The pioneering work of Lippmaa and co-workers on high-resolution 29Si solid-state NMR (SSNMR) of silicates has inspired an enormous amount of research in porous materials. The application of high-resolution 29Si MAS NMR has furthered our understanding of the structures of zeolites and related molecular sieves tremendously. In the last two decades, the field of porous materials has advanced dramatically. Metal-organic frameworks (MOFs) have emerged as a new generation of porous materials. MOFs are crystalline hybrid inorganic-organic solids prepared via self-assembly of metal cations/clusters with organic linkers to form 3D porous networks with novel topologies. MOFs have many unique properties, resulting in numerous important applications. SSNMR spectroscopy remains a key technique for MOF characterization. In this presentation, we use the α-Mg3(HCOO)6 (α-Mg-formate) MOF as an example to illustrate the power of multinuclear high-resolution SSNMR for MOF characterization. α-Mg-formate is a prototypical small pore MOF, which is a very challenging system for SSNMR characterization. It features four NMR-active isotopes: 25Mg (the metal centre), 1H, 13C and 17O (from organic linker). The MOF structure features 4 inequivalent metal centers with very similar local envelopment, 6 chemically equivalent, but crystallographically inequivalent framework protons and carbons. Furthermore, this MOF contains 12 crystallographically unique, yet chemically similar, carboxylate oxygen sites originating from the two different types of binding modes in this MOF, with 6 μ2-COO and 6 μ1-COO sites present in the crystal structure. Using various high-resolution NMR techniques and the state-of-the art magnet technology, we demonstrated that the complete resolution of all inequivalent sites for each framework element can be achieved. The ability to completely resolve multiple chemically and, more importantly, crystallographically inequivalent sites in MOFs, as described in this work, greatly increases the potential for using NMR crystallography to determine the structures of new MOFs and refine the crystal structures of existing MOFs. Furthermore, we have recently made strides in designing new protocols for post-synthetic modification of MOFs. High-resolution SSNMR has played a key role in establishing these new platforms.

High-resolution SSNMR: Insights into the structure and chemistry of metal-organic frameworks

ACS Spring Meeting 2021

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