L7: Cyclopentadienyl ligands and metallocenes

Question 1

Preparation of Cp diene

Answer 1

• Cyclopentadiene is a reactive diene that must be freshly prepared; readily dimerises at room temperature (Diels-Alder reaction)
• Obtained by heating dicyclopentadiene to around 160 degrees
• Acidic hydrogen readily removed

Question 2

Ring whizzing

Answer 2

• Observed in eta-1 Cp ligand
• Very facile and rapid dynamic process, VT-NMR shows that all 5 carbons of the eta-1 Cp ligand take it in turns to be bound to Fe

Question 3

Reactivity of ferrocene

Answer 3

• Metallation
• Electrophilic substitution (Cp rings of ferrocene behave as electron-rich arenes. Fe(Cp)2 undergoes Friedel-Crafts acylation 10^6 times more rapidly than benzene
• However, if electrophile has oxidising properties the substitution fails

Question 4

Pentamethylcyclopentadienyl ligand (qualities)

Answer 4

• Cp* is sterically more demanding than Cp; offers greater steric protection to metal centre
• More electron releasing than Cp (stronger donor)
• The methyl hydrogens of Cp* can show enhanced acidity

Question 5

Indenyl ligand (qualities)

Answer 5

• Can undergo eta5-eta3 hapticity changes very much more readily than Cp or Cp* ligands
• Can switch aromaticity between the two different rings (5 and 6 membered) -> results in rapid rates of ligand substitution reactions in indenyl complexes

Question 6

Ansa-cyclopentadienyl ligands (qualities, 1 use)

Answer 6

• Two Cp-type ligands linked together by a bridging group (handle)
• The ansa bridge allows more control over stereochemistry at the metal centre
• Two ansa-metallocenes used as pre-catalysts for the stereoregular polymerisation of propene