Experimentally probing the chiral recognition mechanism of 1,1′-bi-2-naphthol on a nitrogen enriched chiral metal-organic framework

Abstract

In this article, a micropore CMOF structure with abundant nitrogen species, D-His-ZIF-L (DHZL), was synthesized in water according to the reference with a modification. The performance of DHZL was verified by enantioselective capture of S-1,1′-bi-2-naphthol (S-BINOL), with an enantiomeric excess (ee) value of 60.5% under optimal conditions. To experimentally elucidate the chiral recognition mechanism, the chemical shifts of carbon and nitrogen signals were investigated on the basis of CP MAS NMR spectroscopy by using DHZL (200 mg) with R/S-BINOL of 750 μg mL−1 in CH3CN at room temperature, in which DHZL acts as the chiral shift reagent. The peaks corresponding to the chiral carbon and one imidazole ring carbon resonances in the D-histidine moiety of DHZL exhibit obvious upfield and downfield shifts in the presence of S-BINOL, respectively, indicating that the the strong hydrogen bonding and π-π interactions existed. Clear nitrogen splitting further indicates the hydrogen-bond interaction site between the hydroxyl of the optical S-isomer and the amino of D-histidine in the chiral framework. In addition, the positive shift in the N 1 s peak of DHZL by X-ray photoelectron spectroscopy (XPS) analysis was also proved the described observations when loaded with S-BINOL in contrast with R-one. Furthermore, a control experiment was performed to identify the essential features of the chiral microenvironment in DHZL. Based on these results above, the chiral recognition mechanism was experimentally determined.