Cyclopalladated Schiff's base liquid crystals: The effect of the acac group on the thermal behaviour

Article abstract of DOI:10.1016/S0022-328X(97)00751-1

The cyclopalladation of mesogenic Schiff's bases and subsequent reaction with a β-diketone yields novel metallomesogens. In particular, use of simple alkyl substituted diketones can have a very dramatic effect on both the melting and clearing point.

Full text of DOI:10.1016/S0022-328X(97)00751-1

Journal of Organometallic Chemistry 555 (1998) 81–88
Cyclopalladated Schiff’s base liquid crystals: the effect of the acac
group on the thermal behaviour
Gareth W.V. Cave, Donocadh P. Lydon, Jonathan P. Rourke *
Department of Chemistry, Warwick Uni6ersity, Co6entry, CV4 7AL, UK
Received 22 September 1997
Abstract
The cyclopalladation of mesogenic Schiff’s bases and subsequent reaction with a i-diketone yields novel metallomesogens. In
particular, use of simple alkyl substituted diketones can have a very dramatic effect on both the melting and clearing point.
© 1998 Elsevier Science S.A. All rights reserved.
Keywords: Cyclopalladation; Liquid crystals; Mesogenic
There is currently much interest in the synthesis of
metal-containing liquid crystals due to the perceived
advantages of combining the properties of liquid crystal
systems with those of transition metals. The area has
been well-reviewed recently [1–6], and cyclopalladated
compounds have proved to be a particularly fertile area
of research, with many different examples from many
different groups [7–19].
In previous papers [20,21] we have described the
effect of a cyclopentadienyl group upon the mesogenic
behaviour of a series of mono-cyclopalladated com-
plexes, and the effect of two cyclometallated units on
mesogenic behaviour. Here we present the results of
using a variety of different i-diketonate ligands on a
series of mono-cyclopalladated complexes.
aldehyde and aniline proceeded in high yield and has
been described elsewhere [20], the i-diketones were
synthesised in high yield via a literature route. [22] The
cyclopalladation step to give the intermediate (2) was
essentially quantitative, and (2) was used without fur-
ther purification. The synthesis of the symmetrical dike-
t
tonate derivatives (3) (R=R%=Me, Bu, nBu, nhexyl
n
and octyl) and the unsymmetric derivatives (4) (R=
Me, R%=tBu, nBu, nhexyl and noctyl) proceeded in
good yield, and these complexes were purified by
column chromatography. All homologues of com-
1
pounds (3) and (4) were analysed by H and 13C NMR
and gave good elemental analyses (Table 2). In all, four
different Schiff’s bases and eight different i-diketones
were used, giving rise to 32 new compounds. It should
be noted that reaction of the intermediates (2) with the
unsymmetrical i-diketones to give complexes (4) gives
rise to two isomeric products, Scheme 2. The relative
quantities of the two isomers was determined by proton
NMR. The signals for the immine protons (7.46 and
7.45 ppm) and the signals for the methyl groups of the
diketone (2.06 and 1.90 ppm) in the two isomers are
clearly distinguishable. To the accuracy of the NMR
machine, integration shows both isomers to be present
in equal quantities.
1. Synthesis
The synthesis of the new compounds described here,
(3) and (4), is summarised in Scheme 1. The synthesis of
the 4-alkyloxy-N-(4%-alkyloxybiphenyl)benzylidene lig-
ands (1) via a simple condensation of the appropriate
* Corresponding author. Tel.: +44 1203 523263; fax: +44 1203
524112; e-mail: j.rourke@warwick.ac.uk
0022-328X/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.
PII S0022-328X(97)00751-1

82
G.W.V. Ca6e et al. / Journal of Organometallic Chemistry 555 (1998) 81–88
Scheme 1.
two different ways. Fig. 1 shows the thermal behaviour
2. Thermal properties
of the compounds grouped according to the Schiff’s
base unit present, and only includes the data for the
straight chain diketonates (i.e. the not the data for the
tBu complexes). In contrast, Fig. 2 shows the thermal
behaviour of the compounds grouped according to the
diketone used. Fig. 2 includes all the data present in
Table 1.
The thermal behaviour of the diketonate complexes
(3) and (4) is comprehensively detailed in Table 1 and
summarised in Figs. 1 and 2. Thus, with one exception,
all the complexes with nalkyl chains showed a smectic A
phase and many also exhibited a nematic phase before
clearing. The exception showed only a nematic phase.
The phases were identified on the basis of their optical
texture. The nematic phase exhibited schlieren textures,
showing both 4 and 2 point brushes. The smectic A
phase appeared as a focal-conic fan texture which
separated on cooling as battonets which consisted of
growing focal-conic domains. Though exhaustive differ-
ential scanning calorimetry (DSC) measurements were
not undertaken, DSC data for representative examples
were recorded: this data is incorporated into Table 1.
Figs. 1 and 2 summarise the thermal behaviour in
t
If we ignore the Bu complexes for the moment, it is
immediately apparent from Fig. 1 that any diketone
brings the both the melting and clearing points down
substantially (around 50 K on average, and 100 K in
some cases) compared with the simple acac.
Fig. 1 clearly shows that compounds (4) with unsym-
metrical diketones have melting points substantially
lower (some 50 K) than their symmetrical homologues
(3). Whilst this could be accounted for by the fact that
compounds (4) are in fact mixtures of two isomers

