Some novel nanostructure Schiff base compounds: Antimicrobial and thermal behaviors

Article abstract of DOI:10.1002/poc.3873

In this paper, five new zinc-Schiff base compounds formulated as ZnLX₂ (L is a new N₃-Schiff base ligand obtained by condensation reaction between diethylenetriamine and (Z)-3-(4-(dimethylamino)phenyl) acrylaldehyde and X is (Cl^?, Br^?, I^?, N₃^?, and NCS^?)) were synthesized and characterized by Fourier transform infrared, ¹H and ¹³C NMR, UV–visible, thermal analyses, and molar conductivity measurements. Low molar conductivity values of the compounds in dimethylformamide (DMF) showed nonelectrolyte character of them. Zinc complexes have been also prepared in nanostructure sizes under ultrasonic irradiation confirmed by X-ray powder diffraction and scanning electron microscopy. Thereafter, ZnO nanoparticles were prepared by direct calcination process of zinc iodide complex at 600°C under air atmosphere. Furthermore, thermogravimetric analyses of the complexes were used for the investigation of thermal behavior of the tiled compounds. Based on TG/DTG plots, some kinetics activation parameters of the compounds at all thermal decomposition steps were calculated. In final, antimicrobial activities of the compounds were investigated by the well diffusion technique against the gram-positive bacteria of Staphylococcus aureus and Bacillus subtilis, and the gram-negative bacteria of Escherichia coli and Pseudomonas aeruginosa and the fungi strains of Aspergillus oryzae and Candida albicans. The results showed that all zinc-Schiff base organic compounds are more antimicrobial active than free ligand.

Full text of DOI:10.1002/poc.3873

Received: 24 November 2017
DOI: 10.1002/poc.3873
Revised: 18 May 2018
Accepted: 6 June 2018
R E S E A R C H A R T I C L E
Some novel nanostructure Schiff base compounds:
Antimicrobial and thermal behaviors
S. Khani1
| M. Montazerozohori1
| R. Naghiha2
1 Department of Chemistry, Yasouj
University, Yasouj, Iran
Abstract
In this paper, five new zinc‐Schiff base compounds formulated as ZnLX2 (L is
a new N3‐Schiff base ligand obtained by condensation reaction between
diethylenetriamine and (Z)‐3‐(4‐(dimethylamino)phenyl) acrylaldehyde and
X is (Cl−, Br−, I−, N3−, and NCS−)) were synthesized and characterized by
2 Department of Animal Sciences, Faculty
of Agriculture, Yasouj University, Yasouj,
Iran
Correspondence
Fourier transform infrared, H and 13C NMR, UV–visible, thermal analyses,
1
M. Montazerozohori, Department of
Chemistry, Yasouj University, Yasouj
7591874831, Iran.
and molar conductivity measurements. Low molar conductivity values of
the compounds in dimethylformamide (DMF) showed nonelectrolyte charac-
ter of them. Zinc complexes have been also prepared in nanostructure sizes
under ultrasonic irradiation confirmed by X‐ray powder diffraction and scan-
ning electron microscopy. Thereafter, ZnO nanoparticles were prepared by
direct calcination process of zinc iodide complex at 600°C under air atmo-
sphere. Furthermore, thermogravimetric analyses of the complexes were used
for the investigation of thermal behavior of the tiled compounds. Based on
TG/DTG plots, some kinetics activation parameters of the compounds at all
thermal decomposition steps were calculated. In final, antimicrobial activities
of the compounds were investigated by the well diffusion technique against
the gram‐positive bacteria of Staphylococcus aureus and Bacillus subtilis, and
the gram‐negative bacteria of Escherichia coli and Pseudomonas aeruginosa
and the fungi strains of Aspergillus oryzae and Candida albicans. The results
showed that all zinc‐Schiff base organic compounds are more antimicrobial
active than free ligand.
Email: mmzohory@yahoo.com
Funding information
Yasouj University
KEYWORDS
antimicrobial, nanostructure, Schiff base, thermal, zinc
1 | INTRODUCTION
produce an imine. The basic imine nitrogen exhibits pi
acceptor properties. Conjugated Schiff bases have inter-
The Schiff base ligand is a compound with the common
structure R2C=NR′.[1] Related to their structures, they
can be considered a subclass of imines, being either sec-
ondary ketimines or secondary aldimines. Schiff bases
can be prepared from an aliphatic or aromatic amine
and a carbonyl compound by nucleophilic addition
forming a hemiaminal, followed by a dehydration to
esting optoelectronic properties and have been used in
organic electronics such as organic solar cells,[2,3] organic
field‐effect transistor,[4] and electrochromic devices.[5]
The simple and clean chemistry of coordination com-
pounds make them a promising candidate as a low cost
alternative to be currently used conjugated materials.
Schiff base chemistry is also used to prepare covalent
J Phys Org Chem. 2018;e3873.
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© 2018 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/poc.3873

