Mesogens based on palladium orthometallated complexes with carboxylato bridges: Tuning and shaping a non-planar molecule

Article abstract of DOI:10.1016/j.jorganchem.2004.09.024

The binuclear cyclopalladated compounds [Pd₂(μ-OH) ₂(Lⁿ)₂] (1) derived from imines HLⁿ = p-C_nH_{2n + 1}O-C₆H₄-CHN-C ₆H₄-OC_nH_{2n + 1}-p (n = 6,10) react with carboxylic acids to give the derivatives [Pd₂(μ-ox) ₂(Lⁿ)₂] (2) with a planar core for oxalic acid, and [Pd₂(μ-OOCR)₂(Lⁿ)₂] (3-7) compounds with a non-planar ridge tent structure for other RCOOH acids: (3) R = C_mH_{2m + 1} (m = 1, 3, 5, 7, 9, 11, 13, 15, 17); (4) R = CH₂(OCH₂CH₂)_pOCH₃ (p = 1, 2); (5) R = CH₂-C₆H₄-OC_qH _{2q + 1}-p (q = 2, 4, 6, 8, 10, 12); (6) R = C₆H ₄-OC_rH_{2r + 1}-p (r = 4, 10); (7) R = C*H(OH)CH₃. The acids used were designed to explore the effect on the thermal properties of the compounds prepared of systematic variations in the type of carboxylato ligand, which induce structure, packing, and polarity changes, and in the length of the carboxylato chain. Most of the complexes prepared, even when far from planar, show liquid crystal behavior and display nematic, smectic A and smectic C phases.

Full text of DOI:10.1016/j.jorganchem.2004.09.024

Journal of Organometallic Chemistry 690 (2005) 261–268
www.elsevier.com/locate/jorganchem
Mesogens based on palladium orthometallated complexes
with carboxylato bridges: tuning and shaping a non-planar molecule
*
Laura Dıez, Pablo Espinet , Jesus A. Miguel, M. Paz Rodrıguez-Medina
´
´
´
´ ´
Quımica Inorganica, Facultad de Ciencias, Universidad de Valladolid, 47005 Valladolid, Spain
Received 20 July 2004; accepted 13 September 2004
Abstract
The binuclear cyclopalladated compounds [Pd2(l-OH)2(Ln)2] (1) derived from imines HLn = p-CnH2n + 1O–C6H4–CH@N–C6H4–
OCnH2n + 1-p (n = 6,10) react with carboxylic acids to give the derivatives [Pd2(l-ox)2(Ln)2] (2) with a planar core for oxalic acid, and
[Pd2(l-OOCR)2(Ln)2] (3–7) compounds with a non-planar ridge tent structure for other RCOOH acids: (3) R = CmH2m + 1 (m=1, 3,
5, 7, 9, 11, 13, 15, 17); (4) R = CH2(OCH2CH2)pOCH3 (p = 1, 2); (5) R = CH2–C6H4–OCqH2q + 1-p (q = 2, 4, 6, 8, 10, 12); (6)
R = C6H4–OCrH2r + 1-p (r = 4, 10); (7) R = C*H(OH)CH3. The acids used were designed to explore the effect on the thermal prop-
erties of the compounds prepared of systematic variations in the type of carboxylato ligand, which induce structure, packing, and
polarity changes, and in the length of the carboxylato chain. Most of the complexes prepared, even when far from planar, show
liquid crystal behavior and display nematic, smectic A and smectic C phases.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: Metallomesogens; Palladium; Orthometalated imine; Carboxylato; Liquid crystals
1. Introduction
½Pd2ðl-XÞ Lnꢀ (Ln = orthopalladated imines and azines)
2
2
the X-bridges determine the basic molecular shape,
and consequently have a marked influence on the meso-
genic or lack of mesogenic properties of the material.
Thus, thiocyanato, chloro and bromo bridges produce
planar dimeric structures prone to give mesomorphism
[2g,2h,6–9]. Carboxylato bridges force the molecule into
The tendency for a given molecule to display a given
liquid-crystalline superstructure is usually strongly re-
lated to its shape [1]. Thus, rod-like molecules preferen-
tially assemble into nematic or smectic mesophases,
whereas disc-like molecules tend to display columnar
phases. Notwithstanding that, materials with columnar
phases can be generated also from molecules which are
not disc-shaped, and there are compounds showing cala-
mitic phases with molecular structures rather different
from rod-like.
