comprehensive inorganic chemistry ii || metallomesogens
TRANSCRIPT
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8.21 MetallomesogensJ Torroba and DW Bruce, University of York, York, UK
ã 2013 Elsevier Ltd. All rights reserved.
8.21.1 Preamble 8388.21.2 General Introduction 8388.21.3 Thermotropic Liquid Crystals 8388.21.3.1 Calamitic Mesogens 8398.21.3.2 Mesophases of Calamitic Mesogens 8398.21.3.2.1 The nematic phase 8398.21.3.2.2 The chiral nematic phase 8398.21.3.2.3 The true smectic phases 8408.21.3.3 Discotic Mesogens 8418.21.3.4 Mesophases of Disk-Like Mesogens 8418.21.3.5 Polycatenar Liquid Crystals 8428.21.4 Lyotropic Liquid Crystals 8428.21.5 Physical Properties and Mesophase Characterization 8448.21.5.1 Physical Properties 8448.21.5.2 Mesophase Characterization 8458.21.6 General Overview of Metallomesogens Types 8468.21.6.1 Metal Carboxylate Mesogens 8468.21.6.1.1 Carboxylates of monovalent metals 8468.21.6.1.2 Carboxylates of divalent metals 8468.21.6.1.3 Tetra(carboxylato)dimetal mesogens 8468.21.6.2 Macrocyclic Metallomesogens 8478.21.6.2.1 Metallophthalocyanine mesogens 8478.21.6.2.2 Metalloporphyrin mesogens 8508.21.6.2.3 Other macrocyclic metallomesogens 8538.21.6.3 Complexes of Mono- and Bi-Dentate Ligands 8548.21.6.3.1 b-Diketonato metal complexes 8548.21.6.3.2 Other O,O-donor ligands 8578.21.6.3.3 S,S-Donor ligands 8588.21.6.3.4 Enaminoketones 8598.21.6.3.5 Salicylaldimine derivatives 8608.21.6.3.6 Other N,O-donor ligands 8628.21.6.3.7 Pyrazole-based ligands 8638.21.6.3.8 Pyridines, bipyridines, and related ligands 8648.21.6.3.9 Other N,N-donor ligands 8678.21.6.3.10 Nitriles, isonitriles, and acetylides 8688.21.6.3.11 Miscellaneous organometallic systems 8718.21.6.4 Ferrocene-Containing Metallomesogens 8728.21.6.5 Liquid-Crystalline Metallodendrimers 8748.21.6.6 Miscellaneous 8768.21.7 Ortho-Metallated Metallomesogens 8778.21.7.1 Ortho-Metallated Palladium(II) and Platinum(II) Complexes 8778.21.7.1.1 Ortho-metallated azo, azoxy, and azine complexes 8788.21.7.1.2 Ortho-Metallated imine complexes 8818.21.7.1.3 Ortho-metallated pyrimidine, pyridazine, and pyridine complexes 8868.21.7.1.4 Other ortho-metallated complexes 8908.21.7.2 Octahedral Ortho-Metallated Complexes 8928.21.8 Lanthanide-Containing Liquid Crystals and Magnetic Responses of Metallomesogens 8938.21.8.1 Lanthanide-Containing Liquid Crystals 8948.21.8.1.1 Macrocycles 8948.21.8.1.2 Salicylaldimines 8968.21.8.1.3 N-Donor chelating ligands 8988.21.8.2 Magnetic Properties of Metallomesogens 8998.21.8.2.1 Magnetic alignment and magnetic anisotropy 8998.21.8.2.2 Single-molecule magnets 900
mprehensive Inorganic Chemistry II http://dx.doi.org/10.1016/B978-0-08-097774-4.00824-X 837
838 Metallomesogens
8.21.8.2.3 Spin-crossover compounds 9018.21.8.2.4 Magnetic susceptibility of mesogenic carboxylates and polymeric acetylides 9038.21.9 Conclusion 904Acknowledgments 904References 904
8.21.1 Preamble
The purpose of this chapter is to give an overview of the subject
of metallomesogens. The subject can be traced back to the
nineteenth century, but in reality it was the work of Giroud
and Mueller-Westerhoff1 that sparked more recent interest
at the end of the 1970s, with real momentum developing
from the late 1980s. The subject has been well reviewed2–21
and so to try to offer something original, the chapter is orga-
nized as follows.
Following a short introduction – the introduction is a precis
based on that found in Bruce et al.,21 which will allow the
chapter to be self-contained – an overview of the major struc-
tural types of metallomesogens is given in which the most
salient aspects are highlighted. This is to acquaint the reader
with the wide range of complex types that have been found to
show liquid crystal properties. Then, to show what is possible
with imaginative metal–ligand combinations, a section is
included on the design and mesomorphism of metallomeso-
gens based on ortho-metallated ligands. Inevitably this concen-
trates on palladium(II) and, to an extent, platinum(II), but there
are also examples concerning manganese(I) and rhenium(I), as
well as mercury(II) and iridium(III). Having considered metallo-
mesogens from a structural point of view, the emphasis moves to
consider properties – in fact one, specific property – magnetism.
This section covers diamagnetic anisotropy in lyotropic metallo-
polymers right through d-block systems to lanthanides.
8.21.2 General Introduction22
The liquid crystal state is a true state of matter – the fourth
state of matter, which exists between the solid and liquid states.
In common with the solid state, its molecules possess some
degree of positional and/or orientational order while, in com-
mon with the liquid state, it is a fluid. This combination of
fluidity and order means that liquid crystals are fluids with
anisotropic physical properties. This makes them of funda-
mental interest, for in other fluids physical properties are iso-
tropic, but it is also the basis for their widespread application
in the ubiquitous liquid-crystal display (LCD) among others.
Liquid crystals are organized into two broad categories,
depending on how the order of the solid state is disrupted to
form the liquid crystal state. If this transition is driven by
solvent, then the liquid crystals are termed ‘lyotropic,’ while if
it is achieved in the absence of solvent, most usually by temper-
ature alone, then the liquid crystals are termed ‘thermotropic.’
This latter category is by far the largest in metallomesogens.
The recognition of the liquid crystal state is usually attrib-
uted to the work of Reinitzer23 who, in 1888, published on the
apparent two melting points of cholesteryl esters (Figure 1),
although there is evidence of the recognition of what we now
know to be liquid crystal properties in nerve myelin in the
1850s24,25 and also in the behavior of magnesium soaps
at about the same time.26 Another important event in the
development of understanding of this new state of matter
was the publication in 1922 by Friedel of ‘Les etats mesomorphes
de la matiere,’27 a seminal report that was the very first classi-
fication of the various liquid-crystalline mesophases based on
the order and symmetry of different molecular arrangements.
Since these initial discoveries, liquid crystals have become a
major, multidisciplinary field of research, which have impacted
on society in a major way following the discovery of the cyano-
biphenyl liquid crystals by Gray in the early 1970s28 and their
utilization in the twisted nematic mode display device29 that led
to the birth of the LCD industry, now worth >€100 billion/
annum. Furthermore, the LCD market is ever increasing due to
the invention and optimization of new display modes, the
solution of problems associated with angle of view, displays
capable of operating at 100 Hz, three-dimensional (3D) and
holographic displays, new ferroelectric microdisplays used in
the viewfinders of camcorders, and the advent of picoprojection.
Televisions are now dominated by LCD mode. Thus, the fact
that LCDs can be said to be a ‘mature’ technology should not be
allowed to present any view other than one that shows that work
in materials and device development in this area alone is very
significant, dynamic, and highly competitive. Nonetheless, the
real and potential applications in liquid crystals extend well
beyond the flat-panel display and recent publications30,31 give
indications of some areas with great potential.
Before proceeding further, it will be of use to introduce some
of the terminology commonly associated with liquid crystals.
A material which has liquid crystal properties is referred to as a
‘mesogen’ and is said to exhibit ‘mesomorphism’; something
which is ‘liquid-crystal-like’ is known as ‘mesogenic,’ although
something mesogenic is not necessarily mesomorphic. To avoid
ambiguity, the (conventional) liquid state is referred to as the
‘isotropic’ state. The temperature at which amaterial passes from
the solid state into a mesophase is referred to as the ‘melting
point,’ while the temperature at which the mesophase trans-
forms into an isotropic fluid is called the ‘clearing point.’
8.21.3 Thermotropic Liquid Crystals
Thermotropic liquid crystals can be divided into three princi-
pal types according to shape. While this taxonomy does not
address issues of physical properties and phase behavior, it
does provide a framework by which we may begin to make
sense of the observed behavior. The three types are rod-like
(calamitic), disk-like (discotic), and bent-core liquid crystals.
The liquid crystal state is stabilized by the presence of
intermolecular, anisotropic dispersion forces that result from
the anisotropic nature of the molecules that form the phases.
Metallomesogens 839
Thus, rod-like molecules are much longer than they are broad
and, hence, possess one unique, long axis. Similarly, disk-like
molecules are rather flat and hence possess one, unique short
axis. Such an analysis is a little less obvious for bent-core
materials; yet overall the anisotropy is sufficient to stabilize
mesophases (Figure 2).
8.21.3.1 Calamitic Mesogens
A general structure of calamitic mesogens is often that given in
Figure 3, which follows a design offered by Toyne and which
should be considered indicative rather than prescriptive.32
What the model describes in general is an anisotropic mole-
cule, normally, but by no means exclusively, composed of aro-
matic (or heteroaromatic) rings linked together in some way to
maintain the overall anisotropy. In many cases, there will be
three or more rings and the linking group(s) (B) may preserve
the conjugation of the system, although this is not necessary (e.g.,
–CH¼CH–, –C�C–, –CH¼N–, –N¼N–, –CO2–). The two ter-
minal groups (A and C) can be the same or may be different, but
in almost all cases, one will be an alkyl (or alkyloxy) group that
acts both to enhance the anisotropy of the molecule and to
reduce the melting point. Where the terminal groups are not
the same, then one will often be a small, dipolar group (e.g.,
–CN, –NO2, and –OMe). There are many thousands of liquid
crystal molecules known and examples may be sought in Vill’s
remarkable database.33 Occasionally, the molecule will possess a
lateral group (D). In general, calamitic mesogens do not tolerate
such lateral groups well as they reduce the structural anisotropy.
(a) (b) (c)
Figure 2 The (a) rod-like, (b) disk-like, and (c) bent-coremesogenmotifs.
A B Cn
D
Figure 3 A general molecular structure for calamitic mesogens. FromBruce, D. W.; Deschenaux, R.; Donnio, B.; Guillon, D. Metallomesogens.In Comprehensive Organometallic Chemistry III; Crabtree, R. H., Mingos,D. M. P., Eds.; Elsevier: Oxford, 2006; Vol. 12, pp 195–294.
O
Me
Me
OR
Figure 1 Reinitzer’s cholesteryl liquid-crystalline esters. From Bruce,D. W.; Deschenaux, R.; Donnio, B.; Guillon, D. Metallomesogens. InComprehensive Organometallic Chemistry III; Crabtree, R. H.,Mingos, D. M. P., Eds.; Elsevier: Oxford, 2006; Vol. 12, pp 195–294.
However, used cleverly they can have profound and beneficial
effects, the best example of which is the inclusion of lateral fluoro
substituents, something described beautifully in review form.34
Some examples of ‘typical’ calamitic mesogens are given in
Figure 4. The cyanobiphenyls and derivatives are included in
this list due to their special position in the development of
LCDs, but it should be noted that in so many ways they are not
typical, their liquid crystal mesophases arising from antiparal-
lel molecular correlations.22
8.21.3.2 Mesophases of Calamitic Mesogens
The true liquid crystal mesophases of calamitic mesogens are
divided into two classes – the nematic and smectic phases. There
are, in addition, a series of crystal smectic phases that are not
really liquid crystal phases but which were for many years
classified as such; discussion of these can be found elsewhere.35
8.21.3.2.1 The nematic phaseThe nematic phase has the simplest structure of all of the
mesophases, is very fluid, and is also the least ordered
mesophase – it carries the abbreviation, N. The nematic
phase is characterized by 1D orientational order of the mole-
cules by virtue of correlations of the long molecular axes,
although note that the orientational order is not polar. There
is no translational order within the nematic phase (Figure 5).
8.21.3.2.2 The chiral nematic phaseThere exists a chiral version of the nematic phase that is found
when a nematic phase is shown by a pure enantiomer, or by a
mixture of enantiomers with one in excess, or by a racemic or
CnH2n+1(O) CN
CnH2n+1(O)(O)CnH2n+1
CnH2n+1 CN
CnH2n+1 CN
N
CnH2n+1(O)
(O)CnH2n+1
CnH2n+1(O)(O)CnH2n+1
CnH2n+1(O) (O)CnH2n+1
N N
O
O
N
N
Figure 4 Some typical calamitic mesogens. From Bruce, D. W.;Deschenaux, R.; Donnio, B.; Guillon, D. Metallomesogens. InComprehensive Organometallic Chemistry III; Crabtree, R. H.,Mingos, D. M. P., Eds.; Elsevier: Oxford, 2006; Vol. 12, pp 195–294.
840 Metallomesogens
nonchiral compound doped with a chiral material at, say,
5–10%. This was the phase seen in cholesteryl benzoate by
Reinitzer and so for many years (and still occasionally today)
was known as the cholesteric phase, although the term ‘chiral
nematic’ is much preferred; it takes the abbreviation N*.
A compound will possess either a nematic phase or a chiral
nematic phase, but only in the most exceptional circumstances
of pitch inversion will it show both. Due to the packing con-
straints imposed by the materials being chiral, the molecules
cannot simply align side by side in the phase and the long axis
of one molecule will be slightly offset with respect to that of its
neighbor. The net effect is for the director (n) to precess through
the phase (Figure 6), describing a helix that may be left- or
right-handed depending on both the sense of the chirality
and its position in the molecule36; two enantiomers of the
same material will describe opposite twist senses for the helix.
nn n
nn
Figure 6 Schematic diagram of the chiral nematic phase. From Bruce,D. W.; Deschenaux, R.; Donnio, B.; Guillon, D. Metallomesogens. InComprehensive Organometallic Chemistry III; Crabtree, R. H.,Mingos, D. M. P., Eds.; Elsevier: Oxford, 2006; Vol. 12, pp 195–294.
n
Figure 5 Schematic representation of the molecular arrangement in thenematic phase. From Bruce, D. W.; Deschenaux, R.; Donnio, B.; Guillon, D.Metallomesogens. In Comprehensive Organometallic Chemistry III;Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier: Oxford, 2006; Vol. 12,pp 195–294.
(a) (b)
n n
Figure 7 Schematic representation of the (a) SmA, (b) SmC, and (c) SmB pDeschenaux, R.; Donnio, B.; Guillon, D. Metallomesogens. In ComprehensiveElsevier: Oxford, 2006; Vol. 12, pp 195–294.
The pitch (p) of the helical twist is often of the order of the
wavelength of visible light and is very sensitive to temperature.
Coupled with the fact that the chiral nematic phase exhibits
selective Bragg reflection of light of the wavelength equal to np
(where n is the average refractive index of the material), N*
materials appear to change color with temperature, making
them useful as noninvasive thermometers. A more extensive
description of the properties and applications of N* materials
may be found elsewhere.37
8.21.3.2.3 The true smectic phasesThe true smectic phases35 are more highly ordered than the
nematic phase and are characterized by partial translational
ordering of themolecules into layers, in addition to orientational
correlations. The simplest smectic phase is the smectic A (SmA)
phase which is represented schematically in Figure 7(a).
As in the nematic phase, the long axes of the molecules
are oriented on average in the same direction but in addi-
tion, the molecules are loosely associated into layers, with
the orientational direction perpendicular to the layer nor-
mal; diffusion between the layers occurs readily and the
phase is fluid. In fact, this is a very idealized scheme and
the layers in this (and the following phases) are much less
well defined; a more precise description of the structure
of the smectic phases may be found elsewhere.35,38 If the
SmA phase is modified slightly by tilting the molecules
within the layer plane, then another smectic phase, the
smectic C (SmC) phase, is obtained which is similarly
fluid (Figure 7(b)).
Alternatively, the SmA phase may be modified by introduc-
ing hexagonal symmetry into the layer and increasing the order
slightly so that the molecules sit at sites that describe a hexag-
onal net; this is the smectic B (SmB) phase (Figure 7(c)). As
with all smectic phases, the SmB is fluid and interlayer diffu-
sion of the molecules is facile, although rotation about the
molecular long axes is concerted.
Two other smectic phases are obtained as tilted variations of
the SmB phase, although both are more fluid. Thus, the smec-
tic I (SmI) phase may be regarded as an SmB phase which is
tilted toward a ‘vertex’ of the hexagonal net, while the smectic F
(SmF) phase may be regarded as an SmB phase which is tilted
toward the ‘edge’ of the hexagonal net (Figure 8).
These five phases are true smectic phases, and in normal
phase sequences would be expected to be found in the order
shown in Scheme 1; the nematic and isotropic (I) phases are
included for completeness. Note that the particular phases
shown must be determined experimentally and cannot be pre-
dicted with any great certainty.
(c)
n
hases (viewed from above and from the side). From Bruce, D. W.;Organometallic Chemistry III; Crabtree, R. H., Mingos, D. M. P., Eds.;
(a) (b)
Figure 8 Schematic representation of the (a) SmI and (b) SmFmesophases showing the directions of tilt. From Bruce, D. W.;Deschenaux, R.; Donnio, B.; Guillon, D. Metallomesogens. InComprehensive Organometallic Chemistry III; Crabtree, R. H.,Mingos, D. M. P., Eds.; Elsevier: Oxford, 2006; Vol. 12, pp 195–294.
Increasing order (decreasing temperature)
I N SmA SmC SmI SmF SmB
Scheme 1 Normal thermodynamic ordering of liquid-crystalmesophases.
SmA N
I
I
N
Sequence 2
Sequence 1SmACr
Cr
Scheme 2 Phase sequences illustrating enantiotropic (sequence 1) andmonotropic (sequence 2) mesomorphism.
OOO
OO
O
OO
O
O
O CnH2n+1
CnH2n+1
CnH2n+1
CnH2n+1
CnH2n+1
CnH2n+1
O
Figure 9 Discotic hexaalkanoates of benzene. From Bruce, D. W.;Deschenaux, R.; Donnio, B.; Guillon, D. Metallomesogens. InComprehensive Organometallic Chemistry III; Crabtree, R. H.,Mingos, D. M. P., Eds.; Elsevier: Oxford, 2006; Vol. 12, pp 195–294.
Metallomesogens 841
In most cases, the observed mesophases are found on both
heating and cooling a material, so that sequence 1 in Scheme 2
is fully reversible; such mesophases are termed ‘enantiotropic.’
However, in some cases a particular mesophase may only
appear on cooling a material and is therefore metastable
(sequence 2, Scheme 2); such phases are termed ‘monotropic.’
In addition to the description above, the SmC, SmI, and the
SmF phases may exist as chiral modifications (SmC*, SmI*,
and SmF*) either by doping with a chiral additive or by resolv-
ing a racemic material that shows one or more of the phases.
Because of the low (C2) symmetry in these phases, the molec-
ular dipoles align within the layers that are then ferroelectric.
However, the chirality also requires that the direction of the
ferroelectricity precesses through space from one layer to the
next and so in the bulk sample, the ferroelectricity is lost unless
the helix is unwound by the use of surface anchoring and thin
cells. Note that while there are not chiral equivalents of the
SmA and SmB phases, formally the symmetry of the phases
reduces as the constituent molecules are chiral and so the
terminology SmA* and SmB* is used.
8.21.3.3 Discotic Mesogens
Discotic liquid crystals came to prominence in the late 1970s
when Chandrasekhar, Sadashiva, and Suresh reported the dis-
covery of this new class of liquid-crystalline molecules, which
were found to form columnar phases.39 The first of these, a
hexaalkanoate of benzene, is shown in Figure 9. There then
followed a rather unfortunate confusion of nomenclature in
which the phases formed by discotic molecules were them-
selves referred to as ‘discotic,’ carrying the abbreviation ‘D.’
A liquid-crystal phase must be characterized by its symmetry
and organization and not the shape of the molecules of which
it is composed and this is particularly important in columnar
systems as many nondiscotic molecules exhibit columnar
phases. Indeed, columnar mesophases have been recognized
for many years and studies date back to at least the 1960s with
the work of Skoulios with various metal soaps.40 Columnar
phases therefore take the abbreviation ‘Col’ followed by some
descriptor that describes the symmetry of the phase.
The important feature of discotic mesogens is that now, the
anisotropy is generated by the presence of a unique, ‘short’ axis
for, as their name suggests, the molecules are disk like. Within
reason, design of such mesogens is almost akin to choosing
your favorite disk-like molecule and grafting at least six periph-
eral alkyl chains onto it. Importantly, this disk-like core does
not need to be totally planar and, for example, hexaaza crown
ethers have been shown to form columnar mesophases. Some
examples of discotic mesogens are shown in Figure 10.
8.21.3.4 Mesophases of Disk-Like Mesogens
In contrast to smectic phases, with discotic molecules41 it is the
short axis that correlates and the simplest phase formed is a
nematic phase, which is usually abbreviated ND and referred to
as the discotic nematic phase, although for the reasons out-
lined above, this is somewhat unsatisfactory. In this nematic
phase, there is again only orientational order as illustrated
schematically in Figure 11(a). Materials showing this phase
are rather rare.
More common are the various columnar phases that are
characterized by the symmetry of the side-to-side molecular
arrangement of columns formed by the stacking of the disk-
like molecules. Figure 11(b) shows a side-on view of the
columnar hexagonal phase (Colh) in which the molecules are
arranged in columns which are further organized into a hexag-
onal array; within the columns there is some degree of liquid-
like order. The common lattices of the columnar phases,
namely hexagonal (a), oblique (b), and rectangular (c) and
(d), are represented in Figure 12 as ‘aerial’ views showing
projections of the columns onto a 2D plane; circles represent
disks which are orthogonal within the columns, whereas
ellipses represent disks which are tilted. Examples of so-called
columnar nematic phases have been described by Praefcke42
and by Ringsdorf43; these are nematic arrays of columnar stacks.
(a) (b)
n
Figure 11 Schematic structure of the (a) ND phase and (b) Colh phase. From Bruce, D. W.; Deschenaux, R.; Donnio, B.; Guillon, D. Metallomesogens.In Comprehensive Organometallic Chemistry III; Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier: Oxford, 2006; Vol. 12, pp 195–294.
CnH2n+1OCnH2n+1O
N
N
N N
N N
N
O
O
M
N
CnH2n+1O
CnH2n+1O
CnH2n+1O
CnH2n+1O
CnH2n+1O
CnH2n+1O
CnH2n+1O
CnH2n+1O
CnH2n+1O
CnH2n+1O
CnH2n+1O
OCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1OCnH2n+1
OCnH2n+1OCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1
Figure 10 Representative examples of discotic materials. From Bruce, D. W.; Deschenaux, R.; Donnio, B.; Guillon, D. Metallomesogens. InComprehensive Organometallic Chemistry III; Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier: Oxford, 2006; Vol. 12, pp 195–294.
842 Metallomesogens
8.21.3.5 Polycatenar Liquid Crystals
Detailed descriptions of these materials may be found
elsewhere,44,45 but a brief account is appropriate here as such
materials do appear in the following pages. Polycatenar meso-
gens possess an extended calamitic core, and the total number
of terminal chains is typically between three and six, normally
with at least one chain at each end. For even numbers, these
are normally arranged symmetrically, but this is not necessary;
the number of chains is reflected in the nomenclature (tetra-
catenar¼ four chains, etc. – Figure 13). Tricatenar mesogens
typically show N and SmC phases, while penta- and
hexa-catenar mesogens show columnar phases and so the
mesomorphism changes from that of rods to that of disks
with chain length. The way in which mesogens may be arranged
in the columnar phases is discussed in detail in Ref. 46.
Of the various polycatenar systems, tetracatenar mesogens
where the terminal chains are in the 3,4-positions of the
terminal benzene ring attract the greatest interest; for short
chain lengths, N and SmC phases tend to be found while
when the chain becomes longer, columnar phases are
observed. Such behavior is well discussed elsewhere.47,48
8.21.4 Lyotropic Liquid Crystals
If a surfactant molecule, such as cetyltrimethylammonium bro-
mide (CTAB), is dissolved in water, at some specific concentra-
tion (known as the critical micelle concentration – cmc) the
molecules will organize and form micelles.49 This formation of
micelles (Figure 14) is caused by the hydrophobic effect,50
which is driven entropically. Thus, in solutions of surfactant
monomers, the hydrophobic chains are not molten and are
surrounded by an organized layer of water; when the micelle
forms, the water layer returns to the bulk and the chains
(a) (b)
(c) (d)
Figure 13 Schematic representation of polycatenar mesogens showing different substitution patterns of flexible chains on the terminal rings. FromBruce, D. W.; Deschenaux, R.; Donnio, B.; Guillon, D. Metallomesogens. In Comprehensive Organometallic Chemistry III; Crabtree, R. H.,Mingos, D. M. P., Eds.; Elsevier: Oxford, 2006; Vol. 12, pp 195–294.
Colh (p6mm) Colo (p1)
ah
ah = bh
bh
ao
bo γ
(a) (b)
Colr (c2mm) Colr (p2gg)
ar
br
ar
br
(c) (d)
Figure 12 Representations of the lattices of the (a) hexagonal, (b) oblique, and (c, d) rectangular columnar phases. From Bruce, D. W.;Deschenaux, R.; Donnio, B.; Guillon, D. Metallomesogens. In Comprehensive Organometallic Chemistry III; Crabtree, R. H., Mingos, D. M. P., Eds.;Elsevier: Oxford, 2006; Vol. 12, pp 195–294.
Metallomesogens 843
become molten, greatly increasing the disorder in the system.
Micelles may be formed by cationic (e.g., CTAB), anionic
(e.g., C12H25OSO3Na), or nonionic (e.g., C12H25(OCH2-
CH2)6OH) surfactants, and solvents are not limited to water.
If more surfactant is added above the cmc, the concentra-
tion of micelles increases (rather than the concentration of free
surfactant) until the micelle concentration becomes so high
that they themselves organize to form ordered arrays of lyo-
tropic liquid-crystal phase. There are several well-characterized
lyotropic liquid crystal phases and a host of so-called
intermediate phases whose characterization is not unequivo-
cal. While cmc values are found typically in the range
10�5–10�3 mol dm�3, formation of lyotropic mesophases typ-
ically kicks in around 20 wt% of the surfactant in water.
As it is the aggregation of micelles that leads to the form-
ation of lyotropic mesophases, it is held to be true that the
first mesophase formed is based on the micelles from which
it is derived.51 For example, spherical micelles give rise to
micellar I1 cubic phases that may be regarded as cubic arrays
of spherical micelles. (Note the subscript ‘1’ refers to a so-called
cmc
a = l2b = V2c = V1d = l1Te
mp
erat
ure
Type I:‘Oil-in-water’
Concentration of surfactant
Type II:‘Water-in-oil’
L2 a H2 b H1 L1dLα c
Figure 15 Idealized, theoretical phase diagram for a binary surfactant/water system (L1 is a solution of micelles, L2 is a solution of reversed micelles).From Bruce, D. W.; Deschenaux, R.; Donnio, B.; Guillon, D. Metallomesogens. In Comprehensive Organometallic Chemistry III; Crabtree, R. H., Mingos,D. M. P., Eds.; Elsevier: Oxford, 2006; Vol. 12, pp 195–294.
(a) (b) (c)
Figure 14 Schematic representation of: (a) a ‘normal’ spherical micelle, (b) a ‘normal’ cylindrical micelle, and (c) part of a plate-like micelle. FromBruce, D. W.; Deschenaux, R.; Donnio, B.; Guillon, D. Metallomesogens. In Comprehensive Organometallic Chemistry III; Crabtree, R. H.,Mingos, D. M. P., Eds.; Elsevier: Oxford, 2006; Vol. 12, pp 195–294.
844 Metallomesogens
normal or oil-in-water phase, that is, it is composed of micelles
that have the hydrophobic chains on the interior and the
polar headgroups on the surface). There are, in addition, type
‘2’ phases which are inverse or water-in-oil phases where
this arrangement is reversed; such phases tend to happen
at rather high surfactant concentrations. Rod micelles give
rise to hexagonal mesophases (termed H1), which consist
of a hexagonal array of the rods, while disk micelles give rise
to a lamellar phase (La), which is a solvent-separated bilayer
phase.
A theoretical phase diagram for a lyotropic system is
shown in Figure 15 and reveals, in addition, the V1 and V2
phases which are bicontinuous cubic phases (normal and
reversed, respectively)52 whose structure can be described by
models involving interpenetrating rods53 or periodic minimal
surfaces.54–56 Note that also each pair of phases is separated, at
least in principle, by a cubic phase (a, b, c, d in Figure 15), and
with a biphasic interface (two phases coexisting).
In addition to the lyotropic mesophases formed by surfac-
tant amphiphiles, two other types are generally recognized,
neither of which exhibits a cmc. The first of these are lyotropic
phases of rigid-rod polymers that can formmesophases in both
aqueous and nonaqueous solvents,57 with these mesophases
being of the nematic or hexagonal type. Examples include
polymeric metal acetylide complexes (vide infra) and DNA.58
The other type is formed from, usually, flat and largely aro-
matic molecules which can stack to give lyotropic columnar
phases, also referred to as chromonic phases.59,60 This latter
class is formed from systems with ionic or strongly hydrophilic
peripheral functions and forms mesophases in water, or by
much more thermotropic-like systems which are surrounded
by apolar alkyl chains and which form mesophases in apolar
solvents such as alkanes.61 Here, the stacks of molecules con-
stitute the mesogenic unit which is known to organize into
either a nematic phase where the stacks are well-separated by
water, or a hexagonal phase.
8.21.5 Physical Properties and MesophaseCharacterization
8.21.5.1 Physical Properties
Liquid-crystal phases are anisotropic,62,63 which means that
their physical properties are likewise anisotropic and it is this
feature which is the basis for the widespread application of the
materials and also, in many cases, of their characterization.
Consider, for example, the refractive indices of a nematic
phase (Figure 16).
In the figure, it is assumed that the molecules have a greater
polarizability along their long axis than along either of their
two short axes, so that in case A, the electric vector of the light is
coincident with the direction of greatest polarizability and so
the light is retarded. However, in case B, the electric vector of
the light is coincident with the direction of least polarizability
T TNI
n^Ref
ract
ive
ind
ex
nII
Figure 17 Variation of physical parameters in the nematic phaseof liquid crystals. From Bruce, D. W.; Deschenaux, R.; Donnio, B.;Guillon, D. Metallomesogens. In Comprehensive OrganometallicChemistry III; Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier: Oxford,2006; Vol. 12, pp 195–294.
