Effect of the Polyanion Structure on the Mechanism of Alcohol Oxidation with H₂O₂ Catalyzed by Zr-Substituted Polyoxotungstates

Zr-monosubstituted polyoxometalates (Zr-POMs) of the Lindqvist (Bu₄N)₆[{W₅O₁₈Zr(μ-OH)}₂] (<b>1</b>), Keggin (Bu₄N)₈[{PW₁₁O₃₉Zr(μ-OH)}₂] (<b>2</b>), and Wells-Dawson (Bu₄N)_11.3K_2.5H_0.2[{P₂W₁₇O₆₁Zr}₂(μ-OH)₂] (<b>3</b>) structures catalyze oxidation of alcohols using aqueous hydrogen peroxide as an oxidant. With 1 equiv of H₂O₂ and 1 mol % of Zr-POM, selectivity toward aldehydes and ketones varied from good to excellent, depending on the alcohol nature. Catalytic activity and attainable substrate conversions strongly depended on the Zr-POM structure and most often decreased in the order <b>1</b> > <b>2</b> ≫ <b>3</b>. The reaction mechanism was probed using a test substrate, cyclobutanol, radical and ¹O₂ scavengers, and kinetic and spectroscopic (attenuated total reflectance-Fourier transform infrared (ATR-FT-IR), ³¹P NMR and electrospray ionization-mass spectrometry (ESI-MS)) tools. The results point to heterolytic alcohol oxidation in the presence of <b>1</b> and <b>2</b> and homolytic alcohol oxidation in the presence of <b>3</b>. Kinetic and spectroscopic studies implicated an oxidation mechanism that involves both alcohol and peroxide binding to <b>2</b> followed by an inner-sphere heterolytic H-abstraction from the α-C-H bond by the Zr-hydroperoxo group, leading to a carbonyl compound. The unique capability of <b>1</b> to generate ¹O₂ upon interaction with H₂O₂ complicates the reaction kinetics and improves the product yield. Spectroscopic studies coupled with stoichiometric experiments unveiled that dimeric monoperoxo {Zr₂(μ-η²:η²-O₂)} and monomeric hydroperoxo {Zr(η²-OOH)} species accomplish the transformation of alcohols to carbonyl compounds.