Physicochemical studies on some selected oxaloyldihydrazones and their novel palladium(II) complexes along with using oxaloyldihydrazones as corrosion resistants

Article abstract of DOI:10.1080/24701556.2017.1357595

New palladium (II) oxaloyldihydrazone complexes are reported and characterized. The mode of bonding as well as the structure of the isolated compounds have been ascertained on the basis of data obtained from elemental analyses (CHNM), spectral (UV-Vis., IR, mass and ¹H NMR) and thermal (TG and DTA) measurements. Some optical parameters such as indirect optical band gap and refractive index were estimated for the isolated compounds. The inhibitive properties of all synthesized hydrazones toward the corrosion of carbon-steel in 1?M HCl solutions were tested. The adsorption of hydrazone inhibitors on steel surface proceeded as a spontaneous first order reaction.

Full text of DOI:10.1080/24701556.2017.1357595

Inorganic and Nano-Metal Chemistry
ISSN: 2470-1556 (Print) 2470-1564 (Online) Journal homepage: http://www.tandfonline.com/loi/lsrt21
Physicochemical studies on some selected
oxaloyldihydrazones and their novel palladium(II)
complexes along with using oxaloyldihydrazones
as corrosion resistants
Ayman H. Ahmed, A.M. Hassan, Hosni A. Gumaa, Bassem H. Mohamed &
Ahmed M. Eraky
To cite this article: Ayman H. Ahmed, A.M. Hassan, Hosni A. Gumaa, Bassem H. Mohamed &
Ahmed M. Eraky (2017): Physicochemical studies on some selected oxaloyldihydrazones and
their novel palladium(II) complexes along with using oxaloyldihydrazones as corrosion resistants,
Inorganic and Nano-Metal Chemistry, DOI: 10.1080/24701556.2017.1357595
To link to this article: http://dx.doi.org/10.1080/24701556.2017.1357595
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Physicochemical studies on some selected oxaloyldihydrazones and their novel
palladium(II) complexes along with using oxaloyldihydrazones as corrosion resistants
Ayman H. Ahmed1,2*, A.M. Hassan1, Hosni A. Gumaa1, Bassem H. Mohamed1, Ahmed M.
Eraky1
1Department of Chemistry, Faculty of science, Al-Azhar University, Nasr City, Cairo, Egypt.
2Chemistry Department, College of Science and Arts, Al Jouf University, Algrayuat,
Saudi Arabia.
E-mail: *Author to whom correspondence should be addressed; e-mail address:
ayman_haf532@yahoo.com
Tel: +2022629357
Fax: +2022629358
Abstract
New palladium (II) oxaloyldihydrazone complexes are reported and characterized. The mode
of bonding as well as the structure of the isolated compounds have been ascertained on the basis
1
of data obtained from elemental analyses (CHNM), spectral (UV-Vis., IR, mass and H NMR)
and thermal (TG and DTA) measurements. Some optical parameters such as indirect optical band
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gap and refractive index were estimated for the isolated compounds. The inhibitive properties of
all synthesized hydrazones toward the corrosion of carbon-steel in 1 M HCl solutions were
tested. The adsorption of hydrazone inhibitors on steel surface proceeded as a spontaneous first
order reaction.
Keyword: Hydrazone complexes; Spectral investigation; Optical properties; Inhibition.
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1. Introduction
Actually, ban synthesis of hydrazones and their derivatives may lose much as they have wide
applications in biology [1], medicine [2], catalysis [3], corrosion inhibition [4] and analytical
chemistry [5]. In fact, little papers have been reported for oxaloyldihydrazones or their
complexes due to their poor solubility in most organic solvents. Oxaloyldihydrazones are only
soluble in much more polar solvents such as DMF and DMSO which means much intensive
effort is required to separate such compounds or their complexes in pure crystalline form.
Nevertheless some transition metal complexes of oxaloyldihydrazones have been studied [6,7],
palladium complexes of the selected oxaloyldihydrazones have not been actually isolated yet.
Nowadays, some hydrazone complexes have been accommodated in zeolite-Y by alternative
methods and inferred by various physicochemical characterization techniques [8].
Organic inhibitors usually protect the metal or alloy by forming an interstitial hydrophobic film
on the metal surface. In general, organic compounds containing polar groups including nitrogen,
sulfur, and oxygen have been reported to be good corrosion inhibitors [9]. In fact many Schiff
bases including hydrazones are found to be effective corrosion inhibitors of metals [10,11].
The present work was carried out to study the coordination behavior of PdII ions toward the
selected oxaloyldihydrazones. Furthermore, the work aims to growing up numbers of
semiconductors. For this purpose, the energy gap of used ligands and their relevant complexes
have been determined to describe their electronic behaviors. The study was extended to
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investigate the inhibition efficiency of all ligands toward carbon-steel in hydrochloric acid
solution and some thermodynamic parameters have been calculated.
2. Experimental
2.1. Chemicals and Equipments
The aldehydes were obtained from Merck Company. Palladium (II) acetate, diethyl oxalate,
hydrazine monohydrate were purchased from Sigma-Aldrich. Oxalicdihydrazide was prepared
by
reacting
diethyloxalate
(1
mol)
with
hydrazine hydrate (2 mol) [12], (Exp/Lit. m.p = 240/240 °C). Other chemicals and solvents were
of highest purity and used without further purification. The 1H NMR spectra were recorded on a
º
