metallomesogens: metal complexes in organized fluid phases

Metallomesogens : Metal Complexes in Organized Fluid Phases

By Anne-Marie Giroud-Godquin and Peter M. Maitlis *

Metallomesogens, metal complexes of organic ligands which exhibit liquid crystalline (mesomorphic) character, combine the variety and range of metal-based coordination chemistry with the extraordinary physical properties exhibited by liquid crystals. Thermotropic metallomesogens have been made incorporating many metals, including representatives of s-, p-, d- and even f-block elements. Both rodlike (calamitic) and disklike (discotic) thermotropic metallomesogens are known, and examples of all the main mesophase types are found. Many different varieties of ligand can be used: monodentate (4-substituted pyridines), bidentate (P-diketonates, dithiolenes, carboxylates, cyclometalated aromatic amines), or polydentate (phthalocyanines, porphyrins). As with organic mesogens, molecular shape and intermolecular forces play an important role, i.e. the ligands are important in determining mesophase character. The chief requirement for a metallomesogen is a rigid core, usually unsaturated and either rod- or disklike in shape, bearing several long hydrocarbon tails. The metal atom is usually at or near the center of gravity of the molecule. In some cases the ligands are themselves mesogenic, but this is not a requirement. The presence of one or more metals opens many exciting possibilities : new shapes, not easily generated by organic compounds, and hence new properties are then accessible. The incorporation of d-block metals brings with it features such as color and paramagnetism. Profound effects arise from the large and polarizable concentra- tion of electron density that every metal atom possesses, since the molecular polarizability is a key factor in determining whether a molecule will form liquid crystals. Enhanced physical properties (e.g. high birefringencies), as well as new and unexpected ones, will result. A major requirement for metallomesogens to find applications in new device technology is that the metal-ligand bonds are strong and inert and the complexes stable; this can be accomplished with, for example, chelating ligands and the 5d metals.

1. Introduction

Scientific advances frequently take place at the interfaces between disciplines. Metallomesogens introduce to metal- based coordination chemistry the unusual physical properties exhibited by liquid crystals, so useful in electronic devices and elsewhere. Although metallomesogens have been known for some eighty years, only now is this area, which we have had the pleasure of helping to develop, springing into prominence.

Most work has concentrated on organic materials, but recent developments have highlighted the many new possibilities which can result from the introduction of one or more metal atoms into a liquid crystal; there are, after all, some sixty metals which can in principle be coordinated.

Reasons for putting a metal into a liquid crystal range from satisfying intellectual curiosity (it’s an intriguing com- bination: what will the properties be?) through to the severe- ly practical (novel substances for molecular electronics, new electrical or magnetic switches, conductors, new optical

[*I Prof. P. M. Maitlis FRS Department of Chemistry, The University Shefield S 3 7HF (UK) Dr. A. M. Giroud-Godquin Laboratoire de Chimie, Chimie de Coordination Departement de Recherche Fondamentale Centre &Etudes Nucleaires de Grenoble 38041 Grenoble Cedex (France)

[*I The terms “dithiolenes” and “dithiolene complexes” introduced here are treated as trivial names. Strictly speaking these compounds should be called “enedithiols” and “enedithiolate complexes”, respectively.

devices). Geometries and hence shapes not easily found in organic chemistry can result from coordinating a metal; metals show square planar, square pyramidal, trigonal bipyramidal, octahedral, and a remarkable variety of geometries for coordination numbers 2 7.

Many of the d- and f-block transition metal complexes are in oxidation states which give colored compounds, and many of them have unpaired electrons and exhibit paramagnetism, further extending the range of potentially useful physical properties.

But perhaps the most important effects arise from the large and polarizable electron density which is a feature of every metal atom. Polarizability is one of the most important attributes of molecules which form liquid crystals and any increase will have very profound effects on the physical char- acteristics, with the possibility of a useful but unexpected macroscopic property resulting.

1.1. What are Liquid Crystals?

Liquid crystals form a state of matter intermediate between the solid and the liquid. The terms mesomorph (Greek, mesos morphe, between two states, forms) and mesophase are also used; a mesogen is the molecule which gives rise to a mesophase. Organic liquid crystals have been known for over a century.[’’ They can be divided into two broad fami- lies, the thermotropicd2I and the l y o t r o p i ~ s . ~ ~ ] Thermotropic liquid crystals change phase on heating or cooling; the crystal phase melts to the mesophase, which then clears to the isotropic liquid at a higher temperature. Some exhibit poly-

Angew. Chem. Int. Ed. Engl. 30 (1991) 375-402 0 VCH Verlagsgesellschaft mbH, W-6940 Weinheim. 1991 OS70-0833/91/0404-0375 $3.50+ ,2510 315

morphism: they show several mesophases. Lyotropic phases are formed by molecules in a solvent (generally water) and the appearance of the mesophase is controlled by the concen- tration.

A crystal (denoted by K or C) has defined shape, and most of its physical properties are anisotropic because the constit- uent molecules or ions are ordered in position and orientation. By contrast, the component molecules (or ions) of a fluid lack positional and orientational ordering; hence the physical properties are isotropic (I) and a fluid takes up the shape of the container. In a thermotropic mesophase the positional order is largely absent, giving a fluidity, while some orientational order is retained, giving the anisotropy. The transition from the crystal to the mesophase is often termed the melting point, and that from the highest mesophase to the isotropic, the clearing point, reflecting the fact that mesophases often appear turbid while the isotropic liquid is clear.

One consequence of this anisotropy is that mesophases can have two (or even three) different refractive indices, magnetic susceptibilities, and electric permittivities. As a result, some thermotropic molecules can be oriented by ap- plied electric fields.

Thermotropics can be further sub-divided into calamitic (reed- or rodlike) and discotic (disklike, denoted by D);t41 polymerict5’ liquid crystals are also known. Rodlike thermotropics form two broad classes [nematics (N) and smectics (S)] and several subgroups of discotics are also known.

In the least ordered phase, the nematic, the rodlike molecules line up approximately parallel to each other (Fig. 1); this direction is known as the director. The transition from the crystalline to the nematic phase (K N) is usually associated with an enthalpy change about ten times higher than the nematic to isotropic (N -+ I) transition.

A very useful characteristic of nematic mesophases is the ease with which they are reoriented by a magnetic or electric field. This has found practical application in electronic display devices. Nematic molecules are achiral (or racemic); the introduction of a chiral center into such a molecule gives rise to cholesteric behavior, where the director is now helical.

Fig. 1 . Schematic representation of a calamitic nematic phase.

Smectic liquid crystals form a variety of layered phases, which offer additional order over that found for nematics; some have phases with the molecules perpendicular to the layers (e.g. S,; Fig. 2, top), in others they are inclined to them (e.g. S,; Fig. 2, bottom).

Discotic liquid crystalline phases are obtained with disklike molecules where the director is perpendicular to the molecular plane; columnar and nematic (Fig. 3) as well as

Peter Maitlis is Professor of Inorganic Chemistry at Sheffield University. He was educated in England (PhD with Professor M . J S. Dewar in London, in 1956), and then spent twelve years in North America where he was Professor at McMaster University in Canada before returning to England and taking up the Chair in Sheffield in 1972. His research interests include organotransition metal chemistry and homogeneous catalysis. During the last five years he has become increasingly involved in the design of coordination complexes as Advanced Materials, in particular as liquid crystals, where he has developed the study of metallomesogens with his group. In 1984 he was elected to the Royal Society, and in 1986 he received the Sir Edward Frankland Prize Lectureship of the RSC for his work in organometallic chemistry. He has also been President of the Dalton Division of the RSC and was Chairman of the Chemistry Committee of the Science and Engineering Research Council.

Anne-Marie Giroud-Godquin was born in Casablanca (Morocco). After secondary school (Lycke de Casablanca) she completed her undergraduate studies in physics and chemistry at La Sorbonne (University of Paris). She discovered the excitement of chemical research with Dr. J. Jacques at the Colltge de France during her ThPse troisi2me cyle in i 961. She then entered the Centre National de la Recherche Scientifique, where she gained her PhD (in steroid chemistry) from the University of Grenoble with Professor A. Rassat. After postdoctoral work with Profes- sor G. Luckhurst at the University of Southampton, she returned to Grenoble to develop her original research on metal-containing liquid crystals. Later she spent some months at IBM in California with Professor U. Miiller- Westerhog. She is now Directeur de Recherche at the CNRS Chemistry Laboratory of the Centre d’Etudes Nucl2aires at Grenoble, where she pursues her researches on metallomesogens.

376 Angew. Chem. Int. Ed. Engl. 30 (1991) 37s-402

f

Fig. 2. Schematic representations of calamitic smectic phases. Top: S, phase; bottom: S, phase.

other types are known. Typical organic discotics are benzene derivatives with six long n-alkyloxy tails, 1; many metallo-

R O R V O R

RO OR

1

mesogens (such as those based on phthalocyanine, por- phyrin, or even P-diketonate ligands) also form discotic phases. The two-dimensional network of such a columnar

nn

Fig. 3. Schematic representations of columnar-discotic (left) and nematic-discotic phases (right).

mesophase can be hexagonal, rectangular, or oblique, and the molecules can be either ordered or disordered along the long axes of the columns (Fig. 4).

Thermotropics are held together by weak dipole-dipole and dispersion forces. Their magnitude is critical: when they are too weak or when they are too strong, the liquid crys-

Fig. 4. Schematic representations of columnar-discotic phases. Left: ordered hexagonal, Dko; center: disordered hexagonal Dnd; right: rectangular, D,.

talline character is lost. Hence the molecular features which optimize thermotropic behavior are very important.

To exhibit liquid crystalline properties organic compounds require strongly polarizable groups, such as aromatic rings (A, B in Fig. 5), esters, or other oxy- (X, Y) or nitrile-functions. Rodlike thermotropic mesogens also require a long rigid group (often phenyl rings which form a core), while a rigid conjugated platelike grouping is usually associated with the core of a discotic mesogen. Both types need several long flexible n-alkyl- (or n-alkyloxy-) tails (R). The rod- or disklike shapes, together with the polarizable groups, increase the molecular anisotropy and facilitate formation of liquid crystal phases.

Fig. 5. Schematic representation of a rodlike, thermotropic, organic mesogen.

It is important to note that none of the mesophase structures illustrated is static; each representation is a snapshot; for example, smectic layers can be described by one-dimensional density waves.

The characterization of thermotropic mesophases relies first, on optical microscopy, where the different phases show characteristic textures when viewed between crossed polariz- ers, as the temperature is changed. Mixing with a known mesophase is widely used to confirm the observations, since two identical mesophases are frequently miscible. These optical methods are complemented by Differential Scanning Calorimetry (DSC), and by low angle X-ray scattering in the mesophase.

1.2. Early Work on Metallomesogens

The first thermotropic metal-containing liquid crystals were reported by Vorlander in 1910.[61 He discovered that the alkali-metal carboxylates, R(CH,),COONa, formed classi- cal lamellar phases characteristic of soaps. In 1923 Vorlander also found that the diarylmercurials 2 form smectic phases."]

