Mixed copper-lanthanide metallomesogens

Article abstract of DOI:10.1002/1521-3765(20020301)8:5<1101::AID-CHEM1101>3.0.CO;2-9

This paper describes the first examples of heteropolynuclear metallomesogens that contain both a transition metal ion and a trivalent lanthanide ion. Adducts were formed between a mesomorphic [Cu(salen)] complex (salen = 2,2′-N,N′-bis(salicylidene)ethylen

Full text of DOI:10.1002/1521-3765(20020301)8:5<1101::AID-CHEM1101>3.0.CO;2-9

FULL PAPER
Mixed Copper Lanthanide Metallomesogens
Koen Binnemans,*[a] Katleen Lodewyckx,[a] Bertrand Donnio,[b] and Daniel Guillon[b]
Abstract: This paper describes the first
examples of heteropolynuclear metal-
lomesogens that contain both a transi-
tion metal ion and a trivalent lanthanide
ion. Adducts were formed between a
mesomorphic [Cu(salen)] complex
(salen ˆ 2,2'-N,N'-bis(salicylidene)ethyl-
enediamine) with six terminal tetrade-
cyloxy chains and a lanthanide nitrate
(Ln ˆ La, Gd). Different stoichiome-
tries were found, depending on the
lanthanide ion: a trinuclear copper
lanthanum copper complex [La-
(NO3)3{Cu(salen)}2] and a binuclear cop-
per-gadolinium complex [Gd(NO3)3Cu-
(salen)]. The compounds exhibit a hex-
agonal columnar mesophase (ColH) over
a wide temperature-range with rather
low melting temperatures. Although the
clearing point could be observed for the
parent [Cu(salen)] complex, the mixed
f
d complexes decomposed in the high-
temperature part of the mesomorphic
domain before clearing. On the basis of
X-ray diffraction measurements and
molecular modelling, a structural model
for the mesophase of the metal com-
plexes is proposed.
Keywords: copper ¥ lanthanides ¥
liquid crystals ¥ metallomesogens ¥
supramolecular chemistry
weak external magnetic fields and can find new applications in
Introduction
the display and communication technologies. Examples of
[10]
During recent years, considerable research effort has been
invested in the design of new metal-containing liquid crystals
(metallomesogens) with the aim of combining the unique
properties of anisotropic fluids (anisotropy of physical
properties and fast orientational response to external fields)
with the specific properties of metals (magnetic, electronic).[1]
Furthermore, metals offer multiple structural possibilities for
novel molecular architectures, which in turn may lead to new
types of molecular organisations or mesophases. For example,
mesomorphism has been achieved in nonconventional metal
complexes, with octahedral,[2] square-antiprismatic,[3] trigo-
nal-bipyramidal[4] and tetrahedral[5] coordination geometry.
Recently, the synthesis of mesomorphic metallacrowns,[6]
butterfly compounds,[7] metallodendrimers[8] and polynuclear
systems[9] opened up the field for the construction of original
structures able to contain a large number of metallic centres.
Liquid crystals incorporating paramagnetic metal ions are
of particular interest in the sense that they can be switched by
such paramagnetic metal ions include CuII
um,[11] and the trivalent lanthanide ions.[12]
,
oxovanadi-
Although several mixed f d complexes have been syn-
thesised and their magnetic behaviour studied in detail,[13]
there is still no report on heteropolynuclear metallomesogens
that contain both a transition metal ion and a lanthanide
(rare-earth) ion. Such mixed f d metallomesogens could
represent an intriguing new class of materials, since they
combine specific magnetic interactions of f d coordination
compounds (such as magnetic exchange interactions) with the
properties of liquid crystals. Furthermore, from a materials
processing point of view, such f d metallomesogens could be
advantageous because of the strong tendency of the lantha-
nide-based metallomesogens to form a glassy state on cooling
rather than to crystallise, and thus giving the possibility to
freeze-in the mesomorphic order, whereas common non-
mesomorphic f d coordination complexes are obtained as
crystalline powders.
In this paper, we report on the structural design and on the
thermal behaviour of two novel mixed f d metallomesogens,
based on binuclear or trinuclear copper lanthanide com-
plexes.
