Capturing the Antiaromatic #6094C68 Carbon Cage in the Radio-Frequency Furnace

Although all fullerenes do not satisfy the classical aromaticity condition, as a result of their nonplanar nature, they experience effective stabilization due to extensive cyclic π-electron delocalization and exhibit pronounced "spherical aromaticity". This feature has raised the question of the opposite phenomenon, that is, the existence of antiaromatic carbon cages. Here the first experimental evidence of the existence of antiaromatic fullerenes is reported. The elusive (#6094)C(68) was effectively captured as C(68)Cl(8) by in situ chlorination in the gas phase during radio-frequency synthesis. The chlorinated cage was separated by means of multistage HPLC, and its connectivity unambiguously determined by single-crystal X-ray analysis. Halogen-stripped pristine (#6094)C(68) was monitored by mass spectrometry of the chlorinated C(68)Cl(8) cage. Quantum chemical calculations reveal the highly antiaromatic character of (#6094)C(68), in accordance with all geometric, energetic, and magnetic criteria of aromaticity. Chlorine addition leads to substantial stabilization of the cage owing to aromatization in the resulting C(68)Cl(8), which explains its high abundance in the primary fullerene soot. This work provides new insights into the process of fullerene formation and better understanding of aromaticity phenomena in general.