively. The –NO group
2
-
1
of secnidazole shows two bands (1538, 1367 cm ) due to symmetric and asymmetric
1
3

ACCEPTED MANUSCRIPT
stretches. Similarly we observed Ar-NO group of the M1, M2, M3 and M4 showing
2
-1
two peaks respectively at 1532-1366, 1531-1357, 1531-1377, 1528-1372 cm . The
-1
secnidazole shows two vibrations (1264, 1184 cm ) for the C-N stretches; similar
-1
bands were observed (1270-1188 cm ) for the 4 ester molecules. This region is
common with the vibration region of C-O therefore there is chance of overlapped
bands.
1
13
Fig. 4 H NMR (500 MHz, DMSO-d
6
) and C NMR (125 MHz, DMSO-d
6
) chemical shift values
1
13
The Fig. 4 shows the H NMR (500 MHz, DMSO-d ) and C NMR (125 MHz,
6
DMSO-d , DEPT 90, DEPT 135) characterization for the different positions of the
6
1
M1, M2, M3 and M4 structures. The bold numbers (black) show the H NMR
1
3
chemical shift values while italic (blue) numbers indicate the C NMR chemical shift
values.
The NMR chemical shift values of secnidazole part can be characterized by the
reported NMR chemical shift values of secnidazole (Fig. 4). [36,37] The other part
can be characterized through NMR analysis [38], experimental results (supporting
1
3
information) and comparison with predicted C NMR chemical shift values
(ChemBioDraw Ultra 14.0, MestReNova 6.1) as summarized in Table 3.
1
4

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13
Table 3 C NMR Characterization
1
3
13
Materia
l
Predicted C NMR
Experimental
Predicted C NMR
13
(ChemBioDraw Ultra 14.0)
C NMR
(MestReNova 6.1)
Secni
dazole
M1
M2
M3
1
8.1
NO2
1
29.0
1
37.3
O
138.0
2
2.0 126.7
N
133.0
2
9.5
O
1
29.0
16.9
N
2
2.1
126.7
1
3.9
M4
1
3
The comparison of C NMR predicted chemical shift values with the experimental
(Table 3) showed that ChemBioDraw Ultra 14.0 is more capable of predicting
2
3
aromatic or sp carbons, while MestReNova 6.1 predicts sp hybrid carbons more
accurately. Therefore the two software programs provided useful information. The
1
3
details of NMR chemical shift values are given in the experimental section. The C
NMR chemical shift values of C=O (ester) for M1, M2, M3 and M4 respectively
appear at 164.6, 164.4, 173.1 and 155.3 ppm. The higher variation is due to different
neighbours of C=O (ester) imparting different inductive and mesomeric effects. To
1
3
best of our knowledge C NMR value 155.3 ppm for the C=O (ester, M4) is rarely
1
3
observed in scientific literature. The lower C NMR value of C=O (155.3 ppm) is
likely due to the adjacent oxygen atoms and the anisotropic effect (directionally
dependentπ-electron density distribution) from the adjacent C=O group [39, 40]
making the carbon more shielded, thereby moving the NMR chemical shift values
1
5

ACCEPTED MANUSCRIPT
relatively upfield. The ArC-NO appears in the range 138.5-138.0 ppm which shows
2
very small variation likely because NMR chemical shift values of the imidazole ring
show minor change.
3
.3 Optical and Electrochemical Properties
(
b) 27
0
-1.03 V
(a)
M4
1.03 V
323 nm
-1.00 V
-
27
54
28
0
-
M4
-
1.17 V
-1.05 V
3
24 nm
24 nm
0.91 V
M3
M2
-
28
56
-1.03 V
-
M3
M2
-
1.14 V
2
7
0
-1.05 V
3
00 nm
0.91 V
0.82 V
3
-
27
54
-1.02 V
-
-1.14 V
-1.03 V
2
9
0
324 nm
M1
-
1.02 V
-
29
58
-
M1
-
1.17 V
2
9
0
-
1.03 V
326 nm
0.93 V
Secnidazole
-
1.02 V
-
29
58
Positive Scan
Negative Scan
-
Secnidazole
-
1.18 V
280
300
320
340
360
380
400
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Wavelength (nm)
Potential (V) vs SCE
Fig. 5 UV-vis absorbance spectra (a) and Cyclic voltammograms (b) of the Secnidzole, M1, M2, M3 and M4
Table 4 Summary of UV-vis and Electrochemical Properties
d
0′
e
a
opt
E
Reversibility
(negative scan)
UV-vis
E
g
b
b
E
ox (V)
Ered (V)
c
Reversibility
(positive scan)
c
Formal
Potential
negative Scan)
Molecule
/EHOMO (eV)
negative Scan)
/ELUMO (eV)
(negative Scan)
λ
max (nm)
(eV)
i
p,r /ip,f
(
(
(
V)
-
-
-
-
-
1.104
1.106
1.098
1.101
1.103
0.89
0.84
0.82
0.92
0.95
Secnidazole
326
324
3.229
3.271
3.289
3.280
3.271
0.93/5.33
0.82/5.22
0.91/5.31
0.91/5.31
1.03/5.43
NO
NO
NO
NO
NO
-1.02/3.38
-1.02/3.38
-1.02/3.38
-1.03/3.37
-1.00/3.40
M1
M2
M3
M4
300, 324
324
323
a
b
c
d
e
opt
1
2
3
4
5
.
.
.
.
.
Optical band gap, E
Onset-peak potentials vs. SCE.
Energy levels are estimated from EHOMO/LUMOꢀ=ꢀ−(Eox/redꢀ+ꢀ4.4) eV.
E
i
g
ꢀ=ꢀ1240/λonset
0′
= Ep,f + Ep,r /2; Ep,f and Ep,r represent the forward and reverse peak potentials.
p,r /ip,f = 1, reversible
1
6

