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Syntheses characterization and biological studies of zinc(II) copper(II) and cobalt(II) complexes with Schiff base ligand derived from 2-hydroxy-1-naphthaldehyde and selenomethionine.

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Full Paper
Received: 6 March 2010
Revised: 21 April 2010
Accepted: 22 April 2010
Published online in Wiley Online Library: 28 June 2010
(wileyonlinelibrary.com) DOI 10.1002/aoc.1678
Syntheses, characterization and biological
studies of zinc(II), copper(II) and cobalt(II)
complexes with Schiff base ligand derived
from 2-hydroxy-1-naphthaldehyde
and selenomethionine
Xueguang Rana,b, Lingyun Wanga , Yingcai Linb , Jie Haob and Derong Caoa∗
Novel zinc(II), copper(II), and cobalt(II) complexes of the Schiff base derived from 2-hydroxy-1-naphthaldehyde and D, Lselenomethionine were synthesized and characterized by elemental analysis, IR, electronic spectra, conductance measurements,
magnetic measurements and powder XRD. The analytical data showed the composition of the metal complex to be ML(H2 O),
where L is the Schiff base ligand and M = Co(II), Cu(II) and Zn(II). IR results confirmed the tridentate binding of the Schiff base
ligand involving azomethine nitrogen, naphthol oxygen and carboxylato oxygen atoms. 1 H NMR spectral data of lithium salt
of the Schiff base ligand [Li(HL)] and ZnL(H2 O) agreed with the proposed structures. The conductivity values of complexes
between 12.50 and 15.45 S cm2 mol−1 in DMF suggested the presence of non-electrolyte species. The powder XRD studies
indicated that Co(II) complex is amorphous, whereas Cu(II) and Zn(II) complexes are crystalline. The results of antibacterial and
antifungal screening studies indicated that Li(HL) and its metal complexes are active, but CuL(H2 O) is most active among them.
c 2010 John Wiley & Sons, Ltd.
Copyright Keywords: Schiff base; D, L-selenomethionine; complexes; antibacterial; antifungal
Introduction
Appl. Organometal. Chem. 2010, 24, 741–747
Experimental
Reactants
All the chemicals and solvents were purchased from commercial sources and used as received without any further purifi-
∗
Correspondence to: Derong Cao, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China.
E-mail: drcao@scut.edu.cn
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, China, 510640
c 2010 John Wiley & Sons, Ltd.
Copyright 741
Amino acid Schiff bases derived from o-hydroxyaromatic aldehydes and amino acids as well as their metal complexes receive
much attention for their interesting and important properties. For
example, amino acid Schiff bases exhibit a wide range of biological
activities and applications as markers in the study of biological
processes and pharmacology, e.g. antimicrobial, antifungal, antitumor, decarboxylation, transamination, electron transfer, complexing ability towards some toxic metals.[1 – 6] Furthermore, metal
complexes of amino acid Schiff bases have promising applications
in analytical, catalysis and organic synthesis.[7 – 9]
Schiff base ligands with sulfur and nitrogen donor atoms
in their structures act as good chelating agents for metal
ions.[10,11] Coordination of such compounds with metal ions,
such as copper, nickel and iron, often enhance their biological
activities,[12] as reported for pathogenic fungi.[13] Selenium is
in the same column of the periodic table as sulfur and may
substitute for sulfur in methionine to form selenomethionine.
Selenomethionine has been prepared for a long time for a variety
of biological activities such as predominant antioxidants, selenium
supplementation, chemopreventive agents for different types of
cancer and many other deseases.[14 – 17] Significant progress has
been achieved in understanding the chemistry and biological
activity of selenomethionine in the past decades. However, reports
on the preparation of selenomethionine Schiff base and its metal
complexes are relatively scarce.
Recently, we reported a convenient synthesis of D, L-selenomethionine.[18] As part of extensive primary biological screening
and interest in the medicinal and pharmaceutical role of selenomethionine and its transition metal complexes,[19] herein we
report a synthesis, characterization and biological screening study
of novel lithium salt of the Schiff base ligand [Li(HL)] and its metal
complexes ML(H2 O), where M = Co(II), Cu(II) and Zn(II), and L is the
Schiff base ligand derived from 2-hydroxy-1-naphthaldehyde and
D, L-selenomethionine for the first time. The biological activity of
the lithium salt of the Schiff base ligand and its metal complexes is
tested in vitro against a wide spectrum of bacteria and fungi. The
biological activity results show that selenomethionine Schiff base
metal complexes might be potentially useful in the treatment of
some diseases.
