The discovery of ferrocene in the 1950s was a doorway to the area of organometallic chemistry and this occurrence was followed by the synthesis of massive organometallic complexes. Although the discovery was unexpected, the impact was enormous. These complexes were extensively used as industrial homogenous catalysts and also in organic synthesis where they supplied new synthesis routes to unattainable compounds.
Figure Structure of ferrocene
Historically, there was a debate regarding the structure of ferrocene. The unordinary and unique properties of this compound were contradicted with the expected structure, the aim that Pauson and Kealy want to synthesize by the reaction of cyclopentadienylmagnesiumbromide with FeCl3. It was then come into interest of these three chemists: Geoffrey Wilkinson, Robert Burns Woodward and Emil O. Fisher. The name of “ferrocene” with the “ene” ending implying aromaticity, following the structure of benzene was deduced by Wilkinson and Woodward based on the unusual stability, diamagnetic nature, single C-H stretching in the IR region and nonpolar character of the compound. The sandwich structure was later confirmed by X-ray crystallography. The carbon-metal bond of the ferrocene structure was a new concept of p-bonding of a carbocyclic ring to a metal atom. Following the name of ferrocene, came the term “metallocene”, a major departure of classical model of ligand coordination.
In this particular experiment, studies have been done for the synthesis and the reactivity of ferrocene. In details, this experiment was divided into three sections with their specific aim. The first section is to synthesise and purify ferrocene under inert atmosphere. The second section aims to investigate the redox behaviour of ferrocene. The third section aims to attempt a Friedel-Crafts acetylation to make a mixture of acetyl ferrocenes, as well as to separate and identify the products by column chromatography.
Figure Structure of ferrocene, monoacetyl ferrocene and 1,1′-diacetyl ferrocene respectively
SYNTHESIS & PURIFICATION OF FERROCENE
Potassium hydroxide (KOH, 8g, well crushed) was added to 1,2-dimethoxyethane (20cm3) and mixed well in a 2-necked,round-bottom flasks with a magnetic stirrer under inert condition (N2 flow). Cyclopentadiene (1.8cm3, 0.021mol, monomer density 0.786 g cm-3) was then added slowly against the N2 flow followed by a constant stirring for 10 minutes. The reaction mixture was then added with solution of FeCl2 (2g, 0.01mol) in DMSO (8cm3, dropwise, vigorous stirring) which was prepared earlier. As the addition completed, the reaction mixture continuously stirred for 30minutes, its N2 flow disconnected and then poured into a prepared slurry (HCl (aq), 30cm3, 6moldm-3) and ice (30g)).The slurry mixture then stirred (15 minutes), filtered, washed with water (4 x10cm3) and air-dried. The crude ferrocene obtained was then transferred to a watch glass, dried (steam bath, 5 minutes), broke gently, allowed to cool (5 minutes), weighed and its appearance recorded. It was then purified by sublimation (Silicon oil bath), collected, weighed, get its melting point range determined and submitted. The purified ferrocene obtained appeared as yellow solids (1.3501g, 71.5%); mp 161-165 0C, 760mmHg.
REDOX BEHAVIOUR OF FERROCENE
Provided Ferrocene (100mg) dissolved in acetone (10cm3) forming a yellow-orange solution. Solution of FeCl3.6H2O (0.1g) and H2O (5cm3) was made up and yellow-orange solution was formed as well. Half of the ferrocene solution (5cm3) was then mixed with the Fe (III) solution and dark blue solution was attained. A small spatula of ascorbic acid was then added (with shaking) to the dark blue solution obtained (2cm3) followed by dichloromethane (CH2Cl2, just enough to form discernable layer) and the mixture was shaken well. Yellow suspension was first observed before two layers of solution obtained. The top layer is detected as ferrocene in the dichloromethane while the bottom layer is identified to be the acid layer. Apart from these tests, the remainder of acetonic ferrocene solution was added with few drops of dilute silver nitrate (AgNO3, aq) and the solution (initially yellow-orange) turns dark blue.
