Synthesis, spectroscopic thermal and biological activity studies on azo-containing Schiff base dye and its Cobalt(II), Chromium(III) and Strontium(II) complexes

Article abstract of DOI:10.1016/j.molstruc.2010.08.025

Schiff base ligand is prepared by condensation of o-amino benzoic acid with 5-phenyl azo-salicyladehyde. The characterization of the ligand is based on elemental analysis, mass spectra and IR spectra. The novel Co(II), Cr(III) and Sr(II) metal complexes are reported and characterized by physico-chemical, spectroscopic methods based on elemental analyses, IR, magnetic moment, and thermal analysis (TGA). The molar conductance data reveals that all the complexes are non-electrolytes. The activation thermodynamic parameters, such as ΔE, ΔH, ΔS and ΔG are calculated from the DTA curves using Coats-Redfern method. Optical absorption measurements show that the fundamental absorption edge obeys Tauc's relation for the allowed non-direct transition. Optical band gap (E_g) values equal 2.4, 2.38 and 1.5 eV for Co(II), Cr(III) and Sr(II) metal complexes respectively. The synthesized ligand and its metal complexes were screened for their biological activity against bacterial species, two Gram positive bacteria (Bacillus subtillis and Staphylococcus aureus) and two Gram negative bacteria (Escherichia coli and Pseudomonas aereuguinosa).

Full text of DOI:10.1016/j.molstruc.2010.08.025

Journal of Molecular Structure 983 (2010) 32–38
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, spectroscopic thermal and biological activity studies on azo-containing
Schiff base dye and its Cobalt(II), Chromium(III) and Strontium(II) complexes
b
c
Samir Alghool a, , Hanan F.Abd El-Halim , A. Dahshan
⇑
a Department of Chemistry, Faculty of Science, Port Said University, Port Said, Egypt
b Department of Chemistry, Faculty of Pharmacy–Pharmaceutical, Misr International University, Km 28, Cairo–Ismailia Road, Egypt
c Department of Physics, Faculty of Science, Port Said University, Port Said, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 June 2010
Received in revised form 12 August 2010
Accepted 13 August 2010
Available online 21 August 2010
Schiff base ligand is prepared by condensation of o-amino benzoic acid with 5-phenyl azo-salicyladehyde.
The characterization of the ligand is based on elemental analysis, mass spectra and IR spectra. The novel
Co(II), Cr(III) and Sr(II) metal complexes are reported and characterized by physico-chemical, spectroscopic
methods based on elemental analyses, IR, magnetic moment, and thermal analysis (TGA). The molar
conductance data reveals that all the complexes are non-electrolytes. The activation thermodynamic
parameters, such as D D D D
Eꢀ, Hꢀ, Sꢀ and Gꢀ are calculated from the DTA curves using Coats–Redfern method.
Keywords:
Azo ligand
Metal complexes
Kinetics
Thermodynamic parameters
Bacterial
Optical absorption measurements show that the fundamental absorption edge obeys Tauc’s relation for the
allowed non-direct transition. Optical band gap (Eg) values equal 2.4, 2.38 and 1.5 eV for Co(II), Cr(III) and
Sr(II) metal complexes respectively. The synthesized ligand and its metal complexes were screened for their
biological activity against bacterial species, two Gram positive bacteria (Bacillus subtillis and Staphylococcus
aureus) and two Gram negative bacteria (Escherichia coli and Pseudomonas aereuguinosa).
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
information storage, photo-switching devices and optical comput-
ing [15–18]. Azo groups are relatively robust and chemically stable,
Salen type ligands, one of the oldest classes of ligands in coordi-
nation chemistry, have been used extensively to complex transi-
tion and main group metals [1]. Schiff base complexes containing
different metal ions such as Ni and Cu have been studied in great
details for their various crystallographic features, structure–redox
relationships and enzymatic reactions, mesogenic characteristics
and catalysis properties [2–6]. Transition metal complexes of Schiff
bases are of paramount scientific interest, due to their multiple
uses as biomimetic systems of enzymes involved in the transport,
storage and activation of dioxygen [7–11]. CuN2O2 coordination is
very common in copper chemistry and the redox behavior of a
wide series of mononuclear copper(II) Schiff base complexes was
reviewed some years ago in relation to their structural changes
[12]. Mn complex of Schiff bases have been used as metal cofactors
for the elaboration of artificial metalloenzymes used as new bio-
catalysts for selective oxidation reactions [13,14].
