Engineering of an iron-terpyridine complex with supramolecular gels and mesomorphic properties

Article abstract of DOI:10.1039/b515186d

A new iron-terpyridine complex based on a 3,5-diacylaminotoluene platform bearing two phenyl rings appended with aliphatic chains provides robust and transparent organo-gels and liquid-crystalline materials. The LC mesophase was characterized by small-ang

Full text of DOI:10.1039/b515186d

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LETTER
www.rsc.org/njc | New Journal of Chemistry
Engineering of an iron–terpyridine complex with supramolecular gels and
mesomorphic properties
Franck Camerel,a Raymond Ziessel,*a Bertrand Donniob and Daniel Guillonb
Received (in Montpellier, France) 26th October 2005, Accepted 2nd December 2005
First published as an Advance Article on the web 14th December 2005
DOI: 10.1039/b515186d
A new iron–terpyridine complex based on a 3,5-diacylamino-
toluene platform bearing two phenyl rings appended with aliphatic
chains provides robust and transparent organo-gels and liquid-
crystalline materials. The LC mesophase was characterized by
small-angle X-ray diffraction, and is described as a columnar
phase with a rectangular lattice symmetry. Based on the measured
parameters, a molecular packing model is suggested in which two
complexes aggregate together to form a pseudo elliptical plateau;
the latter then pile up to form the columns.
cells,16 and luminescence devices.1b,17 Immobilization of terpy
ligands and complexes on polymer resins could prove to be of
importance in the field of molecular catalysis.18
Although system-containing terpy offer an enormous struc-
tural diversity and tunability in terms of potential properties,
stability and processability, examples of terpy based organo-
gelators or mesomorphic ligands and complexes are scarce. Up
to now, only dinuclear copper(I) metallo-mesogens involving
segmented and bulky terpy ligands appeared to behave as
liquid crystalline materials,19 thus contrasting with the non-
mesomorphic uncomplexed ligand. Recent studies have de-
scribed terpy-containing carboxylic acids that form hydro-
gels, in which the three-dimensional network is provided by
electrostatic interactions induced by proton transfer from the
acidic to the basic nitrogen atoms of the terpy ligand.20
In the present letter, the design of terpy chelates tethered to
a central 3,5-diacylaminotoluene platform equipped with two
lateral aromatic rings each bearing three appended aliphatic
chains is presented. This new ligand allows both organogels
and mesomorphic materials stabilized by extended hydrogen
bonded networks to be obtained. It is also demonstrated that
the complexation of this functional molecule to iron(II) cation
allows the production of original metallo-organogelators and
metallo–mesogens.
A challenging research area in modern chemistry is the
design and synthesis of multifunctional compounds and soft
materials with desirable or predictable properties such as
luminescence, transport of information, catalytic activity,
nano-structuration and macroscopic ordering.1,2 For this pur-
pose, a steady and progressive interest is devoted to the
synthesis of oligopyridines and an impressive number of such
functional molecules have already been designed. Oligopy-
ridine compounds exhibit interesting synthetic facets and some
of them were found to show important antibiotic and fungi-
cidal character. Moreover, they form stable complexes with
many cations. We recently argued the case that balancing the
rigid core of phenanthroline–copper complexes with flexible
appendages allowed the production of organogels and meso-
phases over a large temperature range.3
Along these lines 2,20:60,200-terpyridine (terpy) ligands ap-
peared to be particularly attractive.4 The resulting complexes
have many potential applications in macromolecular science,
biochemistry, photophysics, and nanoscience. In particular,
terpy and its multiple derivatives have advantageously been
used as building blocks for the engineering of spiral lines,5
dendrimers,6 micelles,7 polymers,8,9 and liquid crystalline ma-
terials.10 Formation of ordered architectures on surfaces11–13
and functional molecular devices for single-atom transistors14
are particularly challenging. Recently, biochemical applica-
tions were reported describing the potential use of terpy-
complexes as sensors in medical research and as polypeptide
binding agents.15 Ruthenium–terpy complexes have received
much attention in the field of energy suppliers such as solar
a
´
Laboratoire de Chimie Moleculaire, Ecole de Chimie, Polymeres,
´
`
´
´
Materiaux (ECPM), Universite Louis Pasteur (ULP), UMR 7509
(CNRS-Universite´ Louis Pasteur), 25 rue Becquerel, 67087
Strasbourg Cedex 02, France. E-mail: ziessel@chimie.u-strasbg.fr;
Fax: 33390242635; Tel: 33390242689
Ligand L was prepared by a convergent strategy adapted
from the already reported preparation of phenanthroline
based 4-methyl-3,5-diacylamidophenyl platforms.21 After
purification, ligand L was obtained as a sticky and soft
material and the observation of a birefringent texture by
polarized optical microscopy points to its mesomorphic state.
