Iron-Substituted Dawson-Type Tungstodiphosphates: Synthesis, Characterization, and Single or Multiple Initial Electronation Due to the Substituent Nature or Position

Isomerically pure α 1 - and α 2 -[Fe(OH 2 )P 2 W 17 O 61 ] 7 - and α 2 [Fe(OH 2 )P 2 Mo 2 W 15 O 61 ] 7 - were synthesized as aqueous-soluble potassium salts. The iron-substituted complexes as well as their immediate lacunary precursors have been characterized by IR, UV−visible, and 31 P NMR spectroscopies. The acid−base equilibria were studied for the iron-substituted complexes. The hydroxo compound α 2 -[Fe(OH)P 2 W 17 O 61 ] 8 - has also been synthesized, and its elemental analysis is correct. Its further characterization and study will be published elsewere. For a given pH value, the comparisons of the cyclic voltammograms reveal the influence of the location of the iron-filled site in the framework. This influence is strong in the α 1 isomer and makes its electrochemistry clearly different from that of the α 2 isomer. This observation is ascribed mainly to the large framework distortion induced by the vacancy in the α 1 site, to subtle variations in the detailed protonation events which ensue, and to the nature of the iron substituent, which can be considered not to fill “completely” the vacancy. The distortion effect is found to favor the coalescence of waves, as also do lower and lower pH media. The combination of these parameters appears to be beneficial for the observation of multiple electronation processes on the first wave of heteropolyanions under very mild conditions.