Synthesis, molecular structures and phase transition studies on benzothiazole-cored Schiff bases with their Cu(II) and Pd(II) complexes: Crystal structure of (E)-6-methoxy-2-(4-octyloxy-2-hydroxybenzylideneamino) benzothiazole

Article abstract of DOI:10.1016/j.molstruc.2011.12.048

Two new homologous series of Cu(II) and Pd(II) complexes with benzothiazole-cored Schiff bases have been synthesised with the aim to study the mesomorphic and thermal properties of ligands upon formation of metal complexes. The molecular structure of title compounds were elucidated with the employment of FT-IR, 1D and 2D FT-NMR spectroscopic techniques. Mesomorphic and thermal behaviour of title compounds have been investigated by differential scanning calorimetry and polarising optical microscope. All the ligands are nematogenic but the corresponding Cu(II) and Pd(II) complexes crystallised in ordinary solid. The conformation of 6-methoxy-2-(4-octyloxy-2-hydroxy- benzylideneamino)benzothiazole was determined by single crystal X-ray diffraction analysis of which the title compound favours more stable (E)-6-methoxy-2-(4-octyloxy-2-hydroxybenzylideneamino)benzothiazole. Crystal structure of the title compound also revealed that the bond length of CN (1.303 ) in the benzothiazole rings very close to that in the exocyclic CN linkage (1.298 ).

Full text of DOI:10.1016/j.molstruc.2011.12.048

Journal of Molecular Structure 1012 (2012) 1–11
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, molecular structures and phase transition studies on benzothiazole-cored
Schiff bases with their Cu(II) and Pd(II) complexes: Crystal structure
of (E)-6-methoxy-2-(4-octyloxy-2-hydroxybenzylideneamino)benzothiazole
a
a
a
b
Guan-Yeow Yeap a, , Boon-Teck Heng , Nur Faradiana , Raihana Zulkifly , Masato M. Ito ,
⇑
Makoto Tanabe c, Daisuke Takeuchi c
a Liquid Crystal Research Laboratory, School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia
b Department of Environmental Engineering for Symbiosis, Faculty of Engineering, Soka University, Hachioji, Tokyo 192-8577, Japan
c Chemical Resources Laboratory (Mailbox R1-03), Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 July 2011
Received in revised form 28 December 2011
Accepted 28 December 2011
Available online 4 January 2012
Two new homologous series of Cu(II) and Pd(II) complexes with benzothiazole-cored Schiff bases have
been synthesised with the aim to study the mesomorphic and thermal properties of ligands upon forma-
tion of metal complexes. The molecular structure of title compounds were elucidated with the employ-
ment of FT-IR, 1D and 2D FT-NMR spectroscopic techniques. Mesomorphic and thermal behaviour of title
compounds have been investigated by differential scanning calorimetry and polarising optical micro-
scope. All the ligands are nematogenic but the corresponding Cu(II) and Pd(II) complexes crystallised
in ordinary solid. The conformation of 6-methoxy-2-(4-octyloxy-2-hydroxy-benzylideneamino)benzothi-
azole was determined by single crystal X-ray diffraction analysis of which the title compound favours
more stable (E)-6-methoxy-2-(4-octyloxy-2-hydroxybenzylideneamino)benzothiazole. Crystal structure
of the title compound also revealed that the bond length of C@N (1.303 Å) in the benzothiazole rings very
close to that in the exocyclic C@N linkage (1.298 Å).
Keywords:
Cu(II) complexes
Pd(II) complexes
FTIR
NMR
Single crystal X-ray diffraction
Mesomorphic behaviour
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
genic. The observation shows that the incorporation of metal ion
reduced or diminished the anisotropic properties of the ligand. This
A considerable number of metallomesogens and their proper-
ties have extensively been studied in recent years [1]. The presence
of metal ions, particularly the transition metals, was found to be
more superior than the free ligands owing to the advantages of
combining the properties of these ligands with transition metals.
In general, the incorporation of metal ions will help to reinforce
the anisotropic behaviour of these ligands which resulted in the
conversion of the non-mesogenic ligands to complexes which pos-
sesses liquid crystalline properties [2]. Besides, the resulting metal
complexes are claimed to be more polarisable and the complex
molecules may become suitably oriented for the fortification
properties [3].
effect may due to the unfavourable orientation and the polarisabil-
ity of the resulting complexes [4].
The introduction of heterocyclic unit within the central core of
calamitic molecules has commonly been regarded as an interesting
strategy to design liquid crystal compounds [5]. The heterocyclic
unit usually involved five or six member ring with heteroatoms
like S, O and N which are considered more polarisable than carbon
[6]. Thus, the heteroatoms can change liquid crystalline phase and
the physical properties of the observed phase [7]. Recently, hetero-
cyclic liquid crystal compounds have been studied for application
such as thiophenyl-based liquid crystalline compounds for non lin-
ear optic materials and benzothiazolyl-based liquid crystalline
compounds for photoconductive materials [8]. In addition, benzo-
thiazole is a kind of heterocyclic fused ring system which has been
widely studied as they exhibit interesting photophysical and fluo-
rescent properties [6]. It has also been proven as an effective core
in mesogens [7]. In this paper, we focus on the studies of 2D NMR
for benzothiazole-cored Schiff bases ligands and the crystal struc-
ture for 6-methoxy-2-(4-octyloxy-2-hydroxybenzylideneami-
no)benzothiazole. We also describe the influence of Cu(II) and
Ni(II) ion upon the mesomorphic and thermal properties of ligands.
In 2005, Patel and Bhattacharya found that even though the free
tetradentate Schiff base exhibit liquid crystalline properties, the
corresponding binuclear Cu(II) and Ni(II) complexes are non-meso-
⇑
Corresponding author. Fax: +60 4 6574854.
