Modulation of Uptake and Reactivity of Nitrogen Dioxide in Metal‐Organic Framework Materials

Abstract We report the modulation of reactivity of nitrogen dioxide (NO2) in a charged metal–organic framework (MOF) material, MFM‐305‐CH3 in which unbound N‐centres are methylated and the cationic charge counter‐balanced by Cl− ions in the pores. Uptake of NO2 into MFM‐305‐CH3 leads to reaction between NO2 and Cl− to give nitrosyl chloride (NOCl) and NO3 − anions. A high dynamic uptake of 6.58 mmol g−1 at 298 K is observed for MFM‐305‐CH3 as measured using a flow of 500 ppm NO2 in He. In contrast, the analogous neutral material, MFM‐305, shows a much lower uptake of 2.38 mmol g−1. The binding domains and reactivity of adsorbed NO2 molecules within MFM‐305‐CH3 and MFM‐305 have been probed using in situ synchrotron X‐ray diffraction, inelastic neutron scattering and by electron paramagnetic resonance, high‐field solid‐state nuclear magnetic resonance and UV/Vis spectroscopies. The design of charged porous sorbents provides a new platform to control the reactivity of corrosive air pollutants.


Introduction
[3][4] It can irritate the respiratory tract, thus increasing the risk of respiratory infections and asthma, particularly for children and people with respiratory problems.Additionally, constant exposure to NO 2 has been linked to an increased risk of heart disease, stroke, and other cardiovascular problems. [5]Although many countries have legislated to restrict the emission of NO x by vehicles, the concentration of NO 2 in the atmosphere continues to increase. [5][8] Alternatively, porous sorbents offer a promising pathways to capture NO 2 under ambient conditions.19][20][21] The first example of reversible adsorption of NO 2 in MOFs was achieved by MFM-300(Al), which shows a high uptake of 14.1 mmol g À 1 at 298 K and 1 bar. [22]UiO-66-NH 2 shows a high adsorption of NO 2 (up to 31.2 mmol g À 1 ) owing to the irreversible chemical reaction with NO 2 to form diazonium species on the aromatic ring of the organic linker, and additionally by reaction with water molecules under humid conditions. [23]However, the molecular details of the reactivity of adsorbed NO 2 molecules within the pore of MOFs have been poorly explored, thus restricting the design of new efficient sorbent materials to mitigate the emission of NO 2 .
Here, we report the modulation of reactivity of NO 2 in a pair of closely related MOFs, MFM-305-CH 3 and MFM-305.MFM-305-CH 3 , [Al(OH)(L)]Cl, (H 2 L)Cl = 3,5-dicarboxy-1methylpyridinium chloride], [24] has an unusual charged structure incorporating cationic (methylpyridinium) and anionic (Cl À ) components to give a zwitterionic-type framework (Figure 1a).MFM-305-CH 3 has an open framework structure comprised of corner-sharing chains of [AlO 4 -(OH) 2 ] ∞ bridged by dicarboxylate ligands.The Al III centres show octahedral coordination defined by four carboxylate oxygen atoms from L 2À ligands and two oxygen atoms from two μ 2 -hydroxyl groups.MFM-305-CH 3 can undergo postsynthetic modification via heating at 180 °C involving the 1methylpyridiniumdicarboxylate ligand undergoing in situ demethylation.This is coupled to the loss of Cl À anion as CH 3 Cl to give the pyridyl-based neutral framework in MFM-305 (Figure 1e), which has the same network topology and metal-ligand coordination as MFM-305-CH 3 .
We sought to monitor the impact of the differences in these materials on their interaction and reactivity with NO 2 .The framework structure in both MFM-305-CH 3 and MFM-305 shows high stability upon adsorption of NO 2 .However, adsorption of NO 2 into MFM-305-CH 3 leads to reaction with Cl À in the pore to give NOCl and NO 3 À , confirmed by in situ infrared (IR), high field solid-state nuclear magnetic resonance (ssNMR) and UV/Vis spectroscopy.The binding domains within the frameworks have been revealed using synchrotron X-ray powder diffraction.A high dynamic uptake of NO 2 of 6.58 mmol g À 1 at 298 K is observed for MFM-305-CH 3 as measured using a flow of 500 ppm NO 2 in He.NOCl can be extracted with CHCl 3 and recovered for use as a reagent for organic synthesis, including N-nitrosation of secondary amines, the structural elucidation of terpenes and the production of aromatic diazonium salts from anilines. [25]Therefore, this work develops a new method for converting NO 2 to useful chemicals, thus converting and utilising a key air pollutant in subsequent chemical processes.MFM-305-CH 3 can be regenerated via ion-exchange and extraction of NO 3 À with Cl À ions.In contrast, the adsorbed NO 2 molecules in MFM-305 do not react with the framework and an uptake of 2.38 mmol g À 1 at 298 K is observed.The host-guest binding interactions in both MOFs have been further studied by in situ electron paramagnetic resonance (EPR) spectroscopy and inelastic neutron scattering (INS) coupled with density functional theory (DFT) modelling.

