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 C 6 H 5 ···C 5 F 4 N···Br – interactions that link the cations into chains, N(C)C–H···Br – interactions that link the chains into sheets, and N 2 C–H···Br – interactions that link the sheets to one another. As a consequence of these, it is polar ( Pna 2 1 ). Density functional theory calculations indicate that the strength of the interaction between a cation and a bromide anion lies in the order N 2 C–H···Br – > N(C)C–H···Br – > C 6 H 5 ···C 5 F 4 N···Br – . Prevention of the N 2 C–H···Br – interaction by substitution of the hydrogen atom with a methyl group leads to dimers linked by two C 6 H 5 ···C 5 F 4 N···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 N 2 C–H···Br – interactions link the chains there are no strong interactions between the sheets. Chains of cations linked by Ar···C 5 F 4 N···Br – interactions also arise when the benzyl group is replaced by 3-phenylbenzyl and 2-naphthylmethyl groups. The former also contains N 2 C–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 ( P 2 1 ). 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 N 2 C–H···Br – and N(C)C–H···Br – interactions persist.