Synthesis, characterization and electrochemistry of the novel Dawson-type tungstophosphate [H4PW18O62]7- and first transition metal ions derivatives

Following the synthesis of pure [H4PW18O62](7-) (PW18), its derivatives monosubstituted with M (M = Mo-VI, V-IV, Mn-II, Fe-III, Co-II, Ni-II Cu-II and Zn-II) were obtained. All compounds were characterized by elemental analysis, IR, UV/visible and P-31 NMR spectroscopy. Their cyclic voltammetry properties were studied as a function of pH and systematically compared with those of their analogs derived from the symmetrical species, [P2W18O62](6-)(P2W18). Comparison of the two unsubstituted precursors revealed that the merging of the first two waves of the monophosphate occurred in a less acidic medium than for the diphosphate. The observations point to the higher basicity of the reduced forms of PW18 compared with those of P2W18. The fingerprint pattern observed for alpha(2)-P2W17M derivatives in media of pH = 3 consisted of the splitting of the third W redox system into two one-electron closely spaced waves which is in contrast to the same system in alpha(1)-P2W17M. This peculiarity was also obtained for several of the present alpha(2)-PW17M systems in media of pH = 3 and confirmed that alpha(2)-substituted derivatives were indeed formed. The absence of this peculiar behavior in some other derivatives is consistent with smooth variations of acid-base properties from one derivative to the next. The electrocatalytic properties of all the compounds are illustrated by the reduction of nitrite by reduced PW18 and of nitrate by reduced alpha(2)-PW17Cu. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.