Synthesis, crystal structure, fluorescence and electrochemical studies of a new tridentate Schiff base ligand and its nickel(II) and palladium(II) complexes

Article abstract of DOI:10.1016/j.saa.2014.02.179

A new unsymmetrical tridentate Schiff base ligand was derived from the 1:1 M condensation of ortho-vanillin with 2-mercaptoethylamine. Nickel and palladium complexes were obtained by the reaction of the tridentate Schiff base ligand with nickel(II) acetate tetrahydrate and palladium(II) acetate in 2:1 M ratio. In nickel and palladium complexes the ligand was coordinated to metals via the imine N and enolic O atoms. The S groups of Schiff bases were not coordinated to the metals and S-S coupling was occured. The complexes have been found to possess 1:2 Metal:Ligand stoichiometry and the molar conductance data revealed that the metal complexes were non-electrolytes. The complexes exhibited octahedral coordination geometry. The emission spectra of the ligand and its complexes were studied in methanol. Electrochemical properties of the ligand and its metal complexes were investigated in the CH₃CN solvent at the 100 mV s^-1 scan rate. The ligand and metal complexes showed both reversible and quasi-reversible processes at this scan rate. The Schiff base and its complexes have been characterized by IR, ¹H NMR, UV/Vis, elemental analyses and conductometry. The crystal structure of nickel complex has been determined by single crystal X-ray diffraction.

Full text of DOI:10.1016/j.saa.2014.02.179

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 363–369
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, crystal structure, fluorescence and electrochemical studies of
a new tridentate Schiff base ligand and its nickel(II) and palladium(II)
complexes
a
a
b
Bita Shafaatian a, , Ahmad Soleymanpour , Nasim Kholghi Oskouei , Behrouz Notash ,
⇑
Seyyed Ahmad Rezvani a
a School of Chemistry, Damghan University, Damghan 3671641167, Iran
b Chemistry Department, Shahid Beheshti University, G.C., Evin, Tehran 1983963113, Iran
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
An unsymmetrical tridentate Schiff
base with SNO donor atoms was
synthesized.
ꢀ
Two new complexes of nickel and
palladium were synthesized with 2:1
(L:M) ratio.
ꢀ
ꢀ
S–S coupling was occurred in these
complexes.
X-ray structure, 1H NMR, IR and
elemental analysis were used for
characterization.
ꢀ
Fluorescence and electrochemistry of
the synthesized compounds were
investigated.
a r t i c l e i n f o
a b s t r a c t
Article history:
AnewunsymmetricaltridentateSchiff baseligandwas derived fromthe1:1 Mcondensation ofortho-vanillin
with 2-mercaptoethylamine. Nickel and palladium complexeswere obtained by the reaction of the tridentate
Schiff base ligand with nickel(II) acetate tetrahydrate and palladium(II) acetate in 2:1 M ratio. In nickel and
palladium complexes the ligand was coordinated to metals via the imine N and enolic O atoms. The S groups
of Schiff bases were not coordinated to the metals and S–S coupling was occured. The complexes have been
found to possess 1:2 Metal:Ligand stoichiometry and the molar conductance data revealed that the metal
complexes were non-electrolytes. The complexes exhibited octahedral coordination geometry. The emission
spectra of the ligand and its complexes were studied in methanol. Electrochemical properties of the ligand
and its metal complexes were investigated in the CH3CN solvent at the 100 mV sꢁ1 scan rate. The ligand
and metal complexes showed both reversible and quasi-reversible processes at this scan rate. The Schiff base
and its complexes have been characterized by IR, 1H NMR, UV/Vis, elemental analyses and conductometry.
The crystal structure of nickel complex has been determined by single crystal X-ray diffraction.
Ó 2014 Elsevier B.V. All rights reserved.
Received 23 November 2013
Received in revised form 30 January 2014
Accepted 24 February 2014
Available online 12 March 2014
Keywords:
Schiff base
Tridentate
Unsymmetrical
Electrochemistry
Nickel(II) complex
Palladium(II) complex
Introduction
Schiff bases represent one of the most widely utilized classes of
ligands in metal coordination chemistry and the chemistry of Schiff
bases is an area of increasing interest [1,2]. Schiff bases are capable
⇑
Corresponding author. Tel./fax: +98 232 5235431.
