Mono and binuclear Ag(I), Cu(II), Zn(II) and Hg(II) complexes of a new azo-azomethine as ligand: Synthesis, potentiometric, spectral and thermal studies

Article abstract of DOI:10.1016/j.saa.2011.01.022

New azo-azomethine dyes were prepared by reaction of p-aminobenzoic acid, o-anisidine, o-nitroaniline, and p-bromoaniline with salicylaldehyde respectively to form azo compounds and then condensation by urea to form 4-(R-arylazo 2-salicylaldene)-urea azo-azomethine derivatives (I_a-d). The complexes of these ligands with Ag(I), Cu(II), Zn(II) and Hg(II) metal ions were prepared. The structure of the free ligands and their complexes were characterized by using elemental analysis (C, H, N), ¹H NMR, IR and UV-Vis-spectra. The proton dissociation constants of the ligands and the stability constant of their complexes have been determined potentiometrically in 40% (v/v) alcohol-water medium as well as the stoichiometry of complexes were determined conductometrically. The data reveal that the stoichiometries for all complexes were prepared in molar ratios (1:1) and (1:2) (M:L). The electrolytic and nonelectrolytic natures of the complexes were assigned based on molar conductance measurements. The thermogravimetric (TG), and differential thermal analyses (DTA) were studied in nitrogen atmosphere with heating rate 10 °C/min. The kinetic and thermodynamic parameters for thermal decomposition of complexes have been calculated by graphical method using Coats-Redfern (CR) method.

Full text of DOI:10.1016/j.saa.2011.01.022

Spectrochimica Acta Part A 78 (2011) 1429–1436
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Mono and binuclear Ag(I), Cu(II), Zn(II) and Hg(II) complexes of a new
azo-azomethine as ligand: Synthesis, potentiometric, spectral and
thermal studies
Ibrahim S. Ahmeda,∗, Moustafa M. Moustafaa, Mohamed. M. Abd El Azizb
a Chemistry Department, Faculty of Science, Benha University, Benha City, Egypt
b Egyptian Electrical Authority, Canal Zone, EL-Arish Power Station, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
New azo-azomethine dyes were prepared by reaction of p-aminobenzoic acid, o-anisidine, o-nitroaniline,
and p-bromoaniline with salicylaldehyde respectively to form azo compounds and then condensation by
urea to form 4-(R-arylazo 2-salicylaldene)-urea azo-azomethine derivatives (Ia–d). The complexes of these
ligands with Ag(I), Cu(II), Zn(II) and Hg(II) metal ions were prepared. The structure of the free ligands and
their complexes were characterized by using elemental analysis (C, H, N), 1H NMR, IR and UV–Vis-spectra.
The proton dissociation constants of the ligands and the stability constant of their complexes have been
determined potentiometrically in 40% (v/v) alcohol–water medium as well as the stoichiometry of com-
plexes were determined conductometrically. The data reveal that the stoichiometries for all complexes
were prepared in molar ratios (1:1) and (1:2) (M:L). The electrolytic and nonelectrolytic natures of the
complexes were assigned based on molar conductance measurements. The thermogravimetric (TG), and
differential thermal analyses (DTA) were studied in nitrogen atmosphere with heating rate 10 ◦C/min. The
kinetic and thermodynamic parameters for thermal decomposition of complexes have been calculated
by graphical method using Coats–Redfern (CR) method.
Received 15 September 2010
Received in revised form 5 January 2011
Accepted 17 January 2011
Keywords:
Azo-azomethine derivatives
Complexes
UV–vis-spectra
TG-DTG/DTA analysis
Elemental analysis (C
H
N)
1H NMR spectra
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
The formation and stability of binary and ternary complexes of
copper(II) with 2,3-pyrazinedicarboxylic acid as a primary ligand
Azo compounds and their metal (II)-complexes are very impor-
tant molecules and have attracted much attention in both applied
and academic research [1–3]. On the other hand, azo compounds
are known to be involved in a number of biological reactions,
such as inhibition of DNA, RNA, and protein synthesis, nitrogen
fixation, and carcinogenesis [4]. The azo dyes and their metal
complexes are more light stable, allow for easier control of the
wavelength by selection of the appropriate substitute groups and
have scarce of thermal studies [5–9]. TG and DTA have been found
wide application in material characterization [10], since these
methods essentially depend on measurements of the changes in
physico-chemical properties as a function of increasing tempera-
ture with time [11–13]. Metal (II)–azo complexes are highly colored
that enjoy widespread use as dyes and pigments in a variety of
applications that include textile dyeing [14] as well as non-linear
and photoelectronics [15], especially in optical information storage
[16–24].
