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

• The epoxidation with H2O2 catalyzed by Zr/Hf-containing POMs was examined.
• At least two different reaction types exist.
• The reactions with 1–4 were brought about by the Venturello complex.
• The reactions with 5–6 proceeded via active species formed on di-Zr/Hf clusters.
• A new mechanism is proposed through the olefin-coordinated species to Zr/Hf atoms.

The homogeneous epoxidation of cis-cyclooctene with H2O2, catalyzed by various Zr/Hf-containing Keggin sandwich polyoxometalates (POMs), i.e., the 1:2 complexes [M(α-PW11O39)2]10− (M = Zr, 1 and M = Hf, 2), the 4:2 complexes [[{M(H2O)}2{M(H2O)2}2(μ-OH)3(μ-OH)2](α-1,2-PW10O37)2]7− (M = Zr, 3 and M = Hf, 4), the 2:2 complexes [{M(μ-OH)(H2O)}2(α-PW11O39)2]8− (M = Zr, 5 and M = Hf, 6), and the 3:2 complex [Zr3(μ2-OH)3(A-α-PW9O34)2]9− (7), was examined. At least two different reaction types exist: (1) one brought about by the highly catalytically active Venturello complex [PO4{WO(O2)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) + H2O2, was explicitly influenced by changing the reaction time of 5 and olefin, before addition of H2O2, 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.

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