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.