Induction of mesomorphism through supramolecular association in coordination Pd (II) compounds of dialkyl 2,2′-bipyridine-4,4′-dicarboxylates

Article abstract of DOI:10.1080/15421400390193864

A scries of three 4,4′-disubstituted-2,2′-bipyridines and the corresponding mononuclear cis-dichloro palladium(II) complexes have been synthesized None of the ligands show mesomorphisrn. Upon their complexation to Pd(II) the peculiar structural arrangement characterized by intermolecular associations of the new derivatives induces mesogenic properties, confirming the new and exciting role of the coordination chemistry in metal-mediated formation of liquid crystals.

Full text of DOI:10.1080/15421400390193864

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Induction of Mesomorphism
Through Supramolecular
Association in Coordination Pd
(Ii) Compounds of Dialkyl 2,2″-
Bipyridine-4,4″-Dicarboxylates
Daniela Pucci a , Giovanna Barberio a , Alessandra
Crispini a , Mauro Ghedini a & Oriano Francescangeli
b
a LASCAMM, Unità INSTM della Calabria,
Dipartimento di Chimica , Università della Calabria ,
Arcavacata, CS, I-87030, Italy
b Dipartimento di Fisica e Ingegneria dei Materiali
e del Territorio and Istituto Nazionale per la Fisica
della Materia , Università di Ancona , Via Brecce
Bianche, Ancona, I-60131, Italy
Published online: 18 Oct 2010.
To cite this article: Daniela Pucci , Giovanna Barberio , Alessandra Crispini ,
Mauro Ghedini & Oriano Francescangeli (2003) Induction of Mesomorphism Through
Supramolecular Association in Coordination Pd (Ii) Compounds of Dialkyl 2,2″-
Bipyridine-4,4″-Dicarboxylates, Molecular Crystals and Liquid Crystals, 395:1,
325-335, DOI: 10.1080/15421400390193864
To link to this article: http://dx.doi.org/10.1080/15421400390193864
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Mol. Cryst. Liq. Cryst., Vol. 395, pp. [325]/155–[335]/165, 2003
Copyright # Taylor & Francis Inc.
ISSN 1542-1406 print/1563-5287 online
DOI: 10.1080/15421400390193864
INDUCTION OF MESOMORPHISM THROUGH
SUPRAMOLECULAR ASSOCIATION IN
COORDINATION Pd (II) COMPOUNDS OF DIALKYL
2,20-BIPYRIDINE-4,40-DICARBOXYLATES
Daniela Pucci*, Giovanna Barberio,
Alessandra Crispini, and Mauro Ghedini
`
LASCAMM, Unita INSTM della Calabria,
Dipartimento di Chimica, Universita della
`
Calabria I-87030 Arcavacata (CS), Italy
Oriano Francescangeli
Dipartimento di Fisica e Ingegneria dei
Materiali e del Territorio and
Istituto Nazionale per la Fisica della Materia,
`
Universita di Ancona, Via Brecce Bianche,
I-60131, Ancona, Italy
A series of three 4,40-disubstituted-2,2 0-bipyridines and the corresponding
mononuclear cis-dichloro palladium(II) complexes have been synthesized. None
of the ligands show mesomorphism. Upon their complexation to Pd(II) the
peculiar structural arrangement characterized by intermolecular associations
of the new derivatives induces mesogenic properties, confirming the new and
exciting role of the coordination chemistry in metal-mediated formation of
liquid crystals.
Keywords: metallomesogens; palladium; dialkyl 2,20-bipyridine-4,40-dicarboxylates; coordina-
tion compounds
INTRODUCTION
The thermotropic metallomesogens form a new class of liquid crystalline
compounds, displaying novel physical properties, whose synthesis was
initially performed starting from mesogenic ligands. The high transition
This work was partly supported by CIPE grants (Clusters 14 and 26) from the Ministero
`
dell’Istruzione, dell’Universita e della Ricerca.
*Corresponding author.
[325]/155

