Liquid crystalline properties of bis (salicylaldiminato) copper (II) complexes: The first columnar discotics derived from salicylaldimine Schiff bases

Article abstract of DOI:10.1039/a707091h

The synthesis and mesomorphic properties of a homologous series of N-(2-hydroxy-4-n-alkoxybenzylidene)-3′,4′-di-n-alkoxylanilines and their copper(II) complexes bis[(N-3′,4′-dialkoxyphenyl)-4-n-alkoxylsalicylaldiminato]copper(II) are reported. The disc-li

Full text of DOI:10.1039/a707091h

J O U R N A L O F
Liquid crystalline properties of bis(salicylaldiminato)copper(II
complexes: the first columnar discotics derived from salicylaldimine
)
Schiff bases
Chung K. Lai,* Chung-Hsiung Chang and Chun-Hsien Tsai
C H E M I S T R Y
Department of Chemistry, National Central University, Chung-L i, Taiwan, ROC
The synthesis and mesomorphic properties of a homologous series of N-(2-hydroxy-4-n-alkoxybenzylidene)-3∞,4∞-di-n-
alkoxylanilines and their copper() complexes bis[(N-3∞,4∞-dialkoxyphenyl)-4-n-alkoxylsalicylaldiminato]copper() are reported.
The disc-like copper complexes exhibited columnar hexagonal disordered mesophases, which were characterized by DSC analysis
and optical polarized microscopy. The structure of the mesophases was confirmed by X-ray powder diffraction (XRD). The
mesomorphic properties for these copper complexes were compared with other similar metallomesogenic
bis(salicylaldiminato)copper() complexes in terms of molecular structural differences.
R
R
There has been extensive research on the synthesis and charac-
terization of metallomesogens1–5 during the past decade.
Introduction of single or multiple metal centers into molecular
systems has allowed the design and generation of a wide
variety of geometric molecules, which were expected to exhibit
interesting mesomorphic and physical properties.3 These
observed novel properties mainly arose from the combination
of rich coordination chemistry and large polarizability, which
are characteristic features5 of transition metal atoms. The
formation of mesophases was often facilitated by intermolecu-
lar dative interactions exhibited in such metallomesogens, since
the metal complexes are generally coordinatively unsaturated.
The relationship between molecular structures and physical
properties still remains a challenging task for material chemists
to explore and understand. In spite of the fact that they seem
to be very promising for development in technology appli-
cations, the status of research using this type of metallomesog-
enic materials3,5 is still in its infancy.
Metal complexes (structures I and II) of Schiff bases derived
from salicylaldimine5,6 and b-diketonates7–9 (III and IV) are
among the best known complexes that exhibit mesogenic
properties in metallomesogenic systems. There have been
numerous investigations of metal b-diketonates complexes, and
almost all of the transition metals (particularly 3d) show
mesomorphism with b-diketonates. A variety of transition
metals (Fe, Rh, Ir, VO, Ni, Co, Cu, Pt, Pd, etc.) show
mesomorphism with salicylaldimine derivative Schiff bases. All
of these complexes derived from salicylaldimine Schiff bases
were more likely of rod- or brick-like shapes with two or four
extended flexible sidechains, and these complexes thus all
exhibited either nematic (N) or various smectic (S) (or chiral
smectic) phases predominantly, regardless of the metal center
incorporated in such systems. Differences in the mesogenic
properties of these Schiff base metal complexes were attributed
to the various types of geometric coordination of the metal
centers.
O
N
N
O
O
O
N
R′
M
M
R
R′
N
R
Z
R
I
II
X
X
Y
Z
Y
X
Y
Z
O
O
O
O
O
O
M
M
O
O
Z
Y
X
Y
X
Y
X
Z
Z
III
IV
H
OR
RO
RO
N
OR
OR
O
Cu
O
N
RO
H
V
R = Cn H2n+1
n = 8,10,12,14,16
The transition of mesophases from rod-like nematic or
smectic phases into disc-like discotics was possible by adjusting
the ratio2,3 of width-to-length (d/l, i.e., aspect ratio) of the
molecular shape. Here, we report the synthesis and meso-
morphic properties of a homologous series of copper()
complexes with the following structure; bis[(N-3∞,4∞-dialkoxy-
phenyl)-4-n-alkoxysalicylaldiminato]copper(). To the best of
our knowledge these complexes derived from salicylaldimines
are the first disc-like molecules to exhibit columnar discotic
phases.
