Synthesis, spectroscopy, electrochemistry and thermal study of Ni(II) and Cu(II) unsymmetrical N2O2 Schiff base complexes

Article abstract of DOI:10.1016/j.saa.2010.06.008

The new tetradentate unsymmetrical N₂O₂ Schiff base ligands, Ni(II) and Cu(II) complexes were synthesised and chracterized by IR, UV-vis, ¹H NMR and elemental analysis. The electrochemical properties of the Ni(II) complexe

Full text of DOI:10.1016/j.saa.2010.06.008

Spectrochimica Acta Part A 77 (2010) 424–429
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, spectroscopy, electrochemistry and thermal study of Ni(II) and Cu(II)
unsymmetrical N2O2 Schiff base complexes
Ali Hossein Kianfara,∗, Liala Keramata, Morteza Dostania, Mojtaba Shamsipurb,
Mahmoud Roushanib, Farzad Nikpourc
a Chemistry Department, Yasouj University, Yasouj, Iran
b Chemistry Department, Razi University, Kermanshah, Iran
c Chemistry Department, Kurdistan University, Sanandj, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
The new tetradentate unsymmetrical N2O2 Schiff base ligands, Ni(II) and Cu(II) complexes were synthe-
sised and chracterized by IR, UV–vis, 1H NMR and elemental analysis. The electrochemical properties of
the Ni(II) complexes were investigated. The thermogravimetry of the Ni(II) and Cu(II) complexes were
carried out in the range of 20–700 ◦C. Decomposition of synthesised complexes is related to the Schiff
base characteristics.
Received 12 March 2010
Received in revised form 15 May 2010
Accepted 4 June 2010
Keywords:
Ni(II)
© 2010 Elsevier B.V. All rights reserved.
Cu(II)
Schiff base complexes
Thermogravimetry
Electrochemistry
1. Introduction
thermodynamic parameters have been calculated using Coats and
Redfern [16] method.
The high stability potential of Schiff base complexes with differ-
ent oxidation states extended the application of these compounds
in a wide range. They were studied as catalysts in organic redox
and electrochemical reduction reactions [1–4]. Knowledge of elec-
tronic and steric effects to control the redox chemistry of these
compounds may prove to be critical in the design of new cata-
lysts. The Ni(I) and Ni(III) species can act as a powerful catalysis on
chemical or electrochemical reduction and hydrogenase reactions
[5–7]. The electronic effects of the functional groups on electro-
chemical properties of Schiff base complexes were investigated
previously [8–11]. Thermogravimetry (TG) and differential ther-
moanalysis (DTA) are valuable techniques for studying the thermal
behaviour of compounds. In view of recent interest in the ther-
mal behaviour of the metal ligand chelates involving Schiff base
ligands [12–15], we started to study the thermal behaviour of
Schiff base complexes derived from tetradentate ligands involving
an N2O2 donor atom. This paper describes the synthesis, spectral,
electrochemical and thermal studies of new unsymmetrical N2O2
tetradentate Schiff base ligands (Fig. 1), and their Ni(II) and Cu(II)
complexes. The electrochemical properties of the Ni(II) complexes
were studied by cyclic voltammetry in DMF solvent. Kinetics and
2. Experimental
2.1. Chemicals and apparatus
All of the chemicals and solvents used for synthesis and elec-
trochemistry were of commercially available reagent grade and
used without purification. The elemental analysis was determined
on a CHN–O-Heraeus elemental analyzer. Infrared spectra were
recorded as KBr discs on a FT-IR JASCO-680 spectrophotome-
ter in the 4000–400 cm−1. UV–vis spectra were recorded on a
JASCO V-570 spectrophotometer in the 190–900 nm. The 1H NMR
spectra were recorded in DMSO-d6 on DPX-400 MHz FT-NMR.
TG and DTA were carried out on a PL-1500. The measurements
were performed in air atmosphere. The heating rate was kept at
10 ◦C min−1
.
Cyclic voltammograms were performed using an autolab
modelar electrochemical system (ECO Chemie, Ultrecht, The
Netherlands) equipped with a PSTA 20 module and driven by GPES
(ECO Chemie) in conjunction with a three-electrode system and
a personal computer for data storage and processing. An Ag/AgCl
(saturated KCl)/3 M KCl reference electrode, a Pt wire as counter
electrode and a glassy carbon electrode as working electrode
(metrom glassy carbon, 0.0314 cm2) were employed for the elec-
trochemical studies. Voltammetric measurements were performed
∗
Corresponding author. Tel.: +98 741 2223048; fax: +98 741 2223048.
E-mail addresses: akianfar@mail.yu.ac.ir, asarvestani@yahoo.com (A.H. Kianfar).
1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.06.008

