Furannic thiosemicarbazone complexes of Co(II), Ni(II), and Cu(II) in ethanol solutions and of Co(II) in the solid state

Article abstract of DOI:10.1080/10426500701519146

Thiosemicarbazone complexes with cobalt(II), nickel(II), and copper(II) metal ions are studied in ethanol solutions. The stability constants of all the identified complexes are calculated, and we found that stability increases from cobalt to copper. The stability of the complex is increased by the presence of a donor group like methyl, whereas it is not affected by the length of the thiosemicarbazone side chain. A further study of the cobalt(II) complexes in the solid state permits the identification of the coordinating atoms. With the help of additional results from electronic spectra structures of the cobalt(II) complexes are proposed. Copyright Taylor & Francis Group, LLC.

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Furannic Thiosemicarbazone
Complexes of Co(II), Ni(II), and
Cu(II) in Ethanol Solutions and
of Co(II) in the Solid State
a
a
El Mostapha Jouad , Gilles M. Bouet , Mustayeen
a
b
Ahmed Khan , Diana Dogaru & Constantin
b
Mateescu
a
Laboratoire de Chimie de Coordination , Angers
Cedex 01, France
b
Faculty of Food Processing Technology , Banat
University of Agricultural Sciences and Veterinary
Medicine , Timisoara, Romania
Published online: 25 Sep 2007.
To cite this article: El Mostapha Jouad , Gilles M. Bouet , Mustayeen Ahmed Khan ,
Diana Dogaru & Constantin Mateescu (2007) Furannic Thiosemicarbazone Complexes
of Co(II), Ni(II), and Cu(II) in Ethanol Solutions and of Co(II) in the Solid State,
Phosphorus, Sulfur, and Silicon and the Related Elements, 182:11, 2705-2718, DOI:
10.1080/10426500701519146
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Phosphorus, Sulfur, and Silicon, 182:2705–2718, 2007
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500701519146
Furannic Thiosemicarbazone Complexes of Co(II),
Ni(II), and Cu(II) in Ethanol Solutions and of Co(II)
in the Solid State
El Mostapha Jouad
Gilles M. Bouet
Mustayeen Ahmed Khan
Laboratoire de Chimie de Coordination, Angers Cedex 01, France
Diana Dogaru
Constantin Mateescu
Faculty of Food Processing Technology, Banat University of Agricultural
Sciences and Veterinary Medicine, Timisoara, Romania
Thiosemicarbazone complexes with cobalt(II), nickel(II), and copper(II) metal ions
are studied in ethanol solutions. The stability constants of all the identified com-
plexes are calculated, and we found that stability increases from cobalt to copper.
The stability of the complex is increased by the presence of a donor group like methyl,
whereas it is not affected by the length of the thiosemicarbazone side chain. A fur-
ther study of the cobalt(II) complexes in the solid state permits the identification of
the coordinating atoms. With the help of additional results from electronic spectra
structures of the cobalt(II) complexes are proposed.
Keywords 3D transition metals; IR spectroscopy; stability constants; thiosemicarbazone
complexes; UV-visible spectroscopy
INTRODUCTION
Metal complexes of oximes, semicarbazones, and thiosemicarbazones
have known biological activities. Their antiviral and cytotoxic activ-
1
−10
ities have been widely reported in the scientific literature.
furan derivatives have been studied in this regard,
Many
and a detailed
1
1−15
study in solution of different semicarbazones as ligands coordinated to
copper(II) was reported by our laboratory.16 This study showed that the
identified complexes were fairly stable, their stability constants being
of the order of 3 ± 1 log units. However, with other metals the same
Received January 9, 2007; accepted May 19, 2007.
Address correspondence to Mustayeen Ahmed Khan, Laboratoire de Chimie de Co-
ordination, SONAS EA 921, UFR Pharmacie, 16 bd Daviers, 49045 Angers cedex 01,
France. E-mail: mustayeen.khan@univ-angers.fr
2705