Results and Discussion
Synthesis
The preparation of copper complexes was straightforward. The
condensation of 1-amino-3,4-dialkoxybenzene and 4-alkoxy-2-
hydroxybenzaldehyde in refluxing CH Cl gave light yellow
2
2
N-(2-hydroxy-4-n-alkoxybenzylidene)-3∞, 4∞-dialkoxyanilines in
J. Mater. Chem., 1998, 8(3), 599–602 599

Table 1 Phase behaviora of copper complexes V
163.2 (13.5) 178.9 (11.4)
high yields. These Schiff base derivatives were then reacted
with copper() acetate to produce the green-to-brown metal
complexes. These copper compounds are all paramagnetic,
and their 1H and 13C NMR spectra display only broad alkoxy
signals. All other proton signals close to the paramagnetic
copper centers are unobserved. Elemental analysis also con-
firmed the bimetallic identity of the complexes. The typical
synthetic pathways for these complexes are summarized in
Scheme 1. Several attempts to prepare vanadyl complexes by
refluxing or stirring the reaction mixture at room temperature
were unsuccessful. Unreacted Schiff base ligands were reco-
vered after work-up.
325.5
n=8
K CCCCCA K ,bbbb) Col ,bbbb) I
1
2
hd
d
133.1 (6.14)
165.7 (13.3)
323.1
313.0
152.4 (10.1)
10 K CCCCCA K ,bbbb) Col ,bbbb) I
1
2
hd
311.4
d
133.1 (10.5)
142.7 (23.4)
292.6 (4.26)
12
14
16
K ,bbbb) Col ,bbbb) I
2 hd
114.1 (19.2)
139.4 (19.8)
290.5 (4.22)
271.7 (3.22)
K ,bbbb) Col ,bbbb) I
2
hd
106.7 (18.7)
135.7 (20.8)
263.9 (2.63)
252.7 (3.22)
Mesomorphic properties
K ,bbbb) Col ,bbbb) I
The copper complexes all exhibit liquid crystalline behavior of
columnar discotics, which were characterized based on DSC
analysis and polarized optical microscopy. The phase-trans-
ition temperature and enthalpies of copper complexes V are
given in Table 1. DSC analyses of the copper complexes showed
typical discotic phase transitions10 of crystal-to-discotic-to-
2
hd
106.2 (19.9)
244.9 (2.55)
aK , K =crystal phase; Col =columnar hexagonal disordered;
1
2
hd
I=isotropic; I =isotropic with decomposition. The transition tem-
d
perature (°C) and enthalpies (in parentheses, kJ mol−1) are determined
by DSC at a scan rate of 10.0 °C min−1.
isotropic
(CryColI).
Additional
crystal-to-crystal
(K K ) transitions were also observed for complexes with
These copper complexes exhibit relatively high clearing
points (325–253 °C), and for a few complexes with lower
carbon numbers (n=8, 10) this is accompanied by partial
decomposition. The melting and clearing points both decrease
as the sidechain length increases. When cooled from their
isotropic phases, they display optical textures (Fig. 1) which
are a mixture of pseudo-focal-conics and mosaic regions with
linear birefringent defects, suggesting hexagonal columnar10
structures. The possibility of rectangular phases was also
readily excluded owing to the absence of mosaic textures with
prominent wedge-shaped8c defect patterns.
1
2
lower carbon lengths (i.e. n=8, 10). Upon heating, the copper
complexes melt to give birefringent fluid phases with columnar
superstructures, as is often observed for disc-shaped molecules.
At crystal-to-discotic transition the flexible side chains undergo
disordering but the central core remains stacked in columns,
however, the core unstacks at discotic-to-isotropic transition.
