A facile route to the new triazene dyes based on substituted pyrazolidin-3,5-dione derivatives

Article abstract of DOI:10.1016/j.dyepig.2011.07.023

Several substituted triazenes dyes were synthesized by coupling functionalized pyrazolidin-3,5-dione derivatives, to various heteroarene azides in excellent yields (98%). Electron delocalization between the two coupled components of these triazene dyes wa

Full text of DOI:10.1016/j.dyepig.2011.07.023

Dyes and Pigments 92 (2012) 1212e1222
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A facile route to the new triazene dyes based on substituted
pyrazolidin-3,5-dione derivatives
,
b
,
c
a,b
Jalal Isaad a , Fouad Malek , Ahmida El Achari
*
a University Lille Nord de France, F-5900 Lille, France
b ENSAIT, GEMTEX, F-59056 Roubaix, France
c Faculté des Sciences, Université Mohamed Premier, Laboratoire de Chimie Organique, Macromoléculaire et Produits Naturels,
Equipe de Chimie Bioorganique et Macromoléculaire, URAC 25, Bd Mohamed VI, BP 717, 60 000 Oujda, Morocco
a r t i c l e i n f o
a b s t r a c t
Article history:
Several substituted triazenes dyes were synthesized by coupling functionalized pyrazolidin-3,5-dione
derivatives, to various heteroarene azides in excellent yields (98%). Electron delocalization between
the two coupled components of these triazene dyes was studied using UVevis spectra and NMR spec-
troscopy. Their thermolysis was investigated and by using an isotopically labeled triazene, the mecha-
nism decomposition reaction was also identified. The protolysis of these triazenes was evaluated and
showed that they are highly stable and even in strongly acidic medium.
Received 14 April 2011
Received in revised form
28 July 2011
Accepted 29 July 2011
Available online 9 August 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Triazene
Dye
Pyrazolidin-3,5-dione
Donoreacceptor
Azide
Spectroscopy
1. Introduction
a result, the scope of compatible substrates and the range of asso-
ciated applications for preparing triazenes are restrictive. These
Triazenes are rather ‘old’ compounds from the organic chemist’s
viewpoint. It was as early as 1862 that Griess described a suitable
way for the synthesis of 1,3-diphenyltriazene as a unique polyazo
compound containing three consecutive nitrogen atoms in an
acyclic form [1e4]. They are known as a versatile tool in organic
synthesis [3]. Although they are studied for their anorectic activity
[5] and potency against specific tumor cell lines [6,7], applied as
protecting groups in natural product synthesis [8,9], or used to
form heterocycles [10e12], most reports describe their applications
as useful linkers in solid-phase organic synthesis [13e15]. We
distinguish two most suitable routes for the synthesis of triazenes
(1) the coupling of aryl diazonium salts to amines [2,16,17] and (2)
the addition of organometallic reagents (RMgX, RLi, etc.) to alkyl
azides [18,19]. However, the reagents used in these synthetic
strategies are extremely reactive, and generally flammable, which
necessitates the use of special equipment and safety measures. As
limitations require the development of new methods for accessing
this important class of donor acceptor systems. Recently, a new,
practical method for preparing triazenes was reported [20,21], the
authors specified that addition of a N-heterocyclic carbene [22e25]
(NHC) to an organic azide afford 1,3-di-substituted triazene in
excellent to moderate yield [26]. Following this study, the NHC/
azide coupling reaction is considered a similar to the copper-
catalyzed [3 þ 2] alkyneeazide “click” coupling [27e30]. In addi-
tion to the practical advantages discussed above, the NHC/azide
coupling reaction presents two features for this purpose: chemical
unsaturation is conserved as reactants are converted to products
and the triazene bridge formed formally conjugates the two organic
components to each other.
On the other hand, nitrogen heterocycles are of special interest
because they constitute an important class of natural and non-
natural products, many of them exhibit useful biological activities
and unique electrical and optical properties [31e35], pyrazolidin-
3,5-dione derivatives are one of these nitrogen heterocycles and
they are in general well-known five-membered nitrogen-containing
heterocyclic compounds. Recently, we developed a new class of
azo- and H-chromophore dyes based on pyrazolidin-3,5-dione
* Corresponding author. ENSAIT, GEMTEX, F-59056 Roubaix, France. Fax: þ33 03
20 27 25 97.
E-mail addresses: jalal.isaad@ensait.fr, jisaadjalal@gmail.com (J. Isaad).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.07.023

J. Isaad et al. / Dyes and Pigments 92 (2012) 1212e1222
1213
Scheme 1. Triazene system synthesis via coupling of pyrazolidin-3,5-dione with its corresponding azide. Reagent and conditions: (a) NaH, THF, RT.
derivatives [36]. The pyrazolidin-3,5-dione was chosen for its elec-
tron donor character because of the removal of the hydrogen atom
in the alpha carbonyl group position (C-4) of pyrazolin-3,5-dione in
basic conditions. Due to the possible electron donor character of
pyrazolidin-3,5-diones, in this paper we report the synthesis of
donoreacceptor triazene systems based on pyrazolidin-3,5-dione
and its corresponding azide as building blocks (Scheme 1).
was used and all samples were studied between 30 and 400 ꢁC.
Literature methods [15] were used to prepare compounds 1aee
(1e, R ¼ Br).
2.2. General procedure A: synthesis of the 4-bromopyrazolidin-3,5-
dione derivatives
The aims of this work were the synthesis of the new class of
“donoreacceptor” triazenes 4aei based on various the pyrazolidin-
3,5-dione NHCPyr derivatives and their coupling to organic azides
AzPyr possessing complementary functional groups. The second
objective was to study the electronic properties of the synthesized
To a suspension of pyrazolidin-3,5-dione derivatives (1 mmol)
in THF (10 mL), NBS (1 mmol) was added and the resulting mixture
was stirred at room temperature for 2 h. The solvent was removed
under reduced pressure and the resulting colorless oil was passed
through short silica column to give crude mixture including the
desired products 2aed and the starting pyrazolidin-3,5-dione
derivatives (the percentages were measured by NMR integral).
conjugated p-triazenes dyes.
