Synthesis, structure and solvatochromic properties of 3-cyano-4,6-diphenyl- 5-(3-and 4-substituted phenylazo)-2-pyridones

Article abstract of DOI:10.2298/JSC091009074A

A series of some new pyridone arylazo dyes was synthesized from the corresponding diazonium salts and 3-cyano-4,6-diphenyl-2-pyridone using the classical reaction for the synthesis of the azo compounds. The structures of these dyes were confirmed by UV-Vis, FT-IR and ¹H-NMR spectroscopic techniques. The solvatochromism of the dyes was evaluated with respect to visible absorption properties in various solvents. The effects of solvent dipolarity/polarizability and solvent/solute hydrogen bonding interactions were analyzed by means of the linear solvation energy relationship concept proposed by Kamlet and Taft. The 2-pyridone/2-hydroxypiridine tautomeric equilibration was found to depend on the substituents as well as on the solvents.

Full text of DOI:10.2298/JSC091009074A

J. Serb. Chem. Soc. 75 (8) 1019–1032 (2010)
JSCS–4027
UDC 667.021+547.821:66.081.2.004.12:
54–145.55:541.621
Original scientific paper
Synthesis, structure and solvatochromic properties of 3-cyano-
-4,6-diphenyl-5-(3- and 4-substituted phenylazo)-2-pyridones
1
1
#
1#
*
ADEL S. ALIMMARI , ALEKSANDAR D. MARINKOVIĆ , DUŠAN Ž. MIJIN ,
NATAŠA V. VALENTIĆ1#, NINA TODOROVIĆ2 and GORDANA S. UŠĆUMLIĆ1#
1Department of Organic Chemistry, Faculty of Technology and Metallurgy, University of
Belgrade, Karnegijeva 4, P.O. Box 3503, 11120 Belgrade and 2Institute for Chemistry,
Technology and Metallurgy, Njegoševa 12, 11000 Belgrade, Serbia
(Received 9 October 2009, revised 10 May 2010)
Abstract: A series of some new pyridone arylazo dyes was synthesized from
the corresponding diazonium salts and 3-cyano-4,6-diphenyl-2-pyridone using
the classical reaction for the synthesis of the azo compounds. The structures of
these dyes were confirmed by UV–Vis, FT-IR and 1H-NMR spectroscopic
techniques. The solvatochromism of the dyes was evaluated with respect to
visible absorption properties in various solvents. The effects of solvent dipola-
rity/polarizability and solvent/solute hydrogen bonding interactions were ana-
lyzed by means of the linear solvation energy relationship concept proposed by
Kamlet and Taft. The 2-pyridone/2-hydroxypiridine tautomeric equilibration
was found to depend on the substituents as well as on the solvents.
Keywords: arylazo pyridone dyes; absorption spectra; solvent effect; substi-
tuent effect; tautomeric equilibration.
INTRODUCTION
Azo dyes are the most widely used compounds in various fields, such as
dyeing of textile fibres, colouring of different materials, in biological–medical
1–6
studies and advanced applications in organic synthesis. The success of azo co-
lorants is due to the simplicity of their synthesis by diazotization and azo coup-
ling, to the almost innumerable possibilities presented by variation on the diazo
compounds and coupling components, to the generally high molar extinction co-
7
efficient and to the medium to high light and wet fastness properties. Pyridone
derivatives are relatively recent heterocyclic intermediates for the preparation of
arylazo dyes. The azo pyridone dyes give bright hues and are suitable for the
8
dyeing of polyester fabrics. The physico-chemical properties of arylazo pyri-
*Corresponding author. E-mail: marinko@tmf.bg.ac.rs
# Serbian Chemical Society member.
doi: 10.2298/JSC091009074A
1019
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ALIMMARI et al.
done dyes are closely related to their tautomerism. Determination of azo-hydra-
zone tautomerism both in the solid state and solution phase is quite interesting
both from theoretical and practical standpoints, since the tautomers have different
technical properties and dyeing performances. Several investigations on substi-
9–11
tuted arylazo pyridones have been performed and reviewed.
It was concluded
that the equilibrium between the two tautomers is influenced by the structure of
the compounds and the solvent used. In previous publications, the absorption
spectra of 3-cyano-6-hydroxy-4-methyl-5-(4-substituted phenylazo)-2-pyridones
in different solvents were studied and the results showed that these dyes exist in
the hydrazone form in the solid state and in the solvent DMSO-d , while equi-
6
12
librium existed between the hydrazone and the azo form in different solvents.
The obtained dyes exhibit their colour hue in the range from orange to red-orange.
