The sterically hindered salicylaldimine ligands with their copper(II) metal complexes: Synthesis, spectroscopy, electrochemical and thin-layer spectroelectrochemical features

Article abstract of DOI:10.1016/j.poly.2007.11.038

The synthesis, structure, spectroscopic and electro-spectrochemical properties of sterically constrained Schiff-base ligands (L_nH) (n = 1, 2, and 3) (L = N-[m-(methylmercapto)aniline]-3,5-di-t-butylsalicylaldimine, m = 4, 3, and 2 positions, respectively) and their copper(II) complexes [Cu(L_n)₂] are described. Three new dissymmetric bidentate salicylaldimine ligands containing a donor set of ONNO were prepared by reaction of different primary amine with 3,5-di-t-butyl-2-hydroxybenzaldehyde (3,5-DTB). The copper(II) metal complexes of these ligands were synthesized by treating an methanolic solution of the appropriate ligand with an equimolar amount of Cu(Ac)₂ · H₂O. The ligands and their copper complexes were characterized by FT-IR, UV-Vis, ¹H and ¹³C NMR and elemental analysis methods in addition to magnetic susceptibility, molar conductivity, and spectroelectrochemical techniques. Analytical data reveal that copper(II) metal complexes possess 1:2 metal-ligand ratios. On the basis of molar conductance, the copper(II) metal complexes could be formulated as [Cu(L_n)₂] due to their non-electrolytic nature in dimethylforamide (DMF). The room temperature magnetic moments of [Cu(L_n)₂] complexes are in the range of 1.82-1.90 B.M which are typical for mononuclear of Cu(II) compounds with a S = 1/2 spin state. The complexes did not indicate antiferromagnetic coupling of spin at this temperature. Electrochemical and thin-layer spectroelectrochemical studies of the ligands and complexes were comparatively studied in the same experimental conditions. The results revealed that all ligands displayed irreversible reduction processes and the cathodic peak potential values of (L₃H) are shifted towards negative potential values compared to those of (L₁H) and (L₂H). It is attributed to the weak-electron-donating methyl sulfanyl group substituted on the ortho (m = 2) position of benzene ring. Additionally, all copper complexes showed one quasi-reversible one-electron reduction process in the scan rates of 0.025-0.50 V s^-1, which are assigned to simple metal-based one-electron processes; [Cu(2+)(L_n)₂] + e^- → [Cu(1+)(L_n)₂]^-. The spectral changes corresponding to the ligands and complexes during the applied potential in a thin-layer cell confirmed the ligand and metal-based reduction processes, respectively.

Full text of DOI:10.1016/j.poly.2007.11.038

Available online at www.sciencedirect.com
Polyhedron 27 (2008) 1024–1032
www.elsevier.com/locate/poly
The sterically hindered salicylaldimine ligands with their copper(II)
metal complexes: Synthesis, spectroscopy,
electrochemical and thin-layer spectroelectrochemical features
a,
a
a
b,
*
*
Esref Tas , Ahmet Kilic , Nazli Konak , Ismail Yilmaz
a Department of Chemistry, University of Harran, 63190 Sanliurfa, Turkey
b Department of Chemistry, Technical University of Istanbul, 34469 Istanbul, Turkey
Received 23 July 2007; accepted 28 November 2007
Available online 14 January 2008
Abstract
The synthesis, structure, spectroscopic and electro-spectrochemical properties of sterically constrained Schiff-base ligands (LnH) (n = 1,
2, and 3) (L = N-[m-(methylmercapto)aniline]-3,5-di-t-butylsalicylaldimine, m = 4, 3, and 2 positions, respectively) and their copper(II)
complexes [Cu(Ln)2] are described. Three new dissymmetric bidentate salicylaldimine ligands containing a donor set of ONNO were pre-
pared by reaction of different primary amine with 3,5-di-t-butyl-2-hydroxybenzaldehyde (3,5-DTB). The copper(II) metal complexes of
these ligands were synthesized by treating an methanolic solution of the appropriate ligand with an equimolar amount of Cu(Ac)2 ꢀ H2O.