A.H. Kianfar et al. / Spectrochimica Acta Part A 77 (2010) 424–429
425
Fig. 1. The structure of Schiff bases and their complexes.
at room temperature in DMF solution with 0.1 M tetrabutylammo-
3. Results and discussion
nium perchlorate as the supporting electrolyte.
3.1. Elemental analysis
2.2. Synthesis of the threedentate Schiff base ligand
The elemental analysis (Table 1) is in good agreement with that
calculated for the proposed formula.
2-Hydroxyacetophenone (1 mmol, 0.2487 g) was added to a
solution of 1,2-phenylendiamine (1.5 mmol, 0.2933 g) dissolved in
20 ml methanol. The solution was refluxed for 3 h and methanol
was evaporated under reduced pressure. Water was added to such a
precipitate to dissolve phenylendiamine. The resulting yellow solid
was collected, washed with water and then recrystallized from
methanol. HL; yield (80%).
3.2. Infrared spectra
The IR spectra of the metal complexes of the Schiff base ligands
were compared with those of the Schiff bases themselves in order to
determine the coordination sites that may be involved in chelation
(Table 2).
The IR spectrum of the threedentate ligand exhibit sharp
medium intensity bands at 3300 and 3400 cm−1, which are
assigned to the NH2 functional group. These bands were disap-
peared in the spectrum of tetradentate ligands. The spectra of the
ligands exhibit broad weak intensity band in the 2500–3500 cm−1
range, which is assigned to the intramolecular hydrogen bond-
ing vibration (O–H· · ·N). This band disappeared in the spectra of
complexes [14,15,17].
2.3. General procedure for Synthesis of tetradentate ligands
All the tetradentate ligands H2L1, H2L2, H2L3, H2L4, were
synthesised in a similar way by stirring a methanolic solu-
tion of HL (1 mmol, 0.2263 g) with salicylaldehyde (1 mmol,
0.12 mml), 5-methoxysalicylaldehyde (1 mmol, 0.81 ml), 5-
bromosalicylaldehyde (1 mmol, 0.1691 g), 5-nitrosalicylaldehyde
(1 mmol, 0.1671 g), respectively. Stirring was continued for 3 h
at room temperature during which a yellow solid compound
separated. It was filtered, washed with methanol, recrystallized
from dichloromethane/methanol and dried over CaCl2.
Table 1
Elemental analysis data for the complexes.
Calculated/found (%)
Compounds
N
H
C
NiL1
65.51(65.4)
60.73(61.81)
54.37(55.25)
55.92(56.43)
64.39(65.29)
62.65(61.15)
53.5(52.12)
57.75(56.36)
4.15(4.17)
4.63(4.87)
3.23(3.63)
3.43(3.22)
4.08(4.19)
4.27(4.7)
7.27(6.95)
6.43(6.19)
6.04(6.04)
9.62(9.45)
7.15(7.16)
6.64(6.24)
5.95(5.55)
9.62(8.98)
2.4. General procedure for synthesis of Ni(II) and Cu(II) complexes
NiL2. H2O
NiL3
The Ni(II) and Cu(II) complexes were synthesised by refluxing
a methanolic solution of the tetradentate Schiff base ligands and
M(II) acetatetetrahydrate. The reaction was continued for 2 h dur-
ing which a green or brown precipitate separated. It was filtered,
washed with methanol and dried in vacuum.
NiL4
CuL1
CuL2
CuL3
3.19(3.36)
3.46(3.73)
CuL4·H2O

