Materials Today Advances 16 (2022) 100287
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Materials Today Advances
journal homepage: www.journals.elsevier.com/materials-today-advances/
The Role of NMR in Metal Organic Frameworks: Deep Insights into
Dynamics, Structure and Mapping of Functional Groups
Sajid ur Rehman a, 1, Shuai Xu a, b, 1, Huangtao Xu a, Tongxiang Tao a, c, Yunyan Li a,
Zhiwu Yu a, Kun Ma a, *, Weihong Xu d, Junfeng Wang a, c, e, **
a
CAS Key Laboratory of High Magnetic Field and Ion Beam Physical Biology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, Anhui,
230031, PR China
Hefei Cancer Hospital, Anhui Province Key Laboratory of Medical Physics and Technology, Institute of Health and Medical Technology, Hefei Institutes of
Physical Science, Chinese Academy of Sciences, Hefei, 230031, Anhui, PR China
c
University of Science and Technology of China, Hefei, 230036, Anhui, PR China
d
Anhui Province Key Laboratory of Biomimetic Sensing and Advanced Robot Technology, State Key Laboratory of Sensor Technology, Institute of Intelligent
Machines, Hefei Institute of Physical Science, Chinese Academy of Sciences, Hefei, 230031, China
e
Institutes of Physical Science and Information Technology, Anhui University, Hefei 230601, Anhui, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 July 2022
Received in revised form
16 August 2022
Accepted 19 August 2022
Available online 29 August 2022
The fundamental understanding of metal-organic frameworks (MOFs) is crucial since the relationship
between the macroscopic properties of these materials and their molecular-level structures allows for
the development of technological applications and improvements in current performance. The metal
centers and attached functional groups dictate MOFs' dynamics, structure, and porosity. The development of the solid-state nuclear magnetic resonance (SSNMR) technique, capable of providing atomiclevel information, enables the non-destructive characterization of the structure and dynamics of MOFs
have become essential step in ameliorating our understanding and are often complementary to traditional methods. This review aims to briefly introduce the concepts of SSNMR and the methods employed
when investigating the dynamics, structure, and mapping of functional groups of microporous materials,
including MOFs. This review highlights the best experimental practices when working with these
complex systems. The article scrutinizes the information on framework structures, active center, surface
position, host-guest interaction, and intermediate interaction through different SSNMR spectrums.
Despite all the recent technological advancements, the SSNMR still faces the challenges of large sample
quantities, long experimental measurements and data analyses, and complex isotopic labeling, which are
enlightened in this review.
© 2022 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/).
Keywords:
Metal organic frameworks
NMR
mapping of functional groups
host-guest interaction
1. Introduction
The field of material characterization is as comprehensive and
diverse as the combination of materials and engineering [1,2].
Many relative methods have been developed for over decades, from
very rough tools to highly complex instruments to the measurement and testing of materials spanning from mechanical, electrical,
* Corresponding author.
** Corresponding author. CAS Key Laboratory of High Magnetic Field and Ion Beam
Physical Biology, Hefei Institutes of Physical Science, Chinese Academy of Sciences,
Hefei, Anhui, 230031, PR China.
E-mail addresses: makun@hmfl.ac.cn (K. Ma), junfeng@hmfl.ac.cn (J. Wang).
1
These authors contributed equally.
and thermal properties; from metals to semiconductors, insulators,
ceramics, polymers, and composites in between; from atomic scale
to nano-, micro-, and macro-scale; from pico-seconds to years of
testing and countless simulations [3]. The materials characterization field is vast and structural characterization is a prerequisite for
a better understanding of the material properties and underlying
phenomena. In 1946, following the initial discoveries of Purcell and
Bloch, the development of nuclear magnetic resonance spectroscopy was considered one of the most important events in the
progress of organic chemistry and material science [2,4]. Nuclear
magnetic resonance (NMR) technology injects new light into many
complex materials [5,6]. Among the spectral techniques that can be
utilized for the characterization, non-destructive and non-invasive
solid-state NMR (SSNMR) spectroscopy has been developed as a
https://doi.org/10.1016/j.mtadv.2022.100287
2590-0498/© 2022 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
S. Rehman, S. Xu, H. Xu et al.
Materials Today Advances 16 (2022) 100287
directly proportional to the distance between the atomic nuclei
that generate the fields and the atomic nucleus that detects these
fields. Utilizing the dipolar coupling in modern NMR research
permits us to obtain qualitative and occasionally even quantitative
information regarding the interatomic distance.
It is imperative to understand the relationship between the
structure and properties of materials at the atomic level [37,38].
NMR produces atomic contact and proximity between some elements in molecules [39], a breakthrough in SSNMR research.
Relevant correlation heteronuclear experiments, such as 29Si-27QAl
NMR and 31Pe-27Al NMR, provide information about the contact
structure of Sie-Oe-Al and Pe-Oe-Al structural units [40]. The
29
Sie-29Si homonuclear NMR correlation spectrum can reveal the
valence point in the crystallization process or the bonding relationship with space, which provides structural constraints for the
molecular sieve powder whose structure cannot be determined by
X-ray diffraction [41]. In addition, it is also proved that spectral
correlation is an ideal tool to study the interaction between porous
framework materials and guest molecules adsorbed in channels.
For example, combining 27Ale-29Si, 1He-27Al, and 1He-13C NMR
spectra revealed the specific position of the interaction between
organic structure-directing agent and zeolite skeleton to reveal the
crystallization mechanism of zeolite [42,43]. The surface reaction
mechanism can be established in the catalytic reaction by detecting
the spatial proximity between the molecules adsorbed in the
catalyst by NMR correlation (such as the 13Ce-13C correlation
spectrum). The development of NMR spectroscopy-related
methods and technologies have dramatically improved the intensity of NMR and enriched its applications for structural and
dynamics study of materials [44].
This introduction succinct that NMR has become an indispensable spectroscopic tool for the atomic-scale study of assortment
compounds. In this review, the first section is the structure elucidation of MOF in the eyes of NMR, including a description of the
interactions between nuclear spins and their environment. Then,
the article scrutinizes the information on the mapping of functional
groups. Section 4 will be discussed about the role of NMR for the
investigation of guest molecules. Section 5 and 6 are about the
defects and dynamics of 17O SSNMR. Further, the 13C NMR, MOF
docking, active center, surface position, transformation of MOFs
discussed. Lastly, the review will conclude with the challenges and
prospects in this field.
powerful tool with atomic-level resolution in the structural examination of materials [7,8]. Compared with X-ray diffraction
(XRD), which requires long-range structure ordering, SSNMR is
very sensitive to short-range and medium-range geometry and
ordering [9,10]. The element-specific feature allows NMR to be a
key spectral tool for detecting the local chemical environment of
target nuclei, including coordination States and electronic structures [11]. In situ/operand SSNMR technology provides an important way to study mechanisms such as molecular identity,
structure, magnetic resonance imaging and catalytic reactions [12].
All NMR methods are used to provide structural information by
using various nuclear spin interactions, including chemical shift
anisotropy (CSA), dipole interaction and quadrupole interaction of
nuclei with spin quantum number >1/2 [13].
