Synthesis and crystal structures of two metal phosphonates, M(HO,PC,H,), (M = Ba, Pb) Damodara M. Poojary,' Baolong Zhang," Aurelio Cabeza,' Miguel A. G. Aranda,b Sebastian Bruque' and Abraham Clearfield*" 'Department of Chemistry, Texas A & M University, College Station, TX 77843, USA 'Departamento de Quimica Inorganica, Cristalografia y Mineralogia, Universidad de Malaga, 29071-Malaga, Spain Divalent metal phosphonates, Ba( H03PC6H5), and Pb( H03PC6H5)', have been synthesized and structurally characterized [crystal data: a=32.18( l),b=5.546(4), c=8.495(4) A,p= 103.21(3)", space group C2/c and 2=4 for Ba(H03PC6H5)2; a= 31.8302( lo), b =5.5997(2), c =8.2935( 3) A,p= 101.875(2)", space group C2/c and 2=4 for Pb(Ho3Pc6HS),]. Their structures are isomorphous. The structure of the barium compound was solved from single-crystal data which was then used to refine the structure of the lead compound by Rietveld methods.In these compounds the metal :phosphonate ratio is 1:2 and the phosphonates use all their oxygens to bridge the metal atoms, which are arranged in two-dimensional layers. One of the phosphonate oxygens is protonated. The phosphonate oxygens are involved in both chelation and bridging interactions. The metal atoms are eight-coordinate; four of the binding sites are due to symmetry-related positions of a single oxygen atom and two each from the remaining two oxygen atoms. Layered metal phosphonate compounds have attracted a great deal of research activity in recent years.'-4 These compounds have potential applications in sorption, catalysis and ion exchange.The layered metal phosphonates can also be easily pillared to yield materials similar to pillared clays, which are amenable to incorporation of specific and selective species and a wide variety of materials properties.'*' Recently it was shown that surface-bound multilayer metal phosphonate structures, analogous to Langmuir-Blodgett films, can be grown through stepwise absorption of metal ions and bisphosphonic acid on metal substrates., Earlier work on metal phosphonates concen- trated on zirconium compounds of composition Zr (O,PR), .2,6 The layered structure of these compounds is built up by oxygen-bridged metal octahedra which are separated by the hydrophobic regions of the organic moieties.The phosphonate groups lie above and below the plane of metal atoms. Recently, studies have shown that a variety of metal ions, including Group 4 and 14 element^,^ divalent'-" and tri~alent'','~ ions form several variations of this type of layered compound. The phosphonate oxygens in these compounds act as unidentate, chelating or bridging ligands depending on the metal :phos-phonate ratio and the coordination requirement of the metal involved. Note also that a variety of organic groups with or without active moieties can be organized in between the inorganic layers of the layered compounds, which is important in designing materials for specific physical properties. The recent discovery of porous structures in metal phosphonate compounds further intensified research activities in this Only a few studies of alkaline-earth-metal phopshonates have been carried out.Magnesium phosphonatesZ2 of the type Mg(03PR)*H,0 have structures similar to the now well known zinc' or rnangane~e'~ phosphonates. Magnesium was also shown to form a different structure type containing [Mg(Hz0)6]2f ions, when the phpsphonic acid is too bulky to fit in the space required (28 A') by the more common layered type compound.22 Calcium was found to form two different layered structures.'* The calcium phosphonates of the type Ca(HO,PR), have a structural motif very similar to the layer structure of zirconium ph~sphonate.