The ligands and their copper complexes were characterized by FT-IR, UV–Vis, 1H and 13C NMR and elemental analysis methods in addi-
tion to magnetic susceptibility, molar conductivity, and spectroelectrochemical techniques. Analytical data reveal that copper(II) metal
complexes possess 1:2 metal–ligand ratios. On the basis of molar conductance, the copper(II) metal complexes could be formulated as
[Cu(Ln)2] due to their non-electrolytic nature in dimethylforamide (DMF). The room temperature magnetic moments of [Cu(Ln)2] com-
plexes are in the range of 1.82–1.90 B.M which are typical for mononuclear of Cu(II) compounds with a S = 1/2 spin state. The complexes
did not indicate antiferromagnetic coupling of spin at this temperature. Electrochemical and thin-layer spectroelectrochemical studies of
the ligands and complexes were comparatively studied in the same experimental conditions. The results revealed that all ligands displayed
irreversible reduction processes and the cathodic peak potential values of (L3H) are shifted towards negative potential values compared to
those of (L1H) and (L2H). It is attributed to the weak-electron-donating methyl sulfanyl group substituted on the ortho (m = 2) position of
benzene ring. Additionally, all copper complexes showed one quasi-reversible one-electron reduction process in the scan rates of 0.025–
0.50 V sꢁ1, which are assigned to simple metal-based one-electron processes; [Cu(2+)(Ln)2] + eꢁ ? [Cu(1+)(Ln)2]ꢁ. The spectral changes
corresponding to the ligands and complexes during the applied potential in a thin-layer cell confirmed the ligand and metal-based reduction
processes, respectively.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Salicylaldimine; Copper(II) complexes; Molar conductivity; Electrochemistry; Thin-layer spectroelectrochemistry
1. Introduction
istics and catalysis properties [1–3]. Recent interest in the
design, synthesis and characterization of dissymmetrical
Schiff base ligands derived from appropriate amines for
transition metal ion complexes have come from the realiza-
tion that the coordinated ligands around central metal ions
in natural systems are unsymmetric [4]. Dissymmetric Schiff
base metal complexes as chiral analogues become more
effective and prevalent in asymmetric catalysis; an inexpen-
sive, large-scale production of these materials would be
highly desirable [4–7]. It has also been reported that transi-
Schiff base metal complexes containing different metal
ions such as Ni, Co and Cu have been studied in great details
for their various crystallographic feature, structure–redox
relationships and enzymatic reactions, mesogenic character-
*
Corresponding authors.
E-mail addresses: etas@harran.edu.tr (E. Tas), iyilmaz@itu.edu.tr
(I. Yilmaz).
0277-5387/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2007.11.038

E. Tas et al. / Polyhedron 27 (2008) 1024–1032
1025
tion metal complexes of Schiff bases containing tetradentate
ligands have anti-microbial activity [8]. Transition metal
complexes with redox active ligands bearing sterically hin-
dered salicylaldimines are also reported to undergo one-
or two-electron transfer. Sterically hindered ligands bearing
salicylaldimines are known to be effective antioxidants and
are widely used in rancidification of fats and oils [9]. The
copper(II) ions play a central role in biological redox meta-
lenzymes like plastocyanin, hemocyanin, azurin, galactose
oxidase an others [10]. In most salicylaldimine, the proton
is localized at the oxygen atom and in some cases is attached
to the nitrogen atom [11]. Proton transfer can occurs both in
solid and solution states, in basic and excited states [11,12].
In recent years, considerable interest has developed in
copper complexes with mixed donor ligands as structural
models for the active site of copper proteins. In the copper
proteins a distorted metal ion environment of low symmetry
with a mixed donor sets is present [13]. This information
prompted us to synthesize copper(II) metal complexes with
a mixed donor atoms environment incorporating nitrogen
and oxygen as ligands donor. The present manuscript is
devoted to the synthesis and spectroscopic characterization
of new salicylideneiminate copper(II) metal complexes con-
taining sterically hindered phenol fragment and electron-
donating OH and methylsulfanyl (S–CH3) substituents,
attached to the salicylaldehyde moiety. Here, we report syn-
thesis, characterization and electro-spectrochemical proper-
ties of three new dissymmetric bidentate salicylaldimine
ligands (LnH) (n = 1, 2, and 3) (L = N-[m-(methylmerca-
pto)aniline]-3,5-di-t-butylsalicylaldimine, m = 4, 3, and 2
positions, respectively) and their copper(II) complexes
[Cu(Ln)2]. A comparative electro-spectrochemical study
was provided an understanding the effect of the position
of methylsulfanyl groups attached on the phenol fragment
on electron transfer mechanism of the ligands and their cop-
per complexes.
spectrophotometer in the wavelength 200–900 nm. Molar
conductivities (KM) were recorded on a 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 exter-
nal PC. A three electrode system (BAS model solid cell
stand) was used for CV measurements in DMSO and con-
sisted 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 solu-
tion by a fritted-glass bridge filled with the solvent/sup-
porting electrolyte mixture. The ferrocene/ferrocenium
couple (Fc/Fc+) was used as an internal standard but all
potentials in the paper are referenced to the Ag/AgCl refer-
ence electrode. Solutions containing (LnH) and [Cu(Ln)2]
were deoxygenated by a stream of high purity nitrogen
for at least 5 min before running the experiment and the
solution was protected from air by a blanket of nitrogen
during the experiment. Controlled potential electrolysis
(CPE) was performed with Princeton Applied Research
Model 2263 potentiostat/Galvanostat. An BAS model elec-
trolysis cell with a fritted-glass to separate the cathodic and
anodic portions of the cell was used for bulk electrolysis.
The sample and solvent were placed into the electrolysis
cell under nitrogen. UV–Vis spectroelectrochemical exper-
iments 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 electrode were used for spectroelectrochemical
cell. Potentials were applied and monitored with a Prince-
ton Applied Research Model 2263 potentiostat. Time- and
applied-resolved UV–Vis spectra were recorded on Agillent
Model 8453 diode array spectrophotometer.
