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Improved Properties of Ferroelectric Liquid Crystals from Palladium -Diketone Complexes.

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[6] Other examples ofcompounds with double-melting behavior: K. Ohta. M.
Yokohama, S . Kusabayashi, H. Mikawa, J Chem. Sor. Chem. Commun.
1980,392; K. Ohta. H. Muroki, K. 1. Hatdda, 1. Yamamoto, K. Matsuzaki. Mol. Cryst. Liy. Cryst. 1985, 130, 249; K. Ohta, H. Muroki. K. I.
Hatada. A. Takagi. H. Ema, I. Yamamoto, K. Matsuzaki, ibid. 1986, 140,
163; K. Ohta. H. Ema. H. Muroki, I. Yamamoto. K. Matsuzaki, &id.
1987. 147, 61.
[7] The nonequivalence of the two halves of the dimer caused by the chiral
center becomes apparent only at - 20 "C in the 'H NMR spectrum: at this
temperature the t w o nonequivalent Haatoms give rise to two very close but
distinct singlets.
Modified complexes of type 4 have been measured. and frequency doubling was observed: J. Zyss, I. Ledoux, J. Martin, J. Buey, P. Espinet,
unpublished.
[a];" of the complexes ( c =1, CH,CI,): 4a, -8.8; 4b, -8.0; 4c, -7.7.
R. Cano. Bull. Soc. Fr. Mineral. Cristallogr. 1967, 90. 333. Cholesteric
mixtures containing about 20 wt% of the chiral additive were placed hetween a flat glass slide and a spherical lens. Both surfaces were rubbed to
provide a uniform planar anchoring. The disclination rings in the sample,
which correspond to steps ofp/2. were observed using a polarizing microscope. The pitch p was calculated from the diameters of these rings.
H. Finkelmann, H. Stegemeyer, Z. Naturforsch. A 1973, 28, 799.
G. Heppke. D. Lotzsch. F. Oestreicher, 2. Naturforsch. A 1987, 42, 279.
L. Komitov, S. T. Lagerwall, B. Stebler. G . Anderson, K . Flatiscler, Ferroelimrics 1991. 114, 167.
A. J. Slaney, I. Nishiyama, P. Styring, J. W. Goodhy, J. Mater. Chem. 1992,
2. 805.
and SmA phases at noticeably lower temperatures. Thus the
monomeric complexes may be taken repeatedly to the
isotropic state without signs of decomposition.[41
Taking these two observations into account, we aimed to
synthesize ferroelectric metallomesogens with low transition
temperatures, which would allow potential applications as
both liquid crystals and in nonlinear optics.16' Our first approach was the introduction of chirality in the alkyl chains
of the above-mentioned acac complexes,[41but this did not
lead to the desired SmC* phases. However, analogues 3 a
(n = 10) and 3 b ( n = 14) with modified 8-diketonate ligands
formed a ferroelectric SmC* liquid crystalline phase at lower
temperatures, showed thermal stability under working conditions, and displayed switching times a thousand times
shorter than that of the first ferroelectric metallomesogen
described.[']
The synthesis of compounds 3 from 1 (Scheme 1) proceeded via the dinuclear complexes 2. The reaction of 2a, bC5]
with thallium(1) P-diketonate['I in CH,Cl, provided the
mononuclear complexes 3a and 3b, respectively. Both complexes gave correct elemental analyses, and 'H NMR and IR
spectra in full agreement with the structure proposed.
Improved Properties of Ferroelectric Liquid
Crystals from Palladium fl-Diketone Complexes**
By Maria Jesus Baena, Pablo Espinet,* Maria Blanca Ros,
Jose Luis Serrano," and Amaya Ezcurra
Ferroelectric liquid crystals (FLCs)"] are one of the main
interests in research on liquid crystals. Although many organic mesogens are known for FLCs, there are few examples
of analogous metal complexes['. 3l and most of them have
high transition temperatures that affect their thermal stability. The first FLC metallomesogen reported,['] a dimeric
ortho-palladated azine complex (a mixture of isomers), displayed some undesirable drawbacks : high transition temperatures (K 102°C SmC* 119°C SmA 149°C I) leading to
thermal instability in the mesophases and a very long switching time (nearly one second). We then discovered that altering the highly symmetrical molecular shape of these dimers
results in lower melting
Whereas dimeric compounds [{(C-N)PdCl),] (C-N = ortho-palladated imine with
long alkyl chains) form SmA and SmC phases at high temp e r a t u r e ~monomeric
,~~~
complexes [ (C-N)Pd(acac)] form N
[*] Dr. P. Espinet, M. J. Baena
["I
Quimica Inorganica, Facultad de Ciencias
Universidad de Valladolid
E-47005 Valladolid (Spain)
Telefax: Int. code + (83)423-013
Dr. J. L. Serrano, Dr. M. 8 . Ros
Quimica Organica, Facultad de Ciencias-I.C.M.A.
