Mesomorphic phenanthroline derivatives: Novel architectures based on hydrogen bonding

Article abstract of DOI:10.1039/b110239g

Strong hydrogen bonding has been observed in mesomorphic phenanthroline compounds engineered from diacylamino-benzene platforms: a columnar organization is found in the free ligands (n = 12 or 16) whereas a smectic arrangement is evidenced in a palladium complex (n = 16).

Full text of DOI:10.1039/b110239g

Mesomorphic phenanthroline derivatives: novel architectures based on
hydrogen bonding†
Guillaume Pickaert,a Laurent Douce,a Raymond Ziessel*a and Daniel Guillonb
a Laboratoire de Chimie Moléculaire, 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 Cambridge, UK) 8th November 2001, Accepted 29th May 2002
First published as an Advance Article on the web 24th June 2002
Strong hydrogen bonding has been observed in mesomorphic
phenanthroline compounds engineered from diacylamino-
benzene platforms: a columnar organization is found in the
free ligands (n = 12 or 16) whereas a smectic arrangement is
evidenced in a palladium complex (n = 16).
pounds and intermediates were purified by careful chromatog-
raphy on silica followed by double recrystallization in adequate
solvents providing analytically pure samples. On the basis of
spectroscopic evidence, the molecular structures of the final
ligand and intermediates were unequivocally authenticated.‡
The thermotropic liquid crystalline behaviour of 1A16 and Ln
(n = 8, 12, 16) was investigated by an interplay of differential
scanning calorimetry, polarizing optical microscopy and X-ray
diffraction. The optical textures observed for the 1A16 and
ligands L12, L16 upon cooling from the isotropic melt clearly
showed the existence of a columnar phase (Col) as indicated by
the appearance of a texture exhibiting birefringent domains and
homeotropic zones. L8 revealed to be non-mesomorphic over
the tested temperature range.
The DSC thermograms registered upon heating and sub-
sequent cooling, showed the presence of peaks characteristic of
first order phase transitions. The melting into the mesophase or
the isotropic liquid involves significant enthalpy changes,
whereas the reverse transitions for the ligands induced by
cooling takes place with a strong hysteresis of about 15 to 35 °C
while the gap is only 5 °C for the precursor 1A16. The high
melting enthalpies found for L12 and L16 are due to the
destruction of strong interaction forces at the clearing transition.
The transition temperatures as well as the enthalpies for all
compounds are gathered in Scheme 2.
The columnar hexagonal mesophase was characterized by X-
ray patterns§ containing two sharp reflections in a ratio 1+1/A3
in the small angle region, and two diffuse reflections in the wide
angle region at ca 4.6 and 3.5 Å corresponding respectively to
lateral distances between molten alkyl chains and to the
classical p–p stacking distance between aromatic rings. It is
assumed that two molecules of Ln form a disk which then pile-
up one on top of the other to form the column. Based on
preliminary X-ray data of a single crystal for a model compound
a drawing representing the hydrogen bonded structures is given
in Scheme 3.13
Phenanthroline (phen) and its derivatives are powerful archi-
tectons for various purposes such as complexation properties,1,2
biological activity3 and more recently as lyotropic or thermo-
tropic liquid-crystalline materials.4,5 While we succeeded in
producing metallomesogens by wrapping non-mesomorphic
bipyridine-based frameworks around copper(
) cations,6 we
I
found this strategy inappropriate for phen scaffoldings. It is
surmized that the lack of internal flexibility around the phen/
imine core provides thermally unstable complexes which are
prejudicial to the engineering of thermotropic mesomorphic
molecules. From a general point of view, mesomorphic
derivatized-phen cores are scarce5 but very interesting in order
to obtain flat systems with extended conjugated orbitals due to
the planar conformation. It was hoped that p–p stacking of the
rigid cores would favour the emergence of mesophases, a
situation found in certain liquid-crystalline materials but more
rarely in metallomesogens.7 Another way to favour mesophases
is to functionalize the molecules with amide groups, auspicious
for stabilization of the supramolecular network via hydrogen
bonding.8,9 It is worthwhile noting that liquid-crystalline
ferrocene derivatives represent rare examples of H-bonded
metallomesogens.10
Putting together these two ideas, we here describe new phen
ligands bearing a 1,3-diacylaminobenzene core which offers a
suitable route for the inclusion of transition metals. Ligands Ln
were prepared from 1n, itself prepared by a multistep procedure
from 3,5-dinitro-p-toluic acid,11 and 2-hydroxy-9-methyl-
1,10-phenanthroline12 by an esterification reaction using
EDC·HCl and dimethylaminopyridine (Scheme 1). Final com-
Evidence for intermolecular amide hydrogen bonding was
deduced from FT-IR measurements especially by taking into
account the N–H and CNO stretching vibrations and N–H
† Electronic supplementary information (ESI) available: FT-IR spectra of
compounds L12 and 1A16 and X-ray diffraction pattern of the palladium
complex of L16. See http://www.rsc.org/suppdata/cc/b1/b110239g/
Scheme 1 (i) CH2Cl2, DMAP (1 equiv.), EDC·HCl (2 equiv.), 15 h, 73% for L8, 76% for L12 and 75% for L16
.
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This journal is © The Royal Society of Chemistry 2002

