Synthesis and fluorescent properties of 2-styryl-6,7-difluoro-8- hydroxyquinoline and its Zn(II) complex

Article abstract of DOI:10.1016/j.jfluchem.2013.03.005

A synthetic approach to 2-methyl-6,7-difluoro-8-hydroxyquinoline, a key intermediate, has been developed. 6,7-Difluoro derivative of 2-styryl substituted 8-hydroxyquinoline and its Zn(II) complex have been obtained. Effects of fluorine atoms in the benzen

Full text of DOI:10.1016/j.jfluchem.2013.03.005

Journal of Fluorine Chemistry 150 (2013) 36–38
Contents lists available at SciVerse ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Short communication
Synthesis and fluorescent properties of 2-styryl-6,7-difluoro-8-
hydroxyquinoline and its Zn(II) complex
a
b
a,b
Emiliya V. Nosova a, , Tatyana V. Stupina , Galina N. Lipunova , Valery N. Charushin
*
a Department of Organic Chemistry, Chemical Technology Institute, Urals Federal University, 19 Mira Str., 620002 Ekaterinburg, Russia
b I. Postovsky Institute of Organic Synthesis, Ural Division of the Russian Academy of Sciences, 22 S. Kovalevskoy Str., 620219 Ekaterinburg, Russia
A R T I C L E I N F O
A B S T R A C T
Article history:
A synthetic approach to 2-methyl-6,7-difluoro-8-hydroxyquinoline, a key intermediate, has been
developed. 6,7-Difluoro derivative of 2-styryl substituted 8-hydroxyquinoline and its Zn(II) complex
have been obtained. Effects of fluorine atoms in the benzene ring on photophysical properties of 2-styryl-
8-hydroxyquinolines and their Zn(II) complexes have been studied.
Received 4 February 2013
Received in revised form 5 March 2013
Accepted 5 March 2013
Available online 15 March 2013
ß 2013 Elsevier B.V. All rights reserved.
Keywords:
2-Methyl-6,7-difluoro-8-hydroxyquinoline
Styrylquinoline
Zn(II) complex
Fluorinated quinolines
UV and FL spectra
1. Introduction
8-hydroxyquinoline has practically no effect on the luminescent
properties. Also it has been reported that in case of metal complexes
8-Hydroxyquinoline and its numerous derivatives and metal
chelates have been attracting attention of researchers for two
decades, since the first data on the aluminum complex of 8-
hydroxyquinoline exhibiting thermal stability, high efficiency of
green luminescence, and rather good electronic mobility have been
published [1]. On the other hand, there are a limited number of
publications in which influence of fluorine atoms in various
positions of the quinoline system on luminescence characteristics
of aluminumcomplexesof 8-hydroxyquinolinehave been described
[2]. Due to specific properties of fluorine atoms metal complexes of
fluorinated 8-hydroxyquinolines proved to have an enhanced
electronic mobility, a low temperature of sublimation, a good
stability in air, and a wide energetic gap. It has been shown [2] that
incorporation of a fluorine atom at the 5-position of 8-hydro-
xyquinoline leads to a red shift of the luminescence maximum
of difluoro substituted8-hydroxyquinolines contacts of an electrode
with organic materials are facilitated [3].
2-Styryl derivatives of fluorinated 8-hydroxyquinolines have
not so far been described. It should be noted that quinoline ligands
are often modified by means of extension of the conjugated chain
by a substituent, for example the styryl fragment, attached to 2- or
4-positions of the pyridine ring [4]. Complexes of 2-styryl-8-
hydroxyquinolines have been shown to possess improved
properties relative to the corresponding complexes 8-hydroxy-
quinoline. They are well soluble in common organic solvents,
thermally stable and more suitable for preparation of films.
Solutions of these complexes in THF exhibit blue-green lumines-
fl
max
cence with a maximum in the range of 513–576 nm, and
l
for
zinc compounds being more red-shifted. Nowadays a great
attention is given to elucidation of photophysical properties of
complexes of styryl substituted 8-hydroxyquinolines with zinc
and aluminum. In this communication we wish to report on the
synthesis of 6,7-difluoro-2-styryl-8-hydroxyquinoline and its
Zn(II) complex, and their characteristics and photophysical
properties in comparison with those of non-fluorinated analogs
have first been discussed.
(lmax = 547 nm, for nonfluorinated analog 515 nm) with a consid-
erable decrease in both intensity (approximately 20 times) and the
quantum yield. At the same time, the aluminum complex of 6-
fluoro-8-oxyquinolate demonstrates a blue shift (lmax = 495 nm)
with a significant enhancement of the luminescence intensity and
2.6-fold increase in quantum yield. Fluorination of the 7-position of
2. Results and discussion
* Corresponding author. Tel.: +7 343 3754501.
We have first obtained 2-methyl-6,7-difluoro-8-oxyquinoline 3
from 2,3-difluoro-6-nitrophenol 1 (Scheme 1). The intermediate 3
E-mail addresses: emily74@rambler.ru, emilia.nosova@yandex.ru (E.V. Nosova),
lipunova@ios.uran.ru (G.N. Lipunova).
0022-1139/$ – see front matter ß 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jfluchem.2013.03.005

