Synthesis, characterization and biological activity of bis(phenylimine) Schiff base ligands and their metal complexes

Article abstract of DOI:10.1016/j.saa.2005.05.044

Metal complexes derived from 2,6-pyridinedicarboxaldehydebis(p-hydroxyphenylimine); L¹, 2,6-pyridinedicarboxaldehydebis (o-hydroxyphenylimine); L², are reported and characterized based on elemental analyses, IR, solid reflectance, ma

Full text of DOI:10.1016/j.saa.2005.05.044

Spectrochimica Acta Part A 64 (2006) 188–195
Synthesis, characterization and biological activity of bis(phenylimine)
Schiff base ligands and their metal complexes
Gehad G. Mohamed
Chemistry Department, Faculty of Science, Cairo University, Giza, Egypt
Received 23 December 2004; accepted 14 May 2005
Abstract
Metal complexes derived from 2,6-pyridinedicarboxaldehydebis(p-hydroxyphenylimine); L1, 2,6-pyridinedicarboxaldehydebis
(o-hydroxyphenylimine); L2, are reported and characterized based on elemental analyses, IR, solid reflectance, magnetic moment,
molar conductance and thermal analysis (TGA). The complexes are found to have the formulae [MX2(L1 or L2)]·nH2O, where M = Fe(II),
Co(II), Ni(II), Cu(II) and Zn(II), X = Cl in case of Fe(II), Co(II), Ni(II), Cu(II) complexes and Br in case of Zn(II) complexes and n = 0–2.5.
The molar conductance data reveal that the chelates are non-electrolytes. IR spectra show that the Schiff bases are coordinated to the
metal ions in a terdentate manner with NNN donor sites of the pyridine-N and two azomethine-N. From the magnetic and solid reflectance
spectra, it is found that the geometrical structure of these complexes are trigonal bipyramidal (in case of Co(II), Ni(II), Cu(II) and Zn(II)
complexes) and octahedral (in case of Fe(II) complexes). The thermal behaviour of these chelates shows that the hydrated complexes
losses water molecules of hydration in the first step followed immediately by decomposition of the coordinated water, anions and ligands
(L1 and L2) in the subsequent steps. The activation thermodynamic parameters, such as, E*, ꢀH*, ꢀS* and ꢀG* are calculated from the
TG curves using Coats–Redfern method. The synthesized ligands, in comparison to their metal complexes also were screened for their
antibacterial activity against bacterial species, Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus and Fungi (Candida).
The activity data show that the metal complexes to be more potent/antibacterial than the parent organic ligands against one or more bacterial
species.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Bis(phenylimine) Schiff base ligands; Transition metal complexes; IR; Conductance; Solid reflectance; Magnetic moment; Thermal analysis;
Biological activity
1. Introduction
1, R = p-phenol) had been designed to function as molecular
turn [10] by incorporating reactive p-phenolic groups in a
bis(phenylimine) ligand.
The preparation of a new ligand was a perhaps the most
important step in the development of metal complexes which
exhibit unique properties and novel reactivity. Since the elec-
tron donor and electron acceptor properties of the ligand,
structural functional groups and the position of the ligand
in the coordination sphere together with the reactivity of
coordination compounds may be the factor for different stud-
ies [1,2]. Schiff bases were important class of ligands, such
ligands and their metal complexes had a variety of appli-
cations including biological, clinical, analytical and indus-
trial in addition to their important roles in catalysis and
organic synthesis [3–9]. A tridentate Schiff base (Structure
Schiff base ligands of general structure 1 had a rich his-
tory, beginning with the preparation of the first examples, the
pyridinal hydrazones, by Stoufer and Busch [11], who were
motivated by the then current interest in the ␲-back-bonding
by unsaturated nitrogen donors. In early work closely related
to the system of interest here, Lions and Martin [12] synthe-
sized Schiff base ligands from 2,6-pyridinedicarboxaldehyde
(PDC) with aniline and benzylamine and noted that the free
ligands were crystalline materials. Therefore, extensive stud-
ies ensued in which this same pair of Schiff base linkages was
incorporated in many different kinds of polydentate ligands,
including various macrocyclic ligands [13–15]. Pursuing the
analogy to the Sauvage template, p-aminophenol was used
E-mail address: ggenidy@hotmail.com.
