A focus on applying 63/65Cu solid-state NMR spectroscopy to characterize Cu MOFs

Metal–organic frameworks (MOFs) are a class of hybrid organic and inorganic porous materials that have shown prospects in applications ranging from gas storage, separation, catalysis, etc. Although they can be studied using various characterization techniques, these methods often do not provide local structural details that help explain their functionality. Zhang et al. (W. Zhang, B. E. G. Lucier, V. V. Terskikh, S. Chen and Y. Huang, Chem. Sci., 2024, https://doi.org/10.1039/D4SC00782D) have recently exploited 63/65Cu solid-state NMR spectroscopy (for the first time) and DFT calculations to elucidate the structures of Cu(i) centers in MOFs. While there are still many challenges in overcoming issues in resolution and sensitivity, this work lays the foundation for further development of solid-state NMR technology in characterizing copper in MOFs or other amorphous solids.

Metal-organic frameworks (MOFs) are a novel class of materials that have gained popularity in recent decades due to their highly porous structures and exceptional tunability, making them attractive candidates for diverse applications such as gas storage, separation, and catalysis. 1A critical factor inuencing their functional properties is the incorporation of metal centers within the MOF structure.Because of the unique properties of Cu + or Cu 2+ , copper-based MOFs have shown uniquely high photocatalytic activity, 2 enhanced luminescence properties, 3 and promising biomedical applications. 4Copper can adopt various coordination environments within an MOF, such as two-coordinate linear, three-coordinate trigonal planar, or fourcoordinate tetrahedral.The coordination number plays a crucial role in determining the electrical conductivity, structural stability, and reactivity of the MOF.Characterization of these copper centers is challenging, especially for Cu(I), because, unlike Cu(II), Cu(I) is 'invisible' to EPR or UV-vis spectroscopy.Moreover, other spectroscopic techniques, such as powder X-ray diffraction, energy dispersive spectroscopy (EDS), etc., oen do not yield high-resolution information on local structural details of the metal centers.Hence, Zhang et al. have exploited 63/65 Cu solid-state nuclear magnetic resonance (NMR) spectroscopy, a powerful method of choice to extract site-specic information of these copper environments in MOFs [(https://doi.org/10.1039/D4SC00782D) 5 and ref .6].
Despite the promising aspects, 63/65 Cu NMR is not commonly employed, mainly because 63 Cu and 65 Cu are both spin-3/2 particles that possess quadrupolar interactions that are oen too large to be averaged by the magic-angle spinning (MAS) technique. 7Hence, their NMR spectra are very broad (>50 kHz) and the poor resolution usually limits the application of 63/65 Cu NMR to materials with a single site.Although it can be applied to materials with multiple well-dened sites, the results are highly dependent on the quality of spectra tting, and the conclusions are sometimes subjective or debatable.Moreover, the broad NMR spectra also inherently result in poor NMR sensitivity, which also limits its use to mostly simple 1D NMR experiments.Nevertheless, the linewidths of the 63/ 65 Cu NMR spectra performed under static (non-spinning) conditions are primarily determined by chemical-shi anisotropy (CSA) and electric-eld gradient (EFG) tensors, which contain rich structural information.Zhang et al. have meticulously performed high-eld (21.1 T) NMR experiments and density functional theory (DFT) calculations to extract the CSA and EFG tensors of 13 different Cu MOFs.For instance, they showed that the experimental 63/65 Cu NMR spectra of [Cu 4 I 4 (DABCO) 2 ] (Fig. 1) can be very well simulated using the tted NMR interactions.Moreover, it was shown that the three different Cu sites in the MOF can be remarkably well distinguished, which is a non-trivial task.The experimental NMR data were effectively combined with DFT calculations, so that specic NMR parameters could be condently assigned to specic copper sites within the MOF structure.This synergy between experiment and theory provides a powerful approach for a comprehensive understanding of the copper environment.
Moreover, Zhang et al. also provided a general tool (Fig. 2a) for estimating the chemical environments of Cu(I) via their quadrupolar coupling constants (C Q ).This allows them to elucidate the structural change in a Cu MOF participating in an anion exchange reaction.Fig. 2b shows that the C Q in the Cu 4 I 4 (DABCO) 2 MOF has increased signicantly when the MOF is soaked in NaNO 3 or NaClO 4 solutions.Using the results obtained earlier (Fig. 2a), it was inferred that the Cu(I) center has transitioned from a distorted tetrahedral conguration to either a two-or three-coordinate structure.The results were then compared with PXRD measurements performed on independently synthesized samples, and it was concluded that the connectivities are similar but not identical.
Although the application of solid-state NMR in characterizing ultra-wideline (UW) nuclei still faces challenges due to poor NMR sensitivity and resolution, we are optimistic that new NMR technologies, i.e., ultra-high-eld NMR and hyperpolarization, will help circumvent these issues.It is known that ultra-high-eld NMR is exceptionally benecial in characterizing half-integer quadrupolar nuclei (e.g., 63/65 Cu, 47/49 Ti, 95 Mo, 91 Zr, 33 S, 67 Zn, etc.) because it grants higherresolution spectra, in addition to higher sensitivity due to the larger Boltzmann population.The latter feature is due to the fact that the linewidth of the NMR central transitions is inversely proportional to the B 0 magnetic eld. 7][13] In conclusion, Zhang et al. have shown that 63/65 Cu NMR spectroscopy is a powerful tool for probing the copper environments within MOFs.By offering site-specic information about the coordination number and geometry of copper centers, it provides crucial insights into the factors governing the properties of MOFs.While challenges remain in overcoming signal broadening, sensitivity limitations, and the need for strategic isotopic enrichment, ongoing advancements in NMR technology, dataprocessing methods, and integration with other techniques hold immense promise for pushing the boundaries of 63/ 65 Cu NMR spectroscopy and further enhancing our understanding of copperbased MOFs.This comprehensive understanding will ultimately pave the way for the rational design of MOFs with tailored properties for specic applications.Moreover, this NMR method could be extended to many different elds involving Cu(I) species, such as catalysis, surface chemistry, solar cells, and biochemistry.

Fig. 2
Fig. 2 (a) The relationship between the chemical environments of Cu(I) and their C Q values.(b) The obvious changes in linewidth and C Q indicate a possible structural change of (Cu 2 (SO 4 )(pyz) 2 (H 2 O) 2 ) upon addition of various aqueous solutions.