A larger enthalpy for the crystal-to-liquid crystal transition at
lower temperatures (136–179 °C) and a lower enthalpy for the
liquid crystal-to-isotropic transition at higher temperatures
(253–325 °C) was observed; this low value of the transition
enthalpy indicated that the mesophases were relatively dis-
ordered. The temperature range of discotic mesophases for
copper complexes was quite wide (between 120 and 150 °C)
and slightly sensitive to the carbon number of sidechains. For
example, the temperature range for complexes decreased with
carbon length (e.g. T =148 °C for n=8 and T =118 °C for n=
16), which is often observed for discotic10 metallogens.
For metallogens3,5 with similar molecular structures, increas-
ing the number of flexible sidechains results generally in a
OR
RO
RO
RO
e
+
NH2
c
OH
d
O
OH
N
OR
OR
OR
RO
HO
f
NO2
b
OH
OH
O
H
N
OR
RO
OR
OR
RO
HO
a
O
Cu
O
RO
N
OR
RO
H
Scheme 1Reaction conditions: a: RBr (2.1 equiv.), K CO (5.0 equiv.),
refluxed in CH COCH , 24 h; b: HNO (conc. 1.1 equiv.), refluxed in
2
3
3
3
3
CH Cl , 16 h; c: H NNH ΩH O (5.0 equiv.), Pd/C (0.1 g, 10%), refluxed
in C H OH, 24 h; d: RBr (1.0 equiv.), KHCO (1.1 equiv.), refluxed
in CH COCH , 24 h; e: CH COOH (few drops), refluxed in CH Cl ,
24 h; f: Cu(OAc) (2.0 equiv.), KOH, refluxed in THF–C H OH, 24 h.
2
2
2
2
2
2
5
3
Fig. 1 Optical texture (100×) observed for copper complex V (n=14).
3
3
3
2 2
Col phase at 245 °C (top); K phase at 60 °C (bottom).
2
2
5
hd
600
J. Mater. Chem., 1998, 8(3), 599–602

lowering of melting and clearing points, and subsequently
increasing the stability of the mesophases. However, in this
system of copper complexes derived from salicylideneamine
derivatives, this lowering in temperatures was not observed.
As a matter of fact the trend was reversed. For complexes V
with six sidechains in total the clearing points were in the
range 325–253 °C from n=8 to n=16, while those for similar
copper complexes with four sidechains in total bis[N-(4-
alkoxyphenyl)-4-alkoxysalicylaldiminato]copper()6 (similar
to structure I) were in the range 165–132 °C. This drastic
increase by ca. 120 °C in clearing temperature suggested that
the type of molecular packing which accounts for the melting
and clearing point in the mesophase for these two similar
series of metal complexes was substantially different. The
bis[N-(4-alkoxyphenyl)-4-alkoxysalicylaldiminato]copper()
complexes which were rod-like formed smectic A and smectic C
phases, however, complexes V are disc-like and exhibit colum-
nar discotic phases as expected.
To further confirm the structure of the mesophases, we
performed powder XRD diffraction experiments. A summary
of the diffraction peaks and lattice constants for the complexes
is given in Table 2. For example, as shown in Fig. 2, the copper
complex with n=12 displays a diffraction pattern of a two-
dimensional hexagonal lattice with an intense peak and two
Fig. 2 X-Ray diffraction data for the columnar hexagonal disordered
phases of copper complexes (n=12)
˚
the rigid cores occur at wider angle regions of 5.11–5.19 A.
The absence of distinct peaks at high angle is consistent with
DSC analysis of low enthalpies for the discotic-to-isotropic
transition, indicative of a highly disordered mesophase; i.e.
there is no long-range order along the columns. The tempera-
ture dependence of the lattice parameters in liquid crystals was
also observed in these copper complexes. We find that the low-
angle reflections of complexes generally shift to lower d-
˚
weak peaks at 32.82, 18.91 and 16.88 A, respectively, at 200 °C.
These are characteristic of a discotic columnar (Col ) phase
hd
with a d-spacing ratio of 1, (1/3)1/2 and (1/4)1/2, corresponding
to Miller indices (10), (11) and (20), respectively. These values
correspond to an intercolumnar distance (a parameter of the
˚
hexagonal lattice) of 37.88 A. Most complexes also display an
interesting additional broad halo peak at mid-angle of d-
˚
spacing ca. 6.00 A. However, liquid-like correlations between
˚
spacings with decreasing temperature (e.g. d=36.78 A at 270 °C
˚
and d=35.08 A at 240 °C for complex V; n=10). The hexagonal
lattices also correlate well with increasing sidechain lengths.
