Rodlike metallomesogens containing nickel(II), palladium(II) and copper(II) based on novel enaminoketonato ligands

Article abstract of DOI:10.1039/b316731c

Several new enaminoketone mesogens were synthesized and coordinated to various metal salts to yield a variety of novel rodlike metal-containing liquid crystals. The mesophases were characterized by polarized optical microscopy (POM), differential scanning calorimetry (DSC) and small-angle X-ray scattering (SAXS). Whereas the free ligands display SmA and nematic mesophases, the Ni ^II and Pd^II complexes give rise to SmC and nematic phases, and the paramagnetic Cu^II mesogens to a narrow temperature-range nematic mesophase only. The magnetic behaviour of the latter was studied by means of EPR and magnetic susceptibility (SQUID) measurements.

Full text of DOI:10.1039/b316731c

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A R T I C L E
Rodlike metallomesogens containing nickel(II), palladium(II) and
copper(II) based on novel enaminoketonato ligands{{
Carsten P. Roll,a Alexander G. Martin,a Helmar Go¨rls,a Guido Leibeling,a
Daniel Guillon,b Bertrand Donniob and Wolfgang Weigand*a
aInstitut fu¨r Anorganische und Analytische Chemie, Friedrich-Schiller-Universita¨t Jena,
August-Bebel-Str. 2, D-07743 Jena, Germany. E-mail: wolfgang.weigand@uni-jena.de
bInstitut de Physique et Chimie des Mate´riaux de Strasbourg, Groupe des Mate´riaux
Organiques, CNRS-ULP (UMR 7504), 23 rue du Loess, BP 43, F - 67034 Strasbourg
Cedex 2, France. E-mail: bdonnio@ipcms.u-strasbg.fr
Received 22nd December 2003, Accepted 31st March 2004
First published as an Advance Article on the web 5th May 2004
Several new enaminoketone mesogens were synthesized and coordinated to various metal salts to yield a variety
of novel rodlike metal-containing liquid crystals. The mesophases were characterized by polarized optical
microscopy (POM), differential scanning calorimetry (DSC) and small-angle X-ray scattering (SAXS). Whereas
the free ligands display SmA and nematic mesophases, the NiII and PdII complexes give rise to SmC and
nematic phases, and the paramagnetic CuII mesogens to a narrow temperature-range nematic mesophase only.
The magnetic behaviour of the latter was studied by means of EPR and magnetic susceptibility (SQUID)
measurements.
Introduction
Since the early reports on metal containing liquid crystals,
work in the area of metallomesogens has developed rapidly and
gained a lot of attention over the last 25 years.1–14 Our objec-
tive in this work is concerned with the design and the synthesis
of some new calamitic b-enaminoketonato ligands in order to
induce and try to control the mesomorphism of the corre-
sponding bischelate metal complexes.
The first examples of mesomorphic complexes with
b-enaminoketonato derivatives were reported by I. V. Ovchinnikov
et al. in 1991.15 Due to their short molecular anisotropy however
such complexes were poorly mesogenic, and essentially mono-
tropic mesophases were seen. Since then, several series of metal-
lomesogens derived from more elongated b-enaminoketonato
ligands have been synthesized and characterized.16–24 These
bischelate complexes of CuII and PdII showed nematic, smectic
Scheme 1 CuII-enaminoketonato complexes with elongated cores.
A and C mesophases depending on the organic substituents of
the N,O chelate unit. The reaction of enaminoketones with
Replacement of alkyloxy chains by branched and
trivalent lanthanide ions also yields new metallomesogens
unbranched aminoalkyl chains was used as a means to depress
showing exclusively broad smectic mesomorphism.25
melting points by steric perturbance of the terminal chains.
As far as complexes 1 and 2 are concerned (Scheme 1), the
Consequently, the mesophase transition temperatures were
core was elongated further by incorporation of azo or azoxy
considerably lowered, especially in the N-(methyl)-alkyl-
moieties.21 Compounds 2 showed monotropic smectic phases
substituted series 1, and the nematic phase was more strongly
(SmC and SmA) along with the nematic phase. In the
favoured over lamellar structures, but at the expense of a large
enaminoketone complexes of type 1, which carry the alkyl
mesophase width. Incorporation of azoxy (trans-N(O)LN)
chain at the nitrogen atom, the less-ordered nematic phase was
versus azo (trans-NLN) groups in the complexes 2 led to lower-
favoured over the SmC and SmA mesophases (the latter being
melting compounds, where the nematic phase was promoted
present only for long chain lengths and over narrow
over smectic phases and clearing temperatures were increased.
temperature ranges). The authors attributed this phenomenon
More sophisticated ligand topologies were used for the
to the non-planar arrangement of the alkyl chain with respect
enaminoketones in order to reduce the transition temperatures
to the chelating ring; such a distortion of the chain resulted in a
geometric perturbation of the mesophase.
more efficiently and avoid problems of decomposition. In one
series of materials based on tetradentate cis-enaminoketone
ligands (3) (Scheme 2),23 the NiII complexes exhibited SmA and
{ Dedicated to Professor Ingo-Peter Lorenz on the occasion of his 60th
birthday.
{ Electronic supplementary information (ESI) available: details of the
synthesis of the ligands and of the complexes, structural analysis,
analytical characterization, and determination of the mesophase and
nematic phases, enantiotropic or monotropic depending on the
structural parameters, whereas a mesomorphism was obtained
for the corresponding VOII complexes. Lai et al. synthesized
oxygen-bridged dicopper(II) complexes which revealed smectic
A phases.26
molecular
b316731c/
structures.