Jeol-FX-90Q Fourier NMR spectrometer at 25 C using DMSO solvent and TMS as an internal
standard. Mass spectra of the ligands were performed by a Shimadzu-GC-MS-QP1000 EX using
the direct inlet system. Metal contents (%wt) were estimated complexometrically by EDTA
using xylenol orange (indicator) and solid hexamine buffer (pH = 6). Elemental analyses (CHN),
spectral (UV-Vis. and FT-IR) and thermal (TG and DTA) measurements were executed as
mentioned elsewhere [13]. The optical band gap energy (Eg) of product compounds was
calculated from Tauc’s equations [14,15]. Corrosion test was performed on freshly prepared
Carbon-steel (C-steel) sheet of the chemical compositions (wt %) depicted in Table 1. Prior to
the experiment, the specimens were polished with emery papers (220-800 grades) until the
surface appears free from any scratches and other apparent defects, then washed with distilled
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water followed by degreasing in absolute ethanol and acetone, dried at room temperature,
weighed and finally stored in a moisture free desiccators prior to use [16].
2.2. Preparations
2.2.1. Preparation of oxaloyldihydrazone ligands
In view of the importance of oxaloyldihydrazones and their chelates, we have undertaken the
synthesis of four ligand structures including bis(salicylaldehyde)oxaloyldihydrazone (L1), bis(2-
hydroxy-1-naphthaldehyde)oxaloyldihydrazone
hydroxyacetophenone)oxaloyldihydrazone
(L2),
bis(2-
(L3),
bis(2-methoxy-
benzaldehyde)oxaloyldihydrazone (L4). Oxalicdihydrazide (0.01 mol) dissolved first in hot water
(20 cm3) followed by adding methanol (40 cm3) was mixed with the appropriate aldehydes
[salicylaldehyde,
2-hydroxy-1-naphthaldehyde,
2-hydroxyacetophenone
and
2-
methoxybenzaldehyde] (0.02 mol) in absolute methanol. The resulting mixture went on refluxing
for 3 hrs under constant stirring. The product was separated out on concentrating the solution to
half of its volume and cooling. The crystals of the desired ligand was collected by filtration
°
through a Buchner funnel and dried in an electric oven at 50 C for 2 h. Afterwards the ligand
was recrystallized from DMF-MeOHaq mixed solvent, collected, washed thoroughly on filter
°
paper by acetone to remove any excess of DMF and then dried in an electric oven at 50 C for 2
h. The molecular structure of the resulting hydrazones was elucidated by elemental analyses and
spectral (IR, mass and 1H NMR) measurements (Table 2,3).
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2.2.2. Preparation of solid complexes
The dihydrazone ligand (1 mmol) was dissolved in 20 ml DMF (20 ml) and then 50 ml
methanol was added. The resulting solution was added slowly to a methanolic solution of
palladium(II) chloride. The metal salt was dissolved first in a minimum amount of distilled water
(20 ml) before making the methanolic solution. Worthy mention, the Pd(II) chloride was mixed
first with NaCl in 1:2 ratio to give a soluble solution. The resulting mixture was heated under
reflux for 3-4 hrs and then reduced to 15 cm3 by gentle heating on hot plate. The resulting
reaction mixture was cooled down to room temperature and the colored solid complexes were
filtered off, washed several times with successive portions of hot solvents (DMF, methanol and
acetone, respectively) to remove any excess of unreacted ligands. Finally, the desired products
were dried in an electric furnace at 80 °C for 6 hrs.
2.3. Dihydrazone ligands as corrosion inhibitors
2.3.1. Solutions
The
aggressive
solutions,
1
M
HCl,
were
prepared
by
dilution of analytical grade HCl with distilled water. Stock solutions of the inhibitors were
prepared in DMF. All test solutions contained 10 ml (20 vol.%) of DMF to ensure solubility. The
concentrations
of
the
used
inhibitors
(L1–L4)
were
5×10-4, 1  10-4, 5  10-5 and 1×10-5 M.
2.3.2 Gravimetric measurements
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A simple test for measuring corrosion is the gravimetric method (weight loss). The method
involves exposing a clean weighed piece of the alloy to the corrosive environment for a specified
time followed by cleaning to remove corrosion products and weighing the piece to determine the
loss of weight.
A
previously
weighed
alloy
(C-steel)
was
completely
immersed
in 50 ml of the test solution (1M HCl) in an open beaker. The beaker was
maintained at a temperature of 301 K. The coupons were suspended in the beakers with the aid
of glass hooks. In the absence of inhibitors, the coupons were retrieved from their corrodent
solutions at 1 h interval for 3 hrs. Further measurements were carried out after introduction of the
additives (L1-L4) at concentrations 5×10-4, 1×10-4, 5×10-5 and 1×10-5 M in the beaker maintained
at the room temperature for the same duration. The coupons were withdrawn from the test
solution
at
1
h
interval progressively for 4 hrs, dipped into saturated ammonium acetate solution at room
temperature to terminate the corrosion reaction, washed by scrubbing with a light brush, rinsed in
°
acetone and finally dried in an oven at 80 C before reweighing. The difference in weight in
grams
was
taken
as
the
total weight loss (W) [17].
W  W  W
i
f
Where: Wi and Wf are the initial and final weights for aluminum, respectively.
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Each reading reported is an average of three experimental reading recorded to the nearest
0.000l g on an Adam184 electronic weighing balance. The value of corrosion rate, ρ (in gm.cm-
2.min-1) was calculated from the following equation [17]:
W
 