2. Experimental
2.2.1. 4-Bromo-1-phenylpyrazolidin-3,5-dione (2a)
2.1. Chemicals and materials
The product 2a was prepared according to the general proce-
dure A using the following quantities: 1-phenylpyrazolidin-3,5-
dione 1a (1.00 g, 5.68 mmol), NBS (0.98 g, 5.68 mmol) in THF
(15 mL) to afford 1a (1.79 g, 95%). 1H NMR (200 MHz, CDCl3):
All chemicals were reagent grade (Aldrich Chemical Co.) and
were used as purchased without further purification. Thin layer
chromatography was carried out on silica gel pre-coated plates
d
¼ 8.02 (s, NH), 7.37e7.35 (m, 4H), 6.89 (m, 1H), 5.32 (s, 1H) ppm.
(Merck; 60 A F254) and spots located with (a) UV light (254 and
13C NMR (50 MHz, CDCl3):
d
¼ 170.4,165.8,134.8,132.3,128.9,128.0,
ꢀ
366 nm), (b) I2 or (c) a basic solution of permanganate [KMnO4
(3 g), K2CO3 (20 g) and NaOH (0.25 g) in water (300 mL)]. Flash
column chromatography (FCC) was carried out on Merck silica
gel 60 (230e400 mesh) according to Still et al. [37]. 1H and 13C
NMR spectra were recorded at 200 MHz with Varian spec-
trometers in deuterated solvents and are reported in parts per
million (ppm) with the solvent resonance used as the internal
reference. Mass spectra were recorded with a Thermo Fisher
LCQ fleet ion-trap instrument (the spectra exported were also
using the ESI þ c technique). Elemental analysis was carried out
with a PerkineElmer 240 C Elemental Analyzer. UV/vis spectra
were recorded with a Cary-4000 Varian spectrophotometer.
Samples for thermogravimetric characterization were located in
65.9 ppm. MS (ESI): m/z ¼ 254.06 [M þ 1]þ.
2.2.2. 4-Bromo-1-(40-methoxyphenyl)pyrazolidin-3,5-dione (2b)
The product 2b was prepared according to the general proce-
dure A using the following quantities: 1-phenylpyrazolidin-3,5-
dione 1b (1.00 g, 5.68 mmol), NBS (0.98 g, 5.68 mmol) in THF
(15 mL) to afford 2b (1.79 g, 95%). 1H NMR (200 MHz, CDCl3):
d
¼ 8.01 (s, NH), 7.30 (m, 2H), 6.92 (m, 2H), 5.33 (s, 1H), 3.84 (s, 3H)
ppm. 13C NMR (50 MHz, CDCl3):
d
¼ 170.3, 165.7, 158.8, 127.3, 125.6,
114.5, 65.8, 55.8 ppm. MS (ESI): m/z ¼ 255.03 [M þ 1]þ.
2.2.3. 4-Bromo-1-(40-hydroxyphenyl)pyrazolidin-3,5-dione (2c)
The product 2c was prepared according to the general procedure
A using the following quantities: 1-phenylpyrazolidin-3,5-dione 1c
(1.00 g, 5.20 mmol), NBS (0.97 g, 5.20 mmol) in THF (15 mL) to
open platinum crucibles and analyzed using
a Rheometric
Scientific TG1000 thermobalance operating under a flowing
argon atmosphere (28 mL minꢀ1). A heating rate of 10 ꢁC minꢀ1
afford 2c (1.78 g, 95%). 1H NMR (200 MHz, CDCl3):
d
¼ 8.02 (s, NH),
Scheme 2. Synthesis of pyrazolidin-3,5-dione azide derivatives (AzPyr). Reagent and conditions: (a) NBS, THF, RT, 2 h, (b) NaN3, acetonitrile, 80 ꢁC, 8 h.

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J. Isaad et al. / Dyes and Pigments 92 (2012) 1212e1222
7.24 (m, 2H), 6.88 (m, 2H), 5.32 (s, 1H) ppm. 13C NMR (50 MHz,
2.4. General procedure C: synthesis of pyrazolidin-3,5-dione
triazenes
CDCl3):
d
¼ 170.4, 165.8, 154.1, 127.4, 126.0, 116.2, 65.8 ppm. MS
(ESI): m/z ¼ 271.04 [M þ 1]þ.
A free pyrazolidin-3,5-dione derivatives (1 equiv) was dis-
solved in THF (approximate concn ¼ 0.3 M), and treated by NaH
(1 mol) for 30 min. Then, azidopyrazolidin-3,5-dione (1 equiv)
was added in a single portion and the resulting reaction mixture
was stirred for up to 8 h at ambient temperature. Solvent
was then removed under reduced pressure to obtain a colorless
solid. The product was purified by flash chromatography
(alumina, ethyl acetate/hexane 9:4) to afford the desired
triazene.
2.2.4. 4-Bromo-1-(4-nitro)phenylpyrazolidin-3,5-dione (2d)
The product 2d was prepared according to the general proce-
dure
A using the following quantities: 1-(4-nitro)-phenyl-
pyrazolidin-3,5-dione 1d (0.99 g, 4.52 mmol), NBS (0.45 mL,
4.52 mmol) in THF (15 mL) to afford 2d (1.55 g, 92%). 1H NMR
(200 MHz, CDCl3):
d
¼ 8.16 (m, 2H), 8.03 (s, NH), 7.09 (m, 2H), 5.33
(s, 1H) ppm. 13C NMR (50 MHz, CDCl3):
d
¼ 170.3, 165.9, 140.9, 143.5,
124.1, 122.5, 65.8 ppm. MS (ESI): m/z ¼ 299.98 [M þ 1]þ.
2.3. General procedure B: synthesis of the 4-azidopyrazolidin-3,5-
dione derivatives
2.4.1. Bis (1-phenylpyrazolidin-3,5-dione) triazene (4a)
The product 4a was prepared according to the general
procedure
5.68 mmol), 3a (1.23 g, 5.68 mmol) in THF (15 mL) to afford 4a
(0.75 g, 99%). 1H NMR (200 MHz, acetone d6):
¼ 12.41 (s, 1H,
NH), 8.13 (s, 2H, NH), 7.37e7.34 (AA0XX0 system, 8H), 6.91 (m,
2H), 4.51 (s, 2H) ppm. 13C NMR (50 MHz, acetone d6):
165.6, 140.1, 129.9, 128.5, 124.2, 120.4, 79.7, 76.1 ppm. MS (ESI):
m/z ¼ 394.47 [M þ 1]þ. C18H15N7O4 (393.36): calcd C, 54.96; H,
3.84; N, 24.93, found, C, 54.99; H, 3.88; N, 24.97.
