Supramolecular helical stacking of metallomesogens derived from enantiopure and racemic polycatenar oxazolines

Article abstract of DOI:10.1021/ja0294313

The present report undertakes a challenge of general interest in supramolecular chemistry: the achievement of helical organizations with controlled structure. To achieve this target we considered the possibility of inducing supramolecular chirality using molecules that were designed to organize into columnar mesophases. The use of oxazoline-derived ligands and metal coordination served as tools to prepare molecules with a phasmidic-like structure, which show columnar organization in the liquid crystalline state. To ensure the formation of chiral mesophases, these complexes bear stereogenic centers in the rigid coordination environment of the metal. X-ray and circular dichroism experiments have revealed that chirality transfer does indeed take place from the chiral molecule to the columnar liquid crystal organization. This chiral columnar organization appears as a helix consisting of stacks of molecules that rotate with respect to one another along the column while maintaining their mean planes parallel to each other. In fact, it has been concluded that packing of these polycatenar molecules must be more efficient upon rotation of a molecule with respect to the adjacent one along the column. Furthermore, the same type of helical supraorganization has been found to be present in the mesophase of the racemic mixture and the mixture of diastereomers prepared from the racemic ligand. In this case, segregation of the optical isomers is proposed to occur to give rise to both types of helix (right-handed and left-handed).

Full text of DOI:10.1021/ja0294313

Published on Web 03/22/2003
Supramolecular Helical Stacking of Metallomesogens Derived
from Enantiopure and Racemic Polycatenar Oxazolines
Joaqu´ın Barbera´,† Emma Cavero,† Matthias Lehmann,‡ Jose´-Luis Serrano,*,†
Teresa Sierra,† and Jesu´s T. Va´zquez§
Contribution from Qu´ımica Orga´nica, Facultad de Ciencias-I.C.M.A., UniVersidad de
Zaragoza-C.S.I.C., 50009-Zaragoza, Spain, Laboratoire de Chimie des Polyme`res CP206/1,
UniVersite´ Libre de Bruxelles, BouleVard du Triomphe, 1050 Bruxelles, Belgium, and
Instituto de Bio-Orga´nica Antonio Gonza´lez, UniVersidad de La Laguna,
Ctra. De la Esperanza, 2, 38206-La Laguna, Tenerife, Spain
Received November 21, 2002; E-mail: joseluis@unizar.es
Abstract: The present report undertakes a challenge of general interest in supramolecular chemistry: the
achievement of helical organizations with controlled structure. To achieve this target we considered the
possibility of inducing supramolecular chirality using molecules that were designed to organize into columnar
mesophases. The use of oxazoline-derived ligands and metal coordination served as tools to prepare
molecules with a phasmidic-like structure, which show columnar organization in the liquid crystalline state.
To ensure the formation of chiral mesophases, these complexes bear stereogenic centers in the rigid
coordination environment of the metal. X-ray and circular dichroism experiments have revealed that chirality
transfer does indeed take place from the chiral molecule to the columnar liquid crystal organization. This
chiral columnar organization appears as a helix consisting of stacks of molecules that rotate with respect
to one another along the column while maintaining their mean planes parallel to each other. In fact, it has
been concluded that packing of these polycatenar molecules must be more efficient upon rotation of a
molecule with respect to the adjacent one along the column. Furthermore, the same type of helical
supraorganization has been found to be present in the mesophase of the racemic mixture and the mixture
of diastereomers prepared from the racemic ligand. In this case, segregation of the optical isomers is
proposed to occur to give rise to both types of helix (right-handed and left-handed).
Introduction
ferro- and antiferroelectric switching and second-order optical
properties)4 in the former and the special optical features (i.e.,
A major challenge in supramolecular chemistry is the design
of simple molecular units that are capable of organizing into
chiral architectures, the essence of which is helicity in different
spatial manifestations. In this context, helical organizations have
been described for supramolecular systems based on metal
coordination,1 H-bonding interactions,2 and mesomorphic (ther-
motropic as well as lyotropic) systems.
These last systems have been found to be a versatile tool to
obtain materials with controlled supramolecular chirality3 that
promotes the appearance of interesting properties for practical
applications. Traditionally, SmC* and cholesteric mesophases
have been the most widely investigated chiral organizations in
liquid crystals. Thus, polar order (and, hence, electrooptical
selective reflection)5 of the latter are the main consequences of
the presence of chiral molecules in the SmC and the nematic
phases, respectively.
Likewise, helical order is also possible in liquid crystals based
on columnar organizations.6 These are currently the target of
studies by several authors, which range from a basic interest in
the knowledge of the helical assembly and how to manipulate
it7 to the achievement of promising properties for practical
applications such as nonlinear optical behavior (i.e., SHG)8 or
electrooptical switching.9,10
(3) Chirality in Liquid Crystals; Kitzerow, H. S., Bahr, C., Eds.; Springer:
New York, 2001.
