Synthesis, characterization and in vitro biological activity of cobalt(II), copper(II) and zinc(II) Schiff base complexes derived from salicylaldehyde and D,L-selenomethionine

Article abstract of DOI:10.1002/aoc.1680

New cobalt(II), copper(II) and zinc(II) complexes of Schiff base derived from D,L-selenomethionine and salicylaldehyde were synthesized and characterized by elemental analysis, IR, electronic spectra, conductance measurements, magnetic measurements and bi

Full text of DOI:10.1002/aoc.1680

Full Paper
Received: 29 December 2009
Revised: 20 April 2010
Accepted: 25 April 2010
Published online in Wiley Online Library: 27 May 2010
(wileyonlinelibrary.com) DOI 10.1002/aoc.1680
Synthesis, characterization and in vitro
biological activity of cobalt(II), copper(II)
and zinc(II) Schiff base complexes derived
from salicylaldehyde and D,L-selenomethionine
Xueguang Rana,b, Lingyun Wanga, Derong Caoa∗, Yingcai Linb and Jie Haob
New cobalt(II), copper(II) and zinc(II) complexes of Schiff base derived from D,L-selenomethionine and salicylaldehyde
were synthesized and characterized by elemental analysis, IR, electronic spectra, conductance measurements, magnetic
measurements and biological activity. The analytical data showed that the Schiff base ligand acts as tridentate towards divalent
metal ions (cobalt, copper, zinc) via the azomethine-N, carboxylate oxygen and phenolato oxygen by a stoichiometric reaction
of M : L (1 : 1) to form metal complexes [ML(H2O)], where L is the Schiff base ligand derived from D,L-selenomethionine and
salicylaldehyde and M = Co(II), Cu(II) and Zn(II). 1H NMR spectral data of the ligand and Zn(II) complex agree with proposed
structures. The conductivity values between 12.87 and 15.63 S cm2 mol−1 in DMF imply the presence of non-electrolyte species.
c
Antibacterial and antifungal results indicate that the metal complexes are more active than the ligand. Copyright ꢀ 2010 John
Wiley & Sons, Ltd.
Keywords: Schiff base; D,L-selenomethionine; complexes; antibacterial; antifungal
Introduction
Meuillet et al. reviewed chemoprevention of prostate cancer with
seleniumtoupdatecurrentclinicaltrialsandpreclinicalfindings.[22]
Significant progress has been achieved in understanding the
chemistry and biological activity of selenomethionine in the
past decades. However, there is comparatively little literature
on the preparation of selenomethionine Schiff base and its metal
complexes.
Amino acid Schiff bases are an important class of ligands because
such ligands and their metal complexes have a variety of appli-
cations including biological, clinical, analytical and industrial in
addition to their important roles in catalysis and organic synthesis.
In particular, transition metal complexes of salicylaldehyde-amino
acid Schiff bases are non-enzymatic models for pyridoxal-amino
acid systems, which are of considerable importance as key inter-
mediates in many metabolic reactions of amino acids catalyzed
by enzymes requiring pyridoxal as a cofactor. Considerable efforts
have been devoted to the preparation and structural characteriza-
tion of Schiff base metal complexes derived from salicylaldehyde
and amino acids such as glycine, alanine, valine, threonine, ser-
ine, methionine, glutamic acid, phenylalanine, tryptophan and
tyrosine.[1–8] Most of the model studies of such metal complexes
have focused upon various binding modes of these ligands and
the X-ray crystal structures of complexes reveal that the Schiff base
ligands mainly act as tridentate moieties, coordinating through
the phenolato oxygen, imine nitrogen and carboxylate oxygen.
