Functionalization of the cyclopentadienyl

As mentioned above, it has recently been demonstrated that ferrocene systems, especially polymers, are potentially excellent NLO materials [4]. Two examples of such materials obtained by polycondensation are described here. New monomers, namely 33 and 34, have been prepared (as shown in Scheme 10-13) by selective functionalization of the cyclopentadienyl rings of ferrocene [40]. 

Parallel to these studies Marks and co-workers have studied the effects of an extensive functionalization of the cyclopentadienyl ligands. It has been observed that substitution of cyclopentadienyl ligands with in-denyl ligands inhibits the photoinduced -hydrogen elimination pro-... 

The interaction of the metal s and dzi orbitals with those of the cyclopentadienyl anion can be illustrated as follows. We begin by placing the metal orbital between the two ligands that have the orbitals oriented to match the sign of the wave function of the metal orbital. Therefore, for the s and dzi orbitals, the combinations are shown in Figure 21.16. 

A comprehensive study of 15 zirconecene monomethyl monochloro compounds and their exchange of chloride for methyl with AlMe3 indicates that the exchange process is a function of the electron deficiency at the metal.62 With low electron densities, the zirconium is more likely to exchange chloride for methyl. The study revealed low electron densities at the metal for the indenyl compounds by comparison to the cyclopentadienyl compounds. The study also revealed that steric effects are minor compared to the electronic effects of the ligands on the zirconium. 

Fig. 4. Sublimation temperatures of base-free strontocenes as a function of molecular weight and steric bulk of the cyclopentadienyl ligands.

One of the best known properties of ferrocene molecules is their ability to lose one electron at potentials that are a function of the electron-donating ability of the substituents attached to the cyclopentadienyl rings [1]. Such electron removal commonly does not involve fragmentation of the original molecular framework. As a typical example, Table 7-1 summarizes the redox potentials and the main structural changes accompanying the one-electron oxidation of some ferrocene molecules. 

Following these synthetic methods a wide number of mono-Cp -amido titanium derivatives have been prepared. An overview of examples organized according to the nature of the cyclopentadienyl-type ligand (Gp, Cp, Ind, Flu) and the bridging group (Si, C, B) connecting the Cp-type and the amido functionalities is presented. 

Surface hydroxyl groups on Ti02 and Si02 are typically less acidic than those on alumina.On silica surfaces, hydroxyl groups are associated with tetrahedrally coordinated silicon atoms. Functionalization of the surface silanols is readily achieved by reaction with chloro- or alkoxysUane reagents. Reactions such as those in equations (2) and (3) may be used to add phosphine, cyclopentadienyl, and other functionalities to the surface. 

A similar shortening of the Si-[M] bond is also obtained for the transition metal fragments which contain the cyclopentadienyl ligand (Figure 30.4). There, detailed relevant experimental data are provided in the work of Malisch et al. ° Further population analysis of the substituted silanes shows that the polarization at the silicon as a function of the distance of the silicon center attached to the transition metal is fairly small. In other words, a change is only obtained in the charge for the neighboring silicon atom. 

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