C using the following quantities: 1a (1.00 g,
To a suspension of 4-bromo pyrazolidin-3,5-dione derivatives
(1 mmol) in acetonitrile (15 mL), NaN3 (1 mmol) was added and the
resulting mixture was stirred at 80 ꢁC for 8 h. The solvent was
removed under reduced pressure and the reaction mixture was
purified by flash column chromatography (EtOAc/hexane 10/7) to
afford the pure products 3aed.
d
d
¼ 170.1,
2.3.1. 4-Azido-1-phenylpyrazolidin-3,5-dione (3a)
The product 3a was prepared according to the general proce-
dure B using the following quantities: 2a (1.00 g, 3.95 mmol),
NaN3 (0.28 g, 3.95 mmol) in acetonitrile (15 mL) to afford 3a (0.84,
2.4.2. 1-(4-Bromophenyl)-4-(3-(3,5-dioxo-1-phenylpyrazolidin-4-
ylidene)triazylidene)pyrazolidin-3,5-dione (4b)
The product 4b was prepared according to the general proce-
dure C using the following quantities: 1e (1.43 g, 3.95 mmol), 3b
(0.86 g, 3.95 mmol) in THF (15 mL) to afford 4b (1.85 g, 99%). 1H
99%). Rf ¼ 0.45. 1H NMR (200 MHz, CDCl3):
d
¼ 8.02 (s, NH),
7.37e7.34 (AA0XX0 system, 4H), 7.12 (s, 1H, NH), 6.89 (m, 1H), 3.22
(s, 1H) ppm. 13C NMR (50 MHz, CDCl3):
d
¼ 170.3, 165.8, 134.7,
NMR (200 MHz, acetone d6):
d
¼ 12.41 (s, 1H, NH), 8.12 (s, 2H, NH),
132.5, 128.8, 128.1, 83.9 ppm. MS (ESI): m/z ¼ 218.11 [M þ 1]þ.
C9H7N5O2 (217.06): calcd C, 49.77; H, 3.25; N, 32.25, found, C,
49.81; H, 3.29; N, 32.28.
7.52e6.71 (AA0XX0 system, 4H), 7.38e7.35 (AA0XX0 system, 4H),
6.91 (m, 1H), 4.52 (s, 2H) ppm. 13C NMR (50 MHz, acetone d6):
d
¼ 170.4, 165.5, 140.6, 139.8, 132.4, 128.7, 124.5, 122.6, 120.4, 118.3,
79.7, 75.2 ppm. MS (ESI): m/z ¼ 472.12 [M þ 1]þ. C18H14BrN7O4
(471.03): calcd C, 45.78; H, 2.99; N, 20.76, found, C, 45.81; H, 3.07;
N, 20.81.
2.3.2. 4-Azido-1-(40-methoxy)phenylpyrazolidin-3,5-dione (3b)
The product 3b was prepared according to the general proce-
dure B using the following quantities: 2b (1.00 g, 3.92 mmol), NaN3
(0.27 g, 3.92 mmol) in acetonitrile (15 mL) to afford 3b (0.83, 99%).
Rf ¼ 0.44. 1H NMR (200 MHz, CDCl3):
d
¼ 8.01 (s, NH), 7.31e6.91
(AA0XX0 system, 4H), 7.15 (s, 1H, NH), 3.83 (s, 3H), 3.24 (s, 1H) ppm.
13C NMR (50 MHz, CDCl3):
d
¼ 170.5, 165.6, 158.8, 127.4, 125.9, 114.7,
83.6, 55.6 ppm. MS (ESI): m/z ¼ 248.11 [M þ 1]þ. C10H9N5O3
(247.07): calcd C, 48.58; H, 3.67; N, 28.33, found, C, 48.61; H, 3.70;
N, 28.38.
2.3.3. 4-Azido-1-(40-hydroxyl)phenylpyrazolidin-3,5-dione (3c)
The product 3c was prepared according to the general proce-
dure B using the following quantities: 2c (1.00 g, 3.70 mmol),
NaN3 (0.25 g, 3.70 mmol) in acetonitrile (15 mL) to afford 3c (0.79,
99%). Rf ¼ 0.49. 1H NMR (200 MHz, CDCl3):
d
¼ 8.01 (s, NH),
7.25e6.88 (AA0XX0 system, 4H), 7.12 (s, 1H, NH), 3.24 (s, 1H) ppm.
13C NMR (50 MHz, CDCl3):
d
¼ 170.7, 165.6, 154.6, 127.6, 126.3,
116.4, 84.6 ppm. MS (ESI): m/z ¼ 234.09 [M þ 1]þ. C9H7N5O3
(233.05): calcd C, 46.36; H, 3.03; N, 30.03, found, C, 46.39; H,
3.07; N, 30.07.
2.3.4. 4-Azido-1-(4-nitro) phenylpyrazolidin-3,5-dione (3d)
The product 3d was prepared according to the general proce-
dure B using the following quantities: 2d (0.99 g, 3.43 mmol), NaN3
(0.23 mL, 3.43 mmol) in acetonitrile (15 mL) to afford 3d (0.75 g,
99%). Rf ¼ 0.51. 1H NMR (200 MHz, CDCl3):
d
¼ 8.15e7.08 (AA0XX0
system, 4H), 8.02 (s, NH), 7.12 (s, 1H, NH), 3.23 (s, 1H) ppm. 13C NMR
(50 MHz, CDCl3):
d
¼ 170.1, 165.6, 140.5, 143.9, 124.8, 122.6,
83.6 ppm. MS (ESI): m/z ¼ 262.05 [M þ 1]þ. C9H6N6O4 (262.12):
Scheme 3. Synthesis of pyrazolidin-3,5-dione triazene. Reagent and conditions: (a)
NaH, THF, RT.
calcd C, 41.23; H, 2.31; N, 32.05, found, C, 41.27; H, 2.35; N, 32.09.

J. Isaad et al. / Dyes and Pigments 92 (2012) 1212e1222
1215
2.4.3. 1-(4-Hydroxyl phenyl)-4-(3-(3,5-dioxo-1-phenylpyrazolidin-
4-ylidene)triazylidene) pyrazolidin-3,5-dione (4c)
The product 4c was prepared according to the general procedure
C using the following quantities: 3c (1.43 g, 5.20 mmol), 1a (0.92 g,
5.20 mmol) in THF (15 mL) to afford 4c (1.85 g, 99%). 1H NMR
81.1, 77.5 ppm. MS (ESI): m/z ¼ 410.16 [M þ 1]þ. C18H15N7O5
(409.11): calcd C, 52.81; H, 3.69; N, 23.95, found, C, 52.85; H, 3.73;
N, 23.98.
