Transition metal complexes of heterocyclic Schiff base: Biological activity, spectroscopic and thermal characterization

Article abstract of DOI:10.1007/s10973-006-7095-3

Metal complexes of Schiff base derived from 2-furancarboxaldehyde and 2-aminobenzoic acid (HL) are reported and characterized based on elemental analyses, IR, ¹H NMR, UV-Vis, solid reflectance, magnetic moment, molar conductance and thermal ana

Full text of DOI:10.1007/s10973-006-7095-3

Journal of Thermal Analysis and Calorimetry, Vol. 86 (2006) 2, 315–325
TRANSITION METAL COMPLEXES OF HETEROCYCLIC SCHIFF BASE
Biological activity, spectroscopic and thermal characterization
M. M. Omar*, G. G. Mohamed and A. M. M. Hindy
Chemistry Department, Faculty of Science, Cairo University, Giza, Egypt
Metal complexes of Schiff base derived from 2-furancarboxaldehyde and 2-aminobenzoic acid (HL) are reported and characterized
based on elemental analyses, IR, 1H NMR, UV-Vis, solid reflectance, magnetic moment, molar conductance and thermal analysis.
The ligand dissociation as well as the metal-ligand stability constants have been calculated pH-metrically at 25°C and ionic strength
m=0.1 (1 M NaCl). The complexes are found to have the formulae [M(HL)2](X)n·yH2O (where M=Fe(III) (X=Cl, n=3, y=4), Co(II)
(X=Cl, n=y=2), Ni(II) (X=Cl, n=y=2), Cu(II) (X=Cl, n=y=2) and Zn(II) (X=AcO, n=y=2)) and [UO2(L)2]·2H2O. The thermal behav-
iour of these chelates is studied and the activation thermodynamic parameters are calculated using Coats-Redfern method. The
ligand and its metal complexes show a biological activity against some bacterial species.
Keywords: biological activity, metal complexes, spectroscopic studies, stability constants, thermal analysis
Synthesis of Schiff base (HL)
Introduction
Hot solution (60°C) of 2-aminobenzoic acid (2.74 g,
10 mmol) in 25 mL ethanol was mixed with hot solu-
tion (60°C) of 2-furancarboxaldehyde (1.92 g,
10 mmol) in the same solvent and the reaction mix-
ture was left under reflux for 2 h. The formed solid
products were separated by filtration, purified by
crystallization from ethanol, washed with diethyl
ether and dried in a vacuum over anhydrous calcium
chloride. The yellow Schiff base product; HL, is pro-
duced in 80% yield.
Many Schiff bases and their complexes have been
widely studied because of their industrial applications
[1]. Some Schiff bases are tested for fungicidal activity,
which is related to their chemical structure [2]. Shiff
base compounds containing an imine group (–RC=N–),
are usually formed by the condensation of a primary
amine with an active carbonyl. The interest in Schiff
base compounds as analytical reagents is increasing,
since they enable simple and unexpensive determina-
tions of different organic and inorganic substances [3].
The present study describes the chelation behav-
iour of Schiff base derived from the condensation of
anthranilc acid with 2-furan carboxaldehyde (HL) to-
wards some d-block elements, which may help in
more understanding of the mode of chelation of HL
towards metals. For this purpose the complexes of
Fe(III), Co(II), Ni(II), Cu(II), Zn(II), and UO2(II) with
HL are studied in solution and in the solid state. The
stability constants are evaluated and structure of the
studied complexes is elucidated using IR, magnetic,
Synthesis of metal complexes
Hot solution (60°C) of the appropriate metal chloride,
nitrate or acetate (1 mmol) in an ethanol-water mixture
(1:1, 25 mL) was added to the hot solution (60°C) of the
HL Schiff base (0.215 g, 2 mmol) in the same solvent
(25 mL). The resulting mixture was stirred under reflux
for one hour whereupon the complexes precipitated.
They were collected by filtration, washed with a 1:1 eth-
anol:water mixture and diethyl ether. The analytical data
for C, H and N were repeated twice.
1
UV-vis diffuse reflectance, mass spectral, and H
NMR measurements, and thermogravimetric analysis.
