Spectrometric study of tautomeric and protonation equilibria of o-vanillin Schiff base derivatives and their complexes with Cu(II)

Article abstract of DOI:10.1016/j.saa.2008.03.029

Electronic absorption and emission properties of a series of Schiff bases derived from 2-hydroxy-3-methoxybenzaldehyde and 2-aminopyridine, 2,3-diaminopyridine, 2,6-diaminopyridine, or 3-aminomethylpyridine were studied in solvents of different polarities. The interconversion of the enolimine to the ketoamine tautomeric form was observed for compound 1, 6-methoxy-2-(3-pyridylmethyliminomethyl)phenol, and the corresponding equilibrium constant was estimated in several solvents. Protonation constants of all the investigated compounds were determined spectrophotometrically in the methanol/water 1/4 system. The effect of copper(II) ions on absorption and on the emission spectra of these ligands was examined in the buffered dioxane/water 1/1 system (pH 5.8). Strong complexation of Cu(II) and formation of a 1:1 complex were observed for the bis-Schiff base derived from 2,3-diaminopyridine. The complex of copper(II) with compound 1 was isolated and characterized by elemental analysis, magnetic susceptibility measurement, UV-vis and IR spectrometry.

Full text of DOI:10.1016/j.saa.2008.03.029

Spectrochimica Acta Part A 71 (2008) 1274–1280
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Spectrometric study of tautomeric and protonation equilibria of
o-vanillin Schiff base derivatives and their complexes with Cu(II)
Nives Galic´ a,∗, Zvjezdana Cimermanb, Vladislav Tomisˇic´ a
a Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102a, 10000 Zagreb, Croatia
b Swiss Federal Institute of Technology Zu¨rich, Tobelhofstrasse 18, CH-8044 Zu¨rich, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
Electronic absorption and emission properties of
a series of Schiff bases derived from 2-
Received 9 January 2008
Received in revised form 14 March 2008
Accepted 19 March 2008
hydroxy-3-methoxybenzaldehyde and 2-aminopyridine, 2,3-diaminopyridine, 2,6-diaminopyridine, or
3-aminomethylpyridine were studied in solvents of different polarities. The interconversion of the
enolimine to the ketoamine tautomeric form was observed for compound 1, 6-methoxy-2-(3-
pyridylmethyliminomethyl)phenol, and the corresponding equilibrium constant was estimated in several
solvents. Protonation constants of all the investigated compounds were determined spectrophotometri-
cally in the methanol/water 1/4 system. The effect of copper(II) ions on absorption and on the emission
spectra of these ligands was examined in the buffered dioxane/water 1/1 system (pH 5.8). Strong com-
plexation of Cu(II) and formation of a 1:1 complex were observed for the bis-Schiff base derived from
2,3-diaminopyridine. The complex of copper(II) with compound 1 was isolated and characterized by
elemental analysis, magnetic susceptibility measurement, UV–vis and IR spectrometry.
Keywords:
Schiff bases
o-Vanillin
Aminopyridine
Spectrophotometry
Spectrofluorimetry
Tautomeric constants
Protonation constants
Copper(II) complexes
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
metal complexes have been found to possess biological activity [19].
Antibacterial activity has also been reported for the ruthenium(II)
Schiff bases have been extensively studied as they possess many
interesting features, including photochromic and thermochromic
properties [1], proton transfer tautomeric equilibria [2], biologi-
cal and pharmacological activities [3–6], as well as suitability for
analytical applications [7,8]. Due to synthetic flexibility and sim-
ple preparation procedure these compounds have received a great
deal of attention as suitable ligands for coordination [9,10] and
determination of various metal ions [7,11–14]. Many Schiff base
metal complexes with a variety of biological activities have been
described in the literature [15–20].