(a) (b)
Figure 16 The interaction of polarized light with a nematic phase. From Bruce, D. W.; Deschenaux, R.; Donnio, B.; Guillon, D. Metallomesogens. InComprehensive Organometallic Chemistry III; Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier: Oxford, 2006; Vol. 12, pp 195–294.
Metallomesogens 845
and so the light is retarded little. The consequence is that the
material has two refractive indices, nk (case A) and n? (case B),
and the difference between these (Dn¼nk�n?) is termed the
birefringence. (Note that as the nematic phase has D1h sym-
metry, the two short axes are effectively equivalent and so the
two possible n? reduce to only one.) Birefringence can either
be positive (Dn>0) or negative (Dn<0).
This anisotropy as illustrated by refractive index extends to
other properties, and common properties of interest would be
the anisotropy in linear polarizability (Da), dielectric permit-
tivity (De), and diamagnetism (Dw). In the nematic phase, these
properties are quite strongly temperature dependent as the
order parameter, S, increases as samples cool from the N–I
transition. This is illustrated in Figure 17 where it is also seen
that the parallel component has the stronger temperature
dependence as it is the orientational correlations that increase
most on cooling. Note that at the transition to the isotropic
phase, there is a discontinuity as there is only a single compo-
nent of each parameter in the isotropic state. Such properties
are determined primarily by molecular features and as such can
be tuned at the molecular level.
8.21.5.2 Mesophase Characterization
The mesophases presented by a liquid-crystalline material must
be determined experimentally and the first technique to be used
is always polarized optical microscopy, which relies on the aniso-
tropic nature of the refractive index in the mesophase. Thus,
plane-polarized light impinges on the sample and the anisotropic
nature of the phases causes the light to become elliptically
polarized – that is, two refracted rays result. These two rays
interfere and the pattern formed is characteristic of the
mesophase. The technique is relatively straightforward to use
and, in the hands (and with the eyes) of a skilled operator, can
be an extremely powerful experiment. It also requires submilli-
gram quantities of material and so does not consume large
amounts of hard-won products. Needless to say, there are many
variables and lots of tricks and guidelines tomake themost of the
effect, and interested readers are directed to further readings.38,64
Complementingmicroscopy is differential scanning calorim-
etry (DSC), which is used to measure the enthalpy (or occasion-
ally heat capacity) change associated with a transition. At
transitions, melting points are strongly first-order thermody-
namically (discontinuity in @G/@T at the transition), whereas
transitions between mesophases or between a mesophase and
the isotropic liquid are generally weakly first order (typically a
few kJ mol�1). The enthalpy change, DH, can be evaluated by
the DSC instrument and, when divided by the temperature,
gives the entropy of transition, DS – a measure of the change
in order, as it is assumed that at the transition, the system is at
equilibrium (DG¼0). Some transitions (mainly SmC–SmA) are
second-order thermodynamically (discontinuity in @2G/@T2 at
the transition) and so DH (and therefore, DS) are zero and the
system experiences a change in heat capacity, DCp.
DSC is best regarded as strictly complementary to optical
microscopy, as all changes in optical texture do not necessarily
correspond to a change in mesophase type and vice versa.
Small-angle x-ray diffraction provides, in general, a defini-
tive way to determine the structure of a liquid-crystalline
phase. Based on the Bragg law, 2dsin y¼nl, x-ray experiments
provide information not only about the inter-planar distance,
but also about the relative orientation and spatial orientational
order of different sets of planes. Typically, layer thickness in
smectic phases or intercolumnar distances in columnar meso-
phases are determined, along with the molecular organization
within each layer or column, respectively.
X-Ray diffraction experiments on liquid crystals provide
much less information than single-crystal experiments owing
to the disordered nature of the phases, and as few as two
reflections may be observed. Thus, in a smectic phase (and
bearing in mind that the experimental data present in recipro-
cal space), there will be a wide-angle reflection corresponding
to the side-to-side interactions of the molten alkyl chains
(2y�20� for CuKa radiation) and a small-angle reflection
corresponding to the apparent layer spacing. Where the sample
is not aligned (e.g., using an external magnetic field), then 1D
data may be recorded, but 2D detectors are required to acquire
the extra data (e.g., relating to tilt angle in an SmC phase)
available when sample alignment is achieved.
846 Metallomesogens
8.21.6 General Overview of Metallomesogens Types
A wide overview of different metallomesogens is presented in
this section. Starting with metal carboxylate mesogens and
then continuing with macrocyclic complexes, a more-or-less
miscellaneous collection of complexes of mono- and bi-
dentate ligands and a brief scope of liquid-crystalline metallo-
dendrimers. The aim of this section is to illustrate the varied
structures and synthetical approaches developed in the field of
metallomesogens. The major exception in coverage relates to
all-inorganic liquid crystals and interested readers are referred
to literature reviews of this subject.65,66
8.21.6.1 Metal Carboxylate Mesogens
A large number of mesomorphic mono- and di-nuclear carbox-
ylates, with the general formula [M(O2CCnH2nþ1)x] (x¼1–3)
and [M2(O2CCnH2nþ1)4], respectively, have been reported since
the pioneering work of Heinz26 in 1855 and Vorlander67 in
1910. Magnesium myristate [Mg(O2CC13H27)2] (1) described
by Heinz is still used nowadays in the cosmetic industry because
of the applications it presents due to its mesomorphic nature. In
general, these materials can show a rich thermotropic behavior
(N, Sm, Cub, and Col), but they may also form lyotropic meso-
phases when dissolved in water or alkanes.
O
O−
O−
O
Mg2+
1
8.21.6.1.1 Carboxylates of monovalent metalsThermotropic smectic and columnar phases have been
observed in alkali metal alkanoates [M(O2CCnH2nþ1)] (n�7)
with lithium,68,69 sodium,70–72 potassium,73–75 rubidium,76,77
and cesium.78,79 Curiously, some other short-chained deriva-
tives such as sodium butanoate and isovalerate,80,81 and as
binary mixtures of the acetates or propionates or butyrates of
lithium and cesium,82 showed liquid-crystalline behavior. In
these cases, it is suggested that the existence of the mesophase
is not due to the anisotropy of the molecular shape, but to the
anisotropy in the distribution of the Coulombic charges.
The lyotropic behavior of concentrated aqueous solutions of
alkali metal alkanoates was also studied widely.40,83,84 Lamellar,
columnar, and even cubic phases were found between amicellar
solution at high temperature and a gel (or coagel) phase at low
temperature. Mesophases were also observed in various sodium
alkanoate–alkane binary mixtures.85
In 2005, Binnemans and coworkers reinvestigated86 the
interesting behavior of some alkali-metal salts of ortho, meta-,
and para-substituted benzoic acids (2: M(O2CC6H4X); M¼Li,
Na, K, Ru; X¼H, F, Cl, Br, I, CH3, OCH3), originally studied by
Vorlander, 85 years before,67 and then a few times since then
by different authors.87–89 Only the materials with the substit-
uent in meta-position were able to form a high-temperature
SmA mesophase (above �250 �C), even if they did not have
the rod-like or disk-like shape of conventional liquid crystals,
while the other isomers (ortho and para-) did not show meso-
morphism. The most stable mesophases were observed for the
sodium salts, up to 450 �C.
M+ −O O
X
2
Thallium(I) alkanoates (Tl(O2CR)) with linear90,91 and
branched92 chains were also studied, and they showed a lamel-
lar mesophase with a head-to-head arrangement of the mole-
cules within the layers. Lamellar lyotropic phases were also
induced in binary systems with the corresponding free acids
(HO2CCnH2nþ1).93–95
8.21.6.1.2 Carboxylates of divalent metalsSeveral reports of alkali-earth and cadmium(II) carboxylates
(3: M(O2CCnH2nþ1)2; M¼Mg,96 Ca,97,98 Sr,99 Ba,100 Cd101)
demonstrated that these materials can exhibit mesomorphism
with lamellar or columnar hexagonal phases.102,103 An addi-
tional body-centered cubic phase was found in some strontium
and barium soaps.53 In contrast, zinc and mercury alkanoates
are not mesomorphic, probably due to insufficient cohesion
between the polar groups to stabilize a mesophase.104
O
OM
O
OR R
3
Different authors have studied lead(II) carboxylates (3:
M¼Pb).105–111 Heating from the initial lamellar crystalline
phase results in a transition to another highly ordered
lamellar phase, sometimes described as ‘condis’ (conforma-
tionally disordered) crystal.112 Further heating induces a
direct transition to the isotropic liquid for the derivatives
with n<8, while for the species with 8�n�12 an SmA
phase is observed before clearing. This mesophase has
been identified in other reports as an SmC phase, but this
seems improbable.113,114
In common with monovalent metal carboxylates, divalent
derivatives (3: M¼Zn, Mn, Pb, Hg, Cd)115,116 can form lyo-
tropic systems in long-chain alcohols, although the lamellar
phase induced appears over small ranges of temperatures and
concentration. Binary mixtures of PbII alkanoates (3: M¼Pb)
with their parent acids were also investigated,117 demonstrat-
ing the induction of a cubic mesophase at high molar ratio of
surfactant (x>0.33).
8.21.6.1.3 Tetra(carboxylato)dimetal mesogensSeveral thermal and characterization studies of the well-known
dimeric copper(II) complexes with four carboxylate groups in a
‘lantern’ or ‘paddle-wheel’ conformation (4: M¼Cu, x¼0,
R¼CnH2nþ1) demonstrated their behavior as metallomeso-
gens.118–122 All the derivatives (n¼2–22) exhibited a Colhmesophase between �120 and 200 �C, with decomposition
Metallomesogens 847
observed at the higher temperatures. Different modifications
were studied, such as inserting carbon–carbon double bonds
in the chains and using branched chains to reduce the melting
points,123,124 or introducing various groups (phenyl, esters,
heterocycles, etc.) into the terminal chains in order to avoid
thermal degradation.125 Lyotropic studies were also carried
out with these materials, and a nematic mesophase was
induced using various hydrocarbon solvents.126,127 In addi-
tion, a polymeric material showing lamello-columnar order
was obtained from the polymerization of a tetracarboxylate–
dicopper complex with diacetylenic moieties in the terminal
chains,128 while some other metallomesogenic polymers
have also been synthesized via addition reactions with
polysiloxanes.129,130z
R
O
O
M
O
O
O
O
O
R
R
R
M
O
None
M
Cu(II)
Rh(II)
Ru(II)
Ru(II)−Ru(III)
x
x+0
0
0
1
0Cr, Mo, W(II)
Y−
Y− = Cl, O2CR, C12H25OSO3
4
x
A significant number of related mesomorphic dimeric com-
plexes containing a single or a multiple metal–metal bond (4)
have also been described with various chain lengths from n¼3
to n¼17.131–144 Of these complexes, only the WII derivatives,
the MoII complexes with n�10, and the RuII/RuIII complexes
with Y¼Cl were found to be nonmesomorphic. Thus, a Colhmesophase was observed in all the remaining cases, but decom-
position appeared frequently before reaching the clearing point.
The mesophase was seen above 85–115 �C for the neutral deriv-
atives, whereas for the ionic RuII/RuIII complexes, the melting
point was slightly higher (120–150 �C) and with a small depen-
dence on the counter-anion used (Y¼O2CCnH2nþ1, n¼5–15;
O3SOC12H25).
Branched, unsaturated, and fluorinated chains (R) were
additionally used, affording similar results (4: M¼Cu, Ru,
Mo, Cr).137,145 Again, only a Colh phase was observed and
decomposition still occurred before complete clearing, but in
this case the mesophase appeared at very different tempera-
tures depending on the nature of the terminal chains (33–
165 �C).In a similar way, very interesting results were achieved by
introducing substituted benzoates in these ‘lantern’ structures
(5: M¼RhII, CuII, RuIII/RuII; R1, R2, R3¼H, OCnH2nþ1).146–149
In this case, different types of mesophases were observed over
very varied transition temperatures. The neutral (x¼0) dirho-
dium(II) complexes (n¼10) showed columnar mesophases
over very different temperature ranges, as a function of the
total number of chains attached to the rigid central core:
four (Cr156Colr203I), eight (Cr45Colh90I), or 12 chains
(Cr55Colh189I). However, the 12-chained dicopper(II)
derivatives exhibited a Colh phase (from 80 to 130 �C) whenn¼8, or a cubic phase (from 85 to 113 �C) when n¼12. For its
part, the cationic mixed-valence diruthenium(II/III) complex
with Cl� as counter-anion (x¼1) presented an unidentified
lamellar phase (�50–157 �C) and a Colh phase (157 �C–dec)
for the eight-chained derivative with R1¼R2¼OC12H25 and
R3¼H, while only a Colh phase was observed for more sym-
metric derivatives, for example, R1¼H, R2¼R3¼OC12H25
(room temperature–302 �C), and R1¼R2¼R3¼OC12H25
(room temperature–220 �C(dec)).
x+
O
O M
M
O
O
O
OO
O
R1
R2
R3
R2
R1
R3
R2
R3
R3
R2
R1
R1
5
Additionally, several interesting characterization studies
were carried out using CuII,150 and, particularly, mixed-valence
RuII/RuIII dinuclear complexes,151,152 because of their unique
magnetic and electronic properties (vide infra).
8.21.6.2 Macrocyclic Metallomesogens
An enormous number of metal-containing mesogens based on
macrocyclic groups such as phthalocyanines or porphyrins
have been reported over the last decades. These disk-shaped
molecules are constituted of extended p-electron systems,
which induce not only stability, but also valuable electronic
and optical properties in the liquid-crystalline material.153–159
While calamitic mesogens tend to self-organize into nematic
and smectic phases, these aromatic, disk-like systems usually
show columnar phases through the combination of the stacking
of the rigid, planar cores and the fluidity of the molten aliphatic
chains. A wide variety of divalent metals and oxometallates have
been used to complex the organic macrocyclic frameworks. Due
to the vast number of complexes of this kind described and the
existence of a previous and thorough review concerning this
subject,20 the aim in this section is only to exemplify the rich
variety of structures reported up to now. Lanthanide-containing
derivatives will be discussed in a later section.
8.21.6.2.1 Metallophthalocyanine mesogensThe phthalocyanines were the first macrocyclic system to be
investigated for their ability to form metallomesogens.160 In
addition, it is probably the most studied type of discotic struc-
ture, probably due to the wide variety of metals that can be
used to render efficiently very stable columnar mesogens. Two
main classes of phthalocyanines (Figure 18) can be distin-
guished depending on the position of the side chains relative
to the rigid core: peripherally (6) or radially (7) substituted
phthalocyanines. Moreover, they can be either uniformly
(R¼R0) or nonuniformly substituted (R 6¼R0).Since Simon et al. reported the first copper(II) complex
bearing a phthalocyanine with eight peripheral dodecyloxy-
methylene chains (6: M¼2H, Cu; R¼CH2OC12H25) in
1982,160 more than a hundred papers have been published
with the aim to study these peripherally substituted systems.
In addition to copper,161–189 various divalent metals or
N N
N
NN N
N N
R R
R
R
R�R�
R
R
MN N
N
NN N
N N
M
R R�
R/R�
R/R�
R/R�R�/R
R�/R
R�/R
6 7
Figure 18 Peripherally (6) and radially (7) substituted metallophthalocyanines.
848 Metallomesogens
oxometallates have also been complexed to evaluate their influ-
ence in the mesomorphic properties. These include Ni, Zn,
Co, Mn, Pb, Sn, Pd, Pt, VO,190–201 TiO,202,203 Si(OH)2,204–208
and Ge(OH)2.209 Moreover, several types of chains have
been used (Figure 1), and in the case of the uniformly, periph-
erally substituted phthalocyanines (6: R¼R0), oxo- and
thioether links,167–175,178,193–201,204–208,210,211 as well as ester
groups,177,209,212 provided generally more ordered mesophases
than the alkyl,165,166,202,203,213 alkoxy-methylene,160–164,190 and
poly(ethyleneoxide)182–187 chains. The transition temperatures
(melting and clearing points) increased in the order ethyle-
neoxide< thioalkyl<alkoxymethyl<alkoxyalkyl. Moreover,
tetra-substituted derivatives (R0 ¼H) showed generally lower
transition temperatures than the related octasubstituted species.
However, the variation of the metal center did not seem to
influence the mesomorphism much, although some exceptions
could be found. A Colh phase was generally observed, and
some examples showed a Colr phase instead. Remarkably, by
introducing branched side chains (6: M¼Cu, Pt, Pb;
R¼R0 ¼OCH2CH(Et)(Bu)),168,198,199 a columnar tetragonal
(Colt) and a nematic discotic (ND) phases were induced in a
copper(II) complex, while a columnar oblique (Colo) or a Coltphase was seen for the related platinum(II) and lead(II) com-
plexes, respectively.
Substitution of phthalocyanines can also take place in the
radial positions of the rigid core (7, Figure 1). This way,
a large number of mesomorphic complexes with divalent
metals have been prepared using radially octasubstituted
phthalocyanines (7: M¼Cu,214–219 Ni,218–220 Zn,216,217,220
Co221–223; R¼R0 ¼R00). Alkyl,214,215,220–223 alkoxyl,218,219 and
alkoxymethylene216,217 chains were used and interestingly, the
complexes exhibited a much richer mesomorphism than the
peripherally substituted isomers described above (6). In gen-
eral, one or two mesophases could be observed, the higher-
temperature phase being assigned always as Colh. At lower
temperatures, a second Colh phase or a Colr phase could be
detected in some materials. Some nonuniformly substituted
materials were also synthesized, but only the derivatives
with alkyl chains were found to be mesomorphic showing
Colr and Colh phases (7: M¼Cu, Ni; R¼CnH2nþ1, R0 ¼Me,
–CmH2mOH; R00 ¼–CmH2mOH)224–226 In general, reducing
the symmetry of the systems led to a destabilization of the
mesophase (monotropic).
The lyotropic behavior of phthalocyanine complexes was
also studied systematically. For example, lyotropic NCol and
Colh mesophases were found for copper and nickel octa-
dodecyloxy complexes (6: M¼Cu, Ni; R¼R0 ¼OC12H25) in
binary mixtures with organic solvents.227 The same phases
were also observed in alkane mixtures of copper and zinc com-
plexes with partial replacement of the chains (from 0 to 7)
by chlorine atoms (6: M¼Cu, Zn; R, R0 ¼Cl, SC8H17),169 and
in other peripherally substituted complexes as well.177,228
Some radially substituted complexes (7: M¼Cu, Ni;
R¼R0 ¼R00 ¼OCnH2nþ1) were able to form a lamello-columnar
mesophase in a contact preparation with the electron-acceptor
trinitrofluorenone (TNF), even if they did not show thermo-
tropic mesomorphism on their own.218,219
Very recent work by Ahmidar et al.229 studied the effect of
the introduction of four Cl, Br, I atoms or 12 F atoms in the
rigid core of a tetracatenar phthalocyanine copper metallome-
sogen (8: X¼H, F, Cl, Br, I; Y¼F, H; n¼5–8), finding colum-
nar mesomorphism for all materials. The phthalocyanines
containing halide atoms did not crystallize as the halide-free
derivative did, rather forming glassy hexagonal columnar
phases. Another remarkable variation carried out is the attach-
ment of crown ether moieties to the core of a phthalocyanine
copper complex (9), to yield a material that exhibited a colum-
nar rectangular phase.230
Some related materials have been prepared based on
expanded phthalocyanine cores. Steric hindrance around the
core could be introduced by replacing the side chains with
mono- or di-alkoxylphenyl rings. Moreover, these two aro-
matic rings were either free (no bond where the dashed line
is) or linked together by a covalent bond (solid line) (10). The
former, less restricted materials with eight chains attached (10:
M¼Cu; R¼OCnH2nþ1, n¼10, 12, 18; R0 ¼H)231 showed high-
temperature columnar mesophases (Colh, Colr), whereas with
16 chains, the derivatives were mesomorphic from room tem-
perature and exhibited one or two Colr phases (10: M¼Cu, Ni;
R¼R0 ¼OCnH2nþ1, n¼10–12).232 Decomposition was
observed above 200 �C in both cases. On the other hand,
zinc derivatives with a more restricted conformation due to
N
N
N
N
N
N
N
NM
R
R
R
RR
R
R
RR�
R�
R�
R�R�
R
R�
R�
10
N
N
N
N
N
N
N
NCu
(1 of 4 regioisomers)
X
SCnH2n+1
SCnH2n+1
H2n+1CnS
H2n+1CnSY
Y
Y
XYY
X
Y
Y
XY
N
N
N
N
N
N
N
NCu
O
OO
OO
O
OO
O O
O
OO
OO
O
OO
O O
8 9
N
N
N
N
N
N
N
NM
R
RR
R
R
RR
R
a
c
b
11
CnH2n
Figure 19 Extended phthalocyanine metallomesogens with polysubstituted
Metallomesogens 849
the linked rings (10: M¼Zn, solid line)233 exhibited a colum-
nar mesophase to temperatures above 300 �C and starting
from 180 �C (R¼OC6H13; R0 ¼H) or 270 �C (R¼H;
R0 ¼OC6H13) for the octacatenar species, or from room tem-
perature in the case of the derivatives with 16 hexyloxy chains
(R¼R0 ¼OC6H13).
In a similar way, some other extended phthalocyaninemetal-
lomesogens have been described bearing polysubstituted phe-
nyl rings, yielding species with 16 (11a), 24 (11b), and 40 (11c)
alkoxy side chains (Figure 19).234,235 These complexes showed,
in general, low-temperature columnar phases (Colh, Colr, Colt).
Surprisingly, the copper complexes (11a: n¼9–14) showed an
additional high-temperature cubic phase (Cub), which is highly
unusual in purely discotic materials.
In addition to some lanthanide derivatives (vide supra), a
few examples of polynuclear phthalocyanine complexes have
been reported so far. A dodecasubstituted bis(phthalocyanine)
dicopper complex (12: R¼CH2CH(Et)(Bu))236,237 showed
R
O
H2n+1CnO OCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1
M
Cu
Zn
Zn
OCnH2n+1
OCnH2n+1
OCnH2n+1OCnH2n+1
+1O
phenyl rings.
850 Metallomesogens
wide-range columnar (Colr, Colt) mesophases, from room
temperature and up to 300 �C. Oligomeric copper complexes
based on radially substituted phthalocyanines (13: M0 ¼2H,
Cu; x¼0, 1: R¼C8H17, C7H15)238–240 presented a similar
behavior, with Colh and Colr mesophases over broad temper-
ature ranges (r.t. to 222–252 �C). Some other nickel dimers
and trimers were reported by Cook and Heeney,241 but while
they showed mesomorphism, the phases were not identified
because of the rapid decomposition of the material.
Several chemical modifications have been made on the
mesogenic core of phthalocyanine-like molecules (Figure 20).
For instance, incorporation of heteroatomic rings was achieved
by replacing one of the benzenoid rings with a fused thiophene
(14)242 or a fused pyridine ring (15: Ni, Zn).243 This
way, new nickel and zinc complexes with columnar meso-
morphism were prepared. The first examples of mesomorphic
naphthalocyanines were described containing a zinc center
(16: R/R0 ¼SC12H25/H, H/CH2OC6H13, H/CH2O(CH2CH2O)
(Bu)).244 Copper, nickel, and cobalt complexes of tetrapyrazino-
porphyrazines (17: M¼Cu, Ni, Co; R¼C12H25, CH2CH(Et)
(Bu))199,245 were also found to be mesomorphic, showing a
Colr phase. Some other extended variations on these last motifs
were also investigated.246–252
N N
N
NN N
N N
Cu
RO OR
ORRO
RO
RO
N N
N
NN N
N N
Cu
RO OR
OR
OR
ORRO
12
N
N
NN
N
R
R
R
M�
N N
N
NN N
N N
C8H17 C8H17
C8H17
C8H17
Me
H17C8
H17C8
Cu
O(CH2)6
O
OO(CH2)n
13
8.21.6.2.2 Metalloporphyrin mesogensTwo different types of porphyrins can be distinguished
depending on the location of the substituents around the
core: at the b-positions of the pyrrole rings (18) or at the
meso positions (19).
Soon after Goodby et al. described in 1980 the first liquid-
crystalline porphyrin (20a),253 Gaspard et al. were the first to
report the lyotropic behavior of a metal-containing porphyrin
system in 1985, observed in mixtures of one-chained meso-
substituted copper(II) species with haloalkanes (20b: n¼12,
22).254 Later in 1987, Gregg et al. discovered the first com-
plexes of this kind showing thermotropic liquid crystallinity
(18: M¼Zn, R¼CO2CnH2nþ1, n¼4, 6, 8).255 Since then, sev-
eral more b-octasubstituted porphyrins have been described
with different metals (18: M¼Zn, Cd, Cu, Ni, Pt)256–259 and
different chain types such as ester (18: R¼O2CCnH2nþ1),256
alkoxymethyl (18: R¼CH2OCnH2nþ1),257,258 or alkyl (18:
R¼C10H21).259 Generally, columnar phases were observed,
and it was demonstrated that the incorporation of the metal
stabilized, or even induced, the mesomorphism in the final
materials compared to the parent free-metal porphyrins.
To induce liquid-crystalline properties to the meso-
tetrasubstituted porphyrins, the extension of the small rigid
core with aryl groups was necessary (19), because simple alkyl
chains directly attached to the meso positions were found not to
be sufficient.259 This way, several mesomorphic complexes
were reported bearing a porphyrin unit functionalized
with four alkoxy (19: M¼Zn,260,261 Cu,262,263 Co,264 Ni,265
Mn(tetracyanoethylene),266,267 Al(OH),268 Fe(OH),269
R¼OCnH2nþ1, R0 ¼H) or alkyl chains (19: M¼Co, Ni,270
Cu, Zn, Pd,271 Al(OH),272 MoOCl,273 MnCl, Mn
(tetracyanoquinodimethane),274 VO,275,276 Si(OH)2, Si
(OMe)2,277 R¼CnH2nþ1, R0 ¼H), and also eight alkoxy
(19: M¼Cu,278 Zn,279 R¼R0 ¼OCnH2nþ1) or alkyl chains
(19: M¼Cu, Ni, R¼R0 ¼CnH2nþ1, n¼8, 12, 18).280 The meso-
morphism observed for these meso-substituted species was
different from the behavior found for the b-substituted porphy-
rins. Thus, most of the mesophases were found to be discotic
N
N
N
R
R
R
NN
N
N NN
NN
C8H17
H17C8
H17C8
Me
C8H17
C8H17
Cu
H17C8
x(CH2)6O
O
OO(CH2)n
N
N N
N
N
N N
N
C6H13
H13C6
C6H13
Ni
S
N
N N
N
N
N N
N
C8H17
M
N
NZn
N
NN
N
N
N
N
R�R
R
R�
R�
R
RR� R�
R
R
R�
R�
R
RR�
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
NM
R
R
R
RR
R
R
R
H13C6
C8H17
14 15
16 17
H13C6
H13C6 H17C8
H17C8 H17C8
H17C8
Figure 20 Examples of phthalocyanine-like metallomesogens.
N M
N
N
N
R R
R
R
RR
R
RN
M NN
N
R,R�
R,R�R�,R
R�,R
18 19
N
N N
N
CO2C12H25
CO2C12H2
CO2C12H25
CO2C12H25
H25C12O2C
H25C12O2C
H25C12O2C
H25C12O2C
H
H
20a
Metallomesogens 851
lamellar (DL), showing, in some cases, two different types of
them in a rather complicated series of phase transitions. It is
worth mentioning that the mesogenic character of the octacate-
nar species was diminished compared to the analogous tetra-
catenar derivatives. Additionally, some of these materials were
found to present interesting photophysical properties such as
fast charge transport under irradiation281–284 and photoelectro-
chemical behavior.285–290
A few examples of extended core species of both b- andmeso-substituted porphyrin complexes were also synthesized
(Figure 21). A room-temperature, enantiotropic Colh phase
was found for a copper complexes with 16 dodecyloxy side
chains (21).291 Some zinc derivatives with four arylalkynyl
substituents showed an unidentified mesophase, followed by
5 NCu
NN
N
OCnH2n+1
20b
NZn
NN
N
OC10H21
OC10H21
OC10H21
OC10H21
C10H21O
C10H21O
C10H21O
C10H21O
21 22
23 24
NCu
N
NN
OC12H25
C12H25O
OC12H25
OC12H25C12H25O
C12H25O
C12H25O
OC12H25
C12H25O
C12H25O
OC12H25
OC12H25
OC12H25C12H25O
OC12H25
OC12H25
N N
NN
R1R2
R3
Zn
R3
R2
R1
R2R1R3
R1
R2
R3
N
N N
NZn
OCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1
H2n+1CnO
H2n+1CnO
H2n+1CnO
H2n+1CnO
OCnH2n+1
Figure 21 Extended core meso-substituted metalloporphyrins.
NN
R R
N
R
RN
N
RR
N
R
RUO O
25
852 Metallomesogens
a Colh phase (22: R1, R2, R3¼OCnH2nþ1 or H).292 Similarly, an
unidentified mesophase was observed for a tri(meso)-substituted
benzoporphyrinatozinc complex (23:n¼10, 12).293,294 Recently,
an octacatenar b-substituted zinc porphycene (24)295 was
reported to have a transient lamellar phase at room temperature
and up to 55 �C, followed by a lamellar columnar (LCol) phase
until the clearing point (90 �C).Sessler et al. described a series of uranium-containing
alaskaphyrin derivatives (25).296 The complexes with n¼10
and 14 exhibited Colh mesophases from 108 to 133 and
106 to 135 �C, respectively, being this way the first uranium-
containing discotic mesogens to be described in the literature.
In 2008, Aida, Tashiro, and coworkers synthesized three
dinuclear copper(II) complexes based on a triply fused por-
phyrin dimer with six tricatenar meso-substituents.297 Among
these materials, only one was liquid crystalline (26) and exhib-
ited a Colr phase from �17 to 99 �C. In a later, very recent
work, they described even wider temperature ranges (up to
220 �C wide) for related materials bearing semifluoroalkyl
side chains (–OC8H16C4F9).298 The authors found prominent
electron transport capability in these materials, classifying
some of them in the rare group of n-type semiconductors and
some others in the p-type group.
