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Effects of Perturbed Symmetry Low-Melting Stable Mesogens Based on ortho-Palladated Imines.

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Effects of Perturbed Symmetry:
Low-Melting Stable Mesogens Based
on ortho-Palladated Imines **
By Maria Jesus Baena, Pablo Espinet,* Maria Blanca Ros,
and Josk Luis Serrano*
Among the rapidly growing family of metallomesogens
(liquid crystals containing coordinated metals),['] a minority
are organometallic compounds. Cyclometalated palladium
complexes derived from azobenzenes,[zl a~omethines,'~]
azin e ~ , [ or
~ ] related ligandsL5]are a distinct subclass mostly
constituted by halogen-bridged dimers, which can be further
elaborated. Thus, replacement of the halide by carboxylate
has provided mesogens based on a novel open-book molecular shape.[41Such mesogens offer a novel approach to introducing ferroelectric properties as well as the possibility of
tuning the mesogenic properties by varying the bridging carb~xylate.~']
A drawback of these cyclopalladated systems is their high
melting and clearing temperatures (usually > 130 and
> 200" C, respectively), which cause thermal decomposition
of the products during prolonged studies. In the azomethine
and azine systems, moreover, there is a clear predominance
of smectic mesophases, and nematic mesophases are a rarity.
Both properties suggest that excellent packing of the molecules in these systems leads to high lattice energies and
ordered mesophases. Consequently, perturbing the highly
symmetric molecular shape might lead to lower melting
points and less ordered mesophases. We describe here a successful application of this idea. A different approach to this
idea of producing asymmetry in the long molecular axis of a
rodlike molecule[*' has yielded low-melting mesogens, but
their properties cannot be compared with the corresponding
symmetrical molecules.
Azomethines 1 were ortho-palladated to give the binuclear
complexes 2 and these were treated with Tl(acac) (acac =
acetylacetonate) to give the mononuclear species 3
(Scheme 1). All the compounds gave elemental analyses, 'H
NMR spectra, and IR spectra in full agreement with the
proposed structures. The chemical transformations result in
changes in molecular shape from the rodlike (or I-shaped)
molecules 1 to the H-shaped (or ladderlike) binuclear complexes 2 and finally to the P-shaped (or "humpbacked")
mononuclear compounds 3.
The thermotropic behavior of 1, 2, and 3 was studied by
optical microscopy and differential scanning calorimetry
(DSC). Their thermal data are summarized in Table 1. The
Tablel. Temperature and enthalpy data for 1-3.
Compound
Transition [a]
T["C]
AH [kJmol-'I
la
c-I
C-N
N-I
N-SA
c-s,
63.9
101.3
115.5
92.6
91.9
111.8
112.7
113.2
91.4
102.9
111.9
185.6
177.8
100.3
172.3
274.0 (dec.)
74.1
146.1
268.2 (dec.)
140.0 [d]
124.2
132.7
204.0 [d]
242.8 (dec.)
104.7
99.1
108.9
117.8
84.0
118.5
124.8
74.3
128.0
45.1
48.6
1.5
-0.7
43.7
lb
lc
PI
sC-sA
S,-N
N-I
Sc& [bl
ld
2n
2b
2c
c-s,
s,-I
c-I
I-S, "Jl
C-C
C'-s,
s,-I
C- C
C-S,
s,-I
sA-sC
2d
Lb1
C-C
C-Sc
sC-sA
s,-I
3a
3b
Q
i
1
3c
n
n
2
3
c-I
Cf C-C+N
C + N-N
N-I
c-s,
S,-N
N-I
36
c-s,
s,-I
7.0 [c]
-2.7
52.0
10.5
59.1
-2.8
32.8
26.9
43.0
24.0
15.1
31.5
46.0
41.0
0.2
0.9
38.4
0.5
1.3
58.3
5.8
[a] C, crystal; N, nematic; S,, smectic A; S,, smectic C ; S,, smectic F;
I, isotropic liquid. [b] Monotropic transition. [c] Combined enthalpies.
[d] Optical microscopy data.
