Dinuclear ortho-metallated palladium(II) azobenzene complexes with acetato and chloro bridges: Influence of polar substituents on the mesomorphic properties

Article abstract of DOI:10.1016/j.jorganchem.2012.04.001

The synthesis, characterization and mesomorphic properties of a new series of acetate and chloro-bridged dinuclear orthopalladated complexes derived from azobenzene with terminal groups of hexadecyloxy moiety at one end and different polar groups (Me, Cl, F, NO₂, CN) at the other end are described. The mesomorphic properties of both the ligands and complexes were investigated by polarizing optical microscopy and X-ray studies to understand the effect of polar group in the nature of mesophase produced. Among the complexes, all chloro-bridged complexes predominantly exhibited SmA phase. However the rare phenomenon of observing mesomorphism in acetato-bridged palladium (II) complexes due its typical open book shape had been realized in substituted acetato-bridged complexes with fluoro and cyano substituents in the ligands. Nitro and cyano substituted ligands only exhibited monotropic Smectic A phase. Model molecular arrangement based on X-ray studies is presented.

Full text of DOI:10.1016/j.jorganchem.2012.04.001

Journal of Organometallic Chemistry 712 (2012) 20e28
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Journal of Organometallic Chemistry
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Dinuclear ortho-metallated palladium(II) azobenzene complexes with acetato
and chloro bridges: Influence of polar substituents on the mesomorphic
properties
,
b
Trirup Dutta Choudhury a, Yongqiang Shen b, Nandiraju V.S. Rao a , Noel A. Clark
*
a Department of Chemistry, Centre for Soft matter, Assam University, Dargakona, Silchar 788 011, Assam, India
b Department of Physics, Liquid Crystal Materials Research Centre, University of Colorado, Boulder, CO 80309, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis, characterization and mesomorphic properties of
a new series of acetate and
Received 12 February 2012
Received in revised form
30 March 2012
chloro-bridged dinuclear orthopalladated complexes derived from azobenzene with terminal groups of
hexadecyloxy moiety at one end and different polar groups (Me, Cl, F, NO2, CN) at the other end are
described. The mesomorphic properties of both the ligands and complexes were investigated by polar-
izing optical microscopy and X-ray studies to understand the effect of polar group in the nature of
mesophase produced. Among the complexes, all chloro-bridged complexes predominantly exhibited
SmA phase. However the rare phenomenon of observing mesomorphism in acetato-bridged palladium
(II) complexes due its typical open book shape had been realized in substituted acetato-bridged
complexes with fluoro and cyano substituents in the ligands. Nitro and cyano substituted ligands only
exhibited monotropic Smectic A phase. Model molecular arrangement based on X-ray studies is
presented.
Accepted 2 April 2012
Keywords:
Liquid crystal
Metallomesogen
Palladium
Azo compound
Ortho-metallation
Polar compound
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
contribution of aromatic core resulted to yield linear [21e27] and
square planar [28e35] complexes exhibiting liquid crystalline
One of the potential applications of cyclopalladated complexes
is ligand transformation using the reactions of PdeC bond [1e5].
The importance of orthopalladated dinuclear complexes derived
from salicylaldimine ligands exhibiting very good catalytic activity,
dependent upon the substituent, towards the hydrogenation of
nitrobenzene and cyclohexene has been demonstrated recently [5].
Mesogenic behaviour is realised in usually unfavourable acetato/
carboxylato bridged ortho-metallated dinuclear Pd(II) complexes of
salicylaldiimine ligands with increased peripheral chains/crown
ethers, in spite of destabilization due to the non-planarity in such
complexes [6e12]. Hence functional properties along with their
potential applications due to the perceived advantage of combining
properties of transition metal with the coordinating molecules of
liquid crystalline compounds promoted basic as well as applied
research activity in this area. This research area has been well
reviewed recently, with the excellent work appearing regularly
[13e20]. The coordination of a metal ion to the mesogenic/
non-mesogenic coordinating organic ligands with an increased
behaviour with enhanced thermal stability. One of the predomi-
nant class of metallomesogens is the organometallic complexes of
“(C, N)Pd” metallacycle viz., orthocyclopalladated imines [36e39],
azines [40], and azobenzenes [41e44].
Cyclopalladated compounds have proved to be an active area of
research with different examples by several groups [45e55] out of
which orthopalladated imine and azobenzene compounds exhib-
iting large thermal range of mesomorphism have been extensively
studied due to their ease of synthesis and thermal stability. Further
the nature of the bridging group as well as polarity of the
substituent played an important role in the mesomorphic charac-
teristics and photo-physical properties. Hence it not only changes
the geometry of the system but also alter the liquid crystalline
properties. A comparative study on the azo-based cyclopalladated
dimers with various bridging systems (X ¼ Cl, Br, I, N3, SCN)
revealed that all the complexes are planar [52e55] and exist in
trans conformation. However the acetato (OAc) bridged complexes
exists in a typical “roof-shape” or “open-book shape” and exists in
cis:trans mixture. The mesomorphism was observed for chloro,
bromo, azido, and oxalato complexes but not for the iodo, thio-
cyanato, or acetate derivatives [18,47,56]. Further the effect of polar
substituent such as H, F, Cl, NO2, CN, OMe etc. either in para
* Corresponding author. Tel.: þ91 94355 22541; fax: þ91 3842 270806.