Tetraazaporphyrins have also been complexed successfully
to yield liquid crystals. Two series of complexes with eight
thioalkyl (27: M¼Ni, Co, Cu, Zn; R¼CnH2nþ1)299–302 or alke-
nylsulfanyl chains (27: M¼Ni, Co, Cu; R¼CnH2n�1)303
showed Colh phases, with a reduced thermal range for the
latter series with respect to the former. A further report
described some other related complexes with 3,6-di- and
3,6,9-trioxaheptylthio chains (27: M¼Co, Ni, Zn; R¼S
NZn
N
NNOCnH2n+1H2n+1CnO
28
NNN
NZn
O
O O
O
O
OO
OC12H25O
C12H25O
C12H25O
O
OOC12H25
OC12H25
OC12H25
2
2
29
N M
N
N
N
SRN N
NN
SR
RS
RS
SR
SR
RS
RS
27
NM
N
NN
RR
RR
H2n+1Cn CnH2n+1
NNi
N
NN
OCnH2n+1CnH2n+1O
OCnH2n+1CnH2n+1O
R1
R2
R1
R2R3
R3
30 31
N N
NN
R1
R1
R1 CuN N
NN
R2
R2
R2
Cu
OC12H25
OC12H25
OC12H25
O(CH2CH2O)3CH3
O(CH2CH2O)3CH3
O(CH2CH2O)3CH3
R1 =
R2 =
26
Metallomesogens 853
(CH2CH2O)xCH3, x¼2, 3),304 and remarkably, the cobalt
complex exhibited a Colh phase from �5.5 �C up to 170 �C.There are only a few examples of calamitic metalloporphyr-
ins. In 1992 Bruce et al. reported the first of these complexes
(28), which exhibited crystal SmB and SmE phases at high
temperature (above 200 �C).305 Further attempts to obtain
more fluid mesophases were executed using different types of
elongated chains, giving rise to materials that showed smectic
(SmA, SmC) and nematic phases.306–308 In this sense, a hex-
acatenar derivative with two additional ‘fly-over’ chains (29)
was found to form a N mesophase from 50 �C and up to the
clearing point 153 �C.
8.21.6.2.3 Other macrocyclic metallomesogensSeveral additional macrocyclic systems have been used to
achieve mesomorphic metal-containing materials. For
instance, complexes of hexasubstituted tetraaza[14]annulenes
(30: M¼Co, Cu, Ni, Pd, Pt; R¼OCmH2mþ1, CO2C8H17,
OCH2CH2OC2H5, O(CH2CH2O)2CH3),272 dibenzo-tetraaza
[14]annulenes (31: R1, R2, R3¼H, OCnH2nþ1),309–312 or tetra-
aza[14]cyclohexadecanes (32: M¼Cu, Ni, Pd)313 showed
columnar mesomorphism, influenced more by the nature and
number (4, 6, 8) of the attached side chains than by the
complexed metal.
N N
NN
OCnH2n+1
CnH2n+1O
CnH2n+1O
CnH2n+1O
OCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1
M
2+
2 [BF4]-
32
An interesting series of metallacrown complex with gold(I)
was reported by Barbera et al. in 1996 (33).314,315 This trinuclear
system, with a variable number of alkoxy chains (12, 15, or 18),
was able to form Colh mesophases over narrow thermal ranges
AuN
N Au N
N N
NAu
R�C10H21O
C10H21O
C10H21OR��
C10H21OR�
OC10H21
OC10H21
OC10H21
OC10H21
R��R�
OC10H21
OC10H21C10H21OC10H21O
R��
R���
R���
R���
33
854 Metallomesogens
around room temperature. A mesogenic tri-gold(I) system
with a related structure to 33, but less substituted, was also
described.316
Different crown systems were additionally used to prepare
macrocyclic mesogenic complexes, such as [18]aneN6 (34:
M¼Ni, Co), [14]aneN4,317,318 [18]aneN2S4,
319 and [9]
aneN3.320–322 Remarkably, an interesting NCol was formed by
the sandwich compound 2:1 (34)/Co(NO3)2.317 Finally, some
calixarenes were found to be mesomorphic (Col) when pur-
posely designed to do so,323as is illustrated with the oxotung-
sten(VI) complex reported by Swager (35: R¼H or
C12H25).324,325
N
N N
N
N N
C14H29O
OC14H29
OC14H29
OC14H29
C14H29O
C14H29O
2[NO3]M
2+
34
NN
OO OOW
O
N NNN
R
NN
OC12H25
OC12H25
OC12H25
OC12H25
R
C12H25O
C12H25O
C12H25OC12H25O
RR
35
8.21.6.3 Complexes of Mono- and Bi-Dentate Ligands
The aim of this miscellaneous section is to collect and show
the wide variety of mono- or bi-dentate ligands and metals
that can be combined to yield disk-like, rod-like, and poly-
catenar mesomorphic molecules. As it was mentioned in
the introduction, discotic structures tend to induce colum-
nar mesophases, while rod-like systems usually show
nematic and smectic phases. Moreover, the polycatenar
liquid crystals are a family of non-disk-like mesogens
which are able to form columnar phases as well. However,
and fortunately, some of the more interesting structures
and mesomorphic behaviors can be found in the frontiers
of this classification.
Thus, in order to have a better overview of the different
approaches reported in the literature to yield mesogenic
metal-containing materials, the discussion will take place by
ligand kind.
8.21.6.3.1 b-Diketonato metal complexesb-Diketones are one of the most used ligands in the prepara-
tion of mesomorphic metal complexes, due not only to the
well-known synthetic method of the organic ligand and its easy
complexation to the metal center but also to the varied sym-
metry these metallomesogens can induce to the final molecule.
Different combinations of substituents can lead to a discotic or
a calamitic shape, thus inducing columnar or smectic meso-
morphism, respectively. Moreover, more than one isomer can
be present as well, which could dramatically influence the
liquid-crystalline behavior.
After some initial studies of the polymorphism of some
palladium(II) and copper(II) b-diketonate derivatives (36:
M¼Pd, R¼OC8H17, n¼1326; M¼Cu, R¼CmH2mþ1, m¼0–12, n¼1327–330), the first mesomorphic complex of this
kind was reported by Giroud-Godquin and Billard in 1981
(37: M¼Cu, R¼R0 ¼C10H21),331 and soon followed by further
studies with different metals and chain lengths (37: M¼Cu,
Ni, Pd; R, R0 ¼CnH2nþ1, n¼8, 10, 12).332,333 These X-shaped
molecules were found to form one or two lamellar discotic
(DL) mesophases from �75–100 to 120–140 �C. Other sys-
tematic studies were carried out on analogous complexes with
symmetrical b-diketones bearing four alkoxy chains (37:
M¼Cu,334–338 Pd339,340; R¼R0 ¼OCnH2nþ1, n¼1–12, 14),
and unsymmetrically substituted (alkylphenyl)(alxoxyphe-
nyl)–b-diketones (37: M¼Cu, R¼CnH2nþ1, R0 ¼OCnH2nþ1,
n¼4–12).341–344 Among the copper complexes, discotic lamel-
lar mesophases (DL) were generally afforded (n�3), those
of the symmetrical systems being more stable (�75–170 �C)than those of the unsymmetrical analog (�80–145 �C).On the other hand, the symmetrical palladium complexes
needed longer chains (n�10) and slightly higher tem-
peratures (�90–170 �C) to form the mesophase. Some
modifications were introduced in the structure (37: M¼Cu),
such as branched chains (R¼OCH(Me)(Bu), OCH(Me)
(Hexyl)),345 alkylthio and alkysulfonyl chains (R¼SC8H17,
O2SC8H17),346 and oligo(ethylene oxide) groups (R¼(OCH2
CH2)3H)).347 Unfortunately, only one of these complexes
exhibited mesomorphism (R¼(OCH2CH2)3H, R0 ¼OC6H13;
Cr103DL146I).347
Metallomesogens 855
OM
O
H2n+1Cn
OO
CnH2n+1
R
R
OM
O
O O
R�
R� R
R
36 37
Curiously, a monotropic nematic phase was described for a
series of asymmetric copper(II) complexes with an alkylcyclo-
hexyl group (36: M¼Cu; R¼C6H10–CmH2mþ1, m¼3, 5, 7;
n¼3–8).348 Similarly, smectic mesomorphism was found for
some vanadyl349 and uranyl350 complexes with long (n>7)
aliphatic chains (36: M¼VO, UO2; R¼OC10H21, C6H10–
C7H15, C6H10C6H10–C7H15, (C6H10¼cyclohexyl)). The
octahedral UVIO2 complex showed an SmC phase (n¼13,
60–75 �C), while the square-pyramidal VIVO derivatives exhib-
ited SmA and SmC phases, and even a N phase for the cyclo-
hexyl derivatives. However, a monotropic Colh phase could be
formed by bis(alkoxyphenyl)-b-diketonate VOIV complexes
(37: R¼OCnH2nþ1, R0 ¼OCmH2mþ1, n, m¼6–14, n¼m or
n 6¼m).340,351 The mesophase presented a long-range order of
the stacking periodicity, probably induced by intermolecular
V¼O V¼O interactions.
The effect of the number of peripheral chains was studied
systematically in several papers. This way, in addition to the
tetracatenar complexes described before, derivatives bearing six
(38: M¼Cu),352–354 eight (38: M¼Cu, Pd, VO),353,355–362 ten
(38: M¼Cu, Pd, VO),359,363 or twelve (36: M¼Cu, Pd,
VO)352,359 alkoxy chains (R¼OCnH2nþ1, n¼7–16) have
been synthesized. From this work, it can be deduced that a
minimum of six chains is needed to induce a columnar meso-
phase (Colh), and that the increase of the number of chains
runs parallel to the reduction in the melting point. The distri-
bution of the chains around the flat core was also investigated,
particularly in the hexa- and octa-substituted derivatives. From
that point of view, a highly unsymmetric configuration could
inhibit mesomorphism completely (i.e., 38: M¼Cu,
R1¼R3¼R5¼OCnH2nþ1, R2¼R4¼R6¼H) while less unsym-
metric structures led to more stable mesophases. It is worth
mentioning that related nickel derivatives with eight alkoxy
chains were found not to be mesomorphic.357
OM
O
O
O
R2
R3
R2
R3
R5
R6
R5
R4
R6
R4
R1
R1
38
Interesting properties were found by Serrano and coworkers
by replacing the alkoxy chains with a chiral chain, in some
ten-chain b-diketonate complexes (38: M¼Cu, Pd, VO;
R1–R5¼O2CC*H(Me)OCnH2nþ1, R6¼H, n¼6, 7).364,365
These complexes showed an enantiotropic Colr phase (r.t. to
125–150 �C), but interestingly, an additional reflection was
seen by x-ray diffraction, revealing the existence of a helical
superstructure formed from the precession of the tilt direction
of the stacking molecules. The electro-optical response of these
materials was found similar to that of classical calamitic ferro-
electric liquid crystals.
Remarkably, a series of nondiscoid copper(II) derivatives
was found to exhibit a Colh phase. These complexes were
prepared with a trialkoxyphenyl group in one of the
b-positions and a thiophene ring or a substituted phenyl ring
with a polar group in the other b-position (38: M¼Cu; R1–
R3¼OCnH2nþ1, n¼10, 12; R5¼H, F, CF3, OMe, N(Me)2, and
R4¼R6¼H, or R4¼F, and R5¼R6¼H).352 A similar study was
later carried out concerning eight-chain copper and palladium
complexes with polar substituents in the meta-positions of
one of the phenyl rings (38: M¼Cu, Pd; R1¼R2¼R3¼R5¼OC8H17, R
4¼H, Me, Et, OMe, Cl, Br, I, CN, R6¼H).366
They showed Colh and Colr phases which, in the case of the
halide and cyano derivatives, started from room temperature
and extended to 150–235 �C.In addition, a large number of examples with extended
b-diketonato systems can be found in the literature (Figure 22).
An interesting, general observation about these compounds is
that the more disk-like materials showed N phases, while rod-
like materials showed columnar phases. In this sense, the
elongated structure of the complexes 39 was able to induce
both columnar and nematic phases. When the lateral chains
were short, a Colr phase was observed (39: M¼Cu;
R¼C16H33,367 OCnH2nþ1, n¼9–18368; m¼1). However, a
nematic phase (monotropic or enantiotropic) was induced
when the lateral chains were longer (39: M¼Cu;
R¼OCnH2nþ1, n¼6–12; m¼2, 3, 4, 8, 12).369–371 SmA and
N phases were observed in similar oxovanadyl derivatives (39:
M¼VO; R¼C8H19, m¼8; –C6H10–C6H10–C7H15, m¼9) that
also exhibited a chiral nematic phase (N*) when a chiral chain
was used instead (R¼CH2C*HMeEt, m¼8).349,372 More
extended central motifs, such as the structures depicted for
40 (M¼Cu, Pd, VO)372–377 and 41378–381 were also used to
prepare smectic and nematic materials. It is worth men-
tioning that one of these complexes (40: M¼Cu, R¼OMe,
R0 ¼–C6H10–C7H15) was reported to form a biaxial nematic
phase (Nb),380,382–386 but it is still an open debate due to some
contradictory re-investigations, and it is likely that the obser-
vation was alignment induced.378 The effect of the introduc-
tion of polar substituents in the different aromatic rings of
both kinds of complexes 40 and 41 was investigated as
well.378–380 Another example reported recently uses a diketone
functionalized with an alkyleneoxycyanobiphenyl group
(42: M¼Cu, VO).387 These materials exhibited a high-
temperature nematic phase, with an additional SmA phase
for the vanadyl derivative.
Dinuclear copper(II) species were prepared using tri- and
tetra-ketonato systems; some examples are collected in
Figure 23. Swager and coworkers first developed the study of
these discotic bimetallic complexes (43, 44),388,389 while Lai
and coworkers later described other related extended com-
pounds (45, 46)390,391 Different lengths and number of chains
(R, R0 ¼OCnH2nþ1, H) were investigated, concluding that all
OM
O
O O
R
R
R�
R�
39 40
41
OM
O
OO
CN
3
OO
NC
3
O O
OO
CN
3
OO
NC
3
42
OM
O
CmH2m+1
O O
R
R
CmH2m+1
OCu
O
O O
C10H21
R
R
C10H21
Figure 22 Examples of extended b-diketonato metallomesogens.
856 Metallomesogens
the prepared complexes exhibited Colh mesomorphism over
varied temperature ranges related to the structure.
Only a few mesogenic complexes bearing a-substituted-b-diketones have been reported. Copper(II) and oxovanadium
(IV) derivatives (47: X¼O; R¼OCnH2nþ1, M¼Cu (n�8),
VO (n�10); R¼CnH2nþ1, M¼Cu (n�5))392–394 displayed a
nematic phase over a narrow temperature range (�10 �C).However, a much wider mesophase range (N, �100–200�C)was observed for related mercaptopropenatonickel(II) com-
plexes (47: X¼S; M¼Ni, R¼OCnH2nþ1 (n�3), R¼CnH2nþ1
(n�2))395 Other complexes with monovalent rhodium(I) and
iridium(I) centers were reported to present nematic and SmC
phases (48: M¼Rh, Ir).396–398 However, contradictory results
exist here, as Barbera et al. described later the absence of
mesomorphism in the complexes 48a and 48c, and the pres-
ence of only a monotropic SmA phase for 48b.399
Other metals have been used to yield mesogenic complexes
with diketonato ligands. To be able to form a Colh phase, the
center thallium(I), dicarbonylrhodium(I), and dicarbonyliri-
dium(I) needed a tetra(alkoxy)- (49: M¼Tl, R¼H)400 or a
hexa(alkoxy)-b-diketonate ligand (49: M¼Rh(CO)2, Ir(CO)2;
R¼OCnH2nþ1),401,402 respectively. Some other complexes with
octahedral geometry, and also showing room-temperature
columnar phases, were described by Swager and coworkers
4443
45
OCu
O
O O
OCnH2n+1
R�
R
O
OCu
R�
RCnH2n+1O
CnH2n+1O
CnH2n+1O
CnH2n+1O
OCnH2n+1
OCnH2n+1
OCnH2n+1
O O
O OO
OCu
O
OCu
CnH2n+1OCnH2n+1O
CnH2n+1O
CnH2n+1O
CnH2n+1OCnH2n+1O
HH
OCnH2n+1OCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1
OOO
OM
R
OO
OO
M
R
R�
R�
R
ROCnH2n+1
OCnH2n+1
CnH2n+1O
CnH2n+1O
R�
R�
OCu
O
O OO
OCu
R
R
OCnH2n+1
OCnH2n+1
OCnH2n+1
CnH2n+1O
CnH2n+1OCnH2n+1O
46
Figure 23 Examples of dinuclear metallomesogens based on polyketonato systems.
X
M SX
S
R
R
O
O
X
O
OMOC
CO
OCnH2n+1
a: X = OOCb: X = COOc: X = CH2
47 48
OM
O
R
OCnH2n+1
OCnH2n+1H2n+1CnO
R
H2n+1CnO
49
MO
O O
O
O
O
OC12H25
OC12H25
OC12H25
OC12H25
OC12H25
OC12H25
R2
C12H25O
R1
R1C12H25O
C12H25O
C12H25O
R1
R2
C12H25O
C12H25O R2
50
Zr
O
O
R2R1
R1
R1
R1
R2
OO
R2
R1R1
R1
R1
R2
O
O
R2
R1 R1
R1
R1
R2
O
O
R2
R1R1
R1
R1
R2Zr
Zr
51
Metallomesogens 857
(50: M¼Fe, Co, Mn, Cr)403–405 following the original work of
Giroud and Rassat in 1982 (50: M¼Fe).406 They contained three
b-diketonato groups, yielding molecules with 12 (R1¼R2¼H),
15 (R1¼OC12H25, R2¼H), or 18 (R1¼R2¼OC12H25) dodecy-
loxyl chains. Note that these octahedral complexes can exist as
two optical isomers (D and L), but are present in a 1:1 ratio.
Moreover, a series of square-prismatic zirconium(IV) derivatives
(51) were also prepared,407 but only the complexes with 24
aliphatic chains (R1¼R2¼OCnH2nþ1 or R-/S-O(CH2)2C*HMe
(CH2)3CHMe2) were found to be mesomorphic (Colh or Colo,
respectively).
8.21.6.3.2 Other O,O-donor ligandsOther chelating ligands with an O,O-donor set that have been
used to yield mesomorphic metallomesogens are tropolones
SS
H2n+1Cn
MS S
CnH2n+1
S S
OCnH2n+1
R
R
OCnH2n+1H2n+1CnO
R
H2n+1CnO
R
MSS
56 57
858 Metallomesogens
(52: M¼Cu, UO2350,408–411; 53: R1, R2, R3, R4¼H,
OCnH2nþ1412,413) and salicylaldehyde derivatives (54).414
While the first compounds exhibited a rich mesomorphism
(52: SmB, SmC, crystal phases E, G, J; 53: SmC, Colh), the
last derivatives (54) displayed only an SmC phase at high
temperature over a narrow range.
O
O
OM
OOCnH2n+1CnH2n+1O
52
OCu
O
O
ON N
R1R2
R3
R4
R1 R2
R4
R3
53
OCu
O
O
O
O
O
O
OCnH2n+1O OCnH2n+1
54
Two discotic compounds were prepared by Kumar and
Naidu by complexation of a penta(dodecyloxy)anthraquinone
(55: M¼Cu, Pd).415,416 Both complexes displayed a Colr phase,
and then a Colh phase before reaching the isotropic state.
OM
O
O O
CnH2n+1O
CnH2n+1O
OCnH2n+1
OCnH2n+1
OCnH2n+1
O
OCnH2n+1
OCnH2n+1
CnH2n+1O
CnH2n+1O
CnH2n+1O O
55
8.21.6.3.3 S,S-Donor ligandsWith a structure similar to the metal b-diketonate derivatives
described previously, there are some example of bis(dithio-
lene) complexes showing liquid-crystalline behavior. The first
compounds of this kind were two series of calamitic derivatives
with nickel and platinum (56: M¼Ni, Pt), showing nematic
(n¼4) or SmC phases (n¼8, 10).1,417–419 The report of these
complexes represented the beginning of the development of
contemporary studies of metallomesogens. Moreover, discotic
mesogens can be obtained by the attachment of more periph-
eral chains to the central core, and in this way, Colh meso-
phases were induced for some octa(alkoxy)dithiolene
complexes (57: M¼Ni,420–422 Pd, Pt423; R¼OCnH2nþ1,
n¼1–12). For this series, it was found that the mesophase
stability decreased as Pd>Ni>Pt. There are contradictory
studies about the mesomorphism of tetrasubstituted deriva-
tives (57: M¼Ni, R¼H). While in some works, liquid-
crystalline behavior (DL) was described,424–427 in other
report428 the authors claimed that the so-called mesophase
was actually a lamellar crystalline phase with some degree of
disorder. Measurements of the electrochemical potential of
some of these complexes were carried out, and charge-transfer
(CT) complexes with tetrathiofulvalene (TFT) derivatives were
also studied.429
Another mesogenic sulfur-donor ligand that has been used is
the 4-alkoxydithiobenzoate. Complexes with nickel(II), palla-
dium(II), or zinc(II) (58)430 were described showing nematic
and smectic mesophases, usually at high temperatures (above
180–200 �C). In-mesophase extended x-ray absorption fine
structure (EXAFS) studies of the related zinc(II) complexes430
showed that an interesting dimeric structure observed in solid
state was retained in the mesophase. Curiously, Ohta reported431
that at around 230 �C, the blue bis(dithiobenzoates) (58)
derivatives of nickel and palladium rearranged to form red,
mixed-ligand (alkoxydithiobenzoato) (alkoxytrithiobenzoato)
complexes (59: M¼Ni, Pd), observation later reproduced by
Bruce et al. Some other additional studies, such as the use of
fluorinated rings432 or chiral chains433 in these systems, were
also developed.
CnH2n+1OS
SM
S
SOCnH2n+1
58S S
C8H17OS
M
SOC8H17
S
59
Very similar calamitic derivatives were achieved using
dithiocarbamates (60: M¼Ni, Pd, Cu).434 They displayed
both a nematic and an SmC phase at elevated temperature
(above 200–260 �C). Another related series was described
later bearing a partly hydrogenated stilbazole and showing
B and SmC phases (61: M¼Ni, Pad, Cu).435,436
S
SM
S
SNN NNCnH2n+1O OCnH2n+1
60
S
SM
S
SNN
OCnH2n+1
H2n+1CnO
61
Metallomesogens 859
A new family of discotic mesogens with platinum(II) was
prepared by complexation of nonmesomorphic dithiooxamide
ligands (62).437,438 Eight (R¼H, n¼12–16) or twelve
(R¼OCnH2nþ1, n¼8, 11, 13) alkoxy chains were needed to
yield columnar mesomorphic materials (Colh). While the octa-
substituted species showed narrow temperature ranges (from
�110 to �120 �C), the dodeca-substituted derivatives pre-
sented slightly wider ranges, and lower transition temperatures
with a strong dependence on the chain length (n¼8:
83–125 �C; n¼13: 42–59 �C).
HN
N
SPt
S
S
S
NH
N
CnH2n+1O
H2n+1CnO
H2n+1CnO
CnH2n+1O OCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1R
R R
R
62
8.21.6.3.4 EnaminoketonesThe structure of enaminoketones is closely related to that of
b-diketones. So, it is not surprising that many metallomesogens
have been reported containing this class of ligand. Only a
few examples of complexes with the structure depicted as
63 (M¼Cu) presented any mesomorphism (monotropic SmA)
64
65
H3CO M
N
NO
R3
O
O
R2
R1
H2n+1CnO
x
66
R1
OCu
N
NR3
R2
R4
NO Cu
N
NO
CmH2m+1
NC6H13XCmH2
Figure 24 Examples of core-extended enaminoketonato metallomesogens.
when the side chain was an alkoxy, alkyl, or alkanoate group.439
However, by elongating the rigid core with an alkyl-cyclohexyl
chain (63: M¼Cu; R¼C6H10–CnH2nþ1, R0 ¼CmH2mþ1),
440–442
wider, enantiotropic SmA phases were observed. Very recently,
palladium(II) complexes with a similar structure to 63a, but
bearing one or two alkoxy chains attached to the aromatic ring
and a propan-diol chain linked to the nitrogen (R0 ¼CH2CH
(OH)CH2(OH)), have been described to exhibit SmA meso-
phases, apparently stabilized by hydrogen bonds.443
RO M
N
NO
R
R�
R� R
OM
N
NO
R
R�
R�
a b
63
Elongated cores have also been investigated, and the meso-
morphism observed for the corresponding complexes was very
similar to the previous derivatives. Some of these complexes are
shown in Figure 24 (64,444–447 65,448 66: M¼Cu, Pd449,450). In
general, N, SmA, or SmC phases were observed, usually highly
influencedby the number, nature, and distributionof the chains.
Columnar mesophases were also observed using this ligand
system, but introducing somemodifications into the rigid core.
This way, different half-disk-shaped complexes were prepared
and found to exhibit Colh phases by linking two iminoketone
moieties with an alkylene bridge (67a: M¼Ni, Cu, Pd, Co, VO;
X¼Me, H; tetra- and hexa-substituted derivatives with R1, R2,
O
O
R2
R1
OCnH2n+1
x = 1, 2x
O
R4
R3
R1
R2
NN XC6H13
ONHN(CH3)
X =m+1
O N
O N
R1
R2R3
R3R2
R1
X
X
M
R4
R4
N
N
N
N
N
NOH
N
N
OC8H17
OC8H17
a: b:
c:
67
860 Metallomesogens
R3¼OCnH2nþ1, H and R4¼H451–453; 67b: M¼Pd; X¼H;
R2¼OCnH2nþ1, R1¼R3¼R4¼H454). However, additional
smectic mesomophism was also observed for a series of
wedge-shaped molecules bearing a bis(octyloxy)phenyl group
linking the two iminoketones groups, and depending strongly
on the substitution pattern (67c: M¼Cu, Co; X¼H; from di-
to hexa-substituted derivatives with R1, R2, R3, R4¼OCnH2nþ1,
H and R4¼OH, OMe).455,456
Using similar mesogenic ligands, homo- and hetero-
polynuclear derivatives were prepared. A few examples are
shown in Figure 25 (68: M, M0 ¼Cu, Ni, Mn, Pd, Co;
X¼C2H4, C3H6388,389; 69: M¼Cu, Ni457,458; 70: M, M0 ¼Ni,
Cu, VO459). Despite the varied shape of these metallomesogens,
all of them exhibited columnarmesomorphism (Colh, Colr), but
over different temperature ranges depending on many factors.
OM�
N
O NOO
M X
CnH2n+1OCnH2n+1O
CnH2n+1O
CnH2n+1O
CnH2n+1O
CnH2n+1O
N
OM�
N
O
C8H17O
C8H17
C8H17
C9H19
C9H19
O
N
NMO
O
68
70
CnH
CnH
Figure 25 Polynuclear enaminoketonato metallomesogens.
8.21.6.3.5 Salicylaldimine derivativesThis family of ligands, related structurally to the previous
enaminoketones, has been extensively used to prepare meso-
genic complexes. In fact, the salicylaldimines are one of the
earliest and most studied class of metallomesogens.20,460
Galyametdinov and coworkers developed the first study in
this area by using the most basic framework of this kind that
is able to induce smectic mesomorphism, containing only two
aromatic rings per ligand and two or four alkoxy chains in total
(71: M¼Cu, R1¼F, OCnH2nþ1, R3¼OCmH2mþ1, R
2¼H).461–468
This was followed by many other reports studying complexes
with different metals (Cu, VO, and Pd) and with different kinds
of chains (R2¼H; R1, R3¼CnH2nþ1, OCnH2nþ1,463,469–488
O2CnH2nþ1,489,490 chiral chains,491–498 fluorinated chains499).
The formation of smectic phases, generally SmA and SmC, was
clearly favored over the nematic phase, but both kinds of meso-
morphism could be observed. On many occasions, ferroelectric
behavior (SmC*) was also found in complexes with chiral
chains.
O M
N
NO
R1
R1R3
R3
R2
R2
71
In contrast, there are only a few examples of columnar
mesogens based on this simple motif. Lai et al. reported in
C8H17
C8H17
C9H19
C9H19
C8H17
N
NO
O
M
69
N OM
N O
NM
2n+1CH2 CH2CnH2n+1
CH2CnH2n+12n+1CH2
O
NO
CnH2n+1 CnH2n+1
CnH2n+1CnH2n+1
Metallomesogens 861
1998 a series of copper(II) complexes bearing an additional
chain per ligand to induce a discoid-like shape (71: M¼Cu;
R1¼R2¼R3¼OCnH2nþ1),500 while Date and Bruce described
later related derivatives with an extended core (71: M¼Pd, Cu,
FeCl; R1¼R2¼R3¼O2CC6H2(OCnH2nþ1)3).18,501
Interestingly, with just the increase in the anisotropy
caused by the shift of one aromatic ring in the ligand, the
tendency to form nematic phases was enhanced significantly.
Thus, several series of copper(II),502–509 nickel(II),502,509,510
palladium(II),509 and oxovanadium(IV)510–513 complexes
with this kind of ligand (72) were found to behave mostly
as nematogens, especially for the short-chain derivatives. An
additional SmC phase was observed for the longer deriva-
tives, whereas the SmA phase was totally absent. Some other
complexes with trivalent metals were prepared (72:
M¼MnCl,514 FeCl515–517), and in those cases, SmA, SmC,
and monotropic N phases were seen over narrower ranges
than the previous complexes. This destabilization could be
caused by the presence of the chloride group reducing the
molecular anisotropy.
It is worth mentioning that chirality was also investigated in
these compounds.518,519 Remarkably, a series of complexes
reported by Galyametdinov and Pyzuk (72: M¼Cu, Ni, Pd;
CmH2mþ1¼CH2C*HMeC2H5)520 showed enantiotropic chiral
nematic (N*) phases, and in some cases one or two blue phases
(BPI, BPII). In fact, the authors claim that a palladium derivative
exhibited the rare blue fog phase (BPIII). A detailed discussion
of blue phases is beyond the scope of this chapter. Blue phases
appear in highly chiral materials and normally over very short
temperature ranges (typically 1 K) between the chiral nematic
(N*) phase and the isotropic phase. BPI and BPII possess cubic
symmetry and are composed of cylinders of highly twisted
structures - the ’blue’ refers to the apparent color, which reflects
the dimensions of the cubic lattices. BPIII is an isotropic phase.
