Synthesis, spectroscopic, photoluminescence properties and biological evaluation of novel Zn(II) and Al(III) complexes of NOON tetradentate Schiff bases

Article abstract of DOI:10.1016/j.saa.2012.06.023

Novel mononuclear Zn(II) and Al(III) complexes were synthesized from the reactions of Zn(OAc)₂·2H₂O and anhydrous AlCl ₃ with neutral N2O2 donor tetradentate Schiff bases; N,N′bis(salicylaldehyde)4,5-dimethyl-1,2-phenylenediamine (H ₂L¹) and N,N′bis(salicylaldehyde)4,5-dichloro-1,2- phenylenediamine (H₂L²). The new complexes were fully characterized by using micro analyses (CHN), FT-IR, ¹H NMR, UV-Vis spectra and thermal analysis. The analytical data have been showed that, the stoichiometry of the complexes is 1:1. Spectroscopic data suggested tetrahedral and square pyramidal geometries for Zn(II) and Al(III) complexes, respectively. The synthesized Zn(II), and Al(III) complexes exhibited intense fluorescence emission in the visible region upon UV-excitation in methylene chloride solution at ambient temperature. This high fluorescence emission was assigned to the strong coordination of the ligands to the small and the highly charged Zn(II) and Al(III) ions. Such strong coordination seems to extend the π-conjugation of the complexes. Thermal analysis measurements indicated that the complexes have good thermal stability. As a potential application the biological activity (e.g., antimicrobial action) of the prepared ligands and complexes was assessed by in-vitro testing of their effect on the growth of various strains of bacteria and fungi.

Full text of DOI:10.1016/j.saa.2012.06.023

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 388–396
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, spectroscopic, photoluminescence properties and biological evaluation
of novel Zn(II) and Al(III) complexes of NOON tetradentate Schiff bases
Ayman A. Abdel Aziz, Ibrahim H.A. Badr ,1, Ibrahim S.A. El-Sayed
⇑
Department of Chemistry, Faculty of Science, Ain Shams University, 11566 Cairo, Egypt
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
Synthetic route of Zn(II) and Al(III) with tetradentate Schiff bases H2L1–2
.
"
Novel Zn(II) and Al(III) salen
complexes were synthesized and
fully characterized.
The complexes exhibited intense
fluorescence at room temperature.
The complexes are promising
candidates as photoactive materials
and anion carriers.
Zn(CH3COO)2.2H2O
[ZnL1-2].H2O
"
"
CH3OH, reflux
H2L1-2
AlCl3
CH3CN, room temp.
[AlL1-2Cl]
"
The novel complexes exhibited
remarkable antimicrobial activities.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 April 2012
Received in revised form 2 June 2012
Accepted 10 June 2012
Available online 28 June 2012
Novel mononuclear Zn(II) and Al(III) complexes were synthesized from the reactions of Zn(OAc)2ꢀ2H2O
and anhydrous AlCl3 with neutral N2O2 donor tetradentate Schiff bases; N,N0bis(salicylaldehyde)4,5-
dimethyl-1,2-phenylenediamine (H2L1) and N,N0bis(salicylaldehyde)4,5-dichloro-1,2-phenylenediamine
(H2L2). The new complexes were fully characterized by using micro analyses (CHN), FT-IR, 1H NMR,
UV–Vis spectra and thermal analysis. The analytical data have been showed that, the stoichiometry of
the complexes is 1:1. Spectroscopic data suggested tetrahedral and square pyramidal geometries for
Zn(II) and Al(III) complexes, respectively. The synthesized Zn(II), and Al(III) complexes exhibited intense
fluorescence emission in the visible region upon UV-excitation in methylene chloride solution at ambient
temperature. This high fluorescence emission was assigned to the strong coordination of the ligands to
the small and the highly charged Zn(II) and Al(III) ions. Such strong coordination seems to extend the
Keywords:
NOON Schiff bases
Zn(II) and Al(III) complexes
Characterization
Photoluminescence
Microbial activities
p
-conjugation of the complexes. Thermal analysis measurements indicated that the complexes have good
thermal stability. As a potential application the biological activity (e.g., antimicrobial action) of the pre-
pared ligands and complexes was assessed by in-vitro testing of their effect on the growth of various
strains of bacteria and fungi.
Ó 2012 Elsevier B.V. All rights reserved.
Introduction
ligands as well as their complexes are widely used for the prepara-
tion of highly selective polymer membrane electrodes [8–12], opti-
Schiff-base ligands have received more and more attention,
mainly because of their wide application in the fields of synthesis
[1], catalysis [2,3], solid phase extraction of metal ions [4], various
types of polymerization [5], biological application as antibacterial,
antifungal, anti-inflammatory herbicidal properties and antitumor
agents [6,7]. Furthermore, Schiff bases and other macrocyclic
cal sensors and biological probes [13–15]. This attention is still
growing and considerable research effort is still devoted to the
synthesis of new Schiff-base complexes with transition [16,17]
and main group metal ions [18–20].
