**Looking into Metal-Organic Frameworks with Solid-State NMR Spectroscopy**

Gregor Mali

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cm401270b

3618 Metal-Organic Frameworks

cphc.201000689

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/64134

#### **Abstract**

Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for characteriza‐ tion of materials. It can detect local structure around selected atomic nuclei and provide information on the dynamics of these nuclei. In case of metal-organic frameworks, NMR spectroscopy can help elucidate the framework structure, locate the molecules adsorbed into the pores, and inspect and characterize the interactions of these molecules with the frameworks. The present chapter discusses selected recent examples of solid-state NMR studies that provide valuable insight into the structure and function of metal-organic frameworks.

**Keywords:** MAS NMR, organic linker, metal centre, molecules within pores, shortrange order, disorder, mixed-linker MOFs

#### **1. Introduction**

Preparing the most efficient metal-organic framework materials (MOFs) for selected appli‐ cations requires not only knowledge about the atomic-scale structures of these MOFs but also understanding of the atomic-scale processes during the action of MOFs in catalysis, gas separation and storage, drug delivery, etc. In order to gain this knowledge and this under‐ standing, it is mandatory that MOFs are inspected by a set of complementary techniques that elucidate short- and long-range structural motifs, static and dynamic properties, interac‐ tions among the frameworks and the adsorbates. That is why, in addition to the very well established thermal, sorption and diffraction analyses, modeling and spectroscopic investi‐ gations are becoming more and more important in the studies of MOFs.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Among the spectroscopic techniques, solid-state nuclear magnetic resonance (NMR) is one of the most powerful characterization techniques, because it can provide element-specific atomicresolution insight into materials. It can be used at many different stages of research connect‐ ed to MOFs; from studies of their formation, their structure determination, to in-situ studies of their performance. As a local spectroscopic tool, solid-state NMR is complementary to diffraction techniques that rely on the existence of long-range order and that provide a picture of an average crystal structure. NMR experiments can prove or disprove the hypotheses proposed by modeling, predicting preferential adsorption sites and estimating strength of interactions between the adsorbed molecules and the MOF matrices. It can follow gradual adsorption or desorption of molecules into pores, locate and quantify these molecules, and thus complement the data obtained by the thermal and sorption analyses. Solid-state NMR is also extremely important for studying dynamics of the frameworks and of the adsorbed molecules. Therefore, employing NMR spectroscopy is crucial for deducing the structure-tofunction relationships of MOFs.

The present chapter begins with a short introduction into solid-state NMR spectroscopy. Here, the basic characteristics of NMR and the most important techniques and recent methodolog‐ ical developments for studying the framework and the adsorbed molecules are briefly mentioned. NMR spectroscopy is particularly important for studying motifs in MOFs that do not exhibit long-range order. Such motifs can be found in the frameworks themselves, especially when one is dealing with mixed-linker or mixed-metal MOFs, and when the molecules within the pores do not occupy equal positions within each unit cell. The second section of the chapter presents some interesting and representative examples of NMR studies of frameworks of the metal-organic materials. For example, in case of mixed-linker MOFs two recent studies demonstrate that solid-state NMR is the only technique, which not only proves or disproves the incorporation of different linkers into the framework, but also provides an answer about the distribution of different linkers within the frameworks. The third, largest section of the chapter is devoted to the application of NMR spectroscopy for studying the molecules adsorbed into the pores of MOFs. These molecules can be solvent molecules, which remain trapped in MOFs after the synthesis and can play a stabilizing role in these materials, or can be molecules adsorbed during the application of MOFs in catalysis, in gas separation and storage, in energy storage, and in drug-delivery.

The chapter does not attempt to present a complete review of the solid-state NMR studies on MOFs, but focuses on prominent recent examples and discusses their impact on the under‐ standing of the properties and functioning of MOFs. Some other, more extensive reviews on NMR spectroscopy of MOFs can be found in literature [1, 2].

