A propeller-like uranyl metallomesogen

Article abstract of DOI:10.1021/ja056664w

Uranyl triflate forms with three imidazo[4,5-f]-1,10-phenanthroline ligands a propeller-like complex that exhibits a hexagonal columnar phase. The ligand is not liquid-crystalline, but a mesophase is induced upon complex formation with the uranyl salt. Th

Full text of DOI:10.1021/ja056664w

Published on Web 11/24/2005
A Propeller-like Uranyl Metallomesogen
Thomas Cardinaels,† Jan Ramaekers,† Daniel Guillon,‡ Bertrand Donnio,*,‡ and Koen Binnemans*,†
Department of Chemistry, Katholieke UniVersiteit LeuVen, Celestijnenlaan 200F, 3001 LeuVen, Belgium, and
Groupe des Mate´riaux Organiques, Institut de Physique et Chimie des Mate´riaux de Strasbourg,
UMR 7504 CNRS-UniVersite´ Louis Pasteur, BP43, 23 rue du Loess, F-67034 Strasbourg Cedex 2, France
Received September 28, 2005; E-mail: Koen.Binnemans@chem.kuleuven.be; bdonnio@ipcms.u-strasbg.fr
The attractiveness of metal-containing liquid crystals (metal-
lomesogens) is that coordination of the mesogenic ligands to the
metal center allows the building of molecular edifices with
geometries that are impossible to achieve by all-organic liquid
crystals.1 Moreover, induction of liquid-crystalline behavior can be
achieved in nonmesomorphic ligands upon coordination to metal
ions. Thanks to intensive research efforts in the field of metal-
lomesogens during the last two decades, nearly every metal of the
periodic system has been incorporated into liquid crystals. Although
lyotropic uranium-containing liquid crystals have been known for
a long time,2 it was not until recently that examples of thermotropic
uranium-containing metallomesogens have been reported. Sinn and
co-workers described calamitic uranyl complexes of â-diketonate3
and tropolonate ligands,3,4 whereas Sessler’s mesomorphic uranium-
alaskaphyrin complexes are the first examples of discotic uranium-
containing metallomesogens.5 Very recently, Aiello et al. reported
on mesomorphic uranyl Schiff base complexes.6 In all these
thermotropic liquid crystals, the uranium is present in the hexavalent
oxidation state as the dioxouranium(VI) or uranyl cation.
The classic design strategy to obtain columnar phases for
metallomesogens is to incorporate the metal ion into the central
cavity of a flat macrocyclic ligand with peripherally attached alkyl
chains (e.g., liquid-crystalline metallophthalocyanines).1 However,
tris-â-diketonate complexes with an octahedral metal center and
an overall propeller-like molecular shape can exhibit columnar
mesophases, as well.7,8
Figure 1. Structure of the uranyl complex. The two noncoordinating triflate
counterions have been omitted.
Here, we present a new approach to the design of propeller-like
metallomesogens, which is based on the ability of the linear uranyl
cation to form complexes by coordination of ligands in the
equatorial plane. The idea was to replace the 1,10-phenanthroline
ligands in the previously reported9,10 [UO2(phen)3][OTf]2 complex
(where OTf ) CF3SO3 or triflate) by an imidazo[4,5-f]-1,10-
phenanthroline moiety bearing three long alkoxy chains. This ligand
was prepared by first oxidizing 1,10-phenanthroline to 1,10-
phenanthroline-5,6-dione,11 followed by reaction with 3,4,5-tris-
(tetradecyloxy)benzaldehyde and ammonium acetate in hot glacial
acetic acid.12 The uranyl complex was synthesized by reaction
between the ligand and uranyl triflate (3:1 molar ratio) in ethanol.
The structure of the uranyl complex is shown in Figure 1. Although
we were not able to obtain single crystals of the uranyl complex,
it is reasonable to assume that its first coordination sphere is
comparable with that observed for the [UO2(phen)3][OTf]2 complex,
with an equal population of right-handed and left-handed helices.9,10
The coordination polyhedron can be described as a bi-end-capped
trigonal antiprism. The choice of the triflate ion as counterion is
not arbitrary because weakly coordinating anions are required to
avoid competition of the anion with the phenanthroline ligands for
binding to the uranyl ion.
Figure 2. Natural optical texture of the uranyl complex at 170 °C (500×
magnification).
The liquid-crystalline properties of the complex were examined
by polarizing optical microscopy (POM), differential scanning
calorimetry (DSC), and X-ray diffraction on a powder sample. The
ligand is not mesomorphic, whereas the uranyl complex melts at
95 °C into a mesophase (∆H ) 69.5 kJ mol-1) and clears into the
isotropic liquid at 181 °C (∆H ) 2.8 kJ mol-1). The fluid and
