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High Coordination Number Calamitic Metallomesogens By Duncan W. Bruce*

The rebirth of research into metal-containing liquid crys- tals (dubbed metallomesogens) dates back to the late 1970s and the reports by Giroud-Godquin and Mueller-Westerhoff of mesomorphism in nickel and platinum complexes of 1,2- dithiolenes. The bulk of the literature on the subject has appeared since the mid-1980s and the field is rapidly expand- ing,['] holding its own small conference every two

The single main feature that identifies a thermotropic liq- uid-crystalline material is that it is structurally anisotropic, having one axis of significantly different dimension to the other two. This describes two main classes of low molar mass material : rods (calamitic systems) and discs (discotic sys- tems). It then becomes relatively straightforward to design a wide variety of organic compounds which will possess meso- genic properties. Over 50000 calamitic materials are now knownr3] and interested readers are referred to excellent ar- ticles by ToyneL4I and DemusL5] and to the contribution by Vi11.[61

A quick glance at this literature reveals that the vast ma- jority of metals which have successfully been incorporated into thermotropic, culumitic (rod-like) liquid-crystalline molecules possess a d8-d" electronic configuration, that is to say, most of the work has been carried out on Rh', Ir', Ni", Pd", Pt", Cu", Ag' and Au"'. Why is this the case? The explanation is rather simple.

In order to use metal complexes to mimic the rather flat, elongated structures of calamitic thermotropic liquid crys- tals, it seemed sensible to use metals which were happy in linear and planar geometries, hence the intense interest in metals from groups 9-11 of the periodic table. Indeed, in designing the bis(l,2-dithiolate) complexes of Ni" and Pt", Giroud-Godquin and Mueller-Westerhoff had postulated that the two five-membered M-S-C-C-S rings might be equivalent to a 1,4-disubstituted phenyl ring (Fig. 1).

Despite the rather small range of metals used, there has been no shortage of interesting results and, paramagnetic nematics,['I ferroelectrics,''] highly birefringentLgl and dichroic["] materials, and even speculation (although ill- founded) about biaxial nematics,[' '] have all turned up.

I 1

[*] Dr. D. W. Bruce'" Centre for Molecular Materials and Department of Chemistry The University Shefield S3 7HF (UK)

1995. ['I Sir Edward Frankland Fellow of the Royal Society of Chemistry 1994/

So what evidence is there that high coordination number metal centers can be incorporated into mesomorphic materi- als? The first such reports concerned the derivatives offer-

Fig. 1. Postulated similarity between a metal dithiolate fragment and a phenyl ring.

rocene, originally synthesized by Malthete and Billard in 1976["] and most recently developed by Deschenaux and co-workers.[' 31 Cyclopentadienes are formally regarded as occupying three coordination sites and so these derivatives can be considered six-coordinate. MalthEte and Billard's complexes all had ferrocene as a terminal group, while Deschenaux has examined 1,l'-, 1,2- and 1,3-disubstituted systems, identifying 1,3-disubstituted systems as those best suited to the formation of mesomorphic compounds. These results are consistent with those of Goodby, Toyne and co- w o r k e r ~ , ~ ' ~ ] who examined a wide variety of 1 ,l'-disubstitut- ed ferrocenes. A key point from Deschenaux's work was that it appeared that at least four aromatic rings were necessary to promote mesomorphism in the 1,3-disubstituted systems (Fig. 2).

Fig. 2. Examples of mesomorphic ferrocenes from Deschendux's group

Another metal system which has been successfully em- ployed in higher coordination number systems is the oxo- vanadium@) moiety, in combination with salicylaldimines and b-diketonates giving five-coordinate complexes of both calamitic and discotic types. Similar shaped complexes have also been described by Ovchinnikov and co-workers based on salicylaldimine complexes of the Fe"'-CI unit,['51 also leading to five-coordinate species (Fig. 3).

Octahedral P-diketonate complexes of Fe(m) were de- scribed in 1982, although they were not shown to be meso- morphic.[16]

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b-,M- 0 ALN. k

Fig 3 Structure of the five-coordinate salicylaldimate complexes of V'" (M = V=O) and Fe"' (M = Fe-CI)

Fig. 5. Non-mesomorphic Mn' complexes.

So what is the problem in trying to use high coordination number metal centers in a routine way? Simply stated, it lies in the fact that in incorporating metals with higher coordina- tion number requirements, the additional ligands are certain to reduce structural anisotropy and so depress mesophase formation. How can this problem be tackled?

