Columnar mesomorphism in hexacatenar tetrahedral (2,2′-bipyridine) zinc complexes and homologous palladium derivatives

Article abstract of DOI:10.1002/ejic.200400528

The synthesis and characterisation of novel liquid crystals which display columnar mesomorphism induced upon complexation of a series of nonmesomorphic hexacatenar 4,4′-disubstituted 2,2′-bipyridines (Lⁿ) are reported. The introduction of different metal centres (Zn, Pd) causes the appearance of mesomorphism in all complexes regardless of the geometry around the metal ion. We therefore report the first examples of mesomorphism in tetrahedral zinc derivatives. The nature of the columnar phases is related to the self-assembly of the half-disc shaped [LⁿMCl₂] (M = Zn, Pd) complexes into full disc-shaped supramolecules. The molecular organisation in the mesophase is mainly driven by intermolecular attractive interactions, as shown by the crystal structure of the model compound [LPdCl₂]. Preliminary measurements of photoconductivity have been performed on samples of [LⁿMCl₂] complexes doped with C₆₀ to increase absorption. Promising results were obtained. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.

Full text of DOI:10.1002/ejic.200400528

FULL PAPER
Columnar Mesomorphism in Hexacatenar Tetrahedral (2,2Ј-Bipyridine)zinc
Complexes and Homologous Palladium Derivatives
Giovanna Barberio,[a] Anna Bellusci,[a] Alessandra Crispini,[a] Mauro Ghedini,[a]
Attilio Golemme,[a] Piotr Prus,[a] and Daniela Pucci*[a]
Keywords: Bipyridine ligands / Zinc / Palladium / Metallomesogens / Columnar structures
The synthesis and characterisation of novel liquid crystals
which display columnar mesomorphism induced upon com-
plexes into full disc-shaped supramolecules. The molecular
organisation in the mesophase is mainly driven by intermol-
plexation of a series of nonmesomorphic hexacatenar 4,4Ј- ecular attractive interactions, as shown by the crystal struc-
disubstituted 2,2Ј-bipyridines (Ln) are reported. The intro-
ture of the model compound [LPdCl2]. Preliminary measure-
duction of different metal centres (Zn, Pd) causes the appear- ments of photoconductivity have been performed on samples
ance of mesomorphism in all complexes regardless of the
geometry around the metal ion. We therefore report the first
examples of mesomorphism in tetrahedral zinc derivatives.
The nature of the columnar phases is related to the self-as-
sembly of the half-disc shaped [LnMCl2] (M = Zn, Pd) com-
of [LnMCl2] complexes doped with C60 to increase absorp-
tion. Promising results were obtained.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
Introduction
disubstituted 2,2Ј-bipyridines in order to induce, upon com-
plexation, a molecular disc shape and columnar meso-
morphism in the corresponding supramolecular metal ag-
gregates. Moreover, since discotic liquid crystal systems
have recently gained interest because of new applications in
molecular electronics and optoelectronics,[10Ϫ12] we wanted
to investigate if the design of supramolecular disc-like met-
allomesogens, with a long-range molecular order, would be
advantageous for the enhancement of pre-selected proper-
ties such as charge transport or luminescence. The attract-
iveness of these systems is also demonstrated by the fact
that during the preparation of the current article an appar-
ently similar study was reported.[13] However, although the
same methodology was used for inducing columnar me-
sophases in hemi-disc like complexes, Donnio et al. carried
out their studies on terdentate ligands such as the 2,6-
The role of metal ions in the formation of molecular mo-
tifs and intermolecular interactions, which are unapproach-
able for organic liquid crystals and promote mesomorphism
in nonmesogenic ligands, continues to make metallomesog-
ens promising materials with novel and interesting
properties.[1Ϫ6] In particular, the design of suitable metal-
ligand combinations represents a challenge for obtaining
new functional liquid crystalline materials by molecular
self-assemblies.[7]
In this context, we recently reported[8] the synthesis of a
series of nonmesomorphic dialkyl 2,2Ј-bipyridine-4,4Ј-di-
carboxylates for which the complexation with various metal
salts resulted in [(Bipy)MX2] (M ϭ Pd, Pt, Ni, Zn) species
which self-assemble in dimers built up from attractive inter-
molecular interactions. This is described by the complemen-
tary shape approach.[9]
Due to the overall shape of the stacked units, these com-
plexes exhibit lamellar mesomorphism which depends
mostly on the geometry of the metal centre. In fact, the zinc
homologues do not exhibit liquid crystalline properties due
to their tetrahedral coordination. In continuation of our
previous studies on the relationship between molecular
structures, symmetry prerequisites and mesomorphism, we
have been interested in the design of a further class of 4,4Ј-
bis[3Ј,4Ј,5Ј-tri(alkoxy)phenyliminomethyl]pyridines
and
consequently the discrete building blocks show very differ-
ent geometric features.
In the present work we report the synthesis of a series of
hexacatenar 4,4Ј-disubstituted 2,2Ј-bipyridine ligands and
related cis-dichloro zinc and palladium derivatives. The
single-crystal X-ray structure of the palladium model com-
plex [LPdCl2] is also reported and discussed. The liquid
crystalline properties of all [LnMCl2] (M ϭ Zn, Pd) com-
plexes which, in contrast to the free ligands, exhibit colum-
nar mesomorphism are also described. It should be noted
that rigid tetrahedral molecules usually seem to prevent me-
sophase formation[14] and the only examples of meso-
morphic zinc derivatives to date exhibit either a trigonal-
bipyramidal geometry with dithiobenzoate[1] or terpyridine
[a]
`
`
LASCAMM, Unita INSTM della Calabria, Dipartimento di
Chimica, Universita della Calabria,
87030, Arcavacata (CS), Italy
Fax: (internat.) ϩ 39-0984-492066
E-mail: d.pucci@unical.it
Eur. J. Inorg. Chem. 2005, 181Ϫ188
DOI: 10.1002/ejic.200400528
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181

D. Pucci et al.
FULL PAPER
ligands[13,15] or a square-planar geometry when the metal
The hexacatenar complexes [LnMCl2] as well as the
centre is incorporated in a porphyrin or a phthalocyanine model complexes [LMCl2], were prepared by treatment of
core.[1,16] Moreover, preliminary measurements of photo- an equimolar amount of the appropriate ligand with ZnCl2
conductivity have been performed on samples of [LnMCl2] (in
dichloromethane
at
room
temperature)
or
doped with C60 to increase absorption. Promising results [Pd(PhCN)2Cl2] (in chloroform at reflux), respectively.
were obtained.
