Extremely high pH stability for a class of heterocyclic azo dyes having the common N2,N6-bis(3-methoxypropyl)pyridine-2,6-diamine coupling component

Article abstract of DOI:10.1016/j.dyepig.2017.06.030

A series of N²,N⁶-bis(3-methoxypropyl)pyridine-2,6-diamine azo dyes, prepared by coupling 2,6-bis((3-methoxypropyl)amino)-4-methylnicotinonitrile and diazotized substituted anilines with distinguishable electron push-pull abilities, have been described in this paper. The new dyes undergoing double functional group transformation (FGT) show extremely high pH stability compared to corresponding mono FGT dyes, which could be ascribed to the introduction of the second 3-methoxypropylamino group forming the new pyridine-2,6-diamine backbone. It is noted that the unusual transformation from D?π?A to A?π?D system has been verified for our multi-substituted phenyl-azo-pyridine FGT products. The adjustment of electron-donating and electron-withdrawing phenyl and pyridine substituted groups narrows the discrepancy of electron push-pull capabilities for dizao and coupling components, which makes possible the transformation for roles of donor and acceptor. It is believed that the achievement of extremely high pH stability for pyridine-2,6-diamine based heterocyclic azo dyes is regarded as a useful exploration for designing new FGT modified pyridone dyes.

Full text of DOI:10.1016/j.dyepig.2017.06.030

Accepted Manuscript
Extremely high pH stability for a class of heterocyclic azo dyes having the common
2
6
N ,N -bis(3-methoxypropyl)pyridine-2,6-diamine coupling component
Xiao-Lei Zhao, Teng Jun, Ya-Nan Feng, Hui-Fen Qian, Wei Huang
PII:
S0143-7208(17)30994-4
10.1016/j.dyepig.2017.06.030
DYPI 6051
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 30 April 2017
Revised Date: 10 June 2017
Accepted Date: 13 June 2017
Please cite this article as: Zhao X-L, Jun T, Feng Y-N, Qian H-F, Huang W, Extremely high pH stability
2
6
for a class of heterocyclic azo dyes having the common N ,N -bis(3-methoxypropyl)pyridine-2,6-
diamine coupling component, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.06.030.
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ACCEPTED MANUSCRIPT
Table of Content
Extremely high pH stability for a
class of heterocyclic azo dyes having
the common
N ,N -bis(3-methoxypropyl)pyridine
2,6-diamine component
2
6
-
Xiao-Lei Zhao, Teng Jun, Ya-Nan
Feng, Hui-Fen Qian, Wei Huang

ACCEPTED MANUSCRIPT
Extremely high pH stability for a class of heterocyclic azo dyes having the
2
6
common N ,N -bis(3-methoxypropyl)pyridine-2,6-diamine coupling component
a
a
b
b
a, ∗
Xiao-Lei Zhao , Teng Jun , Ya-Nan Feng , Hui-Fen Qian , Wei Huang
a
State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of
Microstructures, School of Chemistry and Chemical Engineering, Nanjing University,
Nanjing, Jiangsu Province, 210093, P. R. China
b
College of Sciences, Nanjing Tech University, Nanjing, Jiangsu Province, 210009, P.
R. China
E-mail addresses for all authors: Xiao-Lei Zhao, zhaoxiaolei2016@163.com
Teng Jun, 18648450506@163.com
Ya-Nan Feng, 1179759806@qq.com
Hui-Fen Qian, qhf@njtech.edu.cn
Wei Huang, whuang@nju.edu.cn
∗
Corresponding author.
E-mail address: whuang@nju.edu.cn
1

ACCEPTED MANUSCRIPT
Abstract
2
6
A series of N ,N -bis(3-methoxypropyl)pyridine-2,6-diamine azo dyes, prepared by
coupling 2,6-bis((3-methoxypropyl)amino)-4-methylnicotinonitrile and diazotized
substituted anilines with distinguishable electron push-pull abilities, have been
described in this paper. The new dyes undergoing double functional group
transformation (FGT) show extremely high pH stability compared to corresponding
mono FGT dyes, which could be ascribed to the introduction of the second
3
-methoxypropylamino group forming the new pyridine-2,6-diamine backbone. It is
noted that the unusual transformation from D−π−A to A−π−D system has been
verified for our multi-substituted phenyl-azo-pyridine FGT products. The adjustment
of electron-donating and electron-withdrawing phenyl and pyridine substituted groups
narrows the discrepancy of electron push-pull capabilities for dizao and coupling
components, which makes possible the transformation for roles of donor and acceptor.
It is believed that the achievement of extremely high pH stability for
pyridine-2,6-diamine based heterocyclic azo dyes is regarded as a useful exploration
for designing new FGT modified pyridone dyes.
Keywords: Pyridone dye; Functional group transformation; UV–Vis spectrum;
Push-pull substituent; pH Stability
2

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1
. Introduction
Since their discovery in the 20th century, heterocyclic disperse dyes have spurred
tremendous interest among researchers because of their applications not only in the
dyestuff industry [1−2] as high level-dyeing agents but also in the electronic industry
including nonlinear optical devices [3], colorimetric sensors [4−5] and dye-sensitized
solar cells [6]. Among them, pyridine-2,6-dione based intermediates are considered as
useful coupling components in the syntheses of heterocyclic disperse dyes with a
variety of diazo components [7−12], and the resultant dyes have exhibited bright hues
and excellent performance on versatile fastness [13−16]. For instance, Jang et al. have
investigated the various color range and dyeing properties for several
pyridine-2,6-dione based arylazo dyes on polytrimethylene terephthalate (PTT) fabric,
where the wash and light fastness were improved in comparison with C.I. Disperse
Yellow 241 [17].
Regrettably, the color of these pyridine-2,6-dione based dyes fades quickly under
the basic condition in the dyeing process [18−19], so the fine control of pH values
should be taken into account. The possible reason is suggested to be the following:
Pyridine-2,6-dione based heterocyclic dyes generally exist in the hydrazone
tautomeric form under the neutral and acidic conditions, but they turn to the
deprontonated azo form under the basic condition resulting in the alteration of the
π-conjugated system and the color instability of the whole molecules simultaneously.
The proton transfer is originated from the azo-hydrazone tautomerism in
pyridine-2,6-dione based dyes, and our previous studies have demonstrated that
3

