Synthesis, characterization, antibacterial, and thermal studies of unsymmetrical Schiff-base complexes of cobalt(II)

Article abstract of DOI:10.1080/00958972.2010.532871

Cobalt(II) complexes of a new series of unsymmetrical Schiff bases have been synthesized and characterized by their elemental analyses, melting points, magnetic susceptibility, thermogravimetric analysis, differential scanning calorimetry, infrared (IR), and electronic spectral measurements. The purity of the ligands and the metal complexes are confirmed by microanalysis, while the unsymmetrical nature of the ligands was further corroborated by ¹H-NMR. Comparison of the IR spectra of the Schiff bases and their metal complexes confirm that the Schiff bases are tetradentate and coordinated via N₂O₂ chromophore. The magnetic moments and electronic spectral data support square-planar geometry for the cobalt(II) complexes. The complexes were thermally stable to 372.3°C and their thermal decomposition was generally via the partial loss of the organic moiety. The Schiff bases and their complexes were screened for invitro antibacterial activities against 10 human pathogenic bacteria and their minimum inhibitory concentrations were determined. Both the free ligands and cobalt(II) complexes exhibit antibacterial activities against some strains of the microorganisms, which in a number of cases were comparable with, or higher than, that of chloramphenicol.

Full text of DOI:10.1080/00958972.2010.532871

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Synthesis, characterization,
antibacterial, and thermal studies of
unsymmetrical Schiff-base complexes
of cobalt(II)
Adeola A. Nejo a , Gabriel A. Kolawole a & Ayorinde O. Nejo a
a Department of Chemistry, University of Zululand,
Kwadlangezwa, South Africa
Version of record first published: 18 Nov 2010.
To cite this article: Adeola A. Nejo, Gabriel A. Kolawole & Ayorinde O. Nejo (2010): Synthesis,
characterization, antibacterial, and thermal studies of unsymmetrical Schiff-base complexes of
cobalt(II), Journal of Coordination Chemistry, 63:24, 4398-4410
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Journal of Coordination Chemistry
Vol. 63, No. 24, 20 December 2010, 4398–4410
Synthesis, characterization, antibacterial, and thermal studies of
unsymmetrical Schiff-base complexes of cobalt(II)
ADEOLA A. NEJO*, GABRIEL A. KOLAWOLE and AYORINDE O. NEJO
Department of Chemistry, University of Zululand, Kwadlangezwa, South Africa
(Received 9 July 2010; in final form 23 September 2010)
Cobalt(II) complexes of a new series of unsymmetrical Schiff bases have been synthesized and
characterized by their elemental analyses, melting points, magnetic susceptibility, thermogravi-
metric analysis, differential scanning calorimetry, infrared (IR), and electronic spectral
measurements. The purity of the ligands and the metal complexes are confirmed by
microanalysis, while the unsymmetrical nature of the ligands was further corroborated by
1H-NMR. Comparison of the IR spectra of the Schiff bases and their metal complexes confirm
that the Schiff bases are tetradentate and coordinated via N2O2 chromophore. The magnetic
moments and electronic spectral data support square-planar geometry for the cobalt(II)
complexes. The complexes were thermally stable to 372.3ꢀC and their thermal decomposition
was generally via the partial loss of the organic moiety. The Schiff bases and their complexes
were screened for in vitro antibacterial activities against 10 human pathogenic bacteria and their
minimum inhibitory concentrations were determined. Both the free ligands and cobalt(II)
complexes exhibit antibacterial activities against some strains of the microorganisms, which in
a number of cases were comparable with, or higher than, that of chloramphenicol.
Keywords: Cobalt(II); Unsymmetrical Schiff base; Antibacterial activities, MIC
1. Introduction
Schiff-base complexes of transition metals are of particular interest to inorganic
chemists because their structural, spectral, and chemical properties are often strongly
dependent on the nature of the ligand structure [1–6]. Coordination complexes with
substituted salicylaldehydes have shown diverse structures and properties generating a
variety of stereochemistries and a wide range of bonding interactions [7–10]. Schiff-base
metal complexes are specifically of interest in bioinorganic chemistry because many of
these complexes provide biological models in understanding the structure of biomol-
ecules and biological processes [11]. Naturally occurring dioxygen carriers and storage
proteins contain a transition metal ion to which dioxygen can reversibly bind, e.g., iron
(haemoglobin) and copper (haemocyanin). Such systems have been modeled by a
number of Schiff-base complexes [12–14]. Some cobalt(II) Schiff-base complexes are
also able to bind reversible dioxygen and they have been used as simplified models in
the study of reversible binding of dioxygen by its natural transporters [12–14].
*Corresponding author. Email: anejo@pan.uzulu.ac.za
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958972.2010.532871

