Studies on mononuclear chelates derived from substituted schiff base ligands: Synthesis and characterization of a new 5-methoxysalicyliden-p- aminoacetophenoneoxime and its complexes with Co(II), Ni(II), Cu(II), and Zn(II)

Article abstract of DOI:10.1007/s11173-005-0170-7

New complexes of Co(II), Ni(II), Cu(II), and Zn(II) with new Schiff bases derived by the condensation of p-aminoacetophenoneoxime with 5-methoxysalicylaldehyde are synthesized. The compounds are characterized by elemental analyses, magnetic susceptibility measurements, IR, ¹H and ¹³C NMR spectra, electronic spectral data, and molar conductivity. The thermal stabilities of the compounds are also reported. The Schiff base acts as bidentate O,N-donor atoms, and their metal complexes are supposed to possess a tetrahedral geometry with respect to the central metal ion. The general formula of the 5-methoxysalicyliden-p-aminoacetophenoneoxime Co(II), Ni(II), Cu(II), and Zn(II) complexes is Co(L)₂, Ni(L)₂, Cu(L) ₂, and Zn(L)₂.

Full text of DOI:10.1007/s11173-005-0170-7

Russian Journal of Coordination Chemistry, Vol. 31, No. 11, 2005, pp. 790–794. Translated from Koordinatsionnaya Khimiya, Vol. 31, No. 11, 2005, pp. 834–838.
Original Russian Text Copyright © 2005 by Canpolat, Kaya.
Studies on Mononuclear Chelates Derived from Substituted
Schiff Base Ligands: Synthesis and Characterization
of a New 5-Methoxysalicyliden-p-Aminoacetophenoneoxime
and Its Complexes with Co(II), Ni(II), Cu(II), and Zn(II)
E. Canpolat and M. Kaya
Department of Chemistry, Faculty of Arts and Sciences, Firat University, 23119, Elazii=, Turkey
e-mail: ecanpolat@firat.edu.tr
Received December 12, 2004
Abstract—New complexes of Co(II), Ni(II), Cu(II), and Zn(II) with new Schiff bases derived by the conden-
sation of p-aminoacetophenoneoxime with 5-methoxysalicylaldehyde are synthesized. The compounds are
characterized by elemental analyses, magnetic susceptibility measurements, IR, 1H and 13C NMR spectra, elec-
tronic spectral data, and molar conductivity. The thermal stabilities of the compounds are also reported. The
Schiff base acts as bidentate O,N-donor atoms, and their metal complexes are supposed to possess a tetrahedral
geometry with respect to the central metal ion. The general formula of the 5-methoxysalicyliden-p-aminoace-
tophenoneoxime Co(II), Ni(II), Cu(II), and Zn(II) complexes is Co(L)2, Ni(L)2, Cu(L)2, and Zn(L)2.
There are two most important classes of ligands in Schiff base, as well as its complexes with cobalt, nickel,
the modern coordination chemistry, namely, azome- copper, and zinc.
thines [1] and oximes [2]. A large number of the Schiff
bases and their complexes have been studied in the past
because of their wide application and interesting and
EXPERIMENTAL
important properties [3–5]. Metal complexes of the
Schiff bases derived from substituted salicylaldehydes
and various amines have been widely investigated [4,
5]. They can serve as models of biologically important
species and find applications in biomimetic catalytic
reactions, materials chemistry, and industry [5–7].
The preparation of p-aminoacetophenoneoxime has
been described previously [16]. Elemental analyses (C,
H, N) were carried out using LECO-932 CHNSO at the
Technical and Scientific Research Council of Turkey,
TUBITAK. IR spectra were recorded on a Mattson
1
1000 FT-IR spectrometer as KBr pellets. H and 13C
NMR spectra were recorded on Bruker GmbH
Dpx-400 MHz high-performance digital FT-NMR
spectrometers. Electronic spectra were recorded on a
Shimadzu 1240 UV spectrometer at the Hacettepe
University. Magnetic susceptibilities were determined
on a Sherwood Scientific Magnetic Susceptibility Bal-
ance (model MK1) at room temperature using
Hg[Co(SCN)4] as a standard; diamagnetic corrections
were calculated from Pascal’s constants. The melting
points were determined on a Gallenkamp melting point
apparatus. Molar conductances of the Schiff base
ligand and its transition-metal complexes were deter-
mined at room temperature using a CMD 750 WPA
conductivity meter. TGA curves were recorded on a
Shimadzu TG-50 thermobalance.
