Cyclopalladated acac and cp liquid crystals: A comparative study

Article abstract of DOI:10.1039/a605791h

The cyclopalladation of mesogenic Schiff bases yields two series of novel metallomesogens. The flat acetylacetonate derivatives show both nematic and smectic A phases with extended mesogenic ranges. In contrast, the non-planar cyclopentadienyl complexes s

Full text of DOI:10.1039/a605791h

Cyclopalladated acac and cp liquid crystals: a comparative study
Donocadh P. Lydon, Gareth W. V. Cave and Jonathan P. Rourke*†
Department of Chemistry, Warwick University, Coventry, UK CV4 7AL
The cyclopalladation of mesogenic Schiff bases yields two series of novel metallomesogens. The flat acetylacetonate derivatives
show both nematic and smectic A phases with extended mesogenic ranges. In contrast, the non-planar cyclopentadienyl complexes
show nematic phases at much lower temperatures than those of the acetylacetonate complexes.
There is currently much interest in the synthesis of metal-
containing liquid crystals owing to the perceived advantages
of combining the properties of liquid crystal systems with those
of transition metals. The area has been well reviewed
recently,1–5 with excellent new work appearing constantly.6–11
Cyclopalladated compounds have proved to be a particularly
fertile area of research, with many different examples from
many different groups.10,12–22
and the clearing points are essentially the same. The fact that
acac complexes do not exhibit the more ordered smectic phases
shown by the ligands is not surprising when one considers
that the shape of the complexes will be distorted away from
the calamitic shape of the ligand by the presence of the
Pd(acac) moiety on the side. Presumably this group disrupts
the packing of the molecule within the crystal, resulting in the
lower melting point of the complexes compared with the
ligands. The absence of a nematic phase for compound 3d is
not unexpected given the presence of the two long chains
(seven carbons) at either end of the molecule, which presumably
stabilise the smectic phase. It is interesting to note that the
clearing points of complexes 3a and 3b, which both have four
carbons on the biphenyl group, are very similar, as are those
of 3c and 3d, which both have seven carbons. It is also
interesting to note that the stability of the smectic phase seems
to be related to the length of the chain on the cyclopalladated
ring. Thus, complexes 3b and 3d, which both have seven
carbons on the cyclopalladated ring, locating the bulky
Pd(acac) moiety more towards the centre of the molecule. have
the most stable smectic A phases.
We have been studying a number of cyclopalladated Schiff
base compounds with two very different co-ligands and present
our results here.
Synthesis
The synthesis of the new compounds described here, 3 and 5,
is summarised in Scheme 1. The synthesis of the 4-alkyloxy-
N-(4∞-alkyloxybiphenyl)benzylidene ligands 1 via a simple con-
densation of the appropriate aldehyde and aniline proceeded
in high yield. The cyclopalladation step to give the intermediate
2 was essentially quantitative, and 2 was used without further
purification. The synthesis of the acetylacetonate (acac) deriva-
tives 3 from 2 proceeded in good yield, and these complexes
were purified by column chromatography. The synthesis of the
cyclopentadienyl (cp) complexes 5 from 2 via 4 gave a poor
yield overall, after purification. All homologues of compounds
1, 3 and 5 were analysed by 1H and 13C NMR and gave good
elemental analyses (see Table 2, later).
The thermal behaviour of the cp complexes 5 is listed in
Table 1 and summarised in Fig. 1. Thus, all the complexes
showed only a nematic phase, with the exception of 5d, which
exhibits a smectic A phase too. The phases were identified on
the basis of their optical texture, the nematic and smectic A
phases exhibiting classic textures. The melting and clearing
points of complexes 5 are much lower than those of both the
ligands 1 and the acac complexes 3. The fact that cp complexes
do not exhibit the more ordered smectic phases is not surprising
when one considers the shape of the complexes. Whilst both
the ligands and the acac complexes can be thought of as
essentially planar, the cp ring is perpendicular to the plane of
the ligand. Clearly this group disrupts the packing of the
molecule within the crystal, resulting in the reduced melting
and clearing points of the complexes and the destabilisation
of the smectic phases. The presence of a smectic phase for
compound 5d is not surprising given the presence of the two
long chains (seven carbons) at either end of the molecule, and
this mirrors the behaviour of the acac complex 3d. It is
interesting to note that the melting points of complexes 5a
and 5c, which both have four carbons on the cyclopalladated
ring, are very similar, as are those of 5b and 5d, which both
have seven, and thus the bulky Pd(cp) moiety located more
towards the centre of the molecule.
