Some new Cu(II) complexes containing O,N-donor Schiff base ligands derived from 4-aminoantipyrine: synthesis, characterization, crystal structure and substitution effect on antimicrobial activity

Article abstract of DOI:10.1080/00958972.2021.1900831

A new series of mononuclear Cu(II) complexes, [Cu(L¹)₂]: C1, [Cu(L²)₂]: C2, [Cu(L³)₂]: C3 and [Cu(L⁴)₂]: C4, with bis-O,N-bidentate Schiff base ligands (HL¹

Full text of DOI:10.1080/00958972.2021.1900831

Journal of Coordination Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Some new Cu(II) complexes containing O,N-
donor Schiff base ligands derived from 4-
aminoantipyrine: synthesis, characterization,
crystal structure and substitution effect on
antimicrobial activity
Hadi Kargar, Fariba Aghaei-Meybodi, Mohammad Reza Elahifard,
Muhammad Nawaz Tahir, Muhammad Ashfaq & Khurram Shahzad
Munawar
To cite this article: Hadi Kargar, Fariba Aghaei-Meybodi, Mohammad Reza Elahifard, Muhammad
Nawaz Tahir, Muhammad Ashfaq & Khurram Shahzad Munawar (2021): Some new Cu(II)
complexes containing O,N-donor Schiff base ligands derived from 4-aminoantipyrine: synthesis,
characterization, crystal structure and substitution effect on antimicrobial activity, Journal of
Coordination Chemistry, DOI: 10.1080/00958972.2021.1900831
To link to this article: https://doi.org/10.1080/00958972.2021.1900831
Published online: 25 Mar 2021.
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2021.1900831
Some new Cu(II) complexes containing O,N-donor Schiff
base ligands derived from 4-aminoantipyrine: synthesis,
characterization, crystal structure and substitution effect
on antimicrobial activity
Hadi Kargara , Fariba Aghaei-Meybodib, Mohammad Reza Elahifardc,
Muhammad Nawaz Tahird, Muhammad Ashfaqd and Khurram Shahzad
Munaware,f
aDepartment of Chemical Engineering, Faculty of Engineering, Ardakan University, Ardakan, Iran;
c
bDepartment of Chemistry, Payame Noor University, Tehran, Iran; Department of Metallurgical
d
Engineering, University of Nevada, Reno, NV, USA; Department of Physics, University of Sargodha,
e
f
Punjab, Pakistan; Department of Chemistry, University of Sargodha, Punjab, Pakistan; Department
of Chemistry, University of Mianwali, Mianwali, Pakistan
ABSTRACT
ARTICLE HISTORY
A new series of mononuclear Cu(II) complexes, [Cu(L1)2]: C1,
[Cu(L2)2]: C2, [Cu(L3)2]: C3 and [Cu(L4)2]: C4, with bis-O,
N-bidentate Schiff base ligands (HL1: (E)-4-[(2-hydroxy-5-bro-
mobenzylidene)amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-3H-pyr-
azol-3-one; HL2: (E)-4-[(2-hydroxy-5-chlorobenzylidene)amino]-
1,5-dimethyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one; HL3: (E)-4-
[(2-hydroxy-5-hydroxybenzylidene)amino]-1,5-dimethyl-2-phenyl-
1,2-dihydro-3H-pyrazol-3-one; HL4: (E)-4-[(2-hydroxy-5-nitroben-
zylidene)amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-
one) have been prepared and characterized by elemental (CHN)
analyses, FT-IR and multinuclear NMR (1H and 13C) spectroscopy.
Furthermore, the crystal structures of HL3, HL4 and C4 were ana-
lyzed by single-crystal X-ray diffraction (SC-XRD). The XRD studies
revealed that C4 has 2:1 ligand-to-metal ratio and adapts highly
distorted square planar geometry. The in vitro antibacterial activ-
ities of the synthesized ligands and their respective copper(II)
complexes were elaborated by screening them against
Staphylococcus aureus and Escherichia coli. The results showed a
wide range of antimicrobial activity due to different electronic
environments of the substituents on phenolic moiety.
Received 24 September 2020
Accepted 24 February 2021
KEYWORDS
Aminoantipyrine; Schiff
base complex; x-ray
structure; antibacter-
ial activity
CONTACT Hadi Kargar
h.kargar@ardakan.ac.ir; hadi_kargar@yahoo.com
Department of Chemical Engineering,
Faculty of Engineering, Ardakan University, P.O. Box 184, Ardakan, Iran.
Supplemental data for this article is available online at https://doi.org/10.1080/00958972.2021.1900831.
ß 2021 Informa UK Limited, trading as Taylor & Francis Group

