Synthesis, spectral and electrochemical studies of some Schiff base N 2O2 complexes

Article abstract of DOI:10.1016/j.molliq.2013.10.003

Schiff base metal complexes of different stoichiometries and modes of interactions were synthesized and characterized using different metals Mn, Co, Ni, Cu by elemental analysis, spectral (IR and UV-Vis) methods and magnetic susceptibility studies. Electrochemical studies were performed for some of Schiff base complexes (Cu-salpr, Cu-salen, and Cu-salph·H₂O) and of H₂-salen and its corresponding Ni, Co, and Cu complexes on the corrosion behaviour of steel in 1 M sulphuric acid solution and were studied by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. The stoichiometry of Fe and Mn (salen, salpr and salph) complex was studied by using different spectrophotometric methods.

Full text of DOI:10.1016/j.molliq.2013.10.003

Journal of Molecular Liquids 194 (2014) 149–158
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Synthesis, spectral and electrochemical studies of some Schiff base
N O complexes
2
2
a,
b
b
b
E.E. Shehata ⁎, M.S. Masoud , E.A. Khalil , A.M. Abdel-Gaber
a
Chemistry Department, College of Science, IMSIU (Al-Imam Mohammad Ibn Saud Islamic University), Riyadh 11623, Kingdom of Saudia Arabia
Faculty of Science, Alexandria University, Alexandria, Egypt
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Schiff base metal complexes of different stoichiometries and modes of interactions were synthesized and charac-
terized using different metals Mn, Co, Ni, Cu by elemental analysis, spectral (IR and UV–Vis) methods and mag-
netic susceptibility studies. Electrochemical studies were performed for some of Schiff base complexes (Cu-salpr,
Received 14 October 2012
Received in revised form 27 September 2013
Accepted 4 October 2013
Cu-salen, and Cu-salph·H
behaviour of steel in 1 M sulphuric acid solution and were studied by electrochemical impedance spectroscopy
EIS) and potentiodynamic polarization techniques. The stoichiometry of Fe and Mn (salen, salpr and salph) com-
plex was studied by using different spectrophotometric methods.
2 2
O) and of H -salen and its corresponding Ni, Co, and Cu complexes on the corrosion
Available online 30 October 2013
(
Keywords:
Schiff base
IR
© 2013 Elsevier B.V. All rights reserved.
UV–Vis
Spectrophotometric and electrochemical
studies
1
. Introduction
atom occurs where the proton transfer is the rate determining
step.
Salicyladehyde can be condensed with a series of alkane diamines to
give a series of Schiff bases as follows:
Several Schiff bases have recently been investigated as corrosion
inhibitors for various metals and alloys in acid media [5–9]. The
existing data show that these inhibitors act by adsorption on the
metal/solution interface. This phenomenon could take place via: (i)
electrostatic attraction between the charged metal and the charged
inhibitor molecules, (ii) dipole-type interaction between unshared
electron pairs in the inhibitor with the metal, (iii) p electrons-
interaction with the metal, and (iv) combination of all of the above
[
10].
In the present work, a series of metal Schiff base complexes were
The deprotonated salicyladehyde forms potential quadridentate
ligands bonding via the two N and the two O atoms [1,2] for [N-N′-bis
synthesized using different metals of Mn, Co, Ni and Cu. The free ligand
and its complexes were examined systematically in their solid state
using physicochemical and electrochemical studies.
(
(
salicylidene) ethylenediamine,
salicylidene) propylenediamine, H
of the (CH -by phenyl ring forming [N,N′-bis salicylidene-o-
phenylenediamine], (H -salph), gives a more rigid tetrahedral
2
H -salen; n = 2] and [N-N′-bis
2
-salpr; n = 3]. Replacement
2 n
)
2. Exprimental
2
structure [3]. Two possible mechanisms were proposed for the
complex formation reactions [4]. For the unprotonated com-
pounds, initial bonding to phenolic oxygen atom occurs where
the metal nitrogen bond formation is the rate determining step,
while for protonated compounds, the initial bonding to nitrogen
2
.1. General method for preparation of the Schiff base ligands
The bis-salicyldimine Schiff base ligands: H
salicylidene)-ethylenediamine], H -salpr [N,N′-bis (salicylidene)-
-salph [N,N′-bis (salicylidene)-o-phenyl-
2
-salen [N,N′bis
(
2
propylenediamine] and H
2
enediamine] were prepared as follows [11,12].
A solution of salicyladehyde was added drop wise onto a solution
of the amine in 50ml ethanol in molar ratio of 1:1 or 1:2 for a period
of 1h. The reaction mixture was refluxed for 1h with continuous stir-
ring. The reaction was left to cool where coloured compounds were
filtrated off and washed several times with water and ethanol and
⁎
Corresponding author. Tel.: +966 535260098.
E-mail address: emanmoustafa16@yahoo.com (E.E. Shehata).
0167-7322/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molliq.2013.10.003

150
E.E. Shehata et al. / Journal of Molecular Liquids 194 (2014) 149–158
dried in vacuum desiccator over P
4
O
10. The melting point of H
2
-
vacuum over P
4
O10. The analytical data for the prepared metal com-
salen, H -salpr, H -salph are 120 °C, 65 °C and 165 °C. The elemental
2
2
plexes are collected in Table 1.
analysis and some of physical properties of the compounds are given
in Table 1. A general skeleton I, II and III of the prepared compounds
are given as follows:
2
2
.3. Mode of analysis
.3.1. Metal analysis
Definite weight of the complex was digested with aqua regia and
was determined using two methods: I) The usual complexometric titra-
tion [13,14]. II) Atomic absorption technique (using PerkinElmer 2380
Atomic absorption spectrophotometer).
2.3.2. Carbon, hydrogen, nitrogen contents
The results are collected in Table 1.
2
2
.4. Instruments and working procedures
.4.1. Electronic absorption spectra
Spectrophotometric measurements in the visible and ultra
violet regions were made by PerkinElmer Lambda 4B UV–Vis
spectrophotometer.
