Stable diazonium salts of weakly basic amines—Convenient reagents for synthesis of disperse azo dyes

Article abstract of DOI:10.1016/j.dyepig.2016.08.026

A new synthetic strategy for industrially important deep-shade disperse azo dyes was presented in this study. The key procedure is to prepare stable solid diazonium salts of weakly basic amines in the absence of concentrated sulfuric acid. Diazotization by tert-butyl nitrite in ethyl acetate was allowed to proceed in the presence of equivalent 1,5-naphthalenedisulfonic acid as stabilizer of diazonium salts and donor of hydrogen ion for the reaction. The separated solid diazonium salts exhibited good thermal stability. The corresponding disperse azo dyes were subsequently synthesized through the azo-coupling of the prepared solid diazonium salts with a range of aromatic tertiary amines. The azo dyes were produced in short reaction time, excellent yields, mild reaction conditions, simple experimental procedure and low energy consumption.

Full text of DOI:10.1016/j.dyepig.2016.08.026

Accepted Manuscript
Stable diazonium salts of weakly basic amines—convenient reagents for synthesis of
disperse azo dyes
Jinjing Qiu, Bingtao Tang, Benzhi Ju, Yuanji Xu, Shufen Zhang
PII:
S0143-7208(16)30430-2
10.1016/j.dyepig.2016.08.026
DYPI 5406
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 29 June 2016
Revised Date: 8 August 2016
Accepted Date: 9 August 2016
Please cite this article as: Qiu J, Tang B, Ju B, Xu Y, Zhang S, Stable diazonium salts of weakly basic
amines—convenient reagents for synthesis of disperse azo dyes, Dyes and Pigments (2016), doi:
10.1016/j.dyepig.2016.08.026.
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ACCEPTED MANUSCRIPT
Stable Diazonium Salts of Weakly Basic Amines—Convenient
Reagents for Synthesis of Disperse Azo Dyes
Jinjing Qiu, Bingtao Tang , Benzhi Ju, Yuanji Xu, Shufen Zhang
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, PR China
ABSTRACT
A new synthetic strategy for industrially important deep-shade disperse azo dyes was
presented in this study. The key procedure is to prepare stable solid diazonium salts of
weakly basic amines in the absence of concentrated sulfuric acid. Diazotization by
tert-butyl nitrite in ethyl acetate was allowed to proceed in the presence of equivalent
1,5-naphthalenedisulfonic acid as stabilizer of diazonium salts and donor of hydrogen ion
for the reaction. The separated solid diazonium salts exhibited good thermal stability. The
corresponding disperse azo dyes were subsequently synthesized through the azo-coupling
of the prepared solid diazonium salts with a range of aromatic tertiary amines. The azo
dyes were produced in short reaction time, excellent yields, mild reaction conditions,
simple experimental procedure and low energy consumption.
Corresponding author. Tel.: +86 411 84986267; fax: +86 411 84986264.
E-mail address: tbt1976@sohu.com (B. Tang).
1

ACCEPTED MANUSCRIPT
Keywords: Stable solid diazonium salts; Weakly basic amines; Disperse azo dyes;
Diazotization
1. Introduction
Azo dyes are compounds containing electron-donating (D) and electron-accepting (A)
functional groups through an azo π-conjugated linker [1] and the D–π–A system has
various electronic structures which can bring a full range of colors [2, 3]. Hence, as
industrially important chromophores, azo dyes are widely used in dying of natural and
synthetic fibers [4, 5], and many high technology fields, such as electronic devices, linear
and nonlinear optics, reprography, sensors [6–8]. To date, approximately 3,000 different
azo dyes are used industrially which is the important guarantee for meeting people’s
demand for color [9, 10].
Diazonium salts are precursors of the synthesis of azo dyes [11] and they are also
widely used in organic synthesis such as Sandmeyer reaction [12], Balz–Schiemann
reaction [13], carbon–carbon coupling reaction [14], and modification of polymeric
materials and nanomaterials [15–17]. However, solid diazonium salts are very dangerous
and have the tendency to explode because of their very poor thermal stability [18]. Thus,
the diazotization of amines with sodium nitrite is usually carried out in the presence of
excess hydrochloric or sulfuric acid solution and the obtained diazonium salt solution is
directly poured into the coupling system to synthesize azo dyes resulting in large amounts
of waste water containing strong liquid acid. Considering the above constraints, strong
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solid acids as replacements for liquid acid catalysts [10, 19], nitrite ionic liquid or
nitrite-functionalized graphene quantum dots as effective nitrosonium sources [9, 20]
were reported to synthesize the corresponding azo dyes in excellent yields. Although each
of these methods has specific merits, the substrates are simple aromatic amines or their
derivatives, such as aniline, aminobenzenesulfonic acid and nitroaniline. Clean synthesis
of azo dyes based on weakly basic amines is still a great challenge because they are very
difficult to be diazotized due to their extremely weak alkalinity.