Biological activity
0.5 mL spore suspension (10–6–10–7 spore mL–1) of
each of the investigated organisms was added to a
sterile agar medium just before solidification, then
poured into sterile petri dishes (9 cm in diameter) and
left to solidify. Using sterile cork borer (6 mm in di-
ameter), three holes (wells) were made in each dish,
Experimental
Materials
All chemicals used were of the analytical reagent
grade (AR). Metal solutions were standardized by
recommended procedures [4].
*
Author for correspondence: mmomar27@yahoo.com
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2006 Akadémiai Kiadó, Budapest

OMAR et al.
then 0.1 mL of the tested compounds dissolved in
ity was measured on powdered samples using the Far-
aday method [5]. The diamagnetic corrections were
made by Pascal’s constant and Hg[Co(SCN)4] was
used as a calibrant.
The thermogravimetric analysis (TG and DTG)
was carried out in dynamic nitrogen atmosphere (20
mL min–1) with a heating rate of 10°C min-1 using
Shimadzu TGA-50 H thermal analyzer. The mass
spectra were recorded by the EI technique at 70 eV
using MS-5988 GS-MS Hewlett-Packard instrument
in the Microanalytical Center, Cairo University. The
1H NMR spectra were recorded on a JEOL FX90 Q
NMR Spectrometer at 25 C.
DMF (100 mg mL–1) were poured into these holes. Fi-
nally, the dishes were incubated at 37°C for 48 h
where clear or inhibition zones were detected around
each hole. 0.1 mL DMF alone was used as a control
under the same condition for each organism and by
subtracting the diameter of inhibition zone resulting
with DMF from that obtained in each case, both anti-
bacterial activities can be calculated as a mean of
three replicates [7].
Instrumental methods
The spectrophotometric measurements in solution
were carried out using automated spectrophotometer
UV-vis Perkin-Elmer Model Lambda 20 ranged from
200 to 900 nm. pH measurements were carried out us-
ing 716 DMS Titrino Metrohm connected with 728
Metrohm Stirrer. The molar conductance of solid
complexes in DMF was measured using
Sybron-Barnstead conductometer (Meter-PM.6,
E=3406). Elemental microanalyses of the ligand and
the separated solid chelates for C, H, N and S were
performed in the Microanalytical Center, Cairo Uni-
versity using Perkin-Elmer 2400 CHN Elemental An-
alyzer. The PE 2400 CHN uses a combustion method
to convert the sample elements to simple gases (CO2,
H2O and N2), which are measured as a function of
thermal conductivity. Metal (II) contents were deter-
mined by standard complexometric titration [4]. In-
frared spectra were recorded on a Perkin-Elmer FT-IR
type 1650 spectrophotometer in the wavenumber re-
gion 4000–200 cm–1. The UV-Vis diffuse reflectance
spectra were measured on a Shimadzu 3101 pc
spectrophotometer. The molar magnetic susceptibil-
The potentiometric measurements were carried
out at 25°C and ionic strength m=0.1 by addition of
appropriate amounts of 1 M sodium chloride solution.
The ionization constants of the Schiff base (HL) and
the stability constants of its metal chelates with
Fe(III), Co(II), Ni(II), Cu(II), Zn(II) and UO2(II) ions
were determined potentiometrically using the tech-
nique of Sarin and Munshi [6].
Results and discussion
The results of elemental analyses (C, H, N) with molec-
ular formula and the melting point of the Schiff base;
HL, are presented in Table 1. The results obtained are in
good agreement with that calculated for the suggested
formula and the melting point is sharp indicating the pu-
rity of the prepared Schiff base. The structure of the
Schiff base under study is given in Fig. 1.
The mass spectrum of HL and the possible mo-
lecular ion peaks with their respective relative inten-
sities are shown in Scheme 1. The different pathways
Table 1 Analytical and physical data of HL and its complexes
Found (calcd)/%
Colour/
(yield%)
Lm/
m
eff (B.M.)
