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Journal of Physics and Chemistry of Solids 69 (2008) 1439–1443

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Synthesis and characterization of new potential intercalation hosts—barium arylphosphonates

J. Svobodaa, V. Zimaa,�, L. Benesa, K. Melanovaa, M. Vlceka, M. Trchovab

aJoint Laboratory of Solid State Chemistry of the Institute of Macromolecular Chemistry of Academy of Sciences,

University of Pardubice, Studentska 84, 532 10 Pardubice, Czech RepublicbInstitute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Square 2, 162 06 Prague 6, Czech Republic

Received 29 May 2007; received in revised form 20 September 2007; accepted 30 October 2007

Abstract

New barium phenylphosphonate dihydrate—BaC6H5PO3 � 2H2O—was prepared by precipitation from barium salt and phenylpho-

sphonic acid solutions with pH adjusted to 8.5. The compound was characterized by powder X-ray diffraction, thermogravimetric

analysis, energy-dispersive X-ray analysis and infrared spectroscopy. It was found that this compound reacts with phenylphosphonic acid

to form previously described barium hydrogen phenylphosphonate—Ba(C6H5PO3H)2. This reaction also proceeds in an opposite

direction, i.e., Ba(C6H5PO3H)2 reacts with Ba2+ in diluted ammonia to produce BaC6H5PO3 � 2H2O. Infrared spectra of both barium

compounds are described and compared with those of analogous strontium compounds and phenylphosphonic acid. Preliminary

intercalation experiments indicated that BaC6H5PO3 � 2H2O could be a promising host material for intercalation of amines.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: A. Inorganic compounds; C. Infrared spectroscopy; C. X-ray diffraction

1. Introduction

Metal phosphonates are hybrid inorganic–organic com-pounds in which the nature of the organic moiety may bechanged to confer specific properties or to create requiredstructures.

In case of barium phenylphosphonates, only a smallnumber of these compounds have been reported up to now.Three barium hydrogen phosphonates Ba(C6H5PO3H)2 [1],Ba(HO3PC6H4PO3H) and Ba(HO3PC6H4C6H4PO3H) [2]were prepared. Also, intercalations of n-alkylamines [3] andn-alkyldiamines [4] into barium hydrogen phenylphospho-nates, Ba(C6H5PO3H)2 �H2O and Ba(C6H5PO3H)2, havebeen described.

Recently, we have studied the synthesis and properties ofphenylphosphonates and 4-carboxyphenylphosphonates ofcalcium [5] and strontium [6,7]. We have found thatcompounds with various Me/P ratios (MeQCa, Sr) areformed in dependence on the acidity of the reaction

e front matter r 2007 Elsevier Ltd. All rights reserved.

cs.2007.10.022

ng author. Tel.: +420 46 603 6145; fax: +420 46 603 6011.

ss: vitezslav.zima@upce.cz (V. Zima).

medium. In this paper, we determine interrelations betweenpreviously described barium hydrogen phenylphosphonate,Ba(C6H5PO3H)2 [1], and a newly prepared phenylpho-sphonate with the formula BaC6H5PO3 � 2H2O.

2. Experimental

All starting chemicals were obtained from commercialsources and were used without further purification. Thebarium and phosphorus contents were determined by anelectron-scanning microscope JEOL JSM-5500LV andenergy-dispersive X-ray microanalyzer IXRF Systems(detector GRESHAM Sirius 10). The accelerating voltageof the primary electron beam was 20 kV. The thermogravi-metric measurements were carried out in air between 30and 960 1C at a heating rate of 5 1Cmin�1. Powder X-raydiffraction (XRD) data were obtained with a D8-Advancediffractometer (Bruker AXS, Germany) with Bragg–Bren-tano y–y geometry (40 kV, 40mA) using CuKa radiationwith a secondary graphite monochromator. The diffractionangles were measured at room temperature from 21 to 651(2y) in 0.021 steps with a counting time of 10 s per step.