Results and Discussion
Synthesis and Characterisation
The hexacatenar Ln ligands (where n denotes the number
of carbon atoms in the alkoxy chains) were prepared as
white solids, in high yields, from the reaction of the 4,4Ј-
bis(hydroxymethyl)-2,2Ј-bipyridine with the appropriate
3,4,5-trialkoxy-substituted benzoic acid using a DCC-PPY
esterification (Scheme 1). The model ligand L, without the
six alkoxy chains grafted on the peripherical phenyl rings,
has been synthesised in an analogous manner by treating
the 4,4Ј-bis(hydroxymethyl)-2,2Ј-bipyridine with benzoic
acid.
The structures and purities of all the ligands and com-
1
plexes were confirmed by IR and H NMR spectroscopy
1
and elemental analyses. In the H NMR spectra of these
complexes the H3 and H6 bipyridinic proton resonances
shifted (by about 0.3 ppm) up and down field with respect
to their organic precursors. These observations confirmed
the coordination of the bipyridines to the metal fragments.
Single-Crystal X-ray Diffraction of [LPdCl2]
The [LPdCl2] complex was synthesised and then structur-
ally characterised with the explicit intention of giving more
insight into the molecular organisation in the 3D space of
these new (2,2Ј-bipyridine)metal complexes in which the
4,4Ј-disubstituents are now different from the carboxylates
with long alkyl chains already discussed in a previous pap-
er.[8b] Moreover, in the absence of diffraction-quality crys-
tals for any of the long-chain homologues, [LPdCl2] could
be considered as a model compound and its molecular or-
ganisation taken as being representative for the discussion
of the liquid-crystalline states of similar mesogenic com-
plexes. The structure of [LPdCl2] is shown in Figure 1 and a
selection of bond lengths and angles is reported in Table 1.
Scheme 1. Synthetic route to the hexacatenar Ln ligands
Figure 1. Molecular structure and atomic labelling scheme (50% probability thermal ellipsoids) of complex [LPdCl2]
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Hexacatenar Tetrahedral (2,2Ј-Bipyridine)zinc Complexes
FULL PAPER
˚
The association of molecules into dimers (superimposed
approximately along the c axis) related by an inversion cen-
tre is characterised by the presence of hydrogen bonds be-
tween the carboxylic oxygen atoms and one of the pyridine
Table 1. Selected bond lengths (A) and angles (°) for complex
[LPdCl2]
PdϪN(1)
PdϪCl(1)
N(1)ϪPdϪN(2)
N(1)ϪPdϪCl(1)
N(2)ϪPdϪCl(1)
2.023(5)
2.283(2)
80.9(2)
94.0(1)
174.9(2)
PdϪN(2)
PdϪCl(2)
Cl(1)ϪPdϪCl(2)
N(1)ϪPdϪCl(2)
N(2)ϪPdϪCl(2)
2.034(4)
2.283(2)
90.0(7)
174.4(1)
95.0(1)
˚
hydrogen atoms [C(9a)···O(4) ϭ 3.17(1) A, H(9a)···O(4)
˚
2.54(1) A, C(9a)ϪH(9a)···O(4) 124.8(5)°, a ϭ Ϫx, Ϫy, Ϫz].
The reciprocal orientation of the C(21)/C(26) phenyl ring
with respect to the centrosymmetric C(13)/C(18) ring with
a dihedral angle of 66° is able to create an interesting inter-
molecular contact of the CϪH/Ph type, for which the dis-
tance between H(15a) and the aromatic centroid X of the
˚
C(21)/C(26) ring (2.68 A) and the C(15a)ϪH(15a)···X angle
The palladium atom has a distorted square-planar ge-
ometry and the bond lengths and angles around the metal
ion and within the five-membered chelate ring are similar
to those recently reported by us for the complex (dihexade-
cyl 2,2Ј-bipyridyl-4,4Ј-dicarboxylate)palladium(ii) chlo-
ride.[8b] The two substituents on the 2,2Ј-bipyridine ligand
run in opposite directions as shown by the dihedral angles
C(4)ϪC(3)ϪC(19)ϪO(3) and C(7)ϪC(8)ϪC(11)ϪO(1) of
171.6(5) and Ϫ162.5(5)°, respectively. While the carboxylic
oxygen atom O(2) points toward the internal part of the
bipyridine ligand [C(12)ϪO(1)ϪC(11)ϪC(8) 180.0(5)°],
(147°) are indicative of the presence of a weak hydrogen
CϪH-π bond.[17] Moreover, the disposition of the pyridinic
aromatic rings gives rise to a parallel displaced πϪπ stack-
ing arrangement characterised by a centroid contact of 3.52
˚
A, an angle of 18° between the ring normal and the cen-
troid vector and a corresponding horizontal displacement
[18]
˚
of 1.20 A.
The M···M intermolecular separation of 5.6
˚
A within the dimer is significantly different from any weak
Pd···Pd attractive interaction. The carboxylic oxygen atom
O(2) which is not involved in any significant interaction
within the dimer, assists in holding dimers together through
the formation of hydrogen bonds with one of the hydrogen
atoms of the peripheral phenyl rings (see a in Figure 2). In
3D space, the repetition of dimers which are stacked along
the c direction and separated by layers of interacting aro-
matic rings (see b in Figure 2) is of particular interest in
the discussion of the columnar organisation found for the
analogous complexes [LnMCl2] with long alkoxy chains on
the peripheral aromatic rings in the liquid crystalline state.
Moreover, the absence of solvent molecules in the crystal
packing enables us to view this molecular arrangement as
a fingerprint for identification of future columnar liquid
crystalline organisations.
O(4)
shows
a
very
different
orientation
[C(20)ϪO(3)ϪC(19)ϪC(3) Ϫ76.2(6)°]. The asymmetry in
the conformation around the ‘‘CH2ϪO’’ bond of the two
substituents can be clarified by considering the crystal
packing of molecules of the [LPdCl2] complex.
