Columnar mesophases from tetrahedral copper(I) cores and Schiff-base derived polycatenar ligands

Article abstract of DOI:10.1039/a904245h

In the presence of copper(I), certain di- and tri-topic ligands derived from reaction of substituted anilines with 2-pyridinecarbaldehyde or 2,6-pyridinedicarbaldehyde form a new type of polycatenar metallomesogen around the tetrahedral metallic centre.

Full text of DOI:10.1039/a904245h

Columnar mesophases from tetrahedral copper(i) cores and Schiff-base
derived polycatenar ligands
Laurent Douce,a Abdelkrim El-ghayoury,a Antoine Skouliosb and Raymond Ziessel*a
a Laboratoire de Chimie, d’Electronique et de Photonique Moléculaires, Ecole Européenne de Chimie, Polymères et
Matériaux, Université Louis Pasteur, ESA 7008 au CNRS, 25 rue Becquerel, F-67087 Strasbourg Cedex 2, France.
E-mail: ziessel@chimie.u-strasbg.fr
b Groupe de Matériaux Organiques, IPCMS, UMR 7504 au CNRS, 23 rue du Loess, F-67037 Strasbourg Cedex ,
France
Received (in Basel, Switzerland) 26th May 1999, Accepted 1st September 1999
In the presence of copper(i), certain di- and tri-topic ligands
derived from reaction of substituted anilines with 2-pyr-
idinecarbaldehyde or 2,6-pyridinedicarbaldehyde form a
new type of polycatenar metallomesogen around the tetra-
hedral metallic centre.
for [Cu(L)n)2]BF4 (n = 1–3) 1–3}. Mass spectrometry is
consistent with the isolated products being monomeric without
contamination from polynuclear structures. These products,
therefore, may be assigned the general formula [Cu(Ln)2]BF4.†
A schematic representation of the complexes and a computer
generated CPK model of one of the structures is shown in Fig.
1.
Metal complexes derived from Schiff-bases are known to form
a large variety of molecular architectures, ranging from
macrocyclic helicates to infinite coordination polymers.1,2 In
particular, 2-iminopyridines, readily formed via reversible
reaction between an (achiral or chiral) amine and an appropriate
aldehyde, are attractive building blocks for assembling intricate
supramolecular species3 and active polymerization catalysts.4,5
These ligands can display unusual binding behaviour towards
certain cations, leading to the formation of highly stable
metallohelicates.6 The main advantage of such ligands, how-
ever, relates to the availability of a free imino site that can be
functionalized with a wide variety of appendages. Indeed, the
structural attributes provided by 2-iminopyridines look highly
promising for the assembly of metallomesogens by attaching
flexible paraffinic tails and subsequent rigidification of the
head-group by complexation with copper(i) cations. It is worth
noting that silver complexes constructed from polycatenar
scaffoldings and which exhibit columnar liquid-crystalline
mesophases have previously been studied.7
Fig. 1(a) Idealized pictoral representation of complex 2 representing the
disk-shape and (b) CPK energy-minimised conformation.
Direct involment of the terminal imino groups in the
coordination sphere was confirmed by solid-state and solution-
phase FTIR studies and by both 1H and 13C NMR spectroscopy.
The NMR studies also indicate that the two ligands bound to
Cu(i) are equivalent while the relatively broad signals belonging
to the aromatic protons, together with a broad imine signal (for
complexes 1 and 2) are attributed to a fast exchange process
between coordinated and free imino N atoms. Similar internal
flexibility has been observed previously for related macro-
cyclic10 and terpyridine11 based copper(i) complexes. Solid
state FTIR studies of complex 2 reveal the presence of two
imino stretching vibrations; one lying close to that of the free
ligand (nCNN 1626 cm21) and the second occurring at lower
frequency (nCNN = 1588 cm21) owing to coordination to the
metal centre. X-Ray crystallography made for a single crystal of
complex 3 indicates that two L3 ligands are wrapped around a
single copper cation, each ligand being coordinated via a
pyridine-imine fragment, while the pendant imino arm is
directed away from the metal centre in a trans configuration
(Fig. 2).‡
Ligands L1 and L2 were prepared in 99 and 86% isolated
yield, respectively by refluxing 2-formylpyridine or 2,6-di-
formylpyridine and the corresponding aniline8 in ethanol
containing a trace amount of acetic acid. Each ligand has a well
defined melting point at 58 °C (L1) on 56 °C (L2). Ligand L3,
which serves as a reference compound, was synthesized by a
similar route and has a melting point of 159 °C.
Reaction of either ligand in dichloromethane with 0.5 equiv.
of [Cu(MeCN)4]BF4 leads to immediate formation of a deep-
red colouration indicative of complexation of copper(i) to four
nitrogen donor ligands9 {lmax = 505, 500 (sh) and 500 (sh) nm
with e = 5100, 2600 and 3000 dm3 mol21 cm21, respectively
It is noteworthy that, despite the cis arrangment of the
coordinated py-CHNN- fragment and the trans conformation of
the uncoordinated imino subunit, the overall ligand adopts an
almost planar structure. The four-coordinate copper(i) cation
shows Cu–N(py) and Cu–N(imine) distances of ca 2.085 and
2.021 Å, respectively with chelate bite angles of 81.6 and 81.1°.
The uncoordinated imino fragment lies ca. 4.706 Å from the
cation. There is obvious distortion around the metallic centre
that might explain why the molecule stacks into columns,
favouring a liquid-crystalline phase (vide infra), a situation
which is also clearly observed in the CPK energy-minimised
conformation [Fig. 1(b)].
Chem. Commun., 1999, 2033–2034
This journal is © The Royal Society of Chemistry 1999
2033

