Iron(III) and nickel(II) complexes of O,N,N,O-chelating benzophenone thiosemicarbazone: Electrochemistry and in situ spectroelectrochemistry

Article abstract of DOI:10.1016/j.ica.2012.03.023

New complexes, [Fe(L)Cl] (1 and 2) and [Ni(L)]·C₂H ₅OH (3 and 4), were synthesized by template condensation of 2,4-dihydroxy-benzophenone-S-methyl-thiosemicarbazone with 5-bromo- and 5-chloro-2-hydroxy-benzaldehyde and characterized by elemental analysis, conductivity measurements, IR, NMR and mass spectra. Complex 4 crystallizes in monoclinic space group P2₁/c with unit cell dimensions, a = 13.1091 (3) A, b = 24.2308 (3) A, c = 15.5808 (4) A, β = 102.2632 (11)°, V = 4836.22 (18) A³, Z = 8. The asymmetric unit of 4 contains two main molecules and two ethanol molecules. Electrochemical behaviors of 1-4 were studied using cyclic voltammetry and square wave voltammetry. Voltammetric analysis of the complexes shows a quasi-reversible metal-based reduction processes between -0.55 and -0.78 V followed by an irreversible reduction process within -0.91 and -1.27 V versus SCE. An irreversible oxidation is also observed within -1.10 to -1.23 V versus SCE during anodic potential scans. While the thiosemicarbazone moiety stabilizes +3 and +2 oxidation states for the iron complexes, only Ni^II is stable in solution with the same ligand in dichloromethane and dimethylsulfoxide. In-situ spectroelectrochemical studies were employed to determine the colors and spectra of electrogenerated species of the complexes.

Full text of DOI:10.1016/j.ica.2012.03.023

Inorganica Chimica Acta 388 (2012) 148–156
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Iron(III) and nickel(II) complexes of O,N,N,O-chelating benzophenone
thiosemicarbazone: Electrochemistry and in situ spectroelectrochemistry
b
c
a
Yasemin Kurt a, , Atıf Koca , Mehmet Akkurt , Bahri Ülküseven
⇑
a Department of Chemistry, Istanbul University, 34320 Avcılar, Istanbul, Turkey
b Department of Chemical Engineering, Marmara University, Göztepe 34722, Istanbul, Turkey
c Department of Physics, Erciyes University, Faculty of Sciences, 38039 Kayseri, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
New complexes, [Fe(L)Cl] (1 and 2) and [Ni(L)]ꢀC2H5OH (3 and 4), were synthesized by template conden-
sation of 2,4-dihydroxy-benzophenone-S-methyl-thiosemicarbazone with 5-bromo- and 5-chloro-2-
hydroxy-benzaldehyde and characterized by elemental analysis, conductivity measurements, IR, NMR
and mass spectra. Complex 4 crystallizes in monoclinic space group P21/c with unit cell dimensions,
a = 13.1091 (3) Å, b = 24.2308 (3) Å, c = 15.5808 (4) Å, b = 102.2632 (11)°, V = 4836.22 (18) Å3, Z = 8. The
asymmetric unit of 4 contains two main molecules and two ethanol molecules. Electrochemical behaviors
of 1–4 were studied using cyclic voltammetry and square wave voltammetry. Voltammetric analysis of
the complexes shows a quasi-reversible metal-based reduction processes between ꢁ0.55 and ꢁ0.78 V
followed by an irreversible reduction process within ꢁ0.91 and ꢁ1.27 V versus SCE. An irreversible oxi-
dation is also observed within ꢁ1.10 to ꢁ1.23 V versus SCE during anodic potential scans. While the thi-
osemicarbazone moiety stabilizes +3 and +2 oxidation states for the iron complexes, only NiII is stable in
solution with the same ligand in dichloromethane and dimethylsulfoxide. In-situ spectroelectrochemical
studies were employed to determine the colors and spectra of electrogenerated species of the complexes.
Ó 2012 Elsevier B.V. All rights reserved.
Received 21 October 2011
Received in revised form 29 February 2012
Accepted 8 March 2012
Available online 22 March 2012
Keywords:
Thiosemicarbazone
ONNO complexes
Electrochemistry
Spectroelectrochemical analysis
X-ray analysis
1. Introduction
dihydroxy-benzophenone-S-methylthiosemicarbazone as shown
in Fig. 1. The complexes (1–4) were characterized by means of ele-
After platinum complexes of thiosemicarbazide were found to
have antitumor activities [1], structures and biological potentials
of metal complexes of thiosemicarbazone derivatives have been
intensively studied [2,3]. Research on cytotoxic properties of thio-
semicarbazone complexes have been centralized to ONS and NNS
chelates of copper(II), platinum(II) and palladium(II) obtained
especially from 2-acetyl pyridine derivatives [4–9]. In 2007, we re-
ported the cytotoxic activity against K562 chronic myeloid leuke-
mia of some N2O2 thiosemicarbazone complexes of iron(III) and
nickel(II) [10], and then our research has been focused on the syn-
thesis of N2O2 chelate structures [11].
mental analysis, conductivity measurements, IR, NMR and mass
spectra. The crystal structure of complex 4 was studied by single
crystal X-ray diffraction.
