Bridged 3,5-disubstituted pyrazolate ligands as support of metallomesogens containing [Pd(η3-C3

Article abstract of DOI:10.1016/S0022-328X(03)00669-7

The mesomorphic properties of two new long-chain 3,5-(4-n-alkoxyphenyl) pyrazoles Hpz^R2 (R=C₆H₄OC_n H_2n+1, n=16, 18) have been studied and compared with those of related compounds containing shorter chains (n=4, 6, 8, 10, 12, 14). For liquid crystal purposes, the derivative with 14 carbon atoms in the chain behaves as the best compound on the basis of the lowest melting point and the widest stability range of the mesophases. New Pd(II) complexes of formula [Pd(η³-C₃H₅) (μ-pz^R2)]₂ (R=C₆H₄ OC_n H_2n+1, n=8, 10, 12, 14, 16, 18) (1-6) containing the mesomorphic ligands as pyrazolate-bridging groups have been prepared and characterised. In solution, three conformational isomers (two symmetric and one asymmetric corresponding to different orientations of the allyl groups) have been found. In the solid state, the X-ray crystal structure of 3 (n=12) evidenced the asymmetric isomer. The structure shows a bowlic core drawn on the basis of the boat-like conformation of the six-membered ring Pd(NN)₂Pd. However, a rod-like shape could also be considered by taking the molecular dimensionality into account (47.80 A? in length×4.75 A? in wide). The Pd-complexes 2-6 were found to have liquid crystal properties exhibiting enantiotropic smectic phases consistent with the molecular characteristics. Moreover, the molecular packing of 3 is described as the layer-like type in which the molecules are parallel to the c-axis but tilted off the layer, this fact appearing similar to the molecular ordering in the smectic fluid phases.

Full text of DOI:10.1016/S0022-328X(03)00669-7

Journal of Organometallic Chemistry 682 (2003) 26ꢂ34
/
www.elsevier.com/locate/jorganchem
Bridged 3,5-disubstituted pyrazolate ligands as support of
metallomesogens containing [Pd(h3-C3H5)]ꢀ fragments
X-ray crystal structure of [Pd(h3-C3H5)(m-pzR2)]2×
/
CH2Cl2
(Rꢁ
/
C6H4OC12H25). Part IIIꢀ
M.C. Torralba a, M. Cano a,*, S. Go´mez a, J.A. Campo a, J.V. Heras a, J. Perles b,
C. Ruiz-Valero b
a Facultad de Ciencias Qu´ımicas, Departamento de Qu´ımica Inorga´nica I, Universidad Complutense, E-28040 Madrid, Spain
b Instituto de Ciencia de Materiales, CSIC, Cantoblanco, E-28049 Madrid, Spain
Received 28 March 2003; accepted 28 April 2003
Abstract
The mesomorphic properties of two new long-chain 3,5-(4-n-alkoxyphenyl)pyrazoles HpzR2 (Rꢁ
have been studied and compared with those of related compounds containing shorter chains (nꢁ4, 6, 8, 10, 12, 14). For liquid
crystal purposes, the derivative with 14 carbon atoms in the chain behaves as the best compound on the basis of the lowest melting
point and the widest stability range of the mesophases. New Pd(II) complexes of formula [Pd(h3-C3H5)(m-pzR2)]2 (Rꢁ
C6H4OCnH2nꢀ1, nꢁ8, 10, 12, 14, 16, 18) (1ꢂ6) containing the mesomorphic ligands as pyrazolate-bridging groups have been
prepared and characterised. In solution, three conformational isomers (two symmetric and one asymmetric corresponding to
different orientations of the allyl groups) have been found. In the solid state, the X-ray crystal structure of 3 (nꢁ12) evidenced the
/
C6H4OCnH2nꢀ1, nꢁ
/
16, 18)
/
/
/
/
/
asymmetric isomer. The structure shows a bowlic core drawn on the basis of the boat-like conformation of the six-membered ring
˚
Pd(NN)2Pd. However, a rod-like shape could also be considered by taking the molecular dimensionality into account (47.80 A in
˚
lengthꢃ
/
4.75 A in wide). The Pd-complexes 2ꢂ
/6 were found to have liquid crystal properties exhibiting enantiotropic smectic phases
consistent with the molecular characteristics. Moreover, the molecular packing of 3 is described as the layer-like type in which the
molecules are parallel to the c-axis but tilted off the layer, this fact appearing similar to the molecular ordering in the smectic fluid
phases.
# 2003 Elsevier B.V. All rights reserved.
Keywords: Long-chained 3,5-disubstituted pyrazoles; Mesomorphic behaviour; Palladiumꢂ
/
pyrazolate complexes; X-ray structure; Calamitic Pd(II)
complexes
1. Introduction
Dinuclear bisazolate metal derivatives are among the
complexes extensively characterised with a boat-like
conformation defined by two metal centres and two
Metal-containing liquid crystals are an increasing area
of research which has been commonly developed on
azolate-bridging groups [10ꢂ15]. In this context, since
/
compounds having rod-like or disk-like shapes [1ꢂ
contrast, metallomesogens with bowl-shaped structures
have been scarcely investigated [5ꢂ9].
