Mesogenic copper(II) complexes with [1,2,3]-triazole-based bidentate Schiff bases

Article abstract of DOI:10.1007/s12039-013-0545-0

The first mesogenic Cu(II) complex with [1,2,3]-triazole-based bidentate Schiff bases with flexible terminal alkyl chain, C_nH_2n+1 (even parity of n = 10-18) has been successfully synthesized. The heterocyclic triazole core was introd

Full text of DOI:10.1007/s12039-013-0545-0

c
J. Chem. Sci. Vol. 126, No. 1, January 2014, pp. 247–254. ꢀ Indian Academy of Sciences.
Mesogenic copper(II) complexes with [1,2,3]-triazole-based bidentate
Schiff bases
GUAN-YEOW YEAP∗ and BOON-TECK HENG
Liquid Crystal Research Laboratory, School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden,
Penang, Malaysia
e-mail: gyyeap@usm.my
MS received 25 May 2013; revised 26 September 2013; accepted 29 September 2013
Abstract. The first mesogenic Cu(II) complex with [1,2,3]-triazole-based bidentate Schiff bases with flexi-
ble terminal alkyl chain, CnH2n+1 (even parity of n = 10–18) has been successfully synthesized. The hetero-
cyclic triazole core was introduced into the target compound through the click reaction between azidoalkane
and propargyl aldehyde. All the uncoordinated ligands and target complexes were characterized by elemen-
1
tal analysis, FT-IR, H-NMR and UV-visible spectroscopic techniques. The observation under the polarized
light and differential scanning calometry (DSC) shows that the triazole-based ligands exhibit unstable SmA
phase which are not reproducible upon subsequent heating and cooling. Interestingly, the whole homologues
of Cu(II) complexes show exclusively stable focal conic fan-shaped texture characteristic of SmA phase. This
can be ascribed to the presence of Cu–N and Cu–O coordination modes which enhance the collinearity and
molecular anisotropy. On the other hand, the Cu(II) complexes are thermally more stable as compared to their
corresponding ligands.
Keywords. Cu(II) complexes; smectic A; click reaction; triazole; Schiff bases.
1. Introduction
the conversion from non-mesogenic ligands to meso-
genic complexes.3–7 Besides, the resulting metal com-
plexes are more polarizable and more suitably oriented
for the fortification properties. Such metallomesogens
combine the unique properties of liquid-crystalline
anisotropic fluids with the electronic and optical pro-
perties of metal complexes.5
Since the metal ions are also known to have various
geometries, the ultimate metallomesogens can probably
give rise to different geometrical shapes which may not
be obtained in conventional organic liquid crystals.8,9
Furthermore, metallomesogens have extra properties
such as its richness in oxidation state, colour, mag-
netism possessed by metal ions and redox behaviour.8,9
On the other hand, the coordination of organic liquid
crystal to metal centre can also cause a dramatic effect
on the thermal behaviour of the organic liquid crystal
such as the mesophase modification or suppression.10
These phenomena can result in changes in polarizabil-
ity, molecular anisotropy, modification of the polar or
dipolar characteristics and so on.10
Liquid crystals have been known for more than a decade
but the classes of chemical compounds capable of
exhibiting liquid crystal properties have increased sig-
nificantly only in the last twenty years. For example,
metal containing liquid crystal which is also known as
metallomesogen have received great attention leading
to systematic research and development.1 Generally,
the metallomesogens can be referred to coordination
compounds containing mesogenic or non-mesogenic
ligands.2 Metals which are used for metallomesogen are
cations, for instances, silver(I), rhodium(I), iridium(I),
nickel(II), copper(II), palladium(II) and platinum(II)
ions.3 These cations coordinate to ligands forming li-
near or square planar complexes. Salicylaldimine is one
of the bidentate ligands which have been extensively
studied in coordination compounds.3
The liquid crystal behaviour of these metal-ligand
complexes are claimed to be more prominent than the
free ligands owing to the advantages of combining the
properties of these ligands with those of the transition
metal. For example, the presence of metal ions induces
In this paper, the 1, 2, 3-triazole core system is intro-
duced in metallomesogens. The five-membered [1, 2,
3]-triazole ring is formed by copper catalysed [3 +
2] dipolar cycloaddition reaction between an organic
azides and terminal alkynes, which is also known as
‘click reaction’.11 This heterocyclic five-membered ring
∗For correspondence
247

248
Guan-Yeow Yeap and Boon-Teck Heng
is of great importance as core units in thermotropic microscope equipped with a Linkam LTS 350 hot stage
liquid crystals due to their ability to increase mole- and TMS94 temperature controller. The samples stud-
cular dipoles and dielectric anisotropy combined with ied by optical microscope were prepared in thin film
changes in the molecular shape.11,12 Furthermore, the sandwiched between glass slide and cover. The Elemen-
incorporation of heteroatoms can result in consider- tal (C, H and N) analysis were carried out using a Perkin
able changes in the physical properties of correspond- Elmer 2400 LS Series CHNS/O analyzer. UV-visible
ing liquid-crystalline phases as the nitrogen atoms in the spectra were recorded on a Perkin Elmer, Lambda 25
ring are more polarizable than carbon.12 It is also well- UV/Vis spectrometer.