426
A.H. Kianfar et al. / Spectrochimica Acta Part A 77 (2010) 424–429
3.4. 1H NMR spectra
Table 2
Characteristic IR bands (cm−1) of the ligands and their Ni(II) and Cu(II) complexes.
Compound NH2 (cm−1
)
C
N (cm−1
)
C
C (cm−1
)
C–O (cm−1
)
NO2 (cm−1
)
The chemical shift (ı, ppm) of the different protons of the Schiff
base ligands and their Ni(II) complexes has been recorded in Table 4
(the solubility of NiL4 was low and we could not record its 1H NMR).
The 1H NMR spectra of tetradentate Schiff bases indicate the
presence of methyl group (at about 2 ppm), aromatic hydrogens
(at about 7 ppm), one azomethine (8–9 ppm) and two types of
OH in the range of 11.9–15 ppm. The H2L2 shows an additional
peak (OCH3) at 3.50 ppm. Intramolecular hydrogen bonding also
accounts for the high frequency of the signals for the orthophe-
nolic hydrogens (OH) in all the tetradentate Schiff bases. Due to
the different chemical environments, two signals are recorded in
this region. The signal at about 14 ppm is related to the pheno-
lic hydrogen of 2-hydroxyacetophenone. This signal is nearly a
constant chemical shift with different Schiff bases. The other sig-
nal is related to the phenolic hydrogen of salicylaldehyde and its
derivatives. This signal shows a down field shift via increasing
the electron withdrawing of functional groups. NO2 (15 ppm) > Br
(12.7) > H (12.5 ppm) > OCH3 (11.9 ppm).
HL
3300, 3400 1610
1481
1476
1491
1472
1487
1462
1488
1516
1542
1423
1525
1523
1524
1217
1216
1219
1217
1215
1217
1219
1216
1216
1216
1217
1217
1216
–
–
–
–
H2L1
H2L2
H2L3
H2L4
NiL1
NiL2
NiL3
NiL4
CuL1
CuL2
CuL3
CuL4
–
–
–
–
–
–
–
–
–
–
–
–
1613
1614
1611
1615
1609
1600
1603
1617
1603
1601
1611
1600
1332
–
–
–
1340
–
–
–
1309
The vibration of azomethine group of the free ligands is observed
at 1610–1615 cm−1. For the synthesised complexes this band was
shifted to the lower frequencies, indicating that the nitrogen atom
of the azomethine group is coordinated to the metal [14,15].
In all nickel complexes, no signals are recorded for the pheno-
lic hydrogen in the 11–15 ppm region, as in the case of the Schiff
bases indicating deprotonation of the orthohydroxyl group. The
coordination of all the ligands is also affirmed by almost 0.5 ppm
down field change in chemical shift for methyl group and aromatic
hydrogens.
3.3. Electronic spectra
The spectral data of the free ligands and their complexes are
listed in Table 3.
The electronic spectra of the Schiff base ligands in solu-
tion consists of a relatively intense band in the 300–380 nm
region, involving ␲ → ␲* transition, and a low intense band in the
360–500 nm region, involving n → ␲* excitation [14,15]. Complex-
ation with Ni(II) and Cu(II) results in two significant changes of
the spectra taking place in the UV–vis region (between 300 and
500 nm) of MLCT and ␲ → ␲* absorption bands [14–15]. Moreover,
an intense d → ␲* charge transfer band appears in the 425–475 nm
region in the spectra of the complexes studied here [11,15]. The
absorption band at about 600 nm is related to the d-d transition and
is consistent with the square planar geometry of Ni(II) and Cu(II)
complexes [18–20].
3.5. Thermal analysis
TG and DTA were carried out on the Ni(II) and Cu(II) complexes
in the temperature range of 20–700 ◦C. The thermal decomposi-
tion of the studied complexes presented characteristic pathways,
depending on the nature of the ligands, as can be seen from the
TG/DTA curves in Figs. 2 and 3.
Some of the studied complexes showed weight loss up to 80 ◦C
indicating the present of water molecule in the crystalline solid (for
elemental analysis the complexes were dried in vacuum at 60 ◦C).
In addition, in these complexes, the TG showed no weight loss up
to 150 ◦C indicating the absence of water molecule coordinated
Table 3
UV–vis spectral data (nm) for the ligands and their Ni(II) and Cu(II) complexes.
Compound
␲ → ␲* (phenolic chromophore)
␲ → ␲* (C N and benzene ring)
␲ → ␲* (C N)
d → ␲*
d → d
HL
260
260
257
275
257
250
250
250
238
270
263
270
265
342
342
341
341
300
300
300
310
320
–
–
–
375
–
330
350
350
355
400
375
375
375
375
–
–
–
–
–
425
425
425
420
490
475
470
475
–
–
–
–
–
600
610
605
650
598
605
595
585
H2L1
H2L2
H2L3
H2L4
CuL1
CuL2
CuL3
CuL4
NiL1
NiL2
NiL3
NiL4
–
–
–
Table 4
1H NMR data (ppm) for the ligands and their Ni(II) complexes.
Compounds
(s, 3H, CH3)
(s, 3H, O–CH3)
(m, 11H) Aromatic
(s, 1H, HC N)
(s, 1H, OHa)
(s, 1H, OHb)
H2L1
H2L2
H2L3
H2L4
NiL1
NiL2
NiL3
2.06
2.10
2.06
2.08
2.64
2.64
2.06
–
3.50
–
–
–
6.70–7.10
6.80–7.30
6.80–7.10
6.80–7.50
7.02–8.09
6.50–7.80
6.80–7.80
8.80
8.80
8.85
9.00
8.87
8.60
9.40
12.06
11.90
12.90
15.00
–
14.10
14.15
14.05
14.00
–
3.49
–
–
–
–
–