Compound
M.p./°C
180±2
>300
W–1 mol–1 cm2
C
H
N
M
–
HL
C12H9NO3
yellow
(80)
66.73
(66.98)
4.53 6.32
(4.19) (6.51)
–
–
[Fe(HL)2]Cl3·4H2O
C24H26Cl3FeN2O10
Black
(68)
43.75
(43.34)
3.68 4.05
8.12
(8.43)
5.82
332
(3.91) (4.21)
3.43 4.83
(3.69) (4.70)
[Co(HL)2]Cl2·2H2O
C24H22Cl2CoN2O8
Brown
(63)
>300
48.48
(48.32)
9.57
(9.90)
5.73
4.02
205
215
[Ni(HL)2]Cl2·2H2O
C24H22Cl2N2NiO8
Brown
(73)
>300
48.6
(48.32)
3.39 4.83
10.05
(9.90)
(3.69) (4.70)
3.90 4.42
(3.66) (4.66) (10.57)
[Cu(HL)2]Cl2·2H2O
C24H22Cl2CuN2O8
Deep green
(65)
>300
47.72
(47.96)
10.33
1.93
200
[Zn(HL)2](AcO)2·2H2O
C28H28N2O12Zn
Brown
(72)
>300
51.96
(51.96)
3.54 4.74 9.65
Diam.
Diam.
178
(3.39) (4.31) (10.01)
2.50 3.57
(2.30) (3.81)
[UO2(L)2]·2H2O
C24H20N2O7U
Brown
(77)
>300
38.84
(39.24)
32.50
–
316
J. Therm. Anal. Cal., 86, 2006

TRANSITION METAL COMPLEXES OF HETEROCYCLIC SCHIFF BASE
base has a pKa value of 6.6 which can be attributed to
the deprotonation of the carboxylate group. The free
energy change, DG°, was also calculated and found to
be 37.50 kJ mole–1. The positive value indicates the
nonspontaneous character of dissociation reaction.
The stability constants of the Fe(III), Co(II),
Ni(II), Cu(II), Zn(II) and UO2(II) complexes with HL
are determined potentiometrically using the method
described by Sarin [6] and Bjerrum [9]. The mean
logb1 and logb2 values for the complexes are listed in
Table 2. Three methods are applied for computing
successive stability constants [10] namely Interpola-
tion at half values, Correction-term and Mid-point.
The complex-forming abilities of the transition metal
ions are frequently characterized by stability orders.
The order of stability constants was found to be Co2+<
Ni2+<Cu2+>Zn2+ in accordance with the Irving and
Williams order [11] for divalent metal ions of the 3d
series. It is clear from Table 2 that the stability of
Cu(II) complex is considerably larger as compared to
other metals of the 3d series. Under the influence of
the ligand field, Cu(II) (3d 9) will receive some extra
stabilization [12] due to tetragonal distortion of octa-
hedral symmetry in their complexes. The Cu(II) com-
plex will be further stabilized due to the Jahn-Teller
effect [12]. The free energy of formation, DG°, ac-
Fig. 1 Structure of Schiff base
of the fragments of the parent molecular ion peaks are
also given in Scheme 1. Fragments at m/z = 119
(R. I=100%, base peak) may be due to C7H4NO ion.
The other molecular ion peaks appeared in the mass
spectrum (abundance range from 2–100%) are attrib-
uted to the fragmentation of HL molecule obtained
from the rupture of different bonds inside the mole-
cule as shown below in Scheme 1.
UV-Vis spectra for 2.5·10–3 M solution of HL in
absolute ethanol show three bands (Fig. 2) at 222, 229
and 240 nm beside a broad band at 331 nm with molar
absorptivities (e) 4.25·103, 4.15·103, 4.30·103 and
0.10·103 L mol–1 cm–1, respectively.
Ligand dissociation and metal-ligand stability
constants
The ligand dissociation constant was determined us-
ing Irving and Rossotti [8]. It is found that HL Schiff
Scheme 1 Mass fragmentation pattern of HL
J. Therm. Anal. Cal., 86, 2006
317

OMAR et al.
