Tetraferrocenylallene is synthesized in 49% yield from triferrocenylal
lenylium tetrafluorobate by nucleophilic addition of 1-cuprioferrocene
. Other, more conventional approaches failed, due to steric hindrance
or the known reluctance of a diferrocenyl-substituted sp(2) carbon to
undergo condensation reactions. Side products include triferrocenyl(2-
tetrahydrofuranyl)-allene, 1,1,3,4,6,6-hexaferrocenylhexane-1,2,4,5 an
d 1,1'-bis(triferrocenylallenyl)ferrocene, indicating the involvement
of allenyl radicals in the course of this reaction. X-ray crystal stru
cture analyses show these allenes to be sterically congested cumulenes
with interesting helical propeller conformations. Tetraferrocenylalle
ne can be reversibly oxidized in three consecutive steps to the tetrac
ation, as shown by low-temperature cyclic voltammetry. The expected nu
cleophilic reactivity of the central carbon of the allenic unit is ham
pered by the steric bulk of the metallocenyl substituents and only obs
erved for the smallest electrophile possible, H+, which yields a rathe
r labile tetraferrocenylallylium cation.