Ferrocene (CAS: 102-54-5)

Ferrocene is an organometallic compound with the formula Fe(C5H5)2. It is

the prototypical metallocene, a type of organometallic chemical compound

consisting of two cyclopentadienyl rings bound on opposite sides of a central

 metal atom. Such organometallic compounds are also known as sandwich compounds

. The rapid growth of organometallic chemistry is often attributed to the excitement

arising from the discovery of ferrocene and its manyanalogues.

Structure and bonding

The carbon-carbon bond distances are 1.40 Å within the five-membered rings, and the

Fe-C bond distances are 2.04 Å. Although X-ray crystallography (in the monoclinic space

group) points to the Cp rings being in a staggered conformation, it has been shown through

gas phase electron diffraction and computational studies that in the gas phase the Cp rings

are eclipsed. The staggered conformation is believed to be most stable in the condensed

phase due to crystal packing.

The Cp rings rotate with a low barrier about the Cp(centroid)-Fe-Cp(centroid) axis, as

observed by measurements on substituted derivatives of ferrocene using 1H and 13C nuclear

magnetic resonancespectroscopy. For example methylferrocene (CH3C5H4FeC5H5) exhibits

a singlet for the C5H5 ring.

In terms of bonding, the iron center in ferrocene is usually assigned to the +2 oxidation state,

consistent with measurements using Mössbauer spectroscopy. Each cyclopentadienyl (Cp) ring

is then allocated a single negative charge, bringing the number of π-electrons on each ring to six,

and thus making them aromatic. These twelve electrons (six from each ring) are then shared with

the metal via covalent bonding. When combined with the six d-electrons on Fe2+, the complex

attains an 18-electron configuration.

Synthesis and handling properties

The first reported synthesis of ferrocene used the Grignard reagent cyclopentadienyl magnesium

bromide, which can be prepared by reacting cyclopentadiene with magnesium and bromoethane 

inanhydrous benzene. Iron(II) chloride is then suspended in anhydrous diethyl ether and added to

the Grignard reagent. The reaction sequence is:

2 C5H5MgBr + FeCl2 → Fe(C5H5)2 + MgCl2 + MgBr2

Numerous other syntheses have been reported, including the direct reaction of gas-phase

cyclopentadiene with metallic iron[19] at 350 °C or with iron pentacarbonyl.

Fe + 2 C5H6(g) → Fe(C5H5)2 + H2(g)Fe(CO)5 + 2 C5H6(g) → Fe(C5H5)2 + 5 CO(g) + H2(g)

More efficient preparative methods are generally a modification of the original transmetalation 

sequence using either commercially available sodium cyclopentadienide[21] or freshly 

crackedcyclopentadiene and potassium hydroxide with anhydrous iron(II) chloride in ethereal solvents:

2 NaC5H5 + FeCl2 → Fe(C5H5)2 + 2 NaClFeCl2.4H2O + 2 C5H6 + 2 KOH → Fe(C5H5)2 + 2 KCl + 6 H2O

Direct transmetalation can also be used to prepare ferrocene from other metallocenes, such as manganocene:

FeCl2 + Mn(C5H5)2 → MnCl2 + Fe(C5H5)2

Applications of ferrocene and its derivatives

Ferrocene and its numerous derivatives have no large-scale applications, but have many niche uses

that exploit the unusual structure (ligand scaffolds, pharmaceutical candidates), robustness

(anti-knock formulations, precursors to materials), and redox (reagents and redox standards).

Fuel additives

Ferrocene and its derivatives are antiknock agents used in the fuel for petrol engines; they are

safer than tetraethyllead, previously used. It is possible to buy at Halfords in the UK, a petrol additive

solution which contains ferrocene which can be added to unleaded petrol to enable it to be used in 

vintage cars which were designed to run on leaded petrol. The iron containing deposits formed from

ferrocene can form a conductive coating on the spark plug surfaces.

Pharmaceutical

Some ferrocenium salts exhibit anticancer activity, and an experimental drug has been reported

which is a ferrocenyl version of tamoxifen. The idea is that the tamoxifen will bind to theestrogen 

binding sites, resulting in a cytotoxicity effect.

Materials chemistry

Ferrocene, being readily decomposed to iron nanoparticles, can be used as a catalyst for the production

of carbon nanotubes.The vinyl ferrocene from ferrocene can be made by a Wittig reaction of the aldehyde,

a phosphonium salt and sodium hydroxide. The vinyl ferrocene can be converted into a polymer which can

be thought of as a ferrocenyl version of polystyrene (the phenyl groups are replaced with ferrocenyl groups).

As a ligand scaffold

Chiral ferrocenyl phosphines are employed as ligands for transition-metal catalyzed reactions. Some of them

have found industrial applications in the synthesis of pharmaceuticals and agrochemicals. For example, the 

diphosphine 1,1′-bis(diphenylphosphino)ferrocene (dppf) is a valuable ligand for palladium-coupling reactions.