Thermal properties
The thermal behaviour of the ligands 1 is listed in Table 1 and
summarised in Fig. 1. Thus, all the ligands showed smectic F,
smectic C and nematic phases before clearing. The phases were
identified on the basis of their optical texture, the nematic and
smectic C phases exhibiting schlieren textures, the nematic
showing both four- and two-point brushes, the smectic C only
four-point brushes. The smectic F phase showed a schlieren-
mosaic type texture on cooling from the smectic C phase. The
absence of any point disclinations allowed us to distinguish it
from the smectic I phase.
The thermal behaviour of the acac complexes 3 is listed in
Table 1 and summarised in Fig. 1. Thus, all the complexes
showed a smectic A phase and, with the exception of 3d, a
nematic phase before clearing. The phases were identified on
the basis of their optical texture. The smectic A phase appeared
as a focal-conic fan texture which separated on cooling from
the isotropic as batonnets which consisted of growing focal-
conic domains. The melting and clearing points of complexes
3 neatly mirror those of the ligands from which they are
derived: the melting points of complexes 3 are ca. 10 K lower,
Compounds 5 decompose at ca. 180 °C on transition to the
isotropic liquid, whereas compounds 3 do not decompose at
temperatures almost 100 K higher. Chemically, the major
difference between the two complexes is the electron count on
the palladium: the cp complex has a formal count of 18e−,
whereas the acac complex has one of 16e−. The geometry of
the cp complex is formally trigonal bipyramidal, whilst that of
the acac complex is square planar. The chemistry of pal-
† Email: j.rourke@warwick.ac.uk
J. Mater. Chem., 1997, 7(3), 403–406
403

CnH2n+1
O
N
OCmH2m+1
1
Pd(OAc)2
CmH2m+1
O
N
OCnH2n+1
Pd
Pd
OAc
AcO
N
a n = 4, m = 4
b n = 4, m = 7
c n = 7, m = 4
d n = 7, m = 7
CnH2n+1
O
OCmH2m+1
2
HCl
CmH2m+1
O
N
OCnH2n+1
Pd
Pd
NaAcac
Cl
Cl
CnH2n+1
O
N
OCmH2m+1
4
Tl(C5H5)
O
N
O
Pd
Pd
CnH2n+1
O
CnH2n+1
O
N
OCmH2m+1
OCmH2m+1
5
3
Scheme 1
ladium() is dominated by square-planar 16e− species, with
very few examples of 18e− complexes. Thus the observed
thermal stabilities of our compounds are entirely reasonable.
Compounds 5 represent the only isomerically pure half-
sandwich liquid crystals ever reported. The only other half-
sandwich liquid crystals known are those reported by Ghedini
et al.,10 where an azobenzene cyclometallates to give two
isomeric products. Ghedini’s compounds, which like ours
contain three aromatic rings, only exhibited nematic phases
and at very similar temperatures to ours. Ghedini et al. also
synthesised a couple of compounds with only two aromatic
rings, but observed no mesogenic behaviour for these cp
derivatives. In contrast, some earlier work by Espinet et al.14
had shown that the acac derivatives of a two-ring system did
exhibit mesogenic behaviour, with both smectic A and nematic
phases being observed.
Thus it can be seen that our results are consistent with
established precedent: compared with the acac group, the cp
group brings down both the melting and clearing points of the
complexes and shows a strong preference for the nematic
phase. It is clear that the cp group will become a popular
motif in metallomesogen chemistry.
