RETRACTED ARTICLE: A facile sonochemical protocol for synthesis of 3-amino- And 4-amino-1,2,4-triazole derived Schiff bases as potential antibacterial agents

Article abstract of DOI:10.1371/journal.pone.0229891

A facile method has been developed for the synthesis of Schiff bases derived from substituted and unsubstituted 3-amino- and 4-amino-1,2,4-triazoles. Condensation of the aminotrizoles with a variety of aromatic aldehydes afforded desired Schiff bases in excellent yields in 3-5 minutes of exposure to ultra-sound. The synthesized compounds were characterized by means of IR, 1HNMR and Mass spectrometry. The synthesized compounds were also screened for their antibacterial potential against Gram-negative (Escherichia coli, Shigella sonnei, Pseudomonas aeruginosa and Salmonella typhi) and two Gram-positive (Staphylococcus aureus and Bacillus subtilis) strains.

Full text of DOI:10.1371/journal.pone.0229891

PLOS ONE
RESEARCH ARTICLE
A facile sonochemical protocol for synthesis
of 3-amino- and 4-amino-1,2,4-triazole
derived Schiff bases as potential antibacterial
agents
Aeysha Sultan1, Mian Habib Ur Rehman1, Noreen Sajjad2, Ali Irfan3, Irfan Ullah4,
Muhammad Mustaqeem4, Muhammad Saleem4, Syeda Laila Rubab1, Muhammad Rafiq5,
8
Muhammad Khalid6, Katarzyna Kotwica-Mojzych7, Mariusz Mojzych
*
ID
1 Division of Science and Technology, Department of Chemistry, University of Education, Lahore, Pakistan,
2 Department of Chemistry, University of Lahore, Lahore, Pakistan, 3 Department of Chemistry, University of
Lahore Sargodha campus, Sargodha, Pakistan, 4 Department of Chemistry, University of Sargodha Sub-
campus Bhakkar, Bhakkar, Pakistan, 5 Department of Physiology and Biochemistry, Cholistan University of
Veterinary and Animal Sciences, Bahawalpur, Pakistan, 6 Department of Chemistry, Khwaja Ghulam Fareed
University of Science and Technology, Rahimyar Khan, Pakistan, 7 Chair and Department of Histology,
Embryology and Cytophisiology, Medical University of Lublin, (Collegium Medicum), Lublin, Poland,
8 Department of Chemistry, Siedlce University of Natural Sciences and Humanities, Siedlce, Poland
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* mmojzych@yahoo.com
Abstract
OPEN ACCESS
Citation: Sultan A, Ur Rehman MH, Sajjad N, Irfan
A, Ullah I, Mustaqeem M, et al. (2020) A facile
sonochemical protocol for synthesis of 3-amino-
and 4-amino-1,2,4-triazole derived Schiff bases as
potential antibacterial agents. PLoS ONE 15(6):
e0229891. https://doi.org/10.1371/journal.
pone.0229891
A facile method has been developed for the synthesis of Schiff bases derived from substi-
tuted and unsubstituted 3-amino- and 4-amino-1,2,4-triazoles. Condensation of the amino-
trizoles with a variety of aromatic aldehydes afforded desired Schiff bases in excellent yields
in 3–5 minutes of exposure to ultra-sound. The synthesized compounds were characterized
by means of IR, 1HNMR and Mass spectrometry. The synthesized compounds were also
screened for their antibacterial potential against Gram-negative (Escherichia coli, Shigella
sonnei, Pseudomonas aeruginosa and Salmonella typhi) and two Gram-positive (Staphylo-
coccus aureus and Bacillus subtilis) strains.
Editor: Mohammad Shahid, Aligarh Muslim
University, INDIA
Received: October 8, 2019
Accepted: February 18, 2020
Published: June 4, 2020
Copyright: © 2020 Sultan et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Introduction
Schiff bases are the condensation products of primary amines and carbonyl compounds,[1]
named after Hugo Schiff, who discovered them in 1864.[2] Schiff bases have been well docu-
mented for their wide spectrum potential as chemotherapeutic agents. The biological and
chemical importance of the Schiff bases centers around the presence of azomethine group; it is
the lone pair on nitrogen of azomethine that plays the key role.[3,4]
Data Availability Statement: All relevant data are
within the paper.
Schiff bases derivatives of aliphatic as well as aromatic aldehydes and ketones have been
well reported; however, in general the stability of aliphatic aldehydes derived Schiff bases is
quite inferior to that of aromatic aldehydes. The former being more prone to polymerization
while the latter being more stable due to conjugation. [5]. Heterocyclic Schiff bases moiety is
of special importance in medicinal chemistry and a lot of active research is being done on this
Funding: The authors received no specific funding
for this work.
Competing interests: The authors have declared
that no competing interests exist.
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A facile sonochemical synthesis of 3-amino and 4-amino-1,2,4-triazole Schiff bases as antibacterial agents
pharmacophore. In addition to their importance in medicial chemistry, heterocyclic Schiff
bases find promising potential applications as: sensors (optical and electrical), intermediates in
organic reactions, dyes, pigments and as catalysts [6–9].
Hetereocyclic Schiff bases have been extensively documented for their potential as: antifun-
gal [10,11], antibacterial [12,13], antiproliferative [14], anticoagulant [15], anti-inflammatory
[16], and antiviral [17] agents. These heterocyclic Schiff bases have the added advantages of:
ease of synthesis, electronic properties, higher solubility in organic solvents and most impor-
tantly for their potential role as chelating ligands in coordination chemistry [18].
The 1,2,4-triazole motif is a part of a large number of chemotherapeutic drugs [19,20]
which are useful as: anti-inflammatory [21,22], anti-depressant, antiviral, antifungal, antimi-
crobial [23–27], anticancer properties [28–31], antitubercular [32–35] and analgesic [36],
activities. Owing to the huge possibility of exploration of biologically active molecules contain-
ing this moety, a lot of work has been done on the synthesis of 1,2,4-triazole Schiff base deriva-
tives [37–41].
A lot of work has been done to device strategies for the synthesis of Schiff bases; the most
trivial method involves acid catalyzed condensation of an aldehyde (or ketone) with a primary
amine under refluxing conditions. [42]. Due to attempts towards greener chemistry
approaches, newer and unconventional methods are being explored in the field of synthetic
chemistry. Various non-conventional methods employed in the synthesis of Schiff bases
include: microwave assisted synthesis [43] and click chemistry [44–49].
The presented work is concerned with the synthesis of ultrasound assisted synthesis of
Schiff bases derived from substituted and unsubstituted 3-amino and 4-amino-1,2,4-triazoles.
The reaction outcome has also been compared with that of conventional method. Synthesized
Schiff bases have also been evaluated for their antibacterial potential.
Material and methods
The TLC was carried out on pre-coated silica gel (0.25 mm thick layer over Al sheet, Merck,
Darmstadt, Germany) with fluorescent indicator. The spots were visualized under UV lamps
(λ 365 and 254 nm) of 8 W power or KMnO4 dip and heating. The compounds were purified
either on a glass column packed silica gel (0.6–0.2 mm, 60Å mesh size, Merck) or by crystalli-
zation. All solutions were concentrated under reduced pressure (25 mm of Hg) on a rotary
evaporator (Laborota 4001, Heidolph, Germany) at 35–40˚C. Melting points were determined
using a MF-8 (Gallenkamp, Burladingen, Germany) instrument and are reported uncorrected.
The IR-spectra are recorded on Prestige 21 spectrophotometer (Shimadzu, Japan) as KBr
discs. The LREIMS are carried out on a Fisons Autospec Mass Spectrometer (VG, New Jersey,
USA). The 1H (300, 400 and 500 MHz) and 13C-NMR (75 MHz) are recorded on AM-300, 400
and 500 MHz instruments (Bruker, Massachusetts, USA) in CDCl3 using TMS as internal
standard. In spectroscopic data "A" means yield from conventional method and "B" means
yield from sonochemical method.