and therefore they prompted extensive studies of azobenzene-
based structures as dyes and colorants. Furthermore, the light-in-
duced interconversion allows systems incorporating azo group to
be used as photo switches, effecting rapid and reversible control
over a variety of chemical, mechanical, electronic and optical prop-
erties. Because of the good thermal stability of azo compounds, one
of the most important applications of azo compounds is in the opti-
cal data storage [15,19]. In this work new efficient route for the
synthesis of Schiff base ligand and its metal complexes has been
studied. The thermal decomposition of its complexes is also used
to infer the structure and the different thermodynamic activation
parameters are calculated. The biological activity of this ligand
and its metal complexes are evaluated. Optical absorption mea-
surements have been made on Co(II), Cr(III) and Sr(II) metal com-
plexes. Optical absorption measurements are used to obtain the
band structure and the energy gap of a material, because the anal-
ysis of the optical absorption spectra is one of the most productive
tools for understanding and developing the energy band diagram
of both crystalline and amorphous materials.
On the other hand, Azo compounds are important due to their
applications in dyes, pigments and functional materials. For exam-
ple, azo-containing photochromic organic compounds especially
with liquid crystalline character and azo-conjugated metal com-
plexes have been attracting much attention recently because of
their possible applications in the area of photon-mode high density
2. Experimental
⇑
All the used chemicals are of the analytical reagent grade (AR),
and of highest purity available. CoCl2ꢁ4H2O, CrCl3ꢁ6H2O, SrCl2ꢁ6H2O
Corresponding author. Tel.: +20 02 010 6980924.
E-mail address: alghools@yahoo.com (S. Alghool).
0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.08.025

S. Alghool et al. / Journal of Molecular Structure 983 (2010) 32–38
33
and other chemicals were purchased from Fluka and Merck
companies.
(45 °C) nutrient agar (NA). The homogeneous suspensions were
poured into Petri dishes. The discs of filter paper (diameter
4 mm) were ranged on the cool medium. After cooling on the
formed solid medium, 2 ꢂ 10ꢃ5 dm3 of the investigated com-
pounds were applied using a micropipette. After incubation for
24 h in a thermostat at 25–27 °C, the inhibition (sterile) zone diam-
eters (including disc) were measured and expressed in mm. An
inhibition zone diameter over 7 mm indicates that the tested com-
pound is active against the bacteria under investigation. The anti-
bacterial activities of the investigated compounds were tested
against Escherichia coli and Pseudomonas aeruginosa as Gram nega-
tive, Bacillus subtilis and Staphylococcus aureus as Gram positive.
The concentration of each solution was 1.0 ꢂ 10ꢃ3 mol dm3. Com-
mercial DMSO was employed to dissolve the tested samples.
2.1. Synthesis of ligand and its metal complexes
2.1.1. Synthesis of the Schiff base ligand
The Schiff base ligand was synthesized according to the known
condensation method [20]. The methanol solution (50 ml) of o-
amino benzoic acid (6.85 g, 50 mmol) was mixed with a solution
of 5-phenyl azo-salicyladehyde (11.3 g, 50 mmol). The mixture
was refluxed and stirred magnetically for 2 h at 80 °C on a hot
plate. After cooling, the solution of the Schiff base was filtered,
and the solid was washed several times with methanol. All organic
impurities were then extracted by washing with small portions of
diethyl ether. The ligand was dried under vacuum, and was re-
crystallized several times from ethyl alcohol. The purity of ligand
was evaluated by thin layer chromatography. Elemental analysis
CHN, IR, UV–vis, and mass spectra [20] confirm the composition
of the ligand.
3. Results and discussions
Tridentate complexes were obtained upon reaction between
metal ions and (H2L) ligand at 1:1 M ratio. The synthesized Schiff
base ligand and its complexes are very stable at room temperature
in the solid state. The ligand is insoluble in common organic solvents
on cold. The ligand and its metal complexes are generally soluble in
hot DMF and DMSO. The yields, melting/decomposition points,
elemental analyses, magnetic measurements, and molar conduc-
tance of H2L and its metal complexes are presented in Table 1. The
analytical data are in a good agreement with the proposed stoichi-
ometry of the complexes.
