Video: Structure Determination of Ferrocene-an Organometallic Complex

Structure Of Ferrocene

Przegląd

Source: Tamara M. Powers, Department of Chemistry, Texas A&M University

In 1951, Kealy and Pauson reported to Nature the synthesis of a new organometallic compound, ferrocene.¹ In their original report, Pauson suggested a structure for ferrocene in which the iron is singly bonded (sigma bonds) to one carbon atom of each cyclopentadiene ligand (Figure 1, Structure I).¹^,²^,³ This initial report led to wide-spread interest in the structure of ferrocene, and many leading scientists participated in the structure elucidation of this interesting new molecule. Wilkinson and Woodward were quick to suggest an alternative formulization where the iron is "sandwiched" between two cyclopentadiene ligands, with equal binding to all 10 carbon atoms (Figure 1, Structure II).⁴ Here, we will synthesize ferrocene and decide, based on experimental data (IR and ¹H NMR), which of these structures is observed. In addition, we will study the electrochemistry of ferrocene by collecting a cyclic voltammogram. In the course of this experiment, we introduce the 18-electron rule and discuss valence electron counting for transition metal complexes.

Figure 1. Two proposed structures of ferrocene.

Procedura

1. Cracking the Cyclopentadiene Dimer (Figure 3)

Cyclopentadiene undergoes a Diels-Alder reaction with itself to give dicyclopentadiene. This reaction is reversible, so cracking is accomplished using La Châtelier's principle to drive the reverse reaction by distilling the cyclopentadiene monomer (b.p. 42 °C) away from the dicyclopentadiene dimer (b.p. 170 °C). The dimerization reaction is slow when the cyclopentadiene is kept cold, but it must be

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Wyniki

Ferrocene Characterization:

¹H NMR (chloroform-d, 300 MHz, δ, ppm): 4.15 (s).

The ¹H NMR spectrum of ferrocene clearly shows a single resonance, consistent with structure II.

A CV of ferrocene is given below. The E_1/2 value obtained for the oxidation of ferrocene was +90 mV (acetonitrile, scan rate 100 mV/s, 0.1 M (Bu₄N)PF₆, glassy carbon working electrode). The ferrocene/ferrocenium redox couple is commonly used as a reference in cyclic voltam

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Wniosek i Podsumowanie

In this video, we discussed ferrocene and the role it played in the development of organometallic chemistry. Ferrocene was synthesized and characterized by ¹H NMR and IR spectroscopy. Both spectra are consistent with the 18 e⁻ Structure II, where the iron is "sandwiched" between two cyclopentadiene ligands, with equal binding to all 10 carbon atoms (Figure 1, Structure II). Oxidation of ferrocene to ferrocenium cation was observed electrochemically.

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Odniesienia

1. Kealy, T. J., Pauson, P. L. A New Type of Organo-Iron Compound. Nature. 168 (4285), 1039-1040 (1951).

2. Pauson, P. L. Ferrocene—how it all began. J Organomet Chem. 637, 3-6 (2001).

3. Seeman, J. I., Cantrill, S. Wrong but seminal. Nat Chem. 8 (3), 193-200 (2016).

4. Wilkinson, G., Rosenblum, M., Whiting, M. C., Woodward, R. B. The Structure of Iron Bis-cyclopentadienyl. 74, 2125-2126 (1952).

5. Green, M. L. H., Parkin, G. Application of the Covalent Bond Classification Method for the Teaching of Inorganic Chemistry. J Chem Educ. 91 (6), 807-816 (2014).

6. Press Release. http://www.nobelprize.org/nobel_prizes/chemistry/laureates/1973/press.html.

7. Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, 6th ed. John Wiley & Sons. Hoboken. 2014.

8. Gildner, P. G., Colacot, T. J. Reactions of the 21^st Century: Two Decades of Innovative Catalyst Design for Palladium-Catalyzed Cross-Couplings. Organometallics. 34, 5497-5508 (2015).

Tagi

Structure Determination Organometallic Compounds Ferrocene Kealy And Pauson Wilkinson And Woodward Alternative Structure X ray Crystallography Proton NMR 18 electron Rule Spectroscopic Analysis Electrochemical Analysis Applications Valence Shell Electrons Main Group Elements Transition Metals Valence Orbitals

1. Cracking the Cyclopentadiene Dimer (Figure 3)

Cyclopentadiene undergoes a Diels-Alder reaction with itself to give dicyclopentadiene. This reaction is reversible, so cracking is accomplished using La Châtelier's principle to drive the reverse reaction by distilling the cyclopentadiene monomer (b.p. 42 °C) away from the dicyclopentadiene dimer (b.p. 170 °C). The dimerization reaction is slow when the cyclopentadiene is kept cold, but it must be freshly prepared to successfully synthesize ferrocene.

To a 25-mL round-bottom flask, add a stir bar and 10 mL of dicyclopentadiene dimer.
CAUTION: Be careful to only use dicyclopentadiene and cyclopentadiene in a fume hood because they are both stench chemicals.
Attach the round-bottom flask to a fractional distillation apparatus (see "Fractional Distillation" module in the Essentials of Organic Chemistry series), and place in an oil bath on a stirring hot plate. Be sure to have the collecting flasks in an ice bath. Clamp the apparatus in place.
Set the hot plate to 160 °C and gently stir the solution.
Fractional distil ~ 5 mL of the Cp monomer from the dimer (39-42 °C).

Equation 2
Figure 3. Cracking of dicyclopentadiene.

2. Synthesis of Ferrocene (Figure 4)

To a 100-mL Schlenk flask, add a stir bar and 15 g of finely ground KOH.
CAUTION: KOH is very corrosive and hygroscopic! The KOH should be ground in a well-ventilated hood and used immediately after preparation.
Add 30 mL of 1,2-dimethoxyethane to the flask while stirring.
Connect the flask to nitrogen and place a rubber septum over the neck of the flask.
While stirring under nitrogen, add 2.75 mL of cyclopentadiene via syringe. Allow this to stir for at least 10 min prior to addition of the iron.
While the reaction is stirring, add a stir bar, 3.25 g of ground FeCl₂·4H₂O, and 12.5 mL DMSO to a separate 50-mL Schlenk flask. Place a septum on the neck and stir under nitrogen until all of the iron has dissolved.
Cannula transfer the iron solution to the cyclopentadienyl solution under nitrogen dropwise over the course of 30 min. For a more detailed procedure, please review the "Synthesis of a Ti(III) Metallocene Using Schlenk Line Technique" video in this Inorganic Chemistry series.
Once the addition is complete, stir for an additional 30 min.
Meanwhile, in a beaker cool 45 mL of 6 M HCl by adding crushed ice (50 g) directly to the solution.
Once the reaction is complete, pour the mixture onto the slurry, and stir for a few minutes. Orange crystals should form.
Collect the crystals on a Büchner funnel (fitted with filter paper), and wash the precipitate with water.
Allow the solids to dry in the air.

Equation 3
Figure 4. Synthesis of ferrocene.

3. Purification of Ferrocene. Purify the product by sublimation (for a more detailed procedure, please see the "Purification of Ferrocene by Sublimation" video).

4. Characterization of Ferrocene

Prepare and collect a ¹H NMR spectrum of ferrocene.
Collect an IR spectrum of ferrocene.
Collect a cyclic voltammogram of ferrocene (see the "Cyclic Voltammetry" video in the Essentials of Analytical Chemistry series).

Przejdź do...

0:00

Overview

1:19

Structure Determination of Organometallic Complexes

4:13

Synthesis of Ferrocene

6:37

Results

7:29

Applications

9:33

Summary

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