2

Between 1959 and 1961 Skoulios et al. characterized alkali and alkaline earth salts of carboxylic acids and showed them

Angew. Chem. Int. Ed. Engl. 30 (1991) 375-402 377

to possess a large variety of organizational types : lamellar, ribbonlike, and cylindrical.181 The smectic ferrocenyl Schiff bases 3, the first well-characterized organotransition metallomesogens, were synthesized by MulthCte and Billard in 1976.191

2. Metallomesogens with Monodentate Ligands

2.1. Nitrile Complexes

Nitriles bind readily to a variety of metals. The commer- cially available thermotropic CN-substituted biphenyls 4 and 5 and the bicyclohexyl derivative 6 analogous to 4 (4 = n-CB, 5 n-OCB, 6 = n-CCH; n is the C number of the alkyl or alkoxy substituents) were first used by Muitlis and the Sheffield Liquid Crystal group for the synthesis of metallomesogens with a linear geometry. The (DZh) trans-bis (neutral) ligand complexes 7 , s (L = n-CB and n-OCB), as well as

Pe

4 (n-CB) 5 (n-OCB)

The first to seek advanced materials in this area were Giroud and Miiller- Westerhoff, who in 1977 reported the mesogenic nickel and platinum dithiolenes.l'O1 This work laid the foundations for the study of mesogens containing d-block elements and marked the practical beginnings of interest in the subject. Since then many researchers have entered the field, and many new types of metallomesogen have been synthesized.

This article reviews progress, developments, and problems in the field up to early 1990, and offers indicators to the principles by which new representatives may be constructed, and to the relations between mesomorphism and structure at the molecular level, as they appear at the moment.

Because mesophase formation depends on intermolecular forces and because much of the space around the metal is occupied by the ligand, the properties of metallomesogens are, to a first approximation, dominated by the ligands and their arrangement, in other words by the overall shape of the molecule. Thus, for example, long monodentate ligands will tend to give rodlike nematics and smectics, while flat, disklike polydentate ligands (for example, macrocycles) will give discotics. For this reason the material in this review is arranged by ligand type.

This organization also brings out the exceptions-for example molecules which one might expect to show discotic phases but which have been reported as smectics-and shows how critical the substituents on the perimeter of the ligand to the type of mesophase behavior are. Where X-ray data are available on the single crystals, it also becomes clear that many metallomesogens have interactions which enable the metal to interact axially (though weakly) with a donor atom on a neighboring molecule. Examples of this are found in the dimetal tetracarboxylates, in copper ,!?-diketonates, in zinc, mercury, and palladium dithiocarboxylates, in salicylaldimines and, in a modified sense, in the metallophthalocyanines. This feature appears to have an important, if as yet unquantified, bearing on mesophase formation.

The metallomesogens have therefore been arranged by ligand, starting with the simplest, monodentate, and pro- gressing to more complex bidentate and polydentate types : carboxylates, P-diketonates, dithiolenes, and dithiocarboxylates, polyamines (phthalocyanines, etc), salicylaldimines (Schiff bases with N - 0 chelate formation), and cyclometalated mesogens.

C"H,"+ M C N

6 (n-CCH)

the monoligand complexes, 9 (L = n-OCB), were synthesized." 'I Synthetic routes were by displacement of one nitrile by another for 7 [Eq. (a)], by direct reaction, for 8 [Eq. (b)] or by bridge-splitting, for 9 [Eq. (c)].

[MCI,(PhCN),] + 2 L - L-M-L + 2 PhCN I c1

7 , M = P d ; 8 , M = P t

c1 I

n L-Pt -L I CI 8

[(PtCI,),] + 2 n L +

co I Cl

9

[Rh,(CO),CI,] + 2 L - 2 L-Ah-CO

The palladium alkyl complexes 7 (L = n-CB) showed nematic monotropic behavior (thermodynamically unstable mesophases, which appear only on cooling). Lower homo- logues of the ulkyloxy ligand complexes 7 (L = n-OCB, n = 4-7) showed nematic phases while the higher ones (n = 9) showed enantiotropic (thermodynamically stable) S, and S, phases."', 131 By contrast, platinum complexes, with both n-OCB and n-CB, showed enantiotropic phases.

Detailed comparisons are instructive (Table 1). Thus, while the ligand 4 (n = 5) has a nematic phase (24-35 "C) before clearing, and its PtCI, complex 8 behaves similarly, but at much higher temperature ( K - N 189"C, N - + I 208 "C), the PdCl, complex shows only a monotropic nematic (I -+ N 92°C). The ligand 5 (n = 9) shows an S, phase (64- 77 "C) and a nematic phase (77- 80 "C) before clearing. The PtCI, complex shows an N and an S, phase, but at higher temperatures (K -+ S, 172°C; S, + N 183°C; N -+ I

378 Angew. Chem. Int. Ed. Engl. 30 (199t) 375-402

Table 1. Transition temperatures [ "C] for mesomorphic alkyl- and alkyloxycy- anobiphenyl complexes of palladium(ir) and pIatinum(i1) [13].

Complex Mesophases

analogue, forms monotropic nematic phases (shown in Col- or figure 1 for L = 7-CCH), enantiotropic materials with a nematic range of ca. 5 "C can be prepared if the two complex-

rrans-[PdL,Cl,] 7 L = 5-CB L = 8-CB L = 9-CB L = 9-OCB L = 3-CCH L = 5-CCH

trans-IPtL,Cl,] 8 L = 3-CB L = 4-CB L = 5-CB L = 6-CB L =7-CB L = 8-CB L = 9-CB L = 10-CB L = 1-OCB L = 2-OCB L = 3-OCB L = 4-OCB L = 5-OCB L = 6-OCB L = 7-OCB L = 8-OCB L = 9-OCB L = 10-OCB

-

178 189 192 181 170

168 170.5 157 173 179

245 235 196 197 167 161 174 179

172 183 159 - 194

I 125 120 110 146 181.4 164.4

221 205 208 199 198 190 190 190 248 246 235 229 219 216 [b] 211 [b] 209 [b] 206 [b] 204

Color figure 1. Polarization microscopic photograph of the nematic phase of 7 (L = 7-CCH) at 158 "C obtained by cooling of the isotropic liquid phase.

es are mixed in a 1:l ratio (Fig. 7).I1'] This illustrates an [a] Monotropic (thermodynamically unstable) mesophase, Observed only on cooling. [b] Clears with (some) decomposition.

important aspect of mesogens : even when the correct molecular shape does not produce the desired properties directly, they can often be induced by judicious mixing.

206"C), whereas the PdCI, complex just shows smectic phases (K -+ S, 119; S, + S, 122; S, + I 146°C).['z7131 In some cases the complexes decompose close to the (rather high) clearing temperatures.

The structures of the Pd" complex 7 (L = 5-CB) and the Pt" complexes 8 (L = 5-CB and 8-CB) were confirmed by single crystal X-ray determinations (Fig. 6).["] Since Pd and Pt are nearly equal in size, the 5-CB complexes are virtually identical, even with similar inter- as well as intramolecular dimensions. The ground state structures offer no immediate explanation for the differences in mesophase behavior, which may therefore be due to the generally higher lability of

180

t 170

T I°Cl 160

180

I 70

160

Pd" complexes compared to their Pt" analogs. Fig. 7. Formation of a stable nematic phase by mixing non-mesogenic compo- Dents [PdClz(3-CCH),I (A) with the monotropic complex [PdCl2(5-CCH),] (B),

K t N, + N - I, K + I (after Ref. [12]). Although the bicyclohexylcyanopalladium complex 7

(L = 5-CCH), in contrast to the non-mesomorphic 3-CCH

An unanticipated benefit of complexing to a metal is the sharp increase in the birefringence, An, the difference between the parallel and the perpendicular refractive indices. Thus, while An for the ligands 4-6 is around 0.18, a mixture of PdCI, complexes of 2-OCB, 4-OCB and 6-OCB ligands gave a birefringence, An x 0.45 (at room temperature, by extrap~Iation).''~]

Mesogenic rhodium complexes with one cyanobiphenyl ligand, e.g. 9, (L = 9-OCB) have also been made, but show very short nematic ranges and decompose close to their clearing temperatures.[' 3l

2.2. n-Alkyloxystilbazole Ligands

2 Fig. 6. Crystal structure of 8 (L = 5-CB) (after Ref. [13])

The 4-alkyloxy-4-stilbazoles 10 (n-OST) are 4-substituted pyridines with extra polarizable functions (conjugation) and


a flexible tail. Since pyridines complex well to a large number of transition metals, the stilbazoles have been widely used by the Shefield group as ligands for metallomesogen formation.

CnH,o+lO--@ + C H , = C H G N -

10 (n-OST)

The stilbazoles are made by a Heck reaction (palladium acetate and triethylamine in acetonitrile) between the 4- (alky1oxy)iodobenzene and 4-vinylpyridine [Eq. (d)] . The C=C double bond is trans in the ligand and in all the complexes examined. The ligands themselves show monotropic S, and S, phases for 3- and 4-0ST, and enantiotropic S, and S, phases for n > 5, with clearing points around 85-90 0C.[151

2.2.1. Distilbatole Complexes

Reaction of the stilbazoles with silver salts, AgY, led to a quite unexpected series of mesogenic materials, the ionic bis- ligand silver salts, [Ag(n-OST),]Y 11 (11 a: Y = BF,", 11 b: Y = NO?, 11 c: Y = C,,H,,SOf, l l d : Y = CF,SO:). The tetrafluoroborates 11 a were very sensitive to light, hygro- scopic, and showed high transition temperatures." 61

11

However, the dodecyl sulfate salts 11 c formed mesophases at lower temperatures and showed quite unusual behavior; for example, 11 c (n = 3) gave a nematic phase 146-159°C before clearing." 7 1 Salts with longer alkyloxy chains exhibited complex behavior; for example, l l c (n = 12) showed a lamellar (S,) phase (I 08 - 132 "C), as well as a cubic (M,) and an S, phase before clearing at 178°C to the isotropic liquid." 7* Figure 8 shows a typical phase diagram relating transition temperatures to alkyl chain length. Phase identification was assisted by simultaneous X-ray diffraction and differential scanning calorimetry (XDDSC). The nitrate and triflate salts 11 b and 11 d are broadly similar to the dodecyl sulfates, but do not show such a variety of mesophases.

The existence of ionic nematics among the salts with shorter alkyloxy tails is quite unexpected, given the conflicting presence of both strong isotropic ionic forces and weak, anisotropic dispersion forces which stabilize the liquid crystal phases. The rich and varied polymorphism in these ionic systems may have some electrochemical applications.

A gold complex, [AuC1(8-OST)], which showed a mesophase between 120 and 200"C,t'21 and some very high melting palladium and platinum complexes, [MCI,-

2* 1 1

loo! . I . r . r . I . I . 1

0 2 4 6 8 10 12 n-

Fig. 8. Phase diagrams of [Ag(n-OST),]C,,H,,OSO~ showing the extensive polymorphism of this system. D K + N, 4 K -, S,. n K + MI, 0 K - S,, m N + I. 0 S , -f I, A S , + N, A S +MI , MI -t S , (after Ref. [lS]).

(n-OST),] have also been prepared. Only [PdC1,(12-OST),] showed any mesophases (K + S 280°C; S -+ N 290°C; N + I 315 "C).

