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Preparation and spectrochemical properties of soluble vanadophosphate polyanions with bicapped-Keggin structure
iWy/dnm Vol. 5, No. 3, pp. 811413, Printed in Great Britain
1986 0
0277~5387/86 S3.00 + .OO 1986 Pqqamon Press Ltd
PREPARATION AND SPECTROCHEMICAL PROPERTIES OF SOLUBLE VANADOPHOSPHATE POLYANIONS WITH BICAPPED-KEZGGIN STRUCTURE KENJI
NOMIYA,
KATSUHIKO
MT0
and MAKOTO
MIWA*
Department of Industrial Chemistry, Faculty of Engineering, Seikei University, Musashino, Tokyo 180, Japan (Received 8 July 1985; accepted 2 September 1985)
Abstract-Some soluble forms in aqueous and nonaqueous media ofvanadophosphate with a bicapped-Keggin structure have been prepared. Some isopoly decavanadates derived during such preparations have also been reported. Their spectrochemical data (IR, Uv-vis and MCD) are presented.
Vanadium is an element which can form hetero- and isopolyanions. Such vanadopolyanions are structurally distinct from the polyanions formed by elements such as molybdenum and tungsten, although the coexistence of vanadium with molybdenum, or tungsten, sometimes leads to mixed polyanions structurally related to the molybdo- or the In 1980, Sasaki et aL2v3 tungstopolyani0n.l determined the structure of the vanadophosphate polyanion by X-ray analysis, showing it to be a Keggin framework capped by two V05 trigonal bipyramids. The two caps occupy the bugs-position of six pits on the surface of the Keggin framework. Confirmation of this conclusion has been obtained in the form of an insoluble guanidinium salt, (CN,H,),HCPV,,O,,]
*7H,Q
isolated from aqueous solutions of phosphate and vanadate (P : V = 1: 4) at pH 2.7. This salt, which is easily obtained by sodium salt metathesis, is the most suitable one for the X-ray study. However, other salts, which are soluble in aqueous and nonaqueous media, and suitable for spectrochemical studies, have been poorly characterized. In this paper, we report the preparation of a soluble vanadophosphate with counterions and show their spectrochemical data (IR, UV-vis and MCD). Furthermore,
* Author to whom correspondence should be addressed.
we also report briefly on the isopoly decavanadate, p,,0,s]6-, derived during such preparations. EXPERIMENTAL Preparations (~~H~)sHIPV~~O~~] * Hz0 (1).3NaNO, [4.5 g (37 mmol)] was dissolved in 25 cm3 hot water, to which 6.2 cm3 of 1.5 mol drnm3 phosphoric acid was added. The pH of the deep red solution is adjusted to 2-3 by 3 mol dmw3 nitric acid. Then, an aqueous solution containing 5.0 g (52.3 mmol) guanidine hydrochloride was added with stirring. Brown precipitates were immediately formed, which were insoluble in almost all solvents and could not be recrystallized. They were washed several times with ethanol and then thoroughly with water. Found : C, 5.0; H, 3.1; N, 17.8. Calc. for (CN,H,&H[[PV@,~ H20: C, 5.0; H, 2.7; N, 17.5%. From the coloured aqueous washings, orange needle crystals of isopoly decavanadate are obtained. Found: C, 5.0; H, 3.3; N, 17.6. Calc. for (CN3H6)6[V1002S] *6Hz0 : C, 5.0; H, 3.4;N, 17.7% In the preparation of vanadophosphate, other pr~pi~ting agents could not be used in place of gum&line hydrochloride. For instance, when an aqueous solution containing tetrabutylammonium bromide was added, the tetrabutylarmponium salt of decavanadate was mainly obtained. It could be recrystallized from acetonitrile. Found : C, 33.9 ; H, 6.7; N, 2.5. Calc. for [(C!.,H,),~,H,~,,O,,] : C, 34.2; H, 6.6; N, 2.5%.
