Guest Molecules in a Layered Microporous Tin(IV) Phosphonate

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Guest Molecules in a Layered Microporous Tin(IV) Phosphonate – Phosphate Material: Solid State NMR Studies Vladimir I. Bakhmutov, Douglas W W. Elliott, Aida Contreras-Ramirez, and Abraham Clearfield J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b09144 • Publication Date (Web): 30 Nov 2018 Downloaded from http://pubs.acs.org on December 3, 2018

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The Journal of Physical Chemistry

1

Guest

Molecules

in

a

Layered

Microporous

Tin(IV)

Phosphonate – Phosphate Material: Solid State NMR Studies Vladimir I. Bakhmutov,* Douglas W. Elliott, Aida R. Contreras, and Abraham Clearfield* Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, Texas 77842-3012, United States ABSTRACT: There is little systematic understanding of pore surfaces in layered microporous metal(IV) phosphate-phosphonate materials and their interactions with guest molecules. In this paper we show how to probe mobility of guest molecules in such poorly crystalline systems using multinuclear solid-state NMR and relaxation time measurements. Anisotropic motions of benzene-d6 molecules absorbed on the pore walls of material Sn(O3PC6H4PO3)0.85(O3POH)0.33 (1) have been recognized as the fast in-plane C6 rotation due to metal- interactions with pore walls. The benzene-d6 absorption enthalpy due to Sn… interactions, has been determined as - H = 5.9 kcal/mol. Specific interactions between pyridine and the pore walls of 1 have been observed as immobile pyridine, the population of which grows strongly at low temperatures to show thermodynamic parameters - H of 5.0 kcal/mol and S of -11.0 e.u. It has been suggested that these parameters characterize N…H-OP hydrogen bonding as driving force for accumulation of immobile pyridine molecules in pores of compound 1.

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2

1. INTRODUCTION Layered metal phosphates and phosphonates1 are widely applied as multifunctional materials in many fields of chemistry and industry.2-4 Amongst these materials, the mixed Zr(IV) and Sn(IV) phosphonate / phosphate systems are particularly interesting due to their potential usage for complete separations of lanthanides from actinides in spent nuclear fuel.5-7 It has been suggested that the phosphate groups PO-H in these materials play the role of ion-exchangers. However, some of the experiments with Zr(IV) and Sn(IV) phosphonates5 have, unexpectedly, demonstrated their ability for ion-exchange in the absence of phosphate groups. It has been concluded that the metal phosphonate materials contain phosphonic acid moieties with free OH groups, which may facilitate ion exchanges. Metal(IV) phosphonate/phosphate materials are generally prepared by the hydrothermal method. This method leads to poorly crystalline molecular systems with layered structures and porosity typical of microporous materials.5,8 As accepted now, the formation of pore spaces occurs due to the remaining PO3H2 groups (phosphonic acid moieties) located on pore surfaces;9 however, the pore surface can also contain metal centers with distorted geometries and POH protons of phosphate groups. The activity of these pore spaces and the pore surface is of great interest because the above-mentioned exchange processes may take place namely in pores of these materials. Valuable information about porous materials can be obtained by studying guest molecules and their dynamics within these systems,10 particularly if the guests are capable of specific interactions - like hydrogen bonding. In this paper we report on the experiments carried out on samples of a tin(IV) phosphonate/phosphate material,


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3 Sn(O3PC6H4PO3)0.85(O3POH)0.33 ·6H2O (1) (Figure 1) with the disordered phosphate groups,

containing

guest

molecules

of

benzene-d6

and

pyridine-d5.

Figure 1. A: Portion of the structure of compound 1 with the formula Sn(O3PC6H4PO3)0.85(O3POH)0.33·6H2O.

Deuterated guest molecules allow the application of the well-known 2H NMR techniques that are powerful tools for probing the guest dynamics. In addition, we have also applied solid-state 1H,

13C, 31P,

and

119Sn

NMR. In fact, when probing guest



4 molecules in poorly crystalline materials with unknown pore shapes and surfaces, two aspects should be considered: the spectral/structural characteristics of the compounds containing guest molecules, and molecular dynamics of these guests leading to their absorption energy.

