Synthesis, crystal structure, theoretical calculation, spectroscopic and antibacterial activity studies of copper(II) complexes bearing bidentate schiff base ligands derived from 4-aminoantipyrine: Influence of substitutions on antibacterial activity

Article abstract of DOI:10.1016/j.molstruc.2021.129908

A novel series of Cu(II) complexes, including: [Cu(L¹)₂]: C1, [Cu(L²)₂]: C2, [Cu(L³)₂]: C3, [Cu(L⁴)₂]: C4, with four bis-N,O-bidentate Schiff base ligands (HL¹: (E)-4-[(2?hydroxy-3-methoxybenzylidene)amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one; HL²: (E)-4-[(2?hydroxy-4-methoxybenzylidene)amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one; HL³: (E)-4-[(2?hydroxy-5-methoxybenzylidene)amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one; HL⁴: (E)-4-[(2?hydroxy-6-methoxybenzylidene)amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one have been synthesized and characterized by elemental analyses, FT-IR, ¹H NMR and ¹³C NMR spectroscopic techniques. Furthermore, the crystal structures of HL⁴ and C2 were determined by single crystal X-ray analysis. Single crystal X-ray analyses, the structure of C2 confirmed the bidentate coordination mode. The metal center possesses a distorted tetrahedral geometry with bis-N,O donor atoms coordinating from Schiff base ligand. Theoretical calculation of the synthesized compounds were carried out by DFT using B3LYP method with employing the Def2-SVPD (for ligands) and Def2-SV(P) (for complexes) basis sets. Calculated data are in good accordance with the experimental investigations. The in vitro biological activities of the synthesized ligands and their copper(II) complexes were evaluated against Staphylococcus aureus and Escherichia coli. The activity data showed that the metal complexes have a promising biological potential comparable with the parent Schiff base ligands against bacterial species.

Full text of DOI:10.1016/j.molstruc.2021.129908

Journal of Molecular Structure 1230 (2021) 129908
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
Synthesis, crystal structure, theoretical calculation, spectroscopic and
antibacterial activity studies of copper(II) complexes bearing bidentate
schiff base ligands derived from 4-aminoantipyrine: Influence of
substitutions on antibacterial activity
Hadi Kargara,∗, Fariba Aghaei-Meybodib, Reza Behjatmanesh-Ardakanib,
Mohammad Reza Elahifardc, Vajiheh Torabi b, Mehdi Fallah-Mehrjardib,
Muhammad Nawaz Tahird, Muhammad Ashfaqd, Khurram Shahzad Munaware,f
a Department of Chemical Engineering, Faculty of Engineering, Ardakan University, P.O. Box 184, Ardakan, Iran
b Department of Chemistry, Payame Noor University, 19395-3697 Tehran, Iran
c Department of Metallurgical Engineering, University of Nevada, Reno, NV 89557, United States
d Department of Physics, University of Sargodha, Punjab, Pakistan
e Department of Chemistry, University of Sargodha, Punjab, Pakistan
f Department of Chemistry, University of Mianwali, Mianwali, Pakistan
a r t i c l e i n f o
a b s t r a c t
A
novel series of Cu(II) complexes, including: [Cu(L1)2]: C1, [Cu(L2)2]: C2, [Cu(L3)2]: C3,
Article history:
Received 1 October 2020
Revised 27 December 2020
Accepted 29 December 2020
Available online 7 January 2021
[Cu(L4)2]: C4, with four bis-N,O-bidentate Schiff base ligands (HL1: (E)-4-[(2–hydroxy-3-
methoxybenzylidene)amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one; HL2: (E)-4-[(2–hydroxy-
4-methoxybenzylidene)amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one;
hydroxy-5-methoxybenzylidene)amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one; HL4: (E)-
4-[(2–hydroxy-6-methoxybenzylidene)amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one have
HL3:
(E)-4-[(2–
Keywords:
been synthesized and characterized by elemental analyses, FT-IR, 1H NMR and 13 C NMR spectroscopic
techniques. Furthermore, the crystal structures of HL4 and C2 were determined by single crystal X-ray
analysis. Single crystal X-ray analyses, the structure of C2 confirmed the bidentate coordination mode.
The metal center possesses a distorted tetrahedral geometry with bis-N,O donor atoms coordinating
from Schiff base ligand. Theoretical calculation of the synthesized compounds were carried out by
DFT using B3LYP method with employing the Def2-SVPD (for ligands) and Def2-SV(P) (for complexes)
basis sets. Calculated data are in good accordance with the experimental investigations. The in vitro
biological activities of the synthesized ligands and their copper(II) complexes were evaluated against
Staphylococcus aureus and Escherichia coli. The activity data showed that the metal complexes have a
promising biological potential comparable with the parent Schiff base ligands against bacterial species.
Schiff base complex
X-ray structure
Theoretical calculation
Antibacterial activity
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
heterocyclic compounds which performed an important function in
regulating the biological activities. Among these heterocyclic moi-
During the last few decades utmost attention is given to the
Schiff base metal complexes due to their consideration as stere-
ochemical models in coordination chemistry, their easy mode
of preparation and structural diversity. Schiff bases complexes
behaves as active anticancer drugs with enhanced potential as
compared to free ligands [1,2]. The most promising advancement
in the field of medicinal chemistry is due to the introduction of
eties antipyrines played the leading roles in biotransformation of
drugs within the human body and also showed positive response
in monitoring the patients with chronic liver illness (Hepatitis B
virus (HBC), hepatitis C virus (HCV) and alcohol related disease)
[3,4].
4-aminoantipyrine, through the condensation reaction with
various types of organic reagents such as aldehydes, ketones,
thiosemicarbazides, carbazides, and similar compounds, is able to
form flexible ligand systems which are able to coordinate to a va-
riety of metals [5]. Schiff bases derived from 4-aminoantipyrine
∗
Corresponding authors.
E-mail addresses: h.kargar@ardakan.ac.ir, hadi_kargar@yahoo.com (H. Kargar).
https://doi.org/10.1016/j.molstruc.2021.129908
0022-2860/© 2021 Elsevier B.V. All rights reserved.

H. Kargar, F. Aghaei-Meybodi, R. Behjatmanesh-Ardakani et al.
Journal of Molecular Structure 1230 (2021) 129908
and their respective transition metal complexes exhibit potential
biological activities like antibacterial, antifungal, antitumor, anti-
inflammatory, anti-leishmanial and antiviral [6-10]. These derived
Schiff bases act as corrosion inhibitors of mild steel in acidic me-
dia and also showed solvatochromism depending on polarity of the
solvent [11,12].
