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

Abstract

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 (Pna21). 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 (P21). 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.