(4) Lagerwall, S. T. Ferreoelectric and Antiferroelectric Liquid Crystals; Wiley-
VCH: Weinheim, 1999.
† Universidad de Zaragoza-C.S.I.C.
‡ Universite´ Libre de Bruxelles.
(5) Tamaoki, N. AdV. Mater. 2001, 13, 1135-1147.
§ Universidad de La Laguna.
(6) (a) Maltheˆte, J.; Jacques, J.; Tinh, N. H.; Destrade, C. Nature 1982, 298,
46-48. (b) Green, M. M.; Ringsdorf, H.; Wagner, J.; Wu¨stefeld, R. Angew.
Chem., Int. Ed. Engl. 1990, 29, 1478-1481.
(1) (a) Provent, C.; Williams, A. In Transitions Metals in Supramolecular
Chemistry; Sauvage, J. P., Ed.; John Wiley and Sons Ltd.: Chichester,
1999; Chapter 4, p 135. (b) Jung, O. S.; Kim, Y. J.; Lee, Y. A.; Park, J.
K.; Chae, H. K. J. Am. Chem. Soc. 2000, 122, 9921-9925.
(2) (a) Geib, B. S. J.; Vicent, C.; Fan, E.; Hamilton, A. D. Angew. Chem., Int.
Ed. Engl. 1993, 32, 119. (b) Lightfoot, M. P.; Mair, F. S.; Pritchard, R. G.;
Warren, J. E. Chem. Commun. 1999, 1945-1946. (c) Forman, S. L.;
Fetinger, J. C.; Pieraccini, S.; Gotarelli, G.; Davis, J. T. J. Am. Chem. Soc.
2000, 122, 4060-4067. (d) Fenniri, H.; Mathivanan, P.; Vidale, K. L.;
Sherman, D. M.; Hallenga, K.; Wood, K. V.; Stowell, J. G. J. Am. Chem.
Soc. 2001, 123, 3854-3855. (e) Gandgopadhyay, P.; Radhakrishnan, T.
P. Angew. Chem., Int. Ed. Engl. 2001, 40, 2451-2455.
(7) (a) van Nostrum, C. F.; Bosman, A. W.; Gelink, G. H.; Schouten, P. G.;
Warman, J. M.; Shouten, A. J.; Nolte, R. J. M. Chem. Eur. J. 1995, 1,
171-182. (b) Engelkamp, H.; Middelbeek, S.; Nolte, R. J. M. Science 1999,
284, 785-788. (c) Palmans, A. R. A.; Vekemans, J. A. J. M.; Havinga, E.
E.; Meijer, E. W. Angew. Chem., Int. Ed. Engl. 1997, 36, 2648-2651. (d)
Nuckolls, C.; Katz, T. J. J. Am. Chem. Soc. 1998, 120, 9541-9544. (e)
Nuckolls, C.; Katz, T. J.; Verbiest, T.; van Elshocht, S.; Kuball, H. G.;
Kiesewalter, S.; Lovinger, A. J.; Persoons, A. J. Am. Chem. Soc. 1998,
120, 8656-8660. (f) Nuckolls, C.; Katz, T. J.; Katz, G.; Collings, P. J.;
Castellanos, L. J. Am. Chem. Soc. 1999, 121, 79-88.
9
10.1021/ja0294313 CCC: $25.00 © 2003 American Chemical Society
J. AM. CHEM. SOC. 2003, 125, 4527-4533
4527

A R T I C L E S
Barbera´ et al.
Given the possibilities that metal coordination offers to
molecular design, we have been using it as a means to achieve
molecular shapes capable of organizing into chiral superstruc-
tures. Previous studies were based on discotic metal â-diketo-
nates that showed a rectangular columnar mesophase whose
helical structure was extensively studied, as well as its ferro-
electric behavior.11 The origin of the supramolecular chirality
in these systems was the presence of stereogenic centers in the
peripheral tails (derived from (S)-lactic acid) in the molecule.
Recently, we considered the possibility of inducing the
transfer of chirality from the rigid part of the molecule to the
supramolecular organization in the mesophase. Previous work
carried out in our group on aryloxazolines derived from (S)-â-
amino alcohols demonstrated the great potential of this type of
compounds in the design of mesogenic structures that display
chiral mesophases. For example, SmC* ferroelectric mesophases
with high Ps values were obtained when the chiral oxazoline
moiety was linked to the appropriate mesogenic core.12 Further
work was undertaken based on the ability of phenyloxazolines
to coordinate to metals. Planar mesogenic complexes were
obtained that induced broad cholesteric ranges when used as
chiral dopants in nematic hosts.13
Figure 1. Chemical structure of the polycatenar palladium(II) (Pd(S-C12)2)
and copper(II) (Cu(S-C12)2) oxazoline complexes.