2
H. KARGAR ET AL.
1. Introduction
Nowadays, an emergency medical issue is the increasing resistance of microbes
against commonly available antibiotics which leads to the synthesis of new novel anti-
microbial compounds that can be proved to have potential against these mutated
pathogens [1–3]. Schiff bases are considered promising precursors for the synthesis of
new bioactive compounds of medicinal relevance due to their biological as well as
pharmacological importance and ability to form chelates with transition metals [4–8].
Schiff bases are not only good r-donors but also very excellent p-acid ligands, as
ꢀ
these have the ability to accept the electronic density into empty p -antibonding
molecular orbitals of HC ¼ N (azomethine group) from metal possessing low oxidation
states. The Schiff bases, with the aid of phenolic group at ortho-position, can act as
monobasic bidentate ligands in nature and make the basis of coordination chemistry
with a variety of metals [9–11].
Schiff base derivatives of 4-aminoantipyrine have a dominant position in medicinal
chemistry due to their wide spectrum of applications like antituberculosis, antineoplas-
tic, antimalarial, anticancer, antioxidant, antifungal, anti-inflammatory, anti-proliferative
activity and corrosion inhibition [12–15]. Antipyrine can also coordinate with the metal
ions in a monodentate fashion via its carbonyl oxygen [16]. Coordination of metal
with ligand brings matchless improvement in the biological properties of not only the
ligand but also the metal ion [17–19]. The most suitable metals, specifically for this
motive, are transition metals as they can adapt to extensive types of coordination

JOURNAL OF COORDINATION CHEMISTRY
3
Scheme 1. The synthesis pathways of ligands HL1–HL4 and complexes C1–C4.
numbers, geometries and oxidation states as compared to carbon and other main
group elements.
Copper complexes can exhibit astonishing pharmacological potential, which are not
observed when the ligand or simple inorganic salts of copper are used alone [20–23].
It is revealed from the literature that a very limited number of copper(II) complexes

4
H. KARGAR ET AL.
are reported with O- and N-donor Schiff bases derived from 4-aminoantipyrine [24].
Furthermore, no work is done pointing out the comparative analysis of substitution
effect of various moieties on the salicylaldehydic part of the Schiff base derived from
4-aminoantipyrine and its respective Cu(II) complexes on the antibacterial activities.
This triggers us to prepare a new series of bioactive Cu(II) complexes with four
bidentate (O,N) Schiff bases derived by the condensation of 4-aminoantipyrine with
para-substituted salicylaldehyde. Moreover, these synthesized compounds were char-
acterized by various analytical and spectroscopic techniques and then screened for
their antibacterial activities.
2. Experimental
2.1. Materials and methods
All materials were purchased commercially from Merck and used without purification.
FT-IR spectra were recorded on an IR Prestige-21 spectrophotometer from 400 to
1
4000 cmꢁ1 using KBr pellets. H and 13C NMR spectra of the ligands were recorded at
ambient temperature with a Bruker Avance 400 MHz spectrometer. Elemental (CHN)
analyses were performed using a Heraeus CHN-O-FLASH EA 1112 elemental analyzer.
X-ray data was recorded by a Bruker KAPPA APEXII CCD X-ray diffractometer.
2.2. Synthesis of Schiff base ligands (HLn) (n ¼ 1–4)
For the preparation of bidentate Schiff bases (HLn), an equimolar (1:1) solution of 1-
phenyl-2,3-dimethyl-4-amino-3-pyrazolin-5-one (0.203 g, 1 mM) in ethanol (10 mL) and
respective salicylaldehyde [(0.201 g, 1 mM), 5-bromosalicylaldehyde for HL1 (0.156 g,
1 mM), 5-chlorosalicylaldehyde for HL2 (0.138 g, 1 mM), 5-hydroxysalicylaldehyde for
HL3 and (0.167 g, 1 mM), 5-nitrosalicylaldehyde for HL4] in ethanol (20 mL) were mixed
and stirred for 1 h at room temperature to get a clear yellow solution. Afterwards, the
solution was covered with aluminum foil and kept in air for slow evaporation for a
period of 5 days to collect yellow prism-shaped crystals of HL3 and HL4 and yellow
precipitates of HL1 and HL2. The synthesized products were filtered and washed with
ethanol to remove any impurity and finally dried in air. The elemental analyses, 1H
NMR, 13C NMR and FT-IR data clearly confirmed their composition. The synthesis of
the ligands is illustrated in Scheme 1.
(E)-4-[(2-hydroxy-5-bromobenzylidene)amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-3H-
pyrazol-3-one, HL1; Yield: 78%. Anal. Calcd for C18H16BrN3O2: C, 55.97; H, 4.18; N,
10.88%. Found: C, 55.68; H, 4.09; N, 10.97. FT-IR (KBr, cmꢁ1): m(C ¼ O) 1637, m(C ¼ N)
1
1595, m(C-O) 1124. H NMR (CDCl3, 400 MHz, 298 K), d (ppm): 2.44 (s, 3H, –CH3-C); 3.22
3
4
(s, 3H,–CH3-N); 6.86 (d, J ¼ 8.8 Hz, Hc); 7.37 (dd, 3J ¼ 8.8 Hz, J ¼ 2.4 Hz, Hb); 7.46 (d,
4J ¼ 2.4 Hz, Ha); 7.39–7.54 (m, 5H, antipyrine ring); 9.76 (s, –CH ¼ N, Hi); 13.41 (s, –OH,
Hp). 13C NMR (CDCl3, 100 MHz, 298 K), d (ppm): 10.32; 35.55; 110.63; 115.83; 118.68;
121.75; 124.82; 127.54; 129.41; 133.85; 134.17; 134.35; 149.89; 158.87; 159.44.
(E)-4-[(2-hydroxy-5-chlorobenzylidene)amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-3H-
pyrazol-3-one, HL2; Yield: 81%. Anal. Calcd for C18H16ClN3O2: C, 63.25; H, 4.72; N,
12.29%. Found: C, 63.08; H, 4.67; N, 12.47. FT-IR (KBr, cmꢁ1): m(C ¼ O) 1637, m(C ¼ N)

