Characterization of MetalâOrganic Frameworks: Unlocking the

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Characterization of Metal−Organic Frameworks: Unlocking the Potential of Solid-State NMR Bryan E. G. Lucier, Shoushun Chen, and Yining Huang* Department of Chemistry, The University of Western Ontario, 1151 Richmond Street, London, Ontario, Canada N6A 5B7 CONSPECTUS: An exciting advance in materials science is the discovery of hybrid organic−inorganic solids known as metal−organic frameworks (MOFs). Although they have numerous important applications, the local structures, specific molecular-level features, and guest behaviors underpinning desirable properties and applications are often unknown. Solid-state nuclear magnetic resonance (SSNMR) is a powerful tool for MOF characterization as it provides information complementary to that from X-ray diffraction (XRD). We describe our novel pursuits in the three primary applications of SSNMR for MOF characterization: interrogating the metal center, targeting linker molecules, and probing guests. MOFs have relatively low densities, and the incorporated metals are often quadrupolar nuclei, making SSNMR detection difficult. Recently, we examined the local structures of metal centers (i.e., 25 Mg, 47/49Ti, 63/65Cu, 67Zn, 69/71Ga, 91Zr, 115In, 135/137Ba, 139La, 27Al) in representative MOFs by SSNMR at a high magnetic field of 21.1 T, addressing several important issues: (1) resolving chemically and crystallographically nonequivalent metal sites; (2) exploring the origin of disorder around metals; (3) refining local metal geometry; (4) probing the effects of activation and adsorption on the metal local environment; and (5) monitoring in situ phase changes in MOFs. Organic linkers can be characterized by 1H, 13C, and 17O SSNMR. Although the framework structure can be determined by X-ray diffraction, hydrogen atoms cannot be accurately located, and thus the local structure about hydrogen is poorly characterized. Our work demonstrates that magic-angle spinning (MAS) experiments at very high magnetic field along with ultrafast spinning rates and isotope dilution enables one to obtain ultrahigh resolution 1H MAS spectra of MOFs, yielding structural information truly complementary to that obtained from single-crystal XRD. Oxygen is a key constituent of many important MOFs but 17O SSNMR work on MOFs is scarce due to the low natural abundance of 17O. 17O enriched MOFs can now be prepared in an efficient and economically feasible manner using solvothermal approaches involving labeled H217O water; the resulting 17O SSNMR spectra provide distinct spectral signatures of various key oxygen species in representative MOFs. MOFs are suitable for the capture of the greenhouse gas CO2 and the storage of energy carrier gases such as H2 and CH4. A better understanding of gas adsorption obtained using 13C, 2H, and 17O SSNMR will enable researchers to improve performance and realize practical applications for MOFs as gas adsorbents and carriers. The combination of SSNMR with XRD allows us to determine the number of adsorption sites in the framework, identify the location of binding sites, gain physical insight into the nature and strength of host−guest interactions, and understand guest dynamics. The difference between δ11 and δ33 is the span (Ω) and measures CS anisotropy (CSA); CSA dominates spin-1/2 spectral appearance. The skew (κ) describes the CS tensor axial symmetry and observed powder pattern shape; it ranges from −1 to +1, with either limit representing an axially symmetric tensor. The quadrupole moment (Q) of quadrupolar nuclei (spin (I) > 1/2) interacts with the electric field gradient (EFG) from the surrounding environment (i.e., chemical bonds and local structure) via the QI, broadening SSNMR spectra across tens to thousands of kHz, reducing their signal-to-noise ratio (S/N), and rendering acquisition difficult. The QI also encodes rich information regarding the local electronic and geometric environment. The QI is described using the quadrupolar coupling

1. INTRODUCTION Metal−organic frameworks (MOFs) have diverse applications, arising from their ability to incorporate a plethora of elements in countless structural configurations. In many applications, the connections between local structure and properties of the material are poorly understood. Solid-state nuclear magnetic resonance (SSNMR) can be used to probe MOFs, correlating local structure and environment to key functions and applications. Our research group uses SSNMR to investigate MOF metals,1−9 linkers,1−3,10−12 and guests (Figure 1),1−3,11,13−18 described within sections 2, 3, and 4, respectively. SSNMR spectra are influenced by the chemical shift (CS) and quadrupolar interaction (QI), which alter spectral appearance and provide detailed information on the local environment. CS probes the magnetic environment and is modeled by a tensor with three components: δ11, δ22, and δ33, where δ11 > δ22 > δ33. © XXXX American Chemical Society

Received: July 19, 2017

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DOI: 10.1021/acs.accounts.7b00357 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Figure 1. Different SSNMR targets for MOF characterization.