811
812
K. NOMIYA
(K, H)9[PV14042] - nH,O (2). The deep red aqueous solution obtained in the preparation of 1, the pH of which had been adjusted to 2-3 by nitric acid, was kept at 50°C in a steam bath, to which an aqueous solution at 50°C containing 10 g KC1 was slowly added. After stirring, the solution was left standing overnight at room temperature. Fine black crystals were obtained with a yield of4.8 g. They were very soluble in water, slightly soluble in methanol and ethanol, and almost insoluble in ether, acetone and acetonitrile. In this preparation, the reaction temperature of 50°C was important : if it was lower, compound 2 was contaminated by an orange compound, potassium decavanadate, and, at room temperature, the latter became the main product. C(C4H9)4N14H5CPV140421 - 4H,O (3). Compound 2 (3.5 g) was dissolved in water (below SOY!) and then an aqueous solution containing excess tetrabutylammonium bromide was added. After stirring at 5060°C for a few minutes, dark brown precipitates were collected on a titer which could be recrystallized from acetonitrile. They were very soluble in a&o&rile, slightly soluble in acetone, and insoluble in water, methanol, ethanol and ether. Found : C, 31.3 ; H, 6.2; N, 2.3. Calc. for C(C4~9)4~~4~5~~~14042~ -4~~0: C, 31.2; H, 6.4; N, 2.3%. From the red aqueous filtrate in this preparation, the same precipitates could not be obtained again even by further addition of the aqueous solution of tetrabutylammonium bromide. The filtrate contained salts with different compositions of counterions because the evaporated solids showed the characteristic IR of vanadophosphate. The tetrabutylammonium salt (3) was also obtained by another method. The red aqueous solution containing compound 2 (5 g) was passed through an H+-ion exchange resin column (Dowex50W) and eluted with water. The vanadophosphate was converted to the free-acid form without decomposition. The eluted aqueous solution was evaporated to a small volume and then an aqueous solution containing excess tetrabutylammonium bromide was added. After stirring at 5060°C for a few minutes, dark brown precipitates were filtered and recrystallized from acetonitrile. Found : C, 31.2; H, 6.3; N, 2.3. Calc. for [(C4H9)4N]4H5[PV14042] * 4H,O: C, 31.2; H, 6.4; N, 2.3%. In order to compare IR spectra, sodium decavanadate Nas[V,,,02s] * nH,O was prepared from NaV03 and glacial acetic acid by a modification described in the literature.4 UV-vis absorption spectra were measured on an Hitachi 340~spectrophotometer with a computer keyboard attached. The MCD spectra were recorded by a JASCO J40AS spectropolarimeter with a 1-T electromagnet mounted on it. IR spectra were
et al.
recorded by a JASCO IR-G spectrophotometer. Measurements were made at room temperature. RESULTS
AND DISCUSSION
Some salts of vanadophosphate obtained here, except the guanidinium salt, are soluble in appropriate solvents. The potassium salt (2) especially is very soluble in water and can be used for preparing other salts. The acidity of the free acid of vanadophosphate obtained as a powder from the eluted solution of a cation-exchange resin column was not as strong as that of the Keggin molybdo- and tungstoheteropolyacids. In fact, acid catalysis of the polymerization of benzyl alcohol to polybenzyl could not be observed.5 Generally, the maximum number of bulky tetrabutylammoniums in the counterions is significantly related to the size, rather than to the charge, of the polyanion. For instance, the maximum number is four for the Keggin heteropolyanion and six for the Dawson one. Thus, it should be noted that it is four in the bicapped-Keggin vanadophosphate. IR spectra of vanadophosphate (&-symmetry) with some counterions are shown in Fig. 1. The vibrational structure of the polyanion moiety is
3
I
1100
I
1
900
I
a
700
.
.
.-
500 cn
Fig. 1. IR spectra measured in KBr disk of vanadophosphates with counterions : (a) (CN,H,),H[PV,,O,,] %O,
0-k H)PCPV140421 -nI%O, and C(C~H,)~~,H,CPV,,O~~~-~HZO.
