Metallation of tetradentate N2O2 Schiff base with Mn(II), Co(II), Cu(II) and Zn(II): Synthesis, characterization and formation constants measurement

Article abstract of DOI:10.2478/s11532-009-0147-3

Four Schiff base ligands, salabza-H₂ = N,N′-bis(salicylidene)-2-aminobenzylamine, were synthesized by condensation of one mole of 2-aminobenzylamine and two moles of salicylaldehyde and/or two moles of substituted salicylaldehyde (5-OMe, 5-Br,

Full text of DOI:10.2478/s11532-009-0147-3

Cent. Eur. J. Chem. • 8(2) • 2010 • 291–299
DOI: 10.2478/s11532-009-0147-3
Central European Journal of Chemistry
Metallation of tetradentate N2O2 Schiff
base with Mn(II), Co(II), Cu(II) and Zn(II):
synthesis, characterization and formation
constants measurement
Research Article
Mozaffar Asadi1*, Maryam Mohammadikish1, Khosro Mohammadi2
1 Chemistry Department, College of Sciences, Shiraz University,
Shiraz 71454, I. R. Iran
2 Chemistry Department, Faculty of Sciences, Persian Gulf University,
Bushehr 75169, I. R. Iran
Received 13 June 2009; Accepted 7 October 2009
Abstract: Four Schiff base ligands, salabza-H2 = N,N’-bis(salicylidene)-2-aminobenzylamine, were synthesized by condensation of one mole
of 2-aminobenzylamine and two moles of salicylaldehyde and/or two moles of substituted salicylaldehyde (5-OMe, 5-Br, 5-NO2).
All the four Schiff bases and their Mn(II), Co(II), Cu(II) and Zn(II) complexes are characterized by UV-Vis, FT–IR, 1H NMR spectroscopy,
mass spectrometry and elemental analysis. The formation constants and the Gibbs free energies were measured spectrophotometrically
for 1:1 complexes in methanol in constant ionic strength (I = 0.1 mol dm-3 NaClO ) and at 25 C. The data refinement was carried out
˚
4
with the SQUAD program. The trend of formation constants of H2L1 with M(II) follows the order:
Mn(II) (3.97) < Zn(II) (4.30) < Co(II) (4.89) < Cu(II) (5.73)
Also, the trend of formation constants of ligand toward a given metal (for example Mn) is as follows:
H2L1 (p-OMe) (3.97) > H2L2 (p-H) (3.65) > H2L3 (p-Br) (3.37) > H2L4 (p-NO2) (3.04)
Keywords: Schiff base • Formation constant • Transition metals
© Versita Sp. z o.o.
bases and Cu(II) and Mn(III) complexes, which were
previously synthesized [29-33] but comprehensive
1. Introduction
studies on complexation not achieved. The present
Schiff bases are very important tools for the inorganic
report describes the synthesis and characterization
chemists as these are widely used in designing
of four Schiff bases and their complexation by Mn(II),
molecular ferromagnets [1,2], in catalysis [3,4], in
Co(II), Cu(II) and Zn(II) metals. These compounds were
biological modeling applications [5,6], and as liquid
1
characterized by elemental analysis, FT–IR, H NMR,
crystals [7,8]. Schiff base complexes are once again
topical in connection with self-assembling cluster
Mass and UV-Vis techniques. The effect of the nature of
the equatorial substituents on the ligand upon the UV-
complexes [9-15]. Schiff base complexes are known
Vis bands was studied. Also, the thermodynamic studies
to be biologically important and serve as catalysts in
of complex formation of the ligands with the metals were
various chemical and photochemical reactions [16-20].
described.