### **2. Briefly about solid-state NMR spectroscopy**

Atomic nuclei with nonzero magnetic dipole moment and atomic nuclei with nonzero electric quadrupole moment are extremely sensitive probes capable of detecting tiny differences in local magnetic and local electric fields. Local magnetic field at the position of an atomic nucleus depends on the electric currents in the vicinity of this nucleus and on the number and geometrical arrangement of magnetic moments in its neighborhood. A strong external magnetic field, which is in a modern NMR spectrometer generated by a superconducting magnet, induces electric currentin the cloud of electrons thatis surrounding an atomic nucleus. This electric current gives rise to a local magnetic field that partially shields the external magnetic field. We are talking about chemical shielding and chemical shift, because the extent of shielding and the consequent shift of the spectral line in the NMR spectrum depend on the chemical environment of the atomic nucleus. With chemical environment we mean the nature and the number of neighbors in the first and second coordination shells, and the nature, strength and angles of the nearest chemical bonds. For example, chemical shielding and the resulting local magnetic fields are not equal at the positions of 13C nuclei in CH4 and in CH3OH; consequently, the 13C NMR signals of CH4 and CH3OH resonate at different frequencies.

Among the spectroscopic techniques, solid-state nuclear magnetic resonance (NMR) is one of the most powerful characterization techniques, because it can provide element-specific atomicresolution insight into materials. It can be used at many different stages of research connect‐ ed to MOFs; from studies of their formation, their structure determination, to in-situ studies of their performance. As a local spectroscopic tool, solid-state NMR is complementary to diffraction techniques that rely on the existence of long-range order and that provide a picture of an average crystal structure. NMR experiments can prove or disprove the hypotheses proposed by modeling, predicting preferential adsorption sites and estimating strength of interactions between the adsorbed molecules and the MOF matrices. It can follow gradual adsorption or desorption of molecules into pores, locate and quantify these molecules, and thus complement the data obtained by the thermal and sorption analyses. Solid-state NMR is also extremely important for studying dynamics of the frameworks and of the adsorbed molecules. Therefore, employing NMR spectroscopy is crucial for deducing the structure-to-

The present chapter begins with a short introduction into solid-state NMR spectroscopy. Here, the basic characteristics of NMR and the most important techniques and recent methodolog‐ ical developments for studying the framework and the adsorbed molecules are briefly mentioned. NMR spectroscopy is particularly important for studying motifs in MOFs that do not exhibit long-range order. Such motifs can be found in the frameworks themselves, especially when one is dealing with mixed-linker or mixed-metal MOFs, and when the molecules within the pores do not occupy equal positions within each unit cell. The second section of the chapter presents some interesting and representative examples of NMR studies of frameworks of the metal-organic materials. For example, in case of mixed-linker MOFs two recent studies demonstrate that solid-state NMR is the only technique, which not only proves or disproves the incorporation of different linkers into the framework, but also provides an answer about the distribution of different linkers within the frameworks. The third, largest section of the chapter is devoted to the application of NMR spectroscopy for studying the molecules adsorbed into the pores of MOFs. These molecules can be solvent molecules, which remain trapped in MOFs after the synthesis and can play a stabilizing role in these materials, or can be molecules adsorbed during the application of MOFs in catalysis, in gas separation

The chapter does not attempt to present a complete review of the solid-state NMR studies on MOFs, but focuses on prominent recent examples and discusses their impact on the under‐ standing of the properties and functioning of MOFs. Some other, more extensive reviews on

Atomic nuclei with nonzero magnetic dipole moment and atomic nuclei with nonzero electric quadrupole moment are extremely sensitive probes capable of detecting tiny differences in local magnetic and local electric fields. Local magnetic field at the position of an atomic nucleus

function relationships of MOFs.

38 Metal-Organic Frameworks

and storage, in energy storage, and in drug-delivery.

NMR spectroscopy of MOFs can be found in literature [1, 2].

