Arene-Perfluoroarene-Anion Stacking and Hydrogen Bonding Interactions in Imidazolium Salts for the Crystal Engineering of Polarity

The crystal structure of 1-(2,3,5,6-tetrafluoropyridyl)-3-benzylimidazolium bromide possesses C₆H₅···C₅F₄N···Br^– interactions that link the cations into chains, N­(C)­C–H···Br^– interactions that link the chains into sheets, and N₂C–H···Br^– interactions that link the sheets to one another. As a consequence of these, it is polar (<i>Pna</i>2₁). Density functional theory calculations indicate that the strength of the interaction between a cation and a bromide anion lies in the order N₂C–H···Br^– > N­(C)­C–H···Br^– > C₆H₅···C₅F₄N···Br^–. Prevention of the N₂C–H···Br^– interaction by substitution of the hydrogen atom with a methyl group leads to dimers linked by two C₆H₅···C₅F₄N···Br^– interactions. Prevention of the N­(C)­C–H···Br^– interaction by substitution of the hydrogen with a methyl group permits chains of cations, but because the N₂C–H···Br^– interactions link the chains there are no strong interactions between the sheets. Chains of cations linked by Ar···C₅F₄N···Br^– interactions also arise when the benzyl group is replaced by 3-phenylbenzyl and 2-naphthylmethyl groups. The former also contains N₂C–H···Br^– and N­(C)­C–H···Br^– interactions and is centrosymmetric. The latter does not contain N­(C)­C–H···Br^– interactions and is chiral and polar (<i>P</i>2₁). Exchanging the positions of the aryl and polyfluoroaryl groups results in a crystal structure with no π–π stacking between the aryl and polyfluoroaryl groups although N₂C–H···Br^– and N­(C)­C–H···Br^– interactions persist