[326]/156
D. Pucci et al.
temperatures and the poor thermal stability often found for this class of
materials were the principal drawback which prompted doubts about the
suitability of liquid crystalline metal complexes as materials for practical
applications. Within thirty years numerous strategies were developed in
order to reduce the transition temperatures and improve the knowledge
about new, unconventional structural features of metal-containing liquid
crystals allowing the role of metallomesogens to overturn. [1,2] At the
present time coordination chemistry could be also synonimous of metal-
mediated formation of liquid crystals owing the ability of a metal ion to
induce mesomorphism in non mesomorphic ligands. This phenomenon can
be achieved via changes of molecular conformation, shape and structure
induced both by complexation [3–6] and inter or intramolecular interac-
tions of different nature giving rise supramolecular assemblies. [7–9]
In this context, the so called complementary shape approach has
expecially attracted our interest. 2,20-Bipyridine are very attractive ligands
because of their extensive coordination chemistry and their versatility: the
spectroscopic and chemical properties indeed can be tuned by the choice of
the metal centre and the substituents. Moreover judiciously functionalized
ligands can be metallated and give rise to new mesogenic structures based
on specific molecular correlations. In this paper we explore the possibility
to achieve this target by obtaining attractive dimeric or polymeric asso-
ciations between mononuclear cis-dichloro-palladium complexes by means
intense dipolar forces. Herein we describe the synthesis and characteriza-
tion of a series of dialkyl 2,20-bipyridine-4,40-dicarboxylates, Ln, and of their
square-planar dichloro palladium derivatives [LnPdCl2] 1–3.
EXPERIMENTAL SECTION
Measurements
The infrared spectra were recorded on a Perkin Elmer FT 2000 spectro-
1
photometer and the H NMR spectra were recorded on a Bruker WH-300
spectrometer in CDCl3 solution, with TMS as internal standard. Elemental
analyses were performed with a Perkin-Elmer 2400 analyzer. The textures
of the mesophases were studied with a Zeiss Axioscope polarizing micro-
scope equipped with a Linkam C0 600 heating stage. The transition temp-
eratures and enthalpies were measured on a Perkin-Elmer Pyris 1
Differential Scanning Calorimeter with a heating and cooling rate of 10ꢀC/
min. The apparatus was calibrated with indium. Two or more heating/
cooling cycles were performed on each sample.
The X-ray powder diffraction patterns were obtained by an INEL
CPS120 powder diffractometer equipped with a position sensitive detector

Induction of Mesomorphism through Supramolecular
[327]/157
covering a scattering angle of 120ꢀ, with an angular revolution of 0.018ꢀ.
˚
Monochromatized CuKa radiation (l 1.54 A) impinged on the ꢁ1 mm thick
sample, the temperature of which was controlled to ꢂ 0.1ꢀ by a hot stage
containing electrical resistors.
Synthesis
[Pd(PhCN)2Cl2] was synthesised as reported in the reference 10.
Synthesis of Ln Ligands
4,40-dicarboxy-2,20-bipyridine, I. The synthesis of I was performed
modifying slightly the reported method [11a]. 1.0 g (5.43 mmoles) of 4,40-
dimethyl-2,20-bipyridine was added to a solution of sulfuric acid (98%,
35 mL). 6.34 g (21.56 mmoles) of potassium dichromate were then added in
small portions to the resulting solution, in order to keep the temperature
between 70 and 80ꢀC. The solution was stirred until the temperature fell
below 40ꢀC. The deep green reaction mixture was poured in 160 ml of ice
water and filtered. The solid was washed with water until the filtrate was
colourless and allowed to dry. The resulting light yellow solid was then
further purified by refluxing it in 35 ml of 65% nitric acid for 6 hours. This
solution was poured over ice, diluted with 200 ml of water and cooled to
5ꢀC. The precipitate was filtered, washed with water (5610 ml), then
acetone (2610 ml) and allowed to dry giving 1.28 g (96%) of I as a fine
white solid.
M. p. > 350ꢀC; the1H NMR spectrum was in accordance with that
reported in the literature [11b]. The product was used without further
purification in the next step.
Dioctyl 2,2 0-bipyridine-4,40-dicarboxylate, L1. A mixture of 4,40-
dicarboxy-2,20-bipyridine, I (0,40 g, 1.64 mmol) and thionyl chloride
(15 mL) were refluxed under nitrogen until a clear yellow solution was
obtained. The excess of thionyl chloride was removed and the residue dried
under vacuum for the 2 h. The acid chloride was suspended in toluene
(12 mL) and treated with a slight excess of octanol (0.62 mL, 3.94 mmol).
The mixture was heated at reflux until to give a weakly pink solution.
Chloroform (30 mL) was added and the mixture washed with saturated
sodium hydrogen carbonate solution (30 mL). The aqueous layer was then
shaken with chloroform (3610 mL). The organic extract was washed with
water (100 mL), dried over anhydrous sodium sulphate, filtered and eva-
porated to dryness. Recrystallization from ethanol gave colourless crystals
in a 78% yield. M.p. 64ꢀC. Anal. Calcd. for C28H40N2O4: C, 71.76; H, 8.60; N,
5.98. Found: C, 70.92; H, 8.22; N, 6.46.