See
http://www.rsc.org/suppdata/jm/b3/
When adequately designed, enaminoketonato metal
1 7 2 2
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 1 7 2 2 – 1 7 3 0
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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Scheme 2 Examples of metallomesogens containing tetradentate
cis-enaminoketonato ligands.
complexes can also display columnar mesomorphism charac-
teristic of discotic molecules.27,28 Of particular interest, multi-
nuclear complexes of NiII, CuII and VOII containing
tetradentate enaminoketonato ligands were found to form
discotic mesophases with rectangular columnar and disordered
hexagonal columnar order.29–31
Enaminoketones are therefore very versatile systems, which
coordinate to a large variety of metal ions, and can be readily
structurally modified in various ways to influence the forma-
tion of liquid crystal phases.
In the following work, the synthesis of smectogenic
enaminoketones is described. Additionally the influence of
molecular core elongation was investigated since elongated
enaminoketonato complexes should be well-suited to produce
polycatenar mesogens. The incorporation of polar but flexible
ester groups into the molecular core was chosen as a synthetic
strategy to lower transition temperatures. Another aim in
this work was to study the influence of an additional methyl
group at the b-enaminoketone moiety with regard to the
mesomorphic properties of ligands and their corresponding
complexes.
Results and discussion
Synthesis and characterization of the ligands
The b-enaminoketonato ligands 12–17 were obtained by two
different synthetic routes as shown in Scheme 3. Synthesis of
enaminoketones 12–15 was achieved by a linear synthetic
route, whereas that of 17 was realized by a so-called convergent
procedure. Thereby the synthesis of the alkoxy benzoic acids 5
was performed using standard Williamson alkylation proce-
dures. Ester hydrolysis was carried out using potassium
hydroxide in ethanol/THF, followed by acidification with
hydrochloric acid. The most convenient method for accessing
calamitic para-nitrophenyl esters 7 and 10 consists of the
esterification of the corresponding alkyloxybenzoic acid with
4-nitrophenol using dicyclohexylcarbodiimide (DCC) and
4-dimethylaminopyridine (DMAP)32 as coupling agents. The
acid 8 was prepared from acid 5 by esterification with benzyl-4-
hydroxybenzoate followed by the cleavage of the benzyl ester
group33 with Pd/C under a hydrogen atmosphere. The anilines
9 and 11 were obtained from hydrogenation under atmospheric
pressure using palladium on carbon as catalyst34 in benzene.
Synthesis of ligands 17 could not be performed via the direct
condensation of the 4-aminophenyl(4-alkoxybenzoate)ester
with acetyl acetone. Therefore, 4-N-(4’-hydroxyphenyl)pent-3-
ene-2-one 16 was synthesized by condensation of acetyl acetone
with 4-aminophenol in boiling abs. ethanol (Scheme 3).35
Ligands 17 were obtained from precursor 16 and alkoxy-
benzoic acids 5 by esterification using DCC and DMAP as
coupling agents in moderate yields (27–37%).
Scheme 3 Description of the linear (12–15) and convergent (17)
routes. (i) H2n11CnBr, K2CO3 (3 mol/mol hydroxyl group), CH3CN,
reflux; (ii) KOH, EtOH, reflux, HCl; (iii) p-HO-Ph-NO2, DCC, DMAP,
CH2Cl2; (iv) p-HO-Ph-COOBn, DCC, DMAP, CH2Cl2; (v) Pd/C,
THF; (vi) Pd/C, benzene; (vii) HCl, ethanol, reflux; (viii) acac, EtOH,
reflux; (ix) DCC, DMAP, CH2Cl2.
neighbouring proton is observed. In ligand type 17 the
1H-NMR signals of the enamino protons show a singlet at
12.44 ppm. IR spectra display a broad n(N–H) at 3430 cm21
indicating an intramolecular hydrogen bond.
Synthesis and characterization of the complexes
Refluxing ligands 12, 14 and 17 with palladium(II) acetate in
dry chloroform led to palladium(II) chelates 18, 21 and 23
respectively, which were purified by short column chromato-
graphy, and by crystallization from THF. They were obtained
as yellow solids and were found to be stable in aprotic solvents.
Complex 24 (Scheme 4), which bears a total of four benzoyloxy
units in its molecular core, was obtained in an analogous
synthesis from ligand 15. The 1H-NMR spectra of all PdII
complexes lacked the signal of the amine proton of the
enaminoketonato moiety, and the resonance signal of the
1
The H-NMR signals of the enamino proton in ligands 12–
15 show a broadened (D2O-exchangeable) doublet at approxi-
mately 12 ppm. A trans-coupling of about 12 Hz with the
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 1 7 2 2 – 1 7 3 0
1 7 2 3

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Fig. 1 (a) EPR spectra of complex 24 (n ~ 18) in benzene at room
temperature; (b) after Gauß–Lorentz Transformation.
Crystal structures
By slow cooling from an ethanol–acetone mixture and
methanol, respectively, single crystals of ligands 12 (n ~ 12)
and 17 (n ~ 10) could be obtained with sizes suitable for X-ray
structure analysis. They were isolated as pale yellow 12 (n ~
12) and beige 17 (n ~ 10) needles.
Crystals suitable for structure determination of PdII complex
23 (n ~ 16) were grown from chloroform by slow evaporation.
A summary of the crystal data, data collection and processing
parameters are given in Table 1.