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2005.05.044

G.G. Mohamed / Spectrochimica Acta Part A 64 (2006) 188–195
2. Experimental
189
2.1. Materials and methods
All chemicals used were of the analytical reagent grade.
They included FeCl2·2H2O, CoCl2·6H2O, NiCl2·6H2O,
ZnBr2 and CuCl2·2H2O (BDH). o- and p-Aminophenols
(Sigma) were used as received. The elemental analyses (C, H
and N) were made at the Microanalytical Center at Cairo Uni-
versity. IR spectra were recorded on a Perkin-Elmer FT-IR
type 1650 spectrophotometer. The solid reflectance spectra
were measured on a Shimadzu 3101 PC spectrophotome-
ter. The molar magnetic susceptibilities were measured on
powdered samples using the Faraday method. The molar
conductance measurements were carried out using a Sybron-
Barnstead conductometer. A Shimadzu TGA-50H thermal
analyzer was used to record simultaneously TG and DTG
curves. The experiments were carried out in dynamic nitro-
gen atmosphere (20 ml min−1) with a heating rate of 10 ◦C
min−1 in the temperature range 20–1000 ◦C using plat-
inum crucibles. The sample sizes ranged in mass from
2.21 to 11.92 mg. Highly sintered ␣-Al2O3 was used as
Plate 1.
as the amine in the straightforward Schiff base condensation
reaction with PDC to give the diphenolic ligand. Previous
studies on ligands and metal complexes derived from the
Schiff bases of o- and p-aminophenol are few. In one case,
the reaction between PDC and o-, m- and p-aminophenols
in the presence of cadmium ions was used to differentiate
aminophenol isomers [16] and p-aminophenol had appeared
in some recent publications as a Schiff base precursor. Two
of these studies examined hydrogen-bonding interactions
of the phenolic hydrogen [17,18] while another involved
estrification of the phenolic oxygen of the free Schiff base
ligand to construct potential metallomesogen components
[19]. Although the preparation and X-ray crystal structure
of L1 and its Co(II), Fe(II), Ni(II) and Zn(II) complexes had
been reported by Vance et al. [20], no studies concerning
their molar conductance, thermal behaviour and antibacte-
rial activities are not reported.
a reference. 13C{ H} NMR spectra were recorded using
1
Brucker ARX-300 spectrometer using DMSO-d6 as a sol-
vent. Chemical shifts are reported in parts per million down-
field from tetramethylsilane, and all coupling constants are
reported in hertz. Metal contents were determined by titra-
tion against standard EDTA after complete decomposition
of the complexes with aqua regia in kjeldahl flask several
times.
L2 is a novel ligand and no studies concerning its metal
complexes had been reported. On screening literature, no
thermal studies concerning complexes of bis(phenylimine)
Schiff base ligands (L1 and L2) had been reported.
Recently, the mixed ligand complexes of L1 and L2 and 2-
aminopyridine with some transtion metals (such as Co(II),
Ni(II), Cu(II) and Zn(II)) were reported by Gehad et.
al. [21]. The structures of these ligands are shown in
Fig. 1.
2.2. Synthesis of L1 and L2 ligands
2,6-Pyridinedicarboxaldehyde and 2,6-pyridinedicarbo-
xaldehydebis(p-hydroxyphenyl-imine)(L1) were preparedas
previously reported by the modefied method of Vance et al.
[20].
Therefore, in continuation to our interest in Schiff base lig-
andsandtheirmetalchelates [21–23], thisworkdealswiththe
synthesis and characterization of Schiff base ligands; L1 and
L2, and their complexes. The coordination behaviour of L1
and L2 ligands towards transition metal ions (Fe(II), Co(II),
Ni(II), Cu(II) and Zn(II)) is investigated via the IR, molar
conductance, magnetic moment, solid reflectance and ther-
mal analysis. The thermal decomposition of the complexes
is also used to infer the structure and the different thermo-
dynamic activation parameters are calculated. The biological
activity of these Schiff bases and their metal chelates are
reported.