E-mail address: nandirajuv@gmail.com (N.V.S. Rao).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2012.04.001

T.D. Choudhury et al. / Journal of Organometallic Chemistry 712 (2012) 20e28
21
position of the aldehyde ring [57] or aniline ring [58] of chloro-and
acetato-bridged complexes, derived from benzylidene imine
ligand, had been studied extensively. However no such study was
reported on azo cyclopalladated complexes. One of the important
reasons is that the azo-based system need sufficiently long chain
on both ends to fulfil the structural requirement for liquid crys-
talline behaviour. Most of the azo-based systems with small
aliphatic chain length are non-mesogenic, whereas complexes
show mesomorphic behaviour above 200 ꢀC and decomposed on
clearing point. However greater mobility of the ring having the
polar substituent in para position together with the longer distance
from the central core may allow a more important participation of
the different polar groups in the molecular interactions. As a result,
changes in mesogenic behaviour such as nature of mesomorphic
phase, mesomorphic phase range, etc., can be expected [59], and
except the few contributions from Ghedini et al. [1,40,51,55] on
dinuclear cyclopalladated azo compounds with alkyl end chains at
both ends of the ligand, there are no other report available in
literature with a polar group in the molecule till date to our
knowledge. In this article we report the synthesis and liquid
crystalline properties of azo compounds with a polar group at one
end and long n-hexadecyloxy chain at the other end followed by
complexation with palladium acetate to infer the influence of polar
end group on mesomorphism of both chloro-and acetate bridged
dinuclear azo- palladium complexes.
2. Results and discussion
The chloro, fluoro, cyano, nitro and methyl substituted azo
compounds were synthesized as presented in Scheme 1, following
Scheme 1. Synthesis of the dinuclear Pd(II) ortho-metallated compounds.

22
T.D. Choudhury et al. / Journal of Organometallic Chemistry 712 (2012) 20e28
the known procedures as described in experimental section. The
acetato-bridged palladium (II) complexes were prepared by the
reaction of the ligand with palladium acetate by stirring at room
temperature for 24 h instead of stirring at 50 ꢀC as reported earlier.
The stirring at 50 ꢀC lead to the formation of a black paste and part
of it was found in decomposed form. The acetato bridged
complexes were then converted into the chloro-bridged complexes
by stirring for 1 h with hydrochloric acid in dichloromethane.
Synthesized compounds were characterized by elemental analysis,
UVevisible spectroscopy, FTIR and NMR. Elemental analysis of
ligands and complexes are consistent with the proposed structures,
confirming the binuclear composition of the complexes. The
materials are characterized for liquid crystalline properties by
polarizing optical microscopy (POM) and phase transition temper-
atures are confirmed by differential scanning calorimetry (DSC).
energy band (emax ¼ 10,000e20,000 mol Lꢂ1 cmꢂ1) is associated
with
pe
p*
transition;
the
lower
intensity
band
(emax ¼ 700e3000 mol Lꢂ1 cmꢂ1) corresponds to a transition
involving non-bonding orbital localized on the nitrogen atom, and
leads to direct population of nep*excited states [55]. It was
observed that lmax of ligands gradually shift towards lower energy
(red shift) region as the degree of polarity in the lateral substituent
group increases. Polarity of the group decreases the energy of the
excited state hence absorption occurs at lower wave length.
Maximum red shift was observed for nitro and cyano substituted
ligand due to presence of extended conjugation in these groups.
The electronic absorption spectra of both the acetate and
chloro-bridged orthopalladated complex also exhibit two low
energy bands (Fig. 1b). The high-energy band peaks around
350e380 nm compared well with the high intense band for free
ligand. On this basis, this band (emax ¼ 700e3000 mol Lꢂ1 cmꢂ1) in
the complex can be assigned to an intraligand pe
p* origin. But the
2.1. UV-study
band around 400e480 nm imparts colour of the complex cannot be
compared nep* transition of the free ligand. Because this band has
much higher intensity (emax ¼ 4000e10,000 mol Lꢂ1 cmꢂ1) than
the corresponding nep* transition of the free ligand. Thus this band
finds its origin in electronic transition involving five-membered
ring, i.e. the cyclopalladated ring and correspond to metal ligand
charge transfer. The only difference observed in the UV-spectrum of
the acetate and chloro-bridged complexes was a long tail which
appeared in acetate bridged complexes due to conjugation of eCOO
bridging group.
The optical properties of azobenzene moiety had been under
scrutiny for a long time. The electronic absorption spectra of the
ligand and the corresponding acetate and chloro-bridged
complexes were studied in chloroform solution of same concen-
tration (c ¼ 1ꢁ10ꢂ4e10ꢂ5 mol Lꢂ1) as shown in Fig. 1. (All the
spectral data are summarized in ESI, Table S1) The free ligand
shows two low energy absorption peaks (Fig. 1a) around
(350 w 380) nm and (425 w 455) nm. The more intense high-
a
2.2. FTIR-study
2.0
L1
L2
The FTIR-data cannot distinguish in well defined way between
the ligand and the complexes. Because the eN]N- and eC]C- of
phenyl ring absorption peaks appeared very close to each other and
did not change much before or after complexation. One charac-
teristic difference appeared as peak in case of acetate complex for
bridging C]O bond around 1475 cmꢂ1. The additional peaks from
the stretching vibration of PdeO, PdeC and PdeN bonds appeared
in the region 510e760 cmꢂ1 in all of the complexes. The charac-
teristic frequency for cyano group appeared around 2222 cmꢂ1 for
both the ligand and complexes.
