There have been a few attempts to synthesize cyclo[6]carbon, e.g. by pyrolysis of mellitic anhydride,[2] but without success until 2023, when it was successfully synthesized by atom manipulation of hexachlorobenzene.[1]
Calculations suggest that the alternative cycliccumulene structure, called cyclohexahexaene, is the potential energy minimum of the cyclo[6]carbon framework.[3]
^Fields, Ellis K.; Meyerson, Seymour (October 1966). "Arynes by Pyrolysis of Acid Anhydrides". J. Org. Chem. 31 (10): 3307–3309. doi:10.1021/jo01348a046.
^Zahradník, Rudolf; Hobza, Pavel; Burcl, Rudolf; Andes Hess, B. (October 1994). "Strained unsaturated molecules. Theoretical study of acyclic and cyclic cumulenes and acetylenes". Journal of Molecular Structure: THEOCHEM. 313 (3): 335–349. doi:10.1016/0166-1280(94)85015-1.