Fluorine-19 NMR from retrosynthesis to NMR crystallography

Journal of Fluorine Chemistry 101 (2000) 269±272

Fluorine-19 NMR from retrosynthesis to NMR crystallography Francis Taulellea,*, Corine GeÂrardina, Mohamed Haouasa, Clarisse Huguenarda, Vincent Muncha, Thierry Loiseaub, GeÂrard FeÂreyb

a RMN et Chimie du Solide, UMR 7510, Universite L. Pasteur, 4, rue Blaise Pascal, 67070 Strasbourg Cedex, France Institut Lavoisier UMR CNRS 173 IREM, Universite de Versailles-St Quentin, 45 av. des Etats-Unis, 78035 Versailles Cedex, France

b

Received 9 May 1999; accepted 6 July 1999

Abstract Recently, the need for more accurate structure description that will allow to analyze possible retrosynthesis of microporous compounds has led us to use and develop new structural strategies to access structure determination by solid state NMR techniques. At ®rst, an example, AlPO4-CJ2, between crystallogenesis and crystal structure is explored. AlPO4-CJ2 has been studied by hydrothermal in-situ NMR. A soluble species exists in supersaturated liquid as well as the product of redissolution of AlPO4-CJ2. This species, the prenucleation building units (PNBU) is different from structural buildings units (SBU) the sub-ensembles of the crystal. How this PNBU gets into the structure and becomes therefore a structural building unit (SBU) is our main question. To answer it, an HETCOR experiment between ¯uorine and aluminum has been performed. This experiment shows that when the PNBU enters the solid, it forms at the same time than growing the solid, a ¯uorinated bridge. On ULM-18 after structure elucidation positioning hydrogen, a dynamic charge compensation mechanism is proposed for the solid formation. On TMPGaPO, a set of 2D experiments using radio frequency driven dipolar recoupling (RFDR) on 19 F, Double Quanta on 31 P and HETCOR between 19 F and 31 P allowed analyzing the topology of the network. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Solid state NMR; In-situ crystallogenesis; NMR crystallography

1. Introduction Due to its high sensitivity as an NMR active nucleus 19 F has enjoyed a large popularity in high resolution liquid state NMR. It has a 100% natural abundance, has 94% the resonating frequency of hydrogen and exhibits a considerable chemical shift range of about 400 ppm. Its use in solid state NMR has been limited due to its very strong dipolar interactions, its very large chemical shift anisotropy, its sensitivity to distribution of environment demanding high crystallinity to get reasonably sharp lines. High resolution NMR in solids demands for 19 F, to have, for only ¯uorides, high crystallinity and spin at very high MAS spinning speed. For partially ¯uorinated materials like oxy¯uorinated compounds high resolution spectra are obtained with good crystallinity and long distance through space homonuclear connectivity is expected. Care has to be taken for long or very long T1 relaxation time with increasing crystallinity.

* Corresponding author. Tel.: 33-388-416067; fax: 33-388-607550. E-mail address: [email protected] (F. Taulelle)

In the case of oxy¯uorinated alumino- or gallo-phosphates which is the main subject of this contribution, most of the nuclei are easily observable by NMR. It means the following nuclei: 27 Al, 69;71 Ga, 31 P, 19 F, 1 H, 13 C and 14;15 N. This leads to a wealth of possible NMR experiments permitting a detailed description of the atomic connectivities. It is the aim of this paper to present the expanding potentialities of ¯uorine NMR in the ®eld of crystallogenesis and crystallography of microporous materials. Crystallogenesis of the latter materials proceeds through hydrothermal synthesis within 100±2008C in heterogeneous mixture of aluminum or gallium hydroxide (or alkoxide), phosphoric acid and hydro¯uorhydric acid with a templating agent, usually an amine. Some recent studies [1±4] of NMR characterization using ¯uorine-19 will be reviewed to exemplify some aspects of crystallogenesis and structure elucidation. AlPO4-CJ2 has been studied by in-situ NMR during its synthesis as well as its occupancy factors for the sites involving ¯uorine. Consequences on the nucleation and growth are therefore deduced. A study of ULM-18 polycrystalline sample allows con®rming and describing accurately by NMR the results obtained on a single crystal by XRD. Further coupled

