Nickel versus copper: Enhanced antibacterial activity in a series of new nickel(II) Schiff base complexes

Article abstract of DOI:10.1080/00958972.2015.1117073

Five new Ni(II) Schiff base complexes [NiL^x(Solv)₂] denoted by NiL^x, x = 1-5, were synthesized and characterized. The Schiff base ligands were synthesized from the condensation of 5-bromo-2-hydroxy-3-nitrobenzaldehyde with

Full text of DOI:10.1080/00958972.2015.1117073

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Nickel versus copper: Enhanced antibacterial
activity in a series of new nickel(II) Schiff base
complexes
Mahnaz Honarkar Ashna, Mahdi Behzad & Mehdi Salehi
To cite this article: Mahnaz Honarkar Ashna, Mahdi Behzad & Mehdi Salehi (2015): Nickel
versus copper: Enhanced antibacterial activity in a series of new nickel(II) Schiff base
complexes, Journal of Coordination Chemistry, DOI: 10.1080/00958972.2015.1117073
To link to this article: http://dx.doi.org/10.1080/00958972.2015.1117073
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Date: 14 November 2015, At: 04:03

Nickel versus copper: Enhanced antibacterial activity in a series of new
nickel(II) Schiff base complexes
MAHNAZ HONARKAR ASHNA, MAHDI BEHZAD* and MEHDI SALEHI
Department of Chemistry, Semnan University, Semnan 35351-19111, Iran
Five new Ni(II) Schiff base complexes [NiLx(Solv)2] denoted by NiLx, x = 1-5, were synthesized
and characterized. The Schiff base ligands were synthesized from the condensation of 5-bromo-
2-hydroxy-3-nitrobenzaldehyde with different aliphatic and aromatic diamines. The X-ray crystal
structure of NiL3 was determined. The ligands and complexes were tested as antibacterial agents
against two Gram(+) and two Gram(-) human pathogenic bacteria. The complexes showed
moderate antibacterial activity against both Gram type bacteria. The new Ni(II) complexes
showed enhanced antibacterial activity compared to the previously reported Cu(II) complexes of
the same ligands.
Keywords: Schiff base; Antibacterial; Nickel(II) complexes
1. Introduction
There has been interest in study of the biological applications of coordination compounds [1-3].
The ligand and the metal center cooperatively create a chance for the synthesis of new metallo-
drugs. Indeed, fine tuning of the electronic properties of the central metal ion is possible with the
possibility of introducing different substituents with electron withdrawing and/or electron
donating features on the ligands. Steric factors are also tunable with those substituents, which is
important in some drug-target interactions. The metal center can also modify both the extent and
direction of the pharmacological properties of the ligand [4, 5]. A variety of Schiff base ligands
and their metal complexes have been prepared and studied for their catalytic properties, metal
*Corresponding author. Emails: mahdibehzad@gmail.com; mbehzad@semnan.ac.ir
1

bio-sites modeling, biological properties, etc. [6-10]. Among them, tetradentate N2O2 Schiff base
ligands are perhaps the most widely studied ones.
Nickel is an essential trace element in living systems. Examples include the active site of
at least nine enzymes such as urease and carbon monoxide dehydrogenase [11, 12]. The quest for
understanding the modes of action of nickel in living systems is an active area of research.
Nickel(II) Schiff base complexes have been studied for their biological properties [13-17]. Apart
from the biological importance of nickel Schiff base complexes, these complexes have also been
studied for their catalytic [18-20] and magnetic [21-23] properties. In continuation of our
previous studies, we report herein the synthesis, characterization and antibacterial properties of
five new Ni(II) Schiff base complexes. Our previous studies as well as the study of other
researchers [13, 14] have shown that the presence of more electronegative substituents on the
Schiff base ligand increases the antibacterial activity. Therefore, we deliberately designed a
series of Schiff base ligands derived from condensation of different diamines with 5-bromo-3-
nitro-2-hydroxybenzaldehyde. Nickel(II) complexes of these ligands were prepared and
characterized. Crystal structure of one complex was determined. The complexes have general
formula [NiLx(Solv)2] in which L represents the dianionic Schiff base ligand and Solv represents
the solvent molecules axially coordinated to nickel. Antibacterial activities of these complexes
were investigated against four human pathogenic bacteria, Escherichia coli (Gram negative),
Salmonella typhi (Gram negative), Staphylococcus aureus (Gram positive) and Bacillus subtilis
(Gram positive). These Ni(II) complexes showed higher antibacterial activity against the studied
bacteria than previously reported Cu(II) complexes of the same ligands [13].
2. Experimental
2.1. Materials and methods
All chemicals and solvents were purchased from Merck chemical company, Germany, and were
used as received. 5-Bromo-2-hydroxy-3-nitrobenzaldehyde was synthesized as described [24].
The ligands were synthesized following literature procedure [25-27]. Melting points were
obtained on a thermoscientific 9100 apparatus. Elemental analyses were performed on a Perkin-
Elmer 2400II CHNS-O analyzer. IR spectra were obtained as KBr plates using a Bruker FT-IR
instrument. UV-vis spectra were obtained on a Shimadzu UV-1650PC spectrophotometer. X-ray
diffraction measurements for NiL3 were made on a STOE IPDSII diffractometer with graphite
2

