Halogen Bonding in N-Alkyl-3-halogenopyridinium Salts

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.

Evaluation of Halogenopyridinium Cations as Halogen Bond Donors

“…1%) , can therefore be classified as a tetrel π-hole interaction between the iodide and a positive region on the carbon atom perpendicular to the ring plane. Similar anion···π contacts have also been found in crystal structures of several N -alkyl-3-halogenopyridinium halogenides …”

“…Similar anion•••π contacts have also been found in crystal structures of several N-alkyl-3-halogenopyridinium halogenides. 85 It can be seen that, although in all of the studied structures the C−X•••I − halogen bonding contacts are present, only among the N-methyl-3-bromopyridinium and N-methyliodopyridinium iodides is this halogen bond clearly the dominant interaction in the crystal structures. The cations interact with the iodides not only through the halogen atom (and the N−H group in the protonated pyridine series) but also through fairly short C−H•••I − hydrogen-bonding contacts and even C•••I − π-hole tetrel bonding contacts.…”

“…Similar anion···π contacts have also been found in crystal structures of several N -alkyl-3-halogenopyridinium halogenides. 85 …”

Halogen-Bonded Cocrystals of 1,3,5-Triiodo-2,4,6-trifluorobenzene and Structural Isomers of Benzoylpyridine

“…In the past several decades, halogen bond has been shown to be a ubiquitous tool in supramolecular chemistry for the design of organic, metal–organic, and ionic materials. − The strength and directionality of the halogen bond are generally attributed to the existence of σ-holean area of reduced electron density, and therefore of positive molecular electrostatic potential (MEP), on a covalently bonded halogen atom, which enables it to act as a Lewis acid. − The size of the positive area and the maximum positive value of MEP on a halogen atom ( V max ) are increased when the halogen atom is a part of a positively charged species (cation), − is bonded to an electronegative element (N, O), − or is a part of a molecule which contains electron-withdrawing substituents (fluorine, cyano, or nitro groups). − The latter type includes perfluorinated iodobenzenes which are the most widely used halogen-bond donors in supramolecular chemistry and crystal engineering to date. − …”

Editorial: Advanced Research in Halogen Bonding

“…Zubkov, A. Frontera, and co-workers [3] report results of the experimental and computational analysis of the importance of halogen bonding in two supramolecular assemblies containing four of the most abundant (F, Cl, Br, and I) halogen atoms. L. Fotović and V. Stilinović [4] explore the effects of alkyl substituents on halogen bonding in N-alkyl-3-halogenopyridinium salts. These authors show that N-ethylated ions form stronger halogen bonds (due to more positive σ-hole bonds) than their methylated analogues.…”