Cu(II), Co(II), Ni(II), Mn(II), and Fe(II) metal complexes containing N,N′-(3,4-diaminobenzophenon)-3,5-But2-salicylaldimine ligand: Synthesis, structural characterization, thermal properties, electrochemistry, and spectroelectrochemistry

Article abstract of DOI:10.1016/j.saa.2009.12.002

The synthesis, structure, spectroscopic and electro-spectrochemical properties of steric hindered Schiff-base ligand [N,N′-(3,4-benzophenon)-3,5-Bu^t₂-salicylaldimine (LH₂)] and its mononuclear Cu(II), Co(II), Ni(II), Mn(II) and Fe(II) complexes are described in this work. The new dissymmetric steric hindered Schiff-base ligand containing a donor set of NONO was prepared through reaction of 3,4-diaminobenzophenon with 3,5-Bu^t₂-salicylaldehyde. Certain metal complexes of this ligand were synthesized by treating an ethanolic solution of the ligand with an equimolar amount of metal salts. The ligand and its complexes were characterized by FT-IR, UV-vis, ¹H NMR, elemental analysis, molar conductivity and thermal analysis methods in addition to magnetic susceptibility, electrochemistry and spectroelectrochemistry techniques. The tetradentate and mononuclear metal complexes were obtained by reacting N,N′-(3,4-benzophenon)-3,5-Bu^t₂-salicylaldimine (LH₂) with some metal acetate in a 1:1 mole ratio. The molar conductance data suggest metal complexes to be non-electrolytes.

Full text of DOI:10.1016/j.saa.2009.12.002

Spectrochimica Acta Part A 75 (2010) 811–818
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Cu(II), Co(II), Ni(II), Mn(II), and Fe(II) metal complexes containing
N,Nꢀ-(3,4-diaminobenzophenon)-3,5-But2-salicylaldimine ligand:
Synthesis, structural characterization, thermal properties,
electrochemistry, and spectroelectrochemistry
E. Tasa,∗, A. Kilicb, M. Durgunb, L. Küpecikb, I. Yilmazc, S. Arsland
a Department of Chemistry, University of Siirt, 56100 Siirt, Turkey
b Department of Chemistry, University of Harran, 63190 Sanliurfa, Turkey
c Department of Chemistry, Technical University of Istanbul, 34469, Istanbul, Turkey
d Department of Chemistry, University of Nevsehir, 50300 Nevsehir, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis, structure, spectroscopic and electro-spectrochemical properties of steric hindered
Schiff-base ligand [N,Nꢀ-(3,4-benzophenon)-3,5-But2-salicylaldimine (LH2)] and its mononuclear Cu(II),
Co(II), Ni(II), Mn(II) and Fe(II) complexes are described in this work. The new dissymmetric steric
hindered Schiff-base ligand containing a donor set of NONO was prepared through reaction of 3,4-
diaminobenzophenon with 3,5-But2-salicylaldehyde. Certain metal complexes of this ligand were
synthesized by treating an ethanolic solution of the ligand with an equimolar amount of metal salts.
The ligand and its complexes were characterized by FT-IR, UV–vis, 1H NMR, elemental analysis, molar
conductivity and thermal analysis methods in addition to magnetic susceptibility, electrochemistry and
spectroelectrochemistry techniques. The tetradentate and mononuclear metal complexes were obtained
by reacting N,Nꢀ-(3,4-benzophenon)-3,5-But2-salicylaldimine (LH2) with some metal acetate in a 1:1 mole
ratio. The molar conductance data suggest metal complexes to be non-electrolytes.
Received 9 February 2009
Received in revised form
20 November 2009
Accepted 1 December 2009
Keywords:
Synthesis
Schiff bases
Metal complexes
Thermal properties
Electrochemistry
Spectroelectrochemistry
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
base complexes have potential biological interest and are used as
more or less successful models of biological compounds [10]. Addi-
Tetradentate metal complexes of Schiff bases afford mixed
donor environments and an open equatorial ring, the hole size
of which can in principle accommodate more easily the expected
changes in metal size upon oxidation/reduction [1]. The Schiff base
ligands derived from the reaction of salicylaldehyde with diamines
have been extensively studied. These ligands are of great interest
mainly due to the existence of (O–H–N and N–H–O) type hydrogen
bonds and the possibility of tautomerism [2–5]. Moreover, Schiff
base metal complexes containing different metal ions such as Ni,
Co, Cu, Mn and Fe have been studied in great details for their various
crystallographic features, structure–redox relationships and enzy-
matic reactions, mesogenic characteristics and catalytic properties
[6,7]. Although the magnetic, spectroscopic and catalytic proper-
ties of these Schiff-base complexes are well documented [8,9], it
still seems that there could be new and specific applications for
such a unique class of compound. A considerable number of Schiff-
tionally, some of the salicylidene derivates show photochromism
in the solid state [11,12] most likely due to intramolecular proton
transfer associated with either a change in ␲-electron configura-
tion or isomerization in the non-planar molecule [13]. Sterically
hindered ligands bearing salicylaldimines are known to be effective
antioxidants and are widely used in rancidification of fats and oils
[14]. In most steric hindered salicylaldimine, the proton is localized
at the oxygen atom and in some cases is attached to the nitrogen
atom [15,16]. Proton transfer may occur both in solid and solution
states for basic and excited states [15–20]. It has been reported that
the transition metal complexes with redox active ligands bearing
sterically hindered salicylaldimines undergo one- or two-electron
transfer [21]. Redox-active transition metal complexes that stabi-
lize various oxidation states of the metal centers are of considerable
interest because of their potential significance as model of redox
metalloenzymes [22,23] and as effective redox reactants or cat-
alysts [24]. Furthermore, a recently emerged field of research
concerns the use of redox-active metal complexes such as Cu(II)
complexes with salen and related Schiff bases as synthetic chem-
ical nucleases or DNA damaging agents [25]. For a specific metal
∗
Corresponding author. Tel.: +90 4842232082-1230: fax: +90 4842232086.
