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

Zhenfeng Pang, Kong Ooi Tan

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Zhenfeng Pang, Kong Ooi Tan. A focus on applying 63/65 Cu solid-state NMR spectroscopy to characterize Cu MOFs. Chemical Science, 2024, 15 (18), pp.6604-6607. 10.1039/D4SC90069C. hal-04572260

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

Zhenfeng Pang, Kong Ooi Tan

► To cite this version:

Zhenfeng Pang, Kong Ooi Tan. A focus on applying 63/65 Cu solid-state NMR spectroscopy to characterize Cu MOFs. Chemical Science, 2024, 15 (18), pp.6604-6607. 10.1039/D4SC90069C. hal-04572260

HAL Id: hal-04572260

https://hal.science/hal-04572260v1

Submitted on 10 May 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

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Distributed under a Creative Commons Attribution 4.0 International License
A focus on applying <sup>63/65</sup>Cu solid-state NMR spectroscopy to
characterize Cu MOFs

Zhenfeng Panga and Kong Ooi Tana,*

<sup>a</sup>Laboratoire des Biomolécules, LBM, Département de Chimie, École Normale Supérieure, PSL University, Sorbonne Université, CNRS, 75005 Paris, France

*Corresponding Authors :

Kong Ooi Tan

Laboratoire des Biomolécules, LBM, Département de Chimie,

École Normale Supérieure, PSL University, Sorbonne Université, CNRS,

75005 Paris, France

ORCID:0000-0002-3094-3398

E-Mail: kong-ooi.tan@ens.psl.eu
A focus on applying <sup>63/65</sup>Cu solid-state NMR spectroscopy to
characterize Cu MOFs

Zhenfeng Panga and Kong Ooi Tana,*

<sup>a</sup>Laboratoire des Biomolécules, LBM, Département de Chimie, École Normale Supérieure, PSL University, Sorbonne Université, CNRS, 75005 Paris, France

*Corresponding Authors :

Kong Ooi Tan

Laboratoire des Biomolécules, LBM, Département de Chimie,

École Normale Supérieure, PSL University, Sorbonne Université, CNRS,

75005 Paris, France

ORCID:0000-0002-3094-3398

E-Mail: [kong-ooi.tan@ens.psl.eu](mailto:kong-ooi.tan@ens.psl.eu)
**Abstract**

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 could be studied using various characterization techniques, these methods often do not provide local structural details that help explain their functionality. Zhang *et al*. (Wanli Zhang, Bryan E. G. Lucier, Victor V. Terskikh, Shoushun Chen, and Yining 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.
**Abstract**

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 could be studied using various characterization techniques, these methods often do not provide local structural details that help explain their functionality. Zhang *et al*. (Wanli Zhang, Bryan E. G. Lucier, Victor V. Terskikh, Shoushun Chen, and Yining 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, separations, and catalysis.¹ A critical factor influencing their functional properties is the incorporation of metal centers within the MOF structure. Because of the unique properties of Cu⁺ or Cu²⁺, copper-based MOFs have shown unique high photocatalytic activity,² enhanced luminescent properties,³ and promising biomedical applications.⁴ Copper can adopt various coordination environments within an MOF, such as two-coordinate linear, three- coordinate trigonal planar, or four-coordinate 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 powdered X-ray diffraction, energy dispersive spectroscopy (EDS), etc., often do not yield high-resolution information on local structural details of the metal centers. Hence, Zhang et al. have exploited ⁶³/⁶⁵Cu solid-state nuclear magnetic resonance (NMR) spectroscopy, a powerful method of choice to extract site-specific information of these copper environments in MOFs.⁵,⁶

