Electrochemical generation of high-valent manganese catalysts in aqueous solutions from the sandwich-type polyoxoanion [(MnIII(H2O))3(SbW9O33)2]9−

The selected sandwich-type polyoxometalate (POM), [(MnIII(H2O))3(SbW9O33)2]9−, was found to be stable in various aqueous media with a pH ranging from 2 to 7. Representative electrochemical behaviours are described at pH 6. The reduction of the MnIII centers is represented by two separate waves and generation of the MnII state is achieved quantitatively. Oxidation of the POM, starting from the MnII state goes up to the MnV state through MnIII and MnIV states. To our knowledge, this observation constitutes the first example of MnV state generation within a POM in an aqueous medium in the absence of any organic solvent. The MnV state can be reached also directly by starting the oxidation process from [(MnIII(H2O))3(SbW9O33)2]9− without any prior generation of MnII. The MnV state catalyses water oxidation. Detailed electrochemical quartz crystal microbalance (EQCM) studies indicate that a partial thin film deposition process occurs during the generation of the MnIV state at slow scan rate. The conditions for reaching a steady state of this film were assessed. Finally, the MnIV state has been demonstrated to be efficient in the electrocatalytic oxidation of NO. It allows also a separate detection of NO and NO2- in mixtures where the two substrates are present simultaneously.