Observing Assembly of Complex Inorganic Materials from Polyoxometalate Building Blocks

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.