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-tungstovanadophosphate, [PVVW₁₁O₄₀]4−, (HPA1) and heteropoly 10-tungstodivanadophosphate, [PVVVVW₁₀O₄₀]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 HPAs are converted into the corresponding one-electron reduced heteropoly blues, namely, [PVIVW₁₁O₄₀]5− and [PVIVVVW₁₀O₄₀]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 [PVVW₁₁O₄₀]4−/[PVIVW₁₁O₄₀]5−, [PVVVVW₁₀O₄₀]5−/[PVIVVVW₁₀O₄₀]6− and H₂NCSNH₂/H₂NCS·+NH₂ have been evaluated by Marcus theory.