[328]/158
D. Pucci et al.
0
1H NMR (300 MHz, CDCl3): d 8.95 (2 H, s, H3,3 ), 8.87 (2 H, d, J ¼ 5.0 Hz,
0
0
0
H6,6 ), 7.91 (2 H, dd, J ¼ 5.0, 1.5 Hz, H5,5 ), 4.39 0(4 H, t, J ¼ 6.7 Hz, Ha,a ),
0
1.82 (4 H, q, J ¼ 6.7 Hz, Hb,b ), 1.36 (20 H, m, Hc,c ), 0.88 (6 H, t, J ¼ 6.7 Hz,
0
Hd,d ). IR (KBr): nC¼O 1726 cmꢁ1(s).
The homologous Ln ligands were synthesized following the procedure
described for L1:colors, yields, melting points (M.p.), elemental analyses,
NMR and IR data are as follows.
Dihexadecyl 2,20-bipyridine- 4,40-dicarboxylate, L2. Recrystallized
from chloroform/ethanol. White. Yield 91%. M.p. 81ꢀC. Anal. Calcd. for
C44H72N2O4: C, 76.25; H, 10.47, N 4.04. Found: C, 76.18; H, 10.65; N,
0
3.81.1H NMR (300 MHz, CDCl3): d 8.95 (2H, s, H3,30), 8.87 (2H, d,
0
J ¼ 5.0 Hz, H6,6 ), 7.91 (2H, dd, J ¼ 5.0, 1.5 Hz, H5,5 ), 4.39 (4H, t,
0
0
0
J ¼ 6.7 Hz, Ha,a0), 1.80 (4H, m, Hb,b ), 1.34 (52H, m, Hc,c ), 0.88 (6H, t,
J ¼ 6.7 Hz, Hd,d ). IR (KBr): nC¼O 1715 cmꢁ1 (s).
Didocosyl 2,20-bipyridine- 4,40-dicarboxylate, L3. Recrystallized from
chloroform/ethanol. White. Yield 90%. M.p. 92ꢀC. Anal. Calcd. for
C56H96N2O4: C 78.08; H 11.23; N 3.25. Found: C, 77.83; H, 11.02; N, 3.69. 1H
0
0
NMR (300 MHz, CDCl3): d 8.95 (2H, s, H3,3 ), 8.87 (2H, d,J ¼ 5.0 Hz, H6,6 ),
0
0
7.90 (2H, dd, J ¼ 5.0, 1.8 Hz, H5,5 ), 4.39 (4H, t, J ¼ 6.7 Hz, Ha,a ), 1.80 (4H,
0
0
0
m, Hb,b ), 1.30 (76H, m, Hc,c ), 0.88 (6H, t,J ¼ 6.7 Hz, Hd,d ). IR (KBr): nC¼O
1715 cmꢁ1 (s).
Synthesis of [LnPdCl2] Complexes
(2,20-bipyridyl-4,40-bis-octylcarboxylate)palladium(II)chloride,
[L1PdCl2], 1. A solution of [Pd(PhCN)2Cl2] (0.15 g, 0.39 mmol) and
dioctyl 2,20-bipyridine-4,40-dicarboxylate, L1 (0.18 g, 0.39 mmol) in
chloroform (10 mL) was heated at reflux for 5 h. The solvent was evapo-
rated and the residue recrystallizated from dichloromethane/diethyl ether
to give the complexes as yellow solid. Yield 87%. Thermotropic behaviour
in Table I. Anal. Calcd. for C28Cl2H40N2O4Pd: C 52.06; H 6.24; N 4.34.
Found: C, 52.07; H, 5.86; N, 4.79.1H NMR (300 MHz, CDCl3): d 9.57 (2 H, d,
0
0
0
J ¼ 5.0 Hz, H6,6 ), 8.65 (2 H, s, H3,3 ), 8.12 (2 H, d, J ¼ 5.0 Hz, H5,5 ), 4.48 (4
0
0
0
H, t, J ¼ 6.6 Hz, Ha,a ), 1.84 (4 H, m, Hb,b ), 1.30 (20 H, m, Hc,c ), 0.89 (6 H,
0
m, Hd,d ). IR (KBr): nC¼O 1729 cmꢁ1 (s).
The homologous [LnPdCl2] complexes were synthesized following the
procedure described for 1 starting from the suitable ligands. Colors, yields,
elemental analyses, NMR and IR data are as follows.
(2,20-bipyridyl-4,40-bis-hexadecylcarboxylate)palladiuml(II)chlo-
ride, [L2PdCl2], 2. Yellow. Yield 81%. Thermotropic behaviour in Table I.
Anal. Calcd. for C44Cl2H72N2O4Pd: C 60.72; H 8.34; N 3.22. Found: C, 59.03;
H, 8.58; N, 3.47.1H NMR (300 MHz, CDCl3): d 9.52 (2 H, d, J ¼ 6.3 Hz,
0
0
0
H6,6 ), 8.65 (2 H, s, H3,3 ), 8.10 (2 H, d, J ¼ 6.3 Hz, H5,5 ), 4.47 (4 H, t,