The Schiff bases of salicylaldehyde and aminopyridines
have been characterized in detail in solid state and in solu-
tion [21–25], and proposed as highly sensitive spectrometric
and spectrofluorimetric reagents for Cu(II) [26,27]. However,
analogous heteroaromatic Schiff bases derived from 2-hydroxy-3-
methoxybenzaldehyde (o-vanillin) have not been investigated so
thoroughly. The mono- and bis-Schiff bases of o-vanillin and 2,3-
diaminopyridine have been used as ionophores in a Cu(II) selective
electrochemical sensor [28]. These compounds as well as their
complex of the Schiff base of o-vanillin and 2-aminopyridine [20].
The Schiff bases derived from salicylaldehyde and amino-,
aminoalkyl- and diaminopyridines are unstable in solution, and
are involved in various equilibria like keto-enol or in ring-chain
tautomeric interconversion, hydrolysis or some others [25,27].
Therefore, their successful application requires a detailed study
of their characteristics. In this work a series of known Schiff
bases derived from amino- or diaminopyridines and 2-hydroxy-
3-methoxybenzaldehyde were prepared. A new Schiff base was
synthesized from 3-aminomethylpyridine (Scheme 1). The spec-
troscopic characteristics, tautomeric properties, and protonation
constants of the prepared compounds in the methanol/water mix-
ture are presented and compared to those of analogous Schiff
bases of salicylaldehyde. The effect of copper(II) ions on the
absorption and emission spectra of these compounds is also dis-
cussed.
2. Experimental
2.1. Chemicals
∗
2-Amino-, 2-aminomethyl-, 2,3-diamino-, 2,6-diaminopyridine
(purum), and o-vanillin (puriss) were purchased from Fluka
Corresponding author.
E-mail address: ngalic@chem.pmf.hr (N. Galic´).
1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2008.03.029

N. Gali´c et al. / Spectrochimica Acta Part A 71 (2008) 1274–1280
1275
Scheme 1.
and used as supplied. BeSO4 (The British Drug Houses Ltd.),
Al(NO3)3 and Cu(CH3COO)2 (Merck) were p.a. purity grade. The
solvents used for UV–vis and spectrofluorimetric measurements
were p.a. (diethyl ether, dichloromethane, chloroform, dimethyl-
formamide) or for luminescence spectroscopy (methanol and
dioxane).
product was filtered, washed with ethanol, chloroform and ether,
and dried under vacuum.
Yield: 56%. mp 221–222 ◦C.
Anal. Calcd. for Cu(C14 H13N2O2)2: C, 61.58; H, 4.80; N, 10.26%;
found: C, 61.54; H, 4.82; N, 10.23%.
IR (KBr), ꢀ/cm−1: ꢀ(C N) 1608; ꢀ(C N)pyridine 1590; ꢀ(C–N)
1460; ꢀ(C–O) 1325, 1235; ꢀ(substituted benzene) 740.
UV–vis (CHCl3), ꢁmax/nm (ε × 10−4/dm3 mol−1 cm−1): 284
(3.14), 385 (0.83).
2.2. Preparation of Schiff bases and Cu(II) complex
For the preparation of 6-methoxy-2-(3-pyridylmethylimino-
methyl)phenol (compound 1) 3-aminomethylpyridine (0.50 g,
4.6 mmol), an equimolar quantity of o-vanillin, and a few drops of
piperidine were heated to reflux in absolute ethanol (20 ml) for 2 h.
After the solvent evaporation a dark yellow-brown viscous liquid
was redistilled under a vacuum (reaction yield 89%).
2.3. Apparatus and methods
UV–vis spectra were recorded on
a Varian Cary 3 spec-
trometer. Fluorescence spectra were measured on a PerkinElmer
LS50 spectrofluorometer. Measurements of IR spectra were per-
formed with a PerkinElmer 1725X FT-IR spectrometer. A JOEL
JNM-FX 100 spectrometer was used for recording NMR spec-
tra. For pH measurements a Radiometer pH/mV meter with a
Radiometer combined glass-calomel electrode GK2401C was used.