Recently, the synthesis of seventeen 3-cyano-4,6-dimethyl-5-(3- and 4-subs-
13
tituted phenylazo)-2-pyridones was reported. The 2-pyridone/2-hydroxy-pyridi-
ne tautomeric equilibration was found to depend on the substituents as well as on
the solvents. The obtained results showed that the replacement of a hydroxy group
in position six in the pyridone ring of 3-cyano-6-hydroxy-4-methyl-5-(3- and
4-substituted phenylazo)-2-pyridones by a methyl group influenced a change of
the azo group sensitivity to solvent effects in comparison with 3-cyano-4,6-
-dimethyl-5-(4-substituted phenylazo)-2-pyridones. The obtained dyes exhibited a
light orange to brown-orange colour hue.
In this work, the synthesis of nine new 3-cyano-4,6-diphenyl-5-(3- and 4-sub-
stituted phenylazo)-2-pyridones, their UV–Vis absorption spectra (200–800 nm) in
fifteen solvents of different polarity and the relationship between colour and
constitution of these dyes are reported. The effects of solvent and substituent on
the 2-pyridone/2-hydroxypyridine tautomeric equilibrium are also reported.
RESULTS AND DISCUSSION
All of the investigated arylazo pyridone dyes were synthesized from the cor-
responding diazonium salts and 3-cyano-4,6-diphenyl-2-pyridone using the clas-
14
sical reaction for the synthesis of azo compounds. The chemical structure and
the purity of the obtained compounds (which, to the best of our knowledge, have
1
not been registered in the literature) were confirmed by melting points, and H-
-NMR, FT-IR and UV spectral data. The characterization data are given below.
3-Cyano-5-[(4-methoxyphenyl)azo]-4,6-diphenyl-2-pyridone (1). Yield: 54
–1
%. Colour: red brown crystals; m.p.: 264–268 °C. IR (KBr, cm ): 3436 (N–H),
1
2212 (C–N), 1692 (C=O). H-NMR (200 MHz, DMSO-d , δ / ppm): 3.35 (3H, s,
6
OCH ), 6.88–6.99 (2H, d, ArH), 7.30–7.45 (2H, m, ArH), 7.55–7.66 (6H, m,
3
ArH), 7.70–7.90 (4H, m, ArH), 11.67 (1H, s, pyridone–NH).
3-Cyano-5-[(4-methylphenyl)azo]-4,6-diphenyl-2-pyridone (2). Yield: 29 %.
–1
Colour: dark yellow crystals; m.p.: 243–246 °C. IR (KBr, cm ): 3376 (N–H),
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3-CYANO-4,6-DIPHENYL-5-(3- AND 4-SUBSTITUTED PHENYLAZO)-2-PYRIDONES
1021
1
2212 (C–N), 1681 (C=O). H-NMR (200 MHz, DMSO-d , δ / ppm): 2.28 (3H, s,
6
CH ), 7.08–7.22 (4H, d, ArH), 7.40–7.68 (6H, m, ArH), 7.75–8.08 (4H, m, ArH),
3
11.95 (1H, s, pyridone–NH).
3-Cyano-4,6-diphenyl-5-phenylazo-2-pyridone (3). Yield: 58 %. Colour: oran-
–1
ge crystals; m.p.: 241–242 °C. IR (KBr, cm ): 3371 (N–H), 2212 (C–N), 1666
1
(C=O). H-NMR (200 MHz, DMSO-d , δ / ppm): 7.05–7.20 (3H, m, ArH), 7.25–
6
–7.39 (4H, m, ArH), 7.40–7.49 (6H, m, ArH), 7.50–7.70 (2H, m, ArH), 11.83
(1H, s, pyridone–NH).
3-cyano-5-[(3-methoxyphenyl)azo]-4,6-diphenyl-2-pyridone (4). Yield: 64 %.
–1
Colour: orange crystals; m.p.: 301–303 °C. IR (KBr, cm ): 3387 (N–H), 2212
1
(C–N), 1706 (C=O). H-NMR (200 MHz, DMSO-d , δ / ppm): 3.79 (3H, s, OCH ),
6
3
6.67–6.79 (3H, m, ArH), 7.20–7.33 (3H, m, ArH), 7.45–7.70 (6H, m, ArH), 7.90–
–8.03 (2H, d, ArH), 11.83 (1H, s, pyridone–NH), 14.04 (1H, s, pyridine–OH).
5-[(4-Chlorophenyl)azo]-3-cyano-4,6-diphenyl-2-pyridone (5). Yield: 36 %.
–1
Colour: brown orange crystals; m.p.: 253–255 °C. IR (KBr, cm ): 3355 (N–H),
1
2222 (C–N), 1681 (C=O). H-NMR (200 MHz, DMSO-d , δ / ppm): 7.20–7.39
6
(2H, d, ArH), 7.40–7.55 (4H, m, ArH), 7.60–7.75 (6H, m, ArH), 7.90–8.00 (2H,
m, ArH), 12.27 (1H, s, pyridone–NH), 13.75 (1H, s, pyridine–OH).
5-[(4-Bromophenyl)azo)-3-cyano-4,6-diphenyl-2-pyridone (6). Yield: 69 %.
