Synthesis, characterization, and tautomeric properties of some azo-azomethine compounds

Article abstract of DOI:10.1515/znb-2012-0208

The primary azo compound 1-(3-formyl-4-hydroxyphenylazo)-4-nitrobenzene reacts with some aliphatic and aromatic diamines and yields the corresponding azo-azomethine compounds. These compounds were characterized by elemental analysis, IR, UV/Vis, and NMR spectroscopy. The primary azo compound exists entirely in the azo form in solution as well as in the solid phase. The tautomeric structure of azo-azomethine compounds heavily depends on the solvent and the substituents. Aliphatic diamine-based compounds favor the enol-imine tautomer while aromatic diamine-based compounds have structures that lie between the two enol-imine and keto-amine tautomers due to a relatively strong intramolecular hydrogen bond. The compounds exhibit positive solvatochromism (bathochromic shift) so that their absorption bands move toward longer wavelengths as the polarity of the solvents increases. In addition, UV/Vis spectrophotometry has shown that the studied compounds have molar extinction coefficients larger than 40000.

Full text of DOI:10.1515/znb-2012-0208

Synthesis, Characterization, and Tautomeric Properties
of Some Azo-azomethine Compounds
Kourosh Hamidian, Mohsen Irandoust, Ezzat Rafiee, and Mohammad Joshaghani
Faculty of Chemistry, Razi University, Kermanshah, Iran
Reprint requests to Professor Joshaghani. Fax: +98-8314274559. E-mail: mjoshaghani@razi.ac.ir
Z. Naturforsch. 2012, 67b, 159 – 164; received December 27, 2011
The primary azo compound 1-(3-formyl-4-hydroxyphenylazo)-4-nitrobenzene reacts with some
aliphatic and aromatic diamines and yields the corresponding azo-azomethine compounds. These
compounds were characterized by elemental analysis, IR, UV/Vis, and NMR spectroscopy. The pri-
mary azo compound exists entirely in the azo form in solution as well as in the solid phase. The tau-
tomeric structure of azo-azomethine compounds heavily depends on the solvent and the substituents.
Aliphatic diamine-based compounds favor the enol-imine tautomer while aromatic diamine-based
compounds have structures that lie between the two enol-imine and keto-amine tautomers due to
a relatively strong intramolecular hydrogen bond. The compounds exhibit positive solvatochromism
(bathochromic shift) so that their absorption bands move toward longer wavelengths as the polarity of
the solvents increases. In addition, UV/Vis spectrophotometry has shown that the studied compounds
have molar extinction coefficients larger than 40000.
Key words: Dye, Azo-azomethine, Schiff Base, Hydrogen Bonding, Tautomerism
Introduction
Hydroxyazo compounds can undergo azo/hydraz-
one tautomerization, in which the azo tautomer is the
more stable one (Scheme 1) [10].
Azo compounds are very important molecules and
have received much attention in fundamental and ap-
plied chemistry [1 – 5]. The well-known applications
of azo dyes in acid-base indicators and chemical sen-
sors and as electron transfer catalysts have attracted
the interest of many investigators [6, 7]. Several azo
dyes have been employed in liquid crystal displays
due to their non-ionic character and their solubility
in the liquid crystal hosts [8]. The introduction of a
salicylaldimine leads to azo-Schiff bases or azo-azo-
methine dyes and may result in new and better micro-
optoelectronic devices [9].
On the other hand, α-hydroxy salicylaldimine com-
pounds can undergo enol-imine/keto-amine tautomer-
ization by H-atom transfer from the hydroxyl oxygen
to the imine nitrogen probably via intramolecular hy-
drogen bonding (Scheme 2) [9, 11 – 13].
Solvents and substituents have a remarkable influ-
ence on the relative stability of the two tautomers [14].
Such tautomerizations are very important not only in
fundamental research but also for some applications.
The two tautomers have substantially different color
and chemical properties and hence different applica-
Scheme 1. Azo/hydrazone tautomerization.