The metal-to-ligand ratio in the Co(II), Cr(III) and Sr(II) com-
plexes was found to be 1:1. The ligand decomposed at tempera-
tures higher than 250 °C, while all complexes decomposed at
temperatures higher than 300 °C. The ligand and their metal com-
plexes have dye character due to the high molar extinction con-
stant. Elemental analyses are in good agreement with the
proposed formula.
2.1.2. Synthesis of metal complexes
All complexes were synthesized by adding of the appropriate
metal salts (1.0 mmol, in 20 ml ethyl alcohol–water (1:1) to a hot
solution of the ligand (1.0 mmol, in 30 ml ethyl alcohol (95%))
and few drops of triethylamine. The resulting solutions were stir-
red and heated on a hot plate at 70 °C for 30 min. The volume of
the obtained solution was reduced to one-half by evaporation.
One day later, the colored solid of the complexes formed was fil-
tered, the solids washed with ethanol and diethyl ether, and finally
dried under vacuum. The synthesized complexes were re-crystal-
lized from ethanol–water (1:1). The purity of all complexes was
evaluated by thin layer chromatography. All complexes were pre-
pared by the same method and isolated as powdered material. Ele-
mental analysis (C, H and N), IR, UV–vis, and thermogravimetric
analyses confirm the composition of the complexes.
3.1. Molar conductivity of metal chelates
2.2. Analysis
The metal complexes discussed herein were dissolved in DMF
and the molar conductivities of their 10ꢃ3 M solutions at room
temperature were measured to establish the charge of the metal
complexes. The range of conductance values listed in Table 1 indi-
cates that all the metal complexes have non-electrolyte nature
[22].
These results are confirmed by the chemical analysis (elemental
analysis data) because Clꢃ ions are not precipitated by addition of
AgNO3 solution. This test matches well CHN data. All complexes
did not have electrolytic properties. This fact elucidated that the
Clꢃ ions are absent. Also the molar conductance values indicate
that the anions may be absent or exhibits inside the coordination
sphere as in [Cr(L)ꢁ(2H2O)Cl]ꢁH2O complex, the chloride ion was
detected inside the coordination sphere of the complex by the deg-
radation of the complex using nitric acid and titrated against
AgNO3.
Elemental analyses (C, H and N) were performed using a Perkin-
Elmer CHN 2400 elemental analyzer. The content of metal ions was
calculated gravimetrically as metal oxides. Molar conductance mea-
surements of the ligand and its complexes with 1.0 ꢂ 10ꢃ3 mol/l in
DMSO were carried out using Jenway4010 conductivitymeter. Mag-
netic measurements were carried out on a Sherwood Scientific mag-
netic balance using Gouy method. Electron impact mass spectra
were recorded on a Jeol, JMS, DX-303 mass spectrometer. The UV/
vis spectra were obtained in DMF solution (1.0 ꢂ 10ꢃ3 M) for the
ligand and its metal complexes with a Jenway 6405 spectrophotom-
eter using 1 cm quartz cell, in the range 200–600 nm. IR spectra
(4000–400 cmꢃ1) were recorded as KBr pellets on Bruker FT-IR
spectrophotometer. A double beam (Jasco V-630) spectrophotome-
ter was used to measure the absorption spectra for Co(II), Cr(III) and
Sr(II) metal complexes (using DMF at 25 °C) in the spectral range of
wavelength from 400 to 2500 nm. Thermogravimetric analyses (TG/
DTG) were carried out in the temperature range from 25 to 800 °C in
a steam of nitrogen atmosphere using Shimadzu TGA 50H thermal
analysis. The experimental conditions were: platinum crucible,
nitrogen atmosphere with a 30 ml/min flow rate and a heating rate
10 °C/min.
