Layered calcium phenylphosphonate: Synthesis and properties

Solid State Sciences 3 (2001) 519–525 www.elsevier.com/locate/ssscie

Layered calcium phenylphosphonate: Synthesis and properties Amir H. Mahmoudkhani a,∗ , Vratislav Langer b a Department of Chemistry, Göteborg University, SE-412 96 Göteborg, Sweden b Department of Inorganic Environmental Chemistry, Chalmers University of Technology, SE-412 96 Göteborg, Sweden

Received 7 November 2000; revised 25 January 2001; accepted 7 February 2001

Abstract Calcium phenylphosphonate, Ca(HO3 PPh)2 , was synthesized by the reaction of calcium nitrate and phenylphosphonic acid. A thorough investigation was performed to study the effect of reaction parameters on the synthesis and growth of crystalline product in a high yield. The compound was structurally characterized by single-crystal X-ray diffraction technique. It consists of a layered structure with inorganic framework of CaO8 polyhedra from which phenyl groups are pointing out. The inorganic framework is also stabilized by the O–H· · ·O hydrogen bonds. Results from thermal analysis by thermogravimetry and thermodiffractometry revealed that calcium phenylphosphonate is stable up to 300◦ C. The compound undergoes consequent thermal decomposition and phase transitions above 300◦ C temperatures until it converts to δ-Ca(PO3 )2 at 620◦ C.  2001 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Metal phosphonate; Layered structures; Crystal structure; Hydrogen bonds; Thermal analysis

1. Introduction Chemists have become so qualified at designing and constructing molecules with a defined set of properties. Many of them are now turning their skills to building much larger systems, where natural chemical affinities are used to direct the growth of multi-molecular arrays, well-defined layered structures, clusters and selfassembled macromolecules for use in different areas of fundamental research and industrial applications. Phosphate and phosphite groups have a strong affinity toward many metal ions. Metal phosphonates are a rich class of inorganic-organic hybrid materials and are related to metal phosphites and phosphates, where a (P–)H or (P–)OH is replaced by an organic radical. A combination of the nature of bonding between oxygen and the metal ions and the tetrahedral geometry of the phosphorus atoms controls the orientation of the organic substituents. The general interest in the chemistry of metal * Correspondence and reprints.

E-mail address: [email protected] (A.H. Mahmoudkhani).

phosphonates is mainly due to the unusual compositional and structural diversity, which results in a wide range of applications such as ion exchange, catalysis, sensors, proton conductors, etc. So far several di-, tri- and tetravalent metal phosphonates with a wide variety of chemical and physical properties have been synthesized and structurally characterized, but little attention has been paid to alkalineearth metals. Recently, there has been an interest in the chemistry of calcium phosphonates due to the use of phosphonate-based drugs for diagnosis and therapy of various diseases of bones and calcium metabolism. Calcium ions react with phosphonate groups and a wide variety of compounds with chemical formulas Ca(HO3 PR)2 [1], Ca(HO3 PR)2 ·3H2 O [2] and Ca(O3 PR)·nH2 O [1,3] have been structurally identified. We are going to use the chemical characteristics of the phosphites and phosphonates of alkaline-earth metal ions to obtain chemically modified surfaces and thin films for the protection of different objects such as stones and metals against atmospheric influences, corrosion and decay. In this study,

1293-2558/01/$ – see front matter  2001 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII: S 1 2 9 3 - 2 5 5 8 ( 0 1 ) 0 1 1 6 1 - X

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we wish to report the effects of reaction parameters on the synthesis of calcium phenylphosphonate and elucidate the structural details and properties of this compound.

same manner. The resulting precipitate morphology was studied using an environmental scanning electron microscope (ESEM). 2.2. Crystal structure determination

