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Solid State Sciences 10 (2008) 1533e1542www.elsevier.com/locate/ssscie

New barium 4-carboxyphenylphosphonates: Synthesis,characterization and interconversions

Jan Svoboda a, Vıtezslav Zima a,*, Ludvık Benes a, Klara Melanova a,Miroslava Trchova b, Milan Vl�cek a

a Joint Laboratory of Solid State Chemistry of the Institute of Macromolecular Chemistry AS CR, v.v.i. and University of Pardubice,

Studentska 84, 532 10 Pardubice, Czech Republicb Institute of Macromolecular Chemistry AS CR, v.v.i., Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic

Received 12 December 2007; received in revised form 15 February 2008; accepted 6 March 2008

Available online 15 March 2008

Abstract

Three new barium 4-carboxyphenylphosphonates with formulae Ba(HOOCC6H4PO3H)2, BaH(OOCC6H4PO3), and Ba3(OOCC6H4PO3)2$2H2Owere prepared and characterized by elemental analysis, thermogravimetric analysis, X-ray powder diffraction, energy-dispersive X-ray microanal-ysis, and infrared spectroscopy. It was found that depending on the acidity of the reaction medium, Ba(HOOCC6H4PO3H)2 can be converted toBa3(OOCC6H4PO3)2$2H2O and vice versa. As an intermediate in these reactions, BaH(OOCC6H4PO3) is formed.

The infrared spectra of these compounds were compared with analogous strontium compounds prepared and described previously and theposition of the acidic hydrogen atom in BaH(OOCC6H4PO3) is deduced from the spectra.

The structure of Ba(HOOCC6H4PO3H)2 was solved from X-ray powder diffraction data by an ab initio method using a FOX program withsubsequent Rietveld refinement in the FullProf program. The compound is monoclinic, space group C2/c (No. 15), a¼ 49.382(1), b¼5.5196(1), c¼ 8.4977(2) A, b¼ 127.52(1)�, and Z¼ 4. This compound has a layered structure built up from CPO3 phosphonate tetrahedraand BaO8 distorted tetragonal antiprisms and thus forming layers in the bc plane with organic groups pointing to the interlayer space.� 2008 Elsevier Masson SAS. All rights reserved.

Keywords: Metal phosphonates; Barium; Carboxyphenylphosphonic acid; Inorganiceorganic hybrid compounds; Layered compounds

1. Introduction

Metal phosphonates belong to hybrid inorganiceorganiccompounds in which the metal in the inorganic part is con-nected to the organic part through oxygen atoms of the phos-phonate group. The structural network of metal phosphonatesvaries from open-framework or pillared to layered to one-dimensional. These hybrid materials have potential applica-tions in catalysis [1], proton conductivity [2,3], as sorbents[4,5], ion exchangers [5,6], or as hosts in intercalation reactions[3,7]. Great effort has been devoted to the preparation of porousmetal phosphonates [8] and phosphonates with distinct

* Corresponding author. Tel.: þ420 46 603 6145; fax: þ420 46 603 6011.

E-mail address: vitezslav.zima@upce.cz (V. Zima).

1293-2558/$ - see front matter � 2008 Elsevier Masson SAS. All rights reserved.

doi:10.1016/j.solidstatesciences.2008.03.009

magnetic [9] and optical properties [10]. Metal phosphonatescan also be applied to biotechnology [11] and biology [12].

Despite the extensive recent research in metal phosphonatechemistry, there seem to be left too many unknown factors forcontrolling the structure such as the high coordination flexibil-ity of the phosphonate groups [13]. Thus, the explorativesynthesis still plays a crucial role in the investigation of com-pounds of this type.

Regarding barium arylphosphonates, only a small number ofthese compounds have been reported up to now. Three bariumhydrogen phosphonates Ba(C6H5PO3H)2 [14], Ba(HO3PC6H4-PO3H) [15], and Ba(HO3PC6H4C6H4PO3H) [15], having layerscomposed of eight-coordinate Ba2þ ions and phosphonategroups, were prepared. Barium phenylphosphonate monohy-drate, Ba(HO3PC6H5)2$H2O, was used as a host in intercalationreactions with alkylamines [16] and alkyldiamines [17].

1534 J. Svoboda et al. / Solid State Sciences 10 (2008) 1533e1542

Recently, a nanostructured form of barium hydrogen phenyl-phosphonate, Ba(C6H5PO3H)2, has been described [18].

In our previous papers, we have studied syntheses and prop-erties of phenylphosphonates and 4-carboxyphenylphospho-nates of calcium [19] and strontium [20,21]. In dependenceon the acidity of the reaction medium, compounds withvarious Me/P ratios (Me¼Ca, Sr) are formed, which is givenby the degree of protonation of the phosphonate group (and thecarboxylic group in the case of 4-carboxyphenylphosphonates)in these compounds. To study the influence of pH on thecomposition of the formed phosphonates, we have developeda method for computer-controlled additions of reagents tophosphonates suspensions. This method turned out to bea successful one for the determination of the interrelationsbetween different compounds containing the same metal andthe same phosphonate ligand.

In this paper, this method was used to determine suchinterrelations between barium dihydrogen 4-carboxyphenyl-phosphonate, Ba(HOOCC6H4PO3H)2, and barium 4-carboxy-phenylphosphonate with the formula Ba(OOCC6H4PO3)2$2H2O.In addition, a new barium 4-carboxyphenylphosphonate withthe formula BaH(OOCC6H4PO3) is formed during theinterconversions of the above mentioned compounds.