2.2.1. Synthesis of 2,6-pyridinedicarboxaldehyde
bis(o-hydroxyphenylimine), L2
2,6-Pyridinedicarboxaldehyde (1.00 g, 7.40 mmol) and 2-
aminophenol (1.62 g, 14.80 mmol) were stirred in 20 mL of
absolute ethanol until a yellow precipitate appeared (within
5 min). Stirring was continued for an additional 30 min
and then the yellow powder was collected by vacuum fil-
tration and dried overnight in a vacuum desiccator. Yield
85%. Analyses: calculated: C, 71.92; H, 4.73; N, 13.25.
Found: C, 72.05; H, 4.55; N, 13.00. 1H NMR (DMSO-
d6, 25 ◦C): δ 8.92 (s, OH, 2H), 8.65 (s, CH, 2H), 8.30 (d,
pyH, 2H), 7.98 (t, pyH, H), 7.45–6.97 ppm (m, phH, 8H).
13C NMR (DMSO-d6, 25 ◦C): δ 163 (s, C4, azomethine
(CH N)), 156.35 (s, C1, py), 137.57 (s, C3, py), 122.99
(s, C2, py), 152.82 (s, C6, C O), 138.40, 130.29, 120.34,
116.21 and 115.26 ppm for the 13C resonance of aromatic
groups.
Fig. 1. Structures of L1 and L2 ligands.

190
G.G. Mohamed / Spectrochimica Acta Part A 64 (2006) 188–195
2.3. Synthesis of the complexes
and exhibited a great variety of coordination modes with dif-
ferent coordinating power and these may give rise to varied
bonding and different stereochemical patterns in their coor-
dination complexes as a result for competition between the
phenolic and azomethine nitrogen groups as a coordination
sites [25]. The compounds containing a pyridinium moiety
attached to a heterocyclic systems are important in natural
product chemistry and in organic synthesis [26]. This would
us to obtain some new heterocyclic compounds with expected
wide spectrum of potential applications that were extensively
required for medicinal chemistry program.
The complexes of Schiff base ligands (L1 and L2) were
prepared by the addition of hot ethanolic solution (60 ◦C) of
the appropriate metal chloride or bromide (1 mmol, 0.163 g
FeCl2·2H2O, 0.237 g CoCl2·6H2O, 0.237 g NiCl2·6H2O,
0.1705 g CuCl2·2H2O and 0.225 g ZnBr2) to the hot ethano-
lic solution (60 ◦C) of the Schiff bases (0.317 g, 1 mmol of L1
and L2). The resulting mixtures were stirred under reflux for
2 h, where upon the complexes were precipitated. They were
collected by filtration, washed with 1:1 ethanol: water mix-
ture, then diethylether and dried under vacuum over CaCl2.
The general reaction for the preparation of the complexes
of L1 and L2 is given below:
MX2·nH2O + L1 or L2 → [MX2(L1 or L2)]·nH2O
2.4. Biological activity
0.5 ml spore suspension (10−6–10−7 spore/ml) of each of
theinvestigatedorganismswasaddedtoasterileagarmedium
just before solidification, then poured into stirile petri dishes
(9 cm in diameter) and left to solidify. Using sterile cork borer
(6 mm in diameter), three holes (wells) were made in each
dish, then 0.1 ml of the tested compounds dissolved in DMF
(100 ␮g/ml) were poured into these holes. Finally, 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 conditions for each
organism and by subtracting the diameter of inhibition zone
resulting with DMF from that obtained in each case, both
antibacterial activities can be calculated as a mean of three
replicates.
where M = Fe(II) (X = Cl), Co(II) (X = Cl), Ni(II) (X = Cl),
Cu(II) (X = Cl), Zn(II) (X = Br), n = 0–2.5).
Theresultsoftheelementalanalysesofthechelates, which
are recorded in Table 1, are in good agreement with those
required by the proposed formulae. In all cases, 1:1 (M: L1
or L2) solid complexes are isolated and found to have the
general formulae [MX2(L1 or L2)]·nH2O, where M = Fe(II)
(X = Cl), Co(II) (X = Cl), Ni(II) (X = Cl), Cu(II) (X = Cl) and
Zn(II) (X = Br), n = 0–2.5.
