Effects of Counterions in Heteropoly Electrolyte Chemistry. 1. Evaluations of Relative Interactions by NMR on Kozik Salts.
Evidence is accumulating for the major influence of the identities of counterions on syntheses and properties of various heteropoly anions. An easy and convenient NMR method is presented for evaluating the extent of significant association between heteropoly tungstate species and monoatomic metallic counterions (e.g., alkali metal cations). In diamagnetic [alpha(2)-P(2)W(17)O(61)-Th(4+)-alpha(2)-O(61)W(17)P(2)](16-), the Th linkage is a flexible conducting bridge between the two substituted Wells-Dawson entities. A Kozik complex is a heteropoly species containing added delocalized "blue" electrons exchanging rapidly among several atoms (the belt W atoms of one Wells-Dawson unit in this case) while the same electrons exchange at a much slower rate through a conducting bridge. If a solution of the parent oxidized complex is reduced by 2 electrons per anion, the added electron pairs are distributed among the Wells-Dawson entities present, yielding an equilibrium mixture of oxidized, 4-electron-reduced, and 2-electron-reduced complexes. The (31)P NMR spectrum of this mixture consists, for each of the two structural types of P atoms, of three lines: one for each kind of complex. The signal for the 2-electron-reduced species, which contains a pair of electrons in one heteropoly entity and none in the other, is a coalesced exchange peak located between the other two signals. These Th complexes have anti and syn conformations. When large firmly hydrated Li(+) is the only counterion, the complex remains in the anti conformation and the exchange peak is always exactly midway between the peaks for the oxidized and 4-electron-reduced species. When even small amounts of other alkali metal cations are present, they coordinate in a pocket between the heteropoly lobes of the Th complex, holding it in the syn conformation. In the syn conformation, the P atoms in one heteropoly lobe can sense whether or not the other lobe contains added electrons. This is manifested by the exchange peak moving off-center between the signals from the oxidized and 4-electron-reduced species. The more concentrated the non-Li counterion, the greater the displacement of the coalesced signal. The order of effectiveness in the displacement is Rb > K > Na. These observations are explained, as are small changes in chemical shifts for all the species when K(H(2)O)(m)()(+) progressively displaces Li(H(2)O)(n)()(+) attached to the surface of the heteropoly entities. It is suggested that electron exchange through the Th and the K coordinated in the pocket adds to the stability of the syn-K Kozik complex.
Kirby
James F.
JF
Department of Chemistry, Georgetown University, Washington, D.C. 20057.
Baker
Louis C. W.
LC