Two tridentate Schiff base ligands and their mononuclear cobalt (III) complexes: Synthesis, characterization, antibacterial and antifungal activities

Article abstract of DOI:10.1016/j.saa.2012.03.034

Two Schiff base ligands (HL1, HL2) and their 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-hydroxyethylimino) methyl)-4-bromophenol] were synthesized and cha

Full text of DOI:10.1016/j.saa.2012.03.034

Spectrochimica Acta Part A 94 (2012) 216–221
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Two tridentate Schiff base ligands and their mononuclear cobalt (III) complexes:
Synthesis, characterization, antibacterial and antifungal activities
Elif Gungora, Selma Celenb, Dilek Azazb, Hulya Karaa,∗
a Department of Physics, Faculty of Science and Art, Balikesir University, 10145 Balıkesir, Turkey
b Department of Biology, Faculty of Science and Art, Balikesir University, 10145 Balıkesir, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Two Schiff base ligands (HL1, HL2) and their Co(III) complexes, [Co(HL1)(L1)] (1) and [Co(HL2)(L2)]
Received 6 January 2012
Received in revised form 1 March 2012
Accepted 9 March 2012
(2)
[where
HL1 = 2-((E)-(2-hydroxyethylimino)methyl)-4-chlorophenol
and
HL2 = 2-((E)-(2-
hydroxyethylimino)methyl)-4-bromophenol] were synthesized and characterized using spectroscopic
methods. The crystal structures of 1 and 2 have been re-determined by single crystal diffraction at 100 K.
The ligands and their Co(III) complexes were screened for antibacterial and antifungal activities by the
disc diffusion, microdilution broth and single spore culture techniques. The antimicrobial activity of the
Co(III) complexes and the free ligands exhibit antimicrobial properties and the Co(III) complexes show
enhanced inhibitory activity compared with their parent ligand.
Keywords:
Schiff base
Cobalt(III)
Antibacterial activity
Antifungal activity
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
properties were tested against the 10 bacteria, Campylobacter
jejuni, Enterobacter aerogenes, Escherichia coli, Listeria monocyto-
Schiff bases shown are obtained by condensation of aldehy-
des with primary amines and are versatile ligands in coordination
chemistry because of their wide range of applications [1–10]. Con-
siderable attention has been focused on the biological activities,
such as antibacterial, antifungal, anticancer and herbicidal appli-
cations of Schiff bases and their metal complexes [11–17]. These
studies have shown that the complexation of metals to Schiff base
ligands improves the antimicrobial and anticancer activities of the
ligands [18,19].
genes, Pseudomonas aeruginosa, Proteus vulgaris, Staphylococcus
aureus, Serratia marcescens, Shigella sonnei, Klebsiella pneumoniae,
whereas the antifungal properties were tested against two yeasts,
Candida albicans, C. albicans and five filamentous fungi Aspergillus
flavus, Aspergillus niger, Penicillium expansum, Penicillium lanosum
and Alternaria alternate.
2. Experimental
Cobalt complexes have gained importance for their applicabil-
ity in the biological field [20,21]. Some of the previous reports
have shown that hexamine cobalt complexes induce DNA conden-
sation [22–24]. DNA binding studies and cobalt complexes have
been investigated for their anticancer properties [25,26]. Cobalt(III)
complexes have also been tested as hypoxia-activated anticancer
prodrugs [27].
Although there has been considerable interest in the inves-
tigation of the structural characterization, antimicrobial and
antifungal properties of Salen-type cobalt(II) Schiff base complexes
[28–32], little attention has been paid to systems that contain
Co(III) [33–35]. For this reason, we report here the preparation,
characterization and the antibacterial and antifungal properties
of two Schiff base ligands (HL1 and HL2) and their Co(III) com-
plexes, [Co(HL1)(L1)] (1) and [Co(HL2)(L2)] (2). The antibacterial
2.1. Materials and physical measurements
All chemical reagents and solvents were purchased from Merck
or Aldrich and used without further purification. Elemental (C, H,
N) analyses were carried out by standard methods with a LECO,
CHNS-932 analyzer. UV–vis spectra were obtained at 20 ◦C using a
Perkin Elmer Lambda 25 UV–vis spectrometer. FT-IR spectra over
the 4000–400 cm−1 range were measured using a Perkin–Elmer
Model Bx 1600 instrument with the samples prepared as KBr pel-
lets. 1H and 13C NMR spectra of the HL1 and HL2 ligands were
recorded (in DMSO) using a Bruker Biospin (300 MHz) instrument.
