The synthesis of layered compounds of the general formula Sn(RPO 3)2·xH2O (R = phenyl, 2-carboxyethyl) and Sn(RPO3)2·xH2O (R = dodecyl) are reported. These compounds are formed by the reaction of nitric solutions of SnCl4 with the phosphonic acid or phosphoric ester followed by heating under reflux. The interlayer distances of 13.87, 15.37 and 32.70 Å for 2-carboxyethyl, phenyl and dodecyl derivatives, respectively, are determined by the XRD patterns. A linear relationship between the molar volume and the interlayer distance is observed suggesting an alpha structure similar to α-Sn(RPO4)2·H2O. FTIR spectra show the formation of an Sn- O-P skeleton.2101075107

Alves O.L.

Medeiros M.E.

Repositorio da Producao Cientifica e Intelectual da Unicamp

J. MATER. CHEM., 1992,2(10), 1075-1078 1075 
Tin(iv) Phosphonates with a-Layered Structure: Synthesis and 
Characterization 
Marta E. Medeiros and Oswaldo L. AIves* 
Laboratorio de Quimica do Estado Solido, lnstituto de Quimica, UNICAMP, P.O. Box 6154, 13081, 
Campinas, SP, Brazil 
The synthesis of layered compounds of the general formula Sn(RPO,),.xH,O (R = phenyl, 2-carboxyethyl) and 
Sn(ROPO,),.xH,O (R = dodecyl) are reported. These compounds are formed by the reaction of nitric solutions 
of SnCI, with the phosphonic acid or phosphoric ester followed by heating under reflux. The interlayer distances 
of 13.87, 15.37 and 32.70 8, for 2-carboxyethyl, phenyl and dodecyl derivatives, respectively, are determined 'by 
the XRD patterns. A linear relationship between the molar volume and the interlayer distance is observed 
suggesting an alpha structure similar to a-Sn(HPO,),.H,O. FTIR spectra show the formation of an Sn-0-P 
skeleton. 
Keywords: Layered structure ; Tin(/v) phosphonate; Tin(1v) alkylphosphate 
Layered compounds formed by reaction involving tetravalent 
metals with phosphonic acids have received considerable 
attention, owing to their potential applications in catalysis 
and separation science.'-6 Recent reports have shown that 
the catalytic and separation properties of these bidimensional 
systems arise because the intrinsic site disorder, which has 
been observed in functionalized silica and other solid supports, 
does not appear to O C C U ~ . ~ ~ ~  This structural feature is import- 
ant in chromatography or shape-selective 
The preparation of these materials has been limited to some 
topochemical or exchange reactions of OH groups of layered 
compounds, such as zirconium  phosphate^.^.'^.' I In this case, 
the reaction is not complete because some available anion 
sites are not exchanged. Today, the availability of phosphonic 
acids and esters has opened the way to the preparation of 
new and interesting materials, with desired organic groups, 
by direct reaction with tetravalent  metal^.^"^"^ However, in 
spite of their potential applications in cases where surface 
phenomena are important, the layered phosphonic 
(phosphate) derivatives containing post-transition tetravalent 
metals are incompletely studied. 
In this work we show the preparation of new layered 
compounds of type Sn(RPO,), (R = phenyl, 2-carboxyethyl) 
and Sn(ROP03)x (R =dodecyl) by metathetical reaction from 
acidic solutions of SnCl, with the respective phosphonic acid 
and phosphoric ester. The solids obtained are characterized 
by chemical analyses (P, Sn), infrared (IR) spectroscopy, X-ray 
powder diffraction (XRD) and absolute density. 
Experimental 
Materials 
Tin(1v) chloride (P.A. grade, Fluka) was purified by vacuum 
distillation. Phenylphosphonic acid (PPA) and 2-Carboxy- 
ethylphosphonic acid (CEPA), both from Aldrich, were used 
without further purification. Dodecyl phosphate (DPA) was 
prepared by the reaction of pyrophosphoric acid with dodecyl 
alcohol according to ref. 14. 
Preparation 
The following general procedure was used to prepare the 
samples: to a vigorously stirred solution of tin(1v) chloride 
(1  mol dmP3) in nitric acid (3 mol dm- ,), was added a solution 
of the respective phosphonic acid or phosphate ester (1 mol 
dm-3) also in nitric acid (3 mol dm-3). In the case of DPA 
a small amount of ethanol was used to increase the ester 
solubility. A molar ratio of P: Sn=2.5 was used in all prep- 
arations. The white gel formed immediately after the addition 
of PPA or DPA solutions was redispersed with a few cm3 of 
dilute nitric acid and treated under reflux for 24 h at 85-90 "C. 
