Investigation of zeolitic imidazolate frameworks using 13 C and 15 N solid-state NMR spectroscopy

Abstract

Zeolitic imidazolate frameworks (ZIFs) are a subclass of metal-organic frameworks (MOFs) with extended threedimensional networks of transition metal nodes (bridged by rigid imidazolate linkers), with potential applications in gas storage and separation, sensing and controlled delivery of drug molecules. Here, we investigate the use of 13 C and 15 N solid-state NMR spectroscopy to characterise the local structure and disorder in a variety of singleand dual-linker ZIFs. In most cases, a combination of a basic knowledge of chemical shifts typically observed in solution-state NMR spectroscopy and the use of dipolar dephasing NMR experiments to reveal information about quaternary carbon species are combined to enable spectral assignment. Accurate measurement of the anisotropic components of the chemical shift provided additional information to characterise the local environment and the possibility of trying to understand the relationships between NMR parameters and both local and long-range structure. First-principles calculations on some of the simpler, ordered ZIFs were possible, and provided support for the spectral assignments, while comparison of these model systems to more disordered ZIFs aided interpretation of the more complex spectra obtained. It is shown that 13 C and 15 N NMR are sufficiently sensitive to detect small changes in the local environment, e.g., functionalisation of the linker, crystallographic inequivalence and changes to the framework topology, while the relative proportion of each linker present can be obtained by comparing relative intensities of resonances corresponding to chemically-similar species in cross polarisation experiments with short contact times. Therefore, multinuclear NMR spectroscopy, and in particular the measurement of both isotropic and anisotropic parameters, offers a useful tool for the structural study of ordered and, in particular, disordered ZIFs.

Gas Transport Properties of the Metal–Organic Framework (MOF)-Assisted Polymer of Intrinsic Microporosity (PIM-1) Thin-Film Composite Membranes

“…ZIF-7 shows two resonances for each carbon with significant line broadening, suggesting the presence of two crystallographically different types of linkers and additional disorder. This is consistent with the crystallographic structure of ZIF-7 containing two crystallographically inequivalent linkers. , The spectra clearly show the existence of the solvent molecules trapped in the pores of the ZIF-7 nanoparticles, which is in agreement with the FTIR results. By contrast, a single sharp resonance is seen for each carbon of the Zn 2 (bim) 4 sample, indicating that one type of crystallographically different linker exists in the layered structure.…”

“…This is consistent with the crystallographic structure of ZIF-7 containing two crystallographically inequivalent linkers. 34,35 The spectra clearly show the existence of the solvent molecules trapped in the pores of the ZIF-7 nanoparticles, which is in agreement with the FTIR results. By contrast, a single sharp resonance is seen for each carbon of the Zn 2 (bim) 4 sample, indicating that one type of crystallographically different linker exists in the layered structure.…”

Covalent‐Linking‐Enabled Superior Compatibility of ZIF‐8 Hybrid Membrane for Efficient Propylene Separation

“…[ 37 ] Compared to ZIF‐8, an additional chemical shift associated with the central in‐ring carbon (indicated by purple color) is measured at 145.8 ppm in the NMR spectrum of ZIF‐8 86 ‐CN 14 (Figure 1c). [ 38,39 ] The XPS signal for ZIF‐8 86 ‐CN 14 is slightly different from that for ZIF‐8, and the shadowed binding energy for N 1s can be assigned to the nitrogen of C≡N triple bond in ZIF‐8 86 ‐CN 14 (Figure 1d). The ligand ratio was quantified from the UV–Vis spectra of dissolved samples in mild acidic aqueous solutions during capillary electrophoresis (Figure S1, Supporting Information).…”

Quantitative Analysis of Linker Composition and Spatial Arrangement of Multivariate Metal–Organic Framework UiO-66 through 1H Fast MAS NMR

“…Solid-state NMR spectroscopy was frequently utilized to explore the chemical structures, dynamic behavior, and host–guest interaction in various MOFs. − Figure a–d shows the 1 H MAS NMR spectra of dehydrated MTV-UiO-66 samples acquired at 40 kHz MAS. According to the previous literatures, the resonances at around 7.0–8.4 ppm and 0.5–2.2 ppm are arising from aromatic protons and bridging Zr­(μ 3 -OH) groups, respectively.…”