Charge-Assisted Halogen Bonding in Bromo- and Iodophenylpyridinium Chlorides

Ten N-halophenylpyridinium salts (halogen = Br or I) with chloride (and in one case iodide) counterions have been prepared and structurally characterized in order to examine the degree of solid state halogen bonding exhibited in these systems. N-Halophenylpyridinium salts are easily synthesized from existing pyridines, and the cationic pyridinium group is expected to increase the halogen bond donor ability of attached halophenyl moieties. Halide anions functioning as halogen bond acceptors facilitate formation of charge-assisted halogen bonds. All five substrates featuring iodophenyl or diiodophenyl substituents displayed the expected halogen bonding interactions. In several cases halide ion acceptors were also engaged in complementary hydrogen bonding interactions with C–H groups of pyridinium rings and O–H groups of water solvates. Halogen bond donors containing a diiodophenyl group formed either an extended network or discrete supramolecular macrocyclic constructs through ArI···Cl–···IAr bridging interactions. Significantly fewer and weaker halogen bonding interactions were observed in crystals of N-bromophenylpyridinium salts. The attenuation of halogen bonding in these substrates is attributed to the inability of the activated bromoarene halogen bond donors to compete with hydrogen bond donors (C–H/O–H residues) for the halide anion.