L5
L3
L4
L1
1.5
1.0
0.5
0.0
L5
L4
L2
L3
2.3. NMR-study
Chemical shift (d-value) values of the eight aromatic protons of
300
350
400
450
500
both phenyl rings of the ligands in 1H NMR spectra appeared in the
range of 6e8 ppm and the signals appeared as doublet. The protons
of the terminal CH3 group of alkoxy chain appeared as triplet at
w0.88 ppm. The appearance of only one set of signals for the
aromatic protons revealed that a single isomer is present in the
acetateebridge complexes [55]. The singlet at 2.15 ppm for acetate
bridged palladium complex indicates that both the bridged methyl
groups are chemically equivalent, which further confirm the
formation of a single trans-isomer. However as previously reported
Wavelength(nm)
b
2.0
L5
1-L5
2-L5
1.6
1.2
0.8
0.4
0.0
L5
for similar compounds three different triplets (
d
¼ 3.98, 3.72,
3.47 ppm respectively) [46,55,57] which result from the diaster-
eotropic methylene protons of alkoxy chain are observed in methyl
substituted acetato-bridge complex, 1-L1 (Scheme 1). When the
acetate bridge complex is converted into the chloro-bridged
analogue 2-L1 (Scheme 1), through reaction with HCl, the signal
at 2.15 disappeared due to the loss of bridging methyl group. In
particular attention had been paid related to the proton H7 of the
orthopalladated phenyl ring. Furthermore the 1H NMR signal
correspond to the proton H7 (Scheme 1) of acetate bridged complex
1-L5
2-L5
300
400
500
600
Wavelength nm
(d
w 5.8) shows a strong up-field shift (w1.0 ppm) with respect to
Fig. 1. UVevisible spectra of (a) ligands, (b) cyano ligand L5, acetate bridged complex
1-L5 and chloro-bridged complex, 2-L5.
chloro-bridged complex ( w 6.8). This shift could result from the
d

T.D. Choudhury et al. / Journal of Organometallic Chemistry 712 (2012) 20e28
23
enantiotropic magnetic field created by orthopalladated ring of
other ligand [46,55,57]. The ortho-metallation occurs preferentially
with the alkoxy substituted phenyl ring rather than the phenyl ring
with electron withdrawing Cl or CN or F or NO2 or Me groups
because of higher electron density in this ring due to electron
donating ability of 4-n-hexadecyloxy group.
2.4. Mesomorphic properties
The phase behaviour of the ligand and the complexes are
characterized by a combination of differential scanning calorimetry
Table 1
Mesomorphic phase transition temperatures T ꢀC, associated enthalpies (
D
H kJ/mol)
S J/K/mol) of the phase transitions of all ligands (Lx), acetate 1-Lx]
and chloro bridge2-Lx palladium complex. X ¼ Me, Cl, F, NO2, CN.
and entropies (
D
No Compound Phase Transition Temperature ꢀC
H kJ/mol
D
DS J/mol/K
1.
L1
CreI
IeCr
Cr1eCr2
Cr2eI
IeCr
Cr / SmB
SmBeSmA
SmA / N
N / I
81.9
69.0
49.7
104.0
44.5
33.1
48.3
7.3
38.7
3.9
67.1
26.3
9.8
93.3
141.2
22.6
102.6
12.2
195.6
70.9
25.1
1.9
Fig. 2. Focal conic fan textures of SmA phase observed in nitro compound (L4) at
2.
1-L1
94.3 ꢀC.
3.
2-L1
69.9
97.6
(DSC), polarizing optical microscopy (POM) attached with a hot
stage and powder x-ray scattering experiment on two compounds.
The phase transition temperatures and the associated thermal
parameters viz., enthalpies and entropies for both ligand and
complexes, were determined by differential scanning calorimetry
(representative DSC of L4 is presented Figure S1 in ESI) and are
summarized in Table 1. (The value reported in this table corre-
sponds to first heating scan). The azo-based ligands: with methyl
(L1), chloro (L2) and fluoro (L3) substituents are non-mesogenic,
whereas the ligands with nitro (L4) and cyano (L5) substituents are
mesogenic and exhibit enantiotropic smectic A (SmA) phase which
was identified by the characteristic focal conic fan textures shown
in Fig. 2. All the ligands transformed into the isotropic phase
below 120 ꢀC.
120.8
140.2.
140.2tm
131.9tm
97.7tm
62.5tm
90.6
80.7
92.3
79.2
73.6
0.8.
I / N
N / SmA
SmAeSmB
SmBe Cr
CreI
IeCr
CreI
4.
5.
6.
L2
47.1
51.4
117.8
104.6
4.3
68.1
1.5
5.3
58.5
60.9
69.6
10.7
50.6
5.5
129.5
145.3
322.4
296.9
12.4
186.5
3.5
12.9
167.7
168.3
198.2
31.3
141.9
15.0
e
1-L2
2-L2
IeCr
Cr1 / Cr2
Cr2eSmA
SmA / I
I / SmA
SmAeCr
CreI
92.1
160.0
135.6
75.8
88.7
78.0
7.
8.
L3
The acetate bridged palladium complexes were reported as
non-mesogenic [55] because of its typical “open-book” shaped
geometry (Fig. 3) which restricts the orientation of the molecule
IeCr
1-L3
Cr / LC1
LC1eLC2
LC2 / I
I / LC2
LC2eLC1
LC1eCr
Cr / I
I / SmA
SmAeCr
Cr / SmA
SmA / I
I / SmA
SmAeCr
Cr1 / Cr2
Cr2 / I
IeCr
Cr / SmE
SmE / I
I / SmA
SmAeSmE
Cr / SmA
SmA / I
I / SmA
SmAeCr
Cr / I
I / SmA
SmAeCr
Cr / SmA
SmA / I
I / SmA
SmAeCr
66.8
83.4
94.3
75.5tm
65.5tm
52.2
e
e
e
16.3
10.9
8.7
26.8
53.7
2.3
50.1
24.1
21.1
78.4
148.1
6.3
9.
