Synthesis and characterization of a series of copper(II) complexes with azo-linked salicylaldimine Schiff base ligands. Crystal structure of Cu5PHAZOSALTN·CHCl3

Article abstract of DOI:10.1016/S0277-5387(99)00380-0

Complexes of copper (II) with Schiff bases obtained by condensation of 5-phenylazo salicylaldehyde with di-or tri-amines have been synthesized and characterized by their infrared spectra and elemental analysis. The copper(II) chelates investigated are [bis(5-phenylazo salicylaldehyde)ethylene diiminato]copper(II) (Cu5PHAZOSALEN) (I), [bis(5-phenylazo salicylaldehyde)O-phenylene diiminato]-copper(II) (Cu5PHAZOSALOPHEN) (II), [bis(5-phenylazo salicylaldehyde)trimethylen diiminato]copper(II) (Cu5PHAZOSALTN) (III) and [bis(5-phenylazo salicylaldehyde)diethylene triiminato]copper(II) (Cu5PHAZOSALDETA) (IV). The single-crystal X-ray diffraction is reported for Cu5PHAZOSALTN·CHCl₃. The copper atom lies in a near square-planar coordination with Cu-N bond lengths of 1.962(7) and 1.949(7) A? and Cu-O lengths of 1.903(5) and 1.931(6) A?. Cyclic voltammetry indicates that copper complexes III and IV have a quasi-reversible redox behaviour and complexes I and II undergo irreversible and reversible reduction (at a scan rate 0.05 V s^-1), respectively, under the experimental conditions. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0277-5387(99)00380-0

www.elsevier.nl/locate/poly
Polyhedron 19 (2000) 607–613
Synthesis and characterization of a series of copper(II) complexes with
azo-linked salicylaldimine Schiff base ligands.
Crystal structure of Cu5PHAZOSALTNPCHCl3
*
A.A. Khandar , K. Nejati
Department of Inorganic Chemistry, Faculty of Chemistry, Tabriz University, Tabriz, Iran
Received 5 August 1999; accepted 6 December 1999
Abstract
Complexes of copper (II) with Schiff bases obtained by condensation of 5-phenylazo salicylaldehyde with di-or tri-amines have been
synthesized and characterized by their infrared spectra and elemental analysis. The copper(II) chelates investigated are [bis(5-phenylazo
salicylaldehyde)ethylene diiminato]copper(II) (Cu5PHAZOSALEN) (I), [bis(5-phenylazo salicylaldehyde)O-phenylene diiminato]-
copper(II) (Cu5PHAZOSALOPHEN) (II), [bis(5-phenylazo salicylaldehyde)trimethylen diiminato]copper(II) (Cu5PHAZOSALTN)
(III) and [bis(5-phenylazo salicylaldehyde)diethylene triiminato]copper(II) (Cu5PHAZOSALDETA) (IV). The single-crystal X-ray
diffraction is reported for Cu5PHAZOSALTNPCHCl3. The copper atom lies in a near square-planar coordination with Cu–N bond lengths of
˚
˚
1.962(7) and 1.949(7) A and Cu–O lengths of 1.903(5) and 1.931(6) A. Cyclic voltammetry indicates that copper complexes III and IV
have a quasi-reversible redox behaviour and complexes I and II undergo irreversible and reversible reduction (at a scan rate 0.05 V sy1),
respectively, under the experimental conditions.
q2000 Elsevier Science Ltd All rights reserved.
Keywords: Schiff bases; Copper complexes; X-ray structure; 5-Phenylazosalicylaldehyde; Cyclic voltammetry
1. Introduction
2. Experimental
2.1. Reagents
Schiff base complexes containing different central metal
atoms such as Cu, Ni, Co and Pd have been studied in great
detail for their various crystallographic features, enzymatic
reactions, steric effects [1–4], structure–redox relationships
[5], mesogenic characteristics [6–9], catalysis, magnetic
properties [10,11] and their important role in the understand-
ing of the coordination chemistry of transition metal ions
[12]. CuN2O2 coordination is very common in copper chem-
istry and the redox behaviour of a wide series of mononuclear
copper(II) Schiff base complexes was reviewed some years
ago in relation to their structural changes [5]. We report here
the synthesis, characterizationandcyclicvoltammetryofcop-
per(II) complexes of a series of Schiff base ligands derived
from condensation of 5-phenylazo salicylaldehyde with di-
or tri-amines (Scheme 1). We also report the crystalstructure
of [bis(5-phenylazo salicylaldehyde)trimethylen diiminato]-
copper(II) (Cu5PHAZOSALTN) (III) as determined by
single-crystal X-ray diffraction.