2. Experimental

2.1. Materials and methods

All starting chemicals were obtained from commercialsources and were used without further purification. The bariumand phosphorus contents were determined by an electronscanning microscope JEOL JSM-5500LV and energy-disper-sive X-ray microanalyser IXRF Systems (detector GRESHAMSirius 10). The accelerating voltage of the primary electronbeam was 20 kV. The thermogravimetric measurements werecarried out in air between 30 and 960 �C at a heating rate of5 �C min�1. Powder X-ray diffraction data were obtainedwith a D8-Advance diffractometer (Bruker AXS, Germany)with BraggeBrentano qeq geometry (40 kV, 40 mA) usingCu Ka radiation with secondary graphite monochromator.The diffraction angles were measured at room temperaturefrom 2 to 65� (2q) in 0.02� steps with a counting time of 10 sper step. Infrared spectra in the range of 400e4000 cm�1

were recorded at 64 scans per spectrum at 2 cm�1 resolutionusing a fully computerized Thermo Nicolet NEXUS 870FTIR Spectrometer with a DTGS TEC detector. Measurementsof the powdered samples were performed ex situ in thetransmission mode in KBr pellets. All spectra were correctedfor the presence of moisture and carbon dioxide in the opticalpath.

4-Carboxyphenylphosphonic acid monohydrate was pre-pared according to a previously described procedure [22].

2.2. Preparation of Ba(HOOCC6H4PO3H)2

4-Carboxyphenylphosphonic acid monohydrate, HOOCC6

H4PO3H2$H2O (4 mmol) was added to a mixture of 100 mL

of ethanol and 40 mL of water and heated to dissolution. Tothis solution cooled to 15 �C, a solution of BaCl2$2H2O(2 mmol) in 25 mL of ethanol and 10 mL of water was added.A white precipitate was immediately formed, which was sep-arated by filtration, washed twice with water and twice withethanol and dried at room temperature in a desiccator overP2O5. The yield was 88%. The Ba/P atomic ratio was 0.48,according to energy-dispersive X-ray microanalysis (EDX).Anal. Calcd. for C14H12BaO10P2 (539.51): 31.14% C, 2.22%H; found: 33.58% C, 3.24% H.

2.3. Preparation of Ba3(OOCC6H4PO3)2$2H2O

The acidity of HOOCC6H4PO3H2$H2O (4 mmol) sus-pended in ethanol (100 mL) was adjusted to pH¼ 10.9 withan aqueous 0.4 M solution of NH4OH. The acid dissolvedduring the addition of ammonia. BaCl2$2H2O (6 mmol) wasdissolved in a mixture of ethanol (10 mL) and water(10 mL). Both solutions were mixed. A white precipitate wasimmediately formed, which was separated by filtration andwashed twice with ethanol. The yield of the product dried ina desiccator over P2O5 was 85%. The Ba/P atomic ratio was1.50, according to EDX. Anal. Calcd. for C14H12Ba3O12P2

(846.17): 19.85% C, 1.42% H; found: 21.23% C, 2.43 H.

2.4. Reaction of Ba3(OOCC6H4PO3)2$2H2O with4-carboxyphenylphosphonic acid

The reaction was carried out at room temperature usinga computer-controlled Schott Titronic 97 piston burette. Usingthe burette, an aqueous solution of 4-carboxyphenylphosphonicacid was added to a suspension of the barium compound. Theintervals between additions of the acid were chosen to be suffi-ciently long to ensure that practically all added acids would beconsumed in the reaction with the barium compound. Theacidity of the solutions during the reaction was checked witha glass pH electrode. The value of pH at the end of the intervals(just before another addition of the acid) as a function of theamount of the added acid was then evaluated.

A suspension of Ba3(OOCC6H4PO3)2$2H2O (0.4 mmol) ina mixture of 30 mL of ethanol and 20 mL of water was stirredwith a 0.0465 M aqueous solution of 4-carboxyphenylphos-phonic acid, which was added in 0.5-mL doses with 3200-sintervals between the doses. The values of pH were measuredin 80-s intervals during the reaction. The yield of the solidproduct was 75%.

2.5. Preparation of BaH(OOCC6H4PO3)

A suspension of Ba3(OOCC6H4PO3)2$2H2O (0.4 mmol) ina mixture of 30 mL of ethanol and 20 mL of water was stirredwith a 0.0465 M aqueous solution of 4-carboxyphenylphos-phonic acid, which was added in 0.5-mL doses with 3200-s in-tervals between the doses. The total volume of the added acidwas 8.5 mL (0.4 mmol). A white solid insoluble in water wasobtained in 25% yield. The Ba/P atomic ratio in the product

1535J. Svoboda et al. / Solid State Sciences 10 (2008) 1533e1542

was 1.02, according to EDX. Anal. Calcd. for C7H5BaO5P(337.41): 24.90% C, 1.48% H; found: 26.49% C, 2.54% H.

2.6. Reaction of Ba(HOOCC6H4PO3H)2 withammonia in the presence of barium salt

This reaction was done under the same experimentalarrangement as the reaction of Ba3(OOCC6H4PO3)2$2H2Owith 4-carboxyphenylphosphonic acid. In this case, aqueousammonia was added using the computer-controlled buretteinto the reaction mixture.

Finely ground Ba(HOOCC6H4PO3H)2 (0.5 mmol) wasadded to a solution of BaCl2 (2.5 mmol) in a mixture of etha-nol (30 mL) and water (20 mL). The suspension was reactedwith an aqueous solution of NH4OH (c¼ 0.242 mol L�1),which was added in 0.2-mL doses with 2400-s intervalsbetween the doses. The values of pH during the reactionwere measured in 60-s intervals. The solid product wasobtained in a quantitative yield.

