Combined stabilizing effects of trifluoromethyl groups and semifluorinated side chains on the thermotropic liquid-crystal behavior of β-enamino ketone ligands and their bischelate PdII complexes

Article abstract of DOI:10.1002/ejic.201402489

Several linear β-enamino ketone ligands and their corresponding bischelate Pd^II complexes bearing various combinations of methyl and trifluoromethyl groups at the chelating center as well as peripheral hydrocarbon and perfluorinated side chains were prepared. The effect of these structural modifications on the thermotropic liquid-crystalline behavior of both the ligands and the corresponding metal complexes was studied. The relationships between the molecular structures and the corresponding thermal properties are discussed. As expected, the persistence of liquid-crystalline behavior was highly sensitive to both applied structural manipulations. On the one hand, increased steric congestion around the chelating moiety clearly appears detrimental to the retention of mesomorphism. On the other hand, perfluorinated chains at the molecular extremity can occasionally restore it. Therefore, the regulation of the balance between both structural parameters permits a precise control of the mesomorphic properties of the compounds.

Full text of DOI:10.1002/ejic.201402489

FULL PAPER
DOI:10.1002/ejic.201402489
Combined Stabilizing Effects of Trifluoromethyl Groups
and Semifluorinated Side Chains on the Thermotropic
Liquid-Crystal Behavior of β-Enamino Ketone Ligands and
Their Bischelate PdII Complexes
Alexander Martin,[a] Carolin Mügge,[a] Douglas L. Gin,[b]
Bertrand Donnio,*[c,d] and Wolfgang Weigand*[a,e]
Keywords: Metallomesogens / Palladium / Liquid crystals / Fluorinated ligands / Fluorophobic effects
Several linear β-enamino ketone ligands and their corre-
cussed. As expected, the persistence of liquid-crystalline be-
sponding bischelate PdII complexes bearing various combi- havior was highly sensitive to both applied structural
nations of methyl and trifluoromethyl groups at the chelating
center as well as peripheral hydrocarbon and perfluorinated
side chains were prepared. The effect of these structural
modifications on the thermotropic liquid-crystalline behavior
of both the ligands and the corresponding metal complexes
was studied. The relationships between the molecular struc-
tures and the corresponding thermal properties are dis-
manipulations. On the one hand, increased steric congestion
around the chelating moiety clearly appears detrimental to
the retention of mesomorphism. On the other hand, per-
fluorinated chains at the molecular extremity can occasion-
ally restore it. Therefore, the regulation of the balance be-
tween both structural parameters permits a precise control of
the mesomorphic properties of the compounds.
ties.[2] For instance, several studies have shown that the in-
sertion of electron-withdrawing groups such as halide, cy-
ano, hydroxy, or nitro groups into calamitic[2,6] and other[7]
mesogenic structures often has a stabilizing effect on the
structures and also influences the formation, the degree of
order, and the symmetry of the liquid-crystalline phases. In
addition, the integration of perfluorinated or semifluorin-
ated alkoxy side chains into classical calamitic and discotic
molecules is another means to drastically affect the LC
properties, and this approach has been largely and success-
fully exploited.[8] Indeed, it leads to enhanced microsegrega-
tion, caused by the chemical incompatibility between the
fluoroalkyl and alkyl chains (i.e., the fluorophobic effect),
which often modifies the stability of the mesophase in com-
parison with those of hydrocarbon analogs and, in some
cases, gives rise to the emergence of new types of liquid-
crystalline phases.[9–11]
Introduction
The rational design of functional molecules[1] plays a cru-
cial role in the development of new materials, such as the
engineering of new mesomorphic materials for commercial
applications.[2–5] One approach to partially achieve some of
the demanding and specific requirements for the use of li-
quid crystals as components in various types of devices is
the introduction of local dipole moments into liquid-crystal
(LC) systems.[2,6] The judicious incorporation of dipole mo-
ments within calamitic mesogenic molecules strongly influ-
ences their transition temperatures, the nature and stability
of their mesophases, and some of their physical proper-
[a] Institut für Anorganische und Analytische Chemie, Friedrich-
Schiller-Universität Jena,
Humboldtstraße 8, 07743 Jena, Germany
E-mail: wolfgang.weigand@uni-jena.de
http://www.uni-jena.de/Prof_W_Weigand.html
[b] Department of Chemistry & Biochemistry, and Department of
Chemical & Biological Engineering, University of Colorado,
Boulder, Colorado 80309, USA
For metal-containing liquid crystals,[12] the effect of halo-
genation, particularly fluorination, on the mesomorphic be-
[c] Institut de Physique et Chimie des Matériaux de Strasbourg, havior has been widely studied.[13] In contrast, metallome-
UMR 7504 (CNRS-University of Strasbourg),
sogens bearing perfluorinated chains are scarce, and only a
23 rue du Loess, BP 43, 67034 Strasbourg Cedex 2, France
few examples have been reported to date. Guillevic et al.