~They contain charge-balancing protons bonded to the phosphonate oxygens, making the phosphonate ligand: metal ratio 2: 1, as in the zirconium or titanium compounds.Calcium methylphosphon- ate, Ca(O,PCH,)-H,O is monoclinic, space group P2,/c. The Ca atom is seven-coordinate in the form of a peatagonal bipyramid wit! six Ca-0 bond distances of 2.4A and a seventh of 2.7 A." To gain further insight into the chemistry of alkaline-earth-metal phosphonates we have isolated the barium phenylphosphonate whose synthesis and structure are presented in this paper. We also describe the structural charac- terization of lead@) phenylphosphonate, a first example of a non-transition metal, non-alkaline-earth divalent metal phos- phonate. Interestingly, its structure is isomorphous to the barium compound.Experimental Materials and methods Chemicals of reagent quality were obtained from commercial sources and were used without further purification. Thermal analysis was carried out on a Du Pont thermal analysis unit (model no. 951) and Rigaku Thermoflex apparatus at a heating rate of 10 K min-'. IR spectra were recorded on Digilab Model FTS-40 FTIR and Perkin Elmer 883 spectrometers in the spectral range 4000-400 cm-', using the KBr disk method. Synthesis of barium phenylphosphonate, Ba (HO3PC6H5)2 Phenylphosphonic acid (1.6 g) was first dissolved in deionized distilled water to which 2.5 g of BaCl, .2H20 was added. The cloudy solution thus formed was heated at reflux for one day in a mineral oil bath. The solid formed was filtered off, washed and air dried.Analytical data: C, 31.97; H, 2.63. Calc. for Ba(H0,PC6H5),: c, 31.90; H, 2.66%. Synthesis of lead phenylphosphonate, Pb(HO3PC6H5)2 Lead hydrogen-phenylphosphonate was synthesized by slow addition of a solution of lead acetate (0.1 mol drn-,) over a solution of phenylphosphonic acid (1 mol dm-3) to a final P :Pb molar ratio of 12 :1. The precipitate was heated at reflux for 30 days. The resulting solid was filtered off, washed with water, and air dried. This compound may also be obtained by hydrothermal synthesis. A suspension (prepared as previously) was stirred at 200°C for 4 days in a PTFE-lined reactor at J. Muter. Chem., 1996, 6(4), 639-644 639 autogenous pressure The two synthetic methods led to the same product but single crystals were not obtained A shorter reaction time led to the same product but with slightly lower crystallinity The sample was dissolved in a solution containing an equimolar ratio of HNO, (67% m/m) and H202 (30% m/m) Then, the lead content was determined by atomic absorption spectroscopy (AAS) Carbon and hydrogen con-tents were determined by elemental chemical analysis in a Perkin Elmer 240 analyser The phosphorus content was deduced from the carbon percentage found, assuming a C P molar ratio of 6 1 The results were Pb, 38 0, P 11 67, C, 27 11, H, 2 39 Calc for Pb(H03PC,H,)2 Pb, 39 73, P, 11 90, C, 27 65, H, 2 30% IR spectroscopic study The Ba and Pb compounds display similar characteristics in their IR spectra The spectrum of Pb(Ho,PC,H,), is shown in Fig 1 There are no bands in the 0-H stretching region (3500-3200 cm-') which is consistent with the absence of water molecules in the structure The C-H stretching vibrations of the phenyl ring are present near to 3000crn-' The bands around 2750 and 2340cm-' are characteristics of the 0-H stretching frequencies of the monohydrogen phos- phonate groups The C-C aromatic stretching bands appear near to 1400 cm-' The intense band at around 1160 cm-' is due to the P-C stretching vibrations and the strong bands in the region of 1OOOcm-' are due to the P-0 stretching vibrations of the tetrahedral CP03 group The bands at 695 