Christian G.
Bochet
ab,
Thérèse
Draper
a,
Bernard
Bocquet
a,
Michael T.
Pope
c and
Alan F.
Williams
*a
aSection of Chemistry and Biochemistry, University of Geneva, 30 quai Ernest Ansermet, CH 1211, Genève 4, Switzerland. E-mail: alan.williams@unige.ch; Fax: +41 22 3796830; Tel: +41 22 3796425
bChemistry Department, University of Fribourg, Ch. du Musee 9, CH-1700, Fribourg, Switzerland. E-mail: christian.bochet@unifr.ch
cDepartment of Chemistry, P.O. Box 571227, Georgetown University, Washington, DC 20057, USA. E-mail: popem@georgetown.edu
First published on 7th May 2009
The tungsten-182 Mössbauer spectra of a series of Keggin structure heteropolytungstates, [EW12O40]n− are reported. There is a very considerable variation in quadrupole coupling at the tungsten nucleus indicating considerable asymmetry in the electron distribution for the more electronegative elements E. The quadrupole coupling correlates well with the structural data, in particular with the distance between the tungsten and the oxygen atom of the EO4group. These compounds may be regarded as rigid W12O36 cages interacting more or less strongly with an EO4n−host. The spectra of salts of metatungstate [H2W12O40]6− and [W6O19]2− are also given.
Some years ago, in a preliminary investigation of the application of 182W Mössbauer spectroscopy to inorganic chemistry, we reported the spectrum of a sample of silicotungstic acid, which was remarkable in showing a large quadrupole coupling.7 Since the tungsten atom, formally, has a 5d0 electron configuration and lies in an octahedral environment, this was a surprising observation, and prompted us to investigate a series of these compounds. We report herein the somewhat surprising results of this study.
Mössbauer spectroscopy of tungsten8 has a number of practical disadvantages, including the high γ-ray energy (100 keV), which requires source and absorber to be cooled to liquid helium temperature, and a very weak sensitivity of the isomer shift to changes in the chemical environment. However, the 0 → 2 nuclear spin transition allows complete electric field gradient information (quadrupole coupling, sign and asymmetry parameter, η) to be obtained.
We chose to concentrate our studies on heteropolytungstates of formula [EW12O40]n− with the Keggin structure9 (Fig. 1) although a few other compounds are included for comparison. The Keggin structure has high (432) symmetry such that all tungsten atoms are equivalent, and show a coordination number of six, formed by one terminal oxygen, four co-planar oxygen atoms bridging two tungsten atoms and finally a sixth oxygen, which is formally part of a [EO4]n− anion at the centre of the anion and which bridges three tungsten atoms. Thus, the coordination, although formally octahedral, is quite irregular.
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Fig. 1 The structure of the [PW12O40]3− ion.10 |
Sample | Quadrupole coupling, e2qQ/mm s−1 | Asymmetry parameter, η | Linewidth Γ/mm s−1 | Synthesis Ref. |
---|---|---|---|---|
a Prepared by metathesis from salt or acid.11TBA = tetrabutylammonium. Estimated errors: quadrupole coupling: ± 0.5 mm s−1, asymmetry parameter ± 0.1. | ||||
H3[PW12O40]·9H2O | −16.6 | 0.29 | 2.4 | 12 |
Cs3[PW12O40]·6H2O | −16.4 | 0.20 | 2.2 | a |
(TBA)3[PW12O40] | −16.6 | 0.29 | 2.2 | 11 |
Na3[PW12O40]·9H2O | −16.3 | 0.29 | 2.5 | 12 |
Cs3[AsW12O40]·10H2O | −13.5 | 0.42 | 3.0 | 11 |
(TBA)3[AsW12O40] | −13.7 | 0.40 | 2.4 | 11 |
H4[SiW12O40]·8H2O | −12.7 | 0.41 | 3.0 | 13 |
K4[SiW12O40]·10H2O | −12.0 | 0.22 | 3.0 | 11 |
(TBA)4[SiW12O40] | −12.8 | 0.39 | 2.4 | 11 |
(TBA)4[GeW12O40] | −10.0 | 0.58 | 2.3 | 11 |
K5[BW12O40]·11H2O | −13.1 | 0.33 | 2.8 | 11 |
H5[BW12O40]·15H2O | −12.5 | 0.41 | 2.5 | 11 |
Cs4H [AlW12O40]·6H2O | −6.5 | 0.59 | 3.0 | a |
(TBA)4H[AlW12O40]·3H2O | −7.5 | 0.90 | 2.5 | a |
K5[AlW12O40]·15H2O | −7.4 | 0.51 | 2.7 | 14 |
(TBA)4H[FeW12O40]·4H2O | −5.8 | 0.84 | 3.0 | 15 |
K5[CoW12O40]·11H2O | −6.2 | 0.80 | 2.4 | 16 |
(TBA)4H2[CoW12O40]·6H2O | −6.7 | 0.69 | 2.8 | 17 |
K4H2[CoW12O40]·6H2O | 6.1 | 0.76 | 2.5 | 16 |
(TBA)4H2[ZnW12O40]·6H2O | −6.6 | 0.89 | 3.3 | 18 |
K6[H2W12O40]·2H2O | 6.4 | 1.0 | 3.1 | 19 |
(TBA)2[W6O19] | −9.4 | 0 | 2.3 | 20 |