G.W.V. Ca6e et al. / Journal of Organometallic Chemistry 555 (1998) 81–88
83
Scheme 2.
(Scheme 2) and thus what we are seeing here is a
manifestation of the eutectic effect, it is likely that the
inherent asymmetry of compounds (4) is in fact respon-
sible for their reduced melting points. Compounds (4)
always have a higher clearing point their analogous
compound (3) with the same R% group.
As we move to compounds with longer chains, we see
a stabilisation of the smectic A phase at the expense of
the nematic. This well known effect [23] is shown with
increasing chain length on both the diketone and the
Schiff’s base unit. Thus, both Figs. 1 and 2 show the
dominance of the smectic phase towards the right hand
side of the graphs.
for compounds (4), the (b) and (c) derivatives have
much reduced melting points compared with the (a)
and (d) derivatives. The first observation seems to imply
that within any one compound the combined length of
the alkyloxy chains on the Schiff’s base is more impor-
tant than the distribution. The second observation is
perhaps best accounted for by the fact that compounds
(a) and (d) have their rigid aromatic cores symmetri-
cally substituted by their alkyloxy chains and thus any
asymmetry that arises from head to tail or head to head
packing has minimal effect on the melting point.
t
The effect of using the Bu diketone is very marked.
The sterically demanding group destabilises any meso-
genic behaviour to such an extent that only monotropic
behaviour is seen. Monotropic nematic phases are seen
for all four compounds studied, with the clearing points
around 130°C, some 50–120° lower than the other
compounds. The melting point of these compounds is
substantially higher than their normal chain analogues,
Fig. 2 indicates that the greatest nematic stability is
n
shown by the compounds with butyl chains attached
to the diketone. Thus both compounds (3) (R=R%=
nBu) and compounds (4) (R=Me, R%=nBu) have
greater nematic ranges than the parent simple acac
compounds (3) (R=R%=Me). In particular, these
compounds (4) (R=Me, R%=nBu) all show nematic
phases, with the Schiff’s base derivative with the short-
est chains showing only a nematic phase.
It is interesting to note from Fig. 1 that compounds
(b) (3 and 4) (n=4, m=7) and compounds (c) (3 and
4) (n=7, m=4) show very similar phase behaviour.
Another interesting feature apparent from Fig. 2 is that
t
indicating that the less conformationally disordered Bu
groups result in a better crystal packing. For two of
these compounds a monotropic smectic C phase was
observed below the nematic, the transition occurring
some 30° below the clearing point. It is interesting to
note that this was a tilted C phase, rather than the
smectic A phase shown by the normal chain analogues.

