Pyrazole-based allylpalladium complexes: Supramolecular architecture and liquid crystal behaviour

Article abstract of DOI:10.1016/j.inoche.2006.07.043

The co-ordination of non-mesomorphic 3-substituted pyrazoles Hpz^R to the [Pd(η³-C₃H₅)]⁺ fragment gives rise to four-coordinated complexes [Pd(η³-C₃H₅)(Hpz^R)₂]⁺ (R = C₆H₄OC_nH_2n+1; n = 12 (I₁₂), 14 (I₁₄), 16 (I₁₆), 18 (I₁₈)), which were isolated with BF₄^- as counteranion. The new complexes are proved to have liquid crystal properties exhibiting monotropic or enantiotropic smectic A (SmA) mesophases at low temperatures which range between ca. 40 and 80 °C. The crystal structure of I₁₂ presents a 2D network highly interdigitated, which could be related with the layered structure proposed in the liquid crystalline phase through the X-ray diffraction at variable temperature.

Full text of DOI:10.1016/j.inoche.2006.07.043

Inorganic Chemistry Communications 9 (2006) 1271–1275
www.elsevier.com/locate/inoche
Pyrazole-based allylpalladium complexes: Supramolecular
architecture and liquid crystal behaviour
a
a,
a
a
b
M.C. Torralba , M. Cano *, J.A. Campo , J.V. Heras , E. Pinilla a,b, M.R. Torres
a
Departamento de Quı´mica Inorga´nica I, Facultad de Ciencias Quı´micas, Universidad Complutense, E-28040 Madrid, Spain
Laboratorio de Difraccio´n de Rayos-X, Facultad de Ciencias Quı´micas, Universidad Complutense, E-28040 Madrid, Spain
b
Received 29 May 2006; accepted 22 July 2006
Available online 18 August 2006
Abstract
The co-ordination of non-mesomorphic 3-substituted pyrazoles HpzR to the [Pd(g3-C3H5)]+ fragment gives rise to four-coordinated
complexes [Pd(g3-C3H5)(HpzR)2]+ (R = C6H4OCnH2n+1; n = 12 (I12), 14 (I14), 16 (I16), 18 (I18)), which were isolated with BFÀ as count-
4
eranion. The new complexes are proved to have liquid crystal properties exhibiting monotropic or enantiotropic smectic A (SmA) mes-
ophases at low temperatures which range between ca. 40 and 80 ꢀC. The crystal structure of I12 presents a 2D network highly
interdigitated, which could be related with the layered structure proposed in the liquid crystalline phase through the X-ray diffraction
at variable temperature.
ꢁ 2006 Elsevier B.V. All rights reserved.
Keywords: Palladium(II) complexes; Pyrazole; Metallomesogen; X-ray structure
For several years, we have been interested in the liquid
crystal chemistry of substituted pyrazoles and the related
pyrazolylpyridine ligands, as well as their metal complexes.
In this context, we have proved that, in contrast to the
mesomorphic nature of 3,5-bis(4-alkyloxyphenyl)pyrazole
(HpzR2; R = C6H4OCnH2n+1) or 2-[3,5-bis(4-alkyloxyphe-
nyl)pyrazol-1-yl]pyridine (pzR2py) ligands [1–3], the 3-
substituted homologues HpzR and pzRpy did not exhibit
liquid crystal behaviour [4–6]. However, it was interesting
to note that all of them were able to induce mesomorphism
upon co-ordination to determined palladium(II) fragments
[2,3,5–7]. In particular, we have observed that the co-ordi-
nation of the mesogenic pzR2py ligands to PdCl2 or [Pd(g3-
C3H5)]+ fragments gave rise to metallomesogens [3,7]
(Scheme 1). By contrast, the four-coordinated cis-dichlor-
opalladium complexes [PdCl2(pzRpy)] containing the
related 3-substituted pzRpy ligands were not mesomopo-
rhic, while their co-ordination to the [Pd(g3-C3H5)]+ moi-
ety yielded metallomesogenic derivatives [6] (Scheme1),
this feature suggesting the role of the electron delocalisa-
tion in the mesomorphism.