ACCEPTED MANUSCRIPT
azo-hydrazone tautomerism can be driven by the solvent polarity, pH titration and
metal-ion coordination [20−24].
To avoid the color variation problem for pyridine-2,6-dione based dyes, we have
previously proposed a functional group transformation (FGT) strategy, where the
acidic hydroxyl (−OH) has been transformed into a basic secondary amine
(
−NHCH CH CH OCH ) accomplished by the configuration transformation from the
2 2 2 3
hydrazone form to azo one [25]. As a result, the above-mentioned proton transfer and
azo-hydrazone tautomerism have been successfully obstructed which can be verified
by their increased pH stability. Actually, FGT strategy has been widely used in the
antimicrobial surface modification [26], membrane modifications [27], polymer
grafting and cross linking [28−29], and the introduction of reactive centers for further
reactions [30]. However, there appear to be no references using FGT strategy to
modify pyridine-2,6-dione precursors before us.
In this work, we have selected a special pyridine-2,6-dione precursor, i.e.
6
-hydroxy-4-methyl-2-oxo-1,2-dihydropyridine-3-carbonitrile, to fulfill the double
FGT
for
both
acidic
hydroxyl
groups.
As
a
result,
six
2
6
N ,N -bis(3-methoxypropyl)pyridine-2,6-diamine based azo dyes (Scheme 1) have
been produced via a 2,6-dichloropyridine intermediate, where two basic secondary
amine units (−NHCH CH CH OCH ) are successfully introduced. From the
2
2
2
3
viewpoint
of
molecular
design,
the
targeting
azo
dyes
with
2
6
N ,N -bis(3-methoxypropyl)pyridine-2,6-diamine backbone are no longer pyridone
derivatives, so they exhibit extremely high pH stability (1~13) evidenced by the
4

ACCEPTED MANUSCRIPT
acid-base titration. More interesting, in the studies on structure-performance
relationship for this family of double 3-methoxypropan-1-amine substituted
heterocyclic azo dyes, the uncommon transformation for roles of donor and acceptor
(D−π−A and A−π−D) has been observed because of very close electron
donating/withdrawing capabilities of substituted phenyl rings, which can be deduced
from related electronic absorption spectral comparisons. As far as we are aware, this
2
6
is the first report on N ,N -bis(3-methoxypropyl)pyridine-2,6-diamine heterocyclic
azo dyes prepared from 6-hydroxy-4-methyl-2-oxo-1,2-dihydropyridine-3-carbonitrile
via the FGT strategy.
2
. Experimental section
2
.1.Materials and physical measurements
Analytical grade reagents were purchased from commercial suppliers and used
without any further purification unless otherwise stated. Intermediates 3 and 4 were
prepared according to our previously reported procedure [25]. Column
chromatography was carried out on silica gel (200–300 mesh). All melting points
were measured without corrections. Ultraviolet-Visible (UV–Vis) spectra were
recorded with a Shimadzu UV–3150 double-beam spectrophotometer using a quartz
1
glass cell with a path length of 10 mm at room temperature (25 ℃). H NMR spectral
measurements were performed on a Bruker DMX300 MHz NMR or a Bruker
DMX400 MHz spectrometer, using chloroform-d (CDCl ) as a solvent with
3
tetramethylsilane (TMS) as the internal standard at room temperature (25 ℃). Infrared
5

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–
1
spectra in the region of 4000–500 cm were obtained using a Nicolet FT–IR 170X
spectrophotometer on KBr disks. Electrospray ionization mass spectra (ESI–MS)
were recorded on a Finnigan MAT SSQ 710 mass spectrometer in a scan range of
5
0–500 or 50–1000 amu. Elemental analyses (EA) for C, H and N were performed on
a Perkin-Elmer 1400C analyzer.
2
.2.1. Preparation
2,6-bis((3-methoxypropyl)amino)-4-methylnicotinonitrile)
,6-Dichloro-4-methylnicotinonitrile (1.86 g, 10.0 mmol) was added to
-methoxypropan-1-amine (3.98 g, 44.0 mmol) under stirring, and the mixture was
of
compound
1
(
2
3
refluxed for 3 h. After being cooled to room temperature, 30 mL of water and 30 mL
of ethyl acetate were added into the reaction solution. The aqueous layer was
separated, and the ethyl acetate layer was washed twice by brine and dried with
anhydrous MgSO . The solvent was removed by a rotary evaporator after removing
4
MgSO , and the crude solid was recrystallized by ethyl acetate and hexane (v:v = 4:1)
4
to give compound 1 in a yield of 2.19 g (75%). Mp: 41–43 ℃. Main FT–IR
–
1
absorptions (KBr pellets, ν cm ): 3376 (s), 2923 (m), 2871 (w), 2188 (vs), 1598 (vs),
1
1
5
3
6
526 (s), 1356 (m) and 1119 (s). H NMR (400 MHz, CDCl , ppm): δ = 5.55 (s, 1H),
3
.40 (t, J = 4.8 Hz, 1H), 4.98 (t, J = 5.0 Hz, 1H), 3.51 (m, 6H), 3.41 (m, 2H), 3.37 (s,
H), 3.35 (s, 3H), 2.24 (s, 3H) and 1.86 (m, 4H). Anal. Calcd. For C H N O : C,
15
24
4
2
1.12; H, 8.27; N, 19.16%. Found: C, 60.92; H, 8.48; N, 19.02%. Positive ESI–MS in
+
+
MeOH: m/z = 293.25 (100.0%), [M+H] ; 315.25 (84.0%), [M+Na] . Single crystals of
compound 1 suitable for X–ray diffraction measurement were obtained by slow
6

