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 th e corresponding iron-substituted derivatives have been studied and com pared. For a given pH value, the cyclic voltammograms are influenced b y 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 f avour the merging of waves. The combination of the effects of these tw o parameters is shown to be beneficial for the observation of multiple electronation on the first wave of heteropolyanions in very mild cond itions. Spectroelectrochemistry experiments support completely the con clusion that conditions can be found in which the iron and heteropolya nion waves mix up. Another series of heteropolyanions, alpha(2)P(2) sq uare W15Mo2O6110-, alpha(2)P(2)W(15)Mo(3)O(62)(6-) and alpha(2)P(2)W(1 5)Mo(2)O(61) Fe(OH2)(7-), has permitted the effects of these parameter s to be confirmed and also gives a preliminary insight into their inte rplay with the nature, the number and locations of the substituents in the tungsten framework. (C) 1998 Elsevier Science B.V.