In view of recent interest in the energetic of the
metal ligand bonding in metal chelates involving N, O
donor ligand, we tried to study Schiff base complexes
derived from N,N'-bridged tetradentate ligands involving
N2O2 donor atoms [21-28]. There are some title Schiff
* E-mail: asadi@susc.ac.ir
291

Metallation of tetradentate N2O2 Schiff base
with Mn(II), Co(II), Cu(II) and Zn(II): synthesis,
characterization and formation constants measurement
9
2.4. Synthesis of the complexes
10
11
8
The manganese(II) and cobalt(II) complexes were
prepared under ambient conditions. To a hot solution of
the ligands (1.0 mmol) in appropriate solvent (for H2L1,
H2L2 and H2L3 chloroform and for H2L4 in CH3OH 10 mL:
DMF 10 mL) a hot methanolic solution of M(OAc)2
(1.0 mmol, M = Mn, Co, Cu and Zn) was added drop-
wise. The reaction mixture was refluxed for 2h. The
colored solution was cooled and evaporated to yield
colored powders or crystals. The products were washed
with water and methanol and dried under vacuum.
7'
7
N
N
6'
4'
6
4
5'
5
X
OH'
HO
X
3'
3
H2L1
H2L2
H2L3
H2L4
X =OMe,
X = H,
X = Br,
H2L1: 1H NMR (250 MHz, CDCl3, 25˚C): δ = 12.78 (s, 1H,
OH’), 12.64 (s, 1H, OH), 8.52 (s, 1H, N=CH7’), 8.42 (s,
1H, N=CH7), 7.41 (dd, 7.4 and 1.6 Hz, 1H, H10), 7.38 (d,
1.6 Hz, 1H, H11), 7.35 (d, 1H, 1.5 Hz, H8), 7.31 (dd, 7.4
and 1.5 Hz, 1H, H9), 7.01 (s, 2H, H6,6’), 6.88 (d, 2.5 Hz,
2H, H4,4’), 6.68 (d, 2.5 Hz, 2H, H3,3’), 4.94 (s, 2H, CH2),
3.83 (s, 3H, OCH3’), 3.73 (s, 3H, OCH3) ppm.
X = NO2,
Scheme 1. Schematic representation of the ligands and labels.
2. Experimental Procedure
H2L2: 1H NMR (250 MHz, CDCl3, 25˚C): δ = 13.32 (s, 1H,
OH’), 13.11 (s, 1H, OH), 8.56 (s, 1H, N=CH7’), 8.47 (s,
1H, N=CH7), 7.41 (t, 7.6 Hz, 1H, H10), 7.38 (d, 7.6 Hz,
1H, H11), 7.30 (t, 7.6 Hz, 1H, H9), 7.20 (d, 7.6 Hz, 1H,
H8), 7.15 (d, 7.5 Hz, 2H, H6,6’), 7.06 (d, 7.5 Hz, 2H, H3,3’),
6.96 (t, 7.5 Hz, 2H, H4,4’), 6.85 (t, 7.5 Hz, 2H, H5,5’), 4.96
(s, 2H, CH2) ppm.
H2L3: 1H NMR (250 MHz, CDCl3, 25˚C): δ = 13.23 (s, 1H,
OH’), 13.04 (s, 1H, OH), 8.50 (s, 1H, N=CH7’), 8.36 (s,
1H, N=CH7), 7.50 (s, 2H, H6,6’), 7.40 (d, 6.9 Hz, 1H, H11),
7.34−7.26 (m, 2H, H9, H10), 7.13 (d, 7.2 Hz, 1H, H8), 6.95
(d, 8.6 Hz, 2H, H4,4’), 6.82 (d, 8.6 Hz, 2H, H3,3’), 4.94 (s,
2H, CH2) ppm.
2.1. Reagents
Salicylaldehyde,
5-methoxysalicylaldehyde,
5-nitrosalicylaldehde,
5-bromosalicylaldehyde,
2-aminobenzylamine,Mn(OAc)2•4H2O,Co(OAc)2•4H2O,
Cu(OAc)2•H2O, Zn(OAc)2•2H2O, NaClO4, methanol,
chloroform, dichloromethane, dimethylformamide were
purchased from Merck, Fluka, Aldrich or Acros and used
without further purification.
2.2. Physical measurements
The electronic absorption spectra were recorded using
a Perkin-Elmer Lambda 2 spectrophotometer, while IR
spectra were recorded by Shimadzu FTIR–8300 infrared
spectrophotometer. The NMR spectra were recorded on
a Bruker Avance DPX–250 spectrometer in CDCl3 or
DMSO−d6 solvent using TMS as an internal standard at
250 MHz. The mass spectra were obtained on a Perkin-
Elmer R MU−6E instrument (electron ionization mode-70
electron volt). The elemental analysis was carried out by
Thermo Finnigan-Flash-1200.
