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

Three Ru-induced structural interconversion polyoxometalates (POMs), Na 13 H 5 [Ru 4 (H 2 O) 2 (Cl) 2 (WO 2 ) 4 (AsW 9 O 33 ) 4 ]·43H 2 O ( 1 ), K 5 Na 9 H 8 [Ru 2 (WO 2 ) 4 (AsW 9 O 33 ) 4 ]·50H 2 O ( 2 ), and KNa 13 H 14 [(WO 2 ) 4 (AsW 9 O 33 ) 4 ]·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 {As 4 W 40 O 140 } 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.