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evaporation from a mixture of CHCl and MeOH (v:v = 1:1) in air for one week.
3
2
.2.2. Preparation
of
precursor
preparing
6
-chloro-1,4-dimethyl-2-oxo-1,2-dihydropyridine-3-carbonitrile
for
compound 2
-Hydroxy-1,4-dimethyl-2-oxo-1,2-dihydropyridine-3-carbonitrile (1.64 g, 10.00
6
mmol) was refluxed with phosphorus oxychloride (20 mL, 21.60 mmol) for 3 h,
accompanied by the color change from yellow to red. Excess phosphorus oxychloride
was then distilled under the reduced pressure, and the residue was added to ice
−water
(100 g) with vigorous stirring. The resulting dark red solid was filtered, dissolved in
dichloromethane and washed by brine. The organic layer was dried with anhydrous
Na SO and the solvent was removed by a rotary evaporator. The crude solid was
2
4
recrystallized from toluene in a yield of 1.10 g (61%). Mp: 176–178 ℃. Main FT–IR
–
1
absorptions (KBr pellets, ν cm ): 3074 (m), 2983 (w), 2215 (s), 1644 (vs), 1578 (s),
1
1
1
1
1
533 (vs), 1447 (m) and 1198 (m). H NMR (400 MHz, CDCl , ppm): δ = 6.33 (s,
3
H), 3.70 (s, 3H) and 2.42 (s, 3H). Anal. Calcd for C H ClN O: C, 52.62; H, 3.86; N,
9
9
2
5.34%. Found: C, 52.48; H, 3.91; N, 15.22%. Positive ESI–MS in MeOH: m/z =
+
+
83.08 (72.5%), [M+H] , 205.17 (100.0%), [M+Na] .
2
.2.3. Preparation
of
compound
2
(6-((3-methoxypropyl)amino)-1,4-dimethyl-2-oxo-1,2-dihydropyridine-3-carbonitrile)
The synthetic procedure for compound 2 was analogous to that described for
compound
3
except
that
7

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6
-chloro-1,4-dimethyl-2-oxo-1,2-dihydropyridine-3-carbonitrile (1.82 g, 10.00 mmol)
was
used
as
the
starting
material
to
replace
6
-chloro-1-ethyl-4-methyl-2-oxo-1,2-dihydropyridine-3-carbonitrile. Yield: 1.55 g
–
1
(
70%). Mp: 180–182 ℃. Main FT–IR absorptions (KBr pellets, ν cm ): 3324 (vs),
2
950 (w), 2877 (w), 2195 (m), 1644 (s), 1558 (vs), 1434 (m), 1237 (m) and 1119 (w).
1
H NMR (400 MHz, CDCl , ppm): δ = 6.36 (s, 1H), 5.29 (s, 1H), 3.65 (t, J = 5.1 Hz, J
3
=
10.2 Hz, 2H), 3.41 (s, 3H), 3.38 (s, 3H), 3.34 (m, 2H), 2.31 (s, 3H), 2.01 (m, 2H).
Anal. Calcd for C H N O : C, 61.26; H, 7.28; N, 17.86%. Found: C, 61.12; H, 7.38;
1
2
17
3
2
+
N, 17.69%. Negative ESI–MS in MeOH: m/z = 234.17 (100%), [M−H] .
2
.2.4. Preparation
(E)-5-((4-methoxy-2-nitrophenyl)diazenyl)-2,6-bis((3-methoxypropyl)amino)-4-meth
ylnicotinonitrile)
-Methoxy-2-nitroaniline (0.34 g, 2.00 mmol) was dissolved in a mixture of
of
compound
5
(
4
concentrated hydrochloric acid (1 mL) and water (1 mL) at −5 ℃ in an ice bath.
Sodium nitrite (0.15 g, 2.20 mmol) was dissolved in water (2 mL) and added
dropwise to the reaction mixture for 30 minutes under mechanical stirring. The
diazonium salt solution was obtained and used for the following reaction. Compound
1
(0.58 g, 2.00 mmol) was dissolved in a methanol
−water mixture (16 mL, v:v = 1:1)
immersed in an ice salt bath, and freshly prepared diazonium salt solution was added
−
dropwise for 15 minutes under vigorous mechanical stirring (0–5 ℃ ). After
additional 3 h’s stirring, the precipitate was filtered and dried after thorough washing
with distilled water. The crude product was recrystallized from MeOH in a yield of
8

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4
–1
–1
0
.76 g (81%). Mp: 154–156 ℃. UV–Vis in MeOH, λmax (nm) / ε (×10 L mol cm )
–
1
=
467 / 1.99, 312 / 1.11, 242 / 1.78. Main FT–IR absorptions (KBr pellets, ν cm ):
3
363 (m), 3127 (m), 2208 (m), 1585 (vs), 1519 (m), 1401 (m), 1257 (w), 1191 (m)
1
and 1119 (m). H NMR (300 MHz, CDCl , ppm): δ = 10.91 (s, 1H), 7.86 (d, J = 9.1
3
Hz, 1H), 7.42 (d, J = 2.8 Hz, 1H), 7.17 (dd, J = 9.2, J = 2.8 Hz, 1H), 6.37 (t, J = 4.9
Hz, 1H), 3.92 (s, 3H), 3.69 (m, 4H), 3.56 (t, J = 5.6 Hz, 2H), 3.47 (t, J = 6.2 Hz, 2H),
3
.41 (s, 3H), 3.33 (s, 3H), 2.71 (s, 3H) and 1.96 (m, 4H). Anal. Calcd. For
C H N O : C, 56.04; H, 6.20; N, 20.79%. Found: C, 55.82; H, 6.38; N, 20.65%.
22
29
7
5
-
Negative ESI–MS in MeOH: m/z = 470.33 (100.0%), [M–H] .
2
.2.5. Preparation
(E)-2,6-bis((3-methoxypropyl)amino)-4-methyl-5-((4-nitrophenyl)diazenyl)nicotinoni
trile)
The synthetic procedure for compound 6 was analogous to that described for
of
compound
6
(
compound 5 except that 4-nitroaniline (0.28 g, 2.00 mmol) was used as the starting
material to replace 4-methoxy-2-nitroaniline. Yield: 0.75 g (85%). Mp: 160–162 ℃.
4
–1
–1
UV–Vis in MeOH, λmax (nm) / ε (×10 L mol cm ) = 478 / 1.81, 310 / 1.25, 242 /
–
1
1
1
.71
.
Main FT–IR absorptions (KBr pellets, ν cm ): 3127 (vs), 2877 (w), 2208 (m),
1
683 (w), 1585 (s), 1401(vs), 1283 (m) and 1100 (m). H NMR (300 MHz, CDCl ,
3
ppm): δ = 11.13 (s, 1H), 8.30 (d, J = 9.1 Hz, 2H), 7.78 (d, J = 9.1 Hz, 2H), 6.54 (t, J =
4
.7 Hz, 1H), 3.71 (m, 4H), 3.58 (t, J = 5.6 Hz, 2H), 3.52 (t, J = 6.0 Hz, 2H), 3.42 (s,
H), 3.36 (s, 3H), 2.74 (s, 3H) and 1.95 (m, 4H). Anal. Calcd. For C H N O : C,
3
21
27
7
4
9

