Synthesis and mesomorphic properties of new imines and copper(II) complexes

Article abstract of DOI:10.1016/S0022-328X(02)01425-0

The synthesis and mesomorphic properties of a serie of 4-(alkyloxy)-N-(4-hexylphenyl)-2-hydroxybenzalimine (2) and their copper(II) complexes (3) are reported. The compounds were characterized by spectroscopic methods (IR, ¹H-, ¹³C-NMR and MS). The calamitic liquid crystal series of imines (2) and copper(II) complexes (3) exhibited smectic mesophases, which were characterized by differential scanning calorimetry (DSC) analysis and optical polarizing microscopy.

Full text of DOI:10.1016/S0022-328X(02)01425-0

Journal of Organometallic Chemistry 655 (2002) 105ꢀ110
/
www.elsevier.com/locate/jorganchem
Synthesis and mesomorphic properties of new imines and copper(II)
complexes
Belk z Bilgin-Eran *, Cig˘dem Yorur, Suheyla Uzman
ı ¸
¨ ¨
¨
¨
Department of Chemistry, Y ld z Teknik Universitesi, Davutpasa Yerlesim Birimi, TR-34210 Esenler, Istanbul, Turkey
ı
ı
¸
¸
Received 10 April 2001; received in revised form 6 February 2002; accepted 12 February 2002
Abstract
The synthesis and mesomorphic properties of a serie of 4-(alkyloxy)-N-(4-hexylphenyl)-2-hydroxybenzalimine (2) and their
copper(II) complexes (3) are reported. The compounds were characterized by spectroscopic methods (IR, 1H-, 13C-NMR and MS).
The calamitic liquid crystal series of imines (2) and copper(II) complexes (3) exhibited smectic mesophases, which were characterized
by differential scanning calorimetry (DSC) analysis and optical polarizing microscopy. # 2002 Published by Elsevier Science B.V.
Keywords: Liquid crystals; Metallomesogens; Schiff bases; Copper(II) complexes
1. Introduction
2. Results and discussion
Metal-containing liquid crystals have attracted in-
creasing attention in recent years due to the different
interesting properties they display [1,2]. In literature,
complexes of various metals with salicylaldimines carry-
ing different organic moieties are found [1,2]. The
copper(II) unit is well known for its ability to co-
ordinate four donor atoms, which makes it possible to
obtain structures that favour calamitic and discotic
The synthesis of the ligands were carried out under a
nitrogen atmosphere and in a two-step process. The
starting alkyloxy aldehydes (1) were obtained in the
usual way [9] by the reaction of 2,4-dihydroxybenzalde-
hyde with the appropriate n-alkylbromide, using DMF
instead of acetone. The standard chemicals, 2,4-di-
hydroxybenzaldehyde, 1-bromoalkanes, and 4-hexylani-
line, were used as supplied (Aldrich Chemical Co.).
The imines (2) were prepared in the usual way [10] by
the p-toluensulfonic acid catalyzed condensation of the
corresponding aldehydes (1) with 4-hexylaniline (see
Section 4).
The copper complexes (3) were prepared by reacting
the appropriate ligand with copper(II) acetate dihydrate
in ethanol (see Section 4). The complexes were isolated
in good yields and are soluble in chloroform, toluene,
dioxane and insoluble in ethanol, methanol and hexane.
mesomorphism [1ꢀ
containing alkylanilines have also attracted considerable
attention with regard to structureꢀproperty relation-
ship. These complexes display smectic mesomorphism
and the smectic A phases are especially common [1ꢀ8].
/3]. Schiff base copper (II) complexes
/
/
In this paper the synthesis, characterization and
mesomorphic properties of a new serie of tetracoordi-
nate copper(II) complexes (3) of Schiff bases (2) derived
from aldehydes (1) (see Scheme 1) are presented.
