Palladium(II) and zinc(II) complexes of neutral [N2O 2] donor Schiff bases derived from furfuraldehyde: Synthesis, characterization, fluorescence and corrosion inhibitors of ligands

Article abstract of DOI:10.1016/j.saa.2014.03.127

Metal complexes of Schiff bases derived from furfuraldehyde and 4,5-dimethyl-1,2-phenylendiamine (L¹) or 4,5-dichloro-1,2- phenylendiamine (L²) have been reported and characterized based on elemental analyses, IR, ¹H NMR, UV-Vis, magnetic moment, molar conductance and thermal analysis. The complexes are found to have the formulae [PdL^1-2]Cl₂ and [ZnL^1-2](AcO) ₂·H₂O. The molar conductance data reveal that Pd(II) and Zn(II) chelates are ionic in nature and are of the type 2:1 electrolytes. The spectral data are consistent with a square planar and tetrahedral geometry around Pd(II) and Zn(II), respectively, in which the ligands act as tetradentate ligands. The thermal behavior of some chelates is studied and the activation thermodynamic parameters are calculated using Coats-Redfern method. The corrosion inhibition of stainless steel types 410 and 304 in 1 M HCl using the synthesized Schiff bases as inhibitors have been studied by weight loss method. The obtained data considered these ligands as efficient corrosion inhibitors. The ligands and their metal complexes exhibited considerable antibacterial activity against Staphylococcus aureus, and Escherichia coli and antifungal activity against Candida albicans.

Full text of DOI:10.1016/j.saa.2014.03.127

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 52–60
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Palladium(II) and zinc(II) complexes of neutral [N2O2] donor Schiff bases
derived from furfuraldehyde: Synthesis, characterization, fluorescence
and corrosion inhibitors of ligands
⇑
Omyma A.M. Ali
Chemistry Department, Faculty of Women for Arts, Science and Education, Ain Shams University, Cairo, Egypt
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
ꢀ
The complexes are characterized by
different spectroscopic techniques.
The Schiff base ligands exhibited
efficient corrosion inhibitors.
Zn(II) complexes may have potential
uses as a Zn2+ sensor.
The ligands and Zn(II) complexes
exhibited strong fluorescence.
a r t i c l e i n f o
a b s t r a c t
Metal complexes of Schiff bases derived from furfuraldehyde and 4,5-dimethyl-1,2-phenylendiamine (L1)
or 4,5-dichloro-1,2-phenylendiamine (L2) have been reported and characterized based on elemental anal-
yses, IR, 1H NMR, UV–Vis, magnetic moment, molar conductance and thermal analysis. The complexes are
found to have the formulae [PdL1–2]Cl2 and [ZnL1–2](AcO)2ꢁH2O. The molar conductance data reveal that
Pd(II) and Zn(II) chelates are ionic in nature and are of the type 2:1 electrolytes. The spectral data are con-
sistent with a square planar and tetrahedral geometry around Pd(II) and Zn(II), respectively, in which the
ligands act as tetradentate ligands. The thermal behavior of some chelates is studied and the activation
thermodynamic parameters are calculated using Coats–Redfern method. The corrosion inhibition of
stainless steel types 410 and 304 in 1 M HCl using the synthesized Schiff bases as inhibitors have been
studied by weight loss method. The obtained data considered these ligands as efficient corrosion inhib-
itors. The ligands and their metal complexes exhibited considerable antibacterial activity against
Staphylococcus aureus, and Escherichia coli and antifungal activity against Candida albicans.
Ó 2014 Elsevier B.V. All rights reserved.
Article history:
Received 11 January 2014
Received in revised form 21 February 2014
Accepted 23 March 2014
Available online 2 May 2014
Keywords:
Schiff bases
Furfuraldehyde
Corrosion inhibition
Fluorescence
Introduction
analytical fields [5], organic catalysis [6], oxygen carriers [7] and
as corrosion inhibitors in especially acidic environments for vari-
The Schiff bases are considered a very important class of
ligands, which form complexes with many metals. These com-
plexes have wide applications in some biological aspects [1–4],
ous alloys and metals [8–10]. Schiff base complexes derived from
heterocyclic compounds have found an increased interest in the
context of bioinorganic chemistry [11,12]. Various heterocyclic
Schiff bases and their complexes having nitrogen and also oxygen
donor atoms, have been reported by several scientists [13–15]. The
furan ring, as a part in ligand, has been studied and has attracted
⇑
Tel.: +20 22344032.
