Synthesis, spectroscopic characterization, and antimicrobial activities of Ni(II) and Fe(II) complexes with N -(2-hydroxyethyl)-5-nitrosalicylaldimine

Article abstract of DOI:10.1080/00958972.2013.829568

Metal complexes [Ni(HL1)2H2O] (1) and [Fe(HL1)2] (2), where HL1 is the tridentate Schiff base N-(2-hydroxyethyl)-5-nitrosalicylaldimine, were synthesized and characterized by spectroscopic methods The crystal structures of 1 and 2 have been determined by single crystal diffraction at 100 K Complexes 1 and 2 have a distorted octahedral geometry The ligand and metal complexes were screened for antibacterial and antifungal activities by the disk diffusion, microdilution broth, and single spore culture techniques Antimicrobial activities of the ligand and its complexes have been tested against 10 bacteria, two yeast, and five filamentous fungi The ligand and metal complexes were found to be active against all tested micro-organisms 2013

Full text of DOI:10.1080/00958972.2013.829568

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Synthesis, spectroscopic
characterization, and antimicrobial
activities of Ni(II) and Fe(II)
complexes with N-(2-hydroxyethyl)-5-
nitrosalicylaldimine
Selma Celena, Elif Gungorb, Hulya Karab & A. Dilek Azaza
a Faculty of Science and Art, Department of Biology, Balikesir
University, Balikesir, Turkey
b Faculty of Science and Art, Department of Physics, Balikesir
University, Balikesir, Turkey
Accepted author version posted online: 26 Jul 2013.Published
online: 24 Sep 2013.
To cite this article: Selma Celen, Elif Gungor, Hulya Kara & A. Dilek Azaz (2013) Synthesis,
spectroscopic characterization, and antimicrobial activities of Ni(II) and Fe(II) complexes with N-
(2-hydroxyethyl)-5-nitrosalicylaldimine, Journal of Coordination Chemistry, 66:18, 3170-3181, DOI:
10.1080/00958972.2013.829568
To link to this article: http://dx.doi.org/10.1080/00958972.2013.829568
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Journal of Coordination Chemistry, 2013
Vol. 66, No. 18, 3170–3181, http://dx.doi.org/10.1080/00958972.2013.829568
Synthesis, spectroscopic characterization, and antimicrobial
activities of Ni(II) and Fe(II) complexes with
N-(2-hydroxyethyl)-5-nitrosalicylaldimine
SELMA CELEN†, ELIF GUNGOR*‡, HULYA KARA‡ and A. DILEK AZAZ†
†Faculty of Science and Art, Department of Biology, Balikesir University, Balikesir, Turkey
‡Faculty of Science and Art, Department of Physics, Balikesir University, Balikesir, Turkey
(Received 21 March 2013; accepted 2 July 2013)
Metal complexes [Ni(HL1)2H2O] (1) and [Fe(HL1)2] (2), where HL1 is the tridentate Schiff base
N-(2-hydroxyethyl)-5-nitrosalicylaldimine, were synthesized and characterized by spectroscopic
methods. The crystal structures of 1 and 2 have been determined by single crystal diffraction at
100 K. Complexes 1 and 2 have a distorted octahedral geometry. The ligand and metal complexes
were screened for antibacterial and antifungal activities by the disk diffusion, microdilution broth,
and single spore culture techniques. Antimicrobial activities of the ligand and its complexes have
been tested against 10 bacteria, two yeast, and five filamentous fungi. The ligand and metal com-
plexes were found to be active against all tested micro-organisms.
Keywords: Schiff base; Nickel(II); Iron(II); Antibacterial activity; Antifungal activity
1. Introduction
Schiff bases and their transition metal compounds have numerous biological and medicinal
applications. Schiff base compounds have been used in medicine as antibiotic, antiallergic,
antitumoral [1], antibacterial, and antifungal agents [2–12]. Metal ions present in complexes
accelerate the drug action and the efficacy of the organic therapeutic agents [13, 14]. Devel-
opment of new and effective antibacterial and antifungal drugs has become an urgent task
for infectious diseases [15, 16]. The biological activities of the newly prepared compounds
may be more effective than well-known classes of antimicrobial agents to which many clin-
ically relevant pathogens are resistant. To establish structure-biological activity relation-
ships, numerous transition metal complexes have been studied [17–19]. They exhibit strong
or weak biological activity associated with different coordinations of Schiff bases with
metal ions such as copper, cobalt, nickel, iron, and manganese [2–22]. In the present work,
this field is extended by synthesis of mononuclear Ni(II) and Fe(II) Schiff base complexes.
