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. Nonetheless,
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
π-binding sites were prepared for the enantioseparation of amino
acid derivatives (Fmoc-AAs) with large π-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 π-binding group, while
material 3 displays excellent enantioselective separation
performance for Fmoc-phenylalanine and Fmoc-tryptophan with π-binding
groups at both ends. As evidently demonstrated by our experimental
and density functional theory (DFT) computational results, when the
number of π-binding groups preset in the confined chiral space
of adsorbents matches the number of π-binding groups of chiral
adsorbates, the synergism of π–π or σ–π
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.