{V₆ } Ring Sandwiched Polyoxoniobate as Molecular Electrocatalyst for Oxidant-Free Synthesis of Sulfoxides

Electrocatalytic oxidation of organic molecules to value-added chemicals has attracted recent attention. Although a series of transition metal based electrocatalytic materials have been developed, the lack of precise structure information generates great challenges in understanding the catalytic mechanism at a molecular level. Herein, we present the synthesis and characterization of a molecular electrocatalyst, Na₂ K₆ H₁₄ [(VO)₆ (α-TeNb₉ O₃₃ )₂ ] ⋅ 31H₂ O ⋅ 2.5 C₂ H₈ N₂ (abbreviated as V₆ (TeNb₉ )₂ ), where a reduced {V₆ } ring is sandwiched by two trivacant Keggin-type {α-TeNb₉ O₃₃ }. V₆ (TeNb₉ )₂ as heterogeneous electrocatalyst can selectively convert 95 % of thioanisole to sulfoxide with the Faraday efficiency up to 98 %. Notably, the important role of the embedded {V₆ } ring in the electrocatalytic oxidation was illustrated by comparing with {Nb₆ } ring sandwiched catalyst, Na₅ K₇ H₄ [(NbO)₆ (α-TeNb₉ O₃₃ )₂ ] ⋅ 17H₂ O (abbreviated as Nb₆ (TeNb₉ )₂ ). Mechanism studies reveal that during the electrocatalytic oxidation process water is the only oxygen source and a key intermediate PhCH₃ S⁺ ⋅ is involved.