X. Ran et al.
cation. All metals(II) were used as their acetate hydrates. D,
was synthesized according to a previous
method.[18]
L-Selenomethionine
Analytical and Physical Measurements
The IR spectra were recorded using a Perkin Elmer FT-IR
spectrophotometer 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 spectrophotometer using MeOH
as solvent. Element analyses of C, H, N were determined by
the service Elementar Vario EL. Metal contents were estimated
using standard methods.[20] 1 H NMR spectra were obtained on
a Bruker AV–400 and TMS was used as an internal standard.
Thermogravimetric analysis measurements were carried out
on a TGA 2050 thermogravimetric analyzer, under nitrogen
atmosphere with a heating rate of 10 ◦ C/min in the 20–900 ◦ C
temperature range using a platinum crucible. The magnetic
measurements were carried out on solid complexes using Gouy’s
method on a Sherwood Scientific Magnetic balance MSB-1 at
room temperature; diamagnetic corrections were estimated from
Pascal’s constants; magnetic data were corrected for diamagnetic
contributions of the sample holder. Conductivity measurements
were made on freshly prepared 10−3 mol/L solutions in DMF at
room temperature with a coronation digital conductivity meter.
Powder X-ray diffraction experiments were carried out on a D8
Advance diffractometer with Cu Kα radiation
Preparation of Li(HL)
D, L-Selenomethionine (0.196 g, 1 mmol) in 10 ml of absolute
methanol containing LiOH (0.024 g, 1 mmol) was added to a
solution of 2-hydroxy-1-naphthaldehyde (0.172 g, 1 mmol) in
15 ml of absolute methanol. When the reaction began, anhydrous
Na2 SO4 (0.142 g, 1 mmol) was added to remove the resulting
water. The yellow mixture was stirred for 3 h under reflux. 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 to give Li(HL) as a yellow solid in 68% yield. Anal.
found: C, 53.66; H, 4.94; N, 3.89%. Calcd for C16 H16 NO3 LiSe: C,
53.95; H, 4.53; N, 3.93%. 1 HNMR (DMSO-d6 , δ, ppm): 1.93 (3H, s,
-SeCH3 ), 2.15–2.23 (2H, m, -CH2 -), 2.52–2.55 (2H, m, -SeCH2 -), 4.03
(1H, m, -CH-), 6.62 (1H, d, -ArH), 7.12 (1H, t, -ArH), 7.37 (1H, t,
ArH), 7.56 (1H, d, ArH), 7.65 (H, d, ArH), 7.99 (1H, d, ArH), 8.90
(1H, s, -CH N), 13.63 (1 H, s, -OH). 13 C NMR (DMSO-d6 , δ, ppm):
3.43 (-SeCH3 ), 20.63 (-SeCH2 ), 35.18 (-CH2 ), 65.33 (-CH-), 105.13,
118.05, 121.48, 124.66, 126.78, 127.66, 128.71, 134.92, 136.88,
170.52 (-Ar-C), 156.04 (-C N), 179.11 (-COO).