Induction of Mesomorphism through Supramolecular
[329]/159
TABLE 1 Optical and Thermal Properties of the [LnPdCl2] Complexes 1–3
Complex
Transition[a]
T[ꢀC]
DH[KJmolꢁ1
]
1
Cr- Cr0
Cr0 - I
I- Cr
84.4
180.9
177.8
97.0
3.0
12.2
10.7
7.9
2
3
Cr- Cr0
Cr0 - SmAd
SmAd- I
I- SmAd
SmAd- Cr
Cr- Cr0
Cr0 - SmAd
SmAd- I
I- SmAd
SmAd- Cr0
Cr0 - Cr
130.0
155.9
154.9
80.3
0.2
13.6
14.2
7.9
86.9
97.8
43.0
5.3
139.6
137.8
80.1
12.2
11.9
6.2
68.8
54.5
[a]Cr: crystal; Sm: smectic; I: isotropic liquid.
0
0
0
J ¼ 6.4 Hz, Ha,a0 ), 1.86 (4 H, m, Hb,b ), 1.26 (52 H, m, Hc,c ), 0.88 (6 H, t,
J ¼ 6.4 Hz Hd,d ). IR (KBr): nC¼O 1726 cmꢁ1 (s).
(2,20-bipyridyl-4,40-bis-docosylcarboxylate)palladium(II)chloride,
[L3PdCl2], 3. Yellow. Yield 85%. Thermotropic behaviour in Table I. Anal.
Calcd. for C56Cl2H96N2O4Pd: C 64.75; H 9.31; N 2.70. Found: C, 64.88; H,
9.15; N, 2.86.1H NMR (300 MHz, CDCl3): d 9.58 (2 H, d, J ¼ 5.8 Hz, H6,6 ),
0
0
0
8.64 (2 H, s, H3,3 ), 8.12 (2 H, d, J ¼ 5.8, 1.7 Hz, H5,5 ), 4.47 (4 H, t,
0
0
0
J ¼ 6.7 Hz, Ha,a0), 1.84 (4 H, m, Hb,b ), 1.29 (76 H, m, Hc,c ), 0.88 (6 H, t,
J ¼ 6.7 Hz, Hd,d ). IR (KBr): nC¼O 1735 cmꢁ1 (s).
RESULTS AND DISCUSSION
The preparation of the diesters Ln in which the number of carbon atoms in
the aliphatic chains are gradually increased from 8 to 22, was carried out in
a three step synthesis, as described in Scheme 1.
The oxidation of the 4,40-dimethyl-2,20-bipyridine using potassium
dichromate gives the diacid I, which was reacted with thionyl chloride to
affords the dichloride II. Finally, the esterification of II with the appro-
priate n-alkyl alcohols, in toluene under reflux, gave the ligands Ln as white
crystalline products in very high yields. Reaction of ligands Ln with an
equimolar amount of [Pd(PhCN)2Cl2] produced the square planar cis-
dichloro-palladium complexes [LnPdCl2], according to Scheme 2. All the
ligands and complexes were characterized by elemental analyses, IR and 1H
NMR spectroscopies, whose data are given in the experimental section.