There is a considerable interest in the chemistry of
transition-metal complexes of ligands containing oxy-
gen and nitrogen donor atoms due to the carcinostatic,
antitumor, antiviral, antifungal, and antibacterial activ-
ity and industrial uses of complexes derived from them
[3, 8–10].
Many anticancer and/or antibacterial medicines are
known, on the one hand, to behave as versatile, some of
which exhibit increased anticancer activity when
administrated in the complex form with metal ions [11–
13]. On the other hand, the Schiff base complexes are
known to show antifungal activity, which is increased
in the presence of a hydroxy group in the ligand [14].
The presence of transition metals in human blood
plasma indicates their importance in the mechanism for
accumulation, storage, and transport of transition met-
5-Methoxysalicyliden-p-aminoacetophenoneoxime
als in living organisms [15]. We have prepared and (HL). To a solution of p-aminoacetophenoneoxime
characterized several metal complexes of the Schiff (1.502 g, 10 mmol) in absolute EtOH (20 ml), 5-meth-
base ligands. However, no work has been found con- oxysalicylaldehyde (1.520 g, 10 mmol) and p-toluene-
cerning 5-methoxysalicyliden-p-aminoacetophenone- sulfonic acid (0.01 mg) dissolved in absolute EtOH
oxime and complexes of this compound. We now report (25 ml) were added dropwise at 60°C with continuous
the synthesis and characterization of this O,N-type stirring and monitoring of the course of the reaction
1070-3284/05/3111-0790 © 2005 Pleiades Publishing, Inc.

STUDIES ON MONONUCLEAR CHELATES DERIVED
791
(ν, cm–1): 3350–3400 (oxime O–H), 1615 (phenolic
CH=N), 1595 (oxime C=N), 1290 (C–O), 1005 (N–O);
1H NMR (CDCl3–DMSO-d6, δ, ppm): 10.90 (s, 2H,
oxime OH), 8.53 (s, 2H, azomethine CH=N), 6.83–7.64
(m., 14H, Har), 3.74 (s, 6H, OCH3), 2.18 (s, 6H, CH3);
with IR. The precipitate was filtered, washed with cold
EtOH and Et2O several times, crystallized from ace-
tone–water, and dried at 60°C to a constant weight.
IR spectrum (ν, cm–1): 3350–3400 (oxime O–H),
3250–3295 (phenolic O–H), 1625 (phenolic CH=N),
1595 (oxime C=N), 1265 (C–O), 1005 (N–O); 1H NMR 13C NMR (CDCl3–DMSO-d6, δ, ppm): 172.00
(CH=N), 159.29 (C–OH), 157.06 (C–N), 152.06 (C12),
148.28 (C2), 135.38 (C3), 134.15 (C4 and C8), 126.70
(C13), 120.94 (C11), 120.39 (C5 and C7), 115.16 (C10),
110.26 (C14), 55.66 (C16), 11.51 (C1); λmax (ε, l/(mol cm)):
430 (3.52 × 103) (C–T), 368 (1.64 × 103) (azomethine
(CDCl3–DMSO-d6, δ, ppm): 13.10 (s, 1H, phenolic
OH), 10.91 (s, 1H, oxime OH), 8.67 (s, 1H, azomethine
CH=N), 7.67 (d, 2H, H5ar ), 7.25 (d, 1H, Ha4r ), 6.97 (d,
13
ar
14
ar
11
ar
1H, H ), 6.93 (d, 2H, H ), 6.82 (s, 1H, H ), 3.72 (s,
3H, OCH3), 2.17 (s, 3H, CH3); 13C NMR (CDCl3–
DMSO-d6, δ, ppm): 162.36 (CH=N), 154.83 (C–OH),
152.54 (C–N), 151.89 (C12), 148.02 (C2), 135.66 (C3),
133.88 (C4 and C8), 126.29 (C13), 121.00 (C11), 120.40
(C5 and C7), 115.05 (C10), 110.15 (C14), 55.48
(−OCH3), 11.34 (C1); λmax (ε, l/(mol cm)): 375 (2.01 ×
n
π*).