The molecular organisation is dominated by intermolecu-
lar interactions of two main types: i) hydrogen bonds
(CϪH···O and ‘‘aromatic hydrogen bonds’’ CϪH···π[17]
and ii) πϪπ contacts (Figure 2 a).
)
Thermal Behaviour of [LnMCl2] Complexes
Although hexacatenar 2,2Ј-bipyridine ligands are not
liquid crystals, all [LnMCl2] complexes show enantiotropic
columnar mesomorphism which was investigated using
polarised optical microscopy (POM), differential scanning
calorimetry (DSC) and temperature dependent powder X-
ray diffraction (PXRD) measurements. The thermal data of
the [LnMCl2] series are summarised in Table 2.
For all [LnMCl2] complexes the results from POM sug-
gested the presence of a columnar hexagonal mesophase, as
indicated by the typical pseudo-focal-conics and areas of
homeotropic orientation (Figure 3).
The dodecyloxy homologues feature two mesophases as
shown by DSC and confirmed by subtle textural changes,
such as the appearance in POM of fine disclination lines
and deeper colours at the second order transition. This
could indicate a transition between two hexagonal phases
with different degrees of order within the columns. In the
case of the palladium complexes, cooling from the isotropic
liquid did not lead to crystallization but instead to solidifi-
cation into a glassy state retaining the structure of the hex-
agonal mesophase.
Figure 2. Ball and stick representation of interacting dimers of
[LPdCl2] viewed approximately down the c axis (top view) and rep-
etition of dimers in 3D space with generation of a columnar ar-
rangement (bottom view)
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183

D. Pucci et al.
Table 3. X-ray diffraction data of [[LnMCl2] complexes
FULL PAPER
Table 2. Optical and thermal data of [LnMCl2] complexes
˚
Compound
Transition[a]
T/°C[b]
∆H/kJ·molϪ1
Compound
Mesophase
dobsd./A
Miller
indices
˚
(dcalcd/A)
˚
Lattice constants/A
Cr Ϫ Colh1
Colh1 Ϫ Colh2
Colh2 Ϫ I
46.2
122.6
132.1
131.0
110.6
56.7
58.5
90.6
111.4
111.1
59.3
24.5
15.0
2.7
[L12ZnCl2]
[L12ZnCl2]
Colh1 at 90 °C
a ϭ 40.6
h ϭ 4.1
35.3 (35.3)
20.0 (20.5)
17.7 (17.7)
11.5 (11.8)
8.2
ca. 4.6
ca. 4.1
(1 0)
(1 1)
(2 0)
(3 0)
halo
broad
"
I Ϫ Colh2
2.1
Colh2 Ϫ Colh1
Colh1 Ϫ Cr
Cr Ϫ CrЈ
CrЈ Ϫ Colh
Colh Ϫ I
I Ϫ Colh
Colh1 Ϫ CrЈ
15.2
30.0
73.7
37.7
8.0
n ϭ 2.1[a]
[L16ZnCl2]
[L12PdCl2]
1.3
24.7
102.5
30.0
1.26
2.5
20.8
20.9
9.4
Colh2 at 130 °C
a ϭ 37.5
32.6 (32.6)
18.9 (18.8)
16.1 (16.3)
ca. 4.7
(1 0)
(1 1)
(2 0)
halo
[c]
Cr Ϫ Colh1
76.0
Colh1 Ϫ Colh2
Colh2 Ϫ I
I Ϫ Colh2
Cr Ϫ Colh
Colh Ϫ I
136.4
145.3
145.2
48.6
127.5
112.5
n ϭ 2.1[b]
[L16ZnCl2]
[L12PdCl2]
Colh at 100 °C
a ϭ 42.7
37.0 (37.0)
21.5 (21.4)
18.4 (18.5)
17.5 (16.6)
ca. 4.6
(1 0)
(1 1)
(2 0)
(3 0)
halo
[L16PdCl2]
I Ϫ Colh
nϭ 2.2[b]
[a]
[b]
Cr: crystal; Colh: columnar hexagonal; I: isotropic liquid.
Temperature data as peak onset.[c] Microscopy data.
Colh1 at 90 °C
a ϭ 40.3
h ϭ 3.8
35.0 (35.0)
20.1 (20.2)
17.4 (17.5)
11.5 (11.8)
8.0
(1 0)
(1 1)
(2 0)
(3 0)
halo
"
n ϭ 1.9[a]
ca. 4.6
ca. 3.8
"
Colh2 at 141 °C
a ϭ 38.0
32.7 (32.7)
18.8 (18.9)
16.7 (16.4)
7.3
(1 0)
(1 1)
(2 0)
halo
"
n ϭ 2.0[b]
ca. 4.8
Figure 3. Polarised light optical photomicrograph of the texture
exhibited by [LnMCl2] complexes; plate 1: texture of the hexagonal
columnar mesophase of [L12ZnCl2] at 128 °C; plate 2: texture of
the hexagonal columnar mesophase Colhd1 of [L12PdCl2] at 135 °C
[L16PdCl2]
Colh at 60 °C
43.6 (43.6)
25.6 (25.2)
21.8 (21.8)
14.7 (14.5)
ca. 4.6
(1 0)
(1 1)
(2 0)
(3 0)
halo
"
a ϭ 50.6
h ϭ 3.6
The mesophases were also identified by temperature-de-
pendent powder-XRD experiments. The X-ray diffraction
patterns confirmed the results of optical microscopy. In all
cases, in the small angle region a sharp high intensity peak
and two lower intensity peaks at wider angles were indexed
to the (1 0), (1 1) and (2 0) reflections. This is typical of a
2D hexagonal lattice (Table 3).
n ϭ 2.3[a]
ca. 3.6
[a] Number of molecules in each segment of the columns calculated
with h ϭ 4.1 and 3.6 A. Number of molecules in each segment
of the columns calculated with h in the range 4.7Ϫ4.8 A.