Scheme 4 M ~ Pd: Pd(CH3COO)2, CHCl3, reflux; M ~ Ni: [(Et4N)4-
NiBr2], tert-BuOH, tert-BuOK, reflux; M ~ Cu: Cu(CH3COO)2,
EtOH, reflux.
The horseshoe form of the enaminoketone moiety is
favoured in the solid state (Figs. 2 and 3). The rodlike molecule
˚
12 is 32.5 A long. The dihedral angle between the planes formed
methine proton at the chelate ring was shifted to a higher field
by 0.3 ppm. The upfield shifts of the signals of the carbonyl
group in the 13C-NMR spectra of about 20 ppm confirmed the
formation of the complexes.
by the carbon atoms of the two phenyl rings is 57.6u. The angle
between the plane determined by the non-hydrogen atoms of
the enaminoketone and the plane formed by C5 to C10 equals
10.8u (Fig. 2).
In ligand 17 the angle between the phenyl ring planes is
similar to the one of ligand 12. In contrast to ligand 12(n ~ 12),
the angle between the plane containing the enaminoketone unit
and that of the adjacent phenyl ring amounted to 72.7u (Fig. 3).
The PdII ion center of complex 23 is situated on a crystal-
lographic center of symmetry and adopts a square-planar trans-
[N2O2] environment as could be expected (Fig. 4). As a result
of delocalized p-electrons in the b-enaminoketonato ligand,
The synthesis of CuII and NiII complexes 19, 20, 24 and 25
required the use of polar solvents under strictly anhydrous
conditions. In the case of the copper chelates 20, 24 and 25 the
anhydrous metal acetate was refluxed with two equivalents of
the appropriate ligand (12 and 17 respectively) in abs. ethanol
for one hour. During the reaction the solution became
brownish and the complexes precipitated almost instantly
from solution. They were isolated as brown solids which were
purified by recrystallization to give deep green (compounds 24,
25) or brown (compound 20) products.
The syntheses of the NiII complexes 19 required the use of
a strong non-nucleophilic base (potassium tert-butoxide) and
longer reaction times. Due to its weak nucleophilicity, tert-
butanol was chosen as solvent in order to avoid hydrolysis of
the ester group. Tetraalkylammonium-halogenonickelates were
found to be valuable metal precursors, whereas with the use of
sodium ethoxide/NiII acetate the desired complexes could only
be detected in traces by mass spectrometry. Longer reaction
times led to alcoholysis of the ligand ester group. In the solid
state, the nickel chelates are pale green coloured. Due to the
˚
˚
C1–C2 (1.372(6) A) and C3-N (1.306(6) A) distances are
considerably smaller than the values known from typical single
˚
˚
bonds, whereas O1–C1 (1.276(6) A) and C2–C3 (1.420(7) A)
bond lengths are enlarged.
The angle in the PdII complex 23 between the b-enamino-
ketonato moiety and the adjacent phenyl ring increases from
72.7u in the ligand 17 to 87.0u in the complex 23.
The length-to-width ratio (l/w ratio with l[H-C34 2 H-C34’] ~
67.012 A and w[H-C2 2 H-C2’] ~ 8.384 A) for this complex
was calculated as 8.0, which would be in accordance with
calamitic shape and nematic mesomorphism.
˚
˚
1
presence of paramagnetic species the H-NMR spectra of NiII
complexes 20 showed significant broad signals which were not
suitable for NMR analysis.
Thermal behaviour
The purity of all complexes of PdII, CuII and NiII was
confirmed by mass spectrometry and elemental analysis.
Furthermore, the CuII complexes were characterized by EPR
spectroscopy.36 For example complex 24 (n ~ 18) displays a
typical CuII EPR spectrum in solution (S ~ ½, I ~ 3/2, g #
2.1, Fig. 1).
All ligands containing one alkyl chain as well as their bischelate
NiII, CuII and PdII complexes displayed mesomorphic beha-
viour. In contrast, none of the polycatenar ligands (13 and 14)
and corresponding complexes (21 and 25), based on 3,4-
disubstituted and 3,4,5-trisubstitituted alkoxybenzoic acids,
were mesomorphic, which was attributed to an inappropriate
anisotropic ratio (length/width). The thermal data are collected
in Table 2 for the ligands and Table 3 for the complexes.
Primarily, the different mesomorphism of the ligands 12, 15
and 17 will be discussed with regard to the influence of the
additional methyl group in ligands 17 on the b-enaminoketone
moiety. Subsequently, it will be shown that the decreasing
effect on transition temperatures through the additional methyl
group in the ligands 17 is also observed in PdII complexes 23.
Hyperfine coupling was measured for benzene solution of 24
(n ~ 18). Four hyperfine lines due to coupling of the electron
spin S ~ ½ with the nuclear spin I ~ 3/2 were detected
(coupling constant ACu ~ 90 G). Further hyperfine splitting
into five lines (intensity ratio 1 : 2 : 3 : 2 : 1) resulted from
the coupling with two equivalent nitrogen atoms (I ~ 1, AN ~
11 G). Resolution enhancement using Gauß–Lorentz
a
Transformation (LB ~ 21, GB ~ 0.1) clearly gave five lines
with the expected intensity ratio 1 : 2 : 3 : 2 : 1.
