Halogen bond in separation science: A critical analysis across experimental and theoretical results

Abstract

The halogen bond (XB) is a noncovalent interaction involving a halogen acting as electrophile and a Lewis base. In the last decades XB has found practical application in several fields. Nevertheless, despite the pivotal role of noncovalent interactions in separation science, investigations of XB in this field are still in their infancy, and so far a limited number of studies focusing on solid phase extraction, liquid-liquid microextraction, liquid-phase chromatography, and gas chromatography separation have been published. In addition, in the last few years, our groups have been systematically studying the potentiality of XB for HPLC enantioseparations. On this basis, in the present paper up-to-date results emerging from focused experiments and theoretical analyses performed by our laboratories are integrated with a descriptive presentation of XB features and the few studies published until now in separation science, with the aims to provide a comprehensive and critical discussion of the topic, and account for some still open issues in this field.

Comparative enantioseparation of chiral 4,4’-bipyridine derivatives on coated and immobilized amylose-based chiral stationary phases

“…Moreover, in principle, halogens can also act as HB acceptors through the electron density located on the belt around the σ-hole region (Fig. 3b) [6]. Nevertheless, in compounds 1-7, other more negative regions are present with better capability as HB acceptors.…”

“…XB is a noncovalent interaction occurring between a Lewis base (XB acceptor) and an electropositive region (the so-called σ-hole) located on bound halogens (XB donors) [31]. In our previous studies, we demonstrated by chromatographic and computational analyses that the carbonyl oxygens of Amy-3,5-diMe and cellulose-3,5-diMe are able as Lewis bases to form XBs with the electrophilic σ-hole regions of halogen substituents (XB donors) bound to the 4,4'-bipyridine rings [6,20,26]. Moreover, in principle, halogens can also act as HB acceptors through the electron density located on the belt around the σ-hole region (Fig.…”

“…The polysaccharide phenylcarbamate is arranged in a high-ordered secondary structure forming grooves and ravines containing both polar and hydrophobic regions which accommodate and discriminate enantiomers through an elevated number of noncovalent interaction options (Fig. 1a) [4][5][6]. The supramolecular structure of polysaccharide-based selectors impacts their chiral recognition ability, and for this rea son, the preparation of the CSP is a critical step [1,3].…”

“…More specifically, it is interesting to note that the use of MeOH was unfavourable for retention and selectivity of the halogenated compounds 5 (2'-R = iodine) and 7 (hexachlorinated), and for retention of 6 (2'-R = H). This negative effect on retention and selectivity was expected due to the interaction of MeOH with the carbonyl oxygens of the polysaccharide selectors, thus competing with the XBs which underlie interaction between halogenated 4,4'-bipyridines and these carbonyl oxygens [6].…”

Adsorption of toxic dye Eosin Y from aqueous solution by clay/carbon composite derived from spent bleaching earth

“…Compared with SBE and SBE/C before Eosin Y adsorption, the FTIR peaks of SBE and SBE/C after Eosin Y adsorption decreased or disappeared at 1,705, 1,630, 1,034, 2,923, and 2,854 cm −1 (Figure ), which was caused by the adsorption of Eosin Y. It is worth noting that Eosin Y and two adsorbents both have oxygenated functional groups and aromatic rings, and Eosin Y also contains C‐Br bond, which creates the conditions for hydrogen bonding (Huang, Bin, Bu, Jiang, & Zeng, 2011), halogen bonding (Peluso et al., 2019), and π‐π interaction (Wan et al., 2019). The view was also supported by FTIR spectrum.…”