/
4]. By
the pioneering studies of Trofimenko [16], some allyl-
pyrazolatepalladium derivatives have been investigated,
and the X-ray structures of two of them [Pd(h3-
/
C3H5)(m-pzR2)]2 (Rꢁ
/
CH3, CF3) have been reported
[17,18]. The mentioned complexes showed the charac-
teristic boat-like Pd2N4 core which gave rise to a bowlic
molecular shape.
On the other hand, in a previous work we have
reported the mesomorphic behaviour of a series of 3,5-
ꢀ For Parts I and II, see Refs. [19] and [20].
* Corresponding author. Fax: ꢀ34-91-3944352.
E-mail address: mmcano@quim.ucm.es (M. Cano).
/
0022-328X/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0022-328X(03)00669-7

M.C. Torralba et al. / Journal of Organometallic Chemistry 682 (2003) 26ꢂ
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27
disubstituted pyrazoles HpzR2 containing long-chain 4-
n-alkyloxyphenyl substituents (RꢁC6H4OCnH2nꢀ1
nꢁ4, 6, 12, 14) [19]. By contrast, from these ligands,
the related Rh-complexes of the types [Rh(m-
pzR2)(CO)2]2 [RhCl(CO)2(HpzR2)]
or (Rꢁ
C6H4OCnH2nꢀ1, nꢁ4, 6, 8, 10, 12, 14) did not exhibit
liquid crystal properties [19], as it also occurred for the
homologues [RhCl(CO)2(HpzR)] (Rꢁ
C6H4OCnH2nꢀ1
nꢁ4, 6, 8, 10) containing the non-mesomorphic 3(5)-
2. Experimental
/
,
/
2.1. Materials and physical measurements
/
All commercial reagents were used as received. The
starting Pd-complex [Pd(m-Cl)(h3-C3H5)]2 was pur-
chased from Sigma-Aldrich and used as received.
The 4-n-alkyloxyacetophenone and ethyl 4-n-alkylox-
ybenzoate derivatives were synthesised by alkylation of
4-hydroxyacetophenone or ethyl 4-hydroxybenzoate,
respectively, with the corresponding n-alkyliodide in
acetone solution of potassium carbonate as previously
described for related compounds [20,24,25]. All these
compounds were characterised satisfactorily by analy-
tical and spectroscopic techniques.
/
/
,
/
substituted pyrazoles HpzR [20]. Because Rh-fragments
had been proved to be useful as polar groups to induce
mesomorphism [3,4,21,22], the absence of liquid crystal
properties on our Rh-derivatives was attributed to their
inadequate molecular skeletal characteristics. In fact, we
proved that when we prepared straight molecules t-
[PdCl2(HpzR)2] (Rꢁ
/
C6H4OCnH2nꢀ1, nꢁ6, 8, 10, 12,
/
Elemental analyses for carbon, hydrogen and nitro-
gen were carried out by the Microanalytical Service of
the Complutense University. IR spectra were recorded
on an FTIR Nicolet Magna-550 spectrophotometer
14, 16, 18) with enough length to width relationship,
they behave as calamitic materials in agreement with
their molecular shape [23], although the metallic influ-
ence should not be discarded.
with samples as KBr pellets in the 4000ꢂ
/
400 cmꢄ1
In order to explore the influence of the metal core on
the mesomorphic behaviour of pyrazolate derivatives,
new complexes containing the [Pd(h3-C3H5)]ꢀ fragment
have been prepared. The main goal was directed
towards the evaluation of the ability of rod-like ligands
to induce mesomorphism on a bowlic core.
region.
1H-NMR spectra were performed on a Varian VXR-
300 or on a Bruker DPX-300 spectrophotometers of the
NMR Service of the Complutense University from
solutions in CDCl3. Chemical shifts d are listed in
ppm relative to TMS using the signal of the deuterated
solvent as reference, and coupling constants J are in
hertz. Multiplicities are indicated as s (singlet), d
(doublet), t (triplet), and m (multiplet). The atomic
numbering used in the assignment of the NMR signals is
shown in Scheme 1. The 1H chemical shifts and coupling
constants are accurate to 0.01 ppm and 0.3 Hz,
respectively.
The current report presents the results of complexes
[Pd(h3-C3H5)(m-pzR2)]2
of
the
type
(Rꢁ
6) (Scheme
/
C6H4OCnH2nꢀ1, nꢁ
/
8, 10, 12, 14, 16, 18) (1ꢂ
/
1) which, with the exception of 1, were found to have
liquid crystals properties at difference of the related non-
mesogens [Rh(m-pzR2)(CO)2]2 [19].
The mesomorphic behaviour of these Pd-complexes as
well as of the two new pyrazole ligands HpzR2 (Rꢁ
/
Phase studies were carried out by optical microscopy
using an Olympus BX50 microscope equipped with a
Linkam THMS 600 heating stage. The temperatures
C6H4OCnH2nꢀ1, nꢁ
/
16, 18) (LC16, LC18) (Scheme 1)
has been analysed.
