Novel Carbocyclic Dianions: NMR Study of Charge Delocalization, Paratropicity, and Structure in the Dianions of Acephenanthrylene and Aceanthrylene

The mode of electron delocalization of novel polycyclic dianions, viz. acephenanthrylene dianion (2^-1) and aceanthrylene dianion (3^2_), is deduced from their¹H and¹³C NMR parameters (ID and 2D NMR). While the reduction of acephenanthrylene (2) afforded only the respective dianion (2²“), the reduction of aceanthrylene (3) afforded dianion (3²') followed by monoanion (4). The electron delocalization of the neutral systems (2, 3) as well as the respective dianions (2²~, 3^2_) is discussed. It is concluded that there exists a preferred path of electron delocalization which dominates. Interestingly, the paths of electron delocalization of the charged systems differ from those of the neutral systems. In the neutral systems 1, 2, and 3 the “aromatic” structures that dominate are lb, 2b, and 3b, respectively, whereas the bridge double bond represents only a small perturbation. On the other hand in the dianions l²”, 2^2-, and 3²” the bridges are part of the path of the electron delocalization. These differences of the paths of electron delocalization are accompanied by structural changes as manifested by the X-ray structures and by the coupling constants of the bridge protons and corroborated by calculations. It is demonstrated that in each case the path of electron delocalization that dominates is the one which has the minimum paratropic contribution.