Results and Discussion
Upon introducing NO 2 into MFM-305-CH 3 , a yellow-green liquid was observed on cooling the sample to 77 K (Figure 2a).This behaviour is, however, not observed for NO 2loaded MFM-305.Extracting NO 2 -loaded MFM-305-CH 3 with CHCl 3 gives a dark orange solution, the UV/Vis spectrum of which shows two bands at 472 and 583 nm (Figure 2b) characteristic for NOCl. [25]These bands are absent in the spectra of NO 2 dissolved in CHCl 3 , and are not observed on extraction of pristine MFM-305-CH 3 with CHCl 3 , In addition, CHCl 3 is unreactive towards NÀ Cl species, including NOCl, and these observations suggest a chemical reaction of NO 2 taking place within MFM-305-CH 3 involving the Cl À anions.
In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) was applied to probe the formation of NOCl.[28] Attempts to identify IR bands for NO 3 À near 1360 and 840 cm À 1 [27] were hampered due to overlapping peaks from the framework.The reaction between NO 2 and Cl À has been studied by mixing NO 2 with NaCl as a non-porous material. [27]The reaction at room temperature reaches the equilibrium in 1 h controlled by the limited adsorption of NO 2 onto the external surface of NaCl.In contrast, confined NO 2 molecules within MFM-305-CH 3 react with Cl À immediately (even at 77 K) due to the presence of strong host-guest interactions.Elemental analysis by inductively coupled plasma atomic emission spectroscopy (ICP-AES) was performed on: (i) the as-synthesised MFM-305-  S2, Figure S3).The analysis shows that ca.40 % of the Cl À anion content was converted to NOCl and removed via the reactivation process, and that ca.95 % of MFM-305-CH 3 can be regenerated by ion exchange in NaCl solution.
The conversion of NO 2 in MFM-305-CH 3 can also be observed by dynamic breakthrough experiments using a gas stream containing 500 ppm of NO 2 (diluted in He) at 298 K. From 0 to 840 min g À 1 (quoted as time for breakthrough of NO 2 per gram of porous material, Figure 2d) breakthrough proceeds as normal with NO 2 being expelled from the column.However, from 840 to 2000 min g À 1 , the detected concentration of NO 2 at the outlet remains unchanged at a normalised concentration of C/C 0 = 0.2 consistent with NO 2 molecules introduced to the column being immobilised within the material by reaction with Cl À .Thus, no additional NO 2 was detected at the outlet during this period up to 2000 min g À 1 .Once reaction within the pores is complete, the NO 2 can breakthrough as normal as observed after 2000 min g À 1 (Figure 2d).Thus, MFM-305-CH 3 captures NO 2 by physisorption and then chemisorption via reaction with Cl À anion leading to an observed increase in its dynamic capacity for NO 2 to 6.58 mmol g À 1 compared with MFM-305 (2.38 mmol g À 1 ), despite the latter material having a notably larger surface area (256 and 779 m 2 g À 1 , respectively).Thus, the breakthrough plot of dynamic adsorption of NO 2 in MFM-305-CH 3 has a distinct profile and demonstrates the positive impact of reactivity of NO 2 on the adsorption and capture performance of the porous host. [29,30]  monitor the reaction process, ssNMR spectroscopy was employed at high (23.5 T) and moderate (9.4 T) magnetic fields to investigate the structural changes in MFM-305-CH 3 before and after loading with NO 2 .MFM-305-CH 3 has a 35 Cl NMR peak at δ{ 35 Cl} � 90 ppm, which decreases markedly in intensity on adsorption of NO 2 (Figure 3a).To locate the Cl À ions in the structure of MFM-300-CH 3 , a two-dimensional (2D) 35 Cl-1 H through-space (dipolar) correlation experiment was performed (Figure 3b).Two correlations are observed, with protons at δ{ 1 H} � 8 and 12 ppm, which correspond to μ 2 -OH and the H1 aromatic proton, respectively.This observation implies the presence hydrogen bonding between these protons and the Cl À anion (Figure S12), consistent with the Cl À position as determined by synchrotron X-ray powder diffraction (see below), where Cl À ions are found to hydrogen-bond to these two sites. [24] 1H NMR assignments in Figure 3b were determined from 2D 1 H-1 H, 1 H- 13 C and 1 H- 27 Al dipolar correlation spectra (Figures S9a, b and c, respectively).No correlations were be observed in the corresponding 2D 35   Cl-1 H spectra of NO 2 @MFM-305-CH 3 owing to the reduced 35 Cl signal (Figure S10).