However, the related carboxylato complexes, prepared by reaction of [Pd,(O,CR),] with the n-OST ligand, formed mesophases at much lower temperatures, for example [Pd(0,CC,H,9)2(12-OST),J, K -+ N 144°C; N -+ I 151 0C.['93 Maitlis et al. have found that the replacement of "hard" anions or ligands by softer organic amphiphilic anions or ligands is a generally useful strategy for lowering transition temperatures.

2.2.2. Monostilbazole Complexes

Another way to achieve lower transition temperatures is to incorporate some asymmetry. Since such complexes tend to disproportionate, this may not always be straightforward, but their synthesis has been accomplished in the case of the platinum complexes 12 which have one stilbazole and one q2-olefin (%bonded") in the trans position. The complexes were prepared from Zeise's salt, K[PtCI,(C,H,)] by reaction with n-OST; the ethene can be readily displaced by other a-olefins [Eq. (e)].[201

n-OST K[PtCI,(C,H,)] + trans-[PtCI, (n-OST) (C,H,)]

CmH2m+ ,CH=CH,

Cl ' CH-C,H,,+,

12

The chain lengths n and m were systematically varied (n = 3-12; m = 0-8) and it was found that mesophase behavior (all S,) occurred for those with longer chains (Fig. 9). Complexes with m + n z 8- 11 showed monotropic behavior (SA); those with m + n 2 11 -13 showed enantiotropic S, phases (Color figure 2), while those with m + n 5 8 were non-mesomorphic.t201

The mesogenic complexes 12 had transition temperatures below 100 "C; these were further reduced on mixing. Thus, a 70:30 mixture of the pentene complex 12 (n = 12, m = 3)

380 Angew. Chem. Int. Ed. Engl. 30 (1991) 375-402

Fig. 9. The properties of the mesophases of rrans-[PtCl,(n-0ST)- (CH,=CHC,H,,+,)] 12 as a function of n and m. o non-mesogenic, A monotropic, enantiotropic (after Ref. [20]).

A series of unsymmetric stilbazole rhodium and iridium complexes 13 and 14 were prepared by the route shown in Equation (f). The burgundy iridium complexes 14 became yellow, both in solution and on melting, and showed nematic phases for n = 5-8, and S, phases for n =7-12, in the temperature range 80- 130°C (Table 2).[211 The rhodium com-

Table 2. Transition temperatures [ "C] for mesomorphic alkoxystilbazole-rhodium(1) and -iridium([) complexes [21].

Complex Mesophases

cis-[Rh(n-OST)(CO),CI] 13 n = 5 n = 6 n = 7 n = 8 n = 9 n = 10 n = l l n = 12

cis-[Ir(n-OST)(CO),CI] 14 n = 5 n = 6 n = 7 n = 8 n = 9 n = 10 n = l l n = 1 2

S A

-

87 85 85 82 85 82

sA - -

(87) [a1 92 87 87 88 85

N 110 106 117 123

N (93) [a1 106 92

104.5 -

I 121 [b] 124 [b] 130 [b] 130 [b] 133 [b] 137 [b] 139 [b] 143 [b]

1 108 110 113 113 121 131 133 137

[a] Monotropic mesophase. [b] Clears with (some) decomposition.

plexes 13 behaved similarly but those with shorter alkoxy chains had lower melting and lower clearing temperatures. However for n = 12 the mesophase transitions were identical for the two metals.

Color figure 2. Polarization microscopic photograph of the octene complex 12 during the transition from the isotropic liquid phase into the SA mesophase.

co [{MCI(C,H,,)},] + 2n-OST +

and the related octene complex 12 (n = 12, m = 6) showed a much lower melting temperature (45 "C) than either component, while the clearing temperature (91 "C) remained unaf- fected (Fig. 10).

l o o m

100% B 100%A

Fig. 10. Phase behavior of a mixture of the complex A (12; n = 12, m = 3) and B (12; n = 12, m = 6) (after Ref. [20l).

HJ,ON-M-CO r E Y I co

13, M = Rh; 14, M = Ir

The mean electronic polarizabilities of the iridium complexes 14 (n = 5-12) were very substantially enhanced by comparison with those of the free ligands

2.3. Complexes with Other Pyridine Ligands

Serrano et al. in Zaragoza have used other substituted pyridines as ligands for metallomesogen formation. Thus the cis-iridium complexes 16 containing the short-chain, non-

15, M = Rh; 16, M = Ir


mesomorphic iminopyridine ligand (n-OIP, 4-alkoxy-N-(4- methy1pyridine)aniline) formed nematic phases (mostly monotropic), while those with longer chains (n 2 9) formed S, phases.'23] These again had low transition temperatures, e.g. 16, (n = 10, K-S, 63°C; SA+I 85°C) (Table 3). The related rhodium complexes 16 (n 2 8) are also mesogenic;'241 e.g. 15, (n = 9, K - S, 76°C; S,+I 83°C). It is quite remarkable that mesogenic complexes are formed here with a single organic ligand which is itself non-mesogenic. Further, and quite surprisingly, the trans-[Rh(n-OIP),- (CO)Cl] complexes (n = 4, 8) were very high melting and showed no mesogenic

Table 3. Transition temperatures I "C] for mesomorphic alkyloxyiminopyri- dineiridium(1) complexes [23].

Complex Mesophases

cis-[Ir(n-OIP)(CO),CI] 16 S A N

n = 6 (56.9) [a1 n = 7 (35.3) [a1 (63.1) [a1

n = 5 - - -

n = 8 (66.2) [a1 68.1 n = 9 63.8 - n = l O 62.8 - n = 12 70.1 n = 14 79.6 - n = 16 84.4

[a] Monotropic mesophase.

-

-

I 72.1 88.2 85.0 76.2 75.2 84.8 92.1 99.4

104.1

The silver(1) complexes with n-OIP, [Ag(n-OIP),]Y 17 (Y = BFY, CF,SOF, NO:, and PFF) and with the pyridine carboxylate ligand n-OCP [Ag(n-OCP),]Y 18, also showed mesogenic behavior.[251 Only one salt, 17 (Y = BF:, n = 2), was nematic (194.9-223.8 "C); all the others showed S, or S, phases (for example, 17, Y = BFY, n = 12, K - 6 104°C; S-+N 195°C; N-I 211 "C, with decomposition).

J 17

18

The ester salts 18 showed somewhat wider mesophase ranges and greater smectic polymorphism than the imino salts 17, and smaller anions gave wider mesophases and lower melting points. Low angle X-ray data for four complexes with n = 10 showed the layer thickness in the mesophase to be 41-42 A, i.e. shorter than the fully stretched linear cations (48 8, in 17, 53 8, in 18).[251

2.4. Organometallic Complexes with q'(a)-Bonded Ligands

The first thermotropic metallomesogens were the smectic organomercury compounds 2, described by Vorlander in

1923;"' they were prepared by the condensation of bis(4- aminopheny1)mercury with aromatic aldehydes, RCHO, R = PhCH=CH, p-tolyl, or p-alkyloxyphenyl. All the compounds gave mesophases, and even those from benzaldehyde, R = Ph (mesophase 180-184"C), as well as p- MeOC,H,CH=NC,H,HgX (X = OAc, C1) showed small mesogenic ranges. Unfortunately all these organomercury compounds are thermally unstable.

The very unusual organometallic liquid crystals 19 and 20 (M,M = Pd, Pt or Ni; M = M' or M # M ) have been made by Takahashi et a1,'261 by a copper chloride-triethylamine coupling of the appropriate metal halide with an alkyne [Eq. (g)]. A wide variety of such compounds have been

trans-[ML,(C=C-Z-C=CH),] + truns-[M'L,C1,] - - HCI

(g) L L

[ ( ~ ( - c = c - z - c ~ c - ) ~ , ) " ] I I

L L

PBu, PBu, PBu, I I I

I I I PBu, PBu, PBu,

19

[ ( - C E C - M - C = C - C r C - M - C = C - C ~ C - - - ) ]

20, R = H, Me, Et

made. These materials are very high melting and do not show thermotropic properties; however, they are lyotropic nematic in trichloroethylene solution, and can be aligned in a magnetic field. Whether the particular polymer aligns perpendicular to the magnetic field or parallel to it is determined by its diamagnetic anisotropy Ax', which in turn is dependent on the Ax's of the components of the polymer. Thus, for example, Ax for acetylene is negative, while it is positive for benzene and p-xylene.

2.5. Ferrocenes

A series of mesogenic ferrocenes 3 (which have an overall shape not too dissimilar to the mercury compounds 2) were made by Malth?te and Billard in 1976,I9] by condensation of the appropriate benzaldehyde derivative with 4-aminophenyl ferrocene carboxylate. Recently, it was shown with the ferrocene carboxylic acid building block that the bis-substituted ferrocene [Fe(C,H,-COO-C,H,-C,H,-X),] gave monotropic s, phases for X = C5H,,0, and C,H13, whereas the monosubstituted [Fe(C,H,)(C,H4-COO-C,H,-C6H,-X)] was non-mesogenic.I2 '1

3. Metallomesogens with Carboxylato ligands

Metal soaps have long been known to have mesogenic properties. Thus, in 1910 Vorlunder reported the existence of


lamellar phases in the anhydrous salts of alkali carboxyl- atesJ6I and in 1938 Lawrence noted that copper stearate, for example, melted from a hard crystalline solid to a plastic fluid, which then transformed into a clear liquid.[281

disklike, while above 180 "C it consists of cylinders in a two- dimensional hexagonal array (Fig. 12). This last mesophase is also found for the cadmium soaps.

3.2. Lead, Mercury and Thallium Carboxylates 3.1. Alkali and Alkaline Earth Carboxylates

Structural data, indicating the existence of columnar and even disklike mesophases for the anhydrous alkali, alkaline earth, and cadmium salts of long-chain fatty acids (C,H,,+,COOH, n = 9,11, 13, 15, 17,19), were first reported by Skoulios et al.[29] The transition temperatures increased with increasing chain length, and several types of structure were found, depending on the metal (Fig. 11).

Fig. 11. Structures found in the mesophases of anhydrous alkali-metal carboxylates (after Ref. [29]).

Thus, the potassium salts can be: 1) lamellar, with ordered polar groups and alkyl chains; 2) lamellar with disordered polar groups and alkyl chains; 3) ribbonlike; or 4) disklike in a three-dimensional lattice. In the latter two, the polar groups are ordered while the alkyl chains are disordered.[301 As well as exhibiting thermotropic properties the alkali metal soaps also show lyotropic behavior with solvents.

The calcium carboxylates, for example, showed a lamellar phase below 11 0 "C with the paraffin chains perpendicular to the lamellae: between 120 and 180°C the structure becomes

Fig. 12. Disklike structure adopted by anhydrous calcium carboxylate (after Ref. (301).