Moreover, a wide screening of biological activity has been car-
ried out and radioprotective,[9] antibacterial, fungistatic,[10–12]
DNA cleavable,[13] antipyretic and antidiabetic activities[14] have
been discovered and interactions with different biomolecules
described.[15–18]
Recently, we reported
a convenient synthesis of D,L-
selenomethionine.[23] As part of extensive primary biological
screening and interest in the nutritional and clinical role of se-
lenomethionine and its transition metal complexes,[24] we have
begun to investigate the synthesis, characterization and biological
activity of novel tridentate Schiff base metal complexes [ML(H2O)],
whereListheSchiffbaseligandderivedfromD,L-selenomethionine
and salicylaldehyde, M = Co(II), Cu(II) and Zn(II). The Schiff base
ligandandthreemetalcomplexeshavebeentestedin vitroagainst
a wide spectrum of bacteria and fungi. The results of biological
activity show that the metal complexes are more antibacterial and
antifungal as compared with their uncomplexed Schiff base and
appear to be potentially useful as a novel pharmacological form
for simultaneous metal and selenium supplementation.
∗
Correspondence to: Derong Cao, School of Chemistry and Chemical Engi-
neering, South China University of Technology, Guangzhou 510640, China.
E-mail: drcao@scut.edu.cn
Selenium is in the same column of the periodic table as
sulfur and may substitute for sulfur in methionine to form
selenomethionine. It is currently known as a predominant
antioxidant.[19] Selenomethionine has also been found to be a
very useful form for selenium supplementation.[20,21] Moreover,
selenomethionine has been under intense study as a promising
chemopreventive agent for different types of cancer. Recently,
a
School of Chemistry and Chemical Engineering, South China University of
Technology, Guangzhou 510640, China
b
Institute of Animal Science, Guangdong Academy of Agricultural Sciences,
Guangzhou 510640, China
c
Appl. Organometal. Chem. 2011, 25, 9–15
Copyright ꢀ 2010 John Wiley & Sons, Ltd.

X. Ran et al.
Experimental
added dropwise. The mixture was stirred for 1 h and the colored
precipitate obtained was filtered, washed with water, EtOH and
Et2Oanddriedinvacuo. Theresultingsolidwasre-crystallizedfrom
eitherDMFordimethylsulfloxide(DMSO)(yield=50–55%).Unfor-
tunately only microcrystalline powders could be obtained, which
couldnotbeusedforX-ray structuraldeterminations.ZnL(H2O):1H
NMR (400 MHz; DMSO-d6; δ, ppm): 1.93 (3H, s, -SeCH3), 1.95–2.05
(2H, m, -CH2-), 2.52 (2H, t, J = 7.6 Hz, -SeCH2-), 3.69 (1H, t,
J = 6.7 Hz, -CH-), 6.48 (1H, t, J = 7.1 Hz, -ph), 6.70 (1H, m, -ph), 7.17
(2H, m, -ph), 8.32 (1H, s, -CH N). 13C NMR (DMSO-d6, δ, ppm): 3.39
(SeCH3), 19.38 (-SeCH2), 31.08 (-CH2), 56.56 (-CH-), 117.30, 118.25,
119.21, 132.35, 133.52, 170.89 (-ph), 170.54 (-C N), 174.13 (-COO).
Reactants
All chemicals were purchased from commercial sources (Sigma-
Aldrich Co., Fluka Co.) and used as received without any further
purification. All metal(II) were used as their acetate hydrate.
D,L-Selenomethionine was synthesized according to a previous
method.[23]
Analytical and Physical Measurements
The IR spectra were recorded using a Perkin Elmer FT-
IR spectrometer Spectrum-one Model with KBr disks in the
range 4000–400 cm−1. Electronic spectra were recorded on a
Perkin–Elmer Lambda-25 UV–vis spectrometer using MeOH as
solvent. ElementanalysesofC, H, Nweredeterminedbytheservice
Elementar Vario EL. The metal contents were determined by com-
plexometrictitrationswithEDTA.[25] 1HNMRspectrawereobtained
onaBrukerAV-400andTMSwasusedasaninternalstandard.Ther-
mogravimetric analysis measurements were carried out on a TGA
2050thermogravimetricanalyzerundernitrogenatmospherewith
a heating rate of 10 ◦C/min in the 20–900 ◦C temperature range
using a platinum crucible. The magnetic measurements were car-
ried out on solid complexes using Gouy’s method on a Sherwood
Scientific Magnetic balance MSB-1 at room temperature. Dia-
magnetic corrections were estimated from Pascal’s constants and
magneticdatawerecorrectedfordiamagneticcontributionsofthe
sample holder. Conductivity measurements were made on freshly
prepared 10−3 mol/L solutions in N, N-dimethylformamide (DMF)
at room temperature with a coronation digital conductivity meter.
