Cation-directed structure changes in polyoxometalate chemistry. Equilibria between isomers of bis(9-tungstophosphatodioxouranate(VI)) complexes
The first examples of polyoxometalates incorporating uranyl hetero groups have been synthesized by reaction of Na{sub 9}[A-PW{sub 9}O{sub 34}] with UO{sub 2}(NO{sub 3}){sub 2}. Sodium, ammonium, and potassium salts of [M{sub 2}(UP{sub 2})2(PW{sub 9}O{sub 34}){sub 2}]{sup 12{minus}} (M = Na, NH{sub 4}, K) were characterized by P and W NMR spectroscopy and single-crystal structural analysis. The sodium salt (1) contains an anion of C{sub i} symmetry in which two PW{sub 9}O{sub 34} groups sandwich two UO{sub 2}{sup 2{plus}} and two Na{sup {plus}} and two NA{sup {plus}} cations. The uranium atoms have pentagonal-bipyramidal coordination, achieved by three equatorial bonds to one PW{sub 9}O{sub 34} and two bonds to the other. The sodium cations have approximate tetrahedral coordination geometry. Single-line P and five-line P and five-line W NMR spectra confirm that the anion structure is maintained in solutions containing excess Na{sup {plus}}. The ammonium and potassium salts contain anions in which the UO{sub 2}{sup 2{plus}} cations (pentagonal bipyramids) are unsymmetrically sandwiched between the PW{sub 9}{sub 34} groups, as are two closely associated cations, one internal and one external. In solution both, salts give two-line P and nine-line W NMR spectra that are consistent with the solid-state structures. Solution equilibria are rapidly established between the two structure types and are dependent upon the concentrations and identifies of the cations. Based on integrated P NMR spectra, the equilibrium constants for [Na{sub 2}(UO{sub 2}){sub 2}(PW{sub 9}O{sub 34}){sub 2}]{sup 12{minus}} {leftrightarrow} [M{sub 2}(UO{sub 2}){sub 2}(PW{sub 9}O{sub 34}){sub 2}]{sup 12{minus}} + 2Na{sup plus} are 128 {+-} 12 (M = K{sup plus}) and 1.8 {+-} 0.8 (M = NH{sub 4}{sup plus}). The requirement of the second cation in order to satisfy the mass-action expression for K{sub eq}, coupled with a W NMR spectrum that implies C{sub s} symmetry, demonstrates that the external ion-paired cation undergoes rapid exchange between several surface sites of the heteropolyanion. Solutions of 1 are stable at pH 7 in the presence of excess sodium cations, and the anion is transferable into toluene by phase-transfer techniques. Addition of calcium ions to solutions of 1 generates a new one-line P NMR spectrum of a Ca derivative which is assumed to be isostructural with 1.
- Research Organization:
- Georgetown Univ., Washington, D.C. (US)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- FG07-96ER14695
- OSTI ID:
- 20000124
- Journal Information:
- Journal of the American Chemical Society, Vol. 121, Issue 37; Other Information: PBD: 22 Sep 1999; ISSN 0002-7863
- Country of Publication:
- United States
- Language:
- English
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