S, phases have been reported for the lead@) carboxylates, [Pb(C,H,,,,COO),] where n = 5, 7, 9 and ll.[311 The compounds (with n = 7,9,11,13,15, and 17) investigated further by DSC, optical microscopy, X-ray diffraction and various spectroscopic techniques, were found to transform from a lamellar crystalline to another lamellar highly ordered phase (between 80- 100 "C, depending on chain length) and then to melt (at ca. 100 "C) to S, phases (for n = 7, 9 , l l ) . The alkyl chains melt in more than one step and, on recooling, a crystalline phase different from the original The mercury carboxylates, [Hg(O,CC,H,, + (n = 7 - 17), are not mesogenic, but [TI(O,CR)] with branched alkyls R, gave stable lamellar phases.t331

3.3. Discotic Dinuclear Copper Carboxylates

The liquid crystalline phase obtained on heating copper stearate [Cu,(/*-O,CC, 7H35)4] was reinvestigated in 1964;c341 but it was not until considerably later that it was definitively Giroud-Godquin et al.[361 identified the mesophase of copper laurate as hexagonal columnar (Fig. 13) by DSC, optical microscopy, and X-ray diffraction. This was the first example of a thermotropic hexagonal

Fig. 13. Schematic view of the columnar mesophase of a copper@) soap. Each column is made of stacked dicopper tetracarboxylate units. The polar "inorganic" cores are surrounded by disordered alkyl chains. The column axes define a two-dimensional hexagonal lattice (after Ref. [41]).

discotic mesophase bearing only four alkyl chains. The other carboxylates, [Cu,(/*-O,CC,H,,+ (n = 4-24), showed the same structures. The X-ray studies showed that the complexes crystallize in lamellar structures, in which planes of polar copper carboxylate groups are separated by a double layer of aliphatic chains, slightly tilted with respect to the perpendicular of the plane. The lamellar spacing is a function of chain length. Above 120°C there exists a columnar two-dimensional hexagonal structure in which columns of polar groups are surrounded by disordered alkyl chains. The dinuclear

Angew. Chem. Int. Ed. Engl. 30 (1991) 37s-402 383

[Cu,(p-O,CC,H,, + 1)4] unit is repeated along the column, with a stacking period of 4.7 8, independent of chain length, n; the columns are 15-25 8, apart, depending on n. A sharp increase in the molar volume at the transition temperature, consistent with a first order phase transition, was observed in dilatometric studies of [Cu,(p-O,CC,H,,+,),J (n = 18, 22, 24).[371

X-ray single crystal data of copper carboxylates show them to be dinuclear (Fig. 14) with each copper surrounded by five oxygen atoms at a mean distance of 2.01 8,, one copper atom at a distance of 2.59 8, (its neighbor within a dimer), and another copper atom at 3.19 A, belonging to the closest neighboring dimer.[381 The CU-K edge EXAFS (Ex-

Fig. 14. Crystal structure of a dinuclear copper carboxylate.

tended X-ray Absorbtion fine Structure) data on [Cu,(p- O,CC,H,,+ I)4] (n = 6,7,12,18 and 22) confirmed this and also showed that the bond lengths of the dinuclear core remained the same at 20, 70, and 120°C; i.e., no change was detectable on going from the solid to the me~ophase . [~~] Magnetic susceptibility measurements showed a small but sharp decrease in the magnetic moment of the dicopper unit at the transition temperature (Apeff z -0.04 pB). This was ascribed to a structural deformation of the dinuclear core, as a consequence of which the singlet-triplet gap is higher in the columnar mesophase (- 25 = 310-330 cm-') than in the crystal (290-300 cm- 1).[401 Since the EXAFS data indicate no change in bond lengths, the phase change probably af- fects only the bond angles.[411

Isotopic labeling has facilitated band assignments in the IR spectra of these complexes, allowing the detection of structural variants from changes in the methylene stretching frequency.[421 The dynamic behavior of the alkyl chains in copper carboxylates has been studied by incoherent quasielastic neutron

Copper laurate has also been melt spun into fibers in its mesophase by the Grenoble group;[44] electron microscopy and X-ray diffraction measurements indicate that the fibers have high degrees of orientation both in the crystalline and the columnar phases.

3.4 Discotic Dinuclear Rhodium, Ruthenium, and Molybdenum Carboxylates

The rhodium@) carboxylates, [Rh,(p-O,CC,H,,+ behave very similarly to the copper complexes,[451 and form columnar thermotropic mesophases of the same structures.

Rh K-edge EXAFS measurements of [Rh,(p-0,CC7Hl again show negligible differences between the dimensions in the crystal and in the mesophase [intramolecular, metal- metal bonded, Rh-Rh 2.38 (20°C), 2.37 8, (120°C); intermolecular Rh...Rh 3.168, (20 and 12O"C)]. A band at 350 cm- has been assigned to v(Rh-Rh) in the Raman spectrum; these are the first metal-metal bonded compounds which show a two-dimensional hexagonal columnar lattice.f4l1

The Grenoble group has also shown that the air-sensitive dinuclear Ru" and the mixed valence Ru"-Ru"' carboxylates, [Ru,(p-O,CC,H,,+ l)m] (m = 4 or 5) , form thermotropic columnar me so phase^.^^'^ 461 They are prepared by Cr" reduction of the mixed valence (and non-mesogenic) [Ru,CI(O,CC,H,),], followed by carboxylate exchange. The magnetic moment of [Ru,(~-0,CC,,H,l)4] increased sharply at ca. IOO'C, and a change, corresponding to the transition to the columnar mesophase, was also detected by DSC and X-ray diffraction. The quadruply bonded Mo=Mo carboxylates [Mo,(O,CC,H, + ,),I form analogous discotic m e s ~ p h a s e s . [ ~ ~ ~ l

4. Mesogenic fl-Diketonato Complexes

P-Diketonates form flat bis-ligand complexes with a variety of metals that can take up square planar geometries (including Ni, Pd, and Cu). When appropriate substituents are present, copper(I1) diketonate complexes can give rise to discotic mesophases; nematic and occasionally lamellar mesophases have also been found. Mesogenic behavior seems to be optimized when the molecules have two pairs of p-substituted phenyls, situated 1,3 on each diketone. Some other variations are also tolerated. The complexes are easily made by reaction of the fl-diketone with the appropriate copper salt.*471 Mesogenic P-diketones of other metals appear, so far, to have largely eluded workers in the field, though the synthesis of a discotic vanadyl P-diketonate analogous to 26 has recently been accomplished.[47b1

The first fl-diketonate to be examined for mesophase behavior was the palladium(I1) complex, 21. Bulkin et reported that DSC data for this complex indicated potential mesomorphism, but were unable to confirm this optically because the complex darkened on heating.

Me

H C : Pd :CH

Me

\ czq ,03\

c-o/ \o.z% \


No liquid crystal properties were found for the copper(i1) complexes 22 either.r49. Only crystal-crystal transitions were observed; the polymorphism in the solid depended on chain length and showed an o d d w e n effect, but involved no change in coordination geometry.r511

4.1. Copper Complexes of 1,3-Bis(p-substituted-phenyl)- B-diketones

The 1,3-bis(p-substituted-phenyl)-fl-diketones 23 and 24 are prepared by a standard condensation of thep-substituted acetophenone with the corresponding p-substituted methyl benzoate [Eq. (h)].rs2] Ligands 23 with m + n 2 18 exhibit smectic E phases, and show the unusual feature that the enthalpy changes observed on clearing are larger than those found on melting.[531 This situation is frequently observed for discotics but is exceptional for rodlike m e s o g e n ~ . ~ ~ ~ ]

R e C O M e + MeOOC *R-

23, R = C,H,,+I, R'= C,H,,+, 24, R = C,H,,+,O, R = C,Hzn+,O

The mesogenic copper(1r) fi-diketonates 25, first described by Giroud-Godquin et al.'539 551 in 1981, have a square planar geometry about the and a d9 configuration (one unpaired electron).1471 Both the symmetrically and the un- symmetrically substituted complexes 25 (m = n or m =I= n) exhibit very organized discotic mesophases. The more sym- metric have the higher melting and clearing temperatures and the larger molar enthalpy changes, for example 25 (m =7 and n = 13; 71 -122°C) and 25 (m = n = 10; 85.5- 128.5 oC).r531 Later work on these complexes, particularly NMR investigations of the spin-lattice relaxation time T, ,["I

R R

R

25, R=C,H,,+, ,R'=CnH,,+, 26, R = C,Hzm+,O, R = C,H,,+,O

27, R = C,H,,+,, R '= C,H,,+,O

suggested that a lamellar phase is present, where the disklike molecules are organized in columns as shown in Figure 15. The complexes 25 (m = n = 7- 12) and 26 (m = n = 8) also showed discotic mesophase~ ; [~~* " 7 591 the mesophase range for the alkyloxy ligand 24 with m = n = 8 was narrower

than that for the alkyl ligand 23 with m = n = 8, but the opposite was true for the copper complexes 25 and 26 derived from them. Nickel analogs of 25 showed no mesogenic character.'531

k

Fig. 15. Schematic representation of the tilted discotic lamellar phase of 25 (n = 12) (after Ref. [53]).

Eastman et al.'601 have found that the ESR spectra of single crystals of the symmetrical copper complex 25, (m = n = 8) show features at 298 K associated with a one- dimensional Heisenberg antiferromagnet, while those of complex 26 do not. These workers also followed the phase changes by ESR spectroscopy, and in both cases detected a crystalline to discotic transition. The ESR spectra of complex 25 have been interpreted as showing that exchange interactions are significant and that a degree of long range order is maintained in the discotic phase.

The ligand fl-diketones 24, (m = n) showed triple melting behavior for n = 1, double melting for n = 2-7, and were mesogenic (smectic) only for n = 8-12,16'] but each of the copper complexes 26 derived from them (m = n = 3-12) had only one discotic mesophase. X-ray diffraction established that the mesophase in complex 25 (n = 12) is discotic lamellar, characterized by a tilt of the molecule to the layer and by a random positioning of the molecules within the layer (Fig. 16); there was no columnar structure.1621

Fig. 16: Schematic representation of the structure of the discotic lamellar phase (D,) of 25 (n = 12) (after Ref. [53]).

A reinvestigation of the mesophase originally identified for the unsymmetrical complexes 27 has shown that it is lamellar when m = n = 5-12).r631

A very interesting discovery, by Chandrasekhar et al.[64* 651

is that the mesophases of the copper(r1) complexes of the

Angew. Chem. In(. Ed. Engl. 30 (1991) 375-402 385

dialkyl- and of the alky1,alkyloxy-substituted P-diketonates 28 ( R = Me, Et, OMe, OEt, and OPr) are monotropic nematic, and are miscible with a nematic organic compound, 4-n-pentyl-4-cyano-p-terphenyl (5-CT). These complexes incorporate features of both rodlike and disklike molecules,

and are claimed to be the first examples of biaxial nematic metallomesogens.[66* 671 On addition of small quantities of the uniaxial nematic 5-CT, the sequence of transitions, isotropic + Numiaxial --f Nbiaxia,, was observed on cooling. Magnetic, dielectric, and ESR measurements indicated the existence of antiparallel correlations in the nematic phase.

The biphenylylphenyl-substituted 1,3-propanediones 29, the parent diketones of this series, also showed mesomorphic character.[681 Those with n = 4-7 showed both S, and N phases, the higher ones only S, phases.

"r "9

Fig. 17. Crystal structure of 30 (n = 3, m = 8). Projection along the lattice plane (001) (after Ref. [69]).