Scheme 1.

28
M.C. Torralba et al. / Journal of Organometallic Chemistry 682 (2003) 26ꢂ34
/
Calculated for C100H160N4O4Pd2×
/
/12CH2Cl2: C, 69.5; H,
18):
were assigned on the basis of optic observations with
polarised light.
Measurements of the transition temperatures were
9.3; N, 3.2%. IR (KBr, cmꢄ1): n(CN) 1614. 6 (nꢁ
/
Elemental analyses: Found: C, 71.9; H, 9.6; N, 3.1.
Calculated for C108H176N4O4Pd2: C, 71.8; H, 9.8; N,
3.1%. IR (KBr, cmꢄ1): n(CN) 1613.
As expected, the NMR data of all Pd-complexes are
similar. Therefore, only those of 1 are given as
representative example.
made using a Perkinꢂ
calorimeter with the sample (2ꢂ
sealed in aluminium pans and with a heating or cooling
rate of 5ꢂ
108 minꢄ1
/Elmer Pyris 1 differential scanning
/6 mg) hermetically
/
.
2.2. Synthetic methods
1 (nꢁ
8.06ꢂ7.90m (Ho), 7.04ꢂ
(Hmeso), 2.97d (Hs, Jꢁ
Hz), 4.05t (OCH2, Jꢁ6.4 Hz), 1.90ꢂ
0.90t (CH3, Jꢁ6.6 Hz) ppm. Asymmetric isomer: 8.06ꢂ
7.90m (Ho), 7.04ꢂ6.96 (Hm), 6.60s (H4), 5.05m
(Hmeso), 3.26d (Hs, Jꢁ6.6 Hz), 2.87d (Hs, Jꢁ6.6
Hz), 2.37d (Ha, Jꢁ12.2 Hz), 2.04d (Ha, Jꢁ12.7 Hz),
4.05t (OCH2, Jꢁ6.4 Hz), 1.90ꢂ1.20m (ꢀ(CH2)6ꢀ), 0.90t
(CH3, Jꢁ6.6 Hz) ppm. Symmetric isomer: 8.06ꢂ7.90m
(Ho), 7.04ꢂ6.96 (Hm), 6.55s (H4), 5.05m (Hmeso),
3.16d (Hs, Jꢁ6.6 Hz), 1.93 (Ha, partially overlapped
with the CH2 protons), 4.05t (OCH2, Jꢁ6.4 Hz), 1.90ꢂ
1.20m (ꢀ(CH2)6ꢀ), 0.90t (CH3, Jꢁ6.6 Hz) ppm. Isomer
ratio: ca. 7:9:3.
/
8): 1H-NMR (CDCl3) d: symmetric isomer:
6.96 (Hm), 6.65s (H4), 5.05m
6.6 Hz), 2.50d (Ha, Jꢁ12.4
1.20m (ꢀ(CH2)6ꢀ),
/
/
2.2.1. Preparation of HpzR2 (Rꢁ
16, 18) (LC16, LC18
/
C6H4OCnH2nꢀ1, nꢁ
/
/
/
/
/
/
/
)
/
/
These new pyrazol ligands were prepared following a
similar procedure to that described for related pyrazoles
with shorter chains [19,25]. Due to the low solubility of
the new pyrazoles in relation to those with shorter
/
/
/
/
/
/
/
/
/
chains,
dichloromethaneꢂ
required.
several
recrystallisation
hexane or chloroformꢂ
processes in
hexane were
/
/
/
/
/
LC16: Yield: 83%. Elemental analyses: Found: C, 80.2;
H, 10.6; N 3.8. Calculated for C47H76N2O2: C, 80.5; H,
10.9; N, 4.0%. IR (KBr, cmꢄ1): n(NH) 3431, n(CN)
1615. LC18: Yield: 75%. Elemental analyses: Found: C,
80.5; H, 10.9; N, 3.3. Calculated for C51H84N2O2: C,
80.9; H, 11.2; N, 3.7%. IR (KBr, cmꢄ1): n(NH) 3423,
n(CN) 1614.
/
/
/
/
/
/
2.3. X-ray structure determination
1H-NMR (CDCl3) d: 7.64d (Ho, Jꢁ
(Hm, Jꢁ8.8 Hz), 6.69s (H4), 4.00t (OCH2, Jꢁ
1.80ꢂ1.20m (ꢀ(CH2)n ꢀ), 0.88t (CH3, Jꢁ7.0 Hz) ppm.
/8.8 Hz), 6.96d
/
/6.0 Hz),
Colourless crystals of [Pd(h3-C3H5)(m-pzR2)]2×
(RꢁC6H4OC12H25) (3) were obtained
dichloromethaneꢂ
/
CH2Cl2
from
/
/
/
/
/
/
hexane solution. A summary of the
2.2.2. Preparation of [Pd(h3-C3H5)(m-pzR2)]2 (Rꢁ
C6H4OCnH2nꢀ1, nꢁ8, 10, 12, 14, 16, 18) (1ꢂ6)
/
conditions for data collection is given in Table 1.