ACETYLATION OF FERROCENE
Orthophosphoric acid (0.25cm3,85%)was added to a mixture of ferrocene (0.25g, 1.34mmol) and acetic anhydride (5cm3,density 1.08gcm-3) dropwise with constant stirring. The mixture was protected with freshly-made CaCl2 tube, heated vigorously (water bath, 10 minutes) and poured onto ice (20g) until all the ice melted. Sodium bicarbonate (NaHCO3, solid) was used to neutralise the reaction mixture before it was cooled in ice bath (30 minutes). The semi-sludgy mixture obtained was later extracted with ethyl acetate, get its filtrated collected, isolated (taking the organic layer), dried (MgSO4), filtered by gravity filtration and undergoes rotary evaporation. The compound obtained was later separated by column chromatography and identified using the thin layer chromatography analysis (70: 30 ratio of 40-60 petroleum ether: ethyl acetate as development solvent) as well as the IR and 1H NMR spectrometry. There are two components of acetylated ferrocene compound obtained, monoacetyl ferrocene and diacetyl ferrocene. The monoacetyl ferrocene appeared as dark yellow solid; mp 80 – 84 oC, 760mmHg; Vmax/cm-1 (solution cell, DCM) 1667 (C=O); Î´H (400MHz, DMSO) 4.53 ( , t, H ), 4.80 ( , t, H). The diacetyl ferrocene appeared as dark orange solids mp N/A ; Vmax/cm-1 (solution cell, DCM) 1667 (C=O) ); Î´H (400MHz, DMSO) 4.53 ( , t, H ), 4.80 ( , t, H).
Result & Discussion
Synthesis & Purification of Ferrocene
Initially, white precipitate formed when the well-crushed potassium hydroxide, KOH, dissolve in dichloromethane, DCM. After the addition of cyclopentadiene, the suspension somehow turns brownish red. Meanwhile, the dissolution of Fe(II) Cl2 . 4H2O in DMSO gives yellowish green solution. When both of them mixed, the solution turns dirty greenish brown suspension. Addition of HCl slurry neutralises excess KOH in the mixture and blue solution with yellow floating precipitates was observed after the addition step. As ferrocene is insoluble in water, the yellow crude ferrocene was easily collected from the Hirsch funnel during the filtration using water. The filtrate which obtained as dark blue solution was tested with three heaped spatula of SnCl2 and two layers of yellow solution were formed. This indicates an extra ferrocene left in the filtrate and they were later brought to rotary evaporation to remove the solvent. The crude ferrocene obtained was later assembled and sublimed. The sublimed ferrocene obtained as yellow solid.
(For calculation of yield, please refer appendices.)
Redox behaviour of ferrocene
Reaction with FeCl3:
Yellow- orange solution of ferrocene turned todark blue indicating the Fe (II) of ferrocene has been oxidised to Fe (III). Hence, with FeCl3, ferrocene is a reducing agent.
Reaction with ascorbic acid:
Here, the dark blue solution obtained turned back to yellow-orange colour indicated that the Fe (III) has been reduced back to Fe (II).
Reaction with silver nitrate, AgNO3:
In this case, the yellow-orange solution turned to dark blue showing that ferrocene, which is Fe (II) has been oxidised to Fe (III). Hence, with AgNO3, ferrocene acts as reducing agent.
The three redox tests indicate that ferrocene is a good reducing agent.
Acetylation of ferrocene
At the beginning of the acetylation process, dark brown solution was observed. When the reaction mixture then poured into ice and neutralised by sodium bicarbonate, NaHCO3, gas evolved and identified as CO2 gas. Formation of this gas somehow slows down the neutralisation process and eventually semi-solid i.e sludgy was obtained.
Meanwhile, for thin layer chromatography (TLC) analysis, 70:30 ratio of 40-60 petroleum ether:ethyl acetate was chosen as solvent system for the reaction mixture because this combination gives the best separation. The Rf value calculated using formula as followed.
Rf = Distance travel by compound/ distance travel by solvent
(For sketches of TLC analysis, please refer appendices.)
Separation of acetylation products
The column chromatography method was used to separate the mixture of ferrocene and acetyl ferrocene. In specific, the adsorption chromatography was applied as the compounds are neutral. Separation started by using non polar eluent (100% 40-60 petroleum ether) in order to bring down all the ferrocene as it is non-polar compound. As the non-polar petroleum ether moves through the silica gel adsorbent, the non-polar ferrocene moved with it and could be observed as a yellow band moving down the column. The acetyl ferrocene was left behind. Eventually, the polarity of the eluent used gradually increased with the addition of ethyl acetate to make up mixture of petroleum ether and ethyl acetate as eluents. These polar eluents function to bring down the polar acetyl ferrocene and could be observed as dark yellow band moving down the column. The diacetyl ferrocene is more polar than the monoacetyl ferrocene. Hence, it was only moved after the less polar monoacetyl ferrocene moved and using the more polar eluents (combination of 70:30 ratio of petroleum ether and ethyl acetate). The diacetyl ferrocene moved as a dark orange band. These observations show that polarity is the most important factor in separating the neutral compounds.
Despite of all the interesting observations mentioned, only small amount of monoacetyl ferrocene were obtained after the rotary evaporation which was just enough to make the IR and 1H NMR analysis while the diacetyl ferrocene does not have a good 1H NMR spectrum as the sample was not enough. This is the reason why
Mass of monoacetyl ferrocene obtained = 0.07g
Mass of diacetyl ferrocene obtained = 0.01g