Columnar/smectic metallomesogens derived from heterocyclic benzoxazoles

Article abstract of DOI:10.1016/j.tet.2011.07.024

The synthesis, mesomorphic behavior, and optical properties of two new series of metal complexes 1a,b-M (M=Pd, Cu, Zn) derived from benzoxazoles 2a,b are reported. The crystal and molecular structures of mesogenic 5-decyloxy-2-(6-decyloxybenzooxazol-2-yl)

Full text of DOI:10.1016/j.tet.2011.07.024

Tetrahedron 67 (2011) 7358e7369
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Columnar/smectic metallomesogens derived from heterocyclic benzoxazoles
,
I-Tzu Wu a, Pei-Yi Chaing a, Wen-Jung Chang a, Hwo-Shuenn Sheu b, Gene-Hsiang Lee c, Chung K. Lai a
*
a Department of Chemistry, National Central University and Center for Nano Science Technology, UST, Chung-Li, 32054, Taiwan, ROC
b National Synchrotron Radiation Research Centre, Hsinchu 30077, Taiwan, ROC
c Instrumentation Center, National Taiwan University, Taipei 10660, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 April 2011
Received in revised form 28 June 2011
Accepted 12 July 2011
Available online 4 August 2011
The synthesis, mesomorphic behavior, and optical properties of two new series of metal complexes
1a,beM (M¼Pd, Cu, Zn) derived from benzoxazoles 2a,b are reported. The crystal and molecular
structures of mesogenic 5-decyloxy-2-(6-decyloxybenzooxazol-2-yl)phenol and nonmesogenic bis[5-
octyloxy-2-(6-octyloxybenzooxazol-2-yl) phenol]Pd(II) were determined by means of X-ray structural
analysis. Two benzoxazoles 2a exhibited monotropic SmA phases, and all benzoxazoles 2b were non-
mesogenic. On the other hand, metal complexes 1aeM exhibited distinctly different mesomorphism
from complexes 1beM. Complexes 1aePd formed SmC phases; complexes 1aeCu and 1aeZn formed
crystal phases. In contrast, complexes 1beZn exhibited columnar phases, and complexes 1beCu and
1bePd were nonmesogenic. The difference of the mesomorphism in 1aeM and 1beM was probably
attributed to the geometry and/or the overall molecular shape created by 2a and 2b. The electronic
configuration of metal ion might play an important role in forming the mesophases. The fluorescent
properties of these compounds were also examined.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
metallomesogens (i.e., metal-containing liquid crystals) during the
past decades. Most of such known mesogenic zinc complexes were
Coordination transition metal complexes, particularly for 3d-
metals constitute the largest group among metallomesogenic ma-
terials.1 Transition metal ions often have a variety of oxidation
states and coordination numbers, therefore, numerous metal
complexes with unique structures and physical properties can be
expected. In fact, many metallomesogenic materials were obtained
from nonmesogenic organic moieties. Zinc metal, as a late transi-
tion element constructed a major category of coordination com-
pounds, which are essentially important in catalytic chemistry.2
The importance stems from their chemical stabilities, versatile
structures, and interesting photophysical behavior. A coordination
number (CN) of four is more common over six for Zn2þ ion. Zinc ion
mainly derived from a various types of macrocyclic compounds,
which included, such as phthalocyanines (Pcs),3 porphrins,4 tetra-
benzoporphine,5 and porphycenes.6 Macrocyclic motifs are well
known for their planar structures, rich conjugated
peelectrons,
thermal/photochemical stability, and high polarizability, which are
better induced for the construction of mesogens and metal-
lomesogens. In these metallomesogenic systems,
a preferred
bonding fashion of [N2N2]-donor atoms was often found. Such type
of macrocyclic compounds and/or their transition metal complexes
when substituted with linear or branched alkyl or alkoxy chains
usually formed a major class of disc-like LCs, in which strong in-
termolecular pep interactions were attributed to the formation of
prefers to coordinate both with bidentate ligands (
monodentate ligands (
1-L), and resulting complexes formulated as
neutral Zn(
2-L)2, Zn( 2-L)Cl2 or Zn( 1-L)2Cl2 were then obtained.
h
2-L) and
mesophases. In contrast, mesogenic zinc complexes derived from
other mono-/bidentate ligands, such as salicyldimines,7 2,20-
bipyridines,8 triazoles,9 pyrazoles,10 dipyrins,11 oxazolines,12 tro-
polones,13 bilinones,14 and imidazoles15 were also known. In these
examples, a bonding fashion of [N2O2] and [O2O2] was preferred.
Incorporation of a Zn2þ ion into macrocycles led to square planar
h
h
h
h
In most of zinc complexes, both tetrahedral (Td) and square planar
(SP) geometries at central Zn2þ were often obtained. In general,
metallomesogens incorporated with a square planar metal ion as
core were better induced than those of a tetrahedral one due to its
easier packing in crystal states or liquid crystal states. Zinc ion with
a diamagnetic d10-configuration has been applied to generate
complexes, whereas, other monodentate (h h
1-) or bidentate ( 2-)
structures gave more likely slightly square planar or tetrahedral
complexes. In contrast, a few mesogenic zinc complexes with a five-
coordinated geometry8b,16 were as well reported, and a distorted
trigonal bipyramid geometry (TBP) made of three nitrogen atoms
and two oxygen atoms [N3O2] at central Zn2þ was obtained. Ne-
matic phases, layered structures with smectic phases17 or two-
* Corresponding author. Tel.: þ886 03 4259207; fax: þ886 03 4277972; e-mail
address: cklai@cc.ncu.edu.tw (C.K. Lai).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.07.024

I.-T. Wu et al. / Tetrahedron 67 (2011) 7358e7369
7359
dimensional columnar phases16b,18 were commonly observed. Zinc
2. Results and discussion
complexes exhibiting lyotropic mesomorphism19 was less ob-
served. Zinc complexes based on heterocyclic derivatives also
showed interesting optical properties. Mesogenic zinc complexes
as well as other metal complexes, such as Lanthanides, Ni/Pd/Pt and
Cu/Ag/Au complexes were considered as so-called luminescent
metallomesogens.18d,20
2.1. Synthesis and characterization
The synthetic routes used to prepare benzoxazoles 2a,b and
their metal complexes 1aeM and 1beM were summarized in
Scheme 1. All synthetic procedures were similarly followed by lit-
eratures.22 The final compounds 2a,b were prepared by intracyclic
reaction of intermediates 3a,b with lead acetate Pb(OAc)4 in CHCl3
with a yield ranged from 78 to 90%. A singlet peak appeared at
Known metallomesogens derived from heterocyclic benzox-
azoles21,22 were relatively limited. A few known examples IeII were
prepared and studied by this group.22 Benzoxazoles, as well as
other heterocycles, such as oxadiazoles, benzoxale, and oxazoles
were found to exhibit interesting fluorescent properties21 and their
luminescent behavior were much affected substituent and/or
conjugation length. In this paper, we describe syntheses and me-
somorphic studies of two series of metallomesogens 1a,b derived
from benzoxazoles 2a,b exhibiting smectic A or hexagonal co-
lumnar phases. Three metal ions, Pd2þ, Cu2þ, and Zn2þ were in-
corporated to understand the effect of the central geometry on the
formation of mesophases. Complexes 1aePd formed monotropic
smectic C phases, whereas, all complexes 1aeCu and 1aeZn
formed crystal phases. In contrast, complexes 1beZn exhibited
hexagonal columnar phases (Colh), and all complexes 1beCu and
1bePd were nonmesogenic. The difference of the mesomorphic
properties in compounds 1aeM and 1beM was probably attributed
to the geometry at metal center. The d8-Pd2þ and d10-Zn2þ ions are
all diamagnetic, while d9-Cu2þ ion is paramagnetic. The electronic
configuration of metal ion might also play an important role in
forming mesophase in such electron-deficient heterocyclic com-
pounds. This is the first columnar zinc mesogens derived from
benzoxazoles.
d
8.49e8.51 ppm on 1H NMR spectra is characteristically observed
for imineeH (eC]NHe) group of compounds 3a,b, and this peak
was then disappeared upon formation of final products 2a,b.The
metal complexes were obtained by the reactions of appropriate
benzoxazoles 2a,b and copper(II), zinc(II) or palladium(II) acetate in
refluxing THF/ethanol. Copper, zinc, and palladium complexes were
isolated as a dark-green, off-white and light-yellow compounds. 1H
and 13C NMR spectroscopy were used to characterize all in-
termediates, and the elemental analysis was used to confirm the
purity of the metal complexes.
2.2. Single crystal structures of 5-decyloxy-2-(6-
decyloxybenzooxazol-2-yl)phenol (2a; n[10) and bis[5-
octyloxy-2-(6-octyloxybenzooxazol-2-yl)phenol]Pd(II)
(1aePd; n[8)
In order to understand toe possible correlation between the
molecular arrangements and mesomorphic behavior, two single
crystals of the mesogenic benzoxazole 2a (n¼10) and non-
mesogenic palladium complex 1aePd (n¼8) suitable for crystallo-
graphic analysis were obtained by slow diffusion from CH2Cl2/EtOH
at room temperature and their structures resolved. Fig. 1 shows the
two molecular structures with the atomic numbering schemes.
Table 1 lists their crystallographic and structural refinement data
for the molecules. Compound 2a (n¼10) crystallizes in a triclinic
space group Pꢀ1 with a Z¼2. The overall molecular shape of crystal
2a (n¼10) was considered as a linear structure with an overall
RO
O
RO
X
O
N
N
O
M2+
OR
O
X
M
RO
OH
N
OR
X
O
2a; X = H
2b; X = OR
OR
ꢀ
molecular length of w35.8 A. The benzoxazole and central phenolic
rings were nearly coplanar (dihedral angle¼0.645ꢁ), however, they
were not coplanar with the phenyl ring. As expected, an intra-
molecular H-bond between phenolic N1eH2 atoms was observed,
M = Pd, Cu, M = Zn; R = (CH2)nH
1a; X = H
1b; X = OR
ꢀ
and the distance was measured by ca. 1.890 A. This near coplanarity
X
of two rings was also observed in our other similar structures. The
H-bonding kept the two rings close to coplanar, and the planar
structure was responsible for a better packing both in the solid or/
and the liquid crystal state. In the unit cell, all molecules were
O
OH
N
OC10H21
2c; X = OCH3, F, Cl
arranged by antiparallel and head-to-tail arrangements, shown in
ꢀ
Fig. 2. A
pe
p
interaction with a d w3.65 A (offset face to face
alignment) is observed. No intermolecular H-bonding was formed,
which was probably attributed to a quite short range of mesophase
temperature in compounds 2a.
OR1
O
N
R2O
O
O
In contrast, the palladium complex 1a (n¼8) also crystallizes in
triclinic space group Pꢀ1 with a Z¼1. The geometry at palladium
center was coordinated as nearly perfect square planar. The two
angles of N1ePd1eO1 and N1AePd1-O1 atoms were of 91.248
(114)ꢁ and 88.752 (114)ꢁ, which were close to an angle of 90
expected for a perfect square planar geometry. The overall molec-
O
M
O
O
O
N
OR2
O
R1O
I; M = Pd, Cu
ꢀ
ular shape was considered as lath-like with a length of ca. 33.97 A.
O
O
OC12H25
C12H25
O
O
The two central planes defined by atoms Pd1eN1eC15eC16eO1
and Pd1eN1AeC15AeC16AeC21AeO1A atoms were perfectly co-
planar with a dihedral angle of 0.000 (74)ꢁ. In the unit cell, two
N
Cl Pd Cl
N
molecules were attracted with a weak pep interaction. A distance,
O
O
ꢀ
O
II
OC12H25
w3.500 A of point-to-face alignment between the phenyl ring
C16eC21 and H2A and C16AeC21A and H2 was obtained. All alkoxy
chains were interdigitated and the Van der Waal interaction be-
tween the alkoxy chains was observed. Also a weak interaction
C12H25
O

