DIANIONS AND TETRAANIONS OF BOWL-SHAPED FULLERENE FRAGMENTS DIBENZO[A,G]CORANNULENE AND DIBENZO[A,G]CYCLOPENTA[KL]CORANNULENE

The C28H14 and C30H14 polycyclic aromatic hydrocarbons dibenzo[a,g]cor annulene (3) and dibenzo[a,g]cyclopenta[kl]corannulene (4) both contai n a central corannulene ring system fused on two sides by benzene ring s. Hydrocarbon 4 also has a second five-membered ring in the form of a n etheno bridge on the corannulene. Two-electron reduction of these co rannulene derivatives with lithium metal produces stable solutions of purple dianions that are amenable to study by H-1, C-13, and Li-7 NMR spectroscopy. The dianion of dibenzo[a,g]corannulene (3(2-)) is found to be paratropic, though less so than the dianion of the parent corann ulene 1(2-), whereas the dianion of dibenzo[a,g]cyclopenta[kl]corannul ene (4(2-)) is found to be diatropic. Analogous results were obtained when the two-electron reduction was carried out with potassium metal. Further reduction of the dianion with lithium metal gave the correspon ding trianion radicals but, in contrast to the reduction of unsubstitu ted corannulene with lithium, could not be pushed beyond that stage to the tetraanions. However, with potassium metal, which reduces unsubst ituted corannulene only to the trianion radical, both 3 and 4 could be reduced to tetraanions. The tetraanion of dibenzo[a,g]corannulene (3( 4-)) was found to be diatropic, though less so than the tetraanion of corannulene (1(4-)), whereas the tetraanion of dibenzo [a,g]cyclopenta [kl]corannulene (4(4-)) was found to be weakly paratropic. The differe nt pi topologies, charge distributions. and magnetic properties of the se two large fullerene fragments and their derived anions are discusse d.