Synthesis, optical and thermal behaviour of palladium(II) complexes with 4-(4-alkoxy-2-hydroxybenzylideneamino)azobenzene

Article abstract of DOI:10.1007/s12039-013-0531-6

A series of new Pd(II) complexes derived from the reaction of palladium acetate with 4-(4-alkoxy- 2-hydroxybenzylideneamino)azobenzene having the flexible terminal chain of OCnH2n+1, in which n are even numbers ranging from 8 to 16, has been successfully

Full text of DOI:10.1007/s12039-013-0531-6

c
J. Chem. Sci. Vol. 125, No. 6, November 2013, pp. 1435–1443. ꢀ Indian Academy of Sciences.
Synthesis, optical and thermal behaviour of palladium(II) complexes
with 4-(4-alkoxy-2-hydroxybenzylideneamino)azobenzene
BOON-TECK HENGa, GUAN-YEOW YEAPa,∗ and DAISUKE TAKEUCHIb
aLiquid Crystal Research Laboratory, School of Chemical Sciences, Universiti Sains Malaysia,
11800 Minden, Penang, Malaysia
bChemical Resources Laboratory (Mailbox R1-03), Tokyo Institute of Technology, 4259 Nagatsuta,
Midori-ku, Yokohama, 226-8503, Japan
e-mail: gyyeap@usm.my
MS received 6 April 2013; revised 18 August 2013; accepted 21 August 2013
Abstract. A series of new Pd(II) complexes derived from the reaction of palladium acetate with 4-(4-alkoxy-
2-hydroxybenzylideneamino)azobenzene having the flexible terminal chain of OCnH2n+1, in which n are even
numbers ranging from 8 to 16, has been successfully synthesized. The physical measurement and spectroscopic
1
techniques (FTIR and H-NMR) reveal that the Pd(II) complexes possess the Pd–N and Pd–O coordination
modes in which the central Pd(II) adopts square-planar geometry. The observation under the polarized light
shows that all the ligands and Pd(II) complexes exhibit enantiotropic mesophases. The ligands with n-octyloxy
and n-decyloxy flexible chains exhibit the nematic (N) and smectic A (SmA) phases whilst the Pd(II) complexes
show exclusive SmA phase. The SmA phase observed in Pd(II) complexes can be supported by the presence
of focal conic fan-shaped texture with the presence of curved lines which are prominent during the cooling
process. On the other hand, the comparison studies show that Pd(II) complexes possess exceptional higher
phase transition temperatures as compared to the corresponding Cu(II) and Ni(II) complexes.
Keywords. Pd(II) complexes; smectic A; nematic; 1H-NMR; Cu(II) complexes.
1. Introduction
general, the metallomesogens containing the uncom-
plexed azo moieties are capable of changing the mole-
cular shape by the reversible cis–trans isomerization
under the influence of UV or photoirradiation. These
materials are useful particularly in the development
of information recording system such as high-density
optical data storage, optical switching, display and non-
linear optics.6,7 Another interesting feature of these
compounds lies on its potential to form rod-like mole-
cules leading to the liquid crystalline properties.8
Rezvani and co-workers had earlier reported the
synthesis and liquid crystalline properties of some
metallomesogens containing isomerizable azo moieties
wherein the metal ion such as Cu(II), Ni(II) and VO(IV)
are widely used in formulating the novel metallome-
sogenic system.9,10 In 2002, Nandiraju and co-workers
reported the influence of metal ion on the mesomorphic
properties of the ligands.11 Similar studies had been
carried out by Suste and co-workers.5 In spite of these
studies, the influence of the metal on the mesophase
and correlation of the structure with the anisotropic pro-
perties of metallomesogens are still subject to further
investigation.
