High-Resolution Structural Characterization of Two Layered

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High-Resolution Structural Characterization of Two Layered Aluminophosphates by Synchrotron Powder Diffraction and NMR Crystallographies Boris Bouchevreau,† Charlotte Martineau,*,† Caroline Mellot-Draznieks,‡,# Alain Tuel,§ Matthew R. Suchomel,∥ Julien Trébosc,⊥ Olivier Lafon,⊥ Jean-Paul Amoureux,⊥ and Francis Taulelle*,† †

Tectospin, Institut Lavoisier de Versailles, CNRS UMR 8180, Université de Versailles Saint-Quentin en Yvelines, 45 avenue des États-Unis, 78035 Versailles Cedex, France ‡ Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, U.K. § Université Lyon 1, IRCELYON, Institut de Recherche sur la Catalyse et l’Environnement de Lyon, CNRS UMR 5256, 69626 Villeurbanne, France ∥ Argonne National Laboratory, Advanced Photon Source, Argonne, Illinois 60439, United States ⊥ Université de Lille Nord de France, F-59000 Lille, France and CNRS UMR 8181, Unité de Catalyse et de Chimie du Solide, UCCS, USTL, F-59652 Villeneuve d’Ascq, France S Supporting Information *

ABSTRACT: The syntheses and structure resolution process of two highly complex powdered aluminophosphates with an original 5:7 Al/P ratio are presented: [Al5(OH)(PO4)3(PO3OH)4] [NH3(CH2)2NH3]2 [2H2O], compound 1, and [Al5(PO4)5(PO3OH)2] [NH3(CH2)3NH3]2 [H2O], compound 2. We have previously reported the structure of the periodic part of 1 by coupling synchrotron powder diffraction and solid-state nuclear magnetic resonance (NMR) crystallographies. With a similar strategy, that is, input of large parts of the building blocks determined by analysis of the 27Al−31P correlation pattern of the twodimensional (2D) NMR spectrum in the structure search process, we first determine the periodic structure of 2, using the powder synchrotron diffraction data as cost function. Both 1 and 2 are layered materials, in which the inorganic layers contain five P and seven Al inequivalent atoms, with aluminum atoms that are found in three different coordination states, AlO4, AlO5, and AlO6, and the interlayer space contains the amines and water molecules. In 1, the inorganic layers are stacked on each other with a 42 element of symmetry along the c-axis, while they are stacked with a 180° rotation angle in 2. By analysis of a set of high-resolution 1D and 2D NMR spectra (31P, 27Al, 1H, 15N, 13C, 27Al−31P, 1H−31P, and 1H−14N), the structure analysis of 1 and 2 is extended beyond the strict periodicity, to which diffraction is restricted, and provides localization of the hydroxyl groups and water molecules in the frameworks and an attempt to correlate the presence of these latter species to the structural features of the two samples is presented. Finally, the dehydration/rehydration processes occurring in these solids are analyzed. The methodology of the structure determination for these dehydrated forms uses the same principles, combining X-ray powder diffraction and solid-state NMR data. KEYWORDS: aluminophosphates, synchrotron powder diffraction, NMR, NMR crystallography, layered solids

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exert a major influence on the final architecture and stability of these extraordinary compounds, and therefore they must be described with the highest possible accuracy. Combining multiple diffraction probes (X-ray, electron, neutron), data and information from other physical measurements (electron microscopy, etc.) with the charge flipping algorithm1−4 has been shown to be one possible strategy for the average

INTRODUCTION In the past decades, crystals have been very successfully characterized by diffraction methods within the assumption of strict translational symmetry and periodicity. However, for numerous hybrid organic−inorganic materials like zeolites, metal−organic frameworks (MOFs), or metallophosphates, this approach is not always sufficient since the templating agents (e.g., organic molecules, water molecules, or other free ions) and guest molecules filling the framework pores may be distributed or located on subnetworks that do not necessarily share the periodicity of the host structure. Guest molecules can © 2013 American Chemical Society

Received: February 8, 2013 Revised: May 22, 2013 Published: May 22, 2013 2227

dx.doi.org/10.1021/cm4004799 | Chem. Mater. 2013, 25, 2227−2242

Chemistry of Materials

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Figure 1. (a) Experimental (exp.) and reconstructed (rec.) single-pulse 27Al MAS NMR spectra of 2 recorded at 11.7 T showing contributions of AlIV, AlV, and AlVI (from left to right). (b) 3QMAS 2D NMR spectrum (left) recorded at 18.8 T. The six experimental horizontal slices (blue) along with their simulations (red) are shown on the right part of the figure.

compound 1, and [Al5(PO4)5(PO3OH)2] [NH3(CH2)3NH3]2 [H2O], compound 2, are typical powdered frameworks that traditionally may have confounded attempts at structural determination as, a posteriori, they simultaneously present large unit cells (∼6000 Å3), large number of inequivalent nonhydrogen atoms (∼50, most of them in general position) and subnetworks (hydroxyl groups and water molecules) that do not share the periodicity of the framework. Despite this complexity, we have recently shown that prior knowledge about the constitution of the structural buildings units, extracted from 2D heteronuclear 27Al−31P NMR data,14 drastically reduces the number of independent atomic parameters to be determined, thus the time necessary to converge to a structural model, and leads to a full success rate in the studied aluminophosphate systems. Using this strategy, we could overcome the computability limit for structure determination based on

structure determination of polycrystalline zeolites with complex structures. The atomic character of solid-state nuclear magnetic resonance (ssNMR), the very high-resolution that can be reached in the ssNMR spectra by using high magnetic field, specific pulse sequences, multiple-resonance decoupling and ultrafast magic-angle-spinning (MAS), and the sensitivity of ssNMR to local order make it also an ideal complementary tool for structure elucidation for both periodic framework (usually called the “average structure” or topology) and nonperiodic subnetworks (usually called “structural disorder”) of hybrid materials, as was shown for example in porous or lamellar solids like zeolites,5−7,9 and aluminophosphates.10−12 The two-dimensional (2D) lamellar aluminophosphates13 with a unusual 5:7 Al/P molar ratio, actually the first occurrence of such a Al:P ratio for aluminophosphates, [Al5(OH)(PO4)3(PO3OH)4] [NH3(CH2)2NH3]2 [2H2O], 2228



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(−347.5 ppm from nitromethane),21 a 1 M NH4Cl solution, H3PO4 85%, and a 1 M solution of Al(NO3)3, respectively. The spectra were reconstructed using the Dmfit22 software. All other details about the experimental conditions are given in Supporting Information (SI).

powder diffraction data, and provide an average model for the periodic structure of 1 using high-resolution synchrotron powder diffraction (SPD) data as cost function.15 This analysis was however only the first step in the understanding of the fine structural details, and further NMR data were still required to investigate the nonperiodic subnetworks of this solid, which could not be described from diffraction data only. Therefore, in this contribution, we shall present, in a first part, the determination of the periodic framework (i.e., the topology) of powdered compound 2, using the same strategy, NMR-driven structure resolution from powder diffraction data, that we employed for compound 1,15 essential given the high complexity of this solid. Then, the procedure of assignment of the NMR resonances to the crystallographic sites in the structure of 2 will be detailed, as this step is not straightforward but necessary to interpret all the NMR data. In a third part, by using highly advanced NMR methods for 1H, 14N, 15N, 13C, 31 P, and 27Al nuclei, we will describe the nonperiodic subnetworks (hydroxyl groups and water molecules) in compounds 1 and 2. The data show that the presence of these species in the solid networks can be correlated to structural features of the inorganic layers. Finally, the dehydration/rehydration processes occurring in the solids are analyzed by means of X-ray powder diffraction and solid-state NMR experiments. The diffraction and NMR strategy presented here belongs to what was foreseen years ago by A. L. Mackay,16 when he coined the term of “generalized crystallography” to investigate crystals beyond their periodicity.