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5
7.13; H, 6.16; N, 22.21%. Found: C, 57.02; H, 6.38; N, 22.01%. Negative ESI–MS
-
in MeOH: m/z = 440.25 (100.0%), [M–H] .
2
.2.6. Preparation
(E)-2,6-bis((3-methoxypropyl)amino)-4-methyl-5-((2-nitrophenyl)diazenyl)nicotinoni
trile)
The synthetic procedure for compound 7 was analogous to that described for
of
compound
7
(
compound 5 except that 2-nitroaniline (0.28 g, 2.00 mmol) was used as the starting
material to replace 4-methoxy-2-nitroaniline. Yield: 0.74 g (84%). Mp: 125–127 ℃.
4
–1
–1
UV–Vis in MeOH, λmax (nm) / ε (×10 L mol cm ) = 461 / 2.68, 310 / 1.56, 243 /
–
1
2
2
.28
.
Main FT–IR absorptions (KBr pellets, ν cm ): 3337 (s), 2923 (w), 2877 (w),
1
208 (m), 1591 (vs), 1512 (m), 1283 (w), 1197 (w) and 1119 (m). H NMR (400 MHz,
CDCl , ppm): δ = 11.06 (s, 1H), 7.93 (dd, J = 8.2 Hz, J = 1.3 Hz, 1H), 7.87 (dd, J =
3
8
.3 Hz, J = 1.2 Hz, 1H), 7.61 (m, 1H), 7.37 (m, 1H), 6.49 (t, J = 4.8 Hz, 1H), 3.69 (m,
H), 3.57 (t, J = 5.6 Hz, 2H), 3.47 (t, J = 6.2 Hz, 2H), 3.41 (s, 3H), 3.33 (s, 3H), 2.72
4
(
s, 3H) and 1.96 (m, 4H). Anal. Calcd. For C H N O : C, 57.13; H, 6.16; N, 22.21%.
21 27 7 4
Found: C, 57.00; H, 6.28; N, 22.11%. Negative ESI–MS in MeOH: m/z = 440.25
-
(
100.0%), [M–H] .
2
.2.7. Preparation
(E)-5-((2,3-dimethylphenyl)diazenyl)-2,6-bis((3-methoxypropyl)amino)-4-methylnico
tinonitrile)
of
compound
8
(
1
0

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The synthetic procedure for compound 8 was similar to that described for
compound 5 except that 2,3-dimethylaniline (0.24 g, 2.00 mmol) was used as the
starting material to replace 4-methoxy-2-nitroaniline. Yield: 0.52 g (62%). Mp:
4
–1
–1
1
1
2
20–122 ℃. UV–Vis in MeOH, λmax (nm) / ε (×10 L mol cm ) = 414 / 2.72, 304 /
–
1
.31, 246 / 2.20
.
Main FT–IR absorptions (KBr pellets, ν cm ): 3344 (s), 2923 (w),
1
863 (w), 2202 (m), 1585 (vs), 1539 (s), 1368 (w) and 1119 (m). H NMR (400 MHz,
CDCl , ppm): δ = 10.84 (s, 1H), 7.46 (m, 1H), 7.15 (m, 2H), 6.13 (t, J = 5.1 Hz, 1H),
3
3
.66 (m, 4H), 3.55 (t, J = 5.7 Hz, 2H), 3.46 (t, J = 6.2 Hz, 2H), 3.40 (s, 3H), 3.32 (s,
H), 2.75 (s, 3H), 2.49 (s, 3H), 2.37 (s, 3H) and 1.94(m, 4H). Anal. Calcd. For
3
C H N O : C, 65.07; H, 7.60; N, 19.80%. Found: C, 64.92; H, 7.78; N, 19.59%.
23
32
6
2
-
Negative ESI–MS in MeOH: m/z = 423.25 (100.0%), [M–H] .
2
.2.8. Preparation
(E)-5-((4-bromophenyl)diazenyl)-2,6-bis((3-methoxypropyl)amino)-4-methylnicotino
nitrile)
The synthetic procedure for compound 9 was analogous to that described for
of
compound
9
(
compound 5 except that 4-bromoaniline (0.34 g, 2.00 mmol) was used as the starting
material to replace 4-methoxy-2-nitroaniline. Yield: 0.76 g (80%). Mp: 103–105 ℃.
4
–1
–1
UV–Vis in MeOH, λmax (nm) / ε (×10 L mol cm ) = 435 / 2.43, 306 / 1.20, 244 /
–
1
1
2
.58
.
Main FT–IR absorptions (KBr pellets, ν cm ): 3356 (s), 2926 (w), 2871 (w),
1
195 (m), 1585 (vs), 1539 (s), 1381 (w) and 1112 (m). H NMR (400 MHz, CDCl ,
3
ppm): δ = 10.89 (s, 1H), 7.56 (m, 4H), 6.23 (t, J = 5.1 Hz, 1H), 3.67 (m, 4H), 3.55 (t,
11