Metallomesogens are commonly referred to the coor-
dination compounds which include the complexes con-
taining mesogenic or non-mesogenic ligands, salt of
organic acids or certain organometallic compounds pos-
sessing s, p, d and f block metals.1 The last ten years
have seen a steady increase in the interest towards these
metallomesogens and the recognition of their poten-
tial as advanced materials.2 One of the reasons making
these metallomesogen so essential in the technological
advancement is the ultimate shapes associated with va-
rious geometries of the central metal atom or ion rang-
ing from square planar, folder square, lantern, pyra-
midal and octahedral geometrical shapes.3,4 From the
chemical and physical viewpoints, the central metal
ions can enhance the richness in oxidation state, colour
and magnetism.5
Among these materials, the azo-based polymeric liq-
uid crystals or metallomesogens containing uncom-
plexed azo moieties has been given more attention. In
Recently, we have reported the synthesis and liquid
crystalline character of Cu(II) and Ni(II) complexes
∗For correspondence
1435

1436
Boon-Teck Heng et al.
with ligands derived from azobenzene-cored Schiff homologous compounds (scheme 1). Besides, the dif-
base.12,13 Thus, in this work, we report the synthesis and ferences among the ultimate complexes in terms of
liquid crystalline properties of new bis[4-(4-alkoxy-2- their optical and thermal behaviour are also studied and
hydroxybenzylideneamino)azobenzene]palladium(II) reported.
O
N
+
H
H
O
C
N
NH2
H
O
t ano
E
h
l
H
C
H
O
N
N
N
H
O
+
r
2 CnH 1B
n+
2
K
A
2CO3
cetone
60oC
H
8
C/
H1
0
C/
H1
C/
H
3
C/
H
6
19
C/
C18
C
H4
C/
C16
C15
H12
N
HC
O
C/
H1'
C
C/
H1
3
C/
H2'
C/
H
2C
H
2
N
N
H2
C/
H14
H11
C/
H
C17
C/
9
C/
H
O
H2 '
n Cn
H
C
'
3
H7
C/
H5
C/
n =
' 5
a
1
;
;
;
;
;
+
n =
1b
' 7
n =
' 9
c
1
n =
1d
' 11
+
-
2
2
P
d
H
COO
3
C
(
n =
' 13
e
)
1
t ano
E
h
l
H
C
C/
3
H1
H1
C/
C/
H12
0
H
C/
8
H1
C/
3
C15
C19
H
H4
C/
6
C/
C/
H
C
N
O
H1'
C
C/
C16
H2'
C/
9
C18
H
2C
H
N
N
2
H2
C/
14
H
9
C/
H11
C/
C17
H2 '
n Cn
H
O
C
'
3
H5
O
C/
H7
C/
P
d
H7
H5 C/
C/
H
H
3C Cn
n
2 '
'
H1
'
C/
H2
C/
H
H11
C/
C/
C17
C14
N
H
C/
H
N
2C
C
2
C18
H2
'
H12
C/
H
C
N
O
H4
H
6
C/
H13
C/
C 9
1
C16
C15
C/
H
3
C/
H1
0
C/
H
8
C/
H1
C/
n=
n=
n=
n=
n=
a
2
8
;
;
;
;
;
2b
10
12
14
16
c
2
2d
e
2
Scheme 1. Synthetic routes toward formation of ligands 1a–1e and Pd(II) complexes 2a–2e.

Synthesis, optical and thermal behaviour of palladium(II)
1437
2. Experimental
(KBr)vmax/cm−1: 1206 (C–O ether), 1309 (C–O phe-
nolic), 1470 (N=N), 1587 (C=C), 1605 (C=N),
1
2.1 Materials
2921–2850 (C-H alkyl). H NMR (400 MHz, CDCl3),
δ (ppm): 6.17 (dd, H1, J = 12 Hz), 5.55 (s, H2),
7.08 (d, H3, J = 12 Hz), 7.48–7.62 (m, H4, H5, H10,
H11, H12), 8.02 (d, H6, H7, J = 8 Hz), 7.97 (dd,
H8, H9, J = 4 Hz), 7.66 (s, H13), 3.67 (t, H1ꢁ, J =
4 Hz), 1.20–1.64 (m, H2ꢁ-H7ꢁ), 0.89 (t, H8ꢁ, J = 8 Hz).
13C NMR (75 MHz, CDCl3), δ (ppm): 107.70 (C1),
102.33 (C2), 135.91 (C3), 123.83 (C4,C5), 125.04
(C6,C7), 123.88 (C8, C9), 129.61 (C10,C11), 131.41
(C12), 153.10 (C13), 161.10 (C14), 165.87 (C15),
114.90 (C16), 152.57 (C17), 151.12 (C18), 151.35
(C19), 68.22 (C1’), 32.31-23.10 ( C2’-C7’), 14.53 (C8).
