Multi-wavelength spectrophotometric determination of acidity constants of some salicylaldimine derivatives

Article abstract of DOI:10.1016/j.molliq.2012.10.039

The acidity constants of 9 synthesized derivatives of Schiff base in dimethylformamide/water and ethanol/water (25:75 v/v) at 25 C and an ionic strength of 0.1 M have been determined spectrophotometrically. All of the spectrophotometric data as pure spect

Full text of DOI:10.1016/j.molliq.2012.10.039

Journal of Molecular Liquids 178 (2013) 70–77
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Multi-wavelength spectrophotometric determination of acidity constants of some
salicylaldimine derivatives
b
a
Maryam Bordbar a, , Ali Yeganeh Faal , Mohammad Mahdi Ahari-Mostafavi ,
⁎
Mehrnaz Gharagozlou c, Razieh Fazaeli d
a
Department of Chemistry, University of Qom, Qom, Iran
Department of chemistry, Payame Noor University, Iran
b
c
Department of Nanomaterials and Nanotechnology, Institute for Color Science and Technology, Tehran, Iran
Department of Chemistry, Shahreza Branch, Islamic Azad University, 86145-311, Iran
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 June 2012
Received in revised form 14 October 2012
Accepted 31 October 2012
Available online 21 November 2012
The acidity constants of 9 synthesized derivatives of Schiff base in dimethylformamide/water and ethanol/water
(25:75 v/v) at 25 °C and an ionic strength of 0.1 M have been determined spectrophotometrically. All of the
spectrophotometric data as pure spectra and distribution diagrams calculated with the SQUAD and MCR-ALS
as hard modeling and soft modeling methods, respectively. Also the influence of substituents in the molecular
structure on the ionization constants is discussed.
© 2012 Elsevier B.V. All rights reserved.
Keywords:
Schiff base
Salicylaldimine
Acidity constant
SQUAD
1. Introduction
the chemical equilibrium have distinct spectral responses, their
concentrations can be measured directly, and the determination of
Schiff bases are very important tools for the chemists as these
have been extensively studied as they possess many interesting fea-
tures, including photochromic and thermochromic properties [1],
proton transfer tautomeric equilibria [2], biological and pharmacolog-
ical activities [3–6], as well as suitability for analytical applications
[7]. Among Schiff bases, salicylaldimine Schiff bases have been exten-
sively studied as they possess many interesting features [8–10].
Therefore, their successful application requires a detailed study of
their characteristics. Also the predication of acidity constants of or-
ganic reagents is important for understanding and quantifying chem-
ical phenomena such as reaction rates, biological activity, biological
uptake, biological transport and environmental fate [11]. Proton
transfer reactions in Schiff bases have been studied extensively both
experimentally and theoretically in the last three decades [12–14].
There have been several methods for the determination of acidity
constants, including the use of potentiometric titration, spectropho-
tometry, capillary electrophoresis, and so on. Spectroscopic methods
are in general highly sensitive and as such are suitable for studying
chemical equilibria in solution. When the components involved in
the equilibrium constant is simple [15]. In many cases, the spectral re-
sponses of two and sometimes even more components overlap con-
siderably and their analysis is no longer straightforward. Therefore,
to overcome this problem we have to employ graphical and compu-
tational methods. The most relevant reports are on SPECFIT [16],
SQUAD [17] and HYPERQUAD [18].
In this study the analysis is readily performed with the computer
program SQUAD as the hard modeling method. In comparison, multi-
variate curve resolution-alternating least squares (MCR-ALS) are used
as soft-modeling methods. MCR-ALS is one of the powerful tools for
obtaining information about how the concentration of the species in-
volved in the reactions evolves [19,20].
The MCR-ALS method is based on factor analysis. Ideally, the num-
ber of significant factors should be the same as the number of chem-
ical species involved in the reaction. The physical changes associated
with the unknown equilibriums also that are reflected in the number
of acid–base species, which can be detected from the spectroscopic
monitoring of a titration experiment, were determined. Among the
computational and statistical methods used to solve mixture analysis
problems, the FA, principal component analysis (PCA) [21,22] and
singular value decomposition (SVD) [23] techniques play a key role,
especially in the estimation of the number of species contributing sig-
nificantly to the experimental data variance. In this work the effects
of different substitutes were studied on the dissociation constants
and pure spectrum of these salicylaldimine Schiff bases.
