Engineering UiO-68-Typed Homochiral Metal-Organic Frameworks for the Enantiomeric Separation of Fmoc-AAs and Mechanism Study

Homochiral metal-organic frameworks (HMOFs) have been widely investigated in the application of enantiomeric separation. Nonethe-less, it remains a significant challenge to explore the effect of multiple weak interactions between HMOF adsorbents and chiral adsorbates on enantiomeric separation performance still. In this work, robust chiral amine-alcohol-functionalized UiO-68-typed Zr-HMOFs 1-3 with the same hydrogen-bonding sites but slightly different pi-binding sites were prepared for the enantioseparation of amino acid derivatives (Fmoc-AAs) with large pi- binding groups. As a consequence of multiple host-guest interactions, these Zr-HMOFs exhibit speedy adsorption and high adsorption capacity for Fmoc-L/D-AAs and dissimilar enantioselectivity for the adsorption of their enantiomers. Materials 1 and 2 exhibit excellent enantioselective separation performance for Fmoc-valine with a single terminal pi-binding group, while material 3 displays excellent enantioselective separation performance for Fmoc-phenylalanine and Fmoc-tryptophan with pi-binding groups at both ends. As evidently demonstrated by our experimental and density functional theory (DFT) computational results, when the number of pi-binding groups preset in the confined chiral space of adsorbents matches the number of pi-binding groups of chiral adsorbates, the synergism of pi-pi or sigma-pi interactions will increase enantioselectivity; otherwise, the competition interactions from redundant identical binding sites will weaken enantioselectivity. Our case not only provides a tremendously typical system for investigating the collaborative discrimination of multiple weak interactions and exploring the impact of relatively excessive binding sites of HMOF adsorbents or chiral adsorbates on the enantioselective separation performance but also provides guidance for targeted functional modifications of high-performance chiral porous materials.

Automated summary

In this study, chiral amine-alcohol-functionalized UiO-68-typed Zr-HMOFs 1-3 were prepared to separate amino acid derivatives with large pi-binding groups. The adsorption of Fmoc-L/D-AAs by these Zr-HMOFs exhibited quick adsorption, high adsorption capacity, and different enantioselectivity. The results showed that the matching of pi-binding groups between the confined chiral space of adsorbents and chiral adsorbates increased enantioselectivity, while redundant identical binding sites weakened enantioselectivity.