09
(c)
Vanadophosphate polyanions with bicapped-Keggin structure
813
W Q
MCD +0.5
(a)
6-
1100
900
700
500&l
Fig. 2. IR spectra measured in KBr disk of decavanadates with counterions : (a) (CN,H&V,,O,,] - 6H,O, (b) CGW4NJ3H3cVIo%l, and(4 Na6CVdM *n&O. subtly changed by counterions, especially in the lowfrequency region. The characteristic band at 10501060 cm- ’ is due to P-O stretching and many bands in the region of 1000-500 cm-i are due to the various V-O vibrations. These patterns are more complicated than those of a Keggin heteropolyanion with T,-symmetry. The IR spectra of decavanadate are also changed by counterions, depending upon whether the proton is contained or not (Fig. 2). IR patterns of (CN,H,),[V,,O,s] and Na,[V,,02s] are very close to each other; however, they are different from that of [(C4H,),Nl,H3[V,,02a] in the region of W-600 cm-‘. These are presumably caused by either or both of two possibilities, the existence of an anion-anion interaction in the case of a small-size counterion and the protonation of the surface oxygens of the polyanion.’ Electronic absorption and MCD spectra in acetonitrile solution are shown in Fig. 3 as full lines for [(C4H9)4N]4H5[PV14042] *4Hz0 and as dashed lines for [(C,H,),Nj,H,[V,,,O,,]. All of these absorptions and MCD are due to the V c 0 charge-transfer (CT) transitions. In the absorption spectra, there is a very broad shoulder (E ca 20,000) around 30,000 cm- ’ and a strong broad band (ECQ 60,000) around 43,000 cm-’ for the vanadophosphate, and a shoulder (E ca 10,000) around 30,000 cm- ’ and a broad band (s ca 32,000) around 40,000 cm- ’ for the decavanadate. The intensities of the
25
30
35
40
45
Y/10%,1
Fig. 3. Electronic absorption and MCD spectra in acetonitrile solution of [(C,H&QH5[PV,,0,,1.4H,0 (fullline) and C(c,H9)4N3H3cV~~O~~l @=l=dline).
absorptions of vanadophosphate are larger than those of decavanadate. The absorption position of the lowest-energy CT is almost unchanged in the two vanadopolyanions; however, that of the second CT of vanadophosphate is shifted more to the highfrequency region than that of decavanadate. These tendencies are also observed in the MCD spectra, which consist of two positive components. No negative MCD component appears in this region. The MCD in the first CT region of vanadophosphate contains further components. REFERENCES 1.
M. T. Pope, Heterpoly and Zsopoly Oxometalates. Springer, New York (1983). 2. R. Kato, A. Kobayashi and Y. Sasaki,J. Am. Chem. Sot. 1980,102,6572.
3. R. Kato, A. Kobayashi and Y. Sasaki, Znorg. Chem. 1982,21,240.
4. G. K. Johnson and R. K. Murmann, Znorg. Synth. 1979,19,
140.
5. K. Nomiya, T. Ueno and M. Miwa,Bull.Chem.Sot. Jpn 1980,53,827.
6. C. Rocchiccioli-Deltcheff,M. Fournier and R. Franck, Znorg.Chem. 1983,22,207. 7. H. T. Evans, Jr and M. T. Pope, Znorg. Chem. 1984,23, 501.
1986 0
0277~5387/86 S3.00 + .OO 1986 Pqqamon Press Ltd
PREPARATION AND SPECTROCHEMICAL PROPERTIES OF SOLUBLE VANADOPHOSPHATE POLYANIONS WITH BICAPPED-KEZGGIN STRUCTURE KENJI
NOMIYA,
KATSUHIKO
MT0
and MAKOTO
MIWA*
Department of Industrial Chemistry, Faculty of Engineering, Seikei University, Musashino, Tokyo 180, Japan (Received 8 July 1985; accepted 2 September 1985)
Abstract-Some soluble forms in aqueous and nonaqueous media ofvanadophosphate with a bicapped-Keggin structure have been prepared. Some isopoly decavanadates derived during such preparations have also been reported. Their spectrochemical data (IR, Uv-vis and MCD) are presented.
Vanadium is an element which can form hetero- and isopolyanions. Such vanadopolyanions are structurally distinct from the polyanions formed by elements such as molybdenum and tungsten, although the coexistence of vanadium with molybdenum, or tungsten, sometimes leads to mixed polyanions structurally related to the molybdo- or the In 1980, Sasaki et aL2v3 tungstopolyani0n.l determined the structure of the vanadophosphate polyanion by X-ray analysis, showing it to be a Keggin framework capped by two V05 trigonal bipyramids. The two caps occupy the bugs-position of six pits on the surface of the Keggin framework. Confirmation of this conclusion has been obtained in the form of an insoluble guanidinium salt, (CN,H,),HCPV,,O,,]
*7H,Q
isolated from aqueous solutions of phosphate and vanadate (P : V = 1: 4) at pH 2.7. This salt, which is easily obtained by sodium salt metathesis, is the most suitable one for the X-ray study. However, other salts, which are soluble in aqueous and nonaqueous media, and suitable for spectrochemical studies, have been poorly characterized. In this paper, we report the preparation of a soluble vanadophosphate with counterions and show their spectrochemical data (IR, UV-vis and MCD). Furthermore,
* Author to whom correspondence should be addressed.