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RESULTS AND DISCUSSION Determination of the Periodic Network of 2. The SPD diagram of 2 has been indexed by an orthorhombic unit cell (refined cell parameters a = 15.8800(5) Å, b = 15.9924(4) Å, and c = 24.1508(8) Å) and the observed hkl reflections are compatible with two centrosymmetrical space groups: Pbnn (Pnna in standard setting) and Pbcn. The unit cell content of 2 (number and nature of the aluminum and phosphorus polyhedra) was determined by analysis of high-resolution 27Al and 31P MAS NMR spectra. The 27Al 1D MAS NMR spectrum of 2 (Figure 1a) contains four main signals located around 50, 45, 5, and −15 ppm, characteristic of two 4- (AlIV), one 5- (AlV), and one 6- (AlVI) fold coordinated aluminum species, respectively.23 Increased resolution is obtained in the indirect dimension of the 2D 27Al MQMAS NMR spectrum (Figure 1b), revealing the presence of two aluminum sites for each coordination number, that is, a total of six inequivalent Al atoms. Quantitative analysis of the 27 Al NMR spectrum (Table 1) indicates that four of the Table 1. Line, Isotropic Shift δiso (ppm), Quadrupolar Coupling Constant CQ (MHz), Asymmetry Parameter ηQ, Intensity (%), and Equivalence of the 27Al MAS NMR Spectrum of 2

MATERIALS AND METHODS

Syntheses. 1 was synthesized as reported in ref 15. 2 was synthesized under similar conditions, but with 1,2-diaminopropane (DAP) molecules as a structure directing agent. For the study of the dehydration/rehydration processes by NMR, the samples were first packed in 3.2 mm rotors and then placed in an oven. The ensemble was heated at different temperatures (150 or 250 °C) under 5 mbar vacuum overnight. After return to ambient pressure, the still hot rotors were then rapidly closed. This procedure ensures that no water enters the sample before the NMR measurements are done. The rehydration was performed on the previously heated samples by placing the rotors in a closed desiccator in the presence of a saturated NaCl solution for one day. This procedure was repeated on several samples for several periods of time (ranging from one day to one month). The resulting 1D 1H, 31P, and 27Al NMR spectra (not shown) were always similar. Synchrotron Powder Diffraction Data Collection and Analysis. The high-resolution synchrotron powder X-ray diffraction diagram of 1 and 2 have been recorded on the 11-BM diffractometer at the Advance Photon Source, Argonne National Laboratory. Data were collected over the 0.5−60° 2θ range with a 0.001° step size at room temperature with a wavelength of λ = 0.4139 Å. The samples were contained in 0.8 mm glass capillaries and were spun at 60 Hz during data collection. The Rietveld17 method as implemented in the Fullprof18 program was used for the structure refinement. Solid-State Nuclear Magnetic Resonance. The ssNMR experiments were performed either on an Avance 500 Bruker (static magnetic field B0 = 11.7 T, Larmor frequencies of 500.1, 202.5, 130.3, 125.2, and 50.7 MHz for 1H, 31P, 27Al, 13C, and 15N, respectively) or an Avance 800 Bruker (B0 = 18.8 T, Larmor frequencies of 800.13, 323.9, 208.5, and 57.8 MHz for 1H, 31P, 27Al, and 14N, respectively) using a triple-resonance 1H/31P/27Al 2.5 or 3.2 mm probe or a 1H/X 4 mm probe. In the 2D NMR experiments, phase-sensitive detection in the indirect dimension was obtained using the States19 or States-TPPI20 methods. The 1H, 13C, 15N, 14N, 31P, and 27Al chemical shifts were referenced to proton and carbon signals in TMS, 15N-enriched glycine

line

δiso (±0.1)

CQ (±0.1)

ηQ (±0.1)

intensity

equivalence Al

1 2 3 4 5 6

−17.6 −17.5 12.5 13.2 43.7 48.1

3.3 1.3 5.8 4.5 2.2 1.3

0.1 1.0 0.2 0.3 1.0 1.0

9.8 10.9 19.3 19.2 21.3 19.6

0.5 0.5 1 1 1 1

aluminum sites have similar intensity (60 to −12 ppm, two AlIV and two AlV) while the two hexa-coordinated Al resonances have a half intensity (−10 to −30 ppm). These AlVI atoms (labeled Ala and Alb in Figure 1b, right) are therefore located in special crystallographic positions of multiplicity half that of the other Al atoms. The 31P single-pulse MAS NMR spectrum of 2 (Figure 2a) contains six main signals between −10 and −25 ppm (Table 2) with relative intensities 0.6: 0.4: 2: 1: 1: 2 (from the left to the right part of the spectrum). Under doubleresonance 1H/27Al decoupling (Figure 2b), up to eight lines can be resolved. A similar 31P MAS NMR spectrum was recorded on a sample previously heated for one night at 150 °C under 5 mbar vacuum (see SI, Figure S1). In this spectrum, the line Pa* at −13.8 ppm has disappeared, while the intensity of the line Pa at −12 ppm has increased, indicating that these two signals originate from only one crystallographic site Pa of the periodic framework, which is split because of the presence or absence of water molecule in its vicinity. This type of 31P line splitting, that is, an identical inequivalent crystallographic site of the average structure exhibiting different and well resolved resonances in NMR, was also observed, for example, in compound 115 and in the gallium or zinc phosphates MIL-5024 and MIL-74.25 The same splitting holds for Pb(Pb*) and Pd(Pd*), which arise from only two crystallographic phosphorus sites Pb and Pd of the periodic framework. 2229