E-mail address: shafaatian@du.ac.ir (B. Shafaatian).
http://dx.doi.org/10.1016/j.saa.2014.02.179
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

364
B. Shafaatian et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 363–369
of forming coordinate bonds with many metal ions via azomethine
or phenolic groups, and so they have been used for synthesis of
metal complexes due to their easy formation and strong metal-
binding ability [3–5]. Schiff bases have wide applications in food
and dye industries, analytical chemistry, catalysis, fungicidal, agro-
chemical and biological activities [6–8]. The interest in Schiff base
compounds as analytical reagents is increasing since they enable
simple and inexpensive determinations of different organic and
inorganic substances [9]. Moreover, it was reported that the Schiff
bases containing ONS-donors have an important role in biochemis-
try [10].
Particular attention has been devoted to the metal Schiff base
complexes in the last two decades. Schiff bases have been widely
used as ligands because of the high stability of coordination com-
pounds with different oxidation states [11]. There is a continuing
interest in metal complexes of Schiff bases, because of the presence
of hard nitrogen or oxygen and soft sulfur donor atoms in the back-
bone of these ligands. They readily coordinate with wide range of
transition metal ions yielding stable and intense colored metal
complexes, some of which have been shown to exhibit interesting
physical and chemical properties [12]. Many Schiff base complexes
show excellent catalytic activity in various reactions at high tem-
perature and in the presence of moisture and so over the past
few years, there have been many reports on their applications in
homogeneous and heterogeneous catalysis [13–15]. Moreover,
Schiff-base complexes are considered to be among the most impor-
tant stereochemical models in main group and transition metal
coordination chemistry due to their preparative accessibility and
structural variety [16]. On the other hand, Schiff base metal com-
plexes have numerous applications such as anticancer [17,6], bac-
tericide agents [18,19], antivirus [20–22] and fungicide agents
[23,24]. Many of tridentate Schiff base ligands showed enhanced
carcinostatic activity when complexed to transition metal ions
[25,26]. Therefore a lot of different Schiff base complexes of metal
ions such as Co(II), Cu(II) and Zn(II) have been frequently prepared
and their biochemical properties was studied [27].
The chemistry of nickel and palladium complexes with multiden-
tate Schiff base ligands has attracted huge attention because such
complexes play an important role in bioinorganic chemistry and
redox enzyme systems, and may provide the basis of models for ac-
tive sites of biological systems or act as catalyst [28]. Although, a lot
of Schiff bases with different structures have been synthesized and
characterized, however, little attention has been given to Schiff
bases which include the SNO-donor system. Thus, in this paper we
synthesized a new tridentate Schiff base containing SNO donor
atoms and its relevant nickel and palladium complexes. The Schiff
base ligand and its complexes were characterized by the FT-IR, 1H
NMR, UV–Vis spectroscopy, elemental analysis, magnetic suscepti-
bility, molar conductance and X-ray crystallography. Electrochemi-
cal and emission behavior of the Schiff base and its metal complexes
were studied by cyclic voltammetry and fluorescence spectroscopy.
were performed using a Perkin–Elmer 2400 series II. Melting
points were determined on a Barnstead Electrothermal 9100.
Conductivity was measured in DMSO solution (3 ꢂ 10ꢁ4 M) using
a 712 conductometer (Metrohm).
Cyclic voltammograms were performed using a Metrohm Auto-
lab/PGSTAT302N system equipped with a three-compartment cell
and a personal computer for data storage and processing. An Ag/
AgCl (saturated KCl) reference electrode (Metrohm), a Pt-rod as
counter electrode and a platinum disk electrode (i.d. = 2 mm) as
working electrode (Metrohm Pt-disk, 0.0314 cm2) were employed
for the electrochemical studies. Voltammetric measurements were
performed at room temperature in acetonitrile solution with 0.2 M
tetrabutylammonium perchlorate as the supporting electrolyte.
Syntheses
Synthesis of the tridentate Schiff base ligand, 1
To a solution of 2-mercaptoethylamine (0.050 g, 0.32 mmol) in
methanol (6 mL) was added a solution of ortho-vanillin (0.0986 g,
0.32 mmol) in methanol (6 mL) and the mixture was stirred and
heated on a water bath at 54 °C (optimum obtained temperature)
for 10 h. Then, the solvent was evaporated slowly and a yellow
crystalline product separated and washed twice with methanol,
dichloromethane and n-hexane. Then, the precipitate was dried
in vacuum. (yield: 71%), yellow, m.p: 78 °C, Anal. Calc. for
[C10H13NO2S] (1): (M.W: 211.28), C, 56.85; H, 6.20; N, 6.63%.