Although there has been considerable interest in characterization, antibacterial and anti-
fungal properties of salen-type Schiff base metal complexes [2–22], little attention has been
*Corresponding author. Email: egungor@balikesir.edu.tr
© 2013 Taylor & Francis

Ni(II) and Fe(II) Complexes
3171
paid to systems that contain Fe(II) [17–19]. For this reason here we report the preparation,
characterization, antibacterial and antifungal properties of HL1 and its Ni(II) and Fe(II)
complexes, [Ni(HL1)2H2O] (1) and [Fe(HL1)2] (2), against 10 bacteria, two yeast and five
filamentous fungi.
2. Experimental
2.1. Materials and physical measurements
All chemical reagents and solvents were purchased from Merck or Aldrich and used without
purification. Elemental (C, H, N) analyzes were carried out by standard methods with a
LECO, CHNS-932 analyzer. UV–vis spectra were carried out at 20 °C on a Perkin–Elmer
Lambda 25 UV–Vis spectrometer. FT-IR spectra were measured with a Perkin–Elmer Model
1
Bx 1600 instrument with samples as KBr pellets from 4000 to 400 cm−1. H and 13C NMR
spectra of HL1 were recorded in DMSO on a Bruker Biospin (300 MHz). The melting points
of the ligand and complexes were determined on a Buchi M-560 melting point apparatus.
The synthetic route of HL1, 1 and 2 are outlined in scheme 1.
2.2. Synthesis of HL1
The tridentate Schiff base HL1 was prepared by mixing 2-hydroxy-5-nitrobenzaldehyde
(1 mM) and ethanolamine (1 mM) in 1:1 M ratio in hot ethanol (50 cm3). The solution
obtained was stirred at 78 °C for 30 min. The yellow compound precipitated from the
solution on cooling.
HL1: Yellow crystals, yield 82%. Melting point: 114.8–116.6 °C. Calcd: C, 51.43; H, 4.80;
N, 13.33. Experimentally found composition (in %): C, 51.39; H, 4.82; N, 13.31%. 1H NMR
(DMSO, δ ppm) yielded signals at: 3.83–3.93 (–NCH2CH2OH, 4H), 5.15 (–NCH2CH2OH,
1H), 6.59 (–CH=CHOH–, 1H), 8 (–CHNO2=CH–), 8.4 (–CH(NO2)–CH=CH–, 1H), 8.6
(–CH=N–, 1H). 13C NMR (DMSO, δ ppm) yielded signals at: 54.85 (–N–CH2–), 59.73
(–HOCH2–), 113.79 (–CH=CHOH–), 123.40 (–CH=CH–COH), 139.79 (–CH=CH–COH),
133.30 (–CH=CNO2–), 133.89 (–CH=CNO2–), 168.22 (–C=COH–), 178.74 (–C=N–).
O
O N
2
O N
2
N
OH
H
EtOH
+
HO
NH
2
OH
OH
MeOH
N
H
O
O N
2
O
M
Complex 1
Complex 2
M=Ni
M= Fe
O
O
N
NO
2
H
Scheme 1. The synthetic route of HL1, 1 and 2.

3172
S. Celen et al.
The IR (KBr) spectrum showed signals (in cm−1) at: 3134, 3102, 3098, 3047, 2838, 2742,
1658, 1602, 1319, 1292, 1244, 1175. The UV–Vis (DMF) spectrum had peaks (in nm) at:
335, 417.