Figure 3. 21.1 T 25Mg static and 5 kHz MAS NMR spectra of α-Mgformate are shown in (a), along with the activated 25Mg SPAMMQMAS spectrum (b), and simulated MAS spectra (c). Asterisks (*) denote spinning sidebands. Reproduced with permission from ref 5. Copyright 2013 Wiley-VCH.

Q such as 27Al and 45Sc; (ii) high n.a., high frequency, large Q including 69/71Ga, 115In, and 139La; and (iii) challenging low n.a., low frequency nuclei with medium-to-large Q such as 25Mg, 67 Zn, 91Zr, 135/137Ba, 47/49Ti, and 43Ca. Metals are diluted within low-density MOF structures, compounding n.a. issues. Advances in high magnetic fields and specialized pulse sequences (e.g., MQMAS, WURST-CPMG, etc.) have greatly reduced acquisition times for high-quality SSNMR spectra of metal nuclei. Zinc is incorporated within many MOFs, however, the quadrupolar 67Zn isotope is challenging (I = 5/2, Q = 150 mb,19 n.a. = 4.1%20). ZIFs are good examples of how SSNMR can directly investigate the metal environment, host−guest interactions, and guest distributions.7 ZnN4 tetrahedra in ZIF-8 are symmetrical and yield a narrow 67Zn MAS resonance, whereas increased ZnN4 distortion in ZIF-14 causes a broadened line shape (Figure 2a,b). ZIF-4 features two Zn sites: Zn1 has distorted ZnN4 units, higher CQ(67Zn), and a broader line shape, while more symmetrical ZnN4 units in Zn2 have smaller CQ(67Zn) and a narrower powder pattern (Figure 2c). Metal SSNMR is sensitive to guest distributions and dynamics (Figure 2d). Empty ZIF-8 corresponds to a single narrow 67Zn MAS powder pattern and ordered local environment, while H2O yields broader featureless spectra, suggesting a disordered Zn local structure. MD simulations indicate this arises from irregular

Figure 2. Simulated and experimental 67Zn MAS NMR spectra for (a) ZIF-8, (b) ZIF-14, and (c) ZIF-4 at 21.1 T. Asterisks (*) denote spinning sidebands. In (d), 67Zn MAS spectra and MD simulations of ZIF-8 are depicted. Adapted with permission from ref 7. Copyright 2012 Wiley-VCH.

constant (CQ), where higher values correspond to less spherically symmetric environments and broad spectra. The axial symmetry of the EFG tensor is modeled by the asymmetry parameter, ηQ, which governs the spectral “horn” positions and ranges from 0 in perfectly axially symmetric electronic environments to 1 in axially asymmetric surroundings.

2. SSNMR OF METAL CENTERS Metal atoms play key roles in MOF applications such as guest adsorption, catalysis, drug delivery, sensing, and luminescence. To understand how metals are connected to these applications, the local metal environment must be characterized, and SSNMR is an attractive route.1−8 I = 1/2 nuclides such as 111/113Cd, 119Sn, 195 Pt, and 207Pb often have large CSAs and broad powder patterns. Most metal nuclei are quadrupolar (I > 1/2), which are (i) high sensitivity, high natural abundance (n.a.) with moderate B


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Figure 4. MIL-120(Al) structure is shown in (a), along with BTEC/BDC linkers and corresponding Al coordination (b), and the pXRD patterns of parent and defect MIL-120(Al) (c). 27Al MAS NMR spectra are depicted in (d) (21.1 T, spinning rate: 31.5 kHz), along with MQMAS spectra and NMR parameters of (e) parent and (f) defect MIL-120(Al).9

Figure 5. MIL-68(In) is shown in (a), with In local environments illustrated in (b), along with static (c) as-made and (d) activated MIL-68(In) 115In SSNMR spectra at 21.1 T. Adapted with permission from ref 31. Copyright 2014 Wiley-VCH.