Scheme 2. Enol-imine/keto-amine
tautomerization in azomethine com-
pounds.
c
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K. Hamidian et al. · Azo-azomethine Compounds
Table 1. Analytical data and physical properties of 1 and the azo-azomethine derivatives 3a – 3e.
Compound Empirical formula Formula weight Color
M. p. (◦C) Yield (%)
— Calculated (found) (%) —
1
C13H9N3O4
C28H22N8O6
271.23
566.52
620.61
614.57
659.56
628.59
orange-yellow 194 – 196
92
87
89
94
57
90
57.57 (57.49) 3.34 (3.60) 15.49 (15.45)
59.36 (58.87) 3.91 (4.11) 19.78 (20.04)
61.93 (61.36) 4.55 (4.85) 18.06 (18.43)
62.54 (62.00) 3.61 (3.95) 18.23 (18.62)
58.27 (57.86) 3.21 (3.42) 19.11 (19.52)
63.05 (62.50) 3.85 (4.05) 17.83 (18.33)
3a
3b
3c
3d
3e
brown-red
red
brown
293 – 295
285 – 287
275 – 277
C
C
32H28N8O6
32H22N8O6
C32H21N9O8
33H24N8O6
brown-orange 211 – 214
C
red-brown
288 – 290
Table 2. Characteristic IR
absorption bands of 1 and
the azo-azomethine deriva-
tives 3a – 3e.
IR
1
OH
(cm−1
–
CHaliphatic
CHO
C=N
(cm−1
–
–
–
1635
1631
1614
1618
1616
N=N
Phenol ring
NO2
(cm−1
1342
1342
1342
1342
1342
1342
1344
1340
C–O
(cm−1
1284
1284
1284
1290
1288
1290
1290
1290
)
(cm−1
–
)
(cm−1
)
)
(cm−1
1523
1523
1523
1522
1518
1518
1518
1518
)
(cm−1
1477
1477
1477
1489
1491
1489
1491
1485
)
)
)
1657
2753
2854
–
–
–
–
–
3a
3b
3c
3d
3e
3429
3469
–
–
–
2858 – 2927
2854 – 2912
–
–
–
Scheme 3. Azo-azomethine
compounds.
tions [15]. They display thermochromism and pho- methine compounds 3, the analytical data of which
tochromism [13]. Hydrogen bonding and/or transfer are summarized in Table 1. They are soluble in com-
is also very important in some living metabolism in mon organic solvents such as DMSO, DMF, and
which tautomerization is an essential step [16].
THF.
Keeping these features in mind, we decided to syn-
thesize a series of azo-azomethine compounds and in-
vestigate their electronic and solvatochromic behavior.
Infrared spectroscopy
The characteristic IR absorption bands of 1 and
their azo-azomethine derivatives 3a – 3e are shown
in Table 2. The IR spectrum of 1 exhibits a band
at 1523 cm−1 due to the stretching vibration of the azo
(N=N) group. In addition, a strong band at 1657 cm−1
Results and Discussion
Synthesis of the compounds
The azo-azomethine compounds 3a – 3e were syn- due to the stretching vibration of the aldehydic C=O
thesized in a two-step process (Scheme 3). In the bond and two weak bands at 2753 and 2854 cm−1
first step, salicylaldehyde was coupled with 4-nitro- due to the aldehydic C–H bond vibration confirm the
aniline to produce 1-(3-formyl-4-hydroxyphenylazo)- proposed azo tautomer. A medium to strong band
4-nitrobenzene (1). The reaction of 1 with a se- at 1284 cm−1 may be assigned to the ν(C-O) vibra-
ries of diamines afforded the corresponding azo-azo- tion which also confirms the existence of the azo tau-
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K. Hamidian et al. · Azo-azomethine Compounds
161
Table 3. 1H NMR signals of 1 and the azo-azomethine deriva-
tives 3a – 3e.
NMR
1
CHO
10.06 (s)
OH
11.46 (s)
–
14.10 (s)
10.25 (s)
10.38 (s)
–
C=N
–
–
8.50 (s)
8.85 (d)
9.07 (s)
–
Ar–H
7.14 – 8.41
–
6.99 – 8.40
7.00 – 8.50
6.74 – 8.77
–
3a
–
–
–
–
–
Scheme 4. Intramolecular hydrogen
bonding in salicylaldehyde.