From coordination point of view the Schiff bases of 4-
aminoantipyrine has an advantage that it has two potential donor
sites and is likely to form three types of compounds with metal
ions [13] viz. (i) chelates utilizing both donor atoms, (ii) amine
salts, using only the amino nitrogen atoms and (iii) two types of
complexes, i.e., coordination only from the carbonyl oxygen atom
or amino nitrogen atom. It has been revealed from literature that
there are drastic changes in the biological properties of ligands
took place on chelation with metals [14-16].
As a consequence, 4-aminoantipyrine drew attention of re-
searchers to do theoretical studies revealing its denticity and bind-
ing modes. However, no work has been done on the influence of
substitution on the Schiff bases derived from 4-aminoantipyrine on
their biological activities [17].
Copper is considered a very important metal from the coordi-
nation point of view and it plays a key role in variety of biolog-
ical process like electron transfer reactions for the activation of
many antitumor moieties. It is also an essential micronutrient, re-
quired in small quantities for performing various functions in the
human body. It also acts as co-factor for many enzymes which are
involved in oxidative metabolism [18,19]. Copper complexes with
4-aminoantipyrines derivatives have ability to intercalate with ni-
trogenous bases of DNA and in addition to this Cu(II) is also in-
volved in the causation and treatment of cancer [20-22]. A brief
overview of literature explored that the theoretical calculations are
very useful for predicting the molecular properties and providing
insight into the structural investigations of the compounds [23-
27]. There are only a few reported structure involving the Cu(II)
complexes bearing N,O-bidentate Schiff base ligands derived from
4-aminoantipyrine in cambridge structural database [28,29].
Keeping in mind these facts we have synthesized and char-
acterized a new series of Cu(II) complexes with the heterocyclic
Schiff base derived by the condensation of 4-aminoantipyrine with
methoxy substituted salicylaldehydes (HL1-HL4). Theoretical calcu-
lation of the synthesized compounds were carried out by DFT us-
ing B3LYP method with employing the Def2-SVPD basis set. Fur-
ther these synthesized compounds were tested against various
strains of bacteria to explore the impact of substitution on the bac-
terial activity.
Scheme 1. The synthesis pathways of the HL1–HL4 ligands and C1–C4 complexes.
ratio was dissolved in EtOH. The mixture was stirred for about
1 h at room temperature to give a clear yellow solution. After
that the solution was allowed to slowly evaporate for a period of
5 days to give the products in pure form (HL1-HL3) and yellow
prism-shaped crystals (HL4) suitable for an X-ray crystal structure
analyses. The elemental analyses, 1H NMR, 13C NMR and IR data,
clearly confirmed their composition. The synthesis of the ligands
is illustrated in Scheme 1.
HL1;
(E)−4-[(2–hydroxy-3-methoxybenzylidene)amino]−1,5-
Yield: 89%.
dimethyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one,
2. Experimental
Calculated for C19 H19N3O3: C 67.64, H 5.68, N 12.46%. Analysis
found: C 67.51, H 5.70, N 12.37. IR (KBr, cm−1): ν (C = O) 1662,
ν (C = N) 1599, ν (C O) 1138. 1H NMR (CDCl3, 400 MHz, 298 K)
2.1. Physical measurements
–
–
–
δ/ppm: 2.44 (s, 3H, –CH3 C); 3.21 (s, 3H,–CH3 N); 3.95 (s, 3H,
3
3
Microanalysis of the complexes was done using a Heraeus
–
–CH3 O); 6.86 (t,
J = 7.8 Hz, Hb); 6.95 (dd,
J = 7.8 Hz,
4
3
4
–
CHN O-FLASH EA 1112 elemental analyzer. IR spectra were
J = 1.5 Hz, Hc); 7.01 (dd, J = 7.8 Hz, J = 1.5 Hz, Ha); 7.35–7.53
recorded on a FT-IR Prestige 21 spectrophotometer from 400–
4000 cm−1 using KBr pellets. 1H NMR spectroscopy in CHCl3-d
(400 MHz, Bruker), 13C NMR spectroscopy in CHCl3-d (100 MHz,
Bruker) was carried out by using tetramethylsilane (TMS) as inter-
nal standard. Chemical shifts for proton resonances are reported in
ppm (δ) relative to tetramethylesilane and CHCl3-d.
=
(m, 5H, antipyrine ring); 9.83 (s, –CH N, Hi); 13.93 (s, –OH, Hp).
13C NMR (CDCl3, 100 MHz, 298 K) δ/ppm: 10.18; 35.65; 56.06;
113.48; 116.04; 118.49; 120.14; 123.59; 124.65; 127.34; 129.33;
134.32; 148.09; 149.97; 150.40; 160.26; 160.49.
HL2;
(E)−4-[(2–hydroxy-4-methoxybenzylidene)amino]−1,5-
Yield: 83%.
dimethyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one,
Calculated for C19 H19N3O3: C 67.64, H 5.68, N 12.46%. Analysis
2.2. Syntheses of schiff base ligands (HLn) (n = 1–4)
found: C 67.53, H 5.65, N 12.51. IR (KBr, cm−1): ν (C = O) 1651,
ν (C = N) 1593, ν (C O) 1136. 1H NMR (CDCl3, 400 MHz, 298 K)
–
– –
δ/ppm: 2.42 (s, 3H, –CH3 C); 3.17 (s, 3H,–CH3 N); 3.84 (s, 3H,
For the preparation of bidentate Schiff base ligands,
3
4
HLn,
a
mixture of 4-aminoantipyrine and the respec-
HL1, 4-
–CH3 O); 6.48 (dd,
J = 9.2 Hz,
J = 2.4 Hz, Hb); 6.48 (d,
–
4
3
tive salicylaldehyde (3-methoxysalicylaldehyde
=
J = 2.4 Hz, Hc); 7.28 (d,
J = 9.2 Hz, Ha); 7.33–7.53 (m, 5H,
methoxysalicylaldehyde = HL2, 5-methoxysalicylaldehyde = HL3
antipyrine ring); 9.77 (s, –CH N, Hi); 13.77 (s, –OH, Hp). 13C
NMR (CDCl3, 100 MHz, 298 K) δ/ppm: 10.32; 35.89; 55.40; 101.14;
=
and 6-methoxysalicylaldehyde
=
HL4) with 1:1 stoichiometric
2

H. Kargar, F. Aghaei-Meybodi, R. Behjatmanesh-Ardakani et al.
Journal of Molecular Structure 1230 (2021) 129908
Table 1
2.3. Syntheses of the copper(II) complexes
Crystal data and structure refinement parameters for HL4 and C2 compounds.