Following these results we decided to prove that the
[Pd(g3-C3H5)]+ group can be useful as a candidate to
produce metallomesogenic four-co-ordinated compounds
by co-ordination to 3-(4-alkyloxyphenyl)pyrazole HpzR
ligands (Scheme 2). We have already determined the meso-
morphic properties of the related compounds t-[PdCl2-
(HpzR)2] [5].
In this work we describe the synthesis, characterisation
and mesomorphic properties of a series of cationic
compounds of the type [Pd(g3-C3H5)(HpzR)2]+ (R =
C6H4OCnH2n+1; n = 12 (I12), 14 (I14), 16 (I16), 18 (I18)),
which were isolated with BFÀ4 as counteranion.
The syntheses of [Pd(g3-C3H5)(HpzR)2][BF4] (R =
C6H4OCnH2n+1; n = 12 (I12), 14 (I14), 16 (I16), 18 (I18))
[8] were carried out as it has been described for related
compounds [6,7]. The spectroscopic (IR and 1H NMR)
and analytical data [9,10] allowed us to establish their
identity and they are in agreement with the proposed
formulation.
*
Corresponding author. Fax: +34 91 3944352.
E-mail address: mmcano@quim.ucm.es (M. Cano).
1387-7003/$ - see front matter ꢁ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2006.07.043

1272
M.C. Torralba et al. / Inorganic Chemistry Communications 9 (2006) 1271–1275
N
N
N
N
N
N
pzR2py
R = C6H4OCnH2n+1
pzRpy
n = 14, 16
n = 4, 6, 8, 10, 12, 14, 16, 18
LC
(a)
No LC
(b)
[Pd(η3-C3H5)(pzR2py)][X]
[PdCl2(pzR2py)]
[PdCl2(pzRpy)]
[Pd(η3-C3H5)(pzRpy)][X]
Pd Cl
Cl
Pd Cl
Cl
[X]
[X]
Pd
Pd
-
-
n = 12, 14, 16, 18; X = BF4
n = 12, 14, 16, 18
LC
(c)
LC
(d)
n = 12, 14, 16, 18
No LC
n = 12, 14, 16, 18; X = BF4
LC
n = 14, 16, 18; X = PF6-, CF3SO3
-
-
(f) n = 16; X = PF6-, CF3SO3
Scheme 1. Schematic molecular representation and mesomorphic properties of the pzR2py (a) and pzRpy (b) ligands, and their complexes [Pd(g3-
C3H5)(pzR2py)][X] (c), [PdCl2(pzR2py)] (d), [Pd(g3-C3H5)(pzRpy)][X] (e) and [PdCl2(pzRpy)] (f) [3,6,7].
The knowledge of the molecular structure of the meso-
phases has been a focus of interest in order to understand
[Pd(η3-C3H5)(HpzR)2][BF4]
HpzR
the liquid crystal behaviour of the compounds. In this con-
text, we think that the X-ray structure of the studied com-
plexes will contribute as a support for the establishment of
R = C6H4OCnH2n+1
N
N
[BF4]
Pd
potential structure/properties relationships. On this basis,
we have solved the X-ray single-crystal structure of
[Pd(g3-C3H5)(Hpzddp)2][BF4] (Hpzddp = 3-(4-dodecyloxy-
phenyl)pyrazole) (I12) (Fig. 1), as a representative member
of the homologues studied in this work [12]. The crystal
structure reveals that the complex is comprised of the
BFÀ4 anion and the cation [Pd(g3-C3H5)(Hpzddp)2]+,
bonded through a strong H-bond (Table 1).
H
n = 4, 6, 8, 10, 12, 14, 16, 18
No LC
No LC n = 12 (I12
)
n = 14, 16, 18 (I14-18
)
LC
Scheme 2. Schematic molecular representation of the complexes [Pd(g3-
C3H5)(HpzR)2][BF4] (I12À18) studied in this work.