The Nujol mull electronic absorption spectra were made using
unicam SP 1750 ultraviolet spectrophotometer following the method
described by Lee, Griswold and Kleinberg [15].
2
.2. General method for the preparation of transition metal complexes
2.4.2. Infrared spectra
These were recorded using PerkinElmer spectrophotometer 1430
Manganese (II) chloride, cobalt (II) chloride, nickel (II) chloride and
−
1
model covering the frequency range 200–4000cm
.
copper (II) chloride complexes of Schiff base were prepared as follows:
The required weights of the inorganic salts were dissolved in
3
EtOH (20 cm ) for Schiff base and refluxed with the desired weight
2.4.3. Magnetic susceptibility measurements
3
of the ligand in EtOH (20 cm ) for Schiff base. The reaction mixture
These were determined at room temperature (298 K) using Gouy's
method [16]. The apparatus was calibrated with Hg [Co(SCN) ].
was refluxed with stirring. The corresponding solid complexes
were filtered, washed several times with EtOH and dried under
4
2.4.4. Electrochemical measurements
Electrochemical impedance spectroscopy (EIS) and polariza-
tion curve measurements were collected by connecting the elec-
trochemical cell to Gill AC transfer function analyzer (TFA)
instrument that was controlled by personal computer for data log-
ging and analysis. For EIS measurements, 10 points were measured
for each decade of frequency that ranged between 30 kHz and
Table 1
Elemental analysis of organic ligands and their complexes.⁎
Compound
Colour
Analysis (%)
0
.1 Hz. The amplitude of the superimposed AC potential was
Calculated/found
10 mV. Impedance measurements were carried out at the rest po-
tential vs. saturated calomel electrode. The specimens used for
constructing the working electrode were steel that had the follow-
ing chemical composition wt (%): C, 0.400; S, 0.050; Mn, 1.100; P,
0.030; Si, 0.330, Ni, 0.005 Cr, 0.003; balance Fe. The steel samples
were fixed in polytetrafluoro ethylene (PTFE) rods by an epoxy
resin in such a way that only one surface was left uncovered. The
exposed area 1 cm2 was mechanically abraded with a series of
emery papers of variable grades, starting with a coarse one and
proceeding in steps to the finest (8 00) grade. The samples were
then washed thoroughly with double distilled water followed
with A.R. ethanol and finally with distilled water, just before inser-
tion in the cell.
C
H
N
M
H
2
-salen
Yellow
Brown
Yellow
Green
71.50
6.00
10.40
(10.30)
9.45
(9.70)
8.53
(8.76)
8.49
(9.12)
9.90
(10.20)
7.88
(8.12)
8.75
(9.03)
8.15
(8.33)
8.86
(9.20)
8.40
(8.18)
8.13
(8.52)
8.13
(8.52)
7.08
(
71.40)
64.81
65.20)
59.20
59.70)
58.27
58.50)
72.30
72.50)
57.42
57.81)
63.81
64.12)
59.38
59.94)
75.95
75.80)
71.96
71.56)
69.71
69.82)
69.71
69.62)
60.68
60.82)
(6.20)
5.06
(5.10)
4.31
(4.66)
4.24
(4.70)
6.40
(6.30)
6.19
(6.53)
5.20
(5.52)
4.65
(4.77)
5.10
(5.20)
4.60
(4.39)
4.35
(4.55)
4.36
(4.63)
4.05
Co(H-salen)
2
9.87
(
(9.50)
18.02
(17.61)
19.27
(18.90)
Ni-salen
Cu-salen
(
(
H
2
-salpr
Co(H-salpr)
Ni (H-salpr)
Cu-salpr
-salph
Mn(H-salph)
Co(H-salph)
Ni (H-salph)
Cu-salph·H
Yellow
Dark green
Green
(
2
·5H
2
O
8.04
(
(8.23)
12.20
(12.32)
18.48
(18.10)
2
2
(salpr)
(
Platinum sheet and saturated calomel electrodes were used as coun-
ter and reference electrodes. Polarization curve measurements were
carried out at scan rate 20 mV/min. All measurements were done at
Green
(
H
2
Orange
Brown
Brown
Red
20.0 ± 0.1 °C.
(
2
(H
2
-salph)
5.44
(5.80)
8.17
(8.46)
8.49
(8.87)
16.05
(16.10)
(
3. Results and discussion
2
(
3.1. Infrared absorption spectra of the Schiff base ligands and their com-
2
(
plexes in the solid state
2
O
Brown
(
(4.10)
(7.50)
To determine the mode of bonding of the dibasic tetradentate li-
⁎
All complexes are with m.p (°C) N 300.
2 2 2
gands: H -salen, H -salpr and H -salph in the prepared metal chelates

E.E. Shehata et al. / Journal of Molecular Liquids 194 (2014) 149–158
151
Table 2
Fundamental infrared bands of the free ligands and their metal complexes.
Compound
-salen
νO-H
νC = N
νC = C
νC-H
νC-O
νC-N
νM-O
νM-N
H
2
3456(b)
3581(m)
1633 (w)
1645 (s)
1577 (m)
1534 (vs)
3003 (w)
3120 (m)
3174 (m)
3027 (vw)
3012 (w)
3006 (w)
3012 (vw)
1216 (w)
1207 (m)
1278 (s)
1310(s)
461 (m)
593 (vw)
422 (w)
467 (m)
Co (H-salen)
2
3
388 (m)
Ni-salen
Cu-salen
3421(b)
3434(b)
3456(b)
3400(b)
1622 (s)
1645 (m)
1622 (m)
1606 (s)
1544 (m)
1534 (vs)
1500 (s)
1532 (w)
1200 (m)
1202 (vs)
1218 (m)
1266 (s)
1317 (w)
1327 (w)
1272 (s)
1311 (s)
456 (w)
465 (w)
445 (w)
593 (w)
428 (w)
435 (w)
406 (w)
456 (m)
2
H -salpr
Co (H-salpr) (salpr).5H
2
O
1995 (s)
Ni
Cu-salpr
-salph
Mn (H-salph)
Co (H-salph)
Ni (H-salph)
Cu-salph·H
2
(H-salpr) (salpr)
3444(b)
3422(b)
3443(b)
3391(b)
3435(b)
3413(b)
3421(w)
1611(vs)
1611(s)
1611(m)
1581(s)
1595(m)
1589(s)
1606 (m)
1539 (s)
1522 (s)
1560 (s)
1534 (s)
1527 (s)
1511 (s)
1538 (s)
3012 (vw)
3022 (w)
3044 (w)
3072 (w)
3065 (w)
3065 (w)
3076 (vw)
1247 (s)
1200 (m)
1199 (s)
1198 (m)
1185 (s)
1191 (s)
1196 (m)
1311 (m)
1300 (w)
1272 (m)
13556 (w)
1317 (m)
1322 (m)
1306 (m)
445 (m)
456 (w)
477 (m)
543 (w)
534 (b)
450 (m)
439 (w)
400 (m)
411 (w)
352 (w)
389 (w)
461 (w)
411 (m)
307 (w)
H
2
2
2
(H -salph)
2
2
2
O
Abbreviation: b (broad), m (medium), w (weak), s (strong), vs (very strong) and vw (very weak).