1 7 2 4
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 1 7 2 2 – 1 7 3 0

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Table 1 Crystallographic data for ligands 12 (n ~ 12), 17 (n ~ 10) and the PdII complex 23 (n ~ 16)
Compound
12
17
23 * 2CHCl3
Empirical formula
Formula weight/g mol21
Temperature/K
C29H39NO4
465.61
183(2)
C28H37NO4
451.59
183
C68H96N2O8Pd* 2 CHCl3
1414.60
183
˚
Wavelength/A
0.71073
Monoclinic
P2l
0.71073
Triclinic
P-1
0.71073
Triclinic
P-1
Crystal system
Space group
Unit cell dimensions
˚
a/A
5.4266(3)
9.1466(8)
26.211(2)
5.1776(4)
11.785(1)
20.724(2)
91.927(7)
93.548(6)
91.903(7)
1260.6(2)
2
1.190
7.5241(4)
9.2447(9)
25.918(3)
91.226(3)
90.943(6)
94.782(5)
1795.8(3)
1
1.308
˚
b/A
˚
c/A
a/u
b/u
c/u
92.904(5)
3
˚
Volume/A
1299.3(2)
2
1.190
0.078
504
0.20 6 0.18 6 0.12
3.80 to 27.45
26 ¡ h ¡ 6
211¡ k ¡ 11
233 ¡ l ¡ 33
4639
4639
2645/1/311
1.014
Z
Dc/g cm23
m/mm21
F(000)
0.078
488
0.534
744
Crystal size/mm3
H range/u
Index ranges
0.18 6 0.16 6 0.08
2.58 to 27.43
0 ¡ h ¡ 6
215 ¡ k ¡ 15
226 ¡ l ¡ 26
6389
5749[Rint ~ 0.034]
3193/0/303
2.706
0.12 6 0.12 6 0.04
2.68 to 27.46
27 ¡ h ¡ 9
29 ¡ k ¡ 11
233 ¡ l ¡ 29
9325
6531[Rint ~ 0.082]
2558/0/394
0.759
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I w 2s(I)]
R indices (all data)
Absolute structure parameter
Max. features of final difference Fourier synthesis
R1 ~ 0.0823, wR2 ~ 0.1258
R1 ~ 0.168, wR2 ~ 0.152
23.0(19)
R1 ~ 0.085, wR2 ~ 0.152
R1 ~ 0.171, wR2 ~ 0.157
R1 ~ 0.061, wR2~ 0.121
R1 ~ 0.143, wR2 ~ 0.132
3
3
3
˚
˚
0.479 and 20.496 e A
˚
0.862 and 20.543 e A
0.195 and 20.205 e A
Mesomorphism of the ligands. Enaminoketones 12 melt at
fairly elevated temperatures, around 110–120 uC. In contrast
to previously reported systems,18,37–45 which were found to
display a rich polymorphism of up to six different smectic
phases, only smectic A mesophase (SmA) with a stability range
of 20–42 K is observed as well as a nematic phase (N).
Transition temperatures are only weakly influenced by alkyl
chain-length, particularly the clearing temperature. However,
the increase of the chain-length has a prejudicial effect on the
nematic phase which is strongly destabilised (a few degrees
only), at the expense of the more favoured SmA (Fig. 5). Both
mesophases were identified by optical polarization microscopy.
Samples of the SmA phase displayed textures with large
homeotropic areas and myelinic figures as seen in Fig. 6,
whereas the nematic phase showed highly fluid Schlieren
textures on cooling from the isotropic liquid.
Fig. 2 Molecular structure of ligand 12 (n ~ 12). Selected bonds
˚
distances (A) and angles (u): N(1)–C(4) 1.341(5), N(1)–C(5) 1.415(5),
O(1)–C(2) 1.209(5), C(3)–C(4) 1.361(6), C(2)–C(3) 1.413(6), C(4)–
N(1)–C(5) 126.7(4), N(1)–C(4)–C(3) 126.2(4), C(2)–C(3)–C(4) 122.9(4),
O(1)–C(2)–C(3) 121.0(4), O(1)–C(2)–C(1) 119.8(4) (hydrogen atoms
omitted).49
As a result of the introduction of another methyl group at
the b-enaminoketone moiety (17) the melting and clearing
points decrease dramatically (Table 2) by about 50 K on
average, along with a huge reduction of the liquid crystalline
phase temperature range when compared to the thermal
behaviour of ligands 12. Of all ligands 17, the dodecyloxy
homologue exhibits the broadest enantiotropic mesophase
range with a temperature range of 15 K only. Compounds with
n ~ 10 and n ~ 12 showed a nematic phase in addition to a
smectic A phase, but the mesomorphism of the decyloxy is
monotropic (Fig. 7). The nematic phase is absent for all the
other members of this series. Thus, as in 12, the SmA phase is
still the favoured mesophase in this series. The overall
reduction of the mesophase stability may be linked to the
substantial broadening of the molecule upon the adjunction of
the methyl group, reducing as such the strength of the lateral
interactions permitting more stable arrangements.