E-mail address: omaymaahmed92@yahoo.com
http://dx.doi.org/10.1016/j.saa.2014.03.127
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

O.A.M. Ali / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 52–60
53
great attention for a long time and significant progress has been
made in understanding the structure of its complexes [13,16,17].
Complexes of some transition metal ions with Schiff bases
derived from 2-furancarboxaldehyde and o-phenylenediamine
[18–20], hydrazine hydrate [21], 3,30-diaminobenzidene [22], 3-
aminodibenzofuran [23] and o-tolidine [24] were studied. Furalde-
hyde Schiff bases were found to show the selectivity towards
anions and can act as anion-sensitive membrane electrodes [25],
they also possess the nitrification inhibitory properties [26]. More-
over, luminescent compounds are attracting much current
research interest because of their many applications including
emitting materials for organic light emitting diodes, light harvest-
ing materials for photocatalysis and fluorescent sensors for organic
or inorganic analytes [27–29]. Indeed, Schiff bases derived from
aromatic diamines have received much less attention and are
considered. Here we report, the synthesis and characterization of
heterocyclic Schiff base ligands, N,N0-bis-(2-furancarboxaldimine)-
4,5-dimethyl-1,2-phenylenediamine (L1), and N,N0-bis-(2-furan-
carboxaldimine)-4,5-dichloro-1,2-phenylenediamine (L2) and their
complexes [M(N,N0-bis-(2-furancarboxaldimine)-4,5-dimethyl-1,
2-phenylenediamine)]Cl2 and [M(N,N0-bis-(2-furancarboxaldimine)-
4,5-dichloro-1,2-phenylenediamine)](AcO)2ꢁH2O (M = Zn and Pd).
solid products were filtered, washing with ethanol and dried in
vacuum desiccators. The complexes were purified by crystalliza-
tion from ethanol.
Preparation of Zn(II) complexes
A solution of hydrated zinc acetate (1.0 mmol) in methanol
(10 mL) was mixed with methanolic solution (10 mL) of each
ligand (1.0 mmol) and the mixture was left under reflux for 3 h.
The product was filtered off, washed with cold methanol and hot
petroleum ether, recrystallized from ethanol and finally dried
under vacuum.
Corrosion test
Gravimetric corrosion measurements (weight loss method)
were carried out according to the ASTM standard procedure
described in [30]. In brief, stainless steel specimens in triplicate
were immersed for a period of 4 h in 100 ml 1 M HCl containing
various concentrations of the studied inhibitors. The mass of the
specimens before and after immersion was determined using an
analytical balance accurate to 0.1 mg. Before measurements, the
specimens were abraded with a sequence of emery papers of dif-
ferent grades (400, 800, 1000 and 1200), followed by washing with
double distilled water and finally degreased with ethanol and dried
at room temperature. The composition of the stainless steel types
410 and 304 are in wt.% 0.15C, 11Cr, 75Fe, 1Mn, 0.75Ni, 0.04P,
0.03S, 1Si and 0.08C, 18Cr, 66Fe, 2Mn, 8Ni, 0.04P, 0.03S, 1Si,
respectively.
Experimental
Materials and instrumentation
PdCl2, Zn(CH3COO)2ꢁ2H2O, 4,5-dimethyl-1,2-phenylendiamine
and 4,5-dichloro-1,2-phenylendiamine and furfuraldehyde were
supplied from Aldrich. All solvents were of analytical grade. IR
measurements (KBr pellets) were carried out on a Shimadzu
8000 FT-IR spectrometer. Magnetic susceptibilities of the chelates
were measured at room temperature using a magnetic susceptibil-
ity Cambridge England Sherwood Scientific. NMR measurements
were performed on a Varian-Mercury 300 MHz spectrometer. Sam-
ples were dissolved in (CD3)2SO with TMS as internal reference.
Mass spectra of the solid complexes (70 eV, EI) were carried out
on a Finnigan MAT SSQ 7000 spectrometer. Thermogravimetric
analyses (TG and DTG) were carried out under N2 atmosphere with
a heating rate of 10 °C/min. using a Shimadzu DT-50 thermal ana-
lyzer. Microanalyses were performed using JEOL JMS-AX500 ele-
mental analyzer. All conductivity measurements were performed
in DMF (1 ꢂ 10ꢃ3 M) at 25 °C, by using Jenway 4010 conductivity
meter. Ultraviolet spectra were recorded using a Shimadzu UV
1800 Spectrophotometer in the range of 200–800 nm. The photolu-
minescent properties of all compounds were studied using a
Jenway 6270 Fluorimeter.