2.3. Synthesis of 1 and 2
Complex 1 was prepared by the addition of nickel(II) acetate tetrahydrate (0.248 g, 1 mM) in
40 cm3 of hot methanol to HL1 (1 mM) in 30 cm3 of hot ethanol. This solution was warmed
to 78 °C and stirred for 30 min. A green solution was obtained and allowed to stand at room
temperature. Several weeks of standing led to the growth of green crystals. Complex 2 was
prepared in a similar way using iron(II) chloride tetrahydrate (0.198 g, 1 mM) instead of
nickel(II) acetate tetrahydrate in hot methanol.
In general, iron can form stable compounds in both the divalent and trivalent states
depending on the conditions. Iron(II) complexes with Schiff bases are air sensitive and eas-
ily oxidized to iron(III) complexes by molecular oxygen. In the presence of oxygen, iron(II)
complexes are stable only under acidic conditions (pH < 4); at neutral pH (pH > 4) they are
rapidly oxidized to iron(III) [23, 24]. The synthesis was performed under aerobic conditions
with an acidic pH of 3.2 for complex 2.
Complex 1: Green crystals yield 75%. Melting point: 172.4–183.4 °C. Theoretical com-
position (in %): C, 43.67; H, 4.07; N, 11.32. Experimentally found composition (in %): C,
43.64; H, 4.09; N, 11.34%. The IR (KBr) spectrum showed signals (in cm−1) at: 3201,
2910, 2784, 1652, 1439, 1395, 1378, 647, 588, 550, 536, 513, 474, 450, 429. UV–Vis
(DMF) nm: 256, 395, 602.
Complex 2: Dark-red crystals yield 75%. Melting point: 109.8–110.7 °C. Theoretical
composition (in %): C, 45.59; H, 3.83; N, 11.81. Experimentally found composition (in %):
C, 45.61; H, 3.80; N, 11.83%. The IR (KBr) spectrum showed signals (in cm−1) at: 3067,
2918, 2728, 1616, 1451, 1395, 1342, 651, 562, 544, 493, 474, 435, 413. The UV–Vis
(DMF) spectrum had peaks (in nm) at: 271, 407, 657.
2.4. X-ray structure determination
Diffraction measurements were made on a Bruker ApexII Kappa CCD diffractometer using
graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at 100 K. The intensity data
were integrated using APEXII [25]. Absorption corrections were applied based on equiva-
lent reflections using SADABS [26]. The structures were solved by direct methods and
refined using full-matrix least-squares against F2 with SHELXL [27]. All non-hydrogen
atoms were assigned anisotropic displacement parameters and refined without positional
constraints. Hydrogens were included in idealized positions with isotropic displacement
parameters constrained to 1.5 times the Uequiv of their attached carbon for methyl hydrogens
and 1.2 times the Uequiv of their attached carbon for all others. Hydrogens of water were
located in a difference Fourier map and refined isotropically. Distance restraints were also
applied to hydrogens of water with a set value of 0.90(1) Å. The absolute structure of 1 was
determined on the basis of the Flack [28] parameter x = 0.004 (11). Flack parameter close to
0 is indicative of a non-centrosymmetric structure. Methanol in the crystal lattice of 2
appears to be highly disordered, and it was difficult to model reliable positions. Therefore,
the SQUEEZE function of PLATON [29] was used to eliminate the contribution of the elec-
tron density in the solvent region from the intensity data, and the solvent-free model was
employed for the final refinement.

Ni(II) and Fe(II) Complexes
3173
2.5. Biological activity
HL1, 1, and 2 were subjected to screening for antimicrobial activity by using agar disk
diffusion assay, microdilution broth assay [30, 31], and single spore culture technique (for
filamentous fungi) [32]. The antimicrobial activities of L1, 1, and 2 were evaluated against
10 bacteria Campylobacter jejuni (ATCC 3291), Enterobacter aerogenes (NRRL 3567),
Escherichia coli (ATCC 25292), Listeria monocytogenes (ATCC 7644), Pseudomonas
aeruginosa (ATCC 27853), Proteus vulgaris (NRRL 123), Staphylococcus aureus (ATCC
6538), Serratia marcescens (clinical isolate), Shigella sonnei (ATCC 25931), and Klebsiella
pneumoniae (clinical isolate), two yeasts Candida albicans (ATCC 10231) and C. albicans
(clinical isolate) and five filamentous fungi Aspergillus flavus, Aspergillus niger, Penicillium
expansum, Penicillium lanosum, and Alternaria alternata. (The tested microfungi were iso-
lated from soil in our department.)