b
´
Institut de Physique et Chimie des Materiaux de Strasbourg
(IPCMS), Groupe des Materiaux Organiques (GMO), UMR 7504
´
(CNRS-Universite Louis Pasteur), 23 rue du Loess, BP 43, F-67034
Strasbourg Cedex 2, France
´
ꢀc
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The methyl group bisecting the central phenyl was introduced
to force the amide vectors to point out of the plane and to
extend the dimensionality of the hydrogen bonded network.
Ligand L displays a FT-IR spectrum consistent with hydrogen
bonding through the amide groups: nNH at 3320 cmꢁ1, nCO at
1660 cmꢁ1 and dNH 1517 cmꢁ1 (in the solid state).22 The Ru-
complex was prepared by reacting two equivalents of L with
[Ru(DMSO)4Cl2]23 in refluxing chloroform. The resulting
orange-red complex was isolated as the chloride salt by
column chromatography, with nNH at 3368 cmꢁ1 (nNH), at
1651cmꢁ1 (nCO) and at 1526 cmꢁ1 (dNH), and (ES-MS: m/z
3583.2 [M ꢁ Cl]1 and 1774.2 [M ꢁ 2Cl]21). The Fe-complex
was directly obtained by mixing stoechiometric amounts of the
ligand with Fe(ClO4)2 ꢂ 6H2O under anaerobic conditions.
This complex exhibits a molecular peak at 3601.1 [M ꢁ
ClO4]1 and 1751.2 [M ꢁ 2ClO4]21 in the ES-MS and a solid
amide groups are involved in a tight hydrogen bonding net-
work nNH at 3279 cmꢁ1, nCO at 1641 and 1662 cmꢁ1 and dNH
1519 cmꢁ1 (in the L-gel), and nNH at 3317 cmꢁ1, nCO at 1662
and 1643 cmꢁ1 and dNH at 1519 cmꢁ1 (in the Fe-gel). It is
likely that this supramolecular network is responsible for the
formation of fibers which aggregate in a 3D network of
interlocked fibers leading to the gelation of cyclohexane
(Fig. 2).3,24
From a general point of view it is well-known that iron(II)
salts have a very high affinity for terpyridine ligands and
indeed such chelates are widely used to determine infinitesimal
amounts of iron salts in analytical tests.25 As a consequence,
one interesting aspect of the nanostucturation of L would be
to understand the consequence of iron salt diffusion in the L-
gel. Doping the L-gel by a solution of cyclohexane–THF
(10%) containing around 0.1 equiv. of Fe(II), does not break
the gel but the characteristic violet color of the complex readily
developed in the viscous solution (Fig. 1d). In marked contrast
when iron dust is sprayed over the L-gel, the progressive
formation of the complex (over one week, Fig. 3), irreversibly
destroys the gel, probably due to the presence of excess salts.
Previous studies have shown that the presence of transition
metal ions has a pronounced effect on the gelation ability of
ligands whereas in some cases the formation of the gel is
completely suppressed.26 It is noteworthy to observe the
formation of the complex under these conditions due to the
very low solubility of the salt in cyclohexane.
state FT-IR spectrum with bands located at 3273 cmꢁ1 (nNH
,
very large), at 1660 and 1644 cmꢁ1 (nCO) and at 1520 cmꢁ1
(dNH), showing the presence of two different types of hydrogen
bonds involving the amide functions.
The gelation abilities of the free ligand and associated metal
complexes were tested by heating a cyclohexane solution and
then cooling to room-temperature. L forms a colorless and
transparent organogel (Fig. 1a) which is stable over a long
time (at least during a six months time-scale period).