Recently, blue phases have been shown to be useful in a new
generation of display device. http://www.samsung.com/us/
aboutsamsung/news/newsIrRead.do?news_ctgry¼irnewsrelease
&news_seq¼8351521
N
N
M
O
O
O
O
O
O
C10H21O
C10H21O
R
R�
R�
R
74
O
O
O
OCnH2n+1
CmH2m+1
CmH2m+1
O M
N
NO
CnH2n+1O
O
72
Another significant family of salicylaldimines is represented
by complex 73. In general, relatively stable smectic and nematic
phases were observed for alkyl and alkoxyl derivatives (73:
M¼Cu, Pd, VO, FeCl).511,522–527 However, the introduction of
halogen groups or polar substituents in different positions of the
aromatic rings induced a destabilization in most cases.528–531
Some dinuclear complexes were prepared (74: M¼Cu, VO),532
and when they possessed only one terminal chain per branch
(R¼R0 ¼H) they showed an SmC phase, while when two
(R¼OC10H21, R0 ¼H) or three chains were attached
(R¼R0 ¼OC10H21), Colh mesophases appeared. Recently, a
series of cyanobiphenyl-containing complexes was reported by
Yelamaggad and coworkers (75: M¼Cu, Pd; n¼3–12, m¼0–
10).533 Notably, the complexes with the odd-parity spacer sta-
bilized nematic, uniaxial smectic A (SmA), and biaxial smectic A
(SmAdb) phases, while the even-parity members promoted
nematic and smectic phases. Some other elongated salicylaldi-
mines were complexed to yield mesomorphic materials, but in
general, properties were not so different.120,534–550
O M
N
NO
O
O
O
O
R
R
73
OCnH2n+1
CnH2n+1O
N
N
M
O
O
O
O
OC10H21
OC10H21
O
O
R�
R
R
R�
O M
N
NO
OC10H21
C10H21
O nNC
O On
CN
75
862 Metallomesogens
Finally, note that a few examples with only one salicylaldi-
mine ligand have been reported based on dicarbonylrhodium
(I) (76)551 and allylpalladium(II) (77: R¼Me, H; R0 ¼C8H17,
C6H4OC7H15)552 cores. They generally formed mesophases
(SmA, N) over narrow temperature ranges, and usually with
monotropic nature.
O Rh
N
COOC
O
OOC10H21
C10H21O
76
O
OOC8H17
NO
R�
Pd
R
77
The molecular geometry of this class of complexes can be
modified from the, in principle, more stable trasnsoidal confor-
mation to a cisoidal disposition, by simply linking two salicylal-
dimine units with an alkylene bridge. This way, some systems
have been made combining different spacers and mesogenic
motifs (Figure 26). Conflicting results were reported by
R1, R2
R1
R2
R1
R1
O
N NM
Y
O
Y
−CnH2n+1
CnH2n+1
−OCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1O
N–N
–O
78
a
b
c
d
Figure 26 Salen-like metallomesogens.
different groups about the mesomorphism of some bis-alkyl
and bis-alkoxy-salen (salicylidene-ethylenediamine) derivatives
(78a and 78b: M¼Ni, Cu, Co, VO; Y¼C2H4; R1¼CnH2nþ1,
OCnH2nþ1, R2¼H).553–558 While Paschke et al. reported SmA
mesophases for some of these complexes at high temperature,
Ohta et al. described crystal SmE and SmA phases over different
temperature ranges. The latter group has also been investigating
the mesomorphic properties of V-shaped salen complexes over
the last few years (78b: M¼Ni,559 Cu,560 VO561; Y¼C2H4;
R1¼H, R2¼OCnH2nþ1). Curiously, the nickel(II) and copper
(II) complexes exhibited a lamello-columnar mesophase (ColL)
for the long-chain derivatives, while the oxovanadium(IV) com-
pound formed an unprecedented biomembrane-like bilayer
mesophase with Pa21 symmetry.
The attachment of more rigid substituents to the central
core led to the enhancement of calamitic (N, SmA, SmC)
behavior (Figure 9, 78c: M¼Ni, Cu, VO; Y¼C2H4,
C3H6, CH2C(Me)2CH2; R1¼N2–C6H4–CnH2nþ1, R2¼H,
OCmH2mþ1562,563; M¼Ni, UO2; Y¼(CH2)3NMe(CH2)3;
R1, R2¼H, OC18H37, N2–C6H4–C14H29, C6H4–OC12H25564),
or the induction of a very rich columnar (Colr, Colh) meso-
morphism by using several peripheral chains (Figure 9, 78d:
M¼Cu, VO; Y¼C2H4, C3H6, CH2C(Me)2CH2; R1/R2 or R2/
R1¼O2C–C6H2(OCnH2nþ1)3/H).565–568
In 2009, Pucci et al. synthesized a transoidal, salen-like zinc
derivative by using long spacers between the two salicylaldi-
mine moieties (79: n¼10, 12).569 An interesting intercalated
smectic C mesophase was seen over a narrow temperature
range (�15 �C). Also, in contrast with the free parent ligands,
the zinc derivatives were found to emit an intense blue lumi-
nescence at room temperature.
ZnO
N
OO
O
N
OO
OC12H25
C12H25O
(CH2)n
79
8.21.6.3.6 Other N,O-donor ligandsA good variety of ligands containing a N,O-donor set can also be
used to yield metallomesogens in addition to the previously
OM
N
OM
O
N
O
R
R
N N
MOO
NNOCnH2n+1
H2n+1CnO
R
R
80 81
N
N
O
R
O
RO OM
O
O OO
OC12H25
OC12H25
OC12H25
OC12H25
OC12H25
OC12H25
C12H25O
C12H25O
C12H25O
OO
O
OOO
OO
82
C12H25O
C12H25O
C12H25O
NN OHM
OC16H13
OC16H13
OC16H13OC16H13
C16H13O
C16H13O
Cl Cl
83
Metallomesogens 863
described systems. This way, calamitic complexes were described
bearing groups such as oxo-bridged salicylaldimato (80: M¼Cu,
VO, Pd, FeCl; R¼OCnH2nþ1, O2CH2–C6H4–OCnH2nþ1, OCH2–
C6H4–OCnH2nþ1),570–572 hydrazinato (81: M¼Cu, Ni; R¼H,
Me, Ph, C6H5–OC12H25),573–576 oxo-bridged enaminoketonato
(M¼Cu),577 hydroxoaryl-oxazolinato (M¼Cu),578 hydroxoaryl-
thiodiazolato (M¼Cu),579 hydroxoaryl-benzoxazolato (M¼Cu,
Pd),580 and other, related motifs.581–584
In a similar way, several discotic and polycatenar derivatives
showing columnar mesomorphism have also been synthe-
sized. For instance, eight- and ten-chained hydrazido com-
plexes with a structure similar to 81, but bearing poly
(alkoxy)-aryl substituents, showed Colh phases over narrow
ranges.585 Barbera et al. investigated some extended polycate-
nar oxazoline-derived complexes (82: M¼Pd, Cu) exhibiting
helical stacking in Colh and Colr mesophases.586 Morale et al.
recently reported two bent-core metallomesogens showing
a different behavior depending on the coordinated metal
(83: M¼Zn: Colr; M¼Mn: Colh).587 Note that some other
heterocyclic systems have been explored.588,589
8.21.6.3.7 Pyrazole-based ligandsThe study of pyrazole-based metallomesogens has been mostly
developed over the last decade. The initial work was carried out
by Barbera et al. in 1999 by complexation of rod-like pyrazoles
to yield complexes of cis-dicarbonylrhodium(I),399 and was
soon followed by other studies made by Torralba et al. on
similar rhodium(I) and iridium(I) derivatives.590,591 These
first examples only showed monotropic nematic mesophases
over narrow temperature ranges. Nevertheless, the meso-
morphism was later enhanced by the introduction of a
second pyrazole (84: M¼PdCl2, R¼H, x¼0592; M¼Ag,
R¼H, C6H4–OCnH2nþ1, x¼1593). While the neutral palladium
(II) derivatives showed enantiotropic SmC phases over narrow
temperature ranges (�10 �C), the cationic silver(I) complexes
(Y¼PF6, BF4, or NO3) formed smectic mesophases over a
100 �C range. Additionally, the silver derivatives behaved as
photoluminescent materials even in the liquid-crystalline state.
Note that recently, related square-planar copper(II) complexes
were synthesized by Chen et al.594
With a different molecular shape, a series of pyrazolato-
bridged diallyl-palladium(II) complexes reported by Torralba
et al. were found to form smectic phases, too (85: n¼10–18).595
N
HN
M NNH
OCnH2n+1
CnH2n+1O
R
Rx+
(Y−)x
84
864 Metallomesogens
N
NPd
PdN
N
OCnH2n+1
OCnH2n+1H2n+1CnO
H2n+1CnO
85
Discotic systems were also achieved using these kinds of
ligands (Figure 27). Columnar mesophases could be induced
in 12-chained oxadiazole dichlorido-palladium(II) complexes
(86),596 unsymmetrical eight-chained pyrazole dichlorido-
nickel(II) derivatives (87),597 or half-discotic six-chained cis-
chloridodicarbonylrhodium(I) complexes (88).598
C10H21O
C10H21OOC10H21 OC10H21
OC10H21
OC10H21
CO
Cl
HN N
CO
Rh
H2n+1CnO
H2n+1CnO
NN
NN
O
OCl
ClPd
OCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1
H2n+1CnO
H2n+1CnO
H2n+1CnO
H2n+1CnO
OCnH2n+1
86
88
Figure 27 Discotic metallomesogens based on pyrazole-like ligands.
N
NH
R2
R1
R1
R2
Zn
Cl Cl
N
HN
R2
R1
R1
R2C10H21O
C10H21O OC10H21
OC10H21
a89
Remarkably, two series of tetrahedral zinc complexes were
described by Cavero et al. (89: R1, R2¼H, C10H21).599 The non-
conventional shape of these complexes, together with different
distribution and number of the peripheral chains, was able to
induce either lamellar (SmA) or columnar (Colh) mesomo-
phases. Moreover, interesting near-ultraviolet (UV)-blue lumi-
nescent with large Stokes shifts was observed in these materials.
8.21.6.3.8 Pyridines, bipyridines, and related ligandsPyridine ligands, in particular alkoxystilbazoles, have been
used extensively by Bruce and coworkers over many years and
have, for the most part, been found as complexes of Ag (in
particular), Pd, Pt, Rh, and Ir.600,601 Much of this work has
been of a very systematic nature and a great deal had been
directed to an understanding of factors affecting the formation
of the cubic phase and also of the transition from lamellar to
columnar mesophases in homologous series of tetracatenar
H2n+1CnO OCnH2n+1
H2n+1CnOO
O
NH
HNN
NiCl
ClN
OCnH2n+1
H2n+1CnO OCnH2n+1
H2n+1CnO OCnH2n+1
87
N
N
R2
R1
R1
R2
Zn
Cl Cl
N
N
OC10H21
OC10H21C10H21O
C10H21O
R2
R1
R1
R2
b
Metallomesogens 865
mesogens. It should be noted that none of the stilbazole
ligands possess a true liquid-crystal phase.
cis-Dicarbonylchloridometal(I) (M¼Rh, Ir) complexes of
4-alkoxystilbazoles (90a)602–604 and of some 2- (90b) and
3-fluoro (90c) derivatives605–607 showed the strong ability of
the metal fragment to induce liquid-crystal behavior, with
nematic and SmA phases being shown.
NCnH2n+1O
M CO
Cl
CO
X Ya X = Y = Hb X = H, Y = Fc X = F, Y = H
90
By far the biggest body of work was with silver(I) complexes
of the various alkoxystilbazoles shown in Figure 28 (91).608–622
Early work with 4-alkoxystilbazoles of silver(I) was of interest as
these formally ionic materials showed smectic polymorphism as
0 2 4
Carbon c
Cry
N
N
SA
180
160
T (º
C)
140
120
200
100
CnH2n+1O
–O3
N–Ag+
Figure 28 Silver(I) complexes of alkoxystilbazoles. From Bruce, D. W. Acc.
CnH2n+1O X N
AB
X = HC=CHX = N=CHX = OCOX = N=N
nnnn
A, B = H or FY = BF4, NO3, CF3SO3 (OTf), CmH2m+1O
Ag
91
well as, unexpectedly, a nematic phase. The phases depended
strongly on both the alkoxy chain length and the nature of
the anion. For example, tetrafluoridoborate and nitrate salts
show SmA and SmC phases, while OTf and alkylsulfate salts
show a nematic phase in addition.
However, the most remarkable mesomorphism was shown
by dodecylsulfate salts of the 4-alkoxystilbazoles (Figure 28
shows the phase diagram) where, in addition, several homo-
logs showed a cubic phase623–628; thermotropic cubic phases
were rare at that time. The observation of so many complexes
with a cubic phase initiated much related work from the group
looking at different number of chains on the stilbazoles and
different chain lengths on the alkyl sulfates.
This work was coupled with related studies of palladium(II)
and platinum(II) complexes. Thus, symmetric trans-dichlorido
complexes (92: M¼Pt, Pd; X¼Cl; R1, R2, R3¼H,
OCnH2nþ1)46,629–632 were made along with trans-dicarboxylates
(92: M¼Pd, X¼O2CCnH2nþ1; R1¼H; R2, R3¼H,
I
6
hain length
s
Cub
Sc
SA
8 10 12
OCnH2n+1
SOC12H25
N
Chem. Res. 2000, 33, 831–840.
-OPhVPy-OPhIPy-OPhEPy-OPhAPy
SO3 (m = 12, DOS; m = 8, OS)
OCnH2n+1XN
A B
Y
866 Metallomesogens
OCnH2nþ1)629,633 and unsymmetric trans-chloroacetylides
(93).634,635 Comparison of the behavior of these series of closely
related complexes and comparison with literature materials
allowed the group to postulate that specific intermolecular inter-
actions were required for the observation of cubic phases.48,636
NCnH2n+1O
M
X
XN
OCnH2n+1
R1 R2
R3
R1R2
R3
92
CnH2n+1O
CnH2n+1ON Pt N
Cl OCnH2n+1
OCnH2n+1
CmH2m+1
93
In addition, some dendritic materials were reported using
3,5-disubstituted pyridines, with their silver complexes show-
ing columnar phases.637 Silver complexes of 2- and 3-
stilbazoles were also prepared, along with silver complexes
using dodecylenedisulfate anions.638
Finally, it is worth mentioning that some nematogenic
diruthenium(II) complexes were synthesized, studying the
influence of different structural modifications and their inter-
esting electronic properties (94: R¼H, CF3, Ph, C6H4–OMe,
C6H4–Me).639,640
ON
CnH2n+1O
ORu Ru N
O
O OCnH2n+1
OC OC
O O
CO CO
OO
RR
94
N
R1
La Lb
O
OCnH2n+1R1 = R2 =
O
OR1 = R2 =
Figure 29 4,40-Disubstituted-2,20-bipyridines used in the preparation of var
N
O
O
O
Re
CO
Br
OC
OCnH2n+1O
X X
95
Different unsuccessful attempts of preparing mesomorphic0 0
complexes based on 5,5 -disubstituted-2,2 -bipyridines werereported in the past.641–647 However, by coordination of a
six-ring system, Bruce and Rowe were able to prepare calamitic
materials of this kind (95: X¼H, F),648–650 while El-ghayoury
et al. reported a cationic cyclopalladated derivative (96)651
with a simple disubstituted bipyridines showing a Colr and
an SmAmesophase. In a different way, Hoshino et al. prepared
the chiral ruthenium(II) complex D-[Ru(acac)2(L)], where
L¼5,50-(4-octylphenyloxycarbonyl)-2,20-bipyridine.652 Even
if the authors did not report the liquid-crystalline properties
of this material, it was successfully used as a chiral dopant
with large helical twisting power, for various organic nematic
systems.
−O3SOC12H25
N NCH2OC12H25
+C12H25OCH2
NPd
96
Nevertheless, most of the work in this area was later devel-
oped by Pucci and coworkers, mainly using 4,40-disubstituted-2,20-bipyridines (La and Lb, Figure 29). In this sense, they
prepared cationic silver(I) complexes653,654 and neutral zinc(II),
nickel(II), palladium(II), and platinum(II) derivatives.655,656 As
expected, mono-bipyridine derivatives with the simplest ligand
exhibited lamellar phases ([MLaX2]: M¼Zn, Ni, Pt, Pd; X¼Cl,
Br, N3), while the related complexes with the extended bipyri-
dines exhibited columnar phases ([MLbCl2]: M¼Zn, Pd). All the
silver compounds behaved as disk-like mesogens ([AgL2]Y:
Y¼OTf: Colh, Y¼DOS, Colr). In 2009, a related octahedral
ruthenium(II) complex [Ru(bipy)2Lb](PF6)2 was described as a
N
R2
Lc
OCnH2n+1
OCnH2n+1
OCnH2n+1
OC16H33
OC16H33
OC16H33
R1 =
R2 = CH3
O
O
ious metallomesogens.
N
O
O
O
CO
OOCnH2n+1
X X
Metallomesogens 867
room-temperature liquid crystal, exhibiting columnar meso-
phases up to �200 �C and efficient orange phosphorescence as
well.657 Yam and coworkers have reported a luminescent square-
planar platinum complex [PtCl2Lc] that showed evidence of
smectic mesomorphism over a wide temperature range (from
16 to 169 �C).658
Similarly, some other examples, but using 1,10-phenanthro-
line-based ligands with different substitution patterns, have been
reported. Square-planar palladium(II) [PdCl2(N^N)] (97),659
tetrahedral copper(I) [Cu(N^N)2](BF4),660 octahedral ruthe-
nium(II) [Ru(bipy)2(N^N)]X2 (98),661 and octa-coordinated
dioxouranium(VI) ([UO2(N^N)3](OTf)2)662,663 derivatives
were found to be mesomorphic, thus displaying SmA,
Colo, SmA, or Colh mesophases, respectively. A series of
octahedral ruthenium(II) surfactants, designed with differ-
ent combinations of 4,40-dialkyl-2,20-bypiridines, 4,7-dialkyl-
1,10-phenanthrolines, and the homolog unsubtituted ligands,
was described to show lyotropic mesomorphism in water.664
However, no sign of thermotropic behavior was reported. Note
that other polycatenar rhenium(I)665 and platinum(II)663 com-
plexes have been prepared, but they did not show mesomorph-
ism. Some related lanthanide-containing mesogens were also
described, and they will be discussed later.
N
N
O O
NN
OO
H H
OC16H33
OC16H33
C16H33O
OC16H33
Pd
Cl
Cl
97
Interestingly, in 2009 Pucci et al. reported two pyrrole–
pyridine complexes (99), and they claimed that the formation
of the liquid-crystalline state was based on a phase-segregated
Ru
NN
NN
N
NX2
X = PF6−, Cl−, N(SO2CF3)2
−
N
N
98
structure.666 However, the mesophase could only be induced
over a relatively short temperature range (99: R¼CH3,
174–201; R¼CF3, 150–165�C).
N
Pd
NR
R
O
O
CF3
CF3
R = CH3 or CF3
99
New, related calamitic materials were obtained from the
investigations of Cano and coworkers about pyrazole–pyridine
systems. All the synthesized compounds showed SmA
mesophases. Complexes of palladium(II) and zinc(II)
(100: M¼PdCl2, ZnCl2, x¼0; M¼Pd(�3-allyl), x¼1, Y¼BF4,
PF6, CF3SO3), either with a three- (R¼H)667 or a four-ring
rigid core (R¼C6H4–OCnH2nþ1),668–670 and one or two
peripheral alkoxy chains, respectively, were described. More-
over, cationic silver(I) and zinc(II) complexes bearing two
units of the four-ring pyrazole–pyridine ([M(N^N)2]Yn :
M¼Ag, n¼1, Y¼PF6, SbF6, CF3SO3, NO3; M¼Zn, n¼2,
Y¼NO3) were also reported showing very similar behavior.670
N
NN
MR
OCnH2n+1
xY-
x+
100
8.21.6.3.9 Other N,N-donor ligandsA few other chelating ligands with two N-donor atoms have
been investigated in order to prepare metallomesogens,
such as halotricarbonylrhenium(I)–diazabutadiene complexes
(101: X¼Cl, Br, I),671,672 some bis(2-phenylazopyrrole) nickel
(II) and copper(II) derivatives (102: M¼Ni, Cu; X¼N,
CH; R¼N¼N–C6H4C6H13, C6H10C6H13),673 amidine
N
N N
N SC18H37
SC18H37
N
N N
NHOC18H37
OC18H37
868 Metallomesogens
complexes of tetracarbonylrhenium(I) (103),18 or simple ionic
silver(I) complexes with octyl- or dodecyl-ethylenediamine
(104).674
Particularly, disk-shaped bis(1,2-dioxime) complexes have
resulted to be excellent materials to form columnarmesophases.
Ohta and coworkers are the most important contributors to the
development of this area, reporting many NiII,422,675–677
PdII,676,678,679 and PtII680 complexes with dioximes functiona-
lized with alkoxyl or alkyl chains (105: R¼R0 ¼OCnH2nþ1,
CnH2nþ1). An additional study was carried out to investigate
the transition from columnar to discotic-lamellar behavior that
some asymmetrically substituted nickel dioximes presented
NN
N
OO
ORRO
RO
NN
N
OO
ORRO
RO
PtEt3P
Et3P
OR = 11
O
O
CnH2n+1O
N N
O
O
OCnH2n+1
ReOC
COCO
X
101
X
NM
N
N
X
NR
R
102
Ag
NN
N
N Ag
NN
N
N
CnH2n+1
CnH2n+1
H2n+1Cn
H2n+1Cn
2+
2 NO3−
104
CnH2n+1O
CnH2n+1O
N N
ReOC
COCO
CO
OCnH2n+1
OCnH2n+
103
1
N
OO
RO OR
OR
N
OO
RO OR
OR
PEt3
4 PF6−
PEt3Pt
4+
O
106
(105: M¼Ni; R¼OC12H25, R0 ¼H, OH, OCnH2nþ1,
CnH2nþ1).681
Note that some other structurally related systems to these
dioxime complexes were explored with similar results.682–684
N
N
R
R� R�
R
R
R�
R
R�
MN
OHO
O H O
N
105
In a completely different way, Pecinovsky and coworkers
recently reported an interesting diplatinum(II) complex, based
on large polycatenar trans-azobenzene ligands acting as bridging
groups between two bis(phosphane)platinum(II) fragments
(106).685 This material showed both a thermotropic and a
lyotropic (with polar solvents) Colh phase at room temperature.
Interestingly, photo-conversion to the cis-azobenzene isomer
could be achieved by UV irradiation, with an efficiency
of �10% for the solvent-free material and of �40% in the
lyotropic phase. Moreover, the complexes could be polymerized
by the utilization of a radical thermal initiator (AIBN) and due
to the presence of acrylate functions at the end of the chains.
Even if the optical textures remained unchanged for the cross-
linked polymer, x-ray diffraction studies revealed the loss of
some long-range order.
8.21.6.3.10 Nitriles, isonitriles, and acetylidesOrganonitriles, and in particular benzonitriles, are one of the
most common molecules used in commercial LCDs.686,687
Thus, it is not strange that they have been coordinated to
metal centers to evaluate the properties of the corresponding
materials. In this sense, several trans-dichloridometal(II) com-
plexes of alkyl- or alkoxy-cyanobiphenyl (107: M¼Pd, Pt;
R¼CnH2nþ1, OCnH2nþ1)688–693 were found to show mainly
nematic behavior. Smectic mesophases were also observed for
the materials with longer length chains. However, the
Au C N OCnH2n+1
XF
Y
F F
111
Metallomesogens 869
coordination of 3,4,5-trialkoxybenzonitriles to PdII yielded a
series of complexes showing Colh mesophases over small tem-
perature ranges (108).694 In general, in all of these cases, both
calamitic and discotic materials, the mesomorphism was
clearly enhanced or induced after complexation of the free
ligands.
M
Cl
N
Cl
RCNR C
107
C N Pd N C
CnH2n+1O
CnH2n+1O
CnH2n+1O
OCnH2n+1
OCnH2n+1
OCnH2n+1
Cl
Cl
108
On the other hand, complexes of organoisonitriles have
been more widely studied than those previously described
compounds with the isoelectronic nitriles. This can be due to
the greater stability that, generally, the metal–carbon bond
presents in this kind of complexes, thus enhancing the thermal
stability of the material. Also in this case, trans-dihalometal
complexes were studied thoroughly by Kahan et al. (Figure 30,
109a–109c: M¼Pt, Pd; X¼Cl, Br, I695,696; 109d: M¼Pt, Pd;
X¼ I697). As expected for their rod-like nature, N, SmA, and
SmC phases were commonly observed, while chiral meso-
phases (N*, SmC*) could be detected when chiral chains
were used.698
Half-discotic isonitrile complexes were also described by
Coco et al. using 3,4,5-trialkoxybenzonitriles coordinated in a
cis disposition to a dichlorido- or dibromide-metal(II) center
(Figure 30, 110e: M¼Pd, Pt; X¼Cl, Br), or trans when the
R RN NC CMX
X
109
110
Me
Me
R N C
R
N
C
X
X
M
Figure 30 trans- and cis-bis(isonitrile)dihalometal(II) mesogens.
coordinated halide was iodide (109e: M¼Pd, Pt; X¼ I).699 As
expected for their shape and tail-pattern, all these derivatives
(109e, 110e) showed Colh mesophases, in general, over wide
temperature ranges.
Gold(I) and silver(I) isocyanide complexes have also been
studied and, initially, ionic derivatives with the formula
[M(CNR)2]X (M¼Ag,Au;X¼NO3,PF6,BF4;R¼C6H4OCnH2nþ1,
C6H4–C6H4OCnH2nþ1, C6H2(OCnH2nþ1)3)700 were synthe-
sized, showing a clear stabilization of the mesophases (smectic
or columar) related to the free ligands.
Then, several series of halogold(I) and perhalophenyl-
gold(I) isonitrile complexes were reported. The former com-
plexes (i.e., [AuX(CNR)]: X¼Cl, Br, I; R¼OCnH2nþ1,
C6H4OCnH2nþ1)701–704 behaved as normal calamitic material
and showed lamellar phases (SmA and SmC), with transition
temperature generally depending on the nature and length of
the terminal chains. For their part, the mixed phenylene–
isonitrile gold(I) complexes (111: X¼F, Br; Y¼F, Br,
OCmH2mþ1)705,706 exhibited a similar behavior (N, SmA, and
SmC) at significantly lower temperatures, but remarkably,
when one or two chiral chains were used,707 a much richer
mesomorphism was observed. A chiral nematic phase and a
rare twist-grain boundary A phase (TGBA,521,708 a chiral vari-
ant of the SmA mesophase) were observed when Y¼(R)-2-
butyl, while if both chains were chiral (Y and
OCnH2nþ1¼(R)-2-butyl), the three blue phases BPI, BPII, and
BPIII were detected on cooling between the isotropic liquid
and the N* phase.
OCnH2n+1
O
R
a
b
c
d
e
O
OCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1
O
O
N
Au C N
FF
O
F FR
C8H17O
R
O
O
OO
O
112
870 Metallomesogens
A series of related gold complexes was later described bear-
ing a crown-ether group on the isonitrile ligand (112: R¼H,
OC8H17),709 and showing smectic mesomorphism. However,
the liquid-crystal properties were lost after complexation of
sodium salts.
Recently, Dembinski et al. undertook a wide study of many
complexes of an isocyanide ligand bearing fluorinated chains
(CNR, R¼C6H4O(CH2)4C8F17).710 Some of them were found
to be mesomorphic: [MCl(CNR)] (M¼Au, Cu), [Ag(CNR)2]X,
and trans-[MI2(CNR)2] (M¼Pt, Pd). The authors claimed that
the fluorophobic effect enhanced the microsegregation as com-
pared to their hydrocarbon analogs. Thus, the mesophases
found (SmA) were stable over a wider temperature range and
presented higher transition temperatures.
In a very different and innovative way, Coco et al.
recently reported a series of hybrid organic–inorganic room-
temperature liquid crystals (Colh).711 These materials (113)
were based on a disk-like organic molecule (2,4,6-triaryla-
mino-1,3,5-triazine), linked, only through hydrogen bonds,
to an isocyanide-containing organometallic fragment ([M(CO)n(CNC6H4CO2H)]: M¼Fe, n¼4; M¼Cr, Mo, W, n¼5).
N
NN
N
N
N
H
H
H
C10H21O
C10H21O
C10H21O
C10H21O
OC10H21OC10H21
OC10H21
OC10H21
OC10H21
NO
OHC [M] [M] =
Fe(CO)4
Cr(CO)5
Mo(CO)5
W(CO)5
113
Kaharu et al. investigated themesomorphism of platinum (II)
acetylides. At least three aromatic rings attached to the rigid core
were needed to induce liquid crystallinity in the materials
(114a, b: X, Y¼CO2, O2C, with X¼Y and X 6¼Y). Both
O PtPR
PR
PR3
PR3
OCnH2n+1O
CnH2n+1O X Pt
114
symmetrical and unsymmetrical species formed smectic phases
when the substituent of the phosphane was a methyl group
(R¼Me),712,713 while when triethylphoshine was used instead
(R¼Et),714 nematic phases were observed due to the decrease
in molecular anisotropy. However, Bruce and coworkers stud-
ied later more elongated (six-ring) square-planar PtII and octa-
hedral RhIII trans-bis(acetylides)715–717 and concluded that
increasing the volume of the phosphane reduced the transition
temperatures but complexes of larger phosphanes (such as
PPr3) lacked mesomorphism.
A few examples of mercury(II) acetylides have also been
reported, including calamitic,718,719 polycatenar,720 and the
elegant discotic triphenylene–ethynyl complexes depicted as
115 (R¼OC5H11, O(CH2)2CHMe(CH2)3CHMe2).721
C5H11O
C5H11O
OR
OC5H11C5H11O
Hg
RO OC5H11
OC5H11
OC5H11C5H11O
115
Finally, it is worth mentioning the study of mixed isoni-
trile–acetylide gold(I) complexes.722,723 Different calamitic
systems were explored, as illustrated by the examples 116–
118. A general observation was that this kind of material
tended to decompose on heating; thus, many of the complexes
showed a small stability in the liquid-crystal state.