182W Mössbauer spectra were measured using a constant acceleration spectrometer with source and absorber cooled to liquid helium temperature in an Oxford Instruments cryostat. The source was 182Ta obtained by neutron irradiation of a natural Ta foil. Because of the low recoil-free fractions relatively thick samples containing 110 mg tungsten cm−2 were used. The spectra were fitted using the least square procedure of Stone using five lines of equal width whose positions are determined by the values of the isomer shift δ, the quadrupole coupling e2qQ and the asymmetry parameter η. Isomer shifts were identical within experimental error and are not discussed here.
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Fig. 2 Overlay of the DPP polarograms of phosphotungstic acid as a function of pH. |
Finally, we found that TLC on silica may be used to check the purity of the samples. The eluent used was either ethanol with lithium perchlorate 0.3M or acetone–10% hexane–lithium perchlorate 0.3M. The Rf values decreased with increasing charge: typical values in ethanol were 0.62 for 4− ions, 0.58 for 5− ions and 0.38 for 6− ions. Phosphotungstates with a 3− charge showed a spot at an Rf value of 0.66, but other spots were also present suggesting decomposition in the solution. Paratungstate, a frequent impurity in the preparations, does not migrate under these conditions. An unsatisfactory TLC was inevitably confirmed by other measurements such as polarography or infrared spectroscopy.
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Fig. 3 182W Mössbauer spectrum of K4[SiW12O40]·10H2O. |
The asymmetry parameters increase as the quadrupole coupling falls, to values close to 1 for couplings around 6 mm s−1. In such systems, the sign of the coupling is not well defined, and the apparent change in sign of the gradient is not particularly significant.
The examination of the results leads to two questions. Firstly, why is the quadrupole coupling so large in the phosphotungstates, and why does it fall so sharply as the heteroatom changes? This change is remarkable in that the change of the central atom, from phosphorus to aluminium, for example, drops the quadrupole coupling by a factor of 2 for all twelve tungsten atoms. Secondly, can we explain the variation in the asymmetry parameter? We will discuss the two non-Keggin systems, K6[H2W12O40]·2H2O and (TBA)2[W6O19] later.
As mentioned in the introduction, the tungsten coordination, although it is usually described as octahedral, is in fact highly distorted, as is often the case for the earlier transition metals.21Fig. 4 shows the coordination sphere for two examples, [PW12O40]3− and [FeW12O40]5−22 which correspond to the two extremes of the quadrupole coupling range.
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Fig. 4 The coordination sphere of tungsten in: (a) [PW12O40]3−10 and (b) [FeW12O40]5−. |
The coordination sphere consists of one oxygen atom at a distance of approximately 1.7 Å, the distance corresponding to a tungsten-oxygen double bond,21 and four at roughly 1.9 Å, corresponding to a single bond distance. The sixth site of the octahedron, trans to the WO double bond, is occupied by an oxygen of the EO4 tetrahedron, and the bond distance is much greater, 2.437 Å for the phosphotungstate and 2.229 Å for the ferrotungtstate. This corresponds to a very low bond valence, perhaps not altogether surprisingly given that the bond valence sum for the first five bonds is close to 6 and the sixth oxygen is shared between three tungsten atoms. We may therefore consider the structure to be a W12O36 cage which interacts more or less strongly with the central EO4 unit. Each tungsten is part of a square pyramidal (SP) WO5 unit, which interacts weakly with an oxygen bound to the heteroatom. This structure would be expected to give rise to a large negative quadrupole coupling as observed since the square pyramid may be considered as an octahedron from which electron density has been removed along the z-axis, giving a positive electric field gradient, q, and the quadrupole moment, Q, of the excited state is negative.23
The quadrupole coupling will decrease as the interaction of the tungsten atom with the EO4oxygen increases and charge is donated. The interaction with the EO4 unit can increase for two reasons: (1) an increased basicity of the EO4n− ion; and (2) an increase in the size of E, which will lengthen the E–O bonds, and for simple geometric reasons approach the oxygen to the tungsten.