Weakly basic amines with two or three strong electron-withdrawing groups are the
main raw materials on the preparation of dark disperse azo dyes with excellent color
fastnesses [21]. However, their diazotization reactions using sodium nitrite are difficult to
be carried out in hydrochloric or sulfuric acid solution because of their low reaction
activity and solubility. In industrial production, the diazotization of weakly basic amines
is always conducted in more than 10 times the molar quantity of concentrated sulfuric
acid [22, 23] and has to use nitrosyl sulfuric acid as nitrosating agent which has
remarkable diazotization ability. Subsequently, the concentrated acid solution of
diazonium salt is poured into the coupling system. Due to the strong dilution heat effect
of concentrated sulfuric acid, large amounts of ice were used to maintain low temperature
of the system to avoid the decomposition of the diazonium salt. Meanwhile, it is difficult
to handle the high acid content waste water.
Herein, we present a practical preparation method of disperse azo dyes based on the
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novel stable solid diazonium salts of weakly basic amines. After the reactions of weakly
basic amines with tert-butyl nitrite carried out in organic solution without any sulfuric
acid in the presence of equivalent 1,5-naphthalenedisulfonic acid, the stable solid
diazonium salts were obtained by simple filtration because of their very low solubility in
organic solvent. The related disperse azo dyes were synthesized through the well-known
azo-coupling reaction with aniline derivatives in good yields. Compared with traditional
methods, coupling process can be conducted at room temperature, thus providing high
reaction efficiency, excellent yields, mild reaction conditions, simple experimental
procedure, and low energy consumption.
2. Experimental
2.1. General information
2,6-Dichloro-4-nitroaniline,
2-chloro-4-nitroaniline,
2,6-dibromo-4-nitroaniline,
2-amino-5-nitrobenzonitrile, 2,4-dinitroaniline, N,N-diethylaniline, tert-butyl nitrite and
1,5-naphthalenedisulfonic acid tetrahydrate were purchased from Sinopharm Chemical
Reagent Co., Ltd. 1,5-naphthalenedisulfonic acid tetrahydrate was dried at 130 °C under
high
vacuum
(2.7
kPa)
for
3
h.
3-(N-Ethylanilino)propionitrile,
3,3'-(phenylimino)dipropionitrile and N-[3-(diethylamino)phenyl]acetamide were
purchased from Jiangsu Jihua Chemical Co., Ltd. The solvents were commercial
analytical grade and used as received.
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The diazotization was monitored by thin-layerchromatography on precoated silica gel
glass based plates purchased from Branch of Qingdao Haiyang Chemical Co., Ltd and the
yields of the diazonium salts were obtained by titration with the standard solution of
N,N-diethylaniline at 0 °C–5 °C. The purity of dyes was determined by external standard
method through an Agilent 1260 Infinity HPLC equipment. The diazonium salts and the
dyes were all characterized by spectroscopic data with specified conditions. Tandem mass
spectrometry (MS/MS) was recorded on a Themo Scientific TSQ Quantum Ultra
equipment with suitable collision energies. MS spectra and HRMS spectra were recorded
on a Themo Scientific LTQ Orbitrap XL mass spectrometer. FT-IR spectra were recorded
as Nujol mulls or KBr disks using a Thermo Scientific Nicolet 6700 FT-IR spectrometer.
13
1H NMR spectra and C NMR spectra were recorded on a Bruker Avance III 500 MHz
spectrometer using D2SO4 or DMSO-d6 as the deuterated solvents. The absorbtion spectra
were recorded on an Agilent HP 8453 UV-Vis spectrophotometer. Elemental analysis was
performed on an Element Vario EL III Analyzer. Thermal decomposition temperature of
the diazonium salts were obtained by thermogravimetric analysis (TGA) conducted on a
NETZSCH TG 209 C TGA instrument at 5 °C/min from ambient temperature to 200 °C
under air atmosphere.
2.2. Typical synthesis of stable solid diazonium salt of 2,4-dinitroaniline
2,4-Dinitroaniline (10 mmol) and 1,5-naphthalenedisulfonic acid (10 mmol) were
dissolved in 100 mL ethyl acetate at 50 °C and the mixture was stirred for 10 min.
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Tert-butyl nitrite (15 mmol) was added by dropping to the mixture for about 2 min at
25 °C. The reaction solution was further stirred for 20 min with the temperature
maintained at 25 °C. A salmon pink solid was obtained by suction filtration and air drying.