The new imine compounds 2aꢀ
characterized by various spectroscopic methods, e.g. 1H-
13C-NMR (CDCl3), UVꢀ
vis, IR and mass. The
proposed structures are in full agreement with these
spectroscopic data. The copper(II) complexes 3aꢀd were
characterized by UVꢀvis, IR and mass spectroscopy.
/
d presented here were
,
/
/
/
* Corresponding author. Tel.: ꢁ
2162-064
E-mail address: belkizb@hotmail.com (B. Bilgin-Eran).
/
90-212-4491-751; fax: ꢁ90-212-
/
The data are also consistent with the expected square
planar structure for the complexes.
0022-328X/02/$ - see front matter # 2002 Published by Elsevier Science B.V.
PII: S 0 0 2 2 - 3 2 8 X ( 0 2 ) 0 1 4 2 5 - 0

106
B.-B. Eran et al. / Journal of Organometallic Chemistry 655 (2002) 105ꢀ110
/
Fig. 1. Electronic spectra of the ligand 2a (- - -), and the correspond-
ing Cu(II) complex 3a (*).
/
(DSC) show that they exhibit smectic mesophases (for
the phase transition data see Tables 1 and 2).
The ligands 2aꢀd show similar mesomorphic proper-
/
ties with the previously synthesized analogs [4,6,11]. The
transition temperatures and the thermodynamic data for
the new Schiff base ligands (2) and the previously
reported analogs [6,11] are given in Table 1.
Scheme 1. a: Rꢃ
/
C8H17; b: Rꢃ
/
C10H21; c: Rꢃ
/
C16H33; d: Rꢃ
/
C18H37
.
As seen in Tables 1 and 2, the imine compounds
generally have the S1 mesophase and the complexes have
the S2 mesophase. From optical observations the S1
mesophase shows textures similar to smectic C and the
S2 is similar to smectic A phase (see Fig. 2).
The NMR data of the imine compounds 2aꢀ
definite evidence for the molecular structure. The
d(HCꢀN) value of :8.50 ppm for the ligands (2) is
/
d give
/
/
well within the range expected for imine compounds
[6,8,10].
The melting points of the imine compounds (2)
increase with increasing chain length of the alkyloxy
The electronic spectra of the imines 2aꢀ
absorption band between 341 and 343 nm, whereas the
absorption band of the complexes 3aꢀd shift to 378ꢀ383
nm (see Fig. 1).
The IR spectra of both the ligands 2aꢀ
N stretching at the
/d show the
group. The imine compounds (2aꢀc) show smectic phase
/
/
/
(S1), which exhibit Schlieren and broken fan textures.
On cooling of the ligands from isotropic phase smectic
mesophase (S1) with broken fan-shaped texture can be
/
d and the
complexes 3aꢀ
/
d show the Cꢀ
/
expected values of 1624 and 1617 cmꢂ1, respectively.
Similarly Marcos et al. [7,8] have described the infrared
spectra of the metal complexes analogous for 3, that
show a stretch band around 1614 cmꢂ1 (assigned to
observed. Increase of the chain length further (2d: Rꢃ
/
C18H37) leads only to a monotropic smectic (S1) phase
with Schlieren texture. For the transition temperatures
versus number of carbon atoms in the alkoxy chain of
ligands 2 (see Fig. 3).
All the new copper(II) complexes 3a,c,d show thermo-
tropic mesophases like the previously reported com-
n(Cꢀ
5ꢀ
9 cmꢂ1 as compared to that of the free Schiff bases.
The mass spectra of the complexes 3aꢀd are consis-
/N)) which has shifted to a lower frequency by ca.
/
/
plexes of similar structure [4ꢀ
/
6,8]. The compounds 3aꢀd
/
tent with their proposed structures (see Section 4).