One example of modern organic-inorganic material such as
Metal-Organic Frameworks (MOFs), often features non-uniform
active components of organic frameworks, pores, surfaces, and
interfaces [14e19]. For instance, the zeolite's acid sites (including
Brønsted and Lewis) are heterogeneously dispersed in the zeolite
network [20e22]. In this regard, SSNMR spectroscopy is a powerful
tool for finding the environment around metal centers and
detecting the structure of organic linkers and the behavior of adsorbates and acid centers, which is critically important for many
applications [23,24]. MOFs are well known for removing greenhouse gases such as CO2 and storing fuels such as H2 and CH4 [25].
SSNMR provides information on the location of guest species,
which is very important in practical applications because the
location of guest gas molecules can be directly related to the
location and strength of binding sites [26,27]. Similarly, finding
target species in MOFs is key to understanding their drug delivery
applications and sensors [28,29]. Oxygen is present in various
carboxylic acid ligands, the widely used organic linker and the key
constituent of many important MOFs. Oxygen anions (O2) are
related to the metal clusters of the frameworks (e.g., MOF-5) [24].
On the flip side, Hydroxyl groups are very common linkers bridging
metal clusters (e.g., MIL-53) that exist as part of secondary structural units (e.g., UiO-66) [30]. The SSNMR chemical shifts allow
direct structural distribution of surface groups and organic units.
The oxygen spectra play a critical role in the insight of applications
such as drug encapsulation/release, sensing, gas absorption and
electrocatalysis. For different hydroxyl groups on molecular zeolites, acidic protons (Sie-OHe-Al) and SiOH groups can be easily
eminent by 1H NMR spectra [31]. Meanwhile, 13C SSNMR spectra
deliver info about the structure of organic linkers and enable them
to be quantified [32]. In addition, the peak width of NMR signal
determined by the bond length and angular distribution is an indicator of the degree of disorder in the structure [33] as shown in
Scheme 1.
Atomic nuclei with nonzero magnetic dipole moments and
nonzero electric quadrupole moments are extraordinarily sensitive
probes capable of detecting highly minute fluctuations in their
immediate environment's magnetic and electric fields [34]. A high
external magnetic field, created by a superconducting magnet in a
contemporary NMR spectrometer induces an electric current in the
cloud of electrons surrounding an atomic nucleus [35]. This electric
current generates a magnetic field in the local vicinity, which masks
the effect of the bigger, more potent field, which permits NMR
signal to be sensed and further measured [36]. The chemical shift of
the spectral line in the NMR spectrum relies on the chemical
environment. The phrase “"chemical environment”" refers to the
number of neighbors present in the first and second coordination
shells and the strength and angle of nearby chemical bonds. The
second key factor to the local magnetic field at the position of an
atomic nucleus is the contribution of nearby atomic nuclei with
nonzero magnetic moments. The strength of such magnetic fields is
2. Structure elucidation of MOF in the eyes of NMR
Porous crystalline MOFs are multifunctional materials whose
properties can be efficiently tuned by combining various metalbased junctions with infinitely abundant organic ligands [45]. At
the same time, these tunable properties require fundamental aspects of atomic-scale characterization to explain and control these
effective materials' macroscopic microstructure and dynamic
properties [46,47]. Diffraction methods are very limited in this field
due to the difficulty of single crystal growth and manipulation
during adsorption, and nuclear magnetic resonance (NMR) is useful
for studying the local structure around metals, the structure, kinetics of organic ligands, and the location of invited molecules
[48,49]. The environment provides a powerful alternative. MAS
NMR of open-shell metal-ion MOFs exhibits hyperfine interactions
between NMR-active nuclei and unpaired electrons in paramagnetic metals, which encode important information on the geometry and electronic structure of the metal environment [50].
Blahut et al. [51] showed that with the help of very fast (60 kHz)
MAS rates and tailored radiofrequency (RF) irradiation schemes,
paramagnetic properties can become an advantage for the characterization of open-shell MOFs by 1H -NMR, including sensitive
2
S. Rehman, S. Xu, H. Xu et al.
Materials Today Advances 16 (2022) 100287
Scheme 1. The in-situ ball-stick model illustration of MOF-1005 within an NMR setup. Pink color shows the hydrogen; red are the oxygen; grey are carbon and green are zirconia
atoms.
detection and assessment of 1H and 13C resonances of organic
bonds through 2D 1He-13C and 1He-1H correlations, investigation
of local structures and internal dynamics, and elucidation of the
electronic properties of metals over a wide temperature range.
Blahut's group demonstrated this approach on a switchable
column-layer MOF prototype that provides variable metal substitution, DUT-8(M) containing a (Ni2þ)2 paddle-wheel (PW) in an
open-pore (OP). A strong hysteresis is exhibited in the invited
adsorption/desorption process between OP and closed-pore (CP)
structures including invited molecules such as DMF. This reversible,
client-dependent structural change is associated with a significant
reorientation of the 1,4-diazabicyclo-octane (DABCO) piers relative
to the PW plane [24].
1
H spin-echo spectrum of cp and op-DUT-8(Ni) attained on
700 MHz, 60 kHz-MAS spectrometers (Fig. 1a), demonstrate the
challenges related to 1H detection and paramagnetic NMR in
paddle-wheel constructed frameworks. The combination effect of
1
He-1H homonuclear interactions and unpaired electronic
coupling of hyperfine to paramagnetic metal ions, the spectra
contain highly overlapping central band lines (Fig. 1a inset) and
distinct rotational sideband modes. However, the abundance of
paramagnetic NMR effects allows monitoring dynamic structural
transitions between two states associated with distinct spectral
responses [52]. The two spectra are characterized by a set of unresolved or partially resolved lines (~~9.5 ppm) for the NDC ligands
(H1/H3/H4) and a fundamentally resolved signal shift of 1Hs in the
DABCO (H7) column. The resonance frequencies of 13.5 ppm for the
OP type and 48.8 ppm for the CP type, immediately reveal different
paramagnetic contributions, resulting in different electronic
structures and spin densities of the two distributed samples. But
the linewidths of the two samples differ by a factor of 10 (for
example, the DABCO 1Hs(H7) linewidths in the OP and CP samples
are 440 and 5600 Hz, respectively). Their result shows a larger
inhomogeneous broadening in the latter spectrum, which is
consistent with the more diverse and frequent irregularities of the
cp sample structure revealed by X-ray powder diffraction analysis
[53]. The chemical shift anisotropy (about twice that of the CP
structure) suggests that the magnetic susceptibility of the two
samples is very different, which is consistent with the antiferromagnetic coupling between adjacent Ni2þ ions in the OP consistent
structure. Cross-polarizsation usually achieves the polarization
transfer of protons to near-inhomogeneous nuclei in diamagnetic
samples. In contrast, pulsed techniques such as transfer echo
double resonance recoupling (TEDOR) allow exposure to strong and
fast paramagnetic displacements. TEDOR reintroduces 1Hee13C
dipole coupling over time intervals using P pulses synchronized to
the rotor, otherwise averaged as MAS, and typically performs 13C
detection experiments at moderate MAS rates in the 30 kHz range
[54]. Depending on the TEDOR feedback loop used, 1He-13C correlations observed over short time intervals, corresponding to a
single link (Fig. 1b) or longer distances, including quarterly correlations between 1He-13C.