E-mail address: esreftas@siirt.edu.tr (E. Tas).
1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.12.002

812
E. Tas et al. / Spectrochimica Acta Part A 75 (2010) 811–818
ion, the main factors, which are the nature and the arrangement of
the donor atoms around the metal bonding site or the nature and
the position of substituents, determine redox properties. Among
various ligands, many Schiff bases derived from steric hindered sal-
icylaldehyde and related aromatic aldehydes have been found to
stabilize copper(I), with the Cu(II/I) redox process [26]. Although
the redox behavior of a number of metal complexes containing
Schiff base ligands have been known, the electrochemical prop-
erties of such complexes are not completely clear [27–30].
In the present study, we report the synthesis and characteriza-
tion of the new steric hindered ligand (LH2) involving NONO donor
sites and their different mononuclear complexes. It is known that
the combination of the electrochemical and spectroelectrochemi-
cal methods provides a strong tool to reveal the complementary
nature of the molecular structure and electrochemical informa-
tion of the electroactive molecules [31–40]. The aim of this study
is to establish a comparative electro-spectrochemical study on the
new Schiff-base mononuclear metal complexes based on the differ-
ent molecular structures with NONO donor sites. Also, the thermal
properties of these metal complexes were investigated in this work.
2.2. Synthesis of ligand (LH2)
The synthesis of the ligand (LH2) has been reported previously
[31]. (Scheme 1). The products are soluble in common solvents
such as CHCl3, C2H5OH, DMF and DMSO. 1H NMR (300 MHz, CDCl3,
Me4Si, ppm): ı = 13.36 (s, 1H, −OH, D-exchangeable), 13.32 (s, 1H,
–OH, D-exchangeable), ı = 8.71 (s, 1H, HC N), ı = 8.70 (s, 1H, HC N),
ı = 7.88–7.20 (m, 12H, Ar–CH), ı = 1.44–1.43 (d, 18H, J = 3, C–CH3);
ı = 1.33–1.31 (d, 18H, J = 6, C–CH3), 13C NMR (300 MHz, CDCl3,
Me4Si, ppm): 189.91 (C O); 160.3, 159.9, 153.2, 153.0 (C N);
140.9–112.6 (Ar–C); 25.9, 23.8 vs 29.5, 28.5 (C–CH3).
2.3. General procedure for the synthesis of the metal(II)
complexes
The metal(II) complexes were prepared by the same general
method: 10 mmol of the ligand [N, Nꢀ-(3,4-benzophenon)-3,5-But
-
2
salicylaldimine (LH2)] was dissolved in 45 ml absolute ethanol
at room temperature (Scheme 1). A solution of 10 mmol of the
metal(II) acetate was added dropwise into 20 ml of absolute ethanol
under argon atmosphere with continuous stirring. The stirred mix-
tures were then heated to the reflux temperature and maintained
at this temperature for 8–10 h. Then, the mixtures were evaporated
to a volume of 10 ml in vacuum and left to cool to the room temper-
ature. The compounds were precipitated after adding 5 ml ethanol.
The products were filtered in vacuum and dissolved in chloroform.
The chloroform solution was then filtered to separate inorganic
salts and the products were precipitated by adding ethanol. And
then, they were washed with a small amount of ethanol. The
products were recrystallized from ethanol and they were dried at
100 ◦C. 1H NMR (300 MHz, DMSO, Me4Si, ppm): ı = 8.97–8.95 (d,
2H, J = 6, HC N), ı = 8.32–6.80 (m, 12H, Ar–CH), ı = 1.42–1.10 (m,
36H, C–CH3) for [Ni(L)] complex.
2. Experimental
2.1. Materials and measurements
All reagents and solvents were of reagent-grade quality and
obtained from commercial suppliers (Fluka, Merck and Aldrich).