The redox properties of 1–4 were studied to investigate the
influence of the ligand environment on the relative stability of
the species which formed in solution due to the variable oxida-
tion states of the central metal ions (III, II and I). Besides, the
electrochemical investigation, in situ spectroelectrochemical and
in situ electrocolorimetric studies were firstly employed to deter-
mine the spectra and colors of the electrogenerated species of
the nickel and iron complexes with N2O2 chelating thiosemicar-
bazones. It is well known that certain potential applications (e.g.,
in the design of an efficient catalyst and sensor) of these type
complexes will require an understanding of the nature of their
redox processes. It is well known that many metal N2O2 com-
plexes show excellent electrocatalytic and sensor properties
[16,17]. Therefore, we report here the in situ electrocolorimetric
analysis of the complexes to determine possible application of
the complexes. Because electrocolorimetry technique provides a
more precise way to define color of the electro generated spe-
cies, which are desired properties for the optic sensor applica-
tions [17].
The N2O2 coordination mode based on thiosemicarbazones is
usually obtained from substituted 2-hydroxy benzaldehydes with
Fe(III) [12] and Ni(II) [13], but there are a limited number of such
complexes including
a benzophenone moiety. Some of the
N2O2[14] and N4O2[15] complexes of 2-hydroxy-benzophenone thi-
osemicarbazone has been reported, previously. Herein, we present
four N2O2 complexes of iron(III) and nickel(II) obtained from 2,4-
⇑
Corresponding author. Tel.: +90 212 4737070; fax: +90 212 4737180.
E-mail address: dasdemir@istanbul.edu.tr (Y. Kurt).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.03.023

Y. Kurt et al. / Inorganica Chimica Acta 388 (2012) 148–156
149
HO
OH
NH2
N
N
S CH3
(a)
HO
6
j
a
4
HO
i
20
18
R
Cl
O
R
O
b
22
O
O
c8
k
.
Ni
C2H5OH
Fe
1
4
2
16
15
N
N
N
d
N
1
2
10
N
e
14
N
h
S CH3
12
S CH3
f
g
(b)
Fig. 1. (a) 2,4-Dihydroxy-benzophenone-S-methylthiosemicarbazone. (b) The complexes. M/R: Fe/Cl (1); Fe/Br (2); Ni/Cl (3); Ni/Br (4). In the nickel complex formula, a-k
symbols indicate hydrogen atoms. For clarity, some carbon atoms labeled.
Complex 1 was prepared with the procedure given in previous
paper [14]. 2,4-Dihydroxybenzophenone-S-methylthiosemicarbaz-
one (1 mmol) and 2-hydroxy-benzaldehyde (1 mmol) in 10 mL
absolute ethanol was added dropwise to the solution FeCl3ꢀ6H2O
(1 mmol) in 10 mL absolute ethanol. After a week, dark brown pre-
cipitates were filtered off, washed with a mixture of ethanol and
dried in vacuo over P2O5. Complexes 2–4 were synthesized in sim-
ilar manner. The analytical and spectroscopic data of 1–4 are
below:
2. Experimental
2.1. Materials and physical measurements
All chemicals were of reagent grade and were used as commer-
cially purchased without further purification. The elemental analy-
ses were determined on a Thermo Finnigan Flash EA 1112 Series
Elemental Analyser. Magnetic measurements were carried out at
room temperature with an Sherwood Scientific MK I apparatus
by using CuSO4ꢀ5H2O as calibrant. The molar conductivities of
Complex 1: Dark Brown, m.p. >350 °C, yield 18%,
8.3 Anal. Calc. for C22H16N3O3SCl2Fe
ꢁ1 cm2 molꢁ1
(529.19 g/mol): C, 49.93; H, 3.05; N, 7.94; S, 6.06. Found: C,
49.88; H, 3.04; N, 7.92; S, 6.06%.
max/cmꢁ1: 1607 (C@N1), 1600
leff: 5.8 BM,
the complexes were measured in 10ꢁ3
M DMF solution at
KM
:
X
.
25 1 °C using a digital WPA CMD 750 conductivity meter. Infra-
red spectra were recorded as KBr discs on a Mattson 1000 FT-IR
spectrophotometer in the 4000–400 cmꢁ1 range at room tempera-
ture. The NMR spectra were recorded on Bruker Avance-500 model
spectrometer relative to SiMe4 using DMSO-d6. The ESI-MS analy-
ses were carried out in positive and negative ion modes using a
Thermo Finnigan LCQ Advantage MAX LC/MS/MS.
m
(N4@C), 1584 (N2@C), 1161, 1123
m(C–O)arom. m/z (–c ESI):
529.26 ([M]+, 100%); 528.19 ([Mꢁ1]+, 14.54); 530.25 ([M+1]+,
35.13); 531.26 ([M+2]+, 23.95).
Complex 2: Black, m.p. >350 °C, yield 24%,
ꢁ1 cm2 molꢁ1. Anal. Calc. for C22H16N3O3SBrClFe (573.64 g/
mol): C, 46.06; H, 2.81; N, 7.33; S, 5.59. Found: C, 46.00; H, 2.80;
N, 7.31; S, 5.58%.
max/cmꢁ1: 1607 (C@N1), 1604 (N4@C), 1576
(N2@C), 1161, 1123
leff: 5.5 BM, KM:
1.2
X
m
m
(C–O)arom. m/z (–c ESI): 573.24 ([M]+, 100%),
2.2. Synthesis of the compounds
572.31 ([Mꢁ1]+, 22.36), 571.30 ([Mꢁ2]+, 75.29), 574.23 ([M+1]+,
29.03); 575.24 ([M+2]+, 26.94) .
2,4-Dihydroxy-benzophenone-S-methylthiosemicarbazone was
prepared with small modifications of the literature method [18].
