Synthesis and mesomorphic properties of azo compounds derived from phenyl- and thienyl-1,3,4-thiadiazole

Article abstract of DOI:10.1515/znb-1997-1215

The synthesis and mesomorphic behaviour of new series of liquid crystals containing 1,3,4-thiadiazole and thiophene rings with azo central bond are reported (series 5a and 5b). All compounds of series 5a exhibit enantiotropic nematic mesophase and the hig

Full text of DOI:10.1515/znb-1997-1215

Synthesis and Mesomorphic Properties of Azo Compounds Derived from
Phenyl- and Thienyl-l,3>4-Thiadiazole
M. Parra*, Sh. Villouta, V. Vera, J. Belmar, C. Z úñiga, H. Zunza
Concepcion, D epartam ento de Qufmica Orgänica, Facultad de Ciencias Qui'micas,
Universidad de Concepcion, Casilla 3-C, Concepciön-Chile
Z. Naturforsch. 52b, 1533-1538 (1997); received July 18, 1997
Liquid Crystals, Thiadiazole, M esomorphic Properties
The synthesis and m esom orphic behaviour of new series of liquid crystals containing 1,3,4-
thiadiazole and thiophene rings with azo central bond are reported (series 5a and 5b). All
com pounds of series 5a exhibit enantiotropic nematic m esophase and the higher hom ologues
(« = 9 -1 0 ) also show a m onotropic sm ectic C phase. Series 5b show dimorphism Sc - N (for
« = 5 -7 the Sc is m onotropic). These series are compared with the Schiffs bases analogues,
the imine bond gives rise to similar liquid crystals phase but larger mesomorphic range.
Introduction
Hi5C7-
{
^
t
s ^ - N = N - ^ - O
R
5a
A great num ber of mesomorphic compounds
containing heterocyclic units have been synthe-
sized, and interest in such structures constantly
grow [1,2]. This is not only because of the greater
possibilities with heterocyclics for the design of
new mesogenic molecules, but also because the in-
sertion of heteroatom s strongly influences the for-
mation of mesomorphic phases. The effect of het-
eroatom s can change considerably the polarity,
polarizability and sometimes the geometry of the
molecule, thereby influencing the type of meso-
phase, the phase transition tem peratures, dielectric
constants and other properties of the mesogens
N—N
r° - 0 ^ s^_n=n_<0^_oCioH21
5b
N -N
R C > ~
0
)
-
A
s
^
~
n
=
C
H
~
( 0
)
~
O
C
8
H
^
7
R -C nH2n+i
n = 5-10
Scheme 1. Structures of azo com pounds and Schiff bases.
[3]-
Heterocyclic compounds like pyridine and py-
rimidine derivatives [4] are valuable liquid-crystal-
line materials for technical applications. It has also
been reported that thiadiazole [5-8] or thiophene
ring [9,10] may be incorporated into the molecular
structure of calamitic mesogens.
H ere we report the synthesis and mesomorphic
behaviour of two homologous series of new azo
compounds containing thiophene and 1,3,4-thiadi-
azole rings (5a and 5b). We compare these com-
pounds with the analogous Schiffs bases contain-
ing the same structural units (6 and 7) (Scheme 1).
Synthesis
The reaction pathway used to prepare the azo
compounds (5a,b) is shown in Scheme 2. Com -
pound la was synthesized starting with the acyla-
tion of the thiophene and proceeding with the
reduction of the ketone moiety yielding 2-rc-hep-
tylthiophene [11,12]. The following step was the
metalation with butyllithium followed by carbona-
tion [13], yielding the 5-«-heptyl-2-thiophene car-
boxylic acid in good yields (70%). By esterifica-
tion of the carboxylic acid the methyl ester (la)
was obtained [14]. Compound lb was synthesized
by alkylation of the methyl-4-hydroxybenzoate.
Condensation of la,b with 80% hydrazine hy-
drate [15] yielded the hydrazides 2a,b, which react
*
Reprint requests to Prof. Dr. M. Parra.
0932-0776/97/1200-1533 $06.00 © 1997 Verlag der Zeitschrift für Naturforschung. All rights reserved.
