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

Three Ru-induced structural interconversion polyoxometalates (POMs), Na13H5[Ru-4(H2O)(2)(Cl)(2)(WO2)(4)(AsW9O33)(4)]center dot 43H(2)O (1), K5Na9H8[ Ru-2(WO2)(4)(AsW9O33)(4)]center dot 50H(2)O (2), and KNa13H14[(WO2)(4)(AsW9O33)(4)]center dot 34H(2)O (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.

Automated summary

Three Ru-induced structural interconversion polyoxometalates were synthesized and characterized. The catalytic performances of these compounds in oxidative coupling reactions were found to be strongly dependent on the element contents of the active sites. Compound 1 showed highly efficient photocatalysis, compound 2 exhibited relatively poor efficiency, and compound 3 had no significant contribution to the coupling reaction.