3.2. UV–vis spectra and magnetic moments
The UV–vis spectra of the ligand and its metal complexes are re-
corded in DMF solution in the wavelength range 200–800 nm. The
electronic spectrum of the Co(II) complex, exhibits an intense band
at 684 nm assignable to the 4A2 ? 4T1(P) transition, indicating tet-
rahedral geometry for this complex. Also the magnetic moment of
4.25 BM is a further indication for the tetrahedral geometry.
The electronic spectrum of the dark brown Cr(III) complex
exhibits weak broad bands at 398 and 576 nm, which have been
assigned to 4T1g(F) 4A2g(F) and 4T2g(F) 4A2g(F) transitions,
2.3. Microbiological investigations
For these investigations the filter paper disc method was ap-
plied according to Gupta et al. [21]. The investigated isolates of
bacteria were seeded in tubes with nutrient broth (NB). The seeded
NB (1 cm3) was homogenized in the tubes with 9 cm3 of melted

34
S. Alghool et al. / Journal of Molecular Structure 983 (2010) 32–38
Table 1
Elemental analyses and physical data of H2L ligand and its Co(II), Cr(II) and Sr(II) complexes.
Compound
Calc./(found) (%)
M. wt
leff (BM)
K
(m X
ꢃ1 molꢃ1 cm2)
C
H
N
M
–
H2L
344.34
69.76
(69.44)
52.76
(52.41)
47.87
(47.68)
49.63
4.10
(4.34)
3.98
(4.11)
4.02
(4.21)
3.75
12.20
(12.54)
9.23
(9.52)
8.37
(8.12)
8.68
(8.82)
–
–
C
20H14N3O3
[Co(L)ꢁH2O]ꢁ2H2O
20H18N3O6 Co
[Cr(L)ꢁ(2H2O)Cl]ꢁH2O
20H20N3O7CrCl
[Sr(L)(H2O)]ꢁ2H2O
20H18N3O6ꢁSr
455.31
501.84
483.99
12.94
(13.25)
10.36
(10.64)
18.10
(19.0)
4.25
3.89
–
16
23
16
C
C
C
(49.15)
(3.38)
respectively, with charge transfer bands. The magnetic behavior of
octahedral Cr(III) is independent of the field strength of the ligand.
The Cr(III) complex has the value of 3.89 BM. Thus, the ligand field
bands and magnetic moment value support an octahedral geome-
try around the metal ion in Cr(III) complex.
3.4. Thermal analyses (TG and DTG)
The thermal analysis was realized using a Shimadzu TGA 50H
instrument using 6.235, 2.887, 3.460 and 11.672 mg samples for
3.3. IR spectra and mode of bonding
The IR spectral bands of the ligand and its complexes are listed
in Table 2. The assignments for most of the major peaks are shown
in Fig. 1. The functional groups of the Schiff base ligand and the
metal complexes have been detected by infrared spectra.
A comparison of the infrared spectra of the ligand and the respec-
tive complexes reveals the absence of absorption bands associated
with the stretching OH of the phenolic group, indicating the loss of
phenolic proton on complexation, forming metal–oxygen bond. This
was further supported by the decrease in (CAO) frequency by ca.
ꢄ30 mꢃ1 [23]. On the other hand the absorption bands associated
with the stretching OH of the carboxylic group disappear in IR spec-
tra of complexes, indicating the loss of proton of carboxylic group,
forming metal–oxygen bonds. When the carboxylic acid group in
the ligand is converted to carboxylate, the bands assigned to asym-
metric vibrations
symmetric vibrations
m
as(OCO) at 1539–1556 cmꢃ1 and the bands of
s(OCO) at 1387–1404 cmꢃ1 appear.
m
The band at ꢄ1620 cmꢃ1 was assigned to the stretching vibra-
tion of the (C@N) group of the ligand H2L. This band is shifted in
the complexes toward lower wave numbers by ca. ꢄ20 cmꢃ1 be-
cause of the coordination of the nitrogen atom to the metal ion.
This fact can be explained by the withdrawing of electrons from
nitrogen atom to the metal ion due to coordination process.
The infrared spectra of the complexes exhibited broad band at
3423–3437 cmꢃ1 that are attributed to OH of the crystal water
molecules, while the IR bands of m(H2O) of coordinated water ap-
peared at 834–842 cmꢃ1, indicating the binding of water molecules
to the metal ions. In the far IR spectra of all the complexes, the non-
ligand bands observed at 518–530, 427–437 cmꢃ1 regions can be
assigned to m(MAO), m(MAN), respectively [24]. Therefore, the IR
spectra reveal that H2L coordinated to the metal ions via phenolic
oxygen, carboxylic oxygen and azomethine nitrogen atom as
tridentate.