E.V. Nosova et al. / Journal of Fluorine Chemistry 150 (2013) 36–38
37
O
MeO
F
F
1) SnCl
NO2
2.2HOH, HCl, MeOH
F
F
H
1) CH3-CH=CH-CH(O), HCl
1)
F
F
Ac2O
NaHCO3
2)
NH OH
2)
NH2
4
2) HCl, 00 C
N
3
CH3
OH
1
2
OH
OH
Ar
F
F
1) HCl, EtOH, t
2) Et3N, EtOH
F
F
F
F
Zn(OAc)2
EtOH
N
N
N
O
O
OH
Ar
Zn
O
O
4
5
CH3
F
6
N
OMe
OMe
F
Scheme 1. Compound 6: Ar = 4-methoxyphenyl.
was synthesized from the corresponding nitrophenol
1
by
(Scheme 1). The 1H NMR spectra of complex 6 shows no OH
signals; the signals of H4 and H3 protons of the pyridine ring are
shifted downfield compared to those in ligand 5, while the signal
of H5 proton of the benzene ring is shifted upfield compared to
the corresponding signal of the same ligand. The signal of F7 of
8-hydroxyderivative 5 in 19F NMR spectra is shifted upfield
compared to these in 8-acetoxyquinoline 4 (À147.74 ppm for 4
and À159.05 ppm for 5), the signal of F7 fluorine of complex 6 is
shifted also upfield compared to these in ligand 5 (À159.05 ppm
for 5 and À163.33 ppm for 6).
reduction with SnCl2 followed by the Skraup quinoline synthesis
with crotonic aldehyde in the presence of HCl (Scheme 1). A similar
approach has been used to reduce 2-nitro-4-bromphenol [5],
earlier aminophenol 2 was synthesized by reduction of compound
1 with palladium monocarbide [6], and appropriate condensation
conditions have been reported for the synthesis of 2-methyl-6,7-
difluoroquinoline [7].
The acetoxy derivative 4 was obtained from quinoline 3. In the
NMR 1H spectrum of compound 4 two characteristic doublets of
CH55CH protons (at 7.24 and 7.72 ppm with 3J 16.3 Hz) were
observed, thus indicating that the molecule has the trans-
configuration of the alkenyl fragment. Perkin condensation of 2-
methyl-8-oxyquinolines was reported [8]. Hydrolysis of acetox-
ygroup of 4 was carried out by the heating with hydrochloric acid
in ethanol, for transformation of hydrochloride of 2-styryl-8-
oxyderivative into free ligand 5 reaction mixture was treated with
triethylamine (Scheme 1). The structural evidence for compound 5
has been confirmed by the 1H, 13C NMR and mass spectra.
Complex 6 was obtained by heating of hydroxystyrylquino-
line 5 with zink(II) acetate in the molar ratio 2:1 in ethanol
In the IR spectra of complex 6 there are no bands at 3340–
3375 cmÀ1 (valent fluctuations of O–H) and 1330–1335 cmÀ1
(deformational fluctuations of O–H), while the band at 1229–
1237 cmÀ1 is weak (valent fluctuations C–O) because of coordi-
nation of the oxygen atom with a metal ion. In the UV–vis
absorption spectra of complex 6 batochromic shifts for the long-
wave band of approximately 70 nm is observed in comparison
with these of the corresponding ligand (Table 1).
Absorption and emission spectra of compounds 5 and 6 were
recorded in MeCN solutions at room temperature and compared
with spectra of non-fluorinated analogs 7 and 8 (Scheme 2 and
Table 1).
Table 1
Difluoro derivative 6 showed blue shift of the emission peak in
comparison with the chelate 8 which does not bearing fluorine
atoms in a benzene ring. Difluoro derivative of 8-hydroxy-2-
styrylquinoline showed red shift of the emission peak (compounds
5 and 7) (Table 1). The analysis of literary data [2] allows to assume
that observed shifts of the emission peaks are due to the fluorine
atom at the 6 position.
The absorbtion and fluorescence maxima for compounds 5–8 in acetonitrile at
293 K.
Compound
l
a (nm)
l
f (nm)a
Stokes shift (nm)
5
6
7
8
352
420
320
435
455
559
427
599
103
139
107
164
a
Excitement in the field of an absorption maximum.
3. Conclusion
In conclusion it is worth noting incorporation of fluorine atoms
in styryl substituted quinolines appears to be a good approach to
tune their photophysical properties. Indeed, novel fluorinated
quinolines 3 and 5 proved to be attractive compounds for
preparing of other metal complexes, for example Al(III).
N
N
O
OH
Ar
Ar
O
Zn
4. Experimental
7
N
4.1. General
8
OMe
Difluoro-6-nitrophenol 1 was obtained from 2,3,4-trifluoroni-
trobenzene according to the method described in the literature [9]
and compounds 7 and 8 were synthesized as described [4a].
Scheme 2. Non-fluorinated analogs of
5 and 6. Compound 8: Ar = 4-
methoxyphenyl.