Potassium carbonate was purchased from Systerm while
2,4-dihydroxybenzaldehyde was obtained from Acros
Organic. 1-Bromooctane, 1-bromodecane, 1-bromodo-
decane, 1-bromotetradecane, 1-bromohexadecane and
palladium(II) acetate were purchased from Merck
whereas 4-aminoazobenzene was purchased from
Aldrich-Chemical. All reagents mentioned above are
used without further purification.
2.2 Physical measurements
1
The H-NMR spectra were obtained by using Brucker
2b: Dark green, yield 81%. Elemental analysis/%:
Avance 400 MHz ultrashield spectrometers equipped
with ultrashield magnet. Deuterated chloroform
(CDCl3) and dimethysulphoxide (DMSO-d6) were
used as solvent and TMS as internal standard. The
FTIR spectra were recorded using a Perkin Elmer
2000-FTIR spectrophotometer in the frequency range
4000–400 cm−1 with sample prepared in KBr disc.
Elemental (C, H and N) analysis were carried out using
a Perkin Elmer 2400 LS Series CHNS/O analyzer. The
phase transition temperatures and enthalpy values were
measured by using a Seiko DSC6200R calorimeter
with the heating and cooling rate of 5◦C. The optical
observations were made with a Carl Zeiss Axioskop 40
polarizing 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.
Found
C 68.56, H 6.92, N 8.14; calculated
(C58H68N6O4Pd), C 68.37, H 6.67, N 8.25. IR
(KBr)vmax/cm−1: 1206 (C–O ether), 1310 (C–O phe-
nolic), 1469 (N=N), 1588 (C=C), 1606 (C=N),
1
2923–2852 (C-H alkyl). H NMR (400 MHz, CDCl3),
δ (ppm): 6.17 (dd, H1, J = 12 Hz), 5.56 (s, H2), 7.09
(d, H3, J = 12 Hz), 7.49–7.63 (m, H4, H5, H10, H11,
H12), 8.02 (d, H6, H7, J = 8 Hz), 7.96 (dd, H8, H9,
J = 4 Hz), 7.65 (s, H13), 3.67 (t, H1ꢁ, J = 4 Hz),
1.19–1.63 (m, H2ꢁ-H9ꢁ), 0.89 (t, H10ꢁ, J = 8 Hz).
13C NMR (75 MHz, CDCl3), δ (ppm): 107.70 (C1),
102.34 (C2), 135.97 (C3), 123.82 (C4,C5), 125.09
(C6,C7), 123.87 (C8, C9), 129.61 (C10,C11), 131.40
(C12), 153.15 (C13), 161.08 (C14), 165.88 (C15),
114.91 (C16), 152.57 (C17), 151.13 (C18), 151.34
(C19), 68.22 (C1’), 32.31–23.10 (C2’-C7’), 14.54 (C8).
2c: Dark green, yield 83%. Elemental analysis/%:
2.3 Synthesis of 4-(4-alkoxy-2-hydroxybenzylideneamino)
azobenzene, 1a–1e
Found
C 69.53, H 7.31, N 7.72; calculated
(C62H76N6O4Pd), C 69.27, H 7.07, N 7.82. IR
(KBr)vmax/cm−1: 1206 (C–O ether), 1310 (C–O phe-
nolic), 1469 (N=N), 1587 (C=C), 1606 (C=N),
The synthesis of 4-(4-alkoxy-2-hydroxybenzylidene-
amino)azobenzene, 1a–1e follows the method similar
to that reported earlier.12,13
1
2924–2852 (C-H alkyl). H NMR (400 MHz, CDCl3),
δ (ppm): 6.17 (dd, H1, J = 12 Hz), 5.56 (s, H2), 7.09
(d, H3, J = 12 Hz), 7.49–7.63 (m, H4, H5, H10, H11,
H12), 8.02 (d, H6, H7, J = 8 Hz), 7.97 (dd, H8, H9,
J = 4 Hz), 7.66 (s, H13), 3.67 (t, H1ꢁ, J = 4 Hz),
1.19–1.63 (m, H2ꢁ-H11ꢁ), 0.90 (t, H12ꢁ, J = 8 Hz).