we also report briefly on the isopoly decavanadate, p,,0,s]6-, derived during such preparations. EXPERIMENTAL Preparations (~~H~)sHIPV~~O~~] * Hz0 (1).3NaNO, [4.5 g (37 mmol)] was dissolved in 25 cm3 hot water, to which 6.2 cm3 of 1.5 mol drnm3 phosphoric acid was added. The pH of the deep red solution is adjusted to 2-3 by 3 mol dmw3 nitric acid. Then, an aqueous solution containing 5.0 g (52.3 mmol) guanidine hydrochloride was added with stirring. Brown precipitates were immediately formed, which were insoluble in almost all solvents and could not be recrystallized. They were washed several times with ethanol and then thoroughly with water. Found : C, 5.0; H, 3.1; N, 17.8. Calc. for (CN,H,&H[[PV@,~ H20: C, 5.0; H, 2.7; N, 17.5%. From the coloured aqueous washings, orange needle crystals of isopoly decavanadate are obtained. Found: C, 5.0; H, 3.3; N, 17.6. Calc. for (CN3H6)6[V1002S] *6Hz0 : C, 5.0; H, 3.4;N, 17.7% In the preparation of vanadophosphate, other pr~pi~ting agents could not be used in place of gum&line hydrochloride. For instance, when an aqueous solution containing tetrabutylammonium bromide was added, the tetrabutylarmponium salt of decavanadate was mainly obtained. It could be recrystallized from acetonitrile. Found : C, 33.9 ; H, 6.7; N, 2.5. Calc. for [(C!.,H,),~,H,~,,O,,] : C, 34.2; H, 6.6; N, 2.5%.
811
812
K. NOMIYA
(K, H)9[PV14042] - nH,O (2). The deep red aqueous solution obtained in the preparation of 1, the pH of which had been adjusted to 2-3 by nitric acid, was kept at 50°C in a steam bath, to which an aqueous solution at 50°C containing 10 g KC1 was slowly added. After stirring, the solution was left standing overnight at room temperature. Fine black crystals were obtained with a yield of4.8 g. They were very soluble in water, slightly soluble in methanol and ethanol, and almost insoluble in ether, acetone and acetonitrile. In this preparation, the reaction temperature of 50°C was important : if it was lower, compound 2 was contaminated by an orange compound, potassium decavanadate, and, at room temperature, the latter became the main product. C(C4H9)4N14H5CPV140421 - 4H,O (3). Compound 2 (3.5 g) was dissolved in water (below SOY!) and then an aqueous solution containing excess tetrabutylammonium bromide was added. After stirring at 5060°C for a few minutes, dark brown precipitates were collected on a titer which could be recrystallized from acetonitrile. They were very soluble in a&o&rile, slightly soluble in acetone, and insoluble in water, methanol, ethanol and ether. Found : C, 31.3 ; H, 6.2; N, 2.3. Calc. for C(C4~9)4~~4~5~~~14042~ -4~~0: C, 31.2; H, 6.4; N, 2.3%. From the red aqueous filtrate in this preparation, the same precipitates could not be obtained again even by further addition of the aqueous solution of tetrabutylammonium bromide. The filtrate contained salts with different compositions of counterions because the evaporated solids showed the characteristic IR of vanadophosphate. The tetrabutylammonium salt (3) was also obtained by another method. The red aqueous solution containing compound 2 (5 g) was passed through an H+-ion exchange resin column (Dowex50W) and eluted with water. The vanadophosphate was converted to the free-acid form without decomposition. The eluted aqueous solution was evaporated to a small volume and then an aqueous solution containing excess tetrabutylammonium bromide was added. After stirring at 5060°C for a few minutes, dark brown precipitates were filtered and recrystallized from acetonitrile. Found : C, 31.2; H, 6.3; N, 2.3. Calc. for [(C4H9)4N]4H5[PV14042] * 4H,O: C, 31.2; H, 6.4; N, 2.3%. In order to compare IR spectra, sodium decavanadate Nas[V,,,02s] * nH,O was prepared from NaV03 and glacial acetic acid by a modification described in the literature.4 UV-vis absorption spectra were measured on an Hitachi 340~spectrophotometer with a computer keyboard attached. The MCD spectra were recorded by a JASCO J40AS spectropolarimeter with a 1-T electromagnet mounted on it. IR spectra were
et al.