Magnetic susceptibility measurements of Cu(II) complex were car-
ried out at 298 K on a Gouy balance. Data were corrected for
diamagnetic contributions, which were estimated from Pascal con-
stants.
Anal. Calcd. for C14 H14 N2O2: C, 69.41; H 5.82; N 11.56%; found:
C, 68.24; H 5.83; N 11.56%.
IR, ꢀ/cm−1: ꢀ(C N) 1625; ꢀ(C N)pyridine 1585; ꢀ(C–N) 1460;
ꢀ(C–O) 1265, 1250; ꢀ(substituted benzene) 730.
1H NMR (DMSO, TMS, ı ppm): 13.41 [(OH), s, 1H]; 3.78 [(OCH3),
s, 3H]; 7.07 [(Ar–H), d, 1H, J = 7.94 Hz]; 6.85 [(Ar–H), t, 1H]; 7.08
[(Ar–H), d, 1H, J = 7.94 Hz]; 8.75 [(CH N), s, 1H]; 4.86 [(N–CH2), s,
2H]; 7.77 [(Py–H), d, 1H, J = 7,78 Hz]; 7.41 [(Py–H), dd, 1H]; 8.52
[(Py–H), d, 1H, J = 4.98 Hz]; 8.60 [(Py–H), s, 1H].
For the determination of tautomeric constants, the solutions
of compounds 1–5 in diethylether, dichloromethane, chloroform,
dimethylformamide, dioxane, methanol, and in dioxane/water or
methanol/water mixtures were prepared. The protonation con-
stants were determined in the methanol/water 1/4 system. The
buffer system was a universal buffer mixture. The pH meter
was calibrated and the protonation constants were determined
according to the procedures described in detail elsewhere [25].
The concentrations of Schiff bases in the measuring solutions
were 1 × 10−4 mol dm−3 (compounds 1–3) and 5 × 10−5 mol dm−3
6-Methoxy-2-(2-pyridyliminomethyl)phenol (compound 2), 6-
methoxy-2-(2-amino-3-pyridyliminomethyl)phenol (compound
3),
2,3-pyridinediamine
N,Nꢀ-bis(2-hydroxy-3-methoxyphenylmethylidene)-2,6-
N,Nꢀ-bis(2-hydroxy-3-methoxyphenylmethylidene)-
(compound
4),
and
pyridinediamine (compound 5) were prepared according to
refs. [19,20,29]. The purity of the ligands was checked by 1H NMR,
IR (KBr), and elemental analysis.
The copper(II) complex with compound 1 was prepared by the
following procedure: 2 mmol of 3-aminomethylpyridine was dis-
solved in 5 ml absolute ethanol, mixed with 2 mmol of o-vanillin,
and refluxed for 1 h. Ethanol solution of 1 mmol Cu(CH3COO)2 was
added and the resulting mixture was refluxed for another 2 h. The
(compounds
4 and 5). For the spectrofluorimetric measure-
ments the respective concentrations were 1 × 10−5 mol dm−3
and 5 × 10−6 mol dm−3. Each solution was bubbled with nitro-
gen for 2 min before measurement. The effect of metal ions

1276
N. Gali´c et al. / Spectrochimica Acta Part A 71 (2008) 1274–1280
Table 2
on absorption and on the emission spectra of ligands 1–5
was investigated in the buffered dioxane/water system (acetate
buffer, pH 5.8). Spectroscopic measurements were performed at
25 ◦C.