H2L4: 1H NMR (250 MHz, DMSO−d6, 25˚C): δ = 14.40 (s,
1H, OH’), 14.00 (s, 1H, OH), 8.98 (s, 1H, N=CH7’), 8.82
(s, 1H, N=CH7), 8.35–8.19 (m, 2H, H10,11), 8.03 (dd, 9.5
and 3.0 Hz, 2H, H4,4’), 7.45 (s, 2H, H6,6’), 7.39−7.35 (m,
1H, H9), 7.09 (d, 9.2 Hz, 1H, H8), 6.66 (d, 9.5 Hz, 2H,
H3,3’), 5.02 (s, 2H, CH2) ppm.
1
ZnL1: H NMR (250 MHz, DMSO−d6, 25˚C): δ = 8.46
(s, 1H, N=CH7’), 8.39 (s, 1H, N=CH7), 7.38 (t, 8.1 Hz,
1H, H10), 7.28−7.18 (m, 1H, H9), 7.02–6.82 (m, 2H, H8,11),
6.69 (s, 2H, H6,6’), 6.60 (d, 9.2 Hz, 2H, H4,4’), 6.51 (d, 9.2
Hz, 2H, H3,3’), 4.66 (s, 2H, CH2), 3.66 (s, 3H, OCH3’),
3.63 (s, 3H, OCH3) ppm.
2.3. Synthesis of the ligand
2-aminobenzylamine (1.0 mmol) in methanol (20 mL)
was added drop-wise with continuous stirring to a warm
methanolic solution of appropriate salicylaldehyde
(2.0 mmol). After a few minutes, microcrystalline
solids appeared, which were subsequently isolated via
filtration. The isolated precipitates were washed with
small amounts of cold methanol and dried in vacuum
oven (Scheme 1).
ZnL2: 1H NMR (250 MHz, DMSO−d6, 25˚C): δ = 8.50 (s,
1H, N=CH7’), 8.43 (s, 1H, N=CH7), 7.79 (d, 7.6 Hz, 1H,
H11), 7.42−7.11 (m, 3H, H8, H9, H10), 6.96 (d, 6.9 Hz, 2H,
H6,6’), 6.62 (d, 6.9 Hz, 2H, H3,3’), 6.51 (t, 6.9 Hz, 2H, H4,4’),
6.41 (t, 6.9 Hz, 2H, H5,5’), 4.70 (s, 2H, CH2) ppm.
1
ZnL3: H NMR (250 MHz, DMSO−d6, 25˚C): δ = 8.49
(s, 1H, N=CH7’), 8.43 (s, 1H, N=CH7), 7.56 (s, 2H, H6,6’),
7.45−7.19 (m, 4H, H8, H9, H10, H11), 6.61 (d, 9.1 Hz, 2H,
H4,4’), 6.53 (d, 9.1 Hz, 2H, H3,3’), 4.68 (s, 2H, CH2) ppm.
292

M. Asadi, M. Mohammadikish, K. Mohammadi
1.2
0.9
0.6
0.3
0
out by spectrophotometric titrations of the ligands with
various concentrations of the metal ion at constant ionic
strength (0.10 mol dm-3 NaClO4) and at 25.0(± 0.1)ºC.
The interaction of NaClO4 with the ligands and metals
in methanol was negligible. In a typical measurement,
3 mL solution of ligand (5×10–5 mol dm-3) in methanol
was titrated with methanolic solution of M(OAc)2 (5×10–6
–1×10–4 mol dm-3). The metal concentrations were varied
till 2.0 fold in excess. UV–vis spectra were recorded in
the range 300–500 nm about 5 min after each addition.
The absorption measurements were monitored at
various wavelengths in the 350–410 nm regions where
the difference in absorption between the ligands and
the product was the largest after the equilibrium was
attained. As an example, the variation of the electronic
spectra for [H2L2] titrated with various concentrations of
Zn(II) acetate at 25˚C in MeOH is shown in Fig. 1. The
same procedure was followed for other systems.
