How charging corannulene with one and two electrons affects its geometry and aggregation with sodium and potassium cations

Bowl-shaped mono- and dianions are prepared by reduction of corannulene (C₂₀H₁₀, 1) with sodium and potassium metals in the presence of [18]crown-6 ether. Single-crystal X-ray diffraction studies of two sodium salts, [Na(THF)₂([18]crown-6)]⁺[1^-] (2 a) and [Na([18]crown-6)]⁺[1^-] (2 b), reveal the presence of naked corannulene monoanions 1^- in both cases. In contrast, the potassium adduct, [K([18]crown-6)]⁺[1^-] (3), shows an η²-binding of the K⁺ ion to the convex face of 1^-. For the first time, corannulene dianions have been isolated as salts with sodium, [Na₂([18]crown-6)]²⁺[1^2-] (4 a) and [Na(THF)₂([18]crown-6)]⁺[Na([18]crown-6)] ⁺[1^2-] (4 b), and potassium counterions, [K([18]crown-6)]₂ ⁺[1^2-] (5). Their structural characterization reveals geometry perturbations upon addition of two electrons to a bowl-shaped polyarene. It also demonstrates η⁵- or η⁶-binding of metals to the curved carbon surface of 1^2-, depending on the crystallization conditions. Both mono- and doubly-charged corannulene bowls show the preferential exo binding of Na ⁺ and K⁺ ions in all investigated compounds. Various types of C-H⋯π interactions are found in the crystals of 2-5. The UV/Vis, ESR, and ¹H NMR spectroscopic studies of 2-5 indicate different coordination environment of corannulene anions in solution, depending on the metal ion.