Identification code
HL4
C2
In order to synthesize the copper(II) complexes [Cu(Ln)2] (C1–
C4), where Ln=L1–L4 are bidentate Schiff base ligands (HLn), the
metal precursor Cu(OAc)2.H2O (0.200 g, 1 mmol) was added to
a hot methanolic solution (30 mL) of the corresponding ligand
HLn (0.675 g, 2.0 mmol) the resultant mixture was refluxed for
3 h and then the precipitates were filtered off, washed thoroughly
with methanol and dried in air. Black green crystals of C2 suitable
for X-ray measurements were obtained from methanol solution by
slow evaporation. The synthesis of the complexes is illustrated in
Scheme 1.
Chemical formula
Formula weight
Temperature (K)
C
19H19N3O3
C38H36CuN6O6
736.27
337.37
296
296
˚
Wavelength (A)
0.71073
Monoclinic
P21/c
0.71073
Triclinic
¯
P1
Crystal system
Space group
˚
a (A)
6.8913(4)
28.3676(19)
9.3850(8)
90
10.7079(4)
13.0362(5)
14.4214(8)
105.975(2)
102.344(1)
110.267(2)
1705.55(13)
2
˚
b (A)
˚
c (A)
α (°)
β (°)
γ (°)
119.451(7)
90
C1; [Cu(L1)2], Yield: 81%. Calculated for C38H36CuN6O6: C 61.99,
3
H 4.93, N 11.41%. Analysis found: C 61.87, H 4.97, N 11.52. IR (KBr,
˚
Volume (A )
1715.7(2)
4
cm−1): ν (C = O) 1658, 1604 ν (C = N) 1587, ν (C O) 1215.
–
Z
Calculated Density (Mg/m3)
Absorption coefficient (mm−1
F(000)
1.306
1.434
C2; [Cu(L2)2], Yield: 78%. Calculated for C38H36CuN6O6: C 61.99,
)
0.09
0.70
H 4.93, N 11.41%. Analysis found: C 62.07, H 4.86, N 11.37. IR (KBr,
712
766
cm−1): ν (C = O) 1658, 1610 ν (C = N) 1587, ν (C O) 1247.
–
Crystal Shape
Prism
Prism
C3; [Cu(L3)2], Yield: 73%. Calculated for C38H36CuN6O6: C 61.99,
Crystal color
Yellow
0.32 × 0.23 × 0.22
Black green
0.34 × 0.27 × 0.25
Crystal size (mm)
Data Collection
Diffractometer
H 4.93, N 11.41%. Analysis found: C 62.05, H 4.98, N 11.47. IR (KBr,
cm−1): ν (C = O) 1656, 1622 ν (C = N) 1579, ν (C O) 1249.
–
Bruker KAPPA
APEXII CCD
Bruker KAPPA
APEXII CCD
C4; [Cu(L4)2], Yield: 69%. Calculated for C38H36CuN6O6: C 61.99,
H 4.93, N 11.41%. Analysis found: C 61.88, H 4.95, N 11.45. IR (KBr,
Diffractometer
multi-scan
diffractometer
multi-scan
cm−1): ν (C = O) 1664, 1608 ν (C = N) 1572, ν (C O) 1246.
–
Absorption correction
(SADABS; Bruker,
2007)
(SADABS; Bruker,
2007)
2.4. Computational methods
No. of measured, independent
and observed [I> 2s(I)]
reflections
14,661, 3728, 2564
26,861, 7432, 6320
Theoretical calculations involving geometry optimization in the
gas and solution phases, vibrational frequencies, and NMR chemi-
cal shifts of ligands and complexes were performed with the Gaus-
sian 09 package [30] at B3LYP level of theory [31]. The solution
phase was modeled by using IEFPCM with considering the solvent
[32]. The standard Def2-SVPD and Def2-SV(P) basis set [33] were
used for ligands and complexes, respectively. Geometry optimiza-
tions were tested by frequency analysis to be sure that they are in
the local minimum of potential energy surface (PES). The results
showed no imaginary frequency. The 1H and 13C NMR magnetic
isotropic shielding tensors were calculated by the standard Gauge-
Independent Atomic Orbital (GIAO) approach in the solution phase
[34]. Chemical shift values of HL1_HL4 ligands are calculated by us-
ing B3LYP/Def2-SVPD level and IEFPCM model as implicit model of
solvent and compared with experimental data in chloroform. The
same solvent was used for all IEFPCM calculations for ligands, com-
plexes and TMS. Chemical shifts were calculated by subtracting the
appropriate isotopic part of the shielding tensor from that of TMS
δi = σTMS - σi. The isotropic shielding constants for TMS calcu-
lated in the solution phase at the B3LYP/Def2-SVPD level of theory
were equal to 31.54 ppm and 188.22 ppm for the 1H nuclei and
the 13C nuclei, respectively.
Rint
0.029
0.027
Theta range for data collection
Index ranges
2.429° to 26.998°
−8 ≤ h ≤ 8, −36≤
k ≤ 36, −11≤
l ≤ 10
1.563° to 26.999°
−13≤ h ≤ 13,
−16≤ k ≤ 16,
−18≤ l ≤ 18
0.639
−1
˚
(sin θ/λ)max (A
)
0.639
Refinement
R[F2> 2σ(F2)], wR(F2), S
0.046, 0.129, 1.04
3728
0.035, 0.101, 1.03
7432
No. of reflections
No. of parameters
H-atom treatment
230
466
H-atom parameters
constrained
0.23, −0.20
H-atom parameters
constrained
0.31, −0.38
−3
˚
ꢀρmax, ꢀρmin (e A
)
106.48; 114.09; 116.66; 124.41; 127.11; 129.27; 133.27; 134.50;
149.41; 160.36; 160.53; 162.67; 162.94.