3b
3c
3d
3e
tomer [18]. In addition, the absence of the correspond-
ing bands for the ketone carbonyl (C=O) and hydrazine
(N–H) groups at about 1700 cm−1 and 3300 cm−1 ex-
clude the presence of the hydrazone tautomer.
The IR spectrum of 1 does not show the band
due an OH stretching frequency. This feature has
already been reported for salicylaldehyde itself as
well as for other salicylaldehyde derivatives and sup-
ports the presence of an intramolecular hydrogen bond
(Scheme 4) [19, 20].
The IR spectra of the azo-azomethines 3a – 3e
show the presence of the C=N band while the char-
acteristic bands due to an aldehyde group are ab-
sent. The C–O stretching mode in the range 1288–
1290 cm−1 and the stretching frequency of N=N at
about 1520 cm−1 indicate that the compounds exist
exclusively in the azo form. The absence of bands at
about 1700 and 3300 cm−1 due to ketone carbonyl
(C=O) and hydrazine (N–H) groups also exclude the
keto-amine tautomer.
NMR spectroscopy
The 1H NMR spectra of the compounds were
recorded using [D6]DMSO (3c, 3d) and CDCl3 (3b
and 1) as a solvent. Compounds 3a and 3e were not
sufficiently soluble for 1H NMR investigations.
The 1H NMR spectrum of 1 shows a singlet at
δ = 11.46 ppm which disappears after adding deuter-
ated water, hence it was assigned to the hydroxyl pro-
ton [5, 11, 18]. Another singlet at δ = 10.06 ppm was
assigned to the aldehyde proton (Table 3) [18]. In com-
parison, the corresponding protons of salicylaldehyde
were observed at about 11 and 9.8 ppm. The spec-
trum does not show a broad band at 4 – 6 ppm which is
typical for an NH proton. This result in addition with
the presence of the hydroxyl proton confirms the ex-
istence of the enol-imine tautomer and excludes the
keto-amine form.
The singlet at δ = 10.06 ppm due to the aldehyde
The IR studies strongly suggest that these com-
pounds behave differently regarding the enol-
imine/keto-amine tautomerization. While 3a and 3b
exist exclusively in the enol-imine tautomer, all
phenylenediamine-based compounds 3c – 3e, have
structures that lie between the two enol-imine and
keto-amine tautomers due to a relatively strong
intramolecular hydrogen bond. The stretching fre-
quencies of the imine (C=N) group are observed
at 1614, 1618 and 1616 cm−1 for 3c, 3d and 3e, re-
spectively. Compound 3c has the least C=N stretching
frequency which means it has the strongest hydrogen
bonding. The corresponding bands for 3a and 3b were
proton of 1 disappeared through the reaction with di-
1
amines. Instead, the H NMR spectra of 3b – 3d ex-
hibit signals at about δ = 8.50 – 9.07 ppm due to the
imine proton.
In the 1H NMR spectra of 3c and 3d the salicylic hy-
droxyl proton was observed at 10.25 and 10.38 ppm,
respectively. The OH signal disappeared after the
addition of deuterated water [5, 11, 18]. The chem-
ical shift of the hydroxyl group is a measure of
the hydrogen bonding/transfer ability of azo-azometh-
ine dyes [9, 21, 22]. The presence of the OH signal
at 10 – 11 ppm for 3c and 3d indicates intramolecu-
lar hydrogen bonding [9] while a remarkable shift to-
ward 14 ppm in 3b indicates the presence of the enol-
imine tautomer. This signal disappeared very slowly
and instead, a signal in the range 5 – 6 ppm appeared
due to an NH proton which suggests the enol-imine to
keto-amine tautomerization.
observed at higher frequencies at 1635 and 1631 cm−1
,
respectively, suggesting the double bond character of
the C=N bond.
