Probing Effects of Electron Addition to Polycyclic Conjugated Hydrocarbons Containing Four-Membered Rings

The rectangular cyclobutadiene (CBD, C₄H₄) is a unique moiety for building nonbenzenoid polycyclic conjugated hydrocarbons with interesting electron-accepting properties. Herein, the investigation on chemical reduction of several CBD-containing polycyclic hydrocarbons with increasing conjugation length is reported: biphenylene (C₁₂H₈), dimethyl[2]naphthalene (C₂₂H₁₆), and tetramethyl-dibenzo-[3]phenylene (C₃₀H₂₂). The two-step sequential reduction is first demonstrated by in situ spectroscopic investigation and then confirmed by the isolation of single crystals of the reduced products. The X-ray crystallographic analysis reveals the formation of several mono- and doubly reduced products in solvent-separated and complexed forms. The crystal structures for both neutral parents and corresponding reduced products unravel the changes in bond alternation in each ring of the fused systems. Density functional theory (DFT) and nucleus-independent chemical shift (NICS) scan calculations reveal that the two-electron addition reduces the aromatic character in the benzenoid rings but has minor influence on the antiaromatic CBD rings.