monochromated MoKα radiation. Data were collected at 293(2) K in a series of ω scans in 1°
oscillations and integrated using the Stoe X-AREA software package. A numerical absorption
correction was applied using PLATON software. The data were corrected for Lorentz and
polarization effects. The structure was solved by direct methods using SIR2004. The non-
hydrogen atoms were refined anisotropically by full-matrix least-squares on F2 using SHELXL.
All the hydrogens were placed at calculated positions and constrained to ride on their parent
atoms. Details concerning collection and analysis for NiL3 are reported in table 1.
3

Y
Ligand
H2L1
Complex
NiL1
H2L2
NiL2
H2L3
NiL3
H2L4
H2L5
NiL4
NiL5
Scheme 1. The procedure of the synthesis of the ligands and complexes.
2.2. Synthesis of the complexes
The Ni(II) Schiff base complexes were synthesized by the equimolar reaction of the
corresponding Schiff base ligands and nickel(II) acetate in methanol. The obtained precipitates
were then recrystallized in DMSO to yield pure crystals of the target complexes. The NiLx
complexes were obtained with two coordinated DMSO molecules as shown in scheme 1 except
for NiL3, for which the details are given in results and discussion.
4

2.2.1. Synthesis of NiL1. 1 mmol (0.2 g) of H2L1 was dissolved in 10 mL methanol and was
heated to reflux. To this solution, a methanolic solution of 1 mmol of Ni(OAc)2·H2O (0.07 g in
10 mL) was added. The reaction mixture was refluxed for 4 h. The orange solid product was
collected by filtration, washed with 15 mL of hot methanol and air dried. Recrystallization from
DMSO yielded 0.2 g (89%) of the target complex. Selected FT-IR (KBr, cm-1): 1627(νC=N),
1521(νNO2), 1228(νC-O). UV-vis in DMSO [λmax nm (ε M-1cm-1)]: 268 (187200), 446 (68000),
632 (290). Anal. Calcd. for C20H22Br2N4NiO8S2: C, 32.94; H, 3.02; N, 7.69. Found: C, 32.85; H,
3.11; N, 7.75.
2.2.2. Synthesis of NiL2. This complex was synthesized following a similar procedure as
described for NiL1 except H2L2 was used instead of H2L1. Yield was 0.15 g (82.5%). Selected
FT-IR (KBr, cm-1): 1629(νC=N), 1531(νNO2), 1230(νC-O). UV-vis in DMSO [λmax nm (ε M-1cm-1)]:
267 (13200), 427 (9500), 600 (120). Anal. Calcd. for C21H24Br2N4NiO8S2: C, 33.94; H, 3.23; N,
7.54. Found: C, 33.89; H, 3.18; N, 7.60.
2.2.3. Synthesis of NiL3. This complex was synthesized following a similar procedure as
described for NiL1 except H2L3 was used instead of H2L1. Yield was 0.15 g (83%). Selected
FT-IR (KBr, cm-1): 1639(νC=N), 1528(νNO2), 1230(νC-O). UV-vis in DMSO [λmax nm (ε M-1cm-1)]:
267 (13100), 425 (7200), 595 (100). Anal. Calcd. for C23H29Br2N4NiO8.5S2: C, 35.40; H, 3.72; N,
7.18. Found: C, 35.46; H, 3.68; N, 7.22.
2.2.4. Synthesis of NiL4. This complex was synthesized following a similar procedure as
described for NiL1 except H2L4 was used instead of H2L1. Yield was 0.18 g (97%). Selected
FT-IR (KBr, cm-1): 1612(νC=N), 1523(νNO2), 1205(νC-O). UV-vis in DMSO [λmax nm (ε M-1cm-1)]:
266 (33800), 315 (31500), 445 (19000), 750 (80). Anal. Calcd. for C28H22Br2N4NiO8S2: C,
40.75; H, 2.67; N, 6.79. Found: C, 40.68; H, 2.61; N, 6.72.
2.2.5. Synthesis of NiL5. This complex was synthesized following a similar procedure as
described for NiL1 except H2L5 was used instead of H2L1. Yield was 0.14 g (80%). Selected
FT-IR (KBr, cm-1): 1616(νC=N), 1523(νNO2), 1213(νC-O). UV-vis in DMSO [λmax nm (ε M-1cm-1)]:
5

