Metallomesogens: Synthesis and properties

Article abstract of DOI:10.1039/a801159a

The synthesis, characterization and mesogenic behaviour of the copper(n) and oxovanadium(iv) complexes derived from phenyltetrazole and benzothiazole and their corresponding ligands are reported. The ligands did not exhibit mesomorphism, whereas the compl

Full text of DOI:10.1039/a801159a

J O U R N A L O F
Metallomesogens: synthesis and properties
´
Emerson Meyer, Cesar Zucco and Hugo Gallardo*
Department of Chemistry, Universidade Federal de Santa Catarina, Cep 8800.040–900
Floriano´polis, SC Brazil
C H E M I S T R Y
The synthesis, characterization and mesogenic behaviour of the copper() and oxovanadium() complexes derived from
phenyltetrazole and benzothiazole and their corresponding ligands are reported. The ligands did not exhibit mesomorphism,
whereas the complexes form monotropic smectic A and smectic C mesophases. The mesophases were identified according to their
textures by optical microscopy.
OR
Introduction
Liquid crystals have been known for more than 100 years, but
the number of structural types and classes of chemical com-
R
N
N
pounds capable of exhibiting mesomorphic properties has
increased significantly only during the last 15–20 years, for
example, metal-containing liquid crystals, which are termed
‘metallomesogens’, that combine the variety and range of
metal-based coordination chemistry with the extraordinary
physical characteristics of liquid crystals.1,2
N
N
N
S
N
H
O
H
O
H
O
Coordination compounds, usually complexes of mesogenic
ligands, salts of organic acids, and certain organometallic and
organoelement compounds with representative metals of s-,
p-, d- and even f-block elements, have been made, and fall into
two broad classes: the calamitic, where the metal atoms are
bound to long thin ligands, giving complexes which are long,
thin, and rod-like, and the discotic, where the metal is generally
coordinated in the center of a flat disc-like organic ligand
system. The ligands play a key role in determining the meso-
morphic behavior, since they usually compose the periphery
of the molecule, and hence play a role in controlling the shape.
Schiff bases derived from substituted salicyladehyde (I) are
very versatile ligands which form (N–O) chelates with many
metals.3 Due to the diversity of substituents that can be
introduced, a great variety of these mesogenic complexes has
been reported.4,5
OR
(II)
OR
(I)
OR
(III)
Fig. 1 Schematic of the similarity between the coordination sites of
salicylaldehydes and five-membered heterocyclic rings containing
nitrogen
In this work we study the potentiality of tetrazole (II) and
benzothiazole (III) derivatives in the generation of meso-
morphic behavior. The design of these materials is based on
the similarity of these ligands with Schiff ’s base imines derived
from salicylaldehyde (Fig. 1).
In order to verify whether the structural correlation
approach is suitable in the case of the heterocyclic ligands
tetrazole and benzothiazole, we have studied their com-
plexation behaviour and investigated the thermal behaviour
of both ligands and complexes, with the general structure
shown in Fig. 2.
Synthesis
The synthetic route used to prepare the tetrazole ligands and
the copper() complexes is shown in Scheme 1. The hetero-
cyclic ring was synthesized in several steps, starting from
commercial 2,4-dihydroxybenzaldehyde 1, aldoxime formation
2 and dehydration with acetic anhydride forming the 2,4-
diacetoxybenzaldehyde 3. The 2,4-dihydroxyphenyltetrazole 4
was obtained by treating 3 with sodium azide. Alkylation
reaction with the appropriate alkyl halide furnished the 5-(2-
hydroxy-4-alkoxyphenyl)-2-alkyltetrazoles 5. Treatment of 5
with copper() acetate in ethanol gives the corresponding
copper complexes.