330
350
370
390
410
430
450
470
λ (nm)
Figure 1. Spectrophotometric titration of H2L2 (5.0×10–5 mol dm-3)
with various amounts of Zn(OAc)2•H2O (5×10–6 –1×10–4
mol dm-3) in methanol, at I = 0.10 mol dm-3 (NaClO4)
and at 25ºC.
ZnL4: 1H NMR (250 MHz, DMSO−d6, 25˚C): δ = 8.73 (s,
1H, N=CH7’), 8.68 (s, 1H, N=CH7), 8.09−7.98 (m, 2H,
H10,11), 7.93 (s, 2H, H6,6’), 7.50−7.28 (m, 2H, H8,9), 6.75
(d, 9.5 Hz, 2H, H4,4’), 6.66 (d, 9.5 Hz, 2H, H3,3’), 4.78 (s,
2H, CH2) ppm.
3. Results and Discussion
2.5. Equilibrium measurements
Complexes were obtained from the reaction of the
metals with the Schiff base donors according to Eq. 1
(1)
The analytical and physical data for the complexes
are given in Table 1. The analytical data show that the
metal complexes involve 1:1 metal to ligand ratio and
the complexes are also associated with one or two
molecules of water or methanol.
H2Lx + M(OAc)2
[MLx] + 2HOAc
The formation constant measurements were carried
Table 1. Characteristic and analytical data of Schiff bases and their complexes.
Color
M.P(ºC)
Yield
Found (Calcd.)
H
C
70.47 (70.75)
76.16 (76.34)
51.68 (51.67)
59.67 (60.00)
58.92 (59.17)
59.77 (60.15)
43.34 (43.70)
51.54 (51.34)
59.49 (59.36)
59.22 (59.58)
44.43 (44.79)
48.79 (49.14)
58.43 (58.78)
61.45 (61.53)
45.59 (45.42)
51.73 (51.38)
59.36 (59.33)
58.97 (58.69)
43.22 (42.93)
50.20 (50.43)
N
H2L1
Orange
Yellow
Yellow
Yellow
Brown
Green
Green
Brown
Brown
Green
Brown
Brown
Brown
Green
Green
Green
Yellow
Yellow
Yellow
Yellow
116
94
89
100
83
71
89
77
89
58
72
51
47
87
90
68
97
58
42
44
50
5.74 (5.68)
5.45 (5.49)
3.20 (3.30)
3.88 (3.84)
5.78 (5.56)
4.78 (4.81)
2.91 (3.14)
3.29 (3.28)
4.50 (4.76)
4.47 (4.76)
2.66 (2.86)
3.92 (3.53)
4.47 (4.72)
4.13 (4.43)
2.78 (3.12)
3.04 (3.08)
4.66 (4.98)
4.48 (4.69)
2.88 (3.09)
3.98 (4.05)
7.17 (6.92)
8.43 (8.48)
6.02 (5.74)
H2L2
107
H2L3
146
H2L4
247
13.44 (13.33)
5.18 (5.52)
6.63 (6.68)
4.59 (4.85)
11.14 (11.40)
6.39 (6.02)
6.49 (6.62)
5.25 (4.97)
10.87 (10.91)
5.62 (5.96)
6.53 (6.83)
5.11 (4.82)
11.35 (11.41)
6.01 (5.77)
6.71 (6.52)
4.39 (4.77)
10.23 (10.23)
MnL1•2CH3OH
MnL2• 2H2O
MnL3•2H2O
MnL4•H2O
CoL1•H2O
CoL2•2H2O
CoL3•H2O
CoL4•2H2O
CuL1•H2O
CuL2•H2O
CuL3•CH3OH
CuL4•0.5 H2O
ZnL1•CH3OH
ZnL2•2H2O
ZnL3•2H2O
ZnL4•2CH3OH
113
162
155
>250
168
172
>250
>250
>250
>250
>250
>250
280
>250
>250
>250
293

Metallation of tetradentate N2O2 Schiff base
with Mn(II), Co(II), Cu(II) and Zn(II): synthesis,
characterization and formation constants measurement
Table 2. Mass spectral data of the Schiff bases and their complexes.
