pi-pi interactions between pyrylium and phenyl rings in the crystal

The crystal structures of two isomeric phenyl-substituted pyrylium perchlorates, 2,6-dimethyl-4-phenylpyrylium (1) and 2,4-dimethyl-6-phenylpyrylium (2), have been determined. Both cations are planar in the crystal so that the electron deficient pyrylium rings can interact strongly with the phenyl rings of adjacent molecules forming stacks along the shortest crystallographic axis. The phenyl rings may act as donors in 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 these interactions conforming to the model of Hunter and Sanders [1].