Stabilizing Otherwise Unstable Anions with Halogen Bonding

Xinxing Zhang; Gaoxiang Liu; Sandra Ciborowski; Kit Bowen

doi:10.1002/anie.201705738

Stabilizing Otherwise Unstable Anions with Halogen Bonding

Angew Chem Int Ed Engl. 2017 Aug 7;56(33):9897-9900. doi: 10.1002/anie.201705738. Epub 2017 Jul 18.

Authors

Xinxing Zhang^{1

2}, Gaoxiang Liu¹, Sandra Ciborowski¹, Kit Bowen¹

Affiliations

¹ Department of Chemistry, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD, 21218, USA.
² Noyes Laboratory of Chemical Physics and the Beckman Institute, California Institute of Technology, Pasadena, CA, 91125, USA.

PMID: 28664676
DOI: 10.1002/anie.201705738

Abstract

Both hydrogen bonding (HB) and halogen bonding (XB) are essentially electrostatic interactions, but whereas hydrogen bonding has a well-documented record of stabilizing unstable anions, little is known about halogen bonding's ability to do so. Herein, we present a combined anion photoelectron spectroscopic and density functional theory study of the halogen bond-stabilization of the pyrazine (Pz) anion, an unstable anion in isolation due to its neutral counterpart having a negative electron affinity (EA). The halogen bond formed between the σ-hole on bromobenzene (BrPh) and the lone pair(s) of Pz significantly lowers the energies of the Pz(BrPh)₁^- and Pz(BrPh)₂^- anions relative to the neutral molecule, resulting in the emergence of a positive EA for the neutral complexes. As seen through its charge distribution and electrostatic potential analyses, the negative charge on Pz^- is diluted due to the XB. Thermodynamics reveals that the low temperature of the supersonic expansion plays a key role in forming these complexes.

Keywords: anions; density functional theory; halogen bonding; hydrogen bonding; photoelectron spectroscopy.

Publication types

Research Support, U.S. Gov't, Non-P.H.S.