Cobalt(II) Schiff-base complexes
4399
The synthesis and characterization of symmetrical tetradentate Schiff-base complexes
of cobalt have been thoroughly studied and reported in the literature [1, 15–17], but
only a few unsymmetrical complexes are available [18–20]. We have recently synthesized
unsymmetrical Schiff bases (H2L1–H2L4) from 2-hydroxy-1-naphthaldehyde,
o-phenylenediamine, and salicylaldehyde or substituted salicylaldehyde and character-
ized their oxovanadium(IV) complexes [21]. This investigation was undertaken to study
the effect of the substituents on salicylaldehyde on the stereochemistry and biological
activity of metal(II) complexes. In this article, we report cobalt(II) complexes
(CoL1–CoL4) of the unsymmetrical ligands and their antimicrobial studies. Single
crystal of the complexes could not be isolated from any solutions. However, analytical,
spectroscopic, and magnetic data enable us to propose possible structures.
The complexes investigated are reported for the first time.
2. Experimental
2.1. Materials
All reagents and solvents were of the analar/spectroscopic grades and used as received.
Ethanol, methanol, 5-chlorosalicylaldehyde, 5-nitrosalicylaldehyde, 3-ethoxysalicylal-
dehyde, 2-hydroxy-1-naphthaldehyde, salicylaldehyde, o-phenylenediamine, and
cobalt(II) acetate tetrahydrate were purchased from Aldrich Sigma company.
2.2. Preparation of the compounds
Procedures for the preparation of the unsymmetrical Schiff bases have been reported
in [21]. The cobalt(II) complexes were prepared by the addition of 3 mmol of
Co(CH3COO)2 ꢁ 4H2O (0.53 g), dissolved in hot absolute methanol (60 mL) by stirring
3 mmol of the respective unsymmetrical Schiff base in methanol (40 mL). The color of
the mixture was changed instantly. The mixture was refluxed for 3 h and the
precipitated solids were filtered, washed with cold methanol, and allowed to dry in a
desiccator over silica gel.
2.3. Physical measurements
Microanalytical data were obtained on a Perkin Elmer model 2400 Series II CHNS/O
elemental analyzer for C, H, and N. Infrared (IR) spectra were recorded on a Bruker
FT-IR tensor 27 spectrophotometer directly on small samples of the compounds from
200 to 4000 cmꢂ1. Electronic absorption spectra of the complexes, recorded from 200 to
1100 nm on freshly prepared CHCl3 and DMSO solutions, were measured on a Cary
Model 50 spectrophotometer. Thermogravimetric analysis (TGA) was carried out at
15ꢀC minꢂ1 heating rate using a Perkin Elmer Pyris 6 TGA up to 700ꢀC in a closed
perforated aluminum pan. The differential scanning calorimetry (DSC) analysis was
carried out at 20ꢀC minꢂ1 up to 450ꢀC using a Perkin Elmer DSC 4000 series, which was
calibrated with indium metal. Both TGA and DSC were run under N2. Melting points
were determined on a Barnstead/Electrothermal digital melting point apparatus and are

