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

ABSTRACT

Three Ru-induced structural interconversion polyoxometalates (POMs), Na13H5[Ru4(H2O)2(Cl)2(WO2)4(AsW9O33)4]·43H2O (1), K5Na9H8[Ru2(WO2)4(AsW9O33)4]·50H2O (2), and KNa13H14[(WO2)4(AsW9O33)4]·34H2O (3), were successfully synthesized and thoroughly characterized. Interconversion of structures was accomplished by changing the number of active sites for compounds 1-3. All three compounds contain one {As4W40O140} unit, showing similar structural characteristics except for the active center number (Ru). Interestingly, compound 1 [turnover number (TON)= 486; turnover frequency (TOF)= 20 h-1] showed highly efficient photocatalysis in achieving oxidative coupling of primary amines. Compound 2 (TON = 406, TOF = 17 h-1) was also found to promote the oxidative coupling with relatively poor efficiency; however, compound 3 (TON = 178; TOF = 7.4 h-1) 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.