Found: C, 56.79; H, 6.12; N, 6.64%; IR (KBr,
t
/cmꢁ1): 1629
(OH), 1477,
(SH). 1H NMR (400 MHz, DMSO-d6,
t
(CN)
(imine), 1077
(aromatic), 776
t
t
(CO) (phenolic), 3436
(CS), 2847
t
t(C@C)
t
d/ppm): 13.42 (1H, s, AOH), 8.52 (1H, s, CH@N), 3.27 (1H, s,
SAH), 3.04 (2H, t, SACH2), 3.85 (2H, t, NACH2), 6.80–7.00 (phenylic
hydrogen group), 3.74 (3H, s, AOCH3).
Synthesis of nickel Schiff base complex, 2
A
methanolic solution (5 mL) containing Ni(OAc)2ꢃ4H2O
(0.0176 g, 0.071 mmol) was added slowly to a solution (5 mL) of
Schiff base (0.030 g, 0.142 mmol) and the mixture was stirred
and heated on a water bath at 56 °C (optimum obtained tempera-
ture) for 5 h. A green–brown solution of complex was left to stand
for a night. An oily brown precipitate was washed with methanol
and ether, and then dried in vacuum. (yield: 80%), m.p: 98 °C, Anal.
Calc. for: [C24H30N2NiO8S2] (2): (M.W: 597.33), C, 48.26; H, 5.06; N,
4.69; Ni, 9.83%. Found: C, 48.18; H, 4.97; N, 4.76, Ni; 9.64%. IR (KBr,
t
/cmꢁ1): 1614
t(CN) (imine), 1108
t(CO) (phenolic), 1485,
t(C@C)
(aromatic), 734
1.82 B.M.
t
(CS), 480 (NiAO), 577 t
t
(NiAN). leff (298 K):
Synthesis of palladium Schiff base complex, 3
The synthetic procedure was analogous to that of 2, except that
an ethanolic solution of Pd(OAc)2 (0.030 g, 0.073 mmol) was used
instead of Ni(OAc)2ꢃ4H2O and heated on a water bath at 58 °C
(optimum obtained temperature) for 10 h. A brown precipitate
was obtained and washed with n-hexane, then dried in vacuum.
(yield: 68%), m.p: 150 °C (decomp.), Anal. Calc. for [C24H30N2PdO8-
S2] (3): (M.W: 645.057), C, 44.69; H, 4.69; N, 4.34; Pd, 16.50%.
Experimental
Materials and physical measurements
Found: C, 44.24; H, 4.56; N, 4.28; Pd, 16.20%. IR (KBr,
1622 (CN) (imine), 1112 (CO) (phenolic),1448 (C@C) (aro-
matic), 738 (CS), 405 (PdAO), 487
(PdAN). 1H NMR
t
/cmꢁ1):
All chemicals were reagent grade quality purchased from com-
mercial sources and used as received. Elemental analyses (C, H, N)
and metal analyses (Ni and Pd) were performed on a Perkin Elmer
2400 elemental analyzer and GBS Integra XL ICP–OES, respectively.
UV–Vis and fluorescence spectra were recorded on an Analytik
Jena Specord 205 spectrophotometer and FP-6200 spectrofluorom-
eter, respectively. FT-IR spectra were obtained as KBr pellets on a
Perkin–Elmer spectrum RXI FT-IR spectrophotometer. 1H NMR
spectra were recorded on a Bruker Avance DPX 500 MHz spectro-
photometer, using TMS as an internal standard. The microanalyses
t
t
t
t
t
t
(400 MHz, DMSO-d6, d/ppm):8.17 (1H, s, CH@N), 2.34 (2H, t,
SACH2), 3.87 (2H, t, NACH2), 6.65–7.23 (phenylic hydrogens
group), 2.96 (3H, s, AOCH3). leff (B.M.) (298 K): diamagnetic.