The unreceptive 25Mg (I = 5/2, Q = 199 mb,19 n.a. = 10.0%20) has a narrow CS range, hence, static and MAS techniques cannot resolve nonequivalent sites (Figure 3a). To resolve all four signals, 2D MQMAS can remove the second-order quadrupolar broadening (Figure 3b).23 MAS spectra of all four resonances can then be simulated (Figure 3c) and assigned to crystallographic Mg sites using DFT calculations. Introducing MOF defects by design can improve performance in applications such as adsorption and catalysis.24 Defect characterization is challenging due to their localized nature and

H2O aggregation in the pores. Benzene-loaded ZIF-8 gives rise to a narrow line shape, owing to the even time-averaged distribution and jumping of benzene molecules. 67Zn SSNMR spectra of other MOFs, such as MOF-5, are also exquisitely sensitive to the presence of guest molecules and binding.21 Many MOFs feature nonequivalent quadrupolar metal sites in similar chemical environments, giving rise to overlapping signals that one-dimensional static or MAS experiments cannot resolve, such as the four nonequivalent Mg sites in three α-Mg-formate phases22 (as-made with DMF guests, activated, and benzene-loaded).5 C


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Figure 6. MIL-140A is shown in (a) with ZrO7 SBUs (b) and Zr coordination (c). Experimental (d), simulated (e), and calculated (f, g) 91Zr 21.1 T SSNMR spectra with bond length and angle distributions are included for (h) reported and (i) fully geometry optimized structures, along with (j) calculated pXRD patterns. Reproduced with permission from ref 4. Copyright 2014 American Chemical Society.

Figure 7. MIL-125(Ti) is shown in (a), (b), and (c). In (d), (e), and (f), three possible interior hydrogen configurations are shown. The 47/49Ti SSNMR spectra at 21.1 T of as-made (g) and activated (h) MIL-125(Ti) are included. Reproduced with permission from ref 4. Copyright 2014 American Chemical Society.

irregular distribution. We have intentionally introduced defects into MIL-120(Al)25 (Figure 4a), which features edge-sharing Al(OH4)O2 octahedra. Each 1,2,4,5-benzenetetracarboxylate (BTEC) linker binds eight six-coordinate Al centers; substituting 30% BTEC for the benzenedicarboxylate (BDC) linker yields five-coordinate Al centers as defect open metal sites (OMSs, Figure 4b). The parent and defect MIL-120(Al) pXRD patterns

are similar since the framework structure is preserved (Figure 4c); however, 27Al SSNMR serves as a short-range complementary defect characterization method. 27 Al MAS SSNMR of MIL-120(Al) at 21.1 T yields a signal in the octahedral Al CS range;25 however, the defect MIL-120(Al) MAS spectrum features three signals (Figure 4d). The overlapping 27Al resonances of both MOFs necessitate 27Al MQMAS D


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Figure 8. DMF-loaded α-Mg-formate is shown in (a). Resolution enhancement strategies on activated α-Mg-formate are depicted in (b), along with experimental (red) and deconvoluted (blue) 1H MAS NMR spectra at 21.1 T of 20% 1H samples spinning at 62.5 kHz in (c). 1H−13C CP/MAS spectra of the 20% 1H samples at 18 kHz and 21.1 T are shown in (d). Adapted with permission from ref 11. Copyright 2015 American Chemical Society.