The yield was 99.0%.
2.3. Characterization of the stable diazonium salts
2-Chloro-4-nitrobenzene diazonium salt (Table 1, 2a): MS/MS (10 eV), m/z: calcd for
diazonium cation, 183.99; found, 184.03. 1H NMR (D2SO4) δ (ppm): 9.22 (2.00H, d, J =
8.7 Hz), 9.14–9.06 (3.72H, m), 8.94 (1.86H, dd, J = 9.1, 2.2 Hz), 8.85–8.76 (2.00H, m),
8.25 (2.00H, dd, J = 8.7, 7.5 Hz). FT-IR (Nujol mull, cm-1): 3100, 3071, 2311, 2280,
1569, 1528, 1499, 1461, 1377, 1358, 1298, 1237, 1164, 1098, 1040. Anal. Calcd: C,
40.35; H, 1.88; N, 12.41%. Found: C, 39.63; H, 1.80; N, 12.54%.
2,6-Dibromo-4-nitrobenzene diazonium salt (Table 1, 2b): MS/MS (10 eV), m/z: calcd
for diazonium cation, 307.85; found, 307.63. 1H NMR (D2SO4) δ (ppm): 9.22 (2.00H, d, J
= 8.8 Hz), 9.15 (1.82H, s), 8.79 (2.00H, dd, J = 7.4, 10 Hz), 8.23 (2.00H, dd, J = 8.8, 7.5
Hz). FT-IR (Nujol mull, cm-1): 3084, 2288, 1561, 1539, 1499, 1460, 1377, 1345, 1292,
1231, 1199, 1176, 1152, 1076, 1043. Anal. Calcd: C, 32.29; H, 1.52; N, 7.06%. Found: C,
32.34; H, 1.44; N, 7.22%.
2,6-Dichloro-4-nitrobenzene diazonium salt (Table 1, 2c): MS/MS (5 eV), m/z: calcd
1
for diazonium cation, 217.95; found, 218.18. H NMR (D2SO4) δ (ppm): 9.21 (2.00H, d,
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J = 8.8 Hz), 8.98 (2.72H, s), 8.80 (2.00H, d, J = 7.5 Hz), 8.23 (2.00H, t, J = 8.2 Hz).
FT-IR (Nujol mull, cm-1): 3087, 2285, 1551, 1460, 1415, 1377, 1345, 1293, 1268, 1244,
1220, 1201, 1167, 1110, 1034, 1019. Anal. Calcd: C, 37.30; H, 1.61; N, 9.77%. Found: C,
36.33; H, 1.82; N, 9.11%.
2-Cyano-4-nitrobenzene diazonium salt (Table 1, 2d): MS/MS (8 eV), m/z: calcd for
diazonium cation, 175.03; found, 175.28. 1H NMR (D2SO4) δ (ppm): 9.65 (1.59H, d, J =
1.8 Hz), 9.48–9.41 (3.18H, m), 9.23 (2.00H, d, J = 8.8 Hz), 8.81 (2.00H, d, J = 7.5 Hz),
8.25 (2.00H, t, J = 8.2 Hz), 4.88 (1.58H, q, J = 7.1 Hz), 1.80 (1.05H, d, J = 14.2 Hz).
FT-IR (Nujol mull, cm-1): 3473, 3099, 2309, 2245, 1613, 1549, 1498, 1460, 1377, 1356,
1315, 1236, 1189, 1157, 1101, 1040. Anal. Calcd: C, 43.77; H, 2.23; N, 15.36%. Found:
C, 43.04; H, 2.76; N, 14.79%.
2,4-Dinitrobenzene diazonium salt (Table 1, 2e): MS/MS (10 eV), m/z: calcd for
diazonium cation, 195.01; found, 195.02. 1H NMR (D2SO4) δ (ppm): 9.78 (1.85H, q, J =
2.0 Hz), 9.43 (3.70H, qd, J = 8.8, 5.0 Hz), 9.23 (2.00H, dd, J = 8.8, 2.9 Hz), 8.82 (2.00H,
dd, J = 7.7, 2.9 Hz), 8.26 (2.00H, td, J = 8.3, 3.0 Hz). FT-IR (Nujol mull, cm-1): 3099,
3076, 3043, 2312, 2287, 1612, 1591, 1546, 1498, 1462, 1377, 1348, 1331, 1246, 1227,
1215, 1201, 1163, 1142, 1111, 1090, 1036. Anal. Calcd: C, 39.11; H, 1.81; N, 16.00%.
Found: C, 39.07; H, 1.82; N, 15.83%.