At
Where: W = weight loss to the nearest 0.0001gm, A = area of specimen to the nearest 0.01 cm2
(2 × area of one surface/face) and t = exposure time (minutes).
The inhibition efficiency (%) for each inhibitor and the degree of surface coverage (θ) were
calculated using the equations [17]:


1
2
%  1
100%




1
2
 1
Where ρ1 and ρ2 are the corrosion rates in the presence and absence of inhibitor, respectively.
2.3.2. Kinetic treatment of weight loss results
The corrosion reaction is a heterogeneous one, composed of anodic and cathodic reactions with
the same or different rate [18]. In the present study, the initial weight of steel coupon at time, t is
designated Wi, the weight loss is ΔW and the weight change at time, t is (Wi - ΔW) or Wf. When
log Wf was plotted against time, a linear variation was observed [19]:
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logWf  logWi  Kt
Where: Wi = the initial weight before immersion, K = slope (the rate constant), and t is time.
Regarding the adsorption reaction on steel surface proceeds as first order, the half-life values,
t , of the metal in the test solutions were calculated from the rate constant values using the
½
equation [19]:
ln 2 0.693
t1/ 2


K
K
3. Results and discussion
The entire oxaloyldihydrazones (L1-L4) and their solid complexes with Pd(II) have been
isolated in pure form. Tables 2, 3 declare the analytical, physical and spectroscopic data of the
hydrazones and their isolated palladium complexes. The prepared complexes are non-
hygroscopic,
soluble
in
strong
polar
solvents such as in DMF and DMSO but insoluble in MeOH/EtOH, Et2O, CHCl3, acetone, CCl4
as well as benzene. The calculated and found element percentages (CHNM%) point out that the
compositions of the chelates harmonize with the proposed formulae. Compositions of the solid
complexes have been identified on the basis of spectral UV-Vis., IR and NMR. The solvent
content in the complexes was determined by thermal (TG and DTA) measurements and the
results can be taken as evidence for the suggested structural formulae.
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3.1. IR Spectra
The comparative IR spectral study of the parent ligands and their palladium(II) complexes
demonstrates
the coordination mode of the ligands during the complex formation. The important frequencies
with their possible assignments are listed in Table 1 (S1).
Prepared oxloyldihydrazones can be represented either in the trans- (staggered) or cis- (anti-cis-
or syn-cis-) configuration as revealing in Fig.1, [20]. The structures of the ligands were
authenticated on the basis: (1) The strong bands observed at 1602-1617 cm-1 are assignable to the
azomethine group (νC=N). The observation of these bands provides a direct evidence for the
interaction of dihydrazides with selected aldehydes. (2) The bands centered at (3149, 3204 and
1666, L1), (3476, 3166 and 1705(m) +1660(v.s), L2) and (3448, 3293 and 1689(m) +1651(v.s),
L3) cm-1 are attributed to ν(OH)
ν(NH) and ν(C=O), respectively. Meanwhile, L4
phenolic/naphthoic,
which does not contain ortho-OH group revealed ν(NH) at 3202 cm-1 and ν(C=O) at 1653 cm-1.
(3) Both ν(C=O) and ν(NH) were appeared concurrently in the IR spectra of all ligands giving a
significant reflection to the existence of keto forms. (4) Appearance of ν(C=O) as two bands for
each L2 and L3 ( 1700, 1660 cm-1) and one band for L1 and L4 (1660 cm-1) refers to the mixture
of [cis(syn/anti-cis-structure) + trans(staggered)-structure)] isomers for L2, L3 and trans
(staggered) structure for L1, L4, respectively, (Fig. 1). This suggestion can be pictured by the so
called field effect criteria where the cis isomer is absorbed at higher frequency than trans one
[12]. The band observed at1705 and 1689 cm-1 characterized trans form while the other two
bands located at 1660 and 1653 cm-1 indicated the presence of cis-form. (5) Foundation of OH
(phenolic) group in L1 as sharp at lower position (3149 cm-1) evidenced the persistence of
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intramolecular H-bonding between the phenolic-OH and CH=N group, (O–H….N), and in the
same time ruled out the formation of intermolecular H-bonding between the associated ligand
molecules (O–H….O-H) [21]. On the other hand, the existence of two bands at 3278 cm-1 (L1)
and 3227 cm-1 (L4) may be attributed to ν(OH)enolic in H-bond bonding with azomethine group.
(6) The bands concerning the ν(C-O) in the spectra of L1 – L4 were located at 1275, 1287, 1246
and 964 cm-1, respectively [21,22]. (7) Also, each ligand contains 3-4 bands in the regions 1400-
1600 and 700-800 cm-1 assignable to ν(C=C) and δ(CH)out
of the aromatic ring,
of plane
respectively.