3.1. IR spectra and mode of bonding
In the absence of a powerful technique such as X-ray crys-
tallography, IR spectra have proven to be the most suitable
technique to give enough information to elucidate the way of
bonding of the ligands to the metal ions. The main stretching
frequencies of the IR spectra of the ligands (L1 and L2) and
their complexes are listed in Table 2.
3. Results and discussion
A ligands having phenolic functional groups can be pre-
pared by the general reaction in Fig. 2, diimine molecule,
where R is a potentially reactive functional group.
The IR spectra of L1 and L2 and their chelates are found
to be very similar to each other. Hence significant frequen-
cies are selected by comparing the IR spectra of the ligands
with those of their metal chelates. The strong bands located at
1580 cm−1 are assigned to the ν(C N) stretching vibrations
of pyridyl nitrogen of the L1 and L2 ligands. Coordination of
pyridyl nitrogen is indicted by a 20–42 cm−1 shift to lower
wavenumber, 9–11 cm−1 shift to a higher wavenumber in all
the chelates [27]. On the other hand, from the spectroscopic
behaviour of a simple metallic complexes of pyridine, it is
known that after complexation the vibrations in the high-
frequency region are not appreciably shifted, whereas the
ring deformation found at 604 and 405 cm−1 in the free pyri-
dine are shifted to higher frequencies [28]. Therefore, these
The planarity of L1 and L2 ligands can be attributed to
the absence of alkyl groups at the imine carbon [20]. The
ligands are coordinated meridionaly to produce the desired
orientation of the phenolic oxygens across and away from the
center of the complex. The phenyl rings of L1 and L2 ligands
in the complexes are twisted out of the plane of the pyridine
rings as was observed for other similar complexes [14,24].
L1 and L2 Schiff base ligands are of significant interest from
the point of view that they act as a pincer ligands, often found
as a merdional ligand (i.e. the three nitrogen atoms possess
will be approximately coplanar in metal coordination sphere)
Fig. 2. Preparation of L1 and L2 Schiff base ligands.

G.G. Mohamed / Spectrochimica Acta Part A 64 (2006) 188–195
191
two shifts may be very useful in establishing the participa-
tion of pyridine in complex formation. In the present case, the
in-plane ring deformation bands are found at 638 cm−1 (for
L1) and 641 cm−1 (for L2). After complexation, these bands
are shifted to lower (11–38 cm−1) or higher (11–55 cm−1
)
wavenumbers, indicating coordination via the pyridine nitro-
gen, as previously reported for pyridine complexes [29].
Detecting the shift of the out-of-plane ρ(py) at 405 cm−1 is
difficult because the spectra are rich for the complexes.
The strong and sharp bands at 1618 and 1625 cm−1 are
assigned to the ν(C N) stretching mode of the azomethine
group for L1 and L2, respectively. These bands are shifted to
1583–1652 cm−1 in all complexes. This shift support the par-
ticipation of the azomethine group of these ligands in binding
to the metal ions [23,30].
The other series of weak bands between 3100 and
2800 cm−1 are related to (C H) modes of vibrations. Also,
some weak bands are located between 2000 and 1750 cm−1
can be assigned to overtones of the aromatic rings.
The L1 and L2 ligands also display bands at 3061–3092
and 1344–1337 cm−1 which are assigned to ν(OH) and
ν(C O) stretching vibrations of the phenolic-OH, respec-
tively. These bands are strongly affected by chelation
although the phenolic-OH of the Schiff bases (L1 and L2)
are not involved in chelate formation. The shift to higher or
lower wavenumbers can be attributed to important hydrogen
bonding effects [20].
Most of the band shifts observed at the wave number
region 1150–994 cm−1 are in agreement with the structural
changes observed in the molecular carbon skeleton after com-
plexation, which cause some important changes in (C C)
bond lengths. The bands appeared in the wave number range
419–484 and 500–522 cm−1 [23,29] can be attributed to ν(M-
N) of the pyridine and azomethine nitrogen atoms, respec-
tively [31].
3.2. Molar conductivity
The molar conductivity (Λm) of 10−3 M solutions of the
complexes in DMF at 25 ◦C is listed in Table 1. The com-
plexes are found to be non electrolyte in DMF solution,
implying the coordination of halide anions. Elemental anal-
yses identified the complexes as [MX2(L1 or L2)]·nH2O.