The elemental analyses were carried out in the Laboratory of Inonu
University with a Carlo Erba Strumentaziona Model 1106 appara-
tus. IR spectra were recorded on a Perkin Elmer Spectrum RXI FT-IR
Spectrometer as KBr pellets. 1H NMR spectra were recorded on a
Bruker-Avence 300 MHz spectrometers. Magnetic susceptibilities
were determined on a Sherwood Scientific Magnetic Suscepti-
bility Balance (Model MK1) at room temperature (20 ◦C) using
Hg[Co(SCN)4] as a calibrant; diamagnetic corrections were cal-
culated from Pascal’s constants [41]. Electronic spectral studies
were conducted on a Perkin–Elmer model Lambda 25 UV–vis
spectrophotometer in the wavelength 200–1100 nm. The ther-
mal analyses measurements (TGA and DTA) were carried out on
a Setaram Labsys TG-16 thermobalance under a nitrogen atmo-
sphere. Molar conductivities (ꢀM) were recorded on an Inolab
Terminal 740 WTW Series. Cyclic voltammograms (CV) were car-
ried out using CV measurements with Princeton Applied Research
Model 2263 potentiostat controlled by an external PC. A three
electrode system (BAS model solid cell stand) was used for CV
measurements in CH2Cl2 and consisted of a 1.6 mm diameter of
platinum disc electrode as working electrode, a platinum wire
counter electrode, and an Ag/AgCl reference electrode. Tetra-n-
butylammonium perchlorate (TBAP) was used as a supporting
electrolyte. The reference electrode was separated from the bulk
solution by a fritted-glass bridge filled with the solvent/supporting
electrolyte mixture. The ferrocene/ferrocenium couple (Fc/Fc+) was
used as an internal standard but all potentials in the paper are ref-
erenced to the Ag/AgCl reference electrode. Solutions containing
ligand and the metal complexes were deoxygenated by a stream of
high purity nitrogen for at least 5 min before running the exper-
iment and the solution was protected from air by a blanket of
nitrogen during the experiment. UV–vis spectroelectrochemical
experiments were performed with a home-built thin-layer cell
that utilized a light transparent platinum gauze working electrode.
A platinum wire counter electrode, and Ag/AgCl reference elec-
trode were used for spectroelectrochemical cell. Potentials were
applied and monitored with a Princeton Applied Research Model
2263 potentiostat. Time-resolved UV–vis spectra were recorded on
Agillent Model 8453 diode array spectrophotometer.
3. Results and discussion
The ligand [N,Nꢀ-(3,4-benzophenon)-3,5-But2-salicylaldimine
(LH2)] (Scheme 1) was obtained from the reaction between 3,5-
But2-salicylaldehyde and 3,4-diaminobenzophenon in 2:1 molar
ratio. The Cu(II), Co(II), Ni(II), Mn(II), and Fe(II) complexes were
obtained by a general method using ethanolic solution of metal
acetate salt and solution of ligand in absolute ethanol under argon
gas (Scheme 1). Three chelate rings were formed by metal(II) coor-
dination to tetradentate steric hindered Schiff bases (with four
donor atoms, NONO). The ring formation determines a higher sta-
bility of the compounds [42]. The ligand and its metal complexes
were formed only in the powder form, which was not suitable for
single X-ray diffraction analysis. Further attempts to get crystals
were not successful. The ligand (LH2) on interaction with metal(II)
salts yielded complexes corresponding to the general formula
[M(L)]. The electrochemical and thin-layer spectroelectrochemi-
cal studies of the ligand and its mononuclear metal(II) complexes
were comparatively investigated in the same experimental condi-
tions.
3.1. NMR spectra of the ligand and Ni(II) metal complexes
The principal peaks of the 1H NMR spectra of ligand and its Ni(II)
metal complex are given in Section 2. The 1H NMR spectra of the
ligand in the CDCl3 did not give any signal corresponding to 3,5-
But2-salicylaldehyde and 3,4-diaminobenzophenon protons. The
proton NMR spectra of the free ligand showed two peaks as singlet
at 13.36 and 13.32 ppm, which are characteristic of intramolecu-
lar hydrogen bonded OH proton. These peaks did not appear in the
Ni(II) metal complex as expected. When D2O was added to the lig-

E. Tas et al. / Spectrochimica Acta Part A 75 (2010) 811–818
813
Scheme 1. Synthetic route of ligand (LH2) and the suggested mononuclear metal complexes.
Table 2
and solution, a rapid loss of these signals was observed as expected.
Characteristic IR bands (cm−1) of the ligand and its complexes as KBr pellets.
The peaks at range 7.88–7.20 ppm for ligand and 8.32–6.80 for Ni(II)
metal complex, as multiplet, is assignable to the protons of aro-
matic –CH groups. In the 1H NMR spectra of ligand and Ni(II) metal
complex, the chemical shifts observed at ı = 8.71 and ı = 8.70 ppm
for free ligand as singlet and at ı = 8.97–8.95 for Ni(II) metal com-
plex as doublet are assigned to the proton of azomethine (CH N)
[43,44]. However, the protons of the tert-butyl groups exhibited
two doublet peaks between ı = 1.44–1.43 and ı = 1.33–1.31 ppm
for free ligand and exhibited multiplet peak ı = 1.42–1.10 ppm for
Ni(II) metal complex. In the 13C NMR the imine carbon resonance
is found at 160.30, 159.90, 153.20, 153.0 ppm and carbonyl carbon
resonance is found at 189.91 ppm for free ligand, respectively. In the
spectra of the 13C NMR and 1H NMR of the ligand, the existence of
the signals at 160.30 and 159.90 ppm and at 153.20 and 153.00 ppm
for carbon double resonances, and at 8.71 and 8.70 ppm for proton
double resonances corresponds to the azomethine groups of the
molecule, which indicates that the ligand has cis–trans isomerism
[9].
