Synthesis, structural characterization, and antifungal activity of Schiff bases and their transition metal mixed-ligand complexes

Article abstract of DOI:10.1134/S003602360601013X

Mixed-ligand complexes of Mn(II), Co(II), Ni(II), Cu(II), Zn(II), and Cd(II) have been prepared with biologically active Schiff bases, viz. potassium salt of o-hydroxyacetophenoneglycine [KHL] and bis(benzylidene)ethylenediamine [A¹] or thiophe

Full text of DOI:10.1134/S003602360601013X

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2006, Vol. 51, No. 1, pp. 67–72. © Pleiades Publishing, Inc., 2006.
COORDINATION
COMPOUNDS
Synthesis, Structural Characterization,
and Antifungal Activity of Schiff Bases
and Their Transition Metal Mixed-Ligand Complexes1
Hitesh M. Parekh*, Saurabh R. Mehta**, and M. N. Patel*
* Department of Chemistry and ** Department of Biosciences, Sardar Patel University,
Vallabh Vidyanagar 388120, Gujarat, India
e-mail address: jeenenpatel@yahoo.co.in
Received April 14, 2005
Abstract—Mixed-ligand complexes of Mn(II), Co(II), Ni(II), Cu(II), Zn(II), and Cd(II) have been prepared
with biologically active Schiff bases, viz. potassium salt of o-hydroxyacetophenoneglycine [KHL] and bis(ben-
zylidene)ethylenediamine [A1] or thiophene-o-carboxaldene-p-toluidine [A2]. The synthesized mixed-ligand
complexes have been characterized on the basis of elemental analysis, thermogravimetric analysis, magnetic
measurements, and electronic and infrared spectra. The mixed-ligand complexes show higher antifungal activ-
ity as compared to the free ligands, metal salts, and the control (dimethylsulfoxide) but moderate activity as
compared to the standard fungicides (bavistin and emcarb).
DOI: 10.1134/S003602360601013X
1
INTRODUCTION
until the added component completely dissolved in the
ethanol and KOH solution. Then an ethanolic solution
(100 ml) of o-hydroxyacetophenone (10 mmol, 1.36 g)
was quickly added. The reaction was allowed to reach
completion by stirring at 50°C for 1 hour. The product
was crystallized by diffusion of excess absolute diethyl
ether into the solution. The resulting product was fil-
The field of Schiff base complexes was fast develop-
ing on account of the wide variety of possible structures
for the ligands depending upon the aldehydes and
amines. The mixed-ligand complexes containing N, O,
and/or S donor atoms are important owing to their sig-
nificant antifungal, antibacterial, and anticancer activ-
ity [1]. In view of the importance of the Schiff bases
mixed-ligand complexes and in continuation of our ear-
lier work [2], herein we describe the synthesis, charac-
terization, and antifungal activity of transition metal
mixed-ligand complexes with different Schiff bases.
The structures of the Schiff bases are shown in Fig. 1.
OH
C N CH2 COOK
CH3
KHL
EXPERIMENTAL
Reagents and Solvents
H
H
C N
N C
All the chemicals and solutions that were used were
of analytical grade. The metal nitrates, o-gydroxyace-
tophenone, benzaldehyde, glycine, ethylenediamine,
and p-toluidine were purchased from E. Meark Limited
(Mumbai, India). The thiophene-o-carboxaldehyde was
purchased from Lancaster, England. The organic sol-
vents were purified by recommended methods [3].
Potassium salt of o-hydrosyacetophenoneglycine
(KHL). A solution of potassium hydroxide (10 mmol,
0.56 g) in 30 ml of pure alcohol was added to crystal-
line glycine (10 mmol, 0.75 g). The mixture was stirred
A1
H
S
C
N
CH3
A2
1
Fig. 1. Structures of the Schiff bases.
This text was submitted by the authors in English.
67

68
HITESH M. PAREKH et al.
temperature. Finally the colored crystals that were
CH3
obtained were collected by filtration, washed with
water and ethanol, and dried in air.