[a] Dr. K. Binnemans, K. Lodewyckx
Katholieke Universiteit Leuven, Department of Chemistry
Celestijnenlaan 200F, 3001 Leuven (Belgium)
Fax : (‡32)16-32-79-92
E-mail: koen.binnemans@chem.kuleuven.ac.be
[b] Dr. B. Donnio, Dr. D. Guillon
¬
Institut de Physique et Chimie des Materiaux de Strasbourg
Results and Discussion
¬
Groupe des Materiaux Organiques
23 rue du Loess, 67037 Strasbourg Cedex (France)
E-mail: bertrand.donnio@ipcms.u-strasbg.fr
To obtain these mixed f d metallomesogens, our approach
consisted of modifying the structures of previously described
non-mesomorphic f d complexes in such a way that a
sufficient structural anisotropy was obtained to favour the
formation of mesophases. This can be achieved by extending
Supporting information for this article is available on the WWW under
http://wiley-vch.de/home/chemistry/ or from the author: Experimental
details of the synthesis of the salen ligand L and additional X-ray
diffraction data.
Chem. Eur. J. 2002, 8, No. 5
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1101

K. Binnemans et al.
FULL PAPER
the aromatic parts of the central rigid core on the one hand
and to attach long alkyl chains to the extremities of the core
complex on the other hand. Recently, Kahn et al. described
adducts of lanthanide(iii) nitrates with the [Cu(salen)] com-
plex (salen ˆ 2,2'-N,N'-bis(salicylidene)ethylenediamine).[14]
For the light lanthanides, they obtained heterotrinuclear
[Ln(NO3)3{Cu(salen)}2] compounds, whereas for gadolinium
and for the heavier lanthanides heterobinuclear [Ln(NO3)3-
{Cu(salen)}] complexes were formed. These types of com-
plexes stood model for the mixed copper lanthanide metal-
lomesogens described in this paper. Let us recall that suitably
designed [Cu(salen)] complexes have yielded mesomorphic
properties and have already been previously described,[15]
though they structurally differ from the [Cu(salen)] com-
plexes used here as ligand for the mesomorphic f
complexes.
d
The synthesis of the substituted salen ligand (L) and of the
parent [Cu(salen)] complex 1 is shown in Scheme 1. The
copper lanthanum complex 2 and the copper gadolinium
complex 3 were obtained by reaction of complex 1 with
La(NO3)3 ¥ 6H2O and Gd(NO3)3 ¥ 6H2O, respectively, in ace-
tone. Two different stoichiometries were found, in agreement
with the findings of Kahn et al. for the unsubstituted
complexes.[14]
Observation of the textures by polarised-light optical
microscopy (POM) showed that the copper complex 1 gives
an enantiotropic liquid crystal, melting at 588C and clearing at
1598C. Just above the melting point, despite the high viscosity,
flow could nevertheless be induced by pressing the cover slip
with a needle. However, on further heating, the viscosity was
found to gradually decrease up to the clearing-point temper-
ature, at which the texture appeared totally fluid. On cooling
from the isotropic phase, a characteristic texture of a
columnar phase was formed (as evidence by the growth of
dendritic features). On further cooling, the viscosity gradually
increased, and the mesophase could still be observed at room
temperature; crystallization was observed at around 08C. The
copper lanthanum complex 2 was found by POM to melt at
778C into a viscous mesophase with an atypical texture, with
the viscosity gradually decreasing upon heating. Around
1708C a sharp decrease in viscosity was observed. At this
temperature, a step in the baseline of the differential scanning
calorimetry (DSC) thermogram was detected. Above 2208C,
the compound started to decompose. The copper gadolinium
complex 3 shows a similar mesophase behaviour to that of the
complex 2, with a melting point at 958C. The mesophase
persisted up to 2108C, at which the compound started to
decompose without clearing.
Temperature-dependent X-ray diffraction (XRD) measure-
ments were carried out systematically within the crystalline
and the mesomorphic temperature ranges, every 108C
between 40 and 1808C for 1,
and between 30 and 2008C for 2
and 3. The time exposure was
varied from 30 minutes up to
4 hours, depending on the spe-
cific reflections being sought
(weaker reflections taking lon-
ger to expose). The X-ray dia-
grams were not reproducible on
cooling for the two mixed com-
plexes 2 and 3, due to the fast
decomposition on reaching the
isotropic liquid, the long-time
exposure at high temperature
and the X-ray beam. Neverthe-
less, a good agreement was
found between the tempera-
tures determined by POM and
by DSC, and those determined
by XRD. Practically identical
X-ray patterns were obtained
Scheme 1. Synthesis of copper complex 1, used for adduct formation with the lanthanide nitrates. Reagents and
conditions A) DCC, DMAP, DCM, 24 hours at RT; B) ethylene diamine (1/2 equiv), glacial acetic acid (catalyst),
toluene, Dean-Stark trap, 3 hours at reflux; C) Cu(OOCCH3)2 ¥ H2O (1 equiv), methanol/chloroform, reflux
overnight.
