Synthesis, crystal structure, and electrochemical properties of Ni(II) and Cu(II) complexes with two unsymmetrical N2O2 Schiff base ligands

Article abstract of DOI:10.1007/s13738-013-0365-7

Nickel(II) and copper(II) complexes of two unsymmetrical tetradentate Schiff base ligands [Ni(Me-salabza)] (1), [Cu(Me-salabza)] (2) and [Ni(salabza)] (3), {H₂salabza = N,N′-bis[(salicylidene)-2-aminobenzylamine] and H₂Me-salabza = N

Full text of DOI:10.1007/s13738-013-0365-7

J IRAN CHEM SOC (2014) 11:985–991
DOI 10.1007/s13738-013-0365-7
ORIGINAL PAPER
Synthesis, crystal structure, and electrochemical properties
of Ni(II) and Cu(II) complexes with two unsymmetrical N2O2
Schiff base ligands
•
Soraia Meghdadi Mehdi Amirnasr
•
•
Maryam Bagheri Fatemeh Ahmadi Najafabadi
•
•
Kurt Mereiter Kurt Joß Schenk Fahimeh Ziaee
•
Received: 27 July 2013 / Accepted: 2 October 2013 / Published online: 18 October 2013
Ó Iranian Chemical Society 2013
Abstract Nickel(II) and copper(II) complexes of two
unsymmetrical tetradentate Schiff base ligands [Ni(Me-sal-
abza)] (1), [Cu(Me-salabza)] (2) and [Ni(salabza)] (3),
{H2salabza = N,N0-bis[(salicylidene)-2-aminobenzylamine]
and H2Me-salabza = N,N0-bis[(methylsalicylidene)-2-ami-
nobenzylamine]}, have been synthesized and characterized
by elemental analysis and spectroscopic methods. The
crystal structures of 2 and 3 complexes have been determined
by single crystal X-ray diffraction. Both copper(II) and
nickel(II) ions adopt a distorted square planar geometry in
[Cu(Me-salabza)] and [Ni(salabza)] complexes. The cyclic
voltammetric studies of these complexes in dichloromethane
indicate the electronic effects of the methyl groups on redox
potential.
Introduction
Schiff base complexes have remained an important area of
research due to their simple synthesis, versatility, and
diverse range of applications such as antimicrobial [1–3],
anticancer [4], diuretic [5], antitumor, and biological
activities [6–10].
Many symmetrical tetradentate Schiff bases of 1,2-dia-
mines with o-hydroxyaldehydes/ketones have been pre-
pared and studied intensively. However, much less
attention has been focused on unsymmetrical tetradentate,
but in recent years, there is a growing interest in the
coordination chemistry of transition metal complexes of
unsymmetrical Schiff base ligands, due to the fact that the
ligands around central metal ions in natural systems are
unsymmetrical [11–13]. The unsymmetrical Schiff bases
are obtained by the mono-condensation of diamines with
aldehydes/ketones, or by stepwise condensation of the
appropriate diamine with two different carbonyl com-
pounds [11, 13–16]. Corresponding complexes may serve
as biological models [11, 17–20] or as catalysts in some
chemical processes [21–23] and promising materials for
optoelectronic applications [24–27] and the design of bio-
sensors [28].
Keywords Unsymmetrical N2O2 Schiff base ꢀ
Ni(II) and Cu(II) complexes ꢀ Crystal structure ꢀ
Cyclic voltammetry
Electronic supplementary material The online version of this
article (doi:10.1007/s13738-013-0365-7) contains supplementary
material, which is available to authorized users.
S. Meghdadi ꢀ M. Amirnasr (&) ꢀ M. Bagheri ꢀ
F. A. Najafabadi ꢀ F. Ziaee
Recently, the ease of preparation of metal salen-based
modified electrodes by oxidative electro-polymerisation
of metal complexes in poor coordinating solvents has
prompted their use in heterogeneous electrocatalysis
[29]. The role played by structural electronic effects to
control the redox chemistry of these metal complexes
may prove to be critical in the design of new catalysts
[30]. The electronic properties and steric demands of
these Schiff base ligands can be tuned selectively via
appropriate tailoring and modification of the ligand
structure.