E-mail
addresses:
gyyeap@usm.my,
gyyeap_liqcryst_usm@yahoo.com
(G.-Y. Yeap).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.12.048

2
G.-Y. Yeap et al. / Journal of Molecular Structure 1012 (2012) 1–11
Ethanol
2.2 CnH 2n+1 Br
Acetone
4
11
14
5
1
2
3
H
C
H
C
1'
H
C
8
2n'
n'
2
2
15
CH
3
12
13
9
10
2'
6
7
n'= 5 ; 1a
n'= 7 ; 1b
n'= 9 ; 1c
n'=11 ; 1d
n'=13 ; 1e
Cu2+ (CH2COO- )2.H2O / Pd2+ (CH3COO-)2
Ethanol
M=Pd
M=Cu
n=8 ; 3a
n=10 ; 3b
n=12 ; 3c
n=14 ; 3d
n=16 ; 3e
n=8 ; 2a
n=10 ; 2b
n=12 ; 2c
n=14 ; 2d
n=16 ; 2e
Scheme 1. Synthetic route towards formation of ligands 1a–1e with their complexes 2a–2e and 3a–3e.
The synthetic route to prepare all the intermediate and title com-
pounds are shown in Scheme 1.
2.1. Synthesis of 6-methoxy-2-(2,4-dihydroxybenzylideneamino)
benzothiazole
The synthesis of 6-methoxy-2-(2,4-dihydroxybenzylideneami-
no)benzothiazole is similar to that reported in the literature
published by Ha and co-worker in 2010 [9].
2. Experimental
2,4-Dihydroxybenzaldehyde, copper(II) acetate monohydrate
(Acros), 2-amino-6-methoxybenzothiazole (Aldrich), palladium(II)
acetate (Merck), potassium carbonate (System), 1-bromo-
octane, 1-bromodecane, 1-bromododecane, 1-bromotetradecane,
1-bromohexadecane (Merck) were used without further
purification.
2.2. Synthesis of 6-methoxy-2-(4-alkoxy-2-hydroxybenzylideneamino)
benzothiazole, 1a–1e
0.50 g (1.7 mmol) of 6-methoxy-2-(2,4-dihydroxybenzylide-
neamino)benzothiazole in acetone was treated with two

G.-Y. Yeap et al. / Journal of Molecular Structure 1012 (2012) 1–11
3
equivalent of potassium carbonate. To this mixture, 2.2 equiva-
lent of appropriate 1-bromoalkane were added with the temper-
ature maintained at 60 °C and the reaction mixture was heated
for overnight under continuous stirring. The resulting mixture
was left for evaporation at room temperature. The potassium
carbonate was then dissolved in water and separated from the
crude precipitate via filtration. The product thus obtained was
subsequently purified with n-hexane via recrystallisation.
The yield and elemental analysis data for complexes 1a–1e are
summarised as follow:
3e: Yield 87%. Analysis: calculated for C62H86N4O6S2Pd. C 64.53,
H 7.51, N 4.85%; found C 64.86, H 7.72, N 4.96%.
2.5. FTIR measurement
The FTIR data were recorded using a Perkin Elmer 2000-FTIR
spectrophotometer in the frequency range 4000–400 cmꢀ1 with
sample prepared in KBr discs.
2.6. NMR measurement
The 1H and 13C NMR spectra for ligands 1a–1e and Pd(II) com-
plexes 3a–3e were obtained using Brucker Avance 300 and
400 MHz ultrashield spectrometers equipped with ultrashield mag-
nets. Standard Bruker pulse programs were used throughout the en-
tire experiment [10]. The 1H and 13C NMR data for ligands were
further substantiated with the aids of 2D 1H–1H correlation spec-
troscopy (COSY), 1H–13C heteronuclear multiple quantum correla-
tion (HMQC) and 1H–13C heteronuclear multiple bond correlation
(HMBC). Deuterated chloroform (CDCl3) and dimethysulphoxide
(DMSO-d6) were used as solvent and TMS as internal standard.
1a: Yield 46%. Analysis: calculated for C23H27N2O3S. C 67.12, H
6.61, N 6.81%; found C 67.48, H 6.67, N 6.91%.
1b: Yield 52%. Analysis: calculated for C25H31N2O3S. C 68.30, H
7.11, N 6.37; found C 68.47, H 7.08, N 6.39%.
1c: Yield 50%. Analysis: calculated for C27H35N2O3S. C 69.35, H
7.54, N 5.99%; found C 69.68, H 7.71, N 6.01%.
1d: Yield 53%. Analysis: calculated for C29H39N2O3S. C 70.27, H
7.93, N 5.65%; found C 70.53, H 7.89, N 5.63%.
1e: Yield 50%. Analysis: calculated for C31H43N2O3S. C 71.09, H
8.27, N 5.35%; found C 71.32, H 8.26, N 5.37%.
2.7. Phase transition temperature and enthalpy values
2.3. Synthesis of bis[6-methoxy-2-(4-alkoxy-2-hydroxybenzylidene
amino)benzothiazole]-copper(II) complexes, 2a–2e
The phase transition temperatures and enthalpy values were
measured by using a Seiko DSC6200R calorimeter with the heating
Ligand 1 (1.0 mmol) was dissolved in a round bottom flask con-
taining ethanol solution (50 ml). To this resulting solution, an eth-
anolic solution (10 ml) of copper(II) acetate (1.0 mmol) was then
added dropwise and was refluxed for 6 h at 70 °C. The brown pre-
cipitate formed was collected by filtration and recrystallised from
chloroform–ethanol (1:1).
and cooling rate of 5 °C minꢀ1
.
2.8. Texture observation
The texture was observed using a Carl Zeiss Axioskop 40 polar-
ising microscope equipped with a Linkam LTS 350 hot stage and
TMS94 temperature controller. The samples studied by optical
microscope were prepared in thin film sandwiched between glass
slide and cover.
The yield and elemental analysis data for Cu(II) complexes 2a–
2e are summarised as follow:
2a: Yield 75%. Analysis: calculated for C45H54N4O6S2Cu. C 61.79,
H 6.22, N 6.41%; found C 61.47, H 6.33, N 6.52%.