OCnH2n+1NAuH2m+1Cm
116
EtO
OC10H21NAuCnH2n+1
O
O
117
O Au N C8H17
CnH2n+1OO
118
a
b
3
3
OCmH2m+1
OCnH2n+1Y
Metallomesogens 871
8.21.6.3.11 Miscellaneous organometallic systemsThe study of bis(phenylene)mercury(II) complexes (119)724 was
one of the first investigations of metallomesogens to be carried
out, almost a century ago by Vorlander. These materials were
described to form smectic phases at elevated temperature. Only a
few more similar examples have been reported later, containing
mercury725,726 or the metallic units trimethylgermanium(IV) and
trimethyltin(IV).727 Note that extensive work by Thurmes et al.
exploring varied, new germanium-containing liquid crystals was
published recently.728 Figure 31 shows some of these organoger-
manium metallomesogens (120–124).
Only a few complexes of carbenes have been found to be
mesomomorphic. While Takahashi and coworkers synthesized
some calamitic carbene derivatives with gold(I) (125a)729,730
and platinum(II) (125b),731 Lin reported a series of gold(I)com-
plexes based on heterocyclic carbenes (126a: R¼CmH2mþ1,
CH2CH(OH)CmH2mþ1) with smectic behavior,732 and a dimeric
silver(I) derivative that showed an SmA phase when mixed in a
1:1 ratio with the corresponding imidazolium salt (126b).733
125a
CnH2n+1O O
ONH
OCmH2m+1Au
CI
N Hg NR
R
119
R1
N
NX X
O (CH2)n GeR3
R1 = –OCmH2m+1, –SCmH2m+1,
R3 = –CmH2m+1, –OCmH2m+1
N
N
X = H, F
R = Me, Et
X X
X X
O
X X
R3
R2
O
OX X
O (CH2)n GeR3
(CH2)n GeR3
O (CH2)n GeR3
(CH2)8 GeR3
H2m+1Cm
C4F9C4H8O O
O
–O2C–C6H10–CmH2m+1,
–OCmH2mCpF2p+1,
R2 = –C6H4–C6H4–C8H17,
–O(CH2)mCH=CHCpH2p+1,
–OCH2C*HFC5H11
–OCmH2mCpF2p+1,
–OCH2C*HFC*HFC5H11
120
121
122
123
124
cr
Figure 31 Examples of the germanium-containing, organometallic liquidHN
Pt
HN
NH
O
C8H17
C8H17
II
125b
126a
CnH2n+1
NN
N(NO3)
R
NAu
CnH2n+1
126b
CI
C16H33
C16H33
N
NNAg
C16H33C16H33
C16H33 C16H33
AgN
N CI
CI
N
ystals described by Thurmes et al.
872 Metallomesogens
Some organometallic p-complexes have been developed,
for instance, an interesting series of butadienetricarbonyliron
(0) derivatives (127)734–737 showed chiral mesophases
(SmA* and N*) after resolution, due to the planar chirality
induced by the organometallic fragment. In a different way, a
triangular palladium(0) derivative based on a triolefinic
macrocycle (128)738 was found to show a columnar hexago-
nal phase (54–72 �C).
O
O
OCmH2m+1Fe
OCCO
CONCnH2n+1O
127
N
NN
Pd
SO2
OC12H25
OC12H25
SO2
C12H25O
C12H25O
OC12H25
C12H25O
SO2
128
Similarly, calamitic739,740 and discotic (129)741 Z6-benzene
complexes of tricarbonylchromium(0) were reported. Curi-
ously, despite the presence of the bulky fragment Cr(CO)3
RuO
C2H5OO
O(CH2)2O(CH2CH2O)16Me [CF3SO3]-
+
Fe
O(CH2)n
O
OO
O
H3CO
133
FeO
OOC16H33O
O
134
Figure 32 Representative examples of ferrocene-containing liquid crystals.
130
coordinated to the triphenylene unit, the material could form a
ND mesophase between 37 and 58 �C.
CrOC
CO
OC9H19
OC9H19
OC9H19
C9H19O
C9H19O
C9H19OCO
129
In another remarkable example, an important stabilization
of an SmB phase (44–108 �C) was induced by the coordina-
tion of the fragment Cp*RuII to a long organic mesogenic
molecule (130).742
8.21.6.4 Ferrocene-Containing Metallomesogens
The first reported mesomorphic materials containing a ferro-
cene fragment were synthesized by Malthete and Billard (i.e.,
131),743 and they showed nematic mesophases. After that
pioneering work, not only mono-substituted derivatives have
been published,744–752 but also many di-753–767 and a few tri-
substituted768 materials have been explored. Representative
examples of each of these classes of derivatives are shown in
Figure 32 (131,743 132,764 133,767 and 134768). It is notewor-
thy that, in addition to the common parameters such as
the nature, number, and length of the terminal chains, the
distribution of the substituents around the cyclopentene
group can influence the mesomorphic properties significantly.
O
OO
O
O
OCH3
O
OO OC16H33
OC16H33
O
O
O
OO
(CH2)n
Metallomesogens 873
For instance, the mesogens with the ferrocene unit substituted
in the 1,10-positions exhibited more stable and varied
mesophases than the related mono-substituted derivatives,
while, for example, none of the complexes with a
1,2-disubtituted-ferrocene were mesomorphic. Note that
more systematic reviews concerning this and other aspects
of ferrocene-containing mesogens have already been
published.9,20,769
O
O
NFe
OCnH2n+1
O
O
131
In a similar way, the introduction of different substituents
at the 1,3-positions of one of the Cp rings generated a molecule
with planar chirality (Figure 33, 135).770,771 This complex
exhibited ferroelectric behavior on resolution, and SmC* and
SmA* were seen. Chirality could also be induced by using
chiral substituents,745,772 and remarkably one mono-
Fe
O
O
CnH2n+1OO
O
132
R2
R1
Fe
O
O Fe
H37C18OO
O
135
Figure 33 Unsymmetrical 1,3-disubtituted-ferrocene.
Fe
NR
N
R
1
substituted ferrocene derivative773 showed SmC*, SmA*,
TGBA, N*, and blue phases.
Functionalized ferrocenophanes have been very useful to
understand the mesomorphism of this kind of materials.774,775
Due to the presence of a short linking group between the Cp
groups, these complexes must adopt a U-shape. These studies
confirmed that this conformation was also able to induce
mesophase formation, and, for example, the complex 136a
formed a nematic phase, while the elongated 136b showed
an unidentified smectic phase.775
In addition, a few studies about redox processes on these
ferrocene-containing materials have been carried out. An
interesting study developed by Deschenaux et al. describes
how nonmesomorphic ferrocenes with a rod-organyl frag-
ment attached turned into liquid crystals when the system
was oxidized to ferrocenium with silver tosylate.776,777 This
suggested that the formation of a mesophase could depend
not only on structural factors but also on ionic interactions.
In a similar way, the same group reported the ease of the
oxidation (with I2) of a side-chain liquid-crystal polymer
containing an alkylated ferrocene to the corresponding
ferrocenium polymer (137).778 In this case, the reduced
O
O
OCnH2n+1
O
O
FeR2
R1
O
O
O
O
OC10H21
R
OC12H25
O2C OC12H25
a
b
36
874 Metallomesogens
species showed smectic phases, while the oxidized one
formed nematic phases.
Fe
C8H17
C8H17
O
OO
3
(CH2)6CO2
CCH2
CH3
x
I3137
+
-
8.21.6.5 Liquid-Crystalline Metallodendrimers
Most of the work on dendrimers is based on purely
organic molecules. There is, though, a growing interest on
metallodendrimers, mainly due to specific properties that can
be induced or enhanced by the introduction of metal centers in
dendrimeric systems. However, not so many investigations
have been focused on mesomorphic metallodendrimers.779,780
A series of complexes was prepared by coordinating metal
fragments to classic first- (138)781 or second-generation
(139)782 dendritic ligands. Presumably, these complexes adopt
a disk-like or a cone-like shape, with the metal located in the
center of the molecule. Two possible geometries for the metallic
cores (MX2¼CoCl2, NiCl2, CuY2, Y¼Cl, SCN, NO3) were
NNH
HN
NH
C10H21O
C10H21O
C10
C10H2
C10H2
C10H21O
OC10H21
OC10H21
OC10H21
OC10H21
138
N M
N
N
N
X
H
CH2R
HRCH2
RCH2
H
+
X -
Figure 34 Possible geometries for metallodendrimers based on ligands 138
proposed (Figure 34). Moreover, the presence of bis(alkoxy)
phenyl terminal groups attached to the ligands allowed
the metallodendrimers to exhibit liquid crystallinity. Both
systems formed Colh phases, but the second-generation
dendrimer showed a wider mesomorphic temperature range
(49–140 �C).In an opposite way, the coordination of copper(II) centers
to salicylaldimine-based dendrimeric molecules led a series of
materials with a lamellar behavior (SmC).522,783 Dinuclear
first-generation and tetranuclear second-generation (140) den-
drimers were achieved, but they behaved as liquid crystals only
when long alkoxyl chains were being used (n¼18). Some
other mononuclear oxovanadium(IV)565 and copper(II)784
salicylaldimine-based dendrimers were found to exhibit a sim-
ilar mesomorphism.
A nice piece of work was developed by Deschenaux and
coworkers by studying ferrocene-containing dendrimers.785,786
For example, an elegant second-generation dendrimer (141)787
was described to form a wide-range SmA phase from 52 to
169 �C. Interestingly, a dendrimer (142)788,789 was prepared
containing ferrocene units (electron-donor character) and a
fullerene group (electron-acceptor character). Thus, it was pos-
sible to induce intramolecular photo-electron transfer in this
material in solution (organic solvents).
Very recent examples of metallodendrimeric liquid crystals
were obtained by Cordovilla et al.790 This work described the
H21O
1O
1O
OC10H21
OC10H21
OC10H21
OC10H21
OC10H21
OC10H21
OC10H21
OC10H21
NN
N
N
NHHN
NH
HNHN
NH
O
O
O
O
O
O
139
CH2R
CH2RRCH2
N
M
N N
X
N
X
H
H
H
and 139.
N
ON
OCuN
O
O
N
ON
OCuN
O
O
N
N
ON
OCu N
O
O
N
ON
OCu N
O
O
N
O
O
O
O
O
OO
O
CnH2n+1O
CnH2n+1O
CnH2n+1O
CnH2n+1O
OCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1
140
O
O
OO
O
O
O
O
O
O
O
O
OO
O
O
O
O
O
O
O
O
O
O
OO
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O O
O O
O
O
O
O
O
O
O
O
O
OO
O
O
O
O
O
CO2CholOCO2 (CH2)10
CO2CholOCO2 (CH2)10
CO2CholOCO2 (CH2)10
CO2CholOCO2 (CH2)10
CO2CholOCO2 (CH2)10
CO2CholOCO2 (CH2)10
CholO2C
CholO2C
O O2C
O2C
(CH2)10
Chol =
O (CH2)10
CholO2C O2CO (CH2)10
CholO2C O2CO (CH2)10
CholO2C
CholO2C
O2C
O2C
O (CH2)10
(CH2)10O
Fe
FeFe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe Fe
141
Metallomesogens 875
synthesis and mesomorphism of zeroth-, first- (143), and
second-generation gold(I) dendrimers based on a poly(phenyl
ether) scaffold (Figure 35: only the first-generation materials
are depicted, 143). Depending on the dendritic generation, tri-,
hexa- (143), and dodeca-nuclear complexes were prepared by
complexing AuX moieties (series a, b, and c) through the
isocyanide group at the end of each branch. Interestingly, the
type of mesophase observed depended on the substituent
CO2CholOCO2(CH2)10
CO2CholOCO2(CH2)10
CO2CholOCO2(CH2)10
CO2CholOCO2(CH2)10
O
O
O
O
O
O
O
O
O
O
O
O
CO2
CO2
CO2
CO2
O2C (CH2)6 (CH2)6O CO2O2C
O
O
O
CO2
O
O
O
O
O
O
O
O
OO2C
O2C
O2C
O2C
O
CholO2C O O2C(CH2)10
CholO2C O O2C(CH2)10
CholO2C O O2C(CH2)10
CholO2C O O2C(CH2)10
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
142
X
143
a Cl
C C OC12H25
OC12H25
OC12H25
OC12H25
C C
b
c
N
AuX
C O
O
O
O
11
O
O11
N
Au
X
C
N
Au
X
C
O
O
O
O
N
AuX
C
O
O
11
N
AuX
C
O
O
( )
( )( )
( ) ( )
( )( )
( )
O
O
N
AuX
C
O
O
11
11
11
11
11
11
Figure 35 First-generation mesomorphic gold(I) dendrimer.
876 Metallomesogens
attached to the gold center, while its stability was related to the
size of the dendrimer. Thus, the chlorido (series a) and p-dode-
cyloxyphenyl-acetylide (series b) derivatives formed smectic
phases, while the dendrimers with the tri(dodecyloxy)phenyl-
acetylide substituent showed Colh mesophases; moreover, the
increase in the dendritic generation contributed to a strong
stabilization of the mesophases.
8.21.6.6 Miscellaneous
The liquid crystallinity of a series of pincer-type complexes of
square-planar palladium(II) and nickel(II) was explored by
Espinet et al. (Figure 36, 144: M¼Pd, Ni).791,792 SmC and
nematic mesophases were observed for varied species contain-
ing either an alkoxyl (R¼OCnH2nþ1, n¼4, 8, 12) or a thioalk-
oxyl (R¼SCnH2nþ1, n¼4, 8, 12) chain attached to the 2,6-bis
(thiocarboxylate)pyridine group, and different rod-like pyri-
dines or isocyanide ligands. The corresponding chiral versions
of these mesophases (N* and SmC*) were also observed when
chiral chains were present in the system. Curiously, only one
mesomorphic PdII complex of this kind was found when bear-
ing the related 2,6-dipicolinate group, but a three-chained
pyridine-like auxiliary ligand (L) was required to induce liquid
crystallinity (Colh).
R N M
S
S
O
O
L
OC10H21
OC10H21
OC10H21
OC10H21
OC10H21
N
NN
ON
O
NO
NC
1,2
L
144
Figure 36 Pincer-type metallomesogens of palladium(II) and nickel(II).
Metallomesogens 877
Similarly, Serrette and Swager reported a series of meso-
morphic complexes of pyridinediyl-2,6-dimethanolato bearing
a trialkoxybenzylic mesogenic moiety with dioxomolibdenum
(VI), which showed columnar phases (145: n¼10, 12,
14, 16).549
O N Mo
O
O
H2n+1CnO
H2n+1CnO
H2n+1CnO
O
O
145
Morale et al. explored tri- and tetra-dentate N-donor
ligands in order to yield mesomorphic materials. In this
sense, a series of six-chained, half-discotic complexes (146:
M¼Zn, Co, Mn, Ni) were synthesized in a one-pot con-
densation of 2,6-pyridinedicarboxaldehyde with 2 equiv. of
3,4,5-trialkoxyaniline, in the presence of 1 equiv. of MCl2.793
The complexes exhibited a rich columnar mesomorphism
(Colr, Colh, and Colo) over wide temperature ranges. In
contrast, lamellar behavior was observed for a dimeric zinc
(II) derivative (147) and a palladium(II) complex (148),
both based on the same tetradentate ligand.794 Remarkably,
while the square-planar palladium complex was a monomer
and adopted a hairpin conformation, the dimeric zinc deriv-
ative presented a double-stranded helical structure and a
distorted tetrahedral geometry around the metallic center.
Note that several studies involving mesomorphic metallohe-
licates have been published,795–797 most of them based on
tetrahedral copper(I) complexes and N,N-donor chelate
ligands.798–800
NN N
OCnH2n+1
OCnH2n+1
OCnH2n+1H2n+1CnO
H2n+1CnO
H2n+1CnO
M
Cl Cl
146
N N
N N
N N
N NZn Zn
OC16H33OC16H33
OC16H33OC16H33
H33C16OH33C16O
H33C16OH33C16O
147
N N
N N
OO
OC16H33
Pd
OO
H33C16O
148
8.21.7 Ortho-Metallated Metallomesogens
8.21.7.1 Ortho-Metallated Palladium(II) and Platinum(II)Complexes
Ortho-metallated complexes represent an interesting and
broadly studied class of metallomesogens and, in particular,
square-planar PdII and PtII complexes of this type have
attracted a great deal of attention because of their interesting
and versatile catalytic and photophysical properties. Combin-
ing the properties of these attractive systems with liquid crys-
tallinity is therefore of interest as the properties of the complex
core can be modified, and sometimes enhanced, through the
order and fluidity inherent to mesophases.
Because of the very wide range of metal- and ligand-based
modifications possible with ortho-metallated complexes, such
materials have been selected for a more detailed treatment to
show what is possible.
Table 1 Thermal behavior of complexes 149 ( R¼R1¼OC6H13)and 151
Type Mesomorphism
149, X¼Cl Cr 213 N 221 E 235 dec.149, X¼Br Cr 215 (N 196) I149, X¼ I Cr 211 I149, X¼N3 Cr 186 SmA 190 dec.149, X¼SCN Cr 220 I149, X¼OAc Cr 140 I151 Cr 238 N 246 SmA 290 dec.
From Bruce, D. W.; Deschenaux, R.; Donnio, B.; Guillon, D. Metallomesogens. In
Comprehensive Organometallic Chemistry III; Crabtree, R. H., Mingos, D. M. P., Eds.;
Elsevier: Oxford, 2006; Vol. 12, pp 195–294.
878 Metallomesogens
8.21.7.1.1 Ortho-metallated azo, azoxy, and azinecomplexesOne of the first contributions to the study of metallomesogens
in general, and the ortho-metallated systems in particular, was
the synthesis of the mesogenic palladium(II) derivatives of
azobenzenes by Ghedini and coworkers (149, 150). This repre-
sented as well the first systematic attempt to coordinate metals
to previously known liquid-crystalline organic materials.801
The dinuclear complexes (149) and the related mononuclear
derivatives (150) (R¼OEt and R1¼C4H9CO2, C6H13CO2,
CH2¼CH(CH2)8CO2; X¼Cl; L¼PPh3, pyridine, quinoline,
aniline) were investigated in initial studies,802 showing that
the metallation of the nonsymmetrical ligands occurred always
in the more electron-rich ring. All dinuclear complexes (149)
showed an enantiotropic nematic phase at elevated tem-
perature (165–210 �C) and clearing points in the range
185–215 �C, while the free ligands showed a low-temperature
nematic phase below 125 �C. The role of the bridging halide
was also investigated (149: X¼Cl, Br, I; R¼OEt and
R1¼C6H13CO2)803 and it was found that the melting point
increased in the order Cl<Br< I and that the temperature at
which the nematic phase first appeared increased according to
the same order. On the other hand, none of the mononuclear
derivatives (150) with PPh3 or aniline were mesogenic, while
the pyridine (N,N-cis) and quinoline (N,N-trans) complexes
showed nematic and smectic phases (L¼pyridine: Cr 180 SmB
198 N 235 I; L¼quinoline: Cr 136 SmA 151 N 180 I).
N
R
N
R1
R1
PdN
N
R
PdX
X
R1
NN
R
PdX
L
149 150
The fact that complexation enhances mesophase stability
was demonstrated in further studies of dipalladium(II)
complexes (149) with 4-alkoxyazobenzenes and 4-alkyl-40-alkoxyazobenzenes (X¼Cl; R¼H, CnH2nþ1; R1¼OCm
H2mþ1)804–809 or 4,40-dialkoxyazobenzenes (X¼Cl; R¼OCn
H2nþ1; R1¼OCmH2mþ1).
810–814 All the complexes were found to
exist as equimolecular mixtures of cis- and trans-isomers. Enantio-
tropic nematic phases and, in some cases, additional smectic
phases (SmA and SmC) were observed. In general, for short-
chain-length complexes (nþm¼2–8) the mesophases were
obtained above 200 �C, whereas for the derivatives with long
alkyl and/or alkoxy chains (nþm�13) the melting points
reported are in the range 130–180 �C and the clearing points
around 160–200 �C.In one of these studies (149: R¼R1¼OC14H29),
810 the use of
a chiral acetate bridge (X¼CH3ClC*HCOO) instead of a halide
groupwas found togive rise to a ferroelectric smectic phase (Cr 67
SmC* 84 SmA 91 I). Other ways of inducing ferroelectric meso-
phases consisted of incorporating chiral alkoxy substituents such
as (R)-(�)-2-octanol and (S)-(�)-b-citronellol chains (149:
X¼Cl, I; R¼OCnH2nþ1, n¼7, 10, 12, 14; R1¼OCH2CH2CH2
C*HMeCH2CH2CH¼CMe2, OC*HMeC6H13).811
A complete series of cyclopalladated dimers (149) with var-
ious bridging systems (X¼Cl, Br, I, N3, SCN, SCN, OAc, and
oxalate (151)) and the mesogenic ligand 4,40-di(hexyloxy)azo-benzene (Cr 108N 116 I) has been prepared in order to evaluate
the effectiveness of the bridging group in promoting
mesophases.812,813 All complexes, except the acetate-bridged
one, are planar and in their trans conformation, as can be
observed in the crystalline structures of homologous com-
pounds. For its part, the complex containing the acetate bridging
group possessed a sort of ‘roof-shape’ and existed as a cis:trans
mixture.814 This study revealed thatmesomorphismwas favored
for chlorido, bromide, azido, and oxalato (151) complexes, but
not for the iodido, thiocyanato, or acetato derivatives (Table 1).
The presence of the nematic phase below the more ordered
smectic phases is rather surprising. While such re-entrant behav-
ior is known and possible, the authors tentatively explained this
observation by the dissociation of molecular pairs into single
molecular species of different mesomorphism. In the case of the
oxalato complex, the nematic phase was transient and never
reappeared on successive heating–cooling runs; the extensive
decomposition of the chloride did not allow such an experiment
to be carried out.
NN
Pd
OC6H13
OC6H13
OC6H13
OC6H13
O O
O ON
N
Pd
151
The reaction of the dinuclear chlorido-bridged complexes
(149) with various anionic chelating ligands allowed the
preparation of low-melting mononuclear ortho-palladated
metallomesogens. Thus, complexes (152) combine 4,40-bis(alkoxy)azobenzene (n¼m¼6) with chelating ligands X–Y
such as the O,O-monoanionic acetyl acetonate 1,1,1,5,5,
5-hexafluoro-2,4-pentanedionato anion (hfac), and tropolo-
nate, the N,O-2-aminophenate and 2-amino-2-methyl-1-
Metallomesogens 879
propanoate,815 as well as cyclopentadienyl816 ligands were
prepared. However, none of these materials were mesomor-
phic, probably due to the bulkiness of the co-ligands relative
to the low anisotropy of the complex. In some cases, lumi-
nescence was induced by cyclopalladation.817 In contrast,
some related palladium(II) complexes with an acac co-ligand
and a chiral chain (152: n¼7, 10, 12, 14; OCmH2mþ1¼(R)-
(�)-2-octanol and (S)-(�)-b-citronellol) or amino-acid-
chelated palladated complexes (152: n¼m¼14) exhibited
low-temperature N* and SmA* or SmC* mesophases,
respectively.818 Later, other related complexes (152) bearing
these same chiral chains in the ortho-metallated azobenzene
group, and with the chelate X–Y being either a nonchiral N-
[40-(dodecyloxy)resorcylidene]-4-alkylaniline or a chiral N-[40-(dodecyloxy)resorcylidene]-4-alkoxyaniline, were prepared.
They showed chiral mesophases but only over a narrow temper-
ature range.819
NCnH2n+1ON OCmH2m+1
Pd
XY
152
With the aim of preparing cyclometallated azobenzene
complexes with reliable thermal behavior and improved meso-
morphic properties, the anisotropy of these complexes was
enhanced by using elongated azobenzenes. An early example
of such ortho-palladated complexes was reported by Hoshino
et al. in 1991, containing a three-ring azobenzene group (153:
n¼1–10, 12, 14, 16, 18) in a dinuclear chlorido-bridged
derivative.820 In common with the parent ligands, the com-
plexes were nematic, although melting and clearing points
were raised by 60 and 102 �C, respectively, on complexation,
giving the complexes a much wider nematic range.
NN
PdO
C2H5O
Cl Cl
NN
PdO
OC2H5
OCnH2n+1CnH2n+1O O
O
CH3OO
OOCH3
153
Ghedini and coworkers prepared mesomorphic three- and
four-ring azobenzene ligands and the corresponding
ortho-metallated dinuclear and mononuclear (154: n¼8, 7:
n¼6, 10) complexes containing cyclopentadienyl,816,821
acac,822 tropolonate, and quinolate823 ligands and palladium
(II) and platinum(II) as metallic centers. The new mononu-
clear organometallic species were found to show nematic
phases with clearing temperatures lower than those of the
parent ligands (154: 128–264; 7: 226–224 �C), while the
chlorido-bridged dinuclear complexes presented mesophases
at elevated temperatures (206–263 to 275–342 �C). In these
studies, the first examples of mesomorphic complexes of octa-
hedral PtIV were obtained from the oxidative addition to the
mononuclear square-planar platinum(II) complexes (154,
155) by I2 and MeI (156: R¼OCnH2nþ1, n¼6, 10, X–Y¼acac;
R¼O2C-C6H4-OC8H17, X–Y¼ tropolone, quinolate, hfacac,
A–B¼ I–I, Me–I). These materials presented nematic and
SmC phases, with lower melting points for the MeI derivatives
(108–147 �C) or higher for the I2 analog (125–250 �C) whencompared with the related PtII complexes.
N
CnH2n+1O
N
O
MX
Y
O
OC8H17
OC8H17
OC8H17
OC8H17NN
O
MY
X
O O
O
N
R
N
O
PtX
Y
O
A
B
154 155 156
In addition, the same research group described two unsym-
metrical cyclopalladated complexes (157, 158), but only the
one with longer alkoxy chains and a chiral substituent was
found to be mesomorphic with chiral nematic and SmC phases
appearing at relatively low temperatures (158: Cr 67.1 SmC*
76.7 N* 82.6 I).824,825 These latest compounds were also inves-
tigated as photorefractive materials; thus, substantial responses
were measured in both doped and undoped samples, as pure
materials and as materials dispersed within polymers.826,827
NN
PdN
O
C6H13
OC6H13
NN
PdN
O
OCit*
OC12H25
C14H29O
OCit*
Cit* = citronellyl
157 158
The exploration of mesogens with metallated azoxybenzene
systems began in 1992, when Ghedini reported and patented a
mononuclear cyclopalladated 4,40-bis(hexyloxy)azoxybenzeneacac derivative (159).828 This complex, named Azpac, showed a
N phase between 90 and 105 �C, with temperatures compara-
ble to that of the free azoxy ligand (Cr 80 N 126 I) and was
investigated extensively over following years in order to ana-
lyze some specific properties such as electric829–833 and dielec-
tric834–837 properties, conductivity,838 viscoelasticity,839 and
dynamics.840–842 It is also worth mentioning that, in 1994,
Omenat and Ghedini reported the synthesis and the room-
N
Pd
NN
C
R�
R�
Pd
N
O O
O OC
OR
OR
OR
OR
Figure 37 ‘Open book’ shape observed in the acetato-bridged,azobenzene derivatives. From Bruce, D. W.; Deschenaux, R.; Donnio, B.;Guillon, D. Metallomesogens. In Comprehensive OrganometallicChemistry III; Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier: Oxford,2006; Vol. 12, pp 195–294.
880 Metallomesogens
temperature liquid crystallinity (SmC*) of chiral azoxymercury
(II) complexes (160: n¼6, 10),843 obtained in the form of a
1:1 mixture of two isomers (A and B).
NC6H13ON OC6H13
PdO O
O
159
NH2n+1CnON OC*H(CH3)(C6H13)
Hg
Cl(a)
(b)
O
N(C6H13)(CH3)HC*ON
Hg
Cl
O
OCnH2n+1
160
Othermononuclear PdII complexes (161) with azoxybenzene
groups had been prepared, all of them containing an additional
mesogenic 2-hydroxy-azobenzene (161a: X¼N)844,845 or N-
(salicylidene)-aniline (161b: X¼C)496,846 ligand with alkylic
and/or alkoxylic chains (161a: R¼CnH2nþ1, n¼1–4, 6, 8;
161b: R¼CnH2nþ1, n¼0–4, 6, 8, R¼OR*, R*¼(R)-(�)-2-
octyl, (S)-(�)-b-citronellyl). In the first type of complex (161a),
monotropic mesophases were most commonly observed. How-
ever, SmA and nematic behavior seemed promoted in the second
series (161b). Particularly, the incorporation of the chiral chains
led to the formation of a chiral crystal H* phase in both imine
derivatives bearing the citronellol (Cr 107.2 H* 148.9 I) or the
2-octanol chain (Cr 113 H* 132.3 I), respectively.
NC6H13ON
Pd
N
XOC12H25
R O
OC6H13
O
161a: X = N
161b: X = C
Espinet et al. reported the first series of dinuclear PdII com-
plexes of symmetric azines (162) in 1991.847 They were pre-
pared with different bridging systems, and a mixture of the cis
and trans (X¼SCN, OAc) or only the trans isomer (X¼Cl, Br)
was found, depending on the bridge. For the non-acetato-bridged
dimers, the only mesophase seen was SmC (100–250 �C, forn¼10), whereas for the acetato-bridged complexes a nematic
phase was seen for 6�n�8 and for n�7 and an SmCphase was
also seen (between �100 and 160 �C). In each example, except
when X¼OAc, the complex was assumed to be planar. How-
ever, the acetato derivative was assigned to an ‘open book’ shape
(Figure 37) due to the optical activity observed in 1H NMR
studies (NMR, nuclear magnetic resonance). Regarding this
last result, a derivative was synthesized using the optically pure
(R)-2-chloropropionate (162: X¼O2CC*HMeCl, n¼10).848
The resulting material was found to be a mixture of isomers
(trans-LR,R, 34%; trans-DR,R, 34%; cis-R,R, 32%) and to have
the phase sequence Cr 102 SmC* 119 SmA* 149 I. Interestingly,
the SmC* phase was ferroelectric showing a slow rise time of
�330 ms at a square wave voltage of �17 V and 0.5 Hz, and a
cell thickness of 11 mm. Probably due to the high viscosity of
this chiral phase as a consequence of the molecular shape, the
response time was around three orders of magnitude longer
than those found in standard calamitic SmC* materials. Zhang
et al. extended later the study to a series of (S)-2-
chloropropionate from n¼6 to 16810,849 and revealed a signif-
icant increase in themesophase temperature range by decreasing
the melting point. Another factor also investigated was the
length of the bridging carboxylate (162: X¼O2CCmH2mþ1,
m¼0–11, 13, 15, 17, n¼10)850,851 leading to the conclusion
that the mesomorphic range decreased rapidly as the chain
length was increased, dropping from�40 �C to almost nothing,
although transition temperatures stabilized at around 100 �C,revealing the important perturbation brought to the lateral
molecular packing by the carboxylate.