Interestingly, the Fe-complex also provides a transparent
highly colored gel in cyclohexane (Fig. 1b) with a minimum
gelation concentration (MGC) of 3% (w/w), whereas its Ru-
analogue did not lead to a gel under the same conditions (Fig.
1c). For both complexes the color is imported by the presence
of a strong metal-to-ligand charge transfer state expected by
the presence of the terpyridine ligands lmax ¼ 479 nm for Ru
with e ¼ 15 000 Mꢁ1 cmꢁ1 and lmax ¼ 555 nm for Fe with e ¼
7900 Mꢁ1 cmꢁ1. The organogels exhibited thermally reversible
sol–gel transitions and the FT-IR spectra revealed that the
The ability of L and its corresponding Fe-complex to form
robust and transparent gels made of fibers prompted us to
study their liquid-crystalline properties by heating the solvent-
free materials.
The thermotropic behavior of L was studied by interplaying
thermogravimetric analyses (TGA), differential scanning ca-
lorimetry (DSC), polarized optical microscopy (POM) and X-
ray diffraction. L is liquid crystalline from room temperature
up to 225 1C, the temperature at which a reversible transition
Fig. 1 a) Gelation test in cyclohexane of L in cyclohexane (2.6%
(w/w)); b) Gel obtained with the complex [FeL2](ClO4)2 in cyclohexane
at a concentration of 3.1% (w/w) after heating and cooling cycle; c)
Solution of the ruthenium complex [RuL2](Cl)2 in cyclohexane
(C ¼ 3.3% (w/w)); d) L-gel doped with ca. 0.1 equiv. of Fe(II).
0.1 mL of a solution of cyclohexane–THF (10%) containing 0.1 equiv.
of Fe(ClO4)2 was deposited on the L-gel in cyclohexane–THF (10%)
(6.1% (w/w); 10 mg in 0.2 mL of cyclohexane–THF (10%) mixture).
Gel formation is confirmed when the sample does not flow once the
test tube is turned upside-down.
Fig. 2 SEM picture of freeze dried gel of the Fe–terpy gel from
cyclohexane showing formation of interlocked and elongated objects
(C ¼ 3.2% (w/w)).
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Fig. 3 Solution obtained when Fe(ClO4)2 dust is sprayed over a L-gel (3.3% (w/w)) after 3 days (b), after 5 days (c) and after 7 days (c). The salt
dust is visible in the bottom of the flask.
is detected on both heating and cooling by DSC (DH ¼ 21.8 J
ꢁ1). An X-ray diffraction pattern registered at 120 1C un-
of a lamellar phase (d ¼ 26.1 A at 120 1C). Above 225 1C, this
compound is in the isotropic liquid state.
g
ambiguously confirms that the compound is liquid crystalline
(Fig. 4a). The broad halo A at ca. 4.6 A corresponds to the
average distance between the molten chains. Two slightly less
diffuse halos B and C were observed at 7.25 and 3.8 A,
respectively and were attributed to some liquid-like (short-
range) correlations between neighboring molecules. Three
reflections, a sharp and intense peak followed by two less
intense higher orders, are observed in the small angle region
and indicate the long-range positional ordering of large ob-
jects. The small-angle reflections could be indexed as the
fundamental 001, and two harmonics 002 and 003 reflections
The Fe-complex decomposes above 180 1C as observed by
TGA analysis. No phase transition was detected by DSC
measurements from the ambient up to the degradation tem-
perature. Observations under polarized optical microscope
reveal a birefringent soft and malleable material which is a
good indication of its liquid-crystalline behavior. No recog-
nizable texture could be observed, partly due to the impossi-
bility to reach the isotropic liquid, and thus to grow natural
texture free of paramorphotic defects. In order to elucidate the
molecular organization of these mesostructured materials, a
series of temperature-dependent wide and small-angle X-ray
scattering (WAXS, SAXS) measurements on powder samples
were performed. An X-ray pattern obtained at 150 1C dis-
played in the wide angle regime a diffuse and broad scattering-
halo A centered at 4.6 A, indicative of a liquid-like order of the
aliphatic chains and thus of the fluid-like nature of the phase
(Fig. 4b). Another diffuse scattering halo associated to a