2.2. Synthesis of ligands (L1H, L2H and L3H)
2. Experimental
N-[4-(Methylmercapto)aniline]-3,5-di-t-butylsalicylaldi-
mine (L1H), N-[3-(methylmercapto)aniline]-3,5-di-t-butyl-
salicylaldimine (L2H), and N-[2-(methylmercapto)aniline]-
3,5-di-t-butylsalicylaldimine (L3H) ligands were synthesized
by the reaction of 4.20 mmol (0.98 g) 3,5-di-t-butyl-2-
hydroxybenzaldehyde in 20 ml absolute ethanol and
4.20 mmol (0.58 g) different marcapto aniline derivative in
10 ml ethanol. Also, 3–4 drops of formic acid were added
as catalyst. The mixture was refluxed for 2 h, followed by
cooling to room temperature. The crystals were filtered in
vacuum. Then the products were recrystallized from meth-
anol. The products are soluble in common solvents such as
CHCl3, CH3CH2OH, DMF and DMSO.
2.1. Materials and measurements
All reagents and solvents were of reagent-grade quality
and obtained from commercial suppliers. (Fluka Chemical
Company, Taufkirchen, Germany). 3,5-Di-t-butyl-2-
hydroxybenzaldehyde (3,5-DTB) was synthesized accord-
ing to the literature procedure [14]. The elemental analyses
were carried out in the Laboratory of the Scientific and
Technical Research Council of Turkey (TUBITAK). 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 400 MHz spectrometers.
Magnetic Susceptibilities were determined on a Sherwood
Scientific Magnetic Susceptibility Balance (Model MK1)
at room temperature (20 °C) using Hg[Co(SCN)4] as a cal-
ibrant; diamagnetic corrections were calculated from Pas-
cal’s constants [15]. Electronic spectral studies were
conducted on a Perkin Elmer model Lambda 25 UV–Vis
2.2.1. For (L1H) ligand
1H NMR (400 MHz, CDCl3, Me4Si, ppm): d 13.70 (s, 1H,
–OH, D-exchangeable), d 8.64 (s, 1H, HC@N), d 7.45–7.25
(m, 6H, Ar-CH), d 2.51 (s, 3H, S–CH3), d 1.47 (s, 9H, C–
CH3) and d 1.33 (s, 9H, C–CH3). The 13C NMR results:

1026
E. Tas et al. / Polyhedron 27 (2008) 1024–1032
16.19; 29.43; 31.49; 34.21; 35.15; 118.31; 121.70; 126.76;
127.57; 128.00; 136.76; 136.96; 140.60; 145.86; 158.22 and
162.98 ppm. IR (KBr pellets, mmax/cmꢁ1): 3641–3173
m(OH), 2958- 2864 m(Aliph-H), 1615 m(C@N), 1490–1440
m(C@C), 1172 m(C–O).
(3,5-DTB) from the reaction of 2,4-di-t-butyl phenol with
hexamethylene tetramine. In the second step; the ligands
(L1H), (L2H) and (L3H) were synthesized by the condensa-
tion of 4, 3 or 2-mercapto aniline with 3,5-di-t-butyl-2-
hydroxybenzaldehyde. For the structural characterization
of ligands (L1H), (L2H) and (L3H) and their Cu(II) com-
plexes [Cu(Ln)2], elemental analysis, IR spectra, UV–Vis
2.2.2. For (L2H) ligand
1H NMR (400 MHz, CDCl3, Me4Si, ppm): d 13.59 (s,
1H, –OH, D-exchangeable), d 8.63 (s, 1H, HC@N), d
7.52–6.99 (m, 6H, Ar-CH), d 2.53 (s, 3H, S–CH3), d 1.49
(s, 9H, C–CH3) and d 1.34 (s, 9H, C–CH3). The 13C
NMR results: 14.79; 29.46; 31.49; 34.20; 35.18; 117.48;
118.39; 124.75; 125.24; 126.91; 127.06; 128.28; 134.66;
137.13; 140.51; 145.90; 158.39 and 163.28 ppm. IR (KBr
pellets, mmax/cmꢁ1): 3643–3172 m(OH), 2955–2867 m(Aliph-
H), 1615 m(C@N), 1467–1440 m(C@C), 1173 m(C–O).
spectra, H and 13C NMR spectra, magnetic susceptibility
1
measurements, molar conductivity were used and the corre-
sponding data are given in experimental section and Tables
1 and 2. The metal to ligand ratios in the Cu(II) complexes
were found to be 1:2. The ligands (L1H), (L2H) and (L3H)
on interaction with Cu(II) salt yielded complexes corre-
sponding to the general formula [M(Ln)2].