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KHANI ET AL.
organic framework, which can be used for gas storage.[6]
Pharmaceutical properties of the metal complexes derived
from zinc metal ion and Schiff base ligands have been
widely studied, including various areas of medicinal
chemistry. There are several metal complexes that are
formerly in use for these purposes and this has encour-
aged further research on new metallodrugs such as
metal‐mediated antibiotics and anticancer and antiviral
compounds.[7] Schiff base complexes as catalysts play
an important role in a variety of chemical and photo-
chemical reactions. In recent years, some of the chelate
complexes have been used as a catalyst for solar energy
storage systems. In the past two decades, Schiff bases as
ligands have played a key role in the coordination chem-
istry of transition metals and main group metals.[8,9]
These ligands are often obtained to direct synthesis
routes with good yields and high purity and can easily
create stable complexes with transition metal ions.[10,11]
The reasons for more application of these ligands in
coordination chemistry are related to electronic proper-
ties and good solubility in common solvents, easily
access for their preparation and wide structural variety
of these compounds.[12,13] Metal ions such as zinc (II)
with enzymatic functions and its catalytic role are capable
of important role in bioinorganic chemistry. Schiff base
ligands also include the strong donors such as phenoxo
oxygen atoms like imine nitrogen atoms, the groups
that are excellent in processing the catalyst and biologi-
cal activities.[14,15] Schiff base ligands are very attractive
in terms of medicinal properties such as antibacterial,
antiviral, antifungal, and antitumor activities.[10,11,14,16]
Manganese and iron Schiff base compounds are as addi-
tive factors of resolution in MRI imaging, and because of
electrical characteristics have one‐dimensional and two‐
dimensional applications in the production of new elec-
trical and magnetic devices.[17] The study of fluorescence
properties of Schiff base compounds is very low. Some
Schiff base ligands and their complexes have property
of nonlinear optical, and this feature makes them to be
used in optical devices.[18,19] Recently, zinc‐Schiff base
complexes have been used as precursor materials for
synthesis of metal oxide nanoparticles. Metal oxide
and/or related nanocomposite have various applications
in the fields of catalyst, nanocatalyst, optoelectronic,
and many others.[20–22]
nanostructure zinc complexes were prepared under ultra-
sonic irradiation. Finally, zinc complexes were used as
precursors for preparation of nanostructure zinc oxide.
2 | EXPERIMENTAL
2.1 | Materials and methods
All the chemicals such as (Z)‐3‐(4‐(dimethylamino)phe-
nyl) acrylaldehyde (98%), diethylenetriamine(>99%), and
zinc salts (ZnCl2 (98%), ZnBr2 (99%), ZnI2 (99%)) were
purchased from the Aldrich and/or Merck chemical com-
panies. Zinc thiocyanate and azides were freshly prepared
according to our previous report.[33] Infrared (IR) spectra
were recorded by a JASCO‐Fourier transform (FT)/IR‐
680 instrument on the range of 4000 to 400 cm−1 as
KBr disk. 1H and 13C NMR spectra of the complexes were
obtained using a Bruker DPX FT/NMR‐300 spectrometer
in DMSO as solvent using tetramethylsilane (TMS) as
internal standard. UV–Vis spectra of the compounds in
DMF were obtained from a JASCO‐V570 spectrophotom-
eter in the range of 200 to 800 nm. Kruss instrument was
applied for recording of melting points or decomposition
temperature (°C) of the complexes. Molar conductivity
of the Schiff base ligand and their zinc complexes was
determined in DMF (1.0 × 10−3 M) by a Metrohm 712
conductometer with a dip‐type conductivity cell made of
platinum black at room temperature. A Perkin‐Elmer
Pyris model Thermal gravimetric analysis (TGA) instru-
ment was used for record of thermogravimetric diagrams.
Scanning electron microscopy (SEM) images were cap-
tured on a Philips XL30 field emission scanning electron
microscope using Ac voltage of 20 kV. X‐ray powder dif-
fraction (XRD) spectra were conducted on a STOE‐type
STIDY‐MPGermany X‐ray diffractometer with Cu Ka
radiation (k = 1.5418 Å). The high‐power ultrasonic unit
Bandelin Super Sonorex RK‐100H with ultrasonic peak
output of 320 W and HF power of 80 Weff was handled
for preparation of nanostructure complexes. In the bio-
logical tests, nutrient agar (Merck, Germany) was used
as the solid medium for preparing nutrient plates, while
Mueller Hinton broth (Scharlab) was used as the liquid
culture media. Escherichia coli, Pseudomonas aeruginosa,
Staphylococcus aureus, and Bacillus subtilis were selected
for antibacterial studies and Candida albicans and
Aspergillus oryzae were chosen for antifungal tests.
Herein, in continuation of our previous reports,[23–32]
synthesis and characterization of some new zinc
complexes of a new tridentate Schiff base ligand
entitled as (E)‐N1‐((E)‐3‐phenylallylidene)‐N2‐(2‐((E)‐
((E)‐3‐phenylallylidene) amino) ethyl) ethane‐1,2‐
2.2 | Synthesis of Schiff base ligand
The Schiff base ligand was prepared according to the
previous reports via a condensation reaction between
1 mmol diethylenetriamine dissolved in 5 mL of
ethanol and 2 mmol of (Z)‐3‐(4‐(dimethylamino)phenyl)
diamine
are
presented.
Antibacterial/antifungal
properties and thermal behaviors (TG/DTG/DTA) of all
zinc complexes are described. In addition, the