⁎
Corresponding author. Tel.: +98 251 2906448; fax: +98 251 2916449.
E-mail address: m.bordbar@qom.ac.ir (M. Bordbar).
0167-7322/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molliq.2012.10.039

M. Bordbar et al. / Journal of Molecular Liquids 178 (2013) 70–77
71
N
N
N
N
N
N
H
N
R
R
OH HO
R
OH HO
OH
HO
R
R
R
L1 : R : H
L2 : R : Br
L3 : R : NO2
L4 : R : H
L5 : R : Br
L6 : R : NO2
L7 : R : o-OCH3
L8 : R : p-Br
L9 : R : p-NO2
B
C
Group
Group A
Group
Fig 1. Schematic representation of the ligands and labels.
2. Experimental
2.1. Apparatus and materials
diethylenetriamine (L7), N,N′-bis(4-bromosalicylidene)diethylene-
triamine (L8), N,N′-bis(4-nitrosalicylidene) diethylenetriamine (L9)
were synthesized in our laboratory as shown in Fig. 1.
DMF, HCl, KCl and NaOH were of pro-analysis grade from E. Merck.
These chemicals were used without further purification. All of the so-
lutions were prepared fresh daily. Due to that the salicylaldimines are
unstable in solution, and are involved in various equilibriums like
keto-enol or in ring-chain tautomeric interconversion [25,26], all
solutions were allowed to remain in thermostated sample compart-
ments under stirring for a minimum of 45 min before the spectra
were collected. All calculations were performed in MATLAB 7.5
(Math Works, Cochituate Place, MA).
Absorption spectra were obtained with a Cary-100 UV–Vis spectro-
photometer by using 1 cm path length glass cells and the measurements
were performed at 25 0.1 °C. pH measurements were made with a
Metrohm 729 pH-meter by using a combined glass electrode. A stock
of L1–L6 solutions and L7–L8 containing 10 mg mL−1 of salicylaldimine
derivatives was prepared by dissolving 0.001 g of these reagents in
10 mL ethanol and DMF respectively, and adjusting the volume to 10.0.
The protonation constants and spectral profiles of salicylaldimine
derivatives were obtained in a DMF/water and ethanol/water mixture
(25:75 v/v) by the mixing of stock solutions and water. The recording
pH values in the binary ethanol/water and DMF/water solvents were
corrected by using the following equation:
2.2. Equilibrium measurements
The acidity constants were evaluated from the computer fitting of
the absorbance–pH data to the equations that resulted from substituting
the pH and absorbance values in the mass balances [27,28]. The resulting
equations for diprotic acids are given in Eq. (1):
pHꢀ ¼ pHðRÞ−δpH
where pH* is the corrected pH, pH(R) is the pH-meter reading
obtained in a the binary solvent, and δ is the correction term [24].
The L1–L9 tetradentate Schiff base ligands are N,N′-bis(salicylidene)-
1,2-cyclohexanediamine (L1), N,N′-bis(4-bromosalicylidene)-1,2-
cyclohexanediamine (L2), N,N′-bis(4-nitrosalicylidene)-1,2-cyclo-
hexanediamine (L3), N,N′-bis(salicylidene)ethylenediamine (L4),
N,N′-bis(4-bromosalicylidene)ethylenediamine (L5), N,N′-bis(4-nitro-
salicylidene)ethylenediamine (L6), N,N′-bis(6-metoxysalicylidene)
ꢂ
ꢂ
ꢃꢃ ꢂ
ꢃ
ꢀh
i
ꢁ
h
i
h
i
h
i
A ¼ A0 þ A1 Hþ =K2 þ A2 Hþ 2=K1K2
=
1 þ Hþ =K2
þ
Hþ 2=K1K2: ð1Þ
In these equations, A is the observed absorbance at each titra-
tion point, A0, A1 and A2 are the absorbances of the basic form,
monoprotonated form and diprotonated form, respectively and K1 and
K2 are the first and the second acidity constant.