HL3;
(E)−4-[(2–hydroxy-5-methoxybenzylidene)amino]−1,5-
Yield: 81%.
dimethyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one,
Calculated for C19 H19N3O3: C 67.64, H 5.68, N 12.46%. Analysis
found: C 67.75, H 5.72, N 12.51. IR (KBr, cm−1): ν (C = O) 1653,
ν (C = N) 1573, ν (C O) 1132. 1H NMR (CDCl3, 400 MHz, 298 K)
–
–
–
δ/ppm: 2.43 (s, 3H, –CH3 C); 3.20 (s, 3H,–CH3 N); 3.78 (s, 3H,
–
–CH3 O); 6.88–6.91 (br, 3H, Ha, Hb, Hc); 7.34–7.53 (m, 5H, an-
tipyrine ring); 9.82 (s, –CH N, Hi); 13.87 (s, –OH, Hp). 13C NMR
2.5. Crystallographic methods
=
(CDCl3, 100 MHz, 298 K) δ/ppm: 10.31; 35.67; 55.84; 114.95;
116.31; 117.47; 119.20; 119.91; 124.62; 127.32; 129.32; 129.36;
134.33; 149.94; 152.28; 154.63; 160.31.
The X-ray diffraction measurement of HL4 and C2 compounds
were carried out on Bruker Kappa APEXII CCD X-ray diffractometer
HL4;
(E)−4-[(2–hydroxy-5-methoxybenzylidene)amino]−1,5-
Yield: 76%.
with graphite monochromated Mo-Kα radiation (λ = 0.71073 A).
˚
dimethyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one,
The yellow and black green single crystals of HL4 and C2 suit-
able for X-ray analysis were obtained from methanol solution and
mounted on a glass fiber for data collection on Bruker Apex-
II software [35]. The structures were solved by direct methods
and subsequent difference fourier maps on SHELXS97 [36] and
then refined on F2 by a full-matrix least-squares procedure us-
ing anisotropic displacement parameters. Atomic factors are from
the international tables for X-ray Crystallography [37]. All non-
hydrogen atoms were refined with anisotropic displacement pa-
rameters. Hydrogen atoms were placed in ideal positions and re-
fined as riding atoms with relative isotropic displacement parame-
Calculated for C19 H19N3O3: C 67.64, H 5.68, N 12.46%. Analysis
found: C 67.60, H 5.70, N 12.50. IR (KBr, cm−1): ν (C = O) 1639,
ν (C = N) 1595, ν (C O) 1136. 1H NMR (CDCl3, 400 MHz, 298 K)
–
–
–
δ/ppm: 2.42 (s, 3H, –CH3 C); 3.18 (s, 3H,–CH3 N); 3.84 (s, 3H,
3
3
–
–CH3 O); 6.37 (d, J = 8.3 Hz, Hc); 6.56 (d, J = 8.3 Hz, Ha);
3
7.23 (t, J = 8.3 Hz, Hb); 7.32–7.52 (m, 5H, antipyrine ring); 10.22
(s, –CH N, Hi); 14.24 (s, –OH, Hp). 13C NMR (CDCl3, 100 MHz,
=
298 K) δ/ppm: 10.34; 35.83; 55.62; 100.62; 109.51; 109.74; 116.99;
124.41; 127.07; 129.24; 132.90; 134.53; 149.67; 157.25; 160.04;
160.39; 162.29.
3

H. Kargar, F. Aghaei-Meybodi, R. Behjatmanesh-Ardakani et al.
Journal of Molecular Structure 1230 (2021) 129908
ters. All refinements were performed using the SHELXL-2018/3 and
WinGX-2014.1 programs [38,39]. The method to collect data was
ω-scans and integrated using Bruker SAINT [40] software package.
The crystallographic illustrations for HL4 and C2 were prepared us-
ing ORTEP-3 [41] and platon [42]. Experimental parameters per-
taining to single-crystal X-ray analysis of compounds are given in
Table 1.
2.6. Antimicrobial activity
In vitro antibacterial activities of the Schiff base ligands and
their copper(II) complexes were evaluated against Escherichia
coli (E. coli) ATCC 25,922 and Staphylococcus aureus (S. aur)
ATCC 25,923,
a common referenced gram-negative and gram-
Fig. 1. ORTEP diagram of HL4 drawn at the probability level of 50% with H-atoms
positive bacteria, respectively. The Minimum Inhibitory Concentra-
tion (MIC), the lowest level of antimicrobial agent that greatly in-
hibits the growth of a microorganism, and Minimum Bactericidal
Concentration (MBC), the lowest level of antimicrobial agent re-
sulting in microbial death, were determined by dilution methods.
In 96-well plates the concentrations of 1024, 512, 256, 128, 64,
32, 16, 8, 4 and 2 μg/mL of free ligands and their complexes ac-
company with 5 × 105 CFU/ml of test strains (0.5 McFarland) sus-
pended in Muller-Hinton broth (MHB) were run simultaneously.
It was also included one growth control (MHB) and one sterility
control (MHB + compound). Internal standard, Streptomycin, was
used for comparison under similar condition. After incubation at
37 °C for 18–24 h, the MIC for each tested substance was indicated
by the presence of a white ‘‘pellet” on the well bottom. The dilu-
tion representing the MIC and two 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 demonstrates a pre-determined
reduction (99.9%) in CFU/ml when compared to the MIC dilution.
are shown by small circles of arbitrary radii.
3. Results and discussion
3.1. Syntheses
Condensation of 4-aminoantipyrine with equimolar amounts
of the respective methoxysalicylaldehyde leads to the forma-
tion of an important class of the bidentate Schiff base lig-
ands. The Schiff base ligands HL1, HL2, HL3 and HL4 were
prepared by the reaction of 4-aminoantipyrine with the corre-
sponding 3-methoxysalicylaldehyde, 4-methoxysalicylaldehyde, 5-
methoxysalicylaldehyde and 6-methoxysalicylaldehyde in nearly
76–89% yield in ethanol. Crystal of ligand HL4, suitable for X-ray
structure determination, were crystallized from ethanol by slow
evaporation of the solvents at room temperature over several days.
Treatment of the Schiff base ligands HL1–4 with the respective
Cu(OAc)2.H2O, in a 2:1 ratio under reflux condition led to the cop-
per(II) complexes (C1–C4). Crystals of complex C2 suitable for X-
ray diffraction could be isolated from methanol after slow evap-
oration of the solvent over a period of 5 days. The spectroscopic
data is in good agreement with the chemical formula proposed for
Schiff base complexes. The synthetic procedure of Schiff base lig-
ands and their copper complexes is presented in Scheme 1.