their infrared spectra were recorded and compared with the free ligand
shifted (9 cm−1) in the Ni-salen complex, i.e., the nitrogen atom
is considered to be another centre for complex formation. The rela-
tive change in band position can be explained by the destruction of
the hydrogen bond in the free ligand molecule through coordina-
tion to the metal ion. If the coordinated bond is weaker than the hy-
drogen bond, the band exhibits a red shift whereas for a stronger
coordinated bond the shift is to higher energies. This is based on
the fact that stronger bonds to N- or O- decrease the charge density
on these atoms and hence weakens the bonding to the hydrogen
atoms. This leads to a higher deformation angle.
(Table 2). The following observations are recorded:
1
- H
2 2 2
-salen, H -salpr and H -salph ligands showed broad bands at
−
1
3456, 3456 and 3443 cm , respectively, which may be assigned
to νOH involved in a weak hydrogen bonding [17]. Three types of
hydrogen bonds may exist in equilibrium with each other.
−
1
5
6
7
- The νC-O band at 1216, 1218 and 1199cm for H
and H -salph compounds respectively, affected on complexation
with different degrees [19,20] (Table 2).
- The aromatic νC-H vibrations of weak appearance are identified at
2 2
-salen, H -salpr
2
−
1
3
003, 3006 and 3044 cm
compounds respectively.
- The H
-salen ligand gave very weak bands at 2740 and 2913cm−
corresponding to symmetric and asymmetric vibrations of the CH
group, respectively. The δCH
vibration appeared at 1456 cm−
Table 2). However, increasing the chain length of (CH
n = 2 to 3 leads to an increase in the frequency, where the H
for H
2
-salen, H
2
-salpr and H
2
-salph
1
2
2
1
2
(
2 n
) from
2
-
at 2847, 2924 cm−
1
2 2 2
salpr showed νsym CH , νasym CH and δ CH
and 1457 cm , respectively (Table 2).
The hydrogen bond in this case is of intramolecular type either with: (i)
OH…N or (ii) OH…OH or (iii) OH…N and OH…OH. The predominant
hydrogen bond is probably of the first type due to the formation of a
chelate structure [17,18].
−1
8
- Increasing the chain length of (CH ) from n = 2 to 3 causes a de-
2 n
crease in the νC = N vibration of their metal complexes which
may be due to the decrease in the strain i.e., the flexibility increases
The metal chelates showed a decrease in the intensity of the νOH band
as the length of the alkyl bridge increases. The spectra of H
complexes showed a negative shift in the in-plane stretching of
C_N suggesting coordination through azomethine group.
2
-salph
−
1
within the (3456–3391) cm
3388, 3581), 3400, 3435, 3421, 3444, 3413, 3434, 3422 and 3421 cm
for [Mn (H-salph) (H -salph)], Co(H-salen) Co(H-salpr) ·5H
Co(H-salph) , Ni-salen, Ni (H-salpr) (salpr), Ni(H-salph) , Cu-salen, Cu-
salpr and Cu-salph·H O, respectively. This supports that proton displace-
range e.g., νOH is identified at 3391,
−
1
(
\
2
2
2
,
2
2
O,
9
- The metal-oxygen stretching frequencies are observed at higher
frequencies than those of metal-nitrogen [21]. The νM-O bands
2
2
2
2
2
are identified at 593 in the Co(H-salen)
2
, Co(H-salpr)
2
·5H2O and
ment from the phenolic OH groups through the metal ion occurs, i.e.
M\O bonding is identified.
−1
−1
5
43 cm in Co(H-salph)
2
complexes and at 439–477 cm range
−1
in the copper and nickel complexes and 543 cm
for that of
2
- The Co (H-salen)
2
complex, gave two sharp bands at 3581 and
[
Mn(H-salph)
2
-(H
2
-salph)] complex.
−
1
3388 cm where the ligand contains the OH group of coordinated
0- The νCu-N is identified at 435cm and 411cm−1 in the salen and
−1
1
and non-coordinated behaviour. However, the presence of different
conformations of the C\H groups leads to the appearance of the
salpr complexes respectively. i.e., the shift is decreased with in-
creasing the chain length of alkyl bridge. However, the νNi-N ap-
−
1
two bands at 3120 and 3174cm . However, the Co (H-salpr)
2
·5H2-
−1
peared at 428 cm
in the Ni-salen complex and decreases to
−
1
O complex gave only one band at 3400 cm i.e., at lower frequency
than that of the free ligand (3456 cm ) due to complexation.
- The νC = N band at 1633 cm
−1
4
11 cm
in the Ni (H-salph) complex due to the bulkiness of
2
−
1
the phenyl group.
−
1
3
in the free H
2
-salen ligand,
-salph li-
−
1
1
622cm for H
2
-salpr at higher position compared to H
2
−
1
gand at 1611 cm , may be due to the bulkiness and steric effect
caused by phenyl ring.
