Abstract
Zeolitic imidazolate frameworks (ZIFs) are a subclass of metal-organic frameworks (MOFs) with extended three-dimensional 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 13C and 15N solid-state NMR spectroscopy to characterise the local structure and disorder in a variety of single- and 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 13C and 15N 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.
Highlights
Zeolitic imidazolate frameworks (ZIFs) are a relatively new subclass of metal-organic frameworks (MOFs) with extended three-dimensional networks of transition metal nodes bridged by rigid imidazolate linkers [1,2]
With the exception of ZIF-8 all resonances associated with the linker are found between 100 and 160 ppm, indicating the aromatic nature of the linkers and the presence of the diamagnetic Zn metal centre
In this work we have investigated the 13C and 15N NMR parameters of a series of ZIFs, prepared using one or more functionalised imidazolate and/or benzimidazolate linkers
Summary
Zeolitic imidazolate frameworks (ZIFs) are a relatively new subclass of metal-organic frameworks (MOFs) with extended three-dimensional networks of transition metal nodes (predominantly Zn2þ or Co2þ) bridged by rigid imidazolate linkers [1,2]. The observed Zn-Im-Zn bond angles are similar to that of the ideal Si–O–Si bond angle (~145) found in zeolites, so that many ZIFs exhibit the same framework topologies as zeolites, ZIFs with unique framework topologies have been synthesised [2]. The range of applications can be widened by using functionalised imidazolate linkers, altering the chemical nature of the pores produced, while maintaining the framework topology [10]. More than one type of imidazolate linker can be included into the framework structure [2]
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