Fig. 3 Molecular structure of ligand 17 (n ~ 10). Selected bonds
˚
distances (A) and angles (u): O(1)–C(2) 1.236(4), N(1)–C(4) 1.343(4),
N(1)–C(6) 1.438(4), C(2)–C(3) 1.439(4), C(3)–C(4) 1.363(4), C(4)–
N(1)–C(6) 125.1(3), O(1)–C(2)–C(3) 123.0(3), C(4)–C(3)–C(2) 124.0(3),
N(1)–C(4)–C(3) 122.0(3), C(3)–C(4)–C(5) 120.9(3) (hydrogen atoms
omitted).49
Fig. 4 Molecular structure of PdII complex 23 (n ~ 16). Selected
˚
bonds distances (A) and angles (u). Pd–O(1) 1.970(3), Pd–O(1’)
In order to examine the effect of core length on mesophase
stability, the more elongated enaminoketone 15 was also
synthesized. With the introduction of another 4-hydroxy-
benzoyl moiety the melting and the clearing points of 15 were
raised, as expected, by 13 K and by nearly 100 K respectively
1.970(3), Pd–N 2.017(4), Pd–N’ 2.017(4), O(1)–C(1) 1.276(6), C(1)–
C(2) 1.372(6), C(2)–C(3) 1.420(7), N–C(3) 1.306(6), O(1)–Pd–O(1’)
180.0(2), O(1)–Pd–N 91.63(15), O(1)–Pd–N’ 88.37(15), O(1)–C(1)–C(2)
126.1(5), C(1)–C(2)–C(3) 126.8(5), N–C(3)–C(2) 123.3(5) (hydrogen
atoms omitted).49
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 1 7 2 2 – 1 7 3 0
1 7 2 5

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Table 2 Transition temperatures and thermal data for the ligands 12,
13, 14, 15 and 17
Table 3 Transition temperatures and thermal data for the metal
complexes 18, 19, 20, 22, 23 and 24
Temperature/ DHm
/
DSm/
Temperature/ DHm
/
DSm/
Ligand
n
Transitionb uC kJ mole21 J K21 mole21
Complex n
Transitionb uC kJ mole21 J K21 mole21
12
10 Cr–Cr’
Cr’–SmA
SmA–N
86
129
149
165
91
17.6
37.6
0.6
2.2
21.0
36.5
—
49
93
2
18
10 Cr–Cr’
Cr’–SmX
SmX–Nc
111
187
235
29.7
44.3
—
77
96
—
—
106
102
—
—
48
111
1
—
57
—
2
—
114
55
73
3
N–I
5
N–I 1 dec.c 240
—
12
12
12
12
12 Cr–Cr’
Cr’–SmA
SmA–Nc
N–Ic
14 Cr–Cr’
Cr’–SmA
SmA–N
N–Ic
16 Cr–Cr’
Cr’–SmA
SmA–N
N–Ic
58
92
—
—
118
98
12
—
180
32
12
—
182
15
—
—
—
—
—
—
—
—
114
2
18
18
18
18
12 Cr–Cr’
87
188
216
38.1
47.0
—
122
155
158
105
118
158
159
109
119
156
157
111
153
155
98
101
61
57
135
233
256
86
81
70
Cr’–SmX
SmX–Nc
—
N–I 1 dec.c 219
—
44.5
38.5
5.3
—
68.8
12.5
5.3
—
70.0
6.4
—
—
—
14 Cr–Cr’
61
181
206
246
72
175
213
225
77
162
176
215
231
137
169
183
156
164
172
155
164
166
152
158
164
142
145
139
142
16.2
50.5
0.6
—
19.6
—
Cr’–SmC
SmC–N
N–Ic
16 Cr–Cr’
Cr’–SmCc
SmC–N
N–Ic
1.0
—
18 Cr–SmA
SmA–N
N–Ic
18 Cr–Cr’
39.9
24.1
32.9
1.7
1.6
—
—
—
—
—
—
—
—
—
—
—
—
—
Cr’–Cr@
Cr@–SmC
SmC–N
N–I
13
13
14
14
15
12 Cr–Id
18 Cr–Id
3
8
Cr–Id
—
—
—
—
19
19
19
19
10 Cr–SmXd
SmX–Nd
N–Id
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
12 Cr–Id
12 Cr–SmAd
SmA–Nd
N–Id
12 Cr–SmCd
SmC–Nd
N–Id
—
17
17
10e Cr–I
I–N
41.1
1.0
0.6
36.6
44.2
1.4
1.3
43.2
3.4
63.3
4.7
71.3
5.0
16 Cr–SmCd
SmC–Nd
N–Id
N–SmA
2
SmA–Cr
12 Cr–SmA
SmA–N
N–I
14 Cr–SmA
SmA–I
16 Cr–SmA
SmA–I
18 Cr–SmA
SmA–I
66
67
79
82
75
83
79
85
108
130
4
18 Cr–SmCd
SmC–Nd
N–Id
4
17
17
17
124
10
177
13
200
14
20
20
22
23
16 Cr–Nd
N–Id
—
—
—
—
18 Cr–Nd
N–Id
83
85
12 Cr–SmC 1 200
dec.d
a Heating/cooling rate: 10 K min21, 5 K min21 or 0.2 K min21 b Cr,
.
Cr’ ~ crystalline phases, N ~ nematic, SmA ~ smectic A, I ~ iso-
tropic liquid. c Not observed by DSC. d Not analyzed by DSC.
e Measurements carried out on cooling for the monotropic phases.
10 Cr–Cr’
Cr’–Cr@
Cr@–N
N–I
12 Cr–Cr’
Cr’–N
N–I
14 Cr–N
N–I
16 Cr–N
N–I
120
151
186
221
156
177
202
168
189
158
176
154
167
126
128
123
125
21.8
5.7
48.0
3.2
12.9
46.2
2.6
82.8
2.8
40.9
2.4
38.6
2.7
—
—
—
—
44
11
86
5
24
84
5
153
5
77
4
73
5
—
—
—
—
23
compared to compound 12 with same chain-length. Consequently,
the stability of both phases (SmA and N), and particularly that
of the SmA, was considerably enhanced, a logical consequence
of the increase of the molecular aspect ratio (length/width).