13C NMR (75 MHz, CDCl3), δ (ppm): 107.70 (C1),
102.34 (C2), 135.95 (C3), 123.82 (C4,C5), 125.04
(C6,C7), 123.88 (C8,C9), 129.61 (C10,C11), 131.42
(C12), 153.14 (C13), 161.10 (C14), 165.87 (C15),
114.90 (C16), 152.58 (C17), 151.13 (C18), 151.35
(C19), 68.22 (C1’), 32.31-23.10 ( C2’-C7’), 14.53 (C8).
2.4 Synthesis of the Pd(II) complexes 2a–2e
An ethanolic solution (10 mL) of palladium(II) acetate
(1.0 mmol) was added drop-wise to a hot ethanolic solu-
tion (50 mL) of ligands 1 (1.0 mmol) in round bottom
flask. The mixture solution was refluxed for 6 h and then
cooled to room temperature. The dark green precipi-
tate was collected by filtration and recrystallized from
ethanol.
2a: Dark green, yield 82%. Elemental analysis/%:
Found
C
67.59,
H
6.42,
N
8.60; calculated 2d: Dark green, yield 79%. Elemental analysis/%:
70.31, 7.67, 7.38; calculated
(C54H60N6O4Pd), C 67.36, H 6.23, N 8.73. IR Found
C
H
N

1438
Boon-Teck Heng et al.
(C66H84N6O4Pd), C 70.10, H 7.43, N 7.43. IR
From the FTIR data of ligands 1a–1e and corre-
(KBr)vmax/cm−1: 1206 (C–O ether), 1309 (C–O phe- sponding Pd(II) complexes 2a–2e, it can be observed
nolic), 1470 (N=N), 1589 (C=C), 1605 (C=N), that the broad band assignable to the O–H stretch-
1
2922–2851 (C-H alkyl). H NMR (400 MHz, CDCl3), ing in the free ligands disappears upon complexation
δ (ppm): 6.53 (dd, H1, J = 12 Hz), 5.56 (s, H2), 7.09 with Pd(II) indicating that the OH group is deproto-
(d, H3, J = 12 Hz), 7.49–7.62 (m, H4, H5, H10, H11, nated. As such, the ligand is coordinated to the metal
H12), 8.02 (d, H6, H7, J = 8 Hz), 7.96 (dd, H8, H9, ion via the anionic O−.1,9,10,14 Furthermore, the coordi-
J = 4 Hz), 7.66 (s, H13), 3.67 (t, H1’, J = 4 Hz), nation of O to Pd(II) ion is further supported by the
1.20–1.63 (m, H2’-H13’), 0.91 (t, H14’, J = 8 Hz). stretching vibration of phenolic C–O which has shifted
13C NMR (75 MHz, CDCl3), δ (ppm): 107.72 (C1), to a higher frequency at 1309–1310 cm−1 as compared
102.35 (C2), 135.92 (C3), 123.81 (C4,C5), 125.05 to 1280–1285 cm−1 in uncoordinated ligands.15,16 Com-
(C6,C7), 123.88 (C8,C9), 129.63 (C10,C11), 131.41 paring the stretching frequency of azomethine C=N
(C12), 153.12 (C13), 161.09 (C14), 165.89 (C15), in the free ligand (1624–1625 cm−1) and that in Pd(II)
114.92 (C16), 152.58 (C17), 151.12 (C18), 151.33 complexes (1605–1606 cm−1) suggests the coordina-
(C19), 68.21 (C1’), 32.31–23.09 (C2ꢁ-C7ꢁ), 14.54 (C8). tion of non-bonding electron on N atom to central Pd(II)
ion. This observation can be ascribed to the reduction
of double bond character of the C=N bond and it is
2e: Dark green, yield 85%. Elemental analysis/%:
in agreement with the results obtained from other ana-
Found
C 71.08, H 8.05, N 6.95; calculated
logous complexes.5,9–11,14,17,18 Besides, a weak band at
1471–1473 cm−1 indicates the stretching frequency of
azolinkages of N=N.19 Since this band is not shifted
upon complex formation, therefore, the coordination of
the N atom from the azo-linkages to Pd(II) ion can be
ruled out. All of these observations support the forma-
tion of Pd–O and Pd–N coordination modes in which
the ratio of Pd(II) to ligands is 1:2.
