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A cyclopentadienyl ligand has five molecular orbitals available for interaction with metal orbitals of suitable symmetry. Fig, 1 shows a qualitative bonding scheme for ferrocen e^ ^ . Iron-cyclopentadiene bonding

orbital energy Fe atom -*x ferrocene 2n 3n 11 -àn. In 5n 2 C5H5 ligands Ô7C 57t 45 In J L jL jL J ! _ / 1 1 4n LUMO HOMO \ 11 11 5n \ 1 1 in

\\JLiL3zl

1LJl3n

Figure 1.1 : A qualitative bonding scheme for (Tj^~C^H^)2Fe

ligands and 3d, 4s and 4p atomic orbitals of iron atom. The orbitals that have the same symmetry, good spatial overlap and similar energies interact strongly. Different possible combinations of the ten 2pz atomic orbitals of the Cp ligands are denoted as Itc, 2tc, 3tc, 4 k , 5 k , and 6tc,

sometimes also named as aig, ejg, ej^, C2g and 6 2 u ; each iron

valence atomic orbital is also classified as having one of these symmetry types. When iron and Cp ligand orbitals interact they yield the molecular orbitals of ferrocene, shown in the centre of Fig.l. The HOMO and LUMO in this figure have essentially d'-character.

1.1,4 Chemistry of Ferrocene.

Ferrocene is the most stable and perhaps the most well documented member of the metallocene series. It is very stable towards the attack of air, moisture, and most acids and bases. The cyclopentadienyl rings in ferrocene have essentially aromatic character and behave much like electron-rich arene s. Most of the chemistry of ferrocene and its derivatives can be predicted on this basis. Ferrocene can undergo Friedel Crafts acy latio n ,su lfo n atio n and metallation by butyllithium.28 in Friedel-Crafts acylation it reacts about three million times faster than benzene.9 The effects of ring substituents on ferrocene have been reviewed by Slocum and Emst.29 Introduction of an electron-donating substituent activates the molecule towards electrophilic substitution, preferably at the I'-position, whereas an electron-withdrawing substituent has the opposite effect. It is interesting to note here that as substitution on one ring has its effect on the substitution of the other ring, this means that

some electronic effect must be passed from one ring to the other through iron.

The ferrocenyl group is one of the strongest inductive electron- releasing agents; this is emphasised by the fact that aminoferrocene is found to be a stronger base than aniline by a factor of 2 0 , while

ferrocenecarboxylic acid is a weaker acid than benzoic acid. Ferrocene can be oxidised reversibly, either electrochemically or by oxidising agents such as iodine (and reduced back by common reducing agents such as sodium hydrogen sulfite or ascorbic acid), to give the green-blue paramagnetic ferricenium or ferrocenium cation Cp2Fe+. The standard

electrode potential of Cp2Fe/Cp2Fe+ in dichloromethane is 0.56V. The

Cp2Fe/Cp2Fe+ couple is now used as a standard in cyclovoltametric

experiments. A number of ferrocene derivatives are shown in Scheme 1. Acknowledging the enormous amount of research on ferrocene and related compounds and exponential increase in ferrocene-containing compounds the Journal o f Organometallic Chemistry has published an annual review from the mid seventies to the early nineties on this subject.20 The chemistry of ferrocene is also dealt with on a regular basis in Gmelin’s Handbiich.^i Some of the reactions of ferrocene that are exploited in this thesis are briefly discussed here.

^ ^C H O ^ y - C O R ^ ^ -S O a H Fe Fe Fe Fe Fe Fe COOH Fe Fe 0 0 ^ CHgOH CISO3H A1C13 PhNMeCHO 2BuLi _BuLi TMEDA TMEDA Hg(OAc)2 MeaSiCI MEDA H3PO4 1) B(OMe)3 CH-NRp _Reduction Fe Fe Fe Chealatlng ligand for chiral synthesis and catalysts

(Scheme 1)

1.1.4.1 Mannich Reaction (Aminomethylation).

This reaction is demonstrated by reactive aromatic compounds such as phenol and thiophen. In this reaction an aromatic entity condenses with formaldehyde and amine. Benzene does not undergo this reaction; however, ferrocene on condensation with formaldehyde and dimethylamine gives dimethylaminomethylferrocene^^ (Equation 1.1). That can be used in preparation of many ferrocene derivatives. This reaction also demonstrates that ferrocene is more reactive than benzene towards electrophilic substitution.

CH 2"NM8 2

ft HCHO.HNMe 2 ^ (1.1)

HOAC ^

1.1.4.2 Friedel Crafts Acylation.

The first Friedel Crafts acylation reaction on ferrocene was reported by Woodward et to demonstrate its aromatic character. In this type of reaction ferrocene is reacted with an acyl chloride in the presence of a Lewis acid such as AICI3 or FeClg (Equation 1.2). Ferrocene undergoes

mono-acylation on the first ring about 3x10^ times faster than benzene; however Friedel Crafts acylation on the second ring is slower than on the first ring. Acylation on the first ring reduces the reactivity of the second ring towards acylation by a factor of ca. 2xl04. Both aromatic and aliphatic acylation could be carried out on ferrocene under Friedel Crafts condition. I ^ i I Fe ^ A.CI3 ^ O - c ! ft ^ r " + ft ■■ (1.2) R 1.1.4.3 Metallation.

Another reaction characteristic of aromatic systems is metallation. A wide variety of compounds has been synthesised by mono^s or bis lithiation34 of ferrocene. M onolithiation can be achieved by stoichiometric quantities of n-butyllithium or t-butyllithium in hexane/ether solution. Usually, a yield of about 25-30% is obtained, but use of t-butyllithium in THF at 0^ C was reported by Rebiera et to improve the yield up to 70% (Equation 1.3). However, the 1,T-dilithium

species can be formed exclusively by n-butyllithium with TMEDA^^ (Equation 1.4) The effectiveness of the amine comes from the fact that the lithium atom is strongly complexed with diamine, rendering the organic group even more carbanionic; in addition, the amine breaks down the less reactive RLi tetramer.

Li Ç® ---— ► Fe --- ► Many Pl'oducts (1.3) THF.O-C ^ ^ IM E D A Ll^ Fe ---— ► Fe ^ T M E D A--- Many Products (1 .4 )