Representative conventional procedure for synthesis of triazole based
Schiff bases
An equimolar mixture of amino-1,2,4-triazoles (10 mmol) and respective aldehydes (10
mmol) in methanol (40 mL) were refluxed for ~5 h. The completion of reaction was monitored
by TLC. The reaction mixture was cooled to room temperature and filtered. After an hour,
solid products were obtained from the solution. The precipitated products were recrystallized
with hot ethanol.
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A facile sonochemical synthesis of 3-amino and 4-amino-1,2,4-triazole Schiff bases as antibacterial agents
Representative sonochemical procedure for synthesis of triazole based
Schiff bases
An equimolar mixture of amino-1,2,4-triazoles (1 mmol) and respective aldehydes (1 mmol)
in methanol (4 mL) was subjected to ultra sound for ~5 minutes. The product started to pre-
cipitate within 3–4 minutes of sonochemical procedure. The completion of reaction was moni-
tored by TLC. The solid product thus obtained was centrifuges and isolated by decantation of
mother liquor. The isolated products were recrystallized with hot ethanol.
1-Phenyl-N-(4H-1,2,4-triazol-3-yl)methanimine, 6
Yield: 86% (A), 96% (B), M.p: 196˚C. IR (KBr, cm-1): 3044 (N-H), 1634 (HC = N), 1592
(C = N), 1019 (N-N). 1H-NMR (DMSO-d6, δ): 8.34 (s, 1H, azomethine), 7.81–7.53 (m, 5H,
Ar-H), 6.25 (s, 1H, N-CH = N). 13C-NMR (DMSO-d6, δ): 158.19 (HC = N), 153.83 (CH, tria-
zole), 150.21 (C = N, triazole), 137.17 (C, Ar), 132.50 (C, Ar), 130.24 (CH, Ar), 127.38 (CH,
Ar); EI MS: 178 [M] (67%), 177 [M-H]+ (100%). ESI MS (C9H8N4): 172.074905 (found);
172.074896 (calc).
2-(4H-1,2,4-Triazol-3-yl)iminomethylphenol, 7
Yield: 61% A), 96% (B); M.p: 264–265˚C. IR (KBr, cm-1): 3188 (N-H), 3021 (O-H), 1624
(HC = N), 1605 (C = N), 1021 (N-N). 1H-NMR (DMSO-d6, δ): 8.78 (s, 1H, Azomethine),
7.84–7.13 (m, 4H, Ar-H), 6.25 (s, 1H, N-CH = N). 13C-NMR (DMSO-d6, δ): 159.14 (HC = N),
164.35 (C1, Ar), 153.23 (C5), 150.66 (C2), 134.11(CH, Ar), 121.13(CH, Ar), 116.08 (C, Ar). ESI
MS (C9H8N4O): 188.069834 (found); 188.069811 (calc).
3-[(E)-4H-1,2,4-Triazol-3-yliminomethyl]naphthalen-2-ol, 8
Yield = 75% (A), 98% (B), M.p: 271˚C. IR (KBr, cm-1): 3284 (O-H), 3089 (N-H), 1626
(HC = N), 1528 (C = N), 1023 (N-N). 1H-NMR (DMSO-d6, δ): 8.53 (s, 1H, azomethine), 8.27–
7.35 (m, 6H, Ar-H), 9.24 (s, 1H, N-CH = N). 13C-NMR (DMSO-d6, δ): 159 (HC = N), 156
(C = C�-OH), 153 (N-CH = N), 150 ((N)2 -C = N), 140.1(1C, Ar), 128.15(1C, Ar), 126.08 (2C,
Ar), 122.10 (2C, Ar), 115.93 (1C, Ar), 111.34 (2C, Ar). ESI MS (C13H10N4O): 238.086001
(found), 238.085461 (calc).
3-(4H-1,2,4-Triazol-3-yl)iminomethylbenzene-1,2-diol, 9
Yield = 58% (A), 92% (B). M.p: 282˚C. IR (KBr, cm-1): 3273 (O-H), 3074 (N-H), 1623
(HC = N), 1601 (C = N), 1031 (N-N). 1H-NMR (DMSO-d6, δ): 3.14 (s, 2H, OH), 7.34–7.03
(m, 3H, Ar-H), 8.98 (s, 1H, azomethine), 9.23 (s, 1H, triazole); 13C-NMR (DMSO-d6, δ):
159.97 (HC = N), 154.23 (C, Ar), 153.24 (N-CH = N), 150.65 (C2, triazole), 148.56 (C, Ar),
128.61(C, Ar), 122.45 (C, Ar), 117.38 (CH, Ar). ESI MS (C9H8N4O2): 204.064681 (found);
204.064726 (calc).
N,N-Dimethyl-4-[(E)-(4H-1,2,4-triazol-3-ylimino)methyl]aniline, 10
Yield: 79% (A), 99% (B); M.p: 203˚C. IR (KBr, cm-1): 3092 (N-H), 1623 (HC = N), 1539
(C = N), 1032 (N-N). 1H-NMR (DMSO-d6, δ): 3.15 (s, 6H, (CH3)2-N), 6.23 (s, 1H, azo-
methine), 7.52–6.86 (m, 4H, Ar-H), 9.56 (s, 1H, HC = N); 13C-NMR (DMSO-d6, δ): 159.08
(HC = N), 153.02 (N-CH = N), 150.20 (C2, triazole), 132 (C, Ar), 127.06 (CH, Ar), 121.07 (C,
Ar), 113.58 (CH, Ar), 43.33 [N(CH3)2]; ESI MS (C9H8N4O2): 215.117108 (found); 215.117095
(calc).
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A facile sonochemical synthesis of 3-amino and 4-amino-1,2,4-triazole Schiff bases as antibacterial agents
1-(4-Nitrophenyl)-N-(4H-1,2,4-triazol-3-yl)methanimine, 11
Yield: 67% (A), 97% (B); M.p: 269˚C. IR (KBr, cm-1): 3102 (O-H), 3097 (N-H), 1638 (HC = N),
1595 (C = N), 1011 (N-N); 1H-NMR (DMSO-d6, δ): 6.23 (s, 1H, azomethine), 8.91–8.32 (m,
4H, Ar-H), 9.86 (s, 1H, HC = N); 13C-NMR (DMSO-d6, δ): 164.36 (HC = N), 153.83 (C5, tria-
zole), 150.22 (C, Ar), 151.73 (CH, Ar), 148.06 (C, Ar), 127.66 (CH, Ar), 124.57 (C, Ar). ESI MS
(C9H7N5O2): 217.06009 (found), 217.059975 (calc).
2-Ethoxy-4-[(E)-(4H-1,2,4-Triazol-3-ylimino)methyl]phenol, 12
Yield: 82% (A), 98% (B), M.p: 152˚C. IR (KBr, cm-1): 3073 (N-H), 1633 (HC = N), 1580
(C = N), 1030 (N-N), 1H-NMR (DMSO-d6, δ): 1.15 (t, 3H, CH3), 4.16 (q, 2H, CH2), 7.53–6.96
(m, 3H, Ar-H), 8.38 (s, 1H, azomethone), 9.26 (s, 1H, CH, triazole), 13C NMR (DMSO–d6, δ):
161.18 (HC = N), 153.79 (N-CH = N), 150.26 (C2, triazole), 150.05 (CH, Ar), 148.00 (C, Ar),
132.50 (C, Ar), 125.06 (C, Ar), 117.33 (CH, Ar), 110.82 (C, Ar), 65.65 (CH2), 17.38 (CH3), ESI
MS (C11H12N4O2): 232.09651 (found) 232.096026 (calc).