(C70H92N6O4Pd), C 70.83, H 7.75, N 7.08. IR
(KBr)vmax/cm−1: 1206 (C–O ether), 1309 (C–O phe-
nolic), 1469 (N=N), 1589 (C=C), 1605 (C=N),
1
2920–2851 (C-H alkyl). H NMR (400 MHz, CDCl3),
δ (ppm): 6.16 (dd, H1, J = 12 Hz), 5.56 (s, H2), 7.09
(d, H3, J = 12 Hz), 7.49–7.62 (m, H4, H5, H10, H11,
H12), 8.02 (d, H6, H7, J = 8 Hz), 7.97 (dd, H8, H9,
J = 4 Hz), 7.66 (s, H13), 3.67 (t, H1ꢁ, J = 4 Hz),
1.20–1.64 (m, H2ꢁ-H15ꢁ), 0.89 (t, H16ꢁ, J = 8 Hz). 13C
NMR (75 MHz, CDCl3), δ (ppm): 107.71 (C1), 102.33
(C2), 135.91 (C3), 123.83 (C4,C5), 125.05 (C6,C7),
123.87 (C8,C9), 129.62 (C10,C11), 131.41 (C12),
153.10 (C13), 161.10 (C14), 165.87 (C15), 114.92
(C16), 152.56 (C17), 151.12 (C18), 151.35 (C19),
68.20 (C1’), 32.31–23.10 ( C2’-C7’), 14.53 (C8).
1
The investigation by the H-NMR chemical shifts
(δ) shows that the signals for H1 and H2 (δ 6.51–
6.55 ppm), H3 (δ 7.30–7.33 ppm) and H13 (δ 8.62–
8.63 ppm) in uncoordinated ligands 1a–1e are shifted
upfield upon complexation. The upfield shift of pro-
ton signals shows the increase of shielding effect which
can be explained by the nature of the molecular struc-
tures of 2a–2e wherein the Pd(II) ion is stabilized by
Pd–O and Pd–N coordination modes. The stabiliza-
tion has further been enhanced through the existence of
back bonding from Pd(II) ion to the conjugated system
consisting of H1, H2, H3 and H13.20 Besides, the over-
lapping of chemical shifts for proton H1 and H2 in
3. Results and discussion
3.1 Synthesis
Ligands 1a–1e were obtained from the condensation uncoordinated ligands is not observed in Pd(II) com-
between aromatic amine and 2,4-dihydroxybenzalde- plexes. These two protons have shifted to upper field
hyde wherein the imine thus obtained was alkylated by giving rise to two separate peaks. The large upfield
various bromoalkane (CnH2n+1Br; where n = 8−16) via shift in the chemical shift of azomethine proton H13
has further substantiated the coordination of N atom
from the ligands to the central Pd(II) ion.17,18,20 In addi-
tion, the absence of a singlet at δ = 13.60 ppm indi-
cates that OH group is deprotonated prior to form new
coordination mode of Pd–O. The 13C-NMR chemi-
cal shifts (δ) for azomethine carbon (C13) and OH
group carbon of C14 also support the presence of
new coordination mode of Pd–O whereby the chemi-
cal shift of C13 and C14 at respective δ = 162.38−
162.40 ppm and δ = 164.48 − 164.56 ppm were shifted
the Williamson etherification. The reactions between 1a
and 1e with Pd(II) acetate in stoichiometric ratio have
led to the formation of Pd(II) complexes 2a–2e as dark
green solids. The structure elucidation for compounds
1
1a–1e and 2a–2e were carried out by FT-IR, H-NMR
and elemental analysis. The physical properties of the
compounds thus obtained are summarized in experi-
mental section. These data show that the empirical for-
mula are in accordance with all the complexes 2a–2e of
which the ratio of Pd(II) to ligands is 1:2.