At present, much attention has been paid to complexes of Zn(II)
and Al(III) Schiff base ligands, exhibiting luminescent properties
⇑
Corresponding author. Present address: Taibah University, Faculty of Applied Science, Department of Applied Chemistry, Al Madinah, Saudi Arabia. Tel.: +966
0500213248; fax: +966 048494531.
E-mail address: ibadr1@yahoo.com (I.H.A. Badr).
Chemistry Department, Faculty of Science, Ain Shams University, Cairo, Egypt.
1
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.06.023

A.A. Abdel Aziz et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 388–396
389
[21–27]. Such complexes are promising materials for optoelec-
Syntheses
tronic applications due to their outstanding photo and electrolumi-
nescent (PL and EL) properties [28,29].
Microwave assisted solvent-free synthesis of the Schiff base
Herein, we report the synthesis, spectroscopic characterization,
photophysical properties and thermal analysis of novel Zn(II) and
Al(III) complexes with salen derivatives N,N0bis(salicylalde-
hyde)4,5-dimethyl-1,2-phenylenediamine (H2L1), and N,N0bis(sali-
cylaldehyde)4,5-dichloro-1,2-phenylenediamine (H2L2),(Scheme 1).
Fluorescence emission upon complex formation opens up the
opportunity for photochemical applications of these complexes.
Furthermore, the bio-efficacy of ligands and the synthesized com-
plexes were tested in-vitro against various strains of bacteria and
fungi at different concentration levels to evaluate their utility as
potential antimicrobial agents.
ligand (H2L1–2
)
The ligands H2L1–2 were synthesized as described in literature
[32]. To enhance the yield and reduce the time a microwave syn-
thesis method was used. 0.1 mol of the diamine derivative and
0.2 mol of salicylaldehyde were mixed well in a 50 ml Pyrex beaker
and the mixture was irradiated in a microwave oven for one min-
ute. The yellow product obtained was separated, dried and recrys-
tallized from ethanol. The purity of the ligand was checked by TLC.
Synthesis of Zn(II) complexes
Zn(OAc)2ꢀ2H2O (1.0 mmol) was dissolved in methanol (10 mL)
and heated using a water bath to ensure complete dissolution of
the salt, then added gradually to a hot stirred solution of the ligand
(1.0 mmol) dissolved in methanol (10 mL). The mixture was fur-
ther heated on a hot plate at 60 °C for 2 h to ensure complete pre-
cipitation of the yellow complexes. The precipitated solid was
filtered, washed with cold methanol and hot petroleum ether
40–60 °C, recrystallized from ethanol and finally dried under vac-
uum. The purity of the complexes was checked by TLC.
Experimental
Materials
4,5-Dimethyl-1,2-phenylendiammine, 4,5-dichloro-1,2-pheny-
lendiammine, salicylaldehyde, Zn(CH3COO)2ꢀ2H2O and AlCl3 were
supplied from Aldrich. All solvents were of analytical grade.
Synthesis of Al(III) complexes
Al(III) complexes were obtained from appropriate ligands upon
the reaction with anhydrous AlCl3 in dry acetonitrile at room tem-
perature. This procedure was successfully applied in earlier work
[11]. AlCl3 (0.200 g, 1.5 mmol) was added to a solution of 1 mmol
of free ligands in 20 mL of freshly distilled acetonitrile. A yellow
precipitate was formed immediately. The precipitated product
was filtered off, washed with cold ethyl acetate and dried under
vacuum. The complex was purified by crystallization from ethyl
acetate/ethanol mixture.
Instrumentation
Carbon, hydrogen and nitrogen were determined using JEOL
JMS-AX500 elemental analyzer. Infrared spectra (4000–400 cmꢁ1
)
were recorded with KBr optics on a Unicam-Mattson 1000 FT-IR.
1H NMR measurements were performed on a Spectrospin-Bruker
AC 200 MHz spectrometer using DMSO-d6 as a solvent and TMS
as an internal reference. All conductivity measurements were per-
formed in DMSO (1 ꢂ 10ꢁ3 M) at 25 °C, by using Jenway 4010 con-
Scheme 2 represents the synthetic route of synthesis of Zn(II)
and Al(III) complexes.
ductivity meter. The thermal behavior was monitored by
a
Shimadzu DT-50 thermal analyzer under nitrogen atmosphere
with a heating rate of 10 °C/min. UV–Vis spectra were recorded
using a Shimadzu UV 1800 Spectrophotometer. The UV spectrom-
eter is controlled with a PC by using UVProbe software, version
2.34. The photoluminescence properties of all compounds were
studied using a Jenway 6270 Fluorimeter. All samples were pre-
pared in spectroscopic grade methylene chloride (1 ꢂ 10ꢁ5 M)
and analyzed in a 1 cm optical path quartz cuvette. The photolumi-
nescence quantum efficiencies of the complexes were calculated
by using quinine sulfate as reference quantum yield standard
(kex = 410 nm, quantum yield = 0.54 in 0.1 mol Lꢁ1 H2SO4) [30,31].