2. EXPERIMENTAL 2.1 Materials. Compound 1 was prepared by the hydrothermal method, as it has been described previously.11 A sample of compound 1 was soaked in liquid benzene-d6 (Sigma-Aldrich) for 30-40 min, and was then placed into an oven at 1500 C for 30-45 min. The resulting dry, white powder is designated 1-Ben-0.1 as it contained 0.1 mole of C6D6 per 1 mole of 1. The mole fraction of C6D6 was found using the direct excitation 13C{1H} MAS NMR spectrum with recycle delays affording full nuclear relaxation, which determined the ratio of P-C6H4-P units / benzene was 8.9 to 1. To load pyridine-d5 (Sigma-Aldrich) initial compound 1 was soaked in liquid pyridine for 30-40 min. Then, the sample was placed in an oven at 1500 C for 30-45 min to yield a dry, white powder (1-Py-1.6), containing 1.6 moles of the pyridine per 1 mole of 1 confirmed, again, by the 13C {1H} MAS NMR. It should be emphasized that the experiments with compound 1-Py-1.6 can only be well reproduced on freshly-prepared samples (see the remarks in SI). Therefore, the quantitative data discussed in this paper have been obtained from experiments using freshly-prepared samples of 1-Py-1.6. Compound 1-Py-1-0.6 was obtained by heating a fresh-prepared sample 1-Py-1.6 at 1200 C for 18 hours. 2.2 Solid-state NMR Measurements.


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5 All NMR experiments were carried out with a Bruker Avance-400 solid-state NMR spectrometer (400 MHz for 1H nuclei) equipped with two-channel 7-mm and 4-mm MAS probe heads. The 31P{1H} NMR single pulse spectra, referenced to 1M H3PO4, were obtained with pulse lengths of 2.5 µs (500), 4-8 scans, and recycle delays of 50 s - as needed for the full phosphorus spin-lattice relaxation. Contact times of 6 ms were utilized for the 31P{1H}

CP NMR experiments.

The static

13C{1H}

NMR spectra were referenced to TMS, and obtained with

pulse lengths of 2.5 µs (500), 160 scans, and recycle delays of 10 s. The 13C{1H} MAS NMR spectra required 300-400 scans. In all carbon and phosphorus experiments the standard tppm15 pulse sequence was used for a broad-band 1H decoupling. Contact times of 2 ms were utilized for the 13C {1H} CP MAS NMR experiments. The

119Sn{1H}

MAS NMR spectra, referenced to 1M (CH3)4Sn, were collected

with the 4-mm NMR probe head at a spinning rate of 8.5 kHz. For direct excitation, 500rf-pulses (2.5 s) were applied with relaxation delays of 10 s to optimize the experiment time and signal-to-noise ratio. The static 2H NMR spectra were obtained using the Hahn-echo (900--1800-) pulse sequence ( = 50 s) with a 900-pulse length of 6 µs, recycle delay of 1 s, and a number of scans between 200 and 1500 (more at low temperatures). For the temperatures below 233 K the recycle delay was increased to 3 s. The variable temperature 2H NMR experiments were performed with a thermocouple calibrated using the 1H NMR spectra of liquid methanol ( = (OH) - (CH3)) placed into a static MAS NMR rotor. Following



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6 a change in temperature, a delay of 15-20 min was used before acquisition for the thermal stabilization. 2H

T1 relaxation times were measured by inversion−recovery (180°−τ−90°)

experiments using well-calibrated rf pulses and widely-varied τ delays. The

2H

inversion−recovery data “signal intensity versus τ time” were treated with a standard nonlinear fitting computer program based on the Levenberg−Marquardt algorithm to extract T1. All of the relaxation curves were found to be essentially mono-exponential. According to the statistics afforded from the fit, the errors in 2H T1 time determinations were < 15%. Relaxation (recycle) delays were adjusted based on T1 measurements to provide full nuclear relaxation in each cycle. 2.3 Powder X-ray diffraction (PXRD) patterns were obtained from 4° to 40° (2θ angle) using a Bruker-AXS D8 short arm diffractometer equipped with a multiwire lynx eye detector using Cu (Kα, λ =1.542Å) and operated at a potential of 40 kV and a current of 40 mA.

3. RESULTS AND DISCUSSION Characterization of the host structure of 1 in the presence of guest molecules was done by applying solid-state MAS NMR on

13C, 31P

and

119Sn

nuclei. Furthermore, 2H static

variable-temperature NMR and 2H T1 time measurements were used to describe the mobility of guests. Proceeding from the structure of 1, acidic protons P-OH from both of the phosphate groups and the phosphonic acid moieties were expected in the pore spaces. Since these acidic protons can potentially interact with guests, IR spectroscopy was applied; however, it was found that the room-temperature IR spectra of compound 1 and


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7 of those containing pyridine were indistinguishable are identical (Figure S1 in Supplementary Information). 3.1 The host structure of compounds 1, 1-Ben-0.1 and 1-Py-1.6. Sn(IV) phosphonate/phosphate materials prepared by the hydrothermal method have a layered structure and incorporate mainly micro pores.5,9 Compound 1 used in this study has the same