for all samples, when the ex-
periments were performed in
the mesomorphic temperature
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Chem. Eur. J. 2002, 8, No. 5
1102

Mixed Metallomesogens
1101 1105
ranges. They consist of: a) a diffuse scattering halo (II) in the
wide-angle region, which corresponds to the liquidlike
disorder of the aliphatic chains and rigid parts, at about
4.5 ä (this distance corresponds to hhi); b) another, slightly
less intense diffuse band (I) seen at approximately 9.4 ä,
which could be an indication of a dimeric structure, and thus
of the periodicity along the column, hpi; and c) two or three
sharp, intense reflections in the small-angle region, with the
reciprocal spacings in the ratio 1, 3 and 4, corresponding
to the indexation [hk] ˆ [10], [11] and [20], respectively. Such
features are characteristic of a two-dimensional hexagonal
packing of columns, that is, to a hexagonal columnar
mesophase (ColH). The phase is disordered since there is no
long-range correlation order within the columns, as evident
from the absence of a sharp peak in the wide-angle region
which would have corresponded to a perfect and regular
stacking along the columnar axis. Characteristic data have
been selected and are reported in Table 1, with, in addition, a
representative diffractogram for the copper lanthanum com-
plex 2 (Figure 1).
The systematic observation of a hexagonal columnar
mesophase for the three compounds as well as the similarity
in the measured geometrical parameters suggest strongly that
the local organisation within the mesophase is the same
despite their structural differences. Furthermore, the forma-
tion of what seems likely to be some kind of dimeric structure
is also present in the three cases. Only the thermal stability of
the compounds in the mesophase is different, probably in
relation to the intrinsic structure of each complex. The
hexagonal symmetry of the mesophase implies the necessary
condition that the hard columnar core (i.e., the aromatic
parts) is cylindrical, and, therefore, has a circular cross section
(these hard columns are separated from each other by the
liquid paraffinic continuum–see Figure 2). To generate such
p
p
Table 1. Characteristic XRD data (at 1008C) and mesophase behaviour of
copper complex 1, of the mixed copper-lanthanum complex 2 and of the
mixed copper-gadolinium complex 3.
dexptl [ä]
I[a]
[hk]
dcalcd [ä]
Mesophase and
parameters[b]
1
2
3
36.3
21.1
18.3
9.4
VS
S
[10]
[11]
[20]
36.3
20.95
18.15
Crys 58 ColH 159 I
a ˆ 41.9 ä
M
br
br
VS
S
M
br
br
[10]
S
s ˆ 1521 ä2
Figure 2. Schematic representation of the columnar periodicity d10, the
p
p
3)
lattice parameter a (a ˆ 2d10/ 3), the cross section s (s ˆ ad10 ˆ 2d120
/
4.5
and the apparent hard-core cross section s0, with diameter f0.
36.7
21.2
18.3
9.4
[10]
[11]
[20]
36.7
21.2
18.3
Crys 77 ColH 220 decomp
a ˆ 42.4 ä
s ˆ 1555 ä2
circular cross sections with these large lathlike molecules, the
complexes are likely to be self-organised into molecular
assemblies (clusters) with a flat, more or less square shape,
which will then stack on top of each other to yield the
columns. The rotation of these supramolecular slices, and thus
of the hard columnar core with respect to the columnar axis,
lead to apparent cylindrical rigid columns (with a circular
cross section s0 of diameter f0), as in the case of classical
polycatenar mesogens.[16]
The number of complexes (or dimers) contained in each
molecular cluster can be estimated using the equation for NMol
[Eq. (1) below].[17, 18] In their fluid state, the density of these
complexes can be approximated to be close to 1 gcmÀ3. In this
case, the molecular volumes (VMol) of the three complexes are
3100, 6700, and 3600 ä3, respectively. Thus, in order to
calculate the number of molecules (NMol) contained within the
hexagonal cell of thickness hpi, one just needs to divide the
volume of this cell (Vcell) by the molecular volume [Eq. (1)].
