Ferrocene aromatic character

Due to the aromatic character of Cp2Ee predicted by Woodward and confirmed by the reactivity toward electrophilic substitutions, which proceed with rates comparable to anisole, the name ferrocene was coined in analogy to simple aromatic systems [6]. 

Its aromaticity cannot, of course, be tested by attempted electrophilic substitution, for attack by X would merely lead to direct combination with the anion. True aromatic character (e.g. a Friedel-Crafts reaction) is, however, demonstrable in the remarkable series of extremely stable, neutral compounds obtainable from (15), and called metallocenes, e.g. ferrocene (16), in which the metal is held by n bonds in a kind of molecular sandwich between the two cyclopentadienyl structures ... 

Although the coordinated cyclopentodienyl group resists nucleophilic attack, it does react with electrophiles. Ferrocene resembles free benzene in that it reacts with many electrophilic reagents, but it does so at an even faster rate than benzene. The aromatic character of ferrocene was recognized soon after the complex was identified and has led to a rich literature. Among the numerous reactions that have been studied is acylation in the presence of a Friedel-Crafts catalyst. 

Although dibenzenechromium is thermally quite stable, it is less so than ferrocene and melts with decomposition at 285° to give benzene and metallic chromium. Furthermore, it appears to lack the aromatic character of either benzene or ferrocene as judged by the fact that it is destroyed by reagents used for electrophilic substitution reactions. 

The aromatic character of ferrocene originates in the anionic cydopen-tadienide system (18), which has six 7t-electrons delocalised over a symmetrical cyclic five-carbon system. Coordination of two such ring systems with a metal ion (iron in the case of ferrocene) in a sandwich-like structure (20) gives rise to a remarkably stable compound. 

Ferrocene is diamagnetic in nature. Ferrocene has p-delocalised M.Os and shows aromatic character. (If we consider the formation of ferrocene from Fe2+ and two C5H5- anions containing six -electrons which obeys the An- 2 rule of aromaticity and not the radical.)... 

Figure 7.25 Aromatic character of ferrocene expressed in electrophilic ring functionalization.

Cince the discovery of ferrocene 26) and its aromatic character 67) considerable interest has been attached to the possible function of the metal atom in the reactions of this substance and of its congeners, ruthenocene and osmocene. 

Theoretical studies by E. Ruch (173-177) proved the correctness of this concept of the bonding, with which the retention of the aromatic character of the ring is in harmony. The nature of the bonding in the dicyclopentadienyl metal compounds was discussed by him in terms of molecular and equivalent orbitals, and on the basis of calculated overlap integrals for ferrocene an energy-level diagram was suggested which... 

The chemistry of ferrocene dominates that of the other metallocenes it is commercially available and large numbers of derivatives are known. The rings in (ri -Cp)2Fe possess aromatic character, and selected reactions are shown in Figure 23.22. That protonation occurs at the Fe(II) centre is indicated by the appearance of a signal at S... 

Cyclopentadiene is an acidic hydrocarbon. In 1928 English chemist Christopher Ingold suggested that this was because the cyclopentadienyl anion had an aromatic sextet of electrons. This was the first case of aromatic character being attributed to an ion. An interesting derivative made from this very stable carbanion vizs ferrocene (discovered in 1951). 

The C—C bonds in ferrocene possess similar properties as C—C bonds in benzene. Hence, the reactions shown by benzene, due to aromatic character, are also exhibited by ferrocene. Ferrocene is represented in the following reactions as Fe(77 -C5H5)2/Fe(Cp)2-... 

Ruthenocene and osmocene, however, crystallize in the eclipsed form. The C5H5 rings in w-cyclopentadienyl complexes are planar and all the C—C bond distances are equal. As will be seen later (p 219) they possess considerable aromatic character, and in favourable cases, e.g. ferrocene, undergo electrophilic substitution reactions such as acetylation. 

Many discoveries in synthesis have their own anecdotal histories, one of which—the first synthesis of an organometallic metallocene, ferrocene— is mentioned here. In 1951, the newly appointed Assistant Professor Peter L. Panson, working with an nndergraduate stndent, Thomas J. Kealy, attempted to oxidatively couple cyclopentadienyl magnesium bromide with ferric chloride (52,53). The goal was to prepare fulvalene to test the then recent proposal (54) that it would display aromatic character. Instead, a very stable orange solid was produced, which proved to be the famous ferrocene, about whose structure determination much has been written (Figure 6, (55,56,57)). 

Ferrocene or (r -CjHs)2 Fe is one of the more stable organometallic compounds. It is a representative of the sandwich compounds where a metal is located between two organic ligands below and above the metal ion. The two rings of ferrocene retain most of their aromatic character and hence the extensive range of reactions of ferrocene which resemble organic reactions. 

The only other reaction with an aromatic substance is the C-H insertion into ferrocene [85], giving 41,which illustrates the highly electrophilic character of the phosphinidene complex. Other aromatic C-H insertions have been observed, but these likely occur by means of intermediate P,0- and P,N-ylids,such as the reaction of (0C)5W=PR withbenzophenone and azobenzene that give 42 and 43,respectively [56a, 86]. 

Clearly, ferrocene is an interesting substituent, due to its vast potential for use in biology. It can act by means of specific particularities, whether it be its compact, aromatic, lipophilic character or its ability, in favourable cases, to act as a stable redox agent in biological media. These properties are beginning to be well identified and should find new fields of application in biology in the future. 

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