KHANI ET AL.
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acrylaldehyde dissolved in 5 mL of ethanol under intense
stirring at room temperature[33] for 5 hours. After com-
pletion of the reaction, the Schiff base was obtained as a
yellow powder product. Yield: 87%. M.p.: 118–121°C.
2.6 | Antimicrobial bioassay procedure
In vitro antibacterial activities of the Schiff base ligand
and its zinc complexes as zone of growth inhibitory
effects were surveyed by well diffusion method against
gram‐negative bacteria such as E. coli and P. aeruginosa
and gram‐positive bacteria such as S. aureus and
B. subtillis. C. albicans and A. oryzae were checked as can-
didates for in vitro antifungal tests. For probe into the
effect of concentration, ligand and its zinc complexes
were dissolved in DMSO solvent. Accordingly, the stock
solutions of the compounds were made in three concen-
trations (15, 7.5, and 3.75 mg/mL in DMSO). For DMSO
against above microorganisms, no remarkable effect was
reported.[27] By incubation of the microorganisms in
Muller‐Hinton broth medium at 37°C for 24 hours,
enriched culture of them were prepared. Each sterile petri
plate was treated by 15 mL of sterilized nutrient agar and
Sabouraub dextrose agar for preparation of solid media
for antibacterial and antifungal experiments, respectively.
According to the well diffusion method, after solidifica-
tion of agar gel media, 0.1 mL of suspension of mentioned
microorganism was swabbed on to particular plates by a
sterile glass spreader. Sterile paper disk with diameter
6 mm was loaded with a solution of compounds and
located over media surface on the petri plate and after-
wards they were incubated at 37°C. The diameter of the
inhibition zone produced around each well was evaluated
as antimicrobial activities of the newly synthesized com-
pounds after 24 hours of incubation.
2.3 | Synthesis of zinc complexes
Similarly, in our previous synthetic method,[27,31,33] an
ethanolic solution of the fresh ligand (1 mmol) was drop
by drop added to zinc halide and pseudohalide salts solu-
tion (1 mmol in 10 mL of ethanol), and the mixture was
severely stirred for 4 hours at room temperature. After
this time, the deposited zinc complexes were separated,
washed with ethanol, and dried under air. Finally, they
were identified by various techniques. Physical and spec-
tral data (IR, UV–Visible, and NMR) of the ligand and its
zinc complexes are listed in Tables 1–4.
2.4 | Synthesis of nanostructure zinc
complexes
Synthesis of nanostructured zinc complexes were per-
formed by sonochemical procedure. An ethanolic solu-
tion of the Schiff base ligand (1 mmol in 10 mL) was
gradually added into zinc salts in ethanol (1 mmol in
10 mL) under ultrasonic irradiation. After addition of
ligand, the reaction mixture was kept in the ultrasonic
bath for 60 minutes. The nanostructured zinc complexes
were collected from the solution by filtration, washed
with ethanol, and dried at room temperature.
2.5 | Synthesis of zinc (II) oxide
nanoparticles
3 | RESULTS AND DISCUSSION
3.1 | Physical and analytical data
A definite amount (0.7 g) of zinc iodide complex as pre-
cursor was heated to 600°C for 4 hours under air atmo-
sphere in an electric furnace. The obtained white
powder was washed with a little amount of acetone sol-
vent and then was dried at room temperature to give zinc
oxide nanoparticles in 68% yield. The ZnO nanoparticles
were identified by XRD and SEM methods.
The physical properties and analytical data of the com-
pounds have been listed in Table 1. The reaction between
the tridentate Schiff base ligand (L) and ZnX2 salts (X is
Cl−, Br−, I−, NCS−, and N3−) in 1:1 molar ratio resulted
in formation of favorable zinc complexes with the general
formula of ZnLX2 (Scheme 1). Solubility test of the
TABLE 1 Analytical and physical data of the Schiff base ligand and its Zn (II) complexes
Compound
Color
Dec. P., °C
Yield, %
Λ°M, cm2 Ω−1 M−1
Ligand
Yellow
118‐121 (M. P.)
191‐196
87
74
86
85
78
65
21.7
14.40
20.4
23.0
21.6
20.8
ZnLCl2
Dark orange
Orange
ZnLBr2
241‐243
ZnLI2
Orange
243‐247
ZnL (SCN)2
ZnL (N3)2
Orange
234‐239
Yellow
240‐242
Abbreviations: Dec., decomposition temperature of the compound; M. P., melting point; Λ°M, molar conductivity.