N
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1534
M. Parra et al. ■Mesogens Containing 1,3.4-Thiadiazole Groups
I
—
.
o
Table I. a) Transition temperatures (°C), b) transition
enthalpies (KJ/mol) and yields (%) data for the series
5a.
H15C7—^
c—o c h 3
h15c 7
NHNH,
NHNH,
nh2nh2
1 a
N
•
n
5
C
I
•
•
•
•
•
•
Yield
31
Sc
2b
Rc^O^"
142.8
31.7
147.7
34.1
150.6
35.9
151.9
36.1
152.7
38.3
153.8
35.9
145.7
0.2
a)
b)
a)
b)
a)
b)
a)
b)
a)
b)
a)
b)
•
•
•
-
149.5
0.28
152.8
0.38
157.6
0.42
158.5
0.88
29
6
u ^
fCf—NHNH—Cfl—
H15C7—II S
-
7
24
NH4SCN/H+
2a, 2b
O
S
-
40
8
NHNH—C—NH2
3b
R° - 0 ^ - c - i
•
•
55
9
(129)
(130)
•
•
10
159.8
2.75
24
/
\ \ J - l
h 15c
NH,
4a
4b
1.CH3COCI
2. HCl
N -N
C = Crystal, Sc = Smectic C, N = Nematic, I = Isotropic,
(...) = Monotropic Transition.
NH2
h15c 7-
=n_0 >_or
1.NaNQ2 /H+
4a, 4b
2. ©-OR
N -N
r ° - @ - A 3 - n=n- @ - oc10h21
R —n-CnH2n+i
n = 5 -10
Scheme 2. Synthetic route for azo compounds.
Fig. 1. The phase transition temperatures for series 5a.
with ammonium thiocyanate in concentrated HC1
medium leading to the formation of the thiosemi-
carbazides 3a,b [16].
The amino-thiadiazoles 4a,b were prepared by
dehydration of 3a,b with acetyl chloride and fol-
lowed of hydrolysis of acetamido compounds [16].
The azo compounds 5a,b were prepared by cou-
pling between diazonium salts of the aminothiadi-
azoles with «-alkoxybenzenes [17].
tropic nematic phase. The compounds with n=9
and A 7 = 1 0 also show a m onotropic smectic C phase.
The texture observed with the polarizing micro-
scope for the nematic phase for all compounds in
this series, was marble and for the m onotropic
smectic C phase broken fan-shaped texture was
observed.
The transition tem peratures and enthalpies for
the series 5b are given in Table II and the phase
transition tem peratures are plotted in Fig. 2. The
melting points decrease with the chain length. All
homologues show an enantiotropic nematic phase.
The homologues with n = 5 -7 show a monotropic
smectic C phase, and those with /7=8-10 show an
enantiotropic smectic C phase.
Liquid Crystals Properties
The transition tem peratures and transition heats
were determ ined by DSC measurements and the
transition tem peratures checked using a polarizing
microscope equipped with a hot stage.
The transition tem peratures and enthalpies for
the series 5a are given in Table I, and the phase
transition tem peratures are plotted in Fig. 1. As
can be seen the melting points increase with the
chain length. All homologues show an enantio-
The mesophases exhibited by these compounds
were identified according to their optical textures.
The homologues with n - 5 -7 show the broken fan-
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M. Parra et al. • Mesogens Containing 1,3,4-Thiadiazole Groups
1535
Table II. a) Transition temperatures (°C), b) transition
enthalpies (KJ/mol) and yields (%) data for the series
5b.
mesophases. Using known bond angles, H. G al-
lardo and Y. Favarin reported that for the 2,5-di-
substituted thiophene ring that there is 32° devia-
tion from the hard rod axis of the analogous
compound with a phenyl ring [18].
In order to show the different possibilities that
the 1,3,4-thiadiazole ring can offer in the liquid
crystals field, it is interesting to compare the meso-
genic properties of these series of azo compounds
with the series of Schiffs bases analogues pre-
viously synthesized and reported by us [8] (series
6 and 7).