When the QI is large, metal spectra can be so broad that spinning-based techniques are inapplicable and one must extract information from static “wideline” spectra. An extremely wide spectrum does not mean resolution is low, as demonstrated by MIL-68(In). 115In (I = 9/2, Q = 770 mb,19 n.a. = 95.7%20) is associated with spectral broadening that makes acquisition extremely challenging.29 113In is avoided due to low n.a. As-made MIL-68(In) MOF incorporating DMF (Figure 5a) features two unique In centers30 (Figure 5b). The 115In 21.1 T spectral breadth of as-made MIL-68(In) exceeds 4 MHz (Figure 5c). In1 has more uniform InO6 bond distances, corresponding to CQ(115In) of 248 MHz and the narrower line shape, while InO6 octahedral distortion about In2 leads to a larger CQ(115In) of 300 MHz. Activation causes the large In1 CQ(115In) to decrease to 197 MHz (Figure 5d); DMF removal makes (In1)O6 octahedral bond angles and distances more uniform. SSNMR can refine local metal geometry in MOFs with pXRD crystal structures, such as MIL-140A, which contains NMRactive 91Zr.32 Quadrupolar 91Zr SSNMR is sensitive to the local environment in materials,33 but is challenging due to its properties (I = 5/2, Q = 176 mb,19 n.a. = 11.2%20) and low Larmor frequency. MIL-140A contains ZrO7 SBUs, with four oxygen atoms from BDC linkers and three from bridging μ3-O2− groups connecting SBUs (Figure 6a−c). A CQ(91Zr) value of 35 MHz was obtained from the 91Zr SSNMR spectrum at 21.1 T (Figure 6d).4 Planewave density functional theory (DFT) calculations were used to relate structural features to NMR parameters. The DFT calculated CQ(91Zr) of 62.1 MHz using the MIL-140A pXRD structure with optimized hydrogen positions was a poor match (Figure 6g), prompting optimization of all atoms. The corresponding calculated 91Zr NMR parameters (Figure 6f) matched well with experiment, implying the local Zr environment was more accurately represented. The larger pXRD bond length and angle distributions (Figure 6h)32 yield an overestimated CQ(91Zr), while smaller geometry-optimized ranges (Figure 6i) produce a calculated CQ(91Zr) of 39.8 MHz, near the experimental 35.0 MHz. Figure 6j reveals the two structures yield similar pXRD

Figure 9. 2D 1H−1H DQ BABA spectra at 21.1 T of activated 20% 1 H α-Mg-formate spinning at 18 kHz, using three different excitation/ conversion times and only new correlations shown at longer periods. Reproduced with permission from ref 11. Copyright 2015 American Chemical Society.

experiments. The MQMAS spectrum of parent MIL-120(Al) (Figure 4e) yields the expected two resonances for six-coordinate AlO6,25 but the defect spectrum (Figure 4f) features four resonances. Three high-field resonances are assigned to unique six-coordinate Al sites; the additional octahedral Al site reflects subtle changes in crystal symmetry upon defect introduction. The new high-frequency resonance is assigned to a defect fivecoordinate Al OMS based on CS. This and other examples26−28 illustrate that metal SSNMR is a sensitive probe of local metal coordination for investigating the presence of a defect site in MOFs. E


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Figure 10. Framework, metal coordination, 17O MAS, and 1H−17O CP/MAS SSNMR spectra at 21.1 T of (a) UiO-66 and (b) MIL-53(Al). Asterisks (*) denote spinning sidebands. Reproduced with permission from ref 10. Copyright 2013 American Chemical Society.

Figure 11. Simulated 13C static NMR powder patterns of CO2 at various motional rates undergoing (a) C6 rotation through an angle α = 70°, (b) C2 hopping through an angle β = 35°, and (c) combined C6 and C2 rotation through angles of α = 70° and β = 35°, respectively.

bridging μ2−OH (Figure 7c). The μ2−OH and μ2-O2− assignments are unknown.35 47/49Ti SSNMR sheds insight on Ti local structure in many materials.36 The two quadrupolar NMR-active isotopes of Ti have low Larmor frequencies: 47Ti (I = 5/2, Q = 302 mb, n.a. = 7.4%) and 49Ti (I = 7/2, Q = 247 mb, n.a. = 5.4%).19,20 Due to their similar frequencies, the broader 47Ti signal often overlaps with the narrower, more intense 49Ti signal.37 As-made and activated MIL-125(Ti) yield similar static 47/49Ti SSNMR spectra at 21.1 T consisting of a featureless, 200 kHz broad 49Ti resonance with underlying 47Ti signal (Figure 7g,h). While pXRD indicates high crystallinity, Ti line shapes suggest a