synthesized mesomorphic ReI carbonyl complexes with
benzaldimine-like ligands containing perfluoroalkyl side
chains with different tail lengths.[14] In contrast to their n-
alkyl parent compounds, which only exhibit a nematic
phase, the ReI complexes bearing at least one perfluoro side
chain show a smectic A (SmA) phase. The promotion of
E-mail: bdonnio@ipcms.u-strasbg.fr
http://www.ipcms.unistra.fr/?page_id=8049&lang=en
[d] Complex Assemblies of Soft Matter Laboratory (COMPASS),
UMI 3254 (CNRS-Solvay-University of Pennsylvania), CRTB,
350 George Patterson Boulevard, Bristol, PA 19007, USA
[e] Jena Center for Soft Matter (JCSM),
Philosophenweg 7, 07743 Jena, Germany
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402489.
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the SmA phase over the nematic (N) phase was clearly at- by simultaneous modifications of the core and chains on
tributed to the enhancement of microphase separation, the mesomorphic behavior.
which is strongly favored by tip-to-tip arrangements of ad-
We previously reported a series of calamitic β-enamino
jacent perfluorocarbon segments. Additionally, the melting ketone-based ligands and their corresponding PdII, NiII,
points of over 200 °C are ca. 80 °C higher than those of and CuII complexes and studied the influence of the
their n-alkyl analogs; this corresponds to a great stabiliza- number of methyl groups on the chelating moiety.[19] An
tion of the crystalline phase, but the complexes were prone increase in the number of methyl groups at the chelating
to extensive decomposition after entering the mesophase. enamino ketone moiety resulted in a decrease of the transi-
Mesomorphism was also confirmed in various complexes tion temperatures for both the ligands and complexes. This
containing isocyanide one-ring ligands with a fluorous pon- effect can be attributed to the sterically more demanding
ytail.[15] Other interesting examples of metallomesogens in- methyl groups at the chelate ring and the consequent re-
clude polycatenar salicylaldimine complexes,[16] mono- and duction of lateral interactions. Furthermore, this structural
dinuclear (ortho-metallated imine) PtII and PdII com- modification produced a drastic decrease of the mesophase
plexes,[17] and tetradentate cis-enamino ketone com- stability of both ligands and complexes and, in addition,
plexes.[18] Careful studies on such fluoroalkylated systems provoked mesophase transformation from a smectic to a
have revealed that increasing the number of fluorocarbon nematic phase for the PdII complexes, whereas mesomor-
substituents promotes the formation of columnar and cubic phism was not retained in the CuII complexes.
phases at the expense of smectic phases, and the overall
stability of the mesophase increases owing to rising clearing arising from combining all these mentioned attributes:
temperatures. In almost all listed examples, however, both 1. Is it possible to further increase the steric bulkiness at
In the study presented here, we address the questions
structural modifications, that is, the fluorination of the me- the core by introducing even larger substituents such as CF3
sogenic core to induce dipolar effects and the introduction in place of CH3 and still form stable metal complexes?
of perfluorinated chains to enhance microsegregation, have
2. Can this modification enhance the previously observed
been independently performed despite the benefits gained decrease of transition temperatures in this model system?
Scheme 1. Synthesis of the β-enamino ketone ligands 9–16. Linear synthetic route for the β-enamino ketone ligands 9 and 10: (i)
H2n+1CnBr, K2CO3 (3 equiv./hydroxy group), CH3CN, reflux; (ii) KOH, EtOH, reflux, HCl; (iii) p-HO–Ph–NO2, DCC, DMAP, CH2Cl2;
(iv) Pd/C, H2, benzene; (v) CH3CN, reflux. Convergent synthetic route for β-enamino ketone ligands 11–16: (i) H2n+1CnBr, K2CO3
(3 equiv./hydroxy group), CH3CN, reflux; (ii) KOH, EtOH, reflux, HCl; (v) CH3CN, reflux for 5 and 6 (R2 = C2H5 or CH3); (vi) EtOH,
reflux for 4 and 7 (R2 = H); (vii) DCC, DMAP, CH2Cl2.
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3. Is it possible to control the mesophase stability by
Results and Discussion
carefully tuning the structural features of the whole mo-
lecule by considering not only the steric effects but also the
electronic effects of fluorinated substituents?