and some biological important aliphatic, aromatic and heteroaro-
matic carboxylic acids as secondary ligands were determined by
potentiometric technique [25]. The composition and stability of the
complexes of isatin-thiosemicarbazone with Co(II), Ni(II), Cu(II),
Zn(II), Pd(II), and Hg(II) have been investigated using spectrophoto-
metric method [26,27]. The stoichiometries of Mn(II), Co(II), Ni(II),
Cu(II) complexes with azo sulfonamide derivatives were deter-
mined conduct metrically [28,29]. Azo compounds were the subject
of many spectral studies for purpose of identification or struc-
ture elucidation [30–33], as they are capable of forming metal
chelates with transition metal ions [34,35]. The objective of this
work concern synthesis and characterization of 4-(R-arylazo 2-
salicylaldene)-urea azo-azomethine derivatives as new dyes and
their complexes.
2. Experimental
2.1. Materials and reagents
Chemicals used in this study were of pure grade and of highest
purity available and used without further purification.
∗
Corresponding author. Tel.: +20 122408034; fax: +20 133222875.
E-mail address: isahmed2010@gmail.com (I.S. Ahmed).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.01.022

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I.S. Ahmed et al. / Spectrochimica Acta Part A 78 (2011) 1429–1436
Table 1
Elemental analysis data of the ligands (Ia–d).
Ligand
M.wt.
m.p.
Color
Yield%
%C
%H
%N
Found (calc.)
Found (calc.)
Found (calc.)
Ia
Ib
Ic
Id
312
298
313
346
210
298
266
270
Orange
Yellow
Orange
Brownish yellow
85
78
90
88
56.8 (57.7)
59.0 (60.4)
53.6 (53.7)
48.0 (48.6)
3.6 (3.8)
4.1 (4.7)
3.3 (3.5)
3.1 (3.2)
17.2 (17.9)
17.9 (18.8)
21.6 (22.4)
15.6 (16.2)
Table 2
Assignment and chemical shift (ppm) of different types of protons of ligands (Ia–d).
2.2. Preparation of the 4-(R-arylazo-2-salicylaldene)-urea
derivatives
Compound
Chemical shift (␦), ppm of protons
The azo dye is firstly prepared which in turn is refluxed with
urea to form the azo-azomethine as following:
H (1)OH
H(2)NH
2
H(3)CH=N
H(4)
H(5)
H(6)
Ia
Ib
Ic
Id
10.392
10.37
10.42
6.05
6.04
6.05
6.10
8.1
7.901
8.03
7.84
7.79
7.32
7.24
7.30
7.32
–
3.96
–
50 mmol of aniline derivatives (p-aminobenzoic acid, o-
anisidine, o-nitroaniline, and p-bromoaniline0 were dissolved in
(1:1) of hydrochloric acid (mixture A) and then diazotized in
50 mmol of sodium nitrite is dissolved in bidistilled water (mix-
ture B), 50 mmol of salicylaldehyde was dissolved in 10% sodium
acetate (mixture C), then the usual way put the three mixtures in
ice path, add the contents of mixture B to A to form diazotized
compound, then the diazotized add to mixture C with continu-
ums stirring for one half hour, then leaf the mixture after complete
reaction overnight then filtrate and dry, resulting azo dyes were
crystallized from appropriate solvents.
8.13
8.18
9.1
10.396
–
Cairo University, Egypt. Thermogravimetric analysis (TGA; were
obtained using TA Instruments, SDT2960) of the precursor was
carried out at a heating rate of 10 ◦C/min in nitrogen atmosphere.
2.4. Synthesis of solid complexes
The solid complexes of some selected metal ions were prepared
by mixing 0.01 M of the metal ions solutions with a hot alcoholic
solution of 0.01 M of ligands. The mixture was refluxed on a water
bath for about 6 h and then allowed to cool; the solid complexes
were separated and recrystallized from absolute ethanol, dried and
preserved in desiccators over dried silica gel.
OH
CHO
N
N
12
X
where x = (p-COOH), (o-OCH3), (o-NO2) and (p-Br).
50 mmol of azo dye is dissolved in methanol, then added to
50 mmol of urea in methanol as solvent and reflux for four hours,
the azo-azomethine was obtained. The resulting precipitates were
crystallized from appropriate solvent and subject to, m.p. deter-
mine and elemental analysis (Table 1).