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The UV-vis absorbance spectra of secnidazole and ester molecules were measured
in DMSO (Fig. 5a) and the data is summarized in Table 4. The UV-vis spectra
showed broad bands [41] with λmax = 326, 324, (300, 324), 324, 323 nm for the
secnidazole, M1, M2, M3 and M4 respectively attributed to the π-π* electronic
transition of the aromatic ring. A new band at 300 nm observed for M2 can be
attributed to the dimethoxybenzene. From the onset of spectra we calculated the
opt
optical band gap (Eg ) of these molecules about 3.2 eV.
Electrochemical properties of Secnidzole, M1, M2, M3 and M4 were investigated
by the cyclic voltammetry (CV, DMSO solvent, WE: GC, CE: Pt wire, and Bu NClO4
4
as electrolyte) at the scan rate = 0.1 V/s (Fig.5b, Table 4). It is well known that If a
reduced/oxidized molecule is stable (time scale of the experiment) in solution and
there is reversible electron transfer reactions in both directions (forward/reverse) then
the peak current observed for the return potential scan (i ) should be equal (i /i =
p,r
p,r p,f
1
) to that seen for the forward potential scan (i ) in a cyclic voltammetry experiment.
p,f
All the molecules showed i /i close to 1 (Table 4) for the negative scan, therefore
p,r p,f
we conclude that they undergo reversible reduction process. This occurs due to one
electron reduction of the nitro (–NO ) group attached to the imidazole ring (R-NO +
2
2
−
•−
e
RNO ). [42,43] The metabolism mechanism studies have shown that this
2
is an essential property required for the anti-bacterial (anti-protozoal) activity of the
secnidazole and related molecules. [20]
An irreversible oxidation process (+ve scan, Fig. 5b) was observed (onset oxidation
potentials, E vs SCE) at 0.93, 0.82, 0.91, 0.91 and 1.03 V for the secnidazole, M1,
,
ox
M2, M3 and M4 respectively. From the oxidation potential values, the HOMO energy
levels (EHOMO) were calculated (5.33, 5.22, 5.31, 5.31, and 5.43 eV). Similarly from
1
7

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the onset reduction potentials values (Table 4), the calculated LUMO energy levels
(ELUMO) are 3.38, 3.38, 3.38, 3.37, and 3.40 eV respectively.
4
. Conclusions
In summary, we have synthesized four new secnidazole esters. The reactions provided
moderate to very good yields. The esterification procedure is less toxic, products are
easy to purify, solvent system (toluene/pyridine) can be recycled therefore has higher
environmental safety and experimental simplicity. The structures were studied by the
1
13
EIMS, FTIR, H NMR, C NMR, UV-vis absorption and cyclic voltammetry. The
important EIMS peaks showed unique contribution of heteroatom bonds. These peaks
originate from the loss of single electron, loss of HCN, M-O, M-NO, M-NO , M-
2
C H N O , and M-C H N O . The comparison of EIMS fragmentation patterns
7
10
3
3
8
10
3
4
indicate that nitroimidazole and ester linkages (heteroatom linkages) can be confirmed
through the EIMS. This will greatly help to fast identify and analyze these molecules.
1
3
The comparison of C NMR predicted values with the experimental values showed
2
that ChemBioDraw Ultra 14.0 has advantage of predicting aromatic or sp carbons,
3
while MestReNova 6.1 predicts sp hybrid carbons more accurately. Therefore the
two software programs provided useful information. The electrochemical properties
indicated an irreversible oxidation process and reversible reduction process in the four
ester molecules similar to the parent secnidazole. The HOMO energy levels (EHOMO
)
are 5.33, 5.22, 5.31, 5.31, and 5.43 eV (secnidazole, M1, M2, M3 and M4). Similarly
the LUMO energy levels (ELUMO) are 3.38, 3.38, 3.38, 3.37, and 3.40 eV respectively.
Acknowledgements We highly acknowledge the National Natural Science Foundation of China
Grant Number: 21374044) and HEC Pakistan for supporting this research work.
(
Supplementary Data Spectroscopic Spectral data related to this article is available in the online
supporting information.
1
8

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0

Fig. 1 Chemical structures of common 5-nitroimidazole and clioquinol

1
13
Fig. 4 H NMR (500 MHz, DMSO-d ) and C NMR (125 MHz, DMSO-d ) chemical shift values
6
6

Scheme1General reaction and mechanism of the Steglich esterification

Scheme 2 Synthesis route for the Secnidazole esters (M1, M2, M3 and M4)

Scheme 3 Proposed esterification reaction-mechanism for the secnidazole

Scheme 4

Scheme 4

Scheme 4