Compounds
ꢁ(OH· · ·N)
ꢁ(Aliph C–H)
ꢁ(C O)
ꢁ(C N)
ꢁ(C–O)
Ligand (LH2)
[Cu(L)]
[Co(L)]
[Ni(L)]
[Mn(L)]
[Fe(L)]
2219–3505
2869–2965
2839–2985
2850–2980
2847–2978
2868–2956
2850–2960
1657
1643
1645
1648
1647
1651
1615
1602
1610
1608
1599
1598
1172
1171
1175
1178
1177
1177
–
–
–
–
–
3.2. FT-IR spectra of the ligand and its metal(II) complexes
The main IR characteristic stretching frequencies of the ligand
and its metal complexes along with their proposed assignments
are summarized in Table 2. The IR spectra of the metal complexes
are similar to each other, except for some slight shifts and inten-
sity changes of a few vibration peaks caused by different metal(II)
ions, which indicates that the metal complexes have similar struc-
ture. However, there are some significant differences between the
Table 1
The formula, formula weight, colors, melting points, molar conductivity, yields, magnetic susceptibilities and elemental analyses results of the compounds.
Compounds
F.W. (g/mol)
Color
Melting
ꢀM (ꢂ−1cm2 mol−1
)
Yield (%)
ꢃeff (B.M.)
Elemental analyses % calculated (found)
point (◦C)
C
H
N
Ligand (LH2)C43H52N2O3
[Cu(L)] C43H50N2O3Cu
[Co(L)] C43H50N2O3Co
[Ni(L)] C43H50N2O3Ni
[Mn(L)] C43H50N2O3Mn
[Fe(L)] C43H50N2O3Fe
644
Yellow
146
276
281
286
180
150
–
75
65
60
70
65
70
–
80.09(79.78)
73.11(72.98)
73.59(72.96)
73.62(73.27)
74.01(74.12)
73.92(74.41)
8.13(8.04)
7.12(6.97)
7.18(6.98)
7.18(7.17)
7.22(6.99)
7.21(7.10)
4.34(4.46)
3.97(3.76)
3.99(3.85)
3.99(3.82)
4.01(4.19)
4.01(3.95)
706.4
701.8
701.6
697.8
698.7
Dark Green
Dark Brown
Red
Dark Red
Black
12.6
18.3
9.8
13.8
11.7
1.74
3.35
Dia
3.82
3.90

814
E. Tas et al. / Spectrochimica Acta Part A 75 (2010) 811–818
metal(II) complexes and its free ligand upon chelation, as expected.
The coordination mode and sites of ligand to the metal ions were
investigated by comparing the IR spectra of the free ligand with its
metal complexes. Upon coordination, it is noteworthy that the peak
at 1657 cm−1 for ligand attributed to ꢁ(C O) vibration was shifted
to a lower wavenumber [45]. The coordination of the steric hin-
dered ligand with the different metals through the nitrogen atom is
expected to reduce the electron density in the azomethine link and
lower the ꢁ(C N) absorption frequency. The very strong and sharp
bands located at 1615 cm−1 are assigned to the ꢁ(C N) stretching
vibrations of the azomethine of the ligand. These bands are shifted
by 17–5 cm−1 to lower wavenumber. These shifts support the par-
ticipation of the azomethine group of this ligand in binding to the
Cu(II), Co(II), Ni(II), Mn(II), and Fe(II) ions [11,46,47]. In addition,
the IR spectrum of the ligand revealed a sharp peak at 1172 cm−1
belonging to phenolic C–O stretching which is slightly shifted to
lower or higher frequency range 1171–1178 cm−1 after complex-
ation in all metal complexes, suggesting that the phenolic oxygen
participates in the coordination mode. This has been further sup-
ported by the disappearance of the broad ꢁ(OH· · ·N) peak around
2219–3505 cm−1 in the all metal complexes, indicating deprotona-
tion of the phenolic proton prior to coordination [48].