10
Acid
8
Ligand
6
pH
L-Zn(II)
L-Cu(II)
Hg(II)
where R = p-COOH (Ia), o-OCH3 (Ib), o-NO2 (Ic) and p-Cl (Id).
4
2
0
2.3. Apparatus
Schott Geratte pH meter model CG 710, fitted with a combi-
nation glass-calomel electrode, was used for pH adjustment and
potentiometric measurements. The used apparatus for conduct
metric work was Hanna instrument HI 9032 micro computer con-
ductivity meter.
Ag(I)
The IR spectra were recorded as KBr discs using Matson FT-
IR spectrophotometer in the 4000–200 cm−1 range. The 1H NMR
spectrum of the ligands was obtained using a varian EM 390-90
NMR spectrometer in d6 – DMSO as solvent using tetramethylsi-
lane (TME) as an internal standard. The microanalysis for C, H and
N were performed in the Microanalytical Center, Faculty of Science,
25
30
35
40
45
50
55
60
65
70
75
ml added of 0.01M NaOH
Fig. 1. Potentiometric titration of ligand Ia in presence of metal ions.

I.S. Ahmed et al. / Spectrochimica Acta Part A 78 (2011) 1429–1436
1431
Table 3
Proton ionization constant and metal–ligand stability constant deducted from potentiometric titrations.
Protonionization constants Metal–ligand stability constant
Ligand
pkH1
pkH2
Ag(I)
Cu(II)
Cu(II)
Zn(II)
Log ˇ1
Log ˇ1
Log ˇ1
Log ˇ2
Log ˇ1
Log ˇ2
Log ˇ1
Log ˇ2
Ia
Ib
Ic
Id
7.2
5.5
8.2
7.3
8.4
–
–
3.6
3.9
4.2
3.2
5.6
–
–
3.5
4.3
5.2
3.6
6.6
–
–
4.6
4.4
5.8
3.9
7.1
–
–
4.4
4.1
5.6
3.8
6.8
–
–
–
–
–
–
–
2.5. Solutions
azo-azomethine derivatives of ligands (Ia–d
)
with Ag(I),
Cu(II), Zn(II) and Hg(II) metal ions were determined using
potentiometric titration method where the titration of an
acid, ligand and ligand–metal mixture have been titrated
against 0.01 M NaOH.
0.01 M azo-azomethine solution was prepared by dissolving the
calculated amount of the reagent in the requisite volume of ethanol.
0.1 M stock solution of sodium acetate was prepared by dis-
solving the calculated amount in the necessary volume of distilled
water.
(i) The acid mixture (mixture A)
0.01 M stock solution of Ag(I), Cu(II), Zn(II) and Hg(II) metal ions
was prepared by dissolving the calculated amount of each salt in a
definite volume of distilled water.
1.0 M potassium nitrate was prepared by dissolving the calcu-
lated amount of KNO3 in the appropriate volume of distilled water.
The acid mixture (mixture A) used consisted of 5.0 ml 0.1 M
HNO3, 5.0 ml 1 M KNO3 then completed to 50 ml with 40% alco-
holic water (A) and titrated against 0.01 M sodium hydroxide.
(ii) Ligand mixture (mixture B)
5.0 ml of 0.1 M HNO3, 5.0 ml 0.1 M KNO3, 25 ml of ligand
(1 × 10−3 M) was completed to 50 ml with 40% alcoholic water
and titrated against 0.01 M NaOH.
2.6. Procedures
(iii) The metal–ligand mixture (mixture C)
To mixture B, 5.0 ml metal ion (1 × 10−4 M) is added, and
then titrated against 0.01 M NaOH.
2.6.1. Potentiometric titration
The proton dissociate of the ligands and stability con-
stants of complexes of 4-(R-arylazo 2-salicylaldene)-urea
2.6.2. Conductometric titration
The conductometric titration of 4-(R-arylazo 2-salicylaldene)-
urea azo-azomethine derivatives as ligands (Ia–d) with Ag(I), Cu(II),
Zn(II) and Hg(II) metal ions were carried out in aqueous media
Fig. 2. 1H NMR spectra of 4-(o-methoxyarylazo 2-slicyladene)-urea azo-
Fig. 3. 1H NMR spectra of 4-(R-arulazo 2-slicyladene)-urea azo-azomethine deriva-
azomethine derivatives (Ib).
tives (Ic).

