Activation of H₂O₂ over Zr(IV). Insights from Model Studies on Zr-Monosubstituted Lindqvist Tungstates

Abstract

Zr-monosubstituted Lindqvist-type polyoxometalates (Zr-POMs), (Bu 4 N) 2 [W 5 O 18 Zr(H 2 O) 3 ] (1) and (Bu 4 N) 6 [{W 5 O 18 Zr(μ-OH)} 2 ] (2), have been employed as molecular models to unravel the mechanism of hydrogen peroxide activation over Zr(IV) sites. Compounds 1 and 2 are hydrolytically stable and catalyze the epoxidation of CC bonds in unfunctionalized alkenes and α,β-unsaturated ketones, as well as sulfoxidation of thioethers. Monomer 1 is more active than dimer 2. Acid additives greatly accelerate the oxygenation reactions and increase oxidant utilization efficiency up to >99%. Product distributions are indicative of a heterolytic oxygen transfer mechanism that involves electrophilic oxidizing species formed upon the interaction of Zr-POM and H 2 O 2 . The interaction of 1 and 2 with H 2 O 2 and the resulting peroxo derivatives have been investigated by UV−vis, FTIR, Raman spectroscopy, HR-ESI-MS, and combined HPLC-ICPatomic emission spectroscopy techniques. The interaction between an 17 O-enriched dimer, (Bu 4 N) 6 [{W 5 O 18 Zr(μ-OCH 3 )} 2 ] (2′), and H 2 O 2 was also analyzed by 17 O NMR spectroscopy. Combining these experimental studies with DFT calculations suggested the existence of dimeric peroxo species [(μ-η 2 :η 2 -O 2 ){ZrW 5 O 18 } 2 ] 6− as well as monomeric Zr-hydroperoxo [W 5 O 18 Zr(η 2 -OOH)] 3− and Zr-peroxo [HW 5 O 18 Zr(η 2 - O 2 )] 3− species. Reactivity studies revealed that the dimeric peroxo is inert toward alkenes but is able to transfer oxygen atoms to thioethers, while the monomeric peroxo intermediate is capable of epoxidizing CC bonds. DFT analysis of the reaction mechanism identifies the monomeric Zr-hydroperoxo intermediate as the real epoxidizing species and the corresponding α-oxygen transfer to the substrate as the rate-determining step. The calculations also showed that protonation of Zr-POM significantly reduces the free-energy barrier of the key oxygen-transfer step because of the greater electrophilicity of the catalyst and that dimeric species hampers the approach of alkene substrates due to steric repulsions reducing its reactivity. The improved performance of the Zr(IV) catalyst relative to Ti(IV) and Nb(V) catalysts is respectively due to a flexible coordination environment and a low tendency to form energy deep-well and low-reactive Zr-peroxo intermediates.

Alkene Epoxidation and Thioether Oxidation with Hydrogen Peroxide Catalyzed by Mesoporous Zirconium-Silicates

“…Therefore, in this case too, acid additives favor the formation of active Zr peroxo species (most likely, hydroperoxo ones [52]). This seems to be a general phenomenon that operates with different types of Zr-catalysts, including Zr-silicates and Zr-MOFs.…”

“…On the other hand, Zr-SiO 2 made through a molecular precursor method showed very low activity in both alkene and thioether oxidation [33,51]. Meanwhile, more recent works on Zr-substituted polyoxometalates (Zr-POM) [52] and Zr-based metalorganic frameworks (Zr-MOF) [53,54] proved that Zr-catalysts can serve as highly effective epoxidation catalysts. These recent findings have stimulated our interest in oxidation catalysis with Zr(IV).…”

Synthesis and oxidation catalysis of a difluoride-incorporated polyoxovanadate and isolation of active vanadium alkylperoxo species

“…The catalytic reaction mechanism of polyoxometalates can be discussed in more detail via spectroscopic, X-ray, and theoretical analyses owing to the high molecularity of polyoxometalates designed at an atomic and molecular level. 15–17 In particular, single-crystal X-ray analyses have revealed how reactants interact with the active sites of polyoxometalates, and the results strongly support the reaction mechanism. 18–22 Among various polyoxometalates, vanadium-containing polyoxotungstates, [PW 10 O 38 V 2 (OH) 2 ] 3− exhibit excellent catalytic performance for the oxidation of several organic substrates, such as sulphides, alkanes, alkenes, alkynes, and aromatics.…”

Contrasting Trivalent Lanthanide and Actinide Complexation by Polyoxometalates via Solution-State NMR

“…Finally, open-shell transition-metal substitution and lanthanide substitution provide nearly limitless opportunity for tailoring composition and structure in POM ligands. Studied applications relying on POMs include catalytic reactions, − luminescent or electrochemical sensors, medicine, , electronics, chemical warfare neutralizers, , novel memory devices, qubits, molecular magnets, and model compounds for reactions that occur at mineral–water interfaces. , …”