Crystal structure determination and refinement
The X-ray diffraction measurements were made on a STOE
IPDS-II diffractometer with graphite monochromated Mo K
a radi-
ation. For complex 2, green prismatic shape crystal was chosen

B. Shafaatian et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 363–369
365
using a polarizing microscope and was mounted on a glass fiber
which was used for data collection. Cell constants and orientation
matrices for data collection were obtained by least-squares refine-
ment of diffraction data from 7076 unique reflections. Data were
crystallography data a distorted octahedral geometry was sug-
gested for the nickel complex. In suggested structure the Schiff
base ligands were coordinated to the nickel via the imine N and
enolic O atoms. Moreover, based on the spectroscopy data, we sug-
gested that the structure of the palladium complex is similar to the
nickel complex. It should be noted that when a 1:1 Ligand:Metal
stoichiometry was used, dimer complexes through sulfur bridge
can be formed.
collected to a maximum 2h value of 58.34° in a series of
x scans
in 1° oscillations and integrated using the Stoe X-AREA [29] soft-
ware package. A numerical absorption correction was applied
using X-RED [30] and X-SHAPE [31] software. The data were cor-
rected for Lorentz and Polarizing effects. The structures were
solved by direct methods [32] and subsequent difference Fourier
maps and then refined on F2 by a full-matrix least-squares proce-
dure using anisotropic displacement parameters [33]. The atomic
factors were taken from the International Tables for X-ray Crystal-
lography [34]. All refinements were performed using the X-STEP32
crystallographic software package [35]. A summary of crystal data,
experimental details, and refinement results was given in Table 1.
Single crystals of 2 were grown by slow diffusion of petroleum
ether into a saturated CH2Cl2 solution during 7 days at room temper-
ature, whereas suitable single crystals could not be grown for 1 and 3.
Spectroscopy
IR spectra
In the IR spectra (Fig. 1), all the compounds displayed the char-
acteristic peak in the region 1614–1629 cmꢁ1 indicated the forma-
tion of Schiff base (AHAC@N) [39]. A comparison of the spectra of
the free ligand and the complexes indicated that the ligand was
coordinated to the metal centers. Coordination of the azomethine
Results and discussion
Synthesis and characterization of compounds
nitrogen to the metals was suggested by the shift of the t(C@N)
band to lower frequencies band in the IR spectra of the nickel
and palladium complexes compared with the Schiff base ligand
[40,41]. Additional support for the formation of the complexes
were provided by the existence of weak intensity bands at 577
and 480 cmꢁ1 were attributed to the formation of NiAN and NiAO,
respectively. Also, two new weak bands at 487 and 405 cmꢁ1 were
The unsymmetrical tridentate Schiff base ligand was synthe-
sized by the condensation of 2-mercaptoethylamine with ortho-
vanillin in MeOH, in a 1:1 M ratio under heating on a water bath
and the yellow crystals of the ligand were obtained (Scheme 1).
In order to construct the metal complexes, the Schiff base ligand
was reacted with nickel(II) acetate tetrahydrate and palladium(II)
acetate in 2:1 M ratio in methanol and ethanol respectively. When
a 2:1 Ligand:Metal stoichiometry was used, an interesting product
formed which S groups of the two Schiff bases were coupled. A
strong covalent bond, ASASA, was important in linking two Schiff
bases in nickel and palladium complexes. This linkage was aroused
as a result of the oxidation of the thiol groups in the air during the
reaction. Also, the crystal structure of the nickel complex showed
the formation of ASASA bond [36–38]. Based on the X-ray
attributed to PdAN and PdAO, respectively [42]. The t(CAO) band
in these complexes was shifted approximately 31 cmꢁ1 toward a
higher frequency, compared with the free ligand, which suggested
coordination of the deprotonated phenolic oxygen to the nickel
and palladium ions [43]. A weak band in 2848 cmꢁ1 due to ASH
group in the free Schiff base ligand was not observed in the IR-
spectra of the nickel and palladinum complexes [44], revealed that
thiolate groups in two moles of Schiff base were deprotonated and
coupled to each other in order to form a ASASA covalent bond.
This coupling can be seen in the X-ray structure of the nickel com-
plex. Also, in these compounds the common bands at 620 cmꢁ1 and
about 734 cmꢁ1 were assigned to the SAS and CAS stretches,
respectively [45].
Table 1
Crystallographic and structure refinement data for 2.
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Crystal size (mm3)
a (Å)
C24H30N2NiO8S2
597.33
120(2)
0.71073
Triclinic
1H NMR spectra
The structural assignments were further supported by their 1H
NMR spectra. 1H NMR spectra of the compounds were obtained in
DMSO-d6 at room temperature using TMS as the internal standard.