Fig. 2. Packing diagram of HL4. H-atoms not involve in H-bonding are omitted for
clarity.
Table 2
˚
Hydrogen-bond geometry (A,°) and C
…Cg interaction for HL4.
–
H
D—H···A
D—H
H···A
D···A
D—H···A
O1—H1···N1
0.82
0.93
0.93
0.93
C—H
0.96
0.96
1.82
2.40
2.64
2.56
2.5539 (18)
3.055 (2)
3.283 (2)
3.444 (1)
C···Cg
148
C8—H8···O3
128
C15—H15···O3i
C17—H17···O3ii
C—H···Cg
127
160
H···Cg
2.89
C—H···Cg
123
C12—H12A···Cg2i
C19—H19···Cg2iii
3.512(2)
3.476(2)
2.85
126
Symmetry code: (i)
x
−
1, y, z, (ii)−1
+
x, ½-y, −1/2
+
z),
(iii) x − 1, y, z-1, where Cg2 is the centroid of phenyl ring (C1-C6).
(6)°, respectively. The dihedral angle between 3-methoxyphenol
group
A
(C1-C7/O1/O2) and 5-methyl-4-(methyleneamino)−1H-
3.2. Crystal structures of HL4 and C2 compounds
pyrazol-3(2H)-one group B(C8-C12/N1/N2/O3) shows that these
groups are almost parallel. The C-atom of methyl group attach
In the 4-{(E)-[(2-Hydroxy-6-methoxyphenyl)methylidene]amino}−1,5- to the ring N-atom of 5-methyl-4-(methyleneamino)−1H-pyrazol-
˚
dimethyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one
(Fig.
(C1-C7/O1/O2), 5-methyl-
4-(methyleneamino)−1H-pyrazol-3(2H)-one group B(C8-
C12/N1/N2/O3) and phenyl ring C (C14-C19) is planar with r. m. s
1,
3(2H)-one group is at the distance of −0.4819 (3) A from least
square plane of group B. There exists a strong O H…N intramolecu-
–
Table 1), 3-methoxyphenol group
A
lar bonding between hydroxyl group of phenol moiety and N-atom
of interact with N-atom of 2-(iminomethyl)−3-methoxyphenol
group to form S(6) loop as shown in Fig. 2 and given in Table 2.
˚
deviation of 0.0599, 0.0270 and 0.0062 A, respectively. The dihe-
dral angle between A/B, A/C and B/C is 4.05 (8)°, 50.4 (6)° and 54.4
There exists another intramolecular C H…O bonding to form
–
4

H. Kargar, F. Aghaei-Meybodi, R. Behjatmanesh-Ardakani et al.
Journal of Molecular Structure 1230 (2021) 129908
4
–
Fig. 3. Graphical repsentation of C H…π interaction for HL . H-atoms not involved
˚
–
in C H…π interaction are omitted for clarity. Distances shown are measured in A.
Fig. 5. Packing diagram of C2 with H-atoms not involve in H-bonding are omitted
for clarity.
(C1-C19/N1-N3/O1-O3), the (Z)−3-aminoprop-1-en-1-ol group A
(C1/C7/C8/N1/O1) and 3-methoxyphenol group B (C1-C7/O2) is
˚
planar with r. m. s deviation of 0.0369 and 0.0404 A, respectively
with dihedral angle of 3.6 (8)° between group A and B. This
dihedral angle shows that group A and B are almost parallel.
The 1H-pyrazol-3(2H)-one moiety C (C9/C10/C12/N2/N3/O3) and
phenyl ring D (C14-C19) is planar with r. m. s deviation of 0.0315
˚
and 0.0061 A, respectively with the dihedral angle C/D of 34.23
(9)° between them. The C-atom of methyl group attach to nitrogen
of 1H-pyrazol-3(2H)-one moiety and C-atom of another methyl
group attach to carbon atom of 1H-pyrazol-3(2H)-one moiety is
˚
at the distance of −0.8639 (4) and 0.0368 (4) A, respectively from
least square plane of moiety C. In the second chelating ligand
(C20-C38/N4-N6/O4-O6), the (Z)−3-aminoprop-1-en-1-ol group E
(C20/C26/C27/N4/O4) and 3-methoxyphenol group F (C20-C26/O5)
Fig. 4. ORTEP diagram of C2 drawn at the probability level of 20% with H-atoms
are shown by small circles of arbitrary radii.
˚
is planar with r. m. s deviation of 0.0199 and 0.0252 A, respec-
tively with dihedral angle of 4.7 (1)° between group E and F. This
dihedral angle shows that group E and F are almost parallel but
are not exactly similar to first ligand. The 1H-pyrazol-3(2H)-one
moiety G (C28/C29/C32/N5/N6/O6) and phenyl ring H (C33-C38) is
another S6 loop, where CH is from o-cresol moiety and O-atom is
from 1H-pyrazol-3(2H)-one moiety. The molecules are connected
with each other through C15-H15…O3i bonding to form infinite
chain lying along the a-axis, where CH is from benzene ring
attach to 2,3-dihydro-1H-pyrazole ring and O-atom is from 1H-
pyrazol-3(2H)-one moiety C. The chains are further interlinked by
˚
planar with r. m. s deviation of 0.0425 and 0.0105 A, respectively
with the dihedral angle G/H of 39.61 (9)° between them. The C-
atom of methyl group attach to nitrogen of 1H-pyrazol-3(2H)-one
moiety and C-atom of another methyl group attach to carbon atom
of 1H-pyrazol-3(2H)-one moiety is at the distance of 0.7398 (3)
C17-H17…O3ii bonding. Weak C H-Cg interaction is also present,
–
˚
resulting in a 2D sheet lying in the ac-plane. This weak interaction
and 0.0279 (1) A, respectively from least square plane of moiety G.
–
helps in further stabilization of crystal packing with C Cg distance
The dihedral angle between similar moieties in chelating ligands is
˚
˚
ranges from 3.476(2) A to 3.512(2) A as shown in Fig. 3 given in
Table 2, where Cg is centroid of phenyl ring (C1-C6).