2.7. Structure solution of Ba(HOOCC6H4PO3H)2

Fig. 1. X-ray diffractograms of b-Sr(HOOCC6H4PO3H)2 (a), Ba(HOOCC6H4-

PO3H)2 (b), Ba3(OOCC6H4PO3)2$2H2O (c), and BaH(OOCC6H4PO3) (d).

The structure of Ba(HOOCC6H4PO3H)2 was solved from itsX-ray powder diffraction pattern. Diffraction data (Cu Ka,l¼ 1.5418 A) were collected on a Bruker AXS D8 Advancediffractometer with a BraggeBrentano qeq geometry, equip-ped with parallel (Soller) slits, a secondary beam curved graph-ite monochromator, a Na(Tl)I scintillation detector and pulseheight amplifier discrimination. The generator was operatedat 40 kV and 40 mA. The following slits were used: divergence0.5�, anti-scatter 0.5� and receiving 0.1 mm. The scan wasperformed at room temperature from 3.5 to 90� (2q) in 0.01�

steps with a counting time of 30 s per step.Indexing using a DicVOL method in a WinPLOTR pro-

gram [23] suggested a monoclinic cell of approximate dimen-sions a¼ 49.408, b¼ 5.5225, c¼ 8.5006 A, b¼ 127.535�

with figures of merit M(20)¼ 101.5 [24] and F(20)¼ 222.9(0.0013, 68) [25]. One peak of small intensity (0.5% of themost intensive peak) at 2q¼ 5.7� could not be indexed andwas therefore excluded from the further calculations. Thespace group was determined to be C2/c (No. 15) by meansof the Checkcell program [26].

The structure of Ba(HOOCC6H4PO3H)2 was solved usingan ab initio parallel tempering method implemented in theFOX program [27] with the fragment of 4-carboxyphenylphos-phonic acid taken as a rigid body. The geometry of 4-carbox-yphenylphosphonic acid was first optimized using a PM3semiempirical quantum mechanical calculation implementedin the HyperChem software package [28]. The obtained datawere transformed using BABELWIN [29] into the form ofa FenskeeHall Z-matrix. This matrix was imported intoFOX as a crystal scatterer; all hydrogen atoms data wereremoved. One barium atom was added as another crystalscatterer. The initial position and orientation of the organicfragment and the position of the Ba atom were randomized be-fore optimization. A parallel tempering algorithm was used tofit the calculated diffraction profile with the observed pattern.

A 32-point file was created in WinPLOTR from the X-raypowder diffraction pattern and was used as a background.The x and z coordinates of the Ba atom obtained after the first106 iterations indicated that the atom is in a special positionand were therefore fixed at x¼ 1/2 and z¼ 1/4 for furthercalculations.

The obtained atomic coordinates were used for a Rietveldrefinement using the FullProf program [23]. The 2q range of3.5e90� was used for Rietveld refinement with an excludedregion from 5.6 to 6�. The same 32-point background file asin FOX was used to model the background in FullProf withlinear interpolations between points. A pseudo-Voigt functionwas used to model peak shapes. The restraints on distancesand angles were applied on all carbon atoms and oxygen atomsof the carboxylic group of the 4-carboxyphenylphosphonic acidmolecule. At first, the atomic coordinates were fixed and theprofile parameters were refined. Finally, the atomic coordinatestogether with the profile parameters were refined.

3. Results and discussion

3.1. Synthesis and characterization of barium4-carboxyphenylphosphonates

In the reaction of HOOCC6H4PO3H2$H2O with a bariumsalt without an adjustment of the acidity, a product with theBa/P atomic ratio of 1/2 is formed. Based on this ratio, andon the elemental analysis results, the formula of this compoundcan be written as Ba(HOOCC6H4PO3H)2. The diffractionpattern of this compound contains a series of sharp (00l ) lines,indicating a layered structure, together with some (hkl )diffraction lines with rather low intensity. The basal spacingd¼ 19.583 A is typical of metal dihydrogen 4-carboxyphenyl-phosphonates [19,20]. Nevertheless, the X-ray powder diffrac-tion pattern (Fig. 1b) is the most similar to that of theb modification of Sr(HOOCC6H4PO3H)2 (Fig. 1a) [21].From the pattern, we were able to solve the structure ofBa(HOOCC6H4PO3H)2 as is described below.

On heating, this compound is stable up to 350 �C (Fig. 2a,solid line). Between 350 and 420 �C, first weight loss of about4% occurs, which could be ascribed to a condensation of

Fig. 2. TGA curves of Ba(HOOCC6H4PO3H)2 (a, solid line), Ba-

H(OOCC6H4PO3) (b, dashed line), and Ba3(OOCC6H4PO3)2$2H2O (c, dash-

and-dotted line).

Fig. 3. Course of the reaction of the Ba3(OOCC6H4PO3)2$2H2O suspension

with 4-carboxyphenylphosphonic acid solution.