In case of complexes I-III, obscurity of both υ(NH), υ(OH)phenolic/naphthoic and υ(C=O) after
chelation with splitting of υ(C=N) and positive shift of υ(C-O)phenolic/naphthoic evidenced that C=O
and NH groups did not contribute in coordination whereas the corresponding ligand binds to the
palladium in dienol form via the azomethine and deprotonated o-hydroxyl groups [23]. The
splitting of C=N was occurred due to the enolization of two C=O groups in dihydrazone
molecule assuming dienol skeleton. The appearance of υ(OH)enolic in I and III at lower
frequencies, (3238 and 3194 cm-1, respectively) are due to the formation of H-bond between two
generated enolic (OH) groups [24]. However, the obscure of υ(OH)enolic in II accompanied with
the foundation of υ(M-Oenolic) is most probably due to the deprotonation of enolized carbonyl (C-
OH) upon coordination with the palladium ion. The existence of new bands attributed to υ(M-
Ophenolic) boosts the formation of complexes I-III [25, 26]. In the case of IV, positive shift of
υ(C=O) in addition to the observation of υ(NH) and υ(C=N) at nearly the same locations
excluded the involvement of these groups in coordination and referred to persistence of L4 ligand
in diketo form. The lower positive shift of υ(C-O)methoxy upon chelation indicates the weak
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interaction between PdII ion and ortho-methoxy (-OMe) group. On the other hand, crystalline and
coordinated water molecules in all palladium complexes (I-IV) were presumed based on the
appearance of the broad bands within the range 3300-3500 cm-1, Table 3, [27]. By the way, the
slight changes in the positions of ν(C=C)ph for all free ligands upon chelation arise from the
metal-ligand interaction.
3.2. 1H NMR Spectra
1
The main signals of H NMR spectra of the ligands and some Pd(II) complexes are
summarized in Table 2.
Virtually, NMR spectroscopy is considered a powerful technique employed to identify the
1
skeleton of the ligands. H NMR spectra of all ligands exhibited multiple signals of the aromatic
protons in the 6.5–8.5 ppm region. The signals of equal integration observed in L1, L2 and L3 at δ
(12.6, 11), (12.8, 12.6) and (12.9, 11.8) ppm downfield of TMS have been assigned to NH and
ortho-OH protons, respectively. On the other hand, L4 revealed a signal at 12.3 ppm attributed to
secondary NH group. Upon the addition of D2O then OH and NH signals were obscured.
Furthers, the existence of the δOH (phenolic/naphthoic) at its normal frequency excluded any
intramolecular hydrogen bonding operating between ortho-OH and CH=N group (CH=N…..H-
O). The azomethine signals, δ(CH=N), observed only in L1, L2 and L4 have been assigned at
8.75, 9.73 and 8.95 ppm, respectively. As reported in the literature [24,28], if the dihydrazone
exists in the syn-cis-configuration or staggered configuration, the δOH, δNH and δCH=N,
resonances, each should appear as a singlet. However, the appearance of these signals in the form
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of six signals (doublet of doublet) indicates anti-cis (chair) configuration. Actually, the features
1
of the H-NMR spectra of the dihydrazones are in conformity with syn-cis- or staggered
configuration. According to IR interpretation mentioned above, the staggered configuration is
1
well defined/dominated for all ligands (L1-L4). H NMR spectra of L3 and L4 differ from other
ligands spectra where they showed signals at 2.55 and 3.9 ppm downfield of TMS due the
methyl (CH3) and methoxy (OCH3) protons, respectively [1,9].
1
The assignments of the main signals in H NMR spectra of some diamagnetic palladium
complexes II, III and IV in comparison to that of free ligands (L2, L3 and L4) are given in Table
2 (S2). The data demonstrated the following facts. For II and III complexes: no signal is
detected for a naphthoic/phenolic OH or secondary NH groups. This remark gives a clue for the
deprotonation of both ortho- OH groups upon complexation. Obscurity of secondary NH in the
same complexes ruled out its contribution in coordination where its proton migrated to the
carbonyl oxygen (C=O) forming an enolized carbonyl (C-OH). Appearance of υ(M-Oenolic) in the
IR spectrum of complex II emphasized the deprotonation of the enolized carbonyl by
complexation. Participation of azomethine nitrogen atoms in coordination with the metal centre
is conspicuous in the shift of δ(-CH=N) associated with complex II to higher frequency by 0.246
ppm. In case of sample IV: four signals are observed in the region 12.80 -10.90 ppm and 12.80 -
10.90 ppm assuming doublet doublet of both secondary NH and CH=N protons, respectively
[29]. This result proves the existence of L4 ligand in chair conformation [39] with a cis-
