Reduced corannulenes: 1,8-dicorannulenyloctane anions, a supramolecular octaanion

Abstract

Two corannulenes joined by an octamethylene chain as in 1,8-dicorannulenyloctane (3) can be reduced with lithium, sodium, potassium or cesium in [D₈]THF to give a purple tetraanion. In this state, each corannulene moiety behaves as an independent, strongly paratropic dianion that is best described as an aromatic, 6 π-electron, cyclopentadienyl anion in the core, suspended within an antiaromatic, 16 π-electron, [15]annulenyl anion around the rim. The bowl-to-bowl inversion barrier of the tethered corannulene dianions was determined by variable temperature ¹H NMR spectroscopy to be 8.8 ± 0.3 kcal mol⁻¹ with potassium counter ions and 9.2 ± 0.3 kcal mol⁻¹ with cesium counter ions. It is the first time that the barrier for a bowl-shaped charged species was determined. As expected, charging reduces the energy barrier for bowl-to-bowl inversion relative to that in the neutral hydrocarbon (10.9 ± 0.3 kcal mol⁻¹). Further reduction of the hydrocarbon with lithium, potassium or cesium gives an octaanion, but no further reduction could be achieved with an excess of sodium metal. The bowl-to-bowl inversion barrier of the octaanion is too low to be determined by variable temperature ¹H NMR spectroscopy. Ion diffusion measurements and the failure of 3 to form mixed sandwich compounds with corannulene (1) when it is reduced in the presence of an excess of corannulene are consistent with intramolecular stacking (rather than intermolecular stacking) of the two tethered corannulene tetraanions to form lithium ion-bound sandwiches of the type seen previously for unsubstituted corannulene.