A cyclic voltammetry investigation of the complex formation between Cu2+ and some Schiff bases in binary acetonitrile/dimethylformamide mixtures

Article abstract of DOI:10.1016/j.molstruc.2007.10.007

The complex formation between Cu²⁺ ions and some Schiff base ligands was studied in binary solvent mixtures of acetonitrile (AN)/dimethylformamide (DMF) systems at 25 °C using the cyclic voltammetric technique. The stoichiometry and stability of the complexes were determined by monitoring the shift in the half-wave potential of the CV peaks of the copper against the ligands concentration. The stoichiometry of all the complexes was found to be 1:1 and the complexation constants increased with decreasing amounts of dimethylformamide in these binary systems. In all cases, the variation of the stability constant with composition of the solvents was monotonic and showed good correlation with the inherent solvation ability of the neat solvents which form the mixture.

Full text of DOI:10.1016/j.molstruc.2007.10.007

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Journal of Molecular Structure 885 (2008) 76–81
www.elsevier.com/locate/molstruc
A cyclic voltammetry investigation of the complex formation
between Cu2+ and some Schiff bases in binary
acetonitrile/dimethylformamide mixtures
M. Rahimi-Nasrabadi a,b,*, M.R. Ganjali b, M.B. Gholivand a, F. Ahmadi c,
P. Norouzi b, M. Salavati-Niasari d
a Department of Chemistry, Razi University, Kermanshah, Iran
b Department of Chemistry, University of Tehran, Tehran, Iran
c Pharmaceutical Chemistry Department, Faculty of Pharmacy, Kermanshah University of Medical Science, Kermanshah, Iran
d Department of Chemistry, Faculty of Science, Kashan University, Kashan, Iran
Received 15 May 2007; received in revised form 8 October 2007; accepted 11 October 2007
Available online 22 October 2007
Abstract
The complex formation between Cu2+ ions and some Schiff base ligands was studied in binary solvent mixtures of acetonitrile (AN)/
dimethylformamide (DMF) systems at 25 °C using the cyclic voltammetric technique. The stoichiometry and stability of the complexes
were determined by monitoring the shift in the half-wave potential of the CV peaks of the copper against the ligands concentration. The
stoichiometry of all the complexes was found to be 1:1 and the complexation constants increased with decreasing amounts of dimeth-
ylformamide in these binary systems. In all cases, the variation of the stability constant with composition of the solvents was monotonic
and showed good correlation with the inherent solvation ability of the neat solvents which form the mixture.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Schiff base; Cyclic voltammetry; Mixed solvents; Copper; Stability constants
1. Introduction
considerable attention has been paid to the chemistry of
the metal complexes of Schiff bases containing nitrogen
and other donors [17–22]. This may be attributed to their
stability [23] and potential applications in many fields such
as oxidation catalysis [17] and electrochemistry [18]. It is
well known that some drugs have higher activity when
administered as metal complexes than as free ligands [24].
Schiff bases and their biologically active complexes have
been often used as chelating ligands in the coordination
chemistry of transition metals as radiopharmaceuticals
for cancer targeting, agrochemicals, as catalysts and as
dioxygen carriers.
Copper is an important trace element for plants and ani-
mals, and is involved in mixed ligand complex formation in
a number of biological processes. Copper complexes con-
taining Schiff base ligands are of great interest since they
exhibit numerous biological activities such as antitumor
Schiff base ligands are considered privileged ligands
because they are easily prepared by the condensation
between aldehydes and imines. Stereogenic centers or other
elements of chirality (planes, axes) can be introduced in the
synthetic design. Schiff base ligands are able to coordinate
many different metals [1–5], and to stabilize them in various
oxidation states. Schiff bases can be used to obtain optical
materials and conducting polymers [6]. Thus, new optical
and organic conducting materials can be produced by these
compounds. The Schiff base ligands have been used in con-
struction of membrane sensors [7–14] and as models for
biological systems [15,16]. During the past two decades,
*
Corresponding author.
E-mail address: rahimi1356@gmail.com (M. Rahimi-Nasrabadi).