The synthetic route of the ligands HL1, HL2, complexes 1 and 2 are
outlined in Scheme 1.
2.2. Synthesis of HL1 and HL2 ligands
The tridentate Schiff base ligand HL1 was prepared by mix-
ing 5-chlorosalicylaldehyde (1 mmol) and ethanolamine (1 mmol)
with the molar ratio of 1:1 in hot methanol (40 cm3). The solution
∗
Corresponding author. Tel.: +90 266 6121200; fax: +90 266 6121215.
E-mail addresses: hkara@balikesir.edu.tr, karahulya04@gmail.com (H. Kara).
1386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2012.03.034

E. Gungor et al. / Spectrochimica Acta Part A 94 (2012) 216–221
217
O
R
R
N
OH
H
MeOH
+
HO
NH2
OH
OH
MeOH
N
R
H
O
HL1
HL2
R=Cl
R=Br
Co
O
O
O
N
R
Scheme 1. The synthetic route of the ligands HL1, HL2, complexes 1 and 2 evaluated in this study.
obtained was stirred at 65 ◦C for 30 min. The yellow compound was
2.3. Synthesis of complexes 1 and 2
precipitated from the solution on cooling. HL2 was prepared in a
similar way using 5-bromosalicylaldehyde in hot methanol.
HL1: Yellow crystals, yield 80%. Theoretical composition (in %):
C, 54.15; H, 5.05; N, 7.02. Experimentally determined composition
(in %): C, 54.11; H, 5.07; N, 7.06. 1H NMR (DMSO, ı ppm) yielded
signals at: 3.74–3.88 (–NCH2CH2OH, 4H), 5.17 (–NCH2CH2OH, 1H),
6.69 (–HC COH–, 1H), 7.29 (Cl–C CH–, 1H), 7.50 (Cl–C–CH C–,
1H), 8.54 (–HC N–, 1H). 13C NMR (DMSO, ı ppm) yielded signals at:
60.83 (–N–CH2–), 62.15 (–HOCH2–), 119.95 (–HC COH–), 125.90
(–C–COH), 127.84 (–HC C–Cl), 130.96 (Cl–C–CH–C–), 132.38
(Cl–C CH–), 161.45 (–C–COH), 165.91 (–C N–). The IR (KBr) spec-
trum gave signals (in cm−1) at: 3174, 2907, 2895, 1647, 1278, 1215,
729, 688, 629. The UV/vis (DMSO) spectrum gave signals (in nm)
at: 236 and 325.
Complexes 1 and 2 were prepared by adding cobalt (II) acetate
tetrahydrate (0.249 g, 1 mmol) in 20 cm3 of hot ethanol to the
appropriate ligand (HL1 or HL2) (1 mmol) in 20 cm3 of hot
methanol. This solution was warmed to 78 ◦C and stirred for 15 min.
A red-coloured solution was obtained and then allowed to stand at
room temperature. Several weeks of standing led to the growth of
red-coloured crystals.
During the synthesis performed under aerobic conditions, these
reaction systems exhibited an obvious colour change from light
to dark after the addition of the yellow basic methanol solution
containing the appropriate ligand (HL1 or HL2). These observations
suggest the occurrence of oxidation processes (CoII → CoIII) induced
by the oxygen in air.