The colors, m.p. (°C), yields (%), elemental analysis, FT-IR (KBr disc,
cmꢁ1) and NMR (ppm, 25 °C) data of the compound are as follows:
Colorless, m.p. 184.8–186.0 °C, 61%. Anal. Calc. for C15H15N3O2S
(301.36 g/mol): C, 59.78; H, 5.02; N, 13.94; S, 10.64. Found: C,
Complex 3: Claret red, m.p. >350 °C, yield 21%,
0.06 Anal. Calc. for C22H16N3O3SClNi.C2H5OH
ꢁ1 cm2 molꢁ1
(542.66 g/mol): C, 53.12; H, 4.09; N, 7.74; S, 5.91. Found: C,
53.05; H, 4.08; N, 7.72; S, 5.89%.
max/cmꢁ1 (C@N1) 1615,
(N4@C) 1605, (N2@C) 1584,
(C–O)arom 1161, 1130, dH
leff: 0.30 BM, KM:
X
.
m
: m
m
m
m
59.81; H, 4.98; N, 13.88; S, 10.59%.
m
max/cmꢁ1: 3453 (2-OH),
(500 MHz, DMSO-d6, Me4Si): 10.25 (s, 1H, 4-OH), 8.32 (s, 1H,
N4@CH), 7.98 (d, J = 2.44, 1H, f), 7.56–7.49 (m, 4H, d,e,g,h), 7.33–
7.31 (m, 2H, i,j), 7.10 (d, J = 9.25, 1H, k), 6.71 (d, J = 9.27, 1H, c),
6.37 (d, J = 2.44, 1H, a), 6.17 (d, J = 9.27, 1H, b), 4.39 (s, 1H, alco-
hol–OH), 3.52–3.49 (q, J = 7.32, 2H, –CH2–), 2.23 (s, 3H, S-CH3).
1.12 (t, J = 7.32, 3H, –CH2–CH3). dC (500 MHz, DMSO-d6, Me4Si):
167.82 (1), 159.94 (2), 113.02 (3), 157.41 (4), 104.75 (5), 137.24
(6), 108.45 (7), 131.01 (8), 133.53 (9), 128.39 (10,14), 128.97
(11,13), 129.13 (12), 15.40 (15), 167.11 (16), 124.32 (17), 164.05
(18), 121.09 (19), 135.36 (20), 135.49 (21), 120.06 (22), 56.72
(CH3–CH2–OH), 19.24 (CH3–CH2–OH). m/z (+c ESI): 496.34 ([M]+,
3.39%); 497.32 ([M+1]+, 3.22); 498.40 ([M+2]+, 4.13); 270.56
({[(HO–C6H3–O)(C6H5)CN Ni]}+, 100%).
3345 (NH)as, 3172 (4-OH), 3141 (NH)s, 1628 (C@N1), 1597
(N2@C), 1120 (C–O), 746 (C–S). dH (500 MHz, DMSO-d6, Me4Si):
11.03, 10.46 (cis/trans ratio: 2/1, s, 1H, 2-OH), 9.59 (s, 1H, 4-OH),
6.64 (s, 2H, NH2), 7.60 (d, J = 7.78, 1H, f), 7.45–7.43 (m, 2H, d,h),
7.35–7.33 (m, 2H, e,g), 6.68 (d, J = 8.69, 1H, c), 6.31 (d, J = 2.29
1H, a), 6.24 (d, J = 8.69, 1H, b), 2.41, 2.36 (cis/trans ratio: 3/2, s,
3H, S–CH3). dC (500 MHz, DMSO-d6, Me4Si): 162.98 (1), 160.69
(2), 116.08 (3), 158.78 (4), 105.51 (5), 140.99 (6), 107.23 (7),
132.02 (8), 133.35 (9), 128.45 (10,14), 129.43 (11,13), 129.99
(12), 12.91 (15). m/z (+c ESI): 301.42 ([M]+, 100%); 300.24
([Mꢁ1]+, 19.33), 302.42 ([M+1]+, 18.12); 303.24 ([M+2]+, 7.20);
270.56 ([Mꢁ(–N@C(SCH3)–NH2)]+, 5.66).

150
Y. Kurt et al. / Inorganica Chimica Acta 388 (2012) 148–156
Table 1
Complex 4: Claret red, m.p. >350 °C, yield 26%,
0.10
ꢁ1 cm2 molꢁ1
Anal. Calc. for C22H16N3O3SBrNiꢀC2H5OH
(587.11 g/mol): C, 49.10; H, 3.78; N, 7.16; S, 5.46. Found: C, 48.97;
H, 3.76; N, 7.15; S 5.44.
max/cmꢁ1 (C@N1) 1615, (N4@C) 1607,
(N2@C) 1584,
(C–O)arom 1161, 1130. dH (500 MHz, DMSO-d6,
leff: 0.36 BM, KM:
The crystal data and refinement details of complex 4.
X
.