Dinuclear and mononuclear ortho-palladated com-
pounds are one of the most widely studied class of met-
allomesogens [2]. Some pioneering discoveries in
metallomesogens have been made within this kind of
mesogenic materials [3–5]. In dimeric molecules
a
non-planar ridge-tent (or open-book) structure
(Fig. 1). According to single crystal X-ray diffraction
studies [10], the dihedral angle between the two coordi-
nation planes is about 30ꢁ and there is some Pdꢁ ꢁ ꢁPd
bonding interaction. The aspect ratio of this structure
is not propitious to give rise to mesophases, as empty
spaces in the direction of the ridge of the tent (symbol-
ized as a grey volume in Fig. 1) should produce an inef-
ficient space filling if the alkyl chains remained stretched
(as represented in the Fig. 1). As a consequence, a curl-
ing of the chains must occur to fill that otherwise empty
space, leading to a shorter length/width ratio and disfav-
oring mesogenic behavior.
*
Corresponding author. Tel.: +34 9834 23231; fax: +34 9834 23013.
E-mail address: espinet@qi.uva.es (P. Espinet).
0022-328X/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.09.024

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L. Dıez et al. / Journal of Organometallic Chemistry 690 (2005) 261–268
262
R
R
C
C
O
O
O
O
Pd
Pd
Pd
=orthopalladated imine, azine or diazobenzene with terminal chains
Fig. 1. Non-planar structures in dimeric molecules with carboxylato bridges: the grey volume in the direction of the ridge of the tent highlights the
empty spaces.
The recent availability of ½Pd2ðl-OHÞ Lnꢀ (HLn = p-
rical ethers B were obtained in a Williamson ether
synthesis using the methyl-(4-hydroxy)-phenylacetate
A and the corresponding 1-bromoalkane in acetone.
Finally, saponification with NaOH followed by acid-
ification with cold concentrated hydrochloric acid
afforded the acids C.
2
2
CnH2n + 1O–C6H4–CH@N–C6H4–OCnH2n + 1-p, n = 2, 6
and 10) towards protic substrates [11], has provided us
with an extremely convenient general entry to many
bridged complexes, by reaction with acidic substrates.
Using this preparative approach, we report here the
preparation and thermal behavior of a variety of carb-
The dinuclear complexes 2–7 were obtained from the
hydroxo complexes 1 and the appropriate acid, at room
temperature in dichloromethane (Scheme 2). All the
complexes were isolated in good yield and characterized
oxylato bridged dimers ½Pd2ðl-O2CRÞ Ln2ꢀ (n = 6,10).
2
The carboxylato groups (oxalato, n-alkylcarboxylato,
n-oxaalkylcarboxylato,
p-alkoxyphenylacetato,
p-
1
alkoxybenzoato and chiral carboxylato) are chosen to
produce different structural effects [12].
by IR, H NMR and elemental microanalyses.
The oxalato-bridged group in complexes 2 prefers the
chelating coordination to give five-membered metalacy-
cles, and forces the molecules into a strictly coplanar
Pd(l2-oxalato)Pd geometry, in accordance with struc-
tures found for similar complexes by X-ray diffraction
2. Results and discussion
1
2.1. Syntheses and structures
studies in the literature [13]. The H NMR spectra of
complexes 2 show a mixture of cis (or syn) and trans
(or anti) isomers (relative to the imine arrangement in
each half of the molecule) in a 40:60 ratio. Only the trans
isomer (presumed to be the major component in the
mixtures) is depicted in Scheme 2.
The carboxylato-bridged complexes 3–7 were ob-
tained as the trans isomers, with a small amount (about
4%) of the cis isomers. The 1H NMR parameters (partic-
The complexes ½Pd2ðl-OHÞ Ln2ꢀ were prepared by
2
published methods [11]. The acids employed were ob-
tained from commercial sources, except for p-alkoxy-
substituted-phenylacetic acids, which were synthesized
in a three-step reaction as shown in Scheme 1. Commer-
cial 4-hydroxy-phenylacetic acid was transformed into
its methyl ester using H2SO4 as catalyst. The unsymmet-
O
O
H SO
2
4
+ MeOH
HO
CH
C
HO
CH
C
2
2
OH
OMe
A
CmH2m+1Br
K2CO3
1. NaOH
2. HCl
O
O
H
C O
CH
C
H
C O
CH
C
2
2m+1
m
2
2m+1
m
OMe
OH
C
B
m = 2,4,6,8,10,12
Scheme 1.

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L. Dıez et al. / Journal of Organometallic Chemistry 690 (2005) 261–268
263
OC H
n
2n+1
H
C
N
O
O
O
O
N
C
OH
H C
3
C
H
H
O
Pd
Pd
H C O
2 4
2
C
N
O
O
O
C
L-lactic ac.
