Syntheses, characterization and glass-forming properties of new bis[5-((4-ndodecyloxyphenyl)azo)-N-(4-nalkoxyphenyl)- salicylaldiminato]nickel (II) complex homologues

Article abstract of DOI:10.1016/j.poly.2005.03.096

A series of bidentate Schiff base ligands, 5-((4- ⁿdodecyloxyphenyl)azo)-N-(4-ⁿalkoxyphenyl) -salicylaldimine (ⁿalkoxy = octyloxy, dodecyloxy, hexadecyloxy) homologues, have been synthesized and characterized by IR, NMR, mass spectroscopy and elemental analyses. Nickel (II) complexes of these ligands were synthesized and characterized by elemental analyses, ¹H NMR and IR spectroscopy. DSC measurements, X-ray powder diffraction experiments and optical polarizing microscopy studies indicated that the Ni complexes have a resistance to crystallization and give an amorphous glassy state but after annealing at room temperature for several days transform to an anisotropic glass.

Full text of DOI:10.1016/j.poly.2005.03.096

Polyhedron 24 (2005) 1461–1470
www.elsevier.com/locate/poly
Syntheses, characterization and glass-forming properties of new
bis[5-((4-ndodecyloxyphenyl)azo)-N-(4-nalkoxyphenyl)-
salicylaldiminato]nickel (II) complex homologues
a,
a
b
a
Zolfaghar Rezvani *, Amir Reza Abbasi , Kamellia Nejati , Masoud Seyedahmadian
a
Department of Chemistry, Faculty of Science, Azarbaijan University of Tarbiat Moallem, Km 35 Tabriz-Marageh Road, Tabriz, Iran
b
Department of Chemistry, Payam Noor University-Tabriz Center, Emamieh, Hakim Nezami Street, Tabriz, Iran
Received 10 February 2005; accepted 22 March 2005
Available online 13 June 2005
Abstract
A series of bidentate Schiff base ligands, 5-((4-ndodecyloxyphenyl)azo)-N-(4-nalkoxyphenyl)-salicylaldimine (nalkoxy = octyloxy,
dodecyloxy, hexadecyloxy) homologues, have been synthesized and characterized by IR, NMR, mass spectroscopy and elemental
1
analyses. Nickel (II) complexes of these ligands were synthesized and characterized by elemental analyses, H NMR and IR spec-
troscopy. DSC measurements, X-ray powder diffraction experiments and optical polarizing microscopy studies indicated that the Ni
complexes have a resistance to crystallization and give an amorphous glassy state but after annealing at room temperature for sev-
eral days transform to an anisotropic glass.
Ó 2005 Elsevier Ltd. All rights reserved.
Keywords: Glass-forming; Amorphous materials; Schiff bases; Salicylaldimine; Liquid crystals; Nematic; Nickel (II) complexes
1. Introduction
amorphous glasses above room temperature started in
1980 by Shirota [4]. Some molecular glasses based on twin
molecules containing caracole, dihydrocarbazole, pheno-
thiazine and naphthalimide have recently been reported
by Braun and co-workers [5]. Moreover, Braun et al. [6]
have reported low-molecular-weight glassy fulvalenes.
Bazan et al. [7] have used the binaphtyl framework to syn-
thesize glass-forming organic chromophores. Shirota
et al. [8a] have reported 2,3-tetrakis[4-(N-2-naphthyl-N-
phenyl-amino)phenoxy]-substituted phthalocyanines as
a novel class of amorphous molecular materials.
McKeown et al. [8b] have described the syntheses and
glass-forming properties of phthalocyanine-containing
poly(aryl ether) dendrimers.
In recent years, amorphous molecular materials or
molecular glasses have attracted substantial interest
due to their successful application in organic electrolu-
minescent devices as well as photovoltaic, photochromic
and resist materials [1]. These kinds of materials have
excellent processability, transparency and isotropic
properties [1a]. Low molecular weight organic com-
pounds, polymers and molecularly doped polymer sys-
tems, which are able to form stable glasses with glass
transition temperatures above room temperature have
attracted great attention both from the scientific and
application point of view [2,3].
Systematic studies on such stable low molecular
weight organic compounds that readily form stable
Azo-containing photochromic materials have been
attracting a great deal of attention because of their po-
tential technological applications to optical recording
for data storage and optical switching [9]. For example,
the azobenzene-based compounds, containing an
*
Corresponding author. Tel./fax: +98 412 452 4991.