Table 2 logb1 and logb2 values for complexes of HL with Fe(III), UO2(II), Co(II), Ni(II), Cu(II) and Zn(II)
logb1
logb2
–DG°/
–DG°/
Ion
kJ mol–1
kJ mol–1
A
B
M
A
B
C
M
Fe(III)
UO2(II)
Ci(II)
7.665
7.651
7.535
7.600
7.700
7.645
7.670
7.630
7.440
7.575
7.806
7.610
6.995
7.065
7.075
7.175
7.215
7.190
39.89
40.30
40.35
40.92
41.15
41.02
14.660
14.051
13.935
14.050
14.225
14.145
14.650
14.150
14.065
14.250
14.460
14.285
13.99
14.13
14.15
14.35
14.43
14.38
14.43
14.11
14.05
14.21
14.37
14.27
82.29
80.47
80.12
81.04
81.96
81.39
Ni(II)
Cu(II)
Zn(II)
where A – interpolation at half values method; B – correction-term method; C – midpoint method; M – mean
Molar conductivity measurements
The chelates were dissolved in DMF and the molar
conductivities of 10–3 M of their solutions at 25 2°C
were measured. Table 1 shows the molar conductance
values of the complexes. It is concluded from the re-
sults that Fe(III) chelate is found to have molar con-
ductance value of 332 W–1 mol–1 cm2, indicating the
ionic nature of this complex. It, also, indicates the
non-bonding of the chloride anions to the Fe(III). So,
the Fe(III) chelate is considered as 3:1 electrolyte. On
the other hand, the molar conductivity values of
Zn(II), Cu(II), Co(II) and Ni(II) chelates are found to
be in the range 178–215 W–1 mol –1 cm2. It is obvious
from these data that these chelates are ionic in nature
and they are of the type 2:1 electrolytes.While
UO2(II) complex has Lm=32.50 W–1 mol–1 cm2, which
indicates the non-ionic nature of this complex, it is
considered a non-electrolyte.
Fig. 2 Electronic absorption spectra of 2.5·10–3 M HL absolute
ethanol
companying the complexation reaction has been de-
termined at 25°C. From Table 2, it is apparent that the
negative values of DG° show that the driving ten-
dency of the complexation reaction is from left to
right and the reaction proceeds spontaneously.
IR spectra and mode of bonding
The data of the IR spectra of Schiff base ligand (HL)
and its complexes are listed in Table 3. The IR spectra
of the complexes are compared with those of the free
ligand in order to determine the coordination sites that
may get involved in chelation. Upon comparison it
was found that the n(C=N) stretching vibration is found
in the free ligand at 1663 cm–1. This band is shifted to
higher (21 cm–1) or lower (7–49 cm–1) wavenumbers
in the complexes indicating the participation of the
azomethine nitrogen in coordination (M—N) [13].
The n(OH), n(C=O), nasym(COO) and nsym(COO) stretching vi-
brations are observed at 3319, 1731, 1579 and 1441
cm–1 for HL. The participation of the carboxylate O
atom in the complexes formation was evidenced from
the disappearance or shift in position of these bands to
3275–3309, 1703–1708, 1580–1603 and 1381–1407
in the metal complexes [14]. A medium to sharp band
due to n(C–O–C) stretching vibration of furan appeared
at 1252 cm–1 for HL ligand [15]. This band is shifted
to 1233–1293 cm–1 in the metal complexes [15].
These shifts refer to the coordination through furan O
Composition and structures of Schiff base complexes
The solid complexes under study were subjected to ele-
mental analyses (C, H, N), IR, 1H NMR, magnetic stud-
ies, molar conductance, mass spectra and thermal analy-
sis (TG), to identify their tentative formulae in a trial to
elucidate their molecular structures. The results of ele-
mental analyses; Table 1, are in good agreement with
those required by the proposed formulae. The formation
of these complexes may proceed according to the fol-
lowing equations given below.
MXn+2HL+yH2O®[M(HL)2](X)n×yH2O
M=Fe(III), Co(II), Ni(II), Cu(II), X=Cl, y=2–4
M=Zn(II), X=AcO, n=y=2
UO2(NO3)2+2HL+2H2O®[UO2(L)2]×2H2O+2HNO3
318
J. Therm. Anal. Cal., 86, 2006

TRANSITION METAL COMPLEXES OF HETEROCYCLIC SCHIFF BASE
J. Therm. Anal. Cal., 86, 2006
319

OMAR et al.
atom. New bands are found in the spectra of the com-
trum shows also a band at 25,510 cm–1 which may be
attributed to ligand to metal charge transfer. The
Ni(II) complex has a room temperature magnetic mo-
ment value of 3.02 B.M.; which is in the normal range
observed for octahedral Ni(II) complexes
(meff =2.9–3.3 B.M) [16]. This indicates that, the
Ni(II) complex is six-coordinate and probably octahe-
dral [13, 16]. Its electronic spectrum shows in addi-
tion the p–p* and n–p* bands of free ligand, display
three bands, in the solid reflectance spectrum at
plexes in the regions 463–520 (furan O) and 563–567
(carboxylate O), which are assigned to n(M–O)
stretching vibrations for metal complexes [14, 15].