and 750 cm-' are characteristic of the out-of-plane (monosub- stituted) phenyl ring vibrations Thermal analysis study A thermogravimetry (TG) curve for the barium compound is shown in Fig 2(u) The compound loses 4% of its mass at about 400"C, corresponding to the release of one water molecule due to the condensation of hydrogen phosphonate groups (calculated mass loss 3 990/) The release of the organic group starts around 500°C and is complete at 600°C except for some residual carbon The observed mass loss (3229%) I Fig.1 IR spectrum of Pb(H03PC6H,), 1 I I I I I 11 100 300 500 700 T1°C Fig. 2 (a)TG curve for Ba(H03PC6H5)2 and (b)TG and DTA curves for Pb(HO,PC,H,), agrees well with a calculated value of 34 1% The total mass loss up to 1000°C is 35% which is consistent with the theoretical value of 34 6%, calculated for the conversion of Ba(HO,PC,H,), to Ba(P03)2 The TG curve for Pb(HO,PC,H,), is shown in Fig 2(b) As in the case of the Ba compound, the Pb compound shows two distinctive mass losses in its TG curve, due to the release of a water molecule and the organic groups The release of water in this case, however, takes place at a lower temperature (257 "C) when compared to that from the Ba compound The observed mass loss of 3 5% is in excellent agreement with a calculated value of 345% The second mass loss, due to the release of the phenyl groups, can be seen at around 550°C The observed mass loss for this process is 27 7% (calculated mass loss 2955%) The final thermal decomposition product is an amorphous glass of composition Pb( The overall mass loss, 31 2%, agrees well with the theoretical loss of 29 93% for the decomposition of Pb( HO,PC,H,), to Pb( Note also the thermal behaviour of the Pb compound, as seen in its DTA curve [Fig 2(b)] The curve shows no exotherm and the temperature does not increase abruptly as is usual for the combustion of organic matter There is an endotherm centred at 275 "C with an associated mass loss of 3 5% due to the release of the water molecule During the second mass loss two endotherms take place The first endotherm, centred at 450"C, which is very sharp, is probably due to a phase transition The second endotherm, centred at 550°C, is very broad and intense, and is due to the release of the organic matter (phenyl groups), but not due to combustion 640 J Muter Chem, 1996, 6(4), 639-644 X-Ray structure analysis of Ba (HO,PC,H,), A colourless plate-like crystal with dimensions 0.30 x 0.30 x 0.015 mm3 was mounted on a glass fibre.Most of the crystals scanned were twinned and showed broad reflection profiles. The crystal used for data collection was, however, free from twin contribution. All crystallographic measurements were carried out on a Rigaku AFC5R diffractometer with graphite-monochromated Mo-Ka radiation (A=0.71069 A) and a 12 kW rotating anode generator operated at 50 kV and 180 mA. Cell parameters for data collection were obtained from least-squares refinement of 24 reflections chosen from the 28 =8-35' shell immediately preceding data collection.Intensity data were collected at -110°C using w28 scan mode in shells to a maximum value of 28 =50". Three intensity standards were measured every 150 reflections to monitor crystal decay. Scans of (1.84-tO.3 tan 0)"were made at a rate of 16 degrees min-' in co. The weak reflections [I <10.00(1)] were rescanned (maximum of 3) for good counting statistics. Of the 1491 reflections which were collected, 1462 were unique (Rin,=0.015). A total of 1179 reflections were observed with 1 >341). The data were corrected for Lorentz and polarization effects. Pertinent crystallographic data are presented in Table 1. The structure was solved by the Patterson