1.8
1.5
1.2
0.9
0.6
0.3
0
m/e
H2L1
H2L2
H2L3
H2L4
MnL1
MnL2
MnL3
MnL4
CoL1
CoL2
CoL3
CoL4
CuL1
CuL2
CuL3
CuL4
ZnL1
ZnL2
ZnL3
ZnL4
390, 240, 196, 168, 151, 123, 91, 57
330, 313, 210, 137, 180, 118, 91, 69
488, 289, 209, 180, 152, 118, 91, 57
420, 403, 396, 314, 255, 209, 180, 152, 118, 91, 57
443, 430, 256, 237, 213, 185, 167, 149, 111, 85, 57
383, 222, 194, 167, 149, 129, 97, 71, 55
541, 368, 313, 240, 180, 135, 97, 57
473, 237, 126, 97, 73, 57
5-NO2-Sal-2-abaH2
Mn[5-NO2-Sal-2-aba].H2O
Co[5-NO2-Sal-2-aba].2H2O
Cu[5-NO2-Sal-2-aba].0.5 H2O
Zn[5-NO2-Sal-2-aba].2CH3OH
446, 208, 148, 121, 97, 71, 55
250
300
350
400
450
500
550
λ (nm)
387, 328, 266, 250, 209, 180, 152, 121, 91, 51
545, 289, 180, 91
Figure 2. The electronic spectra of the H2L4 (3.0×10–5 mol dm-3),
MnL4 (5.0×10–5 mol dm-3), CoL4 (1.0×10–5 mol dm-3),
CuL4 (1.0×10–5 mol dm-3) and ZnL4 (5.0×10–5 mol dm-3).
477, 237, 192, 166, 129, 91, 71, 55
451, 388, 314, 239, 196, 168, 151, 111, 83, 57
391, 240, 210, 180, 149, 133, 113, 97, 71, 55
550, 213, 149, 98, 64
3.3. Electronic spectra
With aim to obtain information about the type of the
electronic transition and interaction liable to exist in
solutions, the electronic spectra of ligand and their
complexes were measured in DMF solvent (Fig. 2).
Generally, the recorded spectra of ligands displayed
two main absorption bands (Table 3). The first band was
observed in the 260−262 nm range as a broad and/or a
shoulder band. This band can be ascribed to the π–π*
transitions of the benzenoid system of the compounds
strongly overlapped with n–π* electronic transition that
involves the nonbonding electrons of the azomethine
nitrogen atom. A similar assignment was described by
Jaffe et al. [36].
482, 133, 98, 80, 64
453, 437, 391, 239, 151, 106, 83, 57
394, 368, 314, 283, 236, 211, 195, 166, 118, 97, 79, 52
552, 369, 313, 236, 152, 118, 57
482, 452, 436, 267, 207, 166, 129, 107, 91, 55
3.1. 1H NMR spectra
1
In the H NMR spectra of the ligands, the phenolic
proton was seen at 12−15 ppm. These high frequency
orthophenolic hydrogens in all Schiff base are attributed
to the presence of intramolecular hydrogen bonding. The
1H NMR of the ligands provides compelling evidence of
the presence of two azomethine groups. Due to different
chemical environments two signals were recorded for
azomethine proton. The phenol ring proton signals
resolve in the range 6−8 ppm and CH2 proton was seen
at 4.5−5.0 ppm. A singlet was seen at 3.60–3.90 ppm
that was assigned to those Schiff bases and their
complexes that have OMe group.
By comparing the 1H NMR spectra of the Schiff base
ligands to those of the corresponding zinc complexes,
it is noted that there is an upfield shift in the frequency
of azomethine protons confirming coordination of the
metal ion to both groups. In all the complexes, no signal
is recorded for the phenolic hydrogen in the 12−15 ppm
region, as in the case of the Schiff base indicating
deprotonation of the orthohydroxyl group [34,35].
The longer wavelength band(s) in the recorded
spectra of the ligands observed in the range 316–355 nm
can be ascribed to the transition within the whole
molecule essentially as intramolecular charge transfer
(CT) interaction. The CT bands seem to originate from
the aryl moiety, whereas, the positive charge azomethine
carbon atom is the acceptor center.