A.H. Kianfar et al. / Spectrochimica Acta Part A 77 (2010) 424–429
427
Fig. 4. Coats–Redfern plot of CuL3 complex, step 2, A = log(Wf/(Wf − W))/T2
.
to complexes [14,15], while NiL2 and CuL4 complexes indicate
this property. The first step in the decomposition sequence of
some complexes at 270–325 ◦C corresponded to the loss of methyl,
methoxy, and nitro in the studied complexes, respectively. The NiL3
was decomposed in one step at 460 ◦C. The inflexion of the TG
curve at a temperature range above 390 ◦C indicates the decom-
position of the organic part of the chelate, leaving metallic oxide at
the final temperature [14]. The thermal degradation of the studied
complexes can be depicted as follows.
NIL1
◦
◦
C
T=430
C21H16N2O2NiT=−3→25 CCH20H12N2O2Ni
−→
N O
20 12 2
NiO
–CH
–C
H
3
NIL2
◦
◦
C
T=390
H
C
C
C
C
22H18N2O3Ni · H2OT=−1→20 C22H18N2O3Ni −→ NiO
H
O
C
N O
2
22 18 2 2
NIL3
◦
T=460
C
Fig. 2. The TG and DTA of Ni(II) Schiff base complexes.
21H15N2O2BrNi
−→
NiO
C
H
N
OBr
21 15
2
NIL4
◦
◦
C
21H15N3O4NiT=−3→25 C21H15N2O2Ni −→ NiO
C
T=415
NO
C
H
N O
2
2
21 15
CUL1
◦
◦
◦
T=480 C
C21H16N2O2Cu · H2OT=−8→5 CC21H16N2O2CuT=−2→75 C20H13N2O2Cu −→ CuO
C
H
O
–CH
C
H
N O
2
3
20 13 2
CUL2
◦
◦
◦
T=490 C
C22H18N2O3Cu · H2OT=−9→0 CC21H18N2O3CuT=−2→70 C15H13N2O2Cu −→ CuO
C
H
O
–C
H
C
H
N O
2
6
5
15 13 2 2
CUL3
◦
◦
◦
T=490 C
C22H15N2O2BrCu · H2OT=−8→5 CC21H15N2O2BrCuT=−2→85 C20H12N2O2Cu −→ CuO
C
H
O
–CH
C
H
N
O
2
3
20 12
2
CUL4
◦
◦
◦
C21H15N3O4Cu · H2OT=−1→20 C21H15N3O4CuT=−3→05 C20H12N3O4Cu −→ CuO
C
C
T=395
C
H
O
–CH
C
H
N O
2
3
20 12 2 3
3.6. Kinetics aspects
All the well-defined stages were selected to study the kinetics of
decomposition of the complexes. The kinetics parameters (the acti-
vation energy E and the pre-exponential factor A) were calculated
using the Coats–Redfern equation [16],
ꢀ
ꢁ
ꢀ
ꢁ
g(˛)
T2
AR
ꢀE
2RT
Ea
Ea
log
= log
1 −
−
(1)
2.303RT
where g(˛) = [Wf)/(Wf − W)]. In the present case, a plot of L.H.S. of
this equation against 1/T gives straight line (Fig. 4) whose slope and
intercept are used to calculate the kinetics parameters by the least
square method. The goodness of fit was checked by calculating the
Fig. 3. The TG and DTA of Cu(II) Schiff base complexes.

