Cation-Directed Structure Changes in Polyoxometalate Chemistry. Equilibria between Isomers of Bis(9-tungstophosphatodioxouranate(VI)) Complexes

Abstract

The first examples of polyoxometalates incorporating uranyl hetero groups have been synthesized by reaction of Na 9 [A-PW 9 O 34 ] with UO 2 (NO 3 ) 2 . Sodium, ammonium, and potassium salts of [M 2 (UO 2 ) 2 (PW 9 O 34 ) 2 ] 12 - (M = Na, NH 4, K) were characterized by P and W NMR spectroscopy and single-crystal structural analysis. The sodium salt ( 1 ) contains an anion of C i symmetry in which two PW 9 O 34 groups sandwich two UO 2 2+ and two Na + cations. The uranium atoms have pentagonal-bipyramidal coordination, achieved by three equatorial bonds to one PW 9 O 34 and two bonds to the other. The sodium cations have approximate tetrahedral coordination geometry. Single-line P and five-line W NMR spectra confirm that the anion structure is maintained in solutions containing excess Na + . The ammonium and potassium salts contain anions in which the UO 2 2+ cations (pentagonal bipyramids) are unsymmetrically sandwiched between the PW 9 O 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 identities of the cations. Based on integrated P NMR spectra, the equilibrium constants for [Na 2 (UO 2 ) 2 (PW 9 O 34 ) 2 ] 12 - + 2M + ↔ [M 2 (UO 2 ) 2 (PW 9 O 34 ) 2 ] 12 - + 2Na + are 128 ± 12 (M = K + ) and 1.8 ± 0.8 (M = NH 4 + ). The requirement of the second cation in order to satisfy the mass-action expression for K eq, coupled with a W NMR spectrum that implies C 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 .