0022-2860/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2007.10.007

M. Rahimi-Nasrabadi et al. / Journal of Molecular Structure 885 (2008) 76–81
77
[25], anticandida [26], antimicrobacterial [27] antimicrobial
[28] activities, etc.
In this work, we study the possibility of complex forma-
tion of Cu2+ with Schiff bases. This can affect the concen-
tration of free Schiff bases and therefore the activation of
substituted Schiff bases in biological systems.
quite different donating abilities [30] (DNDMF = 26.6;
DNAN = 14.1). Thus, by the use of different DMF–AN
mixtures, the influence of the solvating ability of the sol-
vent medium on the complexation process can be
investigated.
In this paper, we report the cyclic voltammetry study of
copper complexes with 2,20-(1E,1E0)-(butane-2,3-diyl-
bis(azan-1-yl-1-ylidene))bis(methan-1-yl-1-ylidene)bis(4-
methylphenol) (L1), 2,20-(1E,1E0)-(1,3-phenylenebis(azan-
Among the strategies that can be applied for the study of
binding, the use of potentiometric and spectroscopic tech-
niques deserves special attention, due to the ability of
potentiometry to determine accurate values for the stability
constants and due to the large variety of structural informa-
tion that can be obtained from the different spectroscopic
methods analysis [28]. However, these techniques are lim-
ited by their low sensitivity, which makes their application
impossible when a large concentration of the ligand cannot
be reached (e.g. to prevent complex precipitation). In such
situations, voltammetric techniques can be a suitable alter-
native [29]. In this method the effect of the variables such as
ligand topology, nature and number of binding sites and
type of is studied for the stability complex formation.
Due to the low solubility of the selected Schiff base, we
1-yl-1-ylidene))bis(methan-1-yl-1-ylidene)diphenol
(L2),
2-(1-(2-hydroxyphenyl)ethylideneamino)-3-phenylpropa-
noicacid (L3), (z)-2-(1-(2-mercaptophenylimino)ethyl)phe-
nol (L4), tris(2-(hydroxyamino)ethyl)amine (L5), (z)-2-(1-
(2-methoxyphenylimino)ethyl)phenol (L6) and (N1E,
N2E)-N1,N2-bis(4-chlorobenzylidene)ethane-1,2-diamine (L7)
in binary acetonitrile–dimethylformamide mixtures (see
Fig. 1).
2. Experimental
2.1. Reagents
choose DMF, AN and their mixtures for study of Cu2+
–
Schiff base complexation. DMF and AN have about the
same dielectric constants (eDMF = 36.7; eAN = 37.5), but
The solvents, acetonitrile (AN) and dimethylformamide
(DMF) were refluxed for several days over the dehydration
H3C
N
CH3
N
H
H
H
H
N
N
H3C
OH
HO
CH3
OH
HO
L2
L1
H3C
N
H
N
HS
OH
O
HO
OH
L4
L3
H3C
N
N
HN
NH
OH
OH
HO
NH
MeO
CH3
L5
L6
N
N
Cl
Cl
L7
Fig. 1. Structures of Schiff bases.

78
M. Rahimi-Nasrabadi et al. / Journal of Molecular Structure 885 (2008) 76–81
agent P2O5, and then fractionally distilled. All of these sol-
vents were transferred into a dry box and dried over P2O5.
Reagent grade copper nitrate (from Merck) was used with-
out further purification except for drying over P2O5 in a
vacuum desiccator for 48 h. Tetrabutylammonium perchlo-
rate (TBAP) was prepared by dissolving an equimolar mix-
ture of the tetrabutylammonium chloride (Merck) and
perchloric acid (Merck) in water, and then filtering the
resulting mixture. The filtrate (TBAP) was purified by
recrystallization in distilled water several times and, was
then oven dried at 110 °C for 2 days. Ligands, L1, L2,
L3, L4, L6 and L7 were synthesized, purified and dried
as described elsewhere [31–36].
addition of additional diethyl ether (40 ml) and cooling in
an ice bath for 50 min, the yellow precipitate formed was
filtered off, washed with diethylether and dried in vacuo.
Yield: 80%. Found: C, 80.50%; H, 6.61%; N, 5.95%.
