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Cryst. Res. Technol. 42, No. 4, 333 – 341 (2007) / DOI 10.1002/crat.200610824

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Synthesis and characterization of a layered chlorozincophosphate

templated by protonated 4-methylpiperidine

R. Kefi1, F. Lefebvre

2, and C. Ben Nasr*

1

1 Laboratoire de Chimie des Matériaux, Faculté des Sciences de Bizerte, 7021 Zarzouna, Tunisie 2 Laboratoire de Chimie Organométallique de Surface (LCOMS), Ecole Supérieure de Chimie Physique

Electronique, Villeurbanne Cedex, France

Received 12 September 2006, revised 24 November 2006, accepted 31 November 2006 Published online 10 March 2007

Key words X-ray diffraction, layered compounds, NMR spectroscopy, 4-methyl-piperidine, organic template.

PACS 61.66.Hq

A chlorozincophosphate with the composition Zn(HPO4)Cl.[C6H14N] has been synthesised under mild conditions in water medium in presence of 4-methylpiperidine as organic template. The structure was determinated by single crystal X-ray diffraction. The unit cell is orthorhombic (space group Pcab) with a = 8.743(9), b = 9.592(6), c = 26.573(6) Å, Z = 8 and V = 2228.91(12) Å3. The structure involves a network of ZnO3Cl and PO3(OH) tetrahedra forming macroanionic inorganic layers with four and eight-membered rings. Charge balance is achieved by the protonated amine which is trapped in the interlayers space and interacts with the organic framework through hydrogen bonding. Solid state 31P and 13C MAS-NMR spectroscopies are in full agreement with the X-ray structure.

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

Works on the synthesis and characterization of new microporous materials continue to grow owing to their interesting structural chemistry and potential applications as adsorbents, catalysts, and ion-exchange media, revealing an increasing variety of framework compositions topologies. A large number of these materials have been synthesized in the presence of organic amines as structure-directing agent [1-3]. They are also exploited in other areas such as electronic materials [4], photochemical, and photophysical processes [5]. Since the discovery of AlPO molecular sieves in 1982 [6], a large number of phosphate-based materials have been prepared under hydrothermal conditions. Between those metal phosphates, zincophosphates constitute a large family. So far, zincophosphates with monomeric phases, chains, layers and three-dimensional open-framework have been prepared in the presence of different amines, alkali metal cations or metal complexes as structure directing agent [7-15].

In a continuing theme of research aiming at producing new materials, we investigated the formation of zincophosphates in the presence of a variety of organic amine molecules. During the course of this study, we isolated a new chlorozincophosphate Zn(HPO4)Cl.[C6H14N], with a layered structure in the presence of 4-methylpiperidine cations. The present compound, in which the chlorine is part of the framework, has been characterized by X-ray diffraction, NMR, infrared spectroscopy and TGA-DTA.

2 Experimental

2.1 Chemical preparation

Crystals of the title compound Zn(HPO4)Cl.[C6H14N] have been obtained at room temperature according to the following procedure: a mixture of 1.8 g of 4-methylpiperidine (Acros) dissolved in water and 3.9 g of ____________________

* Corresponding author: e-mail: [email protected]

334 R. Kefi et al.: Synthesis and characterization of a layered chlorozincophosphate

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.crt-journal.org

phosphoric acid ( Fluka 85 % weight H3PO4) has been firstly prepared. An aqueous solution containing 4.0 g of zinc chloride (Prolabo) was then added dropwise to this solution under continuous stirring. A white precipitate formed which was recrystallized in diluted phosphoric acid. Schematically the reaction can be written as follows:

C6H13N + H3PO4 + ZnCl2 → Zn(HPO4)Cl [C6H14N] + HCl

2.2 Investigation techniques

The title compound has been studied by various physico-chemical methods: X-ray diffraction, Solid state NMR, Infrared spectroscopy and Thermal analysis.