4400
A.A. Nejo et al.
left uncorrected. Magnetic susceptibility measurements were made on powdered
samples using a Sherwood Scientific magnetic susceptibility balance. Hg[Co(SCN)4]
was used as the calibrant and corrections for diamagnetism were calculated from
Pascal’s constants.
2.4. Biological studies
2.4.1. Bacterial strains. The metal complexes and ligands were individually tested
against a penal of microorganisms (Gram negative and Gram positive), namely
Enterobacter cloacae (ATCC 13047), Serratia marcescens (ATCC 9986), Acinetobacter
calcaoceuticus (Aci1), Acinetobacter calcaoceuticus anitratus (Aci2), Salmonella spp.,
Staphylococcus aureus (ATCC 6538), Bacillus pumilus (ATCC 14884), Bacillus subtilis,
Staphylococcus epidermidis, and Micrococcus kristinae. The organisms were obtained
from the Department of Microbiology, University of Fort Hare, South Africa.
2.4.2. Disc diffusion assay. Antibacterial activity of the ligands and metal complexes
were carried out using the disc diffusion method described by Bauer et al. [22]. Bacteria
were maintained on Mueller–Hinton nutrient agar at 4ꢀC. Molten Mueller–Hinton agar
was inoculated with a broth culture of the respective bacterial strains and poured over
sterile 90 mm petri dishes. Cobalt(II) acetate, metal complexes, and ligands were
dissolved in DMSO to a final concentration of 10 mg mLꢂ1 and sterile Whatman No. 1
(6 mm) discs were separately impregnated with each sample to be tested at 5.0 mg mLꢂ1
and placed on the inoculated agar. The plates were incubated at 37ꢀC for 24 h and
the zones of inhibition were measured at the end of the incubation period.
Chloramphenicol was used as the standard antibiotic for reference drug.
2.4.3. Minimum inhibitory concentration. The minimum inhibitory concentrations
(MIC) of the ligands, metal complexes, and cobalt(II) acetate that showed antibacterial
activities were determined using the 96-well micro-plate dilution method described by
Eloff [23]. The serial plate concentrations were 5.0, 2.5, 1.25, 0.625, 0.313, 0.157, 0.078,
and 0.039 mg mLꢂ1 for the compounds. Standard antibiotic (chloramphenicol) was
used as positive control. Bacteria were grown for 18 h in nutrient broth and cultures of
108 colony forming unit (cfu) mLꢂ1 were used and incubated for 24 h at 37ꢀC. As an
indicator of bacterial growth, 40 mL of 0.2 mg mLꢂ1 p-iodonitrotetrazolium (INT)
solution was added to each well and incubated at 37ꢀC for 30–120 min. The colorless
tetrazolium salt was reduced to a red-colored product by the biological activity of the
organisms, thereby making the inhibition of bacterial growth visible as a clear well.
MIC values were recorded as the lowest concentration in which there was no bacterial
growth. Each treatment was replicated thrice.
3. Results and discussion
3.1. Synthesis
The synthetic methodology followed for preparing the unsymmetrical Schiff bases was
developed in our laboratory and their formation and characterization have been