All reagents and solvents were used as supplied by Merck.
2.2. Physical measurements
Elemental (C, H and N) analyses were made on a Perkin-
Elmer Model 240B automatic analyser. Electron impact (70
eV) mass spectra were recorded on a Finnigan-mat GC-MS-
DS spectrometer Model 8430. Infrared (IR) spectra were
recorded on an IR-408 Shimadzu 568. The redox properties
of the complexes were studied by cyclic voltammetry. The
voltammetric experiments were carried out in deaerated
(with purged nitrogen) dimethylformamide (DMF) solu-
tionsof the complexescontaining0.1Mtetrabutylammonium
perchlorate (TBAP) as supporting electrolyte.Spectroscopic
grade DMF was distilled and dried on 0.4-nm molecular
sieves. TBAP was dried in an oven and used without further
purification.
Cyclic voltammograms were performed using an AMEL
Instruments Model 2053 as potentiostat connected with a
* Corresponding author. Tel.: q98-41-348917; fax: q98-41-340191;
e-mail: tabrizumail@ark.tabrizu.ac.ir
0277-5387/00/$ - see front matter q2000 Elsevier Science Ltd All rights reserved.
PII S0277-5387(99)00380-0
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the program SHELXTL Plus, version 5, and refined by the
full-matrix least-squares method. In subsequent refinements,
the function Sw(NFoNyNFcN)2 was minimized, where Fo and
Fc are the observed and calculated structure factor ampli-
tudes. The agreement indices RsSNNFoNyNFcNN/SNFoNand
Rws[Sw(NFoNyNFcN)2/SwNFoN)2]1/2 were used to eval-
uate the results. Atomic scattering factors are from the inter-
national tables for X-ray crystallography. Hydrogen atoms
were introduced at theoretical positions with a C–H distance
˚
˚
of 0.95 A for the aromatic H atoms and 0.99 A for methylene
groups.
2.3. Materials [1]
2.3.1. 5-Phenylazo salicylaldehyde
5-Phenylazo salicylaldehyde was obtained as described
elsewhere [13].
2.3.2. Ligands
All ligands were prepared in a similar manner. First, 0.013
mole of related amine and 0.026 mole of 5-phenylazo sali-
cylaldehyde were condensed by refluxing in 80 ml of absolute
ethanol for 1 h. The solution was left at room temperature.
Bis(5-phenylazo salicylaldehyde)ethylene diimine (5PHA-
ZOSALEN) and bis(5-phenylazo salicylaldehyde)tri-
methylen diimine (5PHAZOSALTN) were obtained as
yellow microcrystals, and bis(5-phenylazo salicylalde-
hyde)O-phenylene diimine (5PHAZOSALOPHEN) and
Scheme 1.
function generator (AMEL Model 568).When an iR drop
(ohmic drop) was compensated a multipurpose instrument
from EG&G was used, comprising potentiostat/galvanostat
Model 273 coupled with an IBM personal computer con-
nected to an Epson Model FX-850 printer. In all electrochem-
ical experiments, an aqueous saturated calomel electrode
(SCE) was used as reference electrode and all potential cited
are given versus SCE. A platinum wire was used as counter
electrode and a glassy carbon disc with a diameter of 3 mm
was used as working electrode. The ferricinium/ferrocene
(Fcq/Fc) couple was used as an internal standard; the fer-
rocene solution concentration was 1 mM. All experiments
were carried out at room temperature. NMR spectra were
measured in CDCl3 on a Brucker ACE-300 NMR spectrom-
eter, 300.133 MHz. All chemical shifts are reported in d units
downfield from Me4Si.