distance of 7.5 A (halo B), is also observed and was attributed
to some short-range stacking of the molecules. In the small-
angle region, the XRD pattern displayed four reflections
which can be indexed in the p2gg rectangular symmetry as
(hk) ¼ (11), (20), (21), (22) with lattice parameters a ¼ 51.5 and
b ¼ 69.6 A (S ¼ a.b ¼ 2 ꢃ Scol ¼ 3584.4 A2) at 150 1C. X-ray
patterns obtained at various temperatures between the ambi-
ent and 180 1C are totally consistent with a rectangular
columnar mesophase stable from room temperature to
180 1C. Quite remarkably the structurally related Ru complex
is devoid of mesomorphic behaviour. The absence of gels and
liquid crystal properties for the [Ru(L)2]Cl2 complex might be
related to the nature of the counter-anion. The effects of the
counter-anions in ionic metallomesogens2b have been investi-
gated in detail by several research groups but their influences
on the mesomorphic properties cannot be simply rationalized,
and still remain highly speculative.2b It is admitted that
perchlorate anions are hydrogen bond acceptors and could
be engaged in multiple hydrogen bonds with NH groups,27
contrasting with the behaviour of the chloride ions. Thus, the
presence here of perchlorate ions in the Fe-complex might help
in giving structure to the soft material by rigidifying and
reinforcing the cohesion of the molecular architecture.
In order to get some insight of the packing of the Fe-
complexes inside the columns and of the 2D arrangement of
the latter in the rectangular lattice, a standard geometrical
Fig. 4 a) XRD pattern of L at 120 1C; b) XRD pattern of the Fe
complex at 150 1C.
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To compensate for the width of the complex, greater than the
height of the stratum (w 4 h), an axially shifted packing of
these dimeric species may be envisaged. In this way, an
optimal micro-segregation between the aliphatic parts and
the polar parts inside a plateau is ensured, which is not
achievable in the case of linear conformation of the complex.
The main interest in this class of compounds stems from its
facile synthesis and ready substitution. A vast range of deri-
vatives and complexes could be anticipated with properties
tuned to particular applications. In short, L is representative
of a new class of organo-gelator and liquid crystalline ligand
able to chelate a variety of different metals. The molecular
dimensions of these molecules and their attributes are such
that they are able to make gels with solvents. At this point,
diffusion of iron in ligand gels is feasible to some extent but
detrimental at high concentration. The emergent metallo-
mesogen has many features of merit due to its stability over
a large range of temperature and to the possibility to import
properties of the coordinated metal such as anisotropy, po-
larisability, photo- and electroactivity and magnetic features.
The testing of these supramolecular soft materials in electro-
optics and photonics is currently in progress.
Experimental
Syntheses
Ligand L. A Schlenk flask equipped with a septum and an
argon inlet was charged with 3,5-bis(3,4,5-tridodecyloxyben-
zoylamino)-4-methyl benzoic acid (0.35 g, 1.1 equiv., 0.23
mmol) dimethylaminopyridine (DMAP) (0.06 g, 2.2 equiv.,
0.47 mmol) in anhydrous CH2Cl2 (50 mL). The mixture was
stirred until complete solubilisation of the acid. 1-[3-(dimethy-
lamino)propyl]-3-ethylcarbodiimide hydrochloride (EDCI)
(0.09 g, 2.2 equiv., 0.47 mmol) and 40-aminomethyl-2,20;60200-
terpyridine (0.06 g, 1 equiv., 0.21 mmol) were added to the
clear solution and stirred overnight. After evaporation of the
solvent, purification was performed by flash chromatography
on silica gel with CH2Cl2/MeOH (99.9/0.1 to 98/2) and
followed by crystallisation from CH2Cl2/CH3CN (0.200 g,
Fig. 5 a) CPK model of the Fe complexes in a V-shape conformation
and their arrangement in a dimer to form the elementary plateau. b)
Illustration of the good space filling of the rectangular unit cell with
lattice parameters a ¼ 51.5 and b ¼ 69.6 A.