HL2: Yellow crystals, yield 82%. Theoretical composition (in %):
C, 44.29; H, 4.13; N, 5.74. Experimentally determined composition
(in %): C, 44.37; H, 4.14; N, 5.80. 1H NMR (DMSO, ı ppm) yielded
signals at: 3.71–3.87 (–NCH2CH2OH, 4H), 5.07 (–NCH2CH2OH, 1H),
6.68 (–HC COH–, 1H), 7.38 (Br–C CH–, 1H), 7.63 (Br–C–CH C–,
1H), 8.47 (–HC N–, 1H). 13C NMR (DMSO, ı ppm) gave signals at:
60.83 (–N–CH2–), 61.03 (–HOCH2–), 110.01 (–HC C–Br), 119.77
(–HC COH–), 120.54 (–C–COH), 133.93 (Br–C–CH–C–), 135.21
(Br–C CH–), 161.45 (–C–COH), 165.90 (–C N–). The IR (KBr) gave
a signal (in cm−1) at: 3162, 2946, 2831, 1648, 1289, 1215, 549, 508.
The UV/vis (DMSO) spectrum gave a signal (in nm) at: 236 and 325.
Complex 1: Red crystals, yield 70%. Theoretical composition (in
%): C, 47.50; H, 3.76; N, 6.15. Experimentally found composition (in
%): C, 47.80; H, 3.70; N, 6.18. The IR (KBr) spectrum gave peaks (in
cm−1) at: 3392, 2902, 2864, 1644, 1259, 1243, 698, 663, 615. The
UV/vis (DMSO) gave a signal at: 251 nm, 395 nm and 665 nm.
Complex 2: Red crystals, yield 74%. Theoretical composition (in
%): C, 39.74; H, 3.15; N, 5.15. Experimentally found composition
(in %): C, 40.01; H, 3.18; N, 5.30. The IR (KBr) spectrum showed
signals (in cm−1) at: 3389, 2902, 2861, 1640, 1258, 1243, 588,
546. The UV/vis (DMSO) spectrum had peaks (in nm) at: 261, 395
and 669.
Table 1
Crystal data and structure refinement for 1 and 2.
1
2
Empirical formula
Formula weight (g mol−1
Temperature (K)
Crystal system
Space group
Unit cell dimensions
C
18H16Cl2CoN2O4
C18H16Br2CoN2O4
543.06
100(2)
Trigonal
)
454.16
100(2)
Trigonal
R-3
R-3
a = 18.5657(10) A, ˛ = 90
a = 18.9500(3)Å, ˛ = 90◦
◦
˚
b = 18.5657(10)Å, ˇ = 90◦
b = 18.9500(3)Å, ˇ = 90◦
c = 27.308(4)Å, ꢀ = 120◦
c = 27.0878(10)Å, ꢀ = 120◦
Volume (Å3)
8151.6(7)
18
1.665
1.270
2.0–27.5
8424.1(2)
18
1.927
5.213
2.0–27.5
Z
Density (calculated) (g cm−3
)
Absorption coefficient (mm−1
ꢁ range for data collection (◦)
Index ranges
)
−23 ≤ h ≤ 24, −23 ≤ k ≤ 23, −35 ≤ l ≤ 30
−19 ≤ h ≤ 24, −22 ≤ k ≤ 20, −35 ≤ l ≤ 24
Reflections collected
14 173
15 918
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
R indices [I > 2ꢂ(I)]
4144
2486/2/254
1.05
R1 = 0.0905, wR2 = 0.2250
1.16 and −1.10
4321
3113/1/251
1.02
R1 = 0.0377, wR2 = 0.0769
0.84 and −0.70
Largest diff. peak and hole (eÅ−3
)

218
E. Gungor et al. / Spectrochimica Acta Part A 94 (2012) 216–221
2.5. Biological activity
The tridentate Schiff base ligands HL1 and HL2 and their
mononuclear Co(III) complexes 1 and 2 were subjected to screen-
ing for the antimicrobial activity by using agar disc diffusion assay,
microdilution broth assay [40,41] and single spore culture tech-
nique (for filamentous fungi) [42]. The solvent, DMSO used for
the preparation of compounds did not show inhibition against the
tested organisms. The antimicrobial activity of the tridentate Schiff
base ligands and their mononuclear Co(III) complexes was eval-
uated against the 10 bacteria C. jejuni (ATCC 3291), E. aerogenes
(NRRL 3567), E. coli (ATCC 25292), L. monocytogenes (ATCC 7644), P.