2
706
E. M. Jouad et al.
FIGURE 1 General structure and substitution pattern of the thiosemicar-
bazone ligands.
ligands form very weak complexes. This is expected, however, as it was
shown earlier that the Irving-Williams Order of Stabilities is equally
valid in such systems;17 hence, the stability of the complexes is highest
with copper as compared to other metals.
In the present study, four furan-substituted thiosemicarbazones
were synthesized. Their general structure is presented in Figure 1.
The complexes of these thiosemicarbazones with cobalt(II), nickel(II),
and copper(II) were prepared, their relative stability in solution was de-
termined and compared with that of the corresponding semicarbazone
complexes. As these complexes have shown potential therapeutic prop-
erties, attempts were made to study their structures in the solid state.
EXPERIMENTAL
For the synthesis of the ligands, the experimental procedure given by
West et al. was followed.18 All ligands were synthesized by refluxing the
corresponding furaldehyde with thiosemicarbazide (1:1 molar ratio) in
50 mL absolute ethanol in the presence of pure acetic acid (5 mL) for 1 h.
The mixture was then cooled to ambient temperature, filtered, and the
solid recrystallized from ethanol/water (3:1). The ligands were obtained
as yellow microcrystalline solids. All analytical and spectroscopic data
are in good agreement with the expected data.
For spectrophotometric experiments the final analytical concentra-
−
4
−3
−3
tion of the metal was 2.10 mol dm in case of copper(II), and 2.10
−3
mol dm for solutions of cobalt(II) and nickel(II). Ligand solutions
were kept away from light in order to avoid degradation and were pre-
pared in the concentration range of 10 to 10 mol dm . No analysis
could be done for higher molar ratios due to precipitation. Under such
−
5
−4
−3

Thiosemicarbazone Complexes
2707
experimental conditions, only mononuclear and dinuclear complexes
were envisaged.
The UV-visible spectra between 200 and 500 nm were recorded using
a Perkin Elmer (UV/VIS/NIR) Lambda 19 spectrophotometer equipped
with a thermostat and matched 10 mm quartz cell. The temperature at
◦
which the experiments were performed was maintained at 25.0 ± 0.2 C.
The final solutions of metal and ligand were prepared just before the
measurements and ethanol was used as spectrophotometric reference
solution. The electronic spectra were recorded in the whole range of
UV-VIS-NIR.
The IR spectra were obtained with a Perkin Elmer (Spectrum One
−
1
FTIR) instrument between 4 000 and 400 cm using KBr discs. For
−
1
measurements in the far IR between 400 and 50 cm , a Nicolet FTIR
0F spectrometer with polyethylene disc was used at the Institut des
2
Mat e´ riaux, University of Nantes.
Dichlorobis(furfural Thiosemicarbazone) Cobalt (II)
[
CoCl (FTSC) ] and Dibromobis(furfural Thiosemicarbazone)
2
2
Cobalt (II) [CoBr (FTSC) ]
2
2
These complexes were prepared by mixing 20 mL of a FTSC solution
0.85 g, 5 mmol) in methanol with 20 mL of a methanol solution of
(
cobalt(II) chloride, CoCl2·6H2O (0.59 g, 2.5 mmol), or cobalt(II) bromide,
CoBr2 (0.85 g, 2.5 mmol). The mixture was refluxed for 24 h, and then
cooled to ambient temperature when the complex precipitated.
Dichloro(5-methylfurfural Thiosemicarbazone) Cobalt (II)
[
CoCl (M5FTSC)]
2
The complex [CoCl2(M5FTSC)] was prepared from 1.19 g (5 mmol) of
cobalt(II) chloride, CoCl2.6H2O, dissolved in 10 mL of absolute ethanol.
To this solution M5FTSC (0.92 g, 5 mmol) in 10 mL of absolute ethanol
were added and the mixture was maintained for 24 h at the reflux
temperature of ethanol. The complex precipitated on cooling to ambient
temperature.
Dichlorobis(3-(2-furryl)prop-2-enal Thiosemicarbazone)
Cobalt(II) [CoCl (FATSC) ]
2
2
This complex was obtained by the addition of a solution of CoCl2·6H2O
(
(
0.6 g, 2.5 mmol) in 10 mL of ethanol to a solution of FATSC
0.98 g, 5 mmol) in 20 mL of ethanol with constant stirring under

