Synthesis of polar unique 3d metal-imine complexes of salicylidene anthranilate sodium salt. Homogeneous catalytic and corrosion inhibition performance

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Journal of the Taiwan Institute of Chemical Engineers 88 (2018) 286–304
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Synthesis of polar unique 3d metal-imine complexes of salicylidene
anthranilate sodium salt. Homogeneous catalytic and corrosion
inhibition performance
Hany M. Abd El-Lateef
a , b , ∗, Mohamed Shaker S. Adam
a , b , ∗, Mai M. Khalaf
a , b
a
Department of Chemistry, College of Science, King Faisal University, P.O. Box 380, Al Hufuf 31982, Al Hassa, Saudi Arabia
b
Chemistry Department, Faculty of Science, Sohag University, Sohag 82534, Egypt
a r t i c l e i n f o
Article history:
Received 27 January 2018
Revised 30 March 2018
Accepted 19 April 2018
Keywo rds:
Imine
Catalysis, cyclohexene, inhibition
Carbon steel
Density functional theory
a b s t r a c t
Three polar Ni(II)-, Cu(II)- and Zn(II)-complexes (M-SSA) of salicylidene anthranilate sodium salt ligand
were synthesized. The ligand (H
2
SSA) and its corresponding metal-complexes are characterized by alter-
native physico-chemical tools in which H
2
SSA acts as tridentate bi-basic chelating agent. Catalytic poten-
tial of M-SSA was investigated in the homogenous oxidation of 1,2-cyclohexene at 80 °C in acetonitrile,
water or under solvent-free condition. M-complexes exhibit high catalytic reactivity with high chemose-
lectivity in acetonitrile. Cu-SSA shows the highest catalytic potential for the oxidation of 1,2-cyclohexene
than Ni-SSA or Zn-SSA. The lowest yield of the epoxy-product was obtained in water due to the hydroly-
sis ring opening reaction affording 1,2-cyclohexanediol. The inhibition performance of H
2
SSA and M-SSA
on the carbon steel corrosion (CS) in HCl was studied using electrochemical techniques. The inhibition
capability was increased with increasing inhibitor dose. The adsorption of inhibitors on the surface of
CS obeyed the Langmuir isotherm paradigm. Surface characterizations (SEM/EDX) revealed that the in-
vestigated compounds adsorbed on CS surface and form protective layer that shield the surface from
direct corrosion attack. The experimental data have been completed by density functional theory treat-
ment. The obtained theoretical results are exceedingly in agreement with empirical results catalytically
and inhibitory.
© 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction
Imines, as Schiff base derivatives, are well known as high
coordinated ligands with various transition metals of low and high
valents. Specific chemical features of steric demand, bite angles
and the nature donor centers of imines could control their reactiv-
ity towards complex formation [1,2] . Tridentate imine ligands with
hard donor atoms, alternatively, oxygen and/or nitrogen atoms as
strong σ- and π-donors are widely applied in complexation 3d
transition metals [3,4] , which form very stable and defined stoi-
chiometric structures [5–7] . Complexes of imines are particularly
advantageous because of their ready high applicability in many
fields, e.g. biological [2] , pharmacological [8] and anticancer [6] .
They act as containing enzymes and oxidation catalysts [9,10] . The
type of the central metal ion in the imine complexes has a remark-
able effect on their reactivity of towards catalytic redox processes
∗Corresponding authors at: Department of Chemistry, College of Science, King
Faisal University, P.O . Box 380, Al Hufuf 31982, Al Hassa, Saudi Arabia.
E-mail addresses: hmahmed@kfu.edu.sa , hany_shubra@yahoo.co.uk (H.M.A. El-
Lateef), madam@kfu.edu.sa (M.S.S. Adam), mmkali@kfu.edu.sa (M.M. Khalaf).
and steel corrosion inhibition [11] . Hence, synthesis of novel com-
plexes with the modified ligands of low cost, high coordinately
active and greener reagents [11] , is the motivation in inorganic
chemistry [12] . High applicability of transition metal complexes as
catalysts for redox processes of alkenes in the organic synthesis
is the main object industrially and in laboratories [13] . High
sufficiency and chemoselectivity of metal complexes as catalysts
for such redox processes is the reason for many researchers to
design new classes of metal complexes, especially with cheap and
easily synthesized ligands, e.g. imine-salicylidene ligands [14] . Type
of central metal ion has a remarkable influence in its complex
catalytic activity towards oxidation of alkenes and alcohols [15] ,
which is the challenge to present the catalytic potential of some
various 3d metal ions in the (ep)oxidation of 1,2-cyclohexene.
Corrosion of carbon steel (CS) processes is highlighted in
research and industry, recently, especially in acid solutions due to
the increased industrial implementations [11] . For example, the
crude oil refining products in an assortment of corrosive situations
[16] . Generally, refinery corrosion could take place by the attacking
of equipment surface with a strong acid [17] . The other funda-
mental fields of enforcements are petrochemical processes, acid
https://doi.org/10.1016/j.jtice.2018.04.024
1876-1070/© 2018 Taiwa n Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

H.M.A. El-Lateef et al. / Journal of the Taiwan Institute of Chemical Engineers 88 (2018) 286–304 287
descaling, acid pickling, industrial cleaning and oil well acid in oil
recovery [18] . Among the acid solutions, HCl is one of the most ex-
ceedingly utilized agents [19] . According to the immersion of car-
bon steels in aggressive media, they are liable to various corrosion
types; subsequently, the applied corrosion inhibitors, to protect
the steel surfaces from dissolution, will be unavoidable [20] . Nu-
merous of organic compounds containing N, O and S donor centers
have been applied for protection of metals and alloys from corro-
sion. The presence of –C = N
–linkage (azomethine) in the imine
moiety, as Schiff bases, in the organic compounds, improves their
effective corrosion inhabitation [21] . Imines could be adsorbed on
the metal surface and spontaneously form a monolayer on its sur-
face, therefore, they display good inhibitive properties [22] . Some
imines were used to prevent the corrosion attack, e.g. amino acid
derivatives [11,16] and polydentate imines, which contain donor
nitrogen centers [23] . The high inhibiting potential of imines could
be interpreted by their adsorption on the steel/solution interface.
However, little works about metal complexes with imines, coordi-
nated ligands, as corrosion inhibitors for CS in acidic environments
have been reported previously [24,25] . Various Zn(II)-, Ni(II)-,
Sn(II)- and Co(II)-imines complexes demonstrated good corrosion
inhibiting performance for steel alloy in different aggressive so-
lution at different temperatures [26] . Abdel-Gaber et al. studied
the corrosion inhibition of CS in H
2
SO
4 by various doses (50 to
200 ppm) of a bulky Co(III) Schiff base complex [27] . Transition
metal-imine complexes are predictable to supply better efficiency
because of their compactness, larger size and the synergistic effect
of metal–organic blends [28] . They display improved inhibiting
performance more than their coordinated free ligands [29] .
Quantum chemical calculations have become widely used as an
important method for supported both mechanisms of corrosion in-
hibition and catalytic oxidation of organic compounds [30,31] . The
relationships between structural parameters and their inhibition
and efficiencies would be investigated in these studies, as well
as, the HOMO and LUMO would be used to study the catalytic
potential of metal-complexes in the oxidation processes.
The present study aims to present novel metal-imine com-
plexes as homogenous oxidation catalysts and corrosion inhibitors.
The advantages of these compounds include synthesized from
comparatively cheap raw materials, water soluble and green.
Synthesis and characterization of three novel complexes of Ni(II),
Cu(II) and Zn(II) ions with salicylidene anthranilate sodium salt
(H
2
SSA) ( Fig. 1 ) would be studied. The catalytic performance of
Ni-SSA, Cu-SSA and Zn-SSA could be examined in the homogenous
oxidation of 1,2-cyclohexene by an aqueous H
2
O
2 or tert- butyl
hydroperoxide (TBHP or t BuOOH) at 80 °C in acetonitrile, water or
under solvent-free condition. The ability of H
2
SSA and their Ni(II)-,
Cu(II)- and Zn(II)-complexes as inhibiting agents for the corrosion
of CS in acidic chloride media using electrochemical (EIS and PDP)
and surface characterization (SEM and EDX) methods could be
investigated. DFT approaches are performed on the synthesized
compounds to emphasize empirical results.
2. Experimental part
2.1. Materials and instruments
All applied reagents and materials are supplied from Sigma-
Aldrich and Acros. They are used without further purification or
treatment. Micro-analyses were accomplished by a GMBH VarioEl
model V2.3 CHNS machine. Nuclear magnetic resonating spectra
were evaluated on a multinuclear FT-NMR spectrometer Bruker
ARX40 0 at 40 0.1 for H-proton and 100.6 MHz for C-nucleus at
room temperature. The
1
H and
13
C chemical shifts d are given in
ppm. J
HH and J
CC refer to coupling constants between H and C
nuclei in the NMR. Electronic spectrums were done with 10 mm
quartz cells in a thermostatted holder of a Jasco UV–vis spec-
trophotometer (model V-570) in the range from 190 to 800 nm.
IR spectra (as KBr discs) are estimated by Shimadzu FTIR-8101
Fourier Transform Infrared spectrophotometer from 40 0 0 to
400 cm
−1
. Conductivity values of the polar current complexes
were measured using a Jenway conductivity meter model 4320,
connected with an epoxy bodied conductivity cell (two electrodes,
shiny) and cell constant calibration from 0.01 to 19. 99 at 25 °C.
Magnetic susceptibility of Ni-SSA and Cu-SSA were determined by
a Gouy’s balance, the diamagnetic correction was achieved within
Pascal’s contents and Hg[Co(SCN)
4
] as a calibrant. Electrospray
ionization mass spectroscopy (EIS-MS) was performed on a Waters
API Quatroo Micro Triple Quadrupole electrospray ionization mass
spectrophotometer in the positive and negative modes. All chemi-
cal reactions were taken place with a magnetically stirrer and kept
at the specific temperatures using a thermostated oil bath.
Carbon steel (CS) with the composition (in wt%): 0.04% (S),
0.15% (C),0.70% (Mn), 0.19% (Si), 0.011% (Cr), 0.011% (Ni) and Fe
(rest) was utilized as working electrode (WE) for one time. HCl
(1.0 M, Merck 37%) was prepared in deionized water. H
2
SSA and
its corresponding complexes of Ni-SSA, Cu-SSA and Zn-SSA were
dissolved in bi-distilled at the concentration range 50 to 300 ppm.
2.2. Synthesis of H
2
SSA (2-[(2-Hydroxy-5-sodium sulfonate-
benzylidene)-amino]-benzoate)
In 30 mL water, 5.0 mmol of sodium salicylaldehyde-5-
sulfonate was mixed with 5.0 mmol of anthranilic acid in 30 mL
ethanol at room temperature. A yellow color in the reaction media
was observed just after addition. The reaction mixture was kept
with stirring for 3 h at 25 °C. The reaction was monitored by thin
layer chromatography (TLC). The solvent was removed by vacuum
and the residual washed many times with diethyl ether and dried
in oven. The pure ligand was obtained by recrystallization in
water–ethanol (1:1) to award yellow crystalline solid with yield
1.2 0 g (70%).
1
H NMR (DMSO- d
6
, 400 MHz): δ6.50 (t,
3 J = 6.9 and 7.2 Hz,
1H), 6.74 (d,
3 J = 8.2 Hz, 1H), 6.94 (dd,
3 J = 8.0 and 8.3 Hz, 1H),
7.2 2 (t,
3 J = 7.0 and 7.4 Hz, 1H), 7.70 (dd,
3 J = 7.7 Hz, 2H), 7.92 (s,
1H), 10.2 6 (s, 1H, phenolic-OH), 10.82 ppm (s br, 1H, CH
=
N).
13
C NMR (100 MHz, DMSO- d
6
, dept-135): δ110.18 (C
q
), 115 .05
(CH), 116.83 (CH), 116.96 (CH), 121.48 (CH), 126.92 (CH), 131.61
(CH), 134.20 (CH), 140 .72 (C
q
), 151.94 (C
q
), 161.15 (C
q
), 169.99 (C
q
),
192.11 ppm (CH, CH = N). (
1
HNMR,
13
CNMR and dep135, Figs.
S1–S3, Supplementary materials).
2.3. Synthesis of metal-complexes (M-SSA)
An aqueous solution (20 mL) of copper acetate monohydrate,
nickel acetate hexahydrate or zinc acetate dihydrate (5.0 mmol)
was added dropwisely to an aqueous solution of H
2
SSA at 25 °C.
The reaction mixture was warming up 70 °C for 3 h with stirring.
The color was changed gradually to the corresponding complex
color. The solvent was removed in vacuum and the residual was
washed many times with diethyl ether and dried in oven. The pure
complexes were recrystallized in water–methanol mixture (1:1).
2.3.1. NMR spectra of Zn-SSA
1
H NMR (DMSO- d
6
, 400 MHz): δ1.82 (s, 6H, crystalline 3H
2
O),
3.29 (s, 2H, coordinated H
2
O, overlapped with H
2
O of the solvent),
6.58 (d,
3 J = 8.7 Hz, 1H), 7.2 9 (dd,
3 J = 7.4, 6.7 Hz, 2H), 7.47 (m,
2H), 7.6 4 (d,
4 J = 1.9 Hz, 1H), 7.89 (d,
3 J = 7.0 Hz, 1H), 8.42 ppm
(s br, 1H, CH
=
N).
13
C NMR (100 MHz, DMSO- d
6
, dept-135): δ115.04 (CH), 116. 9 7
(CH), 121.49 (C
q
), 127.20 (C
q
), 131.76 (CH), 132.52 (C
q
), 133.85
(CH), 134.24 (CH), 156.11 (C
q
), 168.55 (C
q
), 192.40 ppm (CH, CH

