Seminar Assignments - Report For Ferrocene Lab PDF

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Report for Ferrocene lab...

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CH 431 – Inorganic Chemistry

Experiment V: Preparation and Purification of Ferrocene Quynh Sam Siri Manjunath, Rafael Williams

Abstract In this experiment, ferrocene was synthesized by reacting FeCl 2∙4H2O with freshly cracked cyclopentadiene in the presence of KOH. The amount of crude ferrocene was obtained at 30.3% yield. The crude ferrocene was then purified by sublimation, after which it was obtained at 19.9% yield. The product was then analyzed using IR spectroscopy. The Fe-Cp bond was characterized by a peak at 473.66 cm-1.

Introduction Ferrocene was first synthesized in 1961 by P.L. Paulson during an attempt to create fulvalene. Although by mistake, the discovery of ferrocene was a major breakthrough in organometallic chemistry. Ferrocene belongs to a class of compounds known as sandwich compounds, where a metal is bonded to two organic aromatic compounds. More specifically, ferrocene is a type of metallocene, a subset of sandwich compounds describing molecules where a metal center is bonded to two cyclopentadienyl anions.1 The cyclopentadienyl anions are flat and aromatic in character, making them very stable. The stability of ferrocene is further attributed to its electron count of 18, which likens it to a noble gas. Its stability, among many other unique characteristics, has made it useful in many applications such as catalysis and medicine.2

In this experiment, ferrocene was synthesized using ferrous chloride and freshly cracked cyclopentadiene in the presence of potassium hydroxide. The reaction occurs according to the following equation: 8 KOH + 2 C5H6 + FeCl2•4H2O → C10H10Fe + 2 KCl + KOH•6H2O The KOH is added to deprotonate the cyclopentadiene and make it reactive toward the Fe2+ ions. The reaction was carried out using Schlenk techniques as Fe2+ ions are readily oxidized to Fe3+ ions in air. The cyclopentadienyl anions are also susceptible to oxidation. The air free technique prevents these reactions, which would greatly decrease the amount of ferrocene obtained if present in substantial amounts. The ferrocene initially obtained contained many impurities. Sublimation was used to purify the ferrocene. This is an effective technique for this particular experiment because ferrocene sublimes at low temperatures relative to its impurities. The ferrocene was analyzed using infrared spectroscopy. IR spectroscopy is used to identify the structure of molecules using their interactions with infrared light. Because this light is very low in energy, it will not cause any major energy changes within a molecule. It is only strong enough to induce vibrations and rotations which are measured by the IR spectrometer. Atoms and bonds can be likened to balls and springs, respectively. The infrared light causes the bonds to stretch and/or bend. Different bonds will move differently, and so the types of bond in a molecule can be deduced from looking at values on an IR spectrum. Light over a wide range of infrared frequencies is shined on the compound sample. The chemical bonds can then absorb between 0 – 100% of the different frequency lights, producing a value called percent transmittance. Over the years, scientists have recorded the frequency of light absorbed by certain types of bonds. Taking the peak values on an IR spectrum and comparing with these literature values allows for the identification of the types of bonds that are present.

In the following procedure, ferrocene will be synthesized by reacting cyclopentadiene with ferrous chloride in the presence of potassium hydroxide.

Experimental Method, Calculations, and Yield Hazards Ferrous chloride, DMSO, DME, cyclopentadiene, ferrocene, potassium hydroxide, and hydrochloric acid are irritants to the skin and eyes, as well as the respiratory and digestive tracts. Cyclopentadiene, ferrocene, DMSO, and DME are flammable. Cyclopentadiene and ferrocene emit noxious fumes.

Method A solution was prepared by adding 2.7518 g FeCl2∙4H2O to 25 mL dimethylsulfoxide and stirring on a stir plate for several minutes. A 250 mL Schlenk flask was filled with 60 mL 1,2dimethoxyethane and a stir bar. The liquid was degassed for several minutes. Against the nitrogen gas, 9.010 g KOH was added along with 2 mL cracked cyclopentadiene. The reaction mixture was stirred for about 30 minutes until the solution turned deep red in color. The ferrous chloride solution was added to the reaction mixture over 25 minutes using a dropping funnel. The mixture was then stirred for another 30 minutes. The apparatus was disassembled and the reaction mixture was poured into a large beaker containing ice and 90 mL HCl. A small aliquot of ice was used to rinse the Schlenk flask of any residual solution and transferred to the large beaker. The ice mixture was then stirred for 15 minutes using a glass stir rod. The solid product was isolated via vacuum filtration, washed with 4 portions of 15 mL distilled water, then left to dry for 1 week.

