Slow proton exchange in aqueous solution. Consequences of protonation and hydration within the central cavity of Preyssler anion derivatives, [{vert{underscore}bar}M(H{sub 2}O){vert{underscore}bar} {contains} P{sub 5}W{sub 30}O{sub 110}]{sup n{minus}}

Journal Article · Wed Dec 08 04:00:00 EST 1999 · Journal of the American Chemical Society

DOI:https://doi.org/10.1021/ja992105b· OSTI ID:20014100

Kim, K C; Pope, M T; Gama, G J; Dickman, M H

Redetermination of the crystal structure of the ammonium salt of the Preyssler anion, [NaP{sub 5}W{sub 30}O{sub 110}]{sup 14{minus}} (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 (NH{sub 4}){sub 13}[Ca(H{sub 2}O)P{sub 5}W{sub 30}O{sub 110}]{center{underscore}dot}35H{sub 2}O (Ca), K{sub 6}H{sub 7}[Ca(H{sub 2}O)P{sub 5}W{sub 30}O{sub 110}]{center{underscore}dot}45H{sub 2}O (HCa), (NH{sub 4}){sub 12}[Y(H{sub 2}O){sub 5}W{sub 30}O{sub 110}]{center{underscore}dot}30H{sub 2}O (Y), K{sub 6}H{sub 6}[Y(H{sub 2}O)P{sub 5}W{sub 30}O{sub 110}]{center{underscore}dot}40H{sub 2}O (HY), and K{sub 6}H{sub 5}[U(H{sub 2}O)P{sub 5}W{sub 30}O{sub 110}]{center{underscore}dot}30H{sub 2}O (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 {sup 31}P 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 protein 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 {sup 1}H and {sup 31}P NMR in pure D{sub 2}O (pD 3.6) for the europium derivative at room temperature followed an A {l{underscore}arrow} B {l{underscore}arrow} C consecutive mechanism with effect rate constants k{sub 1}(HOH {l{underscore}arrow} HOD) = (4.1 {+-}0.2) x 10{sup {minus}4} s{sup {minus}1} and k{sub 2}(HOD {l{underscore}arrow} DOD) = (3.4 {+-} 0.2) x 10{sup {minus}4} s{sup {minus}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 to {sup 31}P and {sup 183}W NMR spectroscopy and measurement of sodium activity released.

• Research Organization: Georgetown Univ., Washington, DC (US)
• Sponsoring Organization: US Department of Energy
• DOE Contract Number: FG07-96ER14695
• OSTI ID: 20014100
• Journal Information: Journal of the American Chemical Society, Journal Name: Journal of the American Chemical Society Journal Issue: 48 Vol. 121; ISSN JACSAT; ISSN 0002-7863
• Country of Publication: United States
• Language: English

Related Subjects

40 CHEMISTRY
AMMONIUM COMPOUNDS
ANIONS
AQUEOUS SOLUTIONS
CRYSTAL STRUCTURE
OXYGEN COMPOUNDS
PHOSPHORUS COMPOUNDS
PROTON TRANSPORT
SODIUM COMPOUNDS
TUNGSTEN COMPOUNDS