Synthesis of the Ligands
The ligands were synthesized according to two different
protocols: the β-enamino ketone ligands were either ob-
tained by a linear synthetic route (9 and 10) or by a conver-
gent method (11–16, Scheme 1). The synthesis of precursors
1–3 followed the standard procedure described pre-
viously.[19] The two series of compounds 9 and 10 were ob-
tained by heating the corresponding 4Ј-aminophenyl-(4-alk-
oxybenzoate) esters 3 with 4-ethoxy-1,1,1-trifluorobut-3-en-
2-one or 4-methoxy-1,1,1-trifluoropent-3-en-2-one, respec-
tively, in acetonitrile under reflux.[20] For the preparation
of compounds 11, the direct condensation of 1,1,1,5,5,5-
hexafluoroacetylacetone and precursor 3 in dry ethanol was
unsuccessful. In contrast, they were prepared indirectly
from the substituted β-oxovinylaminophenol 4 and the ap-
propriate benzoic acids 1. Similarly, this convergent method
was successfully applied for the preparation of compounds
12–16 by coupling the various substituted β-oxovinylamino-
phenols 4–7 and 4-alkoxybenzoic acid 1 or the semifluorin-
ated 4-alkoxybenzoic acid 8.
4. Is there a limit of modification that finally leads to
loss of the mesomorphic properties of such a model system?
We will report on compounds bearing these desired fea-
tures: different ligands 9–12 (Scheme 1) with a stepwise re-
placement of the methyl groups by trifluoromethyl
moieties were synthesized and subsequently reacted with
Pd(OAc)2 to form the corresponding bischelate palladi-
um(II) complexes 17–20 (Scheme 2). Additionally,
a
5,5,6,6,7,7,8,8,9,9,10,10,10-tridecafluorodecyloxy side chain
was introduced in 13–16 and 21–24, which are structural
analogs of 9–12 and 17–20, respectively, to demonstrate
whether the fluorophobic effect observed in common or-
ganic liquid crystals is also applicable to metallomesogens.
The thermal and liquid-crystalline behaviors of these com-
pounds were investigated by polarized optical microscopy
(POM) and differential scanning calorimetry (DSC).
Ligands 11–16 were obtained in good yields with N,N-
dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyr-
idine (DMAP) as coupling reagents for the condensation
reaction. For the preparation of precursors 4 and 7,
1,1,1,5,5,5-hexafluoroacetylacetone
or
acetylacetone,
respectively, were heated to reflux with 4-aminophenol for
several hours in dry ethanol, whereas compounds 5 and 6
were obtained by the dropwise addition of the correspond-
ing trifluoromethyl β-oxovinyl ether to a hot solution of 4-
aminophenol in dry acetonitrile.
1
The H NMR signals of the enamino protons in 9 and
13 are doublets at δ ≈ 11.85 ppm. A trans coupling of 13 Hz
with the neighboring proton is also observed. Enamino
ketones 11 and 15, which have an additional trifluoromethyl
group next to the nitrogen atom, exhibit singlets for the
enamino proton at δ = 11.83 ppm. In contrast to 11 and
15, enamino ketones 10, 12, 14, and 16 with methyl groups
show this signal as a singlet at δ ≈ 12.5 ppm. For all com-
pounds except 12 and 16, the 13C NMR signal of the tri-
fluoromethyl carbon atom exhibits a quadruplet peak at δ
1
≈ 117 ppm with a coupling constant JC,F = 287 Hz. The
Scheme 2. Synthesis of the bischelate PdII complexes 17–24.
carbonyl carbon atom next to the trifluoromethyl group
shows a quadruplet at δ = 179.6 ppm (2JC,F = 34 Hz). The
second trifluoromethyl group in 11 and 15 gives an ad-
On the basis of the data obtained, the liquid-crystalline
behavior of the β-enamino ketones 9–16 will be primarily
discussed with regard to the influence of the different sub-
stitution patterns at the β-enamino ketone moiety (9–12).
Furthermore, it will be shown that the introduction of a
semifluorinated alkoxy side chain (as in 13–16) has a pro-
found stabilizing effect on the mesophases, as deduced from
a comparison with their nonfluorinated parent compounds
(9–12). Subsequently, the thermal behavior of the corre-
sponding bischelate PdII complexes 17–24 will be discussed.
ditional quadruplet carbon signal at δ = 119 ppm (1JC,F
=
277 Hz). In the 19F NMR spectrum of ligand 9 (n = 14), a
singlet at δ = –77.58 ppm indicates the presence of the three
fluorine atoms of the trifluoromethyl group.
