Site-Specific Reduction-Induced Hydrogenation of a Helical Bilayer Nanographene with K and Rb Metals: Electron Multiaddition and Selective Rb⁺ Complexation

The chemical reduction of π-conjugated bilayer nanographene 1 (C₁₃₈ H₁₂₀ ) with K and Rb in the presence of 18-crown-6 affords [K⁺ (18-crown-6)(THF)₂ ][{K⁺ (18-crown-6)}₂ (THF)_0.5 ][C₁₃₈ H₁₂₂ ^3- ] (2) and [Rb⁺ (18-crown-6)₂ ][{Rb⁺ (18-crown-6)}₂ (C₁₃₈ H₁₂₂ ^3- )] (3). Whereas K⁺ cations are fully solvent-separated from the trianionic core thus affording a "naked" 1^{.3 -} anion, Rb⁺ cations are coordinated to the negatively charged layers of 1^{.3 -} . According to DFT calculations, the localization of the first two electrons in the helicene moiety leads to an unprecedented site-specific hydrogenation process at the carbon atoms located on the edge of the helicene backbone. This uncommon reduction-induced site-specific hydrogenation provokes dramatic changes in the (electronic) structure of 1 as the helicene backbone becomes more compressed and twisted upon chemical reduction, which results in a clear slippage of the bilayers.