known that 1,2,3-triazoles have been used as fungicides,
herbicides, light stabilizers, fluorescent whiteners, opti-
2.3 Synthesis of 2-hydroxy-4-(prop-2-ynyloxy)benzal-
dehyde, 1
cal brightening agents, corrosion inhibitors and photo-
stabilizers for fibres, plastics or dyes.12,13 However, to
the best of our knowledge, the study on the relationship
between the structures and mesomorphic properties
of 1,2,3-triazole-based metallomesogen has remained
scarce until now.
To a mixture containing 5 g (36 mmol) of 2,4-
dihydroxybenzaldehyde, catalytic amount of potassium
iodide and 1 equimolar of sodium bicarbonate in ace-
tone, 1 equimolar of propargyl bromide was added. The
reaction mixture was heated for overnight. The reaction
mixture was then filtered and the filtrate was evaporated
to dryness. The precipitate thus obtained was washed
with distilled water and purified by column chromato-
graphy eluting with a mixture of chloroform and hex-
ane (V/V, 1/1). Evaporation of the solvents gave the
product as a white solid.
2. Experimental
2.1 Materials
2,4-Dihydroxybenzaldehyde and propargyl bro-
mide were purchased from TCI (Tokyol Chemical
Industry) while sodium bicarbonate was obtained
from R&M-Chemicals. Copper(II) sulphate pen-
tahydrate and sodium ascorbate were obtained from
Qrec-Chemicals and Sigmal Aldrich, respectively. 1-
Bromodecane, 1-bromododecane, 1-bromotetradecane,
1-bromohexadecane and 1-bromooctadecane were
purchased from Merck whereas 4-fluoroaniline and
copper(II) acetate dihydrate were purchased from
Acros-Chemical. All reagents are used without further
purification.
2.4 Synthesis of 1-Azidoalkane
A mixture containing 6 g of 1-bromoalkane and 2
equimolar of sodium azide in dimethylformide, DMF
was heated overnight at 70◦C. The reaction mixture was
then filtered and the filtrate was extracted twice with
dichloromethane, DCM (50 ml). The extracted solution
was evaporated at room temperature to give the yellow
liquid product.
2.5 Synthesis of 4-((1-alkyl-1H-1,2,3-triazol-4-yl)
methoxy)-2-hydroxybenzaldehyde, 2
2.2 Physical measurement
The chemical structures of intermediates and target ma-
terials were studied by Perkin Elmer 2000-FTIR spec-
trophotometer in the frequency range 4000–400 cm−1
with sample prepared in KBr discs. The Brucker
Avance 300 MHz ultrashield spectrometers equipped
Compound 1 (100 mmol) and 1-azidoalkane (100 mol)
in DMF was treated with 20 mol% of Cu.SO4.5H2O
in DMF and 20 mol% sodium ascorbate in minimum
amount of water. The reaction mixture was stirred
at room temperature for 24 h. The mixture was then
poured into cold water and the resulting yellow pre-
cipitate was then filtered by using Buchner funnel.
