Chem. Mater. 1992,4,661-665
cancy concentration than by the increasing K+ ion concentration. The low K+ occupancy of unsubstituted K6Sb$20m phase4was ascribed as one of the major factors responsible for the high conductivity of the KsSbsP20zo phase compared to the other potassium phosphatoantimonate compounds with different structures.1°
0.70 0.65
-
0.60 -
0.55
I
0.50
2-
661
0
0.45 0.40
1
a
0.0 0.50.00
0.05
0.10
0.15
Conclusions The ionic conductivity of K5Sb6P20zocan be enhanced by substituting small amounta of Ti", Mom, and Wn ions for Sbv ions. The increase of conductivity with increasing x in KHxSbkTi,P20m is due to both the effecta of enlarged bottleneck and the increasing K+ ion concentration. The increase in the preexponential factor with increasing K+ vacancy concentration is attributed to the increase of ionic conductivity in the K6xSb5-xM~xP20m series. In the K6xSb5_xW,P2020series, the effects of increasing preexponential factor and the decreasing activation energy with increasing x are ascribed to the observed increase of conductivity. A comparison of the preexponential factor among the three series K5+,Sb5-,Ti,Pz020, K6-xSb6,M~,P2020,and K5-xSb5xW,P20zosuggests that the preexponential factor is more strongly affected by the increasing K+ vacancy concentration than by the increasing K+ ion concentration.
X Acknowledgment. This work was supported by the National Science Foundation - Materials Chemistry ProFigure 11. (a) Activation energy (E,) vs x in Kb-xSbb-xWxP2020. (b)Preexponential factor ( A ) vs x in Kb-xSbS-xW,P2020. gram DMR 88-08234. Registry No. K5Sb5PzOzo, 98597-35-4;K5.mSb4.gsT&.mPzOzo, remains constant with increasing x for 0.05 I x I 0.15. 140225-43-0; K5.1Sb4.gTiolP20~, 140225-44-1; K5.zSb4,sTb.2PzOm, A comparison of the preexponential factor among the 140225-45-2; K,,g5Sb4,g5Moo.mP2020, 140360-30-1; K4.gSb&00.1three K5+xSb6-xTixP2020, K5-,Sb5-,Mo,P2020, and P2020, 140225-46-3;K4,ssSb~.ssMoo.l5P2020, 140225-47-4;K4.95K+,sb+,WXP2020 series suggests that the preexponentd Sb4,g5WO,O5P2020, 140225-48-5; K4.gSbp.gW0.1P2020, 140225-49-6; K,.,5Sb4,aWoo,l5PzOzo, 140225-50-9. factor is more strongly affected by the increasing K+ va-
Hydrothermal Synthesis and Characterization of Two Vanadium Organophosphonates: VO(CGHSP03)1-y(CH3P03)y~1.5H20, y = 0.50 and 0.75 G. Huan,? A. J. Jacobson,$ J. W. Johnson,* and D. P. Goshorn Corporate Research Laboratories, Exxon Research & Engineering Company, Annandale, New Jersey 08801 Received December 27, 1991. Revised Manuscript Received March 5, 1992 Synthesis of phases in the VO(C6HSP03)ly(CH3P03).xH20 system has been investigated by reaction of VzO3 + C6H6Po3H2+ CH3P03H2under hydrothermd conditions at 200 OC. Two discrete compounds with compositions, VO(C6H5P03~o.50(CH3P03)0.50.1.5H,0 and VO(C6HpP03)o.25(CH3P03~o.,~~1~5H20, have been isolated. A continuous solid solution series of the type found in some similar zirconium organophosphonate systems is not observed under the synthesis conditions used here. Both intermediate compositions are layered compounds with interlayer separations of 11.2 and 19.4 A for y = 0.5 and 0.75 respectively. On the basis of the compositions, the X-ray powder diffraction data, and the reaulta of magnetic susceptibility measurements, structural models for the two compounds are proposed. The data indicate that the y = 0.75 phase is an unusual example of a regularly interstratified inorganic-organiclayered compound. Introduction The metal organophosphonates and organophosphates form a group of layered compounds with alternating or'Present address: Carus Chemical Co., 1001 Boyce Memorial Drive, Ottawa, IL 61350. Present address: Department of Chemistry, University of Houston, Houston, TX 77204-5641.