Synthesis of the Metal Complexes
Enamino ketones 9–16 (2 equiv.) and palladium(II) acet-
In particular, the stronger microphase separation in bischel- ate were heated to reflux in dry acetonitrile to afford the
ate PdII complexes 21–24 results in higher transition tem- corresponding palladium(II) chelates 17–24, which precipi-
peratures.
tated from solution (Scheme 2). The crude complexes were
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collected by filtration through Celite and purified by phases by 19 (for n = 18) to 58 °C (for n = 10). In addition
crystallization from tetrahydrofuran (THF). They were ob- to the increase of the mesomorphic temperature range in
tained as yellow solids that are stable in aprotic solvents. the series, the trifluoromethyl group is also responsible for
1
The H NMR spectra of all of the PdII complexes lack the the suppression of the less-ordered nematic phase that is
signal corresponding to the free NH proton of the enamino present in the analogous methyl compounds (9, X = CH3,
ketonato moiety, and the signal of the methine proton at Y = H).[19]
the chelating ring is shifted to higher field by 0.2 ppm rela-
In an earlier study, we showed that the presence of two
tive to the signal of the respective free ligand. The upfield methyl groups in the β-enamino ketone chelating moiety
shifts of the carbonyl signals of ca. 19 ppm in the 13C NMR instead of only one leads to a substantial decrease of the
spectra confirm the formation of the complexes. Addition- phase transition temperatures and of the mesophase tem-
ally, in the 19F NMR spectrum of 17 (n = 14), the signal of perature ranges without changes to the nature of the me-
the fluorine atoms of the trifluoromethyl group shows an sophases (12, X = Y = CH3, nՆ12, SmA and N phases,
upfield shift of ca. 4 ppm to δ = –73.68 ppm.
Table 1).[19] Quite unexpectedly, the replacement of one
methyl group by a trifluoromethyl group at the β-enamino
ketone unit (10, X = CF3, Y = CH3) or both methyl groups
by two trifluoromethyl groups (11, X = Y = CF3) causes
the complete loss of mesomorphism, but the compounds of
the series still melt at higher temperatures than 12 (Table 1,
Figure 1). Even the use of longer alkyl tails proved to be
Mesomorphism of the Ligands
All members of series 9 (X = CF3, Y = H) display en-
antiotropic mesomorphic behavior, as observed by POM
(cf. Supporting Information). They melt into a single SmA
phase between 92 and 108 °C and are stable up to the clear-
ing temperature at 171–186 °C (Table 1, Figure 2). These
transition temperatures show a substantial dependence on
the alkyl tail length (i.e., a slight increase of the melting
points and a decrease of the clearing points results in a
narrowing of the mesomorphic range with increasing chain
length). The SmA mesophase was easily recognized by
POM through the growth of fan-shaped optical textures
from batônnets and the presence of large homeotropic re-
gions upon cooling from the isotropic liquid (cf. POM
images in the Supporting Information). Thus, as a result of
the replacement of a methyl group by a trifluoromethyl
group at the chelating unit (9, X = CH3 ǞCF3), a decrease
of the melting temperatures with a simultaneous increase of
the clearing temperatures was observed.[19] This leads to a
Figure 1. Phase transition temperature diagram of β-enamino
ketone ligands 13, 14, 15, and 16 with semifluorinated side chains
compared to their nonfluorinated n-alkoxy parent compounds 9,
10, 11, and 12, respectively, all with an overall chain length of 10
net broadening and stabilization of the liquid-crystalline carbon atoms.
Table 1. Thermodynamic data of β-enamino ketonate compounds 9–16. Transition temperatures are given in °C, and the corresponding
enthalpies are shown in square brackets [ΔH in kJmol–1]. Cr: crystalline phase, SmA: smectic A phase, N: nematic phase, I: isotropic
phase, ΔT: mesophase temperature range.
Ligand
Transition properties
ΔT
X
Y
R
9
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CH3
H
H
H
H
C10H21
C12H25
C14H29
C16H33
C18H37
C10H21
C12H25
C14H29
C16H33
C18H37
C10H21
C16H33
C10H21
Cr 92 [38.5] SmA 186 [8.5] I
Cr 100 [45.1] SmA 182 [8.9] I
Cr 105 [55.2] SmA 180 [9.3] I
Cr 107 [63.7] SmA 177 [9.7] I
Cr 108 [70.0] SmA 171 [9.7] I
Cr 98 [75.9] I
Cr 92 [103.0] I
Cr 95 [116.6] I
Cr 92 [129.2] I
Cr 101 [132.7] I
94
82
75
70
63
0
0
0
0
0
9
9
9
9
H
10
10
10
10
10
11
11
12[a]
CH3
CH3
CH3
CH3
CH3
CF3
CF3
CH3
Cr 86 [83.2] I
Cr 67 [74.5] I
Cr 86 [41.1] I
0
0
0
I 81 [1.0] N 70 [0.6] SmA 66 [16.6] Cr
Cr 114 [62.5] SmA 223 [13.1] I
Cr 107 [42.9] SmA 128 [5.8] I
Cr 97 [51.7] I
11/16
109
21
0
13
14
15
16
CF3
CF3
CF3
CH3
H
C4H8C6F13
C4H8C6F13
C4H8C6F13
C4H8C6F13
CH3
CF3
CH3
Cr 95 [51.4] SmA 182 [12.1] I
87
[a] Monotropic behavior, see ref.[19]
.