2.4. Typical procedure for coupling
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N,N-Diethylaniline (10 mmol) was dissolved in 80 mL water with 1.2 mL 37%
hydrochloric acid and then the salmon pink diazonium salt of 2,4-dinitroaniline was
quickly added within 2 min at 0 °C–5 °C followed by adjusting the pH to 4–5 with 2 M
NaOH solution. The reaction endpoint was confirmed by color reaction using H-acid. The
precipitated dye was filtered, washed with water and dried at 80 °C to afford the crude
product. The crude yield was 95.0% and the degree of purity was 97.0%.
2.5. Spectroscopic data of disperse azo dyes
Compound 4a: C16H17ClN4O2: MS, m/z: calcd for [M+H]+, 333.11; found, 333.10.
1
HRMS, m/z: calcd for [M+H]+, 333.1113; found, 333.1115. H NMR (DMSO-d6) δ
(ppm): 8.44 (1H, q, J = 2.5 Hz), 8.25 (1H, dq, J = 9.0, 2.5 Hz), 7.90–7.83 (2H, m),
7.83–7.76 (1H, m), 6.89 (2H, dt, J = 9.0, 2.8 Hz), 3.52 (4H, q, J = 7.2 Hz), 1.18 (6H, t, J
13
= 6.9 Hz). C NMR (126 MHz, DMSO-d6) δ (ppm): 152.49, 151.88, 146.59, 143.12,
132.11, 126.92, 125.67, 123.33, 117.95, 111.53, 44.38, 12.50. FT-IR (KBr, cm-1): 3099,
2972, 2903, 1599, 1579, 1513, 1449, 1407, 1355, 1329, 1307, 1137, 1121, 1074, 1045,
1007, 887, 831, 819. UV (λmax in acetone): 513 nm.
Compound 4b: C16H16Br2N4O2: MS, m/z: calcd for [M+H]+, 456.97; found, 457.01.
1
HRMS, m/z: calcd for [M+H]+, 456.9692; found, 456.9697. H NMR (DMSO-d6) δ
(ppm): 8.56 (2H, s), 7.84–7.78 (2H, m), 6.90–6.83 (2H, m), 3.51 (4H, q, J = 7.1 Hz), 1.17
13
(6H, t, J = 7.0 Hz). C NMR (126 MHz, DMSO-d6) δ (ppm): 155.23, 151.85, 145.67,
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141.65, 127.78, 126.22, 115.16, 111.19, 44.32, 12.44. FT-IR (KBr, cm-1): 3084, 2971,
1601, 1521, 1394, 1355, 1336, 1273, 1253, 1191, 1163, 1131, 1072, 1051, 1007, 888, 819.
UV (λmax in acetone): 441 nm.
Compound 4c: C16H16Cl2N4O2: MS, m/z: calcd for [M+H]+, 367.07; found, 367.08.
1
HRMS, m/z: calcd for [M+H]+, 367.0723; found, 367.0724. H NMR (DMSO-d6) δ
(ppm): 8.43 (2H, s), 7.84–7.78 (2H, m), 6.90–6.83 (2H, m), 3.51 (4H, q, J = 7.0 Hz), 1.17
13
(6H, t, J = 7.0 Hz). C NMR (126 MHz, DMSO-d6) δ (ppm): 153.12, 151.93, 145.21,
142.09, 126.77, 126.28, 124.46, 111.20, 44.33, 12.43. FT-IR (KBr, cm-1): 3089, 2974,
1603, 1521, 1396, 1355, 1340, 1274, 1253, 1192, 1148, 1127, 1061, 1008, 887, 821. UV
(λmax in acetone): 443 nm.
Compound 4d: C17H17N5O2: MS, m/z: calcd for [M+H]+, 324.15; found, 324.13.
1
HRMS, m/z: calcd for [M+H]+, 324.1455; found, 324.1457. H NMR (DMSO-d6) δ
(ppm): 8.83 (1H, s), 8.52 (1H, dd, J = 9.1, 2.6 Hz), 7.95 (1H, d, J = 9.1 Hz), 7.89 (2H, d,
13
J = 8.9 Hz), 7.01–6.90 (2H, m), 3.56 (4H, q, J = 7.0 Hz), 1.19 (6H, t, J = 7.1 Hz). C
NMR (126 MHz, DMSO-d6) δ (ppm): 157.22, 152.71, 145.79, 142.96, 129.50, 128.92,
127.54, 117.43, 115.62, 111.96, 110.29, 44.58, 12.53. FT-IR (KBr, cm-1): 3077, 2975,
2931, 2229, 1597, 1577, 1516, 1414, 1355, 1324, 1271, 1249, 1171, 1136, 1117, 1071,
1007, 907, 838, 823. UV (λmax in acetone): 536 nm.