X-ray diffraction The intensity data collection was performed using a MACH3 Enraf Nonius diffractometer. The experimental conditions of data collection, the strategy followed for the structure determination and the final results are given in table 1. The structure was solved by direct methods using the SIR92 [16] program and refined by full matrix least-squares techniques based on F2

, using SHELXL97 [17]. The structure factors were obtained after Lorentz polarization corrections. The positions of the heavier atoms, including the Zn atom, were located by the direct method. The remaining atoms were found in a series of alternating difference Fourier maps and least-square refinements. The positions of the hydrogen atoms of this hybrid title compound were located directly from the difference Fourier maps. The drawings were made with Diamond [18].

Table 1 Crystal data and experimental parameters used for the intensity data collection strategy and final results of the structure determination.

I.Crystal data Formula : Zn(HPO4)Cl[C6H14N] Crystal system : orthorhombic a = 8.7439(9), b = 9. 592(6), c = 26.573(6) Å, α = β = γ = 90 °, V = 2228.91(12)Å 3. Z = 8 Refinement of unit-cell parameters with ρcal. = 1.8 g.cm-3

Linear absorption factor : µ (Mo Kα ) = 2.577 cm-1 Crystal size (mm) : 0.25 x 0.15 x 0.20 II. Intensity measurements Temperature : 293.2 K Diffractometer : Enraf-Nonius FR590 Monochromator : graphite plate Measurement area : (±h. k. l) Nb of scanned reflections Nb of independent reflections Orientation and control reflections III. Structure determination Lorentz and polarization corrections Program used : SHELX-97 [36] All the hydrogen atoms were located from difference Fourier maps. They are not refined. Unique reflections included : (2011( I >2 σ)) Weighting scheme : σ Residual Fourier density : -0.90 Å 3 < ρ < 0.64 Å 3 Drawings made with Diamond [37]

Fw = 297 Space group : Pcab 25 reflections (7 ° < θ < 10°) F(000) = 1256.00 Morphology : prism Color : transparent Wavelength : MoK

α (0.7107 Å) Scan mode : ω _ 2θ Theta range : 1.53 – 27.85° hmax.= 10; k max. = 12; lmax. = 33 2632 (Rint. = 0.071) 2011 0 3 2 and 0 0 -1 No absorption correction Determination : direct methods

Refined parameters : 124 R = 0.042 ; Rw = 0.1125 Goodness-of-fit on F

2: 1.045 Largest shift/error = 0.000

NMR Spectroscopy All NMR spectra were recorded on a Bruker DSX-300 spectrometer operating at 75.49 MHz for 13C and 121.51 MHz for 31P with a classical 4 mm probehead allowing spinning rates up to 10 kHz. 13C chemical shifts are given relative to tetramethylsilane and 31P ones relative to 85 % H3PO4 (external references, precision 0.5 ppm). Phosphorus spectrum was recorded under classical MAS conditions while the carbon one was recorded by use of cross-polarization from protons (contact time 5ms) and MAS. In

Cryst. Res. Technol. 42, No. 4 (2007) 335

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all cases it was checked that there was a sufficient delay between the scans allowing a full relaxation of the nuclei.

Thermal behavior Thermal analysis was performed using the “multimodule 92 Setaram” analyzer operating from room temperature up to 500°C at an average heating rate of 5 K/min.

IR Spectroscopy Spectra were recorded in the range 4000 - 400 cm-1 with a “Perkin-Elmer FTIR” spectrophotometer 1000 using a sample dispersed in spectroscopically pure KBr pellet.

3 Results and discussion

3.1 Structure description

Final atomic coordinates and thermal parameters for Zn(HPO4)Cl.[C6H14N] are given in table 2. The main geometrical features entities are reported in table 3. All atoms are at general positions. An ORTEP drawing of the asymmetric unit of the chlorozincophosphate templated Zn(HPO4)Cl.[C6H14N] by organic cations is shown in figure 1.