288 H.M.A. El-Lateef et al. / Journal of the Taiwan Institute of Chemical Engineers 88 (2018) 286–304
Comp.
abbreviation
Structure Optimized structure
H2SSA
S
O
O
O
Na
C
OH
N
H
H
H
H
OHO
H
H
H
H
Zn-SSA
S
O
O
O
C
O
N
HH
H
H
OO
H
H
H
H
Zn
OH2
Na
Cu-SSA
S
O
O
O
Na
C
O
N
H
H
H
H
OO
H
H
H
H
Cu
H2O
Ni-SSA
S
O
O
O
Na
C
O
N
H
H
H
H
OO
H
H
H
H
Ni
OH2
Fig. 1. The chemical and optimized structure of H
2
SSA, Zn-SSA, Cu-SSA and Ni-SSA.
= N), (
1
HNMR,
13
CNMR and dep135, Figs. S4–S6, Supplementary
materials).
2.4. Stability and thermodynamic parameters of M-SSA
The stoichiometric molar ratios of M-SSA were estimated by
spectrophotometric continuous variation method [32] . The M-SSA
formation constants K
f were determined using the spectropho-
tometric measurements depending upon temperature (from 20,
25, 30, 35 to 40 °C) [28,29] . The complexes Gibb’s free energy
( ࢞f
G ) could be derived as well as, the thermodynamic parameters,
࢞f
H and ࢞f
S , could derived from the Gibb’s–Helmholtz relation as
function of 1/ T .
2.5. Catalytic procedure
Oxidation of 1,2-cyclohexene (1.0 mmol) carried out by an
aqueous solution of 30% H
2
O
2 (3.0 mmol) or 70% (in water) tert-
butyl hydroperoxide (TBHP or t BuOOH) (1.70 mmol), as terminal
oxidants, catalyzed by Ni-SSA, Cu-SSA or Zn-SSA (0.02 mmol) in
acetonitrile (10 mL), water (10 mL) or under solvent-free condi-
tions at 80 °C for 4 h in homogenous atmosphere. The catalytic
processes were achieved in a 50 mL two necked reaction flask
with a circulated condenser.
The (ep)oxidation products were monitored and analyzed by
using Shimadzu Gas Chromatography mass spectrometer (GC–MS)
model QP2010 SE equipped with Rxi-5 Sil MS capillary column
(30 m length ×0.25 mm ID ×025 um film thickness). The analysis

H.M.A. El-Lateef et al. / Journal of the Taiwan Institute of Chemical Engineers 88 (2018) 286–304 289
was performed using the GC parameters with injector temperature
at 250 °C, initial oven temperature at 40 °C (held for 1 min) and
temperature increase to 200 °C with a rate of 10 °C min
−1
. The
inlet was operated in the split less mode. The temperature of the
MS transfer line was kept at 200 °C. Helium (99.999%) was used
as carrier gas with a flow rate 1 mL min
−1
. The resulted data was
analyzed by LabSolution software, which used also to control the
system.
2.6. Corrosion inhibition measurements
Electrochemical studies were carried out in electrochemical cell
containing three electrodes. The CS, platinum sheet (about 2 cm
2
)
and saturated calomel electrode (SCE) were served as working,
auxiliary and reference electrodes, respectively. The electrochem-
ical tests were probed using the electrochemical measurement
system potentiostat/galvanostat (VersaSTAT4) with a lock-in a
phase-sensitive detector (amplifier) (Model-5208).
The measurements of electrochemical impedance spectroscopy
(EIS) were achieved at the corrosion potential ( E
corr
) within
100 kHz to 0.5 Hz frequency range and amplitude 10 mV. EIS
results were analyzed using the Z-View software and were fitted
to the convenient equivalent circuits. The fitting data gave the
element parameters in the equivalent circuits. Potentiodynamic
polarization curves (PDP) were accomplished in the potential
domain of ±250 mV vs. E
corr
at a sweep rate of 0.2 mV s
−1
. It well
documented that the CS corrosion rate is temperature dependent,
and the highest corrosion rate is detected at 50 °C, the optimized
temperature of corrosion [33] . Therefore, to evaluate the inhibition
performance of the investigated H
2
SSA and M-SSA compounds on
CS in HCl solution, all the experiments were carried out at 50 °C.
2.7. Surface characterization (SEM and EDX)
The CS was immersed in an aqueous solution of HCl (1.0 M)
for 5 day in the absence and presence of 300 ppm of Ni-SSA.
Then, it was detached from the aggressive solution, washed
with bi-distilled water and dried. Consequently, CS sheets mor-
phology was investigated using a JEOL model 5300 scanning
electron microscopy- energy dispersive X-ray analysis (SEM-EDX)
by applying playback voltage of 5.0 kV.
2.8. Details of modeling studies
Density functional theory (DFT) became one of the most popu-
lar method for screening new potential corrosion inhibitors [34] . In
carrying out the computational calculations of the current study,
the input files of imine ligand (H
2
SSA) and its corresponding three
metal complexes of Zn-SSA, Cu-SSA and Ni-SSA used catalysts for
the oxidation of 1,2-cyclohexene and as corrosion inhibitors drawn
using gauss View 5.0.9 [35] . The geometry of the whole active
reagents (H
2
SSA and M-SSA) was performed using Gaussian 09 W
revision C.01 [36] at the DFT level of theory. The minimum-energy
molecular structures of the active reagents were completely op-
timized in the gas and aqueous phases using DFT calculations
[37] . DFT calculations bases on Beck’s three parameter exchange
functional and Lee–Yang–Parr nonlocal correlation functional
(B3LYP) [38] , 6-31G(d,p) orbital basis set for S, C, N, H, O and Na
atoms and efficient core potential SDD basis set [39] for Zn, Cu
and Ni atoms. The conductor-like polarizable continuum model
(CPCM) was applied for water phase calculations. Some electronic
structure descriptors (band gap E , absolute softness σ, absolute
hardness η, nucleophilicity N , electrophilicity index ω, the electron
transferred number N and E
back-donation
) were obtained from
the below equations [40–43] .
E = E
LU MO
−E
HOMO (1)
η=
E
LUMO
−E
HOMO
2
(2)
σ=
1
η(3)
χ= −(
E
HOMO
+ E
LUMO
)
2
(4)
ω =
χ2
2 η(5)
N =
1
ω
(6)
N =
χFe
−χinh
[
2
(
ηFe
+ ηinh
)
]
(7)
E
back −donation
=
−η
4
(8)
using theoretical values of χFe
= 7.0 and of ηFe
= 0 eV mol
−1
for Fe
bulk, because mentioned bulks are softer than the metallic atoms.
3. Results and discussion
3.1. Synthesis and characterization
The novel ligand (H
2
SSA) was synthesized within common
condensation of an aqueous solution of sodium salicylaldehyde-
5-sulfonate [11] with a methanolic solution of anthranilic acid
similar to such condensation of the wide reported salicylaldehyde
with anthranilic acid [5–8] ( Scheme 1 ). The molecular structure of
H
2
SSA was confirmed by EA,
1
H,
13
C NMR, IR and UV–vis. spectra
( Tables 1 and 2 ).
Complexation of H
2
SSA with Ni
2 +
, Cu
2 + or Zn
2 + ion was
carried in an aqueous media. The solubility of the synthesized
ligand H
2
SSA and its corresponding complexes (M-SSA) is high in
water and in the high coordinated organic solvents, DMSO and
DMF. They are partial soluble in high polar organic solvents, e.g.
methanol and acetonitrile, and insoluble in less polar organic sol-
vents, e.g. dichloromethane and chloroform. The novel complexes
are stable with pH from 8.7 to 1.6 (using standard universal buffer
solutions [11] ).
The purity of the suggested structure of M-SSA could be re-
flected from the CHN analyses ( Table 1 ) and thermogravimetric
analyses. The CHN analyses magnitudes agree with the suggested
structure of the M-chelates with very small differences ( ±0.5%).
M-SSA is assigned to be formed in 1:1 mol ratios of metal: ligand
forming chelated dibasic complexes as ternary M-complexes with
presence of a coordinated water molecule, as shown in Scheme 1 .
The highest absorption wavelengths of the characteristic elec-
tronic transitions in M-SSA, λmax (nm) and the corresponding mo-
lar absorpativity, εmax (mol
–1 cm
−1
), are recorded in Table 1 and
presented in Fig. 2 . H
2
SSA shows two high-energy absorption
bands in the UV-region at 250 and 325 nm, which corresponded
to the π→ π∗and n → π∗transitions, respectively. There is an ob-
servable shift of the π→ π∗and n → π∗transitions in H
2
SSA after
complexation to be 298 and 381 nm with Ni
2 +
, 252 and 389 nm
with Cu
2 +
, and 232 and 289 nm with Zn
2 + ions, respectively. The
visible absorption bands occur at 386 and 382 nm for Cu- and
Zn-SSA, which high probably characteristic for a charge-transfer
transition from the ligand to metal ion. The L → MCT band for
Ni-SSA may be overlapped with n → π∗band at 381 nm [44] .
IR spectrum of H
2
SSA and its corresponding M-complexes are
recorded in Table 2 . The disappearance of the broad stretching
vibrational band of the O
–H of the carboxylate group in H
2
SSA