In Vitro Biological Activity
Antibacterial bioassay
The synthesized ligand and corresponding metal(II) complexes
were screened in vitro for their antibacterial activity against E. coli,
B. subtilis, P. vulgaris, S. aureus and E. aerogens bacterial strains
using the agar well diffusion method. Two hour-old bacterial
inocula containing approximately 104 –106 colony forming units
(CFU) ml−1 were used in these assays. The wells were dug into
the media with a sterile metallic borer with centers at least
24 mm part. The recommended concentration (100 µl) of the test
sample (1 mg ml−1 in DMSO) was introduced into the respective
wells. Other wells were supplemented with DMSO and reference
antibacterialstandarddrug(imipenum).Theplateswereincubated
at 37 ◦C for 20 h. Activity was determined by measuring the
diameter of zones showing complete inhibition (mm). In order
to clarify any participating roles of DMSO or metal(II) acetate
monohydrate in the biological screening, separate studies were
carriedoutwithDMSOorsolutionofmetal(II)acetatemonohydrate
alone, and they hardly showed activity against any bacterial and
fungal strains. The tests were carried out in triplicate.
Preparation of the Potassium N-salicylidene-
selenomethioninate (KSal-SeMet)
The potassium salt of N-salicylidene-selenomethioninate was
prepared by the following reaction: to a methanol solution of
D,L-selenomethionine (0.196 g, 1 mmol) and potassium hydroxide
(0.056 g, 1 mmol), thesolutionofsalicylaldehyde(0.122 g, 1 mmol)
in 10 ml of anhydrous methanol was added with stirring. When the
reaction began, anhydrous Na2SO4 (0.142 g, 1 mmol) was added
to remove resulting water. The resulting yellow system was stirred
at 50 ◦C for 24 h. Then the mixture was filtered and the filtrate was
reduced in vacuo using rotary evaporator. Anhydrous ether was
added to deposit the yellowish precipitate and the crude product
was re-crystallized from methanol. Yield: 73%, yellow solid. Anal.
Found: C, 42.71; H, 4.32; N, 3.95%. Calcd for C12H14NO3KSe: C,
42.60; H, 4.17; N, 4.14%. 1H NMR (400 MHz; DMSO-d6; δ, ppm;
s, singlet; d, doublet; t, triplet; m, multiplet): 1.93(3H, s, -SeCH3),
2.13–2.23 (2H, m, -CH2-), 2.51 (2H, t, J = 7.8 Hz, -SeCH2-), 3.80 (1H,
t, J = 6.8 Hz, -CH-), 6.79 (1H, t, J = 7.0 Hz, -ph), 6.53 (1H, m, -ph),
7.20 (2H, m, -ph), 8.35 (1H, s, -CH N). 13C NMR (DMSO-d6, δ, ppm):
3.47 (-SeCH3), 20.10, (-SeCH2), 31.98 (-CH2), 55.12 (-CH-), 116.26,
117.51, 118.50, 130.25, 131.45, 165.77 (-ph), 162.45 (-C N), 172.38
(-COO).
Antifungal activity
Antifungal activities of ligand and corresponding metal (II)
complexes were studied against four fungal cultures, A. flavus,
A. niger, F. solani and Cladosporium. Sabouraud dextrose agar
(Guangzhou, China) was seeded with 105 CFU ml−1 fungal spore
suspensions and transferred to Petri plates. Disks soaked in 20 ml
(10 µg ml−1 in DMSO) of all compounds were placed at different
positions on the agar surface. The plates were incubated at 37 ◦C
for 3 days. The results were recorded as zones of inhibition in
millimeters and compared with standard drug amphotericin B.