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P correlation patterns, confirming that they arise from phosphorus atoms (Pa) in the same crystallographic position (Pa), but with two slightly different local environments. This split of the Pa resonance is correlated to a splitting of the Ald resonance at ∼10 ppm (Figure 1b), as shown by the two wellresolved Ald−Pa and Ald*−Pa* cross correlation peak sets (see SI). From the 2D 31P−27Al NMR correlation spectrum, the number of aluminum atoms linked to the phosphorus atoms can also be determined: at lower chemical shifts (below −22 ppm in this sample) are located the 31P resonances (Pe, Pf, Pg) of the PO4 tetrahedra connected to four Al polyhedra (i.e., four 27 Al−31P cross-correlation peaks per 31P resonance are observed on the 2D map), while at higher chemical shifts are the resonances (Pa−Pd) of the PO4 tetrahedra with one terminal OH group or one O bond (i.e., only three 27Al−31P crosscorrelation peaks per 31P resonance are observed).27 This 31P NMR line assignment is confirmed by the 1H → 31P 2D HETCOR NMR correlation spectrum (Figure 4a), which shows that the correlation peaks of highest intensity with protons from terminal OH groups occur for P resonances in the −21 to −11 ppm range. In the 2D 27Al−31P NMR correlation spectrum (Figure 3a), the 31P line at −23.9 ppm has cross-correlations with more than four 27Al resonances, indicating the presence of at least two overlapping contributions under this 31P resonance (Pf + Pg). The same holds for the line at −15.9 ppm, which contains two P contributions (Pb + Pc). Moreover, one can notice that Pa and Pd are correlated to one of the two AlIV atoms. Therefore, taking into account the Al/P ratio of 5/7, the equality between the number of bridging oxygen atoms coordinated to aluminum atoms, on one hand, and to the phosphorus atoms, on the other hand, implies that the two 6-fold coordinated Al atoms (Ala and Alb) are both connected to six PO4 tetrahedra. Because, as determined from the 27Al NMR experiments, Ala and Alb are located on special positions of half multiplicity (Table 1), only three correlation peaks with distinct phosphorus atoms are then expected for each of these Al atoms on the NMR spectrum (the other three bonds of the AlO6 octahedra being generated by symmetry). The 2D 27Al−31P 2D NMR correlation spectrum shows that only five inequivalent phosphorus atoms (Pb, Pc, Pe, Pf, and Pg) are correlated to Ala/Alb, indicating that these aluminum octahedra share two equivalent phosphorus tetrahedra (one tetrahedron and its symmetrical counterpart). This unambiguously shows that, in the Pbcn space group, the two AlVI atoms occupy a 4c position (0.2. site symmetry), as all other available special positions (4c and 4b, 4c and 4a, or 4b and 4a) would not allow the Al atoms to share phosphorus tetrahedra due to too long Al−Al distances. Finally, in the 2D NMR spectrum of 2, one can notice sets of four peaks forming rectangle, which represent existing alternated P−O−Al−O−P′−O−Al′ connectivity sequences (either forming infinite chains by symmetry or four-membered rings). One of these sequences involves strictly distinct aluminum and phosphorus atoms: Pa−O−AlVc− O−Pd−O−AlVd (rectangle shown in the middle of Figure 3b), therefore, represents a”‘natural tile”, according to the terminology proposed by Blatov and Proserpio in zeolites,28,29 of 2. In the light of the pieces of information extracted from the ssNMR experiments, one 6-member branched chain (alternating (AlO4)PO4 groups with aluminum atoms), one AlVI(PO4)2 and one AlVI(PO4)3 group, both fixed on 4c special position, and two DAP molecules as “relaxed” groups were introduced in the unit cell in the FOX program. Out of the two possible space

Figure 2. 31P one-pulse MAS (15 kHz) NMR spectra of 2 recorded at 11.7 T (a) without decoupling and (b) with 1H/27Al double-resonance decoupling. The reconstruction of this latter spectrum is shown in c, along with the individual contributions (d).

Table 2. Line Number, Isotropic Chemical Shift δiso (ppm), Intensity (%) and Equivalent P Atoms of the 31P MAS NMR Spectrum of 2 line

δiso (±0.1)

intensity (±0.1)

equivalent P (±0.01)

8 7 6 5 4 3 2 1

−12.0 −13.8 −15.2 −15.9 −20.1 −20.6 −23.2 −23.9

9.4 5.9 2.0 26.3 8.1 5.2 10.3 32.9

0.7 0.4 0.15 1.85 0.6 0.4 0.7 2.3

Taking into account these site multiplicities, one can deduce that the asymmetric unit in 2 contains seven inequivalent P atoms (which matched the number of resonances contained on the 31P MAS NMR spectrum of the “dried” spectrum, in which the source of line splitting (water molecules) has been removed) and five aluminum atoms, as expected from the chemical analysis (see SI). As was previously shown in the related compound 1, this knowledge is not sufficient to allow the structure resolution of 2 with the direct space software FOX,26 using SPD data only as cost function. The key step to ensure the success of the structure solution search is to get the maximum of information about the constitution of the Al−P building blocks, that allows the reduction of computing time below the computability limit.15 This can be conveniently done by analysis of a 2D 27 Al−31P NMR spectrum recorded with the MQ-D-R-INEPT14 pulse sequence that we have recently developed.14 In the 2D 27 Al−31P NMR correlation spectrum of 2 (Figure 3a), the observed cross-peaks indicate spatial proximity between neighboring Al and P atoms. One can notice for 2 that the two 31P lines of 0.6 and 0.4 relative intensities on the left part of the 31P NMR spectrum (−12 and −13.8 ppm) have similar Al− 2230



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Figure 3. (a) 2D 27Al−31P MQ-D-R-INEPT MAS (22.222 kHz) NMR correlation spectrum of 2 recorded at 18.8 T. Top and left spectra, on which lines are labeled, are the full projections on the 31P and isotropic 27Al dimensions, respectively. Dash black lines indicate selected 31P−27Al crosscorrelations. (b) Building units as deduced from the 31P−27Al cross-correlations. PO4 tetrahedra are in green, while aluminum polyhedra are in blue.

groups (Pbnn and Pbcn), only Pbcn allowed this starting configuration to converge to an initial model with acceptable agreement factors and was thus retained. Fourier maps were further calculated with the FOX program, providing localization of one water molecule in the interlayer space. Structure Description of 1 and 2. The structural models of 1 and 2 are highly similar and both are built-up from alternated inorganic Al5P7O28 layers, that contain Al atoms in three coordination states (AlIV, AlV, and AlVI), and organic layers, that contain two inequivalent amines (DAE for 1 and DAP for 2) and water molecules. Despite the complexity of the inorganic layer, it can actually be described with a single composite integrant unit (IU) for each compound, containing twelve (IU1 for compound 1) and thirteen (IU2 for compound 2) strictly alternated aluminum and phosphorus polyhedra. In both compounds, two IUs are then assembled forming small ovaloid nanopores of about 300 Å3 volume (Figure 5a for 1 and 5b for 2), with six-membered ring entry windows of ∼5 Å diameter. The nanopores are connected to each other by a side linker made of a pair of PO4−AlO6−PO4 short chains (Figure 5c). The 2D network of the layers formed by nanopores and linkers is a unidodal 4-c net of topology30 sql (see SI). The inorganic layers of 1 are stacked on each other with a 90°rotation around the stacking c-axis, through a very strong fourcenter hydrogen bonding network (Figure 5d), generating a 3D 2-nodal 4,6-c net of topological type fsc (see SI). In 2, the inorganic layers are stacked on each other with a 180°-rotation around the c-axis (Figure 5e). There is only one hydrogen bonding between two terminal P−O(H) groups that generates a 3D net of topological net fsc (see SI), like for 1. Rietveld Refinement of the Structures of 1 and 2. Detailed inspection of the SPD data shows an hkl-dependent peak profile in both compounds 1 (Figure 6a) and 2 (Figure 6b), with varying full width at half-maximum (fwhm) for the diffraction peaks. In 1, a narrower fwhm is mostly observed for the diffraction peaks belonging to the hk0 family. The hkl-