3.1.1. Calculation of the coordination bond length
A study of N\C (8) bond length in several metal [N,N′-
bis(salicylidene) ethylenediamine] complexes has revealed an interest-
ing feature, namely that this bond length falls into two categories. In the
4
- The νC = N in the H
Co (H-salen) and Cu-salen complexes. The same band slightly
-salen ligand is shifted to (+12 cm−1) in the
2
2

152
E.E. Shehata et al. / Journal of Molecular Liquids 194 (2014) 149–158
1
1
1
first, its value ranges from 1.43 to 1.45Å when C (8) is a part of an aro-
matic ring, whereas in the second category its value is ≥1.47Å when C
due to A1g → 1T2g, A1g → 1T1g and, A1g → 3T2g transitions, respec-
tively, in an octahedral geometry [29]. However, the Co (H-salpr)
showed two bands at 402 and 642 nm. On the other hand, the
Co(H-salph) complex recorded two bands at 408 and 690 nm.
2
(
8) is part of a linear chain, a cyclohexyl group, ethylenediamine,
tetramethylenediamine, or a cyclohexanediamine group. The shorter
length in the first group might be due to the conjugation between the
π-orbitals of N with the π-orbitals of the aromatic ring, which is not pos-
sible for the second set of compounds [22].
The coordination bond length (r) can be determined from the
relation [23].
2
1
1
1
3
Both complexes are of
respectively.
In general, the data typified octahedral geometry for all three cobalt
complexes: [Co (H-salen) ], [Co (H-salpr) ·5H O] and [Co (H-salph) ].
All the data are in harmony with the room temperature effective mag-
netic moment values, Table 4.
A
1g
→
T
2g
,
A
1g
→
T2g transitions,
2
2
2
2
ꢀ
ꢁ
ꢀ
ꢁ
pffiffiffiffiffiffiffiffiffiffi
ν
x¼y−νx‐y exp −2π 2r=a
If one symbolizes the ligand as H
sents the reaction:
2
L, the following equation repre-
ð32 π αÞ
Δν ¼
ð1Þ
2
a
1
CoCl þ 2H L→CoðHLÞ þ 2HCl
2
2
2
Where;
The following structure is proposed for the cobalt complex:
α
Δν
a
bond polarisability
shift in the oscillator frequency (ν ligand–ν complex)
lattice constant of the metal salt [a=3.5, 3.524 and 3.614 for
cobalt, nickel and copper, respectively] [24]
ν
ν
l
x − y
frequency of the oscillator with single bond
frequency of the oscillator with double bond
length of the oscillator coordinated to the metal ion
x = y
(
l = 1.950, 1.905, 1.993 and for cobalt [25], nickel [22]
3
.2.3. Nickel complexes (Fig. 1 and Table 4)
The Nujol mull electronic spectra of Ni-salen, showed three
and copper [22], respectively)
bands at 349, 416 and 560 nm. The first band may be assigned to
The α values were computed based on a published data for com-
intraligand charge transfer. The second and third bands may be
plexes with similar structures to the present complexes [26]. These
are 48.61, 46.59 and 42.50 for cobalt, nickel and copper, respectively.
From such data, the r-values between the metal and the nitrogen
atom of the C = N group are computed [27].
Table 3 collects the values of azomethine frequency shift, Δν (ν
ligand–ν complex), log Δν and r (A°).
1
1
1
1
assigned to A1g → A2g, and A1g → B1g transitions, respectively, in a
square planar geometry around nickel atom [30]. Where, the room tem-
perature diamagnetic moment value confirms this configuration
(
2
Table 5). The Ni(H-salph) gave three bands at 402, 456 and 498 nm
1
1
1
1
which may be assigned to A1g→ T2g, A1g→ T1g and visible d–d tran-
sition, in an octahedral geometry [29]. However, the Ni (H-salpr)
salpr) complex gave two bands at 478 and 654 nm which may be
2
2
(
3
.2. Electronic absorption spectra
1
1
1
3
assigned to A1g→ T1g, A1g→ T2g transitions, respectively, in an octa-
hedral structure [29]. To satisfy the coordination number of nickel (6),
two possible approaches could be given:
Nujol mull electronic absorption spectral data (λmax, nm), room
temperature effective magnetic moment values (μeff, 298K) and geom-
etries of the investigated complexes are given in Table 4.
1
2
- Association of the complex takes place.
- The third ligand could be subjected to isomerization and is attached
with the metal.
3
.2.1. Manganese complex (Fig. 1 and Table 4)
The Nujol mull electronic absorption spectra of the [Mn (H-salph)
2
(H -salph)] complex showed two bands at 362 and 470 (b) nm which
2
The following equation is given for 1:1 complex:
1
1
may be due to intra ligand charge transfer type and A1g → T1g, transi-
tion respectively, in an octahedral structure [28]. The datum is in har-
mony with the room temperature effective magnetic moment value,
Table 4.
NiCl þ H L→NiL þ 2HCl
2
2
The possible structure for Ni-salen is probably as follows:
3
.2.2. Cobalt complexes (Fig. 1 and Table 4)
The Nujol mull electronic absorption spectra of Co(H-salen)
2
complex, gave three bands at 404,476 and 620 nm which may be
Table 4
Table 3
Maximum wave length (nm) of the Nujol mull electronic spectra and room temperature
Azomethine frequency shift Δν, log Δν and coordination bond length.r (A°) of Schiff base
effective magnetic moments (μeff·298 K) of Schiff base complexes.
metal complexes.
Complex
λ
max
μeff·(298 K)
Compound
Δν
12
Log Δν
r
Co (H-salen)
Ni-salen
Cu-salen
2
404, 476 (b), 620 (sh)
394, 416, 560 (sh)
368, 584 (sh)
630, 738 (sh)
402, 642
5.12
0
1.75
1.77
4.97
Co (H-salen)
Ni-salen
Cu-salen
2
1.079
0.950
1.079
1.204
1.041
1.041
1.204
1.342
0.698
3.31
3.49
3.20
3.00
3.29
3.25
2.96
2.70
3.86
−9
12
16
11
11
16
22
5
Cu-salpr
Co (H-salpr)
Ni (H-salpr)
Cu-salpr
Co(H-salph)
Ni (H-salph)
Cu-salph·H
2
·5H
2
O
Co (H-salpr)
Ni (H-salpr)
Mn (H-salph)
Co (H-salph)
Ni(H-salph)
Cu-salph·H
2
·5H
(salpr)
(H -salph)
2
O
2
2
(salpr)
2
2
478, 654
2
2
362 (m), 470 (b)
408, 690 (sh)
402, 456, 489
450
5.42
4.95
2
2
2
2
2
O
2
O

E.E. Shehata et al. / Journal of Molecular Liquids 194 (2014) 149–158
153
2 2 2
Fig. 1. Nujol mull electronic absorption spectra of a) H -salen, b) H -salpr and c) H -salph complexes.
