Bowl-shaped carbocations: Easy to produce, hard to reduce

Stable nonplanar polyaromatic carbocation, [C ₂₀H ₁₀CH ₂Cl] ⁺ (1) having the chloromethyl group covalently bound to the hub site of corannulene was prepared by reaction of C ₂₀H ₁₀ with CH ₂Cl ₂ and GaCl ₃ in the presence of SnCl ₂. The X-ray diffraction study of the resulting salt, [1]·[(SnCl)(GaCl ₄) ₂], revealed the presence of two rotational isomers for 1 in the crystal lattice. In the solid state, a complex polymeric inorganic anion of the overall composition [(Sn ₂Cl ₂)·(GaCl ₄) ₄] _∞ is formed with the cationic π-bowls being attached to its backbone. Both X-ray and nuclear magnetic resonance spectroscopic data of 1 indicate the sp ³ hybridization of the C-atom of corannulene at the attachment point. An electrochemical investigation of 1 and the series of related corannulene cations, [C ₂₀H ₁₀R] ⁺ (R=CH _xCl _3-x (x=0-3) and CH ₂CH ₂Cl), was carried out to elucidate the electronic consequences triggered by the above changes. This study revealed that the surface decoration of corannulene core makes the reduction of the resulting [C ₂₀H ₁₀R] ⁺ species significantly more difficult (by about 0.3V) with respect to the parent bowl. To evaluate the consequences of the observed one-electron reduction and conceivable but not seen second-electron reduction on the corannulene core, theoretical calculations at the density functional theory level have been carried out.