3.1. NMR results
1
2.2.3. For L3H ligand
The H NMR spectral results obtained for ligands in
1H NMR (400 MHz, CDCl3, Me4Si, ppm): d 13.43 (s,
1H, –OH, D-exchangeable), d 8.62 (s, 1H, HC@N), d
7.47–6.86 (m, 6H, Ar-CH), d 2.48 (s, 3H, S–CH3), d 1.46
(s, 9H, C–CH3) and d 1.32 (s, 9H, C–CH3). The 13C
NMR results: 15.73; 29.42; 31.48; 34.21, 35.12; 117.63;
118.20; 119.11; 124.41; 126.31; 128.23; 129.61; 137.02;
139.92; 140.66; 149.30; 158.28 and 164.18 ppm. IR (KBr
pellets, mmax/cmꢁ1): 3650–3170 m(OH), 2957–2869 m(Aliph-
H), 1613 m(C@N), 1467–1436 m(C@C), 1169 m(C–O).
CDCl3 together the assignments are given in Section 2.
The H NMR spectra of the (L1H), (L2H) and (L3H) in
1
the CDCl3 do not give any signal corresponding to mer-
capto aniline groups protons and 3,5-di-t-butyl-2-hydroxy-
benzaldehyde protons. The proton NMR spectra of the
free ligands show a peak at 13.70 ppm for (L1H), at
13.59 ppm for (L2H) and at 13.43 ppm for (L3H), charac-
teristic of intramolecular hydrogen bonded OH proton.
The peaks at range 7.45–7.25 ppm for (L1H), at range
7.52–6.99 ppm for (L2H) and at range 7.47–6.86 ppm for
(L3H) are assignable to the protons of Ar-CH as multiplet
2.3. Synthesis of the Cu(II) metal complexes
1
peaks. In the H NMR spectra of ligands, the chemical
(0.53 g, 1.50 mmol), ligands (L1H), (L2H) or (L3H) was
dissolved in absolute methanol (45 cm3). A solution of
0.75 mmol of the metal salt [Cu(CH3COO)2 ꢀ H2O
(0.15 g)] in absolute methanol (25 cm3), was added drop-
wise under a N2 atmosphere with continuous stirring.
The stirred mixtures were then heated to the reflux temper-
ature for 3 h and were maintained at this temperature.
Then, the mixture was evaporated to a volume of 15–
20 ml in vacuum and left to cool to room temperature.
The compounds were precipitated after adding 5 ml etha-
nol. The products were filtered in vacuum and washed with
a small amount of methanol and water. The products were
recrystallized from ethanol. Then they were dried at
100 °C. IR (KBr pellets, mmax/cmꢁ1): 2950–2851 m(Aliph-
H), 1600 m(C@N), 1488–1456 m(C@C), 1170 m(C–O), 522
shifts observed at d 8.64 for (L1H), at d 8.63 for (L2H)
ppm and at d 8.62 for (L3H) is assigned to the proton of
azomethine (CH@N) as singlet [16]. The peak at range
2.53–2.48 ppm for ligands are assignable to the protons
of S–CH3 groups as singlet peaks. Also, the protons of
the t-butyl groups of ligands exhibit singlet peak at d
1.47 and 1.33 ppm for (L1H), d 1.49 and 1.34 ppm for
(L2H), and d 1.46 and 1.32 ppm for (L3H), indicating that
the t-butyl protons of these compounds are magnetically
1
nonequivalent [17]. H NMR and 13C NMR data of the
ligands are given in Section 2. The Cu(II) metal complexes
are paramagnetic, thus their 1H NMR and 13C NMR spec-
tra could not be obtained.
3.2. IR spectra results
m(M–O), 478 m(M–N) for [Cu(L1)2], IR (KBr pellets, mmax
/
cmꢁ1): 2955–2867 m(Aliph-H), 1593 m(C@N), 1471–1460
m(C@C), 1170 m(C–O), 537 m(M–O), 482 m(M–N). for
The observed peaks may be classified into those origi-
nating from the ligand and those arising from the bonds
formed between Cu(II) and coordinating sites. The free sal-
icylaldimine ligands (L1H), (L2H) and (L3H) showed
strong peaks at 1615–1613 cmꢁ1, which are characteristic
of the azomethine m(C@N) group [18]. Coordination of
the salicylaldimine ligands to the metal through the nitro-
gen atom is expected to reduce the electron density in the
azomethine link and lower the m(C@N) absorption fre-
quency. The peak due to m(C@N) is shifted to lower fre-
quencies and appears around 1600–1593 cmꢁ1, indicating
[Cu(L2)2], IR (KBr pellets,
m
max/cmꢁ1): 2957–2869
m(Aliph-H), 1594 m(C@N), 1460–1430 m(C@C), 1166 m(C–
O), 532 m(M–O), 487 m(M–N) for [Cu(L3)2].
3. Results and discussion
The reaction steps for the synthesis of the ligands and
their copper complexes are given in Scheme 1. The first step
is the synthesis of 3,5-di-t-butyl-2-hydroxybenzaldehyde

E. Tas et al. / Polyhedron 27 (2008) 1024–1032
1027
OH
OH
OH
H+
C6H12N4
CH2OH
CHO
-H2O
CH3COOH
: C(CH3)3
R
R
O
CH3CH2OH
N=CH
NH2
4
CH
60-65 o
C
3
2
HO
OH
: C(CH3)3
R: 4, 3 or 2 S-CH3
R
4
N=HC
Cu
R
2
CH3OH, 50 o
C
3
N=CH
4
O
O
2
Cu(CH3COO)2.H2O
3
3
2
HO
4
CH=N
R
R
: C(CH3)3
R: 4, 3 or 2 S-CH3
Scheme 1. Synthetic route for preparation of ligands (LnH) and their copper complexes [Cu(Ln)2].