3.3. Magnetic susceptibility and electronic spectra
As the result of failer to obtain a single crystal for X-ray
analyses to confirm the octahedral and trigonal bipyramidal
structures for these complexes, solid reflectance spectra and
magnetic moment measurements are used for this purpose.
The diffuse reflectance spectra of the Fe(II) com-
plexes give two bands at 12,685–12,820 cm−1 and
5
17,543–17,605 cm−1 which are assigned to 5T2g → Eg tran-
sition, suggesting an octahedral geometry around Fe(II) ion
[32]. The band at 22,321–24,096 cm−1 refers to the charge
transfer band (L → MCT). The observed magnetic moments

192
G.G. Mohamed / Spectrochimica Acta Part A 64 (2006) 188–195
Table 2
Characteristic IR bands (4000–400 cm−1) of L1 and L2 and their complexes
Compound
ν(CH N)
ν(C N)
ρ(py)
ν(OH)
ν(C O)
ν(M N)
ν(M N)
L1
1618sh
1583sh
1592sh
1585sh
1591sh
1592sh
1580sh
1538sh
1544sh
1542sh
1542sh
1548sh
638s
3061br
3055br
3068br
2992brs
3073br
3050br
1344s
–
–
[Fe(L1)Cl2(H2O)]·H2O
617m
600m
600sh
620m
649m
1366m
1365m
1359m
1367m
1370m
522m
500m
500m
501m
500m
479s
424m
419m
436m
422s
[Co(L1)Cl2]
[Ni(L1)Cl2]·2H2O
[Cu(L1)Cl2]·H2O
[Zn(L1)Br2]
L2
1625sh
1652s
1613m
1642s
1596sh
1590m
1580sh
1591sh
1589sh
1593sh
1560sh
1556m
641s
3092br
3166br
3026br
3140br
3249br
3220br
1337s
–
–
[Fe(L2)Cl2(H2O)]·H2O
[Co(L2)Cl2]·2.5H2O
[Ni(L2)Cl2]·H2O
[Cu(L2)Cl2]·2H2O
[Zn(L2)Br2]
664m
652m
596m
660m
630m
1349m
1383m
1375s
1349m
1383m
505m
506m
520m
505s
505s
440m
470s
484s
440m
450s
Sh: sharp, s: small, w: weak and br: broad.
Table 3
Thermoanalytical results (TGA) of L1 and L2-metal complexes
Complex
TG range
(◦C)
DTG (◦C)
Number
of steps
Mass loss
Total mass loss
found (calcd. %)
Assignment
Metallic
residue
[Fe(L1)Cl2(H2O)]·H2O
[Ni(L1)Cl2]·2H2O
30–110
110–800
64
450
1
1
3.95 (3.75)
85.18 (84.58)
Loss of H2O
FeO
81.23 (80.83)
65.79 (65.63)
Loss of H2O, 2HCl and L1
40–140
140–800
78
230
1
1
7.15 (7.45)
73.24 (73.08)
Loss of 2H2O
Loss of L1
NiCl2
[Zn(L1)Br2]
350–650
420, 550
2
85.78 (85.06)
Loss of 2HBr and L1
ZnO
CoO
[Co(L2)Cl2]·2.5H2O
30–150
150–350
350–700
60, 110
270
420, 570
2
1
2
9.17 (9.15)
14.76 (14.84)
84.78 (84.76)
Loss of 2.5H2O
Loss of 2HCl
Loss of L2
60.85 (60.77)
[Cu(L2)Cl2]·2H2O
30–130
130–500
500–900
60
120, 240
520, 620
1
2
2
7.56 (7.39)
14.79 (14.99)
84.43 (83.78)
Loss of 2H2O
Loss of 2HCl
Loss of L2
CuO
ZnO
62.08 (61.40)
55.27 (55.17)
[Zn(L2)Br2]
220–450
450–800
315
615
1
1
30.19 (29.89)
85.46 (85.06)
Loss of 2HBr
Loss of L2
Table 4
Thermodynamic data of the thermal decomposition of L1 and L2 complexes
Complex
Decomposition range (◦C)
E* (kJ mol−1
)
A (S−1
)
ꢀS* (kJ−1 mol−1
)
ꢀH* (kJ mol−1
)
ꢀG* (kJ mol−1
)
[Fe(L1)Cl2(H2O)]·H2O
30–110
110–800
65.85
176.8
3.08 × 107
−45.63
−75.47
65.22
97.78
56.86
135.6
6.40 × 109
[Ni(L1)Cl2]·2H2O
[Zn(L1)Br2]
40–140
140–800
68.73
166.3
6.04 × 1011