Fig. 2. Absorption spectra of L6 at different pH values: (1) 1, (2) 1.5, (3) 3, (4) 2.5, (5) 3, (6) 3.5, (7) 3.8, (8) 4, (9) 4.2, (10) 4.5, (11) 4.75, (12) 5, (13) 5.25, (14) 5.5, (15) 5.75, (16) 6,
(17) 6.25, (18) 6.5, (19) 7, (20) 7.5, (21) 8, (22) 9, (23) 10, (24) 11, and (25) 12 in water/ethanol (75:25% v/v).

72
M. Bordbar et al. / Journal of Molecular Liquids 178 (2013) 70–77
The A0, A1, A2, K1and K2 values were calculated by computer
fitting of the absorbance–pH data to either Eq. (1) by using SQUAD
and MCR-ALS as hard modeling and soft modeling methods, respec-
tively. In order to prevent the protonation of amine groups (in L7–L9)
the pH of the solutions was kept upper than that of the pH of the
ligands.
N
N
N
N
H
H
H
O
H
O
O
O
Fig. 3. Tautomerism between enol-imine and keto-amine forms.
2.3. Synthesis of ligands
2.3.1. Synthesis of L1–L3
N,N′-bis(4-bromosalicylidene)ethylenediamine (L5)
Mp 195–196 °C; 1H NMR (DCCl3) δ 3.96 (s, 4H); 6.86–6.89 (d, J=
8.8 Hz, 2H); 7.29–7.41 (m, 4H), 8.31 (s, 2H), 13.18 (s, 2H)
A
solution of 1,2-diaminocyclohexane (5 mmol) in 20 mL
dichloromethane was added to a 250-mL three-necked round bot-
tomed flask. Salicylaldehyde derivatives (10 mmol) were dissolved
in 30 mL of dichloromethane and placed in the addition funnel. The
solution of aldehyde was added to the stirred solution of diamine
over 15 min. An exothermic reaction occurs; and the reaction mixture
was gently heated upon complete disappearance of the water–
dichloromethane azeotrope. The resulting mixture then was allowed
to cool slowly to room temperature and stirred for 15 min. During the
concentration and cooling period an orange yellow solid precipitated.
The evaporation of the solvent afforded the crude Schiff base ligands
as orange viscous liquids which upon further drying afforded powders
in nearly 100% yield with a trace of excess salicylaldehyde derivates.
The crude product was refluxed with 20 mL of absolute ethanol, cooled,
filtered and vacuum dried.
N,N′-bis(4-nitrosalicylidene)ethylenediamine (L6)
Mp 275–278 °C; 1H NMR (DCCl3) δ 4.03 (s, 4H); 6.77–6.79 (d, J=
9.6 Hz, 2H); 8.10 (dd, J=8.8 Hz, 2.8 Hz, 2H), 8.45 (d, J=2.8 Hz,
2H), 8.79 (s, 2H), 14.17 (s, 2H)
N,N′-bis(6-metoxysalicylidene)diethylenetriamine (L7)
Mp 170–172 °C; 1H NMR (DCCl3) δ 2.01 (t, J=5.2,4 H); 3.64
(t, J=5.2, 4H); 3.92 (s, 6H); 6.90–7.04 (m, 4H); 7.21 (t, J=3.6, 4H);
8.64 (s, 2H), 13.22 (s, 2H)
N,N′-bis(4-bromosalicylidene)diethylenetriamine (L8)
Mp 224–226 °C; 1H NMR (DCCl3) δ 2.70 (t, J=5.6, 4H); 3.80
(t, J=5.6, 4H); 6.97–7.00 (d, J=8.8 Hz, 2H); 7.25–7.40 (m, 2H);
7.40–7.48(m, 2H); 8.59 (s, 2H), 13.06 (s, 2H)
2.3.2. Synthesis of L4–L6
0.1 mol salicylaldehyde derivatives was dissolved in 45 mL ethanol
and heated to 40 °C, and then added dropwise to a solution of 0.05 mol
ethylenediamine in ethanol under vigorous stirring. The mixture solu-
tion turned light yellow, and for a while yellow sheet-like crystals pre-
cipitated out. After 20 min, the mixture was cooled and the precipitate
was collected. The yellow solid was re-crystallized in ethanol and dried
at room temperature under a vacuum.