The resolved 1H resonances are atomically resolved dynamics of
the framework and its local features. Fig. 1c and d shows an
example of 2D 1He-1H correlation obtained by radio frequency
driven recoupling (RFDR) and the cumulative curve of the crossover
signal between NDC and DABCO Ligand. The fast initial slope of the
cp geometry, leads to the shorter distance between the 1H cores of
NDC and DABCO [55]. At the same time, the cross-signal intensity or
correlation in TEDOR is very sensitive to local dynamics, independent of static distortions and difficult to acquire with diffraction
data. Fig. 1e and f shows the experimental accumulation curves of
the 1He-13C correlation op forms within the NDC ligands (H4e-C5)
and the DABCO (H7e-C7). The shape and other parameters sorted
for two spin pairs are obtained by fitting them. The description of
ligand kinetics is crucial for understanding the tunability of guest
adsorption properties [56].
Recently developed MAS NMR probes capable of rapid rotation
at low temperature can extend the sensitivity advantage over a
wide temperature range between 100 K and 305 K [57]. Through
this, MAS NMR can be applied practically to monitor the temperature dependence of the resolved 1H shift, providing microscopic
insight into the magnetic properties of the system [58]. As
mentioned above, this structure is reflected in the low shift
3
S. Rehman, S. Xu, H. Xu et al.
Materials Today Advances 16 (2022) 100287
Fig. 1. NMR properties of DUT-8(Ni), a framework containing Ni2þ paddle-wheel units. (a) 1H MAS NMR spectra of closed-pores (cp) and open-pores (op) DUT-8(Ni) metal-organic
framework. Spinning sidebands and residual solvent signals are marked with an asterisk (*) and hash (#). (b) Heteronuclear Single Quantum Correlation-Transferred Echo Double
Resonance recoupling (HSQC-TEDOR) 1He-13C spectra of cp (red outline) and op DUT-8(Ni) (black outline). (c, d) Radio-Frequency-Driven Recoupling (RFDR) 1He-1H correlation
spectra of cp (red) and op (black). The accumulation of pulse imperfections (the broader spectrum of the cp sample) is responsible for the reduced cp accumulation curve observed
at tRFDR > 0.5 ms. (e, f) 1He-13C TEDOR cumulative curves of heteronuclear correlations of H7e-C7 (E) and H4e-C5 (F) in op DUT-8 (Ni). (g, h) H7 NMR shifts of op and (i) cp DUT8(Ni) samples as a function of temperature. The peak positions (blue þ error bars) were determined by the Heisenberg model (red line) combined. The bar graphs show that the
Boltzmann populations of the electron spin levels at 300 K, JNiNi ¼ ¼ -266 and 52 cm-1 are obtained from the fitting by (h) and (i), respectively. Adapted from Rref. [51] Angew.
Chem. Int. Ed. (Creative Commons CC) Copyright 2021, Wiley-VCH GmbH.
organic linker with different functional groups (FG), such as B(BDCNH2), E (BDH-NO2), F [(BDH-(CH3)2] (Fig. 2a) using a combination of
solid-state NMR measurements and computer simulations. Their
analysis distinguishes the random, alternating and clustering forms
of FG apportionments.
The REDOR NMR technique delivers quantitative estimates of
interatomic distances. Due to the large number of possible connecting structures, pairwise measurements of the distances between two spectrally allowed nuclear spins and the distances
between the corresponding atoms seem unlikely to be used to estimate the distribution of left functions. However, the key to
recovering the distribution of functional groups in this dimension is
how to look at the average distances between points [65].
In the REDOR method, the periods of multinuclear radiation are
gradually increased, thereby developing pairs between active isotopes and NMR, producing "decay curves" [66]. These curves are
administrated by the dipole coupling constant between the two
spins (i and j), where Rij is the distance between the nuclei. Kong
group selectively labeled spins on different linkers, such as 13C on
linker X and 15N on linker Y, to study the intermolecular separation
of these linkers. If the linkers are close, the coupling is high and the
decay is fast; that is, a rapid decay means that most of the linkers
are in close proximity [67]. Fig. 2b shows how these decay curves
depend on the functional distribution of different scenarios that
may arise in MTVMOF-5-BF. The decay is expected to be faster for
alternating, random, and large cluster scenarios (Fig. 2b). To measure the redox curve, in the case of MTV-MOF-5-BF, labelled each
amino group of framework B with 15N and measured coupling with
13
C spins of methyl groups [19 ppm]. Fig. 2c shows the 13C spectrum
measured in the REDOR 13Ce-15N experiment with and without
modified 15N dipole modulation. The apportionment obtained from
this model accepts that linkers with different FG tend to have
different neighbour predilections, and can express these predilections as effective interaction energies [68]. The effective
interaction energy is used as a suitable parameter for redox NMR
curves, describing the most important linker interactions during
MOF formation. The model is used to predict distributions based on
binary interaction parameters, such as the effect of changes in the
anisotropy of the 1H spectrum and the moderate paramagnetic
shift of the DABCO-1Hs. Therefore, the fundamental differences in
the temperature behavior of DABCO-1H -NMR shifts in the two
samples (Fig. 1g). According to Curie's law, the paramagnetic
displacement decreases with increasing temperature, while in OPtype samples, the paramagnetic displacement has the opposite
temperature behavior ("anti-Curie”"). The large temperature range
allows quantitative data analysis using a simple Heisenberg model
that allows extraction of the Heisenberg coupling constant (j) in
nickel ion pairs [59]. Despite some simplifications to the model that
ignore pseudo-contacts, the temperature-dependent adjustment
curves correspond well to the experimental data.
Their results showed the defects created in the CP and 1H NMR
detection reveal the binding properties of metallic compounds and
(super)magnetic exchange, two essential elements for understanding the phase transitions of switchable MOFs at the microscopic and macroscopic scale. The findings demonstrate that
paramagnetic MOF analysis with high MAS rates and carefully
tuned pulse sequences is a step forward [60]. This makes it possible
to use the proton and carbon resonances in the paramagnetic MOFs
to determine the local dynamics of organic entities close to the
paramagnetic center and reveal the electronic properties. The results shown in Fig. 1 are an important complement to conventional
solid-state NMR spectroscopy, especially to reduce the experiment
time for acquiring NMR spectra [61].
