Two-dimensional chiral metal-organic framework nanosheets L-hyp-Ni/Fe@SiO2 composite for HPLC separation

In this study, we have synthesised a chiral l-hyp-Ni/Fe@SiO2 composite as a chiral stationary phase (CSP) for high-performance liquid chromatography (HPLC) for the first time. This was achieved by coating two-dimensional (2D) chiral metal-organic framework nanosheets (MONs) l-hyp-Ni/Fe onto the surface of activated SiO2 microspheres using the "wrapped in net" method. The separation efficiency of the l-hyp-Ni/Fe chromatographic column was systematically evaluated in normal-phase HPLC (NP-HPLC) and reversed-phase HPLC (RP-HPLC) configurations, employing various racemates as analytes. The findings revealed that 16 chiral compounds were separated using NP-HPLC, and five were separated using RP-HPLC, encompassing alcohols, amines, ketones, esters, alkanes, ethers, amino acids and sulfoxides. Notably, the resolution (Rs) of nine chiral compounds exceeded 1.5, indicating baseline separation. Furthermore, the resolution performance of the l-hyp-Ni/Fe@SiO2-packed column was compared with that of Chiralpak AD-H. It was observed that certain enantiomers, which either could not be resolved or were inadequately separated on the Chiralpak AD-H column, attained separation on the 2D chiral MONs column. These findings suggest a complementary relationship between the two columns in racemate separation, with their combined application facilitating the resolution of a broader spectrum of chiral compounds. In addition, baseline separation was achieved for five positional isomers on the l-hyp-Ni/Fe@SiO2-packed column. The effects of the analyte mass and column temperature on the resolution were also examined. Moreover, during HPLC analysis, the l-hyp-Ni/Fe columns demonstrated commendable repeatability, stability and reproducibility in enantiomer separation. This research not only advances the utilisation of 2D chiral MONs as CSPs but also expands their applications in the separation sciences.

A hydroxyl-functionalized homochiral porous organic cage for gas chromatographic separations

A hydroxyl-functionalized homochiral porous organic cage (POC) was synthesized and characterized by FTIR, NMR, thermogravimetric analysis (TGA), MALDI-TOF-MS, and elemental analysis. The synthesized homochiral POC was used as stationary phase to prepare a capillary gas chromatography (GC) column by a static coating method. The fabricated column shows excellent selectivity not only for the separation of positional isomers but also for the resolution of various racemates. Thirty-nine racemates have been resolved on the column, including alcohols, diols, halohydrocarbons, epoxides, esters, lactones, ketones, ethers, and organic acids. Compared to the commercial β-DEX 120 column and previously reported chiral POCs (CC3-R, CC9, and CC10)-coated columns, there are 11, 10, 24, and 15 tested racemates that cannot be resolved on β-DEX 120 column, CC3-R column, CC9 column, and CC10 column, respectively. This reveals that the fabricated column has prominent complementarity or superior separation performance to these columns in enantioseparation. Besides, the fabricated column can achieve some enantioseparations which are not possible using all previously reported chiral POC-based columns. Some positional isomers (xylenes, dichlorobenzenes, dibromobenzenes, nitrochlorobenzenes, and nitrobromobenzenes) were also separated with high-resolution values. The column exhibits good repeatability, reproducibility, and stability. The relative standard deviation (RSD) values of retention times were 0.03-0.18%, 0.11-0.92%, and 2.1-6.6% for run-to-run (n = 5), day-to-day (n = 5), and column-to-column (n = 3), respectively. The experimental results demonstrate the great potential of POCs for practical application in GC. Graphical Abstract A hydroxyl-functionalized homochiral porous organic cage was used as stationary phase for gas chromatography separation of racemates and positional isomers. The resolution of racemates mainly depended on hydrogen bonding, π-interaction, host-guest inclusion, steric fit, etc., while separation of positional isomers by shape-selective guest binding.