2.4.4. 1-(4-Methoxy phenyl)-4-(3-(3,5-dioxo-1-phenylpyrazolidin-
4-ylidene) triazylidene) pyrazolidin-3,5-dione (4d)
The product 4d was prepared according to the general proce-
dure C using the following quantities: 3b (1.17 g, 5.69 mmol), 1a
(1.00 g, 5.69 mmol) in THF (15 mL) to afford 4c (1.85 g, 99%). 1H
(200 MHz, acetone d6):
d
¼ 12.42 (s, 1H, NH), 8.12 (s, 2H, NH),
7.38e7.25 (AA0XX0 system, 4H), 7.23e6.88 (AA0XX0 system, 4H),
6.92 (m, 1H), 4.51 (s, 2H) ppm. 13C NMR (50 MHz, acetone d6):
d
¼ 170.4, 165.6, 153.9, 134.7, 132.6, 129.1, 128.3, 127.8, 126.4, 116.4
Table 1
Structure of the synthesized triazene dyes pyrazolidin-3,5-dione based.a
Entry
1
Dyes
1aed
3aed
Structure of 4aei
Yieldb
98
4a
2
3
4b
4c
98
98
4
5
6
7
4d
4e
4f
98
98
98
98
4g
8
4h
98
98
9
4i
a
General reaction conditions: pyrazolidin-3,5-dione (1.0 equiv) and an organic azide (1.0 equiv), concentration of substrates ¼ 0.3 M, solvent ¼ THF, 25 ꢁC.
b
Isolated yields.

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J. Isaad et al. / Dyes and Pigments 92 (2012) 1212e1222
Fig. 1. Tautomeric forms of the azo-pyrazolidin-3,5-dione triazenes.
NMR (200 MHz, acetone d6):
d
¼ 12.41 (s, 1H, NH), 8.12 (s, 2H, NH),
7.55e6.75 (AA0XX0 system, 4H), 7.27e6.85 (AA0XX0 system, 4H),
5.46 (s, 1H, OH), 4.52 (s, 2H) ppm. 13C NMR (50 MHz, acetone d6):
7.39e7.35 (AA0XX0 system, 4H), 7.29e6.92 (AA0XX0 system, 4H), 6.90
(m, 1H), 4.51 (s, 2H), 3.84 (s, 3H) ppm. 13C NMR (50 MHz, acetone
d
¼ 170.7, 165.6, 153.9, 135.3, 133.3, 127.5, 126.7, 126.2, 122.5, 116.5,
d6):
d
¼ 170.7, 165.3, 158.8, 134.4, 132.5, 128.9, 128.2, 127.5, 125.3,
81.4, 77.6 ppm. MS (ESI): m/z ¼ 488.11 [M þ 1]þ. C18H14BrN7O5
(487.02): calcd C, 44.28; H, 2.89; N, 20.08, found, C, 44.31; H, 2.92;
N, 20.11.
114.6 81.3, 77.8, 55.5 ppm. MS (ESI): m/z ¼ 424.21 [M þ 1]þ.
C19H17N7O5 (423.13): calcd C, 53.90; H, 4.05; N, 23.16, found, C,
53.95; H, 4.09; N, 23.19.
2.4.7. 1-(4-Nitro phenyl)-4-(3-(3,5-dioxo-1-(40-bromo)
phenylpyrazolidin-4-ylidene)triazylidene) pyrazolidin-3,5-dione
(4g)
2.4.5. 1-(4-Nitro phenyl)-4-(3-(3,5-dioxo-1-phenylpyrazolidin-4-
ylidene)triazylidene) pyrazolidin-3,5-dione (4e)
The product 4e was prepared according to the general proce-
dure C using the following quantities: 3d (1.49 g, 5.69 mmol), 1a
(1.00 g, 5.69 mmol) in THF (15 mL) to afford 4e (2.33 g, 99%). 1H
The product 4g was prepared according to the general proce-
dure C using the following quantities: 1e (1.00 g, 4.29 mmol), 3d
(1.14 g, 4.29 mmol) in THF (15 mL) to afford 4g (2.20 g, 99%). 1H
NMR (200 MHz, acetone d6):
8.11e7.08 (AA0XX0 system, 4H), 7.37e7.33 (AA0XX0 system, 4H), 6.90
(m, 1H), 4.51 (s, 2H) ppm. 13C NMR (50 MHz, acetone d6):
165.1, 143.3, 140.1, 134.4, 132.6, 128.7, 127.9, 124.4, 122.6, 81.5,
77.8 ppm. MS (ESI): m/z ¼ 439.18 [M þ 1]þ. C18H14N8O6 (438.10):
calcd C, 49.32; H, 3.22; N, 25.56, found, C, 49.37; H, 3.26; N, 25.59.
d
¼ 12.42 (s, 1H, NH), 8.14 (s, 2H, NH),
NMR (200 MHz, acetone d6):
8.05e7.15 (AA0XX0 system, 4H), 7.57e6.75 (AA0XX0 system, 4H), 4.52
(s, 2H) ppm. 13C NMR (50 MHz, acetone d6):
140.4, 135.4, 133.6, 126.9, 124.6, 122.7, 122.4, 81.6, 77.7 ppm. MS
(ESI): m/z ¼ 517.07 [M þ 1]þ. C18H13BrN8O6 (516.01): calcd C, 41.80;
H, 2.53; N, 21.66, found, C, 41.84; H, 2.57; N, 21.69.
d
¼ 12.41 (s, 1H, NH), 8.13 (s, 2H, NH),
d
¼ 170.5,
d
¼ 170.5, 165.5, 143.6,
2.4.6. 1-(4-Hydroxy phenyl)-4-(3-(3,5-dioxo-1-(40-bromo)
phenylpyrazolidin-4-ylidene) triazylidene) pyrazolidin-3,5-dione
(4f)
2.4.8. 1-(4-Hydroxy phenyl)-4-(3-(3,5-dioxo-1-(40-hydroxy)
phenylpyrazolidin-4-ylidene) triazylidene) pyrazolidin-3,5-dione
(4h)
The product 4f was prepared according to the general procedure
C using the following quantities: 1e (1.00 g, 4.29 mmol), 3c (1.18 g,
4.29 mmol) in THF (15 mL) to afford 4f (2.08 g, 99%). 1H NMR
The product 4h was prepared according to the general proce-
dure C using the following quantities: 1c (1.00 g, 5.20 mmol), 3c
(1.21 g, 4.29 mmol) in THF (15 mL) to afford 4h (2.08 g, 99%). 1H
(200 MHz, acetone d6):
d
¼ 12.41 (s, 1H, NH), 8.12 (s, 2H, NH)
NMR (200 MHz, acetone d6):
d
¼ 12.42 (s, 1H, NH), 8.13 (s, 2H, NH),
7.25e6.88 (AA0XX0 system, 8H), 4.52 (s, 2H) ppm. 13C NMR (50 MHz,
acetone d6):
d
¼ 170.6, 165.4, 154.2, 127.6, 126.8, 116.3, 81.4,
77.6 ppm. MS (ESI): m/z ¼ 426.19 [M þ 1]þ. C18H13N7O6 (425.11):
calcd C, 50.83; H, 3.55; N, 23.05, found, C, 50.87; H, 3.59; N, 23.09.