method.23 The position of the heavy atom was located by deconvolution of the Patterson function.Other non-hydrogen atoms were found in the successive difference Fourier maps. The hydrogen-atom positions were calculated on the basis of geometrical consider- ations and included in the least-squares refinement, but were not refined. The final cycle of full-matrix refinement was based on 1179 reflections and 96 structural parameters. An empirical absorption correction, based on a $-plot, was applied which resulted in transmission factors ranging from 0.36 to 1.0. The weighting scheme was based on counting statistics and included a factor (p=0.03) to downweight the intense reflections. The maximum and minimum peaks on tFe final difference map corresponded to 1.63 and -2.73 e AP3 respectively. These residuals were found near the heavy atom.Neutral scattering factors were taken from Cromer and Waber.24 Final atomic and isotropic thermal parameters are given in Table 2; bond lengths and angles in Table 3. Coordination about the metal atom along with atom labelling is given in Fig. 3. Arrangement of atoms in the layer and the packing down the b axis are shown in Fig. 4 and 5, respectively. Data collection and structure refinement of Pb(HO3PC6H5), Diffraction data for the powder were collected with a Siemens D-501 automated powder diffractometer using graphite-mon- ochromated Cu-Ka radiation. The powder pattern of the sample showed a very high degree of preferred orientation. To reduce this effect on the data, the sample was mixed with Table 1 Crystallographic data for Ba( HO3PC,H5), formula mass 451.51 cqstal system monoclinic 44 32.18( 1) bl+ 5.546 (4) CIA 8.495 (4) PldFgrees 103.21 (3) VIA3 1476(2) Z' 4 space group c2/c Dc[g cm-3 2.04 i/A 0.71069 T/"C -110 absorption coeff/cm-' 29.24 observed data [I >3.00(1)] 1179 no.of parameters 96 R(Fd 0.040 RW(F0) 0.050 GOF 2.6 Table 2 Positional parameters and B(eq) for barium phenyl-phosphonate 0 0.10236(8) 0.25 1.21(2) -0.06690( 5) -0.0554( 2) -0.0582( 2) -0.0423(1) -0.1228( 2) -0.1528( 2) -0.1961(3) -0.2096( 2) -0.1804( 3) -0.1370( 2) -0.1897 -0.3341(3) -0.2973(8) -0.6084( 7) -0.1706( 7) -0.291(1) -0.446( 1) -0.412( 1) -0.224( 2) -0.070( 1 ) -0.101(1) 0.0594 0.0644( 2) 0.2445( 5) 0.0332( 5) -0.0212(5) -0.0173( 7) 0.0227(9) -0.043( 1) -0.147( 1) -0.187( 1) -0.1228( 9) -0.2598 1.25(6) 1.9(2) 1.7(2) 1.5(2) 1.6(2) 2.4( 3) 3.2(3) 3.1(3) 3.1(3) 2.3( 3) 3.8 -0.1170 0.0089 -0.1496 2.7 -0.1433 -0.5780 0.0957 2.9 -0.2164 -0.5222 -0.0142 3.8 -0.2396 -0.2007 -0.1932 3.8 Table 3 Bond lengths(& and angles(degrees) for Ba( H03PC,H,), Ba( 1)-O( 1) 2 x 2.839(5) C( 1)-C(2) 1.39( 1) Ba(1)-0(2) 2 x 2.811(4) C(2)-C(3) 1.39(1)Ba( 1)-0(3) 2 x 2.641(4) C(3)-C(4) 1.38( 1) Ba(1)-0(3)2 x 2.832(4) C(4)-C(5) 1.37( 1) P(1)-O( 1) 1.504(4) C(5)-C(6) 1.39(1)P(1)-0(2) 1.580(4) C(6)-C( 1) 1.39(1)P(1)-0(3) 1.498(4) P( 1)-C( 1) 1.791(7) O(1)-Ba( 1)-O( 1) 77.3(2) O(i)-P( 1)-O(2) 106.8 (3) O(1)-Ba( 1)-0(2)2 x 1414 1) O(1)-P( 1)-0(3) 112.2( 3) O(1)-Ba( 1)-0(2)2 x 96.5( 1) O(1)-P( 1)-C( 1) 111.5( 3) O(1)-Ba( 1)-O( 3)2 x 121.8( 1) O(2)-P(1)-O(3) 11 1.6( 2) 0(1)-Ba(l)-O(3)2x 72.4(1) O(2)-P( 1)-C( 1) 105.4(3) 0(1)-Ba(l)-O(3)2x 52.1(1) O(3)-P( 1)-C( 1) 109.1(3) O(1)-Ba( 1)-0(3)2 x 77.2( 1) C(2)-C(l)-C(6) 118.9(6) O(2)-Ba( 1)-0(2) 110.4(2) C( 1)-C( 2)- C( 3) 120.3( 7) O(2)-Ba( 1)-0(3)2 x 91.3( 1) C( 2)- C( 3)- C( 4) 120.1 (7) O(2)-Ba( 1)-0(3)2 x 79.3( 1) C(3)-C(4)-C( 5) 120.0( 7) 0(2)-Ba(l)-0(3)2 x 164.3(1) C(4)-C( 5)-C( 6) 120.8( 7) O(2)-Ba( 1)-0(3)2 x 69.4( 1) C( 1)-C(6)-C(5) 119.9( 7) O(3)-Ba( 1)-0(3) 163.5(2) O(3)-Ba( 1)-0(3)2 x 73.1(2) O(3)-Ba( 1)-0(3)2 x 116.3(2) O(3)- Ba( 1)- O(3) 11 5.4(2) 02c 02b Fig.3 Coordination geometry about the