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KHANI ET AL.
TABLE 2 Vibrational spectral data (cm−1) of the Schiff base (L) and its zinc (II) complexes
ν (NH)
ν (CH)
ν (CH)
ν (CH)
ν (SCN/
N3)
ν
ν
ν
Compound
(C¼N) ν (C¼C) (C―N) (M―N) λ (nm), ε (M−1 cm
amine
alkene
aliph.
imine
Ligand
3299
3217
3212
3205
3028
3037
3039
3037
3041
3037
2918
2919
2913
2923
2922
2914
2881, 2803
2865, 2814
2867, 2815
2863, 2805
‐
‐
‐
‐
1603
1601
1602
1600
1593
1601
1460,
1442
1161
1165
1165
1167
1165
1165
‐
327 (31600), 339
(38800)
ZnLCl2
ZnLBr2
ZnLI2
1462,
1442
449
451
451
453
451
330 (7800), 385
(33000)
1484,
1442
330 (8400), 384
(39200)
1484,
1432
330 (8400), 385
(39600)
ZnL (SCN)2 3211
2861, 2809 2085
2865, 2815 2057
1483,
1432
330 (8800), 385
(40600)
ZnL (N3)2
3184
1485,
1433
331 (12600), 382
(40000)
TABLE 3 1HNMR spectral data of ligand and its zinc‐Schiff base complexes in DMSO‐d6
Compound 1H NMR Data (δ, ppm)
Ligand
ZnLCl2
ZnLBr2
ZnLI2
7.97 (d, 2Hd,d´, J = 9 Hz), 7.34 (d, 4Hg,g´, J = 8.4 Hz), 6.92 (d, 2Hf,f´, J = 15.9 Hz), 6.66 (d, 4Hh,h´, J = 8.4), 6.60 (dd, 2He,e´
J = 15.9 Hz, 9 Hz), 3.47 (t, 4Hb,b´, J = 6 Hz, 4.8 Hz), 2.94 (s, 12Hi,i´), 2.83 (s, 1Ha), 2.72 (t, 4Hc,c´, J = 4.8 Hz)
,
8.13 (d, 2Hd,d´, J = 6 Hz), 7.58 (dd, 2He,e´, J = 12 Hz, 9 Hz), 7.41 (d, 4Hg,g´, J = 8.4 Hz), 7.09 (bd, 2Hf,f), 6.74 (d, 4Hh,h´, J = 8.1),
3.60 (bt, 4Hb,b), 2.97 (s, 12Hi,i´), 2.86 (bt, 4Hc,c´), 2.64 (bs, 1Ha).
8.23 (d, 2Hd,d´, J = 8.1 Hz), 7.60 (dd, 2He,e´, J = 9 Hz, 15 Hz), 7.41 (d, 4Hg,g´, J = 7.5 Hz), 7.19 (d, 2Hf,f´, J = 15 Hz), 6.74 (d, 4Hh,
h´, J = 6), 3.62 (bt, 4Hb,b), 2.95 (s, 12Hi,i´), 2.89 (t, 4Hc,c´), 2.78 (bs, 1Ha).
8.23 (d, 2Hd,d´, J = 7.2 Hz), 7.60 (dd, 2He,e´, J = 15 Hz, 9 Hz), 7.38 (d, 4Hg,g´, J = 8.7 Hz), 7.20 (d, 2Hf,f´, J = 15 Hz), 6.47 (d, 4Hh,
h´, J = 8.4), 3.39 (bt, 4Hb,b), 3.01 (s, 12Hi,i´), 2.89 (bt, 4Hc,c´), 2.73 (bs, 1Ha).
ZnL (SCN)2 8.22 (d, 2Hd,d´, J = 8.1 Hz), 7.62 (dd, 2He,e´, J = 15 Hz, 8 Hz), 7.55 (d, 4Hg,g´, J = 6.3 Hz), 6.64 (d, 2Hf,f´, J = 15 Hz), 6.74 (d, 4Hh,
h´, J = 7.2), 3.54 (bt, 4Hb,b), 3.00 (bt, 4Hc,c´), 2.95 (s, 12Hi,i´), 2.73 (bs, 1Ha).
ZnL (N3)2
8.23 (d, 2Hd,d´, J = 8.4 Hz), 7.61 (dd, 2He,e´, J = 15 Hz, 8.4 Hz), 7.50 (d, 4Hg,g´, J = 8.7 Hz), 6.75 (d, 2Hf,f´, J = 15 Hz), 6.68 (d,
4Hh,h´, J = 8.7), 3.56 (bt, 4Hb,b), 3.00 (bt, 4Hc,c´), 2.95 (s, 12Hi,i´), 2.83 (bs, 1Ha).
TABLE 4 13C NMR spectral data of ligand and its zinc‐Schiff base complexes in DMSO‐d6
Compound
13C NMR Data (δ, ppm)
Ligand
164.11 (C3,3´), 151.22 (C9,9´), 142.32 (C5,5´), 131.12 (C7,7´), 128.95 (C6,6´),
123.56 (C4,4´), 112.39 (C8,8´), 60.89 (C1,1´), 50.02 (C2,2), 40.70 (C10,10´
167.23 (C3,3´), 154.70 (C9,9´), 151.78 (C5,5´), 131.12 (C7,7´), 129.51 (C6,6´),
123.71 (C4,4´), 112.42 (C8,8´), 56.37 (C1,1´), 48.93 (C2,2), 40.78 (C10,10´
167.93 (C3,3´), 154.81 (C9,9´), 151.94 (C5,5´), 131.13 (C7,7´), 129.76 (C6,6´),
123.67 (C4,4´), 112.39 (C8,8´), 56.28 (C1,1´), 47.23 (C2,2), 40.87 (C10,10´
168.50 (C3,3´), 154.80 (C9,9´), 152.00 (C5,5´), 131.30 (C7,7´), 129.90 (C6,6´),
122.50 (C4,4´), 112.60 (C8,8´), 55.30 (C1,1´), 46.50 (C2,2), 40.80 (C10,10´
)
ZnLCl2
)
ZnLBr2
)
ZnLI2
)
ZnL (SCN)2
ZnL (N3)2
168.60 (C3,3´), 152.00 (C9,9´), 148.00 (C5,5´), 133.90 (CNCS), 131.30 (C7,7´),
130.2 (C6,6´), 122.80 (C4,4´), 112.20 (C8,8´), 56.80 (C1,1´), 47.40 (C2,2), 40.80 (C10,10´
)
168.10 (C3,3´), 152.50 (C9,9´), 151.90 (C5,5´), 131.20 (C7,7´), 130.00 (C6,6´),
123.00 (C4,4´), 112.40 (C8,8´), 56.00 (C1,1´), 47.80 (C2,2), 40.50 (C10,10´
)

