Observing Assembly of Complex Inorganic Materials from Polyoxometalate Building Blocks

Abstract

Understanding the aqueous state of discrete metal-oxo clusters, prenucleation clusters, and even simple ions is valuable for controlling the growth of metal-oxide materials from water. Niobium polyoxometalates (Nb-POMs) are unique in the aqueous metal-oxo cluster landscape in their unusual solubility behavior: specifically, their solubility in water increases with increasing ion-pairing contact with their counterions, and thus provides a rare opportunity to observe these and related solution phenomena. Here, we isolate in the solid state the monomeric and dimeric building blocks, capped Keggin ions, of the extended Keggin chain materials that are now well-known: not only in Nb-POM chemistry, but Mo and V POM chemistry as well. Rb13[GeNb13O41]·23H2O (Rb1), Cs10.6[H2.4GeNb13O41]·27H2O (Cs1) and Cs18H6[(NbOH)SiNb12O40]2·38H2O (Cs2) were characterized by single-crystal X-ray diffraction. Small angle X-ray scattering (SAXS) of solutions of Rb1 and Cs1 in varying conditions revealed oligomerization of the monomers into chain structures: the extent of oligomerization is controlled by pH, concentration, and the counterion. We distinctly observe chains of up to six Keggin ions in solution, with the large alkali cations for charge-balance. This combined solid state and solution study reveals in great detail the growth of a complex material from discrete monomeric building blocks. The fundamentals of the processes we are able to directly observe in this study, ion-association and hydrolysis leading to condensation, universally control the self-assembly and precipitation of materials from water.

An overview of selected current approaches to the characterization of aqueous inorganic clusters

“…Scattering data is dependent on form factors (size, shape, and scattering contrast) and structure factors (interactions between particles). 72 (2) Understanding the mechanism of linking clusters in solution (formation of macromolecular chains) by varying aqueous conditions. 71 This method transforms the reciprocal space data to real space and yields a geometrical representation of the scattering particle as a probability histogram ( p(r) vs. radius).…”

“…75 (Fig. 72 This simple acidification of the cluster led to improved solubility behavior and better thin film quality in addition to providing the correct Li : Nb ratio for the targeted perovskite material. 76 This has inspired the investigation of their dimerization behavior in solution using SAXS.…”

Self-Assembly in Polyoxometalate and Metal Coordination-Based Systems: Synthetic Approaches and Developments

“…These are formed by metals (addenda atoms) such as tungsten, molybdenum and vanadium in their higher oxidation state; however, over the last decades, it has been shown that other metals can serve as the primary atoms within POM clusters, such as niobium [12,13], and tantalum [14,15]. POMs are composed of condensed metal-oxygen {MO x } polyhedra (x = 4 to 7).…”

Direct X‐Ray Scattering Evidence for Metal–Metal Interactions in Solution at the Molecular Level

“…[6] Tr aditionally,m etallophilic contacts exhibiting short metal-metal distances have been characterized in the solid-state by Xray crystallographic studies.M ore importantly,t he interactions in solution have been probed by spectroscopic methods (such as IR, Raman, UV/Vis,NMR, and ESR spectroscopies) and computational techniques have also been important in the study of such contacts. [7] Although small-angle X-ray scattering (SAXS) has been used extensively to probe the size,shape,and interactions of biomolecules, [8] prenucleation clusters, [9] discrete clusters, [10] and nanoparticles [11] in solution, metal-metal interactions have not been explored with this method. While the strong scattering between heavy metals is conducive to this technique,t he small size of molecular species renders interpretation of data challenging.H owever, understanding the relationship between structure in solids and solution is of paramount importance in understanding natural phenomena and controlling synthetic processes.…”