C48H48N3O3 requires: C, 80.64%; H, 6.77%; N, 5.87%;
1 H NMR (400 MHz, CDCl3, internal reference TMS): d
13.25 (br s, 3 H, OH), d 2.17 (m, 6H), 3.06 (t, 6H), 3.66
(t, 6H), 6.95 (15H, s, C6H5), 7.34 (6H, s, naphthyl), 8.43
(6H, s, naphthyl); IR (Nujol mull) cmꢀ1: 3000–2300 (b,
m
m
OAH), 1633.6, 1614.2 (sh), 1580, 1496.6 (s, mC@N and
C@C). Complete condensation of all primary amino groups
is confirmed by the lack of NAH stretching bands in the IR
3150–3450 cmꢀ1 region and the presence of strong C@N
stretching bands for ligand. This conclusion is also sup-
1
ported by the H NMR data which shows not only the
2.2. Synthesis of L5
absence of NAH hydrogen resonances but also the pres-
ence of C@N.
The heptadentate Schiff base ligands were easily pre-
pared by reaction of tris(3-aminopropyl) amine with 3
equivalent of 2-hydroxybenzophenone as shown in Scheme
2.3. Apparatus
1. Tris(3-(2-hydroxybenzophenone)propyl)amine: to
a
solution of 2-hydroxybenzophenone (2.06 g, 12 mmol) in
diethyl ether (12 ml) was added tris(3-aminopropyl)amine
(0.75 g, 4 mmol) in absolute ethanol (40 ml). After the
Cyclic voltammetric measurements were performed with
the aid of a setup, comprising a PC PIII Pentium 300 MHz
microcomputer equipped with a data acquisition board
(PCL-818PG, PC-Labcard Co.) and a custom made poten-
tiostat [37]. A Pt wire auxiliary electrode and a glassy-car-
bon working electrode polished with alumina and
sonicated in deionised water after measurements were used.
A double junction silver–silver chloride reference electrode
was placed in 0.05 M TBAP (with same solvent) and con-
nected to the electrolyzed solution by mean of a bridge con-
N
N
N
OH
HO
N
taining the base electrolyte.
A solution of 0.05 M
tetrabutylammonium perchlorate (TBAP) was used as a
base electrolyte. All solutions were deaerated for 10 min
with pure nitrogen and an inert atmosphere was main-
tained over the solutions during the reduction. All experi-
HO
Scheme 1.
3
2
1
0
-1
-2
-3
-4
4
3 2
1
-400
-200
0
200
400
600
800
1000
1200
1400
mV
Fig. 2. CV polarograms of 5 ꢁ 10-4 M Cu2+ ion in DMF/AN (40% DMF + 60% AN) binary system with different concentrations of L3 ligand: (1) 0.0; (2)
0.3; (3) 1.0; (4) 8 mM.

M. Rahimi-Nasrabadi et al. / Journal of Molecular Structure 885 (2008) 76–81
79
ments were carried out at 25 0.1 °C using a model FK2
9
8
7
6
5
4
3
2
1
0
Haake thermostat with water bath. Concentration of
Cu(II) was 5 ꢁ 10ꢀ4 M and the volume of the sample was
5.0 ml. Redox potentials shown in the paper were obtained
at 5 mV/s of scan rate. All potentials were referenced inter-
nally against the Fe/Fe+ couple.
3. Results and discussion
In cyclic voltammetry investigation, the significant
quantity for studying the stability of the metal–ion com-
plexes is the half-wave potential (E1/2) of the cyclic voltam-
mogram peak potential of the complex and the free metal
ion. The difference in E1/2 between the complex and the
metal ion is used in the calculation of the stability constant
of the complexes. The interaction between L1–L7 with
Cu2+ was studied in acetonitrile/dimethylformamide bin-
-3.2
-3
-2.8
-2.6
-2.4
-2.2
-2
Log [L7]
40AN
0AN
80AN
20AN
60AN
100AN
14
13
12
11
10
9
Fig. 5. Linear plots of DE1/2/(RT/nF) versus log[L7] for the L7–Cu2+
complex in AN and DMF and their binary mixtures.