For CEPA the gel was formed only ca. 2-3 h after heating 
the clear solution under reflux. The solid was separated by 
centrifugation, washed successively with deionized water until 
the pH of the washings was ca. 5 and the test for chloride 
ion proved negative, then dried in air. The yields were > 80%. 
Chemical Characterization 
The samples were heated with a solid mixture of NaOH : N a 2 0  
( 1  : 1 )  at 1000 "C. The resulting solid was disaggregated with 
warm water and dissolved in concentrated hydrochloric acid. 
Phosphorus was then determined spectrophotometrically as 
phosphomolybdate" and tin by flame atomic absorption. 
Physical Measurements 
The XRD patterns were recorded on a Shimadzu powder 
diffractometer, model XD-3A, using Cu-Kac, 1.5418 8, Ni- 
filtered radiation (30 kV and 20 mA) at a scan rate of 2" min- l .  
The measurements were carried out with the sample spread 
on a conventional glass sample holder. IR spectra were 
recorded on a Nicolet 60SX-B FTIR spectrometer using Nujol 
and Fluorolube mull dispersion techniques. Measurements 
of absolute density were carried out on a Micromeritics Multi- 
volume Pycnometer model 1305 using helium as the 
displacement gas. 
Results and Discussion 
Preparation, Composition and Crystallinity 
Nitric solutions of SnCl, react instantaneously with PPA and 
DPA solutions forming a thick white gel. The presence of the 
ionogenic group C 0 2 H  in CEPA could be responsible for the 
differences observed in the gel formation kinetics, as men- 
tioned in the experimental section. However, for the analogous 
material obtained from the ZrOClz solution and CEPA, gel 
formation follows the same pattern of behaviour as that 
observed for PPA and DPA with SnIV. The behaviour of the 
CEPA-Sn" system is similar to that of H3PO4-SnIV, where 
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View Article Online / Journal Homepage / Table of Contents for this issue
1076 J. MATER. CHEM., 1992, VOL. 2 
the precipitation occurs only when the nitric oxide has been 
totally released.16 We believe that the well known ability of 
SnC14 to form six-coordinated complexes with ligands con- 
taining C=O groups,17 prevents access of the P03H2 group 
to the Sn4+ species in solution. Heating under reflux promotes 
the complex decomposition which creates the condition for 
P-0-Sn skeleton formation. Therefore, the gel formation 
for the system CEPA-Sn" can be controlled by the time of 
heat under reflux and temperature. 
Table 1 lists the chemical compositions of the tin@) 
compounds prepared. It can be seen that the ratio P : Sn is 
very close to 2, and the compounds can be therefore formu- 
lated as Sn(C6H5P03)2.1.7H20, Sn(C3H502P03)2.2.4H20 
and Sn(C12H250P03)2.0.1H20. 
As noted previously for several analogous materials, the 
water molecules are located between the layers.2 For these 
compounds it is clear that the amount of water is more closely 
related to the hydrophobic character rather than to the length 
of the organic chain. Fig. 1 displays the XRD patterns for 
PPA, CEPA and DPA tin(rv) compounds. 
The poor definition of the peaks, mainly at 28>10", may 
be attributed to an incipient stage of crystallization. In order 
to obtain materials with a higher degree of crystallization, 
variations in time and concentration of the phosphonic acid 
and phosphoric ester were attempted. For the layered com- 
pounds Zr(HP04)2.H20 and Zr(HPO,),.nH,O, these pro- 
cedures have allowed us to obtain samples with a high degree 
of c r y ~ t a l l i n i t y . ~ . ~ , ~ ~  However, for the compounds described 
in this paper, results were unsuccessful, i.e. the solubilization- 
reprecipitation mechanism, responsible for the increase of 
crystallinity observed for similar zirconium c o m p o ~ n d s , ~ ~ ~ ~ ~  
was not operative. This behaviour is probably due to the low 
solubility of tin(1v) compounds in the reaction medium. 
