Investigation of liquid crystalline behavior of copper(II) complexes derived from Azo-containing salicylaldimine ligands with lateral hydroxyl group

Article abstract of DOI:10.1080/15421400802408360

The one-step metal promoted reaction between 5-(4-alkoxyphenylazo) salicylaldehyde (alkoxy = decyloxy, dodecyloxy, tetradecyloxy) and 2-propanolamine in the presence of Cu (CH₃COO) · 4H ₂O yields new azo-containing salicylaldimine ba

Full text of DOI:10.1080/15421400802408360

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Investigation of Liquid
Crystalline Behavior of
Copper(II) Complexes
Derived from Azo-Containing
Salicylaldimine Ligands with
Lateral Hydroxyl Group
Zolfaghar Rezvani a , Kamellia Nejati b , Masoud
Seyedahmadian a & Baharak Divband a
a Department of Chemistry , Faculty of Science,
Azarbaijan University of Tarbiat Moallem , Tabriz ,
Iran
b Department of Chemistry , Payam Noor University-
Tabriz Center , Emamieh , Tabriz , Iran
Published online: 31 Aug 2012.
To cite this article: Zolfaghar Rezvani , Kamellia Nejati , Masoud Seyedahmadian
& Baharak Divband (2008) Investigation of Liquid Crystalline Behavior of Copper(II)
Complexes Derived from Azo-Containing Salicylaldimine Ligands with Lateral
Hydroxyl Group, Molecular Crystals and Liquid Crystals, 493:1, 71-81, DOI:
10.1080/15421400802408360
To link to this article: http://dx.doi.org/10.1080/15421400802408360
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Mol. Cryst. Liq. Cryst., Vol. 493, pp. 71–81, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802408360
Investigation of Liquid Crystalline Behavior of Copper(II)
Complexes Derived from Azo-Containing Salicylaldimine
Ligands with Lateral Hydroxyl Group
Zolfaghar Rezvani1, Kamellia Nejati2, Masoud
Seyedahmadian1, and Baharak Divband1
1Department of Chemistry, Faculty of Science, Azarbaijan University
of Tarbiat Moallem, Tabriz, Iran
2Department of Chemistry, Payam Noor University-Tabriz Center,
Emamieh, Tabriz, Iran
The one-step metal promoted reaction between 5-(4-alkoxyphenylazo) salicylalde-
hyde (alkoxy ¼ decyloxy, dodecyloxy, tetradecyloxy) and 2-propanolamine in the
presence of Cu (CH3COO) ꢀ 4H2O yields new azo-containing salicylaldimine based
copper(II) bis(chelates) as a result of the [1 þ 1] Schiff base condensation. The
mesogenic properties of these complexes were studied by differential scanning
calorimetry (DSC) and polarizing microscope equipped with a heating and cooling
stage. Copper complexes containing dodecyloxy and tetradecyloxy chain gave
liquid crystalline character, while decyloxy containing copper complex was non-
mesomorphic.
Keywords: azo; copper complexes; metallomesogen; salicylaldimine; schiff bases;
smectic A
INTRODUCTION
Azo-containing polymeric liquid crystal systems and metallomesogens
containing uncomplexed azo moieties have been attracting much
attention recently due to their possible applications in the area of
photon-mode high density data storage and photo-switching devices
[1–5]. In addition, molecules with large conjugated p-systems includ-
ing azo group are very promising active materials for investigation
of nonlinear phenomena [6,7]. The mesogenic properties of dyes are
Address correspondence to Zolfaghar Rezvani, Deparment of Chemistry, Faculty of
Science, Azarbaijan University of Tarbiat Moallem, Tabriz, Iran. E-mail: z_rezvani@
yahoo.com
71