31P{1H}, 13C{1H}, 1H

MAS / static NMR spectra, and XRPD pattern as a

material reported previously.11 On the basis of these data, material 1 can be formulated as Sn(O3PC6H4PO3)0.85(O3POH)0.33·6H2O with the layered structure depicted in Figure 1. Additionally, the

119Sn{1H}

MAS NMR spectrum of 1 has shown two resonances at

chemical shifts () of -788 and -825 ppm, which are typical of tin atoms surrounded by four oxygen atoms.12 The -788 ppm resonance can be well assigned to the Sn-phosphate groups, while the -825 ppm resonance corresponds to the Sn-phosphonate groups. In accordance with these assignments, the Sn-phosphate resonance decreases significantly with H-Sn cross-polarization 119Sn MAS NMR spectrum of 1 due to the presence of only one proton in the phosphate groups. Since the 119Sn NMR spectra of compounds 1 and 1Ben-0.1 were identical, the above-mentioned effects have been shown for 1-Ben-0.1 in Figure 2. It should be noted that in spite of the relatively short recycle delays of 10 s, application of 450-rf pulses gave an integral ratio for the signals at -788 and -825 ppm in the

119Sn{1H}

MAS NMR spectra of 1 and 1-Ben-0.1 close to the formulated structure.

Finally, neither 1H MAS NMR spectrum of 1 (or the static 1H NMR, Figure S2) nor the static 1H NMR inversion-recovery experiments13 show P-OH resonances because of fast exchange with water protons and/or due to very large linewidths of these signals. As



8 shown for apatites,13 POH signals can remain broad even at high spinning rates; and, therefore, be difficult to observe.

Figure 2. The

119Sn{1H}MAS

NMR spectra recorded at a spinning rate of 8.6 kHz from

top to bottom: 1-Ben-0.1 compound with direct excitation of 119Sn nuclei; compound 1Ben-0.1 with H-Sn cross polarization. As has been shown earlier,10 the treatments of a layered zirconium phosphonate with benzene-d6 and toluene-d8 do not change the host’s structure. In accordance with these results, the 31P{1H} and 13C{1H} NMR spectra of 1 did not change in the presence of benzene. For example, the

13C{1H}

CP NMR spectrum of 1-Ben-0.1 showed two


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9 resonances at 135.3 and 130.3 ppm belonging to groups (O3PC6H4PO3). Even the 119Sn{1H}MAS

NMR spectra of 1 and 1-Ben-0.1 remained identical (Figure 2).

Pyridine, as a more aggressive solvent, could, potentially, lead to changes in the host structure. However, according to the XRPD pattern of 1-Py-1.6 (Figure S3), changes in the layered structure were minimal. The 31P{1H} and CP MAS NMR spectra of 1-Py1.6 recorded at a 8.2 kHz spinning rate, were the same as those of the initial compound 1 and compound 1-Ben-0.1: the resonances observed at 3.6 ppm ((O3PC6H4PO3)-groups) and -14.3 ppm (phosphate groups) integrated in agreement with the formula of 1. As in the case of compound 1, the 13C{1H} CP MAS NMR spectrum of 1-Py-1.6 also showed sharp signals from the phenylene rings at 136.4 and 130.7 ppm as illustrated in Figure S4 (middle). As earlier, the direct excitation in

13C{1H}MAS

NMR spectra (Figure S4,

bottom) provided determination of phenylene / guest ratios. It should be emphasized that due to high mobility of benzene and pyridine (see below), their

13C

resonances were

visible only by direct excitation of 13C nuclei in 13C{1H}MAS or static NMR (Figure S4, top). Finally, the room-temperature

119Sn{1H}

MAS NMR spectrum of 1-Py-1.6 has

shown again two resonances belonging phosphate (-775 ppm) and phosphonate (-822 ppm) groups. 3.2 The dynamics of guest molecules. The variable temperature static 2H NMR spectra of compound 1-Ben-0.1, containing benzene-d6 molecules are shown in Figure 3. As seen, at 263 K and higher the 2H resonances are liquid-like / Lorentz shaped indicating fast isotropic reorientations of the guests. Such a liquid-like behavior is not surprising, it has been observed earlier for microporous system MOF-5,14 and mesoporous silica SBA-15.15 The temperature



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10 dependences of the 2H linewidths and the 2H T1 times measured for the isotropic resonance are shown in Table 1 and Figure 4.