2-L3
178.6
144.9
68.6
89.4
96.5
10. L4
95.5
81.1
2.4
6.6
58.5
56.1
15.0
15.8
7.2
16.3
2.7
6.8
165.2
143.9
36.0
42.1
18.9
35.7
6.1
11. 1-L4
12. 2-L4
116.6
143.6
102.8
107.0
182.4
169.0
53.4
106.0
120.0tm
110.5tm
75.4
20.8
105.8
e
13. L5
40.1
e
e
e
47.8
169.9
13.3
142.5
e
137.2
445.0
35.1
402.3
e
14. 1-L5
15. 2-L5
108.9
105.7
81.2
97.3tm
180.7tm
174.2tm
85.6tm
e
e
e
e
e
e
tmmicroscopic observed temperature.
Fig. 3. Open book like structure of acetate bridged orthopalladated complexes.

24
T.D. Choudhury et al. / Journal of Organometallic Chemistry 712 (2012) 20e28
within the layer structure needed for liquid crystalline behaviour.
However the newly synthesized acetate bridged complexes 1-L3
and 1-L5 (fluoro and cyano substituents respectively) exhibit
mesomorphism. POM studies revealed that the complex 1-L3
exhibits an unusual phase sequence of highly ordered monotropic
phase variant LC1eLC2 in cooling cycle as shown in Fig. S2 (ESI).
These two transitions were also observed in heating cycle of DSC at
66.8 ꢀC and 83.4 ꢀC with an enthalpy change of 10.7 kJ/mol and
50.6 kJ/mol respectively and finally become isotropic at 94.3 ꢀC
with small enthalpy change of 5.5 kJ/mol. The fluidity of these two
phases was established by tapping on glass plate. Even though such
anomalous behaviour was reported earlier in Cl bridged cyclo-
palladated symmetrically substituted azo compound which is
related to intermolecular forces coupled with structural modifica-
tions of the mesogenic species, but no such phenomena was
reported for acetate bridged complexes. Further 1-L5 exhibit
monotropic Smectic A phase (Figure S3a in ESI) at 105.9 ꢀC on
cooling, which appeared as a sharp peak in DSC at 105.7 ꢀC with an
enthalpy change of 13.3 kJ/mol (Figure S3b in ESI).
temperature (Figure S5 in ESI). This transition was detected in DSC
as a small peak at 135.6 ꢀC with an enthalpy change of 5.3 kJ/mol.
On further cooling, crystallization of the sample occurred which
appeared as a sharp peak in DSC at 75.8 ꢀC with a large enthalpy
change of 58.5 kJ/mol. Fluoro substituted complex 2-L3, exhibited
monotropic SmA phase (Figure S5 in ESI) in cooling and the tran-
sition was detected in DSC at 144.9 ꢀC with an enthalpy change of
8.7 kJ/mol. On further cooling SmA phase transformed to crystalline
phase with a large enthalpy change of 26.8 kJ/mol at 68.6 ꢀC. Partial
decomposition was observed for chloro-bridged complexes of nitro
and cyano (2-L4 and 2-L5) compounds on heating to isotropic
liquid, which may be due to highly polar nature of NO2 and CN
groups. Compound 2-L4 exhibits SmA and SmE phases. All the
chloro-bridged dinuclear cyclopalladated complexes exhibit
mesomorphism while the acetate bridged complexes did not show
mesomorphism except the cyano (SmA) and fluoro substituted
complexes.
2.5. X-ray studies
The complex 2-L1, with methyl substituent exhibited enantio-
tropic SmB, SmA and nematic phases (Figure S4). In DSC thermo-
graph phase transitions were observed at 69.9 ꢀC, 97.6 ꢀC, 120.8 ꢀC
and 140.2 ꢀC corresponding to crystal to SmB, SmBeSmA,
SmA-nematic and nematic-isotropic phase transitions respec-
tively in the heating cycle only. We could not observe the phase
transformations in the cooling cycle in DSC. However, all the phase
transitions were observed enantiotropically by POM.
The chloro (2-L2) and fluoro (2-L3) substituted chloro-bridged
orthopalladated complexes exhibit SmA phase (Figure S5 in ESI)
similar to those exhibited by 2-L1. Chloro substituted dinuclear
complex 2-L2, exhibit batonnets with large homeotropic areas
resembling SmA phase appeared on cooling from isotropic melt,
which grows as focal conic fan like texture on lowering the
The reported X-ray structure determination of the single crystal
of di-m
-chloro-bis[(4-hydroxy-40–methylazobenzenato-C2, N2)
palladium (II) [54,60] revealed overall “roof-shaped” or 2D H-sha-
ped molecular structure, in which the “(C, N)Pd” chelate rings are
essentially coplanar and the two azo ligands are joined through
central bridging Cl atoms with Pd2Cl2 unit. Interestingly the mes-
ophase behaviour, molecular ordering transition temperatures and
thermal range of mesophases are found to depend on the bridging
moiety [50,55].