The isolated compound thus obtained was recrystallized
several times from ethanol.
1
with ultrashield magnets was used to obtain the H-
NMR spectra. Deuterated chloroform (CDCl3) and
dimethysulphoxide (DMSO-d6) were used as solvent
and TMS as internal standard. Thin-layer chromatogra-
phy was performed with TLC sheets coated with silica
and spots were detected by UV irradiation. The phase
transition temperatures and phase transition enthalpy
values were measured by using a Seiko DSC6200R
calorimeter with the heating and cooling rate of 5◦C.
The liquid crystal textures of all compounds were
observed by Carl Zeiss Axioskop 40 polarizing
2.6 Synthesis of [1,2,3]-triazole-based bidentate
ligands, 3
Compound 2 (100 mmol) was dissolved in ethanol in
a round bottom flask. 4-Flouroaniline was dissolved in

Mesogenic copper(II) complexes with [1,2,3]-triazole
249
ethanol and then added dropwise to the mixture. The Ar-H, J = 8.7 hz), 7.31 (d, 1H, Ar-H, J = 8.4 hz),
resulting mixture was refluxed for 6 h under continuous 7.65 (s, 1H, N-CH), 8.51 (s, 1H, N=CH), 13.7 (s, 1H,
stirring. The mixture solution was then concentrated Ar-OH).
by evaporated out the ethanol at room temperature and
the precipitate thus obtained was recrystallized from (3e): Yellow, yield 56%. Elemental analysis/% : Found
ethanol to yield the desired product.
C 72.58, H 8.73, N 9.90; calculated (C34H49FN4O2),
The yields and the elemental analytical data for C 72.31, H 8.74, N 9.92. IR (KBr) vmax/cm−1: 1248
ligands 3a–3e are shown as follows:
(C–O ether), 1283 (C–O phenolic), 1571 (C=C), 1626
(C=N), 2916–2848 (C–H alkyl), 3442 (O–H). 1H NMR
(300 MHz, CDCL3), δ (ppm): 0.89 (t, 3H, CH3, J =
6.9 hz), 1.32–1.95 (m, 16H, CH2), 4.38 (t, 2H, OCH2,
J = 7.5 hz), 5.26 (s, 2H, OCH2), 6.59–6.62 (m, 2H, Ar-
H), 7.11 (d, 2H, Ar-H, J = 8.7 hz), 7.24 (d, 2H, Ar-H,
J = 8.7 hz), 7.29 (d, 1H, Ar-H, J = 8.4 hz), 7.65 (s, 1H,
N-CH), 8.52 (s, 1H, N=CH), 13.6 (s, 1H, Ar-OH).
(3a): Yellow, yield 43%. Elemental analysis/% : Found
C 69.21, H 7.34, N 12.33; calculated (C26H33FN4O2),
C 69.00, H 7.35, N 12.30. IR (KBr) vmax/cm−1: 1247
(C–O ether), 1283 (C–O phenolic), 1571 (C=C), 1626
(C=N), 2916–2848 (C-H alkyl), 3441 (O–H). 1H NMR
(300 MHz, CDCL3), δ (ppm): 0.89 (t, 3H, CH3, J =
6.9 hz), 1.33–1.95 (m, 16H, CH2), 4.38 (t, 2H, OCH2,
J = 7.5 hz), 5.26 (s, 2H, OCH2), 6.59–6.62 (m, 2H, Ar-
H), 7.11 (d, 2H, Ar-H, J = 8.7 hz), 7.24 (d, 2H, Ar-H,
J = 8.7 hz), 7.30 (d, 1H, Ar-H, J = 8.4 hz), 7.65 (s, 1H,
N-CH), 8.52 (s, 1H, N=CH), 13.6 (s, 1H, Ar-OH).
2.7 Synthesis of copper(II) complexes 4
An ethanolic solution of copper(II) acetate dihydrate
(1.0 mmol) was added dropwise to a hot ethanolic solu-
tion (50 ml) of ligands 3 (1.0 mmol) in round bottom
flask. The mixture was refluxed for 6 h and then cooled
to room temperature. The dark brown precipitate was
collected by filtration and washed with hot ethanol.