4.5
37.2VS
21.45
18.5
9.3
37.2Crys 95 Col
[11]
[20]
H 210 decomp
a ˆ 42.95 ä
21.5
18.6
M
br
br
s ˆ 1598 ä2
4.6
[a] Intensities (I): VS ˆ very strong; S ˆ strong; M ˆ medium; brˆ broad.
[b] Transition temperatures are those obtained from polarised optical
microscopy and expressed in8C (in good agreement with those determined
by DSC and XRD). Crys ˆ solid, crystalline phase; ColH ˆ hexagonal
columnar mesophase; I ˆ isotropic liquid; decomp ˆ decomposition.
p
NMol ˆ Vcell/VMol ˆ shpi/VMol ˆ 2 d210hpi/( 3VMol
)
(1)
Here, s corresponds to the cross section of the hexagonal
columnar phase and hpi to the average columnar repeat unit,
both values being determined by X-ray diffraction (from d10,
Figure 1. XRD pattern of the copper-lanthanum complex 2 at 1008C,
showing the three sharp small angle reflections, d10, d11 and d20 and the two
diffuse halos (I and II).
p
s ˆ 2d210/ 3 and from the centre position of the halo I–given
by a Lorentzian fit–hpi  9.4 ä). The results of these
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1103

K. Binnemans et al.
FULL PAPER
calculations showed that on average, slightly more than four
copper complexes 1 (two dimers), two copper lanthanum
complexes 2 (one dimer) and four mixed gadolinium copper
complexes 3 (or two dimers) are needed to fill a columnar
slice approximately 9.4 ä thick.
A good agreement with a geometrical approach was also
found. The approach implies the calculation of an interface S
associated with the repeat unit along the column (as above the
repeat unit is taken as the columnar slice with the thickness
hpi), and to calculate the number of aliphatic chains (Nch)
radiating out off this interface. Knowing the number of chains
per slice, one can deduce the number of molecules per slice,
NMol (see below). Prior to the calculation, the following
parameters have to be defined (Figure 2): f0 is the diameter of
the apparent circular cross section of the hard column core,
deduced from the length of the rigid molecular part of the
copper complex 1, l0 (l0  27 ä, determined by computer
Figure 3. Molecular organisation of the complexes 1 and 3 (type I) within
the columnar mesophase. The columnar repeat unit is given by hpi, and hhi
is the average distance between adjacent molten chains.
modelling). With the hypothesis of a square cluster, the
p
diameter of s0 equals its diagonal according to f0 ˆ l0 2). G0 is
the hard core perimeter associated to sO (G0 ˆ pf0), and S is
the interface of thickness hpi (S ˆ pf0hpi). From the average
distance between adjacent molten chains, hhi, the cross-
Figure 4. Molecular organisation of complex
2 (type II) within the
columnar mesophase. The columnar repeat unit is given by hpi, and hhi is
the average distance between adjacent molten chains.
section area of one methylene group s can be calculated: s ˆ
p
2hh2i/ 3.[17] Thus, the number of chains radiating out of such
an interface is expressed by: Nch ˆ S/s. Only half of the
interface has to be considered, because, looking at the
molecular shape and the stacking, the chains occupy only
two sides of the square-shaped molecules. The other sides are
filled by the chains of the molecules belonging to adjacent
columns. Of course, the whole clusters of molecules rotate
around the column axis (circular cross section, see Figure 2).
filling of the chains is optimised, and the columns become less
and less ondulating. The columnar phase results from the self-
organisation of the columns into a two-dimensional hexagonal
lattice. The disordered chains ensure at the same time the
gliding of the columns, one with respect to the other, and thus
the fluidity and the liquid-crystalline nature of the mesophase.
A schematic representation of the molecular organisation of
the complexes within the columnar mesophase is shown in
Figure 3 and 4. This is only a schematic representation at one
given moment, as the system is fluid and the molecular
clusters rotate around the column axis (see text above and
Figure 2).
Thus, the number of chains per platelike slice can be
p
expressed by: Nch ˆ pf0 3hpihhÀ2i/4. Knowing the total
number of chains per slice, the number of corresponding
molecules (NMol) forming the cluster is NMol ˆ Nch/n (n ˆ 6 for
p
1 and 3, and 12for 2). Therefore, Nch ˆ pf0 3hpihhÀ2i/(4n).
As mentioned above, comparable results were obtained, in
that more or less four copper or mixed copper gadolinium
complexes (two dimers) and only two copper lanthanum
complexes (or one dimer) form the molecular clusters
approximately 9.4 ä thick.
