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Anderson-type heteropolyanions of molybdenum(VI) and tungsten(VI)
Polyhedron Vol. 6, No. 2, pp. 213-218, Printed in Great Britain
1987 0
0277-5387187 S3.00+.00 1987 Pergamon Journals Ltd
ANDERSON-TYPE HETEROPOLYANIONS OF MOLYBDENUM(W) AND TUNGSTEN(W) KENJI NOMNA,
TAKE0 TAKAHASHI, T AKAHIRO SHIRAI and MAKOTO MIWA*
Department of Industrial Chemistry, Faculty of Engineering, Seikei University, Musashino, Tokyo 180, Japan (Received 7 April 1986 ; accepted 28 May 1986) Abstract-The previously reported preparation of some Anderson-type molybdopolyanions containing divalent metal ions (Zn, Cu, Co or Mn) as a heteroatom has been reinvestigated. The molybdopolyanions of Zn(I1) and Cu(I1) were contied, although the Cu(I1) polyanion was not stable and could not be recrystallized. On the other hand, the polyanions of Co(I1) and Mn(I1) could not be reproduced. Another type of heteropoly compound, D((H,O),_.(MO,O~~)]~- [x = Cu(II), Co(II) or Mn(II)], was isolated as solids, which are not stable thermally. The mixed-type Anderson polyanions, ~i(II)Mos_,WX024H,]4-, which have been questioned as mixtures of species with different x values, were also reinvestigated using IR, UV absorption and MCD spectra. They are single species, but not mixtures, although some positional isomers may be present for the compounds where x = 2-4. The possibility of oxidation of the heteroatom with the Anderson structure maintained was examined. The oxidation of [Ni(II)Mo602fis]4- by the SzOi- ion in aqueous solution gave the Waugh-type ~i(Iv)Mo,032]6polyanion, whereas the oxidation of [Ni(II)W,024Hd4- gave no heteropoly compound.
Anderson-type heteropolyanions, represented by the general formula ~M,O,,H,l”-, possess a heteroatom (X) in a central octahedral cavity of the crown by edge-sharing six octahedral MO6 (M = MO or W).’ These polyanions become a familyforanumberof2+,3+,4+,6+ and7+ metal ions as the heteroatom. They have been classified into A (x = 0) and B (X = 6) types by the number of attached protons, although some polyanions with x other than 0 or 6 have been recently reported.2*3 Most of them have been tabulated in books.‘.4 However, some questionable compounds are also involved there. In this work, we have done three experimental studies. The first is related to members of the family of B-type molybdopolyanions, especially the polyanions containing some divalent metal ions [Zn(II), Cu(II), Co(I1) or Mn(II)] other than Ni(II), which have been tist reported by LaGinestra et al.,’ and later questioned by Malik et al6 The second is a reinvestigation by spectro-
scopic (IR, UV absorption and MCD) methods of the mixed-type Ni(I1) molybdotungstopolyanions previously reported by Matijevic et ~1.’ The last concerns the possibility of the oxidation of Ni(I1) molybdo- and tungstopolyanions leading to the corresponding Ni(IV) ones with the polyanion structure maintained.
EXPERIMENTAL Electronic absorption spectra were measured by a Hitachi 340 spectrophotometer with an attached computer-key board. MCD spectra were recorded by a JASCO J-40AS spectropolarimeter mounted with a lO.O-kG electromagnet. IR spectra were recorded with a JASCO IR-G spectrophotometer. Measurements were made at room temperature.