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal dimensions (mm)
Crystal system
Space group
a (Å)
C24H22BrN3NiO4S
587.12
293
m
:
m
m
m
m
0.71070
0.40 ꢂ 0.30 ꢂ 0.10
monoclinic
P121/c1
13.1091 (3)
24.2308 (3)
15.5808 (4)
90
102.2632 (11)
90
4836.2 (2)
8
Me4Si): 10.18 (s, 1H, 4-OH), 8.25 (s, 1H, N4@CH), 8.03 (d,
J = 2.44 Hz, 1H, f), 7.52–7.46 (m, 4H, d,e,g,h), 7.25–7.23 (m, 2H, i,j),
7.10 (d, J = 9.25 Hz, 1H, k), 6.96 (d, J = 9.27 Hz 1H, c), 6.29 (d,
J = 2.44 Hz 1H, a), 6.10 (d, J = 9.27 Hz 1H, b), 4.32 (s, 1H, alcohol–
OH), 3.44–3.41 (q, J = 7.32, 2H, –CH2–), 2.15 (s, 3H, S–CH3), 1.06 (t,
J = 7.32, 3H, –CH2–CH3). dC (500 MHz, DMSO-d6, Me4Si): 167.81 (1),
159.96 (2), 108.44, 157.40 (4), 104.75 (5), 137.24 (6), 107.23 (7),
131.02 (8), 135.36 (9), 128.39 (10,14), 128.96 (11,13), 129.13 (12),
15.40 (15), 167.30 (16), 124.64 (17), 164.06 (18), 122.04 (19),
135.48 (20), 136.82 (21), 113.03 (22), 56.72 (CH3–CH2–OH), 19.23
(CH3–CH2–OH). m/z (+c ESI): 542.18 ([M+1]+, 100%); 541.24 ([M]+,
15.07%); 540.18 ([Mꢁ2]+, 70.57); 118.08([(C6H5O)CN]+, 40.48). m/z
(–c ESI): 540.31 ([Mꢁ1]+, 100%).
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalc (g/cm3)
1.613
2.582
2384.0
k (Mo K
F(000)
h, k, l range
a
) (mmꢁ1
)
ꢁ18 6 h 6 18
ꢁ34 6 k 6 34
ꢁ21 6 l 6 21
279324
14526
0.031
0.118
0.032
1.146
2.3. X-ray analysis
Reflections collected
Unique reflections
Rint
A red block crystal of complex 4 having approximate dimen-
sions of 0.40 ꢂ 0.30 ꢂ 0.10 mm was mounted on a glass fiber. The
diffraction data was collected on a Rigaku RAXIS RAPID imaging
R [I > 3
Rw [I > 3
Goodness-of-fit on indicator
r(I)]
r
(I)]
plate area detector with graphite-monochromated Mo
Ka
(0.7107 Å) radiation at 293 K. The data were corrected for Lorentz
and polarization effects. An empirical absorption correction was
applied, which resulted in transmission factors ranging from 0.41
to 1.00. The data were corrected for Lorentz and polarization ef-
fects. The molecular and crystal structures were solved by direct
methods using the program SIR92 [19]. Hydrogen atoms were re-
fined using the riding model and the non-hydrogen atoms were re-
fined anisotropically. All calculations were performed using the
Crystal Structure crystallographic software package [20,21]. The
data and refinement details of 4 are presented in Tables 1 and 2.
open-circuit, using the electrolyte solution without complexes un-
der study. During the measurements, readings were taken as a
function of time under kinetic control.
3. Results and discussion
3.1. Synthesis and physical properties
2,4-Dihydroxy-benzophenone-S-methylthiosemicarbazone is in
form of powder crystals and soluble in common solvents. Reactions
of the thiosemicarbazone with 5-bromo- and 5-chloro-2-hydroxy-
benzaldehyde in the presence of iron(III) or nickel(II) in alcohol
yielded stable solid complexes as shown in Fig. 1. The N2O2 com-
plexes, 1–4, form as fine crystal materials, very soluble in common
organic solvents. The complexes are stable in air and their low mo-
lar conductances indicate non-ionic structures. The magnetic sus-
ceptibility values of 1 and 2 are compatible with high-spin d5
iron(III) center. Complexes 3 and 4 are in the formula [Ni(L)]ꢀ-
C2H5OH and recrystallization from alcohol to improve the quality
of the crystals does not changed the composition. The low mag-
netic susceptibility values of 3 and 4 indicate the square planar
environment of nickel(II) center in the N2O2 chelates.
2.4. Electrochemistry and spectroelectrochemistry
Electrochemical and spectroelectrochemical measurements
were carried out with a Gamry Reference 600 potentiostat/galva-
nostat utilizing a three-electrode configuration at 25 °C. For cyclic
voltammetry (CV), and square wave voltammetry (SWV) measure-
ments, the working electrode was a Pt disc with a surface area of
0.071 cm2. The surface of the working electrode was polished with
a diamond suspension before each run. A Pt wire served as the
counter electrode. Saturated calomel electrode (SCE) was em-
ployed as the reference electrode and separated from the bulk of
the solution by a double bridge. Ferrocene was used as an internal
reference. Tetrabuthylammonium perchlorate (TBAP) in dimethyl-
sulfoxide (DMSO) and dichloromethane (DCM) was employed as
the supporting electrolyte at a concentration of 0.10 mol dmꢁ3
.
High purity N2 was used to remove dissolved O2 at least 15 min
prior to each run and to maintain a nitrogen blanket during the
measurements. IR compensation was applied to the CV and SWV
scans to minimize the potential control error.
3.2. Spectroscopic data
The template condensations of the S-methylthiosemicarbaz-
ones and aldehydes can be easily monitored by means of IR and
1H NMR spectra. In the spectra of the complexes, the NH2 and 2-
OH bands of the S-methylthiosemicarbazone disappeared due to
the condensation. Only the (OH) band of the 4-substituted hydro-
xyl group on the thiosemicarbazone were recorded at 3174 cmꢁ1 in
the complex spectra. By the template reaction, the (C@N1) and
(N2@C) bands of the S-methylthiosemicarbazones shifted to lower
energies ca. 13–20 cmꢁ1 and also a new azomethine band, (N4@C),
appeared in 1607–1600 cmꢁ1 region due to condensation of the
thioamide nitrogen (N4) and aldehyde.
