Reactivity of platinum metal with organic radical anions from metal to negative oxidation states.
Résumé
The reaction of platinum metal with an organic molecular radical anion leads to the formation of iono-metallic phases where Pt exists under negative oxidation states. This puzzling transformation of a "noncorrodible metal" was examined using localized electrochemical techniques in dimethylformamide containing different tetra-alkylammonium salts chosen as test systems. Our experiments demonstrate that the platinum metal is locally reduced as soon as the Pt faces relatively moderate reducing conditions, for example, when the Pt is used as a negative electrode or when the metal is in the presence of a reducing agent such as an organic radical anion. Scanning electrochemical microscopy (SECM) analysis, current-distance curves, and transient mode responses provide detailed descriptions of the reactivity of Pt to form negative oxidation states (the key step is the reaction of the metal with a molecular reducing agent), of the insulating nature of the "reduced" solid phases of the thermodynamics and kinetics conditions of the Pt conversion. The passage from the conductor to insulator states controlled the spatial development of the reaction that always remains in competition with the other "natural" roles of a metallic electrode. Formally, the phenomena can be treated by analogy with the C. Amatore's model previously developed for the mediated reduction of the poly(tetrafluoroethylene). Consequences of this general reactivity of Pt are discussed in view of a wide utilization of this metal in reductive conditions and the possible applications of such processes in the micropatterning of metallic surfaces.