The bands at 424–468 in metal complexes, have been
assigned to n(M–N) mode [14, 15].
Therefore, it is concluded that HL Schiff base
behaves as a neutral terdentate ligand in all the com-
plexes except UO2(II) complex, it acts as a uni-nega-
tively coordinated to the UO2(II) ion via its
deprotonated carboxylated O beside the azomethine
N and furan O.
3
3
n1: 13,987 cm–1: A2g®3T2g; n2: 16,155 cm–1: A2g®
3
3
3T1g(F) and n3: 22,522 cm–1 : A2g ® T1g(P). The
spectrum also shows a band at 27,173 cm–1 which
may be attributed to ligand to metal charge transfer.
The electronic spectrum of the Co(II) complex gives
three bands at 13,157, 16,611 and 22,123 cm–1. The
bands observed are assigned to the transitions 4T1g (F)
1H NMR spectra
The chemical shifts of the different types of protons in
the 1H NMR spectra of the Schiff base and its diamag-
netic Zn(II) complex are listed in Table 4. Upon ex-
amination it was found that the COOH signal is found
at 9.62 ppm in the spectra of HL ligand. This signal is
deshielded at 8.25 ppm in case of Zn(II)-HL complex
indicating the participation of the COOH group in
chelation without proton displacement. Coordination
through the lone pair of electrons on the O atom is ex-
pected. New signals are observed at 2.89 ppm for
Zn(II) complex with HL, with an integration corre-
sponding to six protons. The signals are assigned to
two acetate molecules. Also the signals observed at
3.31 ppm with an integration corresponding to four
protons in case of Zn(II) complex with HL ligand, are
assigned to two water molecules.
4
4
® 4T2g (F) (n1), T1g (F) ® 4A2g (F) (n2) and T1g (F)
® 4T2g (P) (n3), respectively, suggesting that there is
an octahedral geometry around Co(II) ion
[13, 14, 17]. The spectrum shows also a band at
27,027 cm–1, which may be attributed to ligand to
metal charge transfer. The magnetic susceptibility
measurement, found to be 5.13 B.M. (normal range
for octahedral Co(II) complexes is 4.3–5.2 B.M.), is
indicative of octahedral geometry [17]. The
reflectance spectrum of Cu(II) chelate consists of a
broad, low intensity shoulder band centered at 16,420
2
and 22,026 cm–1. The 2Eg and T2g states of the octa-
hedral Cu(II) ion (d9) split under the influence of the
tetragonal distortion and the distortion can therefore
2
be such as to facilitate the three transitions B1g
®
2B2g ; 2B1g ® 2Eg and 2B1g ® 2A1g which remain unre-
solved in the spectra [17, 18]. It is concluded that, all
three transitions lie within the single broad envelope
observed at the same range previously mentioned
[17]. The magnetic moment of 1.93 B.M. falls within
the range normally observed for octahedral Cu(II)
complexes [17, 18]. A moderately intense peak ob-
served in the range 26,455 cm–1 is due to ligand –
metal charge transfer transition.