Department of Chemistry, Isfahan University of Technology,
84156-83111 Isfahan, Iran
e-mail: amirnasr@cc.iut.ac.ir
K. Mereiter
Faculty of Chemistry, Vienna University of Technology,
Getreidemarkt 9/164SC, 1060 Vienna, Austria
K. J. Schenk
´
´ ´
Laboratoire de Cristallographie, Ecole Polytechnique Federale
de Lausanne, Le Cubotron-521, Dorigny, 1015 Lausanne,
Switzerland
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J IRAN CHEM SOC (2014) 11:985–991
potentials were calibrated versus internal Fc?/0
(E0 = 0.46 V versus SCE) couple under the same condi-
tions [33].
Synthesis
Synthesis of H2Me-salabza ligand
The new ligand H2Me-salabza was synthesized by adding a
solution of 2-hydroxy acetophenone (0.272 g, 2 mmol) in
ethanol (5 ml) to a stirring solution of 2-aminobenzylamine
(0.122 g, 1 mmol) in ethanol (5 ml). The mixture was
stirred for 1 h at room temperature to give a yellow pre-
cipitate. The product was filtered off and washed with cold
ethanol. Yield 86 %. Anal. Calc. for C23H22N2O2: C,
77.07; H, 6.19; N, 7.82; found: C, 76.84; H; 6.06, N; 7.9 %.
FT-IR (KBr, cm-1) mmax: O–H (3830–3109), C = N (1607,
1571). UV–Vis: kmax (nm) (e, L mol-1 cm-1) (CH2Cl2):
Scheme 1 The chemical formula of the Schiff base ligands
In continuation of our work on the synthesis of the
unsymmetrical Schiff base metal complexes [31, 32], we
report the synthesis, spectral and electrochemical proper-
ties of Cu(II) and Ni(II) complexes of the unsymmetrical
tetradentate Schiff base ligands H2Me-salabza and H2sal-
abza (Scheme 1). The electronic effects of methyl groups
on the redox potentials and the X-ray crystal structures of
[Cu(Me-salabza)] complex (2) and [Ni(salabza)] (3) are
also reported and discussed.
1
229 (101890), 257 (7150), 324 (330). H-NMR (CDCl3,
400 MHz): 2.28 (s, 3H, Hc), 2.32 (s, 3H, Hd), 4.71 (s, 2H,
He), 7.57–6.65 (m, 12H, Haromatic), 14.50 (s, 1H, Henolic),
16.14 (s, 1H, Henolic).
Synthesis of H2salabza ligand
The H2salabza ligand was synthesized by a procedure
reported in the literature [34]. The crystalline product
obtained was characterized by elemental analysis, 1H-
NMR, and spectroscopic methods, and the results are in
good agreement with the literature values.
Experimental setup
Materials and general methods
Synthesis of [Ni(Me-salabza)] complex (1)
All solvents and chemicals were of commercial reagent
grade and used as received from Aldrich and Merck.
Infrared spectra from KBr pellets were collected on a FT-
IR JASCO 680 plus spectrometer in the range
4,000–400 cm-1. UV–Vis absorption spectra were recor-
ded on a JASCO V-570 spectrophotometer. 1H-NMR
spectra were measured with a BRUKER AVANCEDEX
(400 MHz) spectrometer. Proton chemical shifts are
reported in ppm relative to Me4Si as internal standard.
Elemental analyses were performed using a Perkin–Elmer
2400 II CHNS–O elemental analyzer. Electrochemical
properties of these complexes were studied by cyclic vol-
tammetry. Cyclic voltammograms were recorded using a
SAMA 500 Research Analyzer using a three-electrode
system, a glassy carbon working electrode (Metrohm
6.1204.110 with 2.0 0.1 mm diameter), a platinum disk
auxiliary electrode and Ag wire as reference electrode. CV
measurements were performed in dichloromethane with
tetrabutylammonium hexafluorophosphate (TBAH) as
supporting electrolyte. The solutions were deoxygenated
by purging with Ar for 5 min. All electrochemical
To
a
solution of Ni(CH3COO)2ꢀ4H2O (0.0248 g,
0.1 mmol) in ethanol (10 mL), a solution of H2Me-salabza
(0.0353 g, 0.1 mmol) in dichloromethane (15 ml) was
added, and the mixture was stirred for 4 h at room tem-
perature and filtered. Brown crystals were obtained by slow
evaporation of solvents. Yield 78 %. Anal. Calc. for
C23H20N2NiO2: C, 66.55; H, 4.86; N, 6.75; Found: C,
66.19; H, 4.74; N, 6.69 %. FT-IR (KBr, cm-1) mmax: 1597,
1568 (C = N). UV–Vis: kmax (nm) (e, L mol-1 cm-1
(CH2Cl2): 237 (70050), 260 (72890), 350 (123230), 438
)
(8520), 600 (265).