4.2. Copper Complexes of 1,3-Bis(3,4-disubstituted- pheny1)-/ldiketones

Early studies had indicated that at least six n-alkyl tails were needed on a disklike core in order to obtain a columnar m e s ~ p h a s e . ~ ~ ~ ] Giroud-Godquin et al.17 '* 721 reported four columnar copper P-diketonate complexes 31 ; the phenyls at the ends of the p-diketonate were each substituted (3,4) by two alkyloxy substituents, giving a total of eight tails. These complexes were shown to have a hexagonal columnar (D,,) mesophase by miscibility studies with the triphenylene

29 R? OR

X-ray studies of the 4"-alkylcyclohexylphenylalkyl analogs 30 of the Chandrasekhar complexes 28 by Muhlberger and HaaseL6'] showed that the P-diketonate ring was planar and that identical substituents were trans to each

N C"H* n + 1

30

other (Fig. 17). These complexes only gave monotropic nematic phases, but an enantiotropic nematic phase was obtained by mixing 30 (n = 3, m = 8) and the organic nematic 5-CT.

/ \ / \ RO OR RO OR

31 32

hexadecylalkanoate 32 (R = CI5H,,CO), an organic disklike molecule.t73] X-ray diffraction of the complex 31, (R = C,H,,) showed neighboring columns to be 29 8, apart in the mesophase; this value is substantially smaller than the molecular diameter with stretched conformation of the aliphatic chains (42 A).[711

The nickel complex corresponding to 31 showed no mesophase, neither did the copper complexes in which the phenyl was substituted in the 3 3 rather than 3,4 positions. The copper complex of the asymmetric P-diketonato ligand 2Y with R' = C,H,, also showed a D, mesophase, but those with R' = C3H, or C,H,, were not m e s o g e n i ~ . ~ ~ ~ ] Similar complexes were studied by Ohta[741 who found (by optical and DSC measurements) that the P-diketonates with four


pairs of OC,H,, substituents in the 3,4 positions of the phenyl rings showed discotic mesophases. However the corresponding complex with four pairs of n-C,H,, tails was not discotic.

--+ 'q0 wRq0 CH,Br

OR'

Preliminary measurements suggested that the presence of a transition metal in a discotic mesogen such as 31, (R = C,H,,) may enhance the electrical conductivity in the uniaxial columnar (D,,) mesophase; the complex also gels in cy~lohexane.[~

4.3. Other Metal p-Diketonates

The octahedral iron(r1r) tris-fl-diketonate Fe(RC0CH- COR), 33 (R = C,,H,,C,H,) shows endothermic transitions at -10.5"C ( A H = 13.7 kJmol-') and 37°C ( A H =

H

Scheme 1. Synthetic route for the bis(a1kylphenyl)dithiolene metal complexes 34

lined in Scheme l .r10*76-781 A single crystal X-ray diffraction of the nickel complex 34, (M = Ni, n = 8) showed the expected M" square planar geometry, with the substituents in a trans-conformation. The molecules are arranged zig-zag in layers in the unit cell (Fig. 18); this is the arrangement which would be anticipated as the precursor to a smectic C phase.r791

R 33

Fig. 18. The packing of a complex 34 (n = 8) in the unit cell (after Ref. [79]).

40.0 kJ mol- '), and a birefringent structure within this range.["] If confirmed, this would be the first octahedral metallomesogen.

5. Metallomesogens with Sulfur-containing Ligands

The Ni and Pt complexes 34 (M = Ni, Pt; n > 6) formed nematic and smectic mesophases (Table 4); those with n = 4,s showed nematic and those with n > 6 showed only

Table 4. Transition temperatures [ "C] for mesomorphic bis(alkylpheny1)ditho- lent nickel and platinum complexes[78].

5.1. Dithiolene Complexes Complex Mesophases

The first systematic study of d-block metallomesogens was carried out by Giroud and Miiller- Westerhoff on dithiolene complexes of Ni", Pd", and Pt" (d8 electron configuration).

The main series investigated, complexes 34 with two p- alkylphenyl substituents, were synthesized by the route out-

frans-[Ni(C.H,. = ,C6H,C2HS,)zI 34 n = 4 n = 5 n = 6 n =7 n = 8 n = 9 n = 10

fruns-[Pt(C.H,. = ,C6H,CzHS,),I 34 n = 4 n = 5 n = 6 n =7 n = 8 n = 9 n = 10

sA

139 130 121 108 103

sA

169 150 146 140

N 117 133 169

N 158 167 175

I 175 [a] 185 [a] 181 [a] 185 [a] 191 [a] 191 [a] 189 [a]

I 202 202 200 199 209 205 202

34, M = Ni, Pd, Pt

Angew. Chem. Im. Ed. Engl. 30 (1991) 375-402

[a] Clears with (some) decomposition.

387

smectic phases. The platinum complexes had significantly higher melting and clearing temperatures than the nickel analogs, but the clearing temperatures became more similar with larger n. The reason why the palladium analogs do not possess any mesomorphic properties is not clear.

The overall molecular structure of 34 is reminiscent of the organic 4,4"-dialkylterphenyls 35, which form smectic mesophases when n = 3-18.[80' The melting points and the mesophase ranges of 35 are higher than those of the corresponding complex; for example, the smectic range is 161 - 181 "C for 35, (n = lo), 103-189°C for the nickel complex 34, (M = Ni, n = lo), and 140-202°C for the platinum complex 34, (M = Pt, n = 10).

35

The dithiolenes have interesting electron acceptor properties and unusual electronic structures, with strong absorp- tion bands (L,,, > 750 nm, E > 25 000). Smectic and nematic derivatives of the nickel dithiolenes 34 with a strong electron donor, tetrathiafulvalenes 36, were synthesized, but no charge transfer complex resulted."

The complex 34 (M = Ni, n = 4), dissolved in the cyanobiphenyl 4 (n = S), has been shown to be useful as a near-infrared dichroic dye in active and passive devices for lasers operating near 800 nm. The solubility and blocking

extinction of this complex were considerably superior to the structurally similar but non-mesogenic laser dye, [Ni- (3-Me,NC,H3CSCSC,H,),] .t821

Analogous di- and tetrasubstituted nickel dithiolenes 37 (R = p-C,,H,,OC,H,; R = H or R ) prepared by Strzelecka et aLtS3] were originally tentatively described as discotic mesogens. However, recent X-ray data for 37 (R = R = p-C,H,,+,OC,H,, n = 9, 11, and 13) showed that the phase changes were between different crystal forms

37

(K + K') and that these compounds were not m e ~ o g e n i c . ~ ~ ~ ] In 1986 Ohm et al. reported that the tetraalkoxyphenyl compounds 37 (R = R = p-C,H,,OC,H,, and R = R p- C, lH,,OC,H,) also formed discotic mesophases; this report was based on the miscibility of the two compounds giving a

eutectic, and other optical observation^.^^^] No X-ray data are yet available to confirm the claim. However, the related nickel dithiolene 38 with four 3,4-bis(dodecyloxy)phenyl substituents was shown by X-ray diffraction to give rise to a hexagonal disordered columnar (discotic) mesophase (D,).[86'

OR OR

5.2. Dithiocarboxylates

Dithiocarboxylates (RCSF) and xanthates (ROCSF), which react with metals to give MS,C four-membered rings, are related to the dithiolenes, which form five-membered MS,C, rings.

I 1204 ' I ' I . I . I . I . 1

4 5 6 7 8 9 1 0 n-

I

1 5 0 : . I ' I . 1 . I . I . I . 1

3 4 5 6 7 8 9 10 II-

180

t ;: TlOCl .

150 - 140-

3 5 7 9 n-

Fig. 19. Phase diagrams of complexes of the type 39. a) M = Ni; K -+ N, K-S,. & - + I . O N - I . b ) M = P d ; K - N , 4 N - I , n K-S,, 0 S, - I. c) M = Zn; 0 K N, 4 N -t I. Various mesophases occur depending on the metal (after Ref. [87]).


Studies by the Sheffield group on the bis(4-alkyloxy- dithiobenzoato) complexes showed that the Ni" and Pd" complexes 39 (M = Ni, Pd) were mesogenic (Fig. 19a, 19 b).t87- 881 Optical microscopy, combined with an X-ray diffraction and DSC (XDDSC) study, confirmed the occur- rence of s, mesophases for palladium complexes 39 with longer-chain ligands (e.g. n = 8; 214-320°C) and of N phases in the case of shorter-chain ligands (n = 4,5).[18] Sim- ilar results were obtained for the nickel complexes, 39, (M = Ni).

Fig. 21. Structure of [Zn(S,CPh),] (after Ref. 1891).

39

The single crystal X-ray structure analysis of the palladium complex 39 (M = Pd, n = 8; Fig. 20) showed it to be composed of monomeric species, with square-planar coordinated Pd centers (four Pd-S bonds with bond lengths of 2.29-2.35 A), loosely associated into dimers via long intermolecular Pd... S contacts (> 3.38 A).r881

Fig. 20. Crystal structure of a complex 39 (M = Pd, n = 8) (after Ref. [SS]).

The molecular structure of the unsubstituted bis (dithiobenzoato) zinc complex (Fig. 21)[891 suggested that even a tetrahedrally coordinated metal center would induce such compounds to form rodlike mesogens. Mesophases were indeed observed for the dialkyloxy-substituted zinc complexes 39 (Fig. 19c). However, although monomeric in solution, single crystaI X-ray diffraction studies showed that in the solid the molecules are dinuclear with an eight-membered Zn,S,C, ring, making each zinc effectively five-coordinate (Zn-S 2.22-2.77 A, Fig. 22).[879881

Recent single crystal X-ray data on the related bis(4-alkyl- oxydithiobenzoato)mercury complexes Hg(S,CC,H,OR), (R = C,H, and C,H,,; nematic phases at 178-192°C and 137-200 "C, respectively), indicate that these too have a

h

Fig. 22. Crystal structure of a complex 39 (M = Zn, n = 4) (after Ref. [SS]).

more complex coordination geometry than might be expected: two shorter trans Hg-S bonds (2.41 A) and two longer trans Hg-S bonds (2.96 A) in the same plane, with two additional axial intermolecular Hg... S contacts (3.36 A, Fig. 23).[90]

8-g_

8-g

"b,

Fig. 23. Crystal structure of a complex 39 (M = Hg, n = 8).

Again, the metal center, as in the Pd complex (Fig. 20), is planar and not tetrahedrally coordinated. An effect of the alkyloxy chains, which confer mesomorphism, also appears to be to reduce the tendency towards tetrahedral geometry at the metal center.


The use of 39 (M = Ni, Pd) in display and thermal record- ing materials has in the meantime been patented.[g11

The dimeric tetrakis(n-alkyldithiocarboxy1ato)dinickel complexes 40 (R = C5Hll to C,,H,,) have been described; they show monotropic lamellar mesophases with broken fan textures. Identification of the phases has been checked by

X-ray measurements. The corresponding monomeric xanthates, [Ni(ROCS,),] 41 (R = CH, to C,,H,,) exhibit complex double- and triple-melting behavior.t921

6. N,-Macrocycles and Other Polyamine ligands

In view of the range of polyamine ligands, it is not surpris- ing that they also occur as components of metallomesogens. The discotic metallophthalocyanines, first studied by Simon et al. in are especially interesting.

