Generation of metallomesogens using common ligands functionalised with liquid-crystalline moieties

Article abstract of DOI:10.1039/b717428d

Functionalisation of 2,2′-bipyridine or acetylacetone with an alkyleneoxycyanobiphenyl group induces liquid-crystalline properties in simple metal complexes. The Royal Society of Chemistry.

Full text of DOI:10.1039/b717428d

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COMMUNICATION
www.rsc.org/dalton | Dalton Transactions
Generation of metallomesogens using common ligands functionalised with
liquid-crystalline moieties†‡
Indudhara S. Shashikala and Duncan W. Bruce*
Received 12th November 2007, Accepted 11th December 2007
First published as an Advance Article on the web 15th January 2008
DOI: 10.1039/b717428d
Functionalisation of 2,2ꢀ-bipyridine or acetylacetone with an
alkyleneoxycyanobiphenyl group induces liquid-crystalline
properties in simple metal complexes.
gens when complexed to palladium(II) and platinum(II),2 but are
poor ligands to rhodium(I) and silver(I). On the other hand, 4-
alkoxystilbazoles3 (elaborated pyridines) are not ideal ligands for
generating metallomesogens when bound to MCl2 (M = Pd, Pt),
yet they do lead to metallomesogens with RhI, IrI and, especially,
AgI.4 In contrast, di- and tri-alkoxystilbazoles are good ligands
for PdII, PtII and AgI, but not RhI and IrI. Thus, there is a sense
in which ligands are tailor-made for particular complexes (this is
also true in discotic systems) and while this a valid approach to
their study, there is perhaps a need for more generic ligands.
With this in mind, we identified two common and widely used
ligands, namely bipyridine and acetylacetone, and elected to
elaborate each with a cyanobiphenyl, which would be bound to
the core ligand via a flexible alkyl spacer. There is a sense in which
a similar approach has been adopted previously, where Cardinaels
et al.5 described the preparation of extensively derivatised 1,10-
phenanthrolines (imidazo[4,5-f ]-1,10-phenanthrolines, which
were then bound to three alkyleneoxycyanobiphenyls through
a phenyl ring). Indeed, the literature shows6 that almost any
chemical core can be made liquid crystalline if sufficient mesogenic
moieties are attached to it. However, the work described here of-
fers, we believe, a simpler and more generally applicable approach.
The bipyridine ligands were synthesised as described in
Scheme 1. Thus, the hydroxyl function of 1-bromo-11-hydroxy-
undecane was protected with THP to give 1, which was then
The synthesis and study of metallomesogens has been a very
active area of research for more than twenty years and different
approaches have been adopted in order to accommodate different
metal centres.1 For example, in many discotic systems the situation
is reasonably straightforward and it has been sufficient to take
a planar metal–ligand combination and ‘decorate’ the periphery
with six to eight alkyl chains to generate a complex that will stack
to form a columnar mesophase. Examples include, for example, b-
diketonato complexes of copper(II), metallophthalocyanines and,
more exotically, tetrapalladium(II) macrocycles.1
With rod-like (calamitic) mesogens, the situation has always
been a little more delicate and a slightly more careful choice of
metal–ligand combination has been required. For example, from
our own work we know that liquid-crystalline cyanobiphenyls
(really just elaborated benzonitriles) will generate metallomeso-
Department of Chemistry, University of York, Heslington, York, UK YO10
5DD. E-mail: db519@york.ac.uk; Tel: +44 1904 434085
† Dedicated to Professor Ken Wade on the occasion of his 75th birthday.
Truly a scholar and a gentleman.
‡ Electronic supplementary information (ESI) available: Synthesis, elemen-
tal analysis and NMR details. See DOI: 10.1039/b717428d
Scheme 1 Synthesis of the mesomorphic bipyridines and their Pd and Pt complexes: (i) 3,4-dihydro-2H-pyran, CH2Cl2; (iia) LDA, n-BuLi, THF, −70 ◦C;
(iib) 1, −30 ◦C; (iii) p-TSA, EtOH, reflux; (iv) DEAD, CH2Cl2, 0 ◦C; (v) [PdCl2(NCPh)2], CHCl3 or K2[PtCl4], HCl, EtOH.