Preparations
WHWWW@d-bl~ *Author to whom correspondence
(II)Mo,O,,Hd
should be addressed. 213
7W3
and W-bMX-
- 5H,O. B-type molybdopolyanions
214
K. NOMIYA et al.
containing tervalent metal ions (Cr, Al or Fe) and Ni(I1) ion were obtained as crystals by the traditional method :* adding an aqueous solution of metal sulphates or alums (3.1 x low3 mol) in 20 cm3 water into a boiling aqueous solution of (NH.&Mo~O~~ - 4Hz0 (5 g, 4.2 x low3 mol) dissolved in 80 cm3 water, further evaporating on a steam-bath, filtering the hot solution and cooling. In the preparation of the Co(II1) polyanion, a mixed aqueous solution (30 cm3) of CoSO, - 7H,O (4.2 g, 0.015 mol) and 30% aqueous HzOz (2 g) was added into the boiling solution of aqueous heptamolybdate (30.9 g, 2.5 x lo-’ mol in 260 cm3 water). These compounds were recrystallized twice from water. The colour and analytical data are listed in Table 1. Visible absorption spectra of the Co(III), Cr(II1) and Ni(I1) compounds are in good agreement with the previous data.g When the colourless Fe(II1) polyanion was dissolved in water, the solution was a wine-red colour due to the dissociation and/or the hydrolysis. The preparation of B-type Anderson molybdopolyanions containing divalent metal ions (Ni, Zn, Co or Mn) has been reported by LaGinestra et ~1.~However, some questionable points were seen. They stated that such polyanions must be prepared without boiling: however, the Ni(I1) and Zn(I1) compounds were actually obtained from boiling solutions. The Zn(I1) compound was obtained as colourless crystals by adding an aqueous solution of ZnS04 * 7Hz0 (1 g, 3 x 1OT3mol in 20 cm3 water) into a boiled solution of heptamolybdate (5 g, 4.2 x 10e3 mol in 80 cm3 water). It was recrystallized twice from water. Analytical data are listed in Table 1. This compound was very soluble in water. Further, the Cu(I1) compound was obtained by adding Cu(I1) sulphate solution (0.75 g, 3 x 10e3 mol in 20 cm3 water) into the boiled solution of heptamolybdate (5 g, 4.2 x 10e3 mol in 80 cm3
Table 1. Colour and analytical data for (NH&X(III) Mo60xH6] - 7H,O and (NH&X(II)Mo,O,H,] * 5Hz0 Found (%) X Co(II1) Cr(II1) Fe(II1) Al(II1) Ni(I1) Zn(I1)
Colour Blue-green Reddish-violet Colourless Colourless Sky-blue Colourless
NH 2.4 2.4 2.4 2.4 2.7 2.3
Calculated (%) NH
3.6 3.6 3.6 3.7 4.6 4.5
2.7 2.7 2.7 2.7 2.7 2.7
3.5 3.5 3.5 3.6 4.7 4.7
water), filtering the yellow insoluble precipitate produced, cooling the filtrate, and adding excess amounts of acetonitrile. The light blue solid obtained was insoluble in water and not stable thermally. The Co(II) and Mn(I1) compounds were not obtained from boiling solutions. Any changes in the pH of the initial heptamolybdate solution and in the reaction time gave mixtures of the initial materials and the bluish violet [for Co(H)] or yellow [for Mn(II)] insoluble solids. On the other hand, from the experiments without boiling, the Cu(II), Co(I1) and Mn(II) compounds could be obtained, which were insoluble in water and not stable thermally. They were prepared by concentrating the mixed aqueous solutions of the metal sulphates and heptamolybdate without heating and/or adding excess amounts of acetonitrile. However, their spectra were not identical with the characteristic IR spectra of the Anderson molybdopolyanions. (NH&Ni(II)Mo,_,W,O,H,] * 5H20. B-type Ni(II) molybdotungstopolyanions (x = 1,3,5 or 6) were prepared according to two methods of Matijevic et al.’ Method 1 is based on the dropwise addition of an aqueous solution of Ni(II) sulphate into boiling solutions containing appropriate molar ratios of MO and W, where the source of MO is (NH.&Mo70z4 * 4Hz0 for x = 0, Na,MoO, - 2H,O for x = 1, and Moo3 for x = 2-5 ; and the source of W is Na*WO., - 2Hz0 for all x. Method 2 is based on heating above 80°C aqueous solutions containing appropriate molar mixtures of already isolated x = 0 or 6 compounds, and cooling spontaneously to room temperature. All compounds, recrystallized twice. from water, were obtained as sky-blue crystals. Analytical results for all the compounds were in good agreement within the experimental error. However, they do not lead to direct evidence for these mixed species, because they cannot be discriminated from those of mixtures of species with different compositions. (Na, K),[Ni(IV)W,O$ * nHzO. An A-type Ni(IV) tungstopolyanion was obtained by the modification of the preparation of the isostructural Mn(IV) tungstopolyanion.” A solution of Na2W04 * 2H,O (20 g, 0.06 mol) in 100 cm3 water was boiled, into which NiSO,* 6Hz0 (2.6 g, 0.01 mol) in 10 cm3 water was slowly added. Further, a fine powder of K&O,, (5.4 g, 0.02 mol) was added. The boiling was continued for about 15 min, with occasional additions of water. The reaction mixture was poured into an equal amount of hot water and the solution kept at 80°C for about 30 min on the steam-bath. The black crystals formed were filtered, washed with water, and dried (yield 2.5 g). This compound was slightly soluble in water and insoluble in most other solvents.
Anderson-type
heteropolyanions
of molybdenum(V1)
215
and tungsten(W)
Oxidation of Ni(II) molybdo- and tungstopolyanions When an aqueous solution of (NH,),[Ni(II) Mo60z4H6] - 5Hz0 containing (NH&SzOs was heated, a colour change was seen ; from sky-blue it turned gradually to black. After the homogeneous black solution was left standing at room temperature, the colourless precipitates produced were filtered off. The black crystals obtained from the filtrate showed IR spectra identical with that of the Waugh-type heteropoly compound (NH&Ni(IV)Mo,O,,], which was prepared separately.13 On the other hand, in the analogous experiments using (NH,)JNi(II)W,O,H,] * 5Hz0, no heteropoly compound could be obtained. In this reaction, the solution became blackish via yellowish-brown, but then it turned to green. Further addition of (NH&S208 showed no more colour change. Only the mixture of decomposed materials was given. RESULTS AND DISCUSSION B-type molybdopolyanions as a heteroatom
with divalent metal ions
In the Anderson heteropolyanion, each heteroatom forms an octahedral complex of six oxygens for A-type and of six OH groups for B-type. Thus, the visible absorption spectra of B-type polyanions of Co(III), Cr(II1) and Ni(II) are comparable with that of the corresponding hexaaqua complexes,g although the peak intensities of the polyanions are smaller as shown in Table 2. IR spectra of Anderson-type ~(III)Mo,O,,H,]~[x(111) = Co(III), Cr(III), Fe(II1) or AI(III)] are presented in Fig. 1. The bands due to the heteroatom are seen only in the region less than 450 cm-‘. Since the IR spectra in the 950-900- and 65&
1000
Fig.