symmetry, with its binary axis perpendicular to the coordination
plane. This arrangement causes the two methyl groups in the
stereogenic centers to lie on the same side of the molecular
plane. Both complexes have been deeply investigated by X-ray
diffraction and circular dichroism in order to elucidate the
possible helical organization in the mesophase. Taking into
account the geometry of the molecule and the strong influence
of both stereogenic centers in the intermolecular interactions,
we decided to go deeper into the study of supramolecular
organization. Our aim was to assess the influence of both aspects
on the supramolecular organization and also to answer an
intriguing question: Does the helical organization maintain in
the mesophase of the nonoptically active mixtures [Pd(S-C12)2
+ Pd(R-C12)2 and Pd(RS-C12)2]?. For this reason, we extended
our investigation to the enantiomeric complex as well as to
nonoptically active mixtures of complexes. Thus, the enantiomer
Pd(R-C12)2, the racemic mixture, Pd(S-C12)2 + Pd(R-C12)2, and
the palladium(II) complex of the racemic ligand (resulting in a
diastereomeric mixture, Pd(RS-C12)2) were prepared. Palladium-
(II) derivatives were chosen for this study according of
preliminary results that indicated the possibility of producing
well-aligned samples of the palladium(II) complex, Pd(S-C12)2.
Furthermore, palladium(II) complexes yield higher diffraction
intensities due to the presence of the heavier palladium atom.
Studies have recently been expanded to encompass different
mesogenic structures, such as elongated polycatenar molecules
(phasmidic-like) that can give rise to columnar mesomorphism.14
As a first attempt to obtain chiral columnar systems from
aryloxazolines, chiral ligands were prepared derived from 3,4,5-
trialkoxybenzoic acid. Complexation of these ligands with
copper(II) or palladium(II) yielded planar phasmidic-like com-
plexes that did not show mesomorphic behavior unless an
electron acceptor such as TNF was added in certain proportions.
These complexes were, however, shown to have high helical
twisting powers in appropriate nematic hosts.15
On the basis of these results, and in order to achieve
polycatenar metallomesogens that are able to induce helical
columnar organizations, we have designed a new oxazoline-
derived ligand with an extended aromatic core and six dodecyl-
oxy terminal chains. Coordination to palladium(II) or copper(II)
results in phasmidic-like planar complexes, i.e., Pd(S-C12)2 and
Cu(S-C12)2 (Figure 1). X-ray studies of related chiral oxazoline
complexes have demonstrated that upon complexation only the
trans isomer is formed.16 Hence, the molecule should have C2
Results and Discussion
Synthesis.17 The synthetic pathway for all the compounds is
outlined in Scheme 1. The synthesis of the enantiomeric and
racemic 4,5-dihydro-2-(2′,4′-dihydroxyphenyl)-4-methyloxazole
(1) was carried out as reported previously.12 The corresponding
starting (S)-, (R)- and (RS)- amino alcohols were purchased from
Aldrich. The 3,5-bis(3,4,5-tridodecyloxybenzoyloxy)benzoic
acid (4) was prepared by esterification of benzyl 3,5-dihydroxy-
benzoate (3) with 2 equiv of 3,4,5-tridodecyloxybenzoyl chloride
(2). The synthesis of 3 was carried out through a solid-state
phase-transfer reaction between the potassium salt of 3,5-
dihydroxybenzoic acid and benzylbromide, catalyzed by tetra-
butylammonium chloride following the method described by
Keller.18
(8) (a) Verbiest, T.; van Elshocht, S.; Kauranen, M.; Hellemans, L.; Snauwaert,
J.; Nuckolls, C.; Katz, T. J.; Persoons, A. Science 1998, 282, 913-915.
(b) Fox, J. M.; Katz, T. J.; van Elshocht, S.; Verbiest, T.; Kauranen, M.;
Persoons, A.; Thongpanchang, T.; Krauss, T.; Brus, L. J. Am. Chem. Soc.
1999, 121, 3453-3459.
(9) (a) Bock, H.; Helfrich, W. Liq. Cryst. 1992, 12, 697-703. (b) Bock, H.;
Helfrich, W. Liq. Cryst. 1995, 18, 387-399. (c) Bock, H.; Helfrich, W.
Liq. Cryst. 1995, 18, 707-713. (d) Heppke, G.; Kru¨erke, D.; Mu¨ller, M.;
Bock, H. Ferroelectrics 1996, 179, 203-209. (e) Scherowsky, G.; Chen,
X. H. Liq. Cryst. 1994, 17, 803-810. (f) Scherowsky, G.; Chen, X. H. J.
Mater. Chem. 1995, 5, 417-421. (g) Kru¨erke, D.; Rudquist, P.; Lagerwall,
S. T.; Sawade, H.; Heppke, G. Ferroelectrics 2000, 243, 207-220.
(10) Nuckolls, C.; Shao, R.; Jang, W. G.; Clark, N. A.; Walba, D. M.; Katz, T.