72
Z. Rezvani et al.
of great importance for nonlinear applications. Consequently, the kind
of azo dye molecules that are used as side chain moieties in polymers
are of great attraction, and hence are currently subject of intensive
research [8–10].
We recently reported the synthesis and liquid crystalline character
of Cu(II) bis(chelates) based on azo-linked bidentate salicylaldimine
[11–13] and tetra dentate azo-linked N,N0 salicylideneiminato Ni(II),
Cu(II), and VO(IV) complexes [14]. More metallomesogens containing
uncomplexed azo moieties are also reported by other research groups
[11–13,15–17]. Furthermore, the liquid crystalline character of copper
complexes derived from 5-((4-alkoxyphenyl)azo)-N-(ethanol)-salicylal-
dimine homologues as the first example of azo-containing metallome-
sogens with a lateral OH group are also reported by our group [18].
These copper complexes can be attached as an azo-containing meso-
genic side chain to non-liquid crystalline polymers (for example poly-
urethanes) through OH group to induce liquid crystalline character
to these polymers [19].
The goal of this project was to develop new liquid crystalline copper
complex dyes with lateral hydroxyl group capable of attaching as a
side chain to polymers. In this work we report the synthesis and liquid
crystalline properties of new copper complex homologues named bis
[5-((4-nalkoxyphenyl) azo)-N-(2-propanol) salicylaldiminato] copper
(II) complex (see Scheme 1).
SCHEME 1 Synthetic route to the copper complexes.

Investigation of Liquid Crystalline Behavior
73
EXPERIMENTAL
Reagents
All reagents and solvents were used as supplied by Merck chemical
company and used without further purification. 4-Alkoxynitrobenzene
homologues were obtained by reaction between 4-nitrophenol
with 1-bromooctan,1-bromododecane or 1-bromotetradecane in n,n-
dimethylformamide as solvent and K2CO3 as base by refluxing for
three hrs [20], and then crude 4-alkoxy nitro benzene homologues
were purified by recrystallization from ethanol. 4-Alkoxy aniline
homologues prepared by reducing of the corresponding 4-alkoxynitro-
benzene as described in the literature [21].
Physical Measurements
Elemental (C, H, and N) analyses were carried out on a Perkin–Elmer
automatic equipment model 240B. Electron impact (70 eV) mass spectra
were recorded on a Finnegan-mat GC-MS-DS spectrometer model 8430.
Infrared spectra were taken with an FT-IR Bruker, vector 22
spectrometer using KBr pellets in the 400–4000cmꢁ1 range. The DSC
thermograms of the compounds were obtained on a Mettler–Toledo
DSC 822e module, which was calibrated with indium metal
(T ¼ 156.6 ꢂ 0.3, DH ¼ 28.45 ꢂ 0.6 J gꢁ1). Samples of 2–5 mg in solid form
were placed in aluminum pans (40 ml) with a pierced lid, and heated or
cooled at a scan rate of 10ꢃC minꢁ1 under a nitrogen flow. Thermal
Gravimetric Analyses (TGA) was carried out on a Mettler–Toledo TGA
851e at a heating rate of 10ꢃC minꢁ1 under a nitrogen atmosphere. The
optical observations were made with a Zeiss polarizing microscope
equipped with a Linkam THMSG 600 heating and cooling stage and Lin-
kam THMS 93 programmable temperature-controller. 1H NMR spectra
were obtained in deutrated chloroform as solvent on a Bruker FT-
NMR AC-400 (400 MHz) spectrometer. All chemical shifts are reported
in d(ppm) relative to the tetramethylsilane as internal standard.
Materials
All homologue materials were prepared similarly.
5-(4-Decyloxyphenylazo) Salicylaldehyde (1a). This compound
was prepared as described in the literature [22]. Yellow, yield 80%, mp
126ꢃC. MS m=z (relative intensity): 355.4 (M þ 1, 15), 354.3 (M, 40),
241.2 (M-C10H21, 25), 121.0 (M-C10H21OC6H4N2, 100). Anal. Calc. for
C23H30N2O3: C 71.16, H 7.39, N 7.90. Found: C 70.71, H 7.12 N 7.54.