Fig. 2. 1H NMR spectrum of pyrazolidin-3,5-dione triazene 4a.
Fig. 3. Stable tautomer form of the triazene 4aei.

J. Isaad et al. / Dyes and Pigments 92 (2012) 1212e1222
1217
Table 2
Spectroscopic data of triazenes 4aei.
Dyes
1H NMR
FTIR
NeH
Aroms-H
yNeH
yArom-H
yC]O
4a
4b
4c
4d
4e
4f
4g
4h
4i
12.4, 8.13
12.4, 8.12
12.3, 8.12
12.4, 8.12
12.3, 8.14
12.4, 8.12
12.4, 8.13
12.4, 8.13
12.3, 8.15
7.37 (m, 4H), 7.35 (m, 4H), 6.91(m, 2H), 4.51 (s, 2H)
3344, 3339
3343, 3335
3346, 3334
3347, 3335
3345, 3336
3344, 3336
3341, 3335
3346, 3338
3343, 3339
3045
3052
3054
3052
3051
3054
3047
3054
3057
1665
1663
1662
1657
1662
1653
1658
1655
1656
7.52 (m, 2H), 7.37e7.35 (m, 4H), 6.91e6.71 (m, 4H), 4.52 (m, 2H)
7.38 (m, 2H), 7.25e7.23 (m, 4H), 6.91e6.88 (m, 3H), 4.51 (s, 2H)
7.39 (m, 2H), 7.35e7.29 (m, 4H), 6.92e6.90 (m, 3H), 4.51 (s, 2H)
8.11 (m, 2H), 7.37e7.33 (m, 4H), 7.08 (m, 2H), 6.90 (m, 1H), 4.51 (s, 2H)
7.55 (m, 2H), 7.27 (m, 2H), 6.85e6.75 (m, 4H), 4.52 (m, 2H)
8.05 (m, 2H), 7.57 (m, 2H), 7.15 (m, 2H), 6.75 (m, 2H), 4.52 (m, 2H)
7.25 (m, 4H), 6.88 (m, 4H), 4.52 (m, 2H)
8.10 (m, 2H), 7.25 (m, 2H), 7.08 (m, 2H), 6.88 (m, 2H), 4.50 (m, 2H)
2.4.9. 1-(4-Hydroxy phenyl)-4-(3-(3, 5-dioxo-1-(40-nitro)
phenylpyrazolidin-4-ylidene) triazylidene) pyrazolidin-3,5-dione
(4i)
the titled compound was obtained as yellow oil (50% yields). 1H
NMR (50 MHz, acetone d6):
d
¼ 8.11 (s, 2H, NH), 7.69e7.38 (AA0XX0,
4H), 7.37e7.34 (AA0XX0, 4H), 6.92 (m, 2H), 3.9 (s, 1H). 13C NMR
The product 4i was prepared according to the general procedure
C using the following quantities: 1c (1.00 g, 5.20 mmol), 3d (1.36 g,
5.20 mmol) in THF (15 mL) to afford 4i (2.35 g, 99%). 1H NMR
(50 MHz, acetone d6):
d
¼ 170.4, 168.4, 165.4, 163.8, 146.5, 137.8,
134.5, 132.1, 129.8, 128.7, 128.1, 122.4, 120.4, 80.4. MS (ESI): m/
z ¼ 364.18 [M þ 1]þ. C18H13N5O4 (363.10): calcd C, 59.50; H, 3.61; N,
19.28, found, C, 59.55; H, 3.66; N, 19.33.
(200 MHz, acetone d6):
8.10e7.08 (AA0XX0 system, 4H), 7.25e6.88 (AA0XX0 system, 4H), 4.50
(s, 2H) ppm. 13C NMR (50 MHz, acetone d6):
d
¼ 12.42 (s, 1H, NH), 8.15 (s, 2H, NH),
d
¼ 170.7, 165.6, 154.3,
2.4.12. 4-(3,5-Dioxo-1-phenylpyrazolidin-4-ylideneamino)-1-
143.3, 140.5, 127.6, 126.6, 124.2, 122.4, 116.7, 81.6, 77.8 ppm. MS
(ESI): m/z ¼ 455.17 [M þ 1]þ. C18H14N8O7 (454.10): calcd C, 47.58; H,
3.11; N, 24.66, found, C, 47.62; H, 3.14; N, 24.69.
phenylpyrazolidin-3,5-dione (6)
1H NMR (CDCl3):
d
¼ 8.12 (s, 2H, NH), 7.69e7.39 (AA0XX0, 4H),
7.36e7.33 (AA0XX0, 4H), 6.91 (m, 2H), 3.92 (s, 1H), 13C NMR (CDCl3):
d
¼ 170.5, 168.6, 165.3, 163.6, 146.2, 137.4, 134.7, 132.3, 129.7, 128.8,
2.4.10. 4-(2-(2-Hydroxyphenyl) hydrazono)-1-phenyl pyrazolidin-
128.0, 122.2, 120.6, 80.7. MS (ESI): m/z ¼ 365.22 [M þ 1]þ.
C18H13N5O4 (364.10): calcd C, 59.50; H, 3.61; N, 19.28, found, C,
59.55; H, 3.66; N, 19.31.
3,5-dione (8)
1H NMR (200 MHz, acetone d6):
d
¼ 7.62e7.34 (AA0XX0 system,
4H), 7.31e7.33 (m, 2H), 7.03 (s, 1H), 6.92 (m, 3H). 13C NMR (50 MHz,
acetone d6):
d
¼ 168.6, 168.2, 146.4, 137.8, 136.5, 129.8, 128.5, 122.6,
3. Results and discussion
122.1, 120.7, 120.1, 116.4, 115.5. MS (ESI): m/z ¼ 297.17 [M þ 1]þ.