0022-1139/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 1 1 3 9 ( 9 9 ) 0 0 1 6 9 - 4

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CPMAS and 19 F NMR allow demonstrating the localization of H inside a double four ring (D4R) unit, close to ¯uorine without HF bonding formation. This leads to a mechanistic description of the last step of condensation of D4R units to form the crystalline network. On a third example it will be shown that the topology analysis of the ¯uorine subnetwork, the phosphorus one and the ¯uorine phosphorus heteroatomic network can be achieved by some re¯ections on crystallography and NMR suggesting that NMR crystallography of inorganic 3D networks can probably be achieved. From these case studies, some re¯ections will be given on crystallogenesis and crystallography of such materials. 2. Experimental The syntheses of the discussed compounds generally proceed under hydrothermal conditions at 1808C and for periods of few hours to few days. More details are given elsewhere [1,3,5]. NMR has been run on a Bruker DSX 500 spectrometer operating at 11.7 T. For the in-situ work, a Bruker modi®ed high-resolution BBO 10 mm probe allowing temperatures up to 2008C routinely and without aluminum background has been used. For solid state high resolution MAS work a 4 mm H±F±X probe and a 2.5 mm MAS probe have been used. They have respectively maximum spinning speed of 20 and 35 kHz. 3. Results and discussion 3.1. AlPO4-CJ2 crystallogenesis and crystal growth AlPO4-CJ2 (CJ2 as a short form), an open 3D structure has been ®rst synthesized by Yu et al. [6]. Its structure has been reinvestigated and the presence of ¯uorine in the structure demonstrated by combined usage of X-ray diffraction (XRD) chemical analysis and MAS NMR. The structure is displayed in Fig. 1. A detailed study of the synthesis has been conducted, by NMR mainly [4]. To follow the synthesis, in-situ NMR has been used, with special ``hydrothermal NMR'' tubes withstanding temperature, pressure and chemical aggressiveness [7]. A general strategy has been evaluated to characterize the synthesis and the solid formed. The ®rst method is to follow by two different ways the kinetics of solid formation, by insitu and by ex-situ NMR. Ex-situ consists of stopping several syntheses at different time and separating when back at room temperature the solid and liquid phases. Each phase is submitted to an NMR analysis under MAS or liquid high resolution. In-situ analysis consists of measuring the heterogeneous medium in high resolution liquid conditions. The liquid phase is then followed. In both cases, ex- and in-situ, not only spectral analysis is conducted but also but quanti®cation of each

Fig. 1. AlPO4-CJ2 structure.

observed nucleus is done. Mass transfer between liquid and solid phases is obtained from the ex-situ syntheses while evolution of dissolved species is obtained from the in-situ solution. Two additional experiments are made. pH evolution during in-situ kinetics [8] and for in- or ex-situ ®nal solution, temperature exploration of the synthetic medium. The results of these analyses have been reported elsewhere [2,4]. The main conclusions are that aluminum isopropoxyde hydrolyses immediately, followed by ¯uorination of the amorphous phase then by its phosphatation. At this stage the amorphous phase starts to act as a source of soluble species. A ¯uorophosphatoaluminate in a 1 : 1 : 1 ratio is observed and called the primary building unit (PBU). This PBU changes its coordination with temperature from 6 to 5 at high temperature. Furthermore, in in-situ conditions only, a supersaturated species is observed, most probably a PBU dimer. No other species are observed with aluminum. The crystal grows from incorporation of this supersaturated species. The latter is called prenucleation building units (PNBU). The PNBU contains only ®ve-fold coordinated aluminum [2]. In a recent work, quantitative 19 F MAS NMR of CJ2 has allowed to con®rm that the terminal ¯uorine was fully occupied. Demonstration that the bridging site is at least partially occupied by ¯uorine has been obtained by 19 F ! 27 Al Hetcor experiment, and the occupancy factor determined. The latter parameter depends on the sample and is not strictly reproducible. Observed variations have been between 25% and 33% of the site occupied by ¯uorine. The structural building units (SBU) describing CJ2 structure contains Al in ®ve and six coordination. The incorporation of the PNBU into an SBU connects the PNBU to the network and transforms it by isomerization into an SBU. This reorganization of the ef®cient cluster for crystallization in the liquid to enter the solid has been recognized by Ubbelhode [9] but has never been mentioned before in microporous synthesis literature.