JOURNAL OF COORDINATION CHEMISTRY
5
1
1595, m(C-O) 1122. H NMR (CDCl3, 400 MHz, 298 K), d (ppm): 2.44 (s, 3H, –CH3-C); 3.23
3
4
(s, 3H,–CH3-N); 6.91 (d, J ¼ 8.8 Hz, Hc); 7.24 (dd, 3J ¼ 8.8 Hz, J ¼ 2.6 Hz, Hb); 7.32 (d,
4J ¼ 2.6 Hz, Ha); 7.38–7.54 (m, 5H, antipyrine ring); 9.77 (s, –CH ¼ N, Hi); 13.37 (s, –OH,
Hp). 13C NMR (CDCl3, 100 MHz, 298 K), d (ppm): 10.32; 35.55; 110.83; 117.73; 118.22;
124.81; 127.54; 129.40; 130.87; 131.54; 134.38; 134.75; 149.97; 159.01; 160.07.
(E)-4-[(2-hydroxy-5-hydroxybenzylidene)amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-
3H-pyrazol-3-one, HL3; Yield: 69%. Anal. Calcd for C18H17N3O3: C, 66.86; H, 5.30; N,
13.00%. Found: C, 66.64; H, 5.37; N, 12.89. FT-IR (KBr, cmꢁ1): m(C ¼ O) 1651, m(C ¼ N)
1
1587, m(C-O) 1139. H NMR (DMSO-d6, 400 MHz, 298 K), d (ppm): 2.39 (s, 3H, –CH3-C);
4
3.20 (s, 3H,–CH3-N); 6.74 (br, Hc); 6.75 (br, Hb); 6.83 (d, J ¼ 2.0 Hz, Ha); 7.37–7.56 (m,
5H, antipyrine ring); 9.01 (s, –CH ¼ N, Hi); 9.60 (s, –OH, Hq); 12.07 (s, –OH, Hp). 13C NMR
(DMSO-d6, 100 MHz, 298 K), d (ppm): 9.80; 35.15; 115.50; 116.94; 119.34; 120.20; 124.85;
127.18; 129.18; 134.19; 149.65; 150.27; 152.13; 157.12; 159.13.
(E)-4-[(2-hydroxy-5-nitrobenzylidene)amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-3H-
pyrazol-3-one, HL4; Yield: 73%. Anal. Calcd for C18H16N4O4: C, 61.36; H, 4.58; N, 15.90%.
Found: C, 61.12; H, 4.66; N, 15.78. FT-IR (KBr, cmꢁ1): m(C ¼ O) 1649, m(C ¼ N) 1593, m(C-
O) 1139. 1H NMR (CDCl3, 400 MHz, 298 K), d (ppm): 2.47 (s, 3H, –CH3-C); 3.27 (s,
3H,–CH3-N); 7.03 (d, 3J ¼ 9.1 Hz, Hc); 8.20 (dd, 3J ¼ 9.1 Hz, 4J ¼ 2.7 Hz, Hb); 8.32 (d,
4J ¼ 2.7 Hz, Ha); 7.39–7.56 (m, 5H, antipyrine ring); 9.89 (s, –CH ¼ N, Hi); 14.48 (s, –OH,
Hp). 13C NMR (CDCl3, 100 MHz, 298 K), d (ppm): 10.33; 35.37; 117.56; 119.49; 125.08;
127.08; 127, 64; 127.85; 129.51; 133.91; 140.18; 149.64; 157.96; 159.91; 165.95.
2.3. Synthesis of the copper(II) complexes
Copper(II) complexes of the type [Cu(Ln)2] (C1–C4), where Ln ¼ L1–L4, with four
bidentate Schiff base ligands (HLn) were prepared by adding metal precursor
Cu(OAc)2ꢂH2O (0.200 g, 1 mM) to a hot solution of the corresponding ligand HLn
(2.0 mM) in methanol (30 mL). The mixture was refluxed for 3 h and then resultant sus-
pension was filtered, washed thoroughly with methanol and then finally dried in air.
Black-green crystals of C4 suitable for X-ray measurements were obtained from metha-
nol solution. The synthesis of the complexes is also illustrated in Scheme 1.
C1; [Cu(L1)2], Yield: 67%. Anal. Calcd for C36H30Br2CuN6O4: C, 51.84; H, 3.63; N,
10.08%. Found: C, 51.67; H, 3.69; N, 10.29. FT-IR (KBr, cmꢁ1): m(C ¼ O) 1651, 1606;
m(C ¼ N) 1589; m(C-O) 1246.
C2; [Cu(L2)2], Yield: 75%. Anal. Calcd for C36H30Cl2CuN6O4: C, 58.03; H, 4.06; N,
11.28%. Found: C, 57.89; H, 4.14; N, 11.17. FT-IR (KBr, cmꢁ1): m(C ¼ O) 1649, 1606;
m(C ¼ N) 1587; m(C-O) 1315.
C3; [Cu(L3)2], Yield: 58%. Anal. Calcd for C36H32CuN6O6: C, 61.05; H, 4.55; N, 11.87%.
Found: C, 60.87; H, 4.61; N, 11.75. FT-IR (KBr, cmꢁ1): m(C ¼ O) 1654, 1606; m(C ¼ N) 1560;
m(C-O) 1276.
C4; [Cu(L4)2], Yield: 64%. Anal. Calcd for C36H30CuN8O8: C, 56.43; H, 3.95; N, 14.62%.
Found: C, 56.27; H, 4.03; N, 14.56. FT-IR (KBr, cmꢁ1): m(C ¼ O) 1654, 1600; m(C ¼ N) 1543;
m(C-O) 1240.