[b]
˚
˚
Moreover, in the case of [LnMCl2], a transition between ecules in the cross section of the columns. This value, which
hexagonal phases was observed. As shown in Figure 4 for was found to be around 2, was estimated by assuming a
[L12ZnCl2], in the wide angle region of the XRD spectrum density of 1 g·cmϪ3 and a height for the columnar slice of
˚
˚
recorded at 90 °C, the pattern consists of a broad diffuse 4.1 or 4.7 A for M ϭ Zn and 3.8 or 4.6 A for M ϭ Pd
˚
scattering halo (B in Figure 4) at ca. 4.6 A, corresponding (Table 3). The transition Colh1 Ϫ Colh2 is accompanied by
to the liquid-like disorder of the aliphatic chains. Also ob- a decrease in the hexagonal lattice constant and a loss of
˚
served was another broad diffuse band (C in Figure 4) at the broad diffuse band centred at 4.1 or 3.8 A (Table 3).
˚
ca. 4.1 A which is characteristic of the distance between These differences may characterise Colh2 as a more dis-
adjacent cores (also definable as the stacking period in a ordered hexagonal phase than Colh1. Within the tempera-
column)[19] and, finally, a third less intense broad band (A ture range in which the first hexagonal phase may be ob-
˚
in Figure 4) at ca. 8 A possibly indicating a perpendicular served, the lattice parameter was found to be temperature
stacking of the hemi-disc shaped molecules which is neces- dependent. In fact, the low angle reflection shifted to a
sary to produce a disc-shaped dimer for the formation of larger d spacing with decreasing temperature indicating a
the columnar phase.[20]
lattice expansion.[21]
The proposed model of organisation in the hexagonal
A further increase in the number of carbon atoms in the
phase is in accordance with the calculated number of mol- side chains was not accompanied by any change of sym-
184
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Hexacatenar Tetrahedral (2,2Ј-Bipyridine)zinc Complexes
FULL PAPER
which high conduction is required. In order to probe the
effectiveness of our materials as photoconductors, we selec-
ted the longer chain length palladium and zinc derivatives
and performed standard measurements as described in the
Exp. Sect. Since pure samples show no absorption in the
visible region, photogeneration was induced by doping with
a small amount (0.05%) of C60 which is known as a good
acceptor thereby promoting the formation of hole carriers.
Phase transitions were not affected by the small amount of
this dopant. Within our cells, it was not possible to obtain a
homogeneous alignment of the mesophases using standard
techniques and all the measurements were performed on
unaligned samples. Figure 5 shows the ratio of the light and
dark conductivities for [L16PdCl2] at 633 nm over a tem-
perature range which incorporates solid, columnar and iso-
tropic phases.
Figure 4. Powder X-ray diffraction pattern of the columnar hexag-
onal phase (Colh1) at 90 °C for complex [L12ZnCl2]
metry but a significant difference of the lattice parameter
Figure 5. Ratio between the photoconductivity σph and the conduc-
tivity in the dark σd for a 40 µm thick sample of complex
[L16PdCl2] doped with C60 as a function of temperature; the verti-
cal line marks the I Ϫ Colh transition; inset: normalised photocon-
duction measured as a function of temperature for the same
sample; data were collected in an applied field of 8 V/µm and a
wavelength λ ϭ 633 nm
between [L16ZnCl2] and [L16PdCl2] was observed (Table 3).
˚
In fact, the noteworthy increase of a from 42.7 to 50.6 A
on going from the ZnII to the PdII derivative is indicative
of an increase of the size of the disk formed by the associ-
ation of two molecules into a dimeric unit. At this particu-
lar length of the side chains, the geometry around the metal
ion clearly plays an important role in the association of
molecules into dimers. The flatness of the molecules in the
case of the square planar PdII complex compared with the
distorted tetrahedral ZnII derivative encourages close prox-
imity between the cores within the dimer (broad diffuse
It is clearly visible that with decreasing temperature such
a ratio starts to increase at the I-Colh transition and keeps
increasing upon further decreases in temperature. This is a
signature of the reduced contribution of ionic (impurities)
conduction at low temperatures compared with the contri-
bution of electron (hole) hopping. The temperature depen-
dence of the photoconductivity, normalised to the power
density of the incident radiation, is shown in the inset of
Figure 5, while its applied field dependence is shown in Fig-
ure 6. A proper comparison of conductivity with similar
compounds would require mobility measurements which
have not been yet carried out. Nevertheless, although the
ratio of photo-conductivity to dark-conductivity is rather
low, the measured values of the absolute photoconductivity
are quite good, especially considering that the absorption
coefficient for these samples at 633 nm is rather low (α ഠ
16 cmϪ1) and that for the relatively low values of the ap-
˚
band in the XRD spectra at ca. 3.6 A which corresponds
to the distance between adjacent cores). In consequence, a
larger horizontal displacement and an overall rise of the
radius of the disk can be observed.
In both cases, a slight change of the position of the first
peak in the low angle region was observed on increasing the
temperature. A corresponding decrease in the intecolumnar
˚
distance to less than 1 A was also observed. This slow
change corresponds to a very low enthalpy (3.3 and 0.6 kJ/
mol) and the absence of important optical textural changes.
Photoconductivity Measurements
Because of the incremental charge mobility induced by plied fields used, the photogeneration quantum efficiency
molecular self-assembling,[12] columnar mesophases have should also be low.
recently become attractive for their potential applications in
The data in Figure 6 in fact show photoconductivity
organic photovoltaic and electroluminescent devices in which is orders of magnitude higher than that measured for
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185

D. Pucci et al.
FULL PAPER
the overall supramolecular organisation rather than with
the segregation effects of the simple molecular units.
These results show that by a careful choice of molecular
building blocks, it is possible to finely modulate the interac-
tions which determine the organisation of single molecules
(association into dimers) in supramolecular architectures,
giving rise to the desired functionalised metallomesogenic
materials. For this class of derivatives, our goal was the de-
sign of metal-containing columnar liquid crystals in which
the pre-selected property for improvement was the charge
carrier mobility for charge transport. Preliminary measure-
ments of photoconductivity, in samples of [LnMCl2] com-
plexes doped with C60 to increase absorption in the visible,
have given excellent results and further experiments are in
progress.