Fig. 2 Representation of the metal complexes of tetrazole and benzo-
thioazole derivatives
The 5-(2-hydroxy-4-alkoxyphenyl)-2-alkyltetrazole 5 was
prepared by O-alkylation and N-alkylations in one-pot syn-
thesis from 5-(2,4-dihydroxyphenyl)tetrazole 4. The tetrazolate
anion is an ambidentate system in which alkylation reactions
can occur at the N-1 or N-2 positions, the relative proportions
J. Mater. Chem., 1998, 8(6), 1351–1354
1351

CHO
NH2
NOH
b
SH
a
OH
SH
HO
CHO
HO
AcO
CN
OAc
HO
a
+
N
OH
OH
OC10H21
1
2
3
OC10H21
7
c
8
9
b
N
N
N
N
N
N
N
d
HO
H
RO
HO
N
N
S
R
OC10H21
OH
OH
5
4
10
e
c
R
N
S
N
N
N
H
21C10O
N
O
OR
O
N
M
O
Cu
N
RO
O
N
S
OC10H21
N
N
11 M = CuII
12 M = VO
R
6
Scheme 2 a, Pyridine; b, FeCl Ω6H O, ethanol; c, Cu(OAc) ΩH O or
Scheme 1 a, NH OHΩHCl, H O; b, Ac O; c, NaN , NH Cl, DMF; d,
RBr, K CO , cyclohexanone; e, Cu(OAc) , EtOH
3
2
2
2
2
2
2
3
4
VOSO Ω5H O, ethanol
4
2
2
3
2
4
3
2
N
N
N
–
N
N
N
N
5
gen bonding between the 2-hydroxyl group and the N-4
position of the tetrazole ring. This hydrogen bond was clearly
observed both in the solid state by an X-ray crystallographic
study of 5-(2,4-dihydroxyphenyl)tetrazole8,9 4 and in solution
by 1H and IR spectroscopy of 4 and its alkylated products 5.
Complexation was carried out with copper() diacetate
yielding the mesogenic structures 6.
Rotamer complexes are considered forbidden, a priori,
because of steric hindrance of the aliphatic groups which would
impede the ligand bonding (Fig. 4).
The tetrazole ligands and the intermediate compounds were
investigated by a variety of techniques, including IR, 1H NMR,
13C NMR, X-ray crystallography and elemental analyses. The
analyses showed that the structures of all of the materials were
consistent with those expected.
The general reaction pathway used to the target ligand 2-
(4-decyloxy-2-hydroxyphenyl)benzothiazole and its complexes
is shown in Scheme 2.
S
S
N
–
1
Fig. 3 Schematic diagram of canonical tetrazolate anions
N
N
N
( )n
N
( )n
O
OH
HO
N
O
( )n
N
N
( )
n
N
Fig. 4 Schematic diagram of the steric hindrance between rotamers of
the tetrazole ligands (n=0,1,2,3…)
The ligand was synthesized using well known literature
methods in two steps: first, reaction of 2-hydroxy-4-decyloxyb-
enzaldehyde 7 with 2-aminothiophenol 8: secondly, cyclization
of the obtained Schiff base 9 with FeCl Ω6H O. The chelates
of which depend upon the conditions of the alkylation and the
influence of the 5-substituent (Fig. 3).
3
2
11 were prepared by treatment of 2-(4-decyloxy-2-hydroxyphe-
nyl)benzothiazole 10 with CuII and VOII salts. The elemental
and spectral analyses of the complexes and the intermediates
were consistent with their proposed structures.
However, in our case, spectroscopic evidence suggests that
the N-2 anion in the tetrazole ring is the nucleophile in the
alkylation step. The alkyl substituents at the 1- and 2-positions
can be readily distinguished by the 1H and 13C chemical shifts
of the the N-alkyl group. Alkyl groups bonded to N-1 are
more shielded by ca. 0.15–0.35 ppm in the 1H spectra and by
ca. 2–6 ppm in the 13C spectra to their corresponding N-2
isomers.6 The regioselectivity in the alkylation was corrobor-
ated by the analysis of the 13C NMR chemical shifts of the
carbon atom C5, 164.8 ppm, in accordance with Butler and
Garvin,7 that in similar systems there is a perfect distinction
between the isomeric forms N-1 and N-2, in view of the
sensitive difference in the position of such peaks in the spec-
trum. This regioselectivity is due to the large steric hindrance
at the N-1 position considering the bulky and large alkylating
agents used.
Results and Discussion
Tetrazole system
The optical and thermal data of the ligands and complexes
are gathered in Table 1.
Ligands. All the free ligands synthesized were not meso-
morphic. This fact is due in part to a loss of linearity in the
molecule due to the presence of the tetrazole ring, which is
unable to make co-linear disubstitution bondings. Another
contribution to the loss of liquid crystallinity is the strong
intermolecular dipolar repulsions brought about by the pres-
ence of the lateral hydroxide groups.
