First example of coexistence of thermal spin transition and liquid-crystal properties

Article abstract of DOI:10.1002/1521-3773(20011119)40:22<4269::AID-ANIE4269>3.0.CO;2-8

The rodlike Fe^III complex of an N-alkyloxysalicylidenyl-N′-ethyl-N-ethylenediamine ligand, shown as a computer model in the picture, is the first compound in which spin-crossover (SC) and liquid-crystalline (LC) properties coexist. This synergy should allow the magnetic and optical properties of SC compounds to be combined with the sensitivity of the LC state to electromagnetic fields.

Full text of DOI:10.1002/1521-3773(20011119)40:22<4269::AID-ANIE4269>3.0.CO;2-8

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First Example of Coexistence of Thermal Spin
Transition and Liquid-Crystal Properties
Yury Galyametdinov,* Vadim Ksenofontov,
Andrej Prosvirin, Igor Ovchinnikov, Galina Ivanova,
Philipp Gütlich,* and Wolfgang Haase
Dedicated to Professor Weyrich on the occasion
of his 60th birthday
Flexibility in the synthesis of molecular assemblies based on
molecular units is an important factor in achieving synergy of
various interesting physical properties in modern materials.
The fascinating peculiarities of spin-crossover (SC) com-
pounds[1] stimulated the idea of combining liquid-crystalline
(LC) and SC behavior in a single material, which to the best of
our knowledge has not yet been achieved.
To obtain highly ordered materials we chose the LC state as
a basis for the elaboration of supramolecular organized
systems with controlled physical properties.[2] Earlier, similar
approaches were used for the easy alignment by magnetic
fields of LC compounds[3] containing lanthanide ions with
large magnetic moments and anisotropy.[4] Liquid-crystalline
materials with giant magnetic anisotropy were thus obtained.
The realization of synergistic SC and LC properties should
lead to a combination of the sensitivity of the LC state to
electromagnetic fields with the magnetic and optical proper-
ties of SC compounds, with their well-known characteristic
responses to physical perturbation.[1] The practical application
of SC materials with LC properties as active media for
memory storage and optical devices is appealing, and the
whole traditional variety of LC technologies and techniques
can be applied.
Scheme 1. Synthesis of the liquid-crystalline Fe3‡ complexes with an LC
Schiff base ligand.
salt was obtained as black microcrystals by metathesis of the
NO3 salt in CH3OH, and its composition was confirmed by
elemental analysis.
Liquid-crystalline properties were confirmed in the crystal
state and mesophase by polarizing optical microscopy, differ-
ential scanning calorimetry (DSC), and X-ray scattering. The
sample contains rodlike molecules (Figure 1) and, in the range
388 ± 419 K, exhibits the fan-shaped texture usually attributed
to the smectic A mesophase.
As model building blocks for systems with spin-variable
properties we chose a complex of Fe3‡ with an N-alkyloxy-
salicylidenyl-N'-ethyl-N-ethylenediamine ligand. Spin-cross-
over behavior for similar complexes was found and inves-
tigated previously.[5] Pseudooctahedral complexes of Fe3‡ with
an N4O2 coordination core were prepared in a similar way to
that described in ref. [5]. Schiff base ligands with elongated
substituents were employed to provide the rodlike geometry
and hence the LC properties of the complex, which was
prepared from the condensation product of 4-(dodecyloxy-
benzoyloxy)-2-hydroxybenzaldehyde with N-ethylethylenedi-
amine (Scheme 1). The synthesis of the complex was carried
out without isolation of the intermediate products. The PF6
Figure 1. Computer simulation of the molecular structure of the LC Fe3‡
complex (counterion omitted).
Proposed molecular packing in crystal and mesophase
states, based on X-ray scattering data, are shown in Figure 2.