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I.S. Ahmed et al. / Spectrochimica Acta Part A 78 (2011) 1429–1436
with the ligand, giving inflection point, from the titration plots, the
stoichiometry of metal–ligand complexes was determined where
50 ml of metal ions (1 × 10−4 M) was titrated with (1 × 10−3 M) of
the ligands.
3.4. The IR-spectra
When a ligand coordinates to a metal ion Mz+, new modes
of vibrations, are not present in the free compound may become
infrared active. For a coordinated water molecules, rocking, twist-
ing and wagging modes become possible and of course, this is
in addition to the M–O and M–N bond which are due to the
metal ligand bonds stretching vibrations. In general, the frequen-
cies of these new bands not only depend on the ligand involved
but are also sensitive to the nature of the metal ion, its size and
charge.
3. Results and discussion
3.1. 1H NMR spectra
The 1H NMR spectra of ligands (Ia–d) were carried out and chem-
ical shifts of the different signals are recorded in Table 2. It is clear
from 1H NMR spectra of these ligands that signals lying at 10.392,
10.37, 10.42 and 10.396 ppm give evidence that they are due to the
OH group attached to the phenyl ring of ligands Ia–d, respectively.
A signal appeared at 3.96 ppm which attributed to hydrogen pro-
ton of –OCH3 group which confirmed the structure of Ib reagent as
present in Fig. 2. A new multiple signals at range 6.37–7.713 ppm
are due to the protons of the phenyl ring as shown in Fig. 3. The
signals lying in range of 8.18-9.2 ppm are assigned to the proton of
azomethine group of these compounds.
The IR spectra of the ligands show a sharp strong band in the
frequency range 3412–3440 cm−1 which is due to ꢁOH but these
bands become broadened and suffer a frequency shift when the
ligand is bonded with the metal ions to give the chelate. This
indicates the deprotonation of the replaceable hydrogen atom.
The IR spectra of reagents (Ia–d) show the presence band in the
frequency range 1630–1695 cm−1 which is due to ␯ O, these
C
bands become broadened and suffer a frequency shift when the
ligand is bonded with the metal ions to give the chelate. The IR
spectra of reagents (Ia–d) confirmed the presence of azo group
due to the band in the frequency range 1475–1482 cm−1. A weak
band appears in the vicinity of 450 cm−1 which is attributed to an
M–N bond in the reagents chelates. Also, the IR spectra of the solid
complexes confirmed the presence of coordinated water molecules
by the broad band in the frequency range 3400–3600 cm−1. The
IR spectra of these compounds are compared to that of the free
ligand. The data are listed in Table 4. Accordingly, the mode of the
metal ions with the ligand under investigation can be represented
as follow:
3.2. Potentiometric studies
The potentiometric technique has been used in many
fields of chemistry [36–38]. Fig. 1 shows the titration curve
where R = p-COOH (Ia); o-OCH3 (Ib); o-NO2 (Ic) p-Br (Id);
which used in this work is developed by Calvin and Bjerrum and
the proton-reagent stability constants (knH) were determined. The
average number of protons associated with the reagent molecules
nA−, the average number of molecules attached per metal ion
n−, free ligand exponent pL and the values of ionization and sta-
bility constants of ligands and their complexes were determined
Table 3.
M = Ag(I), Cu(II), Zn(II) and Hg(II) metal ions; X = number of water
2−
molecules coordinated to metal ion; Y = OCOCH3, SO4
or Cl−
anions; z = overall charge on the complex; N = Number of crys-
talline water molecules; A = number of anion in outer sphere;
a, y,
x and n are stoichiometric ratio of anions and water
molecules.
3.3. Conductometric and molar conductivities studies
The conductometric titrations were carried out for metal ions
Ag(I), Cu(II), Zn(II) and Hg(II) with the ligands (Ia–d). The changes in
the conductance depend on the change in the number and character
of the conducting species in solution.
The conductivity in nonaqueous solutions in dimethylfor-
mamide (DMF) had frequently been used in the structure studies
of metal complexes within the limits of their solubility. It is clear
from the conductivity data, shown in Table 3, that all the complexes
(1:1) (M:L) present behave as strong electrolytes with values in
range 62.9–92.7 ꢀ−1cm2 mol−1. On the other hand, the conductiv-
ity of complexes (1:2) (M:L) have lower values of conductivity with
non-electrolytic nature of conductivity.
where R = p-COOH (Ia), o-OCH3 (Ib), o-NO2 (Ic), p-Br (Id); M = Ag(I),
Cu(II), Zn(II) and Hg(II) metal ions; z = overall charge on the com-
plex; x = 2; n = 1or 2.