N
O
OH2
M
O
O
Physical Measurement
H
N
The metal content of the mixed-ligand complexes
was analyzed by the EDTA titration [5] technique after
decomposing the organic matter with a mixture of
HClO4, H2SO4, and HNO3 (1 : 1.5 : 2.5). The carbon,
hydrogen, nitrogen, and sulfur were analyzed with a
model 240 Perkin-Elmer elemental analyzer. The
thermogravimetric analyses (TGA) were performed
using a model 5000/2960 analyzer manufactured by
T.A. Instruments, U.S.A. The reflectance spectra of the
mixed-ligand complexes were recorded in the range
1700–350 nm (as MgO) on a Beckman DK-2A spectro-
photometer. Infrared spectra were recorded on an FT-IR
Nicolet 400D spectrophotometer. The magnetic
moments were obtained by Gouy’s method using mer-
cury tetrathiocyanatocobaltate(II) as the calibrant (χg =
C
N
H
C
[M(L)(A1)(H2O)]
CH3
N
O
OH2
M
O
O
S
N
CH3
16.44 × 10–6 c.g.s. units at 20°C). Diamagnetic correc-
tions were made using Pascal’s constant.
[M(L)(A2)(H2O)]
RESULTS AND DISCUSSION
Fig. 2. Suggested structures of the mixed-ligand complexes.
The following reaction describes the formation of
the mixed-ligand complexes:
tered and dried in a vacuum desiccator. Yield: 1.602 g
(62%), M.p. 262°C.
M(NO3)2 · nH2O + KHL + A
[M(L)(A)(H2O)]
+ KNO3 + HNO3 + (n – 1)H2O
Bis(benzylidene)ethylenediamine (A1)
The elemental analysis data, the molecular weights,
the colors, the percentage yields, the melting points,
and the magnetic moment data are represented in
Table 1. The results of the elemental analysis of the
Schiff bases and their mixed-ligand complexes are in
good agreement with those required by the proposed
formula. All the mixed-ligand complexes are insoluble
in methanol, diethyl ether, ethanol, and DMF but solu-
ble in DMSO. The proposed structures of the mixed-
ligand complexes are given in Fig. 2.
The bis(benzylidene) ethylenediamine was synthe-
sized by a published procedure [4].
Thiophene-o-carboxaldene-p-toluidine (A2)
An ethanolic solution (100 ml) of thiophene-o-car-
boxaldehyde (10 mmol, 1.12 g) and an ethanolic solu-
tion (100 ml) of p-toluidine (10 mmol, 1.07 g) in a
molar ratio of 1 : 1 were mixed with constant stirring.
Refluxing was carried out for six hours. The solution
was cooled overnight at room temperature. The yellow
crystals that formed were collected and dried in air.
Yield: 1.27 g (60%), M.p. >360°C.
IR Spectra and Mode of Bonding
The coordinating sites involved in the bonding with
the metal ion have been determined by carefully com-
paring the infrared spectra of the mixed-ligand com-
plexes with those of the free ligands. The IR spectra of
the ligand (KHL) shows a broad band at 3450 cm–1,
which can be attributed to the phenolic OH group. This
band is absent in all the mixed-ligand complexes, thus,
indicating coordination through the phenolic OH group
[6]. The bands observed in the regions 3200–3300,
Preparation of the Mixed-Ligand Complexes
The preparation of the mixed-ligand complexes was
carried out by mixing an aqueous solution (100 ml) of
metal nitrate (10 mmol) and a hot methanolic solution
of potassium salt of o-hydroxyacetophenoneglycine
(10 mmol, 2.31 g) and bis(benzylidene)ethylenedi-
amine (10 mmol, 2.36 g) or thiophene-o-carboxaldene- 1280–1290, 880–850, and 730–700 cm–1 are attributed
p-toluidine (10 mmol, 2.01 g). The complexes were to OH stretching, bending, rocking, and wagging vibra-
formed by heating the mixture on a water bath for two tions, respectively, due to the presence of water mole-
hours at 60°C. The mixture was kept overnight at room cules [7]. The band due to the azomethine (C=N) group
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 51 No. 1 2006

SYNTHESIS, STRUCTURAL CHARACTERIZATION, AND ANTIFUNGAL ACTIVITY
Table 1. Analytical data of the mixed-ligand complexesa
69
% Found (Calcd.)