Summary
Table 2 Variable-temperature XRD diffraction data for copper
complexes (V)
We have prepared and characterized a series of copper com-
plexes derived from salicylaldimine Schiff bases. These disc-
like molecules exhibited enantiotropic columnar hexagonal
phases as expected. While it is still true that rod-like molecules
gave nematic or smectic phases, and disc-like molecules gave
discotic phases, respectively, the results in this paper demon-
strated that the transition of mesophases (nematics to discotics)
can be accomplished from structurally similar molecules by
increasing the numbers of sidechains.
˚
d-spacing/A
obs. (calc.)
mesophase
(temperature/°C)
lattice
spacing/A
Miller
indices
˚
complex
n=10
Col (270)
36.78
31.85 (31.85)
18.39 (18.39)
15.97 (15.93)
5.98 (br)
(10)
(11)
(20)
hd
5.11 (br)
Col (240)
35.08
30.38 (30.38)
17.39 (17.54)
15.70 (15.19)
6.11 (br)
(10)
(11)
(20)
hd
Experimental
5.14 (br)
All chemicals and solvents were reagent grade from Aldrich
Chemical Co. and used without further purification. The
solvents were dried by standard techniques. 1H and 13C NMR
spectra were measured on a Bruker DRS-200 instrument. DSC
thermograms were carried out on a Perkin-Elmer DSC-7
instrument and calibrated with a pure indium sample. All
phase behaviors are determined at a scan rate of 10.0 °C min−1
unless otherwise noted. Optical polarized microscopy was
carried out on a Nikkon MICROPHOT-FXA microscope
with a Mettler FP90/FP82HT hot stage system. X-Ray powder
diffraction (XRD) studies were conducted on an INEL MPD-
diffractometer with a 2.0 kW Cu-Ka X-ray source equipped
with an INEL CPS-120 position sensitive detector and a
variable temperature capillary furnace with an accuracy of
0.10 °C in the vicinity of the capillary tube. The detector
was calibrated using mica and silicon standards. The powder
samples were charged in Lindemann capillary tubes (80 mm
long and 0.01 mm thick) from Charles Supper Co. with a inner
diameter of 0.10 or 0.15 mm. Samples were heated to above
the isotropic temperature and kept at that temperature for
10 min. The sample then was cooled at a rate of 5.0 °C min−1
to the appropriate temperature and diffraction data collected.
n=12
n=14
Col (200)
37.88
37.62
41.76
32.82 (32.82)
18.91 (18.94)
16.88 (16.41)
5.11 (br)
32.58 (32.58)
18.73 (18.81)
16.70 (16.29)
5.09 (br)
36.17 (36.17)
20.74 (20.88)
18.17 (18.08)
13.69 (13.67)
5.19 (br)
36.13 (36.13)
20.73 (20.86)
18.07 (18.07)
13.61 (13.65)
5.13 (br)
(10)
(11)
(20)
hd
Col (170)
(10)
(11)
(20)
hd
Col (200)
(10)
(11)
(20)
(30)
hd
Col (170)
41.72
42.46
(10)
(11)
(20)
(30)
(30)
(10)
(11)
(20)
(30)
(31)
hd
n=16
Col (200)
36.77 (36.77)
21.28 (21.23)
18.89 (18.39)
13.69 (13.89)
11.92 (12.25)
5.13 (br)
hd
J. Mater. Chem., 1998, 8(3), 599–602
601

Preparations
atmosphere. The reaction mixture was refluxed for 24 h. The
solution was concentrated to give a brown solid, which was
purified by column chromatography using CH Cl –hexane
1,2-didodecyloxybenzene. Under a nitrogen atmosphere, cat-
echol (5.0 g, 0.045 mol), anhydrous K CO (25.1 g, 0.18 mol),
2
2
(1/1) as eluent. The product was isolated as golden needles
after recrystallization from acetone (mp 86.5 °C, yield 92%).