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J = 5.7 Hz, 2H), 3.49 (t, J = 6.0 Hz, 2H), 3.40 (s, 3H), 3.35 (s, 3H), 2.71 (s, 3H) and
1
.3 (m, 4H). Anal. Calcd. For C H BrN O : C, 53.06; H, 5.72; Br, 16.81; N, 17.68%.
21 27 6 2
Found: C, 52.82; H, 5.88; N, 16.69%. Negative ESI–MS in MeOH: m/z = 473.25
-
-
(
100.0%), [M–H] ; 475.17 (96.0%), [M+2–H] .
2
.2.9. Preparation
(E)-2,6-bis((3-methoxypropyl)amino)-4-methyl-5-(phenyldiazenyl)nicotinonitrile)
The synthetic procedure for compound 10 was analogous to that described for
of
compound
10
(
compound 5 except that aniline (0.19 g, 2.00 mmol) was used as the starting material
to replace 4-methoxy-2-nitroaniline. Yield: 0.56 g (71%). Mp: 98–100 ℃. UV–Vis in
4
–1
–1
MeOH, λmax (nm) / ε (×10 L mol cm ) = 412 / 3.39, 303 / 1.70, 245 / 2.63. Main
–
1
FT–IR absorptions (KBr pellets, ν cm ): 3337 (s), 2930 (w), 2887 (w), 2202 (s), 1578
1
(
vs), 1546 (vs), 1381 (m) and 1125 (m). H NMR (400 MHz, CDCl , ppm): δ = 10.96
3
(s, 1H), 7.71 (dd, J = 8.4, 1.1 Hz, 2H), 7.45 (t, J = 7.5 Hz, 2H), 7.32 (t, J = 7.3 Hz,
1
H), 6.15 (t, J = 5.0 Hz, 1H), 3.68 (m, 4H), 3.55 (t, J = 5.7 Hz, 2H), 3.50 (t, J = 6.1
Hz, 2H), 3.40 (s, 3H), 3.36 (s, 3H), 2.74 (s, 3H) and 1.93 (m, 4H). Anal. Calcd. For
C H N O : C, 63.62; H, 7.12; N, 21.20%. Found: C, 63.42; H, 7.28; N, 19.99%.
21
28
6
2
-
Negative ESI–MS in MeOH: m/z = 395.25 (100.0%), [M–H] .
2
.2.10. Preparation
of
compound
11
(
(E)-5-((4-methoxy-2-nitrophenyl)diazenyl)-6-((3-methoxypropyl)amino)-1,4-dimethyl
2-oxo-1,2-dihydropyridine-3-carbonitrile)
-
1
2

ACCEPTED MANUSCRIPT
The synthetic procedure for compound 11 was analogous to that described for
compound 5 except that compound 2 (0.47 g, 2.00 mmol) was used as the starting
material to replace compound 1. Yield: 0.51 g (61%). Mp: 170–172 ℃. UV–Vis in
4
–1
–1
MeOH, λmax (nm) / ε (×10 L mol cm ) = 453 / 1.01, 330 / 0.51, 240 / 0.65. Main
–
1
FT–IR absorptions (KBr pellets, ν cm ): 3324 (m), 2877 (w), 2818 (w), 2202 (m),
1
1
651 (s), 1565 (vs), 1434 (m), 1283 (w) and 1112 (m). H NMR (300 MHz, CDCl ,
3
ppm): δ = 12.45 (s, 1H), 7.84 (d, J = 9.2 Hz, 1H), 7.43 (d, J = 2.8 Hz, 1H), 7.21 (dd, J
9.2 Hz, J = 2.8 Hz, 1H), 3.93 (s, 3H), 3.76 (m, 2H), 3.61 (s, 3H), 3.35 (t, J = 5.6 Hz,
H), 3.22 (s, 3H), 2.72 (s, 3H) and 1.99 (m, 2H). Anal. Calcd. For C H N O : C,
=
2
5
1
9
22
6
5
5.07; H, 5.35; N, 20.28%. Found: C, 54.82; H, 5.48; N, 19.99%. Negative ESI–MS
-
in MeOH: m/z = 412.92 (100.0%), [M–H] . Single crystals of compound 11 suitable
for X–ray diffraction measurement were obtained by slow evaporation of a mixture of
CHCl and MeOH (v:v = 1:1) in air for one week.
3
2
.2.11. Preparation
of
compound
12
((E)-1-ethyl-5-((4-methoxy-2-nitrophenyl)diazenyl)-6-((3-methoxypropyl)amino)-4-me
thyl-2-oxo-1,2-dihydropyridine-3-carbonitrile)
The synthetic procedure for compound 12 was analogous to that described for
compound 11 except that compound 3 (0.50 g, 2.00 mmol) was used as the starting
material to replace compound 2. Yield: 0.58 g (68%). Mp: 166–168 ℃. UV–Vis in
4
–1
–1
MeOH, λmax (nm) / ε (×10 L mol cm ) = 453 / 1.55, 347 / 0.78, 241 / 1.01
.
Main
–
1
FT–IR absorptions (KBr pellets, ν cm ): 3120 (vs), 2825 (w), 2818 (w), 2208 (m),
1
3

