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Keywords:

  • alkali metals;
  • carbanions;
  • corannulene;
  • molecular structure;
  • X-ray diffraction

Abstract

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+[12−] (4 a) and [Na(THF)2([18]crown-6)]+[Na([18]crown-6)]+[12−] (4 b), and potassium counterions, [K([18]crown-6)]2+[12−] (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 12−, 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[BOND]H⋅⋅⋅π interactions are found in the crystals of 25. The UV/Vis, ESR, and 1H NMR spectroscopic studies of 25 indicate different coordination environment of corannulene anions in solution, depending on the metal ion.