**2. Briefly about solid-state NMR spectroscopy**

As mentioned above, the second important contribution to the local magnetic field at the position of an atomic nucleus is the contribution of the neighboring atomic nuclei with nonzero magnetic moments. Each atomic nucleus with a magnetic moment acts as a tiny source of magnetic field in a similar way as a bar magnet generates magnetic field in its surroundings. What is particularly important with such magnetic fields is that their strengths depend strictly on the distances between the atomic nuclei that generate the fields and the atomic nucleus that detects these fields. We say that a pair of proximal nuclei is coupled through the magnetic dipolar coupling. With the advanced NMR experiments we can exploit this dipolar coupling for obtaining qualitative or sometimes even quantitative information about the interatomic distance.

**Figure 1.** Summary of the most important interactions, detected by NMR, and of the available information.

Local electric field at the position of an atomic nucleus depends on the arrangement of electric charges in the neighborhood of the nucleus. In fact, NMR spectroscopy detects electric field gradients to which only the nonspherical atomic nuclei are sensitive. These nuclei are often called quadrupolar nuclei and they all have spin quantum number larger than ½. The sensitivity of quadrupolar nuclei to the local electric field gradients can provide useful information on the symmetry of the environments around them. Measurement of the

magnitude of the electric field gradient (the strength of the electric quadrupolar interaction) can also yield an insight into the dynamics of the species in which the atomic nuclei are located, that is, for example, into the dynamics of a framework or a molecule. **Figure 1** schematically shows the most important sources of local fields detectable by solid-state NMR. The figure also briefly summarizes the information that is offered by chemical shifts, dipolar couplings, and electric quadrupolar interactions.

The magnitude and the direction of a local magnetic field do not depend only on the molecu‐ lar or crystalline environment, but also on the orientation of the molecule or the crystal fragment with respect to the direction of the external magnetic field. For example, chemical shieldings in a phenyl ring that is perpendicular to the external magnetic field and in a phenyl ring that is parallel to the external magnetic field will differ from one another, because the induced electric currents in the rings will be different. When studying materials, we are very often dealing with powders in which the particles are oriented in many different directions. This means that, for example, atomic nuclei at equal crystallographic sites but in differently oriented crystallites will detect different local fields. In an NMR spectrum of such a powder, the corresponding NMR signal will reflect the distribution of the local fields and will appear as a broad distributed line, which is called a powder pattern. In this respect, NMR spectra of powders are very much different from the NMR spectra of solutions. In solutions, due to fast motion and reorientation of molecules, atomic nuclei detect orientationally averaged local fields and NMR spectra exhibit very sharp signals. We say that in solutions, NMR detects only the isotropic contributions to the local fields. The resolution of the obtained NMR spectra is excellent and the detection of tiny differences in chemical environments is easy. As opposed to that, the resolution in solid-state NMR spectra of powdered materials is very poor, because the broad powder patterns overlap extensively. In order to improve resolution and thus to gain more information about the inequivalent sites in materials, we try to mimic fast molecu‐ lar reorientations by spinning powdered samples very quickly. Indeed, fast spinning about the axis that is inclined from the direction of the external magnetic field by 54.7° improves resolution of NMR spectra of powders drastically. The special angle mentioned above is called the magic angle and the method is named magic angle spinning (MAS) [3]. MAS is the basis of almost all modern solid-state NMR experiments.

Apart from attempts for improving spectral resolution, solid-state NMR faces two other major technical challenges. The first is how to increase the NMR signal, which is very weak com‐ pared to signals of other spectroscopies. One ofroutinely employed approaches to achieve that is the usage of cross polarization (CP) step [4]. With this step, the signal of the selected atomic nuclei is enhanced via the transfer of spin polarization from atomic nuclei with larger magnetic moments. For example, 1 H nuclei have four- and ten-times as large magnetic moment as 13C and 15N nuclei, respectively, thus the application of <sup>1</sup> H-13C and <sup>1</sup> H-15N CP approach can greatly enhance 13C and 15N NMR signals. The method is most efficient if both types of atomic nuclei have spin quantum number of ½ and if the nuclei are coupled with strong dipolar coupling (i.e., if the nuclei of the two types are proximal one to another). The above listed nuclei <sup>1</sup> H, 13C, and 15N all have spin equal to ½. Because they are the major constituents of the organic linkers

in MOFs and of many molecules that are adsorbed within MOFs (small organic molecules, drug molecules), CP-MAS-based experiments are regularly employed for studying MOFs.