ary mixtures of various compositions at 25 °C. The addi-
tion of the ligands to Cu2+ solutions in 0.05 M TBAP,
shifts the reduction potential E1/2 of the metal ions towards
a more negative value. As an example, the cyclic voltam-
mograms of the Cu2+ ion in the presence of different con-
centrations of L3 in one of the AN/DMF binary systems
is shown in Fig. 2. The shift in half-wave potential towards
more negative values upon the addition of excess ligand
was observed to be in accordance with the simple Lingane
equation:
8
7
6
5
4
-3.2
-3
-2.8
-2.6
-2.4
-2.2
-2
Log [L1]
Fig. 3. Linear plots of DE1/2/(RT/nF) versus log[L1] for the L1–Cu2+
9
8
7
6
5
4
3
2
complex in AN and DMF and their binary mixtures.
13
11
9
7
5
3
0
20
40
60
80
100
120
%AN
1
L3
L4
L7
L1
L2
L5
L6
-3.2
-3
-2.8
-2.6
-2.4
-2.2
-2
Log [L3]
Fig. 4. Linear plots of DE1/2/(RT/nF) versus log[L3] for the L3–Cu2+
complex in AN and DMF and their binary mixtures.
Fig. 6. Variation of log Kf of different Cu2+–Schiff bases complexes with
XAN
.

80
M. Rahimi-Nasrabadi et al. / Journal of Molecular Structure 885 (2008) 76–81
Table 1
Stability constant of Cu2+ complexes with different Schiff bases in AN–DMF mixtures at 25 °C
Composition wt% of DMF in AN
log Kf SD
L1
L2
7.2 0.04
6.67 0.05
6.08 0.09
5.69 0.06
5.31 0.03
4.86 0.03
L3
L4
L5
L6
L7
0
20
40
60
80
7.94 0.05
7.38 0.04
6.96 0.07
6.39 0.03
5.95 0.08
5.55 0.04
7.14 0.05
6.6 0.04
5.92 0.07
5.28 0.07
4.8 0.1
6.87 0.02
6.13 0.02
5.58 0.04
5.14 0.05
4.68 0.03
4.12 0.03
6.21 0.05
5.52 0.05
5.12 0.06
4.78 0.07
4.21 0.09
3.76 0.04
5.61 0.1
5.15 0.07
4.78 0.05
4.25 0.06
3.87 0.03
3.52 0.04
5.61 0.05
5.12 0.05
4.57 0.08
4.12 0.02
3.68 0.09
3.2 0.05
100
4.1 0.08
form the mixtures. In another word, this behavior probably
indicates that the donocity of each solvent will not change
in the presence of another solvent (Fig. 6).
DE1=2 ¼ ðE1=2ÞML ꢀ ðE1=2ÞM ¼ ꢀðRT =nFÞðLnKf þ m ln½LꢂtÞ
ð1Þ
From Table 1 it can be seen that the stability of the
resulting 1:1 complexes of Cu2+ ion with different Schiff
base decreases in the order of L1 > L2 > L3 >
L4 > L5 > L6 > L7.
where (E1/2 ML and (E1/2)M, are the half-wave potentials of
)
the complexed and free metal ion, n is the number of elec-
trons involved in the reaction, Kf is the formation constant
of the complexes, m is the stoichiometry of the complex
and [L]t, is the analytical concentration of the ligand. A lin-
ear graph of DE1/2 versus ln[L]t will have a slope of (ꢀRT/
nF)m and an intercept of ꢀRT/nF ln Kf. The m and Kf,
therefore, can be obtained from the slope and intercept
of the linear plot (Figs. 3–5).
The effect of the ligands structure on the complexes stabil-
ity. Some of the factors, such as the size of the semi-cavity
of the ligand, the number and the nature (based on the
soft and hard theory) of the donor atoms in the ligand,
are important in the stability of the formed complexes.
Among them, however, the most significant factor is the
number of the ligand donor atoms. Therefore, the L1
and L2 ligands are expected to form the most stable com-
plexes (Table 1) with Cu2+ among all the examined
ligands. Owing to the benzene electron withdrawing agent,
the L2 donor ability was lower than that of L1. Therefore,
the L2 complexes were weaker than the L1 complexes.