3. Mapping of functional groups
Mapping functional groups in MOFs is one of the most important achievements in SSNMR applications [62]. A hybrid functional
group framework is intractable for diffraction methods (X-Ray,
electron, or neutron) and is not judged by other characterization
methods [63]. X. Kong and his team experimentally demonstrated
that SSNMR can be used to construct three-dimensional (3D) maps
of the distribution of functional groups within and between pores
of MOF [64]. They produced the heterogeneous spatial distribution
of functional groups in the multivariate metal-organic framework
(MTV-MOF) series, containing BDC (1,4-phenylenedimethoxy)
4
S. Rehman, S. Xu, H. Xu et al.
Materials Today Advances 16 (2022) 100287
Fig. 2. Mapping of heterogeneous functional groups in the multivariate metal-organic framework (MTV-MOF). (a) The structures of the organic linker with variant functional
groups named as B and F. (b) The simulations for 13Ce-15N decay curves for different BF distributions. (c) The SSNMR 13C spectra were obtained from a 13Ce-15N REDOR experiment;
Blue. The spectrum has taken without 15N dipolar modulation; purple. The spectrum with 15N dipolar modulation; red. The alteration spectrum was plotted by subtracting S from S0.
The "*" shows the spinning side bands. Adapted from Rref. [64] with permission of Science. Copyright 2013, American Association for the Advancement of Science.
approach. It can provide a vast array of complementary and
difficult-to-access structural and motional insights that are tough
to obtain with alternative approaches. Advancements in MOFs have
recently reviewed in SSNMR examinations of tiny gas molecules
(e.g., carbon dioxide, carbon monoxide, hydrogen gas, and light
hydrocarbons) [75,76]. These investigations highlight the variety of
information obtainable using SSNMR spectroscopy, including the
number and position of guest adsorption sites, host-guest binding
strengths, and guest mobility. The knowledge gained from these
tests produces a potent instrument for further MOF development
[77e79].
Roztocki et al. investigated in situ technique (13C NMR) for gas
adsorption measurements of JUK-8 to gain a deeper understanding
of CO2 adsorption [80]. It can explore host-guest interactions and
differentiate between distinct adsorbates inside the framework and
the non-adsorbed free gas [75,79]. 13C NMR spectroscopy of
adsorbed CO2 is typically employed to analyze porous materials
such as MOFs. For the investigation of single and mixture adsorption process, By the literature, pure 13CO2 gas at 1 bar (195 K)
produces a narrow signal at 127.8 ppm (Fig. 3) [75]. The CO2induced transition (195 K) from cp CO2@ip CO2@op phases occurs
between p/ p0 ¼ 0.00 and 0.12, according to in situ PXRD (Fig. 3a). In
the case of in situ NMR, the gate opening pressure (gop) shifts to p/
p0 ¼ 0.160.19, presumably as a result of a tiny fluctuation in temperature. In the intermediate phase (CO2@ip), the CO2 molecules
are trapped and have restricted mobility. At p/p0 ¼ 0.09, only a
comprehensive signal (signal I) is recorded from approximately 70
to 180 ppm with maximum strength at 178 ppm (Fig. 3d). The line
form is typical for CO2 and reflects the line shape seen for signals in
powder samples dominated by chemical shift anisotropy with
rotational symmetry. The chemical shift tensor then displays the
two principal values (parallel to the axis of symmetry) and ||
(parallel to the symmetry axis). However, the observed chemical
shift anisotropy (CSA) ¼ || ¼ 110 ppm is significantly less than the
expected value of 355 ppm for totally immobilized CO2 molecules
in samples. This suggests that the pores have restricted mobility,
resulting in a partial averaging of the CSA. In liquid-state NMR
spectroscopy, a comparable characteristic, the so-called persistent
dipolar couplings, is a well-known phenomenon. In the case of CO2
molecules in MOFs, they move fast via the pores. In situ NMR
spectroscopies reveal favored sites of the adsorbed CO2 molecules
by structural analysis. JUK-8cp is a highly selective adsorbent, as
evidenced by comprehensive analyses of one- and multicomponent
equilibrium adsorptions over a wide temperature range.
composition of ternary linkers. This strategy can solve many
problems in the field of materials science, such as the distribution
of defects in crystals and the distribution of functional groups in
block copolymers.
Yuan et al. validate the use of in situ NMR spectroscopy of species to investigate ligand exchange and post-synthetic modification
(PSM) reactions of MOFs [69]. This method makes it possible to
monitor the functionalization process over time without first
deconstructing the result for analysis, which makes response
screening much easier [70]. Yuan group evidenced the ligand that
has been added and the ligand that is leaving the framework. This
in situ approach is demonstrated by analyzing the ligand exchange
and PSM reactions of the zirconium MOF UiO-67 and the ligand
exchange reaction with the aluminum MOF DUT-5. NMR spectroscopy shed light on the reactions that were investigated, and
anticipate that further research utilizing this methodology will
make it possible to investigate a wide range of MOF reactions [71].
In similar pattern, Vinot et al. discussed the structure and dynamic
behavior of zirconium (IV) terephthalate UiO-66(Zr) solids functionalized with Br, OH, and NH2 groups [72]. The zirconium (IV)based porous MOF-type material, whose structure is composed of
Zr6O4(OH)4 oxoclusters bonded to twelve terephthalate (BDC) ligands, has garnered considerable interest at related crystallographic locations. In-depth NMR investigation yields vital
information on the local and long-range structure components of
UiO-66. It has been demonstrated that grafting the organic linker of
the UiO-66(Zr) structure with the NH2 functional group considerably reduces its “"flipping”" capacity. Although grafting such amino
group has been shown to increase the thermodynamic selectivities
of CO2 over gases significantly, it may be unfavorable to follow such
a strategy when the process of interest can be governed by dynamic
considerations, especially in narrow window MOFs such as UiO66(Zr) where it has already been established that a dynamic process governs the rotation of the organic linker in the non-covalent
bond. In future, it is mandatory to conduct quasi-elastic neutron
scattering experiments to establish that grafting a polar function
onto UiO-66(Zr) reduces the diffusivity of the molecules of interest
as a result of a decreased flip rate of the organic linker.
4. Role of SSNMR for the investigation of guest molecules
MOFs are well-known for storing and researching guest molecules, such as CO2, CH4, and others [73,74]. For the characterization
of these functional materials, SSNMR spectroscopy is a promising
5
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Materials Today Advances 16 (2022) 100287
Fig. 3. In situ CO2 adsorption analysis by JUK-8cp: Adsorption/desorption isotherms (a); PXRD patterns observed in parallel with CO2 physisorption (b); unit cell volume changes
during adsorption/desorption (c); and 13C NMR of adsorbed 13CO2 as a function of pressure rise (adsorption) and subsequent desorption (d). Signal I and II changes are illustrated by
violet and red dashed lines, respectively. Adapted from Rref. [80]. of ACS Appl. Mater. Interfaces, Copyright 2021, The authors, American Chemical Society.
M. G. Lopez studied the 13C NMR of MOF-74 and the magnetic
resonance shielding of tiny molecules at various places in the MOF74-Mg cavity [81]. MOFs permit molecules to move through pores
to store and segregate gases such as methane, nitrogen, carbon
dioxide, etc. Therefore, a structure that can accommodate both
short axis and long organic chains is required to comprehend the
dynamics of microporous MOF and the approach to avoiding
penetration difficulties [82]. The hexagonal configuration of SBU
metal oxide rods is coupled by short axis and long axis metaloxygen connections. The resulting product is an extended skeleton consisting of a channel aligned with the secondary axis and an
insulating wall composed of organic chain fillers to prevent penetration beyond the length of the organic chain.