Figure 3. Variable-temperature Hahn-echo static 2H NMR spectra of 1-Ben-0.1 from top to bottom: 343 K, 323 K, 295 K, 285 K, 273 K, 263 K, 253 K, 223 K, 210 K and 193 K. The insert shows quadrupolar pattern at 193 K fit with the CQ constant of 92 kHz and at asymmetry parameter  of 0.02. Table 1. The temperature dependence of linewidths  and 2H T1 times, measured for the Lorentz-shaped resonance of benzene-d6 in compound 1-Ben-0.1, and equilibrium constants Keq, characterizing fractions of bound benzene molecules (see the text). T (K) 343 323 295 273 263

 (Hz) / 2H T1 (s) 469 / 0.059 554 / 0.048 1091 / 0.045 2182 / 0.046 3547 / 0.036


Keq 0.0040 0.0053 0.0137 0.0312 0.0530

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11

Figure 4. The temperature dependences of 2H T1 times in static compound 1-Ben-0.1 (), 1-Py-1.6 ( ) and Zr(IV) phosphonate 10 (o) in the coordinates ln(T1) versus 1000/T; T1 and T are measured in seconds and kelvins, correspondently. As seen in Figure 4, the ln(T1) versus 1000/T changes linearly for 1-Ben-0.1 with a very small slope, corresponding to a low activation energy (less than 1 kcal/mol) of isotropic molecular reorientations where the quadrupole relaxation of C6D6 follows equation (1): 1/T1(D) = (3/10) 2 (DQCC)2 (1+ 2/3) (c / (1 + D2c2) + 4c / (1 + 4D2c 2)) ACS Paragon Plus Environment

(1)


12 Here DQCC is static deuterium quadrupolar constant,  is the asymmetry parameter, C is the molecular motion correlation time expressed as C = 0 exp(E/RT), 0 is a preexponential factor, E is the activation energy, and ωD is the angular deuterium NMR frequency.16 It is obvious that a small E value (< 1 kcal/mol) is not plausible for solids (see the relaxation curve obtained for benzene-d6 in a Zr phosphonate,10 which is Ushaped in accordance with equation (1)). This relaxation behavior can be rationalized by the presence of anisotropic molecular reorientations of the benzene that increase the relaxation time.16 When these motions, both isotropic and anisotropic, are in a fast exchange on the NMR time scale, and the longer T1 time contribution from anisotropic reorientations grows on cooling,10 then the effect of lowering temperatures on observed T1 times will be less pronounced. This interpretation is well supported by the 2H NMR spectra in Figure 3, where the isotropic benzene line broadens strongly at 223 K, and then converts into a Pake pattern at 193 K with a 67 kHz quadrupolar splitting (fitting yielded a quadrupolar constant, , of 92 kHz with an asymmetry parameter of 0.02, see the insert). This quadrupolar pattern is typical of those reported for bound benzene molecules in other porous systems,14,15,11-19 where the benzene molecules experience fast in-plane C6 rotation, as shown in Figure 5.


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13

Figure 5. A stylized pore surface of compound 1: the bound benzene molecules experiencing fast in-plane C6 rotation (A) (an isotropically moving molecule is dashed); a hydrogen-bonded pyridine with rotation around bonds N…H (B); the immobile hydrogenbonded pyridine molecules, located in cavities on the pore surface (C)

Again, only the bound (fast in-plane C6 rotating) benzene molecules were detected in 1Ben-0.1 at 193 K. Then, heating the sample from 193 K to 253 K caused the line shape to



14 evolve, as a typical chemical exchange between the “two resonances separated by 67 kHz”. In these terms, this evolution corresponded to a transformation of the anisotropic motional state to an isotropic motional state that occurred on the T2 NMR time scale. Since the pattern coalesced at 223 K, the transformation rate (k) was formally estimated as 1.5 105 s-1 (via the well-known equation k = 2.22 , where  = 67 kHz in our case). At 253 K, in addition to the wide component, the 2H NMR spectrum showed a prominent isotropic line. In fact, the line shape at this temperature can be best simulated as an exchange on the NMR time scale between the quadrupolar pattern and the isotropic resonance. This exchange becomes very fast at 263 K, and the wide component disappears completely. Finally, from 263 to 343 K the spectrum shows only a sharp isotropic resonance with reducing linewidths on heating. All of these observations support that the increase in the 2H linewidth of the guest benzene (from  470 Hz at 343 K to  3500 Hz at 263 K (Table 1)) was actually caused by increasing the population of bound guest molecules on cooling, and that they were in a very fast exchange with isotropically moving molecules. In fact, the line shape observed between 343 and 263 K can be well simulated as a fast two-center exchange between the sharp (300 Hz10) resonance and the wide (67 kHz) resonance with population of the wide component increasing at lower temperatures. Under these conditions, the linewidth in Table 1 () can be used to determine the fraction of the bound benzene (Kb) via the well-known equation (2):20  = iso + Kb b (2)


