Thesis Defense: Lauren Fullmer

Exploring the Distinct Solution Behavior of Isostructural Polycoltanates

Polyoxometalates are a rapidly growing class of discrete metal oxide clusters with a wide range of applications including functional materials. These anionic clusters of the group V and VI transition metals are inexpensive and non-toxic. Within group V polyoxometalates, niobium and tantalum chemistry has been underdeveloped owing to their unique solubility and speciation in alkaline conditions compared to the acidic group VI polyoxometalates. This thesis compares the solution behavior of analogous hexacoltanate structures [Ta6O19­]8- and [NbO19]8-, their association with counterions, and their potential as thin film precursors. Tetramethylammonium hexametalate salts, [(CH3)4N](8-x)HxM6O19,  have been synthesized in an effort to provide a metal-oxide thin film precursor without incorporating counterions into the final film composition. These structurally identical analogues differ only by a single proton, which affects the behavior of these clusters in solution and in turn, the quality of the thin films that are obtained. Furthermore, these tetramethylammonium hexametalates react with hydrogen peroxide, exchanging terminal multiply bound oxygens with peroxide which yields [(CH3)4N](8-x)[HxM6(O2)6O13]. Small and wide angle x-ray scattering is utilized to analyze the hexametalates in solution and determine any interactions between these highly charged species. Polyoxometalates are ideal to study with x-ray scattering since the high Z metals in the cluster provide excellent electron density contrast with low Z solvents such as water. We observe differences in the solution behavior, indicating that hexatantalate and hexaniobate are not as similar as originally expected. Density functional theory calculations have determined the energetics of peroxide addition as well as preferred sites for substitution. This thesis also compares the ion association of the alkali salts of niobium and tantalum hexametalates. The association of clusters with their counterions precedes crystallization and is therefore an important step in structure elucidation. Hexacoltanates exhibit increased solubility with increased ion association, making them an ideal study to understand the fundamentals of ion pairing. Small and wide angle x-ray scattering as well as conductivity are utilized to investigate ion association as a function of counterion, concentration, solvent polarity, and niobium vs. tantalum. Overall, we observe increased ion association with decreasing solvent polarity. While the increasing ion association with increasing alkali size is expected for hexaniobate, there are some discrepancies with hexatantalate which we attribute to the concomitant effects of protonated clusters. This thesis provides evidence that structurally identical species do not necessarily exhibit the same behavior in solution. Differences in stability, protonation, and electrostatic interactions with counterions provide some insight into the anomalous solubility of niobium and tantalum polyoxometalates.