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. Due to their high reactivity, their isolation in the solid state as a salt has so far failed. We present here the synthesis of the first room temperature-stable Cp cation salt Cp(C6F5)5Sb3F16 (1·Sb3F16) by single electron oxidation of the corresponding Cp radical Cp(C6F5)5∙ (2) with either an excess of XeF2 and SbF5·SO2 or by hydroxide abstraction from Cp(C6F5)5OH (D) with SbF5·SO2 in hexafluorobenzene. 1·Sb3F16 was characterized by sc-XRD, SQUID, UV-vis, and EPR spectroscopy. Although the aromatic triplet state of the Cp(C6F5)5 cation 1 is energetically favored in the gas phase according to quantum chemical calculations, the coordination of the cation by either Sb3F16⁻ (1a·Sb3F16) or C6F6 (1b·Sb3F16) in the crystal lattice stabilizes the antiaromatic singlet state, which is present in the solid state. The calculated hydride and fluoride ion affinities of 1 are higher than those of the tritylium cation C(C6F5)3+. In addition, results from reactions of 1·Sb3F16 with CO, which most likely yields the corresponding carbonyl complex, and 2 with selected model substrates (Cp2Fe, (Ph3C∙)2, and Cp*Al) are presented.