UV–vis spectral data of compound 1 in different solvents
Solvent
ꢁ/nm (ε × 10−4/dm3 mol−1 cm−1
)
334 (0.25)
264 (1.52)
223 (2.31)
334 (0.25)
273 (1.04)
Diethyl ether
3. Results and discussion
Dichloromethane
Chloroform
3.1. Absorption and emission properties of compounds 1–5
336 (0.25)
265 (1.51)
The UV–vis spectral data of compounds 1–5 in protic and apro-
tic solvents of different polarities are summarized in Table 1. As the
slow hydrolysis of the investigated Schiff bases occurred in water
containing solvents, the spectra were recorded immediately after
the preparation of solutions. The bands in the spectral region from
314 nm to 373 nm are associated with the excitation of the whole
molecule while those in the region from 260 nm to 297 nm are
attributed to the excitation of the aldehyde part of the molecule
[25]. The bands which appear at shorter wavelengths (<235 nm)
are not considered because of interference of solvent absorption.
In non-polar and/or aprotic solvents compounds 1–5 are present
predominantly in the enolimine form, as confirmed by NMR spec-
tra taken in dimethyl sulphoxide. The signals of the hydroxyl and
methine protons for Schiff bases 1–5 appear at (12.2–13.4) ppm
and (8.9–9.6) ppm, respectively. Their shapes (singlets) and posi-
tions (low fields) are characteristic of the enolimine form with a
strong intramolecular OH· · ·N C bond. Similar behaviour has been
observed in analogous Schiff bases derived from salicylaldehyde
[23,24].
334 (0.25)
270 (1.33)
Dimethylformamide
Dioxane
332 (0.26)
264 (1.53)
421 (0.10)
329 (0.20)
263 (1.33)
Dioxane/water 1/1
422 (0.07)
329 (0.22)
263 (1.46)
Methanol
422 (0.10)
330 (0.20)
262 (1.35)
Methanol/water 4/1
Methanol/water 1/1
Methanol/water 1/4
421 (0.18)
331 (0.17)
262 (1.17)
416 (0.27)
328 (0.14)
261 (0.93)
In the case of compound 1 the band at ≈420 nm in methanol and
in the dioxane/water mixture is assigned to the ketoamine form
[25]. To investigate the ketoamine–enolimine tautomeric equilib-
rium in more detail, a series of solvents with different polarities
were chosen (Table 2). Assuming that the shift of the UV–vis bands
of compound 1 to shorter wavelengths (e.g. for enolimine from
334 nm in diethylether to 328 nm in methanol/water 1/4 and for
ketoamine from 422 nm in methanol to 416 nm in methanol/water
1/4) is due to the solvent effect and that the molar absorptivities
are not significantly affected by variation of solvent, the molar frac-
tions of the enolimine and ketoamine forms can be deduced and the
Table 1
UV–vis spectral data of compounds 1–5
Solvent
ꢁ/nm (ε × 10−4/dm3 mol−1 cm−1
)
1
2
3
4
5
a
a
334 (0.25)
264 (1.52)
223 (2.31)
361 (0.69)
315 (2.02)
278 (1.04)
226 (2.07)
370 (0.51)
319 (1.77)
279 (0.87)
370b
371 (1.08)
276 (1.42)
228 (2.49)
Diethyl ether
336 (0.25)
265 (1.51)
365 (1.08)
279 (1.32)
339 (2.13)
287 (2.42)
363 (1.84)
290 (2.37)
Chloroform
DMF
334 (0.25)
270 (1.33)
371 (1.10)
277 (1.42)
321 (1.10)
360 (1.86)
290 (2.36)
318 (1.80)
282 (0.98)
332 (0.26)
264 (1.53)
361 (0.63)
317 (1.92)
279 (0.97)
227 (2.04)
373 (0.96)
276 (1.31)
229 (2.33)
337 (2.31)
284 (2.50)
228 (3.85)
360 (1.89)
290 (2.51)
229 (3.79)
Dioxane
421 (0.10)c
329 (0.20)
293b
263 (1.33)
227 (1.33)
Dioxane/water 1/1
315 (1.24)
273 (0.88)
225 (2.05)
363 (1.09)
276 (1.33)
225 (2.38)
339 (2.16)
282 (2.21)
226 (3.76)
350(1.56)
273(1.82)
225 (3.67)
422 (0.07)c
329 (0.22)
297 (0.26)
263 (1.46)
220 (2.23)
Methanol
314 (1.46)
279b
225 (1.79)
362 (1.08)
276 (1.31)
221 (2.29)
207 (1.95)
343 (1.36)
280 (1.87)
220 (3.19)
355 (1.04)
270 (1.51)
221(3.29)
a
Insoluble.