Fig. 1. IR spectra of the H2L ligand and its metal complexes.
Table 2
Infrared spectral data of H2L ligand and its Co(II), Cr(II) and Sr(II) complexes (cmꢃ1).
Compounds
m
(OH) hydrated
m(OH) coord.
m
(C@N)
m
(OH)
m
(OH)
m(CAO) phenolic
m(N@N)
m
as(OCO) COOꢃ
m
s(OCO) COOꢃ
(br)
(m)
(s)
carboxylic
phenolic
(m)
(m)
(m)
(m)
H2L
–
–
1620
1601
1598
1603
3422
3354 (br)
1279
1251
1255
1254
1604
1596
1597
1590
–
–
[Co(L)ꢁH2O]ꢁ2H2O
3424
834
841
842
–
–
–
–
–
–
1544
1556
1539
1397
1404
1387
[Cr(L)ꢁ(2H2O)Cl]ꢁH2O 3435
[Sr(L)ꢁH2O]ꢁ2H2O
3437
br = Broad; s = strong; w = week; m = medium.

S. Alghool et al. / Journal of Molecular Structure 983 (2010) 32–38
35
H2L, [Co(L)ꢁH2O]ꢁ2H2O, [Cr(L)ꢁ3H2O]ꢁH2O, [Sr(L)ꢁH2O]ꢁ2H2O com-
plexes, respectively. The heating rate within the temperature range
from 25 °C to 800 °C is 10 °C/min. The TG and DTG curves are rep-
resented in Fig. 2. The weight losses for each chelate were calcu-
lated for the corresponding temperature ranges and are shown in
Table 3. The metal percentages calculated from the metal oxide
3.5
6
L
Cr
3.0
2.5
2.0
1.5
1.0
5
4
3
2
1
0
0
200
400
600
800
0
200
400
600
800
Temperature (C)
Temperature (C)
12
10
8
2.5
2.0
1.5
1.0
0.5
Sr
Co
6
4
2
0
200
400
600
800
0
200
400
600
800
Temperature (C)
Temperature (C)
Fig. 2. TGA diagrams of H2L ligand and its metal complexes.
Table 3
Thermal data of H2L ligand and its Co(II), Cr(II) and Sr(II) complexes.
Complexes
Temperature range (°C)
DTG peak (°C)
TG weight
Assignments
loss% calc./found
H2L
25–390
390–600
25–140
140–600
600–
300
500
75
39.20(39.61)
60.80(60.39)
7.90(7.51)
C7H4O2N
13H10ON2
2H2O
C
[Co(L)ꢁH2O]ꢁ2H2O
505
3.95(4.12)
H2O
56.75(56.50)
31.40(31.87)
3.58(3.27)
14.24(14.63)
55.09(54.01)
27.09(28.00)
3.71(3.57)
7.43(7.49)
67.46(67.11)
21.40(21.83)
Organic moiety
1/2Co2O3 + 5C
H2O
2H2O + Cl
Organic moiety
1/2Cr2O3 + 5C
2H2O
H2O
Organic moiety
SrO
[Cr(L)ꢁ(2H2O)Cl]ꢁH2O
[Sr(L)ꢁH2O]ꢁ2H2O
25–140
140–350
350–600
600–
25–140
140–220
220–600
600–
80
180
525
80
150
430
H2O
Cl
Cr
N
O
O
O
O
O
O
M
N
H2O
OH2
.H2O
.2H2O
N
N
C
H
N
N
C
H
Where M=Co or Sr
Scheme 1. The proposed structures of metal complexes.

36
S. Alghool et al. / Journal of Molecular Structure 983 (2010) 32–38
residues were compared with those determined by the analytical
metal content determination [25]. The hydrated water molecules
are associated with the complex formation and found outside the
coordination sphere formed around the central metal ion. The
dehydration of this type of water takes place in the temperature
range 25–150 °C, the weight loss corresponds to one or two water
molecules. On the other hand, the coordinated water molecules are
eliminated at higher temperatures than the water molecules of
hydration. The water of coordination is usually eliminated [30] in
the temperature range 120–250 °C. The organic part of the com-
plexes may decompose in one or more steps with the possibility
of the formation of one or two intermediates. These intermediates
may include the metal ion with a part of the Schiff base complexes.