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inefficient to compensate for the likely volume increase of tails (n = 16 and 18) at the expense of the single SmA phase
the chelating moiety upon such a structural modification. apparent in the ligands (Figure 2). The nematic mesophase
The reason for these results is not yet clear, but the relative was identified by its schlieren optical texture with two and
size of the substituents around the chelating ring appears to four brushes on cooling from the isotropic liquid, whereas
be crucial for the retention or emergence of mesomorphism. the SmC phase exhibited a characteristic broken focal conic
Semifluorinated and perfluorinated alkyl chains texture (Figures 3 and 5, left; for more POM images, see the
tend to stabilize thermotropic mesophases.[8–11] Conse- Supporting Information).
quently, we analyzed the mesomorphism of four additional
analogs to 9–12, namely, 13–16, which bear
a
5,5,6,6,7,7,8,8,9,9,10,10,10-tridecafluorodecyloxy side chain
instead of the decyloxy chain. Independently from substitu-
tion at the β-enamino ketone moiety, 13, 14, and 16 show
a stable SmA phase, whereas 15, which has two trifluorome-
thyl groups, does not exhibit any mesomorphic behavior (as
its alkylated analog 11; Table 1, Figure 1). Instead, it di-
rectly melts into an isotropic liquid at 97 °C. This clearly
indicates that the presence of two trifluoromethyl groups at
the chelating ring is an unfavorable situation for the induc-
tion of mesomorphism. Although the SmA phase is stabi-
lized by only 15 °C in the row from 9 to 13, the induction
Figure 3. POM images of the mesophases of 17 (n = 18). The schlie-
ren texture of the N phase (T = 173 °C, left) transforms into the
SmC phase upon cooling (T = 166 °C, right).
The complexation of the non-mesomorphic ligands 10
or stabilization of a broad SmA phase was seen for 14 and (X = CF3, Y = CH3) and 11 (X = Y = CF3) to PdII led
16 with respect to 10 and 12, correspondingly. Quite re- to the non-mesomorphic materials 18 and 19, respectively,
markable is the extensive stabilization of the mesophase for despite the elongation of the molecular core. Only the clear-
16, for which the mesomorphic temperature range reaches ing temperatures increased by ca. 100 °C. Finally, complex
87 °C. The overall stabilization of the mesomorphism in 20, prepared from 12,[19] showed a thermodynamically
these three compounds is clearly connected to an enhance- stable nematic phase (Cr 186 N 221 I).
ment of the microphase separation between the various
A comparison of ligands 9–12 and the corresponding
chemically incompatible parts, which somehow “dilutes” PdII complexes 17–20 shows that the overall behavior fol-
the substitution effect around the chelating ring, and thus lows the same trend. When the ligand itself is mesomorphic,
confirms the presented results.
an important stabilization of the mesophases is observed in
the corresponding metal complexes. However, there is no
induction of mesomorphism upon complexation of non-
mesomorphic ligands. The increase of the transition tem-
peratures and the enhancement of the stability of the me-
sophases are usually expected upon metal complexation,
unlike the diminution of the mesomorphic temperature
range in the metal complexes, which is not predictable and
depends on the metal center.
Such a logical trend of the thermal behavior was, how-
ever, not entirely observed between the ligands and com-
plexes bearing semifluorinated alkyoxy chains. Indeed, only
two complexes (21, X = CF3, Y = H and 24, X = Y = CH3)
out of four exhibited mesomorphism. In contrast, like all
of the systems with X = Y = CF3, 23 is not mesomorphic.
Surprisingly, the PdII complex 22, prepared from the smec-
togen 14, was also deprived of a mesophase.
Mesomorphism of the Complexes
The transition temperatures of the PdII complexes 17 ob-
tained from ligands 9 (X = CF3, Y = H) were considerably
higher than those of the free β-enamino ketone ligands. The
melting temperatures of the complexes were greater than
160 °C (Figure 2), as expected from the extension of molec-
ular anisotropy. In contrast, the mesophase temperature
range was considerably reduced (from ca. 60–70 to ca. 10–
30 °C). Moreover, the nature of the phases changed, and a
nematic phase was observed (for all members) along with a
smectic C mesophase (SmC) for the complexes with longer
Complexes 21 and 24 revealed a quite different meso-
morphic behavior compared to that of their nonfluorinated
alkyloxy parent compounds 17 and 20, respectively
(Table 2). Thus, the N phase was replaced by a SmA phase
in both cases, along with an increase of the melting transi-
tion temperatures to 222 and 240 °C for 21 and 24, respec-
tively. A slight broadening of the mesophase was also ob-
served, and 24 started to decompose after clearing at 284 °C
(Figure 4).
The texture of the SmA phase of 21 is shown in Figure 5
(right picture); it has a typical focal conic texture containing
Maltese crosses. As mentioned, complexes 22 (X = CF3, Y
Figure 2. Phase transition temperature diagrams of ligands 9 and
their corresponding bischelate PdII complexes 17.
