Pyridylpyrazole derivatives. A new type of mesogenic bidentate ligands inducing mesomorphism on their related PdX2 complexes

Article abstract of DOI:10.1016/S1387-7003(03)00053-4

A new family of 3,5-bis(4-n-alkyloxyphenyl)-1-(pyridin-2-yl)-1H-pyrazole ligands (pypz^R2) and their complexes of formula c-[PdCl₂(pypz^R2)] have been synthesised and their mesomorphism was investigated. The effectiveness of the asymmetric molecular design in promoting thermotropic mesophases is greater for complexes than for the related ligands, although the latter exhibit mesophases at lower temperatures. These complexes are the first example of metallomesogens containing N,N′-bidentate ligands coordinated to PdCl₂.

Full text of DOI:10.1016/S1387-7003(03)00053-4

Inorganic Chemistry Communications 6 (2003) 626–629
www.elsevier.com/locate/inoche
Pyridylpyrazole derivatives. A new type of mesogenic
bidentate ligands inducing mesomorphism on their related PdX2
complexes
*
M.J. Mayoral, M.C. Torralba, M. Cano , J.A. Campo, J.V. Heras
Departamento de Quꢀımica Inorgaꢀnica I, Facultad de Ciencias Quꢀımicas, Universidad Complutense, Madrid E-28040, Spain
Received 4 December 2002; accepted 27 January 2003
Abstract
A new family of 3,5-bis(4-n-alkyloxyphenyl)-1-(pyridin-2-yl)-1H-pyrazole ligands (pypzR2) and their complexes of formula c-
[PdCl2ðpypzR2Þ] have been synthesised and their mesomorphism was investigated . The effectiveness of the asymmetric molecular
design in promoting thermotropic mesophases is greater for complexes than for the related ligands, although the latter exhibit
mesophases at lower temperatures. These complexes are the first example of metallomesogens containing N,N0-bidentate ligands co-
ordinated to PdCl2.
Ó 2003 Elsevier Science B.V. All rights reserved.
Keywords: Long-chain 3,5-disubstituted-1-(pyridin-2-yl)pyrazoles; Pd(II) complexes; Mesomorphism
1. Introduction
dated or more scarcely mononuclear derivatives
[1,2,5,6], the latter usually exhibiting nematic and
smectic phases at lower temperatures than the related
symmetrical binuclear compounds.
In the liquid crystal research metallomesogens are a
continuous expansion area [1–3]. However, so far cala-
mitic metallomesogens containing N,N0-bidentate li-
gands are very scarce and the few examples described,
such as Cu(II) or Ni(II) derivatives with 2-substituted
pyrrole ligands, mainly exhibited nematic phases at
temperatures higher than 200 °C[4]. By contrast, a va-
riety of metal derivatives with Schiff base ligands have
been reported as calamitic mesogens [1] and, among
them, the square-planar Pd(II) derivatives exhibited a
higher stability of the mesophases.
The square-planar geometry of metal centre has been
associated to the mesomorphism of a wide variety of
Pd(II) complexes. It appears to favour the molecular
interactions and hence the stability of the mesophases.
In this context, mesogenic PdX2 derivatives based on
bidentate ligands have been centred exclusively on or-
thopalladated azobenzenes and related species. These
are dimers of the type di-(l-halobridged)orthopalla-
In a previous work we reported the mesomorphic
;
properties of the pyrazoles HpzR2 (R ¼ C6H4OCnH2nþ1
n ¼ 4, 6, 8, 10, 12, 14), containing alkyloxyphenyl sub-
stituents at third and fifth positions of the heterocyclic
ring, as well as the chemistry of their complexes
[RhClðCOÞ2ðHpzR2Þ] which were found to be non-liquid
crystal materials [7]. Similar results were obtained using
the non-mesogenic 3-substituted pyrazoles HpzR (R ¼
C6H4OCnH2nþ1; n ¼ 6, 8, 10) [8]. By contrast, we found
that t-[PdCl2ðHpzRÞ2] (R ¼ C6H4OCnH2nþ1; n ¼ 6, 8,
10, 12, 14, 16, 18) were calamitic, their liquid crystal
properties being associated to the strategic molecular
skeletal design [9].
Following these precedents we are now implicated in
the study of a new type of ligands and their Pd-com-
plexes based on the mesogenic pyrazoles HpzR2. The
ligands were pyridylpyrazole type asymmetric deriva-
tives designed by incorporation of a pyridyl group at
first position of the parent pyrazole HpzR2. They should
be appropriate candidates to be co-ordinated to a PdX2
group in a N,N0-bidentate form giving rise to a cis
* Corresponding author. Fax: +34-91-3944352.
E-mail address: mmcano@quim.ucm.es (M. Cano).
1387-7003/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S1387-7003(03)00053-4