Generally, it is evident that addition of a metal ion
to the ligand solution causes distinguishable changes
in the visible absorption spectra of the ligand. This
behavior suggests an instantaneous complex formation
in solution from the reaction of each of the ligands under
investigation with each of the used metal ions.
Bands due to the n–π* transition of the C=N
chromophores occur between 260−262 nm as ligands
shifts to lower energy upon complexation [37]. In the
ultraviolet region, the bands observed in the range
267–299 nm in spectra of metal complexes can be
ascribed to an intraligand electronic transition, because
bands in these regions are sensitive enough to the type
of metal ion used.
3.2. Mass spectra
The mass spectra of all compounds show an intense
molecular ion peak [MLx]+ (Table 2).
The longer wavelength band(s) of each free ligand
at 316–355 nm has acquired a red shift on complex
294

M. Asadi, M. Mohammadikish, K. Mohammadi
Table 3. Electronic spectra data of Schiff bases and their complexes in DMF solvent.
π→π*
Intramolecular CT transition
Charge Transfer
(benzene ring and C=N)
λmax/nm (ε / M–1 cm–1)
262 (21250)sh
260 (29730)
λmax/nm (ε / M–1 cm–1)
355 (12802)
λmax/nm (ε / M–1 cm–1)
H2L1
H2L2
H2L3
H2L4
MnL1
327 (17533)
261 (29834)sh
261 (25114)
325 (17777)
316 (22730)
268 (18673)
331 (8888)sh
440 (4346)
MnL2
MnL3
267 (18916)
267 (16537)
320 (11783)sh
412 (4593)
326 (7518)sh
410 (3376)sh
MnL4
CoL1
CoL2
CoL3
CoL4
CuL1
267 (17890)
287 (10353)
284 (18184)
285 (11350)
299 (5258)
268 (22199)
394 (33807)
425 (3187)
396 (5236)
390 (5502)
381 (11242)
420 (9961)
458 (7135)sh
CuL2
CuL3
270 (25985)
268 (23827)
380 (12902)
412 (10543)sh
388 (8901)
420 (7311)sh
CuL4
ZnL1
ZnL2
ZnL3
ZnL4
272 (6632)
269 (10641)
273 (9453)
286 (9924)sh
267 (16545)
378 (7822)
404 (7171)
378 (7314)
377 (12338)
375 (35071)
sh shoulder
formation with the metal ion. A new band was observed
at longer wavelengths (in the range of 370–460 nm)
in case of the complex solution. This behavior can be
explained as follows.
3.4. IR spectra
To reach a conclusive idea about the structure of the
metal chelates with the ligands studied, infrared spectra
of the free ligands and their complexes were recorded.
The IR spectra of the free ligands and the complexes
exhibit various bands in the 400−4000 cm−1 regions.
Important IR absorption frequencies of ligands and their
complexes along with their assignments are listed in
Table 4. The IR spectra of complexes exhibit absorption
around 3400 cm−1 that is attributed to the presence of
lattice and coordinated water [38]. The weak bands at
2800−3100 cm−1 are related to C−H vibrations.
The longer wavelength band observed in the
spectrum of the free ligands can be assigned to an
intramolecular CT transition liable to take place within
the solute molecule. Thus the red shift observed in
the λmax of this band on complex formation with the
divalent transition metal ions Mn(II), Co(II), Cu(II) and
Zn(II) can be interpreted based on the principle of an
expected easier CT transition within the complexed
Schiff bases rather than within the free ones. This is due
to the positive charge of the coordinating metal ions. On
the other hand, these bands strongly overlap with an
intermolecular transition from the ligands molecules to
the vacant orbitals localized on the coordinated metal
ions, i.e., LMCT.
A strong band in the region 1610−1635 cm−1 is
observed in the IR spectra of all ligands, which may be
assigned to the stretching vibration of the azomethine
group [39]. Generally these bands shifted to lower
frequencies on complexation. Thus, it suggests that
coordination of each of the ligands under investigation
with the metal ions studied takes place through the
nitrogen azomethine. However, the shift of these bands
295

Metallation of tetradentate N2O2 Schiff base
with Mn(II), Co(II), Cu(II) and Zn(II): synthesis,
characterization and formation constants measurement
Table 4. Important IR spectral bands (cm-1) and their assignments.