configuration of dihydrazone. The upfield shift observed for δ(-OCH3) by 0.58 ppm upon
complexation inferred the involvement of this group in bonding without removal of CH3 group.
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The signals concerned to the aromatic protons of all Pd(II) complexes were observed within 6 -
8.4 ppm with minor changes in their positions compared with free hydrazones.
3.3. Electronic spectra and magnetic studies
The assignments of the observed electronic absorption bands of the oxaloylhydrazones as
well as their metal complexes in DMF are shown in Table 3 [26,30,31]. The electronic data
of the investigated ligands exhibited five peaks at 350-392 (n-π*, C=N), 338-375 (n-π*,
C=O), 306-340 (π-π*, C=N), 292-315 (π-π*, C=O), and 280-303 (π-π*, aromatic ring) nm.
It was noticed that the essential features of the spectrum of the complexes were entirely
different when compared with that of the free ligand, which boosted the coordination of
used ligands with the palladium ions. The spectra of all PdII complexes in DMF, Table 3,
revealed three broad peaks at 475-510, 390-470 and 340- 430 nm assigned to (1A1g→3A2g,
3B1g), (1A1g→1A2g) and 390 (1A1g→1Bg) transitions, respectively. Additionally, complex III
exhibited an additional band at 370 nm attributed to (1A1g→1Eg) transition. This suggests a
square-planar environment around the PdII ion. Pd(II) complexes are diamagnetic as
expected.
3.4. Thermal studies
To examine the thermal stabilities of PdII-complexes, thermogravimetric analysis (TG) and
differential thermal analysis (DTA) for all complexes were done (Fig. 2). For complex I, TG
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curve showed four stages of decomposition at 45, 250, 320 and 800 °C. The first stage
corresponds to the loss of 0.5 crystalline water (found/calculated % = 0.5/1.5) and accompanied
by endothermic peak in DT curve. The other three decomposition stages refer to the initial
decomposition temperature of the complex with formation of an expected Pd and/or PdO residue
at the last stage. In DTA curve, the exothermic peak observed at mid point 300 °C reflects the
decomposition while the endothermic peak at mid point 789.9 °C which is not accompanied by
weight loss is most probably attributed to a phase change. This endothermic effect arises from
the crystal structure change of some products resulted from the complex degradation. TG data of
complex II, exhibited three stages of decomposition at 50-100, 250-420 and 550-870 °C. The
first stage which is accompanied by endothermic effects in DTA curve corresponds to expelling
of two crystalline water (found/calculated% = 5.1/5.1) while the other two stages concerned to
the gradual decomposition of the complex with formation of (Pd + PdO) mixture
(found/calculated%= 32.4/32.2) at the last stage. The exothermic peak observed at mid point
equals 330 °C (DTA curve) refers to the decomposition of the solid complex. The TG curve of
complex III showed five stages of decomposition at 40-150, 200-280, 280-425, 425-570 and
570-720 °C, The first stage corresponds to the loss of 2.5 crystalline water molecules
(found/calculated% = 9.0/8.9) while the other four stages corresponds to the gradual
decomposition of the complex with formation of a mixture of Pd and PdO at the last stage
(found/calculated% = 44.9/45.2). DTA curve exhibited an endothermic peak at 60 °C
corresponding to loss of crystalline water molecules meanwhile the remarkable exothermic peak
at 261.99 pointing to the decomposition of the complex. TG and DT thermograms of complex IV
revealed four steps of decomposition where the first step is related to the elimination of two
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crystalline water molecules from the complex formula (found/calculated% = 5.2/6.1) while the
other three steps observed in the regions 250-320, 350-450, 650-900 °C are relevant to the
gradual decomposition with formation of PdO and/or Pd residue at the last stage as occurred with
other analogous complexes (I-III).
On the basis of the various physico-chemical and spectral studies presented above and their
discussion,
the
complexes
may
be
suggested to have structures shown in Fig. 3 (palladium complexes were drawn by ChemDraw
Ultra 8.0 program).
3.5. Optical properties
The optical and the electrical properties of metals complexes are interesting due to great
potential application in Schottky diode, solid state devices and optical sensor [32]. To clarify the
conductivity of the isolated complexes, the optical band gap (Eg) of oxaloyldihydrazone ligands
and their Pd(II) complexes have been calculated with the help of absorption spectra. To calculate
the
optical band gap energy from absorption spectra, Tauc’s relation is used {13,14]:
m