4-chloro-2-[(E)-(1H-1,2,4-triazol-3-ylimino)methyl]phenol, 13
Yield: 81% (A), 98% (B). Light-yellow. M.p: 222–224˚C. IR (KBr, cm-1): 3273 (OH),
3184 (NH), 1636 (HC = N), 1610 (C = N), 1042 (N–N), 810 (C–Cl). 1H NMR (DMSO–d6,
δ, ppm): 7.16 (d, 1H, J = 8.2 Hz, Ar-H), 7.46 (dd, 1H, J = 8.2, 2.4 Hz, Ar-H), 7.89 (d, 1H,
J = 2.4 Hz, Ar-H), 8.44 (s, 1H, triazole C9–H), 9.40 (s, 1H, azomethine), 12.30 (s, 1H, OH),
13.98 (s, 1H, triazole NH). 13C NMR (DMSO–d6, δ, ppm): 118.1 (C3), 120.0 (C1), 129.2 (C5),
132.7 (C6), 134.2 (C4), 153.5 (C9), 156.9 (C8), 161.7 (C7), 162.9 (C2). Anal. Calcd. for
C9H7ClN4O (222.63): C: 48.55; H: 3.17; N: 25.17; Cl: 15.92; Found: C: 48.32; H: 3.21; N: 25.08;
Cl: 16.07%. ESI MS (C9H7N4OCl): 222.030893, 224.027952 (found, 3:1), 222.030839,
224.030839 (calc).
4-bromo-2-[(E)-(1H-1,2,4-triazol-3-ylimino)methyl]phenol, 14
Yield: 80% (A), 97% (B). Bright-yellow. M.p: 239–241˚C. IR (KBr, cm-1): 3267 (OH), 3181
(NH), 1629 (HC = N), 1609 (C = N), 1027 (N–N). 1H NMR (DMSO–d6, δ, ppm): 7.18 (d, 1H,
J = 8.2 Hz, Ar-H), 7.50 (dd, 1H, J = 8.2, 2.4 Hz, Ar-H), 8.00 (d, 1H, J = 2.4 Hz, Ar-H), 8.55 (s,
1H, triazole), 9.39 (s, 1H, azomethine), 12.51 (s, 1H, OH), 14.21 (s, 1H, triazole NH). 13C NMR
(DMSO–d6, δ, ppm): 116.66 (CH, Ar), 119.2 (CH, Ar), 121.4 (CH, Ar), 133.9 (C, Ar), 136.1 (C,
Ar), 153.9 (CH = N), 156.98 (C = N), 161.75 (C2). Anal. Calcd. For C9H7BrN4O (267.08): C:
40.47; H: 2.64; N: 20.98; Br: 29.92; Found: C: 40.29; H: 2.56; N: 21.07; Br: 29.84%.
2-[(E)-{[5-(methylsulfanyl)-1H-1,2,4-triazol-3-yl]imino}methyl]phenol, 15
Yield: 72% (A), 90% (B). Light-yellow. M.p: 183–185˚C. IR (KBr, cm-1): 3265 (OH), 3175
(NH), 1625 (HC = N), 1606 (C = N), 1030 (N–N). 1H NMR (DMSO–d6, δ, ppm): 2.62 (s, 3H,
CH3), 6.95 (ddd, 1H, J = 7.90, 7.81, 2.4 Hz, Ar–H), 7.10 (dd, 1H, J = 7.84, 2.4 Hz, Ar–H), 7.44
(ddd, 1H, J = 7.90, 7.84, 2.35 Hz, Ar–H), 7.81 (dd, 1H, J = 7.81, 2.35 Hz, Ar–H), 9.37 (s, 1H,
azomethine), 12.01 (s, 1H, OH), 14.12 (s, 1H, triazole NH). 13C NMR (DMSO–d6, δ, ppm):
13.9 (CH3), 116.7 (C, Ar), 119.3 (C, Ar), 131.7–134.5 (CH, Ar), 156.2 (C, Ar), 158.7 (C2, tria-
zole), 161.9 (C5, triazole), 163.2 (CH = N). ESI MS (C10H10N4OS): 234.057598 (found),
234.057533 (calc).
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4-chloro-2-[(E)-{[5-(methylsulfanyl)-1H-1,2,4-triazol-3-yl]imino}methyl]
phenol, 16
Yield: 74% (A), 94% (B). Off-white. M.p: 214–215˚C. IR (KBr, cm-1): 3281 (OH), 3183 (NH),
1634 (HC = N), 1610 (C = N), 1038 (N–N), 827 (C–Cl). 1H NMR (DMSO–d6, δ, ppm): 2.48 (s,
CH3), 6.83 (d, 1H, J = 7.92 Hz, Ar-H), 7.30 (dd, 1H, J = 7.94, 2.5 Hz, Ar–H), 7.73 (d, J = 2.5 Hz,
Ar–H), 9.14 (s, 1H, azomethine), 11.89 (s, 1H, OH), 13.98 (s, 1H, triazole NH). 13C NMR
(DMSO–d6, δ, ppm): 13.9 (C, Ar), 118.7 (CH, Ar), 120.8(CH, Ar), 127.6 (CH, Ar), 131.8(C,
Ar), 133.6 (C, Ar), 156.2 (C, Ar), 158.8 (C5, triazole), 162.1(C2, triazole), 163.2 (CH = N).
Anal. Calcd. for C10H9N4SClO (268.72): C: 44.70; H: 3.38; N: 20.85; S: 11.93; Cl: 13.19; Found:
C: 44.34; H: 3.24; N: 20.68; S: 12.04; Cl: 13.23%. ESI MS (C10H9N4SclO): 236.046505,
238.043610 (found, 3:1), 236.046489, 238.043539 (calc).
4-bromo-2-[(E)-{[5-(methylsulfanyl)-1H-1,2,4-triazol-3-yl]imino}methyl]
phenol, 17
Yield: 76% (A), 97% (B). Greenish-yellow. M.p: 210–212˚C. IR (KBr, cm-1): 3276 (OH), 3183
(NH), 1628 (HC = N), 1603 (C = N), 1024 (N–N). 1H NMR (DMSO–d6, δ): 2.60 (s, 3H, CH3),
6.95 (d, 1H, J = 7.89 Hz, Ar-H), 7.62 (dd, 1H, J = 7.89, 2.44 Hz, Ar-H), 8.00 (d, 1H, J = 2.44 Hz,
Ar–H), 9.35 (s, 1H, azomethine), 12.12 (s, 1H, OH), 14.01 (s, 1H, triazole NH). 13C NMR
(DMSO-d6, δ): 13.6 (CH3), 115.7 (C, Ar), 119.1 (CH, Ar), 121.4 (CH, Ar), 135.2 (CH, Ar),
136.3 (C, Ar), 156.4 (C, Ar), 158.1 (C5, triazole), 162.1 (C2, triazole), 163.4 (CH = N). Anal.
Calcd. for C10H9BrN4OS(313.17): C: 38.35; H: 2.90; N: 17.89; S: 10.24; Br: 25.51; Found: C:
38.33; H: 2.63; N: 17.63; S: 10.19; Br: 25.64%.