6
H. KARGAR ET AL.
2.4. Crystal structure determination
The X-ray diffraction measurements of compounds HL3, HL4 and C4 were carried out on
a Bruker Kappa APEXII CCD X-ray diffractometer with graphite monochromated Mo-Ka
radiation (k ¼ 0.71073 Å). The single-crystals suitable for X-ray analysis were obtained
from methanol solution and mounted on a glass fiber for data collection using Bruker
Apex-II software [25]. The structures were solved by direct methods and subsequent dif-
ference Fourier maps on SHELXS97 [26] and then refined on F2 by a full-matrix least-
squares procedure using anisotropic displacement parameters. Atomic factors are taken
from the international tables for X-ray Crystallography [27]. All non-hydrogen atoms
were refined with anisotropic displacement parameters. Hydrogens were placed in ideal
positions and refined as riding atoms with relative isotropic displacement parameters. All
refinements were performed using SHELXL-2018/3 and WinGX-2014.1 programs [28, 29].
The method to collect data was x-scans and integrated using Bruker SAINT [30] software
package. The crystallographic illustrations for HL3, HL4 and C4 were prepared using
ORTEP-3 [29] and Platon [31]. Experimental parameters pertaining to single-crystal X-ray
analysis of compounds are given in Table S1.
2.5. Antibacterial activity
In vitro antibacterial activities of the Schiff base ligands and their copper(II) complexes
were assessed against both gram-positive and gram-negative bacteria. Herein,
Escherichia coli (E. coli, ATCC 25922) and Staphylococcus aureus (S. aureus, ATCC
25923), common referenced gram-negative and gram-positive bacteria, respectively,
were examined. Using dilution method, the lowest level of antimicrobial agent that
greatly inhibits the growth of a microorganism, Minimum Inhibitory Concentration
(MIC), and the lowest level of antimicrobial agent resulting in microbial death,
Minimum Bactericidal Concentration (MBC), were recorded. The mixtures of 1024, 512,
256, 128, 64, 32, 16, 8, 4 and 2 lg/mL of antibacterial agents (free ligands, their com-
plexes and Streptomycin as internal standard) with suspension of 5 ꢃ 105 CFU/mL of
test strains (0.5 McFarland) in Muller-Hinton broth (MHB) were provided in 96-well
plates. It also included one growth control (MHB) and one sterility control
(MHB þ compound). After incubation at 37 ꢄC for 18–24 h, the MIC for each tested sub-
stance was indicated by the presence of a white “pellet” on the bottom of the well.
The dilution representing the MIC and three more concentrated suspensions were
plated and enumerated to determine viable CFU/mL as MBC value. It is the lowest
concentration of a substance necessary to achieve a bactericidal effect that demon-
strates a pre-determined reduction (99.9%) in CFU/mL when compared to the
MIC dilution.
3. Results and discussion
3.1. Synthesis
Condensation of 4-aminoantipyrine with equimolar amounts of the respective para-
substituted salicylaldehydes leads to the formation of an important class of bidentate

JOURNAL OF COORDINATION CHEMISTRY
7
Table 1. Selected bond bond lengths (Å) and angles (ꢄ) for HL3, HL4 and C4.
Selected bond lengths for HL3
Selected bond lengths for HL4
C7-C8
C8-N3
C12-N3
C12-C13
1.442(4)
C7-C8
1.434(2)
1.394(2)
1.272(2)
1.446(2)
1.399(3)
1.292(3)
1.446(4)
C8-N3
C12-N3
C12-C13
Selected bond angles for HL3
Selected bond angles for HL4
C7-C8-N3
C8-N3-C12
N3-C12-C13
129.1(2)
122.2(2)
120.8(3)
C7-C8-N3
C8-N3-C12
N3-C12-C13
129.6(1)
119.89(2)
121.7(1)
Selected bond lengths for molecule I of C4
Selected bond lengths for molecule II of C4
Cu1-O1
1.907(2)
1.878(2)
1.949(2)
1.954(2)
Cu2-O9
Cu2-O13
Cu2-N10
Cu2-N14
1.903(2)
1.884(2)
1.968(2)
1.964(3)
Cu1-O5
Cu1-N2
Cu1-N6
Selected bond angles for molecule I of C4
Selected bond angles for molecule II of C4
O5-Cu1-O1
91.73(10)
148.57(11)
94.66(10)
94.00(10)
143.90(11)
98.66(10)
O13-Cu2-O9
O13-Cu2-N10
O9-Cu2-N10
O13-Cu2-N14
O9-Cu2-N14
N14-Cu2-N10
90.73(10)
153.88(12)
93.02(10)
93.13(10)
150.64(12)
96.12(10)
O5-Cu1-N2
O1-Cu1-N2
O5-Cu1-N6
O1-Cu1-N6
N2-Cu1-N6
Figure 1. ORTEP diagram of HL3 drawn at the probability level of 50%. H-atoms are shown by
small circles of arbitrary radii.
Schiff base ligands. Treatment of the Schiff base ligands HL1–4 with the respective
Cu(OAc)2ꢂH2O in a 2:1 ratio under reflux led to the formation of copper(II) complexes
(C1–C4). The spectroscopic data are in agreement with the chemical formula proposed
for Schiff base complexes. The synthetic procedure for the Schiff base ligands and
their copper complexes is presented in Scheme 1.
3.2. Crystal structures of HL3, HL4 and C4
In HL3 (Figure 1, Table S1), the benzene ring of the 4-amino-1,5-dimethyl-2-phenyl-1H-
pyrazol-3(2H)-one moiety i.e., A (C1-C6), the 4-amino-1H-pyrazol-3(2H)-one ring B (C7-
C9/N1-N3/O1) and 2-methylbenzene-1,4-diol group C (C12-C18/O2/O3) are planar with
r.m.s deviations of 0.0075, 0.0365 and 0.0189 Å, respectively. The C-atoms (C10/C11)