birefringent optical texture of the uranyl metallomesogen (Figure
2, POM) confirms mesomorphism, but the mesophase could not
be characterized only on this basis.
The X-ray pattern of the uranyl complex (Figure 3) is charac-
teristic of a hexagonal columnar phase (Colh). Two Bragg reflections
are observed in the small angle range: an intense and sharp
reflection at 2θ1 ) 2.31° (d1 ) 38.2 Å) and another small signal at
2θ2 ) 4.012° (d2 ) 22.0 Å). The reciprocal d spacings are in the
† Katholieke Universiteit Leuven.
‡ Institut de Physique et Chimie des Mate´riaux de Strasbourg.
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C O M M U N I C A T I O N S
Figure 4. Schematic of the two isomers and stacking of one pure
enantiomer into a polar column.
Experiments to observe photoluminescence for the complex in
the solid state or in solution were unsuccessful. The absence of
photoluminescence in this type of complex is not surprising because
it is known that, for instance, in uranyl complexes of macrocyclic
Schiff base ligands, the luminescence is quenched, as well.14
In conclusion, we presented the first example of a propeller-like
uranyl-containing metallomesogen. The uranyl ion acts as a template
to bring the ligand into the correct position, so that the supramo-
lecular assembly approximates a propeller-like repeating unit of a
hexagonal columnar phase. These metallomesogens stack in such
a way that the polar order is annihilated.
Figure 3. X-ray pattern of the uranyl complex in the hexagonal columnar
phase at 120 °C.
ratio of 1:x3, which allows their indexation (hk) in the hexagonal
2D symmetry as (10) and (11), respectively, the corresponding
lattice parameter of the Colh phase being a ) 44.05 Å. A broad
scattering band, reflecting the liquid-like organization of the
paraffinic chains (h2 ) 4.6 Å), and three other weak signals ascribed
to various intermolecular interactions were also seen. The signal
observed at 3.5-3.6 Å, h1, broad and weak, corresponds to the
average distance between successive stacking molecular cores (taken
hereafter as the stacking periodicity, h1). The broadness of this signal
indicates that such a regular stacking is simply short-range correlated
and also confirms that the bonded ligands are slightly tilted with
respect to the hexagonal lattice plane (expected to be due to the
propeller-like shape of the complex). The Bragg reflection at 6.0-
6.2 Å, h3, is likely due to the strongly scattering uranyl cation
[UO2]2+. Comparison with the crystalline structure of a homologous
compound9,10 leads us to propose that this signal may reflect
reticular planes rich in uranium. Finally, the broad signal at 7.2-
7.4 Å (h4 ≈ 2 × h1) seems to indicate that the stacking is also
alternated (staggered): starting from one molecule, the next one is
rotated by 60° in order to partially “fill” the space left unoccupied
by the first molecule. The next molecule is also rotated by 60°,
therefore, totally superposed with the first one, and so on. The
volume and the dimensions of the complex are in good agreement
with a portion of column defined by h1 × S (the columnar cross-
section S ) 1680 Å2). For one mesogen per such a slice, a density
of ca. 1 is deduced for the complex (i.e., a molecular volume of
ca. 5600 Å3 at 120 °C), reasonable in the present case due the
presence of the heavy metal atom.
The complex, which approximates a D3 symmetry, exists as two
optical isomers, ∆ and Λ, present in a 50:50 ratio (Figure 4). Thus,
packing frustrations can be explained by steric constraints and polar
order within the columns.7,8,13 To accommodate an antiferroelectric
order in a ternary symmetry lattice (Colh phase), the columnar phase
can result from the random piling of these propeller-like mesogens,
however, with important stacking faults all along the columns. For
a more efficient packing, complexes of identical absolute config-
uration about the metal center may stack on top of each other into
polar aggregates. However, to escape from the polar ordering, such
molecular bundles made of a few consecutive stacking enantiomers
are necessarily randomized within the columns.
Acknowledgment. Financial support by the FWO-Flanders
(G.0117.03) and by the K.U.Leuven (GOA 03/03) is acknowledged
gratefully. T.C. is research assistant of the FWO-Flanders. B.D.
and D.G. thank CNRS and ULP for support.
Supporting Information Available: Experimental procedures and
characterization for all compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
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