Synthesis, characterization, and anisotropic properties of 5-alkoxy-2-((4-(phenyldiazenyl)phenylimino)methyl)phenol and their copper(II) complexes

Article abstract of DOI:10.1080/15421406.2011.619097

A new homologous series of bidentate Schiff bases 5-alkoxy-2-((4- (phenyldiazenyl) phenylimino)methyl)phenol and their Cu(II) complexes have been synthesized and characterized by spectroscopic techniques along with studies on the mesomorphic properties. P

Full text of DOI:10.1080/15421406.2011.619097

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Synthesis, Characterization, and
Anisotropic Properties of 5-Alkoxy-2-((4-
(Phenyldiazenyl)Phenylimino)Methyl)phenol
and Their Copper(II) Complexes
Guan-Yeow Yeap a , Boon-Teck Heng a , Makoto Tanabe b & Daisuke
Takeuchi b
a Liquid Crystal Research Laboratory, School of Chemical Sciences ,
Universiti Sains Malaysia , Penang , Malaysia
b Chemical Resources Laboratory , Tokyo Institute of Technology ,
Yokohama , Japan
Published online: 27 Dec 2011.
To cite this article: Guan-Yeow Yeap , Boon-Teck Heng , Makoto Tanabe & Daisuke
Takeuchi (2012) Synthesis, Characterization, and Anisotropic Properties of 5-Alkoxy-2-((4-
(Phenyldiazenyl)Phenylimino)Methyl)phenol and Their Copper(II) Complexes, Molecular Crystals and
Liquid Crystals, 552:1, 217-227, DOI: 10.1080/15421406.2011.619097
To link to this article: http://dx.doi.org/10.1080/15421406.2011.619097
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Mol. Cryst. Liq. Cryst., Vol. 552: pp. 217–227, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2011.619097
Synthesis, Characterization, and Anisotropic
Properties of 5-Alkoxy-2-((4-
(Phenyldiazenyl)Phenylimino)Methyl)phenol
and Their Copper(II) Complexes
GUAN-YEOW YEAP,1,∗ BOON-TECK HENG,1
MAKOTO TANABE,2 AND DAISUKE TAKEUCHI2
1Liquid Crystal Research Laboratory, School of Chemical Sciences, Universiti
Sains Malaysia, Penang, Malaysia
2Chemical Resources Laboratory, Tokyo Institute of Technology, Yokohama,
Japan
A new homologous series of bidentate Schiff bases 5-alkoxy-2-((4-(phenyldiazenyl)
phenylimino)methyl)phenol and their Cu(II) complexes have been synthesized and char-
acterized by spectroscopic techniques along with studies on the mesomorphic properties.
Phase transitions of all the Schiff bases and Cu(II) complexes in this series are enan-
tiotropic. The ligands with n-octyloxy and n-decyloxy flexible chains exhibit the nematic
(N) and smectic A (SmA) phases, the latter of which is characterized by focal conic
fan-shaped texture. In contrast, the Cu(II) complexes show exclusively the SmA phase,
which is ascribed to the presence of Cu O and Cu N coordination modes. For the
same reason, the thermal stabilities of the complexes have also been improved.
Keywords Cu(II) complexes; enantiotropic; nematic; orthogonal smectic A; Schiff
bases
1. Introduction
In recent years, a considerable number of metallomesogens have been synthesized and their
properties documented [1]. The liquid crystal behaviors of these metal–ligand complexes
are claimed to be more prominent than their free ligands owing to the advantages of
combining the properties of these ligands with those of transition metals. One of the factors
can be attributed to the metal ions that enhance the anisotropic properties of these ligands.
As such, the ligands found to be nonmesogenic earlier can change to mesogenic complexes
possessing metal–ligand moieties [2].
Metal ions are generally known to possess diverse geometries. As a result, the metal-
lomesogens thus obtained will exist in new geometrical shapes, which may not be observed
in the metal-free ligands. These metallomesogens are also much superior to uncoordinated
ligands in that the transition metals possess various oxidation states, colors, and magnetisms
∗Address correspondence to Guan-Yeow Yeap, Liquid Crystal Research Laboratory, School of
Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia. Fax: 60-4-6574854.
E-mail: gyyeap@usm.my; gyyeap liqcryst usm@yahoo.com
217