Fig. 1 ORTEP representation of the asymmetric unit of Zn(HPO4)Cl.[C6H14N].

Table 2 Final atomic coordinates in Zn(HPO4)Cl[C6H14N]. Esd are given in parentheses. 1

*3Ueq. = ( . )*ijaUi j a a ai ji j∑ ∑

atomes x y z Ueq Zn(1) Cl(1) P(1) O(1) O(2) O(3) O(4) N(1) C(1) C(2) C(3) C(4) C(5) C(6) H(1) H(2) H(3) H(4) H(5) H(6) H(7)

0.70164(4) 0.73807(15) 0.58279(9) 0.4645(3) 0.7119(2) 0.5037(3) 0.6394(3) 0.6929(5) 0.8494(7) 0.8674(6) 0.8340(5) 0.6748(6) 0.6538(6) 0.8527(9)

0.4006 0.6806 0.6242 0.9594 0.8741 0.7972 0.9535

0.96228(3) 1.04197(13) 1.20546(7) 1.2370(2) 1.1244(2) 1.1165(2) 1.3408(2) 0.9226(4) 0.9808(5) 1.1063(4) 1.0715(4) 1.0109(5) 0.8890(4) 1.1975(6)

1.2891 0.8278 0.9884 0.9095 0.9975 1.1916 1.1399

0.44262(2) 0.36480(4) 0.51406(3) 0.47075(9) 0.48835(10) 0.55377(8) 0.53530(11) 0.60902(17) 0.60353(19) 0.63701(16) 0.69132(15) 0.6948(2) 0.6621(2) 0.7260(2)

0.4801 0.5990 0.5998 0.6178 0.5695 0.6287 0.6377

0.0308(6) 0.0640(3) 0.0305(2) 0.0387(6) 0.0395(6) 0.0387(5) 0.0468(6)

0.0747(13) 0.0699(13) 0.0570(10) 0.0513(9)

0.0659(13) 0.0671(13) 0.095(2)

0.051 0.13 0.079 0.14 0.093 0.048 0.060



H(8) H(9) H(10) H(11) H(12) H(13) H(14) H(15)

0.8934 0.6475 0.5972 0.7173 0.5494 0.9467 0.7855 0.8553

0.9856 0.9781 1.0654 0.8091 0.8377 1.2552 1.2654 1.1946

0.6989 0.7258 0.6829 0.6697 0.6656 0.7273 0.7117 0.7631

0.084 0.064 0.073 0.087 0.071 0.13 0.09 0.16

Table 3 Interatomic distances (Å) and bond angles (°) in Zn(HPO4)Cl[C6H14N]. Esd are given in parentheses. Equivalent positions: (i): -0.5 + x,1.5 – y, z ; (ii): 0.5 + x,1.5 – y, z ; (iii): 1 - x,1 – y, - z.

The HPO42-

P O(1)a O(2) O(3) O(4) O(1)-H(1) The ZnO3Cl tetrahedron Zn O(2) O(3) O(4) Cl Zn-O(2)-P = 129.83(14) Zn-O(4)-P = 152.32(17) The organic group N(1)-C(1) C(1)-C(2) C(2)-C(3) C(3)-C(4) C(4)-C(5) C(5)-N(1) C(3)-C(6) The hydrogen bonds O(N)-H…O O(1)-H(1)…O(2)i N(1)-H(2)...O(1)ii N(1)-H(3)...O(3)iii

O(1)a 1.577(3) 104.81(14) 108.39(14) 108.96(14) O(2) 1.975(2) 108.46(10) 104.99(11) 107.12(8) 1.485(8) 1.505(6) 1.510(6) 1.511(7) 1.469(7) 1.485(8) 1.530(6) O(N)-H 0.79 0.95 0.91