*
ganic and inorganic layers. Several systems have been studied in detail because of their interesting structural chemistry, sorption and catalytic properties. For example, zirconium1-8 and the vanadiumg organophosphonates, (1) Mikulski, C. M.; Karayannis, N. M.; Minkiewicz, J. V.; Pytlewski, L. L.; Labes, M. M. Inorg. Chim. Acta 1969,3,523-526.
0 1992 American Chemical Society
Huan et al.
662 Chem. Mater., Vol. 4, No. 3, 1992
Zr(RP03)2and VORPO,, have been extensively investigated, and systems containing divalent and trivalent metal cations have recently been reported.1° Most of the earlier synthetic studies resulted in the formation of polycrystalline samples but new approaches, including hydrothermal synthesis, have produced single crystals of several phases and detailed structure determinations have been made. As a result, the understanding of this general class of materials has made rapid progress. A specific goal in the synthetic chemistry of layered compounds has been the development of new microporous solids by tailoring the structures of particular metal organophosphonates. Several different approaches have been taken including (1)mixing R groups of different size^,^^,^ (2) pillaring the layers with diphosphonates (03P-RP03),3e,4b19f and (3) mixing phosphonates and phosphates with subsequent removal of some of the phosphate ester (2)(a) Yamanaka, S. Inorg. Chem. 1976,15,2811-2817. (b) Yamanaka, S.; Hattori, M. Chem. Lett. 1979,1073-1076. (c) Yamanaka, S.; Tsuijimoto, M.; Tan&, M. J.Inorg. Nucl. Chem. 1979,41,605-607. (d) Yamanaka, S.;Mateunaga, M.; Hattori, M. J. Inorg. Nucl. Chem. 1981, 43,1343-1346. (e) Yamanaka, S.;Sakamoto, K.; Hattori, M. J. Phys. Chem. 1981,85,1930-1933.(0 Yamanaka, S.;Sakamoto, K.; Hattori, M. J. Phys. Chem. 1984,88,2067-2070. (g) Yamanaka, S.;Yamasaka, K.; Hattori, M. J. Inclusion Phenom. 1984,2,297-304. (3)(a) Dines, M. B.; DiGiacomo, P. Inorg. Chem. 1981,20,92-97.(b) DiGiacomo, P. M.; Dines, M. B. Polyhedron 1982,1,61-68. (c) Dines, M. B.; DiGiacomo, P.; Callahan, K. P.; Griffith, P. C.; Lane, R.; Cooksey, R. E. In Chemically Modified Surfaces in Catalysis and Electrocatalysis; Miller, J., Ed.; American Chemical Society: Washington, DC, 1982;p 223. (d) Dines, M. B.; Griffith, P. C. J.Phys. Chem. 1982,86,571-576. (e) Dines, M. B.; Griffith, P. C. Polyhedron 1983,2,607-611.(0 Dines, M. B.; Griffith, P. C. Inorg. Chem. 1983,22, 567-569. (g) Dines, M. B.; Cooksey, R. E.; Griffith, P. C.; Lane, R. C. Inorg. Chem. 1983, 22, 1003-1004. (4)(a) Cheng, S.;Peng, G.-Z.; Clearfield, A. Ind. Eng. Chem. Prod. Res. Deu. 1984,23,219.(b) Ortiz-Ada, C. Y.; Clearfield, A. Inorg. Chem. 1985, 24,1773-1778. (c) Wan, B.-Z.; Anthony, R. G.; Peng, G.-Z.; Clearfield A., J.Catal. 1986,101,19-27.