Table III and Fig. 3 show the mesogenic proper-
ties of Schiffs bases 6 and Table IV and Fig. 4
show the mesogenic properties of Schiffs bases 7.
These series have the same central core and show
the same behaviour than series 5a,b; the com-
pounds containing the thiophene ring (6) also
show a lower mesophase range than series 7.
n
5
C
:
s
•
•
•
•
•
•
I
•
Yield
73
c
N
•
162.1
51.95
155.6
50.06
153.7
48.49
149.9
37.46
149.5
44.41
146.8
46.92
178.3
0.79
179.0
0.66
176.5
0.58
176.4
1.15
174.0
1.09
(132)
(130)
(125)
a)
b)
a)
b)
a)
b)
a)
b)
a)
b)
a)
b)
•
•
•
•
•
6
•
46
7
•
34
8
156.2
1.13
159.5
2.17
162.0
2.76
•
61
9
•
75
•
10
172.9
1.45
76
C = Crystal, Sc = Smectic C, N = Nematic, I = Isotropic,
(...) = Monotropic Transition.
- • - c
-•-S c
N
Table III. a) Transition temperatures (°C), b) transition
enthalpies (KJ/mol) data for the series 6.
n
5
N
•
I
•
•
•
•
•
•
C
Sc
•
114.2
36.4
116.9
21.9
126.0
24.6
123.4
22.5
121.3
37.0
118.5
39.2
115.8
4.8
127.6
3.25
133.3
2.39
130.5
0.7
136.4
1.1
135.0
1.37
137.4
6.5
138.7
7.1
a)
b)
a)
b)
a)
b)
a)
b)
•
•
•
-
6
•
•
4
5
6
7
8
9
10
11
7
•
•
•
•
n
Fig. 2. The phase transition temperatures for series 5b.
8
•
•
-
9
a)
b)
a)
*
shaped texture in m onotropic smectic C phase and
the nematic phase showed the typical schlieren
texture with characteristic two- and four-brush sin-
gularities. On cooling this phase, its colour grad-
ually changed to finally adopt a homeotropic tex-
ture before passing into the smectic C phase. The
tendency of these compounds to give rise to ho-
m eotropic alignment is noteworthy. The texture
observed for the homologues with «= 8-10 was
schlieren, with only two-brush singularities, on
both heating and cooling for smectic C phase. For
nematic phase a m arble texture was observed.
Series 5b show a larger mesomorphic range than
the series 5a. Probably, the low mesophase sta-
bility of the homologues 5a is due to the presence
of the thiophene ring which produces a additional
deviation from linearity and thereby a molecular
curvature which disturbes the formation of stable
-
10
141.3
8.5
•
b)
C = Crystal, Sc = Smectic C, N = Nematic, I = Isotropic.
Fig. 3. The phase transition temperatures for series 6.
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1536
M. Parra et al. • Mesogens Containing 1,3,4-Thiadiazole Groups
Table IV. a) Transition temperatures (°C), b) transition
enthalpies (KJ/mol) data for the series 7.
FP 800 hot stage. The transition tem peratures and
enthalpies were also determ ined from DSC m eas-
urements using a STA 625 DSC calibrated with
Indium. 'H and 13C NM R spectra were recorded
on a Bruker AC-250 spectrometer.
n
5
N
•
I
•
•
•
•
•
•
C
•
Sc
•
123.5
36.4
116.1
33.4
112.8
31.3
107.8
33.8
106.8
35.4
175.8
2.4
184.3
2.8
189.5
3.6
193.9
4.6
193.5
3.9
193.9
4.1
209.8
1.7
208.7
1.5
206.3
1.4
205.2
1.1
201.1
1.4
198.8
1.3
a)
b)
a)
b)
a)
b)
a)
b)
a)
b)
a)
b)
•
•
•
•
•
6
•
•
•
•
•
•
Methyl-5-n-heptyl-2-thiophenecarboxylate (la) [14]
7
•
To a solution of 5-/z-heptyl-2-thiophenecarbox-
ylic acid (38.3 g, 0.17 mol) in 500 ml of anhydrous
methanol was passed through HC1 gas at room
tem perature for 20 min. The solution was then re-
fluxed for 18 h protected by a drying tube. The
solution was evaporated, the residue was taken up
on 200 ml of dichloromethane, washed with satu-
rated Na2C 0 3 and NaCl solutions, and dried over
Na2S 0 4. The solvent was evaporated and the resi-
due was distilled to yield 34.8 g of la (85%, b.p.:
105 °C/0.1 mmHg).
