Synthesis, spectroscopic and structural characterization, and antimicrobial studies of metal complexes of a new hexadentate Schiff base ligand. Spectrophotometric determination of Fe(III) in water samples using a recovery test

Article abstract of DOI:10.1007/s00706-013-1131-3

New Schiff base complexes were prepared using the bioactive antimicrobial bis-Schiff base ligand synthesized by condensation of 2-aminobenzoic acid and 2,2′-(ethylenedioxy)bis(benzaldehyde). The pK _A value of the bis-Schiff base was determined through visible spectrophotometric experiments and the stability constants of its complexes were also studied. The structural features of the complexes were determined from their elemental analyses, magnetic susceptibility, molar conductance, and spectral (IR and ¹H NMR) studies. Powder XRD indicates that all the complexes are amorphous except the Cr(III) and Cu(II) chelates which are crystalline. The data revealed that the stoichiometries of all prepared complexes are of 1:1 M/L type. The UV-vis absorption spectral data for the complexes suggest an octahedral geometry around the central metal ion. Thermogravimetric data (TG and DTG) were also studied. The kinetic and thermodynamic parameters for thermal decomposition of the complexes were calculated by graphical methods using the Coats-Redfern approach. The antimicrobial activity of the bis-Schiff base and its complexes was tested against a number of bacteria and fungi to assess their inhibiting potential. Most of the complexes exhibit antibacterial activity more than the parent bis-Schiff base. Also Cr(III), Co(II), and Ni(II) complexes have antifungal activity against Candida albicans whereas the parent bis-Schiff base is inactive. A recovery test was also applied for the spectrophotometric determination of Fe(III) in different natural water samples. Graphical abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s00706-013-1131-3

Monatsh Chem
DOI 10.1007/s00706-013-1131-3
ORIGINAL PAPER
Synthesis, spectroscopic and structural characterization,
and antimicrobial studies of metal complexes of a new hexadentate
Schiff base ligand. Spectrophotometric determination of Fe(III)
in water samples using a recovery test
•
Ehab M. Zayed Eman Hamed Ismail
•
•
•
Gehad G. Mohamed Mostafa M. H. Khalil
Ahmed B. Kamel
Received: 28 May 2013 / Accepted: 5 December 2013
Ó Springer-Verlag Wien 2014
Abstract New Schiff base complexes were prepared
using the bioactive antimicrobial bis-Schiff base ligand
synthesized by condensation of 2-aminobenzoic acid and
2,20-(ethylenedioxy)bis(benzaldehyde). The pKA value of
the bis-Schiff base was determined through visible spec-
trophotometric experiments and the stability constants of
its complexes were also studied. The structural features of
the complexes were determined from their elemental
analyses, magnetic susceptibility, molar conductance, and
Schiff base and its complexes was tested against a number
of bacteria and fungi to assess their inhibiting potential.
Most of the complexes exhibit antibacterial activity more
than the parent bis-Schiff base. Also Cr(III), Co(II), and
Ni(II) complexes have antifungal activity against Candida
albicans whereas the parent bis-Schiff base is inactive. A
recovery test was also applied for the spectrophotometric
determination of Fe(III) in different natural water samples.
1
spectral (IR and H NMR) studies. Powder XRD indicates
Keywords Bis-Schiff base ꢀ Metal complexes ꢀ
that all the complexes are amorphous except the Cr(III) and
Cu(II) chelates which are crystalline. The data revealed
that the stoichiometries of all prepared complexes are of
1:1 M/L type. The UV–vis absorption spectral data for the
complexes suggest an octahedral geometry around the
central metal ion. Thermogravimetric data (TG and DTG)
were also studied. The kinetic and thermodynamic
parameters for thermal decomposition of the complexes
were calculated by graphical methods using the Coats–
Redfern approach. The antimicrobial activity of the bis-
Biological activity ꢀ Recovery test ꢀ Spectroscopic studies
Introduction
Schiff bases are important in molecular processes occur-
ring in biochemistry, material science, catalysis,
encapsulation, activation, transport and separation phe-
nomena, hydrometallurgy, etc. [1–5]. Macrocyclic ligands
with additional donor atoms appended to the ring have
attracted considerable interest owing to their capacity to
bind and transport metal ions, for the potential to prepare
and study their mixed-valence forms, and as models for
metalloproteins [6, 7]. There is a continued interest in
synthesizing macrocyclic complexes [2, 8–11] because of
their potential applications in fundamental and applied
sciences [11–13] and importance in the area of coordina-
tion chemistry [14, 15]. The development of the field of
bioinorganic chemistry has been another important factor
in spurring the growth in interest in macrocyclic com-
pounds [16]. An effective way to use the molecular
recognition capability of macrocycles for metal ion sepa-
ration is to incorporate such ligands into solvent extraction
and liquid membrane systems [17]. During the past few
years, the separation of heavy metals and organic
Electronic supplementary material The online version of this
article (doi:10.1007/s00706-013-1131-3) contains supplementary
material, which is available to authorized users.