218
G.-Y. Yeap et al.
[3]. It has been reported that the one-dimensional stacking of metal centers could lead to
materials with interesting properties such as one-dimensional magnetism and conductiv-
ity [4]. Especially, the incorporation of paramagnetic ions in liquid crystalline materials
enhances the application of one-dimensional magnet in order to repair the defects in their
structure [5] and offers important routes to anisotropic materials that can be readily aligned
in the devices [4].
In 1997, Tantrawong and Styring had successfully synthesized the metal containing
liquid crystal with the potential application in optical storage [6]. Hayami et al. have reported
the mesophase and magnetic behavior in Co(II) and Fe(II) compounds [7]. According to
Liu et al., the metallomesogens also possess widespread use as stationary phase in gas
chromatographic application [8].
In 2002, Nandiraju et al. reported the influence of metal on the mesomorphic properties
of the ligands [9]. A similar study has been carried out by Suste et al. [10].
Besides, the azo-based polymeric liquid crystals and their metal-containing derivatives
have also been studied extensively in recent years because these materials are capable
of changing the molecular shape through the reversible cis–trans isomerization under the
influence of UV or photoirradiation. However, it has been claimed that the molecules
with N N group prefer the trans conformation in mesophase as that reported for
photoexcited azo-dye induced torque in N liquid crystals [11].
In spite of these studies, the influence of the metal on the mesophase and correlation
between the structure and anisotropic properties of metallomesogens are still subject to
further investigation. In this study, a new homologous series of bidentate Schiff bases 5-
alkoxy-2-((4-(phenyldiazenyl)phenylimino)methyl)phenol and their Cu(II) complexes have
been synthesized to study the influence of the Cu(II) metal ion upon the mesomorphic
properties.
2. Experimental
1-Bromooctane, 1-bromodecane, 1-bromododecane, 1-bromotetradecane, and 1-bromo-
hexadecane were purchased from Merck. 4-Aminoazobenzene was purchased from Aldrich
Chemical while copper(II) acetate monohydrate and 2,4-dihydroxybenzaldehyde were ob-
tained from Acros Organic. Potassium carbonate was purchased from Systerm.
All the ligands and complexes obtained in this study were characterized by CHN
microanalysis, IR spectroscopy, and NMR spectroscopy. CHN microanalyses were carried
out using a Perkin Elmer 2400 LS Series CHNS/O analyzer. IR spectra were recorded using
a Perkin-Elmer 2000 FT-IR spectrophotometer in the frequency range of 4000–400 cm−1
.
The IR spectra of all compounds were measured for the samples dispersed in KBr pellets.
1H NMR spectra were recorded on a Bruker 400-MHz Ultrashield FT-NMR spectrometer
using CDCl3 as solvent and TMS as the internal standard.
The synthesis of the ligands and corresponding Cu(II) complexes is illustrated in
Scheme 1.
2.1. Synthesis of Ligands 1a–1e
Ligands 1a–1e were synthesized by the general procedure as described below.
The mixture containing 4-aminoazobenzene and 2,4-dihydroxybenzaldehyde was
heated under reflux in ethanol for 4 h. The precipitate thus formed was isolated and
purified with ethanol twice to yield the desired compound. Subsequently, these interme-
diates were reacted with respective 1-bromooctane, 1-bromodecane, 1-bromododecane,

5-Alkoxy-2-((4-(Phenyldiazenyl)Phenylimino)Methyl)phenol
219
N
N
NH2
O
+
HC
OH
HO
Ethanol
N
N
N
C
H
OH
HO
+
2.2 CnH2n+1Br
K2CO3
Acetone
N
N
N
C
H
O
CnH2n+1
HO
+
n = 8; 1a
.
Cu2+(CH3COO-)2 H2O
n = 10; 1b
n = 12; 1c
n = 14; 1d
n = 16; 1e
Ethanol
H
C
N
N
N
O
CnH2n+1
O
Cu
O
O
C
H
N
N
N
H2n+1Cn
n = 8; 2a
n = 10; 2b
n = 12; 2c
n = 14; 2d
n = 16; 2e
Scheme 1. Synthetic route to copper(II) complexes.