I.S. Ahmed et al. / Spectrochimica Acta Part A 78 (2011) 1429–1436
1433
Table 4
Assignment of the important bands in the IR spectra of Ia–d, ligands and their complexes and molar conductivity values.
Complex
Ratio (M:L)
␯
–OH
␦
OH
␯
␯
␯
␯
C–O
␯
Chemical formula
Molecular weight
ꢂM (ꢀ−1cm2 mol−1
)
–C
N
–N N–
C
O
M–O
Ia
–
3412
3680
3738
3742
3427
3440
3424
3424
3398
3360
3741
3429
3742
3742
3314
3691
3742
3741
1378
1376
1375
1384
1378
1366
1370
1366
1393
1394
1375
1394
1344
1344
1346
1380
1374
1384
1573
1576
1576
1527
1574
1586
1585
1586
1530
1528
1582
1593
1576
1579
1622
1619
1617
1617
1455
1424
1423
1425
1427
1482
1482
1484
1478
1480
1482
1463
1480
1479
1472
1475
1472
1471
1660
1665
1620
1645
1630
1685
1610
1650
1670
1665
1650
1625
1690
1620
1615
1695
1630
1670
1156
1150
1155
1150
1159
1151
1153
1151
1152
1152
1162
1151
1145
1144
1150
1143
1160
1160
–
C15H12N4O4
312
–
Zn2+–Ia
Hg2+–Ia
Cu2+–Ia
Ag+–Ia
Ib
(1:1)
(1:1)
(1:1)
(1:1)
–
(1:1)
(1:2)
(1:1)
(1:2)
(1:1)
(1:2)
–
(1:2)
(1:1)
–
(1:1)
(1:2)
408
412
408
404
–
453
454
439
455
449
425
–
449
523
–
407
421
[Zn·L·4H2O]CH3COO·H2O
[Hg·L·H2O]NO3·H2O
[Cu·L·H2O]SO4·H2O
[Ag·L·H2O]NO3
525.5
609.59
506.55
499
298
629.59
783
494.5
629.59
503
843.55
313
838.5
472.5
305.5
644.49
1044.39
67.9
87.5
62.9
87.2
–
67.0
33.0
73.0
25.5
82.8
13.4
–
16.5
88.5
–
84.7
19.2
C15H14N4O3
Hg2+–Ib
Ag+–Ib
Zn2+–Ib
Zn2+–Ib
Ag+–Ib
Cu2+–Ib
Ic
[Hg·L·H2O]SO4·H2O
[Ag·L2·H2O]2H2O
[Zn·L·H2O]SO4.H2O
[Zn·L2·2H2O] 3H2O
[Ag·L·H2O] NO3·H2O
[Cu·L2·2H2O] 3H2O
C14H11N5O4
[Zn·L2·2H2O]3H2O
[Zn·L·H2O]CH3COO·H2O
C14H14N4O2
Zn2+–Ic
Zn+2–Ic
Id
Hg2+–Id
Hg2+–Id
[Hg·L·H2O]NO3·H2O
[Hg·L2·2H2O] 3H2O
3.5. Thermal analysis
attributed to dehydration of one hygroscopic water molecule, the
found weight losses is 3.25% (calc. 3.56%), having broad endother-
mic peak in DTA at 100 ◦C whereas the second step of weight loss
within the range 120–220 ◦C is attributed to release of four water
molecules coordinated with metal ion with endothermic peak, the
third weight loss within the range 220–320 ◦C is attributed to one
acetate ion with exothermic peak in DTA curve at 280 ◦C. The found
weight losses of the complex amount to 14.75 and 13.25% com-
pared with the calculated values (14.65 and 11.95%) for four water
molecule and acetate ion, respectively. The organic part of the com-
plex is decomposed in range 307–523 ◦C, the found weight loss
Thermal methods of analysis introduce a new possibility for
investigation of complexes [39]. The thermal analysis for some
complexes of 4-(R-arylazo 2-salicylaldene)-urea azo-azomethine
derivative with Ag(I), Cu(II) and Zn(II) metal ions were carried
out in nitrogen gas atmosphere with heating rate 10 ◦C/min. They
include suchtechniqueas thermogravimetric(TGA)anddifferential
thermal analysis (DTA). The TGA and DTA curves give interesting
information about the various unexpected phenomena observed
during thermal degradation. It is important to take into account that
the TGA curves reveal the variation in weight, whereas the DTA is
concerned calorimetrically with weight changes which often occur
with observed detection.