Upon adsorption of NO 2 , the 14 N NMR chemical shift of the methylpyridinium nitrogen changes slightly in the 2D 14 N-1 H dipolar correlation spectrum, and another peak is observed upfield (cf. Figure 3c and Figure 3d).Neither peak is observed at the frequency expected for confined NO 3 À , [31] which indicates that the adsorbed species undergoes rapid motion, which averages the guest-host 14 N-1 H dipolar coupling.Nevertheless, the (at least) two 14 N peaks observed for NO 2 @MFM-305-CH 3 indicate that the NO 2 induces structural changes at the methylpyridinium N-centre.Moreover, correlations are observed between these two 14 N resonances and protons at δ{ 1 H} = 4.6 ppm from the methyl group unlike for pristine MFM-305-CH 3 that does not exhibit 14 N-1 H correlations with the methyl group.This indicates that the methyl rotation has slowed, likely from hydrogen bonding with NO 3 À (see below), enabling a more efficient dipolar-based transfer of polarisation between methyl 1 H and framework 14 N centers.Although the NO 3 À is not observed directly by 14 N-1 H NMR spectroscopy, the presence of hydrogen bonded nitric acid is implied through the differences in 1 H spectra between samples and the extremely high-shifted 1 H NMR peak that appears (at δ{ 1 H} � 18 ppm) upon adsorption of NO 2 (Figure S11b) that can only come from acidic protons involved in hydrogen bonding. [32]The framework structure of the MOF undergoes only a slight modification upon NO 2 adsorption and corresponding loss of Cl À , with the ligand carbon environments and the octahedral geometry of the aluminium sites undergoing small perturbations, as observed via 13 C and 27 Al NMR spectroscopy (Figure S9b and Figure S9c, respectively).
A control experiment was designed to monitor how the disproportionation reaction of NO 2 takes place via highresolution synchrotron X-ray powder diffraction (SPXRD) using a sample of MFM-305-CH 3 loaded with a small amount of NO 2 .Rietveld refinement of the data gave the formula [Al(OH)(C 8 H 6 NO 4 )Cl 0.78 •(NOCl) 0.22 •(NO 3 ) 0.22 •(NO 2 ) 0.5 ].
In the refined model (Figure 1b), 22 % of Cl À anions are converted to NOCl, with formation of the same amount of NO 3 À balancing the charge.The remaining 78 % of Cl À ions remain at their initial positions (i.e., in the corner of the channel), and the newly-produced guest NOCl is anchored  1h).The different binding sites also form various monomer-to-monomer, monomer-to-dimer and dimer-to-dimer dipole interactions that stabilise the adsorbed NO 2 molecules in the pore (Figure S6).
In situ INS, coupled with DFT calculations, enabled the direct visualisation of the binding dynamics in NO 2 -loaded MFM-305-CH 3 and MFM-305 with a focus on the À CH groups involved in supramolecular contacts (Figure 4).Addition of NO 2 to MFM-305-CH 3 is accompanied by significant changes to peaks at 14 and 26 meV (peaks I and III) consistent with stiffening and deformation of the lattice modes upon binding of NO 2 .Another notable change in intensity is observed at 19 meV (peak II), indicating the hindrance of rotation motion of À CH 3 groups upon production of NOCl and NO 3 À species in the pore, consistent with the formation of hydrogen bonds as observed in the structural model.Small changes of intensity were also observed in the high energy region (120-200 meV), which correspond to changes of the aromatic À CH groups (twisting/scissoring/wagging modes) (Figure 4a

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the bridging hydroxyl is involved in binding to NO 2 .Changes in CÀ H modes also can be observed above 120 meV (Figure 4b, S14, S16).Thus, the results of INS are in good agreement with the in situ crystallographic analyses.