74
Z. Rezvani et al.
1HNMR (400 MHz, CDCl3) d 11.27 (s, H-8), 10.02 (s, H-9), 8.18
(d, J 3.2 Hz, H-3), 8.14 (dd, J 3.1, 8.4 Hz, H-2), 7.91 (dd, J 3.1,
8.2 Hz, H-4, H-7), 7.10 (d, J 8.7, H-1), 7.01 (dd, J 3.6, 8.3 Hz, H-5,H-
6), 4.05 (t, J 7.1 Hz, H-10), 1.81–1.00 (15H, alkyl chain).
5-(4-Dodecyloxyphenylazo) Salicylaldehyde (1b). Yellow, yield
80%, mp 123ꢃC.MS m=z (relative intensity): 411.6 (M þ 1,15), 410.6
(M, 45), 242.5 (M-C12H25, 25), 121.6 (M-C12H25OC6H4N2, 100). Anal.
Calc. for C25H34N2O3: C 73.15, H 8.35, N 6.83. Found: C 72.73, H
1
8.14 N 6.47. HNMR (400 MHz, CDCl3) d 11.26 (s, H-9), 10.02 (s, H-
8), 8.15 (d, J 2.8 Hz, H-3), 8.13 (dd, J 2.9, 8.2 Hz, H-2), 7.89 (dd, J
3.0, 7.9 Hz, H-4, H-7), 7.11 (d, J 8.1 Hz, H-1), 7.01 (dd, J 3.2, 7.9 Hz,
H-5, H-6), 4.04 (t, J 6.7 Hz, H-10), 1.84-0.87 (23 H, alkyl chain).
5-(4-Tetradecyloxyphenylazo) Salicylaldehyde (1c). Yellow,
yield 80%, mp 123ꢃC. MS m=z (relative intensity): 411.6 (M þ 1,15),
410.6 (M, 45), 242.5 (M-C14H29, 25), 121.6 (M-C12H25OC6H4N2, 100).
Anal. Calc. for C27H38N2O3: C 73.15, H 8.35, N 6.83. Found: C 72.73,
1
H 8.14 N 6.47. HNMR (400 MHz, CDCl3) d 11.26 (s, H-9), 10.02 (s,
H-8), 8.15 (d, J 2.8 Hz, H-3), 8.13 (dd, J 2.9, 8.2 Hz, H-2), 7.89 (dd, J
3.0, 7.9 Hz, H-4, H-7), 7.11 (d, J 8.1 Hz, H-1), 7.01 (dd, J 3.2, 7.9 Hz,
H-5, H-6), 4.04 (t, J 6.7 Hz, H-10), 1.84–0.87 (23 H, alkyl chain).
Syntheses of the Copper Complexes. Copper complexes were
prepared in similar manner using the method described elsewhere
[23]. Thus, an alcoholic solution containing 10 mmol of 2-propanol
amine was added to a solution of 5-alkoxyphenylazo salicylaldehyde
(10 mmol) in 50 ml of boiling absolute ethanol. Then 10 mmol sodium
acetate in 10 ml water was added to the above reaction system. Finally,
50 ml boiling ethanol solution containing 10 mmol Cu(CH3COO)2 ꢀ H2O
was added. The reaction mixture was kept boiling for 30 min. The
crystals precipitated from the solution, and recrystallization was
performed from the ethano=chloroform (1:1 v=v) solution. Copper
complexes were obtained as brown microcrystals. The microcrystals
were filtered off, washed with absolute ethanol, and then recrystal-
lized from ethanol-chloroform (1:3 v=v).
2a:Brown, yield 85%. Anal. Calc. for C52H72N6O6Cu: C 66.42, H 7.66,
N 8.94. Found: C 66.05, H 7.26, N, 8.52.
2b:Brown, yield 80%. Anal. Calc. for C56H80N6O6Cu: C 67.51, H 8.04,
N 8.44. Found: C 67.14, H 7.75, N, 8.14.
2c: Brown, yield 75%. Anal. Calc. for C60H88N6O6Cu: C 68.39, H 8.45,
N 7.97. Found: C 68.42, H 8.11, N 7.80.