Cobalt(II) Schiff-base complexes
4401
1
published in [21]. H NMR data revealed that the unsymmetrical Schiff bases were
isolated, while microanalyses confirmed the purity of all the ligands as formulated.
The formation of the symmetrical N,N0-bis(naphthalidene)-o-phenylenediamine or
N,N0-bis(3- or 5-substituted-salicylidene)-o-phenylenediamine and the unsymmetrical
analogue could be distinguished from the microanalytical data since they have different
molecular masses.
The cobalt(II) complexes were prepared by refluxing the relevant unsymmetrical
Schiff bases with Co(CH3COO)2 ꢁ 4H2O in methanol (figure 1). All the cobalt(II)
complexes formed are brown with melting points greater than 250ꢀC. The complexes
are stable in air, insoluble in water but slightly soluble in chloroform and DMSO. The
purity of the Schiff bases and their metal complexes, as formulated, were established by
microanalyses. The analytical data, color, percentage yields, and melting points of the
compounds are presented in table 1.
3.2. IR spectra
The assignments of the significant IR spectral bands of the ligands and their metal
complexes are presented in table 2. Due to the unsymmetrical nature of the ligands and
the complexes, two bands were observed for each of the following bonds: ꢀ(C¼N),
ꢀ(C–O), ꢀ(Co–N), and ꢀ(Co–O), taking their origin from the different aldehydes [21].
The positions of ꢀ(C¼N) bands of the ligands appeared at 1610–1621 and
1567–1583 cm–1, shifted to lower frequencies at 1603–1606 and 1571–1578 cm–1,
respectively, upon complexation, indicating the involvement of both nitrogens of the
azomethine groups in coordination [24]. On the other hand, the ꢀ(C–O), which occur
at 1313–1333 and 1276–1289 cmꢂ1 for the ligands were moved to higher frequencies
by 12–30 cmꢂ1 after complexation, which indicates that the shifts are due to the
coordination of the phenolic oxygen of the ligand to the metal ion [5]. Thus, it can be
concluded that the Schiff bases are tetradentate ligands coordinating via the azomethine
N and the phenolic O.
Metal–ligand vibrational modes are very sensitive to substituent effects [25, 26].
This was proposed on the basis of isotopic labeling studies (15N- and 18O-labeling). The
substituent effects were based on the position of substitution rather than the nature
of the substituents. The ꢀ(M–O) bands are observed to exhibit higher vibrational
frequencies than ꢀ(M–N) bands for the meta-substituents, while the order is reversed
for para-substituents regardless of the nature of the substituents. It was suggested that
the transmission of the substituent effects in the Schiff-base complexes are propagated
largely by a mesomeric mechanism. The substituents in the complexes reported in this
study are located in the 3- and 5-positions on the salicylaldehyde ring and it is expected
that ꢀ(Co–O) bands would be greater than ꢀ(Co–N) bands, as shown in table 2. The
new bands observed in the complexes in the region 453–580 and 508–554 cmꢂ1 were
assigned to ꢀ(Co–N) while 463–575 and 423–428 cmꢂ1 were attributed to ꢀ(Co–O) [26].
3.3. Electronic spectra
The electronic spectral measurements were used for assigning the stereochemistries of
metal complexes based on the positions and number of d–d transition peaks. For
four-coordinate cobalt(II) complexes, either a square planar or a tetrahedral

4402
A.A. Nejo et al.
O
O
H
H
NH
2
C
C
NH
2
OH
OH
+
+
R
EtOH
R
H
H
H2L1
H2L2
5-Cl
C
N
N
C
5-NO2
R
OH
H2L3
H2L4
3-OCH2CH3
HO
H
MeOH
Co(CH3COO)2
O
O
Co
R
N
C
C
N
H
H
Figure 1. Preparation of the Schiff bases and their metal complexes.
configuration is possible. On the basis of electronic spectra, together with the magnetic
moments, it is possible to differentiate between these two configurations. The electronic
absorption spectra of complexes were recorded in 10ꢂ5 mol Lꢂ1 DMSO and chloroform
in the range of 200–1100 nm at room temperature; the results are summarized in table 3
and figure 2.
Electronic spectra of all the cobalt(II) complexes in CHCl3 are very similar and
consist of three bands, one each at 18,587–18,939, 22,272–22,779, and
24,876–25,575 cmꢂ1, which clearly indicate the low-spin square planar/distorted
square planar geometry of the complexes [27, 28]. This is also corroborated by
the observed effective magnetic moment of the complexes. The other intense bands