(d) Clearfield, A. in Design of New Materials; Clearfield, A,, Cocke, D. L., Eds.; Plenum Press: New York, 1986;p 121. (e) Yang, C Y . ; Clearfield, A. Reactive Polym. 1987,5,13-21.(0 Peng, G.-Z.; Clearfield, A. J.Inclusion Phenomena 1988,6,49-55.(g) Colon, J. L.; Yang, C.-Y.; Clearfield, A.; Martin, C. R. J. Phys. Chem. 1988,92, 5777-5781. (h) Ortiz-Avila, Y.; Rudolf, P. R.; Clearfield, A. Inorg. Chem. 1989,28,2137-2141. (5)(a) Alberti, G.; Costantino, U.; Alluli, S.; Tomassini, J. J. h o g . Nucl. Chem. 1978.40,1113-1117.(b) Alberti, G.: Costantino, U.; Luciani Giovagnotti, M. L. J. Chromatogr. 1979, 180,45-51. (c) Casciola, M.; Costantino, U.; Fazzini, S.;Tosoratti, G. Solid State Ionics 1983,8,27-34. (d) Alberti, G.; Casciola, M.; Costantino, U. J. Colloid Interface Sci. 1985, 107,256-263. (e) Alberti, G.; Costantino, U.; Marmottini, F.; Perego, G. Reactive Polym. 1988,9,267-276. (6)(a) Lee, H.; Kepley, L. J.; Hong, H.-G.; Mallouk, T. E. J. Am. Chem. SOC.1988,110,618-620.(b) Lee, H.; Kepley, L. J.; Hong, H.-G.; Akhter, S.; Mallouk, T. E. J. Phys. Chem. 1988,92,2597-2601. (c) Kreuger, J. S.; Mayer, J. E.; Mallouk, T. E. J.Am. Chem. SOC.1988,110, 8232-8234. (d) Li, Z.;Lai, C.; Mallouk, T. E. Inorg. Chem. 1989,28, 178-182. (e) Hong, H.-G.; Sackett, D. D.; Mallouk, T. E. Chem. Mater. 1991,3,521-527. (7)Kornyei, J.; Szirtes, L.; Costantino, U. J. Radioanal. Nucl. Chem. 1985,89,331-338. (8)Burwell, D. A,; Thompson, M. E. Chem. Mater. 1991,3, 14-17. Burwell, D.A.; Thompson, M. E. Chem. Mater. 1991,3,73C-737. (9)(a) Johnson, J. W.; Jacobson, A. J.; Brody, J. F.; Lewandowski, J. T. Inorg. Chem. 1984,23,3842-3844.(b) Johnson, J. W.; Jacobson, A. J.; Butler, W. M.; Rosenthal, S. E.; Brody, J. F.; Lewandowski, J. T. J. Am. Chem. SOC.1989, 111, 381-383. (c) Johnson, J. W.; Brody, J. F.; Alexander, R. M.; Pilarski, B.; Katritzky, A. R. Chem. Mater. 1990,2,198. (d) Huan, G.; Jacobson, A. J.; Johnson, J. W.; Corcoran Jr., E. W. Chem. Mater. 1990,2,91-93. (e) H u m , G.; Jacobson, A. J.; Johnson, J. W.; Corcoran Jr., E. W., to be published. (0 Huan, G.; Johnson, J. W.; Jacobson, A. J.; Merola, J. S. J. Solid State Chem. 1990,89,220-225. (10)(a) Cunningham, D.; Hennelly, P. J. D.; Deeney, T. Znorg. Chim. Acta 1979,37,95-102. (b) Cao, G.; Lee, H.; Lynch, V. M.; Mallouk, T. E. Solid State Ionics 1988,63-69. (c) Cao, G.; Lee, H.; Lynch, V. M.; Mallouk, T. E. Inorg. Chem. 1988, 27, 2781-2785. (d) Martin, K. J.; Squattrito, P. J.; Clearfield, A. Inorg. Chim. Acta 1989,155,7-9.(e) Cao, G.; Lynch, V. M.; Swinnea, J. S.; Mallouk, T. E. Inorg. Chem. 1990,29, 2112-2117. (0 Cao, G.; Mallouk, T. E. Inorg. Chem. 1991,30,1434-1438. (9) Frink, K. J.; Wang, R.-C.; Colon, J. L.; Clearfield, A. Inorg. Chem. 1991,30, 1438-1441. (h) Bujoli, B.; Palvadeau, P.; Rouxel, J. Chem. Mater. 1990,2,582-589. (i) Bujoli, B.; Palvadeau, P.; Rouxel, J. C. R. Acad. Sci. Paris, Ser. II 1990,310,1213.
