Charge-Assisted Anion-π Interaction and Hydrogen Bonding Involving Alkylpyridinium Cations

Competition and cooperation of charge-assisted anion-π interactions and hydrogen bonding were explored in the solid state and in solutions of 1-ethyl-4-carbomethoxypyridinium iodide, the compound utilized by Kosower to calculate solvent polarity Z-indices. X-ray structural analysis of this salt revealed multiple short contacts of iodide anions with hydrogen atoms and aromatic rings of pyridinium cations. Geometric characteristics, quantum theory of atoms in molecules (QTAIM), and noncovalent interaction (NCI) analysis of these contacts indicated comparable interaction energies of the anion-π and hydrogen bonding between iodide and pyridinium cation. ¹H NMR (indicating the presence of the hydrogen-bonded complexes) and UV-vis measurements (which were consistent with the formation of anion-π associations) pointed out that both these supramolecular interactions also coexist in solutions. The comparable interaction energies (ΔE) of these modes were confirmed by the DFT computations. Also, while the variations of ΔE with the dielectric constant of the solvents for the complexes of iodide with the neutral π-acceptors were related to the increase of the effective radii of hydrogen- or anion-π bonded iodides, the changes in ΔE for the complexes with pyridinium followed interaction energies between two unit charges. However, the distinction of the bonding in hydrogen-bonded and anion-π complexes of iodide with pyridinium led to a switch of their relative energies with an increase of the polarity of the medium.