Structural characterization and reactivity of a room-temperature-stable, antiaromatic cyclopentadienyl cation salt

The singlet states of cyclopentadienyl (Cp) cations are considered as true prototypes of an antiaromatic system. Unfortunately, their high intrinsic reactivity inhibited their isolation in the solid state as a salt, and controlled reactions are also scarce. Here we present the synthesis and solid state structure of the room-temperature-stable Cp cation salt [Cp(C 6 F 5 ) 5 ] + [Sb 3 F 16 ] − . Although the aromatic triplet state of the [Cp(C 6 F 5 ) 5 ] + cation is energetically favoured in the gas phase according to quantum chemical calculations, coordination of the cation by either [Sb 3 F 16 ] − or C 6 F 6 in the crystal lattice stabilizes the antiaromatic singlet state, which is present in the solid state. The calculated hydride and fluoride ion affinities of the [Cp(C 6 F 5 ) 5 ] + cation are higher than those of the perfluorinated tritylium cation [C(C 6 F 5 ) 3 ] + . Reactions of [Cp(C 6 F 5 ) 5 ] + [Sb 3 F 16 ] − with CO, which probably yields the corresponding carbonyl complex, and of radical Cp(C 6 F 5 ) 5 ∙ with selected model substrates (Cp 2 Fe, (Ph 3 C∙) 2 and Cp*Al) are also presented.