Determination of the metal content of the complexes
An accurately weighed portion of the solid complex under
investigation in the range from 10 to 30 mg was placed in Kjeldahl
flask. Concentrated nitric acid (5–10 mL) was added to start the
fast wet oxidation digestion. This mixture was digested by a grad-
ual heating with a concomitant dropping of H2O2 solution. This
treatment was conducted until the entire solid complex was dis-
solved. The obtained clear solution was diluted up to exactly 50
mL by de-ionized water and the metal content was determined
by standard analytical methods described in literature [33].
R
R
Determination of fluorescence quantum yield of Zn(II) and Al(III)
complexes
Determination of the relative quantum yield is generally
accomplished by a comparison of the wavelength-integrated emis-
sion intensity of the unknown sample with that of a reference
material. The fluorescence quantum yields of Zn(II) and Al(III) com-
plexes were determined using quinine sulfate as a reference with a
known UR of 0.546 as measured in 0.5 M sulfuric acid [34]. The
N
N
HO
OH
Zn(CH3COO)2.2H2O
[ZnL1-2].H2O
CH3OH, reflux
H2L1-2
H2L1
H2L2
R = CH3
R = Cl
AlCl3
CH3CN, room temp.
[AlL1-2Cl]
Scheme 1. Chemical formula of the tetradentate Schiff bases H2L1–2
.
Scheme 2. Synthetic route of Zn(II) and Al(III) with tetradentate Schiff bases H2L1–2
.

390
A.A. Abdel Aziz et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 388–396
Table 1
Elemental analyses and some physical properties of the Schiff base ligands (H2L1–2) and their complexes.
Compounds
Yield (%)
M.Wt
Color
(Calculated) Found (%)
Km (X )
ꢁ1 cm2 molꢁ1
C
H
N
Cl
M
H2L1 (C22H20N2O2)
63
70
95
87
79
81
344.41
385.25
425.80
466.64
404.83
445.66
Yellow
Yellow
Yellow
Yellow
Yellow
Yellow
(76.72) 76.21
(62.35) 62.19
(62.05) 61.98
(51.47) 50.98
(65.27) 61.09
(53.90) 53.16
(5.85) 5.72
(3.66) 3.60
(4.26) 4.79
(2.59) 2.48
(4.48) 4.32
(2.71) 2.51
(8.13) 7.98
(7. 27) 7.25
(6.57) 6.13
(6.00) 5.86
(6.91) 6.53
(6.28) 6.17
–
–
–
–
–
H2L2 (C20H14N2O2Cl2)
[C22H18N2O2Zn]ꢀH2O (1)
[C20H12N2O2Cl2Zn]ꢀH2O (2)
[C22H18N2O2ClAl] (3)
[C20H12N2O2Cl3Al] (4)
(18.40) 18.38
-
(15.19) 15.01
(8.75) 8.09
(23.86) 23.72
(15.35) 15.27
(14.01) 13.97
(6.66) 6.24
(6.05) 5.99
13.43
12.47
9.49
10.95
cation, then poured into sterile Petri dishes (9 cm in diameter), and
left to solidify. Using a sterile cork borer (6 mm in diameter), wells
were made in each dish, then 0.1 mL of the tested compounds dis-
solved in DMSO were poured into three wells and the dishes were
incubated at 37 0.1 °C for 48 h (in case of bacteria) and 30 0.1 °C
for 72 h (in case of fungi). The growth of the microorganisms was
inhibited by diffusion of the test solutions and then the inhibition
zones around each well were measured. The effectiveness of an
antimicrobial agent is based on the size of the inhibition zones.
The diameter of the zone is measured to the nearest millimeter
(mm). The antibacterial activity of each compound was compared
with that of standard antibiotics such as Streptomycin, Chloram-
phenicol and Amoxycillin. The antifungal activity of the test com-
pound was compared with Nystatin, Tetracycline and
Chlorometazole as standard antifungal. DMSO was used as a con-
trol under the same conditions for each organism and no activity
was found. The activity results were calculated as a mean of
triplicates.
Table 2
The infrared (cm-1, KBr) data of the Schiff base ligands and their complexes.
Compound
m
(OH)
m
(C@N)
m
(C–O)
m
(M–N)
m
(M–O)
H2L1
H2L2
3428(m, br) 1617(s)
3444(m, br) 1616(s)
1278(m)
1279(m)
1321(m) 443(w)
1322(m) 462(w)
1283(m) 424(w)
1304(m) 479(w)
–
–
–
–
[ZnL1]ꢀ(H2O) (1) 3415(m, br) 1619(s)
466(w)
499(w)
448(w)
591(w)
[ZnL2]ꢀ(H2O) (2) 3411(m, br) 1614(s)
[AlL1Cl] (3)
[AlL2Cl] (4)
–
–
1621(s)
1618(s)
* br, broad; s, strong; m, medium; w, weak.
area of the emission spectrum was integrated and the quantum
yield was calculated according to the following equation [35]:
ꢀ
ꢁꢀ
ꢁ
ꢀ
ꢁ
US
UR
AS ðODÞ
gS2
gR2
AR ðODÞRS
¼
ꢂ
where US and UR are the fluorescence quantum yields of the sample
and reference, respectively, AS and AR are the areas under the fluo-
rescence emission peak of the sample and that of the reference,
respectively. (OD)S and (OD)R are the respective optical densities
of the sample and the reference solution at the wavelength of exci-
tation. gS and gR are the values of refractive indices of the solvent
used for the sample and that of the reference, respectively.