dependency of the peak profile was addressed for both solids in the Rietveld refinement of the SPD diagrams of these solids (Figure 6c and 6d) by including anisotropic line broadening terms (Thompson−Cox−Hastings31 pseudo-Voigt function, see SI for refined parameters), yielding an improved line shape simulation and reliability factors. The shape contribution in the anisotropic line broadening terms is dominant in 1. This is consistent with the observed morphology of the crystallites of 1, which appear as flat pellets, some of them being less than 1 μm thick (Figure 7, left). To the contrary, in 2, the crystallites are much smaller (Figure 7, right) and have a globally more spherical morphology; therefore, the size contribution is most likely to be the dominant factor to the observed diffraction line broadening anisotropy. Finally, because the positions of the H atoms of the water molecules and terminal amine groups are not well-defined, the population parameters of the corresponding oxygen and nitrogen atoms were increased in the Rietveld refinement to account for the two and three electrons missing from the H atoms, respectively.32 For both samples, the final Rietveld refinements are highly satisfactory given the complexity of the SDP diagrams: Rp = 9.71%, Rwp = 11.52%, and RBragg = 3.92% for 1; Rp = 8.83%, Rwp = 10.73%, and RBragg = 3.08% for 2; and all the diffraction lineshapes are properly modeled (Figure 6c and 6d). Atomic positions are given in SI. NMR Resonances Assignment in 2. The step of assigning NMR resonances to the corresponding crystallographic sites of a structure becomes more and more difficult as the number of resonances increases. While some empirical rules for calculating the chemical shift exist, they are not fully reliable for 31P nucleus. NMR line assignment can also be assisted by firstprinciple calculations of NMR parameters. In the case of such complex aluminophosphates, the high number of inequivalent atoms in the asymmetric unit renders the calculations prohibitively time-consuming. Thus, for these solids, the NMR spectra assignment solely relies on the analysis of high2231



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For 1, the assignment of the 31P and 27Al NMR spectra, to the contrary, is unique and has previously been reported.15 It could be performed based on the analysis of the 27Al−31P 2D NMR data only. Non-Periodic Subnetworks: Localization of Water Molecules and Hydroxyl Groups. Evidence for nonperiodicity in 1 and 2 is revealed by the larger number of 27 Al and 31P resonances compared to the number of inequivalent crystallographic positions. The initial structural model yielded by FOX for 1 indeed contains a single AlVI atom and two AlV atoms, which does not match the observations on the 27Al NMR spectra (Figure 8a). As reported in other aluminophosphates, for example, AlPO415,35 AlPO4-21,36 AlPO4-17,37 AlPO4-18,38 AlPO4-40,32 or AlPO4-ERI,39 the presence of a hydroxyl group bridging two aluminum atoms can increase the aluminum coordination. In 1, two hydroxyl groups in special position 4a (00z) were localized in the nanopores, by Fourier difference40 from the SPD data:15 O8H (site occupancy of 75%) bridges two equivalent Al1 atoms, while O9H (site occupancy of 40%) bridges two equivalent Al3 atoms (Figure 5a). Therefore, depending on the position of the OH group, Al1 and Al3 atoms can be either in 5- or 6-fold coordination. These bridging OH groups are not only responsible for the change in coordination of the Al atoms but also for the small split of the 31P NMR resonances of the surrounding P atoms (Figure 8b), since the 31P isotropic chemical shift is highly sensitive to the local environment around the aluminum atoms.41,42 The 1H and 1H-{14N} DHMQC43−45 NMR correlation spectra of 1 were recorded at 18.8 T with ultrafast MAS (νR = 62.5 kHz). The 2D 1H-{14N} D-HMQC NMR spectrum (Figure 9) contains the signatures of the four NH3+ groups of the two DAE molecules and shows the sensitivity of both the 1H and 14N isotropic shifts to the strength of the H-bonding interactions. Indeed, three of the nitrogen atoms (N12, N21, and N22) strongly interact through hydrogen bonding with the P−OH groups from the AlPO4 framework, giving rise to the 14N NMR resonance of stronger intensity. The last nitrogen atom, N11, has no such interaction with P−OH (Figure 9b), thus has a different 14N isotropic shift (Figure 9a). The 1H ultrafast MAS NMR spectrum of 1 (Figure 9c) shows eight contributions. The two 1H resonances at highest chemical shifts (12−13 ppm) are assigned to protons of P−OH groups, on the basis of the 1H−31P CP-HETCOR 2D spectrum (see SI). The two broad peaks in the range 7.2−9.0 ppm contain the signals of the water molecules and the NH3+ protons. The resonance at ∼4 ppm corresponds to CH2 groups. The resonance at 2.1 ppm, which corresponds to OH groups bridging two AlVI atoms, has an area of 2.7% (Table 5), higher than the 1.8% expected, indicating that there is more than one OH group bridging two Al1 or two Al3 atoms per nanopore, that is, finding two OH bridging groups in a same nanopore has a nonzero probability. Thus, while the powder diffraction data have provided an “average” nanoporous subunit, the highresolution 1H NMR spectrum shows a splitting for this crystallographic ensemble into three distinct nanoporous subunits. The resonance at 7.5 ppm is the only contribution from the largest 1H peak that is not connected to a 14N in the 2D 1H-{14N} D-HMQC NMR spectrum (Figure 9a). It is therefore assigned to the protons from the water molecules. The relative area of this line is 13.8%, indicating the presence of three water molecules per IU, but so far, only two of them have been located from the SPD data, Ow1 and Ow2. Ow1 sits in a general position at the layer interface, that is, in the amine

Figure 4. (a) 1H−31P CP-HETCOR MAS NMR spectrum of 2, on which the P−OH groups are circled. Top and right spectra, on which selected lines are assigned, are the full projections on the 31P and 1H dimensions, respectively. (b) 1H MAS (27 kHz) NMR spectrum of 2. The spectra were recorded at 11.7 T.