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15 Here, we considered fast exchange between an isotropic component with linewidth (iso) of 300 Hz10 and an anisotropic component from the bound benzene with a linewidth (b) of 67 kHz (in other words, the resonance observed at low temperatures is approximated to a single line with a 67 kHz linewidth). A bound fraction was determined for each temperature leading to equilibrium constants (K = Kb/(1-Kb)), which gave a straight line in coordinates ln K versus 1000/T corresponding to parameters - H of 5.9 kcal/mol and S of -14.3 e.u. For comparison, an absorption enthalpy of 10 kcal/mol has been reported for benzene molecules in Ca-montmorillonie, which involve the formation of –complexes with Ca2+ centers.21 Thus the interactions of the benzene with the pore walls in compound 1-Ben-0.1, through the Sn… and/or OH… interactions, can be classified as weak. Since even the pyridine does not affect bands POH in the IR spectra of 1 (Figure S1), the Sn… interactions seem to be most probable as a driving force of the benzene absorption. The static, variable-temperature

2H

NMR spectra, characterizing pyridine

molecules in material 1-Py-1.6, are shown in Figure 6. Three features should be noted:



16

Figure 6. Variable-temperature 2H NMR spectra recorded in static compound 1-Py-1.6 from top to bottom: 293 K, 273 K, 243 K, 223 K, 203 K, 183 K, and 175 K; the insert: the 2H NMR spectrum at 175 K simulated as a combination of two sub-spectra corresponding to mobile pyridine (the central isotropic component) and the full size anisotropy of a quadrupolar tensor obtained with static quadrupolar constants of 165, 172 and 175 kHz ( = 0.1) close to 170, 175 and 185 kHz in ref.22

the deuterium resonance in the room-temperature 2H NMR spectrum of 1-Py-1.6 was remarkably broader ( = 3562 Hz, Table 2) than that of the benzene in 1-Ben-0.1; the central component of the spectrum, corresponding to isotropically-moving pyridine molecules, broadened on cooling (Table 2) but remained Lorentz-shaped even at lowest


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17 temperatures; and a lower-intensity quadrupolar pattern appeared at 283 K (i.e. significantly higher than the melting point of pyridine-d5 at 231 K), and grew on cooling (see also the 2H NMR spectra in Figure S5). The anisotropic pattern, Table 2. The temperature dependence of the linewidth () measured for the Lorentzshaped resonance of guest pyridine-d5 in compound 1-Py-1.6. T (K) 363 353 333 295 273

 (Hz) 1483 1535 2245 3562 4124

characterizing motionally-hindered pyridine is considered below. Here, however, it should be emphasized that the exchange between the isotropic and anisotropic resonances is slow on the NMR time scale. As follows from Tables 1 and 2, the pyridine signal in the room-temperature 2H NMR spectrum of 1-Py-1.6 was broad (3.5 kHz) compared to the benzene resonance in 1-Ben-0.1 (1 kHz). Bound pyridine molecules, moving anisotropically, in a fast exchange with “free” pyridine molecules can explain this broadening. Unfortunately, the nature of the bound states and their motions remain unknown because, even at 175 K, the central component in the 2H NMR spectrum of 1-Py-1.6 does not show quadrupolar features (in contrast to benzene in 1-Ben-0.1), indicating that the exchange between free and bound pyridine molecules still remained fast. As it has been shown earlier,10 reducing concentration of benzene in pore spaces of a layered zirconium phosphonate leads to an increase in populations of its bound states due to π–metal interactions with the pore walls. In this connection, we have probed compound 1-Py-0.6, which contained the smaller



18 pyridine amount than that in compound 1-Py-1.6. Indeed, the 175 K static 2H NMR spectrum of compound 1-Py-0.6 exhibited a strongly increased population of the central component (Figure 7). This component was broad enough to include the

Figure 7. The static 2H NMR spectrum of compound 1-Py-0.6 at 175 K (bottom) and compound 1-Ben-0.1 at 193 K (top).

quadrupolar pattern with 67 kHz splitting. However, again, quadrupolar features were not observed. In general, two types of bound pyridine molecules can be assumed: the molecules experiencing fast in-plane C6 rotation due to Sn… interactions (Figure 5A) and/or a hydrogen-bonded pyridine due to interactions with POH protons on the pore surface of 1 shown in Figure 5B. Since the room-temperature IR spectrum of 1 did not change in the presence of the pyridine (Figure S1), a choice between these modes is currently impossible.