8
H. KARGAR ET AL.
Figure 2. ORTEP diagram of HL4 drawn at the probability level of 50%. H-atoms are shown by
small circles of arbitrary radii.
1
Table 2. Characteristic H NMR chemical shift values (ppm), peak multiplicity and coupling con-
stant J (Hz) for the synthesized compounds recorded in CDCl3 (except HL3, which was recorded in
DMSO-d6).
Compound
HL1
–CH3-C
–CH3-N
Hc
Hb
Ha
Antipyrine ring (5 H)
7.39-7.54 m
Hi
Hp
2.44 s
3.22 s
6.86 d
J ¼ 8.8
6.91 d
J ¼ 8.8
6.74 br
7.37 dd
J ¼ 8.8; 2.4
7.24 dd
7.46 d
J ¼ 2.4
7.32 d
J ¼ 2.6
6.83 d
J ¼ 2.0
8.32 d
J ¼ 2.7
9.76 s
13.41 s
HL2
HL3
HL4
2.44 s
2.39 s
2.47 s
3.23 s
3.20 s
3.27 s
7.38-7.54 m
7.37-7.56 m
7.39-7.56 m
9.77 s
9.01 s
9.89 s
13.37 s
12.07 s
14.48 s
J ¼ 8.8; 2.6
6.75 br
7.03 d
J ¼ 9.1
8.20 dd
J ¼ 9.1; 2.7
Table 3. Characteristic 13C NMR chemical shift values (ppm) for the synthesized compounds
recorded in CDCl3 (except HL3, which was recorded in DMSO-d6).
Compound
–CH3-C
–CH3-N
Aromatic and pyrazoline ring carbons
–C ¼ O
–C ¼ N
HL1
HL2
HL3
HL4
10.32
10.32
9.80
35.55
35.55
35.15
35.37
110.63-149.89
110.83-149.97
116.94-152.13
117.56-157.96
158.87
159.01
157.12
159.91
159.44
160.07
159.13
165.95
10.33
are deviated from the plane of ring B and are at the distance of ꢁ0.0127(5) and
ꢁ0.5884(5) Å, respectively, below the plane of ring B. The dihedral angles for A/B, A/C
and B/C are 45.4(8)ꢄ, 47.4(8)ꢄ and 5.47(1)ꢄ, respectively. This dihedral angle investiga-
tion inferred that ring B and group C are almost parallel to each other. The dihedral
angle between phenyl ring (C1-C6) of antipyrine group and central five-membered
ring is 43.8(1)ꢄ as compared to 49.67ꢄ and 36.64ꢄ between similar rings in already
reported similar crystal structures having different substitution at the benzene ring
[32]. Selected bond lengths and angles are given in Table 1. The hydroxyl group in
ortho-position interacts with imine N-atom through intra O-H … N bonding to form an
S(6) loop as shown in Figure S1 and given in Table S2. Similar kind of intramolecular
H-bonding is found in already reported Schiff base compounds having 2-hydroxy-3-
bromo-4-chlorophenyl rings instead of 2,5-dihydroxyphenyl moiety [33]. The molecules
are interlinked through strong O-HꢂꢂꢂO bonding to form C(7) chain, where O-atom of

JOURNAL OF COORDINATION CHEMISTRY
9
Figure 3. ORTEP diagram of C4 drawn at the probability level of 20%. H-atoms are not shown
for clarity.
meta-positioned hydroxyl group acts as donor and O-atom of ortho-positioned
hydroxyl group acts as acceptor. The molecules are also interlinked through compara-
tively weak C-HꢂꢂꢂO bonding to form R12ð7Þ loop, where CH from methyl group that is
directly attached with N-atom of 2,3-dihydro-1H-pyrazole ring acts as donor whereas
acceptor O-atom is from carbonyl group. The CH of other methyl group is also
engaged in C-HꢂꢂꢂO bonding in which acceptor O-atom is from the meta-positioned
hydroxyl group. The crystal packing is further stabilized by the presence of off-set pꢂꢂꢂp
stacking interaction. The five-membered 2,3-dihydro-1H-pyrazole ring at asymmetric
position is engaged in parallel off-set pꢂꢂꢂp stacking interaction with the phenyl ring of
hydroquinone moiety at (-x, 2-y, -z). The centroid-to-centroid distance between these
rings is 4.071 Å with ring offset of 2.087 Å, as shown in Figure S2 and given in Table
S3. Similarly, inversion related phenyl rings of hydroquinone moiety are involved in
parallel off-set pꢂꢂꢂp stacking interaction with centroid-to-centroid separation of 4.159 Å
having ring off-set of 2.388 Å. The crystal packing of a related crystal structure having
2-hydroxy-4-methyoxy substituted phenyl ring instead of 2,5-dihydroxy substituted
phenyl ring is stabilized by off-set pꢂꢂꢂp stacking interaction with inter-centroid separ-
ation ranges from 3.34 Å to 3.83 Å as found in literature [34].
In HL4 (Figure 2, Table S1), the benzene ring of the 4-amino-1,5-dimethyl-2-phenyl-
1H-pyrazol-3(2H)-one moiety i.e., A (C1-C6), the ring B (C7-C9/N1-N3/O1) and o-cresol
group C (C12-C18/O2) are planar with r.m.s deviations of 0.0053, 0.0387 and 0.0074 Å,
respectively. The C-atoms (C10/C11) are deviated from the plane of ring B and are at
the respective distances of 0.0486(2) and 0.4540(2) Å. The O-atoms of nitro group are
disordered over two sets of sites with occupancy ratio 0.886(13): 0.114(13). The major