6.1. Phthalocyanine Complexes

6.1.1. Copper Complexes

Discotic metallophthalocyanines 42 (M often Cu) were originally envisaged as one-dimensional conductors, but also have other potential applications.[931 A typical synthetic path to such compounds is presented in Scheme 2. The copper phthalocyanine 42 with eight dodecyloxymethylene substituents (R = CH2OC,,HZ5) gave a stable mesophase be-

c- B r n C H , - O - R

NC N C x x C H ~ - o - R ' CH,-0-R Br CH,-0-R

CH,-0-R'

bIm CH,-0-R

H,N

+ 44, R = CH,OR'

HN

Scheme 2. Synthetic route for octakis(alkoxymethy1ene)phthalocyanine (after Ref. [lOO]).

tween 53 and ca. 300°C; X-ray data indicated that the macrocycles are stacked at a distance of 4.5 8, in columns, with the columns ca. 31 A apart (Fig. 24).t931

Discotic columnar mesophases (Dho) were also found in the case of complexes 42 (R = CH,OC,H,,+,, and

Fig. 24. Structure of the discotic mesophase of42 (R = CH,OC,,H,,). The zig zag tails from the central unit symbolize the substituents on the phthalocyanine ring.

OC,H,,+ 1 , n = 6-1 1) by van der Pol et a1.1g4] The transition temperatures (as well as the related enthalpies and entropies) are only slightly different from those of the corresponding metal-free molecules. X-ray data showed inter-disk distances

R R

R R 42

within one stack of 3.4 A. Copper phthalocyanines 43 bearing four benzo[l5]crown-5 substituents have also been made; they stack to give channels through which ions may be transported.t951

43

390 Angew. Chem. Inr. Ed. Engl. 30 (1991) 375-402

Although ESR spectra indicated good electron mobility in 42 (R = CH,0C,,H,,),1931 the complexes are semiconduc- tors with mesophase conductivities very similar to that of the unsubstituted and non-mesogenic copper phthalocyanine itself (c = 6 x lo-* Sm-' at 175"C);[941 a slight decrease in conductivity was found on going from the solid to the mesophase. As is very often found with insulating or semi- conducting materials, the conductivity increased by several orders of magnitude on doping with I, .Ig4, 961

Van der Pol et al. examined the luminescence properties of 42 (R = CH,OC,,H,,) as a function of temperature, and found a sharp drop in intensity at the melting point.[971

Interestingly, many of the metal-free phthalocyanines themselves are discotic, for example 44 (R = CH,OC,,H,,) has a structure very similar to that of its copper complex.

r

45

44

The stacking distance here is 4.5 8, and the columns form a hexagonal lattice with the columns 31.2 A apart; X-ray diffraction measurements on the phthalocyanine 44 (R = 2- EtC,H,,OCH,) show a nematic phase.Ig3] Many other discotic metal-free phthalocyanines have also been made.[981

A cholesteric columnar copper complex 42 (R =

C,,H,,OC*H(Me)CH,OCH~CH,), with eight optically active substituents on the phthalocyanine ring, has been reported.Iggl Differences in mesophase behavior between the chiral material and its achiral analog (with racemic C12H,,0CH(Me)CH20CH,CH, substituents) were found; the chiral material showed polymorphism, while the achiral analog showed only one mesophase.

6.1.2. Mn, Co, Ni, and Zn Phthalocyanines

Complexes of Co, Ni, and Pb with the phthalocyanine 44 (R = CH,OC,H,,) exhibit mesophases below 100"C;~lool the cobalt compound reacts with NaCN to give a polymeric material, thought to be 45. Discotic zinc and manganese complexes of 44 (R = CH20C,,H2,) have also been report-

The use of liquid crystalline host materials containing colored compounds, which absorb laser light and cause local heating, has been considered for laser addressed devices.11021 The brightly colored phthalocyanines, their benzo-homo- logues (the naphthalocynines), and eight complexes with Cu, and one each with Ni and Zn, have been investigated for this purpose. Major changes in spectra are achieved by introduction of nuclear alkyloxy substituents; changing the metals offers the opportunity for further spectral fine tuning. More- over, the parent compounds and the metal complexes with

ed.110'1

long chain alkyls dissolve in nematic liquid crystals, which is also useful. A variety of mesophases was found, again below 100°C for longer chain substituents. The textures of one mesophase are consistent with hexagonal columnar, and those of another, with a rectangular columnar discotic phase.

6.1.3. Lutetium Phthalocyanines

Simon et al. found that bis(phtha1ocyanine)lutetium is a molecular semiconductor. Discotic mesophases were found for the complexes of the subtstituted phthalocyanines [LU((ROCH,),C,,H,N,)~] 46 (R = C12H25).11031 Both the

46 't green neutral form and the red oxidized form, as the SbC12 salts, were prepared. The ionic compounds formed the more stable mesophases. X-ray data for the mesophase of 46 (R = C18H3,) indicated an intercolumnar distance of 37 A and a repeating unit of 7.3& twice the thickness of the phthalocyanine ring, as expected (Fig. 25). The X-ray pat-

Fig. 25. Schematic representation of the repeat units of the bis(phtha1ocyan- inatohtetium derivatives 46.


tern remains unchanged on going from ambient temperature to the temperature where the isotropic phase appears, indicating that the mesophase and the solid have similar structures. The ESR spectrum of the neutral species shows broad- ening in the mesophase; there is some uncertainty about the magnetic state of the oxidized form. The electrical properties of the neutral form of 46 (R = C,,H,,) are also very similar in the solid and in the mesophase (conductivity 1.8 x Q-'cm-' at lo4 Hz), but the conductivity in- creases on oxidation. It was estimated that the hopping probability of an electron is lo7 greater within a column than between columns for the same activation energy.

6.1.4. Si, Sn and Pb Phthalocyanines

Columnar phthalocyanines linked by O-Si-O[' 04] or 0- Sn-0['051 spacers have also been described. The tin complex 47 shows a columnar mesophase between 59 and 114"C,

I- 47

characterized by X-ray diffraction measurements as a rectangular packing of columns (column separation 25.2 and 30.7& and with a probable intermetallic spacing of 3.85 ,4.[1051 The fluid isotropic phase is slowly transformed into a highly viscous anisotropic material, which has a polymeric {a-Sn-}, backbone; this in turn becomes isotropic at 290 "C.

The silicon-containing phthalocyanine 48 showed hexagonal-columnar mesophases (&) between ambient and 300 "C

40 i-L for n 2 4. On heating in air at 180 "C loss of water occurred, accompanied by oligomerization to dimers, trimers, and some higher oligomers, e.g. 49.['04]

Also Sn" and Pb", form a series of complexes of the type just The Sn" complex is readily oxidized to the Snl" complex 47. The lead complex forms columnar discotic liquid crystals, with intercolumnar spacings of 26.9, 31 and 36 8, for the C,H,,OCH,, C,,H,,OCH,, and C,,H,,OCH, chains respectively. The mesophase was stable down to

Fig. 26. Side-view of the proposed structure of Pb (phthalocyanine) illustrating the antiferroelectric coupling of the Pb atoms (a) (after Ref. (1071).

- 45 "C; the X-ray data have been interpreted in terms of dimers (Fig. 26) which exhibit antiferroelectric

6.2. Other N, Macrocycles

Mesogenic metal-containing prophyrin complexes 50 (R = CH,OH, M = Cu, Cd, Zn, and Pd) have been reported, as well as some which are not mesogenic but which, on

392 Angew. Chem. Int. Ed. Engl. 30 (1991) 37s-402

CH2OR CH2OR

50

mixing with long-chain alkanes or n-alkyl halides, form monotropic mesophases.['081

The disklike octadodecyltetrapyrazinoporphyrazine 51 (R = C,,H,,) and its copper complex show discotic mesophases (by X-ray).['"] Although the structure of this

R P

51

ligand differs little from that of its copper complex, there is still a change of mesomorphic character. The former shows a hexagonal disordered columnar mesophase (Dhd) between 11 8 and 238 "C, the decomposition temperature, while the copper complex has a rectangular disordered columnar mesophase (Did) between 114 "C and the decomposition temperature 288 "C.

Discotic phases have been reported in a patent for a series of substituted dibenzotetraza[l4]annulenes 52 containing di- valent Ni, Pd, Pt, Co, or Cu. The complexes have been considered for use in electro- or thermooptical displays." lo]

i 52

6.3. Other Amine Complexes

The "annelides" C16H,,CH{CH,NHCH,CH2NH- (CH,CH,O),Me}, form mesogenic copper complexes such

as 53.'' ''I Between 77 and 95 "C, both the hydrophilic and the hydrocarbon chains are in a quasi-liquid state, whereas the polar head groups remain in a two-dimensional crystalline array. These lamellar phases, intermediate between

53

crystalline solids and liquid crystals, have been termed tegma crystals. The ESR spectra of the complex in the mesophase, in the solid, and in solution point in each case to a weak magnetic coupling between the copper

54, X, Y , = CO, CO,CH,, CO,(CH,),,O

The 5,s-linked 2,T-bipyridyls 54 constitute an unusual series of thermotropic polymers ; they complex metals such as Fe", Cu' or Cu" to give metallomesogenic materials.['

7. Salicylaldimine Complexes

Schiff bases derived from substituted salicylaldehydes are very versatile ligands which form (N-0) chelates with many metals. Ligands for thermotropic liquid crystals are usually made by condensing the appropriate 4-substituted ether of 2,4-hydroxybenzaldehyde with a 4-substituted aniline or other primary amine [Eq. (i)]; many of these ligands are

\ OH

OH

themselves mesogenic (nematic). Metal complexes can be prepared in good yield when the salicylaldimine is refluxed in ethanol in the presence of (for example) copper(i1) acetate.

7.1. Copper, Nickel, and Palladium [M(N,O,),J and [M(N202)] Complexes

Most of the paramagnetic salicylaldimine copper@) complexes 55 [n = 1, 3, 5-8; R = CmH2m+l (m = 2, 8, 12), F, O,CC,H,OC,,H,,], first reported by Ovchinnikov et al. in 1984, showed smectic mesophases.[' 14* ' Ho wever, nematic behavior was observed by Galyametdinov et al. for 55 (n = 7 ; R = 0,CC,H,0Cl,H,,).t"61

Angew. Chem. 1111. Ed. Engl. 30 (1991) 37s-402 393

Calorimetric and microscopic investigations revealed that the complex 56 forms S, (142-157°C) and S, (157-165°C) mesophases. ESR spectra showed that it was a slightly distorted square planar complex with trans conformation, both in dilute toluene solution and in the isotropic liquid phase," but that some of the planar molecules had undergone tetrahedral distortion in the mesophase. The data were interpreted as indicating that the planar molecules formed the smectic phase while the tetrahedral ones were concentrated in microscopic isotropic droplets, i.e. that the mesophase was hetero- geneous.

u

56

two Cu-N-0 chelate planes, and a weak axial interaction with the ester carbonyl group of the adjacent molecule

57, M = CU, X = H 58, M = Ni, X = Me 59, M = Cu, X = Me

(Cu ... 0 3.04 A). This compound shows a nematic phase above 223 "C and is also a model for a series of complexes of the type 57 (R = 4-C,H40Et), which show S, and N phases (Table 5). Again, in this series, complexes with smaller n (6-10) form nematic, those with n = 11, 12, 14 form both nematic and S, phases, while those with n = 16,18 form only S, phases.

Table 5. Transition temperatures ["C] of the mesomorphic copper(n) complex 57 (R = 4-Et0C6H,)[118].