M.J. Mayoral et al. / Inorganic Chemistry Communications 6 (2003) 626–629
627
ðHpzRÞ2] complexes should be expected (Fig. 1). In ad-
dition, the asymmetry introduced in both ligands and
complexes is an useful factor in order to induce meso-
morphism [2,7].
In the present communication we describe the syn-
thetic strategy of a family of 3,5-bis(4-n-alkyloxyphenyl)-
1-(pyridin-2-yl)-1H-pyrazole ligands, pypzR2 (R ¼ ddp,
tdp, hdp, odp) (Fig. 2) and their complexes c-[PdCl2
ðpypzR2Þ], which are the first example of calamitic Pd(II)
derivatives containing N,N0-bidentate ligands. Both li-
gands and complexes have been found to be liquid crystal
materials and their mesomorphic properties have been
analysed.
2. Experimental
2.1. Preparation of 3,5-bisð4-n-alkyloxyphenylÞ-1-ðpyri-
din-2-ylÞ-1H-pyrazole, pypzR2 ðR ¼ ddp 1, tdp 2, hdp 3,
odp 4Þ
The new bidentate ligands 1–4 were prepared in a
similar way to that used for previous related ligands [10].
This synthesis required the condensation of the corre-
sponding 1,3-bis(4-n-alkyloxyphenyl)propane-1,3-dione
and 2-hydrazinopyridine in ethanol and using an acidic
media to increase the yield. Yields: 53–62%.
Fig. 1.
distribution of the X groups. Therefore, by using ade-
quate long alkylic chains, no substantial modifications
on the molecular shape related to the mesogenic t-[PdCl2
Fig. 2.
Table 1
1H NMR data of pypzddp2 (1) and c-[PdCl2ðpypzddp2Þ] (5) in CDCl3 at 298 Ka
Pyridine
Pyrazole C6H4OCnH2nþ1
H3
H4
H5
H6
H4
Ho
Ho0
Hm
Hm0
1
7.50d
J3;4 ¼ 7:6
7.74m
7.24m
8.42brs
6.71s
7.86d
Jom ¼ 8:3
7.22d
6.95d
6.84d
CH2-10: 3.98m
0
0
0 0
Jo m ¼ 8:1
J1 2 ¼ 6:5
CH2-20: 1.79m
CH2-30-110: 1.6–1.2m
CH3-120: 0.88t
0
0
J11 12 ¼ 5:8
5
6.81d
J3;4 ¼ 8:0
7.73m
7.26m
9.21d
J5;6 ¼ 5:4
6.63s
7.75d
Jom ¼ 8:5
7.39d
7.07d
6.93d
CH2-10: 4.03m
0
0
0 0
Jo m ¼ 8:8
J1 2 ¼ 6:1
CH2-20: 1.82m
CH2-30-110: 1.6–1.2m
CH3-120: 0.88t
0
0
J11 12 ¼ 5:9
a Analogous 1H NMR data are obtained for the other pypzR2 ligands 2–4 and their related complexes 6–8.