290 H.M.A. El-Lateef et al. / Journal of the Taiwan Institute of Chemical Engineers 88 (2018) 286–304
Scheme 1. The synthetic pathway of H
2
SSA and its corresponding chelates M-SSA.
Tabl e 1
CHN analysis, melting point, color and UV–vis. spectra of H
2
SSA and M-SSA at [complex] = [ligand] = 1. 0 ×10
−5
mol dm
−3
in aqueous media at
25 °C.
Compound MW (g mol
−1
) Microanalyses found %, (calc. %) Color m.p. ( °C) Electronic spectra
C H N λmax
(nm) εmax
(mol
−1
cm
−1
) Assign.
H
2
SSA 343.29 49.25 3.08 3.81 Pale yellow 164 325 5894 n →
π∗
(48.98) (2.94) (4.08) 250 12,436 π→ π∗
Ni-SSA 454.01 36.86 3.30 2.84 Green 225 381 7422 LM-CT
(37.04) (3.11) (3.09) 298 6152 n →
π∗
Cu-SSA 440.85 38.47 3.02 3.38 Green 218 386 5263 LM-CT
(38.14) (2.74) (3.18) 289 6119 n →
π∗
252 14,865 π→ π∗
Zn-SSA 478.72 35.43 3.09 3.26 Yellow 251 382(vbr) 3215 LM-CT
(35.12) (3.37) (2.93) 323 7488 n →
π∗
232 18, 89 4 π→ π∗
Tabl e 2
Structural significant infrared spectral assignments ( ¯
ν, cm
−1
) of H
2
SSA ligand
and M-SSA.
Compound H
2
SSA Ni-SSA Cu-SSA Zn-SSA
Group
O
–H
(water) 3308(m br) 3393(w br) 3352(w br)
3233(m br) 3270(m br) 3296(m br)
3240(w br)
O
–H 3401(w br)
C
–H
ar 3058(w) 3116(w br) 3124(w)
C
=
O 1601 (m ) 1643(m) 1583(s) 1693(m)
C
–O
(phenolic) 149 4( w) 1455(m) 1454 (m ) 1478(w)
C
–O
(carboxylic) 1419(w) 1415(w) 1403(m)
C
=
N
(azomethine) 1577(w) 1589(m) 1526(s) 1612(m )
C
–N 1236(m) 1164 ( s ) 1147 (m) 1162(s)
S
–O
−1382(w) 1402(m) 1377(m) 1378(w)
S
=
O 110 3(m ) 1107( s ) 110 6(m ) 1112 ( s)
M
–O 751(w) 756(w) 749( w)
M
–N 601(w) 604(w) 583(m)
br = broad band, s = strong band, m = moderate band, w = weak band,
ar = aromatic ring.
at 3401 cm
−1 interprets the complexation of the deprotonated
O
–H carboxylate group with the metal ion. New broad bands
¯ν(O
–H)
appeared at 3308 and 3233 cm
−1 for Ni-SSA, at 3393 and
3270 cm
−1 for Cu-SSA, at 3352, 3296 and 3240 cm
−1 for Zn-SSA
are assigned for the coordinated and crystalline water molecules in
the complexes [45] , as shown in Scheme 1 . A stretching vibrational
band at 1577 cm
−1 is characteristic for the imine group ¯ν(CH
=
N)
,
which highly shifted after complexation with Ni
2 +
, Cu
2 + and Zn
2 +
ions to 1589, 1526 and 1612 cm
−1
, respectively. It could be due
to the coordination of imine group (
–CH = N
–) to the central
metal ion, as observed elsewhere [46] . The polar sulfonate group
of S –O
−and S = O bonds gives two weak-medium featured
vibrational bands at 110 3 and 1382 cm
−1
for the ligand. They were
little shifted after complexation to be 1107 and 1402 cm
−1 for
Ni-SSA, 110 6 and 1377 cm
−1 for Cu-SSA, and 1112 and 1378 cm
−1
for Zn-SSA, respectively [47] . M-SSA manifest new weak bands
were detected at 751, 756 and 749 cm
−1 are distinguished for the
coordination bonds of M
–O and 601, 604 and 583 cm
−1 for M
–N
bands of Ni-SSA, Cu-SSA and Zn-SSA, respectively [48] .
To estimate the number of the diagnostic crystalline and coor-
dinated water molecules in M-SSA, TGA was applied for all M-SSA,
which shown in Figs. S7–S9 (Supplementary materials). M-SSA
complexes were heated with rate of 10 °C per min from 30 to
600 °C. The thermogram utilized that M-SSA have an observable
decomposing in two mainly consecutive steps. At 100–130 °C, the
lost endothermic mass was observed with m
rel
= 8.1%, 4.7% and

