Isostructural Co-crystals Derived from Molecules with Different Supramolecular Topologies

In this article, we report the crystal
structures of five halogen
bonded co-crystals comprising quaternary ammonium cations, halide
anions (Cl– and Br–), and one
of either 1,2-, 1,3-, or 1,4-diiodotetrafluorobenzene (DITFB). Three
of the co-crystals are chemical isomers: 1,4-DITFB­[TEA-CH2Cl]­Cl, 1,2-DITFB­[TEA-CH2Cl]­Cl, and 1,3-DITFB­[TEA-CH2Cl]Cl (where TEA-CH2Cl is chloromethyltriethylammonium
ion). In each structure, the chloride anions link DITFB molecules
through halogen bonds to produce 1D chains propagating with (a) linear
topology in the structure containing 1,4-DITFB, (b) zigzag topology
with 60° angle of propagation in that containing 1,2-DITFB, and
(c) 120° angle of propagation with 1,3-DITFB. While the individual
chains have highly distinctive and different topologies, they combine
through π-stacking of the DITFB molecules to produce remarkably
similar overall arrangements of molecules. Structures of 1,4-DITFB­[TEA-CH2Br]Br and 1,3-DITFB­[TEA-CH2Br]Br are also reported
and are isomorphous with their chloro/chloride analogues, further
illustrating the robustness of the overall supramolecular architecture.
The usual approach to crystal engineering is to make structural changes
to molecular components to effect specific changes to the resulting
crystal structure. The results reported herein encourage pursuit of
a somewhat different approach to crystal engineering. That is, to
investigate the possibilities for engineering the same overall arrangement
of molecules in crystals while employing molecular components that
aggregate with entirely different supramolecular connectivity.