N,N′-bis(4-nitrosalicylidene)diethylenetriamine (L9)
Mp 236–238 °C; 1H NMR (DCCl3) δ 2.89 (t, J=5.6, 4H); 3.64
(t, J=5.6, 4H); 6.52–6.55 (d, J=9.6 Hz, 2H); 7.94–7.97 (m, 4H); 8.34
(s, 2H), 13.65 (s, 2H)
2.3.3. Synthesis of L7–L9
3. Results
Salicylaldehyde derivatives (0.1 mol) were dissolved in 75 mL of
methanol, and to this was added a solution of diethylenetriamine
(0.05 mol) in 25 mL of methanol. The reaction mixture thus obtained
was refluxed on a water bath for 1 h. After reducing the volume of the
solvent to ca. 50 mL, the content was transferred into a beaker and the
excess solvent was evaporated under the current of air where a viscous
yellow-red oil was obtained. This was further dried in a vacuum.
All of the known products were identical with authentic samples
by melting points, TLC and NMR determinations.
The absorption spectra that are obtained for the titration of a
4.16×10−5 5.39×10−5 7.44×10−5 7.45×10−5 5.58×10−5
,
,
,
,
,
7.04×10−5, 4.996×10−5, 4.27×10−5 and 4.85×10−5 M of some
salicylaldimine derivative L1–L9 solutions respectively in water/
ethanol and water/DMF mixture by a standard solution of 3 M NaOH
to adjust the pH values at 200–600 nm were recorded. In order to pre-
vent the formation of cationic species of the Schiff base (i.e., H3L+ and
H4L2+) the pH of the solutions was kept upper than that of the pKa1
value. The 3D absorbance–response surfaces representing the mea-
sured multiwavelength absorption spectra of the Schiff base ligands
(L6), on the dependence of pH at 25 °C are plotted in Fig. 2 and
N,N′-bis(salicylidene)-1,2-cyclohexanediamine (L1)
Mp 119–120 °C; 1H NMR (DCCl3) δ 1.48–1.99 (m, 8H); 3.33–3.36
(m, 2H), 6.81–6.84 (m, 2H), 6.88–6.96 (m, 2H), 7.18(dd, J=1.2 Hz,
7.6 Hz, 2H), 7.25–7.31 (m, 2H), 8.29 (s, 2H), 13.36 (s, 2H),
1.8
1.6
1.4
1.2
1
N,N′-bis(4-bromosalicylidene)-1,2-cyclohexanediamine (L2)
Mp 189–191 °C; 1H NMR (DCCl3) δ 1.50–1.60 (m, 2H); 1.73–1.75
(m, 2H); 1.91–1.97 (m, 4H); 3.31–3.36 (m, 2H), 6.81–6.87 (m, 2H),
7.28–7.40 (m, 4H); 8.20(s, 2H), 13.26 (s, 2H)
0.8
0.6
0.4
0.2
0
N,N′-bis(4-nitrosalicylidene)-1,2-cyclohexanediamine (L3)
Mp 217–219 °C; 1H NMR (DCCl3) δ 1.76–1.85 (m, 2H); 2.00–2.19
(m, 2H); 2.22–2.25 (m, 4H); 3.42–3.54 (m, 2H), 6.96–7.00 (m, 2H),
7.74–7.80 (m, 4H); 8.45(s, 2H), 13.45 (s, 2H)
200
250
300
350
400
450
N,N′-bis(salicylidene)ethylenediamine (L4)
Wavelength
Mp 127–129 °C; 1H NMR (DCCl3) δ 3.96 (s, 4H); 6.87–6.98 (m, 4H);
7.24–7.34 (m, 4H), 8.38 (s, 2H), 13.25 (s, 2H)
Fig. 4. Spectra of L1 in ethanol/water mixture (25:75 v/v) during time.

M. Bordbar et al. / Journal of Molecular Liquids 178 (2013) 70–77
73
represent the input data of the regression program SQUAD. A compari-
son of both the SQUAD and MCR-ALS program treatments, along with
the proposed strategy for efficient experimentation in deprotonation
constant determination, is presented. Even though the actual SQUAD
version used has a limited dimension and input can contain 100 spectra
only, an efficient spectra sample 100×55 (ns×nw) was used (Fig. 2a)
for regression analysis. As the changes in the spectra are small within
deprotonation in the range of 240–300 nm, however, both of the vari-
ous deprotonated species L and LH exhibit partly similar absorption
bands. When such small changes in the absorbance spectra are avail-
able, a very precise measurement of the absorbance is then necessary
for the reliable detection of the deprotonation equilibrium studied.