magnitude of the electric field gradient (the strength of the electric quadrupolar interaction) can also yield an insight into the dynamics of the species in which the atomic nuclei are located, that is, for example, into the dynamics of a framework or a molecule. **Figure 1** schematically shows the most important sources of local fields detectable by solid-state NMR. The figure also briefly summarizes the information that is offered by chemical shifts, dipolar couplings,

The magnitude and the direction of a local magnetic field do not depend only on the molecu‐ lar or crystalline environment, but also on the orientation of the molecule or the crystal fragment with respect to the direction of the external magnetic field. For example, chemical shieldings in a phenyl ring that is perpendicular to the external magnetic field and in a phenyl ring that is parallel to the external magnetic field will differ from one another, because the induced electric currents in the rings will be different. When studying materials, we are very often dealing with powders in which the particles are oriented in many different directions. This means that, for example, atomic nuclei at equal crystallographic sites but in differently oriented crystallites will detect different local fields. In an NMR spectrum of such a powder, the corresponding NMR signal will reflect the distribution of the local fields and will appear as a broad distributed line, which is called a powder pattern. In this respect, NMR spectra of powders are very much different from the NMR spectra of solutions. In solutions, due to fast motion and reorientation of molecules, atomic nuclei detect orientationally averaged local fields and NMR spectra exhibit very sharp signals. We say that in solutions, NMR detects only the isotropic contributions to the local fields. The resolution of the obtained NMR spectra is excellent and the detection of tiny differences in chemical environments is easy. As opposed to that, the resolution in solid-state NMR spectra of powdered materials is very poor, because the broad powder patterns overlap extensively. In order to improve resolution and thus to gain more information about the inequivalent sites in materials, we try to mimic fast molecu‐ lar reorientations by spinning powdered samples very quickly. Indeed, fast spinning about the axis that is inclined from the direction of the external magnetic field by 54.7° improves resolution of NMR spectra of powders drastically. The special angle mentioned above is called the magic angle and the method is named magic angle spinning (MAS) [3]. MAS is the basis

Apart from attempts for improving spectral resolution, solid-state NMR faces two other major technical challenges. The first is how to increase the NMR signal, which is very weak com‐ pared to signals of other spectroscopies. One ofroutinely employed approaches to achieve that is the usage of cross polarization (CP) step [4]. With this step, the signal of the selected atomic nuclei is enhanced via the transfer of spin polarization from atomic nuclei with larger magnetic

enhance 13C and 15N NMR signals. The method is most efficient if both types of atomic nuclei have spin quantum number of ½ and if the nuclei are coupled with strong dipolar coupling (i.e., if the nuclei of the two types are proximal one to another). The above listed nuclei <sup>1</sup>

and 15N all have spin equal to ½. Because they are the major constituents of the organic linkers

H nuclei have four- and ten-times as large magnetic moment as 13C

H-15N CP approach can greatly

H, 13C,

H-13C and <sup>1</sup>

and electric quadrupolar interactions.

40 Metal-Organic Frameworks

of almost all modern solid-state NMR experiments.

and 15N nuclei, respectively, thus the application of <sup>1</sup>

moments. For example, 1

NMR-active nuclei in the inorganic metal-oxo vertices in MOFs are typically quadrupolar nuclei, that is nuclei with spin largerthan ½. Among them, 27Al, 45Sc, and 51V are abundant and have moderately large magnetic moments, therefore NMR spectroscopy of these nuclei is very sensitive. In majority of other cases, such as in case of 25Mg, 67Zn, or 91Zr, we are dealing with low-abundance nuclei and/or with nuclei with small magnetic moments, with which NMR spectroscopy is very demanding. Increasing the NMR signal of these nuclei is often attempt‐ ed through the acquisition of the Carr-Purcell-Meiboom-Gill (CPMG) train of echoes [5], and through the usage of strong external magnetic fields and large amounts of samples. Additionally, because spectral lines of quadrupolar nuclei can be very wide, broadbanded WURST (Wideband, Uniform Rate, and Smooth Truncation) or similar type of excitation is often needed [6].