metal atom (Ba, Pb) with atom labelling spherical particles of silica, Cab-oil M-5 (from Fluka), with an approximate size range of 12-45 nm.25 The pattern was scanned over the angular range 28= 12-loo", with a step size of 0.03" at a rate of 15 s per step.The data were transferred to a VAX computer for Rietveld analysis by the GSAS suite of programs.26 The powder pattern was auto-indexed by using the Lattparm program27 based on the Visser algorithm,28 from the position of the first 2,O reflections. The result wasoa monoclinic unit cell [~=31.1$1 A, b=5.592 A, ~=8.285 A, p=93.239", Vr 1441.23 A3, Z=4, and V(non-hydrogen atom)= 17.15 A3 J. Muter. Chem., 1996, 6(4), 639-644 641 Fig.4 Arrangement of the inorganic framework in the layers The carbon atoms are represented by small circles, oxygens by large open circles, P by hatched circles and metal atoms by filled circles Other atoms of the phenyl groups are omitted for clarity Fig.5 Projection of the structure down the b axis showing the arrangement of phenyl groups in the interlayer space atom-1] with a figure of merit M,, of 4429The systematic absences were consistent with the non-standard monoclinic space group 12/u This unit cell was transformed to the standardosetting C2/5 and the oresulting unit-cell dimensions (a=31 80 A, b=5 59 A, c=829 A, p=lOl 92") are very similar to those of Ba( H03PC6H5)2 given above Rietveld refinement was started with the atomic positions of the Ba compound as starting model in space group C2/c After the initial refinement of the overall parameters (scale factor, unit-cell parameters, zero-point error, background function, preferred orientation coefficient and peak-shape parameters), the atomic positions were refined with soft constraints consisting of P-C and C-C bonds, with values of 180(2) and 140(1)A, respectively Finally, one thermal vibration parameter was refined with one parameter for each type of atom The final refinement con- verged with RWP=5 7%, R, =4 O%, and RF=1 8% Positional parameters are given in Table 4, bond parameters in Table 5 and the final observed, calculated and difference profiles are given in Fig 6 642 J Muter Chem, 1996, 6(4), 639-644 Table 4 Atomic positions and thermal parameters for Pb( H03PC6H5)2" atom X Y Z u,,,/A2 0 00 0 0774( 2) 0 25 0 0173(3) -0 0647(2) -0 3267(8) 0 0746( 7) 0 010( 2) -0 0554(3) -0 2736( 15) 0 2569( 12) 0 0086(20) -0 0552(3) -0 5965(22) 0 0449( 10) 0 0086 -0 0406( 3) -01708(16) -00147(11) 0 0086 -0 1213(2) -0 2953(25) -0 0093( 18) 00133(25) -0 1509(4) -04609(20) 0 0280( 16) 0 0133 -0 1942(4) -0 41 57( 27) -0 0404( 20) 0 0133 -0 2083(4) -0 2236(27) -0 1460(21) 0 0133 -0 1777(4) -0 0623( 25) -0 1811(16) 0 0133 -0 1340(4) -0 1007( 25) -0 1142( 18) 0 0133 a Crystal datao space group C2/c, a=31 8302( A,b=5 5997(2) A, c=8 2935(3) A, /I=101 875(2)' V= 1446 60( 12) A3 and 2=4 Table 5 Bond lengths (A)and angles (") for Pb(H03PC6H5)2 Pb-O( 1) 2x 2650(8) Pb-O(2) 2x 2843(10) O(1)-Pb-O( 1) 84 2(4) Pb-0(3) 2x 2605(9) O(1)-Pb-0(2)2 x 142 7(3) Pb-0(3) 2x 2693(9) 0(1)-Pb-O(2)2~ 99 l(3) O(1)-Pb-0(3)2 x 126 9(3) 0(2)-Pb-0(3)2 x 162 3(3) 0(1)-Pb-O(3)2~ 724(3) 0(2)-Pb-O(3)2~ 73 2(3) O(1)-Pb-0(3)2 x 54 9(3) 0(3)-Pb-0(3) 156 8(4) 0(1)-Pb-0(3)2 x 79 O(3) 0(3)-Pb-0(3) 1179(4) 0(2)-Pb-0(2) 100 l(4) 0(3)-Pb-0(3)2 x 1190(3) 0(2)-Pb-0(3)2 x 88 8(3) 0(3)-Pb-O(3)2~ 73 8(3) 0(2)-Pb-0(3)2 x 76 3(2) P-O(1) 1 510( 10) O(l)-P-0(2) llOO(6) P-O(2) 1571(11) O(l)-P-0(3) 1120(6) P-0(3) 1461(9) O(1)-P-C(1) llOO(7) P-C( 1) 1 800(22) 0(2)-P-0(3) ll09(6) 0(2)-P-C(1) 104 l(6) 0(3)-P-C(1) 109 5(7) 20 30 40 50 60 28/10 degrees Fig.6 A portion of the observed and calculated profiles for the Rietveld refinement for Pb(HO,PC,H,), The lower curve is the