Investigation of Liquid Crystalline Behavior
75
RESULTS AND DISCUSSION
Synthesis
5-(4-alkoxyphenylazo) salicylaldehyde homologues (1a–c; see
Scheme 1) were synthesized in a three-step process as described else-
where [14], in which the hydroxy group in 4-nitrophenol is first
replaced by an alkoxy chain followed by reductions of nitro group to
amine. In the third step, salicylaldehyde coupled with the diazonium
chloride obtained from the 4-alkoxyaniline. 1a–c were purified by
repeated crystallization from ethanol to give a single spot on thin layer
chromatography and characterized by IR,1HNMR, mass spectroscopy,
and elemental analyses. Copper complexes were obtained in good yield
and purity, and characterized by C, H, N elemental analysis and IR
spectroscopy. These copper compounds are all paramagnetic, and their
1HNMR spectra display only broad alkoxy signals. All other proton
signals close to the paramagnetic copper centers are unobserved.
The elemental analysis and spectroscopic data for new compounds
gave satisfactory results. Some physical and characterization data
for 5-(4-alkoxyphenylazo) salicylaldehyde homologues, and copper
complexes are given in the experimental section and selected IR data
are tabulated in Table 1.
The IR spectral frequencies of synthesized compounds were carried
out using KBr pellets as described in the experimental section. In the
compounds 1a–c, the hydroxyl group was observed at 3190 cmꢁ1
because the intramolecular hydrogen bonding between OH and formyl
group in compound 1 leads to decreasing the stretching frequency of
OH, and the carbonyl group was observed at 1666 cmꢁ1 [13]. In the
IR spectra of copper complexes(2a–c), the phenolic OH group disap-
peared, and free lateral OH group was observed as a broad peak cen-
tered at 3240–60 cmꢁ1 [23]. In addition, the C N group was observed
=
at 1623 cmꢁ1. It is well characterized that the C N stretching
=
TABLE 1 Selected IR Data for Schiff-Base Ligands and Metal Complexes
n=cmꢁ1
C-H
C-H
=
C N
=
Compound
O-H
(aromatic) (aliphatic)
C-O (etheric) C O
1(a–c)
2(a–c)
3185–90(br,s) 3069–71(m) 2850–2950(s)
3240–3260(br,s) 3040–50(m) 2920,2851(s) 1620–3(s) 1244–75(s)
–
1241 1665-6(s)
–
s: strong, m: medium, br: broad.