2b: Yield 79%. Analysis: calculated for C49H62N4O6S2Cu. C 63.23,
H 6.71, N 6.06%; found C 63.10, H 7.12, N 6.20%.
2.9. X-ray data collection, structure solution and refinement for ligand
1a
2c: Yield 70%. Analysis: calculated for C53H70N4O6S2Cu. C 64.50,
H 7.15, N 5.68%; found C 64.18, H 7.29, N 5.78%.
2d: Yield 80%. Analysis: calculated for C57H78N4O6S2Cu. C 65.64,
H 7.54, N 5.37%; found C 65.43, H 7.74, N 5.41%
2e: Yield 83%. Analysis: calculated for C61H86N4O6S2Cu. C 66.66,
H 7.89, N 5.10%; found C 66.50, H 8.08, N 5.18%.
The X-ray crystal structure analytical data were collected by
using a Rigaku Saturn 70 diffractometer, which uses graphite
monochromated MoK
a radiation. The data were collected at a
temperature of ꢀ159 1 °C to a maximum 2h value of 55.0°. A total
of 720 oscillation images were collected. A sweep of data was done
using
sure rate was 128.0 [s/°]. The detector swing angle was ꢀ20.05°.
second sweep was performed using oscillations from
x
oscillations from ꢀ110.0 to 70.0° in 0.5° steps. The expo-
2.4. Synthesis of bis[6-methoxy-2-(4-alkoxy-2-hydroxybenzylidene
amino)benzothiazole]-palladium(II) complexes, 3a–3e
A
x
ꢀ110.0° to 70.0° in 0.5° steps. The exposure rate was 128.0 [s/°].
The detector swing angle was ꢀ20.05°. The crystal-to-detector dis-
tance is 45.32 mm. Readout was performed in the 0.137 mm pixel
mode. Out of the 7876 reflections that were collected, 4442 were
unique (Rint = 0.0372); equivalent reflections were merged. Data
were collected and processed using CrystalClear (Rigaku). The lin-
An ethanolic solution (10 ml) of palladium(II) acetate (1.0 mmol)
was added dropwise to a hot ethanolic solution (50 ml) of ligand 1
(1.0 mmol) in a round bottom flask. The mixture solution was re-
fluxed for 6 h at 70 °C and then cooled to room temperature. The
brown precipitate was collected by filtration and subsequently
recrystallised from ethanol.
ear absorption coefficient,
l, for MoKa .
radiation was 1.827 cmꢀ1
An empirical absorption correction was applied which resulted in
transmission factors ranging from 0.576 to 0.968. The data were
corrected for Lorentz and polarisation.
The yield and elemental analysis data for Pd(II) complexes 3a–
3e are summarised as follow:
The structure was solved by direct methods [11] and expanded
using Fourier techniques. The non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were refined using the riding mod-
el. The final cycle of full-matrix least-squares refinement on F2 was
based on the 4442 observed reflections, 374 variable parameters
and converged (largest parameter shift was 0.00 times its esd) with
unweighted and weighted agreement factors of
3a: Yield 86%. Analysis: calculated for C46H54N4O6S2Pd. C 59.44,
H 5.85, N 6.03%; found C 59.57, H 5.88, N 6.09%.
3b: Yield 85%. Analysis: calculated for C50H62N4O6S2Pd. C 60.93,
H 6.34, N 5.68%; found C 61.13, H 6.45, N 5.66%.
3c: Yield 85%. Analysis: calculated for C54H70N4O6S2Pd. C 62.26,
H 6.77, N 5.38%; found C 62.49, H 6.88, N 5.47%.
3d: Yield 87%. Analysis: calculated for C58H78N4O6S2Pd. C 63.45,
H 7.16, N 5.10%; found C 63.70, H 7.17, N 5.08%.
X
X
R1 ¼
jjFoj ꢀ jFcjj=
jFoj ¼ 0:0731

4
G.-Y. Yeap et al. / Journal of Molecular Structure 1012 (2012) 1–11
ꢅ
ꢂ
ꢃꢄ
ꢆ
1=2
ꢀ
ꢁ
ꢀ
ꢁ
X
X
2
2
tion. One of the reason for this shifting is the coordination of non-
wR2 ¼
w F2o ꢀ F2c
w F2o
¼ 0:2184
bonding electron from N atom to the Cu(II) ion which led to the
reduction of the double bond character of C@N [4,17,18,22,1,23].
On the other hand, the absence of the OH band of the free ligands
and the shifting of phenolic CAO to a higher frequency at
1313 cmꢀ1 in the complexes indicate the deprotonation of OH group
and the coordination of Oꢀ to the Cu(II) ion [18,19,22,1,23–25]. The
other major bands as observed at 2922–2852 cmꢀ1 can be assigned
to the stretching frequency of CAH bond from alkoxy chain.
The standard deviation of an observation of unit weight was
1.08. Unit weights were used. The maximum and minimum peaks
on the final difference Fourier map corresponded to 0.98 and
ꢀ0.77 e/Å3, respectively. Neutral atom scattering factors were ta-
ken from Cromer and Waber [12]. Anomalous dispersion effects
were included in Fcalc [13]; the values for
D D
f0 and f00 were those
of Creagh and McAuley [14]. The values for the mass attenuation
coefficients were those of Creagh and Hubbell [15]. All calculations
were performed using the Crystal Structure crystallographic soft-
ware package except for the refinement, which were performed
using SHELXL-97 [16].
