Ferrocene

Ferrocene is the chemical compound with the formula Fe(C5H5)2. Ferrocene 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.

History
Ferrocene, like many chemical compounds, was first prepared unintentionally. In 1951, Pauson and Kealy at Duquesne University reported the reaction of cyclopentadienyl magnesium bromide and ferric chloride with the goal of oxidatively coupling the diene. Instead, they obtained a light orange powder of "remarkable stability." This stability was accounted to the aromatic character of the negative charged cyclopentadienyls, but the sandwich structure of the η5 (pentahapto) compound was not recognized by them.

Robert Burns Woodward and Geoffrey Wilkinson deduced the structure based on its reactivity. Independently Ernst Otto Fischer also came to the conclusion of the sandwich structure and started to synthesize other metallocenes such as nickelocene and cobtaltocene. Ferrocene's structure was confirmed by NMR spectroscopy and X-ray crystallography. Its distinctive "sandwich" structure led to an explosion of interest in compounds of d-block metals with hydrocarbons, and initiated the development of the now flourishing study of organometallic chemistry.

Ernst Otto Fischer of the Ludwig-Maximilians-Universität München and Sir Geoffrey Wilkinson of Imperial College London shared a Nobel Prize with for their work on organometalic chemistry. Ferrocene is more efficiently prepared by the reaction of sodium cyclopentadienyl with anhydrous ferrous chloride in ethereal solvents.

Electronic structure
The central iron atom in ferrocene is normally considered to be in the +2 oxidation state (this can be shown using Mossbauer spectroscopy). Each cyclopentadienyl ring is then allocated a single negative charge - this extra electron occupies a π orbital, 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, which, when combined with the six d-electrons on Fe2+, results in the complex having an 18-electron, inert gas electron configuration. This configuration makes ferrocene particularly stable.

Physical properties
Ferrocene is an air stable orange solid that readily sublimes in vacuum, especially upon heating. As expected for a symmetric and uncharged species, ferrocene is soluble in normal organic solvents, such as benzene but is insoluble in water.

Ferrocene sublimates notably around 100 Celsius degree. The following table gives some typical value of vapor pressure of ferrocene at different temperatures :

Reaction with electrophiles
Ferrocene undergoes many reactions characteristic of aromatic compounds, enabling the preparation of derivatives (substituted ferrocenes). The most common substitution patterns are 1-substituted (one substituent on one ring), 1,1'-disubstituted (one substituent on each ring), and 1,2-disubstituted (two substituents next to each other on the same ring). For example the reaction of ferrocene, aluminium chloride and Me2NPCl2 in hot heptane forms dichloroferrocenyl phosphine, while treatment with phenyldichlorophosphine under similar conditions forms P,P-diferrocenyl-P-phenyl phosphine. In common with anisole the reaction of ferrocene with P4S10 forms a dithiadiphosphetane disulfide.

A common undergraduate experiment is the Friedel-Crafts reaction of ferrocene with acetic anhydride (or acetyl chloride) in the presence of phosphoric acid as a catalyst.



Lithiation
Ferrocene is readily deprotonated (e.g. by butyl lithium) to give 1,1'-dilithioferrocene, which in turn is a versatile nucleophile. It has been reported that the reaction of 1,1'-dilithioferrocene with selenium diethyldithiocarbamate forms a strained ferrocenophane where the two cyclopentadienyl ligands are lined by the selenium atom. This ferrocenophane can be converted to a polymer by a thermal ring-opening polymerization (ROP) to form poly(ferrocenyl selenide). Likewise by the reaction of silicon and phosphorus linked ferrocenophanes the poly(ferrocenylsilane)s and poly(ferrocenylphosphines)s can be obtained.



Redox chemistry
Unlike the majority of aromatic hydrocarbons, ferrocene undergoes a one-electron oxidation at a low potential, around 0.5 V vs. a saturated calomel electrode (SCE) (N.B. electron rich aromatic amines such as aniline, the heterocycles pyrrole and thiophene can be oxidized with ease by electrochemical means). The oxidation of ferrocene is a reaction which forms a stable cation which is not prone to decomposition while the oxidation products of amines (such as aniline) and thiophenes tend to form polymers such as polyaniline and polythiophene. By adding groups to the cyclopentadienyl ligands the redox potential of the resulting ferrocene can be altered; by the addition of an electron withdrawing group such as a carboxylic acid the potential can be shifted in the anodic direction (i.e. made more positive), while the addition of electron releasing groups such as methyl groups will shift the potential in the cathodic direction (more negative). Thus, decamethylferrocene is much more easy to oxidise than ferrocene itself. Ferrocene is often used as an internal standard for calibrating redox potentials in non-aqeous electrochemistry.

Ferrocene can be oxidized using FeCl3 to give the blue-colored ferrocenium ion, [Fe(C5H5)2]+, which is often isolated as its [PF6]− salt. Ferrocenium salts are sometimes used as oxidizing agents, in part because the redox product ferrocene is so inert and readily separated from the products.

Applications of ferrocene and its derivatives
Ferrocene itself has few applications. However, known synthetic methods allow the preparation of countless derivatives (above), thus extending the range of applications.

Fuel additives
Ferrocene and its derivatives are antiknock agents used in the fuel for petrol engines; they are considered to be safer than tetraethyl lead, 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. Unfortunately, the iron containing deposits formed from ferrocene can form a conductive coating on the spark plug surfaces leading to spark plug failure.

In diesel-fuelled engines, ferrocene reduces the production of soot.

Medical
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 the estrogen binding sites, resulting in a cytotoxicity effect.

Materials chemistry
Ferrocene, being readily sublimed, can be used to deposit certain kinds of fullerenes, especially carbon nanotubes. Due to the fact that many organic reactions can be used to modify ferrocenes, it is the case that vinyl ferrocene can be made. The vinyl 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.

1,1'-Bis(diphenylphosphino)ferrocene (dppf) is a diphosphine containing a ferrocene moiety; it is a valuable ligand for palladium coupling reactions.

Derivatives and variations
Many other metals can be used in place of iron and many other hydrocarbons can be used instead of cyclopentadiene to form altered Cp ligands which are then attached to iron. For instance indene can be used in place of the cyclopentadiene to form bisbenzoferrocene. .



In addition it is possible by heating Fe(η5-C5H5)(CO)2(η1-pyrrole) in cyclohexane to form the pyridine version (azaferrocene) of ferrocene Fe(η5-C5H5)(η5-C4H4N). . This compound on boiling under reflux in benzene is converted to ferrocene.



Because of the ease of substitution, many structurally unusual ferrocene derivatives have been prepared. For example, the penta(ferrocenyl)cyclopentadienyl ligand, features a cyclopentadiene derivatised with five ferrocene substituents.



In hexaferrocenylbenzene, all six positions on a benzene molecule have ferrocenyl substituents (R). X-ray diffraction analysis of this compound confirms that the cyclopentadienyl ligands are not co-planar with the benzene core but have alternating dihedral angles of +30° and −80°. Due to steric crowding the ferrocenyls are slightly bent with angles of 177° and have elongated C-Fe bonds. The quaternary cyclopentadienyl carbon atoms are also pyramidalized.