Despite the promising aspects, ⁶³/⁶⁵Cu NMR is not commonly employed mainly because they are both spin-3/2 particles that possess quadrupolar interactions that are often too large to be averaged by magic-angle spinning (MAS) technique.⁷ Hence, their NMR spectra are very broad ( > 50 kHz) and the poor resolution usually limits its application to materials with a single site. Although they can be applied on materials with multiple well-defined sites, the results are highly dependent on the quality of spectra fitting, and the conclusions are sometimes subjective or debatable. Moreover, the broad NMR spectrum also inherently results in poor NMR sensitivity, which also limits its use to mostly simple 1D NMR experiments. Nevertheless, the linewidths of the ⁶³/⁶⁵Cu NMR spectra performed at static (non-spinning condition) are primarily determined by chemical-shift anisotropy (CSA) and electric-field gradient (EFG) tensors, which contain rich structural information. Zhang et al. have meticulously performed NMR experiments at high field (21.1 T) 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 ⁶³/⁶⁵Cu NMR spectra of [Cu₄I₄(DABCO)₂] (Fig. 1) can be very well simulated using the fitted 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 specific NMR parameters can be confidently assigned to specific copper sites within the MOF structure. This synergy between experiment and theory provides a powerful approach for a comprehensive understanding of the copper environment.
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, separations, and catalysis.¹ A critical factor influencing their functional properties is the incorporation of metal centers within the MOF structure. Because of the unique properties of Cu⁺ or Cu²⁺, copper-based MOFs have shown unique high photocatalytic activity,² enhanced luminescent properties,³ and promising biomedical applications.⁴ Copper can adopt various coordination environments within an MOF, such as two-coordinate linear, three- coordinate trigonal planar, or four-coordinate 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 powdered X-ray diffraction, energy dispersive spectroscopy (EDS), etc., often do not yield high-resolution information on local structural details of the metal centers. Hence, Zhang et al. have exploited ⁶³/⁶⁵Cu solid-state nuclear magnetic resonance (NMR) spectroscopy, a powerful method of choice to extract site-specific information of these copper environments in MOFs.⁵,⁶

Despite the promising aspects, ⁶³/⁶⁵Cu NMR is not commonly employed mainly because they are both spin-3/2 particles that possess quadrupolar interactions that are often too large to be averaged by magic-angle spinning (MAS) technique.⁷ Hence, their NMR spectra are very broad (> 50 kHz) and the poor resolution usually limits its application to materials with a single site. Although they can be applied on materials with multiple well-defined sites, the results are highly dependent on the quality of spectra fitting, and the conclusions are sometimes subjective or debatable. Moreover, the broad NMR spectrum also inherently results in poor NMR sensitivity, which also limits its use to mostly simple 1D NMR experiments. Nevertheless, the linewidths of the ⁶³/⁶⁵Cu NMR spectra performed at static (non-spinning condition) are primarily determined by chemical-shift anisotropy (CSA) and electric-field gradient (EFG) tensors, which contain rich structural information. Zhang et al. have meticulously performed NMR experiments at high field (21.1 T) 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 ⁶³/⁶⁵Cu NMR spectra of [Cu₄I₄(DABCO)₂] (Fig. 1) can be very well simulated using the fitted 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 specific NMR parameters can be confidently assigned to specific copper sites within the MOF structure. This synergy between experiment and theory provides a powerful approach for a comprehensive understanding of the copper environment.
<figure><img src="image_3.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>(c)</figcaption></figure>

<figure><img src="image_5.png" /></figure>

**Figure 1:** A schematic diagram of [Cu₄I₄(DABCO)₂] featuring three different Cu sites. The experimental (b) ⁶³Cu and (c) ⁶⁵Cu static NMR spectra (blue), cumulative simulations (red), and individual Cu site simulations (black, purple, green).

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 Cu MOF participating in an anion exchange reaction. Figure 2b shows that the $C_Q$ in Cu₄I₄(DABCO)₂ MOF has increased significantly when the MOF is soaked in NaNO₃ or NaClO₄ solutions. Using the results obtained earlier (Fig. 2a), it was inferred that the Cu(I) center has transitioned from a distorted tetrahedral configuration 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 connectives are similar but not identical.
<figure><img src="image_3.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>(c)</figcaption></figure>

<figure><img src="image_5.png" /></figure>

**Figure 1:** A schematic diagram of [Cu₄I₄(DABCO)₂] featuring three different Cu sites. The experimental (b) ⁶³Cu and (c) ⁶⁵Cu static NMR spectra (blue), cumulative simulations (red), and individual Cu site simulations (black, purple, green).