Universidad de Zaragoza-C.S.I.C.
E-50009 Zaragoza (Spain)
Telefax: Int. code + (76)567920
Dr. A. Ezcurra
Fisica Aplicada 11, Facultad de Ciencias
Universidad del Pais Vasco, Aptdo. 644
E-48080 Bilbdo (Spain)
This work was financed by the Comision lnterministerial de Ciencia y
Tecnologia (Projects MAT90-0325, MAT90-0813, and MAT91-0962C0201) and the Consejeria de Cultura de la Junta de Castilla y Leon. M. J. B.
thanks Iberduero for a fellowship. We thank Dr. J. Barhera for the X-ray
measurements and M. M. Zurbano for supplying the 8-diketone. Abbreviations: C = crystalline, SmC' = chiral smectic C phase,
SmA = smectic A phase, I = isotropic, N = nematic, FLC = ferroelectric
liquid crystal.
~
Angen.. Chem. Int. Ed. Engl. 1993, 32, No. 8
0 VCH
d,
-
l a (n : 10)
l b ( n : 14)
Pa (n : 10)
2b (n : 14)
3a (n : 10)
3b (n : 14)
Scheme 1. a) Pd,(AcO),/AcOH; b) HCIIMeOH; c) TI [p-H,,C,,OC,H,CO),.
CH]/CH,CI,.
The thermotropic behavior of the 8-diketonate derivatives
was studied by optical microscopy and differential scanning
calorimetry (DSC) at 10 Kmin-' (Table 1). Smectic A and
chiral smectic C phases were assigned on the basis of their
optical textures and confirmed by X-ray diffraction studies.
The chiral smectic C phase showed a characteristic focalconic texture, and no dechiralization lines were observed.
The ferroelectric properties of these two 8-diketonate complexes-spontaneous polarizations (P,), switching times (T~),
viscosities (y,), and tilt angles (+are
summarized in
Table 1.['I Figure 1 shows the dependence of P, and 0 of the
ferroelectric palladium complexes on temperature.
These metallomesogens have moderate P, values with
maxima at 29 and 22 nCcm-' for 3 a and 3 b respectively,
and maximum optical tilt angles very close to the optimum
tilt angle in FLCs of 22.5". It is noteworthy that the tilt
angles could not be calculated from the X-ray diffraction
measurements, as no significant differences between the layer thickness at the SmA and SmC* phases were detected.['']
The switching times of the new ferroelectric palladium
complexes 3a, b are on the order of milliseconds. Although
this is noticeably slower than the switching times of typical
organic FLCs (microseconds), these new complexes switch
three orders of magnitude faster than the first palladium
Verlagsgesellschafi mbH. 0-69451 Weinheim, 1993
0570-0833/93/0808-1203 $ 10.00+ ,2510
1203
Table 1. Phase transition data and ferroelectric properties of complexes 3.
Complex
Transition
(S)-3a
(S)-3b
T [C]
( A H [kJmol-'I)
[dl
K-SmA
SmA-I
I-SmA
SmA-SmC* [d]
SmC*-K
114.4
118.4 (29.67) [cl
K-I
110.9 (26.X4)
110.3 (-4.55)
99.2 (-0.16)
85.1 (-17.63)
p, (max)
[ K e r n - '1
xr
"Jl
(9 lbl
Iw V iim- 'I
[Pa sl
[;I
PJsin8 [b]
[nCcm-2]
29
9x4
0.120
21.3
-
- 22
1596
0.185
21.5
- 55
-
59
118.2 (-4.66)
110.8 (-0.27)
88.7 (-17.53)
I-SmA
SmA-SmC* [d]
SmC*-K
la] See footnote[**'. [b] Data measured at
transition.
c- T=lO"C ( L :SmA-SmC*
transition temperature determined by DSC). [c] Combined enthalpies. [d] Monotroplc
351
0
0
0
5
x
*
0
0
1 YHz
0
0
e'
b
105 Hz
1 kHz
10 kHz
100 kHZ
74
b
5
0
10
5
T,
- r PCI
-
15
20
3
- ,
80
25
go
iio
too
TPCI
iio
iio
Fig. 2. Temperature dependence of the dielectric permitivity s'ofcompound 3 a
at different frequencies.
t
25$
*:
2 4
t
.ooTable 2. Ferroelectric properties of the binary mixtures of the chiral imines 2 [a].