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19 The temperature behavior of the central Lorentz-shaped component in the 2H NMR spectra of compound 1-Py-1.6 has been additionally characterized by 2H T1 times presented in Table 3 and plotted in Figure 4. Table 3. The 2H T1 time for the isotropic line measured for pyridine molecules in compound 1-Py-1.6 at different temperatures. T (K) 293 273 243 223 213 203 193 183 175

T1 (s) 0.039 0.022 0.017 0.017 0.014 0.013 0.0097 0.0086 0.0087

In contrast to 1-Ben-0.1, the inverse-temperature versus ln T1 time dependence in 1-Py1.6 is U-shaped (Figure 4) and well treated with equation (1) to show a 2H T1min time. As seen, the T1min temperature for pyridine was determined to be lower (183 K) than that for benzene in a Zr phosphonate, where the 2H T1min time was 239 K.10 Fitting gave an activation energy (E) of 1.6 kcal/mol. Such a low E value is typical of isotropic motions in liquids; illustrating, thus, weak interactions of pyridine molecules (contributing to the isotropic component of the 2H NMR spectra in Figure 6) with pore walls of 1. Note that similarly low E values have been also reported for the isotropic motions of benzene molecules in the microporous metal–organic compound Zn4O(O2CC6H4CO2)3, (MOF-5). 14 As mentioned above, immobile pyridine molecules were seen in the 2H NMR spectra of compound 1-Py-1.6 at < 283 K as a quadrupolar resonance with growing



20 intensities at low temperatures (Figure 6, Figure S5). This resonance did not show an exchange on the NMR time scale with the “mobile” pyridine signal, and can be well simulated at 175 K with static quadrupolar constants of 165, 172 and 175 kHz ( = 0.1), are close to 170, 175 and 185 kHz as reported for the deuterons of pyridine in liquid crystals.22 At 243 K, the simulation gave slightly smaller magnitudes of quadrupolar constants (140, 155, 165 kHz at  = 0.05), revealing, thus, low-amplitude motions of pyridine molecules. It is very likely that the pyridine immobility in 1-Py-1.6 is connected with steric hindrance in the pore space, where the immobile molecules are located. Such a hindrance can be manifest when a part of the pore space is narrow, or in cavities on the pore surface with the size commensurable with the size of the pyridine (Figure 5C). To estimate the thermodynamic parameters of the immobile pyridine molecules their fraction was determined by integration of the corresponding signal contributions in the variable temperature 2H NMR spectra of 1-Py-1.6. Note, however, that results at temperatures lower than the melting point of pyridine-d5 could potentially distorted by partial freezing. In fact, at temperatures lower the melting point, the bulk pyridine-d5 2H NMR spectrum shows the full-size anisotropic pattern with a quadrupolar splitting of 121 kHz.23 Therefore, the fraction of immobile pyridine molecules (Kim) in compound 1-Py1.6 was obtained at 273, 253, 243, and 233 K. The corresponding equilibrium constants, Keq, (Keq = Kim/(1-Kim)) in Table 4 give a straight line in coordinates lnKeq versus 1000/T leading to - H of 5.0 kcal/mol (S of -11.0 e.u.).


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21 Table 4. The temperature dependence of the equilibrium constant Keq found via fraction of immobile pyridine molecules in 1-Py-1.6 (see the text). T (K) 273 253 243 233

Keq 0.14 0.28 0.47 0.61

Hydrogen bonds, SiOH…N, between pyridine and surface silanol groups have been well established in mesopores of material MCM-41.24 Since they result in pyridine molecules with limited rotational motions,24 we surmise that N…H-OP hydrogen bonding is driving force for accumulation of immobile pyridine molecules in compound 1-Py-1.6. Note, however, that the pyridine desorption enthalpy in materials MCM-41 is remarkably larger (12.5 – 21.8 kcal24).

