Diels-Alder Reactions of 3-Ferrocenyl-2,4,5-triphenylcyclopentadienone: Syntheses and Structures of the Sterically Crowded Systems C6Ph5Fc, C7Ph6FcH, and [C7Ph6FcH][SbCl6]

Diels-Alder addition of diphenylacetylene or of 1,2,3-triphenylcyclopropene to 3-ferrocenyl-2,4,5-triphenylcyclopentadienone yields, upon thermolysis, C6Ph5Fc (5) or C7Ph6FcH (8), respectively. Subsequent treatment of 8 with triethyloxonium hexachloroantimonate results in the formation of the ferricinium complex [C7Ph6FcH]⁺[SbCl6]^- (13), rather than the anticipated tropylium cation [C7Ph6Fc]⁺. The substituted ferrocene derivatives 5, 8, and 13 have been characterized by X-ray crystallography. From the solid-state structure of 5 it is evident that the peripheral aryl substituents do not adopt a regular propeller type conformation, but instead exhibit an incremental progression of twist angles relative to the central ring. The dynamics of peripheral ring rotations in 5 and 8 have been studied by variable-temperature NMR.