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Figure 2-2: 25Mg static SSNMR spectra of CPO-27-Mg as a function of rehydration degree. All spectra were acquired under the same spectrometer conditions, 16384 scans and a pulse delay of 1 s. The * indicates a small amount of impurity.

As mentioned earlier, a previous powder XRD study suggests that the space group (R-3) of CPO-27-Mg remains unchanged upon dehydration.2 The PXRD patterns of as- made and fully dehydrated CPO-27-Mg obtained in this work (Figure 2-A1, appendix) are in good agreement with those previously reported,2,7 which clearly indicate that the long-range ordering is preserved upon dehydration. 13C MAS NMR spectra of CPO-27- Mg before and after dehydration (Figure 2-A2) also look very similar, confirming that the framework remains intact. However, the corresponding 25Mg static SSNMR spectra acquired at 21.1 T (Figure 2-2) are remarkably different. For as-made CPO-27-Mg, its 25

Mg static SSNMR spectrum has a well-defined second-order quadrupolar powder pattern typical of crystalline systems, which can be well simulated with one set of 25Mg

electric field gradient (EFG) parameters: CQ = 6.4(4) MHz, ηQ = 0.5(1) and δiso = 12(5) ppm. The fact that the spectrum can be well simulated with a single Mg site is in agreement with crystal structure. The nonzero ηQ value is consistent with the low Mg site symmetry (C1). No chemical shift anisotropy was included in the simulation, indicating that the observed spectrum is mainly dominated by the quadrupolar interaction. The δiso falls in the normal range of Mg oxyanion compounds, but the CQ value is rather large for a typical six-coordinated Mg.8 However, the distortion of the MgO6 octahedron is only moderately large (the variations in the Mg–O distances and the O–Mg–O bond angles are in the range of 1.97–2.17 Å and 80.2–99.2°, respectively). Because the quadrupolar coupling is affected by longer-range interactions, the relatively large CQ(25Mg) must reflect the contributions from four organic linkers bound to the Mg and their relative orientations as well as the water molecule.

The 25Mg static spectrum of fully dehydrated CPO-27-Mg looks distinctly different from that of the as-made phase (Figure 2-2), where only a very weak, featureless peak was observed. The most striking observation is that for a fully dehydrated phase more than 80% of the Mg became NMR “invisible” upon dehydration. The large amount of ‘invisible’ Mg is likely due to the fact that these Mg sites have large CQ’s. The Mg ions in the dehydrated phase are indeed expected to have much larger CQ’s compared to that in the as-made sample due to the alteration of the coordination geometry to a distorted square-pyramid. For Mg-containing organic compounds, the CQ’s for the Mg in the square-pyramid geometry are known to be rather large (about 13 MHz) and were only observed using 25Mg-enriched samples.19 The Mg in dehydrated CPO-27-Mg may well have a comparable, if not larger, CQ, yielding a spectrum too broad to be measured at natural abundance even at 21.1 T. Indeed, our density functional theory (DFT) calculation confirms a large CQ(25Mg) of 14.1 MHz for the dehydrated phase. A recent work showed that the “hidden” Mg with a large CQ in a crystalline mineral can be observed at natural abundance by using sensitivity enhancement techniques such as QCPMG.20 Unfortunately, our attempts to acquire QCPMG spectra of various dehydrated samples failed due to the very short T2 (< 1 ms), although we were able to acquire a QCPMG spectrum of the as-made sample (spectrum not shown). The observed signal of

the dehydrated sample does not have a typical line shape arising from the quadrupolar interaction in a crystalline phase. The peak is asymmetrically broadened with a tail at the low-frequency side, which is likely the “tip” of a very broad distribution of CQ due to a range of slightly different Mg environments. The NMR results suggest that although the PXRD pattern shows that the long-range ordering is preserved upon dehydration, the local Mg environment is disordered. The disorder is likely due to the fact that dehydration causes the DOBDC linkers bound to a Mg to slightly change their orientations relative to one other as well as the Mg–O bond length and O–Mg–O bond angles. Such changes appear to vary from site to site, leading to a distribution of CQ. Furthermore, the distortion to the square-pyramid geometry brought about by the changes mentioned above results in a remarkable increase in the CQ(25Mg), leading to the loss of a large amount of 25Mg signal. The situation is somewhat reminiscent of zeolite dehydration. The tetrahedral Al atoms in hydrated zeolites have a rather small CQ(27Al). Dehydration leads to a dramatic increase in CQ(27Al), resulting in a large portion of the 27

Al spins becoming NMR-invisible.27 The PXRD patterns of dehydrated zeolites also indicate high sample crystallinity, whereas 27Al NMR spectra show a distribution of

