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Slow Proton Exchange in Aqueous Solution. Consequences of Protonation and Hydration within the Central Cavity of Preyssler Anion Derivatives, [|M(H2O)|⊃P5W30O110]n-
journal contributionposted on 1999-11-13, 00:00 authored by Kee-Chan Kim, Michael T. Pope, Gennaro J. Gama, Michael H. Dickman
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.