One of the most important behaviors of the salicylaldimine Schiff
base in solution and in solid state is tautomerism. Tautomerism in
salicylaldimine Schiff bases was investigated by using spectroscopy
and X-ray crystallography techniques [29,30]. These Schiff bases
with the OH group in ortho position to the imino group are of interest
mainly due to the existence of either OH....N or O....HN type of hydro-
gen bond and tautomerism between enolimine and keto-amine forms
[25], Fig. 3.
the spectra were determined, because after 45 min the spectra of
these Schiff base didn't change.
After collecting the absorption spectra of the synthesized Schiff
base in the solvent mixture at different pH values and performing
PCA, the PCA results showed that there are three components in
the whole pH range. Then the matrix of the absorption data was
processed by the SQUAD program to obtain the acidity constants
and spectral profiles of the components which participate in the
absorbance data matrix. Distribution diagrams of the different species
of salicylaldimine derivatives (L1–L9) are shown in Fig. 5. The acidity
constants of all the salicylaldimine derivatives studied were eval-
uated by computer fitting of the corresponding absorbance–pH data
to the appropriate equations (i.e., Eq. (1) for diprotic acids). Also
the pure spectra of the salicylaldimine species in water/ethanol and
water/DMF were calculated by the program fitting of SQUAD and
that the corresponding results for L1–L9 are shown in Fig. 6.
In this work about MCR-ALS firstly the number of acid–base species
detected from the singular value decomposition (SVD) as a spectro-
scopic monitoring method [31] and the pure spectra as the initial esti-
mated data were predicted by the evolving factor analysis [32]. Also
the multivariate curve resolution (MCR-ALS) has been implemented
in the MATLAB and it is available in the internet [33].
As it was shown in Fig. 4 because of the tautomerism the UV–vis
spectra of these compounds have changed during time. For the con-
trol of this instability, all solutions were allowed to remain in room
temperature under vigorus stirring for a minimum of 45 min before
Also the estimated acidic constant values for L1–L9 by the two
methods SQUAD and MCR-ALS are shown in Table 1.
Fig 5. Distribution diagrams of different species of salicylaldimine derivatives L1–L9: H2L (solid line), HL (dash line) and L (solid–dash line) in water/DMF and water/ethanol.

74
M. Bordbar et al. / Journal of Molecular Liquids 178 (2013) 70–77
Fig 6. Pure spectra of L1–L9 species: H2L (solid line), HL (dash line) and L (solid–dash line) in water/DMF and water/ethanol.
4. Discussion
(L5), N,N′-bis(4-nitrosalicylidene)ethylenediamine (L6), N,N′-bis(6-
metoxysalicylidene)diethylenetriamine (L7), N,N′-bis(4-bromo-
salicylidene)diethylenetriamine (L8), N,N′-bis(4-nitrosalicylidene)
diethylenetriamine (L9) were identified by using 1H NMR, melting
point and UV spectral data, and these obtained spectral values
were seen to be compatible with literature reports [34–41].
In this study, the structures of the newly synthesized N,N′-
bis(salicylidene)-1,2-cyclohexanediamine (L1), N,N′-bis(4-bromo-
salicylidene)-1,2-cyclohexanediamine (L2), N,N′-bis(4-nitro-
salicylidene)-1,2-cyclohexanediamine (L3), N,N′-bis(salicylidene)
ethylenediamine (L4), N,N′-bis(4-bromosalicylidene)ethylenediamine
4.1. 1H NMR spectra
In the 1H NMR spectra of the ligands, the phenolic proton was
seen at 13–14.5 ppm. These high frequency orthophenolic hydrogens
in all the Schiff bases are attributed to the presence of intramolecular
hydrogen bonding and electron-withdrawing effect of the substitu-
ents on the aromatic phenolic ring. The azomethine protons appear
at 8–9 ppm. The phenol ring proton signals resolve in the range of
6–8 ppm and the aliphatic proton in L1–L9 due to different chemical
environments was seen at 1–4 ppm. A singlet was seen at 3.92 ppm
that was assigned to L7 that has the OMe group.
