Spatial structure of 4nπ helicene dianions

Abstract

Helicene dianions, e.g. phananthrene derivatives, once believed to be non-helical, maintain their chirality as 4nπ dianions. Phenanthrene 5, as well as substituted phenanthrene derivatives, undergo a two-electron reduction to form the respective 4nπ dianion e.g., 5^2–/2Li⁺. Phenanthrene derivatives substituted at the 4- and 5-positions (bay substituents)e.g.6–11, which are helical, afford stable dianions. These dianions are also prepared by a two-electron reduction of the (4n+ 2)π electron hydrocarbons and show, in their ¹H NMR spectra, a quench of the paratropicity compared to 5^2– as well as a line shape dependence on their twist angle. The quench of paratropicity was also observed in the closely related charged helicenes derived from the benzo[g]chrysene system, i.e. anions 12^2– and 13^2–. The twist angles were calculated by MMX and MNDO calculations for the neutral systems, i.e., 5–11, and by MNDO for the dianions. MNDO calculations also included the preferred location of the counter cation. A dynamic NMR spectroscopic study proves experimentally the helicity of anion 11^2– thus shedding light on the behaviour of this novel class of dianions.