1H NMR (CDCl ): d 0.86 (t, J=6.4 Hz, CH , 9 H), 1.16–1.86
2
3
bromododecane (23.7 g, 0.10 mol) and KI (0.5 g) were mixed
in 250 ml of dried acetone. The reaction mixture was refluxed
for 24 h. The reaction solution was filtered to remove any solid
and the filtrate was concentrated to give the yellow
solid product. White needles were obtained after recrystalliz-
ation from CH Cl –CH OH (mp 46.8 °C, yield 86%). 1H NMR
3
3
[m, (CH ) , 60 H], 3.99 (t, J=3.2 Hz, OCH , 6 H), 6.42 (dd,
2 10
J=2.3, 8.9 Hz, C H , 1 H), 6.47 (d, J=2.3 Hz, C H , 1 H),
2
6
3
6 3
6.83 (m, C H , 1 H), 7.21 (d, J=9.0 Hz, C H , 1 H), 8.46 (s,
6
3
6 3
CHN, 1 H), 13.86 (s, C H OH, 1 H). 13C NMR (CDCl ): d
2
2
3
3
6
3
3
(CDCl ): d 0.88 (t, CH , 6 H), 1.26–1.86 (m, CH , 40 H), 3.98
14.06, 22.65, 26.01, 29.06, 29.33, 29.62, 31.90, 68.17, 69.28, 69.62,
101.5, 107.3, 107.4, 112.5, 112.9, 114.3, 133.2, 141.6, 148.0, 149.8,
159.4, 163.4, 163.8.
3
2
(t, OCH , 4 H), 6.88 (s, C H , 4 H). 13C NMR (CDCl ):
d 14.10, 22.68, 26.05, 29.36, 29.67, 31.92, 69.24, 114.1, 121.0,
149.2.
2
6
4
3
Bis[(N-3∞,4∞-didodecyloxyphenyl)-4-n-dodecyloxysalicylal-
diminato]copper(II). N-(2-Hydroxy-4-n-dodecyloxybenzylid-
ene)-3∞,4∞-didodecyloxyaniline (0.50 g, 0.00067 mol) dissolved
in 20 ml of absolute ethanol and 10 ml of dried THF was
added to a hot absolute ethanol solution (5.0 ml) of copper()
chloride dihydrate (0.11 g, 0.00067 mol). The mixture was
refluxed for 24 h. Methanol (10 ml) was added and the solution
was allowed to stand in a refrigerator overnight. The green
solids thus collected were redissolved in 25 ml of THF, and
the solution was then filtered through Celite 545 to remove
any insoluble solids. The THF solution was concentrated to
dryness, and the product recrystallized from THF–methanol.
The product was obtained as fine yellow–green crystals. Yield
56%. Anal. for C H O N Cu; calc. C, 75.36; H, 10.58.
1-Nitro-3,4-didodecyloxybenzene. A solution of 1,2-didodeca-
noxybenzene (5.0 g, 0.011 mol) dissolved in 150 ml of CH Cl
2
2
was added a solution of concentrated nitric acid (1·10 ml) at
0 °C. Then the mixture was refluxed for 16 h. The solution was
extracted three times with distilled water. The organic CH Cl
2
2
solution was dried in anhydrous MgSO and concentrated to
give a white solid. Milky cotton-like crystals were obtained
4
after recrystallization from CH Cl –CH OH (mp 72.0 °C, yield,
2
2
3
86.4%). 1H NMR (CDCl ): d 0.88 (t, J=6.4 Hz, CH , 6 H),
3
3
1.24–1.89 (m, CH , 40 H), 4.04 (2×d, J=6.4, 6.5 Hz, OCH ,
4 H), 6.84 (dd, J=3.0, 8.9 Hz, C H , 1 H), 7.69 (d, J=2.5 Hz,
C H , 1 H), 7.84 (dd, J=2.8, 9.0 Hz, C H , 1 H). 13C NMR
(CDCl ): d 14.07, 22.66, 25.89, 28.89, 29.30, 29.60, 31.89, 69.40,
108.0, 111.0, 117.6, 141.1, 148.6, 154.7.
2
2
6
3
6
3
6 3
98 164 8 2
Found: C, 75.02; H, 10.34%.
3
We thank the National Science Council of Taiwan, ROC for
funds (NSC-87-2113-M-008-007) in generous support of this
work.