appended alkyl chains influence the liquid-crystalline proper-
ties, especially melting temperature, and to establish the
structural and thermodynamic factors that assembles the
aromatic cores into columns. Such information, which is critical
for a proper theoretical description of metallomesogens, is not
yet available for any liquid-crystalline material. We are well
aware that the polycatenar ligand L2 is an attractive substrate for
reaction with metal cations that favour octahedral coordination
geometries.
We thank Professor Anthony Harriman for his critical review
of the manuscript and for the energy-mimimised structure and
Dr. Benoît Heinrich (IPCMS) for fruitful and helpful discus-
sions.
Fig. 2 ORTEP view of complex 3 showing 50% probability thermal
ellipsoids. The hydrogen atoms are omitted for clarity.
Notes and references
Although non-mesomorphic themselves, ligands L1 and L2
produce thermotropic liquid-crystalline complexes when co-
ordinated to copper(i), as demonstrated by differential scanning
calorimetry (DSC) and polarizing optical microscopy (POM).
The DSC thermograms of 2, recorded from 20 to 130 °C,
contain two sharp peaks, each of which indicates a reversible
first-order phase transition. The peak at 49 °C (DH = 149.2 kJ
mol21) corresponds to melting of the crystal into a liquid-
crystalline phase whereas the peak at 117 °C (DH = 3.1 kJ
mol21) can be attributed to clearing of the liquid crystal into an
isotropic melt (values are given for the third cycle). The high
stability of complex 2 was demonstrated by the absence of
significant perturbation of the DSC patterns following several
heating–cooling cycles. The optical textures observed for 2
during slow cooling from the isotropic melt are typical of a
columnar phase (with pseudo focal-conic textures). This
birefringent texture is maintained at room temperature. In
contrast, complex 1 melts into the liquid crystal phase at 48 °C
(DH = 84.6 kJ mol21) and has a clearing point at 75 °C (DH =
10.3 kJ mol21), but only on the first heating stage. It appears,
therefore, that complex 1 is thermally unstable, a feature not
entirely unexpected in view of the lack of substituents at the
6-position.
† Full synthetic details will be given elsewhere. All new compounds were
authenticated by NMR, FTIR, MS and elemental analyses (required values
in parentheses). Selected data: L1: dCNN 8.64 (CDCl3); nCNN 1626 cm21
(KBr pellet, FAB+ m/z 1023.3 [M + H]+, C, 78.59 (78.62), H, 10.62 (10.83),
N, 2.57 (2.74%). L2: dCNN 8.72 (CDCl3), nCNN 1626 cm21 (KBr pellet);
FAB+ m/z 1967.3 [M + H]+, C, 78.43 (78.72), H, 10.53 (11.01), N, 1.99
(2.13%). 1: 92% yield; dCNN 9.18 (CDCl3); nCNN 1589 cm21 (KBr pellet);
FAB+ m/z 2109.2 [M-BF4]+, C, 72.98 (73.24), H, 9.73 (10.09), N, 2.23
(2.55%). 2: 98% yield; dCNN 8.79 (CDCl3); nCNN, 1626, 1588 cm21 (KBr
pellet); FAB+ m/z 3996.8 [M 2BF4]+, C, 75.12 (75.50); H, 10.21 (10.61);
N, 1.53 (2.05%, calculated with one water molecule).
‡ Crystal data for 3: C42H38N6O4Cu•2BF4•H2O•CH2Cl2, M = 1785.26,
¯
triclinic, space group P1, red crystals, a = 12.6180(7), b = 13.075(1), c =
13.783(1) Å, a = 84.437(9), b = 80.643(9), g = 70.631(9), V = 2114.4 Å3,
Z
= 1, m =
0.645 mm21. Data were collected on a Kappa CCD
diffractometer (graphite Mo-Ka radiation, l = 0.71073 Å) at 2100 °C.
15930 reflections collected (2.5 ≤ 2q ≤ 26.36°), 4186 data with I > 3s(I).
The structure was solved using the Nonius OpenMoleN16 package and
refined by full-matrix least squares with anisotropic thermal parameters for
all non-hydrogen atoms except for the solvent molecules (the latter are
disordered). Final results: R(F) = 0.079, wR(F) = 0.105, GOF = 1.189,
542 parameters. CCDC 182/1402.
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The columnar structure of the liquid crystal phases of 1 and
2 was confirmed by X-ray diffraction studies. It is seen that the
rigid aromatic units that comprise the core of the pseudo-
tetrahedral copper(i) complexes are superposed on top of one
another and embedded in a disordered matrix provided by the
molten alkyl chains. These columns are packed laterally into a
two-dimensional hexagonal unit cell having parameters of 60
and 47 Å for 1 and 2, respectively as measured at 60 °C from the
small-angle reflection. The larger value found for 1 suggests
that a ‘phosmidic-type’ of columnar mesophase is formed
(where several individual molecules aggregate to form a disk),
whereas for 2 the individual molecules stack one on top of the
other to form columns as in conventional discoid liquid
crystalline material.
Liquid crystals obtained from purely organic molecules
comprising a tetrahedral carbon atom substituted with four
semi-rigid subunits bearing flexible terminal alkyl chains have
been reported.12 However, complexes 1 and 2 are, to the best of
our knowledge, the first examples of metallomesogens built
around a single tetrahedral metal cation. Many unsuccessful
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tempted in the past.13,14 As such, these materials differ
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