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 Cu MOF participating in an anion exchange reaction. Figure 2b shows that the $C_Q$ in Cu₄I₄(DABCO)₂ MOF has increased significantly when the MOF is soaked in NaNO₃ or NaClO₄ solutions. Using the results obtained earlier (Fig. 2a), it was inferred that the Cu(I) center has transitioned from a distorted tetrahedral configuration 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 connectives are similar but not identical.
<figure><img src="image_6.png" /><figcaption>**Figure 2:** (a) The relation between the chemical environment of Cu (I) and their CQ values. (b) The obvious change in linewidth and CQ indicate a possible structural change of (Cu₂(SO₄)(pyz)₂(H₂O)₂) upon addition of various aqueous solutions.</figcaption></figure>

Although the application of solid-state NMR in characterizing ultrawide line (UW) nuclei still faces challenges due to poor NMR sensitivity and resolution, we are optimistic that new NMR technologies, i.e., ultra-high-field NMR and hyperpolarization, will help circumvent these issues. It is known that ultra-high-field NMR is exceptionally beneficial in characterizing half-integer quadrupolar nuclei (e.g., 63/65Cu, 47/49Ti, 95Mo, 91Zr, 33S, 67Zn, etc.) because it grants higher resolution spectrum, in addition to higher sensitivity due to larger Boltzmann population. The latter feature is due to the fact that the linewidth of the NMR central transitions is inversely proportional to the B₀ magnetic field.⁷ These advantages have recently been exploited to study 95Mo, 33S, and 67Zn using either a commercially available 28.2 T magnet or the world highest-field 35.2 T magnet available in the US national facility (MagLab).⁸⁻¹⁰ On the other hand, dynamic nuclear polarization (DNP) is an NMR sensitivity enhancement technique that can usually boost the NMR signals by several orders of magnitude.¹¹⁻¹³
<figure><img src="image_6.png" /><figcaption>**Figure 2:** (a) The relation between the chemical environment of Cu (I) and their CQ values. (b) The obvious change in linewidth and CQ indicate a possible structural change of (Cu₂(SO₄)(pyz)₂(H₂O)₂) upon addition of various aqueous solutions.</figcaption></figure>

Although the application of solid-state NMR in characterizing ultrawide line (UW) nuclei still faces challenges due to poor NMR sensitivity and resolution, we are optimistic that new NMR technologies, i.e., ultra-high-field NMR and hyperpolarization, will help circumvent these issues. It is known that ultra-high-field NMR is exceptionally beneficial in characterizing half-integer quadrupolar nuclei (e.g., 63/65Cu, 47/49Ti, 95Mo, 91Zr, 33S, 67Zn, etc.) because it grants higher resolution spectrum, in addition to higher sensitivity due to larger Boltzmann population. The latter feature is due to the fact that the linewidth of the NMR central transitions is inversely proportional to the B₀ magnetic field.⁷ These advantages have recently been exploited to study 95Mo, 33S, and 67Zn using either a commercially available 28.2 T magnet or the world highest-field 35.2 T magnet available in the US national facility (MagLab).⁸⁻¹⁰ On the other hand, dynamic nuclear polarization (DNP) is an NMR sensitivity enhancement technique that can usually boost the NMR signals by several orders of magnitude.¹¹⁻¹³
In conclusion, Zhang et al. have shown that ⁶³/⁶⁵Cu NMR spectroscopy is a powerful tool for probing the copper environments within MOFs. By offering site-specific 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, data processing methods, and integration with other techniques hold immense promise for pushing the boundaries of ⁶³/⁶⁵Cu NMR spectroscopy and further enhancing our understanding of copper-based MOFs. This comprehensive understanding will ultimately pave the way for the rational design of MOFs with tailored properties for specific applications. Moreover, this NMR method could be extended to many different fields involving Cu(I) species, such as catalysis, surface chemistry, solar cells, and biochemistry.

**Author contributions**

Z. P. and K.O. T. wrote the manuscript.

**Conflicts of interest**

There are no conflicts to declare.

**Acknowledgements**

This work was supported by fundings obtained from the French National Research Agency (HFPulsedDNP, ANR-21-CE29-0019), RESPORE (n° 339299), and Junior Fellows PSL 2022 (n° 2022-306).

**References**

¹ H. Furukawa, K.E. Cordova, M. O'Keeffe, and O.M. Yaghi, *Science* (80-.). **341**, (2013).

² D. Shi, R. Zheng, M. Sun, X. Cao, C. Sun, C. Cui, C. Liu, J. Zhao, and M. Du, *Angew. Chemie* **129**, 14829 (2017).