Results and Discussion
The problem of instability of the D,L-selenomethionine Schiff base
was encountered, which was resolved by making an equimolar
potassium salt of the Schiff base ligand. KSal-SeMet is a stable
compound, and was successfully prepared by refluxing an
appropriate amount of D,L-selenomethionine with salicylaldehyde
in methanol, in 1 : 1 molar ratio in the presence of KOH and
anhydrous Na2SO4. The synthesis of complexes proved to be
straightforward in a simple one-pot reaction with moderate yields.
In this text, racemic D,L-selenomethionine was used as
the starting amino acid. The potassium salt of Schiff base
ligand (KSal-SeMet) derived from salicylaldehyde and D,L-
selenomethionine as well as its metal complexes were also
Preparation of Complexes: General Procedure
To a warm solution (60–70 ◦C) of the D,L-selenomethionine
(5 mmol) in 10 ml of water, 5 mmol of salicylaldehyde in 10 ml
of ethanol was added. The resulting solution was stirred until D,L-
selenomethionine dissolved. A solution of metal(II) acetate mono-
hydrate (5 mmol) dissolved in a minimum quantity of water was
c
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Copyright ꢀ 2010 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 9–15

Schiff base complexes derived from salicyladeyde and D,L-selenomethionine
O
Se
OK
N
HC
3OH
KOH/CH
OH
O
CHO
OH
Se
+
HO
M
NH2
(O
Ac)
OH2
2.H
EtOH/H
2O
O
M
2
O
O
(M = Co, Cu, Zn)
N
O
Se
Scheme 1. Synthetic route of Schiff base metal complexes [ML(H2O)].
Table 1. Analytical and physical data of the Schiff base ligand and its complexes
Found (calculated) (%)
Compound
KSal-SeMet
Colour
Yellow
C
H
N
M
Molar conductance (S cm2 mol−1
–
)
µeff (µB)
42.71
(42.60)
38.64
4.32
(4.17)
4.24
3.95
(4.14)
3.59
–
–
–
C
12H14NO3SeK
[CoL(H2O)]
12H15NO4SeCo
[CuL(H2O)]
12H15NO4SeCu
Brown
Green
White
15.52
15.63
13.86
12.87
4.05
1.97
Dia.
C
(38.42)
37.89
(4.03)
3.80
(3.73)
3.84
(15.71)
17.03
C
(37.95)
37.47
(3.98)
4.32
(3.69)
3.74
(16.73)
17.04
[ZnL(H2O)]
C12H15NO4SeZn
(37.77)
(3.96)
(3.67)
(17.14)
racemic mixtures. Similar behavior has been reported by
Amendola et al. during the synthesis of the racemic form
of Schiff base of rac5 (trans-N,Nꢁ-bis[1-(8-benzyloxyquinolin-
2-yl)methylidene]cycloexane-1,2-diamine.[26] In that case, the
racemic form of Schiff base was obtained from Schiff base con-
densation of the racemic trans-1,2-cyclohexanediamine with the
pertinent aldehyde and was therefore a mixture of the two enan-
tiomeric R,R and S,S forms. Masood et al. reported that the racemic
mixture of the analogous ligand, obtained from Schiff base con-
densation of trans-1,2-cyclohexanediamine with the pertinent
2-pyridinealdehyde, reacts with copper(I) ions to give only a
racemic mixture of the homochiral double helicate species.[27]
ThecomplexesweresolubleinDMFandDMSO,poorlysolublein
MeOH and insoluble in some common organic solvents. Attempts
to obtain single crystal suitable for X-ray determination were
unsuccessful. The structures of the synthesized ligand and metal
complexes(Scheme 1)wereestablishedwiththehelpofelemental
analyses data, IR and NMR spectra.