We believed that the problem could be approached by learning from studies on laterally substituted mesogens. Lat- eral substituents broaden a molecule, thus reducing its an- isometry and hence the stability of its mesophases (the use of small substituents such as fluorine has become a fine art in generating materials with tunable properties). However, if a more anisometric material is used as the starting point, the effect of lateral broadening will be diminished and therefore mesophases might still result. That this is a reasonable argu- ment can be seen in the mesomorphic ferrocenes, where a particular number of aromatic rings (i.e., structural an- isotropy) is needed to generate mesomorphic complexes. Further, it should also be the case that nematic phases should predominate, as the bulky lateral group would reduce the tendency for the lateral attractions which stabilize smec- tic mesophases.

Our approach was based on imines, which are extensively used in liquid crystals. Some twenty years ago, Stone and co-workers[Z1l had shown that benzylideneaniline could be reacted with [MnMe(CO),] to give the orthometallated spe- cies shown in Figure 4. Given that the [Mn(CO),] fragment

mNa [MnMe(CO)5] - e N G Mn

Fig. 4. Synthetic route to the orthometallated imines

would be a significant perturbation to the structural an- isotropy of such a species, work started with the aim of discovering how long the parent imine needed to be before mesophases were found.

Manganese complexes based on two- and three-ring imines (Fig. 5) showed no liquid-crystalline phases and melt- ed straight to isotropic fluids, with no monotropic phases being observed. It turned out that in order to stabilize enan- tiotropic phases four rings were needed in the backbone (cf. the ferrocenes above) and the complexes illustrated in Fig- ure 6 were shown to have nematic mesophases (as predicted).

Fig. 6 . Structure of the mesomorphic Mn' complexes

It is of interest to compare the phase behavior of the free ligand with that of the related complex. Thus, for n = 8, R = C,H,, and ring = tmns-1,4-cyclohexyl (I), the free lig- and had the following mesomorphism:

Crys .87. J ,137, SmI .143. SmC .216, N ,307, I

where the temperatures are given in degrees Celsius, while for n = 8, R = C,H,,O and ring = 1,4-phenyl (II), the free ligand showed

Crys .63. Crys' ,116, G ,124, SmC ,202. N ,298, I

Very high clearing points indeed. However, compare these with the mesomorphism of the Mn complexes:

[Mn(I)(CO),]Crys . 122. N . 180. I (with decomp) [Mn(II)(CO),]Crys. 154". 190.1 (with decomp)

Thus, the large, lateral [Mn(CO),] fragment had destabilized the nematic phases of the ligands by around 110-120 0C.117]

Several more derivatives of these complexes have now been made, lower melting and clearing complexes have been found, and the same chemistry has now been demonstrated with Re(1) .["I

The design and synthesis of high coordination number complexes with liquid crystal properties is now gathering pace and there are other very recent examples of discotic systems containing six-coordinate metal^.^^^^ As our un- derstanding of the way in which high coordination number complexes can be fabricated into mesomorphic materials in- creases, so more of the periodic table will be opened up. Given the rather exciting physico-chemical properties of many of the elements as yet untried in liquid crystals, this looks like an area to be watched in coming years.

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[I] D. W. Bruce. inInarganicMuterials(Eds: D. W. Bruce, D. OHare), Wiley, Chichester 1992. D. W. Bruce, J Chem. Soc., Dalton Trans. 1993. 2983. P. Espinet, J. L. Serrano, L. A. Oro, M. A. Esteruelas, Coord. Chem. Res. 1992,117,215. P. M. Maitlis, A,-M. Giroud-Godquin, Angew. Chem., Inr. Ed. Engl. 1991,30,402. AngeM,. Chem. 1991, 103,370. S. A. Hudson, P. M. Maitlis, Cliem. Rev. 1993,93,861. A. P. Polishchuk, T. V. Timofeeva, Russ. Cheni. Rev. (Engl. Transl.) 1991, 62, 291.

[2] The first three meetings were held in Sheftield (1989), St Pierre de Chartreuse (1991) and Peiiiscola (1993). The next is scheduled in Calabria in June 1995.

[3] V. Vill, private communication. [4] K. J. Toyne, Thermotropic Liquid Cry.yfals (Ed: G. W. Gray), Wiley,

[5] D. Demus, Liq. Cryst. 1989, 5, 75. [6] V. Vill, in Macroscopic and Technical Properties ofMatter (Ed: J. Thiem),

[7] See, e.g., M. Marcos, J.-L. Serrano, Adv. Muter. 1991, 3, 256. [8] P. Espinet, J. Etxebarria, M. Marcos, J. Perez, A. Remon, J.-L. Serrano,

Angew. Chem., Int. Ed. Engl. 1989, 28, 1065; Angew. Chem. 1989, 101, 1076.