Compounds
M.p.
Mol. Wt.
231.19
Color
Yield, %
Empirical Formula
(°C)
C
H
N
S
–
metal
–
KHL
Yellow
51.92
(51.94)
81.32
4.35
6.07
262
235
62
72
58
62
55
58
63
60
56
53
57
60
64
56
59
C10H10KNO3
(4.32)
6.78
(6.05)
11.83
(11.85)
6.91
A1
236.16
201.20
500.20
504.19
503.95
508.80
510.65
557.67
465.22
469.21
468.97
473.82
475.67
522.69
Yellow
Yellow
Brown
Brown
Green
–
–
–
C6H18N2
(81.36)
71.66
(6.77)
5.43
A2
15.92
(15.94)
–
>360
>360
242
C12H11NS
(71.63)
62.47
(5.46)
5.35
(6.95)
8.32
[Mn(L)(A1)(H2O)]
C26H27MnN3O4
[Co(L)(A1)(H2O)]
C26H27CoN3O4
[Ni(L)(A1)(H2O)]
C26H27NiN3O4
[Cu(L)(A1)(H2O)]
C26H27CuN3O4
[Zn(L)(A1)(H2O)]
C26H27ZnN3O4
[Cd(L)(A1)(H2O)]
C26H27CdN3O4
[Mn(L)(A2)(H2O)]
C22H22MnN2O4S
[Co(L)(A2)(H2O)]
C22H22CoN2O4S
[Ni(L)(A2)(H2O)]
C22H22NiN2O4S
[Cu(L)(A2)(H2O)]
C22H22CuN2O4S
[Zn(L)(A2)(H2O)]
C22H22ZnN2O4S
[Cd(L)(A2)(H2O)]
C22H22CdN2O4S
10.98
(10.98)
11.69
(62.42)
61.91
(5.39)
5.32
(8.39)
8.35
–
–
–
–
–
(61.93)
61.97
(5.35)
5.39
(8.33)
8.32
(11.68)
11.58
>360
229
(61.96)
61.39
(5.35)
5.30
(8.33)
8.31
(11.64)
12.43
Green
(61.37)
61.17
(5.30)
5.28
(8.25)
8.23
(12.48)
12.84
Yellow
Yellow
Brown
Brown
Green
261
(61.14)
55.92
(5.28)
4.86
(8.22)
7.56
(12.80)
20.23
294
(55.99)
56.76
(4.84)
4.74
(7.53)
6.04
(20.15)
11.73
6.87
(6.89)
6.80
>360
>360
>360
272
(56.79)
56.36
(4.72)
4.63
(6.01)
5.94
(11.80)
12.56
(56.31)
56.35
(4.68)
4.71
(5.96)
5.95
(6.83)
6.86
(12.55)
12.50
(56.34)
55.72
(4.69)
4.62
(5.97)
5.94
(6.83)
6.73
(12.51)
13.38
Green
(55.76)
55.56
(4.64)
4.60
(5.90)
5.82
(6.76)
6.74
(13.41)
13.72
Yellow
Yellow
>360
>360
(55.54)
50.56
(4.62)
4.26
(5.88)
5.35
(6.73)
6.15
(13.74)
21.55
(50.55)
(4.20)
(5.35)
(6.13)
(21.50)
a
KHL = Potassium salt of o-hydroxyacetophenoneglycine.
1
A = Bis(benzylidene)ethylenediamine.
A = Thiophene-o-carboxaldene-p-toluidine.