Conclusion
The following molecular organisation within the columnar
mesophase can now be proposed: four molecules of type I
(complexes 1 and 3) self-arrange into a platelike molecular
clusters, while only two molecules of type II (complex 2, itself
having a platelike shape) fulfill the geometrical requirements.
They then self-assemble to form the columns by an alternating
stacking (at right angles, see Figures 3 and 4) in order to
maximise the space occupied by the aliphatic chains. Note that
the orientational molecular ordering in these clusters within
the column and with those of neighbouring columns is not
correlated over a long range, allowing some freedom for the
molecules to fill the space in a more efficient way, as
confirmed by the d10 temperature variation. Indeed, for
complexes 1 and 3, the d spacing decreases linearly with T,
and is almost temperature independent for 2. This is in
agreement with the probable presence of undulations of the
columns in order to compensate the thermal expansion of the
paraffinic chains.[18] On increasing temperature T, the space
In conclusion, we prepared the first examples of heteropoly-
nuclear metallomesogens that contain both a transition metal
ion and a trivalent lanthanide ion. Adducts were formed
between a mesomorphic [Cu(salen)] complex and a lantha-
nide nitrate. Different stoichiometries were found, depending
on the lanthanide ion: a trinuclear copper lanthanum cop-
per complex and a binuclear copper gadolinium complex.
The compounds exhibit an hexagonal columnar mesophase
(ColH) over a wide temperature-range with rather low melting
temperatures. Although the clearing point could be observed
for the parent [Cu(salen)] complex, the mixed f d complexes
decomposed in the high-temperature part of the mesomor-
phic domain before clearing. Further work is in progress to
prepare new complexes by varying some structural parame-
ters, such as varying the metal ions by the use of different
metal combinations, the structure of the ligands and the type
of counterions in order to obtain more thermally stable
compounds and to analyse the magnetic properties.
1104
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Chem. Eur. J. 2002, 8, No. 5

Mixed Metallomesogens
1101 1105
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1555.
Experimental Section
General procedures: CHN elemental analyses were performed on a CE
Instruments EA-1110 elemental analyser. FTIR spectra were recorded on a
Bruker IFS-66 spectrometer, by using the KBr pellet method. Optical
textures of the mesophase were observed with an Olympus BX60 polarised
optical microscope equipped with a Linkam THMS 600 hot stage and a
Linkam TMS 93 programmable temperature controller. The XRD patterns
were obtained with two different experimental set-ups, and in all cases the
crude powder was filled in Lindemann capillaries of 1 mm diameter. A
linear monochromatic CuKa1 beam (l ˆ 1.5405 ä) obtained with a sealed-
tube generator (900 W) and a bent quartz monochromator were used (both
generator and monochromator were manufactured by Inel). One set of
diffraction patterns was registered with a curved counter Inel CPS 120, for
which the sample temperature was controlled within Æ0.058C; periodici-
ties up to 60 ä could be measured. The other set of diffraction patterns was
registered on an image plate. The cell parameters are calculated from the
position of the reflection at the smallest Bragg angle, which was the most
intense in all cases. Periodicities up to 90 ä could be measured, and the
sample temperature was controlled within Æ0.38C.
[3] S. T. Trzaska, H. Zheng, T. M. Swager, Chem. Mater. 1999, 11, 130, and
references therein.
√
[4] a) L. Ziminski, J. Malthete, J. Chem. Soc. Chem. Commun. 1990, 1495;
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ghayoury, A. Skoulios, R. Ziessel, Chem. Commun. 1999, 2 033; d) R.
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The calculation of the rigid aromatic segment length was done by molecular
mechanics with the Discover 3 software from MSI with the esff forcefield.
The length lo was taken between the two opposite oxygen atoms of the
alkoxy chains in the para-position the copper complex 1.
[7] T. Hegmann, B. Neumann, J. Kain, S. Diele, C. Tschierske, J. Mater.
Chem. 2000, 10, 2244.
[8] J. Barbera, M. Marcos, A. Omenat, J. L. Serrano, J. I. Martinez, P. J.
Alonso, Liq. Cryst. 2000, 27, 255.
Synthesis of salen ligand L: A detailed description of the synthesis of the
substituted salen ligand L (salen ˆ 2,2'-N,N'-bis(salicylidene)ethylenedi-
amine) is available as Supporting Information. L melts at 708C, without
forming a mesophase.