T a b l e I. T G A Results for VO(CRHnPOI),_,(CH~P0I),.rHzO first weight loss second weight loss y start, " C end,OC x start,OC end,OC 0 250 200" 1.0 250 550 210 0.50 25 1.5 525 700 0.75 25 180 1.5 500 680 1.0 80 400 1.5 600 780 See text.
groups by hydrolysis in order to separate the remaining R groups by phosphate hydr~xyls.~g Microporous zirconium phosphonates were prepared using the first of these strategies, and we have adapted this approach for the synthesis of analogous vanadium compounds. The layered vanadyl organophosphonates VORP0,xHzO have compositions and structural features controlled by the size of the organic groups and by the method of preparation. For example, when R = CH,, x is 1.5 and the inorganic layers consist of face-sharing V209dimers connected by RPO, groups through corners,h in an arrangement analogous to that found in VO(HP04).0.5H20.11 The layer connectivity can be represented by V01/102/202 (H20)1/2(CH3)P02/201/3(H20). Reaction of vanadium(&) with phenylphosphonic acid at different temperatures results in the formation of two different compositions, VOC6H&'03*2H209a and VOC&6P03.H20,9d both of which have structures different from that of the methylphosphonate phase. The dihydrate, represented by V01,103/2(H20)2/1(C6H5)P03/2, contains isolated V06 octahedra and RPO, tetrahedra in a corner-sharing array, and the phenyl groups from adjacent layers interpenetrate. The structure is analogous to that of newberyite, MgHP04.3H20.12 The monohydrate, represented as V05,,(Hz0)1/1(C6H5)P0,/2, contains one-dimensional - v d V=O- chains connected by the RP03 groups through comers to form a two-dimensional layer with phenyl groups extending into the interlayer space. This structure is analogous to that of a-VO(HP04)-2H20.13The phenyl groups from adjacent layers in the monohydrate cannot interpenetrate but form instead a bilayer arrangement. We have investigated the possibility of obtaining microporous solids in phases containingboth methyl and phenyl groups. However, we observed that reaction of vanadium oxide with mixtures of the two phosphonic acids under hydrothermal conditions at 200 "C does not result in a methyl-phenyl solid solution series but in the formation of two discrete intermediate phases. In this paper we describe the synthesis and characterization of the two intermediates with compositions VO(C6H5P03)1-y(CH3P03),*1.5HzO,y = 0.50 and 0.75.
,-
Experimental Section Synthesis of VO(C6H&'03)I,(CH3P03)y~xHz0.To investigate t h e f o r m a t i o n of the solid solution series, VO(C6H5P03),_y(CH3P03)y.~H20, reactant ratios were selected according to the equation below with incrementa of m equal to 0.1:
+
2.5(1- m)C6H5P03H2 + 2.5mCH3P03H2 VzO3
-
VO(C6H5P03)1,(CH3P03)y*~Hz0 (11)(a) Johnson, J. W.; Johnston, D. C.; Jacobson, A. J.; Brody, J. F.
J.Am. Chem.SOC.1984,106,8123-8128.(b) Leonowicz, M.E.; Johnson,
J. W.; Brody, J. F.; Shannon Jr., H. F.; Newsam, J. M. J. Solid State Chem. 1985,56,370-378.(c) Torardi, C. C.; Calabrese, J. C. Inorg. Chem. 1984,23,1308. (12)Sutor, D. J. Acta Crystallogr. 1967,23, 418-422. Abbona, F.; Boistelle, R.; Haser, R. Acta Crystallogr. 1979,B35, 2514-2518. (13)LeBail. A.: Ferev. G.: Amoroe, P.: Beltran-Porter, D. Eur. J.Solid State Inorg. &em. 1989,26,419-426.
Chem. Mater., Vol. 4, No. 3, 1992 663
Two Vanadium Organophosphonates
E
m
\
m
E
0
m I
0
1.