Pd
Pd
=
2
Pd
N
2
2
N
2
C H
m
CO H
2m+1 2
1
C H
m
2m+1
7
a n = 6
b n = 10
C
O
O
OC H
r
2r+1
OC H
n
2n+1
Pd
N
2
3
m = 1,3,5,7,9,11,13,15,17
OC H
q
2q+1
O
O
HO
OC H
r
2r+1
O
O
OH
p
O
p
H C
2
HO
O
C
N
O
O
2
O
Pd
OC H
q
2q+1
C
N
O
O
6
r = 4,10
Pd
2
CH
2
4
C
q = 2,4,6,8,10,12
p = 1,2
O
O
Pd
N
2
5
Scheme 2.
Table 1
Parameters of 1H NMR for compounds 2–7a
H6 H5
H5´ H6´
H
OCH2
CH´2O
N
H3
Pd
H2´
H3´
0
0
0 0
Compounds
H
H3
H5
H6
H2 6 , H3 5 b
OCH2
4.01
OCH02
Othersc
2
7.88 sd
6.85 d
(2.3)
6.61 dde
(8.3)
7.22 d
7.36, 6.93
(8.9)
3
4
5
6
7
7.45 sd
7.46 sd
7.40 sd
7.55 sd
6.02 d
(2.3)
6.55 dd
(8.3, 2.3)
6.56 dd
7.11 d
(8.3)
7.66, 6.62
(8.9)
3.71 m
3.48 m
3.74 m
3.53 m
3.39 m
3.12 m
2.33 m
2.92 m
3.96 m
3.75 m
3.52 m
3.88 m,
(6.6)
3.88 m
2.12 m O2CCH2;
6.01 d
(2.3)
7.12 d
(8.3)
6.64. 6.62
(9.4)
3.88 m O2CCH2; 3.53 m
O(CH2)2O; 3.38 s OCH3
3.39 m O2CCH2; 7.07,
6.75b (9.0) C6H4; 3.89 m OCH2;
8.01, 6.78 (8.8)
5.73 d
(2.4)
6.49 dd
(8.3, 2.0)
6.50 dd
7.06 d
(8.3)
6.58,6.50
(9.1)
3.89 m
(6.6)
5.97 d
(2.3)
7.14 d
(8.3)
6.70,6.44
(8.8)
6.60 m
3.78 m
3.67 m
(8.3, 2.3)
f
C6H4;3.95 t (6.5) OCH2
4.08 m CH; 3.19 m OH;
1.22 d, 1.18 d (6.8) CH3
7.51 s
7.49 s
5.89 d
5.81 d
(2.2)
7.17 d
7.16 d
(8.3)
a
In CDCl3 at 300.13 MHz; the numbers in parentheses correspond to J(1H–1H) in Hz; s, singlet; d, doublet; t, triplet; m, multiplet.
(AB)2 spin system.
Aliphatic protons appear in the range d = 0.8–1.9 ppm.
b
c
d
e
f
Signals for the cis isomers are observed at 7.89 s (2), 7.59 (3); 7.57 (4); 7.54 (5); 7.54 (6).
The signal for the cis isomer is observed at 6.63 dd.
2060
Overlapped with H , H
3050
.
ularly the H3 proton resonance appearing around
d = 6.0 ppm, see Table 1) support that they a ridge tent
structure, rigid in the NMR time scale as far as inversion
of the folding is concerned [17,14]. This makes the pairs
of hydrogen atoms on each half of the molecule diaster-
eotopic, as can be seen in the methylene groups of the

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L. Dıez et al. / Journal of Organometallic Chemistry 690 (2005) 261–268
264
iminic alkoxy chains and in the first methylene group of
1
the carboxylato bridges. The H NMR parameters of
lateral association, each one sliding over the other neigh-
bors in order to fill the space in the best possible way
[16]. A similar packing is expected for the imine com-
plexes, and indeed mesomorphism appears only when
the flexible alkyl chains in the carboxylate are long en-
ough to orient themselves along the main axis of the
imine ligand (z, molecular length), filling the grey zones
in Fig. 1 and providing a more cylindrical shape to the
molecule (Fig. 3).
In contrast, complexes 3b, with imine L10, are liquid
crystals except when the bridge is acetate, m = 1. Only
when the value of the alkyl carboxylato chain is 5 or 7
are the compounds enantiotropic, with a very short tem-
perature range (2–4 ꢁC) on heating.
Comparing for identical alkoxy chains the properties
of the imine complexes 3b with related azine complexes
reported in our previous study [16], the melting temper-
atures are about 15 ꢁC higher for the former. This is
somewhat surprising since more extended conjugated
aromatic cores usually increase the melting point notice-
ably. Probably this behavior is due to the different iso-
meric compositions for the two types of complexes
(mixtures lower the transition temperatures). The
trans/cis mixture is 3/1 for the azines and 96/4 for the
imines.