Compound 4e: C16H17N5O4: MS, m/z: calcd for [M+H]+, 344.14; found, 344.14.
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1
HRMS, m/z: calcd for [M+H]+, 344.1353; found, 344.1354. H NMR (DMSO-d6) δ
(ppm): 8.89–8.83 (1H, m), 8.50 (1H, ddd, J = 9.2, 6.5, 2.6 Hz), 7.95–7.87 (1H, m), 7.77
(2H, ddd, J = 9.6, 5.0, 2.1 Hz), 6.90 (2H, td, J = 7.1, 5.4, 2.1 Hz), 3.58–3.49 (4H, m),
13
1.21–1.14 (6H, m). C NMR (126 MHz, DMSO-d6) δ (ppm): 152.48, 148.07, 145.99,
145.39, 143.00, 127.55, 127.37, 119.80, 119.18, 111.77, 44.52, 12.48. FT-IR (KBr, cm-1):
3100, 2976, 2931, 1592, 1516, 1411, 1353, 1328, 1271, 1196, 1134, 1113, 1066, 911, 831,
822. UV (λmax in acetone): 529 nm.
Compound 4f: C17H15Br2N5O2: MS, m/z: calcd for [M+H]+, 481.96; found, 482.12.
1
HRMS, m/z: calcd for [M+H]+, 481.9645; found, 481.9649. H NMR (DMSO-d6) δ
(ppm): 8.82–8.33 (2H, m), 7.88–7.79 (2H, m), 7.01–6.94 (2H, m), 3.80 (2H, t, J = 6.8
Hz), 3.57 (2H, q, J = 7.0 Hz), 2.85 (2H, t, J = 6.8 Hz), 1.18 (3H, t, J = 7.0 Hz). 13C NMR
(126 MHz, DMSO-d6) δ (ppm): 155.12, 151.51, 145.82, 142.35, 127.77, 125.94, 119.15,
115.01, 111.80, 45.41, 44.66, 15.72, 12.04. FT-IR (KBr, cm-1): 3100, 3066, 2970, 2933,
2248, 1600, 1522, 1399, 1342, 1280, 1245, 1142, 1077, 1054, 881, 825. UV (λmax in
acetone): 421 nm.
Compound 4g: C17H15Cl2N5O2: MS, m/z: calcd for [M+H]+, 392.07; found, 392.20.
1
HRMS, m/z: calcd for [M+H]+, 392.0676; found, 392.0677. H NMR (DMSO-d6) δ
(ppm): 8.42 (2H, s), 7.83 (2H, d, J = 9.0 Hz), 6.97 (2H, d, J = 9.1 Hz), 3.81 (2H, t, J = 6.8
Hz), 3.58 (2H, q, J = 7.0 Hz), 2.85 (2H, t, J = 6.7 Hz), 1.18 (3H, t, J = 7.0 Hz). 13C NMR
(126 MHz, DMSO-d6) δ (ppm): 152.99, 151.59, 145.40, 142.76, 126.66, 125.99, 124.46,
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119.12, 111.80, 45.41, 44.67, 15.70, 12.03. FT-IR (KBr, cm-1): 3059, 2980, 2252, 1604,
1572, 1529, 1519, 1394, 1336, 1311, 1280, 1148, 1134, 1078, 1065, 884, 822. UV (λmax
in acetone): 423 nm.
Compound 4h: C18H16N6O2: MS, m/z: calcd for [M+H]+, 349.14; found, 349.25.
1
HRMS, m/z: calcd for [M+H]+, 349.1408; found, 349.1412. H NMR (DMSO-d6) δ
(ppm): 8.84 (1H, d, J = 2.6 Hz), 8.54 (1H, dd, J = 9.1, 2.6 Hz), 7.96 (1H, d, J = 9.1 Hz),
7.90 (2H, d, J = 8.9 Hz), 7.04 (2H, d, J = 9.3 Hz), 3.84 (2H, t, J = 6.8 Hz), 3.61 (2H, J =
13
7.1 Hz), 2.87 (2H, t, J = 6.7 Hz), 1.24–1.15 (m, 3H). C NMR (126 MHz, DMSO-d6) δ
(ppm): 156.82, 152.29, 146.18, 143.47, 129.41, 128.92, 127.09, 119.01, 117.54, 115.44,
112.32, 110.76, 45.48, 44.86, 15.74, 12.12. FT-IR (KBr, cm-1): 3080, 2919, 2248, 2231,
1600, 1595, 1577, 1525, 1518, 1411, 1340, 1325, 1309, 1290, 1173, 1133, 1121, 910, 842,
825. UV (λmax in acetone): 511 nm.
Compound 4i: C18H15ClN6O2: MS, m/z: calcd for [M+H]+, 383.10; found, 383.15.