2
708
E. M. Jouad et al.
reflux conditions. The complex precipitated after 3 h of reaction
time.
RESULTS AND DISCUSSION
Complexes of Co(II), Ni(II), and Cu(II) with the ligands FTSC, M5FTSC,
3FTSC, and FATSC were prepared. Since the most stable complexes
are formed with copper, detailed results with this metal ion are pre-
sented here. For the other complexes the results of calculations are
given.
Experimental UV-VIS Spectra
In ethanolic solution copper(II) shows a characteristic maximum at
2
63 nm with a shoulder peak around 245 nm. The principal bands
of the ligands in this solvent in the UV-VIS region are for FTSC at
23 nm, for M5FTSC at 330 nm, for 3FTSC at 295 nm, and for FATSC
3
at 350 nm. The experimental spectra of the copper(II) chloride solutions
with FTSC, M5FTSC, 3FTSC and FATSC are presented in Figures 2-5,
respectively.
FIGURE 2 Absorption spectra of copper(II)-FTSC solutions. [Cu2+] = 10−4 mol
−1
L
; l = 1 cm; C = ligand concentration.

Thiosemicarbazone Complexes
2709
FIGURE 3 Absorption spectra of copper(II)-M5FTSC solutions. [Cu2+] = 10−4
−
1
mol L ; l = 1 cm; C = ligand concentration.
Figure 2 shows that with increasing concentration of FTSC two new
bands are observed: a clear maximum at 350 nm and a shoulder peak
around 295 nm. In the case of M5FTSC, both the metal and the lig-
and maxima undergo a bathochromic shift with the appearance of two
−
6
−3
bands at 267 and 350 nm at concentration as low as 8.10 mol dm
Figure 3). For 3FTSC also, the appearance of two new bands around
75 and 328 nm is observed. They tend to coalesce into one maximum—
that of the ligand, indicating the presence of free ligand in the solution
Figure 4). Finally, in Figure 5 it can be seen, that FATSC solutions
(
2
(
with copper(II) equally exhibit the presence of two new bands at 250
and 375 nm.
Determination of the Stability Constants
The equilibrium under study is:
M2 +L ꢀꢁ [ML]2+
+
,
(1)
where L is the ligand and M = Co, Ni or Cu. The equilibrium is defined
by the stability constant β:
β= [ML +]/[M ][L].
2
2+
(2)

2
710
E. M. Jouad et al.
FIGURE 4 Absorption spectra of copper(II)-3FTSC solutions. [Cu2+] = 10−4
−
1
mol L ; l = 1 cm; C = ligand concentration.
FIGURE 5 Absorption spectra of copper(II)-FATSC solutions. [Cu2+] = 10−4
−
1
mol L ; l = 1 cm; C = ligand concentration.