2.5.1. Agar disk diffusion method. The agar disk diffusion method was employed for
determination of antimicrobial capacities of L1, 1, and 2 [30]. A suspension of the tested
micro-organism (500 μL of 108 CFU/mL) was spread on the solid media plates. Each test
solution was prepared in DMF. Then filter paper disks (6 mm in diameter) were soaked in
20 μL of the stock solutions and placed on inoculated plates. After keeping at 2 °C for 2 h,
they were incubated at 37 °C for 24 h for bacteria and C. albicans (C. jejuni was incubated
at 42 °C, microaerophilic conditions for 48 h). The diameters of the inhibition zones were
measured in millimeters. Each test was carried out in triplicate and the average was
calculated for inhibition zone diameters.
2.5.2. The microdilution broth method. A microdilution broth susceptibility assay was
used for the determination of minimum Inhibitory concentration (MIC) [31]. Serial dilutions
of compounds were prepared in sterile distilled water in 96-well microtitre plates. Freshly
grown bacterial suspension was standardized to 108 CFU/mL (McFarland No. 0.5) in
double strength Mueller-Hinton Broth (L. monocytogenes in Buffered Listeria Enrichment
Broth and yeast suspension of C. albicans in Saboraud Dextrose Broth). Sterile distilled
water served as growth control. Hundred microlitre of each microbial suspension was then
added to each well. The last row containing only the serial dilutions of antibacterial agent
without micro-organism was used as negative control. After incubation at 37 °C for 24 h
(C. jejuni was incubated at 42 °C microaerophilic conditions for 48 h) the first well without
turbidity was determined as the MIC. Chloramphenicol (1000 μg/mL) and ketoconazole
(1000 μg/mL) served as positive controls. Each test was performed in triplicate.
2.5.3. Antifungal studies. In order to obtain conidia, the fungi were cultured on Czapex
Dox Agar and Malt Extract Agar medium (Merck) in 9 cm Petri dishes at 25 °C for
7–10 days. Harvesting was carried out by suspending the conidia in 1% (w/v) sodium
chloride solution containing 5% (w/v) DMSO. The spore suspension was then filtered,
transferred into tubes and, stored at −20 °C, accordingly to Hadecek and Greger [33]. The
1 mL spore suspension was taken and diluted in a loop drop until one spore could be
captured. One loop drop from the spore suspension was applied onto the center of the Petri
dish containing Czapex Dox Agar and Malt Extract Agar. 20 μL of each complex was
applied onto sterile paper disks (6 mm in diameter) and placed in the petri dishes and

3174
S. Celen et al.
incubated at 25 °C for 72 h. Spore germination during the incubation period was followed
using a microscope (Olympus BX51) in 8 h intervals. The inhibition of fungal growth
expressed in percentage was determined on the growth in test plates compared to the
respective control plates [34].
C ꢀ T
Inhibition % ¼ 100 x
C
where C is the diameter of fungal growth on the control, T is the diameter of fungal growth
on the test plate.
The activities of the complexes have been compared with the activity of standard
antifungicide ketoconazole.
3. Results and discussion
3.1. X-ray structure determinations of 1 and 2
Crystallographic data conditions used for intensity data collection and some features of the
structure refinement are listed in table 1. Perspective ORTEP views with atom-labeling
scheme of 1 and 2 are presented in figures 1 and 2, respectively. Selected bond lengths and
angles are given in table 2.
Complexes 1 and 2 crystallize in the orthorhombic space group P212121 and Fddd,
respectively. The asymmetric unit of 1 consists of one chiral monomeric [Ni(HL1)2] unit
with water, while 2 consists of half of the monomeric [Fe(HL1)] unit with methanol.
N-(2-hydroxyethyl)-5-nitrosalicylaldimine is monoanionic, tridentate with ONO donor set
coordinating through one imine nitrogen (Nimi), one alkoxy oxygen (Oalk), and a deproto-
nated phenol (Ophe). The six-coordinate nickel and iron are defined by two imine nitrogens,
Table 1. Crystal data and structure refinement for 1 and 2.