10
H. KARGAR ET AL.
Figure 4. Packing diagram of C4. H-atoms not involved in H-bonding are omitted for clarity.
Benzene rings attached to 1,5-dimethyl-1H-pyrazol-3(2H)-one rings are not involved in H-bonding
are also omitted for clarity.
part of nitro group D (N4/O3A/O4A) and minor part of nitro group E (N4/O3B/O4B)
are twisted at the dihedral angle of 54.9(3)ꢄ with respect to each other. The dihedral
angles for A/B, A/C, B/C, C/D and C/E are 67.6(5)ꢄ, 72.6(5)ꢄ, 5.13(6)ꢄ, 7.74(6)ꢄ and
48.56(2)ꢄ, respectively. The investigation of dihedral angles revealed that ring B and
group C are almost parallel to each other. Selected bond lengths and angles are given
in Table 1. The molecules are first connected with each other in the form of dimers
through strong O-H… O bonding to form R22(18) loop, where acceptor O-atom is from
the carbonyl group as shown in Figure S3. One of the CH of benzene ring (C1-C6) acts
as a donor for both parts of one of the O-atoms of the nitro group to interlink mole-
cules with each other as given in Table S2. The major part of the other O-atom of
nitro group is also involved in comparatively weak C-HꢂꢂꢂO bonding, where CH is from
benzene ring (C1-C6). The minor part of the same O-atom is also involved in C-HꢂꢂꢂO
bonding, where CH is from one methyl group connected with the N-atom of the 4-
amino-1H-pyrazol-3(2H)-one ring. The crystal packing is further stabilized by the pres-
ence of off-set pꢂꢂꢂp stacking interaction. The five-membered 2,3-dihydro-1H-pyrazole
ring (C7-C9/N1/N2) is engaged in parallel off-set pꢂꢂꢂp stacking interaction with the
benzene ring (C1-C6) with centroid-to-centroid distance of 3.632 Å having ring off-set
of 1.032 Å, as shown in Figure S4 and given in Table S3. These rings are connected by
inversion symmetry. The crystal packing of HL4 is entirely different from already
reported very similar crystal structure having 2,4-dihydroxy substituted benzene ring

JOURNAL OF COORDINATION CHEMISTRY
11
Table 4. FT-IR spectral data of the ligands and their corresponding copper(II) complexes (cmꢁ1).
HL1
C1
HL2
C2
HL3
C3
HL4
C4
v(C ¼ N)
v(C–O)
v(Cu–N)
v(Cu–O)
1595
1124
—
1589
1246
410
1595
1122
—
1587
1315
443
1587
1139
—
1560
1276
422
1593
1139
—
1543
1240
420
—
547
—
547
—
549
—
557
Table 5. MIC and MBC of the ligands and their corresponding copper(II) complexes.
MIC (lg/mL)
MBC (lg/mL)
Compound
E. coli
S. aureus
E. coli
S. aureus
HL1
512
512
256
256
128
128
64
256
256
128
64
64
128
64
—
—
—
1024
512
512
256
128
32
—
—
HL2
HL3
1024
512
256
256
128
64
HL4
C1
C2
C3
C4
64
8
32
4
Streptomycin
16
instead of 2,5-dihydroxy substituted benzene ring because in the crystal packing of
the former compound, the substituted benzene ring is stabilized by C-HꢂꢂꢂN bonding
in addition to O-HꢂꢂꢂO bonding whereas in HL4 C-HꢂꢂꢂN bonding is absent [32].
In C4 (Figure 3, Table S1), there are two crystallographically independent molecules
I (C1-C36/N1-N8/O1-O8/Cu1) and II (C37-C72/N9-N16/O9-O16/Cu2) in the asymmetric
unit. Selected bond lengths and angles for molecules I and II are displayed in Table 1.
In both molecules, ligand-to-metal ratio is 2:1, and the coordination sphere around
the central Cu-atom consists of two N-atoms and two O-atoms with both ligands che-
lating the central Cu-atom in a cis arrangement as compared to trans arrangement of
ligands in already reported similar structure having methoxy-substituted phenyl ring
instead of nitro-substituted phenyl ring [35]. Similar ligand-to-metal ratio is also found
in related crystal structure [36]. The distance of Cu1 atom to the basal plane defined
by (N2/N6O1/O5) is 0.66 Å whereas the distance of Cu2 to the basal plane defined by
(N10/N14/O9/O13) is 0.23 Å as compared to the distance value of 0.03 Å and 0.050 Å
of the copper atom from a similar basal plane in already reported similar structures
with different chelating ligands [36, 37]. The geometry of C4 can be computed with
the help of the s4 geometry index, s4 ¼ [360 ꢁ (a þ b)]/141, where a and b are the
two largest angles around the metal center. The value of s4 changes from 0 (perfect
square planar) to 1 (regular tetrahedral) [38–40]. In C4, the geometries of molecules I
and II are highly distorted square planar, as evident from the values of s4, 0.48 and
0.39, respectively. In molecule I, the dihedral angle among nitrobenzene rings A (C1-
C6/N1/O2/O3) and B (C19-C24/N5/O6/O7) is 47.3(1)ꢄ whereas in molecule II, the dihe-
dral angle between similar nitrobenzene rings C (C37-C42/N9/O10/O11) and D (C55-
C60/N13/O14/O15) is 34.8(1)ꢄ. In molecule I, 2,3-dihydro-1H-pyrazole rings E (C8-C10/
N3/N4) and F (C26-C28/N7/N8) are twisted at the dihedral angle of 16.4(1)ꢄ with
respect to each other whereas in molecule II, dihedral angle between similar five-
membered rings G (C44-C46/N11/N12) and H (C62-C64/N15/N16) is 16.2(3)ꢄ. In mol-
ecule I, phenyl rings I (C11-C16) and J (C29-C34) are planar with r.m.s deviations of
0.0036 and 0.0054 Å with dihedral angle I/J is 31.8(3)ꢄ. In molecule II, phenyl ring K