Synthesis of [Cu(Me-salabza)] complex (2)
This complex was prepared by the same procedure used for
1 except that Cu(CH3COO)2ꢀ H2O (0.0199 g, 0.1 mmol)
was used as the metal salt. Brown crystals suitable for
X-ray diffraction were obtained by slow evaporation of
solvents. Yield 80 %. Anal. Calc. for C23H20N2CuO2: C,
65.78; H, 4.80; N, 6.67; found: C, 65.88; H, 4.65; N,
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J IRAN CHEM SOC (2014) 11:985–991
987
6.67 %. FT-IR (KBr, cm-1) mmax: 1598, 1567 (C = N).
UV–Vis: kmax (nm) (e, L mol-1 cm-1) (CH2Cl2): 229
(61010), 262 (29400), 344 (9430), 572 (290).
Table 1 Crystal data and structure refinement for (2) and (3)
Compound
(2)
(3)
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
C23H20CuN2O2
419.95
C21H16NiN2O2
387.07
Synthesis of [Ni(salabza)] complex (3)
294(2)
100(2)
Monoclinic
P21/n
Monoclinic
P21/c
The complex 3 was prepared by a procedure similar to that
of complex 1 except that H2salabza (0.033 g, 0.1 mmol)
was used instead of H2Me-salabza ligand. Dark green
crystals suitable for X-ray diffraction were obtained by
slow evaporation of solvents. Yield 86 %. Anal. Calc. for
NiC21H16N2O: C, 65.16; H, 4.17; N, 7.24. found. C, 65.31;
H, 4.32; N, 6.92 %. FT-IR (KBr, cm-1), mmax: 1609, 1579
(C = N). UV–Vis: kmax (nm) (e, L mol-1 cm-1) (CH2Cl2):
268 (52910), 310 (16360), 342 (119120), 444 (75030), 614
(160).
˚
a (A)
15.526(3)
8.0264(16)
16.517(3)
90.00
11.4528(13)
8.6351(10)
16.8094(19)
90.00
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
115.12(3)
90.00
99.6860(10)
90.00
3
˚
V (A )
1863.5(6)
4
1638.69
4
Z
Dcalc (Mg/m3)
1.497
1.569
l (mm-1
)
1.19
1.202
Crystal structure determination and refinement
Crystal size (mm)
F (000)
0.52 9 0.14 9 0.10
868
0.45 9 0.35 9 0.20
800
X-ray crystallography for 2 and 3
h range (°)
Index ranges
2.7–27.8
2.66–30.00
Suitable single crystals of 2 and 3 were obtained by slow
evaporation of a methanol–dichloromethane (1:1 v/v)
solution at room temperature.
-21 B h B 21,
-9 B k B 10,
-22 B l B 22
-14 B h B 16,
-12 B k B 12,
-23 B l B 23
Absorption
correction
Integration
Multi-scan
X-ray data of 2 were collected at T = 294 K on a STOE
IPDS II diffractometer with graphite monochromatic Mo
Maximum and
minimum
transmission
0.6786 and 0.9083
0.78 and 0.65
˚
Ka (k = 0.71073 A) radiation. Cell refinement and data
reduction were performed with the help of the X-Area
(1.36) program [35], and a numerical absorption correction
was performed with the program XRED32 (1.31) [36].
X-ray data of 3 were collected at T = 100 K, on a
Bruker APEX-II CCD diffractometer with graphite
Reflections
collected
13168/4936
25804/4759
Rint
0.0606
0.0211
Data/restraints/
parameters
4936/0/254
4759/0/235
˚
monochromatic Mo Ka (k = 0.71073 A) radiation. Cell
refinement and data reduction were performed with the
program SAINT [37]. Correction for absorption was car-
ried out with the multi-scan method and program SADABS
[37]. The structures of 2 and 3 were solved with direct
methods using the program SHELXS97 [38] and structure
refinement on F2 was carried out with the program
SHELXL-97. The crystallographic and refinement data of
these complexes are summarized in Table 1.