E-mail address: z_rezvani@yahoo.com (Z. Rezvani).
0277-5387/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2005.03.096

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Z. Rezvani et al. / Polyhedron 24 (2005) 1461–1470
azobenzene chromophore, 4-[di(biphenyl-4-yl)amino]-
azobenzene and 4,40-bis[bis(4-tert-butylbiphenyl-4-yl)-
amino]-azobenzene have been used as photochromic
amorphous molecular materials [10]. Other azoben-
zene-based amorphous molecular materials, with a
spiro-linked bifluorene, have been synthesized and their
photoinduced behavior such as optically induced bire-
fringence, diffraction efficiency and surface relief grat-
ings (SRGs) were investigated in relation to their
unique molecular structures [11].
homologues prepared by reduction of the corre-
sponding 4-alkoxynitrobenzene as described in the
literature [16].
2.2. Physical measurements
Elemental (C, H and N) analyses were carried out on
a Perkin–Elmer automatic equipment model 240B. Elec-
tron impact (70 eV) mass spectra were recorded on a
Finnigan-mat GC–MS–DS spectrometer model 8430.
Infrared spectra were taken with a Bruker FT-IR spec-
trometer model vector 22, using KBr pellets in the
400–4000 cmꢀ1 range. The DSC thermograms of the
compounds were obtained on a Mettler–Toledo DSC
822e module, which was calibrated with indium metal
(T = 156.6 0.3, DH = 28.45 0.6 J gꢀ1). Samples of
2–5 mg in the solid form were placed in aluminum pans
(40 ll) with a pierced lid, and heated or cooled at a scan
rate of 10 °C minꢀ1 under a nitrogen flow. TGA was
carried out on a Mettler–Toledo TGA 851e at a heating
rate of 10 °C minꢀ1 under a nitrogen atmosphere. X-ray
powder diffraction patterns were recorded on a Bruker
Based on our knowledge, only a few coordination or
organometallic compounds have been reported as amor-
phous molecular materials. Platinum 5-fluorouridine
green sulfate is a typical example of an amorphous coor-
dination compound [12]. A polycrystalline sample of
titanyl phthalocyanine was vacuum evaporated at ca.
10ꢀ5 Torr to form an amorphous thin film. Photovoltaic
devices consisting of an amorphous thin film of titanyl
phthalocyanine and N,N-dimethyl-3,4:9,10-perylenebis
(dicarboximide), sandwiched between ITO and Au elec-
trodes, demonstrated a response to light over the whole
visible wavelength region from 400 to 900 nm [13]. Shi-
rota and co-workers [8a] have synthesized copper and
titanyl phthalocyanines containing triarylamine moie-
ties. These compounds are soluble in common organic
solvents and readily form stable amorphous glasses with
high glass transition temperatures. Bazan and co-work-
ers [14] have synthesized Iridium complexes with fluo-
rene-modified phenylpyridine ligands that are resistant
to crystallization and can be used in the fabrication of
single layer light emitting diodes. It is worth mentioning
that the introduction of a metal center into organic sys-
tems in order to form coordination or organometallic
molecular materials can introduce extra parameters such
as color, magnetism and birefringence in amorphous
molecular materials. Besides the metal center can im-
prove the electroluminescent properties of coordination
molecular materials.
˚
D8 powder diffractometer (Cu Ka: 1.541 A). The optical
observations were made with a Zeiss polarizing micro-
scope equipped with a Linkam THMSG 600 heating
and cooling stage and Linkam THMS 93 programmable
1
temperature-controller. H NMR spectra were obtained
in deutrated chloroform as solvent on a Bruker
FT-NMR AC-400 (400 MHz) spectrometer. All chemi-
cal shifts are reported in d (ppm) relative to tetrameth-
ylsilane as an internal standard.
2.3. Materials
5-(4-Dodecyloxyphenylazo)salicylaldehyde (1). This
compound was prepared as described in the literature
[17]. Yellow, yield 80%, m.p. 123 °C. MS m/z (relative
intensity): 411.6 (M + 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,
In this work, we report the synthesis and investiga-
tion of glass-forming properties of new bis[5-((4-ndode-
cyloxyphenyl)azo)-N-(4-nalkoxyphenyl)-salicylaldiminato]-
nickel (II) complex homologues (see Scheme 1).