difference plot on the same intensity scale Results and Discussion The crystal structures of barium and lead phenylphosphonates are isomorphous The structures are layered with the metal atoms (Ba, Pb) located on the two-fold axis and lie in the bc plane at x=O and 1/2 The coordination about the metal atoms is shown in Fig 3 together with the numbering scheme used in the tables The phosphonate group P(l) chelates the metal atom through 0(1) and O(3) There are two such groups related by symmetry (two-fold axis) bonded to the same metal atom The third oxygen of each phosphonate group bridges to the adjacent metal atom along the b direction (Fig 4) To complete the coordination sphere, each O(3) donates an electron pair to the adjacent metal atoms along the c direction In the bc plane the metal atoms are separated by about 5.6 A along the b direction and 4,4A in the c direction. Along the c axis they are arranged in the form of a zigzag chain as shown in Fig.4. Adjacent atoms in this chain are bridged by oxygen atoms [0(3)] which creates four-membered rings. Oxygens 0(1) and O(2) are involved in binding to a single metal atom while O(3) binds to two metal atoms. Thus, 0(1) and O(2) provide two binding sites each to the metal while O(3) provides four sites leading to eight- coordination for the metal atoms in a distorted dodecahedra1 geometry, The Ba-0 distances range between 2.641(4) and 2.839(5) Ab Among the four Ba- O(3) distancesotwo are shorter [2.641(4)A] than the other two [2.832(4)A]. The longer bond lengths correspond to atoms [0(3) and O(3a) in Fig.31 that are involved in chelation to the metal atom. This type of long and short bond distances are common in metal phosphon- ates where phosphonate oxygens are involved in both chelation and bridging. For example, in the structure of Zn(O,PCH,Cl)," the two longest bonds are those that form the chelate ring, and the two shortest are those formed by the same oxygens donating to adjacent zinc atoms. The Ba-0 bond lengths with the :ther two oxygens, O(1) and 0(2), are 2.839(5) and 2.811(4) A respectively. The P-0 bond length (Table 2) jnvolving the O(2) atom is significantly longer [1.580(4)A] than the other two P-0 distances [1.501(4)A], indicating that the proton is bonded to the O(2) atom. The phenyl groups display a regular planar geometry with normal bond parameters.In the case of the lead compound, the Pb-O(1) and Pb-0(2) distances are 2.650(8) and 2.84( 1)A respectively. The twoo types of M-0(3) bond lengths [2.605(9) and 2.693(9) A] have much more similar values in this case when compared to the Ba structure. Again, the proton is bonded to the O(2) atom in the Pb compounds oas indicated by the greater P-0(2) bond length [1.57(1)A]. The geometry of the phosphonate group is regular. The arrangement of the organic groups in the interlayer space is shown in Fig. 5. Th,e phenyl groups are separated by one unit cell or about 5.6A along the b axis. The adjacent rings along the c direction are not in the same plane, instead they are shifted by half a unit along the unique axis, b.This fact is best illustrated in Fig. 4 where the carbon atom [C( l)] bonded to phosphorus is represented by a small circle. The phenyl groups of one layer are pointing towards the centre of two neighbouring phenyl groups of the adjacent layers, as shown in Fig. 5. The phenyl groups are tilted away from the normal to the metal oxygen plane by about 30". The arrange- ment of phenyl groups with respect to the metal-phosphonate layers is similar to that observed for zirconium phenylphos- phonate compo~nd,~ Zr(o3PC6Hs),, that also crystallizes in space group C2/c. However, in the Zr compound the metal atoms are