3.2. NMR spectroscopy
3.2.1. 1H and 13C NMR assignments for ligands 1a–1e
Based on a representative ligand 1c, a complete assignment for
the title compounds can be described. The 1H NMR spectrum
(Table 1) of ligand 1c is observed in the range of d = 0.88–
12.65 ppm. It can be observed that the signal at d = 12.65 ppm is
attributed to hydroxyl proton, H7. The downfield shift of the hy-
droxyl proton indicates the presence of intramolecular hydrogen
bonding [26] and this observation is in agreement with the crystal
structure of 6-methoxy-2-(4-octyloxy-2-hydroxybenzylideneami-
no)benzothiazole in Section 3.4. The chemical shift for aromatic
proton H6 is observed at d = 7.28–7.29 ppm. As for the azomethine
proton H9, it resonates as a singlet at d = 9.07 ppm. Whereas the
proton at d = 3.89 ppm can be assigned for the proton from methyl
group, H8.
3. Results and discussion
3.1. IR spectroscopy
The FT-IR spectrum of ligand 1c shows a broad absorption
assignable to the stretching of OH at 3113–3716 cmꢀ1 while those
of the alkyl groups are observed at 2850–2953 cmꢀ1. A strong
absorption band that is observed at 1604 cmꢀ1 can be ascribed to
the stretching of imine C@N group in the benzothiazole ring. This
band is overlapped with the stretching vibration for C@C [9,17].
Besides, the band at 1592 cmꢀ1 can also be assigned to the stretch-
ing vibration of C@C group in aromatic ring. Another imine C@N
group which is obtained from the condensation reaction between
2,4-dihydroxybenzaldehyde and 2-amino-6-methoxybenzothiaz-
ole gives rise to the stretching vibration at 1638 cmꢀ1 [17,18].
The medium band at 1273 cmꢀ1 is attributed to the stretching
vibration of CAO phenolic group [19] whereas the band assignable
to the stretching vibration of CAO ether is observed as a strong
intensity band at 1226 cmꢀ1 [20]. The presence of stretching vibra-
tion of CAO ether has further confirmed the success of the Wil-
liamson etherification reaction with 1-bromododecane [21]. The
FT-IR spectra for 1a, 1b, 1d and 1e show the similar bands as ob-
served in 1c.
All Cu(II) complexes 2a–2e and Pd(II) complexes 3a–3e show
similar pattern of FT-IR spectra. Hence, the discussion based on a
representative Cu(II) complex 2e will be reported herein. The spec-
trum for complex 2e (Fig. 1) shows the overlapping of the absorp-
tion band for the imine (C@N) group with the absorption band
arising from the C@N stretching in the benzothiazole ring results
in a sharp and strong absorption band at 1609 cmꢀ1. This observa-
tion shows that the stretching frequency of the C@N is shifted to a
lower wavenumber in comparison to the free ligand after coordina-
The correlation of 1H–1H from COSY experiment of ligand 1c is
employed to assign the aromatic protons. From the COSY spectrum
in Fig. 2, the doublet observed at d = 7.81–7.84 ppm suggests the
existence of proton H4 and this proton is correlated with proton
H5 which is observed as doublet of doublet at d = 7.05–7.08 ppm.
Another doublet attributed to proton H3 is observed at d =
7.35–7.37 ppm and is correlated with multipletes observed at
d = 6.50–6.54 ppm for proton H1. Besides, the multipletes that
are observed in this range can also be attributed to proton H2
which is due to the overlap of peaks from these two protons.
Fig. 1 also shows that the ether group of proton H10 is observed
as triplet at d = 4.00–4.03 ppm. This proton shows a correlation
with the adjacent proton H20 which is observed as multipletes. A
similar correlation is observed for the other homologous members.
All ligands 1a–1e show a similar pattern in 13C NMR spectra.
Hence, a representative ligand 1c will be discussed. The 13C NMR
signals have further been substantiated by DEPT 135, 90 and 45
where the assignment can be governed by the additive rules and
substitution effect. Based on the 13C NMR spectroscopy as shown
in Table 2 for 1c, the signal of carbon attached to N atom in the azo-
methine is observed at d = 164.77 ppm. The signals at d = 165.67 and
167.46 ppm can be attributed to C12 and C13, respectively, whereas
the resonances for carbons C7 and C11 appear at d = 165.39 and
157.89 ppm. The signal observed at d = 146.27 ppm is assigned to
carbon C14. The resonances due to the aromatic carbons C1, C2
and C3 are found at d = 109.16, 101.82 and 135.56 ppm, respec-
tively. Other resonances due to aromatic carbons C4, C5 and C6
are observed at d = 123.64, 116.05 and 104.76 ppm, respectively.
The signals appear at d = 135.82 and 112.66 ppm can be assigned
to respective C10 and C15. For the carbon C8 in the methyl group,
(i)
1609
%T
it gives rise to
a peak at d = 56.03 ppm. At d = 14.52 and
(ii)
3439
32.32 ppm, the signals can be assigned as carbon C (methyl) and
C20, respectively. The d = 68.85 ppm indicates methyl carbon C10 of
the alkanoyloxy chain.
The HMQC spectra confirm the attachment of different aromatic
hydrogens to the respective carbons as discussed in the earlier part
on the 1H–1H homonuclear correlation. From Fig. 3, the aromatic
carbon atoms for C1, C2, C3, C4, C5 and C6 show a cross peak with
proton H1, H2, H3, H4, H5 and H6, respectively. In the same way,
the azomethine carbon C9 shows a correlation with proton H9.
1638
4000.0
3000
2000
1500
1000
400.0
cm-1
Fig. 1. Representative IR spectrum of (i) Cu(II) complex 2e and (ii) ligand 1e.

G.-Y. Yeap et al. / Journal of Molecular Structure 1012 (2012) 1–11
5
Table 1
1H NMR chemical shifts, d (ppm) of ligands 1a–1e.