Temperature dependent X-ray diffraction experiments were
systematically performed in the mesophases of the 4-cyano
substituted acetato-bridged complex di-
m-acetato-bis[(4-cyano-
40-n-hexadecyloxyazobenzenato-C2, N2)palladium (II), 1-L5
a
b
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
1-L5 T=99.7oC
1-L5 T=104.6oC
0.1
0.01
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
0.08
0.12
0.16
0.20
q (Å-1
)
q (Å-1
)
c
d
50
48
46
Layer Spacing of 1-L5
1-L5 T=74.8oC
0.1
0.01
32
30
28
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
70
75
80
85
90
95 100 105 110 115
T (oC)
q (Å-1
)
Fig. 4. X-ray diffractograms of a) 1-L5 at109.5 ꢀC, b) 1-L5 at 99.7 ꢀC , c) 1-L5 at 74 ꢀC and d) Temperature variation of layer thickness of 1-L5.

T.D. Choudhury et al. / Journal of Organometallic Chemistry 712 (2012) 20e28
25
(Fig. 4) in order to unequivocally identify the nature of the
mesophase and to obtain additional information about the
molecular packing.
antiparallel arrangement of strongly polar molecules with inter-
digitation of aliphatic chains as depicted in Fig. 5bec with strong
molecular axial fluctuations is proposed.
The XRD patterns of 1-L5 measured between 81 and 105 ꢀC
exhibit a broad scattering in the wide angle part of the diffracto-
gram (at ca. 4.2e4.4 Å) Fig. 4a, which corresponds to the molten
chains in the liquid-like order. In the small angle region only one
fine sharp signal at w48.0 Å (Fig. 4aec) was observed corre-
sponding to layer thickness. Thus, the mesophase was assigned on
the basis of its texture obtained by polarized-light optical micros-
copy and XRD patterns, as smectic A phase (Figure S3a ESI).
The layer spacing (48.0 Å) is significantly smaller than the
calculated molecular length for a fully extended conformation of
the molecule (minimized energy) as shown in Fig. 5a (53.8 Å). To
explain the difference (w5.8 Å) between the theoretical molecular
length of 1-L5 and the layer thickness, a space filling model of an
The variation in the layer spacing (1.0 Å) with temperature
(Fig. 4d) does not show any significant change in SmA phase but as
the phase transition took place a sudden drop in the layer thickness
from 48.2 Å in SmA phase to 30.2 Å upon cooling into the solid
phase was observed in complex 1-L5. Such a large jump in layer
thickness indicates formation of highly ordered crystal phase. The
X-ray data obtained on cooling shows a pattern (Fig. 4c) confirm the
formation of a crystalline species. Hence the reduction in observed
layer thickness in principle suggests to a quasi-crystalline state,
wherein, at temperatures comparatively lower, strong and localized
(because of the low temperature) intermolecular interactions,
exerted by the polar CN end groups, which can afford to a very tight
crystal packing, cannot be discarded.
Fig. 5. a) Minimized molecular structure 1-L5, di-m
-acetato-bis[(4-cyano-40-n-hexadecyloxybenzenato-C2,N2)palladium (II), (b) Model molecule and (c) proposed model of
interdigitated structure in SmA phase.

26
T.D. Choudhury et al. / Journal of Organometallic Chemistry 712 (2012) 20e28
3. Conclusion
unoriented sample contained in 1 mm diameter borosilicate glass
capillary tubes and data were collected using a point detector
A comparative study of the properties of azo-based ligand and
their acetato-bridged and chloro-bridged cyclopalladated complex
revealed a separate phase sequence of polar groups for the melting
point of each series. For azo-based ligand the order of melting point
is eNO2>eCN>eF>eCl>eCH3, for acetate bridge complex
eNO2>eCN>eCH3>eCl>eF and for chloro bridge complex
eF>eNO2>eCl>eCH3>eCN. This indicates that the influence of
the polar groups on the stabilization of the solid state depends on
the central core of the molecule. It was noteworthy that strong
electron withdrawing groups give rise to higher melting point and
also liquid crystalline property. This was reflected in the ligands
where only eNO2 and eCN substituted ligand show the mesogenic
behaviour because of high electron withdrawing nature. The open
book-shaped acetate bridged complexes are generally considered
as non-liquid crystalline but in present case eCN and eF
substituted acetate bridged complexes exhibit mesomorphism.
Further all the planar chloro-bridged complexes show mesogenic
behaviour. From the experimental results in the orthopalladated
azo complexes presented here, the nature of the central core is
mainly responsible for the liquid crystalline properties. The polar
nature of the substituent in the aniline ring has strong influence on
the mesomorphism exhibited by free ligand. The melting point of
the ligand increases with the polarity of the substituent and the
mesomorphism was observed in compounds with a strong electron
withdrawing group (eNO2, eCN). However no such correlation was
observed in case of both the acetate and chloro-bridged complexes.
Most of chloro-bridged complexes exhibited SmA phase with
a characteristic focal conic fan texture with large homeotropic areas
in cooling. Nematic phase was observed only in ligands with
a methyl substituent and a high order phase observed in chloro
substituent compounds. Mesophase textures in these highly
viscous orthopalladated complexes are paramorphotic and are
retained till the room temperature in solid phase.
mounted on a Huber four-circle goniometer at CueK(a) radiation
from a Rigaku UltraX-18 rotating anode generator.
4.1. Synthesis of 4-chloro-40-n-hexadecyloxyazobenzene (L2)
Preparation of 4-hydroxy-40-chloroazobenzene: The ligand L2
was synthesized by the diazocoupling reaction of 4-chloro-aniline
with phenol followed by the alkylation of the hydroxyl group. To 4-
chloro-aniline (1.27 g,10 mmol) were added 10 ml of distilled water
containing hydrochloric acid (12 M, 2.5 ml, 30 mmol) and the
mixture was heated to dissolve the contents. The solution was then
cooled to 0 ꢀC. To the resulting stirred mixture cooled at 0 ꢀC was
added, dropwise, a solution of sodium nitrite (0.69 mg, 10 mmol) in
10 mL of water. The resulting diazonium chloride was consecutively
coupled with an alkaline solution of phenol (0.94 g, 10 mmol) in
10 ml of water containing 0.80 g (20 mmol) of sodium hydroxide
with constant starring. The azo-dye which formed immediately as
yellow precipitate was filtered, washed several times with water
and dissolved in methanol or diethyl ether and the resulting
organic solution dried over anhydrous sodium sulphate. The crude
product obtained after removal of the solvent under reduced
pressure was purified by recrystallization from cold hexane,
precipitate was filtered and washed with water and methanol and
dried in vacuum. Yield 1.74 g (75%).
