Synthesis, 2D NMR and X-ray diffraction studies on Cu(II) and Ni(II) complexes with ligands derived from azobenzene-cored Schiff base: Mesomorphic behaviors of Cu(II)-phenolates and crystal structure of bis[4-(4-alkoxy-2- hydroxybenzylideneamino)azobenzene]copper(II)

Article abstract of DOI:10.1016/j.molstruc.2011.05.036

Two new homologous series of copper(II) and nickel(II) complexes derived from bidentate Schiff bases 4-(4-alkoxy-2-hydroxybenzylideneamino)azobenzene have successfully been synthesized. Their molecular structures were elucidated by FT-IR, ¹H and ¹³C NMR along with ¹H- ¹H COSY, ¹H-¹³C HMQC, ¹H and ¹³C HMBC. Mesomorphic behaviors of ligands and metal complexes have been investigated by differential scanning calorimetry and polarizing optical microscope. All ligands and their copper complexes exhibit exclusively the mesomorphic properties. The X-ray diffraction studies have confirmed the presence of smectic A (SmA) phase in copper(II)-phenolate complexes. The molecular structure of bis[4-(4-alkoxy-2-hydroxybenzylideneamino)azobenzene] copper(II) have been determined by using X-ray crystallographic techniques. It was found that the ligands have square planar configuration around the Cu center ion as evident by the NCuN and OCuO angles of 180°.

Full text of DOI:10.1016/j.molstruc.2011.05.036

Journal of Molecular Structure 999 (2011) 68–82
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, 2D NMR and X-ray diffraction studies on Cu(II) and Ni(II) complexes
with ligands derived from azobenzene-cored Schiff base: Mesomorphic behaviors
of Cu(II)–phenolates and crystal structure of
bis[4-(4-alkoxy-2-hydroxybenzylideneamino)azobenzene]copper(II)
a
b
b
c
d
Guan-Yeow Yeap a, , Boon-Teck Heng , Masaki Kakeya , Daisuke Takeuchi , Ewa Gorecka , Masato M. Ito
⇑
a Liquid Crystal Research Laboratory, School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia
b Chemical Resources Laboratory (Mailbox R1-03), Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
c Department of Chemistry, Warsaw University, Al.Zwirki I Wigury 101, 02-089 Warsaw, Poland
d Department of Environmental Engineering for Symbiosis, Faculty of Engineering, Soka University, Hachioji, Tokyo 192-8577, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new homologous series of copper(II) and nickel(II) complexes derived from bidentate Schiff bases
4-(4-alkoxy-2-hydroxybenzylideneamino)azobenzene have successfully been synthesized. Their molecu-
lar structures were elucidated by FT-IR, 1H and 13C NMR along with 1H–1H COSY, 1H–13C HMQC, 1H and
13C HMBC. Mesomorphic behaviors of ligands and metal complexes have been investigated by differential
scanning calorimetry and polarizing optical microscope. All ligands and their copper complexes exhibit
exclusively the mesomorphic properties. The X-ray diffraction studies have confirmed the presence of
smectic A (SmA) phase in copper(II)–phenolate complexes. The molecular structure of bis[4-(4-alkoxy-
2-hydroxybenzylideneamino)azobenzene]copper(II) have been determined by using X-ray crystallo-
graphic techniques. It was found that the ligands have square planar configuration around the Cu center
ion as evident by the NACuAN and OACuAO angles of 180°.
Received 9 January 2011
Received in revised form 19 May 2011
Accepted 19 May 2011
Available online 13 June 2011
Keywords:
Cu(II)–phenolate complexes
Ni(II) complexes
FTIR
NMR
Ó 2011 Elsevier B.V. All rights reserved.
X-ray diffraction
Mesomorphic behavior
1. Introduction
geometrical shapes [3,4]. Furthermore, the presence of metal ions
can lead to richness in terms of oxidation state, color and magne-
Liquid crystal have been known for many decades but the new
or modified process for the obtainment of new chemical com-
pounds capable of exhibiting mesomorphic properties has only
been accelerated in the last 20 years. The innovative effort in
searching for these compounds has inspired the researchers to
diversify into metal containing liquid crystals or metallomesogens
which possess various ranges of metal-base coordination modes
[1]. The importance from the synthesis of metallomesogens can
generally be associated with the advantage of combining the prop-
erties of liquid crystal system with those of transition metal. The
presence of metal ions can induce the changing of non-mesogenic
compounds to compounds with mesogenic properties [2]. In
general, the metallomesogens have new geometrical shapes as
the metal ions are able to adopt various geometries ranging from
square planar, folder square, lantern, pyramidal and octahedral
tism [5].
In 1997, Tantrawong and co-worker had successfully synthe-
sized metal containing liquid crystal with the potential application
in optical storage [6]. Liu and co-worker have found that metallo-
mesogens have widespread use as stationary phase in gas chro-
matographic application due to the benefits of coupling the usual
analytical strengths of gas chromatography with the unique struc-
ture and shape selectivity properties of the liquid crystalline phase
[7]. Recently, Huang and co-worker synthesized a metallomesogen
of a polycatenar oxazoline copper(II) complex that could exhibit a
columnar mesophase and employed as the stationary phase for the
GC separation with polycyclic aromatic hydrocarbons as model
compound [8].
Besides, the azo-based polymeric liquid crystals and its metal-
containing derivatives have also been studied extensively in re-
cent years because these materials are capable of changing the
molecular shape through the reversible cis–trans isomerization
under the influence of UV or photoirradiation. The thermo- and
photoisomerization of azobenzene derivatives are important for
the development of information recording system such as
⇑
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 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.05.036

G.-Y. Yeap et al. / Journal of Molecular Structure 999 (2011) 68–82
69
high-density optical data storage, optical switching, display and
nonlinear optics [9,10]. However, it has been claimed that the
molecules with AN@NA group in mesophase prefer the trans con-
formation as that/those reported for photoexcited azo-dye in-
feature of azobenzene compound lies on its potential to form
rod-like molecules leading to the materials exhibiting liquid crys-
talline properties [11].
The Schiff bases derived from substituted salicylaldehyde can
serve as ligands which form NAO chelates and have been widely
duced torque in
N liquid crystals [11]. Another interesting
O
N
N
NH2
HC
OH
+
HO
Ethanol
70oC
N
N
N
C
H
OH
HO
+
2.2 CnH2n+1Br
K2CO3
Acetone
60oC
10
11
8
4
6
7
3
1
2
1'
H2
C
2'
Hn'
Cn'
H2
13
15
17
18
19
N
N
N
C
O
CH3
C
12
16
H
5
9
14
n=2 ; 1f
n=3 ; 1g
n=4 ; 1h
n=6 ; 1i
n=8 ; 1j
n=5 ; 1a
n=7 ; 1b
n=9 ; 1c
n=11 ; 1d
n=13 ; 1e
HO
+
Cu2+(CH3COO-)2.H2O / Ni2+(CH3COO-)2.4H2O / Pd2+(CH3COO-)2
Ethanol
70oC
H
N
N
N
C
O
CnH2n+1
O
M
O
Cn
O
C
H
N
N
N
H2n+1
M=Ni
M=Cu
n=8 ; 2a
n=10 ; 2b
n=12 ; 2c
n=14 ; 2d
n=16 ; 2e
n=8 ; 3a
n=10 ; 3b
n=12 ; 3c
n=14 ; 3d
n=16 ; 3e
n=5 ; 2f
n=6 ; 2g
n=7 ; 2h
n=9 ; 2i
n=11; 2j
Scheme 1. Synthetic pathway towards the formation of Cu(II) and Ni(II) complexes.