Figure 6. Normalised photoconduction at λ ϭ 633 nm measured as
a function of applied field for complex [L16PdCl2] doped with C60
standard photoconductors such as polyvinylcarbazole at
the same applied field and with the same absorption coef-
ficients. As a consequence, these results indicate that charge
mobility in [L16PdCl2] could be high and measurements are
in progress to confirm this hypothesis. In the case of the
related [L16ZnCl2] complex, the measured photoconductiv-
ity is about two orders of magnitude lower. Since the meas-
urements are in a preliminary stage, it is not possible at the
moment to rationalise this behaviour.
Experimental Section
Materials and Measurements: All reagents were purchased from the
following and used as received: 4,4Ј-Dimethyl-2,2Ј-bipyridine, ben-
zoic acid, 1-bromododecane, 1-bromohexadecane, zinc chloride, 4-
pyrrolidinopyridine (PPY) from Aldrich, methyl 3,4,5-trihydroxy-
benzoate from Lancaster and N,NЈ-dicyclohexylcarbodiimide
(DCC) from Fluka. The solvents were used as received from com-
mercial sources without further purification and were dried using
standard methods when required. cis-Bis(benzonitrile)dichloro-
palladium,[22] 4,4Ј-bis(hydroxymethyl)-2,2Ј-bipyridine[23] and 3,4,5-
Conclusions
A series of hexacatenar 4,4Ј-disubstituted 2,2Ј-bipyri- trialkoxybenzoic acids[24] were prepared with slight changes follow-
dines (Ln) and the corresponding zinc(ii) and palladium(ii)
ing the methods described in the literature. Infrared spectra were
complexes [LnMCl2] have been synthesised and character-
recorded with a Spectrum One FT-IR PerkinϪElmer spectrometer
and 1H NMR spectra with a Bruker WH-300 spectrometer in
ised. Although the ligands are not mesomorphic, the associ-
[D6]DMSO, CDCl3 or CD3OD with TMS as an internal standard.
ation of the hemi-disc shaped molecules in face-to-face disc-
Elemental analyses were performed with a PerkinϪElmer 2400
shaped dimers induces columnar mesogenic properties for
analyser. The textures of the mesophases were examined with a
all the metal derivatives, regardless of the nature of metal
Zeiss Axioscope polarising microscope equipped with a Linkam
atom. In fact, liquid crystal properties were also induced
CO 600 heating stage. The transition temperatures and enthalpies
for the first time on tetrahedrally coordinated zinc com-
were measured on a PerkinϪElmer Pyris1 Differential Scanning
Calorimeter with a heating and cooling rate of 10 °C/min. The
plexes while for the palladium derivatives [LnPdCl2], the la-
mellar mesophases observed in dialkyl 2,2Ј-bipyridine-4,4Ј- apparatus was calibrated with indium. Two or more heating/cool-
dicarboxylate compounds[8a] are replaced by hexagonal col-
ing cycles were performed on each sample. Spectrofluorimetric
grade dichloromethane (Acros Organics) was used for the photo-
umnar mesomorphism. Hence, it can be suggested that for
all the new [LnMCl2] complexes in which the organic parts
of the molecules have been modified through the introduc-
tion of further aromatic rings, the driving force in produc-
ing a supramolecular columnar arrangement is the existence
of intermolecular interactions (hydrogen bonds, CϪHϪπ
and πϪπ contacts) between the large flat aromatic cores
physical investigations in solution at room temperature. Absorption
spectra were recorded with a PerkinϪElmer Lambda 900 spectro-
photometer. The experimental uncertainty of the band maximum
for the absorption spectra was 2 nm. The ligands and the com-
plexes are stable in CH2Cl2 as demonstrated by the constancy of
their absorption spectra over a week. The powder X-ray diffraction
patterns were obtained using a Bruker AXS General Area Detector
rather than dipolar or metal-metal interactions. The analy-
Diffraction System (D8 Discover with GADDS) with Cu-Kα radi-
sis of the crystal packing in the solid state of the model ation. The highly sensitive area detector was placed at a distance
of 10 cm from the sample and equipped with a CalCTec (Italy)
heating stage. The samples were heated at a rate of 5.0 °C minϪ1
to the appropriate temperature. Measurements were performed by
placing samples in Lindemann capillary tubes with an inner dia-
meter of 0.05 mm. Samples for photoconductivity measurements
were prepared by mixing the complex [L16MCl2] and 0.05% C60 in
toluene and then evaporating the solvent. The resultant material
was then transferred between two ITO-covered conducting glass
slides by using the effect of capillarity within the isotropic phase.
The thickness was controlled with glass spacers. The conductivity
compound [LPdCl2] has given excellent insight into the fu-
ture arrangements of the mesophases of the corresponding
long chain derivatives, at least as far as the polar fragments
of the molecules are concerned. The presence of long alkoxy
chains in the [LnMCl2] derivatives gives rise to mesomorph-
ism without interfering with the promesogenic columnar ar-
rangement of the aromatic cores of the molecules.
Moreover, the thermal behaviour in each series was found
to be insensitive to the nature of the metal atom and slightly
decreases in the order Zn Ͻ Pd. This can be correlated with was measured by monitoring the voltage drop across a resistor in
186 © 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.eurjic.org
Eur. J. Inorg. Chem. 2005, 181Ϫ188

Hexacatenar Tetrahedral (2,2Ј-Bipyridine)zinc Complexes
FULL PAPER
1
series with the sample cell. For temperature dependent measure-
ments, a CalCTec (Italy) hot stage was used. For photoconductivity
observations, an approximately 10Ϫ2 cm2 sample was illuminated
with 5 mW of light from a He-Ne laser at 633 nm.
Yield 0.161 g (82%); m.p. 179 °C. H NMR (300 MHz, CDCl3, 25
°C, TMS): δ ϭ 5.47 (s, 4 H, CH2), 7.49 (m, 2 H, CH), 7.58 (m, 4
H, CH), 7.61 (m, 4 H, CH), 8.13 (dd, 3JH,H ϭ 8.5, 4JH,H ϭ 2.4 Hz,
3
2 H, H-5 bpy), 8.48 (s, 2 H, H-3 bpy), 8.70 (d, JH,H ϭ 8.5 Hz,
2 H, H-6 bpy) ppm. IR (KBr): ν˜ ϭ 1723 cmϪ1 (CϭO). UV/Vis
(dichloromethane): λmax. (ε, molϪ1·dm3·cmϪ1) ϭ 282 nm (17800).