Thiosemicarbazone Complexes
2711
TABLE I Detailed Results for the System Cu(II)-FTSC Using the
SIRKO Program
Model
CuCl2
1
FTSC
0
log β = 0
log β = 0
0
1
1
2
log β = 1.951 Refined
Parameters
λ(nm)
290
310
330
340
350
360
370
ε(3,1) = 16548 ± 4740
ε(3,2) = 15986 ± 5113
ε(3,3) = 18945 ± 6051
ε(3,4) = 20592 ± 6032
ε(3,5) = 19640 ± 5955
ε(3,6) = 17953 ± 5707
ε(3,7) = 14389 ± 5905
log β = 1.951 ± 0.073
5
iterations
R factor = 3.53 ± 0.05%
The calculation of the stability constants is based on the computer
program SIRKO, which was conceived for the determination of these
and related parameters from spectrophotometric, potentiometric and
calorimetric experimental data.19 The essential details of the program
are given in our earlier publication.16
Table I gives the detailed results obtained from the program for
the system CuCl2-FTSC. In this presentation, the theoretical model
proposed is given in the top left-hand corner, CuCl2:1 and FTSC:2,
i.e., M:L = 1:2 complex. The best value obtained for the stability con-
stant of this model is log β = 1.951 with a dispersion of ± 0.073.
The molar absorption coefficients ε given at the left are those of the
complex calculated for the different wavelengths λ. The numbers in
parentheses are programme codes. The first number indicates the na-
ture of the absorbing species, i.e., 1 for the metal, 2 for the ligand,
and 3 for the complex; the second number indicates the code for the
successive wavelengths, e.g. 1 for 290 nm, 2 for 310 nm, etc. These
values are obtained after five iterations with an R factor of 3.53 %.
The R factor defines the compatibility of the theoretical model with
the experimental data and should be as small as possible, 5% or
less.
Table II summarizes the final results obtained for the stability con-
stants of the four thiosemicarbazone ligands with cobalt(II), nickel(II)
and copper(II).

2
712
E. M. Jouad et al.
TABLE II Logarithm of Stability Constants of the Four
Thiosemicarbazone Ligands with Cobalt(II), Nickel(II) and
Copper(II)
Cobalt
II)
Nickel
(II)
Copper
(II)
(
FTSC
M5FTSC
1.00
1.87
2.18
0.83
1.07
2.03
2.39
0.90
1.95
3.82
4.51
1.64
3
FTSC
FATSC
Stability of the Complexes
For each ligand, the complexes formed with different metals increase in
stability from cobalt to nickel and strongly from nickel to copper. This
result is in accord with the Irving–William Order of Stabilities and we
1
7
reported earlier such behaviour in similar systems.
The complexes formed with the ligand 3FTSC have the highest stabil-
ity when compared with the other three ligands. This can be explained
by the position of the thiosemicarbazone part on the furanic cycle. In-
deed, the position 3 of the ring is more favorable for complexation than
the position 2 of the ring because of much less steric hindrance and
consequently gives more stable complexes. Amongst the other three lig-
ands with the thiosemicarbazone part at position 2 of the ring: FTSC,
M5FTSC and FATSC, the stability of the complexes is higher in the case
of M5FTSC than in the case of the other two ligands. This is certainly
due to the donor effect of the methyl group, which increases the elec-
tron density at the coordinating atoms. Finally the only difference in
the structures of the ligands FTSC and FATSC is the length of the side
chain. Since stability constants with these ligands are of the same or-
der, it is postulated that the length of the thiosemicarbazone side chain
has no appreciable effect on the stability of the complexes.
Comparison with Semicarbazone Complexes
The reported results of the semicarbazone complexes with copper(II) 16
can be compared with those obtained for the corresponding thiosemi-
carbazone complexes in this study. The log β values in case of FTSC
(
1.95), FSC (2.02), and M5FTSC (3.82), M5FSC (3.84) are essentially
the same. This is explained by the simple fact, that oxygen and sulfur
are members of the same group of elements in the periodic table and
have, therefore, similar physicochemical properties. However, there is