E. M. Zayed
Green Chemistry Department, National Research Centre,
Dokki, Giza 12622, Egypt
E. H. Ismail ꢀ M. M. H. Khalil
Chemistry Department, Faculty of Science, Ain Shams
University, Ain Shams, Cairo, Egypt
G. G. Mohamed (&) ꢀ A. B. Kamel
Chemistry Department, Faculty of Science,
Cairo University, Giza 12613, Egypt
e-mail: ggenidy68@hotmail.com
123

E. M. Zayed et al.
substances has become more important in water purifica-
tion applications owing to the drastic strengthening of
environmental protection laws. A macrocyclic Schiff base
prepared by reaction of 2,6-diaminopyridine and 2,20-
(ethylenedioxy)bis(benzaldehyde) has been reported. Its
Cu(II), Ni(II), Pb(II), Co(III), and La(III) complexes were
synthesized by exploiting the template effect. Spectral,
magnetic, and electrochemical properties of the new
compounds were studied in detail [18].
hexadentate Schiff base ligand derived from the conden-
sation of 2,20-(ethylenedioxy)bis(benzaldehyde) and
2-aminobenzoic acid. The coordination behavior of the bis-
Schiff base ligand towards metal chlorides was investi-
gated and the data obtained were correlated with their
1
elemental analysis, thermal properties, H NMR, FT-IR,
UV–vis, and molar conductivity measurements. The anti-
microbial efficiency of the bis-Schiff base and its
complexes was screened against different microorganisms.
This work focuses on the use of a hexadentate bisalde-
hyde-type ligand having selective ability to coordinate
transition ions, protect them from deactivation due to
interaction with solvent molecules, and enhance their bio-
logical activity. Structurally characterized bisaldehyde-
type complexes of the 3d block elements are rare and the
coordination mode of the Schiff base ligand (Scheme 1) is
usually proposed on the basis of the composition and
spectroscopic data. Depending on the preparative proce-
dures, different compositions have been reported for
bisaldehyde-type complexes, including [M(L)]Cl (M = Cr
and Fe) and [M(L)] [M = Co, Ni, Cu, and Zn]. In this
context, we have studied the interaction of M(II) or M(III)
chloride (Cr, Fe, Co, Ni, Cu, and Zn ions) with a
Results and discussion
The bis-Schiff base ligand has several potential coordination
sites, of which the two carboxylate oxygen, two azomethine
nitrogen, and two ether oxygen atoms can bond to one metal
ion to produce a 6-membered chelate ring. This work
describesthepreparationofanewmacrocyclicbis-Schiffbase
ligand via the condensation of 2,20-(ethylenedioxy)bis(benz-
aldehyde) with 2-aminobenzoic acid (Scheme 1).
The new bis-Schiff base was characterized on the basis
of elemental analysis, IR, 1H NMR, UV–vis, and mass
spectra. The mass spectrum provides a vital clue for
Scheme 1
O
O
OH
N
HO
N
O
O
2-Aminobenzoic acid
Reflux, 3 h
O
O
O
O
1
2
M(II)/(III) chlorides
Ethanol
Reflux, 2.5 h
O
O
O
O
O
O
O
O
N
N
N
N
Cl
M
M
O
O
O
O
3c
3d
3e
: M = Co
: M = Ni
: M = Cu
: M = Zn
3a
: M = Cr
3b
: M = Fe
3f
123

Synthesis, spectroscopic and structural characterization
using Taneja and Srivastova’s method [20] by measuring
the absorbance of solutions of ligand and metal at fixed
concentration but at distinct pH values ranging from 2 to
12. The complex-forming abilities of the transition metal
ions are frequently characterized by stability orders of
Irving and Williams series [21]. The order of stability
constants is found to be Co(II) \ Ni(II) \ Cu(II) [ Zn(II)
for divalent metal ions of the 3d series. Under the influence
of the ligand field and Jahn–Teller effect [22], Cu(II) (3d9)
will receive some extra stabilization [23] due to tetragonal
distortion of octahedral symmetry in their complexes. The
free energy of formation, DG°, accompanying the com-
plexation reaction was determined at 25 °C; negative
values of DG° show that the driving force of the com-
plexation reaction is from left to right and the reaction
proceeds spontaneously.
Fig. 1 Mass spectrum of bis-Schiff base ligand
elucidating the structures of the Schiff base. The fragment
ion peak at m/z = 506 (relative intensity 8 %) corresponds
to the molecular weight of the Schiff base under investi-
gation. The Schiff base shows a peak at m/z = 78 (100 %)
corresponding to the C6H6 ion, and other fragments
observed were in good agreement with the proposed for-
mula given in Scheme 1 and Fig. 1. The molecular
modeling of the Schiff base ligand shows that the bond
lengths of all bonds in the left- and right-hand sides are
typical. The structure is considered symmetrical and all
bond angles are predicted to be sp3 and sp2 hybridized
according to Chem3D ultra 8.0.
