Probing Crystallization Pathways in Group V Polyoxometalate Solutions

Abstract

Structure elucidation is extremely important to understand and control processes in synthetic chemistry, in drug design, and in biomolecular function; and the most important step of structure elucidation is crystallization. Preceding crystallization of ionic compounds is usually ion-association in solution, which can be induced by a variety of ways. Here we study ion-association of hexaniobate and hexatantalate polyoxometalate salts in mixed water-alcohol solutions. These hexametalate clusters have the unusual characteristic of increased solubility with increased ion-association, which makes them ideal candidates to understand the fundamentals of ion-pairing. We utilize direct (X-ray scattering) and indirect (ion-conductivity) methods to document the ion-association as a function of alkali, concentration, alcohol:water ratio, and Nb versus Ta. The conductivity data coupled with X-ray scattering shows that decreasing solvent polarity increases cluster-alkali association; but decreases any interaction between the alkali-cluster aggregates. Conductivity data show the trend of increasing alkali-cluster association with increasing alkali size as is expected for hexaniobate, but has some discrepancies with hexatantalate. We attribute this to the concomitant effects of protonation of the clusters, with hexaniobate being a stronger base. These studies provide insight into aqueous behaviour of these clusters that exhibit the anomalous behaviour of high solubility with maximum ion-association.