However, the 2:3 stoichiometric reaction is proposed as follows:
The possible structure for the Ni (H-salph)
follows:
2
complex is proposed as
2
NiCl þ 3H L→Ni ðHLÞ L þ 4HCl:
2
2
2
2
2 2
The Ni (H-salpr) (salpr) complex is proposed as follows:
3.2.4. Copper complexes (Fig. 1 and Table 4)
The Nujol mull electronic absorption spectra of the Cu-salen com-
plex showed a band at 368 nm which may be assigned to intraligand
The chemical equation for the 1:2 complex is suggested to occur
based on the following equation:
charge transfer type. The shoulder of Cu-salen complex at 584 nm,
may be due to 2
B
1g → E
2
transition in square planar configuration
g
[26]. The Cu-salpr complex showed an extra band at 630 nm, assigned
NiCl þ 2H L→NiðHLÞ þ 2HCL
to d–d band transition of CuO N2 skeleton [30], and a shoulder at
2
2
2
2
Table 5
pK values and the stoichiometry of the Fe(III), Mn(II) and (ninhydrin-ethylenediamine (ETD) using different spectrophotometric methods.
Methods
Fe-salen
pK
Fe-salpr
pK
Fe-salph
pK
Mn-salen
pK
Mn-salpr
pK
(ninhydrin-EDA)
pK
Molar ratio
1:1
1:1
1:1
1:0.56
1:2.4
1:1.63
1:1
1:1
1:1
1:0.42
1:3.23
1:2.75
1:3.2
1:1, 1:3
1:1
1:1
1:3
1:1, 1:2
1:1
1:1
1:1.7
1:0.9
1:2
1:0.8, 1:2
1:1
1:1
1:2.2
1:1.76
1:2.4
1:1, 1:2
2:1
2.4:1
2:3, 1:3
1:2
4.58, 8.25
6.92
Straight line (simple plot)
Straight line (logarithmic plot)
Continuous variation
Slope ratio
4.32
5.75
3.70
3.69
3.72
6.6
3.60
7.27
5.7
Limiting logarithmic
1:2.77
3:1

154
E.E. Shehata et al. / Journal of Molecular Liquids 194 (2014) 149–158
absorption spectra of (Fe-salen) as representative example. The concen-
tration of iron is kept constant while that of salen is varied.
The positions of λmax for (Fe-salen), (Fe-salpr), (Fe-salph),
(
Mn-salen), (Mn-salpr) and (ninhydrin-ethylenediamine) are
5
00, 510, 320, 400, 480 and 505 nm, respectively.
The stoichiometry of the reaction is obtained by plotting the absor-
bance of the resultant solution against the molar ratio of the variable
concentration of ligand to the constant concentration of the metal and
variable concentration of ninhydrine against constant concentration of
ethylenediamine. If the system forms a stable complex which does not
show appreciable dissociation, such a plot gives a sharp break which
corresponds to the stoichiometry of this complex. Linear extrapolation
of the curve is made where the curved plot becomes parallel to the
molar ratio axis after the addition of an excess of the variable compo-
S
nent [32]. The A -molar ratio plots give the probable stoichiometry 1:
1
for Fe(salen and salpr), (1:3) for Fe-salph, 1:1, 1:2 Mn (salen and
salpr) and (1:1), (1:2) for (ninhydrin-ethylenediamine) complexes,
Table 5.
Fig. 2. Electronic absorption spectra of Fe-salen reaction in 95% ethanol.
3.3.2. The straight line method
The following equation is applied [31]
7
38nm, probably due to 2E
→2T2g transition [31]. The band at 450nm
g
1
1
1
v
CD 1
Kc AS
D
Kc
of the Cu-salph·H O complex due to A1g → T1g transition [31]. The
2
¼
−
ð2Þ
n
electronic spectral data with the room temperature effective magnetic
moment values pointed to formation of square planar geometry. This
leads to assign the following structure for these complexes:
The plottingv1
against 1 taking for numerical values (1,2,3….), then a
n
AS
straight line is obtained only if the value of n is the same as in the equi-
librium equation.
If the complex is strongly dissociated then:
Kc
D V
1
≫1
n
Eq. (2) can be written as:
CD
Kc
n
A ¼
V
s
logCD
Kc
logA ¼
þ nlogv
ð3Þ
s
3
.3. Spectrophotometric methods for tracing the stoichiometry of the Schiff
base complexes in solutions and their dissociation constants
3
.3.1. The molar ratio method
This method depends on constant concentration of the metal ion
and variable concentration of the ligand. Fig. 2 represents the electronic
Fig. 4. Slope–ratio method for Fe-salen reaction in 95%ethanol. a) Variable H
2
-salen con-
Fig. 3. Straight line method for Fe-salen reaction in 95% ethanol.
centration, b) variable Fe concentration.

E.E. Shehata et al. / Journal of Molecular Liquids 194 (2014) 149–158
s
On plotting log A versus log v, a straight line is obtained from which
Table 6
155
logCD
The molecular extinction (ε) and the formation constant (q) for the iron and manganese
n and
values are derived from the slope and the intercept respec-
Kc
complexes using the successive method.
tively. So, the stoichiometry and equilibrium constant of the complexes
are calculated.
Complex
ε
q
 Ã
n
The simple plot relationship between 1 versus
1
where v is the vol-
As
v
Fe-N,N′ salicylidene ethylenediamine
3030.3
7930
ume of ligand added with respect to 3 mls of the metal ion in case of iron
and 5 mls in case of manganese and ninhydrin-ethylenediamine, Fig. 3
represents the straight line method for Fe-salen as representative
example.
