Abstract

The polyoxometalates (POMs) as inorganic species have been known for many years. With the ready access to structural characterization methods such as X-ray diffraction in the 70’s, a blossoming of structural inorganic chemistry in the solid state occurred. The chemistry of the solid state involving polyoxometalates has been of great interest for scientists working in materials chemistry. These molecular systems offer great versatility due to their electronic structure, coordination abilities and synthetic plasticity that make them very attractive as ligands or building blocks in inorganic materials. It is possible to find applications of these systems in catalysis, sorption, magnetism and electronics among others. The following sentence by L. Cronin et al. highlights the interest of POMs for chemists working in the inorganic materials area: “Polyoxometalates (POMs) are a subset of metal oxides that represent a diverse range of molecular clusters with an almost unmatched range of physical properties and the ability to form dynamic structures that can range in size from the nano- to the micrometer scale”. “This versatility makes them excellent candidates for the creation of multifunctional materials”. The structural and magnetic properties of polyoxomolibdates (MoO) and polyoxovanadates (VO) have been studied, but still offer a field full of questions that has been not answered up to the present day. According to Müller et al. in 1998: “These species, most of which are of the diamagnetic, mixedvalence type, show an extreme variety of complicated structures and therefore great significance is attached to structural details. If the ratio Mo(V)/Mo(VI) is high, structures are preferred with Mo(V)-Mo(V) pairs, while if it is smaller, highly delocalized systems with the characteristic darkblue colour (due to intervalence charge-transfer transitions) are favoured”. Another interesting polyoxometalate species, presenting interesting magnetic properties, are the polyoxovanadates (VO). The polyoxovanadates present greater plasticity due to the fact that they can be found as {VO4}, {VO5}, or {VO}. Several polyoxovanadates have been obtained with different numbers of vanadium atoms. Müller, Gatteschi et al. in 1998, presented a magnetic study on several VO species. For a {V6 } cluster, two species with different V(V)/V(IV) ratios were isolated and magnetically studied. It is important to cite: “In fact clusters with 16 and 10 V(IV) ions respectively, were isolated, the complement to 18 being provided by V (V) ions. A comparison of the effective magnetic moments at high temperature immediately shows that something which cannot be expected on the basis of superexchange only is going on.” It is also important to stress that the magnetic studies of these systems are scarce and that the few ones that are very complete, suggest that this is a field full of challenges that should be explored. The hypothesis of the present proposal is that the condensation of diamagnetic atoms such as boron and/or phosphorus to the polyanionic species will permit vanadium and molybdenum polyoxometalates (BVO, BVPO, BMoPO) to be obtained with different paramagnetic/diamagnetic ratios. The presence of the diamagnetic heteroatoms in the polyanion will allow diminished delocalization among the mixed valence species, and therefore a better understanding of the magnetic exchange phenomena among the paramagnetic vanadium(molybdenum) centres. For all the reasons given above, this proposal is completely centred on the synthesis, and structural and magnetic characterization of polyoxometalates of the families BVO, BVPO and BMoPO.