Table 1
The estimated acidic constant values for Schiff Base derivatives by two methods.
No.
SQUAD
pKa1
MCR-ALS
pKa1
pKa2
pKa2
L1
L2
L3
L4
L5
L6
L7
L8
L9
6.92
6.66
5.04
6.75
6.39
5.24
8.19
7.37
5.36
7.45
9.3
6.88
6.25
5.1
6.74
6.42
5.24
8.11
7.31
5.44
7.44
9.28
6.45
8.35
9.00
10.38
10.59
8.91
9.73
6.39
8.28
9.03
10.31
10.54
8.89
9.69
4.2. Electronic spectra
With the aim to obtain information about the type of the electronic
transition and in order to investigate the different effects, a series of

M. Bordbar et al. / Journal of Molecular Liquids 178 (2013) 70–77
75
2.5
2.5
2
3
2.5
2
L4
L5
L6
L7
L8
L9
L1
L2
L3
2
1.5
1
1.5
1
1.5
1
0.5
0.5
0.5
0
200
0
200
0
200
250
300
350
400
450
500
250
300
350
400
450
500
250
300
350
400
450
500
Wave length
Fig. 7. Electronic spectra of ligands were measured in ethanol/water and DMF/water mixture.
the Schiff bases with several bridge lengths were prepared and the
electronic spectra of the ligands were measured in ethanol/water and
DMF/water mixture solvents (Fig. 7). Table 2 presents the color, melting
point and absorption spectra characterization of the L1–L6 and L7–L9 li-
gands in ethanol/water and DMF/water mixture (25:75 v/v) respectively.
Generally, the recorded spectra of the ligands displayed three
main absorption bands (Fig. 7). The first band due to the n→л* tran-
sition of the C_N appear between 300 and 450, 280 and 450 and 320
and 450 in groups A, B and C respectively and in the presence of the
electron-withdrawing substituent an apparent blue shift is observed.
Also the bands between 235and 280 nm are assigned as a π→π*
transition involving C_N. The absorption band between 200 and
235 nm is most probably related to the π–π* transition of the pheno-
lic chromophore [37].
(n–π*) on the increase in the pH of the medium as explained above.
The recorded spectra of all compounds exhibit a clear isosbestic point
within the high pH range revealing the existence of equilibrium between
the neutral and ionic forms of these compounds in such media (Fig. 8).
Fig. 9 shows the plot of the absorbance–pH data for the L1–L9 sys-
tem, and the trend of increasing and decreasing absorbance values at
the selected wavelengths is easily observed. Each figure comprising a
clear inflection, indicates typical dissociation processes. In all com-
pounds the absorbance–pH curve at the selected wavelength displayed
one or two inflections.
One of the outputs of the employed multivariate data analysis method
is the concentration profiles of the acid–base forms of the studied weak
acids. These are shown in Fig. 5. Another output of the employed multi-
variate data analysis method is the pure spectra of the different acid–
base species, which are given in Fig. 6. One can see that for the acidic
form of the most Schiff bases no clear peak maxima are observed whereas
the basic forms represent a clear peak maximum at about 380 nm which
is dependent on the substitution pattern on the Schiff bases.
4.3. Estimation of acidity constants
The acidity constant of the various Schiff bases were calculated by
using the SQUAD program [17], designed to calculate the best value
for the acidity constants of the proposed equation model (Eq. (2))
by employing a non-linear least-squares approach. For the investiga-
tion of validated results we performed this protocol by MCR-ALS as
the soft modeling method and that the comparisons of the results
are shown in Table 1.
In general, the factors that influence the acidity of the compounds
are the inductive effects, steric effects, solvent effects, and hydrogen
bonding and resonance effects. In this work, we observed that with
regard to the acidity of the Schiff bases in water/ethanol and water/
DMF, the inductive, steric and hydrogen bonding effects are strong.
These are discussed below in detail.
H2L þ H2O↔HL−
4.3.1. The electronic effect of para substituted Schiff base ligands
Considering the structures of the Schiff bases L1–L9 we can de-
duce that they have two deprotonation sites (two OH groups of the
phenol rings).