267 (23780), 318 (14760), 445 (16790), 745 (80). Anal. Calcd. for C25H37Br2N4NiO8S2: C,
37.34; H, 4.60; N, 6.97. Found: C, 37.29; H, 4.65; N, 6.92.
2.3. Antibacterial studies
2.3.1. Bacterial strains. The metal complexes were tested against Salmonella typhi
(ATCC 19430, Gram negative), Escherichia coli (ATCC 25922, Gram negative), Staphylococcus
aureus (ATCC 25923, Gram positive) and Bacillus subtilis (ATCC No. 6633, Gram positive).
2.3.2. Determination of minimum inhibitory concentration (MIC) and minimal bactericidal
concentration (MBC). Minimum inhibitory concentration (MIC, μg mL-1) was determined by
the broth micro-dilution method following the procedures recommended by the National
Committee for Clinical Laboratory Standards [28, 29]. MICs were defined as the lowest
concentrations of compounds which inhibit the growth of microorganisms. All tests were
performed in triplicate. Minimal bactericidal concentration (MBC) was also measured following
a standard procedure [30]. 100 µL volumes of all clear (no growth) tubes from a dilution MIC
test was spread onto separate agar plates and incubated at 37 °C for 24 h. The MBC (μg mL-1)
was defined as the lowest concentration of the complex for which no growth occurred.
3. Results and discussion
3.1. Spectroscopic characterization
The most significant signal in the IR spectrum of a Schiff base ligand or complex is the
stretching vibration of the imine group (C=N). This signal usually shifts to lower wavenumbers
upon coordination of the iminic nitrogen to a transition metal ion. In the IR spectra of the
ligands, the presence of an intense band at ~1640 cm-1 was previously assigned to the stretching
vibration of the azomethine (C=N) groups. In the IR spectra of these Ni(II) complexes, these
signals shifted to lower wavenumbers indicating the incorporation of the nitrogens of the C=N
groups in coordination.
In the UV-vis spectra of the ligands with aliphatic diamines, two intense bands were
observed at 270 nm and 480 nm which were assigned to π→π* and n→π* transitions,
respectively. In the UV-vis spectra of the ligands with aromatic diamines, the afore-mentioned
bands were also observed and a new band appeared at 330 nm. These new bands were assigned
6

to the π→π* transitions mainly concentrated on the phenyl rings of the diamine moiety. In the
UV-vis spectra of the complexes, the bands assigned to the π→π* transitions appeared at almost
the same wavelengths while the bands assigned to the n→π* transitions in the ligands were
absent and instead; new bands were observed at wavelengths around 430 nm which were
assigned to the MLCT transitions [26, 31]. The signals due to the d-d transitions in octahedral
Ni(II) Schiff base complexes are usually observed at 400-500 nm (3A2g→3T1g(P)) and 600-
800 nm (3A2g→3T1g(F)). A third signal at above 900 nm (3A2g→3T2g(F)) is sometimes not
observed since it does not fall in the range covered by the commonly used instruments [32]. In
our complexes, the first signals were masked by the intense n→π* signal but the weak signals
due to the 3A2g→3T1g(F) were observed at the expected area.
3.2. Description of the crystal structure of NiL3
Single crystals suitable for X-ray crystallography were obtained from slow evaporation from
NiL3 solution in DMSO in about one month. Unfortunately, attempts to prepare suitable single
crystals from other complexes failed. Figure 1 shows the ORTEP representation of the
[NiL3(DMSO)2] and [NiL3(DMSO)(H2O)] complexes. Table 1 collects a summary of the
crystallographic data and selected bond lengths and angles are collected in table 2. The unit cell
of the NiL3 complex consists of two chemically unequivalent molecules. In both molecules, the
central Ni(II) ion is coordinated to a basal dianionic Schiff base ligand, however, these two
molecules differ in the axial coordination. One of the two molecules contains two axial DMSO
ligands (figure 2, right (A)), while in the other one (figure 2, left (B)) the axial positions are
occupied by one DMSO and one H2O ligand. All the Ni-O and Ni-N bond distances are normal
and are similar to previously reported octahedral Ni(II) complexes. The axial O-Ni-O angle is
176.1(2)° in (A) and 178.91(19)° in (B) which show slight deviations from the ideal octahedron.
The equatorial O-Ni-O and N-Ni-N angles also deviate from ideal octahedral in both (A) and
(B), and hence, the geometry around Ni(II) is considered as slightly distorted octahedron
(table 2). Other bond lengths and angles around the central metal ions collected in table 2
confirm the distorted octahedral geometry. O7 and O8 of one of the NO2 groups in (A) are
disordered and are shown as O7A, O7 B, O8A and O8B in the ORTEP drawing (figure 1). One
uncoordinated DMSO is also present in the unit cell which is not shown in the ORTEP drawing
for clarity. The six-membered chelate ring in the current Schiff base ligand is comprised of a
7

