Cyclic Voltammetric Studies on [SMo12O40]2- and [SMo12O40]3- at Macrodisk Electrodes in Acetonitrile With and Without Added Supporting Electrolyte

The polyoxomolybdate [(C6H13)4N]2[SMo12O40] while soluble in acetonitrile (>2.5 g L−1) precipitates from this solvent when tetraalkylammonium salts are added as the supporting electrolyte at the usual 0.1 M concentration levels used in electrochemical investigations. Although conventional conditions cannot be used, this study demonstrates that application of cyclic voltammetry in the absence of added supporting electrolyte is viable for studies on the [SMo12O40]2–/3– and [SMo12O40]3–/4– reduction processes. Thus, there is excellent agreement between voltammetric experiments and theory based on mass transport by both migration and diffusion, after problems associated with Ohmic drop are overcome by incorporation of the uncompensated resistance into the theory. Unlike the [SMo12O40]2– salts, tetralkylammonium salts of [SMo12O40]3– are soluble in acetonitrile even in the presence of added supporting electrolyte. Consequently, the oxidation of [SMo12O40]3– to [SMo12O40]2– in the presence of 0.1 M (C6H13)4NClO4 electrolyte has been examined by both cyclic voltammetry and electrochemical quartz crystal microbalance methods. As a result of the insolubility of [SMo12O40]2– in the presence of added supporting electrolyte, electrolytic production of this species under conventional conditions generates a precipitation process. The resulting complexities are avoided by employing high scan rates that outrun the precipitation process, or by studying the oxidation process in the absence of supporting electrolyte.