1
6.83. Found: C, 72.73; H, 8.14; N, 6.47%. H NMR
(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 (23H, alkyl chain).
2. Experimental
2.1. Reagents
All reagents and solvents used were supplied by
Merck chemical company and used without further
purification. 4-Alkoxynitrobenzene homologues were
obtained by reaction between 4-nitrophenol with
1-bromoctan or 1-bromododecane in DMF as solvent
and K2CO3 as base by refluxing for 3 h [15] and then
crude 4-alkoxy nitrobenzene homologues were purified
by recrystallization from ethanol. 4-Alkoxyaniline
3. Syntheses of the ligands
All homologue materials were prepared similarly.
Thus, 0.026 mol of the related amine (4-alkoxy aniline)
and 0.026 mol of 5-(4-ndodecyloxyphenylazo)salicylal-
dehyde were dissolved in 100 ml absolute ethanol with
a few drops of glacial acetic acid as a catalyst. The

Z. Rezvani et al. / Polyhedron 24 (2005) 1461–1470
1463
8
H
NH2
CmH2m+1CH2O
O
3
7
4
6
10
Reflux , Ethanol
N
OH
9
N
C11H23CH2O
1
2
5
11
1
14
OCH2CmH2m+1
15
N
8
12
H
13
3
7
6
10
N
OH
9
N
C11H23CH2O
1
2
5
4
L12-m
11
OCH2CmH2m+1
14
15
8
12
H
N
OCH2C11H23
13
Ni
N
O
N
N
3
7
4
6
10
N
O
N
C11H23CH2O
H
1
2
5
CnH2n+1CH2O
Compound No.
Ni12-m
m
L12-8
7
L12-12
L12-16
Ni12-8
Ni12-12
Ni12-16
11
15
7
11
15
Scheme 1.
solution was refluxed for 1 h and then left at room
temperature. After cooling, the ligands were obtained
as yellow micro crystals. The micro crystals were filtered
off, washed with 15 ml of cold absolute ethanol and then
recrystallized several times in ethanol–chloroform (1:3, v/v).
L12-8. Yellow, yield 85%. MS m/z (relative intensity):
614.8 (M + 1,15), 613.8 (M, 65), 443.7 (M ꢀ C12H25,
23), 324.7 (M ꢀ C12H25OC6H4N2, 100). Anal. Calc. for
C39H55N3O3: C, 76.30; H, 9.03; N, 6.85. Found: C,
L12-12. Yellow, yield 87%. MS m/z (relative intensity):
670.7 (M + 1,13), 669.7 (M, 60), 500.6 (M ꢀ C12H25, 25),
380.5 (M ꢀ C12H25OC6H4N2, 100). Anal. Calc. for
C43H63N3O3: C, 78.02; H, 9.59; N, 6.35. Found: C,
1
77.8; H, 9.4; N, 6.2%. H NMR (400 MHz, CDCl3) d
13.99 (s, H-9), 8.73 (s, H-8), 7.97 (dd, J = 3, 6.8 Hz,
H-2), 7.95 (d, J = 2.9 Hz, H-3), 7.88 (dd, J = 3, 6.9 Hz,
H-4, H-7), 7.31 (dd, J = 3.0, 7.8 Hz, H-5, H-6), 7.11
(d, J = 8.8 Hz, H-1), 7.00 (dd, J = 1.9, 8.9 Hz, H-13,
H-15), 6.95 (dd, 1.9, 8.8 Hz, H-12, H-14), 4.04
(t, J = 6.6 Hz, H-10), 3.99 (t, J = 6.5 Hz, H-11), 1.85–
0.87 (46H, alkyl chain).
1
76.0; H, 8.8; N, 6.6%. H NMR (400 MHz, CDCl3) d
13.99 (s, H-9), 8.72 (s, H-8), 7.97 (dd, J = 3, 6.7 Hz,
H-2), 7.95 (d, J = 6.7 Hz, H-3), 7.87 (dd, J = 3, 6.8
Hz, H-4, H-7), 7.30 (dd, J = 3.1, 7.9 Hz, H-5, H-6),
7.10 (d, J = 8.7 Hz, H-1), 6.99 (dd, J = 2, 8.9 Hz,
H-13, H-15), 6.95 (dd, 1.9, 8.8 Hz, H-12, H-14), 4.03
(t, J = 6.5 Hz, H-10), 3.98 (t, J = 6.5 Hz, H-11), 1.85–
0.86 (38H, alkyl chain).