1
The four-co-ordination around the metal is defined by
two nitrogen atoms from two cis-pyrazole groups and the
two allyl C9 and C11 carbon atoms, in an almost square-
planar geometry. It is deduced by the dihedral angle of
4.7(4)ꢀ between the planes defined by the Pd, N1 and N3
atoms, and the Pd, C9 and C11 atoms. The Pd atom devi-
The H NMR spectra of I12À18 at room temperature in
CDCl3 solution show the expected resonances as broad sig-
nals attributed to a dynamic behaviour, which was proved
by the analysis of their spectra at variable temperatures
(from 30 to À50 ꢀC) [10]. This behaviour can be explained
on the basis of the existence of different isomers related
with the relative orientation of the allyl group and the
two pyrazoles, which can interchange through a rotation
of the allyl group or a tautomeric equilibrium. Many exam-
ples in the literature support this proposal [11].
˚
ates 0.044(1) A of the co-ordination plane defined by the
N1, N3, C9 and C11 atoms. The Pd–N distances of ca.
˚
2.08 A are similar to those found in related complexes
[2,5,6,13–17]. The allyl group shows Pd–C distances of
Fig. 1. Perspective ORTEP plot of [Pd(g3-C3H5)(Hpzddp)2][BF4] (I12). Hydrogen atoms, except H2 and H4, have been omitted for clarity. The thermal
˚
ellipsoids are at 30% probability level. Selected bond lengths (A) and angles (ꢀ): Pd1–N1 2.085(8), Pd1–N3 2.079(8), Pd1–C9 2.10(1), Pd1–C10 2.10(2),
Pd1–C11 2.09(1), C9–C10 1.381(5), C10–C11 1.374(5), N1–Pd1–N3 92.9(3), N1–Pd1–C9 99.1(4), N1–Pd1–C10 132.3(4), N1–Pd1–C11 166.6(5), N3–Pd1–
C9 167.9(4), N3–Pd1–C10 131.1(4), N3–Pd1–C11 99.7(5), C9–Pd1–C10 38.4(2), C9–Pd1–C11 68.2(5), C10–Pd1–C11 38.3(2), C9–C10–C11 117(2).

M.C. Torralba et al. / Inorganic Chemistry Communications 9 (2006) 1271–1275
1273
Table 1
whole cation is 26(2)ꢀ. As a consequence, the two NH-
groups interact with two different BFÀ4 counteranions
through strong H-bonds (Table 1). Each BFÀ4 group
bridges two cationic entities, as it had been observed in
related compounds exhibiting polymeric chains and an tor-
sion angle s of ca. 180ꢀ [16–18]. However, at difference to
those, in our compound, two BFÀ4 counteranions bond
the same two cationic unities so generating dimers through
these N–H Á Á ÁF interactions (Fig. 2).
Hydrogen-bond geometries for I12
D–H Á Á ÁA
d(D–H)
(A)
d(HÁ Á ÁA)
d(DÁ Á ÁA)
\(D–HÁ Á ÁA)
(ꢀ)
˚
˚
˚
(A)
(A)
N4–H4Á Á ÁF3
N2–H2Á Á ÁF1a
C5–H5Á Á ÁF1b
a
1.28
1.27
0.93
1.87
1.86
2.46
2.98(1)
3.04(1)
3.31(1)
141.3
151.8
150.5
1 À x, Ày + 1, Àz.
b
x, Ày + 3/2, Àz + 1/2.
New weak C–H Á Á ÁF interactions between the dimers
(Table 1) give rise to layers defining a 2D structure that
is highly interdigitated (Fig. 3).
˚
ca. 2.10 A, in agreement with that observed in other Pd-
compounds containing N-donor ligands [6,13–17].
The C9C10C11 allyl group is slightly deviated from the
ideal geometry, with bond distances of ca. 1.38 A and an
C–C–C angle of 117(2)ꢀ. The central C10 carbon atom pre-
sents a deviation of 0.65(2) A of the co-ordination plane.
The benzene planes of the substituents are slightly
twisted with respect to their own pyrazole planes with dihe-
dral angles of 15.6(4) and 14.2(4)ꢀ. The two alkyl chains
point in an almost opposite direction with an angle of
63.9(1)ꢀ which is defined as that formed between the lines
O2–C29 and O1–C47.