Suitable prismatic crystals of this compound were resin
epoxy coated and mounted on a Bruker Smart CCD
diffractometer equipped with a normal focus, 2.4 kW
/
/
To a solution of [Pd(m-Cl)(h3-C3H5)]2 (0.15 mmol)
and the corresponding pyrazole (0.30 mmol) in dry THF
(15 ml) under nitrogen atmosphere was added dropwise
an excess of NEt3 (2 ml) or a slight excess of 60% NaH.
The mixture of reaction was stirred at room temperature
approximately 3 h. The mixture was then filtered over
Celite to eliminate some metallic palladium formed by
decomposition. The obtained solution was left at low
temperature, giving rise to a white solid which was
sealed tube X-ray source (Moꢂ
/
Ka radiation, lꢁ
/
˚
0.71073 A) operating at 40 kV and 20 mA. Data were
collected over a hemisphere of reciprocal space by a
combination of three sets of exposures. Each set had a
different 8 angle for the crystal and each exposure of 20
s covered 0.38 in v. Unit cell dimensions were deter-
mined by a least-squares fit of 40 reflections with I ꢀ
/
filtered off and dried in vacuo (yield: 35ꢂ
1 (nꢁ8): Elemental analyses: Found: C, 65.2; H, 7.5;
N, 4.3. Calculated for C68H96N4O4Pd2: C, 65.5; H, 7.8;
N, 4.5%. IR (KBr, cmꢄ1): n(CN) 1612. 2 (nꢁ
10):
/50%).
2s(I) and 48B2u B468. The first 30 frames of data
/
/
/
were recollected at the end of the data collection to
monitor crystal decay, and no appreciable decay was
observed.
The structure was solved by Patterson methods and
¯
refined in the triclinic space group P1: Full-matrix least-
/
Elemental analyses: Found: C, 66.8; H, 8.1; N, 4.1.
Calculated for C76H112N4O4Pd2: C, 67.2; H, 8.3; N,
4.1%. IR (KBr, cmꢄ1): n(CN) 1612. 3×
/
CH2Cl2 (nꢁ
Elemental analyses: Found: C, 65.4; H, 8.4; N, 3.6.
Calculated for C84H128N4O4Pd2×CH2Cl2: C, 65.6; H,
8.4; N, 3.6%. IR (KBr, cmꢄ1): n(CN) 1614. 4 (nꢁ
14):
/12):
squares refinement with anisotropic thermal parameters
for all non-H atoms was carried out by minimising w
(F20ꢄ
/
F2c)2. The largest residual peak in the final differ-
/
ence map was 0.49 e Aꢄ3, near the C83 atom. The
refinement converged to an R value of 0.067.
/
˚
Elemental analyses: Found: C, 70.1; H, 9.2; N, 3.5.
Calculated for C92H144N4O4Pd2: C, 69.8; H, 9.2; N,
3.5%. IR (KBr, cmꢄ1): n(CN) 1613. 5×/2
All calculations were performed using: SMART soft-
ware for data collection; SAINT for data reduction [27],
SHELXTLTM to resolve and refine the structure and to
/
1CH2Cl2 (nꢁ
/16):
Elemental analyses: Found: C, 69.2; H, 9.1; N, 3.3.

M.C. Torralba et al. / Journal of Organometallic Chemistry 682 (2003) 26ꢂ
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29
Table 1
Crystal and refinement data for [Pd(h3-C3H5)(m-pzR2)]2×
C6H4OC12H25) (3)
posed formulation (see Section 2). The 1H-NMR spectra
of the complexes were consistent with the presence of a
/
CH2Cl2 (Rꢁ
/
series of eight doublets, four in the range 1.90ꢂ2.50 ppm
/
(J ca. 12 Hz) and other four between 2.95 and 3.30 ppm
(J ca. 7 Hz), and a multiplet at ca. 5.05 ppm,
corresponding to the anti, syn and meso protons of
the allyl group, respectively. In addition, three singlets
for the H4 protons of the pyrazolate ligand were
observed at ca. 6.65, 6.60 and 6.55 ppm. This behaviour
agrees with the presence of three conformational
isomers (two symmetric and one asymmetric) corre-
sponding to those containing both allyl groups pointing
in (isomer a), out (isomer b) or in and out (isomer c)
towards the metallic core of the Pd2N4 boat (Fig. 1).
It was interesting to note that in the original work on
dinuclear Pd(II) allylpyrazolate derivatives reported by
Trofimenko [16], a mixture of the related isomers had
already been proposed for [Pd(h3-CH2C(CH3)CH2)(m-
pzMe2)]2, on the basis of the presence of four types of syn
and four of anti protons, and for [Pd(h3-C3H5)(m-
pzMe2)]2, from very complex syn and anti multiplets
besides four methyl peaks.