resolution 2D NMR correlation spectra. To facilitate this analysis and to list all sets of possible assignments, we made use of the adjacency matrix. Considering cations (or their polyhedra) as vertices of a periodic three-dimensional graph, the full Al−P adjacency matrix in 2 is given in Table 3a. This submatrix also corresponds to a mathematical representation of the heteronuclear correlations observed on a 27Al−31P 2D NMR spectrum.33 By sorting the cations according to their increasing isotropic shifts, the adjacency matrix of 2 has been constructed (Table 3b) from the correlation peaks extracted of the 27Al−31P 2D MQ-D-R-INEPT NMR spectrum. Taking these two matrices as input data, a homemade program returns all secondary submatrices resulting from permutations of all rows and columns34 of the Al−P submatrix, that match the two matrices. Exhaustive details about the calculation procedure and hypotheses are given in SI. For 2, the final calculation yields only four solutions, equivalent in pairs because of the lack of resolution for the lines Pf and Pg. There are thus only two possible line assignments for the thirteen 27Al and 31P resonances of the MAS NMR spectra, which are reported in Table 4. 2232



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Figure 5. Representations of the double IU in 1 (a) and 2 (b) forming ovaloid nanopores. The small red balls are the OH groups bridging two aluminum polyhedral in 1, aluminum and phosphorus atoms are in blue and green, respectively. (c) In both 1 and 2, the ovaloid nanopores (pink color) are linked through Al2P10 linkages (yellow color). Projection of the complete structure of (d) 1 along the [110] direction and (e) 2 along the [100] direction.

form terminal PO4 tetrahedra, no correlations between them and protons from OH groups are observed on the 1H−31P CPHETCOR NMR spectrum (see SI). The presence of correlation peaks with the water molecules confirms that Ow2 and Ow3 share their protons with P4 and P3, respectively: P4O--HOw2H----OP4 and P3O--HOw3H----OP3 (Figure 9d). It is worth noticing that the indirect detection of these water molecules by the modulation of the 31P chemical shift expresses the sensitivity of the valence electrons governing the NMR chemical shift to such water molecule influence, while the core electronic density diffracting the X-rays is much less sensitive to this modulation by water. In 2, the water molecules are also responsible for the larger number of observed resonances compared to the number of inequivalent sites of the periodic-framework. For example, when four 15N resonances are expected (corresponding to the two inequivalent DAP molecules), the 15N CPMAS NMR spectrum of 2 (Figure 10) contains two sets of peaks of equal relative intensity. The first set centered at −357.6 ppm is a single contribution, while that at −361 ppm contains at least three resonances. This implies that this latter line accounts for at least three N atoms; and hence that there are at least six

region (Figure 9b). However, the refined site occupancy was only 60%. Since more 13C and 15N NMR resonances than the number of inequivalent C and N atoms (see SI) are observed, the 40% electronic density of Ow1 not collected from the SPD diagram are very likely because of a distribution of the position of these water molecules between the N21 atoms. Ow2 was found in a special position of 1/2 multiplicity, located at equal distance between two opposite P4 atoms (Figure 9d). However, to ensure stronger hydrogen bonding, this water molecule is very likely off-centered, that is, closer to one or the other P4 atom. The localization of the third water molecule Ow3 is provided by the 31P NMR data. One can indeed observe a large split of about ∼3.5 ppm of the resonances labeled 7 and 7′ (Figure 8b) corresponding to P3. The chemical shift difference between these signals is much larger than the small split of most resonances (below 0.5 ppm), which indicates that the missing water molecule Ow3 sits between two P3 atoms, closer to one of them. A large split is also observed for the P4 resonances, but is not as clear since the P4′ resonance overlaps with that of P5. By analogy to Ow2, only an average position is given for Ow3 on the crystallographic site of half multiplicity located between two opposite P3 atoms. Although P3 and P4 2233



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Figure 6. Observed (red), calculated (black), and difference (blue) SPD diagrams of 1 (a and c) and 2 (b and d) before (a, b) and after (c, d) introduction of anisotropic line broadening in the Rietveld refinement. In inset are displayed expansions of selected hkl diffraction lines.

inequivalent N atoms. The split of the 15N resonances is very likely related to the distribution of a water molecule at the interface of organic and inorganic layers, the presence or absence of the water molecule at the layer interface modifying the 15N chemical shift, as was observed in for the 31P nucleus. The 1 H NMR spectrum of 2 (Figure 9c) can be reconstructed using five contributions (Table 5). The broad 1 H line at 3.4 ppm corresponds to the protons from the CH2 of the DAP molecules, while the lines at higher δiso (13.3 and 14.7 ppm) are characteristic of the presence of P−OH groups (P4− OH, P5−OH, P6−OH, and P7−OH). The remaining 1H lines at 8.8 and 8.2 ppm are the signals of the NH3+ groups and water molecules, respectively. The relative intensity (12.9%) of the line at 8.2 ppm indicates the presence of 2 water molecules per integrant unit. Only one of them could be localized by SPD,

close to P6 and P7 (Pb and Pc). This water molecule cannot explain the splitting of the Pa and Pd resonances. Therefore, the missing water molecule must be located close to the average crystallographic positions of P4 and P5 (Pa and Pd) (Figure 9e). The different hydroxyl groups and water molecules play a critical role in the framework stability of these solids. The lattice energy is indeed mainly dominated by the covalent bonding between the nanopores and the organic linkers. In 1, a 42 screw axis along c is however imposed by the formation of the hydrogen bonds at four centers, which connect inorganic layers by the centers of orthogonal nanopores (see SI). But the inorganic layer is not identical along the length of the nanopores and along its orthogonal linker. Therefore, to keep the tetragonal symmetry of the crystal system (the 42 axis) imposing equality of a and b unit cell parameters, a nonperiodic 2234



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Figure 7. SEM images of 1 (left) and 2 (right).

Table 3. Al−P Adjacency Submatrix of 2 Build (a) from Structural Information and (b) the Correlation Peaks Extracted of the 27Al−31P 2D MQ-D-R-INEPT NMR Correlation Spectrum (a) Al1 Al2 Al3 Al4 Al5 Al6

P1

P2

P3

P4

P5

P6

P7

1 1 0 0 1 1

0 1 1 0 0 1

1 0 0 1 1 0

0 0 1 1 0 1

0 0 1 1 1 0

1 0 0 1 0 1

0 1 1 0 1 0

Pa

Pb

Pc

Pd

Pe

Pf

Pg

0 0 1 1 1 0

0 1 1 0 0 1

1 1 1 1 1 1

0 0 1 1 0 1

1 1 1 0 1 0

1 1 0 1 1 1

1 1 0 1 1 1

(b) Ala Alb Alc Ald Ale Alf

Table 4. Two Assignment Sets (asgn) of the Six 27Al and Seven 31P Resonances Compatible with Both the Al−P Structural Adjacency Matrices in 2 27