Antimicrobial activity
The in vitro activity of ligands and their metal complexes were
tested against the bacterial species Staphylococcus aureus and
Escherichia coli in Mueller Hinton-Agar medium. The antifungal
activity was tested against the fungi Aspergillus flavus and Candida
albicans cultured on YPD-agar medium. The test compounds were
dissolved in DMSO at concentration 2 mg/mL. Antimicrobial
activities of each compound were evaluated by the disc-diffusion
method. The well (8 mm diameter) was then filled with the test
solution and the plates were inoculated at 37 °C for 48 h (for bac-
teria) and 30 °C for 72 h (for fungi). During this period, the growth
of the inoculated microorganisms was affected and then the inhi-
bition zones developed on the plates were measured. The effective-
ness of an antimicrobial agent was assessed by measuring the
zones of inhibition around the well. 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. The antifungal activity of the test
compound was compared that of Chlorometazole as standard
antifungal. DMSO was used as a control under the same conditions
for each organism and no activity was found. The activity results
were calculated as a mean of triplicates.
Preparation of Schiff base ligands
The ligands were obtained from the condensation of furfuralde-
hyde and 4,5-dimethyl-1,2-phenylendiamine or 4,5-dichloro-1,2-
phenylendiamine in 2:1 M ratio, in ethanol for 4 h. The
yellow-orange precipitates obtained were filtered, washed with
ethanol and dried in a vacuum. Recrystallization from ethanol
afforded pure Schiff bases. The infrared and elemental analysis
data of the obtained products were consistent very well with their
formulae.
Results and discussion
The complexes [PdL1]Cl2, [PdL2]Cl2, [ZnL1](AcO)2ꢁH2O, [ZnL2]
(AcO)2ꢁH2O were synthesized by the reaction of ligands and metal
salt in 1:1 M ratio. The results of the elemental analyses of the
complexes, which are recorded in Table 1, are in good agreement
with those required by the proposed formulae. All complexes were
stable at room temperature. The molar conductivity data for 1 mM
solutions of complexes (Table 1) suggest that these chelates are
ionic in nature and they are of the type 2:1 electrolytes. All the pre-
pared complexes are found in diamagnetic character. The solid
Preparation of Pd(II) complexes
A solution of [PdCl2] (0.5 mmol) in ethanol (10 ml) was added to
the solution of ligand (0.5 mmol) in ethanol (10 ml) while stirring.
The resulting mixture was allowed to reflux on a water bath for 2 h
in the presence of a few drops of concentrated HCl. The formed

54
O.A.M. Ali / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 52–60
Table 1
Analytical and physical data of the Schiff bases and their metal chelates.
Compound
C% found (calc)
H% found (calc)
N% found (calc)
m/z
Color (% yield)
m.p (°C)
km (Oꢃ1 molꢃ1 cm2)
L1
73.67 (73.96)
57.93 (57.68)
46.43 (46.04)
37.25 (37.65)
53.87 (53.51)
44.63 (44.93)
5.25 (5.51)
3.35 (3.02)
3.12 (3.43)
2.21 (1.97)
4.55 (4.89)
3.12 (3.39)
9.33 (9.58)
8.12 (8.41)
5.67 (5.96)
5.13 (5.49)
5.23 (5.67)
5.47 (5.24)
292
333
469
510
493
534
Yellow (55)
Yellow (65)
Pale brown (70)
Yellow (78)
Pale yellow (60)
Pale yellow (80)
220
225
>300
>300
>300
>300
–
–
219
212
228
223
L2
[PdL1]Cl2
[PdL2]Cl2
[ZnL1](AcO)2ꢁH2O
[ZnL2](AcO)2ꢁH2O
complexes are soluble in DMF and DMSO. The formation of Schiff
base ligands and their complexes and bonding modes were
inferred from characteristic band positions in FT-IR spectra and
resonance signals in 1H NMR spectra corresponding to coordinated
Schiff base moiety. The geometry around Pd(II), Zn(II) ions in the
complexes were deduced from the positions of absorption bands
observed in the UV–vis spectra.