C26H20N2O4 (424.46): calcd. C 73.57, H 4.57, N 6.60; found C
73.44, H 4.62, N 6.76. All Ln ligands were prepared similarly start-
ing from 4,4Ј-bis(hydroxymethyl)-2,2Ј-bipyridine and the appropri-
ate 3,4,5-trialkoxy-substituted benzoic acid. Therefore only yields,
X-ray Crystallographic Data Collection and Refinement of the
[LPdCl2] Structure: Orange crystals of [LPdCl2] were formed at
room temperature from a DMSO/ethanol mixture. Diffraction
measurements were carried out at room temperature on a Siemens
R3m/V automated four-circle diffractometer equipped with graph-
˚
ite-monochromated Mo-Kα radiation (λ ϭ 0.71073 A). The data
1
melting points, H NMR data, IR data and elemental analyses are
were corrected for Lorentz, polarisation and X-ray absorption ef-
fects using the XABS2 program.[25] Details of the crystal data col-
lection are listed in Table 4. Structure solution (Patterson method)
and full-matrix least-squares refinements based on F2 were per-
formed with the SHELXS/L programs of the SHELXTL-NT
software package (Version 5.10).[26] All nonhydrogen atoms were
refined anisotropically. The hydrogen atoms were included in the
refinement in their observed positions and then fixed as idealised
atoms riding on the respective carbon atom. CCDC-241996 con-
tains the supplementary crystallographic data for this paper. These
data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/
retrieving.html or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: (internat.)
ϩ 44-1223-336-033; E-mail: deposit@ccdc.cam.ac.uk].
reported below.
4,4Ј-Bis[3,4,5-(tridodecyloxy)benzoyloxymethyl]-2,2Ј-bipyridine
(L12): Yield 0.62 g (78%); m.p. 73 °C. 1H NMR (300 MHz, CDCl3,
25 °C, TMS): δ ϭ 0.88 (t, JH,H ϭ 6.6 Hz, 18 H, CH3), 1.26 [m,
3
108 H, (CH2)5CH3], 1.78 [m, 12 H, CH2(CH2)5CH3], 4.03 (t,
3JH,H ϭ 6.3 Hz, 12 H, OCH2), 5.44 (s, 4 H, CH2), 7.33 (s, 4 H,
3
CH), 7.37 (d, JH,H ϭ 4.7 Hz, 2 H, H-5 bpy), 8.49 (s, 2 H, H-3
3
bpy), 8.67 (d, JH,H ϭ 4.7 Hz 2 H, H-6 bpy) ppm. IR (KBr): ν˜ ϭ
1720 cmϪ1 (CϭO). UV/Vis (dichloromethane): λmax. (ε,
molϪ1·dm3·cmϪ1) ϭ 277 nm (39100). C98H164N2O10 (1530.39):
calcd. C 76.91, H 10.80, N 1.83; found C 76.72, H 11.00, N 2.01.
4,4Ј-Bis[3,4,5-(trihexadecyloxy)benzoyloxymethyl]-2,2Ј-bipyridine
(L16): Yield 0.35 g (63%); m.p. 90 °C. 1H NMR (300 MHz, CDCl3,
25 °C, TMS): δ ϭ (t, JH,H ϭ 6.6 Hz, 18 H, CH3), 1.25 [m, 156 H,
3
(CH2)5CH3], 1.77 [m, 12 H, CH2(CH2)5CH3], 4.02 (m, 12 H,
Table 4. Crystal and structure refinements for complex [LPdCl2]
3
OCH2), 5.44 (s, 4 H, CH2), 7.33 (s, 4 H, CH), 7.37 (d, JH,H
5.0 Hz, 2 H, H-5 bpy), 8.49 (s, 2 H, H-3 bpy), 8.68 (d, JH,H
ϭ
3
ϭ
Formula
Mr
C26H20Cl2N2O4Pd
601.7
5.0 Hz, 2 H, H-6 bpy) ppm. IR (KBr): ν˜ ϭ 1704 cmϪ1 (CϭO).
UV/Vis (dichloromethane): λmax. (ε, molϪ1·dm3·cmϪ1) ϭ 278 nm
(88700). C122H212N2O10 (1867.02): calcd. C 78.48, H 11.44, N 1.50;
found C 78.55, H 11.50, N 1.70.
Crystal system
Space group
triclinic
P1
¯
˚
a (A)
9.671(2)
11.329(3)
11.484(3)
99.06(2)
95.20(2)
105.48(2)
1185.7(4)
298
˚
b (A)
˚
c (A)
Zinc Complexes: All zinc complexes were prepared in a similar
manner. The synthesis of [LZnCl2] is described in detail below and
for all other homologues only yields, 1H NMR and IR data and
elemental analyses are reported.
α (°)
β (°)
γ (°)
3
˚
V (A )
T (K)
[4,4Ј-Bis(benzoyloxymethyl) 2,2Ј-bipyridyl]zinc(II
)
Chloride
Z
2
[LZnCl2]: A suspension of ZnCl2 (0.013 g, 0.094 mmol) and L
(0.040 g, 0.094 mmol) in dichloromethane (10 mL) was stirred at
room temperature for four days. By filtration through Celite and
evaporation the final product was obtained as a colourless solid
after recrystallisation from methanol. Yield 0.37 g (70%); m.p. 260
Density (Mg/mϪ3
)
1.685
1.045
604
Absorption coefficient (mmϪ1
F(000)
)
Reflections collected
Unique reflections [I Ͼ 2σ(I)]
R(int)
4483
4213 (2447)
0.037
1
°C. H NMR (300 MHz, CD3OD, 25 °C, TMS): δ ϭ 5.61 (s, 4 H,
Parameters
314
CH2), 7.51 (m, 4 H, CH), 7.65 (m, 2 H, CH), 7.83 (m, 4 H, CH),
[a]
R1 [I Ͼ 2σ(I)]
0.0486
0.1006
0.825
3
8.12 (d, JH,H ϭ 7.3 Hz, 2 H, H-5 bpy), 8.75Ϫ8.68 (m, 4 H, H-3,6
[b]
wR2 (all data)
bpy) ppm. IR (KBr): ν˜ ϭ 1720 cmϪ1 (CϭO). C26H20Cl2N2O4Zn
(560.74): calcd. C 55.69, H 3.60, N 5.00; found C 55.48, H 3.58,
N 4.74.