Results and discussion
Novel mononuclear Zn(II) and Al(III) complexes have been syn-
thesized from the reactions of Zn(OAc)2ꢀ2H2O and anhydrous AlCl3
with neutral [N2O2] donor tetradentate Schiff bases namely:
N,N0bis(salicylaldehyde)4,5-dimethyl-1,2-phenylenediamine
(H2L1), N,N0bis(salicylaldehyde)4,5-dichloro-1,2-phenylenedi-
amine (H2L2). The prepared complexes have been characterized
by several techniques using elemental analyses (CHN), FT-IR, 1H
NMR, UV–Vis spectra and thermal studies. All the metal complexes
have yellow color, stable in air, insoluble in water and soluble in
CHCl3, CH2Cl2 and DMSO. The analytical data obtained suggested
1:1 (M:L) stoichiometry for the complexes. The molar conductivity
Antimicrobial screening
The antimicrobial activity of the synthesized compounds was
tested against various micro-organisms. The in-vitro growth inhib-
itory of the ligands and their complexes were performed against
different types of bacteria; Staphylococcus aureus, Staphylococcus
epidermidis, Bacillus megaterium, Escherichia coli, Pseudomonas aeru-
ginosa and Klebsiella pneumoniae in Mueller Hinton-Agar medium.
The antifungal activity was tested against the fungi; Aspergillus ni-
ger, Aspergillus flavus, Curvularia lunata, Rhizoctonia bataticola, Can-
dida albicans, and Fusarium solani, cultured on YPD-agar medium.
Preliminary screening was performed using two concentrations,
(
Km) of 1ꢂ10ꢁ3 M solutions of the complexes in DMSO at 25 °C fall
in the range 9.49–13.43
X
ꢁ1 cm2 molꢁ1, indicating non-electrolytic
nature of the complexes [38]. Elemental analyses and some phys-
ical properties of the ligands and their complexes are listed in
Table 1.
125 lg/1.0 mL and 250 lg/1.0 mL of the tested compounds dis-
solved in DMSO. The in-vitro antimicrobial activity of the free li-
gands and their complexes were tested using the general
procedure described in the literature [36,37] as follows: 0.5 mL
spore suspension (106–107 spore mLꢁ1) of each of the investigated
organisms was added to a sterile agar medium just before solidifi-
Characterization of N2O2 ligands and their complexes
FT-IR spectra
The FT-IR spectra of the two N2O2 ligands and complexes
are summarized in Table 2. Fig. S1(a) and (b) are given as
Table 3
The 1H NMR spectral data (d, ppm) of the Schiff bases ligands and their complexes.
Compound
OH(phenolic)
CH@N
Aromatic protons
Aliphatic protons (–CH3)
2.33(s)
H2L1
13.12(s)
12.57(s)
–
–
–
–
8.96(s)
9.01(s)
9.03(s)
9.07(s)
9.26(s)
9.37(s)
6.95–7.69(m)
6.98–7.85(m)
6.47–7.70(m)
6.49–7.48(m)
6.83–7.79(m)
6.86–7.69(m)
H2L2
[Zn(L1)]ꢀH2O (1)
[Zn(L2)]ꢀH2O (2)
[Al(L1)Cl] (3)
[Al(L2)Cl] (4)
2.31(s)
2.30(s)
s, singlet; m, multiplet.

A.A. Abdel Aziz et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 388–396
391
Table 4
Absorption and photoluminescent for solutions of Schiff bases ligands and their complexes.
Compound
UV–Vis kmax(nm)
kex
kem
Stoke shift (
D
k)
Quantum yield Uf
p
–
p*
n–p*
H2L1
232
212
244
234
238
218
328
288
364
332
340
320
328
288
364
332
340
320
408
404
494
482
476
460
80
116
130
150
136
140
0.06
0.01
0.35
0.21
0.26
0.15
H2L2
[Zn(L1)]ꢀH2O (1)
[Zn(L2)]ꢀH2O (2)
[Al(L1)Cl] (3)
[Al(L2)Cl] (4)
*
All measurements were carried out at room temperature in 1 ꢂ 10ꢁ5 M CH2Cl2 solution.
2.0
1
140
120
100
80
H L
2
H2L1
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1
[ZnL ].H O
{ZnL1].H2O
[AlL1Cl]
2
1
[AlL Cl]
60
40
20
0
300
350
400
450
500
550
600
650
700
750
800
200
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Fig. 1. UV–Vis spectra of H2L1, [ZnL1]ꢀH2O and [AlL1Cl] in methylene chloride
Fig. 3. Fluorescence spectra of H2L1, [ZnL1]ꢀH2O and [AlL1Cl] in methylene in
methylene chloride (1 ꢂ 10ꢁ5 M).
(1 ꢂ 10ꢁ5 M).