The yields and the elemental analytical data for
complexes 4a–4e are shown as follows:
(3b): Yellow, yield 55%. Elemental analysis/% : Found
C 70.19, H 7.81, N 11.65; calculated (C28H37FN4O2),
C 69.97, H 7.76, N 11.66. IR (KBr) vmax/cm−1: 1248
(C–O ether), 1283 (C–O phenolic), 1572 (C=C), 1626
(C=N), 2917–2850 (C-H alkyl), 3441 (O–H). 1H NMR
(300 MHz, CDCL3), δ (ppm): 0.89 (t, 3H, CH3, J =
6.9 hz), 1.34–1.96 (m, 16H, CH2), 4.38 (t, 2H, OCH2,
J = 7.5 hz), 5.25 (s, 2H, OCH2), 6.59–6.62 (m, 2H, Ar-
H), 7.10 (d, 2H, Ar-H, J = 8.7 hz), 7.24 (d, 2H, Ar-H,
J = 8.7 hz), 7.30 (d, 1H, Ar-H, J = 8.4 hz), 7.64 (s, 1H,
N-CH), 8.52 (s, 1H, N=CH), 13.7 (s, 1H, Ar-OH).
(4a): Dark brown, yield 83%. Elemental analysis/%:
Found C 64.49, H 6.64, N 11.55; calculated (C52H64CuF2
N8O4), C 64.61, H 6.67, N 11.59. IR (KBr) vmax/cm−1:
1250 (C-O ether), 1324 (C-O phenolic), 1590 (C=C),
1601 (C=N), 2917–2850 (C-H alkyl).
(4b): Dark brown, yield 79%. Elemental analysis/%:
Found C 65.98, H 7.02, N 10.97; calculated
(C56H72CuF2N8O4), C 64.61, H 6.67, N 11.59. IR (KBr)
(3c): Yellow, yield 53%. Elemental analysis/% : Found
C 70.96, H 8.09, N 11.03; calculated (C30H41FN4O2),
C 70.84, H 8.12, N 11.01. IR (KBr) vmax/cm−1: 1249
(C-O ether), 1284 (C–O phenolic), 1571 (C=C), 1625
(C=N), 2916–2848 (C-H alkyl), 3440 (O–H). 1H NMR
(300 MHz, CDCL3), δ (ppm): 0.89 (t, 3H, CH3, J =
6.9 hz), 1.33–1.95 (m, 16H, CH2), 4.38 (t, 2H, OCH2,
J = 7.5 hz), 5.25 (s, 2H, OCH2), 6.59–6.62 (m, 2H, Ar-
H), 7.10 (d, 2H, Ar-H, J = 8.7 hz), 7.24 (d, 2H, Ar-H,
J = 8.7 hz), 7.31 (d, 1H, Ar-H, J = 8.4 hz), 7.65 (s, 1H,
N-CH), 8.51 (s, 1H, N=CH), 13.7 (s, 1H, Ar-OH).
v
max/cm−1: 1249 (C-O ether), 1322 (C-O phenolic),
1591 (C=C), 1601 (C=N), 2916–2850 (C-H alkyl).
(4c): Dark brown, yield 88%. Elemental analysis/%:
Found C 67.01, H 7.49, N 10.38; calculated
(C60H80CuF2N8O4), C 66.80, H 7.47, N 10.39. IR (KBr)
v
max/cm−1: 1249 (C–O ether), 1324 (C-O phenolic),
1590 (C=C), 1601 (C=N), 2917–2850 (C-H alkyl).