Synthesis, optical and thermal behaviour of palladium(II)
1439
Table 1. Phase transition temperature (◦C) and associated enthalpy (kJ mol−1) given in
parenthesis of ligands 1a–1d.
Ligands
1a
Cr1
Cr2
SmA
N
I
Heating •
74.2 (5.4)
•
109.7 (30.5)
107.1 (27.7)
•
•
•
•
•
•
•
•
164.3a • 176.5a
161.1a • 173.9a
166.9a • 168.5 (3.3)
159.4a • 162.6a
165.1 (3.2)
•
•
•
•
•
Cooling •
Heating •
Cooling •
Heating •
Cooling •
Heating •
Cooling •
76.1 (−23.5)
1b
99.6 (7.7)
•
80.7 (−31.7)
1c
106.8 (36.3)
96.1 (−34.3)
110.3 (64.2)
91.5 (−66.1)
168.2 (−3.6) •
1d
167.9 (6.5)
•
164.6 (−6.0) •
Cr, crystal; N, nematic; Sm A, smectic A; I, isotropic
aDenotes transition temperature derived from unresolved peaks
Dot means the presence of the phase
to 153.10–153.15 ppm and 161.08–161.10 ppm upon with a heating and cooling hot stage and their enthalpy
complex formation. values are obtained by differential scanning calometry
The central Pd(II) ion in respective complexes 2a– (DSC). The respective phase transition temperatures
2e is stabilized by the two equivalent ligands in which and enthalpy values for ligands 1a–1e and Pd(II) com-
the palladium ion possesses square planar geometry. plexes 2a–2e are illustrated in tables 1 and 2. All li-
This can be supported by the 1H-NMR spectra in which gands with their corresponding Pd(II) complexes
no peak broadening was detected. This observation is exhibit enantiotropic mesophase. The homologous lig-
in agreement with that reported by Blackburn and co- ands with n = 8 (1a) and n = 10 (1b) show the
workers in which they found that the tetrahedral geo- schlieren and the focal conic fan-shaped textures char-
metry of metal complexes caused the peaks broadening acteristic of nematic (N) and smectic A (SmA) phases,
whereas the square planar analogues prevented the peak respectively. However, the N phase is not observed upon
broadening.19 In addition, the square planar geometries further lengthening of the alkoxy chain from ligand 1c
of Pd(II) in complexes 2a–2e can be further evident by (n = 12) to ligand 1e (n = 16). Instead, ligands 1c–
their common stereochemistry as reported by Hudson 1e exhibit only enantiotropic SmA phase. The existence
and co-workers.21
of SmA in ligands 1a–1e have earlier been proven by
X-ray diffraction.12
Optical observation shows that the whole homolo-
gous series of Pd(II) complexes 2a–2e exhibit SmA
phase. Upon heating with the transition rate of
3.2 Liquid crystalline properties
The liquid-crystalline properties of ligands 1a–1e with
their corresponding Pd(II) complexes have been stu-
died by polarizing optical microscope (POM) equipped
Table 2. Phase transition temperature (◦C) for complexes
2a–2e.
Complexes
Cr
SmA
I
2a
2b
2c
2d
2e
Heating
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
•
•
•
•
•
•
•
•
•
•
256.2
158.6
247.4
186.5
224.5
74.3
215.6
84.1
204.9
83.3
•
•
•
•
•
•
•
•
•
•
261.9
260.8
257.4
251.4
249.2
238.5
236.6
228.9
223.4
215.4
•
•
•
•
•
•
•
•
•
•
Figure 1. Photomicrograph showing the SmA phase with
focal conic fan shape texture of complex 2c at 224.5◦C upon
heating.
Dot means the presence of the phase

1440
Boon-Teck Heng et al.
