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

Journal of Molecular Liquids 178 (2013) 70–77

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Multi-wavelength spectrophotometric determination of acidity constants of some salicylaldimine derivatives Maryam Bordbar a,⁎, Ali Yeganeh Faal b, Mohammad Mahdi Ahari-Mostafavi a, Mehrnaz Gharagozlou c, Razieh Fazaeli d a

Department of Chemistry, University of Qom, Qom, Iran Department of chemistry, Payame Noor University, Iran Department of Nanomaterials and Nanotechnology, Institute for Color Science and Technology, Tehran, Iran d Department of Chemistry, Shahreza Branch, Islamic Azad University, 86145-311, Iran b c

a r t i c l e

i n f o

Article history: Received 1 June 2012 Received in revised form 14 October 2012 Accepted 31 October 2012 Available online 21 November 2012 Keywords: Schiff base Salicylaldimine Acidity constant SQUAD

a b s t r a c t 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.

1. Introduction Schiff bases are very important tools for the chemists as these have been extensively studied as they possess many interesting features, including photochromic and thermochromic properties [1], proton transfer tautomeric equilibria [2], biological and pharmacological activities [3–6], as well as suitability for analytical applications [7]. Among Schiff bases, salicylaldimine Schiff bases have been extensively 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 organic reagents is important for understanding and quantifying chemical 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, spectrophotometry, 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

⁎ Corresponding author. Tel.: +98 251 2906448; fax: +98 251 2916449. E-mail address: [email protected] (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

the chemical equilibrium have distinct spectral responses, their concentrations can be measured directly, and the determination of the equilibrium constant is simple [15]. In many cases, the spectral responses of two and sometimes even more components overlap considerably and their analysis is no longer straightforward. Therefore, to overcome this problem we have to employ graphical and computational 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, multivariate 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 involved in the reactions evolves [19,20]. The MCR-ALS method is based on factor analysis. Ideally, the number of significant factors should be the same as the number of chemical 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 significantly 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.

M. Bordbar et al. / Journal of Molecular Liquids 178 (2013) 70–77

N R

N

OH HO

N R

R

N

N

OH HO

71

OH

R

N H

N HO

R

L1 : R : H L2 : R : Br L3 : R : NO2

Group A

L4 : R : H L5 : R : Br L6 : R : NO2

Group B

R

L7 : R : o-OCH 3 L8 : R : p-Br L9 : R : p-NO2

Group C

Fig 1. Schematic representation of the ligands and labels.

2. Experimental 2.1. Apparatus and materials Absorption spectra were obtained with a Cary-100 UV–Vis spectrophotometer 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: pH
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)-1,2cyclohexanediamine (L2), N,N′-bis(4-nitrosalicylidene)-1,2-cyclohexanediamine (L3), N,N′-bis(salicylidene)ethylenediamine (L4), N,N′-bis(4-bromosalicylidene)ethylenediamine (L5), N,N′-bis(4-nitrosalicylidene)ethylenediamine (L6), N,N′-bis(6-metoxysalicylidene)

diethylenetriamine (L7), N,N′-bis(4-bromosalicylidene)diethylenetriamine (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 solutions 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 compartments 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). 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):   h i   h i  h i h i2 þ þ 2 þ þ A ¼ A0 þ A1 H =K2 þ A2 H =K1 K2 =K1 K2 : ð1Þ = 1 þ H =K2 þ H

In these equations, A is the observed absorbance at each titration 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).

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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, respectively. 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 H O

2.3.3. Synthesis of L7–L9 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. 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), 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) 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) N,N′-bis(salicylidene)ethylenediamine (L4) 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)

O

O

N H O

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) 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, 2 H), 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) 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) 3. Results 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 prevent the formation of cationic species of the Schiff base (i.e., H3L + and H4L 2+) the pH of the solutions was kept upper than that of the pKa1 value. The 3D absorbance–response surfaces representing the measured multiwavelength absorption spectra of the Schiff base ligands (L6), on the dependence of pH at 25 °C are plotted in Fig. 2 and

1.8 1.6

Absorbance

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 solution turned light yellow, and for a while yellow sheet-like crystals precipitated 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 H

Fig. 3. Tautomerism between enol-imine and keto-amine forms.