2
in the free ligand is observed in the region 1625– o-hydroxyacetophenoneglycine, but, in the mixed-
1635 cm–1. This band is shifted to the lower frequency
region upon complexation, thus, suggesting coordina-
tion via the azomethine group in all of the prepared
complexes [8]. The asymmetric and symmetric stretch-
ing frequencies of the carboxylate ions are seen at 1595
ligand complexes, they are shifted to lower frequency
regions (~1580 and ~1385 cm–1, respectively) [9]. The
ν(C–S) [10] band of the thiophene-o-carboxaldene-p-
toluidine observed at 765 cm–1, which is shifted to a
lower frequency of about 750 cm–1 in the spectra of the
and 1400 cm–1, respectively, in the potassium salt of mixed-ligand complexes, indicates the participation of
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 51 No. 1 2006

70
HITESH M. PAREKH et al.
Table 2. Electronic parameters of the Co(II) and Ni(II) mixed-ligand complexesa
Observed bands (cm–1)
Complexes
ν2/ν1
B
β
β°
10Dq
ν1
ν2
ν3
[Co(L)(A1)(H2O)]
[Ni(L)(A1)(H2O)]
[Co(L)(A2)(H2O)]
[Ni(L)(A2)(H2O)]
9.125
10.350
9.180
17.990
18.150
18.050
18.070
19.060
24.300
19.000
24.170
1.97
1.75
1.96
1.73
737
760
729
736
0.76
0.74
0.75
0.71
24
26
25
29
10.247
10.350
10.300
10.400
10.400
a
KHL = Potassium salt of o-hydroxyacetophenoneglycine.
1
A = Bis(benzylidene)ethylenediamine.
A = Thiophene-o-carboxaldene-p-toluidine.
2
II
The ligand field splitting energy (10Dq), interelectronic repulsion parameter (B) and covalency factor (nephelauxetic ratio) (β) for the Co
and Ni complexes have been calculated using the secular equations given by E. König [26].
II
For Co(II) complexes
10Dq = 1/2[(2ν – ν ) + (ν2 + ν ν – ν )
For Ni(II) complexes
2
1
1/2
10Dq = ν
]
1
1
3
1 3
3
15B = ν – 2ν + 10Dq
15B = (ν + ν ) – 3ν
1
3
1
2
3
β = B/Bo [Bo(free ion) = 971]
β° = (1 – β) × 100
β = B/Bo [Bo(free ion) = 1030]
β° = (1 – β) × 100
3
the sulfur atom of the thiophene ring in the coordina-
tion. The sharp bands in the range 750–780 and 1525–
1535 cm–1 are due to aromatic ν(C–H) and ν(C=C),
respectively. In the far IR spectra of all the mixed-
ligand complexes, the band observed at the
520−540 cm–1 region can be assigned to M–N stretch-
ing [11]. The bonding of oxygen to the metal ions is
proved by the occurrence of the band at the 430–
440 cm–1 region [12]. The bonding of sulfur to the
metal ion is indicated by the occurrence of the band at
the 410–425 cm–1 region [13].
and A2g
3T2g. For the Co(II) mixed-ligand com-
plexes, the electronic spectra show three bands of
medium intensity at ~19000, ~18000, and ~9200 cm–1,
which are assigned, respectively, to the transitions
4T1g(F)
4T1g(F)
3T1g(P), T1g(F)
4A2g(F), and
4
4T2g(F) of an octahedral geometry [16].
The electronic spectra of the Cu(II) mixed-ligand com-
plexes show an absorption band at ~15500 cm–1 attrib-
uted to the 2Eg
2T2g transition, which are compati-
ble with these complexes having an octahedral struc-
ture [17]. The ground state of the Mn(II) is A1g. The
6
Mn(II) mixed-ligand complexes in an octahedral [18]
field should give three transitions corresponding to
Thermogravimetric Analysis
6
6A1g
4T1g (ν1 ~ 14500 cm–1), A1g
4T2g
The TGA curves were obtained in a nitrogen atmo-
sphere between 50 and 800°C. The mixed-ligand com-
plexes decompose in two steps. The first step is
between 150–180°C and corresponds to the coordi-
nated water molecule [14]. In the temperature range
180–800°C, the ligand molecules are lost. All the
mixed-ligand complexes completely decompose to the
corresponding metal oxides: MnO, Co2O3, NiO, CuO,
ZnO, and CdO. These results are in good agreement
with the composition of the mixed-ligand complexes.