H.M.A. El-Lateef et al. / Journal of the Taiwan Institute of Chemical Engineers 88 (2018) 286–304 291
200 250 300 350 400 450 500 550 600
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
sbA
, nm
H2SSA
Ni-SSA
Cu-SSA
Zn-SSA
Fig. 2. Molecular spectral scan of H
2
SSA ligand and its corresponding M-SSA, [complex] = [H
2
SSA] = 1.0 ×10
−5
mol dm
−3
in an aqueous media at 25 °C.
Tabl e 3
Characteristic magnetic moments (
μ) and molar
conductivity measurements (
m
) of M-SSA ([M-
SSA] = 1.0 ×10
−4
mol dm
−3
) in DMSO, DMF and ace-
tone at 25 °C.
Compound μm
(B.M.) (
−1
cm
2
mol
−1
)
DMSO DMF Acetone
Ni-SSA 2.94 125 15 8 19 4
Cu-SSA 4.17 119 147 17 8
Zn-SSA –107 140 172
10.8%, which agreed with the theoretical mass loss ( m
rel
= 7.9%,
4.0% and 11.2%) of Ni-SSA, Cu-SSA and Zn-SSA, respectively. This
mass loss is referred to the water molecules in the crystal lattice
[29] . In the second step, the TGA diagram of Ni-SSA, Cu-SSA and
Zn-SSA presents an endothermic mass loss at higher temperature
range (180–230 °C), because of the lose percentages of the co-
ordinated water molecule. The measured mass loss magnitudes
( m
rel
= 3.0%, 3.8% and 4.8%) are compatible with the calculated
values ( m
rel
= 3.9%, 4 .0% and 3.8%) of Ni-SSA, Cu-SSA and Zn-SSA,
respectively ( Scheme 1 ).
Molar conductivities ( m
) of M-SSA were measured in highly
coordinated polar solvents (DMSO, DMF and acetone) at 25 °C and
the obtained data are listed in Table 3 . The resulted values illus-
trated that all M-SSA are high electrolytic conducting complexes
and contain two particular ions in the solution, one complex anion
of the
–SO
3
−group and other is the cation, Na
+ ion [49] .
Magnetic susceptibilities of the polar M-SSA ( Table 3 ) clarify
that Ni-SSA and Cu-SSA are highly spin and para -magnetic in a
tetrahedral and square geometrical structures, respectively, with
magnitudes 2.94 and 4.17 B.M. [5] . Those measured data predict
that there is no interaction between metal ions in the solutions
[11,32] .
Mass spectra of all M-SSA are listed in Figs. S10–S12 (Supple-
mentary materials), which have appropriate base peaks with the
tentative complex anion (without the sodium cation). MS records
peak in the negative mode of all metal chelates [M
–Na
+
+ 1].
There are peaks corresponding to the loss of the coordinated
water molecules. M-SSA have peaks with the loss of –SO
3
−anion
in the positive mode [M
–SO
3
−+ 1].
Zn-SSA is diamagnetic with NMR spectral analysis. By com-
parison of the
1
H NMR spectra of H
2
SSA and Zn-SSA, a singlet
signal assigned for the –OH phenolic group at high frequencies
10.26 ppm according to the intramolecular hydrogen bonding
( Scheme 1 ) is disappeared after complexation with Zn
2 + ion. This
is mainly attributable for the coordinated phenolic group within
deprotonatation. The characteristic singlet signal of the imine
group (
–CH = N
–) is appeared at 10.82 ppm with high downfield
shift due to the intermoleuclar hydrogen bonding. The characteris-
tic signal was strongly up field shifted after coordination to Zn
2 +
ion to be 8.42 ppm.
3.2. Stability formation constants and thermodynamic parameters
The stoichiometric molar ratios of M-SSA were estimated by
spectrophotometric continuous variation method [31] . The metal
ion reacted with H
2
SSA in 1:1 molar ratios forming M-SSA in
an aqueous media, see Fig. S13 (Supplementary materials). The
formation constant values, which listed in Table 4 , assign that all
M-SSA are highly stable in the aqueous media. The equilibrium
formation constant ( K
f
) goes in the order as follows: Zn-SSA > Cu-
SSA > Ni-SSA. Consequently, Zn-SSA is the most stable complex
compared to Cu-SSA and Ni-SSA.
K
f of M-SSA was determined from 20 to 40 °C, which used
to calculate the thermodynamic parameters of the complexes
formation. ࢞f
H and ࢞f
S are the thermodynamic parameters, which
derived from the Gibb’s–Helmholtz relation [32] ( Fig. 3 ). The nega-
tive values of the complexes Gibb’s free energy ( ࢞f
G ) refers to that
the formation of current complexes is spontaneous. Moreover, the
negative magnitudes of ࢞f
H assigned that the complexation reac-
tion is exothermal with strong metal-ligand bonds formation [50] .

292 H.M.A. El-Lateef et al. / Journal of the Taiwan Institute of Chemical Engineers 88 (2018) 286–304
Tabl e 4
Stability constants and thermodynamic parameters of M-SSA, [com-
plex] = 1.0 ×10
−5
mol dm
−3
.
Compound T ( °C) K
f
×10
6 −࢞f
G −࢞f
H −࢞f
S
(L mol
−1
) (kJ mol
−1
) (kJ mol
−1
) (J mol
−1
K
−1
)
Ni-SSA 20 6.08 46.08 65.13 37.75
25 5.32 47.20
30 4.10 48.65
35 2.62 50.59
40 1.0 4 53.82
Cu-SSA 20 6.62 45.87 45.61 31.17
25 5.19 47.26
30 3.92 48.76
35 3.16 50.12
40 1.91 52.24
Zn-SSA 20 6.75 45.83 37.76 28.50
25 5.46 47.13
30 4.21 48.58
35 3.56 49.81
40 2.43 51.61
3.3. Catalytic activity
3.3.1. Catalytic (ep)oxidation processes of 1,2-cyclohexene
Alkenes (ep)oxidation processes, which afforded epoxy-
products, as the target products, are essential materials for
many organic syntheses [51,52] . The catalytic potential of M-SSA
was examined in the oxidation of 1,2-cyclohexene by an aqueous
H
2
O
2
or tert- butyl hydroperoxide (TBHP or t BuOOH) in acetonitrile,
in water or under solvent-free conditions. The resulted products
were determined by gas chromatography mass spectra ( Scheme 2 ).
Table 5 reports the percentages of conversion and chemoselectivity
of 1,2-cyclohexene oxidation.
The main target product of the oxidation processes is epoxy-
cylcohexane in presence of other side unwanted products, mainly,
1,2-cyclohexanediol ( Scheme 2 ), as detected form GC–MS [53] . The
reaction media has an observable action on the catalytic processes
in the conversion and chemoselectivity percentages ( Table 5 ).
Comparing of the environmental reaction media, i.e. in ace-
tonitrile, in water or under solvent-free conditions, the catalytic
processes afford various conversion and chemoselectivity per-
centages with either an aqueous H
2
O
2 or TBHP. The oxidation
process did not proceeded in the absent of catalysts. In water,
M-SSA catalysts are more soluble, which afforded the highest con-
version percentages with Ni, Cu- and Zn-SSA catalyst precursors
(95%, 94% and 91%, respectively). However, they gave the lowest
chemoselectivity percentages (64%, 69% and 60%, respectively), see
Table 5 . In acetonitrile, Ni-, Cu- and Zn-SSA catalysts showed little
less conversion percentages (90%, 92% and 87%, respectively) with
the highest target percentages (87%, 90% and 85%, respectively).
By applying solvent-free conditions, the conversion percentages
were the lowest (89%, 90% and 89%, respectively), but with little
better than those processes carried out in water (64%, 69% and
60%, respectively). Similar behave of all catalyst complexes was
observed also with TBHP compared to that with an aqueous H
2
O
2
.
With TBHP, in acetonitrile, the highest yields (92%, 94% and 90%)
and chemoselectivity percentages (85%, 90% and 91%) were de-
tected with Ni-, Cu and Zn-SSA, respectively. In water, the lowest
conversion (85%, 87% and 82%) and chemoselectivity (69%, 73%
and 70%) were obtained with Ni-, Cu and Zn-SSA, respectively. But,
under solvent-free conditions, the catalytic processes gave better
conversion (87%, 89% and 84%) and better chemoselectivity (79%,
84% and 75%) with Ni-, Cu and Zn-SSA, respectively ( Table 5 ).
Conclusively, the solubility of M-SSA catalysts may play a substan-
tial role in the homogeneity of the catalyst complexes. To probe
the catalytic potential in water, Ni-, Cu- and Zn-SSA catalysts
show the highest conversion, however, water as a co-solvent in
such oxidation processes leads to inhibition of the reaction rates,
0.00320 0.00325 0.00330 0.00335 0.00340 0.00345
-21.0
-20.5
-20.0
-19.5
-19.0
-18.5
ln Kf
1/T ( K-1)
Ni-SSA
Cu-SSA
Zn-SSA
Fig. 3. Derivation of thermodynamic data of M-SSA formation from ln K
f magni-
tudes.
Scheme 2. Oxidation of 1,2-cyclohexene by an aqueous H
2
O
2 or tert -butyl hy-
droperoxide catalyzed by M-SSA.
as reported elsewhere [52] . In acetonitrile, they are less soluble
and have less catalytic potential. Under free-solvent conditions,
the catalytic potential of was reduced remarkably, may due to
the heterogeneous catalytic nature, i.e. the less dissolving of the
complex catalysts in the catalytic process, especially with TBHP.
On the other hand, the chemoselectivity was highly reduced in
water with both an aqueous H
2
O
2 and TBHP with detection of
other unwanted side product, 1,2-cyclohexanediol (in high scale)
and other unknown products. This might be resulted from the
hydrolysis ring opening reaction of the chemoselective product
(epoxy-cylcohexane) to 1,2-cyclohexanediol in water [42,51] . This
phenomenon was less observed in acetonitrile or under solvent
free conditions using both oxidant (an aqueous H
2
O
2 or TBHP).
Moreover, the catalytic potential of Cu-SSA is slightly more
advantaged in the oxidation of 1,2-cyclohexene than that of Ni-SSA
or Zn-SSA ( Table 5 ) [49] . According to the highest reversible elec-
trochemical characterization of Cu
2 + species compared to those
Ni
2 + and Zn
2 +
, the catalytic potential of Cu-SSA is more sufficient
than that of Ni-SSA and Zn-SSA under all various catalytic reaction
conditions [54] . Additionally, the more Lewis acid character of
Cu-SSA [55] and the available interchanging of oxidation states for
Cu
2 + ions compared to Ni
2 + and Zn
2 + ions may be the reason for
the highest catalytic potential of Cu-SSA than that of Ni-SSA and
Zn-SSA [54] .
However, under solvent-free condition the catalytic process is
more favored and greener [11,56,57] , the catalytic reaction is more
effective and sufficient in acetonitrile more than under solvent-free
conditions ( Table 5 ), as observed previously [54] . Furthermore,
an aqueous H
2
O
2
, as an oxidant, is more suitable for the green
oxidation processes, but, in particular, tert- butyl hydroperoxide is
more effective for such catalytic oxidation of 1,2-cyclohexene than
an aqueous H
2
O
2 [45] .
3.3.2. A proposed mechanism for the catalytic (ep)oxidation
High oxidizing power of an aqueous H
2
O
2 or TBHP ( t BuOOH)
could be observed due to the rapid change in the color of