[330]/160
D. Pucci et al.
SCHEME 1 Synthesis and proton numbering scheme for Ln ligands.
1
In the H NMR spectrum, for all of the investigated complexes the reso-
0
nance attributable to the H6,6 protons is low-field shifted of about 0.7 ppm
with respect to the ligands, confirming that the metal complexation has
occurred.
None of the 4-40-diesters Ln are liquid crystals but most of the palladium
complexes are and their thermal behaviour, investigated by polarized
optical microscope and differential scanning calorimetry (DSC), is sum-
marized in Table 1.
All the [LnPdCl2] complexes show a crystal to crystal transition fol-
lowed by a smectic phase, except compound 1, which melts directly to an
isotropic liquid at 181ꢀC. Themosaic-like textures observed for complexes
2 and 3 (Fig. 1) is not consistent with a conventional disordered Smectic
phase. When cooled from their isotropic phases they retain the optical
texture of the mesophase giving a glassy solid stable for long time.
As expected, the melting and clearing points decrease with the lenght of
the aliphatic chains on going from 16 (2) to 22 carbon atoms (3) and, as a
consequence, a larger stability of the mesophase is observed. From DSC
analysis very high enthalpy values were measured for the mesophase-
isotropic liquid transitions suggesting an unusual, highly correlated type of
arrangement of the molecules. However, from these data we cannot make a
definitive conclusion about the nature of the mesophase.
X-ray diffraction (XRD) studies on complexes 2 and 3 made possible to
unequivocally assign the nature of the mesophase. The XRD pattern of
sample 2 (Fig. 2) exhibits sharp inner reflections and diffuse outer halos

Induction of Mesomorphism through Supramolecular
[331]/161
SCHEME 2 Synthesis and proton numbering scheme for [LnPdCl2] complexes.
and is consistent with a layered structure of the smectic type. In fact, the
inner reflections consist of a set of 00n peaks centered at q ¼ 2pn/d
(q ¼ 2psiny/l is the modulus of the scattering vector, 2y is the scattering
˚
angle), where n is an integer (n ¼ 1–4) and d ¼ 36.6 A is the smꢁe1ctic layer
˚
spacing. The wide diffuse outer halo centered about q ¼ 1.3 A is asso-
ciated with the short-range liquid-like positional ordering of the hydro-
˚
carbon chains and corresponds to the average interchain distance ꢃ 4.8 A
in the smectic planes. Based on the fact that the spacing of the smectic
layers in this compound are larger than the calculated molecular length in
˚
the fully extended configuration (L ꢃ 28 A), but smaller than 2L, we can
deduce that the molecules are not arranged in single molecular layers but
are organized in semi bilayers partially interdigitated. Thus this smectic
phase can be classified as an semi-bilayer SmAd phase. [12]