Schemel. Syntheses and sketch of molecular shapes of 1-3. a. R = H ; b,
R = OC,H,; c, R = 0-n-C,H,,; d, R = 0-n-C,,H,, .
[*I
[**I
Prof. Dr. Espinet, M. J. Baena
Quimica Inorganica, Facultad de Ciencias, Universidad de Valladolid
E-47005 Valladolid (Spain)
Prof. Dr. J. L. Serrano, Dr. M. B. Ros
Quimica Organica, Faculted de Ciencias-I.C.M.A. Unversidad de
Zaragoza-C S.I.C.
E-50009 Zaragoza (Spain)
This work was supported by the European Community (Twinning Grant
ST2J-0387C) and the Comision Interministerial de Ciencia y Tecnologica
(CICYT, Spain, Project PB86-0028 and MAT88-0324-C02-01)). M . J
Buenu thanks IBERDUERO for a studentship.
Angew. Chem. Int. Ed. Engl. 30 (1991) No.6
0 VCH
nematic phase (N) of complexes 3 was assigned on the basis
of their optical textures. The photomicrograph of the nematic phase of 3c at 321°C (Fig. 1) shows the character is ti^[^]
two- and four-brush isogyres at the termination of disclination lines. Smectic A (S,) and C (S,) phases are observed
showing optical mielinic, focalconic, and schlieren textures
similar to those of classical rodlike organic compounds but
more viscous.
The free ligands 1 are typical calamitic compounds, like
other previously described az~methines,[~. and all except
1 a are mesogenic. Except 3a, all the palladium complexes
are also liquid crystals (Fig. 2). The binuclear complexes 2
show the behavior typical of this type of molecule: they
exhibit high melting ( > 130"C) and clearing (> 180" C) temperatures and decomposition generally occurs at the clearing
point.
A dramatic change in properties is observed in the corresponding mononuclear complexes 3, which give rise to
Verlugsgesellschuft mbH. W-6940 Weinheim, 1991
0570-0833~9lj0606-07~l
$ 3 . S O i .2S/0
73 1
Fig.1. Photomicrograph of 3c in its nematic phase at 121°C viewed between
crossed polars.
mesophases at temperatures below 90”C, about 50”C lower
than the parent binuclear compounds 2. The clearing temperatures are lowered to about 125°C with the consequence
that the compounds can be taken from the crystalline to the
isotropic state repeatedly without decomposition. This is important with respect to their eventual processability.
A second noticeable property of the mononuclear complexes 3 is the consistent occurrence of nematic (N) mesophases, which is unusual in the ortho-palladated binuclear
~ ~ ] accessibility
species 2 we have previously d e ~ c r i b e d .(The
of a nematic phase facilitates ordered arrangements in liquid-crystal-based devices.) Broadly speaking, a decrease of
280
260
1
2LO
220
r loci
200
order in the mesophase is observed such that S, behavior in
2 leads to S, behavior in 3, and S, behavior in 2 leads to N
behavior in 3. Thus, the only previously reportedc5’ acac
mononuclear complex containing an ortho-palladated
phenyl-substituted pyrimidine ligand was monotropic (S,),
whereas its parent binuclear complex displayed an enantiotropic (S,) mesophase.
Moving from I-shaped “one-dimensional’’ molecules 1 to
highly symmetrical H-shaped “two-demensional” structures
2 greatly increases the range of mesogenic behavior, but at
the expense of high transition temperatures and a scarcity of
nematic phases. The new P-shaped “two-dimensional” symmetry-perturbed mononuclear species 3 are an excellent
compromise, since they show mesophase ranges broader
than the free ligands, melting temperatures even lower than
the free ligands, and a less complex mesogenic behavior. The
complete family of I-, H-, and P-shaped mesogens, which can
be extended to many related complexes, offers a very promising horizon in the tailoring of mesogenic materials.
Received: December 18, 1990 [243391E]
German version: Angew. Chem. 103 (1991) 716
~
~~~
[I] Review: A. M. Giroud-Godquin, P. M. Maitlis, Angew. Chem. 103 (1991)
370-398; Angew. Chem. Int. Ed. Engl. 30 (1991) 375-402, and references
cited therein.