The results obtained using the simple plot and the logarithmic plots
are collected in Table 5.
λ
max = 500 nm
Fe-N,N′salicylidene propylenediamine
max = 510 nm
Fe-N,N′ salicylidene phenylenediamine
max = 320 nm
Mn-N,N′ salicylidene ethylenediamine
max = 400 nm
Mn-N,N′ salicylidene propylenediamine
max = 480 nm
1869.15
32,594.52
5281.50
8012.8
2643
λ
215,298.24
12,484.5
35,523.9
569.1
λ
λ
 Ã
n
1
against 1 with different numerical values of n “sim-
On plotting
v
As
λ
ple plot”, all the complexes with different ligands, Table 5 gave (1: 1)
complexes, except for Fe-salph gave (1:1) and (1:3) complexes, howev-
er in case of (ethylenediamine-ninhydrin) complex gave (1:2).
(Ninhydrin-ethylenediamine)
154.79
complex λmax = 505 nm
3
.3.3. The continuous variation method [31]
For calculating the dissociation constant of a complex of the type
In the case of all the systems under investigations (Fe-salen), (Fe-
AB
3
:
salpr), (Fe-salph), (Mn-salen), (Mn-salpr) and (ninhydrin-ethylene-
diamine), the total molar concentration is constant. Table 5 confirmed
the presence of (1:0.65), (1:0.42), (1:1), (1:1.7) and (1:2.2) for (Fe-
salen), (Fe-salpr), (Fe-salph), (Mn-salen) and (Mn-salpr), respectively.
However, the data depict the formation of (ninhydrin-ethylenediamine)
complexes with the stoichiometries 2:3 and 1:3.
−
þ2
−
AB ⇌A þ 3B
3
h
þ2i
ꢀ
ꢁ
ꢀ
ꢁ
−
A
¼ C 1−A
and ½B ꢀ ¼ 3C 1−A=Am
=
Am
For calculating the dissociation constant of a complex of the type
2
:
Where C is the initial concentration
ML
ꢀ
ꢁ
−
½
AB ꢀ ¼ C A
3
=Am
½
ML ꢀ ¼ CðA=A Þ
2
m
3
4
ð6Þ
3
3
27C ð1−A=AmÞ
K1:3 ¼ ½Aꢀ½Bꢀ =½ABꢀ ¼
A=A
Where C is the initial concentration
m
h
þi
2
−
The dissociation constants of the all complexes are calculated by
Eqs. (4)–(6) and are collected in Table 5.
M
¼ Cð1−A=A Þ and ½L ꢀ ¼ 2Cð1−A=AmÞ
m
þ2
−
ML ⇌M þ 2L
2
ð4Þ
ð5Þ
h
i
3.3.4. The slope ratio and the limiting logarithmic methods [32]
The slope ratio method depends on two sets: first set, large constant
concentration of one component and variable concentration for the
other component, the reverse is for the second set. The absorbance is
plotted versus the variable concentration of both sets. The data depict
the formation of (1:2.4), (1:3.2), (1:3), (1:0.9) and (1:1.76), for (Fe-
salen), (Fe-salpr), (Fe-salph), (Mn-salen) and (Mn-salpr) respectively.
The data depicted the formation of (1:1.7) i.e (1:2) in case (ninhydrine-
ethylenediamine complex), Table 5.
þ2
− 2
M
½L
ꢀ
2
3
¼ 4
C ð1−A=A Þ
m
K1:2
¼
½
ML ꢀ
ðA=A Þ
2
m
−
ML1=2→M þ 1=2L
h
þi
.
−
1
M
¼ Cð1−A=A Þ; ½L ꢀ ¼ 2Cð1−A=AmÞ
m
Â
Ã
þ
− 1=2
3=2
3=2
M
½L
ꢀ
¼ 0:707C
ð1−A=A Þ
m
K1:1=2
¼
A=A
A=A
m
m
Fig. 4 was taken for Fe-salen as representative example (slope ratio
method for Fe-salen reaction in 95% ethanol).
2
Fig. 5. Limiting logarithmic method for Fe-salen reaction in 95% ethanol. a) Variable H -
salen concentration, b) variable Fe concentration.
Fig. 6. Successive method for Fe-salen reaction in 95%ethanol.

156
E.E. Shehata et al. / Journal of Molecular Liquids 194 (2014) 149–158
Table 7
Electrochemical polarization parameters for steel in 1.0 M sulphuric acid containing 10%
(v/v) ethanol in the presence and absence of different Cu-salpr concentrations.
4
Additives
Conc. ×10
mol·L
E
mV
corr
Ba
mV
Bc
mV
icorr
−
1
mA/cm2
–
0.0
1.0
−440
−439
−444
−447
−462
−457
43
42
40
42
40
40
102
104
101
104
103
102
0.395
0.362
0.311
0.267
0.232
0.181
[
Cu−salpr]
1
2
3
4
.5
.0
.5
.0
If β is the stability a quotient of ML and K
ligand, it may be shown that:
a
is the dissociation of the
Fig. 7. UV–visible absorption spectra for salpr and its Cu-complex.
Â
Ã
þ
½L−
þ
−
H
ꢀ
HL⇌H þ L ; ∴K ¼
ð9Þ
a
The limiting absorption method was based on the same mode of
½HLꢀ
thinking given before for the slope ratio method, by plotting the logarith-
mic values of the absorbance versus the logarithmic values of the vari-
able concentration. The stoichiometry of the complexes was calculated
based on the ratio of the slopes of the two straight lines. So, the com-
plexes traced are with the stoichiometry (1: 1.63), (1: 2.75), (1: 2.77),
þ
−
½MLꢀ
and M þ L ↔ML ; ∴β ¼
ð10Þ
−
½Mꢀ½L
ꢀ
From subsequent equation:
(
1: 2) and (1: 2.4) for (Fe-salen), (Fe-salpr), (Fe-salph), (Mn-salen)
and (Mn-salpr) respectively. The data depicted the formation of (3.2:1)
i.e (3:1) in case of ninhydrine-ethylenediamine complex. The results
are collected in Table 5. Fig. 5, was taken for Fe-salen (limiting logarith-
mic method for Fe-salen reaction in 95% ethanol).