patterns, while their 91Zr NMR parameters are very different. 91 Zr SSNMR is more sensitive to the Zr local structure than pXRD here, demonstrating how SSNMR can refine the local metal geometry in MOFs; this is highlighted in approaches such as NMR crystallography.34 In MOFs, the metal local environment can be disordered while the long-range framework structure is highly ordered. We recently demonstrated that disorder can be investigated using 47/49Ti SSNMR.4 MIL-125(Ti) features TiO6 polyhedra (Figure 7a,b) connected by BDC linkers. Three oxygen atoms originate from carboxylate groups, two from bridging μ2-O2−, and one from F


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Figure 12. VT static 13C SSNMR spectra of CO2 in PbSDB are shown in (a), with wobbling and hopping in (b). The CO2-loaded PbSDB SCXRD structure is illustrated in (c) with the motional model in (d). 13C SSNMR spectra of CO2 in CdSDB are in (e), with the SCXRD structure in (f), and unique adsorption sites labeled C1A and C1B. Adapted with permission from ref 1. Copyright 2016 American Chemical Society.

overlapping resonances. Using high magnetic fields, fast MAS (i.e., ≥40 kHz), and 1H spin dilution involving partial substitution of 2H, up to six framework protons (not undergoing dynamics) can be resolved within a 0.8 ppm chemical shift range. These ultrahigh resolution MOF 1H SSNMR spectra provide information complementary to XRD data.11 The activated, DMF-loaded, and benzene-loaded α-Mgformate MOF phases reside in the same P21/n space group,22 featuring six chemically similar, crystallographically nonequivalent hydrogen atoms (Figure 8a). The activated static 1H SSNMR spectrum at 21.1 T is ca. 30 kHz wide (Figure 8b). Spinning at 18 kHz narrows the spectrum to a single broad resonance, yet spinning at 62.5 kHz still cannot achieve total resolution. 1 H isotopic dilution and ultrafast MAS at 62.5 kHz distinguishes all six resonances (Figure 8c).22 1 H−1H dipolar coupling can be exploited using rotorsynchronized two-dimensional (2D) 1H−1H double-quantum (DQ) MAS experiments employing the back-to-back (BABA) recoupling pulse sequence, which yields a DQ signal if 1H−1H dipolar coupling exists. Using calculated effective dipolar couplings, 80% 2H dilution, and 2D 1H−1H DQ BABA experiments (Figure 9), all six 1H resonances were unambiguously assigned to chemically similar 1H sites in activated α-Mg-formate (Figure 8c). DMF-, benzene-, and pyridine-loaded α-Mg-formate yield distinct 1H MAS SSNMR spectra (Figure 8c) because 1H SSNMR is sensitive to the local hydrogen environment and host−guest interactions.11 In DMF-loaded α-Mg-formate, H5 and H6 CSs

disordered system and distribution of local environments. To investigate this disorder, DFT calculations were performed on three possible MIL-125(Ti) structures with different μ2−OH and μ2-O2− locations (Figure 7d). 47/49Ti CQ and ηQ values were affected by μ2−OH and μ2-O2− positions, revealing Ti local disorder likely originates from random μ2-OH and μ2-O2− distributions. The sensitivity of metal NMR parameters to structural changes can also be exploited for in situ monitoring of MOF phase changes, as shown by 25Mg/67Zn SSNMR2 of [NH4][M(HCOO)3] (M = Mg, Zn) MOFs as well as 27Al3 and 69/71Ga SSNMR8 of MIL-53.