2. Experimental Chemicals were obtained as reagent grade from commercial sources and used without any further purification. The FT-IR spectrum was recorded on a Nicolet MAGNAIR 560 spectrometer using KBr pellet method. ESEM analysis was performed using a Philips ElectroScan microscope operating at 20 kV. Thermogravimetric analysis was performed using a SETARAM Thermal Analysis System in pure oxygen atmosphere. The sample of calcium phenylphosphonate (0.034 g) was placed into an Al2 O3 crucible and a heating rate of 2◦ C min−1 was applied. 2.1. Synthesis of calcium phenylphosphonate (CAP) Method (a). Hydrothermal synthesis of CAP was studied by reaction of calcium ions (from Ca(OH)2 , CaCO3 or Ca(NO3 )2 ) and phenylphosphonic acid in a stainless steel autoclave at autogenous pressure. In a nominal reaction, Ca(OH)2 (2.25 mmol) was added to a solution of phenylphosphonic acid (4.72 mmol) in 7 ml deionized water in a quartz cell (with the capacity of 10 ml) of an autoclave. The pH was adjusted to 6 (by adding ammonia solution). The autoclave was sealed and kept at 150◦ C for two days. Then, a white precipitate was filtered off and washed with water (20 ml) and THF (20 ml, three times). It was dried in an oven at 80◦ C and kept in a desiccator over silica gel. Method (b). The compound was synthesized by the reaction of calcium nitrate with phenylphosphonic acid in water. A solution of phenylphosphonic acid (0.888 mmol in 20 ml distilled water) was gradually added to a solution of Ca(NO3 )2 (0.428 mmol in 25 ml distilled water) when stirring. The pH was adjusted to 6 by addition of ammonia solution. The mixture was then heated to 80◦ C for about an hour. Colorless thin plate crystals at mm size appeared by a slow evaporation of the solvent after few weeks. Crystals were washed by water and then by THF, separated and kept in a vacuum desiccator for a week (yield 84%). IR spectra (KBr, cm−1 ): 3054m; 2750m,br; 2300w,br; 1593w; 1485m; 1438m; 1223m; 1176s; 1147m; 1105m; 1071m; 1011s; 940m; 923m; 747m; 721m; 693m; 575s; 523s; 430m. The reactions were also performed at pH of 2.1, 8 and 10 in the

All data were collected using a Siemens SMART CCD diffractometer with Mo-Kα radiation (λ = 0.71073 Å, graphite monochromator). A full sphere of reciprocal lattice was scanned by 0.3◦ steps in ω with a crystal-todetector distance of 3.97 cm and exposure time 30 s per frame. Preliminary orientation matrix was obtained from the first frames using SMART [4]. The collected frames were integrated using the preliminary orientation matrix which was updated every 100 frames. Final cell parameters were obtained by refinement on the positions of 7496 reflections with I > 10σ (I ) after integration of all the frames using SAINT [4]. The data were empirically corrected for absorption and other effects using SADABS [5] based on the method of Blessing [6]. The structure was solved by Patterson method and refined by full-matrix least squares on all F 2 data using SHELXTL [7]. The nonH atoms were refined anisotropically. The hydrogen atom of hydroxyl group was located from difference Fourier map and refined isotropically with a restrained bond distance of 0.8 Å, while the other H atoms were constrained to idealized geometries. 2.3. X-ray powder thermodiffractometry The powder thermodiffractometric study on CAP were performed on a Siemens D-5000 automated diffractometer equipped with a PAAR HTK-10 high-temperature camera. The X-ray source was a conventional tube with Cu-Kα radiation. The high-intensity parallel beam (line focus) was obtained by diffraction from a Göbel mirror assembly (Bruker AXS). Data were collected in the range 20–39◦ of 2θ with fixed incident beam angle of 10◦ using detector scan with step size 0.05◦ and exposure time 1 s per step and recorded on a scintillation detector equipped with long Soller slits and a flat secondary monochromator. Data were collected between 30–620◦C in air. The sample was allowed to relax for 2 min at the desired temperature before collecting the data. The powder sample of CAP was placed as a thin layer on a platinum heating filament. Data collections were controlled and processed using the DIFFRACT AT software [8]. Room temperature powder diffraction pattern was obtained with the same diffractometer equipped with a sample spinner with rotation of 30 rpm.