Table 1
1H NMR chemical shift (ppm) of ligands 1a–1j.
Atom
Chemical shift (ppm)
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
H1 and H2
H3
6.52–6.55 (m)
7.32–7.34 (d)
7.41–7.43 (d)
7.46–7.57 (m)
7.94–7.96 (dd)
8.00–8.03 (d)
8.63 (s)
6.51–6.55 (m)
7.31–7.33 (d)
7.40–7.43 (d)
7.45–7.57 (m)
7.93–7.96 (dd)
8.00–8.03 (d)
8.63 (s)
6.51–6.55 (m)
7.30–7.33 (d)
7.40–7.43 (d)
7.46–7.57 (m)
7.93–7.96 (dd)
8.01–8.03 (d)
8.63 (s)
6.52–6.54 (m)
7.28–7.33 (d)
7.40–7.43 (d)
7.45–7.57 (m)
7.93–7.96 (dd)
8.01–8.03 (d)
8.62 (s)
6.52–6.54 (m)
7.28–7.33 (d)
7.40–7.43 (d)
7.45–7.57 (m)
7.93–7.96 (dd)
8.01–8.03 (d)
8.62 (s)
6.51–6.54 (m)
7.31–7.33 (d)
7.41–7.43 (d)
7.46–7.57 (m)
7.93–7.96 (dd)
8.00–8.03 (d)
8.62 (s)
6.51–6.55 (m)
7.31–7.33 (d)
7.41–7.43 (d)
7.46–7.57 (m)
7.93–7.96 (dd)
8.01–8.03 (d)
8.62 (s)
6.51–6.55 (m)
7.30–7.33 (d)
7.40–7.43 (d)
7.46–7.57 (m)
7.93–7.96 (dd)
8.01–8.03 (d)
8.63 (s)
6.52–6.54 (m)
7.28–7.33 (d)
7.40–7.43 (d)
7.45–7.57 (m)
7.94–7.96 (dd)
8.01–8.03 (d)
8.63 (s)
6.51–6.55 (m)
7.30–7.33 (d)
7.40–7.43 (d)
7.45–7.56 (m)
7.94–7.96 (dd)
8.01–8.03 (d)
8.63 (s)
H4 and H5
H10, H11 and H12
H8 and H9
H6 and H7
H13
H14
13.62 (s)
13.60 (s)
13.60 (s)
13.63 (s)
13.63 (s)
13.61 (s)
13.60 (s)
13.61 (s)
13.63 (s)
13.63 (s)
H10
4.02–4.05 (t)
1.31–1.87 (m)
0.88–0.91 (t)
4.02–4.05 (t)
1.29–1.87 (m)
0.88–0.91 (t)
4.02–4.05 (t)
1.25–1.88 (m)
0.88–0.92 (t)
4.02–4.05 (t)
1.25–1.85 (m)
0.89–0.93 (t)
4.02–4.05 (t)
1.25–1.85 (m)
0.89–0.93 (t)
4.02–4.05 (t)
1.25–1.87 (m)
0.88–0.91 (t)
4.02–4.05 (t)
1.29–1.87 (m)
0.88–0.92 (t)
4.02–4.05 (t)
1.25–1.88 (m)
0.88–0.92 (t)
4.02–4.05 (t)
1.25–1.86 (m)
0.89–0.93 (t)
4.02–4.05 (t)
1.25–1.85 (m)
0.89–0.93 (t)
H20AHn00
H (methyl)
TMS as internal standard.
s, singlet; d, doublet; dd, doublet of doublet; m, multiplets.
Table 2
13C NMR chemical shift (ppm) of compound ligand 1a–1j.
Atom
Chemical shift (ppm)
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
C1
C2
C3
108.33
102.10
134.19
122.14
124.64
123.24
129.51
131.23
162.39
164.48
164.44
113.13
153.15
151.16
151.31
68.72
108.35
101.97
134.17
122.12
124.63
123.22
129.50
131.32
162.40
164.49
164.45
113.14
153.14
151.16
151.30
68.74
108.35
101.98
134.18
122.13
124.63
123.23
129.49
131.33
162.40
164.50
164.44
113.14
153.14
151.17
151.29
68.74
108.34
101.98
134.19
122.13
124.62
123.24
129.49
131.34
162.38
164.51
164.43
113.13
153.13
151.18
151.30
68.75
108.35
101.97
134.20
122.12
124.62
123.24
129.48
131.34
162.40
164.56
164.46
113.15
153.14
151.17
151.30
68.75
108.35
101.98
134.17
122.14
124.64
123.23
129.51
131.33
162.40
164.48
164.44
113.13
153.14
151.16
151.31
68.73
108.35
101.97
134.17
122.14
124.64
123.22
129.50
131.32
162.40
164.49
164.44
113.14
153.14
151.16
151.30
68.74
108.35
101.97
134.18
122.13
124.62
123.23
129.51
131.33
162.38
164.49
164.45
113.15
153.13
151.16
151.30
68.74
108.34
101.98
134.18
122.12
124.63
123.23
129.49
131.34
162.38
164.51
164.43
113.13
153.13
151.18
151.30
68.74
108.35
101.97
134.19
122.12
124.62
123.24
129.50
131.34
162.40
164.51
164.46
113.14
153.14
151.18
151.29
68.75
C4 and C5
C6 and C7
C8 and C9
C10 and C11
C12
C13
C14
C15
C16
C17
C18
C19
C10
C20ACn0
C (methyl)
32.24–23.08
14.49
32.31–23.06
14.51
32.32–23.09
14.50
32.30–23.09
14.50
32.32–23.10
14.49
32.20–23.02
14.49
32.21–23.02
14.49
32.21–23.04
14.50
32.30–23.05
14.50
32.32–23.08
14.49

G.-Y. Yeap et al. / Journal of Molecular Structure 999 (2011) 68–82
71
used in the synthesis of metallomesogen. In view of the diversity of
substituents thus introduced, a great variety of mesogenic com-
plexes has been reported [12]. In this paper we report the prepara-
tion of new copper(II) and nickel(II) complexes derived from new
bidentate Schiff bases. The structure for all the title compounds
were characterized by FT-IR and NMR (1H, 13C, COSY, HMQC and
HMBC). The phase transition temperature and enthalpy values of
the title compounds were measured by differential scanning calo-
rimetry (DSC) and the textures of the mesophases were studied
using polarizing optical microscope (POM). The study of the molec-
ular anisotropy in relation to the smectogenic behavior and inter-
molecular interaction has been carried out by X-ray diffraction
analysis.
2.1.2. Synthesis of copper(II) complexes, 2a–2e
An ethanolic solution (10 ml) of copper(II) acetate (1.0 mmol)
was added dropwise to a hot ethanolic solution (50 ml) of com-
pound 1 (1.0 mmol) in round bottom flask. The mixture was re-
fluxed for 6 h at 70 °C and then cooled to room temperature. The
brown precipitate was collected by filtration and recrystallized
from chloroform–ethanol (1:1).
The yield and elemental analytical data for complexes 2a–2j are
summarized as follow:
2a: Yield 80%. Elemental analysis/%: Found C 70.67, H 6.65, N
9.26; calculated (C54H60N6O4Cu), C 70.46, H 6.53, N 9.14.
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.
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.
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.
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.
2f: Yield 82%. Elemental analysis/%: Found C 69.06, H 5.76, N
10.06; calculated (C48H48N6O4Cu), C 68.95, H 5.74, N 10.05.
2g: Yield 79%. Elemental analysis/%: Found C 69.37, H 5.98, N
9.72; calculated (C50H52N6O4Cu), C 69.49, H 6.01, N 9.73.
2h: Yield 81%. Elemental analysis/%: Found C 69.70, H 6.29, N
9.40; calculated (C52H56N6O4Cu), C 70.00, H 6.27, N 9.42.
2i: Yield 80%. Elemental analysis/%: Found C 71.24, H 6.98, N
8.79; calculated (C56H64N6O4Cu), C 70.93, H 6.75, N 8.87.
2j: Yield 81%. Elemental analysis/%: Found C 71.43, H 7.08, N
8.35; calculated (C60H72N6O4Cu), C 71.76, H 7.17, N 8.37.
2. Experimental
1-Bromooctane, 1-bromodecane, 1-bromododecane, 1-bromo-
tetradecane and 1-bromohexadecane were purchased from Merck.
4-Aminoazobenzene was purchased from Aldrich-Chemical while
copper(II) acetate monohydrate and 2,4-dihydroxybenzaldehyde
were obtained from Acros Organic. Potassium carbonate was pur-
chased from Systerm.
Whilst the synthesis of 4-(2,4-dihydroxybenzylideneamino)azo-
benzene was carried out using the method as reported by Ispir [13],
the preparation of ligands 4-(4-alkoxy-2-hydroxybenzylideneami-
no)azobenzene, 1a–1j and their corresponding Cu(II) and Ni(II) will
be described for the first time in this report.
2.1. Synthesis
2.1.1. Synthesis of 4-(4-alkoxy-2-
2.1.3. Synthesis of nickel(II) complexes, 3a–3e
hydroxybenzylideneamino)azobenzene, 1a–1j
An ethanolic solution (10 ml) of nickel(II) acetate (1.0 mmol)
was added dropwise to a hot ethanolic solution (50 ml) of ligands
1a–1e (1.0 mmol) in respective round bottom flask. The mixture
was refluxed for 6 h at 70 °C and then cooled to room temperature.
The brown precipitate was collected by filtration and recrystallized
from chloroform–ethanol (1:1).
To a mixture containing 0.50 g (1.0 mmol) of 4-(2,4-dihydroxy-
benzylideneamino)azobenzene in acetone, two equivalent of
potassium carbonate and 2.2 M of appropriate 1-bromoalkane
were added. The reaction mixture was heated for overnight at
60 °C 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 precip-
itate via filtration. The precipitate was then dried and recrystal-
lized from n-hexane.
The yield and elemental analytical data for complexes 3a–3e
are summarized as follow:
3a: Yield 85%. Elemental analysis/%: Found C 71.10, H 6.57, N
9.20; calculated (C54H60N6O4Ni), C 70.84, H 6.56, N 9.18.
3b: Yield 82%. Elemental analysis/%: Found C 71.92, H 7.02, N
8.72; calculated (C58H68N6O4Ni), C 71.70, H 7.00, N 8.65.
3c: Yield 76%. Elemental analysis/%: Found C 72.86, H 7.57, N
8.16; calculated (C62H76N6O4Ni), C 72.47, H 7.40, N 8.18.
3d: Yield 86%. Elemantal analysis/%: Found C 73.40, H 7.79, N
7.77; calculated (C66H84N6O4Ni), C 73.15, H 7.58, N 7.76.
3e: Yield 84%. Elemental analysis/%: Found C 74.10, H 8.09, N
7.39; calculated (C70H92N6O4Ni), C 73.77, H 8.08, N 7.38.
The yield and elemental analytical data for ligands 1a–1j 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.
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.
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.
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.
1e: Yield 50%. Elemental analysis/%: Found C 77.96, H 8.73, N
7.81; calculated (C35H47N3O2), 1473 (N@N), C 77.63, H 8.69, N
7.76.
The synthetic route to prepare all the intermediate and title
compounds 1a–1j, 2a–2j and 3a–3e are shown in Scheme 1.
2.2. FTIR measurement
1f: Yield 53%. Elemental analysis/%: Found C 74.20, H 6.54, N
10.69; calculated (C24H25N3O2) C 74.43, H 6.46, N 10.85.
1g: Yield 52%. Elemental analysis/%: Found C 74.69, H 6.77, N
10.57; calculated (C25H27N3O2) C 74.82, H 6.72, N 10.47.
1h: Yield 60%. Elemental analysis/%: Found C 75.24, H 7.06, N
9.81; calculated (C26H29N3O2), C 75.19, H 6.98, N 10.12.
1i: Yield 58%. Elemental analysis/%: Found C 75.73, H 7.51, N
9.21; calculated (C28H33N3O2) C 75.86, H 7.44, N 9.48.
1j: Yield 50%. Elemental analysis/%: Found C 76.35, H 7.93, N
8.77; calculated (C30H37N3O2), C 76.44, H 7.85, N 8.91.
The FTIR data were recorded using a Perkin Elmer 2000-FTIR
spectrophotometer in the frequency range 4000–400 cmꢀ1 with
sample prepared in KBr disks.
2.3. NMR measurement
The 1H and 13C NMR spectra for ligands 1a–1j were obtained
and substantiated with the aids of two-dimensional 1H–1H corre-
lation spectroscopy (COSY), 1H–13C heteronuclear multiple