RESULTS AND DISCUSSION
The new Schiff base 5-methoxysalicyliden-p-ami-
noacetophenoneoxime (HL) was synthesized by the
condensation of p-aminoacetophenoneoxime [16] and
5-methoxysalicylaldehyde:
103) (azomethine n
π*).
Bis(p-aminoacetophenoneoxime-5-methoxysalic-
ylaldiminato)cobalt(II) Co(L)2. solution of
A
H3CO
CH3
Co(AcO)2 · 4H2O (0.125 g, 0.50 mmol) in absolute
EtOH (20 ml) was added to a hot (50°C) solution of the
HL (0.285 g, 1.00 mmol) in absolute EtOH (20 ml)
under an argon atmosphere. The complex was obtained
immediately upon the addition of the metal salt solu-
tion. The precipitated complex was filtered off, washed
with H2O, cold EtOH, and cold Et2O several times, and
H N
2
+
N
O
OH
OH
H3CO
16
12 11
dried in vacuo. IR spectrum (ν, cm–1): 3350–3400
(oxime O–H), 1605 (phenolic C=N), 1595 (oxime
C=N), 1280 (C–O), 1005 (N–O); λmax (ε, l/(mol cm)):
13
10
9
5
7
4
8
CH3
Abs. EtOH
–H2O
1
14 15
6
3
2
N
OH
N
676 (189) (d–d), 434 (2.11 × 103) (C–T), 370 (4.43 × 103)
(HL)
OH
(azomethine n
π*).
Bis(p-aminoacetophenoneoxime-5-methoxysali-
cylaldiminato)nickel(II) Ni(L)2. Ni(L)2 was prepared
following the same procedure as that described for
Co(L)2, starting from HL (0.285 g, 1.00 mmol) and
Ni(AcO)2 · 4H2O (0.124 g, 0.50 mmol). IR spectrum
In general, reactions of the ligand with metal salt
occurred at a high rate with good yield of mononuclear
complexes corresponding to the general formula
M(L)2:
(ν, cm–1): 3350–3400 (oxime O–H), 1610 (phenolic
H3CO
C=N), 1595 (oxime C=N), 1275 (C–O), 1005 (N–O);
λ
max (ε, l/(mol cm)): 640 (195) (d–d), 418 (3.00 × 103)
(C–T), 360 (2.30 × 103) (azomethine n
π*).
CH3
N
Bis(p-aminoacetophenoneoxime-5-methoxysali-
cylaldiminato)copper(II) Cu(L)2. Cu(L)2 was pre-
pared following the same procedure as that described
for Co(L)2, starting from HL (0.285 g, 1.00 mmol) and
Cu(AcO)2 · H2O (0.100 g, 0.50 mmol). IR spectrum
O
N
HO
M
N
OH
N
O
(ν, cm–1): 3350–3400 (oxime O–H), 1610 (phenolic
C=N), 1595 (oxime C=N), 1285 (C–O), 1005 (N–O);
H3C
λ
max (ε, l/(mol cm)): 597 (163) (d–d), 418 (1.11 × 103)
(C–T), 374 (3.77 × 103) (azomethine n
π*).
