Longest C─C Single Bond among Neutral Hydrocarbons with a Bond Length beyond 1.8 Å

Based on the intramolecular "core-shell strategy", dihydropyracylene with two spiro(dibenzocycloheptatriene) units was designed, for which theoretical calculations predicted a very weak C-C bond with a bond length beyond 1.8 Å. This bond is expanded by the forced adoption of an eclipsed conformation and by angle strain through a "scissor effect". The highly strained hydrocarbon was isolated as a thermally stable compound with no signs of diradical contribution because the weak C-C bond (core) is protected by the shape-persistent fused-ring structure (shell). A Raman shift corresponding to the C-C stretching vibration (587 cm-1) is much different from that for ethane (993 cm-1). The bond length determined by X-ray [1.806(2) Å] is greater than both the shortest non-bonded intramolecular C…C contact [1.80(2) Å] and theoretically predicted limit for C-C bond (1.803 Å). The long-assumed linear correlation between bond length and bond-dissociation energy for covalent bonding is invalid in greater bond lengths.