The new imines and their copper(II) complexes
exhibit liquid crystalline properties and show thermo-
tropic mesophases. The investigations by polarizing
microscopy and differential scanning calorimetry
(RꢃC8H17, C10H21[5], C16H33, C18H37, respectively) are
/
microcrystalline solids at room temperature that upon
heating and/or cooling from the isotropic liquid, display
smectic mesophase (see Table 2). The mesophases shown

B.-B. Eran et al. / Journal of Organometallic Chemistry 655 (2002) 105ꢀ
/110
107
Table 1
a
a
Phase transition temperatures (8C) and enthalpies (kJ molꢂ1) of imine compounds (2): Cr: crystalline, S: smectic, N: nematic and Iso: isotropic
mesophase
b
Compound
R
Cr
S1
SA
N
Iso
[11]
2a
2b
[6]
2c
C6H13
C8H17
C10H21
m 45
m 76.8
ꢀ
ꢀ/
ꢀ/
/
m 88.8
ꢀ/
ꢀ/
ꢀ/
ꢀ/
ꢀ/
m
m
m
m
m
m
m 35.1 (30.0)
m 42.0 (24.3)
m 51
m 60.6 (40.4)
m 67.8 (40.6)
m 81.4 (2.1)
m 96.4 (5.7)
m 90
m 85.0 (1.5)
{m 56}
C
12H25
C16H33
18H37
m 95
ꢀ/
ꢀ/
c
2d
C
a
Heating rates are 5 K minꢂ1 for the melting and for the clearing processes; the enthalpies are in parentheses. The values of monotropic
transitions are given in {. . .}.
b
S1 mesophase shows textures similar to SC phase from optical observations.
This monotropic phase transition could be detected by hot stage polarizing microscopy only (heating rate 10 K minꢂ1).
c
by the complexes were identified by polarizing micro-
scopy as smectic A by their optical textures (see Fig. 2).
The effect of the alkylchain length on the melting
The formation of the complexes has also effected the
mesophase properties. All the ligands (2aꢀd) exhibit
smectic C phase (S1) by their optical textures. After
complexation, a S2 mesophase is observed which is
identified by polarizing microscope as smectic A by its
optical texture.
/
points of the complexes (3aꢀd) show the opposite trend
/
with respect to the ligands, i.e. a consistent decrease of
melting points is observed. The complexes melt at higher
temperatures than the ligands. On cooling of com-
pounds 3a, 3b [5] and 3c from isotropic phase a smectic
mesophase (S2) with fan-shaped texture can be observed.
The compound 3d presents a crystal-crystal transition
and exhibits only monotropic S2 phase. The transition
temperatures vs. number of carbon atoms in the alkoxy
chain in copper(II) complexes 3 are given in Fig. 4.
4. Experimental
The characterizations of the compounds 2, 3 pre-
sented here are based on various spectroscopic data, e.g.
1H-, 13C-NMR (Varian VXR-400, AM-270 spectro-
meter, respectively, CDCl3 solutions), UVꢀvis (PU
/
3. Conclusion
8740, CHCl3 solutions), FT-IR (Mattson 1000, CHCl3
solutions), MS (Varian MAT 711 or Intectra GmbH,
AMD 402) and EA (CHNS-932, Leco Co.). Only
structurally relavent resonances are given, see below.
The mesophases were investigated by polarizing
microscopy and by DSC. Microscopy was performed
on an Leitz Laborlux 12 Pol polarizing microscope,
equipped with a Linkam THMS 600 hot stage and a
Linkam TMS93 temperature controller. DSC-thermo-
grams were recorded on a Mettler TA 3000/DSC-30 S
The comparison of the synthesized new imines and
their copper(II) complexes with their previously re-
ported analogs [4ꢀ6,8], show that they exhibit similar
mesomorphic properties. The imine compounds and
their copper(II) complexes described in this paper have
presented enantiotropic smectic phases but the com-
/
pound 3a (Rꢃ
/
C8H17) and the longest chained ligand 2d
(RꢃC18H37) and its copper(II) complex 3d have only
/
monotropic behaviour.
with TA 72.5 software or PerkinꢀElmer DSC-7.