shift and intensity change in the spectra of the metal complexes
most likely originate from the metalation which increases the con-
jugation and delocalization of the whole electronic system and
results in the energy change of the ␲ → ␲* and n → ␲* transition of
the conjugated chromophore [53,54]. The results clearly indicate
that the ligand coordinates to Cu(II), Co(II), Ni(II), Mn(II), and Fe(II)
ions, which are in accordance with the results of the other spec-
tral data. Furthermore, in the case of the Cu(II), Co(II), Ni(II), Mn(II),
and Fe(II) complexes, the absorption bands in the visible region are
observedatbetween 460and634 nm forCu(II) complex, at between
452 and 510 nm for Co(II) complex, at between 503 and 559 nm for
Ni(II) complex, at between 494 and 513 nm for Mn(II) complex and
at between 434 and 619 nm for Fe(II) complex (in CHCl3, DMSO
and EtOH) as a low intensity bands. These bands are considered
to arise from the forbidden d–d transition, which is generally too
weak. The nature of the Cu(II), Co(II), Ni(II), Mn(II), and Fe(II) ions
affects the position of absorption band significantly [55]. According
to modern molecular orbital theory [56], any factors that can influ-
ence the electronic density of conjugated system must result in the
bathochromic or hypsochromic shift of absorption bands. Here, in
the case of the metal complexes with same ligand, the main reason
of bathochromic or hypsochromic shifts is generally related with
the electronegativity of the different metal ions [53].
3.3. UV–vis spectra of the ligand and its metal complexes
3.4. Magnetic moments of metal complexes
Electronic spectra of ligand and its mononuclear Cu(II), Co(II),
Ni(II), Mn(II), and Fe(II) complexes have been recorded in the
200–1100 nm range in CHCl3, DMSO and EtOH solutions and their
corresponding data are given in Table 3. The formation of the metal
complexes was also confirmed by electronic spectra. In the elec-
tronic spectra of the ligand and its mononuclear metal complexes,
the wide range bands were observed due to either the ␲ → ␲* and
n → ␲* of C N chromophore or charge-transfer transition arising
from ␲ electron interactions between the metal and ligand, which
involves either a metal-to-ligand or ligand-to-metal electron trans-
fer [49–52]. The electronic spectra of the free ligands in CHCl3 and
DMSO showed strong absorption bands in the ultraviolet region
(263–408 nm), that could be attributed respectively to the ␲ → ␲*
and n → ␲* transitions in the benzene ring or azomethine (–C N)
groups for free ligand. In the electronic spectra of the metal com-
plexes, the band of the high or low wavelength side shows an
inequable bathochromic or hypsochromic shifts relative to its free
ligand in different solutions. The absorption bands between 263
and 408 nm in free ligand change a bit in intensity and remain
essentially slightly changed for metal complexes. The absorption
Magnetic susceptibility measurements provide sufficient data
to characterize the structure of the metal complexes. Magnetic
moments measurements of compounds were carried out at 25 ◦C.
The room temperature magnetic moment of Cu(II) metal complex
was found at 1.74 B.M. which was typical for mononuclear of Cu(II)
compounds with a S = 1/2 spin state and did not indicate antifer-
romagnetic coupling of spin at this temperature. These results are
comparable to the values reported for slightly distorted tetrahe-
dral geometry [20,57]. The room temperature magnetic moment
of the Co(II) complex was found at 3.55 B.M. which was close to
the spin-only magnetic moments (ꢃ = 3.87 B.M.) for three unpaired
electrons. Diamagnetic nature of the Ni(II) complex was also con-
firmed by the magnetic susceptibility measurement. The room
temperature magnetic moment of the Mn(II) complex was found
at 3.82 B.M. and found at 3.90 B.M. for the Fe(II) complex (Table 1).
3.5. Molar conductivity
With a view to studying the electrolytic nature of the mononu-
clear metal complexes, their molar conductivities were measured
in DMF (dimethyl formamide) at 10−3 M. The molar conductiv-
ities (ꢀM) values of these metal complexes are in the range
of 9.8–18.3 ꢂ−1cm2 mol−1 at room temperature, indicating their
almost non-electrolytic nature (Table 1). These low values indicates
that mononuclear metal complexes are non-electrolytes due to
lack of counter ions in the proposed structures of the mononuclear
metal complexes (Scheme 1) [58]. The difference in the number of
protons in the mononuclear Cu(II), Co(II), Ni(II), Mn(II) and Fe(II)
metal complexes probably points the difference in the sizes of the
+2 oxidation state of these metal ions, with the different polarizing
due to its different sizes.
Table 3
UV–vis data of the ligand and its metal complexes.
Compounds
Ligand (LH2)
Solvent
Wavelength [ꢄmax/(nm)]
CHCl3
DMSO
284
263
301
339
408
[Cu(L)]
[Co(L)]
[Ni(L)]
CHCl3
DMSO
C2H5OH
254
260
264
327
324
322
350
452
426
460
634
530
CHCl3
DMSO
C2H5OH
258
266
253
332
318
280
358
423
332
452
510
344
453
CHCl3
DMSO
C2H5OH
267
270
265
305
303
302
396
390
392
519
503
505
559a
3.6. Thermal properties of the ligand and its metal complexes
The thermal properties of the ligand and its metal complexes
were investigated by thermal gravimetric analysis (TGA) and ther-
mal differential analysis (DTA). Fig. 1 shows the recorded TGA and
DTA curves of ligand (a) and its Ni(II) and Mn(II) (b and c) metal
complexes under a nitrogen atmosphere. The samples were heated
up at 1 atm pressure with a heating rate of 10 ◦C min−1 and a tem-
[Mn(L)]
[Fe(L)]
CHCl3
DMSO
255
260
305
320
366a
357
494
479
615a
619a
CHCl3
DMSO
240
260
313
314
434
400
459
a
Shoulder peak.