A/E, B/F, C/G and D/H is 59.30 (6)°, 64.79 (5)°, 21.06 (1)° and 62.26
(8)°, respectively. There exists intramolecular C H…O bonding
–
In
the
bis(3–methoxy-6-((1-(4–1,5-dimethyl-2-phenyl-1,2-
to for S6 loop, where CH is from (Z)−3-aminoprop-1-en-1-ol
dihydro-3H-pyrazol-3-one)imino)methyl)phenolato)-copper(II) (C2)
(Fig. 4, Table 1), coordination sphere around the central Cu-atom
consists of two N-atoms and two O-atoms with both ligands
chelating the central Cu-atom. The chelating nitrogen atoms
group
A
and O-atom is from 1H-pyrazol-3(2H)-one moiety C.
–
There is another intramolecular C H…O bonding is oberved that
connect two ligands with each other, where CH is from methyl
group attach to 1H-pyrazol-3(2H)-one moiety C and O-atom of
1H-pyrazol-3(2H)-one moiety G. The molecules are connected
˚
(N1/N4) are at the distance of 1.9611 (2) and 1.9579 (2) A, respec-
with each other through C H…O bonding to form 2D sheet lying
–
tively from central Cu-atom whereas the chelating oxygen atoms
˚
(O1/O4) are at the distance of 1.8938 (2) and 1.8912 (1) A, respec-
parallel to the [210] direction as shown in Fig. 5 and given in
–
Table 3. There exists weak C H-Cg interaction that helps in further
tively from central Cu-atom. The bond angles in the coordination
spheres ranges from 93.04 (7)° to 101.40 (7)°, the dihehral angle
between (O4-Cu1-N4) and (O1-Cu1-N1) planes is 53.5 (5)° thus
form a distorted tetrahedral geometry. In the first chelating ligand
–
stabilization of crystal packing with C Cg distance ranges from
˚
˚
3.324(3) A to 3.238(3) A as given in Table 3, where Cg is the
centroid of (C20/C26/C27/O4/Cu1) ring.
5

H. Kargar, F. Aghaei-Meybodi, R. Behjatmanesh-Ardakani et al.
Journal of Molecular Structure 1230 (2021) 129908
Table 3
˚
–
Hydrogen-bond geometry (A,°) and C H-Cg interaction for C2.
D—H···A
D—H
H···A
D···A
D—H···A
C4—H4A···O6i
C4—H4C···O1ii
C5—H5···O5iii
C8—H8···O3
0.96
0.96
0.93
0.93
0.96
0.96
0.93
0.93
C—H
0.96
0.96
0.96
2.56
2.49
2.57
2.25
2.47
2.35
2.48
2.61
H···Cg
2.96
2.71
2.57
3.503 (3)
3.225 (3)
3.316 (3)
2.908 (2)
3.203 (3)
3.171 (3)
2.942 (3)
3.235 (3)
C···Cg
166
134
138
128
133
144
111
125
C—H···Cg
98
C11—H11A···O6
C11—H11B···O6iv
C19—H19···O3
C19—H19···O3v
C—H···Cg
C13—H13A···Cg4
C13—H13B···Cg4
C32—H32C···Cg4i
3.238(3)
3.238(3)
3.324(3)
116
136
Symmetry codes: (i) −x
+
2, −y
+
1, −z
+
1;
(ii) −x + 3, −y + 1, −z + 1; (iii) x, y, z − 1; (iv) −x + 1, −y, −z + 1;
(v) −x
+
1, −y, −z, where Cg4 is the centroid of
(C20/C26/C27/N4/O4/Cu1) ring.
3.3. Computational results
Optimized geometry of the compounds are shown in Fig. 6. The
selected structural parameters of the compounds are collected in
Table S1. Data show that theoretical data for selected bond lengths
and bond angles are in good line with the experimental ones. In
addition, root mean square deviation (RMSD) for all bond lengths
and all bond angles (except those belong to hydrogen atoms) were
calculated for ligand HL4 and complex C2 (Table 4). These RMSD
values also show that there is a good agreement between theory
and experiments. Geometries of compounds in the gas phase were
fully optimized at B3LYP level of theory. The coordination geome-
try of the C2 complex is distorted tetrahedral.
3.4. lH NMR, 13 C{1H} nmr spectra
1H NMR spectra of the ligands were recorded in deuterated
chloroform (CDCl3). The results are given in Table 5 and the spec-
tra are shown in Figs. S1-S4. The spectra display three sharp sig-
nals at 2.42–2.44, 3.17–3.21 and 3.78–3.95 ppm with an integra-
tion equivalent to three hydrogens corresponding to the C−CH3,
N−CH3 and O−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 pro-
tons observed between 6.37 and 7.23 ppm indicates the presence
of repeating aromatic units with a different chemical surrounding.
The antipyrine aromatic rings give a group of multi signals at 7.32–
7.53 ppm. The sharp singlet signals at δ = 9.77–10.22 ppm are at-
tributed to the Hi-C = N- proton of imine. The OH hydrogen for
HL1–HL4 resonates at δ = 13.93, 13.77, 13.87 and 14.24 ppm as a
sharp singlet, respectively.
The 13C NMR spectra were recorded in CDCl3 solvent. The re-
sults are given in Table 6 and the spectra are shown in Figs. S5-
S8. The signals due to methyl carbon are observed around 10–
56 ppm. The signals that appeared in region δ = 100.6–149.6 ppm
are assigned to aromatic and pyrazoline ring carbons. The signals
at δ = 154.6–162.9 ppm are assigned to the carbon of C = O and
the carbon of C = N, respectively. For comparison, the NMR chem-
ical shifts (δ) have been calculated in the solution phase at the
B3LYP/Def2-SVPD level of theory for the HL1_HL4 ligands. The ex-
perimental and calculated 1H and 13C NMR chemical shifts (δ) of
the ligands are collected in Tables 5 and 6. The differences be-
tween the theoretical and experimental chemical shifts for ligands
are lower than 4%. The experimental chemical shifts are in good
consistency with the calculated chemical shifts.
Fig. 6. Optimized structures of compounds.
3.5. FT-IR spectra
The measured FT-IR spectra of the ligands and their correspond-
ing copper(II) complexes are shown in Figs. S9-S16. The compari-
son between the selected bands in the IR spectra of Schiff base
ligands and their complexes are presented in Table 7 and Fig.
