Polycatenar complexes

In the palladium(II) macrocyclic complexes, the metal ions were proposed to be coordinated in a square planar geometry, with a disordered smectic mesophase being observed, even for the polycatenar complexes (i. e. those with more than one chain on each terminal ring). Preliminary X-ray diffraction measurements determined the smectic if-spacing to be 53 A which is comparable to the calculated molecular length of the ligand (50-52 A) in the fully extended conformation [83b]. 

Figure 96 Phase diagram for polycatenar complexes of silverfl).

Two series of polycatenar complexes containing the same metallic fragments were also reported [115]. As expected for polycatenar complexes, nematic (54 n=6, 10 and 11), Sc (54 n-lO and 11) and Colh (54 n=12-16) phases were obtained. Resolution of the pure enantiomers gave rise to the equivalent chiral phases for the same n (N and Sc ), and Colh phases (chiral ) for n> 12, with slight differences in the transition temperatures as compared to the racemic mixtures. Small spontaneous polarisations were measured. 

For polycatenar hydrogen bonded complexes with fluorinated chains at both ends (e.g., 138,139, see Fig. 36) formation of columnar phases was observed [246]. However, compound 137, having a branched Rp-chain at one end and three RH-chains at the other has a sequence of three distinct phases in the unusual sequence Cub-Col-SmA-Iso. For the SmA phase of compound 137 a structure with intercalated aromatic cores and RF-chains and separated layers of the hydrocarbon chains was proposed. At lower temperature, when incompatibility rises and the aromatics and Rp-chains disintegrate, all three components form their own layers. However, this produces interface curvature and a columnar phase with square lattice is formed. On further cooling a transition to a cubic phase with Im3m lattice takes place which is most likely of the bicontinuous type [262]. This leads to the unusual phase sequence Cubv-Col-SmA where the positions of the Cubv and Col phases are exchanged with respect to the usually observed phase sequences. The Col-Cub transition at lower temperature could be the result of the decreased conformational disorder of the terminal chains which reduces the steric frustration and hence reduces the interface curvature. 

Having synthesized a number of complexes of monoalkoxystilbazoles, we explored the effect of using various poly(alkoxy)stilbazoles, given the interesting mesomorphism found in polycatenar mesogens (Section VI,A,3). This work is discussed in detail elsewhere 24). Here, we give an overview of the work to enable the reader to see the overall pattern of liquid-crystal behavior and the issues that arise. 

Fig. 47. Polycatenar palladium complexes with lateral alkanoates. The parentheses around the alkoxy groups on the stilbazole indicate that a variety of isomers were studied.

We have pursued the study of polycatenar metallomesogens using poly(alkoxy) stilbazoles [177], and have recently imdertaken a systematic study of the complexes of series of these ligands with palladium(ll), platinum(II) and silveifl) ions. 

Room-temperature tetrahedral zinc(II) complexes with polycatenar pyrazole or bis(pyrazolyl)methane hgands show luminescence in the near ultraviolet and blue spectral regions (Figure 2.42). ° A broad, structureless emission band with a large Stokes shift was observed. The emission bands were found to red-shift with an increase in the number of... 

The study was extended to polycatenar derivatives 7 and the phase diagram for the behavior of the 3,4-tetracatenar materials (7 R = H) is shown in Figure 26. Thus, the addition of the ReBr(CO)4 moiety has suppressed the SmC phase and, due to the increase in the core volume, has ensured that the columnar phases (and now a cubic phase, too, with Ia3d symmetry ) occurred to longer chain length. In common with many other systems, it is assumed that the phase indicated Col is, in fact, Colh, while that indicated Col is one of the Col phases. All complexes of the hexacatenar derivative (7 R = OC Fl2 +i) showed a columnar phase. 