Experimental
General
All chemicals were used as supplied, unless noted otherwise.
All NMR spectra were obtained on either a Bruker AC250 or
404
J. Mater. Chem., 1997, 7(3), 403–406

Table 1 Mesogenic behaviour of compounds 1, 3 and 5
compd.
1a
1b
1c
1d
trans.
T/°C
trans.
S –S
T/°C
trans.
T/°C
trans.
T/°C
C–S
C–S
F
191
160
171
157
197
171
178
161
S –N
201
216
215
217
N–I
N–I
N–I
N–I
272
252
255
254
F
F
C
C
C–S
F
S –S
S –N
F
C
C
S –S
S –N
F
C
C
C–S
S –S
S –N
F
F
C
C
3a
3b
3c
3d
C–S
A
184
162
155
141
S –N
209
241
225
245
N–I
N–I
N–I
260
261
249
A
C–S
A
S –N
A
C–S
S –N
A
A
C–S
S –I
A
A
5a
5b
5c
5d
C–N
C–N
C–N
125
101
125
92
N–I
N–I
N–I
182a
179a
165
145
C–S
S –N
N–I
178a
A
A
*Some decomposition.
on an AC400 in CDCl and are referenced to external SiMe ,
H ], 1.81 (4 H, m, H ), 1.40 (16 H, m, H ), 0.89 [6 H, t,
3
4
n
o,o∞
p,p∞
assignments being made with the use of decoupling, NOE and
the DEPT and COSY pulse sequences. Thermal analyses were
performed on an Olympus BH2 microscope equipped with a
Linkam HFS 91 heating stage and a TMS90 controller, at a
heating rate of 10 K min−1. All elemental analyses were per-
formed by Warwick Microanalytical Service.
3J(HH) 7.0 Hz, H ]; d : 159.2 (C ), 158.5 (C ), 138.1 (C ),
q,q∞
C
a
e,m
i
132.9 (C ), 130.5 (C ), 127.7 (C ), 127.2 (C ), 121.2 (C ), 114.7
b/j
c
h
k
g
(C ), 114.6 (C ), 68.0 (C ), 67.8 (C ), 31.7 (C ), 29.1 (C ),
d/l
d/l
n
n∞
p,p∞
o,o∞
28.9 (C ), 25.9 (C ), 19.2 (C ), 14.0 (C ).
p,p∞
p,p∞
p,p∞
q,q∞
The mesogenic behaviour of all homologues is summarised
in Fig. 1 and detailed in Table 1. Elemental analyses are
detailed in Table 2.
Preparation of 4-heptyloxy-N-(4∞-heptyloxybiphenyl)benzyl-
idene, 1d
Preparation of orthometallated palladium acetate complex, 2d
Compound 1d is described in detail, all other homologues
were prepared similarly. 4-Heptyloxybenzaldehyde (1.61 g,
7.10×10−3 mol) was added to a solution of 4∞-heptyloxy-4-
aminobiphenyl (2.00 g, 7.10×10−3 mol) in toluene (200 ml).
The mixture was heated at reflux for 2 h using a Dean Stark
trap and in the presence of molecular sieves. The solvent was
removed and the product recrystallised from chloroform. Yield
2.47 g (72%, 5.1×10−3 mol).
Compound 2d is described in detail, all other homologues
were prepared similarly. Ligand 1d, (0.41 g, 8.5×10−4 mol)
and palladium acetate (0.191 g, 8.5×10−4 mol) were dissolved
in acetic acid (250 ml) at 60 °C, and stirred (20 h). The solvent
was removed, the crude product dissolved in chloroform,
filtered to remove traces of palladium black and the yellow
solution evaporated to dryness. Yield 0.55 g (98%,
4.2×10−4 mol).