To a suspension of 4-hydroxy-40-chloroazobenzene (1.16 g,
5 mmol) in 100 ml of dry acetonitrile were added dried potassium
carbonate (0.69 g, 5 mmol) and a catalytic amount of tetrabuty-
lammoniumbromide (TBAB). The mixture was stirred for 1 h fol-
lowed by the addition of 1-bromohexadecane (1.5 ml, 5 mmol)
dropwise with constant stirring. The resulting mixture was stirred
under reflux for 16 h, and progress of the reaction was monitored
by TLC. After completion of the reaction, the solid residues were
filtered off, excess of acetonitrile was distilled off and the solution
cooled. To this solution ice cold water was added and the mixture
was acidified with HCl up to pH ¼ 2. The precipitate appeared was
filtered and solid was washed with sodium bicarbonate solution,
dried and finally purified by chromatography on a silica gel column
4. Experimental
All of the chemicals were procured and used as received from
M/S Tokyo Kasei Kogyo Co. Ltd., or Avocado Chemicals or E-Merck.
Silica gel (60e120 mesh, Acme synthetic chemicals) was used for
chromatographic separation. Silica gel G [E-Merck] was used for
thin layer chromatography (TLC). The solvents and reagents are of
AR grade and were distilled and dried before use following stan-
dard procedures. TLC was performed on glass slides pre-coated
with silica gel. The intermediate compounds were purified by
column chromatography using silica gel. UV visible absorption
spectra of the compounds in CHCl3 at different concentrations were
recorded on a Shimadzu UV-1601PC spectrophotometer (lmaxin
nm). All of the compounds were characterized by elemental anal-
ysis, infrared (IR) spectra (PerkineElmer L120-000A spectrometer),
and by 1H nuclear magnetic resonance (NMR) spectroscopy (JEOL
AL 300 FT NMR) in CDCl3 solution, chemical shifts are reported in
eluted with
a mixture of hexane-chloroform (70:30, v/v).
Yield ¼ 78%. Analytical data: 1H NMR (300 MHz, CDCl3),
d: 7.88 (d,
2H, 8.7 Hz, H1,2), 7.47(d, 2H, 8.7 Hz, H3,4), 7.05 (d, 2H, 8.7 Hz, H5,8),
6.98 (d, 2H, 8.7 Hz, H6,7), 4.04 (t, 2H, 6.6), 1.82(q, 2H, 7.5), 1.25
(m, 26H), 0.88 (t, 3H, 6.3). Experimental (Calculated) data for
C28H41ClN2O: C, 73.7 (73.5%); H, 8.98 (9.04%); N, 6.12 (6.13%). IR
(n
cmꢂ1), 2916, 1604, 1583, 1498, 1259.
All analogous compounds were prepared as described for
4-chloro-40-n-hexadecyloxyazobenzene. Yields and analytical data
are as follows:
4.1.1. 4-Methyl-40-n-hexadecyloxyazobenzene (L1)
Yield 67%. Analytical data: 1H NMR (300 MHz, CDCl3),
d: 7.86 (d,
2H, 9.0 Hz), 7.79 (d, 2H, 8.1 Hz), 7.30 (d, 2H, 8.4 Hz), 7.00 (d, 2H,
9.0 Hz), 4.03 (t, 2H, 6.3), 2.42 (m, 3H), 1.79 (d, 2H), 1.26 (m, 26H),
0.88 (t, 3H, 6.9). Experimental (Calculated) data for C29H44N2O: C,
parts per million (d) relative to tetramethylsilane (TMS) as an
internal standard. The liquid-crystal cells were placed in
a computer controlled heating stage (hot and cold stage HCS302,
with STC200 temperature controller configured for HCS302 from
INSTEC) and the optical textures inferring the phase sequences
were investigated by polarizing microscopy (Nikon polarizing
microscope OPTIPHOT-POL2). The phase transition temperatures
and associated enthalpies of transition were determined by
differential scanning calorimetry (DSC; PerkineElmer DSC Pyris1
system that was calibrated previously using pure indium as
a standard). The heating and cooling rates were 5 or 10 ꢀC per
minute. The X-ray diffraction analyses were carried out using
80.0 (79.7); H, 10.1 (10.1); N, 6.4 (6.4). IR (
1253.
n
cmꢂ1), 2916, 1602, 1583,
4.1.2. 4-Fluoro-40-n-hexadecyloxyazobenzene (L3)
Yield 62%. Analytical data: 1H NMR (300 MHz, CDCl3),
d: 7.86 (d,
2H, 8.7 Hz, H1,2), 7.47 (d, 2H, 8.4 Hz, H3,4), 7.05 (d, 2H, 8.7 Hz, H5,8),
6.98 (d, 2H, 8.7 Hz, H6,7), 4.03 (t, 2H, 6.6), 1.82(q, 2H, 7.5), 1.26 (m,
26H), 0.88 (t, 3H, 6.3). Experimental (Calculated) data for
C28H41FN2O, C, 76.4 (76.3); H, 9.42 (9.38); N, 6.88 (6.36). IR (
2916, 1602, 1583, 1251.