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I.-T. Wu et al. / Tetrahedron 67 (2011) 7358e7369
RO
HO
X
HO
a
OH
OH O
X = H or OH
OH
d
b
RO
RO
RO
c
NO2
NH2
X
OH
OH
OH O
X = H or OR
e
OR
O
N
RO
f
N
RO
OR
OH
X
OH
HO
X
2a; X = H
3a; X = H
3b; X = OR
2b; X = OR
g
RO
X
O
O
N
N
O
OR
M
RO
X
O
OR
R = (CH2)nH
M = Pd, Cu, Zn
1a: X = H
1b; X = OR
Scheme 1. Reactions and Reagents. a. RBr (1.1 equiv), KHCO3 (1.5 equiv), refluxing in acetone, 24 h, 62e68%. b. HNO3 (1.2 equiv), NaNO2 (0.20 equiv), stirred at 0ꢁ C in CH2Cl2, 6 h,
25%. c. H2NNH2 (1.20 equiv), Pd/C (0.10 equiv), refluxing in C2H5OH, 4 h, 75e88% d. RBr (1.1 or 2.10 equiv), KHCO3 (2.50 equiv), refluxing in acetone, 48 h, 38e62%. e. Acetic acid
(drops), refluxing in C2H5OH, 24 h, 81e90%. f. Pb(OAc)4 (1.10 equiv), refluxing in CHCl3, 4 h, 78e90%. g. M(OAc)2 (1.10 equiv), refluxing in THF/C2H5OH, 20 h, 88e92%.
between the O3eH14A and O3AeH14 with a distance of 3.0851
(29) A was found. A layered structure for 2a and 1aePd was ob-
served and the molecular arrangement in unit cell was also shown
in Fig. 3.
the mesophases were kinetically unstable. All clearing or melting
temperatures increased with carbon length, indicating that the van
der Waals interaction between alkoxy chains was important. The
two mesogenic derivatives (n¼10, 12) have a relatively short tem-
ꢀ
perature range of mesophase on cooling process,
D
T¼6.80 ꢁC
2.3. Mesomorphic properties of benzoxazoles 2a,b
(n¼10) w11.7 ꢁC (n¼12). This type of mesophase dependence on
the carbon length is quite common in rod-like mesogens. The
mesophases were characterized as smectic A phases by optical
microscope, as their textures shown in Fig. 4. On cooling process,
typical focal-conic textures with some homeotropic domains were
easily observed. Within the layers of a smectic A phase, the long
axes of the molecules lie parallel to one another, their direction
being normal to the plane of the layer. All molecules are free to
rotate about their long axes, and the distribution within the layers
is random. This result was also supported by crystallographic data
The liquid crystalline behavior of benzoxazole derivatives 2a,b
and their copper, zinc and palladium complexes 1aeM and 1beM
was investigated and studied by differential scanning calorimeter
(DSC) and polarizing optical microscope (POM). The phase transi-
tions and thermodynamic data of compounds 2a,b were summa-
rized in Table 2. Compounds 2a formed mesomorphic behavior only
for two derivatives n¼10 and 12, while all others (n¼8, 14, 16) were
not mesogenic. The phases were monotropic, which indicated that