1.
600
600
Solid IR spectra of Anderson-type P(III)Mo~O~~HJ~polyanions.
550-m-’
regions become independent of the sort of heteroatom, they can be said to be in the pattern common to the B-type Anderson molybdopolyanions. In fact, the Ni(II), Zn(I1) and Cu(I1) molybdopolyanions show the characteristic IR spectra (Fig. 2). The IR spectrum previously shown for the Mn(I1) compound by LaGinestra et al.’ is evidently different from these patterns.
Table 2. Spectral data of visible and near-IR absorption of Anderson-type molybdopolyanions of Co(III), Cr(II1) and Ni(I1) I. (x lo-’ cm-‘) (&) [Co(III)Mo,0,H,13[CO(H~O)J~+~
16.5 (18.6) 16.5 (40)
24.2 (18.4) 25.0 (50)
[Cr(III)Mo,O%H J3[Cr(HA%13+ LI
18.5 (7.8) 17.4 (13.3)
25.3 (9.5) 24.6 (15.3) 13.7 (1.2) 13.8 (2.1) 14.5 (6)
[Ni(II)Mo,OxH$-
9.3 (1.6)
lWH@M*’ a
8.5 (2.0)
[SiNiMo, ,0,H,]6-b
8.6 (5.3)
“Ref. 10. ‘Ref. 11.
4 0 (cm-‘)
15.6 (1.8) 15.2 (1.9)
K. NOMIYA et al.
216
1000
Fig.
2.
800
600
8
600
Solid IR spectra of Anderson-type [x(II)Mo,0,H,J4polyanions.
Although they have attributed the disagreement to the different formulae due to the number of hydrates, it presumably accounted for the compound itself. On the other hand, the IR spectrum presented by them as that of the Co(I1) compound is apparently similar to the characteristic patterns. We have occasionally obtained the compound showing such an IR spectrum from the reaction involving boiling with a Co(II) sulphate solution and an aqueous heptamolybdate solution adjusted to pH 4.46. However, such a compound was a minor product. The Cu(II), Co(I1) and Mn(I1) compounds obtained from the experiments without boiling were not of the Anderson type, but like the Mo70z4 polyanion. These compounds showed IR spectra very similar to that of the [Mo~O~~]~-polyanion, except for the slightly broad bands in the 650-550~cn-’ region and the ca 10 cm-’ shift to the high-frequency region of the bands at cu 900 cm-’ (Fig. 3). They were insoluble in water and could not be recrystallized. Heating the aqueous suspension led to decomposition. These compounds are probably the D((Hz0)6_,(M07024)]4--type 1 : 1 complex, which was first proposed by Malik et a1.,6
400
600 (cm?
km-’
Fig. 3. Solid IR spectra of the 1: 1 metal ion (X): Mo,O;; complexes and Mo,O& alone (on the bottom).
from the Job method of continuous variation in an aqueous solution of metal sulphate and heptamolybdate. As a related complex, we have previously isolated the 1 : 2 Ce(II1) : Mo70z4 complex as orange-red crystals from an aqueous solution containing ammonium Ce(II1) nitrate and ammonium heptamolybdate.14,‘5 The preparation was done only at room temperature, and the Ce(II1) complex obtained was also insoluble in water, and not stable thermally. The Anderson heteropolyanion is one of the most well-known polyanions and it constitutes a family with a number of heteroatoms.‘*4 We propose that the Co(I1) and Mn(I1) ions should be excluded from the Anderson family and entered into another category of polyanion. Mixed-type
[Ni(II)M06_,W,o,H6]“-
polyanions
For these mixed polyanions, the possibility has been pointed that they are mixtures of polyanions with different x values.’ However, it was easily confirmed from the IR spectra that the x = 3 compound obtained by method 1 differed from an equimolar mixture of the x = 0 and x = 6 compounds.
