Date Published: September 01, 2019
Publisher: International Union of Crystallography
Author(s): Ruimin Wang, Janine George, Shannon Kimberly Potts, Marius Kremer, Richard Dronskowski, Ulli Englert.
The culprit is the σ-hole: short I⋯N halogen bonds do not necessarily come with high electron density in their (3,−1) critical point. The I—C geometry and Raman spectroscopy complement information from electron density.
The term ‘halogen bond’ denotes a short contact between a Lewis base D and a heavy halogen X (I, Br or Cl) acting as electrophile (Hassel, 1970 ▸; Metrangolo & Resnati, 2001 ▸); a schematic overview is provided in Fig. 1 ▸. More generally, halogen bonds may be understood as a special case of contacts in which a nucleophile approaches the electrophilic region of a neighbouring atom, so-called σ-hole interactions (Brinck et al., 1992 ▸, 1993 ▸; Politzer et al., 2017 ▸; George et al., 2014 ▸).
Not only X, the interaction partner with the σ hole, but also the Lewis base D (the halogen-bond acceptor) in Fig. 1 ▸ may be a halogen atom. Experimental charge–density studies on such short interhalogen contacts will be addressed in §2.1, whereas X⋯D contacts between a heavy halogen and an O or N atom will be discussed in §2.2.
Experimental electron-density studies on compounds with short intermolecular Cl⋯Cl contacts are in agreement with the commonly accepted σ-hole theory. The nucleophile, the electrophile with the σ hole and its covalently bonded partner atom giving rise to this positive region are arranged in a linear fashion. Longer Cl⋯Cl distances are associated with low electron density in the bcp; only little, if any, information about the nature and strength of the interaction can be extracted. Short Cl⋯Cl contacts are associated with clear features in the electron density and derived properties, such as the Laplacian or the ESP: the charge depletion on the electrophile and the polarization of the nucleophile may be perceived, and the electron density in the bcp of the interchlorine contact increases for shorter distances.