Halogen Bonding in N-Alkyl-3-halogenopyridinium Salts

Luka Fotović,Vladimir Stilinović

doi:10.3390/cryst11101240

Abstract

We performed a structural study of N-alkylated halogenopyridinium cations to examine whether choice of the N-substituent has any considerable effect on the halogen bonding capability of the cations. For that purpose, we prepared a series of N-ethyl-3-halopyridinium iodides and compared them with their N-methyl-3-halopyridinium analogues. Structural analysis revealed that N-ethylated halogenopyridinium cations form slightly shorter C−X⋯I− halogen bonds with iodide anion. We have also attempted synthesis of ditopic symmetric bis-(3-iodopyridinium) dications. Although successful in only one case, the syntheses have afforded two novel ditopic asymmetric monocations with an iodine atom bonded to the pyridine ring and another on the aliphatic N-substituent. Here, the C−I⋯I− halogen bond lengths involving pyridine iodine atom were notably shorter than those involving an aliphatic iodine atom as a halogen bond donor. This trend in halogen bond lengths is in line with the charge distribution on the Hirshfeld surfaces of the cations—the positive charge is predominantly located in the pyridine ring making the pyridine iodine atom σ-hole more positive than the one on the alkyl chan.

Highlights

IntroductionThe C−I· · · I− halogen bond lengths involving pyridine iodine atom were notably shorter than those involving an aliphatic iodine atom as a halogen bond donor
Analysing the electrostatic potential plotted on the Hirshfeld surface (Figure 1b), it has been found that ESP on the Hirshfeld surface in the σ-hole area
Using aliphatic dihalogenides to produce bis(halogenopyridinium) dications capable of acting as ditopic halogen bond donors has proven to be somewhat less successful—of the two attempted target cations, only one was isolated, and it decomposed during an attempted ion exchange

Summary

Introduction

The C−I· · · I− halogen bond lengths involving pyridine iodine atom were notably shorter than those involving an aliphatic iodine atom as a halogen bond donor. This trend in halogen bond lengths is in line with the charge distribution on the Hirshfeld surfaces of the cations—the positive charge is predominantly located in the pyridine ring making the pyridine iodine atom σ-hole more positive than the one on the alkyl chan. Various Lewis bases (neutral molecules or charged species) have been employed as halogen bond acceptors in constructing halogenbonded supramolecular assemblies. These have most commonly been organic [13,14]

Methods

Results

Conclusion