23
23
23
24
24
18 Cr–N
N–I
16 Cr–Nd
N–Id
Mesomorphism of the complexes. The transition tempera-
tures of the PdII complexes 18, resulting from the complexation
of the palladium salt to the ligand 12, were considerably raised
as expected due to the elongation of the molecular anisotropy
(Fig. 5). In addition, the mesophase stabilities were enhanced
too. Smectic, along with nematic mesophases, were observed
above 175 uC. At alkyl chain lengths n ¢ 14 the smectic C
phase predominates whereas at lower chain lengths (n ~ 10,
12) another mesophase, presumably of the smectic type,
appears (SmX), but which could not be further identified.
This phase showed a non-characteristic grainy texture and
proved to be immiscible with the SmC phase obtained with the
longer-chain homologues. The SmC mesophase was identified
by its characteristic transition bar texture which appears on
cooling only near the N-to-SmC phase transition (Fig. 6).
Quite remarkably, the thermal behaviour is however very
much influenced by the type of metal. Indeed, while the melting
temperatures are still elevated for NiII and CuII complexes
(but not as much as for the PdII complexes), the mesophase
temperature ranges are strongly reduced, and particularly in
18 Cr–Nd
N–Id
a Heating/cooling rate: 10 K min21, 5 K min21 or 0.2 K min21 b Cr,
.
Cr’, Cr@ ~ crystalline phases; SmX ~ unidentified smectic meso-
phase; SmC ~ smectic C; N ~ nematic phase. c Not observed by
DSC. d Not analyzed by DSC. dec. ~ decomposition.
the copper(II) complexes (Fig. 7). This effect may be related to
the changes in the coordination geometry occurring around the
metal centre at the transition to the mesophase, and con-
sequently to lower intermolecular interactions. NiII complexes
19 were diamagnetic in the solid state, yet NMR in solution
revealed the presence of a paramagnetic species with suppo-
sedly distorted tetrahedral geometry. As for the copper(II)
complexes, EPR experiments seem to point to similar
distortion (vide infra).
Interestingly, and in all cases, the SmA phase of the metal-
free ligand was lost upon metal complexation at the expense of
1 7 2 6
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 1 7 2 2 – 1 7 3 0

View Article Online
Fig. 5 Phase diagram of ligand 12 and corresponding metal complexes
18–20.
Fig. 7 Phase diagram of ligand 17 and corresponding metal complexes
23, 24.
the SmC phase and/or nematic phase. Table 3 summarizes the
transition temperatures and thermal data of all the metal
complexes.
All bischelate PdII complexes 23, based on ligands 17, show a
single enantiotropic nematic phase, at slightly lower transition
temperatures than complexes 18. However, a decrease of the
transition temperatures from 186 uC (n ~ 10) to 154 uC (n ~
18), as well as a decrease of the mesophase stability, with
increasing chain length was observed (Fig. 7). The complex 23
with n ~ 10 exhibits the broadest mesophase range (over 35 K).
Again, the mesomorphic behaviour of the copper(II) complexes
24 is poor with complete suppression of the smectic meso-
morphism and strong reduction of the nematic phase stability
(very narrow stability range of about 2 K). The transition
temperatures of 24 are at least 16 K lower than complexes 20
having the same chain length.
Consequently, the presence of the additional, sterically
demanding methyl group on the b-enaminoketonato unit is
responsible for the suppression of the higher ordered smectic
mesophases, and of the stabilisation of the nematic phase.
Furthermore its presence leads to a decrease in the transition
temperatures, especially for the complexes 23 with longer side
chain length in contrast to complexes 18.
Complex 22, resulting from the coordination of the
elongated ligand 15 with a palladium salt was found to
decompose immediately after entering the SmC mesophase
above 200 uC, a likely consequence of the too large anisotropy.
Solid-state EPR measurements of the solid, nematic and
isotropic phase of nematogenic Schiff’s base CuII complexes by
Martinez and co-workers showed that the coordination sphere
of concentrated (pure) samples of the bischelate copper
complexes have a nearly square planar arrangement in the
solid state and also in the mesomorphic phase.36 Thus, it
was interesting to investigate the CuII complexes containing
Fig. 8 Solid-state EPR spectra of complex 24 (n ~ 18) at variable
temperatures.
b-enaminoketonato ligands by solid-state EPR spectroscopy in
a temperature range of 20 uC in the vicinity of and in the
isotropic liquid.
The EPR-spectra of the CuII complex 24 (n ~ 18) at variable
temperatures are depicted in Fig. 8. At 20 uC a small axial CuII
signal can be seen (lowest spectrum in Fig. 8). With the help of
the simulation program WINEPR SimFonia (Bruker) it is
possible to increase the resolution of the spectra at 20 uC and
the values for g^ ~ 2.0475, DHPP^ ~ 29 G and g, ~ 2.097,
DHPP, ~ 60 G can be determined. The symmetry of the EPR-
signal confirms the square planar environment expected for the
CuII ions at room temperature. When the sample is melted into
the liquid crystal state at 123 uC this signal disappears and at
Fig. 6 Left: Texture of the SmA phase of ligand 12 (n ~ 12) at T = 142 uC. Right: SmC transition bar texture of complex 18 (n ~ 18) at T = 214 uC.
Both on cooling from the isotropic liquid (magnification 100-fold, crossed polarizers).
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 1 7 2 2 – 1 7 3 0
1 7 2 7

View Article Online
124 uC a broad CuII signal (g ~ 2.07) arises. When the sample
is slowly cooled the small signal at g^ ~ 2.0475 and g, ~ 2.097
is restored (uppermost spectrum in Fig. 8).