OCH3
Bis(p-aminoacetophenoneoxime-5-methoxysali-
cylaldiminato)zinc(II) Zn(L)2. Zn(L)2 was prepared
following the same procedure as that described for
Co(L)2, starting from HL (0.285 g, 1.00 mmol) and
Zn(AcO)2 · 2H2O (0.110 g, 0.50 mmol). IR spectrum
M(L)2, M = Co(II), Ni(II), Cu(II), and Zn(II)
These complexes are stable at room temperature and
soluble in DMSO and DMF. Microanalytical, molar
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 31 No. 11 2005

792
CANPOLAT, KAYA
Table 1. Analytical and physical data for the ligand and complexes
Content (calcd/found), %
ΛM,
Com-
Empirical formula
(fr wt, g/mol)
Color
Orange
Mp, °C µeff, µB
Yield, %
–1 cm2 mol–1
pounds
Ω
C
H
N
HL
C16H16N2O3 (284.31)
194
2.5
2.8
5.3
4.2
6.1
74
59
63
69
71
67.44/67.59 5.55/5.67 10.11/9.85
61.21/61.44 5.05/4.83 9.27/8.96
61.23/61.47 4.95/4.84 8.76/8.96
60.71/60.99 4.99/4.80 9.00/8.89
60.65/60.81 4.98/4.78 9.10/8.87
Co(L)2 C32H30CoN4O6 (625.54) Light brown
Ni(L)2 C32H30NiN4O6 (625.30) Dark green
174 3.97
>300 2.89
Cu(L)2 C32H30CuN4O6 (630.15) Mustard yellow >300 1.75
Zn(L)2 C32H30N4O6Zn (631.99) Orange 192 Dia
conductance, and magnetic susceptibility data of the to the Co(II), Ni(II), Cu(II), and Zn(II) ion through the
complexes are given in Table 1. The reactions of the azomethine nitrogen atom is expected to reduce the
transition-metal salts with the Schiff base are described electron density in the azomethine link and, thus, to
by the following equation:
lower the C=N absorption frequency. Hence, this band
undergoes a shift to lower frequency to 1605–1615 cm–1
after complexation indicating the coordination of the
azomethine nitrogen [28] to Co(II), Ni(II), Cu(II), and
Zn(II). A medium-intensity band at 3250–3295 cm–1 in
the spectra of the free ligands due to phenolic O–H was
absent in the spectra of all the complexes, indicating the
deprotonation of the Schiff base prior the coordination
through its oxygen atom. This is further supported by
an increase in the absorption frequency of the phenolic
C–O band from 1265 cm–1 in the spectra of the free
ligand to 1275–1290 cm–1 in the spectra of the com-
plexes, indicating that the other coordination site of the
Schiff base is the phenolic oxygen [29, 30]. The practi-
cally unchanged O–H and C=N of the oxime group
itself do not coordinate to metal atoms by neither oxy-
gen nor nitrogen atoms.
2HL + M(CH3COO)2 · nH2O
M(L)2 + 2CH3COOH + nH2O,
M = Co, Ni, Cu, Zn (n = 4, 4, 1, 2, respectively).
IR spectra of the free Schiff base ligand and the
complexes exhibit various bands in the 400–4000 cm–1
region. The phenolic O–H stretching frequency of the
free ligand is expected at 3250–3295 cm–1; however,
this frequency is generally displaced to 2600 cm–1 due
to the internal hydrogen bridge OH···N=C [17]. The
band becomes broader and sometimes is not detected as
the hydrogen bond becomes stronger. Hydrogen bonds
in the Schiff base are usually very strong. The ligand is
relatively planar with adequate intramolecular dis-
tances that favor the formation of the hydrogen bond.
The presence of an electron-donating group on the
nitrogens also favors intramolecular hydrogen bond
formation [18].
An electron-donating group in the phenolic ring
increases the electron density on the hydroxyl oxygen
making the phenolic O–H bond stronger, and the
absorption usually appears as a broad band in the IR
spectrum. The phenolic O–H stretching appears as two
bands in the spectrum of the free ligand HL: the sharp,
weaker band at 3250 cm–1 is tentatively assigned to the
free O–H stretching frequency, and the broader, more
intense band at 3295 cm–1 is attributed to the intermo-
lecular hydrogen bonded phenolic O–H stretching [19].
The C=N stretching frequency for the ligand is
assigned at 1625 cm–1 in agreement with the assign-
ments reported [20, 21] for this band. An electron-
donating group in the phenolic ring increases the elec-
tron density on the hydroxyl oxygen more than the
azomethine nitrogen. A third characteristic band, due to
the C–O stretching frequency, appears as a weak band
within the 1265 cm–1 reported for similar ligands [22,
23].