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Scheme 2 Preparation of the new b-diketonates: (i) NaHCO3, KI, acetone; (ii) K2CO3, DMF, 80 ◦C; (iii) NaH, DME.
reacted with the dianion of 5,5ꢀ-dimethyl-2,2ꢀ-bipyridine (via reac-
tion with LDA) at low temperature to give the coupled product,
2. Deprotection of the alcohol led to 3 which was then coupled
with 4-hydroxy-4ꢀ-cyanobiphenyl under Mitsonobu conditions to
give bipyridine 4. Bipyridine 4–12 was mesomorphic and melted
at 130.2 ◦C to give a nematic phase that persisted to 149.1 ◦C
when it cleared. Bipyridine 4–8, with the shorter methylene linking
group was, however, not mesomorphic and simply melted directly
to the isotropic state at 214 ◦C. The analogous ligand was also
prepared using 4,4ꢀ-dimethyl-2,2ꢀ-bipyridine and with n = 12, but
it was not mesomorphic, melting directly to the isotropic liquid
at 139 ◦C. The related palladium complex was similarly devoid of
mesomorphism.
Cyanobiphenyl-functionalised derivatives were prepared as
shown in scheme 2. Thus, excess of 1,6-dibromohexane was
reacted with 4-hydroxyacetophenone to give the bromoalkyl-
functionalised acetophenone (7) which, upon reaction with 4ꢀ-
hydroxy-4-cyanobiphenyl, gave intermediate acetophenone 8. A
similar pathway gave the intermediate methyl benzoate 10 via a
bromoalkyl-functionalised methyl benzoate (9). Claisen conden-
sation of 8 and 10 then led to the target b-diketone, 11.
Examination of the thermal properties of 11 and some of its
intermediates showed that acetophenone 8 was mesomorphic,
melting to give a nematic phase at 99.4 ◦C, which cleared to
isotropic liquid at 122.4 ◦C. Similarly, the methyl benzoate 10
showed a nematic phase with transition temperatures rather
similar to those of 8 (Table 1). Considering the complete ligand,
11, heating caused the compound to melt at 152.5 ◦C to give a
nematic phase, which persisted to 201.1 ◦C when clearing was
observed.
Bipyridines 4 were reacted with [PdCl2(NCPh)2] in acetone or
with PtCl2 in the melt,7 to give [PdCl2(4)] (5-n) and [PtCl2(4)] (6-
n), respectively, which were characterised by NMR spectroscopy
and by elemental analysis (details in ESI‡). Complex 5–12 shows
a monotropic SmA phase, so that on heating, it melted directly
to the isotropic state at 179.1 ◦C and on cooling, the SmA
phase was seen to grow in at 156.5 ◦C, persisting until the
complex crystallised, normally around 110 ◦C. Interestingly, 5–
12 was found to be soluble in 4ꢀꢀ-pentyl-4-cyanoterphenyl (T15)
at 128 ◦C at a concentration of 10%. Complex 6–12 was similarly
Considering other common ligand systems, we next turned
our attention to b-diketonates. b-Diketonate ligands have been
used extensively in metallomesogen chemistry,1 and in many
complexes, mesomorphism arises from complexation to diphenyl
b-diketonates functionalised with between two and six alkoxy
chains. Most extensively studied have been 2 : 1 complexes with
IV
CuII, PdII and O V , although 1 : 1 complexes are know with
=
TlI and IrI, 3 : 1 complexes with ions such as CoIII and MnIII and
there is even a 4 : 1 complex with ZrIV. In addition, b-diketones are
co-ligands in a range of complexes, mostly with PdII or PtII bound
to some ortho-metallated such as a Schiff base or a phenylpyridine.
In many of these cases, acac itself is used as a simple ‘spectator’ co-
ligand, but in other cases, more highly functionalised diketonates
are employed, which modify the properties of the parent complex.
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Table 1 Thermal properties of the new liquid crystals
Compound
Transitiona
T/◦C
DH/kJ mol−1
4–12
5–12
6–12
8
Cr–N
N–I
Cr–I
(SmA–I)
Cr–I
(SmA–I)
Cr–N
N–I
Cr–N
N–I
Cr–N
N–I
Cr–N
N–I
Cr–SmA
SmA–N
N–I
130.2
149.1
179.1
(156.5)
176.5
(150.3)
99.4
122.4
125.9
137.4
152.5
201.1
224.6
241.3
130.0
176.0
240.0
202.0
(168.0)
(136.0)
72.3
6.3
54.0
(5.7)
52.3
(2.4)
45.4
3.3
31.8
4.9
70.5
7.7
46.1
6.6
—
—
—
10
11
12
13b
15
Cr–I
(N–I)
(SmA–N)
43.8
(2.6)
(1.9)
Fig. 1 Structures of mesomorphic complexes of the new b-diketonates.
a Reporting monotropic transitions in this way indicates that temperatures
were obtained on re-heating from the lower phase and as such are
thermodynamic temperatures. b Transition temperatures from microscopy.
