Directing the formation of new polyoxometalate architectures through cation control

The primary focus of this thesis is the design and synthesis of novel polyoxometalate clusters by using reactive secondary building units (SBUs) and assembling them in the presence of cations which can direct the formation of new polyoxometalate (POM) architectures. Using lacunary POM species as the SBU with electrophilic linking atoms has proved to be an effective strategy for the formation of higher nuclearity clusters as well as infinite supramolecular network materials. A new reaction system has been developed where lacunary dinuclear transition metal sandwich compounds are formed, ({P2W15}2M2 where M is a transition metal ion), with tetrabutylammonium (TBA) cations from alkali salt precursors of the {P2W15} POM. Normally the tetranuclear sandwich is favoured, ({P2W15}2M4), but by using a combination of bulky TBA cations to stabilise the cluster and protons to balance the charge a family of more reactive dinuclear transition metal sandwiches has been formed. Importantly, this experimental technique serves as a reliable high yielding method for exchanging alkali metal cations for TBA which allows them to be solubilised in less polar solvent. By utilising our knowledge of the reaction forming conditions for tungsten and molybdenum based clusters, carbamate ligands have been used along with hydrothermal reaction conditions to generate two new polyoxoniobate clusters. These clusters both contain pentagonal {M5(M)} units hitherto only seen in tungstate and molybdate systems where they can be combined to form large spherical assemblies such as the Keplerate. One of these compounds, {Nb27} is the largest isopolyoxoniobate reported to date and the second, {Nb31}, is larger but contains a coordinated carbonate ligand, making it a heteropolyoxoniobate. By reaction of the superlacuanry cyclic heteropolyanion {P8W48} with cobalt a series of network materials have been synthesised. The empirical formulae of these networks are very similar but the arrangement of the atoms in the crystal lattice is very different. These reactions show that in POM chemistry small differences in reaction conditions can have a marked effect on the products which are made. By dehydration and subsequent rehydration these compounds undergo a three phase single-crystal to single-crystal transformation with a remarkable 30 % volume change. This single-crystal transformation has the largest volume change for any purely inorganic material and this volume change is comparable to the largest transformations seen in metal organic framework (MOF) compounds. The structural changes occur through the breaking and reforming of the bonds linking the Co(II) ions to the clusters and the removal of coordinated water and subsequent bonding of the cobalt ions to POM ligands. All of these reactions utilise positively charged species to direct the formation of new anionic POM clusters.