As shown in Table 1, the elemental analyses results obtained
are in good agreement with those calculated for the suggested
formulae of ligand and metal complexes. The analytical data show
that the metal to ligand ratio is 1 : 1 in all the complexes. The
molar conductance values in DMF (Table 1) for the complexes
were found to be in the range 12.87–15.63 S cm2 mol−1. The
relatively low values indicate the non-electrolytic nature of these
complexes.[28] This can be accounted for by the satisfaction of the
bivalency of metal by the carboxylate group and deprotonated
phenolato oxygen.
IR Spectra
In the absence of a powerful technique such as X-ray crystal-
lography, IR spectroscopy has proven to be suitable technique to
elucidatethemethodofbondingoftheligandtothemetalion.The
determination of the coordinating atoms is made on the basis of
the comparison of the IR spectra of the ligand and the complexes,
as shown in Fig. 1. The significant data are given in Table 2. The IR
spectrum of the ligand shows the absence of bands at 3245 and
1745 cm−1 due to the ν(NH2) group of D,L-selenomethionine and
ν(HC O) group of salicylaldehyde. Instead, a new strong band
at 1645 cm−1 due to azomethine stretching vibration ν(C N)
appears in the ligand, indicating that condensation between alde-
hyde of salicylaldehyde and amino group of D,L-selenomethionine
has taken place, resulting in the formation of the desired Schiff
base ligand. On complexation, ν(C N) shifts to lower frequency
by 5–25 cm−1 and a new band in the 1620–1640 cm−1 range,
indicating the coordination of the azomethine nitrogen atom to
the central metal ion. Since no data is found in the literature for
D,L-selenomethionine Schiff base metal complexes for the IR spec-
tral region, it was found in this study that new bands at ν = 538,
545, 567 cm−1 can be assigned to Co–N, Cu–N and Zn–N bonds,
respectively. Similar assignments are reported in the literature for
the Co–N, Cu–N and Zn–N bonds at ν = 537, 584 and 580 cm−1
of Co(II), Cu(II) and Zn(II) complexes of the Schiff base derived
from vanillin and D,L-α-aminobutyric acid.[29] Further confirmation
is shown in the literature,[30,31] which shows stretching bands at
c
Appl. Organometal. Chem. 2011, 25, 9–15
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X. Ran et al.
CoL(H2O)
ZnL(H2O)
CuL(H2O)
KSal-SeMet
4000 3600 3200 2800 2400 2000 1600 1200
Wavenumber (cm-1)
800
400
Figure 1. FT-IR spectra of KSal-SeMet and three complexes.
of bending vibration modes of the water molecules; δ(H2O), found
in the range 992–960 cm−1. The other bending vibration of the
water molecules, δ(H2O), is usually around 1600 cm−1, which
always interferes with the skeleton vibration of the benzene ring
(C Cvibration).Theseobservationsindicatethatawatermolecule
occupies the fourth position. Participation of the phenolic oxygen,
carboxylato-oxygen atoms, and phenolic and water molecules is
also confirmed by the appearance of new bands in the spectra
of the complexes in the 438–464 cm−1 regions, which may be
assigned to the ν(M–O) stretching vibration.[36]
Table 2. Infrared spectral data of Schiff base ligand and its complexes
(cm−1
)
ν
ν
νasym
νsym
ν
ν
Compound (C N) (O–H) (COO−) (COO−) (M–N) (M–O)
KSal-SeMet
[CoL(H2O)]
[CuL(H2O)]
[ZnL(H2O)]
1645
1620
1640
1628
3459
3408
3448
3432
1589
1583
1583
1575
1331
1348
1339
1375
–
–
538
545
567
464
457
438
Since Schiff base derived from D,L-selenomethionine and
salicylaldehyde is a four-coordination point ligand, i.e. phenolic
oxygen, azomethine nitrogen, carboxylato oxygen and methyl
seleniumgroups,anypossibleparticipationofthemethylselenium
group in the coordination of Schiff base ligand with Co2+, Cu2+
and Zn2+ was also investigated. Only one weak band at 574 cm−1
assigned to a C–Se stretching vibration band was detected in the
IR spectrum of the ligand. This is based on comparison with the IR
spectral data of selenomethionine,[37] where 577 cm−1 is reported
forstretchingbandsoftheSe–MegroupintheRamanspectrum of
solid selenomethionine. The same band appears in the IR spectra
of the Schiff base metal complexes in the 571–574 cm−1 range,
which shows that no coordination with metal(II) occurred with
methyl selenium group. The spectra of ligand and complexes
appeared in the same region at 1268–1275 cm−1 corresponding
to bending vibration of the -SeCH3 group,[38] further suggesting
that no coordination of -SeCH3 with metal occurred.