General Procedure for the Preparation of Complexes
by One-pot Synthesis
742
To a warm solution (60–70 ◦ C) of D, L-selenomethionine (0.98 g,
5 mmol) in 10 ml of water, 2-hydroxy-1-naphthaldehyde (0.86 g,
5 mmol) in 10 ml of methanol was added. The resulting solution
was stirred until D, L-selenomethionine completely dissolved. A
solution of metal(II) acetate monohydrate (5 mmol) dissolved in
a minimum quantity of water was added dropwise. The mixture
was stirred for 1 h and the colored precipitate was filtered, washed
with water, ethanol and ether, and dried in vacuo. The resulting
solid was re-crystallized from DMSO (yield = 55–58%). ZnL(H2 O):
wileyonlinelibrary.com/journal/aoc
1 H NMR (DMSO-d , δ, ppm): 1.93 (3H, s, -SeCH ), 2.01–2.15 (2H,
6
3
m, -CH2 -), 2.52–2.56 (2H, m, -SeCH2 -), 3.84 (1H, t, J = 7.2 Hz, -CH-),
6.85 (1H, d, J = 9.0 Hz, -ArH), 7.14 (1H, t, J = 7.2 Hz -ArH), 7.39
(1H, m, -ArH), 7.65 (2H, m, -ArH), 8.06 (1H, d, J = 8.4 Hz, -ArH), 9.20
(1H, s, -CH N). 13 C NMR (DMSO-d6 , δ, ppm): 3.43 (-SeCH3 ), 20.34
(-SeCH2 ), 36.07 (-CH2 ), 68.87 (-CH-), 107.82, 118.67, 120.94, 125.13,
126.77, 127.08, 128.56, 134.15, 135.22 (-Ar–C), 162.31 (-C N),
171.55 (-COO).
In Vitro Biological Screening Study
In order to clarify any participating roles of DMSO in the biological
screening, separate study was carried out with DMSO alone, and it
showed hardly any activity against any bacterial and fungal strains.
The tests were carried out in triplicate.
Antibacterial Bioassay
The Li(HL) and Schiff bases metal complexes were screened for
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 (2 mg ml−1 in DMSO) was
introduced into the respective wells. Other wells supplemented
with reference antibacterial standard drug (imipenum). The plates
were incubated at 37 ◦ C for 20 h. Activity was determined by
measuring the diameter of zones showing complete inhibition
(mm).
Antifungal Activity
Antifungal activities of the Li(HL) and Schiff bases metal 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 (2 mg 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 mm 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
lithium salt of the Schiff base ligand. The Li(HL) is a stable
compound prepared by refluxing an appropriate amount of
D, L-selenomethionine with 2-hydroxy-1-naphthaldehyde in 1 : 1
molar ratio in methanol, in the presence of LiOH and anhydrous
Na2 SO4 . The synthesis of complexes is straightforward in simple
by one-pot reactions. The complexes are soluble in DMF and
DMSO, poorly soluble in MeOH and insoluble in some common
organic solvents such as ethanol, acetone, chloroform and water.
Attempts to obtain single crystals suitable for X-ray determination
were unsuccessful. The Li(HL) and metal complexes were subjected
to elemental analyses, IR and 1 H NMR, UV–vis, magnetic studies,
molar conductance and thermal analyses to identify their tentative
formulae in a trial to elucidate their molecular structures. The
biological activity of the Li(HL) and Schiff bases metal complexes
were studied against antibacterial and antifungi organisms.
c 2010 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2010, 24, 741–747
Zinc(II), copper(II) and cobalt(II) complexes with Schiff base ligand
Table 1. Analytical and physical data of Li(HL) and Schiff base metal complexes
Elemental analysis, found (calcd)
Compound
Color
C
H
N
M
Li(HL)
C16 H16 NO3 SeLi
[CoL(H2 O)]
C16 H17 NO4 SeCo
[CuL(H2 O)]
C16 H17 NO4 SeCu
[ZnL(H2 O)]
C16 H17 NO4 SeZn
Yellow
53.66
(53.95)
44.83
(45.20)
44.39
(44.71)
44.48
(44.52)
4.94
(4.53)
4.24
(4.03)
4.28
(3.99)
4.38
(3.97)
3.89
(3.93)
3.58
(3.29)
3.35
(3.26)
3.32
(3.28)
–
–
13.46
(13.86)
14.52
(14.78)
14.95
(15.15)
Brown
Green
Yellow
Molar conductance
(S cm2 mol−1 )
µeff (µB )
–
–
15.45
4.15
13.78
1.89
12.50
Dia.