DSC data not available, see text.
to a nematic phase, characterised by its schlieren and marbled
textures. However, while the complex was pure by 1H NMR
spectroscopy and elemental analysis, insoluble impurities were
observed suspended in the isotropic liquid on cooling. Further,
when the material was heated for a second time, the original
melting point could not be reproduced. Comparison of the
clearing point of the apparently monotropic nematic phase with
that of the two precursors to 11, namely 8 and 10, showed that
TNI was almost exactly the average of the two, implying that on
heating/melting, the complex and its ligand decomposed to give
the precursor methyl benzoate and acetophenone.
mesomorphic, melting at 176.5 ◦C and forming a monotropic SmA
phase at 150.3 ◦C.
Mesomorphic complexes of MCl2 in combination with bipy-
based ligands are rare. Complexes formed from 5,5ꢀ-disubstituted-
2,2ꢀ-bipyridines did not show liquid crystal properties, while
more recently, Pucci et al.8 reported some dichloro-palladium
and -platinum complexes of alkyl esters of 2,2ꢀ-bipyridine-4,4ꢀ-
dicarboxylic acid which were luminescent and which showed an
ill-defined lamellar phase.
Complexes were also made by reacting 4–12 with ZnCl2 and
with [ReCl(CO)5] to give [ZnCl2(4–12)] and [ReCl(CO)3(4–12)],
respectively, but neither was liquid crystalline.
A series of metal complexes was also prepared using the b-
diketonate ligands as indicated in Fig. 1. Thus, a 2 : 1 complexes
was obtained using CuII (12), which was found to be mesomorphic,
melting to the nematic phase at 224.6 ◦C and then clearing at
241.3 ◦C. Complexation to oxovanadium(IV) was also productive
and complex 13 melted at 130 ◦C to form a SmA phase, which
gave way to a nematic phase at 176 ◦C. The nematic phase cleared
at 240 ◦C and on cooling the mesomorphic sequence reversed,
showing no evidence for decomposition. However, we were not
able to obtain DSC data for this complex as no events showed up
during the scan, despite clear evidence for the transitions from
optical microscopy. This is an effect we have seen before, but
methods we have used previously for overcoming it (e.g. using
a pan as a lid in preparing the sample) were to no avail on this
occasion.
Finally, the palladium(II) complex 15 was prepared via the
dinuclear ortho-metallated complex with bridging acetate ligands.
As indicated above, complexes of this type10 normally derive their
liquid crystallinity from the ortho-metallated imine, but in the
present case the intention was to switch things around and to let
mesomorphism be driven by the b-diketonate. This in fact turned
out to be the case and so on heating 15 was found to melt to
the isotropic phase at 202 ◦C which, on cooling, gave way to a
nematic phase at 168 ◦C and then a SmA phase at 136 ◦C. Further
cooling showed no evidence of crystallisation and so it is assumed
that the complex forms a glass on cooling. Re-heating simply
reproduced the SmA–N and N–I transitions showing that there
was no decomposition.
Thus, employing a simple functionalisation of 2,2ꢀ-bipyridine
and acetylacetone, ligands are obtained that can induce liquid
crystal properties in a range of simple metal complexes.
Acknowledgements
We thank the Royal Society for an India Fellowship to ISS and
Johnson Matthey for generous loans of platinum metal salts.
Recalling iridium(I) carbonyl complexes reported by Trzaska
and Swager,9 complexes 14 were prepared by reaction of ligand
11 with [Ir2(l-Cl)2(COD)2] under an atmosphere of CO. The
products were orange in colour and showed the characteristic
two carbonyl stretching vibrations in the infrared spectrum◦at
2069 and 1993 cm−1. On heating, the complexes melted at 150 C
forming an isotropic liquid which, on cooling to 121 ◦C gave way
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