217
Anderson-type heteropolyanions of molybdenum(V1) and tungsten(V1) Further, the IR spectra of the x = 3 compounds obtained by methods 1 and 2 were identical, as shown in Fig. 4. The IR spectra of each mixed polyanion were subtly different, and all of them resembled that of the x = 6 rather than the x = 0 compound as a whole. In aqueous solution, these mixed polyanions showed the behaviour of a single species. Figure 5 shows the W and MCD spectra of the x = 0, 1,3, 5 and 6 compounds obtained by method 1. MCD spectra evidently indicate that they are single species, but not a mixture of species with different compositions, because the peak positions are quite different. We can see some implications for the formation of the mixed Anderson cage in the experiments by method 2, and also in the recrystallization process for the x = 0 and x = 6 compounds. When an aqueous solution containing the Ni(I1) Anderson polyanion is heated above 8O”C, a colour change of the solution is observed from sky-blue to green. During the cooling process to room temperature, it returns to the original sky-blue. The green solution will be due to the Ni(I1) aqua ion produced by dissociation
1200
1000
800
400 600 (cmd)
of the polyanion. Therefore, it seems that, during the cooling process, the Anderson cage is formed around the Ni(II) ion as a core. The mixed Anderson cage will be formed on the basis of the reorientation of randomly distributed MO and W 0x0 ions at the high temperature. Thus, the experiments by two methods do not exclude the possibility that the x = 3 compound, and also the x = 2 and x = 4 compounds, contain some positional isomers. Oxidation of Ni(II) molybdo- and tungstopolyanions
The oxidation of the [Ni(II)Mo60z4HJ4- polyanion by the SzOi- ion in aqueous solution led to the formation of a Waugh-type [Ni(IV)Mo,0,,16polyanion, which was isolated separately.13 The valency of the heteroatom can be changed and the polyanion cage is simultaneously transformed. On the other hand, the oxidation of [Ni(II)W&H6]4gave the decomposition mixture, but no heteropoly compound. The same initial materials have been used in the preparations of [Ni(rv)W@z4]8- and [Ni(II)w6024H6]4- pdyauhs, except for the
600
400 (cm’)
Fig. 4. Solid IR spectra of [Ni(II)Mo,_,W,O,J5$ polyanions; x = 6 (A) and 0 (B) compounds, equimolar mixture (A + B) of them, and x = 3 compounds obtained by methods 1 (C) and 2 (D).
218
K. NOMIYA
et al.
which proceed by the outer-sphere mechanism of an electron transfer. 18*ig In this case, the Anderson structure may be thermally less stable than the Keggin structure.
REFERENCES
.Y
‘0 ;i W
2
5. 6.
t-
7. 8. 9.
o-
’
25
I
30
35
40
45
I
. 50 10Jcm-l
Fig. 5. UV absorption and MCD spectra of [Ni(II)Mo~_xWxOxH,]4polyanions : x = 0 (----), x = 5 ( .e..), and x=1 (-----), x=3 (-), x = 6 (----).
10. 11. 12.
13.
addition of the SzOg- ion in the former. Thus, this oxidation experiment suggests that the formation of the [Ni(IV)W,O,,]“- polyanion does not proceed through the oxidation of the already formed [Ni(II)W6024H6]4- polyanion. In a Keggin-type polyanion, the valency of the central heteroatom can be changed sometimes by chemical oxidation, with the polyanion structure maintained, such as [CO(II)W,,O,]~- + [CO(III)W,~O~]~- 16*17and [CU(I)W~~O~]‘- + [CU(II)W,~O~~]~-,~~ reactions
14. 15. 16. 17. 18. 19.
M. T. Pope, Heteropoly and Zsopoly Oxometalates. p. 21. Springer, New York (1983). U. Lee, A. Kobayashi and Y. Sasaki, Acta Cryst. 1983, C39,817. U. Lee and Y. Sasaki, Chem. Lett. 1984,1297. F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 4th Edn, p. 852. John Wiley, New York (1980). A. LaGinestra, F. Giannetta and P. Fiorucci, Gazz. Chim. Ital. 1968,98, 1197. A. Malik, S. A. Zubaili and S. Khan, J. Chem. SOL, Dalton Trans. 1977, 1049. E. Matijevic, M. Kerker, H. Bayer and F. Theubert, Znorg. Chem. 1963,2, 581. R. D. Hall, J. Am. Chem. Sot. 1907,29,692. Y. Shimura, H. Ito and R. Tsuchida, Nippon Kagaku Zasshi 1954,75,560. D. Sutton, Electronic Spectra of Transition Metal Complexes. McGraw-Hill, New York (1968). S. A. Malik, J. Znorg. Nucl. Chem. 1970,32,2425. V. S. Sergienko, V. N. Molchanov, M. A. PoraiKoshits and E. A. Torchenkova, Koord. Khim. 1979, 5,936; Sov. J. Coord. Chem. (Engl. Trans.) 1979,5, 740. K. Nomiya, R. Kobayashi and M. Miwa, Polyhedron 1985,4, 149. M.-J. Schwing-Weill, Bull. Sot. Chim. Fr. 1972,1754. K. Nomiya, H. Murasaki and M. Miwa, unpublished results. L. C. W. Baker and T. P. McCutcheon, J. Am. Chem. Sot. 1956,78,4503. K. Nomiya, R. Kobayashi and M. Miwa, Bull. Chem. Sot. Jpn 1983,56,2272. A. G. Lappin and R. D. Peacock, Znorg. Chim. Acta 1980,46, L71. P. G. Rasmussen and C. H. Brubaker, Znorg. Chem. 1964,3,977.