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Table 2. Thermodynamic data of palladium(II) complexes 17–24. Transition temperatures are given in °C, and the corresponding enthalp-
ies changes are shown in square brackets [ΔH in kJmol–1]. Cr, CrЈ, CrЈЈ: crystalline phases, SmA: smectic A phase, SmC: smectic C
phase, N: nematic phase, I: isotropic phase, decomp.: decomposition, ΔT: mesophase temperature range.
Ligand
Transition properties
ΔT
X
Y
R
17
17
17
17
17
18
18
18
18
18
19
19
20[a]
21
22
23
24
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CH3
CF3
CF3
CF3
CH3
H
H
H
H
C10H21
C12H25
C14H29
C16H33
C18H37
C10H21
C12H25
C14H29
C16H33
C18H37
C10H21
C16H33
Cr 158 [22.0] CrЈ 189 [52.8] N 221 [4.0] I
Cr 133 [9.2] CrЈ 181 [56.1] N 205 [3.8] I
Cr 174 [59.7] N 192 [3.5] I
Cr 168 [62.9] (SmC 163 [0.6]) N 181 [3.6] I
Cr 160 [58.5] SmC 168 [0.8] N 173 [3.2] I
Cr 84 [27.6] CrЈ 225 [64.3] I
Cr 93 [37.9] CrЈ 211 [64.3] I
Cr 103 [38.1] CrЈ 200 [63.4] I
Cr 106 [38.8] CrЈ 192 [62.4] I
Cr 110 [43.3] CrЈ 186 [64.8] I
Cr 149 [21.7] CrЈ 205 [67.4] I
Cr 89 [33.4] CrЈ 163 [54.8] I
Cr 120 [21.8] CrЈ 151 [5.7] CrЈЈ 186 [48.0] N 221 [3.2] I
Cr 55 [12.2] CrЈ 116 [0.5] CrЈЈ 222 [40.4] SmA 278 [8.7] I
Cr 196 [12.3] CrЈ 233 [42.9] I
Cr 160 [19.0] CrЈ 218 [52.4] I
Cr 240 [44.5] SmA 284 [7.4] I/decomp.
32
24
18
13
8/5
0
0
0
0
0
0
0
35
56
0
0
44
H
CH3
CH3
CH3
CH3
CH3
CF3
CF3
CH3
H
C10H21
C4H8C6F13
C4H8C6F13
C4H8C6F13
C4H8C6F13
CH3
CF3
CH3
[a] Ref.[19]
estingly, the non-mesomorphic complexes 18, 19, 22, and 23
clear, on average, in the same temperature range (Figure 4).
Discussion
At the periphery of the β-enamino ketone moiety, the
appending substituents, that is, the methyl and/or trifluoro-
methyl groups, are located in the plane of the chelating ring.
Therefore, the N,O-units can be regarded as flat disks, and
their diameters depend on the substitution pattern as the
volume of a trifluoromethyl group is approximately 50%
Figure 4. Phase transition temperature diagrams of PdII complexes greater than that of a methyl group.[21] Therefore, if the β-
21, 22, 23, and 24 with semifluorinated side chains and their non-
fluorinated n-alkoxy parent compounds 17, 18, 19, and 20.
enamino ketone moiety is substituted by a trifluoromethyl
group, the diameter of the chelating unit increases com-
pared to that of the methyl analog. The presence of a
methyl group or of a second trifluoromethyl group close to
the primary trifluoromethyl group concomitantly increases
the size of the N,O unit. For liquid-crystalline systems, the
stabilization or formation of mesophases depends on three
main factors: a specific balance between the cross-sections
of the rigid part of the molecule and of the molten alkyl
chains, microphase segregation between incompatible parts,
and dispersive molecular interactions between the mesogens
(Figure 6). Therefore, if the diameter of the rigid part of a
rodlike unit increases, two possibilities may occur. For a
smectic phase, the rigid core will tilt, and for a given volume
of molten alkyl chains, the latter will have to occupy a
larger cross-section to compensate for this apparent in-
crease of the molecular core thickness. This chain deficit
can be counterbalanced by interdigitation of the chains
from neighboring smectic layers, or more precisely by their
folding. This is true up to a certain congestion of the core
diameter, at which point the chains themselves cannot com-
pensate for the tilt of the core and the loss of lateral inter-
molecular interactions. In this ultimate case, the smectic lay-
ers are lost. Another possibility is that the molecules will
= CH3) and 23 (X = Y = CF3) were not mesomorphic,
despite their fluorinated side chains. Furthermore, the SmA
phase exhibited by the ligand 14 was lost in 22. Again, when
the chelating unit is substituted by two CF3 groups (in 18)
or one CH3 and one CF3 group (in 19), mesomorphism is
absent. Notably, the introduction of semifluorinated alkoxy
tails, as in 22 and 23, plays a tangential role and does not
guarantee the formation of liquid-crystalline phases. Inter-
Figure 5. Left: Schlieren texture of the N phase of 17 (n = 10) at
T = 203 °C; right: focal conic texture of the SmA phase of 21 (R
= C4H8C6F13) at T = 245 °C; both obtained by cooling from the
isotropic liquid (magnification 200-fold, crossed polarizers).