Magnetic susceptibility and electronic spectra
measurements
From the diffuse reflectance spectrum, it is observed
that, the Fe(III) chelate exhibits a band at 22,026
6
cm–1, which may be assigned to the A1g® T2g (G)
transition in octahedral geometry of the complexes
[9, 12]. The 6A1g® 5T1g transition appears to be split
into
two
bands
at
16,313–17,636
and
12,626–13,020 cm–1. The observed magnetic moment
of Fe(III) complex is 5.82 B.M. Thus, the complex
formed has the octahedral geometry [14]. The spec-
The complexes of Zn(II) and UO2(II) are dia-
magnetic. In analogy with those described for Zn(II)
complexes containing N–O donor Schiff bases
Table 4 1H NMR spectral data of the Schiff base and its Zn(II) chelate
Compound
HL
Chemical shift (d)/ppm
Assignment
9.62
6.46–8.31
2.6
(s, 1H, COOH)
(m, 8H, 4ArH, 1 azomethine, 3 furan H
(CH3 of solvent)
(s, 2H, COOH)
[Zn(HL)](AcO)2·2H2O
8.25
7.96
6.45–7.77
3.31
2.89
(s, 2H, azomethine H)
(m, 14H, 8ArH, 6furan H)
(s, 4H, 2H2O)
(s, 6H, CH3COO)
(CH3 of solvent)
2.45
320
J. Therm. Anal. Cal., 86, 2006

TRANSITION METAL COMPLEXES OF HETEROCYCLIC SCHIFF BASE
[19–21] and according to the empirical formulae of
these complexes, we propose an octahedral geometry
for the Zn(II) and UO2(II) complexes.
Thermal analyses (TG)
The thermal decomposition of the complexes is used to
obtain information about their properties as well as
about the nature of intermediate and final products [22].
The TG curve of Schiff base HL (Fig. 3A), refers
to two mass loss steps at 180–750°C. These steps in-
volve mass losses of 100.0% (found 100.0%) decom-
position. These mass losses may be due to the succes-
sive losses of C12H9NO3 at the given temperature
ranges, respectively. The curves of Fe(III) chelate
(Fig.3B) shows three decomposition steps within the
temperature range 25–620°C. The first two steps of
decomposition within the temperature range
25–280°C correspond to the loss of water molecules
of hydration, 3HCl, ½H2 and O2 gases with a mass
loss of 26.60% (calcd. 26.86%). The energies of acti-
vation were 25.97 and 64.87 kJ mol–1 for the first and
second step, respectively. The subsequent step
(280–620°C) corresponds to the removal of the or-
ganic part of the ligand leaving metal oxide as a resi-
due with an activation energy of 99.05 kJ mol–1. The
overall mass loss amounts to 88.57% (calcd. 87.96%).
The TG curve of the Ni(II) chelate (Fig. 3C)
shows three steps of decomposition within the tem-
perature range of 30–900°C. The first steps at
30–120°C corresponds to the loss of water molecules
of hydration with an activation energy of
45.45 kJ mol–1. While the subsequent (2nd and 3rd)
steps involve the loss of 2HCl and ligand molecules.
The energies of activation were 53.97 and
195.60 kJ mol–1 for the second and third steps, respec-
tively. The overall mass loss amounts to 87.45%
(calcd. 87.42%). On the other hand, [Cu(HL)2]Cl2·
2H2O chelate exhibits three decomposition steps. The
first step in the temperature range 30–110°C (mass
loss = 6.56%; calcd. for 2H2O; 5.99%) may account
for the loss of water molecules of hydration. The acti-
vation energy of this dehydration step is
66.86 kJ mol–1. The mass losses of the remaining de-
composition steps amount to 80.79% (calcd.
80.77%). They correspond to the removal of 2HCl
and HL molecules leaving CuO as a residue with en-
ergies of activation amount to 155.7 and
254.3 kJ mol–1 for the second and third steps, respec-
tively. The TG curve of the UO2(II) chelate(Fig. 3E)
represents three decomposition steps. The first step of
decomposition within the temperature range
30–120 °C corresponds to the loss of water molecules
of hydration with a mass loss of 4.65% (calcd. for
2H2O; 4.90%) and an activation energy of
Fig. 3 Thermal analysis of HL and its complexes; where
A – ligand HL, B – Fe(III), C – NI(II), D – Cu(II),
E – UO2(II)
56.59 kJ mol–1. The remaining steps of decomposition
within the temperature range 120–650°C correspond
to the removal of HL ligand as gases. The energies of
activation were 168.7 and 242.6 kJ mol–1 for the sec-
ond and third steps; respectively. The overall mass
losses amount to 63.18% (calcd. 63.21%). The Data
are listed in Table 5.
J. Therm. Anal. Cal., 86, 2006
321

OMAR et al.
