Dibenzohomopyracylene, a segment of methanofullerene C61H2, its radical ions and dianion

Dibenzohomopyracylene (2), the 12b,12c-homo derivative of indeno[1,2,3-cd]f luoranthene (1), has been synthesized, Both C-13 NMR and X-ray diffraction data suggest that 2 has a "closed" norcaradiene-like structure (2cl). The r adical ions 2(.+) and 2(.-) were characterized by their H-1 coupling consta nts using ESR, ENDOR, and TRIPLE-resonance spectroscopy. In accord with UB3 LYP/6-31G*//UHF/3-21G* calculations, the radical cation 2(.+) retains the " closed" structure (2cl(.+)), while the structure of the radical anion 2(.-) appears to be "intermediate" (2int(.-)) between the "closed" (2cl(.-)) and the "open" annulene-like (2op(.-)) forms. The C-13 NMR spectrum of dianion 2(2-) is consistent with the "open" structure (2op(2-)). The transition fro m the "closed" to the "open" form in the redox sequence 2cl(.+), 2cl, 2int( .-), 2op(2-)therefore occurs at the radical anion stage. Since 2cl represen ts a segment of the "6-6-closed" methanofullerene (C61H2), the latter may a lso undergo corresponding structural changes in its radical ions and dianio n.