–1
Colour: red brown crystals; m.p.: 292–293 °C. IR (KBr, cm ): 3351 (N–H),
1
2220 (C–N), 1681 (C=O). H-NMR (200 MHz, DMSO-d , δ / ppm): 7.45–7.55
6
(2H, d, ArH), 7.58–7.90 (12H, m, ArH), 11.76 (1H, s, pyridone–NH), 12.75 (1H,
s, pyridine–OH).
3-Cyano-5-[(4-iodophenyl)azo]-4,6-diphenyl-2-pyridone (7). Yield: 62 %.
–1
Colour: dark brown crystals; m.p.: 276–281 °C. IR (KBr, cm ): 3454 (O–H),
1
3356 (N–H), 2222 (C–N), 1691 (C=O). H-NMR (200 MHz, DMSO-d , δ / ppm):
6
7.05–7.22 (2H, d, ArH), 7.25–7.46 (2H, d, ArH), 7.52–7.70 (6H, m, ArH), 7.74–
–7.98 (4H, m, ArH), 11.75 (1H, s, pyridone–NH), 12.57 (1H, s, pyridine–OH).
3-Cyano-5-[(3-nitrophenyl)azo]-4,6-diphenyl-2-pyridone (8). Yield: 65 %.
–1
Colour: dark orange crystals; m.p.: 282–285 °C. IR (KBr, cm ): 3484 (O–H),
1
3361 (N–H), 2222 (C–N), 1688 (C=O). H-NMR (200 MHz, DMSO-d , δ / ppm):
6
7.40–7.70 (8H, m, ArH), 7.76–8.30 (4H, m, ArH), 8.32–8.40 (2H, d, ArH), 11.80
(1H, s, pyridone–NH), 12.15 (1H, s, pyridine–OH).
3-Cyano-5-[(4-nitrophenyl)azo]-4,6-diphenyl-2-pyridone (9). Yield: 60 %.
–1
Colour: dark brown crystals; m.p.: 278–279 °C. IR (KBr, cm ): 3472 (O–H),
1
3350 (N–H), 2232 (C–N), 1686 (C=O). H-NMR (200 MHz, DMSO-d , δ / ppm):
6
7.60–7.74 (4H, m, ArH), 7.76–7.94 (6H, m, ArH), 7.96–8.12 (2H, m, ArH), 8.15–
–8.40 (2H, m, ArH), 12.08 (1H, s, pyridine–OH).
The arylazo pyridone dyes prepared in this work may exist in two tautomeric
forms (Fig. 1). The infrared spectra showed an intense carbonyl band in the
–1
–1
region 1666–1692 cm and an intense N–H band in the region 3376–3495 cm ,
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ALIMMARI et al.
which suggest that these compounds predominantly exist in the 2-pyridone tauto-
1
meric form (Fig. 1, A) in the solid state. The H-NMR spectra of the dyes in
DMSO-d exhibit a signal for the –NH proton at the pyridone ring in the range δ
6
11.67–12.15 ppm. However, for azo dyes with electron-accepting substituents in
1
the phenyl group, the H-NMR spectra showed a signal in the range δ 12.08–
–14.04 ppm, corresponding to the OH proton of enol form (Fig. 1, B).
Fig. 1. Structure of dyes 1–9 and the equilibrium between 2-pyridone form (A) and
2-hydroxypyridine form (B) of 3-cyano-4,6-diphenyl-5-(3- and 4-substituted phenylazo)-
-2-pyridones and the canonical structure (C).
Solvent effects on absorption spectra of the dyes
Since the tautomeric equilibria strongly depend on the nature of the media,
the behaviour of arylazo pyridone dyes in various protic and aprotic solvents was
studied. For this purpose, the absorption spectra of the pyridone dyes (1–9) were
–4
–3
measured in fifteen various solvents at a concentration 10 mol dm , and the
ultraviolet absorption maxima of the electronic transitions involving the free non-
-bonding electrons of the azo group are given in Table I. It was found that the
absorption maxima of these dyes are strongly solvent dependent and vary with
solvent polarity. The dyes generally showed bathochromic shifts as the polarity
of the solvents was increased. The characteristic absorption spectra of investi-
gated azo dyes in ethanol and N,N-dimethylacetamide are shown in Figs. 2 and 3,
respectively.
The equilibrium between the two tautomers is influenced by the structure of
the investigated compound and the solvent used. In protic and non-dipolar aprotic
solvents (alcohols, dioxane, tetrahydrofuran and acetonitrile) azo pyridone dyes
with electron-donating and moderate electron-accepting substituents showed one
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3-CYANO-4,6-DIPHENYL-5-(3- AND 4-SUBSTITUTED PHENYLAZO)-2-PYRIDONES
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absorption band arising from the 2-pyridone tautomeric form. Azo pyridone dyes
with strong electron-accepting substituents exhibited two absorption bands in these
solvents (Fig. 2). In N,N-dimethylformamide, N,N-dimethylacetamide and dime-
thyl sulphoxide solvents, all the investigated azo pyridone dyes existed as two
tautomers, due to the high basicities and high relative permitivities of these sol-
vents (Table I and Fig. 3). The molecular extinction coefficients (ε ) are higher
max
in comparison to the 3-cyano-4,6-dimethyl-5-(4-substituted phenylazo)-2-pyrido-
13
3
–1
–1
nes reported previously, and vary from 6000 to 38000 dm mol cm (not lis-
ted in Table I). The ratios of the molecular extinction coefficients of the 2-pyri-
done form A and the 2-hydroxypyridine form B are listed in Table I for solvents
and substituents where tautomeric equilibrium was found.