Elemental analyses of complexes
The elemental analysis results for the metal complexes are
in good agreement with the calculated values. It is clear
that the complexes have 1:1 metal–ligand stoichiometry of
the type ML. The formation of these complexes may pro-
ceed according to the following equations (H2L = bis-
Schiff base ligand):
The absorption spectra of the bis-Schiff base ligand
(10-4 M) under investigation in buffer solutions of dif-
ferent pH values ranging from 2 to 13 were scanned in the
wavelength range from 200 to 700 nm, against the same
solvent as a blank. The experimental spectrum features
shows two sharp peaks centered at 350 nm (e =
23,820 dm3 mol-1 cm-1) and 390 nm (e = 7,186 dm3
mol-1 cm-1) which can be attributed to p–p* and n–p*
transitions within the bis-Schiff base molecule. The char-
(a) MCl2 ? H2L ? [M(L)] ? 2HCl
M = Ni(II), Co(II), Cu(II), Zn(II)
(b) MCl3 ? H2L ? [M(L)]Cl ? 2HCl
M = Cr(III), Fe(III)
Molar conductance measurements
The metal(II)/(III) complexes were dissolved in DMF and
the molar conductivities of their solutions (10-3 M) at
room temperature were measured. The low conductance
values of the bivalent metal complexes support the non-
bonding of the chloride anions to the metal ions and their
non-electrolytic nature. The Cr(III) and Fe(III) complexes
have molar conductance values of 56.27 and
63.86 X-1 mol-1 cm2, respectively, indicating their elec-
trolytic nature and they are of the type 1:1.
1
acteristic IR and H NMR spectral bands of the bis-Schiff
base will be discussed in detail with the respective metal
complexes.
Ligand dissociation and metal–ligand stability
constants
The ligand dissociation constant was calculated spectro-
photometrically using the half height and limiting
absorbance methods [19]. It is obvious from the data that
one pKa value is obtained, amounting to 5.20, which can be
attributed to the dissociation of the COOH proton. The
positive value of DS° indicates the nonspontaneous char-
acter of dissociation reaction.
IR spectral studies
The spectrum of the free bis-Schiff base ligand showed a
band in the region at 1,597 cm-1 characteristic of the
azomethine stretching mode indicating the formation of the
bis-Schiff base product. This band was shifted toward
lower/higher frequencies in the spectra of metal complexes
(1,590–1,689 cm-1) compared with the parent bis-Schiff
base. This phenomenon appears to be due to the coordi-
nation of the azomethine nitrogen to the metal ion [19, 24].
Calculation of metal complex formation constants
The stability constants of M(II)/(III) complexes with the bis-
Schiff base ligand were determined spectrophotometrically
123

E. M. Zayed et al.
The intense band at 1,686 cm-1 present in the IR spectrum
of the free bis-Schiff base ligand may be assigned to the
carboxylic group. However, the spectra of all the com-
plexes showed variety in the characteristic band
accompanied by the appearance of the two characteristic
bands at 1,469–1,501 and 1,369–1,404 cm-1 attributed to
(COO-)(asym) and (COO-)(sym), respectively, indicating the
participation of the carboxylate oxygen atom in the com-
plex formation. The IR spectra of the complexes clearly
demonstrated that the C–O–C and C–C–O stretching
vibrations are altered compared to those of the bis-Schiff
base ligand owing to conformational changes. The fact that
the C–O–C absorptions of the complexes are shifted to
higher wavenumbers compared to those of the ligand also
confirms the complex formation [19, 25, 26].
Conclusive evidence of the bonding was also shown by
the observation that new bands in the spectra of all metal
complexes appear in the low frequency regions at 463–475
and 513–532 cm-1 characteristic of M–N and M–O
stretching vibrations, respectively, that were not observed
in the spectrum of the free bis-Schiff base ligand. So, it can
be concluded that the bis-Schiff base ligand behaves as a
double negatively charged hexadentate ligand coordinated
to the metal ions via azomethine N, deprotonated carbox-
ylic O, and ether O.
Fig. 2 UV–vis absorption spectra of the bis-Schiff base ligand and its
metal chelates. [M] = [bis-Schiff base ligand] = 5 9 10-4 M,
T = 25 °C
for the free bis-Schiff base. The absorption spectra of the
complexes are given in Fig. 2.
The diffused reflectance spectral data of the complexes,
position of bands, and their transition assignments are of
particular importance as they are highly dependent on the
geometry of the molecule [27]. The electronic spectrum of
the Cr(III) complex shows three absorption bands at
15,556, 21,969, and 27,700 cm-1. The three spin-allowed
transitions for Cr(III) in an octahedral field are as follows:
1H NMR spectra
4
4
4
4A2g (F) ? T2g (F), A2g (F) ? 4T1g (F), and A2g
The formation of the bis-Schiff base ligand was conve-
1
niently monitored by peak ratios in the H NMR spectra
(F) ? T1g (P). The band at 15,556 cm-1 is a direct
4
measurement of the ligand field parameter 10Dq. In addi-
tion, the magnetic moment value (3.61 BM) can be taken
as additional evidence for the octahedral geometry [27].
From the diffused reflectance spectrum of the Fe(III)
complex, the bands at 14,096, 18,625, and 24,585 cm-1
recorded in DMSO-d6 using tetramethylsilane (TMS) as
internal standard. The azomethine proton signal in the
spectrum of the zinc complex (d = 7.95 ppm) was shifted
upfield compared to the free bis-Schiff base ligand
(7.68 ppm), suggesting deshielding of the azomethine
group as a result of the coordination with the metal ion.
