Computational Study into the Effects of Countercations on the [P₈W₄₈O₁₈₄]^40- Polyoxometalate Wheel

Porous metal oxide materials have been obtained from a ring-shaped macrocyclic polyoxometalate (POM) structural building unit, [P₈W₄₈O₁₈₄]^40-. This is a tungsten oxide building block with an integrated "pore" of 1 nm in diameter, which, when connected with transition metal linkers, can assemble frameworks across a range of dimensions and which are generally referred to as POMzites. Our investigation proposes to gain a better understanding into the basic chemistry of this POM, specifically local electron densities and locations of countercations within and without the aforementioned pore. Through a rigorous benchmarking process, we discovered that 8 potassium cations, located within the pore, provided us with the most accurate model in terms of mimicking empirical properties to a sufficient degree of accuracy while also requiring a relatively small number of computer cores and hours to successfully complete a calculation. Additionally, we analyzed two other similar POMs from the literature, [As₈W₄₈O₁₈₄]^40- and [Se₈W₄₈O₁₇₆]^32-, in the hopes of determining whether they could be similarly incorporated into a POMzite network; given their close semblance in terms of local electron densities and interaction with potassium cations, we judge these POMs to be theoretically suitable as POMzite building blocks. Finally, we experimented with substituting different cations into the [P₈W₄₈O₁₈₄]^40- pore to observe the effect on pore dimensions and overall reactivity; we observed that the monocationic structures, particularly the Li₈[P₈W₄₈O₁₈₄]^32- framework, yielded the least polarized structures. This correlates with the literature, validating our methodology for determining general POM characteristics and properties moving forward.