E. Tas et al. / Spectrochimica Acta Part A 75 (2010) 811–818
815
Fig. 1. The TGA and DTA curves of (a) ligand (LH2), (b) [Ni(L)] and (c) [Mn(L)] metal complexes.
perature range of 20–900 ◦C. The thermal decomposition of ligand
and its metal complexes are seen to be irreversible. It can be seen
that TGA curve of the ligand (Fig. 1a) and its Ni(II) and Mn(II) (b
and c) metal complexes shows no mass loss up to 182 ◦C, indi-
cating the absence of water molecules and any other adsorptive
solvents molecules in coordination sphere. As the temperature is
increased, the thermal decomposition of the ligand occurs at three
steps (Fig. 1a). The first degradation step takes place in the range
of 212–395 ◦C which is a weight loss 30.9% and associated with
exothermic peaks at 240 and 261 ◦C with endothermic peak at
276 ◦C. For ligand, the second degradation step takes place in the
range of 424–533 ◦C which is a weight loss 61.4% and associated
with exothermic peaks at 420 and 522 ◦C with endothermic peak
at 447 ◦C. The third degradation step takes place in the range of
579–900 ◦C which is a weight loss 64.4% and associated with two
exothermic peaks at 619 and 679 ◦C. The TGA curves of the metal
complexes give two or three-stage decomposition pattern within
the ranges 20–900 ◦C. The first degradation step takes place in the
range of 287–419 ◦C which is a weight loss 36.6% and associated
with exothermic peaks at 263 and 374 ◦C with endothermic peak
at 290 and 391 ◦C. The second degradation step takes place in the
range of 441–867 ◦C which is a weight loss 60.1% and associated
with exothermic peaks at 646 and 735 ◦C with endothermic peak
at 497 ◦C for Cu(II) metal complexes. For Co(II) metal complex, The
first degradation step takes place in the range of 184–393 ◦C which
is a weight loss 42.7% and associated with endothermic peak at
346 and 382 ◦C, the second degradation step takes place in the
range of 465–900 ◦C which is a weight loss 68% and associated with
exothermic peaks at 533, 751 and 783 ◦C with endothermic peak
at 597 and 819 ◦C. For Ni(II) metal complex, the first degradation
step takes place in the range of 211–316 ◦C which is a weight loss
4.1% and associated with endothermic peak at 327 ◦C, the second
degradation step takes place in the range of 389–494 ◦C which is
a weight loss 54% and associated with exothermic peaks at 538 ◦C
with endothermic peak at 473 ◦C, the third degradation step takes
place in the range of 590–900 ◦C which is a weight loss 77.6%
and associated with exothermic peaks at 617 ◦C with endother-
mic peak at 643 and 679 ◦C (Fig. 1b). For Fe(II) metal complex,
the first degradation step takes place in the range of 182–416 ◦C
which is a weight loss 45.8% and associated with exothermic peaks
at 574 ◦C with endothermic peak at 349 and 379 ◦C, the second
degradation step takes place in the range of 467–900 ◦C which is
a weight loss 68.1% and associated with exothermic peaks at 532,
753 and 784 ◦C with endothermic peak at 602 and 818 ◦C. For Mn(II)
metal complex, the first degradation step takes place in the range
of 247–398 ◦C which is a weight loss 27.4% and associated with
endothermic peak at 363 ◦C, the second degradation step takes
place in the range of 447–586 ◦C which is a weight loss of 45.2%
and associated with exothermic peaks at 536 ◦C with endothermic
peak at 560 ◦C, the third degradation step takes place in the range
of 657–900 ◦C which is a weight loss 55.0% and associated with
exothermic peaks at 684 and 740 ◦C with endothermic peak at 706
and 754 ◦C (Fig. 1c), respectively [59]. As we look at the beginning
to decompose temperature, data also reveal that the most stable
compound is Cu(II) metal complex while the least stable compound
is Fe(II) metal complex. Hence Cu(II) is metal complex decompose
later. [60]. From the TGA and DTA results, mononuclear metal com-
plexes have different thermal stabilities, which may be attributed to
the fact that the M–N and M–O bonds are different highly polarized
[61].

816
E. Tas et al. / Spectrochimica Acta Part A 75 (2010) 811–818
Fig. 4. Cyclic voltammogram (CV) of [Mn(L)] in CH2Cl2 containing 0.1 M TBAP. Scan
rate: 0.010 V s−1. Working electrode: a 1.6 mm diameter of platinum disc electrode.
Fig. 2. Cyclic voltammogram (CV) of [Ni(L)] in CH2Cl2 containing 0.1 M TBAP. Scan
rate: 0.010 V s−1. Working electrode: a 1.6 mm diameter of platinum disc electrode.
c = 4.0 × 10−3 M. The inset figure presents the CV of the ligand (LH2) in the same
experimental conditions.
c = 4.0 × 10−3 M.
sents the CV of the nickel complex [Ni(L)] where the CV of the
ligand is also given as a inset figure. The ligand showed one irre-
versible oxidation at 1.20 V (Epa) and one irreversible reduction
at −1.45 V (Epc) versus Ag/AgCl in the scan rate of 0.010 V s−1
.