2-[(E)-{[5-(methylsulfanyl)-1H-1,2,4-triazol-3-yl]imino}methyl]-
4-nitrophenol, 18
Yield: 75% (A), 98% (B). Deep-yellow. M.p: 222–224˚C. IR (KBr, cm-1): 3290 (OH), 3182
(NH), 1632 (HC = N), 1606 (C = N), 1360 (C–NO2), 1025 (N–N). 1H NMR (DMSO–d6,
δ, ppm): 2.72 (s, 3H, CH3), 7.18 (d, 1H, J = 7.86 Hz, Ar-H), 8.30 (dd, 1H, J = 7.86, 2.45 Hz, Ar-
H), 8.45 (d, 1H, J = 2.45 Hz, Ar-H), 9.40 (s, 1H, azomethine), 12.6 (s, 1H, OH), 14.10 (s, 1H,
NH). 13C NMR (DMSO-D6, δ): 13.98 (CH3), 117.74 (C, Ar), 120.30 (CH, Ar), 125.58 (CH,
Ar), 137.71 (C, Ar), 140.53 (CH, Ar), 156.27 (C, Ar), 158.44 (C2, triazole), 162.28 (C5, triazole),
163.34 (CH = N). Anal. Calcd. for C10H9N5SO3 (279.29): C: 43.01; H: 3.25; N: 25.08; S: 11.48;
Found: C: 42.45; H: 2.98; N: 25.35; S: 11.64%.
2-{(E)-[(5-Amino-1H-1,2,4-triazol-3-yl)imino]methyl}-3-methoxyphenol,
19
Yield: 83% (A), 99% (B); colour (light yellow); M.p: 252–253˚C; IR (KBr, cm-1): 3420 (OH),
3346 (NH2), 3187 (NH), 2910 (OCH3), 1628 (HC = N), 1595 (C = N, triazole), 1022 (N-N);
1H NMR (DMSO–d6, δ, ppm): 2.86 (s, 3H, OCH3), 6.98 (dd, 1H, J = 8.8, 8.9 Hz, Ar-H), 7.12
(d, 1H, J = 8.8 Hz, Ar-H), 7.53 (d, 1H, J = 8.9 Hz, Ar-H), 8.78 (s, 1H, azomethine), 10.22 (s,
1H, OH), 12.18 (s, 1H, NH); 13C NMR (DMSO-D6, δ): 55.4 (OCH3), 119.7 (C, Ar), 121.9
(CH, Ar), 124.2 (CH, Ar), 129.6 (CH, Ar), 150.0 (C, Ar), 151.0 (C, Ar), 156.6 (C2, triazole),
157.2 (C5, triazole), 162.3 (CH = N). LR EIMS (70eV) m/z (%): 233 ([M]+, 13), 218 (100),
202 (32), 177 (9), 171 (19), 164 (7), 150 (12), 134 (27), 123 (14), 104 (20), 77 (22); Anal.
Calcd. for C10H11N5O2 (233.23): C 51.50, H 4.75, N 30.03, O 13.72; Found C 51.47, H 4.72, N,
29.98.
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2-{(E)-[(5-Amino-1H-1,2,4-triazol-3-yl)imino]methyl}-5-chlorophenol, 20
Yield: 73% (A), 97% (B); colour (light yellow); M.p: 222–224˚C; IR (KBr, cm-1): 3428 (OH),
3345 (NH2), 3192 (NH), 1636 (HC = N), 1606 (C = N, triazole), 1032 (N-N), 819 (C-Cl); 1H
NMR (DMSO-d6, δ): 6.04 (s, 2H, NH2), 6.97 (d, 1H, J = 7.8 Hz, Ar-H), 7.48 (dd, 1H, J = 7.8,
2.4 Hz, Ar-H), 7.84 (d, 1H, J = 2.4 Hz, Ar-H), 8.87 (s, 1H, azomethine), 10.26 (s, 1H, OH),
12.27 (s, 1H, NH); 13C NMR (DMSO-d6, δ): 120.27 (CH, Ar), 125.52 (CH, Ar), 131.72 (C, Ar),
133.13 (C, Ar), 156.12 (CH, Ar), 159.63 (C5, triazole), 160.33 (C2, triazole), 163.35 (CH = N);
EIMS: [m/z, (%)]: 237, 235 [M]+ (100, 34), 221 (5), 205 (5), 181 (18), 166 (6), 154 (11), 131
(14), 127 (10), 111 (8), 75 (7); Anal. Calcd. for C9H8ClN5O (237.65): C 45.49, H 3.39, N 29.47;
Found C 45.46, H 3.36, N 29.45.
2-{(E)-[(5-Amino-1H-1,2,4-triazol-3-yl)imino]methyl}- 5-bromophenol, 21
Yield: 77% (A), 95% (B); colour (light yellow); M.p: 227–229˚C; IR (KBr, cm-1): 3424 (OH),
3343 (NH2), 3192 (NH), 1634 (HC = N), 1603 (C = N, triazole), 1027 (N-N), 564 (C-Br); 1H
NMR (DMSO-d6, δ): 6.03 (s, 2H, NH2), 6.97 (d, 1H, J = 8.8 Hz, Ar-H), 7.58 (dd, 1H, J = 8.8,
2.4 Hz, Ar-H), 8.04 (d, 1H, J = 2.4 Hz, Ar-H), 8.83 (s, 1H, azomethine), 10.24 (s, 1H, OH),
12.25 (s, 1H, NH); 13C NMR (DMSO-d6): δ 119.78 (C, Ar), 121.34 (CH, Ar), 133.09 (CH, Ar),
136.53 (C, Ar), 156.12 (CH, Ar), 159.92 (C2, triazole), 158.78 (C5, triazole), 164.89 (CH = N);
LR EIMS: [m/z, (%)]: 282, 284 [M]+ (100), 266 (13), 250 (7), 225 (20), 212 (8), 199 (26), 171
(5), 157 (9), 103 (14), 76 (11); Anal. Calcd. for C9H8BrN5O (282.09): C 38.32, H 2.86, N 24.83;
Found C 38.28, H 2.84, N 24.81.
2-{(E)-[(5-Amino-1H-1,2,4-triazol-3-yl)imino]methyl}-5-nitrophenol, 22
Yield: 67% (A), 95 (B); dark yellow; M.p: 242–244˚C; IR (KBr, cm-1): 3431 (OH), 3347 (NH2),
3199 (NH), 1648 (HC = N), 1617 (C = N, triazole), 1350 (NO2), 1054 (N-N); 1H NMR (DMSO-
d6, δ): 6.07 (s, 2H, NH2), 7.19 (d, 1H, J = 8.7 Hz, Ar-H), 8.24 (dd, 1H, J = 8.7, 2.3 Hz, Ar-H),
8.75 (d, 1H, J = 2.3 Hz, Ar-H), 9.83 (s, 1H, azomethine), 10.31 (s, 1H, OH), 12.30 (s, 1H, NH);
13C NMR (DMSO-d6): δ 120.8 (CH, Ar), 135.6 (CH, Ar), 140.7 (C, Ar), 156.1 (CH, Ar), 160.2
(C, Ar), 162.5 (C5, triazole), 166.1 (C2, triazole), 168.8 (CH = N); EIMS: [m/z, (%)]: 248 ([M]+,
22), 232 (100), 216 (20), 192 (31), 172 (5), 166 (19), 149 (22), 138 (11), 122 (14), 76 (10); Anal.
Calcd. for C9H8N6O3 (248.19): C 43.55, H 3.25, N 33.86; Found C 43.52, H 3.22, N 33.83.