J. Chem. Mater. 2002, 14, 773-776.
(11) (a) Barbera´, J.; Iglesias, R.; Serrano, J.; Sierra, T.; de la Fuente, M. R.;
Palacios, B.; Pe´rez-Jubindo, M. A. J. Am. Chem. Soc. 1998, 120, 2908-
2918. (b) Serrano, J. L.; Sierra, T. Chem. Eur. J. 2000, 6, 759-766.
(12) Serrano, J. L.; Sierra, T.; Gonza´lez, Y.; Bolm, C.; Weickhardt, K.; Magnus,
A.; Moll, G. J. Am. Chem. Soc. 1995, 13, 4374-4381.
(13) Lehmann, M.; Marcos, M.; Serrano, J. L.; Sierra, T.; Bolm, C.; Weickhardt,
K.; Magnus, A.; Moll, G. Chem. Mater. 2001, 117, 8312-8321.
(14) Nguyen, H. T.; Ch. Destrade; Maltheˆte, J. AdV. Mater. 1997, 9, 375-388.
(15) Lehmann, M.; Sierra, T.; Barbera´, J.; Serrano, J. L.; Parker, R. J. Mater.
Chem. 2002, 12, 1342-1350.
(16) Go´mez-Simo´n, M.; Jansat, S.; Muller, G.; Panyella, D.; Font-Bad´ıa, M.;
Solans, X. J. Chem. Soc., Dalton Trans. 1997, 3755-3764.
1
Ligands and complexes were identified by H NMR, 13C
NMR, FT-IR, and elemental analysis. The 1H NMR spectra of
both enantiomeric complexes show that only one isomer is
(17) See Supporting Information.
(18) Keller, P.; Hardouin, F.; Mauzac, M.; Achard, M. F. Mol. Cryst. Liq. Cryst.
1988, 155, 171-178.
9
4528 J. AM. CHEM. SOC. VOL. 125, NO. 15, 2003

Supramolecular Helical Stacking of Metallomesogens
A R T I C L E S
Scheme 1. Synthetic Pathway for Ligands and Complexes
Table 1. Mesomorphic Behavior of Ligands and Complexes.
compound
t, °C(∆H, kJ/mol)a
S-C12
R-C12
RS-C12
Cu(S-C12)2
Pd(S-C12)2
Pd(R-C12)2
Pd(RS-C12)2
Pd(S-C12)2 + Pd(R-C12)2
C
C
C
C
C
C
C
C
69.8 (109.3)
63.7 (104.7)
66.0 (101.8)
-6.5 (43.9)
1.7 (30.1)
2.2 (28.8)
3.9 (25.0)
2.7 (50.8)
I
I
I
Colr
97.2 (34.2)
I
I
I
I
I
Colr 109.5 (53.2)
Colr 110.2 (53.7)
Colh 110.6 (40.6)
Colh 119.3 (50.7)
well as the racemic mixture [Pd(S-C12)2 + Pd(R-C12)2] was
studied by polarizing optical microscopy and DSC. Transition
temperatures and their associated enthalpy changes are gathered
in Table 1. The ligands do not show liquid crystalline properties
in any case. However, all the pure complexes, as well as the
mixtures of the palladium(II) complex isomers, display meso-
morphic behavior at room temperature and over a wide
temperature range. Thus, interactions between the aromatic cores
of the complexes give rise to stacking and hence overcome the
negative steric effect of the methyl groups in the stereogenic
centers, both of which lie out of the planar core.15
Examination of the complexes by polarizing microscopy did
not provide useful information about the type of mesophase
shown by these complexes. When observed between crossed
polarizers, the pure enantiomers appear black (homeotropic-
like texture) on cooling from the clearing temperature (measured
by DSC). In samples prepared without a cover glass, a gray
texture appeared in both cases, but this was not representative
of any particular liquid crystalline phase. Both the racemic
mixture [Pd(S-C12)2 + Pd(R-C12)2] and the diastereomeric
mixture [Pd(RS-C12)2] give rise to a gray texture on cooling,
both with and without a cover glass. The columnar phases shown
by the complexes are designated in Table 1 as rectangular
[Cu(S-C12)2, Pd(S-C12)2, Pd(R-C12)2] or hexagonal [Pd(RS-C12)2,
Pd(S-C12)2 + Pd(R-C12)2] according to X-ray powder diffraction
analysis (see below).
Structure of the Columnar Mesophase. All the complexes
were studied by X-ray diffraction with the aim of elucidating
the type(s) of mesophase and determining the details of their
structures. Most of the experiments were performed at room
temperature; however, in the case of Pd(S-C12)2 high-temperature
experiments were also carried out.
Likewise, the chirality of the supramolecular organization was
studied at different temperatures by circular dichroism in order
to corroborate the conclusions withdrawn from X-ray studies.