Atom
Chemical shift, d (ppm)
1a
1b
1c
1d
1e
H1 and H2
H3
H4
H5
H6
H7
6.52–6.55 (m)
7.34–7.36 (d)
7.81–7.82 (d)
7.06–7.07 (dd)
7.27–7.28 (s)
12.65 (s)
6.51–6.55 (m)
7.35–7.38 (d)
7.81–7.84 (d)
7.06–7.09 (dd)
7.28–7.29 (s)
12.65 (s)
6.50–6.54 (m)
7.35–7.37 (d)
7.81–7.84 (d)
7.05–7.08 (dd)
7.28–7.29 (s)
12.65 (s)
6.50–6.55 (m)
7.36–7.38 (d)
7.81–7.83 (d)
7.06–7.09 (dd)
7.28–7.29 (s)
12.65 (s)
6.52–6.54 (m)
7.35–7.38 (d)
7.81–7.83 (d)
7.07–7.08 (dd)
7.26–7.29 (s)
12.64 (s)
H8
3.91 (s)
3.90 (s)
3.89 (s)
3.90 (s)
3.88 (s)
H9
9.08 (s)
9.07 (s)
9.07 (s)
9.09 (s)
9.06 (s)
H10
4.02–4.05 (t)
1.77–1.83 (m)
0.88–0.91 (t)
4.00–4.03 (t)
1.78–1.82 (m)
0.87–0.90 (t)
4.00–4.03 (t)
1.78–1.82 (m)
0.88–0.89 (t)
4.02–4.05 (t)
1.79–1.85 (m)
0.89–0.93 (t)
3.99–4.03 (t)
1.70–1.80 (m)
0.88–0.90 (t)
H20AHn0
H (methyl)
TMS as internal standard.
s, Singlet; d, doublet; dd, doublet of doublet; m, multiplets; t, triplet.
Fig. 2. 1H–1H connectivities in the COSY spectra for ligand 1c.
Moreover, 13
C
1H NMR spectra also indicate the correlation of car-
C15 with 2J. In addition, the cross peaks of proton H1 and H2 with
C3, C7, C12 and C15 further supports the 2J and 3J long range
connectivities.
bon C8, C10, C20 and C120 with proton H8, H10, H20 and H120.
The HMBC experiment has further confirmed the assignment of
quaternary carbons which is established throughout the connectiv-
ities between the carbon and its neighbouring proton. Fig. 4 show
that the proton H5 is correlated with carbon C10 and C14 with
respective 2J and 4J long range connectivities. Similarly, the HMBC
spectra also indicate that the H4 has 2J connectivities with quater-
nary carbon C11 and 3J long range connectivities with quaternary
carbon C10. However, the same spectra show that the proton H6
is correlated with carbon C14 and C10 with 2J connectivities. Be-
sides, H6 also shows a cross peak with carbon C5 with 3J long range
connectivities. A similar observation can also be observed for azo-
methine proton H9 which shows a correlation between the quater-
nary carbon C15 with carbons C3 and C7 by 2J and 3J long range
connectivities. The proton H3 shows a cross peak with C1 and
3.2.2. 1H and 13C NMR assignment for Pd(II) complexes 3a–3e
The resonances in relation to the 1H and 13C NMR of Pd(II) com-
plexes 3a–3e show a significantly shift to higher or lower field as
compared to the uncoordinated ligands 1a–1e. A representative
Pd(II) complex of 3a will be discussed.
Table 3 shows that the proton H1 is observed as doublet of dou-
blet at d = 6.19–6.21 ppm while the resonance for proton H2 is ob-
tained as singlet at d = 5.55 ppm. The chemical shifts for these two
protons are shifted to upper field as compared to uncoordinated li-
gands. In addition, the overlapping of signals for these two protons
in the free ligands is not observed in complex 3a. Table 3 also
shows that the singlet at d = 8.06 ppm can be attributed to the

6
G.-Y. Yeap et al. / Journal of Molecular Structure 1012 (2012) 1–11
Table 2
13C NMR chemical shifts, d (ppm) of compound ligands 1a–1e.
Atom
Chemical shift, d (ppm)
1a
1b
1c
1d
1e
C1
C2
C3
C4
C5
C6
C7
C8
109.15
101.84
135.52
123.67
116.04
104.74
165.38
56.03
109.16
101.80
135.57
123.62
115.03
104.72
165.39
56.02
109.16
101.82
135.56
123.64
116.05
104.76
165.39
56.03
109.15
101.83
134.57
122.63
116.02
104.74
165.40
56.02
109.15
101.82
135.55
123.64
116.02
104.74
165.41
56.04
C9
164.77
135.83
157.88
165.66
167.45
146.28
112.66
68.82
164.77
135.83
157.87
165.67
167.45
146.28
112.67
68.84
164.77
135.82
157.89
165.67
167.46
146.27
112.66
68.85
164.78
136.81
157.88
165.66
167.45
146.27
112.67
68.85
164.77
135.83
157.88
166.65
167.46
146.27
112.66
68.85
C10
C11
C12
C13
C14
C15
C10
C20ACn0
C (methyl)
32.30–23.09
14.48
32.31–23.06
14.51
32.32–23.09
14.52
32.30–23.09
14.50
32.32–23.10
14.49
Fig. 3. One bond CAH correlation in the HMQC spectra of ligand 1c.
proton H9. As compared with the uncoordinated Schiff base 1a at
d = 9.08 ppm, the chemical shift for this proton is shifted to upper
field. Another doublet of doublet signal at d = 7.10–7.13 ppm indi-
cates the proton H5 while the two doublet signals at d = 7.31–
7.32 ppm and d = 7.90–7.92 ppm can be assigned to H3 and H4,
respectively. The proton H3 and H4 are shifted to upper field
whereas the proton H5 is shifted to the lower side as compared
to the free ligand 1a.
of new PdAN and PdAO bonds in Pd(II) complexes [28,29]. These
results are in agreement with the molecular structure deduced
from FT-IR and CHN data.