322
J. Therm. Anal. Cal., 86, 2006

TRANSITION METAL COMPLEXES OF HETEROCYCLIC SCHIFF BASE
Kinetic studies
ions were confirmed by the elemental analyses, IR,
NMR, molar conductance, magnetic, solid reflec-
tance, UV-Vis, mass and thermal analysis data.
Therefore, from the IR spectra, it is concluded that
HL ligand behaves as a neutral terdentate ligand in all
the complexes except UO2(II) complex, it acts as a
uni-negatively coordinated to the UO2(II) ion via its
deprotonated carbxylated O beside the azomethine N
and furan O. From the molar conductance data, it is
found that the Fe(III) chelate is considered as 3:1
electrolyte, Co(II), Ni(II), Cu(II) and Zn(II) chelates
are of the type 2:1 electrolytes, while UO2(II) com-
The thermodynamic activation parameters of decom-
position processes of dehydrated complexes namely
activation energy (E*), enthalpy (DH*), entropy (DS*)
and Gibbs free energy change (DG*) of the decompo-
sition were evaluated graphically employing the
Coats-Redfern relation [23]:
log{W /(W -W)}
é
ù
f
f
log
=
T 2
ê
ú
ë
û
(1)
é AR
2RT ù
E*
æ
ç
ö
log
1-
-
÷
ê
ú
1
qE*
E*
2.303RT
plex is considered as nonelectrolyte. The H NMR
è
ø
û
ë
spectra of the free ligand and its diamagnetic Zn(II)
complex show that the COOH signal of HL ligand
participate in chelation without proton displacement.
Coordination through the lone pair of electrons on the
O atom is expected. On the basis of the above obser-
vations and from the magnetic and solid diffuse
reflectance measurements, octahedral geometry is
suggested for the investigated complexes.
where Wf is the mass loss at the completion of the re-
action, W is the mass loss up to temperature T; R is the
gas constant, E* is the activation energy in kJ mol–1, q
is the heating rate and (1–(2RT/E*)) @ 1. A plot of the
left-hand side of Eq. (1) vs. 1/T gives a slope from
which E* was calculated and A (Arrhenius factor) was
determined from the intercept. The entropy of activa-
tion (DS*), enthalpy of activation (DH*) and the free
energy change of activation (DG*) were calculated us-
ing the following equations:
As a general conclusion, the investigated Schiff
base behaves as a tridentate and its metal complex
structures can be given as shown in Fig. 4.
DS*=2.303[log(Ah/kT)]R
DH*= E*–RT
(2)
(3)
(4)
Biological activity
All of the tested compounds showed a remarkable bio-
logical activity against different types of Gram-positive
DG*=DH*– TDS*
The data are summarized in Table 6. The activa-
tion energies of decomposition were found to be in
the range 25.97–254.3 kJ mol–1. The high values of
the activation energy reflect the thermal stability of
the complexes. The negative values of S* indicate
that the reactions are slow in nature, hence assisted by
the wide range of decomposition temperature [24].
Structural interpretation
Fig. 4 Structural formulae of metal complexes M=Fe(III),
X=Cl, m=3, y=4. Co(II), X=Cl, m=2, y=2. Ni(II), X=Cl,
m=2, y=2. Cu(II), X=Cl, m=2, y=2. Zn(II), X=AcO,
m=2, y=2
The structures of the complexes of HL Schiff base
with Fe(III), Co(II), Ni(II), Cu(II), Zn(II) and UO2(II)
Table 6 Thermodynamic data of the thermal decomposition of metal complexes of HL
Decomp.