1536 J. Svoboda et al. / Solid State Sciences 10 (2008) 1533e1542

hydrogen phosphonate groups to pyrophosphonate groups [30]with a loss of one molecule of water per formula unit (3.3%theoretically). Further heating causes a decomposition of theorganic part at around 500 �C and most probably a formationof Ba(PO3)2. The product is a glassy material; therefore itsidentity could not be confirmed by X-ray powder diffraction.The total weight loss is about 45%, which is in a good agree-ment with the theoretical weight loss of 45.3% calculated ac-cording to the equation:

BaðHOOCC6H4PO3HÞ2 þ 15O2 / BaðPO3Þ2 þ 14CO2

þ 6H2O: ð1ÞAnother barium 4-carboxyphenylphosphonate was prepared

by the reaction of a barium salt solution with a solution of4-carboxyphenylphosphonic acid, whose pH was adjusted to10.9. The Ba/P atomic ratio was 3/2 in this compound.

The X-ray powder diffraction pattern of this barium com-pound gives the basal spacing d of 10.78 A (Fig. 1c), which isdistinctly smaller than that found for Ca3(OOCC6H4PO3)2$6H2O (d¼ 19.27 A) [19] or Sr3(OOCC6H4PO3)2$5.7H2O(d¼ 19.55 A) [21].

The thermogravimetric curve of the barium compound isshown in Fig. 2c as a dash-and-dotted line. The first massloss observed on heating to 400 �C indicates a release of waterfrom the structure of the product. The weight decrease to95.6% corresponds to two molecules of water per formulaunit. We found that the amount of water varied stronglywith drying conditions (the sample dried only in air at roomtemperature contained about four molecules of water performula unit). Another mass loss above 400 �C is due to thedecomposition of the organic part of the compound. Theproduct of heating to 950 �C is Ba3(PO4)2, as was confirmedby X-ray powder diffraction (PDF No. 25-0028) [31]. In

summary, from EDX and TGA data we can deduce that theformula of this barium 4-carboxyphenylphosphonate can bewritten as Ba3(OOCC6H4PO3)2$2H2O.

The X-ray powder diffraction pattern of this compounddehydrated by heating to 400 �C is identical with that of thehydrate. It indicates that cavities are retained in a three-dimen-sional or pillared structure of Ba3(OOCC6H4PO3)2. Thus, thisbarium 4-carboxyphenylphosphonate can be considered to bea porous material. Unfortunately, the poor quality of theX-ray diffraction pattern did not allow us to solve the structureof this compound. Nevertheless, it can be analogous tocompounds obtained with (2-carboxyethyl)phosphonic acid,e.g., three-dimensional Co3(OOCCH2CH2PO3)2$6H2O [32]or pillared layered Zn3(OOCCH2CH2PO3)2 [33].

3.2. Interconversion reactions

The possibility of mutual interconversion betweenBa3(OOCC6H4PO3)2$2H2O and Ba(HOOCC6H4PO3H)2 wasinvestigated as a reaction of Ba3(OOCC6H4PO3)2$2H2O withHOOCC6H4PO3H2.

The solution of 4-carboxyphenylphosphonic acid wasadded to a suspension of Ba3(OOCC6H4PO3)2$2H2O in smalldoses. The course of the reaction was checked by pH measure-ment. The interval between the additions of the acid waschosen to be sufficiently long to ensure that almost all addedacids were consumed in the reaction. The pH values measuredjust before the additions of the acid were then plotted againstthe amount of the added acid. This amount was expressed asa molar ratio of the added acid to Ba3(OOCC6H4PO3)2$2H2O.The obtained plot is shown in Fig. 3.

Fig. 4. Course of the reaction of the Ba(HOOCC6H4PO3H)2 suspension with

ammonia in the presence of the barium salt.

1537J. Svoboda et al. / Solid State Sciences 10 (2008) 1533e1542

At the beginning of the additions, almost no change in finalpH is observed. It means that all added acids are consumed inthe reaction with Ba3(OOCC6H4PO3)2$2H2O and the value ofpH is maintained at around 6.5. When about 1 mol of the acidper 1 mol of Ba3(OOCC6H4PO3)2$2H2O is added to thereaction mixture (point A in Fig. 3), a steep decrease of pHis observed with further additions of the acid. The pH valuedecreases below 4, up to point B. Further additions of theacid are accompanied by a slight increase of pH, which indi-cates that another reaction takes place. The acid is consumedin this reaction (with the pH around 4) up to point C. At thispoint, about 4 mol of the acid per 1 mol of Ba3(OOCC6H4-PO3)2$2H2O are added to the mixture. Further additions ofthe acid caused a decrease of pH only, which means that noreaction occurs beyond point C. In summary, Fig. 3 indicatesthat the consumption of the acid proceeds in two steps, atpH> 6 and at pH< 4, i.e., two reactions at different pH occur.

In the first step, 1 mol of the acid reacts with 1 mol ofBa3(OOCC6H4PO3)2$2H2O. This reaction was thereforerepeated under the same conditions but the additions werestopped when the n(HOOCC6H4PO3H2)/n(Ba3(OOCC6H4-PO3)2$2H2O) ratio was equal to 1. From the results of EDX(Ba/P atomic ratio of 1.02) and elemental analysis it can bededuced that the formula of this intermediate is BaH(OOCC6H4PO3). Its X-ray powder diffraction pattern is shown inFig. 1d, giving the basal spacing d¼ 10.35 A. The thermogra-vimetric curve of this intermediate is shown in Fig. 2b (dashedline). A small weight decrease (2.5%) between 100 and 420 �Ccan be ascribed to a condensation of the hydrogen phospho-nate groups to diphosphonate groups, according to theequation:

2BaHðOOCC6H4PO3Þ/ Ba2ðOOCC6H4PÞ2O5 þ H2O ð2Þ

with a theoretical weight loss of 2.7%. Around 500 �C, a steepweight decrease due to the decomposition of the organic partwas observed. The final product is Ba2P2O7, according toX-ray powder diffraction (PDF No. 30-0144) [31]. Theobserved total weight loss of 100� 67.5¼ 32.5% is in quitegood agreement with the theoretical one (33.5%) calculatedfrom the decomposition reaction:

2BaHðOOCC6H4PO3Þ þ 15O2 / Ba2P2O7 þ 14CO2

þ 5H2O: ð3Þ

We can therefore write the reaction in the first step as:

Ba3ðOOCC6H4PO3Þ2$2H2O

þ HOOCC6H4PO3H2 / 3BaHðOOCC6H4PO3Þ þ 2H2O:

ð4ÞThe small increase of pH observed beyond point B indi-

cates an occurrence of another reaction. During this reaction,3 mol of the acid are consumed per 1 mol of Ba3(OOCC6H4-PO3)2$2H2O (to point C). The product of this reaction isidentical with Ba(HOOCC6H4PO3H)2 prepared by the directsynthesis from 4-carboxyphenylphosphonic acid and Ba2þ

salt, as was confirmed by EDX, TGA, and X-ray powderdiffraction. Therefore, we propose that this second reactionproceeds according to the equation:

3BaHðOOCC6H4PO3Þþ 3HOOCC6H4PO3H2 / 3BaðHOOCC6H4PO3HÞ2 ð5Þ

and the total reaction of Ba3(OOCC6H4PO3)2$2H2O with theacid can be described as:

Ba3ðOOCC6H4PO3Þ2$2H2O

þ 4HOOCC6H4PO3H2 / 3BaðHOOCC6H4PO3HÞ2þ 2H2O: ð6ÞWe also tested a reverse interconversion, that is a formation

of Ba3(OOCC6H4PO3)2$2H2O from Ba(HOOCC6H4PO3H)2

by the reaction in a basic medium. We accomplished thisreaction by the addition of small amounts of aqueous ammoniato a suspension of Ba(HOOCC6H4PO3H)2 in the presence ofan excess of the Ba2þ salt. As in the previous case, the courseof the reaction was followed by the measurement of pH. Thedependence of pH on the amount of added ammonia,expressed as a n(NH4OH)/n(Ba(HOOCC6H4PO3H)2) molarratio, is given in Fig. 4.

The exponential decrease of pH between the additions ofammonia indicates an occurrence of a reaction. This reactionis finished at n(NH4OH)/n(Ba(HOOCC6H4PO3H)2) around 2(point A). Further additions of ammonia cause a steep increaseof pH due to the increasing concentration of OH� ions. WhenpH overcomes the value of 7, it begins to decrease (point B).Further additions of ammonia are accompanied with anexponential decrease of pH between the additions of ammonia,

1538 J. Svoboda et al. / Solid State Sciences 10 (2008) 1533e1542

which suggests an occurrence of another reaction. This secondreaction proceeds at around pH¼ 6. Another steep increase ofpH at n(NH4OH)/n(Ba(HOOCC6H4PO3H)2)¼ 4 (point C)indicates that this second reaction is completed. Further addi-tions of ammonia cause an increase of pH only.

The curve in Fig. 4 can be explained in the following way:as in the previous experiment, we can presume that the reac-tion proceeds in two steps with a formation of an intermediate.The amount of ammonia needed for the formation of thisintermediate is 2 mol of NH4OH per 1 mol of Ba(HOOCC6H4-PO3H)2. Thus we can presume that this reaction proceedsaccording to the equation:

BaðHOOCC6H4PO3HÞ2 þ 2NH4OH

þ BaCl2 / 2BaHðOOCC6H4PO3Þ þ 2NH4Cl þ 2H2O:

ð7Þ

As in the previous case we repeated the reaction up topoint A only. The X-ray powder diffraction pattern of theproduct of this reaction was identical with that of the interme-diate obtained according to Eq. (4). This behavior is similar tothat observed for strontium dihydrogen 4-carboxyphenyl-phosphonate [21]. The only difference is that the formedstrontium intermediate contained one molecule of water ofhydration and its formula was SrH(OOCC6H4PO3)$H2O.

In the second step, at pH around 6, the BaH(OOCC6H4PO3)intermediate reacts with another 2 mol of ammonia and with thebarium salt (from point B to point C). It was confirmed by pow-der XRD, TGA, and EDX that the product of this reaction is

Fig. 5. The infrared spectra of 4-carboxyphenylphosphonic acid (HOOCC6H4PO

(Sr(HOOCC6H4PO3H)2, 1), barium dihydrogen 4-carboxyphenylphosphonate (B

monohydrate (SrH(OOCC6H4PO3)$H2O, 3), barium hydrogen 4-carboxyphenylpho

rahydrate (Sr3(OOCC6H4PO3)2$4H2O, 5), and barium 4-carboxyphenylphosphona

stretch, ss¼ symmetric stretch, and b¼ bend).

identical with Ba3(OOCC6H4PO3)2$2H2O. The reaction inthis second step can therefore be described by the equation:

2BaHðOOCC6H4PO3Þ þ 2NH4OH

þ BaCl2 / Ba3ðOOCC6H4PO3Þ2$2H2O þ 2NH4Cl: ð8Þ

In conclusion, it was confirmed that Ba(HOOCC6H4-PO3H)2 and Ba3(OOCC6H4PO3)2$2H2O undergo mutual inter-conversions, depending on the acidity of the reaction media,via the intermediate with the formula BaH(OOCC6H4PO3).