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
M.p.
(°C)
KM
Yield
(%)
leff
[B.M.]
Elemental analyses % calculated (found)
(Xꢁ1 cm2 molꢁ1
)
C
H
N
S
L1H
C22H29NSO
Cu(L1)2
C44H56N2S2O2Cu
L2H
C22H29NSO
Cu(L2)2
C44H56N2S2O2Cu
L3H
C22H29NSO
Cu(L3)2
355
771
355
771
355
771
yellow
113
249
93
84
42
65
47
60
41
74.37
(73.83)
68.48
(68.27)
74.34
(74.40)
68.48
(68.21)
74.37
(74.13)
68.48
8.17
(8.43)
7.26
(7.34)
8.17
(8.45)
7.26
(7.33)
8.17
(8.46)
7.26
3.94
(3.72)
3.63
(3.46)
3.94
(3.71)
3.63
(3.46)
3.94
(3.71)
3.63
9.01
(9.06)
8.30
(8.07)
9.01
(9.02)
8.30
(8.06)
9.01
(9.03)
8.30
dark Brown
yellow
19
21
26
1.90
1.87
1.82
brown-green
yellow
244
135
258
dark green
C44H56N2S2O2Cu
(68.32)
(7.29)
(3.52)
(8.09)
coordination of the azomethine nitrogen to the copper
metal [19]. A strong peak observed at range 1173–
1169 cmꢁ1 in the free ligands has been assigned to phenolic
C–O stretching. On the complexation this band is shifted to
a lower frequency range 1170–1166 cmꢁ1, indicating coor-
dination through the phenolic oxygen. This has been fur-

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E. Tas et al. / Polyhedron 27 (2008) 1024–1032
Table 2
UV–Vis data of the ligands (LnH) and their copper complexes [Cu(Ln)2]
Compounds
Solvent
Wavelength [kmax/(nm)](loge)
(L1H)
C2H5OH
DMSO
206(3.27)
253(3.40)
237(4.22)
279(3.62)
276(4.04)
347(3.88)
342(4.16)
365(3.90)
363(4.20)
(L2H)
C2H5OH
DMSO
206(4.07)
253(4.20)
226(4.04)
274(4.40)
272(3.96)
305(4.21)
305(3.72)
355(4.14)
354(3.65)
(L3H)
C2H5OH
DMSO
206(3.61)
273(4.26)
221(3.56)
366(4.10)
242(3.48)
271(3.46)
365(3.22)
[Cu(L1)2]
[Cu(L2)2]
[Cu(L3)2]
C2H5OH
DMSO
213(2.90)
258(4.18)
246(3.82)
317(4.23)
313(3.84)
413(4.03)
415(3.58)
685(3.18)
702(2.14)
C2H5OH
DMSO
216(2.50)
248(3.64)
247(3.47)
260(3.87)
253(3.36)
292(3.85)
292(3.46)
413(3.53)
423(3.31), 624(2.21), 803(2.08)
695(2.29)
C2H5OH
DMSO
210(4.22)
264(4.05)
238(4.33)
295(4.03)
297(4.17)
426(3.76)
429(3.85)
682(2.38)
ther supported by the disappearance of the broad m(OH)
peak around 3650–3170 cmꢁ1 in the Cu(II) metal com-
plexes, indicating deprotonation of the phenolic proton
prior to coordination [18]. The coordination of the azome-
thine nitrogen and phenolic oxygen are further supported
by the appearance of two peaks at 537–522 cmꢁ1 and
487–478 cmꢁ1 due to m(Cu–O) and m(Cu–N) stretching
vibrations that are not observed in the infrared spectra of
the ligands. Thus, it is clear that the ligands (L1H), (L2H)
and (L3H) are bonded to the copper metal ion in a ONNO
fashion through the deprotonated phenolate oxygen and
salicylaldimine nitrogen.
charge-transfer transitions [22]. The absorption bands
below 276 nm in C2H5OH are practically identical and
can be attributed to p?p* transitions in the benzene ring
or azomethine (–C@N) groups for ligands. The absorption
bands observed within the range of 342–365 nm in C2H5OH
are most probably due to the transition of n?p* of imine
group corresponding to the ligands [23]. In the spectra of
Cu(II) metal complexes the bands observed within the range
of 297–317 nm in C2H5OH and DMSO solutions are most
probably due to the transition of n?p* of imine group cor-
responding to the metal complexes [23]. For copper com-
plexes, the absorption bands in the range of 413–429 nm
are assigned to M?L charge-transfer (MLCT) or L?M
charge-transfer (LMCT) [24], respectively. The absorption
bands observed at lower energy and in lower intensities
between 682 and 803 nm may be attributed to the d–d tran-
sitions (dxy ! dx2ꢁy2 and dz2 ! dx2ꢁy2 ). On complex forma-
tion, the electronic spectra of the copper complexes
exhibited significantly different absorption bands compared
to those of the ligands in C2H5OH and DMSO solutions. As
expected, the new bands in the range of 413–429 nm arising
from M?L charge-transfer (MLCT) or L?M charge-
transfer (LMCT) transitions were observed as a result of
coordination of copper metal through azomethine nitrogen.