C15H12N4O3 (296.09): calcd C, 60.81; H, 4.08; N, 18.91, found, C,
60.86; H, 4.11; N, 18.94.
3.1. Synthesis of pyrazolidin-3,5-dione azide derivatives (AzPyr)
The synthetic scheme of the pyrazolidin-3,5-dione azide deriv-
atives was reported in Scheme 2. The treatment of the pyrazolidin-
3,5-dione derivatives 1aed with N-bromosuccinimide in THF as
solvent at room temperature affords the corresponding brominated
derivatives 2aed. The pyrazolidin-3,5-dionyl azides 3aed were
synthesized by treating 2aed with sodium azide in acetonitrile at
80 ꢁC in high yield (99%).
2.4.11. Azamethine 5: 4-(3,5-dioxo-1-phenyl pyrazolidin-4-ylidene
amino)-1-phenyl pyrazolidin-3,5-dione
Bis (1-phenylpyrazolidin-3,5-dione) triazene (4a) (0.1 g,
0.25 mmol) was dissolved in 4 mL of toluene and heated to 150 ꢁC
for 10 h. After removal of solvent under reduced pressure, 0.09 g of
Table 3
UVevis absorbance data for pyrazolidin-3,5-dione triazenes dyes.a
b
Entry
Products
R1
R2
lmax
Log
3
1
2
3
4
5
6
7
8
9
4a
4b
4c
4d
4e
4f
4g
4h
4i
H
H
H
H
OCH3
NO2
OH
NO2
OH
NO2
371
383
379
408
461
411
459
435
477
4.30
4.26
4.25
4.21
4.44
4.37
4.61
4.50
4.58
Br
OH
H
H
Br
Br
OH
OH
a
UVevis spectra were measured in acetone at ambient temperature.
3
Fig. 4. UVevis absorbance spectra of triazene 4aei compounds (from the left to the
right: 4a, 4c, 4b, 4d, 4f, 4h, 4g, 4e, 4i).
b
Corresponds to the molar absorptivity with units of L molꢀ1 cmꢀ1
.

1218
J. Isaad et al. / Dyes and Pigments 92 (2012) 1212e1222
Table 4
3.2. Synthesis of pyrazolidin-3,5-dione triazenes
Electronic effects on triazene stability.
Triazene dyes 4aei were prepared by coupling pyrazolidin-3,5-
dione sodium salt, generated through deprotonation of its pyr-
azolidin-3,5-dione using sodium hydride (1 mol), and then isolated
and used as its free NHCPyr with 1 mol equiv of pyrazolidin-3,5-
dionyl azide (Scheme 3). The reaction was conducted at ambient
temperature using tetrahydrofuran (THF) as solvent at 0.1 M
substrate concentration. The purification of the resulting triazene
4aei requires only a filtration of the reaction mixture through
a PTFE filter followed by evaporation of solvent. Using this work-up,
triazene dyes were obtained in excellent yields (98%) and in high
purity. Table 1 show the synthesized triazenes based on pyr-
azolidin-3,5-dione derivatives and its corresponding azides as
coupling building blocks. Following the nature of the substitutes R1
and R2 (Scheme 3), we can assume that if the triazo linkage enabled
Entry
Products
R1
R2
Mp (ꢁC)a
Td (ꢁC)b
1
2
3
4
5
6
7
8
9
4a
4b
4c
4d
4e
4f
4g
4h
4i
H
H
H
H
OCH3
NO2
OH
NO2
OH
NO2
104
123
125
135
145
127
148
126
148
113
129
133
127
192
136
188
135
179
Br
OH
H
electronic communication,
a bathochromic shift should be
observed by UVevis spectroscopy as the pendent electron-
donating and electron-withdrawing substituents in the resulting
triazenes became more complementary. Similarly, successive
downfield shifts were also expected in the NMR spectra for the
same series of triazenes.
H
Br
Br
OH
OH
a
Melting points (Mp) are uncorrected and were determined under an atmosphere
During the synthesis of triazenes 4aei, the reaction progress
depended essentially on the nature of the substituents R1 and R2 of
the pyrazolidin-3,5-dione and its azide derivatives. To obtain
quantitative information on reaction progress versus time, the
bimolecular rate constant of reaction involving pyrazolidin-3,5-
dione derivatives and a select range of organic azides with
various electronic properties were measured by 1H NMR spec-
troscopy (in CDCl3 as solvent). The coupling reaction between the
free NHCPyr and an electron-poor azide, 4-azido-(40-nitrophenyl)
pyrazolidin-3,5-dione, was extremely fast and measured to be
0.254 L molꢀ1 sꢀ1. In contrast, the rate constant for coupling the
same NHCPyr to relatively more electron-rich azides, 4-azido-
(phenyl) and 4-azido-(40-methoxyphenyl) pyrazolidin-3,5-dione,
were relatively slow and found to be 0.05 L molꢀ1 sꢀ1 and
0.02 L molꢀ1 sꢀ1 respectively. Collectively, these observations were
consistent with a coupling mechanism that involves nucleophilic
attack of a NHCPyr at the terminal nitrogen of a azidopyrazolidin-
3,5-dione.
of air.
b
Decomposition temperature (Td) determined using thermogravimetric analysis
under an atmosphere of nitrogen and defined as the temperature at which 5% mass
loss occurred.
1H NMR spectra of the triazenes 4aei show peaks correspond-
ing to the aryl protons (8.17e6.88 ppm), a characteristic highly
destabilized proton at 12.40 and 8.12 ppm for the hydrogen-
bonded NeH peaks and signals in the 4e6 ppm region corre-
sponding to the proton in position C-4 (Fig. 2).
These NMR data prove that the hydrazo-keto tautomers form A
is the synthesized triazene, the hydrogen borne by the eNHeN]
Ne bond is stabilized by the oxygen of one or both carbonyl groups
present in the pyrazolidin-3,5-dione moiety (Fig. 3).
Furthermore, FTIR spectra of compounds 4aei, show C]O and
NeH stretching vibrations at 3340 (NeH str) and at 1663 (C]O str),
consistent with their existence in the keto-hydrazone tautomeric
form A (Table 2).
3.3. NMR spectroscopic characterization of triazenes 4aei
However, the other non-symmetrical triazenes, there exist six
possible forms. The formation of cyclic hydrogen bond will be
strongly dependent on the nature of substituents in both benzene
nuclei.
