|Jmol-3D images||Image 1|
|Molar mass||186.04 g/mol|
|Appearance||light orange powder|
|Density||1.107 g/cm3 (0°C), 1.490 g/cm3 (20 °C)|
|Melting point||172.5 °C|
|Boiling point||249 °C|
|Solubility in water||Insoluble in water, soluble in most organic solvents|
|Related compounds||cobaltocene, nickelocene, chromocene, ruthenocene, osmocene, plumbocene|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
|(what is: / ?)|
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 many analogues.
- 1 History
- 2 Structure and bonding
- 3 Synthesis and handling properties
- 4 Reactions
- 5 Stereochemistry
- 6 Applications of ferrocene and its derivatives
- 7 Derivatives and variations
- 8 See also
- 9 References
- 10 External links
Ferrocene 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 to prepare fulvalene. Instead, they obtained a light orange powder of "remarkable stability.". A second group at British Oxygen also unknowingly discovered ferrocene. Miller, Tebboth and Tremaine were trying to synthesise amines from hydrocarbons such as cyclopentadiene and ammonia in a modification of the Haber process. They published this result in 1952 although the actual work was done three years earlier.  The stability of the new organoiron compound was accorded to the aromatic character of the negatively charged cyclopentadienyls, but they were not the ones to recognize the η5 (pentahapto) sandwich structure.
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 cobaltocene.
The structure of ferrocene 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 invigorated the development of the flourishing study of organometallic chemistry. In 1973 Fischer of the Technische Universität München and Wilkinson of Imperial College London shared a Nobel Prize for their work on metallocenes and other aspects of organometallic chemistry.
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 resonance spectroscopy. 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 in anhydrous 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
- 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 or freshly cracked cyclopentadiene and potassium hydroxide with anhydrous iron(II) chloride in ethereal solvents:
- 2 NaC5H5 + FeCl2 → Fe(C5H5)2 + 2 NaCl
- FeCl2.4H2O + 2 C5H6 + 2 KOH → Fe(C5H5)2 + 2 KCl + 6 H2O
- FeCl2 + Mn(C5H5)2 → MnCl2 + Fe(C5H5)2
As expected for a symmetric and uncharged species, ferrocene is soluble in normal organic solvents, such as benzene, but is insoluble in water. Ferrocene is an air-stable orange solid that readily sublimes, especially upon heating in a vacuum. It is stable to temperatures as high as 400 °C. The following table gives typical values of vapor pressure of ferrocene at different temperatures:
Ferrocene undergoes many reactions characteristic of aromatic compounds, enabling the preparation of substituted derivatives. 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.
Ferrocene reacts readily with butyl lithium to give 1,1'-dilithioferrocene, which in turn is a versatile nucleophile. But reaction of Ferrocene with t-BuLi produces monolithioferrocene only. These approaches are especially useful methods to introduce main group functionality, e.g. using S8, chlorophosphines, chlorosilanes. The strained compounds undergo ring-opening polymerization.
Many phosphine derivatives of ferrocenes are known and some are used in commercialized processes. Simplest and best known is 1,1'-bis(diphenylphosphino)ferrocene (dppf) prepared from dilithioferrocene. For example, in the presence of aluminium chloride Me2NPCl2 and ferrocene react to give ferrocenyl dichlorophosphine, whereas 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 diferrocenyl-dithiadiphosphetane disulfide.
Unlike the majority of organic compounds, ferrocene undergoes a one-electron oxidation at a low potential, around 0.5 V vs. a saturated calomel electrode (SCE). It has also been used as standard in electrochemistry as Fc+/Fc = 0.64 V vs. SHE. Some electron-rich organic compounds (e.g., aniline) also are oxidized at low potentials, but only irreversibly. Oxidation of ferrocene gives a stable cation called ferricinium. On a preparative scale, the oxidation is conveniently effected with FeCl3 to give the blue-colored ion, [Fe(C5H5)2]+, which is often isolated as its PF6− salt. Alternatively, silver nitrate may be used as the oxidizer.
Ferrocenium salts are sometimes used as oxidizing agents, in part because the product ferrocene is fairly inert and readily separated from ionic products. Substituents on the cyclopentadienyl ligands alters the redox potential in the expected way: electron withdrawing groups such as a carboxylic acid shift the potential in the anodic direction (i.e. made more positive), whereas electron releasing groups such as methyl groups shift the potential in the cathodic direction (more negative). Thus, decamethylferrocene is much more easily oxidised than ferrocene. Ferrocene is often used as an internal standard for calibrating redox potentials in non-aqueous electrochemistry.
A variety of substitution patterns are possible with ferrocene including substition at one or both of the rings. The most common substitution patterns are 1-substituted (one substituent on one ring) and 1,1'-disubstituted (one substituent on each ring). Usually the rings rotate freely, which simplifies the isomerism. Disubstituted ferrocenes can exist as either 1,2-, 1.3- or 1,1'- isomers, none of which are interconvertible. Ferrocenes that are asymmetrically disubstituted on one ring are chiral – for example [CpFe(EtC5H3Me)] is chiral but [CpFe(C5H3Me2)] is achiral. This planar chirality arises despite no single atom being a stereogenic centre. The substituted ferrocene shown at right (a 4-(dimethylamino)pyridine derivative) has been shown to be effective when used for the kinetic resolution of racemic secondary alcohols.
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).
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.
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.
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.
Derivatives and variations
Carbon atoms can be replaced by heteroatoms as illustrated by Fe(η5-C5Me5)(η5-P5) and Fe(η5-C5H5)(η5-C4H4N) ("azaferrocene"). Azaferrocene arises from decarbonylation of Fe(η5-C5H5)(CO)2(η1-pyrrole) in cyclohexane. 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 cyclopentadienyl anion derivatised with five ferrocene substituents.
In hexaferrocenylbenzene, C6[(η5-C5H4)Fe(η5-C5H5)]6, 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.
The synthesis of hexaferrocenylbenzene has been reported using Negishi coupling of hexaiodidobenzene and diferrocenylzinc, using tris(dibenzylideneacetone)dipalladium(0) as catalyst, in tetrahydrofuran:
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- Also, the benzene core has a chair conformation with dihedral angles of 14° and displays bond length alternation between 142.7 pm and 141.1 pm, both indications of steric crowding of the substituents.
- Ferrocene at The Periodic Table of Videos (University of Nottingham)
- NIOSH Pocket Guide to Chemical Hazards (Centers for Disease Control and Prevention)
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