3.3. Infrared spectroscopic study

The prepared barium 4-carboxyphenylphosphonates werefurther characterized by FTIR spectroscopy. In our previouspapers, we have studied syntheses and properties of phenyl-phosphonates and 4-carboxyphenylphosphonates of strontium[20,21]. We discussed the infrared spectra of strontiumphenylphosphonates and 4-carboxyphenylphosphonates, andcompared them with the theoretical infrared spectra of alldiscussed samples obtained from quantum chemical calcula-tions. The attention was concentrated on the positions of thebands connected with the presence of hydrogen atoms onthe PO3 and CO2 groups. The position of the acidic hydrogenatom in SrH(OOCC6H4PO3)$H2O was determined from the IRspectra of the studied compounds [21].

In this paper we employ the fact that the structures of oursamples are analogous to the structures of previously studiedstrontium 4-carboxyphenylphosphonates [21]. In Fig. 5, the

3H2, marked as CPPA), strontium dihydrogen 4-carboxyphenylphosphonate

a(HOOCC6H4PO3H)2, 2), strontium hydrogen 4-carboxyphenylphosphonate

sphonate (BaH(OOCC6H4PO3), 4), strontium 4-carboxyphenylphosphonate tet-

te dihydrate (Ba3(OOCC6H4PO3)2$2H2O, 6) (s¼ stretch, as¼ antisymmetric

Fig. 6. Final Rietveld refinement plot for Ba(HOOCC6H4PO3H)2. The

experimental data are represented in circles, and the calculated pattern is

shown as a solid gray line. The lower line is the difference curve between

observed and calculated pattern. The vertical lines at the bottom of the figure

are the positions of Bragg reflections. In the inset, the detailed Rietveld refine-

ment is given for 2q¼ 12e38�.

1539J. Svoboda et al. / Solid State Sciences 10 (2008) 1533e1542

spectra of barium dihydrogen 4-carboxyphenylphosphonate,Ba(HOOCC6H4PO3H)2, barium 4-carboxyphenylphosphonatewith the formula Ba(OOCC6H4PO3)2$2H2O and the spectrumof a new barium 4-carboxyphenylphosphonate with the for-mula BaH(OOCC6H4PO3) are compared with the spectra ofthe previously prepared samples of a-Sr(HOOCC6H4PO3H)2,SrH(OOCC6H4PO3)$H2O, Sr3(OOCC6H4PO3)2$4H2O, andwith the spectrum of 4-carboxyphenylphosphonic acid.

In the spectra of all samples, several absorption bands canbe assigned to the vibrations of phenyl ring: the CeH stretch-ing vibrations in the region of 3090e3000 cm�1 and thearomatic C]C stretching vibrations observed at about 1438cm�1. The CH and CeC out-of-plane bending vibrationappears as three absorptions at about 780, 720 and 700 cm�1

and peaks of the stretching mode of PeC6H5 are situated atabout 450 cm�1 in the spectrum of carboxyphenylphosphonicacid. All these vibrations are found very close to thesepositions in the spectra of all other samples.

We can see that the spectra of Ba(HOOCC6H4PO3H)2 anda-Sr(HOOCC6H4PO3H)2 are very similar to each other and tothe spectrum of 4-carboxyphenylphosphonic acid. In all ofthese spectra we observe a strong band of the stretching vibra-tion of the C]O bond of the carboxylic group at about1690 cm�1, with a shoulder at the higher-wavelength side ofthe spectrum due to the transition dipole coupling (TDC) ofthe carbonyl groups in the solid state [21]. This band ismissing in the spectra of the SrH(OOCC6H4PO3)$H2O andBaH(OOCC6H4PO3) intermediates, and also in the spectra ofBa3(OOCC6H4PO3)2$2H2O and Sr3(OOCC6H4PO3)2$4H2O,where two bands (at about 1585 and 1400 cm�1) characteristicof the carboxylic anion are observed [21]. Other bandscorresponding to the carboxylic group are: the OeH stretchingvibration at about 3000 cm�1, the overtone bands at about2678 and 2555 cm�1, the OeH in-plane bending vibration at1430 cm�1, the CeO stretching vibration at 1295 cm�1 andthe OeH out-of-plane deformation vibration at 938 cm�1

[34]. These bands are also missing in the spectra of intermedi-ate samples SrH(OOCC6H4PO3)$H2O and BaH(OOCC6

H4PO3), and also in the spectra of Ba3(OOCC6H4PO3)2$2H2Oand Sr3(OOCC6H4PO3)2$4H2O supporting the presence ofcarboxylic anion in their structure [21].

The symmetric and antisymmetric modes relative to RPO3

tetrahedra are situated in the spectral range 1300e850 cm�1

(Fig. 5). The bands, evident in the spectra of Ba(HOOCC6H4-PO3H)2 and a-Sr(HOOCC6H4PO3H)2 at about 1217 cm�1, canbe assigned to the OPO stretching vibration of the delocalizedP]O double bond [21]. The spectra of these samples and thespectra of the intermediate samples SrH(OOCC6H4PO3)$H2Oand BaH(OOCC6H4PO3) are very similar in this region indi-cating clearly the presence of the PO3H groups [20]. All thesesamples exhibit a strong band at about 939, 913, 902 and880 cm�1, assigned to PeOH stretching vibrations, indicatingalso the presence of PO3H groups [35]. These bands are miss-ing in the spectra of the samples Ba3(OOCC6H4PO3)$2H2Oand Sr3(OOCC6H4PO3)$4H2O, supporting the presence ofthe PO3

2� group [20]. The OePeO bending vibrations in theregion 600e400 cm�1 have also a similar shape as those found

for analogous barium and strontium 4-carboxyphenylphospho-nates [20]. The bands around 2750 and 2340 cm�1 character-istic of the OeH stretching frequencies of the monohydrogenphosphonate groups [14] are overlapped by the overtone bandsof the carboxylic group in the spectra of Ba(HOOCC6H4-PO3H)2, a-Sr(HOOCC6H4PO3H)2, and of the 4-carboxyphe-nylphosphonic acid. The presence of sharp and intensebands observed at about 3600 cm�1 in the spectra of SrH(OOCC6H4PO3)$H2O, Sr3(OOCC6H4PO3)$4H2O and also inBa3(OOCC6H4PO3)$2H2O indicates an occurrence of a singlewell-defined kind of a water molecule most probably coordi-nated to a metal [35].

