Slow Proton Exchange in Aqueous Solution. Consequences of Protonation and Hydration within the Central Cavity of Preyssler Anion Derivatives, [|M(H₂O)|⊃P₅W₃₀O₁₁₀]^n-

Redetermination of the crystal structure of the ammonium salt of the Preyssler anion, [NaP₅W₃₀O₁₁₀]^14- (Na), at low temperature, reveals that a water molecule is coordinated to the encrypted (sodium) cation, a feature that had been previously observed in structures of normal and acid salts of the europium(III) derivatives (Eu and HEu) and the normal salt of the uranium(IV) derivative (U). The crystal structures of (NH₄)₁₃[Ca(H₂O)P₅W₃₀O₁₁₀]·35H₂O (Ca), K₆H₇[Ca(H₂O)P₅W₃₀O₁₁₀]·45H₂O (HCa), (NH₄)₁₂[Y(H₂O)P₅W₃₀O₁₁₀]·30H₂O (Y), K₆H₆[Y(H₂O)P₅W₃₀O₁₁₀]·40H₂O (HY), and K₆H₅[U(H₂O)P₅W₃₀O₁₁₀]·30H₂O (HU) have also been determined. The structures reveal that the encrypted cations become more displaced from the equator of the anion as the charge of the cation increases. Evidence that these heteropolyanions can be protonated inside the central cylindrical cavity is provided by observation of two ³¹P NMR lines, one in acidic and one in less acidic solution. At intermediate acidities both lines are observed, indicating that proton exchange involving the internal proton is slow on the NMR time scale. The relative intensities of the two lines for the europium derivative as a function of pH could be fitted to a simple acid−base equilibrium expression. The internal water molecule of the unprotonated anion undergoes slow H/D exchange with solvent water. Isotopomers with internal HOH, HOD, and DOD are readily distinguished by P NMR (and for the first two, by H NMR) especially for derivatives with paramagnetic internal cations (Eu, U). The exchange reaction followed by ¹H and ³¹P NMR in pure D₂O (pD 3.6) for the europium derivative at room temperature followed an A → B → C consecutive mechanism with effective rate constants k₁(HOH → HOD) = (4.1 ± 0.2) × 10^-4 s^-1 and k₂(HOD → DOD) = (3.4 ± 0.2) × 10^-4 s^-1. The rates increased as the pH was lowered, while the rate decreased to a limit of no observable exchange at pH 7. Hydrothermal treatment of the sodium derivative in strongly acidic solutions (>2 M HCl) releases the encrypted cation, according to³¹P and ¹⁸³W NMR spectroscopy and measurement of sodium activity released.