162
CnH2n+1O
CnH2n+1O
OCnH2n+1
OCnH2n+1
N
N N
Pd
Pd
XX
N
N
O O
O
O O
N
OO
O
OO
Pd PdX
X N
O O
O
O O
N
OO
O
OO
Pd PdX
X
164a: X = OAc 164c: X = OAc
164b: X = Cl 164d: X = Cl
Metallomesogens 881
8.21.7.1.2 Ortho-Metallated imine complexesOrtho-palladated mesogenic imine compounds have also been
systematically and broadly studied, together with rather fewer
platinum(II) examples. In a similar way to the systems just
described, bothmono- and di-nuclear complexes were prepared.
In 1987, the first examples of imine-contaning metallome-
sogens were reported based on dinuclear cyclopalladated com-
plexes with various bridging groups (163: X¼OAc, Cl, Br,
SCN; Z¼H: R¼C10H21, OC10H21, R1¼OC10H21; Z¼Me:
R¼C10H21, R1¼OC10H21).852 Mixtures of cis–trans-isomers
were obtained when using thiocyanate, while only trans-
compounds were found for the acetato- and halo-bridged
derivatives. Except for the nonmesomorphic OAc complexes,
SmA phases were observed for the remainder along with an
additional SmC phase for two examples where X¼Cl. Wide
mesophase ranges of 80–100 �C were observed for the halide
derivatives (clearing points above 200 �C), but shorter ranges(30 �C) and higher transition points were observed for the
thiocyanato-bridge compounds. Interestingly, the lateral
methyl group (Z) did not lead to significantly different meso-
morphic behavior.
NRPd
NR1
R1
PdXX
R
Z
Z
163a: Z = H
163b: Z = Me
Several later studies investigated the influence of the chain
length (163a: X¼Cl, R¼R1¼OCnH2nþ1, n¼2, 4, 6, 8,
10),810,853 the chain type (163a: X¼Cl, Br; R/R1¼OCnH2nþ1/
CmH2mþ1, n¼1, 2, 6, 10, m¼2, 6, 10),854 and the presence of a
polar group either in the cyclometallated ring (163a: X¼Cl, OAc;
R¼C8H17, OC8H17; R1¼H, F, Cl, Br, CN, NO2, Me, OMe, CF3,
OCOMe, OCOC6H5, CO2Me)855 or in the aniline ring (163a:
X¼Cl, OAc; R1¼OC8H17; R¼H, Cl, CN,NO2, Me, OMe856; and
R1¼OMe, R¼C4H9857). The conclusions can be summarized as
follows: (1) none of the acetato-bridged systems were mesomor-
phic, while SmC and SmA phases were seen for the chlorido-
bridged derivatives, and SmA phases (and N for a few cases) for
the bromide analogs; (2) the longer the terminal chains, the
lower the melting points and the more ordered the mesophases
(smectic phases in place of the nematic phase); (3) the complexes
with solely alkoxy chains hadmore stablemesophases than those
with both alkyl and alkoxy chains; and (4) in the study of the
examples with a polar substituent, its location did not affect the
mesomorphic behavior, that consisted of an SmA phase (above
140 �C) for all the complexes except R¼H, and an additional N
phase for those with the cyano group or the shortest chain.
The use of imines, functionalized with one or two
chiral 2-octanol chains, allowed the preparation of new
ortho-palladated dinuclear metallomesogens (163a: X¼Cl, R/
R1¼ OCnH2nþ1/OC*HMeC6H13, n¼6, 8, 10, 14; OC*H-
MeC6H13/OC8H17)858,859 exhibiting a ferroelectric SmC*
phase along with an SmA phase. This way, Lopez de Murillas
et al. reported later some isomerically related dinuclear com-
plexes with the same chiral 2-octanol chains (164).548 The
corresponding parent ligands showed a very rich mesomorph-
ism with SmA*, SmC*, andmonotropic antiferroelectric SmC*
and SmI* phases (SmC*A, SmI*A). Interestingly, the m-Cl com-
plexes (164b, 164d) retained the SmA* phase of the ligand,
but, in addition, showed a monotropic SmC* (164b) or an
enantiotropic SmC*A phase (164d). Both complexes decom-
posed in their SmA* phases between 240 and 250 �C, but thespontaneous polarization of 164d was measured to be
15 nC cm�2 at 35 V mm�1 in the SmC*A phase.
Espinet’s group has been undertaking a thorough study of the
influence of different bridging systems in promoting meso-
morphism in dipalladated dialkoxybenzylidene complexes. In
addition to the complexes described before, many examples
were reported containing a symmetric bridge (m-X)2 (Figure 38,
165a: X¼OH, O2CR, NHR, SR, oxalate)853,860–862 as well as a
mixed-bridge m-X/m-Y (Figure 38, 165b: X/Y¼Cl/SR, OH/NHR,
O2CR/SR,NHR/SR,O2CR/NHR),861–865 with R being alkyl (–Cm
H2mþ1), alkoxyl (–OCmH2mþ1), (benzyl)alkoxyl (–C6H4–
CmH2mþ1), or oligo(ethyleneoxide) (–CH2(OCH2CH2)mOCH3)
chains of different lengths. Themesomorphic behavior found for
these species depends mostly on the overall combination of
chain lengths present in both cyclometallated and bridging
groups (the longer the chains, the lower the melting points and
the more ordered the mesophases). In general, SmA, SmC, and,
in fewer situations, nematic phases were observed. Remarkably,
one of the complexes with a m-carboxylato/m-thiolato bridge
(165b: n¼6, X/Y¼SC6H13/O2CC*HClMe)864 resulted to be
the first metallomesogen to show a chiral nematic phase, and
later, another similar complex (165b: n¼2, X/Y¼SC18H37/
O2CC*HClMe)865 exhibited a monotropic blue phase (BPI)
along with a N* phase. In 2010, two azocarboxylato-bridged
complexes (Figure 38, 165c: R¼H, OC10H21)862 were described
NPd
Y
X
N
PdPd
X
X
CnH2n+1O
CnH2n+1O
CnH2n+1O
CnH2n+1O
CnH2n+1O
CnH2n+1O
N
Pd
N
OCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1
a
c
d
b
O
C10H21O
C10H21O
N
Pd Pd
N
OC10H21
OC10H21
O
N
R
N
R
R
OO
N
R
N
R
R
NPd
X
S
N
Pd
ArN
H2n+1CnO
Pd
165
Figure 38 Examples of mesomorphic, dipalladated dialkoxybenzylidene complexes.
882 Metallomesogens
as photoresponsive materials as they suffered a trans–cis isomer-
ization of the azobenzene moiety induced by UV light, in solu-
tion and in condensed phases.
Notably, trinuclear, ortho-palladated imine complexes with
unsymmetrical m3-S/m3-X bridges (Figure 38, 165d: n¼2, 6,
10; X¼OH, O2CEt, SC4H9)866 were obtained from the parent
dinuclear di-hydroxo complex, but no information was given
about their thermal behavior.
Among these imine-based dipalladium complexes, some
chlorido-bridged derivatives (163a: R¼CnH2nþ1, R1¼OCmH2mþ1) and a mixed m-acetato/m-thiolato-bridged complex
(165b: X¼SC6H13, Y¼OAc, n¼6) were found to show lyo-
tropic mesomorphism when in contact with apolar organic
solvents.867,868 In this sense, the dichlorido complexes formed
lamellar phases while the m-acetato/m-thiolato derivative showed
an inducednematic phase. Remarkably, a chiral nematic phasewas
observed for the mixed bridged complex (165b) when the chiral
solvent (R)-(þ)-limonene was used for the contact preparation.
Espinet and coworkers869 also developed the investigation of
the effect of oligo(ethyleneoxide) terminal chains (R¼O
(CH2CH2O)nEt, n¼2, 3) in a series of dinuclear PdII benzylidene
complexes with a structure similar to those shown in Figure 38
(165a and 165b) with different bridging systems (di(m-X): X¼Cl,
Br, OAc; m-X/m-Y: X¼SCnH2nþ1, Y¼Cl, OAc), as well as mono-
nuclear derivatives with a nonmesogenic co-ligand (acac and
alanine, similar to structure 167). While none of the precursor
imines or the acetato-bridged complexes were found to be meso-
morphic, some of the materials did show liquid-crystalline prop-
erties (SmC, SmA, and N phases), and with clearing points lower
than those of the symmetric alkoxy derivatives described before.
Similar systematic studies were also carried out with cyclo-
metallated platinum complexes of 4,40-dialkoxybenzylidene(166: R1¼R2¼OCnH2nþ1, n¼2, 6) with symmetric bridges
di(m-chlorido), di(m-acetato), di(m-thiolato), and di(m-chlorido-propionato) (166: X¼Y¼Cl, OAc, SCnH2nþ1 n¼4, 6, 16,
O2CC*HClMe) or unsymmetric bridges m-chlorido/m-thiolato,
Metallomesogens 883
m-acetato/m-thiolato, and m-chloropropionato/m-thiolato (166:
X¼SCnH2nþ1, n¼4, 6, 16; Y¼Cl, OAc, O2CC*HClMe).870,871
The substitution of palladium by platinum resulted in an
overall increase in the mesophase stability: all the platinum
complexes were mesomorphic, except the acetato-bridged mate-
rial, and they exhibited, in general, more ordered mesophases
than their palladium analogs (e.g., an SmA phase was induced
in place of the nematic phases), and overall, the transition tem-
peratures, particularly the clearing temperatures, were slightly
higher for Pt than Pd. The effect of the attachment of one or
two 2-octanol chiral chains to the imine group was also investi-
gated through dinuclear ortho-platinated derivatives (166: R1,
R2¼OC*HMeC6H13, OCnH2nþ1 n¼8, 10; X¼Y¼Cl), as well
as mononuclear species with b-substituted-phenyldiketonate co-ligands (167).872,873 SmA* and/or SmC* mesophases were
found for all compounds, over wide ranges (�100 �C) and
high transition temperatures for the dinuclear derivatives, and
with lower clearing points (about 100 �C) for the mononuclear
complexes. Ferroelectric measurements were carried out, observ-
ing that the spontaneous polarization increasedwith the number
of stereogenic centers, but at the expense of a shortening of the
mesophase range. However, no general trends concerning the
influence of the metal atom (Pd vs. Pt) on the ferroelectric
behavior could be identified.
NPt
Y
X
R2R2
R1 R1R1
N
R2
PtN
PtO
O
OC10H21
OC10H21
166 167
Mononuclear ortho-palladated benzylidene derivatives have
also been investigated by various research groups. Cleavage of
the corresponding dinuclear chlorido-bridged complexes (163:
Z¼H, X¼Cl) with bidentate chelate ligands led to various
series of mononuclear derivatives (168). Several studies were
carried out using b-diketones (168a: R1/R2¼OCnH2nþ1/
OCmH2mþ1 nþm¼4–20, R3/R4¼Me, Et, Pr, C10H21, CF3,
C2F5, C3F7)853,874–879 and b-enaminoketones (168b: R1/
R2¼OCnH2nþ1/OCmH2mþ1 n¼m¼2, 4, 6, 8, 10),853 showing
that lowering the symmetry of the complex appeared to be an
R1
N
R2
PdX
Y
O
R3
HN
Me
H2N
R5
X Y
168
excellent strategy for reducing the transition temperatures in
relation with the dinuclear predecessors, and simultaneously
preserving a large mesomorphic range. Monotropic nematic
phases (for the short-chain length compounds), both enantio-
tropic SmA and N phases (intermediate-chain length), and
only an SmA phase (the derivative 168a with nþm¼20),
were generally found. However, the nematic phase became
destabilized when the trifluoroacetylacetonate ligand was
used (168a: n¼m¼6, R¼CF3)875,876,878,879 probably due
to the steric impediment introduced by the CF3 group, and
the existence of an equimolar mixture of isomers. This way,
mesomorphism disappeared totally for the complexes
with larger perfluoroalkyl substituents (R3/R4¼C2F5, C3F7).
Also, the use of aliphatic diketones (168a: R3¼R4¼C10H21,
n¼m ¼10)877 induced a remarkable suppression of the
mesomorphism.
A family of complexes was prepared with imines bearing
hexyl (168c: R1¼R2¼C6H13), hexyloxy (168c: R1¼R2¼OC6H13), or mixed hexyl-hexyloxy chains (168c: R1/R2
and R2/R1¼C6H13/OC6H13), and b-diphenylketonate ligands
substituted by polar groups in meta- and para-positions (168-
c: Z/Z0 ¼H, 4-CN, 4-F, 3-F, 4-CF3, 4-Me).879 Clearly, the presence
of polar groups tends to stabilize the SmAphase at the expense of
the nematic phase, and the largest temperature ranges are
obtained for the complexes of the dihexyloxy series.
Moreover, several studies of the influence of attaching
chiral chains to both the imine or the auxiliary ligand in the
mesomorphism of this kind of mononuclear derivative were
also carried out. One complex bearing a 2-octanoyl chain and
a b-aminoenonate group (168b: R1¼OC*HMeC6H13,
R2¼OC8H17) exhibited monotropic BPII, BPI, and N* phases
at a reasonably low temperature.865 An SmC* phase (168d:
R5¼Me, R1¼R2¼OC14H29) and a broad SmA phase (168d:
R5¼CH2Ph, CHMe2, CH2CHMe2; R1¼R2¼OC14H29) were
obtained in related mixed complexes with chiral amino acids,
again at very accessible temperatures.880 Espinet and coworkers
investigated in detail the chiral derivatives of mononuclear
ortho-palladated complexes (168e) incorporating a bis(alkoxy)
benzylidene and a bis(alkoxyphenyl)-b-diketonato ligand.881–883
Using again a 2-octanol chain attached onto the fixed ortho-
metallated ring (168e: R1¼OC*HMeC6H13, R2¼OC10H21,
OC14H29, R6, R7¼OC10H21),
881 they prepared two complexes
showing monotropic SmC* phases. These materials presented
low Ps values (–22 nC cm�2), but a switching time in the milli-
second time regime three orders of magnitude faster than the
time reported for the dinuclear azine derivatives described in the
previous section.848 With a view to finding an enantiotropic
SmC* phase, the effects of the position and number of chiral
O
R4
O
Me
O
O
a
b
d
O O
Z¢Z
O O
R7R6
c
e
NPd
CnH2n+1O
OO
R R
N OCnH2n+1Pd
O O
RR
170
N
PdYX
OCmH2m+1
CnH2n+1O YX
O O
CpH2p+1 CqH2q+1
OH2N
OR
a
b
c
169
884 Metallomesogens
chains on the ferroelectric behavior of this system were investi-
gated (168e: R1, R2, R6, R7¼OC*HMeC6H13/OC10H21).882
Note the compounds with unsymmetrical diketones exist as a
cis-/trans-isomeric mixture, and none of the complexes with three
and four chiral chains, or two chiral chains on the b-diketone,were liquid crystalline. In addition, the number and position
of these chains drastically influenced the ferroelectric prop-
erties, particularly spontaneous polarization and nonlinear
optical responses.883 Compounds with a chiral chain on the
imine ligand exhibited monotropic behavior, as well as all
the compounds with two chiral chains in general. However,
when one chiral chain was on the diketone, the behavior
was enantiotropic.
It is worth mentioning that one of these mononuclear
complexes (168e: ¼ R1, R2, R6, R7¼OC10H21) was also inves-
tigated as a lyotropic mesogen.867,868 A nematic phase was
observed in contact preparations with linear alkanes, showing
an increasing temperature range with the elongation of the
solvent chain. By using chiral limonene though, a chiral
nematic phase could be observed from 65 to 136 �C.Another complete series of mononuclear palladium com-
plexes using also b-diketones as co-ligands were described
earlier by Cave et al., but in that case attaching one additional
aromatic ring to the phenyl-benzylidene group (169a: (n,m)¼(4, 4), (4, 7), (7, 4), (7, 7); p, q¼1, 4, 6, 8).884 All the com-
plexes were mesomorphic, the majority showing both SmA
and nematic phases typically in the range 70–250 �C depend-
ing on the chain lengths, but with the absence of the nematic
phase for the derivatives with long aliphatic chains in the
ketonate. Other types of co-ligands were used to prepare
related complexes, such as cyclopentadienyl (169b: (n, m)¼(4, 4), (4, 7), (7, 4), (7, 7))885 and amino acids (169c: (n,m)¼(4, 4), (4, 7), (7, 7), (10, 10); R¼Me (alanine), iPr (valine), iBu
(leucine), sBu (isoleucine)).886 Interestingly, the first series
(169b) exhibited mainly a nematic phase (except an SmA
when n¼m¼7), while for the amino acid derivatives the
SmA was the only mesophase observed. Dinuclear complexes
containing di-metallated imines were also described (170).887
These complexes presented only nematic phases, with high
transition temperatures (greater than 200 �C) if they containedshort chains (170: R¼Me, n¼4–8), and lower temperature
ranges (100–150 �C) when increasing the chain lengths (170:
R¼butyl, hexyl; n¼8) or even suppressed when R was longer
(170: R¼octyl; n¼8) or bulkier (170: R¼ tBu; n¼8).
The study of ortho-metallated imine systems also led to the
first organometallic complexes showing the nematic phase of
disk-like molecules, ND.888 Praefcke et al. reported a series of
disk-shaped dipalladium and diplatinum complexes (171)
bearing four-chained benzalimine ligands and different bridg-
ing systems (171: R1¼OC6H13, R2¼C6H13; M¼Pd: X¼Cl, Br,
I, SCN, OAc; M¼Pt: X¼Cl, SCN).888–891Particularly, all the
flat halogeno-bridged derivatives, together with the
thiocyanato-palladium complex, exhibited a ND phase. How-
ever, neither the acetate-bridged palladium complex nor the
thiocyanato-bridged platinum derivative was mesomorphic.
Interestingly, unlike the chlorido-bridged palladium com-
plexes, the platinum complex existed as an isomeric syn/anti
mixture in solution (in the ratio 17:83), and attempts to sep-
arate the two isomers were unsuccessful because of decompo-
sition processes; the thiocyanato platinum complex was
obtained as a single antiparallel isomer. Later, in 2006 Bilgin-
Eran et al. synthesized related disk-shaped chlorido-bridged
dipalladium complexes carrying semiperfluorinated alkyl
chains at the imines (171: M¼Pd; X¼Cl; R2¼C6H13; R1¼O
(CH2)6C4F9 (Cr 101 Colh 164 I), O(CH2)4C6F13 (Cr 115 Colh227 I)).892 Incorporating the fluorinated chains led to the
appearance of more ordered (ND phases are replaced by Colhphases) and more stable mesophases (enantiotropic and
strong increase in the clearing points was observed by increas-
ing the degree of fluorination).
R1O
OR1R1O
N R2
M
NOR1
MX X
OR1R1O
R2
171
These disk-like metallomesogens were found to be capable of
forming CT complexes when doped with the strong electron
acceptor 2,4,7-TNF. Moreover, the bridge and the chain type
seemed to influence the type of mesophase induced. Particularly,
for the first series of compounds (171: R1¼OC6H13, R2¼C6H13;
M¼Pd: X¼Cl, Br, I, SCN, OAc; M¼Pt: X¼Cl, SCN),893,894 the
Metallomesogens 885
binary mixtures of chlorido- and bromide-bridged palladium
complexes induced the suppression of the ND phase above 10%
of TFN, leading to enantiotropic Colh phases instead. Neverthe-
less, the iodido-palladium derivative showed both the Colh and
ND phases, but at various TNF concentrations – above 45 mol%
TNF – a monotropic ND phase was induced. Both palladium and
platinum thiocyanato-bridged complexes showed a stabilized ND
phase, and once again, the acetato-bridged complex did not show
mesomorphism. Contact preparations of the chlorido-bridged
platinum complex with TNF also resulted in an induced Colhphase, with a higher thermal stability than its palladium analog.
The different mesophases were characterized by x-ray methods,
which confirmed intercalation of TNFmolecules between succes-
sive planar complexes in the columnar phases, while no such
stacking was implied in the case of the nematic phase. The differ-
ences in the mesomorphism observed for the pure compounds
and in the binary mixtures were explained from unequal core
dimensions caused by the bridging groups, as well as space-filling
(steric) and electronic effects.
Four homologous complexes with chiral substituents were
also prepared (171: R1¼OC6H13; M¼Pd: R2¼(S)-b-citronel-lol, X¼Cl, Br, SCN; M¼Pt: R2¼(S)-2-methylbutyloxy,
X¼Cl),895 but none of the palladium complexes showed
mesomorphic properties. However, a monotropic ND* phase
was observed for the platinum complex. All of the complexes
again formed CT complexes with TNF. A Colh phase was
induced for all the halo-bridged complexes, but the chiral
nematic phase of the platinum compound was suppressed. At
low TNF content, a chiral ND* phase was stabilized for the
thiocyanato-bridged compound along with a nonchiral ND
phase at higher concentration.
Mononuclear palladium and platinum complexes with
different numbers of hydrocarbon and fluorocarbon chains
were obtained (172: M¼Pd, Pt; R1¼C10H21, (CH2)6C4F9,
(CH2)4C6F13; R2¼H, OR1; R3¼H, OC10H21)896 from the
parent dinuclear complexes (171). SmA and SmC (five
peripheral chains), Colh (six and seven peripheral chains),
and nematic (five and six peripheral chains) phases were
observed, with different factors influencing the mesomorphic
behavior, namely total number of chains, the fluorocarbon:
hydrocarbon ratio, the degree of chain fluorination, and, to a
lesser extent, the metal. Interestingly, the change from smectic
to columnar phases as a function of the number of chains was
rather abrupt and did not involve examples that showed both
phases.
C12H25O OC12H25
OC12H25
C12H25O
C12H25O
C12H25O
N
Pd
N
Pd
X X
174
R1O
R2
R1O
N
C6H13
MO
O
OC10H21
OC10H21
R3
172
Interesting thermotropic and lyotropic tetrametallomesogens
[M4(m-X)4L2] with palladium61,888,897,898 and platinum890,899
were reported by Praefcke and coworkers (173: M¼Pd, Pt;
n¼6, 8, 10, 12, 14, 16, 18; X¼OAc, Cl, Br, I, SCN, N3; 174:
X¼Cl, Br, I, OAc, SCN). The structure of these large, lipophilic
tetrametallaorganyls was confirmed by osmometry and single-
crystal x-ray crystallography, in addition to classic spectroscopic
techniques. They exhibited, as expected from their molecular
shape, broad temperature-range columnar mesophases (�50–
100 up to 250–300 �C), with either a rectangular or oblique
lattice.900,901 The transition temperatures and phase symmetry
were influenced by different factors: the metal ion (PdII vs. PtII)
seemed to have only minor effects, whereas the nature of the
bridging group and the chain length did show some influence. In
this sense, the expansion of the rigid spacer (from 173 to 174)
caused a decrease in thermal stability. Additionally, a reduction
of the number of chains (from 12 to 8) was investigated for the
first kind of complexes (173), and the mesomorphism remained
columnar but with important modifications in the transition
temperatures and phases sequences.
CnH2n+1O CnH2n+1O
CnH2n+1O
CnH2n+1O
CnH2n+1O
CnH2n+1O OCnH2n+1 OCnH2n+1
OCnH2n+1
OCnH2n+1OCnH2n+1
OCnH2n+1
N N
M M
N N
M M
X X XX
173
C12H25O
OC12H25
OC12H25
OC12H25
OC12H25
N
Pd
N
C12H25O
Pd
X X
C12H25O
C12H25O
C12H25O
OC12H25
OC12H25
OC12H25
Pd
O O
O O
N NPd
175
886 Metallomesogens
Interestingly, these complexes formed lyotropic mesophases
when dissolved not only in lipophilic solvents such as alkanes,
but also in choloroform, benzene, octanol, octadecanol, and
stearic acid.894,902–904 In alkanes, the mesophases were stable
over wide ranges of temperature and concentration. In general,
at high complex concentration, a columnar phase was observed,
while at lower concentration, a nematic phase was usually
induced. Here again, the mesophase behavior depended
strongly on some intrinsic structural parameters, such as the
type of bridges and the nature of the metal ions. Thus, while
no mesophase was stabilized in any of the m-acetato complexes,
a Colh phase was seen for the iodido- and azido-bridged deriv-
atives, and both a Colh and a nematic phase were oberserved in
the thiocyanato analog. Remarkably, both the chlorido-
and bromide-bridged complexes (173: M¼Pd, Pt; X¼Cl, Br;
n¼6–14) exhibited two lyotropic nematic phases (N1, N2) and
a Colh phase. Increasing the chain length of the complexes and
that of the alkanes seemed to favor the occurrence of Colh andN
phases. For its part, extension of the rigid spacer (from 173 to
174) clearly inhibited the nematic phase induction. The transi-
tion between two nematic phases, being a unique case, was
investigated thoroughly (173: M¼Pd, X¼Cl, n¼6, 10, 12,
14) in various alkanes. While the high-temperature nematic
phase, N2, was present in all mixtures, the appearance of the
low-temperature nematic phase, N1, seemed to be dependent on
the chain length of the alkane solvent and that of the terminal
chains on the complex. A columnar structure was proposed for
these two nematic phases,905 with the solvent located between
the complexes (NCol) and the columns arranged with only weak
inter-columnar order. Thus, the large and flat metallaorganyls
were stacked on top of each other, and arranged perpendicular
to the axis of the columns forming the N2 phase. In the N1
phase, however, the complexes were tilted with respect to the
columnar axis.
These tetrametallated complexes (173 and 174), except
those with an acetate bridge, also formed CT complexes with
TFN, and gave rise to a viscous type of columnar phase on
heating (transformation of oblique/rectangular symmetry into
a hexagonal lattice). The stability of the lyotropic nematic
phase was enhanced (higher clearing points) in all systems
compared to the behavior of the pure complexes in pentade-
cane. The columnar phase was still present in all cases. Addi-
tionally, a large temperature-range, chiral lyotropic nematic
phase was induced in a binary system composed of equimole-
cular amounts of 173 and the chiral p-electron acceptor TAPA
(20-(2,4,5,7-tetranitro-9-fluorenylideneaminooxy)-propionic
acid) in heptane, pentadecane, and eicosane.906 A ligand-
exchange reaction between acac and the chlorido-bridging
group of one of these tetranuclear complexes (173: M¼Pd;
X¼Cl; n¼12) led to a dipalladium organyl complex (175).902
Despite the fact that it was mesomorphic neither in its pure
state nor in binary mixtures with alkanes, it formed a CT
complex with TNF. That association also induced a thermo-
tropic mesophase (Col), as well as lyotropic behavior in several
compositions (two Colh and a NCol phase).
Some other interesting ortho-metallated imine complexes
have been reported. Galyametdinov and coworkers have inves-
tigated ferrocene-containing cyclopalladated metallomesogens.
Initially, a series of di- and tri-heteronuclear derivatives were
described (176, Figure 39)907 showing enantiotropic SmA
mesophases with transition temperatures and temperature
ranges strongly dependent on the nature of the enaminoketo-
nate co-ligand. Recently in 2008, new trinuclear complexes were
synthesized (177, Figure 39),908 now presenting nematogenic
behavior over a broad temperature range (177a: Cr 71–74 N
148–205 I; 177b: Cr 140–148 N 148–200 I), probably induced
by the introduction of phenylenecarboxyl units in the rigid core.
Arias et al. carried out the ortho-palladation of crown-
derivatized benzilidenes, affording dinuclear chlorido- and
acetato-bridged derivatives and mononuclear diketonate spe-
cies (178: n¼4, 6, 8, 10, 12).909 Only an SmA mesophase was
seen for the chlorido-bridged dimer at high temperatures
(170–230 �C) and at lower temperatures (60–135 �C) for themononuclear derivatives. Additionally, experiments were
undertaken to see if sodium/potassium picrate could be
extracted from aqueous media by these complexes, but the
transport observed was rather modest.
N
Pd
C12H25O OC12H25
OCnH2n+1O
O O
O
O
OO
178
8.21.7.1.3 Ortho-metallated pyrimidine, pyridazine, andpyridine complexesThe well-known liquid-crystalline 2-phenylpyrimidines were
initially ortho-metallated by Ghedini, to give rise to a series of
trans-dipalladium complexes with a systematic study of the
bridging groups (179: X¼Cl, Br, I, OAc) and ligand chain
lengths (179: (m,n)¼(6,1), (9,1), (6,11), (9,9)).910,911 Some
of these complexes were found not to be mesomorphic (all the
aceto-bridged derivatives among them), but all the remaining
mesogenic materials had a broad SmA phase between 100 and
200 �C, and two materials (X¼Cl with (m,n)¼(9,1) and X¼ I
with (m,n)¼(9,9)) were reported to have an additional smec-
tic phase (SmX). Later studies912 showed that what had been
identified as SmX was, in fact, SmA, while the phase identified
as SmA was actually an ordered smectic phase.
In another report by Guang et al.,913 the influence of the
carboxylate bridging group was investigated (179: n¼6;
X¼O2CR, R¼Me, CH2Cl, CH2Br, CHBrMe, CH2CH2Br). For
m¼6, all the derivatives (except the nonmesomorphic m-acetato) showed an SmC phase, with the m-chloroacetato
N
PdNO
Fe
C12H25O
C12H25O
C12H25O
C12H25O
C18H37
C18H37C12H25O
NO
a
bN
O
Fe
NO
NO
c
176
N
PdNO
RN
OFe
NO
Fe
NO
FeO
Oa
b
OC12H25
O2C-C6H4-OC12H25R =
177
OC10H21
Figure 39 Di- and tri-nuclear, ferrocene-containing ortho-palladated mesogens.
CnH2n+1O
OCnH2n+1
N
Pd
N
Pd
X X
N
N
CmH2m+1
CmH2m+1
179
Metallomesogens 887
complex showing an SmA phase too. Clearing points varied
widely, but the bromosubstituted bridges consistently gave the
lowest values. Several derivatives were prepared by varying
m from 6 to 12 for the m-chlorido- and bromide-acetato com-
plexes. There was a marked odd–even effect so that for m¼7, 9,
11, only SmA was seen, while for m¼8, 10, 12, both SmC and
SmA were observed.
