Reduction-Induced C─C Cleavage and Site-Specific Hydrogenation of a Highly Strained Bilayer Spironanographene

The chemical reduction of a bilayer spironanographene, spiro-NG (C₁₃₇H₁₂₀), with Na and K metals in the presence of [2.2.2]cryptand to yield [Na⁺(2.2.2-cryptand)](C₁₃₇H₁₂₁ ^-) (1) and [K⁺(2.2.2-cryptand)](C₁₃₇H₁₂₁ ^-) (2), respectively, is reported. X-ray crystallography reveals the formation of a new "naked" anion (spiro-NG_H ^-), in which spirocyclic ring cleavage and subsequent hydrogenation have occurred. Density Functional Theory (DFT) calculations suggest that the generation of the radical anion of the parent nanographene (spiro-NG^{• -}), upon electron acceptance from Na and K metals, induces the cleavage of the strained spirobifluorene core. The resulting spin density localizes on a particular carbon atom, previously attached to the spiranic sp³ carbon atom, facilitating a site-specific hydrogenation to afford (spiro-NG_H ^-). The electrostatic potential map of this anion reveals electron density concentrated at the five-membered ring of the readily formed indenyl fragment, thus enhancing the aromaticity of the system. Furthermore, nuclear magnetic resonance (NMR) and UV-vis absorption spectroscopy experiments allowed to follow the in situ reduction and hydrogenation processes in detail.