1,3-diamino-2,2-dimethylpropane fragment (neopentandiamine). The conformation of the six-
membered chelate ring in both structures is very clearly a chair somewhat flattened at the
N3-Ni-N4 site as taken from the structure of [NiL3(DMSO)2] and at the N7-Ni-N8 site as taken
from [NiL3(DMSO)(H2O)] (figure 2) [33]. The dihedral angles between the planes defined by the
two phenyl rings of the salicylaldehyde moieties in (A) and (B) are 51.79(2)° and 67.10(2)°,
respectively. The six-membered rings in the diamine moieties have distorted chair structure.
3.3. Antibacterial studies
The presence of more electronegative substituents on the Schiff base ligands increases the
antibacterial activity of ligands and their metal complexes [13, 14]. To further explore this idea,
we have synthesized a salicylaldehyde derivative with two electronegative substituents,
i.e. bromo and nitro substituents at the 5- and 3-position of the salicylaldehyde. Copper(II)
complexes of these ligands were previously synthesized and their antibacterial effect was studied
[13]. In this work, the synthesis of a series of the corresponding Ni(II) complexes is being
reported. The synthesized Ni(II) complexes were screened for their in vitro antibacterial activity
against two Gram positive (S. aureus and B. subtilis) and two Gram negative (E. coli and S.
typhi) human pathogenic bacteria according to literature protocols [28-30]. The results of the
antibacterial studies of the NiLx complexes as well as kanamycin as the standard drug are
collected in table 3. The Ni(II) complexes showed remarkable antibacterial activity against
studied bacteria. Comparison of these data with those of the Cu(II) analogues revealed that the
Ni(II) complexes showed increased antibacterial activity [13]. For example, the [CuL2] complex
in the reported work, which was the most active antibacterial agent, had MIC values 525, 375,
325 and 450 μg mL-1, respectively, for E. coli, S. typhi, S. aureus and B. subtilis. The
corresponding Ni(II) complex (NiL3) showed values of 175, 125, 75 and 75 μg mL-1 for the same
bacteria, indicating involvement of the central metal in the antibacterial properties of transition
metal complexes. These complexes are octahedrally surrounded by the ligands, while the
corresponding copper(II) complexes were square planar. By increasing the number of
substituents (i.e. the presence of two axial ligands) the lipophilicity of the complexes could be
increased and, hence, the antibacterial properties of the complexes increased [31, 34]. The
ligands [13] and Ni(OAc)2 did not show activity, which confirms that complexation is important
in the observed antibacterial activity. Complexes with aliphatic diamines were more active than
8

the aromatic analogues, which suggest contribution of the ligands and supports the idea of
cooperation of the ligands and central metals in such biological activities.
4. Conclusion
Five new Ni(II) Schiff base complexes were synthesized and characterized by various
spectroscopic and analytical methods. The X-ray crystal structure of NiL3 was obtained. The new
complexes were screened for potential antibacterial activity against four human pathogenic
bacteria. Comparison with the results of the previously reported copper(II) complexes showed
that, by changing from Cu(II) to Ni(II), the antibacterial properties of these complexes was
considerably improved.
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11

