The Interaction Modes of Haloimidazolium Salts in Solution

Abstract

We performed a comparative study on the interaction modes of 2-haloimidazolium salts with anions in solution, particularly with regard to halogen bonding, hydrogen bonding and anion-π interactions. The syntheses and solid-state analyses of a series of sterically and electronically modified 2-haloimidazolium structures are presented. Detailed isothermal titration calorimetry (ITC) measurements, quantum mechanics/molecular mechanics (QM/MM), classical molecular dynamics simulations (MD) and free-energy calculations together with NMR spectroscopy were used to elucidate the binding modes in solution. Our work reveals the absence of a potential anion-π interaction between the cationic imidazolium ring and the Lewis basic counteranion, and corroborates a formation of halogen bonding via the Lewis acidic iodine moiety and hydrogen bonding via the backbone hydrogen atoms, with repercussions in the field of organocatalysis.

Synthesis and Evaluation of Azolium‐Based Halogen‐Bond Donors

“…Finally, achiral triazolium iodide 5 underwent salt metathesis with the sodium salts of three chiral phosphoric acids, producing three achiral iodinated triazoliums with chiral counterions ( 20 – 22 ). The further inspection of the crystal structures for catalysts 15 and 20 confirmed that the counterions were not tightly associated with the σ‐hole as was observed for the corresponding iodides which is also a feature shared with iodinated imidazoles (Figure ) . We observed no additional solid state interactions with the triazolium cores of all compounds characterized, suggesting that replacement of the imidazolium C4 with nitrogen is an effective means to suppress previously observed competitive H‐bonding with C4/C5 C−H bonds.…”

“…Compounds 5 , 6 , and 10 were additionally characterized by X‐ray crystallography, and the structures all featured a close coordination of the iodide counterion to the σ‐hole of the heteroaromatic 2‐iodo substituent, an interaction similar to one which was observed by Huber with imidazolium counterparts . Due to this presumably strong association, we hypothesized that less coordinating counterions would be necessary to facilitate the possible coordination to a Lewis basic substrate for activation (Table ).…”

“…We observed no additional solid state interactions with the triazolium cores of all compounds characterized, suggesting that replacement of the imidazolium C4 with nitrogen is an effective means to suppress previously observed competitive H‐bonding with C4/C5 C−H bonds. These undesired interactions in previous azolium‐I structures required blocking substituents at these positions …”

Preorganization: A Powerful Tool in Intermolecular Halogen Bonding in Solution

“…Up to this point, only examples for halide abstraction reactions were discussed, because it is known from many publications in the field of anion recognition, that XB donors strongly interact with halide anions.…”

Thermodynamics of Anion Binding by Chalcogen Bonding Receptors

“…These observations suggest that Cl − is not binding in the cleft between both SeCH 3 units, but is predominantly interacting in the vicinity of the cationic pyridinium nitrogen atom of both receptors, possibly driven by Coulombic attraction and HB interactions with H b and H c . Indeed, the possibility of competing HB or to a much lesser extent, anion–π interactions, between anions and cationic aromatic receptor frameworks bearing XB donor groups has been recently demonstrated by Huber and co‐workers, providing support for this mode of interaction between Cl − and 2.Se / 3.Se . To provide further evidence, an analogous Cl − titration experiment under identical conditions with the bis‐fluorotriazole acyclic host analogue 2.F was undertaken (see Figure for structure).…”