220
G.-Y. Yeap et al.
1-bromotetradecane, and 1-bromohexadecane in the presence of potassium carbonate as
base and acetone as the solvent under reflux for overnight. The crude products thus obtained
were purified through recrystallization from hexane.
2.2. Synthesis of Copper Complexes 2a–2e
The Cu complexes 2a–2e were prepared using the following method.
Copper(II) acetate monohydrate was dissolved in ethanol and added to a solution of
free ligand (1a–1e) in ethanol. The reaction mixture was stirred and refluxed for 6 h. The
brownish crystals were filtered and recrystallized from a solvent mixture of ethanol and
chloroform (1:1 v/v).
The analytical data, i.e., IR, 1H NMR spectra data for ligands (1a–1e) and Cu(II)
complexes (2a–2e), are summarized as follows.
1a: Yield 46%. Elemental analysis/%: found C 75.92, H 7.32, N 9.80; calculated
(C27H31N3O2) C 75.52, H 7.23, N 9.79. IR (KBr)vmax/cm−1: 1625 (C N), 2848–2954
(C H alkyl), 1194 (O CH2), 1592, 1571 (C C), 3114–3712 (O H). 1H NMR (400 MHz,
CDCL3) δ/ppm: 0.88–0.91 (t, 3H, CH3), 1.31–1.87 (m, 12H, CH2), 4.02–4.05 (t, 2H, CH2),
6.52–6.55 (m, 2H, Ar), 7.32–7.34 (d, 1H, Ar), 7.41–7.43 (d, 2H, Ar), 7.46–7.57 (m, 3H,
Ar), 7.94–7.96 (d, 2H, Ar), 8.00–8.03 (d, 2H, Ar), 8.63 (s, 1H, CH N), 13.62 (s, 1H,
Ar OH).
1b: Yield 52%. Elemental analysis/%: found C 76.45, H 7.68, N 9.21; calculated
(C29H35N3O2) C 76.15, H 7.66, N 9.19. IR (KBr)vmax/cm−1: 1625 (C N), 2954–2848
(C H alkyl), 1194 (O CH2), 1592, 1571 (C C), 3115–3714 (O H). 1H NMR (400 MHz,
CDCL3) δ/ppm: 0.88–0.91 (t, 3H, CH3), 1.29–1.87 (m, 12H, CH2), 4.02–4.05 (t, 2H, CH2),
6.51–6.55 (m, 2H, Ar), 7.31–7.33 (d, 1H, Ar), 7.40–7.43 (d, 2H, Ar), 7.45–7.57 (m, 3H,
Ar), 7.93–7.96 (d, 2H, Ar), 8.00–8.03 (d, 2H, Ar), 8.63 (s, 1H, CH N), 13.60 (s, 1H,
Ar OH).
1c: Yield 50%. Elemental analysis/%: found C 77.01, H 8.14, N 8.70; calculated
(C31H39N3O2), C 76.70, H 8.04, N 8.66. IR (KBr)vmax/cm−1: 1624 (C N), 2953–2848
(C H alkyl), 1193 (O CH2), 1592, 1571 (C C), 3115–3714 (O H). 1H NMR (400 MHz,
CDCL3) δ/ppm: 0.88–0.92 (t, 3H, CH3), 1.25–1.88 (m, 12H, CH2), 4.02–4.05 (t, 2H, CH2),
6.51–6.55 (m, 2H, Ar), 7.30–7.33 (d, 1H, Ar), 7.40–7.43 (d, 2H, Ar), 7.46–7.57 (m, 3H,
Ar), 7.93–7.96 (d, 2H, Ar), 8.01–8.03 (d, 2H, Ar), 8.63 (s, 1H, CH N), 13.60 (s, 1H,
Ar OH).
1d: Yield 53%. Elemental analysis/%: found C 77.59, H 8.58, N 8.19; calculated
(C33H43N3O2), C 77.19, H 8.38, N 8.18. IR (KBr)vmax/cm−1: 1625 (C N), 2953–2849
(C H alkyl), 1193 (O CH2), 1591, 1571 (C C), 3115–3714 (O H). 1H NMR (400 MHz,
CDCL3) δ/ppm: 0.89–0.93 (t, 3H, CH3), 1.25–1.85 (m, 12H, CH2), 4.02–4.05 (t, 2H, CH2),
6.52–6.54 (m, 2H, Ar), 7.28–7.33 (d, 1H, Ar), 7.40–7.43 (d, 2H, Ar), 7.45–7.57 (m, 3H,
Ar), 7.93–7.96 (d, 2H, Ar), 8.01–8.03 (d, 2H, Ar), 8.62 (s, 1H, CH N), 13.63 (s, 1H,
Ar OH).
1e: Yield 50%. Elemental analysis/%: found C 77.96, H 8.73, N 7.81; calculated
(C35H47N3O2), C 77.63, H 8.69, N 7.76. IR (KBr)vmax/cm−1: 1625 (C N), 2953–2849
(C H alkyl), 1192 (O CH2), 1591, 1571 (C C), 3114–3714 (O H). 1H NMR (400 MHz,
CDCL3) δ/ppm: 0.89–0.93 (t, 3H, CH3), 1.25–1.85 (m, 12H, CH2), 4.02–4.05 (t, 2H, CH2),
6.52–6.54 (m, 2H, Ar), 7.28–7.33 (d, 1H, Ar), 7.40–7.43 (d, 2H, Ar), 7.45–7.57 (m, 3H,
Ar), 7.93–7.96 (d, 2H, Ar), 8.01–8.03 (d, 2H, Ar), 8.62 (s, 1H, CH N), 13.63 (s, 1H,
Ar OH).

