Ferrocene: Preparation, Properties, and Structure
Ferrocene, [Fe(η-C5H5)2], is an organometallic compound where an iron atom is "sandwiched" between two cyclopentadienyl (Cp) rings. The chemical name of ferrocene is bis(η-cyclopentadienyl) iron(II), the symbol η denotes that all carbon atoms of the ring are bonded identically to the metal. In short, we can write it as Cp2 Fe. Ferrocene exhibits aromatic properties and is thermally very stable. It is also resistant to acidic and basic reagents. Ferrocene is generally an orange-colored crystalline solid with a camphor-like odor.
Preparation of Ferrocene
From Grignard Reagent
The formation of ferrocene was a serendipity. In 1951, Kealy and Paulsion attempted to prepare bicyclopentadienyl by reacting FeCl2 and C5H5 MgCl. Instead of expected product, this reaction produce ferrocene.
Laboratory Method
Sodium or potassium cyclopentadienide reacts with Iron(II) chloride in an ethereal solvent like tetrahydrofuran (THF) or dimethoxyethane (DME) under an inert atmosphere (nitrogen) yields ferrocene.
Physical Properties of Ferrocene
Ferrocene is a diamagnetic and an orange crystalline solid at room temperature having an odor like camphor. It is soluble in organic solvents like benzene and ether, but insoluble in water. It is a stable organometallic compound having melting points of 172-174 °C and boiling points of 249°C. Ferrocene does not decompose upto 400°C. Ferrocene readily sublimes, especially upon heating in a vacuum.
Chemical Properties of Ferrocene
Ferrocene, with its aromatic-like cyclopentadienyl rings, readily undergoes electrophilic substitution reactions. It also involves in redox reactions, forming the ferricinium ion (blue) upon oxidation.
Nitration and Halogenation of Ferrocene
Direct nitration and halogenation leads to the decomposoition of ferrocene. Nitration and halogenation takes place indirectly as given below-
Carboxylation of Ferrocene
Ferrocene can also be carboxylated as shown below-
Vilsmeir Reaction of Ferrocene
Ferrocene undergoes Vilsmeir reaction to yield ferrocene carboxyaldehyde which is a useful starting for the preparation of other ferrocene derivatives.
Mannich Condensation of Ferrocene
Ferrocene also undergoes Mannich condensation as shown below-
Alkylation of Ferrocene
Alkylation of ferrocene can also be carried out as shown below. Since the presence of an alkyl group increases the electron density on the Cp− ring, hence, in the presence of excess of ligand, the disubstitution of alkyl group also occurs in the same ring.
Friedel Craft Reaction of Ferrocene
Ferrocene readily undergoes acylation as illustrated below-
Ferrocene can also be alkylated in a similar manner.
Structure of Ferrocene
Structure of ferrocene is a highly symmetrical and stable arrangement where the iron atom is effectively coordinated to the π-electron clouds of the two cyclopentadienyl rings and the hydrogen atoms are slightly tilled towards the iron atom.
Ferrocene has two isoenergetic conformations viz. eclipsed and Staggered conformation. The point group of eclipsed ferrocene is D5h whereas the point group of staggered ferrocene is D5d possessing a center of inversion. Electronic diffraction and X-ray crystallographic studies show that ferrocene has eclipsed conformation in the gas phase (at high temperature) and staggered conformation in the solid phase (at low temperature). Staggered conformation in the solid phase exist because of the crystal packing forces so that C-C and H-H repulsions between the two rings are minium. The barrier to rotation between the two rings (cyclopentadienyl, Cp) is only 4±1 kJ/mole and this allows the free rotation of rings.
The carbon-carbon bond distances within each cyclopentadienyl ring are equal (around 1.389Å, almost equal to that in benzene), consistent with their aromaticity. The iron-carbon bond distances are also equal (around 2.04 Å) and the distance between two rings is 3.25Å.