L12-16. Yellow, yield 87%. MS m/z (relative inten-
sity): 726.8 (M + 1,12), 725.8 (M,63), 556.7 (M ꢀ
C12H25, 26), 433.7 (M ꢀ C12H25OC6H4N2, 100). Anal.
Calc. for C47H71N3O3: C, 78.62; H, 9.97; N, 5.85.
1
Found: C, 77.1; H, 9.5; N, 5.5%. H NMR (400 MHz,

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Z. Rezvani et al. / Polyhedron 24 (2005) 1461–1470
CDCl3) d 13.99 (s, H-9), 8.73 (s, H-8), 7.97 (dd, J = 3,
6.8 Hz, H-2), 7.95 (d, J = 6.8 Hz, H-3), 7.89 (dd,
J = 3, 6.8 Hz, H-4, H-7), 7.30 (dd, J = 3.0, 7.6 Hz,
H-5, H-6), 7.12 (d, J = 8.8 Hz, H-1), 7.00 (dd, J = 1.9,
8.8 Hz, H-13, H-15), 6.96 (dd, 2.0, 8.7 Hz, H-12,
H-14), 4.04 (t, J = 6.6 Hz, H-10), 3.99 (t, J = 6.5 Hz,
H-11), 1.85–0.87 (54H, alkyl chain).
Syntheses of the nickel complexes. The nickel com-
plexes were prepared in a similar manner as described
by Nejati et al. [17]. Thus, a solution of 4 mmol of
Ni(CH3COO)2 Æ4H2O in 10 ml of ethanol was added
to an ethanol–chloroform (1:1 v/v) solution containing
8 mmol of ligand, and the resulting solution was re-
fluxed for 2 h. The obtained solution was left at room
temperature. The nickel complexes were obtained as yel-
low-green micro crystals. The micro crystals were fil-
tered off, washed with absolute ethanol and then
recrystallized in ethanol–chloroform (1:3 v/v).
4. Results and discussion
4.1. Synthesis
The Schiff base ligands were synthesized in a four-step
process, in which the hydroxy group in 4-nitrophenol is
first replaced by an alkoxy chain followed by reduction
of nitro group to an amine. In the third step, salicylalde-
hyde is coupled with the diazonium chloride obtained
from the 4-dodecyloxyaniline and finally the Schiff base
ligands were obtained by reaction of 5-(4-dodecyloxyph-
enylazo)salicylaldehyde with an appropriate 4-alkoxyan-
iline (Scheme 1) by refluxing in absolute ethanol, using a
few drops of acetic acid as a catalyst. The Schiff-bases,
2a–2c, were purified by repeated crystallization in an
ethanol/chloroform mixture. The 5-(4-ndodecyloxyphe-
nylazo)salicylaldehyde and ligand homologues were
1
characterized by IR, H NMR, mass spectroscopy and
elemental analyses. The Ni complexes were character-
Ni12-8. Yellow-green, yield 80%. Anal. Calc. for
C78H108N6O6Ni: C, 72.94; H, 8.48; N, 6.54. Found: C,
1
ized by C, H, N elemental analyses, H NMR and IR
1
72.4; H, 8.0; N, 6.2%. H NMR (400 MHz, CDCl3) d
spectroscopy. Some physical and characterization data
for the ligands and the complexes are given in the exper-
imental section and selected IR data are reported in
Table 1. The IR spectra of the nickel complexes show
that the stretching frequency of the C@N bond was
shifted to lower wavenumbers (ca. 15 cmꢀ1) in compar-
ison with the free ligand after coordination. This shift is
due to the reduction of the double bond character of the
C@N bond, which is caused by the coordination of
nitrogen to the metal center and is in agreement with
the results obtained from the other similar complexes
described previously [17–19]. On the other hand, the dis-
appearance of the OH band of the free ligands in the
spectra of the metal complexes indicates that the OH
group has been deprotonated and coordinates to the me-
8.13 (s, H-8), 7.97 (dd, J = 2.9, 6.9 Hz, H-2), 7.94
(d, J = 6.6 Hz, H-3), 7.84 (dd, J = 3, 6.8 Hz, H-4, H-7),
7.28 (dd, J = 3.1, 7.9 Hz, H-5, H-6), 7.13 (d, J = 8.5 Hz,
H-1), 7.00 (dd, J = 2, 8.8 Hz, H-13, H-15), 6.93 (dd, 1.9,
8.7 Hz, H-12, H-14), 4.24 (t, J = 6.1 Hz, H-11), 4.02
(t, J = 6.5 Hz, H-10), 1.81–0.84 (76H, alkyl chain).