X-ray Diffraction Studies. In the cases of Pd(S-C12)2,
its enantiomer Pd(R-C12)2, the racemic mixture [Pd(S-C12)2 +
Pd(R-C12)2] and the diastereomeric mixture [Pd(RS-C12)2], we
were able to obtain oriented X-ray patterns at room temperature.
The samples were aligned by shearing with a metal or glass
rod on the inner wall of the capillary when the sample was held
at a temperature close to its clearing point. The use of this
technique allowed us to obtain fiber patterns with the fiber axis
parallel to the stretching direction. The patterns revealed that
the columns are oriented parallel to the capillary axis and hence
perpendicular to the X-ray beam.
present in each case. It is expected that this is the trans isomer
on the basis of reported X-ray diffraction studies.16 The 1H NMR
spectrum of the complex prepared from the racemic ligand RS-
C12 shows different chemical shifts for some aromatic hydro-
gens. These hydrogens belong to the different diastereomers
formed upon coordination of the racemic ligand RS-C12 with
palladium(II) (see Supporting Information). Analysis of the
spectrum reveals a ratio of 0.25:0.25:0.5 of Pd(S-C12)2/Pd(R-
C12)2/Pd(S-C12)(R-C12), respectively, within a diastereomeric
mixture that, hereafter, will be referred to as Pd(RS-C12)2. This
result indicates that the formation of Pd(RS-C12)2 is statistical,
as expected from the same reactivity of both enantiomeric
ligands against palladium(II). The racemic mixture Pd(S-C12)2
+ Pd(R-C12)2 was prepared by dissolving equal amounts of the
two enantiomeric complexes, Pd(S-C12)2 and Pd(R-C12)2, in
methylene chloride and evaporating the solvent. Characterization
of the copper(II) complex Cu(S-C12)2 was carried out by FT-IR
and elemental analysis. NMR spectroscopy was not a suitable
technique given the paramagnetic nature of the metal center.
Mesomorphic Behavior. The thermal and thermodynamic
behavior of the ligands (S-C12, R-C12, RS-C12) and their
complexes [Cu(S-C12)2, Pd(S-C12)2, Pd(R-C12)2, Pd(RS-C12)2] as
(a) Mesophase Structure of the Pure Enantiomers. In the
oriented patterns of pure enantiomer Pd(S-C12)2, or Pd(R-C12)2,19
a set of spots is observed at low angles (Figure 2a), the intensity
of which is maximum in the equator (plane perpendicular to
9
J. AM. CHEM. SOC. VOL. 125, NO. 15, 2003 4529

A R T I C L E S
Barbera´ et al.
Table 2. X-ray Results for the Mesophases of the Complexesa
temp meso-
(oC) phase
dobs
(Å)
d
(Å)
lattice
constants (Å)
calc
compound
hkl
c
Pd(S-C12)2
25 Colr 200, 110
41.0
30.1
23.2
18.5
40.5
30.6
23.4
18.6
15.3
10.1
a ) 81
210
310, 020
410
510, 420, 130 15.3
800, 440
930, 060
b ) 46.8
c ) 3.8
10.2
7.8
7.80
4.4b
3.8
001
3.8
41.0
31.0
23.7
18.8
15.5
10.2
80 Colr 200, 110
41.9
30.4
23.4
18.5
a ) 82
b ) 47.3
c ) 3.85
210
310, 020
410
510, 420, 130 15.5
800, 440
930, 060
10.4
8.1
7.88
4.5b
3.85
41.0
23.5
15.9
11.7
9.5
7.84
6.74
6.74
4.4b
3.8
001
3.85
40.7
d
Pd(RS-C12)2
25 Colh 100
a ) 47
c ) 3.8
110
210
220
320
330
430
600
23.5
15.4
11.75
9.34
7.83
6.69
6.78
001
25 Colr 200, 110
210
3.8
41.5
31.4
Cu(S-C12)2
41.5
31.9
23.6
a ) 83
b ) 47.9
310, 020
23.95 c ) 3.75
510, 420, 130 15.2
15.7
10.4
7.98
800, 440
930, 060
10.6
7.9
4.5b
3.75
001
3.75
Figure 2. Oriented X-ray pattern (a), rectangular lattice parameters (b),
and proposed helical model (c) for the complex Pd(S-C12)2.
a The columns of the table show, respectively, the compound code, the
temperature, the type of mesophase, the proposed indexation, the observed
and calculated spacings, and the lattice constants. b Broad, diffuse maximum.
cData corresponding to Pd(R-C12)2 are identical to those of its enantiomer,
Pd(S-C12)2. Data corresponding to the racemic mixture [Pd(R-C12)2 + Pd(S-
C12)2] are identical to those of the mixture of diastereomers, Pd(RS-C12)2.
d
At middle angles (Figure 2a), a pair of diffuse spots appears
at both sides of the meridian (direction parallel to the column
axis). This is indicative of a modulation of the electronic density
and suggests the existence of a superlattice periodicity in the
direction of the stacks of molecules. The periodicity of this
modulation measured along the column axis is 11.4 Å. It is
only possible to account for this modulation if we assume that
the molecules adopt a helical stacking in the column.21 The
absence of scattering at the meridian itself is a consequence of
the helical structure with its axis parallel to the column axis.