recorded by a JASCO IR-G spectrophotometer. Measurements were made at room temperature. RESULTS
AND DISCUSSION
Some salts of vanadophosphate obtained here, except the guanidinium salt, are soluble in appropriate solvents. The potassium salt (2) especially is very soluble in water and can be used for preparing other salts. The acidity of the free acid of vanadophosphate obtained as a powder from the eluted solution of a cation-exchange resin column was not as strong as that of the Keggin molybdo- and tungstoheteropolyacids. In fact, acid catalysis of the polymerization of benzyl alcohol to polybenzyl could not be observed.5 Generally, the maximum number of bulky tetrabutylammoniums in the counterions is significantly related to the size, rather than to the charge, of the polyanion. For instance, the maximum number is four for the Keggin heteropolyanion and six for the Dawson one. Thus, it should be noted that it is four in the bicapped-Keggin vanadophosphate. IR spectra of vanadophosphate (&-symmetry) with some counterions are shown in Fig. 1. The vibrational structure of the polyanion moiety is
3
I
1100
I
1
900
I
a
700
.
.
.-
500 cn
Fig. 1. IR spectra measured in KBr disk of vanadophosphates with counterions : (a) (CN,H,),H[PV,,O,,] %O,
0-k H)PCPV140421 -nI%O, and C(C~H,)~~,H,CPV,,O~~~-~HZO.
09
(c)
Vanadophosphate polyanions with bicapped-Keggin structure
813
W Q
MCD +0.5
(a)
6-
1100
900
700
500&l
Fig. 2. IR spectra measured in KBr disk of decavanadates with counterions : (a) (CN,H&V,,O,,] - 6H,O, (b) CGW4NJ3H3cVIo%l, and(4 Na6CVdM *n&O. subtly changed by counterions, especially in the lowfrequency region. The characteristic band at 10501060 cm- ’ is due to P-O stretching and many bands in the region of 1000-500 cm-i are due to the various V-O vibrations. These patterns are more complicated than those of a Keggin heteropolyanion with T,-symmetry. The IR spectra of decavanadate are also changed by counterions, depending upon whether the proton is contained or not (Fig. 2). IR patterns of (CN,H,),[V,,O,s] and Na,[V,,02s] are very close to each other; however, they are different from that of [(C4H,),Nl,H3[V,,02a] in the region of W-600 cm-‘. These are presumably caused by either or both of two possibilities, the existence of an anion-anion interaction in the case of a small-size counterion and the protonation of the surface oxygens of the polyanion.’ Electronic absorption and MCD spectra in acetonitrile solution are shown in Fig. 3 as full lines for [(C4H9)4N]4H5[PV14042] *4Hz0 and as dashed lines for [(C,H,),Nj,H,[V,,,O,,]. All of these absorptions and MCD are due to the V c 0 charge-transfer (CT) transitions. In the absorption spectra, there is a very broad shoulder (E ca 20,000) around 30,000 cm- ’ and a strong broad band (ECQ 60,000) around 43,000 cm-’ for the vanadophosphate, and a shoulder (E ca 10,000) around 30,000 cm- ’ and a broad band (s ca 32,000) around 40,000 cm- ’ for the decavanadate. The intensities of the
25
30
35
40
45
Y/10%,1
Fig. 3. Electronic absorption and MCD spectra in acetonitrile solution of [(C,H&QH5[PV,,0,,1.4H,0 (fullline) and C(c,H9)4N3H3cV~~O~~l @=l=dline).
absorptions of vanadophosphate are larger than those of decavanadate. The absorption position of the lowest-energy CT is almost unchanged in the two vanadopolyanions; however, that of the second CT of vanadophosphate is shifted more to the highfrequency region than that of decavanadate. These tendencies are also observed in the MCD spectra, which consist of two positive components. No negative MCD component appears in this region. The MCD in the first CT region of vanadophosphate contains further components. REFERENCES 1.
M. T. Pope, Heterpoly and Zsopoly Oxometalates. Springer, New York (1983). 2. R. Kato, A. Kobayashi and Y. Sasaki,J. Am. Chem. Sot. 1980,102,6572.
3. R. Kato, A. Kobayashi and Y. Sasaki, Znorg. Chem. 1982,21,240.
4. G. K. Johnson and R. K. Murmann, Znorg. Synth. 1979,19,
140.
5. K. Nomiya, T. Ueno and M. Miwa,Bull.Chem.Sot. Jpn 1980,53,827.
6. C. Rocchiccioli-Deltcheff,M. Fournier and R. Franck, Znorg.Chem. 1983,22,207. 7. H. T. Evans, Jr and M. T. Pope, Znorg. Chem. 1984,23, 501.