aeruginosa (ATCC 27853), P. vulgaris (NRRL 123), S. aureus (ATCC
6538), S. marcescens (clinical isolate), S. sonnei (ATCC 25931), K.
pneumoniae (clinical isolate). The antifungal activity of the ligands
and complexes were evaluated using the two yeasts C. albicans
(ATCC 10231), C. albicans (clinical isolate) and five filamentous fungi
A. flavus, A. niger, P. expansum, P. lanosum and A. alternate. The micro-
fungi used during the experiments were isolated in our Department
from soil.
Fig. 1. ORTEP drawing of complex 1 with atom labelling. Thermal ellipsoids have
been drawn at the 30% probability level.
2.4. X-ray structure determination
Diffraction measurements were made using a Bruker ApexII
kappa CCD diffractometer using graphite monochromated Mo–K␣
radiation (ꢃ = 0.71073 A) at 100 K for both ligands 1 and 2. The inten-
2.5.1. Agar disc diffusion method
The agar disc diffusion method was employed for the deter-
mination of antimicrobial capacities of the free ligands and their
Co(III) complexes in questions [40]. Briefly, a suspension of the
tested microorganism (500 ␮L of 108 CFU/mL) was spread on the
solid media plates. All test solutions were prepared in DMSO. Then,
filter paper discs (6 mm in diameter) were soaked with 20 ␮L of the
stock solutions and placed on the 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, under microaerophilic
conditions for 48 h. The diameter of inhibition zones were mea-
sured in millimeters. Each test was carried out in triplicate and the
average inhibition zone diameters were calculated.
˚
sity data were integrated using the APEXII program [36]. Absorption
corrections were applied based on equivalent reflections using
SADABS [37]. The structures were solved by direct methods and
refined using full-matrix least-squares against F2 using SHELXL
[38]. All non-hydrogen atoms were assigned anisotropic dis-
placement parameters and refined without positional constraints.
Hydrogen atoms were included in idealized positions with isotropic
displacement parameters constrained to 1.5 times the Uequiv of
their attached carbon atoms for methyl hydrogens, and 1.2 times
the Uequiv of their attached carbon atoms for all others. A possi-
ble disorder in the ethanolamine portion of the ligand for 1 and 2
has been considered. All of the crystallographic computations were
carried out using SHELXTL [38] and MERCURY [39] programs.
Details of the data collection parameters and crystallographic
information for the complexes are summarized in Table 1. ORTEP
diagrams of 1 and 2 are displayed in Fig. 1 and Fig. S1 and packing
diagrams are displayed Fig. 2 and Fig. S2.
2.5.2. The microdilution broth method
A microdilution broth susceptibility assay was used for the
determination of Minimum Inhibitory Concentration (MIC) [43].
The serial dilutions of compounds were prepared in sterile dis-
tilled 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. albi-
cans in Saboraud Dextrose Broth). Sterile distilled water served as
growth control. 100 ␮L of each microbial suspension were then
added to each well. The last row containing only the serial dilu-
tions of antibacterial agent without microorganism 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 minimal inhibitory
concentration. Chloramphenicol (1000 ␮g/mL) and ketoconazole
(1000 ␮g/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 a 1% (w/v) sodium chloride solution containing 5%
(w/v) DMSO. The spore suspension was then filtered and trans-
ferred into tubes and stored at −20 ◦C, accordingly to the method
reported by Hadecek and Greger [44]. The 1 mL spore suspension
was taken, diluted in a loop drop until one spore could be captured.
One loop drop from the spore suspension was applied onto the cen-
tre of the petri dish containing Czapex Dox Agar and Malt Extract
Agar. 20 ␮L of each complex was applied onto sterile paper discs
Fig. 2. Perspective view of the star-shaped hydrogen bonded hexameric assembly
of complex 1. Dotted lines indicate hydrogen bonds.

