Mono-reduced Corannulene: To Couple and Not to Couple in One Crystal.

One-electron reduction of corannulene, C20 H10 , with Li metal in diglyme resulted in crystallization of [{Li+ (diglyme)2 }4 (C20 H10 .- )2 (C20 H10 -C20 H10 )2- ] (1), as revealed by single-crystal X-ray diffraction. This hybrid product contains two corannulene monoanion-radicals along with a dianionic dimer, crystallized with four Li+ ions wrapped by diglyme molecules. The dimeric (C20 H10 -C20 H10 )2- anion provides the first crystallographically confirmed example of spontaneous radical dimerization for C20 H10 .- . The C-C bond length between the two C20 H10 .- bowls of 1.588(5) Å is consistent with the single σ-bond character of the linker. The trans-disposition of two bowls in the centrosymmetric (C20 H10 -C20 H10 )2- dimer is observed with the torsion angle around the central C-C bond of 180°. Comprehensive theoretical analysis of formation/decomposition processes of the dimeric dianion has been carried out in order to evaluate the nature of bonding and energetics of the C20 H10 .- coupling. It is found that such σ-bonded dimers are thermodynamically unstable due to large preparation energy and repulsive Pauli component of the bonding, but kinetically persistent due to a high energy barrier provided by the existing spin-crossing point.