The coordinate structures and conformational profiles of
adsorbed spices (As an example, adenosine mono-/diphosphate
(AMP) is used) in ideal and defective UiO-66 are depicted in Fig. 4a.
The AMP has a bigger octahedral acquisition with a volume of
0.73 nm3. It does not prefer the decreased tetrahedral cage with
AMP (0, 43 nm3) size over the tetrahedral pore size (0.18 nm3).
When a m-O defect is introduced, the AMP phosphate group tilts
towards the Zre-O cluster. The m-O defect would not result in
chemical absorption. The chemical link between the phosphate
group of AMP and the exposed zirconium side is observed when a
BDC defect is introduced. When BDC and m-O are absent, the
phosphate group of AMP moves closer to the Zre- O cluster and
exchanges positions with m-O. The interaction energy is divided
into two distinct contributions, Coulombic and Van der Waals, to
better understand the hosteguest interactions. The Coulombic
interaction is primarily a result of the chemical coordination between the phosphate group and zirconium, whereas the Van der
Waals interaction results from the non-bonded components
[89,90]. In perfect UiO-66 or missing m-O defects, the contribution
of Van der Waals interaction predominates (shown in green in
Fig. 4b). In contrast, for missing BDC defects or defects with both
BDC and m-O lacking, the contribution of Coulombic interaction
predominates for AMP (shown in red, Fig. 4b). In the drug loading
process, AMP interact with defect-compensating species such as
water, acetate, hydroxyl, and dangling BDC [28]. The Coulombic
contact energy of AMP is significantly more than water, dangling
5. Study of defects in MOFs through NMR
Understanding defects is necessary for the applications of
metal-organic framework (MOF). The interaction may be different
for MOF with or without defects [83]. Fu et al. showed that the
defects within MOF play a key role in loading many pharmaceuticals with phosphate or phosphonate couplings [84,85]. The guest
interaction is dominated by the defective sides, which damages the
loading capacity [86,87]. They introduce SSNMR and molecular
simulations to trigger the mechanization scans for drug load
transfer. For similar molecules without defects, the loading capacity
is significantly reduced [88].
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Materials Today Advances 16 (2022) 100287
Fig. 4. Investigation of defects and docking in MOFs through NMR. (a) Minimum energy configurations of adenosine mono-/diphosphate (AMP) adsorbed in perfect and defective
UiO-66. Zr, cyan; P, green; m-O, yellow; H, white; O, red. The adenosine part of AMP is shown in yellow, while BDC linkers are shown in greay. (b) AMP dynamics in UiO- 66 with
divergent defects. green for van der Waals interaction and Red for Coulombic interaction. (c) Coulombic interaction energy of coordinated molecules in UiO-66 with BDC defects. dgBDC means BDC with an attached carboxylate. (d) 1H NMR spectrum of the digested defective UiO-66 with different load densities of AMP. Adapted from Rref. [84] with permission
of Angew. Chem. Int. Ed. Copyright 2021, Wiley-VCH GmbH.
chemistry impacted ZIFs' phase transition behavior, it had no effect
on the short-range disorder [94]. Recently, Ma and Horika published an excellent review article on MOF-based glasses, which can
help a lot to the interested readers in MOF-forming glass [94].
BDC, and marginally greater than hydroxyl (Fig. 4c). The comparatively strong Coulombic interaction energy of AMP suggests that it
is capable of occupying a significant proportion of the defect sites
[91]. It is experimentally validated by employing 1H NMR spectroscopy on samples with varying AMP loading densities. When
AMP dosing is increased, the amount of remaining acetate decreases, confirming that AMP has replaced acetate (Fig. 4d).
Although defects are widespread throughout the crystal; however, locating or creating a specific defect in the organic part of
MOFs is a tough undertaking. R. Pallach recently introduced the
concept of flexibility in MOFs, which is caused by intra-framework
dispersion forces regulated by linker functionalization [92]. The
accompanying SSNMR results revealed a top dog's role, which
helped grasp this method. The structural changes in MOF-5-CX
during the crystalline-to-amorphous-to-crystalline transition are
depicted in Fig. 5. Cross polarization (CP) 13C magic angle spin
technique, nuclear magnetic resonance (MAS NMR) spectroscopy,
X-ray spectroscopy, and total infrared scattering were used to
investigate the crystal structure of MOF-5-CX (Fig. 5bee). The
expanded signals of the carbon atoms belonging to the organic
backbone of the MOFs are seen in the NMR spectrum of CP 13C,
indicating the heterogeneous local structure of non-crystalline dry
MOF-5-CX (Fig. 5a). Individual properties may be easily determined
from a small number of basic structures spectroscopic due to the
variability of dry MOF-5-C6 (Fig. 5 c). Their findings not only
confirm the NMR results but also show that all forms of MOFs play a
role in defect creation at the atomic level. The chemical shift of the
uncoordinated carboxyl group was estimated to be 180e-185 ppm.
Fig. 5c depicts the Zn and Zn bonds between six nearby nodes
[Zn4O(O2C)6], which corresponds to the SSNMR data. Similar
SSNMR and XPDFs images can be obtained from infrared spectrometer data by evaluating the features of the carboxylate coordination model. Talking about ZIF glass and their defects, Madsen
et al. used ultrahigh-field 67Zn SSNMR spectroscopy to detect shortrange disorder [93]. The melting of the parent crystals resulted in
the transformation of two different Zn sites into one tetrahedral site
with a wide range of structural characteristics. Although the ligand
6. Dynamics of MOF using
17
O SSNMR
Oxygen is ubiquitous in almost every field of science. Therefore,
it is very important to characterize the local electronic and geometric environment of oxygen [95]. Since 17O is sensitive to
chemical shifts and quadrupole interactions, 17O SSNMR spectroscopy has emerged as an ideal tool for characterizing specific sites,
has a wide range of diagnostic chemical shifts and is affected
through the coupling environment by NMR active nuclei (1H, 15N,
etc.) [96,97]. In recent years, NMR methods and techniques have
made great progress. But the potential of 17O SSNMR to discover
detailed structural and bonding information in oxygenates is
limited by its inherent low sensitivity and low resolution, which are
due to its extremely low natural abundance [98]. (0.038%), relatively low gyromagnetic ratio (g ¼ ¼5.774 MHz T11), and 17O
quadrupolar spin (I ¼ ¼5/2). Isotopic enrichment can alleviate
sensitivity issues associated with low natural abundance of 17O
[99]. To deal with the relatively low g value and the quadrupole of
17
O, NMR measurements can be achieved at a higher magnetic
field; this increases the sensitivity and reduces the line broadening
associated with the second-order quadrupole interaction [100].
According to the basic NMR theory, the higher magnetic field
increases the NMR signal sensitivity and decreases the issue of line
broadening related with the second-order quadrupole interaction.