Complex Mesophases

The X-ray single crystal structure of mesogenic 57 (n = 4, R = p-C,H,C,H,,) showed that, owing to a twist of the N-phenyl and the benzoyl moieties relative to each other, the molecule adopts a lathlike rather than a disklike structure (Fig. 27)" The coordination about copper is distorted square-pyramidally, with a torsion angle of 22" between the

trans-[Cu(C,H,,. n = 6 n = 7 n = 8 n = 9 n = l O n = l l n = 12 n = 14 n = 1 6 n = 18

:,C,H,(O)CH=NR},] 57 S,

-

181 178 1 70 162 156

N I 222 274[a] 211 272 [a] 201 275 [a] 192 262[a] 189 271 [a] 185 268 [a] 204 264 [a] 230 260[a] - 256

251 -

[a] Clears with (some) decomposition.

Both copper(I1) and nickel(@ complexes 57 and 58 (R = 4- C,H,OC,H,,+,; n = 1-6; 8, 10, 14, and R = CmHZm+,; m = 1-10) derived from the n-alkylamines (C,H,,+ ,NH,) show mainly nematic mesophases, while those derived from arylamines show largely smectic me so phase^.["^* I2O1 The nickel complexes were more stable thermally but the copper complexes had lower melting points. The nickel complexes were diamagnetic, and hence square planar.

All the copper complexes 57 (X = H, Me; R = Me, n- C,,H,, , p-C,H,OC,,H,,) derived from 2,4-dihydroxybenzaldehyde showed nematic or S, phases, whereas those from 2,5-dihydroxybenzaldehyde were only mesogenic when the imine (R" =) was derived from methylamine.c'21J The stability of the nematic phase of 57 (R = Me) was found to decrease with increasing n, until, for n = 14, only an S, phase is Nematic mesophases have also been reported for complexes of type 57'.t'231

X-ray studies on the mesophase have been carried out for 55 (R = C,H,, n =7, 12); the complexes show smectic phases, whereas the free ligands form only nematic

interdigitated layers of partially melted chains, and a pos-

Fig. 27. Crystal structure of a complex 57 (n = 4, R = p-C,H,C6H,,) showing weak axial coordination (dotted line) of the carbonyl 0-atom (black) to the phases'"241 There is evidence favoring the presence Of

neighboring copper center (after Ref. [ll8]).


sible coupling of copper atoms in pairs inside the smectic layers. X-ray studies of the nematic and smectic phases of 57 (R = Me, C,,H,,, and C,,H,,OC,H,) have also been carried out."2s1

57'

Other salicylaldimines have also been used for the synthesis of mesogenic copper and nickel complexes. Thus, the bis(salicy1idene)ethylenediamine (H,salen) ligands (derived from 5-alkoxy-2-hydroxybenzaldehydes), form complexes 60 (M = Cu, Ni, n = 4-8), all of which show highmelting S , By contrast, related complexes with the N-methylalkyloxysalicylaldimine ligands, [M{ROC,H,(O)CH=NMe),] were not mesogenic.

A

60

The copper and palladium complexes 61 have also been shown to exhibit smectic mesomorphism, forming mainly S , phases." The palladium complexes have much higher melting and clearing temperatures than the corresponding copper complexes. The crystal structures of representatives

0

+ I

0 61

of each series have been determined; both in the case of Pd" as well as in the case of Cu", the coordination is square planar (Fig. 28).['281 The prosmectic packing of the copper complexes in the crystal (in layers with their major axes inclined at ca. 60" to the layer plane) explains why these complexes do not show discotic behavior. Copper, nickel, and palladium complexes 57 and 58 have been compared. None of the nickel complexes show mesomorphic behavior, whereas the corresponding copper complexes show nematic (and some smectic) phases, and the two palladium complexes synthesized show both smectic and nematic phases." 271

Fig. 28. Molecular structure of the palladium salicylaldimine complex 61 (m = 6, n =7) (according to [128]).

7.2. Vanadyl and Iron [M(N,O,),I Complexes

Of the complexes of type 61 with esterified salicylaldimi- natoligands (M = Zn, Co, Ni, Cu, VO, and Pd)[1291 the zinc, the cobalt, and the nickel complexes showed no mesomorphic behavior, but the copper, the vanadyl, and the palladium complexes (m = 8, n =7) all showed smectic mesophases: S , phases (Cu: 142-156; VO: 151.8-170; Pd: 164.5-198°C) and S, phases (Cu: 156-165; VO: 170- 179.8; and Pd: 198-209°C). The absence of mesophases for the nickel and the cobalt complexes was ascribed to the tetrahedral geometry about the metal, as shown by ESR and electronic spectroscopy ; the copper and the palladium centers are square planar, while the vanadyl unit is square pyramidal.['301

The first mesogenic Schiff base-vanadyl complex was reported in 1984.['301 The complexes 62 (R = Me, n-C,H,,, C,,H,, , and p-alkylOC,H,), form nematic (e.g. for R = n- C,H,, 109-131 "C) as well as smectic mesophase~.[ '~~] The low viscosity of the mesophases allowed easy orientation of the molecules by a magnetic field; the orientation remained unaltered in the solid state.


The mesogenic iron(II1) complex 63, recently reported by Galyametdinov et al.[' 321 is paramagnetic (ESR parameters, g , = 2.096, g , = 4.30), and forms an S, phase (85-151 "C).

63

Although the available data are still patchy, it appears that mesophases are formed when the molecular geometry of the salicylaldimine moiety of the ligand about the metal atom is planar or planar with an axial interaction (square pyramidal), but not for tetrahedral geometries. Within the series most closely examined, those of Cu", nematic phases tend to be favored with short alkyloxy chains, and smectic phases with longer ones.

7.3. Polymeric Liquid Crystals Based on Salicylaldimine Ligands

Curfugna et al.['331 reported the synthesis and properties of mixed ethylene oxide-dodecanediol-Schiff base polymers of the type 64, as well as of the monomeric complex 65. Complex 65 forms a clear S, phase, the polymers, on the other hand, are monotropic, and presumably show S, phases. However, in solution there is some evidence for de-

polymerization. Related polymeric copper complexes based on salen-type units have also been found to be me~ogenic.[ '~~'

8. Cyclometalated Mesogens

Up to now, only palladium-containing cyclometalated mesogens have been reported. However, since many other metals form stable cyclometalates this is likely to be a fruitful area for future development.

8.1. Complexes Derived from Azobenzenes

Ghedini et al. in Calabria prepared the first cyclopalladated liquid crystals 66; (R = EtO or Et; R = C,H,, C,H,, ,

C,H,,, and (CH,),CH = CH,), by cyclopalladation of the appropriate 4,4'-substituted azobenzene derivative with K,PdCl, in aqueous dioxane (25"C, one week) [Eq. (j)J.['351 These dinuclear chloro-bridged complexes formed nematic phases on melting, typically at ca. 200"C, i.e. at much higher

0

0YR 0

66

temperature and in a narrower range than the free ligands; for example, while the nematic range for the ligand EtOC,H,N = NC,H,O,CC,H,, was 59-112"C, that for the complex derived from it was 190-205 "C. The monomeric complexes 67 (L = PPh,) obtained by cleaving 66 with PPh, are not esogenic; however, the amine adducts 67 (L = pyridine, or quinoline) form smectic and nematic

0

d' R

67

me so phase^.['^^] The adduct with the planar quinoline showed an S, phase already at 136-151 "C, and a nematic phase from 151-180°C; the importance of the geometry of the added ligand L was reinforced by the observation that the aniline adduct is not mesogenic and simply decomposes

03-'C6H 13

0 68

396 Angew. Chem. Int . Ed. Engl. 30 (1991) 375-402

at 173 - 175 "C. The dimeric complexes 66 show a high birefringence in the rnes~phase. ' '~~]

All three p-halogeno-complexes 68 show similar nematic phases, but the melting temperatures decrease in the sequence X = I > Br > C1.11361 The chloride showed only a nematic phase (190-205 "C), while the bromide showed enantiotropic S, (210-215°C) and N (215-250°C) phases as well as a monotropic S, phase, and the iodide showed enantiotropic S, (220-225 "C) and nematic (225-230°C) phases.

The Calabria group also found that mesogenic azobenzene ligands were not essential for the synthesis of mesogenic complexes; however, in this case only monotropic materials are formed." 3 7 1 Thus, the complex derived from C,,H,,OC,H,N = NC,H, showed a smectic phase (150- 136 "C) on cooling the isotropic liquid.

Other nematogenic cyclopalladated 4,4'-azobenzenes have been reported ;[1381 and a non-mesogenic complex derived from 4,4'-dimethoxyazobenzene has been characterized by a single-crystal X-ray structure determination (Fig. 29).[' 391

d

Fig. 29. Structure of the non-mesomorphic, cyclopalladated complex formed from the PdCl adduct of 4,4'-dirnethoxyazobenzene and 8-hydroxyquinoline (after Ref. [139]).

8.2. Complexes Derived from Arylimines

Espinet et al. in Spain have prepared the mesogenic complexes 69 and 70 from the corresponding Schiff bases and Pd,(OAc), (Z = H, R = C,,H,, or C,,H,,O; Z = Me, R = C,,H,, for 69, and X = H, halide, OCOR, CN, NO,, CF,, etc for 70)11401

R 69

Angew. Chem. In!. Ed. Engl. 30 (1991) 375-402

The complexes 69 showed smectic phases (usually S,) when X = C1, Br, and SCN, (Table 6) but the acetato-bridged complexes were generally non-mesogenic. It was suggested that this was due to the nonplanarity usually found for complexes with di-yacetato bridges in contrast to the planar arrange- ments with other bridging ligands. Again for this series, the

Table 6. Transition temperatures [ "C] for mesomorphic orthopalladated imi- noarenes 69 (R = C,,H,,, Z = Me)[140].

Komplex Mesophases

69 s c S A I x = CI 112.4 135.0 231.6 [b] X = Br - 102.6 241.2 X = SCN - 166.6 219.6 X = OAc (160.0) [a] 169.9

[a] Monotropic mesophase. [b] Clears with (some) decomposition.

melting and especially the clearing temperatures of the complexes are very significantly higher than those of the free ligands; e.g. in the case of C,,H,,OC,H,CH = NC,H,C,,H,, the phase changes occur at 53.3 (K-tS,), 84.4 (SF'S,), and 89.1 "C (S,+I), while the corresponding di-p-chloro complex showed phase changes at 112.4 (K'+Sc), 135.0 (Sc+S,), and 237.6"C (SA+I, decomposition; Table 6). The chloro complexes 70 showed similar behavior; again the ac- etates were non-mesogenic.

8.3. Complexes Derived from Diarylazines

In contrast to the above-mentioned compounds the di-p-acetato complexes 71 (X = MeCO,) derived from the symmetrical 4,4'-bis(a1kyloxybenzylidene)azines C,H,,+,OC,H,CH = N-N = CHC,H,OC,H,,+, (n =7- 10,12,14) showed S, (and in two cases, also N) mesophases. The p-acetato ligands constrain the complexes to be nonpla- nar, and a novel type of structure based upon "open-book"- shaped molecules (Fig. 30) has been proposed.['411

Fig. 30. The proposed structure of the optically active complex 71 reminiscent of an "open book". The complex exhibits a ferroelectric phase (after Ref. [142]).