Table 2. Infrared spectral data of Li(HL) and Schiff base metal complexes (cm−1 )
Compound
ν(C N)
ν(O–H)
νasym (COO− )
νsym (COO− )
ν (H2 O)
ν(M–N)
ν(M–O)
Li(HL)
[CoL(H2 O)]
[CuL(H2 O)]
[ZnL(H2 O)]
1630
1623
1622
1620
3429
–
–
–
1589
1583
1583
1584
1376
1366
1360
1369
3423
3230
3306
–
562
545
567
–
454
457
460
Elemental Analysis and Molar Conductance Measurements
As shown in Table 1, the elemental analyses results are in
good agreement with those calculated for the suggested
formulae of Li(HL) and metal complexes. All the complexes
have 1 : 1 metal–ligand ratios. The molar conductance values
in DMF for the complexes were found to be in the range
12.50–15.45 S cm2 mol−1 . The relatively low values indicate
the non-electrolytic nature of these complexes and they are
considered as non-electrolytes.[21]
IR Spectra
Appl. Organometal. Chem. 2010, 24, 741–747
1 H NMR Spectra
The 1 H NMR spectrum of the diamagnetic Zn(II) complex is examined in comparison with that of Li(HL). The azomethine proton of
Li(HL) resonated as a sharp singlet at 8.90 ppm. On complexation,
c 2010 John Wiley & Sons, Ltd.
Copyright wileyonlinelibrary.com/journal/aoc
743
In the absence of a powerful technique such as X-ray crystallography, IR spectroscopy is a suitable technique to provide enough
information to elucidate the method of bonding of the ligand to
the metal ion. The determination of the coordinating atoms was
made on the basis of the comparison of the IR spectra of Li(HL)
and the complexes. The significant IR spectral data are given
in Table 2. A strong band at 1630 cm−1 indicated the presence
of C N stretching band in Li(HL). On complexation, ν(C N)
was shifted to 1620–1623 cm−1 , indicating the coordination of
the azomethine nitrogen atom to the central metal ion.[22] The
coordination of nitrogen to the metal ion would be expected to
reduce the electron density of the azomethine link and thus cause
a shift in the ν(C N) group.[23]
The mode of coordination of carboxylate group has often been
deduced from the magnitude of the observed separation between
the νas (COO− ) and νs (COO− ). The differences between νas (COO− )
and νs (COO− ) for the Co(II), Cu(II) and Zn(II) complexes in the
present study are 217, 223 and 215 cm−1 , respectively. This value
compares favorably with that of 196–266 cm−1 , characteristic for
the monodentate coordination of the carboxylato group.[24 – 26]
In this study, Li(HL) displays two bands at 1589 and 1376 cm−1
ascribed to νas (COO− ) and νs (COO− ), which shifts to lower frequencies in the spectra of metal complexes in the regions 1584–1583
and 1360–1366 cm−1 , respectively. This suggests the coordination
of metal ions through the carboxylate oxygen atom of the ligand.
A shift in ν(C–O) of naphthol group at 1336 cm−1 for Li(HL) to
lower frequencies in the region 1282–1335 cm−1 for complexes
was observed, indicating the participation of the oxygen atom
of phenolic group in coordination to the metal ion. Moreover,
according to properties of non-electrolytes and 1 : 1 metal–ligand
ratios of all complexes, it can be inferred that deprotonated
naphthol oxygen coordinates to metal ions due to the satisfaction
of the bivalency of the metal ions.
The spectra of the complexes show broad bands in the
3306–3423 cm−1 range, attributed to OH stretching vibration of
water molecules. The appearance of bending vibration modes of
the water molecules in the range 992–960 cm−1 further suggests
the presence of the lattice water. These observations indicate
that a water molecule occupies in the fourth position, which is
supported by thermal analysis study discussed later.
The Li(HL) displays a weak band at 574 cm−1 ascribed to stretching band of -SeCH3 ,[27] which shows no shift in the spectra of metal
complexes, suggesting no coordination of metal(II) ions with
methyl selenium group. Moreover, the spectra of Li(HL) and complexes showed bands in the same region at 1268–1275 cm−1 corresponding to bending vibration of the -SeCH3 group,[28] further
suggesting that no coordination of -SeCH3 with metal occurred.
