Evaluation of Halogenopyridinium Cations as Halogen Bond Donors

Abstract

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 σ-hole, it is generally not the most positive site on the cation, allowing for alternate binding sites.

Evaluation of 6‐Halogenated 2‐Pyridone Moieties as Halogen Bond Donors

“…From the higher binding affinity of HÀ I 2, a stronger XB interaction cannot be concluded with certainty, as bromide binding via hydrogen bonding is also possible. Such a dual binding mode was found by Cinčić et al [29] and Stilinović et al [30] in co-crystals of halide anions with halopyridinium species where both HB and XB short contacts are visible. A titration using simple 2-pyridone 1 was therefore performed under the same conditions, which resulted in an even higher binding affinity of 69 m À 1 .…”

“…Such a dual binding mode was found by Cinčić et al [29] . and Stilinović et al [30] . in co‐crystals of halide anions with halopyridinium species where both HB and XB short contacts are visible.…”

Towards hydrogen and halogen bonded frameworks based on 3,5-bis(triazolyl)pyridinium motifs

“…To put these results into a broader context, we also compared the lengths of the short iodotriazole⋯Cl − interactions observed in 2 I Bn ·Cl 2 with those of chloride and other iodine containing halogen bond donor motifs such as perfluoroiodioarenes, 62,63 iodopyridiniums, 64–69 iodotriazoliums 70 and iodoimidazoliums. 71,72 While there are a limited number of such structures, it is notable that the I⋯Cl − interaction in 2 I Bn ·Cl 2 is as short as the shortest of the 25 iodopyridinium⋯Cl − contacts in the CSD despite the fact that the pyridinium ring in these systems is cationic while the iodotriazole group in 2 I Bn ·Cl 2 is neutral.…”

“Click”‐Like η6‐Metalation/Demetalation of Aryl Iodides as a Means of Turning “ON/OFF” Halogen Bond Donor Functionality

“…Concerning regulation of the XB donor, it is clear that the installation of electron withdrawing groups (e.g., nitro [13] or fluoro, [14] Scheme 1a) onto aryl halides amplifies XB donor functionality. Similarly, the XB donor strength of N ‐heterocyclic aryl halides (e.g., halopyridines) can, in principle, be amplified with charge‐assistance via simple protonation, [15a] alkylation, [15b] or metal coordination [16] . Nonetheless, to our knowledge, no formal examples of fully reversible amplification/suppression of XB donor functionality have yet been reported.…”