ARTICLE IN PRESSJ. Svoboda et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1439–14431440

Infrared spectra in the range 400–4000 cm–1 were recordedat 64 scans per spectrum at 2 cm–1 resolution using afully computerized Thermo Nicolet NEXUS 870 FTIRSpectrometer with a DTGS TEC detector. Measurementsof the powdered samples were performed ex situ in thetransmission mode in KBr pellets. All spectra werecorrected for the presence of moisture and carbon dioxidein the optical path.

2.1. Preparation of Ba(C6H5PO3H)2

Barium hydrogen phenylphosphonate, Ba(C6H5PO3H)2,was prepared according to the procedure previouslydescribed [1]. The relative amounts of barium andphosphorus were 67 at% of P and 33 at% of Ba, accordingto energy-dispersive X-ray analysis (EDX). Its powderXRD pattern fully corresponded with the diffractionpattern calculated using a PowderCell program [8] fromcoordinates determined by a single-crystal XRD forBa(C6H5PO3H)2 [1].

2.2. Preparation of BaC6H5PO3 � 2H2O

A concentrated solution of ammonia was added to asuspension of phenylphosphonic acid (2.5� 10�2mol) inwater (50mL) to adjust pH to 8.5. A solution ofBaCl2 � 2H2O (2.5� 10�2mol) in water (25mL) was addedunder stirring to the solution of phenylphosphonic acid.The reaction mixture was further stirred for about 30minat room temperature. The white precipitate was separatedby filtration, washed twice with boiling water and thentwice with ethanol. The product was dried in air at roomtemperature. The yield was 3.11 g (38%). Elementalanalysis calcd. (%) for C6H9BaO5P (329.4): C 21.86, H,2.73; found: C 21.77, H 3.00. The relative amounts ofphosphorus and barium were 52.5 and 47.5 at%, respec-tively, according to EDX.

2.3. Reaction of Ba(C6H5PO3H)2 with ammonia in the

presence of the Ba2+ ions

The reaction was carried out at room temperature usinga computer-controlled Schott Titronic 97 piston burette asdescribed previously [5]. In this reaction, an aqueoussolution of ammonia was added using the burette to anaqueous suspension of Ba(C6H5PO3H)2. The intervalsbetween additions of ammonia were chosen to be suffi-ciently long to ensure that practically all added bases wouldbe consumed in the reaction with the barium compound.The acidity of the solutions during the reactions waschecked with a glass pH electrode. The value of pH atthe end of the intervals, just before another addition,was then evaluated depending on the amount of addedammonia.

The starting compound, Ba(C6H5PO3H)2 (1� 10–3 mol),was added to a solution of BaCl2 � 2H2O (2� 10–3mol) in amixture of water (20mL) and ethanol (30mL). The

suspension was titrated with an aqueous solution ofNH4OH (c ¼ 0.398mol L–1), which was added in 0.08-mLdoses with 1800-s intervals between the doses. The valuesof pH during the reaction were measured in 45-s intervals.The solid white product (m ¼ 0.4 g) was separated byfiltration, washed with water and ethanol and dried in airat room temperature.

2.4. Reaction of BaC6H5PO3 � 2H2O with phenylphosphonic

acid

This reaction was conducted using the same experi-mental arrangement as that of Ba(C6H5PO3H)2 withammonia.A suspension of BaC6H5PO3 � 2H2O (1� 10–3mol) in a

mixture of water (35mL) and ethanol (35mL) was titratedwith a 0.1M aqueous solution of phenylphosphonic acid,which was added in 0.2-mL doses with 1800-s intervalsbetween the doses. The values of pH during the reactionwere checked every 45 s. The solid white product(m ¼ 0.135 g) was separated by filtration, washed withwater and ethanol and dried in air at room temperature.

2.5. Intercalation of BaC6H5PO3 � 2H2O with n-butylamine

A suspension of solid BaC6H5PO3 � 2H2O (1� 10–3mol)was shaken in 5mL of n-butylamine at room temperaturefor 40 h. The white solid intercalate was separated by quickvacuum filtration and the powder XRD pattern of the wetsample was measured under protection foil.