From TG curves, one can calculate the percentage of water
molecules and metal ion in the complexes, whereas those of DTA
make it possible to characterize, thermographically, the process of
phase transformation in the examined system. To understand the
thermal decomposition of the complexes under investigation, the
mechanism of the decomposition has to be established from TG-
DTA data. The kinetic and thermodynamic parameters: activation
energy (Ea) and pre-exponential factor (A), enthalpy (ꢃH#), entropy
(ꢃS#) and free energy (ꢃG#) are calculated from TG/DTA curves
in the temperature range 30–1000 ◦C using Coats–Redfern method
[40].
3.5.1. The thermal degradation of mono and binuclear Ag(I),
Cu(II), and Zn(II) complexes
TG/DTA analysis for [Ag–Ia·H2O] NO3. H2O complex (1:1), shows
that the water molecules were expelled after raising temperature
up to 216 ◦C with weight loss 14.11% (theoretically 13.36%) cor-
responding to the loss of four molecules of water. This step is
accompanied by an endothermic peak in the DTA curve, the second
weight loss (found 11.90%) takes place between 216 and 336 ◦C hav-
ing endothermic peak in the DTA curve at 336 ◦C which is attributed
to elimination of nitrate ion (theoretically 11.45%) the final step
which is shown in the TG curve in the range 350–850 ◦C with weight
loss 54.50% (theoretically 54.93%) and this corresponding to the
decomposition of the organic part of the complex having exother-
mic peak in DTA curve at 750 ◦C. The metal oxide is formed at 850 ◦C
and the activation energy of degradation process is 32.39 kJ mol−1
.
Thermal analysis has been carried out on [Zn·Ic·4H2O]
CH3COO·H2O complex. From TG-curves as shown in Fig. 4, the
weight losses were calculated for the different overlapping steps
and compared with those theoretically calculated for the sug-
gested formula. The first weight loss at temperature up to 120 ◦C is
Fig. 4. TG/DTA analysis of 4-(R-arylazo 2-salicylaldene)-urea azo-azomethine (Ic)
with Zn2+ ion in ratio (1:1) (M:L) in a nitrogen gas with a heating rate of 10 ◦C/min.

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I.S. Ahmed et al. / Spectrochimica Acta Part A 78 (2011) 1429–1436
Table 5
TG-DTG/DTA data of Ag(I), Cu(II) and Zn(II) complexes with 4-(R-arylazo 2-salicylaldene)-urea azo-azomethine derivative.
Complex
Temperat ure range (◦C)
Weight loss %
Calc.
Type of peak
Assignment
Found
30–360
13.41
11.54
54.93
8.22
13.35
12.41
70.41
2.57
5.14
80.16
4.98
2.49
81.60
13.66
11.90
54.50
8.75
13.25
12.36
70.36
2.30
4.95
80.25
5.12
2.60
82.43
Endo
Endo
Exo
Endo
Endo
Exo
Four water molecules and nitrate ion
Nitrate ion
Organic part
2H2O
3H2O
Acetate ion
Organic part
One molecule of H2O
Two coordinate of water
Organic part
Ag(I)–Ia (1:1)
Zn(II)–Ic (1:1)
Zn(II)–Ia (1:2)
120–220
220–850
35–180
180–300
300–350
350–600
40–150
150–350
350–600
40–125
120–250
250–600
Exo
Endo
Endo
Exo
Endo
Endo
Exo
2H2O
Cu(II)–Ib (1:2)
One molecule of coordinate H2O
Organic part
of organic part is 56.16% (theoretically 53.41%), and having sharp
exothermic peak at 500 ◦C. The activation energy of degradation
amount to 10.55% compared with the calculated values (10.26%) for
four water molecules. The organic part of the complex is decom-
posed in range 120–523 ◦C, the found weight loss of organic part is
77.16% (cal. 77.86%), and having sharp exothermic peak at 500 ◦C
with forming zinc oxide 11.21 (cal. 11.59%). The activation energy
process is 46.7 kJ mol−1
.
