Density functional theory and thermodynamics analysis of MAl12 Keggin substitution reactions: Insights into ion incorporation and experimental confirmation

Polyaluminum cations, such as the MAl12 Keggin, undergo atomic substitutions at the heteroatom site (M), where nanoclusters with M = Al3+, Ga3+, and Ge4+ have been experimentally studied. The identity of the heteroatom M has been shown to influence the structural and electronic properties of the nanocluster and the kinetics of ligand exchange reactions. To date, only three epsilon -analogs have been identified, and there is a need for a predictive model to guide experiment to the discovery of new MAl12 species. Here, we present a density functional theory (DFT) and thermodynamics approach to predicting favorable heteroatom substitution reactions, alongside structural analyses on hypothetical epsilon -MAl12 nanocluster models. We delineate trends in energetics and geometry based on heteroatom cation properties, finding that Al3+-O bond lengths are related to heteroatom cation size, charge, and speciation. Our analyses also enable us to identify potentially isolable new epsilon -MAl12 species, such as FeAl127+. Based upon these results, we evaluated the Al3+/Zn2+/Cr3+ system and determined that substitution of Cr3+ is unfavorable in the heteroatom site but is preferred for Zn2+, in agreement with the experimental structures. Complimentary experimental studies resulted in the isolation of Cr3+-substituted delta -Keggin species where Cr3+ substitution occurs only in the octahedral positions. The isolated structures Na[AlO4Al9.6Cr2.4(OH)(24)(H2O)(12)](2,6-NDS)(4)(H2O)(22) (delta -CrnAl13-n-1) and Na[AlO4Al9.5Cr2.5(OH)(24)(H2O)(12)](2,7-NDS)(4)(H2O)(18.5) (delta -CrnAl13-n-2) are the first pieces of evidence of mixed Al3+/Cr3+ Keggin-type nanoclusters that prefer substitution at the octahedral sites. The delta -CrnAl13-n-2 structure also exhibits a unique placement of the bound Na+ cation, which may indicate that Cr3+ substitution can alter the surface reactivity of Keggin-type species.

Automated summary

Polyaluminum cations, such as MAl12 Keggin, undergo atomic substitutions at the heteroatom site, with nanoclusters of M = Al3+, Ga3+, and Ge4+ being experimentally studied. The identity of the heteroatom M influences structural and electronic properties, as well as ligand exchange kinetics. Predicting favorable heteroatom substitutions using density functional theory and thermodynamics reveals trends in energetics and geometry based on cation properties. New isolable epsilon -MAl12 species have been identified, such as FeAl127+, with Cr3+ substitution shown to be unfavorable but Zn2+ preferred in experimental structures.