A remarkable variation in thermal properties is pro-
duced when some methylene groups in the carboxylato
bridges are substituted for oxygen. Thus complexes 4
with 3,6-dioxaheptanoic (p = 1) or 3,6,9-trioxadecanoic
acids (p = 2) display enantiotropic SmA phases. Com-
paring the two compounds 4a in Fig. 2 (p = 1, 2), a de-
crease in the melting and the clearing temperature is
observed when the length of the polyethers chains in-
creases. A very interesting increase in range of the smec-
tic A phase (>50 ꢁC) occurs when the length of the imine
chains changes from six (4a) to ten (4b) carbon atoms,
which decreases the melting point and increases the
clearing temperature for 4b.
the imine moiety (Table 1) are very similar for all the
compounds. The larger difference in chemical shifts is
observed for the iminic proton and for H3, which are
very sensitive to structural changes [17,11]. The assign-
ments given in Table 1 for 5 were assessed by COSY
and NOESY experiments.
Complexes 7 were made from the commercially avail-
able form of (S)-lactic acid. The IR band observed at
3605 cmꢂ1, due to m(OH), and the chemical shift of
H3 (around d = 5.8 ppm) are typical of dimers with
carboxylato bridges. Several examples of this type of
structure have been confirmed by X-ray diffraction for
a-hydroxycarboxylates [15]. Since the rigid ridge tent
structure is chiral (K and D enantiomers) [17,14], the
use of (S)-lactic acid gives rise to trans-D(S,S) and
trans-K(S,S) diastereomers, which are observed in a
1
40:60 in the H NMR spectrum.
2.2. Mesogenic behavior
The family of complexes studied here offers a com-
plete and systematic panorama of the effects of changes
in the carboxylato bridges. This had been studied only
partially on some particular azine or imine systems
[4,16–18]. The thermal properties of the complexes were
studied using polarized light optical microscopy and dif-
ferential scanning calorimetry (DSC). The data are sum-
marized in Table 2 and are plotted for direct comparison
in Fig. 2.
The bridging oxalate imposes a planar structure to
the central bimetalic core of the dimer. Thus the inter-
molecular interactions are very strong leading to melting
points of 2a–b above 230 ꢁC, That is even higher than
for the hydroxo and chloro bridged analogues, which
are also planar. Due to the very high transition temper-
ature the compounds decompose at, or just above, their
clearing point.
It is known for organic systems that the substitution
of one methylene by oxygen in the alkoxylic chains of
imines derived from benzylideneanilines lowers the
clearing point of smectic and nematic phases [19]. Also,
the biphenyl derivatives (4-hexyl,40-hexyloxy)biphenyl
and (4-hexyl,40-nonyloxy)biphenyl, with alkyl chains,
are liquid crystals but the oligoethylene glycol 4,40-
bis[2-(2-methoxy-ethoxy)ethoxy]biphenyl with compa-
rable length is not mesomorphic [20,21], whereas the
replacement of alkyl chain linkages by polyethers causes
an increase in range of mesophase in biphenylophane
derivatives [21], and a decrease in the case of 2- or 5-phe-
nylpyrimidine para-cyclophane [22]. This has been ex-
plained considering the influence of these structural
changes on the amphiphilicity of the molecules. The
increase in range of mesophase reported here for
complexes 4 can also be explained by the effect
that replacing alkyl by polyether chains has on the
Compounds 3a (m = 1, 3, 5, 7, 9, 11, 13, 15, 17), with
imine L6, exhibit thermotropic behavior only for the
largest chains (m P 11). They display a smectic A phase,
with spherulites and homeotropic zones, and a nematic
phase (schlieren texture) in a very short temperature
range (about 2 ꢁC only). In addition, a SmC phase is ob-
served on cooling for m P 13. A very sharp decrease in
melting point is observed on going from m = 1 to m = 3,
which was expected as the increase in length of the carb-
oxylato chain, for short chains, leads to an increase in
molecular width and a less efficient packing of the mol-
ecules in the crystal state.