t a
6
t.9
18
e1 TWO
-
sa THETn
36
1I
4s
51
a
88
CDEGREESI
Figure 1. Powder X-ray diffraction data for VO(C6H5P03)I,(CH3P03),.1.5Hz0 with y = 0.5 and 0.75. The phosphonic acids and the VzO3 were obtained from Alfa. The reactants were transferred into a 23-mL Teflon-lined autoclave (Parr Instruments), and distilled water was added to fill -70% of the total volume. The reactants were heated at 200 "C and autogenous pressure for 2 days and then cooled to room temperature. The products were filtered, washed several times with distilled water, and air dried. The preparations of the two end members (y = 0, 1) have been described previously?d*e Powder X-ray diffraction patterns (see below) indicated that only two additional phases with compositions corresponding to x = 1.5, y = 0.50 and to x = 1.5, y = 0.75 were formed and there was no evidence for a solid solution. To prepare pure single phases of the two intermediate compounds, the ratios of the starting materials were adjusted to C6H5P03Hz:CH3P03Hz:Vz03= 1.05:1.95:1.00and 0.502.501.00, respectively, and the reaction time was reduced to 12 h. Light-blue crystals in the form of thin sheets were obtained as the only product of each reaction. The compositions were determined by chemical analysis (Galbraith Laboratories, Inc.). For the phase withy = 0.50 C, 19.08%;H, 3.15%;V, 23.04%; P, 14.41%, calculated for C3.5H,VP05,5: C, 19.19%;H, 3.22%, V, 23.26%, P, 14.14%. For the phase with y = 0.75: C, 13.69%,H, 3.14%;V, 25.26%;P, 15.65%,calculated for C2,&6,5VP05,5: C, 13.28%;H, 3.22%;V, 25.03%;P, 15.22%. Powder X-ray diffraction patterns were obtained with a Siemens D500 diffractometer. Thermogravimetric analyses were performed in flowing helium at a rate of 10 "C/min on a Du Pont 900 thermal analyzer. Magnetic susceptibility measurements were made from 5 to 300 K using a Quantum Design Model MPMS SQUID magnetometer with an applied magnetic field of 6.0 kG.
L
1
b O
.
O
.
.
.
I
.
.
-
l
.
.
.
200 T
2.0
.
I00
0
300
(K)
r-----7
E m
\
m
E 0
Lo
0
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. . 1 1
Results The TGA data for the compounds with y = 0,0.5,0.75 and 1.0 are summarized in Table I. For VOC6H5P03.H20 (y = 0), the first weight loss in He flow was much greater than that expected for the release of one water molecule because of accompanying loss/reaction of the organic component. However, at a heating rate of 2 "C/min in air, the weight change due to loss of one H 2 0 per formula unit is resolved and occurs between 200 and -250 "C. Above 250 OC, the loss and/or reaction of the organic component begins and is complete by 550 "C. The other compositions corresponding toy = 0.5,0.75, and 1.0 behave differently than the y = 0 phase but are generally similar to each other. The first weight loss begins at lower temperature and corresponds to 1.5 H20/formula unit. A second weight loss associated with decomposition of the organic component is well resolved and does not start until 2500 "C. The first weight losses for the compounds withy = 0.5 and 0.75 phases are complete by -200 "C in contrast to -400 "C for VOCH3P03.1.5H20(y = 1). The water contents and the similar TGA behavior suggest a possible structural similarity between the compounds with y = 0.5,0.75, and
C
0.0 0
. . .
1
.
.
.
1
200
100 T
.
.
1
300
(K)
Figure 2. Magnetic susceptibility data as a function of temwithy = 0.5 (a), perature for VO(C6HSP03)ly(CH3P03)y'1.5Hz0 0.75 (b), and 1.0 (c).
1.0 and a distinction from the y = 0.0 phase. The powder X-ray diffraction patterns of the two intermediate phases displayed in Figure 1indicate that both phases have layered structures. It is noteworthy that the X-ray powder pattern for the phase with y = 0.50 shows a normal progression in the intensities of the (001) lines, while in the powder pattern of the y = 0.75 compound the first and third lines are less intense than the second, suggesting that the unit cell of this compound is doubled due to some minor differences between adjacent layers. The magnetic susceptibility data for the compounds with y = 0.5, 0.75, and 1.0 are shown in Figure 2. The
664 Chem. Mater., Vol. 4, No. 3, 1992
Huan et al.