Another interesting point to be noted is a diffuse, nearly
isotropic ring corresponding to a mean distance of 4.4 Å and,
in the outermost part, a relatively sharp arc centered in the
meridian at 3.8 Å. The diffuse ring is characteristic of the
liquidlike order between the aliphatic chains, whereas the strong
arc corresponds to the intracolumnar stacking distance (inter-
molecular Pd-Pd distance).
Figure 3. Powder X-ray pattern of the Colr mesophase of Pd(S-C12)2 taken
at room temperature. Inset: small-angle pattern with the proposed indexation
(410 and 510 reflections are very weak)
the capillary axis). These maximums are related to a two-
dimensional (2D) array of columns. This array is better
characterized from the powder patterns (Figure 3), which provide
more accurate spacings that can be indexed in a 2D rectangular
lattice (hk0 spots, Table 2). The lattice constants of this
rectangular lattice are a ) 81 Å, b ) 46.8 Å. The absence of
mixed reflections (reflections with h or k and l simultaneously
different from 0) confirms the mesomorphic nature of this
structure and rules out a 3D organization.20
From density considerations it can be deduced that the 2D
unit cell contains two columns, one located at the center and
the other at the corner of the rectangle (Figure 2b). It is
interesting to note that the lattice constants a and b are in the
(19) In the following discussion, we will refer only to the S-enantiomer, Pd(S-
C
12)2, since X-ray results are identical and reveal equal parameters for both
enantiomers.
(20) An X-ray pattern of Pd(S-C12)2 was taken at high temperature. The structure
of the mesophase at 80 °C is the same as at room temperature, except for
a slight increase in the lattice parameters.
(21) (a) Levelut, A. M. J. Phys. Lett. 1979, 40, 81-84. (b) Levelut, A. M.;
Oswald, P.; Ghanem, A.; Maltheˆte, J. J. Phys. 1984, 745-754. (c) Levelut,
A. M.; Maltheˆte, J.; Collet, A. J. Physique 1986, 47, 351-357.
9
4530 J. AM. CHEM. SOC. VOL. 125, NO. 15, 2003

Supramolecular Helical Stacking of Metallomesogens
A R T I C L E S
Figure 4. Powder X-ray pattern of the Colr mesophase of Cu(S-C12)2 taken
at room temperature. Inset: small-angle pattern with the proposed indexation
(the 210 reflection is partially overlapped with the intense 200 reflection)
ratio x3, which is characteristic of a hexagonal structure.
However, the presence of the reflections indexed 210 and 410
is not consistent with a hexagonal symmetry. A hexagonal cell
is equivalent to a C-centered orthorhombic cell with lattice
constants a and b in the ratio x3. A C-centered orthorhombic
symmetry implies that the molecule located at (1/2,1/2,0) has the
same orientation as the molecule located at (0,0,0), and thus all
the (hk0) reflections with an odd value for h + k should be
systematically absent. This is not the case here, as revealed by
the presence of the extra reflections.
Figure 5. Oriented X-ray pattern for the racemic mixture Pd(S-C12)2
Pd(R-C12)2 (a) and the mixture of diastereomers Pd(RS-C12) (b), and
hexagonal lattice parameter (c).
+
Hence, it can be concluded that, although the column axes
are located at the nodes of a hexagonal lattice, the molecules
must be tilted and there is an alternation in the tilt direction.
The resulting space group is P21, and the rectangular symmetry
arises from a heringbone-like arrangement of the elliptical
sections of the columns when viewed along the column axes
(Figure 2b).21b The same kind of pseudohexagonal arrangement
has been found in a number of nonchiral22 and chiral11a columnar
mesophases.
When we compare the periodicity of the helix, measured from
the middle-angle scattering, and the stacking distance, measured
from the meridional reflection, we find a ratio between the two
periods of 11.4/3.8 ) 3. This means that one helix period
contains three stacked molecules. As this kind of complex has
2-fold symmetry, in fact three molecules make half a turn and
thus the helical pitch is 11.4 × 2 ) 22.8 Å (Figure 2c). All
these data are consistent with a columnar structure in which
the stacked molecular cores are tilted with respect to the column
axis and parallel to each other; however, the molecules do rotate
around the normal to their mean planes. Thus, two neighboring
molecules are mutually rotated by an angle of 360°/6 ) 60°.
The fact that the 3.8-Å reflection is centered in the meridian
suggests that the palladium atoms are located approximately
on top of each other, meaning that they are arranged in a row
along the column axis.