The donoreacceptor pyrazolidin-3,5-dione triazenes (4aei)
were studied by 1H and 13C NMR spectroscopy. In general, signals
attributed to the aryl protons on the pyrazolidin-3,5-dione azide-
containing moieties shifted up field whereas the aryl protons on the
pyrazolidin-3,5-dione shifted downfield upon coupling. This result
was consistent with charge transfer from the electron-rich
component to the relatively electron-poor component. The
magnitude of these shifts was strongly dependent on the nature of
the substitute on the pyrazolidin-3,5-dionyl azide with a smaller
dependency on the pendent functional group on the pyrazolidin-
3,5-dione, although overall changes were small for all compounds
studied (1H NMR: <0.1 ppm; 13C NMR: <1.0 ppm). The most
pronounced shifts were observed in the series (1H NMR, sol-
vent ¼ CDCl3): OHeNO2 (
Ar-H meta to NO2), HeNO2
¼ 8.19, 7.18 ppm).
d
¼ 8.10 ppm, Ar-H ortho to NO2; 7.09 ppm
(d
¼ 8.15, 7.13 ppm), and BreNO2
(d
3.4. Tautomerism of pyrazolidin-3,5-dione triazenes 4aei
The symmetrical triazenes 4a and 4h can exist in three different
possible tautomeric forms, namely the azo-keto form A, the azo-
enol form B and the azo-keto-enol form C in Fig. 1.
Fig. 5. Thermal decomposition of triazene via loss of nitrogen.

J. Isaad et al. / Dyes and Pigments 92 (2012) 1212e1222
1219
Fig. 6. Proposed mechanism of thermally triazene decomposition based on a 15N-labeling experiment.
3.5. UVevis absorption spectroscopic characterization of
pyrazolidin-3,5-dione triazenes 4aei
and 1,3-diene (C]CeC]C) linkages. As reported in the literature, the
electronic properties of donoreacceptor azines studied by NMR
spectroscopy [38e40], demonstrate that the electronic communica-
tion across the NeN bond was minimal or extensively delocalized and
under some conditions exhibited nonlinear optical (NLO) responses
[41]. In this sense, it is important to study the electronic properties
The triazene linkage (CeNHeN]Ne) formed in the NHCPyr/
AzPyr reaction presents structural similarities to azine (C]NeN]C)
of four-electron, conjugated
consecutive nitrogen atoms.
p-triazene system containing three
As shown in the UVevis absorption spectra reported in Table 3.
A gradual bathochromic shift in the lmax of the triazene chromo-
phore was observed as the electron-donating/electron-with-
drawing character of its peripheral functional groups became more
complementary (Fig. 4). Close analysis of the data suggested that
while electronic communication between the terminal functional
group on pyrazolidin-3,5-dione and the pyrazolidin-3,5-dionyl
azide was observed, the interaction was relatively minor. For
example, only 4e12 nm bathochromic shifts were observed in the
Scheme 4. Probable protolysis mechanism of triazene 4a.
Fig. 7. Protolysis spectral of the 4a decomposition at pH ¼ 4, RT, intervals 1 h.

1220
J. Isaad et al. / Dyes and Pigments 92 (2012) 1212e1222
lmax upon replacing one para hydrogens in the phenyl of pyr-
azolidin-3,5-dione to either hydroxyl or methoxy groups (i.e., H/
H / OH/H or Br/H; entries 1e3). These observations can be
explained by the high degree of bond polarization observed in the
triazene formed between pyrazolidin-3,5-dione and the azide
moiety, due to the strong electron-donating character of the
substituents present in the pyrazolidin-3,5-dione derivatives. In
contrast, significant effects on lmax were observed when the
terminal functional group on the pyrazolidin-3,5-dionyl azide
coupling partner was varied. For example, replacing a methoxy
group with a nitro group resulted in a 53 nm bathochromic shift in
the lmax (i.e., H/OCH3 / H/NO2; entries 4 and 5). Similar spectro-
scopic changes were observed with triazenes containing electron-
poor (i.e., Br/OH / Br/NO2, Dlmax ¼ 48 nm; entries 6 and 7) and
electron-rich (i.e., OH/OH / OH/NO2, Dlmax ¼ 42 nm; entries 8 and
9) pyrazolidin-3,5-dione components. As expected, triazene
OHeNO2, prepared by coupling a highly electron-rich pyrazolidin-
3,5-dione with a highly electron-deficient azide, exhibited the
longest lmax at 477 nm for any of the triazenes studied (entry 9).
Collectively, these results suggested that the triazeno linkages
(CeNHeN]Ne) were effective in delocalizing electronic charge
between complementary NHCPyr and organic azide components.
Fig. 8. Spectral record of the decomposition of triazene 4a in HCl, pH 4 in the presence
of phenol in large excess at room temperature.
of these results, we suggest the following thermal decomposition
mechanism of triazene 4a as shown in Fig. 6.
Interesting observation was that the addition of HCl solution
(1 N) in the aqueous solution of ethanol (10/1) reduced the time of
decomposition of triazene 4a from 90 min to 30 min. Therefore we
found it necessary to study the stability of the triazene 4aei in
acidic media.
3.6. Thermogravimetric analysis of triazene 4aei
The thermal stability of triazenes 4aei has been studied in the
solid-state using thermogravimetric analysis to investigate how the
size of the substituents (R1 and R2) as well as electronic effects of
the pyrazolidin-3,5-dione component affect triazene stability. The
thermal stabilities of various triazenes evaluated in the solid-state
using thermogravimetric analysis (TGA) are illustrated in Table 4.
The results show that the material suffers thermal degradation at
temperatures above about 110 ꢁC.
Following the results reported in Table 4, for example the Tg of
the triazene 4e which possessed an electron-deficient p-nitro
substitute decomposes at 192 ꢁC (entry 5), and was 65 ꢁC higher
than an analog containing a p-methoxy substitute 4d (Tg ¼ 127 ꢁC,
entry 4). We can conclude that increasing the size of the substitute
positively influenced triazene stability. The role of pyrazolidin-3,5-
dione electronics in influencing triazene thermal stability was
found to be relatively major. These results suggested that coupling
pyrazolidin-3,5-dione possessing phenyl substituents with elec-
tron deficiency affords triazenes with the highest thermal stabili-
ties. Melting points (Mp) were also determined and found to be
below the decomposition temperature (Td) for all compounds
studied.