The disappearance of the COOH signal in the spectrum of
the Zn(II) complex relative to free bis-Schiff base
(10.31 ppm) indicates the involvement of the COOH group
in chelation with proton displacement. In addition, multiple
signals observed at 4.59 and 6.49–7.67 ppm (free bis-
Schiff base ligand) and 4.58 and 7.10–7.67 ppm (Zn(II)
complex) are assignable to the aromatic and OCH2 protons,
respectively.
6
6
5
may be assigned to the A1g ? T2g(G) and A1g ? T1g
transitions in octahedral geometry of the complex [28]. The
spectrum shows also a band at 27,777 cm-1 which may be
attributed to ligand to metal charge transfer [27, 29].
The diffused reflectance spectrum of the Cu(II) complex
displayed the d–d transition band in the region 13,606 cm-1
2
2
which is due to Eg ? T2g transition. This d–d transition
band strongly favors a distorted octahedral geometry around
the metal ion. The spectrum of the Ni(II) complex displayed
three d–d bands at 13,150, 14,749, and 23,310 cm-1. These
3
3
correspond to 3A2g (F) ? T2g (F), 3A2g (F) ? T1g (F), and
Magnetic susceptibility and electronic spectral studies
3A2g (F) ? T1g (P) transitions, respectively, being char-
3
acteristic of an octahedral geometry [27, 29]. This geometry
is further supported by its magnetic susceptibility value
(2.76 BM). The diffused reflectance spectrum of the Co(II)
complex displayed the d–d transition bands in the region
15,017, 16,723, and 23,566 cm-1 which are assigned to 4T1g
The electronic absorption spectra of the chelates were
recorded at room temperature using DMF/ethanol mixture
as solvent. The electronic spectra of the free ligand in this
solvent showed strong absorption bands in the ultraviolet
region that could be attributed to the p–p* and n–p* tran-
sitions in the benzene ring or azomethine or C=O groups
4
4
4
(F) ? A2g (F), 4T1g (F) ? T2g (F), and 4T1g (F) ? T1g
123

Synthesis, spectroscopic and structural characterization
Fig. 3 Thermal analyses (TG and DTG) of a bis-Schiff base ligand and b Fe(III), c Co(II), and d Cu(II) bis-Schiff base complexes
Table 1 Thermoanalytical results (TG and DTG) of bis-Schiff base ligand and its metal complexes
Compound
TG range/°C
DTGmax/°C
n
Mass loss
Calcd (estim)/%
Total mass loss
Assignment
Metallic residue
2
150–375
375–970
25–175
291
1
2
1
1
1
1
2
1
1
2
2
29.33 (30.09)
70.67 (69.91)
18.72 (18.72)
9.55 (9.53)
100 (100)
Loss of 3CO2 and NH3
–
414, 711
160
Loss of C27H21N
3b
89.28 (89.73)
Loss of 3HCl and 1/2O2
Loss of 2NO and 2H2
Loss of CO2
FeO
175–370
370–475
475–765
30–285
372
472
6.56 (6.53)
619
54.45 (54.95)
25.24 (24.86)
14.42 (14.69)
48.60 (48.76)
22.57 (22.84)
65.06 (64.73)
Loss of C29H17
3c
3e
110, 219
291
88.26 (88.31)
87.63 (87.57)
Loss of 2HCl, 2NO and CO
Loss of CO2 and 3CH4
Loss of C25H10
CoO
CuO
285–480
480–810
25–465
655
161, 388
505, 639
Loss of 2HCl, CO2 and CO
Loss of O2, 2NH3 and C28H16
465–760
n number of decomposition steps
(P) transitions, respectively [27, 29]. The transitions corre-
spond to the octahedral geometry of the complex, which is
also supported by its magnetic susceptibility value
(5.25 BM). The complex of Zn(II) is diamagnetic. Accord-
ing to the empirical formula, an octahedral geometry is
proposed for this complex.
Schiff base and their metal(II)/(III) complexes. The ther-
mogravimetric analysis (TG) (Table 1) and curves were
obtained at a heating rate of 10 °C/min in a nitrogen
atmosphere over a temperature range of 20–900 °C. The
bis-Schiff base ligand was stable up to 150 °C and its
decomposition started at this temperature. In the decom-
position process of the ligand, the mass losses
corresponded to 3CO2, NH3, and C27H21N molecules. The
decomposition of the bis-Schiff base ligand is irreversible.
The Fe(III) complex starts its decomposition at 25 °C.
In the decomposition process of the Fe(III) complex, the
Thermal analysis studies (TG and DTG)
Figure 3 and Table 1 show the TG and DTG results
introducing a new possibility for investigation of the bis-
123

E. M. Zayed et al.
mass losses corresponded to 3HCl, 1/2O2, 2NO, 2H2, CO2,
and C29H17 molecules leaving FeO in the last stage of the
decomposition. Sharp peaks were found at 160, 372, 472,
and 619 °C in the DTG curve which demonstrated the
decomposition of the complex in four steps. The overall
weight loss of 89.73 % (theoretically 89.28 %) may be in
accordance with the proposed structure of the complex
shown in Scheme 1.