The sharp reflection in the small-angle region (d ˆ 21.5 Š)
detected at room temperature corresponds to the length of
l ˆ 22.6 Š of the rigid part of the molecule. Drastic changes
[*] Prof. Dr. Y. Galyametdinov, Dr. A. Prosvirin, Prof. Dr. I. Ovchinnikov,
Dipl.-eng. G. Ivanova
Kazan Physical Technical Institute, Russian Academy of Science
Sibirsky Tract 10/7, 420029 Kazan (Russia)
Fax : (‡ 7)84-32-76-50-75
E-mail: galyametdinov@sci.kcn.ru
Prof. Dr. P. Gütlich, Dr. V. Ksenofontov
Institut für Anorganische Chemie und Analytische Chemie
Johannes-Gutenberg-Universität
Staudinger Weg 9, 55099 Mainz (Germany)
Fax : (‡ 49)6131-39-22990
E-mail: p.guetlich@uni-mainz.de
Figure 2. Proposed molecular packing of the Fe3‡ complex in the crystal
and mesophase states at different temperatures: a) starting material, crystal
state: Tˆ 303 K, d ˆ 21.5 Š; b) SA phase, Tˆ 391 K, d ˆ 33.6 Š.
Prof. Dr. W. Haase
Institute of Physical Chemistry, Darmstadt University of Technology
Petersenstraûe 20, 64287 Darmstadt (Germany)
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appear at 398 K. At this temperature the formation of the
smectic layers is complete. A sharp reflection in the small-
angle region (d ˆ 33.6 Š) is detected at this temperature. This
X-ray pattern indicates a disordered smectic phase. The
length of the Schiff base complex, calculated by molecular
modeling, is 55.3 Š. The difference between the calculated
and experimental data can be explained by interpenetration
of alkyl chains of the layers (Figure 2). Alternatively, the
smaller experimental smectic layer distances can be explained
by strong thermal motion of the alkyl chains on heating in the
mesophase and/or bilayer lamellar structure of the meso-
phase.
corresponds to the HS state only, whereas the Mössbauer
spectrum measured at Tˆ 4.2 K (Figure 4c) reveals two kinds
of Fe3‡ species and confirms the incompleteness of spin
crossover at this temperature. The LS fraction of 86% is in
good agreement with that derived from magnetic measure-
ments; the rest is the HS fraction. The isomer shift and
1
quadrupole splitting values at 4.2 K are d ˆ 0.26 mms ,
1
1
DEq ˆ 0.72 mms
for the HS state and d ˆ 0.01 mms ,
1
DEq ˆ 2.90 mms for the LS state. The fractions of the
HS and LS components, derived from the spectrum at Tˆ
125 K (Figure 4b), are 52 and 48%, respectively. This means
that the spin transition temperature T1/2, at which both spin
states are present to 50%, is around 125 K. Equal areas of
both components of the LS spectra rule out the Goldanskii ±
Karyagin[6] effect or texture influence. The observed pro-
nounced asymmetry of the LS quadrupole doublet is caused
by spin ± spin relaxation, for which the observed temperature
independence of the asymmetry is typical in the range 1.5 ±
4.2 K. The relaxation effects are responsible for the broad-
ening of lines of the HS component, too. The large quadrupole
splitting for Fe3‡ (LS) indicates strong distortion of the local
N2O4 coordination environment of iron, which contributes to
the electric field gradient in addition to a noncubic valence
electron distribution in the split t2g manifold.
Magnetic properties were investigated in the temperature
range 4.5 ± 460 K (Figure 3). Clearly, the compound exhibits a
gradual spin-crossover transition. According to the magnetic
A specific feature in the magnetic behavior of the studied
system is the jump in the magnetic moment at Tˆ 380 K
associated with the crystal ± liquid crystal phase transition. To
clarify the reason for the observed magnetic anomaly, the
compound was studied in the temperature range 293 < T<
425 K. The sample was heated in an external magnetic field of
1.5 T through the crystal ± smectic phase transition. The jump
of meff from 5.04 to 5.14 mB occurs in the range of crystal ±
smectic phase transition. Further heating leads to a decrease
of meff to 5.05 mB in the isotropic phase. This value was
preserved when the sample was cooled to room temperature
in the presence of a magnetic field. Cooling in zero magnetic
field led to the value of the magnetic moment at room
temperature measured at the beginning of the experiment.
