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 (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.