5-Alkoxy-2-((4-(Phenyldiazenyl)Phenylimino)Methyl)phenol
221
2a: Yield 80%. Element analysis/%: found C 70.67, H 6.65, N 9.26; calculated
(C54H60N6O4Cu), C 70.46, H 6.53, N 9.14. IR (KBr)vmax/cm−1: 1610 (C N), 2924–2852
(C H alkyl), 1206 (O CH2).
2b: Yield 75%. Elemental analysis/%: found C 71.66, H 7.03, N 8.73; calculated
(C58H68N6O4Cu), C 71.35, H 6.97, N 8.61. IR (KBr)vmax/cm−1: 1611 (C N), 2921–2850
(C H alkyl), 1206 (O CH2).
2c: Yield 70%. Elemental analysis/%: found C 72.42, H 7.57, N 8.16; calculated
(C62H76N6O4Cu), C 72.13, H 7.37, N 8.14. IR (KBr)vmax/cm−1: 1611 (C N), 2921–2851
(C H alkyl), 1206 (O CH2).
2d: Yield 80%. Elemental analysis/%: found C 73.21, H 7.87, N 7.88; calculated
(C66H84N6O4Cu), C 72.82, H 7.72, N 7.72. IR (KBr)vmax/cm−1: 1610 (C N), 2923–2851
(C H alkyl), 1206 (O CH2).
2e: Yield 80%. Elemental analysis/%: found C 73.87, H 8.17, N 7.46; calculated
(C70H92N6O4Cu), C 73.46, H 8.05, N 7.35. IR (KBr)vmax/cm−1: 1611 (C N), 2921–2850
(C H alkyl), 1206 (O CH2).
3. Results and Discussion
3.1. Phase Transition and Mesomorphic Properties of Schiff Bases and Related Cu(II)
Complexes
Under the polarizing microscope, all the uncoordinated ligands (1a–1e) and their Cu(II)
complexes (2a–2e) exhibit the phase transitions which are reproducible upon heating and
cooling. The transition temperatures and associated enthalpies (kJ mol−1) obtained from
differential scanning calorimetry (DSC) are given in Tables 1 and 2, respectively.
When the ligands 1a and 1b are cooled down from isotropic liquid, the schlieren texture
characteristics of N phase [12] are observed at 173.9◦C and 162.6◦C, respectively. Further
cooling of 1a and 1b to respective temperatures of 161.1◦C and 159.4◦C, has led to the
appearance of well-defined focal conic fan-shaped texture characteristics of SmA phase
[13]. It is clearly shown in Fig. 1 that the nematic droplets appear and coalesce (Fig. 1(a))
Figure 1. Photomicrographs of ligand 1a upon cooling. (a) The small droplets indicate characteristics
of the nematic phase; and (b) the schlieren texture of the nematic phase with two-brush and four-brush
defects.

