On the basis of the characters that PW₁₁O₃₉Cr^III(H₂O)^4－ is oxidized or reduced in the inner-sphere electron transfer manner and that the sixth coordinate water bound to Cr(III) center is readily replaced by other ligand, the electrocatalytic oxidation of 4-methylpyridine by PW₁₁O₃₉Cr^III(H₂O)^4－ was investigated. Vis spectrum confirmed the existence of PW₁₁O₃₉Cr^III(NC₆H₇)^4－ due to a ligand exchange reaction between 4-methylpyridine and H₂O in PW₁₁O₃₉Cr^III(H₂O)^4－. Cyclic voltammetric and potential-controlled electrolyte experiments indicated that the molecule of 4-methylpyridine modified onto the Cr(III) active center was oxidized via a two electron transfer to form pyridine-4-methanol, pyridine-4-carboxaldehyde and 4-picolinic acid in turn. A novel electrocatalytic mechanism involving in intramolecular electron transfer was proposed, and thus a possible application to the electrocatalytic oxidation of substrate by transition metal-substituted heteropolyanions was provided according to this model.

Key words: Cr(III)-substituted heteropolytungstate, electrocatalytic oxidation, 4-methylpyridine, cyclic voltammetry