Introduction
Polyoxometalates (POMs) have attracted extensive and rising attention
because of their particular chemical and physical properties, which have
great potential applications in wide-ranging fields, such as
medicine11J. T. Rhule, C. L. Hill, D. A. Judd and R. F.
Schinazi, Polyoxometalates in medicine, Chem. Rev., 1998, 98,
327–358. catalysis22N. Mizuno and M. Misono, Heterogeneous
catalysis, Chem. Rev., 1998, 98, 199–217.
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Lv, Y. V. Geletii, C. Zhao, J. W. Vickers, G. Zhu, Z. Luo, J. Song, T.
Lian, D. G. Musaev and C. L. Hill, Polyoxometalate water oxidation
catalysts and the production of green fuel, Chem. Soc. Rev., 2012, 41,
7572–7589. DOI:10.1039/C2CS35292C,
electrochemistry44M. Sadakane and E. Steckhan, Electrochemical
properties of polyoxometalates as electrocatalysts., Chem. Rev., 1998,
98, 219–238.
http://pubs.acs.org/doi/pdf/10.1021/cr960403a
and electronic devices5517 A. M. Douvas, E. Makarona, N. Glezos,
P. Argitis, J. A. Mielczarski and E. Mielczarski,
Polyoxometalate-based layered structures for charge transport control
in molecular devices, ACS Nano, 2008, 2, 733–742.
http://pubs.acs.org/doi/pdf/10.1021/nn700333j,
additionally, new potential applications in materials science are based
on unusual magnetic66Lehmann, J.; Gaita-Arino, A.; Coronado, E.;
Loss, D. Spin Qubits with Electrically Gated Polyoxometalate
Molecules. Nat. Nanotechnol. 2007, 2, 312–317., electrical,
optical77Xu, B.; Lu, M.; Kang, J.; Wang, D.; Brown, J.; Peng, Z.
Synthesis and Optical Properties of Conjugated Polymers Containing
Polyoxometalate Clusters as Side-Chain Pendants. Chem. Mater. 2005,
17, 2841–2851.
http://pubs.acs.org/doi/pdf/10.1021/cm050188r
and photochromic88Zhang, G.; Yang, W.; Yao, J. Thermally
Enhanced Visible- Light Photochromism of Phosphomolybdic Acid-
Polyvinylpyrrolidone Hybrid Films. Adv. Funct. Mater. 2005, 15,
1255–1259. doi:10.1002/adfm.200500179 properties of some POMs, and
potential medical applications such as antitumor and antiviral
uses99Nan Gao, Hanjun Sun, Kai Dong, Jinsong Ren, Taicheng Duan,
Can Xu & Xiaogang Qu. Transition-metal-substituted polyoxometalate
derivatives as functional anti-amyloid agents for Alzheimer’s disease.
Nature Communications 5, Article number: 3422 (2014)
doi:10.1038/ncomms4422., have been studied extensively. Hybrid POMS
Materials for charge storage devices such as lithium batteries and
supercapacitors are one of the most recent and useful
applications1010Jesús Muñiz, Ana Karina Cuentas‑Gallegos, Miguel
Robles and Maximiliano Valdéz. Bond formation, electronic structure,
and energy storage properties on polyoxometalate–carbon
nanocomposites. Theor Chem Acc (2016) 135:92 DOI
10.1007/s00214-016-1855-3,1111D.A.
Baeza-Rostro, A. K. Cuentas-Gallegos. )“Capacitance Improvement of
Carbon Aerogels by the immobilization of Polyoxometalates
Nanoparticles” Journal of New Materials for Electrochemical Systems
16(3), 203-207, 2013
The use of POMs is very wide due to their high structural, chemical and
electronic versatility. Many of these properties depend strongly of POMs
molecular structure and the charge distribution everywhere in the
molecule.
The huge number of structural types in polyoxometalate
chemistry1212M. T. Pope, Isopolyanions and Heteropolyanions,
Comprehensive Coordination Chemistry, ed. G. Wilkinson, R. D. Gillard,
and J. A. McCleverty, Pergamon Press, Oxford, 1987, vol. 3, p. pp.