1
2
Empirical formula
Formula weight (g mol−1
Temperature (K)
Crystal system
C18H20N4NiO9
495.09
100(2)
Orthorhombic
C18H18N4FeO8
474.21
100(2)
Orthorhombic
)
Space group
P212121
Fddd
Unit cell dimensions
a = 9.5566(11) Å, α = 90°
a = 10.2723(3) Å, α = 90°
b = 12.8963(16) Å, β = 90°
b = 23.7900(9) Å, β = 90°
c = 16.047(2) Å, γ = 90°
1977.7(4)
4
1.663
1.044
c = 36.5054(12) Å, γ = 90°
8921.1(5)
16
1.412
0.726
Volume (Å3)
Z
Density (calculated) (g/cm−3
)
Absorption coefficient (mm−1
θ range for data collection (°)
Index ranges
)
2.0–27.5
2.0–27.6
–12 ≤ h ≤ 9, –13 ≤ k ≤ 16, –20 ≤ l ≤ 14 –13 ≤ h ≤ 13, –30 ≤ k ≤ 30, –47 ≤ l ≤ 47
Reflections collected
10,147
37,296
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
R indices [I>2σ(I)]
4448
3762/4/301
1.02
R1 = 0.0305, wR2 = 0.0668
0.44 and –0.47
0.004 (11)
2591
2408/1/145
1.18
R1 = 0.0370, wR2 = 0.0881
0.35 and –0.51
–
Largest diff. peak and hole (eÅ−3
Flack parameter
)

Ni(II) and Fe(II) Complexes
3175
two phenoxo oxygens, and alcoholic oxygens. The average bond lengths are Ni–Ophe = 2.02
(17) Å, Ni–Oalk = 2.12(17) Å, and Ni–Nimi = 2.00(2) Å for 1, and Fe–Ophe = 1.94(15) Å,
Fe–Oalk = 2.01(13) Å, and Fe–Nimi = 2.12(15) Å for 2, respectively. The angles around the
metal in the six-membered chelate ring are slightly distorted. Therefore, 1 and 2 have a
distorted octahedral geometry being chelated by two tridentate Schiff base ligands. The
angles O–M–O (M=Ni, Fe) are 87.81(7)– 91.59(7)° for 1 and 90.34(5)–91.41(6)° for 2 and
the angles dealing with O–M–N are 81.44(8)–96.11(8)° for 1 and 86.07(6)–90.37(6)° for 2;
the N–M–N angle is 175.51(8)° for 1 and 165.39(7)° for 2. Selected bond lengths and
angles, listed in table 1, lie within the range of reported values for corresponding bond
lengths and angles of other mononuclear nickel(II) [35, 36] and iron(II) complexes [37, 38].
In 1 and 2, adjacent molecules are linked through intermolecular O–H⋯O hydrogen
bonds (table 3, figures 3 and 4). Molecules interacting through O–H⋯O hydrogen bonds in
1 give a 3-D coordination network. The structure of 1 is observed in the bc-plane, stacked
orthogonally to the a-axis. Intermolecular hydrogen bonding of 2 assembles the molecular
units leading to ordered zigzag chain structures. This intermolecular interaction results in
the formation of 1-D networks. The zigzag chain structures propagate along the crystallo-
graphic a axis.
3.2. FTIR spectroscopy
The IR spectra of 1 and 2 were analyzed in comparison with that of their free ligand HL1
from 4000–400 cm−1. The strong sharp absorption at 1661 cm−1 in spectra of HL1 can be
assigned to C=N stretch; in the complexes, these bands are shifted to lower frequencies
attributed to the coordination of the imine nitrogen. Bands at 1652 and 1597 cm−1 are
assigned to the imine stretch in 1 and 2, respectively [39]. IR spectra of HL1 show a broad
band at 3134–3047 cm−1 due to O–H stretch, which disappears in the respective complexes
indicating deprotonation of the Schiff base upon complexation. Several weak peaks
observed for the complexes at 3201–2728 cm−1 are due to aromatic and aliphatic C–H
stretches. The C–N stretches for 1 and 2 are in the range 1451–1342 cm−1. Aromatic nitro
compounds of HL1 and complexes have absorptions due to asymmetric and symmetric
stretches of C–NO2 at 1597–1549 and 1395–1378 cm−1, respectively. Ligand coordination
Figure 1. ORTEP drawing of 1 with atom labeling. Thermal ellipsoids have been drawn at 50% probability level.