2.3. Synthesis of ligands 2.3.1. Synthesis of L1–L3 A solution of 1,2-diaminocyclohexane (5 mmol) in 20 mL dichloromethane was added to a 250-mL three-necked round bottomed 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 H

1.4 1.2 1 0.8 0.6 0.4 0.2 0 200

250

300

350

400

450

Wavelength 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

represent the input data of the regression program SQUAD. A comparison 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 various deprotonated species L and LH exhibit partly similar absorption bands. When such small changes in the absorbance spectra are available, 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 hydrogen bond and tautomerism between enolimine and keto-amine forms [25], Fig. 3. As it was shown in Fig. 4 because of the tautomerism the UV–vis spectra of these compounds have changed during time. For the control of this instability, all solutions were allowed to remain in room temperature under vigorus stirring for a minimum of 45 min before

73

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 evaluated 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 spectroscopic monitoring method [31] and the pure spectra as the initial estimated 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]. 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.

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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 In this study, the structures of the newly synthesized N,N′bis(salicylidene)-1,2-cyclohexanediamine (L1), N,N′-bis(4-bromosalicylidene)-1,2-cyclohexanediamine (L 2), N,N′-bis(4-nitrosalicylidene)-1,2-cyclohexanediamine (L3), N,N′-bis(salicylidene) ethylenediamine (L4), N,N′-bis(4-bromosalicylidene)ethylenediamine

(L5), N,N′-bis(4-nitrosalicylidene)ethylenediamine (L6), N,N′-bis(6metoxysalicylidene)diethylenetriamine (L7), N,N′-bis(4-bromosalicylidene)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]. 4.1. 1H NMR spectra

Table 1 The estimated acidic constant values for Schiff Base derivatives by two methods. No.

L1 L2 L3 L4 L5 L6 L7 L8 L9

SQUAD

MCR-ALS

pKa1

pKa2

pKa1

pKa2

6.92 6.66 5.04 6.75 6.39 5.24 8.19 7.37 5.36

7.45 9.3 6.39 8.28 9.03 10.31 10.54 8.89 9.69

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

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 substituents 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. 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

L4

L1

2.5

Absorbnace

2.5

2.5

3

L7

2

2 1.5

L3

1.5

L8

L5

L2

2

75

1.5

L6

1

1

0.5

0.5

L9

1 0.5

0

0 200

250

300

350

400

450

500

200

0 250

300

350

400

450

500

200

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 ligands 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* transition 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 phenolic chromophore [37]. 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 investigation 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. H2 L þ H2 O↔HL− − − HL þ H2 O↔L2 :

ð2Þ

Generally, for all compounds studied by increasing the pH of the medium 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

(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 system, 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 compounds 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 multivariate 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. 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. 4.3.1. The electronic effect of para substituted Schiff base ligands Considering the structures of the Schiff bases L1–L9 we can deduce 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 logical when one thinks about the inductive electron-donating effect of

Table 2 UV–vis spectral data of ligands. MW

colour

m. p. (°C)

π–π* (benzen ring)

π–π* n–π* C_N λ (nm)

L1 L2 L3 L4 L5 L6 L7

C20H22N2O2 C20H20Br2N2O2 C20H20N4O6 C16H16N2O2 C16H14Br2N2O2 C16H14N4O6 C20H25N3O4

L8 L9

C18H19Br2N3O2 C18H19N5O6

322.4 480.19 412.4 268.31 426.1 358.3 371.42

Fulvous Yellow Orange Yellow Yellow Yellow Dark Yellow 469.17 Orange 401.36 Dark Yellow

119–120 189–191 217–219 127–129 195–196 275–278 170–172

210 223 224 213 222 – 204

224–226 – 236–238 –

256 253 247 256 252 250 263

323 340 357 324 341 309 352

253 252

340 307

2

Absorbance

Ligand Formula

2.5

1.5 1 0.5 0 250

300

350

400

450

Wavelength Fig. 8. Absorption spectra of the L9 (4.99 × 10−5 M) in different pH-values (4–12).

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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 reduced derivatives in an increasing acidity order for the deprotonation process we get the following sequence: 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): H > Br > NO2 :

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 molecules 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.

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 results 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.

M. Bordbar et al. / Journal of Molecular Liquids 178 (2013) 70–77

R

N

N

H O

H O

pKa 1

R

O H

H

N

pKa 2

N

O H

R

R

N

N

O-

R

77

R

O-

N -O

R

HN H

O

R NH HN N

N H

R

O H

-O

R

O

O

R

R

Scheme 1. Possible deprotonation patterns for investigated Schiff bases.

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