Rational design of an unusual 2D-MOF based on Cu(i) and 4-hydroxypyrimidine-5-carbonitrile as linker with conductive capabilities: a theoretical approach based on high-pressure XRD

Capillary electrochromatography synergistic enantioseparation system for racemate malic acid based on a novel nanomaterial synthesized by chiral molecularly imprinted polymer and chiral metal-organic framework

BACKGROUND

Chirality is one of the most fundamental features of nature. In terms of biological activities, pharmacological effects, etc., enantiomers often show great differences among each other. Therefore, it is important to develop highly efficient enantioseparation and analysis methods. Capillary electrochromatography (CEC) is one of the most popular methods in the field of enantioseparation. In the chiral stationary phase of CEC, chiral molecularly imprinted polymers (CMIPs) and chiral metal-organic frameworks (CMOFs) have shown great potential of enantioseparation. However, the enantioseparation performance of CMOFs and CMIPs alone as chiral separation media is less satisfactory.

RESULTS

In this work, a novel nanomaterial synthesized by CMOFs and CMIPs was used as stationary phase in CEC synergistic enantioseparation system and the relevant reports have not been internationally found by authors. As a proof-of-concept demonstration, a coated capillary column was prepared by a one-step method using l-malic acid (template), [Cu2(D-Cam)2Dabco] (Cu-MOF) and dopamine (functional monomer/cross-linking agent), which greatly simplified the modification process of the capillary columns. Compared with Cu-MOF and CMIP alone, the CEC synergistic enantioseparation system based on Cu-MOF@MIP has significantly better enantioseparation performance of malic acid enantiomers (resolution: 1.03/0.58 → 4.22), and there is also a satisfactory performance in the quantitative analysis in real samples. Finally, through molecular docking and adsorption experiments, it was systematically proved that Cu-MOF@MIP had a significantly stronger binding ability for l-malic acid than d-malic acid.

SIGNIFICANCE

Cu-MOF with chiral recognition ability have synergize with CMIPs to greatly improve the chiral selectivity of Cu-MOF@MIP, which is firstly used for the construction of the CEC chiral separation system. This pioneering synergistic chiral separation system creates a potential direction for efficient enantioseparation. Considering the diversity of CMOFs and CMIPs, the stationary phases hold great promise in chiral separation science.

Understanding Cu(i) local environments in MOFs via63/65Cu NMR spectroscopy

The field of metal-organic frameworks (MOFs) includes a vast number of hybrid organic and inorganic porous materials with wide-ranging applications. In particular, the Cu(i) ion exhibits rich coordination chemistry in MOFs and can exist in two-, three-, and four-coordinate environments, which gives rise to many structural motifs and potential applications. Direct characterization of the structurally and chemically important Cu(i) local environments is essential for understanding the sources of specific MOF properties. For the first time, 63/65Cu solid-state NMR has been used to investigate a variety of Cu(i) sites and local coordination geometries in Cu MOFs. This approach is a sensitive probe of the local Cu environment, particularly when combined with density functional theory calculations. A wide range of structurally-dependent 63/65Cu NMR parameters have been observed, including 65Cu quadrupolar coupling constants ranging from 18.8 to 74.8 MHz. Using the data from this and prior studies, a correlation between Cu quadrupolar coupling constants, Cu coordination number, and local Cu coordination geometry has been established. Links between DFT-calculated and experimental Cu NMR parameters are also presented. Several case studies illustrate the feasibility of 63/65Cu NMR for investigating and resolving inequivalent Cu sites, monitoring MOF phase changes, interrogating the Cu oxidation number, and characterizing the product of a MOF chemical reaction involving Cu(ii) reduction to Cu(i). A convenient avenue to acquire accurate 65Cu NMR spectra and NMR parameters from Cu(i) MOFs at a widely accessible magnetic field of 9.4 T is described, with a demonstrated practical application for tracking Cu(i) coordination evolution during MOF anion exchange. This work showcases the power of 63/65Cu solid-state NMR spectroscopy and DFT calculations for molecular-level characterization of Cu(i) centers in MOFs, along with the potential of this protocol for investigating a wide variety of MOF structural changes and processes important for practical applications. This approach has broad applications for examining Cu(i) centers in other weight-dilute systems.