GOF on F2
[a]
[b]
2
2 2
R1 ϭ Σ(|Fo| Ϫ |Fc|)/Σ|Fo|.
wR2 ϭ {Σ[w(Fo Ϫ Fc ) ]/
Σ[w(Fo ) ]}1/2
.
2 2
{4,4Ј-Bis[3,4,5-tridodecyloxy)benzoyloxymethyl]-2,2Ј-bipyridyl}-
zinc(II) Chloride [L12ZnCl2]: Recrystallisation from dichlorometh-
ane/diethyl ether. Yield 0.160 g (86%); thermotropic behaviour in
Table 2. 1H NMR (300 MHz, CDCl3, 25 °C, TMS): δ ϭ 0.88 (t,
3JH,H ϭ 6.4 Hz, 18 H, CH3), 1.26 [m, 108 H, (CH2)5CH3], 1.79 [m,
Synthesis
Model Ligand: 4,4Ј-Bis(benzoyloxymethyl)-2,2Ј-bipyridine (L): 4,4Ј-
Bis(hydroxymethyl)-2,2Ј-bipyridine (0.099 mg, 0.46 mmol), benzoic 12 H, CH2(CH2)5CH3], 4.03 (m, 12 H, OCH2), 5.52 (s, 4 H, CH2),
3
acid (0.112 g, 0.92 mmol) and DCC (0.208 g, 1.01 mmol) were sus-
pended in dry dichloromethane (20 mL). PPY (0.013 g, 0.09 mmol)
was added and the reaction was stirred at room temperature and
under N2 for a week. The colourless precipitate was removed by
filtration and the solvent was evaporated. The crude product was
recrystallised from chloroform/methanol giving a colourless solid.
7.29 (s, 4 H, CH), 7.78 (d, JH,H ϭ 5.3 Hz, 2 H, H-5 bpy), 8.24 (s,
3
2 H, H-3 bpy), 8.85 (d, JH,H ϭ 5.3 Hz, 2 H, H-6 bpy) ppm. IR
(KBr): ν˜ ϭ 1718 cmϪ1 (CϭO). UV/Vis (dichloromethane): λmax.
(ε, molϪ1·dm3·cmϪ1) ϭ 282 (30500), 297 (31400), 307 nm (27700).
C98H164Cl2N2O10Zn (1666.67): calcd. C 70.62, H 9.92, N 1.68;
found C 70.31, H 9.63, N 1.90.
Eur. J. Inorg. Chem. 2005, 181Ϫ188
www.eurjic.org
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
187

D. Pucci et al.
FULL PAPER
{4,4Ј-Bis(3,4,5-tris[hexadecyloxy)benzoyloxymethyl]-2,2Ј-bipyridyl}-
zinc(II) Chloride [L16ZnCl2]: Recrystallisation by chloroform/ace-
[1]
[2]
J. L. Serrano, Metallomesogens, Wiley-VCH, Weinheim, 1996.
B. Donnio, D. W. Bruce, in Structure and Bonding (Ed.: D. M.
P. Mingos), Springer, Berlin, 1999, vol. 95.
tone. Yield 0.12 g (91%); thermotropic behaviour in Table 2. 1H
3
NMR (300 MHz, CDCl3, 25 °C, TMS): δ ϭ 0.87 (t, JH,H
ϭ
[3]
[4]
R. W. Date, E. F. Iglesias, K. E. Rowe, J. M. Elliott, D. W.
Bruce, Dalton Trans. 2003, 1914Ϫ1932.
6.7 Hz, 18 H, CH3), 1.25 [m, 156 H, (CH2)5CH3], 1.80 [m, 12 H,
CH2(CH2)5CH3], 4.03 (m, 12 H, OCH2), 5.52 (s, 4 H, CH2), 7.28
´
R. Gimenez, D. P. Lydon, J. L. Serrano, Curr. Opin. Sol. State
3
(s, 4 H, CH), 7.77 (d, JH,H ϭ 5.1 Hz, 2 H, H-5 bpy), 8.24 (s, 2 H,
Mat. Sci. 2002, 6, 527Ϫ535.
3
H-3 bpy), 8.86 (d, JH,H ϭ 5.1 Hz, 2 H, H-6 bpy) ppm. IR (KBr):
[5] [5a]
I. Aiello, M. Ghedini, M. La Deda, D. Pucci, O. Frances-
ν˜ ϭ 1720 cmϪ1 (CϭO). UV/Vis (dichloromethane): λmax. (ε,
molϪ1·dm3·cmϪ1) ϭ 282 (25600), 296 (27400), 307 nm (24600).
C122H212Cl2N2O10Zn (2003.32): calcd. C 73.15, H 10.67, N 1.40;
found C 73.38, H 10.90, N 1.48.
[5b]
cangeli, Eur. J. Inorg. Chem. 1999, 1367Ϫ1372.
C. Tschier-
ske, Angew. Chem. Int. Ed. 2000, 39, 2454Ϫ2458. [5c] H. Richtz-
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[5d]
Schröder, Chem. Commun. 2001, 2580Ϫ2581.
M. Benouaz-
zane, S. Coco, P. Espinet, J.-M. Martin-Alvarez, J. Barbera, J.
Palladium Complexes: All palladium complexes were prepared in a
similar manner. The synthesis of [LPdCl2] is described in detail
below. For all other homologues only yields, 1H NMR and IR data
and elemental analyses are reported.
[5e]
Mater. Chem. 2002, 12, 691Ϫ696.
L. Plasseraud, L. Gon-
zales Cuervo, D. Guillon, G. Suss-Fink, R. Deschenaux, D. W.
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M. Huck, H. Loc Nguyen, H. Coles, M. Hursthouse, B. Don-
nio, D. W. Bruce, J. Mater. Chem. 2002, 12, 2879Ϫ2886. [5g] L.
[4,4Ј-Bis(benzoyloxymethyl)-2,2Ј-bipyridyl]palladium(II
)
Chloride
´
Douce, T.-H. Diep, R. Ziessel, A. Skoulios, M. Cesario, J.