These intermediates may finally decompose to stable metal oxides.
The decomposition of all complexes ended with the stoichiometric
oxide formation and carbon residue. The metal content percent-
ages of the complexes are thus calculated and compared with
those obtained from the metal content analytical determination
[26]. The average metal percentages were found to be in good
agreement with those calculated from the tentative formulae
based on the elemental analyses. The initial decomposition and
inflection temperatures have been used as an indication on the
thermal stability of complexes [27].
The proposed structures of the synthesized complexes which
are presented in Scheme 1 are consistent with each other in term
of chemical, spectroscopic data and thermal analysis.
3.5. Kinetic studies
Coats–Redfern [28] is the method mentioned in the literature
related to decomposition kinetics studies; this method is applied
in this study. From the TG curves, the activation energy, E, pre-
exponential factor, A, entropies,
DS, enthalpy, DH, and Gibbs free
energy, G, were calculated by Coats–Redfern method; where
D
D
H ¼ E ꢃ RT and G ¼ H ꢃ T
D
D
DS
The linearization curves of Coats–Redfern method is shown in
Fig. 3. Kinetic parameters are calculated by employing the Coats–
Redfern equations, are summarized in Table 4. The Coats–Redfern
equation [28] may be written in the form:
ꢀ
ꢁ
ꢀ
ꢂ
ꢃꢁ
ðwf =ðwf ꢃ wÞÞ
AR
hEa
2RT
Ea
Ea
2:303RT
log
¼ log
1 ꢃ
ꢃ
ð1Þ
T2
Fig. 3. Coats–Redfern (CR) of third step of the H2L ligand and its metal complexes.
Table 4
Thermodynamic data of the thermal decomposition of H2L ligand and its Co(II), Cr(II) and Sr(II) complexes.
Complex
Parameter
r
E (J molꢃ1
)
A (sꢃ1
)
D
S (J molꢃ1 Kꢃ1
)
D
H (J molꢃ1
)
D )
G (J molꢃ1
H2L
120
92.8
372
179
1.60 ꢂ 106
43.3
ꢃ131
ꢃ220
287
ꢃ131
ꢃ4.25 ꢂ 103
ꢃ5.06 ꢂ 103
ꢃ4.56 ꢂ 103
ꢃ6.25 ꢂ 103
6.45 ꢂ 104
0.99401
0.99407
0.99650
0.99681
[Co(L)ꢁH2O]ꢁ2H2O
[Cr(L)ꢁ(2H2O)Cl]ꢁH2O
[Sr(L)ꢁH2O]ꢁ2H2O
1.31 ꢂ 105
1.14 ꢂ 1028
ꢃ1.75 ꢂ 105
9.47 ꢂ 104
2.34 ꢂ 106

S. Alghool et al. / Journal of Molecular Structure 983 (2010) 32–38
37
Table 5
where Wf is the mass loss at the completion of the reaction, W the
mass loss up to temperature T (Wr = Wf ꢃ W), R the gas constant, Ea
the activation energy in J molꢃ1 and h is the heating rate. Since 1–2
RT/Eaꢄ = 1, a plot of the left-hand side of the above equation against
1/T was constructed (Fig. 3) and Eꢀ was calculated from the slope
and A (Arrhenius constant) was found from the intercept. The acti-
vation entropy Sꢀ, the activation enthalpy Hꢀ and the free energy of
activation Gꢀ were calculated using the following equations:
Antibacterial activity data of H2L ligand and its Co(II), Cr(II) and Sr(II) complexes
inhibition zone (mm).