Cobalt(II) Schiff-base complexes
4403
Table 1. Physical properties, analytical data, and magnetic moment of the compounds.
Microanalysis (Calcd)
Empirical
formula
Formula
weight Yield (%)
m.p.
(ꢀC)
ꢁeff
%N (B.M.)
Compound
Color
%C
%H
H2L1
CoL1
H2L2
CoL2
H2L3
CoL3
H2L4
CoL4
C24H17N2O2Cl
400.58
91.5
93.2
86.2
89.5
71.9
94.7
78.4
91.8
Orange-yellow 194–195 71.60 4.73
6.60
(71.90) (4.28) (6.99)
4250 63.17 3.20 5.81
(62.95) (3.30) (6.12)
C24H15N2O2ClCo 457.5
Brown
2.61
2.59
2.23
2.36
C24H17N3O4
411.12
468.04
410.16
467.08
366.14
423.05
Orange-yellow 134–135 69.69 4.23 10.12
(70.05) (4.17) (10.22)
C24H15N3O4Co
C26H22N2O3
Brown
4250 61.97 3.10
(61.53) (3.23) (8.98)
Orange-yellow 135–136 76.34 5.51 6.79
(76.07) (5.41) (6.83)
8.63
C26H20N2O3Co
C24H18N2O2
Brown
4250 66.62 4.30 5.91
(66.80) (4.32) (6.00)
Orange-yellow 187–188 78.41 4.81
7.62
(78.66) (4.95) (7.65)
C24H16N2O2Co
Brown
4250 68.31 3.65 6.16
(68.08) (3.81) (6.62)
Table 2. Selected IR spectral bands of the compounds.
Complex
ꢀ(C¼N)
ꢀ(CꢂO)
ꢀ(MꢂN)
ꢀ(MꢂO)
CoL1
H2L1
CoL2
H2L2
CoL3
H2L3
CoL4
H2L4
1604, 1578
1611, 1583
1603, 1574
1621, 1567
1606, 1577
1610, 1569
1603, 1571
1615, 1568
1326, 1288
1313, 1276
1363, 1319
1333, 1289
1342, 1314
1316, 1283
1337, 1313
1313, 1286
453, 508
ꢂ
575, 423
ꢂ
454, 539
ꢂ
568, 428
ꢂ
475, 548
ꢂ
568, 428
ꢂ
580, 554
ꢂ
463, 428
ꢂ
Table 3. Electronic spectral data of the cobalt(II) complexes.
d–d Transitions (cmꢂ1
(" cmꢂ1 molꢂ1
)
Compound
CoL1
Solvent
CHCl3
)
C.T.
Liganda
18,939 (77), 22,523 (205)
24,876 (273)
25,126 (236)
18,587 (59), 22,272 (153)
25,575 (227)
25,063 (333)
18,868 (93), 22,523 (233)
24,876 (279)
25,063 (228)
18,832 (94), 22,779 (250)
25,189 (337)
25,445 (263)
31,949 (228)
40,323 (606)
DMSO
CHCl3
29,586 (183)
CoL2
CoL3
CoL4
31,447 (202)
40,816 (364)
36,101 (363)
40,323 (675)
DMSO
CHCl3
28,902 (270)
30,675 (274)
29,412 (238)
DMSO
CHCl3
31,546 (277)
41,494 (668)
DMSO
29,586 (215)
aThe spectra were noisy in the UV and some of the bands in the region could not be extracted.

4404
A.A. Nejo et al.
Figure 2. Electronic spectra of cobalt(II) complexes: (A) [complexes] ¼ 10ꢂ5 mol Lꢂ1 in CHCl3;
(B) [complexes] ¼ 10ꢂ5 mol Lꢂ1 in DMSO.
between 28,902 and 41,494 cmꢂ1 are due to charge-transfer transitions. The electronic
spectra of all the complexes in DMSO displayed a single d–d transition at
25,063–25,445 cmꢂ1 and a charge-transfer transition at 29,412–36,101 cmꢂ1. The
appearance of a single d–d transition in DMSO is attributed to the effect of
coordination of solvent which alters the stereochemistry to form low-spin
six-coordinate distorted octahedral [29]. The slight differences in the peaks observed
in the spectra of these complexes in both solvents are due to the substituent effects
of the different substituents on the salicylaldehyde ring.