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Figure 6. Top: schematic representation of relative sizes of the central unit and peripheral chains in 17–23 following systematic substitu-
tion. Below: models of the arrangement of the n-alkoxy complexes 17 and 20 in the liquid-crystal phase and complexes 21 and 24 bearing
semifluorinated tails in the SmA phase.
escape from a lamellar arrangement to a nematic type of ever, the overall width/length ratio might be slightly less fa-
arrangement. However, if the overall aspect ratio (length/ vorable in the complexes compared to those of their ligands.
width) is not ideal, the phase is also lost.
Therefore, it is worthwhile to note that PdII complexes 21
The ligands bearing one methyl[19] or trifluoromethyl and 24 formed by ligands 13 and 16 (showing the broadest
group (9) at the β-enamino ketone moiety show broad and mesophase range of all semifluorinated ligands), respec-
stable mesophases. A slight enhancement of the diameter of tively, exhibit broad and stable mesophases. This also holds
the chelate unit in ligand 12 by the introduction of two true for 17 and 20 derived from ligands 9 and 12, although
methyl groups leads to a dramatic decrease of mesophase the nature of the mesophase is altered for the latter com-
stability. Further stepwise enhancement of the diameter pounds (Figure 6). Complex 22 (from ligand 14 with the
through the insertion of one trifluoromethyl group and an smallest mesophase stability of the semifluorinated ligands)
additional methyl group (10) or two trifluoromethyl groups did not show any liquid-crystalline behavior. This means
(11) at the chelating units gives non-liquid-crystalline com- that the diameter of the chelating unit is too large and the
pounds. Even the use of longer tails in these ligands does semifluorinated chains cannot fill the available space. The
not lead to liquid-crystalline materials; this leads us to the filling was still possible in the corresponding ligand 14, but
assumption that the alkyl chains cannot compensate for the the enlargement of the rigid central unit by complexation
discrepancy.
to a PdII ion makes the inaccessible volume too large. This
Through the replacement of hydrocarbon chains by semi- may also be responsible for the absence of mesomorphism
fluorinated chains in the ligands, mesophase induction and in PdII complex 23 with two trifluoromethyl groups per che-
stabilization were observed, particularly for ligands 14 and lating unit and would be in agreement with the behavior of
16.
the corresponding non-liquid-crystalline ligand 15. Again,
In addition to an increasing microphase separation ten- in these two cases, owing to the incompatible parts, which
dency of the incompatible components in the molecules, it show a strong tendency to segregate, the nematic phase is
is conceivable that the semifluorinated alkyl chain is able to not favored.
fill the increased available space as it has a larger diameter
than the respective alkyl chain.[21] However this is not the
case for ligand 15 with two trifluoromethyl groups. Thus,
the formation of the smectic phase is unfavorable for this
Conclusions
compound, as is the formation of the nematic phase as the
fluorinated chains still mix with the aromatic parts.
Several β-enamino ketone compounds with methyl and/
or trifluoromethyl groups at the chelating unit and their
For the bischelate palladium(II) complexes, the diameter corresponding bischelate PdII complexes have been synthe-
of the rigid central unit formed by two β-enamino ketonato sized to evaluate the effects of such structural modifications
moieties and the PdII ions is more than twice that of the on their mesomorphic properties. The progressive attach-
corresponding single ligand, owing to the size of the two ment of bulky groups at the chelating center is concomitant
ligand cores plus the effective metal radius. Due to the coor- with a decrease of the mesomorphism tendency in both li-
dination of the ligands in a trans configuration, the length gands and complexes. The position of the modified substit-
of the whole extended molecule also almost doubles; how- uents seems to be crucial for the final effect: the introduc-
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performed with a Perkin–Elmer DSC7 instrument calibrated
against a standard of high-purity Indium. Microscopic studies were
performed with a Carl-Zeiss Axiopol microscope with a Linkam
THMS 600 hot stage and a CI-93 controller. MS spectra were re-
corded with a Finnigan SSQ710 and MAT95XL spectrometer.
tion of larger substituents at the
X position (e.g.,
CH3 ǞCF3 in ligands 12 vs. 10 and complexes 20 vs. 18)
leads to a loss of mesomorphic properties. Here, the size
effect can be considered stronger for the ligands than for
the respective complexes, as the trans geometry in the com-
plexes partially compensates for the increased bulkiness of
the free ligand (Scheme 2). An increase in the size of sub-
stituent Y has a similar destabilizing effect. However, in the
ligands, the introduction of a semi-perfluorinated side chain
can to some extent balance out this widening of the ligand
core (and restore mesomorphic behavior in ligands 14 com-
pared to 10), whereas in the respective complexes (e.g., 22
compared to 18), this is not possible. This might in part be
due to the Y substituent pointing away from the longest
axis of the complexes when the core is considered as a flat
disc.