2.0
100
H2L2
1.8
2
H L
2
2
90
80
70
60
50
40
30
20
10
0
[AlL Cl]
[ZnL2].H2O
2
[ZnL ].H O
1.6
2
2
[AlL Cl]
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
200
300
400
500
600
700
800
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Fig. 2. UV–Vis spectra of (H2L2), [ZnL2]ꢀH2O and [AlL2Cl] in methylene chloride
Fig. 4. Fluorescence spectra of H2L2, [ZnL2]ꢀH2O and [AlL2Cl] in methylene chloride
(1 ꢂ 10ꢁ5 M).
(1 ꢂ 10ꢁ5 M).
representative examples of the FT-IR spectra of Zn(II) and Al(III)
complex of H2L2. The FT-IR spectra of the complexes are compared
with those of the free ligands in order to determine the coordina-
tion sites which are involved in chelation. The position and/or the
intensities of the peaks of the chelation sites are expected to
change upon coordination. New peaks are also guiding peaks as
well as peaks of water of chelation. The discussion is confined to
the most important vibrations of the 4000–400 cmꢁ1 region in
relation to structure. It was clearly seen that the absence of the
characteristic aldehydic carbonyl stretching bands, and the appear-
ance of the azomethine –C@N– bands, at 1617 cmꢁ1 and
1616 cmꢁ1 for H2L1 and H2L2, respectively, indicating the forma-
tion of the Schiff bases. The broad band of medium intensity in
the region 3428–3444 cmꢁ1 was attributed to the intra-molecular
hydrogen-bonded O–H group [39]. The broadness of this band was
attributed to the internal hydrogen bond OHꢀꢀꢀN@C [40]. As the
hydrogen bond becomes stronger, the bandwidth increases, and
this band sometimes cannot be detected [41]. This band was

392
A.A. Abdel Aziz et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 388–396
Table 5
Thermogravimetric characteristics of Zn(II) and Al(III) complexes.
Complex
Temp. range (°C)
Mass loss (%) (calc.) found
Assignment
H2O
Residue
ZnO
[C22H18N2O2Zn]ꢀH2O (1)
187–245
373–683
182–250
326–508
218–268
400–513
591–768
219–243
368–454
500–683
(4.23) 4.19
(76.65) 76.54
(3.86) 3.67
(78.69) 78.47
(8.75) 8.65
(25.72) 25.69
(58.85) 58.67
(7.95) 7.83
C
22H18N2O
H2O
[C20H12N2O2Cl2Zn]ꢀH2O (2)
ZnO
Al
C
20H12N2OCl2
[C22H18N2O2ClAl] (3)
½Cl2
C8H8
2C7H5NO
½Cl2
C6H2Cl2
2C7H5NO
[C20H12N2O2Cl3Al] (4)
Al
(32.53) 32.39
(53.45) 53.37
missing in the spectra of Al(III) complexes due to deprotonation
upon complexation. On the other hand, the FT-IR spectra of Zn(II)
complexes exhibited a broad band at 3411–3415 cmꢁ1 which ren-
ders it difficult to be attributed to the involvement of phenolic-OH
group in coordination. However the deprotonation of phenolic –OH
groups of the ligands upon complexation with Zn(II) was approved
by the disappearance of the signal of the –OH group in the 1H NMR
spectra of Zn(II) complexes as it will be discussed later. Coordina-
tion of the Schiff base to the metal through the nitrogen atom is ex-
pected to reduce electron density in the azomethine as a result of
the withdrawal of electron density from the nitrogen atom and
should lower the C@N absorption frequency [40]. Surprisingly,
the FT-IR spectra of the complexes indicated that chelation of
N2O2 ligands to Al(III) and Zn(II) strengthened the C@N bonds ex-
cept for [ZnL2]ꢀH2O (see data in Table 2). This could be due to the
effect of different substituents in the diamine bridge, which may
alter conjugation of the ligands. Although the change in the FTIR
peak of C@N does not support the involvement of this group in
chelation, other FTIR peaks and the 1H NMR spectra strongly sup-
port the chelation of this group to the metal ion center (see discus-
sion below).
Furthermore, the involvement of the deprotonated phenolic-OH
group in chelation was confirmed by clarifying the effect of chela-
tion on the m(C–O). The medium intensity band in the region 1278–
1279 cmꢁ1 was ascribed to the phenolic C–O stretching vibration
in the case of salicylideneanilines [42]. On complexation this band
was shifted to higher frequency in the range 1283–1322 cmꢁ1 for
complexes. The shift of (C–O) band to higher frequency suggested
the strengthening of the (C–O) bond upon deprotonation of the o-
OH group of Schiff base moiety [43]. Further evidences of the co-
ordination of the azomethine nitrogen and phenolic oxygen were
supported by the appearance of two non-ligand weak bands at
448–591 cmꢁ1 and 424–479 cmꢁ1 which were assigned to
O) and (M–N) [44], respectively. This supports that the bonding
m(M–
m
of the ligands to the metal ions was achieved by the phenolic oxy-
gen atoms and the azomethine nitrogen atoms of the ligands.
Therefore, the above arguments together with the elemental anal-
yses indicated that, the Schiff bases ligands behaved as a dibasic
tetradentate ligands coordinated to the metal ions with NOON do-
nor sites of the azomethine –N and phenolate –O.