/
Table 2
Phase transition temperatures a (8C) and enthalpiesa (kJ molꢂ1) of copper(II) complexes (3): Cr: crystalline, S: smectic and Iso: isotropic mesophase
b
Compound
R
Cr1
Cr
S2
Iso
[5]
3a
3b
[6]
3c
3d
C6H13
C8H17
ꢀ
ꢀ/
ꢀ/
ꢀ/
/
m 153.9 (45.3)
m 139.5 (31.9)
m 131.7 (26.6)
m 119
m 82.4 (19.8)
m 94.4 (40.5)
{m 145.8}
{m133}
m 144.3 (10.1)
m 140
m
m
m
m
m
m
c
d
C
10H21
C12H25
C16H33
C18H37
ꢀ/
m 112.5 (6.6)
c
m 41.4 (17.6)
{m 85}
a
Heating rates are 5 K minꢂ1 for the melting and for the clearing processes; the enthalpies are in parentheses. The values of monotropic
transitions are given in {. . .}.
b
S2 mesophase shows textures similar to SA phase from optical observations.
This monotropic phase transition could be detected by hot stage polarizing microscopy only (heating rate 10 K minꢂ1).
c
d
In Ref. [5] for compound 3b: Cr 130.5 8C (31.3)0SA 145.1 8C (10.1)0Iso.

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B.-B. Eran et al. / Journal of Organometallic Chemistry 655 (2002) 105ꢀ110
/
Fig. 3. Plot of transition temperatures versus number of carbon atoms
(n) in the terminal chain (R) for the imine compounds (2). (") CrꢀS1
or Iso; (j) S1 ꢀIso, N, SA or Cr; (m) SA Iso; (k) NꢀIso.
/
/
ꢀ/
/
Fig. 4. Plot of transition temperatures versus the number of carbon
atoms (n) in the terminal chain (R) for the copper(II) compounds (3).
(") Crꢀ
/
S2 or Iso; (j) Cr1 ꢀ
/
Cr; (m) S2 ꢀIso or Cr.
/
68.27 (t; OCH2 group), 35.52 (t; a-CH2 group). UVꢀ
vis: (nm)ꢃ341.9. IR: gꢃ N).
1624 cmꢂ1 (Cꢀ
409.2988
/
l
/
/
/
C27H39NO2 (409.61); MS (HR): m/z (%)ꢃ
/
[M]ꢁ, Calc. 409.2981.
Fig. 2. Optical texture (200ꢄ) observed on cooling from the isotropic
/
phase for imine ligand 2c: S1 (SC) phase at 70.2 8C (top) and
copper(II) complex 3c: S2 (SA) phase at 106.4 8C (bottom).
4.1.2. 4-(Decyloxy)-N-(4-hexylphenyl)-2-
hydroxybenzalimine (2b)
Yield: 1.56 g (71%) of yellow crystals. H-NMR: dꢃ
1
/
4.1. Synthesis of the imine compounds 2
13.88 (s; OH), 8.51 (s; HCꢀ
8.5 Hz each; 1, 2 and 2 arom. Hs, respectively), 6.49ꢀ
6.45 (m; 2 arom. Hs), 3.99 (t, J :6.5 Hz; OCH2 group),
2.62 (t, J :
6.5 Hz; a-CH2 group). 13C-NMR: dꢃ
164.13, 163.48, 145.92, 141.33, 113.04 (5s; 5 arom. Cs),
160.58 (d; HCꢀN), 133.32, 129.25, 120.73, 107.49,
101.67 (5d; 1, 2, 2, 1 and 1 arom. CH, respectively),
68.22 (t; OCH2 group), 35.47 (t; a-CH2 group). UVꢀvis:
l (nm)ꢃ343.2. IR: gꢃ N). C29H43NO2
1624 cmꢂ1 (Cꢀ
(437.66); MS (EI): m/z (%)ꢃ
437 (100) [M]ꢁ, 297 (95)
[437ꢂC10H20]; Anal. Calc.: C, 79.59; H, 9.90; N, 3.20.