TABLE I. UV–Vis spectral data (λmax / nm and ε1/ε2) of dyes 1–9 in different solvents
Solvent or
Substituent
Methanol
4-OCH3 4-CH3
H
3-OCH3 4-Cl 4-Br 4-I 3-NO2 4-NO2
No.
1
370
361 375
358
356 355 360, 340,
443 473
372,
551
–
373
–
–
–
360
–
–
2.24 1.15
1.71
371,
562
2
3
4
5
6
7
8
9
Ethanol
364 374
359 356 359, 353,
456 484
–
375
–
–
–
359
–
–
1.94 0.69
1.60
373,
565
1.56
371,
572
1.58
374,
566
1.64
370,
585
3.32
370,
578
1.80
370,
420
2.05
364,
571
Propan-1-ol
Propan-2-ol
Butan-1-ol
361 374
355 354 361, 342,
452 484
–
356 356 360, 355,
448 487
–
356 346 362, 353,
456 485
–
356 357 357, 342,
439 403
–
358 357 360, 340,
448 485
–
355 360 360, 330,
429 403
–
–
375
–
–
–
360
–
1.68 0.79
360 371
–
375
–
–
–
360
–
2.02 0.63
364 373
–
372
–
–
–
360
–
1.43 0.74
2-Methyl-2-
-propanol
360 375
–
373
–
–
–
361
–
1.93 0.87
Butan-2-ol
Dioxane
360 353
–
368
–
–
–
353
–
1.79 0.84
358 350
–
364
–
–
–
350
–
1.08 1.06
Ethyl acetate
357 348
348 351 355, 332,
431 414
–
365
–
–
–
353
–
–
1.25 2.46
2.90
375,
602
10 Tetrahydrofuran
356 352
355 354 350, 330,
439 412
–
–
–
–
–
–
0.99 1.18
1.47
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ALIMMARI et al.
TABLE I. Continued
Solvent or
No.
4-OCH3 4-CH3
H
369
–
3-OCH3 4-Cl 4-Br 4-I 3-NO2 4-NO2
Substituent
11
12
13
14
15
Acetonitrile
368
356
–
352
351 352 341, 341, 368,
467 483 575
1.20 0.73 0.79
352, 354, 355, 340, 375,
488 479 461 490 588
–
–
–
–
Acetone
365,
505
357, 350,
493 439
353,
476
9.40
364,
493
4.11
362,
488
4.47
350,
485
5.51
21.50
376,
495
20.02 2.42
368, 380,
495
4.55 6.49 1.11 0.71 0.79
360, 361, 364, 348, 386,
Dimethyl
sulphoxide
493
495 494 495 495
592
14.58
16.00 10.12
3.18 2.45 0.98 0.53 0.78
403, 406, 372, 405, 521,
N,N-Dimethyl- 377,510 370, 383,
formamide
493
493
493 493 497 495
588
8.69
373,
505
14.2 8.99
360, 370,
500
2.67 1.98 0.42 0.94 1.35
357, 355, 364, 390, 526,
N,N-Dimethyl-
acetamide
502
493 492 500 497
595
10.62
14.40 10.46
2.65 3.03 0.38 0.51 0.93
Fig. 2. Absorption spectra of the arylazo pyridone dyes (1–9) in ethanol.
By considering the equilibrium between the 2-pyridone and 2-hydroxypyri-
15
dine tautomeric forms in solvents of different polarity, it was found that in-
16
creasing solvent polarity shifted the equilibrium towards the pyridone form.
This form is more dipolar than the hydroxy form due to the contribution of the
charge-separated mesomeric form (Fig. 1, C). The present results are in agre-
ement with this explanation. Furthermore, the hydrogen-bonding ability of the sol-
vent plays an important role since hydrogen-bond donors tend to stabilize the oxo
form (Fig. 1, A), whereas hydrogen-bond acceptors stabilize the hydroxy form
(Fig. 1, B).
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3-CYANO-4,6-DIPHENYL-5-(3- AND 4-SUBSTITUTED PHENYLAZO)-2-PYRIDONES
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Fig. 3. Absorption spectra of the arylazo pyridone dyes (1–9) in N,N-dimethylacetamide.