H.M.A. El-Lateef et al. / Journal of the Taiwan Institute of Chemical Engineers 88 (2018) 286–304 293
Tabl e 5
Oxidation of 1,2-cyclohexene by an aqueous H
2
O
2
or TBHP ( t BuOOH) catalyzed by M-SSA at 80 °C for
4 h in various reaction conditions.
Complex
a Conversion, % (Selectivity, %)
An aqueous H
2
O
2 TBHP
Acetonitrile
b Water
c Solvent-free
d Acetonitrile
b Water
c Solvent-free
d
Ni-SSA 90 (87) 95 (64) 89 (70) 92 (85) 85 (69) 87 (79)
Cu-SSA 92 (90) 94 (69) 90 (65) 94 (90) 87 (73) 89 (84)
Zn-SSA 87 (85) 91 (60) 89 (62) 90 (81) 82 (70) 84 (75)
a The oxidation process carried out for 1,2-cyclohexene (1.0 mmol) with catalyzed by M-SSA
(0.02 mmol).
b The solvent for the oxidation process is acetonitrile (10 mL).
c The solvent for the oxidation process is water (10 mL).
d The oxidation process carried out under solvent-free conditions.
Scheme 3. The proposed catalytic cycle for the (ep)oxidation of 1,2-cyclohexene.
M-SSA after its addition to the substrate (1,2-cyclohexene) and
the M-SSA catalyst in the reaction media. Electron transfer from
the M-SSA catalyst to H
2
O
2 or TBHP affording new active species
of higher valent central metal ion (M
3 +
) in the complex [50] , or
a higher charged metal complex ( A ) ( Scheme 3 ) [47] . The higher
charged metal complex could be more acceptable for the catalytic
mechanism especially for the high stable M
2 + ion in its complex,
i.e. Zn
2 + ion in Zn-SSA, because its probability for oxidation to
Zn
3 + could be not logical. Replacement of the labile coordinated
water molecule in M-SSA catalyst by another oxidizing agent
molecule in A could take place to give an active intermediate
complex ( B ) [58] . The coordinated H
2
O
2 or TBHP molecule to
central metal ion in B could cause Oxygen transfer mechanism
affording active oxo- or peroxo-intermediate ( C ), as reported
elsewhere [59,60] . The 1,2-cyclohexene approach to the active
species of oxo- or peroxo-intermediate ( C ) [60] results oxidation of
1,2-cyclohexene to the chemoselective product [61] with liberating
of the active complex ( A ) in another catalytic cycle ( Scheme 2 ).
3.4. Corrosion inhibition activity
3.4.1. EIS studies
Nyquist plots for CS electrode in HCl (1.0 M) in the absence
and presence of various doses of (a) H
2
SSA, (b) Zn-SSA, (c) Cu-SSA
and (d) Ni-SSA inhibiting reagents at 50 °C are shown in Fig.
4 (a)–(d). The Nyquist curve shows a depressed semicircle with the
size depending upon the concentration of inhibitors. The Nyquist
plots reveal comparable behavior in HCl (1.0 M) with and without
the inhibiting reagents, which refer to that CS corrosion inhibition
in HCl by the investigated inhibitors does not alter the corrosion
mechanism process [62] .
The diameters of the semicircles in the solution containing
the inhibiting reagent are generally larger than that of the un-
inhibited solution. Hence, the CS electrode/HCl systems display
higher impedance in the presence of inhibiting reagent, i.e. H
2
SSA
and M-SSA. Nevertheless, the Nyquist plots ( Fig. 4 ) are not ideal
semicircle, because of the porosity and non -homogeneity of CS
surface [63] .
From the Nyquist plots, R
ct
, C
dl
, n , Y
o and E
EIS could be derived
by Z-View impedance fitting software and listed in Table 6 . The
equivalent circuit (EC) presented in Fig. 5 was used to analyze the
EIS results. In this model, R
s represents the solution resistance and
R
ct represents the resistance of charge transfer. Constant phase
element (CPE) was applied in place of the “ideal” capacitance. The
impedance of CPE was estimated by the below equation [19] :
Z
CPE
= Y
−1
o (
Jω
)
−n (9)
where n and Y
o represent the exponent and CPE constant, respec-
tively. ω = 2 πf and represents the angular frequency, and f is the
frequency in Hz, and J = ( −1)
1/2 is an imaginary number. CPE is
equivalent to the pure capacitance if n = 1. CPE is equivalent to
the Warburg impedance, if n = 0.5. Finally, CPE is equivalent to

294 H.M.A. El-Lateef et al. / Journal of the Taiwan Institute of Chemical Engineers 88 (2018) 286–304
0 50 100 150 200 250 300 350
0
50
100
150
200
250
300
350
(d) Ni-SSA
10 Hz
0.5 Hz
0.5 Hz
100 kHz
C steel bare
50 ppm
100 ppm
150 ppm
200 ppm
300 ppm
Simulated
0 2 4 6 8 1012141618
0
2
4
6
8
10
12
14
16
18
C steel bare
Simulated
-Z//
ga
m
I/ Ω cm2
Z/
Real/ Ω cm2
0 30 60 90 120 150 180 210 240 270
0
30
60
90
120
150
180
210
240
270
(c) Cu-SSA
0.5 Hz
10 Hz
0.5 Hz
100 kHz
C steel bare
50 ppm
100 ppm
150 ppm
200 ppm
300 ppm
Simulated
024681012141618
0
2
4
6
8
10
12
14
16
18
C steel bare
Simulated
-Z//
g
a
mI / Ω cm2
Z/
Real/ Ω cm2
0 20 40 60 80 100 120 140 160 180 200
0
20
40
60
80
100
120
140
160
180
200
(b) Zn-SSA
024681012141618
0
2
4
6
8
10
12
14
16
18
C steel bare
Simulated
25 Hz
10 Hz
0.5 Hz
100 kHz
-Z//
ga
m
I/ Ω cm2
Z/
Real/ Ω cm2
C steel bare
50 ppm
100 ppm
150 ppm
200 ppm
300 ppm
Simulated
0 20 40 60 80 100 120 140 160 180
0
20
40
60
80
100
120
140
160
180
(a) H2SSA
024681012141618
0
2
4
6
8
10
12
14
16
18
C steel bare
Simulated
25 Hz
10 Hz
0.5 Hz
100 kHz
-Z//
gamI / Ω cm2
Z/
Real/ Ω cm2
C steel bare
50 ppm
100 ppm
150 ppm
200 ppm
300 ppm
Simulated
Fig. 4. Nyquist plots for CS in HCl (1.0 M) in the absence and presence of various concentrations of the synthesized inhibitors (a) H
2
SSA, (b) Zn-SSA, (c) Cu-SSA and (d)
Ni-SSA at 50 °C.
Tabl e 6
Electrochemical impedance parameters for CS in HCl (1.0 M) without and with various concentrations of the synthe-
sized inhibitors at 50 °C.
Inhibitor C
inh
/ppm by weight R
ct
/
cm
2 C
dl
/μF cm
−2 Q
CPE θE
EIS
/%
Y
o
/
μΩ
−1
s
n
cm
−2 n
Blank 0.0 16.82 ±1.4 148.58 4.13 0.899 ––
H
2
SSA 50 30.89 ±1.9 75.28 2.05 0.801 0.455 45.54
100 41.02 ±2.8 53.97 1.2 3 0.803 0.589 58.99
150 72.04 ±3.7 29.35 0.85 0.814 0.766 76.65
200 110 . 37 ±6.2 18.54 0.44 0.817 0.847 84.76
300 180 .84 ±8.3 10.23 0.22 0.829 0.906 90.69
Zn-SSA 50 34.64 ±2.2 61.01 1.71 0.801 0.514 51.44
100 45.33 ±2.9 43.74 1.03 0.809 0.628 62.89
150 79.78 ±3.5 23.79 0.71 0.818 0.789 78.91
200 127.89 ±5.3 15.03 0.37 0.827 0.868 86.84
300 193.33 ±7.5 8.29 0.19 0.836 0.912 91.29
Cu-SSA 50 40.14 ±2.1 45.67 1.31 0.808 0.580 58.09
100 60.66 ±2.7 30.42 0.79 0.802 0.722 72.27
150 130. 69 ±4.3 14. 35 0.54 0.819 0.871 87.12
200 188.37 ±4.2 8.92 0.27 0.827 0.910 91.07
300 252.12 ±7. 8 5.70 0.14 0.845 0.933 93.32
Ni-SSA 50 54.53 ±2.6 35.31 1.01 0.827 0.691 69.15
100 70.64 ±3.7 26.88 0.61 0.836 0.761 76.18
150 159.92 ±5.3 11.4 2 0.42 0.827 0.894 89.48
200 300.10 ±7.4 6.02 0.22 0.863 0.943 94.39
300 343.71 ±11.2 4.15 0.12 0.881 0.951 95.10