Structural Model and Interlayer Distances 
The main features of the XRD patterns (Fig. 1) are the 
presence of well defined peaks in the region of 28 < 10". If we 
consider similar, better known layered phosphonates involving 
tetravalent metals other than Sn, the first reflection is straight- 
forwardly assigned to the 002 family of planes, which is related 
Table 1 Chemical composition of tin(1v) derivatives of phosphonic 
acids and phosphate ester 
R P:Sn H20/mol" 
phenyl 2.10 1.7 
2-carbox yethy 1.90 2.4 
do d e c y I 1.95 0.1 
" Estimated from TG measurements.'* 
r 
x 
v) 
Q) 
c .- 
c 
.- 
1 1 1 1  I 1 1  I I J  
5 10 15 20 25 30 35 40 45 50 
2Wdegrees 
Fig. 1 XRD patterns for tin(1v) layered compounds: 
(a) Sn(C3H502P03)2; (b)  Sn(CfjH5P03)2 and (c) Sn(C12H250P03)2 
Table 2 Density and calculated and observed interlayer distance for 
Sn(RP03), and Sn(ROP03), compounds 
d(002) d(002) 
density/ calculated/ observed/ 
R g cm-3 A A A(%)(I 
- hydroxyl 3.12b 7.80b - 
2-carboxyethyl 2.20' 13.10 13.87' 5.9 
phenyl 1.92' 14.70 15.37' 4.6 
dodecyl 1.32' 32.70 33.54' 2.7 
" A( %) = (dabs - dcalc)/dobs x 100; from ref. 2 1; ' this work. 
to the separation of layers. Table 2 shows the interlayer 
distances for tin(rv) phosphonates and phosphates and 
for Sn(HOPO3),*H20, i.e. a-SnP, which are included for 
comparison. 
The interlayer distances shown in Table 2 are in agreement 
with those reported for the analogous zirconium and titanium 
c o r n p o ~ n d s , ~ , ~ ~ , ~ ~  and reveal a dependence on the length of 
the organic group. Fig. 2 shows a plot of interlayer distance 
versus molar volume (molecular weight over density) for the 
tin(rv) compounds. The straight line which fits the points has 
a slope of 0.0665, very close to 0.0683 obtained for similar 
well established layered compounds.12 This result can be 
explained in terms of the ideas of Alberti et al.,12 who argued 
that in the case of zirconium(rv) phosphonate derivatives, the 
structures of the compounds are similar to that of Zr(HP04)2, 
that is the OH of the phosphate group is replaced by R or 
0-R groups. For the tin(1v) compounds studied here this 
assumption can also be applied. By analogy we would have 
the same type of layered structures i.e., an a-type structure 
as represented in Fig. 3 for a dodecyl phosphate compound. 
Table 2 shows estimated interlayer distances for the tin(1v) 
phosphonates (phosphate), considering the length of the 
organic groups using the Korney-Szirtes adapted 
for tin(1v) compounds. Although the deviation (A) can be 
accepted it is important to note that the estimated value does 
not take into account the possible orientations of the pendant 
group with respect to the plane of the tin(1v) atoms. Further- 
more, the samples are not well crystallized, so that the value 
of the (001) reflections can exhibit variations. For example, in 
the case of Zr(HP03), values from 5.99 to 5.57 8, are observed 
on going from a semicrystalline to a crystalline phase.lg The 
35.0 r 1 
25.0 3 -01 
5.0 
01 I I I I I 1  
0 100 200 300 400 500 
molar v,/cm3 
Fig. 2 Relationship between the interlayer distance and the molar 
volume for tin(1v) -layered compounds (see Table 2) 
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View Article Online
J. MATER. CHEM., 1992, VOL. 2 1077 
/ -  
I 
H3 
'C 
c: 
C:" 
C:" 
c? 
C:" 
C:" 
I 
H3\ 
C:" 
c:" 
C:" 
C:" 
c: 
C:" 
H3\ 
C:" 
C:" 
c:" 
C:" 
C:" 
c< 
I 
- I-- 
Fig. 3 Schematic representation of the a-structure for 
Sn(C12H250P0,)2 (adapted from ref. 2; d = interlayer distance) 
same behaviour was observed for Zr(H02CCH2CH2P03)2, 
obtained both in the presence and absence of hydrofluoric 
acid as a crystallization p r ~ m o t e r . ~  In both cases values of 
interlayer distances for the poorly crystallized samples are 
larger than for the well crystallized ones. 
