Evaluation of Halogenopyridinium Cations as Halogen Bond Donors

We have performed a database survey and a structural and computational study of the potential and the limitations of halogenopyridinium cations as halogen bond donors. The database survey demonstrated that adding a positive charge on a halogenopyridine ring increases the probability that the halogen atom will participate in a halogen bond, although for chloropyridines it remains below 60%. Crystal structures of both protonated and N-methylated monohalogenated pyridinium cations revealed that the iodo- and bromopyridinium cations always form halogen-bonding contacts with the iodide anions shorter than the sum of the vdW radii, while chloropyridinium cations mostly participate in longer contacts or fail to form halogen bonds. Although a DFT study of the electrostatic potential has shown that both protonation and N-methylation of halogenopyridines leads to a considerable increase in the ESP of the halogen sigma-hole, it is generally not the most positive site on the cation, allowing for alternate binding sites.

Automated summary

A database survey and structural study reveal that protonation and N-methylation increase the probability of halogen atoms participating in halogen bonds in halogenopyridinium cations, although chloropyridines have a lower probability. Crystal structures show that iodo- and bromopyridinium cations form shorter halogen-bonding contacts, while chloropyridinium cations mostly form longer or fail to form halogen bonds. DFT study indicates an increase in the electrostatic potential of the halogen sigma-hole after protonation and N-methylation, but it is not always the most positive site, allowing for alternate binding sites.