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 exte nt of significant association between heteropoly tungstate species and monoatomic metallic counterions (e.g., alkali metal cations). In diam agnetic [alpha(2)-P2W17O61-Th4+-alpha(2)-O61W17P2](16-) the Th linkage is a flexible conducting bridge between the two substituted Wells-Daw son 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 sa me electrons exchange ata much slower rate through a conducting bridge . If a solution of the parent oxidized complex is reduced by 2 electro ns per anion, the added electron pairs are distributed among the Wells -Dawson entities present, yielding an equilibrium mixture of oxidized, Celectron-reduced, and 2-electron-reduced complexes. The P-31 NMR spe ctrum of his 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 an ti and syn conformations. When large firmly hydrated Li+ is the only c ounterion, the complex remains in the anti conformation and the exchan ge peak is always exactly midway between the peaks for the oxidized an d Celectron-reduced species. When even small amounts of other alkali m etal cations are present, they coordinate in a pocket between the hete ropoly lobes of the Th complex, holding it in the syn conformation. In the syn conformation, the P atoms in one heteropoly lobe can sense wh ether or not the other lobe. contains added electrons. This is manifes ted by the exchange peak moving off-center between the signals from th e oxidized and 4-electron-reduced species. The more concentrated the n on-ii 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 f or all the species when K(H2O)(m)(+) progressively displaces Li(H2O)(n )(+) attached to the surface of the heteropoly entities. It is suggest ed that electron exchange through the Th and the K coordinated in the pocket adds to the stability of the syn-K Kozik complex.