NMR data: d : 8.44 (1 H, s, H ), 7.87 (2 H, AA∞XX∞, H ),
H
a
c
7.57 (2 H, AA∞XX∞, H ), 7.55 (2 H, AA∞XX∞, H ), 7.28 (2 H,
h
k
NMR data: d : 7.59 (1 H, s, H ), 7.51 (2 H, AA∞XX∞, H ),
AA∞XX∞, H ), 6.96 (2 H, AA∞XX∞, H ), 6.95 (2 H, AA∞XX∞, H ),
H
a
m
g
d
l
7.35 (2 H, AA∞XX∞, H ), 7.18 [1 H, d, 3J(HH) 8.4 Hz, H ], 6.95
4.03 [2 H, t, 3J(HH) 7.0 Hz, H ], 4.01 [2 H, t, 3J(HH) 7.0 Hz,
n∞
j
g
(2 H, AA∞XX∞, H ), 6.78 (2 H, AA∞XX∞, H ), 6.59 [1 H, dd,
n
i
3J(HH) 8.4 Hz, 4J(HH) 2.3 Hz, H ], 6.03 [1 H, d, 4J(HH)
f
2.3 Hz, H ], 4.05 [2 H, t, 3J(HH) 6.4 Hz, H ], 4.00 [2 H, t,
d
p∞
3J(HH) 6.7 Hz, H ], 1.90 (3H, s, H ), 1.81 (4 H, m, H ), 1.40
p
t
q,q∞
(16 H, m, H ), 0.88 [6 H, t, 3J(HH) 7.1 Hz, H ].
r,r∞
s,s∞
Table 2 Elemental analysis data for compounds 1, 3 and 5
found (expected)
compd.
n
m
C (%)
H (%)
N (%)
1a
1b
1c
1d
3a
3b
3c
3d
5a
5b
5c
5d
4
4
7
7
4
4
7
7
4
4
7
7
4
7
4
7
4
7
4
7
4
7
4
7
80.9 (80.8)
81.2 (81.2)
81.6 (81.2)
81.3 (81.6)
63.4 (63.4)
64.5 (64.9)
64.7 (64.9)
66.3 (66.0)
69.9 (67.2)
68.1 (68.5)
68.2 (68.5)
69.5 (69.6)
7.8 (7.8)
8.4 (8.4)
8.4 (8.4)
8.7 (8.9)
6.2 (6.2)
6.6 (6.7)
6.7 (6.7)
7.2 (7.3)
6.2 (6.2)
6.6 (6.7)
6.9 (6.7)
7.6 (7.2)
3.4 (3.5)
3.1 (3.2)
3.2 (3.2)
3.0 (2.9)
2.3 (2.3)
2.0 (2.2)
2.3 (2.2)
1.9 (2.0)
2.4 (2.5)
2.1 (2.3)
2.2 (2.3)
2.2 (2.1)
Fig. 1 Phase behaviour of compounds 1, 3 and 5
J. Mater. Chem., 1997, 7(3), 403–406
405

Preparation of palladium acetylacetonate complex, 3d
NMR data: d : 7.86 (1 H, s, H ), 7.54 (4 H, m, H ), 7.52
H
a
i,m
j
[1 H, d, 3J(HH) 8.4 Hz, H ], 7.35 (2 H, AA∞XX∞, H ), 7.24
Compound 3d is described in detail, all other homologues
were prepared similarly. Sodium acetylacetonate (0.038 g,
3.08×10−4 mol) was added to a solution of the acetate bridged
palladium complex 2d (0.200 g, 1.54×10−4 mol) in acetone
(150 ml) at room temperature and stirred (2 h). The solvent
was removed and the product was purified by column chroma-
tography on silica, eluting with a 50550 mixture of dichloro-
methane and hexane. Yield 0.135 g (64%, 1.96×10−4 mol).