The effects of solvent dipolarity/polarizability and hydrogen-bonding on the
absorption spectra of 3-cyano-4,6-diphenyl-5-(3- and 4-substituted phenylazo)-2-
17,18
-pyridones, and many other compounds,
can be interpreted by means of the
linear solvation energy relationship (LSER) concept using a Kamlet-Taft Equa-
19
tion of the type:
ν = ν + sπ* + bβ + aα
(1)
0
where π* is a measure of the solvent dipolarity/polarizability, β is the scale of the
solvent hydrogen-bond acceptor (HBA) basicities, α is the scale of the solvent
hydrogen-bond donor (HBD) acidities and ν is the regression value of the solute
0
property in the reference solvent cyclohexane. The regression coefficients s, b
and a in Eq. (1) measure the relative susceptibilities of the solvent-dependent so-
lute property (absorption frequencies) to the indicated solvent parameters. The
20
solvent parameters are given in Table II. The correlations of the spectroscopic
data for 2-pyridone tautomeric form were realised by means of multiple linear
regression analysis. It was found that absorption frequencies ν for the azo dyes
1
(Table I) in ten selected solvents (Table III) showed satisfactory correlation with
the π*, β and α parameters. The results of the multiple regressions are presented
in Tables III and IV, and coefficients ν , s, b and a (Table III) have confidence
0
intervals at a level of significance of 95 %. The degree of success of Eq. (1) is
shown in Fig. 4 by means of a plot of ν
calculated vs. ν
observed in dif-
max
max
ferent solvents (the value of ν
for 4-NO substituted azo dye in ethyl acetate
max
2
was excluded from correlation).
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ALIMMARI et al.
TABLE II. Solvent parameters17
No.
Solvent
π*
α
β
1
2
3
4
5
6
7
8
Methanol
Ethanol
Propan-1-ol
Propan-2-ol
Butan-1-ol
0.60
0.54
0.52
0.48
0.47
0.41
0.40
0.55
0.55
0.58
0.75
0.88
0.71
1.00
0.88
0.62
0.77
0.83
0.95
0.88
1.01
0.80
0.37
0.45
0.55
0.31
0.76
0.48
0.76
0.69
0.93
0.83
0.78
0.76
0.79
0.68
0.69
0
0
0
0.19
0
0.08
0
2-Methyl-2-propanol
Butan-2-ol
Dioxane
Ethyl acetate
Tetrahydrofuran
Acetonitrile
9
10
11
12
13
14
15
Acetone
Dimethyl sulphoxide
N,N-Dimethylformamide
N,N-Dimethylacetamide
0
TABLE III. Regression fits to the solvatochromic parameters (Eq. (1))
v0×10-3 s×10-3
b×10-3
a×10-3
Solvents used in the
correlationsd
Substituent
4-OCH3
Ra
Sb
Fc
cm-1
cm-1
cm-1
cm-1
28.67
–1.49
–0.75
–0.68 0.990 0.05 102 2–6, 9–11, 13, 15
(±0.11) (±0.14) (±0.11) (±0.08)
29.71 –1.64 –1.10 –0.44 0.934 0.13 17
(±0.32) (±0.33) (±0.25) (±0.17)
31.41 –3.86 –1.48 –1.71 0.993 0.13 142 1–5, 8–10, 13, 15
(±0.23) (±0.34) (±0.33) (±0.17)
30.00 –1.54 –1.31 –0.43 0.995 0.04 204
(±0.09) (±0.11) (±0.07) (±0.05)
29.84 –1.36 –0.87 –0.40 0.991 0.04 111 1, 3–6, 9, 11–13, 15
(±0.10) (±0.11) (±0.07) (±0.06)
35.92 –8.65 –5.03 0.99
(±1.00) (±1.14) (±0.75) (±0.43)
30.85 –1.22 –2.89 –0.29 0.933 0.25 14 1–3, 5, 7, 10–13, 15
(±0.60) (±0.67) (±0.53) (±0.34)
37.62 –8.68 –6.56 0.89
(±0.93) (±1.19) (±0.89) (±0.59)
41.88 –19.49 –7.68 1.63
(±0.90) (±1.00) (±0.86) (±0.45)
4-CH3
H
1–7, 10–13
3-OCH3
4-Cl
4-Br
4-I
1–6, 9, 11–13
0.970 0.36 28
1–6, 9, 10, 14, 15
3-NO2
4-NO2
a
0.979 0.46 46
1, 2, 4, 5, 7–9,
11, 14, 15
0.996 0.37 235 1–6, 9, 10, 14, 15
b
c
d
Correlation coefficient; standard error of the estimate; Fischer’s test; solvent numbers are given in Table I
The negative signs of the s, b and a coefficients for all the azo dyes (except
the coefficient a for 4-Br, 3-NO and 4-NO substituted azo dyes, Table III) indi-
2
2
cate a bathochromic shift with increasing solvent dipolarity/polarizability and
solvent hydrogen-bond acceptor basicity and hydrogen-bond donor acidity. This
suggests stabilization of the electronic excited state relative to the ground state.