The chemical shifts of 13C NMR for Pd(II) complex 3c are collated
in Table 4. The resonance at d = 167.24 ppm can be ascribed to the
carbon C7, in which carbon C7 is the carbon bonded to O and coor-
dinated to Pd(II) ion. The resonance for carbon C1 appears at
d = 109.04 ppm and resonances for C2, C3, C4, C5 and C6 are ob-
served at d = 102.60, 136.78, 123.84, 115.51 and 104.73 ppm,
respectively. These are the resonances due to the aromatic CAH car-
bons. The resonance observed at d = 56.23 ppm can be attributed to
the carbon C8. The resonance obtained at d = 167.11 ppm is identi-
fied as the carbon C9 whereas the resonance for carbon C10 from
The large upfield shifting of the protons (H1, H2, H3 and H9)
close to the Pd(II) centre ion are due to the increase of shielding ef-
fect that is presumably caused by the
p back-bonding from Pd(II)
ion [27]. Besides, the absence of phenolic proton H7 and the larger
upper shift in chemical shift for H9 further confirm the formation

G.-Y. Yeap et al. / Journal of Molecular Structure 1012 (2012) 1–11
7
Fig. 4. Long range CAH correlation in the HMBC spectra for ligand 1c.
Table 3
1H NMR chemical shifts, d (ppm) of Pd(II) complexes, 3a–3e.
Atom
Chemical shift, d (ppm)
3a
3b
3c
3d
3e
H1
H2
6.19–6.21 (dd)
5.55 (s)
6.18–6.20 (dd)
5.54 (s)
6.19–6.21 (dd)
5.55 (s)
6.18–6.21 (dd)
5.53 (s)
6.19–6.22 (dd)
5.55 (s)
H3
H4
H5
H6
7.31–7.32 (d)
7.90–7.92 (d)
7.10–7.13 (dd)
7.15 (s)
7.31–7.32 (d)
7.90–7.91 (d)
7.10–7.13 (dd)
7.15 (s)
7.30–7.32 (d)
7.91–7.93 (d)
7.11–7.14 (dd)
7.14 (s)
7.31–7.33 (d)
7.90–7.93 (d)
7.10–7.14 (dd)
7.13 (s)
7.30–7.32 (d)
7.90–7.93 (d)
7.10–7.13 (dd)
7.14 (s)
H7
–
–
–
–
–
H8
3.91 (s)
3.91 (s)
3.92 (s)
3.92 (s)
3.91 (s)
H9
8.06 (s)
8.05 (s)
8.06 (s)
8.05 (s)
8.07 (s)
H10
3.62–3.65 (t)
1.22–1.69 (m)
0.90–0.93 (t)
3.61–3.65 (t)
1.21–1.68 (m)
0.91–0.93 (t)
3.60–3.64 (t)
1.22–1.69 (m)
0.90–0.93 (t)
3.61–3.64 (t)
1.21–1.67 (m)
0.91–0.93 (t)
3.62–3.65 (t)
1.22–1.69 (m)
0.91–0.93 (t)
H20AHn0
H (methyl)
TMS as internal standard.
s, Singlet; d, doublet; dd, doublet of doublet; m, multiplets; t, triplet.
ether group is observed at d = 68.35 ppm. The resonance at
d = 14.18 ppm can be assigned to the methyl carbon C120. On the
other hand, the resonances for quaternary aromatic carbons C10,
C11, C12, C13, C14 and C15 are observed at d = 137.16 ppm,
158.14 ppm, 167.57 ppm, 169.38 ppm, 146.67 ppm and 115.28
ppm, respectively. The other homologous members of Pd(II) com-
plexes show the similar pattern of 13C NMR as described for com-
plex 3c.
(kJ molꢀ1) of the ligands (1a–1e) with their corresponding Cu(II)
and Pd(II) complexes are shown in respective Tables 5–7.
All the ligands are nematogenic. The thermal behaviours of the
ligands 1a–1e are similar as we can infer from the DSC curve.
Therefore, the ligand 1b will be discussed as a representative
example. The heating cycle of ligand 1b exhibits two endothermic
peaks at 81.5 °C and 127.7 °C, respectively. The first transition cor-
responds to the transition of crystal to nematic phase (CrAN) with
the associated enthalpies 29.96 kJ molꢀ1 followed by the transition
to isotropic phase which gives the second endothermic peak. The
cooling cycle of ligand 1b exhibits two exothermic transitions at
125.9 °C and 58.0 °C corresponding to the transitions of IAN and
NACr.
3.3. Mesomorphic behaviour
The observation from polarising microscope and DSC traces re-
vealed that all the uncoordinated ligands are enantiotropic whereas
all the corresponding Cu(II) and Pd(II) complexes are non meso-
genic. The transitions temperatures and associated enthalpies
Texture observation under polarising microscope reveals that
the ligands 1a–1e exhibit schlieren textures of N phase with

8
G.-Y. Yeap et al. / Journal of Molecular Structure 1012 (2012) 1–11
Table 4
13C NMR chemical shift (ppm) of Pd(II) complexes 3a–3e.
Atom
Chemical shift, d (ppm)
3a
3b
3c
3d
3e
C1
C2
C3
C4
C5
C6
C7
C8
109.02
102.58
136.86
123.83
115.41
104.69
167.24
56.18
109.00
102.56
136.88
123.85
115.44
104.70
167.26
56.20
109.04
102.60
136.78
123.84
115.51
104.73
167.32
56.23
109.02
102.60
136.79
123.83
115.52
103.70
167.25
56.28
108.95
102.58
136.79
123.79
115.42
104.74
167.24
56.13
C9
162.14
137.10
158.14
167.55
169.33
146.68
115.29
68.37
167.10
137.13
158.13
167.56
169.35
146.68
115.30
68.34
167.11
137.16
158.14
167.57
169.38
146.67
115.28
68.35
167.17
137.14
158.13
167.53
169.33
146.68
115.30
68.35
167.15
137.13
158.14
167.55
169.35
146.67
115.29
69.15
C10
C11
C12
C13
C14
C15
C10
C20ACn0
C(methyl)
22.88–32.07
14.19
22.88–32.06
14.20
22.86–32.03
14.18
22.88–32.09
14.20
22.87–32.05
14.19
Table 5
Phase transition temperatures (°C) and associated enthalpies (kJ molꢀ1) of ligands 1a
(n = 8) to 1e (n = 16).