Conclusive evidence of the bonding is also shown by the
observation that new bands in the spectra of all metal complexes appear in the low frequency regions at 454–460 and
562–567 cm−1 characteristic to ν(M–O) and ν(M–N) stretching
vibrations, respectively, that are not observed in the spectrum
of Li(HL). From the IR results, it may be concluded that Li(HL) is
tridentate (Scheme 1) and coordinates with the metal ion through
the deprotonated phenolic oxygen, azomethine nitrogen and
carboxylato oxygen atoms.
X. Ran et al.
O
Se
OLi
N
LiOH
CHO
O
OH
OH
/ CH 3
OH
Se
HO
NH2
Li(HL)
+
M(O
Me
Ac)
OH
2 .H
2O
/H
2O
Se
N
O
C
C
O
M
O
O
H
H
[ML(H2O)]
(M = Co, Cu, Zn)
Scheme 1. Synthetic route of Li(HL) and Schiff base metal complexes.
Figure 1. Electronic spectra of Li(HL) and Schiff base metal complexes.
this signal was shifted to 9.20 ppm, indicating the involvement
of -CH N group in chelation. The signals assigned to naphthyl
protons were observed between 6.62–7.99 ppm in the 1 H NMR
spectrum of Li(HL). These signals were found between 6.85 and
8.06 ppm in the 1 H NMR of Zn(II) complex. The upfield shift of the
methylidyne proton in Zn(II) complex at 3.84 ppm, which appeared
at 4.03 ppm Li(HL), supports the involvement of azomethine and
carboxylato groups in coordination with the metal ions. The shifts
were caused by methylene protons from 2.15–2.23 ppm for Li(HL)
to 2.01–2.15 ppm for Zn(II) complex. Comparatively, protons of
methylene selenium at 2.51–2.56 ppm and methyl at 1.93 ppm
were practically unaffected by complexation, indicating no coordination of selenium atom with Zn(II) ion.
UV–vis Spectral Analysis and Magnetic Susceptibility
Measurements
744
The UV–vis spectra of Li(HL) and its metal complexes in methanol
are shown in Fig. 1. Li(HL) exhibits a band at 231 nm assignable to
naphthyl ring transition π –π ∗ , which remains slightly changed in
the spectra of their metal complexes. In addition, in the spectrum
of Li(HL) there are two medium intensity bands at 307 nm and
400, 420 nm assigned to the phenol-imine and keto-amine forms,
which may be attributed to n–π ∗ and π –π ∗ type transitions,
respectively. The bands at 400 and 420 nm can be assigned to the
NH form of Li(HL) in methanol, characteristic of proton transferred
form (Fig. 2).[29,30] Although the band at 317–318 nm exists in the
spectra of all complexes, the other bands at 400 and 420 nm are
shifted to shoulder bands. This means that in the complex Schiff
bases exist only in the phenol-imine form and coordinate metal
ions with phenolic oxygen and imine nitrogen, as confirmed by IR
and 1 H NMR analyses.
The cobalt(II) complex exhibits one main band at 645 nm due to
4
A2 (F) → 4 T1 (F) transition in the tetrahedral ligand field. There is
also a shoulder at about 550 nm due to spin-orbital coupling. Also,
the µeff value of 4.15 BM is additional evidence for tetrahedral
geometry.[31]
The electronic spectrum of copper(II) complex gives one broad
band centered at 625 nm due to the 2 B1g → 2 A1g transition,
which indicates a square-planar geometry. In general, due to
Jahn–Teller distortion, the square planar Cu(II) complex displays a
broad absorption band between 600 and 700 nm and the peak at
510 nm merges with the broad band, which is due to 2 B1g → 2 Eg
wileyonlinelibrary.com/journal/aoc
Li
Se
C
N
Phenol Imine
Li
Se
C
N
H
H
O
O
Keto Amine
Figure 2. Structures of phenol-imine and keto-amine forms.
and 2 B1g → 2 A1g with the respective absorption bands.[32] The
magnetic moment of Cu(II) complex is 1.89 µB , indicating the
presence of one unpaired electron.[33 – 35] This is consistent with
the electronic spectral results that square planar geometry for the
Cu(II) complex contains one unpaired electron and the µeff value
would be in the range of 1.8–2.1 µB.