g
[1 H, d, 4J(HH) 2.3 Hz, H ], 6.97 (2 H, AA∞XX∞, H ), 6.59
d
n
[1 H, dd, 3J(HH) 8.4 Hz, 4J(HH) 2.3 Hz, H ], 5.80 (5 H, s,
f
H ), 4.02 [2 H, t, 3J(HH) 6.9 Hz, H ], 4.01 [2 H, t, 3J(HH)
t
p∞
6.9 Hz, H ], 1.84 (4 H, m, H ), 1.40 (16 H, m, H ), 0.90
p
q,q∞
r,r∞
[6 H, t, 3J(HH) 8.2 Hz, H ]; d : 164.3 (C ), 158.9 (C ), 157.7
s,s∞
C
a
e
(C ), 139.6 (C
), 138.9 (C
), 138.4 (C
), 138.1 (C
),
o
b/c/h
b/c/h
b/c/h
b/c/h
j/m
132.4 (C ), 131.8 (C ), 130.5 (C ), 128.0 (C ), 127.9 (C ),
k
d
g
i
l
j/m
125.6 (C ), 123.0 (C ), 114.8 (C ), 110.5 (C ), 95.8 (C ), 68.0
n
f
t
r
(C ), 67.8 (C ), 31.8 (C ), 31.3 (C ), 29.2 (C ), 26.0 (C ), 19.2
p
r,r∞
p∞
q
q∞
r,r∞
(C ), 14.1 (C ), 13.9 (C ).
s/s∞
s/s∞
The mesogenic behaviour of all homologues is summarised
in Fig. 1 and detailed in Table 1. Elemental analyses are
detailed in Table 2.
We thank the University of Warwick for financial support
(D.P.L.), and Johnson-Matthey for loan of chemicals.
NMR data: d : 8.03 (1 H, s, H ), 7.58 (2 H, AA∞XX∞, H ),
H
a
j
7.51 (2 H, AA∞XX∞, H ), 7.46 (2 H, AA∞XX∞, H ), 7.31 [1 H, m,
m
i
3J(HH) 8.1 Hz, H ], 7.14 [1 H, d, 4J(HH) 2.3 Hz, H ], 6.97
g
d
[2 H, AA∞XX∞, 3J(HH) 8.4 Hz, H ], 6.60 [1 H, dd, 3J(HH)
n
8.3 Hz, 4J(HH) 2.3 Hz, H ], 5.36 (1 H, s, H ), 4.08 [2 H, t,
f
v
References
3J(HH) 6.5 Hz, H ], 4.02 [2 H, t, 3J(HH) 6.5 Hz, H ], 2.10
p∞
p
(3H, s, H ), 1.91 (3H, s, H ), 1.84 (4 H, m, H ), 1.40 (16 H,
1
S. A. Hudson and P. M. Maitlis, Chem. Rev., 1993, 93, 861.
A-M. Giroud-Godquin and P. M. Maitlis, Angew. Chem., Int. Ed.
Engl., 1991, 30, 375.
x
t
q,q∞
m, H ), 0.90 [6 H, t, 3J(HH) 8.2 Hz, H ]; d : 188.3 (C ),
2
r,r∞
s,s∞
C
u/w
185.7 (C ), 172.4 (C ), 160.4 (C ), 158.7 (C ), 146.3 (C ), 139.7
u/w
a
e
o
h
3
4
5
6
7
8
9
P. Espinet, M. A. Esteruelas, L. A. Oro, J. L. Serrano and E. Sola,
Coord. Chem. Rev., 1992, 117, 215.
(C ), 138.8 (C ), 132.6 (C ), 129.5 (C ), 127.9 (C ), 127.6
b/c
l
b/c
k
g
m
(C ), 126.5 (C ), 123.6 (C ), 115.7 (C ), 114.7 (C ), 111.4 (C ),
j
i
d
n
f
D. W. Bruce, in Inorganic Materials, ed. D. W. Bruce and D.
O’Hare, Wiley, Chichester, 1992.
100.1 (C ), 67.7 (C ), 67.5 (C ), 31.2 (C ), 31.1 (C ), 29.2
v
p
p∞
q,q∞
q,q∞
(C ), 27.8 (C ), 27.4 (C ), 25.9 (C ), 19.1 (C ), 13.8 (C ).