six-coordinate and all the oxygen atoms are bonded to a single metal atom. The Zr atoms in the plane are arranged in a regular hexagonal manner and the P atoms lie above and below this plane.This arrangement of metal and P atoms is slightly distorted in the Ba and Pb compounds owing to the geometrical requirements for higher coordination number for the metals. Nevertheless, there are six metal atoms surrounding each metal atom as in the Zr compounds but they do not occupy the corners of a regular hexagon. Similarly, there are six phosphonate groups about each of the metal atoms, among which three are above the plane and the other three are located below the plane. All of the 1:2 metal phosphonates cited above have unit- cell dimensions within the plane that are very similar. In this regard they mimic the parent compound a-zirconium phos- phate, Zr( HP04), H2O3' whose cell dimepions within the layer are a =9.6060(2) and b=5.297(1)A.The interlayer dimension is variable and depends upon the nature of the organic pendant group and its orientation to the plane. To the best of our knowledge the lead phenylphosphonate represents the first example of a non-transition, non-alkaline- earth-metal layered phosphonate to be structurally charac- terized. Among the alkaline-earth-metal compounds, as already mentioned, two calcium phosphonate structures have been reported,15 one of which is that of Ca(O,PCH,)*H,O, the other is of Ca(HO3PC6HI3),. The former crystallizes in the monoclinic space group P2,/c, with a =8.8562( 13), b= 6.6961( lo), c=8.1020( 10)A, and p=96.910( 11)'.The structure is layered, and the phosphonate oxygens are involved in both chelation and bridging. The calcium atoms have an approxi- mately pentagonal-bipyramidal coordination. In terms of met-al :phosphonate ratio, the compound Ca( HO$C6H13)2 resembles very closely the Ba structure presented here, although the organic group in the Ca structure is a primary alkane. This calcium compound crystallizes in the sFace group Pi, with a= 5.606(2), b= 7.343(3), c =21.158(7) A, a =97.31(3), ,8=96.98(3), y= 90.43(4)". Again, this structure is layered and the phosphonate oxygens are involved in both bridging and chelation, as in the Ba and Pb compounds. Unlike the title compounds, the calcium atoms in this structure are octa-hedrally coordinated.One further difference between the pre- sent structures and this Ca structure is that the phosphonate oxygen carrying the proton does not coordinate to the Ca atom. The alkyl chains are oriented towards the interlayer space, as the phenyl groups in the Ba and Pb compounds. We thank the National Science Foundation (grant no. DMR- 9107715) and DGICYT (Ministerio de Educacion y Ciencias, research project PB-93/1245) for financial support. References 1 A. Clearfield, in Design of New Materials, ed. D. L. Cocke and A. Clearfield, Plenum, New York, 1986. 2 A. Clearfield, Comments Znorg. Chem., 1990,10, 896. 3 G. Cao, H. Hong and T. E. Mallouk, Acc. Chem. Res., 1992, 25, 420. 4 M. E. Thompson, Chem. Muter., 1994,6,1168. 5 M. B.Dines and P. DiGiacomo, Inorg. Chem., 1981, 20, 92; M. B. Dines, P. DiGiacomo, K. P. Callahan, P. C. Griffith, R. Lane and R. E. Cooksey, in Chemically Modijied Surfaces in Catalysis and Electrocatalysis, ed. J. Millar, ACS Symp. Ser. 192, American Chemical Society, Washington, DC, 1982,p. 223. 6 G. Alberti, U. Costantino, S. Allulli and N. Tomassini, J. Znorg. Nucl. Chem., 1978, 40, 113; G. Alberti and U. Costantino, in Inclusion Compounds, ed. J. L. Atwood, J. E. D. Davis and D. D. MacNicol, Oxford University Press, London, 1991, vol. 5, ch. 5. 7 D. M. Poojary, H-L. Hu, F. L. 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