I.-T. Wu et al. / Tetrahedron 67 (2011) 7358e7369
7361
Fig. 1. Two ORTEP plots for 2a (n¼10; top) and 1aePd (n¼8) with the numbering scheme, and the thermal ellipsoids of the non-hydrogen are drawn at the 50% probability level.
Selected bond lengths of 2a; N1eC1, 1.3011 (21), O1eC1, 1.3811 (24), N1eC13, 1.4037 (24), O1eC8, 1.3814 (19), O2eH2, 0.8397 (12). Selected bond angles and bond lengths in 1aePd:
N1ePd1eO1, 91.248 (114)ꢁ; O1ePd1eN1A. 88.752 (114)ꢁ; N1AePd1eO1A, 91.248 (114)ꢁ; O1AePd1eN1, 88.752 (114)ꢁ; Pd1eN1, 2.0348 (27); Pd1eO1, 1.9648 (24); Pd1eN1A,
2.0348 (27); Pd1eO1A, 1.9648 (24).
Table 1
to-isotropic transition (Cr/I) with a larger enthalpy was all ob-
served. The lack of mesomorphism was probably attributed to the
Crystallographic and experimental data for compound 2a and 1aePd
shape effect. The overall molecular shape of compounds 2b was
considered as a lath-like, between rod-like and disc-like molecules,
which might not facilitate the formation of the mesophases.
Compound
2a (n¼10)
C33H49NO4
523.73
1aePd (n¼8)
C56H74N2O8Pd
1009.57
Empirical formula
Formula weight
T/K
100 (2)
293 (2)
Crystal system
Space group
Triclinic
Pꢀ1
Triclinic
Pꢀ1
2.4. Mesomorphic properties of compound 1a,b
ꢀ
a/A
6.7337 (3)
12.2682 (4)
19.2682 (7)
106.2964 (15)
97.5945 (16)
93.7590 (16)
1505.56 (10)
2
5.5372 (2)
14.3557 (5)
18.2610 (6)
73.437 (2)
84.119 (2)
88.383
ꢀ
b/A
In order to understand the effect of the metal ion incorporated
on the formation of the mesomorphic properties, a variety of
complexes Cu2þ, Pd2þ and Zn2þ were prepared and studied. All
metal complexes of 1aeM with four alkoxy chains are more rod-
like molecules, while metal complexes 1beM with six alkoxy
chains are more likely disc-like molecules. The Pd2þ and Zn2þ ions
are d8- and d10-diamagnetic, while Cu2þ ion is d9-paramagnetic.
Pd2þ ion prefers a square planar geometry. Meanwhile, Pd2þ or
Cu2þ ion prefers a square planar or a slightly square planar geom-
etry, and Zn2þ ion favors only a tetrahedral geometry.
The phase transitions and thermodynamic data of metal com-
plexes 1aeM and 1beM were summarized in Table 3. Complexes
1aePd (n¼10, 12, 14) are mesogenic, showing monotropic proper-
ties. On the other hand, both complexes 1aeCu and 1aeZn were
not mesogennic. The clearing temperatures of three metal com-
plexes 1aeM were all higher than their precursor compounds 1a as
ꢀ
c/A
ꢁ
ꢁ
a
/
b
/
ꢁ
/
g
3
ꢀ
U/A
1384.02 (8)
1
534
Z
F(000)
572
1.155
Dc/Mg mꢀ3
1.211
Crystal size/mm3
Range for data collection/ꢁ
Reflection collected
Data, restraints, parameters
Independent reflection
Final R1, wR2
0.40ꢂ0.25ꢂ0.10
1.11e25.00
16,988
5236/0/346
5236 [R(int)¼0.0550]
R1¼0.0490, 0.1134
0.33ꢂ0.13ꢂ0.04
1.17e28.43
24,588
6862/0/306
6862 [R(int)¼0.0310]
R1¼0.0536, 0.1435
discussed above. The crystallographic data indicated that all mol-
ecules were in fact antiparallel arranged (head-to-tail arrange-
ment) between the neighboring layers. The benzoxazole was
relatively considered as a larger electron-deficient core, and gen-
erally more extended terminal chains were required to induce the
phase. Also in order to understand the effect of the polar groups on
the formation of the mesophase, three derivatives 2c (X¼OCH3, F,
Cl) with a polar group substituted on the opposite end of the
benzoxazole were also studied. The eOCH3, eF, and eCl groups
expected, by a
D
D
Tcl¼40.9 ꢁC (n¼16)ꢀ86.7 ꢁC (n¼8) for 1aePd,
Tcl¼40.2 ꢁC (n¼16)ꢀ94.6 ꢁC (n¼8) for 1aeCu and
D
Tcl¼26.8 ꢁC
(n¼16)ꢀ109.3 ꢁC (n¼8) for 1aeCu. This was attributed to the larger
molecular size or/and a more rigid core. All complexes 1aePd
(n¼10, 12, 14) except two derivatives (n¼8, 16) exhibited meso-
morphic behavior. The clearing temperature decreased slightly
with the terminal carbon length; by Tcl¼154.2 ꢁC (n¼8)>136.2 ꢁC
(n¼12)>129.1 ꢁC (n¼16). The influence of terminal alkoxy chains
was not apparent in 1aePd. Alkoxy chain lengths often has a more
influence on the formation of rod-like liquid crystals; shorter
chains prefer nematic and/or smectic A phase, while longer chains
length form smectic C and/or higher Smectic X phase. An in-
termolecular interaction enhanced by van der Waals force of
were considered as strong p-donors and also electron-withdrawing
groups. However, their mesomorphic properties were not im-
proved. Only a crystal-to-isotropic transition was observed at
Tcl¼83.8 ꢁC, 65.3 ꢁC and 65.9 ꢁC for derivatives X¼eOCH3, eF and
eCl. In contrast, all compounds 2b (n¼8, 12, 16) substituted with
two more flexible terminal chains were nonmesogenic. A crystal-

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I.-T. Wu et al. / Tetrahedron 67 (2011) 7358e7369
Fig. 2. The
pe
p
interactions observed in the unit cell of 2a (top) and 1aePd (bottom).
alkoxy chains increases the interdigitation of alkoxy chains,
often leading to higher order of mesophases observed
(Cr/N/SmA/SmC/SmX). The phases were identified and
confirmed as smectic C phases by polarized microscope, and typical
optical schlieren textures, shown in Fig. 5 were observed when
cooling from isotropic states. The temperature range was relatively
respectively. The clearing temperatures for complexes 1aeCu and
1aeZn decreased with carbon lengths; Tcl¼162.1 ꢁC (n¼8)>
140.0 ꢁC (n¼12)>128.4 ꢁC (n¼16) for 1aeCu and Tcl¼176.8 ꢁC
(n¼8)>142.1 ꢁC (n¼12)>115.0 ꢁC (n¼16) for 1aeZn (n¼8). The
1aeZn has a slightly higher clearing temperature than that of
1aeCu, indicating that the intermolecular force is stronger in
1aeZn than 1aeCu. Zinc complexes often have tetrahedral geom-
etries, while copper complexes have slightly square planar or
square planar geometries for such bidentate ligands as
benzoxazoles.
Interestingly, metal complexes 1beM showed distinctly differ-
ent mesomorphic properties from metal complexes 1aeM. All
complexes 1beZn exhibited hexagonal columnar phase (Colh) and
other complexes 1beCu and complexes 1bePd formed crystal
phases. Compounds 2b have an extra alkoxy chains, giving to a total
number of six alkoxy chains for complexes 1beM. For complexes
1beZn, the DSC analysis showed a typical columnar phase transi-
tion, crystal-to-columnar-to-isotropic (Cr/Col/I). All compounds
a
short,
D
TSmC¼7.8 ꢁC (n¼8), w7.9 ꢁC (n¼12), and w4.7 ꢁC (n¼14).
The monotropic behavior and their shorter temperature ranges also
indicated that the mesophases were kinetically unstable. A few
other benzoxazoles and metal complexes I were previously repor-
ted in this group; palladium complexes also formed a relatively
short temperature range (
D
Tn¼8.3e39.8 ꢁC). In general, in order for
a rod-like molecule to form nematic, SmA or/and SmC phase, an
appropriate aspect ratio (d/l) is often needed. All these Pd com-
plexes are not truly rod-like, but a lathe-like molecule. However, all
complexes 1aeCu and 1aeZn complexes were not liquid crystals. A
transition of crystal-to-isotropic was only observed, for example;
Tcl¼162.1 ꢁC and Tcl¼176.8 ꢁC for 1aeCu (n¼8) and 1aeZn (n¼8),

I.-T. Wu et al. / Tetrahedron 67 (2011) 7358e7369
7363
Fig. 3. The molecular arrangements for 2a (top) and 1aePd (bottom) in unit cell.
1beZn (except for the shorter derivative n¼8) exhibited enantio-
tropic behavior. The only complex with the shortest chain (n¼8)
was not mesogenic due to the insufficient chain flexibility. Both
melting and clearing temperatures decreased with carbon chain
length, Tmp¼160.6 ꢁC (n¼10)>127.1 ꢁC (n¼16) and Tcl¼163.9 ꢁC
(n¼10)>142.0 ꢁC (n¼16). On the other hand, the temperature range
Table 2
Phase transitions and enthalpiesa of compounds 2a,b
67.5 (38.3)
2a; n = 8
Cr
I
I
47.1 (32.2)
70.5 (44.2)
of columnar phase stayed close to
D
TCol¼18.5 ꢁC (n¼8)ꢀ15.0 ꢁC
10
12
Cr
Cr
SmA
60.8 (2.73)
79.1 (55.6)
49.1 (31.7)
65.6 (38.8)
(n¼12) on the cooling process. In addition, a smaller enthalpy
(DH¼3.23e6.03 kJ/mol on heating cycle) for the Colh/I transition
I
I
SmA
Cr
72.4 (7.03)
84.1 (69.0)
was observed, indicating that the molecular order in the columnar
phase was slightly ordered. The mesophase was first characterized
by optical microscope. Under POM, for example, compound 1beZn
(n¼12) displayed an optical texture of pseudo focal-conics with
linear birefringent defects (Fig. 5), suggesting a columnar structure.
A small area of homeotropic domains was also observed. In co-
lumnar mesophases, the disc-like molecules stack into columns in
a hexagonal (Colh) or rectangular (Colr) superstructure. Within
a column the molecules can be either periodically or aperiodically
stacked. However, all complexes 1beCu and 1bePd were not
mesogenic. A single transition from crystal phase to isotropic state
(Cr/I) was obtained, at Tcl¼117.5 ꢁC and Tcl¼136.2 ꢁC for 1beCu
and 1bePd, respectively. The observed mesomorphism was dis-
tinctly different from compounds 1aeM. The lack of mesomorphic
properties in this series of copper and palladium complexes might
be attributed to the central geometry. Zinc ion often prefers
14
16
72.6 (64.6)
88.2 (89.5)
Cr
Cr
Cr
Cr
I
76.7 (86.7)
75.5 (52.8)
2b; n = 8
I
51.2 (54.8)
90.0 (27.2)
I
I
12
78.9 (56.8)
89.3 (98.1)
16
77.9 (101)
a
Phase transition temperature (ꢁC) and enthalpies (KJ/mol) were determined by
DSC at a scan rate of 20ꢁ C minꢀ1. Cr¼crystalline, SmA¼smetic A and I¼isotropic
phase.

