Ferrocene

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ImageFileL1 = Ferrocene-2D.png ImageSizeL1 = 80 px
ImageFileR1 = Ferrocene-3D-balls-B.png ImageSizeR1 = 120 px
IUPACName = ferrocene, bis(η⁵-cyclopentadienyl)iron
OtherNames = ferrocene, iron cyclopentadienyl
Section1 = Chembox Identifiers
CASNo = 102-54-5
PubChem = 11985121
SMILES =
Section2 = Chembox Properties
Formula = C₁₀H₁₀Fe
MolarMass = 186.04 g/mol
Appearance = light orange powder
Density = 2.69 g/cm³ (20 °C)
MeltingPt = 174 °C
BoilingPt = 249 °C
Solubility = Insoluble in water, soluble in most organic solvents
Section3 = Chembox Hazards
MainHazards =
FlashPt =
Autoignition =
Section8 = Chembox Related
OtherCpds = cobaltocene, nickelocene, chromacene, bis(benzene)chromium

Ferrocene is the 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. [cite journal|author=R. Dagani |url=http://pubs.acs.org/isubscribe/journals/cen/79/i49/html/7949sci1.html | title=Fifty Years of Ferrocene Chemistry| journal=Chemical and Engineering News |date=3 December 2001|volume= 79 |issue= 49 | pages = 37–38 |format=Subscription required] The rapid growth of organometallic chemistry is often attributed to the excitement arising from the discovery of ferrocene and its many analogues.

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 to prepare fulvalene. Instead, they obtained a light orange powder of "remarkable stability." [cite journal
author = T. J. Kealy, P. L. Pauson
title = A New Type of Organo-Iron Compound
journal = Nature
year = 1951
volume = 168
pages = 1039
doi = 10.1038/1681039b0] This stability was accorded 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. [cite journal
author = G. Wilkinson, M. Rosenblum, M. C. Whiting, R. B. Woodward
title = The Structure of Iron Bis-Cyclopentadienyl
journal = Journal of the American Chemical Society
year = 1952
volume = 74
pages = 2125–2126
doi = 10.1021/ja01128a527] Independently Ernst Otto Fischer also came to the conclusion of the sandwich structure and started to synthesize other metallocenes such as nickelocene and cobaltocene. [cite journal
author = E. O. Fischer, W. Pfab
title = Zur Kristallstruktur der Di-Cyclopentadienyl-Verbindungen des zweiwertigen Eisens, Kobalts und Nickels
journal = Z. Naturforsch. B
year = 1952
volume = 7
pages = 377–379
doi = ] Ferrocene's structure was confirmed by NMR spectroscopy and X-ray crystallography. [cite journal
author = J. Dunitz, L. Orgel, A. Rich
title = The crystal structure of ferrocene
journal = Acta Crystallographica
year = 1956
volume = 9
pages = 373–5
doi = 10.1107/S0365110X56001091] [cite journal
author = Pierre Laszlo, Roald Hoffmann,
title = Ferrocene: Ironclad History or Rashomon Tale?
journal = Angewandte Chemie International Edition
year = 2000
volume = 39
pages = 123–124
doi = 10.1002/(SICI)1521-3773(20000103)39:1<123::AID-ANIE123>3.0.CO;2-Z] 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 Ludwig-Maximilians-Universität München and Wilkinson of Imperial College London shared a Nobel Prize with for their work on metallocenes and other aspects of organometalic chemistry. [cite web |url=http://nobelprize.org/nobel_prizes/chemistry/laureates/1973/press.html |title= Press Release: The Nobel Prize in Chemistry 1973 |year= 1973 |publisher= The Royal Swedish Academy of Sciences] Ferrocene is efficiently prepared by the reaction of sodium cyclopentadienyl with anhydrous ferrous chloride in ethereal solvents::2 NaC₅H₅ + FeCl₂ → Fe(C₅H₅)₂ + 2 NaCl

Bonding and Structure

The iron atom in ferrocene is normally assigned to the +2 oxidation state, as can be shown using Mössbauer spectroscopy. Each cyclopentadienyl 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, which, when combined with the six "d"-electrons on Fe²⁺, results in the complex having an 18-electron, noble gas electron configuration. The carbon-carbon bond distances are 1.40 Å within the five membered rings, and the bond distances between the sandwiched iron and the carbons of the rings are 2.04 Å.

Physical properties

Ferrocene is an air-stable orange solid that readily sublimes, especially upon heating in a vacuum. As expected for a symmetric and uncharged species, ferrocene is soluble in normal organic solvents, such as benzene, but is insoluble in water. It is stable to temperatures as high as 400 °C. [Solomons, Graham, and Craig Fryhle. Organic Chemistry. 9th ed. USA: John Wiley & Sons, Inc., 2006.]