The X-ray pattern of the copper(II) complex [Cu(S-C12)2] has
the same appearance, but in this case, good single domains could
not be obtained. However, powder experiments performed at
room temperature (Figure 4) led us to conclude that the
mesophase of Cu(S-C12)2 is rectangular columnar with P21
symmetry and lattice constants a ) 83 Å, b ) 48 Å (Table 2).
These constants are similar to those determined for Pd(S-C12)2
and the a/b ratio is still x3. As we were unable to obtain well-
oriented patterns, we could not unambiguously establish that
there is a helical stacking of the cores of the molecules.
However, a diffuse maximum can be discerned at middle angles.
This halo is similar to that observed for Pd(S-C12)2, which
appeared split in oriented patterns and was believed to be related
to the helical organization. The distance between neighboring
molecules in a stack is still defined and has a value similar to
that found for the palladium(II) complexes.
(b) Mesophase Structure of Nonoptically Active Mixtures.
It is interesting to compare the structure of the mesophases of
the pure enantiomer Pd(S-C12)2 to that of its racemic mixture,
Pd(S-C12)2 + Pd(R-C12)2, and that of its diastereomeric mixture,
Pd(RS-C12)2. In the case of the mixtures (Figure 5), the columnar
mesophase is hexagonal with a lattice constant a of ∼47 Å
(Figure 5c), as deduced from the powder experiments. The
meridional region of the oriented patterns (Figure 5a and b)
contains the same features as the pure enantiomer, which shows
that the stacked molecules generate the same type of helical
configuration. The measured pitch period and stacking distance
have, within the experimental error, practically the same values
as for Pd(S-C12)2. The middle-angle scattering is clearly visible
in the original patterns. From these results, we propose a model
in which segregation between optical isomers takes place and
this would account for the subsistence of this modulation in
the mixtures.21b However, it is difficult to assess whether
segregation takes place to give different columns (each column
containing only one optical isomer)23 or within each column
(segregated domains of each optical isomer along the column
axis).
It is interesting to note that in the mixtures of optical isomers
of [Pd(RS-C12)2 and Pd(R-C12)2 + Pd(S-C12)2] despite the fact
that the symmetry of the mesophase is hexagonal rather than
rectangular, the dimensions of the unit cell are practically
equivalent to those of the pure S-enantiomer: A hexagonal
lattice with a constant a ) 47 Å is equivalent to a C-centered
rectangular lattice with a ) 81.4 Å, b ) 47 Å (a/b ) x3) and
two molecules per unit cell. However, the difference between
a rectangular lattice with a P21 symmetry and a hexagonal lattice
(23) Trzaska, S. T.; Hsu, H.-F.; Swager, T. M. J. Am. Chem. Soc. 1999, 121,
4518-4519.
(22) Weber, P.; Guillon, D.; Skoulios, A. Liq. Cryst. 1991, 9, 369-382.
9
J. AM. CHEM. SOC. VOL. 125, NO. 15, 2003 4531

A R T I C L E S
Barbera´ et al.
is the fact that in the rectangular lattice the director (axis
perpendicular to the molecular planes) has a fixed direction in
each column and the mutual orientation of the directors of
neighboring columns is defined by the symmetry of the lattice,
in particular by the 2-fold screw axis. Due to the similarity of
the measured lattice constants, the molecules of the isomer
mixtures in the hexagonal columnar mesophase are also
probably tilted, but a correlation does not exist between the tilt
direction of different columns.21b A hexagonal symmetry
therefore results, as confirmed by X-ray diffraction (all the
observed reflections are consistent with a 2D hexagonal lattice).
It can be concluded that the difference between the mesophases
of the pure enantiomers and of the mixtures does not concern
the organization within each column and is due only to
modifications in the correlations between the columns.
Circular Dichroism Studies. Circular dichroism (CD)
experiments were carried out in order to study the effect of the
proposed helical organization on the optical activity of the
materials. Experiments were performed for the corresponding
isolated molecule, i.e., in hexane solution, and for thin films of
neat samples prepared by sandwiching the material in question
between two quartz plates. The sample thickness was not
rigorously controlled, and therefore, ellipticity values are not
significant for the spectra of neat samples. Experiments on the
bulk material were performed at different relevant temperatures
(according to the DSC trace) in order to establish whether the
observed optical activity was dependent upon the organization
within the mesophase. During the experimental procedure, and
in order to avoid linear effects due to a possible anisotropy of
the orientation in the mesophase, several CD spectra were
recorded as the sample was rotated through successive 60°
increments around the light beam, while maintaining a constant
temperature. Each CD spectrum shown here is the averaged
result of all the measured spectra for a given sample.
Figure 6. (a) CD spectral comparison of the complex Pd(S-C12)2 in the
mesophase, in the isotropic liquid and in solution (10-mm cell, 1.07 × 10-5
M in hexanes). (b) Uv spectrum in the region under study of compound
Pd(S-C12)2, in the mesophase.