5◦C min−1, these complexes exhibit the SmA phase a complete photomicrograph attributed to the presence
with focal conic fan-shaped texture as illustrated in of SmA phase could not be observed. Besides, the
figure 1. However, the transition is accompanied by fast decomposition behaviour of Pd(II) complexes has
partial decomposition and the continuous heating has also affected the studies using X-ray diffraction and
led to isotropization.
DSC.
A notable feature among these complexes 2a–2e is
Upon cooling with the same rate (5◦C min−1), the
batonnets thus appeared in complex 2c (figure 2) coa- that the mesophases thus observed are reproducible
lesce to form the SmA phase (figure 3). It is also clearly upon heating and cooling processes even though these
shown from figure 3 that the focal conic fan-shaped tex- complexes are partially decomposed. However, the
ture filled with curved lines which exist over a large presence of arc in the SmA phase is only observed upon
smectogenic temperature range but diminished upon cooling process.
crystallization. As in the Pd(II) complexes 2a–2e, the
Figure 4 illustrates that the Pd complexes 2a (n = 8)
presence of irregular shape of black spots observed in and 2b (n = 10) exhibit only SmA phase in comparison
figures 1, 2 and 3 indicate fast decomposition. Hence, to uncoordinated ligands exhibiting both N and smectic
A phases. This could plausibly be due to the Pd(II) com-
plexes which posses new coordination modes of Pd–O
and Pd–N with greater polarity leading to the suppres-
sion of N phase.5,22 The other factor which enhances
the smectogenicity of the complexes can be associated
with the increase in the number of aromatic rings in the
complexes which has resulted in the lateral interaction
between the molecules.23
In addition, the correlation study upon heating pro-
cess shows that the phase transition temperatures (Cr-
SmA and SmA-I) of the Pd(II) complexes 2a–2e are
relatively higher than those in 1a–1e (figure 4). This
indicates that the introduction of Pd(II) increases the
phase transition temperature of the corresponding li-
gands. This observation can be rationalized by the
fact that the molecular weights as well as the num-
ber of interacting sites of 2a–2e are higher in com-
parison to the ligands.3 Further study upon these data
also reveals that the transition temperature decreases
when the number of carbon atom in the flexible alkoxy
chain increases. This can be exemplified by the SmA-I
Figure 2. Photomicrograph showing the batonnets of com-
plex 2c with the presence of arced lines at 242◦C during the
cooling process.
Figure 4. A set of plots of phase transition temperature
upon heating versus the number of carbon atoms in alkoxy
chain for the ligands 1a–1e, L (solid line) and their Pd(II)
complexes, PdL2 (dotted line). Cr, crystal; SmA, Smectic A;
N, Nematic; I, Isotropic; CP, Clearing point.
Figure 3. Photomicrograph showing the focal conic fan
shape texture of complex 2c with the presence of arced lines
on mesophase upon cooling.

Synthesis, optical and thermal behaviour of palladium(II)
temperature for 2a (261.9◦C) which has decreased to
1441
The similar observation in mesomorphic region for
257.4◦C, 249.2◦C, 236.6◦C and 223.4◦C in 2b, 2c, 2d Cu(II) and Pd(II) complexes suggests the similar coor-
and 2e, respectively.
dination geometry between these two complexes.12,24
Thus, the Pd(II) complexes are suggested to have square
planar geometry according to the common stereochem-
istry of this kind of compound.21
3.3 The comparative study of existing Pd(II) complexes
2a–2e with earlier reported Cu(II) and Ni(II) complexes.
Ligands 1a 1e
In order to rationalize the importance of different metal
ion resulted from the liquid crystalline and thermal
properties of 4-(4-alkoxy-2-hydroxybenzylidene-
amino)azobenzene ligands, the present Pd(II) com-
plexes are compared with our earlier reported Cu(II)
and Ni(II) complexes.12 Below is a summary of the
mesophases for ligands 1a–1e with their correspond-
ing Cu(II), Pd(II) and Ni(II) complexes. All the Pd(II)
complexes 2a–2e show the SmA phase as observed in
homologous Cu(II) complexes.12 However, the coor-
dination of ligands 1a–1e to Pd(II) metal ion forms a
less stable metal complexes which has been evident by
the little decomposition of the complexes at melting
temperature. In addition, there is obvious variation of
the phase transition temperature in Pd(II) complexes
as compared to Cu(II) and Ni(II) complexes (figure 5).