E. Gungor et al. / Spectrochimica Acta Part A 94 (2012) 216–221
219
Table 2
FT-IR and electronic spectral data of HL1, HL2, 1, 2 and their assignments.
Complexes
ꢄ(O–H)
ꢄ(C–H)
ꢄ(C N)
ꢄ(C–O)
ꢄ(C–Cl)
ꢄ(C–Br)
d–d (nm)
Charge transfer band (nm)
HL1
HL2
1
3174
3162
–
2907, 2895
2946, 2831
2902, 2864
2902, 2861
1647
1648
1644
1640
1278, 1215
1289, 1215
1259, 1243
1258, 1243
729, 688, 629
–
–
–
665
669
236, 325
236, 325
251, 395
261, 395
–
698, 663, 615
–
549, 508
–
588, 546
2
–
˚
˚
(6 mm in diameter) and placed in the petri dishes and incubated
at 25 ◦C for 72 h. Spore germination during the incubation period
was followed using a microscope (Olympus BX51) at 8 h intervals.
The inhibition of fungal growths, expressed in percentage terms,
was determined on the growth in test plates compared with the
respective control plates as given inhibition [45].
and Co–Nimi = 1.898(6) A for 1, and Co–Ophe = 1.875(3) A,
˚
˚
Co–Oalk = 1.921(3) A and Co–Nimi = 1.887(3) A for 2, respectively.
The angles O–Co–O are in the range from 87.7(2)◦ to 91.4(2)◦ for
1 and 88.55(11)◦–91.02(12)◦ for 2 and the angles dealing with
O–Co–N range from 86.1(2)◦ to 94.9(2)◦ for 1 and from 85.94(13)◦
to 95.12(13)◦ for 2 while the angle of N–Co–N is 177.5(3)◦ for 1
and 177.36(12)◦ for 2. Selected bond lengths and angles are listed
in Table S1, which lie well within the range of reported values
for corresponding bond lengths and angles of other mononuclear
cobalt (III) complexes [46–50].
In the crystal structure, the adjacent molecules are linked
through intermolecular O–H···O hydrogen bonds (Table S2, Fig. 2).
In the complexes O–H···O hydrogen bondings assemble the molec-
ular units leading to ordered supramolecular architectures. Both
complexes 1 and 2 are a unique star-shaped cyclic hexamer gener-
ated through intermolecular hydrogen bonding.
C − T
Inhibition % = 100 ×
C
where C is the diameter of fungal growth on the control and T is the
diameter of fungal growth on the test plate.
The activities of the complex were then compared with the
activity of the standard antifungicide ketoconazole.
3. Results and discussion
3.1. X-ray structure redeterminations of 1 and 2
3.2. FTIR spectroscopy
The crystal structure of 1 was reported previously at room tem-
perature [46]. The crystal structure of 2 was reported previously at
room temperature [47] and low temperature [48]. We report here a
redetermination of 1 and 2, at 100 K, with improved precision. The
The IR spectra of the ligands and the Co(III) complexes were
recorded using KBr discs. The FT-IR characteristic frequencies of
HL1, HL2, complexes 1 and 2 are summarized in Table 2. The FTIR
spectra of HL1 and HL2 show ꢄ(O–H) and ꢄ(C N) vibrations in
the range 3174–3162 cm−1 and 1647–1648 cm−1, respectively. The
phenolic C–O stretching are present at 1278 and 1215 cm−1 for HL1
and at 1289 and 1215 cm−1 for HL2. The observed bands in the
range of 729–615 cm−1 are characteristic of ꢄ(C–Cl) vibrations for
HL1 and 1. The bands in the range of 588–508 cm−1 are character-
istic of ꢄ(C–Br) vibrations for HL2 and 2. Chemical structure of HL1
and HL2 which are likely to be a superposition of bands from alcohol
–OH and phenol –OH groups. The spectrum of 1 reveals the bands
for ꢄ(O–H) at 3392 cm−1, for ꢄ(C–H) at 2902 and 2864 cm−1, for
ꢄ(C–O) at 1259 and 1243 cm−1, and for ꢄ(C N) at 1644 cm−1. The
spectrum of 2 had a signal for ꢄ(O–H) at 3389 cm−1, for ꢄ(C–H)
at 2902 and 2861 cm−1, for ꢄ(C–O) at 1258 and 1243 cm−1 and
for ꢄ(C N) at 1640 cm−1. The ꢆ(C N) band (ca. 1647 cm−1 for HL1
and 1648 cm−1 for HL2) of the free ligands are shifted to a slightly
lower frequency (ca. 1644 cm−1 for 1 and 1640 cm−1 for 2) upon
complexation, suggesting that the imino nitrogen is coordinated to
the cobalt ion. The infrared spectrum of complexes 1 and 2 are very
much consistent with the structural data presented in this paper.