As stated in Section 1, the non-planar structure of the
complexes is expected to disfavor an efficient molecular
packing. X-ray diffraction studies in the mesophase on
complexes with orthopalladated azines and carboxylato
bridges have shown that these molecules tend to pack by

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L. Dıez et al. / Journal of Organometallic Chemistry 690 (2005) 261–268
265
Table 2
Phase transition temperatures (ꢁC) and enthalpies (kJ molꢂ1) (in parentheses) for compounds 2–7a
Compounds
m/p/q/r
Heating scan
2a
2b
3a
3a
3a
3a
3a
3a
3a
Cr 266bSmA 277b,c
I
Cr 231.8 (36.1) SmA 257.b,c
Cr 217.0c (56.2) I
Cr 156.9 (37.8) I
I
1
3
5
Cr 146.7 (33.1) I
Cr 143.6 (33.9) I
7
9
Cr 142.4 (59.2) I
Cr 51.8 (6.2) Cr0 130b SmA 132.0 (39.6)d
I
11
13
Cr 118.8 SmA 121b N 122.9e (34.9)d
I
I 121.3 N 120b(ꢂ4.9)d SmA 98b (SmC)f
3a
3a
15
17
Cr 62.9 (ꢂ12.7) Cr0 108.8 SmA 111b (41.5)d N 116.8e (3.9) I
I 115.1 N 114b (ꢂ4.4)d SmA 92 (SmC)f
Cr 72.4 (ꢂ9.4) Cr0 108.1 (46.3) I
I 108.1e (N)f 108b (ꢂ4.3)c (SmA)f 86.1 (ꢂ0.3) (SmC)f
3b
3b
1
3
Cr 205.5c (62.7) I
Cr 114.0 (9.2) Cr0 135.7 (31.9) I
I 129.2 (ꢂ5.3) (SmA)f
3b
3b
3b
5
7
9
Cr 128b SmA 131.7e (25.9)d
Cr 128b SmA 130.4e (31.8)d
I
I
Cr 95.5 (ꢂ13.6) Cr0 129.9 (50.2) I
I 119.6 (ꢂ5.4) (SmA)f
3b
3b
3b
3b
11
13
15
17
Cr 77.5 (ꢂ9.8) Cr0 127.1 (56.0) I
I 116.3 (ꢂ5.4) (SmA)f
Cr 81.4 (9.5) Cr0 97.7 (ꢂ15.8) Cr00126.6 (63.9) I
I 115.3 (ꢂ5.4) (SmA)f
Cr 81.7 (ꢂ15.3) Cr0 117.3 (64.3) I
I 114.1 (ꢂ5.4) (SmA)f100.8 (ꢂ1.5) (SmC)f
Cr 85.4 (ꢂ21.6) Cr0 113.5 (64.7) I
I 112.2 (ꢂ5.6) (SmA)f 98.4 (ꢂ1.6) (SmC)f
Cr 119.6 (18.4) SmA 133.3 (5.3) I
Cr 93.1 (16.9) SmA 103.8 (3.7) I
Cr 91.6 (9.7) SmA 145.8 (5.8) I
Cr 67.8 (13.2) SmA 127.9 (6.7) I
4a
4a
4b
4b
5a
5a
5a
1
2
1
2
2
4
6
Cr 172b SmA 175.8 (37.6)d
I
Cr 164.7 (37.2) SmA 170.5 (5.6) I
Cr 136.2 (38.4) SmA 168.1 (8.2) I
I 167.9e (ꢂ7.4) SmA 139.3 (ꢂ0.7) (SmC)f
Cr 122.9 (23.6) SmC 131.6 (0.6) SmA 162.4 (7.7) I
Cr 115.6 (19.8) SmC 123.9 (0.3) SmA 153.3 (7.5) I
Cr 115.0 (21.4) SmC 125.8 (0.5) SmA 152.9 (8.7) I
Cr 138.5 (20.5) SmA 174.0 (9.7) I
5a
5a
5a
5b
8
10
12
2
I 173.9 (ꢂ9.7) SmA 134b (SmC)f
5b
4
Cr 125.4 (21.1) Cr0 145.6 (12.5) SmA 165.6 (8.0) I
I 163.3 (ꢂ8.7) SmA 140b (SmC)f
5b
5b
5b
5b
6a
6a
6b
6b
6
8
Cr 131.2 (28.2) SmC 138.9 (1.4) SmA 159.5 (6.0) I
Cr 128.8 (32.8) SmC 133b SmA 153.2 (7.8) I
Cr 123.7 (30.3) SmC 129b SmA148.9 (7.4) I
Cr 121.6 SmC 1289b SmA 147.1 (7.3) I
Cr 140.8 (30.9) Cr0 174.9 (36.9) I
10
12
4
10
4
Cr 128.6 (33.9) I
Cr 65.6 (ꢂ8.7) Cr0 114.7 (21.9) SmC 131b (4.3) SmA 148b,c
I
10
Cr 133.3 (56.3) I
I 128.6 (SmA)f
7a
7b
a
Cr 102.8 (14.2) SmA 176.0 (6.6) I
Cr 85.1 (4.0) SmA 162.9 (7.0) I
Cr, Cr0, Cr00 = crystalline phase; SmA = smectic A; SmC = smectic C; I = isotropic liquid; N = nematic.
b
c
d
e
f
Observed by polarized light microscopy.