At first sight, the CD spectrum corresponding to the meso-
phase appears significantly different from those of the isolated
molecule (solution) and the molecule in an isotropic environment
(isotropic state) for both of the complexes studied, i.e., Pd(S-
C12)2 (Figure 6) and Cu(S-C12)2 (Figure 7). In each case, the
spectra taken from the isotropic melt and from the hexane
solution exhibit Cotton effects at the same wavelength and with
identical signs. These become negative exciton couplets when
the spectra are registered at the temperature of the mesophase.
Overlapping of CD signals at this wavelength region may be
responsible for a slight shift of the deflection points of the
exciton couplets with respect to the maxima of absorption in
the UV spectra.
The stereogenic centers of the corresponding molecules are
contained in the rigid coordination plane of the complex, and
hence, a change in the chiral skeleton of the molecule would
not be expected to account for the observed change in the CD
spectra. Indeed, this change should arise from the modification
of the chiral environment of the chromophores due to some sort
of chiral supramolecular organization, i.e., a helical superstruc-
ture, as deduced from the X-ray experiments discussed above.
Moreover, we can confirm the suggestion that the copper
complex should adopt the same type of helical organization in
the mesophase as the palladium analogue, as deduced from
X-ray experiments, even in the absence of sufficient data from
oriented X-ray patterns.
Figure 7. (a) CD spectral comparison of the complex Cu(S-C12)2 in the
mesophase, in the isotropic liquid, and in solution (10-mm cell, 6 × 10-5
M in hexanes). (b) UV spectrum in the region under study of compound
Cu(S-C12)2 in the mesophase.
As expected, the two enantiomers [Pd(S-C12)2 and Pd(R-C12)2]
give rise to CD spectra of opposite sign (Figure 8). Moreover,
the S-configuration of the stereogenic centers of complexes Pd-
(S-C12)2 and Cu(S-C12)2 seems to bias their helixes toward a
left-handed sense (M), as evidenced by the negative exciton CD
couplets centered at 380 and 350 nm, respectively.24 In contrast,
9
4532 J. AM. CHEM. SOC. VOL. 125, NO. 15, 2003

Supramolecular Helical Stacking of Metallomesogens
A R T I C L E S
plex, despite the fact that an oriented X-ray diffraction pattern
could not be obtained.
Rotation of one molecule with respect to the adjacent one
within the column is the method by which these molecules fill
empty space, thus improving molecular packing within the
mesophase. This process allows a better arrangement of the
chains around the central core of the column and thus prevents
the steric hindrance that could otherwise occur between the
chains of neighboring molecules due to the difference in the
preferred core-core (3.8 Å) and side-chain (4.4 Å) distances.
This occurs in the chiral derivatives as well as in the nonoptically
active mixtures. In the former, the rotation gives rise to a helix
whose sense (right or left) is determined by the configuration
of the corresponding stereogenic centers. In the nonoptically
active mixtures, the helical organization deduced from X-ray
diffraction is proposed to form through spatial segregation of
the optical isomers, which organize themselves into both types
of helix.
Figure 8. CD spectra of the pure enantiomers Pd(S-C12)2 and Pd(R-C12)2
showing opposite signs.
the CD spectrum taken in the mesophase of Pd(R-C12)2 shows
a positive exciton CD couplet that corresponds to a right-handed
helix (P).
Conclusion
The use of metal coordination has allowed the preparation
of polycatenar oxazoline complexes that show columnar meso-
phases with a helical organization within the column. The
diffraction patterns of enantiomerically pure Pd(S-C12)2 [and
hence of Pd(R-C12)2], as well as of their racemic mixture Pd-
(S-C12)2 + Pd(R-C12)2 and the mixture of diastereomers Pd-
(RS-C12)2, are consistent with a structural model in which an
assembly of helical columns adopts a 2D packing. One full
helical turn occurs every six molecules: a rotation of 60° by
each molecule, spaced 3.8 Å apart, produces a pitch of 22.8 Å.
These results, in conjunction with CD experiments, reveal that
chirality transfer takes place from the stereogenic centers in the
rigid core to the supramolecular organization. The same
conclusion can be extended to the oxazoline-copper(II) com-
The work described here represents a significant example of
a successful combination of coordination chemistry and liquid
crystal self-organization processes to achieve and control
supramolecular helical architectures.
Acknowledgment. This work was supported by the C.I.C.Y.T.
(Projects MAT1999-1009-CO2-02 and MAT2000-1293-CO2-
01) and by the European Project FMRX-CT97-0121 (Dr. M.
Lehmann’s fellowship).
Supporting Information Available: Additional information
as noted in text. This material is available free of charge via
the Internet at http://pubs.acs.org.
(24) Nakanishi, K.; Berova, N.; Woody, R. W. Circular Dichroism: Principles
and Applications; VCH Publishers: New York, 1994.
JA0294313
9
J. AM. CHEM. SOC. VOL. 125, NO. 15, 2003 4533