4. CONCLUSIONS. The first comprehensive study of layered microporous Sn(IV) phosphonate / phosphate material 1 with benzene-d6 and pyridine-d5 within pores has demonstrated how to probe pore surfaces in poorly crystalline systems on the basis of multinuclear solid state NMR and relaxation time measurements. The various methods (13C, 31P, 119Sn solid state NMR, IR spectroscopy and XRPD) did not reveal macroscopic changes in the host structure of 1 in the presence of benzene and pyridine molecules. Anisotropic motions of benzene-d6 molecules absorbed on the pore walls have been recognized as the fast in-plane C6 rotation due to metal- interactions with pore walls. Absorption energy parameters have been determined: - H = 5.9 kcal/mol, S = 14.3 e.u.. In addition to the mobile pyridine molecules in compound 1-Py-1.6, the variable temperature 2H NMR spectra have shown a population of immobile pyridine,



22 which grows strongly at low temperatures, according to thermodynamic parameters - H of 5.0 kcal/mol and S of -11.0 e.u. It has been suggested that driving force for accumulation of immobile molecules is N…H-OP hydrogen bonding with the surface POH groups. ASSOCIATED CONTENT Supporting Information. Detailed description of sample preparations; NMR spectra for the studied compounds; and XRPD and TGA characterizations. (PDF) AUTHOR INFORMATION Corresponding Author (V.I.B.) E-mail: [email protected] ORCID Vladimir I. Bakhmutov: 0000-0002-5011-0385 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the Robert A. Welch Foundation Grant A-0673 (Metal Phosphonates as Crystal Engineered Solids and platforms for drug delivery) and the Nuclear Energy University Program (DOE) Grant Award # DE-NE0000746 (Mixed Metal Phosphonate-Phosphate Resins for Separation of Lanthanides from Actinides), for which grateful acknowledgement is made.


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23

REFERENCES. (1) Kumar, C. V.; Bhambhani, A.; Hnatiuk, N. Layered α-Zirconium Phosphates and Phosphonates. In Handbook Layered Materials, ed. Auerbach, S. M.; Carrado, K. A.; Dutta, P. K. Marcel Dekker, Inc.: New York, 2004, pp 313−367. (2) Diaz, A.; Saxena, V.; Gonzalez, J.; David, A.; Casanas, B.; Batteas, J. D.; Colon, J. L; Clearfield, A.; Hussain, M. D. Zirconium Phosphate Nano-Platelets: a Novel Platform for Drug Delivery in Cancer Therapy. Chem. Commun. 2012, 48, 1754-1756. (3) Díaz, A.; González, M. L.; Pérez, R.; David, A.; Mukherjee, A.; Báez, A.; Clearfield, A.; Colón, J. L. Direct Intercalation of Cisplatin into Zirconium Phosphate Nanoplatelets for Potential Cancer Nanotheraly. Nanoscale 2013, 5, 11456-11463. (4) Costantino, F.; Vivani, R.; Bastianini, M.; Ortolani, L.; Piermatti, O.; Nocchetti, M.; Vaccaro, L. Accessing Stable Zirconium Carboxyaminophophonate Nanosheets as Support for Highly Active Pd Nanoparticles.Chem. Commun. 2015, 51, 15990-15993. (5) Silbernagel, R.; Shehee, T. C.; Martin, C. H., Hobbs, D. T.; Clearfield, A. Zr/Sn(IV) Phosphonates as Radiolytically Stable Ion-Exchange Materials Chem. Mater. 2016, 28, 2254−2259. (6) Cahill, R. S.; Shpeizer, B.; Peng, G.-Z.; Bortun, L.; Clearfield, A. In Separation of FElements; ed. Nash, K. L.; Choppin, G. R. Plenum Press: New York, 1995; 165−176. (7) Burns, J. D.; Clearfield, A.; Borkowski, M.; Reed, D. T. Pillared Metal(IV) Phosphate-Phosphonate Extraction of Actinides. Radiochim. Acta 2012, 100, 381−387. (8) Wang, J.; Wang, R.; Zi, H.; Wang H.; Xia, Y.; Liu, X. Porous Organic Zirconium Phosphonate as Efficient Catalysts for the Catalytic Transfer Hydrogenation of Ethyl



24 Levulinate to γ-Valerolactone without External Hydrogen. J. Chin. Chem. Soc. 2018, 65, 750-759. (9) Clearfield, A.; Wang, Z. J. Organically Pillared Microporous Zirconium Phosphonates, Chem. Soc., Dalton Trans., 2002, 2937–2947. (10) Contreras, A.R.; Bakhmutov, V. I.; Elliott, D. W.; Clearfield, A. Benzene-d6 and Toluene-d8 as Guest Molecules in Micro Pores of a Layered Zirconium Phosphonate: 2H, 13C{1H}

and

31P{1H}

Solid-state NMR, Deuterium NMR Relaxation and Molecular

Motions. Magn. Reson. Chem. 2018, 56, 1158-1167. (11) Sheikh, J. A.; Bakhmutov, V. I.; Clearfield, A. Layered Metal(IV) Phosphonate Materials: Solid‐State 1H,