Shoulder.
b
c
Bands corresponding to the absorption of the ketoamine form.

N. Gali´c et al. / Spectrochimica Acta Part A 71 (2008) 1274–1280
1277
Fig. 1. Emission spectra of compound
5
at different pH values. c(5) = 5.0 ×
10−6 mol dm−3; solvent: dioxane/water 1/1; ꢁex = 333 nm; slit with = 2.5 nm.
Fig. 2. UV–vis spectra of compound 1 in methanol/water 1/4 mixture at different
pH values.
tautomeric constants, defined as Kt = [ketoamine]/[enolimine] can
be calculated (Tables 2 and 3). As can be seen, an increase in sol-
vent permittivity gives rise to an increase in the value of Kt. The
exception is dimethylformamide, an aprotic polar solvent.
The Schiff bases of salicylaldehyde with amino- and diaminopy-
ridines are not fluorescent in non-polar solvents such as hexane,
diethyl ether and chloroform. Weak fluorescence has been observed
in polar solvents: methanol, DMF and dioxane/water mixtures [27].
Like salicylaldehyde derivatives the compounds 1–5 derived from
o-vanillin are not fluorescent in most organic solvents (diethyl
ether, chloroform, dimethylformamide, methanol). In the diox-
ane/water 1/1 mixture fluorescence was observed for compounds
2 and 5 immediately after the preparation of solutions and relative
intensity of emission increased with time or as a result of lower pH.
For compounds 1, 3, and 4 no fluorescence was observed for freshly
prepared solutions. However, solution spectra of compounds 3 and
4 recorded about 24 h after preparation showed weak fluorescence.
As an example, the emission spectra of compound 5 at different pH
values are given in Fig. 1.
The increase in fluorescence intensity at lower pH can be
explained by the protonation of pyridine nitrogen which influences
the electronic density of the whole molecule. In water contain-
ing solvents slow hydrolysis of compounds 1–5 took place, so the
increase in fluorescence with time was probably due to the forma-
tion of starting fluorescent aminopyridines.
Fig. 3. Dependence of absorbance of compound 1 at 390 nm on pH (ꢀ, measured;
—, calculated).
titration was used in this work as the most convenient experi-
mental method. The protonation constants were determined in the
methanol/water 1/4 mixture. To gain rough information about the
rate of hydrolysis, it was estimated that, depending on the com-
pound, this reaction in unbuffered MeOH/H2O mixture was almost
completed in 10 min or more. Therefore, the spectra were recorded
immediately after the preparation of solutions, or more precisely,
after addition of the methanolic ligands solutions into the buffered
aqueous media. As an example, the dependence of the absorption
spectra of compound 1 on pH is shown in Fig. 2 and the corre-
sponding plot of the measured and calculated absorbances is given
in Fig. 3. The average values of protonation constants from three
determinations are listed in Tables 4 and 5. It should be noted that,
3.2. Protonation constants of compounds 1–5
Difficulties in obtaining reliable protonation constants of Schiff
bases 1–5 are due to their poor solubility (it is necessary to work at
low concentrations) as well as slow hydrolysis in mixed organic sol-
vent/water solutions. For that reason the batch spectrophotometric
Table 3
Tautomeric constants of compound 1 in different solvents and relative permittivities
of pure solvents at 20 ◦C [30]
Solvent
Kt
εr
Table 4
Protonation constants of compounds 1 and 1a in methanol/water 1/4 mixture
Diethyl ether
Dichloromethane
Chloroform
Dimethylformamide
Dioxane
Dioxane/water 1/1
Methanol
Methanol/water 4/1
Methanol/water 1/1
Methanol/water 1/4
0
0
0
0
4.27
8.93a
4.81
Protonation constant
Compound 1
Compound 1aa
lg K1
10.19 0.06
9.94
9.83
8.01 0.02
7.65
7.76
10.38
10.07
10.09
8.47
8.18
8.16
38.25
2.22
lg K1(A)
lg K1(B)
lg K2
lg K2(A)
lg K2(B)
lg K3
0
0.22
0.12
0.21
0.45
0.78
33.0
5.40 0.01
5.47
a
a
The value at 25 ◦C.