KHANI ET AL.
5 of 12
major reason for the wave numbers shift in the complexes
spectra.[37] The broad vibrational band at the 3299 cm−1
observed in the spectrum of free ligand is related to
N―H vibration of secondary amine group of ligand. Sig-
nificant shift in vibrational frequency of secondary amine
group N―H confirms the involvement of the aminic
nitrogen of ligand in chelation to zinc ion. Furthermore,
in complexes spectra, the appearance of the weak vibra-
tion peaks at 449 to 453 may be due to the stretching
vibrations of M―N in the complexes.[25,27,36] In IR spec-
trum of ZnL (NCS)2, vibration of the thiocyanate ligand
becomes visible at 2085 cm−1 that shows N‐coordination
of thiocyanate ion.[34,38] The stretching vibrational fre-
quency ν (NCS) in this compound is near to reported zinc
N‐bonded thiocyanate complexes (2077 cm−1, 2073 cm
−1).[39–41] Finally, the appearance of a very strong and
sharp peak 2057 cm−1 is considered as potent witness
supporting the coordination of azide anions to zinc
ion.[41,42]
SCHEME 1 The structure of the zinc complexes
complexes and ligand in different solvents exhibited that
dimethylformamide and dimethylsulfoxide are suitable
solvents for them, whereas chloroform and dichlorometh-
ane dissolves slightly them. The metal complexes are
nonsoluble in alcoholic media such as methanol and eth-
anol. All compounds are stable under air atmosphere for
several months at room temperature. The ligand was
obtained in 87% yields and is melted at the temperature
range of 118°C to 121°C. The zinc complexes are
decomposed in the temperature range of 191°C to
247°C. The yields of metal complexes were obtained in
65% to 86%. The molar conductivity values of ligand and
its zinc complexes were evaluated in DMF (10−3 M) in
the range of 14.40 to 23.00 cm2 Ω−1 mol−1 at room tem-
perature confirming the nonelectrolytic nature of
them.[34,35] The low molar conductivities indicate that
the halide/pseudohalide ions and Schiff base have been
bonded to zinc ion and are resistant to be dissociated in
DMF solution.
3.3 | UV–visible spectra
The electronic spectral data of the Schiff base ligand and
its zinc complexes at DMF (10−5 M) have been tabulated
as Table 2. The electronic spectrum of ligand shows two
notable absorption bands, one at 327 nm as a shoulder
and the other at 339 nm, that may are considered as
π–π* intra‐ligand electronic transition of phenyl rings
and olefinic and azomethine bonds, respec-
tively.[25,26,28,36] In the zinc (II) complexes electronic spec-
tra, the first band showed no remarkable shift (3‐4 nm)
while the later band is shifted toward longer wavelengths
that the observed red shifts attributing to elongation in
the ligand conjugated system after coordination to zinc
ion via azomethine nitrogens.
3.2 | FT/IR spectra
Table 2 summarizes some selected absorption frequencies
of the Schiff base ligand and its zinc complexes. In IR
spectrum of ligand, the lack of bands at 1661 and
3336 cm−1 related to carbonyl ν (C¼O) and ν (NH2)
stretching vibrations (as the starting materials) and
appearance of a new strong band at 1603 cm−1 assigned
as azomethine ν (C¼N) vibration indicates the formation
of ligand.[25,27,36] The likely coordination sites involving
in chelation are supported by the comparison of the IR
spectra of the free ligand and its zinc complexes. The
sharp peak appearing at 1603 cm−1 assigned to the
azomethine vibration is shifted to lower frequencies[25,27]
in the range of 1593 to 1602 cm−1 The weak vibrational
bands at 3028, 2918, and 2881 cm−1 in IR spectrum of
ligand are assigned to stretching vibrations of olefinic, ali-
phatic, and iminic C―H bonds, respectively. The move-
ment of these signals and also the enhancement of their
intensity in IR spectra of all zinc complexes well demon-
strate the involvement of azomethine nitrogens in coordi-
nation to the zinc center.[26,27,30,36] The π‐back bonding of
metal (d10) to π* of azomethine bond of ligand may be the
3.4 | NMR spectra
The NMR spectra of the Schiff base ligand and its com-
plexes in DMSO‐d6 as solvent have been given in experi-
mental part (Tables 3 and 4) based on Scheme 1. The
1H NMR spectra of the ligand and zinc chloride complex
are illustrated in Figure 1 as typical ones. In the 1H NMR
spectrum of the ligand, the formation of the Schiff base
ligand was confirmed by appearance of the azomethine
proton signal (Hd, Hd′) as a doublet with a coupling con-
stant of 9.0 Hz at 7.97 ppm. After complexation, this sig-
nal indicates downfield shift to new position in the range
of 8.13 to 8.23 ppm with a coupling constants in the range
of 6.0 to 8.4 Hz that well supports change in the electron
density of azomethine hydrogens after linkage to zinc
ion.[23–33,43–45] The ligand spectrum shows the signals of
aromatic ring protons (Hg, Hg′ and Hh, Hh′) as doublets

6 of 12
KHANI ET AL.
2.83 ppm. This signal appears in the range of 2.64 to
2.83 ppm at the zinc complexes spectra.
In the 13CNMR spectrum of the Schiff base ligand,
azomethine carbon peak of (C3,3´
)
appears at
164.11 ppm. This signal notably shifts to 167.23 to
168.60 ppm in its zinc complexes that is a mainspring to
confirm the complex formation via azomethine nitrogen
linkage to zinc ion.[23–33,43–45] The peak at 151.22 ppm
attributes to (C9,9´) that undergoes a red shift to 152.00
to 154.81 ppm in the zinc complexes spectra. According
to Schiff base ligand spectrum, the signal at 142.23 ppm
belongs to (C5,5´) that shifts to 148.00 to 152.00 ppm in
all complexes spectra. Based on ligand spectrum, the peak
at 131.12 ppm assigns to (C7,7´) in ligand spectrum that
shifts to 134.76 to 135.36 ppm in the zinc complexes.
(C6,6´) peak appear in 128.95 ppm that shifts to the range
of 129.51 to 130.2 ppm when the Schiff base ligand attach
to the zinc center. The signals that appear at 123.56,
112.39, 60.89, and 50.02 ppm belonged to other free
ligand carbons of (C4,4´), (C8,8´), (C1,1´), and (C2,2´), and
these signals shift to the ranges of 122.50 to 123.71,
112.20 to 112.60, 55.30 to 56.80, and 46.50 to 48.93 ppm
at zinc complexes spectra, respectively. In the ligand spec-
tra, (C10,10´) peak is found at 40.70 ppm that shifts to the
range of 40.50 to 40.87 ppm when the ligand joins to
the zinc ion. With respect to other complexes, an extra
peak appears at 133.90 ppm in zinc thiocyanate complex
FIGURE 1 1HNMR of the A, ligand and B, zinc chloride complex
1
at 7.34 and 6.66 ppm with coupling constants of 8.4 and
8.4 Hz, respectively. After coordination of ligand to zinc
metal ion, these signals exhibit downfield shift to new
chemical shifts as shown in Table 3. In the 1H NMR spec-
dedicates to C(NCS). Regarding the spectral data from H
and 13C NMR spectra and a comparison between peak
chemical shifts in free ligand and its complexes well sup-
port the suggested structure for the zinc complexes
(Scheme 1).
trum of Schiff base ligand, olefinic hydrogens (Hf,
)
f′
observe at 6.92 ppm with a large coupling constant
15.9 Hz as a doublet due to coupling with other olefinic
proton (He, e′). Based on the suggested structure for these
complexes, the signal of Hf and Hf´ appeared as a doublet
with coupling constant in range of 14 to 16 Hz as down-
field peaks in the range of 7.09–7.20 ppm except for ZnL
(SCN)2 and ZnL (N3)2 complexes. Olefinic hydrogens of
the ligand (He, e′), after splitting by neighboring hydrogen
atoms, appear as doublet of doublet signal at 6.60 ppm.
After coordination of ligand, these signals remarkably
indicate downfield shift to the range of 7.58–7.62 ppm.
3.5 | XRD analysis
X‐ray powder diffraction patterns of zinc chloride, iodide,
thiocynate, and azide complexes prepared by the
sonochemical process are represented respectively in
Figure 2A‐D in the 2θ range of 0°C to 90°C to evaluate
the nanostructure sizes. The notable expanding of the
peaks shows that the particles are of nanometer dimen-
sions. Some 2θ angles for the zinc chloride complex are
7, 11, 13, 17, 19, 20.5, 21, 25, and 28 in degree, for the zinc
iodide complex are 17, 30, and 40 in degree, for the zinc
thiocyanate complex are 7.5, 17.5, 27, 28, and 42 in
degree, and finally for the zinc azide complex are 6, 8,
10, 125, 18, 20, 26, and 31 in degree. The average sizes
of the particles were calculated by the Scherer formula,
D = kλ/βCosθ, where D is the average grain size, k is
Blank's constant (0.891), λ is the X‐ray wavelength
(0.15405 nm), and θ and β are the diffraction angle and
full‐width at half maximum of an observed peak, respec-
tively.[46] Accordingly, the average sizes of the particles
In free ligand spectrum, aliphatic hydrogens of (Hb, b′
)
and (Hc, c′) appear as triplet signals in 3.47 and
2.72 ppm that after coordination of the ligand to the zinc
metal ion, these signals were downfielded to 3.54 to
3.62 ppm except for ZnLI2 and 2.86 to 3.00 ppm,
respectively. In the ligand spectrum, aliphatic hydrogens
(Hi, i′) appear as a singlet signal at 2.94 ppm. In all zinc
complexes spectra, this signal slightly shifts to the down-
field regions with respect to the free ligand. Secondary
amine hydrogen in ligand spectrum appears at