2-{(E)-[(5-Amino-1H-1,2,4-triazol-3-yl)imino]methyl}phenol, 23
Yield: 75% (A); 98% (B); colour (yellow); M.p: 181–183˚C; IR (KBr, cm-1): 3420 (OH), 3350
(NH2), 3190 (NH), 1631 (HC = N), 1594 (C = N, triazole), 1025 (N-N); 1H-NMR (DMSO) δ:
6.01 (s, 2H, NH2), 6.98 (t, 1H, J = 8.4 Hz, Ar-H), 7.12 (d, 1H, J = 7.8 Hz, Ar-H), 7.41 (t, 1H, J =
7.4 Hz, Ar-H), 7.63 (d, 1H, J = 7.9 Hz, Ar-H), 8.81 (s, 1H, azomethine, Ar-H), 10.31 (s, 1H,
OH), 12.22 (s, 1H, NH); 13C NMR (CDCl3): δ 117.7 (CH, Ar), 121.2 (CH, Ar), 130.3 (CH, Ar),
133.2 (C, Ar), 158.1 (C2, triazole), 159.8 (C5, triazole), 160.6 (CH = N),; LR EIMS: [m/z, (%)]:
203 ([M]+, 100), 186 (85), 172 (48), 161 (36), 147 (81), 132 (72), 120 (47), 104 (70), 91 (31), 77
(84); Anal. Calcd. for C9H9N5O (203.20): C 53.20, H 4.46, N 34.47; Found C 53.16, H 4.43, N
34.45.
4-[(4-Chloro-benzylidene)-amino]-5-(2,3,5-trichloro-phenyl)-4H-[1,2,4]
triazole-3-thiol, 24
Yield: 46%, 92% (B), M.p: 148˚C, IR (KBr, cm-1): 3082 (Ar-H), 3071 (NH/SH), 1617 (CH = N),
1594 (C = N), 1493 (C = C), 1019 (N-N). 1H-NMR (DMSO) δ: 7.33 (d, 2H, J = 8.7 Hz,
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4-chlorophenyl), 7.53 (d, 2H, J = 8.4 Hz, Ar-H), 7.66 (d, 1H, J = 2.7 Hz, Ar-H), 7.91 (d, 1H,
J = 2.4 Hz, Ar-H), 9.8 (s, 1H, azomethine), 14.29 (s, 1H, SH). LR EIMS: [m/z, (%)]: 418.8 (48),
417 (100), 415.2 (72), 387 (10), 335 (5), 307 (20), 289 (15), 281 (15), 232 (40).
4-[(4-Fluoro-benzylidene)-amino]-5-(2,3,5-trichloro-phenyl)-4H-[1,2,4]
triazole-3-thiol, 25
Yield: 53% (A), 88% (B), M.p: 154˚C; IR (KBr, cm-1): 3075 (NH), 3075 (Ar-H), 2910 (C-H),
1624 (HC = N), 1601 (C = N), 1612, 1486 (C = C), 1018 (N-N), 785 and 628 (C = Cl). 1H NMR
(DMSO–d6, δ, ppm): 7.1–7.14 (t, 2H, J = 10.4 Hz, Ar-H), 7.474 (d, 1H, J = 2.7 Hz, Ar-H), and
7.68 (d, 1H, J = 2.4 Hz, Ar-H), 7.7–7.74 (m, 2H, Ar-H), 10.34 (s, 1H, N = CH), 11.29, (s, 1H,
SH). LR EIMS: [m/z, (%)]: 404, 402, 400 [M+], (100, 100, 29), 403, 401, 399 [M-H] (100, 100,
30), 383 (20), 307 (30), 280 (50).
N-(thiophen-2-ylmethylidene)-1H-1,2,4-triazol-3-amine, 26
Yield: 71% (A), 99% (B). Off-white. M.p: 172–174˚C. IR (KBr, cm-1): 3175 (NH), 1628
(HC = N), 1611 (C = N), 1570, 1540 (C = C), 1020 (N–N), 960 (C–S). 1H NMR (DMSO–d6,
δ, ppm): 7.24 (dd, 1H, J = 4.6, 4.0 Hz, thienyl–H), 7.8 (d, 1H, J = 4.0 Hz, thienyl–H), 7.89 (d,
1H, J = 4.6 Hz, thienyl–H), 8.25 (s, 1H, triazole–H), 9.30 (s, 1H, azomethine), 13.98 (s, 1H, tria-
zole NH). 13C NMR (δ, ppm): 126.7 (CH, thienyl), 129.5 (CH, thienyl), 132.6 (C, thienyl),
153.5 (C5, triazole), 156.2 (C2, triazole), 159.0 (CH = N). Anal. Calcd. for C7H6N4S (178.21):
C: 47.17; H: 3.39; N: 31.44; S: 17.99; Found: C: 47.30; H: 3.41; N: 31.35; S: 17.80%. ESI MS
(C7H6N4S): 178.031359 (found) 178.031318 (calc).
N-[(5-methylthiophen-2-yl)methylidene]-1H-1,2,4-triazol-3-amine, 27
Yield: 73% (A), 98% (B). Off-white. M.p: 168–170˚C. IR (KBr, cm-1): 3185 (NH), 1632
(HC = N), 1612 (C = N), 1575, 1545 (C = C), 1020 (N–N), 965 (C–S). 1H NMR (DMSO–d6,
δ, ppm): 2.5 (s, 3H, CH3), 6.96 (d, 1H, J = 3.0 Hz, thienyl-H), 7.60 (d, 1H, J = 3.0 Hz, thienyl–
H), 8.20 (s, 1H, triazole–H), 9.20 (s, 1H, azomethine), 13.95 (s, 1H, triazole NH). 13C NMR
(δ, ppm): 15.4 (CH3), 128.2 (C, thienyl), 130.9 (CH, thienyl), 137.6 (C, thienyl), 142.9 (CH,
thienyl), 153.6 (C5 triazole), 156.5 (C2 triazole), 159.2 (CH = N). Anal. Calcd. for C8H8N4S
(192.24): C: 49.98; H: 4.19; N: 29.14; S: 16.68; Found: C: 50.10; H: 4.10; N: 29.20; S: 16.61%. ESI
MS (C8H8N4S): 192.047008 (found) 192.046968 (calc)
N-[(3-methylthiophen-2-yl)methylidene]-1H-1,2,4-triazol-3-amine, 28
Yield: 74% (A), 96% (B). Light-brown. M.p: 171–173˚C. IR (KBr, cm-1): 3180 (NH), 1626
(HC = N), 1610 (C = N), 1568, 1540 (C = C), 1020 (N–N), 970 (C–S). 1H NMR (DMSO–d6,
δ, ppm): 2.46 (s, 3H, CH3), 7.06 (d, 1H, J = 4.5 Hz, thienyl–H), 7.79 (d, 1H, J = 4.5 Hz, thienyl–
H), 8.20 (s, 1H, triazole–H), 9.29 (s, 1H, azomethine), 13.90 (s, 1H, triazole NH). 13C NMR
(δ, ppm): 14.4 (CH3), 128.4 (C, thienyl), 129.3 (CH, thienyl), 132.9 (C, thienyl), 140.6 (CH,
thienyl), 153.1 (C5 triazole), 156.8 (C2, triazole), 159.2 (CH = N). Anal. Calcd. for C8H8N4S
(192.24): C: 49.98; H: 4.19; N: 29.14; S: 16.68; Found: C: 49.80; H: 4.00; N: 29.30; S: 16.57%. ESI
MS (C8H8N4S): 192.047001 (found), 192.046968
N-[(5-chlorothiophen-2-yl)methylidene]-1H-1,2,4-triazol-3-amine, 29
Yield: 59% (A), 91 (B). Light-brown. M.p: 178–180˚C. IR (KBr, cm-1): 3190 (NH), 1629
(HC = N), 1610 (C = N), 1570, 1540 (C = C), 1020 (N–N), 970 (C–S), 820 (C–Cl). 1H NMR
(DMSO–d6, δ, ppm): 7.39 (d, 1H, J = 3.5 Hz, thienyl–H), 7.70 (d, 1H, J = 3.5 Hz, thienyl–H),
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8.5 (s, 1H, triazole–H), 9.24 (s, 1H, azomethine), 13.99 (s, 1H, triazole NH). 13C NMR
(δ, ppm): 128.9 (CH, thienyl), 132.5 (CH, thienyl), 135.6 (C, thienyl), 146.2 (C, thienyl), 153.3
(C5, triazole), 156.9 (C2, triazole),159.3 (CH = N). Anal. Calcd. for C7H5ClN4S (212.66): C:
39.53; H: 2.37; N: 26.35; S: 15.08; Cl: 16.67; Found: C: 39.70; H: 2.34; N: 26.26; S: 15.13; Cl:
16.80%. ESI MS (C7H5ClN4S) = 211.992408, 213.989609 (found, 3:1), 211.992346, 213.989396
(calc).