1H NMR spectra
Further evidence of the bonding mode of the ligands was also
provided by the 1H NMR spectra of the Schiff base ligands and their
diamagnetic Zn(II) and Al(III) complexes. The chemical shifts of dif-
ferent types of protons in the 1H NMR spectra of the N2O2 ligands
and their complexes are given in Table 3. 1H NMR spectra of Zn(II)
and Al(III) complexes with H2L1 is shown in Fig. S2 (a) and (b) as
representative examples for the 1H NMR data obtained for the pre-
pared compounds. The 1HNMR spectra of the parent ligands H2L1–2
showed singlet signal at very downfield in the region d 12.57–
13.12 ppm, which was attributed to two phenolic -OH protons.
The OH signals disappear with addition of the D2O [45]. The ligands
also showed singlet in the region d 8.96–9.01 ppm which was
attributed to the azomethine (–CH@N–) protons [46]. The 1H
NMR spectra of the ligands revealed multiplets in the range
6.95–7.85 ppm which was attributed to aromatic protons [47]. A
sharp singlet appeared at d 2.33 ppm in the spectra of H2L1 which
was assigned to methyl protons attached to the benzene ring of the
diamine. A comparison of 1H NMR spectra of the free Schiff base li-
gands with that of the corresponding diamagnetic complexes of
Zn(II) and Al(III) revealed that the chemical shifts observed for
the OH protons of the ligands have disappeared in all complexes.
The absence of –OH signals indicated deprotonation of the hydro-
xyl group of the Schiff bases and confirmed the bonding of oxygen
to the metal ions (C–O–M) [48] and supports the FTIR findings. It is
well-know that the 1H NMR spectra can provide compelling evi-
dence for the presence of either one or two azomethine groups. In-
deed, the presence of only one sharp singlet for the –C(H)@N
proton clearly indicated that the magnetic environment is equiva-
lent for all such protons, suggesting the presence of a planar ligand
in these complexes [49]. In case of Zn(II) and Al(III) complexes, the
position of azomethine signal was shifted to downfield region
TGA
6.0
(a)
5.0
4.0
3.0
2.0
1.0
400
0.0
200
600
800
1000
Temp. (oC)
TGA
10.0
(b)
8.0
6.0
4.0
2.0
400
0.0
200
600
800
1000
Temp. (oC)
Fig. 5. TGA curve of: (a) [ZnL1]ꢀH2O complex; (b) [AlL1Cl] complex.

A.A. Abdel Aziz et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 388–396
393
187-245 oC
-(H2O) (4.19 %)
[Zn(C22H18N2O2)]
[Zn(C22H18N2O2)].H2O
425.80
407.78
N
N
Zn
Al
. H2O
N
N
Cl
Scheme 5. The proposed structure of Zn(II) and Al(III) complexes.
Photophysical properties
ZnO
81.38
The optical properties of the Schiff base ligands and their com-
plexes were explored in methylene chloride solution (1 ꢂ 10ꢁ5 M)
using UV–Vis and photoluminescence. Table 4 summarizes the
absorption and emission data for the ligands and their complexes.
The UV–Vis absorption spectra of the ligands and their complexes
are shown in Figs. 1 and 2. As can be seen the absorption spectra of
the Schiff base ligands exhibited two high intensity bands around
(19.17 %)
Scheme 3. Thermal decomposition steps of [ZnL1]ꢀH2O complex.
218-268 oC
212–232 nm and 288–328 nm, designated as p–
p⁄ and n–p⁄ tran-
[Al(C22H18N2O2)]
369.38
[Al(C22H18N2O2)Cl]
404.83
-(1/2 Cl2) (8.65 %)
sitions, respectively, for the electrons localized on the C@N chro-
mophore. The absorption spectra of Zn(II) and Al(III) complexes
have similar pattern to that of the free ligands yet their spectra
indicated a bathochromic shift, which confirms the formation of
Schiff base metal complexes. As expected for the diamagnetic
Zn(II) (d10) and Al(III) (d0) configuration, ligand field band due to
d–d electronic transitions is not expected [51] and the trends ob-
served for the ligands were maintained after coordination.
The study of photoluminescence (PL) of Zn(II) and Al(III) com-
plexes is not only of fundamental interest, but also significant for
different applications (optical amplifiers, optical waveguides,
OLED, etc.) [52]. In this work steady-state fluorescence studies
have been employed as an independent evidence of complexation
between the ligands and the metal ions. Figs. 3 and 4 represent the
luminescence spectra of the ligands and their corresponding com-
plexes. Photoluminescence studies were carried out at room tem-
perature in methylene chloride solution. The emission spectra
were obtained by excitation at the longest wavelength of the
absorption peaks. All the prepared Schiff bases, and their Zn(II),
Al(III) complexes emit in the visible region of spectrum upon the
UV excitation. The photoluminescence spectra of the Zn(II) and
Al(III) complexes were assigned to energy transfer between the
HOMO and LUMO of the deprotonated ligands with the HOMO
591-768 oC
-(2C7H5NO) (58.67 %)
Al
[Al(C14H10N2O2)]
265.23
26.98
(6.99%)
Scheme 4. Thermal decomposition steps of [AlL1Cl] complex.
