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Ion‐Pair Halogen Bonds in 2‐Halo‐Functionalized Imidazolium Chloride Receptors: Substituent and Solvent Effects
Author(s) -
Nunes Rafael,
Costa Paulo J.
Publication year - 2017
Publication title -
chemistry – an asian journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.18
H-Index - 106
eISSN - 1861-471X
pISSN - 1861-4728
DOI - 10.1002/asia.201601690
Subject(s) - chemistry , halogen bond , chloride , substituent , aqueous solution , halogen , solvent , chloroform , solvent effects , ion , computational chemistry , ab initio , hydrogen bond , molecule , stereochemistry , organic chemistry , alkyl
The interaction of 2‐halo‐functionalized imidazolium derivatives ( n ‐X + ; X=Cl, Br, I) with a chloride anion through ion‐pair halogen bonds ( n ‐X⋅Cl ) was studied by means of DFT and ab initio calculations. A method benchmark was performed on 2‐bromo‐1 H ‐imidazol‐3‐ium in association with chloride ( 1‐Br⋅Cl ); MP2 yielded the best results when compared with CCSD(T) calculations. The interaction energies (Δ E ) in the gas phase are high and, although the electrostatic interaction is strong owing to the ion‐pair nature of the system, large X⋅⋅⋅Cl − Wiberg bond orders and contributions from charge transfer (nCl-→σ*C−X) are obtained. These values drop considerably in chloroform and water; this shows that solvent plays a role in modulating the interaction and that gas‐phase calculations are particularly unrealistic for experimental applications. The introduction of electron‐withdrawing groups in the 4,5‐positions of the imidazolium (e.g., −NO 2 , −F) increases the halogen‐bond strength in both the gas phase and solvent, including water. The effect of the substituents on the 1,3‐positions (N−H groups) also depends on the solvent. The variation of Δ E can be predicted through a two‐parameter linear regression that optimizes the weights of charge‐transfer and electrostatic interactions, which are different in vacuum and in solvent (chloroform and water). These results could be used in the rational design of efficient chloride receptors based on halogen bonds that work in solution, in particular, in an aqueous environment.