UV–Vis absorption spectra and chromaticity diagrams were
measured by an OceanOptics QE65000 diode array spectrophotom-
eter. In situ spectroelectrochemical and in situ electrocolorimetric
measurements were carried out by utilizing a three-electrode con-
figuration of thin-layer quartz spectroelectrochemical cell at 25 °C.
The working electrode was Pt tulle. Pt wire counter electrode and a
SCE reference electrode separated from the bulk of the solution by
a double bridge were used. The standard illuminant A with 2°
observer at constant temperature in a light booth designed to ex-
clude external light was used. Prior to each set of measurements,
background color coordinates (x, y and z values) were taken at
In the 1H and 13C NMR spectra of the thiosemicarbazone, the
expected chemical shift values were monitored for the aromatic,

Y. Kurt et al. / Inorganica Chimica Acta 388 (2012) 148–156
151
Table 2
molecular ion peaks {[MꢁH], [Mꢁ2H], [M+H], [M+2H], [M], etc.}.
The mass spectrum of 3 and 4 showed a molecular ion (M+) peak
at m/z 496.34 and 541.24, corresponding to the N2O2 chelate com-
plexes [C22H16N3O3SClNi]+ and [C22H16N3O3SBrNi]+, respectively.
Consequently, the analytical and spectral data become evident
that the chelating N1,N4-diarylidene-S-methylthiosemicarbazida-
to ligands are bonded to metal atom through ONNO donor set
and so it can be proposed the template structures in Fig. 1.
Selected bond lengths (Å) and bond angles (°) for molecules
asymmetric unit of 4.
A and B in the
Ni1–O2
Ni1–O3
Ni1–N3
Ni1–N1
Ni2–O6
Ni2–O5
Ni2–N4
Ni2–N6
S1–C14
S1–C15
S2–C39
S2–C38
1.8239 (11)
1.8399 (13)
1.8388 (14)
1.8291 (15)
1.8399 (13)
1.8243 (13)
1.8259 (15)
1.8376 (14)
1.7463 (19)
1.785 (2)
O2–Ni1–O3
O2–Ni1–N1
O2–Ni1–N3
O3–Ni1–N1
O3–Ni1–N3
N1–Ni1–N3
N4–Ni2–N6
O5–Ni2–N6
O5–Ni2–O6
O5–Ni2–N4
O6–Ni2–N6
S1–C14–N3
S1–C14–N2
85.98 (6)
95.10 (6)
177.87 (6)
174.43 (6)
94.99 (6)
84.13 (6)
83.97 (6)
179.14 (6)
85.64 (6)
95.24 (6)
95.16 (6)
120.93 (13)
120.33 (14)
3.3. Crystallography
1.743 (2)
1.791 (2)
Single crystal of nickel complex (4), suitable for X-ray diffrac-
tion studies, was grown by slow evaporation of the alcoholic solu-
tion of complex. Crystal parameters and refinement results of the
complexes are summarized in Table 1. The selected bond distances
and angles are presented in Table 2. An ORTEP diagram of the mol-
ecule 4, along with the atom numbering scheme, is shown in Fig. 2.
The complex is formed by chelating of a doubly deprotonated
thiosemicarbazone molecule having ONNO donor set which struc-
tured in template reaction (Fig. 1). In crystal structure, each mole-
cule (A with Ni1 and B with Ni2) in the asymmetric unit of 4, each
molecules A and B is consisting by one mononuclear nickel salen-
type complex and one molecule of ethanol. The nickel atom lies in
a four-coordinate environment and adopts a slightly distorted
square-planar geometry (Fig. 2). Coordination of the symmetric
[N2O2] ligands to the nickel atom generates three chelate rings,
two of them composed by six atoms, NiONC3 (from the keto block
or salicyl aldehyde moiety) and the other one by five atoms (the
isothiosemicarbazidic NiN3C ring), in the sequence 6:5:6. In the
chelate structure of 4, there are two diagonal O-Ni-N angles of
95.10 (6) and 94.99 (6) (for A), 95.24 (6) and 95.16 (6) (for B),
barely deviating from linearity. The Ni–N and Ni–O bond distances
are in the normal range for Ni-salen type complexes [22–24]. The
Ni–N distances are 1.8291 (15), 1.8388 (14) Å for molecule A, and
1.8259 (15), 1.8376 (14) Å for molecule B and the Ni–O distances
are 1.8239 (11), 1.8399 (13) Å for molecule A and 1.8243 (13),
1.8399 (13) Å for molecule B. The chelate rings [O(2)–N(1)] and
[O(3)–N(3)] are approximately planar.
azomethine, S-methyl protons and even the cis–trans isomer peaks
of these protons [14,18]. The ratios of signal integrations of 2-hy-
droxyl group are 2:1 because of cis–trans isomerism, and also S-
methyl groups gave cis and trans peaks in 3:2 ratio. In the 3–4
spectra, the proton signals of 2-OH and N4H2 groups of Ni(II) com-
plexes disappear by chelation. The absence of these N4H2 hydro-
gens indicates their deprotonation and arising the N4@CH signal
which is a singlet and equivalent to one proton integral value con-
firms the template formation around nickel(II). The 1H NMR data of
the nickel templates did not show any isomer peak, probably as
these moieties have been partially fixed because of the template
reaction.