Table 1
Crystallographic data for Cu5PHAZOSALTNPCHCl3 (III)
Empirical formula
Formula weight
Temperature (K)
Space group
C29H24CuN6O2PCHCl3
671.45
293(2)
¯
P1 (No. 2)
Unit cell dimensions
˚
a (A)
9.646(4)
10.961(5)
15.791(6)
˚
b (A)
˚
c (A)
a (8)
b (8)
g (8)
82.12
84.68(3)
64.54(3)
1492.0(11)
3
˚
Volume (A )
Z
2
Density (calc.) (Mg my3
)
1.495
1.040
686
Absorption coefficient (mmy1
F(000)
)
2.2.1. X-ray crystallography
Crystal size (mm)
0.1=0.4=3
Crystallographic data for complex III are given in Table 1
as a typical example. Single crystals of complex were
acquired by slow evaporation from a chloroform–ethanol
(5:1) solution at room temperature and mounted in sealed
glass capillaries. Diffraction data werecollectedonaSiemens
P3/PC diffractometer (four-circle geometry) at 293 K.
Intensity data were obtained using Mo Ka radiation (0.7107
A) monochromatized from graphite; the v–2u scan mode
was used to a maximum 2u value of 47.97. The data were
reduced and the structure was solved by direct methods using
u Range for data collection (8)
Limiting indices
2.07 to 23.96
0FhF10, y11FkF11, y17FlF17
4079
3760 (Rints0.0436)
1980
3745/0/388
1.099
0.0758
0.1653
Reflections collected
Independent reflections
Reflections observed
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I)2s(I)]
Rw
˚
Largest difference peak and hole 0.484 and y0.454
y3
˚
(e A
)
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609
d
n
n
n
n
n
n
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bis(5-phenylazo
salicylaldehyde)diethylene
triimine
to the IR spectra of the ligand. Therefore complex IV is also
(5PHAZOSALTEDA) were obtained as red microcrystals;
the microcrystals were filtered off, washed with 10 ml of
absolute ethanol and then recrystallized from ethanol–
chloroform (1:3, v/v).
four-coordinate via ONNO of the ligand atoms.
3.2. Crystal structure of [bis(5-phenylazo salicylaldehyde)-
trimethylen diiminato]copper(II) (Cu5PHAZOSALTN) (III)
2.3.3. Complexes
The crystal structure and unit cell diagram of complex III
are illustrated in Figs. 1 and 2, respectively. The selected
bond distances and angles are given in Table 4.
All complexes were prepared in a similar manner. A solu-
tion of 0.004 mole of Cu(CH3COO)2P4H2O in 10 ml of
ethanol was added to an ethanol–chloroform (1:1, v/v) solu-
tion containing 0.004 mole of ligand and refluxed for 1 h.
The obtained brown solution was left at room temperature
and brown microcrystals were collected by filtration, washed
with 15 ml of ethanol and then recrystallized from ethanol–
chloroform (2:1, v/v).
The coordination geometry around copper can be
described as a near square-planar. The Cu–O(1) and Cu–
˚
O(19) distances of 1.903(5) and 1.931(6) A are similar
˚
to the corresponding value [Cu–O(2), 1.915(7) A] in
N,N9-trimethylene bis(salicylaldehyde imine)copper(II),
Cu(sal2tn), as reported by Xiong et al. [12], and the differ-
ences are not significant, but the differences between Cu–
˚
O(1) and Cu–O(19) with Cu–O(1) [1.840(9)A] in
3. Results and discussion
Cu(sal2tn) are probably just significant.
The Cu–N(1) and Cu–N(19) distances are 1.949(7) and
˚
3.1. Syntheses
1.962(7) A. The corresponding values of complex
˚
Cu(sal2tn) are 1.954(7) and 1.995(8) A.
The 5-phenylazo salicylaldehyde(pre-ligand),Schiffbase
ligands and related copper complexes were obtained in good
yield and purity. The pre-ligand and Schiff base ligands were
characterized by 1H NMR and IR and mass spectroscopy.