IR spectra and mode of bonding
In the absence of a powerful technique such as X-ray crystallog-
raphy, IR spectra has 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 metal complexes
are tabulated in Table 2 The IR spectra of the complexes are
compared with those of the free ligands in order to determine
the coordination sites that may get involved in chelation. Spectra
of ligands as well as their complexes showed absence of band at
1735 cmꢃ1 due to mm(C@O) stretching frequency [22]. Upon com-
Fig. 1. The IR spectrum of [ZnL1](AcO)2ꢁH2O complex.
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 Pd(II) and Zn(II) complexes. The chemical shifts of the
different types of protons in the 1H NMR spectra of the N2O2
ligands and their metal complexes are listed in Table 3. The 1H
NMR spectra of L1 and L2 is shown in Figs. 2 and 3. The 1H NMR
spectra of the parent ligands L1–2 showed singlet in the region
7.95–8.12 ppm which was attributed to the azomethine
(ACH@NA) protons. The ligands also revealed multiplets in the
range 5.68–8.03 ppm which was attributed to aromatic and furan
ring protons. On complexation, the position of azomethine signal
was shifted to higher region 7.69–7.93 ppm in comparison with
that of the free ligands, inferring coordination through the azome-
thine nitrogen atom of the ligand [35]. The multiplets, assigned to
the aromatic and furan ring protons were displaced upper field
around the region 5.58–7.83 ppm, indicating the involvement of
oxygen of furan ring in the coordination [18].
parison it was found that the m(C@N) stretching vibration is found
in the free ligands L1 and L2 at 1625 and 1631 cmꢃ1, respectively.
These bands were shifted to lower frequency in the complexes
indicating the participation of azomethine nitrogen in the coordi-
nation to metal ion. This shift is due to the reduction of the double
bond character of the C@N bond, which is caused by the coordina-
tion of nitrogen to the metal center and is in agreement with the
results obtained from the other similar complexes described previ-
ously [31,32]. A medium intensity band due to m(CAOAC) stretch-
ing vibration of furan appeared at 1261 and 1294 cmꢃ1 in the
ligands L1 and L2, respectively [19]. These bands shifted in metal
complexes suggesting a coordination through oxygen of furan moi-
ety. Appearance of new bands in the spectra of complexes in the
regions 478–544 cmꢃ1 and 433–440 cmꢃ1 were assigned to
m
(MAO) and m(MAN) stretching vibration [33]. The bands appear-
ing in the region 1522–1507 cmꢃ1, 1175–1143 cmꢃ1 and 758–
747 cmꢃ1 were usual modes of phenyl ring vibration. The infrared
spectra of Zn(II) complexes exhibited a broad band in the region
3450–3395 cmꢃ1, confirming the presence of water molecules in
complexes [34]. Fig. 1 shows the IR spectrum of [ZnL1](AcO)2ꢁH2O
complex.
Mass spectroscopy
Mass spectrometry is an effective method for investigation of
molar weight and structure of the ligands and complexes. The
Table 2
IR data of the Schiff bases and their metal chelates (cmꢃ1).a
Compound
m
(OH)
m
(C@N)
m
(CAOAC)
m
(MAO)
m
(MAN)
Phenyl ring vibration
L1
–
–
–
–
1625(m)
1631(m)
1616(m)
1612(s)
1592(s)
1583(s)
1261(w)
1294(m)
1278(w)
1302(m)
1258(w)
1298(w)
–
–
–
–
1507(s), 1143(m), 753(s)
1518(m), 1168(w), 750(s)
1508(m), 1147(m), 750(s)
1516(s), 1175(w), 755(s)
1512(w), 1174(w), 758(m)
1522(sh), 1173(w), 747(m)
L2
[PdL1]Cl2
544(w)
486(m)
521(w)
478(w)
438(w)
435(m)
440(w)
433(w)
[PdL2]Cl2
[ZnL1](AcO)2ꢁH2O
3450(b)
3395(b)
[ZnL2](AcO)2ꢁH2O
a
s, strong; m, medium; w, weak; b, broad.

