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.  State University of New York at Binghamton, Binghamton, New York 13902 (Funded by the National Science Foundation) (Submitted to J. Chemical Education) -------------------------------------------------- ------------------------------ 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. -------------------------------------------------- ------------------------------ 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 nitrogen flow is reduced and drop-by-drop addition of the FeCl2 solution is begun. The rate of addition is adjusted so that the entire solution is added in 30 minutes. Then the dropping funnel stopcock is closed and vigorous stirring of the dark green solution is continued for an additional 30 minutes. Finally, the nitrogen flow is stopped and the mixture is added to a mixture of 6M HCl (45 mL) and crushed ice (50 g). Some of the resulting slurry may be used to rinse the reaction flask to maximize the product yield.
The slurry is stirred for about 15 minutes and the orange precipitate is collected on a Buchner or Hirsch funnel and washed with four 5-mL portions of water. The moist solid is spread out on a large watch glass and dried in the air. The compound is then purified through sublimation in a large glass petri dish that is placed on a warm hot plate (100 C). Care is used to avoid charring the ferrocene. The purified ferrocene is then characterized by melting point determination, UV-Vis and IR spectroscopies, and cyclic voltammetry.
We are incorporating a bulk electrolysis to generate the ferrocenium cation. Preparation of Acetylferrocene Acetylferrocene is synthesized under mild conditions with a modification of the procedure reported by Bozak (4). The students are supplied with ferrocene during the second laboratory period so that the acetylation of ferrocene may take place concurrently with the purification of ferrocene. This encourages students to develop multi-tasking skills. A mixture of ferrocene (1.5 g) and acetic anhydride (5 mL) is prepared in a small Erlenmeyer flask. To this mixture, 85% H3PO4 (1 mL) is added dropwise with constant stirring. This addition is exothermic and is accompanied by a change in color.
Following the addition of the phosphoric acid, the Erlenmeyer flask is fitted with a CaCl2 drying tube. The dark green solution is then heated in a beaker of water on a hot plate for ten minutes (50 C). During this time, the solution becomes rose colored. The mixture is then poured over ice (20 g) into a large beaker that will accommodate the gas (CO2) formed during the NaHCO3 neutralization. Water is used to rinse the reaction flask and maximize the product yield. When the ice has melted, small quantities of sodium bicarbonate are added until gas evolution stops.
The pH may be tested with pH paper to insure that neutrality is achieved. This is followed by cooling the resulting orange solution in an ice bath for 30 minutes during which time a brown precipitate forms. This precipitate is collected by suction filtration using a coarse fritted funnel. The dark brown solid is then washed with distilled water to remove impurities until it is pale orange in color. It is then dried in air for 15 minutes.
Thin layer chromatography is used to optimize the conditions for column chromatography of acetylferrocene. TLC plates (silica gel) are provided for student use. Alternatively, microscope slides may be used as TLC plates by applying a slurry that consists of silica gel (40 g) and chloroform (100 mL). A small amount of the crude acetylferrocene, which is a mono- and diacetylferrocene/ferrocene mixture, is dissolved in a vial in toluene (2-3 drops). A small amount of ferrocene is also dissolved in a separate vial in toluene. A line is penciled on each slide approximately 1 cm from the bottom of the TLC plate.
The plates are spotted using a fine capillary applicator approximately on the pencil line. Each plate will contain two spots, one is ferrocene and one is crude acetylferrocene. The spots are allowed to air dry and then a second spot is applied at the same location to obtain a concentrated area of compound. The identity of the spot is indicated with a pencil mark. The plates are individually placed with the spotted end in the solvent in five developing chambers.
The chambers contain the following solutions: petroleum ether, toluene, ethyl ether, ethyl acetate and a mixture of 10% ethyl acetate and 90% petroleum ether. The pencil mark should be above the solvent level. The solvent containers are covered while the plates are developing. The plates are removed when the solvent front has traveled approximately 3/4 of the distance of the plate. The plates are air dried.
The TLC plates may be developed in an iodine chamber. This will result in brown spots that can be marked and identified so that the plates may be included in a laboratory report. The solutions that provide maximum separation of the two components are chosen as column chromatography solutions. For instance, ferrocene may elute with toluene while the acetylferrocene remains on the column and is then eluted with a toluene/ethyl acetate mixture. The color of the spots is helpful to discern the individual bands that elute from the column. The crude acetylferrocene is dissolved in the solution that is selected to elute the first component.
The column is assembled by placing a small piece of glass wool into the bottom of the column (50 mL buret). The glass wool is then covered with a small amount of sand and the buret is filled with the solvent that was chosen to dissolve the crude mixture. A powder funnel is used to slowly fill the column with dry silica gel to a height of approximately 30 cm. The column is never allowed to dry. Alternately, the column may be prepared by the traditional slurry method.
A small amount of silica gel may be added to the crude acetylferrocene solution to make a slurry that is then added to the top of the column and covered with a small amount of sand. The two solutions (or mixtures) are then used to purify the crude acetylferrocene. The ferrocene band is discarded and the solvent is removed from the acetylferrocene band by rotary evaporation. It may then be recrystallized from chloroform. The acetylferrocene is characterized by melting point determination, IR and UV-Vis spectroscopies, and cyclic voltammetry. -------------------------------------------------- ------------------------------ Discussion The experimental procedure for the synthesis of ferrocene provided above was adopted after several failed attempts to incorporate newer microscale techniques that utilize ethy ...