ACCEPTED MANUSCRIPT
1
1
664 (s), 1565 (s), 1408 (vs), 1270 (m), 1204 (m) and 1112 (m). H NMR (400 MHz,
CDCl , ppm): δ = 12.96 (s, 1H), 7.82 (d, J = 9.1 Hz, 1H), 7.43 (d, J = 2.8 Hz, 1H),
3
7
.21 (dd, J = 9.1 Hz, J = 2.8 Hz, 1H), 4.22 (m, 2H), 3.93 (s, 3H), 3.79 (m, 2H), 3.40 (t,
J = 5.6 Hz, 2H), 3.26 (s, 3H), 2.70 (s, 3H), 2.02 (m, 2H) and 1.42 (t, J = 7.0 Hz, 3H).
Anal. Calcd. For C H N O : C, 56.07; H, 5.65; N, 19.62%. Found: C, 55.82; H, 5.88;
2
0
24
6
5
-
N, 19.49%. Negative ESI–MS in MeOH: m/z = 426.83 (100.0%), [M–H] .
2
.2.12. Preparation
of
compound
13
((E)-1-(3-isopropoxypropyl)-5-((4-methoxy-2-nitrophenyl)diazenyl)-6-((3-methoxypro
pyl)amino)-4-methyl-2-oxo-1,2-dihydropyridine-3-carbonitrile)
The synthetic procedure for compound 13 was analogous to that described for
compound 11 except that compound 3 (0.64 g, 2.00 mmol) was used as the starting
material to replace compound 1. Yield: 0.75 g (75%). Mp: 131–133 ℃. UV–Vis in
4
–1
–1
MeOH, λmax (nm) / ε (×10 L mol cm ) = 454 / 1.66, 352 / 0.84, 243 / 1.08. Main
–
1
FT–IR absorptions (KBr pellets, ν cm ): 3107 (w), 2970 (w), 2930 (w), 2864 (w),
1
2
208 (m), 1664 (vs), 1565 (vs), 1278 (s), 1198 (s) and 1119 (m). H NMR (300 MHz,
CDCl , ppm): δ = 12.88 (s, 1H), 7.81 (d, J = 9.1 Hz, 1H), 7.42 (d, J = 2.7 Hz, 1H),
3
7
.20 (dd, J = 9.1 Hz, J = 2.8 Hz, 1H), 4.29 (t, J = 7.1 Hz, 2H), 3.92 (s, 3H), 3.82 (m,
2
H), 3.50 (m, 1H), 3.40 (m, 4H), 3.23 (s, 3H), 2.70 (s, 3H), 2.00 (m, 4H) and 1.09 (d,
J = 6.1 Hz, 6H).. Anal. Calcd. For C H N O : C, 57.59; H, 6.44; N, 16.79%. Found:
2
4
32
6
6
C, 57.39; H, 6.64; N, 16.59%. Negative ESI–MS in MeOH: m/z = 498.92 (100.0%),
-
[
M–H] . Single crystals of compound 13 suitable for X–ray diffraction measurement
1
4

ACCEPTED MANUSCRIPT
were obtained by slow evaporation of a mixture of CHCl and MeOH (v:v = 1:1) in
3
air for one week.
2
.2.18. X–ray data collection and solution
Single-crystal samples of 1, 11 and 13 were covered with glue and were mounted
on glass fibers for data collection on a Bruker SMART 1K CCD area detector at
91(2) K, using graphite mono-chromated Mo Kα radiation (λ = 0.71073 Å). The
2
collected data were reduced by using the program SAINT [31] and empirical
absorption corrections were done by SADABS [32] program. The crystal systems
were determined by Laue symmetry and the space groups were assigned on the basis
of systematic absences by using XPREP. The structures were solved by direct
method and refined by least-squares method. All non-hydrogen atoms were refined
2
on F by full-matrix least-squares procedure using anisotropic displacement
parameters. The hydrogen atom (H3) bonded to the nitrogen atom of
3-methoxypropylamino moiety in compound 11 was located in the difference Fourier
synthesis and refined isotropically. All the other hydrogen atoms were inserted in the
calculated positions assigned fixed isotropic thermal parameters at 1.2 times of the
equivalent isotropic U of the atoms to which they are attached (1.5 times for the
methyl groups) and allowed to ride on their respective parent atoms. In the case of
compound 1, the solvent molecules are highly disordered, and attempts to locate and
refine the solvent peaks were unsuccessful. So contributions to scattering due to
these solvent molecules were removed using the SQUEEZE routine of PLATON. The
1
5

ACCEPTED MANUSCRIPT
structure was then refined again using the data generated. The contents of the solvent
region were not represented in the unit cell contents in the crystal data. The carbon
atoms (C7, C8 in compound 11 and C62 in compound 13) were refined as the
disordered mode with the site occupancy factors as 0.717(6):0.283(6) and
0.796(7):0.204(7), respectively. All calculations were carried out on a PC with the
SHELXTL [33] PC program package and molecular graphics were drawn by using
XSHELL, Diamond and ChemBioDraw software. Details of the data collection and
refinement results for 1, 11 and 13 are listed in Table 2. Selected bond distances and
bond angles are given in Table SI1, while the hydrogen bonding interactions are
listed in Table 3.
3
.
Results and discussion
3
.1. Syntheses and spectral characterizations
The introduction of functional groups into the pyridone moieties is regarded as an
essential method for designing new library of heterocyclic dyes. As shown in Scheme
FGT strategy was used to prepare new type of
1
,
a
2
6
N ,N -bis(3-methoxypropyl)pyridine-2,6-diamine azo dyes 5−10, where the two
acidic hydroxylic groups of 6-hydroxypyridin-2(1H)-one precursor were successfully
replaced by 3-methoxypropan-1-amine via a 2,6-dichloropyridine intermediate
(
Scheme SI1) prior to the coupling reactions. In addition, three new
-(3-methoxypropylamino)pyridin-2-one based dyes 11−13 with different pyridine
N-substituted tails were synthesized as the structural analogues of
6
1
6

ACCEPTED MANUSCRIPT
2
6
N ,N -bis(3-methoxypropyl)pyridine-2,6-diamine dye 5 for comparisons (Scheme 1),
where the same 4-methoxy-2-nitroaniline diazo component was adopted with
distinguishable electron-pushing (-OCH ) and electron-pulling (-NO ) groups
3
2
simultaneously.
In our
experiments,
the
common
coupling
based dyes
component
of
2
6
N ,N -bis(3-methoxypropyl)pyridine-2,6-diamine
5−10,
i.e.
2
2
,6-bis((3-methoxypropyl)amino)-4-methylnicotinonitrile (1), was synthesized from
,6-dichloro-4-methylnicotinonitrile according to double nucleophilic substitutions.
Similarly, mono 3-methoxypropan-1-amine substituted intermediate 2 was prepared
from new precursor of
-chloro-1,4-dimethyl-2-oxo-1,2-dihydropyridine-3-carbonitrile. Compound 1 was
used for the following diazo reactions with a variety of arylazo components
a
6
(4-methoxy-2-nitroaniline, 4-nitroaniline, 2-nitroaniline, 2,3-dimethylaniline,
2
6
4
-bromoaniline and aniline) to yield N ,N -bis(3-methoxypropyl)pyridine-2,6-diamine
based dyes 5−10. In addition, compound 2 was coupled with the diazo salt of
-methoxy-2-nitroaniline to access 6-(3-methoxypropylamino)pyridin-2-one based
dye 11 for comparison, together with two structural analogs of 12 and 13.
4
2
6
It is noted that the yields of N ,N -bis(3-methoxypropyl)pyridine-2,6-diamine dyes
are generally higher than those of 6-(3-methoxypropylamino)pyridin-2-one based
dyes because of the better solubility and facile post-treatment of the former ones.
Further
solubility
data
of
5–13
and
five
previously
reported
6
-(3-methoxypropylamino)pyridin-2-one modified azo dyes 6a−10a with the same
1
7

