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CHEM 443L Inorganic Chemistry Laboratory Revision 1.1 The Synthesis and Acetylation of Ferrocene In this laboratory we will synthesize Ferrocene, or bis(5-cyclopentadienyl)iron, (C5H5)2Fe; an 18 electron sandwich type complex and first of the metallocenes to be synthesized. We will then acetylate the cyclopentadienyl ligand of the complex using a Friedel-Crafts Acylation technique, demonstrating the ligands of the complex are truly aromatic. Ferrocene, (C5H5)2Fe, was first discovered in 1951. Until this time organometallic compounds containing Metal-Carbon bonds were restricted to Grignard Reagents (R-MgBr), Zeise's Salt and a few miscellaneous others. Attempting to prepare Fulvalene via a Grignard Reagent, T.J. Kealy and P.L. Paulson formed Dicyclopentadienyl Iron, or Ferrocene, instead: 2 (C5H5)-MgBr + FeCl2 (C5H5)-Fe-( C5H5) + MgBr2 + MgCl2 Preparing Ferrocene via an alternate route; Miller et al noted: Compounds containing only carbon, hydrogen, and iron have not hitherto been described, and the direct replacement of hydrogen attached to carbon by iron would not have been expected to be feasible. It has now been found that reduced iron, in the form of the well-known "doubly-promoted synthetic ammonia catalyst," can be made to react with cyclo-pentadiene in nitrogen at 300o and at atmospheric pressure, to give a yellow crystalline compound of composition C 10H10Fe. The iron is in its bivalent form, since treatment of a carbon tetrachloride solution of the material with bromine in the same solvent gave a blue solution containing ferrous and bromide ions. The structure of the compound has not been further elucidated, but by analogy with the well-known cyclopentadienyl potassium, it is believed that substitution has occurred in the methylene group, and that the compound has the structure inset. Samuel A. Miller, John A. Tebboth and John F. Tremaine "Dicyclopentadienyliron" Journal of the Chemical Society,1952 Cyclopentadiene Miller et al's Depiction of "Dicyclopentadienyliron" Wilkinson, Rosenblum, Whitting and Woodward soon corrected this view of Ferrocene's structure and proposed the currently accepted "picture"of its bonding: Geoffrey Wilkinson, M. Rosenbluth, M.C. Whiting, R.B. Woodward "The Structure of Iron Bis-Cyclopentadienyl Journal of the American Chemical Soceity, 1952 This structure is quite amazing. As pointed out by Huheey: Organometallic chemistry leaped forward in the early 1950s when the structure of ferrocene, Fe(5-C5H5)2, was elucidated. Prior to that, ideas regarding metal-ligand interactions included only the coordinate covalent bond (e.g., M-CO) and the covalent bond (e.g., M-CH3). It was revolutionary in bonding theory to porpose a metal-ligand bond between a metal and the orbitals of C5H5. Ferrocene was the first of many complexes which came to known as metallocenes, a name which arose because they participated in reactions similar to those of aromatic molecules. For obvious reasons complexes in which a metal atom was found between two parallel carbocyclic rings became known as "sandwich" compounds. James E. Huheey, Ellen A. Keiter and Richard L. Keiter Inorganic Chemistry: Principles of Structure and Reactivity 1993 First to the bonding in the ligand; Cyclopentadienyl Anion. Because of the resonance stabilization of its anion, Cyclopentadiene is unusually acidic: C5H6 C5H5- + H+ or In accord with Huckel's 4n+2 (n = 0, 1, 2, …) Rule, Cyclopentadienyl Ion (n = 1) is an aromatic planar carbocyle. As can be shown rather easily using Huckel MO calculations, the molelcular orbitals for the conjugated system of the Ion are as depicted below: Group theory can then be employed (cf. Cotton) to determine how these MOs interact with the orbitals of the valence shell of the Fe2+ atom to give MOs of the entire molecule with the correct molecular symmetry, D5d. For example, the interaction of the dyz orbital of the metal with E1 MOs of the rings is as pictured. Based on this type of analysis, a crude MO diagram for Ferrocene can be constructed. (See figure on next page.) Note that Ferrocene is an extremely stable complex, stable in Air to temperatures of 500oC, because its 18 valence electrons occupy only bonding and non-bonding MOs. Twelve of these electrons are contributed to the structure by the cyclopentadienyl rings and occupy the lowest six Ferrocene MOs. The six electrons of the Fe2+ contribute to the Ferrocene metal-like