3. SSNMR OF LINKERS Organic MOF linkers incorporate the NMR-active isotopes 1 H, 13C, 14/15N, 17O, and 19F. SSNMR targeting linker atoms1−3,10−12 can provide information on the local environment that is unavailable from other techniques such as XRD. Hydrogen atoms are invisible to XRD and placed with questionable accuracy at idealized positions, but characterization is important because these atoms are often involved in adsorption, guest binding, and catalysis. 1H SSNMR yields detailed structural data truly complementary to XRD, but is challenging due to strong 1H−1H homonuclear dipolar interactions that severely broaden spectra. Using techniques including fast MAS, CRAMPS, and specialized pulse sequences,38 1H resonances from chemically different species (e.g., phenyl, methyl, amine, hydroxyl groups) can be resolved. Chemically identical, but crystallographically nonequivalent hydrogens have similar CSs and G


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Figure 13. CO2 dynamics at bridging −OH groups (blue circles) in MIL-53 are shown in (a), with a perpendicular hopping perspective in (b). Static 1 H−13C CP SSNMR spectra are shown of (c) CO2-loaded, (d) CO2-loaded deuterium exchanged, and (e) empty MIL-53 frameworks. Asterisks (*) denote the adsorbed CO2 resonance. Reproduced with permission from ref 17. Copyright 2016 The Royal Society of Chemistry.

Figure 14. CO2 wobbling in Mg-MOF-74 is depicted (a), alongside nonlocalized 6-fold hopping motion between OMSs (b, c). VT 17O SSNMR spectra of C17O2 are shown in (d), with motionally simulated42 spectra: (e) wobbling and hopping, (f) only wobbling, and (g) only hopping. Rates are in Hz. Reproduced with permission from ref 15. Copyright 2014 American Chemical Society.

change due to C−H···O formate−DMF hydrogen bonds. Benzene guests deshield H3 while shielding H2 due to ring current effects. The H5 proton in pyridine-loaded α-Mg-formate is deshielded due to hydrogen bonding, while pyridine C2 π-flipping averages out ring current effects. 1H−13C cross-polarization (CP)/MAS SSNMR experiments (Figure 8d) and 2D 1H−13C heteronuclear correlation (HETCOR) experiments can identify the six unique formate carbon atoms in all three α-Mg-formate phases. 13C SSNMR experiments also have other uses; for example, rotational-echo double-resonance (REDOR) can be used to map linker distributions in MOFs.39

Oxygen is an ideal choice for linker characterization, but the NMR-active 17O n.a is only 0.037%.20 17O enrichment of porous materials is possible;40 we used a solvothermal approach and 17 O-enriched H217O vapor to embed 17O within MOFs.10 17O SSNMR can establish structural solutions and connectivity alongside XRD. 17O MAS and 1H−17O CP/MAS SSNMR can distinguish chemically unique oxygen species, and even crystallographically nonequivalent oxygen atoms under ideal circumstances. UiO-66 features three chemically unique oxygens: carboxylate (COO−) alongside μ3-O2− anions and μ3−OH groups that cannot be distinguished using XRD, but have unique 17O CSs H


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Figure 15. Static 2H SSNMR spectra of D2 in MOF-74 (a−c), with the motion of D2 in Mg-MOF-74 illustrated in (d−f). Adapted with permission from ref 13. Copyright 2016 The Royal Society of Chemistry.

(Figure 10a). 1H−17O CP/MAS experiments assign the 65 ppm resonance to μ3−OH and the 386 ppm resonance to μ3-O2−. 17 O SSNMR is sensitive to phase transitions. Linker carboxylate oxygens in flexible MIL-53 are equivalent in the large-pore (lp) phase, giving rise to a single NMR signal, but nonequivalent in the narrow-pore (np) phase, yielding two resonances (Figure 10(b)). The two nonequivalent carboxylate groups, bridging μ2−OH, and free H2O generate four 17O resonances that are assignable using NMR parameters. The oxygen groups bridging AlO6 were assigned as μ2−OH using 1H−17O CP/MAS SSNMR. 17O SSNMR can also probe structure in paramagnetic MOFs.12