8
•
9
•
10
103.3
38.2
•
C = Crystal, Sc = Smectic C, N = Nematic, I = Isotropic.
‘H NMR (CDC13, TMS, 250 MHz): (3 = 7.55 (d,
7=4.1 Hz, 1H, arom. H); 6.75 (d, 7=4.1 Hz, 1H,
arom. H); 3.80 (s, 3H, CH3 of the ester function);
2.75 (t, 7=6.45 Hz, 2H, CH 2 joined to thiophene
ring); 1.65-1.30 (m, 10H, 5~CH2); 0.85 (t, 7=6.65
Hz, 3H, CH3 of the aliphatic chain).
13C NM R (CDC13, TMS, 62.9 MHz): (5 = 160.10
(C =0); 148.82; 130.10; 127.35; 120.80 (arom. C);
50.82 (CH3 of the ester function); 30.20-13.80
(aliph. C).
Fig. 4. The phase transition temperatures for series 7.
5-n-H eptyl-2-thienohydrazide (2a) [15]
A solution of la (34.75 g, 0.14 mol) and excess
of a solution of hydrazine hydrate (80%) in 20 ml
of ethanol was heated to reflux during 8 h. The
mixture was then cooled to room tem perature, and
the solid obtained was filtered, washed with water
and recrystallized from ethanol/w ater (1:1), yield-
ing 31.43 g of 2a (90%, m.p.: 76 °C).
’H NMR (CDC13, TMS, 250 MHz): 6 = 8.25 (s,
1H, N -H ); 7.45 (d, 7=4.25 Hz, 1H, arom. H); 6.70
(d, 7=4.25 Hz, 1H, arom. H); 4.15 (s, 2H, NH 2);
2.75 (t, 7=6.40 Hz, 2H, CH? joined to thiophene
ring); 1.65-1.25 (m, 10 H, 5~CH2); 0.85 (t, 7=6.60
Hz, 3H, CH-,).
However, some significant differences between
both central linkages can be noted, for the imine
series (6 and 7) the lower melting point and
broader mesomorphic range can be observed, see
Table III and IV. However, on the contrary the azo
central linkage in the series 5a,b, give rise the
higher melting point and lower mesomorphic
range, see Table I and II.
Probably, the high thermal stability of the
Schiffs bases is due to that the imine linkage gives
rise to a more planar structure than the azo link-
age, allowing for stronger molecular interactions
in the liquid crystalline phase which could explain
the higher clearing tem peratures of the Schiffs
bases derivatives.
13C NM R (CDC13, TMS, 62.9 MHz): 6 = 163.70
(C =0); 151.78; 133.39; 128.80; 124.95 (arom. C);
31.84; 31.48; 30.31; 29.28; 29.20; 22.65; 14.09
(aliph. C).
p-n-A lkoxybenzhydrazides (2b)
Experimental
The transition tem peratures were determ ined
by optical microscopy using an Ortholux Pol-BK-
11 polarizing microscope equipped with a M ettler
The series 2b were obtained from Methyl-/?-/?-
alkoxybenzoate (lb), using the same procedure
given for 2a. A fter recrystallization from ethanol.
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1537
M. Parra et al. ■Mesogens Containing 1,3,4-Thiadiazole Groups
collected, washed several times with water and
recrystallized from 2-ethoxyethanol, yielding
10.6 g of the acetamido compound (41%, m.p.:
220-222 °C). This compound (10.0 g, 0.031 mol)
was mixed with concentrated hydrochloric acid (52
ml) and 2-ethoxyethanol (200 ml) and then was
refluxed for 18 h. The solvent was evaporated un-
der reduced pressure and the residue was poured
into concentrated sodium hydroxide solution (80
ml). The solid was filtered, washed several times
with water and recrystallized from ethanol/water
(1:1) yielding 4.76 g of 4a (55%, m.p.: 140 °C).