(4d): Dark brown, yield 80%. Elemental analysis/%:
Found C 68.04, H 7.83, N 9.90; calculated
(C64H88CuF2N8O4), C 67.73, H 7.82, N 9.87. IR (KBr)
(3d): Yellow, yield 49%. Elemental analysis/% : Found
C 71.96, H 8.50, N 10.49; calculated (C32H45FN4O2),
C 71.61, H 8.45, N 10.44. IR (KBr) vmax/cm−1: 1249
(C–O ether), 1283 (C–O phenolic), 1570 (C=C), 1625
(C=N), 2916–2849 (C–H alkyl), 3441 (O–H). 1H NMR
(300 MHz, CDCL3), δ (ppm): 0.89 (t, 3H, CH3, J =
6.9 hz), 1.33–1.96 (m, 16H, CH2), 4.37 (t, 2H, OCH2,
J = 7.5 hz), 5.25 (s, 2H, OCH2), 6.59–6.63 (m, 2H,
Ar-H), 7.10 (d, 2H, Ar-H, J = 8.7 hz), 7.24 (d, 2H,
v
max/cm−1: 1251 (C-O ether), 1323 (C-O phenolic),
1590 (C=C), 1602 (C=N), 2916–2849 (C-H alkyl).
(4e): Dark brown, yield 78%. Elemental analysis/%:
Found C 68.75, H 8.10, N 9.42; calculated
(C68H96CuF2N8O4), C 68.57, H 8.12, N 9.41. IR (KBr)

250
Guan-Yeow Yeap and Boon-Teck Heng
v
max/cm−1: 1250 (C-O ether), 1324 (C-O phenolic), 4-(prop-2-ynyloxy)benzaldehyde. The triazole derivative
1591 (C=C), 1602 (C=N), 2917–2850 (C-H alkyl).
thus obtained was subjected to condensation reaction
with aromatic amine to yield the target Schiff bases
3a–3e. Finally, the reactions between 3a–3e with
Cu(II) acetate dihydrate was carried out to form Cu(II)
complexes 4a–4e as dark brown solids. The chemical
structure of the target ligands and their correspond-
ing complexes were elucidated by FT-IR, H-NMR,
UV-visible spectrometer and elemental analysis.
The carbon, hydrogen and nitrogen composition for
Cu(II) complexes 4a–4e were determined via the CHN
microanalysis which were then compared with the cal-
culated values. The results showed that the percentages
3. Result and discussion
3.1 Synthesis
1
The mesogenic Cu(II) complexes 4a–4e and their
bidentate ligands 3a–3e were prepared through the syn-
thetic route as described in scheme 1. The intermediate
[1,2,3]-triazole ring compound was synthesized from
the Cu(I) catalysed ‘click reaction’ of 1-azidoalkane
(CnH2n+1N3; n = 10, 12, 14, 16 and 18) and 2-hydroxy-
CnH2n+1Br
HO
CHO
+
+
Br
OH
2NaN3
NaHCO3
Acetone
DMF
70oC
O
CHO
CnH2n+1-N3
+
CuSO4.5H2O
OH
1
Sodium Ascorbate
DMF
N
N
N
C
H2N
F
+
C
H
O
CHO
H2n+1Cn
2
OH
Ethanol
N
C
N
N
3a ; n=10
C
H
O
CH
H2n+1Cn
3b ; n=12
3c ; n=14
3d ; n=16
3e ; n=18
F
N
OH
3
+
Cu(CH3COOH)2.2H2O
Ethanol
N
N
N
C
C
O
CH
H2n+1Cn
H
N
O Cu O
N
F
4a ; n=10
F
4b ; n=12
4c ; n=14
4d ; n=16
4e ; n=18
H
C
CnH2n+1
HC
O
N
C
4
N
N
Scheme 1. Synthetic routes toward formation of ligands 3a–3e and Cu(II) complexes 4a–4e.

Mesogenic copper(II) complexes with [1,2,3]-triazole
251
(i)
3442
1626
.
%T
(ii)
1602
1800
4000.0
3600
3200
2800
2400
2000
1600
1400
1200
1000
800
600
400.0
cm-1
Figure 1. Representative IR spectra of (i) ligand 3e and (ii) Cu(II) complex 4e.
of C, H and N in the respective complexes are in agree- complexes indicates that the OH group is deprotonated
ment with its empirical formula of which the ratio of to form O− ion which entails the formation of new Cu-
ligand:Cu is 2:1.