The O-alkylation is also regioselective at the 4-hydroxyl
group, because of the existence of strong intramolecular hydro-
1352
J. Mater. Chem., 1998, 8(6), 1351–1354

Table 1 Optical and thermal data (in °C) of the tetrazolic ligands and complexes of CuII
ligand
mp
complex
Cryst
SmA
SmC
Iso
n=10
n=12
n=14
40–42
50–52
60–62
n=10
n=12
n=14
Ω
Ω
117.0
110.0
116.0
Ω
Ω
Ω
(106.7)
(103.4)
(92.7)
Ω
—
—
(67.4)
—
—
Ω
Ω
Ω
Complexes. The CNN stretching vibration of the ligands is
located in the 1630–1634 cm−1 region and is shifted to lower
wavenumbers (aproximately 20 cm−1) upon chelation, indicat-
ing that the tetrazolic N atom is involved in metal–nitrogen
bond formation.
As can be observed, all the complexes exhibit mesogenic
behavior. The mesophases were identified according to their
textures which were observed by optical microscopy.10–12
All the complexes n=10–14 show monotropic SmA meso-
phases. Complex n=10, in addition, gives a monotropic SmC
phase. On cooling the isotropic liquid, the SmA phase appears,
via batoˆnnets, forming a focal-conic textures.
The different mesomorphic behaviour when compared to
the Schiff bases complexes is unsurprising, given that one
would expect them to be not effectively isostructural, but in
the coordination geometry only small differences in metal–
ligand bond lengths and angles can be expected.
The difference in mesomorphic behavior compared to the
Schiff bases is not surprising considering the very different
electronic and steric character of the tetrazole heteroaromatic
system.
polarizability of transition metals), sufficiently strong to cause
three dimensional order and consequently the absence of a
mesophase.
The addition of a long aliphatic chain to the rigid nucleus
furnishes both the anisotropy and irregular packing needed to
promote mesomorphism.
Based on these considerations the absence of mesomorphic
behavior in the free ligand 10 and the complexes 11 and 12
can be attributed to the small number of aliphatic substituents
permitting interactions that favor only three dimensional order.
Experimental
The transition temperatures for all compounds were deter-
mined by optical microscopy using a Leitz Ortholux polarizing
microscope in conjunction with a Mettler FP-52 heating stage.
The purity of the compounds was evaluated by thin layer
chromatography and elemental analysis. The IR spectra were
recorded using the KBr disc method with a Perkin-Elmer
model 283 spectrometer, and the 1H NMR and 13C NMR
spectra were recorded at 80 MHz (Bruker WP-80) or 270 MHz
(Bruker HX-270).
The low thermal stability of the mesophases can be inter-
preted as being due to the reduced anisotropy of the complexes
because of the deviation from linearity of the aliphatic chains
caused by the heterocyclic ring (Fig. 5).
Materials
2,4-Dihydroxybenzaldehyde (Aldrich), 2-hydroxybenzaldehyde
(Merck), hydroxylamine hydrochloride (Aldrich), sodium azide
(Aldrich), 1-bromodecane (Aldrich), 1-bromododecane
(Aldrich), 1-bromotetradecane (Aldrich), 2-aminothiophenol
(Aldrich) and resorcinol (Aldrich) were used as received.
Anhydrous sodium sulfate (Aldrich), potassium carbonate
(Merck), copper acetate (Aldrich), iron() chloride hexahy-
drate (Merck) and vanadyl sulfate pentahydrate (Fluka) were
used without purification. Acetic anhydride (Merck), N,N-
dimethylformamide (Merck), ethanol (Merck), methanol
(Merck), acetic acid (Merck), acetone (Aldrich), cyclohexanone
(Carlo Erba) and pyridine (Merck) were purified by distillation
prior to use.
Benzothiazole system
The thermal data of the ligand and complexes are gathered
in Table 2.
The IR spectra show a shift of the ligand CNN stretching
band (1632 cm−1) to lower wavenumbers (1608 cm−1, cop-
per() complex; 1605 cm−1, oxovanadium() complex), which
indicates that the N atom of benzothiazole participates in the
formation of the complex. For the oxovanadium() complex
the stretching band characteristic of the VNO group was
observed at 976 cm−1.