7364
I.-T. Wu et al. / Tetrahedron 67 (2011) 7358e7369
Fig. 4. The optical textures observed. SmA phase by 2a (n¼10; left) at 60 ꢁC; SmA phase by 2a (n¼12; right) at 72 ꢁC.
Table 3
1beZn>1bePd>1beCu. The molecular interaction in 1bePd and
1beCu might not be strong enough for complexes 1bePd and
1beCu to be induced for the formation of the mesophases. How-
ever, the benzoxazole is often considered as an electron-deficient
heterocycle. The electronic configuration of the central ions, d9-Cu
and d8-Pd might be another important factor for the lack of any
mesophase formed.
Phase transitions and enthalpiesa of compounds 1aeM and 1beM
154.2 (39.7)
I
I
I
1a-Pd; n = 8
Cr
132.4 (36.2)
140.9 (45.4)
10
Cr
Cr
Cr
SmC
SmC
118.4 (10.7)
136.2 (57.2)
110.6 (33.8)
109.3 (40.8)
12
14
2.5. Powder X-ray diffraction of compounds 1b
117.2 (9.96)
130.3 (83.4)
I
I
I
Variable-temperature powder XRD diffraction experiments
were conducted to confirm the structures of the mesophases of
compounds 1beZn (n¼16). A typical diffraction pattern of a two-
dimensional hexagonal lattice with one very strong diffraction
peak at lower angle and two much weaker diffraction peaks was all
observed. These are characteristic of hexagonal columnar phases
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. For example, a diffraction
SmC
Cr
110.4 (11.2)
129.1 (95.0)
105.7 (70.1)
16
109.7 (93.5)
162.1 (45.5)
1a-Cu; n = 8
Cr
144.3 (44.6)
140.0 (49.4)
117.6 (14.9)
Cr1
Cr1
Cr2
I
12
131.0 (48.6)
128.4 (89.0)
115.7 (13.7)
61.7 (27.9)
ꢁ
ꢀ
ꢀ
ꢀ
pattern at 130 C with a d-spacing at 28.2 A, 16.3 A, 14.1 A and
Cr2
Cr
I
I
I
I
I
I
I
I
I
ꢀ
16
a broad diffuse peak (w4.71 A) at wide-angle region was observed
120.7 (83.4)
176.8 (22.4)
52.1 (22.8)
for compound 1beZn (n¼16) and this diffraction pattern corre-
sponded to a hexagonal columnar arrangement (see Fig. 6). This
diffraction pattern corresponds to an inter-columnar distance or
lattice constant (i.e., a parameter of the hexagonal lattice) of
1a-Zn; n = 8
150.6 (20.3)
142.1 (19.6)
Cr
12
116.1 (16.3)
115.0 (20.9)
ꢀ
a¼32.6 A. This lattice constant is slightly than the length of mole-
78.7 (14.5)
73.9 (13.4)
cule calculated by MM2, indicating that the alkoxy chains might be
slightly ꢁ interdigitated. At lower nonmesogenic temperature
Cr1
Cr2
Cr
16
99.4 (18.0)
156.2 (8.26)
ꢀ
(T¼100 C) the first peak moved to smaller angle (d w29.3 A). The
1b-Zn; n = 8
ꢀ
137.6 (5.79)
163.9 (3.23)
presence of a relatively weak peak at wide angles (d w3.60 A) in-
160.6 (3.87)
dicated that all molecules might be slightly ordered along the
columns. However, liquid-like correlations between the rigid cores
Colh
Cr
10
160.7 (6.01)
157.0 (3.26)
142.2 (7.15)
153.0 (0.94)
ꢀ
occurred at wide-angle regions of 4.71 A. The hexagonal lattices
Colh
Colh
Colh
Cr
Cr
Cr
Cr
12
14
were correlated well with side-chain alkoxy carbon lengths.
152.0 (5.20)
152.4 (6.34)
137.2 (3.47)
143.7 (5.54)
2.6. Optical properties
128.3 (1.44)
127.1 (2.89)
145.9 (5.03)
142.0 (4.18)
Heterocyclic benzoxazoles have been an excellent candidate as
potential light emitting materials due to their photophysical and
fluorescent properties. The luminescent properties observed by this
type of heterocyclic molecules were often affected by polar sub-
stituents, position and/or conjugation length. 2-(2-Hydroxyphenyl)
benzoxazole (HBO)23 has been extensively studied due to its dual
emission via the excited-state intramolecular proton transfer
(ESIPT).22 In contrast, known luminescent metallomesogens20 were
relatively rare. A few ions, particularly of d8-Pd2þ/Pt2þ, d10-Cuþ and
16
137.9 (4.13)
117.5 (90.2)
121.7 (1.60)
I
I
1b-Cu; n = 10
100.0 (94.2)
143.5 (108)
Cr
1b-Pd; n = 10
133.5 (107)
a
n is the carbon numbers in the alkoxy chains. Cr¼crystal phase, SmC¼smectic C
and Colr¼rectangular columnar phase. The transition temperature (ꢁC) and en-
thalpies (in parenthesis, KJ/mol) are determined by DSC analysis at a scan rate of
10.0 ꢁC/min.
d
10-Zn2þ are in fact considered as promising ions to be potentially
luminescent metallomesogens. Absorption and emission spectra of
compounds 2a,b (n¼12), 1aeZn (n¼12) and 1beZn (n¼12) mea-
sured in CH2Cl2 at 25 ꢁC are shown in Fig. 7. The highest absorption
peaks of all compounds were found to be insensitive to the carbon
a
tetrahedral geometry, while copper and palladium favors
a slightly square planar or square planar geometry. The clearing
temperatures of compounds 1beM were ranged in order of

I.-T. Wu et al. / Tetrahedron 67 (2011) 7358e7369
7365
Fig. 5. The optical textures observed. SmC phase by 1aePd (n¼10; top left) at 112 ꢁC, SmC phase by 1aePd (n¼12; top right) at 114 ꢁC, 1beZn (n¼12; bottom left), and Colh phase at
150 ꢁC and Cr phase at 70 ꢁC (bottom right).
18000
length. Compounds 2a,b showed two, low-energy
pep*; absorp-
tion at 328e329 nm (as lmax) and 344e345 nm (see Table 4). The
former band was assigned to the anti-enol, and the later band
corresponded to the syn-enol. On the other hand, complexes 1aeZn
and 1beZn occurred at 328e329 nm (as lmax) and 344e345 nm.
The emission peaks for 2a and 2b occurred at 467.0 nm and
370.4 nm, while 1aeZn and 1beZn occurred at 407.1 nm and
440.8 nm, respectively. Compound 2b showed a blue-shift of ca.
96.6 nm over 2a due to a promotion effect of ESIPT. Compound 2b
has a second alkoxy group substituted on meta-position to the
16000
14000
12000
10000
8000
6000
4000
2000
0
d
10 ∼ 28.2 Å
4.71 Å
∼ 3.60 Å
d
20 ∼ 16.3 Å
11 ∼ 14.1 Å
benzoxazole ring. The fluorescence emission originates a
pep*;
d
transition. The emission fluorescent spectra measured in different
solvents and solid state were also studied. A slight shift, for example
for 2a (n¼12) was observed; lmax¼466 nm as solid, 467 in CH2Cl2,
476 in hexane and 461 nm in ethanol. The emission wavelength in
the solid state remained the same as that in CH2Cl2 with lower
emission intensity. In general, benzoxazole, when measured in
nonpolar solvent emitted long wavelength fluorescence exclusively
via the ESIPT process of the excited state of the intramolecular
hydrogen-bonded enol. On the other hand, compound 1beZn
showed a slight red-shift of ca. 33.7 nm over 1aeZn. The zinc
complexes emit more intensity in the same spectral region than
those of free ligands, and also, non-planar geometry at zinc(II) often
prevents the formation of excimers.20b The quantum yields esti-
mated with anthracene as a standard (4f¼0.27 in hexane) were
ranged from ca. 0.02e0.03% and 0.10e0.18% for 2a,b and 1a,b,
respectively.
-2000
5
10
15
20
25
30
2 Theta
25000
20000
15000
10000
5000
0
d∼ 29.3 Å
3. Conclusions
Two series of metallomesogens derived benzoxazoles 1a,b were
prepared and their mesomorphic properties studied. Compounds
2a (n¼10, 12) exhibited monotropic smectic A phases. Complexes
1beM exhibited distinctly different mesomorphism from com-
plexes 1aeM. Complexes 1aePd (n¼10, 12, 14) formed monotropic
smectic C phases, whereas, all complexes 1aeCu and 1aeZn
formed crystal phases. In contrast, complexes 1beZn (n¼10, 12, 14,
5
10
15
20
25
30
2 Theta
Fig. 6. The XRD diffraction plots for 1beZn (n¼16) at 130 ꢁC (top) and at 100 ꢁC
(bottom).