In the 13C NMR spectra, the chemical shifts of some carbon
atoms of the nickel complexes are in different values compared
to the thiosemicarbazone as starting material due to the formation
of a new conjugated backbone by the condensation reaction. Sig-
nificantly, carbon atoms of thioamide (1) and methyl (15) groups
were recorded in the lower field at 2.50 and 4.84 ppm, respectively,
and the chemical shift value of carbon atom (6) binding 4-OH sub-
stituent was observed at 4.84 ppm in higher field.
Characteristic peaks were observed in the ESI-mass spectra of
ligand and its metal complexes. The mass spectra of iron
complexes 1 and 2 revealed the protonated and deprotonated
Fig. 2. View of the two molecules (A with Ni1 and B with Ni2) in the asymmetric unit of 4 with the atom numbering scheme. Displacement ellipsoids for non-H atoms are
drawn at the 30% probability level.

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Y. Kurt et al. / Inorganica Chimica Acta 388 (2012) 148–156
Fig. 3. (a) CV of the complex 4 at various scan rates (inset: CV of the complex 4 at
Fig. 5. (a) CV of the complex 2 at various scan rates (inset: CV of the complex 2
recorded with different negative switching potentials) in TBAP/DCM electrolyte
system on Pt working electrode. (b) SWV of the complex. (SWV parameters: pulse
size = 100 mV; pulse wide = 5 mV; frequency: 25 Hz).
0.100 Vsꢁ1 scan rate with different switching potentials) and (b) SWV of the
complex
4 in TBAP/DCM electrolyte system on Pt working electrode (SWV
parameters: pulse size = 100 mV; pulse wide = 5 mV; Frequency:25 Hz) rates (inset:
Repetitive CV of the complex
4 recorded with all potential window of the
electrolyte system at 0.100 Vsꢁ1 scan rate.).
3.4. Electrochemistry
The redox properties of the complexes (1–4) were studied using
CV and SWV measurements in DCM and DMSO containing TBAP as
supporting electrolyte on a Pt electrode (Fig. 3–8). All complexes
show one 1-electron oxidation response during the anodic poten-
tial scan and two 1-electron reduction responses during the anodic
potential scan. The one-electron nature of these processes has been
established by comparing their current height with that of the
standard ferrocene/ferrocenium couple under identical experimen-
tal conditions. Table 3 lists the assignments of the redox couples
and the electrochemical parameters, which included the half-wave
peak potentials (E1/2), anodic to cathodic peak potential separation
(DEp), and ratio of the anodic to cathodic peak currents (Ipa/Ipc). E1/2
values are in agreement with the reported data for redox processes
in similar iron and nickel complexes in the literature [25–32].
The nickel complexes, 3 and 4, give very similar CV and SWV re-
sponses, thus electrochemical responses of 4 is given in Fig. 3 as a
representative of the nickel complexes. CV responses of 3 and 4
(Fig. 3a) indicate a metal-based 1-electron irreversible reduction
(at ꢁ1.08 V for 3 and at ꢁ0.94 V for 4) in addition to the ligand-
based oxidation process (at 1.19 V for 3 and at 1.17 V for 4) in
TBAP/DCM electrolyte system. Moreover, an irreversible ligand-
based reduction process is recorded with SWV at the end of the
Fig. 4. CV of the complex 4 at various scan rates in TBAP/DMSO electrolyte system
on Pt working electrode (inset: Repetitive CV of the complex 4 recorded with all
potential window of the electrolyte system at 0.100 Vsꢁ1 scan rate.).

Y. Kurt et al. / Inorganica Chimica Acta 388 (2012) 148–156
153
Fig. 6. (a) CV of the complex 2 at various scan rates (inset: CV of the complex 2
recorded with different negative switching potentials) in TBAP/DMSO electrolyte
system on Pt working electrode. (b) SWV with SWV parameters: pulse
size = 100 mV; pulse wide = 5 mV; frequency: 25 Hz.
Fig. 7. In-situ UV–Vis spectral changes of 4. (a) Eapp = ꢁ1.30 V. (b) Eapp = ꢁ1.80 V. (c)
Chromaticity diagram of 4. (each symbol represents the color of electro-generated
species; h: [NiIIL], s: [NiIL]ꢁ1, 4: [NiILꢁ1]]ꢁ2
.
negative potential window of the electrolyte system. It is reported
that thiosemicarbazone ligand gave a 1-electron irreversible oxida-
tion and a 1-electron irreversible reduction processes at the end of
the DCM or DMSO/TBAP electrolyte windows [33–35] and coordi-
nation of the ligands to the metal ion did not affect the redox po-
tential of ligands considerably. Thus, the first reduction process of
the complexes is easily assigned to the reduction of the central me-
not increases with repetitive CV cycles. These voltammetric re-
sponses indicate the presence of the chemical reaction succeeding
the oxidation and also reduction processes. Besides, Red.(CP) wave
disappeared when the positive potentials are scanned.
Although both oxidation and reduction of NiII center of NNS
type complexes were reported in the literature [36,37], reduction
of NiII to NiI is only recorded in this study for both nickel com-
plexes. Moreover NiII/NiI reduction peak shifts to the more negative
potentials with respect to the similar complexes in the literature.