in CH2Cl2/CH3CN afforded the analytically pure mononuclear
complex [Pd(L16)Cl2] in 76% isolated yield.‡ Preliminary
microscopic and X-ray diffraction measurements suggests that
this complex is mesomorphic near rt and exhibits a Smectic A
mesophase. From X-ray data obtained at 70 °C, a layer
thickness of 34.0 0.2 Å was found in good agreement with the
length of the complex in its most extended conformation (Fig.
S3†).¶This monolayer type of organization is in strong contrast
with the columnar arrangement found in the free ligand and
might suggest that within the pallado-mesogen the hydrogen
bonding is not the driving force. Finally, we also note some
decomposition of this Pd-complex during melting in the
isotropic phase. In order to reduce the transition temperature we
are working on the synthesis of doubly substituted phen ligands.
Further experiments will afford a deeper insight into this
interesting set of mesomorphic compounds.
Scheme 2 Phase transition temperatures and corresponding enthalpies were
determined from the second heating and cooling scans of the DSC; Cr =
crystalline phase, Col = columnar phase, I = isotropic phase.
Notes and references
‡
Full synthetic details will be given elsewhere. Selected data are for
ligands: L8 FAB+ m/z 1093.7 ([M + H]+, 100), 716.7 ([M 2 C23H38NO3 +
H]+, 30); Anal. Calcd for C68H92N4O8·H2O: C, 73.48; H, 8.52; N, 5.04;
Found: C, 73.15; H, 8.27; N, 4.75%. L12 1317.4 ([M + H]+, 100), 828.6 ([M
2 C31H54NO3 + H]+, 10); Anal. Calcd for C84H124N4O8·H2O: C, 75.52; H,
9.51; N, 4.19; Found: C, 75.36; H, 9.15; N, 4.09%. L16 1542.6 ([M + H]+,
100), 940.5 ([M
2 C39H70NO3 +
H]+, 25); Anal. Calcd for
C100H156N4O8·H2O: C, 76.98; H, 10.21; N, 3.59; Found: C, 76.69; H, 10.05;
N, 3.38%. [Pd(L16)Cl2] MALDI-TOF 1683.2 ([M 2 Cl]+, 30), 1646.2 ([M
2 2Cl], 18); Anal. Calcd for C100H156N4O8·PdCl2: 69.85; H, 9.14; N, 3.26;
Found: C, 69.59; H, 8.83; N, 2.86%.
A16
§ Measured Bragg spacing lattice for 1
d
100
16
= 24.9 and d110 = 14.1 Å,
12
for L
d
100
= 29.2 and d110 = 16.7 Å, for L
d
100
= 32.9 and d110 = 18.6
Å, at 90 °C.
Scheme 3
¶ Measured Bragg spacing lattice for [Pd(L16)Cl2] d100 = 34.0 and d200
16.5 Å, at 70 °C.
=
bending vibration. Variable-temperature spectra have been
performed with KBr pellets in the various physical states and in
chloroform solutions at rt. Spectra of L12 and L16 are similar
and characterized during the heating cycle by a broadening and
a weakening of N–H stretching vibrations with shifts from 3229
cm21 in the crystalline phase to 3290 cm21 in the isotropic
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phase. In contrast the chloroform solutions (9.3 3 1023 M21
)
afford single bands at 3426 (nNH,st), 1700 (nCOO), 1677 (nCNC),
and 1525 (nNH,ben) cm21. Strong NH–hydrogen bonding is
clearly evidenced by comparing the NH stretching vibration in
the mesomorphic state versus the solution (DnNH,st = 200
cm21). The evolution of C = O stretching and N–H bending
vibrations during heating of L12 from 40 to 180 °C is provided
in Fig. S1.† In the crystalline phase the CNO stretching vibration
is characterized by a single band at 1645 cm21 and after melting
in the mesomorphic phase a shoulder clearly appears at 1655
cm21 with a simultaneous decrease of the original band. In the
isotropic phase (over 120 °C) we still observe a splitting of the
band with a concomitant shift to 1657 and 1673 cm21. These
values are in agreement with previous studies on mesogenic
hydrogen bonded amide aggregates.14 As expected, no global
weakening or shift of the CNC and COO bands are observed
during the heating process. It is worthwhile to note that similar
studies performed with the 1A16 precursor revealed the absence
of any significant perturbations of the characteristic stretching
and bending vibrations (see Fig. S2†).
13 M. Cesario and T. Prangé work in progress.
14 T. Kato, T. Kutsuna, K. Hanabusa and M. Ukon, Adv. Mater., 1998, 10,
606; R. M. Silverstein, G. C. Bassler and T. C. Morrill, Spectrometric
Identification of Organic compounds, Third edition, Wiley, New York,
USA, 1976.
When L16 was allowed to react with [PdCl2(CH3CN)2] a
deep-yellow complex was isolated which after recrystallization
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