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–Al_o bond lengths to known heteroatom reactivity trends and anion pK_a 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 pK_a 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.