On the other hand, it has been established that in related
[Pd(g3-C3H5)(Hpz)2]+ derivatives, the two cis-pyrazole can
adopt different orientations of the NH-groups which can be
determinant for the supramolecular architecture [16,17]. In
the current compound, the pyrazole planes form an dihe-
dral angle of 71.9(3)ꢀ, and the torsion angle s defined by
the four N-atoms of the two pyrazoles involved in the
The thermal behaviour of the new complexes I12À18 was
studied by differential scanning calorimetry (DSC), polar-
ised light optical microscopy (POM) and X-ray diffraction
at variable temperature (XRD). The phase transition tem-
peratures and thermodynamic data are given in Fig. 4.
Complex I12 was non-mesomorphic showing only a melting
process at 73 ꢀC. The remaining compounds I14À18 dis-
played monotropic (I14 and I16) or enantiotropic (I18)
liquid crystal properties exhibiting smectic A (SmA) meso-
phases (Fig. 5), which were identified from the battonets
and homeotropic textures observed by POM.
For compounds I14 and I16, the transition from the mes-
ophase to crystal could not be determined since the crystal-
line phase appears at lower temperatures than 40 ꢀC. Then,
on reheating from 40 ꢀC, a simple clearing event at 60 and
71 ꢀC, respectively, is observed by POM and DSC. The
third heat–cool cycle reproduces the second exactly.
˚
˚
74 (92.5)
I14
Cr
Cr
I
I
SmA
T < 40
59 (−3.2)
69 (−2.5)
96 (65.7)
I16
SmA
T < 40
57 (58.2)
77 (2.2)
I18
Cr
SmA
I
54 (−21.3)
79 (−2.2)
Fig. 4. Phase properties on the first heat–cool cycle of I14À18. Onset
temperatures (ꢀC) and enthalphies (kJ molÀ1, in parentheses) are given.
Fig. 2. Dimeric unit of I12 showing the N–HÁ Á ÁF hydrogen bonds.
Fig. 3. 2D network of I12 viewed in the ac plane.

1274
M.C. Torralba et al. / Inorganic Chemistry Communications 9 (2006) 1271–1275
Universidad Complutense (UCM2005-910300) is gratefully
acknowledged. We also thank Comunidad de Madrid for
financial support (Project GR/MAT/0511/2004) and the
contract to M.C.T.
References
[1] M.C. Torralba, M. Cano, J.A. Campo, J.V. Heras, E. Pinilla, M.R.
Torres, J. Organomet. Chem. 654 (2002) 150.
´
[2] M.C. Torralba, M. Cano, S. Gomez, J.A. Campo, J.V. Heras, J.
Perles, C. Ruiz-Valero, J. Organomet. Chem. 682 (2003) 26.
[3] M.J. Mayoral, M.C. Torralba, M. Cano, J.A. Campo, J.V. Heras,
Inorg. Chem. Commun. 6 (2003) 626.
[4] M.C. Torralba, M. Cano, J.A. Campo, J.V. Heras, E. Pinilla, M.R.
Torres, J. Organomet. Chem. 633 (2001) 91.
[5] M.C. Torralba, M. Cano, J.A. Campo, J.V. Heras, E. Pinilla, M.R.
Torres, Inorg. Chem. Commun. 5 (2002) 887.
[6] M.C. Torralba, M. Cano, J.A. Campo, J.V. Heras, E. Pinilla, M.R.
Torres, J. Organomet. Chem. 691 (2006) 765.
Fig. 5. Texture of the smectic A mesophase, observed on cooling, for I16
at 71 ꢀC.
[7] M.C. Torralba, J.A. Campo, J.V. Heras, D.W. Bruce, M. Cano,
Dalton Trans. (2006) 3918.
The enantiotropic behaviour of I18 was confirmed by the
DSC trace observed in the first heating, which shows two
phase transitions related with the melting Cr–SmA and
clearing SmA–I processes (Fig. 4). Successive heat–cool
cycles reproduce the same behaviour. The enantiotropic
SmA phase in I18 can be related to a larger molecular isot-
ropy produced by the presence of longer chains.