While in classic organic structures, the basic question is,
frequently, to increase intermolecular contact to induce meso-
morphism, in coordination compounds the problem is to avoid
intermolecular contacts (whenever a coordination site is access-
ible), and strong dipolar interactions (associated with the high
2,4-Dihydroxybenzaldoxime 2. To a solution of hydroxylam-
ine hydrochloride (0.05 mol) and sodium acetate (0.05 mol) in
water (30 ml), at 40 °C, 2,4-dihydroxybenzaldehyde (0.05 mol)
was added and the mixture was stirred for 10 min. The mixture
was cooled and filtered. The solid was collected and recrys-
tallized from water (92% yield). Mp 194–196 °C; IR (KBr):
3359, 1624, 1525 cm−1.
2,4-Diacetoxybenzonitrile 3. 2,4-Dihydroxybenzaldoxime
(0.05 mol) was added slowly to acetic anhydride (30 ml) at
room temperature. After an additional 30 min at room tem-
perature (rt) the reaction mixture was warmed slowly to 100 °C
over a 2 h period. Removal of the solvent gave a solid which
was recrystallized from methanol–water (351) (75% yield).
Mp 72–73 °C; IR (KBr): 2229, 1780, 1186 cm−1.
Fig. 5 Sketch of geometry of copper complexes
Table 2 Transition temperatures (°C)
5-(2,4-Dihydroxyphenyl)tetrazole 4. A suspension of 2,4-
diacetoxybenzonitrile (0.05 mol), sodium azide (0.15 mol) and
ammonium chloride (0.15 mol) in dimethylformamide (50 ml)
was heated to 150 °C over a 6 h period. The reaction mixture
was allowed to come to rt with stirring over 1 h. The reaction
mixture was poured into ice cold water (100 ml), and the
resulting material was filtered and washed with cold water.
structure
C
I
ligand
copper complex
oxovanadium complex
Ω
Ω
Ω
97
216
194
Ω
Ω
Ω
J. Mater. Chem., 1998, 8(6), 1351–1354
1353

The crude product was recrystallized from ethanol–water (351)
to give 5-(2,4-dihydroxyphenyl)tetrazole as colorless crystals
(85% yield). Mp 304 °C; IR (KBr): 3348, 1612, 1490 cm−1;
Anal. Calc. for C H N O : C, 47.19; H, 3.37; N, 31.46. Found:
temperature. The pyridine was then removed by evaporation
in vacuo, and the resulting residue was added to a hot ethanolic
solution of FeCl Ω6H O. The mixture was then allowed to
cool and filtered. The solid was collected and recrystallized
from ethanol (82% yield). Mp 97 °C; IR (KBr): 3400, 2919,
3
2
7
6 4 2
C, 47.20; H, 3.20; N, 31.92.
1632 cm−1. 1H NMR (200 MHz, CDCl ): 12.71 (s, 1H),
3
7.96–6.52 (m, 7H), 4.02 (t, J=6.4 Hz, 2H), 1.82 (t, J=6.75 Hz,
General procedure for preparation of 5-(2-hydroxy-4-alkoxy-
phenyl)-2-alkyltetrazoles 5. To
2H), 1.30 (m, 14H), 0.90 (t, 3H) ppm. 13C NMR (50 MHz,
CDCl ): 169.28, 163.03, 159.87, 151.87, 131.87, 129.51, 126.44,
a suspension of 5-(2,4-
dihydroxyphenyl)tetrazole (0.03 mol) and anhydrous potass-
ium carbonate (0.06 mol), in cyclohexanone (50 ml), was added
the appropriate alkyl halide (0.065 mol). The reaction mixture
was stirred for 72 h at 150 °C. The mixture was then allowed
to cool and filtered. The filtrate was poured into ice cold water
(100 ml) and the residue formed was recrystallized from
ethanol.
3
124.89, 121.58, 121.32, 110.15, 108.05, 101.77, 68.23,
31.82–14.04 ppm. Anal. Calc. for C H NO S: C, 71.96; H,
7.56; N, 3.65. Found: C, 71.66; H, 7.55; N, 4.00.
23 29
2
Bis[2-(2-hydroxy-4-decyloxyphenyl)benzothiazole]
copper(II) 11. An ethanolic solution (10 ml) of 2-(2-hydroxy-4-
decyloxyphenyl)benzothiazole (2 mmol) was added to a hot
ethanolic solution (10 ml) of copper() acetate (1 mmol). The
solution was refluxed for 20 min and then cooled. The precipi-
tate was collected by filtration and recrystallized from cyclohex-
ane–ethanol (153) (68% yield). The brown copper complex
was characterized by IR spectroscopy and elemental analysis.