12
H. KARGAR ET AL.
(C47-C52) is planar with r.m.s deviation of 0.0073 Å. The phenyl ring L (C65-C70) is dis-
ordered over three sets of sites with occupancy of 1/3 for each site. The molecules of
type I are connected with each other in the form of dimers through C-HꢂꢂꢂO bonding
to form a R12ð6Þ loop as shown in Figure 4 and given in Table S2. In this C-HꢂꢂꢂO bind-
ing, O-atom attached to a 2,3-dihydro-1H-pyrazole ring E acts as a bifurcated acceptor.
Molecules I and II are interlinked through C35-H35BꢂꢂꢂO15, C57-H57ꢂꢂꢂO1 and C57-
H57ꢂꢂꢂO5 bonding whereas the molecules of type II are connected with each other
through C43-H43ꢂꢂꢂO16, C61-H61ꢂꢂꢂO12 and C72-H72CꢂꢂꢂO15 bonding as specified in
Table S2. No off-set p … p stacking interaction is found in the crystal packing of C4.
Crystal packing of already reported similar copper complex having methoxy-substi-
tuted benzene ring instead of nitro-substituted benzene was stabilized by the pres-
ence of strong O-HꢂꢂꢂO bonding but in C4, no strong intermolecular O-HꢂꢂꢂO bonding
is found [36].
3.3. NMR spectra
1H NMR spectra of the ligands were recorded in deuterated chloroform-d (with the
exception of HL3, which was recorded in deuterated DMSO-d6). The results are given
in Table 2 and the spectra are shown in Figures S5-S8. The spectra displayed two
sharp signals at 2.39–2.47 and 3.20–3.27 ppm with an integration equivalent to three
hydrogen atoms corresponding to the C ꢁ CH3 and N ꢁ CH3 groups, respectively. The
absorption peaks of salicylaldehyde ring protons (Ha, Hb and Hc) appeared within the
expected range. The broadening in signals for the aromatic protons observed between
6.24 and 8.32 ppm indicate the presence of repeating aromatic units with a different
chemical surrounding. The antipyrine aromatic rings give a group of multi signals at
7.37–7.56 ppm. The sharp singlets at d ¼ 9.01–9.89 ppm are attributed to the Hi-C ¼ N-
proton of imine. The OH hydrogen for HL1–HL4 resonates at d ¼ 13.41, 13.37, 12.07
and 14.48 ppm as a sharp singlet, respectively.
The 13C NMR spectra were recorded in the CDCl3 (with the exception of HL3, which
was recorded in deuterated DMSO-d6). The results are given in Table 3 and the spectra
are shown in Figures S9–S12. The signals due to methyl carbons are observed around
10–35 ppm. The signals that appeared in the region d ¼ 110.6–157.9 ppm are assigned
to aromatic and pyrazoline ring carbons. The signals at d ¼ 157.1–165.9 ppm are
assigned to the carbonyl (C ¼ O) and azomethine (HC ¼ N) carbons, respectively.
3.4. FT-IR spectra
The measured FT-IR spectra of the ligands and their corresponding copper(II) com-
plexes are shown in Figures S13–S21. Comparison between the selected bands in the
FT-IR spectra of Schiff base ligands and their complexes are presented in Table 4 and
Figure S21. In the FT-IR spectra for C1-C4, the m(C ¼ O) remains un-modified, indicating
that the exocyclic ketonic oxygen of the antipyrine ring is not involved in the coordin-
ation while the m(HC ¼ N) and m(C–O) bonds shifted to lower and higher wavenum-
bers, respectively, in comparison with their corresponding free ligands, thereby
indicating a coordinative interaction of the iminic nitrogen and phenolic oxygen atoms

