Experimental and theoretical methods to investigate extraframework species in a layered material of dodecaniobate anions

The highly charged dodecaniobate Keggin ions [XNb₁₂O ₄₀]^-16 (X = Si, Ge) and [XNb₁₂O ₄₀]^-15 (X = P) serve as building blocks of self-assembled, low-dimensional anionic framework materials. In addition to its high charge, the Keggin ion provides optimal binding geometries that render these materials as attractive metal sorbents and ion exchangers. We describe here the synthesis and single-crystal X-ray structure of K_10-x[Nb₂O ₂][H_xGeNb₁₂O₄₀]·11H ₂O (GeNb₁₂-2d; x = ∼1-1.5), a phase featuring 2D linkage of [GeNb₁₂O₄₀]^-16 Keggin ions interlayered with charge-balancing K⁺ cations and water molecules. Thermogravimetry, infrared spectroscopy (IR), ¹H MAS NMR, and D ₂O exchange experiments as well as computational studies were used to describe the location and behavior of these interlayer, extraframework species. To model the basicity of the different types of framework oxygen sites appropriately, atomic-centered partial charges were derived from density functional theory (DFT) calculations to model the electrostatic potential. This model enabled the locations and bonding of K⁺ cations associated with the framework, as well as K⁺ cations bound predominantly to water in the interlayer space, to be accurately computed via Monte Carlo simulation. The poorest agreement between experimental and simulation results was observed for potassium sites that were associated with disordered portions of the framework, namely, the [Nb₂O₂]⁶⁺ bridge between Keggin ions. Finally, through grand canonical Monte Carlo (GCMC) calculations, saturation water loadings consistent with experimental measurements were computed.