The same phenomenon was observed for the L3 and L4
ligands. The previous ligands formed a semi-cavity,
enhancing the formation power of the complexes. Never-
theless, L5 could not form such a cavity and the L5 com-
plexes were weaker than the L1–L4 complexes. The
number of the donor atoms in L6 and L7 was smaller
compared with the number of the other ligands. As a con-
sequence, these ligands formed weaker complexes than the
complexes of the other ligands.
The variations of DE1/2 as a function of log[L]t for com-
plex formation between L1, L3, L7 and Cu2+ ions in aceto-
nitrile/dimethylformamide binary systems are shown in
Figs. 3–5. Similar linear plot was obtained for other sys-
tems which is an indication of the formation of a single
complex in solution. The slope of these linear plots gave
a value of m ꢃ 1, which confirms the formation of a 1:l
complex in solution. The formation constants were
obtained by fitting the voltammetric data to Eq. (1) by
computer. The results are given in Table 1. The data col-
lected in this table show that the values of the stability con-
stant of all of the complexes increase as the concentration
of dimethylformamide is lowered in acetonitrile/dimethyl-
formamide binary systems. DMF is a solvent of high sol-
vating ability (DN = 26.6) [38], which can compete
strongly with the ligands for Cu2+ ion. Thus, it is not sur-
prising that the addition of DMF to the AN as a low donic-
ity solvent (DN = 14.1) [36] will decrease the extent of
interaction between the ligand donating atoms and Cu2+
ion. It is immediately obvious that the nature of the med-
ium plays an important role in the complexation process,
and it seems that the donor ability of the solvents as
expressed by the Gutmann donor numbers [39] plays a very
important role in determining the stabilities of these com-
plexes. The variations of the formation constants of
ligands–Cu2+ as a function of solvent composition in ace-
tonitrile/dimethylformamide solutions are shown in
Fig. 6. Investigation of these figures shows that the com-
plexation process in these mixed non-aqueous solvents is
sensitive to the solvent composition. In all cases, the varia-
tion of the stability constant with composition of the sol-
vents is monotonic and shows a good correlation with
the inherent solvating ability of the neat solvents which
References
[1] S.M.E. Khalil, Chem. Pap. 54 (2000) 12.
[2] A.H. Osman, Transition Met. Chem. 31 (2006) 35.
[3] S.A. Sallam, Transition Met. Chem. 31 (2006) 46.
´
[4] M. Cindric, N. Strukan, V. Vrdoljak, T. Kajfezˇ, B. Kamenar,
Croatica Chim. Acta 76 (2003) 157.
[5] C. Sousa, C. Freire, B. de Castro, Molecules 8 (2003) 894.
[6] M.H. Habibi, M. Montazerozohori, A. Lalegani, R.W. Harrington,
W. Clegg, J. Fluorine Chem. 127 (2006) 769–773.
[7] T. Poursaberi, L. Hajiagha-Babaei, M. Yousefi, S. Rouhani, M.
Shamsipur, M. Kargar-Razi, A. Moghimi, H. Aghabozorg H, M.R.
Ganjali, Electroanalysis 13 (2000) 1513.
[8] M.R. Ganjali, T. Poursaberi, L. Hajiagha-Babaei, S. Rouhani, M.
Yousefi, M. Kargar-Razi, A. Moghimi, H. Aghabozorg, M. Shamsi-
pur, Anal. Chim. Acta 440 (2001) 81.
[9] H.A. Zamani, Gh. Rajabzadeh, M.R. Ganjali, S. Mola Khatami,
Electroanalysis 17 (2005) 2260.
[10] M.R. Ganjali, M. Rezapour, P. Norouzi, M. Salavati-Niasari,
Electroanalysis 17 (2005) 2032.

M. Rahimi-Nasrabadi et al. / Journal of Molecular Structure 885 (2008) 76–81
81
[11] M.B. Gholivand, M. Rahimi-Nasrabadi, M.R. Ganjali, M. Salavati-
Niasari, Talanta 73 (2007) 553.
[26] G. Majella, S. Vivienne, M. Malachy, D. Michael, M. Vickie,
Polyhedron 18 (1999) 2931.
