A unique coordination environment for an ion: EXAFS studies and bond valence model approach of the encapsulated cation in the Preyssler anion

Abstract

X-Ray absorption spectroscopy was used to probe the coordination of different encrypted cations in the Preyssler anions [Mⁿ⁺P₅W₃₀O₁₁₀]^(15−n)− (Mⁿ⁺ = Sr²⁺, Am³⁺, Eu³⁺, Sm³⁺, Y³⁺, Th⁴⁺, U⁴⁺ in decreasing order of ionic radius, IR), hereafter abbreviated [Mⁿ⁺PA]^(15−n)−. The increase of the M–W distance and the decrease of the M–P distance with increasing M ionic radius reveal that the M cation is displaced along the C₅ axis within the Preyssler cavity. The slight change (0.07 Å) of the M–O distance that does not correspond to the IR difference of 0.27 Å confirms that the cavity retains its rigidity upon cation substitution. Geometric modeling of the encapsulated cation in the channel was performed for comparison to the EXAFS results. The position of the cation in the cavity was calculated as well as the M–O₁₀, –W₅ and –P₅ distances. This modeling confirms the cation displacement toward the center of the Preyssler anion as the cation size increases, which is understood in terms of the non-homogenous electrostatic potential present within the cavity. The bond valence model approach was applied to obtain experimental bond valences. Only the bond valence sum (BVS) of Am³⁺ is close to its actual charge. Sums smaller than the actual valences of the +3 and +4 ions (2.39–2.63 for +3 cations, Y, Sm, Eu; 3.17 and 3.38 for +4 cations, U and Th, respectively) were obtained, and a larger sum (2.89) was obtained for Sr²⁺. The deviations from the formal M sums of the encapsulated ions are attributed to the rigidity of the Preyssler framework. The tendency toward coordinative unsaturation for electroactive cations, such as Eu³⁺, is thought to be the driving force for facile reduction. Unlike other inorganic chelating ligands, the Preyssler anion provides a unique redox system to stabilize an electroactive cation in a low oxidation state.