It is well documented that DMSO is easily coordinated with
many transition metal ions [38]. Fig. 4 illustrates the CV and
SWV responses of 4 in DMSO/TBAP electrolyte system. The CV re-
sponses of 4 are approximately same in both solvents (DCM and
DMSO) in cathodic potential scan, but irreversible L/L+ wave shifts
to negative potentials and get reversible in DMSO. As shown in
Fig. 4b, voltammetric responses of the complex indicate presence
of the chemical reaction following the L/L+ process in DMSO during
repetitive CV scans like in DCM. Differently peak current of the
wave assigned to the product of the chemical reaction increases
continuously during the repetitive CV scans.
tal ions. Even though
DEp values of these peaks were in quasi-
reversible range especially at very slow scan rates, very small Ipa
/
Ipc values and deviation of Ip versus m1/2 from linearity may assign
the irreversible character of the redox processes to the chemical
reactions succeeding the electron transfer processes. The product
of the chemical reaction gives an irreversible oxidation process
(Red.(CP)) at ꢁ0.13 V. When the potential is switched before the
NiII/NiI process (from ꢁ1.0 V), Red.(CP) process disappeared, which
support the existence of the chemical reaction succeeding the
NiII/NiI process and instability of the reduced NiIL species (Fig. 3a
inset). Similarly, the oxidation process (L/L+) of the complex is fol-
lowed with a chemical reaction, whose product gives a reduction
wave at –0.43 V and an oxidation wave at 0.86 V (Fig. 3b inset).
When the potential is scanned from 0.0 to ꢁ1.70 V, the complex
gives a reduction couple (NiII/NiI) at ꢁ1.17 V and Red.(CP) at
ꢁ0.13 V in addition to L/L+ process at 1.17 V during the reverse
scan from ꢁ1.70 to 1.50 V (blue CV: first cycle in Fig. 3b inset).
Then when the CV scans continue repeatedly, new waves are
recorded at ꢁ0.43 and 0.86 V, but peak current of these waves do
The iron complexes (1 and 2) give very similar CV and SWV re-
sponses with each other. As shown in Fig. 5a, CV responses of 2
indicate a 1-electron metal-based reversible redox couple at
ꢁ0.24 V in addition to the ligand-based reduction process at
ꢁ1.26 V in TBAP/DCM electrolyte system. Moreover, an irreversible

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Y. Kurt et al. / Inorganica Chimica Acta 388 (2012) 148–156
the ligand orbitals and those in the visible region are probably due to
metal to ligand charge-transfer (MLCT) transitions. In situ spectro-
electrochemical studies were employed to determine the spectra of
electrogenerated species of the complexes to perform assignments
of the redox processes, and to predict the redox mechanismof the pro-
cesses. The nickel complexes (3 and 4) give very similar spectroscopic
changes in DCM and DMSO electrolyte under applied potentials. Fig. 7
shows in situ UV–Vis spectral changes of 4 in TBAP/DCM electrolyte as
a representative of the nickel complexes during the controlled poten-
tial reductions and oxidation processes. Under the applied potential at
ꢁ1.30 V, the broad MLCT band at 566 nm increases in intensity, while
the intraligand transitions band at 425 nm shifts to 394 nm with
increasing in intensity (Fig. 7a). These spectroscopic changes indicate
a metal-based redox process and assign the couple to [NiIIL]/[NiIL]ꢁ1
process. Well-defined isosbestic points at 345, 417, and 494 nm dem-
onstrate that the reduction proceeds to give a single reduced species
underappliedpotentialatꢁ1.30 V.Applying0.0 Vafterthefirstreduc-
tion process regenerate the original spectra of [NiIIL] species, which
indicates the chemical reversibility of the process. Isosbestic points
are commonly met when electronic spectra are taken on a solution
inwhichareactionisinprogressinwhichcasethetwoabsorbingcom-
ponents concerned are a reactant and a product and the sum of the
concentrations of two principal absorbing components is constant. If
absorption spectra of the types considered above intersect not at one
ormoreisosbesticpointsbutoverprogressivelychangingwavelength,
this is prima facie evidence for the formation of a reaction intermedi-
ateinsubstantialconcentration. Spectroscopic changesgiven inFig. 7b
are recorded under applied potential at ꢁ1.70 V. During this process,
whilethe bandat 394 nmshifts to 430 nmwith increasing inintensity,
the band at 566 nm decreases in intensity. These spectral changes are
indicator of a ligand-based redox reaction. Well-defined isosbestic
points at 343, 394, and 467 nm are recorded during this process. The
spectrum did not return to the initial spectrum of the [NiIL]ꢁ1 species
when ꢁ1.30 V was applied to the working electrode, which indicates
the irreversibility of the process chemically. Under applied potential
at 1.50 V, the spectra of the complex do not change considerably.
The color changes of the compounds during the redox processes were
recordedusingin situ colorimetricmeasurements. Without any poten-
tial application, the solution of [NiIIL] is orange (x = 0.419 and
y = 0.387). As the potential is stepped from 0 to ꢁ1.30 V, the color of
the neutral [NiIIL] gets purple (x = 0.314 and y = 0.2824) at the end of
thefirstreduction. Similarlycolorofthe dianionicspeciesarerecorded
as yellow (x = 0.406 and y = 0.420) at the end of the second reduction
process (Fig. 7c). Measurement of the xyz coordinates allows quantifi-
cation of each color of the redox species that is very important to de-
cide their possible electrochromic application.
