Unravelling dispersion forces in liquid-phase enantioseparation. Part II: Planar chiral...

In the first part of our study on possible contribution of dispersion forces in liquid-phase enantioseparations, the enantioseparation of the axially chiral 3,3'-dibromo-5,5'-bis-ferrocenylethynyl-4,4'-bipyridine with an amylose tris(3,5-dimethylphenylcarbamate)-based chiral column appeared reasonably consistent with a picture of the enantioselective recognition based on the interplay between hydrogen bond (HB), π-π stacking and dispersion interactions.

In the second part of this study, we evaluated the impact of analyte and chiral stationary phase (CSP) structure, mobile phase and temperature on the enantioseparations of planar chiral 1-(iodoethynyl)-3-arylferrocenes (3-aryl = phenyl, 2-naphthyl, 4-methylphenyl, 4-t-butylphenyl) with polysaccharide-based chiral columns. The main aim of the present study was to understand the molecular bases of the high affinity observed for the second eluted (R_p)-enantiomer of some of these analytes toward amylose phenylcarbamate-based selectors when methanol-containing mixtures were used as mobile phases. Significantly, higher affinity of the second eluted (R_p)-enantiomer toward the selector could be also observed for the sterically hindered 1-(iodoethynyl)-3-(4-t-butylphenyl)ferrocene (k₂ = 6.21) compared to the smaller 1-(iodoethynyl)-3-(4-methylphenyl)ferrocenes (k₂ = 4.07) as 2.5% methanol was added to the n-hexane-based mobile phase.

This study reasonably showed that the contribution of dispersion forces may explain the unusually large retention of the second eluted enantiomers observed for the enantioseparation of some planar chiral 1-(iodoethynyl)-3-arylferrocenes with amylose-based selectors. Based on the obtained results, we can conclude that in liquid-phase enantioseparation steric repulsion can be turned into attraction depending on the features of analyte, selector, and mobile phase.