3176
S. Celen et al.
Figure 2. ORTEP drawing of 2 with atom labeling. Thermal ellipsoids have been drawn at 50% probability level.
Table 2. Selected bond lengths [Å] and angles [°] for 1 and 2.
1
2
Bond lengths
Ni1–O1
Ni1–O2
Ni1–O3
Ni1–O4
2.01(16)
2.09(16)
2.15(19)
2.03(18)
2.01(2)
Fe1–O1
Fe1–O2
Fe1–O1*
Fe1–O2*
Fe1–N1
Fe1–N1*
2.01(13)
1.94(15)
2.01(13)
1.94(15)
2.12(15)
2.12(15)
Ni1–N1
Ni1–N2
2.00(2)
Bond angles
O1–Ni1–O3
O1–Ni1–O4
O2–Ni1–O3
O2–Ni1–O4
O1–Ni1–N1
O1–Ni1–N2
O2–Ni1–N1
O2–Ni1–N2
O3–Ni1–N1
O3–Ni1–N2
O4–Ni1–N1
O4–Ni1–N2
N1–Ni1–N2
90.31(7)
91.42(7)
91.59(7)
87.81(7)
91.51(8)
91.17(8)
81.44(8)
95.95(8)
96.11(8)
80.27(8)
93.01(8)
90.53(8)
175.51(8)
O1–Fe1–O1*
O1–Fe1–O2*
O2–Fe1–O1*
O2–Fe1–O2*
O1–Fe1–N1
O1–Fe1–N1*
O2–Fe1–N1
O2–Fe1–N1*
O1*–Fe1–N1
O2*–Fe1–N1
O1*–Fe1–N1*
O2*–Fe1–N1*
N1–Fe1–N1*
90.34(5)
91.41(6)
91.41(6)
90.61(6)
79.30(6)
90.37(6)
86.07(6)
104.32(6)
90.37(6)
104.32(6)
79.30(6)
86.07(6)
165.39(7)
Note: The asymmetric unit of 2 consists of half of the monomeric [Fe(HL1)] unit. The other half
of the molecule is symmetric. Therefore, symmetrical atoms in the molecule are shown with ‘*’
symbol.
Table 3. Hydrogen bond geometry (Å, °) of 1 and 2.
D–H⋯A
D–H
H⋯A
D⋯A
D–H⋯A
1
O7–H7A⋯O9(i)
O7–H7A⋯O1(ii)
O2–H19⋯O4(iii)
O3–H20⋯O7(iv)
O1–H1A⋯O1*(v)
0.8800
0.8600
0.8800
0.8400
0.8100
2.02
2.33
1.83
1.87
1.72
2.866
3.113
2.683
2.700
2.464
162.00
150.00
162.00
167.00
153.00
2
Notes: Symmetry codes: (i)1 + x, 1 + y, z; (ii)1 − x, −1/2 + y, 3/2 − z; (iii) −1/2 + x, 1/2 − y, 2 − z; (iv) −1 + x, y, z;
(v)
3/4 − x, 3/4 − y, z. The asymmetric unit of 2 consists of half of the monomeric [Fe(HL1)] unit. The other half of
the molecule is symmetric. Therefore, symmetrical atoms in the molecule are shown with ‘*’ symbol.
to the metal center is substantiated by bands at 647–474 and 450–413 cm−1, attributable to
υ(M–N) and υ(M–O), respectively, observed in all the complexes.

Ni(II) and Fe(II) Complexes
3177
Figure 3. A 3-D network structure formed by hydrogen bonds in the bc-plane of 1. Dotted lines indicate
hydrogen bonds.
Figure 4. A 1-D network structure formed by hydrogen bonds of 2. Dotted lines indicate hydrogen bonds.