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C SSNMR of guest CO2, supported by single-crystal XRD (SCXRD), reveals the number of adsorption sites, guest dynamics, and differences in host−guest binding strengths at separate adsorption sites. Microporous PbSDB features linear channels, the V-shaped 4,4′-sulfonyldibenzoate (SDB) linker, and PbO7 coordination. PbSDB 13C VT SSNMR spectra (Figure 12a) feature a sharp free CO2 resonance at 125 ppm, accompanied by a broad powder pattern indicative of one unique adsorption site.1 Motional simulations reveal two CO2 motional components: a localized “wobbling” modeled by a C6 rotation and nonlocalized C2 hopping (Figure 12b). Complementary SCXRD studies situate the CO2 motional model in the V-shaped SDB “π-pocket” (Figure 12c,d). The Cd analogue of PbSDB, CdSDB, has 6-coordinate Cd atoms and sinusoidal-shaped channels. There are two powder patterns in 13C VT SSNMR spectra of CdSDB (Figure 12e). One resonance resembles the PbSDB 13C resonance, indicating a CO2 adsorption site exists at a common location, while the other CdSDB adsorption site must be in located elsewhere due to the distinct 13C line shape. SCXRD reveals two unique adsorption sites in CdSDB (Figure 12f), with the site common to both PbSDB and CdSDB located in the V-shaped SDB π-pocket. The second CdSDB adsorption site lies within a Cd-based π-pocket that is structurally prohibited in PbSDB. Stronger guest binding and restricted guest motion corresponds to increased 13C CSA. CO2 at adsorption site 1 (C1A, Figure 12f) exhibits span values 66−77% versus adsorption site 2 CO2 (C1B, Figure 12f),

4. SSNMR OF GUEST SPECIES MOF guests can be probed using 2H,11,13,14 13C,1,3,15−18 17O,15 and 14N2 SSNMR. Capture and storage of CO2 within MOFs carries tremendous importance. CO2 is an excellent example of how SSNMR can investigate the number of adsorption sites, adsorption locations, guest dynamics, and host−guest interactions in MOFs. 13C (I = 1/2, n.a. = 1.1%20) SSNMR of isotopically enriched 13CO2 can unravel CO2 dynamics in MOFs; any motion reorienting the 13C CS tensor alters spectral breadth and shape. With knowledge of the static41 and observed variabletemperature (VT) anisotropic NMR parameters, spectral simulations42 can reveal the routes of guest motion (Figure 11), along with motional angles and rates. I


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5. FUTURE OUTLOOK The scientific community has just begun to realize the structural insights and dynamic information available from SSNMR of MOFs, and the future is promising. A major SSNMR problem is sensitivity; however, there have been exciting advances such as the increased availability of high magnetic fields, innovative pulse sequences, and emergence of techniques such as dynamic nuclear polarization.50,51 These developments will enable more detailed SSNMR study of currently accessible isotopes and many additional challenging nuclei across the periodic table, particularly those of low frequency and low n.a. In the coming years, we envision SSNMR will shed a brighter light on short-range MOF structure and its connections with applications, spanning topics including guest adsorption mechanisms and guest dynamics, host−guest interactions, the roles of metal centers in processes such as catalysis and adsorption, as well as linker−metal and linker−guest interactions.