9.03–9.37 ppm in comparison with that of the free ligands, infer-
ring coordination through the azomethine nitrogen atom of the li-
gand [50]. This suggested deshielding of the azomethine proton
and proved the coordination of the azomethine group to the cen-
tral metal ion. The multiplets, assigned to the aromatic protons,
were displaced upper field around the region d 6.47–7.79 (Table 3).
It can be also noticed that the proton signal of –CH3 at 2.33 ppm (in
case of the complexes of the ligand H2L1) was slightly affected by
chelation.
mainly being
p⁄ antibonding in character. To sum up, the luminescence has been
caused by the
p⁄ transition between the ligand orbitals (ligand
p-bonding in character and the LUMO mainly being
p?
Table 6
Antibacterial activity of the Schiff base ligands (H2L1–2) and their complexes.
Compound
Antibacterial activity of zone of inhibition, mm
S. aureus
S. epidermidis
B. megaterium
E. coli
P. aeruginosa
K. pneumoniae
A
B
A
B
A
B
A
B
A
B
A
B
H2L1
7
8
8
6
9
18
21
24
15
17
27
29
24
0
7
10
16
18
21
16
15
26
28
24
0
8
10
12
14
11
12
21
21
18
0
10
13
17
20
16
16
24
29
22
0
6
10
11
14
11
12
20
22
18
0
10
12
17
20
16
18
24
27
23
0
8
11
17
16
21
21
20
26
29
25
0
H2L2
11
21
26
17
19
28
29
27
0
16
13
17
10
12
19
21
20
0
12
13
15
11
12
20
22
19
0
14
10
18
16
16
17
20
19
0
[Zn(L1)]ꢀH2O (1)
[Zn(L2)]ꢀH2O (2)
[Al(L1)Cl] (3)
[Al(L2)Cl] (4)
Streptomycin
Chloramphenicol
Amoxycillin
DMSO
17
21
12
16
24
23
22
0
A, 125
lg/1.0 mL; B, 250
lg/1.0 mL.

394
A.A. Abdel Aziz et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 388–396
Table 7
Antifungal activity of the Schiff base ligands (H2L1–2) and their complexes.
Compound
Antifungal activity of zone of inhibition, mm
A. niger
A. flavus
C. lunata
R. bataticola
C. albicans
F. solani
A
B
A
B
A
B
A
B
A
B
A
B
H2L1
12
12
13
15
13
14
17
21
24
0
14
13
17
20
16
18
21
28
29
0
10
14
16
18
12
15
17
18
23
0
13
16
20
22
17
19
21
23
27
0
9
12
18
20
11
13
19
22
23
0
11
16
23
27
13
19
22
29
28
0
9
11
15
18
11
11
18
19
22
0
11
13
20
23
13
15
22
24
27
0
10
15
18
19
13
12
18
17
24
0
14
18
21
25
18
17
22
21
28
0
8
13
16
19
11
11
17
19
22
0
11
15
19
24
14
16
21
25
26
0
H2L2
[Zn(L1)]ꢀH2O (1)
[Zn(L2)]ꢀH2O (2)
[Al(L1)Cl] (3)
[Al(L2)Cl] (4)
Nystatin
Tetracycline
Chlorometazole
DMSO
A, 125
lg/1.0 mL; B, 250
l
g/1.0 mL
to ligand charge transfer) [53]. The enhancement of the fluores-
cence of the complexes compared to their respective ligands is
due to CHEF (chelation enhancement of fluorescence emission)
[54]. Factors like a simple binding of the ligand to the metal ions
[55], an increase in rigidity of structure [56], a restriction in the
photo induced electron transfer (PFT) [54,57] etc. are assigned to
the appearance and enhancement of the PL. In the present case,
the first two factors seem to be responsible for the enhancement
PL.
signed to the elimination of C22H18N2O organic moiety to give fi-
nally ZnO residue with a net weight of 19.15% (Calc. 19.11%). The
TGA plot [ZnL2]ꢀH2O complex (2) exhibited a similar pattern but
the first decomposition step occurred in the range 182–250 °C
and the second step occurred in the range 326–508 °C.
On the other hand the TG plots [AlL1Cl] and [AlL2Cl] complexes
displayed a similar decomposition pattern with three resolved
decomposition steps (see data in Table 5 and Fig. 5). The first
decomposition is consistent with evolution of ½ Cl2, whereas the
second and third steps involved removal of organic ligand moiety,
leaving Al as a final residue as presented in Scheme 4.
The position of emission maximum and the quantum yield of
luminescence depend on the structure of the complexes and their
ligands. The observed differences in the PL properties of the pre-
pared compounds may be caused by structural variations in the
conjugation systems. The effect of the substituents in the diamine
bridge has been also studied. Indeed, complexes with Schiff bases
bearing electrondonating substituents (–CH3) are shifted in the
bathochromic direction compared to those with electronwith-
drawing substituents (–Cl). It is plausible that the –CH3 group in
H2L1 tends to activate the benzene ring and thus increases the free-
Thermal studies have shown that the stability of the prepared
complex is in the following the order: [ZnL1]H2O > [ZnL2]ꢀH2O >
[AlL1Cl] > [AlL2Cl]. This order could be explained based on the fact
that electron withdrawing group (–Cl) of the ligand reduces the
stability of the complexes, while the electron donating group (–
CH3) increases the stability of the complexes [50].