O(2) 2.463(3) 1.532(2) 111.04(14) 111.37(14) O(3) 3.185(0) 1.951(2) 114.79(10) 108.08(7) Zn-O(3)-P = 132.00(14) C(1)- N(1)-C(5) C(1)-C(2)-C(3) C(2)-C(3)-C(4) C(2)-C(3)-C(6) C(4)-C(3)-C(6) C(5)-C(4)-C(3) N(1)-C(1)-C(2) C(4)-C(5)-N(1) H...O 1.86 2.33 2.03

O(3) 2.514(3) 2.518(3) 1.523(2) 111.95(15) H(1)-O(1)-P 110.8 O(4) 3.080(1) 3.249(7) 1.907(2) 113.02(9) 112.8(4) 111.6(4) 108.8(4) 112.4(4) 111.4(5) 112.6(4) 109.8(4) 111.1(4) O(N)…O 2.620(3) 2.955(5) 2.890(5)

O(4) 2.504(3) 2.504(3) 2.505(3) 1.500(2) Cl 3.385(2) 3.386(5) 3.453(3) 2.227(6) O(N)- H…O 161(4) 123(9) 158(1)

Each Zn atom is tetrahedrally coordinated by three phosphate groups and has one terminal Zn-Cl vertex. The Zn-O bond lengths are in the range of 1.907(2)-1.975(2) Å, Zn-Cl distance is 2.227(6) Å, and the O-Zn-O(Cl) angles are between 104.99(11)° and 114.79(10)°. On the other hand, each phosphorus atom is linked to three Zn atoms through three oxygen atoms with the fourth coordination site being a terminal P-OH group. The P-O distance is in the range 1.500(2)-1.577(3) Å and O-P-O angles are in the range 111.95(15)-104.81(14)°.

The structure consists of two-dimensional neutral sheets of Zn(HPO4)Cl.[C6H14N] parallel to the ab plane, situated at z = 0 and z = ½ with an interplane of 13.3 Å (Fig. 2). Each sheet shows that the inorganic entities have a layered organization along the c-axis (Fig. 3), and are constructed from strictly alternating ZnO3Cl and HPO4 tetrahedra sharing vertices. The covalent connectivity in the layer produces four-membered {Zn2P2} rings (approximate dimensions 3.62 x 4.45 Å2) and eight-membered {Zn4P4} rings (approximate dimensions 4.33 x 9.27 Å2), which fuse to propagate the network structure.

The terminal chlorine atoms, hanging from the zinc center, project in a direction perpendicular to the layer. As with many two-dimensional structures, the amine molecules occupy the inter-lamellar space and participate in hydrogen bond interactions with the oxygen atoms of the framework through N-H…O hydrogen bonds (Fig. 2). Both inter- and intra-layer hydrogen bond interactions are observed. The most important observed hydrogen bond interactions are listed in table 3.



Fig. 2 Projection of the Zn(HPO4)Cl.[C6H14N] structure in the plane (a, c) (hydrogen atoms and layer-diamine H-bonds not shown).

Fig. 3 Polyhedral representation of the framework Zn(HPO4)Cl.[C6H14N], viewed down the c direction.

Fig. 4 31P MAS-NMR spectrum of Zn(HPO4)Cl.[C6H14N].

Fig. 5 13C CP-MAS-NMR spectrum of Zn(HPO4)Cl.[C6H14N].

It can be pointed out that the structure of Zn(HPO4)Cl.[C6H14N] is closely related to those of the two zincophosphates Zn(HINT)(HPO4) and [C10N2H10][ZnCl(HPO4)]2 [19,20]. The former has a similar inorganic framework consisting on a two-dimensional network constructed from edge-sharing 4-rings and 8-rings, with a 4.82 topology, but in this case the Zn is tetrahedrally coordinated by three phosphate groups and one carboxylate group of an HINT unit in a monodendate fashion. The second, having also a 2D anionic network,



consist of corrugated tetrahedral layers composed solely of 6-rings to give a 63 topology, while the compound described in this paper shows 4-rings and 8-rings; it can be pointed out that these two types of 4.82 patterns are commonly observed: one with elongated 8-rings in a herringbone pattern [21] and one with 8-rings that more closely approximate octagons [22]. It is evident that the 4.82 pattern of the synthesized compound has elongated 8-rings.