Cobalt(II) Schiff-base complexes
4405
Table 4. Thermoanalytical results (TGA, DSC) of the cobalt(II) complexes.
Endothermic
Exothermic
b
b
Weight lossa
(Calcd) (%)
tmin (ꢀC) DHb
tmax (ꢀC) DHb
(kJgꢂ1
Decomposition
assignment
Compounds
T a (ꢀC)
(kJgꢂ1
)
)
CoL1
CoL2
CoL3
CoL4
422.0
409.8
372.3
398.3
17.7 (16.6)
13.5 (13.7)
27.8 (28.3)
19.1 (18.0)
420.2
—
253.2
340.3
0.0206
—
0.0439
0.0833
422.8
420.1
382.6
399.0
ꢂ0.1282
ꢂ0.3448
ꢂ0.0552
ꢂ0.0406
C6H4
C5H4
C6H4, HCN, C2H5
C6H4
T a: Onset decomposition temperature from TGA.
aData obtained from TGA; 15ꢀC minꢂꢂ11 under N2 gas.
bData obtained from DSC; 20ꢀC min under N2 gas.
3.4. Magnetic moment
The observed magnetic moments for cobalt(II) complexes are generally diagnostic of
the coordination geometry about the metal ion. The room temperature magnetic
moments of low-spin square-planar cobalt(II) complexes are between 1.9 and 2.9 B.M.,
arising from one unpaired electron plus an apparently large orbital contribution. Both
tetrahedral and high-spin octahedral cobalt(II) complexes possess three unpaired
electrons but may be distinguished by the magnitude of the deviation of effective
magnetic moment from the spin-only value. The octahedral and tetrahedral cobalt(II)
complexes are reported to have magnetic moments between 4.9–5.2 and 4.2–4.8 B.M.,
respectively [30]. The effective magnetic moments of all the cobalt(II) complexes
reported here lie in the range of 2.23–2.61 B.M. (table 1), corresponding to one unpaired
electron for square-planar stereochemistry around d7 cobalt(II) ion [27].
3.5. Thermal analysis
The thermal decomposition characteristics of the cobalt complexes were studied using
thermal analytical techniques (TGA and DSC) under nitrogen. The thermal behaviors
of the complexes are summarized in table 4. The results show good agreement with the
formulas suggested from the analytical data (table 1). Their thermal behavior is typified
by CoL3 (figure 3).
In the TGA, decomposition was generally via partial loss of the organic moiety. The
complexes were thermally stable up to 372.3ꢀC, which indicates that the complexes have
excellent stability. The TGA data showed that CoL1 was stable up to 422.0ꢀC. Above
this temperature, the thermogram showed a weight loss of 17.7% (Calcd 16.6%),
accompanied by an exothermic peak at 422.8ꢀC on the DSC thermogram, which
reasonably accounted for the loss of C6H4. The decomposition curve of CoL2
proceeded with single degradation step at 409.2ꢀC displaying a weight loss of 13.5%
(Calcd 13.7%) corresponding to the removal of C5H4. The DSC thermogram of the
complex exhibits an exothermic peak at 420.1ꢀC corresponding to the loss of C5H4. The
TGA thermogram of CoL3 showed a mass loss of 27.8% (Calcd 28.3%) with a
corresponding exothermic peak at 382.6ꢀC on the DSC curve, which could be attributed
to the loss of C6H4, HCN, and C2H5. The thermogram of CoL4 showed a mass loss
of 19.1% (Calcd 18.0%) at the onset decomposition temperature at 398.3ꢀC with a
corresponding exothermic peak at 399.0 on the DSC curve, which indicates the loss

