PI-PI INTERACTIONS BETWEEN PYRYLIUM AND PHENYL RINGS IN THE CRYSTAL

The crystal structures of two isomeric phenyl-substituted pyrylium per chlorates, 2,6-dimethyl-4-phenylpyrylium (1) and 2,4-dimethyl-6-phenyl pyrylium (2), have been determined. Both cations are planar in the cry stal so that the electron deficient pyrylium rings can interact strong ly with the phenyl rings of adjacent molecules forming stacks along th e shortest crystallographic axis. The phenyl rings may act as donors i n a charge transfer interaction which is more pronounced in 1 than in 2. Charge transfer appears as a non-mandatory consequence of the pi-pi stacking. The structures demonstrate the electrostatic nature of thes e interactions conforming to the model of Hunter and Sanders [1].