3.8. Protolysis of triazenes dyes 4aei
The azo coupling reaction at nitrogen takes place in neutral to
weakly alkaline media [16]. In acidic media, the azo coupling
reaction at nitrogen is reversible, therefore the triazenes are not
stable in acidic medium. Triazenes are protonated by acid at the
amino group and decompose back to the aniline and diazonium ion
[43e45]. This cleavage may be followed by azo coupling reaction in
the nucleus of the aniline derivatives formed (Scheme 4).
Typically, experiments were carried out in buffereethanol
mixtures in the presence of 5-fold concentration of HCl compared
to the initial concentration of triazene (5 ꢂ10ꢀ5 M). The decrease of
triazene with time was measured at room temperature in different
buffer solutions. A characteristic example for a protolysis experi-
ment followed by UVevisible spectra measurements are reported
in Fig. 7.
The protolysis UVevis spectra reported in Fig. 7 show firstly
a very slow decrease in the absorbance of the absorption band at
lmax ¼ 371 nm. If the decomposition of the triazene 4a in the
presence of HCl solution at pH ¼ 4 affords the diazonium salt 6a,
it is very probable that this diazonium salt reacts with an
3.7. Thermo analysis stability of triazenes 4aei
To determine the thermal decomposition process of the tri-
azenes synthesized in this study, 4a was heated to 150 ꢁC in
aqueous ethanol for 90 min, the azamethine (5) has been observed
by 1H NMR spectroscopy and isolated in 50% yield via flash chro-
matography. The difference between the molecular weight of the
triazene 4a and the azamethine (5) was 28 Da which corresponds
to the molecular weight of the molecular nitrogen (Fig. 5).
It is reported in the literature that aromatic triazenes decom-
pose thermally via radical intermediate following breakage of the
N2eN3 bond [42]. To confirm this hypothesis, pyrazolidin-3,5-
dionyl azides containing an isotopically enriched nitrogen atom
has been synthesized using 15N-labeled sodium azide. After, this
15N-labeled azide has been coupled with pyrazolidin-3,5-dione 1a
15
and the resulting N-labeled triazene 4a was heated to 150 ꢁC in
aqueous ethanol. The resulting azamethine 6 was obtained in 50%
yield and it was characterized by mass spectrometry. On the basis
Fig. 9. 1H NMR spectrum of the protolysis resulting azo dye 8.

J. Isaad et al. / Dyes and Pigments 92 (2012) 1212e1222
1221
Fig. 10. Protolysis of 4a in the presence of HCl (pH 4) and a large excess of phenol.
electrophilic site to form its corresponding azo dye. Therefore, the
protolysis UVevis spectra of triazene 4a at pH 4 in the presence of
a phenol will lead to a decrease in the absorbance at lmax ¼ 371 and
an increasing of a new band which corresponds to the formed azo
dye. For this purpose and to confirm that the absorbance decrease
of the triazene 4a is due to its reverse decomposition to the dia-
zonium ion 6a and 4-amino-1-phenyl, pyrazolidin-3,5-dione 6b
(Scheme 4), the same experiment was carried out in the presence of
phenol in a large excess. We have found that the decrease in
absorbance at the triazene lmax ¼ 371 nm is accompanied by an
increase in absorbance at lmax ¼ 446 nm, the spectra intersecting at
an isosbestic point (Fig. 8).
To characterize the formed dye after the protolysis of the triazene
4a at pH 4, the dye corresponding to the absorbance band at
lmax ¼ 446 nm have been isolated by flash chromatography (AcOEt/
PE: 10/3, Rf ¼ 0.53) and characterized by 1H NMR spectroscopy (Fig. 9).
The 1H NMR spectrum reported in Fig. 9 indicates that the iso-
lated compound resulting from the protolysis of the triazene 4a in
the presence of HCl (pH 4) in an a large excess of phenol corresponds
to the azo dye 8 and not its para-isomer because of the hydrogen of
the phenol is stabilized by an intramolecular H-bond with the
nitrogen of the azo bond which explains the absence of the eOH
signal on the 1H NMR spectrum (Fig. 10).
To explain the very slow decrease in the absorbance of the
absorption band at lmax ¼ 371 nm observed when 4a was treated
with HCl (pH 4), we have attempted to synthesize dye 7 by coupling
of 4-amino-1-phenyl pyrazolidin-3,5-dione 6b with itself in the
presence of NaNO2 in HCl/H2O: 1/10 as solvent. We have found that
the dye 7 (Fig. 10) was synthesized but in 8% as yield. However, its
isomer, the triazene 4a, was obtained in 91% as yield. As reported in
Fig. 3, the major formation of triazene 4a may be due to the exis-
tence of intramolecular hydrogen bonds in 4a between the ]
NeNeHeN] bond which is stabilized by the oxygen of one of both
carbonyl groups present in the pyrazolidin-3,5-dione moiety, and
its absence in the azo derivative 7 which favors the coupling
between the diazonium salt 6a and the amine formed 6b.
4. Conclusions
A novel class of pyrazolidin-3,5-dione triazenes have been
prepared by coupling N-heterocyclic pyrazolidin-3,5-dione NHCPyr
with various pyrazolidin-3,5-dionyl azides in excellent yields (98%).
The structure of these triazenes were confirmed by UVevis spec-
troscopy, NMR spectroscopy, mass spectroscopy and thermogravi-
metric analysis. The electronic properties of the new triazene dyes
were dominated by the substituent nature of the coupled NHCPyr
and AzPyr and its complementarities in the formed triazene dye.
Triazene thermal stability was found to be also governed by
the electronic substituent nature of the pyrazolidin-3,5-dione
moiety in particular, a pyrazolidin-3,5-dione triazene possessing
an electron-deficient phenyl substituent affords triazenes with the
highest thermal stability. The decomposition mechanism was also
suggested by using an isotopically labeled azide and found to afford
molecular nitrogen and an azamethine as thermal decomposition
product. The protolysis in acidic environment of triazene 4a shows
that these dyes are stable in the presence of a strong acid. This
acidic stability is due to the existence of intramolecular hydrogen
bonds in 4a between the ]NeNeHeN] bond which is stabilized
by the oxygen of one of both carbonyl groups present in the pyr-
azolidin-3,5-dione moiety, and its absence in the corresponding
azo derivative which favor the formation of the triazene dye and
not its corresponding azo dye.
Acknowledgments
This work is financially supported by GEMTEX LaboratorydFrance.
We thank Mr Christian Catel (technician of GEMTEX laboratory) for its
kind disponibility.

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J. Isaad et al. / Dyes and Pigments 92 (2012) 1212e1222
[23] Crabtree RH. NHC ligands versus cyclopentadienyls and phosphines as spec-
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