A. Weitz et al., DIANIONS AND TETRAANIONS OF BOWL-SHAPED FULLERENE FRAGMENTS DIBENZO[A,G]CORANNULENE AND DIBENZO[A,G]CYCLOPENTA[KL]CORANNULENE, Chemistry, 4(2), 1998, pp. 234-239
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.