[Ni(L)] exhibited two anodic waves (IIa and IIIa) with corresponding
cathodic waves (IIc and IIIc) and one cathodic wave (Ic) with-
out corresponding anodic wave. The anodic wave (IIa) belonging
to the first oxidation is assigned to ligand-based oxidation. The
cathodic peak waves of the first oxidation and reduction processes
of the complex [Ni(L)] appeared almost at the same position with
those of the ligand, indicating that the first oxidation and reduc-
tion processes of the complex is based on the ligand. The first
and second oxidation processes of [Ni(L)] are quasi-reversible with
the corresponding anodic–cathodic peak separations (ꢅE) for the
(L)/(L)+ couples (0.12 V at scan rate 0.010 V s−1) and Ni(II)/Ni(III)
couples (0.17 V at scan rate 0.010 V s−1). Half-wave potentials (E1/2
)
of (L)/(L)+ and Ni(II)/Ni(III) couples were calculated as the aver-
age of the anodic and cathodic peak potentials of the processes,
which are given in Table 4. Fig. 3 represents the CV of the copper
complex [Cu(L)] where the inset figure exhibits differential pulse
voltammogram (DPV) of the complex. [Cu(L)] showed two quasi-
reversible oxidation and one irreversible reduction processes. This
electrochemical behavior is similar to that of the [Ni(L)] in the same
experimental conditions. The combined anodic waves (IIa and IIIa)
observed for the CV of [Cu(L)] could be separated by differential
pulse voltammetry. No considerable difference was observed for
the voltammetric data of [Cu(L)] compared with those of the [Ni(L)].
Fig. 4 presents the CV of the manganese complex [Mn(L)] which
exhibits considerably different electrochemical behavior compared
with [Ni(L)] and [Cu(L)] complexes. [Mn(L)] gave two oxidation
and two reduction processes. The first and second oxidation pro-
cesses are based on the ligand and metal center, respectively as
observed for the [Ni(L)] and [Cu(L)] complexes. The first oxidation
which has no corresponding cathodic wave is irreversible while
the second oxidation with a corresponding anodic wave is quasi-
reversible. The cathodic wave (IIc) belongs to a reduction of metal
center Mn(II)/Mn(I). This reduction process has irreversible char-
acter with the large anodic–cathodic peak separation (ꢅE) for the
Mn(II)/Mn(I) couples (0.455 V at scan rate 0.010 V s−1), probably
due to a low electron transfer on the electrode surface. The second
reduction process is based on the ligand as observed for [Ni(L)] and
[Cu(L)] complexes. The half-wave potential of the oxidation process
based on the Mn(II)/Mn(III) couples shifted toward more positive
value while the cathodic peak potential (Epc) of the reduction pro-
Fig. 3. Cyclic voltammogram (CV) of [Cu(L)] in CH2Cl2 containing 0.1 M TBAP. Scan
rate: 0.010 V s−1. Working electrode: a 1.6 mm diameter of platinum disc electrode.
c = 4.0 × 10−3 M. The inset figure presents the DPV of the complex in the same exper-
imental conditions.
3.7. Electrochemistry and spectroelectrochemistry
Electrochemistry of the ligand (LH2) and its metal complexes,
[Ni(L)], [Cu(L)], and [Mn(L)], were studied by cyclic voltammetry
in scan rate of 0.010 V s−1 in dichloromethane (CH2Cl2) solution
containing 0.1 M TBAP supporting electrolyte. Cyclic voltammo-
grams (CVs) of the ligand and complexes are given in Figs. 2–4
and their voltammetric data are listed in Table 4. Fig. 2 repre-
Table 4
Voltammetric data for the ligands and their copper complexes versus Ag/AgCl in
CH2Cl2–TBAP.
Compounds
(L)/(L)−a
(L)/(L)+b
M(II)/M(I)a
M(II)/M(III)c
Ligand (LH2)
[Ni(L)]
[CuL)]
−1.45
−1.41
−1.39
−1.66
1.20a
1.10c (0.12)
1.11c (0.18)
1.25d
1.32 (0.17)
1.30 (0.20)
1.43 (0.19)
[Mn(L)]
−0.62
a
Epc: cathodic peak potential for reduction for irreversible processes.
ꢅEp = Epc − Epa at 0.010 V s−1 scan rate (the values given in parentheses).
E1/2 = Epc + Epa/2.
b
c
d
Epa: anodic peak potential for oxidation for irreversible processes.