H.M.A. El-Lateef et al. / Journal of the Taiwan Institute of Chemical Engineers 88 (2018) 286–304 295
Fig. 5. Equivalent circuit used to fit the impedance spectra obtained for CS in HCl solution in the absence and presence of investigated inhibitors.
the inductance if n = −1[16]. The Y
o and n values of were derived
from the Z -View software, and C
dl
was calculated from CPE and R
ct
parameters according to Eq. (10 ) [18] , as presented in Table 6 .
C
dl
=
Y
o
R
1 −n
ct 1 /n (10)
R
ct
is obtained from EIS measurements, which used to calculate
the inhibition efficiency ( E
EIS
, %) according to Eq. (11 ) [62] ;
E
EIS
(
%
)
= 100 ×R
ct
−R
o
ct
R
ct = 100 ×θ(11)
where R
ct and R
o
ct
are the resistance of charge transfer in the
presence and absence of the studied inhibiting reagents. It is
noticed from Table 6 that, the n values were near to unity, sug-
gesting the pseudo-capacitive features of the CS/HCl systems.
The slightly lower values of n in the presence of investigated
inhibitors compared to the free HCl indicate that the surface of CS
is comparatively more heterogeneous, which may be because of
non-uniform inhibitor species adsorption on the surface of metal.
The values of E
EIS and R
ct are increased while Y
o decreased with
the increasing inhibitor dose. This behavior was possible due to
the protective film formation on the CS surface, which significantly
prevent the charge transfer and the dissolution of metal. However,
C
dl
, values are decreased, which is related to the denser protective
film [17] . According to Mallaiya et al. [64] , the adsorbed inhibitor
molecules thickness is increment with decreasing C
dl
. Moreover,
the inhibitor molecules could minimize the capacitance by in-
crement the thickness of double layer based on the Helmholtz
pattern ( Eq. (12 )) [11] :
δorg
=
ε ε
o
A
C
dl
(12)
where δorg
, A, ɛ
o and ɛ and are the protective layer thickness,
surface area of electrode, vacuum permittivity and the dielectric
constant of the medium, respectively. Value of C
dl
was constantly
smaller in the solution containing inhibitor than in its absence,
which could be resulted from the efficient adsorption of H
2
SSA
and M-SSA.
The Bode phase angle curves ( Fig. 6 (a)–(d)) manifest single
maximum at intermediate frequencies, expanding of the maximum
in the solution containing inhibitors, attributed to a protective
layer formation on the surface of CS electrode. Moreover, the
value of impedance in the solution containing inhibitors is larger
than that in its absence and also the impedance value increment
with increasing inhibitor dose. The phase angle value < −80 °and
a straight relationship between log Z with log f with as lope
value around −1 has been spotted, whereas an ideal capacitor is
described by a value of phase angle −90 °and the slope is equal
to −1 [65] .
Finally, Inhibition efficiencies observed from EIS study agree
with the data obtained from the PDP measurements. As expected,
[Ni-SSA], [Cu-SSA] and [Zn-SSA] give higher E
EIS
% than the studied
imine ligand. The high P % of the metal complexes compared to
H
2
SSA ligand may be attributed to more active sites for adsorption
and their larger size. The P % order for the tested ligand and its
complexes is:
Ni-SSA > Cu-SSA > Zn-SSA > H
2
SSA
The above sequence of E
EIS % could also be demonstrated in
terms of the central metal size Ni(II) > Cu(II) > Zn(II).
3.4.2. PDP results
Fig. 7 (a)–(d) displays the PDP plots of CS electrode in HCl
(1.0 M) containing different dose of H
2
SSA and M-SSA, as in-
hibitors, at 50 °C. The addition of those inhibiting reagents shifts
both the cathodic and anodic branches to depress current density
than those registered in solutions without the inhibiting reagents.
Particularly, the four applied inhibiting reagents had considerable
influence on both cathodic and anodic reactions of corrosion
process. Additionally, lower corrosion rates of CS electrode were
remarked in the presence of investigated compounds contrasted to
that in free HCl (1.0 M). The corrosion electrochemical indices, i.e.
corrosion current density ( J
corr
), cathodic and anodic Tafel slopes
( βc and βa
), corrosion potential ( E
corr
), and corrosion inhibition
efficiency ( E
PDP
) were calculated from PDP curve and mentioned
in Table 7 . The βc
and βa
slopes show approximately values in the
absence or presence of metal imine complexes inhibiting reagents,
which indicates that the studied inhibitors do not change the
mechanisms of the hydrogen evolution and iron dissolution. This
means that the investigated H
2
SSA and M-SSA compounds act as
adsorptive inhibitors, leading to block the corrosion active sites.
Notably, the E
PDP of the inhibitors was estimated from J
corr
values in the existence and absence of the studied inhibiting
reagents ( Eq. (13 )) [18] :
E
PDP
(
%
)
=
J
cor r
(
0
)
−J
cor r
(
i
)
J
cor r
(
0
) ×100 = θ×10 0 (13)
where J
corr(i)
and J
corr(o)
are the inhibited and uninhibited J
corr
,
respectively. From the data recorded in Table 7 ( Fig. 7 ), no orderly
change in E
corr is observed with the addendum of inhibitor. The
maximum change is seen to be ±0.021 V with regard to E
corr
in both directions. The recorded displacement in E
corr is lower
than ±0.085 V and subsequently these ligand and its complexes
cannot be classified as cathodic or anodic corrosion inhibitors [66] .
Therefore, H
2
SSA and its corresponding Cu-, Zn- and Ni-SSA are
mixed type inhibitors [67] .
According to the data in Table 7 , J
corr decrease on increasing
the dose of the all tested inhibitors and maximum decrease was

296 H.M.A. El-Lateef et al. / Journal of the Taiwan Institute of Chemical Engineers 88 (2018) 286–304
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
-0.8
-0.4
0.0
0.4
0.8
1.2
1.6
2.0
2.4
0
-10
-20
-30
-40
-50
-60
-70
Phase angle/ degree
(a) SSA
log frequency/ Hz
C steel bare
50 ppm
100 ppm
150 ppm
200 ppm
300 ppm
Simulated
/|Z|gol Ω cm 2
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
-0.8
-0.4
0.0
0.4
0.8
1.2
1.6
2.0
2.4
0
-10
-20
-30
-40
-50
-60
-70
Phase angle/ degree
(b)Zn-SSA
log frequency/ Hz
C steel bare
50 ppm
100 ppm
150 ppm
200 ppm
300 ppm
Simulated
/|Z|go
lΩ cm 2
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
-0.8
-0.4
0.0
0.4
0.8
1.2
1.6
2.0
2.4
0
-10
-20
-30
-40
-50
-60
-70
Phase angle/ degree
(c)Cu-SSA
log frequency/ Hz
C steel bare
50 ppm
100 ppm
150 ppm
200 ppm
300 ppm
Simulated
/|Z|gol Ω cm 2
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
-0.8
-0.4
0.0
0.4
0.8
1.2
1.6
2.0
2.4
0
-10
-20
-30
-40
-50
-60
-70
-80
Phase angle/ degree
(d)Ni-SSA
log frequency/ Hz
C steel bare
50 ppm
100 ppm
150 ppm
200 ppm
300 ppm
Simulated
/|Z|gol Ω cm 2
(a) H2SSA
Fig. 6. Bode and Phase angle plots for CS electrode in HCl (1.0 M) in the absence and presence of different concentrations of the synthesized inhibitors (a) H
2
SSA, (b) Zn-SSA,
(c) Cu-SSA and (d) Ni-SSA at 50 °C.
Tabl e 7
Electrochemical parameters and inhibition efficiency obtained from PDP studies for CS electrode in HCl (1.0 M) with
presence or absence of various concentrations of studied inhibitors at 50 °C.
Inhibitor C
inh
/ppm by weight J
corr
/ −E
corr
/ βa
/ −βc
/ θE
PDP
/%
mA cm
−2 V (SCE) mV dec
−1 mV dec
−1
Blank 0.0 2.764 ±0.089 0.462 104 182 ––
H
2
SSA 50 1.187 ±0.10 0.466 107 173 0.570 57.02
100 1.0 34 ±0.06 0.465 107 185 0.625 62.58
150 0.704 ±0.041 0.467 104 188 0.745 74.52
200 0.532 ±0.0225 0.469 105 166 0.807 80.73
300 0.429 ±0.017 0.474 110 18 1 0.844 84.45
Zn-SSA 50 1.112 ±0.101 0.468 111 179 0.59 59.74
100 0.951 ±0.071 0.470 111 191 0.655 65.57
150 0.606 ±0.047 0.466 108 194 0.780 78.07
200 0.426 ±0.032 0.454 109 171 0.845 84.58
300 0.318 ±0.022 0.483 11 4 186 0.884 88.48
Cu-SSA 50 1.0 0 4 ±0.091 0.462 111 193 0.636 63.67
100 0.789 ±0.058 0.469 115 187 0.714 71.45
150 0.487 ±0.037 0.475 111 182 0.823 82.37
200 0.354 ±0.028 0.465 113 183 0.871 87.19
300 0.285 ±0.017 0.477 114 186 0.896 89.68
Ni-SSA 50 0.911 ±0.086 0.481 109 194 0.670 67.04
100 0.668 ±0.061 0.466 112 17 8 0.758 75.81
150 0.414 ±0.035 0.476 111 18 7 0.850 85.02
200 0.312 ±0.024 0.472 110 188 0.887 88.70
300 0.260 ±0.011 0.480 111 193 0.905 90.58

H.M.A. El-Lateef et al. / Journal of the Taiwan Institute of Chemical Engineers 88 (2018) 286–304 297
-0.70 -0.65 -0.60 -0.55 -0.50 -0.45 -0.40 -0.35 -0.30 -0.25 -0.20
-6.5
-6.0
-5.5
-5.0
-4.5
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
gol Jm
c.
A
/-2
C steel bare
50 ppm
100 ppm
150 ppm
200 ppm
300 ppm
(b) Zn-SSA
-0.70 -0.65 -0.60 -0.55 -0.50 -0.45 -0.40 -0.35 -0.30 -0.25 -0.20
-6.5
-6.0
-5.5
-5.0
-4.5
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
C steel bare
50 ppm
100 ppm
150 ppm
200 ppm
300 ppm
gol J / mc.A -2
-0.70 -0.65 -0.60 -0.55 -0.50 -0.45 -0.40 -0.35 -0.30 -0.25 -0.20
-7.0
-6.5
-6.0
-5.5
-5.0
-4.5
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
E/ V vs. (SCE)
C steel bare
50 ppm
100 ppm
150 ppm
200 ppm
300 ppm
(c) Cu-SSA
gol J / m
c.A -2
-0.70 -0.65 -0.60 -0.55 -0.50 -0.45 -0.40 -0.35 -0.30 -0.25 -0.20
-7.5
-7.0
-6.5
-6.0
-5.5
-5.0
-4.5
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
E/ V vs. (SCE)
(d)Ni-SSA
gol J/ m
c.A -2
C steel bare
50 ppm
100 ppm
150 ppm
200 ppm
300 ppm
(a) H2SSA
Fig. 7. PDP curves of CS electrode in HCl (1.0 M) without and with various concentrations of (a) H
2
SSA, (b) Zn-SSA, (c) Cu-SSA and (d) Ni-SSA at 50 °C.
Tabl e 8
Adsorption indices for H
2
SSA, Zn-SSA, Cu-SSA and Ni-SSA inhibitors calculated from
Langmuir adsorption isotherm for CS electrode in HCl (1.0 M) at 50 °C.
Inhibitor Slope values K
ads Regression coefficient ( R
2
) G
o
ads
(mol
−1
L) kJ mol
−1
H
2
SSA 0.853 4784 0.9954 −31.46
Zn-SSA 0.895 7581 0.9962 −32.62
Cu-SSA 0.9928 10,752 0.9980 −33.50
Ni-SSA 0.949 14, 970 0.9981 −34.33
observed at 300 ppm at the investigated temperature. In addition,
the inhibition efficiency ( E
PDP
) values increase as a function of
compound concentration. The inhibition action is related to in-
crement of the adsorption of the inhibitor molecules on the CS
electrode, which causes an increment the covered surface area
of the electrode ( θ). The higher surface coverage value ( θ≈1)
indicates nearly a full coverage of the CS surface with adsorbed
inhibitor ( Table 7 ). The inhibitor adsorption on the CS surface
forms a protective film, which minimizes the contact between the
corrosive solution and CS surface, and so decreases the lethal in-
fluence of aggressive solution on the surface of CS. This adsorption
could be demonstrated by an electrostatic interaction among polar
groups in ligand, its corresponding metal-complexes and the CS
surface active sites [68] .
The suggested mechanism of CS oxidation in acidic solution
containing chlorides (Cl
−=
X
−and Fe
=
M) could be described in
general as the follows [69] :
M + X
−↔
M X
−ads
(14a)
M X
−ads
↔
(
MX
)
ads
+ e
−(14b)
(
MX
)
ads
↔ MX
+
ads
+ e
−(14c)
MX
+
ads
↔ M
2+
+ X
−(14d)
M + H
+
↔
M H
+
ads
(14e)
M H
+
ads
+ e
−↔
(
MH
)
ads
(14f)
(
MH
)
ads
+ H
+
+ e
−↔ M + H
2 (14g)
The E
PDP of the studied inhibitors was increment in the
following sequence: Ni-SSA > Cu-SSA > Zn-SSA > H
2
SSA. The

