Slow proton exchange in aqueous solution. Consequences of protonation and hydration within the central cavity of Preyssler anion derivatives, [vertical bar M(H2O)vertical bar superset of P5W30O110](n-)

Redetermination of the crystal structure of the ammonium salt of the Preyss ler anion, [NaP5W30O110](14-)(Na), at low temperature, reveals that a water molecule is coordinated to the encrypted (sodium) cation, a feature that h ad been previously observed in structures of normal and acid salts of the e uropium(III) derivatives (Eu and HEu) and the normal salt of the uranium(IV ) derivative (U), The crystal structures of (NH4)(13)[Ca(H2O)P5W30O 101]. 3 5H(2)O (Ca), K6H7[Ca(H2O)P5W30O110]. 45H(2)O (NCa), (NH4)(12)[Y(H2O)P5W30O1 10]. 30H(2)O (Y), K6H6[Y(H2O)P5W30O110]. 40H(2)O (HY), and K6H5[U(H2O)P5W30 O110]. 30H(2)O (HU) have also been determined. The structures reveal that t he encrypted cations become more displaced from the equator of the anion as the charge of the cation increases. Evidence that these heteropolyanions c an be protonated inside the central cylindrical cavity is provided by obser vation of two P-31 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 func tion of pH could be fitted to a simple acid-base equilibrium expression. Th e internal water molecule of the unprotonated anion undergoes slow H/D exch ange with solvent water. Isotopomers with internal HOH, HOD, and DOD are re adily distinguished by P NMR (and for the first two, by H NMR) especially f or derivatives with paramagnetic internal cations (Eu, U). The exchange rea ction followed by H-1 and P-31 NMR in pure D2O (pD 3.6) for the europium de rivative at room temperature followed an A -> B -> consecutive mechanism wi th effective rate constants k(l)(HOH -> HOD) = (4.1 +/- 0.2) x 10(-4) s(-1) and k(2)(HOD -> DOD) = (3.4 +/- 0.2) x 10(-4) s(-1). The rates increased a s 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 strong ly acidic solutions (> 2 M HCl) releases the encrypted cation, according to (31)P and W-183 NMR spectroscopy and measurement of sodium activity release d.