428
A.H. Kianfar et al. / Spectrochimica Acta Part A 77 (2010) 424–429
Table 5
Thermal and kinetics parameters for Ni(II) and Cu(II) complexes.
Compounds
NiL1
ꢁT
% calculated (found)
E =/ (kJ mol−1
)
A =/ (s−1
)
S =/ (kJ mol−1 K−1
)
H =/ (kJ mol−1
)
G =/ (kJ mol−1
)
250–350
350–470
3/5(5)
70(65)
157.61
108.00
2.89 × 1011
−31
152.80
102.21
170.71
207.30
1.82 × 105
−151
90–145
320–500
7/1 (6/2)
76(73)
172.32
188.10
3.59.1010
−45.1
−33
169.10
184.40
187.00
199.00
NiL2
NiL3
1.74 × 1011
400–550
83(79)
111.93
1.17 × 105
−155
105.92
217.98
290–320
355–390
390–540
3/2(3/2)
9.9(10)
64(67)
97.63
143.76
111.90
1.19 × 106
2.58 × 109
1.04 × 105
−134.
−71.2
−156
92.77
138.40
105.89
170.90
184.19
218.67
NiL4
CuL1
CuL2
CuL3
270–300
350–475
3.5(4)
70(69)
228.67
135.05
6.17 × 1018
110
−67.8
224.08
129.24
163.36
177.19
3.76 × 109
425–345
410–535
17/5(18)
60(58)
92.24
71.70
1.13 × 106
1.55 × 102
−134
−211
87.79
65.46
159.48
223.92
255–285
400–590
3(3.1)
71(75)
206.65
73.01
2.06 × 1017
82
−184
202.35
65.99
157.07
205.41
4.41 × 103
80–125
285–315
320–435
3/9(4)
3/3(3)
74(70)
60.27
12.25
25.28
4.38 × 105
1.09 × 100
5.62 × 100
−139
−250
−237
57.13
7.48
19.70
109.45
150.73
178.71
CuL4
correlation coefficient. The other systems and their steps show the
same trend.
The entropy of activation S =/ was calculated using the equation
kTs
h
=/
/R
A =
eS
(2)
where k, h and Ts are the Boltzmann constant, the Planck’s con-
stant and the peak temperature respectively. The enthalpy and free
energy of activation were calculated using equations
Ea = H =/ + RT,
G =/ = H =/ − TS =/
(3)
(4)
The various kinetics parameters calculated are given in Table 5.
The activation energy (Ea) in the different stages is in the range
12–228 kJ mol−1. The respective values of the pre-exponential fac-
tor (A) vary from 1.09 to 6.17 × 1018 s−1. The corresponding values
of the entropy of activation (S =/ ) are in the range of −283 to
+110 J mol−1. The corresponding values of the enthalpy of activa-
tion (H =/ ) are in the range of 7–224 kJ mol−1. The corresponding
values of the free energy of activation (G =/ ) are in the range of
112–223 kJ mol−1. There is no definite trend either in the values
of activation parameters among the different stages in the present
series. But the activation energy for the second stage is smaller
than that of the first one due to the unstable intermediate occur-
ring at the later stages. However, the negative values of entropy of
activation indicate that the activated complex has a more ordered
structure than the reactants [14].
Fig. 5. Cyclic voltammogram of NiL3 in DMF at room temperature (reduction pro-
cess) Scan rate: 100 mV/s.
is attributed to quasi-reversible one electron oxidation of Ni(II)
to Ni(III) process while cathodic process is attributed to quasi-
reversible one electron reduction Ni(II) to Ni(I) process, similar to
that of other Ni(II) Schiff base complexes [15]. The Epan value for the
Ni(II) complexes increases via increasing the electron-withdrawing
property of functional group, while the Epca does not show any
trend.
3.7. The electrochemical study of Ni(II) complexes
The electrochemical properties of nickel complexes were
investigated in DMF. The Nickel(II) complexes exhibit a one
electron quasi-reversible oxidation and reduction process. The
electrochemical data are summarized in Table 6. The cyclic voltam-
mograms for NiL3 are shown in Figs. 5 and 6. The anodic process
Table 6
Redox potential data of Ni(II) complexes in DMF solution.
compound
Epa (II → III)
Epc (III → II)
Epc (II → I)
Epa (I → II)
NiL1
NiL2
NiL3
NiL4
0.900
0.900
0.972
1.064
0.783
–
0.820
0.677
−1.370
−1.324
−1.243
−1.396
−1.315
−1.261
−1.190
−1.280
Fig. 6. Cyclic voltammogram of NiL3 in DMF at room temperature (oxidation pro-
cess) Scan rate: 100 mV/s.

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429
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