NO 2 is a free radical and thus EPR spectroscopy was used to investigate the host-guest interactions in these two systems under high NO 2 loading.EPR spectra of both NO 2loaded MFM-305-CH 3 and NO 2 -loaded MFM-305 at 10 K show signals from immobilised NO 2 with resolution of the anisotropic electronic g-factor and 14 N hyperfine interaction (Figure 5a). [33]These results confirm the presence of immobilised NO 2 , and for MFM-305-CH 3 , NO 2 is adsorbed into the pores after the disproportionation reaction to form NOCl and NO 3 À has reached equilibrium.Under identical NO 2 loading conditions, the intensity of NO 2 signal is much lower in MFM-305-CH 3 than in MFM-305 due to the disproportionation reaction in MFM-305-CH 3 (Figure 5b).
The interaction between NO 2 and the framework and other guest molecules is revealed by Davies ENDOR (Electron-Nuclear DOuble Resonance) [34] and 1 H and 14  distance is approximately in this direction.Calculated ENDOR spectra [35] based on the SPXRD refined structure including the nearest CH 3 and aromatic 1 H positions gave couplings that are too large (Figure S19).The ENDOR spectra were measured at 10 K, and the SPXRD at room temperature, and it appears that on cooling the NO 2 molecules move towards the centre of the pores.Calculated ENDOR spectra based on trial-and-error movement of the NO 2 in the structural model gave good agreement on moving and rotating the NO 2 to the centre of the pore with the nearest protons (H3) along the molecular C 2 axis of NO 2 (Figure 5c, Figure S18), although this model may not be unique.The ENDOR spectra of NO 2 -loaded MFM-305 is similar to that of NO 2 -loaded MFM-305-CH 3 but with a slightly larger 1 H coupling, indicating the relative position of NO 2 in these two materials are similar but the N•••H-(aromatic) distances in MFM-305 are slightly shorter due to the absence of the methyl group (Figure 5d, Figure S21).
1 H HYSCORE spectra for the above materials were collected at identical magnetic field positions and could be simulated with the same models (Figure S28, S29).In addition, weak hyperfine couplings were observed to 14 N and 27 Al nuclei in the HYSCORE, corresponding to remote nuclei (Figure 5e).In the spectra of NO 2 @MFM-305-CH 3 , a point-like signal at ca. (3.8, 3.8) MHz represents the interaction between NO 2 molecules and 27 Al metal centres from the framework.For NO 2 @MFM-305, a similar 27 Al signal is observed but, in addition, intense cross-peaks at ca. (2.8, 1.9) and (1.9, 2.8) MHz are observed corresponding to double-quantum signals from a weakly coupled 14 N nucleus.These could be reproduced with a point-dipolar 14 N hyperfine coupling j A( 14 N) j = [1.3,1.3, À 2.0] MHz, corresponding to an electron••• 14 N distance of 1.85 Å (Table S7).These 14 N signals are not observed in MFM-305-CH 3 , and hence cannot be from the pyridyl group but from other guest molecules, consistent with the higher NO 2 /N 2 O 4 concentration in MFM-305 (Figure 5f).

Conclusion
Our studies reveal a new type of "regenerable reactive adsorption" of NO 2 by modulating the charge in robust MOF materials.MFM-305-CH 3 with a charged pore environment enables the in situ conversion of NO 2 to the usable chemical species NOCl.A full investigation of the reactivity of adsorbed NO 2 molecules, complex and dynamic hostguest binding and detailed structural investigation reveal key molecular details of the conversion of adsorbed NO 2 molecules.The effective control of reactivity of NO 2 achieved by MFM-305-CH 3 provides new insights into the design of efficient protocols for abatement of corrosive air pollutants.@MFM-305-CH 3 and NO 2 @MFM-305 under identical conditions; Xband Davies ENDOR spectrum of c) NO 2 @ MFM-305-CH 3 and d) NO 2 @MFM-305 (black) at 5 K and the static magnetic fields indicated, shown by the arrows in (a), selecting the NO 2 x, y and z axes (bottom to top), respectively.ENDOR gives pairs of transitions separated by the effective hyperfine coupling for the orientations selected, centred on the Larmor frequency of the nucleus being probed (14.9 MHz for 1 H at 350 mT), and its simulated spectra (magenta for sum; red, green and blue for different protons); view of binding site of c) NO 2 in MFM-305-CH 3 and d) NO 2 in MFM-305 after movement and rotation attached; X-band (9.7368 GHz) 14 N HYSCORE spectra of e) NO 2 @MFM-305-CH 3 measured at static fields 342.8, 346.7 and 354.0 mT, and of f) NO 2 @MFM-305 measured at static fields 343.0, 346.9 and 354.3 mT; g) simulated spectra (red) with parameters in Table S7.The anti-diagonal dashed lines cross the diagonal at the Larmor frequencies for 14 N and 27 Al (black: 14 N; magenta: 27 Al).