However, the zinc complex only gives a high-intensity band
at 317–318 nm due to the absence of d–d transition, which
is assigned to a ligand–metal charge transfer besides the
characteristic of ligand.[36] At the same time, the zinc(II) complex
is found to be diamagnetic,[37] as expected for d10 configuration.
The proposed structures of metal(II) complexes are shown in Fig. 3.
Thermal Analyses
Thermogravimetry (TG) is an efficient method to determine
complex stoichiometries. The thermoanalytical data of the
complexes are given in Table 3. The weight losses for each
complex are calculated within the corresponding temperature
ranges. In general, all complexes are stable up to 90 ◦ C. Above
this temperature, mass loss in thermal decomposition reactions of
complexes is observed. The detailed data are discussed as follow.
The first step of decomposition of [CoL(H2 O)] within the
temperature range 98–125 ◦ C corresponds to the loss of water
molecule of coordination with a mass loss of 3.99% (calcd 4.24%)
in the expected complex stoichiometry. The second step of
decomposition corresponds to the removal of the organic part
of the ligand (mass loss = 62.98%; calcd = 63.33%) leaving CoSe
as a residue.
c 2010 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2010, 24, 741–747
Zinc(II), copper(II) and cobalt(II) complexes with Schiff base ligand
O
O
C
CH3SeCH2CH2CH
O
N
N
Co
O
OH2
(a)
O
C
CH3SeCH2CH2CH
CH3SeCH2CH2CH
O
Cu
C
O
N
Zn
OH2
O
O
(b)
OH2
(c)
Figure 3. Proposed structures of (a) [CoL(H2 O)]; (b) [CuL(H2 O)]; (c) [ZnL(H2 O)].
Table 3. Thermal analytical data for Schiff base metal complexes
Molecular formula
Molecular weight
[CoL(H2 O)]
C16 H17 NO4 SeCo
[CuL(H2 O)]
C16 H17 NO4 SeCu
[ZnL(H2 O)]
C16 H17 NO4 SeZn
425.20
429.82
431.66
Decomposition
temperature (◦ C)
Mass loss found
(%, calcd)
Eliminated species
Solid residue
mass found (%, calcd)
98–125
215–850
98–130
205–845
102–138
210–848
3.99 (4.24)
62.98 (63.33)
3.91 (4.19)
62.23 (62.65)
3.86 (4.17)
62.21 (62.68)
H2 O
C16 H15 NO3
H2 O
C16 H15 NO3
H2 O
C16 H15 NO3
CoSe
32.85 (32.43)
CuSe
32.94 (33.16)
ZnSe
33.15 (33.44)
On the other hand, [CuL(H2 O)] exhibits the first decomposition
step in the temperature range 98–130 ◦ C (mass loss = 3.91%; calcd
= 4.19%) accounting for the loss of a coordinated water molecule.
The mass loss of the remaining decomposition steps amounts to
62.23% (calcd 62.65%). They correspond to the removal of the
ligand molecule leaving CuSe as a residue.
[ZnL(H2 O)] shows that the first step of decomposition within
the temperature range 102–138 ◦ C corresponds to the loss of
coordinated water with a mass loss of 3.86% (calcd 4.17%). The
subsequent stages involve the loss of ligand with a mass loss of
62.21% (calcd 62.68%). ZnSe as the final thermal decomposition
product is observed.
X-ray Diffraction Analysis
Although single crystal X-ray crystallographic investigation is the
most precise source of information regarding the structure of a
complex, the difficulty of obtaining crystalline complexes in proper
symmetric form renders this method unsuitable for such a study.
However, a variety of other spectroscopic techniques could be
used with good effect for characterizing the metal complexes.
To obtain further evidence about the structure of the metal
complexes, power X-ray diffraction recorded at 2θ = 10–80◦ was
performed. The diffractograms obtained for the Schiff base metal
complexes are given in Fig. 4. The Co(II) complex only shows weak
peaks at 2θ = 15–25◦ , indicating its amorphous nature. However,
Zn(II) and Cu(II) complexes show sharp peaks and have nearly
similar crystallinity.