[LPdCl2]: A solution of [Pd(PhCN)2Cl2] (0.045 g, 0.12 mmol) and
L (0.20 g, 0.17 mmol) in chloroform (10 mL) was heated to reflux
for 7 h. Filtration and washing with methanol gave the final prod-
uct as a pale yellow solid. Yield 0.08 g (80%); m.p. 322 °C. 1H
NMR (300 MHz, [D6]DMSO, 25 °C, TMS): δ ϭ 5.59 (s, 4 H, CH2),
7.56 (m, 4 H, CH), 7.70 (m, 2 H, CH), 7.90 (d, 4 H, CH), 8.105
Mater. Chem. 2003, 13, 1533Ϫ1539.
[6]
[7]
J. L. Serrano, T. Sierra, Chem. Eur. J. 2000, 6, 759Ϫ766.
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6, 579Ϫ587.
[8] [8a]
D. Pucci, G. Barberio, A. Crispini, O. Francescangeli, M.
[8b]
Ghedini, Mol. Cryst. Liq. Cryst. 2003, 395, 325Ϫ335.
D.
4
(dd, 3JH,H ϭ 6.85, JH,H ϭ 1.4 Hz, 2 H, H-5 bpy), 8.68 (s, 2 H, H-
Pucci, G. Barberio, A. Crispini, O. Francescangeli, M. Ghedini,
M. La Deda, Eur. J. Inorg. Chem. 2003, 19, 3649Ϫ3661.
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8879Ϫ8880.
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7, 343Ϫ354.
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2002, 6, 569Ϫ578.
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2003, 9, 2484Ϫ2501.
3
3 bpy), 9.075 (d, JH,H ϭ 6.85 Hz, 2 H, H-6 bpy) ppm. IR (KBr):
[9]
ν˜ ϭ 1720 cmϪ1 (CϭO). C26H20Cl2N2O4Pd (601.78): calcd. C 51.89,
H 3.35, N 4.66; found C 51.63, H 3.35, N 4.62.
[10]
[11]
{4,4Ј-Bis[3,4,5-tri(dodecyloxy)benzoyloxymethyl]-2,2Ј-bipyridyl}-
palladium(II) Chloride [L12PdCl2]: Recrystallisation from chloro-
form/methanol. Yield 0.21 g (93%); thermotropic behaviour in
[12]
[13]
1
Table 2. H NMR (300 MHz, CDCl3, 25 °C, TMS): δ ϭ 0.87 (m,
18 H, CH3), 1.26 (m, 108 H, (CH2)5CH3], 1.78 (m, 12 H,
CH2(CH2)5CH3], 4.02 (m, 12 H, OCH2), 5.52 (s, 4 H, CH2), 7.29
3
(s, 4 H, CH), 7.48 (d, JH,H ϭ 5.2 Hz, 2 H, H-5 bpy), 8.18 (s, 2 H,
[14]
[15]
A. Pegenau, T. Hegmann, C. Tschierske, S. Diele, Chem. Eur.
J. 1999, 5, 1643Ϫ1660.
3
H-3 bpy), 9.10 (d, JH,H ϭ 5.2 Hz, 2 H, H-6 bpy) ppm. IR (KBr):
ν˜ ϭ 1723 cmϪ1 (CϭO). UV/Vis (dichloromethane): λmax. (ε,
molϪ1·dm3·cmϪ1) ϭ 268 (36600), 304 (24700) 314 nm (23000).
C98H164Cl2N2O10Pd (1707.71): calcd. C 68.93, H 9.68, N 1.64;
found C 69.15, H 9.50, N 1.37.
´
E. Terazzi, J.-M. Benech, J.-P. Rivera, G. Bernardinelli, B. Don-
nio, D. Guillon, C. Piguet, Dalton Trans. 2003, 769Ϫ772.
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[16]
[17]
[18]
[19]
{4,4Ј-Bis[3,4,5-tri(hexadecyloxy)benzoyloxymethyl]-2,2Ј-bipyridyl}-
palladium(II) Chloride [L16PdCl2]: Yield 0.20 g (91%); thermotropic
1
behaviour in Table 2. H NMR (300 MHz, CDCl3, 25 °C, TMS):
ˆ
Malthete, Chem. Mater. 2002, 14, 375Ϫ384.
[20] [20a]
3
S. T. Trzaska, T. M. Swager, Chem. Mater. 1998, 10,
δ ϭ 0.87 (t, JH,H ϭ 6.5 Hz, 18 H, CH3), 1.25 [m, 156 H,
[20b]
438Ϫ443.
T. Hegmann, J. Kain, S. Diele, B. Schubert, H.
(CH2)5CH3], 1.78 [m, 12 H, CH2(CH2)5CH3], 4.02 (m, 12 H,
3
Bögel, C. Tschierske, J. Mater. Chem. 2003, 13, 991Ϫ1003.
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2067Ϫ2077.
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1938, 60, 882Ϫ885.
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53Ϫ56.
OCH2), 5.52 (s, 4 H, CH2), 7.29 (s, 4 H, CH), 7.49 (d, JH,H
ϭ
[21]
[22]
[23]
3
5.8 Hz, 2 H, H-5 bpy), 8.16 (s, 2 H, H-3 bpy), 9.11 (d, JH,H
ϭ
5.8 Hz, 2 H, H-6 bpy) ppm. IR (KBr): ν˜ ϭ 1721 cmϪ1 (CϭO). UV/
Vis (dichloromethane): λmax. (ε, molϪ1·dm3·cmϪ1) ϭ 268 (43900),
304 (26400), 314 nm (22800). C122H212Cl2N2O10Pd (2044.35):
calcd. C 71.68, H 10.45, N 1.37; found C 71.83, H 10.22, N 1.65.
[24]
[25]
Acknowledgments
This work was partly supported by CIPE grants (Clusters 14 and
[26]
SHELXTL/NT crystal structure analysis package, Bruker
`
26) from the Ministero dell’Istruzione, dell’Universita e della
AXS, Inc.: Madison, WI, 1999.
Ricerca. P. P. is pleased to acknowledge the Marie Curie develop-
ment host fellowship Contract HPMD-CT-2001-00073 for finan-
cial support.
Received June 17, 2004
Early View Article
Published Online November 10, 2004
188
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Inorg. Chem. 2005, 181Ϫ188