/
N), 7.24, 7.21, 7.17 (3d, J :
/
/
The imines 2aꢀd were prepared in the usual way [10]
/
/
by a p-toluensulfonic acid (40 mg) catalyzed condensa-
tion of 4-alkyloxy-2-hydroxybenzaldehyde (5 mmol)
with 4-hexylaniline (6 mmol) in toluene (25 ml) and
purified by crystallization from acetone/ethanol.
/
/
/
The melting points of the imines are given in Table 1.
/
/
/
/
4.1.1. 4-(Octyloxy)-N-(4-hexylphenyl)-2-
hydroxybenzalimine (2a)
/
1
/
Yield: 1.65 g (80%) of yellow crystals. H-NMR: dꢃ
/
Found: C, 79.44; H, 9.83; N, 3.34%.
13.74 (s; OH), 8.49 (s; HCꢀ
8.5 Hz each; 1, 2 and 2 arom. Hs, respectively), 6.55ꢀ
6.44 (m; 2 arom. Hs), 3.99 (t, J :6.5 Hz; OCH2 group),
2.61 (t, J :
6.5 Hz; a-CH2 group). 13C-NMR: dꢃ
164.09, 163.56, 145.66, 141.40, 112.90 (5s; 5 arom. Cs),
160.46 (d; HCꢀN), 133.32, 129.27, 120.69, 107.57,
101.61 (5d; 1,2,2,1 and 1 arom. CH, respectively),
/
N), 7.24, 7.20, 7.17 (3d, J :
/
/
/
4.1.3. 4-(Hexadecyloxy)-N-(4-hexylphenyl)-2-
/
/
hydroxybenzalimine (2c)
Yield: 1.98 g (76%) of yellow crystals. H-NMR: dꢃ
1
/
/
13.86 (s; OH), 8.50 (s; HCꢀ
/
N), 7.22, 7.19, 7.16 (3d, J :
/
8.5 Hz each; 1, 2 and 2 arom. Hs, respectively), 6.47ꢀ
/

B.-B. Eran et al. / Journal of Organometallic Chemistry 655 (2002) 105ꢀ
/110
109
6.43 (m; 2 arom. Hs), 3.98 (t, J :
2.60 (t, J :
6.5 Hz; a-CH2 group). 13C-NMR: dꢃ
164.16, 163.51, 145.77, 141.38, 112.95 (5s; 5 arom. Cs),
160.52 (d; HCꢀN), 133.31, 129.29, 120.73, 107.51,
101.57 (5d; 1, 2, 2, 1 and 1 arom. CH, respectively),
68.22 (t; OCH2 group), 35.49 (t; a-CH2 group). UVꢀvis:
l (nm)ꢃ343.0. IR: gꢃ N). C35H55NO2
1624 cmꢂ1 (Cꢀ
(521.83); MS (HR): m/z (%)ꢃ
521.4231 [M]ꢁ, Calc.
521.4233.
/
6.5 Hz; OCH2 group),
4.2.3. bis[(N-4?-Hexylphenyl)-4-n-
hexadecyloxysalicylaldiminato]copper(II) (3c)
/
/
Yield: 175 mg (63%) of greenish brown crystals. UVꢀ
vis: (nm)ꢃ378.5. IR: gꢃ N).
1617 cmꢂ1 (Cꢀ
C70H108N2O4Cu (1105.19); MS (EI): m/z (%)ꢃ1105
(4) [M]ꢁ, 522 (100) [C35H55NO2], 297 (75) [522ꢂ
/
/
l
/
/
/
/
/
/
/
/
/
C16H33].
/
4.2.4. bis[(N-4?-Hexylphenyl)-4-n-
octadecyloxysalicylaldiminato]copper(II) (3d)
Yield: 210 mg (72%) of greenish brown crystals. UVꢀ
vis: (nm)ꢃ378.4. IR: gꢃ N).