Antibacterial and Antifungal Bioassay
Appl. Organometal. Chem. 2010, 24, 741–747
Compound (zone of inhibition in mm)
Bacteria
E. coli
B. subtillis
S. aureus
P. vulgaris
E. aerogens
Li(HL) [CoL(H2 O)] [CuL(H2 O)] [ZnL(H2 O)] Imipenum
+
0
+
+
0
+
+
+
0
+
++
++
++
++
++
+
++
++
++
0
+++
++
++
++
++
a
Imipenum was used as a standard antibacterial agent. Well diameter,
0.6 cm; +, inhibition values = 0.1–0.5 cm beyond control; ++,
inhibition values = 0.6–1.0 cm beyond control; +++, inhibition values
= 1.1–1.5 cm beyond control; 0, not detected.
fungi to different extents, and they are found to be more effective
against fungi than bacteria species. Generally, the inhibitation
zone of the metal complexes is higher than that of Li(HL).
This enhancement of metal complexes in the activity can be
explained on the basis of chelation theory.[39] 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 the cell membrane and blocking the metal binding sites on
enzymes of the microorganism. Furthermore, the mode of action
of the compounds may involve the formation of hydrogen bonds
through the azomethine group of the complexes with the active
centers of cell constituents, resulting in interference with normal
cell processes.[40]
The variation in the effectiveness of different metal complexes
against different organisms depends either on the impermeability
of the cells of the microbes or on differences in ribosomes of
microbial cells.[41] There is a marked increase in the bacterial and
fungi activities of the Cu(II) complex as compared with the other
c 2010 John Wiley & Sons, Ltd.
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745
Li(HL) and its metal complexes have been screened for their
antibacterial and antifungal activities according to the respective
literature protocol[38] and the results are presented in Tables 4
and 5. Both imipenum and amphotericin B were used as standard
antibacterial and antifungal agents, respectively.
Both Li(HL) and its metal complexes have the capacity of
inhibiting the metabolic growth of the investigated bacteria and
Table 4. Results of antibacterial bioassaya
X. Ran et al.
Figure 4. X-ray diffraction patterns of Zn(II), Cu(II) and Co(II) complexes.
Table 5. Results of antifungal bioassaya
Compound (zone of inhibition in mm)
Organism
A. flavus
A. niger
F. Solani
Cladosporium
Li(HL)
[CoL(H2 O)]
[CuL(H2 O)]
[ZnL(H2 O)]
amphotericin B
+
+
+
0
++
+
0
+
++
+
+
++
++
+
+
0
++
+++
++
++
a
Amphotericin B was used as a standard f antifungal agent. Well diameter: 0.6 cm; +, inhibition values = 0.1–0.5 cm beyond control; ++, inhibition
values = 0.6–1.0 cm beyond control; +++, inhibition values = 1.1–1.5 cm beyond control; 0, not detected.
two complexes. This result reflects the differences in the method
and mechanism of interaction of the complexes with bacteria and
fungi due to the difference in metal ions. It is believed that a trace
of Cu(II) destroys the microbe; however, a recent study on the
mechanism suggests that activated oxygen in the surface of metal
Cu kills the microbe because Cu(II) activity is weak. This is also in
agreement with the antifungal and antibacterial properties of a
range of Cu(II) complexes evaluated against several pathogenic
fungi and bacteria.[42]
ligand increases the biological activity of the molecule. In addition
to role of selenomethionine as a promising chemopreventive
agent, these complexes could be potentially applied in medicinal
and pharmacy field.
Acknowledgments
The financial support by the National Natural Science Foundation
of China (20904010) and the Fundamental Research Funds for the
Central Universities (2009ZM0170) is gratefully acknowledged.
Conclusion
746
Three complexes, Co(II), Cu(II) and Zn(II) with a tridentate O,N,Odonor Schiff base derived from 2-hydroxy-1-naphthaldehyde and
D, L-selenomethionine, were synthesized and characterized. The
results demonstrate that Co(II) complex probably has tetrahedral
geometry, while the Cu(II) complex probably has square planar
geometry. The results of antimicrobial and antifungal activities
show that the metal complexes are more active than Li(HL), which
is in accordance with the fact that the chelating of metal to the
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biological, complexes, cobalt, zinc, schiff, ligand, selenomethionyl, base, naphthaldehyde, synthese, characterization, derived, coppel, studies, hydroxy
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