The Synthesis, Purification, and Characterization of Ferrocene Dr. Nisha Sharma

Ferrocene The compound Fe(ɳ5-C5H5)2, bis (cyclopentadienyl) iron, popularly known as ferrocene was discovered in 1951. It has famous sandwich structure.

Preparation of Ferrocene By treating iron(II) chloride with grignard reagent 2 (C5H5)-MgBr + FeCl2 → (C5H5)-Fe-( C5H5) + MgBr2 + MgCl2 By treating iron halide with sodium cyclopentadienide (inTHF) C5H6 + Na → C5H5Na + ½ H2 2C5H5Na + FeCl2 → (C5H5)-Fe-( C5H5) + 2NaCl By reaction of iron halide with cyclopentadiene in presence of strong base 2C5H6 + FeCl2 + 2(C5H5)2NH → Fe (C5H5)2 + 2(C5H5)2NH2Cl Laboratory preparation

Experimental Details Cyclopentadiene, is obtained from a light oil derived from coal tar distillation. Because Cyclopentadiene is such a reactive system, at Room Temperature it exists as a stable dimer, Dicyclopentadiene. This is a Diels-Alder adduct of two molecules of the diene. Thus, generation of Cyclopentadiene involves heating the dimer to initiate a "retro-" or "reverse-Diels-Alder" reaction.

Cracking of dicyclopentadiene Thermometer 5. Condenser 2. Vigreux column 6. Nitrogen bubbler 3. Heating mantle 7. Variac 4. 3 Necks round bottom flask 8. Ice-water 9 If the distillation is delayed and the actual preparation of ferrocene Cracking of dicyclopentadiene

1. 100-mL 3 necks round bottom flask. 2. Dropping funnel with pressure-equalization arm. 3. Nitrogen bubbler. 4. T connection

Description The preparation of ferrocene in a teaching laboratory setting can be accomplished by the use of potassium hydroxide with cyclopentidiene, to form potassium cyclopentidiene, which is then reacted with Iron (II) chloride and the resulting slurry purified by sublimation. The final product obtained is then characterized using FTIR and melting point information.

Chemical reactions of ferrocene Acetylation

Mechanism of Acetylation The electrophile first coordinate with metal atom and oxidise it from iron (II) to iron (III). Then it is transferred to C5H5 ring. A proton is expelled from C5H5 ring and iron (III) is reduced to iron (II).

Reaction with mercuric acetate

Friedel Craft’s alkylation Mannich condensation

Sulphonation Reaction with alkyl lithium

Structure of Ferrocene The staggered configuration is due to crystal packing forces so that carbon carbon and hydrogen hydrogen repulsions between the two rings are minimum.

Ferrocene: the cyclopentadienyl anion ligand Ferrocene contains the cyclopentadienyl anion ligand, (Cy-) which contributes five electrons for the 18-electron rule, which is to be expected from the presence of two double bonds (4 electrons) and a negative charge (1 electron). The anion is stable because it is aromatic, which requires 4n + 2 electrons in the π–system. Cy- has 5 electrons in the π–system from the five sp2 hybridized C-atoms, plus one from the negative charge, giving six electrons in the π–system. Cyclopentadienyl anion (Cy-)

Ferrocene: the cyclopentadienyl ligand Ferrocene is a remarkable molecule. It can be sublimed without decomposition at 500 ºC. The 18-electron rule works for ferrocene as follows: Fe(0): d8 2 Cy- 10e 18e Ferrocene =‘sandwich compound’

Valence Bond Theory Explain diamagnetic character and stable 18 electron outer electronic configuration of molecule.

Molecular Orbital Theory Group theory can then be employed (cf. Cotton) to determine how these MOs interact with the orbitals of the valence shell of the Fe2+ atom to give MOs of the entire molecule with the correct molecular symmetry, D5d. For example, the interaction of the dyz orbital of the metal with E1 MOs of the rings is as pictured.

Ferrocene is an extremely stable complex, stable in Air to temperatures of 500oC, because its 18 valence electrons occupy only bonding and non-bonding MOs. Twelve of these electrons are contributed to the structure by the cyclopentadienyl rings and occupy the lowest six Ferrocene MOs. The six electrons of the Fe2+ contribute to the Ferrocene metal-like MOs. The strictly anti-bonding MOs of Ferrocene are not occupied

Carbonyl Complexes (CO) Bonding

Characteristics of CO complexes Infrared Spectroscopy Free CO stretch n = 2143 cm-1 Cr(CO)6 CO stretch n = 2000 cm-1 because p-back donation from metal weakens the CO bond by adding e- to antibonding p* orbital Negative charge on complex further weakens CO bond: [V(CO)6]- n = 1858 cm-1 [Mn(CO)6]+ n = 2095 cm-1 d) Bridging CO further weakened by extra p-back donation (e- count = 1/M) X-Ray Crystallography Free CO bond length = 112.8 pm M—CO carbonyl bond length = 115 pm

Synthesis and Reactions of CO Complexes Carbonyl complexes of most metals exist. Most obey the 18-electron rule Bridging decreases down the periodic table as d-orbitals become larger. Synthesis Direct reaction: Ni + 4 CO Ni(CO)4 Toxic, used to purify Ni Reductive Carbonylation: CrCl3 + CO + Al Cr(CO)6 + AlCl3 Thermal/Photochemical: 2 Fe(CO)5 + hn Fe2(CO)9 + CO Reactions: useful for the synthesis of other compounds by substitution of CO Cr(CO)6 + PPh3 Cr(CO)5(PPh3) + CO Re(CO)5Br + en Re(CO)3(en)Br + 2 CO