In this regard, Martins and his group used a hybrid magnet strength
of 35.2T (1H Larmor frequency, 1.5 GHz) for 17O-SSNMR in biomolecules and minerals [101]. In their work, they use existing
magnet (35.2 T) to target a-Mg3(HCOO)6 MOF in an “"activated”"
and “"as-made”" form and obtained very high spectral resolution of
17
O-SSNMR at 35.2 T. Fig. 6a shows the 3D structure of aMg3(HCOO)6 micropores formed by octahedral MgO6. These micropores have common angles which are connected with the bond
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Materials Today Advances 16 (2022) 100287
Fig. 5. Visual percept of structure by solid-state NMR, XPDF and infrared spectroscopy. (a) The 13C NMR spectra of MOF-5-CX correspond to carboxylic acid, phenyl or alkyl carbon
atoms. (b) Representation of XPDFs dry- and as-MOF-5-C3. (c) Structural representation of [Zn4O(O2C)6]. (d) Superposition of XPDFs of dry MOF-5-CX material, focusing on the
change of correlation length (left, vertical offset XPDF) and low R area (right). (e) Extracted from the infrared spectrum region, showing the multiple vibration bands of dry MOF-5CX material. Adapted from Rref. [92] of Nat. Commun. (Creative Commons CC).
gravity (dd2) using the Eq. (1&2) [101].
of formic ligands and zigzag channels containing the molecules of
DMF (remaining of synthesis solvent). Activated a-Mg3(HCOO)6
contains 12 oxygen centers at different crystal sites, which are
connected with the carboxylate group. The bond length of C-m1-O is
shorter and has more double-bond properties. Resulting, the 17O
1D-MAS a-Mg3(HCOO)6 spectrum of 21.1T has only two sets of
signals (Fig. 6b, black line). Meanwhile, NMR spectra of as-made
and activated a-Mg3(HCOO)6 phases obtained at 35.2 T (Fig. 6b,
blue line) has considerably narrower resonance of both phases due
to the reduction of the second-order quadrupolar broadening. The
spectrum containing all overlapping signals of m1-O group is now
completely separated from m2-O, and several spectral features
appear, including “"edge”" and “"angle”" of single 17O-SSNMR
resonance. Fig. 6c and d shows the NMR spectrum of 17O-2D 3QMAS to eliminate the second-order expansion of the 17O quadrupolar broadening and discrete overlapping signals observed in
the 1D MAS spectra. Considering that the number of signals
allowed in isotropic dimensions are less than 12 oxygen points in
the crystal framework, some of the signals in the F1 dimension
must correspond to very similar signals from several oxygen points
with almost identical NMR parameters [102]. The isotropic chemical shift ddiso (in ppm) and the quadrupole product PQ ¼ CQ
(1 þ hQ2/3)1/2 (in MHz) can be obtained from the spectral center of
diso ¼
17
10
d þ
d
27 1 27 2
(
)1=2
170 ½4I ð2I 1Þ2
ðd d2 Þ
PQ ¼
yo 103
81 ½4IðI þ 1Þ 3 1
Where I is a spin quantum number and no is the Larmor frequency.
The PQ and ddiso can be derived from these equations for each peak
along the F1 dimension as shown in Fig. 6b. For peaks along the
isotropic measurement corresponding to an oxygen point, the CQ
and hQ values can be extracted by adjusting the F2 cross-section. If
the peak of the F1 measurement comes from several oxygen points,
ddiso and PQ are averages. The investigation of these weak interactions is important for MOF applications in various fields, such
as biomedical applications of drug delivery systems [103]. Martins
and his group published a review article specifically on a variety of
organic and inorganic compounds are currently viable targets for
17
O SSNMR [104]. In a similar manner, Bignami et al. employed 17O
SSNMR to probe cationic disorder in MOFs containing two different
types of metal cations (e.g. For Al, Ga MIL-53) [105]. SSNMR
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Materials Today Advances 16 (2022) 100287
Fig. 6. NMR spectra and dynamics of 17O-enriched a-Mg3(HCOO)6 at 21.1 T and 35.2 T magnetic field strength. (a) Structural representations of the as-made and activated (empty
pores) a-Mg3(HCOO)6. Color coding: O, red; N, blue; Mg, turquoise; C, grey; m2-O, pink m1-O, orange; (b) 17O 1D NMR spectra at fields of 21.1 T (black) & 35.2 T (blue) and frequency of
18 kHz. The asterisk (*) indicates spinning sidebands (SSBs). NMR spectra of (c) as-prepared and (d) active a-Mg3(HCOO)6. The black dashed line corresponds to the section
examined. The red and blue lines represent the experimental and simulated spectra, respectively. Adapted with permission from Rref. [101] of J. Am. Chem. Soc. Copyright 2020,
American Chemical Society.
provided information about the final composition of the material
(significantly different from the initial starting material) and the
preference for aggregation/ordering of cations within the MOF. For
Al, Ga MIL-53, the distribution of cations results in a mixed pore
shape when exposed to water. 17O provides the evidence of hydroxyl groups linked to the Al and Ga center, confirming the formation of mixed-metal material. Their research showed that 17O
SSNMR is an invaluable tool for studying these important functional materials.
Wang et al. demonstrated that 17O SSNMR experiments provide
unique information on dynamics and have the potential to provide
a comprehensive picture of CO2 mobility within MOFs [106]. These
experiments can be performed independently or in conjunction
with complimentary 13C SSNMR experiments. CPO-27 is a suitable
test system to investigate the factors that led to the reduced CO2
adsorption capacity in CPO-27 and to demonstrate how sensitive
17
O and 13C NMR spectra to the metal- CO2 bound together.
7.
13
were carried out on the N-functionalized MOF compound (In)-MIL68-NH2, a partially functionalized variation of terephthalate and a
10% proline-functionalized derivative (In)-MIL-68-NH-Pro. Despite
the fact that the pore size of the MOFs is much smaller (ca. 1.6 nm)
than the other mesoporous materials, the effective sensitivity
enhancement factors obtained for 1He-13C CPMAS experiments.
The DNP technology's 10- to 30-fold reduction in experimental
time enables for the rapid recording of two-dimensional 1He-13C
correlation spectra and 1He-15N CPMAS NMR spectra at natural
abundance.
The one-dimensional 1He-13C CPMAS spectra recorded on the
three MOF materials with or without microwave (MW) irradiation
to induce DNP are shown in Fig. 7aec. Notably, S accounts for the
fact that the signal increase provided by DNP is partially compensated by signal intensity reductions caused by various paramagnetic phenomena. Fig. 7aec shows the carbon resonances'
assignments, which are based on chemical shift values and comparisons of various spectra [111]. Because only 20% of the linkers
contained an amine moiety, the resonances corresponding to carbons 2 and 5 of the aminoterephthalate are easily distinguished
[112]. The spectra of (In)-MIL-68-NH-Pro are consistent with the
replacement of proline ligands for 10% of the amine functionality
[113]. The resonances from the carbon nuclei of proline are barely
detectable in the aliphatic area of the 1D 13C CPMAS spectrum
(Fig. 7c) and overlap with the spinning sidebands of the aromatic
resonances. However, the associated correlations can be seen
clearly in the 2D dipolar 1He-13C heteronuclear correlation (HETCOR) spectrum (Fig. 7d).