N-[(5-nitrothiophen-2-yl)methylidene]-1H-1,2,4-triazol-3-amine, 30
Yield: 79% (A), 89 (B). Brownish-green. M.p: 202–204˚C. IR (KBr, cm-1): 3175 (NH), 1629
(HC = N), 1611 (C = N), 1560, 1540 (C = C), 1370 (C–NO2), 1020 (N–N), 980 (C–S). 1H NMR
(DMSO–d6, δ, ppm): 7.72 (d, 1H, J = 4.1 Hz, thienyl–H), 7.87 (d, 1H, J = 4.1 Hz, thienyl-H),
8.30 (s, 1H, triazole–H), 9.27 (s, 1H, azomethine), 14.00 (s, 1H, NH). 13C NMR (δ, ppm): 129.9
(C, thienyl), 133.8 (CH, thienyl), 141.2 (CH, thienyl), 147.0 (C, thienyl), 153.6 (C5, triazole),
156.9 (C2, triazole), 159.1 (CH = N). Anal. Calcd. for C7H5N5SO2 (223.21): C: 37.67; H: 2.26;
N: 31.38; S: 14.37; Found: C: 37.78; H: 2.34; N: 31.26; S: 14.45%. ESI MS (C7H5N5SO2):
223.016724 (found), 223.016397 (calc).
N-[(E)-(4-bromothiophen-2-yl)methylidene]-1H-1,2,4-triazol-3-amine, 31
Yield: 69% (A), 98% (B). Off-white. M.p: 215–217˚C. IR (KBr, cm-1): 3184 (NH), 1630
(HC = N), 1608 (C = N), 1570, 1545 (C = C), 1025 (N–N), 965 (C–S). 1H NMR (DMSO–d6,
δ, ppm): 7.85 (d, 1H, J = 3.5 Hz, thienyl–H), 8.0 (d, 1H, J = 3.5 Hz, thienyl–H), 8.55 (s, 1H, tria-
zole–H), 9.30 (s, 1H, azomethine), 14.00 (s, 1H, triazole NH). 13C NMR (δ, ppm): 116.7 (C4),
123.9 (C5), 125.2 (C3), 144.6 (C2), 153.4 (C8), 156.2 (C7), 159.5 (C6). Anal. Calcd. for
C7H5N4SBr (257.11): C: 32.70; H: 1.96; N: 21.79; S: 12.47; Br: 31.08; Found: C: 33.06; H: 1.89;
N: 22.10; S: 12.34; Br: 30.94%. ESI MS (C7H5N4SBr): 255.941909, 257.939824 (found, 1:1),
255.941829, 257.939783 (calc).
N-[(E)-(3-methylfuran-2-yl)methylidene]-1H-1,2,4-triazol-3-amine, 32
Yield: 68% (A), 95% (B). Light-yellow. M.p: 106–108˚C. IR (KBr, cm-1): 3190 (NH), 1630
(HC = N), 1609 (C = N), 1575, 1545 (C = C), 1090 (C–O), 1020 (N–N). 1H NMR (DMSO–d6,
δ, ppm): 2.55 (s, 3H, CH3), 7.00 (d, 1H, J = 3.0 Hz, furanyl–H), 7.7 (d, 1H, J = 3.0 Hz, furanyl–
H), 8.4 (s, 1H, triazole–H), 9.25 (s, 1H, azomethine), 13.95 (s, 1H, triazole, NH). 13C NMR
(δ, ppm): 15.2 (CH3), 115.2 (CH, furanyl), 118.9 (CH, furanyl), 139.9 (C, furanyl), 144.6 (C,
furanyl), 154.5 (C2, triazole), 156.5 (C2, triazole), 160.4 (CH = N). Anal. Calcd. for C8H8N4O
(176.18): C: 54.54; H: 4.58; N: 31.80; Found: C: 54.75; H: 4.46; N: 31.97%. ESI MS (C8H8N4O):
176.069872 (found), 176.069811 (calc).
N-[(E)-(5-chlorofuran-2-yl)methylidene]-1H-1,2,4-triazol-3-amine, 33
Yield: 68% (A), 97% (B). Light-yellow. M.p: 176–178˚C. IR (KBr, cm-1): 3180 (NH), 1632
(HC = N), 1608 (C = N), 1565, 1546 (C = C), 1086 (C–O), 1025 (N–N), 830 (C–Cl). 1H NMR
(DMSO–d6, δ, ppm): 7.38 (d, 1H, J = 3.5 Hz, furanyl–H), 7.49 (d, 1H, J = 3.5 Hz, furanyl–H),
8.5 (s, 1H, triazole–H), 8.90 (s, 1H, azomethine), 13.97 (s, 1H, triazole NH). 13C NMR
(δ, ppm): 116.2 (CH, furanyl), 120.0 (CH, furanyl), 135.4 (C, furanyl), 142.6 (C, furanyl), 153.9
(C5, triazole), 155.5 (C2, triazole), 160.5 (CH = N). Anal. Calcd. for C7H5N4OCl (196.59): C:
42.77; H: 2.56; N: 28.50; Cl: 18.03; Found: C: 42.90; H: 2.43; N: 28.30; Cl: 18.11%. ESI MS
(C7H5N4OCl): [M]+ = 196.015213, 198.012287 (found, 3:1), 196.015189, 198.012239.
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N-[(E)-(5-nitrofuran-2-yl)methylidene]-1H-1,2,4-triazol-3-amine, 34
Yield: 76% (A), 91% (A). Gray. M.p: 212–214˚C. IR (KBr, cm-1): 3185 (NH), 1629 (HC = N),
1607 (C = N), 1570, 1548 (C = C), 1370 (C–NO2), 1080 (C–O), 1030 (N–N). 1H NMR
(DMSO–d6, δ, ppm): 7.60 (d, 1H, J = 4.1 Hz, furanyl–H), 7.85 (d, 1H, J = 4.1 Hz, furanyl–H),
8.62 (s, 1H, triazole–H), 9.16 (s, 1H, azomethine), 14.20 (s, 1H, triazole NH). 13C NMR
(δ, ppm): 120.6 (C, furanyl), 123.0 (C, furanyl), 143.2 (CH, furanyl), 147.0 (C, furanyl), 153.6
(C5, triazole), 156.7 (C2, triazole), 160.8 (CH = N). Anal. Calcd. for C7H5N5O3 (207.15): C:
40.59; H: 2.43; N: 33.81; Found: C: 40.23; H: 2.34; N: 34.20%. ESI MS (C7H5N5O3): 207.03991
(found), 207.03924 (calc).