Transition with decomposition.
Combined enthalpies.
Peak data.
Monotropic transition (data for the cooling cycle).

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L. Dıez et al. / Journal of Organometallic Chemistry 690 (2005) 261–268
266
amphiphilicity of the molecules, and the fact that the
microsegregation [23] of the polar regions (rigid cores
and polyether chains) from the lipophilic chains should
become more efficient.
Complexes 5, derived from 4-alkoxyphenyl acetic
acid (p-CmH2m + 1OC6H4CH2CO2H, m = 2, 4, 6, 8, 10,
12), are enantiotropic liquid crystals, The influence of
the chain length on the mesogenic properties of the di-
mer complexes can be seen in Fig. 2. Smectic A phases
were found for these complexes. In addition, SmC
phases appeared for the complexes with L6 and the long-
est alkoxy chains (m P 6) in the 4-alkoxyphenyl acetate,
and for all the complexes with L10 (for m = 2, 4 the SmC
phases are monotropic). The absence of nematic phases
is not unexpected, considering the presence of the two
additional aryl rings that stabilize the smectic phase.
Longer chains, whether on the 4-alkoxyphenyl acetate
or on the imine, lower the melting and clearing points.
The influence of the chain length of the carboxylato
on the thermal behavior is larger for L6 than for L10.
For the compounds with L6 the increase in the length
of the chain from 2 to 12 carbons produces a bigger de-
crease in melting (59 ꢁC) than in clearing points (21 ꢁC)
and greater ranges of mesogenic behavior are produced.
For L10 the melting and clearing points are similarly re-
duced and the mesophase ranges are maintained.
The thermal behavior observed is consistent with the
model proposed in Fig. 1. In the p-alkoxyphenylacetate
bridged complexes (compounds 5, Fig. 4a) we presume
that the aryl moiety (depicted as an ellipse) and the long
chain can immediately arrange more o less parallel to
the molecular long axis, improving the mesogenic prop-
erties. When this methylene group is lacking (derivatives
6, Fig. 4b) the rigidity of the aryl group on the carboxy-
lic fragment imposes a less favorable molecular shape.
Finally, the complexes with 2-hydroxopropionate (L-
lactate) bridges, 7a and 7b, exhibit a wide range of smec-
Fig. 2. Thermotropic behavior of Pd complexes 3–7. The number (m,
p, q or r) in the x-axes refers to the labels used in Scheme 2. When
monotropic transitions are present two bars are shown, one for the
heating (h) and one for the cooling (c) cycle; m, monotropic transition.
C
O
C
O
O
O
Pd
z
Pd
Fig. 3. Schematic representation with the alkyl chains of the carboxylate filling the grey zones.
C
O
C
O
C
C
O
O
O
O
O
O
Pd
Pd
Pd
Pd
(a)
(b)
Fig. 4. Schematic representation of the ridge tent-shaped derivatives: (a) [Pd(l-alkoxyphenyl acetate)Ln]2 (5) and (b) [Pd(l-alkoxybenzoate)Ln]2 (6).

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L. Dıez et al. / Journal of Organometallic Chemistry 690 (2005) 261–268
267
tic A phase. Compared to the lack of mesomorphism for
the corresponding acetato-bridged complexes, this
might look is somewhat surprising, since L-lactate is
bulkier than acetate and should disturb more the molec-
ular packing in the mesophase. This positive effect on
mesogenic properties for bulkier carboxylates has been
observed also in 2-halopropionato bridged complexes
of palladium [17] and platinum [5] with imines and can
be the result of a combination of factors such as the pos-
itive influence of the new dipole moment introduced by
C–OH bonds, the formation of isomeric mixtures (the
chiral carboxylates as bridges gives rise to a number of
diastereomers cis-S,S:trans-DS,S:trans-KS,S (4:38:58)),
and a more efficient space filling in the melt: the halo
or hydroxo substituents can be oriented into the empty
region close to the hinge of the molecule, thus helping
to partially fill this unfavorable gap, while not increasing
the width of the molecule. Comparison of the mesogenic
behavior of 7a and 7b with the chloropropionate ana-
logues of palladium shows that the effect of replacing
Cl by OH in the propionate bridges is thermally benefi-
cial, yielding an important decrease in the melting tem-
peratures (up to 50 ꢁC) with only a slight decrease in
the clearing temperatures, and producing a wider meso-
genic range. Although the phases obtained are not chiral
in this case, the use of lactato bridges looks very
promising.
rials). IR spectra were recorded on a Perkin–Elmer
FT-1720X spectrometer using Nujol mulls between
polyethylene plates. 1H NMR spectra were recorded
on Bruker AC-300 or ARX-300 MHz spectrophotome-
ters. The textures of the mesophases were studied with a
Leitz microscope with polarizers equipped with a Met-
tler FP82HT hot stage and a Mettler FP90 central proc-
essor, at a heating rate of approximately 10 ꢁC minꢂ1
.