3.3. Electronic spectra
UV–Visible spectra of HL1, 1, and 2 were carried out in DMF. The electronic transitions
due to the ligand in 1 and 2 showed the absorptions of the π→π* and n→π* transitions of
C=N at 253–407 nm regions. These values are lower than the corresponding absorption for
HL1, at 335 and 417 nm, respectively. This may be due to the coordination of nitrogen and
oxygen. The UV–Vis spectrum of 1 exhibits two absorptions at 256 and 395 nm, while
these are at 253 and 407 nm for 2 (figures S1 and S2). It was not possible to identify d–d

3178
S. Celen et al.
transitions due to a strong charge transfer band tailing from UV to the visible region
[40, 41].
3.4. Antibacterial and antifungal activities
The antimicrobial activities of ligand and complexes have been tested in vitro by agar disk
diffusion (table 4), microdilution broth susceptibility assay (table 5), and single spore
culture technique (table 6) against micro-organisms which are known to cause food spoilage
and infections in humans. The activity data show that the complexes are more potent
antimicrobial agents than the free ligand. Chloramphenicol (1000 μg/mL) and ketoconazole
(1000 μg/mL) served as positive controls. Chloramphenicol and ketoconazole showed
strong antimicrobial activity against all the micro-organisms tested.
The results of agar disk diffusion assay (table 4) exhibit that Ni(II) and Fe(II) complexes
and the free ligand affect bacteria and yeasts because they show clear inhibition zones with
the tested bacteria and yeast on solid agar media. At the end of incubation, the diameters of
the inhibition zones are in the range 7–10 mm. If the inhibition zone measures 2–3 mm,
then the complex has good antimicrobial activity. If the inhibition zone is more than 3 mm
across, then it is considered very effective, but if there is no inhibition zone then the
complex has no activity on the bacterium [42]. Ni(II) and Fe(II) complexes have high
antimicrobial activity against P. aeruginosa, E. coli, and S. aureus with inhibitions zones of
10 mm.
The Ni(II) and Fe(II) complexes exhibited similar levels of antimicrobial activity. Fe(II) is
most effective against P. vulgaris and S. marcescens with an MIC value of 125 μg/mL. Ni(II)
and Fe(II) complexes have higher antibacterial activity than the free ligand. This enhance-
ment in activity may be due to an efficient diffusion of the metal complexes into the bacterial
cell and/or interaction with the bacterial cell [43]. Previously we studied the antibacterial and
antifungal activities of Co(III) complexes [Co(HL1)(L1)](1) and [Co(HL2)(L2)](2) [where
HL1 = 2-((E)-(2-hydroxyethylimino)methyl)-4-chlorophenol and HL2 = 2-((E)-(2-hydroxy-
ethylimino)methyl)-4-bromophenol] using agar disk diffusion and microdilution broth
susceptibility assay. The results obtained showed that Co(III) complexes demonstrated
antimicrobial activity against all tested micro-organisms [22] as well as Ni(II) and Fe(II)
complexes showed higher antibacterial activities than Co(III) complexes. The variation in the
Table 4. Inhibition zones of the ligand and complexes according to agar disc diffusion method [mm].
Microorganisms
Sources
1
L1
2
Standard
Campylobacter jejuni
Enterobacter aerogenes
Escherichia coli
Listeria monocytogenes
Pseudomonas aeruginosa
Proteus vulgaris
Staphylococcus aureus
Serratia marcescens
Shigella sonnei
Klebsiella pneumoniae
Candida albicans
ATCC 33291
NRRL 3567
ATCC 25292
ATCC 7644
ATCC 27853
NRRL 123
ATCC 6538
Clinical isolate
ATCC 25931
Clinical isolate
ATCC 10231
Clinical isolate
9
9
10
9
10
9
10
9
9
9
8
9
8
8
9
7
9
8
9
8
8
8
8
8
9
9
10
9
10
9
10
9
9
9
9
9
10C
9C
11C
10C
10C
10C
10C
9C
9C
9C
9K
Candida albicans
9K
Notes: CChloramphenicol (1000 μg/mL).
KKetoconazole (1000 μg/mL).