1617 cmꢂ1 (Cꢀ
C74H116N2O4Cu (1161.30); MS (EI): m/z (%)ꢃ1162
(2) [M]ꢁ, 550 (100) [C37H59NO2], 297 (77) [550ꢂ
/
4.1.4. 4-(Octadecyloxy)-N-(4-hexylphenyl)-2-
hydroxybenzalimine (2d)
Yield: 2.22 g (81%) of yellow crystals. H-NMR: dꢃ
l
/
/
/
1
/
/
/
13.87 (s; OH), 8.51 (s; HCꢀ
8.5 Hz each; 1, 2 and 2 arom. Hs, respectively), 6.50ꢀ
6.44 (m; 2 arom. Hs), 3.99 (t, J :6.5 Hz; OCH2 group),
2.62 (t, J :
6.5 Hz; a-CH2 group). 13C-NMR: dꢃ
163.48, 158.54, 145.84, 141.37, 126.60 (5s; 5 arom. Cs),
160.54 (d; HCꢀN), 133.31, 129.29, 120.81, 107.50,
101.61 (5d; 1, 2, 2, 1 and 1 arom. CH, respectively),
68.22 (t; OCH2 group), 35.49 (t; a-CH2 group). UVꢀvis:
l (nm)ꢃ342.8. IR: gꢃ N). C37H59NO2
1624 cmꢂ1 (Cꢀ
(549.88); MS (HR): m/z (%)ꢃ
549.4549 [M]ꢁ, Calc.
549.4546.
/
N), 7.24, 7.21, 7.18 (3d, J :
/
C18H37].
/
/
/
/
Acknowledgements
/
Financial support of our liquid crystal studies was
¨
provided by the Y ld z Teknik Universitesi Research
/
ı ı
Fond and Devlet Planlama Teskilati (DPT) (State
/
/
/
/
Planning Organization Turkey). We are grateful to
Professor C. Tschierske, Martin-Luther Universitat
¨
Halle-Wittenberg, Germany, as well as to Dr. M.
Horn, University of California, USA, for valuable
information, DSC measurements and fruitful discus-
sions.
4.2. Synthesis of the copper(II) complexes 3
To a suspension of corresponding imine (0.5 mmol) in
abs. ethanol (5 ml) stirred at room temperature (r.t.)
were added potassium hydroxyde (0.5 mmol) in abs.
ethanol (10 ml) and copper(II) acetate dihydrate (0.25
mmol). The mixture was stirred at room temperature for
4 h; the greenish or greenish brown solid formed was
filtered and recrystallized from chloroform/ethanol.
The melting points of the copper(II) compounds are
given in Table 2.
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(nm)ꢃ383.2. IR: gꢃ N). C54H76N2-
1617 cmꢂ1 (Cꢀ
O4Cu (880.76); MS (HR): m/z (%)ꢃ
879.5107 [M]ꢁ,
Calc. 879.5101. MS (EI): m/z (%)ꢃ
879 (44) [M]ꢁ, 409
(100) [C27H39NO2], 297 (23) [409ꢂC8H16].
/vis: l
/
/
/
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(nm)ꢃ380.8. IR: gꢃ N). C58H84N2-
1617 cmꢂ1 (Cꢀ
O4Cu (936.86); MS (EI): m/z (%)ꢃ
935 (15) [M]ꢁ, 437
(100) [C29H43NO2], 297 (85) [437ꢂC10H20]; Anal. Calc.:
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/
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/
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75.
/
/
C, 74.36; H, 9.04; N, 2.99. Found: C, 74.35; H, 8.90; N,
2.89%.

110
B.-B. Eran et al. / Journal of Organometallic Chemistry 655 (2002) 105ꢀ110
/
[5] M. Marcos, P. Romero, J.L. Serrano, C. Bueno, J.A. Cabeza,
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