1987 0
0277-5387187 S3.00+.00 1987 Pergamon Journals Ltd
ANDERSON-TYPE HETEROPOLYANIONS OF MOLYBDENUM(W) AND TUNGSTEN(W) KENJI NOMNA,
TAKE0 TAKAHASHI, T AKAHIRO SHIRAI and MAKOTO MIWA*
Department of Industrial Chemistry, Faculty of Engineering, Seikei University, Musashino, Tokyo 180, Japan (Received 7 April 1986 ; accepted 28 May 1986) Abstract-The previously reported preparation of some Anderson-type molybdopolyanions containing divalent metal ions (Zn, Cu, Co or Mn) as a heteroatom has been reinvestigated. The molybdopolyanions of Zn(I1) and Cu(I1) were contied, although the Cu(I1) polyanion was not stable and could not be recrystallized. On the other hand, the polyanions of Co(I1) and Mn(I1) could not be reproduced. Another type of heteropoly compound, D((H,O),_.(MO,O~~)]~- [x = Cu(II), Co(II) or Mn(II)], was isolated as solids, which are not stable thermally. The mixed-type Anderson polyanions, ~i(II)Mos_,WX024H,]4-, which have been questioned as mixtures of species with different x values, were also reinvestigated using IR, UV absorption and MCD spectra. They are single species, but not mixtures, although some positional isomers may be present for the compounds where x = 2-4. The possibility of oxidation of the heteroatom with the Anderson structure maintained was examined. The oxidation of [Ni(II)Mo602fis]4- by the SzOi- ion in aqueous solution gave the Waugh-type ~i(Iv)Mo,032]6polyanion, whereas the oxidation of [Ni(II)W,024Hd4- gave no heteropoly compound.
Anderson-type heteropolyanions, represented by the general formula ~M,O,,H,l”-, possess a heteroatom (X) in a central octahedral cavity of the crown by edge-sharing six octahedral MO6 (M = MO or W).’ These polyanions become a familyforanumberof2+,3+,4+,6+ and7+ metal ions as the heteroatom. They have been classified into A (x = 0) and B (X = 6) types by the number of attached protons, although some polyanions with x other than 0 or 6 have been recently reported.2*3 Most of them have been tabulated in books.‘.4 However, some questionable compounds are also involved there. In this work, we have done three experimental studies. The first is related to members of the family of B-type molybdopolyanions, especially the polyanions containing some divalent metal ions [Zn(II), Cu(II), Co(I1) or Mn(II)] other than Ni(II), which have been tist reported by LaGinestra et al.,’ and later questioned by Malik et al6 The second is a reinvestigation by spectro-
scopic (IR, UV absorption and MCD) methods of the mixed-type Ni(I1) molybdotungstopolyanions previously reported by Matijevic et ~1.’ The last concerns the possibility of the oxidation of Ni(I1) molybdo- and tungstopolyanions leading to the corresponding Ni(IV) ones with the polyanion structure maintained.
EXPERIMENTAL Electronic absorption spectra were measured by a Hitachi 340 spectrophotometer with an attached computer-key board. MCD spectra were recorded by a JASCO J-40AS spectropolarimeter mounted with a lO.O-kG electromagnet. IR spectra were recorded with a JASCO IR-G spectrophotometer. Measurements were made at room temperature.