(ν2 ~19800 cm–1), and 6A1g
4A1g (ν3 ~24400 cm–1)
in increasing order of energy. The ligand field splitting
energy (Dq), the Racah interelectronic repulsion
parameter (B), the nephelauxetic ratio (β), and the ratio
ν2/ν1 for the Ni(II) and Co(II) mixed-ligand complexes
are presented in Table 2.
Magnetic Measurements
The magnetic moments of the mixed-ligand com-
plexes were measured at room temperature. In the
present case, the magnetic moment values of the
[Mn(L)(A1)(H2O)] and [Mn(L)(A2)(H2O)] are 6.01 and
6.04 µB, respectively, which are within the limit of the
spin-free value (5.92 µB) for five unpaired electrons,
thus, indicating that the complexes are high-spin
Electronic Spectra
The electronic spectra of the Ni(II) complexes are
consistent with the formation of an octahedral geome-
try with the appearance of three bands at ~24000,
~18000, and ~10500 cm–1 corresponding to the follow-
ing transitions [15]: 3A2g
3T1g(P), 3A2g
3T1g(F),
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 51 No. 1 2006

SYNTHESIS, STRUCTURAL CHARACTERIZATION, AND ANTIFUNGAL ACTIVITY
71
% Inhibition
120
A. niger
F. oxysporum
100
A. flavus
80
60
40
20
0
Compounds
Fig. 3. Plot of antifungal activity.
d5-systems with an octahedral geometry [19]. The mag-
netic moment values for the Co(II) mixed-ligand com-
plexes have been used as criteria to determine the type
of coordination around the metal ion. Due to the intrin-
sic orbital angular momentum in the ground state, there
is consistently a considerable orbital contribution. The
The antifungal activities of the ligands, mixed-
ligand complexes, metal nitrates, fungicides (bavistin
and emcarb), and the control (dimethylsulfoxide) were
screened using the plate poison technique [24]. Seven-
day-old cultures of Aspergillus niger, Fusarium
oxysporum, and Aspergillus flavus were used as test
organisms. A stock solution of 500 µg/ml was made by
dissolving 50 mg of each compound in dimethylsul-
phoxide (100 ml). The sterilized medium with the
added stock solution was poured into 90 mm sterile
petri plates and allowed to solidify. They were inocu-
lated with a 5 mm actively growing mycelial disc and
incubated at 27°C for 72 h. After 72 h of inoculation,
the percent reduction in the radial growth diameter over
the control was calculated. The growth was compared
with dimethylsulfoxide as the control. The results of the
antifungal studies are given in Table 3 and Fig. 3.
The results show that the mixed-ligand complexes
are more toxic than their parent ligands and metal
nitrate against the same microorganisms. The increase
in the antifungal activity of the mixed-ligand com-
plexes may be due to the effect of the metal ion on the
normal cell processes. A possible mode for the toxicity
increase may be considered in light of Tweede’s chela-
tion theory [25]. Chelation considerably reduced the
polarity of the metal ion because of the partial sharing
of its positive charge with the donor groups and the
π-electron delocalization over the whole chelate ring.
effective
magnetic
moment
values
of
[Co(L)(A1)(H2O)] and [Co(L)(A2)(H2O)] are 4.06 and
4.09 µB, respectively, which are slightly greater than the
spin only value (3.87 µB), which suggest an octahedral
geometry for the Co(II) mixed-ligand complexes [20] in
the high-spin state. The magnetic moment values of the
[Ni(L)(A1)(H2O)] and [Ni(L)(A2)(H2O)] are 2.89 and
2.77 µB, respectively, which are within the range
expected for similar hexacoordinated [21] Ni(II) ions.
