Dianions and Tetraanions of Bowl-Shaped Fullerene Fragments Dibenzo[a,g]corannulene and Dibenzo[a,g]cyclopenta[kl]corannulene Amir Weitz, Elad Shabtai, Mordecai Rabinovitz,* Matthew S. Bratcher, Casey C. McComas, Michael D. Best, and Lawrence T. Scott* Abstract: The C 28 H 14 and C 30 H 14 poly- cyclic aromatic hydrocarbons diben- zo[a,g]corannulene (3) and diben- zo[a,g]cyclopenta[kl]corannulene (4) both contain a central corannulene ring system fused on two sides by benzene rings. Hydrocarbon 4 also has a second five-membered ring in the form of an etheno bridge on the corannulene. Two- electron reduction of these corannulene derivatives with lithium metal produces stable solutions of purple dianions that are amenable to study by 1 H, 13 C, and 7 Li 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 corannulene 1 2 , whereas the dianion of dibenzo[a,g]cy- clopenta[kl]corannulene (4 2 ) is found to be diatropic. Analogous results were obtained when the two-electron reduc- tion was carried out with potassium metal. Further reduction of the dianion with lithium metal gave the correspond- ing trianion radicals but, in contrast to the reduction of unsubstituted corannu- lene with lithium, could not be pushed beyond that stage to the tetraanions. However, with potassium metal, which reduces unsubstituted 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 corannu- lene (1 4 ), whereas the tetraanion of dibenzo[a,g]cyclopenta[kl]corannulene (4 4 ) was found to be weakly paratropic. The different p topologies, charge dis- tributions, and magnetic properties of these two large fullerene fragments and their derived anions are discussed. Keywords: annulenes ´ diatropism ´ magnetic properties ´ paratropism ´ ring currents Introduction Corannulene (1), with its bowl-shaped equilibrium geometry, represents the smallest subunit of C 60 that retains a curved surface. The lowest unoccupied molecular orbital (LUMO) of corannulene lies unusually low in energy and is doubly degenerate; this accounts for the ease with which this C 20 H 10 hydrocarbon can accept up to four electrons. [1] Renewed interest in the bowl-shaped corannulene following the dis- covery of C 60 led to the development of a convenient new synthesis for corannulene and its derivatives [2] as well as to further investigation of their electronic and geometric struc- ture. [3] Our discovery of the tetraanion 1 4 on lithium reduction [1] and its dimerization [4] to 2 prompted further investigation of the various reduction states. [5] Attention turned to the nature of the dianion, being either para- or diatropic, since it had not been detected previously. We were also concerned with the question of the reduction stage in which the formation of dimers occurs. Under the right conditions, [5] the dianion 1 2 was observed by 1 H NMR spectroscopy, in the form of one line at d  5.6. [5] This significant paratropic shift confirms its annulene nature. [4] A reduction path for corannulene was also suggest- ed, indicating that the formation of the dimer takes place only at the tetraanionic stage of the reduction. [5] The questions that arose from our work on the anions of corannulene (1) and other anions are: a) to what extent does [*] Prof. M. Rabinovitz, A. Weitz, E. Shabtai Department of Organic Chemistry The Hebrew University of Jerusalem Givat Ram, Jerusalem 91904 (Israel) Fax: Int. code  (972) 2 65-27547 Prof. L. T. Scott, M. S. Bratcher, C. C. McComas, M. D. Best Department of Chemistry, Boston College Eugene F. Merkert Chemistry Center Chestnut Hill, MA 02167 3860 (USA) FULL PAPER  WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1998 0947-6539/98/0402-0234 $ 17.50+.50/0 Chem. Eur. J. 1998, 4, No. 2 234