Site-Directed Dimerization of Bowl-Shaped Radical Anions to Form a σ-Bonded Dibenzocorannulene Dimer

Designed site-directed dimerization of the monoanion radicals of a π-bowl in the solid state is reported. Dibenzo[a,g]corannulene (C₂₈ H₁₄ ) was selected based on the asymmetry of the charge/spin localization in the C₂₈ H₁₄^.- anion. Controlled one-electron reduction of C₂₈ H₁₄ with Cs metal in diglyme resulted in crystallization of a new dimer, [{Cs⁺ (diglyme)}₂ (C₂₈ H₁₄ -C₂₈ H₁₄ )^2- ] (1), as revealed by single crystal X-ray diffraction study performed in a broad range of temperatures. The C-C bond length between two C₂₈ H₁₄^.- bowls (1.560(8) Å) measured at -143 °C does not significantly change upon heating of the crystal to +67 °C. The single σ-bond character of the C-C linker is confirmed by calculations. The trans-disposition of two bowls in 1 is observed with the torsion angles around the central C-C bond of 172.3(5)° and 173.5(5)°. A systematic theoretical evaluation of dimerization pathways of C₂₈ H₁₄^.- radicals confirmed that the trans-isomer found in 1 is energetically favored.