7366
I.-T. Wu et al. / Tetrahedron 67 (2011) 7358e7369
16) exhibited hexagonal columnar phases, and all other complexes
2a (n = 12)
0.8
0.6
0.4
0.2
0.0
1beCu and 1bePd were nonmesogenic. The difference of the
mesomorphic properties in compounds 1aeM and 1beM (M¼Pd,
Zn, Cu) was probably attributed to the geometry at metal center.
Zinc(II) complexes 1a,beZn emitted typical large luminescent
2b (n = 12)
1a-Zn (n = 12)
1b-Zn (n = 12)
bands originating from
pep*; transitions, centered at ca.
407.1e440.8 nm in CH2Cl2 at room temperature, while palladium
and copper complexes 1a,beM (M¼Pd, Cu) were in fact non-
emissive. The quantum yields were relatively lower.
4. Experimental section
4.1. General
270
300
330
360
390
All chemicals and solvents were reagent grades from Aldrich
Chemical Co. Tetrahydrofuran (THF) was distillated from Na metal
or CaH2, and CH2Cl2 was distillated from P2O5. 1H and 13C NMR
spectra were measured on a Bruker DRS-200. DSC thermographs
were carried out on a Mettler DSC 822 and calibrated with a pure
indium sample. All phase transitions are determined by a scan rate
of 10.0 ꢁC/min. Optical polarized microscopy was carried out on
Zeiss Axioplan 2 equipped with a hot stage system of Mettler FP90/
FP82HT. The UVevis absorption and fluorescence spectra were
obtained using a Jasco V-530 and Hitachi F-4500 spectrometer. All
spectra were measured in CHCl3 at room temperature, and the
excitation wavelength of the fluorescent spectra was 397 nm. Ele-
mental analyses were performed on a Heraeus CHNeO-Rapid ele-
mental analyzer, and their mass spectra were also measured. The
powder diffraction data were collected from the Wiggler-A beam
line at National Synchrotron Radiation Research Center (NSRRC)
Wavelength [nm]
2a (n = 12)
2b (n = 12)
1a-Zn (n = 12)
1b-Zn (n = 12)
2000
1600
1200
800
400
0
ꢀ
with a wavelength of 1.3263 A. Diffraction patterns were recorded
in
q/2
q
geoꢀm1etry with step scans normally 0.02ꢁ in
2q
¼1e10ꢁ step sꢀ1 and 0.05ꢁ in 2
q
¼10e5ꢁ stepꢀ1 sꢀ1 and a gas
350
420
490
560
630
flow heater was used to control the temperature. The powder
samples were charged in Lindemann capillary tubes (1.00 mm)
from Charles Supper. The synthetic procedures for compounds, 3-
alkoxyphenols, 5-alkoxy-2-nitrophenols, 2-amino-5-alkoxy phe-
nols, 4-alkoxyxy-2-hydroxybenzaldehydes, and 2-hydroxy-4-
alkoxy-N-(2-hydroxy- 4-alkoxybenzylidene)anilines were fol-
lowed by literatures.24
Wavelength [nm]
300
240
180
120
60
2a (n=12) solid
2a (n=12) in DCM
2a (n=12) in Hex
2a (n=12) in EtOH
4.1.1. 3-Dodecyloxyphenol. White solid; yield 52%. 1H NMR (CDCl3):
d
0.86 (t, 3H, eCH3, J¼6.4 Hz), 1.25e1.40 (m, 18H, eCH2), 1.67e1.78
(m, 2H, eOCH2CH2), 3.90 (t, 2H, eOCH2, J¼6.6 Hz), 6.39e6.49 (m,
3H, AreH), 7.10 (t, 1H, AreH, J¼8.5 Hz). 13C NMR (CDCl3):
d 14.12,
22.68, 26.01, 29.19, 29.34, 29.38, 29.57, 29.63, 31.90, 68.02, 101.99,
107.00, 107.51, 130.05, 156.66, 160.48.
4.1.2. 5-Dodecyloxy-2-nitrophenol. Yellow solid; yield 42%. 1H NMR
(CDCl3):
d
0.85 (t, 3H, eCH3, J¼6.3 Hz), 1.24e1.45 (m, 18H, eCH2),
1.71e1.82 (m, 2H, eOCH2CH2), 3.99 (t, 2H, eOCH2, J¼6.5 Hz),
6.45e6.51 (m, 2H, AreH), 8.00 (d, 1H, AreH, J¼10.1 Hz), 11.03 (s, 1H,
0
AreOH). 13C NMR (CDCl3):
d 14.10, 22.66, 25.83, 28.79, 29.24, 29.32,
400
450
500
550
600
29.49, 29.54, 29.60, 31.88, 69.11, 101.73, 109.83, 114.54, 126.89,
157.99, 166.71.
Wavelength [nm]
4.1.3. 2-Amino-5-dodecyloxyphenol. Off-white solid; yield 86%. 1H
NMR (CDCl3): 0.86 (t, J¼6.7 Hz, eCH3, 3H), 1.24e1.38 (m, eCH2,
18H), 1.67e1.71 (m, eOCH2, 2H), 3.84 (t, J¼6.5 Hz, eOCH2, 2H),
6.42e6.45 (d, J¼7.6 Hz, eArH, 2H), 6.74 (s, eArH, 1H), 11.03(s, eOH,
1H). 13C NMR (CDCl3): 14.10, 23.12, 28.81, 29.25, 29.26, 29.32, 29.5,
29.49, 29.61, 31.91, 72.28, 102.17, 107.60, 117.23, 125.64, 144.53,
Fig. 7. UV (top) and PL spectra (center and bottom) of the compounds 2a,b.
Table 4
A summary of UV and PL peaksa for compounds 2a,b and 1a,beZn
Compds
UV (nm)
PL (nm)
2a (n¼12)
329 (lmax), 319 (sh), 345
328 (lmax), 314 (sh), 344
329 (lmax), 320 (sh), 345
328 (lmax), 313 (sh), 344
467.0
370.4
407.1
440.8
2b (n¼12)
162.17. IR (neat): 3352, 3268 (eNH2) cmꢀ1
.
1aeZn (n¼12)
1beZn (n¼12)
4.1.4. 4-Dodecyloxy-2-hydroxybenzaldehyde. White solid; yield
54%. 1H NMR (CDCl3):
0.86 (t, 3H, eCH3, J¼6.8 Hz), 1.24e1.43 (m,
a
Spectra measured in CH2Cl2 at rt.
d