The 1H NMR spectrum of the complex exhibits large shifts
relative to the free ligand, and all signals are broadened as a
result of the paramagnetic metallic ion. Mp 216 °C; IR (KBr):
2922, 1608, 1468 cm−1. Anal. Calc. for C H N O S Cu: C,
homologue
melting point/°C
yield (%)
C H
10 21
C H
12 25
C H
14 29
40–42
50–52
60–62
66
70
70
For the homologue C H : IR (KBr): 3314, 2954, 1634,
1472 cm−1. 1H NMR (200 MHz, CDCl ): 9.81 (s, 1H),
2H), 2.05 (q, 2H), 1.79 (q, 2H), 1.44–1.19 (m, 44H), 0.87 (t,
14 29
3
7.90–6.57 (m, 3H), 4.64 (t, J=7.1 Hz, 2H), 3.98 (t, J=6.5 Hz,
46 56
2 4 2
66.71; H, 6.76; N, 3.38. Found: C, 66.76; H, 6.67; N, 3.48.
6H) ppm. 13C NMR (50 MHz, CDCl ): 164.18, 162.50, 158.01,
128.31, 108.16, 104.18, 102.11, 68.21, 58.30, 31.95–14.15 ppm.
3
Bis[2-(2-hydroxy-4-decyloxyphenyl)benzothiazole]oxo-
vanadium(IV ) 12. An ethanolic solution (10 ml) of 2-(2-
hydroxy-4-decyloxyphenyl)benzothiazole (2 mmol) was added
to a hot ethanolic solution (10 ml) of vanadyl sulfate pentahy-
drate (1 mmol). The solution was refluxed for 20 min and then
cooled. The precipitate was collected by filtration and recrys-
tallized from cyclohexane–ethanol (153) (65% yield). The
green oxovanadium complex was characterized by IR spec-
troscopy and elemental analysis. The 1H NMR spectrum of
the complex exhibits large shifts relative to the free ligand, and
all signals are broadened as a result of the paramagnetic
metallic ion. Mp 194 °C; IR (KBr): 2920, 1605, 1469, 976 cm−1.
Anal. Calc. for C H N O S V: C, 66.44; H, 6.73; N, 3.37.
Anal. Calc. for C H N O : C, 73.56; H, 10.86; N, 9.80. Found:
C, 73.58; H, 11.05; N, 9.52.
35 62
4 2
General procedure for preparation of bis[5-(2-hydroxy-4-
alkoxyphenyl)-2-alkyltetrazole]copper(II ) 6. An ethanolic solu-
tion (10 ml) of the appropriate tetrazole (2 mmol) was added
to a hot ethanolic solution (10 ml) of copper() acetate
(1 mmol). The solution was refluxed for 20 min and then
cooled. The precipitate was collected by filtration and recrys-
tallized from chloroform–ethanol (153). The green copper
complexes were characterized by IR spectroscopy and elemen-
tal analysis. The 1H NMR spectra of the complexes exhibit
large shifts relative to the free ligands, and all signals are
broadened as a result of the paramagnetic metallic ion.
46 56
2 5 2
Found: C, 66.43; H, 6.58; N, 3.44.
The authors gratefully acknowledge the financial support of
CNPq, CAPES and PRONEX.
homologue
yield (%)
C H
10 21
C H
12 25
C H
14 29
62
65
60
References
1
2
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14 29
3
4
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70 122
8 4
5
6
7
8
9
2-Hydroxy-4-decyloxybenzaldehyde 7. To a suspension of
2,4-dihydroxybenzaldehyde (0.03 mol) and anhydrous potass-
ium carbonate (0.03 mol) in acetone (50 ml) was added 1-
bromodecane (0.035 mol). The reaction mixture was refluxed
for 56 h. The mixture was then allowed to cool and filtered.
The solvent was removed and the residue was distilled at
reduced pressure giving the product as a clear liquid (63%
yield): bp 185 °C (0.5 mm); IR (thin film): 3300, 2920,
1650 cm−1.
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2-(2-Hydroxy-4-decyloxyphenyl)benzothiazole
10.
2-
Hydroxy-4-decyloxybenzaldehyde (0.02 mol) was dissolved in
10 ml of dry pyridine containing 2-aminothiophenol (0.02 mol)
under an atmosphere of nitrogen and was stirred 5 h at room
Paper 8/01159A; Received 9th February, 1998
1354
J. Mater. Chem., 1998, 8(6), 1351–1354