In similar, Ramakrishna et al. [110] provided a thorough multinuclear (13C, 15N, 25Mg) NMR research on ABX3 perovskite-like MOF
C NMR of functionalized metal organic frameworks
SSNMR spectroscopy is the best option to characterize the molecular structure and dynamics when X-ray diffraction is insufficient to determine the topology of the framework or the molecular
properties of MOF materials [107]. Rossini et al. [108] recently
demonstrated how dynamic nuclear polarization (DNP) can be
used to boost NMR sensitivity for surface organic functions in
hybrid nanoporous materials. The rapid and precise structural
characterization of surface bonding patterns and local conformations was enabled by the dramatic reduction in experiment time
given by DNP 13C or 29Si SSNMR spectroscopy [109]. Their studies
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Materials Today Advances 16 (2022) 100287
Fig. 7. 13C NMR of Functionalized Metal-Organic Frameworks. (ae-c) 1D 1He-13C CPMAS spectra of (In)-MIL-68-NH2, functionalized with terephthalate variant and prolinefunctionalized derivative of (In)-MIL-68-NH-Pro, respectively. Black and red spectrum discriminate microwave irradiation to induce dynamic nuclear polarization (DNP). Sample
temperatures 105 K and spinning frequency (NROT) was 12 kHz. Asterisks are used to indicate spinning sidebands. (d) A three-dimensional 1He-13C HETCOR spectrum of (In)-MIL68-NH-Pro with 1 s recycle delay and a 1 ms CP contact pulse. Adapted with permission of Angew. Chem. Int. Ed from Rref. [108]. Copyright 2012, Wiley-VCH Verlag GmbH & Co.
KGaA, Weinheim. (e) 13C chemical shift of Mg(HCOO)3 at 150.8 MHz near its phase transition temperature (Tc ¼ 265 K), peaks 1 and 3 correspond to low temperatures, whereas
peak 2 corresponds to high temperature. (f) 2D 13Ce-13C EXSY spectra for methyl carbons at 262.5 K. (g) Room-temperature crystal structure of Mg(HCOO)3 MOF with Dimethylammonium (DMAþ) cation. Adapted with permission from J. Phys. Chem. C ref. [110]. Copyright 2021, American Chemical Society.
progressively increase, while the intensity of the peak at 39.2 ppm
(corresponding to the high temperature phase) steadily diminishes
at lower temperatures. The peak at 39.2 ppm eventually vanishes
below the phase transition temperature, and is replaced by a peak
doublet attributable to two (methyl 13C) peaks at 39.8 and 38.4, as
seen in Fig. 7e. The appearance of the methyl carbon doublet in the
low-temperature phase implies that the DMAþ cation is displaced
from its cavity center, causing the two methyl groups in the
DMA þ cation to become chemically inequivalent at this temperature. Fig. 7f displays the 13C 2D EXSY spectra of methyl and formate
at 262.5 K, near the phase transition. The Fourier transform of the
signals recorded at t1 (vertical) and t2 (horizontal) times is represented by the two axes in the image. The coexistence of hightemperature (red contours) and low-temperature (blue contours)
and the phase transition is confirmed by the diagonal peaks. The
13
C atoms of both the methyl and formate moieties have multiple
sites (shown in Fig. 7g) due to different chemical environments, the
peaks further indicate that the cluster formation near the phase
transition is not due to the temperature gradient instead indicating
a slow chemical exchange between them and confirming that the
DMAþ cation is hopping in the MOF framework below the phase
transition [116]. Cross peaks indicate that they have been
of [(CH3)2NH2]Mg(HCOO)3, where A is the organic dimethylammonium (DMAþ) cation [(CH3)2NH2]þ, B is the divalent metal
cation, and X is the organic linker (Fig. 7g). Mg(HCOO)3 demonstrates a dielectric phase change at 270 K. The mechanism driving
this phase shift is unknown, several researchers attribute it to
either the order-disorder dynamics in the cavity or to the
contraction of the metal formate framework. By examining the
dynamics of nitrogen and carbon, SSNMR can elucidate the process
of this phase shift. The correlation time for nitrogen hopping is 109
s, and the activation energy is 28.62 kJ/mol, according to spinlattice relaxation time (T1) and BPP theory [114,115]. The phase
transition exhibits order-disorder and displacive properties. The
change in DMA þ cation dynamics causes a change in line width in
25
Mg NMR spectra, but there is no change in its chemical shift,
meaning that its immediate environment remains unchanged. The
13
C CP/MAS NMR spectra of Mg-MOF are shown in Fig. 7e. Despite
the fact that each DMAþ cation has two methyl groups, only one
resonance at 39.2 ppm is appeared above the phase transition
temperature, showing that DMAþ cations are in a disordered
motional state that averages out the orientational disparities in
their chemical shifts. Additional two peaks at 39.8 and 38.4 ppm
begin to develop as the temperature drops, and their strengths
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Materials Today Advances 16 (2022) 100287
(Fig. 8bed). The thermal stability of the TCPP-MOF could be
inhibited from the DPA-MOF. Photosynthetic CPP containing a large
number of carboxylic acids facilitates the coordination of the Zr6
group, which DPA partially replaced. When the TCPP is placed inside the frame, the air perforation of the DPA-MOF window combined with Zre-COO bonds creates a “"bottle cap”" effect that can
prevent leaching of the TCPP. Images of elemental mapping of C, N,
Zr in TCPP@DPA-MOF shows the core-shell structure in configuration (Fig. 8e1-e4). In MOF, ligands are isolated in the framework and
retain molecular properties. This feature can maximize its potential
applications by optimizing the scope of the MOF. Technically, the
formation of MOF undergoes nucleation and growth crystal, simply
as a bond between Hydrogen and carbon atoms (Fig. 8f). A new
peak at the center of 84 ppm appears upon exposure to 1O2,
consistent with peroxidic bond in EPO-MOF as shown in Fig. 8g.
Overall, Fig. 8 shows that DPA-MOF can capture 1O2 and transforms
to EPO-MOF, which can be recovered by UV irradiation in the
original DPA-MOF. A strong coordination interaction between Zr6
clusters and the linkers are well explained through the NMR. Cao
et al. did an excellent job of controlling the substituent length of
organic ligands, resulting in a framework transition after linker
exchange [121]. The digested samples' 1H NMR spectroscopy
revealed complete ligand exchange in bulk Cd-MOF samples. All of
the ligand substituents in the target Cd-MOFs orient to the inside of
the hexagonal open channels. Similar manner, Song and his team
successfully converted MOFs from an ionic insulator to an ionic
conductor through structural change [122]. MOFs can have
reversible proton conduction modulation due to their reversible
structural transition from amorphous to crystalline phases. 2H NMR
analysis elucidates the proton conduction mechanism of the ionic
conductor phase.
chemically swapped. Ramakrishna group shed light on the phase
transition and the dynamics of molecules in metal-organic frameworks, piqueing interest in using high-resolution solid-state NMR
to investigate phase transitions [110].