298 H.M.A. El-Lateef et al. / Journal of the Taiwan Institute of Chemical Engineers 88 (2018) 286–304
1.0x10
-4 2.0x10
-4 3.0x10
-4 4.0x10
-4 5.0x10
-4 6.0x10
-4 7.0x10
-4 8.0x10
-4 9.0x10
-4
3.0x10
-4
4.0x10
-4
5.0x10
-4
6.0x10
-4
7.0x10
-4
8.0x10
-4
9.0x10
-4
1.0x10
-3
1.1x10
-3
1.0x10-4 2.0x10-4 3.0x10-4 4.0x10-4 5.0x10-4 6.0x10-4 7.0x10-4
2.0x10-4
3.0x10-4
4.0x10-4
5.0x10-4
6.0x10-4
7.0x10-4
8.0x10-4
1.0x10
-4 2.0x10-4 3.0x10-4 4.0x10-4 5.0x10-4 6.0x10-4 7.0x10-4 8.0x10-4
2.0x10-4
3.0x10-4
4.0x10-4
5.0x10-4
6.0x10-4
7.0x10-4
8.0x10-4
1.0x10-4 2.0x10-4 3.0x10-4 4.0x10-4 5.0x10-4 6.0x10-4 7.0x10-4 8.0x10-4
1.0x10-4
2.0x10-4
3.0x10-4
4.0x10-4
5.0x10-4
6.0x10-4
7.0x10-4
8.0x10-4
Cinh./)L/lom( Cinh./)L/
l
om(
R2=0.9954
(a) H2SSA
Linear Fit of Data
R2=0.9962
(b) Zn-SSA
Linear Fit of Data
R2=0.9980
(c) Cu-SSA
Linear Fit of Data
Cinh.
(mol/L)
Cinh.
(mol/L)
R2=0.9981
(d) Ni-SSA
Linear Fit of Data
Fig. 8. Langmuir adsorption isotherm of (a) H
2
SSA, (b) Zn-SSA, (c) Cu-SSA and (d) Ni-SSA in HCl (1.0 M) solution at 50 °C.
Tabl e 9
Calculated quantum chemical parameters of H
2
SSA, Zn-SSA, Cu-SSA and Ni-SSA molecules in gas and aqueous
phase calculated at B3LYP/6-31G (d, p)/SDD level of theory.
Gas phase
Compound
E
HOMO
/eV E
LUMO
/eV E /eV η/eV σ/eV
−1 ω/eV N E
Back −donation
H
2
SSA −5.92 −1.77 4.15 2.08 0.48 3.56 0.76 −0.52
Zn-SSA −5.86 −2.14 3.72 1.86 0.54 4.31 0.80 −0.46
Cu-SSA −5.89 −2.22 3.67 1.83 0.55 4.50 0.80 −0.45
Ni-SSA −5.81 −2.29 3.52 1.76 0.56 4.65 0.83 −0.44
Aqueous phase
H
2
SSA −6.19 −1.8 5 4.34 2.17 0.46 3.72 0.69 −0.54
Zn-SSA −5.87 −2.07 3.80 1.90 0.52 4.15 0.79 −0.47
Cu-SSA −6.02 −2.25 3.77 1.88 0.53 4.54 0.76 −0.47
Ni-SSA −5.95 −2.36 3.59 1.7 9 0.56 4.81 0.79 −0.45
best inhibitor under this investigation was Ni-SSA, because the
electronegativity of these transition metals is follows the order:
(Ni
2 +
= 1.88 > Cu
2 +
= 1.85 > Zn
2 +
= 1.59 according to Allen scale
[70] ) and the E
PDP increment with an increase in electronegativity
of these ions. Ni
2 + has considerable electronegativity, which incre-
ments their attraction for the surface of CS electrode; subsequently
their complex with H
2
SSA could be polar in kind. The polarity
could increment the adsorption of M-SSA on the CS surface and
accordingly E
PDP increased.
When we compared between our current inhibitors reactivity
and the previous reported studies, e.g. Tawfik and Zaky [20] , who
studied the inhibition performance of imine anionic surfactant
(L) and its Co(II), Cu(II), and Zn(II)-complexes of on CS in HCl
(1.0 M). The corrosion rate of CS was decreased with increment of
dose of the synthesized imine surfactant and its metal complexes
and E
PDP values were found in the following sequence: L
–Cu
(85.7%) > L
–Co (79.5%) > L
–Zn (73.5%) > L (66.9%) with dose
400 ppm. Kiruthikajothi and Chandramohan [71] also synthesized

H.M.A. El-Lateef et al. / Journal of the Taiwan Institute of Chemical Engineers 88 (2018) 286–304 299
——
5kV ×2,000 10 µm 0000 JSM -5500LV
5kV ×2,000 10 µm 0000 JSM -5500LV
——
Spectrum 1
Spectrum 1
Spectrum 2
Spectrum 2
KeV
——
a
b
c
d
Fig. 9. SEM micrographs of CS electrode exposure to HCl (1.0 M) (a) and containing 300 ppm Ni-SSA (b) after immersion time 12 0 h. EDX analysis at the same conditions (c
and d).
four copper (II) amino acid complexes and studied their inhibition
action on the corrosion of mild steel in HCl (1.0 M) by weight
loss measurements. The results showed that, methionine amino
acid copper complex has better efficiency than serine amino
acid copper complex and the maximum E
PDP obtained was 75%.
Mishra et al. [72] prepared complexes of Ni(II) and Cu(II) with
some acylhydrazine imine ligands, namely, 2-hydroxy-benzoic acid
(1-phenyl-propylidene)-hydrazide (Hhbh) and 2-amino-benzoic
acid (1-phenyl-propylidene)-hydrazide (Habh). They probed their
inhibition characteristics on mild steel in H
2
SO
4 (0.5 M). They
deduced that, in the presence of 50 ppm of Habh and Hhbh and
their M-complexes, the E
PDP was around 74.3%.
In the present study, E
PDP % values increases for H
2
SSA and
M-SSA with increase of the inhibitor concentration, which man-
ifest the highest value at 300 ppm of the inhibiting agent. The
E
PDP % progress in the following order: Ni-SSA (90.58%) > Cu-SSA
(89.68%) > Zn-SSA (88.48%) > SSA (84.45%). The high molecular
weight, complexation and the large size of H
2
SSA and its corre-
sponding M-SSA could contribute to enhance their E
PDP % values.
3.4.3. Adsorption isotherms
The adsorption isotherms could be estimated by supposing that
inhibition performance, which due fundamentally to the adsorp-
tion at CS electrode/HCl interface. Important features about the in-
hibitors adsorption on the surface of CS could be provided from
adsorption isotherms [11] . In order to define the type of isotherm,
the values of surface coverage ( θ) as a function of C
inh
, should be
gained. The θvalues can be readily determined from EIS studies
( Eq. (11 )). To comprehend the adsorptive nature of H
2
SSA and its
Zn-, Cu- and Ni-SSA on CS, the empirical data were tested to differ-
ent isotherms such as Flory–Huggins, Temkin, Frumkin, Freundlich
and Langmuir. Thus, the adsorption isotherm of Langmuir was
found to display the best fit of the adsorption data of the present
inhibiting reagents, as presented by the below equation [16] :
C
inh
θ=
1
K
ads
+ C
inh (15)
where C
inh
and K
ads
are the inhibitor concentration in molar and
the equilibrium constant of adsorption process, respectively. The
Langmuir adsorption isotherms of H
2
SSA and Zn-SSA, Cu-SSA and
Ni-SSA are presented in Fig. 8 . The linear correlation coefficients
( R
2
) are almost 0.998 near to unity ( Table 8 ), stressing that the
adsorption of H
2
SSA and M-SSA in the studied medium follows
the adsorption isotherm of Langmuir’s.
The K
ads
values were measured from the inverted intercept of
Fig. 8 . Large K
ads
values were deduced ( Table 8 ) for all studied
compounds referring to more efficient adsorption and hence
preferable E
%
. The standard adsorption free energy ( G
o
ads
) values
were calculated based on K
ads
values as shown in the below
equation [17] :
G
o
ads
= −RT ln
(
55 . 5 K
ads
) (16)
The value of 55.5 is the molar concentration of water. In-
spection data in Table 8 , G
o
ads
values are −31.46 kJ mol
−1
(H
2
SSA), −32.62 kJ mol
−1 (Zn-SSA), −33.50 kJ mol
−1 (Cu-SSA) and
−34.33 kJ mol
−1 (Ni-SSA), which is less than −40 kJ mol
−1 but
more than −20 kJ mol
−1
, showing that the adsorption of SSA and
its Zn-, Cu- and Ni-chelates on CS surface are mixture between
physical and chemical adsorption [18,19] .
3.4.4. Corrosion attack morphology
In order to support the formation of coverage inhibiting layer
on the CS surface SEM and EDX studies were utilized. The SEM