S17. In the FT-IR spectra for C1-C4 complexes, the ν (C = O) re-
mains unmodified, indicating that the exocyclic ketonic oxygen
of the antipyrine ring is not involved in the coordination while
the ν (C = N) and ν (C–O) bonds shifted to lower and higher
wave numbers, respectively, in comparison with their correspond-
ing free ligands thereby indicating a coordinative interaction be-
tween the iminic nitrogen and phenolic oxygen atoms with Cu(II)
central metals. Theoretical data for ligands and complexes confirm
this subject. The iminic nitrogen and phenolic oxygen coordination
could also be confirmed by appearance of weak bands located at
the low wavenumbers which assigned to (M–N) and (M–O) at 416–
436 cm−1 and 530–551 cm−1 respectively. The vibrational frequen-
cies of ligands were calculated at B3LYP/Def2-SVPD and vibrational
frequencies of complexes were calculated at B3LYP/Def2-SV(P) (two
6

H. Kargar, F. Aghaei-Meybodi, R. Behjatmanesh-Ardakani et al.
Journal of Molecular Structure 1230 (2021) 129908
Table 4
The root mean square deviation (RMSD∗) for the bond lengths and bond angles of the HL4, C2 compounds.
HL4
C2
Exp
Calc
࢞
Exp
Calc
࢞
Bond lengths
C = N
1.291(2)
1.291
1.351
1.232
1.363
1.400
–
0
1.304(3), 1.301(2)
1.300(2), 1.302(2)
1.219(2), 1.224(2)
1.361(2), 1.360(2)
1.403(2), 1.406(2)
1.8942(14)
1.312, 1.314
1.287, 1.289
1.228, 1.227
1.354, 1.354
1.401, 1.400
1.9392
0.008, 0.013
−0.013,−0.013
0.009, 0.003
−0.007,−0.006
−0.002, −0.006
0.045
C–OH
1.351(2)
0
C = O
1.2322(19)
−0.0002
C–OCH3
1.363(2)
0
N–N
1.3999(19)
0.0001
Cu1–O1
–
–
–
–
–
Cu1–O4
–
–
1.8915(13)
1.9364
0.0449
Cu1–N4
–
–
1.9581(16)
2.0038
0.0457
Cu1–N1
–
–
1.9615(16)
2.0086
0.0471
RMSD∗∗
0.0004
0.0193
Bond angles
N4–Cu1–N1
O4–Cu1–O1
O1–Cu1–N4
O4–Cu1–N1
O1–Cu1–N1
O4–Cu1–N4
C5–O2–C7
C8–N1–C9
C10–N2–N3
C10–N2–C13
N3–N2–C13
C11–N3–N2
C11–N3–C14
N2–N3–C14
O1–C1–C2
O1–C1–C6
–
–
–
101.40(7)
101.49
0.09
–
–
–
93.05(6)
95.89
2.84
–
–
–
137.35(7)
142.76
5.41
–
–
–
144.73(7)
140.73
−4
–
–
–
95.55(7)\
93.33
−2.22
–
–
–
94.99(6)
93.98
−1.01
118.33
122.53
107.35
125.44
118.08
109.74
125.98
120.75
118
118.34
122.55
107.34
125.44
118.08
109.76
125.98
120.73
118.02
120.83
0.01
0.02
−0.01
0
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
0
–
0.02
0
–
–
−0.02
0.02
0
–
–
120.83
0.0238
–
RMSD∗∗∗
1.3048
ꢀ
n
ꢁ
(Xicalc − Xiexp)2, i= variable, n= number of data, Xicalc= theoretical bond lengths or bond angles,
1
n
∗
RMSD=
i
Xiexp= experimental bond lengths or bond angles.
∗∗
RMSD calculated for all bond lengths except those containing hydrogen atom.
∗∗∗
RMSD calculated for all bond angles except those containing hydrogen atom.
Table 5
Characteristic 1H NMR chemical shift values (ppm), peak multiplicity and coupling constant J (Hz) for the synthesized compounds recorded in CDCl3.
antipyrine ring
compound -CH3-C
-CH3-N
-CH3-O
Ha
Hb
Hc
(5 H)
Hi
Hp
HL1
2.44 s (2.49)∗
3.21 s (3.21)
3.95 s (4.00)
7.01 dd (7.09) 6.86 t
6.95 dd (6.97) 7.35–7.53 m
9.83 s (10.09)
13.93 s (14.43)
J = 7.8;1.5
(7.02)
J = 7.8; 1.5
(7.29–7.79)
J = 7.8
HL2
HL3
HL4
2.42 s (2.49)
2.43 s (2.46)
2.42 s (2.46)
3.17 s (3.18)
3.20 s (3.21)
3.18 s (3.19)
3.84 s (3.95)
3.78 s (3.89)
3.84 s (3.97)
7.28 d (7.50)
J = 9.2
6.48 dd (6.50) 6.48 d (6.50)
7.33–7.53 m
(7.28–7.78)
7.34–7.53 m
(7.29–7.79)
7.32–7.52 m
(7.29–7.79)
9.77 s (10.03)
9.82 s (10.10)
13.77 s (14.37)
13.87 s (13.81)
J = 9.2;2.4
6.88–6.91br
(7.01)
J = 2.4
6.88–6.91br
(6.86)
6.88–6.91br
(7.01)
6.56 d (6.37)
J = 8.3
7.23 t
6.37 d (6.61)
J = 8.3
10.22 s (10.44) 14.24 s (14.96)
(7.43)
J = 8.3
∗
The calculated 1H NMR chemical shifts are reported in parenthesis.
Table 6
Characteristic 13 C NMR chemical shift values (ppm) for the synthesized compounds recorded in CDCl3.
compound -CH3-C
-CH3-N
-CH3-O
aromatic and pyrazoline ring carbons -C–OMe
-C–OH
-C = O
-C = N
HL1
HL2
HL3
HL4
10.18 (9.01)∗ 35.65 (32.16) 56.06 (53.56) 113.48–148.09 (109.52–149.33)
10.32 (8.97) 35.89 (32.33) 55.40 (53.87) 101.14–149.41 (97.59–148.83)
10.31 (9.02) 35.67 (32.13) 55.84 (53.36) 114.95–134.33 (109.08–149.23)
10.34 (9.16) 35.83 (32.24) 55.62 (54.37) 100.62–149.67 (95.62–149.02)
149.97 (145.64) 150.40 (148.62) 160.26 (157.27) 160.49 (154.74)
160.36 (160.60) 160.53 (160.68) 160.26 (157.32) 162.94 (153.71)
149.94 (148.51) 152.28 (153.03) 154.63 (157.31) 160.31 (154.09)
157.25 (157.84) 160.04 (160.78) 160.39 (157.43) 162.29 (149.55)
∗
The calculated 13 C NMR chemical shifts are reported in parenthesis.