Although different strategies were followed to prepare single
crystals of the prepared compounds, we were unable to prepare
single crystals for X-ray studies. Tetrahedral and square pyramidal
geometry are proposed for the Zn(II) and Al(III) complexes, respec-
tively (Scheme 5). Such geometries are suggested based on the
interpretation of analytical, conductance, infrared and 1H NMR
spectral data.
dom of the
tivates the ring by withdrawing the
p
-electrons. On the other hand –Cl group in H2L2 deac-
-electrons from the ring thus
p
reducing their freedom by increasing their localization [58]. Liter-
ature data showed that both conjugation enhancement and the
introduction of electrondonating substituents should lead to a con-
siderable decrease in the energy difference between the HOMO
and LUMO [59,60].
Biological activity
Thermogravimetric analysis studies
The Schiff base ligands H2L1–2 and their complexes (1–4) were
tested for their inhibitory effects on the growth of different bacte-
ria types: S. aureus, S. epidermidis, B. megaterium, E. coli, P. aerugin-
osa K. pneumoniae and different fungi types: A. niger, A. flavus, C.
lunata, R. bataticola, C. albicans and F. solani. Such organisms can
achieve resistance to antibiotics through biochemical and morpho-
logical modification [62]. The antibacterial and antifungal activities
obtained for the prepared compounds are listed in Tables 6 and 7,
respectively. The results indicated that most of the tested com-
plexes displayed more activity against the same microorganisms
compared to the parent ligands under identical experimental con-
ditions. The observed antibacterial and antifungal activities are
presented graphically in Fig. 6(a) and (b). The increase in the activ-
ity of the complexes compared to that of the ligands could be ex-
plained on the basis of Overtone’s concept [62] and Tweedy’s
Chelation theory [63]. According to Overtone’s concept of cell per-
meability, the lipid membrane that surrounds the cell favors the
passage of only lipid-soluble materials. This makes liposolubility
is an important factor that controls the antifungal activity. On che-
lation, the polarity of cation will be reduced to a greater extent due
to the overlap of the ligand orbital. Further, the chelation increases
The thermal studies of the reported complexes provided further
insight into the proposed structures. The reported complexes were
found to be air stable and have high thermal stability. The thermal
stability is an important feature of a compound that could be uti-
lized as EL material [61]. The thermal behavior of the complexes
under investigation was assessed using thermogravimetric analy-
sis (TGA). The samples were analyzed in a platinum pan under
N2 and the temperature was linearly increased at 10 °C minꢁ1 over
a temperature range 20–1000 °C.
The TG data for the synthesized complexes are summarized in
Table 5 and TG plots of [Zn(L1)]ꢀH2O and [AlL1Cl] complexes, as
representative example, are shown in Fig. 5. As can be seen the
TG plot of [Zn(L1)]ꢀH2O complex (1) displayed two resolved and
well-defined decomposition steps (Scheme 3). The first decomposi-
tion occurred in the temperature range 187–245 °C with a net
weight loss of 4.20% (Calc. 4.23%) which is consistent with the
elimination of H2O molecule. The second decomposition step oc-
curred in the temperature range 373–683 °C with a net weight loss
of 76.54% (Calc. 76.65%). This decomposition step could be as-

A.A. Abdel Aziz et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 388–396
395
with neutral N2O2 donor tetradentate Schiff bases namely:
N,N0bis(salicylaldehyde)4,5-dimethyl-1,2-phenylenediamine
(H2L1), and N,N0bis(salicylaldehyde)4,5-dichloro-1,2-phenylenedi-
amine (H2L2). Spectral characterizations of the new complexes
showed that Zn(II) forms four coordinate tetrahedral complexes
with 1:1 (metal:ligand) stoichiometry. On the other hand, Al(III)
forms five coordinate square pyramidal complexes. Thermal stud-
ies showed that the complexes have good thermal stabilities and
follow the order: [ZnL1]H2O > [ZnL2]ꢀH2O > [AlL1Cl] > [AlL2Cl].
Furthermore, the obtained research results indicated that Zn(II)
complexes exhibit strong fluorescence at room temperature
compared to Al(III) complexes. The PL behavior of Zn(II) and Al(III)
complexes is particularly important for their potential application
as photoactive materials. Interestingly the prepared complexes
exhibited biological activity comparable to the well-known
antibiotics.
(a)
35
30
25
20
15
10
5
0
K. pneumonia
P. aeruginosa
E. coli
B. megateriu
S. epidermidis
(4)
Amoxycillin
(3)
(2)
(1)
Streptomycin
Chloramphenicol
Acknowledgements
m
S. aureus
H2L2
H2L1
The authors are grateful to the Scientific Research Sector of Ain
Shams University (Grant 08-010) for providing financial support to
undertake this work.
Bacteria strains
Appendix A. Supplementary data
(b)
35
30
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2012.06.023.
25
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