‘H NM R (CDC13, TMS, 250 MHz): d = 7.15 (d,
white crystals were obtained, m.p.: 94-95 °C, yield
70-78% .
Spectroscopic characterization of 2b with n=5
IR (KBr disk): c m '1: 3310 (N -H ), 3210 and
3180 (NH2), 2950 (Csp3-H), 1600 (benzene ring).
13C NMR (CDC13, TMS, 62.9 MHz): d = 168.28
(C =0); 161.94; 128.74; 124.56; 114.21 (arom. C);
68.11; 28.3; 24.99; 22.10; 13.99 (aliph. C).
5-n-Heptyl-2-thiophenecarbonyl-thiosemicarbazide
(3a) [16]
5-n-Heptyl-2-thienohydrazide (2a) (30.0 g, 0.125 7=4.10 Hz, 1H, arom. H); 6.70 (d, 7=4.10 Hz, 1H
mol) was suspended in 11 ml of alcoholic hy- arom. H); 5.95 (s, 2H, N H 2); 2.80 (t, 7=6.40 Hz,
drogen chloride solution and evaporated under re- 2H, CH 2 joined to thiophene ring); 1.65-1.30 (m,
duced pressure, the residue dried by evaporation
of several small amounts of alcohol and heated
under reflux for 18 h with a solution of dry ammo-
niumthiocyanate (10.2 g, 0.14 mol) in ethanol ab-
solute. The solid was filtered, washed several times
with water and recrystallized from ethanol, yield-
ing 24.87 g of 3a (67%, m.p.: 177-179 °C).
10 H, 5 C H 2); 0.85 (t, 7=6.60 Hz, 3H, C H 3).
13C NM R (CDC13, TMS, 62.9 MHz): (5 = 164.87;
155.76; 149.02; 130.38; 127.67; 124.67 (arom. C);
31.75; 31.60; 31.47; 30.23; 29.00; 22.64; 14.08
(aliph. C).
5-(p-n-Alkoxy)phenyl-2-amino-l,3,4-thiadiazoles
*H NMR (CD 3COCD 3, TMS, 250 MHz): <3 =
9.65 (s, 1H, NH joined to C =0); 8.65 (s, 1H, NH
joined to C=S); 7.60 (d, 7=4.20 Hz, 1H, arom. H);
7.40 (s, 2H, NH2); 6.90 (d, 7=4.20 Hz, 1H, arom.
H); 2.55 (t, 7=6.40 Hz, 2H, CH2 joined to thio-
phene ring); 1.70-1.30 (m, 10 H, 5 C H 2); 0.85 (t,
7=6.60 Hz, 3H, CH3).
(4b)
The homologues of amino-thiadiazoles 4b were
obtained using the same procedure given for com-
pounds 4a. These compounds were recrystallized
from ethanol/water (4:1), m.p.: 165-170 °C, yield
70-80% .
13C NMR (CD3COCD 3, TMS, 62.9 MHz): Ö =
206.38 (C =0); 185.46 (C=S); 153.40; 135.14;
130.55; 126.24 (arom. C); 32.49; 32.35; 30.81; 30.73;
28.97; 23.27; (6 C, sp3); 14.34 (CH3).
Spectroscopic characterization of 4b with n=5
IR (KBr disk): cm "1: 3260, 3100 (NH2), 1600
(benzene ring), 2920 (Csp3-H).
!H NM R (DM SO-d6, TMS, 250 MHz): Ö = 7.70
(d, 7 = 8.80 Hz, 2H, 2 arom. H); 7.35 (s, 2H, N H 2);
7.10 (d, 7 = 8.80 Hz, 2H, 2 arom. H); 4.00 (t, 7 =
6.50 Hz, 2H, O CH 2); 1.80-1.40 (m, 6H, 3CH2);
0.95 (t, 7 = 6.60 Hz, 3H, C H 3).