Al

Ala Alb Alc Ald Ale Alf

asgn 1

asgn 2

Al1 Al2 Al3 Al4 Al6 Al5

Al2 Al1 Al4 Al3 Al5 Al6

31

P

Pa Pb Pc Pd Pe Pf + Pg

asgn 1

asgn 2

P4 P7 P6 P5 P2 P1 + P3

P5 P6 P7 P4 P3 P1 + P2

Figure 8. (a) Projection of the 27Al isotropic dimension of the MQMAS NMR spectrum of 1. (b) 31P MAS NMR spectrum of 1 recorded without decoupling (top) under 1H/27Al double-resonance decoupling (bottom). Both NMR spectra were recorded at 18.8 T.

film, which insures additional cohesion via an electrostatic nondirectional interaction between the cation/water inter sheets layer and the negative inorganic layers. In 2, the inorganic layer is also not identical along the length of the nanopores and along its orthogonal linker. However, here the inorganic layers are stacked, through a two-bond center hydrogen-bonding network, with a 180° rotation along c (Supporting Information Figure S7). There is therefore no

distribution of OH groups bridging the Al octahedra within the pores is required. The modulation of the bond angles and distances along the (a or b) axis of the layer brings a considerable extra lattice energy by connecting the layers between them. The additional water molecules Ow2 and Ow3 stabilize further the network by completing the hydrogen bonding of the remaining terminal PO4 tetrahedra. Finally, the last part of the lattice energy is the NH3+/H2Ow1 interlayer 2235



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Figure 9. (a) 1H-{14N} MAS (62.5 kHz) D-HMQC 2D NMR correlation spectrum of 1. Dash lines indicate N−H spatial proximities. (b) Representation of the layer interface region in 1. Dash lines represent hydrogen bonds between oxygen atoms (black) of two consecutive layers and nitrogen atoms (blue) of the DAE molecule. (c) Experimental and reconstructed 1H high-field (B0 = 18.8 T) and ultrafast MAS (62.5 kHz) NMR spectra of 1 on which lines are assigned. Representation of two neighboring nanopores (pink color) of 1 (d) and 2 (e), showing the possible location of the water molecules as determined by diffraction and NMR data. In 1, the water molecules are very likely off-centered, to ensure stronger hydrogen bonding with the P atoms.

need to make the a and b dimensions equals, and hence the symmetry of the crystal is orthorhombic. The absence of

bridging OH groups in 2 confirms that these groups participate actively in the layer stability of 1. 2236



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Table 5. Line Number, Isotropic Chemical Shift δiso (ppm), Intensity (%), Equivalent Number of Protons (Equiv H) and Assignment of the 1H MAS NMR Spectra of 1 and 2 line

δiso (±0.1)

intensity (±0.1)

1 2 3 4 5 6 7 8

2.1 3.8 7.2 7.5 8.6 9.0 12.2 13.1

2.7 31.8 20.3 13.8 12.0 11.9 4.0 3.6

1 2 3 4 5

3.4 8.2 8.8 13.3 14.7

38.4 12.9 41.2 5.0 2.5

equiv H

assignment

0.7 8 6 4 3 3 1 1

Al−OH−Al CH2 NH3 H2O NH3 NH3 (P5/P7)−OH P5−OH

The thermodiffraction patterns (recorded on a laboratory diffractometer using the Cu Kα radiation) of 1 and 2 are shown in SI. Above 250 °C, the diffraction lines broaden before the irreversible collapse of the structures, because of the degradation of the organic molecules. No major changes are observed in the temperature range 30−250 °C, indicating that, despite the departure of the water molecules, both compounds seem to retain their average structure and symmetry until they collapse. The cell parameters evolve, as expected, with the temperature (see SI). For 1, in parallel with a slight increase of the stacking cell parameter c (about 0.01 Å every 50 °C), there is a small jump of cell parameters a and b of about 0.05 Å between 100 and 150 °C, leading to an overall increase of 0.7% of the cell volume. For 2, in a first step, the cell parameters a and c, thus the cell volume, decrease linearly with temperature (about 22 Å3 every 50 °C) to 100 °C. In a second step, as for 1, the cell volume increases slightly with the temperature. Finally, before the collapse of the structure, one observes at 250 °C a somewhat larger jump of the cell parameters (−0.02 Å for a, −0.03 Å for b, and +0.08 Å for c) at constant cell volume. Compound 2. The 1H, 31P, and 27Al NMR spectra of compound 2 recorded on the as-synthesized sample and on the sample previously heated at 150 °C are shown in Figure 11. In the dehydrated sample, the nonperiodic species (water molecules) have left, leading to a fully periodic inorganic framework, and the 31P spectrum can be deconvoluted with the five resonances of relative intensities 1:2:1:1:2 (see SI) expected from the average periodic structural model determined earlier. One can notice that the 1D 1H and 27Al spectra present only little or no notable changes, except for a shift of the P−OH group as the hydrogen bonding with water molecules is modified upon heating. These NMR spectra indicate that in compound 2, the water molecules leave the framework without inducing significant changes in the compound topology. The 2D 27Al−31P MQ-D-R-INEPT (Figure 12), 1H → 31P CP-HETCOR and 31P−31P DQ-SQ (see SI) NMR spectra of 2 are also spectacularly simplified upon heating, as the 31P lines are no longer splitted. The 27Al−31P and 31P−31P crosscorrelation peaks observed are similar to those of the assynthesized compound (see SI), indicating that the topology and symmetry of the inorganic layers have not been changed

1

2 11.6 3.9 12.4 1.5 0.7

CH2 H2O NH3 P−OH P−OH*

Figure 10. 1H−15N CPMAS (10 kHz) NMR spectrum of 2 on which lines are labeled.

Dehydration/Rehydration Processes in 1 and 2. The TGA curves of 1 and 2 (see SI) show two main events occurring in the temperature range 150−200 °C and at ∼350 °C associated at first to the departure of water molecules and OH groups and to the destruction of the organic moieties, respectively.

Figure 11. 1H (left), 27Al (middle), and 31P (right) NMR spectra of 2 recorded on the “as-synthesized” sample (a) and on the sample previously heated at 150 °C (b). Panel c shows the NMR spectra after rehydration of the previously heated sample. The stars indicate the 31P resonances splitted because of distribution of the water molecules. 2237



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or absence of water molecules around the PO4, and smaller shifts of about 0.5 ppm corresponding to the distribution of the OH groups that bridge two AlVI octahedra (Figure 5a, absent in 2). Upon heating of the sample, the 31P NMR spectrum of 1 becomes less crowded (Figure 13b), in agreement with the nonperiodic subnetworks (water molecules and bridging OH groups) leaving the framework. The departure of the OH groups is confirmed by the 27Al NMR spectra: the AlV contribution grows while that of the AlVI decreases as the bridging OH groups are removed and by the disappearance of the narrow resonance at 2.1 ppm on the 1D 1H NMR spectrum of the heated spectrum. Removal of most of the OH-groups is confirmed by the AlVI−OH cross-correlation peaks on the 2D 1 H{27Al} D-HMQC NMR spectrum (Figure 14a) which have very low intensities. Along with the departure of the water molecules, the interlayer hydrogen bonding with the terminal P−OH groups has changed. This is illustrated by the large shift of the 1H resonances assigned to the P−OH groups: from ∼13 ppm on the hydrated sample, to 9.7 and 10.7 ppm for the “dried” sample (Figure 14b and SI). The 1H chemical shift is known to be related to the length of the O−H···O bond46 as follows:

Figure 12. 27Al−31P MQ-D-R-INEPT of compound 2 previously heated at 150 °C overnight. Dash lines indicate through space correlations. B0 = 18.8 T, νMAS = 21.348 kHz.

d(O···O) (pm) =

upon heating. The spectra also confirm the partial line assignment proposed in the previous section for the assynthesized sample. Upon rehydration of compound 2, the splits of the 31P resonances in the 31P MAS NMR spectrum corresponding to the phosphorus tetrahedra exposed to the interlayer space reappear (see Figure 11), indicating that water molecules have reintegrated the framework. The 1H, 27Al, and 31P MAS NMR spectra, as well as the PXRD diagram (see SI) of the rehydrated sample 2 are pretty similar to those of the as-synthesized phase, showing that the rehydration process is mostly reversible, and that the water molecules go back to the initial positions, without disturbing the main periodicity of the framework. Compound 1. The 1H, 31P, and 27Al NMR spectra of the assynthesized compound 1 are shown Figure 13. As observed for 2, more 31P resonances than the seven expected from the average structural model are observed on the 31P MAS NMR spectrum of the as-synthesis solid 1. Two types of splitting are observed: split of about 3 ppm, corresponding to the presence

79.05 − δiso (ppm) 0.255

(1)

where δiso is the 1H isotropic chemical shift. From this relation, one can estimate the differences in the O···O distance between the P−OH groups upon hydration/ dehydration. Between the as-synthesized and dehydrated samples of compound 1, the 1H δiso of the protons involved in the hydrogen bonds are indeed shifted downfield by 2.55 ppm (see SI), corresponding, according to eq 1, to an increase of the P−O−H···O distance of about 0.1 Å, which is consistent with the magnitude of the unit cell expansion upon heating and dehydration. Despite the departure of the water molecules that simplifies the 31P MAS NMR spectrum, there is still more resonances than crystallographic sites for both phosphorus and aluminum atoms (seven and five crystallographic sites, respectively). This is particularly visible in the 2D 27Al−31P MQ-D-R-INEPT spectrum (Figure 15), in which up to four resonances for both tetra- and penta-coordinated aluminum atoms can be distinguished, when the average structure is described with

Figure 13. 1H (left), 27Al (middle), and 31P (right) NMR spectra of 1 recorded on the “as-synthesized” sample (a) and on the sample previously heated at 150 °C (b). Panel c shows the NMR spectra after rehydration of the previously heated sample. Examples of the resonance splits generated by the distributions of H2O (∼3 ppm) and OH (∼0.5 ppm) are shown by arrows. 2238



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Figure 14. (a) 1H-{27Al} D-HMQC and (b) 1H → 31P CP-HETCOR NMR spectra of 1. The Al resonances are labeled. The hydroxyl group bridging two Al1 and the terminal P5-OH and P7-OH groups are shown.

lines in Figure 15), indicating that all three inequivalent nanopores are within the same inorganic layer (i.e., the symmetry within a layer has been changed while the symmetry elements between the layers have not been modified). Since these symmetry modifications were hardly detected on the laboratory X-ray diffraction data (see SI), we have used the NMR data to explore in more details the changes that have occurred upon dehydration as follows: (1) The structure of the dehydrated solid is first described in a new space group of lower symmetry compared to the space group of the as-synthesized solid. If the number of crystallographic sites for the phosphorus and aluminum nuclei is consistent with that determined from the NMR experiments, this candidate structure is selected for the next step. (2) The compatibility of the lower symmetry topology with the 2D NMR experiments is checked. In other words, the heteronuclear P−Al adjacency matrix of the new structure is compared, through all possible permutations of rows and columns, to that determined from the 2D 27Al−31P MQ-D-R-INEPT NMR spectrum (Figure 15). This can be done with the whole or partial matrices, for faster calculations. If at least one of the adjacency matrix permutations matches the NMR correlation matrix, the candidate structure is compatible with the experimental data. According to this procedure, the structure of 1 has been, in a first step, described in the three maximum nonisomorphic subgroups of the original space group (P42bc No. 106): space groups number 32, 37, and 77. However, none of these new topologies were consistent with the correlation Al−P pattern observed on the 2D 27Al−31P NMR spectrum. The structural description in subgroups of P42bc of still lower symmetry was not considered since it would have further increased the numbers of crystallographic sites, beyond those enumerated by the NMR experiments. Therefore, in a second step, a pseudosymmetry search has been performed on the inorganic part of the structure (i.e., the hydroxyl groups, water and organic molecules have been neglected), with the help of the program PSEUDO.47 The search of pseudosymmetry could be helpful for predicting, for example, a more symmetric structure at higher temperature. If

Figure 15. 27Al−31P MQ-D-R-INEPT NMR spectrum of 1 recorded on the sample previously heated at 150 °C. The colored solid lines correspond to the spatial proximities between the Al and P atoms belonging to one nanopore. The dark solid lines correspond to the internanopore correlations.

only two crystallographic sites for each of these coordination states. In total, fourteen 31P and eleven 27Al resonances arising from difference crystallographic sites can be resolved, that is to say about twice the expected values. Moreover, the 27Al−31P correlation patterns no longer match the Al−P structural adjacency matrix of the hydrated solid, which may indicate a lowering of the average crystal symmetry upon dehydration. A step-by-step analysis of the 2D 27Al−31P MQ-D-R-INEPT spectrum (Figure 15) allows distinction of three inequivalent nanopores (when in the hydrated solid, all nanopores are equivalent). Correlation peaks between Al and P atoms belonging to different nanopores are also observed (dash 2239



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Figure 16. Representation of the tree search for compatible subgroups of the identified supergroup. The evaluated subgroups are circled by a solid line. The subgroups circled by a dashed line were not evaluated as they are the minimal nonisomorphic subgroups of No. 133 and only act as intermediates in the chains of symmetry lowering.

Figure 17. Representation of the inorganic layers of 1: (a) as-synthesized structure and (b) sample heated at 150 °C. The colored volumes represent the nanopores (see text for more details): one equivalent nanopore in panel a and three inequivalent nanopores in panel b. The blue and dark gray spheres represent, respectively, the aluminum and phosphorus polyhedra linking the nanopores. Unit cells are represented with blue lines. Oxygen atoms are omitted for sake of clarity.

the existence of pseudosymmetry in the crystal structure is established, one can extend the range of descriptions of the structure in all possible subgroups of the pseudosuper-group when returning to room temperature. This pseudosymmetry search applied to compound 1 has led to the centrosymmetric P42/nbc (No. 133). Starting from this supergroup, a tree search of all possible space groups that describe the structure with a consistent number of crystallographic sites has been performed. Twenty-six inequivalent structure descriptions in twenty-three different space groups have been retained (Figure 16). The topologies of all twenty-six structures have then been confronted to the heteronuclear correlation matrix obtained from the 2D 27Al−31P NMR experiment. Only one of these structures, the one described in the noncentrosymmetric P42212 (No. 94) space group, matches fully the NMR data. It must be noted that there may be different chains and transformation matrices that give lead to the same subgroup: for example, two path ways are possible to go from space group 133 to 77: 133 → 106 → 77 or 133 → 86 → 77. In these cases, the two resulting structure descriptions are different.