Transition temperatures and enthalpies were measured
by differential scanning calorimetry, with a Perkin–El-
mer DSC-7 apparatus operated at a scanning rate of
10 ꢁC minꢂ1 on heating. The apparatus was calibrated
with indium as standard (156.6 ꢁC, 28.5 J gꢂ1), the sam-
ples were sealed in aluminum capsules in air, and the
holder atmosphere was dry nitrogen.
Typical preparation procedures and physical data are
given below. All analogous ligands and complexes were
prepared in the same way.
4.2. Methyl-(4-hydroxy)phenylacetate (A)
To a nitrogen flushed flask containing, p-hydroxy-
phenylacetic acid (5.00 g, 33 mmole) dissolved in 100
ml of anhydrous MeOH was added a catalytic amount
of H2SO4, and the mixture was refluxed for 7 h. The
reaction mixture was poured into water and extracted
three times with ether. The organic layer was separated,
acidified with NaHCO3, and dried over MgSO4. The sol-
vent was evaporated to yield a white solid. Yield: 82%.
1H NMR (CDCl3): d 7.09, 6.76, (AB system JAB = 8.7
C6H4); 3.69 (s, CH3); 3.56 (s, CH2).
3. Conclusions
This study shows that, once the rules of the molecular
system are understood, metal-containing compounds
with unfavorable molecular shapes can be manipulated
and tuned to obtain liquid crystalline compounds. In
this case the features of the carboxylato bridges in di-
mers [Pd2(l-OOCR)2(Ln)2] are a key factor to control
of their thermal behavior. They can be manipulated to
shape an essentially inadequate ridge tent-shaped mole-
cule into a more cylindrical form, or to provide extra
interactions easing microsegregation.
4.3. 4-(Decyloxy)phenylacetic acid (C, m = 10)
To a mixture of 100 ml of acetone-DMSO (90/10) was
added methyl-(4-hydroxy)phenylacetate (A) (4.5 g, 27.1
mmol), K2CO3 (3.7 g, 27.1 mmole) and 1-bromodecane
(5.62 ml, 27.1 mmole). The suspension was refluxed for
18 h and then allowed to cool down. The solution was
then poured into 500 ml of H2O, acidified with diluted
formic acid until pH = 5, and then extracted with
dichloromethane (3 · 30 ml) The organic phase was sep-
arated, dried over MgSO4, filtered and the solvent evap-
orated on a rotary evaporator. The residue was purified
by column chromatography (silica gel, hexane/ether 9:1
as eluent). The product obtained after evaporation was a
white solid, which was added to a solution of NaOH (5
g) in 100 ml of water. The suspension was refluxed for 2
h until the solid got dissolved. The resulting solution
was cooled and acidified with HCl. A white precipitate
was formed which was filtered from the solution,
washed thoroughly with water and dried. Yield: 5.83 g
(60%).1H NMR (CDCl3): d 7.18, 6.85, (AB system,
JAB = 8.7 Hz, C6H4); 3.93 (t, 6.7 Hz, OCH2); 3.58 (s,
CH2), 1.50–0.85 (m, C9H19). IR (Nujol, cmꢂ1): m(CO)
1703; m(OH) 3446.
4. Experimental
4.1. Materials and general methods
Literature methods were used to prepare the imines
[Pd2(l-OH)2(Ln)2]
(1)
[11].
Aliphatic
acids
HO2CCmH2m + 1, oxalic acid and L(+)lactic acid were
obtained from commercial sources and were used with-
out further purification. 3,6-Dioxaheptanoic and 3,6,9-
trioxadecanoic acids were supplied by Hoechst AG. C,
H, N analyses were carried out on a Perkin–Elmer
2400 microanalyzer. All new compounds gave satisfac-
tory elemental analyses (Table S1, Supplementary Mate-

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L. Dıez et al. / Journal of Organometallic Chemistry 690 (2005) 261–268
268
4.4. [Pd2(l-O2CC5H11)2(L6)2] (3a, m = 5)
(f) M.J. Baena, P. Espinet, M.B. Ros, J.L. Serrano, Angew.