Several mesomorphic neutral and cationic mononuclear
species (180) were also prepared from the related chlorido-
bridged complexes (179) with different bidentate chelating
ligands such as acac (180a: x¼0, n¼11, m¼6)910 and
substituted 2,20-bipyridines (180b: x¼1, n¼11, m¼6; R¼H,
CO2C22H45, CH2OH).910,914 Remarkably, the complex with an
unsubstituted bipyridine (R¼H) and BF4 as counter-anion is
one interesting example of the rare ionic materials showing a
thermotropic nematic phase. However, the derivatives bearing
substituted bipyridines (180b: x¼1, n¼11, m¼6;
R¼CO2C22H45, CH2OH) behaved differently and were
found to show SmC or SmA phases, respectively (decomposing
rapidly in the mesophase above 200 �C). Chiral complexes
with various combinations of chains (180c: (n,m)¼(1,6),
(1,9), (11,6), (9,9); R*¼S-(�)-b-citronellyl)496 were also pre-
pared. All these complexes exhibited an SmA* phase between
�100 and 130 �C, losing the SmC* of the free ligand.
As for the previous types of cyclometallated metallomeso-
gens described in this section, b-diketones were also used
extensively as co-ligands to prepare mononuclear complexes
with an ortho-metallated phenylpyrimidine group. Mesogenic
molecules resulting from the combination of a calamitic
2-phenylpyrimidine unit and a half-discotic 1,3-diketonato
moiety (181) were investigated by Hegmann et al.915,916 The
number of side chains on the diketonate fragment was
increased stepwise from 4 to 8, so that the overall molecular
structure changed continuously from a rod- to a disk-like
molecular shape (Table 2). Thus, this study offers a possible
insight into the evolution from smectic phases to columnar
phases as a function of shape, allowing a better understand-
ing of the intimate relationship between these different
mesophases.
Thus, the first compound of the series 181a (M¼Pd, Pt; R1,
R2, R3, R4¼H/OC10H21), with a total number of four chains
(181ai), showed both SmC and SmA phases. Increasing the
number of chains to five (181aii) led to the destabilization of
the mesophase and monotropic behavior (probably due to the
reduction in the symmetry), whereas with six chains (181aiii),
the mesomorphism was totally suppressed (not yet under-
stood). Mesomorphism was then regenerated with the further
increase in the number of chains (181aiv), and Colh phases
were formed. Moreover, the mesophase stability was enhanced
on increasing the number of chains from seven to eight (181av,
181avi). Interestingly, a binary phase diagram between the
two unsymmetric compounds (181aii, 181av) revealed the
induction of another birefringent mesophase at the contact
Table 2 Mesomorphism of complexes 181a
i ii iii iv v vi
181a SmC and/or SmA (SmA, N) – Colh(2) Colh Colh181b SmA (SmA, N) – Colh(2), NCol Colh Colh181c SmA SmC, SmA, N (SmC, SmA) Colh(2) ColX, Colh(2) Colh
From Bruce, D. W.; Deschenaux, R.; Donnio, B.; Guillon, D. Metallomesogens. In Comprehensive Organometallic Chemistry III; Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier:
Oxford, 2006; Vol. 12, pp 195–294.aColh(2) represents a columnar hexagonal phase in which a repeat unit consists of two molecules of complex, ColX an unidentified columnar phase, – complex not mesomorphic;
monotropic phases are in parentheses.
N
CnH2n+1O
Pd
CmH2m+1
N
X Y
X Y
x+
a
b
N
O O
N
R R
OC12H25
NO
R*Oc
180
M
O
O
OC10H21
OC10H21
R2
R3
R4
R
N N
R�
R1
a: M = Pd, Pt; R = C7H15, R� = OC10H21
b: M = Pd; R = C10H21,R� = OC8H17
c: M = Pd; R = C6H4–C5H11, R� = C9H19
181
888 Metallomesogens
region with the destabilization of both mesophases of the pure
compounds. Note that no significant differences were found
between the palladium and platinum series. In addition, the
study of these complexes was later followed by reports of the
behavior of related PtII and PdII complexes (181b,c),916 in
which the authors focused on investigating the transition from
a calamitic-like behavior to that associated with more disk-like
materials. The results, summarized in Table 2, showed that it is
both the number and distribution of chains on the b-diketonatethat determine the transition to disk-like behavior. Interestingly,
none of the complexes showed mesomorphic behavior charac-
teristic of both rods and disks as can be found in polycatenar
systems.44,47 Moreover, the platinum complexes were found to
be luminescent, with their optical properties strongly dependent
on the number, position, and length of the chains.
The existence of the biaxial SmA phase (SmAb),917 also
known as the McMillan phase, as demonstrated by textural
observations and x-ray investigations in some CT complexes,
formed with the palladium metallomesogens 181a (i–iii) and
TNF. Two novel mesophases were induced systematically. At
low TNF concentration, a (probably) columnar mesophase is
induced and remains stable at high temperature and up to
�60 mol% concentration of TNF. However, it is at higher
concentrations of TNF when these CT complexes form the
biaxial SmAb phase that is self-organized into layers, with the
flat molecules arranged parallel to each other and orthogonal
to the layers, with a long-range face-to-face organization, and
short side-by-side correlations. This face-to-face interaction
hindered the molecular rotation around the long axis, reducing
the symmetry, and thus giving rise to the biaxiality.
In order to create novel types of metallomesogens with
unusual molecular shape, Tschierske et al. reported two dinuc-
lear cyclopalladated complexes derived from macrocyclic
2-phenylpyrimidine derivatives.918 Enantiotropic SmA and
N phases (182: X¼Y¼O; Cr 168 SmA 208 N I) or only a
monotropic N phase (182: X¼Y¼OCH2; Cr 118 (SmA 91)
I) were found for these complexes, showing this way a meso-
phase stabilization or induction after cyclopalladation. In this
sense, they later investigated macrocyclic molecules combining
the rod-like molecular architecture of the para-cyclophanes
with two half-disk-like 1,3-b-diketonato units (183: R1,
R2¼H/OC10H21; x, y¼1–3).919 These novel complexes were
mesomorphic, with a smectic-to-columnar phase cross-over
observed on increasing the chain number. Differences in the
mesophase stability were observed as a function of the poly-
ether chain length, while the number of chains influenced the
nature of the mesophase.
CnH2n+1O
OCnH2n+1
N
N
N
N
Pd
Pd
O
CmH2m+1O
OCmH2m+1
O
O
O
RR
186
O
N
Pd
N
O
Pd
Cl
Cl
O
N
N
O
O
O
O
X
Y
O
O
O
O
O
O
Y
X
O
O
O
O
O
O
O
O
O
O
O
NN NN
Pd PdO
OO
O
R2
R2 R2
R2
R1
R1
R1
R1
C10H21O
C10H21O
OC10H21
OC10H21
y
x
182 183
Metallomesogens 889
Pyridazine groups have also been used to prepare cyclome-
tallated metallomesogens. Slater et al. synthesized mono-
(184: R¼Me, Bu; n¼4–10) and di-palladium complexes
(185: R¼Me, Bu; n¼4–10) bearing a three-ring pyridazine
system and again diketonates as co-ligands.920 The related
platinum species could not be isolated as their acac
derivatives.921 All the monoclear complexes exhibited a single
SmA mesophase, with lower transition temperatures for
the acac derivatives (184: R¼Me, 180–300 �C) than those
with the bulkier diketonate (184: R¼Bu, 100–190 �C).Among the dinuclear complexes, only the cis-dicyclopalladated
acac derivative (185: R¼Me) showed an SmA phase (well
above 200 �C for the shorter homologs) and with extensive
decomposition.
CnH2n+1O CnH2n+1O
N
N
OCnH2n+1 OCnH2n+1
PdO
O
R
R N
N
Pd
O
OR
Pd
O
O
R
RR
184 185
A series of dicarboxylato-bridged dinuclear palladium com-
plexes were prepared by Guang et al. (186: R¼Me, CH2Cl,
CH2Br). While the simple m-acetato complex was not meso-
morphic at all chain lengths reported, the other two series
always showed an SmA phase (except no mesomorphism
observed when n¼6, m¼10, and R¼CH2Br).
Initially, Tschierske prepared a series of ortho-metallated
phenylpyridine derivatives with a similar structure to the
phenylpyrimidine derivatives described before (181a: M¼Pd,
Pt; R1, R2, R3, R4¼H/OC10H21).916,922 Unfortunately,
they were only poorly mesomorphic, so that only the eight-
chain palladium complex exhibited a Colh phase. Nevertheless,
Venkatesan et al. later reported an extensive study of
related mesomorphic mononuclear platinum complexes with
substituted 2-phenylpyridines (187: R1¼H, C12H25, OC12H25;
R2, R3¼H, OC12H25) and 2-thienylpyridines (188).923 In gen-
eral, by increasing the number of chains attached to the central
core (187: from 4 to 9; 188: from 4 to 8), a transition from
lamellar (SmA) to columnar (Colh or Colr) organization was
observed. Also, the distribution of the chains around the cyclo-
metalating ligand seemed to have a significant effect on the
mesophase stabilization. Remarkably, the influence of the dif-
ferent ortho-metallated system (phenyl- vs. thienyl-pyridine)
was inappreciable. However, that influence was important for
the optical properties of the materials. All the complexes were
highly photoluminescent, but with green-yellow emissions for
the phenylpyridine derivatives (187) and orange-red for the
thienylpyridine species (188) (Figure 40).
In a recent study, Bruce and coworkers prepared different
series of mononuclear platinum complexes (189a,b: n¼6, 8,
10, 12) bearing an ortho-metallated 2,5-diphenylpyridine
group.924 The influence of a fused cyclopentene ring in the
aromatic mesogenic group was also evaluated (b series).
Despite the rich polymorphism shown by the parent ligands
without the fused ring (n¼6: SmA, SmC, SmF, SmI, crystal J),
the acac complexes 189a presented a simple SmAmesophase at
high temperatures (melting points: 170–200, clearing points:
230–258 �C). For the related complexes with the fused cyclo-
pentene ring, it was found that all were monotropic, showing
nematic (189b: n¼6, 8) and/or SmA phases (189b: n¼8, 10,
12). Related series of intermediate complexes prepared in this
work [Pt(C^N)Cl(dmso)] were also found to be mesomorphic,
PtN
O
O
O
O
C12H25
OC12H25
OC12H25
OC12H25
OC12H25
OC12H25
OC12H25
C12H25O
C12H25O
C12H25
C12H25
R1
R3
R2
O
O
PtN
O
O
S
187
188
O
O
a b
Figure 40 Mesomorphic ortho-platinated complexes with b-diketones described by Venkatesan et al.
X–
N NCH2OC12H25C12H25OCH2
N
Pd
+
190
890 Metallomesogens
with a similar behavior to the final acac derivatives. All these
complexes also showed phosphorescence, and luminescent
studies in solution, solid, and thin films were carried out.
Remarkably, the acac derivatives presented the highest emis-
sion quantum efficiencies ever reported for materials of this
type (189a: F¼0.49; 189b: F¼0.57; n¼12).
Pt
N
OCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1
O
OPt
N O
O
189a 189b
8.21.7.1.4 Other ortho-metallated complexesOrtho-palladated quinolines coordinated to nonmesomorphic
2,20-bipyridines (190: X¼BF4, O3SOC12H25) were found to
exhibit liquid crystallinity.651 However, only the dodecyl sul-
fate (DOS) salt presented a reasonable thermal stability (Cr
131 Colr 168 SmA 216 I). The Colr phase was identified by
x-ray diffraction at low temperature, and resulted from the
stacking of the flat aromatic cores on top of each other, but
with the columns not completely surrounded by aliphatic
chains; hence, the polar centers of the columns were in lateral
contact which one another, forming layers which are separated
by layers of molten chains.
New multifunctional liquid-crystalline materials based on
cyclopalladated mesogens and curcumin have been described
by Ghedini and coworkers (Figure 41). A red-emitting chromo-
phore bearing a 9-diethylamino-5H-benzo[a]phenoxazine-5-one
group and a mesogenic curcumin-based ligand (Figure 41, 191)
was reported to show a Colr phase from room temperature
up to 173 �C.925 Moreover, a cholesteric-functionalized 2-
phenylquinoline was cyclopalladated to form chiral metallome-
sogens with the help of auxiliary bidentate ligands (Figure 41,
192).926 Particularly, the complex with curcumin (192b: R¼H)
showed promising biological activity based on preliminary in
vitro anticancer screening against human prostatic cancer cell
lines. While the tropolone derivative (192a) presented a N*
phase from 135 to 270 �C, the curcumin derivatives showed a
Colr (192b, 140–220�C) or a Colh phase (192c, 65–200 �C).
Two series of related chiral dinuclear complexes derived from
oxazoline-based ligands were synthesized by Lehmann et al.
They found a broad SmA phase for planar dichlorido-bridged
species (193: X¼Cl; x¼1: R¼CHMe2; x¼1, 2: R¼C*HMeEt),
while for the nonplanar acetato-bridged complexes, only the
biphenyl derivative showed an SmA phase at elevated tempera-
ture (193: X¼OAc; x¼2: R¼C*HMeEt).578 Using chiral dop-
ants (�10 mol%), a chiral nematic phase was induced
systematically in both the chlorido- and acetato-bridged series,
with the suppression of the SmA phase in the chlorido-bridged
systems (X¼Cl, x¼1). New complexes with six terminal chains
were described in a subsequent paper,927 but none of the pure
dinuclear compounds (194: X¼OAc, Cl) were mesomorphic
NPd
O
O
N
OO
OMeO
O
C14H29O
OC14H29
OC14H29
OC14H29
OC14H29
C14H29O
OMe
O
O
191
Pd
OO
N
OCol*O
OO
OMe
OROMe
ROOO
O O
a
b: R = Hc: R = C22H45
192
Figure 41 Multifunctional liquid crystals based on cyclopalladated complexes, described by Ghedini and coworkers.
N
PdN
PdX
X
O
R
O
R
OO
OO
OC10H21
C10H21Ox
x
O
N
PdN
O
PdX
X
O
C12H25O
C12H25O
C12H25O
OC12H25
OC12H25
OC12H25
O
Me
O
MeH
O
193 194
H
Metallomesogens 891
and most were room-temperature oils or glassy materials.
However, whenmixed with TNF, they all formed CT complexes.
Only two examples of terdentate ortho-metallated systems
have been used to prepare metallomesogens. First, Ghedini and
coworkers reported some mesomorphic C,N,N-cyclometallated
chloridopalladium(II) complexes (195: n¼6, 8, 12).928,929 They
exhibited high-temperature mesophases, all of which were
monotropic (N when n<12 and SmA when n¼12), and
interestingly, they presented attractive photophysical properties,
and particularly electroluminescent emission was found.930
Related cationic ortho-metallated iridium(III) complexes were
also prepared, but were not mesomorphic.931
Later, Kozhevnikov et al. reported the synthesis of two
series of N,C,N-cycloplatinated metallomesogens bearing a
five-ring hexacatenar ligand (196a: n¼4, 6, 8, 10, 12), or a
modified system with two fused cyclopentene ring (196b:
N
N
OPdCl
O
O
OOCnH2n+1
195
892 Metallomesogens
n¼10, 12).932 The ligands themselves were not liquid crys-
talline, but all the complexes (except 196a when n¼4) were
mesomorphic and showed columnar mesophases (196a:
Colh, 196b: Colr). Comparison of the transition temperatures
between the two series shows that longer chains are required
in the second series (196b) before mesomorphism is
observed, but the mesophase is stabilized strongly by the
introduction of the cyclopentene group. These platinum com-
plexes were found to be phosphorescent. The emission in the
liquid-crystal phase was found characteristic of the monomer
complex, when the emission from the material in solid state
was exciplex-like. More than that, this emission was subject to
tribological control, with the initial state re-obtained by ther-
mal cycling.
N
N
PtCl
OCnH2n+1
OCnH2n+1 OCnH2n+1
OCnH2n+1
H2n+1CnO H2n+1CnO
H2n+1CnO
H2n+1CnO
H2n+1CnO
H2n+1CnO
H2n+1CnO
H2n+1CnO
N
N
PtCl
196a 196b
Lu et al. studied phenylbipyridine (197a)933 and terpyri-
dine (197b)934 complexes of Pt as gelating agents and in both
cases, polarized optical microscopy of the gels shows a bire-
fringent texture, although it is not possible to say if this arises
from a liquid-crystalline gel state in the absence of other char-
acterization. However, in both examples low-angle x-ray pow-
der patterns of the evaporated gels are recorded, and in the case
of the terpyridine complex a pattern was observed that could
be indexed within a rectangular lattice and that showed broad
features at 2y¼19� (¼4.7 A) and 25� (¼3.6 A). These features
correspond to the separation of fluid alkyl chains (4.7 A) and
of flat cores (3.6 A), and the fact that they are broad may
suggest some sort of liquid-crystalline organization, although
because this measurement is made on the dried material, this
does not infer anything directly about the gel state. The dried
material obtained from the gel prepared using phenylbipyri-
dine showed a mixture of sharp and broad reflections at wider
angles that were much less easy to interpret.
n+
n [ClO4]−
m
197a
NN
CN
N
F
FN
Pt Pt
197b
Pt Pt
N
O
N N
N
2+
2 [PF6]-
N
N
N
N
8.21.7.2 Octahedral Ortho-Metallated Complexes
Ortho-metallation of mesogenic ligands was not limited to
square-planar PdII and PtII, and Bruce and coworkers demon-
strated mesomorphism in benzylideneaniline complexes
bound to octahedral MnI and ReI. A nematic phase, which
cleared below 190 �C with decomposition, was seen for two
series of manganese derivatives (198a: M¼Mn, n¼5, 7; 198b:
M¼Mn, X¼Y¼H, n¼m¼8), while the free parent ligands
showed smectic and N phases at temperature up to
300 �C.935,936 The related Re(I) complexes presented very sim-
ilar transitions, but with enhanced thermal stability because
the decomposition was not observed at the clearing point.937
N
CO
O
OC OOC8H17
O
M
CO
CO O
CnH2n+1
198a
N
CO
O
OC OOCmH2m+1
O
M
CO
CO O
CnH2n+1O
X X
Y Y
198b
Several studies evaluated systematically the variation of both
terminal chain lengths938 and the influence of lateral fluorination
on the rhenium complexes 198b.939 Generally, the effect of the
chain length on themesomorphismof the resultingmaterials was
insignificant, and the nematic phase appeared between �130–
155 and 140–200 �C.Nevertheless, the fluorination reduced con-
siderably the stability of the nematic mesophase with increasing
fluorine substitution (198b: M¼Re; X, Y, ¼ H, F).
Different modifications were introduced on the structure of
complexes 198b to investigate their influence on the meso-
morphism. Thus, a nematic phase was observed when hexyl
chains were at each end of the rhenium complex (198b:
Metallomesogens 893
M¼Re, X¼Y¼H, n¼m¼6). However, when the hexyl chains
were replaced by one or two perfluorinated chains (CpFpþ1, p¼6–
8, 10), the mesophase changed to an SmA phase, and occurred at
higher temperatures with decomposition taking place in the
mesophase.940,941 It is worth mentioning that some of these
perfluorinated imines showed thermotropic cubic mesophases
before metallation. Another study revealed that when chiral ali-
phatic chains (citronellyloxy and its hydrogenated analog) were
substituted for one of the hexyl chains, the final material pre-
sented an N* phase (120–160 �C), but that when both chains
were chiral, mesomorphism was suppressed.942
Various reports indicated that complexes based on two-ring
imine ligands were not mesomorphic,943 and neither the dinuc-
lear rhenium derivatives with a four- or a five-ring system.944
Recently, a dirhenium(I) complex (199) was obtained by cyclo-
metallation of a bent-core mesogen.945 Despite the mesomor-
phic properties found in the parent ligand (Cr 144 B4 159 B1174 I), the complex proved not to be mesomorphic, melting
directly to an isotropic fluid at 98 �C.
N
OCCO
Re
OC
CO
C6H13O
O
ON
COOC
Re
CO
CO
O
O
OC6H13
199
The first example of a liquid-crystalline platinum(III) mate-
rial was reported in 2010 by Santoro et al.946 Supported dimeric
Pt(III) complexes (200: n¼12, 14) were synthesized with two
cyclometallated diphenylpyridine fragments linked by two acet-
ato bridges and a short Pt–Pt bond. The octahedral coordination
sphere of the Pt(III) centers was completed with a chloride
group for each metal occupying the axial positions. The meso-
morphism of this new stable system was investigated for n¼14.
From room temperature to 170 �C, the complex existed in a
mesophase that was assigned as a ribbon lamellar phase with a
structure analogous to an SmA phase (namelymodulated SmA),
except that this was much more ordered. Interestingly, the
lamellar spacing found by small-angle x-ray diffraction
(37.5 A) was less than the molecular length (�51 A), which
suggested either a tilted phase or an interdigitated one, or
both. Moreover, above 170 �C, the mesophase transformed to
show a smectic arrangement with two orders of reflection.
Pt
Pt
O
O
O
O
Cl
Cl
N
OR
OR
N
OR
RO
R = CnH2n+1
200
A new ionic iridium(III) mesogen was synthesized by Ghe-
dini and coworkers by using ortho-metallated phenylpyridine
groups and a functionalized 2,20-bipyridinic ligand (201:
R¼H, OC8H17).947 Thermal studies were performed on both
derivatives, but only the half-disk-like hexacatenar derivative
(R¼OC8H17) was found to be mesomorphic. At room tem-
perature it was a solid that, on the first heating, showed only
the transition to the isotropic liquid at 184 �C. However, in the
second cycle, the behavior differed depending on the cooling
process from the isotropic state. A rapid cooling to room
temperature led to the formation of a liquid-crystalline phase
(Colh) that persisted down to room temperature, whereas a
crystalline phase was obtained by cooling slowly. On reheat-
ing, the texture remained unmodified up to isotropization. As
expected, the complex was found to be an effective phospho-
rescent emitter, but interestingly, a dynamic response could be
obtained by external stimuli. In this sense, the complex in the
solid state showed a bright green emission, while in the kinet-
ically favored Colh phase, it emitted an intense yellow phos-
phorescence. In thin film, a fully reversible color tuning from
an orange-red emission (amorphous-like state) to a blue-
shifted green emission (crystalline state) could be induced by
surface stress and heating.
Ir
N
N
N
N
O
O
R
R
R
O
O
R
R
RPF6
−
+
201
In a very recent paper, Santoro et al. reported some other
neutral and ionic, octahedral iridium(III)-containing meso-
gens, but in this case the mesomorphism was driven by the
substituted 2-phenylpyridine ligands instead by the auxiliary
neutral ligand (Figure 42).948 The observation of liquid crys-
tallinity in these derivatives was found to be a balance of
the number and disposition of aliphatic chains on the
ortho-metallated ligands. Remarkably, a neutral complex with
a chloride and a dmso group was found to be mesomorphic
(showing probably a ribbon phase) when two-chained diphe-
nylpyridines were used (202). On the other hand, hexacatenar
ligands were needed to induce mesomorphism (Colh) in an
acac derivative (203). In a similar way, columnar mesophases
were also observed in a 20-chained, dichlorido-bridged dinuc-
lear complex (204) and in an ionic derivative with two-chained
diphenylpyridines and acetonitrile (205). Note that these
materials exhibited emissive properties which were also stud-
ied thoroughly.
8.21.8 Lanthanide-Containing Liquid Crystals andMagnetic Responses of Metallomesogens
The aim of this section is, first, to illustrate the varied structures
of liquid-crystalline materials containing lanthanide ions
described in the literature, and second, to introduce one of
Ir
N
N
Cl
dmso
OR
RO
RO
OR
R = C12H25
Ir
N
N
O
O
OR
RO
RO
OR
OR
ORRO
OR
OR
RO
OROR
Ir
N
N
Cl
Cl
OR
RO
RO
OR
OR
ORRO
OR
OR
RO
Ir
N
N
RO
OR
OR
RO
OR
ROOR
RO
OR
OR
Ir
N
N
NCMe
NCMe
OR
RO
RO
OR
PF6−
+
203202
205204
Figure 42 Octahedral iridium(III)-containing mesogens reported by Santoro et al.
894 Metallomesogens
the most relevant types of physical properties that some metal-
lomesogens present: magnetic response.
8.21.8.1 Lanthanide-Containing Liquid Crystals
In more recent years, lanthanide-containing mesogens have
been very well investigated. This is due not only to their high
coordination numbers and the varied geometries the lantha-
nide ions present, but also to themetal-centered properties that
they bring to the liquid-crystalline phases.
For instance, these materials usually exhibit lumines-
cence,949–952 so that metal-centered phosphorescence (f–f
transitions) through the visible region (Tm3: blue light;
Sm3þ: orange; Tb3þ: green; Eu3þ: red) and the near-infrared
region (Yb3þ, Nd3þ, and Er3þ) is well known. However,
because the light absorption of these ions is weak, the intensity
of the emissions is also weak. Fortunately though, the ‘antenna
effect’ due to the organic ligands may, in some cases, reduce
this issue.953–955
Another interesting property of these mesogens containing
lanthanides is that they can exhibit high values of magnetic
anisotropy,949 often one or two orders of magnitude greater
than simple organic liquid crystals. This aspect and other mag-
netic responses are discussed in the next section.
Note that in addition to other recent reviews,956,957 a com-
prehensive review of lanthanide-containing mesogens was
published by Binnemans and Gorller-Walrand in 2002.16
Thus, the purpose of this section is to give an overview of the
different approaches reported to prepare this type of materials.
Moreover, recent examples are collected in order to show the
development of this area over the last years. However, alkano-
ates, phosphonates, or surfactants with lanthanide ions are not
discussed in this chapter; interested readers are referred to the
literature.16,956,958–963
8.21.8.1.1 MacrocyclesMany lanthanide complexes of poly-substituted phthalocya-
nines and porphyrins have been investigated as mesomorphic
materials. The trivalent ions can easily be complexed by two
units of these aromatic macrocycles leading to a neutral sand-
wich compound with a distorted square antiprism coordina-
tion geometry around the central ion. These compounds are
very stable, and they generally form columnar mesophases.
In fact, the first liquid crystals containing a lanthanide
metal (lutetium(III)) were based on phthalocyanines, and
[Lu(Pc)2]+ [Lu(Pc)2] [Lu(Pc)2]− [Lu(Pc)2]2− [Lu(Pc)2]3−e− e− e− e−
Orange Green Light blue Dark blue Violet
[Lu(Pc)2]+ [Lu(Pc)2] [Lu(Pc)2]−e− e−
Orange Green Blue(a)
(b)
Scheme 3 One-electron redox processes of bis(phthalocyanato)lutetium(III) mesogens with alkoxymethyl (a) or alkythio (b) peripheral chains.
NNN
NN
N
N
N
RRR
R
RRR
R
R
a
c
b
N
NNN
N
N
N
N RR
RR
RR
RR
Ln
CH2OCnH2n+1
OCnH2n+1
SCnH2n+1
CnH2n+1
C6H4(OCnH2n+1)
O(CH2CH2O)3CH3
OC6H3(OCnH2n+1)2
d
e
f
g
206
Figure 43 Bis(phthalocyanato)lanthanide(III) mesogens.
Metallomesogens 895
reported by Piechocki et al. in 1985 (Figure 43, 206a: Ln¼Lu,
Lu[SbCl6]).964 They exhibited low-temperature Colh phases
over a narrow range when Ln¼Lu (�6 �C), but with a much
larger stability for the oxidized complexes with Ln¼Lu[SbCl6]
(80–140 �C).After that pioneering work, many other bis(phthalocyanato)
lanthanide(III) mesogens were reported with almost the entire
lanthanide series and bearing different peripheral substituents
such as alkoxy (206b: Ln¼Lu965–967 Nd, Eu, Er,968 Pr, Gd, Tb,
Dy, Ho, Er, Tm, Yb969), alkyl (206c: Ln¼Lu),970 p-alkoxyphenyl
(206d: Ln¼Lu),971 poly(oxyethylene) (206e: Ln¼Lu),228 and
thioalkyl chains (206f: Ln¼Eu, Tb, Lu).972,973 Most of the com-
plexes showed a columnar hexagonal mesophase with, in a few
cases, an additional rectangular (206b: Ln¼Lu, n¼18) or tetrag-
onal (206e: Ln¼Lu; 206f: Ln¼Eu Tb, Lu, n¼10, 12) phase.
Curiously, the lutetium(III) derivative with octadecyl chains
(206c: Ln¼Lu, n¼18) displayed initially a room-temperature
discotic lamellar phase (DL), but after the first heating–cooling
cycle, amixture of DL andColh phases was seen. The temperature
range of themesophases seemed to depend on both the type and
the length of the peripheral chains attached to the central core. In
this sense, the largest temperature ranges were found for the
alkoxy and thioalkyl derivatives, with widths around 120 and
140 �C, respectively. In the opposite way, some alkyl derivatives
were mesomorphic only over a very narrow range of 3 �C (206c:
Ln¼Lu, n¼8).
In one report, Binnemans et al. studied systematically
the effect of the metal ion and the chain length within a
series of bis(alkoxyphthalocyanato)lanthanide(III) complexes
(206b).969 They concluded that for a fixed metal (Ln¼Er),
both melting and clearing temperatures decrease similarly
when the chain length increased (n¼4–18, with Cr 202 Colh
>280 dec when n¼4, Cr 65 Colh 151 I when n¼18). However,
changing the metal did not influence the mesophase much,
and only a slight reduction in the temperature range was seen
from the light lanthanide ions to the heavy ones (n¼12;
Ln¼Pr (Cr 74 Colh 208 I), Nd, Eu, Gd, Tb, Dy, Ho, Er, Tm,
Yb, Lu (Cr 92 Colh 151 I)).
A very nice piece of work was developed by Ohta and
coworkers during the last decade. In an initial study,974 they
found that attaching 3,4-dialkoxyphenoxy groups to the
peripheral positions of the phthalocyanine, cubic phases
could be formed by a lutetium(III) sandwich complex (206g:
Ln¼Lu, n¼12, 13). Interestingly, the mesophase sequence
Colh–Cub1–Cub2–Colt was observed between the crystalline
and the isotropic state (from �30 to 230 �C). Later, they also
found a similar rich mesomorphism, including cubic phases,
in many other complexes based on similar phthalocyanines
and different lanthanide ions (206g: Ln¼La, Ce, Eu, Gd, Tb,
Yb, Lu; n¼8–16).189,975–979
Note that several physical studies were carried out for many
of these complexes, such as electrochemical behavior,980–982
and photoconductivity along the columns.983,984 Remark-
ably, the bis(phthalocyanato)lanthanide(III) mesogens can
exhibit electrochromism, showing more stability and versatil-
ity under electrochemical cycling than related, non-liquid-
crystalline compounds. For example, some derivatives
with lutetium(III) and octaalkoxymethyl- (206a)985,986 and
octaalkythio-phthalocyaninate moieties (206f)987 were found
to show intense changes by reversible oxidation/reduction pro-
cesses, both in solution or in thin films, thus presenting different
colors as a function of the applied potential (Scheme 3).