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271

Fig. 3. TMPGaPO SBU unit.

Fig. 2. ULM-18 structure.

3.2. ULM-18: an example of infinite condensation of anions with simultaneous charge compensation ULM-18 [1] is a network made of D4R units linked by a phosphorus tetrahedron, as can be seen in Fig. 2. In the center of the D4R is a ¯uorine that is linked to the three gallium atoms like in cloverite [10]. 71 Ga DOR experiments 31 as well as P Double quantum (DQ) experiments have shown that assignments between XRD and NMR agreed and the correspondence for each inequivalent atom was clearly established. As the templates are present under the form of protonated species (ammonium form), a remaining proton is present for charge balancing the network. A detailed analysis of the proton to phosphorus cross polarization curves shows that the proton resides unexpectedly within the D4R. However, several positions are possible within the latter unit to house it. They are probably all accessed in the crystal and the ¯uorine spectrum exhibits a distribution of signals in good agreement with multiple locations within the unit. Quantum calculations on this case support this description. The ¯uorine has in the unit a tetrahedral geometry and is linked to three gallium atoms and points towards a fourth too far to be considered as linked. If the proton in the unit would form a HF molecule within the D4R unit, it would lie between the same last ¯uorine to gallium line, which is unlikely. Last, there will be in this case, not several but a single location of the hydrogen inside the D4R, which is not the case. If ¯uorine can be, by analogy with polyanions, considered as a templating agent of the D4R and not HF here, it implies that proton is included in the D4R after its formation.

Whether proton inclusion in the unit occurs before or after condensation does not change its essential role. For in®nite condensation, the network demands the charge to stay at zero. As the templates only, are not providing enough charges to completely compensate the network negative charge, a dynamic inclusion of proton during condensation must occur. It is therefore a noticeable feature, of this network formation, to condense anionic species with external charge compensation by templating cations and internal charge compensation provided by proton stabilization within the D4R unit. 3.3. TMPGaPO topology TMPGapO structure is built of D4R units having an open side. Three ¯uorine atoms are located in this unit. Two of them are bridging gallium atoms and one is terminal. Its structure has been investigated twice [3,11] to have a better determination of the presence of ¯uorine in the network and its SBU is displayed in Fig. 3. A method of topology determination has been proposed [3] that allow determining by dipolar interactions spatial proximity of atoms in their homoatomic sub-network. This has been achieved by RFDR on 19 F NMR at very high speed (35 kHz) and by double quantum experiment on 31 P NMR. These determinations have allowed counting the number of inequivalent ¯uorine and a partial assignment per type, bridging and not bridging for 19 F and by groups (P1, P2) and (P3, P4) for phosphorus. Nevertheless, a complete assignment is only achieved when these ®rst results are compared to an Hetcor 19 F !31 P experiment. On the experiment, the slices on f2 dimension show contrasted intensities that reveal the proximities of P with F. A direct assignment is allowed based upon distances from XRD. The slices on f1 dimension allow also assignment of F by their F to P proximities. For the terminal ¯uorine, its assignment is further con®rmed by its anisotropy, which is stronger than both bridging ones. This method has proved to demonstrate the effective presence of ¯uorine in the network, con®rmed by elemental analysis, to quantify the number of inequivalent sites, prove that their occupancy factor is 1.0 in all the sites and to get their XRD site assignment by dipolar proximity analysis.