The described feature was completely reproducible. Heating
the sample in the temperature range 300 < T< 360 K in a
magnetic field of 1.5 T caused a gradual increase in the
magnetic moment. This may be
Figure 3. Temperature dependence of the effective magnetic moments
meff [mB] of the Fe3‡ complex.
susceptibility data, the transition should be almost complete
both at high and low temperatures. The value of the effective
magnetic moment per Fe ion, meff, is 5.14 mB at 400 K, instead
of the 5.9 mB expected for an Fe3‡ complex in a high-spin state.
A fraction of about 88% of low-spin Fe3‡ follows from the
magnetic susceptibility measured at Tˆ 4.6 K. The 57Fe
Mössbauer spectra measured in the range 4.2 < T< 293 K
reveal Fe3‡ in high-spin (HS) and low-spin (LS) states and
confirm the occurrence of thermal spin crossover between
these states. The room-temperature spectrum (Figure 4a)
explained by increasing orienta-
tion of microcrystalline particles
with anisotropic paramagnetic
susceptibility, as the viscosity of
intergrain boundaries in pre-
transitional regions decreases.
This effect is similar to the
known phenomenon of magnet-
ic field induced texture forma-
tion (thermomagnetic process-
ing) in paramagnetic powder
samples.[7] The orientation ef-
fect reaches its maximum in the
vicinity of the transition from
the crystalline (Cr) to smectic
(SA) phase because of a drastic
decrease in viscosity in the pre-
Figure 4. Mössbauer spectra at room temperature (a), 125 K (b), and 4.2 K (c). Shaded subspectra correspond to
3‡
Fe3‡(HS) (
) and Fe (LS) (- - - -).
* * * *
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control of better than Æ0.05 K. Differential scanning calorimetry measure-
ments were carried out on a Perkin-Elmer DSC-2M differential scanning
calorimeter (scan rate of 5 Kmin 1).
transitional temperature range. In the smectic phase increas-
ing temperature leads to the disappearance of long-range
order, and hence the susceptibility decreases. The behavior of
the magnetic susceptibility on cooling in a magnetic field of
1.5 T, with a change from the isotropic to liquid-crystalline
phase (Figure 5) may be explained by magnetic anisotropy
The high-temperature X-ray measurements where obtained with a STOE
STADI 2 diffractometer, equipped with a linear position-sensitive detector
(STOE mini PSD). Monochromatic CuKa radiation was obtained by using a
curved germanium detector (111 plane). The temperature-dependent
susceptibilities in the range 4.2 ± 450 K of the powdered metallomesogen
were recorded by using a Faraday-type magnetometer, equipped with an
enhanced heating device operating in the range 300 ± 450 K and an applied
field of 1.5 T.[8]
Received: July 3, 2001 [Z17417]
[1] P. Gütlich, A. Hauser, H. Spiering, Angew. Chem. 1994, 106, 2109 ±
2141; Angew. Chem. Int. Ed. Engl. 1994, 33, 2024 ± 2054.
[2] A. Reichert, H. Ringsdorf, P. Schuhmaher, W. Baumeister, T. Schey-
bani in Comprehensive Supramolecular Chemistry, Vol. 5 (Eds.: J. L.At-
wood, J. E. D. Davies, D. D. MacNicol, F. Vogtle, K. S. Suslick),
Pergamon, Oxford, 1996, pp. 313 ± 350.
[3] Yu. Galyametdinov, M. Athanassopoulou, K. Griesar, O. Kharitonova,
E. Soto-Bustamante, L. Tinchurina, I. Ovchinnikov, W. Haase, Chem.
Mater. 1996, 8, 922 ± 926.
[4] K. Binnemans, Yu. Galyametdinov, R. Van Deun, D. Bruce, S.
Collinson, A. Polishchuk, I. Bikchantaev, W. Haase, A. Prosvirin, L.
Tinchurina, I. Litvinov, A. Gubajdullin, A. Rakhmatullin, K. Uytterho-
even, L. Van Meervelt, J. Am. Chem. Soc. 2000, 122, 4335 ± 4344.
[5] M. S. Haddad, M. W. Lynch, W. D. Federer, D. N. Hendrickson, Inorg.
Chem. 1981, 20, 123 ± 132.