[8] To
a
colourless acetone solution (20 mL) of [Pd(g3-C3H5)(ace-
tone)2][BF4], prepared from [Pd(l-Cl)(g3-C3H5)]2 (100 mg,
0.273 mmol) and AgBF4 (106.3 mg, 0.546 mmol) according to
described procedures [6,7], was added the stoichiometric amount
(0.546 mmol) of the corresponding pyrazole HpzR ligand [4,5] in
acetone (40 mL). After 24 h of stirring under nitrogen at room
temperature, the solvent was removed in vacuo and the solid
crystallised from dichloromethane/hexane leading to the precipitation
of a colourless solid, which was filtered off, washed with hexane and
dried in vacuo.
As a representative example, X-ray diffraction experi-
ments at variable temperature were undertaken for I14 in
order to examine the mesomorphic structure of these com-
pounds. At 50 ꢀC on cooling, the diffractogram exhibits
[9] I12: Yield: 79%. Elemental analyses: found C 60.2, H 7.5, N 6.3%;
calculated for C45H69BF4N4O2Pd: C 60.6, H 7.5, N 6.3%. IR(KBr,
cmÀ1): 3330 m(NH), 1616 m(CN), 1096-1006 m(BF), 519 d(FBF). I14
:
˚
two small-angle peaks at 40.9 and 21.0 A, and a broad halo
Yield: 80%. Elemental analyses: found C 60.3, H 7.8, N 5.7%;
calculated for C49H77BF4N4O2Pd Æ 1/2CH2Cl2: C 60.1, H 7.9, N 5.7%.
IR(KBr, cmÀ1): 3323 m(NH), 1617 m(CN), 1099-1020 m(BF), 521
d(FBF). I16: Yield: 50%. Elemental analyses: found C 62.5, H 8.1, N
5.3%; calculated for C53H85BF4N4O2Pd Æ 1/3CH2Cl2: C 62.2, H 8.4, N
5.4%. IR(KBr, cmÀ1): 3326 m(NH), 1615 m(CN), 1099-1016 m(BF), 521
d(FBF). I18: Yield: 84%. Elemental analyses: found C 64.3, H 8.4, N
5.6%; calculated for C57H93BF4N4O2Pd: C 64.6, H 8.8, N 5.3%.
IR(KBr, cmÀ1): 3331 m(NH), 1615 m(CN), 1094-1035 m(BF), 525
d(FBF).
˚
at ca. 4.5 A corresponding to the fluid behaviour of the
alkyl chains. The former ones from the (001) and (002)
reflections agree with a lamellar structure from the SmA
phases observed by POM. At 25 ꢀC, on cooling, the pattern
observed corresponds neither to a crystalline phase nor to a
liquid crystal phase. This result could be attributed to the
presence of a smectic glassy phase in accordance with the
no evidence of crystallisation established from the POM
observations.
[10] 1H NMR data of I12 (CDCl3, 248 K; d, ppm; J, Hz): Major signals:
12.00, 11.90, 11.82 (NH); 7.91 (3J = 8.7), 7.56 (3J = 8.7), 7.54
(3J = 8.6) (Ho of C6H4); 7.67, 7.42, 6.78 (H5 of pyrazole); 7.02
(3J = 8.7), 6.94 (3J = 8.6) (Hm of C6H4); 6.54, 6.51, 6.34 (H4 of
pyrazole); 5.80 (Hmeso of allyl); 4.07 (3J = 6.6), 4.03 (3J = 6.4), 4.01
(3J = 6.6) (Hsyn of allyl); 3.35 (3J = 12.2), 3.32 (3J = 12.4), 3.22
(3J = 12.5) (Hanti of allyl); 3.95 (OCH2); 1.8-1.1 (CH2); 0.85
(3J = 6.6) (CH3). Minor signals: 11.70 (NH); 7.33 (3J = 8.4) (Ho of
C6H4); 6.77 (3J = 8.4) (Hm of C6H4); 6.43 (H4 of pyrazole); 3.86
(3J = 6.8) (Hsyn of allyl); 3.05 (3J = 12.3) (Hanti of allyl); the
remaining resonances are masked by the major signals.
The length of the cationic part in the most extended
crystalline conformation, calculated on the basis of the
X-ray structure of I12 commented above, was estimated
˚
to be of ca. 52 A, and the value of the spacing was
˚
40.9 A. Therefore these results reveal some interdigitation
of the layers, as it has been observed in the packing of
I12 (Fig. 3).
