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

Abstract

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.

Examining a Transition from Supramolecular Halogen Bonding to Covalent Bonds: Topological Analysis of Electron Densities and Energies in the Complexes of Bromosubstituted Electrophiles

“…The M062X/6-311+G(d,p) method was used in our earlier computations of XB complexes. 12 − 15 Previous theoretical analyses indicated that it produces excellent geometries and energies at a reasonable computational cost. 34 , 35 Importantly, our earlier studies demonstrated that the characteristics of the XB complexes between RBr electrophiles obtained via such computations were in good agreement with the experimental data.…”

“… 36 This moderately polar solvent represents a good media for modeling solid-state ionic compounds. 14 , 15 …”

“…Indeed, the crystallographic literature contains examples of the XB associations with interatomic distances ranging from the corresponding van der Waals separations to those which are within 15–20% of the relevant fully developed covalent bond. 1 , 2 , 9 − 15 This raises questions about the relationship and distinctions between such diverse bonds and boundaries between supramolecular and covalent bonding, as well as the nature of XB and its relation to a covalent bond.…”

“… b From ref ( 13 ) and references therein. c From ref ( 15 ) and references therein. d From ref ( 14 ) and references therein.…”

“…Structural and thermodynamic characteristics of these complexes were reproduced reasonably well by quantum-mechanical computations. 13 − 15 Importantly, these series contain (experimental and/or calculated) complexes with characteristics covering the whole array of values, eliminating any large changes and leaving no substantial gaps between successive entries. Accordingly, they allow QTAIM analysis of the bonding in the full range of the Br···Y lengths, from weak complexes with XB lengths near the van der Waals separations to covalent bonds.…”