Figure 5. Effective magnetic moment meff versus temperature for the Fe3‡
*
*
complex in an applied magnetic field of 1.5 T. heating run; cooling run
&
at 1.5 T; cooling run at zero magnetic filed. Cr: crystal, SA: smectic A, I:
isotropic phases.
caused by magnetic field induced orientation of the mole-
cules. During the alignment of the molecules, the axis of
maximum magnetic anisotropy orients itself parallel to the
director in the smectic phase, and a macroscopic orientation
of the sample appears. One cannot exclude the possibility that
this process is accompanied by a change of the HS/LS ratio
because of possible changes of elastic properties of the
compound that occur during the rearrangement of the
molecular packing.
[6] V. I. Goldanskii, E. F. Makarov in Chemical Applications of Mössbauer
Spectroscopy (Eds.: V. I. Goldanskii, R. H. Herber), Academic Press,
New York, 1968, pp. 102 ± 107.
Â
[7] D. L. Nagy, K. Kulcsar, G. Ritter, H. Spiering, H. Vogel, R. Zimmer-
Â
Â
mann, I. Dezsi, M. Pardavi-Horvath, J. Phys. Chem. Solids 1975, 36,
759 ± 767.
[8] L. Merz, W. Haase, J. Chem. Soc. Dalton Trans. 1980, 875 ± 879.
In conclusion, the possibility of coexistence of spin cross-
over and liquid-crystalline properties in a single compound
has been demonstrated. Our efforts are directed towards
enhancing the synergy between SC and LC phenomena. To
achieve this we intend to chemically modify the system to
increase the SC transition temperature and simultaneously
decrease the transition temperature to the liquid-crystalline
state. Furthermore, SC compounds containing thermochromic
Fe2‡ will be studied. Due to the presence of paramagnetic
ions, the material reported here exhibits enhanced magnetic
anisotropy and can be aligned by a magnetic field in the
mesophase of a liquid crystal.
Assembly of Encapsulated Transition Metal
Catalysts**
Vincent F. Slagt, Joost N. H. Reek,* Paul C. J. Kamer,
and Piet W. N. M. van Leeuwen
There is considerable interest in the encapsulation of guest
molecules in the hollow framework of spherical host mole-
cules. The first examples of containerlike molecules were
obtained by performing the synthesis in the presence of the
guest molecule.[1] More recent strategies involve the con-
struction of spherical hosts consisting of two self-complemen-
tary units held together by hydrogen bonds[2±4] and multi-
component assembly of capsules by using metal ± ligand
interactions.[5, 6] Initially, only small guest molecules were
Experimental Section
N-ethylethylenediamine, Fe(NO3)3 ´ 9H20, and KPF6 were used as received
from Aldrich. Aldehyde 1 was prepared according to the literature
procedure.[3]
2: N-Ethylethylenediamine (0.046 g, 0.52 mmol) was added in small
portions to 1 (0.208 g, 0.52 mmol) in EtOH (30 mL). The mixture was
heated for 20 min at 1008C, and NaOH (0.021 g, 0.52 mmol) in H2O (1 mL)
was added. Fe(NO3)3 ´ 9H20(0.1 g, 0.53 mmol) in EtOH (5 mL) was added
to the resulting orange solution with stirring. A twofold excess of KPF6 in
methanol was added to the resulting suspension of the complex, which was
then heated to reflux for 15 min. The product was filtered from the hot
mixture. The brown precipitate was washed with methanol and dried in
vacuo. Yield: 0.25 g (42%). Elemental analysis (%) calcd for
C56H78N4O8PF6Fe: C 59.21, H 6.87, N 4.93; found: C 59.02, H 6.94, N 5.11.
[*] Dr. J. N. H. Reek, V. F. Slagt, Dr. P. C. J. Kamer,
Prof. Dr. P. W. N. M. van Leeuwen
Institute of Molecular Chemistry, University of Amsterdam
Nieuwe Achtergracht 166, 1018 WV Amsterdam (The Netherlands)
Fax: (‡31)20-525-6456
E-mail: reek@anorg.chem.uva.nl
[**] The NRSCC is kindly acknowledged for financial support.
Supporting information for this article is available on the WWW under
http://www.angewandte.com or from the author.
The temperatures and textures of phase transitions were determined with a
polarization microscope, equipped with a hot stage and with temperature
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