In our complexes, the eight very well-resolved doub-
lets of the syn and anti protons of the allyl group, four
for the asymmetric and the other four from the two
symmetric isomers, as well as the three H4 proton
signals of the pyrazolate ligand allowed us to identify
undoubtedly the presence of the three isomers and
suggest that no exchange among them should occur at
room temperature. The 1H-NMR spectra in C6D6 in the
Empirical formula
Formula weight
Crystal system
C
1555.63
84H128N4O4Pd2×CH2Cl2
/
triclinic
¯
P1
Space group
/
Unit cell dimensions
˚
a (A)
˚
10.0(1)
17.0(2)
28.2(3)
90.0(2)
87.9(2)
90.0(2)
4788(80)
2
b (A)
˚
c (A)
a (8)
b (8)
g (8)
3
V (A )
˚
Z
T (K)
F(0 0 0)
rcalc (g cmꢄ3
293(2)
1652
1.079
)
m (mmꢄ1
)
0.474
Crystal dimensions (mm3)
Scan technique
Data collected
0.30ꢃ
v scans
(ꢄ9, ꢄ16, ꢄ
3.23ꢂ20.81
/0.15ꢃ
/0.08
/
/
/28) to (9,14,25)
u range (8)
/
Reflections collected
Independent reflections
Data/restraints/parameters
13 391
8645 (Rint
ꢁ/0.096)
8645/0/879
3618
0.814
Observed reflections [I ꢀ2s(I)]
/
GOF on F2
a
R
0.067
0.138
b
RwF
a
a½DFj/ajFoj.
b
a[w(DF)2]/a[w(Fo)2].
25ꢂ708 range showed broadening of all signals with the
/
prepare material for publication [28], and ATOMS for
molecular graphics [29].
Different crystals of 3 were used to solve the X-ray
structure. However in all cases the same structural
results, corresponding to the asymmetric isomer, were
observed (see below Section 3.2).
temperature. At 708, the H4 protons are observed close
to coalescence as well as the syn and anti protons are
also close to equivalence. Therefore, a fluxional beha-
viour of complexes yielding a rapid isomeric interchange
is suggested on heating.
The proton ratios indicated that the major isomer in
the mixture at room temperature was the asymmetric
one, being the relative areas among them of ca. 7:9:3.
3. Results and discussion
3.1. Synthetic and characterisation studies
3.2. Crystal structure of [Pd(h3-C3H5)(m-pzR2)]2×
CH2Cl2 (RꢁC6H4OC12H25) (3)
/
/
The 3,5-di(4-n-hexadecyloxyphenyl)pyrazole (LC16
and 3,5-di(4-n-octadecyloxyphenyl)pyrazole (LC18
)
)
Suitable crystals of
3
were obtained from
have been synthetised by an analogous procedure to
that reported for related pyrazoles [19,26,30,31], and
fully characterised by spectroscopic and analytical
techniques (see Section 2).
dichloromethaneꢂhexane. Fig. 2a illustrates the mole-
cular structure with the atom numbering scheme, and
Table 2 lists selected bond distances and angles. The
/
¯
complex crystallises in the P1 space group with two
The reaction of [Pd(m-Cl)(h3-C3H5)]2 in tetrahydro-
furan with two equivalents of pyrazolate ligand (pzR2),
prepared in situ by deprotonation of HpzR2 with NaH
or NEt3 in the same solvent, yields to the dimeric
[Pd(h3-C3H5)(m-pzR2)]2 unities and two dichloro-
methane molecules in the unit cell. No short contacts
between them are observed. The geometry about the
Pd(II) atoms is slight distorted square-planar with the
[Pd(h3-C3H5)(m-pzR2)]2
8, 10, 12, 14, 16, 18) (1ꢂ
(Rꢁ
6). The
/
˚
Pd1 and Pd2 0.38(1) and 0.30(1) A out of their
coordination planes (N1N3C80C82 and N2N4C83C85,
complexes
C6H4OCnH2nꢀ1, nꢁ
analytical and spectroscopic data agree with the pro-
/
/
respectively).

30
M.C. Torralba et al. / Journal of Organometallic Chemistry 682 (2003) 26ꢂ34
/
Fig. 1. Conformational isomers for [Pd(h3-C3H5)(m-pzR2)]2.
The allyl groups, C80C81C82 and C83C84C85, de-
groups have also been found in other complexes
containing [Pd(h3-allyl)]ꢀ fragments [32ꢂ
35].
The main structural feature was that related with the
molecular central core, which adopts a bowl-shaped
structure, as expected. The bowl is determined by two
viate from the ideal allyl geometry (Cꢀ
/
C distances of ca.