MOs. The strictly anti-bonding MOs of Ferrocene are not occupied. Now to the synthesis of Ferrocene. Ferrocene is easily prepared by reaction of Cyclopentadienyl Anion and Ferrous Cation: 2 C5H5- + Fe2+ (C5H5)2Fe A strong base such as Potassium Hydroxide, KOH, will react with the acidic Cyclopentadiene to produce the needed Cyclopentadienyl Anion: C5H6 + OH- C5H5- + H2O Our source of Fe2+ will be Ferrous Chloride Tetrahydrate; FeCl2•4H2O. Overall, the synthetic reaction is: 8 KOH + 2 C5H6 + FeCl2•4H2O Ferrocene + 2 KCl + KOH•6H2O This reaction requires a solvent which is sufficiently polar to allow for the dissociation of the Potassium Hydroxide and yet allow for solvation of the hydrocarbon. Two solvents suitable for these purposes are Glyme (1,2-dimethoxyethane) and DMSO (dimethylsulfoxide). Additionally, the solvent must allow for the removal of the Water produced when the proton is abstracted from Cyclopentadiene otherwise the Iron will be oxidized to Fe3+. Glyme and DMSO both allow for the complexation of excess KOH with Water, effectively "removing" the latter from the reaction mixture. Finally, because of the aromatic nature of the Cyclopentadienyl rings, the Ferrocene compound exhibits chemistry similar to Benzene. In fact, it is this chemistry which provides an important clue as to the nature of the Cyclopentadienyl ligand in the compound. In 1877, Charles Friedel and James Crafts discovered that an acyl halide reacts with Benzene in the presence of an aluminum halide AlX3 (X = Cl, Br). The products of this reaction are the corresponding acylbenzene and hydrogen halide (HCl or HBr). This reaction, which can be carried out in the presence of other Lewis acids catalysts, is called the Friedel-Crafts Acylation reaction. In a like manner, we can acylate Ferrocene. Thus, in this exercise we will synthesize and purify Ferrocene. This compound will then be acetylated using a Friedel-Crafts Acylation to demonstrate the aromatic character of its ligands. Procedure Cracking Dicyclopentadiene Cyclopentadiene, is obtained from a light oil derived from coal tar distillation. Because Cyclopentadiene is such a reactive system, at Room Temperature it exists as a stable dimer, Dicyclopentadiene. This is a Diels-Alder adduct of two molecules of the diene. Thus, generation of Cyclopentadiene involves heating the dimer to initiate a "retro-" or "reverse-Diels-Alder" reaction. This procedure may be performed collectively by the Laboratory Instructor as the diene is particularly noxious. 1. Measure 20 mL of Dicyclopentadiene into a 100 mL round bottom flask and arrange for fractional distillation into an ice-cooled receiver. 2. Heat the dimer with a heating mantle until it refluxes briskly and at a rate such that the monmeric diene begins to distill in about 5 minutes and soon reaches a steady boiling between 40oC and 42oC. Do not exceed the boiling point of 42oC. Synthesis of Ferrocene 1. Grind 0.750g of KOH in a Mortar as rapidly as possible. Wear gloves and work in a fume hood while performing this operation. 2. Add 1.24 mL of Glyme (1,2-dimethoxyethane) and a magnetic stir bar to a 5 mL round bottom flask. Add the powdered KOH to this. 3. Cap the flask with a good septum and pass nitrogen into the flask and through the solution for about 1 minute. 4. Begin stirring the mixture so that as much of the solid as possible dissolves. 5. To a 10 mL Reaction Tube add 0.350g of finely powdered Iron (II) Chloride Tetrahydrate and 1.5 mL of DMSO (Dimethyl Sulfoxide). 6. Cap the tube with a good rubber septum and pass nitrogen into the tube for about 1 minute. Now shake the tube vigorously until all the Iron Chloride dissolves. Some warming may be needed. 7. Using an accurate syringe, add 0.30 mL of freshly distilled cyclopentadiene directly to the mixture of KOH in Glyme. Do not grasp the body of the syringe, because the heat of your hand will cause the cyclopentadiene to volatilize. Stir the flask vigorously. The solution will turn brown in color because of the formation of potassium cyclopentadienide salt. 8. After waiting about 5 minutes for the anion to form, pierce the septum with an empty needle for pressure relief and inject the Iron Chloride solution in six 0.25 mL portions over a 10 minute period. Stir the mixture well. After all the Iron Chloride has been added, rinse the reaction tube with 0.25 mL of DMSO and add this to the flask. Continue to stir of shake for about 15 minutes to complete the reaction. 