"Anti-electrostatic" Halogen Bonding between Ions of Like Charge

Halogen bonding occurs between molecules featuring Lewis acidic halogen substituents and Lewis bases. It is often rationalized as a predominantly electrostatic interaction and thus interactions between ions of like charge (e. g., of anionic halogen bond donors with halides) seem counter-intuitive. Herein, we provide an overview on such complexes. First, theoretical studies are described and their findings are compared. Next, experimental evidences are presented in the form of crystal structure database analyses, recent examples of strong "anti-electrostatic" halogen bonding in crystals, and the observation of such interactions also in solution. We then compare these complexes to select examples of "counter-intuitive" adducts formed by other interactions, like hydrogen bonding. Finally, we comment on key differences between charge-transfer and electrostatic polarization.

The crystal structure of 2,4-dimethylimidazole monohydrate, C5H10N2O

C5H10N2O, monoclinic, Ia (no. 9), a = 4.7212(4) Å, b = 11.1424(8) Å, c = 12.8211(9) Å, β = 94.349(3)°, V = 672.52(9) Å3, Z = 4, R gt (F) = 0.0280, wR ref (F 2) = 0.0821, T = 200 K.

Supramolecular assemblies of 2-hydroxy-3-naphthoic acid and N-heterocycles via various strong hydrogen bonds and weak X⋯π (X = C–H, π) interactions

Hydrogen bonds and weak X⋯π (X = C–H, π) interactions in a series of multi-component molecules constructed from 2-hydroxy-3-naphthoic acid with N-heterocycles are discussed in context.

Robustness of thioamide dimer synthon, carbon bonding and thioamide–thioamide stacking in ferrocene-based thiosemicarbazones

The role of thioureas in crystal engineering as robust supramolecular synthons is now recognized, but their analogs, namely thiosemicarbazones/N-iminothioureas, have not received the attention they deserve.

Using halogen⋯halogen interactions or C/N–H⋯Cl hydrogen bonding to direct crystal packing in tetrachlorophthalic acid with N-heterocyclic compounds

Hydrogen bonding patterns and halogen⋯halogen interactions, C/N–H⋯Cl hydrogen bonding in a series of multi-component molecular structures constructed by tetrachlorophthalic acid with N-heterocycles were discussed.