4.83 × 1014
−43.06
60.23
185.3
94.71
180.3
−78.98
350–430
430–600
53.20
153.9
4.66 × 108
−63.34
−96.56
−28.33
−83.17
−108.2
−45.52
−35.93
−55.33
−65.43
−85.69
76.53
169.5
36.77
185.9
2.05 × 1011
[Co(L2)Cl2]·2.5H2O
30–150
150–350
350–700
40.23
49.30
187.3
1.77 × 105
5.08 × 109
3.97 × 107
98.28
206.5
246.3
66.95
217.0
237.4
[Cu(L2)Cl2]·2H2O
30–90
54.23
87.57
157.9
166.4
212.6
3.50 × 108
6.09 × 107
2.39 × 1012
6.89 × 1014
4.37 × 1019
87.55
101.2
142.9
187.9
201.5
96.53
63.03
195.2
202.6
254.9
90–160
200–300
400–550
550–700
[Zn(L2)Br2]
220–450
450–800
98.79
197.7
4.92 × 109
−48.99
76.42
122.4
76.06
189.32
5.92 × 1011
−96.69

G.G. Mohamed / Spectrochimica Acta Part A 64 (2006) 188–195
193
• 85.72% elemination of 2HBr and L1 ligand for Zn(II)-L1
complex within the temperature range 350–650 ◦C (calcu-
lated 85.06%).
of the Fe(II) complexes lie in the range 5.22–5.43 B.M. which
are an indicative of octahedral geometry.
The electronic spectra of the Co(II) complexes resem-
bles those of other five coordinate Co(II) complexes
[33], where three bands are observed at 12,820–12,987,
15,673–15,772 and 17,391–17,452 cm−1. The fourth band at
21,786–23,255 cm−1 refers to the charge transfer band. The
low magnetic moment values (µeff = 4.95–5.06 B.M.) range
compared with those observed for octahedral or tetrahedral
complexes, may support the five coordinate geometry [33].
The reflectance spectra of the Ni(II) complexes show rec-
ognizable spectral bands at 13,570–13,789, 17,561–17,905
and 21,091–21,756 cm−1. The positions of these spectral
bands are quite consistent with those predicted for five coor-
dinate Ni(II) complex whose structure had been established
by X-ray crystallographic measurements [24]. The bands can
3.5. Kinetic studies
The kinetic parameters such as activation energy (ꢀE*),
enthalpy (ꢀH*), entropy (ꢀS*) and free energy change of the
decomposition (ꢀG*) were evaluated graphically by employ-
ing the Coats–Redfern relation [36]:
ꢀ
ꢁ
log{Wf/(Wf − W)}
log
T2
ꢀ
ꢁ
ꢂ
ꢃ
AR
θE∗
2RT
E∗
2.303RT
= log
1 −
−
(1)
E∗
3
3
3
3
be assigned respectively [24] to B1 → E, B1 → A2 and
where Wf is the mass loss at the completion of the reaction,
W is the mass loss up to temperature T; R is the gas constant,
3B1 → E transitions assuming the effective symmetry to
3
be C4V. The bands at 27,569–28,450 cm−1 may assign to
E is the activation energy in kJ mol−1, θ is the heating rate
L → M CT band. The Cu(II) complexes exhibit a broad bands
*
∼
and (1 − (2RT/E )) 1. A plot of the left-hand side of Eq. (1)
=
at 12,658–12,903, 15,600–15,649 and 19,193–19,569 cm−1
.
against 1/T gives a slope from which E* was calculated and A
(Arrhenius constant) was determined from the intercept. The
entropy of activation (ꢀS*), enthalpy of activation (ꢀH*) and
the free energy change of activation ((ꢀG*) were calculated
(Table 4).