Temp./°C
DS*/
DH*/
Complex
E*/kJ mol–1
A/s–1
DG*/kJ mol–1
JK–1 mol–1
kJ mol–1
[Fe(HL)2]Cl3·4H2O
25–110
160–280
450–560
25.97
64.87
99.05
3.79·106
4.05·109
7.61·1013
–118.2
–106.7
–194.6
63.98
71.69
48.99
72.25
94.10
178.40
[Ni(HL)2]Cl2·2H2O
[Cu(HL)2]Cl2·2H2O
30–120
330–460
550–680
45.45
53.97
195.6
2.26·105
7.53·107
2.09·1012
6.22·1010
4.98·1011
5.89·1017
–106.9
–148.9
–183.0
74.82
91.47
205.9
82.60
151.00
352.30
30–110
150–380
600–750
66.86
155.7
254.3
–36.83
–169.8
–204.6
51.23
94.6
152.0
54.10
132.50
287.00
30–120
180–430
460–620
56.59
168.7
242.6
3.09·109
5.27·1013
3.47·1012
–41.59
–65.64
–93.49
66.77
186.3
247.6
69.50
213.20
246.90
[UO2(L)2]·2H2O
J. Therm. Anal. Cal., 86, 2006
323

OMAR et al.
Table 7 Biological activity of HL and its metal complexes
Staphylococcus
piogenes
Pseudomonas
aeruginosa
Fungus
(candida)
Escherichia
coli
Sample
C/mg L–1
5
2.5
1
5
2.5
–
1
5
2.5
1
5
2.5
1
HL
+
+
+
–
–
+
–
–
+
+
–
+
+
–
–
+
+
+
+
+
–
+
+
+
+
+
+
+
+
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
++
+++
++
+
++
++
+
+
+
+
–
+
–
+
–
+
[Fe(HL)2]Cl3·4H2O
[Co(HL)2]Cl2·2H2O
[Ni(HL)2Cl2·2H2O
Cu(HL)2Cl2·2H2O
[Zn(HL)2(AcO)2·2H2O
[UO2(L)2]·2H2O
Traivid
++
++
++
+
++
+
++
++
+
++
+
+++
++
++
++
+
++
++
+
++
++
++
+
+++
++
++
++
+
++
+++
++
++
+
++
++
+
Travinic
+++
++
+++
++
+++
++
The test was done using the diffusion agar technique.
Inhibition values = 0.1–0.5 cm beyond control=+
Inhibition values = 0.6–1.0 cm beyond control=++
Inhibition values = 1.1–1.5 cm beyond control=+++
and Gram-negative bacteria. The data are listed in Ta-
ble 7. On comparing the biological activity of the Schiff
base and its metal complexes with the standard (Traivid
and Tavinic), the following results are obtained:
• Fe(III) complex is considered as 3:1 electrolyte,
Co(II), Ni(II), Cu(II) and Zn(II) complexes are
ionic in nature and they are of the type 2:1 electro-
lytes. While UO2(II) complex is non-ionic in nature
and is considered as non-electrolyte.
The biological activity of HL is less than Tavinic
and comparable with that of Traivid (standard). The
biological activity of all metal complexes is higher
than that of the free HL ligand, equal to that of stan-
dard Traivid and less than that of standard Tavinic.
The biological activity of the complexes follow the
order UO2(II)>Zn(II)>Co(II)>Fe(III)=Cu(II)=Ni(II).
Also the data in Table 7 show that E. Coli was inhib-
ited by Fe(III) complex. The importance of this lies in
the fact that these complexes could be applied fairly
in the treatment of some common diseases caused by
E. Coli e.g. Septicaemia, Gastroenteritis, Urinary
tract infections and hospital acquired infections
[25, 26]. However, Ni(II), Zn(II) and UO2(II) com-
plexes were specialised in inhibiting Gram-positive
bacterial strains (Staphylococcus pyogenes and Pseu-
domonas aeruginosa). The importance of this unique
property of the investigated Schiff base complexes
lies in the fact that, it could be applied safely in the
treatment of infections caused by any of these partic-
ular strains. In addition, all metal complexes inhibit-
ing Fungi at high concentration (5 mg L–1) more than
the parent ligand and standards.
• HL Schiff base behaves as a neutral terdentate
ligand in all the complexes except UO2(II)
complex. It acts as a uni-negatively coordinated to
the UO2(II) ion via its deprotonated carboxylated O
beside the azomethine N and furan O.
• All studied complexes have octahedral geometry.
• Thermal decomposition of complexes gave the
possibility to establish the number and nature of
water molecules, the composition of complexes
and also the intervals of thermal stability.
• The biological activity of all complexes is higher
than that of the free HL ligand and follow the order:
UO2(II)>Zn(II)>Co(II)>Fe(III)=Cu(II)=Ni(II).
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325