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3.4. Reflections on NMR crystallography In crystallography, one considers atomic geometrical arrangement of a structure. Once this is done, it can be described in one of the 3D space groups. Atoms are considered as points located in a reduced unit cell and all the possible atoms in the unit cell are generated from application of symmetry elements to the set of inequivalent atoms. Physical determination from XRD proceeds along other lines. From the diffraction lines, one attempts to access the space group, unit cell parameters and positions of atoms. The inversion problem is limited by the phase indetermination. Moreover, identi®cation of the nature of each atom relies only on electronic density differences, which in several cases is not possible (OH/F, Si/Al as examples). It is therefore necessary to introduce some prior knowledge, especially chemical composition into the diffraction analysis. On a different ground, crystallogenesis of a material ought to be described not simply like a geometrical construction of points of atoms, but it should describe all chemical objects that can be observed and measured and describe the steps and relations leading to the ®nal crystalline arrangement. Among all possible geometrical constructions of a lattice not many correspond to realistic nucleation and growth scheme. In-situ, ex-situ, liquid and solid NMR measurements permit speciation of the different nuclei, allow kinetics and dynamics characterization as well as they can characterize the inequivalent atoms of a structure as well as their topological relation. It is our conviction that usage of NMR will allow a common description of crystallogenesis and crystallography. Indeed a chemical paving of a solid (crystallogenesis) is also a geometrical paving of a structure (crystallography). However to reach this goal, a patient collection of how units do organize, by limited condensation, then by in®nite condensation, implying probably often, as seen twice in this contribution other steps that the simple in®nite iterative addition of units. Such chemical facts and different structural construction approaches [12±14] may in the near future converge to form a single frame to account for geometrical and chemical formation of solids. 4. Conclusion Fluorine NMR has found today, for polycrystalline samples, their experimental conditions (high ®eld, high MAS

spinning speed and high RF power) that allow full high resolution NMR information to be collected. With the advent of multidimensional NMR techniques, many aspects of structure description and solid formation will be elucidated in the very near future. Crystallogenesis and crystallography will probably develop common methods to follow the chemical construction of the solid. Acknowledgements For this micro-review of some recently studied samples, M. Pruski (Ames Laboratory), J.P. Amoureux (Lille University) are thanked for the many detailed discussions for AlPO4-CJ2 interpretation. A.K. Cheetham, G. Stucky and S. Weigel (University of Santa Barbara) have been at the origin of the TMPGaPO reinvestigation. References [1] F. Taulelle, A. Samoson, T. Loiseau, G. FeÂrey, J. Phys. Chem. B 102 (1998) 8588. [2] F. Taulelle, M. Haouas, C. Gerardin, C. Estournes, T. Loiseau, G. Ferey, Colloids and Surfaces A, in press. [3] V. Munch, F. Taulelle, T. Loiseau, G. FeÂrey, A.K. Cheetham, S. Weigel, G.D.Stucky, Magnetic Resonance in Chem., in press. [4] M. Haouas (Ed.), Etude RMN de la syntheÁse hydrothermale des aluminophosphates micropreux oxyfluoreÂs: AlPO4-CJ2, ULM-3 et ULM-4, University Louis Pasteur, Strasbourg, France, 1999. [5] G. Ferey, T. Loiseau, P. Lacorre, F. Taulelle, J. Solid State Chem. 105 (1993) 179. [6] L. Yu, W. Pang, L. Li, J. Solid State Chem. 87 (1990) 241. [7] M. Haouas, C. GeÂrardin, F. Taulelle, C. Estournes, T. Loiseau, G. FeÂrey, J. Chim. Phys. 95 (1998) 302. [8] C. Gerardin, M. In, L. Allouche, M. Haouas, F. Taulelle, Chemistry of Materials, May 1999. [9] A.R. Ubbelhode, Molten State of Matter, Melting and Crystal Structure, Wiley-Interscience, New York, 1978. [10] M. Estermann, L.B. McCusker, C. Baerlocher, A. Merrouche, H. Kessler, Nature 352 (1991) 320. [11] S.J. Weigel, G.D. Stucky, A.K. Cheetham, T. Loiseau, G. FeÂrey, V. Munch, F. Taulelle, R.E. Morris, Hydrothermal synthesis and structural study of a new fluorinated gallophosphate Ga4(PO4)3(HPO4)F3. T (Tˆamine), Baltimore, MD, 1998. [12] I.D. Brown, Acta Crystallographica B 53B (1997) 381. [13] R.W. Grosse-Kunstleve (Ed.), Zeolite Structure Determination from Powder Data: Computer-Based Incorporation of Crystal Chemical Information, Dissertation ETH 11422, ETH, Zurich, 1996. [14] K.J. Andries, J.V. Smith, Acta Crystallographica A 50 (1994) 317.