Table 11. Magnetic Data for VO(C6HQ08)ly(CH8POs)r SH20 Compounds N
1.0
Cd, cm3K/g 2.00 X
2J/k,, K
Ci, cm3K/g
e, K
x0, cm3/g kffr
WB
-43.8 5.26 X -5.3 -4.44 X 1.75
0.75 1.68 X -43.1 8.61 X -3.4 -6.28 X lo-’ 1.69
0.50 1.15 X -43.7 5.65 X 10”’ -21.3 -7.27 X 1.73
magnetic susceptibility data for the y = 0 composition shows no departure from Curie-Weiss behavior until much lower temperature ( 15 K)than the other compositions. The behavior of the y = 0.5,0.75, and 1.0 phases is similar to that of VO(HP04)2~0.5H2010a and VOSe03.H2014and indicates the presence of exchange coupled V, dimers. The data were analyzed as described previouslyloausing the expression N
X
= Xo
+ C i / ( T - 6) + 4Cd/(T(3 -k exp(-2J/kBT)))
Least-squares fits to the data over the entire temperature range are shown in Figure 2. Values of xo, the temperature-independent contribution; c d , the Curie constant associated with the V2 dimers; Ci and 6, the constants associated with the magnetic impurities; and J, the coupling constant within the V2 pairs are reported in Table 11.
Discussion The compounds VOCH3P03-1.5H20and VOC6H5P03-H20do not form a continuous solid solution, presumably due to the differences between their structures. The structure of VOC6H5P03-H20is based on chains of V06 octahedra sharing trans vertices. The chains are connectsd to form layers by the phenylphosphonate groups. The connectivity within the layer is similar to that found in a-V0(HPO4)-2H20.l3The phenyl groups from adjacent layers form a bilayer between the inorganic layers. In contrast, the structure of VOCH3P03-1.5H20is analogous to that of VO(HP04)2*0.5H20.11The vanadium oxygen octahedra form face-shared dimers in the inorganic layer, and the methyl groups, which point out from the layer surface into the interlayer region, interdigitate significantly with the methyl groups from the adjacent layer. The structure of the methyl derivative has been obtained in a moderate level of detail from a partial single-crystal structure determination.* The magnetic susceptibility data indicate the presence of exchange coupled V(1V) d1 dimers and are similar to thw of the hydrogen phosphate, as is expected from the structural similarity. Although no range of solid solution is observed between the methyl and phenyl end members, two discrete intermediate compounds with methyl/phenyl ratios of 1/1and 3/1 are formed. Powder X-ray data indicate that no compounds with other ratios are formed and that the intermediate phases have narrow ranges of stoichiometry. The X-ray data for the intermediate compositions indicate that they are layered compounds with layer separations of 11.2 and 19.4 A for the 1/1 and 3/1 compounds, respectively. The layer lines are strong and sharp and the interlayer separations are well defined. However, the crystals formed during hydrothermal synthesis are very small and consequently even the best patterns give an (14)Huan, G.;J o h n , J. W.; Jacobson, A. J.; Goshom, D.P.; Merola, J. S.Chem. Mater. 1991,3, 539-541.
ououou 1
QQooQQ
Figure 3. Structural models for VO(CBH6P03)o,6(CHsPOS)o,6, 1.5Hz0.
insufficient number of in-plane reflections to unambiguously determine the complete unit cell. The intralayer structure can only be inferred from indirect evidence. Thermogravimetric analysis of both compounds indicates the presence of 1.5 H20/formula unit, implying that the structures of the intermediate compositions are similar to that of the methyl compound. The magnetic susceptibility of both the 1/1and the 3/1 compounds shown in Figure 2 indicates the presence of the antiferromagnetically coupled vanadium(IV) dimers as found in both VOCH3P03.1.5H20and VO(HP04)2.0.5H20.The compounds with y = 1and 0.75 gave small values of Ci consistent with the presence of a small number (2.6 and 4.9%) of uncoupled spins. The much larger value of Ci observed for the phase with y = 0.5 suggested the presence of an impurity in the specific sample used in the magnetic measurements, which was different from the first sample on which the X-ray diffraction, TGA, and elemental analysis had been performed. Powder X-ray diffraction confirmed that the sample used in the magnetic susceptibility measurement contained in addition to the phase with y = 0.5 an impurity that could be identified as VOC6H5P03*H20.Elemental analysis for P, V, and C each gave an estimate of the amount of the second phase. An average value of 36 f 5% was obtained in agreement with the value of 33% estimated from the relative values of Ci and C,. The magnetic susceptibility of VOC6H6PO3.H20has previously been shown to follow Curie-Weiss behavior over the temperature range of the present measurements.gd Thus, although the magnetic susceptibility