1-Amino-3,4-didodecyloxybenzene. A mixture of 1-nitro-3,4-
didodecanoxybenzene (5.0 g, 0.01 mol) and palladium (0.10 g,
10% on carbon) was added to 150 ml of absolute ethanol in a
250 ml three-necked round bottomed flask. A solution of
hydrazine hydrate (5.0 ml) diluted in 20 ml of absolute ethanol
was slowly added to the flask and the reaction mixture was
refluxed for 24 h. Black solids were filtered off by use of Celite
545. The solution was reduced in volume to ca. 25 ml, and
then was left in a refrigerator overnight. The product was
obtained as white crystals (mp 55.0 °C, yield 86%). 1H NMR
(CDCl ): d 0.86 (t, J=6.4 Hz, CH , 6 H), 1.24–1.84 (m, CH ,
References
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2
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3
3
2
40 H), 3.40 (s, NH , 2 H), 3.89 (2×d, J=6.5, 6.5 Hz, OCH ,
2
2
4 H), 6.19 (dd, J=2.6, 8.2 Hz, C H , 1 H), 6.28 (d, J=2.5 Hz,
6
3
C H , 1 H), 6.71 (d, J=8.3 Hz, C H , 1 H). 13C NMR (CDCl ):
d 14.08, 22.68, 26.04, 29.32, 29.46, 29.65, 31.90, 68.93, 70.92,
102.7, 106.9, 117.3, 140.9, 142.0, 150.6.
6
3
6
3
3
7
8
4-Dodecyloxy-2-hydroxybenzaldehyde. 2,4-Dihydroxyben-
zaldehyde (5.0 g, 0.036 mol), KHCO (4.0 g, 0.04 mol), KI (cata-
lytic amount) and 1-bromododecane (8.1 g, 0.035 mol) were
3
mixed in 125 ml of dried acetone under a nitrogen atmosphere.
The mixture was refluxed for 24 h. The reaction mixture was
filtered while hot to remove insoluble solids. Dilute HCl (1.0 )
was added to neutralize the warm solution. The solution was
extracted twice with CHCl (100 ml). The combined CHCl
9
3
3
extract solution was concentrated to give a purple solid which
was purified by chromatography eluting with chloroform–
hexane (151, v/v). Evaporation of the solvent gave the product
as a white solid. Yield 87–93%, mp 39.0 °C. 1H NMR (CDCl ):
d 0.86 (t, J=6.4 Hz, CH , 3 H), 1.14–1.83 [m, (CH ) , 20 H],
3.97 (t, J=6.47 Hz, OCH , 2 H), 6.38 (d, J=2.3 Hz, C H , 1
H), 6.50 (dd, J=2.4, 5.5 Hz, C H , 1 H), 7.38 (d, J=8.7 Hz,
C H , 1 H), 9.67 (s, CHO, 1 H), 11.45 (s, C H OH, 1 H).
13C NMR (CDCl ): d 14.08, 22.67, 25.88, 28.91, 29.29, 29.52,
29.60, 29.72, 31.90, 68.57, 101.0, 108.8, 114.9, 135.2, 164.5,
166.4, 194.3.
3
3
2 10
2
6 3
6
3
6
3
6 3
3
10 (a) A. G. Serrette, C. K. Lai and T. M. Swager, Chem. Mater., 1994,
6, 2252; (b) C. K. Lai, A. G. Serrette and T. M. Swager, J. Am.
Chem. Soc., 1992, 114, 7948; (c) Chung K. Lai, Min-Yi Lu and Fun-
Jane Lin, L iq. Cryst., 1997, 23, 313; (d) Chung K. Lai and Fun-Jane
Lin, J. Chem. Soc., Dalton T rans., 1997, 17.
N-(2-Hydroxy-4-n-alkoxybenzylidene)-3∞,4∞-dialkoxyanilines.
4-Dodecoxy-2-hydroxybenzaldehyde (2.0 g, 0.0065 mol), 3,4-
didodecyloxyphenylamine (3.0 g, mol) and acetic acid (a few
drops) were mixed in 100 ml of dried CH Cl under a nitrogen
Paper 7/07091H; Received 10th October, 1997
2
2
602
J. Mater. Chem., 1998, 8(3), 599–602