After the drying period, the crude product was weighed on a piece of filter paper. The product was then transferred to a covered petri dish. The petri dish was heated on a hot plate at 110 °C to sublime the ferrocene. The pure ferrocene was weighed and stored in a capped vial.

Calculations

Results and Discussion The practicality and novelty of ferrocene is owed to its unusual stability for a compound consisting only of a transition metal and hydrocarbon. The cyclopentadienyl ligands have a 1charge, making the iron take on a 2+ oxidation state with a d 8 electron count. Because each cyclopentadienyl ligand contributes 5 electrons to the entire structure and Fe 2+ contributes 8, ferrocene has a highly stable electron count of 18. Staggered ferrocene has a point group of D 5d while eclipsed ferrocene has a point group of D 5h. When it is in its solid crystal state, ferrocene is in its staggered form. In the gas phase, the cyclopentadiene ligands spin as the rotation energy barrier is fairly low.3 The synthesis of ferrocene was carried out using ferrous chloride and cyclopentadiene in the presence of KOH. The KOH was used to deprotonate the cyclopentadiene in order to make it reactive towards the Fe2+ ions. While ferrocene is quite stable in air, the cyclopentadiene anion and Fe2+ ions used to create the ferrocene are both susceptible to oxidation by oxygen. For this reason, it was necessary to use Schlenk techniques to create an inert nitrogen atmosphere in the reaction flask. After the reaction between the iron and cyclopentadienyl, the reaction mixture was poured into a mixture of ice and hydrochloric acid. The hydrochloric acid was needed to neutralize any remaining KOH. Because acid-base neutralizations are exothermic, ice was needed to cool the reaction mixture as ferrocene sublimes at relatively low temperatures. The ability of ferrocene to sublime at lower temperatures allows it to be purified by sublimation. This technique separates ferrocene from its less volatile impurities. Sublimation is a very effective purification technique when the desired compound and its impurities vary significantly in vapor pressure. A major advantage of sublimation is that not as much product loss occurs as with other methods such as recrystallization and extraction. With recrystallization

and extraction, different solvents are used which increase product loss because some traces of product will always remain in the solvents. The percent yield prior to sublimation was 30.3% while the percent yield after sublimation was 19.9%. Because sublimation separates impurities effectively, a large majority of the mass lost can be attributed to impurities. There were several potential sources of product loss. It is possible that there was poor conversion of reactants to products due to the speed at which the steps of the experiment were carried out – the dropwise addition of the ferrous chloride solution to the cyclopentadiene, for example. This issue could be solved by allowing more time for the experiment to be carried out so that each step can be given more time complete. Another possible source of product loss is that not all of the cyclopentadiene was deprotonated. Without deprotonation, the cyclopentadiene would not have been able to react to form ferrocene. In order to remedy this, the KOH should be stirred for longer periods of time. An IR spectrum of ferrocene was obtained for analysis purposes. Peaks at 785.68 cm -1 and 814.04 cm-1 indicate aromatic C-H bending. The peaks at 1631.49 cm -1 and 1694.67 cm-1 correspond to aromatic C=C bonds. The aromatic C-H stretches are given by peaks at 3084.78 cm-1, 3094.83 cm-1, and 3107.11 cm-1. The Fe-Cp bond has been reported to present itself around 471 cm-1.3 A very strong peak at 473.66 cm-1 on the spectrum obtained likely represents the FeCp bond.

Conclusion The aim of this experiment was to prepare and purify ferrocene. The preparation was done by reacting ferrous chloride with cyclopentadiene in the presence of KOH. The purification was done by sublimation. The crude ferrocene was obtained at 30.3% yield while the pure

ferrocene was obtained at 19.9% yield. Possible sources of product loss were adding the reagents too quickly and insufficient deprotonation of the cyclopentadiene. Both of these issues can be improved upon by allotting more time to complete the corresponding steps in the experiment. In order to analyze the ferrocene, an IR spectrum was obtained. The Fe-Cp bond was indicated by a peak at 473.66 cm-1.

References 1. Jaffe, H.H. Electronic structure of ferrocene. Journal of Chemical Physics, 1953, 21, 156 – 157. 2. Seibold, E.A.; Sutton, L.E. Structure of ferrocene. Journal of Chemical Physics, 1955, 23, 1967. 3. Mohammadi, N.; Ganesan, A.; Chantler, C.; Wang, F. Differentiation of ferrocene D5d and D5h conformers using IR spectroscopy. Journal of Organometallic Chemistry, 2012, 713, 51 – 59....