Halogen Bonding in the Complexes of Brominated Electrophiles with Chloride Anions: From a Weak Supramolecular Interaction to a Covalent Br–Cl Bond

• Main Authors: Cody Loy, Matthias Zeller, Sergiy V. Rosokha
• Format: Article
• Language: English
• Published: MDPI AG 2020-11-01
• Series: Crystals
• Subjects: halogen bonding; bromosubstituted electrophiles; chloride; X-ray crystallography; DFT computations; chemical bonding
• Online Access: https://www.mdpi.com/2073-4352/10/12/1075

The wide-range variation of the strength of halogen bonds (XB) not only facilitates a variety of applications of this interaction, but it also allows examining the relation (and interconversion) between supramolecular and covalent bonding. Herein, the Br^…Cl halogen bonding in a series of complexes of bromosubstituted electrophiles (R-Br) with chloride anions were examined via X-ray crystallographic and computational methods. Six co-crystals showing such bonding were prepared by evaporation of solutions of R-Br and tetra-n-propylammonium chloride or using Cl⁻ anions released in the nucleophilic reaction of 1,4-diazabicyclo[2.2.2]octane with dichloromethane in the presence of R-Br. The co-crystal comprised networks formed by 3:3 or 2:2 halogen bonding between R-Br and Cl⁻, with the XB lengths varying from 3.0 Å to 3.25 Å. Analysis of the crystallographic database revealed examples of associations with substantially longer and shorter Br^…Cl separations. DFT computations of an extended series of R–Br^…Cl⁻ complexes confirmed that the judicious choice of brominated electrophile allows varying halogen Br^…Cl bond strength and length gradually from the values common for the weak intermolecular complexes to that approaching a fully developed covalent bond. This continuity of halogen bond strength in the experimental (solid-state) and calculated associations indicates a fundamental link between the covalent and supramolecular bonding.