Ni12-12. Yellow-green, yield 85%. Anal. Calc. for
C86H124N6O6Ni: C, 74.26; H, 8.95; N, 6.02. Found: C,
1
73.9; H, 8.6; N, 5.8%. H NMR (400 MHz, CDCl3) d
8.12 (s, H-8), 7.97 (dd, J = 3, 6.8 Hz, H-2), 7.93
(d, J = 6.7 Hz, H-3), 7.85 (dd, J = 2.9, 6.7 Hz, H-4, H-
7), 7.27 (dd, J = 3.0, 7.8 Hz, H-5, H-6), 7.12 (d, J = 8.3
Hz, H-1), 7.01 (dd, J = 2.1, 8.6 Hz, H-13, H-15), 6.94
(dd, 2, 8.5 Hz, H-12, H-14), 4.24 (t, J = 6.1 Hz, H-11),
4.02 (t, J = 6.5 Hz, H-10), 1.82–0.82 (92H, alkyl chain).
Ni12-16. Yellow-green, yield 80%. Anal. Calc. for
C94H140N6O6Ni: C, 74.82; H, 9.35; N, 5.57. Found: C,
tal ion as –Oꢀ. In addition, a comparison of the H
1
NMR spectra of the ligands and the Ni complexes indi-
cates that the proton signal corresponding to the OH
group of the free ligands has disappeared in the nickel
complexes and a large shielding occurrs for the imine
proton H-8 (Dd = dligand ꢀ dcomplex = 0.7 ppm). Based
on these observations and elemental analyses results,
we concluded that the Schiff-base ligands are coordi-
nated to the metal center as bidentate (N–O) ligands in
a 2:1 ratio.
1
74.4; H, 8.9; N, 5.2%. H NMR (400 MHz, CDCl3) d
8.13 (s, H-8), 7.96 (dd, J = 3, 6.9 Hz, H-2), 7.92
(d, J = 6.8 Hz, H-3), 7.84 (dd, J = 3, 6.8 Hz, H-4, H-7),
7.28 (dd, J = 3.0, 7.7 Hz, H-5, H-6), 7.13 (d, J = 8.1
Hz, H-1), 7.02 (dd, J = 2.2, 8.5 Hz, H-13, H-15), 6.95
(dd, 2, 8.6 Hz, H-12, H-14), 4.23 (t, J = 6.2 Hz, H-11),
4.01 (t, J = 6.4 Hz, H-10), 1.83–0.81 (108H, alkyl chain).
Table 1
Selected IR data for the Schiff base ligands and the metal complexes
Compound
m (cmꢀ1
)
O–H
C–H (aromatic)
C–H (aliphatic)
C@N
C–O (etheric)
C@O
1
3185(br, m)
3445-60(br, m)
3070-75(m)
3050-55(m)
3040-47(m)
2950-2850(s)
2950-2865(s)
2922-2848(s)
1241-4(s)
1250-80(s)
1240-48(s)
1663(s)
L12-m
Ni12-m
1625-30(s)
1615-18(s)
s, strong; m, medium; br, broad.

Z. Rezvani et al. / Polyhedron 24 (2005) 1461–1470
1465
We recently reported the crystal structure of bis-
[5-((4-npropyloxyphenyl)azo)-N-(n-pentyl-salicylaldiminato)]
Ni(II) [20], which has a similar coordination environ-
ment to that of the title complex of this work. In these
kinds of complexes, the Schiff base ligands have an
approximately trans-planar configuration around the
Ni central ion and the structure is square planar.
Therefore, we propose that the Ni(II) complexes have
square planar or nearly square planar coordination
geometry.
Fig. 1. DSC thermograms of: (a) L12-8, (b) L12-12 and (c) L12-16.