1
HRMS, m/z: calcd for [M+H]+, 383.1018; found, 383.1021. H NMR (DMSO-d6) δ
(ppm): 8.45 (1H, ddd, J = 7.4, 4.6, 2.4 Hz), 8.26 (1H, ddt, J = 8.9, 6.0, 2.2 Hz), 7.88 (2H,
ddd, J = 9.4, 4.5, 2.3 Hz), 7.81–7.73 (1H, m), 7.09 (2H, dt, J = 9.4, 2.2 Hz), 3.89 (4H, t, J
13
= 7.0 Hz), 2.86 (4H, t, J = 6.9 Hz). C NMR (126 MHz, DMSO-d6) δ (ppm): 152.11,
150.78, 147.27, 144.25, 132.64, 126.29, 125.73, 123.37, 118.99, 118.24, 112.55, 45.68,
15.40. FT-IR (KBr, cm-1): 2958, 2250, 1600, 1514, 1404, 1362, 1339, 1314, 1151, 1121,
1046, 888, 832. UV (λmax in acetone): 459 nm.
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Compound 4j: C18H14Cl2N6O2: MS, m/z: calcd for [M+H]+, 417.06; found, 417.06.
1
HRMS, m/z: calcd for [M+Na]+, 339.0448; found, 439.0447. H NMR (DMSO-d6) δ
(ppm): 8.43 (2H, dd, J = 6.4, 1.9 Hz), 7.85 (2H, d, J = 9.2 Hz), 7.09 (2H, d, J = 9.2 Hz),
13
3.89 (4H, t, J = 6.8 Hz), 2.86 (4H, t, J = 6.8 Hz). C NMR (126 MHz, DMSO-d6) δ
(ppm): 152.88, 150.97, 145.59, 143.38, 126.55, 125.78, 124.49, 119.00, 112.38, 45.66,
15.36. FT-IR (KBr, cm-1): 3079, 2918, 2247, 1603, 1568, 1518, 1406, 1377, 1345, 1249,
1208, 1147, 1062, 1026, 900, 831. UV (λmax in acetone): 405 nm.
Compound 4k: C18H19Br2N5O3: MS, m/z: calcd for [M+H]+, 513.99; found, 513.99.
1
HRMS, m/z: calcd for [M+H]+, 513.9907; found, 513.9913. H NMR (DMSO-d6) δ
(ppm): 10.23 (1H, s), 8.54 (2H, s), 7.93 (1H, d, J = 3.0 Hz), 7.72 (1H, d, J = 9.4 Hz), 6.67
(1H, dd, J = 9.5, 2.8 Hz), 3.51 (4H, q, J = 7.1 Hz), 2.15 (3H, s), 1.20 (6H, t, J = 7.0 Hz).
13C NMR (126 MHz, DMSO-d6) δ (ppm): 169.25, 153.78, 153.28, 145.15, 139.97,
130.28, 128.06, 123.12, 116.07, 107.94, 100.07, 44.76, 25.06, 12.51. FT-IR (KBr, cm-1):
3091, 2981, 2930, 1690, 1622, 1616, 1529, 1513, 1494, 1336, 1291, 1184, 1167, 1125,
1074, 1055, 1005, 890, 873, 858. UV (λmax in acetone): 491 nm.
Compound 4l: C18H19Cl2N5O3: MS, m/z: calcd for [M+H]+, 424.09; found, 424.23.
1
HRMS, m/z: calcd for [M+H]+, 424.0938; found, 424.0950. H NMR (DMSO-d6) δ
(ppm): 10.30 (1H, s), 8.41 (2H, d, J = 1.0 Hz), 7.91 (1H, d, J = 2.7 Hz), 7.71 (1H, d, J =
9.5 Hz), 6.67 (1H, dd, J = 9.4, 2.7 Hz), 3.51 (4H, q, J = 7.1 Hz), 2.14 (3H, s), 1.20 (6H, t,
13
J = 7.0 Hz). C NMR (126 MHz, DMSO-d6) δ (ppm): 169.26, 153.34, 151.91, 144.66,
12

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139.95, 130.92, 127.42, 124.66, 123.25, 108.00, 100.16, 44.78, 24.94, 12.51. FT-IR (KBr,
cm-1): 3353, 3103, 2980, 2935, 2901, 1704, 1607, 1552, 1531, 1495, 1419, 1350, 1330,
1256, 1172, 1111, 1064, 1005, 901, 886, 855. UV (λmax in acetone): 496 nm.