76
Z. Rezvani et al.
frequency in bidentate salicylidenic schiff base ligands with N-alkyl
structure appears at 1635–1640 cmꢁ1 region [11–14,24], so the appear-
ance of C N at 1623 cmꢁ1 (in the lower wave numbers than free schiff
=
=
base ligands) in copper complexes demonstrates that the C N has
been formed by [1 þ 1] condensation reaction and coordinated to the
copper center. On the other hand, the lack of phenolic OH group in
copper complexes indicates that the OH group has been deprotonated
and coordinated to the metal ion as ꢁOꢁ. Reduction of the double bond
=
character of the C N bond, which caused by the coordination of nitro-
gen into the metal center and is in agreement with the results
obtained from the other similar complexes described previously
[11–14,24]. Based on these observations and elemental analysis
results, we concluded that the Schiff-base ligands are coordinated to
metal atoms as bidentate (N-O) ligands in 2:1 ratio.
On the basis of literature data for copper complexes with similar
coordination environment, the OH group is free of coordination [23]
and has not been bonded to the metal center. This fact may be related
to the strong John–Teller effect in d9 Cuþ2, which avoid the coordi-
nation of OH to the metal center. Therefore, it is suggested that the
Cu(II) have square planar or nearly square planar coordination
according to the common stereochemistry of this kind of compounds
[25].
Mesomorphism
The thermal stability of these complexes was checked by thermogravi-
metric analysis (TGA) at first. All other thermal characterizations
were then carried out below the decomposition temperature. Figure 1
shows the TGA and DSC thermograms of 2a as a representative
example. All the copper complexes have similar TGA thermograms
at a heating rate of 10ꢃC=min and illustrates that decomposition starts
at 250ꢃC.
The liquid crystalline character of 5-(4-nalkoxyphenylazo) salicylal-
dehyde homologues (1a–c) and Cu(II) complexes were investigated by
polarizing optical microscopy (POM) observations using a heating–
cooling stage. The phase transition temperatures and enthalpies
were measured by carrying out DSC thermal analysis. Phase tran-
sition temperatures along with the corresponding enthalpy values
for copper complexes are summarized in Table 2 Aldehyde homolo-
gues (1a–c) did not show any liquid crystalline character. These
compounds clearly melted and transformed into isotropic liquids
and no mesophase was observed on the heating or cooling cycle on
optical observations.

Investigation of Liquid Crystalline Behavior
77
FIGURE 1 TGA and DSC thermograms of 2a.
Figure 2 illustrates the DSC curves of copper complexes for the
period of the first heating cycle. Two endothermal peaks were
observed for 2a at 162.8ꢃC and 184.2ꢃC. The peak at 162.8ꢃC
(DH ¼ 20.95 kJ molꢁ1) corresponds to a solid–solid transition, and the
one at 184.2ꢃC is assigned to a crystal-isotropic liquid transition.
The compounds 2b and 2c exhibit one endothermic peak at 165.0ꢃC
and 157.9ꢃC, respectively. These peaks refer to a crystal-isotropic
liquid transition. These compounds clearly melted and transformed
TABLE 2 Transition Temperatures and Enthalpy Changes of Copper(II)
Complexes
Compound
Transitiona
Tb=ꢃC
DHb=KJ molꢁ1
2a
Cr1---Cr2
Cr2---I
162.8
184.2
(157.7)
165.0
(152.1)
(146.1)
157.9
(145.5)
(132.2)
20.95
59.78
48.65
86.35
4.36
36.91
64.36
2.17
(I---Cr)c
Cr---I
2b
2c
(I---SmA)
(SmA---Cr)
Cr---I
(I---SmA)
(SmA---Cr)
42.00
aCr: Crystal, SmA: Smectic A, I: Isotropic liquid.
bData obtained from first heating DSC cycle.
cData in parenthesis obtained from first cooling DSC cycle.

78
Z. Rezvani et al.
FIGURE 2 DSC therograms of copper complexes for the period of the first
heating cycle. Heating and cooling rate: 10 K minꢁ1
.
into isotropic liquids, and no mesophase was observed on the heating
cycle, neither on optical observations nor in DSC measurements. The
melting points of the complexes decrease with increasing the alkyl
chain length (n, see Scheme 1).
Figure 3 shows the DSC curves of copper complexes in the first cool-
ing cycle. Only one exothermal peak was observed for 2a at 157.7ꢃC
(DH ¼ 48.65 kJ molꢁ1) which corresponds to an isotropic liquid–solid
transition. Compound 2b exhibits two exothermal peaks at 152.1ꢃC,
146.1ꢃC and compound 2c at 145.5ꢃC, 132.2ꢃC. The peaks at 152.1
(DH ¼ 4.36 kJ molꢁ1) and 145.5 (DH ¼ 2.17 kJ molꢁ1) are corresponding
to isotropic liquid–mesophase transitionsꢃfor 2b and 2c, but the peaks
at 146.1 (DH ¼ 36.91 kJ molꢁ1) and 132.2 C (DH ¼ 42.00 kJ molꢁ1) are
corresponding to mesophase–solid transitions for 2b and 2c, respecti-
vely. The mesophase was also studied by hot-stage polarized optical
microscopy (POM). As an example, the texture under crossed polari-
zers for 2c is shown in Figure 4. As it is apparent from Figure 4, a typi-
cal batonnet texture was observed on cooling from the isotropic liquid.
Based on the optical observations and DSC measurements, the meso-
phase structure assigned to a monotropic smectic A phase [26].
The copper complexes reported in this work are the further
examples of metallomesogens with two ring salicylidenic Schiff-base
ligand system with an azo moiety and a polar OH on the ligands
(N-CH2 CH(OH)CH3 ). Some other mesogenic copper complexes with
similar structure to that of the title complexes of this work have been