/
˚
1.36 A and Cꢀ
/
Cꢀ
/
C angles of 1208). So, each allyl group
presents two types of Cꢀ
/C distances of ca. 1.40 and 1.28
˚
A and a Cꢀ
/
Cꢀ
/
C angle of ca. 1518. Similar distorted allyl
Fig. 2. (a) Perspective ORTEP plot of 3. Hydrogen atoms and solvent molecule have been omitted for clarity, and thermal ellipsoids are at 50%
probability level. (b) Molecular schematic representation of the asymmetric isomer showing bond lengths and angles. (c) Schematic representation of
the distorted ‘H’ shape, showing the numbering of the chains on the pyrazolate rings. (d) View of 3 showing the bowl-shaped core and including its
length to width dimensionality.

M.C. Torralba et al. / Journal of Organometallic Chemistry 682 (2003) 26ꢂ
/34
31
Table 2
Selected bond distances (A) and angles (8) for [Pd(h -C3H5)(m-pzR2)]2×
CH2Cl2 (RꢁC6H4OC12H25) (3)
C6H4OC12H25 on the metal core. It was also observed
for other parameters involved in the boat. So, the Pdꢀ
bond lengths were 2.03(2) and 2.23(2) A for Pd1 and
3
˚
/
/N
/
˚
˚
2.13(2) and 2.42(2) A for Pd2 (Fig. 2b), slightly longer
that those for [Pd(h3-C3H5)(m-pzR2)]2 (Rꢁ
CH3, CF3)
2.111 A, respectively) [17,18],
and the intersection angles between the two PdꢀNꢀNꢀ
Pd planes were 113.6(6)8 for 3, and 103.28 and 106.68 for
[Pd(h3-C3H5)(m-pzR2)]2 (Rꢁ
CH3, CF3) [17,18].
˚
Bond distances (A)
/
Pd1ꢀ
Pd1ꢀ
Pd1ꢀ
Pd1ꢀ
Pd1ꢀ
C80ꢀ
C81ꢀ
/
N1
2.23(2)
2.03(2)
1.94(2)
1.90(2)
2.23(3)
1.40(3)
1.27(3)
3.293
˚
/N3
(2.067ꢂ
/
2.083 and 2.101ꢂ
/
/
C80
C81
C82
/
/
/
/
/
/
/
C81
C82
The other relevant structural feature of 3 is concerned
with the conformational isomer found which was
associated with the asymmetric one, according to the
major isomer detected in solution. So, the Pdꢀ
distances involving the terminal atoms of the allyl group
˚
were 2.23(3) and 1.94(2) A for Pd1 and 2.13(2) and
/
Pd1ꢀ ꢀ ꢀPd2
Pd2ꢀ
Pd2ꢀ
Pd2ꢀ
Pd2ꢀ
Pd2ꢀ
C83ꢀ
C84ꢀ
/
N2
2.42(2)
2.13(2)
2.13(2)
2.24(2)
2.55(2)
1.42(2)
1.28(2)
/N4
/C
/
C83
C84
C85
/
/
˚
2.55(2) A for Pd2, whereas those with the central carbon
˚
atom were 1.90(2) and 2.24(2) A respectively (Fig. 2b).
/
C84
C85
/
Bond angles (8)
The asymmetric isomer was also proposed by Trofi-
menko [16] for some pyrazolate derivatives and found
for [Pd(h3-C3H5)(m-pzCF3)]2 from the X-ray crystal
structure [18].
An interesting result emerges by considering the
whole molecular geometry. The molecule presents a
distorted ‘H’ form with the four chains almost parallel
but those on the same side of the pyrazolate groups (3
and 3?, and 5 and 5?; see Fig. 2c) slightly shifted. Then
the diagonal line linking the terminal carbon atoms of
opposite chains (3 and 5?) is longer than that from 3? and
5, and so the molecule presents a very elongated shape
N1ꢀ
N1ꢀ
N1ꢀ
N3ꢀ
N3ꢀ
C80ꢀ
C80ꢀ
N2ꢀPd2ꢀ
N2ꢀPd2ꢀ
N2ꢀPd2ꢀ
N4ꢀPd2ꢀ
N4ꢀPd2ꢀ
C83ꢀPd2ꢀ
C83ꢀC84ꢀ
/
Pd1ꢀ
Pd1ꢀ
Pd1ꢀ
Pd1ꢀ
Pd1ꢀ
Pd1ꢀ
C81ꢀ
N4
/
N3
102.7(7)
91.6(9)
167.9(5)
165.7(6)
89.4(7)
76.3(9)
/
/
C80
C82
C80
C82
/
/
/
/
/
/
/
/
C82
/
/
C82
151(3)
/
/
91.1(7)
102.2(8)
169.5(4)
166.1(5)
99.3(7)
/
/
C83
C85
C83
C85
/
/
/
/
/
/
/
/
C85
67.3(8)
/
/
C85
152(2)
˚
with the distance between C39 and C61 of 47.80 A being
˚
extremely long related to the width of 4.75 A (deter-
mined by the distance between two parallel planes
containing the C80 and C48 atoms, respectively) (Fig.
2d).