72
G.-Y. Yeap et al. / Journal of Molecular Structure 999 (2011) 68–82
Table 4
Table 3
2D 1H–13C HMQC and HMBC correlation for compound ligands 1a–1j.
1H–1H correlation from 2D COSY for ligands 1a–1j.
Ligands
Atom
HMQC
1J
HMBC
2J
Ligands
Atom H
COSY
3J
1a–1j
H1
H3
H3
H1
1a–1j
H1 and H2
H3
H4 and H5
H8 and H9
H6 and H7
H13
C1 and C2
C3
C4 or H5
C8 or C9
C6 and C7
C13
C3, C14 and C15
C16 and C1
C6, C17 or C7
C10, C19 or C11
C18, C4 or C5
C16
C16
H4 and H5
H6 and H7
H8 and H9
H10 and H11
H12
H6 or H7
H4 or H5
H10 or H11
H8 or H9 and H12
H10 or H11
C12
C3, C14
quantum correlation (HMQC) and 1H–13C heteronuclear multiple
bond correlation (HMBC). All data were recorded using Brucker
Avance 300 and 400 MHz ultrashield spectrometers equipped with
ultrashield magnets. Deuterated chloroform (CDCl3) and dim-
ethysulphoxide (DMSO-d6) were used as solvent and TMS as inter-
nal standard.
TMS94 temperature controller. The samples were prepared in thin
film sandwiched between glass slide and cover.
2.6. X-ray data collection
The X-ray diffraction patterns were performed with a Bruker D8
Discover and GADDS systems. The homeotropic alignment of the
samples were obtained by slow cooling of a drop of the isotropic
liquid below the clearing temperature. The X-ray beam was direc-
ted almost parallel to the substrate surface and the temperature of
the sample was controlled by an accuracy of 0.1°. Finally, the data
obtained were analyzed by using Topas software.
2.4. Phase transition temperature and enthalpy values
The phase transition temperature and enthalpy values were
measured by Seiko DSC6200R differential scanning calometry at
heating and cooling rate of 5 °C minꢀ1 and ꢀ5 °C minꢀ1
,
respectively.
2.7. X-ray data collection, structure solution and refinement for Cu(II)
2.5. Texture observation
complex 2f
The texture was observed using a Carl Zeiss Axioskop 40 polar-
izing microscope equipped with a Linkam LTS 350 hot stage and
The X-ray crystal structure analytical data were collected
using a Rigaku Saturn 70 diffractometer, which uses graphite
Fig. 1. 1H–1H connectivities in the COSY spectra for ligand 1e.

G.-Y. Yeap et al. / Journal of Molecular Structure 999 (2011) 68–82
73
Fig. 2. One bond CAH correlation in the HMQC spectra of ligand 1e.
h
.
i
X
X
1=2
monochromated Mo K
a
radiation. The data were collected at a
2
2
wR2 ¼
ðwðF2o ꢀ F2c Þ Þ
wðF2oÞ
¼ 0:1801
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
The standard deviation of an observation of unit weight was
1.11. Unit weights were used. The maximum and minimum peaks
on the final difference Fourier map corresponded to 1.23 and
ꢀ1.02 eꢀ/Å3, respectively. Neutral atom scattering factors were ta-
ken from Cromer and Waber [15]. Anomalous dispersion effects
was done using
The exposure rate was 128.0 (s/°). The detector swing angle was
oscillations
x
oscillations from ꢀ110.0° to 70.0° in 0.5° steps.
ꢀ20.05°. A second sweep was performed using
x
from ꢀ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 distance was 45.32 mm. Readout was performed in
the 0.137 mm pixel mode. Out of the 15,291 reflections that were
collected, 4708 were unique (Rint = 0.0411). Data were collected
and processed using CrystalClear (Rigaku)1. The linear absorption
were included in Fcalc [16]; the values for
D D
f0 and f00 were those
of Creagh and McAuley [17]. The values for the mass attenuation
coefficients were those of Creagh and Hubbell [18]. All calculations
were performed using the crystal structure crystallographic soft-
ware package except for the refinement, which was performed
using SHELXL97 [19].
coefficient,
l, for Mo Ka
radiation was 5.778 cmꢀ1. An empirical
absorption correction was applied which result in transmission
factors ranging from 0.462 to 0.966. The data were corrected
for Lorentz and polarization effects.
3. Results and discussion
The structure was solved by direct methods [14] and expanded
using Fourier techniques. The non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were refined using the riding
model. The final cycle of full-matrix least-squares refinement on
F2 was based on 4708 observed reflections, 269 variable parame-
ters and converged (largest parameter shift was 0.00 times its
esd) with unweighted and weighted agreement factors of
3.1. Physical characterization
3.1.1. 1H and 13C NMR spectra assignment for ligands 1a–1j
Assignment of the ligands 1a–1j can be based on representa-
tive ligand 1e owing to all of these compounds show similar char-
acteristics as inferred from NMR spectra. The 1H NMR data from
Table 1 shows that the chemical shifts of the aromatic protons
are observed at d = 6.53–8.03 ppm. Proton H1 and H2 appear as
.
X
X
R1 ¼
kFoj ꢀ jFck
jFoj ¼ 0:0621