Figure 5 shows that the Pd(II) complexes have an
exceptionally high phase transition temperature. The
whole homologous series of Pd(II) complexes have
relatively higher transition temperatures of I-SmA as
compared to those in Cu(II) complexes. Besides, the
clearing temperatures for Pd(II) complexes are also
significantly higher as compared to Cu(II) complexes
especially for Pd(II) complexes with terminal alkoxy
chain from n = 8 to n = 12. The differences may
be due to the difference of the size of metal ion and
the possibility of weak metal–metal intermolecular
interaction between the complexes.24
1a and 1b (n= 8 and 10)
Cr
1c, 1d and 1e (n= 12, 14 and 16)
Cr SmA
SmA
N
I
I
Cu(II) complexes (n= 8, 10, 12, 14 and 16)
Cr SmA
Ni(II) complexes (n= 8, 10, 12, 14 and 16)
Cr
I
I
Pd(II) complexes 2a 2e (n= 8, 10, 12, 14 and 16)
Cr SmA
I
3.4 The comparative study of different core systems
on optical and thermal properties of Pd(II) complexes
The present Pd(II) complexes series derived from
azobenzene-cored Schiff bases ligands 1a–1e are com-
pared to the structurally related compounds reported by
Yeap and co-workers in 2012 with general structure as
shown below.25 Both series have almost similar mole-
cular structure but with different types of cored systems.
Thus, general rules for the effect of the chemical con-
stitution in liquid crystalline and thermal properties can
be deduced.
The general molecular structure of the two Pd(II)
complexes are shown below where Pd(II) complexes
2a–2e consist of azobenzene-cored system whereas the
Pd(II) complexes synthesized by Yeap and co-workers
consist of benzothiazole-cored system.25
A comparison in term of optical or liquid crys-
talline properties between the azobenzene-cored Pd(II)
complexes 2a–2e and benzothiazole-cored Pd(II) com-
plexes reveals that the differences are pronounced. All
the homologous members of Pd(II) complexes 2a–2e
exhibit enantiotropic SmA phase whereas the DSC
traces and optical observation of benzothiazole-cored
Figure 5. A set of plots of phase transition temperatures
upon heating versus the number of carbon atoms in alkoxy
chain for the Cu(II), Ni(II) and Pd(II) compexes. Cr, Crystal;
SmA, Smectic A; I, Isotopic; CuL2,Cu(II) complexes; NiL2,
Ni(II) complexes; PdL2, Pd(II) complexes.

1442
Boon-Teck Heng et al.
Pd(II) complexes show that these complexes are non- Besides, the Pd(II) complexes also have an exceptional
liquid crystalline. higher phase transition temperatures as compared to
However, a comparison with regard to the thermal Cu(II) and Ni(II) complexes. Comparative studies show
properties for both Pd(II) complexes show that all that the azobenzene-cored Pd(II) complexes are more
the benzothiazole-cored Pd(II) complexes undergo a stable and favour the liquid crystalline formation as
considerable amount of decomposition right after the compared to the benzothiazole-cored Pd(II) complexes
isotropic phase. Similarly, all the Pd(II) complexes 2a– which are fully decomposed and non-mesogenic.
2e also exhibit thermal decomposition behaviour. But,
these Pd(II) complexes just decompose slightly during
Acknowledgements
the transition from crystal to SmA phase and the SmA
phase is reproducible upon the subsequent heating and
cooling processes.
One of the authors (GYY) would like to thank Uni-
versiti Sains, Malaysia for the RU Grant No. 1001/
PKIMIA/811223.
The above observations show that azobenzene-cored
system favours the mesophase formation and forms
Pd(II) complexes 2a–2e which are more stable as com-
pared to the benzothiazole-cored Pd(II) complexes that
are fully decomposed. By substituting a core system
with another one, the polarity, core length, rigidity and
fractional residue volume are also altered. As a conse-
quence, the differences in liquid crystalline and ther-
mal behaviour of these two Pd(II) complexes can be
distinguished.
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