1
3
56%). H NMR (CDCl3, 300 MHz) d 0.89 (t, J ¼ 4.5 Hz,
treatment was applied.28 Thus, the packing may be character-
ized by two structural parameters:29 the columnar cross-sec-
tion, Scol, and the stacking periodicity h along the columnar
axis, both parameters being analytically linked through the
hScol ¼ NVm relation, where N is the number of molecules
within a columnar stratum h-thick and Vm is the molecular
volume. Considering a density close to 1 (ca. 0.9 at 150 1C), the
molecular volume was found to be about Vm ¼ 6725 A3.
Taking h ¼ 7.5 A as the columnar stratum thickness (and
corresponding to the halo B), an exact value of two is obtained
for N, indicating that there are two molecular equivalents of
the Fe complex per such defined plateau. With these additional
data in hand, a CPK model was constructed, as illustrated in
Fig. 5, which allows for a good description of the space filling
of the rectangular mesophase unit cell. The counter-anions are
arbitrarily located in the unoccupied volumes created by this
molecular arrangement. The complexes likely adopt a V-shape
in order to introduce interactions between the terpyridine
fragments facilitating their aggregation into pseudo dimers.
18H, CH3), 1.26 (m, 108 H, CH2), 1.72 (m, 12H, CH2), 2.21 (s,
3
3H, CH3), 3.95 (m, 12H, OCH2), 4.40 (d, J ¼ 3.96 Hz, 2H,
3
4
CH2), 7.20 (m, 6H, H arom.), 7.66 (td, J ¼ 7.7 Hz, J ¼ 1.7,
2H, CH), 7.90 (s, 2H, CH), 8.01 (s, 2H, CH), 8.26 (d, 3J ¼ 7.9
Hz, 2H, CH), 8.49 (d, 3J ¼ 4.3 Hz, 2H, CH), 8.65 (s, 3H, NH).
IR (KBr): 3437, 3271, 2923, 2853, 1640, 1584, 1521, 1494,
1468, 1426, 1406, 1384, 1334, 1263, 1232, 1115 cmꢁ1. MS
(FAB1, mNBA): m/z (%) ¼ 1724.2 (100) [M þ H]1. Anal.
Calcd (%) for C110H174N6O9: C, 76.61; H, 10.17; N, 4.87;
Found (%): C, 76.52; H, 10.04; N, 4.75.
[Ru(L)2](Cl)2. Ligand L (0.066 g, 0.038 mmol) and Ru(DM-
SO)4Cl2 (0.013 g, 0.026 mmol) were heated at 80 1C during 6 h
in a mixture of CHCl3 (10 mL) and EtOH (10 mL). During the
course of the reaction the color turned progressively brown.
The complex was isolated by column chromatography on
silica using a gradient of methanol (1 to 10%) in dichloro-
methane. The deep orange complex was recrystallized by slow
evaporation of dichloromethane from dichloromethane–
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acetonitrile mixture (0.060 g, 87%). IR (KBr): 3369, 2923,
2853, 1719, 1651, 1582, 1526, 1494, 1465, 1434, 1378, 1334,
1222, 1114 cmꢁ1. Anal. Calcd (%) for C220H348N12O18RuCl2:
C, 72.97; H, 9.69; N, 4.64; Found (%): C, 72.39; H, 9.31; N,
4.29.
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[Fe(L)2](ClO4)2. To an argon degassed dichloromethane
solution (20 mL) of ligand L (0.082 g, 0.047 mmol), a degassed
methanol solution (5 mL) containing Fe(ClO4)2 ꢂ 6H2O (0.009
g, 0.026 mmol) was progressively added and the solution
turned instantaneously deep-violet. The complex was isolated
by slow evaporation of dichloromethane from dichloro-
methane–acetonitrile mixture (0.080 g, 92%). IR (KBr):
3273, 2922, 2853, 1660, 1645, 1583, 1526, 1520, 1495, 1469,
1456, 1427, 1379, 1335, 1230, 1114 cmꢁ1. Anal. Calcd (%) for
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C
220H348N12O18FeCl2O8: C, 71.34; H, 9.47; N, 4.54; Found
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¨
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