Conformation and Aromaticity Switching in a Curved Non-Alternant sp2 Carbon Scaffold

Abstract

A curved sp2 carbon scaffold containing fused pentagon and heptagon units (1) was synthesized by Pd-catalyzed [5+2] annulation from a 3,9-diboraperylene precursor and shows two reversible oxidation processes at low redox potential, accompanied by a butterfly-like motion. Stepwise oxidation produced radical cation 1.+ and dication 12+. In the crystal structure, 1 exhibits a chiral cisoid conformation and partial π-overlap between the enantiomers. For the radical cation 1.+, a less curved cisoid conformation is observed with a π-dimer-type arrangement. 12+ adopts a more planar structure with transoid conformation and slip-stacked π-overlap with closest neighbors. We also observed an intermolecular mixed-valence complex of 1⋅(1.+)3 that has a huge trigonal unit cell [(1)72(SbF6)54⋅(hexane)101] and hexagonal columnar stacks. In addition to the conformational change, the aromaticity of 1 changes from localized to delocalized, as demonstrated by AICD and NICS(1)zz calculations.