500–581 cm−1 in the spectra of the complexes corresponding to
M–N vibration bands.
The participation of the phenolic group is deduced by clarifying
the effect of chelation on the ν(C–O) stretching vibration. The
shift in ν(C–O) of the phenolic group from 1336 cm−1 in the
free ligand to 1282–1335 cm−1 in the complexes indicates
the participation of the phenolic group in complex formation.
In contrast to other vibrations, the positions of symmetric
carboxyl stretching νs(COO−) and asymmetric carboxyl stretching
νas(COO−) are distinct, and the difference between two values
indicates the scale of electron delocalization of carboxyl group
and thus the possible bridging function of the carboxyl group is
evident. As compared with the values of νas(COO−) and νs(COO−)
assigned to 1589 and 1331 cm−1 in the ligand, respectively, the
νas(COO−) is shifted to a lower frequency in the 1575–1583 cm−1
range and the νs(COO−) is shifted to a higher frequency in the
1339–1375 cm−1 range, indicating the linkage between the metal
ion and carboxylato oxygen atom.[32] The differences between
νas(COO−) and νs(COO−) for the Co(II), Cu(II) and Zn(II) complexes
in the present study are 235, 244 and 200 cm−1, respectively.
From the IR results, it may be concluded that the ligand is
tridentate (Scheme 1) and coordinates with the metal ion through
the phenolic oxygen, azomethine nitrogen and carboxylato
oxygen atoms.
This value compares favorably with the values of 196–266 cm−1
,
1H NMR Spectra
characteristicforthemonodentatecoordinationofthecarboxylato
group.[33–35]
The spectra of the complexes show broad bands in the
3408–3448 cm−1 range, attributed to the stretching vibration
of the O–H group of water molecules. The presence of water
molecules in the complexes is also ascertained by the appearance
The 1H NMR spectra of ligand and Zn(II) complex were recorded
in DMSO-d6. All the protons were found to be in their expected
regions and numbers of protons calculated from the integration
curves agreed with those obtained from the values of the C, H,
N element analyses. In the spectra of diamagnetic Zn(II) complex,
c
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Schiff base complexes derived from salicyladeyde and D,L-selenomethionine
Figure 2. UV–vis spectra of KSal-SeMet and three complexes.
which is assigned to 4A2 (F) → 4T1(F) transition. This prefigures
Table 3. Electronic spectral data (nm) of the ligand and its complexes
tetrahedral geometry for the Co(II) complex. The electronic
spectrum of Cu(II) complex exhibits a broad band centered at
675 nm, indicating square planar geometry. In general, due to
Jahn–Teller distortion, square planar Cu(II) complex displays a
broad absorption band between 600 and 700 nm and the peak at
Compound
σ → σ∗
π → π∗
n → π∗
d–d
KSal-SeMet
[CoL(H2O)]
[CuL(H2O)]
[ZnL(H2O)]
200, 220
201, 225
202, 224
214, 231
264
270
271
267
357
352
356
352
–
550
675
–
2
510 nm merges with the broad band, which is due to 2B1g → Eg
2
and B1g
→
2A1g with the respective absorption bands.[40]
However, the zinc complex only gives a high-intensity band at
352 nm due to absence of d–d transition, which is assigned to a
ligand-metal charge transfer besides the characteristic ligand.[41]
The proposed structures of metal(II) complexes are shown in Fig. 3.
signals shifted upfield, as compared with that the ligand. The
-CH N- proton of ligand resonated as a sharp singlet at 8.35 ppm.