Chem. Int. Ed. Engl. 30 (1991) 711;
(g) M.J. Baena, P. Espinet, M.B. Ros, J.L. Serrano, A. Ezcurra,
Angew. Chem. Int. Ed. Engl. 32 (1993) 1203;
´
(h) M.J. Baena, J. Barbera, P. Espinet, A. Ezcurra, M.B. Ros,
A mixture of [Pd2(l-OH)2(L6)2] (1a) (0.10 mmol) and
hexanoic acic (0.025 ml, 0.20 mmole) was stirred in dry
dichloromethane (20 ml) for 1 h. The yellow solution
was evaporated to ca. 3 ml under reduced pressure.
Addition of methanol afforded 3a as a yellow solid,
which was filtered of, washed with cold acetone (2 · 3
ml) and vacuum dried. Yield: 80%.
J.L. Serrano, J. Am. Chem. Soc. 116 (1994) 1899;
(i) M. Ghedini, D. Pucci, F. Neve, J. Chem. Soc. Chem. Commun.
(1996) 137;
(j) D.P. Lydon, G.W.V. Cave, J.P. Rourke, J. Mater. Chem. 7
(1997) 403;
(k) K. Praefcke, D. Singer, B. Gundogan, Mol. Cryst. Liq. Cryst.
¨
223 (1992) 181;
(l) B. Neumann, T. Hegmann, R. Wolf, C. Tschierske, J. Chem.
Soc. Chem. Commun. (1998) 105.
4.5. Pd2(l-O2CCH(CH3)OH)2(L6)2] (7a)
´
´
´
[3] P. Espinet, J. Etxebarrıa, M. Marcos, J. Perez, A. Remon, J.L.
Serrano, Angew. Chem. Int. Ed. Engl. 28 (1989) 1065.
[4] M.J. Baena, J. Buey, P. Espinet, H.S. Kitzerow, G. Heppke,
Angew. Chem. Int. Ed. Engl. 32 (1993) 1201.
To a suspension of [Pd2(l-OH)2(L6)2] (0.10 mmol) in
dichloromethane (20 ml) was added of L(+)lactic acid
(0.22 mmole). After stirring for 1 h, the solvent was
evaporated and the residue was extracted with 15 ml
of acetone/ether 1:1. Then, the solution was refrigerated
at ꢂ4 ꢁC and the resulting orange solid was collected on
a frit, and dried in vacuum. Yield: 60%.
[5] J. Buey, L. D´ıez, P. Espinet, H.S. Kitzerow, J.A. Miguel, Chem.
Mater. 8 (1996) 2375.
´
[6] J. Barbera, P. Espinet, E. Lalinde, M. Marcos, J.L. Serrano, Liq.
Cryst. 6 (1987) 833.
[7] M. Marcos, M.B. Ros, J.L. Serrano, Liq. Cryst. 8 (1988) 1129.
[8] M.B. Ros, N. Ruiz, J.L. Serrano, Liq. Cryst. 9 (1991) 77.
[9] M.J. Baena, P. Espinet, M.B. Ros, J.L. Serrano, J. Mater. Chem.
6 (1996) 1291.
Acknowledgement
[10] (a) See for example M.T. Pereira, J.M. Vila, E. Gayoso, M.
Gayoso, W. Hiller, J. Strahle, J. Coord. Chem. 18 (1988) 245;
´ ´
(b) R. Mosteiro, E. Perille, A. Fernandez, M. Lopez-Torres, J.M.
We gratefully acknowledge financial support by the
´
Spanish Comision Interministerial de Ciencia y Tec-
nologıa (Project MAT2002-00562), and the Junta de
´ ´
Vila, A. Suarez, J.M. Ortigueira, M.T. Pereira, J.J. Fernandez,
Appl. Organomet. Chem. 14 (2000) 634;
´
´
Castilla y Leon (Project VA050/02).
(c) R.B. Bedford, C.S.J. Cazin, S.J. Coles, T. Gelbrich, M.B.
Hursthouse, V.J.M. Scordia, Dalton Trans. (2003) 3350.
[11] L. D´ıez, P. Espinet, J.A. Miguel, J. Chem. Soc. Dalton Trans.
(2001) 1189.
Appendix A. Supplementary material
[12] (a) For the use of carboxylates as modifiers of liquid crystal
properties in very different systems see: J.P. Rourke, F.P. Fanizzi,
N.J.S. Salt, D.W. Bruce, D.A. Dunmur, P.M. Maitlis, J. Chem.
Soc. Chem. Commun. (1990) 229;
Table of yields, elemental analysis and relevant IR
data for complexes 2–7. Supplementary data associated
with this article can be found, in the online version, at
doi:10.1016/j.jorganchem.2004.09.024.
(b) C.P. Roll, B. Donnio, W. Weigand, D.W. Bruce, Chem.
Commun. (2000) 709.
[13] (a) T. Kawato, T. Uechi, H. Koyama, H. Kanatomi, Inog. Chem.
23 (1984) 764;
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