0
o o
0
1
0
,
I
4
, o , ,,
8
r, - rpcj
,
,
16
20
-
12
.
complex described,['] and are even faster than polymeric
FLCS'"~ and in the range of FLCs from dimeric organic
compounds.['21
The dielectric behavior of 3a, b was also studied a t different frequencies (Fig. 2) by using a method described else"] The SmC* phase is characterized, as usual, by
an additional contribution to E', which is mainly due to the
helical structure. This additional contribution decreases and
finally disappears when the frequency increases, as the responsible mechanism relaxes.
It is worth noting that none of the Schiff bases used as
ligands are liquid crystals ( l a , C-I 60.3"C and l b , C-I
44.3 "C). The ferroelectric behavior of these chiral organic
compounds was evaluated in 15% binary mixtures['41 in
which the SmC* phase was induced. As can be seen in Table 2,
the PJsinQ values extrapolated to 100% 1 a and 1 b are 77 and
64 nCcm-*, respectively, at temperatures 10 K below the
SmA-SmC* transition of the corresponding binary mixture.
If we compare the PJsinO values of compounds 1 and 3
(data independent of T ) , it seems that the P, values of the
chiral organic ligands slightly decrease on complexation.
VCH V e r l u , q . ~ ~ ~ , s e l l nzhH,
. ~ ~ h a0-69451
~l
Weinhelm, 1993
T["C] [b] P. [nCcm-']
P, (ext)
B(T) ["C]
[nCcm-']
P, (ext)/slnO (7')
(S)-la
(S)-lb
57.0
58.7
-33
-24
77
64
-5.5
-3.5
[C]
ICI
25.4
21.8
[a] 15% binary mixture of 1[14]. [b] T = c-10-C (T, = SmA-SmC* transition temperature determined by optical microscopy). [c] Maximum value.
Fig. 1. Temperature dependence of the spontaneous polarization P, and tilt
angle0ofcomplexes3a(o)and3b(o). T,is the temperatureoftheSmA-SmC*
transition determined by DSC.
1204
Cmpd.
However, since complexation leads to dramatic structural
changes, which undoubtedly modify the arrangement of the
molecular dipoles at the SmC* mesophase, this may be a
misleading effect and further studies will be needed before
any conclusions about the effect of the metal atom can be
reached.
E-xperimental Procedure
3a. b : To a stirred solution of the corresponding dimeric complex 2 (0.05 mmol)
in 20 mL of CH'CI, was added a stoichiometric amount of thallium ,&diketonate. A white precipitate of TIC1 immediately formed. After stirring at room
temperature for 1 h, the precipitate was filtered off and the solution was filtered
through a 20-cm silicagel column using CH,CI, as an eluent. The resulting
solution was concentrated to dryness, and the residue was triturated in a mixture of acetonejethano] (113) to obtain a yellow solid, which was filtered and
washed with ethanol. Complexes 3a and 3b gave correct elemental analyses.
Representative spectroscopic data of the palladium complexes: 3a IR (Nujol),
i.[cm-'l=1608(C=N), 1591 (C=O). 1544 (C=C), 1254 (C-0), 1023 (C-0);
' H N M R (300 MHz. CDCI,): 6 = 0.83-0.95 (m. 12H, CH,), 1.24-1.60 (m,
SOH, CH,), 1.39 (d, 3 H , J(H.H) = 6.0 Hz, CH,). 1.72-1.90 (m, 8H. CH,),
3.9-4.0 (m. 6 H . CH,), 4.59 (m. 1 H. CH), 6.60 (dd, 1 H, J(H,H) = 8.24 Hz,
J(H.H) = 2.40 Hr, CH). 6.61 (s. 1 H, CH), 6.79 (d. ZH, J(H,H) = 8.97 Hz.
CH), 6.90 (d, 2H, J(H,H) = 8.97 Hz, CH), 6.93 (d, 2H. J(H,H) = 8.97 Hr.
CH), 7.29 (d, 1 H, CH), 7.31 (d, 2H, J(H,H) = 8.79 Hz. CH), 7.42 (d, ZH,
J(H,H) = 8.79 Hz. CH), 7.71 (d. 2H. J(H,H) = 8.79 Hz, CH), 7.97 (d. 2H,
J(H,H) = 8.97 Hz. CH), 7.99 (s. 1 H, CH).