O.A.M. Ali / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 52–60
55
ligand L2 displayed the parent molecular ion [M]+ at m/z = 333
which is an agreement with the formula C16H10N2O2Cl2 and is
equivalent to its molecular weight, Fig. 5. The ligand L2 shows
some important peaks at m/z 266 corresponding to [C12H7N2OCl2]+
(obtained by loss a furfural molecule [C4H3O]), at m/z 158 belongs
to [C7H3Cl2]+ and at m/z 79 corresponding to [C5H3O]+. The mass
spectra of the complexes showed a peak at m/z value correspond-
ing to their molecular weight, Table 1.
Table 3
1H NMR spectral data of the Schiff bases and their metal chelates.
Compound
L1
Chemical shift, (d) ppm
Assignment
7.95
5.68–7.53
2.35
(s, 2H, azomethine H)
(m, 8H, 2ArH, 6 furan H)
(s, 6H, methyl H)
L2
8.12
5.80–8.03
7.69
5.58–7.58
2.31
(s, 2H, azomethine H)
(m, 8H, 2ArH, 6 furan H)
(s, 2H, azomethine H)
(m, 8H, 2ArH, 6 furan H)
(s, 6H, methyl H)
[PdL1]Cl2
Magnetic measurements and electronic absorption spectra
[PdL2]Cl2
7.93
5.77–7.83
7.88
5.65–7.45
2.31
(s, 2H, azomethine H)
(m, 8H, 2ArH, 6 furan H)
(s, 2H, azomethine H)
(m, 8H, 2ArH, 6 furan H)
(s, 6H, methyl H)
[ZnL1](AcO)2ꢁH2O
Magnetic susceptibility measurements showed diamagnetic
properties for Pd(II) and Zn(II) complexes. In addition, these com-
plexes gave 1H NMR signals (Table 3) confirming its diamagnetism.
So, no molar magnetic susceptibility data were recorded for dia-
magnetic complexes whereas that for paramagnetic complexes
were recorded. The electronic absorption spectra of the Schiff base
ligands and their complexes were explored in DMF solution
(1 ꢂ 10ꢃ5 M). The data are summarized in Table 4. The UV–Vis
absorption spectra of the ligands and their complexes are shown
in Figs. 6 and 7. The absorption spectra of the two ligands (L1
and L2) exhibited four bands. The first two bands around 230–
1.81
(s, 6H, CH3COO)
[ZnL2](AcO)2ꢁH2O
7.80
5.60–7.78
1.79
(s, 2H, azomethine H)
(m, 8H, 2ArH, 6 furan H)
(s, 6H, CH3COO)
244 and 256–264 nm may be attributed to
p–
p⁄ transitions and
the second two bands observed at 318–320 nm and 332–334 nm
are assigned to n–p⁄ transitions, for the electrons localized on
the C@N chromophore. The shift of these bands in the spectra of
complexes indicated the formation of their metal complexes. As
expected for the diamagnetic Zn(II) (d10) configurations, ligand
field band due to d–d electronic transitions is not expected [36]
and the trends observed for the ligands were maintained after
coordination. The electronic absorption spectra of the Pd(II) com-
plexes exhibited broad bands at about 408–422 nm assignable to
the transition (1B1g 1A1g). These data and the diamagnetic nature
of the complexes are in accordance with square planar geometry
for the Pd(II) complexes [37,38].
Fig. 2. 1H NMR spectrum of L1 ligand.
Fluorescence spectra studies
The fluorescence properties of the Schiff base ligands and their
complexes were studied at room temperature in DMF solutions
(Figs. 8 and 9). The L1 and L2 ligands displayed the maximum emis-
sion bands at 359 and 356 nm when excited at 318 and 334 nm,
respectively. [PdL1]Cl2 complex upon excitation at 308 nm dis-
played a maximum emission band at 361 nm and a weak broad
band at 463 nm with fluorescence intensity 21.7 and 1.23, respec-
tively. Also, [PdL2]Cl2 complex exhibited a maximum emission
band at 359 nm and a weak broad band at 481 nm upon excitation
at 324 nm. The intensity of the emission bands in Pd(II) complexes
is decreased in comparison with ligands. The emission spectral
shapes of the Zn(II) complexes closely resembled that of the
ligands (Table 5). The emission intensity of the Zn(II) complexes
is stronger than that of the ligands [39]. These results suggest that
Fig. 3. 1H NMR spectrum of L2 ligand.
Mass spectrum of the L1 exhibited a molecular ion peak at m/z 292
which is equivalent to its molecular mass (Fig. 4). The ion peaks at
225, 120, 80 and 68 referred to the ions [C14H13N2O]+, [C8H10N]+,
[C5H4O]+ and [C4H4O]+, respectively. The mass spectrum of the
Fig. 4. Mass spectrum of the ligand L1.

