expt10-3820

7/27/2019 Expt10-3820

1/5

Experiment 10

Chemistry 3820 Laboratory Manual Page 10 - 1

Cyclopentadiene complexes: the preparation andelectrochemical characterization of ferrocene

Introduction

In 1950, the known transition metal organometallic compounds included several metal

carbonyls (i.e. M-CsO compounds) and a variety of very unstable alkyl complexesanalogous to Grignard and organozinc compounds (i.e. M-CH2-(-CH2-)n-CH3

compounds. The latter were much less stable than the main-group analogues. Whenferrocene was discovered by Kealy and Pauson and independently by Miller and co-workers, its structure was unknown, but its properties were strikingly different. It was

stable in air, and resisted decomposition if heated in vacuo above 500 C; it could in factbe sublimed in the open atmosphere.

These properties are explained by the physical and electronic structure of ferrocene. Thephysical structure is that of a metal atom "sandwiched" between two planar aromatic

C5H5 rings.The electronic structure requires a molecular orbital description (ref.3), but requires theinvolvement of metal d orbitals which overlap with the p orbitals of the aromatic ring.

The cyclopentadiene anion is a 6-electron aromatic ring which obeys the Hckel 2N + 2rule, and since the molecule is overall neutral, this implies that the formal oxidation state

of the iron atom is +2.

In this laboratory you will prepare ferrocene and purify a sample by sublimation in air.You will obtain infrared, UV-visible and NMR spectral data on this compound anddecide whether this data is consistent with the proposed structure. Further information on

the electronic structure of this complex will be achieved by studying its redox chemistryby a technique called cyclic voltammetry.

H

H

HH

H

C CC

C

C

Fe

Ferrocene

Figure 10-1 Structures of the cyclopentadienide ion and ferrocene

Schematic representation 3-dimensional structure

Fe

C

C H5

-5

7/27/2019 Expt10-3820

2/5

Experiment 10


Instructional goals:

Properties of the following elements are highlighted: Fe, C and H.

(1) Experience with the synthesis of a transition-metal organometallic compound.

(2) Use sublimation to purify a volatile organometallic compound.(3) Learn to distinguish the electronic absorption spectra of organometallic

complexes from traditional coordination complexes.(4) Experience with NMR for the characterization of diamagnetic organometallic

compounds.(5) Learn to prepare KBr disks for IR spectroscopy.

Pre-lab exercise

1. Write balanced equations for the synthesis of ferrocene from the indicated startingmaterials.

2. What is the role of the KOH in this experiment?

3. What range of IR frequencies need to be recorded in this experiment?4. What is a good starting concentration to use in the UV-visible spectral

measurement of ferrocene?5. What is sublimation, and how can it be used to purify a compound? What

properties must the compound posses to make it suitable for this type of

purification?6. Map out the timing of this project. Several steps, e.g. the cracking of

dicyclopentadiene and the sublimation of ferrocene must be performed outside ofregular lab hours, but require minimal supervision. Use free gaps of time to doother operations. Be realistic in the time allotted for each operation!

Procedure

Preparation of cyclopentadiene, C5H6

This procedure must be started several hours before the laboratory begins, using thepreassembled apparatus for the cracking of dicyclopentadiene. Charge the distillation

flask with 25 mL of dicyclopentadiene. Warm the recirculating heating bath to 60C.While running warm water through the column of the still, and cold through the

collection condenser, slowly distil C5H6 from the dimer, collecting only the fraction

boiling below 44C. (the b.p. of C5H6 is 42.5C, whilst that of C10H12 is 142C.) The

freshly distilled C5H6 must be used within 2 to 3 hours, or stored at -78 C until use,because slow dimerization occurs at room temperature.

Preparation of [Fe(5-C5H5)2], ferrocene

Using the blender provided, grind up 60g of KOH pellets. Store the ground material inthe jar provided, which should be kept capped as much as possible since finely ground

KOH is deliquescent.

7/27/2019 Expt10-3820

3/5

Experiment 10


In a fume hood with nitrogen outlets, assemble the apparatus shown in Figure 1. Charge

the 500 mL flask with an egg-shaped stirring bar, 120 mL of 1,2-dimethoxyethane andthe powdered KOH. One side arm is stoppered and other is connected via a bubbler to

the nitrogen outlet. While the mixture is slowly stirred the flask is flushed with a strong

stream of nitrogen gas. After stirring and flushing for 15 minutes, 11.0 mL ofcyclopentadiene is added. Flushing is continued for a further 5 minutes. The 100 mL

dropping funnel is now attached to the central neck with the stopcock open. Afterallowing the funnel to be thoroughly flushed, the stopcock is closed and the funnel is

charged with a solution of 13.0 g of FeCl2.4H2O in 50 mL of dimethyl sulfoxide (the ironchloride should be ground to a fine powder with a mortar and pestle before attempting toprepare this solution.)