Some of the simplest systems reported raise a possible point of semantics as to whether these systems are, in fact, polycatenar in nature or perhaps ought to be described as discotic. The first of these 77 was based simply on the coordination of 3,4,5-trialkoxybenzonitriles to Pd(ll), following a motif for the formation of simple, calamitic systems "" described some years earlier. Only two compounds with different chain lengths were studied ( =10, 18). The ligands were themselves not mesomorphic, but on complexation, a Col), phase was observed over small temperature ranges, i.e., 73-91 °C for the complex with six tridecyloxy chains, and between 58 and 80 °C for that with six octadecyloxy chains. 

Praefcke and co-workers have carried out important research on macroheterocyclic tetrapalladium" " and tetraplatinum organyls 118 and 119, derived from polycatenar bis(imine)phenylene, and the nematogenic bis(imine)-stilbenylene ligands. These [M4(/r -X)4L2] complexes were prepared by ort4o-metallation of the appropriate diimine with palladium acetate or di-/r-chloro-bis-[(77 -2-methylallyl)platinum], followed by an acidic treatment to form the chloro-bridged compormds. The other complexes were obtained by ion-exchange reactions with KBr, KI, and... 

The study of 126 was later followed up by reports of the behavior of the related Pd(ll) and Pt(ii) complexes 127-in which the authors set out to investigate the transition from the behavior associated with calamitic materials to that associated with more disk-like materials (Figure 38). The results, summarized in Table 23, show that it is both the number and distribution of chains on the /3-diketonate that determine the transition to disk-like behavior. Interestingly, none of the complexes show behavior characteristic of both rods and disks as found in polycatenar systems2 2" (see 12.05.3.5). 

The same type of columnar structure is also proposed for other polycatenar materials, such as tetrone derivatives [51], phasmidic macrocyclic liquid crystals [52], silver (I) complexes [53], or 2,2 -bipyridine derivatives [54]. 

There are, in fact, rather few such compounds. We have recently reported the thermal behaviour of several series of polycatenar metal complexes, 40-45, and observed very interesting relationships between the structure of the metal complexes and their thermal behaviour [81]. 

Fused-polycatenar metallomesogens is the name given to metallomesogens resulting from metal-complexation to polycatenar ligands, by analogy to... 

The ( -arene)tricarbonylchromium complexes 55 were reported by De-schenaux et al. [117]. The uncomplexed ligands showed a mesomorphism characteristic of polycatenar mesogens, progressing from N and Sc phases at shorter chain lengths to Colob and Colh phases at longer chain lengths. 

The division of the subject matter concentrates primarily on the type of liquid crystal phase formed and, to a large degree, this is a reflection of molecular shape, although as the reader will see this is not such a clear-cut distinction where polycatenar liquid crystals are concerned. However, within these broader classifications, material is then organized paying attention to the ligand types used to form the complexes. This is not a unique classification, but it does provide some sort of framework within which the field may be viewed. 

Results were also described from studies which introduced lateral substituents into these complexes and in one paper the effect of introducing lateral alkanoate groups was reported (113). " The use of lateral alkanoates had been described previously by Maitlis and co-workers in palladium complexes of 4-alkoxystilbazoles (Section 7.9.14.3.6(iv)) and had been shown to induce the formation of liquid crystal nematic phases. In the present situation where the complex was polycatenar, it was not clear whether the chains would act to increase the volume of the core of the polycatenar mesogen or simply as extra chains, so rendering the complex more classically discotic. In fact, the answer was neither and at all chain lengths (n and m) and degrees of substitution (R = H or OC H2 +i) where mesomorphism was observed, a nematic phase was seen at or near room temperature. This behavior was somewhat puzzling and is so far without explanation. 

Dianionic platinum(II) complexes ((119) n =10, 14, 18) with sulfur-rich dithiolato ligands based on tetrathiafulvalene have been reported.These potentially interesting complexes for their unique electronic properties, having a polycatenar structure unfortunately did not form a mesophase. The absence of mesomorphism may be simply due to the bulkiness of the counter ion. 

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