Investigation of Liquid Crystalline Behavior
79
FIGURE 3 DSC therograms of copper complexes for the period of the first
cooling cycle. Heating and cooling rate: 10 K minꢁ1
.
reported. We have reported the liquid crystalline properties of
bis[5-((4-alkoxyphenyl) azo)-N- (ethanol)-salicylaldiminato]copper(II)
complex homologues. These complexes exhibit enantiotropic smectic
C (thermodynamically stable) liquid crystalline character [14]. We also
FIGURE 4 Optical texture observed for compound 2c through crossed
polarizers at 140ꢃC.

80
Z. Rezvani et al.
have reported the liquid crystalline character of bis[5-((4-alkoxyphe-
nyl)azo)-N- (alkyl)-salicylaldiminato] copper(II) complex homologues
[12]; metallomesogens which are based on two ring salicylidenic
Schiff-base ligand with an azo moiety and an alkyl group on the
ligands (N-alkyl). These complexes exhibit monotropic nematic (ther-
modynamically unstable) liquid crystalline character. Furthermore,
Hoshino [27], Cruso [28–30], Marcos [31,32], and Barbera [33] have
reported the nematic liquid crystalline properties of bis [4-((4-alkoxy)
benzoyloxy)-salicylaldimine-N-alkyl] copper(II) complexes. In com-
parison with similar complexes with no lateral hydroxyl group, the
complexes with an OH lateral group reported in this study exhibit a
high order mesophase (SmA). The comparison of the mesomorphic
behavior of N-CH2CH(OH)CH3 complexes with N-alkyl analogous
reveals the importance of the lateral OH group on the mesomorphic
character. Due to this, we can conclude that in this type of metallome-
sogenic systems, the presence of the OH group on the ligand consider-
ably improves the liquid crystalline behavior from monotropic nematic
to monotropic smectic A phase. This fact may be related to the attract-
ive intermolecular interactions between HO---Cu and=or HO---HO.
Meanwhile, the comparison of liquid crystalline character of N-CH2
CH(OH)CH3 with N-CH2CH2OH complexes shows that due to branch-
ing of lateral group, the order and thermodynamic stability of
mesophase have been reduced from enantiotropic smectic C to
monotropic smectic A.
CONCLUSION
In this work we synthesized bis[5-((4-alkoxyphenyl)azo)-N-(2-
propanol) salicylaldiminato] copper(II) complex homologues. Their
mesomorphic properties were examined and compared with copper
complexes reported previously. Decyloxy containing copper complexes
do not show mesomorphic properties but other compounds exhibit
monotropic smectic A mesophase. Mesomorphic temperature range
was found to be controlled by side chain length (n) and increases with
increasing of n. The lateral hydroxyl group had an important role in
thermodynamic stability and order of mesophase.
ACKNOWLEDGMENT
The authors sincerely appreciate the financial support from
Azarbaijan University of Tarbiat Moallem research Vice Chancellor
Office and Faculty of Basic Science.

Investigation of Liquid Crystalline Behavior
81
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