Unsymmetrically 1,1-Disubstituted Ferrocene Derivatives

Unsymmetrically disubstituted complex 20 [16], obtained during the preparation of 9 (n = 10) [16], seemed to show a nematic phase. The latter did not reform during the cooling process. A mixture of the two derivatives 20 (4 parts) and 9 (n = 10) (1 part) gave rise to a broad nematic phase (C 110 N 170 I). 

Comparison of the nematic to isotropic liquid transition of related systems. 

In conclusion the ferrocene unit is a poor component of a mesogen but can be tolerated if the moleeule is long enough to compensate for the reduetion of mesogenic properties induced by its inclusion. This statement is in agreement with the findings obtained for the 1,3-disubstituted ferroeene derivatives (compare 15 (Ijd = 8.3) and 16 l/d = 7.2) with 17 l/d = 4.6)). 

We designed and studied [27] a family of unsymmetrically l,l -disubstituted ferrocene derivatives obtained by combining the organic units A and B (above), used to prepare the families 13 and 14, respectively within the same molecular framework. Structures 25 (Fig. 9-21) led to remarkable mesomorphic properties. All derivatives exhibited liquid crystal properties. Compound 25 (n = 11) gave rise to an enantiotropic smectic A phase. Complex 25 (n = 12) showed an enantiotropic 

However, one can already mention that the introduction of dissymmetry had two positive effects on the mesomorphic properties a depression of the melting points was observed and ferrocene derivatives exhibiting smectic C and smectic A phase were obtained for the first time. 

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Fig. 9-19. Unsymmetrically l,l -disubstituted ferrocene derivatives reported by Thompson, Goodby and Toyne.

For the preparation of unsymmetrical ferrocenes, two ways by which only mono-substituted derivatives are produced have been suggested. One route starts from iron tetracarbonyl and a substituted cyclopenta-diene the other from monocyclopentadienyl iron dicarbonyl bromide, which on treatment with a substituted cyclopentadienyl lithium is finally converted into the corresponding mono-substituted ferrocene. Experience shows the first method to be more suitable for the preparation of aryl, and the latter method for the preparation of alkyl, derivatives (57). Corresponding work already carried out on substitution in Ru(C5H6)2 and Os CsH5)2 has also been fruitful. It is found that in the Friedel-Crafts reaction with acetyl or benzoyl chloride there is a distinct predominance of mono- over disubstitution as the atomic weight of the central atom increases (47, 72). 

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