Substituent effect in the electrochemistry of Dawson-type tungstodiphosphates: Some strategic parameters for apparently multiple electronation on the first wave of heteropolyanions

The lacunary species alpha(1) and alpha(2)P(2) square W17C6110- and the corresponding iron-substituted derivatives have been studied and compared. For a given pH value, the cyclic voltammograms are influenced by the location of the vacant W site in the framework. The influence is strong in the alpha(1) but hardly exists in the alpha(2) substituted derivative. This observation is ascribed mainly to the large framework distortion induced by the vacancy in the alpha(1) site. The effect is found to favour coalescence of waves. Lower and lower pH media also favour the merging of waves. The combination of the effects of these two parameters is shown to be beneficial for the observation of multiple electronation on the first wave of heteropolyanions in very mild conditions. Spectroelectrochemistry experiments support completely the conclusion that conditions can be found in which the iron and heteropolyanion waves mix up. Another series of heteropolyanions, alpha(2)P(2) square W15Mo2O6110-, alpha(2)P(2)W(15)Mo(3)O(62)(6-) and alpha(2)P(2)W(15)Mo(2)O(61) Fe(OH2)(7-), has permitted the effects of these parameters to be confirmed and also gives a preliminary insight into their interplay with the nature, the number and locations of the substituents in the tungsten framework. (C) 1998 Elsevier Science B.V.