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3-CYANO-4,6-DIPHENYL-5-(3- AND 4-SUBSTITUTED PHENYLAZO)-2-PYRIDONES
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The positive sign of the a coefficient for the azo dyes with electron-accepting sub-
stituents, 4-Br, 3-NO and 4-NO , indicates a hypsochromic shift with increasing
2
2
solvent hydrogen-bond donor acidity. These results show that the solvent effect
on UV–Vis absorption spectra of pyridone azo dyes is very complex and strongly
dependant on the nature of the groups present on the pyridone nucleus.
TABLE IV. Percentage contribution of the solvatochromic parameters
Substituent
Pπ* / %
Pβ / %
Pα / %
4-OCH3
4-CH3
H
3-OCH3
4-Cl
4-Br
4-I
3-NO2
4-NO2
51
51
55
47
52
59
28
54
67
26
35
21
40
33
34
66
41
27
23
14
24
13
15
7
6
5
6
Fig. 4. The plot of vmax observed against vmax calculated from Eq. (1) for the
3-cyano-4,6-diphenyl-5-(3- and 4-substituted phenylazo)-2-pyridones.
Substituent effect on the absorption spectra of the dyes in various solvents
The data from Table I confirm that the positions of the ultraviolet absorption
frequencies depend on the nature of the substituents in the phenyl ring of the azo
dyes. The percentage contributions of the solvatochromic parameters (Table IV)
for the azo dyes with electron-donating and moderate electron-accepting substi-
tuents (4-OCH , 4-CH , H, 3-OCH and 4-Cl) in the phenyl group show that
3
3
3
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ALIMMARI et al.
most of the solvatochromism is due to solvent dipolarity/polarizability and sol-
vent basicity rather than on the solvent acidity. These results are in accordance
with the canonical structure of the 2-pyridone form of these dyes (Fig. 1, C). The
effect of strong electron-accepting substituents (3-NO and 4-NO ) and 4-Br and
2
2
4-I is slightly different and the percentage contributions of the solvatochromic
parameters show decreased proton-donating solvent effects. These results can be
explained by the positive charge on the azo group in the 2-hydroxypyridine tauto-
mer with electron-accepting substituents in the phenyl group (Fig. 5, E), and with
a decreasing hydrogen-bond donating solvent effect when the more dipolar tauto-
+
–
meric form NH –CO transforms into the N=C–OH tautomeric form.
Fig. 5. Resonance effect of electron-donating (structure D) and electron-accepting
(structure E) substituents of the arylazo component of the azo dyes.
In order to explain these results, the absorption frequencies were correlated
or σ + substituent constants:
21
by the Hammett Equation, Eq. (2), using σ
m/p
m/p
v = v + ρσ
(2)
where ρ is the proportionality constant reflecting the sensitivity of the absorption
frequencies to the substituent effects. The substituent constants, σ
and σ +
0
m/p
,
m/p
m/p
measure the electronic effect of the substituents (in a given position, meta or para).
The plot v vs. the σ substituent constants gave a correlation which
max
m/p
showed deviations from the Hammett Equation in all dipolar aprotic solvents
(N,N-dimethylacetamide, excluding the H and 4-NO substituted dyes). How-
2
ever, a linear Hammett correlation was obtained in protic solvents (excluding the
H and 4-NO substituted dyes). A better correlation of v
was obtained with
2
max
the σ + substituent constants than with the σ
constants in all solvents (Figs.
22
m/p
m/p
6 and 7), which indicates extensive delocalization in the azo group (–N=N–). The
existence of the correlation presented in Fig. 6 was interpreted as evidence of the
significant role of substituent effects on the 2-pyridone/2-hydroxypyridine tauto-
merism. Electron-donating and moderate electron-accepting substituents stabilize
2-pyridone tautomeric form, while electron-accepting substituents stabilize the
2-hydroxypyridine tautomeric form. The azo group is an electron-acceptor, hence
the azo group is stabilized by the more electron-donating substituents. These re-
sults are in accordance with the resonance structure of these dyes as shown in
Fig. 5, D.
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3-CYANO-4,6-DIPHENYL-5-(3- AND 4-SUBSTITUTED PHENYLAZO)-2-PYRIDONES
1029
Fig. 6. Hammett correlation of vmax vs. σm+ /p in ethanol.
Fig. 7. Hammett correlation of vmax vs. σm+ /p in N,N-dimethylacetamide.