In the 1H NMR spectra, the integral intensities of each signal were
found to agree with the number of different types of protons pres-
ent in the Schiff base ligand and its palladium complex. The most
important peak in the Schiff base ligand was attributed to a singlet
signal due to azomethine proton at the 8.52 ppm that confirmed
the formation of Schiff base. Chemical shift of the imine in the
Schiff base ligand comparing with the CH@N value of the imines
in palladium complex at 8.17 ppm supported nitrogen coordina-
tion to the metal center [46]. In the Schiff base ligand and its pal-
ladium complex, the protons peaks of OACH3 were appeared in
3.74 and 2.96 ppm, respectively. The singlet signal at 13.42 ppm
showed the presence of OH proton in the Schiff base ligand and
was not observed in palladium Schiff base complex. Moreover, a
singlet signal at 3.27 ppm confirmed the presence of SH proton
in the Schiff base ligands and was disappeared in the metal Schiff
base complex which revealed deprotonation of ASH groups due
to the easily SAS coupling. The multiplets of the aromatic protons
in 1 and 3 appeared in the range of 6.65–7.23 ppm with different
multiplicity and coupling constants. On the other hand, 1H NMR
spectrum of Schiff base and its palladium complex indicated the
aliphatic protons of ASACH2A as a triplet signal at 2.49 and
Pı
¯
0.17 ꢂ 0.15 ꢂ 0.15
8.5826(17)
10.957(2)
15.287(3)
75.25(3)
b (Å)
c (Å)
a
(°)
b (°)
89.65(3)
c
(°)
72.33(3)
1320.7(5)
2
Volume (Å3)
Z
Densitycalc (g cmꢁ1
)
1.502
h Ranges for data collection (°)
F (000)
2.50–29.17
624
0.943
ꢁ11 6 h 6 11
ꢁ15 6 k 6 15
ꢁ18 6 l 6 20
14808
Absorption coefficient (mmꢁ1
Index ranges
)
Data collected
Unique data, (Rint
)
7076, (0.1161)
346/0
0.0839/0.1143
0.1603/0.1337
1.039
Parameters/restrains
a
Final R1/wR2 (I > 2
r(I))
a
Final R1/wR2 (all data)
Goodness-of-fit on F2 (S)
Largest diff. peak and hole (e Åꢁ3
)
0.605, ꢁ0.485
h
ꢀ
ꢁ
i
1=2
P
P
a
R1
=
R
||Fo|–|Fc||/
R
|Fo|, wR2
¼
wðF2o ꢁ F2c Þ2
=
wðF2o Þ2
.

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B. Shafaatian et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 363–369
O
CH
OH
N
SH
C
H
2N
H
MeOH
+
SH
OH
OMe
OMe
Scheme 1. The synthetic route of the tridentate Schiff base ligand.
Fig. 1. Infrared spectra of (a) Schiff base, (b) nickel(II) and (c) palladium(II) complex.
3.04 ppm, respectively and also exhibited a triplet signal due to
ANACH2A resonances at 3.74 and 3.85 ppm which showed a down
field shift [47].
lower energy region (>200 nm). The stronger and higher energy
peak (Table 2) was attributed to the
methine chromophore and the benzene ring, while the weaker
and less energetic peak is assigned to the n ? pꢄ transition involv-
ing the promotion of the lone pair electron of azomethine nitrogen
p ?
pꢄ transition of the azo-
Electronic spectra
atom to the antibonding
p orbital associated with the azomethine
The electronic spectra of the Schiff base ligand and its com-
plexes were investigated in DMSO solution (6 ꢂ 10ꢁ5 M). The Schiff
base ligand and its complexes exhibit two absorption peaks at a
group [48]. The absorption spectra of the nickel and palladium
complexes bands were observed at 390 and 419 nm which was a

B. Shafaatian et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 363–369
367
Table 2
rates 100 mV/s within the potential range +2.0 to ꢁ2.0 V
(Figs. 3–5). All solutions were nitrogen-purged prior to experiment.