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3. Results and discussion 3.1. Synthesis Several routes have been used for the synthesis of metal phosphonates. Among them, hydrothermal and soft-chemical methods are the most frequent ones. Nevertheless, the quest for structural characterization of the final product requires a preparation route, which provides crystalline material suitable for diffraction study. Therefore, choice of starting materials and reaction conditions are of great importance. The reaction of calcium ions with phenylphosphonic acid was first reported by Cao et al. [9,10]. They attributed the formula Ca(HO3 PPh)2 to the product, however, poor crystallinity hampered the structure elucidation of the material by the diffraction techniques. In 1992, Murakami and Imai [11] made an attempt to synthesize and investigate thermal properties of calcium phenylphosphonate. Surprisingly, by using another synthetic route, they obtained a white precipitate which was attributed to a chemical composition of CaO0.5 (H2 O3 PPh)2 ·2H2 O. This seems to be rather odd, since metal phosphonates generally have stoichiometric chemical composition. We suspect the product described by Murakami and Imai was probably contaminated from a high pH media and extrinsic water. In this study, we have examined the effect of reaction parameters on the preparation and crystallization of calcium phenylphosphonate (CAP). The reactions at high pH (i.e. 8 and 10) resulted in relatively low yield and difficulties in isolation of the desired product. However, our results show that CAP could be obtained by the reaction of different calcium sources (i.e. CaCO3 , Ca(OH)2 and Ca(NO3 )2 ) and phenylphosphonic acid at moderately acidic media (pH = 2.1–6). Nevertheless, effect of pH is critical to obtain the desired product with suitable crystallinity for diffraction studies. Fig. 1 gives a comparison of the morphology of the products obtained at pH 6 either by method (a) or by method (b). As mention in the experimental part, large plate-like crystals at mm size were grown by the reaction at pH = 6 using method (b), while method (a) only provides polycrystalline material at µm size. The formation of plate crystals could be attributed to the intrinsic nature of layered structures. 3.2. Crystal structure study of calcium phenylphosphonate (CAP) The compound crystallizes in monoclinic system with space group C2/c (No. 15). Crystallographic and refine-

(a)

(b) Fig. 1. A comparison of ESEM microphotograph of polycrystalline CAP (a) and the microphotograph of the CAP crystals suitable for single-crystal X-ray diffraction (b).

ment data are summarized in Table 1 and atomic coordinates in Table 2. As shown in Fig. 2, the calcium ion is eight coordinated by oxygen atoms from the phosphonate groups in a distorted dodecahedron fashion. Thus,

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A.H. Mahmoudkhani, V. Langer / Solid State Sciences 3 (2001) 519–525 Table 1 Crystallographic and refinement data of CAP Empirical formula

C12 H12 CaO6 P2

Formula weight

354.24

Crystal system

Monoclinic

Space group

C2/c (No. 15)

a (Å)

31.5414(7)

b (Å)

5.6696(1)

c (Å)

7.7985(5)

β (◦ )

101.755(1)

V (Å3 )

1365.34(5)

Z

4

T (K) Fig. 2. Molecular structure and atom labeling scheme for calcium phenylphosphonate (CAP). Thermal ellipsoids are shown at 50% probability.

CAP is isomorphous with already known barium and lead phenylphosphonates [12], but it is different in comparison with the structure of Ca(HO3 PC6 H13 )2 where calcium is octahedrally coordinated [1]. The Ca–O bond distances range between 2.3641(14) and 2.6966(13) Å (see Table 3). The phosphonate group is chelated to calcium ion through O(1) and O(2) atoms. The structure exhibits twodimensional sheets formed by edge sharing polyhedra of calcium phosphonate aligned parallel to the ac plane. The layered structure of CAP is illustrated in Fig. 3. Within the layers, the O(3) atom is involved in a hydrogen bond of the type O–H· · ·O with the O(2) atom of the hydroxyl group which further stabilizes the inorganic framework (see Fig. 4). The geometry of hydrogen bonding is given in Table 4. 3.3. Thermal behavior of calcium phenylphosphonate First, a comparison of simulated powder diffraction of the compound from its single crystal data with the observed pattern of the bulk product was made to be sure that a pure material was received (see Fig. 5). Thermodiffractogram of the sample heated up to 620◦ C in air is presented by Fig. 6. The thermogravimetry (TG) curve is represented in Fig. 7. According to TG analysis, there is a weight change (0.1%) at about 140◦ C. This can be attributed to the loss of adsorbed water and sample drying, since thermodiffractogram of the sample heated up to 300◦C reveals no changes except thermal dilatation. The condensation is starting at about 300◦ C,