The thermal decomposition of [Zn·2Ia] 4H2O complex in ratio
(1:2) (M:L) has been carried out in temperature range 30–1000 ◦C
in nitrogen atmosphere with heating rate of 10 ◦C/min. From TGA
curve in Fig. 5, the weight losses were calculated and compared
with those theoretically calculated for the suggested formula. The
first weight loss at temperature up to 120 ◦C is attributed to elim-
ination of four water molecules, having broad endothermic peak
in DTA at 100 ◦C whereas the second step of weight loss within
the range 120–550 ◦C is attributed to organic part with exothermic
peak in DTA curve at 499 ◦C. The found weight losses of the complex
of thermal degradation process is 41.33 kJ mol−1
.
TGA/DTA analysis of the [Cu·2Ib] H2O complex in ratio (1:2)
(M:L) has been carried out with heating rate 10 ◦C/min. in nitro-
gen atmosphere. The found weight losses of the complex amount
to 2.12% compared to calculated 2.62% for one water molecule hav-
ing endothermic peak in DTA at 120 ◦C. The organic part of the
complex is decomposed in range 120–423 ◦C, the found weight
loss of organic part is 87.16% (cal. 88.14%), and exothermic peak
observed at 400 ◦C with forming copper oxide 11.21 (cal. 11.58%).
The activation energy of degradation process is 33.2 kJ mol−1
.
3.5.2. Coats–Redfern method
Coats–Redfern [40] simplified Van Krevelen’s interpretation
[41] according to the following:
ꢀ
ꢁ
ꢂ
ꢃ
c−1
T2
ZR
Ea
ln − ln
= ln
−
(1)
RT
Eaˇ
With c = (w∞ − w)/w , where w is the total mass loss for a par-
∞
∞
ticular stage, w is the mass loss of absolute temperature T, Z the
frequency factor and ˇ the heating rate.
Also, Coats–Redfern suggested an asymptotic expansion of the
temperature integral, giving:
ꢂ
ꢃ
f (˛)
T2
AR
Ea
ln
= ln
−
(2)
RT
Eaˇ
Plotting ln(−ln c−1/T2) and ln f(˛)/T2) → 1/T for all the steps gives
the value of activation energy Ea from the slope and Z and A from
the intercept.
Tables 5 and 6 demonstrated the thermal behaviors of the
complexes, the activation kinetic and thermodynamic parame-
ters; activation energies (Ea), pre-exponential factor (A), entropy
(ꢃS#), enthalpy (ꢃH#) and free energy (ꢃG#) were calculated from
TG–DTG/DTA-curves in the temperature up to 1000 ◦C in nitrogen
atmosphere by using the following equations:
Ah
ꢃS# = R ln
(3)
kT
Ah
kT
ꢃS# = R ln
(4)
(5)
ꢃG# = ꢃH# − TꢃS#
where k and h are the Boltzman and Plank constants, respectively.
The correlation coefficients of the Arrhenius plots of the thermal
Fig. 5. TG/DTA analysis of 4-(R-arylazo 2-salicylaldene)-urea azo-azomethine (Ic)
with Zn(II) (1:2) (M:L) in a nitrogen gas with a heating rate of 10 ◦C/min.

I.S. Ahmed et al. / Spectrochimica Acta Part A 78 (2011) 1429–1436
1435
Table 6
The activation kinetic and thermodynamic parameters of thermal decomposition of Ag(I), Cu(II) and Zn(II) complexes with 4-(R-arylazo 2-salicylaldene)-urea azo-azomethine
derivative.
Complex
Step
T, K
Correlation coefficient, r
Ea kJ mol−1
ꢃH# kJ mol−1
Pre-exponential factor, A Sec−1
-ꢃS# kJ mol−1k−1
ꢃG# kJ mol-1
Ag(I)–Ia (1:1)
1
2
3
1
2
3
1
2
3
1
2
3
4
580
775
1151
441
574
766
496
556
700
363
530
648
788
0.998
0.966
0.909
0.996
0.992
0.99
0.962
0.956
0.998
0.929
0.995
0.928
0.991
61.27
17.07
42.66
31.77
22.48
33.07
28.20
21.17
34.87
17.05
33.9
56.16
10.63
33.09
28.10
17.71
27.24
24.08
16.55
29.05
14.03
29.49
10.04
41.97
2.13 × 106
4.02 × 103
2.10 × 104
3.50 × 104
1.20 × 104
4.06 × 104
2.63 × 104
1.03 × 104
4.54 × 104
1.02 × 102
2.8 × 102
129.28
183.91
173.42
161.21
172.29
164.56
164.57
173.30
162.88
212.06
185.21
146.48
158.71
7.5 × 104
14.3 × 104
20.0 × 104
7.1 × 104
9.9 × 104
12.6 × 104
8.14 × 104
9.64 × 104
11.0 × 104
7.7 × 104
9.8 × 104
9.5 × 104
15.5 × 104
Zn(II)–Ia (1:2)
Cu(II)–Ib (1:2)
Zn(II)–Ic (1:1)
14.92
48.52
2.26 × 103
8.43 × 104
degradation stages were found to lie in range 0.928–0.998, showing
a good fit with the linear function.
