Ferrocene - Derivatives and Variations

Derivatives and Variations

Ferrocene analogues can be prepared with variants of cyclopentadienyl. For example, bisindenyliron and bisfluorenyliron.

Carbon atoms can be replaced by heteroatoms as illustrated by Fe(η5-C₅Me₅)(η5-P₅) and Fe(η5-C₅H₅)(η5-C₄H₄N) ("azaferrocene"). Azaferrocene arises from decarbonylation of Fe(η5-C₅H₅)(CO)₂(η1-pyrrole) in cyclohexane. This compound on boiling under reflux in benzene is converted to ferrocene.

Because of the ease of substitution, many structurally unusual ferrocene derivatives have been prepared. For example, the penta(ferrocenyl)cyclopentadienyl ligand, features a cyclopentadienyl anion derivatised with five ferrocene substituents.

In hexaferrocenylbenzene, C₆₆, all six positions on a benzene molecule have ferrocenyl substituents (R). X-ray diffraction analysis of this compound confirms that the cyclopentadienyl ligands are not co-planar with the benzene core but have alternating dihedral angles of +30° and −80°. Due to steric crowding the ferrocenyls are slightly bent with angles of 177° and have elongated C-Fe bonds. The quaternary cyclopentadienyl carbon atoms are also pyramidalized.

The synthesis of hexaferrocenylbenzene has been reported using Negishi coupling of hexaiodidobenzene and diferrocenylzinc, using tris(dibenzylideneacetone)dipalladium(0) as catalyst, in tetrahydrofuran:

The yield is only 4%, which is further evidence consistent with substantial steric crowding around the arene core.