Compound 4m: C18H20N6O5: MS, m/z: calcd for [M+H]+, 401.16; found, 401.22.
1
HRMS, m/z: calcd for [M+H]+, 401.1568; found, 401.1573. H NMR (DMSO-d6) δ
(ppm): 10.69 (1H, s), 8.83 (1H, d, J = 2.5 Hz), 8.47 (1H, dd, J = 9.1, 2.5 Hz), 8.18 (1H, d,
J = 9.1 Hz), 7.95 (1H, d, J = 2.7 Hz), 7.75–7.63 (1H, m), 6.73 (1H, dd, J = 9.5, 2.8 Hz),
13
3.53 (4H, q, J = 7.1 Hz), 2.21 (3H, s), 1.20 (6H, t, J = 7.0 Hz). C NMR (126 MHz,
DMSO-d6) δ (ppm): 169.70, 153.93, 148.27, 145.13, 144.54, 132.33, 127.39, 123.76,
119.95, 119.40, 109.02, 100.47, 9.48, 45.02, 24.88, 12.58. FT-IR (KBr, cm-1): 3448, 3095,
2975, 2935, 1693, 1624, 1593, 1567, 1520, 1488, 1419, 1323, 1279, 1254, 1193, 1172,
1137, 1117, 1078, 897, 858, 845, 831. UV (λmax in acetone): 546 nm.
3. Results and discussion
3.1. Preparation and characterization of stable solid diazonium salts
The solid diazonium salts of aromatic amines were conveniently obtained by simple
filtration after the reactions of weakly basic amines with tert-butyl nitrite which were
carried out in organic solution in the presence of equivalent 1,5-naphthalenedisulfonic
acid (Scheme 1). The structures of diazonium salts were confirmed by FT-IR and tandem
mass spectrometry according to references [24, 25].
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---Insert Scheme 1---
Optimization of the diazotization conditions was investigated with different molar
ratios of the aromatic amine, 1,5-naphthalenedisulfonic acid and tert-butyl nitrite and the
optimal ratio was 1:1:1.5. The new diazonium salts of weakly basic amines were formed
by the diazonium cation and 1,5-naphthalenedisulfonic anion. Diazonium salts exist in
two forms, i.e., structure A and structure B (Scheme 1). The ratio of each structure form
1
was further confirmed by manual integration in H NMR (Fig. 1). The corresponding
results are summarized in Table 1. For 2,4-dinitrobenzene diazonium salt, the integral
1
ratio in the H NMR at 8.25 and 9.42 was about 1:1.85, which are corresponding to the
1,5-naphthalenedisulfonic acid anion and 2,4-dinitrobenzene diazonium cation (Fig. 1,
2e). Accordingly, the ratio of structures A and B is 0.15:0.85; thus, structure B accounts
for 85% of the whole diazonium salt structures (Table 1, entry 5). This result can also be
verified by elemental analysis (see Experimental section).
---Insert Fig. 1---
---Insert Table 1---
Reaction temperatures were stipulated in accordance with different substrates (Table 2).
Although diazotization proceeded without any concentrated sulfuric acid or nitrosyl
sulfuric acid, the reaction efficiency was very high. As a result, the very low basic amine,
2,4-dinitroaniline, could be diazotized completely within a short time (22 min, Table 2,
14

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entry 5). The diazotization times of three substrates (Table 2, entries 3-5) were much less
than that of the traditional methods [28–30]. Meanwhile, the yields of the diazonium salts
were excellent (Table 2). The high reaction efficiency may be attributed to the formation
of nitrosonium cation (+NO) [31] in the presence of 1,5-naphthalenedisulfonic acid and
tert-butyl nitrite.
---Insert Table 2---
3.2 Thermal stability of solid diazonium salts
---Insert Fig. 2---
TGA was used to characterize the thermal stability of the diazonium salts. The initial
thermal decomposition temperatures of 2a, 2b, 2c, 2d, and 2e are 164.4, 175.5, 125.7,
115.9, and 121.9 °C, respectively (Table 2 and Fig. 2). Thus, the prepared diazonium salts
of weakly basic amines are thermostable and can be stored at room temperature in dried
state without disintegration.
3.3. Synthesis of disperse azo dyes
The disperse azo dyes were synthesized by the well-known azo-coupling reaction
between the as-prepared stable diazonium salts and the tertiary anilines at 0 °C–5 °C. The
pH of the coupling system was adjusted and maintained at 4–6 by 2 M NaOH solution.
The crude yields and purities of the relevant disperse azo dyes are shown in Table 3.