O coordination mode.15,21–27 Furthermore, the stretch-
Based on the FT-IR spectra of ligands 3a–3e, a strong ing vibration of phenolic C–O is shifted to a higher
band observed at 1625–1626 cm−1 is assigned to the frequency (1322–1324 cm−1) which can support the
stretching of imine, C=N group.14–16 The presence of coordination of O− to Cu(II) ion.18,22,23,28 The other
the C=N absorption and the absence of N–H absorp- major bands at 2849–2917 cm−1 can be assigned to the
tion bands of 4-aminoazobenzene at 3473–3382 cm−1 stretching frequency of C–H bond from alkoxy chain.
indicates the formation of the desired Schiff base link- The band with strong intensity in the IR spectra at
age. A strong band at 2848–2917 cm−1 can be assigned 1249–1251 cm−1 is assignable to the stretching of the
to the stretching vibration of C–H bond from alkoxy C–O ether group.
chain. The band with strong intensity in the spectra at
the range of 1247–1249 cm−1 (O–CH2) can be ascribed
1.000
to the ether group.17 A medium band at the frequency of
1570–1572 cm−1 can be attributed to the C=C stretch-
ing in the phenyl ring.16 The broad band at 3440–
Complex 4c
0.800
0.600
0.400
0.200
0.000
3441 cm−1 can be assigned to the stretching frequency
of O–H group whereas the stretching of the phenolic
C–O is observed as a medium band at the stretching
frequency of 1283–1284 cm−1.18–23
Coordination of the ligands 3a–3e to Cu(II) ion
involves a salicylidene fragment and imine group. The
corresponding changes can be described by compar-
ing the relevant IR bands associated with 3e and 4e
(figure 1). The strong band observed at 1601–
1602 cm−1 corresponds to the stretching of imine C=N.
The C=N frequency in the complex is shifted to a lower
stretching frequenct by 24–25 cm−1 as compared to free
ligands 3a–3e. This observation is due to the reduction
in the bond order of C=N bond and hence suggested
the coordination of N atom from the ligands to the cen-
tre metal atoms.15,21–27 Besides, the disappearance of the
O–H band of free ligands in IR spectruam for Cu(II)
Ligand 3c
270.00
400.00
600.00
800.00
nm.
Figure 2. UV-visible spectra for ligand 3c and Cu(II) com-
plex 4c.

252
Guan-Yeow Yeap and Boon-Teck Heng
the broad signals assignable to the alkyl group as all
of these proton signals are in close proximity. How-
ever, the signal for the paramagnetic Cu(II) centre is
not detectable. The electronic spectra of the ligands 3a–
3e and their Cu(II) complexes were recorded in chloro-
form. Figure 2 shows the electronic spectra for ligands
3c and Cu(II) complex 4c. The UV-visible absorption
spectra of the ligands consist of an intense band at 337–
338 nm which can be related to n–π* transition of the
non-bonding valence shell electron pairs from the li-
gands. Another weak band at 287–288 nm in the higher
energy region can be attributed to the π–π* transition
of benzene rings. Upon complexation with Cu(II) ion,
these bands are red shifted to the longer wavelength
(figure 2). The expected characteristic of the d–d tran-
sitions in the visible region at about 600 nm for the
complexes were not detectable even in the concentrated
solutions. This observation is due to the forbidden tran-
sition of the highly symmetrical Cu(II) complexes. The
UV-visible data are summarized in table 1. All of these
Table 1. UV-visible data for ligands 3a–3e and complexes
4a–4e.
Absorption maximun,nm
Compounds
π–π*
n–π*
3a
4a
3b
4b
3c
4c
3d
4d
3e
4e
288
303
289
302
288
303
289
301
287
303
338
379
338
382
338
379
337
381
338
380
The magnetic susceptibility measurement for all
Cu(II) metal complexes, 4a–4e, in which the electronic
configuration is d9, show that these complexes are para-
1
magnetic and their H NMR spectra displayed only
Table 2. Phase transition temperature (◦C) and associated enthalpy (kJ/mol)
of ligands 3a–3e.