The spectroscopic parameters g-tensor, hyperfine coupling
constant A and the line-width DHPP depend on the symmetry
and the strength of the crystal field in the vicinity of the
paramagnetic CuII centre. EPR spectra show that at the
transition temperature to the liquid crystalline phase at 123 uC,
the symmetry and the vicinity of the CuII centre change
dramatically. A rise of symmetry around the copper(II) ion
seems to occur due to the heating process in the solid-state.
Consequently, an approximately tetrahedral coordination at
the central CuII ion results. Moreover, the line shape of the
EPR-spectra in the isotropic phase does not change during
the transition to the nematic phase. The coordination state of
the CuII ion seems therefore to have a great effect on inter-
molecular interactions.
Fig. 10 X-ray diffraction pattern of complex 19 (n ~ 16) at T =
160 uC.
Additionally, complexes 20 (n ~ 16, 18) and 24 (n ~ 16, 18)
were investigated by SQUID measurements.
optical texture, to a confirmation of the SmA nature of the
mesophase.
In Fig. 9 the magnetic behaviour of a CuII enaminoketonato
complex 20 (n ~ 18) as a function of temperature is presented.
The magnetic moment increases weakly with temperature
staying below meff ~ 1.4 BM. On heating the sample over
117 uC, a sudden increase in the magnetic moment to 1.69 BM
is observed. On cooling, magnetization remains at a higher
level (above 1.54 BM). The calculated spin-only moment for an
S ~ 1/2 system would be 1.73 BM at T = 20 uC.46 Due to
experimental limitations the complex could not be heated to the
clearing point.
A
selection of complexes described above was also
investigated by small-angle X-ray scattering. In all cases only
the first-order Bragg reflex in the small angle region was
observed corresponding to the smectic periodicity d001. In the
˚
wide angle region the broad reflex centred around 4.5–4.6 A
was seen, indicating the quasi-liquid-like behaviour of the
molecules within the layers (Fig. 10). The layer spacing
obtained from the X-ray experiments was in all cases smaller
than the molecular length of the mesogens dcalc., which was
calculated using simple modelling procedures. This is coherent
with the increase of the molecular broadness upon complexa-
tion, and with the tilt of the core with respect to the layer
normal, in accordance with a SmC phase.
Magnetic behaviour can be explained by the breaking of
intermolecular interactions and magnetic couplings as the
complex passes from the crystal into the liquid-crystal state. In
the nematic phase the orientation of individual magnetic
moments seems enhanced and remains at a higher level even on
recrystallization. This can be attributed to alignment of the
molecules by the magnetic field.47,48 The non-mesomorphic
polycatenar copper(II) complex 25 did not exhibit a comparable
jump in the magnetic moment on heating which indicates that
the orientation of spin moments is closely associated with
nematic mesomorphism.
In order to finally identify the mesophases, the molecular
area, Amol, was calculated for these compounds. Let us recall
that the molecular area is directly deduced from the molecular
volume and the layer periodicity, the molecular volumes, Vmol
being estimated considering a density of 1 g cm23 for the
molecules in the mesophase, and then corrected by
,
a
temperature factor since the volume increases with tempera-
M
VCH ðTÞ
0:6022 VCH 2ðT0Þ
ture (Vmol
~
where M is molecular weight,
2
VCH ðTÞ~26:5616z0:02023T, T in uC, T0 ~ 22 uC) (Table 4).
2
The value of Amol, for the ligand, deduced from these calcula-
tions, corresponds very precisely to the expected cross-section
X-Ray studies
2
of molten alkyl chain in the SmA phase (ca. 24 A ). As for the
˚
In order to confirm the nature of the mesophases obtained in
these series of materials, powder X-ray diffraction experiments
were carried out on some samples.
The mesophases of assorted ligands 12 (n ~ 10, 12) were
investigated by small-angle X-ray scattering (SAXS). In the
small angle region of ligand 12 (n ~ 12) (T = 135 uC), one reflex
at 2h ~ 1.21u was observed which corresponds to a smectic
layer spacing of 36.5 A, whereas in the wider angle, the
˚
presence of a broad scattering at ca. 4.5 A confirmed the liquid-
like order of the smectic phase. This correlates fairly well with
complexes, the large value of Amol is consistent with the larger
breadth of the complexes with respect to the ligand, as well as
confirming the substantial molecular tilt expected for the SmC
phase. Applying the formula a ~ cos21(d001/dcalc.) allows
calculation of the tilt angle a of the smectic C phase, which was
found to be on average 60u, a normal value. Table 4 gives an
overview of the results.
˚
˚
the molecular length which was determined as 32.5 A from the
single crystal structure (see Fig. 2), and on the basis of the
Conclusion
We have synthesized different new series of b-enaminoketone
ligands and their corresponding bischelate NiII, PdII and CuII
complexes. Exclusively the b-enaminoketone ligands with only
one alkoxy side chain in the 4-position to the ester linkage and
their metal complexes exhibit liquid crystalline behaviour.
Thereby the ligands reveal smectic A and nematic phases, the
bischelate complexes show smectic C and nematic phases with
higher transition temperatures. EPR studies at different
temperatures show that the CuII complexes exhibit a change
in the coordination environment at the transition from the
crystal to the liquid crystal phase. The magnetic behaviour of
the CuII enaminoketonato complexes display a jump in
magnetic moment if the sample is heating in the liquid crystal
phase attributed to a possible alignment of the molecules by the
Fig. 9 m vs. T plot of complex 20 (n ~ 18) (B ~ 1 T).