³ L. Chen, X. Chen, R. Ma, K. Lin, Q. Li, J.-P. Lang, C. Liu, K. Kato, L. Huang, and X. Xing, *J. Am. Chem. Soc.* **144**, 13688 (2022).

⁴ M. Al Sharabati, R. Sabouni, and G.A. Husseini, *Nanomaterials* **12**, 277 (2022).

⁵ W. Zhang, B.E.G. Lucier, V. V Terskikh, S. Chen, and Y. Huang, *Chem. Sci.* **12** (2024).
In conclusion, Zhang et al. have shown that ⁶³/⁶⁵Cu NMR spectroscopy is a powerful tool for probing the copper environments within MOFs. By offering site-specific 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, data processing methods, and integration with other techniques hold immense promise for pushing the boundaries of ⁶³/⁶⁵Cu NMR spectroscopy and further enhancing our understanding of copper-based MOFs. This comprehensive understanding will ultimately pave the way for the rational design of MOFs with tailored properties for specific applications. Moreover, this NMR method could be extended to many different fields involving Cu(I) species, such as catalysis, surface chemistry, solar cells, and biochemistry.

**Author contributions**

Z. P. and K.O. T. wrote the manuscript.

**Conflicts of interest**

There are no conflicts to declare.

**Acknowledgements**

This work was supported by fundings obtained from the French National Research Agency (HFPulsedDNP, ANR-21-CE29-0019), RESPORE (n° 339299), and Junior Fellows PSL 2022 (n° 2022-306).

**References**

¹ H. Furukawa, K.E. Cordova, M. O'Keeffe, and O.M. Yaghi, *Science* (80-.). **341**, (2013).

² D. Shi, R. Zheng, M. Sun, X. Cao, C. Sun, C. Cui, C. Liu, J. Zhao, and M. Du, *Angew. Chemie* **129**, 14829 (2017).

³ L. Chen, X. Chen, R. Ma, K. Lin, Q. Li, J.-P. Lang, C. Liu, K. Kato, L. Huang, and X. Xing, *J. Am. Chem. Soc.* **144**, 13688 (2022).

⁴ M. Al Sharabati, R. Sabouni, and G.A. Husseini, *Nanomaterials* **12**, 277 (2022).

⁵ W. Zhang, B.E.G. Lucier, V. V Terskikh, S. Chen, and Y. Huang, *Chem. Sci.* **12** (2024).
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⁷ R.W. Schurko, *Acc. Chem. Res.* **46**, 1985 (2013).

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⁹ A.J. Stirk, S.T. Holmes, F.E.S. Souza, I. Hung, Z. Gan, J.F. Britten, A.W. Rey, and R.W. Schurko, *CrystEngComm* **1219** (2024).

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⁷ R.W. Schurko, *Acc. Chem. Res.* **46**, 1985 (2013).

⁸ Z.J. Berkson, R. Zhu, C. Ehinger, L. Lätsch, S.P. Schmid, D. Nater, S. Pollitt, O. V. Safonova, S. Björgvinsdóttir, A.B. Barnes, Y. Román-Leshkov, G.A. Price, G.J. Sunley, and C. Copéret, *J. Am. Chem. Soc.* **127**, 2023.

⁹ A.J. Stirk, S.T. Holmes, F.E.S. Souza, I. Hung, Z. Gan, J.F. Britten, A.W. Rey, and R.W. Schurko, *CrystEngComm* **12**, 1979 (2014).

¹⁰ E. Bellan, F. Maleki, M. Jakoobi, P. Fau, K. Fajerwerg, D. Lagarde, A. Balocchi, P. Lecante, J. Trébosc, Y. Xu, Z. Gan, L. Pautrot-D'Alençon, T. Le Mercier, H. Nagashima, G. Pacchioni, O. Lafon, Y. Coppel, and M.L. Kahn, *J. Phys. Chem. C* **127**, 17809 (2023).

¹¹ A.J. Rossini, A. Zagdoun, M. Lelli, A. Lesage, C. Copéret, and L. Emsley, *Acc. Chem. Res.* **46**, 1942 (2013).

¹² P. Berruyer, L. Emsley, and A. Lesage, *J. Phys. Chem. C* **127**, 93 (2013).

¹³ K.O. Tan, L. Yang, M. Mardini, C. Boon Cheong, B. Driesschaert, M. Dincă, and R.G. Griffin, *Chem. – A Eur. J.* **28**, 22 (2022).