n
cmꢂ1),

T.D. Choudhury et al. / Journal of Organometallic Chemistry 712 (2012) 20e28
27
4.1.3. 4-Nitro-40-n-hexadecyloxyazobenzene (L4)
Yield 48%. Analytical data: 1H NMR (300 MHz, CDCl3),
4.3. Synthesis of chloro-bridged complexes 2-L2
d: 8.34
(d, 2H, 8.7 Hz, H1,2), 7.96(m, 4H, H3,4,5,8), 7.00 (d, 2H, 9.0 Hz, H6,7),
4.06 (t, 2H, 6.3), 1.83(q, 2H, 7.5), 1.26 (m, 26H), 0.88 (t, 3H, 6.3).
Experimental (Calculated) data for C28H41N3O3: C, 72.2 (71.9); H,
9.01 (8.84); N, 9.18 (8.99). IR (
1249.
A solution of hydrochloric acid in methanol (Pd: HCl, 1:1) was
cautiously added to a stirred solution of 0.5 g (0.3 mmol) of acetate
bridge complex in 30 ml of dichloromethane. As the reaction pro-
ceeded, the initially dark red colour solution turns to orange. After
1 h the solution was evaporated to dryness, and the residue was
recrystallized from methanol and dichloromethane mixture to give
orange colour product. Yield ¼ 48%. Analytical data: 1H NMR
n
cmꢂ1), 2916, 1602, 1581, 1519, 1342,
4.1.4. 4-Cyano-40-n-hexadecyloxyazobenzene (L5)
Experimental (Calculated) data for C29H41N3O: C, 77.9 (77.8); H,
(300 MHz, CDCl3), d: 7.91 (d, 4H, 9.0 Hz, H1,2),7.83 (d, 4H, 8.7 Hz,
9.21 (9.23); N, 9.41 (9.39). IR (
n
cmꢂ1), 2920, 2220, 1598, 1581, 1500,
H3,4), 7.47 (d, 2H, 9.0 Hz, H5), 6.98 (d, 2H, 8.7 Hz, H6), 6.87 (d, 2H,
8.7 Hz, H7), 4.04 (t, 4H, 6.3), 1.82 (m, 4H), 1.26 (m, 52H), 0.88 (t, 6H,
6.6). Experimental (calculated) data for C56H80Cl4N4O2Pd2; C, 56.3
1269.
4.2. Synthesis of acetate bridged complexes 1-L2
(56.2%); H, 6.67 (6.74%); N, 4.55 (4.68%). IR (
1583, 1502, 1469, 1257.
n
cmꢂ1), 2918, 1598,
To a suspension of 0.76 g (3 mmol) of palladium acetate in15 ml
of acetic acid, 1.37 g, (3 mmol) of L2 was added and the mixture was
stirred for 12 h at 50 ꢀC and then allowed to cool the dark brown
precipitate of acetate bridged complex was filtered off,washed with
cold acetone and air dried. No trace of metallic palladium was
observed. Yield ¼ 70%. Analytical data: 1H NMR (300 MHz, CDCl3),
All analogous chloro-bridged complexes were prepared as
described for palladium complex of 4-chloro-40-n-hexadecylox-
yazobenzene 2-L2. Yields and analytical data are as follows:
4.3.1. 2-L1
Yield ¼ 44%, Analytical data: 1H NMR (300 MHz, CDCl3),
d: 7.81
d
: 7.43 (d, 2H, 9.0 Hz, H5), 7.80 (d, 4H, H3,4), 7.87 (d, 4H, 6.9 Hz, H1,2),
(d, 4H, 7.8 Hz, H3,4), 7.74 (d, 2H, 7.8 Hz, H5), 7.00 (d, 4H, 6.9 Hz, H1,2),
6.72 (d, 2H, 8.7 Hz, H6), 6.85 (d, 2H, 6.0 Hz, H7), 4.05 (t, 4H, 6.3), 2.43
(m, 6H), 1.81 (m, 4H), 1.26 (m, 52H), 0.88 (t, 6H, 6.9). Experimental
(Calculated) data for C58H86Cl2N4O2Pd2: C, 60.2 (60.3); H, 7.61
6.97 (d, 2H, 9.0 Hz, H6), 5.88 (d, 2H, 6.0 Hz, H7), 4.03 (t, 4H, 6.3), 2.09
(s, 6H), 1.80 (m, 4H), 1.26 (m, 52H), 0.88 (t, 6H, 6.9). Experimental
(Calculated) data for C60H86Cl2N4O6Pd2: C, 58.3 (57.9%); H, 7.05
(6.97%); N, 4.35 (4.51%). IR (
1238.
n
cmꢂ1), 2918, 1585, 1570, 1500, 1471,
(7.50); N, 4.99 (4.85). IR (n
cmꢂ1), 2916, 1602, 1581, 1500, 1471, 1253.