Goodness of fit on
F2
0.981
1.035
Final R indices
[I [ 2r(I)]
R1 = 0.055,
wR2 = 0.152
R1 = 0.0222,
wR2 = 0.0598
Largest difference
peak and hole
0.369 and -0.611
0.445 and -0.458
-3
˚
(e.A
)
P
P
P
P
[*]R1 = ||Fo|-|Fc||/ |Fo|, wR2 = { [w(F2o-Fc2)2]/ [w(Fo2)2]}1/2
complexes suitable for X-ray crystallography were
obtained in good yield (about 80 %).
Results and discussion
Description of structures
Synthesis and characterization
Crystal structure of [Cu(Me-salabza)] (2)
The tetradentate Schiff base ligands, H2salabza or H2Me-
salabza, have been derived from condensation of 2-ami-
nobenzylamine with aldehyde/ketone. All the complexes
(Scheme 2) have been prepared by the direct reaction of
the corresponding ligand, H2salabza or H2Me-salabza with
metal salt at room temperature. Crystals of these
The crystal structure of [Cu(Me-salabza)] complex is
shown in Fig. 1. The crystallographic and refinement data
for complex 2 are summarized in Table 1, and selected
bond distances and angles are listed in Table 2.
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J IRAN CHEM SOC (2014) 11:985–991
Scheme 2 The chemical formula of 1, 2, and 3 complexes
Fig. 2 The molecular structure of [Ni(salabza)] with its atom
labeling scheme
Complex 2 crystallizes in monoclinic space group
P 21/n in this complex, the Schiff base ligand acts as tet-
radentate chelating ligand coordinated to Cu(II) center. The
central Cu(II) ion has the distorted square planar geometry.
However, the whole molecule is not planar since the two
halves are folded with an angle of 18.08°. The average Cu–
˚
˚
O bond length of 1.882 A and Cu–N distance of 1.975 A
agree well with the corresponding distances in related
copper(II) complexes [13].
Crystal structure of [Ni(salabza)] (3)
The crystal structure of [Ni(salabza)] complex is shown in
Fig. 2. The crystallographic and refinement data for com-
plex 3 are summarized in Table 1, and selected bond dis-
tances and angles are listed in Table 2.
Fig. 1 The molecular structure of [Cu(Me-salabza)] with its atom
labeling scheme
Complex 3 crystallizes in monoclinic space group
P21/c in this complex, the Schiff base ligand acts as tet-
radentate chelating ligand coordinated to Ni(II) center. The
angle O–Ni–N = 164.35(4)° indicates that the coordina-
tion geometry of the nickel atom is distorted square planar.
The two halves of this complex are folded with respect to
each other with an angle of 21.18°. The average Ni–O and
˚
Table 2 Selected bond lengths (A) and angles (°) for (2) and (3)
Complex
(2)
(3)
Bond lengths
Cu1–O1
1.908(3)
1.857(3)
1.975(3)
1.976(3)
Ni1–O1
Ni1–O2
Ni1–N1
Ni1–N2
1.8435(8)
1.8563(8)
1.8800(9)
1.8919(9)
˚
Cu1–O2
Ni–N bond lengths are 1.845(2) and 1.885(9) A, in
Cu1–N1
agreement with the corresponding distances reported for
the related Ni(II) complexes [39].
Cu1–N2
Bond angles
O2–Cu1–O1
O2–Cu1–N1
O1–Cu1–N1
O2–Cu1–N2
O1–Cu1–N2
N1–Cu1–N2
Spectral characterization
87.85(13)
168.51(13)
87.46(13)
92.89(12)
164.76(15)
94.39(12)
O1–Ni1–O2
O1–Ni1–N1
O2–Ni1–N1
O1–Ni1–N2
O2–Ni1–N2
N1–Ni1–N2
82.53(3)
94.39(4)
164.35(4)
164.36(4)
93.16(4)
93.63(4)
The prepared metal complexes are all air-stable solids and
have good elemental analyses. The FT-IR data of the
ligands and their complexes are listed in ‘‘Experimental
setup’’. The FT-IR spectra of the free ligands exhibit the
characteristic bands of two different imine (C=N) groups in
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J IRAN CHEM SOC (2014) 11:985–991
989
the unsymmetrical Schiff base, which appear at 1,607 and
1,571 cm-1 for H2Me-salabza and at 1,633 and
1,614 cm-1 for H2salabza. Similar features are observed in
the IR spectra of the 1–3 complexes, with the C=N
stretching vibrations appearing in the 1,567–1,598 cm-1
region. The m(C=N) bands are shifted to lower wave
numbers with respect to the free ligand, indicating a
decrease in the C=N bonds order due to the coordination of
the imine nitrogen to the metal and back bonding from the
metal to the p* orbital of the azomethine groups.