Preparations
WHWWW@d-bl~ *Author to whom correspondence
(II)Mo,O,,Hd
should be addressed. 213
7W3
and W-bMX-
- 5H,O. B-type molybdopolyanions
214
K. NOMIYA et al.
containing tervalent metal ions (Cr, Al or Fe) and Ni(I1) ion were obtained as crystals by the traditional method :* adding an aqueous solution of metal sulphates or alums (3.1 x low3 mol) in 20 cm3 water into a boiling aqueous solution of (NH.&Mo~O~~ - 4Hz0 (5 g, 4.2 x low3 mol) dissolved in 80 cm3 water, further evaporating on a steam-bath, filtering the hot solution and cooling. In the preparation of the Co(II1) polyanion, a mixed aqueous solution (30 cm3) of CoSO, - 7H,O (4.2 g, 0.015 mol) and 30% aqueous HzOz (2 g) was added into the boiling solution of aqueous heptamolybdate (30.9 g, 2.5 x lo-’ mol in 260 cm3 water). These compounds were recrystallized twice from water. The colour and analytical data are listed in Table 1. Visible absorption spectra of the Co(III), Cr(II1) and Ni(I1) compounds are in good agreement with the previous data.g When the colourless Fe(II1) polyanion was dissolved in water, the solution was a wine-red colour due to the dissociation and/or the hydrolysis. The preparation of B-type Anderson molybdopolyanions containing divalent metal ions (Ni, Zn, Co or Mn) has been reported by LaGinestra et ~1.~However, some questionable points were seen. They stated that such polyanions must be prepared without boiling: however, the Ni(I1) and Zn(I1) compounds were actually obtained from boiling solutions. The Zn(I1) compound was obtained as colourless crystals by adding an aqueous solution of ZnS04 * 7Hz0 (1 g, 3 x 1OT3mol in 20 cm3 water) into a boiled solution of heptamolybdate (5 g, 4.2 x 10e3 mol in 80 cm3 water). It was recrystallized twice from water. Analytical data are listed in Table 1. This compound was very soluble in water. Further, the Cu(I1) compound was obtained by adding Cu(I1) sulphate solution (0.75 g, 3 x 10e3 mol in 20 cm3 water) into the boiled solution of heptamolybdate (5 g, 4.2 x 10e3 mol in 80 cm3
Table 1. Colour and analytical data for (NH&X(III) Mo60xH6] - 7H,O and (NH&X(II)Mo,O,H,] * 5Hz0 Found (%) X Co(II1) Cr(II1) Fe(II1) Al(II1) Ni(I1) Zn(I1)
Colour Blue-green Reddish-violet Colourless Colourless Sky-blue Colourless
NH 2.4 2.4 2.4 2.4 2.7 2.3
Calculated (%) NH
3.6 3.6 3.6 3.7 4.6 4.5
2.7 2.7 2.7 2.7 2.7 2.7
3.5 3.5 3.5 3.6 4.7 4.7
water), filtering the yellow insoluble precipitate produced, cooling the filtrate, and adding excess amounts of acetonitrile. The light blue solid obtained was insoluble in water and not stable thermally. The Co(II) and Mn(I1) compounds were not obtained from boiling solutions. Any changes in the pH of the initial heptamolybdate solution and in the reaction time gave mixtures of the initial materials and the bluish violet [for Co(H)] or yellow [for Mn(II)] insoluble solids. On the other hand, from the experiments without boiling, the Cu(II), Co(I1) and Mn(II) compounds could be obtained, which were insoluble in water and not stable thermally. They were prepared by concentrating the mixed aqueous solutions of the metal sulphates and heptamolybdate without heating and/or adding excess amounts of acetonitrile. However, their spectra were not identical with the characteristic IR spectra of the Anderson molybdopolyanions. (NH&Ni(II)Mo,_,W,O,H,] * 5H20. B-type Ni(II) molybdotungstopolyanions (x = 1,3,5 or 6) were prepared according to two methods of Matijevic et al.’ Method 1 is based on the dropwise addition of an aqueous solution of Ni(II) sulphate into boiling solutions containing appropriate molar ratios of MO and W, where the source of MO is (NH.&Mo70z4 * 4Hz0 for x = 0, Na,MoO, - 2H,O for x = 1, and Moo3 for x = 2-5 ; and the source of W is Na*WO., - 2Hz0 for all x. Method 2 is based on heating above 80°C aqueous solutions containing appropriate molar mixtures of already isolated x = 0 or 6 compounds, and cooling spontaneously to room temperature. All compounds, recrystallized twice. from water, were obtained as sky-blue crystals. Analytical results for all the compounds were in good agreement within the experimental error. However, they do not lead to direct evidence for these mixed species, because they cannot be discriminated from those of mixtures of species with different compositions. (Na, K),[Ni(IV)W,O$ * nHzO. An A-type Ni(IV) tungstopolyanion was obtained by the modification of the preparation of the isostructural Mn(IV) tungstopolyanion.” A solution of Na2W04 * 2H,O (20 g, 0.06 mol) in 100 cm3 water was boiled, into which NiSO,* 6Hz0 (2.6 g, 0.01 mol) in 10 cm3 water was slowly added. Further, a fine powder of K&O,, (5.4 g, 0.02 mol) was added. The boiling was continued for about 15 min, with occasional additions of water. The reaction mixture was poured into an equal amount of hot water and the solution kept at 80°C for about 30 min on the steam-bath. The black crystals formed were filtered, washed with water, and dried (yield 2.5 g). This compound was slightly soluble in water and insoluble in most other solvents.
Anderson-type
heteropolyanions
of molybdenum(V1)
215
and tungsten(W)
Oxidation of Ni(II) molybdo- and tungstopolyanions When an aqueous solution of (NH,),[Ni(II) Mo60z4H6] - 5Hz0 containing (NH&SzOs was heated, a colour change was seen ; from sky-blue it turned gradually to black. After the homogeneous black solution was left standing at room temperature, the colourless precipitates produced were filtered off. The black crystals obtained from the filtrate showed IR spectra identical with that of the Waugh-type heteropoly compound (NH&Ni(IV)Mo,O,,], which was prepared separately.13 On the other hand, in the analogous experiments using (NH,)JNi(II)W,O,H,] * 5Hz0, no heteropoly compound could be obtained. In this reaction, the solution became blackish via yellowish-brown, but then it turned to green. Further addition of (NH&S208 showed no more colour change. Only the mixture of decomposed materials was given. RESULTS AND DISCUSSION B-type molybdopolyanions as a heteroatom
with divalent metal ions
In the Anderson heteropolyanion, each heteroatom forms an octahedral complex of six oxygens for A-type and of six OH groups for B-type. Thus, the visible absorption spectra of B-type polyanions of Co(III), Cr(II1) and Ni(II) are comparable with that of the corresponding hexaaqua complexes,g although the peak intensities of the polyanions are smaller as shown in Table 2. IR spectra of Anderson-type ~(III)Mo,O,,H,]~[x(111) = Co(III), Cr(III), Fe(II1) or AI(III)] are presented in Fig. 1. The bands due to the heteroatom are seen only in the region less than 450 cm-‘. Since the IR spectra in the 950-900- and 65&
1000
Fig.
