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(C₆F₅)₅]⁺[Sb₃F₁₆]⁻. Although the aromatic triplet state of the [Cp(C₆F₅)₅]⁺ cation is energetically favoured in the gas phase according to quantum chemical calculations, coordination of the cation by either [Sb₃F₁₆]⁻ or C₆F₆ 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(C₆F₅)₅]⁺ cation are higher than those of the perfluorinated tritylium cation [C(C₆F₅)₃]⁺. Reactions of [Cp(C₆F₅)₅]⁺[Sb₃F₁₆]⁻ with CO, which probably yields the corresponding carbonyl complex, and of radical Cp(C₆F₅)₅∙ with selected model substrates (Cp₂Fe, (Ph₃C∙)₂ and Cp*Al) are also presented.