Modeling of MAl12 Keggin Heteroatom Reactivity by Anion Adsorption

Heteroatom-substituted Keggins, of general formula MAl12 (M = Al, Ga, Ge), are a class of nanoclusters whose properties are sensitive to changes in composition and intermolecular interactions. Previous studies have shown that they display significant differences in oxygen-isotope exchange rates, depending on the identity of heteroatom M. By exploring the intermolecular interactions of these nanoclusters with a series of anions using density functional theory (DFT) calculations, we find bond length changes and adsorption energy values that track with experimentally measured oxygen exchange reactivity trends: Ga < Al < Ge. We compare elongations in μ4O–Alo bond lengths to known heteroatom reactivity trends and anion pKa properties, suggesting a window for producing isolable products. To yield insights into the atomistic interactions that dictate the crystallization process, we investigate trends in adsorption energy, DFT-optimized geometries, and vibrational modes and calculate the distribution of charge in polyoxometalates through an analysis of electronic structure. We find that the pKa of the anion is related to nanocluster reactivity and thus the relative tendency to deprotonate the MAl12 Keggin nanocluster. Our analyses provide a quantitative comparison of the three analogues which explains differences in oxygen exchange rates and why certain combinations of heteroatoms and anions will or will not form isolable crystalline products.