74
G.-Y. Yeap et al. / Journal of Molecular Structure 999 (2011) 68–82
Fig. 3. Long range CAH correlation in the HMBC spectra for ligand 1e.
multipletes at d = 6.52–6.54 ppm whilst the proton pairs in aro-
matic rings H4 and H5, H6 and H7, H8 and H9, H10 and H11
are equivalent. The doublet at d = 7.28–7.33 ppm can be ascribed
to proton H3 and the appearance of multiplets at d = 7.45–
7.57 ppm can be ascribed to protons H10, H11, and H12. A singlet
observed at d = 8.62 ppm is corresponded to the azomethine pro-
ton H13. The two triplets at d = 0.89–0.93 ppm and d = 4.02–
4.05 ppm can be ascribed to the methyl proton H160 and proton
H10, respectively. The group of protons H20 to H150 are confirmed
by the appearance of multiplets at d = 1.25–1.85 ppm. The hydro-
xyl protons H14 gives rise to the resonance at d = 13.60–
13.63 ppm. This observation is in accordance with the analytical
results obtained by Ostrovskii et al. [20] whom had reported that
the down-field shifts of the hydroxyl proton to d = 13–14 ppm
confirmed the presence of intramolecular hydrogen bonding
[21]. The presence of singlet within this range of chemical shift
supports the Williamson reaction wherein various alkyl groups
from bromoalkanes replaced the H atom from hydroxyl group
at para position.
carbons attached to N atom in the azobenzene. The signal appears
at lower field (d = 164.50 and 164.45 ppm) can be assigned to C15
and C14 while the signal at d = 162.40 and 153.14 ppm can be
attributed to C13 and C17, respectively. The resonance due to aro-
matic carbon C4 or C5, C6 or C7, C8 or C9 and C10 or C11 can be
located at d = 122.13, 124.63, 123.23 and 129.49 ppm, respectively.
The resonances recorded at d = 131.33, 134.18, 108.35, 101.98 and
123.34 ppm can be assigned to C12, C3, C1, C2 and C16, respec-
tively. The signal observed in high field at d = 68.74 and
14.50 ppm can be assigned to C10 and C120 of the methyl group
in the alky chain while signal at d = 23.08–32.31 ppm can be attrib-
uted to the carbon from C20 to C110.
The 1H–1H COSY NMR has further substantiated the correlation
between the equivalent proton pairs with the adjacent protons
wherein the cross peak resulting from the correlations appear at
the same region (Table 3). Fig. 1 for ligand 1e exhibits the correla-
tion between H1 with the respective proton H3. The proton H4 or
H5 is correlated with proton H6 or H7 and a similar phenomenon
can also be found on H8 or H9 which has correlation with H10 or
H11. The COSY spectra also reveal that the H10 which is adjacent
to O atom in ether group is correlated with H20 and the methyl pro-
ton H160 is correlated with H150. A similar correlation can also be
observed for ligands 1a–1j.
All ligands 1a–1j show a similar pattern of 13C NMR spectra (Ta-
ble 2). Hence, the discussion based on representative compound 1c
will be reported herein. The signal observed at d = 151.17 and
151.29 ppm can be attributed to the C18 and C19 which are the

G.-Y. Yeap et al. / Journal of Molecular Structure 999 (2011) 68–82
75
Table 7
Table 5
Phase transition temperatures (°C) and associated enthalpies (kJ molꢀ1) of Ni(II)
complexes 3a–3e.
Phase transition temperature (°C) and associated enthalpies (kJ molꢀ1) of ligand
1a–1j.
Complexes
K1
K2
K3
I
Ligand
K1
K2
SmA
N
I
3a
Heating
Heating
Heating
Heating
Heating
ꢁ
145.4
(23.5)
133.6
(21.4)
135.8
(16.5)
124.8
(24.5)
108.6
(21.3)
ꢁ
263.9
(47.8)
244.9
(43.8)
229.5
(40.1)
221.7a
ꢁ
1a
Heating
Heating
Heating
Heating
Heating
Heating
Heating
Heating
Heating
Heating
ꢁ
74.2
(5.4)
99.6
(7.7)
ꢁ
109.7
(30.5)
107.1
(27.7)
106.8
(36.3)
110.3
(64.2)
112.5
(64.3)
112.8
(28.8)
113.9
(23.3)
110.6
(24.5)
107.4
(42.8)
106.7
(33.9)
ꢁ
164.3a
166.9a
ꢁ
176.5a
ꢁ
3b
3c
3d
3e
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
1b
1c
1d
1e
1f
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
168.5
(3.3)
165.1
(3.2)
167.9
(6.5)
164.5
(5.8)
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
155.4
(34.5)
112.9
(23.1)
ꢁ
ꢁ
213.9
(19.1)
83.05
(3.3)
ꢁ
ꢁ
165.9
(1.8)
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
181.0
(0.6)
K, crystal; N, nematic; Sm A, smectic A; I; isotropic.
a
Denotes transition temperature derived from unresolved peaks.
1g
1h
1i
157.1
(1.1)
193.2a
2J. A similar phenomenon can also be found on H6 or H7 which is
correlated with C18 and C4 or C5 with 2J. The H8 or H9 is also cor-
related with carbons C19, C10 or C11 with 2J and C12 with 3J long
range connectivities. Similarly, H3 is correlated with C1 and C16
with 2J. The H13 and C16 are correlated with 2J long range conduc-
tivities. HMBC spectra also show the correlation between H13 with
C14 and C3 with the 3J long range connectivities. The HMBC spec-
tra show that H1 and H2 are correlated with C3, C14, C15 with 2J
and C16 with 3J long range connectivities. The emergence of the
cross peaks associated with the correlations of C10, C20 and C160
with respective H10, H20 and H160 has confirmed the presence of
carbon nuclei from the aliphatic chain.
75.8
(5.8)
151.9a
164.8a
ꢁ
ꢁ
ꢁ
166.9
(1.9)
165.1
(1.9)
174.2
(0.5)
1j
166.4a
K, crystal; N, nematic; Sm A, smectic A; I; isotropic.
a
Denotes transition temperature derived from unresolved peaks.
The 1H–13C HMQC spectra of compounds 1a–1j provide infor-
mation on the interaction between the protons and the carbon
atoms which are directly attached to each other (Table 4). As a rep-
resentative, the discussion will be based on the spectrum for ligand
1e (Fig. 2). It can be deduced from Fig. 2 that the H1 or H2, H4 or
H5, H8 or H9, H6 or H7 in aromatic ring are correlated with C1
or C2, C4 or C5, C8 or C9 and C6 or C7, respectively. Moreover,
the one-bond 13C–1H spectra also indicate the correlation between
H10, H20 and H10 with C10, C20 and C160, respectively. A similar phe-
nomenon can also be found on H3 and H13 which are correlated
with C3 and C13.
The aromatic carbons in all compounds were established via the
connectivities between the carbon and its neighboring proton by
HMBC correlation spectra (Table 3). The aliphatic protons for all ti-
tle compounds exhibit no correlation with any aromatic carbon.
However, the HMBC spectra of ligand 1e (Fig. 3) has shown that
the H4 or H5 is correlated with the carbons C17 and C6 or C7 with
The 1H NMR spectra for Cu(II) and Ni(II) complexes display only
broad alkoxy signals. All other proton signals close to the paramag-
netic Cu(II) and Ni(II) centers are not observable [3,22,23].
3.1.2. FT-IR spectral data
It is clearly shown from the FT-IR spectra of ligands 1a–1j that
they exhibit similar pattern of FT-IR spectra. A strong band observed
at 1625 cmꢀ1 corresponds to the stretching of azomethine, C@N
group [3,24,25]. A strong band at 2953–2849 cmꢀ1 can be assigned
to the stretching vibration of CAH bond from alkoxy chain. The band
with strong intensity in the spectra at the range of 1192–1194 cmꢀ1
(CAO) can be ascribed to the ether group [26]. Whereas, the stretch-
ing of phenolic CAO at 1282–1285 cmꢀ1 is observed as a medium
band [27]. A weak band that is observed at the frequency range of
1471–1473 cmꢀ1 is assignableto thestretchingvibrationofazolink-
ages, N@N [28]. Two bands which appear at respective frequency of
Table 6
Phase transition temperature (°C) and associated enthalpies (kJ molꢀ1
complexes 2a–2j.
)
of Cu(II)
Complexes
K1
K2
SmA
I
Ligands 1a-1j:
2a
Heating
Heating
Heating
Heating
Heating
Heating
Heating
Heating
Heating
Heating
ꢁ
209.4
(45.1)
207.8
(50.6)
183.8
(39.2)
125.9
(14.3)
130.1
(20.0)
253.7
(31.9)
241.1
(42.4)
220.5
(41.8)
208.9
(45.0)
191.3
(38.7)
ꢁ
236.7
(7.6)
239.0
(9.7)
237.5
(10.3)
233.0
(10.5)
225.7
(11.6)
261.3a
ꢁ
1a,1b, 1f-1j (n=5 to n=11)
2b
2c
2d
2e
2f
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
SmA
N
K
I
1c,1d and 1e (n=12,14,16)
SmA
I
K
ꢁ
ꢁ
179.5
(36.1)
171.4
(22.9)
Cu(II) Complexes 2a-2j (n=5-12, 14, 16):
K
I
SmA
2g
2h
2i
250.2a
239.8a
Ni(II) Complexes, 3a-3e:
237.8
(9.0)
239.2
(7.9)
K
I
2j
K, Crystal; SmA, Smectic A; N, Nematic; I, Isotropic.
K, crystal; N, nematic; Sm A, smectic A; I; isotropic.
Fig. 4. Mesomorphic phase of ligands 1a–1j, Cu(II) complexes 2a–2j and phase
transition temperature of Ni(II) complexes 3a–3e.
a
Denotes transition temperature derived from unresolved peaks.