0S70-0833/93/0808-1204B 10.00f-2510
Received: February 9, 1993 [Z 5858 IE]
German version: An,qnr. Chem. 1993, 105. 1260
Angew. Chem. In,. Ed. En,ql. 1993, 32. No. 8
For an introduction to FLCs see: S. T. Lagerwall, B. Otterholm, K. Skarp,
Mol. C r w . Liq. Crysr. 1987, 152. 503; L. A. Beresnev. L. M. Blinov.
M. A. Osipov, S. A. Pikin. ibid. 1988,158A. 3;D. M.Walba, Adv. Synth.
Rrucr. Solids 1991.1. 173;L. A. Beresnev, S. A. Pikin, W. Hasse, Condens.
Mutter News 1992,1, 13.
P. Espinet, J. Etxebarria, M. Marcos, J. Perez, A . Remon, J. L. Serrano,
A i i x i ~ Cliem.
.
1989,I01,1076;Angew. Chem. Inr. Ed. Engl. 1989,28,1065.
Review o n metallomesogens: A. M. Giroud-Godquin. P. M. Maitlis.
A n p i r . Chem. 1991,103.370-398; Angew. Chrm. In(. Ed. Engl. 1991.30,
375.
M Marcos, J. L. Serrano, T. Sierra, M. J. Gimenez, Angew. Chem. 1992,
I04. 1523;Angen.. Chem. Inl. Ed. Engl. 1992,31,1471.
M. J. Baena, P. Espinet, M. B. Ros, J. L. Serrano, Angen. Chem. 1991.103,
716. Angrw. Chem. Inf. Ed. EngI. 1991,30,711.
J. Barberi. P. Espinet, E. Lalinde. M. Marcos, J. L. Serrano, Liq. Cryst.
1987.2, 833.
D. Walba, M. B. Ros, N. A. Clark, R. Shao. M. G. Robinson, J.-Y. Liu, K.
Johnson, D. Doroski. J. Am. Chem. Soc. 1991,1I3, 5471. and references
therein.
J. Barberi. C. Cativiela, J. L. Serrano, M. M. Zurbano, Adv. Muter. 1991,
3. 602.
P,.r e and 7 were measured simultaneously using the triangular-wave
method [9] with cells of 10 pm thickness and polyimide-coated indium tin
oxide electrodes. For these measurements fields of 50 Hz and peak-to-peak
voltages of 30-40 V were used. The optical evaluations were performed
using the same cell and sample and applying electrical fields of 30 V.
a) K . Miyasato, S. Abe, H. Takezoe. A. Fukuda, E. Kuze, Jpn. J. Appl.
Plijs. 1983,22. 661 ; b) M. R. De la Fuente, A. Ezkurra, M. A. Perez-Jubindo. J. Zubia. Li4. Crvsf. 1990, 7, 51.
[lo] R. Bartolino, J. Doucet. G. Durand, Ann. Phys. 1978,3, 389; R. Halfon,
E. N. Keller. E. Nachatiel, D. Ddvldov, C. Escher, Ferroelerlrics 1991,
II4,354.
1111 M. Dumon, H. T. Nguyen. M. Mauzac, C. Destrade, M. F. Achard, H.
Gasparoux, Macromolecules 1990. 23. 355; H.Poths. A. Schonfeld. R.
Zentel, F. Kremer, K. Siemensmeyer, Adv. Muter. 1992,4, 351.
[12] M. Marcos, A. Omenat, J. L. Serrano, T. Sierra, A. Ezcurra. Adv. Murer.
1992,4. 285.
[13] M. A. Perez-Jubindo, A. Ezcurra, M. R. de la Fuente, C. Santamaria, J.
Etxebarria, J. L. Serrano, M. Marcos, Ferroelectrics 1988,81, 405.
