Christopher Imrie

Monosubstituted thermotropic ferrocenomesogens: an overview 1976–1999

Research in the late 1940s in academic institutions, most notably the University of Hull in the UK, led to the development of liquid crystal display technology in the 1970s, which has subsequently had a large impact throughout the world. The liquid crystal technology is based on simple organic molecules. Since the late 1980s, some liquid crystal scientists have turned their attention to investigating the effect of introducing a metal atom into the systems. This review focuses on the major developments in the field of ferrocenyl-containing liquid crystals in which the organometallic group is situated in a terminal position with respect to the core of the molecule. Metallomesogens with terminal metal atoms are not very common, since it can be deduced from the theory of organic liquid crystals that bulky terminal groups would not be conducive to the stabilization of liquid crystal phases. Nevertheless, a terminal ferrocenyl group can stabilize a nematic liquid crystal phase and examples of this will be discussed in this review.

The synthesis and liquid crystal behaviour of monosubstituted ferrocenomesogens

The synthesis and characterization of several new series of monosubstituted ferrocenyl-containing liquid crystals has been achieved. The results indicate that a terminal ferrocenyl group can promote stabilization of a nematic liquid crystal state. At least three phenyl rings are required in the molecular core in order to provide nematic properties and addition of a fourth ring substantially enhances it. Bulky lateral substituents or the introduction of linker groups that introduce kinks inhibit liquid crystal phase formation. Short highly polarising terminal groups or terminal groups that support hydrogen bonding support liquid crystal behaviour.

Esterification reactions in ionic liquids. The efficient synthesis of ferrocenyl esters in the ionic liquids [bmim][BF4] and [bmim][PF6]

The synthesis of ferrocenyl esters has been achieved using the DCC/DMAP protocol in the ambient temperature ionic liquids 1-butyl-3-methylimidazonium tetrafluoroborate ([bmim][BF4]) and 1-butyl-3-methylimidazonium hexafluorophosphate ([bmim][PF6]). Yields are very high and efficient recycling of solvent was achieved.

Synthesis of ferrocenylphenyl derivatives including biphenylferrocenes, arylferrocenylphenyl ethers and arylferrocenylphenyl amines

The preparation of a series of substituted biphenylferrocenes via a modified Suzuki reaction between 4-bromophenylferrocene and substituted phenylboronic acids is described. The X-ray crystal structure of 4′-formyl-4-biphenylferrocene is reported and this compound is then incorporated into the first series of ferrocenomesogens containing the ferrocenylbiphenyl unit. The synthesis of a series of arylferrocenylphenyl ethers and arylferrocenylphenyl amines is also described.

Thermal properties of monosubstituted ferrocene derivatives: a series of new ferrocenomesogens

The preparation of a series of 1-substituted ferrocene derivatives is described. The structure of the substituent was varied systematically in order to determine the structural limits within which mesogenic behaviour is retained. The compounds are particularly useful for stabilizing the nematic phase, the range of which was increased substantially by extending the aromatic core of the substituent. Selective lateral fluorination was successfully used to lower the nematic temperatures as well as to suppress any smectic phases. Derivatives with no more than two rings in the core only display the crystal smectic B phase. The significance of substituents on the cyclopentadienyl rings is discussed with reference to their electronic properties and their effect on the stability of the compounds.

A liquid crystalline ferrocene derivative with a chiral smectic c phase

The synthesis of the first liquid crystalline derivative of ferrocene with a chiral smectic C phase is reported.

The use of a modified Suzuki reaction for the synthesis of monoarylferrocenes

A modification of the Suzuki cross-coupling reaction proved to be a clean and useful method for the preparation of monosubstituted arylferrocenes. Iodoferrocene was reacted with a series of substituted arylboronic acids in the presence of sodium carbonate and palladium acetate in aqueous ethanol at room temperature to produce a range of substituted monoarylferrocenes. A systematic investigation undertaken to determine optimal reaction conditions indicated that scrupulous deoxygenation of the solvent is critical. The use of stronger bases such as barium hydroxide and potassium carbonate is favourable and gives rise to better yields of monoarylferrocenes. The reactions also proceed efficiently in aqueous DMF, broadening the scope of the reaction allowing efficient reactions with boronic acids that show low solubility in organic solvents.

Syntheses and crystal structures of neutral oxorhenium(V) complexes of N2O2-donor tripodal ligands

The neutral distorted octahedral complexes [ReOCl(L)] {H2L = N,N-bis(2-hydroxybenzyl)-2-(2-aminoethyl)dimethylamine (H2had); N,N-bis(2-hydroxybenzyl)-aminomethylpyridine (H2hap); N,N-bis(2-hydroxybenzyl)-2-(2-aminoethyl)pyridine (H2hae)} were prepared by the reaction of trans-[ReOCl3(PPh3)2] with a twofold molar excess of H2L in ethanol. X-ray structure determinations of [ReOCl(had)] (1) and [ReOCl(hap)] (2) were performed, and the structures compared. In both complexes the choride is coordinated trans to the tripodal tertiary amino nitrogen, with a phenolate oxygen trans to the oxo oxygen.

The synthesis and X-ray crystal structure of [(4-ferrocenylphenylimido)]trichlorobis(triphenylphosphine)rhenium(V) and related ferrocenyl–rhenium(V) compounds

The reaction of 4-ferrocenylaniline with trans-[ReOCl3(PPh3)2] gave [(4-ferrocenylphenylimido)]trichlorobis(triphenylphosphine)rhenium(V) (1). The molecular structure of 1 has been determined by X-ray diffraction. The X-ray crystal structure of [(4-ferrocenylphenylimido)]tribromobis(triphenylphosphine)rhenium(V) (2) has also been determined and its molecular structure is compared with that of 1. The complexes 1 and 2 have also been investigated by cyclic voltammetry and UV-visible spectroscopy. The results of the cyclic voltammetry suggest that there is insignificant electron delocalization from the ferrocenyl group towards rhenium in 1 and 2. However, both 1 and 2 show a long wavelength absorption in their electronic spectra which is characteristic of conjugated ferrocenyl derivatives. The synthesis and characterization of two further similar imido-complexes but in which the ferrocenyl-phenyl group is part of a more extended unit is described (complexes 5 and 6). The synthesis of a rhenium oxo-complex, fac-oxotrichloro[1,1′-bis(diphenylphosphine)ferrocene]rhenium(V) (3) has also been achieved and 3 has been further derivatized with two different ferrocenylamines to provide rhenium complexes containing two ferrocenyl groups (4 and 7).