Ferrocene derivatives properties

There are many organic and inorganic materials available which exhibit significant non-linear optical properties. In the present context, however, we underline that ferrocene derivatives were the first organo-metallic materials observed possessing non-linear optical properties. In fact, in 1987 it was reported that ci s-ferrocenyl-2-(4-nitrophenyl)ethylene has a 62-fold greater efficiency than urea in generating the second harmonic, Scheme 13.71... 

Ferrocene-containing co-polyesters, preparation, 12, 350 Ferrocene-containing liquid crystals dendrimers, 12, 235 disubstituted ferrocenes, 12, 227 hydrogen-bonded ferrocene derivatives, 12, 234 monosubstituted ferrocenes, 12, 222 polycatenar ferrocenes, 12, 234 properties, 12, 221... 

As already mentioned, in contrast to the cobaltocenium moiety, the ferrocene derivatives represent neutral redox-active receptors for anions. As we are losing coulombic interactions, the complexation is mediated solely via hydrogen bonding. Consequently, the corresponding association constants can be evaluated only in HB non-competitive solvents and they are usually much lower when compared with those of cobaltocenium derivatives. This apparent disadvantage is compensated by the much higher synthetic potential of ferrocene (commercial availability of many derivatives, much easier synthetic handling) and the excellent electrochemical properties of this compound. 

Ferrocene derivatives coupled with heterocyclic systems have attracted special attention in recent years because of their interesting organic and inorganic properties. Recently, an efficient and rapid route for the synthesis of 4-aryl-2-ferrocenyl-quinolines 70 has been described by Tu and co-workers [116] through a microwave-assisted MCR of acetylferrocene with an aromatic aldehyde and dimedone in the presence of ammonium acetate in DMF. This novel procedure provides the target hetero-metallic compounds in excellent yields without the need of any purification (Scheme 54). 

A chiral C2-symmetric bridged ferrocene was constructed from a synthetic ligand containing two cyclopentadiene units (Fig. 4-12d). The key step in the synthesis of the ligand is a diastereoselective Diels — Alder reaction of anthracene with bis[(S)-l-ethoxycarbonylethyl]fumarate. When oxidized to the ferrocinium salt, the ferrocene derivative has Lewis acid properties and catalyzes Diels — Alder reactions with some enantioselectivity [85]. 

Two papers published in 1991 described the thermal properties of the diester ferrocene derivatives 8 [15] and 9 [16] (Fig. 9-4). Complexes 8 (n = 3, 4, 6, 8) were found to be non-mesomorphic and 8 (n = 10) seemed to give rise to a smectic phase (unidentified) between 85 °C and 88 °C during the second heating cycle. When 9 n = 1) was cooled from the isotropic melt at a rate of 2 °C/min, a liquid crystalline texture was observed (transition temperatures and liquid crystalline range not given) that, according to the authors, was indicative of a smectic phase. The latter was not identified. Derivative 9 n = 10) formed a nematic phase between 78 °C and 79 °C. This mesophase did not reform on cooling from the isotropic melt. 

None of the ferrocene derivatives 13 showed liquid crystal properties on heating. They all melted into an isotropic melt. When cooled from the isotropic liquid, the first members of the series [n = 1 — 6) exhibited a monotropic nematic phase. A representative example of a nematic schlieren texture is shown in Fig. 9-10. 

The thermal characteristics exhibited by the series 11 [18] and 14 [20] proved that interesting liquid crystal properties can be obtained with symmetrically 1,T-disubstituted ferrocene derivatives. The structure of the organic unit, which is combined with the ferrocenyl moiety, must be carefully chosen. 

Ferrocene derivatives 15 exhibited remarkable liquid crystal properties (Fig. 9-13). Indeed, they all gave rise to enantiotropic mesophases. Structures with n = 1 to 11 showed nematic phases. From n = 12 a smectic C phase formed. The latter was monotropic only for 15 (n = 12). The smectic C domain increased from n = 13 to n = 16, and, inversely, the nematic range narrowed. The last member of this series (n = 18) presented one smectic C phase between 159 °C and 179 °C. A nematic to smectic C transition and a focal-conic texture of a smectic C phase are presented in Figs. 9-14 and 9-15, respectively. 

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. 

After our investigations on low molar mass ferrocene derivatives, we examined the thermal properties of isomeric polymeric structures. 

Bard and co-workers have reported on the attainment of equilibrium between the nanosized particles and an electrode in the presence of a redox mediator [25a]. The study refers to the production of a mediator (methyl viologen radical cation) that reduces water in the presence of colloidal gold and platinum metal catalyst. An electrochemical model based on the assumption that the kinetic properties are controlled by the half-cell reactions is proposed to understand the catalytic properties of the colloidal metals. The same authors have used 15 nm electrodes to detect single molecules using scanning electrochemical microscopy (SECM) [25b]. A Pt-Ir tip of nm size diameter is used along with a ferrocene derivative in a positive feedback mode of SECM. The response has been found to be stochastic and Ear-adaic currents of the order of pA are observed. 

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