The following table gives typical values of vapor pressure of ferrocene at different temperatures: [Monte, M. J. S.; Santos, L. M. N. B. F.; Fulem, M.; Fonseca, J. M. S. & Sousa, C. A. D., New static apparatus and vapor pressure of reference materials: Naphthalene, benzoic acid, benzophenone, and ferrocene, J. Chem. Eng. Data, 2006, 51, 757-766]

Chemical properties

Reaction with electrophiles

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. The preparation of phosphorus derivatives of ferrocenes are illustrative. In the presence of aluminium chloride Me₂NPCl₂ and ferrocene react to give ferrocenyl dichlorophosphine, [cite journal
title = Ferrocene derivatives. 27. Ferrocenyldimethylphosphine
author = G.R. Knox, P.L. Pauson and D. Willison
journal = Organometallics
volume = 11
issue = 8
pages = 2930 &ndash; 2933
year = 1992
doi = 10.1021/om00044a038] while treatment with phenyldichlorophosphine under similar conditions forms "P,P"-diferrocenyl-"P"-phenyl phosphine. [cite journal
author = G.P. Sollott, H.E. Mertwoy, S. Portnoy and J.L. Snead
title = Unsymmetrical Tertiary Phosphines of Ferrocene by Friedel-Crafts Reactions. I. Ferrocenylphenylphosphines
journal = J. Org. Chem.
year = 1963
volume = 28
pages = 1090 &ndash; 1092
doi = 10.1021/jo01039a055] In common with anisole the reaction of ferrocene with P₄S₁₀ forms a dithiadiphosphetane disulfide. [cite journal
title = 2,4-Diferrocenyl-1,3-dithiadiphosphetane 2,4-disulfide; structure and reactions with catechols and [PtCl₂(PR₃)₂] (R = Et or Bun)
author = Mark R. St. J. Foreman, Alexandra M. Z. Slawin and J. Derek Woollins
journal = J. Chem. Soc., Dalton Trans.,
year = 1996
pages = 3653 &ndash; 3657
doi = 10.1039/DT9960003653]

Lithiation

Ferrocene reacts readily with 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 linked by the selenium atom. [Ron Rulkens, Derek P. Gates, David Balaishis, John K. Pudelski, Douglas F. McIntosh, Alan J. Lough, and Ian Manners, "J. Am. Chem. Soc.", 1997, 119, 10976] 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. [Paloma Gómez-Elipe, Rui Resendes, Peter M. Macdonald, and Ian Manners, "J. Am. Chem. Soc.," 1998, 120, 8348] [Timothy J. Peckham, Jason A. Massey, Charles H. Honeyman, and Ian Manners, "Macromolecules", 1999, "32", 2830]

Redox chemistry

Unlike the majority of hydrocarbons, ferrocene undergoes a one-electron oxidation at a low potential, around 0.5 V "vs". a saturated calomel electrode (SCE). Some electron rich hydrocarbons (e.g., aniline) also are oxidized at low potentials, but only irreversibly. Oxidation of ferrocene gives a stable cation called ferrocenium. 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. [cite journal|author=N. G. Connelly, W. E. Geiger| title=Chemical Redox Agents for Organometallic Chemistry|journal=Chemical Reviews|date= 1996| volume= 96| pages= 877–910| doi=10.1021/cr940053x] Substituents on the cyclopentadienyl ligands alters the redox potential in the expected way: electron withdrawing group 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 easy to oxidise than ferrocene itself. Ferrocene is often used as an internal standard for calibrating redox potentials in non-aqeous electrochemistry.

tereochemistry

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- and 1,1' isomers, which are not interconvertable. 1,2-Heterodisubstituted ferrocenes are chiral.

Applications of ferrocene and its derivatives

Ferrocene and its numerous derivatives have no large-scale applications, but have many niche uses that exploit the unsual 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 tetraethyl lead, previously used. [ [http://www.osd.org.tr/14.pdf Application of fuel additives] ] 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. [US patent|4104036] The iron containing deposits formed from ferrocene can form a conductive coating on the spark plug surfaces.

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

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 the estrogen binding sites, resulting in a cytotoxicity effect. [cite journal|journal=Chemical and Engineering News | date=16 Sep 2002| title= The Bio Side of Organometallics | author= Ron Dagani| volume = 80| issue= 37| pages = 23–29| url=http://pubs.acs.org/cen/science/8037/8037sci1.html] [cite journal|author=S. Top, B. Dauer, J. Vaissermann and G. Jaouen| journal=Journal of Organometallic Chemistry| title= Facile route to ferrocifen, 1- [4-(2-dimethylaminoethoxy)] -1-(phenyl-2-ferrocenyl-but-1-ene), first organometallic analogue of tamoxifen, by the McMurry reaction | doi=10.1016/S0022-328X(97)00086-7 |date=1997| volume=541| pages= 355–361] cite journal|author=S. Top, A. Vessières, G. Leclercq, J. Quivy, J. Tang, J. Vaissermann, M. Huché and G. Jaouen| title=Synthesis, Biochemical Properties and Molecular Modelling Studies of Organometallic Specific Estrogen Receptor Modulators (SERMs), the Ferrocifens and Hydroxyferrocifens: Evidence for an Antiproliferative Effect of Hydroxyferrocifens on both Hormone-Dependent and Hormone-Independent Breast Cancer Cell Lines| journal=Chemistry, a European Journal| date=2003| volume=9| pages=5223–36 | pmid=14613131 | doi=10.1002/chem.200305024]