precision of the unit-cell dimensions was improved by an order of
3
˚
magnitude. The unit-cell volume decreased by ca. 183.1 A for 1
3
3
˚
˚
[46] and 176.9 A [47], 27.2 A [48] for 2, respectively.
Both the HL1 and HL2 are tridentate ligands with the Ophen
,
Nimi and Oalk donor sites. The strong ꢅ acceptor ability of the
Schiff base ligands favours oxidation of the cobalt(II) centre in the
presence of methanol, allowing the formation of [CoIII(HL1)(L1)]
(1) and [CoIII(HL2)(L2)] (2) species. Complexes 1 and 2 are
isostructural and crystallize in the trigonal space group R3 with
18 molecules per unit cell. In the complexes, the cobalt(III) centre
is coordinated to two tridentate Schiff base ligands of which one
ligand molecule is in dianionic form (L)2− after deprotonation
of both the phenolic and alkoxy OH− groups; while the other
ligand is monoanionic (HL)− because of the deprotonation of
only the phenolic OH group. The geometry around the CoIII ion
can best be described as a distorted octahedron with two imine
nitrogen atoms (N1 and N2), two phenoxo oxygen atoms (O2
and O4) and alcoholic oxygen atoms (O1 and O3). The average
˚
˚
bond lengths are Co–Ophe = 1.882(5) A, Co–Oalk = 1.927(5) A
Table 3
Inhibition zones of the ligands and complexes according to agar disc diffusion method [mm].
Microorganisms
Sources
1
2
HL1
HL2
Control
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
8
8
9
8
9
8
9
8
8
8
7
8
8
8
9
7
8
7
8
8
8
8
8
8
25c
22c
22c
24c
23c
24c
22c
24c
25c
22c
24k
27k
10
10
9
10
9
10
9
9
10
10
8
10
9
10
9
9
9
8
9
9
8
8
Candida albicans
c, chloramphenicol; k, ketoconazole.

220
E. Gungor et al. / Spectrochimica Acta Part A 94 (2012) 216–221
Table 4
Minimum inhibitory concentration [␮g/mL] of the ligands and complexes.
Microorganisms
Sources
1
2
HL1
HL2
Control
c
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
500
250
250
500
250
500
250
500
500
250
500
500
500
250
250
500
500
500
250
500
500
500
500
500
1000
500
500
1000
500
1000
500
1000
1000
500
1000
1000
1000
500
500
–
c
–
c
–
c
1000
1000
1000
500
1000
1000
1000
1000
1000
–
c
–
c
–
c
–
c
–
c
–
c
–
k
–
k
Candida albicans
–
c, chloramphenicol; k, ketoconazole; –, no turbidity.
3.3. Electronic spectrum
flavus, A. niger, P. expansum, P. lanosum and Alternaria alternata
(Table 5). The results showed that P. lanosum was much more sen-
sitive against all the tested compounds compared with the other
filamentous fungi. On the other hand, complex 1 showed more tox-
icity compared with the other tested compounds against A. niger, P.
expansum, P. lanosum and A. alternata under identical experimental
conditions. A. flavus was more sensitive against the complex 2.