These bands are generally consistent with a five coordi-
nate geometry for Cu(II) complexes [32,34a,36]. The spec-
tra also show a band at 30,078–30,576 cm−1 which may
assign to L → Cu CT band. The magnetic moment values
of 2.07–2.16 B.M. are indicative of trigonal bipyramidal
structure [23,32a,34]. In analogy with those described for
Zn(II) complexes containing N–O donor Schiff bases [35]
and according to the empirical formulae of these complexes,
we proposed trigonal bipyramidal geometry for the Zn(II)
complexes.
According to the kinetic data obtained from DTG curves,
all the complexes have negative entropy, which indicates that
activated complexes have more ordered systems than reac-
tants.
3.6. Structural interpretation
From all of the above observations, the structure of these
complexes may be interpreted in accordance with complexes
of ligands with a similar distribution of like coordination
sites [12,14,15,20]. Vance et al. [20] and Curry [37] (based
on X-ray structural data) and Gehad et al. [21], reported
that L1 and 2,6-diacetylpyridinebis(p-methoxyphenylimine)
were coordinated to Fe(II), Co(II), Ni(II) and Zn(II) through
the pyridine-N and azomethine-N. Thus their coordinating
behaviour described in the crystal structure of their complex
is relevant for the interpretation of L1 and L2 coordinating
capacity. The structural information from these complexes
3.4. Thermogravimetric analysis (TGA)
Thermal data of the complexes are given in Table 3, and
the activation thermodynamic data associated with the loss
of water of crystallization (dehydration process) and ligand
molecules are listed in Table 4. The thermal analysis of the
chelates under study evinces the following weight losses:
• 3.95–9.17% loss of water molecules of hydration over the
range 30–150 ◦C interval (calculated 3.75–9.15%) for the
Fe(II) and Ni(II)-L1 and Co(II)- and Cu(II)-L2 complexes.
• 14.76–30.19% elimination of 2HX gases (X = Cl or Br)
over the 130–500 ◦C interval (calculated 14.84–29.89%)
for Co(II)-, Cu(II)- and Zn(II)-L2 complexes.
• 55.27–62.08% elimination of L2 ligand over the temper-
ature range 350–900 ◦C (calculated 55.17–61.40%) for
Co(II)-, Cu(II)- and Zn(II)-L2 complexes.
• 81.23% elimination of H2O, 2HCl and L1 ligand for Fe(II)-
L1 complex over the temperature range 350–650 ◦C (cal-
culated 80.83%).
• 65.79% elimination of L1 ligand for Ni(II)-L1 com-
plex over the temperature range 140–800 ◦C (calculated
65.63%).
Fig. 3. Structure of L1 and L2 complexes.

194
G.G. Mohamed / Spectrochimica Acta Part A 64 (2006) 188–195
Table 5
Antibacterial activity of Schiff base ligands (L1 and L2) and their complexes
Compound
Gram positive
Gram negative
Staphylococcus aureus
Bacillus subtillis
Pseudomonas aereuguinosa
Escherichia coli
5a
2.5
++
1
5
2.5
1
5
2.5
+
1
5
2.5
1
L1
++
+++ ++
+++ ++
++
+++ ++
++
++
+++ ++
+++ ++
++
++
++
nd
+
+
+
+
nd
+
+
+
nd
+
++
++
++
++
+
+
+
+
nd
+
+
nd
nd
+
nd
nd
+
+
nd
+
+
nd
+
nd
+
nd
+
nd
+
++
++
++
++
+++ ++
+++ ++
++
++
++
++
+++ ++
+++ ++
+
+
+
+
nd
+
+
nd
+
+
nd
+
[[Fe(L1)Cl2(H2O)]·H2O
+++ ++
+++ ++
++
+++ ++
++
+
++
+++ ++
+++ ++
++
++
++
++
nd
[Co(L1)Cl2]
[Ni(L1)Cl2]·2H2O
[Cu(L1)Cl2]·H2O
[Zn(L1)Br2]
+
+++ ++
+
++
++
+
+++ ++
++
++
+++ ++
+++ ++
+++ +++ ++
++
+
+
+
+
+
+
+
L2
+
++
+
[Fe(L1)Cl2(H2O)]·H2O
[Co(L2)Cl2]·2.5H2O
[Ni(L2)Cl2]·H2O
[Cu(L2)Cl2]·2H2O
[Zn(L2)Br2]
+
+
+
+
++
++
+
++
++
+
++
+
+
+
++
Chloroamphinicol (standard) ++
+++ +++ ++
++
++
Inhibition values = 0.1–0.5 cm beyond control = + (less active); inhibition values = 0.6–1.0 cm beyond control = ++ (moderate active); inhibition val-
ues = 1.1–1.5 cm beyond control = +++ (highly active).