3.4. Structure of Ba(HOOCC6H4PO3H)2

The structure of Ba(HOOCC6H4PO3H)2 was solved fromits X-ray powder diffraction pattern using a combination ofFOX (estimation of atomic coordinates) and FullProf (finalRietveld refinement) programs. The plotted output from theRietveld refinement is shown in Fig. 6. The basic crystallo-graphic data are given in Table 1. The atomic coordinatesand the selected geometric parameters are listed in Tables 2and 3, respectively.

The Ba atoms lie in the bc plane and are eight-coordinatedby the O11, O12 and O13 oxygen atoms forming distorted te-tragonal antiprisms (Fig. 7). The BaeO distances vary from2.81 to 2.84 A, which is in the range 2.63e2.83 A observedfor another barium phosphonate, [Ba3(O3PCH2NH2CH2-PO3)2(H2O)4]$3H2O, whose structure was characterized bysingle crystal diffraction [36]. The Ba atoms are connectedthrough the phosphonate groups lying above and below theBa atoms’ plane. Each phosphonate group is bonded to oneBa atom through two oxygen atoms (O11 and O13) and to

Table 1

Crystallographic data and structure refinement parameters for Ba(HOO

CC6H4PO3H)2

Formula Ba(HOOCC6H4PO3H)2

Formula weight, g mol�1 539.5

Calculated density, g cm�3 1.907

Crystal system Monoclinic

Space group C2/c (No. 15)

a, A 49.382(1)

b, A 5.5196(1)

c, A 8.4977(2)

b, � 127.52(1)

V, A3 1837.06(7)

Z 4

2q range, � 3.5e90

T, �C 25

l, A 1.5418

Rpa 12.8

Rwpa 16.0

RBa 3.82

RFa 3.41

a Criteria of fit as defined in Ref. [38]. The values are given in %.

Table 3

Selected bond distances (A) and angles (�) with their standard deviations for

Ba(HOOCC6H4PO3H)2a

BaeO11 2.814(12) BaeO12 2.84(1)

BaeO11 2.680(15) BaeO13 2.841(12)

C1eC2 1.406(10) P8eO11 1.503(15)

C2eC3 1.412(17) P8eO12 1.501(13)

C3eC4 1.410(17) P8eO13 1.484(18)

C4eC5 1.411(10) C7eO10 1.237(16)

C5eC6 1.425(16) C7eO9 1.347(18)

C3eC7 1.523(10) C6eP8 1.862(8)

O11eP8eO12 112.87(72) O13dBadO11A 121.06(41)

O12eP8eO13 110.7(7) O13eBaeO11B 70.86(43)

O11eP8eO13 109.48(81) O13eBaeO11C 79.02(33)

O11eBaeO11A 73.32(36) O13eBaeO12A 64.07(26)

O11eBaeO11B 114.72(35) O13eBaeO13A 80.01(32)

O11eBaeO11C 115.41(33) O11AeBaeO11B 165.90(39)

O11eBaeO12 29.44(26) O11AeBaeO11C 114.72(35)

O11eBaeO12A 87.35(26) O11AeBaeO12A 99.50(28)

O11eBaeO13 51.09(33) O11AeBaeO13A 70.86(43)

O11eBaeO13A 79.02(33) O11BeBaeO11C 73.32(36)

O12eBaeO11A 92.61(28) O11BeBaeO12A 92.61(28)

O12eBaeO11B 99.50(28) O11BeBaeO13A 121.06(41)

O12eBaeO11C 87.35(26) O11CeBaeO12A 29.44(26)

O12eBaeO12A 61.75(20) O11CeBaeO13A 51.09(33)

O12eBaeO13 28.64(27) O12AeBaeO13A 28.64(27)

O12eBaeO13A 64.07(26)

a See Fig. 7 for the atom marks.

1540 J. Svoboda et al. / Solid State Sciences 10 (2008) 1533e1542

the other Ba atom through the remaining oxygen atom (O12),as shown in Fig. 8. In the c direction, the connection of theneighboring Ba atoms is made through two O11 atoms, whichare therefore m3 atoms, being coordinated by two bariumatoms and a phosphorus atom. The Ba ions thus form zigzagchains of edge-sharing polyhedra in the c axis direction. Thesezigzag chains are connected to each other by O12 atoms. Insuch a way, a layer in the bc plane, composed of the Ba, O,and P atoms, is built up. From this layer the 4-carboxyphenylgroups jut out above and below the bc plane are shown inFig. 9. These groups are slightly tilted with regard to the bcplane. The benzene rings are oriented parallel to each otherin a row along the b direction, whereas in the c direction theplanes of two adjacent benzene rings are roughly perpendicu-lar to each other. From the distance of the O9 and O10 oxygenatoms from two nearest carboxyl groups of the neighboringlayers (2.60 A) the presence of a hydrogen bond can bepresumed. This O]CeOeH/O]CeOeH hydrogen bond

