{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, Na2K6H14[(VO)6(α‐TeNb9O33)2] ⋅ 31H2O ⋅ 2.5 C2H8N2 (abbreviated as V6(TeNb9)2), where a reduced {V6} ring is sandwiched by two trivacant Keggin‐type {α‐TeNb9O33}. 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, Na5K7H4[(NbO)6(α‐TeNb9O33)2] ⋅ 17H2O (abbreviated as Nb6(TeNb9)2). Mechanism studies reveal that during the electrocatalytic oxidation process water is the only oxygen source and a key intermediate PhCH3S+⋅ is involved. A recyclable molecular electrocatalyst, sandwich‐type Na2K6H14[(VIVO)6(α‐TeNb9O33)2] ⋅ 31H2O ⋅ 2.5 C2H8N2 has been successfully assembled, which can efficiently promote the oxidation of sulfides to sulfoxides with high selectivity and Faraday efficiency.