Study Notes On Ferrocene(Chemical Science)

Study Notes On Ferrocene

Ferrocene is an orange, crystalline solid that exists in the monoclinic form at room temperature. Between 110 K and 164 K, it is triclinic, while below 110 K, it exists in orthorhombic form. It melts at 446 K, sublimes at 673 K and decomposes above 743 K. It is diamagnetic in nature, and dissolves in the most organic solvents but is insoluble in water. It remains stable in the air.

Structure: In ferrocene, the iron atom is sandwiched between the two cyclopentadienyl rings. The ᴨ electron cloud of each ring is delocalized over all five carbon atoms. The two cyclopentadienyl rings have staggered configurations relative to each other. In the gaseous state, the two cyclopentadienyl groups acquire eclipsed configuration with respect to each other. The energy barrier to ring rotation is very small.

Bonding: With the help of an electrostatic model, the sequence of the chemically relevant frontier orbitals of metallocenes with axial symmetry can be easily obtained. According to this model, the crystal field generated by two negatively charged Cp rings splits the energy levels of the metal d orbitals. The reason behind the splitting is the presence of unequal electrostatic repulsions between the metal d electrons and the charge loops. Refer to the figure given below:

So, the correct electronic configuration of the frontier orbital region in the ferrocene is (e2)4(a1)2(e1)0. In the chemically relevant frontier orbital region, the order is e2 < a1 < e1. This order is the same as obtained from the crystal field model. The energies of these frontier orbitals with reference to the basic orbitals can be regarded as bonding (e2), nonbonding (a1) and antibonding (e1). When oxidation of ferrocene to ferrocenium ion takes place, no significant change will be observed in bond length as the frontier orbitals are neither strongly bonding nor strongly antibonding. Also, the proximity of the frontier orbitals among themselves indicates that metallocenes with 17, 19 or 20 electrons can also exist with stabilities not too far off from the 18 electron metallocenes. By considering the chemical structure of ferrocene, its redox property can be understood; ferrocene can be understood in terms of its chemical structure and from the orbital point of view; oxidation corresponds to the removal of an electron from a nonbonding a1 orbital. So, due to this, bond lengths can be correlated with the electronic occupancy in the bonding and antibonding orbitals. Ferrocene gets readily oxidized to the ferrocenium ion even by I2, so the electrophile reacting with ferrocene should not be an oxidizing agent.

As compared to other organometallic compounds, ferrocene is inexpensive, has high thermal stability and high tolerance to oxygen as well as moisture. In reactions, it behaves like an electron-rich compound. Also, it can stabilise and stabilize carbocations at their benzylic-like position are properties which pave the way to preparing functionalized substituted ferrocenes. A major interest in ferrocene chemistry is centred around developing chiral chelating ligands with planar and lateral chirality and their use as prochiral ligands in asymmetric catalysis. The use of ferrocenophanes as precursors for ferrocene-based polymers is another important application.

As compared to benzene, toluene, or phenol, the 6π electron system of ferrocene is more electron-rich due to which it can undergo aromatic substitution reactions rather easily. In the Friedel-Crafts acetylation, ferrocene is 3.3 × 106 times more reactive than benzene and about 1 × 103 times more reactive than mesitylene. Friedel-Crafts acetylation which uses acetyl chloride and aluminium chloride results in the formation of the 1,1′-diacetyl compound. Only one isomer will be formed which points to free rotation of the Cp rings around Fe. In the presence of an excess acetylating agent, 1,1′ and 1,2-derivatives (same ring substitution) are formed in a 60: 1 ratio. This is because deactivation of the Cp ring will take place once an acetyl group is substituted. Mono acetylation of ferrocene will take place under mild conditions or when acetic anhydride and phosphoric acid are used.

The most widely used derivatives are alkynyl ferrocene and N, N-dimethyl-1-ferrocenylethylamine (Ugi’s amine). This compound was first prepared by Ivar Ugi, opening diverse synthetic routes for introducing both planar and central chirality on ferrocene derivatives.

When ferrocene undergoes Friedel crafts type reactions with AlCI3 and phenylphosphorus chlorides result in the formation of air-stable ferrocene derived tertiary phosphines. Ferrocene undergoes a wide variety of electrophilic substitution reactions. Electrophiles should not be oxidizing in nature as ferrocene is prone to oxidation and the oxidized Cp2Fe+ (ferrocenium cation) will repel the electrophile. Therefore, nitration, halogenation and similar reactions cannot be carried out directly on ferrocene. The 17-electron ferrocenium ion complex is an excellent radical trap. This unique property enables the ferrocene to stabilize an adjacent carbocation centre. It is mostly prepared in situ by reagents such as I2 or H2SO4, air-stable ferrocenium salts with an intense blue colour can readily be isolated by the reaction of ferrocene with oxidants such as p-benzoquinone or FeCI3.