Activation of tert-Butyl Hydroperoxide by Zr(IV) Stabilized by Polyoxotungstate Scaffolds

Zr-monosubstituted polyoxometalates (Zr-POMs) of the Keggin (Bu₄N)₈[{PW₁₁O₃₉Zr(μ-OH)}₂] (Zr-K), Lindqvist (Bu₄N)₆[{W₅O₁₈Zr(μ-OH)}₂] (Zr-L), and Wells-Dawson (Bu₄N)₁₁H₃[{P₂W₁₇O₆₁Zr(μ-OH)}₂] (Zr-WD) structures are capable of heterolytic activation of the environmentally benign oxidant tert-butyl hydroperoxide (TBHP) and catalyze epoxidation of alkenes and oxidation of alcohols to carbonyl compounds. Catalytic activity of corresponding Ti-POMs is much lower. Among Zr-POMs, Zr-K revealed higher epoxide yields. All Zr-POMs do not catalyze unproductive TBHP degradation, and epoxide yields with both aqueous and anhydrous TBHP are generally higher than with aqueous H₂O₂. Regioselectivity of the Zr-K-catalyzed limonene epoxidation with TBHP is different from that with H₂O₂: the more substituted and nucleophilic double bond is preferably epoxidized, pointing to an electrophilic oxygen transfer mechanism. The oxidation rates are first order in catalyst (Zr-K) and substrate (cyclooctene or cyclohexanol) and show a saturation behavior with increasing concentration of TBHP. Studies by HR-ESI-MS, ATR-FT-IR, and ³¹P NMR spectroscopic techniques implicated the formation of zirconium alkylperoxo species upon interaction of Zr-POMs with TBHP. HR-ESI-MS revealed the existence of monomeric and dimeric alkylperoxo complexes, [{PW₁₁O₃₉Zr}((CH₃)₃COO)]^4- and [{PW₁₁O₃₉Zr((CH₃)₃COO)}₂]^8-, with predomination of the former, which is most likely the active species responsible for the selective oxidations.