The OH stretching frequency is observed at 3429
(3a) and 3469 cm−1 (3b). In contrast, the correspond-
ing band does not appear in the spectra of phenylene-
diamine and 1. This result confirms our postulate re-
In the 1H NMR spectrum of 3c, the imine proton sig-
garding the presence of a relatively strong intramolec- nal splits into a doublet due to a coupling with the hy-
ular hydrogen bond in compounds 3c – 3e. A simi- droxyl proton. Indeed, as a result of a relatively strong
lar situation has already been reported in salicylalde- hydrogen bond, this proton is sensed by the hydroxyl
hyde [19, 20].
proton which is now located mainly on the imine ni-
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162
K. Hamidian et al. · Azo-azomethine Compounds
trogen. In other words, 3c prefers the keto-amine tau-
tomer over the enol-imine tautomer.
UV/Vis spectrophotometry
Band assignment
The UV/Vis spectrum of 1 displays mainly two
bands at r. t. The first band located at 258 – 270 nm
can be assigned to the π → π∗ transition of the aro-
matic ring [11]. The second band located at 360 –
370 nm corresponds to a π → π∗ transition involving
the π electrons of the azo group [12, 23]. The latter
band confirms the existence of the azo tautomer since
in case of the keto-amine tautomer, there should be an
absorption band at about 450 nm or higher [18]. These
two absorption bands shift to higher wave lengths in
the azo-azomethines 3a – 3e due to the extension of the
conjugated system as a result of coupling.
The electronic absorption spectra of the azo-azo-
methine compounds were investigated in organic sol-
vents of different polarity. The results show that
there are three characteristics absorption bands in
all studied solvents except DMSO. The first band at
about 230– 290 nm is due to a π → π∗ electron tran-
sition of the aromatic ring [11, 12]. The second band
at 370 – 395 nm corresponds to a π → π∗ transi-
tion involving the π-electrons of the azo and azome-
thine groups [12, 23]. The broad band observed in
the range 470 – 580 nm can be assigned to an in-
tramolecular charge transfer interaction involving the
whole molecule [12]. The strong broadness of the in-
termolecular CT band supports the existence of an
enol-imine/keto-amine tautomeric equilibrium origi-
nating from the OH group in ortho-position to the ni-
trogen of the imine group. Thus the CT band may be
Fig. 2. Absorption spectra of the prepared dyes 3a – 3e in
DMF.
considered as a composite band resulting from the ab-
sorption of the two equilibrium species. The absorp-
tion in the lower energy region is due to the enol-imine
form while the one in the higher energy region can be
attributed to the keto-amine species [12]. This behav-
ior seems to be quite common for azo or azomethine
compounds having a hydroxy group in ortho-position
to the N=N or C=N bond on the aromatic ring [11, 12].
For azo-azomethines 3a and 3b, the second absorp-
tion band is more intense than the third one indicating
the existence of the enol-imine tautomer. The relative
intensity of this band decreases slowly after a long pe-
riod of time indicating a relatively slow tautomeriza-
tion into the corresponding keto-amine tautomer. The
presence of an isosbestic point in the UV/Vis spectrum
of 3b strongly supports such a slow tautomerization
(Fig. 1).
For azo-azomethines 3c – 3e, the third band at
about 470 – 580 is more intense with respect to the sec-
ond one indicating an intramolecular hydrogen bond
for these compounds (Fig. 2).
Solvatochromic behavior
The electronic absorption spectra of the com-
pounds were recorded in four organic solvents, DMSO,
DMF, CH2Cl2, and THF, at a concentration of ap-
proximately 2 × 10−5 M in the range from 200
to 700 nm. The results indicate that the absorp-
tion bands at 370 – 395 nm are solvent-dependent
and shift toward higher wavelengths with increas-
ing solvent polarity (positive solvatochromism) in
all solvents, except DMSO (Table 4). The different
behavior in DMSO may be due to its higher ba-
sicity and/or the change of solvation of the solute
Fig. 1. The spectral change of 3b and the occurrence of an
isosbestic point due to slow tautomerization in DMF solu-
tion.
