The Synthesis and Characterization of Ferrocene
A Modern Iterative Approach to a Classical Organometallic Laboratory Experiment
Pamela S. Tanner, Gennady Dantsin, Stephen M. Gross, Alistair J. Lees,
Clifford E. Myers, M. Stanley Whittingham and Wayne E. Jones, Jr. [1]

State University of New York at Binghamton, Binghamton, New York 13902

(Funded by the National Science Foundation)
(Submitted to J. Chemical Education)



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Since ferrocene is credited with the rapid acceleration of modern organotransition metal chemistry (1,2) and the cyclopentadienyl group is extensively used as a stabilizing ligand, it is only fitting that the synthesis of ferrocene be incorporated into an advanced undergraduate inorganic laboratory. In our four credit course, the students work in pairs and have the opportunity to select six experiments from a total of nine. Three of these experiments must be selected from the area of materials chemistry and the topics include the synthesis of anhydrous CrCl3, a high temperature superconductor, the ZSM-5 zeolite and the lithium intercalation of WO3. Three wet experiments are also selected. These include the synthesis of W(CO)4, metal complexes of DMSO, a tris(bipyridyl)ruthenium complex, ferrocene, and the acetylation of ferrocene. If ferrocene is selected, it must be done in conjunction with the acetylation of ferrocene and these labs make up two of the three wet labs that are done by the student. Each lab incorporates an open ended question that the student may answer with the aid of library research or CAChe molecular modeling software with the Project Leader extension. This iterative approach builds confidence in the students ability to explore the unknown and reinforces the basic idea of the scientific method.

The ferrocene synthesis has been an extremely successful and popular selection. The students enjoy the diverse technical skills acquired during this experiment. These are techniques that a student may not be introduced to again as an undergraduate and include the use of air-less glassware while working on a vacuum line, cyclic voltammetry, bulk electrolysis, thin-layer and column chromatography. In addition, the compounds are characterized by standard methods such as melting point determination, IR and UV-Vis spectroscopies.



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Experiments
Preparation of Ferrocene
Ferrocene is synthesized with a modification of the preparation reported by Jolly (3). The yield in the reported synthesis was 93% (3). Cyclopentadiene undergoes a 4+2 cycloaddition to form dicyclopentadiene. For this reason, cyclopentadiene is usually purified before use. Dicyclopentadiene boils at 170C and cyclopentadiene boils at 42.5 C. For efficiency, the dicyclopentadiene dimer is thermally cracked using a fractional distillation apparatus in advance by the teaching assistant. While this is usually done on the day of the experiment, we have found that cyclopentadiene may be stored without significant dimerization in a foil covered container in a freezer for several days. At the beginning of the lab period, the students grind KOH in a mortar and quickly transfer it to a tared vial. KOH is hygroscopic and should be ground in small portions (2 g). A nitrogen glove bag is a worthwhile investment for this step in the procedure. In addition to protecting the students from the corrosive KOH, it ensures that the KOH is dry. The FeCl2.4H20 will also go into solution more effectively if it is finely ground. It is then placed in a tared vial.
The pre-weighed KOH (15 g) is placed in a 100 mL (14/20) three-neck round bottom flask equipped with a magnetic stirring bar.

1,2-Dimethoxyethane (30 mL) is added with stirring to the KOH. One side of the neck is stoppered and the other is connected to a vacuum line through a gas adapter. While the mixture is slowly stirred and the flask is being purged with a stream of nitrogen, the cyclopentadiene (2.75 mL) is added. The resulting solution is rose colored. The main neck is then fitted with a pressure equalizing dropping funnel (25 mL) with its stopcock open. In a second one neck round bottom flask that is fitted with a septum, FeCl2.4H20 (3.25 g) and DMSO (12.5 mL) are stirred under a nitrogen atmosphere to dissolve the FeCl2.4H20.

After about five minutes, the stopcock is closed and the FeCl2 solution is added to the pressure equalizing dropping funnel. The reaction mixture in the three-neck flask is stirred vigorously and the purging with nitrogen is continued. After about ten minutes, the stopper is placed on the dropping funnel, the