υ O−H
υ C−H
υ C=N
υ C=C
υ C−O
υ M−N
υ M−O
H2L1
H2L2
H2L3
H2L4
MnL1
MnL2
MnL3
MnL4
CoL1
CoL2
CoL3
CoL4
CuL1
CuL2
CuL3
CuL4
ZnL1
ZnL2
ZnL3
ZnL4
3425
3009
2835
1635
1609
1566
1489
1161
3445
3414
3425
3421
3445
3433
3441
3441
3437
3441
3441
3441
3441
3441
3441
3433
3425
3425
3433
3055
2881
1632
1612
1566
1481
1149
1176
1176
1161
1141
1172
1099
1157
1153
1165
1099
1034
1145
1168
1107
1157
1149
1169
1103
3067
2912
1635
1616
1562
1473
3055
2920
1620
1610
1578
1485
3032
2854
1620
1608
1543
1465
771
763
756
752
752
756
756
756
783
756
768
756
794
756
756
756
675
621
648
650
678
617
636
644
667
602
636
652
671
698
636
640
3046
2854
1623
1608
1539
1446
3056
2920
1620
1605
1527
1454
3065
2932
1620
1608
1554
1466
3024
2854
1620
1608
1535
1466
3055
2931
1612
1608
1535
1450
3032
2851
1620
1608
1520
1454
3044
2920
1613
1608
1551
1470
3065
2920
1631
1608
1531
1466
3035
2920
1623
1608
1531
1443
3024
2854
1627
1608
1515
1454
3074
2932
1612
1601
1551
1462
3032
2843
1623
1608
1542
1473
3036
2920
1630
1610
1531
1446
3065
2920
1630
1610
1524
1454
3054
2920
1620
1601
1551
1481
to lower frequencies reveals that the bond order of the lower frequency in the IR spectra of the corresponding
−C=N− linkage is lowered upon their coordination to metal chelates confirming its coordination to the metal
metal ion.
ion [40-42]
The two bands appeared in the IR spectra of the
The coordination mode of ligands is further supported
free ligands and the corresponding metal chelates in by the new frequencies occurring in the 600−795 cm−1
the regions 1520−1580 cm−1 and 1445−1485 cm−1 are range and which have been assigned to M−O and M−N,
assigned to C=C stretching vibrations of the aromatic respectively [43].
systems. The observed shifts to lower frequencies
may be due to the involvement of C=C band in some
mesomeric interaction upon the formation of chelate
rings.
Furthermore, the phenolic C−O stretching vibrations
appeared at 1145−1175 cm−1 in the spectra of the
ligands and was found to have an appreciable shift to
296

M. Asadi, M. Mohammadikish, K. Mohammadi
Table 5. The formation constants, log K, for the complexes of ligands with M(II), in methanol at 25˚C (I = 0.10 mol dm-3).
Mn
Zn
Co
Cu
H2L1
H2L2
H2L3
H2L4
3.97 (± 0.04)
3.65 (± 0.03)
3.37 (± 0.04)
3.04 (± 0.07)
4.30 (± 0.01)
3.90 (± 0.03)
3.54 (± 0.02)
3.34 (± 0.02)
4.89 (± 0.03)
4.36 (± 0.02)
3.79 (± 0.02)
3.67 (± 0.13)
5.73 (± 0.10)
5.54 (± 0.03)
5.10 (± 0.02)
4.88 (± 0.07)
Table 6. The free energy, ΔG˚, for the complexes of ligands with M(II), in methanol at 25˚C (I = 0.10 mol dm-3).