Ni(II) and Fe(II) Complexes
Table 5. MIC [μg/mL] of the ligand and complexes.
3179
Microorganisms
Sources
1
L1
2
Standard
Campylobacter jejuni
Enterobacter aerogenes
Escherichia coli
Listeria monocytogenes
Pseudomonas aeruginosa
Proteus vulgaris
Staphylococcus aureus
Serratia marcescens
Shigella sonnei
Klebsiella pneumoniae
Candida albicans
ATCC 33291
NRRL 3567
ATCC 25292
ATCC 7644
ATCC 27853
NRRL 123
ATCC 6538
Clinical isolate
ATCC 25931
Clinical isolate
ATCC 10231
Clinical isolate
125
125
125
125
125
250
125
250
125
125
250
125
250
250
250
250
125
250
250
250
250
250
250
250
125
125
125
125
125
125
125
125
125
125
250
125
250C
250C
125C
125C
250C
125C
125C
250C
250C
250C
250K
250K
Candida albicans
Notes: CChloramphenicol (1000 μg/mL).
KKetoconazole (1000 μg/mL).
Table 6. Antifungal activities for the tested complexes (inhibition %).
Filamentous fungi
1
HL1
2
Ketoconazole
Aspergillus flavus
Aspergillus niger
Penicillium expansum
Penicillium lanosum
Alternaria alternata
20
12.5
21.6
19
9.5
20.4
25
83.63
40
65
54
82
35.1
33.3
28.5
22.2
29.7
23.8
28.5
22.8
activity of the metal complexes against different organisms depends on the impermeability of
the micro-organisms’ cells or on differences in the ribosome of the microbial cells [44].
The antifungal activities of ligand and complexes are shown in table 6. The inhibition of
fungal growths expressed in terms of percentage was determined on the growth in test plates
compared to the respective control plates as a percent of inhibition. A. niger was more
sensitive (35.1%) against all the tested compounds compared with other filamentous fungi.
The Ni(II) complex is most effective against A. niger and P. expansum. On the other hand,
A. flavus and A. alternata were more sensitive to Fe(II) complex. The Ni(II) and Fe(II)
complexes are more effective against filamentous fungi than the free ligand. Previous studies
have shown that the Ni(II) and Fe(II) complexes have antimicrobial activity [17, 45, 46] and
metal chelates have higher antimicrobial activity than the free ligand [45, 47]. Such an
increased activity of the complex can be explained on the basis of overtone’s concept and
chelation theory [48]. The complex may disturb the respiration process of the cell and thus
block the synthesis of proteins [49].
4. Conclusion
Tridentate Schiff base HL1 and its Ni(II) and Fe(II) complexes were synthesized and
characterized. The single crystal X-ray structures of 1 and 2 show a distorted octahedral
geometry around Ni(II) and Fe(II). HL1 is a monobasic neutral tridentate ligand coordinat-
ing through alkoxy oxygen, imine nitrogen, and phenolic oxygen via deprotonation. The
free ligand and complexes were screened for the antibacterial and antifungal activities by

3180
S. Celen et al.
disk diffusion, microdilution broth, and single spore culture techniques. The antimicrobial
activities show that the free ligand and its metal complexes exhibit antimicrobial properties.
In comparison to the ligand, the metal complexes were more biologically active.
Supplementary material
Crystallographic data for the structure reported in this paper have been deposited with the
Cambridge Crystallographic Data Center (The Director, CCDC, 12 Union Road, Cam-
bridge, CB2 1EZ, UK; E-mail: deposit@ccdc.cam.uk; www: http://www.ccdc.cam.ac.uk;
Fax: +44 1223 336033) and are available free of charge on request, quoting Deposition
Nos. CCDC 8866731 and 866732.
Acknowledgments
The financial support from the Scientific and Technical Research Council of Turkey (TUBITAK)
Grant No: TBAG-108T431 and the Research Funds of Balikesir University (BAP-2010/122) is
gratefully acknowledged. Dr Kara also thanks the Nato-B1-TUBITAK for funding and Prof. Guy
Orpen (School of Chemistry, University of Bristol, UK) for his hospitality.
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