[9] D. W. Bruce, D. A. Dunmur, P. M. Maitlis, M. R. Manterfield, R. Orr, .I Mater. Chem. 1991, I , 255.

Chichester, UK 1987.

Liquid Crystals, Vol. 7, Springer, Berlin 1992.

[I01 D. W. Bruce, D. A. Dunmur, S. E. Hunt, P. M. Maitlis, R. Orr, J Mufer.

[ I l l S. Chdndrasekhar, B. K. Sadashiva, B. R. Ratna, V. N. Raja, Pramana

[I21 J. Malthete, J. Billard, Mol. Cryst. Liq. Cryst. 1976, 34, 117. [13] R. Deschenaux, J.-L. Mareudaz, J Cliern. Soc., Chm. Commun. 1991,909.

R. Deschenaux, J. Santiago, J Muter. Chem. 1993.3,219. R. Deschenaux, J.-L. Marendaz, J. Santiago, Helv. Chim. Acta 1993. 76, 865. R. Deschenaux, J. Santiago, Tetrahedron Lett. 1994, 35, 21 69.

[I41 N. J. Thompson, J. W. Goodby, K. J. Toyne, Liq. C r y f . 1993, 13, 381.

[I51 Y. G. Galydmetdinov, G. I. Ivanova, I. V. Ovchinnikov, Izv. Akad. Nauk SSSR, Ser. Khim. 1989, 1931.

[I61 A.-M. Giroud-Godquin, A. Rassat, C. R. S4ances Acad. Sci. Ser. C 1982, 294, 2.

[I71 D. W. Bruce, X.-H. Liu, J Chem. Soc., Chem. Commun. 1994, 729. [IS] D. W. Bruce, X.-H. Liu, unpublished. [I91 H. Zheng, T. M. Swager, .I Am. Chem. Soc. 1994, 116, 761. [20] S. Schmidt, G. Lattermann, R. Kleppinger, J. C. Wendorff, Liq. Cryst.

[21] R. L. Bennett, M. I. Bruce, B. L. Goodall, M. Z. Iqbal, F. G. A. Stone, J .

Chem. 1991, 1, 857.

1988, 30, L491.

1994, 16, 693.

Chem. Soc., Dalton Trans. 1972, 1787.

Research News

Continuous Polymer Fractionation By Bernhard A. Wolf*

1. Introduction

In both basic research and industrial applications it is often necessary to use polymers of a certain minimum molec- ular uniformity. For example, the energy stored in a sheared melt depends approximately on the seventh power of the molecular weight M , i.e., traces of high molecular weight material in a broadly distributed sample dominate the effect, and also in pharmaceutical products too short and too long chains are often harmful.

In order to provide access to large amounts of good frac- tions in cases where it is impossible to synthesize polymers with narrow molecular weight distribution, continuous poly- mer fractionation (CPF) has been developed in recent years.“ -’I In this method the low molecular weight material is removed from a concentrated polymer solution in a con- tinuous counter-current extraction. The two liquid phases required for that purpose are only formed in the presence of the polymer, i.e., the different polymer species need not “de- cide” between two chemically dissimilar solvents, but only between a dilute and a concentrated phase. The chromato-

[*I Prof. B. A. Wolf Institut fur Physikalische Chemie, Universitat Mainz J.-Welder-Weg 13, D-55099 Mainz (FRG)

graphically working CPF allows a relatively sharp cut to be made through the molecular weight distribution of a given sample at a predetermined position; this process can easily be implemented on a technical scale. By means of model calculations[61 it is possible to assess the efficiency of a cer- tain CPF experiment theoretically; the thermodynamic sim- ulation of the process starting from the equilibrium behavior of the system allows the number of theoretical plates that were realized in a certain continuous counter-current frac- tionation to be determined.

2. The Principle of CPF

CPF functions in the following way. A moderately con- centrated (typically 10-20 wt- %) homogeneous solution of the starting polymer is used as feed (FD) and extracted by a homogeneous (single or mixed) solvent (EA: extracting agent). The two phases required for extraction form within the separation device, which can consist of a sieve-bottom column, a packed column, a mixer-settler system, or a cen- trifugal separator. The coexisting phases are transported due to the differences in their density. In the course of this coun- ter-current motion the better soluble material (i.e., that with

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