ACCEPTED MANUSCRIPT
phenyl substituents reveal the obvious increase of solubility for compounds with
multiple long alkyl chains in the pyridine rings (Table 1). However, the improvement
of solubility of one compound sometimes means the decrease of crystallinity, so the
attempts to access single-crystal structures for double 3-methoxypropan-1-amine
substituted
dyes
5−10
are
unsuccessful.
Instead,
two
6
-(3-methoxypropylamino)pyridin-2-one based dyes 11 and 13 are finally
characterized by X−ray diffraction, together with a double 3-methoxypropan-1-amine
substituted intermediate 1.
In FT–IR spectra, the N–H stretching vibration is found as one single peak in the
–
1
–1
cases of 1 (3376 cm ) and 2 (3324 cm ), respectively, indicative of successful
hydroxyl substitution by 3-methoxypropan-1-amine. In addition, the disappearance of
–
1
C=O stretching vibration at 1644 cm in compound 1 could further confirm the
successful reaction between 2,6-dichloro-4-methylnicotinonitrile and
-methoxypropan-1-amine. Similarly, the C=O stretching vibration in compounds
3
–
1
5
−10 disappears and one strong single peak at 1578–1591 cm can be observed,
suggesting the formation of N=N structure. In contrast, the C=O stretching vibration
in 6-(3-methoxypropylamino)pyridin-2-one modified dyes 11−13 emerges at
–1
1
644–1664 cm . Additionally, typical adsorptions corresponding to the C≡N
–
1
stretching vibration in all compounds 1−13 are located at 2201–2225 cm .
1
In their H NMR spectra, the new peak at 12.45–12.96 ppm after FGT is assigned
as the pyridyl secondary amine proton in compounds 11−13 [25]. In contrast, the two
secondary amine hydrogen atoms in azo dyes 5−10 display high-field shifts to
1
8

ACCEPTED MANUSCRIPT
different extents. One is in the range of 6.13–6.54 ppm ascribed to the NH proton
adjacent the azo unit, and the other close to the cyano group falls within 10.84–11.13
ppm. This phenomenon demonstrates the alterations of chemical environments after
mono and double 3-methoxypropan-1-amine substitution. Moreover, ESI–MS spectral
approach is proved to be effective in analyzing the molecular structures of our azo
dyes, where the molecular-ion peak for every compound appears undoubtedly with
1
00% abundance in methanol.
Generally speaking, the variations of dye structures through FGT strategy would
lead to the shifts of molecular energy levels, which could be reflected by their
UV–Vis electronic absorption spectra. Thus, the electronic spectra of
2
6
N ,N -bis(3-methoxypropyl)pyridine-2,6-diamine
based
dyes
5−10
and
6
-(3-methoxypropylamino)pyridin-2-one modified dyes 11−13 have been determined
in methanol for comparisons. As we have mentioned in our previous studies [25,34],
the alteration of pyridine N-substituted tails with slight discrepancy in the electron
withdrawing/donating capabilities would not significantly impact the π–conjugated
system of the whole dye molecule. The UV–Vis spectra of azo dyes 11−13 are very
similar and only dye 11 is selected as a representative to plot with dyes 5−10 for
clarity (Fig. 1). Instead, dyes 12−13 have been moved to the Supporting Information
as Fig. SI37. All our mono and double FGT products dyes 5−13 and 6a−10a [25] can
be divided into two categories depending on the phenyl substitutents. Namely dyes
5
−7, 9, 11−13, 6a, 7a and 9a bearing the electron-withdrawing
4
-methoxy-2-nitroaniline, 4-nitroaniline, 2-nitroaniline and 4-bromoaniline moieties
1
9

ACCEPTED MANUSCRIPT
and dyes 8, 10, 8a and 10a having the electron-donating 2,3-dimethylaniline and
aniline moieties. By comparing the mono and double FGT products, it is found that
different bathochromic shifts (14 nm from 11−13 to 5, 15 nm from 6a to 6, 15 nm
from 7a to 7 and 4 nm from 9a to 9) and hypsochromic shifts (11 nm from 8a to 8 and
6
nm from 10a to 10) can be observed for the afore-mentioned two classes of dyes, as
can be seen in Table 1.
On the other hand, further inspections of the λmax values for double FGT products
reveal that the position of λmax is strongly relied on the nature of the electron
push-pull abilities of phenyl substituents. In comparison with the λmax peak of dye 10
centered at 412 nm, introduction of two additional electron-donating methyl groups in
8
leads to a slight hypsochromic shift of 2 nm, whereas implanting of the
electron-withdrawing groups produce strong bathochromic shifts of 23~66 nm for
−7 and 9 (Table 1). In addition, the same electron push-pull tendency is observed for
all the mono FGT products 11 and 6a−10a. With regard to four
-methoxy-2-nitroaniline based dyes (5 and 11−13), the UV–Vis spectra imply that
5
4
contribution of the electron-withdrawing nitro group overwhelms that of the
electron-donating methoxyl group in the phenyl ring.
It should be pointed out that the variations of UV–Vis spectra for all the mono and
double FGT products can be elucidated by the formation of different D−π−A and
A−π−D system depending on the introduced phenyl substituents with distinguishable
electron-donating and electron-withdrawing capabilities. In general, as for
non-functionized aryl pyridone dyes with typical D–π–A system, the introduction of
2
0