222

223

224
G.-Y. Yeap et al.
Figure 2. Photomicrographs of ligand 1d upon cooling. (a) The battonets coalesce to form the focal
conic fan-shaped texture of SmA phase as observable in (b).
to form the schlieren texture with two-brush and four-brush defects (Fig. 1(b)) [14]. From
Table 1, the mesomorphic range for N phase decreases when the number of carbon atoms
in the flexible alkoxy chain increases from n = 8 to n = 10.
The N phase is not observed upon further lengthening of the alkoxy chain in the
ligand ranging from 1c (n = 12) to ligand 1e (n = 16). Instead, these ligands exhibit only
enantiotropic SmA phase. The presence of SmA phase can be evident by the formation
of battonets that coalesce to form the fan-shaped texture (Fig. 2) [15]. This shows that
with the increase in the length of the flexible terminal chain, the nematogenic properties
decrease leading to the formation of the SmA phase. This phenomenon can be explained
by the hydrophobic interaction between the long flexible terminal chains leading to their
intertwining, which facilitates the lamellar packing and is essential for the smectic phase
[16].
The smectogenic properties of the ligands can also be ascribed to the presence of a
hydroxyl group in the ortho position, which enhances the transverse dipole moment and
intramolecular as well as intermolecular interactions. As such, it helps to stabilize the
molecule and promotes ordered mesophase [9].
Correlation of phase transition temperatures and the carbon number of the flexible
chain in uncoordinated ligands is illustrated in Fig. 3. In this figure, it is shown that the
clearing points of the ligands are almost independent of the length of the flexible alkoxy
chain in which the carbon numbers are even.
All the Cu(II) complexes (2a–2e) show enantiotropic SmA phase with focal conic
fan-shaped texture as shown in Fig. 2. The lower homologues of complexes with n = 8 and
n = 10 exhibit only SmA phase in comparison to the uncoordinated ligands with the same
n exhibiting both N and SmA phases. The suppression of N phase can be caused by the
greater polarity of the coordination bonds in CuL2 that enhances transverse intermolecular
association, promoting the formation of SmA phase [3]. This can also result from the
differences of molecular geometry between the ligand and the Cu(II) complex [3]. Although
the Cu(II) complexes with n = 12, 14, and 16 retain the same mesophases as those appear
in the free ligands, the mesomorphic range shifts to the higher temperature side upon
complexation.

5-Alkoxy-2-((4-(Phenyldiazenyl)Phenylimino)Methyl)phenol
225
Figure 3. A plot of phase transition temperature upon heating versus the number of carbon atoms in
the alkoxy chain for the ligands 1a–1e, L (solid line), and their copper(II) complexes, CuL2 (dotted
line) (Color figure available online).
Figure 4. Representative IR spectra of (i) Cu(II) complex 2e; and (ii) ligand 1e, shifted vertically.

