Thioether oxidation with tert-butyl hydroperoxide catalysed by Zr(IV)-substituted polyoxometalates

Area of Science:

• Inorganic Chemistry
• Catalysis
• Materials Science

Background:

• Polyoxometalates (POMs) are versatile inorganic clusters with tunable properties.
• Zirconium-substituted POMs (Zr-POMs) offer potential for catalytic applications.
• Oxidation of thioethers to sulfoxides and sulfones is an important chemical transformation.

Purpose of the Study:

• To investigate the catalytic activity of Zr-POMs with different structures (Lindqvist, Keggin, Wells-Dawson) for thioether oxidation.
• To explore the influence of Zr-POM structure on catalytic activity and selectivity.
• To elucidate the reaction mechanism of thioether oxidation catalyzed by Zr-POMs.

Main Methods:

• Synthesis and characterization of Zr-POMs with Lindqvist, Keggin, and Wells-Dawson structures.
• Catalytic oxidation of thioethers using tert-butyl hydroperoxide (TBHP) as the oxidant.
• Kinetic, isotopic (H218O), and spectroscopic studies to probe the reaction mechanism.
• Characterization of intermediate alkylperoxo complexes using elemental analysis, HR-ESI-MS, ATR-FT-IR, and NMR spectroscopy.

Main Results:

• Zr-POMs efficiently catalyze thioether oxidation with TBHP, outperforming Ti- and Nb-substituted POMs.
• Catalytic activity and selectivity are highly dependent on the Zr-POM structural type (Keggin, Wells-Dawson, Lindqvist).
• Reaction rate decreases in the order Zr-K ≈ Zr-WD > Zr-L.
• Zr-K predominantly yields sulfoxides, while Zr-L favors sulfones with 1 equiv. TBHP.
• Monomeric alkylperoxo complexes were formed and characterized.
• Both Zr-K and Zr-L alkylperoxo complexes show activity via an electrophilic oxygen transfer mechanism.
• Under turnover conditions, sulfoxide binding to Zr-L influences selectivity towards sulfone formation.
• Solvent nature significantly impacts the oxygen transfer mechanism and selectivity.

Conclusions:

• Zr-POMs are effective catalysts for thioether oxidation, with structural type dictating activity and selectivity.
• The reaction proceeds via an electrophilic oxygen transfer mechanism, influenced by intermediate complex formation and solvent effects.
• This study highlights the potential of tailored Zr-POMs for selective oxidation reactions.