1.
600
600
Solid IR spectra of Anderson-type P(III)Mo~O~~HJ~polyanions.
550-m-’
regions become independent of the sort of heteroatom, they can be said to be in the pattern common to the B-type Anderson molybdopolyanions. In fact, the Ni(II), Zn(I1) and Cu(I1) molybdopolyanions show the characteristic IR spectra (Fig. 2). The IR spectrum previously shown for the Mn(I1) compound by LaGinestra et al.’ is evidently different from these patterns.
Table 2. Spectral data of visible and near-IR absorption of Anderson-type molybdopolyanions of Co(III), Cr(II1) and Ni(I1) I. (x lo-’ cm-‘) (&) [Co(III)Mo,0,H,13[CO(H~O)J~+~
16.5 (18.6) 16.5 (40)
24.2 (18.4) 25.0 (50)
[Cr(III)Mo,O%H J3[Cr(HA%13+ LI
18.5 (7.8) 17.4 (13.3)
25.3 (9.5) 24.6 (15.3) 13.7 (1.2) 13.8 (2.1) 14.5 (6)
[Ni(II)Mo,OxH$-
9.3 (1.6)
lWH@M*’ a
8.5 (2.0)
[SiNiMo, ,0,H,]6-b
8.6 (5.3)
“Ref. 10. ‘Ref. 11.
4 0 (cm-‘)
15.6 (1.8) 15.2 (1.9)
K. NOMIYA et al.
216
1000
Fig.
2.
800
600
8
600
Solid IR spectra of Anderson-type [x(II)Mo,0,H,J4polyanions.
Although they have attributed the disagreement to the different formulae due to the number of hydrates, it presumably accounted for the compound itself. On the other hand, the IR spectrum presented by them as that of the Co(I1) compound is apparently similar to the characteristic patterns. We have occasionally obtained the compound showing such an IR spectrum from the reaction involving boiling with a Co(II) sulphate solution and an aqueous heptamolybdate solution adjusted to pH 4.46. However, such a compound was a minor product. The Cu(II), Co(I1) and Mn(I1) compounds obtained from the experiments without boiling were not of the Anderson type, but like the Mo70z4 polyanion. These compounds showed IR spectra very similar to that of the [Mo~O~~]~-polyanion, except for the slightly broad bands in the 650-550~cn-’ region and the ca 10 cm-’ shift to the high-frequency region of the bands at cu 900 cm-’ (Fig. 3). They were insoluble in water and could not be recrystallized. Heating the aqueous suspension led to decomposition. These compounds are probably the D((Hz0)6_,(M07024)]4--type 1 : 1 complex, which was first proposed by Malik et a1.,6
400
600 (cm?
km-’
Fig. 3. Solid IR spectra of the 1: 1 metal ion (X): Mo,O;; complexes and Mo,O& alone (on the bottom).
from the Job method of continuous variation in an aqueous solution of metal sulphate and heptamolybdate. As a related complex, we have previously isolated the 1 : 2 Ce(II1) : Mo70z4 complex as orange-red crystals from an aqueous solution containing ammonium Ce(II1) nitrate and ammonium heptamolybdate.14,‘5 The preparation was done only at room temperature, and the Ce(II1) complex obtained was also insoluble in water, and not stable thermally. The Anderson heteropolyanion is one of the most well-known polyanions and it constitutes a family with a number of heteroatoms.‘*4 We propose that the Co(I1) and Mn(I1) ions should be excluded from the Anderson family and entered into another category of polyanion. Mixed-type
[Ni(II)M06_,W,o,H6]“-
polyanions
For these mixed polyanions, the possibility has been pointed that they are mixtures of polyanions with different x values.’ However, it was easily confirmed from the IR spectra that the x = 3 compound obtained by method 1 differed from an equimolar mixture of the x = 0 and x = 6 compounds.