Figs. 3, 4 respectively. The irreversible oxidation process
observed at 1.03 and 1.285 V is attributed to the oxidation
of the NiII/NiIII center in the [Ni(Me-salabza)] and
[Ni(salabza)] complexes, respectively. The oxidation
potential in [Ni(Me-salabza)] is less positive than that of
the [Ni(salabza)].
The cyclic voltammogram of the [Cu(Me-salabza)]
complex in CH2Cl2 solution is shown in Fig. 5. The quasi-
reversible reduction process observed at -1.20 V is
attributed to the reduction of the CuII/CuI center. The
reduction potential for CuII/CuI in [Cu(salabza)] is
-1.014 V [33], which is less negative than that of the
[Cu(Me-salabza)].
The UV–visible spectra of the ligands and their com-
plexes were recorded in dichloromethane solution in the
200–800 nm region. The UV–Vis data of compounds are
presented in ‘‘Experimental setup’’. The absorption spectra
of H2Me-salabza and H2salabza ligands consist of an intense
band centered at 324 nm, attributing to n ? p* transitions.
Further, two intense bands at 257 and 229 nm for H2Me-
salabza and at 261 and 231 nm for H2salabza were related to
p ? p* transitions of aromatic rings and the azomethine
groups. These transitions are also found in the spectra of the
complexes, but they shifted toward lower frequencies, con-
firming the coordination of the ligand to the metal ion.
The UV–visible spectra of the Ni(II) complexes show
The variation in the redox potentials of Me-salabza
complexes relative to those of salabza analogs correlates
1
1
two absorption bands corresponding to A1g ? B1g and
1
1A1g ? A2g transitions and appear at 600 and 438 nm for
[Ni(Me-salabza)], and 614 and 448 nm for [Ni(salabza)].
The blue shift in the position of the ligand field transitions
in [Ni(Me-salabza)] relative to the [Ni(salabza)] is due to
the higher ligand field strength of Me-salabza as compared
to that of salabza, originating from the inductive electron-
donating properties of methyl substituents. The copper(II)
complex shows two bands at 572 and 344 nm, assignable
2
2
2
2
to B1g ? A1g and B1g ? B2g transitions [40]. These
data are indicative of an approximate square planar
geometry of the ligand around the Ni(II) and Cu(II) ions.
The 1H NMR spectral measurements were performed in
CDCl3 solution for H2Me-salabza and H2salabza ligands
and the corresponding data are given in ‘‘Experimental
setup’’. The chemical formula of ligands is presented in
Scheme 1. The main features of the 1H NMR spectra of the
unsymmetrical Schiff base ligands are the signals due to
two different CHimine groups in H2salabza appearing at
8.47 and 8.58 ppm and the signals due to two different CH3
substituents in H2Me-salabza appearing at 2.28 and
2.32 ppm. The enolic signals appear at 14.50 and
16.14 ppm for H2Me-salabza and at 13.13 and 13.35 ppm
for H2salabza. The aromatic protons of these compounds
appear in the appropriate region (5.8–7.8 ppm).
Fig. 3 Cyclic voltammogram of the [Ni(Me-salabza)] in CH2Cl2 at
298 K, c &4 9 10-3, scan rate = 100 mV s-1
Electrochemical studies
The cyclic voltammograms of the [Ni(Me-salabza)] and
[Ni(salabza)] complexes in CH2Cl2 solution are shown in
Fig. 4 Cyclic voltammogram of the [Ni(salabza)] in CH2Cl2 at
298 K, c &4 9 10-3, scan rate = 100 mV s-1
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J IRAN CHEM SOC (2014) 11:985–991
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In this work, we have reported the synthesis of new Ni(II)
and Cu(II) complexes with two unsymmetric Schiff base
ligands, H2Me-salabza and H2salabza. The crystal struc-
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have been determined by single crystal X-ray diffraction.
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Acknowledgments Partial support of this work by the Isfahan Uni-
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