Thiosemicarbazone Complexes
2713
significant difference in the values with FATSC (1.64) and FASC (4.52).
This exception is most probably due to different coordination sites.
Cobalt(II) Complexes in the Solid State
The thiosemicarbazone complexes have shown important biological ac-
tivities and consequently, for a better understanding of the chemistry
of these complexes, studies in the solid state were carried out, as well.
Some of the results have already been reported.[20−22] In this study the
results of the cobalt(II) complexes are presented and a comparison is
made with the results obtained in solution. It must be mentioned that in
contrast to the corresponding Ni(II) and Cu(II) complexes, no structure
determination by X-ray diffraction on single crystals of the cobalt(II)
complexes could be performed. The four cobalt complexes synthesized
were studied in the solid state by spectroscopic methods.
IR Spectra
Analysis and a comparative study of the IR spectra of the ligands and
those of their complexes with cobalt(II) show that the ligands undergo
deprotonation at the nitrogen atom of the hydrazine moiety N(2), as
shown below:
C = N(3) − N(2)H − C(1)(= S) − N(1)H2 → C = N(3) − N(2)
=
C(1)(−S) − N(1)H2.
(3)
Following X-ray diffraction results with metals other than cobalt, a
numbering scheme was devised to identify readily the different atoms.
These numbers are placed within brackets and the detailed schemas
were given in our earlier publications.2
0−22
It is further revealed that the coordination of the metal occurs
through the nitrogen atom of the imine moiety and the sulfur atom.
In case of sulfur, this is possible due to the delocalisation of the double
bond and the resulting increase of electron density on the sulfur atom.
−
1
Indeed, the bands which appear at 1609, 1600, and 1597 cm in the
IR spectra of FTSC, M5FTSC, and FATSC, respectively, corresponding
to the vibration ν(C N), are shifted to lower wave numbers in the case of
−
1
the complexes. This shift is on the average about 43 cm for complexes
−
1
with FTSC and with M5FTSC, and about 15 cm for complexes with
FATSC. These shifts indicate the participation of the nitrogen atom of
the imine moiety, N(3) in the numbering scheme, in the coordination to
2
3−26
the metal.

2
714
E. M. Jouad et al.
−1
The intense band that appears around 3150 cm in the spectra of
the ligands, which is attributed to the NH vibration of the hydrazine
moiety N(2)H, disappears in the spectra of all complexes. This shows
the loss of this hydrogen atom on complexation. The presence of a well-
−1
resolved band around 1625 cm in the spectra of these compounds
confirms the deprotonation. The band corresponds to the formation of
the double bond after the loss of hydrogen in the thiosemicarbazone
moiety between the carbon atom numbered 1, C(1) and the nitrogen
atom numbered 2, N(2), as shown in the scheme above.
Regarding the two characteristic ν(C S) vibration bands in the spec-
−
1
tra of the ligands, it is observed that the band appearing at 1280 cm
disappears in the spectra of all complexes, whereas the band at around
−1
8
40 cm undergoes a shift to lower wave numbers. These two phe-
nomena are indicative of the participation of the sulfur atom in the
2
3–26
coordination to the metal in these complexes.
In the literature, the bands appearing between 160 and 300 cm are
−
1
2
3,26−31
assigned to the vibration of the Co–X bond, where X = Cl or Br.
In
this study, the frequencies ν(Co-X) are in accordance with the reported
−1
data and appear between 220 and 300 cm
.
Electronic Spectra
The calculation of the parameters 10 Dq, B and of the theoretical posi-
tions of the bands in the electronic spectra, were made with the program
3
2
Quick Basic, which is based on K o¨ nig’s relations. The cobalt(II) com-
plexes have three bands, viz., ν1, ν2, ν3, which are presented in Table III.
Table IV gives the mean values of the calculated 10 Dq and B.
The values of 10 Dq are quite similar in all the hexacoordinated
complexes as well as in the two cobalt salts. These values are in accord
with those found for the corresponding complexes of semicarbazones of
CoX2(L)2] type, which have an octahedral geometry.[
31,33]
[
TABLE III Electronic Spectra Data of Cobalt(II) Complexes in
Ethanol Solution
ν1
ν2
(cm
ν3
(cm 1)
−
−1
)
−1
(cm )
Complex
[CoCl2(FTSC)2]
[CoBr2(FTSC)2]
[CoCl2(M5FTSC)]
[CoCl2(FATSC)2]
7350
7360
4108
7548
15,112
15,604
6820
17,822
17,620
14,213
17,953
15,625