These results are contrary to the results published in a previous work for
13
3-cyano-4,6-dimethyl-5-(4-substituted phenylazo)-2-pyridones. The obtained re-
sults show that replacement of methyl groups in the pyridone ring of 3-cyano-
4,6-dimethyl-5-(3- and4-substituted phenylazo)-2-pyridones by phenyl groups in-
fluences the change of the sensitivity of the azo group to solvent and substituent
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ALIMMARI et al.
effects compared to 3-cyano-4,6-diphenyl-5-(3- and 4-substituted phenylazo)-2-
-pyridones. Additionally, two phenyl electron rich groups at the 4- and 6-position
of the pyridine ring significantly changes the electronic structures of the highest
populated molecular orbitals, and influences a different spatial arrangement of
the investigated arylazo pyridone dyes in comparison to 3-cyano-4,6-dimethyl-5-
(3- and 4-substituted phenylazo)-2-pyridones. Thus, the absorption maxima of 3-
cyano-4,6-diphenyl-5-(3- and 4-substituted phenylazo)-2-pyridones showed ba-
thochromic effects in comparison with analogous dyes containing two methyl
groups in the pyridone ring. The pyridone azo dyes system, investigated in this
work, absorbed at a higher absorption maxima due to the mobility of the electro-
nic densities across the resonating systems (structures D and E, Fig. 5). The dye
system comprising the phenyl moiety absorbed bathochromically compared to
the other analogues due to the longer conjugation, which stabilized charge sepa-
ration in the corresponding stabilized hybrids (Fig. 5). The obtained dyes exhibit
their colour hue in the range from dark-yellow to brown.
EXPERIMENTAL
General
The chemicals used in the synthesis of all the dyes were obtained from the Fluka
Chemicals Company and were used without further purification. The chemical structure and
the purity of the azo pyridone dyes were confirmed by melting points, and by UV–Vis, FT-IR
and 1H-NMR spectroscopic techniques. All melting points are uncorrected.
The IR spectra were recorded on a Bomem FTIR spectrophotometer, MB-series, in the
form of KBr pellets.
1
The H-NMR spectral measurements were performed on a Varian Gemini 2000 (200
MHz) instrument. The spectra were recorded at room temperature in deuterated dimethyl
1
sulphoxide (DMSO-d6). The chemical shifts in the H-NMR spectra are expressed in ppm
values referenced to TMS (δH = 0 ppm).
The UV–Vis absorption spectra were recorded in the range 200–800 nm on a Shimadzu
1700 spectrophotometer in spectroquality solvents (Fluka) using a concentration of 10-4 mol dm-3.
Preparation of 3-cyano-4,6-diphenyl-5-(3- and 4-substituted phenylazo)-2-pyridones (1–9)
All the investigated azo pyridone dyes were synthesized from the corresponding diazo-
nium salts and 3-cyano-4,6-diphenyl-2-pyridone using the classical reaction for the synthesis
of azo compounds.14
3-Cyano-4,6-diphenyl-2-pyridone was prepared from dibenzoyl methane and cyanoace-
tamide using a modified literature procedure.23 Equimolar amounts of dibenzoyl methane and
cyanoacetamide (10 mmol) were heated under reflux in a 50 % water/ethanol mixture (20
cm3) in the presence of a few drops of piperidine as catalyst for 4 h. The product was purified
by crystallization from ethanol (yield: 35 %, m.p.: 310–311 °C; lit.21 m.p.: 312–313 °C).
The yields of the dyes were in the range 29–69 %. The obtained compounds were puri-
fied by crystallization from acetone and then analyzed.
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3-CYANO-4,6-DIPHENYL-5-(3- AND 4-SUBSTITUTED PHENYLAZO)-2-PYRIDONES
1031
CONCLUSIONS
A series of new pyridone arylazo dyes was synthesized from the correspond-
ing diazonium salts and 3-cyano-4,6-dipenyl-2-pyridone. The solvatochromic
behaviours and substituent effects in various solvents were evaluated. The results
indicated that these dyes were strongly dependent on solvents and generally
showed bathochromic shifts as the polarity of solvents was increased. These dyes
exist dominantly in the 2-pyridone tautomeric form in the solid state. The results
show that electron-donating and moderate electron-accepting substituents stabi-
lized the 2-pyridone tautomeric form in protic and non-dipolar aprotic solvents,
while in dipolar aprotic solvents the existence of two tautomers was observed.
Strong electron-accepting substituents stabilized the 2-hydroxypyridine tautomer
and existence of two tautomers was determined in all investigated solvents.
The satisfactory correlation of the ultraviolet absorption frequencies of the
investigated arylazo pyridone dyes with the general solvatochromic equation in-
dicates that the correct model was selected. It was demonstrated that a solvato-
chromic equation with three solvatochromic parameters π*, β and α can be used
to evaluate the effects of both types of hydrogen bonding and of the solvent di-
polarity/polarizability effect. The obtained results showed that the replacement of
the methyl groups in the pyridone ring of 3-cyano-4,6-dimethyl-5-(3- and 4-subs-
tituted phenylazo)-2-pyridones by phenyl groups in 3-cyano-4,6-diphenyl-5-(3-
and 4-substituted phenylazo)-2-pyridones influenced a change in the sensitivity
of the azo group to solvent and substituent effects. The absorption maxima of the
3-cyano-4,6-diphenyl-5-(3- and 4-substituted phenylazo)-2-pyridones showed ba-
thochromic effects in comparison with analogous dyes containing two methyl
groups in pyridone ring, and exhibited their colour hue in the range from dark
yellow to brown.
