Activation-controlled outer-sphere electron transfer reactions: reduction of heteropoly 11-tungstovanadophosphate and heteropoly 10-tungstodivanado phosphate by thiourea in aqueous acid medium

摘要

The kinetics of reduction of heteropoly 11 -tung-stovanadophosphate, [PV~VW_(11)O_(40)]~(4-), (HPA1) and heteropoly 10-tungstodivanadophosphate, [PV~VV~VW_(10)O_(40)]~(5-), (HPA2) by thiourea has been investigated in HClO4/ phthalate/acetate buffer solutions spectrophotometrically at 25 °C in aqueous medium. The stoichiometry of the reaction is 1:1 in both cases. The HP As are converted into the corresponding one-electron reduced heteropoly blues, namely, [PV~(IV)W_(11)O_(40)]~(5-) and [PV~(IV)V~VW_(10)O_(40)]~(6-), and thiourea is oxidised to formamidine disulphide. The reaction shows first-order dependence in both [HPA] and [thiourea] at constant pH. The rate-pH profile shows the participation of both the neutral and deprotonated forms of thiourea in the reaction. The reaction proceeds through an outer sphere electron transfer mechanism in which activation-controlled electron transfer is the rate-determining step. Self-exchange rate constants for the couples [PV~VW_(11)O_(40)]~(4-)/[PV~(IV)W_(11)-O_(40)]~(5-), [PV~VV~VW_(10)O_(40)]~(5-)/[PV~(IV)V~VW_(10)O_(40)]~(6-) and H2NC-SNH2/H2NCS~(·+)NH2 have been evaluated by Marcus theory.