h  A h  Eg
Where hν is the photon energy, h is Planck’s constant, α is the absorption coefficient, Eg is the
optical energy gap,
A is the constant, m is equal to 1/2 and 2 for direct and indirect transitions, respectively.
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The absorption coefficient (α) was calculated from to the relation:
1
 
d ln

1/T

Where d is the width of the cell and T is the measured transmittance.
We plot a graph between (αhʋ)2 versus hν, the extrapolation of the straight
line to (αhʋ)2 = 0 axis gives optical band gap. Figure 4 shows Tauc’s plot for all the samples.
Band gap for ligands and PdII-complexes in DMF solvent lies in the ranges 3.06-3.42 eV and
2.38-3.39 eV, respectively (Table 2). Careful inspection of data mentioned in Table 2 revealed
that the optical band gap values of all ligands (Eg) were minimized upon complexation. As
reported in the literature [33] it is suggested that after complexation, metal leads to raise
mobilization of the ligand electrons by accepting them in its shell. It can be evaluated that after
formation of the complex, the chemical structure of the ligands is changed, the width of the
localized levels is expanded and in turn, the band gap is smaller. This result is very significant in
applications of electronic and optoelectronic devices, because of the lower optical band gap of
the materials [34]. Small band gap facilitates electronic transitions between the HOMO-LUMO
energy levels and makes the molecule more electro-conductive [35]. Upon excitation, some of
the electrons get enough energy to jump the band gap in one go. Once they are in the conduction
band, they can conduct electricity, as can the hole they left behind in the valence band. The hole
is an empty state that allows electrons in the valence band some degree of freedom. In essence,
the obtained band gap values suggest that these complexes are semiconductors and lie in the
same range of highly efficient photovoltaic materials. So, the present compounds could be
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considered potential materials for harvesting solar radiation in solar cell applications [33,36].
The little difference in the optical band gap Eg values between all studied complexes may be due
to their synonymous chemical structures.
As the refractive index is an important parameter for optical applications, it is important to
determine it for ligands and their complexes. The evaluation of refractive indices of optical
materials is of considerable importance for application in integrated optic devices such as
switches, filters and modulators. Besides, the refractive index is an important physical parameter,
which is widely used in chemistry to identify the liquid, or its purity. For this purpose, optical
properties were investigated for the compounds by spectrophotometric measurement of
transmittance, T, and reflectance, R, at normal incidence in the wavelength range of 190–900
nm. The optical reflectance spectra (R) for oxaloyldihydrazone ligands and their Pd(II)
complexes as a function of wavelength (λ,nm) are shown in Fig. 5. The reflectance has been
estimated using the relationship, R + T + A=1, for normal reflectance, the refractive index value
(n) of the compounds may be approximately expressed by the relation [37],

1 R


n 
,where R is the normal reflec tance.