The magnetic moment values for the [Cu(L)(A1)(H2O)]
and [Cu(L)(A2)(H2O)] are 1.89 and 1.76 µB, res-
pectively, which are consistent with an octahedral
structure [22]. The magnetic moment determinations
show that the Zn(II) and Cd(II) mixed-ligand com-
plexes are diamagnetic (as expected for the d10 system)
and presumably octahedral.
ANTIFUNGAL ACTIVITY
Isolation of Fungi
The dilution plate method [23] was used for isola- Such chelation could enhance the lipophilic character
tion of fungi. Selected and isolated fungi were main- of the central metal atom, which subsequently favors its
tained on potato dextrose agar plates at 4°C for further permeation through the lipid layer of the cell mem-
experimental work.
brane. Although there is a sufficient increase in the fun-
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 51 No. 1 2006

72
HITESH M. PAREKH et al.
Table 3. Antifungal activities of the compoundsa
2. P. K. Panchal, D. H. Patel, and M. N. Patel, Synth. React.
Inorg. Met.-Org. Chem. 34, 1223 (2004).
Growth diameter in mm
3. A. I. Vogel, Textbook of Practical Organic Chemistry,
(% of inhibition)
5th ed. (Longman, London, 1989).
Compounds
4. P. P. Dholakiya and M. N. Patel, Synth. React. Inorg.
F. oxy-
sporum
A. niger
A. flavus
Met.-Org. Chem. 32, 819 (2002).
5. A. I. Vogel, Textbook of Practical Organic Chemistry,
DMSO (control)
Bavistin
29
46
39
3rd ed. (Longman, London, 1975).
00(100)
00(100)
23(21)
22(23)
20(31)
18(38)
20(31)
22(23)
19(34)
22(23)
20(31)
13(55)
12(59)
13(55)
14(52)
13(55)
12(59)
14(52)
13(55)
14(52)
13(55)
14(52)
13(55)
00(100)
00(100)
33(28)
34(27)
31(32)
28(40)
29(37)
31(32)
28(40)
30(35)
31(32)
24(48)
28(40)
27(41)
24(48)
23(50)
22(52)
24(48)
19(59)
20(57)
28(40)
27(41)
21(54)
00(100)
00(100)
28(28)
26(32)
29(25)
29(25)
27(30)
29(25)
28(28)
27(30)
27(30)
21(46)
19(51)
18(54)
20(49)
22(44)
19(51)
22(44)
23(41)
22(44)
23(41)
24(38)
21(46)
6. B. R. Pandya, S. K. Menon, and Y. K. Agrawal, Synth.
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Emcarb
7. M. Tumer, B. Erdogan, H. Koksal, et al., Synth. React.
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KHL
A1
A2
8. E. Tas, A. Cukurovali, and M. Kaya, J. Coord. Chem. 44,
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Mn(NO3)2 · 4H2O
Co(NO3)2 · 6H2O
Ni(NO3)2 · 6H2O
Cu(NO3)2 · 3H2O
Zn(NO3)2 · 6H2O
Cd(NO3)2 · 4H2O
[Mn(L)(A1)(H2O)]
[Co(L)(A1)(H2O)]
[Ni(L)(A1)(H2O)]
[Cu(L)(A1)(H2O)]
[Zn(L)(A1)(H2O)]
[Cd(L)(A1)(H2O)]
[Mn(L)(A2)(H2O)]
[Co(L)(A2)(H2O)]
[Ni(L)(A2)(H2O)]
[Cu(L)(A2)(H2O)]
[Zn(L)(A2)(H2O)]
[Cd(L)(A2)(H2O)]
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15. R. Carbello, A. Cartineiras, W. Hiller, and J. Strahle,
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16. G. Wilkinson and F. A. Cotton, Advanced Inorganic
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