3. Results and discussion

Poojary et al. [1] have reported the synthesis ofBa(C6H5PO3H)2 by refluxing a solution of phenylpho-sphonic acid and BaCl2 � 2H2O, i.e., a reaction in an acidicmedium. This barium hydrogen phenylphosphonate is alayered compound with the basal spacing d(2 0 0) ¼ 15.7 A(Fig. 1a). When the reaction of phenylphosphonic acidwith BaCl2 � 2H2O is carried out in a moderately basicenvironment, a new compound with a Ba/P molar ratio of1/1 is formed, as found by EDX.On heating, this compound loses weight in two steps

(Fig. 2). The first weight loss of 11.0% is in the temperaturerange 50–250 1C; the second one of roughly 20.6% occursfrom 400 to 550 1C. The product of heating to 950 1C isBa2P2O7, as confirmed by powder XRD (PDF no. 83-0990)[9]. Thus, the first weight loss corresponds to the releaseof two molecules of water from BaC6H5PO3 �

2H2O (calculated weight loss 10.9%), the second loss ofweight is due to the decomposition of the organic part(calculated weight loss 21.0%). This dihydrate also easilyloses one water molecule by being kept in a dessicatorover P2O5.The most intensive peak in the X-ray powder pattern of

BaC6H5PO3 � 2H2O corresponded to a basal spacing of13.4 A (Fig. 1b). By dehydration, the value of the

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Fig. 1. X-ray diffractograms of Ba(C6H5PO3H)2 (a), BaC6H5PO3 � 2H2O

(b), and BaC6H5PO3 � 2H2O intercalated with n-butylamine (c).

Fig. 2. Thermogravimetric curve of BaC6H5PO3 � 2H2O.

Fig. 3. Course of the reaction of the Ba(C6H5PO3H)2 suspension with

ammonia in the presence of BaCl2 solution. (Inset) Time dependence of

pH at the beginning of the reaction.

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basal spacing increases to 15.4 A, a phenomenontypical for this type of compound as described pre-viously [3,10].

We studied the possibility of preparing BaC6H5

PO3 � 2H2O from Ba(C6H5PO3H)2 in a basic medium. Forthis purpose, we used the method described in theExperimental section. Between the additions of ammonia,the pH value decreases in an exponential way (see inset inFig. 3), which indicated that ammonia was consumed in thereaction. The values of pH at the ends of the intervalsbetween ammonia additions (depicted as circles in theinset) are plotted in Fig. 3 against the amount of added

ammonia, expressed as a molar amount of ammonia per1mol of Ba(C6H5PO3H)2. When the value of n(NH4OH)/n(Ba(C6H5PO3H)2) is close to 2, the pH values stopdecreasing between the additions of ammonia and onlyan increase in pH is observed with the increasing amount ofammonia. This indicates an end of the reaction, whichcould be described by the equation

BaðC6H5PO3HÞ2 þ 2NH4OHþ BaCl2

! 2BaC6H5PO3 � 2H2Oþ 2NH4Cl: (1)

The product of the reaction is identical to BaC6H5-

PO3 � 2H2O prepared by the synthesis, as confirmed bypowder XRD, thermogravimetric analysis (TGA) andEDX. This behavior of Ba(C6H5PO3H)2 is similar to thebehavior of Sr(C6H5PO3H)2 in a basic medium [6].A reverse reaction was also investigated that is the

formation of Ba(C6H5PO3H)2 from a suspension of BaC6

H5PO3 � 2H2O to which an aqueous solution of phenylpho-sphonic acid was added. At the beginning of the additions,only a decrease in pH was observed due to the increasedconcentration of the H+ ions. When the pH valuedecreased below 7, the reaction started as follows fromthe exponential increase in pH with time between theadditions of acid (see inset in Fig. 4). The pH value reachedaround 7 before another addition of the acid. This value ofpH was then maintained roughly the same up to the molarratio n(C6H5PO3H2)/n(BaC6H5PO3 � 2H2O) ¼ 1. Furtheradditions of the acid caused a rapid decrease in pH(Fig. 4). The XRD pattern of the product corresponded tothat of Ba(C6H5PO3H)2. It can be deduced that the

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Fig. 4. Course of the reaction of the BaC6H5PO3 � 2H2O suspension with

the phosphonic acid solution. (Inset) Time dependence of pH at the

beginning of the reaction.