[12] M.R. Ganjali, M.B. Gholivand, M. Rahimi-Nasrabadi, B. Maddah,
M. Salavati-Niasari, F. Ahmadi, Sens. Lett. 4 (2006) 1.
[13] M.B. Gholivand, F. Ahmadi, E. Rafiee, Electroanalysis 18 (2006)
1620.
[27] D.K. Saha, U. Sandbhor, K. Shirisha, S. Paddye, D. Deobagkar, C.E.
Ansond, A.K. Powell, Bioorg. Med. Chem. Lett. 14 (2004) 3027.
[28] M.A. Zoroddu, S. Zanetti, R. Pogni, R. Basosi, J. Inorg. Biochem. 63
(1996) 291.
[14] M.R. Ganjali, A. Ghesmi, M. Hosseini, M.R. Pourjavid, M.
Rezapour, Sens. Actuators B: Chem. 105 (2005) 334.
[15] D. Chen, A.E. Martel, Inorg. Chem. 26 (1987) 1026.
[16] J. Costamagna, J. Vargas, R. Latorre, A. Alvarado, G. Mena, Coord.
Chem. Rev. 119 (1992) 67.
[17] S.S. Djebbar, B.O. Benali, J.P. Deloume, Transition Met. Chem. 23
(1998) 443.
[18] Y.J. Hamada, IEEE Trans. Electron Devices 44 (1997) 1208.
[19] R. Ramesh, M. Sivagamasundari, Synth. React. Inorg. Met.-Org.
Chem. 33 (2003) 899.
[29] B. Maddah, K. Alizadeh, M. Shamsipur, A. Moghimi, M.R. Ganjali,
Pol. J. Chem. 80 (2006) 737.
[30] Vogel’s, Textbook of Practical Organic Chemistry, fourth ed.,
Longman, 1032 (1978).
[31] M.R. Ganjali, M. Golmohammadi, M. Yousefi, P. Norouzi, M.
Salavati-Niasari, M. Javanbakht, Anal. Sci. 19 (2003) 223.
[32] M.R. Ganjali, M. Yousefi, T. Poursaberi, L. Naji, M. Salavati-
Niasari, M. Shamsipur, Electroanalysis 15 (2003) 1476.
[33] M. Salavati-Niasari, H. Banitaba, J. Mol. Catal. A: Chem. 201 (2003)
43.
[20] J.C. Wu, N. Tang, W.S. Liu, M.Y. Tan, A.S.C. Chan, Chin. Chem.
Lett. 2 (2001) 757.
[34] M. Salavati-Niasari, P. Salemi, F. Davar, J. Mol. Catal. A: Chem. 238
(2005) 215.
[21] D. Chatterjee, A. Mitra, J. Coord. Chem. 57 (2004) 175.
[22] G. Minu, H. Bhowon, A. Li Kam Wah, M.O. Dosieah Ridana, D.
Ramalingum Lacour, Synth. React. Inorg. Metal-Org. Chem. 34
(2004) 1.
[35] M. Salavati-Niasari, S.N. Mirsattari, J. Mol. Catal. A: Chem. 268
(2007) 50.
[36] M. Salavati-Niasari, M. Shaterian, M.R. Ganjali, P. Norouzi, J. Mol.
Catal. A: Chem. 261 (2007) 147.
[23] C.M. Liu, R.G. Xiong, X.Z. You, Y.J. Liu, K.K. Cheung, Polyhe-
dron 15 (1996) 4565.
[37] A.S. Baranski, T. Krogulek, J. Nelsson, P. Norouzi, Proc. Electro-
chem. Soc. 9 (1996) 41.
[24] S.A. Abdel-Latif, H.B. Hassib, Y.M. Issa, Spectrochim. Acta Part A
67 (2007) 950.
[38] V. Gutmann, The Donor–Acceptor Concept of Molecular Interac-
tion, Plenum, NY, 1978.
[25] J.D. Ranford, P.J. Sadler, J. Chem. Soc. Dalton Trans. (1993)
3393.
[39] V. Gutmann, Coordination Chemistry in Nonaqueous Solutions,
Springer-Verlag, New York, 1960.