1 R
The variations in the refractive index (n) with wavelength (λ,nm) is presented in Fig. 6. The
variation of n values with frequency range indicated that some interactions took place between
photons and electrons [38]. Moreover, Fig. 6 clarified that the complexeation affects the
refractive index of the free ligands where there is a difference in the n values upon connection.
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3.6. Application
3.6.1. Corrosion inhibition of hydrazones
The inhibitive actions of ligands on the dissolution of C-steel in 1 M HCl are investigated and
the inhibition efficiency values of the hydrazide derivatives are depicted in Table 4. Careful
inspection
increasing
of
these
results
showed
both
that,
by
the
inhibitor
concentration,
(θ)
and
(%) are increased while the weight loss is reduced. So, the dissolution of C-steel in the presence
of the investigated inhibitor can be
interpreted on the basis of interface inhibition mode where the adsorbed species mechanically
screen the coated part of the metal surface from the action of the corrosive medium. This
adsorption is related to some discriminated features of the investigated oxaloyldihydrazones like
the functional groups (OH, NH, C=O and C=N), quite large molecular size, high molecular
weight, aromaticity, electron density at the donor atoms (N:, :O:) and also the -orbital
character of azomethine, carbonyl and phenyl groups. Adsorption and desorption of inhibitor
molecules continuously occur at the metal surface and an equilibrium exists between the two
processes at a particular temperature. The inhibition achieved by dihydrazone compounds
decreases
in
the
sequence L4 > L3 > L1 > L2 and this behavior can be discussed in what follows. The difference in
the inhibition efficiencies of the four compounds lies in their steric factor and electron density at
the donor atoms. Nevertheless, all hydrazone ligands are capable to attach to the metal surface,
the persistence of methoxy (OMe) group in (L4) which is an efficient electron donating group
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than phenolic-OH group present in other compounds facilitates the adsorption on the metal
surface. Positive mesomeric effect (+M effect) and negative inductive effect (–I effect) of -
OCH3 leads to increase the electron density over phenyl ring which pushes the electrons towards
the donor atoms in O=C-NH-N=CH part by virtue of resonance and subsequently enhances the
bond strength between the molecule and the corroding metal [39]. Under these circumstances,
the inhibition efficiency of (L4) should be higher than other ligands. L3 came after L4 as the
electron donating nature of the methyl group attached to the azomethine (-N=C(CH3))
maximized the electron density on the azomethine group subsequently the ligand to metal
surface became more strong. Apparently, L1 comes before L2 because the electron pair on the
ortho-OH group reaches directly to the azomethine group by delocolization with one phenyl ring
while in case of L2 the naphthyl ring withdraws the electron pair to themselves. Furthers,
existence of two naphthyl rings in the structure of L2 may cause crowding that encourages
incomplete coating. The obtained data, Fig. 7, boosted the application of the utilized ligands
especially L4 as plausible corrosion pickling inhibitors whereas the maximum protection
efficiency of L4 was 47%. It is evident that the inhibition efficiency values are not extremely
high owing to the lower ligand concentrations used.
3.6.2. Kinetic treatment
Regarding the experimental section, when logWf was plotted against time, Fig. 8, with respect
to the abovementioned sheets in HCl solutions, a linear variation was perceived, which
confirmed a first-order reaction kinetics [39]. Table 4 gives the rate constants and half life
values. It is evident that the rate constants come down with raising the inhibitor concentrations
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(L1-L4) sustaining the inhibition of carbon steel in 1M HCl by the additives. The increase in half
life indicates more protection of the metals by the oxaloyldihydrazones [39].
4. Conclusion
In the present study, we have prepared mono and binuclear palladium complexes of
dihydrazones
and
characterized
them
on
the
basis
of