The acidity of the Schiff bases is influenced by the substituents on
the aromatic phenolic ring. This order of increasing acidity seems log-
ical when one thinks about the inductive electron-donating effect of
ð2Þ
−
HL− þ H2O↔L2
:
Generally, for all compounds studied by increasing the pH of the me-
dium within pHs 2.0–12.0, the absorbance band at the 350–450 range
band increases until a more or less constant value is reached (Fig. 8).
This behavior can be ascribed to the expected easier electron transfer
Table 2
2.5
2
UV–vis spectral data of ligands.
Ligand Formula
MW
colour
m. p.
(°C)
π–π*
(benzen
ring)
π–π* n–π*
C_N
1.5
1
λ (nm)
L1
L2
L3
L4
L5
L6
L7
C20H22N2O2
322.4
Fulvous
119–120 210
189–191 223
217–219 224
127–129 213
195–196 222
256
253
247
256
252
250
263
323
340
357
324
341
309
352
C20H20Br2N2O2 480.19 Yellow
C20H20N4O6
C16H16N2O2
412.4
268.31 Yellow
C16H14Br2N2O2 426.1
Orange
0.5
Yellow
Yellow
C16H14N4O6
C20H25N3O4
358.3
275–278
170–172 204
–
0
250
371.42 Dark
Yellow
C18H19Br2N3O2 469.17 Orange
C18H19N5O6 401.36 Dark
Yellow
300
350
400
450
L8
L9
224–226
236–238
–
–
253
252
340
307
Wavelength
Fig. 8. Absorption spectra of the L9 (4.99×10−5 M) in different pH-values (4–12).

76
M. Bordbar et al. / Journal of Molecular Liquids 178 (2013) 70–77
Fig. 9. Change of absorbance vs pH at λmax for L1–L9 (L1:255 nm; L2,L3,L5,L6,L7,L9:390 nm; L4:378 nm and L8:395 nm).
the OMe group and the strong electron-withdrawing effect of the NO2
and Br groups in the para position of the phenol ring.
About pKa1 when we arrange the studied Schiff bases and their re-
duced derivatives in an increasing acidity order for the deprotonation
process we get the following sequence:
4.3.2. Effect of the diamine bridge length
Also group C behaves in a similar manner to group B but because
of the distance between the two hydroxyl groups, the intramolecular
hydrogen bonding ability of the molecules L7 and L8 is very weak than
that of L9, while in group A it seems that because the rigidity of mol-
ecules is not promising for hydrogen bonding to occur, the pKa1 and
pKa2 values are closer to each other. The electronic absorption spectra
and obtained acidity constants for the deprotonation process are
depicted in Table 1.
NO2 > Br > H > OMe:
About the second deprotonation (pKa2), according to the results
founded, hydrogen bonding seems to be more effective than the other
factors and the formation of hydrogen bonding is dependent to the
effects of different substitutes and geometry of molecules, directly.
The results showing the presence of the strong electron-withdrawing
groups in the para position of the phenol ring lead to an increase in
the hydrogen bonding effect. In detail about group A (L4–L6), the
lower acidities can be attributed to the strong hydrogen bonding
due to the presence of the nitro group in para position of the phenol
ring. It is observed that the decrease in the electronegativity of L4–L6
molecules gives rise to the increase of acidity (pKa2) as follows
(Scheme 1):
5. Conclusion
In the present study, the acid dissociation constants of a series of
substitutive derivatives of the salicylaldimine derivatives have been
carried out by using ultraviolet visible absorption spectroscopy in all
pH regions. This study showed that the proposed program (SQUAD)
is useful in obtaining acidic constants and concentration and spectral
profiles of the components which exist in a suitable pH range for a
diprotic Schiff base and the results are in good agreement with the re-
sults obtained by the MCR-ALS method. So we proposed this method
to determine the equilibrium characteristics of the diprotic acids in a
short time with good accuracy.
H > Br > NO2:

M. Bordbar et al. / Journal of Molecular Liquids 178 (2013) 70–77
77
N
N
pKa2
pKa1
N
N
N
N
H
H
O
H
O-
-O
O-
O
O
R
R
R
R
R
H
R
N
HN
H
O
H
O
R
R
NH HN
N
N
O
O
H
R
R
-O
O
R
R
H
Scheme 1. Possible deprotonation patterns for investigated Schiff bases.
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