Fig. 8 shows in situ UV–Vis spectral changes of 2 in TBAP/DCM
electrolyte as a representative of the iron complexes. Under ap-
plied potential at ꢁ0.50 V, a new band at 555 nm is recorded, while
the intraligand transitions band at 300 nm decreases in intensity
slightly (Fig. 8a). These spectroscopic changes indicate a metal-
based redox process and assign the couple to [FeIIIL]/[FeIIL]ꢁ1 pro-
cess. Well-defined isosbestic points at 284, 320, and 780 nm dem-
onstrate that the reduction proceeds to give a single reduced
species. This process is chemically reversible with in the time scale
of the spectroelectrochemical measurement, since applying 0.0 V
after the first reduction process regenerate the original spectra of
[FeIIIL] species. Spectroscopic changes given in Fig. 8b are recorded
under applied potential at ꢁ1.70 V. During this process, intensity of
the band at 394 nm increases, whereas the band at 555 nm
decreases in intensity and disappear completely. These spectral
changes are indicator of a ligand-based redox reaction. Under
applied potential at 1.50 V, the spectra of the complex do not
change considerably. The color changes of the compounds during
the redox processes were recorded using in situ colorimetric mea-
surements and given as chromaticity diagram in Fig. 8c.
Fig. 8. In-situ UV–Vis spectral changes of 2. (a) Eapp = ꢁ0.50 V. (b) Eapp = ꢁ1.70 V. (c)
Chromaticity diagram of 2. (each symbol represents the color of electro-generated
+1
species; h: [FeIIIL], s: [FeIIL]ꢁ1, 4: [FeIILꢁ1
] ] .
ꢁ2, [FeIIIL+1
ligand-based oxidation process is recorded at 1.33 V during the
anodic potential scan. SWV measurements clearly illustrate these
processes and reversibility of the first reduction process (Fig. 5b).
D
Ep values of the FeIII/FeII couple is in reversible range especially
at very slow scan rates and unity of the Ipa/Ipc ratios and linearity
of Ip versus
m
1/2 changes indicate the reversible character of this re-
dox process. As shown in Fig. 5a inset, peak character of the first
reduction process does not change with different switching poten-
tials. As compared with the similar FeIII complexes in the literature
[39,40], FeIII/FeII reduction peak shifts to the negative potential due
to the electron releasing ability of the ligand. For example, L.R.
Teixeira and coworkers reported FeIII/FeII reduction peak at –
0.12 V followed with a reduction process at ꢁ1.20 V for FeIII com-
plexes of N4-para-tolyl-thiosemicarbazones [39].
Redox properties of complex 2 change when coordinating solvent
DMSO is used instead of DCM. Fig. 6 illustrates the electrochemicalre-
sponsesof2inDMSO/TBAPelectrolytesystem. Whileredoxprocesses
of 2 shift to the negative potentials, reversibility of L/Lꢁ process in-
creasesinDMSOwith respect to those inDCM. AsshowninFig. 6a (in-
set), different switching potentials do not also affect the electron
transfer character of the redox processes in DMSO like in DCM.
3.5. Spectroelectrochemical studies
Neutral form of each complex shows several intense absor-
ption bands in the visible and ultraviolet region in solution. The
absorptions in the ultraviolet region are assigned to transitions within

Y. Kurt et al. / Inorganica Chimica Acta 388 (2012) 148–156
155
Table 3
Voltammetric data of complexes 1–4.
Complexes
Peak parameters
Redox processes
L/L+
M reduction
L/Lꢁ
1 (in DCM)
aE1/2
1.35d
–
–
ꢁ0.22
65
0.92
ꢁ1.24
60
0.32
bDEp (mV)
cIpa/Ipc
2 (in DCM)
2 (in DMSO)
3 (in DCM)
4 (in DCM)
4 (in DMSO)
a
aE1/2
1.33d
–
–
ꢁ0.24
74
0.96
ꢁ1.26
97
0.44
bDEp (mV)
cIpa/Ipc
aE1/2
0
0.97d
–
–
1.21d
–
–
ꢁ0.32
66
0.97
ꢁ1.39
90
0.45
bDEp (mV)
cIpa/Ipc
aE1/2 vs. SCE
bDEp (mV)
cIpa/Ipc
ꢁ1.06
77
0.34
ꢁ1.75d
–
–
aE1/2 (V)
bDEp (mV)
cIpa/Ipc
1.30d
–
–
ꢁ1.10
78
0.30
ꢁ1.79d
–
–
aE1/2
0.71
320
0.85
ꢁ1.12
60
0.23
ꢁ1.93d
bDEp (mV)
cIpa/Ipc
–
–
E1/2 = (Epa + Epc)/2 at 0.100 Vsꢁ1
.
b
c
D
Ep = |Epc ꢁ Epa|.
Ipa/Ipc for reduction, Ipc/Ipa for oxidation processes at 0.100 Vsꢁ1 scan rate.
E1/2 values were derived from SWV measurements.
d
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.ica.2012.03.023.
4. Conclusion
We synthesized many N2O2 chelate complexes of S-alkyl-thio-
semicarbazones due to cytotoxic activities [10,11,14], antidiabetic
and also protective effects on pancreas [41], in previous years.
However, we could reported a limited number of crystal structure
of these complexes obtained from benzophenone thiosemicarba-
zone because of difficulties in crystal growth [14,15]. In this paper,
we present new N2O2 complexes of iron(III) and nickel(II) obtained
from 2,4-dihydroxy-benzophenone-S-methylthiosemicarbazone
(1–4), single crystal analysis of square-planar nickel complex,
[Ni(L)]ꢀC2H5OH (4), and first electrochemical studies of these struc-
tures in a comprehensive manner.
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