Examples of UV–Vis spectra of the ligands and their copper
metal complexes were presented in Fig. 1.
3.3. UV–Vis spectra results
Electronic spectra of ligands (L1H), (L2H) and (L3H)
and their copper complexes [Cu(Ln)2] have been recorded
in the 200–900 nm range in C2H5OH and DMSO solutions,
and presented in Fig. 1. The corresponding data are also
listed in Table 2. Each ligands and their copper complexes
show several intense absorptions in the visible and ultravio-
let region. The UV–Vis spectra of the ligands in C2H5OH
and DMSO showed five absorption bands between 206
and 365 nm, and two or four absorption bands between
253 and 366 nm, respectively. The electronic spectra of the
Cu(II) metal complexes in C2H5OH and DMSO solutions
showed four-seven absorption bands between 216 and
698 nm. In the electronic spectra of the ligands and their
copper complexes, the wide range bands seem to be due
to both the p?p* and n?p* of C@N chromophore or
change-transfer transition arising from p electron interac-
tions between the metal and ligand which involves either a
metal-to-ligand or ligand-to-metal electron transfer and
d–d transitions [20,21]. In the electronic spectra of ligands
and their copper complexes, the absorption bands below
366 nm for ligands and the absorption bands below
426 nm for copper complexes have high extinction coeffi-
cients in C2H5OH and DMSO solutions and are almost cer-
tainly associated with intraligand p?p* and n?p* or
3.4. Magnetic moments
Magnetic susceptibility measurements provide sufficient
data to characterize the structure of the metal complexes.
Magnetic moments measurements of compounds carried
out at 25 °C. The room temperature magnetic moments
of Cu(II) metal complexes in the range of 1.82–1.90 B.M
which were typical for mononuclear of Cu(II) compounds
with a S = 1/2 spin state and did not indicate antiferromag-
netic coupling of spin at this temperature. These results are
comparable to the values reported for slightly distorted tet-
rahedral geometry [25].

E. Tas et al. / Polyhedron 27 (2008) 1024–1032
1029
Cu(II) metal complexes, the results indicate that Cu(II)
metal complexes are very poor in molar conductivity.
3.6. Electrochemistry
The electrochemistry of the ligands (LnH) and their cop-
per complexes [Cu(Ln)2] (n = 1, 2, 3) were studied by the
cyclic voltammetry in the scan rates of 0.025–0.50 V sꢁ1
in DMSO solution containing 0.1 M TBAP supporting
electrolyte. The cyclic voltammograms (CVs) of the ligands
are depicted in Figs. 2 and 3 at 0.025 V sꢁ1 scan rate. As
seen, all ligands exhibited one irreversible reduction pro-
cesses without corresponding anodic waves. The redox pro-
cesses of the ligands are based on the reduction of the
azomethine nitrogen in the presence of the phenolic proton
in the molecule. Mostly, the process results in the reductive
0
(L1H)
-5
-10
-15
(L2H)
(L3H)
-2.0
-1.5
-1.0
-0.5
0.0
Potential / V vs. Ag/AgCl
Fig. 2. Cyclic voltammograms (CVs) of the ligands (LnH) in DMSO
containing 0.1 M TBAP. Scan rates: 0.025 V sꢁ1. Working electrode: a
1.6 mm diameter of platinum disc electrode. c = 9.4 ꢂ 10ꢁ3 M.
0.025 V s-1
0.050 V s-1
10
0.10 V s-1
0.25 V s-1
Fig. 1. UV–Vis spectra of ligands (LnH) (a); Cu(II) metal complexes
[Cu(LnH)2] in DMSO (b).
0.50 V s-1
0
3.5. Solubility and molar conductivity
The Ligands (L1H), (L2H) and (L3H), and their Cu(II)
metal complexes [Cu(Ln)2] are soluble in C2H5OH, CH3OH,
DMSO and DMF solvents. With a view to studying the elec-
trolytic nature of the Cu(II) metal complexes, their molar
conductivities were measured in DMF (dimethyl formam-
ide) at 10ꢁ3 M. The molar conductivities (KM) values of
these Cu(II) metal complexes are in the range of 19–
26 Xꢁ1 cm2 molꢁ1 at room temperature [26], indicating their
almost non-electrolytic nature. Due to not free ions in
-10
[Cu(2+)(L1)2] + e-
-1.0 -0.8
[Cu(+)(L1)2]-
-0.6
-1.2
-0.4
-0.2
Potential / V vs. Ag/AgCl
Fig. 3. Cyclic voltammograms (CVs) of the copper complexes [Cu(Ln)2] in
DMSO containing 0.1 M TBAP. Scan rates: between 0.025 and 0.50 V sꢁ1
Working electrode:
c = 4.3 ꢂ 10ꢁ3 M.