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. [Liu, Wan-yi; Xu, Qi-hai; Ma, Yong-xiang; Liang, Yong-min; Dong, Ning-li; Guan, De-peng,"J. Organomet. Chem.", 2001, 625, 128 - 132] 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).

Ferrocene is also used as a nano-sized "loom" in the manufacture of ultra-high molecular weight polyethylene's very long fibers, which are used to manufacture newer types of bulletproof vest fabric.Fact|date=March 2008

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

Many other hydrocarbons can be used instead of cyclopentadienyl. For example, indenyl can be used in place of the cyclopentadienyl to form bisbenzoferrocene. [B.R. Waldbaum and R.C. Kerber, "Inorg. Chim. Acta", 1999, 291, 109 - 126.] .

The carbon atoms can be replaced by heteroatoms as illustrated by Fe(η⁵-C₅Me₅)(η⁵-P₅ and Fe(η⁵-C₅H₅)(η⁵-C₄H₄N) ("azaferrocene"). The latter arises from decarbonylation of Fe(η⁵-C₅H₅)(CO)₂(η¹-pyrrole) in cyclohexane.(a [J. Zakrzewski and C. Gianotti, "J. Organomet. Chem.", 1990, 388,175 - 180.] . This compound on boiling under reflux in benzene is converted to ferrocene. [A. Efraty, N. Jubran and A. Goldman, "Inorg. Chem.", 1982, 21, 868 - 873.]

Because of the ease of substitution, many structurally unusual ferrocene derivatives have been prepared. For example, the penta(ferrocenyl)cyclopentadienyl ligand [cite journal|author=Y. Yu, A.D. Bond, P. W. Leonard, K. P. C. Vollhardt, G. D. Whitener| title=Syntheses, Structures, and Reactivity of Radial Oligocyclopentadienyl Metal Complexes: Penta(ferrocenyl)cyclopentadienyl and Congeners| journal= Angewandte Chemie International Edition| volume =45 | issue=11| pages= 1794–1799|date=2006| doi=10.1002/anie.200504047] , features a cyclopentadiene derivatised with five ferrocene substituents.

In hexaferrocenylbenzene, all six positions on a benzene molecule have ferrocenyl substituents (R) [cite journal|title=Hexaferrocenylbenzene|author= Yong Yu, Andrew D. Bond, Philip W. Leonard, Ulrich J. Lorenz, Tatiana V. Timofeeva, K. Peter C. Vollhardt, Glenn D. Whitener and Andrey A. Yakovenko| journal=Chemical Communications| date=2006| pages= 2572–2574 |doi=10.1039/b604844g] . 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. [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.]

References

Further reading

;Announcement of the discovery of ferrocene, but with wrong structure
*cite journal|author=Kealy, T. J., Pauson, P. L.|title=A New Type of Organo-iron Compound|journal= Nature| date= 1951 | volume= 168| pages =1039–40 | doi = 10.1038/1681039b0
*;Announcement of the correct 'sandwich' structure
*cite journal|author=Wilkinson, G., Rosenblum, M., Whiting, M. C., Woodward, R. B.|title= The Structure of Iron Bis-Cyclopentadienyl| journal=Journal of the American Chemical Society|date= 1952| volume=74|pages=2125–2126 | doi = 10.1021/ja01128a527
*cite journal |author=Fischer, E. O., Pfab, W.|title=Cyclopentadien-Metallkomplexe, ein neuer typ metallorganischer Verbindungen |journal=Zeitschrift für Naturforschung B|date=1952| volume=7 |pages=377–379;Others
*cite journal|author=Dunitz, J. D., Orgel, L. E.|title=Bis-Cyclopentadienyl - A Molecular Sandwich|journal= Nature |date=1953| volume=171 |pages= 121–122 | doi = 10.1038/171121a0
*cite journal|author=Pauson, P. L.|title=Ferrocene-how it all began|journal=Journal of Organometallic Chemistry|date=2001|pages=637–639 | volume = 637-639 | doi = 10.1016/S0022-328X(01)01126-3
*cite book|author= Gerard Jaouen (ed.)| title=Bioorganometallics: Biomolecules, Labeling, Medicine| publisher=Wiley-VCH| location= Weinheim| year= 2006 | id=ISBN 978-3-527-30990-0 (discussion of biological role of ferrocene and related compounds)