CQ(27Al). A recent 67Zn SSNMR study also showed that the desolvation led to the distortion of Zn environment in IRMOF-1.22

To better understand the effect of coordinated water on the Mg environment, we systematically examined the rehydration. Figure 2-2 shows the 25Mg static SSNMR spectra of CPO-27-Mg as a function of a degree of rehydration. At the beginning of rehydration (0.6H2O/Mg), although the PXRD pattern (Figure 2-A1) looks identical to that of fully dehydrated phase, the corresponding 25Mg NMR spectrum exhibits observable changes. Specifically, it now shows a featureless peak (similar to that observed in the fully dehydrated phase) superimposed on top of a very broad resonance with a breadth of about 110 kHz. This very broad new signal is assigned to the Mg atoms whose open sites are now occupied by the added H2O. Because local geometries of these Mg ions are octahedral, they have smaller CQ(25Mg) and start becoming detectable. It is worth mentioning that the breadth of this signal from newly formed MgO6 is larger than that of the as-made phase, suggesting that the spatial arrangement of the ligands around

the metal has not fully relaxed back to the state of fully hydrated phase. The water exchange between Mg sites may also exist.

Figure 2-3: The plot of calculated (a) CQ(25Mg) and (b) δiso(25Mg) as a function of the Mg–OH2 distance.

When the rehydration degree increases to approximately one water molecule per metal center (1H2O/Mg), the broad new peak observed in the previous (0.6H2O/Mg) sample now has a breadth similar to that of the as-made sample, implying that for these Mg ions, the local Mg environments gradually revert back to that in the hydrated phase. It is noticed that although in this sample every Mg, in principle, can adsorb one water molecule, the observed 25Mg intensity only counts for 57% of that seen in the as-made sample. There are several possible reasons for this observation. First, under the experimental conditions employed, the water molecules may not be homogenously distributed within the channels. It is possible that the water initially coordinated to the metal interacts with the water molecules subsequently entering the channel via hydrogen bonding, leaving a significant number of five-coordinated Mg unaffected. Second, even if, on average each Mg adsorbs one water molecule, there will be a distribution of the EFG parameters and chemical shift if the Mg–OH2 bond length varies from site to site. To verify this argument, we carried out the DFT calculations on the Mg(OH2)(DOBDC)43- cluster (Figure 2-3). Specifically, we calculated the NMR parameters as a function of the

Mg–OH2 bond length and the results show that (1) while the δiso changes little, the CQ varies in a very wide range, consistent with the disordering, and (2) the large Mg–OH2 distances can lead to very large CQ(25Mg)’s, resulting in the “hidden” Mg. When the degree of hydration is increased to 4 and 5H2O/Mg, the number of 25Mg spins observed and their line shape change back to those of the as-made sample. The results indicate that when a large amount of water is available, the local Mg environment can be completely restored.

Figure 2-4: 25Mg static SSNMR spectra of CPO-27-Mg loaded with different guest species. All spectra were acquired under the same spectrometer conditions, 16384 scans and a pulse delay of 1 s.

Recently, MOFs have been used as sensors to detect VOCs.1 CPO-27-Mg certainly has a potential for such application due to the strong interactions between analyte and the open Mg site. Therefore, understanding the effect of adsorption of VOCs on the local Mg geometry is important. We acquired 25Mg static SSNMR spectra of CPO- 27-Mg loaded with acetone and acetonitrile (Figure 2-4). In order to maximize the adsorbate effect, saturated adsorption conditions were used. While the PXRD patterns of CPO-27-Mg loaded with two organics (Figure 2-A1) look identical to that of the as-made

phase, the corresponding 25Mg SSNMR spectra are markedly different. The spectrum of the sample loaded with CH3CN looks similar to that of CPO-27-Mg with 1H2O/Mg, indicating that adsorption of CH3CN also leads to partial line narrowing due to the formation of some octahedral Mg sites. The line shape suggests a distribution of Mg coordination environments. Similarly, adsorption of acetone also increases the intensity of the Mg, while the Mg environment remains disordered. The computational modeling indicates that the EFG parameters are sensitive to both the orientation of the acetone (Figure 2-A5) and its distance to the Mg (Figure 2-5), the observed disorder originates mainly from the variation in the Mg–OC(CH3)2 distance from site to site. A 2H SSNMR study of CPO-27-Mg is currently performed in our group to understand the dynamics of water and the organics adsorbed at Mg.

Figure 2-5: The plot of calculated (a) CQ(25Mg) and (b) δiso(25Mg) as a function of the Mg–OC(CH3)2 distance.