On the basis of the above observations, the strucutres of the
complexes are shown in Scheme 1.
Biological activity
Because of the many biological roles played by these metal
ions efforts have been made to synthesize metal complexes
to study their structure, spectral, electrochemical, and
magnetic properties. The synthesized ligand and the com-
plexes were tested for their in vitro antimicrobial activity
against bacteria (Staphylococcus aureus and Escherichia
coli) and fungi (Aspergillus flavus and Candida albicans).
The inhibition zone diameter (mm/mg sample) values of
the investigated compounds are summarized in Table 2. A
comparative study of inhibition zone diameter (mm/mg
sample) values of the bis-Schiff base ligand and the com-
plexes indicates that the metal complexes exhibit higher
antimicrobial activity than the free bis-Schiff base ligand
(Table 2; Fig. 4). Such increased activity of the complexes
can be explained on the basis of Tweedy’s chelation theory
[31, 32]. Chelation reduces the polarity of the metal ion
considerably because of the partial sharing of its positive
charge with the donor group and also due to p-electron
delocalization on the whole chelate ring. The lipids and
polysaccharides are some important constituents of the cell
wall and membranes which are preferred for metal ion
interaction. Apart from this, the cell walls also contain
many phosphates, carbonyl, and cysteinyl ligands which
maintain the integrity of the membrane by acting as a
diffusion barrier and also provide suitable sites for binding.
Furthermore, the reduction in polarity increases the lipo-
philic character of the chelates and an interaction between
the metal ion and the lipid is favored. This may lead to the
breakdown of the permeability barrier of the cell resulting
in interference with the normal cell processes. Besides this,
the complexes may also form hydrogen bonds through the
coordinated anions and azomethine group with the active
centers of the cell constituents [31, 32]. Factors capable of
increasing the lipophilic nature are expected to enhance the
antimicrobial properties. The synthesized Schiff base
ligand has moderate inhibitory effects on the growth of the
tested microorganisms. This is due to the presence of
azomethine groups which have chelating properties. These
properties may be used in metal transport across the bac-
terial membranes or to attach to the bacterial cells at a
specific site from which it can interfere with their growth.
The antimicrobial activity of metal complexes shows
greater bactericidal and fungicidal activities as compared to
their corresponding Schiff base. It is clear from Table 2
that the Cr(III), Co(II), and Ni(II) complexes show anti-
fungal activity against C. albicans in comparison with the
free bis-Schiff base ligand which has no activity. Also, the
Zn(II) complex shows a higher antibacterial activity than
The Co(II) complex starts decomposition at 30 °C. In
the decomposition process of the Co(II) complex, the mass
losses corresponded to 2HCl, 2NO, CO, CO2, 3CH4, and
C25H10 molecules leaving CoO at the end of the decom-
position. The Cu(II) complex was stable up to 25 °C and its
decomposition started at this temperature. In the decom-
position process of the Cu(II) complex, the mass losses
corresponded to 2HCl, CO2, CO, O2, 2NH3, and C28H16
molecules leaving CuO in the fourth stages of the
decomposition. The decompositions of the Fe(III), Co(II),
and Cu(II) complexes are irreversible. The total weight
losses for Fe(III), Co(II), and Cu(II) complexes were found
to be approximately the same, when expressed as the
percentages calculated stoichiometrically from their
chemical formulas given in Table 1.
X-ray diffraction
X-ray powder diffraction crystallographic investigation is
the most precise source of information regarding the
structure of the bis-Schiff base and its complexes. The data
were recorded over the 2h = 0°–80° range. The trend of
the curves gives a clear idea about the crystallinity of the
complexes. It indicates that all the metal complexes are
amorphous except the Cr(III) and Cu(II) chelates, which
are crystalline.
Calculation of activation thermodynamic parameters
The thermodynamic activation parameters of decomposi-
tion processes of dehydrated complexes are evaluated
graphically by employing the Coats–Redfern relation [30]
and the data are summarized in Table S1 (Supplementary
Material). The high values of the activation energies reflect
the thermal stability of the complexes. The entropy of
activation is found to have negative values in all the
complexes which indicates that the decomposition reac-
tions proceed with a lower rate than the normal ones.
Structural interpretation
Single crystals of the complexes could not be isolated from
any solutions; thus no definitive structure could be
described. However, the analytical, spectroscopic, and
magnetic data enable us to propose the possible structures.
123

Synthesis, spectroscopic and structural characterization
Table 2 Biological activity of
Sample
Inhibition zone diameter/mm/mg sample-1
the bis-Schiff base ligand and its
metal complexes
Escherichia
coli (G-)
Staphylococcus
aureus (G?)
Aspergillus
flavus (fungus)
Candida
albicans (fungus)
Control: DMSO
Standard
0.0
0.0
0.0
0.0
Tetracycline
(antibacterial agent)
33
–
30
–
–
–
Amphotericin B
(antifungal agent)
18
19
2
13
12
11
16
14
13
20
14
12
11
15
15
14
28
0
0
0
0
0
0
0
0
3a
3b
3c
3d
3e
3f
11
0
12
14
0
0
Fig. 4 Percent of inhibition
zone diameter (mm/mg sample)
relative to using standard vs. the
bis-Schiff base ligand and its
metal complexes
the free bis-Schiff base ligand and the remaining
complexes.
recommended conditions. They are pH 3.5, temperature
25 °C, time 45 min, and 1:1 [Fe(III)]/[L] ratio.