Hydrogen bonding plays an important role in stabilizing the Zn(HPO4)Cl.[C6H14N] structure. The 4-methyl-

piperidinium cations occupy the interlayer sites and interact with zincophosphate layers by way of N−H---O

hydrogen bonds as N−H(2)···O(1) [d H…O = 2.33 Å] and N−H(3)···O(3) [d H…O = 2.03 Å]. An interesting feature

is the absence of hydrogen bonding between the chloride atom and the organic molecule. The P−OH groups

participate in sheets H−bonds: O(1)−H(1)···O(2) [d H…O = 1.86 Å]. N-C and C-C distances and C-C-N, C-N-C, and C-C-C in the template molecule are comparable with those found in the literature [20].

Fig. 6 DTA curve of Zn(HPO4)Cl.[C6H14N] during two heating and cooling runs.

Fig. 7 X-ray powder patterns of Zn(HPO4)Cl.[C6H14N]: not heated (a); heated and then cooled to room temperature (b).

3. 2 NMR spectroscopy

The 31P MAS NMR spectrum of the crystalline chlorozincophosphate Zn(HPO4)Cl.[C6H14N] is shown in Figure 4 and is in good agreement with the X-ray structure. Indeed, it exhibits a single resonance peak corresponding to only one crystallographic site. The chemical shift value (-2.5 ppm) is in accordance with those corresponding to monophosphates (between -10 and + 5 ppm) [23-29].

The 13C CP-MAS-NMR spectrum of the title compound is given in figure 5. The spectrum contains four resonances corresponding to the six carbon atoms of the organic cation. The first signal at 22.5 ppm is assigned to the methyl carbon atom (C6). The resonance at 30.6 ppm is related to the tertiary carbon atom (C3) of the piperidinium ring. The peak at 32.1 ppm corresponds to the secondary carbon atoms (C2 and C4) and the large

signal at 47.5 is attributed to the CH2 groups in α position of NH2+ (C1 and C5). This result proves the

presence of only one organic moiety in the asymmetric unit of the compound in agreement with the X-ray diffraction data.

3.3 Thermal analysis

Examination of the DTA and TGA curves of the title compound shows two kinds of transformations.

Transformation without weight loss The sample of Zn(HPO4)Cl.[C6H14N] was treated thermally in the temperature range from 325 K to 473 K. It was heated under argon flow at a heating rate of 1 K/min. The DTA curve exhibits an endothermic peak at about 433 K (Fig. 6) when rising the temperature and an exothermic peak at 420 K when cooling.



With the aim to examine the nature of this transformation, the initial sample was carried at 470 K for some minutes. The resulting product has a crystalline aspect. An investigation by X-ray diffraction (Fig. 7) and IR absorption (Fig. 8) does not shows any difference with the initial compound Zn(HPO4)Cl.[C6H14N]. This thermal behaviour can then be assigned to a reversible phase transition.

Transformations with weight loss Crystals of Zn(HPO4)Cl.[C6H14N] are transformed by heating into diphosphates according to the following reaction:

2[Zn(HPO4)Cl.[C6H14N] → Zn2P2O7 + H2O + 2HCl + 2C6H13N

Two curves corresponding to DTA and TGA analysis, in open air are reported in Fig.9. The DTA curve shows first the phase transition peak at 433 K and then a significant endothermic peak at about 563 K, followed by an endothermic peak and a large exothermic one in a wide temperature range (560-770K). The TGA curve shows an important weight loss corresponding to those peaks. The first phenomenon produced at 542 K corresponds to the loss of H2O molecule from the condensation of two molecules of Zn(HPO4)Cl.[C6H14N] as indicated above by the reaction scheme (weight loss calculated 3.04 %, experimental 3.05 %). The second weight loss, between 547 K and 585 K, corresponds to the loss of HCl from the molecule Zn(HPO4)Cl.[C6H14N] (weight loss calculated 12.16 %, experimental 11.40 %). Finally the weight loss, between 585 K and 775 K, corresponds to the fusion of the compound at 639 K followed by the degradation of the template molecule (organic entities). A grey solid is obtained containing probably a residual organic carbon. This latter can be eliminated by calcination of the sample at 1100 K in air. The remaining compound has an X-ray diffractogramm in good agreement with that Zn2P2O7.

Fig. 8 IR spectra of Zn(HPO4)Cl.[C6H14N]: not heated (a); heated and then cooled to room temperature (b).

Fig. 9 Thermal analysis (DTA and TG) under argon of Zn(HPO4)Cl.[C6H14N].

3.4 IR spectroscopy investigations

The infrared spectrum (Fig. 9) of [Zn(HPO4)Cl.[C6H14N] contains characteristic bands of the 4-methyl-piperidinium and HPO4

2- anions. The stretching and bending modes of the (NH2+) group appears as weak and

large bands at 3251 and 1602 cm-1 respectively. They are indicative of the presence of the 4-methyl-piperidinium molecules in its protonated form [30, 31]. The stretching vibration (-CH2-) groups appear in the 2970-2830 cm-1 range and the bending modes are observed in the 1580-1400 cm-1 region. As in other

phosphate groups [32,33], the IR bands in the range 2520-2250 cm-1 are assigned to the ν(P–OH), whereas the

bands observed in the 1250-1090 cm-1 and 930-810 cm-1 regions are ascribed to the δP–O–H in plane bending and

the γ P–O–H out of plane bending modes, respectively. The vibrational modes of the PO4 tetrahedra anions show different groups of bands between 1200 and 300 cm-1 [34]. In this case, the stretching vibration bands originate

from both symmetric ν1(A1) and asymmetric ν3(F2) modes and are observed respectively in the ranges [1000-

800] cm-1 and [1200-1000] cm-1. The bending modes symmetric ν4(F2) and asymmetric ν2(E) appear respectively in the frequencies ranges [500-300] cm-1 and [650-500] cm-1.



4 Supplementary material

Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre (Deposition No. CCDC 616816).

5 Conclusion

The use of 4-methylpiperidinium as organic-structure directing agent leads to the 2-D zincophosphate of chemical formula: Zn(HPO4)Cl.[C6H14N]. Its framework topology is similar to that of Zn(HPO4)Cl.[C5H12N] synthesized in the presence of piperidinium as structure-directing agent. The anionic part is built up from a two-dimensional network of vertex-linked HPO4

2- and ZnO3Cl5- building units. The template molecule (4-methylpiperidine) occupies the space between the anionic layers and interacts by hydrogen bonding with the

inorganic framework. The existence of terminal Cl atoms and −OH groups leads to a porous sheets containing

four- and eight-membered rings within [Zn(HPO4)Cl]− layers. Compared to other metalphosphates, these layers can be related to the structure type 488, which was previously seen in many zincophosphates [20,35,36], aluminophosphates (AlPO-12) [37] and gallophosphates (GaPO4-14) [38]. Upon heating, this new zincophosphate is stable until 530 K. It undergoes a reversible phase transition at 433 K, the loss of H2O molecule at 542 K while the loss of HCl and 4-methylpiperidine molecules occurs between 570 K and 775 K. Above 775 K, the resulting amorphous material is Zn2P2O7 characterized elsewhere. Solid state 31P and 13C MAS-NMR spectroscopies are in agreement with the X-ray structure.

Acknowledgements This work was supported by the Secretary of Tunisian State for Scientific Research and Technology. We would like to thank it.

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