13C, 31P

NMR Spectra and NMR Relaxation. Magn Reson

Chem. 2018, 56, 276-284. (12) Gomez-Alcantara, N. M.; Cabeza. A.; Olivera-Pastor, P.; Fernandez-Moreno, F.; Sobrados, I.; Sanz, J.; Morris, R. E.; Clearfield, A.; Aranda, M. A. G. Layered Microporous Tin(IV) Bisphosphonates, Dalton Trans., 2007, 2394–2404. (13) Garcia, S. D. PhD Thesis, University Pierre and Marie Curie - Paris VI, 2012. (14) Gonzalez, J.; Devi, R. N.; Tunstall, D. P.; Cox, P. A.; Wright, P. A. Deuterium NMR Studies of Framework and Guest Mobility in the Metal-organic Framework Compound MOF-5, Zn4O(O2CC6H4CO2)3. Microporous and Mesoporous Materials 2005, 84, 97–104. (15) Gedat, E.; Schreiber, A.; Albrecht, J.; Emmler, T.; Shenderovich, I.; Findenegg, G. H.; Limbach, H.-H.; Buntkowsky, G. Evidence for Surface and Core Phases of Benzened6 Confined in Mesoporous Silica SBA-15 as Studied with 2H-NMR. J. Phys. Chem. B 2002, 106, 1977-1984.


Page 24 of 27

Page 25 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25 (16) Bakhmutov, V. I. NMR Spectroscopy in Liquids and Solids; CRC Press: Boca Raton, Fl, 2015, 59- 172. (17) Gonzalez, J.; Devi, R. N.; Wright, P. A.; Tunstall, D. P. ; Cox, P. A. Motion of Aromatic Hydrocarbons in the Microporous Aluminum Methylphosphonates AlMePO- and AlMePO- β. J. Phys. Chem. B, 2005, 109, 21700-21709. (18) Xu, J.; Sinelnikov, R.; Huang, Y. Capturing Guest Dynamics in Metal–Organic Framework CPO-27-M (M = Mg, Zn) by 2H Solid-State NMR Spectroscopy, Langmuir, 2016, 32, 5468−5479 (19) Buntkowsky, G.; Breitzke, H.; Adamczyk, A.; Roelofs, F.; Emmler, T.; Gedat, E.; Grunberg, B.; Xu, Y.; Limbach, H.-H.; Shenderovich, I.; et al. 2H-Solid-State NMR Study of Benzene-d6 Confined in Mesoporous Silica SBA-15. Phys. Chem. Chem. Phys., 2007, 9, 4843–4853 (20) Martin, M. L.; Delpuech, J. J.; Martin, G. J. Practical NMR Spectroscopy, Heyden & Sons, London, 1980, p. 303. (21) Xiong, J.; Maciel, G. E. Deuterium NMR Studies of Local Motions of Benzene Adsorbed on Ca-Montmorillonite, J. Phys. Chem. B 1999, 103, 5543-5549. (22) Ambrosetti, R.; Catalano, D.; Forte, C.; Veracini, C. A. Deuterium Quadrupolar Parameters from

1H

and

2H

NMR Spectra for Pyridine-d5, Benzonitrile-d5 and

Chlorobenzene-d5 Using Liquid Crystal Solvents. Z. Naturforsch. 1986, 41a, 431-435. (23) Aliev, A. E., Harris, D. M.; Mahdyarfar, A. Dynamics of Benzene and Pyridine Guest Molecules in their TOT Inclusion Compounds: Solid-State 1H NMR Studies. J. Chem. Soc. Faraday Trans. 1995, 91, 2017-2026.



26 (24) Zhao, X. S.; Lu,G. Q.; Whittaker, A. K.; Millar, G. J.; Zhu, H. Y. Comprehensive Study of Surface Chemistry of MCM-41 Using

29Si

CP/MAS NMR, FTIR, Pyridine-

TPD, and TGA. J. Phys. Chem. B 1997, 101, 6525-6531


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Guest

Molecules

in

a

Layered

Microporous

Tin(IV)

Phosphonate – Phosphate Material: Solid State NMR Studies Vladimir I. Bakhmutov,* Douglas W. Elliott, Aida R. Contreras, and Abraham Clearfield*

Guest molecules and their motions in pore spaces of layered compound 1, found by the variable-temperature deuterium NMR spectra.


Guest Molecules in a Layered Microporous Tin(IV) Phosphonate

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