β
½MLꢀ
¼
ð11Þ
1 þ ð½H ꢀ=K Þ Mð½HLꢀ þ ½L−ꢀÞ
þ
a
If the reaction is measured at constant pH, the left hand-side of
Eq. (11) is constant and symbolized by q. By applying Beer's law for
3
.3.5. The successive method [31]
This method was constructed depending on the changes of both the
1 cm optical path length, A = ε C
s
.
As
concentrations of the ligand (salen, salpr and salph) and the metal ion
Mn and Fe). So, the stability constant of the formed complex can be cal-
Ccomplex
¼
ε
(
culated, assuming the reaction to take place as follows:
½
½
Mꢀ ¼ C −C
M
complex
n
mM þ nL⇌M L :
m
n
−
−
L
ꢀ ¼ C −C
L
complex
The charge on the complex could be determined by knowing the
values of m and n. However, these values are omitted for simplicity.
The concentration of Mn+ and the L are calculated:
So, the q value in Eq. (11) becomes:
−
.
As
ε
q ¼ ꢀ
. ꢁꢀ
. ꢁ
ð12Þ
CM ¼ ½Mꢀ þ ½MLꢀ
ð7Þ
ð8Þ
As
As
C −
CL −
−
M
ε
ε
−
−
C
¼ ½HLꢀ þ ½L ꢀ þ ½MLꢀ
L
Fig. 8. Potentiodynamic polarization curves for steel in 1.0 M sulphuric acid containing
Fig. 9. Nyquist plots for steel in 1.0 M sulphuric acid containing 10% (v/v) ethanol in the pres-
−
4
−4
−4
−4
−4
−4
1
2
0% (v/v) ethanol in the presence of [1], 0.0 M; [2], 1.0 × 10 M [3], 1.5 × 10 M; [4],
× 10 M and [5], 4.0 × 10 M of Cu-salpr.
ence of [1], 0.0M; [2], 1.0×10 M; [3], 1.5×10 M; [4], 2×10 M; [5], 3.5×10 M and
−
4
−4
−4
[6], 4 × 10 M of Cu-salpr.

E.E. Shehata et al. / Journal of Molecular Liquids 194 (2014) 149–158
157
where A
cient of the formed complex.
s
is the absorbance due to [ML], ε is the molar extinction coeffi-
Table 9
, Rct, and Cdl for steel in 1.0 M sulphuric acid containing 10% (v/v) ethanol containing
R
s
2
different concentration of H -salen and its Ni, Co, and Cu complexes at 30 °C.
.
As
4
Additive
Conc. ×10
mol·L
R
s
R
ct
C
dl
ε
−1
q ¼
.
ð13Þ
À
ꢂ Á
As
2
As
C C
−
þ
−
M
L
ε
–
–
0.78
0.58
1.02
0.62
1.22
0.819
0.82
0.88
1.78
0.68
0.72
0.80
0.65
0.79
0.61
23.11
24.23
25.46
26.90
27.17
28.44
30.16
27.16
23.55
28.46
28.80
29.27
34.22
38.47
45.21
338.4
297.4
309.3
407.1
728.3
815.6
608.6
486.3
626.6
440.5
689.2
643.8
472.4
474.0
513.0
ðCMþCL− Þ
2
H -salen
0.8
1.0
2.0
À
ꢂ Á
2
As
The value
C C
is neglected with no appreciable error so:
Þ=εꢀ
ε
3.0
1
qε
−
−½A ðC þ CL
−
M
L
s
M
4.0
3.0
¼
A
Ni-salen
s
4
5
.0
.0
C C
−
1
qε
1
M
L
∴
¼
þ ε ðCM þ CL
−
Þ
Co(H-salen)
2
1.0
3.0
As
4.0
−
Cu-salen
1.0
1.5
Plotting the left hand side of the above equation versus (C
straight line is obtained with a slope equals to and an intercept of . The
value of the overall formation constant and molecular extinction coeffi-
cients for these systems is collected in Table 6. Fig. 6 represents the suc-
cessive method for Fe-salen reaction in 95% ethanol.
M
+C
L
), a
1
ε
1
qε
2.0
decreased with increasing Cu-salpr concentration. The decrease in the
double layer capacitance is attributed to the adsorption of salpr moiety
that results from dissociation of Cu-salpr complex in acid media.
The obtained stoichiometries for the (Fe-salen) are (1: 1), (1:2) and
1:0.56), in case of (Fe-salpr) are (1:1), (1:0.42) and (1:3). However in
(
case of (Fe-salph) are (1:3) and (1:1) but in case of Mn (salen and salpr)
are (1:1 and 1:2). In conclusion from the discussed spectrophotometric
methods the stoichiometric of (ninhydrin-ethylenediamine) complexes
are (1:1), (1:2), (2:3), (1:3), (2:1) and (3:1), Table 5.
3
2
.4.2. Effect of H -salen and its corresponding Ni, Co, and Cu complexes on
the corrosion behaviour of mild steel in 1 M sulphuric acid
The addition of ligand solution or its Ni and Co complexes did not
show any significant effect on the polarization behaviour and inhibition
of steel corrosion in acidic solution. The Cu complex shifts the corrosion
potential into anodic direction and slightly affect both cathodic and an-
odic parts of the polarization curve.
The powerful non-destructive EIS technique is useful in such case
and can give more information about the effect of these additives on
the corrosion behaviour of steel in acidic solution. The Nyquist plots of
impedance data for steel in 1M sulphuric acid containing different con-
3
3
.4. Electrochemical behaviour of Schiff base complexes
.4.1. Effect of Schiff base complex Cu-salpr on the corrosion behaviour of
mild steel in 1 M sulphuric acid
The stability of Cu-salpr complex was studied spectrophotometrical-
ly. It is clear that the Cu-complex decomposes readily in the test solution
to give the corresponding salpr ligand, Fig. 7.