[332]/162
D. Pucci et al.
FIGURE 1 Optical texture (100x) observed for complex [L3PdCl2]. SmAd phase
at 138ꢀC on cooling from the isotropic phase.
However, differently from the typical XRD pattern of the smectic phase
of conventional calamitic liquid crystals, this pattern shows two additional
˚
˚
peaks, corresponding to spacings d1 ¼ 10.5 A and d2 ¼ 3.4 A, which are
significantly broader than those of the 00n series. These peaks could arise
from Pd-Pd in-plane correlations along mutually-orthogonal directions
parallel and orthogonal, respectively, to the long side of the molecular core
(i.e. the axis of the benzene rings). A similar XRD pattern was observed in
the high temperature mesophase of 3, which is consistent with the same
semi-bilayer SmAd molecular arrangement of 2, with a smectic layer spa-
˚
˚
cing d ¼ 45.0 A (calculated molecular length (L ꢃ 36 A).

Induction of Mesomorphism through Supramolecular
[333]/163
˚
FIGURE 2 XRD pattern (l 1.54 A) of the high temperature smectic phase of
[L2PdCl2] recorded at T ¼ 140ꢀC. The inset reports the same pattern expanded in
the vertical axis in order to evidence the weak reflections.
The molecular geometry of a single molecule of complex 2 has been
minimised using the Cerius 3.0 molecular modelling software package
(UFF force field). The overall geometry is typical of a molecule with a rod-
like shape, since the hydrophobic forces between the flexible chains tend
˚
to keep them parallel and at a distance of about 5 A (Fig. 3a). On the basis
of the XRD experimental data and the geometry of the minimised single
FIGURE 3 Molecular packing of two minimised molecules (a) and the formation of
the smectic A arrangement (b) of [L2PdCl2] complex.

[334]/164
D. Pucci et al.
molecule (L < d < 2L), the molecular organization can be hypothesized as
illustrated in Figure 3b, even in absence of crystal packing minimization.
Thus, we can assume that the large dipole moment adopted by the
bipirydine ligand forced in the cisoid conformation by means the com-
plexation [13], will induce favorable dipole-dipole interactions and anti-
parallel correlations of the nearest neighbour molecules, characterised by
short intermetallic Pd—Pd distances. The flexible chains of the resulting
dimers tend to align leading to a global rod-like molecular shape and a
lamellar supramolecular organization of the interdigitated pairs.
CONCLUSIONS
We have prepared and characterized a new series of three bifunctionalized
dialkyl 2,20-bipyridine-4,40-dicarboxylates and the corresponding mono-
nuclear cis-dichloro palladium(II) complexes [LnPdCl2]. None of the
ligands show mesomorphism while the coordination to a PdCl2 moiety has
led to metallomesogenic species confirming the role of the coordination
chemistry in the metal-mediated formation of liquid crystals. All the
[LnPdCl2] complexes show a crystalline polymorphism and, for the highest
homologous, an enantiotropic lamellar mesophase. On the basis of XRD
studies in the liquid crystalline state and from molecular modelling simu-
lation, we have proposed a model in which the promotion of a dipolar
moment upon complexation in [LnPdCl2] molecules induces self-assem-
bling in dimers, with antiparallel disposition and short Pd—Pd inter-
molecular distances. Moreover, the most efficient space filling has been
reached by aggregation of each pair of molecules into a bilayered structure.
The lamellar spacing is 1.3 times the molecular length of a single molecule
giving rise to a semi-bilayered SmAd phase.
The thermotropic phase behaviour and the supramolecular organization
of [LnPdCl2] complexes make them promising candidate for design new
metallomesogens with tunable properties. Future research will be planned
in order to change both metal centre and substituents to use coupling of
dipoles as molecular architecture tools.
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