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K. Hamidian et al. · Azo-azomethine Compounds
163
Table 4. Absorption spectral data of 3a – 3e in various or-
ganic solvents; λmax (nm) (ε dm−1 in mol−1 cm−1).
and hence will be more stabilized in more polar sol-
vents [12].
Compound
DMSO
DMF
CH2Cl2
THF
In this work, five novel azo-azomethine compounds
shown in Scheme 3 were synthesized from the reac-
tion of 1-(3-formyl-4-hydroxyphenylazo)-4-nitrobenz-
ene with a series of diamines. According to the spec-
troscopic data, the primary azo compound exists in
the azo form in solution and in the solid. Aliphatic
diamine-based azo-azomethine compounds favor the
enol-imine tautomer while aromatic diamine-based
compounds have structures that lie between the two
enol-imine/keto-amine tautomers due to a relatively
strong hydrogen bond between the nitrogen atom of
the azomethine group, and the hydroxyl proton. In ad-
dition, spectrophotometric studies in different solvents
indicate a positive solvatochromism.
3a
273 (37900) 271 (37900) 233 (55000) 238 (41900)
393 (33300) 392 (51100) 281 (39500) 277 (30100)
469 (19800) 485 (33900) 382 (69700) 382 (55800)
477 (8300) 473 (14400)
3b
3c
3d
3e
258 (44300) 275 (26100) 233 (58500) 487 (5200)
382 (44500) 389 (38000) 282 (47700) 280 (25400)
477 (14700) 484 (22100) 384 (75900) 382 (52700)
484 (25100) 487 (5200)
275 (27600) 298 (14300) 234 (44700) 436 (27800)
382 (40000) 387 (18800) 277 (32600) 305 (65000)
554 (37500) 384 (48400) 436 (27800)
547 (6600)
277 (44000) 279 (32700) 236 (69500) 238 (40200)
385 (76000) 393 (49400) 276 (35700) 272 (41800)
581 (68300) 382 (77900) 370 (83600)
511 (5800) 509 (7100)
275 (41200) 266 (29200) 230 (30300) 238 (36500)
377 (55400) 393 (34000) 278 (21400) 278 (39100)
543 (43600) 384 (35200) 382 (57900)
512 (5600) 509 (8200)
Experimental Section
Materials and measurements
All reagents and solvents were used as supplied by Merck
and were used without further purification. 1-(3-formyl-
4-hydroxyphenylazo)-4-nitrobenzene (1) was prepared ac-
cording to a literature procedure [17]. NMR spectra were
recorded on a Bruker 200 MHz spectrometer. Elemental
analyses were performed on an EURO 3000 instrument. Ab-
sorbance spectra were recorded using a spectrophotometer
Agilen 8453 equipped with a thermostated bath (Huber poly-
stat cc1); th◦e temperature of the cell holder was maintained
at 25 0.1 C. FT-IR spectra were recorded on a WQF-510
spectrophotometer in the region of 3000 – 400 cm−1 on KBr
pellets. Melting points of all compounds were determined on
an Electrothermal apparatus.
General procedure for the synthesis of 3a – 3e
Fig. 3. Effect of solvent on the relative stability of the in-
A solution of diamine 2 (2a – 2e) (2 mmol) in absolute
EtOH (10 mL) was added to a stirring solution of 1-(3-form-
yl-4-hydroxyphenylazo)-4-nitrobenzene (1) (4 mmol) in ab-
solute EtOH during a period of 30 min at 50 ◦C. The mixture
was heated in a water bath for 2 h at 80 ◦C with stirring, then
cooled and let to stand at ambient temperature. The product
was collected by filtration, washed successively with diethyl
ether and dried in air. Physical and spectroscopic data: Ta-
tramolecular hydrogen bonding in 3a.
molecules on going from the ground to the excited
state [24]. A similar red shift has also been ob-
served for the intramolecular CT band appearing in
the range 470 – 580 nm (Fig. 3). This positive solva-
tochromism may be explained on the principle that
the excited state is more polar than the ground state bles 1 – 4.
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