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O.A.M. Ali / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 52–60
Fig. 5. Mass spectrum of the ligand L2.
with ligands, since Zn(II) ion is difficult to oxidize or reduce due to
Table 4
its stable d10 configuration [41]. Enhancement of fluorescence
through complexation is, however, of much interest as it opens
up the opportunity for photochemical applications of these com-
plexes [42].
UV–Vis data of the Schiff bases and their metal chelates.
Compound
kmax (nm)a
p–
p⁄
n–p⁄
d–d transition
L1
230, 256
244, 264
236, 262(sh)
236
252
246
318, 332(sh)
320, 334
308
324, 338(sh)
318, 334(sh)
324, 334
–
–
L2
Thermal analysis (TGA and DrTGA)
[PdL1]Cl2
408(b)
374(sh, b), 422(b)
–
–
[PdL2]Cl2
[ZnL1](AcO)2ꢁH2O
The presence of water and thermal behavior of the complexes
was further investigated by thermal analysis. The thermal analysis
data for [PdL1]Cl2 and [ZnL1](AcO)2ꢁH2O complexes are listed in
Table 6 and the calculated thermodynamic parameters from DrTGA
are listed in Table 7. One of the features in TGA data concerning the
associated water molecules within the complexes supports the ele-
mental analyses. The TGA curve of [PdL1]Cl2 (Fig. 10) showed a
decomposition step within the temperature range of 194–490 °C
[ZnL1](AcO)2ꢁH2O
a
sh, shoulder; b, broad.
L1 and L2 may be a suitable agent to detect zinc ion. Therefore, this
kind of compounds may have potential uses as a Zn2+ sensor [40].
The Zn(II) complexes exhibited strong fluorescence in comparison
Fig. 6. Electronic absorption spectra of: (a) Schiff base ligand L1; (b) [ZnL1](AcO)2ꢁH2O; and (c) [PdL1]Cl2 complexes in 10ꢃ4 M DMF.
Fig. 7. Electronic absorption spectra of: (a) Schiff base ligand L2; (b) [ZnL2](AcO)2ꢁH2O; and (c) [PdL2]Cl2 complexes in 10ꢃ4 M DMF.

O.A.M. Ali / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 52–60
57
Fig. 8. Emission spectra of:(a) Schiff base ligand L1; (b) [PdL1]Cl2; and (c) [ZnL1](AcO)2ꢁH2O complexes in 10ꢃ4 M DMF solution.
Fig. 9. Emission spectra of:(a) Schiff base ligand L2; (b) [ZnL2](AcO)2ꢁH2O; and (c) [PdL2]Cl2 complexes in 10ꢃ4 M DMF solution.
Table 5
acetate moieties and C13H8NO species. The third step involved
removal of organic ligand moiety to give finally ZnO residue with
a net weight loss of 16.33% (Calc. 16.63%).
Fluorescence data of the Schiff bases and their metal chelates.
Compound
kexcitation
kemission
L1
318
334
308
324
318
334
359, 378(sh)
356, 373(sh)
361, 377(sh), 463(b)
359, 376(sh), 481(sh, b)
358, 373(sh)
L2
Kinetic data
[PdL1]Cl
[PdL2]Cl2
The kinetic and thermodynamic activation parameters of
decomposition processes of complexes namely activation energy
(E⁄), enthalpy (H⁄), entropy (S⁄) and Gibbs free energy change of
the decomposition (G⁄) were evaluated graphically by employing
the Coats–Redfern relation [43].