Ligands
Cr1
Cr2
N
I
1a
1b
1c
1d
1e
Heating
Cooling
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
88.5 (42.67)
73.7 (ꢀ36.9)
81.5 (29.96)
58.0 (ꢀ19.3)
81.5 (14.92)
68.3 (ꢀ31.6)
72.4 (12.43)
69.8a
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
145.0a
142.9a
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
Heating
Cooling
127.3a
125.9a
122.7a
121.8a
120.9a
118.7a
104.0a
103.2a
Heating
Cooling
ꢁ
ꢁ
ꢁ
ꢁ
84.7 (30.5)
96.1a
Heating
Cooling
87.2 (22.15)
72.4a (ꢀ34.1)
Heating
Cooling
ꢁ
ꢁ
93.2 (34.83)
79.8 (ꢀ34.0)
ꢁ
ꢁ
ꢁ
ꢁ
Cr, crystal; N, nematic; I, isotropic.
a
Denotes transition temperature derived from unresolved peaks.
Table 6
Phase transition temperatures (°C) and associated enthalpies (kJ molꢀ1) of complexes
2a–2e.
Complexes
Cr1
I
2a
2b
2c
2d
2e
Heating
Heating
Heating
Heating
Heating
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
189.2 (49.3)
181.7 (33,5)
172.5 (18.9)
163.4 (35.9)
154.4 (49.5)
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
Cr, crystal; I, isotropic.
Table 7
Phase transition temperature (°C) and associated enthalpies (kJ molꢀ1) of complexes
3a (n = 8) to 3e (n = 16).
Complexes
Cr
I
3a
3b
3c
3d
3e
Heating
Heating
Heating
Heating
Heating
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
220.36 (26.6)
215.62 (25.1)
199.51 (31.7)
189.35 (25.8)
180.5a
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
Fig. 5. Photomicrographs of ligand 1c upon cooling. (a) The small droplets
characteristics of the nematic phase; and (b) the schlieren texture of nematic
phase with two-brush and four-brush defects. Cr, crystal; N, nematic; I, isotropic; L,
ligands; CuL2; Cu(II) complexes; PdL2; Pd(II) complexes.
Cr, crystal; I, isotropic.
a
Denotes transition temperature derived from unresolved peaks.
two-brush and four-brush defects [9]. Upon cooling from isotropic
liquid, the nematic droplets appear and coalesces (Fig. 5a) to form
the classical schlieren texture with two-brush and four-brush
defects (Fig. 5b) [9]. This nematic phase is reproducible upon con-
tinuous heating and cooling.

G.-Y. Yeap et al. / Journal of Molecular Structure 1012 (2012) 1–11
9
Fig. 6 illustrates the correlation of phase transition tempera-
tures and the carbon number of the flexible chain in uncoordinated
ligands. The clearing points of the ligands are considerably reduced
upon increasing the number of carbon in the alkoxy chain from
n = 8 to n = 16. This observation is resulted from the dilution of
core system induced by the increasing of the flexibility of the ter-
minal alkoxy chain [30,31]. On the other hand, this figure clearly
shows that the nematic phase range begins to drop upon lengthen-
ing of the alkoxy chain from n = 8 to n = 16 [31].
of N phase which are different from schlieren textures observed
in 1a–1e. Comparing the transition temperatures and mesophase
range of compounds 8MHBABTH to 16MHBABTH and 1a–1e re-
veals that compounds 8MHBABTH to 16MHBABTH possess higher
clearing temperatures. However, 1a–1e have relatively larger N
phase range as compared to compounds 8MHBABTH to
16MHBABTH. All these observation are due to the greater polarity
of ester linkages compared to ether linkage.
However, all the Cu(II) complexes 2a–2e and Pd(II) complexes
3a–3e do not exhibit liquid crystalline properties even though their
ligands are nematogenic. Optical observation shows that these
complexes are clearly melted and transformed into isotropic phase.
Besides, continuous heating after the melting point causes the
immediate decomposition which appears to be extinct black under
the polarising light. On the other hand, the first heating cycle of
Compound
Transition
temperatures (°C)
Mesophase
range (°C)
8 MHBABTH
10 MHBABTH
12 MHBABTH
14 MHBABTH
16 MHBABTH
Cr 133.2 N 145.8 I
Cr 127.5 N 139.5 I
Cr 124.9 N 136.8 I
Cr 123.6 N 133.6 I
Cr 123.2 N 129.7 I
12.6
12.0
11.9
10.0
6.5
O
C
N
3.4. Crystal structure of 6-methoxy-2-(4-octyloxy-2-hydroxybenzyl
ideneamino)-benzothiazole, 1a
H
C
H3C
Cn-1H2n-1
N
O
O
S
HO
The molecular structure with the atom-numbering scheme of
6-methoxy-2-(4-octyloxy-2-hydroxybenzylideneamino)benzothi-
azole, 1a is shown in Fig. 7. Crystal data and refinement of ligand
1a are collated in Table 8.
nMHBABTH
where n= 8, 10, 12, 14, 16
The molecule of 6-methoxy-2-(4-octyloxy-2-hydroxybenzy-
lideneamino)-benzothiazole, 1a crystallines into triclinic lattice
with the space group of P-1. Fig. 7 shows that the discrete molecule
of ligand 5a consists of a benzothiazole moieties linked by an imine
linkage to the ortho-hydroxyl phenyl ring bearing an alkoxy chain
of OC8H17. The bond lengths for exocyclic imine N2AC9 and imine
group N1AC8 in the benzothiazole ring are 1.298 Å and 1.303 Å,
respectively. There is no remarkable difference between these
two C@N bond distances which resemble the value reported for
2-(2,4-dimethylpyrolly)benzothiazole by Rajnikat and co-worker
in which the C@N bond length was 1.284 Å (C@N) [33].