Examination of the data supports this argument. As the formal positive charge on the central E atom falls, the basicity of the EO4n− increases, and quadrupole coupling drops, as shown in the series P > Si > Al. The size effect is shown by comparing two elements in the same column of the periodic table where we see P > As, Si > Ge, and B > Al. The strength of the interaction between tungsten and EO4oxygen may also be measured by the W–O bond distance. Table 2 gives the average W–OEO3 distance for the heteropolytungstate anions extracted from the data in the Cambridge Structural Database (version 5.29, update of January 2009). Fig. 5 shows that the average distances correlate very well with the observed quadrupole couplings for distances greater than 2.25 Å. For shorter distances the quadrupole coupling is both rather small and the environment is distinctly non-axial.
Atom E | EO–W/Å | Bond order21 | E–W/Å |
---|---|---|---|
P | 2.443 | 0.195 | 3.556 |
As | 2.362 | 0.249 | 3.567 |
Si | 2.356 | 0.253 | 3.520 |
Ge | 2.298 | 0.302 | 3.539 |
B | 2.359 | 0.251 | 3.482 |
Al | 2.253 | 0.346 | 3.504 |
Co | 2.180 | 0.432 | 3.496 |
Fe | 2.210 | 0.394 | 3.515 |
Zn | 2.160 | 0.459 | 3.499 |
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Fig. 5 Correlation between quadrupole coupling and W–O bond distance. |
The behaviour of the asymmetry parameter is also consistent with this explanation. Fig. 4 shows that the EO4oxygen is displaced from the axis of the WO double bond, giving non-axial symmetry. This is the origin of the small but non-zero asymmetry parameter in the phosphotungstate. As the size of the hetero atom increases the EO4oxygen is moved progressively further away from the axis, and the asymmetry parameter increases. As can be seen from Fig. 4b in the ferrotungstate, the EO4oxygen is much further from the W
O axis.
If we consider these compounds as containing W12O36 cages encapsulating [EO4]n− anions, we may ask to what extent the W12O36 cage can shrink or expand to fit in the anion. We may estimate the size of the cage from the mean E–W distance in [EW12O40]n− and the averaged values extracted from the Cambridge Structural Data base are shown in Table 2. Although the values do vary as a function of E, the variation is much less than that of the O–W distances, which would tend to suggest that the cage is rather rigid. A similar conclusion was obtained from recent theoretical calculations on these systems.24
Finally, we may look at the two non-Keggin systems in Table 1. Both are similar to the Keggin systems in that they contain a WO5 square pyramidal unit with a sixth oxygen weakly bound. The metatungstate K6[H2W12O40]·2H2O may be regarded as a Keggin system in which the central heteroatom is missing. The four oxygen atoms that remain of the EO4 tetrahedron are thus formally oxide ions and are quite basic. The two hydrogen atoms are located inside the O4 cavity but cannot be observed by X-ray crystallography. The interaction with the tungsten atoms is thus strong, with a low quadrupole coupling. The average O–W distance is 2.212 Å in agreement with the correlation shown in Fig. 5. As for the Keggin systems a high asymmetry parameter is predicted and observed.
The [W6O19]2− ion in (TBA)2[W6O19] has a central oxygen atom octahedrally coordinated by six WO5 units composed of one terminal oxygen and four oxygen atoms shared with neighbouring tunsgten atoms. The coordination is thus similar to the polytungstate except for the fact that the tungsten atom lies on a four-fold axis. We observe the expected zero asymmetry parameter with a negative coupling of −9.4 mm s−1. The average W–O distance for the central oxygen is 2.323 Å, and this lies close to the correlation in Fig. 5.
Footnotes |
† In memory of Alfred Gavin Maddock, 15th August 1917–5th April 2009. |
‡ Electronic supplementary information (ESI) available: Sample characterisation data. See DOI: 10.1039/b904101j |
This journal is © The Royal Society of Chemistry 2009 |