Ref. [25].

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N. Gali´c et al. / Spectrochimica Acta Part A 71 (2008) 1274–1280
Table 5
for the Schiff base of salicylaldehyde with 3-aminomethylpyridine
(1a) are given as well [25].
Protonation constants of compounds 2–5 and 2a–5a in methanol/water 1/4 mixture
The introduction of the methoxy group in the aldehyde part of
the Schiff base has a remarkable effect on the crystallization prop-
erties of compound 1 as compared to 1a (at room temperature
compound 1 is a viscous liquid while compound 1a crystallizes eas-
ily) [24]. However, the influence on protonation constants is less
prominent. The small difference between the lg K1 values of com-
pounds 1 and 1a (10.19 and 10.38) can be explained by an inductive
effect of the –OCH3 group. The effect is more pronounced in the
case of pure starting materials, e.g. o-vanillin and salicylaldehyde
(the corresponding lg K values, determined by the same procedure,
are 8.02 and 8.59). The equilibrium constant assigned to the proto-
nation of azomethine nitrogen is considerably lower for compound
1 (lg K2 = 8.01) than for compound 1a (lg K2 = 8.47). Such an effect of
the –OCH3 group is unexpected as the group is situated in the meta
position to the imine group and no resonance effect is possible. The
protonation of pyridine nitrogen is not influenced by substitution
because resonance through the whole molecule is disrupted by the
methylene group on the pyridine ring at position 3. The correspond-
ing lg K values for compounds 1 and 1a are 5.40 and 5.47, and are
comparable to the unsubstituted pyridine (lg K = 5.34).
The protonation pathway for compounds 2–5 is shown in
Scheme 3 and the calculated constants are listed in Table 5. For com-
parison, the protonation constants of salicylaldehyde analogues
(2a–5a) are also given [25]. The protonation sequence for com-
pounds 2–5 is changed with respect to compound 1 due to the
imino nitrogen directly bounded to the pyridine ring at position
2. The resonance involving imino and electron-withdrawing pyri-
dine nitrogen atoms causes a drastic decrease in the basicity of the
imino group and an increase in the basic character of the pyridine
nitrogen [25]. For that reason the protonation constant K3 for com-
pounds 2 and 5 (derived from 2-amino and 2,6-diaminopyridines)
cannot be determined by the method applied.
Compound
lg K1
9.34 0.03
8.82
9.67 0.004
10.01
9.45 0.02
9.71
9.81 0.03
8.75
lg K2
lg K3
2
7.48 0.05
6.92
5.73 0.004
5.92
6.89 0.04
6.90
6.52 0.05
7.09
2aa
3
3.98 0.03
4.12
4.31 0.07
4.40
3aa
4
4aa
5
5aa
a
Ref. [25].
because of hydrolysis reaction, some uncertainty is embedded in
those values.
The protonation sequence of compound 1 is presented in
Scheme 2. As the absorption bands of the pure enolimino and
ketoamino forms can be deduced from the UV–vis spectra of com-
pound 1 in solvents of different polarities (Table 2), the individual
constants K1(A) and K1(B) as well as K2(A) and K2(B) can be obtained
by means of the following equations [25]:
[ketoamine]
Kt =
[enolimine]
K1(B)
K1(A)
K2(A)
=
K2(B)
Kt =
K1 = K1(A) + K1(B)
K2 = K2(A) + K2(B)
The calculated values of the macroscopic and individual constants
are presented in Table 4. For comparison, the corresponding values
The protonation constants of OH groups in compounds 2a and
5a (Table 5) are similar to those of salicylaldehyde (lg K = 8.59) [25].