KHANI ET AL.
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FIGURE 2 X‐ray powder diffraction patterns of A, zinc chloride, B, iodide, C, thiocynate, and D, azide complexes
for zinc chloride, iodide, thiocynate, and azide were eval-
uated to be 62.49, 66.14, 38.37, 16.85 nm, respectively.
3.8 | Thermal analyses of the zinc complexes
The thermal stability of the Schiff base and its zinc coordi-
nation complexes were investigated based on thermogravi-
metric method from room temperature to 900°C at a
heating rate of 10°C/min under nitrogen atmosphere.
The related thermal plots of complexes (TG/DTG/DTA)
are demonstrated in Figure 5A‐F. All thermogravimetric
data including thermal decomposition steps, mass loss
(%), and kinetic activation parameters of each thermal
decomposition step of the ligand and its zinc complexes
and final residues are listed in Tables 5 and 6. Since the
TG plots indicates no weight loss below the 200°C, lack of
water molecules in the complex structures is confirmed.
A proposed thermal decomposition pathway for the Schiff
base ligand is exhibited in Scheme 2. As suggested in
Scheme 2, decomposition of the ligand is occurred in two
successive steps corresponding to the elimination of the
following fragments: C18H22N2 and C8H13N3. According
to thermal analyses data, the zinc complexes demonstrate
quite different thermal behavior. ZnLI2 is destructed dur-
ing two steps while ZnLCl2, ZnLBr2, ZnL (NCS)2, and
ZnL (N3)2 are decomposed thermally in three thermal
steps. Zinc chloride and azide complexes are decomposed
with residuals of Zn, but zinc bromide complex is
completely destructed without any residual. Zinc iodide
and thiocynate complexes leave out a little amount of zinc
as residuals. The suggested segments eliminated from the
complexes structures have been organized in Table 5.
3.6 | SEM of the compounds
Scanning electron microscopy images of zinc (II) com-
plexes were recorded to show the morphologies of the
nanostructure complexes. From the SEM images
(Figure 3), it seems that particles are well resolved. The
nanoparticles are agglomerated, and a rod‐like morphol-
ogy is suggested for these complexes.
3.7 | Characterization of zinc oxide
nanoparticles
Zinc oxide nanoparticles were prepared with direct calci-
nation of the zinc iodide complex. After the formation,
the nanoparticles were identified by XRD. The XRD pat-
tern of zinc (II) oxide (Figure 4) is in agreement with
the standard pattern of ZnO (hexagonal phase, space
group P63mc, with lattice constants a = 3.2490 Å,
c = 5.2050 Å, Z = 2, JCPDS card number 36‐1451).[26]
The XRD pattern ZnO shows that zinc oxide nanoparti-
cles are readily prepared under mentioned synthetic con-
ditions. Peaks for any other impurities in the XRD
spectrum were not observed. The average crystallite size
of ZnO nanoparticles by using the Scherer equation was
obtained to be 42.81 nm.