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Z. Rezvani et al. / Polyhedron 24 (2005) 1461–1470
4.2. Glass formation
ties of the Schiff base ligands and the related Ni(II) com-
plexes were studied by differential scanning calorimetry,
polarizing optical microscopy (POM) observations uti-
lizing a heating–cooling stage and X-ray diffraction
(XRD). The transition temperatures of the ligands and
the complexes were measured by DSC. Fig. 1(a)–(c)
The thermal stability of these materials was checked
by thermogravimetric analysis (TGA) at first. All other
thermal characterizations were then carried out below
the decomposition temperature. Glass forming proper-
Fig. 2. DSC thermograms of: (a) Ni12-8, (b) Ni12-12 and (c) Ni12-16, heating and cooling rate: 10 K minꢀ1
.

Z. Rezvani et al. / Polyhedron 24 (2005) 1461–1470
1467
shows the DSC curves of the ligands. The thermal
behaviors of the ligands were similar as it is apparent
from the DSC curves; therefore we describe here the
behavior of compound L12-12 as a representative. In
the first heating scan, L12-12 showed four endothermal
peaks at 99.1, 102.9, 114.3 and 120.5 °C (Fig. 1(b)). In
the second heating scan, four endothermal peaks at
98.1, 101.0, 113.4 and 120.2 °C were observed. When
the isotropic melt of L12-12 was cooled at a cooling rate
of 10 K minꢀ1, two exothermal peaks at 108.7, and
99.5 °C were observed (see Fig. 2). The powder XRD
patterns of L12-12 recorded at room temperature prior
to heating and after cooling from the isotropic liquid
indicated a typical crystalline phase. Therefore, the
peaks at 120.5 °C (in the first and second heating scan)
are attributed to melting, the peak at 108.7 °C on the
cooling step due to crystallization and other peaks
may be related to solid–solid phase transitions. The
melting peak is narrow and the enthalpy of fusion
amounts to 40 kJ molꢀ1. Such a high value of the en-
thalpy of fusion is in agreement with the presence of a
perfect crystalline state of the compound as found by
X-ray analyses [3a]. Melting to the isotropic liquid
and crystallization of the isotropic liquid of L12-12
are confirmed by polarizing optical microscopy and
are in agreement with the DSC measurements. On the
basis of these observations we can conclude that, due
to little difference between melting and crystallization
temperatures, the Schiff base ligands do not have a ten-
dency towards supercooling and do not form a glassy
state. The tendency of Ni(II) complexes towards glass
formation was analyzed using differential scanning calo-
rimetry, POM and XRD. We here again describe the
glass-forming behavior of Ni12-12 as a representative;
because the other complexes behave similarly. When a
polycrystalline sample of Ni12-12 obtained by recrystal-
Fig. 3. X-ray diffraction patterns of Ni12-12; (a) virgin sample crystallized from ethanol/chloroform, (b) after heating the sample above the melting
point and cooling to room temperature.

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Z. Rezvani et al. / Polyhedron 24 (2005) 1461–1470
state of Ni12-12, the XRD pattern of a sample upon
cooling from the isotropic liquid was recorded at room
temperature. The XRD pattern of Ni12-12 after cooling
from the isotropic liquid is given in Fig. 3(b). No sharp
Bragg peaks were observed in the diffraction pattern of
Ni12-12 and only a broad diffraction hump was ob-
served, indicating an amorphous state. The crystalliza-
tion process of Ni12-12 was confirmed by XRD. XRD
patterns of Ni12-12 after annealing the amorphous
material at 120 °C for 30 min. and virgin sample are
similar, which indicates that the two crystalline phases
are identical. Moreover, the high value of the enthalpy
of fusion of Ni12-12 (29.9 kJ molꢀ1) is in agreement
with the presence of a perfect crystalline state of Ni12-
12 as confirmed by the X-ray analyses.
Upon cooling the isotropic liquid of the nickel com-
plexes, only a black field under POM was observed
throughout the cooling process even when the tempera-
ture reached below ꢀ20 °C, which confirms the existence
of the amorphous state. However, anisotropic glass
phases were found while annealing samples for a long
time. When the samples were annealed at room temper-
ature for several days, a schlieren optical texture was
formed. Fig. 4 shows the photomicrograph of Ni12-12
as an example, indicating the formation of a discotic
nematic (Nd) mesophase [21]. To further confirm the
structure of the mesophase, we carried out XRD exper-
iments. The XRD pattern obtained exhibits only a peak
in the small angle area and a broad halo with a wide an-
gle (Fig. 5). On the basis of these observations we can
conclude that an anisotropic glass has been formed.
Formation of a discotic nematic mesophase in the Ni
complexes is in agreement with the discotic structure
of the complexes [22]. Thermodynamic data for the
Schiff-base ligands and the related Ni complexes are tab-
ulated in Table 2.
