{V 6 } 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, Na 2 K 6 H 14 [(VO) 6 (α-TeNb 9 O 33 ) 2 ] ⋅ 31H 2 O ⋅ 2.5 C 2 H 8 N 2 (abbreviated as V 6 (TeNb 9 ) 2 ), where a reduced {V 6 } ring is sandwiched by two trivacant Keggin-type {α-TeNb 9 O 33 }. V 6 (TeNb 9 ) 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 {V 6 } ring in the electrocatalytic oxidation was illustrated by comparing with {Nb 6 } ring sandwiched catalyst, Na 5 K 7 H 4 [(NbO) 6 (α-TeNb 9 O 33 ) 2 ] ⋅ 17H 2 O (abbreviated as Nb 6 (TeNb 9 ) 2 ). Mechanism studies reveal that during the electrocatalytic oxidation process water is the only oxygen source and a key intermediate PhCH 3 S + ⋅ is involved.