The present investigations of antimicrobial activity data
indicated that all of the newly synthesized complexes
exhibited different antimicrobial activity as compared to
that of the control drugs. According to these data, it can be
concluded that these complexes might be recommended
and/or established as new candidates for the search for new
antibacterial and antifungal agents [28, 33].
Effect of foreign ions
In order to assess the applicability of the proposed method
to recover Fe(III) added to water samples, the effect of
some foreign ions was investigated. These foreign ions
were selected on the basis that they are normally present in
fresh and saline waters. Solutions containing various
amounts of foreign ions, Fe(III), and the ligand were sub-
jected to the described spectrophotometric procedure. The
tolerable amounts of each ion, given a maximum error of
2 % in the recovery, are summarized in Table S2 (Sup-
plementary Material). As can be seen, all the investigated
foreign ions with a relatively high concentration have no
adverse effect on the ferric ions to be determined. There-
fore, the recommended procedure may be applied to
natural water samples.
Application: spectrophotometric determination
of Fe(III) ions in natural water
In this work, the bisaldehyde-type ligand was utilized for
the spectrophotometric determination of Fe(III) in different
natural water samples. This reaction produces a stable
colored product which can be measured spectrophotomet-
rically at 335 nm. Different analytical parameters were
optimized in order to achieve maximum recovery under the
123

E. M. Zayed et al.
their complexes were determined. The activation kinetic
and thermodynamic parameters were calculated in different
stages of thermal decomposition. We also evaluated the
in vitro activities of the new bis-Schiff base and its metal
complexes; the results demonstrated that the compounds
have good to moderate antibacterial and antifungal activity
against the bacterial and fungal strains tested.
Table 3 Recovery percent of Fe(III) in spiked natural water samples
Water samples
(location)
[Fe(III)] [Fe(III)] Recovery/ SD
spiked/
RSD/
%
found/
mg cm-3 mg cm-3
%
Distilled water
(Cairo
University)
2.63
6.44
2.58
6.41
98.10
99.53
0.060 2.33
0.078 1.22
A new analytical application was developed to measure
Fe(III) in aqueous solution, based on the reaction of Fe(III)
with 2,20-[1,2-ethanediylbis[oxy(2,1-phenylenemethylidyn
enitrilo)]]bis(benzoic acid). This reaction produces a stable
color, which can be measured by absorbance at 330 nm.
The selectivity of the proposed method for the determina-
tion of Fe(III) was studied in the presence of various
concentrations of cations and anions. The tolerance limits
of the ions do not cause a deviation of more than 2 % in
the absorbance of the Fe(III) complex. The method is
simple and does not require any difficult or time-consum-
ing operations. It can be performed by using standard
laboratory instruments (spectrophotometer). Therefore, the
synthesis of these complexes might provide a new
approach for the determination of Fe(III) ions in natural
water.
Tap water (Cairo
University)
2.63
6.44
2.55
6.40
2.60
6.37
2.59
6.39
2.57
6.39
2.61
6.38
96.96
99.38
98.86
98.91
98.48
99.22
97.72
99.22
99.24
99.07
0.047 1.84
0.062 0.97
0.055 2.12
0.042 0.66
0.064 2.47
0.036 0.56
0.028 1.09
0.058 0.91
0.031 1.19
0.064 1.00
Nile water (Egypt) 2.63
6.44
Sea water
(Gamasah)
2.63
6.44
Underground water 2.63
(Belqas)
6.44
Well water (sixth
October City)
2.63
6.44
Conditions: 5 9 10-4 mol/cm3 bis-Schiff base ligand, t = 45 min,
pH 3.5, T = 25 °C, and kmax = 315 nm
Application
The proposed analytical procedure was applied to different
types of water samples in order to assess its accuracy.
Recovery tests were carried out on different types of water
samples. The selection of these samples was done in a way to
provide a wide variety of sample matrices characterized by
different types of interferents. Solutions of pre-filtered water
samples(10 dm3 each)containingFe(III) with a concentration
of 5 or 10 mg/cm3 were recovered under the recommended
conditions. ThedatainTable 3showasatisfactoryrecoveryof
Fe(III). So, the synthesis of these complexes might provide a
new approach for the search for organic ligands for the
removal of ferric ions from natural water.
Experimental
All the chemicals used were of high purity analytical grade.
CuCl2ꢀ2H2O (Sigma), CoCl2ꢀ6H2O, and NiCl2ꢀ6H2O
(BDH), CrCl3ꢀ6H2O and ZnCl2ꢀ2H2O (Ubichem), and
FeCl3ꢀ6H2O (Prolabo) were used. Organic solvents like
absolute ethyl alcohol, diethyl ether, and dimethylform-
amide were supplied by BDH. 2-Aminobenzoic acid was
supplied by Aldrich. 2,20-(Ethylenedioxy)bis(benzalde-
hyde) was prepared as previously described [34].