1H and 13C NMR spectra. In order to identify
structures of the Schiff base ligand and complexes in
solution, the 1H and 13C NMR spectra were recorded in
1
CDCl3–DMSO-d6. The H and 13C NMR assignments
are also given in Experimental. Similar spectra were
obtained for the ligand. It is important to emphasize the
1H NMR resonance of the OH group at 13.10 ppm, due
to the presence of intramolecular hydrogen bonding
[31]. The singlet at 10.91 ppm can be assigned to the
proton of the oxime OH proton [32, 33]. The signal due
to the OH protons disappears in a D2O solution. The
single-proton resonance in the 1H NMR spectra of the
ligand accruing near 8.67 ppm has been assigned to the
azomethine group proton. The multiplets at 6.82–
7.67 ppm can be assigned to the protons of the aromatic
ring. In addition, the proton of the methoxy group is
observed at 3.72 ppm as a singlet.
The 13C NMR spectral data of the ligand confirms
the results of the 1H NMR spectra. The carbon atoms of
the CH=N, C–OH, and C–N are observed in the exten-
sive 162.36, 154.83, and 152.54 ppm ranges. The signal
at 55.48 ppm can be attributed to the carbon atom of the
The infrared spectrum of HL bands at ca. 3350–
1
3400 and 1595 cm–1 are assigned to O–H (oxime) and
methoxy (–OCH3) group. As can be seen in the H
C=N (oxime) [24–27]. Coordination of the Schiff base NMR spectra of the Zn(II) complex, there is no OH
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 31 No. 11 2005

STUDIES ON MONONUCLEAR CHELATES DERIVED
Table 2. TGA data of Co(II), Ni(II), Cu(II), and Zn(II) complexes with HL
793
Decomposition step
temperature
range, °C
weight loss
(calcd/found), %
temperature
range, °C
weight loss
(calcd/found), %
residue
(calcd/found), %
complexes
final product
Co(L)2
Ni(L)2
Cu(L)2
Zn(L)2
271–313
230–296
247–299
243–335
12.5/12.8
12.4/13.1
12.3/12.5
21.7/22.6
313–597
296–555
299–500
335–688
75.6/76.2
75.8/75.5
75.1/75.5
65.4/65.3
CoO
NiO
CuO
ZnO
11.9/11.0
11.8/11.4
12.6/12.0
12.9/12.1
peaks expected. The absence of the phenolic OH proton (P) transition in a tetrahedral ligand field [41]. Also, the
signal (at 13.10 ppm in the ligand HL) in the Zn(II) value of the magnetic moment (3.97 µB) is additional
evidence for a tetrahedral structure around the Co2+ ion.
complex indicates the coordination by the phenolic
oxygen to the metal ion after deprotonation. The coor-
dination of the azomethine nitrogen is inferred by the
upfield shift of the CH=N proton signal from 8.67 ppm
in the ligand to 8.53 ppm in the complex [34]. The aro-
matic proton signals and –OCH3 proton signal are seen
at 6.83–7.64 and 3.74 ppm, respectively.
The magnetic moment value of the Ni(II) complex
in the 2.89 µB corresponds to two unpaired electrons
and suggests a four-coordinate tetrahedral structure.
The spectra of the Ni(II) complex show a less intense
shoulder at 640 nm (ε = 195 l/(mol cm)), which is
assigned to d–d transition of the metal ion. The former
1
Thus, the H NMR data confirm the monobasic
band is probably due to the 3A2
3T2 (F) transition of
bidentate (ON) nature of the ligand, which already has
been suggested from the IR spectral data. 13C NMR
spectrum of the Zn(II) complex for the CH=N, C–OH,
and C–N carbon atoms are observed at 172.00, 159.29,
and 157.06 ppm, respectively. The rest of carbon atoms
likewise showed similar diagnostic features for the free
ligand, as well as its complex as expected. The signals
corresponding to the N–OH proton and C=N–OH car-
bon (both in oxime) [35–38] groups are unchanged in
the 1H and 13C NMR spectra of the complexes indicat-
ing that these oxime groups do not take part in com-
plexation.