.
a 1.6 mm diameter of platinum disc electrode.

1030
E. Tas et al. / Polyhedron 27 (2008) 1024–1032
coupling of the reduced species as previously observed for
some Schiff bases such as Salen or its derivatives, indicating
irreversible fashion of the processes [27]. However, the
reduction process is probably assigned to the electro-
deprotonated ligand anion at the platinum surface. The
ligands (L1H), (L2H), and (L3H) displayed the cathodic
peak potentials at Epc = ꢁ1.60, ꢁ1.60, and ꢁ1.64 V versus
Ag/AgCl, respectively. It is known that the introduction of
electron-donating groups to the electroactive molecules is
expected to lead to an increase of electron density in the
molecule, thereby making the molecule easier to oxidize
and harder to reduce [28–32]. On the other hand, the posi-
tions of the substituent on the benzene ring may also effect
peak potentials in different extent. This effect was clearly
demonstrated by the fact that the cathodic peak potential
of (L3H) was shifted towards more negative potentials
compared to those of (L1H) and (L2H). It is attributed to
the weak-electron-donating methyl sulfanyl group substi-
tuted on the ortho position of benzene ring. An example
of the cyclic voltammograms (CVs) of the copper com-
plexes [Cu(Ln)2] in the scan rates of 0.025–0.50 V sꢁ1 are
exhibited in Fig. 3. The data corresponding to the cyclic
voltammograms (CVs) of [Cu(Ln)2] are listed in Table 3.
All copper complexes showed one quasi-reversible one-
electron reduction process in the scan rates of 0.025–
0.50 V sꢁ1, which are assigned to simple metal-based one-
electron processes; [Cu(2+)(Ln)2] + eꢁ ? [Cu(1+)(Ln)2]ꢁ.
The half-wave potentials (E1/2) of the reversible Cu(2+)/
Cu(1+) redox couples were calculated as the average of
the cathodic and anodic peak potentials of the processes.
The E1/2 of the reduction processes of [Cu(L1)2],
[Cu(L2)2], and [Cu(L3)2] are ꢁ0.65, ꢁ0.65, and ꢁ0.68 V ver-
sus Ag/AgCl (scan rate 0.025 V sꢁ1), respectively. As seen,
E1/2 of [Cu(L3)2] was negatively shifted compared to those
of [Cu(L1)2], [Cu(L2)2], as observed for the ligands.
Although the reduction processes had low ratio of ano-
dic-to-cathodic peak currents (ipa/ipc) (Table 3), the ano-
dic–cathodic peak separations (DE) for the Cu(2+)/
sion controlled with the anodic current function (Ipa/v1/2
)
independent of the scan rate (v) over the scan range
0.025–0.500 V sꢁ1 [33–37].
3.7. In situ UV–Vis behaviors of the electro-reduced ligands
and complexes
‘‘The spectroelectrochemical behaviors of ligands (LnH)
and their copper complexes [Cu(Ln)2] were investigated
using a thin-layer UV–Vis spectroelectrochemical tech-
nique in DMSO solution containing 0.2 M TBAP. The
UV–Vis spectral changes for the reduced species of (Ln)
and [Cu(Ln)2] were obtained in a thin-layer cell during
applied potentials. The absorption spectra of electrochem-
ically generated species, (L1H)ꢁ and [Cu(+)(L1)2]ꢁ, are
given in Figs. 4 and 5. The convenient values of applied
potentials for in situ spectroelectrochemical experiment
were determined for the reduction processes by taking
CVs of the complexes in the thin-layer cell.
358 nm
0.4
465 nm
0.2
0.0
300
350
400
450
500
550
600
650
Wavelength / nm
Fig. 4. Time-resolved UV–Vis spectra of (L1H) during the first reductions
at Eapp = ꢁ1.80 V in DMSO containing 0.2 M TBAP.
Cu(1+) couples are 0.100–105 V (scan rate 0.025 V sꢁ1
)
which assign to the quasi-reversible processes. The slow
electron transfer could be associated with a structural rear-
rangement of the molecule during the reduction process.
The reduction processes of (LnH) and [Cu(Ln)2] are diffu-
[Cu(2+)(L1)2] + e-
[Cu(+)(L1)2]-
1.5
460 nm
416 nm
1.0
0.5
0.0
Table 3
Voltammetric data for the ligands and their copper complexes vs. Ag/
AgCl (Fc/F+c ): values in parentheses vs. Fc/Fc+ in DMSO–TBAP
Complexes
L/Lꢁ Eapc (V)
M2+/M+ Eb1/2 (V)
DEc (V)
Ipa/Idpc
(L1H)
(L2H)
(L3H)
[Cu(L1)2]
[Cu(L2)2]
[Cu(L3)2]
a
ꢁ1.60 (ꢁ2.10)
ꢁ1.60 (ꢁ2.10)
ꢁ1.64 (ꢁ2.14)
ꢁ0.65 (ꢁ1.15)
ꢁ0.65 (ꢁ1.15)
ꢁ0.68 (ꢁ1.18)
0.103
0.100
0.105
0.54
0.60
0.55
300
350
400
450
500
550
600
650
Epc: Cathodic peak potential for reduction for irreversible processes.