N HYSCORE (Hyperfine Sublevel Correlation) spectroscopies at 10 K.The ENDOR spectra of NO 2 -loaded MFM-305-CH 3 were taken at different static magnetic fields (Figure 5b) corresponding to different orientations of the NO 2 molecules.Each spectrum contains features of multiple doublets of protons centred at the proton Larmor frequency.The spectra are dominated by frequencies of ca. 2 MHz which correspond to the nearest e••• 1 H distances of 3.4 Å in a point dipolar approximation.When MFM-305-CH 3 was synthesised with deuterated methyl groups (MFM-305-CD 3 ), some weaker 1 H couplings disappear (ca. 1 MHz; corresponding to distances of ca.4.3 Å), and so these peaks must arise from the methyl protons (Figure S23).The orientationselective ENDOR measurements confirm that the largest 1 H coupling to the nearest protons are observed along the C 2 (z) axis of the NO 2 molecule, hence the nearest NO 2 ••• 1 H

Figure 4 .
Figure 4. INS spectroscopic data.Comparison of the experimental INS spectra (left) and views of the corresponding structural model (right) for bare and NO 2 -loaded a) MFM-305-CH 3 and b) MFM-305.

Figure 5 .
Figure 5. EPR spectroscopic data.a) Continuous-wave X-band(9.72GHz) EPR spectrum of NO 2 @MFM-305-CH 3 at 10 K (blue) and simulation (light blue) with g x = 2.0063, g y = 1.9910 and g z = 2.0030 and 14 N nuclear hyperfine interactions (nuclear spin, I = 1) of A x = 148, A y = 133 and A z = 194 MHz, where x, y and z define the NO 2 molecular axes (inset).NO 2 has C 2v point symmetry with the z axis along the C 2 rotational axis, y parallel to the OÀ O vector and x normal to the NO 2 plane; continuous-wave X-band (9.72 GHz) EPR spectrum of NO 2 @MFM-305 at 10 K (red) and simulation (light red) with g x = 2.006, g y = 1.9913 and g z = 2.0022 and 14 N nuclear hyperfine interactions (nuclear spin, I = 1) of A x = 144, A y = 133 and A z = 188 MHz; b) comparison of intensity of EPR signal of NO 2 @MFM-305-CH 3 and NO 2 @MFM-305 under identical conditions; Xband Davies ENDOR spectrum of c) NO 2 @ MFM-305-CH 3 and d) NO 2 @MFM-305 (black) at 5 K and the static magnetic fields indicated, shown by the arrows in (a), selecting the NO 2 x, y and z axes (bottom to top), respectively.ENDOR gives pairs of transitions separated by the effective hyperfine coupling for the orientations selected, centred on the Larmor frequency of the nucleus being probed (14.9 MHz for 1 H at 350 mT), and its simulated spectra (magenta for sum; red, green and blue for different protons); view of binding site of c) NO 2 in MFM-305-CH 3 and d) NO 2 in MFM-305 after movement and rotation attached; X-band (9.7368 GHz)14 N HYSCORE spectra of e) NO 2 @MFM-305-CH 3 measured at static fields 342.8, 346.7 and 354.0 mT, and of f) NO 2 @MFM-305 measured at static fields 343.0, 346.9 and 354.3 mT; g) simulated spectra (red) with parameters in TableS7.The anti-diagonal dashed lines cross the diagonal at the Larmor frequencies for 14 N and 27 Al (black: 14 N; magenta: 27 Al). 2