Thiosemicarbazone Complexes
2715
TABLE IV Mean Values of 10 Dq and of B for the Cobalt(II)
Complexes and Their Salts
1
(cm
0 Dq
B
(cm
−
1
−1
Salt/Complex
)
)
CoCl2·6H2O
CoBr2·6H2O
8030
8200
7990
8290
4110
8265
710
680
760
750
590
755
[
[
[
[
CoCl2(FTSC)2]
CoBr2(FTSC)2]
CoCl2(M5FTSC)]
CoCl2(FATSC)2]
Ferguson et al.34 showed that the existence of an asymmetric band in
−
1
the UV-VIS spectrum centred at around 17 000 cm is typical of octahe-
dral complexes of cobalt(II), and that a shoulder band situated towards
the side of higher energies corresponds to forbidden spin transitions.
The complexes [CoCl2(FTSC)2], [CoBr2(FTSC)2] and [CoCl2(FATSC)2]
−
1
present this type of bands between 17,600 and 18,000 cm . With re-
gards to Racah’s B parameter for the same three complexes, the calcu-
lated values are close to those of the corresponding semicarbazones as
reported by Ibrahim et al., 3
1,33
and they are also comparable to the val-
2
9,35
ues of other cobalt(II) complexes derived from thiosemicarbazones.
On the other hand, the tetracoordinated structure of the
CoCl2(M5FTSC)] complex and its blue color, indicate tetrahedral ge-
ometry for this compound. The values of 10 Dq and of the B pa-
rameter confirm this environment. This explains the unique value
of this complex, different from the others in Table IV. The results of
[
36
these calculations (Table IV), are comparable to those of other com-
plexes of tetrahedral geometry.1
5,29
Consequently, it can be assumed,
that[CoCl2(M5FTSC)] also adopts this geometry.
Regarding the ionic character of the complexes, one of the criteria
proposed by Lever 36 is the value of the ratio of ν1 and ν2, which is
generally around 2. For non-ionic complexes this ratio is about 2.1 and
according to Lever it reaches 2.2 for compounds having a high ionic
character. In the present study, this ratio varies between 2.06 and 2.12.
The cobalt(II) thiosemicarbazone complexes are therefore non-ionic or
feebly ionic in character and this is confirmed by measurements of their
molar conductivity.
Structures
The coordinating atoms in the complexes are identified by infrared stud-
ies. The results of structural studies based on X-ray diffraction of Ni(II)

2
716
E. M. Jouad et al.
2 2 2 2
FIGURE 6 Proposed structure for [CoCl (FTSC) ] and [CoBr (FTSC) ].
and Cu(II) complexes with the same ligands20,22 show, that the coordi-
nating atoms are in trans configuration, with respect to the C(1) N(2)
bond. By analogy, it would be the same in the case of these cobalt(II)
complexes. In the process of complex formation, all ligands lose the hy-
drogen atom of the NH group of the hydrazine moiety. The study of
the electronic spectra in ethanol helps in elucidating the geometry of
each complex. Taking into consideration all these structural aspects,
the structures of the complexes proposed are given in Figures 6–8. It
may be emphasized here that these structures are proposed after con-
sidering the spectroscopic data only, which in itself is not sufficient and
should be complemented with crystallographic investigations. However,
as pointed out earlier, single crystals of the cobalt(II) complexes could
not be obtained and therefore, a profound crystallographic study could
not be undertaken to confirm these propositions. Nevertheless, these
structural propositions are supported by analogy to the X-ray diffrac-
tion results of Ni(II) and Cu(II) complexes with identical ligands. It is
hoped that knowledge of the structures of these complexes will help in
a better understanding of their biological activities.
2
FIGURE 7 Proposed structure for [CoCl (M5FTSC)].

Thiosemicarbazone Complexes
2717
2 2
FIGURE 8 Proposed structure for [CoCl (FATSC) ].
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