Fig. 5. FTIR spectra of phenylphosphonic acid C6H5PO3H2 (a), barium

hydrogen phenylphosphonate (Ba(C6H5PO3H)2) (b), strontium hydrogen

phenylphosphonate (Sr(C6H5PO3H)2) (c), barium phenylphosphonate

dihydrate (BaC6H5PO3 � 2H2O) (d), and strontium phenylphosphonate

dihydrate (SrC6H5PO3 � 2H2O) (e).

J. Svoboda et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1439–14431442

reaction proceeds according to the equation

BaC6H5PO3 � 2H2Oþ C6H5PO3H2

! BaðC6H5PO3HÞ2 þ 2H2O: (2)

As in Eq. (1), no formation of an intermediate wasobserved.

To further characterize BaC6H5PO3 � 2H2O, we haveanalyzed the prepared barium phenylphosphonates byFTIR spectroscopy. We compared the obtained spectraof barium compounds, Ba(C6H5PO3H)2 and BaC6H5PO3 �

2H2O, with those of phenylphosphonic acid (C6H5PO3H2),strontium hydrogen phenylphosphonate (Sr(C6H5PO3H)2),and strontium phenylphosphonate dihydrate (SrC6H5PO3 �

2H2O) described in our previous paper [6]. All spectra aredepicted in Fig. 5. The strontium compounds haveanalogous formulae like the barium ones and it can bepresumed that they also have analogous structures.

The spectra of all compounds have some commonfeatures: (i) the C–H stretching vibrations of the phenylring in the region of 3090–3000 cm�1 (not shown in Fig. 5),(ii) the aromatic CQC stretching vibrations at 1438 cm�1,(iii) the peaks at about 695 and 745 cm�1 representing theout-of-plane bending of the mono-substituted phenyl ring,(iv) the P–O stretching vibrations in the region 1200–1000 cm�1, (v) the O–P–O bending vibrations in the region600–410 cm�1 [11]. The infrared spectra of (Ba(C6H5

PO3H)2) and (Sr(C6H5PO3H)2) are similar and they areclose to the spectrum of phenylphosphonic acid (spectra a,b and c in Fig. 5). There are no bands in the O–H stretchingregion (3500–3200 cm�1), which is consistent with the

absence of water molecules in all these structures [1]. Thebands around 2750 and 2340 cm�1 are characteristic of theO–H stretching frequencies of the monohydrogen phos-phonate groups [1]. All spectra exhibit strong bands atabout 939 (913) cm�1, assigned to P–OH stretchingvibrations, indicating also the presence of PO3H groups[12]. The spectra of these three samples differ in the regionof the PO3 stretching vibrations where a high number ofbands are observed. This is connected with the absence ofwater in the structure of these samples which causes aninternal stress leading to a deformation of the phosphonategroup [6]. The presence of PQO stretching vibration at1220 cm�1 in the spectrum of phenylphosphonic acid(shifted to 1254 and 1215 cm�1 in the spectra of (Ba(C6H5

PO3H)2) and (Sr(C6H5PO3H)2)) support the existence ofthe mentioned group. This vibration is missing in thespectra of BaC6H5PO3 � 2H2O and SrC6H5PO3 � 2H2O(spectra d and e in Fig. 5) because the PQO double bondis delocalized in the PO3

2� group. The bands at about 939(913) cm�1 of P–OH stretching vibrations are also missingin the spectra, supporting the presence of the PO3

2� group.By shaking solid BaC6H5PO3 � 2H2O with n-butylamine,

a compound with an increased basal spacing d ¼ 18.75 A isformed. The X-ray powder pattern (Fig. 1c) shows that thelayered structure of the compound is retained, whichindicates an intercalation of the amine into the layeredstructure of the phenylphosphonate. Unfortunately, theintercalation compound spontaneously deintercalates un-der ambient condition, forming several different phasesduring this process so that we were not able to determine itscomposition. Preliminary tests show that other amines alsocan be intercalated into these barium phenylphosphonates.A more thorough study of these intercalations is now underway.

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Acknowledgment

This work was supported by the Grant Agency of theCzech Republic (GA 203/05/2306).

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