Mn
Zn
Co
Cu
H2L1
H2L2
H2L3
H2L4
–22.65 (± 0.10)
–20.82 (± 0.07)
–19.22 (± 0.10)
–17.33 (± 0.17)
–24.52 (± 0.03)
–22.24 (± 0.07)
–20.19 (± 0.05)
–19.05 (± 0.05)
–27.90 (± 0.07)
–24.88 (± 0.05)
–21.62 (± 0.05)
–20.93 (± 0.32)
–32.70 (± 0.25)
–31.60 (± 0.07)
–29.09 (± 0.05)
–27.83 (± 0.17)
no ligand field stabilization, but Co(II)-d7 and Cu(II)-d9
have additional ligand field stabilization energy due to
Jahn-Teller effect. Also, Cu(II)-d9 has more Jahn-Teller
effect than Co(II)-d7.
3.5. The formation constant and the
thermodynamic
calculations
free
energy
The formation constant of the various Schiff bases
with the metal ions were calculated using SQUAD
program [44,45], designed to calculate the best
value for the formation constant of the proposed
equation model (Eq. 1) by employing a non-linear,
least-squares approach. Also the free energy change
ΔG˚ of the complexes formed were calculated from
ΔG˚ = –RT ln K at 25˚C.
3.7. The electronic effect of para substituted
Schiff base ligands
The stability of each metal complexed with the different
ligands used decreases according to the sequence:
5-OMe > 5-H > 5- Br > 5- NO2
This is somewhat in agreement with decreasing
electron-releasing character of the substituents in the
same direction, which results in a decrease in the basicity
of the azomethine nitrogen and, the phenolic oxygen
groups of the ligand and, consequently, the tendency
toward complex formation is expected to decrease. For
example, the 5-OMe substituted ligand has the tendency
to act as a good σ–donor as a result of the high electron
releasing power of the OMe groups in the para position
in HL1 relative to that of the non-substituted HL2 and
electron-withdrawing groups in para position HL3, HL4.
The withdrawing functional groups makes the Schiff
base a poor donor ligand and decreases the formation
constants while the electron donor group increases the
formationconstants.Therefore,theligandshavingBrand
NO2 groups have the smallest formation constant while
the ligands with OMe group have the highest [50,51].
3.6. Metal effect
The formation constant and the free energy parameter
for the metal-ligand complexes are presented in Tables
5 and 6. On the basis of the results, the formation of the
complexes follows the sequence below:
Mn(II) < Zn(II) < Co(II) < Cu(II)
This is in agreement with the general order of the
stability of the complexes of these metal ions which
has been reported before [46]. The electronegativity
(Pauling Scale) of the transition metals selected, 1.55,
1.88, 1.90 and 1.65 for manganese, cobalt, copper and
zinc respectively, helps us to interpret the above trend
in the formation of complexes. The above trend also is
confirmed by considering the M(II)–O and M(II)–N bond
distances [47,48]. Mean metal ligand bond distances for
isothiocyanate, pyridine, imidazole, water, and chloride,
bound to Mn, Co, Cu and Zn in their +2 oxidation states
have been investigated by See et al. [47], and were as
follows: Mn > Zn > Co ≈ Cu. The order of bond distances
is in agreement with the electronegativity order. The
more electronegative the metal is, the more it withdraws
electrons and therefore the formation constants are
higher [49].
4. Conclusion
Using a tetradentate N2O2 Schiff base ligand system,
we have succeeded in preparing four coordinate
Mn(II), Co(II), Cu(II) and Zn(II) complexes containing
2-aminobenzylamine and substituted salicylaldehyde.
By considering the formation constants and the ΔG˚
of the complex formation for Schiff base as donor and
Moreover ligand fields have additional stabilization
that confirms this trend. Mn(II)-d5 and Zn(II)-d10 have
297

Metallation of tetradentate N2O2 Schiff base
with Mn(II), Co(II), Cu(II) and Zn(II): synthesis,
characterization and formation constants measurement
the M(II) as acceptor, the following conclusions were
drawn:
Acknowledgment
1. The formation constant for a given Schiff bases (H2L1)
changes according to the following trend:
Mn(II) (3.97) < Zn(II) (4.30) < Co(II) (4.89) < Cu(II) (5.73)
2.TheformationconstantsforagivenM(II)(Mn)acceptor
toward the Schiff base donors changes according to the
following trend:
We are grateful to Shiraz University Council for their
financial support.
5-OMe (3.97) > 5-H (3.65)> 5- Br (3.37)> 5- NO2 (3.04)
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