The molecular packing is recovered in Fig. 3. It could
be described as layer-like type with the layers formed by
stacking of two sheets of molecules with opposite
orientation. Each sheet in turns has molecules with the
same orientation and presents interdigitation between
neighbours. Because of the molecular shape, the layers
adopt a zigzag or folded structure.
palladium atoms and the four nitrogens of two pyrazo-
late rings bridging the metal centers. Then the six-
membered ring Pd(NN)2Pd, in which the N1ꢀ
and N2ꢀPd2ꢀN4 planes intersect with an angle of
83.2(7)8, generates a boat-like conformation. The open-
ess of the bowl is primarily determined by the N1ꢀPd1ꢀ
N3 and N2ꢀPd2ꢀN4 angles which were 102.7(7)8 and
/
Pd1ꢀ
/
N3
/
/
/
/
/
/
91.1(7)8, respectively. The flat pyrazolate rings further
extend the size of the bowl spreading out upwards to
make the top of the bowl. The C6H4OC12H25 substi-
tuents spread out sideways forming an almost flat plane.
Each pyrazolate ligand carries two alkyloxyphenyl
chains in the 3- and 5-positions having both phenyl
groups almost coplanar (12.9(8)8 and 15.0(7)8) but
twisted from their own pyrazolate ring angles with
values ranging from 36.8(8)8 to 46.6(7)8. Because the
dihedral angle between planes containing the pyrazo-
late-bridging groups is 46.6(7)8, the corresponding
phenyl groups of different pyrazolates form dihedral
angles from 28.1(7)8 to 42.7(7)8.
However by considering the molecule as rod-like type,
the structure suggests an ordering in which all the
molecules are approximately parallel to the c-axis
(Fig. 2d), but taking as molecular axis the longest
˚
C61
distance of 47.80 A corresponding to the C39ꢀ
/
diagonal mentioned above the molecules are deviated
108 from the layer. This ordering suggests some resem-
blance with the layered molecular organisation observed
in the smectic mesophases.
3.3. Thermal studies
˚
The Pdꢀ ꢀ ꢀPd distance in the boat is 3.293 A similar to
that found in [Pd(h3-C3H5)(m-pzR2)]2 (Rꢁ
(3.343 and 3.2169 A, respectively) [17,18], indicating the
/
CH3, CF3)
The thermal behaviour of the Pd-complexes 1ꢂ
well as the two new pyrazoles HpzR2 (Rꢁ
C6H4OCnH2nꢀ1, nꢁ
16, 18) (LC16, LC18) was studied
/6 as
˚
/
slight incidence of sterically demanding substituents
/

32
M.C. Torralba et al. / Journal of Organometallic Chemistry 682 (2003) 26ꢂ
/34
Fig. 3. Molecular packing of 3 along the c-axis.
by polarised light microscopy and differential scanning
calorimetry.
increasing from LC4 to LC14 and then lowering for LC16
and LC18. Then it is of interest to note that for liquid
crystal purposes, the best ligand was LC14 which gives a
lower melting point at 100 8C and also shows a wider
mesophase stability range of 60 8C. This fact merits
further comment. The chains contribute to the structural
anisotropy but because they do not easily pack in the
crystalline form they act to reduce the melting point
(until a determined chain length). That is, it should exist
a compromise between the increase of the chain length
and the disorder produced by them.
The two new pyrazole ligands behave as liquid
crystals like the related counterparts HpzR2 previously
described containing side chains with 4 (LC4), 6 (LC6), 8
(LC8), 10 (LC10), 12 (LC12) and 14 (LC14) carbon atoms
[19,25]. LC16 and LC18 exhibit enantiotropic smectic C
mesophases at lower transition temperatures and over a
narrower temperature range as it was observed by
polarised light microscopy (LC16: Cr, 111 8C; SmC,
151 8C I; LC18: Cr, 115 8C. SmC, 147 8C I). Fig. 4
displays a bar graph showing the phase behaviour of all
of those ligands for comparative purposes. A decrease in
the melting temperature accompanying an increase in
the side chain length is observed from LC4 to LC14, while
LC16 and LC18 show the opposite behaviour. The
stability range displayed by the mesophases was also
found to be dependent on the number of carbon atoms
in the side chains and following the same sequence, i.e.
The Pd-complexes 1ꢂ6, with the exception of that
/
with the shortest chain 1, exhibited enantiotropic
smectic phases at temperatures between 43 8C and
108 8C, which were smectic A for 2, smectic A and
smectic C for 3 and smectic C for 4ꢂ6. Table 3 lists the
/
onset temperatures observed for these compounds on
the cooling. The melting temperature varied between
43 8C and 60 8C and the clearing temperatures were in
the range 95ꢂ108 8C. For this class of complexes, it was
/
observed that the increase of the number of carbon
atoms of the alkyloxy chains does not introduce
significant modifications on the melting and clearing
points. Fig. 5 shows the textures of the mesophases of
compounds 2ꢂ4 as representative examples.
/
In relation to the mesogenic free ligands [19,26], it can
be observed that the corresponding pyrazolate Pd-
complexes show liquid crystal phases at lower tempera-
tures.