[ZnL1](AcO)2ꢁH2O
[ZnL2](AcO)2ꢁH2O
353, 371(sh)
ꢀ
ꢁ
ꢀ
ꢂ
ꢃꢁ
with a mass loss of 73.20%; (calculated 73.06%) leaving the metal
oxide (PdO) as a residue. The thermal decomposition of [ZnL1]
(AcO)2ꢁH2O (Fig. 11) complex proceeds with three main degrada-
tion steps. The first stage of decomposition occurred in the temper-
ature range 33–128 °C with a net weight loss of 3.90% (Calc. 3.66%)
which is consistent with the elimination of H2O molecule. The sec-
ond decomposition step occurred in the temperature range 128–
290 °C with a net weight loss of 63.00% (Calc. 63.22%). This
decomposition step could be assigned to the elimination of two
logðW =ðW1 ꢃ WÞÞ
AR
2RT
Eꢄ
1
log
¼ log
1 ꢃ
ꢃ
ð1Þ
/Eꢄ
Eꢄ
2:303RT
T2
where W is the mass loss at the completion of the decomposition
1
reaction, W is the mass loss up to temperature T, R is the gas con-
stant and / is the heating rate. Since 1–2RT/E⁄ ffi 1, the plot of the
left-hand side of Eq. (1) against 1/T would give a straight line. E⁄
was then calculated from the slope and the Arrhenius constant, A,
was obtained from the intercept. The other kinetic parameters;

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O.A.M. Ali / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 52–60
Table 6
Thermogravimetric data of [PdL1]Cl2 and [ZnL1](AcO)2ꢁH2O complexes.
Complex
TG range (°C)
DTG (°C)
Mass loss%
Calcd
Assignment
Metallic residue
Found
[PdL1]Cl2
194–490
33–128
128–290
290–492
316
84
241
417
73.20
3.90
63.00
16.33
73.06
3.66
63.22
16.33
Loss of Cl2 + L1
Loss of H2O
Loss of 2 (CH3COO) + C13H8ON
Loss of C5H8N
PdO
ZnO
[ZnL1](AcO)2ꢁH2O
Table 7
The thermodynamic data of the thermal decompositions of [PdL1]Cl2 and [ZnL1](AcO)2ꢁH2O complexes.
Complex
Decomposition range (°C)
E⁄ (kJ molꢃ1
)
A (Sꢃ1
)
S⁄ (Kꢃ1 J molꢃ1
)
H⁄ (kJ molꢃ1
)
G⁄ (kJ molꢃ1
)
[PdL1]Cl2
194–490
128–290
366–492
109.67
72.98
185.27
7.1 ꢂ 108
2.8 ꢂ 106
5.95 ꢂ 1012
ꢃ75.85
ꢃ119.55
ꢃ3.32
107.04
70.975
181.72
131.01
99.78
183.14
[ZnL1](AcO)2ꢁH2O
Gꢄ ¼ Hꢄ ꢃ TSꢄ
ð4Þ
where (k) and (h) are the Boltzman and Planck constants, respec-
tively. The kinetic parameters are listed in Table 7. From the
obtained results, it is apparent that G⁄ values of the complexes
acquire highly positive magnitudes. The high value of the energy
of activation of the complexes revealed the high stability of the
investigated complexes due to their covalent character [44]. The
negative values of S⁄ for the degradation process indicates that
more ordered activated complex than the reactants and the decom-
position reaction is slow [45].
Structural interpretation
The structures of the complexes of Schiff bases L1 and L2, with
Pd(II) and Zn(II) ions were confirmed by elemental analyses, IR,
NMR, molar conductance, magnetic, UV–Vis, mass, and thermal
analysis data. Therefore, from the IR spectra, it is concluded that
L1 and L2 ligands behave as a neutral tetradentate ligand, coordi-
nated to the metal ions via azomethine N and furan O. From the
molar conductance data, it was found that all complexes are ionic
in nature and they are of the type 2:1 electrolytes. On the basis of
the above observations square planar and tetrahedral geometries
are suggested for Pd(II) and Zn(II) complexes (Scheme 1).
Fig. 10. TG and DTG curves of [PdL1].
Corrosion inhibition
The weight loss method for monitoring corrosion rate is useful
because of its simple application and reliability. The values of cor-
rosion rate (Rcorr) obtained from weight loss measurements at dif-
ferent concentrations of Schiff bases at 25 °C (Tables 8 and 9). From
the weight loss values, the inhibition efficiency (h) and percentage
inhibition efficiency
g of each concentration was calculated using
the following equations [8]:
R1 ꢃ R2
Fig. 11. TG and DTG curves of [ZnL1](AcO)2ꢁH2O.
