Halogen-Bonded Mono-, Di-, and Tritopic N-Alkyl-3-iodopyridinium Salts

Halogen bonding interactions of 15 crystalline 3-iodopyridinium systems were investigated. These systems were derived from four N-alkylated 3-iodopyridinium salts prepared in this study. The experimental results in the solid state show that halogen bonding acts as a secondary intermolecular force in these charged systems but sustains the high directionality of interaction in the presence of other intermolecular forces. Halogen bonds donated by polytopic 3-iodopyridinium cations are also sufficient to enclose guest molecules inside the formed supramolecular cavities. The experimental data were supplemented by computational gas-phase and solid-state studies for selected halogen-bonded systems. Calculations of the model systems with the increasing number of halogen bond donors and acceptors showed the halogen bond strengths to be exaggerated for the smallest of model systems. The agreement between experimental and calculated structures improved for larger systems that were able to account for the influence of other intermolecular interactions. The best agreement between experimental and calculated structural parameters were found for solid-state calculations with periodic boundary conditions. Comparison of the halogen bond interaction strengths with the strength of other lattice interactions showed the halogen bonds to come second to electrostatic interactions in stabilizing the structures but having a major role in directing the packing of the solid-state structures.