Table 7
IR spectral data of ligands and their corresponding copper(II) complexes (cm−1).
HL1
C1
HL2
C2
HL3
C3
HL4
C4
Exp
1593
1138
–
Calc
1652
1314
–
Exp
Calc
1507
1387
561
Exp
1593
1136
–
Calc
1653
1267
–
Exp
Calc
1644
1354
539
Exp
1579
1132
–
Calc
1664
1306
–
Exp
Calc
1638
1361
526
Exp
1595
1136
–
Calc
Exp
Calc
1631
1375
549
C = N
C–O
1587
1215
416
1587
1247
418
1573
1249
420
1652
1354
1572
1246
436
M–N
M–O
–
–
551
559
–
–
530
592
–
–
551
561
–
530
570
7

H. Kargar, F. Aghaei-Meybodi, R. Behjatmanesh-Ardakani et al.
Journal of Molecular Structure 1230 (2021) 129908
Table 8
increases the lipophilicity of the complexes. It boosts their perme-
ation through the lipid layer of the microorganism, thus destroying
them more efficiency [47,48].
MIC and MBC of the Schiff base ligands and their cor-
responding Cu(II) complexes.
MIC (μg/ml)
MBC (μg/ml)
Compound
E. coli
S. aur
E. coli
S. aur
4. Conclusion
HL1
HL2
HL3
HL4
512
256
128
256
128
64
256
128
64
–
–
–
1024
1024
1024
128
64
In conclusion, we have succesfuly synthesized a series of Schiff
base ligands derived from 4-aminoantipyrine and their copper(II)
complexes. The single crystal X-ray structure of C2, illustrates that
the steric effect imposed by the methyl group has a profound in-
fluence on the Cu(II) geometry, rearranging into a distorted tetra-
hedral geometry. Theoretical calculation results were found to be
in good agreement with experimental results. Affecting the abil-
ity of salicylaldehyde ring to form hydrogen bond with the active
center of cell, the position of methoxy group brings different an-
timicrobial activity. The in vitro biological screening experiments
showed higher activities for the complexes compared to the free
Schiff base ligands.
1024
–
256
32
C1
512
128
128
128
32
C2
32
C3
64
32
64
C4
64
64
128
16
Streptomycin
8
4
different basis sets were used: the smaller basis set of Def2-SV(P)
for complexes which have more atoms than ligands, and the larger
basis set of Def2-SVPD for ligands). Some of the most important
experimental and calculated frequencies for ligands and complexes
are collected in Table 7. The calculated DFT frequencies are in close
agreement with the experimental values.
Compliance with ethical standards
Conflict of interest: The authors declare that they have no con-
3.6. Antibacterial activities
flict of interest.
Using MIC and MBC measurements, the in vitro antibacterial ac-
tivities of the Schiff base ligands and their copper(II) complexes
were screened. The results (Table 8) show the significant antibac-
terial activity of all complexes against both S. aur and E. coli, while
the free ligands resulted in a moderate activity under the same
experimental conditions. All tested compounds, however, show
greater bacterial activities against gram positive than gram nega-
tive strains. Having thicker peptidoglycan layer, gram positive bac-
teria absorb antibiotics and cleaning products easily. However, ow-
ing to thin peptidoglycan layer, gram negative bacteria are pro-
tected from certain physical assaults because they do not absorb
foreign materials that surround it.
Author’s statement
We would like to make an statement about the author’s main
contribution: HK, FAM and MFM synthesized all the compounds,
and characterized them by different techniques; RBA and VT de-
signed the model and the computational framework and anal-
ysed the data.; MRE was responsible for biological data acquisi-
tion and analysis; MNT, MA and KSM collected the single-crystal
X-ray diffraction data and determined the structures. HK wrote the
manuscript with input from all authors. All authors discussed the
results and commented on the manuscript.
Dislocating only the methoxy site, a wide range of inhibitory
effect of the synthesized Schiff base ligands (MIC values in 128–
512 μg/mL against E. coli and 64–512 μg/mL against S. aur) on the
growth of the strains appeared eccentrically; a high influence of
substitutions on antibacterial activity. It is claimed that the forma-
tion of a hydrogen bond with the active centres of cell constituents
disturbs/inhibits the normal cell process [43]. Accordingly, the dif-
ferent activity of the free ligands (HL3 > HL2 & HL4 > HL1) can
be discussed over the ability of methoxy group to form hydro-
gen bond; in HL1, spatial barrier limits the involvement of hydroxy
and methoxy groups to form hydrogen bond leading the highest
MIC value; in HL2 and HL4, whereas methoxy groups are electron
donor substituent reducing their ability, low spatial barrier to hy-
droxy group leads a moderat antibacterial activity; in HL3 where
methoxy group is an electron acceptor substituent with the lowest
spatial barrier, the most antibacterial activity was observed. The
MBC results of free ligands, however, show ignorable bactericidal
activities.
Declaration of Competing Interest
The authors declare that they have no conflict of interest.
Acknowledgement
The support of this work by Ardakan University and Payame
Noor University are gratefully acknowledged.
Appendix A. Supplementary data
CCDC No. 2031,693 (for C2) and CCDC No. 2031,694 (for HL4)
contain the supplementary crystallographic data for this contri-
bution. Copies of the data may be obtained free of charge on
application to CCDC, 12 Union Road, Cambridge CB2 1 EB, UK
(Fax: +44–1223–336–033; E-mail: deposit@ccdc.cam.ac.uk or http:
//www.ccdc.cam.ac.uk).
Compared to their ligands, the complexes provide greater bac-
tericidal activities and inhibitory effects in a small range; MBC
and MIC values of 128 and 64 μg/mL against E. coli and 64 and
32 μg/mL against S. aur, respectively. It can be explained out of
coordination bond formation on the basis of Overtone’s concept
[44] and Tweedy’s chelation theory [45]. 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 mem-
branes which are highly sensitive towards the charged particle
[46]. Moreover, the delocalization of π-electrons over the chelate
ring system created by the donor groups during the coordination
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