Copper complexes were characterized by elemental anal-
ysis and IR spectroscopy. All physical and characterization
data for the ligands and complexes are given in Tables 2 and
3, respectively. In the mass spectra of 5-phenylazo salicyl-
aldehyde and the related Schiff base (5PHAZOSALO-
PHEN), [Mq1], M and [MyC6H5N2] for aldehyde and
[My2*C6H5N2] for Schiff base were observed. For the IR
spectra of the four title complexes, n(C_N)shiftedtoalower
wavenumber by 10–30 cmy1 upon coordination. On theother
hand, the disappearance of the OH band of free ligands in
copper(II) complexes indicates that the OH group has been
deprotonated and bonded to the metal ions as –Oy.
The bond angles O(19)–Cu–O(1), N(19)–Cu–N(1),
N(1)–Cu–O(19) and N(19)–Cu–O(1) are 79.8(2),
95.7(3), 171.9(3) and 171.8(3)8, respectively. The two for-
mer bond angles are similar to the values in Cu(sal2tn)
[79.8(4) and 95.5(4)8], but on the basis of the two latter
bond angles [(171.9(3) and 171.8(3)8] we can conclude
that the coordination geometry around copper in the title
complex is near to square-planar in comparison to
Cu(sal2tn), which has corresponding values of 159.7(4) and
158.2(4)8, respectively. The central carbon atom in the tri-
methylene diamine chelate ring (labelled C(1) and C(19);
see Fig. 1) is disordered over two positions, one of which is
above and another below the coordinationplane. Thedisorder
is quite common and is observed, for example, in some of the
transition metal complexes with Schiff base ligands [14].
Complex III has been prepared and recrystallized in eth-
anol solution. The elemental analysis confirms the formula-
tion of the complex. Recrystallization of complex III for
preparing a single crystal was carried out in ethanol–chloro-
form. The crystal structure shows the presence of a CHCl3
On the basis of these results it can be deduced that in
complexes I, II and III the Schiff bases are coordinated to
copper atom as tetradentate ONNO ligands. The IR spectra
of complex IV shows no shift for N–H bond in comparison
Table 3
Physical and characterization data for complexes
Compound
I
II
III
IV
Formula
C28H22CuN6O2
94
C32H22CuN6O2
92
C29H24CuN6O2
94
C30H27CuN7O2
92
Yield (%)
IR (cmy1
)
n(C_N)
n(N–H)
1630
1600
1605
1625
3250
Anal.: found (calc.) (%)
C
H
N
62.3 (62.50)
4.0 (4.09)
15.5 (15.62)
65.4 (65.58)
3.7 (3.75)
14.1 (14.34)
62.8 (63.09)
4.3 (4.35)
15.1 (15.23)
61.8 (62.01)
4.6 (4.65)
16.7 (16.88)
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A.A. Khandar, K. Nejati / Polyhedron 19 (2000) 607–613
611
Fig. 1. Structure of Cu5PHAZOSALTNPCHCl3 (III).
Fig. 2. Stereoview of the unit cell of the Cu5PHAZOSALTNPCHCl3 (III).
Table 4
corresponding characteristic data are given in Table 5. Note
that the free ligands are electroinactive in the working poten-
tial region.
˚
Selected bond lengths (A) and angles (8) for Cu5PHAZOSALTNPCHCl3
(III)
On the basis of the cyclic voltammograms in Fig. 3, com-
plex I undergoes an irreversible reduction process under the
experimental condition (Fig. 3a). The peak separations DE
(sEpayEpc) for complexes II, III and IV were 82, 103 and
227 mV, respectively, at a scan rate of 0.05 V sy1, and
decreased to 62, 78 and 204 mV when the iR drop was
compensated by the cyclic voltammetryset-up;thustheredox
process for complex II is reversible and quasi-reversible for
complexes III and IV [15]. Note that when a similar iR
compensation is applied to the cell, the ferrocene behaved
ideally and the peak separation for the ferricinium/ferrocene
couple was at 60 mV, which can be used as a criterion for
electrochemical reversibility in the present experimental
conditions.