[2] M. Ghedini, M. Longeri, R. Bartolino, ,4401. Crysf. Liq. Cryst. 84 (1982)
207-211; M. Ghedini, S. Armentano, R. Bartolino, F. Rustichelli, G.
Torquati, N. Kirov, M. Petrov, ibid. 151 (1987) 75-91.
[3] J. Barbera, P. Espinet, E. Lalinde, M. Marcos, J. L. Serrano, Liq.Cryst. 2
(1987) 833-842; M. Marcos, M. B. Ros, J. L. Serrano, ibid. 3 (1988) 11291136; M. B. Ros, N. Ruiz, J. L. Serrano, P. Espinet, ibid. 9 (1991) 77-86.
[4] P. Espinet, E. Lalinde, M. Marcos, J. Perez, J. L. Serrano, Organomefallics
9 (1990) 555-560.
[5] M. Ghedini, D. Pucci, J. Organomet. Chem. 395 (1990) 105-112.
[6] P. Espinet, J. Etxebarria, M. Marcos, J. Perez, A. Remon, J. L. Serrano,
Angew. Chem. 101 (1989) 1076-1078; Angew. Chem. Inl. Ed. Engl. 28
(1989) 1065-1066.
(71 P. Espinet, J. Pirez, M. Marcos, M. B. Ros, J. L. Serrano, J. Barbera,
A. M. Levelut, Organomerallics 9 (1990) 2028-2033.
[8] J. P. Rourke, F. P. Fanizzi, N. J. S. Salt, D. W. Bruce, D. A. Dunmur, P. E.
Maitlis, J. Chem. Soc. Chem. Commun. i990,229.
[9] D. Demus, L. Ritcher: Textures of Liquid Crystals, Verlag Chemie, Weinheim 1978.
[lo] D. Demus, H. Demus, H. Zaschke: Flussige Krisfalle in Tabellen I , VEB
Deutscher Verlag fur Grundstofindustrie, Leipzig 1974, p. 97-98; D. Demus, H. Zaschke: Nussige Kristalle in TabefknII, VEB Deutscher Verlag
fur Grundstofindustrie, Leipzig 1984, p. 199.
0
f
I
I
180
I
A
I
160
Chemoenzymatic Syntheses
of w-Ureido D-Amino Acids **
1LO
By Karlheinz Dram,* Matthias Kottenhahn,
Kyriakos Makryaleas, Herbert Klenk, and Michael Bernd
120
Dedicated to Professor Heinz A . Staab
on the occasion of his 65th birthday
100
The D-amino acids are of great importance in the synthesis
of biologically active substances. Compounds such as D-
80
[*] Dr. K. Drauz, Dr. M. Kottenhahn, Dr. K. Makryaleas, Dr. H. Klenk
o
i
6
10
n-
Fig.2. Transition temperatures of the palladium complexes 2 (---) and 3
(..*.) as a function of the organic ligand 1 (n is the number of C atoms in the
substituent R). l a , n = 0; 1 b, n = 2; lc, n = 6 ; Id, n = 10. m , C-I; o,C-N; +,
C-S,;
712
A,
C-S,;
0,S,-N; 0,S&,;
0,
N-I; A, S,-I.
0 VCH Verlagsgesellschaf~mbH,
W-6940 Weinheim, 1991
[**I
Degussa AG, Fachbereich Organische und Biologische Chemie
Postfach 1345, W-6450 Hanau 1 (FRG)
Dr. M. Bernd
Asta Pharma AG, Abteilung CFW
Amino Acid Transformation, Part 7. This work was supported by Recordati-De. Bi., Milan (donation of the Agrobacterium radiobacter culture).
Part 6 : K. Drauz, U. Groeger, M. Schafer, H. Klenk, Chern. Zrg. 115
(1991). in press.
0570-0833/91/0606-0712$3.50+ ,2510
Angew. Chem. Int. Ed. Engl. 30 (1991) No. 6
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