Isostructural Halogen Exchange and Halogen Bonds: The Case of N-(4-Halogenobenzyl)-3-halogenopyridinium Halogenides.

Luka Fotović,Vladimir Stilinović,Nikola Bedeković

doi:10.1021/acs.cgd.1c01285

Abstract

Six N-(4-halogenobenzyl)-3-halogenopyridinium cations were prepared by reacting meta-halogenopyridines (Cl, Br, and I) with (4-halogenobenzyl) bromides (Br and I) and were isolated as bromide salts, which were further used to obtain iodides and chlorides. Sixteen compounds (out of 18 possible cation/anion combinations) were obtained; two crystallized as hydrates and 14 as solvent free salts, 11 of which belonged to one isostructural series and 3 to another. All crystal structures comprise halogen-bonded chains, with the anion as an acceptor of two halogen bonds, with the pyridine and the benzyl halogen substituents of two neighboring cations. The halogen bonds with the pyridine halogen show a linear correlation between the relative halogen bond length and angle, which primarily depend on the donor halogen. The parameters of the other halogen bonds vary with all three halogens, indicating that the former halogen bond is the dominant interaction. This is also in accord with the calculated electrostatic potential in the σ-holes of the halogens and the thermal properties of the solids. The second isostructural group comprises combinations of the best halogen bond donors and acceptors, and features a more favorable halogen bond geometry of the dominant halogen bond, reaffirming its significance as the main factor in determining the structure.

Highlights

The goal of crystal engineering is the deliberate design of crystals with planned structures and predictable physical and chemical properties.[1−6] In attempting to achieve this, one must take into account both the geometric properties of the constituent molecules, as well as their proclivity to participate in intermolecular interactions
As the halogen bond energy greatly increases with the donor atom size,[36] in such isostructural crystals, the only significant difference between the two crystals will be the strength of the halogen bond
The cations were prepared by reacting meta-halogenopyridines (Cl, Br, and I) with (4-halogenobenzyl) bromides (Br and I), which yielded a series of six bromide salts of N-(4halogenobenzyl)-3-halogenopyridinium cations

Summary

■ INTRODUCTION

The goal of crystal engineering is the deliberate design of crystals with planned structures and predictable physical and chemical properties.[1−6] In attempting to achieve this, one must take into account both the geometric properties of the constituent molecules, as well as their proclivity to participate in intermolecular interactions. This can be achieved in several ways; halogen atoms can act as halogen bond acceptors either of part of the neutral molecule (type II XB) or as halogenide anions (in their “free” form or coordinated as ligands to metal centers) The latter approach was very successfully employed by the Brammer group for the study of the hierarchy of intermolecular interactions in 3-halogenopyridinium tetrahalogenometalates, demonstrating that the structure type is dependent on both the hydrogen and the halogen bond strength.[37] halogenopyridinium halogenides were shown to be extremely prone to isostructurality: among both the ortho- and para-halogenopyridinium halogenides (halogen = Cl, Br, I), there are groups of six isostructural salts, while among meta-halogenopyridinium halogenides there are two groups of four.[45−48]. This would enable us to study in more detail how halogen bond affects the structures and properties of the crystals

■ RESULTS AND DISCUSSION

■ CONCLUSION

■ ACKNOWLEDGMENTS

■ REFERENCES