1023– 1058 can be roughly divided into three classes: (i)
Hetero-polyanions: these are metal oxide clusters that include hetero
anions such as SO4 2+, PO4 3+. (ii)
) Iso-polyanions: these are composed of a metal-oxide framework, but
without the internal hetero-atom/hetero-anion. As a result, they are
often much more unstable than their hetero-polyanion
counterparts1313L. Cronin, in High Nuclearity Clusters, ed. J. A.
McCleverty and T. J. Meyer, Elsevier, Amsterdam, 2004. (iii) Mo-blue
and Mo-brown reduced POM clusters1414De-Liang Long, Eric
Burkholder and Leroy Cronin. Poly-oxo-metalate clusters,
nanostructures and materials: From self-assembly to designer materials
and devices. Critical review. Chem. Soc. Rev., 2007, 36, 105–121.
DOI: 10.1039/b502666k.
Iso-polyanions have been extremely well investigated in the case of
molybdenum, vanadium and tungsten. However, the tungsten cases are
limited due to the time required for the system to equilibrate (of the
order of weeks)1515M. T. Pope, Heteropoly and Isopoly
Oxometalates, SpringerVerlag, Berlin, 1983.. Hetero-polyanions are
another class of cluster that can be synthetized when hetero atoms are
introduced, and these are extremely versatile. Indeed, hetero-anions
based on tungsten have been used in the assembly of very large
clusters1616K. Wassermann, M. H. Dickman and M. T. Pope, Angew.
Chem., Int. Ed. Engl., 1997, 36, 1445..
Generally, the approaches used to produce high nuclearity POM based
clusters are really simple, consisting of acidifying an aqueous solution
containing the relevant metal oxide anions. Next, we suggest a set of
electrochemical reaction for obtain the
[SiW12O40]4-
anion. Beginning from sodium silicate in aqueous solution:
Na4SiO4
+4H+→H4SiO4+
4Na+ (1)
Which reacts to form silicate ions:
[H4SiO4] ↔
[SiO4]4- + 4H+ (2)
Equivalent reactions are possible for obtaining the tungstenate ions:
[WO4]2-.
Silico-tungstenic acid is obtained by mixing previous ions:
[SiO4]4- +
12[WO4]2- + 28H+
↔ [H4SiW12O40] +
12H2O (3)
[H4SiW12O40] ↔
[SiW12O40]4- +
4H+ (4)
Structurally,
[SiW12O40]4-
should be formed with Si-O-W, W=O, W-O-W, O-Si-O and there are four
Oxygen charged atoms (-0-). But, which of the forty
oxygen atoms available correspond with the four oxygen charged atoms?
In order to give a plausible answer to previous question, in Figure 1 we
represent a sketch of twenty four isomers for
[SiW12O40]4- anion
evaluated in this work. An additional quite different structure,
represented as
[SiO4:W12O36]4-
that we called “neutrum structure” also was calculated. Isomers
were labeled from letter A to L, and double labeled as AA to LL. Silicon
atoms in structures single labeled from A to L, have a tetragonal
structure. Silicon atoms is bonded to four oxygen atoms. By other hand,
one of these oxygen atoms is joined to three tungsten atoms (as a
tetravalent specie) located in top triangle. Whereas, each of the other
three oxygen atoms are joined to two contiguous tungsten atoms (as a
trivalent specie) located inside hexagon. In this first block (labels A
to L), is assumed that Charges are located on oxygen atoms single bonded
with tungsten atoms represented by black circles. In this scheme, some W
atoms are hexavalent and other ones are heptavalent (for example those
which are in hexagon and not joined to oxygen charge atoms) or
pentavalent (those in bottom triangle with oxygen charge atoms).
In bottom part of figure 1, we represent the isomers double labeled,
from AA to LL. In this second group of suggested isomers, Silicon atoms
have not a tetragonal structure. Instead of that, they are joined to
only one W atom by oxygen bond (Si-O-W bonds). All Oxygen atoms are
considered as divalent species. Again, the four oxygen charged atoms are
indicated as black circles (W-O-). Twelve W atoms are
hexavalent species.