That was explained by resonance stabilization of the planar conju-
gate bases of pyridyliminomethylphenols which is similar to that
of the salicylaldehyde anion. As compound 5 in solid state is also
planar molecule [29] the K1 value was predicted to be lower than
the one observed. The same could be concluded for compound 2.
The unexpectedly high K1 values of the two Schiff bases could be
tentatively explained by the assumption that compounds 2 and 5
are present in solution in non-planar conformations. As a conse-
quence, delocalization of the free electron pair through the entire
molecule is diminished or is no longer possible.
Scheme 2.
Scheme 3.

N. Gali´c et al. / Spectrochimica Acta Part A 71 (2008) 1274–1280
1279
Fig. 4. UV–vis spectra of free compound 1 and in presence of Cu(II) in diox-
ane/water 1/1 mixture. c(1) = 1.0 × 10−4 mol dm−3; — 1; . . . c(Cu(II))/c(1) = 1; - - -
c(Cu(II))/c(1) = 10.
Fig. 5. UV–vis spectra of free compound 3 and in presence of Cu(II) in diox-
—
ane/water 1/1 mixture. c(3) = 1.0 × 10−4 mol dm−3
- - -c(Cu(II))/c(3) = 10.
;
3; . . . c(Cu(II))/c(3) = 1;
findings strongly suggest that among the compounds investigated
ligands 3 and 4 form the most stable complexes with copper(II).
Therefore, complexation of Cu(II) with compound 4 having four
binding sites was studied in more detail spectrophotometrically
(Fig. 6a and b). As can be seen in Fig. 6b, an almost linear rela-
The lower K1 values of compounds 3 and 4 than of the analogues
3a and 4a (Table 5) can be attributed to the inductive effect of the
–OCH3 group. Although two hydroxyl groups in compound 4 have
a different basicity, the difference is too small to allow determina-
tion of the protonation constant of each group. The K1 value listed
in Table 5 is an average value of constants of both groups. The proto-
nation of pyridine nitrogen is not influenced by the presence of the
–OCH3 group. For example, the lg K2 of compound 4 is 6.89 which
agrees very well with the salicylaldehyde analogue (lg K = 6.90). The
protonation constant K3, determined only for compounds 3 and 4,
is related to the imino group at position 3 of the pyridine ring. As
with compound 1 the protonation of azomethine nitrogen is not
influenced by the presence of the –OCH3 group so the correspond-
ing equilibrium constants are similar to those of analogous Schiff
bases derived from salicylaldehyde (Table 5).
3.3. Influence of metal ions on spectroscopic characteristics of
compounds 1–5
Copper(II) is known to form stable complexes with the Schiff
bases derived from salicylaldehyde and aminopyridines [26]. In this
work its influence on the absorption spectra of compounds 1–5 was
investigated in the buffered dioxane/water 1/1 mixture (acetate
buffer, pH 5.8). In order to get a qualitative insight into the com-
plexation affinities of the ligands towards copper(II) ion, UV–vis
spectra were recorded at two different Cu(II):ligand molar ratios,
i.e. 1:1 and 10:1. In the case of compounds 2 and 5, the equimo-
lar addition of Cu(II) did not cause any significant change in the
free ligands spectra. Upon addition of a tenfold excess of metal ion,
only a low intensity shoulder appeared in the visible region of the
spectrum (≈400 nm). On the contrary, after addition of an equimo-
lar amount of Cu(II) in the solution of compound 1 a new band
emerged with a maximum at ≈379 nm, and an upward trend in
absorbance was observed upon increasing the molar ratio to 10:1
(Fig. 4). That can be attributed to the moderate complexation affin-
ity of compound 1 towards copper(II) under the conditions used.