84
G.W.V. Ca6e et al. / Journal of Organometallic Chemistry 555 (1998) 81–88
Table 1
Mesogenic Behaviour of Compounds (3) and (4)
n
m
R
R%
trans
T (°C)
DH (J g−1
)
trans
T (°C)
DH (J g−1
)
trans
T (°C)
DH (J g−1
)
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
4
4
4
4
4
4
4
4
7
7
7
7
7
7
7
7
4
4
4
4
4
4
4
4
7
7
7
7
7
7
7
7
Me
Me
nBu
Me
nhex
Me
noct
tBu
Me
Me
nBu
Me
nhex
Me
noct
tBu
Me
Me
nBu
Me
nhex
Me
noct
tBu
Me
Me
nBu
Me
nhex
Me
noct
tBu
Me
K–SA
K–SA
K–N
K–SA
K–SA
K–SA
K–SA
K–I
K–SA
K–SA
K–SA
K–SA
K–SA
K–SA
K–SA
K–I
K–SA
K–SA
K–SA
K–SA
K–SA
K–SA
K–SA
K–I
K–SA
K–SA
K–SA
K–SA
K–SA
K–SA
K–SA
K–I
184
130
131
130
139
139
137
205
162
101
129
71
121
88
134
199
155
76
SA–N
SA–N
N–I
SA–N
SA–N
SA–N
SA–I
209
147
195
182
158
198
176
127a
241
164
134
201
178
198
199
124a
225
184
138
201
176
210
198
101a
245
199
161
208
179
206
190
107a
N–I
N–I
260
225
nBu
nBu
nhex
nhex
noct
noct
tBu
31.9
34.4
2.6
1.3
N–I
N–I
N–I
210
188
210
1.6
1.2
28.5
1.6
N–I
Me
SA–N
SA–N
SA–N
SA–N
SA–N
SA–N
SA–I
N–I
N–I
N–I
N–I
N–I
N–I
261
206
198
220
182
208
nBu
nBu
nhex
nhex
noct
noct
tBu
34.3
(33.2
38.0
1.2
1.1
1.8
1.4
—b)
1.9
27.4
3.4
N–I
Me
SA–N
SA–N
SA–N
SA–N
SA–N
SA–N
SA–I
SC–N
SA–I
SA–N
SA–N
SA–I
N–I
N–I
N–I
N–I
N–I
N–I
249
211
181
205
183
212
nBu
nBu
nhex
nhex
noct
noct
tBu
117
76
116
85
30.0
30.2
1.4
1.3
1.4
1.3
32.6
(3.0
—b)
135
224
141
78
109
130
115
118
128
205
N–I
131a
Me
nBu
nBu
nhex
nhex
noct
noct
tBu
N–I
N–I
212
187
28.9
35.7
36.5
(2.7
2.9
1.6
—b)
2.4
SA–N
SA–I
SA–I
N–I
N–I
182
34.7
3.8
SC–N
134a
a Monotropic tranisition.
b Combined enthalpies.
pounds (where decomposition can set in at 200°C). In
contrast, when a Bu group is attached to the acac core,
3. Conclusions
t
the mesophases are destabilised very substantially, to
the extent that all mesophases exhibited are
monotropic. The Bu group is a compact group that
These results clearly show the advantages of using
short chain length alkyl diketones: both melting and
n
t
clearing points can be brought down substantially. In
addition, it appears that the effect of a short (four
carbon, nBu) alkyl chain substantially stabilises the
nematic phase at the expense of the smectic A phase.
This can be understood in terms of the short chain
freely moving at the side of the rigid core disrupting
both the crystal packing and the lateral interactions
that are necessary for the formation of a smectic phase.
These short chained alkyl diketones bring the melting
and clearing points down to a similar extent to that
observed with the cyclopentadienyl group, which we
used in analogous compounds [20] (the effect is to make
the molecule non-planar, disrupting the packing and
thus bringing down the melting and clearing points).
The short chain diketones are to be preferred, however,
as the thermal stability of their palladium complexes is
much greater than that of the cyclopentadienyl com-
cannot move freely, and will not disrupt the packing
forces.
Other analogous work has used both simple acac and
phenyl substituted [24] i-diketonate complexes of pal-
ladium Schiff’s base derivatives. At the time, these
simple acac compounds were unusual in that they
showed nematic phases [9]. Our new results comple-
ment these older results, with our short chain diketones
showing a greatly stabilised nematic phase. Indeed, we
have also shown this effect with some of our di-metal-
lated complexes [21].
Though, one report [25] is solely concerned with the
synthesis of 3-substituted pentane-2,4-diones for the
purposes of liquid crystal research, apart from two
recent papers [19,21] there have been no other reports
of the use of the simple alkyl diketones that we have
employed to our very great advantage.

G.W.V. Ca6e et al. / Journal of Organometallic Chemistry 555 (1998) 81–88
85
Fig. 1. Phase behaviour of compounds (3) and (4) grouped by Schiff’s base.
4. Experimental
benzylidenes were synthesised via literature routes [20].
All i-diketones with the exception of 2,2,6,6-te-
tramethyl-3,5-heptanedione (which is commercially
available) were synthesised via literature routes [22].
4.1. General
All chemicals were used as supplied, unless noted
otherwise. All NMR spectra were obtained on either a
Bruker AC250 or on an AC400 in CDCl3 and are
referenced to external TMS, assignments being made
with the use of decoupling, nOe and the COSY pulse
sequence. 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, and a Perkin–Elmer Pyris 1 DSC.
All elemental analyses were performed by Warwick
Analytical Service. 4-Alkyloxy-N-(4%-alkyloxybiphenyl)
4.2. Preparation of ortho-metallated palladium acetate
complexes (2)
(2d) is described in detail, all other homologues were
prepared
similarly.
4-Heptyloxy-N-(4%-heptyloxy-
biphenyl)benzylidene (0.41 g, 8.5×10−4 mol) and palla-
dium 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.