On complexation, this signal was shifted to 8.32 ppm. The phenyl
protons (7.20, 6.79 and 6.53 ppm in the ligand) were recorded at
7.17, 6.70 and 6.48 ppm in the complex. The significant shift of
the -CH- proton from 3.80 ppm in the free ligand to 3.69 ppm in
the complex was observed. The shifts were caused by methylene
protons from 2.13–2.23 ppm in the ligand to 1.95–2.05 ppm in
the complex. Comparatively, methylene selenium at 2.51 ppm
and methyl protons at 1.93 ppm were practically unaffected by
complexation. The conclusions drawn from these studies lend
further support to the mode of bonding discussed in IR spectra.
Magnetic Susceptibility Measurements
The magnetic moments of the complexes determined at room
temperature are given in Table 1. The Co(II) complex has a
magnetic moment value of 4.05 µB, which compares favorably
with that of 4.1–4.8 µB expected for the Co(II) complex with
three unpaired electrons.[42,43] As a result, in agreement with the
electronic spectral studies, Co(II) complex may have tetrahedral
geometry. The magnetic moment of Cu(II) complex is 1.97 µB,
indicating the presence of one unpaired electron.[44–46] This is
consistent with the electronic spectral result that square planar
geometry for the Cu(II) complex contains one unpaired electron
and the µeff value would be in the range 1.8–2.1 µB. The zinc (II)
complex is found to be diamagnetic,[47] as expected.
UV–vis Spectral Analysis
As shown in Fig. 2, UV–vis spectra of ligand and complexes
measuredintherangeof200–825 nmdisplaytwodistinctregions.
Thelowerwavelengthintherangeof200–400 nmisspecificforthe
electronic intra-ligand transitions. The higher wavelength region
is specific for d–d transition. The observed absorption bands and
their assignments are shown in Table 3.
The spectra of complexes generally show the characteristic
bands of the free ligand with some changes both in wavelengths
(λmax) and intensity together with appearances of new bands
at longer wavelengths. The spectra of the KSal-SeMet and the
complexes exhibit bands in the regions of 200–231, 264–271
and 352–357 nm, which may be due to the transition of σ → σ∗,
π → π∗ or n → π∗, respectively.[39] In addition, the Co(II) complex
shows only one absorption band in the visible region at 550 nm,
Thermal Analyses
The Thermogravimetry (TG) measurements of all the complexes
were performed in nitrogen over the temperature range of
20–900 ◦C.Ingeneral,allcomplexeswerestableupto90 ◦C.Above
thistemperature,thesampleweightdecreasedupto164 ◦C,which
is probably connected with the elimination of the coordinated
water molecule, leading to the formation of intermediate ML. The
intermediate was stable within the interval of 175–248 ◦C. The
next decay proceeded in two steps without formation of thermally
stable intermediates up 800 ◦C. Afterwards, a plateau could be
c
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X. Ran et al.
OH2
O
OH2
O
OH2
Cu
O
O
O
Co
Zn
O
N
N
N
O
O
O
Se
Se
Se
(c)
(a)
(b)
Figure 3. Proposed structures of (a) [CoL(H2O)]; (b) [CuL(H2O)]; (c) [ZnL(H2O)].
Table 4. Thermal analytical data for the complexes
Decomposition temperature
Solid residue Mass
found (%, calcd)
Molecular formula
[CoL(H2O)]
Molecular weight
375.1
(◦C)
Mass loss found (%, calcd)
Eliminated species
80–172
248–615
78–175
4.98 (4.81)
58.18 (58.44)
4.71 (4.74)
H2O
CoSe
36.84 (36.76)
CuSe
C12H15NO4SeCo
C12H13NO3
H2O
[CuL(H2O)]
379.8
381.6
C12H15NO4SeCu
[ZnL(H2O)]
250–650
80–174
58.05 (57.74)
5.01 (4.72)
C12H13NO3
H2O
37.24 (37.52)
ZnSe
C12H15NO4SeZn
250–625
57.75 (57.45)
C12H13NO3
36.24 (37.83)
seen above 800 ◦C and the weight of the residue in the range of
36.24–37.24% was found to be in consistent with the formation of
MSe as a final thermal decomposition product. The detailed data
are shown in Table 4.