9. Prepare a mixture of 5.0g Ice and 4.5 mL of 6M HCl in a graduated xylinder and add this mixture to a 30 mL beaker. To isolate the Ferrocene, pour the dark reaction slurry into the beaker with the ice and HCl. Stir the mixture thoroughly to neutralize any remaining KOH. (Test the mixture with pH paper. If needed, add more HCl.) 10. Filter the orange crystals using a Hirsch funnel. Wash the crystals with two 1 mL portions of chilled water. Draw air through the Hirsch funnel for 5 minutes. Dry the crystals between the folds of filter paper. Finally dry the product on a filter paper. 11. Purify the crude product via sublimation. 12. Weight the product. 13. Determine the melting point in a sealed capillary; since the product sublimes. 14. Determine the Theoretical Yield and Percentage Yield of your product. Acetylation of Ferrocene 1. Place 0.5 mmole of sublimed Ferrocene in a 5 mL round-bottom flask with a magnetic stir bar. Add 0.35 mL of Acetic Anhydride using a dry Pastuer pipet. Add 0.1 mL 85% Phosphoric Acid using a 1 mL syringe. Cap the flask with a septum and push a clean needle through the septum. This will minimize the amount of moist Air that enters the flask, and allow for release of any gas evolved. 2. Dissolve the Ferrocene by gently heating the flask on a heating the flask on a sand bath while carefully stirring and agitating the flask. 3. Continue heating for an additional 10 minutes and then place the flask in an ice-bath. Remove the needle and spetum, and carefully add 0.5 mL of ice-cold Water dropwise with a thorough mixing. 4. Neutralize the resulting solution against litmus by dropwise addition of a 3M aqueous solution of Sodium Hydroxide. Avoid an excess of base. 5. Filter the mixture by vacuum filtration, rising out the flask and washing the solid collected with cold water. Remove the solid from the filter and place it on a filter paper. Press the product as dry as possible between sheets of filter paper. 6. Separate the crude product via column chromatography. Use Alumina as the packing material and form your initial slurry using Pet Ether. Apply your solid to the column by dissolving it in a minimum amount of Dichloromethane, adding 300 mg Alumina, heating to remove the Dicholormethane and then applying the mixture to the column. Wash the compound onto the column using a small amount of Pet Ether. Elute the compound with several small portions of Pet Ether. Elute any Ferrocene with Pet Ether. Then, elute the Acetyl Ferrocene with a 50:50 mixture of Pet Ether and t-Butanol. 7. Collect the Acetyl Ferrocene, evaporate the solvent and weigh the product. Determine your percentage yield. Post Lab Questions 1. Both Cobaltocene and Nickelocene are easily oxidizable. Vanadocene and Chromocene will add additional ligands. Explain these observations using an MO diagram for each compound similar to that of Ferrocene. (A word of caution; the details of each MO diagram will differ slightly due to differences in the nature of the metal center. However, there are enough similarities that we can draw some simple conclusions concerning the above compounds by referencing the Ferrocene MO diagram.) 2. Provide a mechanism for the Friedel-Crafts Acylation of Benzene with Acetyl Chloride using AlCl3 as a catalyst. 3. Why can we use Acetic Anhydride in our acetylation of Ferrocene, as opposed to using a Acetyl Chloride? Reference your above mechanism. Why is Phosphoric Acid an acceptable catalyst? Again, reference your above mechanism. 4. Draw five resonance structures for the Cyclopentadienyl Anion. References Cotton, F. Albert Chemical Applications of Group Theory Wiley-Iterscience, New York, 1971. Huheey, James E.; Keiter, Ellen A. and Keiter, Richard L. Inorganic Chemistry: Principles of Structure and Reactivity Harper Collins, 1993. Kealy, T.J. and Pauson, P.L. "A New Type of Organo-Iron Compound" Nature 168 (1951) 1039. Miessler, Gary L. and Tarr, Donald A. Inorganic Chemistry Prentice Hall, Boston, 2011. Miller, Samuel A.; Tebboth, John A. and Tremaine, John F. "Dicyclopentadienyliron" J. Chem. Soc. (1952) 623. Purcell, Keith F. and Kotz, John C. An Introduction to Inorganic Chemistry Saunders College Publishing, Philadelphia, 1980. Streitwieser, Andrew, Jr. and Heathcock, Clayton H. Introduction to Organic Chemistry Macmillan Publishing Co., 1981 Wilkinson, Geoffrey; Rosenblum, M.; Whiting, M.C. and Woodward, R.B. "The Structure of Iron Bis-Cyclopentadienyl" J. Am. Chem. Soc. 74 (1952) 2125.