of VO(C6H5P03)o.6(CH3P03)o,5.1.5H20 was not measured using a single-phase sample, X-ray diffraction, elemental analysis, and analysis of the magnetic susceptibility as a function of temperature combine to give a good description of the sample employed in the magnetic measurements as a mixture of and VOCGH$O~.H~O. Assuming that the layer connectivity in the 1/1 composition is the same as in the methyl compound, there are two simple possibilities for the distribution of the methyl and phenyl groups on the layer surface. In one (Figure 3a) equal numbers of methyl and phenyl groups are distributed, possibly in an ordered arrangement, on each side of the VOP03 layer. Methyl and phenyl groups from adjacent layers then interlock to give a layer spacing corresponding to the sum of the methyl and phenyl van der Waals dimensions together with an additional amount to accommodate the interlayer water. The interlayer separation of 11.20 A is intermediate between that of vanadyl methylphosphonate (8.30 A) and that of vanadyl phenylphosphonate (14.14 A). A second arrangement (Figure 3b) in which all of the methyl groups are on one side of the layer and all of the phenyl groups are on the other would be expected to give a similar interlayer separation but a noncentrosymmetric structure. Such an arrangement
Two Vanadium Organophosphonates n
n
n
QQQQQQ
000000 iQAA
Figure 4.
Structural models for VO(CsH5P03)o.z6(CH~PO~)O.,~*~.~HZO.
cannot be ruled out by the present data. The structure of the 3/1 compound is also layered but the layer spacing (19.4 A) is approximately twice the value expected from an arrangement of methyl and phenyl groups on a single layer. It is also difficult to propose an ordered arrangement at this composition which could interlock to minimize the free interlayer space. The intensities of the layer lines are such that the 002 line is much stronger than either the 001 or the 003 lines, suggesting that the doubled unit cell arises from a regular alternation of two similar types of layers, a phenomenon known as interstratification in clay chemistry or staging in the intercalation chemistry of graphite or transition metal dichalcogenides. The interlayer spacing of 19.4 A is close to the sum of the value for the methyl and the 1/1compounds (8.3 + 11.2 A). An equal combination of pure methyl layers and 1/1 methyl-phenyl layers gives the correct overall stoichiometry, though again based on the layer spacing two different arrangements of the methyl and phenyl groups in the 1/1layer are possible. Packing all methyl layers with layers where the methyl and phenyl
Chem. Mater., Vol. 4, No. 3, 1992 665
groups were arranged on either side of the layer would minimize any free interlayer volume, and consequently the noncentrosymmetric structure shown in Figure 4b is more likely. The driving force for the crystallization of ordered phases in preference to single-phase solid solutions over a range of y is the enhanced enthalpy provided by more efficient packing within the organic layer of these inorganic/organic alternating layered systems. The driving force for the formation of structures involving ordering of this type diminishes as the c axis periodicity of the ordered structures increase. It is possible that other ordered structures of the series VO (C6H6P03)ly(CH3P03)y.1.5H20 with 0.75 < y < 1 could exist, but the energy difference between them and the structures with y = 0.75 and y = 1 is likely to be small, so that they may be difficult to obtain experimentally. The vanadium organophosphonate system behaves differently from the tetravalent metal organophosphonates where solid solutions have been reported. For example, a continuous variation in the interlayer separation was observed3ffor Th(C6H6P03),(C6H5C6H4Po~)z-~ from 1c = 0 to 1.4, which was interpreted either as resulting from partial interdigitation or random interstratification. In the initial reports of the mixed R phases of the tetravalent metal systems, compounds were prepared by rapid precipitation at ambient temperature. However, Clearfield has observed regular interstratification in the Zr(HP04)z/Zr(C6H5P03)z system under conditions similar to those used here for the vanadium phases.4d The hydrothermal results suggest that in mixed R systems the formation of compounds with regular layers is preferred and that synthesis by rapid precipitation at lower temperatures is necessary for the formation of disordered phases. A more detailed study is required to determine whether random interstratification is the source of the disorder. Solids organized in this way would not be expected to show permanent microporosity. Registry No. Vz03,1314-34-7; C6H5P03H2,1571-33-1; CH3P03H2, 993-13-5.
