Halogen and Hydrogen Bond Architectures in Switchable Chains of Di- and Trihaloimidazoles

Halogen bonds have been employed
for controlling the structure
of NH···N hydrogen-bonded chains in halogenated imidazoles;
the CH···halogen contacts stabilize the coplanar arrangement
of molecules, whereas the halogen···halogen contacts
twist the molecules around the NH···N bonds and stretch
their N···N distances. The interplay of halogen···halogen
and NH···N hydrogen bonds leads to isostructural relations
of 4,5-dihaloimidazoles as well as between trichloro- and tribromoimidazole.
However, the increasing steric hindrance excludes the triiodoimidazole
from this isostructural series, and the triiodo derivative forms an
unusual structure with three symmetry-independent molecules. All symmetrically
substituted 4,5-dichloro-, dibromo-, and diiodoimidazoles as well
as trichloro-, tribromo-, and triiodoimidazoles have been synthesized
and crystallized and their structures determined. The derived rules
also apply to the co-crystal structures of 4,5-diiodoimidazole:2,4,5-triiodoimidazole
as well as to 4-iodoimidazole:2,4-diiodoimidazole. The halogen interactions
prove an efficient means for engineering and tuning new materials
with desired dielectric properties relying on highly polarizable NH···N
hydrogen bonds.