Mono- and bis(allenes) with three to seven ferrocenyl substituents

Tetraferrocenylallene is synthesized in 49% yield from triferrocenylallenylium 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 and 1,1'-bis(triferrocenylallenyl)ferrocene, indicating the involvement of allenyl radicals in the course of this reaction. X-ray crystal structure analyses show these allenes to be sterically congested cumulenes with interesting helical propeller conformations. Tetraferrocenylallene can be reversibly oxidized in three consecutive steps to the tetracation, as shown by low-temperature cyclic voltammetry. The expected nucleophilic reactivity of the central carbon of the allenic unit is hampered by the steric bulk of the metallocenyl substituents and only observed for the smallest electrophile possible, H+, which yields a rather labile tetraferrocenylallylium cation.