Compound
Bacillus
Staphylococcus Escherichia Pseudomonas
subtilis G+ aureus G+
coli Gꢃ
aeruginosa Gꢃ
H2L
20
22
17
20
22
20
19
22
24
16
16
18
22
21
[Co(L)ꢁH2O]ꢁ2H2O
[Cr(L)ꢁ(2H2O)Cl]ꢁH2O 24
[Sr(L)ꢁH2O]ꢁ2H2O
20
ꢂ
ꢃ
Ah
KT
Sꢀ ¼ 2:303 log
R; Hꢀ ¼ Eꢀ ꢃ RT; Gꢀ ¼ Hꢀ ꢃ TSSꢀ
ð2Þ
Key to interpretation: less than 10 mm = inactive, 10–15 mm = weakly active,
15–20 mm = moderately active; more than 20 mm = highly active.
where K and h are the Boltzman’s and Plank’s constants, respec-
tively. The calculated values of
D
Eꢀ, Sꢀ, Hꢀ and Gꢀ for the dehy-
D
D
D
coefficient (
tra using the relation:
a) can be calculated from the optical absorption spec-
dration and the decomposition steps are given in Table 4. The
activation energies of the decomposition were found to be in the
range 134–208 J molꢃ1
.
a
¼ 1=d ln A
ð3Þ
According to the kinetic data obtained from DTG curves, the
negative values of activation entropies
activated complex than the reactants and/or the reactions are slow
[29].
Sꢀ indicate a more ordered
where d is the width of the cell. According to Tauc’s relation [30,31]
for allowed non-direct transitions, the photon energy dependence
of the absorption coefficient can be described by:
D
1=2
ða
hm
Þ
¼ Bðh
m
ꢃ EgÞ
ð4Þ
3.6. Optical band gap
where B is a parameter that depends on the transition probability
and Eg is the optical energy gap. Fig. 5 shows the absorption coeffi-
The absorption spectra for Co(II), Cr(III) and Sr(II) complexes, re-
corded using DMF at 25 °C, are shown in Fig. 4. The absorption
cient in the form of (ahm) m for the Co(II), Cr(III) and Sr(II)
1/2 versus h
complexes. The intercepts of the straight lines with the photon en-
ergy axis yield values of the optical band gap.
4
From Fig. 5, it is clear that the values of Eg equal 2.4, 2.38 and
1.5 eV for Co(II), Cr(III) and Sr(II) metal complexes respectively.
There is no trend between the atomic numbers of the central atoms
and Eg values. However, it is clear that Eg depends on the stereo-
chemistry of the complexes. The band gap values reveal that these
complexes are semi-conductors. Also, the values of Eg are in the
range reported for the high efficient photovoltaic materials. So,
the present compounds could be considered potential materials
for harvesting solar radiation in solar cell applications [32].
Ligand
Co
Cr
3
2
1
0
Sr
3.7. Microbiological investigation
The biological activity of H2L ligand and its metal complexes
were tested against bacteria; we used more than one test organism
to increase the chance of detecting antibiotic principles in tested
materials. The organisms used in the present investigations in-
cluded two Gram positive bacteria (Bacillus subtillis and Staphylo-
coccus aureus) and two Gram negative bacteria (Escherichia coli
and Pseudomonas aereuguinosa). The results of the bactericidal
screening of the synthesized compounds are recorded in Table 5.
An influence of the central ion of the complexes in the antibacterial
activity against the tested Gram positive and Gram negative organ-
isms show that the complexes have an enhanced activity compared
to the ligand itself.
400
600
800
1000
Wavelength (nm)
Fig. 4. The optical absorption spectra of the H2L ligand and its metal complexes.
2.0
Ligand
Co
Cr
Sr
1.5
1.0
0.5
0.0
4. Conclusion
The coordination ability of 5-phenyl azo-salicyladehyde has
been proved in complexation reaction with Co(II), Cr(III) and Sr(II)
ions. The elemental analysis and (TGA) confirmed the compositions
of the compounds. IR, UV–vis spectra and magnetic measurements
of the ligand and its metal complexes confirmed the suggested
coordination of the ligand through phenolic oxygen, oxygen of
OH carboxylic group and nitrogen of the azomethine group as tri-
dentate. Under experimental conditions employed, only 1:1 (M:L)
complexes have been found. Allowed non-direct electronic transi-
tions are mainly responsible for the photon absorption in the
investigated complexes.
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
hν (eV)
1/2
Fig. 5. The dependence of (
a
h
m)
on photon energy (hm) of the H2L ligand and its
metal complexes from which the optical band gap (Eg) is estimated (Tauc
extrapolation).

38
S. Alghool et al. / Journal of Molecular Structure 983 (2010) 32–38
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