The obtained data from the cyclic voltammograms of the Schiff
base ligand and its metal complexes were given in Table 3. A typ-
ical cyclic voltammogram of Schiff base ligand, 1, was shown in
Fig. 3. The cyclic voltammogram showed one quasi-reversible pro-
cess at Epc = +0.70 V and Epa = +0.78 V due to the reduction of Schiff
base ligand with ipa/ipc close to unity (ipa/ipc = 0.91). One anodic
peak was also detected at Epa = +0.96 V which can be attributed
to the oxidation of ASH group in the ligand.
The cyclic voltammogram of Ni(II) complex (Fig. 4) showed
a well-defined reversible reduction peak at Epc = ꢁ1.26 V and
Epa = ꢁ1.17 V which correspond to Ni(II) ꢀ Ni(I) [53–56]. The
two other peaks at Fig. 4 were attributed to the Schiff base ligand
which revealed a little potential shift due to the complexation. On
the other hand, Epa = +0.96 in the cyclic voltammetry of ligand was
disappeared due to the SAS coupling in resulted complex and so no
oxidation peak of ASH was observed in this region.
The electronic spectra data (nm) of Schiff base and relevant metal co mplexes.
Compounds
p
?
pꢄ
n ? pꢄ
d ? d
1
2
3
270
280
266
330
390
419
–
525
522
higher wavelength shifting of the azomethine band of n ? pꢄ
showing the coordination of the nickel and palladium with CH@N.
Weak bands observed in the 525 nm (
e
= 1300 Mꢁ1 cmꢁ1) and
522 nm (
e
= 670 Mꢁ1 cmꢁ1) were due to d–d transition in nickel
and palladium complexes, respectively [49].
Fluorescence spectral studies
The emission spectra of the Schiff base ligand and its nickel
complex (1 and 2) were investigated at room temperature
(298 K) in methanol solution (Fig. 2). The Schiff base ligand was
characterized by emission band around 429 and 489 nm upon
photo excitation at 424 nm. The emission observed in the Schiff
The cyclic voltammogram of the Pd(II) complex (Fig. 5) showed
a well-defined redox process corresponding to the formation of
Pd(II)/Pd(I) couple at Epc = ꢁ0.09 V and Epa = ꢁ0.41 V. This couple
is found to be quasi-reversible process. The peaks of Schiff base
base was assigned to the
p ?
pꢄ intraligand fluorescence. The fluo-
rescence of the ligand was probably quenched by the occurrence of
a photoinduced electron transfer (PET) process due to the presence
of a lone pairs of electrons of the donor atoms in the ligand (N and
O) [50]. On the other hand, the Ni(II) complex was characterized by
the emission band at 530 nm with a higher emission intensity than
the Schiff base. Since, PET process is prevented by the complexa-
tion of the Schiff base ligand with metal ion; thus the fluorescence
intensity can be greatly enhanced by the coordination of Ni(II).
Moreover, the complex was coordinated with two Schiff base li-
gands and the
p ?
pꢄ intramolecular electron transition was
accordingly much higher [50,51] and the chelation of the ligands
to metal ion increases their rigidity and thus reduces the loss of
energy by thermal vibration decay [52].
Eelectrochemical studies of the Schiff base and its complexes
Cyclic voltammetry was performed with 2 ꢂ 10ꢁ3 M solutions
of Schiff base and its complexes in CH3CN with 0.2 M tetrabutyl-
ammonium perchlorate (TBAP) as supporting electrolyte and scan
Fig. 3. Cyclic voltammetry of Schiff base (2.0 mM) in acetonitrile (0.20 M TBAP) at a
stationary platinum-disk electrode (i.d. = 0.3 cm) with a scan rate of 0.1 V sꢁ1, at
25 °C.
Fig. 4. Cyclic voltammetry of nickel(II) complex (2.0 mM) in acetonitrile (0.20 M
TBAP) at a stationary platinum-disk electrode (i.d. = 0.3 cm) with a scan rate of
0.1 V sꢁ1 at 25 °C: (A) nickel(II) complex (2.0 mM); (B) supporting electrolyte.
Fig. 2. Fluorescence spectra of (a) Schiff base ligand (kexc = 424 nm) and (b)
nickel(II) complex (kexc = 524 nm) in methanol.

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B. Shafaatian et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 363–369
Table 3
Electrochemical parametersa for the Schiff base, nickel(II) and palladium(II) com-
plexes in acetonitrile solution containing TBAP (0.20 M).
