Metallocenes cobaltocenes

Because of their reversible electrochemical properties, ferrocene [biscyclopentadie-nyl-iron(II), FeCp2 and cobaltocenium [biscyclopentadienyl-cobalt(III), CoC p2 1 I are the most common electroactive units used to functionalize dendrimers. Both metallocene residues are stable, 18-electron systems, which differ on the charge of their most accessible oxidation states zero for ferrocene and + 1 for cobaltocenium. Ferrocene undergoes electrochemically reversible one-electron oxidation to the positively charged ferrocenium form, whereas cobaltocenium exhibits electrochemically reversible one-electron reduction to produce the neutral cobaltocene. Both electrochemical processes take place at accessible potentials in ferrocene- and cobaltocenium-containing compounds. 

Other derivatives such as CpCo(indenyl) and Co(indenyl)2 have been prepared. These compounds exhibit properties similar to that of cobaltocene but are of lower thermal stability. There have been numerous studies of linked metallocenes with a focus on the nature of metal-metal interactions. The bis(fulvalene)dicobalt complex was found to be diamagnetic, either because of direct metal-metal coupling or by electron coupling through the fluked five-membered ring (Scheme 30). 

Reductive intercalation compounds of /3-ZrNCl with cobaltocenes have been reported where the metallocene molecules are believed to be oriented with the molecular axes parallel to the layers. The metallocene intercalation compounds also show superconductivity at 14K. 

Reactions of Cyclopentadienylthallium with Transition Metal Compounds. Numerous Metallocene Complexes have been prepared using CpTl as a Cp-transfer reagent, including Ferrocene, Cobaltocene, and Nickelocene, as well... 

Metallocenes are useful electron donors as judged by their low (vertical) ionization potentials in the gas phase and oxidation potentials in solution (see Table 2). In fact, the electron-rich (19 e ) cobaltocene with an oxidation potential of E°ox = -0.9 V relative to the SCE [45] is commonly employed as a very powerful reducing agent in solution. Unlike the alkylmetals (vide supra), the HOMOs of metallocenes reside at the metal center [46] which accounts for two effects (i) Removal of an electron from the HOMO requires minimal reorganization energy which explains the facile oxidative conversion from metallocene to metallocenium. (ii) The metal-carbon bonding orbitals are little affected by the redox process, and thus the resulting metallocenium ions are very stable and can be isolated as salts. 

Other metallocenes have similar structures but do not necessarily obey the rule. For example, cobaltocene and nickelocene are structurally similar 19- and 20-electron species. 

Also in 1952, Woodward et al. 291) reported that ferrocene would undergo a number of substitution reactions typical of aromatic compounds and that the rings had overall electrical neutrality—a fact that would have to be considered in drawing up any bonding picture. Finally, they proposed the name ferrocene itself. This has now become the commonly used term for (CgHgjaFe and has spawned other names such as metallocene and cobaltocene. 

Some catalyst could be recovered after completion of the reaction but the nature of catalyst deactivation requires further study [79]. The cobaltocene chemistry reported above (Sect. 4.1) can also be made catalytic with respect to the metallocene by using Al or Hg as the terminal reductant [74]. 

The first metallocene radical isolated was cobaltocene—the importance of which is highlighted by its inclusion in an early preparative organometallic text.12 This 19 e complex remains a reagent of choice for electron transfer reactions in nonaqueous solvents.13 A key question in delocalized organometallic radical complexes is where is the unpaired electron. An early example of this is RhCp2 (Cp = T 5-C5H5), which... 

Besides the substances mentioned so far, functionalized fuUerenes like the simple Bingel adduct can be intercalated into nanotubes as well (Section 2.5.5.2). The formation of peapods has further been described for metallocenes (e.g., ferrocene), porphyrines (e.g., erbium phthalocyanine complex) and small fragments of nanotubes. The most important prerequisite for the feasibility of inclusion is always a suitable proportion of sizes of both the tube and the structure to be embedded. For example, this effect can be observed for the intercalation of different cobaltocene derivatives into SWNT. The endohedral functionalization only takes place at an internal diameter of 0.92nm or above (Figure 3.100). But there is also an upper limit to successful incorporation. When the diameter of the nanotube is too large, the embedded species can easily diffuse away again from the host. Only few molecules are consequently found inside such a wide tube. 

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