Experimental and Theoretical Charge Density Study of the I-N Halogen Bond and F-F Interactions in 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, including the topological analysis of the X-ray multipole refined charge density [1]. Fluorine-Fluorine interactions [2] 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 on two iodotetrafluoroethylimidazole derivatives, whose crystal structure is dominated by 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∙∙∙ and ∙∙∙) interactions. The experimental charge densities have been derived from X-ray data collected at 100 K, using the aspherical atom formalism of Stewart [3] as implemented in VALTOPO [4], as well as by accurate M062X/6-311++G(d,p) and MP2/6-311++G(d,p) calculations in gas phase, and M062X/6-311G(d,p) calculations in solid state [5]. The topological analysis of charge density and its Laplacian allowed to elucidate the role exerted on the XB properties by the alkyl/aryl moiety bonded to the iodine atom through comparison with previous results [1a]. Moreover, an Interacting Quantum Atoms (IQA) analysis [6] on the M062X/6-311++G(d,p) optimized halogen bonded dimer revealed the nature essentially electrostatic for this interaction, though the dispersive contribution resulted to be significant. Selected dimers extracted from the crystal structure were as well submitted to IQA analysis in order to investigate the nature of the F∙∙∙F interactions and in particular the relative electrostatic/exchange contributions. Both Type I (θ1  θ2) and Type II (θ1  180° and θ2  90°, where θ1=C–X1•••X2 and θ2=X1•••X2–C angles) geometries have been considered. All interactions in the presently investigated systems follow the relationship recently suggested by Spackman et al [7].