# Nonuniform Chiralization of Metal–Organic Frameworks Using Imine Chemistry

## TL;DR

Scientists used imine chemistry to create nonuniform chiral metal-organic frameworks, which could improve enantioseparation in chromatography.

## Contribution

The study introduces a novel postsynthetic chiralization method using imine chemistry to create nonuniform chiral MOFs.

## Key findings

- Using (R)-2,2-dimethyl-1,3-dioxolane-4-carboxaldehyde on UiO-66 NH2 yields the best reactivity and stability.
- Imine chemistry leads to covalent and chiral surface modification, forming enantioselectors.
- Modified MOFs show good retention of resolving power in chromatography due to restricted diffusion.

## Abstract

Homochiral metal–organic frameworks (MOFs) are
exceptional
media for heterogeneous enantiodifferentiation processes. Modifying
available achiral structure-bearing MOF scaffolds is a preferred method
to extend this class of materials. Reported postsynthetic covalent
chiralizations generally lead to uniform, site-specific modifications.
The use of chemically versatile modifying agents, like aldehydes,
may instead result in the statistical formation of chemically nonuniform
anchored products. In addition, the use of such modifying agents gives
rise to spatial nonuniformities in the radial direction, due to prohibited
diffusion through the MOF bulk. The advantageous grain structure formation
plus molecular nonuniformity greatly increase the complexity of such
systems. The use of such modifying agents, therefore, necessitates
a broader holistic characterization. The present work explores the
adaptation of imine chemistry for postsynthetic chiralization. A chiral
aldehyde and a chiral ketone are probed on two amine-functionalized
MOF substrates—MIL-125 NH2 and UiO-66 NH2. The UiO-66 NH2 modified with the natural product-derived
(R)-2,2-dimethyl-1,3-dioxolane-4-carboxaldehyde ((R)-1 aldehyde) is found to have the best performance
in terms of reactivity and MOF stability. A comprehensive toolbox
of methods was demonstrated to robustly characterize the obtained
material. This includes high-resolution accurate mass electrospray
ionization mass spectrometry (HRAM-ESI-MS) to reveal the competing
reactions that yield a set of oligomer-rich structures. In
silico modeling correctly predicts the localization of the
modification. The modification is found to be covalent and chiral
and mainly proceeds through imine formation, resulting in a surface
enantioselector display formation. Restricted diffusion lengths in
the solid phase infer good retention of resolving power in ascending
van Deemter régimes in chromatography. Meeting this criterion
makes the yielding material a promising potential stationary phase
candidate for performant chromatographic enantioseparations.