226
G.-Y. Yeap et al.
From Tables 1 and 2, and Fig. 3, a general trend can be noted. The transition tempera-
tures for all the Cu(II) complexes are significantly higher than those of the free ligands and
decrease as the number of carbon atoms in flexible alkoxy chain increases. Besides, the
clearing temperatures of Cu(II) complexes increase by 60◦C–70◦C upon complexation. The
enthalpy change associated with the clearing point for the Cu(II) complex, which is higher
than that for the corresponding free ligand, increases with the increase in the molecular
length.
3.2. Spectroscopic Studies
Upon complexation with Cu(II) acetate, the signals attributed to all the protons in close
proximity with the paramagnetic Cu(II) centers are not observed and their 1H NMR spectra
display only a broad signal indicating the presence of alkoxy group [1]. These results
confirm the coordination of the ligand to the central Cu(II) atom. The representative IR
spectra are shown in Fig. 4. All the IR spectra of metal complexes show the stretching mode
of the C N bond in the range of 1610–1611 cm−1, which is shifted to lower frequency than
the free ligand (1624–1625 cm−1) [1, 3, 5, 9, 11]. This observation can be ascribed to the
reduction of double bond character of the C N bond and this supports the coordination of
the ligand N atom to the central Cu(II) atom. On the other hand, the stretching frequency of
the phenolic OH group in the Cu(II) complexes is absent. This indicates that the OH group
is in the deprotonated state as a result of the coordination of this ligand to the Cu(II) center
through negatively charged O.
4. Conclusions
A new homologous series of bidentate Schiff base 5-alkoxy-((4-(phenyldiazenyl)phenylim
ino)methyl)phenol and their Cu(II) complexes with flexible terminal alkoxy chain in even
parity ranging from n = 8 to n = 16 have been synthesized and characterized. The phase
transitions of all ligands and their Cu(II) complexes are enantiotropic. The ligands with
n-octyloxy and n-decyloxy flexible chains are found to exhibit N and the SmA phase
(the latter of which is suggested by focal conic fan-shaped texture), while all the Cu(II)
complexes exhibit the SmA phase. The transition temperatures and clearing points for all
the Cu(II) complexes are significantly higher than those of the free ligands.
Acknowledgments
The main author (G.-Y. Yeap) would like to thank Universiti Sains Malaysia for the FRGS
Grant No. 203/PKIMIA/6711192 and Incentive Grant No. 1001/PKIMIA/822208.
References
[1] Rezvani, Z., Divband, B., Abbasi, A. R., & Nejati, K. (2006). Polyhedron, 25, 1915–1920.
[2] Lee, C. K., Ling, M. C., & Ivan Lim, J. B. (2003). J. Chem. Soc. Dalton Trans., 4731–4737.
[3] Suste, A., & Sunjic, V. (1996). Liq. Cryst., 20, 219–224.
[4] Trzaska, S. T., & Swager, T. M. (1998). Chem. Mater., 10, 438–443.
[5] Pucci, D., Aiello, I., Bellusci, A., Callipari, G., Crispini, A., & Ghedini, M. (2009). Mol. Cryst.
Liq. Cryst., 500, 144–154.
[6] Tantrawong, S., & Styring, P. (1997). Liq. Cryst., 22, 17–22.
[7] Hayami, S., Kojima, Y., Urakami, D., Ohta, K., & Inoue, K. (2009). Polyhedron, 28, 2053–2057.

5-Alkoxy-2-((4-(Phenyldiazenyl)Phenylimino)Methyl)phenol
227
[8] Liu, C. Y., Chen, J. L., Shiue, C. C., & Liu, K. T. (1999). J. Chromatogr. A., 862, 65–83.
[9] Nandiraju, V. S., Singha, D., Das, M., & Paul, M. K. (2002). Mol. Cryst. Liq. Cryst., 373,
105–117.
[10] Suste, A., & Sunjic, V. (1996). Liq. Cryst., 20, 219–224.
[11] Saad, A., Galstyan, T. V., Denariez-Roberge, M. M., & Dumont, M. (1998). Opt. Commun.,
151, 235–240.
[12] Ha, S. T., Ong, L. K., Wong, P. W., Yeap, G.-Y., Lin, H. C., Ong, S. T., & Koh, T. M. (2009).
Phase Transitions, 5, 387–397.
[13] Ha, S. T., Ong, L. K., Lee, S. L., Lin, H. C., Win, Y. F., & Ong, S. T. (2010). Chin. Chem. Lett.,
21, 637–640.
[14] Ha, S. T., Koh, T. M., Yeap, G.-Y., Lin, H. C., Beh, J. K., Win, F. Y., & Boey, P. L. (2009). Chin.
Chem. Lett., 20, 1081–1084.
[15] Ha, S. T., Ong, L. K., Ong, S. T., Yeap, G.-Y., Wong, P. W., Koh, T. M., & Lin, H. C. (2009).
Chin. Chem. Lett., 20, 767–770.
[16] Gray, G. W. (1962). Molecular Structure and Properties of Liquid Crystals, Academic Press:
London.