A. P. Polishchuk and T. V. Timofeeva, Russ. Chem. Rev., 1993,
62, 291.
r,r∞
t
x
r,r∞
r,r∞
s,s∞
The mesogenic behaviour of all homologues is summarised
in Fig. 1 and detailed in Table 1. Elemental analyses are
detailed in Table 2.
A. Omenat and M. Ghedini, J. Chem. Soc., Chem. Commun., 1994,
1309.
R. Ishii, T. Kaharu, N. Pirio, S-W. Zhang and S. Takahashi,
J. Chem. Soc., Chem. Commun., 1995, 1215.
J. P. Rourke, D. W. Bruce and T. B. Marder, J. Chem. Soc., Dalton
T rans., 1995, 317.
Preparation of chloro-bridged palladium complex, 4d
Compound 4d is described in detail, all other homologues
were prepared similarly. One equivalent of 0.4 mol dm−3
methanolic hydrogen chloride was added to the acetato bridged
palladium complex 2d, (0.356 g, 3.1×10−4 mol) dissolved
in chloroform (250 ml) at room temperature causing the
clear yellow solution to become cloudy. The solvent was
removed and the crude product was washed with acetone
(15 ml) and filtered to collect the product. Yield 0.21 g (54%,
1.72×10−4 mol).
R. Deschenaux, I. Kosztics and B. Nicolet, J. Mater. Chem., 1995,
5, 2291.
10 M. Ghedini, D. Pucci and F. Neve, Chem. Commun., 1996, 137.
11 R. Deschenaux, M. Schweissguth and A-M. Levelut, Chem.
Commun., 1996, 1275.
12 J. Barbera´, P. Espinet, E. Lalinde, M. Marcos and J. L. Serrano,
L iq. Cryst., 1987, 2, 833.
13 P. Espinet, J. Pe´rez, M. Marcos, M. B. Ros, J. L. Serrano,
J. Barbera´ and A. M. Levelut, Organometallics, 1990, 9, 2028.
14 M. J. Baena, P. Espinet, M. B. Ros and J. L. Serrano, Angew.
Chem., Int. Ed. Engl., 1991, 30, 711.
Preparation of palladium cyclopentadienyl complex, 5d
15 M. J. Baena, J. Barbera, P. Espinet, A. Ezcurra, M. B. Ros and J. L.
´
Serrano, J. Am. Chem. Soc., 1994, 116, 1899.
16 M. Ghedini, S. Morrone, O. Francescangeli and R. Bartolino,
Chem. Mater., 1992, 4, 1119.
Compound 5d is described in detail, all other homologues
were prepared similarly. Thallium cyclopentadienide (0.200 g,
7.64×10−4 mol,
4
equiv.) was added to
a
solution of
17 M. Ghedini, S. Morrone, O. Francescangeli and R. Bartolino,
Chem. Mater., 1994, 6, 1971.
the chloro-bridged palladium complex 4d (0.239 g,
1.91×10−4 mol) in THF (250 ml) and heated at reflux (1 h).
The mixture was filtered and the solvent removed. The crude
product was purified by column chromatography on silica,
eluting with chloroform to yield a red solid. Yield 0.040 g
(28%, 5.16×10−5 mol).
18 M. Ghedini, D. Pucci, N. Scaramuzza, L. Komitov and S. T.
Lagerwall, Adv. Mater., 1995, 7, 659.
19 L. Zhang, D. Huang, N. Xiong, J. Yang, G. Li and N. Shu, Mol.
Cryst., L iq. Cryst., 1993, 237, 285.
20 N. Hoshino, H. Hasegawa and Y. Matsunaga, L iq. Cryst., 1991,
9, 267.
21 M. Marcos, J. L. Serrano, T. Sierra and M. J. Gime´nez, Chem.
Mater., 1993, 5, 1332.
22 K. Praefcke, S. Diele, J. Pickardt, B. Gundogan, U. Nutz and D.
¨
¨
Singer, L iq. Cryst., 1995, 18, 857.
Paper 6/05791H; Received 20th August, 1996
406
J. Mater. Chem., 1997, 7(3), 403–406