Unravelling the Nature of I-N Halogen Bond and F-F Interactions by Charge Density Studies on Iodoperfluoroalkylimidazoles

Halogen bonding (XB), namely any noncovalent interaction involving halogens as electrophilic sites, is a relatively new item in the supramolecular toolbox and shares numerous properties with the better known hydrogen bonding. The X∙∙∙N(O) halogen bond has been thoroughly investigated by means of several experimental and theoretical techniques. Fluorine-Fluorine interactions [1] are much less explored intermolecular interactions, though F∙∙∙F contacts below or just above the sum of van der Waals radii of the fluorine atoms are ubiquitous in fluorinated organic structures. We present here the results obtained by experimental and theoretical charge density studies on two iodotetrafluoroethylimidazoles, whose crystal structure is dominated by the formation of I∙∙∙N halogen bonds between equivalent molecules, and stabilized by the presence of F∙∙∙F and other weak (C-H∙∙∙F, C-H∙∙∙π and π ∙∙∙π) interactions. The charge density has been obtained by multipolar refinement against single-crystal X-ray diffracted intensities collected at 100 K [3] and by M06-2X/6-311++G(d,p) and MP2/6-311++G(d,p) calculations in gas phase, and M06-2X/6-311G(d,p) calculations in solid state [2]. An Interacting Quantum Atom (IQA) [4] analysis on the M06-2X/6-311++G(d,p) halogen bonded dimer at the experimental geometry has been carried out confirming the essentially electrostatic nature for this interaction, though the exchange-correlation contribution resulted to be significant. Moreover, the dispersion contribution into halogen bonding interaction energy has been estimated through the D2 version of Grimme’s dispersion [5], as implemented in the ωB97XD functional, and compared with the corresponding counterpoise-corrected interaction energy, revealing the importance of dispersion in this mainly electrostatic interaction. Selected dimers extracted from the crystal structure were as well submitted to IQA analysis in order to investigate the nature of F∙∙∙F interactions and in particular the relative electrostatic/exchange-correlation contribution.