The inorganic layer of compound 1 dehydrated is highly similar to that of the as-synthesized solid, but presents three inequivalent nanopores (all nanopores were equivalent in this latter solid): one is identical to that of the initial structure, the other two are located at the intersection of three 2-fold axes, making them both more symmetric than the first one (Figure 17). As a result of the higher symmetry of two nanopores, two phosphorus and two aluminum atoms are located in special position (identified in Table 6 with an asterisk). It should be mentioned that the pecular 42 screw axis, which induces a 90° rotation between two successive inorganic layers, is still present in the dried compound. In the as-synthesized form of compound 1, the NMR data have provided the crystallographic positions of the water molecules, but have also shown that these positions are not fully occupied by the water molecules. The fact that, on average, all nanopores appears identical at the scale of the NMR in the as-synthesized compound, while they are different in the “dried” phase suggests a homogeneous water loading in the former compound rather than separate domains (i.e., the surface of the crystallite may have been partially 2240



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Table 6. Two Possible Assignment Sets of the Eleven 27Al and Fourteen 31P Resonances Compatible with the Al−P Structural Adjacency Matrix in the Dehydrated Form of Compound 1 Pa Pb Pc Pd Pe Pf Pg Ph Pi Pj Pk Pl Pm Pn

set 1

set 2

P31 P4 P5 P3 P51 P32 P52 P7 P2 P11 P12 P61* + P1 P6 P62

P32 P3 P7 P4 P52 P31 P51 P5 P1 P12 P11 P62* + P2

Ala Alb Alc Ald Ale Alf Alg Alh Ali Alj Alk

set 1

set 2

Al42 Al4 Al5 Al41 Al3 Al11 Al12 Al1 Al22* Al2 Al21*

Al41 Al5 Al4 Al42 Al1 Al12 Al11 Al3 Al21*

coordination environments, AlO4, AlO5, and AlO6, while the interlayer space contains the amines and water molecules. Taking into account the number of discrete 31P and 27Al NMR resonances, much higher than the expected periodic-framework sites present in the average structural model, and into account high-resolution 1D and 2D NMR spectra (31P, 27Al, 15N, 1H, 1 H−31P, and 1H−14N), the hydroxyl groups and water molecules, which do not have the periodicity of the inorganic frameworks, could be localized. An attempt to decompose the lattice energy into its main geometrical components has also been proposed. Through these two aluminophosphates, we have shown the particularly high sensitivity of the 27Al and 31P chemical shifts to the presence of neighboring water molecules or hydroxyl groups, which were used to provide localization of these species, which was not possible from the diffraction data. Describing accurately this nonperiodic part of a crystal is a very important demand in porous solids in general, as water is an almost unavoidable impurity in these systems. The dehydration and rehydration processes, which lead to space group modification in one sample, have been further investigated by analysis of an ensemble of 1 and 2D NMR spectra and PXRD data. These two solids show that a general strategy involving a combination of powder diffraction and NMR data all throughout the structure elucidation process, from the determination of the average topology to the localization of the nonperiodic subnetworks, can provide, even for highly complex solids, a structure description with a level of accuracy that would not have been reached by using these two techniques separately.

Al22*

P61

dehydrated), which would have led to two distinct sets of 27 Al−31P correlation patterns. The search for peak assignments leads to two possible solutions, presented in the Table 6. The labeling of peaks and crystallographic sites respects that from the original structure, with addition of indices 1 and 2 for the description of each of the more symmetric nanopores. Besides the symmetry modification of the dried solid 1, the other major differences between compounds 1 and 2 is that the rehydration process in 1 is not at all reversible as evidenced by the NMR spectra (Figure 13). Rehydration and rehydroxylation do occur as shown by the reappearance of the AlVI contribution on the 27Al NMR spectrum and by the shift of the 1H resonances of the P−OH groups back to their initial position, in agreement with the modification of the hydrogen bond network. Mostly, the topology seems to remain unchanged, as the 31P and 27Al chemical shift ranges and lines relative intensity compare well to those of the as-synthesized sample. However, contrary to 2, the water molecules do not reintegrate the framework in their initial position but are most likely statistically distributed in the solid, as shown by the broadness of all 31P resonances that indicate a large degree of local disorder over the whole solid, which is not detected on the PXRD diagram of the rehydrated sample (see SI). This last feature is completely consistent with the polar structure of the as-synthesized sample. It would have been very surprising that a rehydration process would regenerate spontaneously a polar order of the layers.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental conditions, selected bond angles and distances in 2, reconstructed 1D NMR spectra, selected 2D NMR spectra, and CIF file of 2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.M.); francis.taulelle@ uvsq.fr (F.T.). Fax: +33139254476. Present Address #

Laboratoire de Chimie et Biologie des Métaux, Université Joseph Fourier, Grenoble, CNRS UMR 5249, CEA, DSV/ iRTSV, 17 rue des Martyrs, 38054 Grenoble cedex 9, France Notes

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The authors declare no competing financial interest.

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CONCLUSION The syntheses and structures of two highly complex powdered aluminophosphates with the original 5:7 Al/P ratio, [Al5(OH)(PO4)3(PO3OH)4] [NH3(CH2)2NH3]2 [2H2O], compound 1, and [Al5(PO4)5(PO3OH)2] [NH3(CH2)3NH3]2 [H2O], compound 2, have been presented. The average structure of 2 was solved from powder synchrotron diffraction data by using an NMR-driven structure resolution from synchrotron powder diffraction data strategy, ensuring quick and successful structure resolution. Both 1 and 2 contain layers, 1 as component of a 3D material, 2 as a truly layered crystal. Their inorganic frameworks contain five inequivalent PO4 groups and seven inequivalent Al atoms, which are found in three different

ACKNOWLEDGMENTS Financial support from the TGE RMN THC FR3050 for conducting the research is gratefully acknowledged. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0206CH11357. Pr. G. Férey was the source of inspiration for this study that has been approached, at the periodic structural level only, in a previous unpublished study by Drs. J. Dutour, C. Mellot-Draznieks, and N. Guillou during J. Dutour’s PhD. J.P.A., O.L., and J.T. are grateful for funding provided by Region Nord/Pas de Calais, Europe (FEDER), CNRS, French 2241



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Minister of Science, USTL, ENSCL, CortecNet, Bruker BIOSPIN, and contract No. ANR-2010-JCJC-0811-01.

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