Stock solution of the bis-Schiff base ligand
(5 9 10-4 M) was prepared by dissolving the appropriate
amount of 0.395 g in 100 cm3 of doubly distilled water.
Working solutions were obtained from the stock solution
by further dilution with doubly distilled water. Stock
solutions (5 9 10-4 M) of the metal salts (Cr(III) 0.341 g/
dm3, Fe(III) 0.117 g/dm3, Co(II) 0.119 g/dm3, Ni(II)
0.119 g/dm3, Cu(II) 0.085 g/dm3, and Zn(II) 0.086 g/dm3)
were prepared by dissolving the accurately weighed
amounts of the metal salts in the appropriate volume of
doubly distilled water. A mixture of 0.04 M phosphoric,
acetic, and boric acids was prepared to obtain universal
buffer solutions of the required pH values from 2 to 13
adjusting using NaOH or HCl (0.1 M) prior to dilution of
the mixture to 100 cm3 with water. The reference buffer
was prepared analogously. Zinc oxide stock solution
(0.05 M) was prepared and the 0.01 M dilute solution was
prepared by accurate dilution. The 0.049 M disodium salt
Conclusions
The new metal(II)/(III) complexes synthesized in this work
were characterized using different physicochemical and
spectroscopic methods. The interaction of the metal ions
with the bis-Schiff base ligand was studies with UV–vis
spectroscopy. The geometry can be described as octahedral
and the thermal study indicates the coordination with the
metal ion. Magnetic moment, electronic spectra, and con-
ductance studies provide further evidence of the geometry
of the complexes. All these studies support the proposed
1
structures. The results of H NMR studies were in good
agreement with those obtained from IR studies. XRD
analysis established the crystalline and morphological
nature of the complexes. The proton dissociation constants
of the bis-Schiff base ligand and the stability constant of
123

Synthesis, spectroscopic and structural characterization
of EDTA solution (MW = 372 g/mol) was prepared by
dissolving 4.557 g in 250 cm3 doubly distilled water. This
solution was standardized compleximetrically. Ammonium
chloride (54 g) was mixed with 350 cm3 of ammonia
solution (33 % v/v). This mixture was diluted up 1 l to
prepare an ammoniacal buffer of pH 10. Sodium hydroxide
solution (10 %) was prepared by dissolving 25 g in
250 cm3 doubly distilled water.
distinct pH values ranging from 2 to 12. The degree of
formation of the complex is obtained from the
relationship
nꢁ ¼ ðAx ꢂ ALÞ=ðAML ꢂ ALÞ
ð1Þ
where Ax, AL, and AML are the absorbances of the partially
formed complex at a specific pH, the free ligand, and the
fully formed complex, respectively. Then, the negative
logarithm of the concentration of non-protonated ligand
(pL) was obtained using Eq. 2:
Elemental microanalyses of the separated solid chelates
for C, H, N, and Cl were performed at the Microanalytical
Center, Cairo University using a CHNS-932 (LECO) Vario
Elemental analyzer. The molar conductance of the com-
plexes in DMF was measured using a Sybron-Barnstead
conductometer (Meter-PM.6, E = 3,406). Infrared spectra
were recorded on a Perkin-Elmer FT-IR type 1,650 spec-
trophotometer. The spectra were recorded as KBr pellets.
The UV–vis absorption spectra were measured on a Shi-
madzu 3101pc spectrophotometer. The molar magnetic
susceptibility was measured on powdered samples using the
Faraday method. The diamagnetic corrections were made by
Pascal’s constant and Hg[Co(SCN)4] was used as a calibrant.
The mass spectra were recorded by the EI technique at 70 eV
using an MS-5988 GS-MS Hewlett-Packard instrument. The
1H NMR spectra were recorded using a 300-MHz Varian-
Oxford Mercury. The thermal analyses were carried out in a
dynamic nitrogen atmosphere (20 cm3 min-1) with a heat-
ing rate of 10 °C min-1 using a Shimadzu TG-60H thermal
analyzer. XPD analyses were carried out by using a Philips
Analytical X-ray BV, diffractometer type PW 1840. Radia-
tion was provided by a copper target (Cu anode 2,000 W)
high intensity X-ray tube operated at 40 kV and 25 mA. All
spectrophotometric measurements were performed using a
Unicam UV-2 spectrophotometer using 10-mm quartz cell
and a blank solution as a reference. The pH measurements
were made using a HANNA pH meter.
