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

Two corannulenes joined by an octamethylene chain as in 1,8-dicorannulenylo ctane (3) can be reduced with lithium, sodium, potassium or cesium in [D-8] THF to give a purple tetraanion. In this state, each corannulene moiety beh aves as an independent, strongly paratropic dianion that is best described as an aromatic, 6 pi-electron, cyclopentadienyl anion in the core, suspende d within an antiaromatic, 16 pi-electron, [15]annulenyl anion around the ri m. The bowl-to-bowl inversion barrier of the tethered corannulene dianions was determined by variable temperature H-1 NMR spectroscopy to be 8.8 +/- 0 .3 kcal mol(-1) with potassium counter ions and 9.2 +/- 0.3 kcal mol(-1) wi th cesium counter ions. It is the first time that the barrier for a bowl-sh aped charged species was determined. As expected, charging reduces the ener gy barrier for bowl-to-bowl inversion relative to that in the neutral hydro carbon (10.9 +/- 0.3 kcal mol(-1)). Further reduction of the hydrocarbon wi th lithium, potassium or cesium gives an octaanion, but no further reductio n could be achieved with an excess of sodium metal. The bowl-to-bowl invers ion barrier of the octaanion is too low to be determined by variable temper ature H-1 NMR spectroscopy. Ion diffusion measurements and the failure of 3 to form mixed sandwich compounds with corannulene (1) when it is reduced i n the presence of an excess of corannulene are consistent with intramolecul ar stacking (rather than intermolecular stacking) of the two tethered coran nulene tetraanions to form lithium ion-bound sandwiches of the type seen pr eviously for unsubstituted corannulene.