JOURNAL OF COORDINATION CHEMISTRY
13
with Cu(II). The coordination of iminic nitrogen and phenolic oxygen could also be
confirmed by the appearance of weak bands located at the low wavenumbers which
are assigned to v(Cu–N) and v(Cu–O) at 410–443 cmꢁ1 and 547–557 cmꢁ1, respectively.
These results are consistent with structurally related compounds already reported [32,
34, 41].
3.5. Antibacterial activities
Having MIC and MBC measurements, antibacterial activities of the Schiff base ligands
and their copper(II) complexes were executed and reported in Table 5. It is obvious
from the results that a greater antibacterial activity of all tested compounds was
observed against gram positive as compared to gram negative strains that is a nor-
mal trend of these kinds of antibacterial agents. Due to thinner peptidoglycan layers,
gram-negative bacteria are protected from certain physical assaults because they do
not absorb foreign materials that surround it, juxtaposition to this, thicker peptido-
glycan layer of gram positive bacteria absorb antibiotics and cleaning prod-
ucts easily.
However, in comparison with a moderate antibacterial activity of free ligands, all
complexes showed significant antibacterial activity against both S. aureus and E. coli
under the same experimental conditions. The complexes provide bactericidal activities
and inhibitory effects in a big range; MBC/MIC values of 512–128/128–64 lg/mL
against E. coli and 256–64/128–32 lg/mL against S. aureus. Replacing R-group with
methoxy to produce 1-phenyl-2,3-dimethyl-4-(N-2-hydroxy-4-meth-oxy-benzaldehyde)-
3-pyrazolin-5-one, Rosu et al. [34] synthesized and evaluated antibacterial activity of
six corresponded copper(II) complexes. In line with our results, they reported that the
Schiff base has an inhibitory effect (MIC values in the range 128–512 lg/mL) on the
growth of the E. coli and S. aureus strains and greater bactericidal activities against E.
coli (MIC ¼ 16–512 lg/mL) and S. aureus (MIC ¼ 4–128 lg/mL) was observed for the
complexes. Similarly, some complexes of Cu(II), Ni(II), VO(II) and Mn(II) with Schiff base
derived from 4-amino-2,3-dimethyl-1-phenyl-3-pyrazolin-5-one showed a large range
of inhibitory effects on the growth of the E. coli (MIC ¼ 32–512 lg/mL) and S. aureus
bacterial strains (MIC ¼ 16–512 lg/mL) [42].
Herein, the MIC values of the synthesized free ligands are 512 and 128 lg/mL
against E. coli and 256–64 lg/mL against S. aureus. Also, the MBC results of free
ligands showed ignorable bactericidal activities, except HL4. This behavior can be
explained due to the formation of coordinate covalent bonds on the basis of
Overtone’s concept [43] and Tweedy’s chelation theory [44]. Coordination reduces the
polarity of the central metal ion, leading its larger atomic radius and electronegativity
that decreases its effective positive charges and facilitates the interaction of complexes
with cellular membranes which are highly sensitive towards the charged particle [45].
Moreover, the delocalization of p-electrons over the chelate ring system created by
the donor groups during the coordination increases the lipophilicity of the complexes.
It boosts their permeation through the lipid layer of the microorganism, thus destroy-
ing them more efficiently [46, 47].

14
H. KARGAR ET AL.
Substituting only the R site, a wide range of inhibitory effects of the synthesized
Schiff base ligands and corresponding complexes on the growth of the strains
appears. The same trend of the different activity of the free ligands (HL4 > HL3 > HL2
and HL1) and their complexes (C4 > C3 > C2 and C1) can be based on the electron-
accepting feature of the substituents; stronger the electron acceptors (NO2 > OH > Br
and Cl), the higher will be the antibacterial activity.
4. Conclusion
A series of Schiff base ligands derived from 4-aminoantipyrine and their copper(II)
complexes were synthesized and characterized by various physicochemical techniques.
The exact molecular structures of ligands HL3 and HL4 and complex C4 were deter-
mined by single-crystal X-ray crystallography. The X-ray structure of C4 illustrates that
the steric effect imposed by the methyl group has a profound influence on the Cu(II)
geometry, rearranging into a distorted square planar shape. Even though the ligands
are tridentate in nature, they coordinate with Cu(II) ion in a bidentate fashion through
deprotonated phenolic oxygen and nitrogen of the azomethine chromophore, leaving
the antipyrine exocyclic ketonic oxygen free. Having different electron-accepting fea-
tures of substitutes, ligands and their complexes show a wide range of antimicrobial
activities. The in vitro biological screening experiments resulted in higher activities for
the complexes compared to the free Schiff base ligands. The activity data showed that
the nitro-substituted Schiff base complex has a promising biological potential against
all bacterial species.
Acknowledgements
The support of this work by Ardakan University and Payame Noor University are gratefully
acknowledged. H.K. thanks Prof. Mehmet Akkurt from Erciyes University for the X-ray analysis of
complex C4.
Disclosure statement
No potential conflict of interest was reported by the authors.
ORCID
Hadi Kargar
http://orcid.org/0000-0002-7817-0937
Khurram Shahzad Munawar
http://orcid.org/0000-0001-9055-2519
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