N-[(E)-1H-pyrrol-2-ylmethylidene]-1H-1,2,4-triazol-3-amine, 35
Yield: 61% (A), 92% (B),. Light-brown. M.p: 190–192˚C. IR (KBr, cm-1): 3185 (NH), 3120
(NH), 1631 (HC = N), 1610 (C = N), 1570, 1540 (C = C), 1025 (N–N). 1H NMR (DMSO–d6,
δ, ppm): 6.24 (dd, 1H, J = 4.6, 4.0 Hz, pyrrolyl–H), 6.86 (d, 1H, J = 4.0 Hz, pyrrolyl–H), 7.11
(d, 1H, J = 4.6 Hz, pyrrolyl–H), 8.30 (s, 1H, triazole–H), 8.89 (s, 1H, azomethine), 11.92 (s, 1H,
pyrrolyl NH), 13.69 (s, 1H, triazole NH). 13C NMR (δ, ppm): 114.5 (CH, pyrrolyl), 116.6 (C,
pyrrolyl), 120.7 (CH, pyrrolyl), 132.0 (C, pyrrolyl), 153.7 (C5, triazole), 155.8 (C2, triazole),
159.9 (CH = N). Anal. Calcd. for C7H7N5 (161.16): C: 52.17; H: 4.38; N: 43.45; Found: C:
52.39; H: 4.46; N: 43.72%. ESI MS (C7H7N5): 161.070185 (found), 161.070145 (calc).
N-[(E)-(1-methyl-1H-pyrrol-2-yl)methylidene]-1H-1,2,4-triazol-3-amine,
36
Yield: 69% (A) 94% (B). Light-brown. M.p: 132–134˚C. IR (KBr, cm-1): 3195 (NH), 1630
(HC = N), 1609 (C = N), 1024 (N–N). 1H NMR (DMSO–d6, δ, ppm): 3.30 (s, 3H, CH3), 6.19
(dd, 1H, J = 4.4, 4.1 Hz, pyrrolyl-H), 6.88 (d, 1H, J = 4.1 Hz, pyrrolyl–H), 7.18 (d, 1H, J = 4.4
Hz, pyrrolyl–H), 8.36 (s, 1H, triazole–H), 8.98 (s, 1H, azomethine), 13.62 (s, 1H, triazole NH).
13C NMR (δ, ppm): 31.2 (CH3), 113.6 (C, pyrrolyl), 117.8 (C, pyrrolyl), 122.5 (CH, pyrrolyl),
130.3 (CH, pyrrolyl), 153.6 (C5, triazole), 155.8 (C2, triazole), 159.4 (CH = N). Anal. Calcd. for
C8H9N5 (175.19): C: 54.85; H: 5.18; N: 39.98; Found: C: 54.39; H: 5.27; N: 40.14%. ESI MS:
(C8H9N5): 175.085816 (found), 175.085795 (calc).
Results and discussion
For the synthesis of 1,2,4-triazole based Schiff bases, variously substituted and unsubstituted
3-amino and 4-amino-1,2,4-triazoles were employed. Condensation of these aminotriazoles
with various aromatic aldehydes under ultrasound conditions would afford the desired Schiff
bases (S1 File). For comparison purpose, the conventional method of synthesis of Schiff bases
was also employed in parallel to ultrasound assisted synthesis.
S1 File. Synthesis of target molecules
The conventional method involved refluxing the amine and aldehyde in ethanol for 4–5 hours.
Upon cooling the reaction mixture at ambient temperature (and / or in ice bath under certain
cases) led to the precipitation of the Schiff bases as solid products. In some cases the precipi-
tates of the products were formed even during reaction process.
When the reaction was carried out in the presence of aq. HCl, a decrease in product yield
was observed. Same was the case with acetic acid catalysed reaction. It is believed that the
amino group of the triazole system is sufficiently nucleophilic to require any catalyst for the
reaction. Furthermore, the use of acid catalysts tends to protonate and thereby reduces the
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nucleophilicity of the N of amino group. Conversely the reaction worked well in the absence of
any catalyst.
The ultrasound mediated reaction was carried out by placing the ingredients of the reaction
in a screw cap tube which was then placed in a sonicator. Subjecting the reaction contents to
ultrasound resulted in immediate formation of product. The progress of reaction was moni-
tored after every minute by means of TLC. The disappearance of both of the reactants on TLC
was considered the completion of reaction. Majority of reactions got completed in 3 minutes
and some took 4 minutes for completion.
The purity of the synthesized compounds was checked by TLC using a EtoAc/n-hexane
(1:2) as mobile phase. All of the synthesized Schiff bases were soluble in DMSO, DMF while
were soluble upon heating in methyl alcohol and ethyl alcohol. All products were stable to air
as well. The synthesized compounds were characterized by means of FTIR, 1H NMR, 13
C
NMR and mass spectrometry.
The disappearance of the aldehydic carbonyl group as well as the stretching frequency corre-
sponding to NH2 group of the triazole and appearance of characteristic azomethine (imine)
absorption signal at 1580–1630 cm-1 in IR spectrum further strengthened the evidence of success
of the reaction. In case of imines derived from 3,5-diamino-1,2,4-triazoles, the products exhibited
an NH2 signal that appeared from 3343–3350 cm-1. In case of Schiff bases derived from 3-amino-
1,2,4-trizoles and 4-amino-1,2,4-trizoles, the N-H of triazole appears 3187–3199 cm-1 (Fig 1).
The 1H NMR of the synthesized products exhibited presence of imine (CH = N) proton at
8.78–8.93 ppm. The appearance of the azomethine (aka imine) proton confirms the successful
condensation and hence the formation of desired product.
The characteristic absorption frequencies and chemical shift values of azomethine proton
of the Schiff bases are tabulated in Table 1.
Fig 1.
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Table 1. Melting points and characteristic spectroscopic/spectrometric signals of synthesized Schiff bases.
Compound No.
R5
Ar
M.P (C)
FTIR (absorption frequencies in cm-
NMR (chemical
shift in ppm)
[M]+
1
)
azomethine str C = N str
N-N str
1021
1022
1023
1030
1029
1014
1025
1030
1028
1030
1030
1025
1025
1022
1032
1027
1054
1025
CH = N
9.24
9.24
9.24
8.99
9.55
9.85
9.24
9.40
9.39
9.35
9.15
9.35
9.40
8.78
8.87
8.83
9.83
8.81
CH = N
158.17
159.13
159.00
159.98
159.00
164.37
161.17
161.67
161.75
161.93
162.11
162.13
162.28
157.19
160.33
158.78
166.05
160.63
6
H
H
Ph
196
263–264
271
1632
1622
1626
1622
1621
1636
1628
1631
1632
1625
1630
1627
1632
1628
1636
1634
1648
1631
1590
1604
1528
1602
1540
1596
1572
1608
1610
1606
1607
1605
1606
1595
1606
1603
1617
1594
M.P (C)
-
7
2-OHPh
-
8
H
2-OHnaph
3,4-(OH)2Ph
4-(NMe2)Ph
4-(NO2)Ph
-
9
H
182
-
10
11
12
13
14
15
16
17
18
19
20
21
22
23
H
203
-
-
H
269
H
3-OEt,4-OHPh
2-OH,5-ClPh
2-OH,5-BrPh
2-OHPh
151
-
H
222–224
239–241
183–185
214–215
210–212
222, 224 (3:1)
266,268 (1:1)
234.29
H
SMe
SMe
SMe
2-OH,5-ClPh
2-OH,5-BrPh
268, 270 (3:1)
312.17,314.21 (1:1)
279.29
SMe 2-OH,5-NO2Ph 222–224
NH2 2-OH,6-OMePh 252–253
233
NH2
NH2
2-OH,4-ClPh
2-OH,4-BrPh
222–224
227–229
235, 237 (3:1)
282, 284 (1:1)
248
NH2 2-OH,4-NO2Ph 242–244
NH2 2-OHPh 181–183
203
Compound No
X
FTIR (absorption frequencies
in cm-1)
NMR (chemical
shift in ppm)
[M]+
azomethine str C = N str N-N str CH = N CH = N
24
25
Cl
F
142
158
1617
1624
1594
1601
1019
1018
9.80
-
-
418, 420 (3:1)
401
10.34
Compound No.