86
G.W.V. Ca6e et al. / Journal of Organometallic Chemistry 555 (1998) 81–88
Fig. 2. Phase behaviour of compounds (3) and (4) grouped by diketone.
Yield 0.55 g (98%, 4.2×10−4 mol)
NMR data:
7,9-Pentadecanedione (0.081 g, 3.39×10−4 mol) and
triethylamine (0.034 g, 3.39×10−4 mol) were 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 chromatography
lH: 7.59 (1H, s, Ha), 7.51 (2H, AA%XX%, Hg), 7.35 (2H,
AA%XX%, Hf), 7.18 (1H, d, 3J(HH) 8.4 Hz, Hb), 6.95 (2H,
AA%XX%, Hh), 6.78 (2H, AA%XX%, He), 6.59 (1H, dd,
3J(HH) 8.4 Hz, 4J(HH) 2.3 Hz, Hc), 6.03 (1H, d, 4J(HH)
3
2.3 Hz, Hd), 4.05 (2H, t, J(HH) 6.4 Hz, Hi%), 4.00 (2H,
on silica, eluting with
dichloromethane and hexane.
a
50/50 mixture of
t, 3J(HH) 6.7 Hz, Hi), 1.90 (3H, s, Hm), 1.81 (4H, m, Hj,j%),
3
Yield 0.208g (76%, 2.34×10−4 mol)
NMR data (3):
1.40 (16H, m, Hk,k%), 0.88 (6H, t, J(HH) 7.1 Hz, Hl,l%)
lH: 8.06 (1H, s, Ha), 7.55 (2H, AA%XX%, Hf), 7.52 (2H,
AA%XX%, Hg), 7.45 (2H, AA%XX%, He), 7.32 (1H, d,
4
3J(HH) 8.5 Hz, Hb), 7.19 (1H, d, J(HH) 2.3 Hz, Hd),
4.3. Preparation of i-diketonate palladium complexes
(3) and (4)
6.97 (2H, AA%XX%, Hh), 6.63 (1H, dd, 3J(HH) 8.5,
4J(HH) 2.3 Hz, Hc), 5.33 (1H, s, Hm), 4.09 (2H, t, 3J(HH)
3
(3d) with R=R%=nhex is described in detail, all other
homologues were prepared similarly.
6.0 Hz, Hi%), 4.00 (2H, t, J(HH) 6.0 Hz, Hi), 2.32 (2H,
t, 3J(HH) 7.5 Hz, Hn%), 2.14 (2H, t, 3J(HH) 7.5 Hz, Hn),