8. The NMR study in transformation of MOFs
Endoperoxides (EPO) are the focus of fundamental research due
to their interesting optical, thermodynamic, and photochemical
features, which have applications in reversible oxygen storage and
chemical sources for reactive oxygen species [117]. Several polycyclic aromatic hydrocarbon (PAH) derivatives have the ability to
scavenge singlet oxygen (1O2) and convert to EPOs [118]. EPO is
usually aroused to a condition by heat or UV light, showing that it
has a high potential to release oxygen. When exposed with light or
heat, EPO can bind to oxygen [119]. Zeng and his colleagues
described how they were able to manage the synthesis, structure,
and spectrum features of anthracene-based oxygen MOF, as well as
reversible oxygen binding [120]. NMR plays a crucial role in the
research of MOF transformations because this type of transmission
is impossible to detect using XRD, despite Raman and FTIR studies.
The NMR may be used to examine the photo-oxygenation of
anthracene MOF to endoperoxide MOF by coupling a [4 þ 2]cycloaddition on the electronegative carbon atom.
Zheng et al. envisioned chemical changes between DPA-MOF
and EPO-MOF, as shown in Fig. 8a. An anthracene-based MOF can
pass through UV-VIS irradiation using a photosensitive system in
the frame. Accordingly, tetra (4-carboxyphenyl)porphyrin (TCPP),
which can convert 3O2 into 1O2, is incorporated into DPA-MOF
through the ability to coordinate the Zr6 group [120]. TEM and
SEM images of TCPP@DPA and MOF show ca. 510 nm core-shell
nanoparticles with high uniformity and narrow size distribution
Fig. 8. NMR investigation in terms of structural transformation in MOFs (a) graphical diagram of TCPP@DPA-MOF used to reversibly bind oxygen. SEM and TEM images (inset) of (b)
DPA-MOF, (c, d) TCPP@DPA-MOF, scale ¼ 400 nm. (e) HAADF-STEM image and its corresponding EDS mapping, scale ¼ 200 nm. (f, g) Scheme and chemical shift illustration of
reversible binding of O2 with DPA-MOF and EPO-MOF (O red, C black, Zr blue). CP / MAS 13C NMR spectra of active DPA-MOF before and after photo- oxygenation. Adapted with
permission of Angew. Chem. Int. Ed from Rref. [120] Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
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Materials Today Advances 16 (2022) 100287
9. Final discussion / challenges
10. Conclusions
Despite recent technological and methodological advances,
NMR still falls short in many respects, necessitating large sample
volumes, extensive experimental measurements and data analysis,
and often sophisticated isotope labeling devices [123e125]. A
common issue of NMR is samples concentration, leading to the low
sensitivity and second one is the magnetic field drift, which is
highly detrimental to NMR spectra.
Direct detection of 1H signals is the most obvious way to
circumvent these obstacles by enhancing the sensitivity of NMR
investigations due to the high gyromagnetic ratio g of 1H spins,
their inherent isotopic abundance, and their widespread presence
in MOF organic materials [126]. However, the similar properties of
1
H spins form a dense network of strong dipolar couplings in solids,
broadening 1H resonances and making them difficult to exploit
constructively [127]. Although significant proof-of-concept discoveries on the direct detection of 1H resonances in MOFs have
been made, spectra have often lacked the resolution required for
the comprehensive identification of individual 1H sites. By weakening the 1H dipolar coupling networks and sharpening their NMR
lines, fast magic-angle spinning (MAS) at high magnetic fields
overcomes the problem [128]. However, the current MAS rates
available with commercially available NMR probes are insufficient
to produce fully resolved 1H spectra of MOFs, and the only siteresolved 1H-detected NMR structural studies relied on 1H spin
dilution, which required the (complex and costly) partial replacement of 1Hs with 2Hs, severely limiting the approach's benefits
[129].
The addition of open-shell metal ions to MOFs for MAS NMR
adds another level of complication to the experiment [130,131]. The
hyperfine interactions in these samples involve NMR active nuclei
and unpaired electrons of paramagnetic nuclei [132]. Metals present major spectrum capture and interpretation issues. On the
other hand, these interactions provide critical information
regarding the geometry and electrical structure of metal environments. Correlation NMR spectroscopy, in conjunction with other
NMR techniques, is critical in revealing the structure of MOFs. The
usefulness and impact of correlation NMR spectroscopy in materials research has been strengthened and magnified by the
impending development of NMR hardware and pulse sequences
[133].
The research community has just recently begun to recognize
the structural and dynamic information supplied by SSNMR, but
the future is promising [134]. Despite the fact that sensitivity is a
major issue in SSNMR, exciting advances have occurred, such as the
availability of greater magnetic fields, innovative pulse sequences,
and the introduction of approaches such as dynamic nuclear polarization [135]. These advances will enable more extensive SSNMR
studies of currently available isotopes as well as many more difficult nuclei from all throughout the periodic table, particularly those
with low frequency. Improving sensitivity has long been an
objective of solid-state NMR for material characterization. Using
isotope-enriched materials can significantly improve NMR sensitivity and enable multidimensional correlation testing. This is
frequently hampered by the prohibitively expensive and occasionally unavailable NMR-active isotopes. The advancements in
NMR instrumentation provide exciting opportunities. In the coming years, we anticipate that SSNMR will shed more light on shortrange MOF structure and applications, covering topics such as guest
adsorption mechanisms and dynamics, host guest interactions, the
roles of metal centers in processes such as catalysis and adsorption,
and linker-metal and linker-guest interactions.
Solid-state nuclear magnetic resonance (SSNMR) has become an
invaluable spectroscopic instrument for the atomic-scale analysis
of a variety of substances, as outlined in this introduction. The ultimate objective of material design is to optimize its functionality
and performance for a particular application. Heterogeneous MOFs
are notoriously difficult to characterize owing to the variety of their
constituents, structures, and characteristics. SSNMR has progressed
to a point where atomic-level insights of the local structure in the
framework and on the surface and interface, as well as host-guest
interactions, can be obtained by a combination of complex experiments. Based on the chemical shift, which provides direct information about the coordination state, structure symmetry, and local
chemical environment, NMR correlation spectroscopy is a valuable
and versatile technique for studying the structural and electronic
information from the same sample on the medium-range order,
which is essential for gaining a deeper understanding of the
structure and properties.
This review article summarizes the structure elucidation of MOF,
their guest molecules, their interactions, and their defects. The
article examines the facts about functional groups, MOF docking,
active center, surface location and intermediate interaction under
in situ conditions. SSNMR reveals many new possibilities for characterizing material properties at the atomic level. We hope that
SSNMR will bridge the gap between low-resolution techniques and
achieve a high degree of complementarity.
Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Acknowledgements
This work was made possible by the grantsgrants that were
awardedawarded by the National Natural Science Foundation of
China (3190110313 to K.M., 42177444 and U1932218 to Z.Y.), the
Special Foundation of President of the Chinese Academy of Sciences
(YZJJ2022QN44 to S.X.), the Alliance of International Science Organizations (ANSO-VF-2021-03 to S.R.), and the HFIPS Director's
FundFund (YZJJZX202014 to J.F W). The Steady High Magnetic Field
Facility and the High Magnetic Field Laboratory at CAS were both
used in the performance of certain aspects of this work.
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