indicating site 2 is the stronger binding site. In agreement, SCXRD studies reveal that CO2 prefers site 2, and only populates the weaker binding site 1 at higher loading levels. For many MOFs, high-quality single crystals can be difficult to obtain, instead VT 13C and 1H−13C CP SSNMR can be used to identify guest adsorption locations. In Ga-MIL-53, Al-MIL-53, and linker-functionalized Ga-/Al-MIL-53-NH2, CO2 presents a broad 13C NMR powder pattern indicative of a single adsorption site.17 Dynamic simulations42 indicate CO2 undergoes localized rotational wobbling and 2-fold hopping between equivalent adsorption sites in MIL-53, thought to be bridging −OH linking MO6 octahedra (Figure 13a,b).43 VT 1H−13C CP SSNMR experiments and deuteration confirm CO2 adsorption takes place upon bridging −OH groups. CP between framework 1H and 13CO2 was successful (Figure 13c), indicating that MIL-53 hydrogen and 13CO2 carbon are proximate. To determine if the CP source was MIL-53 phenyl linkers or bridging −OH groups, bridging −O1H groups were deuteriumexchanged in D2O, yielding −O2H groups. 1H−13C CP SSNMR spectra of MIL-53-O2H lack a CO2 signal (Figure 13d), indicating the 1H CP source and 13CO2 adsorption site is bridging −OH groups, since phenyl 1H does not exchange for 2H in D2O. CO2 locations and dynamics obtained from SSNMR can be verified using computational methods, as demonstrated in the α-Mg3(HCOO)6 MOF.18 It is also possible to calculate activation energies for CO2 motion44 and reveal CO2 dynamics in mixed-metal MOFs via SSNMR.45 Ambiguity regarding CO2 motion can exist. For example, 13C VT spectra of CO2 in Mg-MOF-74 can be simulated using two different motional models: either CO2 undergoes uniaxial rotation upon the OMS46 or CO2 performs localized wobbling on the OMS and C6 nonlocalized hopping between OMSs.47 Smit et al. demonstrated MD simulations can validate the wobbling and hopping model.47 We have alternately shown a SSNMR approach examining the other NMR-active isotope in CO2, 17O, encodes unambiguous dynamic data.15 17 O SSNMR spectra are influenced by the independent anisotropic QI and CS, and simulated spectra must account for both. 17O SSNMR of labeled C17O2 in Mg-MOF-7415 confirms CO2 undergoes both localized C6 rotational wobbling (Figure 14a) and nonlocalized C6 hopping between OMSs (Figure 14b−d). Using 17O motional parameters, simulated 13C SSNMR spectra fit experimental spectra.15 The opposite approach fails: 17O spectra generated using 13C C6 rotation data46 differ from experimental 17O spectra (Figure 14f). 17O SSNMR seems to be a technique that can unambiguously determine CO2 guest motion in MOFs. SSNMR can also be used to study gases in MOFs, including H213 and CH4.48 Understanding H2 behavior is necessary to hasten MOF adoption as greener, safer H2 storage materials. XRD cannot detect hydrogen atoms and 1H SSNMR is challenging due to 1H−1H dipolar coupling and guest/linker resonance overlap. VT 2H SSNMR and deuterated H2 (D2) can probe adsorbed H2 within MOFs (Figure 15a−c).13 In Mg-MOF-74, H2 undergoes fast C6 wobbling on the OMS and C6 hopping between OMSs below 163 K (Figure 15d−f). In Ni-MOF-74, the interaction of D2 with Ni becomes significant at 233 K, when a new 2H signal appears from D2 directly interacting with Ni. However, the 2H signal of D2 in Zn-MOF-74 remains a single sharp line at all temperatures, suggesting little interaction between Zn and D2. 2H SSNMR can also reveal the motion of other species in MOFs, such as dynamic interlocked components.49

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Web page: http://publish.uwo.ca/ ~yhuang/index.htm. ORCID

Bryan E. G. Lucier: 0000-0002-9682-4324 Yining Huang: 0000-0001-9265-5896 Notes

The authors declare no competing financial interest. Biographies Bryan E. G. Lucier received his B.Sc. (2007) and Ph.D. (2013) from the University of Windsor, researching SSNMR of insensitive metal nuclei in inorganic materials. He is currently a postdoctoral associate at the University of Western Ontario, where his interests lie in using SSNMR to unravel the structure of porous materials and examine host−guest interactions. Shoushun Chen received his B.Eng. from Liaoning University (2010) and M.Sc. from Xiamen University (2013). He is currently a Ph.D. candidate at the University of Western Ontario. His interests include using SSNMR and single crystal XRD to explore gas adsorption mechanisms and postsynthetic modification in MOFs. Yining Huang obtained his B.Sc. and M.Sc. from Peking University, earned his Ph.D. from McGill University, was a postdoctoral fellow at the University of British Columbia, and was an Assistant Professor at Laurentian University. He is currently Full Professor and Chair of the Department of Chemistry at the University of Western Ontario. His current research interests focus on characterization of layered materials, zeolites, and metal−organic frameworks by solid-state NMR and vibrational spectroscopy.

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ACKNOWLEDGMENTS Y.H. thanks the Natural Science and Engineering Research Council (NSERC) of Canada for a Discovery grant and a Discovery Accelerator Supplements Award. The authors thank Drs. V. V. Terskikh, T. Woo, A. Zheng, W. Wang, and J. Dong. Access to the 900 MHz NMR spectrometer was provided by the National Ultrahigh Field NMR Facility for Solids (Ottawa, Canada). J


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