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KHANI ET AL.
FIGURE 3 Scanning electron microscopy images of A, zinc chloride, B, iodide, C, thiocynate, and D, azide complexes
of ΔS* for other thermal steps represent more ordered
activated complex than the reactants or a slower reaction
rate than normal decomposition processes.[26,48] In addi-
tion, the positive values ΔH* in the range of 11.61 to
142.12 kJ/mol reflects the endothermic character of all
thermal degradation steps, and the positive values of the
Gibbs free energy of activation (ΔG*) in the range of
(1.25 to 3.47) × 102 kJ/mol reveals the nonspontaneous
nature of thermal decomposition processes.
3.9 | Antibacterial bioassay (in vitro)
Biological tests of some zinc complexes by well diffusion
method were evaluated for antibacterial activity against
two bacteria gram‐positive (B. subtilis and S. aureus)
and two bacteria gram‐negative (E. coli and
P. aeruginosa). The collected results from experiments
have been listed in Table 7 that zone diameter of inhibi-
tion from the growth (mm) are measured as antibacterial
activities. According to the screening results, zinc com-
plexes showed antimicrobial nature against various
microorganisms. Generally, zinc complexes represented
higher activities against microorganisms than free ligand.
Based on data in Table 7, ZnLI2 complex is the most effec-
tive compound against B. subtilis whereas ligand exhibits
the weakest activity. ZnLCl2 complex showed maximum
activity against S. aureus. Other compound showed simi-
lar activity against S. aureus. Zinc iodide complex showed
more activity against P. aeruginosa than ligand and other
FIGURE 4 The X‐ray powder diffraction pattern of ZnO
nanostructure obtained from thermolysis of zinc iodide complex
Moreover, the thermodynamic activation parameters of
decomposition processes of the compounds such as activa-
tion energy (ΔE*), enthalpy of activation (ΔH*), entropy of
activation (ΔS*), and Gibbs free energy change (ΔG*) of the
decomposition were evaluated graphically by plotting of
data based on the Coats‐Redfern relation[47] and tabulated
in Table 6. In all complexes, the high values of activation
energy propose thermal stability of the compounds within
the studied temperature range.[31]
Positive value of the activation entropy (ΔS*) at the
first decomposition step of ZnLCl2 complex shows the
dissociation nature of the process and the negative values

KHANI ET AL.
9 of 12
FIGURE 5 TG/DTG/DTA plots of A, ligand, B, ZnLCl2, C, ZnLBr2, D, ZnLI2, E, ZnL (NCS)2, F, and ZnL (N3)2 complexes
complexes. ZnLCl2 and ZnL (NCS)2 complexes have been
observed to be more effective compounds against E. coli
whereas ZnL (N3)2 had minimum bactericidal against
E. coli. In comparison with standard drugs, the complexes
showed acceptable activities against microorganisms.[36]
more inhibitory effect against C. albicans whereas ZnLCl2
was more impressive than other complexes against
A. orayzea. Among the complexes and the free ligand,
ZnLI2 complex showed higher antifungal activity against
C. albicans. The antifungal activities of the Schiff base
ligand and its zinc complexes against A. orayzea can be
ordered as following the general trend:
3.10 | Antifungal bioassay (in vitro)
Antifungal activities of the ligand and its zinc complexes
were evaluated in vitro against C. albicans and A. orayzea
fungal strains. The diameter of inhibition zone from the
growth (mm) was measured using the well diffusion
method as the antifungal activity and the bioassay results
have been tabulated as Table 8. The antifungal activities
data exhibited that the zinc complexes are more effective
as compared with the free ligand under similar experi-
mental conditions. Among the complexes, ZnLI2 showed
ZnLðNCSÞ2>ZnLCl2>ZnLI2>ZnðN3Þ2 Ligand:
e
According to previous reports,[26,36] the coordination of
ligand to metal causes a decrease in the positive charge of zinc
ion and therefore occurs π‐electron delocalization on the
entire complex, which leads to the biological properties of
the zinc complexes that are more notable as compared with
the free ligand. This results have been explained by the
Overtone's concept and Tweedy's chelation theory.[49] It

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KHANI ET AL.
TABLE 5 Thermal analysis data of the zinc complexes including temperature range, differential thermal gravimetry (DTG) peak, mass
loss, proposed segment, and final residuals
Temperature
Range, °C
Mass Loss Found,
% (Calculated)
DTG Peak,
°C
Proposed
Segment
Final
Residue
Compound
Ligand
190‐430
430‐816
65.55 (63.79)
34.03 (36.21)
317
582
C18H22N2
C8H13N3
‐
ZnLCl2
ZnLBr2
139‐220
220‐437
437‐816
6.2 (5.43)
28.41 (28.93)
59.64 (59.74)
181
327
595
C2H6
C10H12N2
C14H17Cl2N3Zn0.5
Zn
77‐214
214‐478
478‐815
0.82 (0.93)
28.77 (28.66)
69.65 (70.40)
160
329
665
6H
‐
C12H12N2
C14H17Br2N3Zn
ZnLI2
179‐485
485‐814
30.34 (29.36)
58.31 (58.32)
326
561
C14H20N2
C12H15I1.8N3
Zinc salt
Zinc salt
ZnL (NCS)2
168‐388
388‐509
509‐813
44.21 (44.46)
7.72 (8.69)
25.36 (26.24)
315
437
633
C18H22N2
C4H4
C5H9N4S
ZnL (N3)2
153‐373
373‐509
509‐815
32.26 (33.56)
15.13 (15.72)
39.30 (39.20)
256
452
671
C12H18N2
C7H5
C7H12N9
Zn
TABLE 6 Thermokinetic parameters of the thermal decomposition steps of the zinc complexes
Decomposition
Compound
Step, °C
E*, kJ/mol
A*, 1/S
ΔS*, kJ mol K
ΔH*, kJ/mol
ΔG*, kJ/mol
Ligand
190‐430
430‐816
101.94
145.03
2.88 × 106
2.97
−1.27 × 102
−2.45 × 102
97.03
137.92
1.72 × 102
3.47 × 102
ZnLCl2
ZnLBr2
139‐220
220‐437
437‐816
145.90
51.71
93.03
5.34 × 1014
5.12 × 101
7.23 × 102
3.35 × 101
−2.18 × 102
−1.99 × 102
142.12
46.72
85.82
1.27 × 102
1.78 × 102
2.59 × 102
77‐214
214‐478
478‐815
67.79
75.24
58.46
4.20 × 105
5.35 × 103
1.83
−1.40 × 102
−1.79 × 102
−2.49 × 102
64.19
70.23
50.66
1.25 × 102
1.78 × 102
2.85 × 102
ZnLI2
179‐485
485‐814
65.60
71.92
6.95 × 102
2.72 × 101
−1.96 × 102
−2.26 × 102
60.61
64.99
1.78 × 102
2.53 × 102
ZnL (NCS)2
168‐388
388‐509
509‐813
96.43
34.46
19.15
1.01 × 106
8.33 × 10‐1
1.85 × 10‐2
−1.36 × 102
−2.54 × 102
−2.87 × 102
91.53
28.56
11.61
1.71 × 102
2.09 × 102
2.72 × 102
ZnL (N3)2
153‐373
373‐509
509‐815
110.72
44.13
30.19
1.36 × 108
3.41
4.37 × 10‐2
−9.40 × 101
−2.42 × 102
−2.81 × 102
106.32
38.10
22.34
1.56 × 102
2.14 × 102
2.87 × 102
SCHEME 2 The proposed
decomposition pathway for the ligand