217
Anderson-type heteropolyanions of molybdenum(V1) and tungsten(V1) Further, the IR spectra of the x = 3 compounds obtained by methods 1 and 2 were identical, as shown in Fig. 4. The IR spectra of each mixed polyanion were subtly different, and all of them resembled that of the x = 6 rather than the x = 0 compound as a whole. In aqueous solution, these mixed polyanions showed the behaviour of a single species. Figure 5 shows the W and MCD spectra of the x = 0, 1,3, 5 and 6 compounds obtained by method 1. MCD spectra evidently indicate that they are single species, but not a mixture of species with different compositions, because the peak positions are quite different. We can see some implications for the formation of the mixed Anderson cage in the experiments by method 2, and also in the recrystallization process for the x = 0 and x = 6 compounds. When an aqueous solution containing the Ni(I1) Anderson polyanion is heated above 8O”C, a colour change of the solution is observed from sky-blue to green. During the cooling process to room temperature, it returns to the original sky-blue. The green solution will be due to the Ni(I1) aqua ion produced by dissociation
1200
1000
800
400 600 (cmd)
of the polyanion. Therefore, it seems that, during the cooling process, the Anderson cage is formed around the Ni(II) ion as a core. The mixed Anderson cage will be formed on the basis of the reorientation of randomly distributed MO and W 0x0 ions at the high temperature. Thus, the experiments by two methods do not exclude the possibility that the x = 3 compound, and also the x = 2 and x = 4 compounds, contain some positional isomers. Oxidation of Ni(II) molybdo- and tungstopolyanions
The oxidation of the [Ni(II)Mo60z4HJ4- polyanion by the SzOi- ion in aqueous solution led to the formation of a Waugh-type [Ni(IV)Mo,0,,16polyanion, which was isolated separately.13 The valency of the heteroatom can be changed and the polyanion cage is simultaneously transformed. On the other hand, the oxidation of [Ni(II)W&H6]4gave the decomposition mixture, but no heteropoly compound. The same initial materials have been used in the preparations of [Ni(rv)W@z4]8- and [Ni(II)w6024H6]4- pdyauhs, except for the
600
400 (cm’)
Fig. 4. Solid IR spectra of [Ni(II)Mo,_,W,O,J5$ polyanions; x = 6 (A) and 0 (B) compounds, equimolar mixture (A + B) of them, and x = 3 compounds obtained by methods 1 (C) and 2 (D).
218
K. NOMIYA
et al.
which proceed by the outer-sphere mechanism of an electron transfer. 18*ig In this case, the Anderson structure may be thermally less stable than the Keggin structure.
REFERENCES
.Y
‘0 ;i W
2
5. 6.
t-
7. 8. 9.
o-
’
25
I
30
35
40
45
I
. 50 10Jcm-l
Fig. 5. UV absorption and MCD spectra of [Ni(II)Mo~_xWxOxH,]4polyanions : x = 0 (----), x = 5 ( .e..), and x=1 (-----), x=3 (-), x = 6 (----).
10. 11. 12.
13.
addition of the SzOg- ion in the former. Thus, this oxidation experiment suggests that the formation of the [Ni(IV)W,O,,]“- polyanion does not proceed through the oxidation of the already formed [Ni(II)W6024H6]4- polyanion. In a Keggin-type polyanion, the valency of the central heteroatom can be changed sometimes by chemical oxidation, with the polyanion structure maintained, such as [CO(II)W,,O,]~- + [CO(III)W,~O~]~- 16*17and [CU(I)W~~O~]‘- + [CU(II)W,~O~~]~-,~~ reactions
14. 15. 16. 17. 18. 19.
M. T. Pope, Heteropoly and Zsopoly Oxometalates. p. 21. Springer, New York (1983). U. Lee, A. Kobayashi and Y. Sasaki, Acta Cryst. 1983, C39,817. U. Lee and Y. Sasaki, Chem. Lett. 1984,1297. F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 4th Edn, p. 852. John Wiley, New York (1980). A. LaGinestra, F. Giannetta and P. Fiorucci, Gazz. Chim. Ital. 1968,98, 1197. A. Malik, S. A. Zubaili and S. Khan, J. Chem. SOL, Dalton Trans. 1977, 1049. E. Matijevic, M. Kerker, H. Bayer and F. Theubert, Znorg. Chem. 1963,2, 581. R. D. Hall, J. Am. Chem. Sot. 1907,29,692. Y. Shimura, H. Ito and R. Tsuchida, Nippon Kagaku Zasshi 1954,75,560. D. Sutton, Electronic Spectra of Transition Metal Complexes. McGraw-Hill, New York (1968). S. A. Malik, J. Znorg. Nucl. Chem. 1970,32,2425. V. S. Sergienko, V. N. Molchanov, M. A. PoraiKoshits and E. A. Torchenkova, Koord. Khim. 1979, 5,936; Sov. J. Coord. Chem. (Engl. Trans.) 1979,5, 740. K. Nomiya, R. Kobayashi and M. Miwa, Polyhedron 1985,4, 149. M.-J. Schwing-Weill, Bull. Sot. Chim. Fr. 1972,1754. K. Nomiya, H. Murasaki and M. Miwa, unpublished results. L. C. W. Baker and T. P. McCutcheon, J. Am. Chem. Sot. 1956,78,4503. K. Nomiya, R. Kobayashi and M. Miwa, Bull. Chem. Sot. Jpn 1983,56,2272. A. G. Lappin and R. D. Peacock, Znorg. Chim. Acta 1980,46, L71. P. G. Rasmussen and C. H. Brubaker, Znorg. Chem. 1964,3,977.