13C NM R (DM SO-d6, TMS, 62.9 MHz): d =
167.93; 159.8; 157.36; 127.84; 123.52; 114.95 (arom.
C); 67.89; 28.52; 25.51; 22.12; 13.97 (aliph. C).
p-n-Alkoxybenzoylthiosemicarbazides (3b)
This series was prepared according to the pro-
cedure given for 3a. The homologues were puri-
fied by recrystallization from ethanol, m.p.: 196-
198 °C, yield 64-70% .
IR (KBr disk): cm“1: 3550 (N -H ), 3220 and
3160 (NH2), 1650 (C = 0), 1600 (benzene ring),
2920 (Csp3-H).
These compounds are insoluble in common or-
ganic solvents for this reason is not possible to ob-
tain their !H NMR and 13C NMR spectra.
2-(p-n-Alkoxyphenyl-azo)-5-(5'-n-heptyl-2' -
thienyl)-!,3,4-thiadiazoles (5a)
G e n e r a l m e t h o d [17]
Com pound 4a (0.50 g, 1.78 mmol) was dissolved
by heating and stirring in 8 ml of 85% phosphoric
acid. The solution was cooled to 0 °C in an ice
5-(5'-n-Heptyl-2'-thienyl)-2-amino-l,3,4-
thiadiazole (4a) [16]
Compound 3a (24.0 g, 0.08 mol) and acetyl chlo- bath, and then concentrated nitric acid (4 ml) and
a solution of sodium nitrite (0.13 g, 1.87 mmol) in
ride (250 ml) were mixed ice-cold and afterwards
2 ml of water was added. The mixture was stirred
cautiously heated under reflux. W hen the vigorous
reaction subside, heating was continued or 15 min- vigorously and maintained at below 5 °C during
10 minutes. Afterwards «-alkoxybenzene (1.78
utes, and water added. The insoluble material was
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1538
M. Parra et al. ■Mesogens Containing 1,3,4-Thiadiazole Groups
2- (p-n-Decanoxyphenyl-azo)-5-(p-n-alkoxy)phenyl-
1,3,4-thiadiazoles (5b)
mmol) was added dropwise with stirring. The yel-
low solid was filtered, washed several times with
water and recrystallized from toluene.
Compounds 5b were obtained from 5-(p-n-a\-
koxy)phenyl-2-amino-l,3,4-thiadiazoles (4b) and
n-decanoxybenzene, using the same procedure
given for series 5a. The transition tem peratures
and yields are given in Table II.
These compounds are insoluble in common or-
ganic solvents for this reason is not possible to ob-
tain their !H NM R and 13C NM R spectra.
The transition tem peratures and yields of the
series 5a are given in Table I.
Spectroscopic characterization o f 5a with n=5
!H NM R (CDC13, TMS, 250 MHz): d = 8.01 (d.
7=8.80 Hz, 2H, benzene ring); 7.45 (d, 7=4.05 Hz,
1H, thiophene ring); 7.05 (d, 7=8.80 Hz, 2H, ben-
zene ring); 6.85 (d, 7=4.05 Hz, 1H, thiophene ring);
4.10 (t, 7=6.60 Hz, 2H, O CH 2); 2.85 (t, 7=6.35 Hz,
2H, C H 2 joined to thiophene ring); 1.75-1.45 (m,
16H, 8 CH 2); 0.95 (t, 7=6.50 Hz, 6H, 2 CH 3).
13C NM R (CDCI3, TMS, 62.9 MHz): (3 = 168.85;
164.60; 162.39; 153.18; 145.51; 131.22; 130.85;
127.40; 126.25; 115.88 (arom. C); 69.39; 32.42;
32.11; 31.12; 29.68; 29.46; 28.80; 23.32; 23.11; 14.76;
14.68 (aliph. C).
Acknowledgements
This work was supported by FONDECYT
(G rant 1930548) and the “D irection de Investiga-
tio n ” of the University of Concepcion (Grant
95.23.05-1 and G rant 96.023.008-1.2).
[9] J. A. Sagitdinov, H. Schubert, Zh. Org. Khim. 14,
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