[9] J. Szydlowska, A. Krowczynski, E. Gorecka, D. pociecha, Inorg.
Chem. 2000, 39, 4879.
[10] a) Yu. G. Galyametdinov, I. V. Ovchinnikov, B. M. Bolotin, N. B.
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¬
Chem. Sci. 1984, 33, 2174; b) J. Barbera, A. M. Levelut, M. Marcos, P.
Synthesis of 1: A hot solution of Cu(OOCCH3)2 ¥ H2O (0.20 g; 1 mmol) in
methanol was added dropwise to a hot solution of L (1.78 g; 1 mmol) in
chloroform. The reaction mixture was heated at reflux overnight. After
allowing the solution to cool to room temperature, the solvent was removed
under reduced pressure. The crude product was crystallised from ethyl
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¬
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acetate, washed with methanol, and dried in vacuo. Yield: 81% (1.50 g); IR
À1
ˆ
À
(KBr): nÄ ˆ 1640 (C N), 1206 cm (C O); elemental analysis (%) calcd for
C114H190O12N2Cu (1844.28): C 74.24, H 10.38, N 1.52; found: C 74.38, H
10.59, N 1.30.
Synthesis of 2: A solution of La(NO3)3 ¥ 6H2O (0.143 g; 0.33 mmol) in
acetone was added to a solution of 1 (0.553 g; 0.3 mmol) in acetone. The
reaction mixture was stirred at room temperature for 24 hours. The
precipitate was filtered, washed with cold methanol and dried in vacuo.
À1
ˆ
À
Yield: 96% (578 mg); IR (KBr): nÄ ˆ 1639 (C N), 1198 cm (C O);
elemental analysis (%) calcd for C228H380O33N7Cu2La (4013.48): C 68.23, H
9.54, N 2.44; found: C 68.19, H, 9.70, N 2.47.
Synthesis of 3: The same procedure as for compound 2 was used: 1 (0.553 g;
[13] a) A. Bencini, C. Benelli, A. Caneshi, R. L. Carlin, A. Dei, D.
Gatteschi, J. Am. Chem. Soc. 1985, 107, 8128; b) A. Benelli, A.
Caneshi, A. Dei, D. Gatteschi, Inorg. Chem. 1986, 25, 572; c) M.
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Inorg. Chem. 2000, 39, 165.
0.3 mmol), Gd(NO3)3 ¥ 6H2O (0.149 g; 0.33 mmol). Yield: 97% (635 mg);
À1
ˆ
À
IR (KBr): nÄ ˆ 1647 (C N), 1198 cm (C O); elemental analysis (%) calcd
for C114H190O21N5CuGd (2187.54): C 62.59, H 8.75, N, 3.20; found: C 62.40,
H 9.15, N 2.93.
Acknowledgements
K.B. is a Postdoctoral Fellow of the Fund for Scientific Research-Flanders
(Belgium). Funding by the K.U.Leuven (GOA 98/3 and PhD grant to K.L.)
and by the F.W.O.-Vlaanderen (G.0243.99) is gratefully acknowledged.
K.B. and K.L. wish to thank Prof. C. Gˆrller-Walrand for providing
laboratory facilities. CHN microanalyses were performed by Ms. P.
Bloemen. B.D. and D.G. would like to thank Dr C. Bourgogne for his
help in the computer molecular modelling.
¡
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R. Acad. Sci. II C 2000, 3, 131.
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Blake, P. G. Nelson, G. W. Gray, Mol. Cryst. Liq. Cryst. Lett. 1988, 6,
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Synthesis, Properties and Applications (Ed. J. L. Serrano), VCH,
Weinheim, 1996; c) D. W. Bruce in Inorganic Materials, 2nd ed.
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p. 429,; d) B. Donnio, D. W. Bruce, Struct. Bond. 1999, 95, 193.
√
[16] H. T. Nguyen, C. Destrade, J. Malthete, Adv. Mater. 1997, 9, 375.
¬
[17] M. Marcos, R. Gimenez, J. L. Serrano, B. Donnio, B. Heinrich, D.
Guillon, Chem. Eur. J. 2001, 7, 1006.
[18] B. Donnio, B. Heinrich, T. Gulik-Krzywicki, H. Delacroix, D. Guillon
and D. W. Bruce, Chem. Mater. 1997, 9, 2951.
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Received: September 13, 2001 [F3554]
Chem. Eur. J. 2002, 8, No. 5
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