The solution is now stirred vigorously while the iron chloride solution is added dropwiseat a rate adjusted so that the entire solution is added over 45 minutes. After addition iscomplete, the stopcock is closed and stirring is continued for a further 30 minutes.

Then the nitrogen flow is stopped, and the mixture is added to a slurry of 180 mL of 6 M

HCl and 200g of crushed ice. Use the resulting liquid to rinse out the flask if necessary.

The slurry is stirred for about 15 minutes, and warmed if necessary to melt any remainingice. The precipitate is collected on a Bchner funnel and washed with four 25 mL

portions of distilled water. The moist solid is spread out on a large watch glass and driedin the air overnight.

To N2 cylinder

500 mL three-necked flask

100 mL dropping funnel

Stirrer

Figure 10-2 Reaction flask set-up

Mineral-oil bubler

7/27/2019 Expt10-3820

4/5

Experiment 10


Purification of ferrocene

An extremely pure product can be obtained by sublimation. The material to be sublimedis placed in the inverted cover of a Petri dish so that none of the material is within 2 mm

of the wall of the cover. The layer should not be more than 5 mm deep. The Petri dish

itself is inverted and carefully placed in the cover (Figure10-3).

The apparatus is then placed on a hot plate, which is gradually warmed up until the topsurface of the apparatus is almost too hot to touch. After 4 to 10 hours, the ferrocene

should be completely sublimed onto the upper glass surface and should be completelyseparated from the small amount of residue on the bottom by a gap of several mm. If anyof the ferrocene crystals are touching the residue, the temperature of the hot plate should

be increased, and more time allowed for complete sublimation.

Characterization

Record the melting point of the sublimed material. If all the material did not fit in thesublimation apparatus, a crude yield can be reported, and from the amount sublimed an

estimate obtained of the true yield. Measure the IR spectrum as a KBr disk. Record theUV-visible spectrum in 95% ethanol (make up a quantitative solution of knownconcentration in a small volumetric flask) in the range 600-300 nm. Record the 1H and13C NMR spectra of ferrocene in CDCl3.

Report

Hand in your product as well as all original spectra. Discuss the spectral data as

supporting or contradicting the proposed "sandwich" structure of ferrocene. What otherstructures are possible? For background on the IR spectrum consult reference 2. Interpret

the UV-visible spectrum in terms of the likely chromophore in this complex, and withreference to the ligand field splitting and d-electron populations.

Your Discussion should address at least the following points:1. What is the structure of dicyclopentadiene?2. X-ray diffraction data indicate that the ferrocene molecule is centrosymmetric, but

that ruthenocene is not. What can be said about the structures of these two

Surface of hot plate

Pure ferrocene

Residue

Figure 10-3 Apparatus for the sublimation of ferrocene

7/27/2019 Expt10-3820

5/5

Experiment 10


compounds in the solid as a consequence of these observations? What is the gas-phase structure of ferrocene? Assign the point groups of the structures you

mention.3. Describe two other methods for the preparation of ferrocene.

4. Use a molecular orbital energy-level diagram to interpret the electronic absorption

spectra.

References

1. F.A. Cotton and G. Wilkinson, Advanced inorganic chemistry, 5th edition (Wiley,N.Y., 1988), p. 1171. [QD151.2.C68]

2. K. Nakamoto, Infrared spectra of inorganic and coordination compounds (NY:

Wiley, 1970). [QC457.N3]3. D.F. Shriver, P.W. Atkins and C.H. Langford, Inorganic chemistry (Freeman,

N.Y., 1990), p. 522.4. Charles Lukehart, Fundamental transition metal organometallic chemistry

(Monterey, Calif.: Brooks/Cole, 1985), p. 85ff. [QD411.L857.1985]

5. Allen J. Bard, Electrochemical methods: fundamentals and applications (NewYork : Wiley, 1980), p. 215ff. [QD 553 B37]

6. Daniel C. Harris, Quantitative chemical analysis (San Francisco: W.H. Freeman,1982) p. 452. [QD 101.2 H37 1982]

7. J.B. Headridge, Electrochemical techniques for inorganic chemistry (NY:

Academic Press, 1969), p.42ff. [QD553.H4]

expt10-3820

Documents