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Since the discovery and structural characterization of the compound ferrocene [Fe(η-C5H5)2] in the 1950s, there was a large amount of research done on metal sandwich compound chemistry. Geoffrey Wilkinson and Ernst Otto Fischer, who defined the appropriate structure of ferrocene (Figure 1), received the Nobel Nobel Prize in Chemistry in 1973 for their work on sandwich complexes chemistry.

Ferrocene is a p-complex in which reactions between the d-orbitals of the Fe2+ metal centre with the p-orbitals of the two planar cyclopentadienyl ligands (C5H5-) form the metal-ligand bonds. Hence there is equal bonding of all the carbon atoms in the cyclopentadienyl rings to the central Fe2+ ion.

Ferrocene shows aromatic properties and is very stable. This article covers the synthesis of ferrocene and several reactions done with it as well as characterizing the products with NMR spectroscopy.

Figure 1. Ferrocene [Fe(η-C5H5)2]

This experiment is done in two parts:

Both parts are done in the fumehood.

Most of the compounds, dicyclopentadiene, cyclopentadiene and 1,2-dimethoxyethane (DME) are toxic. While working with these compounds, one must take precautions to prevent contact with skin and breathe vapour.

The method of synthesis is described below:

Figure 2. Experimental setup for ferrocene synthesis

Figure 3. (a)-(b). Purification of crude ferrocene via sublimation. a) Crude ferrocene (b) Sublimed ferrocene

The Spinsolve NMR spectrometer is used to monitor the different synthetic stages for preparing ferrocene. By studying the different aliquots collected during the synthetic procedure, the disappearance of reactants and formation of products can be observed. In Figure 4, the monomeric nature of cyclopentadiene (a and b), the formation of the cyclopentadienyl anion (c) and its disappearance and formation of ferrocene (d and e) are confirmed.

Figure 4. Overlay of 1H NMR spectra of reactants and reaction mixtures during the synthesis of ferrocene.

The 1H NMR spectrum of ferrocene (Figure 5) shows ten equivalent aromatic protons as a singlet at 4.16 ppm.

Figure 5. 1H NMR spectrum of ferrocene, [Fe(η-C5H5)2], CDCl3

This experiment shows the Friedel-Crafts acylation reaction to obtain acetylateferrocene.

The synthesis of acetyl ferrocene is as follows:

Figure 6 shows the 1H NMR spectrum of acetylferrocene with five equivalent aromatic protons as a singlet at 4.19ppm for the unsubstituted cyclopentadienyl ring.

A singlet is observed at 2.39ppm (3H) corresponding to the acetyl methyl group. The substituted cyclopentadienyl ring protons are seen as a second order AA’BB’ system, with two multiplets centred at 4.49 and 4.77ppm, integrating for two protons each.

Figure 7 shows the COSY spectrum of acetylferrocene that protons at position 2 and 3 are in the same spin system, as in the substituted cyclopentadienyl ring.

Figure 6. 1H NMR spectrum of acetylferrocene, CDCl3

Figure 7. COSY spectrum of acetylferrocene, CDCl3. Refer to Equation 3 for the annotated structure.

This involves a ligand exchange reaction between one of the cyclopentadienyl rings in ferrocene and benzene to form a cationic iron p complex, which is then precipitated as the hexafluorophosphate (PF6-)salt.

Equation 4. Ligand exchange of ferrocene with benzene

Figure 8. [Fe(η-C5H5)(η-C6H6)]PF6

Figure 9. 1H NMR spectrum of [Fe(η-C5H5)(η-C6H6)]PF6 complex, acetone-d6.

The reaction of the iron benzene p-complex prepared in part 3 with LiAlH4 and LiAlD4, sources of H and D- ions respectively. Arenes, for instance benzene, are more susceptible to attack by electrophiles than nucleophiles. However, associating with a metal often alters the reactivity of organic ligands. Thus, the reactivity of the benzene ligand in the iron p-complex towards the nucleophiles H- and D- is examined.

This information has been sourced, reviewed and adapted from materials provided by Magritek.

For more information on this source, please visit Magritek.

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