Metal–organic frameworks (MOFs) are a unique class of porous materials synthesized via the combination of organic linkers and inorganic metal containing nodes to form 3D porous frameworks. MOFs have many applications such as gas storage/adsorption, sensing, catalysis, drug delivery and energy storage. Solid-state NMR (SSNMR) is an important technique for MOF characterization. Vast majority of NMR active isotopes of nuclides in MOFs are quadrupolar. Consequently, their NMR spectra often suffer from the quadrupolar broadening. Furthermore, many quadrupolar nuclei in MOFs are also unreceptive, resulting in low sensitivity. In this presentation, we will present the recent progress in SSNMR of quadrupolar nuclei for MOF characterization made in our labs.

1) Ultrahigh-resolution SSNMR
We report a new approach in overcoming sensitivity challenges in ⁶⁷Zn SSNMR. Combing new cryogenic CPMAS probe technology and preforming NMR experiments at a high magnetic field of 18.8 T results in significant signal enhancement, leading to enhanced spectral resolution. With boosted sensitivity, ⁶⁷Zn 3QMAS spectra can be obtained at natural abundance, allowing multiple chemically and crystallographically inequivalent Zn sites in several challenging MOFs to be resolved. Oxygen exists in various species in numerous MOFs. Using α-Mg₃(HCOO)₆ as an example we show that ¹⁷O 3QMAS at 35.2 T can provide very high resolution, unlocking identification of 12 inequivalent oxygen sites in this MOF.

2) Ultra-wideline NMR
Recently, Bi-based MOFs have attracted much attention. Characterization of Bi local environments by ²⁰⁹Bi SSNMR is challenging as ²⁰⁹Bi has an enormously large quadrupole moment, resulting in very large quadrupolar interaction. We characterized the Bi centers in several MOFs via ²⁰⁹Bi ultra-wideline NMR using the 36 T SCH magnet. We demonstrate that although these spectra are extremely wide with spectral breadths ranging from several to 40 MHz, they are very sensitive to the change in Bi local environment. Some important MOFs contain metal-iodide clusters as secondary building units. We have examined several such MOFs by ¹²⁷I ultra-wideline NMR at 36 T. Finally, ^63/65Cu SSNMR has been used to investigate a variety of Cu(I) sites and local coordination geometries in MOFs. NMR experiments at 21.1 T, supported by complementary plane-wave DFT calculations, illustrate how ^63/65Cu wideline NMR is a remarkably sensitive probe of local environment in Cu(I) MOFs.

Wanli Zhang

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