´
[11] F.A. Jalon, B.R. Manzano, B. Moreno-Lara, Eur. J. Inorg. Chem.
Supplementary material
(2005) 100, and References therein.
[12] Colourless prismatic single-crystals (0.60 · 0.07 · 0.06 mm3) were
obtained from dichloromethane/hexane solution. Crystal data:
Crystallographic data of I12 have been deposited with
the Cambridge Crystallographic Data Centre (CCDC
deposition number 609242).
˚
C45H69BF4N4O2Pd; Mr 891.25; T 293(2) K; k 0.71073 A; Bruker-
Smart CCD diffractometer (operating at 50 kV and 30 mA); mono-
clinic, space group P21/c, a = 27.562(2) A, b = 9.3437(8) A,
˚
˚
3
˚
˚
c = 18.190(2) A, b =
91.203(2)ꢀ, V = 4683.5(7) A , Z = 4,
Acknowledgements
Dc = 1.264 g cmÀ3, l = 0.451 mmÀ1. The data were collected over a
hemisphere of the reciprocal space by combination of three exposure
sets. The cell parameters were determined and refined by least-squares
fit of all reflections collected [h range = 1.48–25.00ꢀ; index
Financial support from the DGI of the Ministerio de
´
Educacion y Ciencia (Project No. BQU2003-07343) and

M.C. Torralba et al. / Inorganic Chemistry Communications 9 (2006) 1271–1275
1275
range = (À32, À11, À18) to (29, 11, 21)]. Each exposure of 20 s
covered 0.3ꢀ in x. The first 50 frames were recollected at the end of
the data collection to monitor crystal decay. No appreciable decay in
the intensities of standard reflections was observed. The structure was
solved by direct methods and refined by full-matrix least-squares on
F2 [19]. All non-hydrogen atoms were refined anisotropically with the
exception of the BF4 and allyl groups, which have been refined
isotropically and in the last cycles, the thermal parameters were fixed.
The carbon atoms of the allyl group were refined with geometrical
restraints and a variable common carbon–carbon distance. All
hydrogen atoms were included in calculated positions, except H2
and H4 bonded to N2 and N4, respectively, which were located in a
Fourier synthesis. All hydrogen atoms were refined as riding on their
respective bonded atoms. The final R indices with I > 2r(I) was 0.072
for 2620 observed reflections, while wR2 for all data (8231 indepen-
dent reflections, Rint = 0.1416) was 0.231. The GOF (F2) was 0.855,
and the largest residual peak and hole in the final difference map were
[13] M.C. Torralba, M. Cano, J.A. Campo, J.V. Heras, E. Pinilla, M.R.
Torres, J. Perles, C. Ruiz-Valero, J. Organomet. Chem. 691 (2006)
2614.
´
´
[14] F. Gomez-de la Torre, A. de la Hoz, F.A. Jalon, B.R. Manzano, A.
´
´
´
Otero, A.M. Rodrıguez, M.C. Rodrıguez-Perez, Inorg. Chem. 37
(1998) 6606.
´
´
[15] F. Gomez-de la Torre, A. de la Hoz, F.A. Jalon, B.R. Manzano,
A.M. Rodr´ıguez, J. Elguero, M. Mart´ınez-Ripoll, Inorg. Chem. 39
(2000) 1152.
[16] M. Cano, J.V. Heras, M.L. Gallego, J. Perles, C. Ruiz-Valero, E.
Pinilla, M.R. Torres, Helv. Chim. Acta 86 (2003) 3194.
[17] R.M. Claramunt, P. Cornago, M. Cano, J.V. Heras, M.L. Gallego, E.
Pinilla, M.R. Torres, Eur. J. Inorg. Chem. (2003) 2693.
[18] M.L. Gallego, M. Cano, J.A. Campo, J.V. Heras, E. Pinilla, M.R.
Torres, Helv. Chim. Acta 88 (2005) 2433.
[19] G.M. Sheldrick, ‘SHELXL97, Program for Refinement of
Crystal Structure’, University of Go¨ttingen, Go¨ttingen, Germany,
1997.
1.018 and À0.834e AÀ3, respectively.
˚