Infrared Spectra 
The IR spectra in the region of 4000-1400cm-' and 
1400-300 cm-' are shown in Fig.4 and 5. The first obser- 
vation is that the broad bands in the region of 3700- 
2250 cm- ', due to the stretching and bending modes of the 
P-OH groups in the free acid2, are absent in the spectra of 
the tin(1v) derivatives. The second observation of note is the 
presence of bands near 3386,3420 and 3520 cm-', attributable 
to OH stretching modes, as well as bands at 1638, 1637 and 
1632 cm-', due to the bending OH, modes of water for the 
PPA, CEPA and DPA derivatives, respectively. The relative 
intensities of these bands confirm the data shown in Table 1 
with respect to water content. 
In comparison with the free acids, the region 
1250-900 cm- ', where the P=O and P-O(H) stretching 
modes23 are expected, shows marked modifications. For all 
samples broad bands centred at 1088, 1079 and 1101 cm- ' 
for phenyl, 2-carboxyethyl and dodecyl groups, respectively, 
are observed. These findings show clearly that formation of 
the R-P-0-Sn skeleton occurs in the system and reflect 
the formation of the PO3 group by transformation of P=O 
to P -0  and ionization of PO-H bonds as follows: 
2RP(O)(OH), + Sn4+ -+Sn(RP03)2 +4H+ 
or 
2ROP(O)(OH), + Sn4f+Sn(ROP03)2 +4H+ 
- 
I0 3420 2 
n 
n 
l I I 1 I  
.O 2260 1680 
wave n u m be r/c m-' 
Fig. 4 IR spectra (4000-1400 cm-') for tin@) layered compounds: 
n = nujol bands 
(a) Sn (C3H502P03)2; (b) Sn(C6HSP03)2 and (c) Sn(C12H250P03)2; 
n 
I I  , , I , ,  
1510 1250 990 730 470 
wave nu mbe r/cm-' 
Fig. 5 IR spectra (1510-340 cm-') for tin(1v) layered compounds: 
n = nujol bands 
Similar features in the IR spectra in the region of P -0  
absorptions have been observed for other layered compounds 
involving tetravalent metals and phosphorous acid (H,PO,), 
and can be attributed to the same effect.lg Furthermore, the 
band assigned to the C=O stretching at 1695cm-' in the 
(a)  Sn(C3H502P03)2; (b) Sn(C6H5P03)2 and (c) Sn(C12H250P03)2 
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View Article Online
1078 J. MATER. CHEM., 1992, VOL. 2 
free 2-carboxyethylphosphonic acid23 is little perturbed, 
indicating that the chemical nature of the carboxy group is 
preserved upon formation of the layered structure. 
Conclusions 
Layered compounds of type Sn(RP03)2 and Sn(ROP03)2 can 
be readily prepared by the reaction of nitric solutions of SnC1, 
with the respective phosphonic acid or phosphoric ester. The 
formation of these compounds depends on the nature of the 
organic group R being more rapid for PPA and DPA than 
for CEPA. A linear dependence of the molar volume uersus 
the interlayer distance is observed and hence an a structure 
is proposed for the tin(1v) derivatives, analogous with similar 
zirconium(1v) compounds. 
Preliminary results from the "'Sn Mossbauer spectra of 
Sn(C12H250P0,)2 show that an order-disorder transition 
occurs at low  temperature^.'^ An in-depth study of this 
transition and the thermal behaviour of this family of 
compounds is in progress in our laboratory. 
The authors gratefully acknowledge the Financiadora de 
Estudos e Projetos (FINEP/PQME) and the Conselho 
Nacional de Desenvolvimento Cientifico e Technologico 
(CNPq) for financial support. We also thank Dr S. F. Fonseca 
for very useful discussions on DPA synthesis. 
References 
1 A. Clearfield, in Znorganic Zon Exchange Materials, CRC Press, 
Boca Raton, 1982. 
2 G. Alberti and U. Costantino, in Intercalation Chemistry, ed. 
M. S. Whittingham and A. J. Jacobson, Academic Press, New 
York, 1982, pp. 147-80. 
3 P. R. Rudolf and A. Clearfield, Znorg. Chem., 1989, 28, 1706. 
4 D. A. Burwell and M. E. Thompson, Chem. Muter., 1991, 3, 14. 
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Paper 2/01030E; Received 27th February, 1992 
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Tin(iv) Phosphonates With α-layered Structure: Synthesis And Characterization

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Tin(iv) Phosphonates With α-layered Structure: Synthesis And Characterization

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