I.-T. Wu et al. / Tetrahedron 67 (2011) 7358e7369
7367
18H, eCH2), 1.74e1.84 (m, 2H, eOCH2CH2), 3.97 (t, 2H, eOCH2,
J¼6.6 Hz), 6.38 (s, 1H, AreH), 6.49 (dd,1H, AreH, J¼1.8, 8.7 Hz), 7.38
(d, 1H, AreH, J¼8.5 Hz), 9.67 (s, 1H, AreCHO), 11.45 (s, 1H, AreOH).
163.30. Anal. Calcd for C29H41NO4: C, 74.48; H, 8.84; N, 3.00. Found C,
74.45; H, 8.84; N, 2.93.
13C NMR (CDCl3):
d
14.12, 22.70, 25.93, 25.98, 26.06, 28.20, 28.79,
4.1.9. 5-Dodecyloxy-2-(6-dodecyloxybenzoxazol-2-yl)phenol
(2a;
28.94, 29.32, 29.36, 29.46, 29.55, 29.56, 29.59, 29.64, 31.93, 32.87,
33.99, 68.61, 101.06, 108.76, 115.02, 135.17, 164.55, 166.47, 194.25.
n¼12). White solid; yield 75%. 1H NMR (CDCl3):
d 0.86 (t, 6H, eCH3,
J¼6.9 Hz), 1.25e1.46 (m, 36H, eCH2), 1.73e1.84 (m, 4H, eOCH2CH2),
3.95e4.00 (m, 4H, eOCH2), 6.51e6.58 (m, 2H, AreH), 6.91 (q, 1H,
AreH, J¼2.4 Hz), 7.06 (d, 1H, AreH, J¼2.1 Hz), 7.50 (d, 1H, AreH,
4.1.5. 2-Hydroxy-4-dodecyloxy-N-(2-hydroxy-4-dodecyloxybenzyli-
dene)aniline (3a; n¼12). Light yellow solid; yield 78%. 1H NMR
J¼8.7 Hz), 7.82 (d, 1H, AreH, J¼8.7 Hz). 13C NMR (CDCl3):
d 14.13,
(CDCl3):
d
0.88 (t, 6H, eCH3, J¼6.9 Hz), 1.27e1.46 (m, 36H, eCH2),
22.70, 26.00, 26.05, 29.09, 29.24, 29.37, 29.60, 29.65, 31.93, 68.25,
68.93, 96.23, 101.66, 103.88, 107.98, 113.21, 118.64, 127.77, 133.73,
149.88, 157.52, 160.07, 162.31, 163.31. Anal. Calcd for C37H57NO4: C,
76.64; H, 9.91; N, 2.42. Found C, 76.59; H, 9.92; N, 2.26.
1.74e1.80 (m, 4H, eOCH2CH2), 3.94 (t, 2H, eOCH2, J¼6.6 Hz),
3.99 (t, 2H, eOCH2, J¼6.6 Hz), 6.47e6.50 (m, 3H, AreH), 6.56
(s, 1H, AreH), 7.04 (d, 1H, AreH, J¼8.8 Hz), 7.24 (t, 1H, AreH,
J¼4.6 Hz), 8.51 (s, 1H, AreCH]NeAr). 13C NMR (CDCl3):
d 13.92,
22.58, 25.94, 25.99, 29.06, 29.21, 29.24, 29.27, 29.31, 29.47, 29.50,
29.55, 29.57, 31.84, 68.38, 68.49, 101.73, 101.83, 107.68, 107.86,
113.37, 118.12, 129.19, 133.55, 150.95, 159.65, 160.51, 162.76,
163.69.
4.1.10. 5-Tetradecyloxy-2-(6-tetradecyloxybenzoxazol-2-yl)phenol
(2a; n¼14). White solid. 1H NMR (CDCl3):
d 0.86 (t, 6H, eCH2,
J¼6.9 Hz), 1.24e1.46 (m, 44H, eCH2), 1.73e1.85 (m, 4H, eOCH2CH2),
3.97e4.01 (m, 4H, eOCH2), 6.52e6.58 (m, 2H, AreH), 6.91 (q, 1H,
AreH, J¼2.4 Hz), 7.07 (d, 1H, AreH, J¼1.8 Hz), 7.50 (d, 1H, AreH,
4.1.6. 2-Hydroxy-4-dodecyloxy-N-(2-hydroxy-3, 4-didodecyloxy-
benzylidene)aniline (3b; n¼12). The mixture of 2-amino-5-
dodecyloxyphenol (2.0 g, 6.82 mmol), 3,4-didodecanoxy-2-
hydroxybenzaldehyde (2.8 g, 6.20 mmol) and a few drops of
acetic acid was refluxed in 100 mL of absolute ethanol for 24 h
under nitrogen atmosphere. The orange-brown crude product
was collected. The product isolated as yellow-orange solids was
obtained after recrystallization from CH2Cl2/hexane (1/1). Yield
J¼8.7 Hz), 7.83 (d, 1H, AreH, J¼8.7 Hz). 13C NMR (CDCl3):
d 14.13,
22.70, 26.00, 26.06, 29.10, 29.24, 29.38, 29.60, 29.68, 31.93, 68.25,
68.93, 96.23, 101.66, 103.88, 107.98, 113.20, 118.64, 127.77, 133.73,
149.88, 157.52, 160.07, 162.31, 163.30. Anal. Calcd for C41H65NO4: C,
77.43; H, 10.30; N, 2.20. Found C, 77.24; H, 10.30; N, 2.08.
4.1.11. 1-Hexadecyloxy-2-(6-hexadecyloxybenzoxazol-2-yl)phenol
(2a; n¼16). White solid. 1H NMR (CDCl3):
d 0.84 (t, 6H, eCH3,
85%. 1H NMR (CDCl3):
54H, eCH2), 1.70e1.81 (m, 6H, eOCH2), 3.90 (t, 2H, eOCH2,
J¼6.50 Hz), 3.99e4.05 (m, 4H, eOCH2), 6.42e6.60 (m, 3H, AreH),
7.04e7.09 (m, 2H, AreH), 8.49 (s, 1H, AreCH]NeAr). 13C NMR
d
0.87 (t, 9H, eCH3, J¼6.9 Hz), 1.26e1.41 (m,
J¼7.2 Hz), 1.22e1.46 (m, 52H, eCH2), 1.72e1.83 (m, 4H, eOCH2CH2),
3.95e3.99 (m, 4H, eOCH2), 6.50e6.57 (m, 2H, AreH), 6.90 (q, 1H,
AreH, J¼2.1 Hz), 7.06 (d, 1H, AreH, J¼1.2 Hz), 7.49 (d, 1H, AreH,
J¼8.70 Hz), 7.82 (d, 1H, AreH, J¼8.70 Hz). 13C NMR (CDCl3):
d 14.13,
(CDCl3):
d
14.09, 22.67, 26.02, 26.05, 29.34, 29.40, 29.57, 29.63,
22.70, 26.00, 26.05, 29.09, 29.24, 29.38, 29.60, 29.70, 31.93, 68.25,
68.93, 96.24, 101.66, 103.89, 107.98, 113.21, 118.65, 127.77, 133.75,
149.89, 157.52, 160.07, 162.31, 163.30. Anal. Calcd for C45H73NO4: C,
78.10; H, 10.63; N, 2.02. Found C, 78.16; H, 10.59; N, 1.86.
31.90, 68.27, 68.76, 73.28, 101.65, 104.26, 107.44, 114.41, 117.88,
127.77, 135.98, 151.13, 156.26, 156.57, 159.30, 159.59. IR (neat):
2982, 2921, 2852, 1633, 1601, 1442, 1276, 1189, 1110, 922,
786 cmꢀ1
.
4.1.12. 2,3-Didodecyloxy-6-(6-dodecyloxybenzoxazol-2-yl)phenol (2b;
n¼12). The solution of 2-hydroxy-4-dodecyloxy-N-(2-hydroxy-3, 4-
didodecyloxybenzylidene)aniline (1.0 g, 1.31 mmol) dissolved in
warm CHCl3 was slowly added lead acetate Pb(OAc)4 (0.69 g,
1.57 mmol), and the solution was gently refluxed for 4 h. The solution
was filtered off and the solution was extracted twice with CH2Cl2/
H2O. The organic layers were combined and dried over MgSO4. The
product was purified by flash chromatography (silica gel) eluting
with hexane/CH2Cl2 (5/1). The product isolated as white solids was
obtained by recrystallization from CH2Cl2/CH3OH. Yield 80%. 1H NMR
4.1.7. 5-Decyloxy-2-(6-decyloxybenzoxazol-2-yl)phenol (2a; n¼12).
The solution of 2-decyloxy-4-decyloxy-N-(2-hydroxy-4-decyloxy-
benzylidene)aniline (1.00 g, 1.90 mmol) dissolved in 150 mL of
CHCl3 was added lead acetate (1.01 g, 2.30 mmol) under nitrogen
atmosphere. The solution was gently refluxed for 4 h. The solution
was concentrated under reduced pressure, and the residue was
abstracted twice with CH2Cl2/H2O (1/5). The organic layers were
combined and dried over MgSO4. The product was purified by
passage to a short column of silica gel eluting with hexane/EA (1:1).
The product isolated as white solids was obtained after re-
crystallization from CH2Cl2/CH3OH. Yield 78e83%. 1H NMR (CDCl3):
(CDCl3):
d
0.86 (t, 9H, eCH3, J¼6.8 Hz), 1.25e1.50 (m, 54H, eCH2),
1.77e1.83 (m, 6H, eOCH2), 3.99 (t, 2H, eOCH2, J¼6.5 Hz), 4.02e4.06
(m, 4H, eOCH2), 6.55 (d, 1H, AreH, J¼8.9 Hz), 6.91 (dd, 1H, AreH,
J¼2.2 Hz), 6.93 (dd, 1H, AreH, J¼2.23 Hz), 7.07(s, 1H, AreH), 7.51 (d,
1H, AreH, J¼8.7 Hz), 7.62 (d, 1H, AreH, J¼8.8 Hz). 13C NMR (CDCl3):
d
0.85 (t, 6H, eCH3, J¼6.90 Hz), 1.25e1.45 (m, 28H, eCH2), 1.74e1.81
(m, 4H, eOCH2CH2), 3.95e3.99 (m, 4H, eOCH2), 6.50e6.57 (m, 2H,
AreH), 6.90 (q, 1H, AreH, J¼2.1 Hz), 7.05 (d, 1H, AreH, J¼2.1 Hz),
7.49 (d, 1H, AreH, J¼8.7 Hz), 7.81 (d, 1H, AreH, J¼8.7 Hz). 13C NMR
d
14.11, 22.69, 26.05, 26.09, 29.12, 29.23, 29.30, 29.35, 29.38, 29.41,
(CDCl3):
d
14.12, 22.69, 26.01, 26.06, 29.10, 29.24, 29.34, 29.39,
29.53, 29.59, 29.64, 29.66, 29.70, 29.73, 30.28, 31.93, 68.95, 73.38,
96.23, 104.83, 105.47, 113.28, 118.81, 121.77, 133.71, 136.49, 149.93,
152.90, 156.25, 157.63, 162.23. IR (neat): 2917, 2850, 1651, 1505, 1457,
1295, 1237, 1117, 1082 cmꢀ1. MS (FAB): calcd for MHþ C49H82NO5:
764.6. Found: 764.2.
29.58, 31.91, 68.25, 68.92, 96.22, 101.66, 103.88, 107.96, 113.20,
118.63, 127.76, 133.72, 149.88, 157.52, 160.07, 162.31, 163.30. Anal.
Calcd for C33H49NO4: C, 75.68; H, 9.43; N, 2.67. Found C, 75.76; H,
9.44; N, 2.60.
4.1.8. 5-Octyloxy-2-(6-octyloxybenzoxazol-2-yl)phenol (2a; n¼8).
4.1.13. 2,3-Dioctyloxy-6-(6-octyloxybenzoxazol-2-yl)phenol (2b; n¼8).
White solid; yield 72%.1H NMR (CDCl3):
d
0.86 (t, 6H, eCH3, J¼6.9 Hz),
White solid; yield 75%.1H NMR (CDCl3):
d
0.85(t, 9H, eCH3, J¼6.8 Hz),
1.27e1.52 (m, 20H, eCH2), 1.75e1.82 (m, 4H, eOCH2CH2), 3.95e4.00
(m, 4H, eOCH2), 6.50e6.57 (m, 2H, AreH), 6.90 (q, 1H, AreH,
J¼2.4 Hz), 7.06 (d, 1H, AreH, J¼2.1 Hz), 7.49 (d, 1H, AreH, J¼8.7 Hz),
1.25e1.47 (m, 30H, eCH2), 1.73e1.86 (m, 6H, eOCH2), 3.95e4.08 (m,
6H, eOCH2), 6.55 (d, 1H, AreH, J¼8.8 Hz), 6.89 (dd, 1H, AreH,
J¼2.3 Hz), 6.94(dd,1H, AreH, J¼2.3 Hz), 7.06(s,1H, AreH), 7.51(d,1H,
AreH, J¼8.7 Hz), 7.62 (d, 1H, AreH, J¼8.8 Hz). 13C NMR (CDCl3):
7.82 (d, 1H, AreH, J¼8.7 Hz). 13C NMR (CDCl3):
d 14.11, 22.67, 26.01,
26.07, 29.10, 29.25, 29.35, 31.82, 68.25, 68.29, 96.22, 101.65, 103.88,
107.96, 113.19, 118.63, 127.77, 133.73, 149.88, 157.52, 160.07, 162.31,
d 14.11, 22.68, 26.03, 29.28, 29.35, 29.58, 29.65, 30.25, 31.82, 31.87,
68.88, 73.35, 96.15, 104.71, 105.42, 113.22, 118.78, 121.75, 133.67,