Overall, the introduction of a semifluorinated side chain
leads to the stabilization or induction of liquid-crystalline
behavior in some of the resulting materials, essentially be-
cause of the enhancement of microphase separation. The
mesomorphic ligands reveal a smectic A phase, whereas the
complexes with n-alkyl tails show smectic C and nematic
mesophases. The SmA phase in the complexes is favored if
the tails are replaced by semifluorinated chains. This behav-
ior highlights the increasing tendency of microphase segre-
gation of incompatible molecular parts, also present in
these metallomesogens.
General Procedure for the Synthesis of β-Enamino Ketone Ligands
9 and 10: The corresponding 4-aminophenyl 4-alkoxybenzoate 3
(1 equiv.) was dissolved in a minimum amount of boiling dry aceto-
nitrile.
A
solution of 4-ethoxy-1,1,1-trifluorobut-3-en-2-one
(1.1 equiv.) for ligands 9 or 4-methoxy-1,1,1-trifluorpent-3-en-2-
one (1.1 equiv.) for ligands 10 in dry acetonitrile (2 mL) was added
dropwise. The solution was heated at reflux for 4 h and then cooled
to room temperature. The solvent was evaporated, and the resulting
residue was crystallized from ethyl acetate to give the products as
pale yellow solids.
General Procedure for the Synthesis of β-Enamino Ketone Ligands
11, 13, 14, 15, and 16: The respective substituted 4-anilinophenol
4–7 (1 equiv.), 4-decyloxybenzoic acid (1, 1 equiv.) or 4-
(5,5,6,6,7,7,8,8,9,9,10,10,10-tridecafluorodecyloxy)benzoic acid (8,
1 equiv.), and DMAP (10 mol-%) were dissolved in dry dichloro-
methane (150 mL). To the stirred solution, DCC (1 equiv) was
added. The solution was stirred for 20 h at ambient temperature,
after which it was filtered, and the solvent was evaporated. The
crude product was purified by column chromatography (SiO2/
CH2Cl2). The products were recrystallized from methanol to give
pale beige crystals.
General Procedure for the Synthesis of the Bischelate β-Enamino
Ketonato Palladium(II) Complexes 17–24: The ligand 9–16
(2 equiv.) and PdII acetate (1 equiv.) were heated to reflux in dry
acetonitrile (25 mL). After 4 h, the solvent was removed under vac-
uum, and the brown residue was dissolved in dichloromethane. The
solution was filtered through 3 cm of Celite, and the solvent was
evaporated. The resulting crude product was recrystallized from
ethyl acetate and tetrahydrofuran (80:20 v/v) to give a yellow solid.
Apparently, the most important criterion in maintaining
or favoring mesomorphism is the molecular aspect ratio,
which is tuned by both the substitution pattern of the che-
lating ring and the chemical nature of the side chains. Both
play a significant role and stand in delicate balance to each
other. By careful choice of both parameters, the modifica-
tion of the mesomorphic properties of the compounds is
possible.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental details, analytical data for all described com-
pounds, POM images of various compounds.
Acknowledgments
Experimental Section
A. M. thanks the Studienstiftung des deutschen Volkes for personal
financial support. C. M. wishes to thank the Carl Zeiss Stiftung
for a scholarship. Support for chemicals and materials for work
performed at CU Boulder was provided by the National Science
Foundation through a grant to D. L. G. (DMR-0111193). B. D.
thanks the Centre National de la Recherche Scientifique (CNRS)
for support.
General: All reactions were performed under an argon atmosphere
by standard Schlenk techniques. Solvents were dried and distilled
according to standard procedures and stored under an argon atmo-
sphere. Palladium(II) acetate was purchased from Aldrich.
All alkoxybenzoic acids 1,[19] 4Ј-nitrophenyl 4-alkoxybenzoate es-
ters 2,[19] 4Ј-aminophenyl 4-alkoxybenzoate esters 3,[19] 4-ethoxy-
1,1,1-trifluorobut-3-en-2-one,[20] 1,1,1-trifluoro-4-methoxypent-3-
en-2-one,[20] 4-[(4-hydroxyphenyl)amino]pent-3-en-2-one (7),[22] 4-
[(4-oxopent-2-en-2-yl)amino]phenyl 4-(decyloxy)benzoate (12),[19]
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Received: June 1, 2014
Published Online: October 1, 2014
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