Zirconium(IV)- and hafnium(IV)-containing polyoxometalates as oxidation precatalysts: Homogeneous catalytic epoxidation of cyclooctene by hydrogen peroxide

Abstract The homogeneous epoxidation of cis -cyclooctene with H 2 O 2 , catalyzed by various Zr/Hf-containing Keggin sandwich polyoxometalates (POMs), i.e., the 1:2 complexes [M(α-PW 11 O 39 ) 2 ] 10− (M = Zr, 1 and M = Hf, 2 ), the 4:2 complexes [[{M(H 2 O)} 2 {M(H 2 O) 2 } 2 (μ-OH) 3 (μ-OH) 2 ](α-1,2-PW 10 O 37 ) 2 ] 7− (M = Zr, 3 and M = Hf, 4 ), the 2:2 complexes [{M(μ-OH)(H 2 O)} 2 (α-PW 11 O 39 ) 2 ] 8− (M = Zr, 5 and M = Hf, 6 ), and the 3:2 complex [Zr 3 (μ 2 -OH) 3 (A-α-PW 9 O 34 ) 2 ] 9− ( 7 ), was examined. At least two different reaction types exist: (1) one brought about by the highly catalytically active Venturello complex [PO 4 {WO(O 2 ) 2 } 4 ] 3− , which was generated by reactions of the Zr/Hf-containing POMs with hydrogen peroxide, and (2) the reaction with active species formed on Zr/Hf clusters in the POMs. In the first type, the original sandwich structures are not maintained during the reactions (as shown in 1 , 2 , 3 , and 4 ), but in the second type, the sandwich structures are maintained (as seen in 5 and 6 ). 7 was inactive. The reaction, ( 5  +  cis -cyclooctene) + H 2 O 2 , was explicitly influenced by changing the reaction time of 5 and olefin, before addition of H 2 O 2 , indicating a significant interaction between 5 and olefin, such as coordination of olefin to the POM. Thus, for homogeneous olefin epoxidation by hydrogen peroxide catalyzed by di-Zr/Hf clusters, a new mechanism is proposed through olefin-coordinated species to di-Zr/Hf centers.