The comparison of the mesomorphic properties on
the Pd-complexes [Pd(h3-C3H5)(m-pzR2)]2 with the non-
mesomorphism of the related Rh-derivatives [Rh(m-
Fig. 4. Bar graph showing the thermal behaviour of the HpzR2
compounds.

M.C. Torralba et al. / Journal of Organometallic Chemistry 682 (2003) 26ꢂ
/34
33
Table 3
Phase properties of compounds 1ꢂ
allyl groups on the molecule (see Section 3.1). The
reduced molecular symmetry should give rise to a less
favourable packing in the solid state reducing the
corresponding melting points. In agreement with this
proposal, the observed melting points of the Rh-com-
/
6 determined by DSC on cooling a
DH (kJ molꢄ1
Transition
T (8C)
)
1
2
I0
I0
/
Cr
124.9
107.5
ca. 60
95.8
ꢄ
/
31.7
plexes [Rh(m-pzR2)(CO)2]2 (Rꢁ
143.8 8C, 133.4 8C and 126.0 8C for nꢁ
/
C6H4OCnH2nꢀ1
) of
10, 12, 14,
/SmA
ꢄ
/
10.4
b
SmA0
I0
SmA0
SmC0
I0
SmC0
I0
SmC0
I0
SmC0
/
Cr
/
3
/
SmA
ꢄ
/
21.0
respectively [19] were slightly higher than the clearing
temperatures for the Pd-derivatives (120.5 8C, 103.9 8C
and 98.9 8C, respectively, measured on the heating).
Therefore, both the skeletal molecular characteristics
and the packing arrangement observed for the Pd-
derivatives seem to favour that they exhibit liquid
crystal properties.
/
SmC
Cr
77.1
43.5
ꢄ0.4
/
c
/
4
5
6
/
SmC
101.7
58.6
98.5
ꢄ
/
12.9
/Cr
ꢄ
/
56.7
/
SmC
ꢄ
/
4.8
c
/Cr
46.9
/
SmC
95.5
61.3
ꢄ
/
9.6
/Cr
ꢄ/55.5
a
Onset temperatures are given. If heating temperatures are used,
melting and clearing points appear overlapped in a broad peak. By
contrast they were clearly observed using the cooling data and agree
with the visual observations by microscopy.
4. Concluding remarks
b
Observed only by polarised light microscopy.
It is not possible to determine the enthalpy variation of this
transition because it is in the low limit of the calorimeter.
It has been established that in most cases the shape of
the mesogen determines the symmetries of the resulting
thermotropic mesophases. So, ellipsoidal molecules give
rise to nematic and lamellar phases whereas discoid
molecules display columnar mesomorphism [1]. In our
Pd-complexes, the molecular structure indicate that
these mesogens should be considered as rod-like type,
c
pzR2)(CO)2]2 [19] suggests some comments. This beha-
viour could be due to a lower symmetry of the Pd-
complexes resulting from the different orientation of the
Fig. 5. Textures of the smectic mesophases on cooling of (a) 2 at 92.9 8C (SmA), (b) 5 at 80.6 8C (SmC), (c) 3 at 90.0 8C (SmA), and (d) 3 at 57.6 8C
(SmC).

34
M.C. Torralba et al. / Journal of Organometallic Chemistry 682 (2003) 26ꢂ34
/
[10] R. de la Cruz, P. Espinet, A.M. Gallego, J.M. Mart´ın-Alvarez,
J.M. Mart´ınez-Ilarduya, Polyhedron 663 (2002) 108 (and refer-
ences therein).
which is established on the basis of the overall molecular
geometry as well as the molecular dimensionality
determined by the position of the alkyloxy substituents
(3 and 5) on the pyrazolate ligands and by the number of
carbons (higher than 8). The arrangement of this type of
rod-like molecules results preferentially in the formation
of a layer-like structure in the solid state, and these
Pd(II) complexes appear to arrange in a similar layered
structure in the mesophases.
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5. Supplementary material
[19] M.C. Torralba, M. Cano, J.A. Campo, J.V. Heras, E. Pinilla,
M.R. Torres, J. Organomet. Chem. 654 (2002) 150.
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Chem. Soc. Chem. Commun. (1989) 55.
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC No. 206256 for compound 3.
Copies of this information may be obtained free of
charge from The Director, CCDC, 12 Union Road,
[22] D.W. Bruce, D.A. Dunmur, M.A. Esteruelas, S.E. Hunt, R. Le
Lagadec, P.M. Maitlis, J.R. Marsden, E. Sola, J.M. Stacey, J.
Mater. Chem. 1 (1991) 251.
Cambridge CB2 1EZ, UK (Fax: ꢀ44-1223-336033; e-
/
mail: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk).
[23] M.C. Torralba, M. Cano, J.A. Campo, J.V. Heras, E. Pinilla,
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Acknowledgements
Financial support from the DGES of Spain is grate-
fully acknowledged (Project No. PB98-0766).
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SMART system, Siemens Analytical X-ray Instrument, Inc.,
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