Ligands
Cr1
Cr2
•
I
3a
3b
3c
3d
3e
Heating
•
•
•
•
•
•
•
•
•
•
81.8 (2.4)
80.7 (8.1)
79.0 (2.6)
88.6 (7.1)
90.9 (3.7)
115.7 (16.0)
109.7 (−12.8)
115.0 (22.9)
85.0 (−3.3)
•
•
•
•
•
•
•
•
•
•
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
•
•
118.8 (24.8)
105.3 (−26.3)
116.8 (26.3)
101.6 (−38.9)
115.9 (42.5)
97.15 (−45.0)
Cr1, crystal 1; Cr2, crystal 2; I, isotropic.
Table 3. Phase transition temperature (oC) and associated enthalpy (kJ/mol)
of complexes 4a–4e.
Complexes
Cr
SmA
I
4a
4b
4c
4d
4e
Heating
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
•
•
•
•
•
•
•
•
•
•
157.9 (36.0)
127.1a
•
•
•
•
•
•
•
•
•
•
111.1 (−8.0)
96.2 (−3.3)
•
•
•
•
•
157.5 (39.9)
114.0a
156.3 (41.4)
126.6 (−5.6)
155.9 (89.0)
130.4 (−6.4)
154.8 (93.6)
133.5 (−7.1)
101.9 (−18.2)
107.5 (−18.7)
102.5 (−19.3)
Cr, crystal; Sm A, smectic A; I; isotropic
aDenotes transition temperatures determined via polarizing microscopy, unde-
tected via DSC

Mesogenic copper(II) complexes with [1,2,3]-triazole
253
Figure 5. Photomicrograph shows the SmA phase with
focal conic fan-shaped texture of complex 4d at 130.4◦C
during the cooling process.
Figure 3. Photomicrograph shows the existence of bat-
tonets and crystal phase of ligand 3d at 101.6◦C during the
cooling process.
observations support the formation of Cu–O and Cu–
N coordination modes in which the ratio of Cu(II) to
ligands is 1 : 2.
molecular anisotropic and resulted in lower clearing
temperature.23,29,30
Optical observation shows that the whole homo-
logous series of Cu(II) complexes 4a–4e exhibit
monotropic SmA phase. Upon cooling, the liquid
phases of these complexes are gradually changed to
the SmA phases with focal conic fan-shaped texture
as illustrated in figure 5. The formation of mesophase
is reproducible upon subsequent heating and cooling.
3.2 Phase transition and mesomorphic properties
of [1,2,3] triazole Schiff bases and related Cu(II)
complexes
The transition temperatures and associated enthalpies The formation of stable SmA phase in complexes as
(kJ mol−1) obtained from differential scanning compared to the free ligands can be attributed to the
calorimetry (DSC) are tabulated in tables 2 and 3, metal ions which increases the colinearity and thus
respectively. The first cooling on ligands 3a–3e from enhances the anisotropic properties of these ligands.3–7
isotropic liquid with transition rate of 1◦C min−1 shows On the other hand, the formation of Cu(II) complexes
the existence of battonets of SmA phase and crystal 4a–4e can also increase the aspect ratio of aromatic core
phase. Figure 3 shows that the presence of battonets to terminal flexible chain which enhances the thermal
is suppressed very quickly by the formation of crystal stability of both crystal and liquid crystal phases.31
phase and these two phases occurred almost at the
same time. Besides, the battonets of SmA phase is not
reproducible upon subsequent heating and cooling.
This phenomenon can be explained by the alkyl chain
of [1, 2, 3]-triazole ring which shows non-colinearity to
the rigid core segment leading to the poor mesophase
stability.11,23 It also shows clearly in figure 4 that the
alkyl chain attached to the triazole ring is ‘out of line’
with the molecular long axis and therefore reduced the
N
N
C
N
C
H
O
CH
C
H2
F
N
OH
Figure 6. A plot of clearing temperature versus the number
Figure 4. The molecular structure of ligand with the dotted of carbon atoms in alkyl chain for the ligands 3a–3e and their
line indicating the molecular axis. Cu(II) complexes 4a–4e.

