Amazing Molecules: Ferrocene

Ferrocene, C10H10Fe, also known by its full name of dicyclopentadienyl iron is a diamagnetic, orange solid that was produced unintentionally in 1951 by Thomas Kealy and Peter Pauson. The discovery of ferrocene made the concept of hydrocarbon ringsπ-bonding to a metal atom known, which was described as “a breakthrough and a major departure from the classical model of ligand coordination that had prevailed until that time”.[1]

The first known synthesis of ferrocene had been made purely by accident, by Kealy and Pauson, in the quest to make a fulvalene (Figure 1).[2] They had begun by coupling two molecules of magnesium cyclopenta-1,3-diene bromide, a cyclopentadienide anion ( [C5H5]─ ) [3], in the presence of iron chloride (FeCl3) in order to make an intermediate in the formation of pentafulvalene. However, what they had produced was an unusually stable compound containing iron. Though, the two scientists had not correctly determined the structure of the compound.

In the same year, an industrial laboratory reported making ferrocene using a completely different method – by passing cyclopentadiene over a liquid ammonia catalyst with iron(II) oxide at high temperatures.3

Even though ferrocene had been synthesised in two dissimilar methods, the true structure of ferrocene had still not been correctly identified. The following year two scientists succeeded, Robert Burns Woodwardand Geoffrey Wilkinson. The pair were analysing the molecule's reactivity along with its Infra-red (IR) and Nuclear Magnetic Resonance (NMR) spectra. The pair stated that “the equal unsaturation of each of the carbon atoms of the cyclopentadienyl anion suggested that two such units might form covalent bonds to ferrous iron symmetrically”.[4] The structure proposed stated that ferrocene was in fact a tridentate ligand with six (three paired) π electrons from each organic ring being donated to the central metal ion . Ferrocene was the first of many molecules discovered with a central metal ion, which have since been named metallocenes.

The construction of the aromatic structure of ferrocene was aided by Whiting and Rosenblum who noticed the structure undergoes Friedel-Crafts acylation , and so it was these who first named the compound ‘ferrocene', named with the -ene suffix suggesting its aromaticity.4

The ferrocene molecule is considered to be an iron(II) ion ‘sandwiched' between the two cyclopentadienyl rings. Ferrocene was the first of many molecules containing a central metal ion between two π-bonded hydrocarbon ligands, now known as sandwich complexes.[6] These complexes are often difficult to deduce using chemical reactions and methods as ferrocenes are soluble in organic solvents but not water and they are unaffected by oxygen, water, aqueous bases and non-oxidising agents in ambient conditions. So, it was not until 1954 when Ernst Otto Fischer's X-ray crystallography[7] confirmed the predictions made by Woodward and Wilkinson and their structure became accepted as standard structure for ferrocene. Since this approval ferrocene chemical formula began to be considered as Cp2Fe rather than C10H10Fe.

Woodward and Wilkinson's structure of ferrocene was also accepted as it obeys the 18-electron rule, in which low oxidation state organometallic complexes tend to fill the transition metal's nine valence orbitals.7 The molecule is considered as an Fe(II) compound containing two [Cp]─ ligands. However, for electron counting, and to fulfil the 18-electron rule, it considered as a combination of two neutral Cp● ligands (donating 5 electrons each) and an Fe(0) centre (with 8 valence electrons)[8]

Ferrocene has become one of the most important metallocenes as it is commercially available and a large number of derivatives have been made. Much of the chemistry of ferrocene involves electrophilic substitution reactions to the two cyclopentadienyl ligands as they behave similarly to benzene rings and other arenes.

Ferrocene's aromaticity and stability to 400 °C without decomposing[10] makes the molecule ideal as an anti-knocking fuel additive, preventing car engines from damage as auto-ignition occurs as the fuel-air mixture compresses. Ferrocene therefore acts to increase the octane rating of fuels, which means the probability of knocking is reduced.[11] Today, ferrocene is widely used as it is considered to be more stable and safer than the toxic lead ligand tetraethyl lead, (CH3CH2)4Pb, which was previously used.[12]

The derivatives of ferrocene are used in ‘ExacTech pen' to measure glucose levels in people with diabetes, with the electron transfer between the glucose and glucose oxidase being assisted by ferrocene's iron centre.[13] Additionally, the molecule's ability to rapidly redox, this process being reversible, and remain stable made ferrocene an ideal compound. This property enables ferrocene to be used in electrochemistry to calibrate redox potentials.[14]

Another use of ferrocene is replacing the tamoxifen analogues in anti-breast cancer drugs to produce ferrocifens (ferrocenium salts). These, unlike pure ferrocene, bind to estradiol, which is commonly referred to and classed as an estrogen, and block its activity to cause cancer cells and tissue to grow.[15]

In conclusion, since ferrocene's unexpected discovery nearly sixty years ago the structure has been found to have unusual properties, resulting in a new group of molecules to be formed. Even though the compound's structure was difficult to determine, it is clear that Kealy and Pauson's accidental discovery of the first sandwich structure complex was worthwhile. As these molecules and ferrocene derivatives have subsequently had widespread applications, from the petrochemical industry as anti-knocking agents to the pharmaceutical industry as anticancer drugs.

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[1] R. Dagani, Fifty Years of Ferrocene Chemistry, CENEAR, 2001, 79 49, pp. 37-38.

[2] P.L. Pauson, Journal of Organometallic Chemistry: Ferrocene - how it all began, Elsevier Science, 2001, 637-639, pp. 3-6.

[3] J.C. Kotz,P. Treichel,J.R. Townsend, Chemistry and chemical reactivity, Thomson Brooks/Cole, Belmont, 2008, pp. 1052.

[4] G. Wilkinson, M.Rosenblum, M.C. Whiting, R.B.Woodward, The Structure of Iron bis-cyclopentadienyl , J. Am. Chem. Soc., 1951, pp. 2125.

[5] P. Štěpnička et al, Ferrocenes: Ligands, Materials and Biomolecules, John Wiley & Sons Ltd, Chichester, 2008, Part I:1, pp. 5.

[6] C.E. Housecroft and A.G. Sharpe, Inorganic Chemistry, Pearson Education Ltd, Harlow, 2008, Chapter 24, pp. 841.

[7] D. Astruc, Organometallic chemistry and catalysis, Springer, Grenoble, 2007, Chapter 1, pp 7-8.

[8] C.E. Housecroft and A.G. Sharpe, Inorganic Chemistry, Pearson Education Ltd, Harlow, 2008, Chapter 24, pp. 815.

[9] A.J. Elias, A Collection of Interesting General Chemistry Experiments, Orient Blackswan, Hyderabad, 2002, Chapter 20, pp. 114.

[10] E.S. Goulde, Inorganic Reactions and Structure, Holt, Rinehart and Winston, New York, 1955, pp. 403

[11] G. Burton et al, Salters Advanced Chemistry: Chemical Storylines, Heinemann, Oxford, 2000, DF, pp. 26,

[12] K. Terao, Irreversible phenomena: ignitions, combustion, and detonation waves, Springer, New York, 2007, Chapter 6, pp. 87.

[13] C.E. Housecroft and A.G. Sharpe, Inorganic Chemistry, Pearson Education Ltd, Harlow, 2008, Chapter 24, pp. 842.

[14] G. Jaouen, Bioorganometallics: biomolecules, labeling, medicine, John Wiley & Sons Ltd, Wienheim, 2006, Chapter 1, pp. 13-14.

[15] A. Vessières et al, 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, John Wiley & Sons Ltd, Wienheim, 2003, pp. 5223-5236.

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