G.W.V. Ca6e et al. / Journal of Organometallic Chemistry 555 (1998) 81–88
87
Table 2
Elemental Analysis data for Compounds (3) and (4)
n
m
R
R%
C %
H %
N %
Found
(Expected)
Found
(Expected)
Found
(Expected)
4
4
4
4
4
4
4
4
4
4
4
4
4
4
7
7
7
7
7
7
7
7
7
7
7
7
7
7
4
4
4
4
4
4
4
7
7
7
7
7
7
7
4
4
4
4
4
4
4
7
7
7
7
7
7
7
Me
nBu
nBu
nhex
nhex
noct
noct
tBu
64.4
65.8
65.4
67.5
66.5
67.0
65.7
65.9
67.2
66.8
69.1
67.6
69.5
67.4
66.0
67.3
66.7
68.4
67.3
69.2
67.3
67.0
68.7
67.9
69.4
68.3
67.0
68.5
(64.9)
(66.1)
(65.7)
(67.6)
(66.5)
(67.5)
(66.1)
(66.1)
(67.2)
(66.9)
(68.6)
(67.6)
(69.7)
(67.2)
(66.1)
(67.3)
(66.9)
(68.6)
(67.6)
(69.7)
(67.3)
(67.3)
(68.3)
(67.9)
(69.4)
(68.6)
(70.5)
(68.3)
6.8
7.1
7.1
7.7
7.5
7.6
7.1
7.3
7.5
7.5
8.3
7.5
8.5
7.6
7.2
7.5
7.5
8.0
7.6
8.2
7.8
7.7
7.8
7.7
8.3
8.0
8.6
7.7
(6.7)
(7.2)
(7.0)
(7.7)
(7.3)
(7.7)
(7.2)
(7.2)
(7.6)
(7.4)
(8.1)
(7.7)
(8.5)
(7.6)
(7.2)
(7.6)
(7.4)
(8.1)
(7.7)
(8.5)
(7.6)
(7.6)
(7.9)
(7.8)
(8.4)
(8.1)
(8.8)
(7.9)
2.2
2.2
2.1
1.9
2.0
2.0
2.0
2.1
1.7
2.0
1.5
1.7
1.5
2.0
2.2
2.1
1.8
1.8
2.1
1.5
1.7
2.0
1.7
1.7
1.7
1.7
1.5
1.9
(2.2)
(2.0)
(2.1)
(1.9)
(2.0)
(1.9)
(2.0)
(2.0)
(1.9)
(2.0)
(1.8)
(1.9)
(1.7)
(1.9)
(2.0)
(1.9)
(2.0)
(1.8)
(1.9)
(1.7)
(1.9)
(1.9)
(1.8)
(1.8)
(1.7)
(1.8)
(1.6)
(1.8)
nBu
Me
nhex
Me
noct
tBu
Me
nBu
nBu
nhex
nhex
noct
noct
tBu
nBu
Me
nhex
Me
noct
tBu
Me
nBu
nBu
nhex
nhex
noct
noct
tBu
nBu
Me
nhex
Me
noct
tBu
Me
nBu
nBu
nhex
nhex
noct
noct
tBu
nBu
Me
nhex
Me
noct
tBu
1.77 (4H, m, Hj,j%), 1.5–1.2 (32H, m, Hk,k%,o,o%), 0.90 (6H,
AA%XX%, He/e%), 7.45 (2H, AA%XX%, He/e%), 7.32 (1H, d,
3J(HH) 8.0 Hz, Hb/b%), 7.28 (1H, d, 3J(HH) 8.0 Hz,
Hl,l%), 0.80 (6H, Hp,p%
)
NMR data (4):
lH: 8.05 (1H, s, Ha/a%), 8.04 (1H, s, Ha/a%), 7.56 (2H,
AA%XX%, Hf/f%), 7.55 (2H, AA%XX%, Hf/f%), 7.53 (2H,
AA%XX%, Hg/g%), 7.52 (2H, AA%XX%, Hg/g%), 7.46 (2H,
4
Hb/b%), 7.17 (1H, d, J(HH) 2.3 Hz, Hd/d%), 7.16 (1H, d,
4J(HH) 2.3 Hz, Hd/d%), 6.98 (4H, AA%XX%, Hh,h%), 6.63

88
G.W.V. Ca6e et al. / Journal of Organometallic Chemistry 555 (1998) 81–88
3
4
[7] J. Barbera´, P. Espinet, E. Lalinde, M. Marcos, J.L. Serrano, Liq.
Cryst. 2 (1987) 833.
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(1H, dd, J(HH) 8.0, J(HH) 2.3 Hz, Hc/c%), 6.62 (1H,
dd, 3J(HH) 8.0, 4J(HH) 2.3 Hz, Hc/c%), 5.35 (2H, s,
3
Hm,m%), 4.10 (4H, t, J(HH) 6.0 Hz, Hi/i%), 4.01 (4H, t,
3J(HH) 6.0 Hz, Hi,i%), 2.32 (2H, t, 3J(HH) 7.5 Hz, Ho/o%),
3
2.14 (2H, t, J(HH) 7.5 Hz, Ho/o%), 2.06 (3H, s, Hn/n%),
[10] M.J. Baena, J. Barbera´, P. Espinet, A. Ezcurra, M.B. Ros, J.L.
Serrano, J. Am. Chem. Soc. 1994 (1899) 116.
[11] M. Ghedini, S. Morrone, O. Francescangeli, R. Bartolino, Chem.
Mater. 4 (1992) 1119.
[12] M. Ghedini, S. Morrone, O. Francescangeli, R. Bartolino, Chem.
Mater. 1994 (1971) 6.
1.90 (3H, s, Hn/n%), 1.77 (8H, m, Hj,j%), 1.5–1.2 (48H, m,
Hk,k%,p,p%), 0.85 (18H, m, Hl,l%,q,q%
)
The mesogenic behaviour of all homologues is de-
tailed in Table 1 and summarised in Figs. 1 and 2.
Elemental analyses are detailed in Table 2.
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Acknowledgements
We thank the University of Warwick for financial
support (DPL), and Johnson–Matthey for loan of
chemicals.
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