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 1 7 2 2 – 1 7 3 0
1 7 2 8

View Article Online
Table 4 Summary of SAXS results
3
Vmol/A (at T)
2
˚
M/g mol21
Amol/A
˚
d001/A
˚
Compound
n
M
x
12
18
18
19
22
12
14
16
16
12
/
1
1
1
1
2
466
1090
1150
1100
1275
36.5
43.5
45.9
45.5
48.3
840 (135 uC)
2040 (190 uC)
2150 (190 uC)
2015 (160 uC)
2400 (200 uC)
23.0
46.9
46.8
44.3
49.7
Pd
Pd
Ni
Pd
12 K. Binnemans and C. Gorller-Walrand, Chem. Rev., 2002, 102,
2303–2345.
magnetic field. Additionally X-ray studies confirm the
character of the ascertaining mesophases and allow the deter-
mination of tilt angles of the smectic C phases of the
palladium(II) and nickel(II) complexes.
13 B. Donnio, D. Guillon, D. W. Bruce and R. Deschenaux,
Metallomesogens, in Comprehensive Coordination Chemistry II:
From Biology to Nanotechnology, ed. J. A. McCleverty and
T. J. Meyer, Elsevier, Oxford, UK, 2003, vol. 7, chapter 7.9, ed.
M. Fujita and A. Powell, pp. 357–627.
Acknowledgements
14 R. W. Date, E. F. Iglesias, K. E. Rowe, J. M. Elliott and
D. W. Bruce, Dalton Trans., 2003, 1914–1931.
We thank Dr M. A. Athannassopoulou (TU Darmstadt) for
performing SAXS of ligands well as Dr Manfred Friedrich for
recording EPR spectra and Mrs Petra Weiss for recording
DSC, respectively. We are indebted to Mr Henning Schirmer
for his help in the preparation of some ligands. Generous
support by the Fond der Chemischen Industrie and Degussa
AG is acknowledged. A. Martin thanks Studienstiftung des
deutschen Volkes for personal financial support.
15 Y. G. Galyametdinov, G. I. Ivanova and I. V. Ovchinnikov, Zh.
Obshch. Khim., 1991, 61, 234–237.
16 W. Pyzuk, E. Gorecka and A. Krowczynski, Liq. Cryst., 1992, 11,
797–802.
17 W. Pyzuk, E. Gorecka and A. Krowczynski, SPIE Vol.1845 Liquid
and Solid State Crystals, 1992, 277–282.
18 W. Pyzuk, E. Gorecka, A. Krowczynski and J. Przedmojski, Liq.
Cryst., 1993, 14, 773–784.
19 W. Pyzuk, E. Gorecka and A. Krowczynski, Mol. Cryst. Liq.
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20 A. Krowczynski, J. Szydlowska, D. Pociecha and E. Gorecka, Pol.
J. Chem., 1996, 70, 32–35.
21 J. Szydlowska, W. Pyzuk, A. Krowczynski and I. Bikchantaev,
J. Mater. Chem., 1996, 6, 733–738.
Appendix
22 A. Krowczynski, J. Szydlowska, D. Pociecha, J. Przedmojski and
E. Gorecka, Liq. Cryst., 1998, 25, 117–121.
23 A. Krowczynski, J. Szydlowska and E. Gorecka, Liq. Cryst., 1999,
26, 685–689.
Experimental section and methods of
characterization
The synthesis of the ligands and of the complexes, their
structural analysis, and the various techniques used for their
analytical characterization, the structural determination of the
mesophase structures and molecular structures have been
deposited in the electronic supplementary information.{
24 O. A. Turanova, Y. G. Galyametdinov and I. V. Ovchinnikov,
Russ. Chem. Bull., 2001, 50, 805–808.
25 I. Bikchantaev, Y. G. Galyametdinov, O. Kharitonova,
I. V. Ovchinnikov, D. W. Bruce, D. A. Dunmur, D. Guillon
and B. Heinrich, Liq. Cryst., 1996, 20, 489–492.
26 C. K. Lai, R. Lin, M. Y. Lu and K. C. Kao, J. Chem. Soc., Dalton
Trans., 1998, 1857–1862.
27 C. D. Yang, Y. S. Pang and C. K. Lai, Liq.Cryst., 2001, 28, 191–195.
28 U. Pietrasik, J. Szydlowska, A. Krowczynski, D. Pociecha,
E. Gorecka and D. Guillon, J. Am. Chem. Soc., 2002, 124,
8884–8890.
29 W. Pyzuk, A. Krowczynski, L. Chen, E. Gorecka and
I. Bickczantaev, Liq. Cryst., 1995, 19, 675–677.
30 A. Krowczynski, D. Pociecha, J. Szydlowska, J. Przedmojski and
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31 J. Szydlowska, A. Krowczynski, E. Gorecka and D. Pociecha,
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32 X. H. Liu, D. W. Bruce and I. Manners, J. Organomet. Chem.,
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33 R. P. Tuffin, K. J. Toyne and J. W. Goodby, J. Mater. Chem.,
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34 T. Kaharu and S. Takahashi, Chem. Lett., 1992, 1515–1516.
35 A. S. Kudryavtsev and I. A. Savich, Zh. Obshch. Khim., 1963, 33,
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36 J. I. Martinez, M. Marcos, J. L. Serrano, V. M. Orera and
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