Wavelength / nm
b
c
E1/2 = Epc + Epa/2.
DEp = Epc ꢁ Epa at 0.025 V sꢁ1 scan rate.
ipa/ipc for reduction at 0.025 V sꢁ1 scan rate.
Fig. 5. Time-resolved UV–Vis spectra of [Cu(L1)2] during the first
reductions at Eapp = ꢁ1.00 V in DMSO containing 0.2 M TBAP.
d

E. Tas et al. / Polyhedron 27 (2008) 1024–1032
1031
Fig. 4 shows changes in the electronic spectrum of (L1H)
during reduction process in the thin-layer cell. The well-
defined isosbestic point is observed at 408 nm, confirming
that the electrode reaction proceeds in a quantitative fash-
ion and therefore the absence of any coupled chemistry
[28,29,33,37–39]. When the potential (Eapp = ꢁ1.80 V)
was applied in the thin-layer cell, the broad band at
358 nm belonging to the n?p* transitions of the azome-
thine (CH@N), disappeared and new bands at 465 and at
315 nm formed during the reduction process. The 465 nm-
peak was observed at longer wavelength compared with
the neutral ligand, which clearly indicates a reduced prod-
uct having extended conjugation. The original spectrum
corresponding to the neutral ligand was 60% recovered
when the potential was applied at Eapp = ꢁ1.80 V for the
re-oxidation process, indicating decomposition or chemical
change of the electro-generated reduced species at the some
extent in the time scale of the reduction process. This behav-
ior confirms the irreversible nature of the reduction process
observed for the CV of the ligands. Similar spectral changes
were observed for (L1H) and (L2H) during their reduction
processes in the same experimental conditions.
thesized and their electro-spectrochemical properties were
investigated using cyclic voltammetric and thin-layer spec-
troelectrochemical techniques in DMSO. Besides the classi-
1
cal methods such as FT-IR, UV–Vis, H and 13C NMR,
elemental analysis, magnetic susceptibility, and molar con-
ductivity for structural characterization, the combination
of the electrochemical and spectroelectrochemical 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 Schiff-bases
and their Cu(II) complexes. So the comparative electro-
spectrochemical studies revealed that [Cu(Ln)2] complexes
showed four coordination with the ligand (LnH) having
N and O donor sites, and a shift on their half-wave poten-
tials or band positions because of different positions of
methylsulfanyl groups attached on benzene ring. The elec-
tro-reduced species of [Cu(Ln)2] complexes remained stable
during spectroelectrochemical time scale although the cor-
responding ligands exhibited an irreversible reduction pro-
cesses in their CV measurements. Finally, the CVs and the
well-defined spectral changes observed during applied
potentials in the thin-layer cell spectral data supported
the corresponding structures of the copper complexes.
Fig. 5 shows the time-resolved spectra of [Cu(L1)2] dur-
ing the applied potential at Eapp = ꢁ1.00 V. The copper
complexes showed two main bands at about 316 nm and
416 nm the former of which was referred to the n?p*
transitions of the azomethine (CH@N) group and the
charge-transfer transitions. As expected, the latter band
was red-shifted compared to those of the ligands, indicating
coordination of the azomethine nitrogen to the copper
metal. After reduction, the Cu(II)(d9)?Cu(I)(d10) process
was formed by adding one electron in the metal system. This
change resulted in an appearance of the absorption band at
460 nm which was attributed to a metal to ligand charge-
transfer transitions (MLCT) in Cu(I) complex. Also, the
intensity of the band at 316 nm was decreased during the
reduction process. All complexes exhibited similar spectral
changes during the reduction processes in the thin-layer cell.
The spectral changes were similar to what were observed for
a metal-centered reduction Cu(II)?Cu(I) processes in the
literature [40–42]. The original spectra of the complexes
were obtained upon re-oxidation during spectroelectro-
chemical measurement. It is concluded that the singly-
reduced species remains stable and is not companied by a
chemical reaction in the time scale of the spectroelectro-
chemical measurement. The controlled potential coulomet-
ric (CPC) study indicated that the number of electrons
transferred for the forward and reverse electrochemical
reaction of the complexes was one for the reduction and
re-oxidation processes based on the Cu(II)/Cu(I) couples.
Acknowledgements
This work have been supported, in part, by the HUBAK
Fund (Project No. 649) of Harran University (Sanliurfa,
Turkey). The supports of State Planning Organization
(DPT, Project No. 90177) is gratefully acknowledged. This
work has also been supported, in part, by the Turkish
Academy of Sciences in the framework of the Young Scien-
_
¨
tist Award Program (TUBA-GEBIP).
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