7368
I.-T. Wu et al. / Tetrahedron 67 (2011) 7358e7369
136.38, 149.89, 152.85, 156.20, 157.58, 162.19. IR (neat): 3088, 2917,
2850, 1632, 1503, 1462, 1295, 1235, 1104, 1082 cmꢀ1. MS (FAB): calcd
for MHþ C37H57NO5: 596.4. Found: 596.0.
6.81e6.96 (m, 2H, AreH), 7.10(d, 1H, AreH, J¼7.5 Hz), 7.69 (d, 1H,
AreH, J¼8.7 Hz), 8.07 (t, 1H, AreH, J¼7.8 Hz). 13C NMR (CDCl3):
d
14.13, 22.69, 26.05, 29.21, 29.34, 29.40, 29.58, 31.91, 68.95,
96.18,104.31, 104.64, 107.32, 107.63, 113.71, 119.10, 128.26, 128.41,
133.29, 149.98, 158.02, 159.99, 160.17, 161.36, 163.94, 167.27.
4.1.14. 2,3-Didecyloxy-6-(6-decyloxybenzoxazol-2-yl)phenol
(2b;
n¼10). White solid; yield 85%. 1H NMR (CDCl3):
d 0.85e0.88 (m, 9H,
eCH3), 1.24e1.50 (m, 42H, eCH2), 1.78e1.82 (m, 6H, eOCH2), 3.99
(t, 2H, eOCH2, J¼6.5 Hz), 4.02e4.06 (m, 4H, eOCH2), 6.55 (d, 1H,
AreH, J¼8.9 Hz), 6.91 (dd, 1H, AreH, J¼2.2 Hz), 6.92 (dd, 1H, AreH,
J¼2.2 Hz), 7.06 (s, 1H, AreH), 7.51 (d, 1H, AreH, J¼8.7 Hz), 7.62 (d,
4.1.20. Bis[5-decyloxy-2-(6-decyloxybenzooxazol-2-yl)phenol]Pd(II)
(1aePd; n¼12). The solution of 5-decyloxy-2-(6-decyloxyb-
enzooxazol-2-yl)phenol (0.15 g, 0.28 mmol) dissolved in 30 mL of
warm dry C2H5OH/THF (10:1) and palladium(II) acetate (33 mg,
0.147 mmol) dissolved in 10 mL dry C2H5OH was mixed. The solu-
tion was gently refluxed for 4 h. The dark brown solids were col-
lected, and the product isolated as yellow solids was obtained after
recrystallization from THF/CH3OH. Yield 61e65%.
1H, AreH, J¼8.8 Hz). 13C NMR (CDCl3):
d 14.11, 22.69, 26.05, 26.09,
29.24, 29.42, 29.57, 29.58, 29.65, 29.70, 30.28, 31.92, 31.94, 68.94,
73.37, 96.22, 104.82, 105.47, 113.27, 118.81, 121.76, 133.72, 136.49,
149.93, 152.91, 156.24, 157.63, 162.23. IR (neat): 2917, 2850, 1632,
1503, 1466, 1295, 1237, 1117, 1080 cmꢀ1. MS (FAB): calcd for MHþ
C43H70NO5: 680.5. Found: 680.1.
Anal. Calcd for 1a (n¼8) C58H80N2O8Pd: C, 67.00; H, 7.76; N, 2.69.
Found C, 67.18; H, 7.95; N, 2.73.
Anal. Calcd for 1a (n¼10) C66H96N2O8Pd: C, 68.82; H, 8.40; N,
2.43. Found C, 69.15; H, 8.50; N, 2.50.
4.1.15. 2,3-Ditetradecyloxy-6-(6-tetradecyloxybenzooxazol-2-yl)phe-
nol (2b; n¼14). White solid; yield 87%. 1H NMR (CDCl3):
d
0.86
Anal. Calcd for 1a (n¼12) C74H112N2O8Pd: C, 70.31; H, 8.93; N,
(t, 9H, eCH3, J¼6.9 Hz), 1.24e1.49 (m, 66H, eCH2), 1.78e1.83 (m,
6H, eOCH2), 3.99 (t, 2H, eOCH2, J¼6.6 Hz), 4.02e4.06 (m,
4H, eOCH2), 6.55 (d, 1H, AreH, J¼8.9 Hz), 6.91 (dd, 1H, AreH,
J¼2.2 Hz), 6.93 (dd, 1H, AreH, J¼2.2 Hz), 7.07 (s, 1H, AreH), 7.51 (d,
1H, AreH, J¼8.7 Hz), 7.62 (d, 1H, AreH, J¼8.9 Hz). 13C NMR
2.22. Found C, 70.07; H, 8.95; N, 2.26.
Anal. Calcd for 1a (n¼14) C82H128N2O8Pd: C, 71.56; H, 9.37; N,
2.04. Found C, 71.71; H, 9.35; N, 2.02.
Anal. Calcd for 1a (n¼16) C90H144N2O8Pd: C, 72.62; H, 9.75; N,
1.88. Found C, 72.58; H, 9.80; N, 1.95.
(CDCl3):
d 14.12, 22.70, 26.05, 26.09, 29.24, 29.31, 29.38, 29.40,
29.42, 29.59, 29.67, 29.68, 29.71, 29.74, 30.28, 31.94, 68.94, 73.37,
96.22, 104.82, 105.47, 113.27, 118.81, 121.76, 133.72, 136.49, 149.93,
152.90, 156.24, 157.63, 162.23. IR (neat): 2913, 2848, 1634, 1505,
1470, 1293, 1237, 1111, 1082 cmꢀ1. MS (FAB): calcd for MHþ
C55H94NO5: 848.7. Found: 848.2.
4.1.21. Bis[5-decyloxy-2-(6-decyloxybenzoxazol-2-yl)phenol]Cu(II)
(1aeCu; n¼12). The solution of 5-decyloxy-2-(6-decyloxyb-
enzooxazol-2-yl)phenol (0.20 g, 0.34 mmol) dissolved in 30 mL of
warm dry C2H5OH/THF (10:1) and copper(II) acetate (36 mg,
0.18 mmol) dissolved in 10 mL dry C2H5OH was mixed. The solution
was gently refluxed for 4 h. The dark brown solids were collected,
and the product isolated as brown solids was obtained after re-
crystallization from THF/CH3OH. Yield 68e75%. Anal. Calcd for 1b
(n¼12) C74H112N2O8Cu: C, 72.78; H, 9.24; N, 2.29. Found C, 72.87; H,
9.26; N, 2.26.
4.1.16. 2,3-Dihexadecyloxy-6-(6-hexadecyloxybenzooxazol-2-yl)phe-
nol (2b; n¼16). White solid; yield 82%. 1H NMR (CDCl3):
d 0.86 (t,
9H, eCH3, J¼6.9 Hz), 1.24e1.49 (m, 78H, eCH2), 1.79e1.83 (m, 6H,
eOCH2), 3.99 (t, 2H, eOCH2, J¼6.6 Hz), 4.02e4.06 (m, 4H, eOCH2),
6.55 (d,1H, AreH, J¼9.0 Hz), 6.91 (dd,1H, AreH, J¼2.3 Hz), 6.93 (dd,
1H, AreH, J¼2.3 Hz), 7.07 (s,1H, AreH), 7.51 (d,1H, AreH, J¼8.7 Hz),
4.1.22. Bis[5-decyloxy-2-(6-decyloxybenzoxazol-2-yl)phenol]Zn(II)
(1aeZn; n¼12). The solution of 5-decyloxy-2-(6-decyloxyb-
enzooxazol-2-yl)phenol (0.15 g, 0.28 mmol) dissolved in 30 mL of
warm dry C2H5OH/THF (10:1) and zinc(II) acetate (30 mg,
0.145 mmol) dissolved in 10 mL dry C2H5OH was mixed. The solu-
tion was gently refluxed for 4 h. The off-white solids were collected,
and the product isolated as white solids was obtained after re-
crystallization from THF/CH3OH. Yield 65e77%. Anal. Calcd for
C74H112N2O8Zn: C, 72.67; H, 9.23; N, 2.29. Found C, 72.4; H, 9.27; N,
2.24.
7.62 (d, 1H, AreH, J¼8.9 Hz). 13C NMR (CDCl3):
d 14.11, 22.70, 26.05,
26.09, 29.24, 29.31, 29.37, 29.40, 29.60, 29.65, 29.68, 29.71, 29.73,
30.28, 31.94, 68.95, 73.37, 96.22, 104.82, 105.48, 113.27, 118.81,
121.76, 133.73, 136.49, 149.93, 152.91, 156.25, 157.63, 162.24. IR
(neat): 2917, 2850,1638,1501,1468,1300,1273,1115,1082 cmꢀ1. MS
(FAB): calcd for MHþ C61H106NO5: 932.8. Found: 932.3.
4.1.17. 5-Chloro-2-(6-decyloxybenzoxazol-2-yl)phenol (2c; X¼Cl). 1H
NMR (CDCl3):
d
0.86 (t, 3H, eCH3, J¼6.0 Hz), 1.26e1.47 (m, 14H,
eCH2), 1.76e1.85 (m, 2H, eOCH2CH2), 3.99 (t, 2H, eOCH2, J¼6.6 Hz),
6.95 (m, 2H, AreH), 7.10 (m, 2H, AreH), 7.55 (d,1H, AreH, J¼9.0 Hz),
4.1.23. Bis[2,3-didodecyloxy-6-(6-dodecyloxybenzoxazol-2-yl)phe-
nol]zinc(II) (1beZn, n¼12). The solution of 2,3-didodecyloxy-6-(6-
dodecyloxybenzooxazol-2-yl)phenol (0.2 g, 0.26 mmol) dissolved
in 5.0 mL of dried THF was slowly added a solution of zinc(II) ace-
tate (34.0 mg, 0.16 mmol) dissolved in 3.0 mL of absolute C2H5OH/
THF (10/1). The mixture was refluxed for 20 h. Brown solids were
collected, and the product isolated as a light yellow as obtained
after recrystallization from THF/CH3OH. Yield 85%. MS (FAB): calcd
for C98H161N2O10Zn: 1589.1. Found: 1589.0 [MþH]þ. Anal. Calcd for
C98H160N2O10Zn: C, 73.95; H, 10.13; N, 1.76. Found: C, 73.83; H,
10.17; N, 1.70.
7.85 (d, 1H, AreH, J¼8.7 Hz). 13C NMR (CDCl3):
d 14.13, 22.70, 26.05,
29.20, 29.34, 29.40, 29.58, 31.91, 68.95, 96.14, 109.61, 113.88,
117.56,119.24, 120.10, 126.97, 127.51, 133.32, 138.52, 150.06, 158.19,
158.73, 161.18.
4.1.18. 2-(6-Decyloxybenzoxazol-2-yl)-5-methoxyphenol (2c;X¼OCH3).
1H NMR (CDCl3):
d
0.86 (t, 3H, eCH3, J¼6.3 Hz), 1.26e1.46 (m,
14H, eCH2), 1.75e1.85 (m, 2H, eOCH2CH2), 3.83 (s, 3H, eOCH2), 3.98
(t, 2H, eOCH2, J¼6.6 Hz), 6.55 (d, 1H, AreH, J¼8.7 Hz), 6.60 (s, 1H,
AreH), 6.91(d, 1H, AreH, J¼8.7 Hz), 7.07(s, 1H, AreH), 7.51 (d, 1H,
AreH, J¼9.0 Hz), 7.84 (d, 1H, AreH, J¼8.4 Hz). 13C NMR (CDCl3):
Compound 1beZn (n¼8) Anal. Calcd for C74H112N2O10Zn: C,
70.81; H, 8.99; N, 2.23. Found: C, 70.23; H, 10.07; N, 2.29.
Compound 1beZn (n¼10) Anal. Calcd for C86H1136N98O10Zn: C,
72.57; H, 9.63; N, 1.97. Found: C, 72.26; H, 9.87; N, 1.76.
d
14.13, 22.69, 26.06, 29.23, 29.34, 29.41, 29.58, 31.91,55.49, 68.94,
96.25, 101.19, 104.09, 107.57, 113.30, 118.67, 127.86, 149.89, 157.59,
160.14, 162.23, 163.74, 183.86.
Compound 1beZn (n¼14) Anal. Calcd for C110H184N2O10Zn: C,
75.06; H, 10.54; N, 1.59. Found: C, 75.23; H, 10.65; N, 2.12.
Compound 1beZn (n¼16) Anal. Calcd for C122H208N2O10Zn: C,
75.99; H, 10.87; N, 1.45. Found: C, 75.92; H, 10.03; N, 1.25.
4.1.19. 2-(6-Decyloxybenzoxazol-2-yl)-5-fluorophenol (2c; X¼F). 1H
NMR (CDCl3):
d
0.86 (t, 3H, eCH3, J¼6.6 Hz), 1.26e1.46 (m, 14H,
eCH2), 1.75e1.85 (m, 2H, eOCH2CH2), 3.98 (t, 2H, eOCH2, J¼6.6 Hz),

