Anion templated crystal engineering of halogen bonding tripodal tris(halopyridinium) compounds

In this work four new tripodal tris(halopyridinium) receptors containing potentially halogen bonding groups were prepared. The ability of the receptors to bind anions in competitive CD₃CN/d⁶-DMSO was studied using ¹H NMR titration experiments, which revealed that the receptors bind chloride anions more strongly than more basic acetate or other halide ions. The solid state self-assembly of the tripodal receptors with halide anions was investigated by X-ray crystallography. The nature of the structures was dependent on the choice of halide anion, as well as the crystallisation solvent. Halogen bond lengths as short as 80% of the sum of the van der Waals radii were observed, which is shorter than any halogen bonds involving halopyridinium receptors in the Cambridge Structural Database.