All analogous acetate bridged complexes were prepared as
described for palladium complex of 4-chloro-40-n-hexadecylox-
yazobenzene 1-L2. Yields and analytical data are as follows:
4.3.2. 2-L3
Yield ¼ 38%, Analytical data: 1H NMR (300 MHz, CDCl3),
d: 7.79
(d, 4H, 9.0 Hz, H1,2), 7.76 (d, 4H, 8.1 Hz, H3,4), 6.91 (d, 2H, 8.4 Hz, H5),
6.81 (d, 2H, 8.7 Hz, H7), 6.73 (d, 2H, 8.7 Hz, H6), 4.04 (t, 4H, 6.3), 1.80
(m, 4H), 1.26 (m, 52H), 0.88 (t, 6H, 6.6). Experimental (Calculated)
data for C56H80Cl2F2N4O2Pd2, C, 57.8 (57.8); H, 7.00 (6.93); N, 4.90
4.2.1. 1-L1
Yield ¼ 62%. Analytical data:, 1H NMR (300 MHz, CDCl3),
d: 7.56
(d, 2H, 7.8 Hz, H5), 7.34 (d, 4H, 7.2 Hz, H3,4), 6.67 (d, 4H, 6.9 Hz, H1,2),
6.59 (d, 2H, 6.9 Hz, H6), 5.88 (d, 2H, 6.0 Hz, H7), 4.03 (t, 4H, 6.3),
2.05e2.37 (m, 12H), 1.80 (m, 4H), 1.26 (m, 52H), 0.88 (t, 6H, 6.9).
Experimental (Calculated) data for C62H92N4O6Pd2: C, 62.0 (61.9);
(4.82). IR (
n
cmꢂ1), 2916, 1577, 1259.
4.3.3. 2-L4
Yield ¼ 32%, Analytical data: 1H NMR (300 MHz, CDCl3),
d: 8.30
H, 7.99 (7.71); N, 4.80 (4.66). IR (
n
cmꢂ1), 2918, 1636, 1602, 1500,
(d, 4H, 7.2 Hz, H1,2), 7.91 (d, 2H, 7.5 Hz, H5), 7.84 (d, 4H, 8.4 Hz, H3,4),
6.77 (m, 4H, H6,7), 4.07 (t, 4H, 6.3), 1.81 (m, 4H), 1.26 (m, 52H), 0.88
(t, 6H, 6.6). Experimental (Calculated) data for C56H80Cl2N6O6Pd2: C,
1259.
4.2.2. 1-L3
55.1 (55.2); H, 6.60 (6.63); N, 7.10 (6.91). IR (
1529, 1402, 1315, 1273.
n
cmꢂ1), 2916, 1581,
Yield ¼ 58%. Analytical data 1H NMR (300 MHz, CDCl3),
d: 7.89
(d, 4H, 6.9 Hz, H1,2), 7.88 (d, 4H, H3,4), 7.86 (d, 2H, 9.0 Hz, H5),
6.97 (d, 2H, 9.0 Hz, H6), 5.88 (d, 2H, 6.0 Hz, H7), 4.03 (t, 4H, 6.3),
2.09 (s, 6H), 1.79 (m, 4H), 1.26 (m, 52H), 0.88 (t, 6H, 6.6).
Experimental (Calculated) data for C60H86F2N4O6Pd2, C, 59.3
4.3.4. 2-L5
Yield ¼ 32%, Analytical data: 1H NMR (300 MHz, CDCl3),
d: 8.30
(d, 4H, 7.2 Hz, H1,2), 7.91 (d, 2H, 7.5 Hz, H5), 7.84 (d, 4H, 8.4 Hz, H3,4),
6.77 (m, 4H, H6,7), 4.04 (t, 4H, 6.3), 1.80 (m, 4H), 1.26 (m, 52H), 0.88
(t, 6H, 6.6). Experimental (Calculated) data for C58H86Cl2N4O2Pd2: C,
(59.5); H, 7.07 (7.16); N, 4.60 (4.63). IR (
1474, 1256.
n
cmꢂ1), 2918, 1602, 1585,
58.9 (59.1); H, 6.83 (6.85); 7.11 (7.14). IR (
1581, 1500, 1465, 1249.
n
cmꢂ1), 2916, 2220, 1598,
4.2.3. 1-L4
Yield ¼ 44%. Analytical data: 1H NMR (300 MHz, CDCl3),
d: 8.03
Acknowledgements
(d, 4H, 9.0 Hz, H1,2), 7.70 (d, 2H, 8.7 Hz, H5), 7.47 (d, 4H, 9.0 Hz, H3,4),
6.63 (d, 2H, 6.6 Hz, H6), 5.83 (d, 2H, 2.4 Hz, H7), 4.03 (t, 4H, 6.3), 2.15
(s, 6H), 1.79 (m, 4H), 1.26 (m, 52H), 0.88 (t, 6H, 6.6). Experimental
(Calculated) data for C60H86N6O10Pd2: C, 57.3 (57.0); H, 6.95 (6.86);
This research was supported by DAE, DST, DRDO, NRBD and UGC
India.
N, 6.50 (6.65). IR (n
cmꢂ1), 2920, 1637, 1581, 1406, 1319, 1263.
Appendix A. Supplementary material
4.2.4. 1-L5
Yield ¼ 64%. analytical data: 1H NMR (300 MHz, CDCl3),
d: 7.89
Supplementarymaterialassociated with this article canbe found,
in the online version, at doi:10.1016/j.jorganchem.2012.04.001.
(d, 4H, 6.9 Hz, H1,2), 7.88 (d, 4H, H3,4), 7.86 (d, 2H, 9.0 Hz, H5), 6.97
(d, 2H, 9.0 Hz, H6), 5.88 (d, 2H, 6.0 Hz, H7)), 4.03 (t, 4H, 6.3),
2.05e2.37 (m, 12H), 1.80 (m, 4H), 1.26 (m, 52H), 0.88 (t, 6H, 6.9).
Experimental (calculated) data for C62H92N4O6Pd2: C, 60.9 (60.8);
H, 7.03 (7.08); N, 6.80 (6.86). IR (
1502, 1474, 1249
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