ACCEPTED MANUSCRIPT
an electron-sufficient group at the phenyl ring would induce a bathochromic shift,
while the implanting of an electron-deficient one would cause a hypsochromic shift
[
35−36]. The same phenomenon has been observed in the cases of 8, 8a, 10 and 10a
with the electron-donating 2,3-dimethylaniline and aniline diazo components.
However, it is not the case for all the other azo dyes with electron-withdrawing
substituents. This aberrant behavior is suggested to be explicated by the
transformation from D−π−A to A−π−D system for our multi-substituted
phenyl-azo-pyridine skeleton. On the one hand, introducing the electron-withdrawing
substituents at the phenyl ring weakens the electron density of donor. On the other
hand, implanting the electron-donating secondary amine into the pyridine unit via
FGT strategy strengthens the electron density of acceptor. It is therefore reasonably
understood that the balance of close electron push-pull capabilities for both donor and
acceptor in our compounds is responsible for the uncommon transformation from
D−π−A to A−π−D system. Actually, we have tried to check the literature on this kind
of transformation but we are not successful yet. We think the main reason may be the
difficulties in finding a suitable example where similar electron push-pull capabilities
for both donor and acceptor are present and different substituent effects are
investigated simultaneously.
What
impact
on
the
pH
stability
can
be
brought
for
2
6
N ,N -bis(3-methoxypropyl)pyridine-2,6-diamine backbone after double FGT? To
gain a better insight on the effects of pH stability, acid-base titration has been done
2
6
for N ,N -bis(3-methoxypropyl)pyridine-2,6-diamine based dyes and our
2
1

ACCEPTED MANUSCRIPT
experimental results demonstrate that double FGT products have extremely high pH
stability in comparison with 6-(3-methoxypropylamino)pyridin-2-one based azo dyes.
For example, a comparative plot for a pair of double and mono FGT products with
the same 4-methoxy-2-nitroaniline diazo component (5 and 11) indicates that they
both have high pH stability and the former reaches an extremely high value of 13
(Fig.
2).
Similarly,
compared
with
our
previous
prepared
6-(3-methoxypropylamino)pyridin-2-one dyes 8a and 9a [25], their counterparts 8
and 9 can keep stable in a wider pH range of 0.44~13.09 and 0.45~13.23 (Fig. 3). It
is no doubt that the replacement of the second carbonyl group in
6-hydroxy-4-methyl-2-oxo-1,2-dihydropyridine-3-carbonitrile
by
the
basic
secondary amine unit can further enhance the pH stability for resultant double FGT
dyes.
3
.2. Structural descriptions of compounds 1, 11 and 13
As we have mentioned above, the solubility of one compound is closely related to
its crystallinity. In this paper, only two single-crystal structures of mono FGT dyes
11 and 13 have been finally obtained on account of their worse solubility but better
crystallinity, together with one double 3-methoxypropan-1-amine substituted
intermediate 1. X-ray single-crystal diffraction analyses reveal that all the
non-hydrogen atoms in compound 11 except the 3-methoxypropylamino tail are
ideally coplanar with the ZERO dihedral angle between the phenyl (A) and pyridine
(B) rings, as can be seen in Fig. 4. In contrast, the dihedral angles between rings A
2
2

ACCEPTED MANUSCRIPT
o
and B vary from 1.4(2) to 12.6(2) for four crystallographically independent
molecules in the asymmetric unit of compound 13, which could be ascribed to the
steric hindrance between two adjacent long alkyl chains (−NHCH CH CH OCH
3
2
2
2
and −(CH ) OCH(CH ) ) bonded to the pyridine ring. Strong intramolecular
2
3
3 2
N−H···N hydrogen bonds can be observed in 11 and 13 forming six-membered rings
Fig. 4 and Table 3), in which the H···A distances are in a range of 1.79(2)~1.93(3)
Å.
(
Moreover, offset dimeric π–π stacking interactions are found in the crystal packing
of 11 and 13. As shown in Fig. 5, neighboring two molecules in every dimeric unit in
compound 11 adopt the head-to-tail packing fashion with the centroid-to-centroid
separations of 3.603(2) Å between the pyridine and phenyl rings, but no π–π stacking
interactions can be found between the dimeric units. In compound 13, effective π–π
stacking interactions can only be observed among one set of molecules forming a
different head-to-head dimeric packing mode (Fig. 5). The centroid-to-centroid
distance between adjacent phenyl rings in every dimeric unit is calculated to be
3.617(3) Å, and all the four long alkyl chains belonging to each molecule point
outside the dimeric plane to minimize the spatial crowding effects.
4
. Conclusion
To extend the synthetic strategy of FGT, we have synthesized a library of double
and mono 3-methoxypropan-1-amine substituted azo dyes from classical
pyridine-2,6-dione hydrazone dyes with different pyridine N−H and N−R tails.
2
3

ACCEPTED MANUSCRIPT
Moreover, different aniline with distinguishable electron-donating and
electron-withdrawing substituents in the diazo components are introduced into double
and mono FGT products, respectively, in order to better understand their
structure-performance relationship. Interestingly, the uncommon transformation for
roles of donor and acceptor (D−π−A and A−π−D) can be achieved for our
functionalized azo dyes because of the fine tuning of electron push-pull abilities for
both phenyl and pyridine rings by adopting different substituents. Comparative
studies on the UV−Vis spectra of all azo dyes clearly support the formation of
different D−π−A and A−π−D system according to their respective λmax values. As
expected, double basic secondary amine substituted pyridine-2,6-diamine azo dyes
display extremely high pH stability up to 13 owning to the replacement of the second
carbonyl group. From the current study, one can see that both the FGT strategy and
structure-performance relationship can throw some light on the rational design of new
dyes and effective improvements of as-synthesized dyes with better dyeing
performance.
Supplementary material
o
1
Tables of selected bond distances (Å) and angles ( ), FT–IR, ESI–MS, H NMR and
UV–Vis spectra as well as synthetic route for related compounds are attached to this
paper. CCDC reference numbers 1547049-1547051 for compounds 1, 11 and 13
contain the supplementary crystallographic data for this paper. These data can be
obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from the
2
4

ACCEPTED MANUSCRIPT
Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK;
Fax: (internat.) +44-1223/336-033; E-mail: deposit@ccdc.cam.ac.uk].
Acknowledgements
This work was financially supported by the Major State Basic Research
Development Programs (No. 2013CB922101), the National Natural Science
Foundation of China (No. 21171088), and the project of scientific and technological
support program in Jiangsu province (No. BE2014147-2).
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