76
G.-Y. Yeap et al. / Journal of Molecular Structure 999 (2011) 68–82
K, crystal; SmA, Smectic A; N, Nematic; I, Isotropic; CP, Clearing point
Fig. 5. A plot of phase transition temperature upon heating versus the number of carbon atoms in alkoxy chain for the ligands 1a–1j, L (solid line) and their copper(II)
complexes, CuL2 (dotted line).
major bands observed at 2921–2850 cmꢀ1 can be assigned to the
stretching frequencyfor CAHbond fromalkoxychain. Thebandwith
strong intensity in the spectra at the range of 1206–1200 cmꢀ1 is
assignable to the stretching of the CAO ether group. Besides, the
weak stretching frequency of azo linkages, N@N is observed at
1470–1471 cmꢀ1 [28] which is similar to the stretching vibration
of the uncoordinated ligands. This observation shows that N atom
of azo linkages is not coordinated to the Cu(II) metal ion. The stretch-
ing frequency for the coordination bonds MAN and MAO in the fin-
gerprint area are too weak and difficult to be identified.
Fig. 6. X-ray pattern of SmA phase of ligand 1a.
1591 cmꢀ1 and 1571 cmꢀ1 can be attributed to the C@C stretching in
phenyl ring [29]. The broad band at 3114–3714 cmꢀ1 can be as-
3.2. Mesomorphic behavior
signed to the stretching frequency of OAH group.
FT-IR spectra data for metal complexes 2a–2j and 3a–3e show
the similar vibration frequency. The strong band observed at
1610–1611 cmꢀ1 corresponds to the stretching of azomethine,
C@N. The reduction of C@N frequency as compared with free ligands
suggests the coordination of N atom from the ligand to the central
metal atom [22,24,5,30,31]. On the other hand, the disappearance
of the OAH band of free ligands in the Ni(II) and Cu(II) metal com-
plexes indicates that OH group is deprotonated and then it is coordi-
nated to the metal ion [22,24,29,5,30]. Furthermore, the
coordination of oxygen to metal ion is further confirmed by the
stretching vibration of phenolic CAO shifted to higher frequency at
1311–1313 cmꢀ1 for Cu(II) and Ni(II) complexes [27,32]. The other
All ligands and their Cu(II) complexes are enantiotropic as re-
vealed by DSC and polarizing optical microscope. The transitions
temperatures and associated enthalpies (kJ molꢀ1) of the ligands
1a–1e and their corresponding Cu(II) and Ni(II) complexes are
shown in Tables 5–7, respectively. All the Cu(II) complexes exhibit
smetic A (SmA) phase due to the appearance of nematic phase with
schlieren texture followed by SmA phase characterized by well de-
fined focal conic fan shape texture in the ligands with n-octyloxy
and n-decyloxy flexible chains. However, all Ni(II) complexes of
this series do not exhibit mesomorphic properties. This can be in-
ferred from the observation wherein the Ni(II) experienced the di-
rect isotropization upon heating and cooling without showing the
Fig. 7. Dependence between thickness of the layer in SmA phase and the length of alkoxyl chain for ligands 1a–1e (solid line) and their corresponding Cu(II) complexes 2a–2e
(dotted line).

G.-Y. Yeap et al. / Journal of Molecular Structure 999 (2011) 68–82
77
phase formation [3]. Whereas, all the Ni(II) complexes 3a–3e have
the inclination toward tetrahedral geometry [3]. This is evident
from the 1H NMR of paramagnetic Ni(II) complexes 3a–3e which
shows only broad signals from the alkoxy chains [33]. The tetrahe-
dral geometry of Ni(II) and crossed alignment of the coordinated li-
gands are unfavorable for mesophase formation [3]. All the Ni(II)
complexes exhibiting mesophase properties are diamagnetic,
which have been interpreted to indicate a square planar coordina-
tion about the metal [23]. Tuning the molecular geometries is
important to achieve a balance in the magnitude of molecular inter-
action as it optimizes the mesomorphic behavior [24].
3.2.1. Chemical structure–mesormorphic relationship
In order to rationalize the importance of different cored system
resulted from the mesormorphic and thermal properties of Cu(II)
complexes, the present azobenzene-cored Cu(II) complexes 2a–
2b and 2h–2i (A) are compared with fluorenes-cored (B), biphe-
nyls-cored (C) and stilbenes-cored (D) Cu(II) complexes synthe-
sized by Reddy and co-worker in 1991 [34]. The general
molecular structure for these four different core complexes are
shown below. It can be noticed that Cu(II) complexes A exhibit
enantiotropic SmA phase and thermally stable up to clearing tem-
perature in the range of 33–261 °C. Thus, comparison between the
mesophase of Cu(II) complexes A with B, C and D reveal that all the
Cu(II) complexes B, C and D also exhibit enantiotropic SmA phase,
but with the exception for terminal alkoxyl chain n = 5. Besides, the
clearing temperature for Cu(II) complexes B, C and D are in the
range of 240–260 °C, 250–278 °C and 272–277 °C, respectively.
Thus, the clearing temperatures can be arranged in the increasing
order of A < B < C < D. Therefore, these observations deduced that
the introduction of azobenzene cored can induce the formation
of mesophase in lower homologous member of Cu(II) complexes,
n = 5 and lower the clearing temperature of the complexes.
CHAINS INTERCALATION
Fig. 8. Schematic drawing of local arrangement of molecules for Cu(II) complexes,
2a–2e.
mesophase. Fig. 4 shows the summary of mesomorphic phase for
ligands and their Cu(II) and Ni(II) complexes.
In conventional organic liquid crystal, the intention is to increase
intermolecular contact to induce the mesomorphic properties. As
for the coordination compound, it is to avoid intermolecular con-
tacts and strong dipolar interaction that are sufficiently strong to
cause three-dimensional order in the formation of mesophase
[1,24]. Due to the difference of coordination geometry between
nickel and copper complexes, the magnitude of intermolecular
interaction in copper complexes is also different from nickel com-
plexes. Thus, this shows that the absence of mesophase in Ni(II)
complexes can be attributed by the different coordination geometry
between Ni(II) and Cu(II) complexes [3,24]. The crystal data in Sec-
tion 3.5 shows the square planar geometry of Cu(II) complex 2f.
Thus, it is suggested that the other homologous member of Cu(II)
complexes have the similar geometrical shape as Cu(II) complex
2f. The square planar geometry of Cu(II) complexes can increase
the geometrical anisotropy of molecules and is favorable for meso-
CnH2n
O
+
1
N
X
O
Cu
O
N
X
O
H2n 1Cn
+
n= 5-10
X
Complexes
N
A
N
Table 8
Summary of crystal data for Cu(II) complexes 2f.
Empirical formula
Formula weight
Crystal color, habit
Crystal dimensions
Crystal system
C48H48CuN6O4
836.49
Orange, prism
0.520 ꢂ 0.380 ꢂ 0.060 mm
Monoclinic
B
C
D
Lattice type
Lattice parameters
C-centered
a = 51.044(3) Å
b = 10.5834(6) Å
c = 7.7308(5) Å
b = 94.993(4)°
V = 4160.5(5) Å3
C2/c (#15)
Space group
Z value
Dcalc
4
C
C
H
1.335 g/cm3
1756.00
H
F000
l
(Mo K
a
)
5.778 cmꢀ1

