ABSTRACT
Each step is an overall two-electron reduction of MoVI centers, during which two of them are converted to MoV.
In terms of utilization as electrocatalysts, most studies have been related to the promotion of reductions of inorganic species. A means of employing POMs as oxidation catalysts is to modify them into transition metal-substituted analogues, TMSPs [31]. Using a reported procedure to modify phosphotungstic acid [32], we removed a tungsten center and its terminally bonded oxygen from the Keggin structure and replaced them with a center hypothesized to serve as an electrochemical catalyst, dirhodium acetate [33]. The resulting compound, Rh2POM, was of interest for two reasons. The molecular size of the TMSP suggested that as a dopant in a sol-gel material, it would be stable against leaching. The basis of this prediction is that the molecular size of the TMSP is greater than the width of the interstitial spaces in a microporous silica sol-gel (described below). As a result, it is trapped in pockets within the matrix and cannot diffuse through the pores. This model has been established for the immobilization of enzymes in sol-gel materials [34]. Second, the reversible electron transfer in POMs suggests that that these compounds will be effective electron-transfer mediators. In this regard, there is a virtual absence of major bond-length changes during redox of these compounds, so the electron transfer rates are rapid.