data obtained from physicochemical and spectroscopic studies. The oxaloyldihydrazone ligands
(L1- L4) can be synthesized by usual condensation of oxaloyldihydrazide with salicylaldehyde, 2-
hydroxy-1-naphthaldhyde, 2-hydroxyaceto-phenone and 2-methoxy-benzaldehyde, respectively,
in a 1:2 molar ratio. The dihydrazone is bonded in I and III as a dibasic tetradentate giving
mononuclear complexes while in complex II, the dihydrazone acts as a tetrabasic hexadentate
providing binuclear complex. The ligand behaves in dibasic bidentate manner in the remaining
complex IV. The obtained complexes may tentatively be suggested to have square planar
structures. Indirect band gaps of all samples lie in the range of wide band gap semiconductors.
Corrosion test reflects the capability of the investigated ligands to mitigate the corrosion of C-
steel in HCl solution especially L4.
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Table captions
Table 1.Specifications of C-steel coupons used for weight loss measurements.
Specimen C
C- steel
Si
0.07 0.01 -
As Sn Mn
P
S
Cr
Ni
Pb
Zn Cu
Al
Fe Mo
Co
Nb
Ti
V
W
-
0.39 0.022 0.0001 0.04 0.04 0.003 -
0.073 0.03 -
0.009 0.016 0.0006 0.003 0.017 .013
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Table 2. Analytical, physical and spectroscopic data of the oxaloyldihydrazones and their related metal complexes.
M.p(oC)
Color
Found(Cald.)%
Compound.
Symbol
1H-NMR Chemical shift (δppm)
μeff.
M+ Found/calcd.
Eg (eV)
3.10
3.06
3.32
3.42
2.25
2.55
2.38
3.25
C
H
N
M
>300
59.1
(58.9)
68.1
(67.7)
59.5
(61)
5.4
16.2
(17.2)
12.9
(13.2)
14.7
(15.8)
15.7
(15.8)
13.1
(12.7)
8.2
-
C16H14N4O4
L1
L2
L3
L4
I
12.6(NH, s), 10.98(OH, s), 8.75(CH=N, s), 6.6-8.40 (aromatic protons, m)
12.76(NH, s), 12.57(OH, s), 9.74(CH=N, s), 7.0-8.8 (aromatic protons, m
12.85(NH, s), 11.85(OH, s), 6.6-8.0(aromatic protons, m), 2.48(CH3, s)
12.3(NH, s), 11.9(OH, s), 8.95(CH=N, s), 6.8-8.5(aromatic protons, m), 3.93(OCH3, s)
very weak bands
-
326.0/ 326.0
Yellow
>300
(4.3)
5.3
-
-
C24H18N4O4
-
427.1/426.4
Yellow
>300
(4.3)
6.1
-
-
C18H18N4O4
-
356.4/354.2
Pale Yellow
>300
(5.1)
5.8
-
59.2
(61)
-
C18H18N4O4
-
355.0/354.4
White
(5.1)
3.7
-
>300
44.0
(43.7)
41.4
(40.7)
43.2
(42.9)
39.1
(38.1)
23.2
(24.2)
30.2
(30.1)
21.5
(21.1)
19.7
(18.7)
[Pd(L1-2H)].0.5H2O
[Pd2(L2-4H)(H2O)2].2H2O
[Pd(L3-2H)].2.5H2O
[Pd(L4)(Cl)2].2H2O
Diamg
Diamg
Diamg
Diamg
-
-
-
-
Orange
>300
(3.0)
3.4
II
9.98(CH=N, s), 6.8-8.6 (aromatic protons, m)
Orange Red
>300
(3.1)
4.3
(7.9)
11.2
(11.1)
10.2
(9.9)
III
IV
6.6-8.2(aromatic protons, m), 2.55(CH3, s)
Pale Yellow
>300
(4.2)
4.2
12.8-10.8(NH, q), 9-8.1(CH=N, s), 6.4-8 (aromatic protons, m), 3.35(OCH3, s)
Orange
(3.9)
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Table 3. Significant IR and electronic absorption data of oxaloyldihydrazones and their palladium complexes.
ν(OH)
ν(C-H)
δ(C-H) aromatic
ν(C=C) out of plane
ν(C-O)
phenolic OMe)
ν(C-
ν(M-O)
λmax, nm
Symbol phenolic ν(H2O) ν(NH) aromatic
ν(C=O) ν(C=N)
phenolic/enolic (assignments)
(enolic)
3149
(aliphatic)
3204 3062
(carbonyl/methoxy)
390(n-π*, C=N),
350(n-π*,
C=O), 340(π -
π*, C=N), 303
(π -π*, C=O),
292(π -π*,
Phenyl)
392(n-π*, C=N),
375(n-π*, C=O),
L1
L2
L3
L4
-
-
-
-
1666
1620
1621
1607
1600
1275
1287
1246
-
-
1403
1458
756
-/-
(3278)
(2924)
(-/-)
1486
1465
1541
3476
(-)
3166 3043
(2926)
1705
1660
-
741
746
760
-/-
330(π -π*,
C=N), 315(π -
π*,
(-/-)
C=O), 303(π -
π*, Phenyl)
372(n-π*, C=N),
352(n-π*,
C=O), 336(π -
π*, C=N), 305
(π -π*, C=O),
290(π -π*,
Phenyl)
350(n-π*, C=N),
338(n-π*,
C=O), 306(π -
π*, C=N), 292
(π -π*, C=O),
1574
1449
1486
-
3293 3048
(2923)
1689
1651
-
-/-
(3227)
(-/-)
1512
1464
-
3202 3041
(2940)
1653
964
-/-
(3238)
1486
1572
(-/-)
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280(π -π*,
Phenyl)
475 (1A1g→3A2g,
3B1g)
I
-
3473
3442
-
-
3060
-
-
1621
1638
1287
1296
-
-
1410
756
750
550/-
(-/-)
(-)
(2925)
1457
1495
390(1A1g→ 1A2g
(
340 (1A1g→1B1g)
500(1A1g→3A2g,
3B1g)
II
-
3053
1617
1644
1452
589/552
(-/-)
450
(-)
(2924)
1530
1600
1433
(1A1g→1A2g),
425 (1A1g→1Bg)
475(1A1g→3A2g,
3B1g)
III
IV
-
3442
3451
-
3065
-
1632
1638
1262
-
759
760
555/-
(-/-)
440(1A1g→1A2g)
,093
(3194)
(2927)
1484
1513
1466
(1A1g→1Bg), 370
(1A1g→1Eg)
510(1A1g→3A2g,
3B1g)
-
3205 3060
(2923)
1665
1600
-
967
-/-
470(1A1g→1A2g),
430 (1A1g→1Bg)
(-)
1486
1527
(-/559)
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