ρcalc

(g cm−3 )

173(2) 1.723

Crystal dimensions (mm)

0.30 × 0.25 × 0.04

µ (mm−1 )

0.718

Min. and max. transmission

0.8134/0.9719

F (000)

728

θ range (◦ )

1.32–32.72

Index ranges

−47 ≤ h ≤ 47 −8 ≤ k ≤ 8 −11 ≤ l ≤ 11

Reflections collected

11 374

Independent reflections

2414

Rint

0.0599

Reflections observed (I > 2σ (I ))

1977

Data/restraints/parameters

2414/1/100

GooF (Fo2 ) R1 /wR2 1 for obs. refl.

1.035 0.0507/0.1254

R1 /wR2 for all data

0.0645/0.1323

Weighting scheme2 x/y

0.0912/0.000

Larg. res. peak/hole (e Å−3 )

1.876/−1.077

1 R =
F | − |F /
|F |, c o
1
o wR2 = { [w(Fo2 − Fc2 )2 ]/ (Fo2 )2 }1/2 . 2 w = 1/[σ 2 (F 2 ) + (xP )2 + yP ], P = (F 2 + 2F 2 )/3. o o c

where a water molecule leaves and the material condenses to a pyrophosphonate compound. This seems to be in consistence with thermal behavior of Ba(HO3 PPh)2 and Pb(HO3 PPh)2 , in which no dehydration takes place below 400 and 270◦ C, respectively [12]. The sample undergoes other changes (loss of organic part) during the

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Table 2 Atomic coordinates (× 104 ) and equivalent isotropic displacement parameters (Å2 × 103 ) for CAP. U (eq) is defined as one third of the trace of the orthogonalized Uij tensor x

y

z 2500

U (eq)

Ca(1)

5000

6090(1)

P(1)

4394(1)

1600(1)

817(1)

9(1)

O(1)

4643(1)

3218(2)

−160(2)

12(1)

O(2)

4489(1)

2718(2)

2733(2)

12(1)

O(3)

4503(1)

−1017(2)

864(2)

12(1)

C(1)

3819(1)

1963(3)

−44(2)

12(1)

C(2)

3667(1)

3994(4)

−1023(3)

17(1)

C(3)

3221(1)

4310(4)

−1690(3)

21(1)

C(4)

2925(1)

2610(4)

−1360(3)

21(1)

C(5)

3074(1)

591(4)

−386(3)

22(1)

C(6)

3517(1)

254(4)

264(3)

16(1)

Fig. 3. Representation of the layered structure of calcium phenylphosphonate (CAP). Layers are parallel to ac plane.

9(1)

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Table 3 Selected bond distances (Å) and angles (◦ ) of calcium phenylphosphonate (CAP). P1–O1

1.5111(14)

O1–P1–O2

102.35(8)

P1–O2

1.5942(14)

O1–P1–O3

117.77(8)

P1–O3

1.5215(14)

O2–P1–O3

111.52(8)

P1–C1

1.812(2)

O1–P1–C1

109.14(8)

Ca1–O1

2.6966(14)

O2–P1–C1

106.50(8)

Ca1–O2

2.5321(14)

O3–P1–C1

108.91(8)

Ca1–O1a

2.3641(13)

O1–Ca1–O2

Ca1–O1b

2.3641(13)

Ca1–O1e

2.6966(14)

Ca1–O2b

2.5321(14)

Ca1–O3c

2.4413(14)

Ca1–O3d

2.4413(14)

55.02(4)

Symmetry transformations used to generate equivalent atoms: a x, −y + 1, z + 1/2; b −x + 1, −y + 1, −z; c x, y + 1, z; d −x + 1, y + 1, −z + 1/2; e −x + 1, y, −z + 1/2. Fig. 5. Comparison of the calculated (top) and observed (bottom) powder diffraction pattern for polycrystalline CAP.

