Solid-State NMR Spectroscopy: a Powerful and Comprehensive Technique to Characterize Porous Metal-Organic Frameworks

Solid-state NMR (ssNMR) is a unique tool for characterising Metal-Organic Frameworks (MOFs), a new and emerging class of porous hybrid crystalline materials. While the crystal structures are usually determined by diffraction techniques, ssNMR can give a comprehensive insight into the dynamic of the molecular moieties comprising the material and the gases diffusing inside the pores. Multi-nuclear and multi-dimensional high- resolution ssNMR techniques are used to demonstrate structural purity, activation completeness and structural changes in the material. 1H and 13C T1 spin-lattice relaxation times collected at variable temperatures provide information regarding the motional behaviour of the molecular moieties, elucidating the presence of molecular rotors and their related activation energies. It is also possible to study the interaction between the framework and adsorbed guests by sealing the NMR rotors under the desired conditions, for example, in the presence of vapours or under gas pressure, and monitor the consequent variation in the mobility of the molecular rotors. This is of particular interest for studying flexible crystalline materials that undergo structural changes depending on the adsorbed guest. Finally, the permanent porosity of the MOFs can be studied by hyperpolarised (HP) 129Xenon NMR. This laser-assisted technique exploits 129Xe nuclei as a probe to explore the shape and size of the cavities in the porous materials. Due to the high sensitivity of the HP 129Xe nuclei, it is possible to work in extremely diluted conditions (2% Xe in He/N2 gas mixture) to prevent room temperature Xe-Xe interactions and under flow conditions, thus proving direct accessibility of the pores and preferential sites of the framework. [1] Perego et al. Angew. Chem. Int. Ed. 2023, 62, e202215893 [2] Orfano et al. Nature Photonics 2023, DOI: 10.1038/s41566-023-01211-2 [2] Sozzani et al. Hyperpolarised Xenon-129 Magnetic Resonance: Concepts, Production, Techniques and Applications, RSC (2015)