a
Concentration in mg/ml. nd: non detected.
is in agreement with the data reported in this paper based
on the IR, molar conductivity, solid reflectance and mag-
netic moment measurements. Consequently, the structures
proposed are based on trigonal bipyramidal (Co(II), Ni(II),
Cu(II) and Zn(II)) complexs. L1 and L2 ligands always coor-
dinate via the pyridine-N and two azomethine-N forming a
three binding chelating sites. The trigonal bipyramidal com-
plexs, the two extra positions are provided by the two-halide
ions. While in octahedral Fe(II) complexes, the extra position
is provided by the coordinated water molecule. According to
the above data and similar to those proposed previously, the
structures of the complexes are shown in Fig. 3.
may play an important role in the antibacterial activity [40],
as well as the presence of two imine groups which imports
in elucidating the mechanism of transformation reaction in
biological systems [38].
Antibacterial activity of all complexes at low concentra-
tions towards gram positive and negative bacteria are not
detected or low. The activity of the two schiff base ligands
and their complexes increases as the concentration increases
because it is a well known fact that concentration plays a vital
role in increasing the degree of inhibition [41]. Against all the
organisms, ligands L1 and L2 did not exhibit any remarkable
activity compared with standard chloroamphinicol, whereas
all the complexes showed moderate to high activities. It is
suggested that the complexes having antimicrobial activ-
ity may act either by killing the microbe or by inhibiting
multiplication of the microbe by blocking their active sites
[42]
3.7. Antibacterial activity
The biological activity of the two Schiff base ligands (L1
and L2), their complexes and chloroamphenicol (as a stan-
dard compound) were tested against bacteria because bacteria
can achieve resistance to antibiotics through biochemical and
morphological modifications [38]. The organisms used in
the present investigations included Staphylococcus aureus
and Bacillus subtillis (as gram positive bacteria) and Pseu-
domonas aereuguinosa and Escherichia coli (as gram neg-
ative bacteria). The diffusion agar technique was used to
evaluate the antibacterial activity of the synthesized mixed
ligand complexes [39].
The results of the bactericidal screening of the synthe-
sized compounds are recorded in Table 5. The data obtained
reflect that the two Schiff base ligands; L1 and L2, have mod-
erate activity in comparison with Staphylococcus aureus,
Escherichia coli and less active in comparison with Pseu-
domonas aeruginosa. L1 ligand shows a moderate activity
towards Bacillus subtillis while L2 ligand is less active. The
remarkable activity of the two Schiff base ligands may be
arise from the pyridyl-N and the hydroxyl groups, which
4. Conclusion
The design and synthesis of the first member of a new
family of tridentate Schiff base ligands for use in trigonal
bipyramidal complexes have been successfully demonstar-
taed. As anticipated, the ligand coordinates merdionaly to six
coordinate transition metal ions to give trigonal bipyramidal
oriented molecular turns around the metal ion anchor. The
vacant position is occupied by the water molecules to give
octahedral oriented molecular turns around Fe(II) ion anchor.
The syntheses of the ligands and their complexes proved
to be as straightforward as expected, giving high yields of
either the free ligands or their complexes in simple, one-pot
reactions. Upon chelation, the phenolic groups are directed
across and beyond the metal ion center, and, unlike earlier
ligands of this general type which lacked reactive moieties,

G.G. Mohamed / Spectrochimica Acta Part A 64 (2006) 188–195
[24] A.J. Lavery, M. Schroder, Acta Crytallogr. CS2 (1996) 37.
195
substitution reactions may be carried out at the free pheno-
lic groups of octahedral complex. These reactions must yet
be optimized in order to achieve the long-term goal of prov-
ing their value as precursors for supramolecular species. This
optimization, along with the preparation and study of com-
plexes with different functional groups, represents the focus
of ongoing studies.
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