38
E.V. Nosova et al. / Journal of Fluorine Chemistry 150 (2013) 36–38
NMR, MS and analytical data for compounds obtained are
water (15 mL). The solid obtained was dissolved in ethanol
presented in Supporting information.
(12 mL), triethylamine (2.5 mL) was added, and the mixture was
stirring at room temperature during 3 h, and then concentrated in
vacuo. The residue obtained was recrystallized from hexane:etha-
nol (1:1). Yield of (E)-2-[2-(4-methoxyphenyl)vinyl]-6,7-difluoro-
8-hydroxyquinoline is 0.16 g (46%), mp. 130–132 8C.
4.2. Synthesis of 2-amino-5,6-difluorophenol (2)
To a solution of SnCl2Á2H2O (32.1 g, 142.8 mmol) in methanol
(100 mL) a concentrated hydrochloric acid (70 mL) and 2,3-
difluoro-6-nitrophenol 1 (5.0 g, 28.5 mmol) at 0 8C were added.
The reaction mixture was stirred at room temperature for 48 h, the
solution has decolorated. Solution was neutralized with water
NaHCO3 to pH 7, the formed colorless solid was filtered, dried at
room temperature and extracted with ethyl acetate for several
times. Water filtrate was extracted with ethyl acetate, combined
organic lays were dried over anhydrous Na2SO4 and concentrated
in vacuo. Yield of 2-amino-5,6-difluorophenol is 3.5 g (84%), mp.
128–130 8C is in full agreement with the literature [6].
4.6. Synthesis of (E)-2-[2-(4-methoxyphenyl)vinyl]-6,7-difluoro-8-
hydroxyquinolinate Zn(II) (6)
To a solution of quinoline 5 (0.125 g, 0.4 mmol) in ethanol
(20 mL) zink(II) acetate (0.04 g, 0.2 mmol) was added, the reaction
mixture was refluxed for 6 h, and the formed solid was filtered off.
Yield of Zn(II) complex 6 is 0.062 g (45%), mp. >300 8C.
Acknowledgments
4.3. Synthesis of 2-methyl-5,6-difluoro-8-hydroxyquinoline (3)
This work was supported by the Russian Foundation for Basic
Research (Grants 11-03-00718, 12-03-33144, 13-03-96039) and
Russian Federation Ministry of Science and Education
(14.A18.21.0817) as well as Ural Division of Russian Academy of
Sciences (program «Development of methods of receiving chemi-
To 2-amino-5,6-difluorophenol 2 (2 g, 13.8 mmol) 6 N hydro-
chloric acid (10 mL) was added, the mixture was heated to 100 8C,
then crotonic aldehyde (1.2 mL, 14.3 mmol) was added slowly. The
mixture was refluxed for 16 h, after cooling NH4OH was added
under stirring to pH 8. Then ethyl acetate (50 mL) was added, and
the reaction mixture was stirred during 30 min, the formed
colorless solid was filtered. Filtrate was extracted with ethyl
acetate, organic solution was concentrated in vacuo, and residue
was recrystallized from hexane–ethanol (1:1). Yield of 2-methyl-
5,6-difluoro-8-hydroxyquinoline is 1.36 g (51%), mp. 102–104 8C.
cals and creation of new materials», project „. 12-A-1020).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.jfluchem.2013.03.005.
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