KHANI ET AL.
11 of 12
TABLE 7 Antibacterial activity of the zinc complexes as diameter of zone of inhibition (mm) around constructed wells (saturated with 15,
7.5, and/or 3.75 mg/mL) against some bacteria
Gram‐Positive
Gram‐Negative
Compound
Bacillus Subtilis
Staphylococcus Aureus
Pseudomonas Aeruginosa
Escherichia Coli
15.00
16.00
24.00
25.60
19.40
25.00
7.50
14.30
16.50
20.00
19.40
16.70
3.75
13.30
11.40
16.60
19.40
12.40
15.00
14.00
16.00
14.40
14.70
15.00
7.50
11.00
14.50
13.40
13.20
13.00
3.75
10.50
11.00
12.00
12.40
6.00
15.00
13.60
12.00
15.50
13.00
14.50
7.50
9.50
3.75
9.50
15.00
17.30
20.00
18.00
20.00
14.00
7.50
15.60
15.50
14.50
17.50
13.30
3.75
11.00
13.10
13.80
16.00
8.60
Ligand
ZnLCl2
11.40
14.00
15.00
12.50
10.00
12.00
12.50
6.00
ZnLI2
ZnL (NCS)2
ZnL (N3)2
TABLE 8 Antifungal activity of the compounds as diameter of zone of inhibition (mm) around constructed disks (saturated with 15, 7.5,
and/or 3.75 mg/mL) against two fungi
Candida Albicans
Aspergillus Orayzea
15 7.5
6.00 6.00
20.0
Compound
15
7.5
3.75
3.75
6.00
Ligand
16.4
25.0
32.0
22.3
34.0
14.3
24.0
30.0
22.0
22.3
13.0
24.0
29.0
18.3
11.4
ZnLCl2
20.0
18.0
23.0
20.0
15.0
20.0
ZnLI2
16.2
21.0
ZnL (NCS)2
ZnL (N3)2
6.00
6.00
6.00
seems that the lipophilic character of the tested complexes
can be a considerable factor in diffusion into cellular mem-
brane of microorganisms. By binding the metal ion to the
ligand, the lipophilicity nature of complexes is increased, that
causes facilitating their passage from microorganism's cell
membrane. Finally, the active site of enzyme of microorgan-
isms is blocked by binding to the metal coordination sites and
lead to preventing growth of the bacteria and/or fungi.[34,50]
The zinc complexes showed various thermal behaviors.
The ligand and ZnLBr2 were decomposed without any
final residue via two and three thermal steps. ZnLCl2 and
ZnLI2complexes lose their organic segments and Zn and
a little amount of zinc salt are suggested as the residuals
at final, respectively. Also ZnL (N3)2 and ZnL (NCS)2 loses
86.69% and 77.29%, respectively, of their initial mass at
three steps and leaving out Zn and a little amount of zinc
salt at the end. The values of some thermokinetic activa-
tion parameters such as activation enthalpy (ΔH*) and
Gibbs free energy (ΔG*) at all steps were positive demon-
strating endothermic character for thermal decomposition
of the complexes. Finally, the antimicrobial activities of the
free ligand and its zinc complexes were evaluated in vitro
against different bacteria and fungi. The results of antimi-
crobial tests revealed that the zinc complexes have higher
antimicrobial activity with respect to free ligand.
4 | CONCLUSIONS
In this research, N3‐Schiff base ligand and its zinc com-
plexes formulated as ZnLX2 (L is a Schiff ligand and X
is a halide or pseudohalide) were synthesized and identi-
fied by various spectral and physical techniques such as
1
FT/IR, UV–visible, H and 13C NMR, thermal analyses,
melting points, and molar conductivity. Also, the nano-
structure forms of the zinc compounds have been pre-
pared by the sonication process. The morphology and
size of nanostructures were investigated SEM images
and XRD patterns. In addition, the nanoparticles of ZnO
were prepared by the calcination of ZnLI2 complex at
600°C under air atmosphere that characterized by XRD.
The results of thermogravimetric analysis of the ligand
and its zinc complexes revealed that decomposition pro-
cess happens during two to three successive thermal steps.
ACKNOWLEDGEMENT
Partial support of this work by Yasouj University is
acknowledged.
ORCID
S. Khani http://orcid.org/0000-0001-5273-2295
M. Montazerozohori http://orcid.org/0000-0001-8897-4683

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KHANI ET AL.
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How to cite this article: Khani S,
Montazerozohori M, Naghiha R. Some novel
nanostructure Schiff base compounds:
Antimicrobial and thermal behaviors. J Phys Org
Chem. 2018;e3873. https://doi.org/10.1002/
poc.3873
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Acta 2016, 443, 207.
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Dusek, M. Kucerakova, Spectrochim. Acta, Part a 2015, 137, 389.
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McArdle, M. S. Shalamzari, Mater. Sci. Eng., C 2016, 61, 809.