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. http://pubs.acs.org/doi/pdf/10.1021/cr960401q,33H. 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.