78
G.-Y. Yeap et al. / Journal of Molecular Structure 999 (2011) 68–82
Fig. 9. The molecular structure with atom-numbering scheme of Cu(II) complex 2f.
Fig. 10. The molecular packing of Cu(II) complex 2f.
The azobenzene-cored Cu(II) complexes 2a–2j are also used to
make a comparison with azobenzene-cored Cu(II) complexes syn-
thesized by Khandar and Rezvani in 1998 [35]. The general molec-
ular structure of the ligands for the reported Cu(II) complexes
synthesized by Khandar and Rezvani are shown below. To ensure
that the comparison is relevant, only complexes with m = 5, 6
and 7 are chosen to make a comparison with Cu(II) complexes
2f–2h (n = 5, 6, 7).
N
O
CmH2m+1
N
CH
C7H15
O
N
OH
4a ðm ¼ 5Þ; 4b ðm ¼ 6Þ; 4c ðm ¼ 7Þ
Table 9
Summary of intensity measurements for Cu(II) complexes 2f.
Table 10
Diffractometer
Radiation
Saturn70
Mo K (k = 0.71070 Å) graphite
Summary of structure solution and refinement for Cu(II) complexes 2f.
a
monochromated
50 kV, 100 mA
ꢀ159.8 °C
Structure solution
Refinement
Function minimized
Direct methods
Voltage, current
Temperature
Detector aperture
Data images
Full-matrix least-squares on F2
R
w (F2o ꢀ F2c )2
70 ꢂ 70 mm
w = 1/[
r
2(F2o) + (0.0917 ꢃ P)2 + 5.2142 ꢃ P],
Least squares weights
720 exposures
ꢀ110.0° to 70.0°
128.0 s/°
where P = (Max(F2o , 0) + 2Fc2)/3
x
Oscillation range
Exposure rate
Detector swing angle
2hmax cutoff
55.0°
ꢀ20.05°
Anomalous dispersion
No. observations (all reflections)
No. variables
All non-hydrogen atoms
4708
269
x
Oscillation range
ꢀ110.0° to 70.0°
Exposure rate
128.0 s/°
Detector swing angle
Detector position
Pixel size
ꢀ20.05°
Reflection/parameter ratio
17.50
45.32 mm
Residuals: R1 (I > 2.00
r(I))
0.0621
0.0762
0.1801
1.114
0.137 mm
55.0°
Total: 15,291
Unique: 4708 (Rint = 0.0411)
Lorentz-polarization absorption
(trans. factors: 0.462–0.966)
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
2hmax
No. of reflections measured
0.000
1.23 eꢀ/Å3
ꢀ1.02 eꢀ/Å3
Corrections

G.-Y. Yeap et al. / Journal of Molecular Structure 999 (2011) 68–82
79
Table 11
Atomic coordinates and Biso/Beq and occupancy.
Atom
x
y
z
Beq
occ
Cu1
O1
O2
N1
N2
N3
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
C11
C12
C13
C14
C15
C16
C17
C18
C19
C20
C21
C22
C23
C24
0.2500
0.2500
0.5000
0.3230(3)
1.35(2)
1.66(4)
1.94(4)
1.44(4)
4.81(9)
5.58(10)
1.43(5)
1.30(4)
1.54(5)
1.67(5)
1.58(5)
1.63(5)
1.45(5)
2.27(6)
2.49(6)
3.17(7)
4.38(9)
5.78(13)
1.60(5)
1.85(6)
2.57(7)
2.99(7)
2.62(6)
1.99(5)
5.58(13)
6.0(2)
1/2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0.22310(4)
0.15814(4)
0.22688(4)
0.11497(8)
0.10550(10)
0.23711(5)
0.26410(5)
0.22806(5)
0.20222(5)
0.18331(5)
0.19045(5)
0.21696(5)
0.13701(6)
0.11175(6)
0.08768(6)
0.06233(7)
0.03788(7)
0.19924(5)
0.18371(6)
0.15671(7)
0.14476(6)
0.16008(6)
0.18729(6)
0.07526(8)
0.05928(10)
0.03277(9)
0.02183(7)
0.03771(8)
0.06522(8)
0.2129(2)
0.3910(2)
0.1767(2)
0.0958(4)
0.1811(4)
0.1353(3)
0.1396(3)
0.4309(3)
0.4375(3)
0.3694(3)
0.2913(3)
0.2859(3)
0.3341(4)
0.3896(4)
0.3396(4)
0.3962(5)
0.3501(6)
0.1577(3)
0.2550(3)
0.2431(4)
0.1321(4)
0.0342(4)
0.0460(3)
0.1512(6)
0.2347(6)
0.2279(5)
0.1348(5)
0.0470(6)
0.0548(6)
ꢀ0.0879(3)
0.6743(3)
0.5991(4)
0.5319(5)
0.8249(4)
0.8890(4)
ꢀ0.0401(4)
ꢀ0.1050(4)
ꢀ0.0216(4)
0.1170(4)
0.1888(4)
ꢀ0.0053(4)
ꢀ0.0877(5)
ꢀ0.0090(5)
ꢀ0.0908(7)
ꢀ0.0129(7)
0.6403(4)
0.5633(4)
0.5416(5)
0.5948(4)
0.6665(4)
0.6872(4)
0.5471(7)
0.4572(10)
0.4686(8)
0.5643(5)
0.6512(6)
0.6445(6)
4.95(11)
3.69(8)
4.68(10)
5.59(13)
Beq = 8/3
p
2(U11(aaꢄ)2 + U22(bbꢄ)2 + U33(ccꢄ)2 + 2U12(aaꢄbbꢄ)cos + 2U13(aaꢄccꢄ)cos b + 2U23(bbꢄccꢄ)cos
c a.
Comparison in terms of mesomorphic properties between Cu(II)
complexes derived from ligands 4a–4c and the presence of the
Cu(II) complexes 2f–2h reveal that the difference are pronounced.
Cu(II) complexes 2f–2h exhibit enantiotropic SmA phase during
heating and cooling process. Whilst only Cu(II) complexes derived
from ligand 4b show a monotropic SmA phase during cooling pro-
cess. On the other hand, optical observation also reveals that the
SmA phase for Cu(II) complexes derived from ligand 4b exhibits
different size of ‘‘Maltese Crosses’’ texture. These observation are
different from the existence of SmA phase with focal conic fan
shape texture for Cu(II) complexes 2f–2h. Besides, the isotropiza-
tion temperature of Cu(II) complexes 2f–2h are found to be signif-
icantly higher than those complexes derived from ligands 4a–4b.
3.3. The influence of molecular structure on thermal stability
A distinct odd–even effect of the clearing temperature of li-
gands 1a–1f is illustrated in Fig. 5. The values of clearing temper-
ature tend to decrease in a regular, zig-zag fashion with the
number of carbon atoms in alkoxy chain (n) ranges from n = 5 to
n = 12. However, the odd–even effect becomes less distinctive
when the number of carbon in the alkoxyl chain, n is larger than
8. This is due to the increase in flexibility of the terminal alkoxyl
chain [36–38]. These observations strongly show that ligands with
even number of terminal alkoxy chain have higher clearing point
than those with odd numbers. Such pattern of regular change often
been observed in compounds consisting of central mesogenic
structure having variable lengths of terminal alkyl chain [38–40].
Generally, the odd–even effect can be explained from a consid-
eration of molecular structure. Homologous with the even member
alkoxy chain exhibits colinearity to the rigid core segment in the
preferred trans conformation geometry [36,37]. On the other hand,
the odd member alkoxy chain shows non-colinearity to the rigid
core segment [36,37]. The presence of alkoxyl groups of even parity
at the terminal are ‘in line’ with the molecular long axis and there-
fore enhance the molecular anisotropic and result in high clearing
temperature, whereas the odd numbers have the opposite effect
[36,37].
Table 12
Atomic coordinates and Biso involving hydrogen atoms.
Atom
x
y
z
Biso
occ
H1
H3
H4
H6
0.2253
0.2410
0.1972
0.1775
0.1389
0.1371
0.1102
0.1124
0.0893
0.0870
0.0633
0.0607
0.0382
0.0373
0.0223
0.1918
0.1462
0.1520
0.1978
0.0664
0.0216
0.0033
0.0303
0.0766
0.0986
0.4746
0.4873
0.2406
0.3523
0.2414
0.3703
0.4826
0.3582
0.2466
0.4892
0.3762
0.3785
0.2576
0.3845
0.3297
0.3099
0.8990
1.71
1.84
2.00
1.95
2.72
2.72
2.98
2.98
3.80
3.80
5.26
5.26
6.94
6.94
6.94
2.23
3.08
3.14
2.38
7.20
5.94
4.42
5.62
6.71
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
ꢀ0.0981
ꢀ0.2046
0.1649
H8A
H8B
H9A
H9B
H10A
H10B
H11A
H11B
H12A
H12B
H12C
H14
H15
H17
H18
H20
H21
H22
H23
H24
0.1208
ꢀ0.0217
ꢀ0.2135
ꢀ0.0745
0.1170
ꢀ0.0232
ꢀ0.0789
ꢀ0.2163
0.1080
ꢀ0.0168
ꢀ0.0794
0.5257
0.4907
0.7017
0.7333
0.3868
0.4093
0.5698
0.7155
0.7051
ꢀ0.0412
ꢀ0.0223
0.2973
0.2887
0.1315
From Fig. 5, a general trend can be noted. The transition temper-
atures 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. This is due to the
ꢀ0.0184
ꢀ0.0044