Compound
Schiff base
Epc (V)
Epa (V)
E1/2
0.74
ipa/ipc
0.70
–
0.78
0.96
0.91
–
–
Nickel(II) complex
0.60
ꢁ1.26
0.81
ꢁ1.17
0.71
ꢁ1.22
2.24
0.98
Palladium(II) complex
0.56
ꢁ0.41
0.72
ꢁ0.09
0.64
ꢁ0.25
0.76
1.94
a
Measured at a stationary platinum-disk electrode (i.d. = 0.3 cm) with a scan rate
of 0.1 V sꢁ1 at 25 °C.
Description of the molecular structure of 2
Green crystals of 2 were obtained by slow diffusion of petro-
leum ether into a saturated CH2Cl2 solution during 7 days at room
temperature. Crystallographic data and parameters for complex
were summarized in Table 1. This Table reveals that complex 2
Fig. 5. Cyclic voltammetry of palladium(II) complex (2.0 mM) in acetonitrile
(0.20 M TBAP) at a stationary platinum-disk electrode (i.d. = 0.3 cm) with a scan
rate of 0.1 V sꢁ1 at 25 °C.
crystallize in the triclinic (space group, Pı) crystal system. The
¯
asymmetric unit of the complex consists of one crystallographi-
cally independent Ni(II) species. An ORTEP drawing of the complex
2 was presented in Fig. 6. The coordination geometry around nickel
ion can be described as distorted octahedral geometry. The dis-
torted octahedral geometry can rationalize the reduced effective
magnetic moment value (1.82 B.M). As shown in Fig. 6, four coor-
dination sites were occupied by doubly deprotonated Schiff base li-
gand and two other by two acetic acid molecules [58]. Nickel(II)
has six-coordination number in which two deprotonated phenolic
oxygen atoms occupy trans positions and two acetate and two
imine nitrogen atoms are located in cis position relative to each
other. Selected bond lengths and angles with their standard devia-
tions for complex 2 were given in Table 4. Structural parameter
ligand at +0.56 and +0.72 V were again observed in the voltammo-
gram of palladium complex.
Molar conductivity measurements
The molar conductivity of 3 ꢂ 10ꢁ4 M complexes was measured
in DMSO at room temperature. The molar conductivity values of
both complexes were in the range of 1.68–3.21 ohmꢁ1 cm2 molꢁ1
,
indicating the non-electrolytic nature of these complexes [57].
The conductivity value of complex 2 in DMSO can be explained
with respect to the crystal structure of this compound which
showed it did not have any ionic structure.
Fig. 6. The labeled diagram of complex 2 (thermal ellipsoids are at 30% probability level).

B. Shafaatian et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 363–369
369
Table 4
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Selected bond distances (Å) and bond angles (°) for 2.
Ni(1)AO(2)
2.010(3)
Ni(1)AO(3)
2.012(3)
2.049(4)
2.131(3)
1.820(5)
1.313(6)
1.294(6)
97.58(15)
87.25(13)
89.70(14)
86.76(15)
Ni(1)AN(1)
2.067(4)
Ni(1)AN(2)
Ni(1)AO(5)
2.180(3)
Ni(1)AO(7)
S(1)AC(10)
1.825(5)
S(2)AC(11)
S(1)AS(2)
N(1)AC(8)
2.0335(19)
1.290(6)
O(8)AC(23)
N(2)AC(13)
O(2)ANi(1)AO(3)
O(7)ANi(1)AO(5)
N(2)ANi(1)AO(7)
O(3)ANi(1)AO(5)
172.69(15)
86.10(13)
172.41(15)
88.03(13)
N(2)ANi(1)AN(1)
O(3)ANi(1)AO(7)
N(1)ANi(1)AO(7)
N(2)ANi(1)AO(5)
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Conclusion
The Schiff base ligand and its nickel and palladium complexes
were synthesized and characterized using microanalysis, 1H
NMR, IR, UV–Vis spectral data and conductometry. The Schiff base
ligand was coordinated as a bidentate ligand through NO donor
atoms of both phenolic and azomethine groups in nickel and
palladium complexes. Due to using 1:2 Metal:Ligand stoichiome-
try, the S donor atoms in the Schiff bases were not coordinated
to the metals and ASASA coupling was occurred which stabilized
the structure of the complexes. The elemental analysis and also X-
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Appendix A. Supplementary material
CCDC 908058 contains the supplementary crystallographic data
for complex 2. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: +44 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2014.02.179.
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