Table 1. Crystallographic data for NiL3.
C46H58Br4N8Ni2O17S4
Formula
Formula weight
Crystal system
Space group
T (K)
1560.26
Monoclinic
P21/ c
293
λ (Å)
0.71073
Unit cell dimensions
a (Å)
12.073(2)
b (Å)
10.848(2)
c (Å)
48.674( ) [au: the esd is missing]
90
α ()
102.13(3)
90
β ()
 ()
Volume (Å3)
Z
6232(2)
4
3.37
µ (mm−1)
Radiation type
Crystal description
Crystal color
Crystal size (mm)
Calculated density (g cm-3)
Absorption coefficient (mm-1)
F(000)
Mo Kα
Block
Dark-orange
0.18 × 0.12 × 0.10
1.663
3.372
3144
θ range for data collection (°)
Limiting indices
1.9 to 25
-14≤ h ≤ 14
-12≤ k ≤ 12
-57≤ l ≤ 57
10812 / 23 / 753
31293
Data / restraints / parameters
Total reflections
Unique reflections (Rint)
Completeness
10812 (Rint = 0.120)
98.4% (θ = 25.00)
MULTI-SCAN
1.067
R1 = 0.0880, wR2 = 0.1664
R1 = 0.1511, wR2 = 0.1975
0.688 and -0.724
Refinement method
Goodness-of-fit on F2
Final R index [I > 2σ(I)]
R index [all data]
Largest difference peak and hole (e Å3)
12

Table 2. Selected bond lengths (Å) and angles (°) in the two chemically unequivalent
molecules of NiL3.
[NiL3(DMSO)2]
Bond lengths (Å)
[NiL3(DMSO)(H2O)]
Bond lengths (Å)
Ni1—O6
2.007(5)
2.023(5)
2.109(5)
2.111(5)
2.042(6)
2.055(6)
Ni2—O11
Ni2—O13
Ni2—O16
Ni2—O12
Ni2—N7
Ni2—N8
2.062(5)
2.069(4)
2.025(5)
2.116(5)
2.029(6)
2.036(6)
Ni1—O3
Ni1—O4
Ni1—O5
Ni1—N4
Ni1—N3
Bond angles (°)
Bond angles (°)
O16—Ni2—N7
O6—Ni1—O3
O6—Ni1—N4
O3—Ni1—N4
O6—Ni1—N3
O3—Ni1—N3
N4—Ni1—N3
O6—Ni1—O4
O3—Ni1—O4
N3—Ni1—O4
O6—Ni1—O5
O3—Ni1—O5
N4—Ni1—O5
N3—Ni1—O5
O4—Ni1—O5
87.96(19)
173.8(2)
88.1(2)
88.6(2)
175.6(2)
95.5(2)
92.8(2)
89.3(2)
88.1(2)
90.8(2)
89.1(2)
84.3(2)
93.7(2)
176.1(2)
91.9(2)
90.4(2)
93.0(2)
O16—Ni2—N8
N7—Ni2—N8
O16—Ni2—O11
N7—Ni2—O11
N8—Ni2—O11
O16—Ni2—O13
N8—Ni2—O13
O11—Ni2—O13
O16—Ni2—O12
N7—Ni2—O12
N8—Ni2—O12
O11—Ni2—O12
O13—Ni2—O12
90.35(19)
177.5(2)
88.2(2)
89.27(19)
179.7(2)
91.95(18)
178.91(19)
88.5(2)
90.6(2)
89.28(19)
89.72(19)
13

Table 3. MIC and MBC values for the Ni(II) complexes against the studied bacteria (μg mL-1).
NiL1 NiL2 NiL3 NiL4 NiL5
Kanamycin
MIC MBC MIC MBC MIC MBC MIC MBC MIC MBC MIC MBC
E. coli
100 125 225 225 175 175 250 275 275 475
75 125 175 225 125 150 225 225 225 275
4
3.8
3.2
3
S. typhi
3.3
3.2
4
S. aureus
B. subtilis
100 125 100 100
150 175 225 225
75
75
100 325 350 325 325
125 525 525 375 475
3.6
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Figure 1. ORTEP representation of the two related molecules in the unit cell of NiL3,
[NiL3(DMSO)2] (left) and [NiL3(DMSO)(H2O)] (right). Displacement ellipsoids are drawn at the
50% probability level. Uncoordinated and disordered solvent of crystallization (DMSO) and
hydrogens are omitted for clarity.
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Figure 2. The conformation of the six-membered chelate ring (neopentandiamine fragment) in
[NiL3(DMSO)2] (left) and [NiL3(DMSO)(H2O)] (right).
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Graphical abstract
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