628
M.J. Mayoral et al. / Inorganic Chemistry Communications 6 (2003) 626–629
2.2. Preparation of c-½PdCl2ðpypzR2ފ ðR ¼ ddp 5, tdp 6,
hdp 7, odp 8Þ
Table 2
Phase properties of pypzR2 (1–4) and c-[PdCl2ðpypzR2Þ] (R ¼ ddp, tdp,
hdp, odp) (5–8) determined by DSC
DH (kJ molÀ1
)
To a solution of the corresponding pypzR2 (1–4) li-
gand (0.015 mmol) in 20 ml of dichloromethane was
added [PdCl2ðPhCNÞ2] (0.015 mmol). After 1 h stirring
at ca. 40 °C, the solvent was reduced and the addition of
hexane led to the precipitation of a yellow solid that was
isolated by filtration, washed with hexane and dried in
vacuo. Yields: 74–85%.
Transition
T (°C)
1
2
Crys ! I
71.0
47.3
I ! SmA
44.1
30a
)4.4
)5.7
SmA ! Crys
3
I ! SmA
48.8
37a
SmA ! Crys
4
5
I ! Crysb
52.1
)59.6
Crys ! Crys0
Crys0 ! SmA
SmA ! I
89.7
132.0
144.1
36.8
31.7
4.1
3. Results and discussion
The ligands pypzR2 (1–4) and their related com-
plexes c-[PdCl2ðpypzR2Þ] (5–8) were characterised by
analytical and spectroscopic (1H NMR and IR) tech-
niques, all data being in agreement with the proposed
molecular formulation. Additionally, thermal studies
by differential scanning calorimetry (DSC) and pola-
rising light microscopy were also made for all com-
pounds.
6
7
8
Crys ! Crys0
Crys0 ! SmA
SmA ! I
Crys ! Crys0
Crys0 ! SmA
SmA ! I
Crys ! Crys0
Crys0 ! SmA
SmA ! I
95.8
129.4
148.8
32.4
31.7
1.4
95.6
124.9
153.9
27.7
42.8
3.4
97.8
119.4
147.8
9.9
43.4
3.1
1
The H NMR spectra of 5–8, at room temperature
a Data obtained by polarising light microscopy.
in CDCl3 solution (Table 1), show all the expected
ligand signals. Related to the pyrazole group, the
H(4) and alkyloxyphenyl protons are scarcely shifted,
and the non-equivalence of the two lateral chains
observed on the free ligands is maintained on the
complexes. The general behaviour of the pyridyl
protons deals with small shifts upfield and downfield
of the H(3) and H(6) protons, respectively, in
agreement with a N co-ordination of the pyridyl
group.
The IR spectra of the complexes in solid state show
the characteristic bands of the pyridylpyrazol groups,
slightly modified upon co-ordination. As a general fea-
ture, the c(CH) band of the pyridyl group at ca.
788 cmÀ1 is shifted ca. 20 cmÀ1 to lower frequencies
than in the free ligand, according to a N-pyridyl co-or-
dination.
b A SmA mesophase could be observed near the solidification point
by polarising light microscopy.
detected by polarising light microscopy in a narrow
temperature range of ca. 2 °C.
The above ligands 1–4 were co-ordinated to the PdCl2
group giving rise to the complexes c-[PdCl2ðpypzR2Þ] (5–
8), all of them exhibiting an enantiotropic SmA meso-
phase (Fig. 3(b)). This is a new example of induction of
mesomorphism by co-ordination to a metal centre. The
stability of the mesophases increases smoothly as the
alkyloxy chain length is longer.
By comparison of the mesomorphic properties of
t-[PdCl2ðHpzRÞ2] and c-[PdCl2ðpypzR2Þ], we can deduce
that lowering the symmetry of related molecular struc-
tures as synthetic strategy has been successful, increasing
the stability temperature range of the mesophases.
However, the accessibility of mesophases on c-[PdCl2
ðpypzR2Þ] (5–8) was not appreciably modified in relation
to t-[PdCl2ðHpzRÞ2] [9]. This fact could suggest that the
structural relationship (width to length) in both types of
complexes operate in opposite sense to that of the mo-
lecular asymmetry. However, in both cases it was ob-
served that an increase in the length of lateral chain led
to a decrease in the stability of the crystal phase, which
is reflected by the lowering in the melting points. At the
same time, the clearing temperatures also raised,
thereby increasing the range of existence of the liquid
crystal behaviour. This fact is more pronounced in 5–8
due to the high asymmetry of the pyridylpyrazole
complexes.
3.1. Thermal studies
The transition temperatures and enthalpies deter-
mined by DSCare listed in Table 2 and Fig. 3 shows the
textures of the mesophases.
The ligands 2 and 3, as their counterpart 3,5-disub-
stituted pyrazoles HpzR2 [7], were found to be mesogens,
the transition temperatures being lower for the former
compounds. So 2 and 3 exhibit a monotropic SmA
mesophase in the temperature range of 44–48 °C(Fig.
3(a)), at difference of the 100–200 °Crange observed for
HpzR2. By contrast, compound 1, containing the
shortest chain (R ¼ ddp, n ¼ 12), does not exhibit me-
somorphic behaviour and 4 (R ¼ odp, n ¼ 18), with the
longest substituents, shows a SmA mesophase only

M.J. Mayoral et al. / Inorganic Chemistry Communications 6 (2003) 626–629
629
Fig. 3. Textures of the SmA mesophases on cooling of: (a) ligand 2 at 38 °Cand (b) complex 7 at 137 °C.
Acknowledgements
[4] (a) A. Krowczynski, W. Pyzuk, E. Gorecka, Polish J. Chem. 68
(1994) 281;
(b) A. Krowczynski, W. Pyzuk, I. Bikchantaev, J. Szydlowska, E.
Gorecka, 15th International Liquid Crystal Conference, Budapest,
Hungary, 1994.
Financial support from the DGES of Spain (Project
no. PB98-0766) and the Comunidad de Madrid (Grant
to M.C. Torralba) are gratefully acknowledged.
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