Fig. 6. TG curve for calcium phenylphosphonate (CAP), m0 = 0.034 g and heating rate of 2◦ C min−1 .

Fig. 4. Representation of cooperative hydrogen bonding within the inorganic part of CAP. Phenyl groups are omitted for clarity. Table 4 Hydrogen bonding geometry (Å, ◦ ) in CAP D–H· · ·A

d(D–H )

d(H· · ·A)

d(D· · ·A)

<(DHA)

O2–H1· · ·O3a

0.800(18)

1.87(2)

2.6173(18)

155(3)

Symmetry transformation used to generate equivalent atoms: a x, −y, z + 1/2.

heating process according to TG curve. These changes are reflected in diffraction patterns at 340, 380, 420, 460 and 500◦ C. The X-ray powder diffraction pattern of the sample heated to 540◦ C shows the originally crystalline material turned to be amorphous. Thermal analysis curve shows the complete loss of organic component (phenyl groups) occurs at 538◦ C. The material crystallizes again by heating to 620◦ C and exhibits diffraction peaks corresponding to δ-Ca(PO3 )2 , PDF No. 09-0363 [13]. Our results for thermal behavior of calcium phenylphosphonate

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with the Cambridge Crystallographic Center as supplementary publication No. CCDC-144289 (CAP). Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44) 1223-336-033; E-mail: [email protected]. uk).

Acknowledgements We wish to thank Anders Järdnäs for performing thermal analysis.

References Fig. 7. Thermodiffractogram of CAP in the range 30–620◦ C.

exhibit a remarkable difference in comparison with the results presented by Murakami and Imai [11] that calcium phenylphosphonate thermally decomposes to α-Ca2 P2 O7 and P2 O5 at 580◦C. In conclusion, this work has demonstrated the structure details of layered calcium phenylphosphonate (CAP), Ca(HO3 PPh)2 , and its thermal behavior in the range of 30–620◦C. The reaction conditions were investigated to obtain crystalline product at high yield. This colorless material is stable up to 300◦ C and is not soluble in organic solvents and water at pH higher than 2. These properties of CAP make it suitable for application in surface treatment of different objects. Application of CAP for the surface treatment of calcerous stones is currently under investigation.

4. Supplementary data Crystallographic data (excluding structure factors) for the structure reported in this paper have been deposited

[1] G. Cao, V.M. Lynch, J.S. Swinnea, T.E. Mallouk, Inorg. Chem. 29 (1990) 2112. [2] K.J. Langley, P.J. Squattrito, F. Adani, E. Montoneri, Inorg. Chim. Acta 253 (1996) 77. [3] P.R. Rudolf, E.T. Clarke, A.E. Martell, A. Clearfield, Acta Cryst. Sec. C 44 (1988) 796. [4] SMART & SAINT: Area detector control and integration software, Siemens AXS, Madison, WI, USA, 1995. [5] G.M. Sheldrick, SADABS , Program for empirical absorption correction of area detectors, University of Göttingen, Germany, 1996. [6] R.H. Blessing, Acta Cryst. Sec. A 51 (1995) 33. [7] G.M. Sheldrick, SHELXTL (Version 5.10), Structure determination software programs, Bruker AXS Inc., Madison, WI, USA, 1997. [8] Siemens, DIFFRACT AT Software, Version 3.3, Socabim, 1993. [9] G. Cao, H. Lee, V.M. Lynch, T.E. Mallouk, Solid State Ionics 26 (1988) 63. [10] G. Cao, H. Lee, V.M. Lynch, T.E. Mallouk, Inorg. Chem. 27 (1988) 2781. [11] Y. Murakami, H. Imai, J. Ceramic Soc. Jpn. 100 (1992) 439. [12] D.M. Poojary, B. Zhang, A. Cabeza, M.A.G. Aranda, S. Brugue, A. Clearfield, J. Mater. Chem. 6 (1996) 639. [13] PDF-1 Database (Release 1999 with 124 564 phases), International Centre for Diffraction Data, Pennsylvania, USA, 1999.