Metal‐Organic Frameworks: NMR Studies of Quadrupolar Nuclei

Abstract

Metal‐organic frameworks (MOFs) are a novel type of porous materials with many current and potential applications. Solid‐state NMR (SSNMR) spectroscopy is an excellent technique for MOF characterization. It can provide nuclide‐specific information on structure and dynamics, which is complementary to that obtained from X‐ray diffraction. The NMR‐active isotopes of vast majority of metal centers including 27 Al, 71 Ga, 45 Sc, 67 Zn, 25 Mg, and 115 In as well as some framework elements such as 17 O are quadrupolar nuclei. Compared to spin ‐1/2 nuclei such as 13 C, SSNMR spectroscopy of quadrupolar nuclei has traditionally considered very challenging because of the quadrupolar interaction that makes spectral acquisition and interpretation difficult. This is particularly true for those so‐called unreceptive quadrupolar nuclei (i.e., the nuclei with low gyromagnetic ratios, low natural abundances, and large quadrupole moments). However, in recent years, significant progresses have been made in developing SSNMR spectroscopy of quadrupolar nuclei. This chapter begins with a brief introduction of NMR spectroscopy of quadrupolar nuclei; followed by discussion of several very recent examples to illustrate that SSNMR spectroscopy of quadrupolar nuclei can be used as an effective tool to characterize the nuclear environments in MOF‐based materials. It is hoped that this chapter will encourage more researchers to use SSNMR spectroscopy for investigation of MOF properties by interrogating quadrupolar nuclei.

Complete multinuclear solid‐state NMR of metal‐organic frameworks: The case of α‐Mg‐formate

“…This is an excellent example of how, even when NMR signals overlap, metal SSNMR can be used to resolve and assign the resonances from crystallographically non‐equivalent metal sites in MOFs, and this technique holds promise for determining the structures of MOFs where high‐quality XRD data are difficult or impossible to obtain. We have also shown in recent years that ultra‐wideline SSNMR techniques can be employed to probe unique metal centers in MOFs …”

“…Although metal centers are ubiquitous within MOFs and should serve as a convenient spectroscopic handle for SSNMR characterization, many MOFs feature multiple non‐equivalent metal sites that reside in similar chemical environments, which give rise to overlapping signals and pose a severe obstacle to metal SSNMR characterization. When overlapping signals are combined with the presence of quadrupolar metal centers with high quadrupole moments and low gyromagnetic frequencies, it is apparent that SSNMR of quadrupolar metal centers can be quite complicated …”

“…Research in our group has been focused on MOF characterization by multinuclear NMR in several areas: interrogating the metal centres, probing linker atoms, monitoring the behavior of guest molecules in the MOFs, and understanding interactions between guest species and the framework . In this article, we aim to use the α‐Mg‐formate (Mg 3 (HCOO) 6 MOF as an example to illustrate the power of SSNMR for MOF characterization, as it features metal ( 25 Mg), linker ( 1 H, 13 C, 17 O), and guest ( 13 C, 2 H) isotopes that are all NMR‐active.…”

Spectroscopy, microscopy, diffraction and scattering of archetypal MOFs: formation, metal sites in catalysis and thin films

“…Despite the significance of this field and the number of research reports, an overview and perspective of advanced MOF characterization tools is, to the best of our knowledge, lacking. Several review articles focused on a single or narrow selection of characterization techniques have been published, including: transmission electron microscopy (TEM), 6 magnetic, [7][8][9][10][11][12] X-ray [13][14][15] and vibrational 16,17 spectroscopies. One example of a broader view was provided by Howarth et al, 18 in which a selection of routinely used characterization tools was discussed, such as X-ray diffraction (XRD), thermogravimetrical analysis (TGA) and N 2 physisorption, responding to the need of establishing good practices for a fair comparison of MOF reports in the field.…”

Exploring Structural Disorders in Aluminum-Containing Metal–Organic Frameworks: Comparison of Solid-State 27Al NMR Powder Spectra to DFT Calculations on Bulk Periodic Structures

“…Here, we take the advantage of having the NMR-active quadrupole 27 Al (spin 5/2, 100% natural abundance) to probe the coordination environment of the metal centers in Al-MOFs with solid-state NMR and thus to estimate the deviation from the expected bulk structures. The approach of observing directly the metal centers of MOFs using solid-state NMR is relatively rare, and we have found only a few of such studies in the literature. − One such application is magic angle spinning (MAS) NMR measurements of the principal components of the electric field gradient (EFG)the quadrupolar constant C Q and the asymmetry parameter η, which are then compared with the theoretical values calculated using idealized MOF structures. − Another relevant application is the use of a multiple quantum MAS NMR to identify nonequivalent metal species in MOFs through the correlation between their quadrupolar parameters and chemical shifts. − …”