4406
A.A. Nejo et al.
(a)
100
90
80
70
60
50
40
CoL3
50
150
250
350
450
550
650
Temperature (°C)
(b)
0
5
CoL3
10
15
20
25
30
35
40
45
50
0
100
200
300
400
500
Temperature (°C)
Figure 3. Thermal analysis of CoL3: (A) TGA and (B) DSC.
of C6H4. The order of thermal stability of the cobalt(II) complexes, CoL1 4
CoL2 4 CoL4 4 CoL3, showed that the complexes with electron-withdrawing substit-
uents on the salicylaldehyde were more stable than the other complexes.
The thermal behaviors, including melting temperature and enthalpy of fusion, were
determined by DSC. DSC data (table 4) showed a melting process (endothermic peaks)
at 420.2ꢀC, 253.2ꢀC, and 340.3ꢀC for CoL1, CoL3, and CoL4, respectively; CoL2
decomposed without melting. The DSC thermogram of CoL1 showed a sharp
endothermic peak and one exothermic peak in which the melting and the onset of
decomposition are partially superimposed. All complexes investigated decomposed
(exothermic peaks) with DH values ranging from as low as ꢂ0.040.6 kJ gꢂ1 for CoL4 to
as high as ꢂ0.344.8 kJ gꢂ1 for CoL2 (table 4).
3.6. Biological properties
Antibacterial activity of the ligands and complexes were tested in vitro against 10
human pathogenic bacteria. The activities were compared with that of chloramphenicol,

Cobalt(II) Schiff-base complexes
4407
a standard broad-spectrum antibiotic for bacterial strains. The effect of cobalt(II)
acetate was also reported. The compounds were tested at a concentration of 5 mg mLꢂ1
in DMSO using the paper disc diffusion method. The growth inhibition zones were
measured in diameter (mm) and the results are reproduced in table 5 and figure 4. The
susceptibility zones measured were clear zones around the discs killing the bacteria. All
the Schiff bases and their complexes exhibit varying degrees of inhibitory effects on the
growth of the tested bacterial species.
H2L1 and H2L2 were effective against all the bacteria, with inhibitory zones of 8–12
and 9–14 mm, respectively. H2L3 was effective against eight organisms, with inhibitory
zones of 8–10 mm, whereas H2L4 was effective against five organisms with inhibitory
zones of 9–10 mm. The cobalt(II) complexes of all the Schiff bases were highly effective
against all the bacterial strains with zones of inhibition of 10–24 mm. The cobalt(II)
acetate used for the synthesis of cobalt complexes was effective against six organisms,
with inhibitory zones of 8–10 mm. Consequently, the enhanced activity of the metal
complexes arises from coordination.
MIC values of the 10 bacteria strains used in this study are presented in table 6.
Antibacterial MIC less than 1 mg mLꢂ1 is considered to be an acceptable antibacterial
activity [31]. When the Schiff bases and the corresponding complexes have the same
MIC, the complexes are considered to be more active because of the lower molar
concentration of the complexes due to higher molecular mass. Both the free ligands and
cobalt(II) complexes exhibit pronounced antibacterial activities against some bacterial
strains, in a number of cases comparable or even higher with that of chloramphenicol.
The Schiff bases were biologically active. However, the metal complexes, CoL2 and
CoL3, showed enhanced antibacterial activity against more bacterial strains than their
corresponding ligands. The increased activity of the metal chelates can be explained by
Overtone’s concept [32] and Tweedy’s theory [33]. Metal complexes also disturb the
respiration process of the cell and thus block the synthesis of proteins, which
restricts further growth of the organism [32]. The enhanced activity of these cobalt(II)
complexes are in good agreement with previously published data on cobalt complexes
[34–36].
The biological activities of the complexes follow the order: CoL2 4 CoL3 4
CoL1 4 CoL4. From the above results, it seems that the substitution on the
salicylaldehyde ring enhances the biological activity of the complexes because the
complex without a substituent has the lowest antibacterial effect. The order of
activity observed in these complexes is reversely obtained for oxovanadium(IV)
complexes of this same ligand system. The insulin mimetic activity of the
oxovanadium(IV) complexes reported earlier showed that the substitution on
the salicylaldehyde lowers the insulin-enhancing activity of the complexes [21]. The
antibacterial result of the cobalt(II) acetate show activity against only Bacillus
pumilus while six of the bacteria strains were resistant to it. In general, the
antibacterial results show that the majority of the metal complexes were more active
than their respective Schiff bases. However, the Schiff bases are, in a few cases,
more active than the metal complexes. Cobalt(II) acetate shows low antibacterial
activity in comparison with the ligands and the metal complexes. Chloramphenicol
showed the lowest MIC value of 50.0098 mg mLꢂ1 against eight bacterial strains
used in this study and was more active than all the compounds tested when their
MIC values are compared.

