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.