397

Reaction of 71, (n = 10, X = C1) with (R)-MeCH- CIC0,Na furnished the optically active complex 71 (n = 10, R = C*Me(H)Cl), which was shown by NMR spectroscopy to contain a mixture of the trans-AR,R. trans-AR,R and cis- R,R The complex showed an enantiotropic S, phase (102-119"C), making it the first organometallic fer-

?C"H* n + 1

A v OC"% n + 1

71

roelectric. However, the electrooptic response is substantially slower than for organic calamitic SF phases, due to the high viscosity of the material. The high viscosity may be due to the bulkiness of the open-book molecular shape.

Further pcarboxylato-complexes of the type 71, (n = 10, R = CmHZm+J have been It was found that short chains (m = 1-3) gave rise to S, (and N) mesophases, while for m = 4-6 enantiotropic N and for m =7-9 monotropic N mesophases are observed. A more complex behavior was found with m 2 10: monotropic S, and N as well as enantiotropic N mesophases were formed. It is ar- gued that these effects arise from a perturbation of the molecular packing, a point which is reinforced by X-ray data.

9. Structure-Property Relationships

The study of metallomesogens is very much in its infancy and considerable information must still be accumulated before relationships similar to those now available for organic mesogens can be drawn. Many metallomesogens are colored and this, together with their somewhat high melting temperatures, can make optical phase identification difficult. Ow- ing to their high viscosity, smectic, discotic, and disordered crystal phases can sometimes be confused; this makes the use of additional techniques for mesophase identification imper- ative. Phase behavior needs to be definitively established, and this will require more low angle X-ray determinations in the mesophases.

Further, the detailed molecular geometries of many quite simple metal complexes are not yet known, and single crystal X-ray structures are also necessary. The assignment of structures by spectroscopy, extrapolation and analogy, so widely used in both organic and inorganic chemistry, may not always be reliable for metallomesogens.

The information gathered in this review allows the follow- ing generalizations to be made.

Both disk- and rodlike metallomesogens of many different types are known. Most of the mononuclear systems have the metal at or near the center of gravity of the molecule; this is true of the discotic mesogens (e.g. phthalocyanines, 8-diketonates, and of the dinuclear carboxylates) as well as for the nematic and smectic calamitic mesogens.

The basic requirements for a metal complex to show mesomorphism are not dissimilar to those for many organics. Thus, a calamitic mesogen will typically have a long rigid group, which, frequently, but not essentially, corresponds to the core of the molecule containing the metal atom and two trans-ligands containing aromatic rings, carrying n-alkyl (or n-alkyloxy) tails in the p-positions. Shorter tails tend to give rise to nematic mesogens, longer ones to smectic mesogens, while the stretched S-shape with the "outboard dipoles" characteristic of many organic compounds forming S, phases (Fig. 31) is also found in many S, metallomesogens with n-alkyloxyphenyl tails. In some cases it is possible to construct calamitic mesogens with only one long organic ligand; these often show S, phases (e.g. 12-16).

1

1

7

J Fig. 31. Molecules forming S , phases with S shape.

A discotic molecule will usually have a flattish platelike form with the metal in the center surrounded by a highly unsaturated organic ligand bearing at least four and prefer- ably six or eight n-alkyl (or n-alkyloxy) substituents. A great many of the metallomesogens known to date are of the columnar discotic type, including carboxylates, b-diketonates, and phthalocyanines (cf. Sections 3.3, 4 and 6.1). By contrast, the complexes of salicylaldiminato ligands (Section 7), especially those with longer hydrocarbon tails, tend to give smectic phases. Both for calmitic compounds as well as for discotic compounds, the tails usually need to be at least five atoms long to show mesomorphism; and the presence of an extra phenyl in the tail is often advantageous.

Many metallomesogens are composed of ligands which are not mesomorphic. When the free ligands are themselves

398 Angew. Chem. Int . Ed. Engl. 30 (1991) 375-402

mesomorphic, the derived metallomesogens show similar phase behavior when incorporation of the metal changes the molecular shape to only a small extent, for example in the phthalocyanines (Section 6.1). In other cases, the considerable difference in structure which the organization of ligands by the metal makes to the overall molecular shape ensures quite different mesophase behavior, for example in the copper P-diketonates.

Thermotropic metallomesogens have been made which incorporate alkali metals, alkaline earths, many 3d metals (V, Mn, Fe, Co, Ni, Cu), some 4d and 5d metals (Mo, Rh, Pd, Ag, Ir, Pt, Au), several p-block metals (Zn, Cd, Hg, Pb, TI), and the lanthanide Lu.

Linear, square planar, and 5-coordinate (usually distorted square pyramidal) geometries have so far been found for the metals in mesogenic complexes. It seems that most metallomesogens need at least one “vacant” coordination site on the metal, which, in the crystal, loosely associates with a donor atom in a neighboring molecule. Thus, for example, the X-ray single crystal data of mesogenic square planar complexes show such axial interactions between the metal (M) and a liganding atom (X) on a neighboring molecule (cf. e.g., the structures in Figures 13, 20, 22, 23, 24, 27 and in Section 6.1). No complexes having an unequivocal tetrahedral geometry about the central metal have yet been found to be mesomorphic. This may be because intermolecular M ... X interactions are not compatible with a tetrahedral geometry, but that they are needed to confer mesomorphism. Furthermore, for dithiobenzoate or salicylaldimine mesogens (Sections 5.2 and 7.1) it appears that attempts to impose a tetrahedral geometry lead to distortion.

The tails needed to confer mesomorphism on a complex may even be able to change the structure subtly, at least in the solid-state. For example, in the bis(dithiobenzoat0) zinc complex (Section 5.2) : while the unsubstituted complex shows a clear tetrahedral geometry about the zinc, the 4-octyloxy and the 4-butyloxy complexes have a loose dinuclear structure with a 5-coordinated metal.

Only one possible molecular metallomesogen with a pre- sumed octahedral arrangement of six oxygen ligand atoms about the metal has so far been reported (see 33). It is important to confirm this and to make other similar molecules.

These generalizations all depend on the assumption that the structure in the crystal is closely related to that in the mesophase, but this may not always be valid. In a few cases it has been checked: thus, in the columnar discotic dicopper and dirhodium tetracarboxylates there is no detectable change in bond lengths (by EXAFS) on going from the crystal to the mesophase (Section 3.3).

It is found that ligands with n-alkoxy tails often have beneficial effects in stabilizing mesophases; for example while the palladium alkoxy-CB complexes 7 show thermodynamically stable (enantiotropic) phases, the closely related alkyl-CB complexes at best only form the unstable (monotropic) phases (Section 2.1). Similarly, whereas the copper B-diketonates with 1,3-phenyls bearing p-C,H1,O substituents gave discotic mesophases, those where the phenyls had p-CsH,, substituents did not (Section 4.2).

The introduction of a metal can lead to rather high-melting mesophases, perhaps due to strong axial (M.-X) intermolecular interactions, This can be counteracted i) by intro-

ducing asymmetry into the molecule (Sections 2.2.2 and 2.3), which makes the molecular packing more difficult; ii) by introducing more amphiphilic coligands (e.g. carboxylate in place of halide, Section 2.2.1); or iii) by mixing two or more related complexes (this lowers the melting point without seri- ously affecting the clearing temperature, Figs. 7 and 10). These methods have been used to bring transition temperatures close to ambient, and further decreases are certainly possible.

In order to be useful as advanced materials, metallomesogens must be reasonably inert and quite thermally robust. Since complexes often have the metal as center of reactivity, for example towards air or moisture, inertness and stability must be added. This can be done by using very strongly binding ligands or by incorporating the heavier 5d metals, or both. Suitable ligands are those with donor atoms having large stability constants (e.g. S or P in compounds of the heavy Group VIII metals) and especially chelates.

Apart from those needed to characterize the materials, rather few serious physical measurements have yet been un- dertaken on metallomesogens. However, considerable in- creases in polarizability have been found (as expected),[”] as well as (unanticipated) high birefringen~es.~’~’ 351 This latter observation parallels the use of heavy metal oxides in high refractive index glasses.

Currently available metallomesogens may have long electrooptic response times (see Ref. [142]), which could be a disadvantage for some electronic displays. However, there are many other potential applications and their uses as passive NIR blockersts2] in laser addressable or thermal record- ing materialstg1* l r o l have already been proposed. New and interesting is the suggestion that, since the incorporation of alkali-metal cations into cholesteric crown ethers causes a change in the helical pitch, this can be made the basis of an alkali-metal sensor.t1441 Another is the observation by scien- tists at the 3M-Company in the USA that salts such as the fluorescent pink compounds [Pt(RNC),][Pd(CN),] (RNC, e.g., decylphenyl isocyanide), turn blue when exposed to the vapors of many hydro- or fluorocarbons.t1451 Although not specifically identified as liquid crystals, such materials must at least be closely related to metallomesogens.

10. Outlook

Metallomesogens now form a viable and challenging in- terdisciplinary topic which offers much scope to the imagina- tion of both the synthetic chemist, for the design of new ligands and their metal complexes, and the materials scientist and engineer for new uses for these advanced materials. The knowledge is now at hand to design complexes offering rodlike nematic or smectic mesophases or, by suitably modify- ing the ligands, a variety of discotic phases. Colors can be incorporated by the use of suitable metals and ligands, as can magnetic, ferroelectric, and many more exotic properties The organized stacks of some of the metallomesogens, for example the dimetal tetracarboxylates, suggest that, with suitable manipulation, electrically conducting materials may result.[7’.94T 1031

Angem,. Chem. Int. Ed. Engl. 30 (199i) 375-402 399

Although a substantial number of metals have already been incorporated into metallomesogens, many still remain to be explored. The design of ligands for metallomesogen character is also in its infancy and new applications will require new types of complexes. This is one challenge to workers in the field.

Possible device applications for metallomesogens, which may presently be anticipated, are as new thermal or nonlin- ear optical materials, and in electrochemistry and sensors.

The effects of organic mesogens on organic reactions are just beginning to be Metal-promoted reactions are in general more sensitive to steric factors than organic ones, thus there appears considerable scope for using mesophases and metallomesogens to promote stereoselectiv- ity both in stoichiometric organometallic reactions and in metal catalyzed processes. However, great care will be needed in the design of properly controlled experiments and their careful execution to avoid spurious results.

Thus far one can speculate, but we suspect the best application has not yet been thought of. What a challenge for the future!

We thank all our many colleagues and co-workers who have contributed to metallomesogens and who made this review possible. We are especially grateful to Dr. N . A . Bailey, Dr. D . FV Bruce and Dr. D . A. Dunmur (Sheffield), Professor P . Es- pinet (Valladolid), Dr. D. Guillon (Strasbourg), Dr. .I C. Marchon, and Dr. P . Maldivi (Grenoble), and Professor J . L. Serrano (Zaragoza) for most fruitful collaborations. We also thank the SERC, the Royal Society, the EC Science Progam- me (Grant ST2J-O387-C), CNRS, CEA, and BDH Chemicals plc for their strong support, and M Aubert for assistance with the drawings.

Received: June 25, 1990 [A 811 IE] German version: Angew. Chem. 103 (1991) 370

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metallomesogens: metal complexes in organized fluid phases

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