E. Tas et al. / Spectrochimica Acta Part A 75 (2010) 811–818
817
Fig. 6. Time-resolved UV–vis spectra of [Cu(L)] during the reduction at
Eapp = −1.65 V.
ing the oxidation process. The original spectrum of the neutral
complex could be recovered when the potential was applied at
Eapp = 0.40 V for the re-reduction process, indicating that the dou-
bly electro-reduced products remain stable in the thin layer cell
throughout the experiment. Fig. 6 shows changes in the electronic
spectra of [Cu(L)] during the reduction process in the thin-layer cell
(Eapp = −1.65 V). The main bands at 455 and 332 nm disappeared
Fig. 5. Time-resolved UV–vis spectra of [Cu(L)] in CH2Cl2 containing 0.4 M TBAP
(a) during the first oxidation at Eapp = 1.30 V and (b) during the second oxidation at
Eapp = 1.60 V.
cess based on the ligand (L)/(L)− negatively shifted compared with
those of [Ni(L)] and [Cu(L)].
The spectroelectrochemical behaviors of the ligand (LH2) and its
metal complexes were investigated using a thin-layer UV–vis spec-
troelectrochemical technique in CH2Cl2 solution containing 0.4 M
TBAP. The UV–vis spectral changes for the reduced and oxidized
species were obtained in a thin-layer cell during applied poten-
tials. The absorption spectra of electrochemically generated species
are given in Figs. 5–7. The convenient values of applied potentials
for in situ spectroelectrochemical experiment were determined for
the reduction and oxidation processes by taking CVs of the metal
complex in the thin-layer cell.
Fig. 5a shows changes in the electronic spectra of [Cu(L)] during
the first oxidation process in the thin-layer cell. When the poten-
tial (Eapp = 1.30 V) was applied in the thin layer cell, Intensity of the
main band at 451 nm belonging to the n → ␲* transitions of the
azomethine (CH N) decreased. The similar behavior was observed
for the ligand in the same experimental conditions but where isos-
bestic points could not clearly seen probably due to a fast chemical
decomposition or a chemical coupling of the electro-generated
oxidized species in the time scale of spectroelectrochemical mea-
surements. These results confirm the ligand-based first oxidation
process of the complex. Fig. 5b exhibits changes in the elec-
tronic spectra of [Cu(L)] during the second oxidation process (the
applied potential (Eapp) = 1.60 V). As seen, two main bands at 451
and 317 nm disappeared and a new band formed at 372 nm dur-
Fig. 7. Time-resolved UV–vis spectra of [Ni(L)] in CH2Cl2 containing 0.4 M TBAP
(a) during the second oxidation at Eapp = 1.60 V and (b) during the reduction at
Eapp = −1.65 V.

818
E. Tas et al. / Spectrochimica Acta Part A 75 (2010) 811–818
and a new band evolved at 407 nm during the reduction process.
The original spectrum corresponding to the neutral complex could
not be recovered when the potential was applied at Eapp = −0.60 V
for the re-oxidation process which indicates that the reduced
species is not stable during the process. Fig. 7a shows changes in the
electronic spectra of [Ni(L)] during the second oxidation process in
the thin-layer cell (Eapp = 1.60 V). The well-defined isosbestic points
were observed at 377, 420, 472, and 609 nm, confirming that the
electrode reaction proceeds in a quantitative fashion and there-
fore the absence of any coupled chemistry [9,31,36–38]. During
the oxidation process, the bands at 393 and 510 nm disappeared
and a new bands formed at 358, 425, and 770 nm. It was also
seen that intensity of band at 297 nm increased. The main band
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at 393 nm belonging to the n → ␲ transitions of the azomethine
(CH N) shifted to a shorter wavelength. This spectral changes is
similar to what is observed for [Ni(L)]. Fig. 7b presents changes
in the electronic spectra of [Ni(L)] during the reduction process in
the thin-layer cell (Eapp = −1.65 V). The main band at 425 nm disap-
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New steric hindered ligand (LH2), [N, Nꢀ-(3,4-benzophenon)-
3,5-But2-salicylaldimine (LH2)] and its mononuclear Cu(II), Co(II),
Ni(II), Mn(II), and Fe(II) complexes were synthesized and their
electro-spectrochemical properties were investigated using the
cyclic voltammetric and thin-layer spectroelectrochemical tech-
niques in CH2Cl2. Besides the classical methods such as FT-IR,
UV–vis, 1H NMR spectra, elemental analysis, magnetic suscepti-
bility, thermal properties and molar conductivity for structural
characterization, the combination of the electrochemical and spec-
troelectrochemical methods provided a power tool to reveal the
complementary nature of the molecular structure and electron-
transfer reactions of the reduced species of new electroactive steric
hindered Schiff-bases and its mononuclear Cu(II), Ni(II), Mn(II)
complexes. As we look at the begins to decompose temperature,
data also reveal that the most stable compound is Cu(II) complex
while the least stable compound is Fe(II) complex. The molar con-
ductivities (ꢀM) values of these metal complexes are in the range
of 9.8–18.3 ꢂ−1cm2 mol−1 at room temperature, indicating their
almost non-electrolytic nature. The analytical results agree with
the expected structure and other data are given in Section 2 and
Scheme 1.
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Acknowledgements
This work have been supported, in part, by the Technological
and Scientific Research Council of Turkey TUBITAK (TBAG Project
No.: 106T085) and by the Harran University Research Fund (HUBAK,
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