80
G.-Y. Yeap et al. / Journal of Molecular Structure 999 (2011) 68–82
Table 14
Bond lengths (Å).
significant higher molecular weight upon complexation and the
increasing number of interacting site [3]. Besides, the clearing tem-
peratures of Cu(II) complexes increase by 60–70 °C upon complex-
ation indicate that the introduction of metal atom increases the
isotropic temperature of the corresponding ligands. These phe-
nomena can be associated with the thermal stability upon
coordination.
Atom
Atom
Distance
Atom
Atom
Distance
Cu1
Cu1
O1
O2
N1
N2
C1
C2
C4
O1
N1
C7
C8
1.8932(19)
2.022(3)
1.309(4)
1.433(4)
1.427(4)
1.572(6)
1.424(4)
1.421(4)
1.405(4)
1.418(4)
1.513(5)
1.513(6)
1.393(5)
1.402(6)
1.390(5)
1.392(8)
1.380(7)
1.412(6)
Cu1
Cu1
O2
N1
N2
N3
C2
C3
C5
O11
N11
C5
C1
N3
C19
C31
C4
C6
C9
1.8932(19)
2.022(3)
1.360(4)
1.309(4)
1.129(6)
1.590(7)
1.415(4)
1.371(4)
1.378(4)
1.507(5)
1.513(5)
1.400(4)
1.380(5)
1.384(5)
1.353(8)
1.366(7)
1.370(7)
C13
C16
C2
C71
C5
3.4. X-ray diffraction study on smectic A phase for ligands 1a–1e and
Cu(II) complexes 2a–2e
C6
C9
C7
C8
C10
C12
C18
C16
C18
C24
C22
C24
C10
C13
C14
C16
C19
C20
C22
C1
C11
C13
C15
C17
C19
C21
C23
C14
C15
C17
C20
C21
C23
In order to investigate the liquid crystalline structure of ob-
tained materials the X-ray studies have been carried out for aligned
samples. Ligands 1a–1e and their corresponding Cu(II) complexes
2a–2e show mesomorphism with SmA phase below isotropic
phase as confirmed by X-ray diffraction, thus they show similar
X-ray patterns. Low angle Bragg reflections further confirmed the
existence of layers (Fig. 6). The diffused signal at equatorial posi-
tion reflects the lack of molecular long range order inside each
smectic layer or the lateral distribution of the molecules within
each layer is in random manner. For ligands 1a–1e the layer spac-
ing increases by 1.25 Å (Fig. 7) with the elongation of alkoxyl chain
length by one carbon atom. However for their corresponding Cu(II)
complexes 2a–2e, the layer spacing increases only by 1.04 Å
(Fig. 7). The layer spacing for the ligands corresponds well to their
molecular length while the layer thickness of the complexes is sig-
nificantly smaller as compared to their molecular length. This
show the presences of some interdigitation of molecular alkoxy
chains between neighboring layers. The presences of the interca-
lated structure is resulted due to the molecular shape. The complex
molecule has tri-cylindrical shape (Fig. 8) with a much larger cross
section for the mesogenic core than the molecular tails when it ro-
tate. This shows the contrary between the tri-cylindrical shape
with the rod-like ligands. In order to fill the space efficiently the
Symmetry operators: (1) ꢀx + 1/2, ꢀy + 1/2, ꢀz + 1.
molecules interdigitate their tails when they are packed into
layers.
3.5. Crystal structure of Cu(II) complex 2f
The molecular structure with the atom-numbering scheme and
packing of Cu(II) complex 2f are shown in Figs. 9 and 10, respec-
tively. Crystal data, intensity measurement and structure solution
and refinement of Cu(II) complexes are collated in Tables 8–10,
respectively. The atomic coordinates, Biso/Beq, Biso involving hydro-
gen atoms and anisotropic displacement parameter are given in
Tables 11–13, respectively. Tables 14 and 15 show the selected
bond lengths and bond angles whereas Tables 16 and 17 show
the bond lengths and bond angles involving hydrogen.
Table 13
Anisotropic displacement parameters.
Atom
U11
U22
U33
U12
U13
0.0002(2)
0.0005(8)
ꢀ0.0017(8)
0.0028(9)
ꢀ0.016(2)
0.033(3)
0.0046(10)
0.0028(10)
0.0021(11)
ꢀ0.0010(11)
ꢀ0.0009(10)
0.0034(11)
ꢀ0.0001(10)
0.0001(12)
0.0003(13)
ꢀ0.001(2)
0.006(2)
U23
0.0015(2)
0.0028(8)
0.0018(9)
ꢀ0.0002(9)
ꢀ0.006(2)
0.002(2)
Cu1
O1
O2
N1
N2
N3
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
C11
C12
C13
C14
C15
C16
C17
C18
C19
C20
C21
C22
C23
C24
0.0190(3)
0.0226(10)
0.0207(10)
0.0207(11)
0.107(4)
0.137(4)
0.025(2)
0.0207(13)
0.025(2)
0.026(2)
0.0205(13)
0.024(2)
0.025(2)
0.025(2)
0.024(2)
0.022(2)
0.022(2)
0.023(2)
0.0175(13)
0.025(2)
0.027(2)
0.022(2)
0.026(2)
0.026(2)
0.035(3)
0.033(3)
0.035(3)
0.020(2)
0.036(2)
0.039(3)
0.0183(3)
0.0243(10)
0.0301(11)
0.0170(11)
0.041(2)
0.036(2)
0.0141(13)
0.0148(12)
0.0167(13)
0.020(2)
0.022(2)
0.021(2)
0.0182(13)
0.036(2)
0.034(2)
0.061(3)
0.066(3)
0.129(5)
0.028(2)
0.028(2)
0.049(3)
0.065(3)
0.048(2)
0.028(2)
0.123(5)
0.089(4)
0.072(3)
0.071(3)
0.095(4)
0.126(5)
0.0137(3)
0.0162(10)
0.0225(10)
0.0172(11)
0.031(2)
ꢀ0.0013(2)
ꢀ0.0028(8)
0.0027(9)
0.0013(9)
0.028(2)
ꢀ0.007(3)
ꢀ0.0008(10)
0.0013(10)
ꢀ0.0001(10)
0.0005(11)
0.0010(11)
ꢀ0.0022(11)
0.0023(11)
ꢀ0.0019(13)
0.0014(13)
ꢀ0.006(2)
0.006(2)
ꢀ0.012(3)
0.0013(11)
0.0055(11)
0.0162(13)
ꢀ0.001(2)
ꢀ0.010(2)
ꢀ0.0034(12)
ꢀ0.030(3)
ꢀ0.015(3)
ꢀ0.008(2)
ꢀ0.001(2)
0.011(3)
0.044(2)
0.0155(12)
0.0141(12)
0.0165(13)
0.0173(13)
0.0170(13)
0.0169(13)
0.0120(12)
0.025(2)
0.036(2)
0.037(2)
0.079(3)
0.067(3)
0.0157(12)
0.017(2)
0.021(2)
0.027(2)
0.026(2)
0.021(2)
0.058(3)
0.110(5)
0.084(4)
ꢀ0.0026(10)
ꢀ0.0013(10)
0.0007(11)
0.0011(11)
ꢀ0.0051(11)
ꢀ0.0014(11)
ꢀ0.0032(11)
0.0013(13)
ꢀ0.002(2)
ꢀ0.001(2)
0.000(3)
ꢀ0.003(3)
ꢀ0.0034(11)
ꢀ0.0034(11)
ꢀ0.0113(13)
ꢀ0.014(2)
ꢀ0.009(2)
ꢀ0.0021(12)
ꢀ0.043(3)
ꢀ0.009(4)
0.000(3)
0.001(2)
0.0036(10)
ꢀ0.0000(12)
ꢀ0.0034(13)
0.0040(13)
0.0053(12)
0.0036(11)
0.022(2)
0.027(3)
0.021(3)
0.007(2)
0.010(2)
0.049(3)
0.049(3)
0.045(3)
ꢀ0.003(2)
0.009(3)
0.032(3)
ꢀ0.010(2)
ꢀ0.018(3)