Ohta and coworkers also explored porphyrin systems to yield
cerium-containing mesogens.988,989 Double- (207) and triple-
(a)
CnH2n+1ON+ CmH2m+1
O– H
Ln(b)
Figure 45 (a) Crystalline structure of a neodymium complex with theformula [Nd(LH)3](NO3)3 and (b) proposed coordination of the ligand inlanthanide-containing complexes of salicylaldimines.
896 Metallomesogens
decker (208) sandwich derivatives were synthesized with disub-
stituted (R¼C6H3(OCnH2nþ1)2, C6H3(C6H4(OCnH2nþ1))2,
C6H3(C6H3(OCnH2nþ1)2)2; R0 ¼H) and tetrasubstituted porphy-
rins (R¼R0 ¼C6H3(OCnH2nþ1)2). The disubstituted systems
showed mainly DL mesophases, and also Colh and Colr arrange-
ments were observed in some cases. However, when four bulky
substituents were present, only Colr phases were seen for the
double deckers and none for the nonmesomorphic triple-decker
derivatives.
NN
NN
R�
R�
R
R
NNN
N
R�
R�
R�
RR
CeN
NN
N
R�
R�
R
R
NNN
N
R'
RR
Ce
NNN
N
R�
R'
RR
Ce
207
208
8.21.8.1.2 SalicylaldiminesGalyametdinov and coworkers developed the area of meso-
morphic salicylaldimine derivatives with lanthanide ions, and
many complexes were prepared with the ligands shown in
Figure 44 and using almost the entire lanthanide series. Ini-
tially, the formulation [Ln(LH)2(L)]X2 was proposed, where
LH is the neutral salicylaldimine, L its deprotonated form, and
X an anion.990 However, a single-crystal x-ray diffraction study
of one of the nitrate derivatives (Figure 45(a)), and selective
NMR decoupling experiments,991,992 later suggested that the
ions are nine-coordinated, and that three molecules of the
ligand in a zwitterionic form are linked to the ion only through
the oxygen atom (Figure 45(b)). Thus, the correct formulation
could be [Ln(LH)3]X3.
This way, many calamitic complexes were reported based on
the one-ring salicylaldimine ligand (209a) with different anions,
CnH2n+1O
CnH2n+1O
CnH2n+1O
[Ln(LH)3]X3
[Ln(LH)2(L)]X2
or
209
a:
b:
c:
LH
Figure 44 Lanthanide complexes of mesogenic salicylaldimines.
such as X¼NO3,562,563,991,993–998 Cl,999,1000 DOS,1001–1003
alkysulfate,1004,1005 and perfluoroalkylsulfate.1006 In a similar
way, nitrate and dodecylsulfate derivatives with elongated sali-
cyldimines were also explored (209b990,1007 and 209c1008). As
expected from their rod-like shape, all these complexes exhibited
smectic mesophases (SmA, SmC, and an unidentified smectic
phase). In general, the influence of both chain lengths was found
not to be very important, whereas the lanthanide affected ther-
mal behavior much more, decreasing the clearing point and
increasing the melting point when going from the lighter ions
to the heavier ones.
Some modifications of the structure induced different
liquid-crystalline behavior. For example, by attaching an addi-
tional peripheral chain on the ligand in complexes 209b
CmH2m+1
CmH2m+1
CmH2m+1
NOH
ON
OH
O
ON
OH
OO
O
Metallomesogens 897
(Figure 44), a Colh mesophase was observed.1009 Recently,
Yelamaggad et al. reported a series of chiral complexes with
La(III), Gd(III), and Yb(III), and a cholesteric group attached to
the salicylaldimine ligand a.1010 All these compounds, with the
formula [Ln(LH)2(L)](NO3)2, exhibited SmA* mesophases.
A series of dinuclear complexes with Nd(III), Sm(III), and
Gd(III) was synthesized by complexation of a specifically
designed Schiff base by Binnemans et al. (210).1011 Neutral
complexes with the formula Ln2L2 were achieved; thus, coun-
teranions were not needed in this case. These materials dis-
played a very stable Colr mesophase, with melting points
above 100 �C and over a 120–140 �C range.
H2n+1CnO
H2n+1CnO
H2n+1CnO
H2n+1CnO
H2n+1CnO
O
O
O
H2n+1CnO
O
OOCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1
OM
Ln (NO3)3
O
O
O
N N
N N
O
O
OM
N
NN N
N N
211
210
O
N
O
O
O
N
O
O
O
OC14H29
C14H29O
C14H29O
C14H29O
C14H29O
O
N
O
O
O
N
O
O
O
OC14H29 OC14H29
OC14H29
OC14H29
OC14H29
OC14H29
OC14H29
LnLn
Salen-like ligands were also investigated to prepare
lanthanide-containing mesogens. In addition to mononu-
clear derivatives,992,1012 heteropolynuclear f–d systems were
also achieved.568,1013,1014 A dinuclear Cu–Gd derivative and
various trinuclear complexes (211: M¼Cu, Ni; Ln¼Gd, La)
were synthesized by reaction of the copper or nickel salen
complex with the corresponding lanthanide nitrate. In all the
cases, the materials formed Colh mesophases over large tem-
perature ranges, always wider than the mesophase range of
the mononuclear d- or f-metal salen complexes exhibited by
their own.
All attempts to prepare complexes with lanthanide metals
and only mesogenic b-diketones failed. However, various
mixed mesomorphic salicylaldimine-b-diketone compounds
were synthesized successfully (212). Almost the entire
lanthanide series (except Ce and Pm) was complexed to two
units of a zwitterionic Schiff base and three molecules of bis
(phenyl)-b-diketonate (212a).1015,1016 Unfortunately, these
materials showed poor mesomorphism, and only the com-
plexes of the lightest ions (from La to Eu) showed monotropic
SmA phase. However, the substitution of the phenyl rings on
the b-diketone by a CF3 or a thiophene group (212b, 212c)1017
promoted a significant stabilization of the SmA mesophase,
now enantiotropic and possible over a slightly wider tempera-
ture range (15–45 �C) and for heavier ions (La–Er). Moreover,
the physical properties of some of these liquid crystals were
studied, such as the luminescence of the Eu, Sm, Nd, and Er
derivatives,1017,1018 and a pressure-induced phase transition of
one of the Eu complexes.1019
-O
N
C18H37
OC14H29
H+
LnO
O
R1
R23
2
a: R1, R2 = Ph, Ph
b: R1, R2 = Ph, CF3
c: R1, R2 = thiophene, CF3
212
Finally, it is worth mentioniong that, with a very similar
structure to the salicylaldimine complexes, b-enaminoketonate
898 Metallomesogens
derivatives were also investigated (213: Ln¼La, Dy, Gd, Er, Eu,
Tb; X¼NO3, Cl).1020–1022 These complexes showed SmA
mesophases, but over small temperature ranges.
CnH2n+1OO H
N CmH2m+1LH
[Ln(LH)(L)2]X2
213
8.21.8.1.3 N-Donor chelating ligandsGalyametdinov and coworkers reported recently a series of
novel complexes consisting of three b-diketonate ligands and
one substituted 2,20-bipyridine (214: R¼C7H15, Ln¼La, Nd,
Eu, Tb1023; R¼CnH2nþ1, n¼1–8, Ln¼La1024). All these mate-
rials showed an SmA mesophase followed by a N phase. How-
ever, the interest resides in the photophysical properties of the
europium derivative as polarized luminescence from aligned
samples was achieved. Moreover, when the complex was
dissolved in an eutectic mixture of 4-cyanobiphenyls and
4-cyanoterphenyls, it was possible to induce a preferential
alignment by application of an electric potential in a cell.
OO
R
NN
Ln
3
H35C17 C17H35
C3H7
214
Similarly, tris(b-diketonate)phenanthroline–lanthanide(III) complexes were explored (215: Ln¼La, Nd, Eu, Gd, Tb,
Dy, Er, Yb).1025 However, only poorly stable monotropic SmA
phases were observed for the derivatives from Eu to Yb.
216
N
N N
NHO
O
OF3C
Ln
S
3
LnN
NN
NHH29C14O
H29C14O
H29C14O
Cl C
O CH2
217
OO
NN
Ln
3H33C16O
215
OC16H33
In contrast, by functionalization with different motifs
phenantroline-based ligands and their coordination to lantha-
nide cores, nematic (216: Ln¼Y, La, Nd, Sm, Eu, Er, Yb)1026
and cubic (217: Ln¼La, Pr, Nd, Sm, Eu)663 phases were
achieved. While the nematic phases of the former complexes
existed over a �40 �C range and around a temperature of
100 �C, the cubic phases of the other derivatives appeared
at much higher temperatures (above 200 �C) and over a
30–80 �C range.
In an extensive study, Piguet and coworkers reported inves-
tigations of lanthanide-containing mesogens based on terden-
tate N,N,N-donor set ligands (Figure 46). They analyzed
thoroughly the influence on the mesomorphic properties of
the different substitution patterns (on the 5- or 6-position) and
a varied set of substituents (a, b, c) and also described the
changes induced by the coordination of the lanthanide frag-
ments to the ligands. Thus, in their free state, the ligands exist
in a trans–trans conformation, whereas once complexed, the
cis–cis geometry (as depicted in Figure 46) is adopted.
Neither the five- nor the six-substituted derivatives with
the smallest core (218a, b) and one or two terminal chains per
substituent (218b: R1, R2¼H, OC12H25, R3¼H) were found
to be mesomorphic.1027,1028 Interestingly, the introduction of
an additional alkoxy chain (218b: R1¼R2¼R3¼OC12H25)
promoted a very rich mesomorphism of LCol, Cub, and Colhphases depending on the size of the ion.1029–1031 In an oppo-
site way, but as expected from their calamitic shape, deriva-
tives with very elongated substituents (218c: X¼H, CH3) were
found to form bilayer SmA phases.1032,1033 Remarkably, the
attachment of the methyl group to the cyanobiphenyl moiety
(CH2)10O CN
O(CH2)10 O CN
N
N N
NH
OC14H29
OC14H29
OC14H29l
l
N
N
NN
N[Ln]
O
OR2
R1
R3
[Ln] = LnX3
X = NO3,CF3CO2
66N
N
NN
N[Ln] 55
OOC12H25
X
CN
O
OCO2(CH2)10O CO2
OO(CH2)10O
OO(CH2)10O
CO2
X
CNCO2
ba
c
218
Figure 46 Lanthanide-containing mesogens with terdentate N,N,N-donor ligands described by Piguet and coworkers.
Metallomesogens 899
seemed to induce the formation of an additional interdigi-
tated SmA phase or a nematic phase in the case of the smallest
ions. It is worth mentioning that very detailed photophysical
studies of these complexes were also included in all these
reports.
By changing the ethyl group of the imidazole moiety by
introducing an elongated, three-chained fragment, some
dendritic-like complexes were prepared (219: [Ln]¼LnX3,
X¼NO3, CF3CO2).1034 Again, the materials showed a very
rich mesomoprhism (SmA, Colh, and Cub phases), but in
this case, with low melting temperatures (between �43 and
�25 �C) as a consequence of the presence of 12 aliphatic
chains in the system.
N
N
NN
N OO
OOOC12H25
OC12H25
OC12H25
C12H25O
C12H25O
C12H25O
O
O
OC12H25
OC12H25
OC12H25O
O
OC12H25
C12H25O
C12H25O
[Ln]
219
8.21.8.2 Magnetic Properties of Metallomesogens
8.21.8.2.1 Magnetic alignment and magnetic anisotropyIn the liquid-crystalline state, the molecules can be aligned by
application of an external magnetic field.949,1035 This effect is
based on the intrinsic anisotropy of the molecules constituting
the material, which tend to orient in a direction where their
magnetic energy is minimum. While for discrete molecules this
behavior can be cancelled by thermal agitation, the cooperative
effect of the molecules in a mesophase can overcome the
thermal factor and create a preferential arrangement driven
by the applied magnetic field.
An aligned mesophase is sometimes necessary for carrying
out measurements of anisotropic physical properties such as
x-ray diffraction, dielectric permittivity, and electrical conductiv-
ity. But for an effective alignment, a significant magnetic anisot-
ropy (Dw) is required. In this sense, the presence of metal atoms
in the liquid crystal could enhance this property, and in partic-
ular, paramagnetic lanthanide-containing mesogens are able to
exhibit high values, sometimes a few orders of magnitude larger
than those of simple organic materials. Diamagnetic calamitic
metallomesogens usually present a positive magnetic anisot-
ropy, while diamagnetic discotic materials have a negative mag-
netic anisotropy. This is an important parameter because the
HO
OOR
[Mn12O12(OAc)16(OH2)4] + 16 HL [Mn12O12(L)16(OH2)4] + 16 HOAc
OR
OR CN
(CH2)11b
LH = R
C12H25
O
a
Figure 47 Functionalization of the mixed valence manganese clusterwith mesogenic carboxylates.
900 Metallomesogens
orientation of the molecules in a magnetic field is driven by the
sign of their magnetic anisotropy. Thus, a positive value induces
a parallel orientation of the director of the mesophase with the
magnetic field, whereas a negative magnetic anisotropy involves
a perpendicular arrangement. One interesting example was
reported by Pate et al. in the form of metalloporphyrazine
with diamagnetic first-row transition metals (220: M¼Ni,
Zn).302 During the cooling process from the isotropic state, the
diamagnetic discotic zinc and nickel derivatives are arranged in a
columnar mesophase with the director perpendicular to an
applied magnetic field as low as 0.50 T.
N M
N
N
N
SC10H21N N
NN
SC10H21
C10H21S
C10H21S
SC10H21
SC10H21
C10H21S
C10H21S
220
On the other hand, the orientation of mesogens with
paramagnetic d-metal centers is difficult to predict. The rea-
son is the existence of a competition between the paramag-
netic and the diamagnetic contribution to the overall
magnetic anisotropy of the molecule. This way, in the same
example shown before, paramagnetic porphyrazine com-
plexes were also studied (220: M¼Co, Cu), and while the
columns of cobalt derivatives oriented perpendicularly just
under a 0.25 T magnetic field, the copper complexes never
showed a preferential orientation. This can be explained by
the fact that the paramagnetic and diamagnetic moments in
the Co(II) complex are both oriented in the same direction,
thus ‘increasing’ the negative value of the magnetic anisot-
ropy. In contrast, in the Cu(II) derivatives the paramagnetic
moment is directed along the molecular z axis while the
diamagnetic contribution is in the xy plane, and because the
values are comparable, there is not a preferential orientation
of the columnar director.
In an earlier series of papers, and as mentioned above,
Galyametdinov and coworkers studied several series of calamitic
paramagnetic salicylaldimine-copper(II) and -oxovanadium(IV)
complexes (221: M¼Cu, VO; R¼F, OCnH2nþ1).461–464,467–
469,1036,1037 The copper derivatives were described as the first
liquid crystals to form a paramagnetic nematic phase. Several
EPR studies carried out by Marcos and coworkers about this
kind of mesogens473,475,476 gave rise to the knowledge that the
magnetic anisotropy of the vanadyl compounds was positive,
while the copper-containing materials had a negative value.
O M
N
NO
R
RH2n+1CnO
OCnH2n+1
221
On the other hand, when lanthanide ions are present (par-
ticularly the heaviest members: Tb3þ, Dy3þ, Ho3þ, Er3þ, andTm3þ), the diamagnetic contribution can be neglected in com-
parison with the paramagnetic moment. Therefore, the
lanthanide-containing mesogens exhibit a very large magnetic
anisotropy and an easy orientation under an external magnetic
field. The sign of the magnetic anisotropy of the material
cannot be predicted,1038–1040 but curiously, it is possible to
create two groups of ions: Ce3þ, Pr3þ, Nd3þ, Sm3þ, Tb3þ, Dy3þ
and Ho3þ on the one hand, and Eu3þ, Er3þ, Tm3þ, and Yb3þ
on the other. Complexes from different groups would always
show opposite signs for the magnetic anisotropy: if Dw<0 for
the first series of complexes, we could expect Dw>0 for the
second series, and vice versa.
As shown in the previous section, many lanthanide-
containing mesogens have been reported. However, only the
magnetic anisotropy of calamitic salicylaldimine derivatives
(209a) was investigated thoroughly.991,1041 In agreement
with the classification mentioned above, either a perpendicular
(Dw<0: Ce3þ, Pr3þ, Nd3þ, Sm3þ, Tb3þ, Dy3þ, Ho3þ) or a
parallel alignment (Dw>0: Eu3þ, Er3þ, Tm3þ, Yb3þ) to the
applied magnetic field was achieved. However, the high viscos-
ity of the smectic phases was found to be an issue in achieving a
good alignment in short times or low temperatures. Thus, slow
cooling rates from the isotropic state were necessary to create a
significant magnetic alignment.
8.21.8.2.2 Single-molecule magnetsA single-molecule magnet (SMM) is a discrete, molecular-sized
system that is able to retain a magnetized state even after
the external magnetic field is gone.1042,1043 The material main-
tains the molecular magnetization only below a certain
temperature, known as the blocking temperature (TB). Differ-
ent systems with this capacity have been explored, such as
oxometallaclusters1044,1045 or polynuclear cages1046 among
others, but all of them are based on metal ions bound together
by organic ligands.1047–1049 The interest of these materials
resides in the potential applications for areas like quantum
computation and molecular magnetic memory devices.
Nevertheless, only a few liquid-crystalline SMMs have been
reported so far. One of the most studied nonmesomorphic
SMMs is a mixed-valence dodeca-manganese cluster with the
formula [Mn12O12(OAc)16 (OH2)4].1050 Terrazi et al. found a
way to functionalize the cluster by the substitution of the acetate
group with trisubstituted-benzoate ligands (Figure 47).1051 This
way, the material with the tris(dodecyloxy)benzoate (ligand a)
was able to form a room-temperature cubic phase (from
�11.5 �C), while the cyano-biphenyl derivative (ligand b)
HO
OO
O
O
X = H, CNLH =
X(CH2)10
O
X(CH2)10
O
(NBu4)2[Mo6Br8F6] + 6 HL (NBu4)2[Mo6Br8F6] + 6 HF
X(CH2)10 O
Figure 48 Functionalization of a molybdenum manganese cluster withmesogenic carboxylates.
Metallomesogens 901
exhibited a rare lamellar mesophase called ‘filled random mesh
smectic phase’1052 from 40.5 �C. Both materials decompose
before reaching the clearing point at 150 �C. Remarkably, the
magnetic properties of the cluster were well retained in the novel
liquid-crystalline material, and the values of 2.4 and 1.8 K were
found for the blocking temperatures of the cyanobiphenyl and
the tris(dodecyloxy) derivatives, respectively.
Similarly, Molard and coworkers have recently investi-
gated the functionalization of a the molybdenum cluster
(n-Bu4N)2[Mo6Br8F6] with three-chained carboxylate groups
(Figure 48).1053,1054 Smectic behavior was found for the
novel mesogenic clusters and their light-emissive properties
were investigated.
Following a different approach, a double-decker terbium
complex with peripheral chiral aliphatic chains (222) was
investigated as a SMM.1055 The authors found that the com-
pound existed in a Colh mesophase at room tempera-
ture. Remarkably, by kinetically trapping the system at low
temperatures they managed to measure and compare different
magnetic properties of the complex in a disordered state, a
partially disordered state, and an ordered crystalline state.
The most interesting finding was the coexistence of two very
different relaxation times, and that was attributed to the pres-
ence of two molecular conformations for the sample at low
temperature.
NNN
NN
N
N
N
RRR
R
RRR
R
N
NNN
N
N
N
N RR
RR
RR
RR
Tb
R = CH2C
O (CH2)11–CH3
H3C H
222
8.21.8.2.3 Spin-crossover compoundsSpin-crossover materials are d-metal-containing products
that display labile and switchable electronic spin configura-
tions.1056–1059 The change between high-spin (HS) and low-
spin (LS) states always involves not only effective changes
in the magnetic properties but also changes in the electronic
properties (color and dielectric constant) and even in the
structure of the material. The crossover between the two
spin states can be induced by variations of temperature or
pressure, and sometimes by the effect of light, too. The
metal ions implied in spin-crossover phenomena are those
with a d4 to d7 electronic configuration. Octahedral com-
plexes of Fe(II) are the most studied class of spin-crossover
materials, but also complexes of Fe(III), Co(II), Cr(II), or
Mn(III) have been thoroughly investigated in this sense.
As in other areas, bifunctional materials have been developed
through the combination of both the spin-crossover behavior
and the inherent order of liquid crystals. Properties such as
change of color (photo- and thermo-chromism) can be brought
to the liquid-crystal area, at the same time as the enhancement of
spin-transition signals and the processing of thematerials in thin
films constitute useful tools for the study of spin-crossover phe-
nomena. There are recent and systematic reviews covering the
field of spin-crossover metallomesogens.949,1060,1061 Therefore,
the aim of this section is only to present different approaches
used so far to create this class of materials.
The first example to be reported was an iron(III) complex
based on tridentate salicylaldimine-like ligands and was syn-
thesized by Galyametdinov et al. in 2001 (223).1062 This ionic
octahedral complex exhibited an SmA phase over a moderately
wide temperature range (115–146 �C). Unfortunately, the spin
transition temperature (T1/2) was found around �148 �C,hence by far outside the liquid-crystalline range. This parame-
ter was obtained from the analysis of the magnetic moment
as a function of the temperature, and confirmed by 57Fe
Mossbauer spectroscopy.
Later, Hayami et al. reported two different series of iron(II)
metallomesogens (224)1063,1064 with a behavior similar to that
observed in the previous iron(III) derivative. Some of the iron
(II) complexes (224: n¼12, 14, 16, 18, 20) displayed what
seemed to be an SmA mesophase over a temperature range
reduced with the increasing length (n¼12: 45–131 �C;n¼20: 74–119 �C). For their part, the spin-crossover transition
temperatures varied from �43 �C (n¼12) to �68 �C (n¼20),
which implied a clear increase compared to those found
for nonmesomorphic analogs (<�96 �C), but still signifi-
cantly far from the liquid-crystalline state range. Some of
these complexes also exhibited light-induced spin transition
(LIESST).
O
N
O
O
OC12H25
NHC2H5
FeO
N
O
O
C12H25O
HN C2H5
(PF6)
223
902 Metallomesogens
N
N
H2n+1CnOOCnH2n+1
OCnH2n+1
Fe
N
NH2n+1CnO
H2n+1CnO
H2n+1CnO
NCS
NCS
224
However, by playing with different combinations of chain
length and branched systems, the same authors achieved a bis
(terpyridine)cobalt(II) complex (225)1065 that presented a
coupling between the mesophase transition and the spin tran-
sition: an SmA phase was visible from 15 to 250 �C, while thelow-to-high spin transition was found within a thermal hyster-
esis loop of T1/2¼15 and T1/2¼11 �C.
Fe NN
N
NN
N
N2+
2A−
A = ClO4, Cl, F, Br, I
N
RH2n+1CnO
226
Figure 49 Structure of iron(II) mesogens and their ligands, showing differe
NN
N
N
N
N
M(CH 2)4H21C10
CH OH25C12
225
In a very interesting piece of work, Seredyuk et al. achieved
a series of mesogenic iron(II)-containing materials showing
varied spin-crossover behavior in close interplay with their
mesomorphic properties.1066–1069 They described three classes
of materials as a function of the synergy between the spin-state
transition and the solid to liquid-crystal transition: (1) systems
with coupled spin and phase transitions; (2) systems with coex-
istence of both transitions in the same temperature region, but
uncoupled; and (3) systems with uncoupled transitions in
different regions. Remarkably, in one particular study,1066 they
managed to synthesize a series of octahedral complexes
(Figure 49, 226) that were able to present those three different
behaviors only by modification of the chain lengths and the use
of different counter-anions.Moreover, the authors demonstrated
that the solid-to-liquid-crystal phase transition could be the
driving force of the spin-state transition in some of these cases.
In another piece of work, the same authors have also stud-
ied polymeric 1D1069,1070 and 2D1071,1072 systems based on
iron complexes with both spin-crossover and liquid-crystalline
behavior. In particular, two different 2D lattices were formed
by heteropolymetallic frameworks of iron(II) and metal(I)
N
N
R = H, CH3
N
R
OCnH2n+1
N
N
N
R OCnH2n+1
nt spin-crossover behavior as a function of the chain length.
O (CH 2)4C10H21
C12H25
CH (BF4)2
Fe NN
L
LN
NM
M N Fe
L
LN
M N Fe
L
L
N
M
Fe N
L
LN
Fe N
L
L
N
228
227
R3
R2
R1
NL =
Fe NN
L
LN
N
M
M
M
M
Fe
L
LN
Fe
L
L
NN
N
Fe
L
L
Fe
L
L
Figure 50 Two-dimensional, heteropolynuclear mesogens showing spin-crossover behavior.
Metallomesogens 903
(227: M¼Ag, Au) or metal(II) (228: M¼Ni, Pd, Pt) centers
connected by cyanide groups (Figure 50). The coordination of
pro-mesogenic mono-, bis-, or tris-(alkoxy)phenylpyridines
(L: R1, R2, R3¼H, OCnH2nþ1) to the iron atoms led to the
formation of smectic mesophases above about 100 �C. How-
ever, the spin-crossover was found to occur over an incomplete
and continuous transition at T1/2��100 �C.
8.21.8.2.4 Magnetic susceptibility of mesogeniccarboxylates and polymeric acetylidesSeveral studies of the magnetic properties of transition metal
carboxylates have been reported. One of the earliest was the
measurement of high-sensitivity magnetic susceptibility
(SQUID) of a series of thermotropic columnar dicopper(II)
carboxylates (229: R¼C11H22, C17H25, CH2CH(C9H19)2).150
Giroud-Godquin et al. observed a slight drop of the suscepti-
bility near the phase transition temperature, indicating a
decrease in the paramagnetic contribution as the sample went
from the crystal to the columnar mesophase. This way, the
magnetic susceptibility was found to be a highly sensitive
probe of the reversible phase transitions.
229
R
O
O
Cu
O
O
O
O
O
R
R
RCu
O
The magnetic behavior of ruthenium carboxylates was
investigated thoroughly. Following theoretical and experimen-
tal studies of dimeric ruthenium systems developed by Cotton
et al.,1073 the Grenoble group carried out investigations of
the magnetic properties of some mesomorphic Ru(II)–Ru(II)
carboxylates (230: R¼C11H23, CH2CH(CH3)(CH2)2CH¼C
(CH3)2, (CH2)8CH¼CH2)135,137 and mixed-valence Ru(II)–
Ru(III) derivatives (231: X¼Cl, DOS, RCO2; R¼CnH2nþ1,
C6H3(OCnH2nþ1)2).151 This work gave rise to a detailed inter-
pretation of the electronic structure of the dinuclear, bonded
system and the interdimer interactions present in the com-
pounds. The magnetic features of the dimetallic moities seemed
to be essentially independent of the carboxylate substituent.
However, a discontinuity in the magnetic susceptibility curve
at a temperature identical to the transition temperature to
the columnar mesophase was observed for all the mesomorphic
complexes. The authors attributed this break to some slight
modification of the electronic structure due to the structural
reorganization during the transition to the liquid-crystalline state.
231
R
O
O
Ru
O
O
O
O
O
R
R
RRu
O
230
R
RuO
O O
O
O
O
O
R
R
RRu
O
X
Takahashi and coworkers investigated the magnetic anisot-
ropy of polymeric acetylide complexes with lyotropic nematic
behavior.1074–1079 For instance, homo- (232: M¼M0 ¼Pd, Pt)
and heterometallic (232: M¼Pt, M0 ¼Pd or Ni; M¼Pd,
M0 ¼Ni) polymers of 1,4-butadiyne showed a negative diamag-
netic anisotropy (Dw<0) when dissolved in concentrations of
about 36 wt% in trichloroethene, whereas a related palladium
(II) copolymer of 1,4-butadiyne and 1,4-diethynylbenzene
(233) displayed a positive magnetic anisotropy (Dw>0). The
change in sign of Dw was explained by the fact that in such an
arrangement carbon–carbon triple bonds and benzene groups
show opposite sign for Dw (parallel and perpendicular to the
molecular axis, respectively), with the negative value from the
ring clearly dominating.
M
PBu3
PBu3
M�
PBu3
PBu3 n
Pd
PBu3
PBu3
Pd
PBu3
PBu3 n
232 233
904 Metallomesogens
8.21.9 Conclusion
One of us (DWB) used the original volumes of Comprehensive
Inorganic Chemistry a great deal as a young academic. His work
at that time was not so much about pushing forward the
frontiers of synthetic inorganic chemistry, rather finding
what others had done and elaborating their complexes to
produce new materials with properties of interest – in his
case liquid crystals. Therefore, to be able to contribute to a
rewrite is a great thrill.
What has been instructive to both of us as we have written
this chapter is to bear in mind that many subjects included in
this new edition did not exist or were hardly thought of when
the first edition was prepared. That this is the case is a testament
to the vigor of the subject, but it also gave us cause to consider
what it was that we wanted to do for there are many reviews of
the subject and a repeat of all those is not very helpful. There-
fore, we chose to focus on one series of materials with a com-
mon structural feature (ortho-metallation) and another with a
common property (magnetism), hoping in this way to show the
breadth of imagination brought to bear on the subject. Whether
we have succeeded is for others to judge.
However, what we have tried to do is capture some of the
diversity of the subject and in this way we hope to inspire many
others to take up the challenge. For a related chapter in this
Comprehensive, we refer to Chapter 8.20.
Acknowledgments
It is appropriate to thank gratefully Linda McAllister and
Dr Alvaro Diez who read the final version of the manuscript
and commented critically. DWB wishes to acknowledge his co-
author, Javier Torroba who was the one that made this review
possible by writing almost all of it. JT wants to thank Comu-
nidad Auntonoma de La Rioja (Spain) for the financial support
of his postdoctoral fellowship and, gratefully, Duncan Bruce
for sharing and spreading his wide knowledge of the subject.
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