Such increased activity of the complex can be explained on the
basis of Overtone’s concept and chelation theory [53] according
to which chelation reduces the polarity of the central metal atom,
mainly because of partial sharing of its positive charge with the
ligand. Consequently this favours the penetration of the complex
through the lipid layer of the cell membrane and blocking of the
metal binding sites in the enzymes of microorganisms. This com-
plex may disturb the respiration process of the cell and thus block
the synthesis of proteins [54].
UV–visible spectra of the Schiff base ligands and the Co(III) com-
plexes were performed after dissolution of the materials in DMSO.
The data are summarized in Table 2. The Schiff base ligands HL1
and HL2 show broad bands at 236 and 325 nm which may arise
because of the conjugated system. The absorption bands of the com-
plexes 1 and 2 are shifted to longer wavelength region compared
with their parent ligand. The electronic spectra of 1 exhibit three
absorption bands at 251, 395 and 665 nm while these are at 261,
395 and 669 nm for material 2. The d–d bands at 665 nm and 669 nm
originate from the 1A1g → T1g transition for the distorted octahe-
1
dral cobalt(III) ion in complexes 1 and 2 [51,52]. The higher energy
band 1A1g → T2g of 1 and 2 is obscured by ligand to metal charge
1
transfer bands obtained in the broad range 251–395 nm. For com-
plexes 1 and 2, the intense high energy bands at 251 and 395 nm
may be assigned to the intra-ligand ꢅ → ꢅ*and n → ꢅ* transitions,
respectively.
4. Conclusion
Tridentate Schiff base ligands (HL1, HL2) and their Co(III) com-
plexes (1, 2) were synthesized and characterized by spectroscopic
methods. The crystal structures of complexes 1 and 2 have been
determined by single crystal diffraction at 100 K. The Schiff base
ligands (HL1 and HL2) and the Co(III) complexes (1 and 2) were
screened for the antibacterial and antifungal activities by the disc
diffusion, microdilution broth and single spore culture techniques.
The results of antimicrobial activity show that the Co(III) com-
plexes and the free ligands exhibit antimicrobial properties and it is
important to note that Co(III) complexes show enhanced inhibitory
activity compared with their parent ligand.
3.4. Antibacterial and antifungal activity
The antibacterial and antifungal activities of the Schiff base lig-
ands HL1 and HL2 and the Co(III) complexes 1 and 2 were tested
in vitro against 10 pathogenic bacteria, 5 filamentous fungi and
2 yeast by the Agar Disc Diffusion Method (Table 3), Microdilu-
tion Broth Susceptibility Assay (Table 4) and Single Spore Culture
Technique (Table 5). The Co(III) complexes generally have higher
activities than the corresponding free ligands against the tested
microorganisms.
The activities of the Co(III) complexes and the parent lig-
ands were compared with the activity of the standard antibiotic
chloramphenicol (1000 ␮g/mL) and the standard antifungicide
ketoconazole (1000 ␮g/mL). Generally, the Co(III) complexes, 1
and 2 exhibited similar levels of antimicrobial activity. The results
revealed that complex 1 is most effective against P. aeruginosa and
K. pneumoniae with MIC (Minimum Inhibitory Concentration) value
of 250 ␮g/mL. All compounds also showed positive results against
C. albicans (500 ␮g/mL–1000 ␮g/mL) (Table 4).
Acknowledgements
The financial support of the Scientific and Technical Research
Council of Turkey (TUBITAK) Grants No: TBAG-108T431 and Balike-
sir University 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.
At the same time all compounds were screened in order to
evaluate their antifungal activity against the filamentous fungi A.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2012.03.034.
Table 5
Antifungal activity data for the tested complexes (inhibition %).
Filamentous fungi
1
2
HL1
HL2
Ketoconazole
References
Aspergillus flavus
Aspergillus niger
Penicillium expansum
Penicillium lanosum
Alternaria alternata
24
22
38
38
16
32
18
28
33
16
5.4
9
23
28
13
5.4
6.8
19
23
15
83.63
40
65
54
82
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