I.-T. Wu et al. / Tetrahedron 67 (2011) 7358e7369
6. Stepien, M.; Donnio, B.; Sessler, J. L. Chem.dEur. J. 2007, 13, 6853e6863.
7369
4.1.24. Bis[2,3-didecyloxy-6-(6-decyloxybenzoxazol-2-yl)phenol]cop-
per(II) (1beCu, n¼10). The procedures were similar to above Zn
complex (1a) except that copper(II) acetate hydrate was used. Yield
80%. Anal. Calcd for C98H160N2O10Cu: C, 74.03; H, 10.10; N, 1.76.
Found: C, 74.17; H, 10.16; N, 1.79.
7. (a) Singh, A. K.; Kumari, S.; Kumar, K. R.; Sridhar, B.; Rao, T. R. Polyhedron 2008,
27, 181e186; (b) Singh, A. K.; Kumari, S.; Kumar, K. R.; Sridhar, B.; Rao, T. R.
Polyhedron 2008, 27, 1937e1941; (c) Aiello, I.; Bellusci, A.; Crispini, A.; Ghedini,
M.; Pucci, D.; Spataro, D. Mol. Cryst. Liq. Cryst. 2008, 481, 1e13.
8. (a) Crispini1, A.; Ghedini, M.; Pucci, D. Beilstein J. Org. Chem. 2009, 5, 1e6; (b)
Morale, F.; Date, R. W.; Guillon, D.; Bruce, D. W.; Finn, R. L.; Wilson, C.; Blake, A.
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J.; Schrder, M.; Donnio, B. Chem.dEur. J. 2003, 9, 2484e2501; (c) Pucci, D.;
Barberio, G.; Crispini, A.; Francescangeli, O.; Ghedini, M.; Deda, M. L. Eur. J.
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4.1.25. Bis[2,3-didodecyloxy-6-(6-dodecyloxybenzoxazol-2-yl)phe-
nol]palladium(II) (1bePd, n¼10). The procedures were similar to
above Zn complex (1a) except that palladium(II) acetate was used.
Yield 75%. Anal. Calcd for C86H136N2O10Pd: C, 70.53; H, 9.36; N, 1.91.
Found: C, 70.07; H, 9.30; N, 1.90.
9. Seredyuk, M.; Gaspar, A. B.; Ksenofontov, K.; Reiman, S.; Galyametdinov, Y.;
Haase, H.; Rentschler, E.; Gutlich, P. Chem. Mater. 2006, 18, 2513e2519.
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P.; Valien, M.; Pinilla, E.; Torres, M. R. Mol. Cryst. Liq. Cryst. 2008, 481, 34e55.
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Acknowledgements
12. Lehmann, M.; Sierra, T.; Barbera, J.; Serrano, J. L.; Parkerb, R. J. Mater. Chem.
2002, 12, 1342e1350.
13. (a) Mori, A.; Mori, R.; Takemoto, M.; Yamamoto, S. I.; Kuribayashi, D.; Uno, K.;
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64, 10598e10604.
We thank the National Science Council of Taiwan, ROC (NSC 98-
2113-M-008-003-MY2) in generous support of this work.
Supplementary data
15. Lee, C. K.; Ling, M. J.; Lin, I. J. B. Dalton Trans. 2003, 4731e4737.
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Supplementary data associated with this article can be found in
the online version at doi:10.1016/j.tet.2011.07.024.
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