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 center dot center dot center dot C5F4N center dot center dot center dot Br- interactions that link the cations into chains, N(C)C-H center dot center dot center dot Br- interactions that link the chains into sheets, and N2C-H center dot center dot center dot Br- interactions that link the sheets to one another. As a consequence of these, it is polar (Pna2(1)). Density functional theory calculations indicate that the strength of the interaction between a cation and a bromide anion lies in the order N2C-H center dot center dot center dot Br- > N(C)C-H center dot center dot center dot Br- > C6H5 center dot center dot center dot C5F4N center dot center dot center dot Br-. Prevention of the N2C-H center dot center dot center dot Br- interaction by substitution of the hydrogen atom with a methyl group leads to dimers linked by two C6H5 center dot center dot center dot C5F4N center dot center dot center dot Br- interactions. Prevention of the N(C)C-H center dot center dot center dot Br- interaction by substitution of the hydrogen with a methyl group permits chains of cations, but because the N2C-H center dot center dot center dot Br- interactions link the chains there are no strong interactions between the sheets. Chains of cations linked by Ar center dot center dot center dot C5F4N center dot center dot center dot Br- interactions also arise when the benzyl group is replaced by 3-phenylbenzyl and 2-naphthylmethyl groups. The former also contains N2C-H center dot center dot center dot Br- and N(C)C-H center dot center dot center dot Br- interactions and is centrosymmetric. The latter does not contain N(C)C-H center dot center dot center dot Br- interactions and is chiral and polar (P2(1)). Exchanging the positions of the aryl and polyfluoroaryl groups results in a crystal structure with no pi-pi stacking between the aryl and polyfluoroaryl groups although N2C-H center dot center dot center dot Br- and N(C)C-H center dot center dot center dot Br- interactions persist.