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.