Functionalization of the cyclopentadienyl rings

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]. 

Figures 5-41 and 5-42 compare CpTiCp (centroid) bond angles in titanium cyclopentadienyl dichloride complexes from PM3 and BP/ 6-31G calculations, respectively, with experimental values for these and other compounds dealt with in this section from X-ray crystallography. Due to practical limitations, the data used for comparison with the density functional calculations are a subset of that used in comparison with PM3. Both models perform well in separating those systems where the cyclopentadienyl rings are spread far apart from those where they are closer together.

Metallocene derivatives may be named by either standard organic suffix (functional)7 or prefix nomenclature. Substituents are given the lowest numerical locants in the usual manner on the equivalent cyclopentadienyl rings of the ocene entity. The first ring is numbered 1 to 5 and the second ring is numbered V to 5. In metallocenes composed of multiple ocene groupings the cyclopentadienyl rings are further numbered 1" to 5", V" to 5", etc. The radical names -ocenyl, -ocenediyl, -ocenetriyl, etc. are used. For examples see Table 25. 

We recently published synthetic methodology to selectively functionalize the cyclopentadienyl rings of ferrocene (4) based on the Seyferth transmetal ation reaction (5). Starting from complex 1 (4) monomer 4 was prepared in 65% overall yield. Ferrocene 4 is isolated as long purple needles (mp 81-2 °C) after crystallization from chloroform/pentane [Amax in CH2C12, 515 nm (e = 2.24 x 103) and 322 nm (e = 1.37 x 104)]. 

Furthermore, we could show that this reaction mechanism to produce Cp-complexes of "mTc is not limited to carboxylate derivatives of Cp but that also further functionalities can be conjugated to receive, e.g., amides (49 and 50) [125]. This reactivity implies that, comparable to the SAAC approach, Thiele s acid derivatives can be incorporated in, e.g., a peptide strand and directly be labeled with [99mTc(OH2)3(CO)3]+. The preparation of cyclopentadienyl complexes with a variety of functionalities attached to the cyclopentadienyl ring stands for a more routine introduction of this important tridentate ligand in radiopharmaceutical organometallic complexes. 

The cyclopentadienyl ring in the even less sterically bulky tris(cyclopentadienyl) lanthanide complexes, (C5H5)3Ln and (CH3CsH4)3Ln, has been found to be able to functionalize by a direct nucleophilic addition reaction under suitable reaction conditions. Reaction of (C5H5)3Ln... 

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. 

A number of approximate molecular orbital calculations on ferrocene were reviewed by Cotton and Wilkinson in 1959 and the result of an ab-initio calculation was published in 1972 The latter work also contains references to approximate molecular orbital calculations published between 1959 and 1972. It seems that the particular stability of ferrocene is due to its closed-shell structure all occupied molecular orbitals are bonding between the metal and the ligands, the cyclopentadienyl rings function as five-electron ligands, and the iron atom attains an inert gas configuration of 18 electrons in the valence shell. 

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