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---Insert Scheme 2---
---Insert Table 3---
The coupling time of the present method was all much shorter than those reported in
the literatures (a few to dozens of minutes, Table 3, entries 1, 6, and 9). The reaction
ability of the tertiary anilines is restricted by concentrated sulfuric acid in traditional
methods. However, large amounts of concentrated acid were replaced in the present
method by equivalent organic acid in contrast to amines during diazotization.
Consequently, the coupling reaction could be conducted with short time at low
concentration acid.
---Insert Fig. 3---
Compared with traditional coupling technology, the present method does not employ
the dilution process of concentrated sulfuric acid because of the usage of solid diazonium
salts. Thus, the generation of a large amount of heat produced from the dilution of
sulfuric acid in the traditional coupling process was avoided. Meanwhile, the thermal
analysis results of solid diazonium salts show that their thermal stabilities are sufficient.
Therefore, it is not needed to use a lot of ice to keep the system at low temperature or
even the reaction can be carried out at normal temperature without any cooling process.
Compounds 4a, 4c, and 4d were chosen as objects of study and prepared with different
temperatures. The yields of the dyes are shown in Fig. 3, which indicate no significant
16

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reducing of the yield during temperature increase. These results show that this new
technology is of great significance for the realization of energy saving and emission
reduction in the production of disperse azo dyes based on weakly basic amines.
4. Conclusion
A novel strategy for synthesis of industrial disperse azo dyes based on weakly basic
amines is presented. The key advantage of this new procedure is the formation and simple
separation of stable solid diazonium salts of weakly basic amines. This new procedure
can avoid the strong heat effect coming from the dilution of concentrated sulfuric acid in
the traditional coupling process. Large amounts of ice were saved and the generation of
highly concentrated acidic waste water was prevented. For these reasons, the efficient,
rapid and economical synthesis of disperse azo dyes employing stable solid diazonium
salts has a very good prospect in the industrial application.
Acknowledgments
This work was supported by the National Natural Science Foundation of China
(21276042, 21576039 and 21421005), Program for Innovative Research Team in
University (IRT_13R06). Fundamental Research Funds for the Central Universities
(DUT14YQ209), Program for New Century Excellent Talents in University
(NCET130080), Program for Liaoning Excellent Talents in University (LJQ2013006).
References
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Scheme 1. Preparation of stable diazonium salts in organic solvent.
Scheme 2. Azo-coupling of stable diazonium salts with tertiary aniline derivatives.
Fig. 1. 1H NMR spectra of the diazonium salts in D2SO4.
Fig. 2. Thermogravimetric curves of five stable solid diazonium salts.
Fig. 3. Yields of three representative dyes at different coupling temperatures (Yield = Crude yield
× purity).

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Table 1
Structures of the stable solid diazonium salts
Structure
number
Ratio of
structures A and B
Entry
1
Amine
1a
Diazonium salt
2a
0.14:0.86
1.00:0.00
2
3
1b
1c
2b
2c
0.64:0.36
4
5
1d
1e
2d
2e
0.41:0.59
0.15:0.85
Table 2
Preparation of stable diazonium salts
Reaction time (min)
Structure Temperature
Entry
Yield
(%)
Thermal decomposition
temperature (°C)
number
(°C)
Found
60
Reported [Ref.]
1
2
3
4
5
2aa
2b
2c
2d
2e
0
60[26]
92.8
93.1
98.0
89.7
99.0
164.4
175.5
125.7
115.9b
121.9
50
22
0
120
17
120[27]
120[28]
120[29]
120[30]
60
25
22
a The 2-chloro-4-nitroaniline was added slowly to the reaction system after tert-butyl nitrite.
b There was a continuous lower decline of the TG curves before 115.9 °C.

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Table 3
Synthesis of azo dyes using stable diazonium salts
Reaction time (min)
Crude
yield
(%)a
Diazonium Coupling
Structure
number
Purity
(%)
Entry
1
Product
Reported
Found
salt
2a
reagent
[Ref.]
4a
4b
4c
4d
4e
4f
5
120 [32]
92.6
90.8
88.5
90.9
95.0
93.5
96.1
94.7
94.8
94.0
95.9
97.9
97.7
88.8
97.0
96.7
96.0
87.3
96.6
90.9
2
2b
2c
2d
2e
2b
2c
2d
2a
2c
5
--
3
5
--
4
5
--
5
5
--
6
12
10
16
40
19
120 [27]
7
4g
4h
4i
--
8
--
9
300 [26]
10
4j
--

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11
12
2b
2c
4k
4l
6
--
--
94.3
95.9
91.6
91.5
19
13
2e
4m
34
--
95.9
89.3
a The crude yields of disperse azo dyes were calculated based on the initial weakly basic amines.

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