Elemental analyses and molar conductance. The
general characteristic properties of the complexes and
the Schiff base are shown in Table 1. The elemental
analysis results agree with the calculated values show-
ing that the complexes have the 1 : 2 metal/ligand
ratios. The elemental analyses confirmed the composi-
tions of the above-synthesized compounds.
The observed molar conductance of the complexes
in 10–4 molar solutions in DMSO lies in the range 2.8–
6.1 cm2/(Ω mol). This is consistent with the nonelectro-
lytic nature of these complexes [39].
UV–Vis spectra and magnetic moment. A solu-
tion spectrum (DMF) of Schiff base ligand and its com-
plexes, as well as d–d bands in solution, is presented in
Experimental. HL exhibits a band at 270 nm assigned
tetrahedral geometry [42]. The Cu(II) complex exhibits
a magnetic moment value close to the spin-only value
for an unpaired at 1.73 µB. In visible region, the Cu(II)
complex exhibits a high-intensity shoulder at 418 nm
(ε = 1.11 × 103 l/(mol cm)) and a weak broad shoulder
at 597 nm (ε = 163 l/(mol cm)) assignable to charge
transfer (C–T) and 2T2
2E (G) in a tetrahedral field,
respectively [43]. The Zn(II) complex was found to be
diamagnetic as expected. The Zn(II) complex shows an
intense band at 368 nm (ε = 1.64 × 103 l/(mol cm)),
which is assigned to n
π* transition associated with
azomethine linkage. The spectra of the Zn(II) complex
show an intense band at 430 nm (ε = 3.52 × 103 l/(mol cm)),
which can be assigned to C–T transition of tetrahedral
geometry [44].
Thermal studies. The thermal behavior was deter-
mined by TGA curves for the Schiff base ligand and its
complexes at a heating rate of 10°C/min in a nitrogen
atmosphere over a temperature range of 25–800°C. The
thermal stability data are listed in Table 2. Thermo-
gravimetric studies of all the complexes showed no
weight loss up to 230°C, indicating the absence of
water in the complexes. The TGA curves showed that
the thermal decomposition of the complexes occurs in
two steps. Possibly, the different groups in the ligand
affect the stability of all the complexes. Furthermore, it
is known that the electronegativity and atomic radius of
the central metal atom affect also the thermal stability.
As a result of decomposition, the observed weight
losses for all complexes are in good agreement with the
calculated values. All these complexes undergo com-
plete decomposition to the corresponding thermody-
namically stable metal oxides, CoO (residue 11.0%),
NiO (residue 11.4%), CuO (residue 12.0%), and ZnO
(residue 12.1%). In addition, the thermal stability of all
to the phenyl ring transition π
π*, which remains
nearly unchanged in the spectra of its metal complexes.
The intense band at 375 nm (ε = 2.01 × 103 l/(mol cm))
in the spectrum of the ligand is assigned to the n
π*
transition, which shifted to higher energy on complex-
ation [40]. The electronic spectra of the Co(II) complex
in DMF solution are similar and show the main band at
676 nm (ε = 189 l/(mol cm)) assigned to the 4A2
4T1
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 31 No. 11 2005

794
CANPOLAT, KAYA
21. Kukushkin, Y.N., Krylov, V.K., Kaplan, S.F., et al.,
Inorg. Chim. Acta, 1999, vol. 285, no. 1, p. 116.
complexes increases in the order Cu < Ni < Co < Zn
[45, 46].
22. Percy, G.C. and Thornton, J.J., Inorg. Nucl. Chem.,
1973, vol. 35, no. 7, p. 2319.
ACKNOWLEDGMENTS
23. Felicio, R.C., Synth. React. Inorg. Met.-Org. Chem.,
1999, vol. 29, no. 2, p. 171.
The support of the Management Unit of Scientific
Research Projects of Firat University (FUBAP) under
research project no. 835 is gratefully acknowledged.
24. Canpolat, E., Kaya, M., and Gorgulu, A.O., Pol.
J. Chem., 2002, vol. 76, no. 5, p. 687.
25. Canpolat, E.. and Kaya, M., J. Coord. Chem., 2002,
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26. Canpolat, E., Kaya, M., and Yazc, A., Russ. J. Coord.
Chem., 2004, vol. 30, no. 2, p. 87.
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