[I41 Achirdl host: 4-hexyloxyphenyl-4-decyloxybenzoate. K 62.5"C/SmC
78.2 C/SmA 84.5 "C/N 90.5T / I .
1,3-Bis(triphenylphosphoranylidene)allene and
Tribenzyl(4-methyldiphen y lphosphoniobutadiynyl)borate, Stabilized C, and C, :
Confirmation of a Hypothesis
By Huns Jiirgen Bestmum,* Dariusch Hadawi,
Harald Behl, Matthias Bremer, and Frank Humpel
Different hypotheses have been put forth for the formation of fullerenes. According to one proposal, for example,
small carbon units arise from C , and grow into long chains,
which in turn then form rings.['.21 The fragmentation of
fullerenes apparently proceeds by the continuous cleavage of
C, units.I3I C, is found in interstellar space.141The chemical
reactivity of short-lived C , , C,, C,, and C, is being investiThe question of whether
gated in particular by Skell et al. .Is1
these small, highly reactive species can be stabilized can be
answered as follows:[61In hexaphenylcarbodiphosphorane
1, a compound known for many years, a C, unit is stabilized
by two triphenylphosphane donor molecules.[71In complex
2, which has a betaine structure, stabilized C, was prepared
by attaching a donor molecule (tertiary phosphane) to one C
atom and an acceptor molecule (borane) to the other C
We
that C, should also be stabilized by
two donor molecules in 3 and C, with a donor molecule and
an acceptor molecule at either end of the C, chain as in 4.We
report here on our verification of this postulate.
-
t
,PR3
I +
Ph3c'3;P;
h,
R,P-CEC-B%
Angeii.. Chem. Inr. Ed. Engl. 1993,32,No. 8
0 VCH
3
- 2
1+
R,P-C=C-C=C-BR,
4
A compound of type 4 could be prepared quite easily,
since its synthesis follows that of 2. Treatment of diacetylene
5F8Iwith ethylmagnesium bromide (6) provided the Grignard
compound 7, which was allowed to react with chlorodiphenylphosphane (8) to give the light-sensitive butadiynyldiphenylphosphane( 9 ) . L 9 3 O1 The reaction of 9 with
butyllithium (10) in T H F provided the corresponding Li
which was converted into the borate comcompound 11,[111
plex 13 by treatment with tribenzylborane (12).["' The reaction of 13 with methyl iodide led to the desired product,
tribenzyl(4-methyldiphenylphosphoniobutadiynyl)borate
(15).['31The crystals that formed from the reaction mixture
were recrystallized from dimethylformamide. They are colorless, light sensitive, and can be stored in a refrigerator only
for a limited time. The spectroscopic data of 15 are listed in
the reference^."^] Two C=C stretching bands are found in
the IR spectrum. In the I3C spectra the values of the coupling constants J(P,C) decrease as expected from C, to C,
(Ca: 'J(P,C) = 193.8 Hz, C,: 'J(P,C) = 38.1 Hz); C, does
not couple with the P atom. The signal for C, cannot be
recognized due to the effects of the quadrupole moment of
the neighboring B atom, similar to the case for compounds
of type 2.@'The positions of the 31P NMR and "B NMR
signals indicate that both atoms are tetracoordinate, as in 2.
Recrystallization of 15 from dimethylformamide provided
crystals suitable for an X-ray structure determination; the
resulting structure is shown in Figure
The solvent
dimethylformamide is included in the crystal lattice. The
diacetylene character of the betaine compound is apparent.
The distances between C1 and C2, and C3 and C4 (120.5(6)
and 121.1(7) pm, respectively) correspond to those of triple
bonds. The C2-C3 separation, 136.1(6) pm, is in accord
with that of a single bond between two sp-hybridized C
The P1 -C4 distance is 171.7(5) pm and that between B1 and C1 159.5(7) pm. These bond lengths are almost
identical to the analogous data for compound 2 with
R' = C,H,, CH,, R2 = C,H,. The structure of 15, like that
of 2, is also not linear but slightly bent.
EtMgBr 6
tt-CEC-CEC-H
5
BULI
Ph,P-C=C-CEC-H
9
CIPPh
-
7
THF, -30°C. 56%
10
(PhCH,),B
Ph,P-CFC--C-C-Li
THF, -60°C
Verlugsgesellschaftm b H , 0.69451 Weinheim, I993
11
Li
14
THF. -30°C. 48%
+ a
p u 6 PhzP-C~C--C3-E(CH,Ph),
I
CH,
'
12
THF. -60°C
I+
13
CH,I
8
2
BrMg-CEC-CEC-H
THF, -10°C
-
Dr.M. Bremer. Dip].-Chem. F. Hampel
Institut fur Organische Chemie der Universitlt Erlangen-Nurnberg
Henkestrasse 42. D-91054 Erlangen (FRG)
Telefax: I n t . code + (9131)856864
R3P
2
1
~,P-C?C-C~C-E(CH2Ph)3
[*I Prof. Dr.H.J. Bestmann, DipLChem. D. Hadawi, Dr.H. Behl,
+p=c=c-
- 2
15
8 10.00+ .25iO
0570-0X33/93/0808-1205
1205
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