300 H.M.A. El-Lateef et al. / Journal of the Taiwan Institute of Chemical Engineers 88 (2018) 286–304
Fig. 10. Frontier molecular orbitals of H
2
SSA, Zn-SSA, Cu-SSA and Ni-SSA molecules in gas phase.
images and EDX spectrum of the surface of CS after 120 h immer-
sion in HCl (1.0 M) only, and containing 300 ppm of Ni-SSA were
presented in Fig. 9 (a)–(d).
In Fig. 9 (a), in HCl (1.0 M) without inhibitors, the roughness of
surface was very elevated, and CS surface was strongly corroded
and damaged area along with cracks and pits which is attributed
due to aggressive acid attack of metal. EDX spectra ( Fig. 9 (c))
shows the peaks of (Mn, Fe, C, Si, Cl and O) referring to the for-
mation of iron oxides and the accumulation of corrosion products
on the surface of CS. Consequently, the surface of CS was highly
corroded in hydrochloric acid without inhibitors. In contrast,
the presence of Ni-SSA (300 ppm) in HCl solutions reduced the
corrosion rate remarkably and a smoother surface was observed
( Fig. 9 (b)) compared to that in Fig. 7 (a). The smoothed surface of
the CS substrate in the inhibited solution is attributed due to the
adsorption of inhibitor molecule on the surface of metal. In the
presence of Ni-SSA inhibitor, the surface of CS was undamaged and
Ni-SSA inhibitor could protect the surface from Cl
−ion attack in
the corrosive solution. The corresponding EDX spectrum ( Fig. 9 (d))
display the peaks of Mn, Fe, C, S, Cl and O in addition to Na and
Ni peaks from inhibitor molecule suggesting the adsorption of
Ni-SSA inhibitor on the CS surface. Additionally, there is a decrease
in the corrosion products amount where the peaks related to salts
and oxides formed due to the process of corrosion are reduced.
Accordingly, SEM and EDX investigations of the surface of CS
support the electrochemical investigations.
3.5. Theoretical studies: DFT approach
The perfected geometries of H
2
SSA, Zn-SSA, Cu-SSA and Ni-SSA
are exhibited in Fig. 1 ; also studied theoretical calculations are
given within Table 9 . The energy of highest occupied molecular
orbital ( E
HOMO
) and the energy of lowest unoccupied molecular or-
bital ( E
LUMO
) are extremely critical theoretical parameters in order
to portend the chemical stability and its reaction efficiency. HOMO
is often linked with the electron donating capability of a catalyst
and an inhibitor species [73] . The great E
HOMO values specify
the higher receptivity of molecule to give electrons to suitable
acceptor molecules by low energy empty molecular orbital. From
Table 9 , for gas and aqueous phase together, that the molecule
having the greatest ( E
HOMO
) is Ni-SSA in gas phase and Zn-SSA
in aqueous phase. The ( E
HOMO
) trend can be arranged in the gas

H.M.A. El-Lateef et al. / Journal of the Taiwan Institute of Chemical Engineers 88 (2018) 286–304 301
Fig. 11. Molecular electrostatic potential contour maps of compounds in gas phase.
phase as, Ni-SSA > Zn-SSA > Cu-SSA > H
2
SSA. The agreement be-
tween experimental the catalytic potential ( Table 5 ) and inhibition
efficiency ( Tables 6 and 7 ) of M-SSA, and theoretical study on the
other side is weak. E
LUMO describes the inhibitor or the catalyst
particles capacity as electron acceptors. Lower E
LUMO
values exhibit
the great electron affinity by inhibitor species [74] , but this might
be the reason for reducing of M-SSA catalytic potentials.
The distributions of frontier molecule orbital density on the
investigated reagents H
2
SSA and M-SSA are presented in Fig. 10 .
The electron density of the HOMO in reagent molecule is predom-
inantly distributed over the entire molecule, which is accounted
by the existence of delocalized electrons in the aromatic system.
The density of LUMO for H
2
SSA is disseminated over –SO
3
Na
group, while the other molecules are distributed mostly over
entire molecule except
–SO
3
Na group.
Molecular electrostatic potential (MEP) maps indicate that
which heteroatom is more active. The MEP maps of studied
reagents M-SSA are given in Fig. 11 . The negative sites of H
2
SSA
and M-SSA could be easily seen from MEP maps.
Low values of the energy band gap ( E ) gives well catalytic
potential and inhibition capacities, as for the energy to illuminate
an electron from the last occupied orbital shall be low, especially
in the central metal ion in the complex reagents (Ni
2 +
, Cu
2 + and
Zn
2 +
) [75] . The energy gap ( E ) is an essential character of the
active molecule in respect of catalytic oxidation and the adsorption
on the surface of carbon steel. E
LUMO energy in gas and aquatic
media provide a compatible result with calculated parameters
of H
2
SSA reactivity and its complexes as inhibiting reagent and
homogeneous catalysts ( Table 9 ).
Absolute softness ( σ) and hardness ( η) are fundamental fea-
tures to estimate the reactivity and stability of a molecule. It is
obvious that the σmainly indicates that the resistance towards
the polarization or distortion of the electron cloud of the com-
pounds under small disorder of chemical reaction. A soft molecule
has a small E and a hard molecule has a large E [76] . Both η
and σvalues of studied current reagents (H
2
SSA and M-SSA) are
reported in Table 9 . The calculated results indicated that ηvalue
of Ni-SSA (1.76 eV) is mostly smaller than other studied inhibitors.
ηand σvalues of investigated current reagents will be the same
with the classification obtained based on the values of E .
Electrophilicity indicator ( ω) was inserted by Parr as a measure
of molecule tendency to gain electron [77] . Nucleaophilicity is
physically the inverse of electrophilicity. A perfect, more reactive,
nucleophilic is described by low-value of ω and reciprocally a
good electrophilic is distinguished by a high-value of ω. The rank-
ing of the ω values are in agreement with experimental result,
catalytically and inhibition reactivity ( Table 9 ).
The number of electrons transferred ( N ) from inhibitor
species to CS surface displays that the inhibition efficiency ( E %)
increment with increasing N values [78] If N lower than 3.60,
the E % increment by increasing the electron-donating capacity of
these compounds to contribute electrons to the surface of steel
[79] . N values of the inhibitors are agreement in gas phase with
experimental result but in aqueous the agreement is weak, which
support the high catalytic potential of M-SSA.
The back-donation ( E
b-d
) shows that when η> 0 and
E
b- d
< 0 the charge transmits to an inhibitor species, followed
by a back-donation from the molecule, is strongly favored. In this
regard, it is potential to compare the stabilization between the
reactive molecules (H
2
SSA and M-SSA), since there would be a
reaction with the same metal. Then, it is predictable that it will
diminish as the hardness increment. The uses of Mulliken pop-
ulation analysis to fulfillment the adsorption centers of catalysts
and inhibitors have been exceedingly mentioned [52,75] . The more
negatively charged hetero-atom in the inhibiting species is strongly
adsorbed on the surface of steel during donor-acceptor reaction
type [80] . Considerably, electrophiles attack molecules at negatives
charged sites [81] , which could be evaluated from atomic charges
in its molecule. Fig. 12 shows the value of Mulliken charges
of studies reagents as inhibitors. It is observed that oxygen in
–SO
3
Na and
–C
=
O groups present considerable excess of negative
charges.

302 H.M.A. El-Lateef et al. / Journal of the Taiwan Institute of Chemical Engineers 88 (2018) 286–304
Fig. 12. Mulliken atomic charges of the compound molecules.

H.M.A. El-Lateef et al. / Journal of the Taiwan Institute of Chemical Engineers 88 (2018) 286–304 303
4. Conclusions
Complexation of novel polar tridentate monobasic 2-[(2-
hydroxy-5-sodium sulfonate-benzylidene)-amino]-benzoate
(H
2
SSA) with Ni
2 +
, Cu
2 + and Zn
2 +
-acetate in aqueous media
afforded new complexes (Ni-SSA, Cu-SSA and Zn-SSA, respectively)
within deprotonation of hydroxyl-carboxylic and hydroxyl-phenolic
groups. H
2
SSA and its corresponding complexes were character-
ized by UV–vis., IR and mass spectra, EA, TGA, conductivity and
magnetic measurements. H
2
SSA and Zn-SSA were characterized
additionally by
1
HNMR and
13
CNMR. The catalytic potential of
M-SSA was investigated in the homogenous oxidation of 1,2-
cyclohexene at 80 °C for 4 h by an aqueous H
2
O
2 or TBHP, as
oxygen source, in acetonitrile, in water or under solvent-free
condition. All complexes showed alternative catalytic potential
in different reaction media, however, acetonitrile was the best
suitable solvent in the reaction media for the 1,2-cyclohexene
using either an aqueous H
2
O
2 or TBHP. Cu-SSA exhibited a little
more catalytic activity compared to those of Ni-SSA and Zn-SSA.
The inhibition performance of the synthesized H
2
SSA ligand and
their Zn, Cu and Ni complexes has also been investigated for CS
corrosion in HCl (1.0 M). The results obtained from PDP and EIS
methods showed that, H
2
SSA and their M-complexes (M-SSA) act
as effective and mixed-type inhibitors. The adsorption of com-
pounds on CS surface could be exemplified by Langmuir isotherm.
SEM/EDX studies supported the development of an inhibitive layer
by the adsorbed inhibitor molecules on the CS surface. Moreover,
most theoretical results are in agreement with the empirically
determined catalytic and corrosion inhibition performance.
Acknowledgment
The authors greatly thank Vice-Presidency of Graduate Studies
and Academic Research in King Faisal University for its financial
support and encouragement to produce this work as a scientific
project (project number 17122005 ).
Supplementary materials
Supplementary material associated with this article can be
found, in the online version, at doi: 10.1016/j.jtice.2018.04.024 .
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