◦
(2) [Zn–2L · 2H2O]150–−3→50 C[ Zn–2L] + 2H2O
3.5.3. The thermal decomposition can be formulated as the
following:
(a) The thermal degradation Ag(I)–Ia complex (1:1)
◦
(3) [Zn–2L]350–−6→00 Corganic part + ZnO
◦
(1) [Ag–L · 2H2O] NO3 · 2H2O30–−3→60 C[Ag–L]NO3 + 4H2O
(d) The thermal degradation Cu(II)–Ib complex (1: 2)
◦
360–750
−
−→ C[Ag–L] + NO3
◦
(1) [Cu–2L2H2O] H2O 40–−1→25 C [Cu–2L · H2O] + 2H2O
(2) [Ag–L · 2H2O] NO3
◦
(3) [Ag–L]+750–950
−→ CAg2O + organic part
◦
(2) [Cu–2L · H2O]125–−2→50 C[Cu–2L] + H2O
(b) The thermal degradation Zn(II)–Ic complex (1:1)
(1)
[Zn–L · 4H2O] CH3COO · H2O
◦
(3) [Cu–2L]250–−5→00 Corganic part + CuO
◦
35–180
−→ C[Zn–L · 3H2O] CH3COO + 2H2O
3.6. Electronic spectra
(2)
[Zn–L · 3H2O] CH3COO
◦
180−300
−→ C[Zn–L · 3H2O] CH3COO + 3H2O
The electronic absorption spectra of the prepared solid com-
plexes are measured in nujol mull and in DMF solution within
the range 29411.46–12,500 cm−1, based on the fact that the elec-
tronic absorption spectra are very diagnostic of the stereochemistry
of complex. The spectra of the free ligands (Ia–d) in DMF indi-
cate that the wave number ꢁ− (cm−1) of the CT band are 21,186,
26,041, 23,547 and 25,687 cm−1 for ligands (Ia–d), respectively
whereas those taken in nujol mull are 24,752, 28,011, 27,472 and
28,818 cm −1 respectively. The value ꢁ− (cm−1) of the CT bands of all
complexes displays a red or blue shift due to substituent effect com-
pared to that of the free ligands. The spectral complexes Ag(I), Cu(II)
and Zn(II) metal ions in comparison to those of the corresponding
free ligands are listed in Table 7.
◦
(3) [Zn–L · 3H2O]+300–−3→50 C[Zn–L]+ + CH3COO−
◦
(4) [Zn–L]+350–600
−→ Corganic part + ZnO
(c) The thermal degradation Zn(II)–Ia complex (1:1)
◦
(1) [Zn–2L] 4H2O40–−1→50 C[ Zn–2L · 2H2O] + 2H2O
Table 7
Electronic spectra of Ag(I), Cu(II) and Zn(II) complexes with 4-(R-phenylazo 2-salicylaldene)-urea azo-azomethine derivative in DMF and nujol mull.
Ligand/complex
Ratio (M:L)
Band Maxima in range (cm−1
DMF
)
Assignment
Nujoll mull
Ia
–
1:1
–
1:2
–
1:1
1:2
21,186, 24,212, 28,222
21,834, 25,232, 29,154
20,234, 22,315, 26,040
21,254, 24,568, 27,454
18,154, 21,232, 23,547
21,881, 22,421, 25,125
24,224, 25,212, 26,354
24,752, 26,224, 29,235
25,773, 27,585, 30,395
19,978, 24,587, 28,011
24,154, 26,464, 29,411
20,289, 25,221, 27,472
21,598, 26,986, 28,254
22,587, 25,462, 27,771
␲–␲*, n–␲*, CT
␲–␲*, n–␲*, CT
␲–␲*, n–␲*, CT
Ag(I)–Ia
Ib
Cu(II)–Ib
Ic
2T2g
→
2Eg
␲–␲*, n–␲*, CT
␲–␲*, n–␲*, CT
␲–␲*, n–␲*, CT
Zn(II)–Ic

1436
I.S. Ahmed et al. / Spectrochimica Acta Part A 78 (2011) 1429–1436
4. Conclusions
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