X
R3’
R4’
R5’
M.P (C)
FTIR (absorption frequencies
in cm-1)
NMR (chemical
shift in ppm)
[M]+ (amu)
azomethine str C = N str N-N str CH = N CH = N
26
27
28
29
30
31
32
33
34
35
36
S
H
H
H
H
H
H
Br
H
H
H
H
H
H
172–174
168–170
171–173
178–180
202–204
215–217
106–108
176–178
212–214
190–192
132–134
1628
1632
1626
1629
1629
1630
1630
1632
1629
1631
1630
1611
1612
1610
1610
1611
1608
1609
1608
1607
1610
1609
1020
1020
1020
1020
1020
1025
1090
1025
1080
1025
1024
9.30
9.20
9.29
9.24
9.27
9.30
9.25
8.90
9.16
8.89
8.97
156.0
156.5
156.8
156.9
156.9
156.2
156.5
155.5
156.7
155.8
159.3
178.21
192.24
S
H
Me
H
S
Me
H
192.00
S
S
Cl
212.66, 214.80 (3:1)
223.21
H
NO2
H
S
H
255.00, 257.18 (1:1)
176.18
0
Me
H
H
0
Cl
196.59, 198.64 (3:1)
207.15
0
H
NO2
H
NH
NMe
H
161.16
H
H
175.19
https://doi.org/10.1371/journal.pone.0229891.t001
The appearance of [M]+. in LR EIMS further confirmed the formation of Schiff bases. The
fragmentation pattern was consistent with that of desired product. The base peak was observed
by loss of H as radical fragment in majority of cases (Fig 2).
Antibacterial activities
Some representatives of the synthesized Schiff bases were evaluated for anti-bacterial activity.
These compounds were tested for their effectiveness as antibacterial agents against gram
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A facile sonochemical synthesis of 3-amino and 4-amino-1,2,4-triazole Schiff bases as antibacterial agents
Fig 2. The significantly higher yields of the product under ultra-sound mediated conditions may be attributed to the
phenomenon of cavitations. The passing ultra-sound waves generate small bubbles in the reaction medium. These
bubbles have energy trapped in them. These bubbles when combine form larger bubbles and hence large energy. When
these bubbles rupture, the release energy which is sufficient enough to allow the reactant molecules overcome the energy
barrier required for a specific reaction. The reaction under ultra-sound condition was significantly fast and all reactions
afforded desired compounds as exclusive products in excellent yields.
https://doi.org/10.1371/journal.pone.0229891.g002
positive (S. aureus, B. subtilis) and gram negative bacteria (E. coli, S. flexneri, P. aeruginosa and
S. typhi). Imipenem was used as positive control in this study while filter paper disc dipped in
10% DMSO was used as negative control. In order to determine the antibacterial activity, 5 mg
of each selected representatives were dissolved in 5 mL of DMSO to make up concentration of
1000 μg/mL. 0.1 mL of this solution was used for the determination of zone of inhibition. The
zone of inhibition of different compounds was calculated in mm and mean SEM of triplicate
data were calculated. The antibacterial activity was considered significant if the zone of inhibi-
tion was greater than 16 mm, the Schiff bases producing zone of 11–15 mm were considered
moderately active while those with less than 10 mm zone were considered weakly active as
antibacterial agents. The antibacterial activities of the selected Schiff bases are summarized in
Table 2:
Establishing Structure activity relationship (SAR) from the zone of inhibition data is diffi-
cult however, some generalizations can be made. In case of 3-amino trizole derived Schiff
bases, the presence of a bulky +R group promotes antibacterial activity while presence of a–R
group decreases the activity (see entry 10 & 11, Table 2). Presence of a hydroxyl at ortho posi-
tion of the benzene ring does not give any significant activity unless there is a +R group at
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A facile sonochemical synthesis of 3-amino and 4-amino-1,2,4-triazole Schiff bases as antibacterial agents
Table 2. Antibacterial activities of some selected Schiff bases.
Schiff base
Zone of inhibition (mm)
Bacterial strains
Gram positive
Gram negative
S. aureus
11
14
16
12
11
20
22
17
19
16
18
18
20
16
8
B. subtilis
E. coli
12
13
12
15
9
S. flexneri
P. aeruginosa
S. typhi
11
14
15
13
10
19
15
18
19
16
21
12
11
22
13
11
15
15
21
15
15
31
6
8
10
12
18
11
10
22
20
20
16
21
19
18
22
17
6
11
9
7
12
18
14
11
18
11
17
18
19
16
8
10
17
13
12
25
10
18
19
17
17
24
12
16
11
6
11
15
16
24
16
17
18
20
18
15
18
19
12
6
17
19
20
22
23
24
25
16
22
11
10
15
11
19
9
26
27
28
6
8
29
14
6
12
8
16
10
21
13
8
18
11
19
10
10
33
30
31
22
12
12
31
24
15
10
37
35
36
11
27
Imipenem
30
https://doi.org/10.1371/journal.pone.0229891.t002
position 5 of 6 of the aromatic ring. Presence of an SMe and /or NH2 group at C5 of the tria-
zole ring promotes antibacterial activity. Any substituent present of C3 of aromatic ring has no
influence on antibacterial activity. The 4-aminotriazole derived imines showed significant
activity which may be attributed to presence of Cl and F. however, presence of a F atom at
4-position of the benzene ring gave more broader and efficient response than Cl atom at the
same position; the later gave significant response against Gram positive bacteria. Among the
schiff base derived from heterocyclic aldehydes, thienyl moiety exhibited better response than
its oxygen and nitrogen counterparts. Again the presence of a halogen atom resulted in better
activity. Since these studies were of preliminary nature, therefore further structure activity rela-
tionship studies require extensive exploration of the compounds.
Conclusion
A series of 30 Schiff bases was synthesized by employing ultra-sound from sonicator as energy
sourse. For comparison purpose, a conventional setup was also used. It was observed that for-
mer method afforded products with higher yields (88–99%) and higher purity in less time
(almost 5 minutes). The yields of conventional method were significantly lower and took
almost 5 hours for completion. These results are encouraging enough to explore the ultra-
sound mediated synthetic protocol for synthesis of other compounds of medicinal interests as
well. some selective Schiff bases were subjected to preliminary antibacterial screening against
Gram positive and Gram negative bacteria; the results exhibited promising results. The
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A facile sonochemical synthesis of 3-amino and 4-amino-1,2,4-triazole Schiff bases as antibacterial agents
compounds that yielded good results will be further evaluated for their toxicology, MIC and
MBC profile.
Supporting information
S1 File. Scheme 1.
(CDX)
S1 Data.
(CDX)
Acknowledgments
The authors are thankful to Higher Education Commission of Pakistan for Startup Research
Grant (SRGP) to Dr Aeysha Sultan. We are also thankful to University of Lahore, Lahore,
Pakistan for assistant and spectroscopic analysis.
Author Contributions
Conceptualization: Muhammad Rafiq, Mariusz Mojzych.
Formal analysis: Mariusz Mojzych.
Investigation: Aeysha Sultan, Mian Habib Ur Rehman, Noreen Sajjad, Ali Irfan, Irfan Ullah,
Muhammad Mustaqeem, Muhammad Saleem, Syeda Laila Rubab, Muhammad Rafiq,
Muhammad Khalid.
Methodology: Muhammad Rafiq, Mariusz Mojzych.
Supervision: Mariusz Mojzych.
Writing – original draft: Aeysha Sultan, Muhammad Rafiq.
Writing – review & editing: Katarzyna Kotwica-Mojzych, Mariusz Mojzych.
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