Acknowledgements. The authors acknowledge the financial support by the Ministry of
Science and Technological Development of the Republic of Serbia (Project Number 142063).
И З В О Д
СИНТЕЗА, СТРУКТУРА И СОЛВАТОХРОМНА СВОЈСТВА 3-ЦИЈАНО-4,6-ДИФЕНИЛ-5-
(3- И 4-СУПСТИТУИСАНИХ ФЕНИЛАЗО)-2-ПИРИДОНА
ADEL S. ALIMMARI1, АЛЕКСАНДАР Д. МАРИНКОВИЋ1, ДУШАН Ж. МИЈИН1, НАТАША В. ВАЛЕНТИЋ1,
НИНА ТОДОРОВИЋ2 и ГОРДАНА С. УШЋУМЛИЋ1
1Katedra za organsku hemiju, Tehnolo{ko–metalur{ki fakultet, Univerzitet u Beogradu, Karnegijeva 4,
p. pr. 3503, 11120 Beograd i 2Institut za hemiju,tehnologiju i metalurgiju, Njego{eva 12, 11000 Beograd
Серија нових пиридонских арилазо боја је синтетисана реакцијом одговарајућих диазо-
нијум соли и 3-цијано-4,6-дифенил-2-пиридона применом класичне синтезе азо једињења.
Структура синтетисаних боја је потврђена на основу података добијених из UV, FT-IR и 1H-
-NMR спектара. Солватохромна својства боја су процењена у односу на њихову апсорпцију
у видљивом делу спектра у различитим растврачима. Ефекти растварача, диполарност/по-
ларизабилност и водоничне интеракције растварач/растворак, су анализиране применом ли-
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ALIMMARI et al.
неарне корелације солватохромних ефеката предложене од стране Kamlet-а и Taft-а. Тауто-
мерна равнотежа 2-пиридон/2-хидроксипиридин зависи како од ефеката супституента тако и
од утицаја растварача.
(Примљено 9. октобра 2009, ревидирано 10. маја 2010)
REFERENCES
1. H. Zollinger, Color Chemistry; synthesis, properties and application of organic dyes and
pigments, 3rd revised ed., VCH, Weinheim, 1987
2. H. S. Bhatti, S. Seshadri, Color. Technol. 120 (2004) 151
3. K. Tanaka, K. Matsuo, A. Nakanishi, M. Jo, H. Shiota, M. Yamaguchi, S. Yoshino, K.
Kawaguchi, Chem. Pharm. Bulletin 32 (1984) 3291
4. G. J. Hallas, J. Soc. Dyers Colour. 95 (1979) 285
5. A. D. Towns, Dyes Pigm. 42 (1999) 3
6. K. Krishnankutty, M. B. Ummathur, P. Ummer, J. Serb. Chem. Soc. 74 (2009) 1273
7. C. C. Chen, I. J. Wang, Dyes Pigm. 15 (1991) 69
8. A. Cee, B. Horakova, A. Lyčka, Dyes Pigm. 9 (1988) 357
9. P. Y. Wang, I. Y. Wang, Textile Res. J. 60 (1990) 519
10. Q. Peng, M. Li, K. Gao, L. Cheng, Dyes Pigm. 15 (1991) 236
11. N. Ertan, F. Eyduran, P. Gurkan, Dyes Pigm. 27 (1995) 313
12. G. Ušćumlić, D. Mijin, N. Valentić, V. Vajs, B. Sušić, Chem. Phys. Lett. 397 (2004) 148
13. D. Mijin, G. Ušćumlić, N. Perišić-Janjić, N. Valentić, Chem. Phys. Lett. 418 (2006) 223
14. Vogel’s Textbook of Practical Organic Chemistry, 4th ed., Longman, London, 1978, p. 715
15. P. Beak, J. Couvngton, J. M. White, J. Org. Chem. 45 (1980) 1347
16. C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, Wiley-VCH, Weinheim,
2004, p. 113
17. N. D. Divjak, N. R. Banjac, N. V. Valentić, G. S. Ušćumlić, J. Serb. Chem. Soc. 74
(2009) 1195
18. S. Ž. Drmanić, A. D. Marinković, B. Ž. Jovanović, J. Serb. Chem. Soc. 74 (2009) 1359
19. M. J. Kamlet, J. M. Abboud, R. W. Taft, Prog. Phys. Org. Chem. 13 (1981) 485
20. M. J. Kamlet, J. L. M. Abboud, M. H. Abraham, R. W. Taft, J. Org. Chem. 48 (1983)
2877
21. L. P. Hammett, J. Am. Chem. Soc. 59 (1937) 96
22. H. C. Brown, Y. Okamoto, J. Am. Chem. Soc. 80 (1958) 4979
23. J. M. Bobbit, D. A. Skola, J. Org. Chem. 25 (1960) 560.
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