Fig. 7 also shows that the two bulky groups situated at the
opposite side of the imine C@N and this led to formation of E iso-
mer. The E conformation is preferred because it helps to reduce the
crowdiness thus forming a more stable (E)-6-methoxy-2-(4-octyl-
oxy-2-hydroxybenzylideneamino)benzothiazole. The bond lengths
of two CAS bonds in the five member thiazole ring are 1.734 Å
(S1AC3) and 1.745 Å (S1AC8), respectively. On the other hand,
the C4AN1AC8 angle in the benzothiazole ring is 109.3 °C. These
values are similar to those values reported for 2-(2,4-dimethylpy-
rolly)benzothiazole [33].
DSC curve for this two series of complexes exhibit only one endo-
thermic transition whereas during the cooling cycle there is no
exothermic transition observed due to the thermal decomposition
occurs during the first heating cycle.
The greater polarity of the coordination bond and the differ-
ences in the molecular geometry between ligands and metal com-
plexes could probably be the reason for the suppression of N phase
in the Cu(II) and Pd(II) complexes [23]. Besides, another reason is
the structural orientation and hydrophobicity of the Cu(II) and
Pd(II) complexes which do not favour the mesogenic character
[4]. On the other hand, Fig. 6 shows that the clearing temperatures
of Cu(II) and Pd(II) complexes are significantly higher in compari-
son with the free ligands 5a–5e. This could be resulted from the in-
crease in their molecular weights and the number of interacting
sites upon complexes formation [32]. However, Pd(II) complexes
3a–3e have relatively higher clearing temperatures as compared
to those values in Cu(II) complexes 2a–2e (Fig. 6). This is not sur-
prising as the molecular weight of Pd(II) ion is higher than Cu(II)
analogue and is also due to the possibility of the presence of weak
PdAPd intermolecular interaction [32].
3.3.1. Chemical structure–mesomorphic property relationship
The homologous series of benzothiazole-cored Schiff bases 1a–
1e are compared to the structurally related compounds reported
by Ha and co-worker in 2010 with general structure as shown below
[9]. Only compounds 8MHBABTH to 16MHBABTH are chosen to
compare with 1a–1e to ensure that the comparison is relevant.
The comparison will be discussed in term of transition temperature,
type of mesophase, mesophase range and molecular structures.
Both series have almost similar molecular structure but with a
different functional group at the terminal chain. Compounds
8MHBABTH to 16MHBABTH contain an ester linkage at the termi-
nal flexible chain whereas present ligands 1a–1e contain an ether
linkage. It is found that although both series exhibit N phase but
compounds 8MHBABTH to 16MHBABTH show a marble texture
Fig. 6. A plot of phase transition temperature upon heating versus the number of
carbon atoms in alkoxy chain for the ligands 1a–1e (solid line) and their
corresponding metal complexes (dotted line). Cr, crystal; N, nematic; I, isotropic;
L, ligands; CuL2; Cu(II) complexes; PdL2; Pd(II) complexes.

10
G.-Y. Yeap et al. / Journal of Molecular Structure 1012 (2012) 1–11
Fig. 7. The molecular structure with the atom-numbering scheme of 6-methoxyl-2-(4-octyloxy-2-hydroxybenzylideneamino)benzothiazole, 1a.
Table 8
Crystal data and refinement of ligand 1a.
Crystal data
Values
C23H28N2O3S
Empirical formula
Formula weight
Crystal colour, habit
Crystal dimensions
Crystal system
412.55
Yellow, prism
0.270 ꢃ 0.220 ꢃ 0.180 mm
Triclinic
Lattice type
Primitive
Lattice parameters
a = 5.82880(10) Å
b = 13.000(3) Å
c = 15.085(6) Å
a = 73.97(3)°
b = 79.67(4)°
g = 72.12(4)°
V = 1039.9(5) Å3
P-1 (#2)
Space group
Z value
Dcalc
2
1.317 g/cm3
440
F000
m(MoK
a
)
1.827 cmꢀ1
Refinement
Function minimised
Full-matrix least-squares on F2
P
2
wðF2o ꢀ F2c Þ
2
w ¼ 1=½r2ðFo2Þ þ ð0:1117 ꢂ PÞ þ 0:0000 ꢂ Pꢄ where P ¼ ðMaxðFo2; 0Þ þ 2F2c Þ=3
Least squares weights
2qmax cutoff
55.0°
Anomalous dispersion
No. observations (All reflections)
No. Variables
All non-hydrogen atoms
4442
374
Reflection/parameter ratio
11.88
Residuals: R1 (I > 2.00
r
(I))
0.0731
0.1
0.2184
1.076
Residuals: R (all reflections)
Residuals: wR2 (all reflections)
Goodness of fit indicator
Max shift/error in final cycle
Maximum peak in final diff. map
Minimum peak in final diff. map
0.001
0.98 e/Å3
ꢀ0.77 e/Å3
In addition, the existence of intramolecular hydrogen bond
O2AH2ꢂ ꢂ ꢂN2 in which O2 acts as donor and N2 acts as acceptor
is further confirmed by the crystal structure of 6-methoxyl-2-(4-
octyloxy-2-hydroxybenzylidene-amino)benzothiazole, 1a.
COSY, 1H–13C HMQC and HMBC correlation for ligand 1c. The
intensity measurement of 1a are listed in Table 10. Tables 11, 12
and 13 show the atomic coordinates, Biso/Beq, Biso of ligand 1a
which involved hydrogen atoms and anisotropic displacement
parameters. Whilst the selected bond lengths and bond angles
are given in Tables 14 and 16, bond lengths and bond angles
involving hydrogen are listed in Tables 15 and 17. Supplementary
data associated with this article can be found, in the online version,
at doi:10.1016/j.molstruc.2011.12.048.
Acknowledgements
The main author (G.-Y. Yeap) would like to thank Universiti
Sains Malaysia for the RU Grant No. 1001/PKIMIA/811159 and
Incentive Grant No. 1001/PKIMIA/822208.
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