Â
Â
Ã
Â
Ã
Ã
2
pL ¼ log ð1 þ BH1 Hþ þB2H Hþ þ ꢀ ꢀ ꢀÞ=ðTL ꢂ nTMÞ
ð2Þ
where BH1 and BH2 are the reciprocals of the acid
dissociation constants of the ligand, i.e., [LH1]/
[L][H] and [LH2]/[L][H]2, respectively. TL and TM
represent the stoichiometric concentration of the ligand
and metal. The stability constant (K) can be calculated
from Eq. 3:
nꢁ þ ðnꢁ ꢂ 1ÞKðLÞ ¼ 0
2,20-[1,2-Ethanediylbis[oxy(2,1-phenylenemethyl-
ð3Þ
idynenitrilo)]]bis(benzoic acid) (2, C30H24N2O6)
The bis-Schiff base was prepared by refluxing a mixture of
3.654 g 2-aminobenzoic acid (13.34 mmol) and 1.6 g 2,20-
(ethylenedioxy)bis(benzaldehyde) (6.67 mmol) in etha-
nolic medium on a water bath for 3 h. The resulting
yellow precipitate was filtered and purified by crystalliza-
tion from ethanol, then dried in vacuum to give the desired
bis-Schiff base ligand. Yellowish brown solid; yield 95 %;
ꢀ
m.p.: 96 °C; IR: m = 1,686 (C=O), 1,485 (COO)(asym)
1,388 (COO)(sym), 1,597 (HC=N)azomethine, 1,045 (C–O–
C)ether cm-1 1H NMR (DMSO-d6): d = 4.59 (m, 2H,
,
;
OCH2), 6.49–7.67 (m, 4H, ArH), 7.68 (s, 1H, CH=N),
10.31 (s, 1H, COOH) ppm; 13C NMR (DMSO-d6):
d = 68.9 (O–CH2–CH2–O), 112.0, 120.4, 125.1, 127.3,
131.6, 158.5 (O–C6H4), 128.6, 130.2, 130.3, 133.9
(N–C6H4), 160.8 (–CH=N), 169.3 (–COOH) ppm; pKa
(-DG°) = 5.20 (31.52 kJ mol-1).
Spectrophotometric determination of ionization
constant
The absorption spectra of the bis-Schiff base ligand
(1.0 9 10-4 M) were scanned within the wavelength range
from 200 to 700 nm in a universal buffer of different pH
values ranging from 2 to 13. From the absorption spectra of
the bis-Schiff base ligand, the dissociation constants were
calculated by applying half height and limiting absorbance
methods [19].
Synthesis of metal complexes
To a solution of 0.395 g Schiff base ligand (1 mmol) in
30 cm3 ethanol a solution of metal(II)/(III) chlorides
(1 mmol) in 30 cm3 ethanol was added. The resulting
mixture was refluxed for 2.5 h on a water bath whereupon
the complexes precipitated, which were then filtered off
and washed thoroughly with ethanol and diethyl ether for
further purification and dried in vacuum. The metal content
of the complexes was analyzed complexometrically
[29, 36].
Calculation of metal complex formation constants
Chelate stability constants were obtained spectrophoto-
metrically [20, 35] by measuring the absorbance of
solutions of ligand and metal at fixed concentration but at
123

E. M. Zayed et al.
[2,20-[1,2-Ethanediylbis[oxy(2,1-phenylenemethyl-
idynenitrilo)]]bis(benzoato)] chromium(III) chloride
tetrahydrate (3a, C30H22ClCrN2O6)
125.2, 127.3, 131.4, 158.4 (O–C6H4), 128.5, 130.3, 130.3,
133.8 (N–C6H4), 160.9 (–CH=N), 169.4 (–COO) ppm; log
b (-DG°) = 3.67 (222.4 kJ mol-1).
Dark green solid; yield 80 %; m.p.: 130 °C;
2
leff = 3.61 BM; Km = 56.27 X-1 mol-1 cm ; IR: m =
Biological activity
ꢀ
1,640 (C=O), 1,489 (COOasym), 1,369 (COOsym), 1,689
(HC=Nazomethine), 1,049 (C–O–Cether), 532 (M–O), 463
(M–N) cm-1; log b (-DG°) = 2.56 (155.2 kJ mol-1).
[2,20-[1,2-Ethanediylbis[oxy(2,1-phenylenemethyl-
idynenitrilo)]]bis(benzoato)]iron(III) chloride trihydrate
(3b, C30H22ClFeN2O6)
The standardized disk-agar diffusion method [37] was
followed to determine the antibacterial and antifungal
activity of the synthesized compounds. The tested com-
pounds were dissolved in DMF (which has no inhibition
activity) to get concentrations of 100 lg/cm3. Uniform size
filter paper disks (3 disks per compound) were impregnated
by equal volume (0.1 cm3) from the specific concentration
of dissolved tested compounds and carefully placed on the
incubated agar surface. After incubation for 48 h at 37 °C,
inhibition of the organisms evidenced by a clear zone
surrounding each disk was measured and used to calculate
the mean inhibition zones.
Dark green solid; yield 83 %; m.p.: 196 °C;
2
leff = 5.30 BM; Km = 63.86 X-1 mol-1 cm ; IR: m =
ꢀ
1,655 (C=O), 1,496 (COOasym), 1,383 (COOsym), 1,600
(HC=Nazomethine), 1,055 (C–O–Cether), 529 (M–O), 474
(M–N) cm-1; log b (-DG°) = 3.15 (190.9 kJ mol-1).
[2,20-[1,2-Ethanediylbis[oxy(2,1-phenylenemethyl-
idynenitrilo)]]bis(benzoato)]cobalt(II) tetrahydrate
(3c, C30H22CoN2O6)
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2
Km = 12.40 X-1 mol-1 cm ; IR: m = 1,639 (C=O), 1,474
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123

Synthesis, spectroscopic and structural characterization
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123