Fig. 4. Optical texture of Ni12-12 after annealing the isotropic glass
phase at room temperature for 7 days between crossed polarizers.
lisation from ethanol/chloroform (1:1 v/v) was heated,
two endothermic peaks at 90 and 150 °C due to solid–
solid transition and melting were observed. In the cool-
ing cycle no exothermal peaks were observed. When the
isotropic liquid was cooled down at the same cooling
rate by DSC under polarizing microscopy, a transparent
glass was formed via a supercooled liquid. When the
amorphous glass sample was again heated, a glass tran-
sition phenomenon was observed at 64 °C. On further
heating above the glass transition, an exothermic peak
was observed at 120 °C due to crystallization producing
the same crystals obtained by recrystallization from
solution, which melted at 150 °C. Fig. 3 shows the
XRD patterns of Ni12-12. The powder XRD pattern
from virgin crystals of Ni12-12 (Fig. 3(a)) indicates a
crystalline state. In order to determine the amorphous
Fig. 5. X-ray diffraction pattern after annealing Ni12-12 at room temperature for 7 days.

Z. Rezvani et al. / Polyhedron 24 (2005) 1461–1470
1469
Table 2
Thermal transitions and thermodynamic data of the ligands and the Ni complexes, heating and cooling rate: 10 °C minꢀ1
Compound
Phase transitions (°C) and corresponding enthalpies (kJ molꢀ1, in parentheses)a
First heating scan
First cooling scan
Second heating scan
L12-8
L12-12
L12-16
Ni12-8
Ni12-12
Ni12-16
a
K104.0,105.5(2.7)bK107.7(7)K118.2(61.3)I
K99.1,102.9b(18.6)K114.3(21.3)K120.5(26.9)I
K104.9,108.1b(6.1)K113.4(31.3)I
K86.8(8.5),105.1(11.6)b,160.8(35.6)I
K73.4(16.1),83.2(0.2),149.1(29.9)I
K107.7(9.7),136.2(41.5)I
I112.3(59.3)K99.4(15.1)K
I108.7(56.0)K99.5(14.5)K
I107.7(34.6)K102.2(10.7)K
K102.4(16.7)K116.7(58.70)
K98.1,99.6,101.1b(10.6)K113.4(17.0)K120.2(40.30)I
K107.1(10.9)K112.0(33.7)I
G65.1c,126.4(ꢀ24.2)K159.4(33.3)I
G66.5c,119.9(ꢀ19.7)K147.7(25.8)I
G58.1c,83.7(ꢀ19.3)K107.7(1.0)K135.1(39.4)
K, crystal; G, glass; I, isotropic.
b
c
Overlapped with a previous transition.
Glass transition.
From the above results, it is apparent that the Schiff
Acknowledgements
base ligands do not show any tendency towards glass
formation and go to a crystalline phase when cooled
from the isotropic liquid, but the related Ni(II) com-
plexes have a great tendency for glass forming and after
several days change to an anisotropic glassy state. Some
structural requirements such as non-planarity of molec-
ular structure, bulkiness and weight of substitutions and
number of conformers of the molecule in the fluid state
are responsible for glass formation in organic com-
pounds. Furthermore, the enlargement of molecular size
is an important factor for the achievement of the stabil-
ity of a glassy state [1a]. On the other hand, another
requirement is that the molecular system should have
a weak tendency towards crystallization despite strong
intermolecular attractions [5]. From a structural point
of view, the nickel complex is essentially non-planar. It
is well known that azobenzene and its p,p0-substituted
derivatives are essentially planar, but the azomethine
part of the structure is non-planar. As the X-ray analy-
ses indicates in the crystalline state, in this kind of com-
pound, the two benzene rings (phenylimine and
salicylidenic ring) are twisted with respect to the azome-
thine group plan [23]. The Ni(II) complexes, in compar-
ison with the ligands, have all of the structural
requirements to form a glassy state. It is notable that
the phenomenon of polymorphism, which is observed
for the Ni(II) complexes and suggests the existence of
different conformers, is responsible for the formation
of the glassy state [1a].
We thank professor Yasuhiko Shirota, The Univer-
sity of Osaka, Japan for useful comments and sugges-
tions. Financial support from Azarbaijan University of
Tarbiat Moallem research council is greatly appreciated.
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