h ¼
R1
g
¼ h ꢂ 100
the entropy of activation (S⁄), enthalpy of activation (H⁄) and the
free energy change of activation (G⁄) were calculated using the
following equations:
where R1 and R2 are the corrosion rates in the absence and presence
of inhibitors, respectively. It has been found that the inhibition effi-
ciency of all of these compounds increases with increasing of con-
centration of inhibitors. The maximum efficiencies for L1 ligand is
91% for the concentration of 1 ꢂ 10ꢃ4 M on SS410 and 92% for the
concentration of 1 ꢂ 10ꢃ4 M on SS304. Also, the maximum efficien-
cies for L2 ligand is 83% for the concentration of 1 ꢂ 10ꢃ3 M on
ꢂ
ꢃ
Ah
KT
Sꢄ ¼ 2303 log
R
ð2Þ
ð3Þ
Hꢄ ¼ Eꢄ ꢃ RT

O.A.M. Ali / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 52–60
59
Scheme 1. The proposed structure of Pd(II) and Zn(II) complexes.
to a large extent and this may be due to the similarity of all utilized
ligands in their chemical compositions, the number of donor atoms
(nitrogen and oxygen) and structures. The inhibition efficiency of
the ligands can be illustrated on the basis of their larger molecular
sizes and electron donating nature lead to a larger surface coverage
of the metal and a high bond strength between the molecule and
the metal surface. This was indicated from the fact that, by increas-
Table 8
Corrosition parameters of stainless steel 410 in 1 M HCl at different L1 and L2
concentrations.
Inhibitor
Conc. (M)
R (mg cmꢃ2 hꢃ1
)
h
g
(%)
Blank
L1
–
0.73
0.45
0.09
0.06
0.32
0.21
0.12
–
–
1 ꢂ 10ꢃ5
5 ꢂ 10ꢃ5
1 ꢂ 10ꢃ4
1 ꢂ 10ꢃ5
1 ꢂ 10ꢃ4
1 ꢂ 10ꢃ3
0.38
0.86
0.91
0.56
0.70
0.83
38
86
91
56
70
83
ing the inhibitor concentration, both (h) and (g%) were increased
L2
while (R) was reduced. So, the dissolution of stainless steel in the
presence of the investigated inhibitors can be interpreted on the
basis of interface inhibition mode, the inhibitors act effectively at
the metal solution interface [10].
Table 9
Corrosion parameters of stainless steel 304 in 1 M HCl at different L1 and L2
concentrations.
Biological activity
Inhibitor
Conc. (M)
RCorr (mg cmꢃ2 hꢃ1
)
h
g
(%)
The in vitro biological screening effects of the investigated com-
pounds were tested against two bacterial strains; Escherichia coli
(E. coli) as Gram-negative and Staphylococcus aureus (S. aureus) as
Gram-positive and also antifungal screening was studied against
two species, C. albicans and A. flavus microorganisms. The antibac-
terial and antifungal activities obtained for the prepared com-
pounds are listed in Table 10. The ligands and Pd complexes
show a moderate activity against Gram-negative and Gram-positive
microorganisms while Zn complexes have higher antibacterial
effect than Schiff bases. This enhancement in the activity can be
explained on the basis of chelation theory [46]. On the other hand,
the ligands and the tested complexes show a moderate activity
against C. albicans fungi and no antifungal activity against A. flavus.
Blank
L1
–
4.28
2.99
1.35
0.34
2.7
–
–
30
5 ꢂ 10ꢃ6
1 ꢂ 10ꢃ5
1 ꢂ 10ꢃ4
1 ꢂ 10ꢃ5
2 ꢂ 10ꢃ5
5 ꢂ 10ꢃ5
0.30
68
92
36
42
90
0.68
0.92
0.36
0.42
0.90
L2
2.46
0.38
SS410 and 90% for the concentration of 5 ꢂ 10ꢃ5 M on SS304. This
indicates a good corrosion inhibition for stainless steel 410 and
304. The inhibition efficiencies of all ligands seem to be comparable

60
O.A.M. Ali / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 52–60
Table 10
Biological activities of ligands and their complexes.
Compound
Diameter of inhibition zone (mm)
Escherichia coli (Gꢃ)
Staphylococcus aureus (G+)
Aspergillus flavus (Fungus)
Candida albicans (Fungus)
L1
10
11
10
9
16
18
21
–
12
10
11
9
19
17
24
–
0.0
0.0
0.0
0.0
0.0
0.0
–
10
9
10
9
10
9
–
L2
[PdL1]Cl
[PdL2]Cl2
[ZnL1](AcO)2ꢁH2O
[ZnL2](AcO)2ꢁH2O
Streptomycin antibacterial agent
Chlorometazole antifungal agent
23
20
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