4408
A.A. Nejo et al.

Cobalt(II) Schiff-base complexes
4409
35
30
25
20
15
10
5
E. cloacae
S. marcescens
A. calcaoceuticus
A. anitratus
Salmonella spp.
B. pumilus
B. subtilis
S. epidermidis
S. aureus
M. kristinae
0
Compounds
Figure 4. Histogram showing the comparative activities of the compounds.
Table 6. Antibacterial MIC of the compounds.
MIC (mmol mLꢂ1
)
H2L1 CoL1 H2L2 CoL2 H2L3 CoL3 H2L4 CoL4 Co(Ac)2 Chloramphenicol
Bacterial strains
10ꢂ3 10ꢂ3 10ꢂ3 10ꢂ3 10ꢂ3 10ꢂ3 10ꢂ3 10ꢂ3
10ꢂ3
10ꢂ3
E. cloacae
0.40 0.35 0.39 0.09 0.76 0.34 0.11 0.09
1.57 0.68 1.53 0.66 1.54 1.35 0.85 0.73
R
R
R
R
R
0.90
R
7.06
14.13
7.06
3.87
50.03
50.03
50.03
50.03
50.30
3.87
50.03
50.03
0.06
S. marcescens
A. calcaoceuticus
A. calcaoceuticus anitratus 1.57 1.38 1.53 0.66 0.76 0.66 0.85 1.49
Salmonella spp.
B. pumilus
B. subtilis
S. epidermidis
S. aureus
M. kristinae
3.12 0.68 3.04 0.66
R
0.66 0.85 0.73
3.12 1.38 1.53 0.66 3.05 1.35 3.69 2.95
0.20 0.70 0.39 1.35 0.10 1.35 0.22 1.49
1.57 1.38 1.53 1.35 1.54 1.35 1.72 1.49
0.40 2.73 0.78 1.35 1.54 2.86 0.87 2.95
1.57 1.38 1.53 1.35 0.78 0.69 1.72 0.76
1.57 1.38 3.04 1.35 3.29 1.35 3.41 2.95
R ¼ resistant; Ac ¼ acetate.
4. Conclusion
A series of cobalt(II) complexes involving unsymmetrical tetradentate Schiff bases
derived from 2-hydroxy-1-naphthaldehyde, phenylenediamine, and salicylaldehyde or
substituted salicylaldehyde have been synthesized and characterized. Comparison of the
IR spectra of the Schiff bases and their metal complexes indicate that the Schiff bases
are tetradentate coordinating via the azomethine N and the phenolic O. The electronic
spectra of the complexes indicate low-spin square-planar geometry, further

4410
A.A. Nejo et al.
corroborated by the observed effective magnetic moment of 2.23–2.61 B.M. The order
of thermal stability of the cobalt(II) complexes, CoL1 4 CoL2 4 CoL4 4 CoL3, shows
that the complexes with electron-withdrawing substituents on the salicylaldehyde were
more stable than the other complexes. The antibacterial (MIC) results showed that the
majority of the Co(II) complexes were more active than the Schiff bases, arising from
the coordination of cobalt(II) to the ligand.
Acknowledgment
G.A. Kolawole and A.A. Nejo acknowledge support from the National Research
Foundation (NRF), South Africa, under the IRDP funding.
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