The potentiodynamic polarization curves for steel in 1.0M sulphuric
acid solution containing different concentration of Cu-salpr, at 30 °C
Fig. 8, shows that addition of Cu-salpr shifts the corrosion potential
centrations of H
formed at the rest potential of steel, are characterized by a depressed
semicircle of a capacitive type. The values of R , Rct and Cdl obtained
2
-salen and its Ni, Co, and Cu complexes at 30 °C, per-
s
(Ecorr) to more negative values. Moreover, Cu-salpr has an inhibiting ef-
from the analysis of these figures are shown in Table 9.
fect on the cathodic part of the polarization curves and accelerating ef-
fect on the anodic part. This indicates that the presence of copper in
the test solution increases the anodic dissolution of steel.
The obtained electrochemical parameters from polarization mea-
surements, Table 7, show that the corrosion current decreases with in-
creasing Cu-salpr concentration indicating that the inhibitive effect of
salpr moiety predominate the accelerating effect of copper ions in
solution.
The data indicated that a slight increase in the charge transfer resis-
2
tance takes place with increasing H -salen concentration. Increasing Ni-
salen complex concentration leads to unexpected decrease in the
charge transfer resistance, clarifying that Ni acts as a catalyst in the dis-
solution process. On the other hand, increasing Co (H-salen)
tration has no significant effect on the inhibition efficiency. Whereas,
the Cu-salen complex shows the maximum increase of the Rct
2
concen-
.
Fig. 9 shows the Nyquist plots of steel in 1 M sulphuric acid in the
presence and absence of different concentrations of Cu-salpr. It is clear
that the size of capacitive depressed semicircles increased with increas-
ing Cu-salpr concentration indicating inhibitive effect for the added
complex.
Table 8 shows the values of Rct and Cdl for steel in 1M sulphuric acid
containing different Cu-salpr concentrations. The data indicated that the
charge transfer resistance increased and the double layer capacitance
3.4.3. Comparative study for the effect of Cu-Schiff base complexes
(Cu-salpr, Cu-salen and Cu-salph·H
of mild steel in 1 M sulphuric acid
2
O) on the corrosion behaviour
Table 10 collects the values of Rct and Cdl for steel in 1 M sulphuric
acid containing different Cu-salph·H O concentrations. The data indi-
2
cated that the charge transfer resistance increases and the double
layer capacitance decreases with increasing Cu-salph concentration.
From Tables 8–10, on comparing the charge transfer resistance at a
Table 8
Rct, Cdl, and %P for steel in 1.0 M sulphuric acid containing 10% (v/v) ethanol in the
presence of different Cu-salpr concentrations.
Table 10
R
ct, Cdl and %P for steel in 1.0M sulphuric acid containing 10% (v/v) ethanol in the presence
4
ct, Ω cm2
Conc. ×10
mol·L
R
C
μF
dl
,
%P
of different Cu-salph·H
2
O concentrations.
−
1
4
Rct, Ω cm2
Conc. ×10
mol·L
Cdl, μF
%P
Cu-salpr
0.0
23.11
34.5
45.1
68.2
91.6
99.2
338.4
280.4
274.4
272.7
256.6
134.9
–
−1
1.0
1.5
2.0
3.5
4.0
33.0
48.8
66.1
74.8
76.7
Cu-salph·H
2
O
1.0
2.0
3.0
5.0
28.13
38.67
46.79
57.48
332.9
287.5
254.6
286.3
17.8
40.2
50.6
59.8

158
E.E. Shehata et al. / Journal of Molecular Liquids 194 (2014) 149–158
concentration 2 × 10−4 M of the added complex, one could conclude
that the inhibition efficiency of the added complex is in the order.
[9] M. Elayyachy, A. El Idrissi, B. Hammouti, Corros. Sci. 48 (9) (2006) 2470–2479.
10] A.W. Adamson, Physical Chemistry of Surfaces, John Wiley & Sons, New York, NY,
USA, 1990.
[
[
11] M.S. Masoud, A. Akelah, S.S. Kandil, Indian J. Chem. 24A (1985) 855–859.
Cu ꢁ salpr > Cu ꢁ salen > Cu ꢁ salph·H O
[12] A.G. Ligtenbarg, R. Hage, A. Meetsma, B.L. Feringa, J. Chem. Soc. Perkin Trans. 2
1999) 807–812.
2
(
[
[
13] G. Schwurzenbach, Complexometric Titration, 3rd edition Longmans, London, 1961.
14] A.I. Vogel, A Text Book of Quantatitive Inorganic Analysis, 3rd edition Longmans,
London, 1964.
4
. Conclusions
[
15] R.H. Lee, E. Griswold, J. Kleinberg, Inorg. Chem. 3 (9) (1964) 1278–1283.
Schiff base metal complexes of different stoichiometries and modes
[16] B.N. Figgis, R.S. Nyholm, J. Chem. Soc. (1959) 331–337.
[
[
17] H.H. Hammud, A. Ghannoum, M.S. Masoud, Spectrochim. Acta A 63 (2006) 255–265.
18] L.J. Bellamy, The Infrared Spectra of Complex Molecules, 2nd edition, Chapman and
Hall Publishers, London and, New York, 1980.
of interactions were synthesized and characterized by elemental analy-
sis, spectral (IR and UV–Vis) methods and magnetic susceptibility stud-
ies. The stoichiometry of Fe and Mn (salen, salpr and salph) complexes
were evaluated using different spectrophotometric methods. The % of
inhibition of some ligands and their complexes is increased with
increasing the concentration depending on the molecular structure.
The inhibition efficiency of the added complex is in the order: Cu-
[19] E. Canpolat, M. Kaya, Russ. J. Coord. Chem. 31 (6) (2005) 415–419.
[
[
20] R.E. Ernst, M.J. O' Conner, R.H. Holm, J. Am. Chem. Soc. 90 (21) (1968) 5735–5744.
21] H.W. Richard, W.E. Hatfield, H.J. Stokosa, J.R. Wasson, Inorg. Chem. 12 (9) (1973)
2
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