Table 5. Results of antibacterial bioassay (concentration used in
1 mg ml−1 of DMSO)
Compound (zone of inhibition in mm)
Bacteria
K Sal-SeMet [CoL(H2O)] [CuL(H2O)] [ZnL(H2O)] SD
Antibacterial and Antifungal Bioassay
E. coli
8
7
17
19
18
18
16
21
23
22
21
20
14
15
17
14
18
26
25
24
25
22
The ligand and its metal complexes were screened for their
antibacterial and antifungal activities according to the respective
literature protocol[48] and the results obtained are presented
in Tables 5 and 6. The results were compared with those of
the standard drug. All the metal complexes were more potent
bactericides and fungicides than the ligand. Co(II) and Zn(II)
complexes were much less microbially active than the Cu(II)
complex. From Table 5, it can be seen that the highest inhibition of
growthoccurredonCu(II)complexagainstthebacterium B.subtilis
(23 mm). Ontheotherhand, Cu(II)complexshowdthebestactivity
towards fungi against A. niger (26 mm) and the lowest against
Cladosporium (15 mm), as shown in Table 6. There was a marked
increase in the bacterial and fungi activities of the Cu(II) complex
as compared with the free ligand and other complexes under
test, which is in agreement with the antifungal and antibacterial
properties of a range of Cu(II) complexes evaluated against several
pathogenic fungi and bacteria.[49] For many years it was believed
that a trace of Cu(II) destroys the microbe; however, a more recent
mechanism is that activated oxygen in the surface of metal Cu kills
the microbe because Cu(II) activity is weak.
This enhancement of metal complexes in the activity can be
explained on the basis of chelation theory.[50] Chelation reduces
the polarity of the metal atom mainly because of partial sharing of
its positive charge with the donor groups and possible π electron
delocalization within the whole chelate ring. Such a chelation
also enhances the lipophililic character of the central metal atom,
which subsequently favors its permeation through the lipid layers
of cell membrane and the blocking of the metal binding sites on
enzymes of microorganism. The variation in the effectiveness of
different compound against different organisms depends either
on the impermeability of the cell of the microbes or differences in
the ribosomes of microbial cells.
B. subtillis
S. aureus
P. vulgaris
E. aerogens
9
12
15
SD = imipenum. Ligand: >15 mm = significant activity; 7–14 mm =
moderate activity; <7 mm = weak activity.
Table 6. Results of antifungal bioassay (concentration used
200 µg ml−1
)
Compound (zone of inhibition in mm)
Organism
K Sal-SeMet [CoL(H2O)] [CuL(H2O)] [ZnL(H2O)] SD
A. flavus
A. niger
F. Solani
7
12
11
13
18
15
17
14
24
26
25
15
17
19
17
16
29
28
27
29
Cladosporium
SD = amphotericin B. Ligand: >15 mm = significant activity; 7–14 mm
= moderate activity; <7 mm = weak activity.
Conclusion
Three complexes, Co(II), Cu(II) and Zn(II), with a tridentate
O,N,O-donor Schiff base derived from salicylaldehyde and D,L-
selenomethionine were synthesized and characterized, and their
biological activity was evaluated by antibacterial and antifungal
bioassay. The results demonstrate that Co(II) complex probably
have tetrahedral geometry, while the Cu(II) complex probably
c
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Copyright ꢀ 2010 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 9–15

Schiff base complexes derived from salicyladeyde and D,L-selenomethionine
has square planar geometry. The non-electrolytic nature of the
complexes is shown by the low values of the molar conductance
of the complexes. The elemental analysis suggests that all
metal complexes possess a coordinated water molecule, which
is further evidenced by IR spectra and thermal analysis. The results
of antimicrobial and antifungal activities show that the metal
complexes are more active than the ligand; furthermore, Cu(II)
complex is more active than Zn(II) and Co(II) complexes.
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