Cu(1)–O(1)
Cu(1)–N(19)
N(1)–C(2)
C(2)–C(1)
C(29)–C(1)
1.903(5)
1.949(7)
1.482(10) N(19)–C(29)
1.44(2)
1.46(3)
Cu(1)–O(19)
Cu(1)–N(1)
1.931(6)
1.962(7)
1.459(10)
1.36(3)
C(2)–C(19)
C(29)–C(19)
1.36(3)
O(1)–Cu(1)–O(19)
O(19)–Cu(1)–N(19)
O(19)–Cu(1)–N(1)
C(5)–O(1)–Cu(1)
C(2)–N(1)–Cu(1)
79.8(2)
92.4(3)
171.9(3)
130.5(5)
123.3(6)
O(1)–Cu(1)–N(19) 171.8(3)
O(1)–Cu(1)–N(1)
N(19)–Cu(1)–N(1)
C(3)–N(1)–Cu(1)
C(59)–O(19)–Cu(1) 130.6(5)
C(29)–N(19)–Cu(1) 124.4(6)
92.1(3)
95.7(3)
124.8(6)
C(39)–N(19)–Cu(1) 123.4(6)
molecule in the crystal structure, with the empirical formula
C29H24CuN6O2PCHCl3 (formula weight 671.45).
As seen in Table 5, the peak:current ratio decreases with
increasing scan rate, indicating a weak adsorption of the reac-
tants on the electrode [16]. For N2O2 Schiff base complexes
of copper the redox potential is correlated with the degree of
3.3. Cyclic voltammetry
The cyclic voltammograms recorded (without iR compen-
sation) for complexes I–IV are shown in Fig. 3 and the
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Fig. 3. Cyclic voltammograms (recorded without iR compensation) for (a) Cu5PHAZOSALEN (I), (b) Cu5PHAZOSALOPHEN (II), (c)
Cu5PHAZOSALTN (III), (d) Cu5PHAZOSALDETA (IV) in DMF (0.002 M) with 0.1 M TBAP as supporting electrolyte at a scan rate 0.1 V sy1
.
Table 5
Cyclic voltammetry data (without iR compensation) for complexes I–IV
Scan rate (V sy1
)
Ec (V)
Ea (V)
DE
Ic (mA)
Ia (mA)
Ia/Ic
Cu5PHAZOSALEN (I)
0.05
0.1
0.2
0.3
0.4
0.5
y0.991
y1.005
y1.009
y1.015
y1.021
y1.031
30
42
57
69
78
84
Cu5PHAZOSALOPHEN (II)
0.05
0.1
0.2
0.3
0.4
0.5
y0.970
y0.974
y0.981
y0.993
y1.005
y1.025
y0.888
y0.871
y0.865
y0.856
y0.848
y0.840
0.082
0.103
0.116
0.137
0.157
0.185
25.4
30.8
42.5
51.9
60.1
66.2
19.3
23.1
30.8
36.9
41.6
44.7
0.77
0.75
0.72
0.71
0.69
0.67
Cu5PHAZOSALTN (III)
0.05
0.1
0.2
0.3
0.4
0.5
y0.851
y0.863
y0.873
y0.881
y0.890
y0.900
y0.748
y0.733
y0.725
y0.718
y0.710
y0.701
0.103
0.130
0.148
0.163
0.180
0.199
28.6
38.6
50.5
59.8
67.8
77.1
22.6
29.3
37.9
43.9
47.9
51.9
0.79
0.76
0.75
0.73
0.71
0.67
Cu5PHAZOSALDETA (IV)
0.05
0.1
0.2
0.3
0.4
0.5
y0.785
y0.798
y0.820
y0.825
y0.852
y0.860
y0.558
y0.553
y0.549
y0.541
y0.531
y0.511
0.227
0.245
0.271
0.284
0.321
0.349
23.9
33.2
43.9
51.9
58.5
67.8
21.3
28.6
37.2
43.9
47.9
53.2
0.89
0.86
0.85
0.85
0.82
0.78
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A.A. Khandar, K. Nejati / Polyhedron 19 (2000) 607–613
613
tetrahedral distortion of the copper coordinationenvironment
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nation environment is near square-planar; therefore the redox
potential of this complex is less negative than the planar
analogue. On the basis of the redox potential, we conclude
that complex IV has a structure deviating from square-planar
with N2O2 and not N3O2 coordination [5,19].
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