254
Guan-Yeow Yeap and Boon-Teck Heng
8. Marcos M, Omenat A, Barberá J, Durán F and Serrano
The correlation study between thermal stability of
J L 2004 J. Mater. Chem. 14 3321
free ligands and their corresponding complexes has
shown that the Cu(II) complexes 4a–4e are thermally
more stable than free ligands 3a–3e. Figure 6 shows that
the clearing temperatures of Cu(II) complexes increases
by 37–42◦C upon complexation. This phenomenon can
be ascribed to the fact that the molecular weights as well
as the number of interacting sites of Cu(II) complexes
are higher in comparison to the ligands.14
9. Abe Y, Nakabayashi K, Matsukawa N, Takashima H,
Iida M, Tanase T, Sugibayashi Mukai H and Ohta K
2006 Inorg. Chim. Acta. 359 3934
10. Plasseraud L, Cuervo l G, Guillon D, Fink G S,
Deschenaux R, Bruce D W and Donnio B 2002 J. Mater.
12 2653
11. Srividhya D, Manjunathan S, Thirumaran S and
Saravanan C 2009 J. Mol. Struct. 927 7
12. Gallardo H, Bortoluzzi A J and Oliveira Santos M P D
2008 Liq. Cryst. 6 719
13. Gallardo H, Ely F, Bortoluzzi A J and Conte G 2005 Liq.
Cryst. 32 667
4. Conclusion
14. Chae H W, Kadhin O N and Choi M G 2009 J. Liq.
Cryst. 36 53
15. Rezvani Z, Divband B, Abbasi A R and Nejati K 2006
Polyhedron 25 1915
16. Yeap G Y, Hng T C, Kamil Mahmood W A, Ito M M,
Youhei Y, Takanishi Y and Takezoe H 2010 Liq. Cryst.
33 979
17. Pretsch E, Buhlmann P and Affolter C 2000 Struc-
ture determination of organic compound (Spring–Verlag
Berlin Heidelberg, New York)
A series of Cu(II) complexes derived from [1,2,3]-
triazole bidentate Schiff bases with various alkyl groups
of even parity (n = 8–16) were successfully synthe-
sized and characterized. All the Cu(II) complexes show
monotropic SmA phase which are reproducible upon
subsequent heating and cooling. The non-collinearity of
the ligands has led to the formation of monotropic SmA
phase with poor stability. The clearing temperatures of
Cu(II) complexes are significantly higher as compared 18. Mustafaa I M, Hapipaha M A, Abdullab M A and Warda
T R 2009 Polyhedron 28 3993
19. Mahalakshmi N and Rajavel R 2011 Asian J. Biochem.
Pharm. Res. 1 2231
to the uncoordinated ligands and these values decrease
with the increase in the length of the alkyl chain.
20. Liu J N, Wu B W, Zhang B and Liu Y 2006 Turk. J.
Chem. 30 41
Acknowledgements
21. Yeap G Y, Heng B T, Kakeya M, Takeuchi D, Gorecka
E and Ito M M 2011 J. Mol. Struct. 999 68
22. Yeap G Y, Heng B T, Tanabe M and Takeuchi D 2011
Mol. Cryst. Liq. Cryst. 552 217
The first author would like to thank Universiti Sains
Malaysia for the RU Grant No. 1001/PKIMIA/811159
and 1001/PKIMIA/811223.
23. Yeap G Y, Heng B T, Faradiana N, Zulkifly R, Ito M M,
Tanabe M M and Takeuchi D 2012 J. Mol. Struct. 1012 1
24. Rezvani Z, Ahar L R, Nejati K and Seyedahnadian S M
2004 Acta Chim. Slov. 51 675
25. Suste A and Sunjic V 1996 Liq. Cryst. 20 219
26. Pucci D, Aiello I, Bellusci A, Callipari G, Crispini A and
Ghedini M 2009 Mol. Cryst. Liq. Cryst. 500 144
27. Nandiraju V S, Singha D, Das M and Paul M K 2002
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29. Okamato K, Kawamura T, Sone M and Ogino K 2007
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