According to the structural characteristics, number, and arrange-
ment of the donor groups, the most efficient coordination was
expected for compounds 3 and 4. Indeed, the most prominent spec-
tral changes were observed upon addition of Cu(II) into solutions
of these Schiff bases. The bands which can be attributed to the
metal–ligand complex formed (≈425 nm and ≈453 nm for com-
pound 3; ≈433 nm for compound 4) were quite intensive, and, in
addition, no significant difference between the spectra of solutions
with 1:1 and 10:1 (Cu(II):ligand) ratios was observed (Fig. 5). These
Fig. 6. (a) Spectrophotometric batch titration of compound 4 (c = 1 × 10−4 mol dm−3
)
with Cu(CH3COO)2. (b) Dependence of absorbance at 433 nm on n(Cu2+)/n(4) ratio.

1280
N. Gali´c et al. / Spectrochimica Acta Part A 71 (2008) 1274–1280
tionship of absorbance vs. n(Cu2+) was observed up to the ratio
n(Cu2+)/n(4) ≈ 1 (n denotes total amount), followed by a break in the
titration curve. That indicates a rather strong complexation under
conditions used, and formation of a complex of 1:1 stoichiome-
try. In order to check whether the hydrolysis of compound 4 in the
complex also occurs (as it does in the free ligand solution), spectra
of an equimolar reaction mixture were recorded during 15 h. No
significant changes were observed indicating that hydrolysis of the
carbon-nitrogen double bond was suppressed by binding to copper
through the azomethine nitrogen atom.
The reaction of analogous Schiff bases derived from salicy-
laldehyde with Al(III) and Be(II) ions in the dioxane/water system
induced small shifts in the excitation and emission maxima, and
an increase in fluorescence intensities [27]. However, in the case
of fluorescent compounds 2 and 5, the addition of those ions in
equimolar amount and in excess had no influence on the emis-
sion spectra under the conditions used (dioxane/water 1/1; acetate
buffer, pH 5.8). The presence of Cu(II) ions slightly reduced the fluo-
rescence intensity, in agreement with the data obtained by UV–vis
measurements.
aprotic solvents all Schiff bases appeared predominantly in the
enolimine form. A shift to the ketoamine tautomer was observed
only for compound 1 in polar protic solvents and the corresponding
tautomeric constants were determined. Protonation constants for
all compounds were determined spectrophotometrically in the
methanol/water 1/4 system. Fluorescence was observed in the
case of compounds 2 and 5 in the dioxane/water 1/1 mixture. The
effect of copper(II) ions on the absorption spectra of the Schiff
bases in the dioxane/water 1/1 system was most pronounced for
compounds 3 and 4 singling them out as potential ionophores in
optical sensors. The Cu(1)2 complex was isolated and coordination
of the Schiff base to the metal ion through azomethine nitrogen
and phenolic oxygen was proposed according to the data obtained
by IR spectroscopy.
Acknowledgements
This work was supported by the Ministry of Science, Education
and Sports of the Republic of Croatia (projects 119-1191342-2960
and 119-1191342-1083). We thank Prof. Marina Cindric´ for perform-
ing magnetic measurements.
3.4. Isolation of Cu(1)2 complex
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ˇ
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A
series of Schiff bases were prepared: 6-methoxy-2-(3-
pyridylmethyliminomethyl)phenol (compound 1), 6-methoxy-
2-(2-pyridylimino-methyl)phenol (compound 2), 6-methoxy-2-
(2-amino-3-pyridyliminomethyl)phenol (compound 3), N,Nꢀ-bis
(2-hydroxy-3-methoxyphenylmethylidene)-2,3-pyridine-diamine
(compound 4) and N,Nꢀ-bis(2-hydroxy-3-methoxyphenylmethyl-
idene)-2,6-pyridinediamine (compound 5). In non-polar and
ˇ
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