Controlled Assembly of Ru-Containing Polyoxometalates for Photocatalytic Activity of the Primary Amine Coupling Reaction.

Three Ru-induced structural interconversion polyoxometalates (POMs), NaH[Ru(HO)(Cl)(WO)(AsWO)]·43HO (), KNaH[Ru(WO)(AsWO)]·50HO (), and KNaH[(WO)(AsWO)]·34HO (), were successfully synthesized and thoroughly characterized. Interconversion of structures was accomplished by changing the number of active sites for compounds . All three compounds contain one {AsWO} unit, showing similar structural characteristics except for the active center number (Ru). Interestingly, compound [turnover number (TON)= 486; turnover frequency (TOF)= 20 h] showed highly efficient photocatalysis in achieving oxidative coupling of primary amines. Compound (TON = 406, TOF = 17 h) was also found to promote the oxidative coupling with relatively poor efficiency; however, compound (TON = 178; TOF = 7.4 h) had no obvious contribution to the coupling reaction system, and a chain of evidence indicates that the catalytic performances are strongly dependent on element contents of active sites. Furthermore, the Ru-containing POM-based photocatalysts are conveniently recyclable and reusable during the photocatalytic processes. This study demonstrates the possibility of tuning the catalytic efficiency and stability of POM-based photocatalysts by well designing and controlling their structures. The possible reaction mechanism for the photocatalysis synthesis of imine product is also proposed based on experimental studies.