G.-Y. Yeap et al. / Journal of Molecular Structure 999 (2011) 68–82
81
Table 15
Bond angles (°).
Table 17
Bond angles involving hydrogens (°).
Atom Atom
Atom Angle
Atom Atom
Atom Angle
Atom
Atom
Atom
Angle
Atom
Atom
Atom
Angle
O1
O1
Cu1
Cu1
Cu1
O1
N1
N1
N3
C2
C2
C4
O11
N11
N11
C7
180.00(13)
O1
Cu1
Cu1
Cu1
O2
N1
N
C1
C2
C3
C5
N1
88.48(9)
N1
C1
C3
C4
C6
C8
C8
C8
C9
H1
H3
H4
H6
117
119
121
120
110
110
109
109
109
109
109
108
109
109
109
109
109
120
120
120
120
120
119
120
120
121
C2
C4
C5
C7
O2
C9
C8
C10
H9A
C9
C11
C10
C12
H11A
C11
H12A
H12B
C15
C16
C18
C17
C21
C22
C23
C24
C23
C1
C3
C4
C6
C8
C8
C9
C9
H1
H3
H4
H6
117
119
121
120
110
110
109
109
108
109
109
109
109
108
109
109
109
120
120
120
120
120
119
120
120
121
O11
N1
N1
91.52(9)
180.00(12)
119.0(3)
123.81(17)
99.8(4)
C21
C3
91.52(9)
88.48(9)
123.99(18)
120.68(18)
115.4(3)
100.7(5)
117.8(3)
119.2(3)
118.9(3)
124.8(3)
120.5(3)
119.2(3)
107.3(3)
112.8(4)
119.3(3)
119.3(3)
119.7(3)
108.8(3)
120.2(4)
112.2(5)
121.5(5)
121.4(5)
119.4(5)
O11
Cu1
Cu1
C1
N11
C8
C5
Cu1
N3
C5
O2
C9
H8A
C8
C1
C13
C16
C2
H8A
H8A
H8B
H9B
H9B
H10A
H10A
H10B
H11B
H11B
H12A
H12C
H12C
H14
H15
H17
H18
H20
H21
H22
H23
H24
H8B
H8B
H9A
H9A
H9B
H10B
H10B
H11A
H11A
H11B
H12B
H12B
H12C
H14
H15
H17
H18
H20
H21
H22
H23
H24
C13
C19
C31
C71
C5
C6
C7
C6
C9
C11
C14
C18
C16
C17
C18
C20
C24
C22
C24
N2
N1
126.7(3)
123.0(3)
121.5(3)
113.9(3)
121.3(3)
122.5(3)
118.3(3)
112.9(3)
114.3(4)
121.3(3)
120.5(3)
131.1(4)
120.0(3)
120.1(3)
126.3(4)
119.3(6)
119.9(4)
118.5(5)
C1
C1
C71
C4
C31
C3
C21
O2
C10
C9
C9
C9
C4
C10
C10
C10
C11
C11
C12
C12
C12
C14
C15
C17
C18
C20
C21
C22
C23
C24
C10
C10
C11
C11
C11
C12
C12
C12
C14
C15
C17
C18
C20
C21
C22
C23
C24
O2
C5
O1
O2
C9
N1
C14
C14
N2
C16
N3
C20
C20
C22
C5
C6
C7
C8
C4
C5
C7
C7
C9
C6
C11
H10A
C10
C12
C11
C11
H12A
C13
C14
C16
C13
C19
C20
C21
C22
C19
O1
C21
C6
C21
C8
C10
C12
C18
C15
C15
C17
C17
C24
C21
C23
C23
C10
C13
C13
C15
C16
C17
C19
C19
C21
C23
C10
N1
C13
N2
C15
C13
N3
C19
C21
C19
C11
C13
C14
C16
C16
C18
C19
C20
C22
C24
Symmetry operators: (1) ꢀx + 1/2, ꢀy + 1/2, ꢀz + 1.
Symmetry operators: (1) ꢀx + 1/2, ꢀy + 1/2, ꢀz.
Table 16
2.006 (CuAN) and 1.893 (CuAO) for copper(II) complexes derived
from 2-[(Z)-[furan-2-ylmethyl]imino]methyl]-6-methoxyphenol
[42]. The elongated CuAN bonds have also been observed for other
Schiff base complexes [41]. Thus, the Cu(II) complex of 2f can also
be described as distorted square planar geometries as evident from
the CuAN1(2.022 Å) bond being slightly longer than the corre-
sponding CuAO1(1.893 Å) bond.
Bond lengths involving hydrogens (Å).
Atom
Atom
Distance
Atom
Atom
Distance
C1
C4
C8
C9
C10
C11
C12
C12
C15
C18
C21
C23
H1
H4
0.950
0.950
0.990
0.990
0.990
0.990
0.980
0.980
0.950
0.950
0.950
0.950
C3
C6
C8
C9
C10
C11
C12
C14
C17
C20
C22
C24
H3
H6
0.950
0.950
0.990
0.990
0.990
0.990
0.980
0.950
0.950
0.950
0.950
0.950
H8A
H9A
H10A
H11A
H12A
H12C
H15
H18
H21
H23
H8B
H9B
H10B
H11B
H12B
H14
H17
H20
H22
H24
4. Conclusion
Two new series of Cu(II) and Ni(II) complexes derived from 4-
(4-alkoxy-2-hydroxybenzylideneamino)azobenzene homologous
have been successfully synthesized and characterized by 13C
NMR, 1D and 2D 1H NMR. Mesomorphic behavior is found to be
controlled by the number of carbon atoms in alkoxy chain (n)
and nature of central metal ion. All ligands and Cu(II) complexes
show liquid crystalline character while all Ni(II) complexes are
non-mesomorphic. XRD studies confirmed the existence of SmA
phase for ligands and Cu(II) complexes. The X-ray crystallographic
confirmed the square planar geometry of bis[4-(4-alkoxy-2-
hydroxybenzylideneamino)azobenzene]copper(II) complex.
Fig. 10 shows the discrete molecule of Cu(II) complex 2f consist-
ing of two ligands coordinated to one Cu(II) metal. This crystal
structure further confirmed that the azo moties does not involve
in coordination which is in agreement with FTIR spectra and ele-
mental analysis data.
The Cu(II) complexes crystalline into C-centered monoclinic cell
with a space group C2/c with unit cell paraneters a = 51.0443(3) Å,
b = 10.5834(6) Å, b = 94.993(4)°, V = 4160.5(5) Å3. Table 12 shows
that the cisoid angles of N1ACuAO1 and O1ꢄACuAN1 are 91.52°
and 88.48°, respectively. While both the transoid angles of
O1ꢄACuAO1 and N1ꢄACuAN1 are 180°. The cisoid and transoid an-
gles show that the ligands have an approximately square planar
configuration around the Cu center ion. The cisoid and transoid an-
gles are similar to the corresponding value reported by Ünvera and
Hayvalib which were 91,48°, 88.52° and 180° for copper(II) com-
plexes derived from 2-[(Z)-[furan-2-ylmethyl]imino]methyl]-6-
methoxyphenol [42]. Besides, these observations are similar to
the value (91.62°, 88.38° and 180°) reported by Mandal and Rout
for bis(N-2-hydroxybenzyl-2-furylmethylimine)copper(II) [41].
The bond lengths of CuAN1(2.022 Å) and CuAO1(1.893 Å) are
similar to the values reported by Ünvera and Hayvalib which were
Acknowledgements
The main author (G.-Y. Yeap) would like to thank Universiti
Sains Malaysia for the RU Grant No. 1001/PKIMIA/811127 and
Incentive Grant No. 1001/PKIMIA/822208.
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