Even WURST-CPMG and strong magnetic fields are usually not sufficient for a successful NMR spectroscopy of oxygen nuclei. Because the NMR-active isotope 17O is a very rare isotope of oxygen, only 0.04% abundant in nature, practical 17O NMR spectroscopy relies on isotopic enrichment of samples. Isotopic enrichment can be useful or even necessary also when measuring 13C or 15N NMR spectra of adsorbed molecules, especially if these molecules are present in small concentrations within MOFs and if CP via <sup>1</sup> H nuclei is not possible (either because the molecules do not contain hydrogen atoms, or because the dipolar coupling with 1 H nuclei is motionally averaged out). Typical examples of NMR studies of isotopically enriched molecules are studies of 13CO2 molecules within the pores of MOFs. **Table 1** lists selected physical properties for atomic nuclei (isotopes) that are most often employed as probes in NMR spectroscopy of MOFs.


**Table 1.** Selected properties of nuclear isotopes, which are frequently encountered in MOFs.

The third technical challenge of solid-state NMR spectroscopy is how to extract the informa‐ tion about the selected internuclear distances. In case of MOFs, these could be the distances

among the atomic nuclei of the adsorbed molecules and the atomic nuclei within the frame‐ works. As mentioned above, the information on the distances is contained in the magnitude of the dipolar coupling between these nuclei. However, because dipolar coupling is an anisotropic interaction, it is very efficiently suppressed or entirely averaged out by MAS. To keep the enhanced resolution of NMR spectra obtainable by MAS, but still to be able to detect the magnitude of the dipolar coupling among the selected nuclei, several recoupling techni‐ ques were developed and were included into different types of homonuclear and heteronu‐ clear correlation experiments. Homonuclear correlation experiments, like <sup>1</sup> H-1 H correlation experiments, most often exploit spin diffusion enhanced by RFDR (Radio-Frequency-driven Dipolar Recoupling) [7], or recoupling of the dipolar interaction by BABA or POST-C7 pulse sequences [8, 9]. Heteronuclear dipolar couplings, like <sup>1</sup> H-13C couplings, can be probed qualitatively by various two-dimensional HETCOR (HETero-nuclear CORrelation) experi‐ ments [10, 11] or quantitatively by the REDOR-type pulse sequences [12].

In the above discussion of local magnetic fields, we have skipped a contribution that is often quite important in MOFs. It is the contribution of paramagnetic centers, such as Cr, Mn, Fe, Co, Ni, and Cu centers. Unpaired electrons of these centers have much larger magnetic moments than atomic nuclei and therefore drastically affect the NMR spectra of neighboring nuclei. In powders, the strong dipolar interaction with unpaired electrons leads to very fast nuclear spin-lattice relaxation and to huge line broadening of NMR signals. These effects depend on the geometrical arrangement of the unpaired electrons around an atomic nucleus and again offer some information on the distances between the paramagnetic centers and atomic nuclei. If electronic spin polarization is through bonds transferred to the position of an atomic nucleus, the so-called hyperfine electron-nucleus interaction has to be taken into account. This interaction can be very strong and can severely shift NMR lines. For example, for nuclei that are two bonds apart from the paramagnetic metal centre (e.g., for 13C in the –C– O–Cu motif), the shifts can be several hundred or even several thousand ppm. Because of the difficulties connected with the measurement of extremely broad, severely shifted NMR signals and because of very quick nuclear spin relaxation, NMR measurements in paramagnetic MOFs are relatively rare.