Table 2

Positional parameters and Wyckoff positions for Ba(HOOCC6H4PO3H)2

Atom Wyck. x y z

Ba 4e 1/2 0.8936(4) 1/4

C1 8f 0.3885(2) 0.110(3) 0.580(2)

C2 8f 0.3536(2) 0.075(2) 0.427(2)

C3 8f 0.3316(2) 0.249(2) 0.417(2)

C4 8f 0.3410(2) 0.453(2) 0.540(2)

C5 8f 0.3761(2) 0.463(2) 0.701(2)

C6 8f 0.3997(2) 0.281(2) 0.736(2)

C7 8f 0.2958(2) 0.257(2) 0.220(2)

P8 8f 0.446(1) 0.338(1) 0.9321(8)

O9 8f 0.2745(2) 0.402(2) 0.226(2)

O10 8f 0.2870(2) 0.084(2) 0.106(2)

O11 8f 0.4655(2) 0.166(2) 0.898(2)

O12 8f 0.4527(2) 0.599(2) 0.917(2)

O13 8f 0.4542(3) 0.288(2) 1.128(2)

Fig. 7. Fragment of Ba(HOOCC6H4PO3H)2, viewed down the b axis, showing

the coordination about the Ba atom and the numbering scheme used in the

tables.

Fig. 8. Structure of Ba(HOOCC6H4PO3H)2 viewed towards the (100) plane.

The 4-carboxyphenyl groups are omitted for clarity.

1541J. Svoboda et al. / Solid State Sciences 10 (2008) 1533e1542

system in the interlayer space contributes to the stabilizationof the crystal structure.

From the comparison of the structure of Ba(HOOCC6H4-PO3H)2 with the structures of Ca(C6H5PO3H)2 [37], Sr(C6

H5PO3H)2 [30], Ba(C6H5PO3H)2 and Pb(C6H5PO3H)2 [14]we found that these compounds are isomorphous in their metalphosphonate parts. Thus, this structural pattern seems to betypical for metal hydrogen organophosphonates in which themetal atom is eight-coordinated.

It is worth noting the difference between this barium com-pound and the previously reported analogous calcium [19] andstrontium [20] compounds. In these compounds, the Ca and Sratoms are also eight-coordinated by the oxygen atoms of thephosphonate groups creating structural motifs similar to thosefound in Ba(HOOCC6H4PO3H)2. The smaller CaeO (2.29e2.84 A) and SreO (2.51e2.71 A) distances do not allow aninterconnection in both b and c directions and thus theCa(Sr), P, and O atoms create only one-dimensional ribbons.

Fig. 9. Layer arrangement of Ba(HOOCC6H4PO3H)2 as viewed along the

b axis.

On the other hand, the greater BaeO distance in Ba(HOOCC6H4PO3H)2 allows two-dimensional interconnections ofthe Ba, P, and O atoms in the bc plane.

4. Conclusions

The composition of barium 4-carboxyphenylphosphonatesdepends on the acidity of the medium in which the reactionproceeds. In an acidic environment, Ba(HOOCC6H4PO3H)2

is formed whereas Ba3(OOCC6H4PO3)2$2H2O can be synthe-sized under basic conditions. An intermediate with formulaBaH(OOCC6H4PO3) can be obtained during the interconver-sions of Ba(HOOCC6H4PO3H)2 and Ba3(OOCC6H4PO3)2$2H2O. These interconversions are the reason why an attemptto intercalate basic guests, for instance amines, into Ba(HOOCC6H4PO3H)2 or BaH(OOCC6H4PO3) failed due to theirtendency to form Ba3(OOCC6H4PO3)2$2H2O.

It follows from the infrared spectra of these compounds thatin the intermediate the acidic hydrogen is present at thephosphonate group rather than at the carboxylic group.

The structure of Ba(HOOCC6H4PO3H)2, as determinedfrom its X-ray powder diffraction pattern, shows that this com-pound is a layered one, with the carboxylic groups pointingabove and below the layers which are formed from phosphonategroups and barium atoms. The structure of Ba(HOOCC6H4

PO3H)2 is stabilized by hydrogen bonds between adjacentcarboxylic groups. Its BaeO3P part has characteristic structuralfeatures similar to those found in several other metal hydrogenarylphosphonates.

We were not able to determine the structure of Ba3

(OOCC6H4PO3)2$2H2O and BaH(OOCC6H4PO3), neverthe-less the shape of the X-ray diffraction patterns and the basalspacings around 10.5 A indicate that these compounds arelayered with the carboxyphenyl fragments interdigitated inthe interlayer space with bonds between barium atoms andboth the phosphonate and carboxylic groups.

The computer-controlled addition of reagents has beenshown to be a successful method for the study of chemicalbehavior and the preparation of new arylphosphonates of alka-line earth metals, as shown in this and our previous paper. Animplementation of this method to phosphonates of othermetals is under way.

5. Supplementary information

The supplementary material regarding the structure ofBa(HOOCC6H4PO3H)2 has been sent to Cambridge Crystal-lography Data Centre (CCDC), 12 Union Road, Cambridge,CB2 1EZ, UK, and can be obtained by contacting the CCDC (deposition number: CCDC 670128).

Acknowledgment

This work was supported by the Grant Agency of the CzechRepublic (GA 203/06/P127).

1542 J. Svoboda et al. / Solid State Sciences 10 (2008) 1533e1542

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